14TH EDITION
Shlomo Melmed, MB ChB, MACP
Executive Vice President and Dean of the Medical Faculty
Cedars-Sinai Medical Center
Los Angeles, California
Richard J. Auchus, MD, PhD
Professor
Departments of Pharmacology and Internal Medicine
Division of Metabolism, Endocrinology, and Diabetes
University of Michigan
Endocrinology Service Chief
Ann Arbor VA Healthcare System
Ann Arbor, Michigan
Allison B. Goldfine, MD
Associate Physician, Division of Endocrinology, Diabetes, and 
Hypertension
Brigham a

docrinology, Diabetes, and 
Brigham and Women’s Hospital 
Lecturer, Part-Time
Harvard Medical School
Boston, Massachusetts
Director, Translational Medicine Cardiometabolic Disease
Novartis Institute of Biomedical Research
Cambridge, Massachusetts
Williams Textbook of 
Endocrinology
Ronald J. Koenig, MD, PhD
Professor
Department of Internal Medicine
Clifford J. Rosen, MD
Professor of Medici

Michigan
Professor of Medicine
Tufts University School of Medicine
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iii
John C. Achermann, MB, MD, PhD
Wellcome Trust Senior Research Fellow in 
Clinical Science and Professor of Pediatric 
Department of Genetics and Genomic 
Medicine
UCL GOS Insititute of Child Health
University College London
London, Great Britain
Andrew J. Ahmann, MD, MS
Division of Endocrinology, Diabetes, and 
Clinical Nutrition
Director, Harold Schnitzer Diabetes Health 
Center
Oregon Health & Science Un

Diabetes Health 
Center
Oregon Health & Science University
Portland, Oregon
Lloyd P. Aiello, MD, PhD
Professor of Ophthalmology
Director, Beetham Eye Institute
Joslin Diabetes Center
Erik K. Alexander, MD
Chief, Thyroid Section
Brigham and Women’s Hospital
Rebecca H. Allen, MD, MPH
Associate Professor
Department of Obstetrics and Gynecology
Warren Alpert Medical School of Brown 
Univer

logy
University
Providence, Rhode Island
Bradley D. Anawalt, MD
Professor and Vice Chair
Department of Medicine
University of Washington
Seattle, Washington
Mark S. Anderson, MD, PhD
Professor
Diabetes Center
University of California, San Francisco
San Francisco, California
Ana María Arbeláez, MD, MSCI
Department of Pediatrics
Washington University School of Medicine
Pediatrician, St. Louis Children’s Hospital
St. Louis, Missouri
Mark A.

is Children’s Hospital
Mark A. Atkinson, PhD
American Diabetes Association Eminent 
Scholar for Diabetes Research
Departments of Pathology and Pediatrics
Director, University of Florida Diabetes 
Institute
Gainesville, Florida
Professor
Departments of Pharmacology and Internal 
Medicine
Division of Metabolism, Endocrinology, and 
Diabetes
Endocrinology Section Chief
Jennifer M

A Healthcare System
Jennifer M. Barker, MD
University of Colorado
Aurora, Colorado
Rosemary Basson, MD, FRCP(UK)
Clinical Professor
Department of Psychiatry
University of British Columbia
Director, University of British Columbia 
Sexual Medicine Program
British Columbia Centre for Sexual Medicine
Vancouver, British Columbia, Canada
Contributors
iv
iv
Sarah L. Berga, MD
Professor and Director
Division of Reproductive En

Division of Reproductive Endocrinology and 
Infertility
University of Utah School of Medicine
Salt Lake City, Utah
Sanjay K. Bhadada, MBBS, MD, DM
Professor
Department of Endocrinology
Nehru Hospital
Post Graduate Institute of Medical Education 
and Research
Chandigarh, India
Arti Bhan, MD
Division Head, Endocrinology, Diabetes, 
Bone and Mineral Disorders
Henry Ford Health System
Detroit, Michigan
Shalender Bhasin, MB, BS
Direc

Director, Research Program in Men’s Health: 
Aging and Metabolism
Director, Boston Claude D. Pepper Older 
Americans Independence Center
Dennis M. Black, PhD
Department of Epidemiology and Biostatistics
Andrew J.M. Boulton, MD, FACP, FRCP
Professor
Centre for Endocrinology and Diabetes
University of Manchester
Manchester, Great Britain

niversity of Manchester
Visiting Professor
Division of Endocrinology, Metabolism, and 
Diabetes
University of Miami
Miami, Florida
Glenn D. Braunstein, MD
Professor of Medicine Emeritus
The David Geffen School of Medicine at 
University of California, Los Angeles
Annabelle Brennan, MBBS, LLB (Hons)
Department of Obstetrics and Gynaecology
Royal Women’s Hospital
Melbourne, Australia
Gregory A. Bren

en’s Hospital
Gregory A. Brent, MD
Professor of Medicine and Physiology
Chief, Division of Endocrinology, Diabetes, 
and Metabolism
F. Richard Bringhurst, MD
Physician and Associate Professor of Medicine
Endocrine Unit
Massachusetts General Hospital
Juan P. Brito, MD, MS
Division of Endoc

Professor
Division of Endocrinology
Knowledge and Evaluation Research Unit in 
Mayo Clinic
Rochester, Minnesota
Todd T. Brown, MD, PhD
Professor of Medicine and Epidemiology
Metabolism
Johns Hopkins University
Baltimore, Maryland
Michael Brownlee, MD
Anita and Jack Saltz Chair in Diabetes 
Research Emeritus
Professor Emeritus, Medicine and Pathology
Associate Director for Biomedical Sciences 
Emeritus
Einstein Diabete

for Biomedical Sciences 
Emeritus
Einstein Diabetes Research Center
Albert Einstein College of Medicine
Bronx, New York
Serdar E. Bulun, MD
JJ Sciarra Professor of Obstetrics and 
Gynecology and Chair
Northwestern University Feinberg School of 
Medicine
Chicago, Illinois
v
v
David A. Bushinsky, MD
Professor of Medicine and of Pharmacology 
and Physiology
University of Rochester School of Medicine
Rochester, New York
Chri

hester School of Medicine
Christin Carter-Su, ScB, PhD
The Anita H. Payne Distinguished University 
Professor of Physiology
The Henry Sewall Collegiate Professor of 
Physiology
Professor of Molecular and Integrative 
Professor of Internal Medicine
University of Michigan Medical School
Associate Director
Michigan Diabetes Research Center
Yee-Ming Chan, MD, PhD
Associate Physician
Boston Children’

iatrics
Boston Children’s Hospital
Assistant Professor of Pediatrics
Ronald Cohen, MD
Department of Medicine 
University of Chicago
David W. Cooke, MD
Johns Hopkins University School of Medicine
Mark E. Cooper, AO, MB BS, PhD, FRACP
Professor and Head
Department of Diabetes
Central Clinical School
Monash University
Melbourne

entral Clinical School
Melbourne, Australia 
Ewerton Cousin, MSc
Postgraduate Program in Epidemiology
Universidade Federal do Rio Grande do Sul
Porto Alegre, Brazil
Philip E. Cryer, MD
Physician
Barnes-Jewish Hospital
Eyal Dassau, PhD
Director, Biomedical Systems Engineering 
Research Group
Senior Research Fellow in Biomedical 
Engineering in the Harvard John A. 
P

 Biomedical 
Paulson School of Engineering and Applied 
Sciences
Harvard University
Mehul T. Dattani, MD, MBBS, ­DCH, 
FRCPCH, FRCP
Professor
Department of Paediatric Endocrinology
Great Ormond Street Hospital for Children 
NHS Foundation Trust
Genetics and Genomic Medicine Programme
University College London Institute of Child 
Health
Francisco J.A. de Paula, MD, PhD
Associate Professor of Endocrinology and 
Meta

PhD
Ribeirão Preto Medical School
University of São Paulo
Ribeirao Preto, Brazil
Marie B. Demay, MD
Physician and Professor of Medicine
Sara A. DiVall, MD
Departments of Pediatrics
vi
vi
Bruce B. Duncan, MD, MPH, PhD
Dep

vi
Department of Social Medicine and 
School of Medicine
Eva L. Feldman, MD, PhD
Russell N. DeJong Professor of Neurology
Director, Program for Neurology Research 
and Discovery
Ele Ferrannini, MD
Institute of Clinical Physiology
National Research Council
Pisa, Italy
Heather 

ogy
Heather A. Ferris, MD, PhD
Assistant Professor of Medicine
University of Virginia
Charlottesville, Virginia
Sebastiano Filetti, MD
Full Professor of Internal Medicine
Department of Translational and Precision 
Medicine
Sapienza University of Rome
Rome, Italy
Laercio J. Franco, MD, MPH, PhD
Professor of Social Medicine
Ribeirão Preto Medical School–University of 
São Paulo
Ribeirão Preto, Brazil
Evelien F. Gevers, MD, PhD
Department of Pediatric Endocrino

 Gevers, MD, PhD
Department of Pediatric Endocrinology
Royal London Children’s Hospital
Barts Health NHS Trust
Centre for Endocrinology
William Harvey Research Institute
Queen Mary University of London
London, Great Britian
Ezio Ghigo, MD
Professor of Endocrinology
Division of Endocrinology, Diabetology, and 
University of Turin 
Turin, Italy
Ira J. Goldberg, MD
Clarissa and Edgar Bronfman Jr. Professor
New York University School of Medicine
Director, Division of Endocrinology, Diabet

dicine
Director, Division of Endocrinology, Diabetes, 
New York University Langone Health
New York, New York
Associate Physician, Division of Endocrinology, 
Diabetes, and Hypertension
Harvard Medical School 
Director, Translational Medicine 
Cardiometabolic Disease
Peter A. Gottlieb, MD
Professor

A. Gottlieb, MD
Professor
Steven K. Grinspoon, MD
Chief, Metabolism Unit
Sabine E. Hannema, MD, PhD
Paediatric Endocrinologist
Department of Paediatrics
Leiden University Medical Centre
Leiden, The Netherlands
Erasmus Univeristy Medical Centre
Rotterdam, The Netherlands
Frances J. Hayes, MB BCh, B

erdam, The Netherlands
Frances J. Hayes, MB BCh, BAO
Associate Professor of Medicine
Clinical Director, Endocrine Division
vii
vii
Martha Hickey, MD, BA(Hons), MSc, 
MBChB, FRCOG, FRANZCOG
Professor
University of Melbourne
Joel. N. Hirschhorn, MD, PhD
Chief, Division of Endocrinology
Concordia Professor of Pediatrics and 
Professor of Genetics
Har

essor of Pediatrics and 
Ken Ho, MD, FRACP, FRCP (UK), 
FAHMS
Emeritus Professor
St. Vincent’s Hospital
Garvan Institute of Medical Research
University of New South Wales
Sydney, Australia
Anthony Hollenberg, MD
Sanford I. Weill Chair
Joan and Sanford I. Weill Department of 
Medicine
Physician-in-Chief
New York Presbyterian/Weill Cornell Medical 
Center
Ieuan A. Hughes, MD, MA
Emeritus Pro

ork, New York
Emeritus Professor of Paediatrics
University of Cambridge
Cambridge, Great Britain
C. Ronald Kahn, MD
Chief Academic Officer
Mary K. Iacocca Professor of Medicine
Ursula Kaiser, MD, FACP
and Hypertension
George W. Thorn, MD, Distinguished 
Professor in Endocrinology
Brigham an

in Endocrinology
Harshini Katugampola, PhD, BSc, MBBS, 
MRCPCH, MSc
Health
Andrew M. Kaunitz, MD, FACOG
Professor and Associate Chairman
Department of Obstetrics and Gyne

sociate Chairman
University of Florida College of Medicine–
Jacksonville
Jacksonville, Florida
Professor
Diabetes
Peter A. Kopp, MD
Molecular Medicine
Center for Genetic Medicine
Northwestern University
Feinberg School of Medicine

ity
Henry M. Kronenberg, MD
Lori Laffel, MD, MPH
Chief, Pediatric, Adolescent and Young Adult 
Section
Senior Investigator and Head, Section on 
Clinical, Behavioral and Outomes Research
Professor of Pediatrics
viii
viii
Steven W.J. Lamberts, MD, PhD
Professor of Inter

i
Erasmus Medical Center
Fabio Lanfranco, MD, PhD
Department of Medical Sciences
University of Turin
P. Reed Larsen, MD, FRCP
Senior Physician
Sophie Leboulleux, MD, PhD
Department of Nuclear Me

ophie Leboulleux, MD, PhD
Department of Nuclear Medicine and 
Endocrine Oncology
Gustave Roussy
Villejuif, France
Ronald M. Lechan, MD, PhD
Tufts Medical Center
Amit R. Majithia, MD
Assistant Professor
Departments of Medicine and Pediatrics
University of California San Diego School of 
Medicine
La Jolla, California
Spyridoula Maraka, MD, MSc
Division of Endocrinology and M

fessor of Medicine
Division of Endocrinology and Metabolism
University of Arkansas for Medical Sciences
Central Arkansas Veterans Healthcare System
Little Rock, Arkansas
Eleftheria Maratos-Flier, MD
Professor Emerita
Beth Israel Deaconess Medical Center
Director, Translation Medicine
Cardiovascular-Metabolic Disease
N

lation Medicine
Novartis Institutes of Biomedical Research
Andrea Mari, PhD
Institute of Neuroscience
Padua, Italy
Alvin M. Matsumoto, MD
University of Washington School of Medicine
Dayna E. McGill, MD
Research Associate, Section on Clinical, 
Behavioral, and Outcomes Research
Pediatric Endocrinologist, Pediatric, 
Adolescent, and Young Adult Section
Joslin Diabetes Cente

ent, and Young Adult Section
Instructor of Pediatrics
Executive Vice President and Dean of the 
Medical Faculty
Victor Montori, MD, MS
Nutrition
Martin G. Myers, Jr., MD, PhD
Professor
Department of

tin G. Myers, Jr., MD, PhD
Professor
ix
ix
John D.C. Newell-Price, MA, PhD, FRCP
Professor of Endocrinology, Oncology, and 
The Medical School
University of Sheffield
Sheffield, England
Paul J. Newey, MBChB (Hons), BSc (Hons), 
DPhil, FRCP
Senior Lecturer in Endocrinology
Division of Molecular and Clinical Medicine
Jacqui Wood Cancer Centre
Ninewells Hospital and Medical School
University of Dundee

s Hospital and Medical School
Dundee, Scotland
Joshua F. Nitsche, MD, PhD
Division of Maternal-Fetal Medicine
Wake Forest School of Medicine
Winston Salem, North Carolina
Kjell Öberg, MD, PhD
Professor
Department of Endocrine Oncology
University Hospital
Uppsala, Sweden
David P. Olson, MD, PhD
Brian T. O’Neill, MD, PhD
Assistant 

bor, Michigan
Fraternal Order of Eagles Diabetes Research 
Center
University of Iowa
Iowa City, Iowa
Naykky Singh Ospina, MD, MS
University of Florida
Jorge Plutzky, MD
Director, Preventive Cardiology
Director, The Vascular Disease Prevention 
Program
Division of Cardiovascular Medi

revention 
Program
Division of Cardiovascular Medicine
Kenneth S. Polonsky, MD
Richard T. Crane Distinguished Service 
Professor
Dean of the Division of the Biological 
Sciences and the Pritzker School of 
Medicine
Executive Vice President for Medical Affairs
Sally Radovick, MD
Rutgers Robert Wood Johnson Medical 
School
New Brunswick, New Jersey
Ajay D. Rao, 

al 
School
Ajay D. Rao, MD, MMSc
Section of Endocrinology, Diabetes, and 
Lewis Katz School of Medicine at Temple 
Philadelphia, Pennsylvania
Sudhaker D. Rao, MBBS
Section Head, Bone and Mineral Disorders
Director, Bone and Mineral Research 
Laboratory
x
x
Matthew C. Riddle, MD
Professor of Med

tributors
x
Clinical Nutrition 
Rene Rodriguez-Gutierrez, MD, MS
Plataforma INVEST Medicina UANL-KER 
Unit (KER Unit Mexico)
Facultad de Medicina
Endocrinology Division
Hospital Universitario “Dr. José E. Gonzalez”
Universidad Autónoma de Nuevo León
Monterrey, México
Roc

on Research Unit in 
Evan D. Rosen, MD, PhD
Institute Member
Broad Institute of Harvard and MIT
Stephen M. Rosenthal, MD

thal, MD
Division of Endocrinology and Diabetes
Medical Director, Child and Adolescent 
Gender Center
Benioff Children’s Hospital
Mahmoud Salama, MD, PhD
Adjunct Assistant Professor
Domenico Salvatore, MD, PhD
Department of Public Health
University of Naples “Federico II”
Naples, Italy
Victoria Sandl

 Naples “Federico II”
Victoria Sandler, MD
Clinical Assistant Professor
NorthShore University Health System
Evanston, Illinois
Maria Inês Schmidt, MD, PhD
Professor
Clay F. Semenkovich, MD
Irene E. and Michael M. Karl Professor
Chief, Division of Endocrinology, 
Me

 of Medicine
Metabolism, and Lipid Research
Washington University
Patrick M. Sluss, PhD
Associate Director, Clinical Pathology Core
Pathology Service
Associate Professor of Pathology
Christian J. Strasburger, MD
Chief, Division of Clinical Endocrinology
Department of Endocrinology, Diabetes, and 
Nutritional Medicine
Charité Universitaetsmedizin

Berlin, Germany
xi
xi
Dennis M. Styne, MD
Yocha Dehe Chair of Pediatric Endocrinology
University of California
Davis, California
Jennifer K. Sun, MD, MPH
Associate Professor of Ophthalmology
Chief, Center for Clinical Eye Research and 
Trials
Beetham Eye Institute
Investigator, Research Division
Vin Tangpricha, MD, PhD
D

ts
Lipids
Emory University School of Medicine
Atlanta, Georgia
Staff Physician
Atlanta VA Medical Center
Decatur, Georgia
Rajesh V. Thakker, MD, ScD, FRCP, 
­FRCPath, FRS, FMedSci
Academic Endocrine Unit
Radcliffe Department of Medicine
University of Oxford
Oxford Centre for Diabetes, Endocrinology 
and Metabolism (OCDEM)
Churchill Hospital
Oxford, United Kingdom
Robert L. Thomas, MD, PhD
Resident Physician

ngdom
San Diego, California
Christopher J. Thompson, MD, MBChB, 
FRCP, FRCPI
Academic Department of Endocrinology
Beaumont Hospital/Royal College of 
Surgeons in Ireland Medical School
Dublin, Ireland
R. Michael Tuttle, MD
Clinical Director
Endocrinology Service
Memorial Sloan Kettering Cancer Center
Annewieke W. van den Beld, 

ter
Annewieke W. van den Beld, MD, PhD
Groene Hart Hospital
Gouda, The Netherlands
Joseph G. Verbalis, MD
Georgetown University
Chief, Endocrinology and Metabolism
Georgetown University Hospital
Washington, District of Columbia
Anthony P. Weetman, MD, DSc
Emeritus Professor of Medicine
Department of Human Metabolism
Sheffie

f Human Metabolism
Sheffield, Great Britain
Wilmar M. Wiersinga, MD, PhD
Department of Endocrinology and 
Academic Medical Center
Amsterdam, The Netherlands
Teresa K. Woodruff, PhD
Thomas J. Watkins Professor of Obstetrics and 
Gynecology
William F. Young, Jr., MD, MSc
Professor of Medicine, Tyson Family 
Endocrinology Clinical Professor
Division of Endocrinology, Diabetes, 
Metabolism, and Nu

on of Endocrinology, Diabetes, 
Metabolism, and Nutrition
xii
Preface
The Editors are delighted to welcome you to the 69th anniversary 
14th edition of Williams Textbook of Endocrinology. In this new edi­
tion, we have strived to maintain Robert Williams’ original 1950 
mandate to publish “a condensed and authoritative discussion 
of the management of clinical endocrinopathies based upon the 
application of fundamental information obtained from chemical 
and phys

ental information obtained from chemical 
and physiological investigation.” With the passing of the decades, 
our scholarly goal has been enriched by the addition of genetic, 
molecular, cellular, and population sciences, together forming 
the basis of multiple new insights into both the pathogenesis and 
management of endocrine disorders. Editors of this textbook aim 
to provide a cogent navigation through the wealth of information 
emanating from novel medical discoveries that advance the fiel

om novel medical discoveries that advance the field 
and bring new therapeutic approaches to patients with endocrine 
diseases. Our challenge remains to be both concise and didactic, 
while comprehensively covering relevant translational and clinical 
endocrine science in an accessible fashion.
With these goals in mind, we have once again assembled a 
team of outstanding authorities who each contribute their unique 
expertise to synthesize current knowledge in their respective topic 
area. For t

t knowledge in their respective topic 
area. For this edition, we have added new chapters on the global 
burden of endocrine disease and the navigation of the prolific 
expert endocrine guidelines, as well as chapters devoted to trans­
gender endocrinology, and osteomalacia. The section on diabetes 
mellitus has been expanded with dedicated chapters on the physi­
ology of insulin secretion, as well as a comprehensive update on 
therapeutics of type 2 diabetes mellitus. These new contributions 
r

pe 2 diabetes mellitus. These new contributions 
reflect the changing emphasis of endocrine practice today and the 
availability of a wealth of new knowledge and therapeutic options 
that together affect clinical care. Each section has undergone sig­
nificant revision and updating to bring the most current informa­
tion to our readers.
We are deeply appreciative of the valued co-workers in our 
respective offices, including Shira Berman, and Grace Labrado for 
their dedicated efforts. We also th

e Labrado for 
their dedicated efforts. We also thank our colleagues at Elsevier—
Rae Robertson and Nancy Duffy—for shepherding the produc­
tion process so professionally. The final product of this exemplary 
text is due to their skilled navigation of the medical publishing 
world. We are confident that our combined efforts have succeeded 
in achieving the high standards set by previous editions that have 
made Williams the classic “go to” book for all those interested in 
endocrinology.
1 
CHAP

or all those interested in 
1 
CHAPTER OUTLINE 
The Evolutionary Perspective, 3 
Endocrine Glands, 4 
Transport of Hormones in Blood, 5 
Target Cells as Active Participants, 6 
Control of Hormone Secretion, 7 
KEY POINTS 
Endocrinology is a scientific and medical discipline with a 
unique focus on hormones that features a multidisciplinary 
approach to understanding normal and pathologic hormone 
production and action, as well as diseases related to abnormal 
hormone signaling. 
E

iseases related to abnormal 
Endocrine and paracrine systems differ in important respects 
that illustrate the evolutionary pressures on these distiact cell-
signaling strategies. 
Differentiated hormone-secreting cells are design.ed to ef-
ficiently synthesize hormones and secrete them in a regu ated 
way. 
Hormones in the bloodstream often are associ~ 
with binding 
proteins to enhance their solubility, 12rotect them from degra-
dation and renal excretion, and regula e thei

gra-
dation and renal excretion, and regula e their-stability in the 
extracellular space. 
A
bout a hundred years ago, Starling coined the term hor-
mone to describe secretin, a substance secreted by the 
small intestine into the bloodstream to stimulate pancre-
atic secretion. In his Croonian Lectures, Starling considered the 
endocrine and nervous systems as two distinct mechanisms for 
coordination and control of organ function. Thus, endocrinology 
found its first home in the discipline of 

nology 
found its first home in the discipline of mammalian physiology. 
Over the next several decades, biochemists, physiologists, and 
clinical investigators characterized peptide and steroid hormones 
secreted into the bloodstream from discrete endocrine glands 
2 
Hormone Measurement, 10 
Endocrine Diseases, 10 
Future Perspectives, 12 
JS 
HCrmones either act on receptors on the plasma membranes of 
target cells or move into cells to bind to intracellular receptors; 
in either case, the tar

 intracellular receptors; 
in either case, the target cell is not a passive recipient of signals 
but rather has key roles in regulating hormonal responses. 
Control of hormone secretion involves integrated inputs from mul-
tiple distant targets, nervous system inputs, and local paracrine and 
autocrine factors, all leading to complex patterns of circadian secre-
tion, pulsatile secretion, secretion driven by homeostatic stimuli, or 
stimuli that lead to secular changes over the lifespan. 
Endoc

 lead to secular changes over the lifespan. 
Endocrine diseases fall into broad categories of hormone 
overproduction or underproduction, altered tissue response to 
hormones, or tumors arising from endocrine tissue. 
Hormones and synthetic molecules designed to interact with 
hormone receptors are administered to diagnose and treat 
diseases. 
or other organs. Diseases such as hypothyroidism and diabetes 
could be treated successfully for the first time by replacing specific 
hormones. These in

rst time by replacing specific 
hormones. These initial triumphs of discovery formed the foun-
dation of the clinical specialty of endocrinology. 
Advances in cell biology, molecular biology, and genetics over 
the ensuing years began to reveal mechanisms underlying endo-
crine disease pathogenesis, hormone secretion, and action. Even 
though these advances have embedded endocrinology in the 
framework of molecular cell biology, they have not changed the 
essential subject of endocrinology-the s

nged the 
essential subject of endocrinology-the signaling that coordinates 
3
Chapter 1  Principles of Endocrinology
and controls functions of multiple organs and processes. Herein 
we survey general themes and principles that underpin diverse 
approaches used by clinicians, physiologists, biochemists, cell 
biologists, and geneticists to understand the endocrine system.
The Evolutionary Perspective
Hormones are broadly defined as chemical signals secreted into 
the bloodstream that act on dist

ls secreted into 
the bloodstream that act on distant tissues, usually in a regulatory 
fashion. Hormonal signaling represents a special case of the more 
general process of signaling between cells. Even unicellular organ­
isms such as baker’s yeast, Saccharomyces cerevisiae, secrete short 
peptide mating factors that act on receptors of other yeast cells 
to trigger mating between the two cells. These receptors resemble 
the ubiquitous family of seven membrane-spanning mammalian 
receptors that

 seven membrane-spanning mammalian 
receptors that respond to diverse ligands such as photons and gly­
coprotein hormones. Because these yeast receptors trigger activa­
tion of heterotrimeric G proteins just as mammalian receptors do, 
this conserved signaling pathway was likely to have been present 
in the common ancestor of yeast and humans.
Signals from one cell to adjacent cells, termed paracrine sig­
nals, often use the same molecular pathways used by hormonal 
signals. For example, the sev

ys used by hormonal 
signals. For example, the sevenless receptor controls the differen­
tiation of retinal cells in the Drosophila eye by responding to a 
membrane-anchored signal from an adjacent cell. Sevenless is a 
membrane-spanning receptor with an intracellular tyrosine kinase 
domain that closely resembles signaling by hormone receptors 
such as the insulin receptor tyrosine kinase. As paracrine factors 
and hormones can share signaling machinery, it is not surpris­
ing that hormones can

achinery, it is not surpris­
ing that hormones can, in some settings, act as paracrine factors. 
Testosterone, for example, is secreted into the bloodstream but 
also acts locally in the testes to control spermatogenesis. Insulin-
like growth factor 1 (IGF1) is a polypeptide hormone secreted 
into the bloodstream from the liver and other tissues but is also 
a paracrine factor produced locally in most tissues to control cell 
proliferation. Furthermore, one receptor can mediate actions of a 
hor

ermore, one receptor can mediate actions of a 
hormone, such as parathyroid hormone (PTH), and of a paracrine 
factor, such as parathyroid hormone–related protein. In some 
cases, the paracrine actions of “hormones” exhibit functions quite 
unrelated to the hormonal functions. For example, macrophages 
synthesize the active form of vitamin D, 1,25-dihydroxyvitamin 
D3 (1,25[OH]2D3), which can then bind to vitamin D receptors 
in the same cells and stimulate production of antimicrobial pep­
tides

d stimulate production of antimicrobial pep­
tides.1 This example illustrates that the vitamin D 1α-hydroxylase 
(P450 27B1) responsible for activating 25-hydroxyvitamin D is 
synthesized in tissues in which its function is unrelated to the cal­
cium homeostatic actions of the 1,25(OH)2D3 hormone.
Target cells respond similarly to signals that reach them from 
the bloodstream (hormones) or from adjacent cells (paracrine 
factors); the cellular response machinery does not distinguish 
between sit

sponse machinery does not distinguish 
between sites of origin of hormone signals. The shared final com­
mon pathways used by hormonal and paracrine signals should 
not, however, obscure important differences between hormonal 
and paracrine signaling systems (Fig. 1.1). Hormone synthesis 
occurs in specialized cells designed specifically for their produc­
tion, and the hormone then travels in the bloodstream and dif­
fuses in effective concentrations into tissues. Therefore, hormones 
must be pr

ions into tissues. Therefore, hormones 
must be produced in much larger amounts to act as hormones 
relative to the amounts needed to act as paracrine factors, which 
act at specific local locations. Hormones must be able to travel 
to and be protected from degradation in transit from the site of 
production to the distant site of action. Therefore, for example, 
lipophilic hormones bind to soluble proteins that allow them to 
travel in the aqueous media of blood at relatively high concentra­
ti

us media of blood at relatively high concentra­
tions. The ability of hormones to diffuse through the extracellu­
lar space implies local hormone concentrations at target sites will 
rapidly decrease when glandular secretion of the hormone ceases. 
As hormones quickly diffuse throughout extracellular fluid, hor­
monal metabolism can occur in specialized organs, including liver 
and kidney, in a manner that determines the effective hormone 
concentration in other tissues.
Paracrine factors have r

tration in other tissues.
Paracrine factors have rather different constraints. Paracrine 
signals do not travel very far; consequently, the specific site of ori­
gin of a paracrine factor determines where it will act and provides 
specificity to that action. When the paracrine factor bone mor­
phogenetic protein 4 (BMP4) is secreted by cells in the developing 
kidney, BMP4 regulates differentiation of renal cells; when the 
same factor is secreted by cells in bone, it regulates bone forma­
tion.

d by cells in bone, it regulates bone forma­
tion. Thus the site of origin of BMP4 determines its physiologic 
role. In contrast, because hormones are secreted into the blood­
stream, their sites of origin are often divorced from their func­
tions. Like BMP4, thyroid hormone, for example, acts in many 
tissues but the site of origin of thyroid hormone in a gland in 
the neck is not functionally relevant to the sites of action of the 
hormone.
Because specificity of paracrine factor action is so 

ause specificity of paracrine factor action is so dependent 
on its precise site of origin, elaborate mechanisms have evolved to 
regulate and constrain the diffusion of paracrine factors. Paracrine 
factors of the hedgehog family, for example, are covalently bound 
to cholesterol to constrain diffusion of these molecules in the 
extracellular milieu. Most paracrine factors interact with binding 
proteins that block their action and control their diffusion. For 
example, chordin, noggin, and man

 diffusion. For 
example, chordin, noggin, and many other distinct proteins bind 
to various members of the BMP family to regulate their action. 
Proteases such as tolloid then destroy the binding proteins at spe­
cific sites to liberate BMPs so that they can act on appropriate 
target cells.
Thus, hormones and paracrine factors have several distinct 
strategies regulating biosynthesis, sites of action, transport, and 
metabolism. These differing strategies may partly explain why a 
hormone such

 strategies may partly explain why a 
hormone such as IGF1, unlike its close relative insulin, has multi­
ple binding proteins to control its action in tissues. IGF1 exhibits 
a double life as both a hormone and a paracrine factor. Presumably 
the IGF1 actions mandate an elaborate binding protein apparatus 
to enable appropriate hormone signaling.
All the major hormonal signaling programs—G protein–cou­
pled receptors, tyrosine kinase receptors, serine/threonine kinase 
receptors, ion channels, 

serine/threonine kinase 
receptors, ion channels, cytokine receptors, nuclear receptors—are 
also used by paracrine factors. In contrast, several paracrine signal­
ing programs are used only by paracrine factors and not by hor­
mones. For example, Notch receptors respond to membrane-based 
ligands to control cell fate, but no known blood-borne ligands use 
Notch-type signaling. Perhaps the complex intracellular strategy 
used by Notch, which involves cleavage of the receptor and subse­
quent nuc

lves cleavage of the receptor and subse­
quent nuclear actions of the receptor’s cytoplasmic portion, is too 
inflexible to serve the purposes of hormones.
Analyses of the complete genomes of multiple bacterial species, 
the yeast Saccharomyces cerevisiae, the fruit fly Drosophila mela­
nogaster, the worm Caenorhabditis elegans, the plant Arabidopsis 
thaliana, humans, and many other species have allowed a compre­
hensive view of the signaling machinery used by various forms of 
life. S. cerevis

chinery used by various forms of 
life. S. cerevisiae uses G protein–coupled receptors; this organism, 
4
SECTION I    Hormones and Hormone Action
however, lacks tyrosine kinases, used in the insulin signaling path­
way, and nuclear receptors that resemble the estrogen/thyroid 
receptor family. In contrast, the worm and fly share with humans 
the use of each of these signaling pathways, although with sub­
stantial variation in numbers of genes committed to each pathway. 
For example, the Drosoph

mmitted to each pathway. 
For example, the Drosophila genome encodes 21 nuclear recep­
tors, the C. elegans genome encodes about 284, and the human 
genome encodes 48 such receptors. These patterns suggest ancient 
multicellular animals must already have established the signaling 
systems that are the foundation of the endocrine system as we 
know it in mammals.
Our understanding of endocrine systems and novel physio­
logic biology continues to expand. Even before the sequencing of 
the human ge

xpand. Even before the sequencing of 
the human genome, sequence analyses had made clear that many 
receptor genes are found in mammalian genomes for which no 
clear ligand or function was known. Analyses of these “orphan” 
receptors broadened current understanding of hormonal sig­
naling. For example, the liver X receptor (LXR) was one such 
orphan receptor found when searching for unknown putative 
nuclear receptors. Subsequent experiments found oxygenated 
derivatives of cholesterol are the l

d oxygenated 
derivatives of cholesterol are the ligands for LXR, which regu­
lates genes involved in cholesterol and fatty acid metabolism.2 
The examples of LXR and many others raise the question of what 
constitutes a hormone. The classic view of hormones is that they 
are synthesized in discrete glands and have no function other 
than activating receptors on cell membranes or in the nucleus. 
Cholesterol, which is converted in cells to oxygenated deriva­
tives that activate the LXR receptor,

ated deriva­
tives that activate the LXR receptor, in contrast, uses a hormonal 
strategy to regulate its own metabolism. Other orphan nuclear 
receptors similarly respond to ligands such as bile acids and fatty 
acids. These “hormones” have important metabolic roles quite 
separate from their signaling properties, although hormone-
like signaling permits regulation of the metabolic function. The 
calcium-sensing receptor is an example from the G protein–coupled 
receptor family that responds to

 protein–coupled 
receptor family that responds to a nonclassic ligand, ionic cal­
cium. Calcium is released into the bloodstream from bone, kid­
ney, and intestine and acts on the calcium-sensing receptor on 
parathyroid cells, renal tubular cells, and other cells to coordinate 
cellular responses to calcium. Thus, many important metabolic 
factors have hormonal properties as part of a regulatory strategy 
within complex organisms. Broadened understanding of these 
new metabolic factors is lead

erstanding of these 
new metabolic factors is leading to new therapeutic approaches 
to treat or prevent human diseases. 
Endocrine Glands
Hormone formation may occur either in the endocrine glands, 
which are localized collections of specific cells, or in cells that have 
additional roles. Many protein hormones, such as growth hor­
mone (GH), PTH, prolactin (PRL), insulin, and glucagon, are 
Regulation of signaling: paracrine
Source: adjacent cell
•  Major determinant of target
•  Synthesis/sec

ll
•  Synthesis/secretion
Target cell
•  Receptor:
      Specificity and sensitivity
      Diffusion barrier
      Determinant of gradient
•  Induced inhibitory pathways,
      ligands, and binding proteins
Distribution: matrix
•  Diffusion distance
•  Binding proteins: BMP, IGF
•  Proteases
•  Matrix components
Regulation of signaling: endocrine
Source: gland
Distribution: bloodstream
Nontarget organ
•  No contribution to
     specificity of target
•  Synthesis/se

tion to
•  Universal—almost
•  Importance of dilution
•  Receptor: source of specificity
•  Responsiveness:
      Number of receptors
      Downstream pathways
      Other ligands
      Metabolism of ligand/receptor
      All often regulated by ligand
•  Metabolism
•  Liver
• Fig. 1.1  Comparison of determinants of endocrine and paracrine signaling. BMP, bone morphogenetic 
protein; IGF, insulin-like growth factor.
5
Chapter 1  Princi

F, insulin-like growth factor.
5
produced in dedicated cells by standard protein synthetic mecha­
nisms common to all cells. These secretory cells contain specialized 
secretory granules designed to store large amounts of hormone 
and to release the hormones in response to specific signals. Hor­
mones made in these glands and specialized cells are considered to 
be classic endocrine systems. Formation of small hormone mole­
cules initiates with commonly fou

ll hormone mole­
cules initiates with commonly found precursors, usually in specific 
glands such as the adrenals, gonads, or thyroid. In the case of the 
steroid hormones, the precursor is cholesterol, which is modified 
by various cytochrome P450–based hydroxylations and carbon-
carbon bond cleavages and by specific oxidoreductases to form 
the glucocorticoids, androgens, estrogens, and their biologically 
active derivatives.
However, not all hormones are formed in dedicated and spe­
cialized 

ormones are formed in dedicated and spe­
cialized endocrine glands; the adipose, enteroendocrine, and 
other systems are now also recognized to be complex endocrine 
systems. Thus with the discovery of novel peptides and amino 
acid or ­steroid-based molecules and their regulatory functions, the 
field of endocrinology and metabolism has recently been greatly 
expanded. For example, the protein hormone leptin, which regu­
lates appetite and energy expenditure, is formed in adipocytes, 
providing

y expenditure, is formed in adipocytes, 
providing a specific signal reflecting the nutritional state of the 
organism to the central nervous system. The enteroendocrine 
system comprises a unique hormonal system in which peptide 
hormones that regulate metabolic and other responses to oral 
nutrients are produced and secreted by specialized endocrine cells 
scattered throughout the intestinal epithelium. The cholesterol 
derivative, 7-­dehydrocholesterol, the precursor of vitamin D3, 
is conver

olesterol, the precursor of vitamin D3, 
is converted in skin keratinocytes to previtamin D3 by a photo­
chemical reaction.
Thyroid hormone synthesis occurs via a unique pathway. The 
thyroid cell synthesizes a 660,000-kDa homodimer, thyroglobu­
lin, which is then iodinated at specific tyrosines. Some iodotyro­
sines combine enzymatically to form the iodothyronine molecule 
within thyroglobulin, which is then stored in the lumen of the 
thyroid follicle. For tyrosine iodination to occur, the thy

ollicle. For tyrosine iodination to occur, the thyroid 
cell must concentrate trace quantities of iodide from the blood 
and oxidize it via a specific peroxidase. Release of thyroxine (T4) 
from thyroglobulin requires phagocytosis and cathepsin-catalyzed 
digestion by the same cells.
Hormones are synthesized in response to biochemical signals 
generated by various modulating systems. Many of these systems 
are specific to the effects of the hormone product; for example, 
PTH synthesis is regulat

ne product; for example, 
PTH synthesis is regulated by the concentration of ionized cal­
cium, and insulin synthesis is regulated by the concentration of 
glucose. For others, such as gonadal, adrenal, and thyroid hor­
mones, control of hormone synthesis is achieved by the homeo­
static function of the hypothalamic-pituitary axis. Cells in the 
hypothalamus and pituitary monitor circulating hormone con­
centrations and secrete trophic hormones, which activate specific 
pathways for hormone synt

which activate specific 
pathways for hormone synthesis and release. Typical examples are 
GH, luteinizing hormone (LH), follicle-stimulating hormone 
(FSH), thyroid-stimulating hormone (TSH), and adrenocortico­
tropic hormone (ACTH).
These trophic hormones increase rates of hormone synthe­
sis and secretion, and they may induce target cell division, thus 
causing enlargement of the various target glands. For example, 
in hypothyroid individuals living in iodine-deficient areas of the 
world, TS

living in iodine-deficient areas of the 
world, TSH secretion causes a marked hyperplasia of thyroid 
cells. In such regions, the thyroid gland may be 20 to 50 times its 
normal size. Adrenal hyperplasia occurs in patients with genetic 
deficiencies in cortisol formation. Hypertrophy and hyperplasia 
of parathyroid cells, initiated by an intrinsic response to the stress 
of hypocalcemia, occur in patients with renal insufficiency or cal­
cium malabsorption.
Hormones may be fully active when rele

absorption.
Hormones may be fully active when released into the blood­
stream (e.g., GH or insulin), or they may require activation in 
specific cells to produce biologic effects. These activation steps are 
often highly regulated. For example, T4 released from the thyroid 
cell is a prohormone that must undergo specific deiodination to 
form the active 3,5,3'-triiodothyronine (T3). This deiodination 
reaction can occur in target tissues, such as in the central nervous 
system; in thyrotrophs, w

 in the central nervous 
system; in thyrotrophs, where T3 provides feedback regulation of 
TSH production; or in hepatic and renal cells, from which it is 
released into the circulation for uptake by all tissues. A similar 
postsecretory activation step catalyzed by a 5α-reductase causes 
tissue-specific activation of testosterone to dihydrotestosterone 
in target tissues, including the male urogenital tract, prostate, 
genital skin, and liver. Vitamin D undergoes hydroxylation at 
the 25 positi

itamin D undergoes hydroxylation at 
the 25 position in the liver and in the 1 position in the kidney. 
Both hydroxylations must occur to produce the active hormone, 
1,25-dihydroxyvitamin D. Activity of 1α-hydroxylase, but not 
25-hydroxylase, is stimulated by PTH and hypophosphatemia but 
is inhibited by calcium, 1,25-dihydroxyvitamin D, and fibroblast 
growth factor 23 (FGF23).
Most hormones are synthesized as required on a daily, hourly, 
or minute-to-minute basis with minimal storage, but t

minute-to-minute basis with minimal storage, but there are 
significant exceptions. One is the thyroid gland, which contains 
enough stored hormone to last for about 2 months. This storage 
permits a constant supply of thyroid hormone despite significant 
variations in the availability of iodine. However, if iodine defi­
ciency is prolonged, normal T4 reservoirs can be depleted.
Feedback signaling systems exemplified earlier enable the hor­
monal homeostasis characteristic of virtually all endoc

 homeostasis characteristic of virtually all endocrine systems. 
Regulation may include the central nervous system or local sig­
nal recogniton mechanisms in the glandular cells, such as the 
calcium-sensing receptor of the parathyroid cell. Disruption of 
hormonal homeostasis due to glandular or central regulatory sys­
tem dysfunction has both clinical and laboratory consequences. 
Recognition and correction of disorders of these systems are the 
essence of clinical endocrinology. 
Transport of

Transport of Hormones in Blood
Protein hormones and some small molecules, such as catechol­
amines, are water soluble and readily transported via the circu­
latory system. Others are nearly insoluble in water (e.g., steroid 
and thyroid hormones), and their distribution presents spe­
cial problems. Such molecules are tightly bound to 50-kDa to 
60-kDa carrier plasma glycoproteins such as thyroxine-binding 
globulin (TBG), sex hormone–binding globulin (SHBG),

obulin (TBG), sex hormone–binding globulin (SHBG), and 
­corticosteroid-binding globulin (CBG) or weakly bound to abun­
dant albumin. Ligand-protein complexes serve as hormone reser­
voirs, ensure ubiquitous distribution of water-insoluble ligands, 
and protect small molecules from rapid inactivation or excretion 
in urine or bile. Protein-bound hormones exist in equilibria with 
the often-minute quantities of hormone in the aqueous plasma, 
with the “free” fraction of the circulating hormone ta

 the “free” fraction of the circulating hormone taken up by the 
target cell. For example, if tracer thyroid hormone is injected into 
the portal vein in a protein-free solution, it binds to hepatocytes at 
the periphery of the hepatic sinusoid. When the same experiment 
is repeated with a protein-containing solution, uniform distribu­
tion of the tracer hormone occurs throughout the hepatic lobule.3 
Despite the very high affinity of some binding proteins for their 
6
SECTION I    Hormones and 

g proteins for their 
6
respective ligands, one specific protein may not be essential for 
hormone distribution. For example, in humans with congenital 
TBG deficiency, other proteins—transthyretin (TTR) and albu­
min—subsume its role. As the affinity of these secondary thyroid 
hormone transport proteins is several orders of magnitude lower 
than that of TBG, it is possible for the hypothalamic-pituitary 
feedback system to maintain free thyroid hormone 

feedback system to maintain free thyroid hormone in the nor­
mal range at a much lower total hormone concentration. The fact 
that the free hormone concentration is normal in individuals with 
TBG deficiency indicates that the hypothalamic-pituitary axis 
defends the free, active hormone.4
Availability of gene-targeting techniques allows specific assess­
ments of the physiologic roles of hormone-binding proteins. For 
example, mice with targeted inactivation of the vitamin D–bind­
ing protein (

 inactivation of the vitamin D–bind­
ing protein (DBP) have been generated.5 Although the absence 
of DBP markedly reduces circulating concentrations of vitamin 
D, the mice are otherwise normal. However, they have enhanced 
susceptibility to a vitamin D–deficient diet due to the reduced 
reservoir of this sterol. In addition, the absence of DBP markedly 
reduces the half-life of 25-hydroxyvitamin D by accelerating its 
hepatic uptake, making the mice less susceptible to vitamin D 
intoxication.

 mice less susceptible to vitamin D 
Protein hormones and some small ligands (e.g., catechol­
amines) produce their effects by interacting with cell surface 
receptors. Others, such as steroid and thyroid hormones, must 
enter the cell to bind to cytosolic or nuclear receptors. In the past, 
it has been thought that much of the transmembrane transport of 
hormones was passive. Evidence now demonstrates specific trans­
porters involved in cellular uptake of thyroid and some steroid 

ed in cellular uptake of thyroid and some steroid 
hormones,7 providing yet another mechanism for regulating the 
distribution of a hormone to its site of action. Studies in mice 
devoid of megalin, a large, cell surface protein in the low-density 
lipoprotein (LDL) receptor family, suggest estrogen and testos­
terone bound to SHBG use megalin to enter peripheral tissues 
while still bound to SHBG.8 In this scenario, the hormone bound 
to SHBG, rather than “free” hormone, is the active moiety th

ather than “free” hormone, is the active moiety that 
enters cells. It is unclear how frequently this apparent exception to 
the “free hormone” hypothesis occurs.
MicroRNAs (miRNAs) have recently also been shown to 
elicit remote metabolic actions. For example, exosomal miRNA 
derived from adipose tissue regulates distant tissue gene expres­
sion, glucose tolerance, and circulating fibroblast growth factor 21 
(FGF21) levels. MiRNAs may thus function as circulating adipo­
kines.9 Other small lip

tion as circulating adipo­
kines.9 Other small lipid signaling molecules are being discovered, 
especially for their role in activating or suppressing what were pre­
viously designated as orphan receptors. 
Target Cells as Active Participants
Hormones determine cellular target actions by binding with 
high specificity to receptor proteins. Whether a peripheral cell is 
hormonally responsive depends to a large extent on the presence 
and function of specific and selective hormone receptors and th

of specific and selective hormone receptors and the 
downstream signaling pathway molecules. Thus receptor expres­
sion and intracellular effector pathways activated by the hormone 
signal are key determinants for which cells will respond, and how. 
Receptor proteins may be localized to the cell membrane, cyto­
plasm, or nucleus. Broadly, polypeptide hormone receptors are 
cell membrane associated, but steroid hormones selectively bind 
soluble intracellular proteins (Fig. 1.2).
Membrane-associa

ntracellular proteins (Fig. 1.2).
Membrane-associated receptor proteins usually consist 
of extracellular sequences that recognize and bind ligand, 
transmembrane-anchoring hydrophobic sequences, and intracel­
lular sequences, which initiate intracellular signaling. Intracellu­
lar signaling is mediated by covalent modification and activation 
of intracellular signaling molecules (e.g., signal transducers and 
activators of transcription [STAT] proteins) or by generation of 
small molecule secon

roteins) or by generation of 
small molecule second messengers (e.g., cyclic adenosine mono­
phosphate) through activation of heterotrimeric G proteins. Sub­
units of these G proteins (α-subunits, β-subunits, and γ-subunits) 
activate or suppress effector enzymes and ion channels that gener­
ate the second messengers. Some of these receptors (e.g., those 
for somatostatin) may in fact exhibit low constitutive activity and 
have been shown to signal in the absence of added ligand.
Several growth 

al in the absence of added ligand.
Several growth factors and hormone receptors (e.g., for insulin) 
behave as intrinsic tyrosine kinases or activate intracellular protein 
tyrosine kinases. Ligand activation may cause receptor dimeriza­
tion (e.g., GH) or heterodimerization (e.g., interleukin 6), fol­
lowed by activation of intracellular phosphorylation cascades. 
These activated proteins ultimately determine specific nuclear 
gene expression.
Both the number of receptors expressed per cell and

oth the number of receptors expressed per cell and their 
responses are regulated, thus providing a further level of con­
trol for hormone action. Several mechanisms account for altered 
receptor function. Receptor endocytosis causes internalization of 
cell surface receptors; the hormone-receptor complex is subse­
quently dissociated, resulting in abrogation of the hormone signal. 
Receptor trafficking may then result in recycling back to the cell 
surface (e.g., as for the insulin receptor) or

ll 
surface (e.g., as for the insulin receptor) or the internalized recep­
tor may undergo lysosomal degradation. Both these mechanisms 
triggered by activation of receptors effectively lead to impaired 
hormone signaling downregulation of the receptors. The hor­
mone signaling pathway may also be downregulated by receptor 
desensitization (e.g., as for epinephrine); ligand-mediated receptor 
phosphorylation leads to a reversible deactivation of the recep­
tor. Desensitization mechanisms can be 

the recep­
tor. Desensitization mechanisms can be activated by a receptor’s 
ligand (homologous desensitization) or by another signal (heter­
ologous desensitization), thereby attenuating receptor signaling 
in the continued presence of ligand. Receptor function may also 
be limited by action of specific phosphatases (e.g., Src homology 
phosphatase [SHP]) or by intracellular negative regulation of the 
signaling cascade (e.g., suppressor of cytokine signaling [SOCS] 
proteins inhibiting Janus k

kine signaling [SOCS] 
proteins inhibiting Janus kinase/signal transducers and activators 
of transcription [JAK-STAT] signaling). Certain ligand-receptor 
complexes may also translocate to the nucleus.
Mutational changes in receptor structure can also determine 
hormone action. Constitutive receptor activation may be induced 
by activating mutations (e.g., TSH receptor) leading to endocrine 
organ hyperfunction, even in the absence of ligand. Conversely, 
inactivating receptor mutations may lea

nversely, 
inactivating receptor mutations may lead to endocrine hypofunc­
tion (e.g., testosterone or vasopressin receptors). These syndromes 
are well characterized (Table 1.1) and are well described in subse­
quent chapters.
The functional diversity of receptor signaling results in overlap­
ping or redundant intracellular pathways. For example, GH, PRL, 
and cytokines each activate JAK-STAT signaling, whereas the dis­
tal effects of these stimuli clearly differ. Thus, despite common 
upstream

uli clearly differ. Thus, despite common 
upstream signaling pathways, hormones can elicit highly specific 
cellular effects. Tissue-type or cell-type genetic programs or recep­
tor-receptor interactions at the cell surface (e.g., hetero-oligomer­
ization of dopamine D2 with somatostatin receptor, or insulin 
with IGF1 receptor) may also confer a specific cellular response to 
a hormone and provide an additive cellular effect.10 In addition, 
effector protein expression may differ in select cell

ector protein expression may differ in select cells to modulate 
7
hormonal response. For example, the glucose ­transporter-4 pro­
tein, which leads to insulin-mediated glucose uptake, is most 
abundantly expressed in muscle, hepatic and ­adipose tissues, 
causing these tissues to be the most sensitive ­tissues for insulin-­
mediated glucose disposal.
A final mechanism of nuclear receptor modulation is prerecep­
tor regulation via intracellular enzymes that

cep­
tor regulation via intracellular enzymes that convert the circulat­
ing molecules to more or less potent hormones. In addition to 
the activation of T4 and testosterone described earlier, selective 
hormone inactivation occurs in some cells. In the distal nephron, 
the enzyme 11β-hydroxysteroid dehydrogenase type 2 converts 
the mineralocorticoid-receptor ligand cortisol to inactive corti­
sone, thus preventing receptor activation. This mechanism allows 
aldosterone, which is not a substrat

anism allows 
aldosterone, which is not a substrate for the enzyme, to regulate 
mineralocorticoid activity in the kidney despite circulating aldo­
sterone concentrations 1000 times lower than those of cortisol. 
Control of Hormone Secretion
Anatomically distinct endocrine glands are composed of highly 
differentiated cells that synthesize, store, and secrete hormones. 
Circulating hormone concentrations are a function of glandular 
secretory patterns and hormone clearance rates. Hormone secre­

terns and hormone clearance rates. Hormone secre­
tion is tightly regulated to attain circulating levels that are most 
conducive to elicit the appropriate target tissue response. For 
example, longitudinal bone growth is initiated and maintained 
by exquisitely regulated levels of circulating GH, yet mild GH 
hypersecretion results in gigantism, and GH deficiency causes 
growth retardation. Ambient circulating hormone concentrations 
are not uniform, and secretion patterns determine appropriate

form, and secretion patterns determine appropriate 
physiologic function. Thus, insulin secretion occurs in short pulses 
elicited by nutrient and other signals; gonadotropin secretion is 
episodic, determined by a hypothalamic pulse generator; and PRL 
ss
s
s
s
s
Biological responses
Proteins
mRNAs
Target gene
Progesterone
IGF1
cAMP
ATP
AAAAA
PKA
AC
G
R
LH
R
R
R
P
O
O
O
R
O
O
XTyr
XTyr
P
P
TFTyr
TF
• Fig. 1.2  Hormonal signaling by cell surface and intracellular receptors. The receptors for the

and intracellular receptors. The receptors for the water-soluble 
polypeptide hormones, luteinizing hormone (LH), and insulin-like growth factor 1 (IGF1) are integral mem­
brane proteins located at the cell surface. They bind the hormone-utilizing extracellular sequences and 
transduce a signal by the generation of second messengers: cyclic adenosine monophosphate (cAMP) 
for the LH receptor and tyrosine-phosphorylated substrates for the IGF1 receptor. Although effects on 
gene expression are in

eptor. Although effects on 
gene expression are indicated, direct effects on cellular proteins (e.g., ion channels) are also observed. In 
contrast, the receptor for the lipophilic steroid hormone progesterone resides in the cell nucleus. It binds 
the hormone and becomes activated and capable of directly modulating target gene transcription. AC, 
adenylyl cyclase; ATP, adenosine triphosphate; G, heterotrimeric G protein; mRNAs, messenger RNAs; 
PKA, protein kinase A; R, receptor molecule; TF, t

PKA, protein kinase A; R, receptor molecule; TF, transcription factor; Tyr, tyrosine found in protein X; X, 
unknown protein substrate. (From Mayo K. Receptors: molecular mediators of hormone action. In: Conn 
PM, Melmed S, eds. Endocrinology: Basic and Clinical Principles. Totowa, NJ: Humana Press; 1997:11.)
8
secretion appears to be relatively continuous, with secretory peaks 
elicited during suckling.
Hormone secretion also adheres to rhythmic patterns

ormone secretion also adheres to rhythmic patterns. Circa­
dian rhythms serve as adaptive responses to environmental signals 
and are controlled by a circadian timing mechanism.11 Light is the 
major environmental cue adjusting the endogenous clock. The ret­
inohypothalamic tract entrains circadian pulse generators situated 
within hypothalamic suprachiasmatic nuclei. These signals sub­
serve timing mechanisms for the sleep-wake cycle and determine 
patterns of hormone secretion and action. Dist

ne 
patterns of hormone secretion and action. Disturbed circadian 
timing results in hormonal dysfunction and may also be reflective 
of entrainment or pulse generator lesions. For example, adult GH 
deficiency due to a damaged hypothalamus or pituitary is associ­
ated with elevations in integrated 24-hour leptin concentrations 
and decreased leptin pulsatility, yet preserved circadian rhythm of 
leptin. GH replacement restores leptin pulsatility, promoting loss 
of body fat mass.12 Sleep is an 

, promoting loss 
of body fat mass.12 Sleep is an important cue regulating hormone 
pulsatility. About 70% of overall GH secretion occurs during 
slow-wave sleep, and increasing age is associated with declining 
slow-wave sleep and concomitant decline in GH and elevation 
of cortisol secretion.13 Most pituitary hormones are secreted in a 
circadian (day-night) rhythm, best exemplified by ACTH peaks 
before 9 am, whereas ovarian steroids follow a 28-day menstrual 
rhythm. Disrupted episodic rhyth

28-day menstrual 
rhythm. Disrupted episodic rhythms are often a hallmark of endo­
crine dysfunction. For example, loss of circadian ACTH secretion 
with high midnight cortisol levels is a feature of Cushing disease.
Hormone secretion is induced by multiple specific bio­
chemical and neural signals. Integration of these stimuli results 
in the net temporal and quantitative secretion of the hormone 
(Fig. 1.3). Signals elicited by hypothalamic hormones (growth 
hormone–releasing hormone [GHRH], s

mones (growth 
hormone–releasing hormone [GHRH], somatostatin), periph­
eral hormones (IGF1, sex steroids, thyroid hormone), nutri­
ents, adrenergic pathways, stress, and other neuropeptides all 
converge on the somatotroph cell, resulting in the ultimate pat­
tern and quantity of GH secretion. Networks of reciprocal inter­
actions allow for dynamic adaptation and shifts in environmental 
signals. These regulatory systems involve the hypothalamic, pitu­
itary, and target endocrine glands, as wel

, pitu­
itary, and target endocrine glands, as well as the adipocytes and 
lymphocytes. Peripheral inflammation and stress elicit cytokine 
signals that interface with the neuroendocrine system, resulting 
in hypothalamic-pituitary axis activation. Parathyroid and pan­
creatic secreting cells are less tightly controlled by the hypothala­
mus, but their functions are tightly regulated by the distal effects 
they elicit. For example, PTH secretion is induced when serum 
calcium levels fall and the

is induced when serum 
calcium levels fall and the signal for sustained PTH secretion is 
abrogated by rising calcium levels, whereas insulin secretion is 
induced when blood glucose rises but suppressed when glucose 
concentrations fall.
Several tiers of control subserve the ultimate net glandular secre­
tion. First, central nervous system signals, including afferent stim­
uli, neuropeptides, and stress, signal the synthesis and secretion of 
hypothalamic hormones and neuropeptides (Fig. 1.4). 

pothalamic hormones and neuropeptides (Fig. 1.4). Four hypo­
thalamic-releasing hormones (GHRH, ­corticotropin-releasing 
hormone [CRH], thyrotropin-releasing hormone [TRH], and 
gonadotropin-releasing hormone [GnRH]) traverse the hypotha­
lamic portal vessels and impinge upon their respective transmem­
brane trophic hormone-secreting cell receptors. These distinct 
cells express GH, ACTH, TSH, and gonadotropins, respectively. 
In contrast, hypothalamic somatostatin and dopamine suppress 
GH or 

halamic somatostatin and dopamine suppress 
GH or PRL and TSH secretion, respectively. Trophic hormones 
  Diseases Caused by Mutations in G Protein–Coupled Receptors.
Conditiona
Receptor
Inheritance
ΔFunctionb
Retinitis pigmentosa
Rhodopsin
AD/AR
Loss
Nephrogenic diabetes insipidus
Vasopressin V2
X-linked
Loss
Familial glucocorticoid deficiency
ACTH
AR
Loss
Color blindness
Red/green opsins
X-linked
Loss
Familial precocious puberty
LH
AD (male)
Gain
Familial hypercalcemia
Ca2+ sensing
AD
Loss
N

Gain
AD
Loss
Neonatal severe parathyroidism
AR
Loss
Dominant form hypocalcemia
AD
Gain
Congenital hyperthyroidism
TSH
AD
Gain
Hyperfunctioning thyroid adenoma
TSH
Somatic
Gain
Metaphyseal chondrodysplasia
PTH-PTHrP
Somatic
Gain
Hirschsprung disease
Endothelin-B
Multigenic
Loss
Coat color alteration (E locus, mice)
MSH
AD/AR
Loss and gain
Dwarfism (little locus, mice)
GHRH
AR
Loss
aAll are human conditions with the exception of the fin

are human conditions with the exception of the final two entries, which refer to the mouse.
bLoss of function refers to inactivating mutations of the receptor, and gain of function to activating mutations.
ACTH, Adrenocorticotropic hormone; AD, autosomal dominant inheritance; AR, autosomal recessive inheritance; FSH, follicle-stimulating hormone; GHRH, growth hormone–releasing hormone; LH, lutein­
izing hormone; MSH, melanocyte-stimulating hormone; PTH-PTHrP, parathyroid hormone and parathyroid 

e; PTH-PTHrP, parathyroid hormone and parathyroid hormone–related peptide; TSH, thyroid-stimulating hormone.
From Mayo K. Receptors: molecular mediators of hormone action. In: Conn PM, Melmed S, eds. Endocrinology: Basic and Clinical Principles. Totowa, NJ: Humana Press; 1997:27.   
TABLE 1.1
9
maintain the structural and functional integrity of endocrine 
organs, including the thyroid and adrenal glands and the gonads. 
Target hormones, in turn, serve as p

 the gonads. 
Target hormones, in turn, serve as powerful negative feedback 
regulators of their respective trophic hormone, often also sup­
pressing secretion of hypothalamic-releasing hormones. In certain 
circumstances (e.g., during puberty), peripheral sex steroids may 
positively induce the hypothalamic-pituitary-target gland axis. For 
example, LH induces ovarian estrogen secretion, which feeds back 
positively to induce further LH release. Pituitary hormones them­
selves, in a short feedb

 Pituitary hormones them­
selves, in a short feedback loop, also regulate their own respec­
tive hypothalamic-controlling hormone. Hypothalamic-releasing 
hormones are secreted in nanogram amounts and have short half-
lives of a few minutes. Anterior pituitary hormones are produced 
in microgram amounts and have longer half-lives, but peripheral 
hormones can be produced in up to milligram amounts daily, with 
much longer half-lives.
A further level of secretion control occurs within the gland 

vel of secretion control occurs within the gland 
itself. Intraglandular paracrine or autocrine growth peptides serve 
to autoregulate pituitary hormone secretion, as exemplified by 
epidermal growth factor (EGF) control of PRL or IGF1 con­
trol of GH secretion. Molecules within the endocrine cell may 
also subserve an intracellular feedback loop. For example, corti­
cotrope SOCS-3 induction by gp130-linked cytokines serves to 
abrogate the ligand-induced JAK-STAT cascade and block pro-­
opiomel

d-induced JAK-STAT cascade and block pro-­
opiomelanocortin (POMC) transcription and ACTH secretion. 
This rapid on-off regulation of ACTH secretion provides a plastic­
ity for the endocrine response to changes in environmental signal­
ing and serves to maintain homeostatic integrity.14
External/internal
environmental
signals
Uterine
contraction
Lactation
(oxytocin)
Water
balance
(vasopressin)
Central nervous
system
Electric or chemical
transmission
Hypothalamus
Axonal transport
Oxytocin, vasopr

ion
Oxytocin, vasopressin
Neurohypophysis
Release
Releasing hormones (ng)
Adenohypophysis
Target glands
Ultimate hormone (µg-mg)
Hormonal response
Anterior pituitary
hormones (µg)
Short
feedback
loop
Fast
feedback
loop
Long
feedback
loop
• Fig. 1.3  Peripheral feedback mechanism and a millionfold amplifying cascade of hormonal signals. Envi­
ronmental signals are transmitted to the central nervous system, which innervates the hypothalamus, 
which responds by secreti

vates the hypothalamus, 
which responds by secreting nanogram amounts of a specific releasing hormone. These are transported 
down a closed portal system, pass the blood-brain barrier at either end through fenestrations, and bind to 
specific anterior pituitary cell membrane receptors to elicit secretion of micrograms of specific anterior pitu­
itary hormones. These hormones enter the venous circulation through fenestrated local capillaries, bind 
to specific target gland receptors, trigger rele

to specific target gland receptors, trigger release of micrograms to milligrams of daily hormone amounts, 
and elicit responses by binding to receptors in distal target tissues. Peripheral hormone receptors enable 
widespread cell signaling by a single initiating environmental signal, thus facilitating intimate homeostatic 
association with the external environment. Arrows with a large dot at their origin indicate a secretory pro­
cess. (From Normal AW, Litwack G. Hormones. 2nd ed. New York: A

ormal AW, Litwack G. Hormones. 2nd ed. New York: Academic Press; 1997:14.)
10
In addition to the central nervous system–neuroendocrine 
interface mediated by hypothalamic chemical signal transduction, 
the central nervous system directly controls several hormonal 
secretory processes. Posterior pituitary hormone secretion occurs 
as direct efferent neural extensions. Postganglionic sympathetic 
nerves also regulate rapid changes in renin, insulin, and glu

 regulate rapid changes in renin, insulin, and glucagon 
secretion, while preganglionic sympathetic nerves signal to adre­
nal medullary cells eliciting epinephrine release. 
Hormone Measurement
Endocrine function can be assessed by measuring levels of basal 
circulating hormone, evoked or suppressed hormone, or hormone-
binding proteins. Alternatively, hormone function can be assessed. 
When a feedback loop exists between the hypothalamic-pituitary 
axis and a target gland, the circulating leve

ary 
axis and a target gland, the circulating level of the pituitary trophic 
hormone, such as TSH or ACTH, is typically an exquisitely sensi­
tive index of deficient or excessive function of the thyroid or the 
adrenal cortex, respectively. Meaningful strategies for timing hor­
monal measurements vary from system to system. In some cases, 
circulating hormone concentrations can be measured in randomly 
collected serum samples. This measurement, when standardized 
for fasting, environmental stre

when standardized 
for fasting, environmental stress, age, and gender, is reflective of 
true hormone concentrations only when levels do not fluctuate 
appreciably. For example, thyroid hormone, PRL, and IGF1 levels 
can be accurately assessed in fasting morning serum samples. On 
the other hand, when hormone secretion is clearly episodic, timed 
samples may be required over a defined time course to reflect hor­
mone bioavailability. Thus, early morning and late evening cortisol 
measurements ar

morning and late evening cortisol 
measurements are most appropriate. A 24-hour sampling for GH 
measurements, with samples collected every 2, 10, or 20 minutes, 
is expensive and cumbersome, yet may yield valuable diagnostic 
information. Random sampling may also reflect secretion peaks or 
nadirs, thus confounding adequate interpretation of results.
In general, confirmation of failed glandular function is made 
by attempting to evoke hormone secretion by recognized stimuli. 
Testing of pituita

cretion by recognized stimuli. 
Testing of pituitary hormone reserve may be accomplished by 
injecting appropriate hypothalamic-releasing hormones. Injec­
tion of trophic hormones, including TSH and ACTH, evokes 
specific target gland hormone secretion. Pharmacologic stimuli 
(e.g., metoclopramide for induction of PRL secretion) may also 
be useful tests of hormone reserve. In contrast, hormone hyperse­
cretion can best be diagnosed by suppressing glandular function. 
Failure to appropriately su

g glandular function. 
Failure to appropriately suppress GH levels after a standardized 
glucose load implies inappropriate GH hypersecretion. Failure to 
suppress insulin secretion during hypoglycemia indicates inappro­
priate hypersecretion of insulin and should prompt a search for 
the cause, such as an insulin-secreting tumor.
Radioimmunoassays use highly specific antibodies that 
uniquely recognize the hormone, or a hormone fragment, to 
quantify hormone levels. Enzyme-linked immunosorbent 

ntify hormone levels. Enzyme-linked immunosorbent assays 
(ELISAs) use enzyme-conjugated antibodies, and enzyme activity 
is reflective of hormone concentration. Immunometric assays use 
two antibodies directed to different epitopes of a polypeptide hor­
mone: one “capture” antibody that isolates the hormone to a solid 
support and one “signal” antibody coupled to a signal-generating 
molecule such as acridinium ester or an enzyme. These sensitive 
techniques have allowed ultrasensitive measurem

e 
techniques have allowed ultrasensitive measurements of physi­
ologic hormone concentrations. Hormone-specific receptors may 
be used in place of the antibody in a radioreceptor assay. However, 
all antibody-based assays may be subject to artifacts, which should 
be kept in mind especially when the assay results are discordant 
with the clinical picture. 
Endocrine Diseases
Endocrine diseases fall into four broad categories: (1) hormone 
overproduction, (2) hormone underproduction, (3) altered

oduction, (2) hormone underproduction, (3) altered tissue 
responses to hormones, and (4) tumors of endocrine glands. An 
additional albeit atypical fifth category is exemplified by one kind 
of hypothyroidism in which overexpression of a hormone-inacti­
vating enzyme in a tumor leads to thyroid hormone deficiency. 
Other disorders of inadequate hormone inactivation include 
apparent mineralocorticoid excess, vitamin D 24-­hydroxylase 
deficiency, and X-linked hypophosphatemic rickets (PHEX 
def

, and X-linked hypophosphatemic rickets (PHEX 
deficiency).
Hormone Overproduction
Occasionally, hormones are secreted in increased amounts 
because of genetic abnormalities that cause abnormal regulation 
of hormone synthesis or release. For example, in glucocorticoid-­
remediable hyperaldosteronism, an abnormal chromosomal 
crossover event creates a fusion gene that encodes a protein with 
aldosterone synthase activity under the control of the ACTH-
regulated 11β-hydroxylase promoter. More oft

ACTH-
regulated 11β-hydroxylase promoter. More often, diseases of hor­
mone overproduction are associated with an increase in the total 
number of hormone-producing cells. For example, hyperthyroid­
ism associated with Graves disease, in which antibodies mimic 
TSH and activate the TSH receptors on thyroid cells, is accompa­
nied by dramatic increase in thyroid cell proliferation and increased 
synthesis and release of thyroid hormone from each thyroid cell. 
In this example, the increase in thy

hyroid cell. 
In this example, the increase in thyroid cell number represents 
a polyclonal expansion of thyroid cells, in which large numbers 
of thyroid cells proliferate in response to an abnormal stimulus. 
However, most endocrine tumors are not polyclonal expansions 
but instead represent monoclonal expansions of a single mutated 
cell. Pituitary and parathyroid tumors, for example, are usually 
monoclonal expansions in which somatic mutations in multiple 
CNS Inputs
Pituitary trophic
hormo

ns in multiple 
CNS Inputs
hormone
Target gland
Pituitary
Tier II
Paracrine cytokines
and growth factors
Tier III
Peripheral
hormones
Tier I
Hypothalamic
hormones
• Fig. 1.4  Model for regulation of anterior pituitary hormone secretion by 
three tiers of control. Hypothalamic hormones impinge directly on their 
respective target cells. Intrapituitary cytokines and growth factors regu­
late trophic cell function by paracrine (and autocrine) control. Peripheral 
horm

aracrine (and autocrine) control. Peripheral 
hormones exert negative feedback inhibition of respective pituitary trophic 
hormone synthesis and secretion. CNS, central nervous system. (From 
Ray D, Melmed S. Pituitary cytokine and growth factor expression and 
action. Endocr Rev. 1997;18:206–228.)
11
tumor suppressor genes and proto-oncogenes occur. These muta­
tions lead to an increase in proliferation or survival of the mutant 
cells. Sometimes this prol

survival of the mutant 
cells. Sometimes this proliferation is associated with abnormal 
secretion of hormone from each tumor cell. For example, mutant 
Gsα proteins in somatotrophs can lead to both increased cellular 
proliferation and increased secretion of GH from each tumor cell. 
Hormone Underproduction
Underproduction of hormone can result from a wide variety of 
processes, ranging from surgical removal of parathyroid glands 
during neck surgery, to tuberculous destruction of adrenal gland

rgery, to tuberculous destruction of adrenal glands, 
to iron deposition in pancreatic beta cells of islets in hemochro­
matosis. A frequent cause of destruction of hormone-producing 
cells is autoimmunity. Autoimmune destruction of beta cells in 
type 1 diabetes mellitus or of thyroid cells in chronic lympho­
cytic (Hashimoto) thyroiditis are two of the most common dis­
orders treated by endocrinologists. Recently a direct passage of 
insulin fragments by exocytosis from pancreatic islets to ly

agments by exocytosis from pancreatic islets to lym­
phoid tissue has been shown to trigger autoimmune diabetes in 
mice.15 Multiple genetic abnormalities can also lead to decreased 
hormone production. These disorders can result from abnormal 
development of hormone-producing cells (e.g., hypogonadotropic 
hypogonadism caused by KAL gene mutations), from abnormal 
synthesis of hormones (e.g., deletion of the GH gene), or from 
abnormal regulation of hormone secretion (e.g., the hypoparathy­
roi

n of hormone secretion (e.g., the hypoparathy­
roidism associated with activating mutations of the parathyroid 
cell’s ­calcium-sensing receptor). Drugs are important causes of 
endocrine gland dysfunction as exemplified by immune check­
point inhibitors leading to multiple endocrinopathies. 
Altered Tissue Responses to Hormones
Resistance to hormones can be caused by a variety of genetic dis­
orders. Examples include mutations in the GH receptor in Laron 
dwarfism and mutations in the Gsα gene 

 in Laron 
dwarfism and mutations in the Gsα gene in the hypocalcemia of 
pseudohypoparathyroidism type 1A. Insulin resistance in muscle 
and liver central to the cause of type 2 diabetes mellitus is com­
plex in origin, resulting from inherited variations in many genes, 
as well as from theoretically reversible physiologic stresses. Type 2 
diabetes is also an example of a disease in which end-organ insen­
sitivity is worsened by signals from other organs, in this case by 
signals originating i

her organs, in this case by 
signals originating in fat cells. In other cases, the target organ of 
hormone action is more directly abnormal, as in PTH resistance 
occurring with renal failure.
Increased end-organ function can be caused by mutations in 
signal reception and propagation. For example, activating muta­
tions in TSH, LH, and PTH receptors can cause increased activity 
of thyroid cells, Leydig cells, and osteoblasts, even in the absence 
of ligand. Similarly, activating mutations in 

ce 
of ligand. Similarly, activating mutations in the Gsα protein can 
cause precocious puberty, hyperthyroidism, and acromegaly in 
McCune-Albright syndrome. 
Tumors of Endocrine Glands
Tumors of endocrine glands often result in hormone overproduc­
tion. Some endocrine gland tumors produce little if any hormone 
but cause disease by local, compressive symptoms or by meta­
static spread. Examples include so-called nonfunctioning pitu­
itary tumors, which are usually benign but can cause a variet

s, which are usually benign but can cause a variety of 
symptoms due to compression of adjacent structures, and thyroid 
cancer, which can metastasize without causing hyperthyroidism. 
Excessive Hormone Inactivation or Destruction
Although most enzymes important for endocrine systems activate 
a prohormone or precursor protein, there are also those whose 
function is to inactivate the hormone in a physiologically regulated 
fashion. An example is the type 3 iodothyronine deiodinase (D3), 
which 

 the type 3 iodothyronine deiodinase (D3), 
which inactivates T3 and T4 by removing an inner ring iodine 
atom from the iodothyronine, blocking its nuclear receptor bind­
ing. Large infantile hepatic hemangiomas express high D3 levels, 
causing “consumptive hypothyroidism,” because thyroid hormone 
is inactivated at a more rapid rate than it can be produced.16,17 
Furthermore, D3 may also be induced in other tumors by tyrosine 
kinase inhibitors. In theory, accelerated destruction of other hor­

 In theory, accelerated destruction of other hor­
mones could occur from similar processes as yet to be determined. 
Diagnostic and Therapeutic Uses of 
Hormones
In general, hormones are used pharmacologically for their replace­
ment or suppressive effects. Hormones may also be used for diag­
nostic stimulatory effects (e.g., hypothalamic hormones) to evoke 
target organ responses or to diagnose endocrine hyperfunction 
by suppressing hormone hypersecretion (e.g., T3). Ablation of 
endocrine gla

ersecretion (e.g., T3). Ablation of 
endocrine gland function due to genetic or acquired causes can 
be restored by hormone replacement therapy. Thyroid hormones 
and some steroids can be replaced orally, whereas peptide hor­
mones and analogues (e.g., insulin, PTH, GH) are administered 
parenterally or absorbed through mucous membranes (inhaled 
insulin, intranasal desmopressin). Gastrointestinal absorption and 
first-pass kinetics determine oral hormone dosage and availability. 
Physiologic re

l hormone dosage and availability. 
Physiologic replacement can achieve both appropriate hormone 
levels (e.g., thyroid) and approximate hormone secretory patterns 
(e.g., GnRH delivered intermittently via a pump). Hormones can 
also be used to treat diseases associated with glandular hyperfunc­
tion. Long-acting depot preparations of somatostatin receptor 
ligands suppress GH hypersecretion in acromegaly and hypersecre­
tion of diarrhea-causing mediators from neuroendocrine tumors 
of the pancr

mediators from neuroendocrine tumors 
of the pancreas and small intestine. Estrogen receptor antagonists 
(e.g., tamoxifen) are useful for some patients with breast cancer, 
and GnRH analogues may downregulate the gonadotropin axis 
and benefit patients with prostate cancer.
Novel formulations of receptor-specific hormone ligands are 
now being clinically developed (e.g., estrogen agonists/antago­
nists, somatostatin receptor subtype-specific ligands, or peroxi­
some proliferator-activated recep

ands, or peroxi­
some proliferator-activated receptor alpha [PPARα] ligands), 
resulting in more selective therapeutic targeting. Modes of hor­
mone injection (e.g., for PTH) may also determine therapeutic 
specificity and efficacy. Improved hormone delivery systems, 
including computerized minipumps, intranasal sprays (e.g., for 
desmopressin), pulmonary inhalers, depot intramuscular injec­
tions, and orally bioavailable peptide formulations, will also 
enhance patient compliance and improve ea

ll also 
enhance patient compliance and improve ease of administration. 
Cell-based therapies using the reprogramming of human cells to 
perform differentiated functions, either through differentiation 
of induced pluripotent stem cells or directed differentiation of 
one somatic cell type into another, are under active investiga­
tion.18 Novel technologies offer promise of marked prolongation 
in the half-life of peptide hormones, thereby requiring infre­
quent administration. For example, a on

ing infre­
quent administration. For example, a once weekly preparation of 
­glucagon-like peptide-1 (GLP-1) analogues is used in the treat­
ment of type 2 diabetes.
12
Important progress has been made in the therapeutic use of 
hormones. Although delivery of insulin usually still relies on fre­
quent administration by injection and close monitoring by the 
patient, purity of the insulin preparations, as well as novel deliv­
ery devices, has enhanced pati

ell as novel deliv­
ery devices, has enhanced patient compliance and quality of life. 
Preparations with differing pharmacokinetics allow the normal 
physiology of insulin secretion to be more closely mimicked. 
Continuous administration via subcutaneous pump infusion 
enhances therapeutic effectiveness in carefully selected patients. 
These include closed-loop systems, in which the dose of insulin 
is automatically adjusted depending on continuously monitored 
interstitial glucose concentration

usly monitored 
interstitial glucose concentrations. Implementation of such sys­
tems has the potential to substantially reduce the burden of this 
disease. However, hormones are biologically powerful molecules 
that exert therapeutic benefit and effectively replace pathologic 
deficits. They should not be prescribed without clear-cut indica­
tions and should not be administered without careful evaluation 
by an appropriately qualified medical practitioner. 
Future Perspectives
An introduction t

actitioner. 
An introduction to the principles underlying endocrinology 
should end by emphasizing the rapidly changing dynamics of 
discovery in this field and attempting to foresee what remains 
to be discovered. New hormones are continually being discov­
ered, from the recent focus on major regulators of metabolism 
and phosphate homeostasis (FGF19, FGF21, and FGF23) to the 
continued quest to identify ligands for orphan nuclear and G 
protein–coupled receptors.19 Presumab

clear and G 
protein–coupled receptors.19 Presumably other equally impor­
tant hormones remain to be discovered. The observation that 
nuclear receptors, like most transcription factors, bind to thou­
sands of specific sites within the cell’s nucleus stresses how little 
we understand about hormone action. Even the name “nuclear 
receptors” may be viewed in the future as misleading, since 
there is an increasing appreciation of extranuclear, rapid actions 
of nuclear receptors. Many of our diagn

d actions 
of nuclear receptors. Many of our diagnostic tests are severely 
limited by both technology and our inability to foresee novel 
diagnostic targets. For example, the “disappearance” of isolated 
GH deficiency when many children with that diagnosis achieve 
adulthood means either that we have little understanding of the 
etiology/pathogenesis of that childhood deficiency or that our 
diagnostic tools today yield many false-positive results. Although 
endocrinologists pride themselves wi

ts. Although 
endocrinologists pride themselves with having logical treatments 
for many diseases, these treatments seldom address their under­
lying causes. We have no satisfactory tools for preventing auto­
immune endocrine deficiencies or for preventing the benign 
tumors that underlie many diseases characterized by hormone 
excess. Treatments for diseases such as type 1 diabetes, although 
highly effective, are still very obtrusive in the lives of patients 
with this disease.
This new editio

es of patients 
This new edition communicates major advances that have been 
made in our field over the past 5 years, yet gaps in our knowledge 
about endocrinology remain. Importantly, debilitating chronic 
endocrine illnesses with significant morbidity (e.g., diabetes and 
Cushing disease) still pose significant diagnostic and therapeutic 
challenges.
References
The complete list of references is available online at ExpertConsult.com.
12.e1
	 1.	 Liu PT, Stenger 

-cell. 
Development. 140:2472–2483.
	19.	 Evans RM, Mangelsdorf DJ. Nuclear receptors, RXR, and the Big 
Bang. Cell. 157:255–266.
2 
Introduction to Hormone Signaling, 13 
Ligands That Act Through Cell Surface Receptors, 14 
Binding Properties of Cell Surface Receptors, 16 
Cell Surface Hormone Receptors, 16 
Coupling of Cell Surface Receptors to Intracellular Signaling, 27 
KEY POINTS 
Hormones signal to target cells via receptors on the cell surface 
or in the cell nucleus. 

ors on the cell surface 
Polypeptide hormones act at the cell surface and trigger a 
cascade of events in the cytoplasm as well as in the nucleus that 
alter the function of their target cells. 
In addition to polypeptide hormones, many nonpolypeptide 
hormones such as catecholamines signal via cell surface rece -
tors. 
There are multiple classes of cell surface receptors, including 
ligand-gated ion channel receptors, G protein-couped re ep-
tors, receptors with intrin

 protein-couped re ep-
tors, receptors with intrinsic enzymatic activity, and receptors 
that associate with enzymes. 
Some of the cell surface receptors have intrinsic catal tic activ-
ity, whereas others depend on interaction with other signaling 
proteins to exert their actions. 
Introduction to Hormone Signaling 
The evolution of multicellularity enabled specialization of organs 
and tissues. As organs took on distinct functions, mechanisms 
were required to allow communication between tissu

were required to allow communication between tissues; this is the 
fundamental purpose of hormones. Hormones encode informa-
tion about environmental or developmental conditions in one 
location and transmit that information to a separate location. This 
process ultimately requires that information move from outside of 
the target cell to its interior, so that cellular function can be altered 
to meet the needs of the organism. Specifically, the concentration 
of the substance must be detected b

concentration 
of the substance must be detected by the target cell and converted 
into a change in cellular activity, a process known as signal trans-
duction. The strategies used by hormones to affect cellular func-
tion are analogous and, in many cases, identical to those used by 
Disease Caused by Defective Cell Surface Receptors, 31 
Ligands That Act Through Nuclear Receptors, 31 
Nuclear Receptor Signaling Mechanisms, 34 
Receptor Regulation of Gene Transcription, 37 
Ligand binaing to the

n of Gene Transcription, 37 
Ligand binaing to the extracellular domain of cell surface receptors 
causes con~ rmational changes in the receptors that activate enzy-
matic activity and recruitment of cytoplasmic signaling proteins. 
Stemi and thyroid hormones signal via nuclear receptors. 
Some nuclear receptors transduce signals from vitamins, me-
ta olites, and drugs acting as ligands to regulate reproduction, 
growth, and metabolism. 
Nuclear receptors work directly in the cell nucleus to reg

receptors work directly in the cell nucleus to regulate 
gene transcription, acting at the genome and recruiting coregu-
lator proteins called corepressors and coactivators. 
Hormone binding to nuclear receptors causes a conformational 
change in the receptor that favors the recruitment of coactiva-
tors to the specific genes that are regulated. 
Some nuclear receptors may work through additional pathways 
that involve nongenomic mechanisms. 
other extracellular agents such as neurotransmitters,

er extracellular agents such as neurotransmitters, drugs, and 
metabolites. However, classic endocrinology defines itself as the 
process by which extracellular signaling molecules use the blood-
stream to travel from the organ of origin to the target tissue. By 
its nature, this process invariably results in dilution of the secreted 
molecule in the intravascular space, and thus, with rare exception, 
the target cell must be capable of detecting and responding to very 
low concentrations of hor

 and responding to very 
low concentrations of hormone. 
In spite of the vanishingly small concentrations of hormones 
present in the circulation, classic endocrine organs are usually 
uniquely equipped to secrete substantial amounts of hormone. 
Much of the history of endocrinology is defined by purification of 
hormones from these specialized secretory tissues. In the earliest 
days, the discovery of a hormone usually followed a stereotypical 
13 
14
co

13 
14
course of events: (1) A syndrome, often resembling some human 
disease, was associated with removal of an endocrine gland; (2) the 
abnormal phenotype would be corrected by the reimplantation 
of the absent organ; (3) the same cure would be accomplished by 
administration of an extract from the organ of interest; and (4) the 
active agent would be purified from the organ. The discovery of 
insulin represents the prototype for this series of observa

epresents the prototype for this series of observations, but 
the same process led to the identification of other hormones such 
as thyroid hormone and cortisol.
Hormones can be divided into two groups on the basis of where 
they function in a target cell. The first group includes hormones 
that interact with receptors at the cell surface. All polypeptide 
hormones (e.g., growth hormone [GH], insulin), monoamines 
(e.g., serotonin), and prostaglandins (e.g., prostaglandin E2) use 
cell surface r

ndins (e.g., prostaglandin E2) use 
cell surface receptors. The second group includes hormones that 
can enter cells. These hormones bind to intracellular receptors that 
function in the nucleus of the target cell to regulate gene expres­
sion. Classic hormones that use intracellular receptors include 
thyroid and steroid hormones.
It is worth noting that many molecules behave both as clas­
sical hormones that use the bloodstream to travel from their site 
of production to their site of action a

heir site 
of production to their site of action and as signaling molecules 
that do not meet that strict definition. For example, insulin-like 
growth factor 1 (IGF1) is produced and secreted by the liver 
under the positive influence of GH and circulates to target tissues 
like bone, but it is also produced locally by some tissues (e.g., 
chondrocytes at bone growth plates) to exert effects on neighbor­
ing cells. Similarly, norepinephrine is a neurotransmitter that is 
released at nerve endin

 neurotransmitter that is 
released at nerve endings and binds to cell surface receptors at 
postsynaptic membranes, but it is also secreted into the blood by 
the adrenal medulla, allowing it to act as a classic endocrine hor­
mone. Testosterone is a nuclear receptor ligand that is produced 
by the Leydig cells of the testis; it can circulate as a hormone and 
act on muscle, bone, and other tissues, but it also acts as a para­
crine agent on neighboring seminiferous tubules. Finally, many 
secr

ghboring seminiferous tubules. Finally, many 
secreted molecules that are not regarded as classic hormones meet 
virtually all of the criteria used to define such agents. For exam­
ple, cytokines are released by immune cells at the site of inflam­
mation, but they also circulate in plasma and bind to cell surface 
receptors in the brain, evoking fever. In this sense, many circu­
lating molecules, including those produced exogenously (i.e., 
obtained from the diet or synthesized by commensal bact

ned from the diet or synthesized by commensal bacteria), 
could be regarded as having hormonal properties. The key point 
is that a complete understanding of cell surface and nuclear recep­
tor biology requires a more inclusive perspective than is typically 
achieved by adhering to a strict set of definitional criteria estab­
lished decades ago. Having said that, in the interest of brevity, this 
chapter focuses primarily on receptors that bind classic hormonal 
ligands, with examples drawn from

lassic hormonal 
ligands, with examples drawn from other systems as needed to 
provide a more comprehensive picture that reflects our current 
understanding of receptor biology. 
Ligands That Act Through Cell Surface 
Receptors
The impermeability of the plasma membrane to peptides and 
small, water-soluble, charged molecules requires that recep­
tors that recognize such substances be located on the outer sur­
face of the cell. The limiting membrane of a typical eukaryotic 
cell is a 5-nm to 8-nm

ne of a typical eukaryotic 
cell is a 5-nm to 8-nm structure composed of proteins embed­
ded in a bilayer of phospholipids and cholesterol, forming the 
fluid-mosaic membrane. The phospholipid polar head groups face 
outward from the membrane, interacting with the hydrophilic 
milieu that comprises the extracellular fluid and the cytoplasm. 
Buried between these two charged surfaces are the hydrophobic 
lipid tails of the phospholipids made up of acyl groups, which are 
long chains of hydrocarbo

 acyl groups, which are 
long chains of hydrocarbons derived from fatty acids. The strongly 
nonpolar environment prevents the diffusion of water-soluble 
molecules, including many hormones, across the membrane. Thus 
surface proteins are needed to detect the presence of extracellu­
lar ligands that cannot diffuse and are not transported into the 
cell. Information from this hormone-binding process must then 
be transmitted across the plasma membrane so that intracellular 
signaling can commence

rane so that intracellular 
signaling can commence.
Classic Peptide Hormones
Most notable among the hormones that bind to cell surface recep­
tors are the peptide hormones, which vary in size from a hand­
ful to hundreds of amino acids. Examples of peptide hormones 
include the glycoproteins and the GH family of proteins secreted 
by the pituitary, the pancreatic hormones glucagon and insulin, 
and numerous peptides secreted from nonglandular organs, such 
as leptin from adipocytes and atrial na

ans, such 
as leptin from adipocytes and atrial natriuretic peptide from the 
heart (Table 2.1).
A hormone’s rate of secretion is closely tailored to its lifetime 
in the circulation and to its time course of action. In general, pep­
tide hormones are released from endocrine glands quickly, as they 
are preformed and stored in secretory vesicles or granules. In the 
  Hormones That Work on the Cell Surface
Peptides and Proteins
Adrenocorticotropic hormone (ACTH)
Antidiuretic hormone (ADH)
Atria

ic hormone (ACTH)
Atrial natriuretic peptide (ANP)
Calcitonin
Cholecystokinin
Corticotropin-releasing hormone (CRH)
Follicle-stimulating hormone (FSH)
Gastrin
Glucagon
Gonadotropin-releasing hormone (GnRH)
Growth hormone (GH)
Growth hormone–releasing hormone (GHRH)
Insulin
Insulin-like growth factor 1 (IGF1)
Luteinizing hormone (LH)
Oxytocin
Parathyroid hormone (PTH)
Prolactin (PRL)
Secretin
Somatostatin (SS)
Thyrotropin-releasing hormone (TRH)
Thyrotropin or thyroid-s

n-releasing hormone (TRH)
Thyrotropin or thyroid-stimulating hormone (TSH)
Molecules Derived From Amino Acids
Dopamine (inhibits prolactin)
Epinephrine (also called adrenaline)
Norepinephrine (also called noradrenaline)
Serotonin
Eicosanoids
Prostaglandins: PGA1, PGA2, PGE2
TABLE 2.1
15
Chapter 2  Principles of Hormone Action
course of synthesis, peptide hormones are diverted to secretory 
vesicles via a regulated secretory pathway (Fig. 2.1). The cyto­
plasm of endocrine glands containing such 

e cyto­
plasm of endocrine glands containing such secretory vesicles, such 
as the endocrine pancreas, the anterior pituitary, and the parathy­
roid glands, is filled with 200-nm electron-dense granules that 
represent the packaged hormone awaiting secretion. Just as secre­
tion of hormones stored within vesicles can be evoked quickly, 
often within milliseconds, release can usually be terminated 
abruptly with great efficiency. Peptide hormones tend to have 
very short half-lives within the cir

tend to have 
very short half-lives within the circulation, which allows blood 
levels to change rapidly in response to changes in secretion. Like 
the rapid changes in secretion and blood concentrations, initia­
tion of signaling tends to be rapid, which is facilitated by high on 
rates for hormone binding to receptors. In contrast, the off rate is 
often slow, which results in a high equilibrium binding constant 
that enables the receptors to detect the relatively low levels of hor­
mone in bl

etect the relatively low levels of hor­
mone in blood. However, a slow off rate is not compatible with 
the relatively rapid transient nature of peptide hormone–initiated 
signaling, suggesting mechanisms must exist for turning off the 
hormonal signal other than simple diffusion of the hormone off 
of the receptor.
A notable exception to the general rule that peptide hormones 
turn over quickly and have short durations of action is provided 
by IGF1. Unlike most peptide hormones, IGF1 circulate

IGF1. Unlike most peptide hormones, IGF1 circulates in the 
bloodstream bound to one or more binding proteins, which has 
two important consequences. First, the concentration of total 
IGF1 in blood is much greater than that of the unbound, bio­
logically active hormone. Second, the lifetime of IGF1 is greatly 
extended, such that circulating levels of the hormone change 
slowly over the course of hours or days. As predicted by these 
properties, IGF1 primarily influences phenotypes that are mod

 IGF1 primarily influences phenotypes that are modi­
fied over extended periods, such as growth and differentiation, 
and in marked contrast to its cousin insulin, most of the cellular 
targets of IGF1 are transcriptional. 
Nonpeptide Hormones That Act at Cell Surface 
Receptors
In addition to peptide hormones, there are small, hydrophilic 
hormones related to monoamine neurotransmitters that bind cell 
surface receptors. These include adrenergic agents such as nor­
epinephrine as well as other 

 agents such as nor­
epinephrine as well as other amino acid–derived water-soluble 
molecules such as melatonin, serotonin, and histamine. Like pep­
tide hormones, these hormones can be stored in dense secretory 
vesicles, but they are more typically packaged into small, approxi­
mately 50-nm electron-lucent vesicles that are similar morpho­
logically to vesicles in neural and neuroendocrine cells. The major 
difference is that in the presynaptic cleft, the vesicles are arranged 
in a tightly pa

 cleft, the vesicles are arranged 
in a tightly packed array at the membrane.
Interestingly, while most lipophilic molecules have intracellular 
receptors, there is at least one class of lipid that breaks this rule. 
The eicosanoids are a group of extracellular signaling molecules 
derived from 20-carbon fatty acids that includes the leukotrienes 
and prostaglandins. Many biologically active eicosanoids bind to 
cell surface receptors, which initiate their functions.1
The recent expansion of mes

iate their functions.1
The recent expansion of messenger types and the novel modes 
of interorgan communication have dramatically changed the tra­
ditional view of endocrinology such that all cell types can poten­
tially both send and receive messages. One of the more interesting 
recent additions to the assortment of hormone-like molecules has 
Nucleus
Cisternae
4
3
2
1
Transition elements
Secretory vesicles
Immature secretory granules
Mature secretory granules
Lysosome
RER
SER/Golgi
Capillary 

cretory granules
Lysosome
RER
SER/Golgi
Capillary lumen
Plasma membrane
• Fig. 2.1  Subcellular organelles involved in transport and secretion of polypeptide hormones or other 
secreted proteins within a protein-secreting cell. (1) Synthesis of proteins on polyribosomes attached to 
rough endoplasmic reticulum (RER) and vectorial discharge of proteins through the membrane into the 
cisterna. (2) Formation of shuttling vesicles (transition elements) from endoplasmic reticulum followed by 
their t

s) from endoplasmic reticulum followed by 
their transport to and incorporation by the Golgi complex. (3) Formation of secretory granules in the Golgi 
complex. (4) Transport of secretory granules to the plasma membrane, fusion with the plasma mem­
brane, and exocytosis resulting in the release of granule contents into the extracellular space. Notice that 
secretion may occur by transport of secretory vesicles and immature granules or by transport of mature 
granules. Some granules are taken up 

t of mature 
granules. Some granules are taken up and hydrolyzed by lysosomes (crinophagy). Golgi, Golgi complex; 
SER, smooth endoplasmic reticulum. (From Habener JF. Hormone biosynthesis and secretion. In: Felig P, 
Baxter JD, Broadus AE, et al, eds. Endocrinology and Metabolism. New York: McGraw-Hill; 1981:29–59.)
16
been circulating metabolites such as lactate, ketone bodies, and 
succinate, as well as ions such as calcium2 (Table 2.2). An even 
more 

 ions such as calcium2 (Table 2.2). An even 
more distant modification of the original definition of hormone is 
the idea that metabolites produced by microbes in the gut, such 
as short-chain fatty acids, can signal by binding to cell surface 
receptors.3 
Binding Properties of Cell Surface Receptors
When a hormone or hormone-like molecule arrives at a target 
cell, at least three critical components are required to induce the 
appropriate biologic response. First, there has to be recognition 

gic response. First, there has to be recognition 
of the hormone as different from all other components of the 
extracellular milieu. This is an issue of specificity (the ability to 
distinguish the hormone from other structurally related mol­
ecules). Second, the receptor must be able to recognize the low 
concentration of hormone found in the blood, which is an issue 
of sufficient affinity of the receptor for the hormone. Third, the 
initial recognition step mediated by the receptor must be c

ecognition step mediated by the receptor must be con­
verted into a single action or a defined set of cellular events. 
Studies of the binding properties of hormones and neurotrans­
mitters crystallized into a fundamental rule governing the action 
of extracellular agents: A biologic effect is directly proportional 
to the ligand occupancy of the receptor. A subtle but important 
modification to occupancy theory is the notion of spare recep­
tors, which describes the situation in which a maximal

, which describes the situation in which a maximal biologic 
response is achieved by occupancy of only a portion of the avail­
able receptors. One consequence of the existence of spare recep­
tors is that a decrease in the number of cellular receptors results in 
a change in the ED50 (effective dose for eliciting a 50% response) 
for a hormone but does not necessarily alter the maximal biologic 
response, as detailed later for insulin.4,5
As noted, a fundamental characteristic of a cell surface 

d, a fundamental characteristic of a cell surface recep­
tor is the ability to bind hormone with high specificity and high 
affinity. In addition, because there is a limited number of recep­
tors, binding is saturable, such that adding additional ligand 
above a certain level results in no additional binding and no fur­
ther increment in downstream biologic activity. Specificity, affin­
ity, and saturability can be established experimentally by assessing 
the binding of ligands to receptors, usi

ssessing 
the binding of ligands to receptors, using radioactive ligands in a 
variety of in vitro binding assays.6 Authentic physiologic receptors 
for a given hormone will display a greater affinity for the cognate 
hormone than other potentially competing circulating molecules. 
In addition, the half maximal binding for a hormone to its real 
receptor will always be in the range of the circulating free concen­
tration of that hormone. 
Cell Surface Hormone Receptors
Cell surface receptors can

rface Hormone Receptors
Cell surface receptors can be grouped conveniently into four 
classes: ligand-gated ion channel receptors, G protein–coupled 
receptors, receptors with intrinsic enzymatic activity, and recep­
tors that associate with enzymes.
Ligand-Gated Ion Channels
The simplest form of a cell surface signaling system is one in which 
both hormone-binding and signal-generating functions are pro­
vided by a single protein or complex of proteins. Ligand-gated 
ion channels fall into this

roteins. Ligand-gated 
ion channels fall into this category. They are made up of two key 
components: a ligand-binding domain accessible from the surface 
of the cell and a transmembrane domain containing a channel. 
Binding of ligand to the exofacial surface of the receptor generates 
a conformational change that results in the opening of a pore, 
allowing specific ions to travel through the channel across the 
plasma membrane (Fig. 2.2).
The prototype and founding member of the family of ligan

ototype and founding member of the family of ligand-
gated ion channels is the nicotinic acetylcholine receptor, which 
is present on some neurons and on the postsynaptic membrane 
of the neuromuscular junction.7 When a nerve impulse arrives 
at the presynaptic terminal, depolarization leads to an increase 
in cytosolic calcium and secretion of acetylcholine. The secreted 
acetylcholine binds to its receptor on the muscle, which elicits a 
conformational change that opens the pore and allows sod

mational change that opens the pore and allows sodium 
and potassium ions to pass in and out of the cell, respectively. 
This leads to depolarization and muscle contraction. The struc­
ture of the acetylcholine receptor, which is made up of four dif­
ferent peptides that constitute five subunits, defines a family 
of receptors that also includes the 5-hydroxytryptamine type 
3 (5HT3R), glycine, and inhibitory GABA type A receptors. 
Another shared characteristic of pentameric receptors is a con­

d characteristic of pentameric receptors is a con­
served 15-amino acid dicysteine loop in the extracellular ligand-
binding domain (LBD), giving this family its alternative name, 
the cys-loop receptors.8
Most ligand-gated ion channels, which when activated can 
elicit microsecond changes in signal transduction, serve as neu­
rotransmitter receptors rather than receptors for classic hormones. 
A notable exception involves the receptors for some hypothalamic 
releasing factors, which are dischar

hypothalamic 
releasing factors, which are discharged from hypothalamic neurons 
into the portal circulation to regulate the secretion of hormones 
from the anterior pituitary. For example, it is thought that sero­
tonin regulates release of prolactin by binding to the 5HT3R recep­
tor in lactotrophs of the anterior pituitary.9 Glycine and GABA 
receptors are present in the pituitary gland, but their physiologic 
functions appear complex and remain imperfectly understood. 
Another class of ligan

in imperfectly understood. 
Another class of ligand-gated ion channels, the purinergic cation 
receptors, are also expressed in the pituitary and most likely func­
tion in an autocrine/paracrine fashion in response to extracellular 
adenosine triphosphate (ATP). 
G Protein–Coupled Receptors
The largest family of cell surface receptors is defined by their use 
of heterotrimeric G proteins for signaling, leading to their des­
ignation G protein–coupled receptors (GPCRs). These receptors 
  Recept

pled receptors (GPCRs). These receptors 
  Receptors for Metabolites and Ions
Metabolite
Receptor
Lactate
GPR81
Ketone bodies
GPR109A
3-Hydroxyoctanoate
GPR109B
Succinate
GPR91
α-Ketoglutarate
GPR80/99
Long-chain fatty acids
GPR40, GPR120
Medium-chain fatty acids
GPR84
Short-chain fatty acids
GPR41, GPR43
Calcium
CASR
TABLE 2.2
17
have seven 25–amino acid α-helical segments that pass through 
the plasma membrane, with the amino (N)-terminus and carboxy 
(

mbrane, with the amino (N)-terminus and carboxy 
(C)-terminus outside the cell and in the cytoplasm, respectively, 
leading to the name seven transmembrane (7TM) proteins.10 There 
are more than 800 GPCR family members, with the vast majority 
being olfactory receptors. The diversity of ligands capable of bind­
ing to GPCRs is remarkable, ranging from a single photon to large 
proteins and including ions, odorants, amines, peptides, lipids, 
nucleotides, and metabolic intermediates. The smaller 

eotides, and metabolic intermediates. The smaller hormones, 
including catecholamines, bind to their GPCRs within the trans­
membrane-spanning region, oriented parallel to the cell surface; 
larger hormones bind to the extracellular N-terminus, which itself 
can range in size from 10 to 600 amino acids,11 in addition to 
interacting with the transmembrane-spanning region (Fig. 2.3). 
The GPCR family has been divided into five subfamilies based 
on primary sequence and phylogeny, named the glutam

n primary sequence and phylogeny, named the glutamate, rho­
dopsin, adhesion, frizzled/taste2, and secretin families.12 Many 
hormones, including some hypothalamic releasing factors, the 
glycoprotein hormones secreted by the pituitary, and the amines, 
bind to members of the rhodopsin-like family. On the other hand, 
glucagon, parathyroid hormone (PTH), calcitonin, and some 
hypothalamic hormones, such as GH-releasing factor and cortico­
tropin-releasing factor, bind to members of the secretin-

releasing factor, bind to members of the secretin-like fam­
ily. For many GPCRs, the endogenous ligand and its function are 
not known; these GPCRs are known as orphan receptors. 
Signaling by Heterotrimeric G Proteins
An important advance in the understanding of GPCRs occurred 
when Bourne and associates took advantage of the lethality of 
cyclic adenosine monophosphate (cAMP) toward lymphoma cells 
to select mutant lines resistant to the actions of the β-adrenergic 
agent isoproterenol.13 Beca

s of the β-adrenergic 
agent isoproterenol.13 Because the mutant cell lines lost respon­
siveness to a number of nonadrenergic agonists, it was clear that 
the genetic lesion did not reside in the β-adrenergic receptor but 
in a downstream component. When the signaling module that 
restored hormone responsiveness to the deficient membranes was 
purified, it turned out to be a heterotrimeric G protein complex, 
now known as Gs.14 Gs binds a single guanosine triphosphate 
(GTP) to its α-subunit, w

 guanosine triphosphate 
(GTP) to its α-subunit, which causes the α-subunit to dissociate 
from its other two (β, γ) subunits. The GTP-bound α-subunit 
of Gs is necessary and sufficient for activation of its downstream 
target, adenylyl cyclase. Like Gs, all G protein complexes are 
composed of one member each of the α-subunit, β-subunit, and 
γ-subunit families. Which exact subunit family member deter­
mines the downstream effector(s). Sixteen distinct genes encode 
about 20 different G protein

istinct genes encode 
about 20 different G protein α-subunits, which can be divided 
into four groups based on both structure and function: Gαs, Gαi, 
Gαq/11, and Gα12.10 The Gαs family has only two members, Gαs 
and the G protein for the olfactory receptor, Gαolf; both couple 
to activation of adenylyl cyclase. The Gαi group of eight includes 
three Gαi proteins, all of which inhibit adenylyl cyclase; two Gα0 
proteins that are abundant brain proteins whose multiple targets 
are still not compl

oteins whose multiple targets 
are still not completely defined; two Gαt proteins that couple pho­
toreceptors to cAMP phosphodiesterase (PDE); and Gαz, which 
inhibits potassium channels. The Gαq/11 subfamily consists of six 
members, all of which activate the enzyme phospholipase C beta 
(PLCβ), generating the second messengers diacylglycerol (DAG) 
and inositol trisphosphate (IP3). Gα12 and Gα13, which inhibit 
and activate the guanine nucleotide exchange factor, RhoGEF, 
respectively, form t

ide exchange factor, RhoGEF, 
respectively, form the final group. The combinational possibilities 
are complex, with 5 β-subunit isoforms and over 12 γ-subunit 
isoforms.
The key operational feature of G protein signaling is that the 
system behaves like a timed switch. Engagement of hormone with 
its cognate receptor promotes its association with a heterotrimeric 
G protein comprised of subunits Gα, Gβ, and Gγ (Fig. 2.4). This 
stimulates dissociation of guanosine diphosphate (GDP) from the 
α-

iation of guanosine diphosphate (GDP) from the 
α-subunit, allowing GTP to bind to the unoccupied site as a result 
of its greater intracellular concentration compared to GDP. The 
occupied receptor then detaches from the G protein. GTP loading 
of the G protein also induces the trimeric G protein complex to 
dissociate into the α-subunit and a dimeric β/γ-subunit, at least 
in vitro; it is not clear that dissociation actually occurs in an intact 
cell. In most cases, the α-subunit modulates an 

cell. In most cases, the α-subunit modulates an associated ampli­
fier, which in the case of Gs is adenylyl cyclase, but other targets 
of α-subunits include those referred to previously. The β/γ-dimer 
can also interact with and regulate downstream signaling mol­
ecules. For example, the β/γ-dimer activates potassium channels 
following ligand binding to the muscarinic acetylcholine receptor. 
Na+
Na+
Na+
Na+
Ligand
Ligand-gated
ion channel
Na+
Na+
Na+
Na+
Na+
• Fig. 2.2  Ligand-gated ion cha

+
Na+
Na+
Na+
Na+
• Fig. 2.2  Ligand-gated ion channels are transmembrane proteins that comprise at least two domains, a 
ligand-binding domain and a membrane-spanning domain capable of functioning as a pore. When a ligand 
binds, it induces a conformational change in the receptor such that the pore opens to the passage of ions, 
in this case sodium ions, down their electrochemical gradient.
18
A
B
C
D
E
F
Light
Biogenic amines
Peptides
Glycoprotein hormo

Light
Peptides
Glycoprotein hormones
Family 1
Family 2
Family 3
Peptides
Ca2+, Glutamate
• Fig. 2.3  The G protein–coupled receptor (GPCR) superfamily: diversity in ligand binding and structure. Each panel depicts members of the GPCR superfamily. 
The seven-membrane-spanning α-helices are shown as cylinders, with the extracellular amino (N)-terminus and three extracellular loops above them and the 
intracellular carboxy (C)-terminus and three intracellular loops below. The super

nus and three intracellular loops below. The superfamily can be divided into three subfamilies on the basis of amino acid sequence 
conservation within the transmembrane helices. Family 1 includes the opsins (A), in which light (arrow) causes isomerization of retinal covalently bound within the 
pocket created by the transmembrane helices (bar); monoamine receptors (B), in which agonists (arrow) bind noncovalently within the pocket created by the 
transmembrane helices (bar); receptors for pepti

transmembrane helices (bar); receptors for peptides such as vasopressin (C), in which agonist binding (arrow) may involve parts of the extracellular N-terminus 
and loops and the transmembrane helices (bar); and glycoprotein hormone receptors (D), in which agonists (oval) bind to the large extracellular N-terminus, acti­
vating the receptor through undefined interactions with the extracellular loops or transmembrane helices (arrow). (E) Family 2 includes receptors for peptide hor­
mones such a

2 includes receptors for peptide hor­
mones such as parathyroid hormone and secretin. Agonists (arrow) may bind to residues in the extracellular N-terminus and loops and to transmembrane helices 
(bar). (F) Family 3 includes the extracellular Ca2+-sensing receptor and metabotropic glutamate receptors. Agonists (circle) bind in a cleft of the Venus flytrap–like 
domain in the large extracellular N-terminus, activating the receptor through undefined interactions with the extracellular loops or tra

d interactions with the extracellular loops or transmembrane helices (arrow).
Critical to signal transduction by G proteins is that they remain 
active as long as GTP is bound. The rate of conversion of nucleo­
tide GTP to GDP determines both the timing for inactivation 
of signaling and reassembly of subunits. Thus the G protein can 
exist in two distinct states: bound to GTP and active or bound 
to GDP and inactive; the time spent in each condition defines 
the strength of signaling. G protein

tion defines 
the strength of signaling. G protein α-subunits have low levels of 
intrinsic GTPase activity, but this can be enhanced by association 
with the regulators of G protein signaling (RGS) proteins.15 Thus 
RGS proteins, which function as GTPase accelerating proteins 
19
(GAPs), serve to shorten the duration of signaling by G proteins, 
providing another important site of regulation. Many members 
of the large family of RGS proteins contain withi

of the large family of RGS proteins contain within their primary 
sequences canonical domains indicative of other functions and 
undergo complex post-translational modification. Modulation 
of the levels of RGS proteins affords a mechanism for signaling 
pathways to communicate with each other. For example, both 
thyroid-stimulating hormone (TSH, thyrotropin) and PTH sig­
nal through a Gs-cAMP pathway to increase expression of RGS2, 
which feeds back to inhibit Gs and to antagonize other pathwa

 back to inhibit Gs and to antagonize other pathways 
that depend on Gq.
Another GPCR regulatory system involves a family of proteins 
called arrestins (see Fig. 2.4). Two of the four arrestins (1 and 4) 
have been designated visual arrestins because they are expressed 
only in photoreceptor cells, while two arrestins (2 and 3) are 
expressed ubiquitously; the latter two are also called β-arrestins 
1 and 2. Ligand binding to a GPCR not only signals the dis­
sociation of the G protein complex as

als the dis­
sociation of the G protein complex as described earlier, but also 
promotes a conformational change in the GPCR that often leads 
to phosphorylation of the receptor by a G protein receptor kinase 
(GRK).16 GRKs are represented by a family of seven related 
kinases. Phosphorylation of the GPCR at serine and threonine 
residues by GRKs allows the binding of an arrestin, which steri­
cally uncouples the GPCR from the G protein, terminating the 
signal. Binding to the receptor also alte

ing the 
signal. Binding to the receptor also alters the conformation of the 
arrestin such that it interacts with components of the endocytosis 
system such as clathrin.17 The GPCR is escorted to the sorting 
endosome where it either recycles back to the cell surface or is 
targeted to the lysosome for degradation. This system provides 
an efficient mechanism for homologous desensitization, in which 
there is receptor-specific downregulation of signaling pathways. 
This mechanism stands in cont

signaling pathways. 
This mechanism stands in contrast to negative regulation by sec­
ond messenger–dependent protein kinases, which phosphorylate 
and inhibit all susceptible GPCRs regardless if occupied by ligand. 
In addition to its role in the modulation of G protein signaling, 
β-arrestin has a well-defined function as a signaling intermediate. 
Effector
Receptor
activation
by agonist
G protein
Synthesis
and targeting
of components
GTPase
and receptor
resensitization
GTP
GDP
Rece

s
GTPase
GTP
GDP
Receptor
Agonist
γ
β
α
Arrestin
Receptor
kinase
Effector
GDP
P
Receptor
γ
β
α
Effector
GTP
Receptor
Agonist
γ
β
α
Arrestin
Receptor
kinase
Arrestin
Receptor
kinase
• Fig. 2.4  The G protein guanosine triphosphatase (GTPase) and G protein–coupled receptor (GPCR) 
desensitization-resensitization cycle. In each panel, the shaded area denotes the plasma membrane, with 
the extracellular region above and the intracellular region below. In the basal state,

he intracellular region below. In the basal state, the G protein is a 
heterotrimer with guanosine diphosphate (GDP) tightly bound to the α-subunit. The agonist-activated 
GPCR catalyzes release of GDP, which permits guanosine triphosphate (GTP) to bind. The GTP-bound 
α-subunit dissociates from the βγ-dimer. Arrows from the α-subunit to the effector and from the βγ-dimer 
to the effector indicate regulation of effector activity by the respective subunits. The arrow from effector to 
the α-subun

 subunits. The arrow from effector to 
the α-subunit indicates regulation of its GTPase activity by effector interaction. Under physiologic condi­
tions, effector regulation by G protein subunits is transient and is terminated by the GTPase activity of the 
α-subunit. The latter converts bound GTP to GDP, thereby returning the α-subunit to its inactivated state 
with high affinity for the βγ dimer, which reassociates to form the heterotrimer in the basal state. In the basal 
state, the receptor 

he basal state. In the basal 
state, the receptor kinase and arrestin are shown as cytosolic proteins. Dissociation of the GTP-bound 
α-subunit from the βγ-dimer permits the dimer to facilitate binding of receptor kinase to the plasma mem­
brane (arrow from βγ-dimer to receptor kinase). Plasma membrane binding permits the receptor kinase to 
phosphorylate the agonist-bound GPCR (P, depicted here as occurring on the carboxy-terminal tail of the 
GPCR, although sites on intracellular loops are als

PCR, although sites on intracellular loops are also possible). GPCR phosphorylation facilitates arrestin 
binding to the GPCR, resulting in desensitization. Endocytic trafficking of arrestin-bound GPCR and recy­
cling to the plasma membrane during resensitization are not shown.
20
β-arrestin is now known to bind multiple members of the Src fam­
ily of tyrosine kinases as well as other proteins, such as mitogen-
activated protein kinases (MAPKs, also known

ogen-
activated protein kinases (MAPKs, also known as extracellular 
regulated kinases [ERKs]), phosphoinositide 3-kinase (PI3K), 
Akt, PDE4, and c-Jun N-terminal kinase-3.18,19
One of the most interesting aspects of GPCR signaling is the 
ability of GPCRs to undergo functional selectivity (also known as 
biased signaling), defined as the ability of ligands to stimulate dis­
tinct downstream signaling pathways, presumably due to stabiliza­
tion of distinct conformational states of the receptor.2

f distinct conformational states of the receptor.20 For GPCR 
receptors that can activate multiple G proteins, biased signaling 
refers to the ability to preferentially activate pathways downstream 
of a subset of the G proteins. For GPCR receptors that can acti­
vate arrestins, biased signaling usually refers to the ability of the 
receptor to favor the G protein response and minimize the arrestin 
response. Most of the research activity around biased signaling has 
taken place in the pharmaceu

biased signaling has 
taken place in the pharmaceutical industry, where the principle 
has been used in attempts to develop more specific therapeutics. 
For example, attempts have been made to develop opioid agonists 
that activate the analgesic effects mediated by G protein signal­
ing but are devoid of arrestin-dependent desensitization and tol­
erance.21,22 A similar strategy is being attempted to dissociate G 
protein–mediated opioid analgesia from arrestin-mediated consti­
pation and respir

a from arrestin-mediated consti­
pation and respiratory depression.21,22 Which downstream path­
ways are initiated by activation of a given GPCR is affected by the 
type and concentration of the ligand itself, and also the recruit­
ment of specific GRKs, the subcellular location of the GPCR, and 
the time after ligand exposure.23 
Receptor Tyrosine Protein Kinases as Cell Surface 
Receptors
The receptors that make up the receptor tyrosine protein kinase 
(RTK) family use a number of strategies t

 kinase 
(RTK) family use a number of strategies to accomplish the same 
goal: to convert the binding of ligand to the exofacial portion of 
the receptor to a change in the activity of a tyrosine protein kinase 
domain residing in the interior of the cell. All of these receptors 
are type I transmembrane proteins with an N-terminal hormone-
binding domain on the outside, a 25–amino acid hydrophobic 
segment that spans the membrane (the transmembrane domain), 
and a carboxy portion of the protein

ane domain), 
and a carboxy portion of the protein containing a kinase domain 
extending into the cytoplasm24 (Fig. 2.5). The intracellular cata­
lytic domain transfers phosphate from ATP to tyrosine residues in 
proteins, including the receptor itself. The 58 RTKs expressed in 
humans can be divided into about 20 subfamilies based on struc­
tural features. One of these groups is exemplified by the insulin 
receptor, which, unlike other RTKs, exists as a disulfide-linked 
tetramer in the basal s

sts as a disulfide-linked 
tetramer in the basal state. Receptors in all other subgroups of 
RTKs, including receptors for fibroblast growth factor, platelet-
derived growth factors (PDGFs), and epidermal growth factor 
(EGF), exist as monomers, though there is evidence that many 
associate noncovalently into larger structures in the basal state.
Biochemical experiments involving affinity cross-linking and 
biosynthetic labeling identified the structure of the insulin recep­
tor and that of the 

ructure of the insulin recep­
tor and that of the highly related IGF1 receptor as a heterotet­
ramer, composed of two 125-kDa α-subunits and two 90-kDa 
β-subunits linked by disulfide bonds25,26 (Figs. 2.5 and 2.6). 
The receptor is synthesized as a single peptide with a cleavable 
signal sequence directing insertion cotranslationally into the 
membrane, and it is glycosylated and cleaved into the α and β 
chains in the Golgi complex.27 Even though they exist as two 
separate peptides in the mat

gh they exist as two 
separate peptides in the mature protein, each pair of α and β 
chains behaves much like a receptor monomer found in other 
growth factor receptors. Affinity labeling by insulin shows cross-
linking to both the α-subunit and β-subunit, indicating that 
both are accessible to substances at the surface of the cell. Insulin 
binding has been long recognized to exhibit negative cooperativity, 
which means that the affinity for additional hormone decreases 
as the population of r

ditional hormone decreases 
as the population of receptors binds more ligands.28 In structural 
terms, this is explained by the presence of four binding sites on 
EGF
receptor
PDGF
receptor
Insulin
receptor
Cys-rich
domain
Tyrosine
kinase
domain
Ig-like
domain
• Fig. 2.5  Receptor tyrosine kinases. Three of the 16 families of receptor tyrosine kinases are represented. 
All receptor tyrosine kinases possess an extracellular domain containing the ligand-binding site, a sin­
gle transmembrane domai

igand-binding site, a sin­
gle transmembrane domain, and an intracellular portion containing the tyrosine kinase domain. Several 
structural motifs (i.e., cysteine-rich domain, immunoglobulin-like domain, tyrosine kinase domain) in these 
receptor tyrosine kinases are indicated on the right side of the figure. Dotted lines indicate disulfide bonds. 
Cys, cysteine; EGF, epidermal growth factor; Ig, immunoglobulin; PDGF, platelet-derived growth factor.
21
ea

tor.
21
each holoreceptor—two of low affinity and two of high affin­
ity. Insulin initially binds to a low-affinity site before binding 
to a high-affinity site on the contralateral α/β-dimer, thus effec­
tively cross-linking the two halves of the receptor such that the 
stoichiometry of this high-affinity complex is one insulin mol­
ecule per insulin receptor. This stable structure prevents bind­
ing of hormone to the second high-affinity site. This struc

rmone to the second high-affinity site. This structural 
organization for binding is largely conserved in the association 
of IGF1 with its receptor.29 Other classes of RTKs use alterna­
tive strategies for ligand binding. For example, activation of the 
EGF receptor appears to require binding of one EGF molecule 
to the outer surface of one of two noncovalently associated EGF 
receptors,30,31 while PDGF binds as a dimer to two noncovalently 
associated PDGF receptors.32
In general, activation o

ociated PDGF receptors.32
In general, activation of RTKs requires the formation of a 
receptor dimer. In some cases, such as the insulin, IGF1, FGF, 
and EGF receptors, the unbound receptors appear to consist 
of preformed dimers. In other cases, bivalent ligand binding to 
two receptor monomers is thought to promote dimer formation. 
Examples of receptors thought to act this way include the recep­
tors for PDGF, vascular endothelial growth factor, and nerve 
growth factor.
Because some of the R

r, and nerve 
Because some of the RTK receptors exist as a dimer in the 
basal state, it is clear that dimerization alone is not sufficient to 
activate RTKs; there must also be some fundamental change in 
the interaction between the two halves of the receptor. In the 
case of the insulin and IGF1 receptors, the extracellular por­
tions of the unbound receptor exist in an inverted V conforma­
tion formed by the α-subunits and part of the β-subunits.33 The 
base is continuous with 

of the β-subunits.33 The 
base is continuous with and anchored by the transmembrane 
domains of the β-subunits. Insulin or IGF1 binding to its low-
affinity site removes a brake on a molecular hinge, allowing the 
V to close and bring the transmembrane domains closer to each 
other34,35 (see Fig. 2.6). This conformational change is trans­
mitted to the cytoplasmic domains, where it has the effect of 
bringing the two kinase domains into closer proximity. In the 
unbound state, each kinase domain

oximity. In the 
unbound state, each kinase domain is inactive due to an intra­
molecular peptide, the activation loop, which is buried in the cat­
alytic cleft and sterically hinders entry of substrates.36 When the 
two cytoplasmic portions of the receptor domains are brought 
sufficiently close together, the kinase domain of one β-subunit 
phosphorylates the other on a cluster of tyrosine residues in 
the activation loop, forcing the loop out of the catalytic cleft, 
thus activating the kinase

f the catalytic cleft, 
thus activating the kinase domain.37 This is possible because 
of the kinetic nature of the receptor’s inactive state, in which 
the catalytic site is always alternating between open and closed 
conformations, though in the basal state the activation loop is 
inaccessible most of the time. However, when the contralateral 
kinase domain is brought sufficiently close, it can phosphorylate 
the activation loop during the brief period it is in the extended 
Extracellular
TM
J

f period it is in the extended 
TM
JM
KIN
A
B
Intracellular
IGF-1
Fn1
Fn2
Fn3
Fn3*
> 100 Å
L2
L2
L1
L1*
L1
L1*
Fn3*
CR
CR
Fn1
Fn3
Fn2
• Fig. 2.6  How insulin-like growth factor 1 (IGF1) activates its receptor. Each IGF1 receptor is made up of 
two half-receptors, which are linked by disulfide bonds (not shown). The six domains in the extracellular 
region of the first half-receptor (orange) are L1, CR, L2, Fn1, Fn2, and Fn3; the domains in the second 
half-receptor (green) are the 

ains in the second 
half-receptor (green) are the same and labeled with an asterisk. The L1, CR, L2, and Fn1 are in the α-chain, 
and the Fn3 and the transmembrane and intracellular domains make up the β-chain of each half-receptor; 
the Fn2 domain is made up of contributions from both chains. The intracellular region comprises the juxta­
membrane region (JM) and the tyrosine kinase domain (KIN). Sites of transphosphorylation are shown as 
circles. (A) When IGF-1 is not bound to the receptor, an

s. (A) When IGF-1 is not bound to the receptor, an interaction between L1* of the second half-receptor 
and Fn2 and Fn3 of the first half-receptor (and vice versa) is thought to maintain a large separation between 
the transmembrane (TM) helices (double arrow). (B) When IGF1 binds to L1* (or to L1), it disrupts the L1*-
Fn2 (or L1-Fn2*) interaction. This allows Fn2 and Fn3 of each half-receptor to pivot (curved arrows) toward 
each other (the previous positions of Fn2 and Fn3 are shown semitrans

vious positions of Fn2 and Fn3 are shown semitransparently). This in turn facilitates 
the dimerization of the TM helices in the membrane, which juxtaposes the kinase domains for efficient 
transphosphorylation (black arrows). Binding of a single IGF1 molecule (shown as binding to the left side) is 
sufficient to activate the receptor, but exactly how this asymmetry affects the conformational changes in the 
receptor is unclear. It is believed that the same mechanism also applies to activation o

at the same mechanism also applies to activation of the insulin receptor. 
(Modified from Hubbard SR, Miller WT. Closing in on a mechanism for activation. eLife. 2014;3:e04909.)
22
position, converting this to the more stable conformation. In 
this way, phosphorylation of one-half of the receptor increases 
its kinase activity, allowing the kinase in that half of the receptor 
to phosphorylate the activation loop in the other half and, ulti­
mately, exoge

on loop in the other half and, ulti­
mately, exogenous substrates.38 Proximity-driven phosphoryla­
tion and activation of one monomer by the other are common 
features of RTK activation, but the precise strategies utilized 
to achieve this vary. Thus, although the active conformations 
of all tyrosine protein kinases are similar, the configurations of 
the inactive states differ enormously. An exception to the rule 
of activation by transphosphorylation is provided by the EGF 
receptor, in which

ylation is provided by the EGF 
receptor, in which activation depends on allosteric regulation 
of the kinase domain of one monomer by the other monomer, 
once again brought about by a conformational change bring­
ing the two domains into adjacency. The critical interaction is 
between the C lobe of the activator kinase and the N lobe of 
the receiver kinase, which disrupts an autoinhibitory interaction 
present in the inactive monomer.39 
Signaling by Receptor Tyrosine Protein Kinases
Because t

ing by Receptor Tyrosine Protein Kinases
Because the insulin receptor is an enzyme with catalytic activity 
residing on the cytoplasmic surface of the plasma membrane, it 
stands to reason that it would transmit its signal by phosphory­
lating protein substrates within the cell. Nonetheless, though 
autophosphorylation sites within and outside the cytoplasmic 
kinase domain of the β-subunit have been long recognized, it 
proved difficult to identify robust, physiologically significant 
phosphory

ify robust, physiologically significant 
phosphorylation of tyrosine residues in other proteins. This 
seeming paradox is partially explained by the underlying mecha­
nism of activation of signaling pathways by RTKs, which signal 
by recruiting a variety of signaling proteins to the different phos­
phorylated tyrosines in the receptor. These signaling proteins 
contain motifs such as the Src homology 2 (SH2) domain and 
the phosphotyrosine binding (PTB) domain that bind to phos­
phorylated tyros

g (PTB) domain that bind to phos­
phorylated tyrosines in specific contexts. In the case of the SH2 
domain, a phosphorylated tyrosine residue in concert with some 
amino acids C-terminal to the phosphotyrosine serves as the 
binding interface for SH2 domains and therefore provides much 
of the specificity of the interaction.40 For example, after PDGF 
binds to its receptor, autophosphorylation of tyrosines within the 
PDGF receptor in a context defined by the sequence tyrosine–
methionine–any a

defined by the sequence tyrosine–
methionine–any amino acid–methionine (YMXM) generates a 
binding site for the SH2 domains of the regulatory subunit of 
PI3K.41 PI3K comprises a regulatory subunit that contains two 
SH2 domains in tandem and a catalytic subunit. Recruitment of 
PI3K to a phosphorylated receptor present in the plasma mem­
brane both activates PI3K and brings the PI3K into proximity 
to its major physiologic substrate, the lipid phosphatidylinositol 
4,5-bisphosphate (PI4,5P2), w

hosphatidylinositol 
4,5-bisphosphate (PI4,5P2), which resides on the inner surface 
of the plasma membrane. PI3K phosphorylates PI4,5P2 on the 
3′-position of its inositol ring, generating phosphatidylinositol 
3,4,5-trisphosphate (PIP3), a potent signaling molecule by virtue 
of its ability to recruit protein kinases and other signaling mol­
ecules to the membrane. This illustrates an important principle 
governing RTK signaling: The initiation of intracellular events is 
often driven primaril

 of intracellular events is 
often driven primarily by the spatial relationship of proteins and 
lipids rather than changes in the specific activity of assembled 
components. Although in some cases the hormone-bound recep­
tor will modulate the activity of target protein by phosphory­
lation, the more important event is often the establishment of 
adjacency between two or more critical signaling molecules, such 
as PI3K and its substrate, PI4,5P2. An additional example of this 
signaling mechani

. An additional example of this 
signaling mechanism is provided by activation of another proto-
oncogene, c-Ras. In this case, signaling is initiated by recruit­
ment of the adapter protein SH2 domain-containing protein 
(SHC) or growth factor receptor-bound protein 2 (GRB2) via 
their SH2 and/or PTB domain. When SHC is recruited, it is in 
turn tyrosyl phosphorylated by the RTK, enabling it to recruit 
GRB2 via its SH2 domain. GRB2 contains two Src homology 
3 (SH3) domains that remain constit

 Src homology 
3 (SH3) domains that remain constitutively bound to a poly­
proline sequence in the son of sevenless (SOS) protein, which is 
thus, in turn, carried to the plasma membrane.42,43 Association 
of SOS with the plasma membrane is necessary and sufficient 
for activation of the small G protein Ras.44 SOS is a guanine 
nucleotide exchange factor (GEF) protein that activates Ras by 
catalyzing the removal of GDP from inactive Ras to allow bind­
ing of GTP. As noted earlier, the critical 

 bind­
ing of GTP. As noted earlier, the critical event that determines the 
activity of Ras is the positioning of SOS in proximity to Ras.45,46
The insulin and IGF1 receptors signal using a variation of the 
strategy described previously for the PDGF receptor (Fig. 2.7). 
Rather than assembling a signaling complex on the cytoplasmic 
domain of the receptor, they assemble the complex on members 
of a family of scaffolds called insulin receptor substrate (IRS) pro­
teins.47 There are at least thr

bstrate (IRS) pro­
teins.47 There are at least three members of this family in humans, 
but IRS1 and IRS2 are thought to be the most important to physi­
ologic signaling by insulin and IGF1. Like other members of the 
group, IRS1 and IRS2 lack intrinsic enzymatic activity; they serve 
solely as docking proteins to bring signaling molecules together 
into a multimeric complex. IRS1 and IRS2 are heavily tyrosine 
phosphorylated by activated insulin receptor, generating binding 
sites for the SH2 d

 receptor, generating binding 
sites for the SH2 domains of PI3K, GRB2, and the phosphotyro­
sine phosphatase SHP2. A pleckstrin homology (PH) domain and 
PTB domain located at the N-terminus of IRS1/2 are instrumental 
in bringing the protein to the receptor.48 Upon ligand engagement 
of the insulin or IGF1 receptor, IRS1/2 is rapidly phosphorylated 
on tyrosine residues and more slowly on serine/threonine residues, 
the latter by a number of cytoplasmic kinases, including protein 
kinase C (PK

toplasmic kinases, including protein 
kinase C (PKC), c-Jun N-terminal kinase (JNK), and pp70 S6 
protein kinase. Serine/threonine phosphorylation of IRS proteins 
provides a strong negative feedback signal as it is thought to block 
further tyrosine phosphorylation and in some cases induces deg­
radation of the protein.
There is some evidence that the insulin receptor is capable of 
signaling through scaffolds other than the IRS proteins, though 
the physiologic significance of these pathways r

the physiologic significance of these pathways remains unclear. 
The insulin receptor recruits SHC to a phosphotyrosine motif 
via SHC’s PTB domain and phosphorylates SHC to generate a 
docking site for the SH2 domain of GRB2; this leads to activa­
tion of Ras as described earlier.49 GRB10, and most likely its close 
relative GRB14, is an SH2 domain–containing protein that binds 
to the insulin receptor with high affinity.50 However, unlike the 
IRS proteins, GRB10 binds to the three phosphory

IRS proteins, GRB10 binds to the three phosphorylated tyro­
sine residues in the activation loop and blocks the activity of the 
insulin receptor, inhibiting the insulin-dependent production of 
PIP3.51 GRB10 is stabilized via phosphorylation by mammalian 
(mechanistic) target of rapamycin complex 1 (mTORC1), itself 
activated downstream of insulin, providing another form of nega­
tive feedback.52,53 Disruption of GRB10 in mice yields embry­
onic overgrowth, consistent with its role as a negat

ic overgrowth, consistent with its role as a negative regulator 
of IGF1 signaling.54 Both SH2B1 and SH2B2 (formerly known 
as APS) bind directly to the phosphorylated insulin receptor and 
both enhance the actions of insulin in vivo. However, while insu­
lin sensitivity is decreased in SH2B1-deficient mice, consistent 
23
with SH2B1 enhancing the actions of insulin, insulin sensitivity is 
modestly increased in SH2B2-deficient mice, suggesting that the 
S

ed in SH2B2-deficient mice, suggesting that the 
SH2B2 gene product(s) may negatively regulate insulin sensitivity 
in animals.51 
Receptor Serine/Threonine Protein Kinases
One of the more interesting variants on signaling by intracellular 
protein kinases is provided by a class of integral membrane recep­
tors possessing intrinsic serine/threonine protein kinase activ­
ity. Ligands for these receptors are members of the transforming 
growth factor-β (TGFβ) family of first messengers. These 42 a

or-β (TGFβ) family of first messengers. These 42 ago­
nists encoded in the human genome can be classified into distinct 
groups typified by TGFβ itself, activin, inhibin, bone morpho­
genetic protein (BMP)/growth and differentiation factor (GDF), 
nodal, myostatin, and antimüllerian hormone. Each ligand is 
composed of a dimer of two peptides joined by hydrophobic 
interactions and often disulfide bonds. The hormone inhibin was 
isolated as an activity produced by gonadal tissue that blocks the 

tivity produced by gonadal tissue that blocks the 
secretion of follicle-stimulating hormone (FSH) from the pitu­
itary.55 Like other members of the TGFβ family, it is composed of 
two chains, an α-subunit and one of two related β-subunits. The 
hormone activin, which promotes the release of FSH, is formed 
by the assembly of homodimers of the β-subunit.56 Like inhibin, 
activin was originally identified as a product of the gonads but is 
now known to be secreted by many tissues and to function 

wn to be secreted by many tissues and to function in an 
autocrine or paracrine manner as well. The first indication that the 
TGFβ family of ligands exerts its actions via membrane protein 
kinases arose from the cloning of a complementary DNA encod­
ing the activin receptor and recognition of a canonical kinase 
domain.57 Like all receptors for ligands in the TGFβ superfamily, 
the activin receptor is composed of four transmembrane glycopro­
teins, two type 1 receptors and two type 2 receptors

ins, two type 1 receptors and two type 2 receptors. Type 1 and 
2 receptors have a similar primary structure, the major difference 
being an insertion of a conserved 30–amino acid sequence rich 
in glycine and serine (the GS domain) in the type 1 cytoplasmic 
domain preceding the kinase domain, which binds the immu­
nophilin FKBP12. Activin interacts initially with type 2 recep­
tors, which brings the type 1 and type 2 receptors into proximity 
so that the type 2 receptors can phosphorylate the 

o that the type 2 receptors can phosphorylate the GS domain of 
the partner type 1 receptors. This alleviates steric hindrance of the 
type 1 receptor kinase catalytic site and releases FKBP12, the two 
changes working in concert to activate the type 1 receptors, which 
allows the receptors to phosphorylate target substrates.58 Inhibin 
exerts its inhibitory action by recruiting the transmembrane gly­
coprotein betaglycan (also called the type III receptor) to form a 
IRSs 1 and 2
Shc
Phosphoryl

I receptor) to form a 
IRSs 1 and 2
Shc
Phosphorylation
substrates
SH2 domain-
containing proteins
Effector
proteins
Downstream
effectors
Insulin
receptor
Insulin
PI 3-kinase p85
GRB-2
PI 3-kinase p110
m-SOS
PDK 1, 2
Akt
Sgk
PKC-λ/ξ
Ras
MAP kinase
• Fig. 2.7  Simplified model of signaling pathways downstream from the insulin receptor. Insulin binds to the 
insulin receptor, activating the receptor tyrosine kinase to phosphorylate tyrosine residues on insulin recep­
tor substrates (IRSs), includi

s on insulin recep­
tor substrates (IRSs), including IRS1 and IRS2. The phosphotyrosine residues in the IRS molecules bind 
to Src homology 2 (SH2) domains in molecules such as growth factor receptor–binding protein 2 (GRB2) 
and the p85 regulatory subunit of phosphoinositide (PI) 3-kinase (PI3K). These SH2 domain–containing 
proteins initiate two distinct branches of the signaling pathway. Activation of PI3K leads to activation of 
phosphoinositide-dependent kinases (PDKs) 1 and 2, which activa

ide-dependent kinases (PDKs) 1 and 2, which activate multiple protein kinases, including 
Akt/protein kinase B, atypical protein kinase C (PKC) isoforms, and serum-induced and glucocorticoid-
induced protein kinases (Sgk). GRB2 interacts with m-SOS, a guanine nucleotide exchange factor that 
activates Ras. Activation of Ras triggers a cascade of protein kinases, leading to activation of mitogen-
activated protein (MAP) kinase. Shc, Src, homology domain–containing protein.
24
SECTION I    Hormone

domain–containing protein.
24
stable complex with type 2 receptors, thus sequestering them and 
preventing activation of the partner type 1 receptors.59
The major intracellular signaling mechanism utilized by all 
members of the TGFβ family involves SMAD proteins, which 
function as the major substrates for type I receptors (Fig. 2.8). 
There are eight human genes coding for SMAD proteins. Five 
of the human SMAD proteins, termed the receptor regulated 
S

an SMAD proteins, termed the receptor regulated 
SMADs or R-SMADs (SMADs 1, 2, 3, 5, and 8/9), contain a 
Ser-X-Ser phosphorylation site at their C-terminal tail and serve as 
substrates for the type I receptors. The activin receptor phosphory­
lates SMAD3 (and possibly SMAD2). Two R-SMADs then form a 
trimer with the common partner SMAD or co-SMAD (SMAD4) 
and are transported to the nucleus.60 It is likely that other SMAD 
isoforms contribute to activin regulation of gene expression 
in vivo in

 activin regulation of gene expression 
in vivo in a tissue-specific manner. Upon import into the nucleus, 
SMAD proteins are modified at their so-called linker domains 
by a complex set of phosphorylation events that serve both to 
enhance binding of SMAD proteins to transcriptional regulatory 
proteins and to target SMAD proteins for ubiquitin-dependent 
proteasomal degradation. SMAD proteins bind directly to DNA 
through a conserved N-terminal domain and interact with other 
transcription fac

 domain and interact with other 
transcription factors, which, in concert with the SMAD proteins, 
exert control over a transcriptional network defined by the cell 
type and activating ligand. A third class of SMADs, the inhibitory 
or I-SMADs (SMADs 6 and 7), can bind to the activated receptor 
and promote ubiquitination and degradation of the receptor.
One particularly interesting member of the TGFβ family is 
the hormone myostatin, formerly known as GDF8. Myostatin 
is secreted by skeletal mu

own as GDF8. Myostatin 
is secreted by skeletal muscle and negatively regulates muscle 
growth through binding to a type II (ActR-IIB) receptor and type 
1 receptors (ALK4 and ALK5), which phosphorylate SMAD2 
and SMAD3.61 A deficiency of myostatin is responsible for the 
“double-muscled” phenotype of Belgian Blue and Piedmontese 
cattle, and deletion of its gene in mice and humans leads to mas­
sive muscle hypertrophy and hyperplasia.62 
Signaling by Receptors That Associate With 
Enzymes
Anoth

ng by Receptors That Associate With 
Enzymes
Another mode of signal transduction across the plasma membrane 
is provided by receptors that possess no intrinsic catalytic activity 
but that associate with a cytoplasmic, non–membrane-spanning 
tyrosine kinase. The best example of this is the family of class I 
and class II cytokine receptors, which are type 1 transmembrane 
proteins with the N-terminus on the outside of the cell and a 
cytoplasmic C-terminus (Fig. 2.9). As for RTKs, dimerization o

C-terminus (Fig. 2.9). As for RTKs, dimerization or 
higher order oligomerization appears important for activation of 
the receptor. In many cases, including the GH receptor, a single 
ligand molecule contains two distinct recognition sequences. The 
initial binding is to a high-affinity site, which is followed by a sec­
ond lower affinity association with a site located on a second, asso­
ciated monomer. The two monomers that compose the activated 
receptor make significant contact with each ot

ed 
receptor make significant contact with each other, again in the 
exofacial domain close to where the receptor inserts in the mem­
brane. For GH, prolactin, leptin, thrombopoietin, and erythro­
poietin (EPO), the receptor is a homodimer with two identical 
subunits. However, for some cytokines, the receptors consist of a 
ligand-specific monomer and one or more transmembrane chains 
shared with other cytokine receptors (see Fig. 2.9). For example, 
the interleukin 2 (IL2) receptor consists of

ple, 
the interleukin 2 (IL2) receptor consists of an IL2 receptor–specific 
P
P
P
P
P
Nucleus
Endosome
Regulate gene expression
DNA
Co-SMAD
R-SMAD
RI
RII
R-SMAD
P
Co-SMAD
R-SMAD
I-SMAD
Smurf
Receptor
ubiquitination
and degradation
R-SMAD
SARA
Ligand
• Fig. 2.8  Mechanism of action for receptor serine kinases. Binding of dimeric ligand to the type II receptor 
(RII) subunit triggers assembly of the receptor into the heterotetrameric [(RI)2(RII)2] state. RII transphos­
phorylates the type I recep

 state. RII transphos­
phorylates the type I receptor (RI), thereby activating phosphorylation of the receptor-regulated SMAD 
(R-SMAD) protein that is bound to the SMAD anchor for receptor activation (SARA) in endosomes. The 
phosphorylated R-SMAD associates with a comediator SMAD (Co-SMAD). Eventually, the R-SMAD is 
translocated into the nucleus, where it binds to DNA, enabling it to regulate gene transcription. The inhibi­
tory SMAD (I-SMAD) can also bind to the activated receptor, promoting

can also bind to the activated receptor, promoting ubiquitination and degradation of 
the receptor. P, phosphorylation; Smurf, SMAD ubiquitination regulatory factor.
25
subunit (IR2Rα), a second subunit shared with the IL15 receptor 
(IL2Rβ), and a γc subunit that is shared with the receptors for 
IL4, IL7, IL9, and IL15. The IL6 receptor has a unique subunit 
but shares a glycoprotein 130 (GP130) subunit with at least five 
other receptors. As with RTKs, 

ith at least five 
other receptors. As with RTKs, oligomerization appears impor­
tant for activation of these receptors, as indicated by the observa­
tion that bivalent, but not monovalent, antibodies are capable of 
activating the receptors. However, also like RTKs, dimerization 
alone is insufficient to activate this class of receptors. This was 
recognized when the EPO receptor and subsequently the GH and 
prolactin receptors were examined in situ and found to exist as 
preformed dimers even 

situ and found to exist as 
preformed dimers even in the unbound state.63 The importance 
of dimerization of the GH receptor is illustrated by the effective­
ness of the GH antagonist pegvisomant in treating acromegaly, a 
disease of excess GH secretion. Pegvisomant competes with native 
GH for its receptor and prevents functional dimerization.64
Proximal to the membrane on the inside of the cell, the class 
I and class II cytokine receptors have a conserved sequence that 
is critical to binding

 a conserved sequence that 
is critical to binding a protein tyrosine kinase of the JAK family. 
There are four members of this family: JAK1, JAK2, JAK3, and 
tyrosine kinase 2 (TYK2), with JAK3 largely restricted to cells of 
the hematopoietic lineage.65 For those receptors that function as 
homodimers, JAK2 is the predominant isoform involved in sig­
naling. Cytokine receptors that function as heterodimers or higher 
order oligomers tend to bind more than one JAK family member. 
For example, t

d more than one JAK family member. 
For example, the IFNγR1 subunit of the IFNγ receptor binds 
JAK1 and the IFNγR2 subunit binds JAK2, while the IL2 recep­
tor recruits JAK1 to the IL2Rβ subunit and JAK3 to the γc sub­
unit (see Fig. 2.9). The JAK proteins associate with a cytoplasmic, 
juxtamembrane portion of the cytokine receptors via a conserved, 
N-terminal domain structure called the FERM domain (named 
for its presence in Band 4.1 protein, ezrin, radixin, and moesin).66 
The carboxy half

, ezrin, radixin, and moesin).66 
The carboxy half of JAK consists of two homologous regions in 
tandem, a pseudokinase domain followed by a kinase domain. The 
former has many of the conserved sequences that define a protein 
kinase, but it also has mutations of amino acids that are essential 
for catalytic activity. It is believed that in the non–ligand-bound 
receptor, the intracellular portions of two monomers are arranged 
in a way such that each pseudokinase domain binds to and sup­
presse

 each pseudokinase domain binds to and sup­
presses the catalytic activity of the kinase on the other subunit, 
and vice versa. Binding of GH to its receptor results in a con­
formational change in the extracellular domain of the receptor, 
which induces a motion intracellularly like the opening of scissors, 
causing sliding of the two subunits of JAK in opposite directions. 
This relieves the allosteric inhibition of the kinases67,68 (Fig. 2.10).
The major consequence of releasing the block to 

.
The major consequence of releasing the block to GH receptor-
associated JAK2 activity is the JAK2-catalyzed transphosphory­
lation of the contralateral receptor subunit and its associated 
JAK2.65 This allows binding of the SH2 and/or PTB domains of 
a number of signaling molecules, including IRS1/2 and PLCγ, 
thus recruiting them to the receptor and plasma membrane.69 
However, more important than these to the actions of GH on 
growth are members of the signal transducers and activators of th

ers of the signal transducers and activators of the 
transcription (STAT) family (Fig. 2.11). The GH receptor binds 
a number of STAT family members, but STAT5b is most critical 
to its growth-promoting actions. There are seven STAT proteins 
with a shared domain structure. The N-termini are composed of 
four helical coils that function in binding to other proteins, fol­
lowed by a DNA-binding domain (DBD).70 The carboxy half of 
the proteins consists of a linker region, an SH2 domain, and a 
tr

sists of a linker region, an SH2 domain, and a 
transcriptional transactivation domain. Several of the tyrosine res­
idues in the GH receptor that undergo phosphorylation by JAK2 
in response to ligand binding serve as docking sites for STAT5b. 
Once recruited to the receptor, STAT5b is itself phosphorylated, 
resulting in dimerization, with each STAT5b protein binding via 
its phosphorylated tyrosine to its partner’s SH2 domain. At the 
same time, STAT5b dissociates from the receptor and transl

e, STAT5b dissociates from the receptor and translo­
cates into the nucleus where it can regulate gene transcription. In 
addition to this basic pathway, there are numerous other layers of 
regulation. Serine/threonine protein kinases such as members of 
the MAPK and PKC families also phosphorylate STAT proteins; 
in some cases, this latter phosphorylation is required for maximal 
GHR
GHR
IFNγR1
IFNγR2
IL2-Rα
IL2-Rβ
IFNγR1
IFNγR2
γc
A
B
C
GH
IL2
IFNγ
IFNγ
JAK2
JAK2 JAK2
JAK1
JAK1
JAK3
JAK1
JAK2

IFNγ
IFNγ
JAK2
JAK2 JAK2
JAK1
JAK1
JAK3
JAK1
JAK2
• Fig. 2.9  Cytokine receptors are composed of multiple subunits and bind to one or more members of 
the Janus kinase (JAK) family of tyrosine kinases. (A) Growth hormone (GH), such as prolactin and leptin, 
binds to growth hormone receptor (GHR) homodimers and activates JAK2. (B) Interferon-γ (IFNγ) homodi­
mers bind to their ligand-binding γR1 subunits. The γR2 subunits are then recruited, leading to activation 
of JAK1, which binds to the γR1 

ng to activation 
of JAK1, which binds to the γR1 subunit, and JAK2, which binds to the γR2 subunit. Both subunits and 
both JAKs are necessary for responses to IFNγ. (C) Interleukin 2 (IL2) binds with high affinity to receptors 
composed of three subunits: a γc subunit shared with receptors for IL4, IL7, IL9, IL15, and IL21; an IL2Rβ 
subunit shared with the IL15 receptor; and a noncytokine receptor subunit, IL2Rα. IL2 activates JAK3, 
bound to the γc subunit, and JAK1, bound to IL2Rβ. Extracel

the γc subunit, and JAK1, bound to IL2Rβ. Extracellular regions of homology are indicated by the 
black lines and colored patterns. Intracellular regions of homology are indicated by the small white boxes. 
Identical subunits are indicated by identical colors.
26
Transmembrane
domain
Plasma
membrane
Extracellular domain
Intracellular domain
Transactivation
Activation loop
Pseudo-
activation loop
Pseudokinase domain
Kinase domain
Catalytic do

oop
Catalytic domain
Juxtamembrane
domain
Growth hormone
Growth hormone receptor
(inactive)
(active)
JAK2 (inactive)
JAK2 (active)
• Fig. 2.10  Scissor model for activation of the human growth hormone (hGH) receptor. In the basal state, 
the hGH receptor exists as an inactive dimer in which the two subunits are held together through weak 
interactions in the transmembrane membrane domain (TMD) and poised in the inactive stat

brane domain (TMD) and poised in the inactive state through 
electrostatic repulsion in the extracellular juxtamembrane domain (JMD) and pseudokinase inhibition in the 
associated JAK2 dimer (left). Binding of hGH to the receptor (right) clamps the JMD such that it avoids 
the electrostatic repulsion and mechanically alters the TMD such that the intracellular domain is splayed 
outward. Splaying pulls on the JAK2 molecules to align their kinase domains. This triggers a wave of 
phosphorylation e

omains. This triggers a wave of 
phosphorylation events, including the STAT proteins critical to receptor signaling. (Modified from Wells JA, 
Kossiakoff AA. New tricks for an old dimer. Science. 2014;344:703.)
Cytokine
Recruitment
Dimerization
Translocation
GLE
STATs
Cytoplasm
Nucleus
P- -P
P- -P
-P
-P
P-
JAK
• Fig. 2.11  Cytokines activate signal transducers and activators of transcription (STATs). STAT proteins are 
latent cytoplasmic transcription factors. STATs bind through 

plasmic transcription factors. STATs bind through Src homology 2 (SH2) domains to one or more 
phosphorylated tyrosines (P) in activated receptor–Janus kinase (JAK) complexes. Once bound, STATs 
themselves are tyrosyl phosphorylated, presumably by the receptor-associated JAKs. STATs then dissoci­
ate from the receptor-JAK complex, homodimerize or heterodimerize with other STAT proteins, move to 
the nucleus, and bind to gamma-activated sequence-like elements (GLEs) in the promoters of cytokine-

ike elements (GLEs) in the promoters of cytokine-
responsive genes. P, phosphorylation. (Modified from J. Herrington, used with permission.)
27
transcriptional activation. Using a different mechanism, SH2B1 
binds to and enhances JAK2 activity.69,71 STAT proteins can also 
heterodimerize with other STAT proteins or other transcription 
factors. For example, STAT5b has been shown to dimerize with 
the glucocorticoid receptor, with the latter acting as a coa

orticoid receptor, with the latter acting as a coactivator 
for STAT5b to promote expression of GH-regulated genes (e.g., 
IGF1) implicated in body growth.72
Another important hormone that uses the JAK/STAT signaling 
pathway is leptin. Leptin is secreted by adipocytes; it acts on the 
arcuate nucleus of the hypothalamus as well as other regions in the 
brain to suppress appetite and, in rodents, increase metabolic rate. 
Humans deficient in leptin display massive obesity early in life.73 
Like 

in display massive obesity early in life.73 
Like GH, leptin binds to homodimers of a class I cytokine recep­
tor and activates JAK2.74 However, in contrast to the GH recep­
tor, the leptin receptor recruits STAT3 as its primary signaling 
molecule, which binds to phosphotyrosines in a YXXQ motif. The 
phosphorylated leptin receptor also binds STAT5 and the SH2-
containing protein tyrosine phosphatase 2 (SHP2; PTPN11). The 
latter is thought to act as a positive signaling module by mediat­
ing t

ct as a positive signaling module by mediat­
ing the first step in the activation of the ERK cascade.75 On the 
other hand, the tyrosine phosphatase PTP1B dephosphorylates 
the leptin receptor and inhibits leptin action, and thus its deletion 
in mouse brain leads to obesity and insulin resistance.76 JAK2 also 
phosphorylates IRS proteins, thereby engaging the PI3K pathway. 
The roles of the different signaling pathways activated down­
stream of leptin and JAK2 have been investigated using mice 

leptin and JAK2 have been investigated using mice in 
which specific tyrosine residues in the receptor have been mutated. 
Replacement of tyrosine 1138 by serine completely blocks recruit­
ment of STAT3, generating mice similar in their degree of obesity 
to those lacking leptin receptors, showing that STAT3 signaling 
is critical to the regulation of appetite and energy metabolism.77
Termination of class I cytokine signaling occurs in response to 
dephosphorylation of key phosphotyrosines; it i

o 
dephosphorylation of key phosphotyrosines; it is also promoted 
by the transcriptional induction of the suppressors of cytokine sig­
naling, or SOCS proteins. The eight members of the SOCS family 
are direct targets of the STAT transcription factors and provide 
a potent negative feedback signal by binding to phosphorylated 
tyrosines in the receptors via the SOCS SH2 domain. Upon inter­
acting with the receptors, SOCS proteins inhibit their action by 
reducing JAK activity, by competing for 

ction by 
reducing JAK activity, by competing for binding of other signaling 
molecules, and/or by inducing the degradation of receptor via the 
ubiquitin pathway due to the conserved SOCS box located at the 
C-terminus of the protein.78 Mice deficient for SOCS2 appear 
normal when young but after weaning grow substantially larger 
than their wild-type littermates, consistent with enhanced GH 
signaling.79 Cytokine signaling via STAT proteins can also be 
downregulated by members of the protein 

 also be 
downregulated by members of the protein inhibitor of activated 
STAT (PIAS) family, which have been shown to regulate transcrip­
tion through several mechanisms, including blocking the DNA-
binding activity of transcription factors, recruiting transcriptional 
corepressors, and promoting protein sumoylation. 
Coupling of Cell Surface Receptors to 
Intracellular Signaling
Downstream Signaling by Cyclic Adenosine 
Monophosphate
For the many hormones that bind exclusively to the outer sur

ny hormones that bind exclusively to the outer surface 
of cells to carry out their actions, there must be some means of 
translating the extracellular signal into an intracellular response. 
The first example of a transduction system that was understood 
in some detail derived from investigating one of the key features 
of the fight-or-flight response, the mobilization of stored carbo­
hydrate in the liver. The physiologic response to stress requires a 
supply of readily consumable energy, best

uires a 
supply of readily consumable energy, best provided in the form of 
blood glucose, which is stored as polysaccharide glycogen primar­
ily in the liver. β-Adrenergic stimulation of hepatocytes by epi­
nephrine leads rapidly to the hydrolysis of glycogen and the release 
of free sugar; glucagon also stimulates the breakdown of hepatic 
glycogen. The mechanism used to transmit this response is the 
prototypical example of a second messenger system, in which the 
agonist that interacts with 

system, in which the 
agonist that interacts with the outside of the cell, in this case glu­
cagon or epinephrine, is considered a first messenger, and a soluble, 
intracellular signaling molecule generated by hormone-receptor 
association is called a second messenger.80 For hepatic glycogen 
breakdown in response to glucagon or β-adrenergic agents, the 
second messenger is cAMP, which is produced by a plasma mem­
brane enzyme, adenylyl cyclase, from ATP (Fig. 2.12). Adenylyl 
cyclase is a direc

from ATP (Fig. 2.12). Adenylyl 
cyclase is a direct target of GαS, which becomes GTP loaded and 
active in response to receptor occupancy.
The scope and diversity of hormones and other extracellular 
signals that activate adenylyl cyclase and increase the level of intra­
cellular cAMP are remarkably extensive. Included in the long list 
of hormones that signal through this mechanism are β-adrenergic 
agents, glycoprotein hormones such as TSH, glucagon, adreno­
corticotropic hormone (ACTH), hypot

cagon, adreno­
corticotropic hormone (ACTH), hypothalamic hormones, and 
antidiuretic hormone. Moreover, the range of physiologic and 
biochemical events modulated by cAMP is equally vast. Thus, 
although the second messenger cAMP defines a commonly used 
mechanism for transducing signals from extracellular hormones, it 
also presents another problem in signaling: How do cells maintain 
selectivity in the way they respond to a given hormone? Much of 
this is accomplished by the subcellular compa

 of 
this is accomplished by the subcellular compartmentalization of 
signaling complexes. A-kinase anchoring proteins (AKAPs), which 
are scaffolds localized to distinct intracellular sites, bind a number 
of proteins that modulate the actions of cAMP, including degrad­
ing enzymes and target kinases.81 The regulated assembly of higher 
order structures confers a spatiotemporal resolution to cAMP sig­
naling that can allow multiple biologic responses to exist within 
the same cell. For example,

onses to exist within 
the same cell. For example, β-adrenergic agents and prostaglandin 
E1 both act on the heart through elevations in cAMP, but each reg­
ulates a different cardiac function. This is accomplished through 
stimulation of distinct populations of cAMP target kinases, such 
that β-adrenergic agents are more potent than prostaglandins in 
their effects on the particulate fractions of the heart cell.82 It is 
likely that AKAPs confer this specificity to the cardiomyocyte.
cAMP is de

 this specificity to the cardiomyocyte.
cAMP is degraded to AMP and phosphate by a specific PDE, 
and the balance between synthesis and degradation of cAMP 
determines the levels of the cyclic nucleotide. Although hormones 
generally use adenylyl cyclase as the means for modulating cAMP 
levels within the cell, the PDEs provide an additional site of reg­
ulation.83 The cyclic nucleotide PDEs are a large and complex 
family of enzymes, whose diversity in both tissue and subcellular 
localization 

sity in both tissue and subcellular 
localization has made them favorite targets for the development of 
therapeutics. Caffeine and theophylline were two of the first drugs 
recognized to be inhibitors of PDE, but more recently, selective 
inhibitors of PDE5, an enzyme that degrades cyclic guanosine 
monophosphate (cGMP), have been widely used for the treat­
ment of erectile dysfunction. In addition, PDE inhibitors are 
either currently being used or are in development for the treat­
ment of a w

d or are in development for the treat­
ment of a wide variety of diseases, including asthma, neurologic 
diseases, and pulmonary hypertension.
28
Glycogen metabolism in liver and muscle provided the initial 
example of a common mode of signaling initiated by second mes­
sengers—activation of a cascade of intracellular protein kinases. 
Signal transduction by protein phosphorylation stands as one of 
the most critical regulatory mechanisms in biology. The 

st critical regulatory mechanisms in biology. The state of a 
phosphoprotein is regulated dynamically, determined by the rela­
tive rates of phosphorylation and dephosphorylation by protein 
kinases and phosphatases, respectively. In most cases, the turnover 
of the phosphate is rapid, allowing regulation by either the kinase 
or phosphatase, or in many instances both coordinately. Numer­
ous endocrine signals exert control over intracellular metabolism, 
growth, and other functions via modulati

abolism, 
growth, and other functions via modulation of protein kinase 
activity. Originally, protein kinases were found to phosphorylate 
proteins on serine and threonine residues, but, as described ear­
lier, tyrosine phosphorylation has emerged as another mode of 
signaling.
One advantage of such series of kinases is signal amplifica­
tion. Amplification occurs because each individual kinase mol­
ecule can modify many downstream target proteins. When these 
downstream targets are also kinases

s. When these 
downstream targets are also kinases that become activated upon 
phosphorylation, each one of them can in turn modify and acti­
vate many more proteins. In the case of cAMP-initiated signal­
ing, one receptor:ligand pair creates multiple cAMP molecules. 
The multiple cAMP molecules activate the serine/threonine kinase 
protein kinase A (PKA), which in turn phosphorylates multiple 
downstream effector proteins. In the case of glycogen metabolism, 
PKA phosphorylates and activates gl

n metabolism, 
PKA phosphorylates and activates glycogen phosphorylase kinase, 
which in turn phosphorylates and activates glycogen phosphory­
lase, which releases glucose-1-P from glycogen. In muscle, phos­
phorylase kinase is also stimulated by calcium, which is released 
from the sarcoplasmic reticulum during electrical stimulation 
and contraction. The mechanism by which cAMP activates PKA 
illustrates another theme in signal transduction: displacement or 
dissociation of intramolecular pseu

splacement or 
dissociation of intramolecular pseudosubstrates or substrates as 
a means to activate protein kinases, a mechanism also used by 
such protein kinases as PKC and myosin light chain kinase. cAMP 
binds to the two regulatory subunits of the heterotetrameric PKA, 
which causes them to dissociate from two catalytic subunits. A 
domain in the regulatory subunit resembles a PKA phosphoryla­
tion sequence but with the critical serine replaced by an alanine, 
which lacks the hydroxyl group

ced by an alanine, 
which lacks the hydroxyl group required for transfer of the phos­
phate from ATP. When PKA is assembled into a heterotetramer of 
two regulatory subunits and two catalytic subunits, this pseudo­
substrate interacts with the catalytic subunit, preventing it from 
phosphorylating target proteins.84
In addition to enhancing glycogen breakdown, PKA mediates 
the effects of a number of hormones in various tissues, including 
the positive inotropic and chronotropic effects of epine

sitive inotropic and chronotropic effects of epinephrine on 
the heart, the trophic effects of the anterior pituitary hormones 
TSH and ACTH, and the effects of antidiuretic hormone on the 
permeability of the renal collecting duct to water. PKA also trans­
locates into the nucleus to regulate gene transcription.85 The best 
studied nuclear target of PKA is the cAMP-response element–
binding protein (CREB), though it is still not clear how many 
of the physiologic actions of cAMP require this tr

of the physiologic actions of cAMP require this transcription fac­
tor to be phosphorylated. PKA also phosphorylates a number of 
coregulatory proteins, which also contribute to transcriptional 
outputs.
Importantly, cAMP also has actions that are independent 
of PKA. One of these is the direct regulation of ion channels; 
Phosphorylates
targets
P
1
2
3
4
5
6
7
8
P
Adenylate
cyclase
γ β
GTP
ATP
β-adrenergic receptor kinase
(BARK)
Shuts down
β-adrenergic receptor
Inactive PKA
cAMP

huts down
cAMP
C
R
R
Active PKA
C
α
β-Arrestin 
Epinephrine
β-Adrenergic
receptor
Plasma
membrane
BARK
cAMP
cAMP
• Fig. 2.12  Adenylate cyclase, protein kinase A (PKA), and β-adrenergic receptor kinase (BARK) activation 
by epinephrine. Step 1: Upon binding of epinephrine to the β-adrenergic receptor, Gs is activated. Step 
2: Gsα binds to and stimulates adenylate cyclase. Step 3: Adenylate cyclase catalyzes the conversion of 
ATP to cAMP. Step 4: cAMP binds to

 conversion of 
ATP to cAMP. Step 4: cAMP binds to the regulatory subunit (R) of PKA, releasing free catalytic subunit 
(C), which is active. Step 5: C phosphorylates a number of intracellular substrates in a manner determined 
by its location in the cell. Step 6: C phosphorylates serine and threonine residues on BARK. Step 7: 
BARK, itself a serine/threonine kinase, phosphorylates serine and threonine residues on the β-adrenergic 
receptor. Step 8: β-Arrestin binds to the phosphorylated recepto

 8: β-Arrestin binds to the phosphorylated receptor, which blocks further activation of Gs. 
β-Arrestin also initiates signaling cascades, which are not shown.
29
another involves the exchange protein activated by cAMP 
(EPAC), which functions as a guanine nucleotide exchange fac­
tor (GEF) for the small GTP-binding protein Rap1.86 Regulation 
of insulin secretion from pancreatic beta cells by glucagon-like 
peptide-1 (GLP1) and stabilization of the endoth

peptide-1 (GLP1) and stabilization of the endothelial barrier by 
β-adrenergic agents are two processes thought to be mediated by 
EPAC. 
Regulation by the Second Messengers Calcium 
and PKC
Many additional second messengers have been identified since 
the discovery of cAMP. These include calcium, cGMP, inosi­
tol polyphosphates, DAG, and nitric oxide. The calcium ion 
(Ca2+) is one of the most common second messengers utilized 
by diverse cell types, and one that plays a particularly impor­
t

 types, and one that plays a particularly impor­
tant role in the regulated secretion of hormones.87 Ca2+ is 
maintained at low micromolar concentrations in the cytoplasm 
such that opening channels that lead to the outside of the cell 
or intracellular storage organelles results in a rapid increase in 
cytosolic Ca2+. The heterotrimeric G proteins containing Gαq 
or Gα11 cause increases in intracellular calcium by targeting the 
membrane-associated enzyme PLC. PLC catalyzes the hydro­
lysis of 

ted enzyme PLC. PLC catalyzes the hydro­
lysis of phosphatidylinositol 4’,5’-bisphosphate into DAG and 
IP3. Hormones that signal through G protein–dependent acti­
vation of PLC include angiotensin II, α-adrenergic catechol­
amines, growth hormone–releasing hormone (GHRH), and 
vasopressin. IP3 binds to a receptor located on the cytoplas­
mic face of the endoplasmic reticulum, leading to the release 
of stored Ca2+ from that organelle. Ca2+ also interacts with the 
IP3 receptor, further stimulat

interacts with the 
IP3 receptor, further stimulating calcium discharge from the 
endoplasmic reticulum and providing a strong positive feed­
back loop. Another source of cytoplasmic Ca2+ is entry through 
receptor-operated channels in the plasma membrane, such as 
those activated by noradrenaline, endothelin, or histamine via 
heterotrimeric G proteins.
Ca2+ transmits its signal via a number of effectors, including 
protein kinases, in most cases through the intermediary bind­
ing protein, calm

s through the intermediary bind­
ing protein, calmodulin, or its relative, troponin C. Calmodulin 
is a small, acidic protein that contains four copies of a canoni­
cal calcium-binding motif.88 Calmodulin associates with and 
regulates in a Ca2+-dependent manner glycogen phosphorylase 
kinase, myosin light chain kinase, and members of the family 
of calcium/calmodulin-dependent kinases. In addition to protein 
kinases, other calcium/calmodulin-dependent enzymes include 
the serine/threonine prot

pendent enzymes include 
the serine/threonine protein phosphatase, calcineurin, some 
adenylate cyclase and PDE isoforms, and nitric oxide synthase. 
Calcium interacts directly and independently of calmodulin with 
targets such as the protease calpain, synaptotagmin (a regulator 
of neurotransmitter and hormone exocytosis), and cytoskeletal 
proteins.
An important group of protein kinases directly activated by 
calcium is the PKC family. PKC, originally identified as the target 
of the tumor pro

ginally identified as the target 
of the tumor promoter phorbol ester, is a cyclic nucleotide-inde­
pendent protein kinase regulated by the direct binding of DAG 
and calcium, two second messengers produced by the activation 
of PLC. The PKC family has been divided into three groups: clas­
sic (regulated by DAG, phosphatidylserine, and calcium), novel 
(regulated by DAG and phosphatidylserine), and atypical. All 
PKC proteins have a conserved kinase domain in their C-terminal 
portion and regula

ase domain in their C-terminal 
portion and regulatory sequences in their N-terminal domain. For 
classic PKCs, the latter consist of a C1 domain, which binds DAG 
or phorbol ester, followed by a C2 domain, which associates with 
anionic lipids in a Ca2+-dependent manner89 (Fig. 2.13). Novel 
isoforms have a modified form of the C1 domain that confers 
a higher affinity for DAG than in the classic isoforms but lack 
the C2 domain, explaining the absence of Ca2+ regulation. Atypi­
cal PKCs have a

absence of Ca2+ regulation. Atypi­
cal PKCs have alterations in the C1 domain that eliminate DAG 
binding and also lack a site for Ca2+ binding. The regulation of 
PKC isoforms is complex, involving such covalent modifications 
as phosphorylation and proteolysis, as well as interaction with lip­
ids and hydrophilic molecules other than those traditionally asso­
ciated with activation of classic PKCs.90 
Regulation of Protein Kinases by PI3K
Another important signaling pathway involves a family o

er important signaling pathway involves a family of related 
proteins that catalyzes phosphorylation of phosphoinositides on 
C1B
C2
C1A
Pseudosubstrate
Isoenzyme
Regulatory
Catalytic
Ligand
Conventional:
α, βI, βII, γ
Activation
loop
Turn
motif
Hydrophobic motif
novel C2
C1A
C1B
Novel:
δ, ε, θ, η
PB1
atypical C1
Atypical:
ζ, ι/λ
+
DAG
++
–
+
Ca2+
–
–
+++
Phorbol
esters
+++
–
• Fig. 2.13  Domain structure and ligands of protein kinase C (PKC). The PKC family can be divided into 
three classes: t

e PKC family can be divided into 
three classes: the conventional, or classic, PKCs (cPKCs); the novel PKCs (nPKCs); and the atypical PKCs 
(aPKCs). The C1 domains bind diacylglycerol (DAG) or phorbol ester; the C2 domain binds calcium. A 
novel C2 domain in nPKCs does not bind calcium but mediates protein-protein interactions. Similarly, a 
PB1 domain in aPKCs is involved in protein-protein interactions. The aPKCs possess only one C1 domain 
and thus do not bind diacylglycerol. The conserved ps

 thus do not bind diacylglycerol. The conserved pseudosubstrate motif is represented by the white 
boxes in the regulatory domain. The activation loop and the turn and hydrophobic motifs are sites of 
regulatory phosphorylation.
30
the 3′ position of the inositol ring.91 All class I PI3Ks are com­
prised of a catalytic protein associated with a regulatory subunit 
and use PI4,5P2 as a preferred substrate; these isoforms are most 
important to signaling by

these isoforms are most 
important to signaling by RTK, GPCRs, and tyrosine kinase 
oncogenes. Class II PI3Ks phosphorylate PI and PI4P in vivo 
and lack stable regulatory subunits but probably associate with 
other proteins as modulating factors. They have been impli­
cated in a diverse set of physiologic responses, but the down­
stream targets are largely unknown. Class III PI3K, which has 
one catalytic member also known as vacuolar protein sorting 
34 (Vps34), binds tightly to the regulatory

rting 
34 (Vps34), binds tightly to the regulatory protein Vps15, uses 
exclusively PI as a substrate, and is involved primarily in mem­
brane protein trafficking related to endocytosis, phagocytosis, 
and autophagy.
Class IA PI3Ks are defined by regulatory subunits containing 
SH2 domains, which target them to activated RTKs. The hetero­
trimeric G protein subunit pair Gβγ, when free, activates those 
class I PI3Ks containing regulatory subunits not bearing SH2 
domains.
Class IA PI3Ks are thou

 not bearing SH2 
domains.
Class IA PI3Ks are thought to be the most important PI3Ks 
for the actions of hormones, particularly insulin and IGF1.92 
Activation of the receptors for either hormone leads to phos­
phorylation of IRS1 or IRS2 at sites specialized for docking with 
SH2 domains in the p85 regulatory subunit associated with the 
p110α catalytic subunit of PI3K. The bound PI3K catalyzes the 
production of PIP3 and possibly PI3,4P2, which serve to recruit 
additional proteins (including 

 serve to recruit 
additional proteins (including protein kinases) to the membrane 
by binding their PH domains.93 PH domains are best known 
for their ability to bind phosphoinositides with high affinity and 
specificity, although only a small portion have been proven to do 
so. The serine/threonine protein kinase Akt, also named protein 
kinase B because of its structural similarities to PKA and PKC, 
contains an N-terminal PH domain that preferentially binds to 
PIP3 and PI3,4P2.94 When insul

rentially binds to 
PIP3 and PI3,4P2.94 When insulin acts upon a target cell, the PH 
domain of Akt associates with the PIP3 generated on the cyto­
plasmic face of the plasma membrane. The binding of the PH 
domain to PIP3 serves two purposes: to recruit Akt to the mem­
brane and to relieve steric hindrance of Akt’s phosphorylation sites 
and catalytic domain by the PH domain. Also at the plasma mem­
brane via its own PH domain is the enzyme 3-phosphoinositide-
dependent protein kinase (PDK1), w

osphoinositide-
dependent protein kinase (PDK1), which phosphorylates Akt on 
a threonine in its activation loop (Fig. 2.14). mTORC2 also phos­
phorylates Akt on a serine in its C-terminus. Together, the PDK1 
and mTORC2 phosphorylation events confer full activity to Akt. 
mTORC2 appears to be regulated by insulin, but the mechanism 
is unknown.
Akt is essential to many of the metabolic actions of insulin and 
growth effects of IGF1.95,96 There are three Akt isoforms, each 
encoded by separate g

re three Akt isoforms, each 
encoded by separate genes. Akt1 is the most widely expressed iso­
form and seems to be critical to the regulation of growth; Akt2 is 
enriched in insulin target tissues and is more important to the control 
of metabolism; and Akt3 is expressed primarily in the brain, where 
it controls growth of that tissue.97 Indirect activation of mTORC1 
by Akt and suppression of forkhead box (FOX)O–driven transcrip­
tion are two of the critical targets for promoting organ growth,

f the critical targets for promoting organ growth, the 
Akt/mTORC1 pathway being particularly engaged in the regula­
tion of cell size.98 Members of the Rab GTPase-activating protein 
family, TBCD4 (also known as AS160) in fat cells and TBC1D1 
in both muscle and fat, are phosphorylated and inhibited by Akt, 
contributing to the activation of glucose transport.99 
Regulation of Protein Kinases by Ras
Routes to activation of Ras by GRB-SOS include both RTKs and 
GPCRs acting through β-arrestin.10

 both RTKs and 
GPCRs acting through β-arrestin.100 GTP-bound Ras recruits 
several receptors to the plasma membrane, including the serine/
threonine kinase Raf, which is activated by dimerization and a 
series of phosphorylation/dephosphorylation events.101 Raf then 
phosphorylates MAPK/ERK kinase (MEK1), a tyrosine and ser­
ine/threonine–dual specificity protein kinase, initiating a protein 
kinase cascade centered on extracellular signal-regulated kinases 
1 and 2 (ERK1/2). This represents on

ated kinases 
1 and 2 (ERK1/2). This represents one of four MAPK cascades, 
Plasma
membrane
PtdIns(4,5)P2
mTORC2
PtdIns(3,4,5)P3
PH domain
Ser473
Thr308
P
P
PI3K
PDK1
AKT
Active
• Fig. 2.14  Mechanism of AKT activation. When phosphatidylinositol-3,4,5-trisphosphate (PtdIns(3,4,5)P3) 
levels are low in the plasma membrane, AKT is in an inactive conformation in the cytoplasm and cannot 
be phosphorylated by the upstream activating 3-phosphoinositide-dependent protein ki

activating 3-phosphoinositide-dependent protein kinase 1 (PDK1) (not 
shown). PtdIns(3,4,5)P3 levels increase in the plasma membrane following the insulin-dependent recruit­
ment to IRS1 and IRS2 of phosphoinositide 3-kinase (PI3K), which phosphorylates phosphatidylinosi­
tol-4,5-bisphosphate (PtdIns(4,5)P2). AKT binds PIP3 through its pleckstrin homology (PH) domain and 
induces a conformational change within the AKT kinase domain, allowing PDK1 to phosphorylate the 
critical residue in the act

 to phosphorylate the 
critical residue in the activation loop required for AKT kinase activity, threonine 308 (Thr308). Mammalian 
target of rapamycin complex 2 (mTORC2) also phosphorylates AKT at the carboxy-terminal serine 473 
(Ser473) site to fully activate its kinase activity. PDK1 has a PH domain that can bind PtdIns(3,4,5)P3, but 
this interaction is not essential for PDK1 catalytic activity. (Redrawn from Finlay D, Cantrell DA. Metabolism, 
migration and memory in cytotoxic T cells. Nat

m, 
migration and memory in cytotoxic T cells. Nat Rev Immunol. 2011;11:109.)
31
the others involving c-Jun N-terminal kinase (JNK), the 38-kDa 
stress-activated kinases (p38), and ERK5. Specificity for MAPKs 
is conferred by scaffold proteins that bind most or all members 
of a given pathway, ensuring that each member phosphorylates 
only its appropriate target kinase.102 Gonadotropin-releasing hor­
mone, PTH, GH, angiotensin, and gastrin are just a few o

PTH, GH, angiotensin, and gastrin are just a few of the 
many hormones believed to signal at least in part through regula­
tion of MAPKs. 
Disease Caused by Defective Cell Surface 
Receptors
Numerous diseases develop as a result of dysfunctional binding to 
or signaling by hormone receptors. These hormone resistance syn­
dromes invariably mimic the phenotype of the hormone-deficient 
state but present with high levels of biologically active hormones 
in the circulation.
Insulin Resistance Syndro

nes 
Insulin Resistance Syndromes
The best studied inherited disease of hormone resistance is 
that caused by mutations in the insulin receptor. In addi­
tion to hyperinsulinism and the expected abnormalities in 
metabolism, patients with severe insulin resistance also display 
acanthosis nigricans (hyperpigmentation primarily in the skin 
folds) and often hyperandrogenism.103 Beyond that, there is 
a range of syndromes that correlate with the degree of insulin 
signaling imp

orrelate with the degree of insulin 
signaling impairment. The strongest loss-of-function muta­
tions result in leprechaunism, in which there are severe devel­
opmental defects presenting at birth. Some mutations of the 
insulin receptor gene cause a decrease in the number of recep­
tors in the plasma membrane, in some cases accompanied by a 
decrease in the mRNA. Other mutations adversely affect hor­
mone binding or the function of the kinase domain.103 In con­
trast to insulin resistance cause

main.103 In con­
trast to insulin resistance caused by mutations in the receptor 
gene, sometimes referred to as type A insulin resistance, type B 
resistance presents at middle age, often with signs of autoim­
munity such as vitiligo, alopecia, and arthritis. This syndrome 
is defined by the presence of antibodies directed against the 
insulin receptor; the levels of antibody correlate with the sever­
ity of the disease.
In many ways, the use of insulin resistance to describe the 
common syndro

 insulin resistance to describe the 
common syndrome associated with obesity or polycystic ovary 
syndrome (PCOS) is a misnomer. The term resistance was origi­
nally coined to describe the situation of hyperglycemia in the face 
of elevated concentrations of insulin in the blood.105,106 However, 
the recognition that insulin has numerous physiologic actions in 
addition to those on carbohydrate metabolism has led to ambi­
guity in nomenclature. On the one hand, the term insulin resis­
tance is o

n the one hand, the term insulin resis­
tance is often applied to abnormalities in insulin signaling to all 
outputs from the receptor; this typically occurs with mutations 
of the insulin receptor. However, in the insulin resistance of obe­
sity or PCOS, some actions of insulin are preserved. This is dem­
onstrated by a comparison of the phenotype of individuals with 
type 2 diabetes mellitus to those with genetically encoded partial 
defects in insulin receptor function.107 Both groups share h

 insulin receptor function.107 Both groups share hyper­
glycemia, but only those with type A insulin resistance display 
defects in the regulation of hepatic lipid metabolism by insulin. 
Thus the metabolic phenotype associated with type A inherited 
insulin resistance is not faithfully phenocopied by the insulin 
resistance of obesity. Consistent with this, numerous pathologic 
mechanisms have been proposed to account for the insulin resis­
tance associated with obesity, almost all of which inv

e associated with obesity, almost all of which involve a “post­
receptor defect.” 
Defects in Cell Surface Receptors That Control 
Growth
One of the most clinically recognizable syndromes is resistance to 
the actions of GH, which results in shortness of stature. An inability 
to respond to GH results in Laron syndrome, characterized by high 
levels of circulating GH, very low levels of IGF1, and short stat­
ure.108 Diverse molecular causes have been reported, including large 
deletions as well 

been reported, including large 
deletions as well as missense, frameshift, and splicing mutations 
in the GH receptor. Similar syndromes of decreased growth can 
also result from mutations in STAT5b and deficiency in IGF-1 or 
defects in IGF-1 signaling. Recently, a syndrome has been described 
in which mutations in the PIK3R1 gene, which encodes the p85α 
regulatory subunit of class I PI3K, lead to SHORT syndrome, 
which includes dysmorphic facial features and defects in growth 
(short stature,

al features and defects in growth 
(short stature, hyperextensibility, ocular depression, Rieger anom­
aly, and teething delay).109 As might be expected by the similarities 
in IGF-1 and insulin signaling, individuals with SHORT syndrome 
also display lipodystrophy and insulin resistance.110 
Diseases Caused by Mutations in GPCRs and G 
Proteins
A number of endocrine diseases can be attributed to mutations in 
the GPCR–G protein signaling system.111,112 For GPCRs, many 
mutations are associated 

111,112 For GPCRs, many 
mutations are associated with some degree of loss of function and 
are inherited in a recessive manner (Table 2.3). Some examples 
include hypothyroidism from mutations in the thyrotropin-
releasing hormone or TSH receptor, glucocorticoid deficiency 
from mutations in the melanocortin 2 receptor, extreme obesity 
from dysfunction of melanocortin 4 receptor, and infertility due 
to mutations in the receptor for luteinizing hormone or FSH. 
Gain-of-function mutations inclu

 hormone or FSH. 
Gain-of-function mutations include those in the TSH receptor 
causing hyperthyroidism, in the α2-adrenergic receptor, leading 
to diabetes mellitus, and in the calcium-sensing receptor result­
ing in hypoparathyroidism. Somatic activating mutations have 
been reported in the luteinizing hormone and TSH receptors.111 
A limited number of heterotrimeric G proteins are known to have 
mutations that cause human disease, and in all cases they affect 
the α-subunit. Mutation of the g

ases they affect 
the α-subunit. Mutation of the gene encoding the Gαt subunit of 
transducin is associated with night blindness. Dominant, activat­
ing mutations of Gαs cause pituitary adenomas, most often secret­
ing GH, and more rarely, tumors of the thyroid, parathyroid, and 
adrenal glands.112 Patients who inherit a loss of a functional allele 
in Gαs develop Albright hereditary osteodystrophy (AHO); those 
who inherit the mutant allele from their mothers also have pseu­
dohypoparathyroidis

 their mothers also have pseu­
dohypoparathyroidism type 1a in addition to AHO. This is due to 
imprinting of the Gαs gene, such that it is expressed preferentially 
from the maternal allele in a number of hormone target tissues, 
but biallelically in most other cell types. 
Ligands That Act Through Nuclear Receptors
Many signaling molecules share with thyroid and steroid hor­
mones the ability to function in the nucleus to convey intercel­
lular and environmental signals. Lipophilic signaling m

 and environmental signals. Lipophilic signaling molecules 
that use nuclear receptors include derivatives of vitamins A and 
D, endogenous metabolites such as oxysterols and bile acids, and 
32
chemicals not naturally encountered in the environment (i.e., 
xenobiotics). These molecules are referred to as nuclear receptor 
ligands. The nuclear receptors for all of these signaling molecules 
are structurally related and are collectively referred to as the 

y related and are collectively referred to as the nuclear 
receptor superfamily. They are all transcription factors, serving to 
activate or repress specific gene sets that mediate their physiologic 
effects. The study of these receptors is a rapidly evolving field, and 
more detailed information can be obtained by visiting the Nuclear 
Receptor Signaling Atlas website.113,114
General Features of Nuclear Receptor Ligands
Unlike polypeptide hormones that function through cell surface 
receptors, 

es that function through cell surface 
receptors, no ligands for nuclear receptors are directly encoded in 
the genome. All nuclear receptor ligands are small (molecular mass 
<1000 Da) and lipophilic, enabling them to enter cells by passive 
diffusion, although in some cases a membrane transport protein is 
involved. For example, several active and specific thyroid hormone 
transporters have been identified, including monocarboxylate 
transporter 8 (MCT8), MCT10, and organic anion transporting 

r 8 (MCT8), MCT10, and organic anion transporting 
polypeptide 1C1 (OATP1C1).115
All naturally occurring nuclear receptor ligands are derived 
from dietary, environmental, or metabolic precursors. In this 
sense, the function of these ligands and their receptors is to trans­
late cues from the external and internal environments into changes 
in gene expression. Their critical role in maintaining homeostasis 
in multicellular organisms is highlighted by the fact that nuclear 
receptors are found 

ted by the fact that nuclear 
receptors are found in all vertebrates and insects but not in single-
cell organisms such as yeast.116
Because nuclear receptor ligands are lipophilic, most are readily 
absorbed from the gastrointestinal tract. This makes nuclear recep­
tors excellent targets for pharmaceutical interventions. In addition 
to natural ligands, many drugs in clinical use target nuclear recep­
tors, ranging from drugs used to treat specific hormone deficien­
cies to those used to treat

ific hormone deficien­
cies to those used to treat common multigenic conditions such as 
inflammation, cancer, and type 2 diabetes. 
Subclasses of Nuclear Receptor Ligands
One classification of nuclear receptor ligands is outlined in Table 
2.4 and is described in the following paragraphs.
Classic Hormones
The classic hormones that use nuclear receptors for signaling are 
thyroid hormone and steroids. Endogenous steroid hormones 
include cortisol, aldosterone, estradiol, progesterone, and testos

, aldosterone, estradiol, progesterone, and testos­
terone. In some cases (e.g., thyroid hormone receptor α and β 
genes [THRA and THRB], estrogen receptor α and β genes [ESR1 
and ESR2]), more than one gene exists for a given type of hor­
mone receptor. Each receptor gene may in turn encode additional 
receptors for the same hormone by alternative promoter usage or 
by alternative splicing (e.g., THRB1 and THRB2).
Some receptors mediate the signals of multiple hormones. 
For example, the minera

als of multiple hormones. 
For example, the mineralocorticoid receptor, also known as the 
  Diseases Caused by G Protein–Coupled 
Receptor Loss-of-Function Mutations
Receptor
Disease
V2 vasopressin
X-linked
ACTH
Familial ACTH resistance
AR
GHRH
Familial GH deficiency
AR
GnRH
Hypogonadotropic hypogonadism
AR
GPR54
AR
Prokineticin 
receptor 2
ADa
FSH
Hypergonadotropic ovarian dysgen­
esis
AR
LH

a
FSH
esis
AR
LH
46 XY, intersex
AR
TSH
Familial hypothyroidism
AR
Ca2+ sensing 
receptor
Familial hypocalciuric hypercalcemia
AD
Neonatal severe primary hyperpara­
thyroidism
AR
Melanocortin 4
Obesity
AR
PTH/PTHrP
Blomstrand chondrodysplasia
AR
aWith incomplete penetrance.
ACTH, Adrenocorticotropic hormone; AD, autosomal dominant; AR, autosomal recessive; 
FSH, follicle-stimulating hormone; GH, growth hormone; GHRH, growth hormone–releasing 
hormone; GnRH, gona

HRH, growth hormone–releasing 
hormone; GnRH, gonadotropin-releasing hormone; GPR54, orphan G protein–coupled recep­
tor 54; LH, luteinizing hormone; PTH, parathyroid hormone; PTHrP, parathyroid hormone–
related protein; TSH, thyroid-stimulating hormone.
   
TABLE 2.3
  Nuclear Receptor Ligands and Their 
Receptors
Ligand
Receptor
Thyroid hormone
Thyroid hormone receptor (TR), subtypes α, β
Estrogen
Estrogen receptor (ER), subtypes α, β
Testosterone
Androgen receptor (AR)
Proge

pes α, β
Progesterone receptor (PR)
Aldosterone
Mineralocorticoid receptor (MR)
Cortisol
Glucocorticoid receptor (GR)
Vitamins
1,25-(OH)2-vitamin D3
Vitamin D receptor (VDR)
All-trans-retinoic acid
Retinoic acid receptor, subtypes α, β, γ
9-cis-retinoic acid
Retinoid X receptor (RXR), subtypes α, β, γ
Metabolic Intermediates and Products
Fatty acids
Peroxisome proliferator-activated receptor 
(PPAR), subtypes α, δ, γ
Oxysterols
Liver X receptor (L

), subtypes α, δ, γ
Liver X receptor (LXR), subtypes α, β
Bile acids
Bile acid receptor (BAR, also called FXR)
Heme
Rev-Erb subtypes α, β
Phospholipids
Liver receptor homologue-1 (LRH1)
Steroidogenic factor-1 (SF1)
Xenobiotics
Pregnane X receptor (PXR)
Constitutive androstane receptor (CAR)
TABLE 2.4
33
aldosterone receptor, has equal affinity for aldosterone and cortisol 
and probably functions as a glucocorticoid receptor in some tissues, 
suc

as a glucocorticoid receptor in some tissues, 
such as the brain.117 Likewise, the androgen receptor binds and 
responds to both testosterone and dihydrotestosterone (DHT).118 
Vitamins
Vitamins are essential constituents of a healthful diet. Two fat-
soluble vitamins, A and D, are precursors of important signaling 
molecules that function as ligands for nuclear receptors.
Precursors of vitamin D are synthesized and stored in skin 
and activated by ultraviolet light; vitamin D can also be derive

by ultraviolet light; vitamin D can also be derived 
from dietary sources. Vitamin D is then converted in the liver 
to 25(OH)D (25-hydroxyvitamin D, calcidiol), which is subse­
quently converted by the kidney to 1,25(OH)2D3 (1,25-dihy­
droxyvitamin D3, calcitriol), the most potent natural ligand of the 
vitamin D receptor (VDR).119 The 1-hydroxylation of calcidiol is 
tightly regulated, and calcitriol acts as a circulating hormone, aris­
ing in the kidney and circulating through the bloodstream

the kidney and circulating through the bloodstream to act 
on target tissues such as intestine and bone.
Vitamin A is stored in the liver and is activated by metabolism 
to all-trans-retinoic acid, which is a high-affinity ligand for retinoic 
acid receptors (RARs).120 Retinoic acid functions as a signaling 
molecule in both a paracrine and an endocrine manner. Retinoic 
acid is also converted to its 9-cis-isomer, which is a ligand for 
another nuclear receptor, the retinoid X receptor (RXR).121

uclear receptor, the retinoid X receptor (RXR).121 These 
retinoids and their receptors are essential for normal development 
of multiple organs and tissues, and they have pharmaceutical util­
ity for conditions ranging from skin diseases to leukemia.122 
Certain nuclear receptors respond to naturally occurring endogenous 
metabolic products. The peroxisome proliferator-activated receptors 
(PPARs) constitute the best-defined subfamily of metabolite-sensing 

the best-defined subfamily of metabolite-sensing 
nuclear receptors.123 All three PPAR subtypes are activated by poly­
unsaturated fatty acids, and although specific lipid species may act as 
selective PPAR ligands, the PPARs may also function as integrators of 
the concentration of a number of fatty acids.124
The natural ligand for PPARα has not been clearly identi­
fied, but may be a fatty acid derived from lipolysis.125,126 The 
fibrate class of lipid-lowering pharmaceuticals are potent ligan

of lipid-lowering pharmaceuticals are potent ligands 
for PPARα, and the very name of this receptor is derived from 
its ability to induce the proliferation of peroxisomes in the liver, 
organelles that break down very long-chain fatty acids through 
β-oxidation.127 Indeed, stimulation of fatty acid oxidation is an 
important physiologic role of PPARα.
The other PPARs (δ and γ) are structurally related to PPARα 
but do not induce proliferation of peroxisomes when activated by 
their respective l

 peroxisomes when activated by 
their respective ligands. PPARδ, also known as PPARβ, is ubiqui­
tous, and its ligands—other than fatty acids—are not well charac­
terized. Activation of PPARδ increases oxidative metabolism in fat 
and skeletal muscle.128 PPARγ is expressed primarily in adipocytes 
and is necessary for differentiation along the adipocyte lineage.129 
PPARγ is also expressed in other cell types, including colonic epi­
thelial cells, macrophages, and vascular endothelial cells, whe

, macrophages, and vascular endothelial cells, where it 
may play physiologic and pathologic roles. The natural ligand for 
PPARγ is not known, but PPARγ is a major tissue target of thia­
zolidinedione (TZD) antidiabetic drugs that improve insulin sen­
sitivity.130,131 These pharmaceutical agents bind to PPARγ with 
nanomolar affinities. Non-TZD PPARγ ligands are also insulin 
sensitizers, further implicating PPARγ in this physiologic role.
Another metabolite-responsive nuclear receptor, the liv

er metabolite-responsive nuclear receptor, the liver 
X receptor (LXR), is activated by oxysterol intermediates in 
cholesterol biosynthesis. Mice lacking LXRα have a dramatically 
impaired ability to metabolize cholesterol.132 A related receptor 
known as farnesyl X receptor (FXR) binds and is activated by bile 
acids, and it plays a role in the regulation of bile synthesis and 
circulation in normal conditions and in disease states.132 
Endobiotics and Xenobiotics
Other nuclear receptors appea

tics and Xenobiotics
Other nuclear receptors appear to function as integrators of 
exogenous environmental signals, including natural endobiot­
ics (medicinal agents and toxins found in plants) and xenobiot­
ics (compounds that are not naturally occurring). In these cases, 
the role of the activated nuclear receptor is to induce cytochrome 
P450 enzymes that facilitate detoxification of potentially danger­
ous compounds in the liver. Receptors in this class include sterol 
and xenobiotic recepto

 this class include sterol 
and xenobiotic receptor (SXR), also known as pregnane X recep­
tor (PXR), and constitutive androstane receptor (CAR).133 Unlike 
other nuclear receptors that have high affinity for specific ligands, 
xenobiotic receptors have low affinity for a large number of ligands, 
reflecting their function in defense against a varied and challeng­
ing environment. Although these xenobiotic compounds are not 
hormones in the classic sense, the function of these nuclear recep­
tor

ic sense, the function of these nuclear recep­
tors is consistent with the general theme of helping the organism 
to cope with environmental challenges. 
Orphan Receptors
The nuclear receptor superfamily is one of the largest families of 
transcription factors, with 48 members in humans. The hormones 
and vitamins just described account for the functions of only a 
fraction of the nuclear receptors. The remaining receptors have 
been designated as orphan receptors because their putative ligands 

s orphan receptors because their putative ligands 
are not known.134
From analyses of mice and humans with mutations in various 
orphan receptors, it is clear that many of them are required for 
life or for the development of specific organs, ranging from brain 
nuclei to endocrine glands. Some orphan receptors appear to be 
active in the absence of any ligand (i.e., constitutively active) and 
may not respond to a natural ligand. Nevertheless, all of the recep­
tors known to respond to metaboli

ll of the recep­
tors known to respond to metabolites and environmental com­
pounds were originally discovered as orphans. Therefore future 
research will likely find that additional orphan receptors function 
as receptors for physiologic, pharmacologic, or environmental 
ligands. For example, the nuclear receptor NR1D1 (also known as 
Rev-Erbα), which is a regulator of circadian rhythms,17 has been 
shown to be a receptor for heme,135,136 although the physiologic 
significance of this remains t

gh the physiologic 
significance of this remains to be determined. 
Variant Receptors
The C-terminal domain of the nuclear receptors is responsible for 
hormone binding. In a few nuclear receptors, including THRA and 
the glucocorticoid receptor, alternative splicing produces variant 
receptors with unique C-termini that do not bind ligands.137,138 
These variant receptors are normally expressed, but their biologic 
relevance is uncertain. They may modulate the action of the classic 
receptor to

ay modulate the action of the classic 
receptor to which they are related by inhibiting its function.
Other normally occurring variant nuclear receptors lack a 
classic DBD (discussed later). These types include NR0B1 (also 
known as DAX1), which is mutated in human disease,139 and 
PTPN6 (also known as SHP1).140 Their ligands have not been 
identified, and it is likely that NR0B1 and PTPN6 bind to and 
repress the actions of other receptors.
34
Rare, nat

Rare, naturally occurring mutations of hormone receptors can 
cause hormone resistance in affected patients. Inheritance of the 
hormone resistance phenotype is dominant if the mutant recep­
tor inhibits the action of the normal receptor, as occurs with 
resistance to thyroid hormone or PPARγ ligands.141 Inheritance 
is recessive if the mutation results in a complete loss of receptor 
function, as with the syndrome of hereditary calcitriol-resistant 
rick

 syndrome of hereditary calcitriol-resistant 
rickets, which is caused by mutations in the VDR.142 Inheritance 
can also be X-linked, as with the mutated androgen receptor in 
androgen insensitivity syndromes.143 
Regulation of Ligand Levels
Ligand levels can be regulated in several ways (Table 2.5). A dietary 
precursor may not be available in required amounts (e.g., hypo­
thyroidism due to iodine deficiency). Pituitary hormones (e.g., 
TSH) regulate the synthesis and secretion of classic thyro

ulate the synthesis and secretion of classic thyroid and 
steroid hormones. If the glands that synthesize these hormones 
fail, hormone deficiency can occur.
Many nuclear receptor ligands are enzymatically converted from 
inactive prohormones to biologically active hormones; examples 
include the 5′ deiodination of thyroxine (T4) to triiodothyronine 
(T3) (see Chapter 11). This can occur in the target cell itself or 
within other tissues that subsequently release T3 to the circulation 
for actio

sequently release T3 to the circulation 
for action elsewhere in the body. In other cases, one hormone is a 
precursor for another, as illustrated by the aromatization of testos­
terone to estradiol. Biotransformation may occur in a specific tissue 
that is not the main target of the hormone, as with renal 1-hydrox­
ylation of vitamin D (see Chapter 29), or it may occur primar­
ily in target tissues (e.g., 5α-reduction of testosterone to DHT; 
see Chapter 19). Deficiency or pharmacologic inhibit

e Chapter 19). Deficiency or pharmacologic inhibition of the 
enzymes responsible for these reactions can reduce hormone levels.
Transport into the target cell can also be a regulated process. 
T3 and T4, for example, do not penetrate the hydrophobic mem­
brane by themselves; they require a transporter such as MCT8 or 
OATP1. Mutations in MCT8, for example, lead to neurologic 
issues, including severe intellectual disability and movement dis­
orders with elevated serum T3.144 In this condition, 

ers with elevated serum T3.144 In this condition, it seems likely 
that the pathology is secondary to the inability of T3 to enter neu­
rons. Steroid hormones, by contrast, are believed to traverse the 
membrane by passive diffusion, although it remains possible that 
undiscovered binding proteins play a role.
Nuclear receptor ligands can be inactivated by hepatic or renal 
clearance or by more specific enzymatic processes. Mutations in 
genes encoding inactivating enzymes, or pharmacologic agen

coding inactivating enzymes, or pharmacologic agents 
that inhibit these enzymes, can result in symptoms of hormone 
excess such as renal deactivation of cortisol by 11β-hydroxysteroid 
dehydrogenase (11βHSD). Because cortisol can activate the min­
eralocorticoid receptor, insufficient 11βHSD activity due to lico­
rice ingestion, gene mutation, or extremely high cortisol levels 
causes syndromes of apparent mineralocorticoid excess.145 
Nuclear Receptor Signaling Mechanisms
Nuclear receptors are

eceptor Signaling Mechanisms
Nuclear receptors are multifunctional proteins that transduce the 
signals of their cognate ligands. General features of nuclear recep­
tor signaling are illustrated in Fig. 2.15.
For hormone action, the ligand and the nuclear receptor 
must both get into the nucleus. The nuclear receptor also must 
bind its ligand with high affinity. Because a major function of 
the receptor is to selectively regulate target gene transcription, 
it must recognize and bind to promote

nscription, 
it must recognize and bind to promoter and enhancer elements 
in appropriate target genes. One discriminatory mechanism 
is dimerization of a receptor with a second copy of itself or 
with another nuclear receptor. The DNA-bound receptor must 
work in the context of chromatin to signal the basal transcrip­
tion machinery to increase or decrease transcription of the tar­
get gene. In the regulation of signaling by nuclear receptors, 
some basic mechanisms are used by many or all memb

some basic mechanisms are used by many or all members of 
the nuclear receptor superfamily, whereas other mechanisms 
impart the specificity that is crucial to the vastly different bio­
logic effects of the many hormones and ligands that use these 
related receptors.
Domain Structure of Nuclear Receptors
Nuclear receptors are proteins with molecular masses between 
50,000 and 100,000 Da. They share a common series of 
domains, referred to as domains A through F (Fig. 2.16). This 
linear depictio

ins A through F (Fig. 2.16). This 
linear depiction is useful for describing and comparing recep­
tors, but it does not capture the roles of protein folding and 
tertiary structure in mediating various receptor functions. The 
structures of individual domains have now been solved for 
many receptors, as has the full-length structure of a more lim­
ited number of nuclear receptors. 
  Mechanisms Regulating Ligand Levels
Precursor availability
Synthesis
Secretion
Activation (prohormone → active h

thesis
Secretion
Activation (prohormone → active hormone)
Transport
Deactivation (active hormone → inactive hormone)
Elimination (hepatic, renal clearance)
TABLE 2.5
Prohormone
(ligand precursor)
Hormone
(ligand)
Phenotypic change
(hormone effect)
Transcription
mRNA
Protein
Hormone-responsive gene
Nucleus
HRE
• Fig. 2.15  Mechanism of signal transduction by hormones and other 
ligands that act through nuclear receptors. HRE, hormone response ele­
ment; mRNA, messenger ribonucleic acid.
35
Chapte

35
Nuclear Localization
The nuclear receptors, like all cellular proteins, are synthesized on 
ribosomes that reside outside the nucleus. Import of the nuclear 
receptors into the nucleus requires the nuclear localization signal 
(NLS), which is located near the border of the C and D domains (see 
Fig. 2.16). As a result of their NLSs, most of the nuclear receptors 
reside in the nucleus, with or without their ligan

reside in the nucleus, with or without their ligands. A major exception 
is the glucocorticoid receptor; in the absence of hormone, it is teth­
ered in the cytoplasm to a complex of chaperone molecules, including 
heat shock proteins (HSPs). Hormone binding to the glucocorticoid 
receptor induces a conformational change that results in dissociation 
of the chaperone complex, allowing the hormone-activated glucocor­
ticoid receptor to translocate to the nucleus by means of its NLS. 
Hormone Bindi

to the nucleus by means of its NLS. 
Hormone Binding
High-affinity binding of a lipophilic ligand is mediated by the 
C-terminal ligand-binding domain (LBD), domains D and E in 
Fig. 2.16. This region of the receptor has many other functions, 
including induction of dimerization and transcriptional regula­
tion (see later discussions).
The structure of the LBD has been solved for a number of recep­
tors. All share a similar overall structure consisting of 12 α-helical 
segments in a highly folde

isting of 12 α-helical 
segments in a highly folded tertiary structure (Fig. 2.17A). The 
ligand binds within a hydrophobic pocket composed of amino acids 
in helices 3, 4, and 5 (H3, H4, and H5, respectively). The major 
structural change induced by ligand binding is an internal folding of 
the most C-terminal helix (H12), which forms a cap on the ligand-
binding pocket (see Fig. 2.17B). Although the overall mechanism of 
ligand binding is similar for all receptors, the molecular details are 
e

ar for all receptors, the molecular details are 
essential for determining ligand specificity.146,147 Ligand binding is 
the most critical determinant of receptor specificity. 
Target Gene Recognition by Receptors
Another crucial specificity factor for nuclear receptors is their 
ability to recognize and bind to the subset of genes that is to be 
regulated by their cognate ligand. Target genes contain specific 
DNA sequences that are called hormone response elements (HREs). 
Binding to the HRE i

ne response elements (HREs). 
Binding to the HRE is mediated by the central C domain of the 
nuclear receptors (see Fig. 2.16). This region is typically composed 
of 66 to 68 amino acids, including two subdomains that are called 
zinc fingers because the structure of each subdomain is maintained 
by four cysteine residues coordinated with a zinc atom.
The first of these zinc-ordered modules contains basic amino 
acids that contact DNA; as with the LBD, the overall structure of 
the DBD is simila

e LBD, the overall structure of 
the DBD is similar for all members of the nuclear receptor super­
family. The specificity of DNA binding is determined by multiple 
factors (Table 2.6). All steroid hormone receptors, except for the 
estrogen receptor, bind to the double-stranded DNA sequence 
AGAACA (Fig. 2.18).
By convention, the double-stranded sequence is described by 
the sequence of one of its complementary strands, with the bases 
ordered from the 5′ to the 3′ end. Other nuclear receptors 

rom the 5′ to the 3′ end. Other nuclear receptors recognize 
the sequence AGGTCA. The primary determinant of this specific­
ity is a group of amino acid residues in the P box of the DBD (see 
Fig. 2.18). These hexamer DNA sequences are referred to as half-
sites. The only difference between these hexameric half-sites is the 
central two base pairs (underlined in Fig. 2.18). For some nuclear 
receptors, the C-terminal extension of the DBD contributes speci­
ficity for extended half-sites containi

tes speci­
ficity for extended half-sites containing additional, highly specific 
DNA sequences 5′ to the hexamer. Another source of specificity 
for target genes is the spacing and orientation of these half-sites, 
which in most cases are bound by receptor dimers. 
Receptor Dimerization
The nuclear receptor DBD has affinity for the hexameric half-site 
or for extended half-sites; however, many HREs are composed 
of repeats of the half-site sequence, and most nuclear receptors 
bind these HREs a

nce, and most nuclear receptors 
bind these HREs as dimers.148 Steroid receptors, including estro­
gen receptors, function primarily as homodimers, which prefer­
entially bind to two half-sites oriented toward each other (i.e., 
inverted repeats [IRs]) with three base pairs in between (IR3) (see 
Fig. 2.18A). The major dimerization domain in steroid receptors 
is within the C domain, although the LBD contributes. Ligand 
Activation function-1 (AF-1)
DNA-binding (DBD)
P box (half-site specificity

-1)
P box (half-site specificity)
Nuclear localization
signal (NLS)
C-terminal extension (flanking
DNA binding specificity)
Ligand binding domain (LBD)
Ligand-dependent activation
Activation function 2 (AF-2)
helix 12
Repression
A/B
C
D
E
F
• Fig. 2.16  Domain structures of nuclear receptors.
36
binding facilitates dimerization and DNA binding of steroid 
hormone receptors. Most other receptors, including THR, RAR, 
PPAR, LX

ost other receptors, including THR, RAR, 
PPAR, LXR, and VDR, bind to DNA as heterodimers with RXR 
(see Fig. 2.18B).
Heterodimerization with RXR is mediated by two distinct 
interactions, one involving LBDs and the other involving DBDs. 
The receptor LBD mediates the strongest interaction, which 
occurs even in the absence of DNA. These receptor heterodimers 
bind to two half-sites arranged as direct repeats (DRs) with a vari­
able number of base pairs in between.
The spacing of the half-sites 

e pairs in between.
The spacing of the half-sites is a major determinant of target gene 
specificity; it results from the second receptor-receptor interaction, 
which involves the DBDs and is highly sensitive to the spacing of 
the half-sites. For example, VDR/RXR heterodimers bind preferen­
tially to DRs separated by three bases (DR3 sites), TR/RXR binds 
DR4, and RAR/RXR binds DR5 with highest affinity.149
Studies of isolated DBD binding to DNA have shown that 
these spacing requirements are r

 have shown that 
these spacing requirements are related to the fact that the RXR 
binds to the upstream half-site (i.e., farthest from the start of tran­
scription). As a result of the periodicity of the DNA helix, each 
base pair separating the half-sites leads to a rotation of about 36 
degrees of one half-site relative to the other. Subtle differences in 
the structure of the receptor DBDs make the interactions more 
or less favorable at different degrees of rotation.150 Solution of 
the cry

erent degrees of rotation.150 Solution of 
the crystal structures of full-length nuclear receptor heterodimers 
bound to DNA has demonstrated remarkable diversity in the pre­
cise relationship between heterodimeric partners. For example, 
the PPARγ-RXR heterodimer forms a nonsymmetric complex, 
allowing the LBD of PPARγ to cooperate with both DBDs to 
enhance response element binding,151 whereas the LXR-RXR het­
erodimer binds symmetrically to its target sequence.152 Additional 
structures will 

s target sequence.152 Additional 
structures will be required to better understand the spectrum of 
RXR heterodimer binding to DNA.
The discovery of nuclear receptor binding sites has been largely 
empiric, based on the finding of binding sites in small num­
bers of known target genes. Unbiased analysis of thousands of 
H11
H12
H3
A
C
D
Helix 4-5
(H4-5)
H4-5
Helix 11
(H11)
H11
Helix 12
(H12)
H12
Helix 3
(H3)
H3
Corepressor
(CoR)
Coactivator
(CoA)
CoRNR box
region of CoR
NR box
region of CoA
Acti

CoRNR box
NR box
Activating ligand
(agonist)
(agonist)
Dissociation
of CoR
H4-5
H11
H12
H3
H4-5
B
• Fig. 2.17  Structural basis of nuclear receptor ligand binding and cofactor recruitment. (A and C) Apo-
receptor (no ligand bound). (B and D) Ligand-bound receptor. (C and D) Structures showing the positional 
binding of a corepressor (CoR) (in C) or coactivator (CoA) (in D). NR, nuclear receptor.
37
nuclear rec

hapter 2  Principles of Hormone Action
nuclear receptor binding locations in living cells using chroma­
tin immunoprecipitation followed by next generation sequencing 
has confirmed the canonical binding sequences for many nuclear 
receptors, including those of the estrogen receptor,153 the andro­
gen receptor,154,155 the glucocorticoid receptor,156 and PPARγ-
RXR heterodimers.157, 158 The complete set of cellular binding 
sites is referred to as the cistrome.159 Although the sequence of the 
ge

 the cistrome.159 Although the sequence of the 
genome is the same in nearly all cells of the body, cistromes are 
context dependent, owing to cooperation with factors that open 
chromatin in a cell type or developmentally specific way, allowing 
the receptors to bind. 
Receptor Regulation of Gene Transcription
Nuclear receptors mediate a variety of effects on gene transcrip­
tion. The most common modes of regulation are ligand-dependent 
gene activation, ligand-independent repression of transcr

tivation, ligand-independent repression of transcrip­
tion, and ligand-dependent negative regulation of transcription 
(Table 2.7). Much of this regulation is mediated by interactions of 
nuclear receptors with proteins called coregulators, which include 
coactivators and corepressors.160
Ligand-Dependent Activation
Ligand-dependent activation is the best understood function of 
nuclear receptors and their ligands. The ligand-bound receptor 
increases transcription of a target gene to which it i

eases transcription of a target gene to which it is bound. The 
DBD brings the receptor domains that mediate transcriptional 
activation to a specific gene. Transcriptional activation itself is 
mediated primarily by the LBD, which can function as an inde­
pendent unit even when it is transferred to a DNA-binding pro­
tein that is not related to nuclear receptors. The activation function 
(AF) of the LBD is referred to as AF2 (see Fig. 2.12).
Gene transcription is mediated by a large complex of 

e transcription is mediated by a large complex of factors 
that ultimately regulate the activity of ribonucleic acid (RNA) 
polymerase, the enzyme that uses the chromosomal DNA tem­
plate to direct the synthesis of messenger RNA. Most mamma­
lian genes are transcribed by RNA polymerase II using a large set 
of cofactor proteins that include basal transcription factors and 
associated factors collectively referred to as general transcription 
factors (GTFs). Details about GTFs are of fundamental 

-specific 
factors that open chromatin to permit 
receptor binding based on receptor-
intrinsic properties above)
DBD, DNA-binding domain; RXR, retinoid X receptor.
   
TABLE 2.6
A
B
Dimer
interface
P box
Steroid hormone receptor homodimer
Nuclear receptor (NR)-RXR heterodimer
P box
NR
N
RXR
Dimer
interface
A G A A C A
T C T T G T
T C T T G A
A G A A C T
N
A G G T C A
T C C A G T
A G G T C A
T C C A G T
• Fig. 2.18  Structural basis for nuclear receptor (NR) DNA-binding speci­
ficity is shown in

eceptor (NR) DNA-binding speci­
ficity is shown in the ribbon diagrams of receptor DNA-binding domains 
(DBDs). (A) Steroid hormone receptor binding as a homodimer to the 
inverted repeat (arrows) of the AGAACA half-site. The central base pairs 
are underlined. (B) RXR-NR heterodimer binding to the direct repeat of 
AGGTCA. The position of the P box, the region of the DBD that makes 
direct contact with DNA, is shown. N, number of base pairs between the 
two half-sites; RXR, retinoid X receptor.

TABLE 2.7
38
specifically recognize the ligand-bound conformation of the 
LBD and bind to the nuclear receptor on DNA only when an 
activating (agonist) hormone or ligand is bound. A number of 
coactivator proteins that bind to liganded nuclear receptors have 
been described (Table 2.8).161
The most important determinant of coactivator binding is the 
position of H12, which changes dramatically when activating 
ligands bind receptors (see Fig. 2.17B). Al

ating 
ligands bind receptors (see Fig. 2.17B). Along with H3, H4, and 
H5, H12 forms a hydrophobic cleft that is bound by short polypep­
tide regions of the coactivator molecules.162–164 These polypeptides, 
called NR boxes (see Fig. 2.17D), have characteristic sequences of 
LxxLL, in which L is leucine and xx can be any two amino acids.165
Coactivators increase the rate of gene transcription. This is 
accomplished by enzymatic functions, including histone acet­
yltransferase (HAT) activity; so

ing histone acet­
yltransferase (HAT) activity; some coactivators possess intrinsic 
HAT activity, while others act as scaffolds to recruit HAT pro­
teins.166 HAT activity is critically important for activation because 
chromosomal DNA is tightly wrapped in nucleosomal units com­
posed of core histone proteins. Acetylation, as well as some other 
histone modifications, opens up this chromatin structure.
The best understood class of coactivator proteins is the p160 
family, whose name is based on

oteins is the p160 
family, whose name is based on their protein size (approximately 
160 kDa). The family contains at least three molecules, each with 
many names (see Table 2.8).167 These factors possess HAT activ­
ity and recruit other coactivators, such as CREB-binding protein 
(CBP) and p300, which are also HATs. A third HAT, p300/CBP-
associated factor (PCAF), is also recruited by liganded receptors. 
These HATs, along with a complex of SMARC molecules (SWI/
SNF-related, matrix-associated,

RC molecules (SWI/
SNF-related, matrix-associated, actin-dependent regulators of 
chromatin) that direct ATP-dependent DNA unwinding, create a 
chromatin structure that favors transcription (Fig. 2.19).168
Recruitment of multiple HATs may reflect different specifici­
ties for core histones and, potentially, for some nonhistone pro­
teins. Some HATs also interact directly with GTFs and further 
enhance their activities. The mediator complex, which has also 
been called the thyroid hormone recepto

 has also 
been called the thyroid hormone receptor–associated protein 
(TRAP) complex, and the vitamin D receptor–interacting protein 
(DRIP) complex link nuclear receptors to GTFs.169 The HATs 
and TRAP factors are recruited to the liganded, target gene–
bound receptor in an ordered manner that also involves cycling on 
and off the target receptor with a time scale of minutes.170 Nuclear 
receptor interactions with the genome are even more complex, 
with on-off rates that have been measured to

lex, 
with on-off rates that have been measured to be on the order of 
milliseconds.171 
Repression of Gene Expression by 
Unliganded Receptor
Although DNA binding is ligand dependent for steroid hormone 
receptors, other nuclear receptors are bound to DNA even in the 
absence of their cognate ligand. In many cases, the unliganded, 
DNA-bound receptor actively represses transcription of the target 
gene. By reducing the expression of the target gene, this repressive 
function of the receptor amp

ene, this repressive 
function of the receptor amplifies the magnitude of the subse­
quent activation by hormone or ligand. For instance, if the level 
of gene transcription in the repressed state is 10% of the basal level 
in the absence of a receptor, hormone activation to 10-fold above 
that basal level represents a 100-fold difference of transcription 
rate between hormone-deficient (repressed) genes and hormone-
activated genes (Fig. 2.20).172 This phenomenon helps to explain 
why loss of h

72 This phenomenon helps to explain 
why loss of hormone production can result in a much more pro­
found phenotype than loss of the receptor. For example, hypothy­
roidism due to thyroid gland dysfunction or ablation or iodine 
HRE
HDAC3
Other HDACs
(N-CoR, SMRT)
HRE
HAT Coactivators
p160
CBP/p300
PCAF
CoRNR
CoRNR
LBD
LBD
DBD DBD
Ligand
HDAC
HRE
GTFs
TRAP/DRIP
Coactivators
NR box
NR box
NR box
NR box
HAT
• Fig. 2.19  Coactivators and corepressors in transcriptio

.19  Coactivators and corepressors in transcriptional regulation by nuclear receptors. CBP, CREB-
binding protein; CoRNR, coreceptor nuclear receptor box; DBD, DNA-binding domain; DRIP, vitamin D 
receptor–interacting protein; GTFs, general transcription factors; HAT, histone acetyltransferase; HDAC, 
histone deacetylase; HRE, hormone response element; LBD, ligand-binding domain; N-CoR, nuclear 
receptor corepressor; NR, nuclear receptor; PCAF, CBP/p300-associated factor; SMRT, silencing media­

BP/p300-associated factor; SMRT, silencing media­
tor of retinoid and thyroid receptors; TRAP, thyroid hormone receptor–associated protein.
  Nuclear Receptor Coregulators
Chromatin remodeling
SWI/SNF complex
Histone acetyltransferase p160 family (Srcs)
p300/CBP
PCAF (p300/CBP-associated factor)
Mediator
Corepressors
NCoR (nuclear receptor corepressor)
SMRT (silencing mediator for retinoid and thyroid hormone receptors)
CBP, CREB-binding protein (CREB = cAMP-response element–bindin

nding protein (CREB = cAMP-response element–binding protein).
   
TABLE 2.8
39
deficiency leads to severe consequences, up to and including cre­
tinism, coma, and death. This is true in mice as well as people. 
On the other hand, mice lacking all thyroid hormone receptors 
are relatively mildly affected, with only moderate growth and fer­
tility issues.173,174 This discrepancy can be attributed to the fact 
that an unliganded receptor in the hormone defici

that an unliganded receptor in the hormone deficient state exerts 
unchecked repressive activity, which has more severe consequences 
than loss of both repressive and activating functions of THR.
In many ways, the molecular mechanism of repression is the 
mirror image of ligand-dependent activation. The unliganded 
nuclear receptor recruits negatively acting coregulators, called 
corepressors, to the target gene. The two major corepressors 
are large (approximately 270 kDa) proteins: nuclear re

large (approximately 270 kDa) proteins: nuclear receptor 
corepressor (NCoR) and silencing mediator for retinoid and 
thyroid receptors (SMRT, also known as NCoR2).175 NCoR 
and SMRT specifically recognize the unliganded conformation 
of nuclear receptors and use an amphipathic helical sequence 
similar to the NR box of coactivators to bind to a hydrophobic 
pocket in the receptor.
For corepressors, the peptide responsible for receptor bind­
ing is called the CoRNR box (see Fig. 2.17C), and it c

is called the CoRNR box (see Fig. 2.17C), and it contains the 
sequence (I or L)xx(I or V)I, in which I is isoleucine, L is leu­
cine, V is valine, and xx represents any two amino acids.176 The 
receptor uses helices 3 to 5 to form the hydrophobic pocket, as in 
coactivator binding, but H12 does not promote and even hinders 
corepressor binding. This highlights the role of the ligand-depen­
dent change in the position of H12 as the switch that determines 
repression versus activation by nuclear 

termines 
repression versus activation by nuclear receptors (see Fig. 2.19).177
The transcriptional functions of NCoR and SMRT are the 
opposite of those of the coactivators. The corepressors themselves 
do not possess enzyme activity, but they recruit histone deacety­
lases (HDACs) to the target gene, thereby reversing the effects 
of histone acetylation described earlier and leading to a compact, 
repressed state of chromatin. Although the mammalian genome 
contains multiple HDACs, several of 

alian genome 
contains multiple HDACs, several of which may play a role in 
nuclear receptor function, the main one involved in repression 
is HDAC3, whose enzyme activity depends on interaction with 
NCoR or SMRT.178 The ability of NCoR to bind and activate 
HDAC3 is required for normal metabolic and circadian physi­
ology.179 The corepressors interact directly with GTFs to inhibit 
their transcriptional activities, and they also exist in large, multi­
protein complexes whose range of functions

 multi­
protein complexes whose range of functions is not fully under­
stood. Biology is complex, however, and recent studies suggest 
that in brown adipose tissue, HDAC3 foregoes its corepressor 
role and acts as a coactivator in concert with the nuclear receptor 
ERRα. In this case, the effect of HDAC3 is mediated at least in 
part by deacetylation of the key coactivator protein PGC-1, rather 
than a histone.180 
HRE
HRE
HRE
Activated state
Repressed state
100-fold
x
0.

ator
100-fold
x
0.1x
10x
Basal state
of
coactivator
of
corepressor
Relative level of gene transcription
• Fig. 2.20  Repression and activation functions augment the dynamic range of transcriptional regulation by 
nuclear receptors. The magnitudes of activation and repression were arbitrarily set at 10-fold for this theo­
retical example. In cells, these magnitudes vary as a function of coactivator and corepressor concentration 
and in a ta

tivator and corepressor concentration 
and in a target gene–specific manner. HRE, hormone response element.
40
Ligand-Dependent Negative Regulation of Gene 
Expression: Transrepression
The ligand-dependent switch between repressed and activated 
receptor conformations explains how nuclear receptor ligands 
activate gene expression. However, many important gene targets 
of such hormones are turned off in the presence of the ligand. 
This is referred to as 

e presence of the ligand. 
This is referred to as ligand-dependent negative regulation of tran­
scription, or transrepression, to distinguish it from the repression of 
basal transcription by unliganded receptors.
The mechanism of negative regulation is less well under­
stood than ligand-dependent activation, and there may in fact 
be several mechanisms. One mechanism involves nuclear recep­
tor binding to DNA-binding sites that reverse the paradigm of 
ligand-dependent activation (i.e., negativ

igm of 
ligand-dependent activation (i.e., negative response elements). 
For example, when the unliganded thyroid receptor binds to 
the negative response element of the genes for the β-subunit of 
TSH or TSH-releasing hormone, transcription is activated,181 
although more recent studies suggest that the role and recruit­
ment of coregulators in this process are complex.182 In other 
cases, negative regulation may result from nuclear receptors that 
bind to, and inhibit, other transcription fact

at 
bind to, and inhibit, other transcription factors without bind­
ing DNA. This interaction leads to redistribution of coactiva­
tors from the other transcription factors that positively regulate 
the gene. Recent evidence supports this model, whereby inhibi­
tion of the activity of the positively acting factors results in the 
observed negative regulation.183,184 Nuclear receptors can also 
lead to inhibition of gene expression indirectly by activating a 
gene that encodes a transcriptional r

ctivating a 
gene that encodes a transcriptional repressor. 
Roles of Other Nuclear Receptor Domains
The N-terminal A/B domain of the nuclear receptors is the 
most variable region among all members of the superfamily in 
terms of length and amino acid sequence. Subtypes of the same 
receptor often have completely different A/B domains, and the 
function of this domain is the least defined. It is not required 
for unliganded repression or for ligand-dependent activation. 
In many receptors, the 

and-dependent activation. 
In many receptors, the A/B domain contains positive transcrip­
tional activity, often referred to as activation function 1 (AF1) 
(see Fig. 2.16). Its activity is ligand independent, but it prob­
ably interacts with coactivators and may influence the magni­
tude of activation by agonists or partial agonists. This AF is 
tissue specific and tends to be more important for steroid hor­
mone receptors, whose A/B domains are notably longer than 
those of other members of th

 notably longer than 
those of other members of the superfamily.185 The F domain of 
the nuclear receptors is hypervariable in length and sequence, 
and its function is not known. 
Cross-Talk With Other Signaling Pathways
Hormones and cytokines that signal through cell surface recep­
tors also regulate gene transcription, often by activating protein 
kinases that phosphorylate transcription factors such as CREB. 
Such signals can also lead to phosphorylation of nuclear recep­
tors. Multiple sign

sphorylation of nuclear recep­
tors. Multiple signal-dependent kinases can phosphorylate nuclear 
receptors, leading to conformational changes that regulate func­
tion.186 Phosphorylation can lead to changes in DNA binding, 
ligand binding, or coactivator binding, depending upon the spe­
cific kinase, receptor, and domain of the receptor that is phos­
phorylated. The properties of coactivators and corepressors are 
also regulated by phosphorylation.187 
Receptor Antagonists
Certain ligands funct

on.187 
Certain ligands function as receptor antagonists by competing with 
agonists for the ligand-binding site. In the case of steroid hormone 
receptors, the position of H12 in the antagonist-bound receptor is 
not identical to that in the unliganded receptor or in the agonist-
bound receptor. In antagonist-bound steroid receptors, H12, which 
has a sequence that resembles the NR box, binds to the coactivator-
binding pocket of the receptor and thereby prevents coactivato

et of the receptor and thereby prevents coactivator 
binding.188 This antagonist-bound conformation of the receptor 
also favors corepressor binding to steroid hormone receptors. 
Tissue Selectivity of Ligands Interacting With 
Nuclear Receptors
Many endogenous hormones that act through nuclear receptors do so 
in a tissue-specific manner. The most obvious mechanism is differen­
tial expression of the receptors, both in space (e.g., cell type specific­
ity)189 and time (e.g., circadian variation

cific­
ity)189 and time (e.g., circadian variation).190 Ligand levels may be 
regulated intracellularly (see earlier discussion and Table 2.5), and 
post-translational modification (e.g., phosphorylation) influences 
cell-specific receptor function. Although nuclear receptors bind at 
thousands of sites on genomic DNA, the specific binding sites are 
regulated in a cell type–specific manner. For example, the estrogen 
receptor binds to overlapping but clearly different sets of genomic 
sites in 

g but clearly different sets of genomic 
sites in the uterus and in the breast, probably because of the differ­
ential actions of so-called pioneer transcription factors, which open 
tightly compacted chromatin in a tissue-specific manner and enable 
nuclear receptor and other transcription factors to bind.191
Some ligands function as antagonists in certain tissues but as 
full or partial agonists in others. These selective receptor modula­
tors include compounds such as tamoxifen, a selective e

include compounds such as tamoxifen, a selective estrogen 
receptor modulator (SERM). SERMs are estrogen receptor antag­
onists with respect to the functions of AF2, including coregulator 
binding, and they require the AF1 for their agonist activity.192 
This agonism, like AF1 activity, tends to be tissue specific and 
therefore has great therapeutic utility.193 Table 2.9 summarizes 
factors contributing to the tissue specificity of receptor activity. 
Nongenomic Actions of Nuclear 
Receptor Lig

vity. 
Receptor Ligands
Some actions of steroids and other nuclear receptor ligands occur 
within seconds or minutes, far quicker than should be possible 
  Factors Modulating Receptor Activity in 
Different Tissues
Concentration of receptor
Cell specificity
Variation within a given cell type
Post-translational modification of receptor (e.g., phosphorylation)
Regulation of intracellular ligand levels (see Table 2.5)
Tissue-specific factors that open chromatin
Func

)
Function of ligand
Agonist
Partial agonist
Antagonist
Concentration and types of coregulators
TABLE 2.9
41
using the transcriptional mechanisms described in the chapter. 
This suggests that some traditional nuclear receptor ligands may 
have a discrete set of nongenomic actions. There is now reasonable 
evidence that thyroid hormone, estrogen, androgen, and possibly 
other ligands can 

trogen, androgen, and possibly 
other ligands can bind and activate receptors outside of the nucleus. 
In most cases, these extranuclear receptors are splice variants of the 
same genes that encode the traditional nuclear receptor, often with 
loss of the DNA binding domain and NLS. Binding of ligands to 
these receptors causes activation of many classic signaling pathways, 
such as Src, ERK, and Akt pathways.194 In some cases there may 
be binding of ligands to receptors that are completely unr

ng of ligands to receptors that are completely unrelated to 
the nuclear receptors, though this is less clear. The nongenomic and 
genomic actions of ligands may act cooperatively (e.g., by causing 
phosphorylation of the nuclear form of the receptor).195 
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The Role of Genetics in Endocrinology, 42 
Principles of Genetics, 43 
Genetics of Endocrine Diseases, 47 
KEY POINTS 
The genetic basis of each heritable endocrine disease/trait is 
quantified by its genetic architecture (1) the number of genetic 
variants/genes, (2) their frequency in the population, and (3) their 
respective contribut

he population, and (3) their 
respective contributions to disease risk/phenotypic variation. 
Mendelian endocrine disorders are caused by variants found 
rarely in the population, usually from a relatively small number 
of genes, and each variant has a large individual effect on dis-
ease risk so that in any one individual, most of the disease risk is 
explained by variants in a single gene. Mendelian variants can 
be highly penetrant, but this is not always the case. 
Common endocrine diseases/

s not always the case. 
Common endocrine diseases/traits such as stature, type 2 dial:5e-
tes, and serum lipids are polygenic-the result of combined, i-
The Role of Genetics in Endocri ology 
The sequencing of the human genome has~
hered in an era of 
genomic medicine. The catalog of protein-coding genes in humans is 
essentially complete, and the number of associations between genes 
and specific diseases is growing rapidly. Moreover, it is now feasible 
to identify nearly every genetic variant

feasible 
to identify nearly every genetic variant in an individual's protein-cod-
ing genes (whole-exome sequencing [WES]) or in his or her entire 
genome (whole-genome sequencing) due to revolutionary advances 
in sequencing technologies (collectively referred to as next genera-
tion sequencing [NGS]). The ability to interpret this variation is less 
advanced but is improving, as databases of variants and their clinical 
associations increase in both size and accuracy. Founded in 1982, 
the Na

n both size and accuracy. Founded in 1982, 
the National Center for Biotechnology Information's GenBank now 
contains more than 200 million sequences and includes worldwide 
contributions from the DNA Data Bank of Japan and the European 
Nucleotide Archive. 1 Ongoing projects include the Genome Aggre-
gation Database, Exome Aggregation Consortium (ExAC), United 
Kingdom 100,000 Genomes Project, and the National Center for 
Biotechnology Information's Single Nucleotide Polymorphism 
Database. Ava

on's Single Nucleotide Polymorphism 
Database. Available sequencing data are growing exponentially, and 
the volume of information places increasing the demand for meth-
ods to distinguish pathologic variants from benign ones. 
42 
Considerations for Clinical Use of Genetic Information and 
Sequencing in Endocrinology, 56 
multaneous effects of many variants in many genes, often found 
frequen Ix in the general population, and with each variant 
contributing a small individual effect so that the

contributing a small individual effect so that the phenotype in 
any one indiv'dual results from variants in many different genes. 
Genetic information enables endocrinologists to personalize 
therapy or patients. 
Comprehensive genetic testing (i.e., genome sequencing) can 
be standardized and automated, but drawing valid and clinically 
useful conclusions requires integration with patient history, 
physical examination, and other laboratory examinations. 
Genetic information is most likely to 

minations. 
Genetic information is most likely to be of direct clinical use in 
patients with suspected mendelian syndromes. 
With the expanding reach of precision medicine-individualized 
diagnosis and therapy informed by genetics-we anticipate that 
increasing numbers of patients will have clinical indications for exome 
or genome sequencing, and others will come to clinical encoun-
ters with their sequences already in hand. Clinicians will be asked 
to interpret these genetic data to shed lig

asked 
to interpret these genetic data to shed light on an individual's risk 
of developing disease, on diagnosis and prognosis for those already 
affected, on implications to family members, and on individualiza-
tion of therapy. As such, it is critical that clinicians are able to draw 
valid and clinically useful connections between DNA sequence varia-
tion and human traits and diseases. Perhaps even more important, it 
is critical that clinicians understand the limits of such information. 
In

ans understand the limits of such information. 
In this chapter, we present a guide to help clinicians appreciate 
and critically interpret the relationship between a DNA sequence 
(genotype) and an individual's clinical presentation (phenotype). 
We first discuss principles of genetics to provide the framework 
for understanding and interpreting DNA variation in patients. We 
then focus on endocrine disorders, providing an overview of the 
genetics of endocrine diseases, with illustrative examp

ics of endocrine diseases, with illustrative examples from both 
mendelian disorders (caused by mutations in single genes) and poly-
genie disorders (in which variation in many genes influences dis-
ease risk). Finally, we examine scenarios for clinical uses of genetic 
information in endocrinology and provide recommendations. 
43
Chapter 3  Genetics of Endocrinology
Most diseases, including endocrine disorders, are heritable, 
meaning that genetic variation contributes to disease risk in 
a pop

ic variation contributes to disease risk in 
a population. These diseases range across the spectrum of rare, 
single-gene disorders, such as multiple endocrine neoplasia (see 
Chapter 42), Carney complex (see Chapter 15), and congenital 
adrenal hyperplasia (CAH) (see Chapter 24) to polygenic diseases, 
such as type 2 diabetes (see Chapter 34 and 35), Graves disease 
(see Chapter 12), and osteoporosis (see Chapter 29). The detailed 
discussions of the genetics of these and other disorders can be

f the genetics of these and other disorders can be 
found throughout this textbook; this chapter provides illustrative 
examples that illuminate key concepts and refers the reader to the 
appropriate chapters for additional detail. 
Principles of Genetics
A Brief Historical Perspective
In Western conception, the relationship between inheritance and 
physical characteristics (disease and nonpathologic) has been rec­
ognized since the time of Aristotle (323 bc). But it was not until 
1865 that the

otle (323 bc). But it was not until 
1865 that the Austrian abbot Gregor Mendel, after decades of 
careful experimentation in pea plants, posited and provided evi­
dence for the modern genetic concept of genes (as coined by the 
botanist Wilhelm Johannsen in 1909).2 Mendel deduced certain 
rules governing the passage of genotype (the collective versions 
of multiple genes in an individual) from parent to offspring, 
enabling the prediction of the resulting physical characteristics 
(phenotype) o

 resulting physical characteristics 
(phenotype) of the offspring. It was recognized in the early 20th 
century that certain human phenotypes, including diseases, were 
inherited according to the same rules that Mendel had described; 
these diseases are called mendelian.
Over the course of the next century, numerous breakthroughs 
established that genes were composed of DNA, physically con­
nected on chromosomes, and encoded proteins. The first descrip­
tion of the molecular basis of a mendelian

escrip­
tion of the molecular basis of a mendelian disease was made for 
sickle cell anemia, which involved a mutation in a single gene. In 
the 1970s, the ability to sequence DNA revealed natural and heri­
table sequence variation (genetic polymorphisms) in any given gene 
among different individuals. It was appreciated that the molecular 
basis of variation in the genotypes of individuals resulted from DNA 
sequence polymorphisms, which in turn effected alterations in phe­
notype. By tracing t

 effected alterations in phe­
notype. By tracing the transmission of these polymorphisms in fam­
ilies, it became possible to identify genes causing mendelian human 
disorders (those caused by altered function in a single gene and that 
consequently show distinctive patterns of inheritance in families).3
However, most human diseases and phenotypes are not men­
delian. Biometricians had appreciated in the early 1900s that 
most continuous and commonly varying traits (e.g., height and 
blood press

only varying traits (e.g., height and 
blood pressure) did not follow mendelian patterns of inheritance. 
In 1919, R.A. Fisher4 provided a general framework explaining 
continually varying traits as the consequence of polygenic inheri­
tance—that is, polygenic phenotypes are a result of combined, 
small, and additive effects of variation in many genes simultane­
ously. In this framework, monogenic/mendelian traits were a spe­
cial case. Despite this recognition, only a few genetic variants were 

his recognition, only a few genetic variants were 
convincingly connected with polygenic diseases/traits over the next 
80 years. It would take a series of technological advances, includ­
ing the sequencing of the human genome (Human Genome Proj­
ect 1990–2003) and the systematic cataloging of DNA sequence 
polymorphisms across diverse human populations (International 
HapMap Project 2002–2005 Phase I), to systematically identify 
the genetic causes for common polygenic diseases.5 
Heritability:

ses for common polygenic diseases.5 
Heritability: An Estimate of the Importance of 
Genetic Factors to Disease Causation
Relatives resemble each other in many ways. Resemblance with 
respect to traits such as height or to diseases such as multiple endo­
crine neoplasia type 1 (MEN1) could be explained by shared geno­
types passed down through generations, shared environments, and 
nonlinear interactions between genes and environment. Heritability 
quantifies, as a proportion, how much of this f

y 
quantifies, as a proportion, how much of this familial resemblance is 
due to genetic factors. A trait that has no genetic influence would have 
a heritability of 0%; a trait that is completely determined by inher­
ited factors would have a heritability of 100%. Most clinically impor­
tant traits have heritabilities ranging from 20% to 80% (Table 3.1). 
Appreciating the heritability of a trait is important when interpreting 
the contribution of genetic risk factors in disease: genetic factors

f genetic risk factors in disease: genetic factors are 
less influential for traits with low heritability and are likely to have 
more predictive or explanatory power for traits with high heritability.
In the past, the gold standard for heritability estimation was 
the comparison of monozygotic and dizygotic twin concordance 
rates for diseases/traits. Such studies relied on the rationale that 
an excess of disease correlation between genetically identical indi­
viduals (monozygotic twin pairs) 

 identical indi­
viduals (monozygotic twin pairs) compared with those who share 
only 50% of their genes (dizygotic twin pairs) pointed to the role 
of genetic factors. However, the validity of comparing twin con­
cordance rates across different families relied on the assumption 
that the effect of environment was the same for the twin pairs, 
regardless of whether they were monozygotic or dizygotic twins. 
More recent methods for heritability estimation can overcome 
some of these limitations b

timation can overcome 
some of these limitations by leveraging subtle fluctuations in 
genetic similarities between sibling pairs.6
Regardless of the methodology employed, it is critical to appre­
ciate that heritability is not a fixed property of a disease/trait. The 
heritability estimate from any study must be interpreted in the 
context of the population in which it is being measured, including 
the historical period, and variability in environmental factors such 
as socioeconomic status and

onmental factors such 
as socioeconomic status and nutrition. These factors likely explain 
the wide range of heritability estimates for type 2 diabetes, ranging 
from 40% in Finland7 to 80% in Japan.8 An illustrative example 
of the importance of history can be drawn by examining type 1 
diabetes rates across the Scandinavian region of Karelia. In 1940, 
this region was divided between Finland and the former Soviet 
Union with little contact between the two sections over the next 
60 years. Fin

ween the two sections over the next 
60 years. Finnish Karelians have a sixfold increased rate of type 
1 diabetes compared with Russian Karelians.9 As a result, herita­
bility for type 1 diabetes will be different when estimated in the 
combined Karelian populations than when estimated in Finnish 
or Russian Karelians alone. The difference in diabetes rate is likely 
due to environmental factors, because both Karelian populations 
recently originated from a common ancestry and therefore likely 

nated from a common ancestry and therefore likely 
have similar genetic risk factors for type 1 diabetes.10 
Human DNA Sequence Variation: Molecular 
Forms and Biologic Effects
Each human has two versions of his or her genome (one from 
each parent); each version consists of a sequence of approximately 
3 billion DNA bases. When comparing two versions of a human 
genome, either within the same person or between two different 
people, about 1/1000 of these bases vary (i.e., 99.9% of them are 
the

 of these bases vary (i.e., 99.9% of them are 
the same) (Table 3.2). There are many possible ways in which 
DNA sequences can vary; several specific types of DNA sequence 
variants are frequently observed (Fig. 3.1).
44
The most frequent form of variation, the single nucleotide 
polymorphism (SNP), refers to the situation in which a single base 
in the sequence of one individual is different from the base seen 
at the same position in the sequence of ano

 seen 
at the same position in the sequence of another individual. SNPs 
can exert a wide range of biologic effects, depending on where 
the variant occurs and whether it alters the function of the DNA 
sequence. Some SNPs occur within the portions of genes that are 
transcribed into RNA and then translated into proteins (protein-
coding regions). Synonymous SNPs occur in the protein-coding 
portion of DNA but both versions (alleles) of the SNP encode 
the same amino acid, and so this sort of va

ncode 
the same amino acid, and so this sort of variation usually does 
not affect function. SNPs can be missense changes (alteration of 
a single amino acid in a protein-coding gene), as is the case of 
the C282Y mutation in the HFE gene responsible for autosomal 
recessive hereditary hemochromatosis (see Chapter 19). Some 
missense SNPs greatly alter function, whereas others appear to 
have no consequences. SNPs can also alter splice sites, disrupt­
ing the structure of the mRNA that is transc

rupt­
ing the structure of the mRNA that is transcribed from the DNA 
during gene expression. For example, the most common cause of 
autosomal dominant isolated growth hormone (GH) deficiency is 
single-base mutations that inactivate a splice donor site of intron 3 
in the GH1 gene, causing skipping of exon 3 in GH1 (see Chapter 
25). SNPs can also introduce stop codons, leading to premature 
termination of translation and a truncated protein product. These 
nonsense variants typically dramatica

duct. These 
nonsense variants typically dramatically impair or eliminate the 
function of the protein.
Changing the protein sequence is not the only way that SNPs 
(and other types of genetic variations) can alter gene function. Most 
of the human genome does not code for proteins (see Table 3.2), 
and most genetic variation occurs in this noncoding portion of the 
genome. For example, noncoding variants can alter the level, tim­
ing, or location of gene expression, without changing the sequenc

n of gene expression, without changing the sequence 
of the encoded protein. Noncoding variants often result in more 
  Heritable Endocrine Traits and Diseases
Common Form
Heritability
Referencea
Selected Mendelian Forms
Type 1 diabetes
80%
132
KCNJ11, ABCC8 (permanent neonatal diabetes)
Type 2 diabetes
40–80%
23, 28, 133
AGPAT2 (congenital generalized lipodystrophy), LMNA (familial partial lipodystrophy 1) HNF4A, GCK, 
HNF1A (MODY1-3)
Obesity
40–70%
134, 135
MC4R, POMC
30–70%
136

0–70%
134, 135
MC4R, POMC
30–70%
136
MEN1, RET (MEN2A/B), VHL, SCNN1A (Liddle syndrome), CYP17A1 (17OHD), HSD11B2 (AME)
Height
80%
24, 72
GH1, FGFR3 (achondroplasia), SHOX1 (Ullrich-Turner syndrome), FBN1 (Marfan syndrome)
Pubertal timing
50–80%
137
KAL1, KISS1R, FGFR1 (hypogonadotropic hypogonadism)
Hyperthyroidism
80%
138
TSHR (familial nonautoimmune hyperthyroidism)
Hypothyroidism
67%
139
TSHR, SLC5A5, TG, TPO, and TSHB (congenital hypothyroidism)
Osteoporosis
50–85%
140, 141
COL

l hypothyroidism)
50–85%
140, 141
COL1A1, COL1A2, IFITM5 (osteogenesis imperfecta)
Serum calcium
40%
142, 143
CASR (familial hypocalciuric hypercalcemia), HRPT2 (hyperparathyroid jaw-tumor syndrome)
Lipids
40–60%
85, 86
Low-density lipoprotein: LDLR (familial hypercholesterolemia)
High-density lipoprotein: CETP
Triglycerides: APOE (familial dysbetalipoproteinemia)
Kidney stones
56%
144, 145
CLCN5 (X-linked recessive nephrolithiasis), NKCC2 (Bartter syndrome)
aNumbers in this column 

 NKCC2 (Bartter syndrome)
aNumbers in this column indicate references listed at the end of the chapter.
   
TABLE 3.1
  Characteristics of Human Genome 
Sequence Variation
Characteristic
Frequency
Length of the human genome sequence (base 
pairs)
3 billion
Number of human genes (estimated)
20,000
Fraction of base pairs that differ between the 
genome sequence of a human and a chim­
panzee
1.3% (1:80)
Fraction of base pairs that vary between the 
genome sequence of any two humans
0.1% (1:1000)
F

Fraction of coding region base pairs that vary in a 
manner that substantially alters the sequence 
of the encoded protein
0.02% (1:5000)
Number of sequence variants present in each 
individual as heterozygous sites
3 million
Number of amino acid–altering variants present 
in each individual as heterozygous sites
12,000
Number of sequence variants in any given human 
population with frequency >1%
10 million
Number of amino acid polymorphisms pres­

0 million
ent in the human genome with a population 
frequency >1%
75,000
Fraction of all human heterozygosity attributable 
to variants with a frequency >1%
98%
Adapted from Altshuler D. The inherited basis of common diseases. In Goldman L, Schafer AI, 
eds. Goldman’s Cecil Medicine, 24th ed. Philadelphia, PA: WB Saunders; 2012.
   
TABLE 3.2
45
subtle biologic effects, and the mechanisms are still being uncov­
ered. 

, and the mechanisms are still being uncov­
ered. For example, some SNPs subtly influence type 1 diabetes risk 
and lie in enhancers (noncoding DNA segments that activate gene 
transcription at a distance) that appear to affect gene expression 
only in lymphoid cells.11
Insertions and deletions (collectively called indels) respectively 
refer to the addition or removal of one or more bases in the DNA 
sequence. Indels in protein-coding sequences are called frameshift 
mutations, as long as the n

are called frameshift 
mutations, as long as the number of bases inserted or deleted is 
not a multiple of three. Because the genetic code is composed of 
Common SNP
Indel
polymorphism
Recombination
hotspot
A
T
G
G
G
G
G
G
G
G
G
G
G
G
G
G
A
A
A
A
A
A
C
C
C
C
C
C
C
C
G
G
G
G
G
G
C
C
C
C
C
C
C
C
C
C
G
G
G
G
C
C
C
C
G
G
C
C
G
G
G
G
AT T C AT T C
AT T C AT T C
AT T C AT T C
AT T C AT T C
AT T C
AT T C
AT T C
AT T C
AT T C AT T C
AT T C AT T C
AT T C AT T C
AT T C AT T C
AT T C
AT T C
AT T C AT T C
A

AT T C
AT T C AT T C
AT T C
AT T C
AT T C AT T C
AT T C AT T C
AT T C
AT T C
AT T C
AT T C
G
G
G
G
C
C
C
C
G
G
G
G
C
G
G
C
C
C
C
A
A
A
A
C
C
C
C
A
A
A
A
C
A
A
C
C
C
C
T
T
T
T
C
C
C
C
T
T
T
T
C
T
T
C
C
C
C
C
C
C
C
T
T
T
T
C
C
C
C
     )
T
C
C
T
T
T
T
C
C
C
C
C
C
C
C
C
C
C
C
C
C
A
A
A
A
A
A
A
A
A
A
A
A
A
A
T
T
T
T
(      
T
A
A
A
A
A
A
T
T
T
T
T
T
T
T
A
A
A
A
A
A
T
T
T
T
T
T
( G )
( G )
( G )
( G )
( G )
( G )
( G )
( G )
(    )
(    )
(    )
(    )
(    )
(    )
( G )
( G )
( G )
( G )
( G )
( G 

)
(    )
(    )
( G )
( G )
( G )
( G )
( G )
( G )
C
1
2
(
)
3
4
5
6
7
8
9
10
11
12
Strong correlation
No correlation
Repeat
Low-frequency
variant
• Fig. 3.1  DNA sequence variation in the human genome. Common and rare genetic variation in 10 indi­
viduals, carrying 20 distinct copies of the human genome. The amount of variation shown here is typical for 
a 5-kb stretch of genome and is centered on a strong recombination hotspot. The 12 common variations 
include 10 single-nucleoti

e 12 common variations 
include 10 single-nucleotide polymorphisms (SNPs), an insertion-deletion polymorphism (indel), and a 
tetranucleotide repeat polymorphism. The six common polymorphisms on the left side are strongly corre­
lated. Although these six polymorphisms could theoretically occur in 26 possible patterns, only 3 patterns 
are observed (indicated by pink, orange, and green). These patterns are called haplotypes. Similarly, the 
six common polymorphisms on the right side are strongly 

mmon polymorphisms on the right side are strongly correlated and reside on only two haplotypes 
(indicated by blue and purple). The haplotypes occur because there has not been much genetic recombi­
nation between the sites. By contrast, there is little correlation between the two groups of polymorphisms 
because a hotspot of genetic recombination lies between them. In addition to the common polymor­
phisms, lower-frequency polymorphisms occur in the human genome. Five rare single-nucleotide poly

the human genome. Five rare single-nucleotide poly­
morphisms are shown, with the variant nucleotide marked in red and the reference nucleotide not shown. 
In addition, on the second to last chromosome, a larger deletion variant is observed that removes several 
kilobases of DNA. Such larger deletion or duplication events (i.e., copy number variants) may be common 
and segregate as other DNA variants. (Redrawn from Altshuler D, Daly MJ, Lander ES. Genetic mapping 
in human disease. Science. 2008

. Genetic mapping 
in human disease. Science. 2008;322[5903]:881-888.)
46
triplets (every three bases encode one amino acid), a frameshift 
mutation alters how every subsequent base in the sequence is 
translated into a protein, resulting in profound molecular and 
clinical consequences. For example, classic salt-wasting CAH is 
often caused by frameshift deletions in the CYP21A2 gene that 
ablate its function (see Chapter 15 and 24). Repeat polymor­
phis

tion (see Chapter 15 and 24). Repeat polymor­
phisms (often referred to as copy number variants [CNVs] if 
the repeats are large) are a special case of indels in which DNA 
sequences are repeated in tandem, and the number of copies of 
the repeated sequence varies. For example, the AR gene (encod­
ing the androgen receptor [AR]) contains a repeat polymorphism 
in which a CAG codon, encoding glutamine, is repeated 11 
to 31 times (see Chapter 24). Structural variation can include 
both insertions

 Structural variation can include 
both insertions and deletions, as well as rearrangement of large 
chunks of DNA sequence (translocations and other complex 
forms of genomic variation). Structural variation causes famil­
ial hyperaldosteronism type 1; the adrenocorticotropic hormone 
(ACTH, corticotropin)-responsive promoter of the CYP11B1 
gene is incorrectly located adjacent to the aldosterone synthase 
gene (CYP11B2), causing aldosterone to be produced by ACTH 
stimulation (see Chapter 16).

be produced by ACTH 
stimulation (see Chapter 16). 
Factors Influencing the Biologic Impact of 
Genetic Variants in a Particular Gene
As discussed previously, the impact of a genetic variant on gene 
function will depend on the type of variant and its location with 
respect to the gene. For example, frameshift deletions in the 
CYP21A2 gene eliminate 21-hydroxylase activity, whereas mis­
sense variants in CYP21A2 often retain partial 21-hydroxylase 
activity (see Chapter 24). However, even a sin

se 
activity (see Chapter 24). However, even a single, specific vari­
ant may have different effects in different individuals. The effect 
of any given genetic variant (genotype) on the phenotype can be 
modified by variants in other genes (gene-gene interactions) or 
by environmental factors (gene-environment interactions) or by 
random chance. It is usually not possible to measure or quantify 
these factors in any one person, but their combined effect can be 
quantified on a population level a

 effect can be 
quantified on a population level as penetrance, the proportion of 
individuals carrying a genetic variant who exhibit the phenotype. 
The penetrance of a genetic variant is highly context dependent 
with respect to phenotypic definition. For example, the hemo­
chromatosis-associated C282Y allele in the HFE gene exhibits 
high penetrance for the biochemical phenotype of high ferritin 
(>60% of homozygous carriers manifest increased ferritin levels) 
but only 2% penetrance for the 

 ferritin levels) 
but only 2% penetrance for the clinical phenotype of liver cirrho­
sis. Temporal context is also an important consideration, as disease 
incidence often increases with age. Carriers of mutations causing 
MEN1 have nearly 100% penetrance for parathyroid adenomas 
by age 40 but only 20% penetrance at age 20.
A common observation in members of a family carrying the 
same disease-causing genetic variant is that not all members of 
the family are equally affected. This range of phe

the family are equally affected. This range of phenotypic expres­
sion resulting from a particular genotype is referred to as vari­
able expressivity and, as with penetrance, arises from the range 
of impacts of specific variants, as well as modifying influences of 
genetic background (gene-gene interactions), environment (gene-
environment interactions), and random chance. For example, 
the same mutation in the AR (encoding an S703G substitution) 
resulted in a spectrum of clinical androgen ins

) 
resulted in a spectrum of clinical androgen insensitivity such that 
some individuals were raised as 46,XY females and others as males; 
other mutations in AR have different ranges of phenotypic effects 
(see Chapter 24).
Mosaicism, whereby cells within a single individual have differ­
ent genotypes, is another mechanism that leads to variable expres­
sivity. Most mutations known to influence disease are germline 
mutations—inherited from the sperm or egg and present in every 
cell—but some d

sperm or egg and present in every 
cell—but some diseases can be caused by somatic mutations that 
occur after fertilization and are present in only some cells, lead­
ing to mosaicism. In these cases, which tissues or organs carry 
the mutation will influence the clinical outcome. The most famil­
iar class of disease caused in large part by somatic mutations is 
neoplasia, including endocrine tumor syndromes such as Conn 
syndrome and Cushing disease. Another classic example from 
endocrinology 

ease. Another classic example from 
endocrinology is the McCune-Albright syndrome, in which the 
same activating mutation in GNAS1 exhibits variable expressiv­
ity because of postzygotic mosaicism. The phenotype of patients 
with McCune-Albright syndrome depends on which tissues and 
what fraction of cells carry the GNAS1 mutation. A minority of 
affected individuals (24%) display the classic triad of café au lait 
spots, polyostotic fibrous dysplasia, and gonadotropin-releasing 
hormone (GnRH)-

lasia, and gonadotropin-releasing 
hormone (GnRH)-independent precocious puberty; the major­
ity express two or fewer features of the classic triad (see Chapter 
26). The mechanism of variable expressivity likely maps to the 
zygotic stage in which the mutation arose: a mutation earlier in 
embryogenesis is present in more tissue lineages. Because muta­
tions in a mosaic individual are not present in every cell, they can 
be hard to detect in DNA isolated from a blood sample if the cell 
in whic

 isolated from a blood sample if the cell 
in which the mutation occurred does not give rise to blood leuko­
cytes. The GNAS1 mutation responsible for the McCune-Albright 
syndrome is detected in only 8% to 46% of blood samples from 
affected individuals but is found in 90% of affected tissue sampled 
irrespective of clinical presentation (see Chapter 26). Conversely, 
blood cells can contain somatic variation that is absent in other 
tissues or the germline.12
It is important to remember that t

the germline.12
It is important to remember that the base pair composition of 
a DNA sequence is not the only molecular determinant of phe­
notypic expression (Table 3.3). DNA is subject to other forms of 
modification besides sequence variation (termed epigenetic varia­
tion), such as cytosine methylation or packaging into nucleosomes 
  Origins of DNA Sequence Variation in 
Human Populations: Common Versus Rare 
Variants
The type of genetic variant (missense, frameshift, noncoding, etc.) pro­

riant (missense, frameshift, noncoding, etc.) pro­
vides clues to its possible consequences. In addition, the population 
frequency of a variant, whether it is common or rare, can also provide 
information about its likely impact on phenotype. The relative balance 
between common and rare genetic variation is strongly influenced 
by evolution and human demographic history. Modern humans likely 
originated from a small population residing in Africa that had been 
evolving over millions of years. 

a that had been 
evolving over millions of years. Within the past 50,000 years, mem­
bers of this ancestral population migrated “out of Africa,” settled the 
globe, and only recently, over the past 5000 to 10,000 years, multi­
plied exponentially.127 As a consequence of this demographic history, 
most of the 3 million genetic variants an individual inherits from his 
or her parents are common (typically >1% frequency in the popula­
tion), can be traced back to the ancient African population, and

traced back to the ancient African population, and are 
shared in many unrelated individuals in the population. Individuals 
also inherit thousands of genetic variants unique to themselves and 
their relatives. These rare genetic variants arose more recently from 
spontaneous mutation in the past 10 millennia, after the migration of 
many humans out of Africa, and are typically observed infrequently 
(<0.1% of all chromosomes) in the population.
TABLE 3.3
47

TABLE 3.3
47
with various biochemically modified histones. Thus, the same 
molecular form of DNA sequence variation can vary in its cellular 
and phenotypic effect through epigenetic modifications. Indeed, 
epigenetic modification is a normal part of development and is the 
reason different cells have different properties even though they 
share the identical DNA sequence. A striking example of the effect 
of epigenetics is imprinting, the expression of a gen

epigenetics is imprinting, the expression of a genetic variant in a 
parent-of-origin specific manner. For paternally imprinted genes, 
the copy that is inherited from the father is silenced, and only the 
mother’s copy is expressed in the offspring. Imprinting can affect 
the impact of disease-causing mutations. Inactivating mutations in 
SDHD cause familial paraganglioma type 1. SDHD is maternally 
imprinted, so the mutation does not cause disease when inherited 
from the mother but is highly 

ase when inherited 
from the mother but is highly penetrant when inherited from the 
father. Imprinting can also be tissue specific. A paternally inher­
ited inactivating mutation in GNAS1 causes Albright hereditary 
osteodystrophy (pseudopseudohypoparathyroidism; see Chapter 
29). The same mutation, when maternally inherited, manifests 
not only with Albright hereditary osteodystrophy but also with 
hypocalcemia secondary to parathyroid hormone resistance (pseu­
dohypoparathyroidism type 1a [PH

resistance (pseu­
dohypoparathyroidism type 1a [PHP1a]), because only the mater­
nal copy of GNAS1 is expressed in renal proximal tubules.
Evolution influences the frequency of variants that affect 
human phenotypes (e.g., endocrine diseases) through the process 
of natural selection. Variants that greatly increase the risk of a 
disease that is deleterious from a reproductive standpoint are less 
likely to be passed on to offspring and will be rare in the popula­
tion (unless they have a compen

are in the popula­
tion (unless they have a compensatory benefit like malaria resis­
tance in carriers of sickle cell disease). If a disease is at least mildly 
evolutionarily deleterious, then most common variants associated 
with that disease will only modestly increase disease risk. This is 
because those common variants, if they had strongly increased 
disease risk, would have then been subject to strong negative evo­
lutionary selection and never would have risen in frequency to 
become com

never would have risen in frequency to 
become common in the first place. By contrast, it is more plau­
sible for rare/recent variants to exert strong effects on phenotype 
and greatly increase disease risk.
Finally, the number of genes that contribute to disease in a 
single individual (mendelian or polygenic disease) will be related 
to the strength of effect of any one variant on disease risk. By 
definition, variants that cause mendelian disorders have strong 
effects, whereas variants contr

rders have strong 
effects, whereas variants contributing to risk of polygenic diseases 
will typically have more modest effects. Thus, most variants with 
strong effects on disease will be rare, especially for those diseases 
that are clearly deleterious from an evolutionary standpoint (lethal 
before reproductive age). By contrast, common polygenic diseases 
and traits will have a much more substantial contribution from 
common genetic variants, although these considerations do not 
rule out a

, although these considerations do not 
rule out an important role for rarer variants in polygenic pheno­
types. As we will see later in this chapter, these patterns of genetic 
variation have important implications for identifying genetic 
variants that underlie disease and for interpreting the impact of 
genetic variation on disease. 
Summary
To summarize this introductory section, we have briefly described 
several basic principles of genetics. Heritability describes the pro­
portion of a dis

. Heritability describes the pro­
portion of a disease/trait that can be explained by genetic fac­
tors; the heritability of most endocrine diseases ranges between 
20% and 80% (see Table 3.1). Genetic variants can take many 
forms ranging from single-base changes (SNPs) to translocations 
of entire chromosomes (see Fig. 3.1). The biologic effect of these 
variants depends on the type of variant; where in the DNA they 
are located (e.g., within coding sequence, splice sites, enhancers); 
how sev

oding sequence, splice sites, enhancers); 
how severely the variant affects function; and for somatic muta­
tions, the cells and tissues that carry the mutation. Biologic impact 
can also be modified by the presence of genetic variants in other 
genes (gene-gene interactions), the individual organism’s envi­
ronment (gene-environment interactions), and random chance. 
The demographic history of modern human populations explains 
the presence of common and rare genetic variants in the human 
geno

ommon and rare genetic variants in the human 
genome (see Table 3.2). Common variants are mostly ancient and 
typically have relatively modest clinical effects, whereas rare vari­
ants tend to have arisen more recently and can exert larger clinical 
effects (Table 3.4). 
Genetics of Endocrine Diseases
As described earlier, heritable diseases and traits, including endo­
crine phenotypes, span a range of genetic architectures ranging 
from single-gene mendelian disorders to common, polygenic dis­

ene mendelian disorders to common, polygenic dis­
eases and traits. Mendelian and polygenic disorders represent two 
ends of a spectrum (Fig. 3.2) of genetic architectures. Although 
we distinguish between these two extremes of genetic architec­
ture, it is important to appreciate that many disorders lie between 
these two extremes: rare variants of moderate effect can affect the 
common form of the disease, and genetic and nongenetic modi­
fiers can strongly influence the outcome of mendelian d

 can strongly influence the outcome of mendelian disorders. 
Furthermore, many polygenic endocrine disorders also have rare 
mendelian forms (see Table 3.1).
The genes for a wide range of mendelian endocrine diseases 
have been mapped, revealing great mechanistic insight. Although 
mendelian diseases have offered valuable insights into pathophys­
iology, not all insights gained from mendelian forms of disease 
translate directly to the common forms of disease. For example, 
mendelian obesity cau

ms of disease. For example, 
mendelian obesity caused by recessive inactivating mutations in 
the leptin receptor could be well treated by exogenous leptin, but 
this clinical insight did not apply to most obese individuals who 
actually demonstrate elevated leptin levels and do not respond 
to exogenous therapy with leptin (see Chapter 40). Obesity as 
a common trait is highly heritable (heritability 40–80%), and 
genome-wide association studies (GWASs) analysis has begun to 
identify risk vari

 (GWASs) analysis has begun to 
identify risk variants for the common form.13 Although some of 
the risk variants overlap with those causing mendelian syndromes 
(as is also true for other diseases), GWASs have pointed to addi­
tional genetic contributions outside the mendelian genes. And, of 
course, the variants that have strong effects on quite rare genetic 
syndromes do not explain much, if any, of the risk of the com­
mon forms of disease. Thus, genetics of both mendelian forms 
and common 

hus, genetics of both mendelian forms 
and common polygenic forms will have important and often 
complementary impacts on our understanding of disease and on 
patient care.
The sections that follow discuss representative examples of 
mendelian and polygenic endocrine disorders that illustrate 
important concepts in gene discovery, understanding of the 
impact of genetic variation on disease, and implications for clini­
cal care and insights into new biology. We discuss several classes of 
mendel

new biology. We discuss several classes of 
mendelian diseases and highlight three polygenic endocrine dis­
eases/traits: (1) type 2 diabetes, (2) stature, and (3) serum lipids. 
In each section, we discuss what is known about the underlying 
genetic contributors, the impact of genetics on our understand­
ing of disease biology, and the translation into clinical care in the 
short and long term.
48
Mendelian Endocrine Diseases
Genetic Architecture
Mendeli

an Endocrine Diseases
Mendelian diseases represent one extreme of a spectrum of pos­
sible genetic architectures (see Fig. 3.2). The alleles causing men­
delian diseases are found in a small number of genes, are typically 
rare (<1:1000), can be (although not always) highly penetrant, and 
follow simple patterns of dominant and recessive inheritance. 
They are considered monogenic in that a mutation in a single 
gene causes disease in an individual or family. But as differen

isease in an individual or family. But as different 
families segregating the same mendelian disease are identified and 
the causal genetic variants mapped, genetic heterogeneity is often 
observed: different alleles in different genes can cause the same 
disease. Some mendelian disorders (e.g., MEN2) demonstrate 
recurrent mutations in the same gene but of different molecular 
forms and locations, a phenomenon termed allelic heterogeneity. 
For other disorders (e.g., familial paraganglioma), mu

other disorders (e.g., familial paraganglioma), multiple genes 
across different chromosomes are implicated, each causing the 
same/similar disease in different individuals. This phenomenon, 
variants in different genes causing the same disease, is termed locus 
heterogeneity. It is important to bear in mind that locus hetero­
geneity is intrinsically tied to the precision of disease definition. 
For example, CAH can be caused by defects in multiple genes 
encoding steroid biosynthetic enzymes (

ple genes 
encoding steroid biosynthetic enzymes (CYP21A2, CYP11B1, 
CYP17A1, HSD3B2, POR, StAR; see Chapter 15). However, if 
the CAH phenotype is refined to include biochemical measure­
ments (mineralocorticoid, sex hormone, and electrolyte levels), 
individual subtypes emerge, each of which possesses a simpler 
genetic architecture (i.e., decreased locus heterogeneity).
When contrasted with common polygenic diseases, mendelian 
disorders exhibit relatively less locus heterogeneity. In other w

it relatively less locus heterogeneity. In other words, 
an appreciable fraction of mendelian disease cases can be largely 
explained by mutations in one or a few genes. For example, recur­
rent mutations in a single gene (the eponymous MEN1) account 
for 70% of families segregating the MEN1 clinical syndrome. Even 
in this classic mendelian case, however, the genetic architecture 
remains incompletely defined, as 30% of cases have no mutation 
in MEN1. Thus, much of the genetic architecture of 

n MEN1. Thus, much of the genetic architecture of mendelian 
diseases remains uncharted territory for genetic mapping. Modern 
sequencing technologies have facilitated a renaissance in mendelian 
disease gene mapping and will help improve our understanding of 
the genetic basis of mendelian disorders. By exome sequencing two 
individuals in a kindred with familial combined hypolipidemia 
(see Chapter 41), investigators identified two nonsense mutations 
in ANGPTL3 that segregated with low serum 

ations 
in ANGPTL3 that segregated with low serum lipoproteins when 
genotyped in other family members.14 These mutations and the 
ANGPTL3 gene were contained in the region identified by tradi­
tional linkage mapping14 and could be quickly identified because 
the sequence of all exons in that region had been determined. 
Disease Biology
Every endocrine organ ranging from the pituitary to adrenal is 
affected by well-described and less-described mendelian disorders. 
Mechanistic insight into dise

endelian disorders. 
Mechanistic insight into disease biology has been gained from 
discovering the identities of the genes that lead to disease. When 
mutations in several different genes can all cause a disease (locus 
heterogeneity), additional mechanistic insight into molecular 
pathophysiology becomes possible. This makes intuitive sense in 
the context of a molecular understanding of genes as encoding 
proteins that act in concert to accomplish cellular functions. For 
example, Noonan synd

lish cellular functions. For 
example, Noonan syndrome (characterized endocrinologically by 
variable short stature, delayed puberty, and cryptorchidism in the 
  Performing and Interpreting Genetic 
Studies
For any heritable disease, the success of genetic mapping efforts, the 
strategy employed, and the clinical utility of any resulting genotype-
phenotype map depend on its genetic architecture: (1) the number of 
genetic variants/genes, (2) their frequency in the population, and (3) 
their r

heir frequency in the population, and (3) 
their respective contributions to risk (i.e., penetrance). On one end 
of the spectrum lie mendelian diseases, such as multiple endocrine 
neoplasia type 1, characterized by (1) few variants often in a single 
gene, (2) extremely rare frequency in the population (<1:1000), and 
(3) potentially high penetrance (>50-fold risk). On the other end of 
the spectrum lie the so-called common diseases, such as type 2 
diabetes, characterized by (1) many variants

pe 2 
diabetes, characterized by (1) many variants in many genes (poly­
genic), (2) high frequency in the population (>1:20), and (3) often low 
penetrance (<1.5-fold risk) (see Fig. 3-2).
Because of their simple genetic architectures, mendelian endocrine dis­
orders were ideally suited for genetic mapping using the techniques 
of familial linkage mapping developed in the 1980s.3 Because they 
are rare and have strong effects on phenotype, mendelian variants 
were typically identified in familie

ian variants 
were typically identified in families. As a result, the genotype-pheno­
type correlations for these variants could not be generalized to the 
population at large. For example, penetrance of mendelian variants 
could be accurately estimated only if these variants were ascertained 
in the general population rather than in selected families with a 
specific genetic background. Large-scale sequencing studies in the 
general population, which can identify all variants, rare and common, 

which can identify all variants, rare and common, 
are now enabling such estimates. Such studies have found that, 
when ascertained in the general population, the so-called mendelian 
variants are less penetrant than was estimated from family-based 
studies.146
By contrast, the variants for common polygenic disorders have been 
identified through genetic association studies in the general popula­
tion. Genetic association studies do not require the identification of 
rare families segregating di

he identification of 
rare families segregating disease, because they simply compare the 
frequency of a given genetic variant in disease cases and controls. 
Thus, they can be applied to identify genetic factors underlying dis­
eases occurring in a population of unrelated individuals (i.e., common 
diseases). Unlike clinical risk factors/biomarkers association studies, 
correlation in genetic association studies implies causation, because 
genotype always precedes phenotype. Through the 1980s, 

ype always precedes phenotype. Through the 1980s, genetic 
association studies were performed using single-nucleotide poly­
morphisms at candidate genes selected by educated guessing. Such 
studies yielded several common disease associations but were poorly 
reproducible and confounded by false-positive results arising from 
population stratification. The development of modern sequencing 
and genotyping technologies along with the cataloging of more than 
10 million common variants (the Internat

ore than 
10 million common variants (the International HapMap project147) 
enabled genome-wide association studies (GWASs), a systematic 
approach to simultaneously test all genes for associations that could 
account for population-based confounding.5 GWASs have yielded a 
large number of reproducible genetic associations for diverse com­
mon/polygenic diseases/traits,148 yielding insight into disease biology 
and genetic architecture.
When interpreting a result from any genetic study, it is im

rpreting a result from any genetic study, it is important to bear 
in mind that the actual variant (usually a single-nucleotide poly­
morphism) tested in the study marks a haplotype (a combination of 
genetic variants inherited together) that can span millions of bases. 
The causal variant, in the sense that it is molecularly responsible for 
alteration in gene function leading to cellular and disease phenotype, 
may lie anywhere on this haplotype. As with the chromosomal link­
age studies of th

e. As with the chromosomal link­
age studies of the past, identifying the causal variants/genes on a 
haplotype necessitates a combination of further association analysis 
(fine-mapping)149,150 and functional experimentation in model 
systems.151
TABLE 3.4
49
setting of dysmorphic features and variable cardiac defects [see 
Chapter 24]) is typically caused by activating the RAS-MAPK 
(mitogen-activated protein kinase) signaling pathway. Domi­
nant gain-of-fun

 kinase) signaling pathway. Domi­
nant gain-of-function mutations in multiple pathway members 
(PTPN11, SOS1, KRAS, RAF1, BRAF, NRAS) have all been 
shown to cause Noonan syndrome. For other disorders, a more 
complex picture emerges in which multiple molecular pathways 
are implicated. For example, Kallmann syndrome (see Chapter 
26), which arises from failure of migration of GnRH neurons 
during fetal development, demonstrates X-linked (KAL1), auto­
somal dominant (FGFR1), and autosomal recess

auto­
somal dominant (FGFR1), and autosomal recessive (PROK2) 
inheritance. The gene product of KAL1, a secreted protein called 
anosmin, is thought to interact with the fibroblast growth factor 
(FGF) receptor, whereas the gene product of PROK2, the secreted 
protein prokineticin 2, interacts with a different receptor. Both 
signaling pathways are required for GnRH neuronal migration.
At the level of a single gene/locus, genotype-phenotype correla­
tions mapping allelic heterogeneity to phenoty

la­
tions mapping allelic heterogeneity to phenotypic heterogeneity 
can provide detailed insight into how alterations in gene function 
affect disease severity. CAH caused by CYP21A2 deficiency is a 
classic case. Several genetic variants including frameshifting dele­
tions, splice site alterations, and missense mutations have been 
identified (see Chapter 24) in CYP21A2. This spectrum of alleles 
has been mapped to a biochemical spectrum of 21-hydroxylase 
enzyme activity, which in turn maps t

hydroxylase 
enzyme activity, which in turn maps to a spectrum of clinical fea­
tures along the axes of mineralocorticoid sufficiency, androgen 
excess, and ACTH elevation (see Chapter 24). In this disorder, it 
is possible to make predictions about clinical phenotype (catego­
rized as salt wasting, simple virilizing, and nonclassic) based on 
genotype. Notably, the positive predictive value (PPV, the strength 
of the genotype-phenotype correlation) is strongest for variants 
that severely affec

on) is strongest for variants 
that severely affect CYP21A2 gene function and are predicted to 
cause severe disease (salt wasting, PPV ∼100%). Predictive power 
is weaker for genetic variants that are expected to have more mod­
erate effects on gene function and therefore result in milder dis­
ease (nonclassic, PPV ∼60%). Some of this complexity is due to 
the potentially compensatory 21-hydroxylase enzyme activity of 
CYP2C19 and CYP3A4, a form of gene-gene interaction. Geno­
type-phenotype co

 of gene-gene interaction. Geno­
type-phenotype correlations must be established empirically and 
are not possible in many cases. Even when mutations of varying 
molecular severity are identified, they may not predictably affect 
phenotype. For example, many individuals with genetic variants 
in the SRD5A2 gene (encoding 5α-reductase type 2) of differing 
molecular severity and location have been identified (see Chap­
ter 24), but no correlation between the genotype and the clinical 
degree of v

between the genotype and the clinical 
degree of virilization is apparent.
A second example of synergy between NGS and classical 
genetic study design revealed gain-of-function mutations in 
the KCNJ5 and CACNA1D genes as causes of hyperaldosteron­
ism.15,16 By sequencing exomes in a series of aldosterone-produc­
ing adenomas from individuals with primary hyperaldosteronism 
(Conn syndrome), investigators identified missense mutations in 
the KCNJ5 gene in about one-third of tumors from unrelate

J5 gene in about one-third of tumors from unrelated 
individuals. They also identified a separate KCNJ5 missense 
mutation in a mendelian family with hypertension, primary 
hyperaldosteronism, and massive adrenal hyperplasia (familial 
hyperaldosteronism type 3; see Chapter 16). Follow-up bio­
chemical and electrophysiologic studies showed that this series 
of somatic and inherited missense mutations eliminated ion 
selectivity in the KCNJ5 gene product, a potassium channel. 
The increased sodiu

product, a potassium channel. 
The increased sodium conductance through these mutant chan­
nels caused membrane depolarization of adrenal cortical cells in 
the zona glomerulosa, stimulating aldosterone release and cell 
proliferation.
As demonstrated previously, modern genome sequencing tech­
nologies facilitates the identification of pathogenic mutations. 
Translation of mutations into a disease mechanism, however, 
requires relevant experimental model systems and close cor­
relation with huma

al model systems and close cor­
relation with human phenotype. PHP1a (see Chapter 29) with 
gonadotropin-independent sexual precocity provides a classic 
example of the iterative relationship between mutations, human 
phenotypes, and laboratory experiments. Some individuals with 
PHP1a harbor the Ala366Ser missense mutation in a stimulatory 
G protein (Gs). This mutation destabilizes the protein, causing 
loss of function in most body tissues and thus hormone resis­
tance. But these mutation car

 thus hormone resis­
tance. But these mutation carriers also exhibited paradoxical tes­
totoxicosis consistent with gain of function in Gs. The paradox 
of the same mutation causing both loss and gain of function was 
Very rare
0.001
Penetrance
Mendelian
disease
Hard to identify
genetically
variants with
intermediate
penetrance
Highly unusual
for common
diseases
Most variants
identified by
GWAS
Uncommon
0.1
Common
Allele
frequency 
Rare
Low
Modest
Intermediate
High
0.01
• Fig. 3.2 

Rare
Low
Modest
High
0.01
• Fig. 3.2  Genetic architecture of common and mendelian diseases. On one end of the spectrum are men­
delian diseases caused by few variants in few genes, each with a large individual effect on disease risk. On 
the other end of the spectrum are common diseases and traits caused by the combined effects of many 
variants, observed frequently in the population, each with a modest individual effect. GWAS, genome-wide 
association. (Redrawn from McCarthy MI, A

me-wide 
association. (Redrawn from McCarthy MI, Abecasis GR, Cardon LR, et  al. Genome-wide association 
studies for complex traits: consensus, uncertainty and challenges. Nat Rev Genet. 2008;9[5]:356-369.)
50
resolved with a series of experiments showing that the Ala366Ser 
mutation caused a temperature-sensitive effect physiologically 
relevant to the normal temperature for Leydig cells. In most body 
tissues maintained at 37° C, the mutant Gs protein 

issues maintained at 37° C, the mutant Gs protein was destabi­
lized, whereas in Leydig cells (within the testes, maintained at a 
3–5° C lower temperature), the mutant Gs demonstrated increased 
activity. Elucidating this mechanism required an appreciation of 
the discordant phenotype (testotoxicosis) and biochemical char­
acterization in the relevant physiologic system (Leydig cells at a 
lower temperature). 
Clinical Translation
Target discovery, risk prediction, and the tailoring of pharmaco

ry, risk prediction, and the tailoring of pharmaco­
therapy based on genotype are potential clinical applications of 
genotype-phenotype correlation. The existence of loss-of-function 
and gain-of-function variants in an allelic series and their concor­
dance with opposing phenotypes can provide a rationale for thera­
peutically modulating gene function.17 For example, inactivating 
mutations in the KISS1R receptor cause hypogonadotropic hypo­
gonadism, whereas an Arg386Pro missense variant in K

nadism, whereas an Arg386Pro missense variant in KISS1R is 
associated with central precocious puberty. Kisspeptin, the agonist 
ligand of the KISS1R receptor, has shown promise as a fertility 
treatment.18
Genotype-phenotype correlation can be used to predict risk 
of disease in asymptomatic carriers. Prior to the identification 
and cloning of the RET proto-oncogene, MEN2 kindreds were 
monitored for evidence of medullary thyroid cancer (MTC) 
by calcitonin stimulation tests. Once mutations in

by calcitonin stimulation tests. Once mutations in RET were 
established as causing MEN2A/B and familial MTC, it became 
apparent that specific mutations could be mapped to the dif­
ferent syndromes. The RET gene product encodes a cell surface 
receptor tyrosine kinase. Mutations in the extracellular domain 
predispose to MEN2A (characterized by MTC, pheochromo­
cytomas, and hyperparathyroidism), whereas mutations in the 
intracellular tyrosine kinase domain predispose to MEN2B 
(characterized b

inase domain predispose to MEN2B 
(characterized by MTC, pheochromocytomas, and muco­
sal neuromas) (see Chapter 42). The clinical aggressiveness of 
MTC, the sine qua non of all three syndromes, is greatest in 
MEN2B, then less in MEN2A, with familial MTC demon­
strating the least propensity to grow and metastasize. A well-
defined genotype-phenotype correlation between specific RET 
mutations and clinical aggressiveness of MTC now dictates 
the timing of lifesaving prophylactic thyroidectomy i

 timing of lifesaving prophylactic thyroidectomy in carriers 
of RET mutations.19 The key to establishing clinically robust 
risk prediction based on genotype-phenotype correlations is 
a well-differentiated allelic series derived from multiple indi­
viduals/families. The consensus genotype-phenotype correlation 
for prophylactic thyroidectomy in RET mutation carriers was 
derived from analysis of more than 200 individuals from more 
than 100 families (see Chapter 42).
A genetic diagnosis in sev

ilies (see Chapter 42).
A genetic diagnosis in several mendelian disorders can also 
directly inform pharmacotherapies. A classic example includes 
obesity caused by leptin deficiency (see Chapter 40), which can 
be treated by exogenous leptin injections. Other examples include 
HNF1A MODY (maturity-onset diabetes of the young) and neo­
natal diabetes (discussed in detail later) caused by genes whose 
properties predict excellent response to sulfonylureas. In the case 
of congenital hyperinsulin

onylureas. In the case 
of congenital hyperinsulinism, autosomal recessive mutations in 
ABCC8 or KCNJ11 correlate with diffuse disease on spectroscopic 
imaging and lack of responsiveness to medical therapy (diazoxide); 
such individuals require near-total pancreatectomy for control of 
hypoglycemia.20 
Type 2 Diabetes
Type 2 diabetes (T2D) is a multifactorial, polygenic disor­
der for which manifestation depends on multiple interacting 
genetic and environmental risk facto

 interacting 
genetic and environmental risk factors. Heritability estimates 
show strong evidence of familial clustering, ranging from 
40%7 to 80%.8 Approximately 5% of diabetes cases that may 
be classified as nonautoimmune arise from a single-gene dis­
order, follow mendelian patterns of inheritance, and cluster 
into clinically defined syndromes. These mendelian diabetes 
syndromes include neonatal diabetes, MODY, and congenital 
lipodystrophies.21 To date, familial linkage studies have suc

hies.21 To date, familial linkage studies have suc­
cessfully implicated approximately 30 genes as monogenic 
causes of diabetes.22
The genetic factors underlying the majority of T2D cases (95% 
of cases) fit a polygenic model; genetic variants in multiple genes 
independently contribute to disease risk, each with a modest 
effect. The partial elucidation of these genetic risk factors required 
the advent of genetic association studies/GWASs and the assem­
bly of cohorts of thousands of cases an

the assem­
bly of cohorts of thousands of cases and control subjects.23 As of 
2014, about 70 loci have been identified from aggregated analysis 
on about 150,000 case controls. Taken together, these loci account 
for about 6% of heritability for T2D.23 Of these loci, an SNP at 
TCF7L2 (with the risk-increasing allele present at a frequency of 
∼30%) has the largest overall effect on risk, conferring a 1.4-fold 
increase in risk per allele.23
By contrast, type 1 diabetes shows a somewhat differe

contrast, type 1 diabetes shows a somewhat different 
genetic architecture, with common loci of large effect (a variant 
at the HLA locus found in 61% of the population confers a five­
fold increase in risk,24 and a common variant at the insulin gene 
confers a threefold increase in risk). This finding was consistent 
with prior studies from the 1980s estimating that 50% of the 
heritability of type 1 diabetes was explained by common hap­
lotypes at the HLA locus.25,26 Notably, the genes implica

 at the HLA locus.25,26 Notably, the genes implicated in 
monogenic causes of diabetes also contribute to polygenic forms 
but through distinct genetic variants. Genes associated with men­
delian diabetes syndromes, such as KCNJ11 (neonatal diabetes), 
HNF1A (MODY2), and PPARG (familial partial lipodystrophy 
3),27 were found to harbor common variants that conferred risk 
for common T2D.21,28 Conversely, genes first found associated 
with diabetes through GWASs have subsequently been identified 

s through GWASs have subsequently been identified 
as having rare, highly penetrant alleles. For example, noncoding 
common variants pointed to the MTNR1B gene (encoding the 
melatonin receptor) as a T2D-associated locus (1.15-fold risk).29 
Subsequently, large-scale resequencing studies identified mul­
tiple, rare coding variants of the same gene (present in <1:1000 
individuals) that conferred a greater than fivefold increased risk 
of T2D.30
Mapping studies performed across various population

apping studies performed across various populations reveal 
both similarities and differences in the genetic risk factors contrib­
uting to diabetes among different ancestral groups. T2D GWASs 
performed in multiple populations/ancestries (European, South 
Asian, East Asian, Latino, African American)31 reveal that many 
common variants are shared across populations with equivalent 
effects on disease risk, regardless of ancestry. This pattern is consis­
tent with the origin of most common varian

consis­
tent with the origin of most common variants in an ancestral Afri­
can population (see Table 3.3), but remarkable ancestry-specific 
effects have also been identified. A T2D GWAS performed in indi­
viduals of Latino and Mexican ancestry identified a common SNP 
at a locus containing the genes SLC16A11/13 that confers a 1.25-
fold increased risk of diabetes.32 The same locus was identified by 
51
another T2D GWAS performed in Japanese individuals.33 Be

r T2D GWAS performed in Japanese individuals.33 Because 
the associated SNPs were rare in Europeans, the locus had not been 
detected in GWASs in European-ancestry populations. Similarly, a 
common variant in TBC1D4 in individuals from Greenland (pres­
ent in 17% of Greenland’s population) strongly increases risk of 
T2D (10-fold increased risk).34 This variant, which causes a prema­
ture truncation and is associated with elevated muscle insulin resis­
tance, is extremely rare in continental Eur

resis­
tance, is extremely rare in continental Europe and likely became 
common in Greenland because it was present in the founding 
ancestors of Greenland’s current population.
In summary, genetic mapping studies over the past three 
decades have revealed a genetic architecture for T2D with wide­
spread locus and allelic heterogeneity. With regard to effect size 
and allele frequency, T2D genetic architecture so far consists of 
some very rare variants of large effect, some common variants with

ariants of large effect, some common variants with 
small to moderate effects (1.2- to 1.5-fold increased risk), and a 
larger number of common variants with even more modest effects 
on disease risk, with rare and common genetic variants spread out 
across multiple loci in the genome. This genetic architecture has 
proved to be typical for other common diseases24 (see Fig. 3.2) 
and reflects both the underlying genetic architecture of the disease 
and the ability of large GWAS to more readily d

e 
and the ability of large GWAS to more readily detect common 
variants of modest effect. 
The past three decades of genetic discoveries in T2D have nucle­
ated a molecular understanding of disease mechanisms, high­
lighted differences between glycemia and T2D, and implicated 
previously unknown physiology in disease pathogenesis.
Supporting the current physiologic conception of T2D as a dis­
order of decreased insulin production, as well as decreased insulin 
sensitivity, genet

, as well as decreased insulin 
sensitivity, genetic mapping has pointed to a molecular basis for 
both axes. Prediabetic individuals harboring T2D-associated vari­
ants in beta-cell genes (SLC30A8, HNF1A) and cell survival genes 
(CDKAL1) manifest with decreased insulin secretion (homeo­
static model assessment B; see Chapter 34).28 However, predia­
betic individuals harboring T2D-associated variants in adipocyte 
genes (PPARG, KLF14) tend toward increases in homeostatic 
model assessment insul

d increases in homeostatic 
model assessment insulin resistance (see Chapter 34).28 About 
30% of T2D-associated SNPs point to insulin secretion/beta-cell 
function, and 15% point to insulin resistance.22 Interestingly, the 
SNPs associated with insulin secretion in prediabetic individu­
als predict incident T2D, but the SNPs associated with insulin 
resistance do not.35 These findings from genetic epidemiology are 
consistent with beta-cell failure being a final common pathway 
for manifestatio

ure being a final common pathway 
for manifestation of hyperglycemia and diagnosis of T2D. Impor­
tantly, more than half of the associated SNPs and the genes they 
point to cannot be connected with either insulin secretion or sen­
sitivity. Their pathogenic mechanisms remain to be elucidated by 
physiologic and functional investigation.
Even without a full understanding of their molecular/cellular 
mechanism of causation, the large number of T2D-associated loci 
(∼70 as of 2014) have been deploy

associated loci 
(∼70 as of 2014) have been deployed to refine disease classifica­
tion. By examining quantitative glycemic traits (insulin produc­
tion, sensitivity, processing, and fasting glucose) in nondiabetic 
individuals genotyped for 37 T2D-associated common genetic 
variants, investigators were able cluster genes with glycemic traits 
to define unique diabetes subtypes.36 For example, individuals har­
boring variants in MNTR1B and GCK manifested a combination 
of fasting hyperglycemia a

nifested a combination 
of fasting hyperglycemia and decreased insulin secretion, whereas 
those harboring variants in SLC30A8, CDKN2A/B, TCF7L2, and 
other genes manifested primarily with decreased insulin secretion. 
Notably, many genes did not cluster with predefined glycemic 
traits, again suggesting that the current physiologic description of 
T2D remains incomplete.
Genetic mapping has also corroborated the epidemiologi­
cally identified intersection between T2D and obesity. An SNP 
in the

tersection between T2D and obesity. An SNP 
in the second intron of the FTO gene was identified in parallel 
in GWASs for T2D 37 and obesity.38-40 The association signal for 
T2D entirely disappeared with correction for body mass index 
(BMI), indicating that this SNP increased T2D risk by increasing 
BMI. Interestingly, this locus illustrates some of the difficulties in 
proceeding from GWAS signal to function. Although this SNP 
was initially thought to exert its effect on BMI by affecting FTO

hought to exert its effect on BMI by affecting FTO 
gene function, detailed mechanistic studies have revealed that it 
may function by altering expression levels of IRX3, a gene over 
a million bases away.41 Although initial studies in mice showed 
that increasing Fto gene dosage increased food intake leading to 
increased fat mass,42 no connection has been found between the 
disease-associated SNPs and Fto expression level or function.43
Even more recently, splice acceptor variants that generat

re recently, splice acceptor variants that generate pre­
mature stop codons and loss of function in ADCY3 (a gene highly 
expressed in visceral adipose tissue) has been identified as causal 
for increased BMI and T2D risk. Concordant functional studies 
in mice with loss of function causing obesity, hyperphagia, and 
insulin resistance suggest that ADCY3 may be a new therapeutic 
target.44
Whereas T2D is diagnosed on the basis of hyperglycemia, 
genetic mapping has revealed that the genes that d

genetic mapping has revealed that the genes that determine fast­
ing glucose are partly distinct from those that are associated with 
T2D. Comparison of GWASs performed for blood glucose levels 
in nondiabetics versus T2D case-control studies revealed that gly­
cemia and T2D have distinct genetic associations.45 Some genes 
harbor variants that increase blood glucose levels and T2D risk, 
whereas others alter blood glucose levels but do not confer T2D 
risk. Thus, the two phenotypes have both co

r T2D 
risk. Thus, the two phenotypes have both common and distinct 
biology. Additionally, it is important to bear in mind that the 
genetic basis for surrogate measures of glycemia do not always 
point to genes specifically altering glycemic physiology. A particu­
larly salient example is the association of hexokinase 1 (HK1) with 
hemoglobin A1c levels but not with fasting or dynamic glycemia.46 
It is thought that the genetic variant in HK1 alters hemoglobin 
A1c levels as a result of the al

lters hemoglobin 
A1c levels as a result of the alteration of the red blood cell life span 
and anemia.46 
The principle of genetics pointing to important therapeutic targets 
in T2D is well validated. Both rare and common genetic variants 
link PPARG, the drug target of thiazolidines, to syndromic and 
common T2D.27 Similarly, rare variants in the sulfonylurea recep­
tor (encoded by ABCC8) cause neonatal diabetes.47 Although 
these oral hypoglycemics were identified in a pr

these oral hypoglycemics were identified in a pregenetic era, they 
point to an optimistic future of genetically guided drug discov­
ery, one that will require detailed mechanistic understanding of 
the genes mapped by studies to date. A particularly attractive 
target nominated by genetics is SLC30A8, a gene that encodes 
a zinc transporter expressed almost exclusively in the endocrine 
pancreas (ZnT8). The common R325W missense variant in the 
protein encoded by the SLC30A8 gene (present in 

protein encoded by the SLC30A8 gene (present in ∼1:3 individu­
als in most continental populations) was found to associate with 
protection from T2D (1.18-fold decreased risk).48 Rare, protein-
truncating variants in SLC30A8 (present in ∼2:1000 individu­
als) have also been associated with protection from T2D with a 
larger effect size (2.6-fold decreased risk).49 The finding of human 
52
heterozygous knockouts for SLC30A8 who are protected from 
T2D an

ockouts for SLC30A8 who are protected from 
T2D and have no other deleterious phenotype offers a tantalizing 
therapeutic hypothesis that a chemical or antibody inhibitor of 
SLC30A8 could treat diabetes and minimize side effects.
In the arena of risk prediction, genetics has not yet made a 
major impact on T2D because existing clinical risk factors already 
predict disease well. Common genetic variants, by virtue of their 
relatively high frequency, can explain a large part of heritability 
for

ncy, can explain a large part of heritability 
for a trait/disease but have small effects on the individual. For 
example, the common P12A variant in PPARG is associated with 
1.25-fold risk of T2D.50 Based on the high population frequency 
of the risk variant (85% P), if one were to theoretically substitute 
every P to A in the population, 20% of diabetes would be elimi­
nated. Despite this incredible population-attributable risk, any 
given individual carrying the P variant only has a 25% incr

ividual carrying the P variant only has a 25% increased 
risk of diabetes when compared with someone carrying the A 
variant. Given the high population frequency of common vari­
ants, many disease susceptibility variants are found in the same 
individual, each of which confers modest increases in risk. The 
progressive accessibility of genome sequencing makes it feasible 
to ascertain all known risk-conferring variants in an individual at 
once and combine these for potentially more clinically m

nd combine these for potentially more clinically meaningful 
risk prediction. Investigators have attempted to combine common 
variants into a genetic risk score with modest success. For T2D, 
a risk score combining 18 common variants (including PPARG 
P12A) demonstrated 2.6-fold elevated risk in the high-risk versus 
low-risk group.51 By contrast, simply reporting a family history of 
diabetes increases risk by threefold to sevenfold.52
Tailoring pharmacotherapy based on genotype has been suc­
c

 pharmacotherapy based on genotype has been suc­
cessful in monogenic diabetes; the genome-sequencing era prom­
ises to bring this benefit to a wider group of individuals. The 
classic example of genotype guiding pharmacotherapy is that of 
individuals with MODY2 caused by autosomal dominant muta­
tions in the GCK gene. Such individuals meet diagnostic criteria 
for diabetes but are able to regulate glycemia at a higher set-point, 
thus avoiding all secondary complications (see Chapter 35). Thus

all secondary complications (see Chapter 35). Thus, 
a genetic diagnosis of GCK-related diabetes can allow such indi­
viduals to avoid pharmacotherapy. Individuals with permanent 
neonatal diabetes caused by ABCC8 or KCNJ11 mutations can 
be safely treated with high-dose sulfonylureas in place of insu­
lin.53,54 It is likely that individuals with functional mutations in 
ABCC8 or KCNJ11 will be found who do not present with the 
classic neonatal syndrome and are classified as common T2D but 
who

syndrome and are classified as common T2D but 
who may still respond preferentially to sulfonylureas. Proving this 
prediction will require identifying such individuals and perform­
ing prospective clinical trials. Prospective genotype-based inter­
vention trials have been performed in individuals with MODY3 
caused by HNF1A mutations and have shown the superiority of 
sulfonylureas over metformin.55 Interestingly, exome sequencing 
of Latin American T2D case-control groups revealed the HNF1A 
E

ican T2D case-control groups revealed the HNF1A 
E508K variant, previously annotated as MODY3, as conferring a 
fivefold risk of T2D in approximately 1:1000 individuals. These 
data demonstrate that the E508K is not fully penetrant; neverthe­
less, the individuals who carry it may still benefit preferentially 
from sulfonylurea treatment, as do their counterparts with clini­
cally defined MODY3.
Large-scale sequencing of the known monogenic diabetes 
genes has uncovered many novel protein-coding

tes 
genes has uncovered many novel protein-coding variants; these 
present both an opportunity for pharmacogenetics and a chal­
lenge. As a case in point, sequencing of PPARG in approximately 
20,000 T2D case controls identified 49 rare protein-coding vari­
ants. Three of the 49 had ever been identified before, were known 
to cause loss of function, and had been associated with mendelian 
lipodystrophy56; 46 variants were novel and of unknown func­
tion. Functional testing of each of the varian

nc­
tion. Functional testing of each of the variants with respect to 
their ability to rescue adipocyte differentiation revealed nine of 
the novel variants causing loss of PPARG function, with the rest 
being indistinguishable from wild type.56 The variants causing loss 
of function conferred a sevenfold increased risk of T2D, whereas 
the wild-type–like variants conferred no additional risk of T2D. 
To address the challenge of interpreting novel protein-coding vari­
ants at the scale they are 

l protein-coding vari­
ants at the scale they are being discovered (1:500 individuals in the 
general populations carry a rare protein-coding variant in PPARG) 
and which ones might be amenable to pharmacotherapy, investi­
gators applied synthetic biology and high throughput sequencing 
to synthesize and functionally test all possible (∼10,000) protein-
coding variants in PPARG.57 Using the resulting “lookup table” 
for PPARG function, clinical investigators identified two PPARG 
mutations (R308

nvestigators identified two PPARG 
mutations (R308P and A261E) in patients affected by diabetes 
and partial lipodystrophy and demonstrated in vivo responsive­
ness to thiazolidinediones.58 These studies demonstrate a proof of 
concept for the use of genetic information and high throughput 
functional characterization to guide pharmacotherapy in other 
monogenic disease genes. 
Short Stature
Adult height is a polygenic quantitative trait with a heritabil­
ity of 80%.6 Many m

tative trait with a heritabil­
ity of 80%.6 Many mendelian syndromes manifest (see Chapter 
25) large phenotypic variations in stature, and more than 150 
genes are associated with monogenic short stature or overgrowth. 
The effect sizes for these rare, highly penetrant alleles are typi­
cally large, up to 300 mm (with 1 standard deviation [SD] for 
height equal to 55–60 mm). The genetic factors underlying most 
of human height variation are polygenic. GWASs aggregating 
more than 690,000 Europe

genic. GWASs aggregating 
more than 690,000 European samples have identified more than 
3290 independent loci associated with height59 through common 
variant association. Common variants account for about 60% of 
heritability.59,60 The effect sizes for these common alleles are in the 
range of <1 to 15 mm (<0.02–0.20 SD). Taken together, these 
genetic mapping studies show that the genetics of height consists 
of the additive effects of common alleles with modest individual 
effects except in v

lleles with modest individual 
effects except in very short individuals (>2 SD below the mean).61 
In extremely short individuals, it is likely that rare alleles of large 
effect play a larger role.61 Many of the genes (more than expected 
by chance) in height-associated loci identified by GWASs contain 
rare monogenic alleles. These height-associated genes frequently 
also harbor common alleles that contribute to polygenic stature 
variations.59 A classic example is GH1 (isolated GH deficiency 

 A classic example is GH1 (isolated GH deficiency 1a; 
see Chapter 25). This substantial overlap suggests that the genes in 
associated loci that have yet to demonstrate rare, large-effect alleles 
may be prime candidates for resequencing studies to discover new 
monogenic causes of alterations in stature.62
Genetic causes of short stature include Turner syndrome, muta­
tions of the GH/insulin-like growth factor (IGF) pathway, and 
SHOX mutations. Previous GWASs have identified 700 common 
varia

. Previous GWASs have identified 700 common 
variants that explain approximately 20% of height variation in the 
population, with an additional 1.7% caused by rare coding vari­
ants. After excluding environmental factors such as nutrition, the 
cause of 60% to 80% of height variation remains unknown, but 
much of it likely is attributable to common variants that have not 
yet been identified as associated with height.62 Short stature, but 
not tall stature, has been associated with deletions in 

ll stature, has been associated with deletions in the genome. 
53
By systematically comparing CNVs across the genomes of more 
than 4000 individuals with developmental delay and congenital 
abnormalities with 7000 population-based control subjects, inves­
tigators observed that individuals with short stature harbored an 
excess number of low-frequency (found in <5% of the popula­
tion) deletions.63 Individuals in the cohort clinically diagnosed 
with short st

 in the cohort clinically diagnosed 
with short stature demonstrated an average loss of 900,000 bp 
from their genomes.63
In summary, the genetic architecture of height is consistent 
with classical theory and animal models64 for a polygenic, quan­
titative trait: thousands of genetic variants in hundreds of genes 
contribute to the genetic variability in height. Most of the genetic 
variation in height (97%) occurs from the additive effects of com­
mon variants, each contributing a modest effec

om­
mon variants, each contributing a modest effect. Rare variants of 
large effect often causing mendelian syndromes cluster at the short 
extreme of height variation. 
Genetic mapping of genes influencing both normal variation and 
the extremes of stature have revealed diverse molecular pathways 
operating in multiple tissue types that work through both endo­
crine and cell-autonomous mechanisms.
Genetic mapping in individuals with low GH levels and short 
stature have delineat

ith low GH levels and short 
stature have delineated multiple components of the GH/IGF1 
axis as a key endocrine pathway in human height regulation (see 
Chapter 25). These components include hormones, their recep­
tors, binding proteins, and intracellular signaling proteins, such 
as GH1, GH receptor (GHR), GH-releasing hormone receptor 
(GHRHR), STAT5B, IGF acid-labile subunit (IGFALS), IGF1, 
and IGF1 receptor (IGF1R). IGF bioavailability varies according 
to its association with IGF binding 

es according 
to its association with IGF binding proteins including IGFBP3. 
Recent WES in two families with short stature and microcephaly 
revealed the critical role of PAPP-A regulation in IGF signaling. 
PAPP-A is a metalloproteinase that cleaves IGF binding pro­
teins to release the active hormone near IGF receptors. Inher­
ited homozygous loss-of-function mutations in PAPP-A resulted 
in progressive growth failure in several family members despite 
elevated total circulating IGF.65 A subs

 despite 
elevated total circulating IGF.65 A subsequent meta-analysis of 
exome array association data for height identified low-frequency 
coding variants in STC2 (Stanniocalcin-2) with profound effects 
on height. STC2 covalently binds and inhibits PAPP-A.66 Rare 
missense mutations in STC2 decrease PAPP-A binding, resulting 
in increased PAPP-A activity, increased IGF availability, and an 
associated average 2 cm height increase. The same study identi­
fied rare AR, CRISPLD2, and IHH gene mu

dy identi­
fied rare AR, CRISPLD2, and IHH gene mutations, all of which 
decreased average height by approximately 2 cm. Remarkably, 
these rare IGF pathway gene variants have effect sizes that are 
often greater than 10 times the magnitude of common height 
variants.67
Genetics has also pointed to the importance of the pituitary 
gland as a key endocrine organ regulating height, as mutations 
in genes that encode transcriptional regulators of pituitary devel­
opment, such as HESX1, PITX1, PITX2

tuitary devel­
opment, such as HESX1, PITX1, PITX2, PROP1, POU1F1, and 
LHX3, also lead to short stature when mutated in humans.68 At 
the paracrine and cell-autonomous levels, genetic mapping has 
highlighted the importance of cellular proliferation, extracellular 
matrix deposition, and cartilage/bone development. These cel­
lular processes were first implicated by monogenic short stature 
syndromes caused by mutations in genes such as FGFR3 (achon­
droplasia) and SHOX (Langer mesomelic dyspla

chon­
droplasia) and SHOX (Langer mesomelic dysplasia) or tall stature 
syndromes, such as FBN1 (Marfan syndrome) and EZH2 (Weaver 
syndrome).
GWASs for height as a polygenic trait, combined with sys­
tematic clustering of genes in associated loci, have refined and 
expanded insights gained from mendelian genetics. Common 
variants associated with multiple members of the GH/IGF1 axis 
demonstrate the importance of this pathway in height regulation 
within the normal range.69 The association of T

n 
within the normal range.69 The association of TGF-β itself and of 
its binding proteins, LTBP1 through LTBP3, complements the 
finding of FBN1 mutations in Marfan syndrome in highlighting 
TGF-β signaling.70 Similarly, the FGF signaling pathways are also 
highlighted, as common variants near FGF4 are associated with 
height, complementing the finding of monogenic mutations in 
FGFR3 causing achondroplasia. Clustering of GWAS-associated 
genes has also implicated hedgehog signaling (GLI2, LAMA

has also implicated hedgehog signaling (GLI2, LAMA5), 
Wnt signaling (CTNNB1, FBXW11, WNT4, WNT5A), and 
mammalian target of rapamycin (mTOR) signaling (SMAD3, 
MTOR) cascades in genetic control of height. Many associated 
loci do not cluster with the pathways above or other known tis­
sue/cellular processes such as bone/cartilage formation, implicat­
ing previously unknown biology in height regulation. A notable 
example is the microRNA cluster MIR17HG, which was also 
identified as a syndromic

MIR17HG, which was also 
identified as a syndromic cause of short stature.71 By cross refer­
encing GWAS-associated genes with gene-expression microarrays 
from thousands of human tissue samples, investigators have found 
that height-associated genes are predominantly expressed in the 
growth plate and to a lesser extent in cartilage, bone, and endo­
crine organs.59,72 
Gene mapping and functional characterization in monogenic 
stature disorders have motivated therapeutic ad

c 
stature disorders have motivated therapeutic advances for dis­
eases of overgrowth and short stature. A classic example is that 
of Marfan syndrome, in which excess TGF-β signaling resulting 
from the effects of disruptive mutations in FBN1 has led to the 
development of TGF-β antagonist therapies.73 Human trials of 
the angiotensin receptor blocker losartan, which exhibits TGF-β 
antagonist properties in in vivo preclinical models, were begun in 
cohorts of Marfan syndrome patients.74 Result

 in 
cohorts of Marfan syndrome patients.74 Results 3 years after treat­
ment showed a decrease in aortic root diameter but not more so 
than conventional therapy with β-blockers.75 The identification 
of the activating G380N mutation in FGFR3 that causes 95% of 
achondroplasia has motivated the development of inhibitors of 
the tyrosine kinase activity of FGFR3 through small molecules,76 
as well as through analogues of C-type natriuretic peptide.77
Due to the magnitude of its effect on height,

e.77
Due to the magnitude of its effect on height, STC2 repre­
sents an intriguing novel therapeutic target for short stature 
and IGF-associated disease. Transgenic STC2 overexpression in 
mice causes severe postnatal growth retardation that is abolished 
when STC2 is replaced with a mutant protein lacking the ability 
to bind PAPP-A. STC2 overexpression may treat IGF excess in 
conditions including acromegaly and malignancies while knock­
ing down STC2 with siRNA may help promote growth to tre

own STC2 with siRNA may help promote growth to treat 
short stature.66,78 The diagnosis of short stature is an important 
application of genetic testing, particularly in pediatric popula­
tions. For example, the genotype-phenotype correlation of SHOX 
deficiency shows a clinical spectrum ranging from homozygous 
loss-of-function mutations causing the severe syndrome Langer 
mesomelic dysplasia to heterozygous defects found in patients 
with milder syndromic disease (Léri-Weill dyschondrosteosis,

r syndromic disease (Léri-Weill dyschondrosteosis, 
Ullrich-Turner syndrome) or idiopathic short stature.79 Genetic 
diagnosis qualifies these patients for GH therapy. Genetic diag­
nosis also enables directed screening for comorbid conditions. For 
example, males with the likely underdiagnosed 3-M syndrome 
54
(caused by mutations in CUL7, OBSL1, or CCDC8), clinically 
defined by severe postnatal growth retardation, characteristic 
facies, and radiograph

etardation, characteristic 
facies, and radiographic findings of skeletal abnormalities, are at 
high risk of primary hypogonadism and need to be monitored.79 
Similarly, individuals with short stature secondary to Noonan 
syndrome are also often underdiagnosed and are at higher risk 
for cardiac disease.80 Recent work added WES to targeted genetic 
testing to test whether sequencing would substantially improve 
genetic diagnosis. Of 565 patients with short stature referred for 
genetic evaluati

 with short stature referred for 
genetic evaluation, targeted genetic testing revealed the underly­
ing genetic etiology in 13.6% of patients. Of the remaining 491 
patients without a known diagnosis, 200 underwent WES, and 
a variant in a gene known to cause short stature was found in an 
additional 16.5% of patients in this population, suggesting that 
adding WES can increase diagnostic yield.62 Genetic diagnosis 
could improve care for short stature by facilitating early identifica­
tion and

stature by facilitating early identifica­
tion and intervention prior to puberty and growth plate closure, 
screening for known comorbidities associated with the diagnosis, 
and potentially predicting the efficacy of interventions such as 
GH therapy.
Tailoring pharmacotherapy based on genotype can be a use­
ful adjunct to biochemical testing in monogenic stature disorders. 
Physiologic stimulation and serum biochemistry are the gold 
standards for assessing GH sensitivity and resistance guiding

or assessing GH sensitivity and resistance guiding its 
pharmacologic use. But genetic information plays an important 
role in solidifying diagnoses suggested by biochemical testing 
and suggesting alternative pharmacologic therapy. For example, 
children with defects in the GH receptor or post-GH receptor 
signaling (STAT5B) will be candidates for treatment with recom­
binant IGF1 (mecasermin).81 Children with defects in IGFALS, a 
serum protein that stabilizes IGFs, respond poorly to both medi

 that stabilizes IGFs, respond poorly to both medi­
cations.82 GH therapy is used to treat short stature arising from 
certain genetic defects outside the GH/IGF1 axis but is costly and 
not indicated in many others. Given the variable expressivity in 
many syndromic stature disorders, clinically distinguishing among 
syndromes presenting with short stature can be challenging and 
imprecise. Genetic diagnosis can resolve ambiguity, particularly 
in children before all the features of a syndrome 

in children before all the features of a syndrome are present. For 
example, GH is contraindicated in chromosomal breakage disor­
ders. Bloom syndrome (caused by loss-of-function mutations in 
BLM, which encodes a DNA helicase) is one such disorder pre­
senting with short stature. In the absence of genetic diagnosis with 
only clinical and biochemical testing, there are case examples of 
children being treated for years with GH until their clinical pre­
sentation evolved and Bloom syndrome was d

al pre­
sentation evolved and Bloom syndrome was diagnosed.83 
Lipids and Coronary Artery Disease
Serum lipid levels are a complex polygenic trait significantly influ­
enced by environmental factors such as diet. Heritability estimates 
suggest a large role for genetic factors: approximately 40% to 60% 
for high-density lipoprotein cholesterol (HDL-C), about 40% to 
50% for low-density lipoprotein cholesterol (LDL-C), and about 
35% to 48% for triglycerides (TG).84 Monogenic

ut 
35% to 48% for triglycerides (TG).84 Monogenic disorders caus­
ing extremes of dyslipidemia have been associated with around 20 
genes. Mendelian dyslipidemia syndromes can present with single 
or combined lipoprotein abnormalities (see Chapter 41). On 
the hyperlipidemic side, these abnormalities include syndromes 
of elevated LDL (familial hypercholesterolemia, sitosterolemia), 
elevated TG (lipoprotein lipase deficiency, APOCII deficiency), 
elevated HDL (CETP deficiency), and combined LD

, 
elevated HDL (CETP deficiency), and combined LDL/TG 
elevation (familial combined hyperlipidemia, dysbetalipoprotein­
emia). Monogenic disorders manifesting with extremely low lipid 
levels have also been identified and include low LDL (familial 
hypobetalipoproteinemia, PCSK9 mutations), low HDL (famil­
ial hypoalphalipoproteinemia, lecithin cholesterol acyltransferase 
deficiency, Tangier disease), and combined low cholesterol/TG 
(abetalipoproteinemia, chylomicron retention syndrome).
The 

proteinemia, chylomicron retention syndrome).
The genetic factors underlying most serum lipid variations are 
polygenic. GWASs aggregating approximately 200,000 multi­
ethnic samples have identified more than 150 independent loci 
associated with serum lipids through common variant associa­
tion accounting for about 15% of heritability.85 The effect sizes of 
common variants on lipid levels range from less than 1 mg/dL to 
about 15 mg/dL (SNP rs964184 at APOA1 and TG).86
Among the loci identifie

64184 at APOA1 and TG).86
Among the loci identified, many alter one of the lipoproteins 
identified, a few (CETP, TRIB1, FADS1-2-3, APOA1) alter all 
lipoprotein levels, and a subset alters various combinations of 
lipoproteins.85 These overlaps are consistent with observations in 
mendelian disorders and corroborate the shared metabolism and 
lipoprotein constituents of LDL, HDL, and TG.
Many of the genes identified as monogenic causes of dyslipid­
emia also alter lipid levels in the general po

id­
emia also alter lipid levels in the general population through both 
common and rare variants. LDLR, encoding the LDL receptor, 
provides a case in point of how the allelic spectrum of genetic vari­
ation ranges from rare mendelian alleles to common small-effect 
alleles with effect sizes inversely proportional to variant frequency. 
Disruptive mutations (those that frameshift or prematurely termi­
nate the protein) in LDLR like the ones found in familial hyper­
cholesterolemia are found in 

d in familial hyper­
cholesterolemia are found in 2:1000 to 7:1000 individuals and 
increase LDL levels by 150 to 200 mg/dL.87 Missense variants 
that decrease function of LDLR in cell models are found in about 
1:100 individuals and increase LDL levels by about 100 mg/dL.88 
A common intronic SNP in LDLR found in 1:10 individuals 
decreases LDL levels by 7 mg/dL.86
In summary, the genetic architecture of serum lipids consists of 
a full spectrum of rare and common alleles in hundreds of genes 

 of rare and common alleles in hundreds of genes 
throughout the genome acting in a polygenic fashion. Different 
variants at the same locus, based on varying impact on gene func­
tion, can alter lipid levels in a broad range. Variant frequency in 
the population is inversely correlated with the magnitude of effect 
on serum lipid levels. Many loci have an impact on multiple lipo­
protein levels simultaneously. 
Genetic mapping for lipid traits has a rich history of synergy with 

r lipid traits has a rich history of synergy with 
biochemical and physiologic investigation to reveal the molecular 
mechanisms of lipoprotein metabolism and its relationship to car­
diovascular disease in humans.
Mendelian hyperlipidemia syndromes were the first to yield 
pathophysiologic insight, starting with Brown and Goldstein’s 
classic studies showing that LDL failed to suppress HMG-CoA 
(3-hydroxy-3-methyl-glutaryl coenzyme A) reductase activity in 
fibroblasts from subjects with famili

activity in 
fibroblasts from subjects with familial hypercholesterolemia.89 
Subsequent studies of families with severe hypercholesterolemia 
(LDLR, APOB, ABCG5, ABCG8, ARH, PCSK9) yielded insights 
into basic mechanisms of cholesterol absorption and biliary excre­
tion and contributed to basic biologic understanding of receptor-
mediated endocytosis, recycling, and feedback regulation.90,91 
The association between high LDL-C and increased rates of 
myocardial infarction (MI) was also noticed 

s of 
myocardial infarction (MI) was also noticed in these families 
and complemented by observations of low rates of coronary dis­
ease in families segregating unusually low LDL-C levels (familial 
55
hypobetalipoproteinemia: APOB, PCSK9, ANGPTL3).91 Epide­
miologic association and the success of statin therapy in prevent­
ing coronary heart disease (CHD) extended the relationship of 
LDL-C and CHD to the general population,92 as have common 
LDL-C–associate

eral population,92 as have common 
LDL-C–associated SNPs identified by GWASs.86
The large number of associated loci for LDL-C, HDL-C, and 
TG combined with clinical outcome data on population-size 
cohorts of genotyped individuals has enabled causality testing for 
epidemiologic associations with important public health conse­
quences. Elevated HDL-C levels have been associated with protec­
tion from MI, but the causality of this association is controversial. 
Should public health efforts be mad

ontroversial. 
Should public health efforts be made to raise HDL levels in the 
population? Should drugs that increase HDL-C levels continue to 
be developed after early clinical failures?93 By leveraging common 
variants associated with HDL-C levels, the causality of HDL-C 
in heart disease can be tested using an approach called mendelian 
randomization.94 As described earlier, genetic associations imply 
causality because genotype precedes phenotype, mitigating epi­
demiologic issues of confou

otype, mitigating epi­
demiologic issues of confounding, bias, and reverse causation. 
Mendelian randomization can be conceived of as a clinical trial 
performed by nature in which the subjects are randomized at con­
ception to genetic variants associated with a risk factor (e.g., SNPs 
increasing levels of HDL-C). The subjects randomized to geno­
type are assessed for the outcome (e.g., MI), and their relative risk 
for the outcome is compared in patient groups with and without 
the genetic var

n patient groups with and without 
the genetic variants in question.
Although earlier observational studies showed a strong correla­
tion between elevated HDL-C and CHD,95 investigators tested 
the causality of this association using a mendelian randomization 
approach with SNPs quantitatively associated with lipid levels. 
They found that genetic variants increasing HDL-C were not pro­
tective for MI, whereas genetic variants decreasing LDL-C were 
protective of MI.96,97 These findings are cons

re 
protective of MI.96,97 These findings are consistent with drug trials 
showing that LDL-C–lowering agents protect from MI, whereas 
multiple agents aimed at increasing HDL-C do not.98 These 
results indicate that targets with support from human genetics are 
more likely to be clinically effective in humans.99,100
For TG, however, multiple lines of genetic evidence have sup­
ported a causal connection with CHD. First, rare, loss-of-function 
mutations in APOC3 are associated with low levels o

utations in APOC3 are associated with low levels of TG and are 
also protective for ischemic cardiovascular disease in European101 
and American cohorts.102 Second, a mendelian randomization 
study showed that SNPs that raised serum TG levels also increased 
rates of CHD.103 Finally, because of the interrelatedness of TG, 
LDL-C, and HDL-C, investigators have systematically examined 
all lipid-associated loci in cohorts phenotyped for CHD to dis­
sect the contribution of TG to CHD risk apart fro

sect the contribution of TG to CHD risk apart from LDL-C and 
HDL-C.85 By constructing a statistical framework to account for 
the pleiotropic effects of SNPs on all three lipoprotein levels, they 
demonstrated that (1) SNPs that alter LDL-C and TG in the same 
direction of effect are associated with CHD risk, (2) SNPs that 
exclusively alter TG levels are also associated with CHD, and (3) 
the strength of an SNP’s effect on TG levels independently are 
correlated with the magnitude of effect o

tly are 
correlated with the magnitude of effect on CHD risk.85 
In the area of therapeutics, mapping of genes that affect lipid levels 
has exemplified a promising approach to drug target identifica­
tion: genes that protect from disease when inactivated by nature 
might be useful pharmacologic targets. Statins, which inhibit 
HMG-CoA reductase (encoded by HMGCR), are among the 
most therapeutically successful drugs in lowering LDL-C levels 
and decreasing risk for CHD in b

ing LDL-C levels 
and decreasing risk for CHD in both primary and secondary pre­
vention settings. GWAS did identify HMGCR as a locus altering 
LDL-C levels (with an effect size of about 3 mg/dL),86 but how 
would this particular locus be prioritized as a therapeutic candi­
date among more than 100 other associated loci? In some cases, 
experiments of nature (i.e., an allelic series) can be used to infer a 
dose-response curve of gene function that indicates how enhance­
ment or suppression of t

at indicates how enhance­
ment or suppression of the encoded protein’s activity raises or low­
ers disease risk. In the well-known case of PCSK9, for instance, 
loss-of-function mutations decrease LDL-C and cardiovascular 
disease risk, whereas gain-of-function mutations increase LDL-C 
and cardiovascular disease risk.104 Early clinical trials suggest that 
inhibiting the protein encoded by PCSK9 is a promising strat­
egy for lowering LDL-C and preventing cardiovascular disease.105 
Identifying 

reventing cardiovascular disease.105 
Identifying genes that, when inactivated by nature, protect from 
disease offers several advantages for therapeutic targeting: (1) the 
target is already validated in humans, (2) designing inhibitors of 
gene/protein function is more tractable than increasing gene/pro­
tein function, and (3) nature has performed a lifelong clinical trial 
of inhibiting gene function, and the side effects of doing so are 
known. In the case of PCSK9, individuals with loss-of-

n. In the case of PCSK9, individuals with loss-of-function 
mutations demonstrated no phenotypic abnormalities other than 
low LDL-C levels and decreased risk of heart attack. In a simi­
lar example, naturally occurring mutations that disrupt NPC1L1 
function, the inhibitory target of ezetimibe, were found to be asso­
ciated with reduced plasma LDL-C levels and a reduced risk of 
CHD.106
Genetic risk predictors of CHD based on lipid-associated 
loci alone have historically added little to the ex

oci alone have historically added little to the excellent risk pre­
diction already provided by clinical risk factors, but recent work 
has shown significant promise in using combinations of SNPs to 
identify individuals at high risk. Working in the UK Biobank, 
investigators validated a polygenic risk score that contained SNP 
combinations conferring threefold risk in CHD, equivalent to 
the risk conferred by familial hypercholesterolemia. These high-
risk polygenic combinations were identified

 high-
risk polygenic combinations were identified in 8% of the study 
population, making them 20 times more prevalent than familial 
hypercholesterolemia. Given the broad indications for statins in 
primary and secondary CHD prevention,107 genetics can help 
predict treatment efficacy and elucidate side effects. GWASs have 
identified a few reproducible loci (APOE, LPA, SLCO1B1, and 
SORT1/CELSR2/PSRC1) for the trait of LDL-C response follow­
ing statin therapy.108 SLCO1B1 encodes the organic a

g statin therapy.108 SLCO1B1 encodes the organic anion trans­
porter, OATP1B1, which has been shown to regulate the hepatic 
uptake of statins.109 When exposed to a single dose of simvastatin, 
individuals carrying a missense variant in SLCO1B1 (V174A), 
which causes loss of function, have up to a 2.5-fold increase in 
plasma levels of statin.109 A GWAS for statin-induced myopathy 
performed in a secondary prevention cohort receiving high-dose 
simvastatin identified the same genetic signal (via

imvastatin identified the same genetic signal (via a noncoding 
SNP within the same haplotype) at SLCO1B1, which conferred 
a 4.5-fold risk of myopathy for one allele and a 16.9-fold risk of 
myopathy in homozygous individuals.110 The investigators esti­
mated that 60% of the myopathy cases in their cohort were due 
to this variant at SLCO1B1.110 They performed a GWAS on only 
200 case controls for myopathy, and it is likely that additional 
pharmacogenetically relevant loci like SLCO1B1 will be

macogenetically relevant loci like SLCO1B1 will be identified 
in future studies with larger sample sizes. Several observational 
and intervention studies have also associated statin therapy with 
increased T2D risk. An important question is whether this risk 
is mediated by an off-target effect of statins or an on-target effect 
via HMG-CoA reductase. An off-target effect suggests that new, 
56
more specific statins could be developed without this side e

fic statins could be developed without this side effect, 
whereas an on-target effect implies that the development of more 
potent and specific statins would increase the risk of T2D. Genet­
ics has begun to shed light on this question. Taking a mendelian 
randomization approach, investigators have found that SNPs at 
HMGCR that decrease LDL-C levels also increase BMI, insulin 
resistance, and risk for T2D, suggesting an on-target mechanism 
for statin-mediated T2D risk. 
Considerations for Clin

statin-mediated T2D risk. 
Considerations for Clinical Use of 
Genetic Information and Sequencing in 
Generally speaking, medical application of laboratory testing 
requires three components. First, a validated test must be estab­
lished with sufficient accuracy and precision for consistent mea­
surement across testing centers and over time (clinical grade 
assay). Second, representative control populations of sufficient 
size must undergo testing to establish a normal clinical ref

undergo testing to establish a normal clinical reference 
range. Third, mapping between test values and phenotypic out­
come must be established using robust clinical studies. Ideally, 
these take the form of prospective randomized controlled trials. 
Expensive in their own right, these studies were prohibitive given 
early sequencing technology. NGS has provided the first require­
ment, furnishing high throughput, high-fidelity sequencing at an 
increasingly reasonable cost. The second requirem

increasingly reasonable cost. The second requirement is currently 
under way. Genomics databases are growing around the world, 
providing a “reference range” for common gene sequences in 
healthy individuals.
As detailed previously, the clinical applications of genetic infor­
mation include diagnosis, prognosis, risk prediction, and person­
alized therapy (e.g., pharmacogenetics). With the barriers of cost 
and feasibility diminishing every year, we believe that the use of 
genetic information 

, we believe that the use of 
genetic information will become commonplace in clinical practice. 
To maximize benefits to patients while minimizing the burden 
of false-positive and false-negative results, clinicians will have to 
select the right patients, deploy the appropriate genetic testing 
technology, and properly interpret the results. Specific clinical 
algorithms for patient/genetic test selection are being proposed for 
various endocrine diseases (e.g., short stature),79 but they will 

e diseases (e.g., short stature),79 but they will take 
time to validate. With this in mind, we present a series of broad 
patient scenarios ranging from those with no clinically apparent 
disease to affected individuals with clinically identifiable genetic 
syndromes, summarizing benefits and caveats of genetic testing in 
each scenario. We subsequently review issues related to targeted 
and genome-wide testing and provide some guidance for patient 
and test selection. Finally, we provide an ov

ent 
and test selection. Finally, we provide an overview of disease-rele­
vant classification of genetic findings, examine the interpretation 
of genetic information as presented in a clinical laboratory report, 
and make suggestions for clinical decision making (Fig. 3.3).
Genome Screening in the General Population
Population-based screening for mendelian mutations known 
to cause disease would seem to be an advantageous application 
of genome sequencing. As mentioned earlier, certain mutations

equencing. As mentioned earlier, certain mutations 
in the RET gene predispose to aggressive MTC with such high 
penetrance that the American Thyroid Association recommends 
prophylactic thyroidectomy in infants under 1 year of age.19 As 
genome sequencing becomes commonplace, it seems reasonable 
that genomes from the general population (e.g., as part of new­
born screening) might be examined for RET mutations causing 
this rare but potentially fatal disease. Cases of cancer might be 
prevented

fatal disease. Cases of cancer might be 
prevented and lives saved. However, one must consider that the 
clinical data on RET gene mutations have been obtained from 
kindreds affected by MEN2A/B and familial MTC (see Chapter 
42). Are genotype-phenotype correlations between RET muta­
tions and MTC, or between other gene mutations and the risk of 
other diseases, applicable to someone from the general population 
with no family history of disease?
Clinical interpretation
Bioinformatic filtering
R

Raw results
Genetic testing
Clinical assessment
Affected
individual
Defined
monogenic
syndrome
Single gene/
10 gene
panel
Exome
sequencing
Exome
Whole
genome
1–10
variants
1000s
10,000
None
Benign
Pathogenic
Benign
VUS
VUS
All results
returned
Not all
reported
Reported
and
actionable
Not all
reported
Reported
and
Reported,
NOT
Reported,
NOT
Limited
utility
Undefined
monogenic
syndr

tionable
Limited
utility
Undefined
monogenic
syndrome
Not likely
monogenic
• Fig. 3.3  The suggested use of targeted and genome-wide genetic testing is shown in individuals sus­
pected of harboring a monogenic/mendelian disease. VUS, variants of uncertain significance (see text).
57
An investigation of mendelian diabetes mutations ascertained 
in the general population suggests that genotype-phenotype cor­
relations identified in mendelian disease families ma

ations identified in mendelian disease families may not gener­
ally hold for the population at large. As a case in point, genome 
sequencing of a population-based US cohort phenotyped longitu­
dinally for diabetes identified 25 individuals with mutations pre­
viously known to cause autosomal dominant diabetes (MODY). 
Despite harboring mutations from a curated catalogue of disease 
genes (Human Gene Mutation Database Professional111), only one 
of these individuals met the clinical criteria for 

f these individuals met the clinical criteria for MODY, and overall 
this group of MODY mutation carriers developed diabetes at a rate 
no different from that in the general population. As population-
based sequencing becomes prevalent, it will become necessary to 
re-examine estimates of penetrance and genotype-phenotype corre­
lations in the context of differing genetic backgrounds (e.g., ances­
try) and environment. The current evidence and state of knowledge 
do not support genome sequencing

ate of knowledge 
do not support genome sequencing for population-based screening. 
Individual Patients
Asymptomatic Individuals
Endocrinologists may be referred individuals with no apparent 
symptoms in whom risk of disease may be increased: (1) those 
with a family history of known genetic disease who have not 
yet been tested, and (2) those without family history who were 
tested and found with an apparently pathogenic mutation (i.e., a 
“genetic inciden

tly pathogenic mutation (i.e., a 
“genetic incidentaloma”). At present, this family history serves as 
a proxy for genetic predisposition or risk used to calculate pretest 
probability for screening. As with any medical test, the implica­
tion of genetic testing with regard to the likelihood of developing 
disease in the individual depends on a combination of inherent 
test characteristics (quantified by sensitivity/specificity) and the 
pretest likelihood of disease. For individuals with a fami

likelihood of disease. For individuals with a family his­
tory of genetic disease, the pretest probability of disease can be as 
high as 1 in 2 for a highly penetrant autosomal dominant disorder 
or 1 in 4 for a sibling of an individual with a recessive disorder. 
Thus, for individuals with a family history of mendelian endo­
crine disease, genetic testing is often warranted (depending on the 
risks and benefits for the individual patient at hand, including 
psychosocial factors); this fits well

, including 
psychosocial factors); this fits well with current clinical practice for 
asymptomatic individuals from families with mendelian disorders.
However, for individuals with no family history presenting with 
a mendelian-like “genetic incidentaloma,” the pretest likelihood of 
disease is that of the population (1:10,000 to 1:100,000). Even 
with a mutation conferring 50-fold increased risk, the individual 
is far more likely to remain free of disease. So what reassurance can 
be offered 

e of disease. So what reassurance can 
be offered to such individuals who carry mendelian mutations? 
Population-based sequencing surveys reveal that, on average, the 
genome of an apparently healthy individual contains approxi­
mately 100 mendelian-like disruptive mutations (i.e., frameshift­
ing indels and SNPs resulting in premature stop codons), up to 20 
of which are homozygously inactivated.112 Thus, even the protein-
coding portion of the human genome (comprising only 1–2% of 
the total g

human genome (comprising only 1–2% of 
the total genome) contains unexpected redundancy that protects 
from disease. Depending on the severity of clinical consequences 
and the estimate of penetrance in the general population, watchful 
waiting can be a prudent course of action. 
Symptomatic Individuals
Individuals with symptomatic disease can present with clini­
cally defined or unknown syndromes. In both cases, a genetic 
diagnosis can be of psychological benefit, inform family plan­
ning, and

ychological benefit, inform family plan­
ning, and sometimes can direct therapeutic screening/interven­
tion. NGS has the potential to improve diagnostic efficiency in 
these symptomatic individuals. Techniques including GWASs and 
WES shift the diagnostic workflow from a sequential search for 
candidate genes to parallel sequencing panels. By simultaneous 
testing a host of target genes, NGS avoids a lengthy process of 
diagnosis by exclusion. In addition, NGS panels can be designed 
to simulta

n addition, NGS panels can be designed 
to simultaneously ascertain second-order disease modifying muta­
tions that are missed by classic sequential analysis.
The genotype-phenotype correlation of endocrine tumor syn­
dromes (MEN1, MEN2; see previous discussion) are a classic 
example of the benefits of genetic diagnosis for directed screen­
ing and prophylactic interventions in both affected individuals 
and close relatives. For example, the genotype-phenotype corre­
lation of RET oncogene muta

otype-phenotype corre­
lation of RET oncogene mutations dictates the urgency of pro­
phylactic thyroidectomy (ranging from <1 year old to >5 years 
old) in children.19 Thus, even with a clinical diagnosis, genetic 
testing can have great prognostic value. Another example is the 
3-M syndrome (clinically defined by short stature, facial dysmor­
phism, and skeletal abnormalities) caused by mutations in the 
CUL7 gene (among others). Males with this syndrome are at high 
risk for hypogonadism and s

 syndrome are at high 
risk for hypogonadism and should be screened more vigilantly as 
a result of the genetic diagnosis to prepare for future infertility. 
Sometimes a genetic diagnosis can allow a patient to avoid life­
long treatment and testing. Individuals with diabetes from GCK 
mutations (MODY2) manifest hyperglycemia but are still able 
to regulate their blood sugar levels such that they neither require 
hypoglycemic therapy nor are at elevated risk for secondary com­
plications of diab

levated risk for secondary com­
plications of diabetes. More generally, and especially for hetero­
geneous endocrine diseases such as diabetes, genetic classification 
has the potential to guide pharmacotherapy. An existing example 
of genetically guided pharmacotherapy can be found in individu­
als with diabetes caused by HNF1A mutations. In clinical trials, 
these HNF1A mutation carriers were much more sensitive to sul­
fonylureas than noncarriers and maintained more durable glyce­
mic control

ers and maintained more durable glyce­
mic control without additional agents.55
As in the cases described previously, genetic testing of affected 
patients is most likely to be informative when a high penetrance 
genetic variant is identified (i.e., for rare mendelian-type dis­
eases). We offer the following criteria and mnemonic (SSSS), 
which should raise clinical suspicion for a mendelian disorder: (1) 
severe—more severe than usual for the disease; (2) segregating—
multiple affected family m

sease; (2) segregating—
multiple affected family members or family history of consanguin­
ity; (3) syndromic—features such as dysmorphism, developmental 
abnormalities, and unusual biochemical phenotypes; and (4) too 
soon—early age of onset. The diagnostic yield of genetic testing is 
highly variable both due to technological reasons and underlying 
genetic (locus) heterogeneity. Even for well-described mendelian 
syndromes such as MEN1, 30% of kindreds have no identifiable 
mutations identifie

kindreds have no identifiable 
mutations identified by MEN1 gene sequencing. For affected indi­
viduals with unknown syndromes, or for whom targeted sequenc­
ing has failed to make a diagnosis, systematic gene sequencing 
using NGS methods would be required, and diagnostic yields are 
typically lower. A National Institutes of Health study of exome 
sequencing for rare disorders found a diagnosis in 20% of cases.113 
Avoiding Invasive Diagnostics
As ultrasound, computed tomography, and magnetic r

As ultrasound, computed tomography, and magnetic resonance 
imaging continue to improve, the number of incidental endo­
crine findings also increases. NGS provides a tool to evaluate these 
58
findings without resorting to invasive resections. Thyroid nod­
ules present a classic diagnostic dilemma for the endocrinologist. 
Although most nodules are benign, fine-needle aspiration identi­
fies 100,000 nodules with indeterminate cytology in the United 
State

s with indeterminate cytology in the United 
States each year.114 Several endocrine societies recommend surgi­
cal diagnosis via lobectomy or total thyroidectomy for indetermi­
nate cases, although only 5% to 30% of these nodules prove to be 
malignant.115 In these cases, genetic diagnosis may help reduce 
the need for surgery and lifelong thyroid replacement. A recent 
study applied genetic testing to 176 thyroid biopsies with inde­
terminate cytology. For these nodules, typical workup includes

tology. For these nodules, typical workup includes 
lobectomy, total thyroidectomy, or close observation. Instead, 
fine-needle aspiration biopsies were analyzed using NGS to check 
for mutations associated with thyroid malignancy. The resulting 
analysis demonstrated a specificity of 91% and prevented diag­
nostic hemithyroidectomy in 49 patients. Of 46 patients who pre­
sented for subsequent ultrasound follow-up, 45 had nodules that 
remained stable in size, and the exception was benign on rep

table in size, and the exception was benign on repeat 
biopsy. Successful clinical application of genetic screening tools 
may thus save patients from the morbidity and cost of invasive 
surgical diagnosis.116
NGS can also help distinguish papillary thyroid carcinoma 
(PTC) from benign adenomatoid nodules. By analyzing patients 
with coincident benign nodules and PTC, Ye and colleagues were 
able to identify BRAF somatic mutations (22/32) that are only 
found in PTC, whereas mutations in SPOP (4

e only 
found in PTC, whereas mutations in SPOP (4/38), ZNF148 
(6/38), and EZH1 (3/38) were enriched in adenomatoid nod­
ules.117 Several of the adenomatoid markers were found to be 
mutually exclusive, and phylogenetic tree analysis demonstrated 
that PTCs develop independently rather than progressing from 
adenomatoid nodules. This model reveals a distinct genetic origin 
for invasive thyroid neoplasms that differs from the progressive 
dysplasia evident in tissues such as colon. The data sug

sia evident in tissues such as colon. The data suggest that 
thyroid sequencing studies may rule out malignant potential in 
adenomatous nodules and reduce the need for further surveil­
lance. In the future, specific genetic signatures may increase the 
specificity of aspiration biopsies and reduce the cost, care burden, 
and lifelong morbidity associated with thyroidectomy.117
Selection of Genetic Tests: Targeted Versus Genome-wide 
Approaches
If cost were no object, one might consider deployin

f cost were no object, one might consider deploying whole-
genome sequencing to maximize sensitivity and find the “smoking 
gun” mutation in every case. Using genome sequencing in this 
way is tantamount to performing thousands of genetic tests at 
once and clinically analogous to ordering every possible hormone 
level for every endocrine patient. Intuitively, this approach is not 
pursued because every test carries with it the possibility of false-
positive results. The more tests one orders, t

se-
positive results. The more tests one orders, the more likely at least 
one result will be false. The same logic applies to genetic tests and 
genome sequencing. Even if genetic variants are identified with 
100% analytical sensitivity and specificity, their clinical sensitivity 
and specificity for disease risk are much less, due to incomplete 
penetrance, variable expressivity, and our incomplete knowledge 
of genotype-phenotype correlations. 
Limitations of Genomic Testing
The molecular ty

. 
The molecular type of genetic variation (single-base changes or 
more complex variation, e.g., insertions/deletions/duplications) 
strongly affects the analytic sensitivity and specificity. The most 
commonly used gene-panel and genome-wide tests rely on NGS 
technology, which is currently well suited for detecting single 
or multiple base variations but poorly optimized for the detec­
tion of structural variants (chromosomal rearrangements), large 
indels, trip

 (chromosomal rearrangements), large 
indels, triplet repeat expansions, poly T and A sequences, novel 
breakpoints, and copy number variation.118 For example, all 
false-positive genetic variants identified in a pilot clinical-grade 
genome sequencing study were indels or were near repetitive 
DNA stretches.119 As NGS technology and genome reconstruc­
tion algorithms improve, so will the ability to accurately detect 
these more complex genetic variants, reducing both false-negative 
and false-p

ariants, reducing both false-negative 
and false-positive results. Currently, alternative modes of testing, 
such as chromosomal microarrays (array CGH), are used to iden­
tify large (>50 kb) structural and CNVs.120
Even as technology addresses sequencing limitations, proper 
classification of gene variants limits deployment of sequencing in 
clinical practice. Recent data from the ExAC database focused on 
analysis of rare germline coding region, nonsynonymous, single-
nucleotide variants (SNVs

, nonsynonymous, single-
nucleotide variants (SNVs) in 38 endocrine disease genes. The 
study concluded that current variant prediction tools suffer from 
ascertainment bias and dependence on in silico pathogenicity pre­
diction. Ascertainment bias results from dependence on studies 
that identified rare SNVs in disease cohorts. As genetic screen­
ing expands to the general population, the baseline prevalence 
of “pathogenic” SNVs must be validated in large healthy control 
populations. Again, a

ed in large healthy control 
populations. Again, a truly pathogenic variant should be enriched 
in affected individuals and absent in controls. At this stage, too 
many variants have been called pathogenic using disease cohorts 
without being appropriately excluded due to their presence in 
healthy controls. Meanwhile, in silico prediction for functional 
effects of mutations is much less robust for missense mutations 
than more dramatic frameshift and nonsense mutations. As a 
result of these t

ft and nonsense mutations. As a 
result of these two factors, genetic prediction models frequently 
misclassify benign variants as pathogenic, and the number of 
identified pathogenic gene variants far exceed reported disease 
prevalence. Several such examples emerge from the ExAC study 
in which individuals with mendelian disease are not included. 
For MEN1, apparently “pathogenic” missense mutations occur 
in 1:2000 individuals sequenced, whereas disease prevalence is 
1:30,000.121 Ultimately,

as disease prevalence is 
1:30,000.121 Ultimately, healthy individuals carry a large number 
of misclassified “pathogenic alleles” initially identified as rare vari­
ants in individuals with disease. As genetic testing becomes more 
widespread, our definition of pathogenic variations and clinically 
actionable mutations will require increasing rigor.121
Targeted testing (either measuring variation in a requested set 
of genes or masking/only reporting back variation in the requested 
set of gene

rting back variation in the requested 
set of genes) is one solution to decrease false-positive results. In 
addition, clinical laboratories that focus on testing specific sets of 
genes may be particularly well versed in interpretation of variation 
in these genes, potentially improving sensitivity and specificity. Of 
course, when targeted testing is negative, then more comprehen­
sive testing may be required to make a diagnosis. However, even 
beyond the issue of false-positive findings, targ

beyond the issue of false-positive findings, targeted testing may 
offer superior analytic performance. Detection of variants can be 
highly variable in current versions of genome-wide sequencing, 
in which millions of genetic variants are identified simultane­
ously.122 There is often a trade-off between the number of vari­
ants identified and the sensitivity/specificity for detecting and 
calling any individual variant. This analytic sensitivity/specificity 
relates to coverage, which is the 

ty/specificity 
relates to coverage, which is the depth of sequencing performed, 
or the number of independent times a particular nucleotide has 
been sequenced in a single test. Depth can vary greatly across the 
genome. For example, a clinical trial of whole-genome sequencing 
reported clinical-grade genome sequencing at “30× coverage on 
59
average and at least 8× coverage for more than 95% of bases.”119 
This means that of the 3 billion bases in the human

his means that of the 3 billion bases in the human genome, 
each base was sequenced independently 30 times on average, and 
more than 95% of bases were observed at least 8 times. However, 
5% of the human genome was still poorly observed or missing. 
Thus, if clinical suspicion motivated examination of certain genes 
or genomic regions, a targeted approach might well have higher 
analytic sensitivity and specificity. The clinical-grade genome 
described previously is sufficient for detecting var

scribed previously is sufficient for detecting variants at a fre­
quency found with germline heterozygosity (on average 50% of 
the molecules sequenced would contain the variant base), but for 
disorders requiring detection below germline heterozygosity, such 
as somatic mutation testing in tumors, higher coverage would be 
required. 
Interpretation of Identified Genetic Variants
Once genetic variants are identified (from sequencing or other­
wise), they must be interpreted for their impact on h

e), they must be interpreted for their impact on health and 
disease. This interpretation requires the integration of popula­
tion data (to know whether a variant is seen at higher frequency 
than expected for a pathogenic variant), computational predic­
tions, experimental evidence, and familial comparisons. Curated 
databases are being established to begin to accurately catalogue 
this information and assist in interpretation. The ClinVar archive 
aggregates information about genomic variation

ve 
aggregates information about genomic variation and its relation­
ship to human health.123
Identified variants can be classified into three broad clinical 
categories: benign variants, pathogenic variants, and variants of 
uncertain significance (VUS), but intermediate categories have 
also been proposed.124 In single-gene or disease-targeted testing, 
the number of identified variants is small enough to allow for the 
individual assessment of all variants in each patient, once com­
mon benig

 all variants in each patient, once com­
mon benign variations are curated. However, exome sequencing 
identifies tens of thousands of variants, and genome sequencing 
identifies several million in each individual.125 Thus, automated 
filtering is necessary to point out a few pathogenic variants in a 
haystack of benign ones. Based on the assumption that the test­
ing was done for a mendelian variant with high penetrance, most 
genetic variants can be filtered as benign based on having been 
obs

an be filtered as benign based on having been 
observed at a frequency greater than 1% (or even lower thresholds 
for rare disorders) in suitable reference populations. Computa­
tional analysis can support a benign classification by showing a 
variant is silent (e.g., encodes the same amino acid as the reference 
base). This assumption, however, misses rare exceptions such as 
the LMNA Gly608Gly mutation in Hutchinson-Gilford proge­
ria syndrome. Although mutating cytosine to thymine at position

 Although mutating cytosine to thymine at position 
1824 still codes for glycine, this variant introduces a new splice site 
in exon 11 that eliminates 50 amino acids from the final progerin 
protein.126 Conversely, experimental data can provide evidence 
that a missense variant does not actually alter protein function 
in disease-relevant bioassays. In addition, family-based data can 
be extremely powerful for interpretation; for example, most vari­
ants for dominant diseases can be classified 

ari­
ants for dominant diseases can be classified as benign if they are 
also observed in healthy relatives. Typically, the filtering of benign 
variants results in a 100- to 1000-fold reduction in the number of 
variants, requiring further analysis for 30 to 300 variants.119,127
Among these, a similar hierarchy of evidence can support a 
pathogenic designation, or the evidence can remain inconclusive, 
resulting in a VUS designation. Computational analysis showing 
predicted gene disruption (pr

al analysis showing 
predicted gene disruption (premature stop or frameshift variants) 
in a gene known to cause disease when inactivated provides strong 
evidence of pathogenicity. Experiments in disease-relevant bio­
assays can demonstrate deleteriousness of a variant, and family-
based data can increase the conclusiveness of findings by showing 
that a proposed pathogenic variant segregates with disease or that 
it is absent from both parents in de novo cases of disease. Presence 
of the vari

in de novo cases of disease. Presence 
of the variant in databases such as the Human Gene Mutation 
Database can also provide supportive evidence, but the quality 
of evidence can vary widely among variants. Before final report­
ing, variants classified as pathogenic are typically validated by 
resequencing using traditional methods (i.e., Sanger sequenc­
ing),124 although this practice may shift depending on the state 
of sequencing technology. In a recent pilot case-control study of 
20 cardio

In a recent pilot case-control study of 
20 cardiomyopathy patients and controls subjected to genome 
sequencing, two to four variants per individual (in both cases and 
controls) were classified as pathogenic,125 illustrating the chal­
lenges in interpreting genetic variation in isolation. 
Using a Genetics Laboratory Report to Make 
Clinical Decisions
In the clinic, genetic testing is used to identify or confirm the 
cause of disease and to help the physicians make individualized 
treatment de

p the physicians make individualized 
treatment decisions (Table 3.5). Given the complexity of genetic 
testing, especially at the genomic (exome or genome) scale, physi­
cians and clinical laboratories will have to work collaboratively to 
achieve useful results. For example, when a laboratory finds a rare 
or novel variant in the course of genomic sequencing, the director 
cannot assume it is relevant to a patient just because it is rare, or 
novel. The context of the patient’s history, physic

ovel. The context of the patient’s history, physical examinations, 
and previous laboratory examinations are key to distinguishing 
among causal variants for the patient’s disorder, incidental find­
ings, and benign variants.
How should identified variants be used in clinical decision 
making? Variant analysis contains uncertainties that are implicitly 
defined in the three-part classification described earlier: benign, 
pathogenic, and VUS. American College of Medical Genetics 
guidelines124 re

ican College of Medical Genetics 
guidelines124 recommend that variants classified as pathogenic can 
be used in clinical decision making. However, genetic evidence 
should not be the only evidence of disease and should be used in 
conjunction with complementary clinical information when pos­
sible, especially as some pathogenic variants may be misclassified. 
Examples of corroborative clinical data include enzyme assays, 
physical findings, and imaging studies. VUS should generally not 
be used

imaging studies. VUS should generally not 
be used in clinical decision making. Efforts to resolve the classifica­
tion of the variant as pathogenic or benign should be undertaken, 
and interpretation in the context of the patient’s clinical scenario 
is critical. Variants classified as benign can usually be assumed not 
to cause the patient’s disorder. Detection of pathogenic variants 
incidental to the diagnostic motivation for sequencing, but of 
potential clinical relevance, will be an inevi

of 
potential clinical relevance, will be an inevitable consequence of 
genomic testing. This scenario is analogous to the inevitable detec­
tion of adrenal masses in computed tomography scans or thyroid 
nodules on physical examination.
A clinician ordering genomic testing should be aware of labora­
tory policies and current ethical guidelines regarding such inciden­
tal or secondary findings. Current recommendations are to offer 
the patient the option not to receive such incidental findings, 

e option not to receive such incidental findings, and 
laboratories may vary in their reporting of such incidental find­
ings. From both the clinician and patient perspectives, incidental 
findings can also be specifically requested or declined. The labora­
tory should provide clear information about what constitutes a 
reportable incidental finding and how a finding may be requested 
60
or declined. Guidelines have been set forth in the American Col­
leg

lines have been set forth in the American Col­
lege of Medical Genetics recommendations for reporting of inci­
dental findings in clinical exome and genome sequencing.128 
Future Perspectives and Summary
In the future, we anticipate that a genome sequence will become 
a standard accompaniment to the medical chart; thus, the ques­
tion “Should we sequence?” will transmute to “What part of the 
sequence should we look at?” A rational clinical approach will 
require the discipline to not look at al

ach will 
require the discipline to not look at all of it, or at least to rigor­
ously interpret sequence data in the clinical context. As detailed 
previously, every human genome is littered with thousands of VUS 
and multiple variants classified as pathogenic; clinical suspicion is 
essential to help direct where to look and how to interpret genetic 
variation. This approach recapitulates current clinical algorithms 
for genetic testing. For example, familial paraganglioma shows 
locus heterog

ample, familial paraganglioma shows 
locus heterogeneity as a mendelian disorder with cases attributable 
to mutations in multiple genes encoding proteins for the succinate 
dehydrogenase complex. The current genetic testing algorithm is 
hierarchical, starting with the gene that best fits the clinical pre­
sentation of the index case (e.g., sequencing of the SDHB gene if 
the neoplasm is abdominal or malignant vs. SDHD if located in 
the head or neck). If no mutations are found in the most like

 neck). If no mutations are found in the most likely 
candidate gene, other complex members are tested (e.g., SDHC). 
In a genome-sequencing era, a clinical algorithm might hierarchi­
cally look up mutations in the succinate dehydrogenase complex 
members from a sequenced individual with familial paragangli­
oma syndrome. No additional sequencing costs would be incurred 
as each gene is subsequently tested, but honing in on the appropri­
ate and interpretable areas of the genome will reduce the 

interpretable areas of the genome will reduce the clinical 
burden of false-positive results.
As mentioned earlier, changes in DNA sequence are not the 
only means of passing on inherited information. Looking beyond 
primary sequencing data, epigenetic changes including DNA base 
methylation and histone modification are likely to become essen­
tial for accurate genetic diagnosis. Epigenetic changes can perma­
nently and heritably alter the expression of genes and thus clinical 
phenotypes. Methy

sion of genes and thus clinical 
phenotypes. Methylation of cytosine residues in CpG dinucleo­
tides of DNA is so ubiquitous in the genome that it has been 
referred to as the “5th base.”129 Hypermethylation at promoters 
reduces gene expression by decreasing affinity for transcription 
factors and increasing recruitment of methylated DNA-binding 
gene repressors. DNA methyltransferases reproduce these meth­
ylation patterns in daughter cells and generate new methylation 
patterns according to e

 generate new methylation 
patterns according to environmental influences and extracellular 
The clinical impact of inherited methylation on genetic dis­
ease is just beginning to be understood. Recent work assessed the 
genome-wide methylation state in more than 5000 individuals 
with respect to T2D and obesity in a process analytically similar 
to GWASs. Of the 187 methylation sites associated with increased 
adiposity and T2D, 38 were found in loci harboring genes such 
as ABCG1, a

re found in loci harboring genes such 
as ABCG1, a known regulator of lipid homeostasis and insulin 
resistance. These methylation loci predicted obesity and T2D 
independently of SNP variation and outperformed inflammatory 
markers such as C-reactive protein.130
In summary, genetic information is most likely to be of clinical 
use in individuals with suspected mendelian syndromes (see the 
SSSS criteria enumerated earlier). For individuals with a clinically 
defined syndrome, for which targeted

a clinically 
defined syndrome, for which targeted panels exist and are well 
validated, a targeted approach (single-gene or gene panel testing) 
is currently recommended as an initial approach. For example, 
genome-wide sequencing is likely not needed when MEN2B is 
suspected on clinical grounds; sequencing RET will usually make 
a diagnosis. If results from targeted genetic testing are uninfor­
mative, and the suspicion of a genetic disorder is high, exome 
or genome sequencing will make addit

 high, exome 
or genome sequencing will make additional diagnoses in some 
patients (see Fig. 3-3). We recommend primary genome-wide 
approaches that assess both structural variation and sequence 
variation for individuals with clinically unclassifiable genetic syn­
dromes or when targeted panels are not available or well validated. 
Depending on technological progress, this may simply be an 
unmasking of data that had not been reported back in a targeted 
test or may require new sequencing. The

 targeted 
test or may require new sequencing. The exome comprises 1% 
to 2% of the genome yet contains nearly 85% of known disease-
causing mutations.131 Thus, given current technologies, exome 
sequencing supplemented with or following array CGH is a rea­
sonable initial genome-wide approach.
  Examining a Clinical Genetics Laboratory 
Report
A clinician considers several issues when examining a clinical genetics 
laboratory report.
Technical Summary of Genetic Testing
For targeted tests, the

Summary of Genetic Testing
For targeted tests, the list of genes tested and coverage for each gene 
can be found at the Genetic Testing Registry (http://www.ncbi.nlm.
nih.gov/sites/GeneTests). For genomic tests, average coverage (e.g., 
>30×) and minimum coverage (e.g., >8× for 95% of bases) should 
be considered in evaluating test quality. Genes with poor coverage or 
no coverage should be listed. For both targeted and genomic testing, 
the specific limitations of detection for different molecu

ific limitations of detection for different molecular classes 
of genetic variants given the technology utilized (e.g., copy number 
variants are not well captured by next generation sequencing) should 
be described.
Clinical Interpretation
With regard to the clinical indication for testing, was the test positive 
(an explanatory variant identified for the patient’s condition), negative 
(no explanatory variants identified), or inconclusive (only variants of 
uncertain significance identified)? 

 variants of 
uncertain significance identified)? Does the potential explanatory 
variant fit with the clinical scenario or at least explain some of the 
patient’s phenotypes? With regard to incidental findings, is there a 
carrier risk to future progeny or future risk of monogenic disease to 
the patient? A compendium of monogenic diseases and their patterns 
of inheritance can be found at the Online Mendelian Inheritance in 
Man website (https://www.omim.org/).
Variant Reporting and Classifica

//www.omim.org/).
Variant Reporting and Classification
When applicable, the gene name, transcript, molecular form of variant 
(single-nucleotide polymorphism, indel, etc.), base changes, amino 
acid change, zygosity, population frequency, and classification 
(benign, pathogenic, variants of uncertain significance) should be 
provided. Naming conventions are determined by the HUGO Gene 
Nomenclature Committee (http://www.genenames.org). Variant 
frequencies in the population can be found at the 1

requencies in the population can be found at the 1000 Genomes 
Project (http://www.1000genomes.org) and for exomes at the 
Exome Aggregation Consortium (http://exac.broadinstitute.org/). The 
justification for variant classifications should be provided, taking into 
account family-based data if available and information from clinical 
databases. Previously reported relationships among variants and 
phenotypes can be found at ClinVar (http://www.ncbi.nlm.nih.gov/
clinvar).123 The analytic accurac

cbi.nlm.nih.gov/
clinvar).123 The analytic accuracy of each reported variant in terms 
of coverage and validation (e.g., Sanger resequencing) should be 
reported.
TABLE 3.5
61
Many other best practices will improve the outcome of genetic 
diagnosis through sequencing. Ideally, DNA from both parents 
should be obtained, if possible, and DNA from additional rela­
tives may also aid in interpretation. If unaffected and affected 
tissues are identified, paired af

ed and affected 
tissues are identified, paired affected tissue-blood samples should 
be obtained when possible. Identified variants can be classified 
as benign, pathogenic, or VUS, based on cross referencing with 
databases of diseased and undiseased individuals, as well as fam­
ily members; computational analysis; and experimental evidence. 
Indeed, to maximize interpretability of any genomic approach, 
it will be vital to interpret variation in the context of massive 
numbers of genome seque

in the context of massive 
numbers of genome sequences obtained in healthy individuals 
and in patients with disease. Finally, accurate classification will 
require physicians and clinical laboratories to work collaboratively, 
and the resulting genetic information should always be used in 
conjunction with complementary data (chemistries, imaging, etc.) 
for clinical decision making.
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62
4
Laboratory Techniques for Recognition 
of Endocrine Disorders
PATRICK M. SLUSS AND FRANCES J. HAYES
E
ndocrinology is a practice of medicine that is highly depen­
dent on accurate laboratory measureme

ighly depen­
dent on accurate laboratory measurements. Small changes 
in hormone levels, biomarkers, or molecular markers often 
may be more specific and more sensitive for early disease detec­
tion (or risk) than the classic physical signs and symptoms. Most 
endocrinologists no longer have facilities to develop and validate 
laboratory assays. They must rely on centralized hospital or com­
mercial reference laboratories. Understanding the nuances of labo­
ratory testing can greatly aid the cli

es of labo­
ratory testing can greatly aid the clinician in working with the 
laboratory, particularly when faced with disparate clinical observa­
tions and laboratory results.
Laboratory testing as practiced today contributes significantly, 
both directly and indirectly, to the cost of care, which, over the 
past decade or so, has increased faster than improvements in 
clinical outcomes.1 Current treatment guidelines, especially in 
endocrine practice, rely heavily on early laboratory testing. 

actice, rely heavily on early laboratory testing. Thus 
clinicians and pathologists are increasingly required to understand 
the inner workings of laboratory medicine and work as a team in 
determining optimal management strategies to contain the costs 
of care without compromising quality.
This chapter provides an overview of the analytic techniques 
typically used for diagnosing and monitoring the progress of 
endocrine disorders. Historically the quantitative measurement of 
endocrine factors

the quantitative measurement of 
endocrine factors, protein, and steroid hormones and related fac­
tors, such as steroid binding proteins, in blood and urine has been 
the primary goal. More recently, wide arrays of disease biomarkers, 
particularly with respect to endocrine cancers, have become valu­
able targets for measurement in the clinical laboratory. Analytic 
validation is then discussed. The parameters of analytic validation 
are not method specific. Principles are presented to help end

hod specific. Principles are presented to help endocri­
nologists better assess the performance of the analytic systems that 
they are using. Techniques used by clinical laboratories to control 
CHAPTER OUTLINE
Laboratory Methods, 63
Analytic Validation, 78
Quality Control, 88
Quality Assurance, 89
Classes of Assays, 90
Conclusion, 90
KEY POINTS
 • 
The practice of endocrinology relies heavily on accurate labora­
tory measurements. Small changes in hormone levels, biomark­
ers, or molecular mar

 in hormone levels, biomark­
ers, or molecular markers are often more specific and earlier in­
dicators of disease than the appearance of physical symptoms.
 • 
Analytic methods for assessing endocrine problems are continu­
ally expanding. Traditional measurement of endocrine factors, 
protein, and steroid hormones and related factors has been 
supplemented by a wide array of disease biomarkers, particu­
larly with respect to endocrine cancers.
 • 
Newer systems are often manufactured outside 

 • 
Newer systems are often manufactured outside the laboratory. 
Although the configurations are generally more user friendly, 
they also become more of a “black box,” concealing most of the 
details of the system. Numeric values, especially when reported 
to several decimal places, can falsely suggest levels of accuracy 
and reproducibility beyond the technical limits of the technol­
ogy used.
 • 
Understanding the basic principles of method validation and 
quality control is essential if en

validation and 
quality control is essential if endocrinologists are to assess the 
reliability and robustness of numeric values reported and to 
work effectively with the laboratory to reconcile test values that 
do not match clinical presentations.
 • 
both directly and indirectly, to the cost of care, which over the 
past decade or so has increased faster than improvements in 
clinical outcomes. Clinicians and pathologists are

clinical outcomes. Clinicians and pathologists are increasingly re­
quired to understand the inner workings of laboratory medicine 
and work as a team in determining optimal management strate­
gies to contain the costs of care without compromising quality.
63
Chapter 4  Laboratory Techniques for Recognition of Endocrine Disorders
and assure quality testing results and services follow to provide 
guidance in appreciating the reliability and robustness of numeric 
values reported and in working wi

ness of numeric 
values reported and in working with the laboratory to reconcile test 
values that do not match clinical presentations. Finally, especially 
for the academic practitioner, the classes of assays are discussed to 
provide some clarity on the regulatory requirements laboratories 
are required to meet in providing test results for patient care, fed­
erally supported human studies, and federally regulated clinical 
trials.
Laboratory Methods
Historically laboratory methods unique to t

ethods
Historically laboratory methods unique to the clinical practice of 
endocrinology were directed at the measurement of peripheral lev­
els of hormones or hormone metabolites in serum, plasma, and/
or urine. This measurement is analytically challenging because 
concentrations of most hormones are much lower than those of 
general chemistry analytes. Specialized techniques are necessary to 
measure these low concentrations that can be reported in molar 
units, mass units, or standardized uni

d in molar 
units, mass units, or standardized units, such as the World Health 
Organization (WHO) international unit (IU). Fig. 4.1 illustrates 
the concentrations of representative hormones in plasma from 
healthy individuals. Expressed in molar units to allow direct 
comparisons, peripheral hormone levels range from 10−6 to 10−12 
mol/L (i.e., micromolar to picomolar concentrations). Thus 
clinically useful analytic methods must have exquisite sensitivity. 
Furthermore, as also illustrated in

 sensitivity. 
Furthermore, as also illustrated in Fig. 4.1, the range of concentra­
tions is very broad (often several orders of magnitude), necessitat­
ing methods with a very wide dynamic range of measurement. 
Antibody-based methods are ideally suited to achieve sensitivity 
and wide dynamic ranges and were the first methods successfully 
used both to define endocrine systems and to be applied clinically 
in patient care. Because of their cost effectiveness, suitability for 
high throughput 

t effectiveness, suitability for 
high throughput implementation, and potential for automation, 
antibody methods replaced earlier chromatographic/mass spec­
trometric methods that were used in the discovery and charac­
terization of hormones, particularly steroid hormones. Initially, 
competitive binding assays using polyclonal antibodies were uti­
lized; then with the development of monoclonal antibody tech­
nology in the 1980s immunometric, or double antibody, methods 
were utilized. Both of 

 double antibody, methods 
were utilized. Both of these analytic designs are automated and 
are in widespread use today: Competitive binding assays are used 
for measuring small molecules and immunometric assay is used 
for measuring antigens containing multiple antibody-binding epi­
topes (i.e., protein hormones and biomarkers).
As discussed in detail later, antibody-based assays are subject 
to interference and lack of specificity that can result in inaccurate 
measurements. Even when a given 

lt in inaccurate 
measurements. Even when a given assay has been well validated 
and reference intervals are known (see discussion under “Analytic 
Validation”), this limitation is manifest as producing measure­
ments that are method specific, vitiating the ability of clinicians to 
compare measurements reported using different assays (e.g., assays 
T4
Cortisol
Vitamin D-25-OH
DHEA
T3
Testosterone—male
Norepinephrine
FSH
Prolactin
Testosterone—female
Vitamin D 1,25-OH
Estradiol—fema

stosterone—female
Estradiol—female
Estradiol—male
LH
TSH
FT4
Insulin
FT3
1
100
10,000
1,000,000
Concentration in picomoles/L
• Fig. 4.1  Six-logarithm range of normal plasma concentrations in endocrine tests. DHEA, dehydroepian­
drosterone; FSH, follicle-stimulating hormone; FT4, free thyroxine; FT3, free triiodothyronine; LH, luteinizing 
hormone; T3, triiodothyronine; T4, thyroxine; TSH, thyrotropin.
64
SECTION I    Hormones and Hormone 

thyrotropin.
64
from different laboratories) for the same hormone or biomarker. 
Although preanalytic methods such as extraction and chromatog­
raphy have been tried to improve the accuracy of immunoassays 
used in research settings, these methods are seldom utilized in 
clinical laboratories today because of their high cost, complexity, 
and lack of commercial availability; all are by definition labora­
tory-developed tests (see “Classes of Assays”).
Sin

ory-developed tests (see “Classes of Assays”).
Since the early 2000s, technologic advances in mass spectrome­
try–based assay systems have led to the rapid and ongoing replace­
ment of antibody-based methods for the clinical measurement 
of hormones and biomarkers relevant to the endocrine practice. 
Currently these more complex and expensive methods are utilized 
primarily by commercial reference and large academic hospital 
laboratories, but as the technology becomes more cost effective 
and u

 the technology becomes more cost effective 
and user friendly, its use will clearly increase. Thus it is impor­
tant for clinicians to appreciate the principles of these assays as 
well as those of the older, albeit still widely used, antibody-based 
methods.
The final technologies considered in this section are molecular-
based assays. These methods are not specifically designed for endocrine 
practice but are generic for identifying and in some cases quantifying 
genetic variance. Subsequent 

e cases quantifying 
genetic variance. Subsequent to the sequencing of the human genome 
and the continuing evolution of molecular methods and knowledge, 
these methods are rapidly penetrating endocrine practice. Although 
these methods are still in the early stages of clinical use and generally 
require specialized informatics and interpretative support, laborato­
ries are increasingly providing molecular-based testing with respect to 
determining endocrine cancers, inherited disease, and indiv

ng endocrine cancers, inherited disease, and individualized 
therapeutics.
Antibody-Based Methods
Classic Competitive Binding Immunoassays
The term competitive binding assay refers to a measurement 
method in which an analyte (e.g., a hormone or biomarker) in a 
specimen competes with a labeled reagent analyte for a limited 
number of binding sites on a binding protein. The earliest clini­
cal assays used for the measurement of circulating concentrations 
of endocrine hormones utilized radioisot

trations 
of endocrine hormones utilized radioisotope-labeled analytes 
and antibodies in the classic radioimmunoassay format illus­
trated in Fig. 4.2. The three basic components of a competitive 
immunoassay are antibody, labeled analyte, and unlabeled 
analyte.2,3 The basic principle of this methodology is to allow an 
equilibrium or steady-state condition (e.g., competition) to be 
established between a labeled analyte and the unlabeled analyte 
in calibrators or specimens binding to the ant

te 
in calibrators or specimens binding to the antibody. The reac­
tion obeys the law of mass action and is driven by the affinity 
of the antibody (Ab) for the target analyte, or antigen (Ag), as 
shown in Fig. 4.2. If the concentrations of antibody and labeled 
analyte are held constant, the amount of labeled analyte bound 
is inversely proportional to the concentration of the compet­
ing unlabeled analyte, as illustrated in Fig. 4.3. By comparing 
the percentage of bound antigen (% [bound/tot

ing 
the percentage of bound antigen (% [bound/total]) generated by 
an unknown specimen to the dose-response curve generated by 
known concentrations of analyte (see Fig. 4.3B), the amount of 
analyte in a specimen can be quantified.
Competitive antibody-based assays are referred to generically 
as immunoassays. The analytic sensitivity of a competitive immu­
noassay is approximately inversely related to the affinity of the 
antiserum, such that an antiserum with an affinity constant of 
109 L/

 an antiserum with an affinity constant of 
109 L/M can be used to measure analytes in the nanomolar con­
centration range. This methodology has evolved significantly since 
the development of the prototypical radioimmunoassay in the late 
1950s. Currently, although radioimmunoassays still have a role in 
the research laboratory, the immunoassays most widely utilized in 
clinical endocrine testing are fully automated, nonisotopic instru­
ment systems whose manufacture and reagents are regulated 

tems whose manufacture and reagents are regulated by 
the Food and Drug Administration (FDA). Each of the compo­
nent parts of the competitive immunoassay is discussed in detail 
next.
Antibody
Antibodies are ideal as the binding component in a competitive 
binding assay that is highly specific and can measure very low 
concentrations of analyte in complex mixtures such as serum or 
plasma. Antibodies are inherently specific, and both their speci­
ficity and affinity can be manipulated in develo

]
Analyte-bound
antibody [AbAg]
“Free”
reagents
125I-Analyte
“Competition” for
Ab binding sites
Separate to measure
analyte-bound antibody
K1
K1
• Fig. 4.2  Components of a radioimmunoassay; a prototype competitive binding methodology.
65
A polyclonal antiserum represents a composite of many immu­
nologic clones, with each clone having a different affinity and 
different antigenic epitope specificity. Most clones have affin

igenic epitope specificity. Most clones have affinities 
in the range of 107 to 109 L/M. The affinity of the antiserum 
or purified immunoglobulins for the analyte (i.e., the immuno­
gen) is the sum of the affinities of all the various clones. Antisera 
used in immunoassays typically have affinity constants above the 
1012-L/M range and can easily measure picomolar concentra­
tions of analytes in biologic fluids. Various techniques are used 
to develop a specific antiserum. For example, the anti

evelop a specific antiserum. For example, the antigen may be 
altered chemically to block cross-reacting epitopes either before or 
after immunization. Historically immunoaffinity purification of 
antiserum to obtain epitope-specific immunoglobulins has been 
effectively used. This technology can also be applied to preana­
lytic assay steps to enhance the specificity of immunoassays as well 
as enrich specimens for chromatographic or mass spectrometric 
assays by selecting or eliminating cross-r

metric 
assays by selecting or eliminating cross-reacting factors.4–6 The 
major disadvantage of a polyclonal antiserum is the limited quan­
tity produced. Commercial manufacturers require large quantities 
of immunoassay reagents to support high-volume clinical testing 
performed by a large number of laboratories, and these reagents 
require rigorous validation. Thus the majority of commercial 
immunoassay systems available today are based on monoclonal 
antibodies that can be produced in virtu

noclonal 
antibodies that can be produced in virtually limitless quantities.
Monoclonal antibodies are used in most current immunoas­
says and are required for immunometric assays because they are 
epitope, as opposed to antigen, specific and can be produced vir­
tually without limit. These antibodies are obtained by immuniz­
ing animals using techniques similar to those used for polyclonal 
antisera. Instead of harvesting the antisera from the blood, lym­
phocytes from the spleen are fused with

lood, lym­
phocytes from the spleen are fused with myeloma cells to make 
cells (hybridomas) that will grow in culture continuously and 
produce monospecific antibodies.7–10 These fused cells are sepa­
rated into clones by means of dilution plating techniques similar 
to those used in subculturing bacteria. The supernatant of pure 
monoclonal cell lines (or ascites fluid if the cells are transplanted 
into carrier mice) contains monoclonal antibodies. The selection 
processes used to separate th

dies. The selection 
processes used to separate the initial clones can be targeted to 
identify specific clones, producing antibodies with high affinities 
and low cross-reactivity to related compounds.11
The epitope specificity of monoclonal antibodies allows assays 
to be designed for large analytes (containing multiple nonover­
lapping epitopes), which do not depend on competition; these 
immunometric assays are often referred to as two-site or sandwich 
assays (see later). However, the high 

r sandwich 
assays (see later). However, the high specificity of monoclonal-
based assays can cause problems for some endocrine assays. 
Many hormones circulate in the blood as heterogeneous mix­
tures of multiple biologically active forms. Some of these forms 
are caused by genetic differences in patients; others are related to 
metabolic precursors and degradation products of the hormone. 
Genetic differences cause some patients to produce variant forms 
of a hormone, such as luteinizing hormo

ant forms 
of a hormone, such as luteinizing hormone (LH). These genetic 
differences can cause marked variations in measurements made 
using assays with specific monoclonal antibodies, compared with 
more uniform measurements made using assays with polyclonal 
antisera that tend to cross-react with the multiple forms.12 Well-
characterized monoclonal antibodies can be mixed together to 
produce engineered polyclonal antibodies with improved sensi­
tivity and specificity.13
Cross-reactivity with

i­
Cross-reactivity with precursor forms of the analyte and with 
metabolic degradation products can cause major differences 
in assays. For example, the degree of cross-reactivity among six 
molecular forms of human chorionic gonadotropin (hCG) causes 
differences in hCG assays. Similarly, cross-reactivity among vari­
ous circulating metabolic fragments causes method-specific dif­
ferences in parathyroid hormone (PTH) assays.14,15 Cortisol is 
an example of an analyte 

ssays.14,15 Cortisol is 
an example of an analyte for which major cross-reactivity with 
other steroids, such as corticosterone, 11-deoxycortisol, cortisone, 
and numerous synthetic steroids, causes significant immunoassay 
interferences.16 Matrix effects with albumin also can cause major 
differences in cortisol immunoassays (see “Mass Spectrometry” for 
a more robust method for measuring steroids).17 
Labeled Antigen
In radioimmunoassays, radioactive iodine (125I or 131I) was orig­
inally used

active iodine (125I or 131I) was orig­
inally used to label the antigen. Subsequently a large variety of 
methods have been developed to label the analytes.18–23 Today 
most commercial kits and all automated immunoassays use non­
isotopic signaling systems to measure hormone concentrations. 
These assays use colorimetric, fluorometric, or chemiluminescent 
signals rather than radioactivity to quantify the relative amount 
of antigen bound to the antibody used in the assay. The advan­
tages of th

antibody used in the assay. The advan­
tages of these nonisotopic labeling technologies include biosafety, 
Competitive binding
Calibration of standards
8 Ab
Ab
A
B0
B
NSB
Concentration
% Bound labeled antigen
+
+
+
+
+
Ag*
Ag0
Ab • Ag*
Ag*
Ab • Ag0
Ag0
8
16
0
8
8
0
0
8
16
36
2
14
6
30
8
16
12
4
12
4
8
8
16
4
6
10
2
2
Bound
Free
Constant
Variable
Bound
Free
2 Ab • Ag0
6 Ab • Ag*
2 Ag0
10 Ab*
+
+
+
+
+
16 Ag*
4 Ag0
0 0
0 0
0
0
0
0
60
50
40
30
20
10
0
0
4
8
12
16
20
24
28
32
36
• Fig. 4.3  Quantit

0
0
0
4
8
12
16
20
24
28
32
36
• Fig. 4.3  Quantitation using competitive binding assays. (A) Principles of 
competitive binding assays. Ab, antibody; Ag*, labeled antigen; Ag0, native 
antigen. See text for details. (B) Typical dose-response curve. The point on 
the curve labeled B0 represents the percentage binding of the radiolabeled 
antigen when zero native antigen is present. The nonspecific binding (NSB) 
level is the minimal binding level of radiolabeled antigen at high concentra­
tions 

 of radiolabeled antigen at high concentra­
tions of native antigen.
66
longer reagent shelf life, ease of automation, and reduced cost. On 
the other hand, they can be more subject to matrix interferences 
than radioactive detection systems. Radioactivity is not affected 
by changes in protein concentration, hemolysis, color, or drugs 
(except for other radioactive compounds), whereas many of the 
current signal systems can yield spurious results when su

 signal systems can yield spurious results when such inter­
ferences are present. Later in this chapter, potential troubleshoot­
ing steps are outlined to help clinicians evaluate the integrity of 
test measurements when spurious results are suspected.
The specificity and accuracy of labeled antigen assays (e.g., 
competitive assays) depend on the purity of the labeled anti­
gen. Especially with respect to labeling small molecules, such 
as steroid hormones, purification of the labeled antigen c

id hormones, purification of the labeled antigen can be 
challenging and certainly contributes to lot-to-lot variance in 
assay performance. Additionally, sensitivity in this assay design 
is influenced by the specific activity of the labeled product (i.e., 
the amount of label incorporated into the antigen on a molar 
basis). An alternative design for competitive assays is to attach 
the antigen to a solid phase and label the antibody. A competi­
tive binding assay is then achieved by allowing 

­
tive binding assay is then achieved by allowing unlabeled anti­
gen to compete with solid-phase antigen for labeled antibody 
binding. Although this addresses issues associated with antigen 
labeling, this format is subject to similar issues associated with 
modification of antibody binding characteristics as the antigen 
is chemically attached to a solid phase or due to restricting its 
conformation once the antigen is attached. 
Unlabeled Antigen
Labeled and unlabeled antigens must compete f

igen
Labeled and unlabeled antigens must compete for a limited num­
ber of binding sites. The competition is not always equal because 
the labeled antigen (tracer) and the native antigen may react differ­
ently with the antibody. This disparity in reactivity may be caused 
by alteration of the antigen due to labeling, as discussed earlier, or 
by differences in the endogenous antigen compared with the form 
of the antigen labeled for use as an assay reagent. The latter is a 
problem often encoun

say reagent. The latter is a 
problem often encountered with protein hormones or biomark­
ers that often exhibit a wide range of isoforms and degradation 
products in peripheral circulation. For this reason, immunometric 
assay designs are preferable for proteins with multiple antibody-
binding epitopes.
Because an assay can be calibrated with certified reference 
materials having known concentrations, differences in reactivity 
of labeled compared to unlabeled antigens do not prevent obtain­
in

ed to unlabeled antigens do not prevent obtain­
ing useful clinical measurements as long as the reactions are repro­
ducible and appropriate reference intervals are established. Such 
differences do, however, result in method-specific measurements, 
and in this case assay results cannot be extrapolated across assays 
using different reagents. 
Separation of Reactants/Automation
As illustrated in Figs. 4.2 and 4.3, immunoassays depend on 
detecting only the labeled antigen bound by the antibody. 

g only the labeled antigen bound by the antibody. Thus 
the entire antibody-bound antigen component of the assay must 
be recovered and separated from any unbound reactants (i.e., 
labeled or unlabeled antigens not bound to antibody). Over the 
years since the introduction of radioimmunoassays, vast arrays of 
technologies have been developed to accomplish this separation. 
Approaches vary from methods to precipitate immunoglobulins 
and recover them by centrifugation or filtration to innovative

them by centrifugation or filtration to innovative ways 
to create solid-phase antibodies (i.e., antibodies attached to solid 
surfaces that can be washed to remove unreacted reagents after the 
binding process is completed).
Separation of immune complexes by precipitation and centrif­
ugation is labor intensive and, like the use of radioactivity itself, 
not amenable to full automation. This approach is still widely used 
in research applications but seldom utilized in clinical testing. In 
con

s but seldom utilized in clinical testing. In 
contrast, solid-phase approaches are widely used and can be batch 
or fully automated. Three frequently used solid-phase materials 
are microtiter plates, polystyrene or latex beads, and paramagnetic 
particles. Most recently the use of immunoassay systems at the 
point of care and miniaturized assay systems are driving the devel­
opment of novel solid-phase antibody systems.21,23–25 Separation 
of solid-phase immune complexes from the unbound moiet

olid-phase immune complexes from the unbound moieties is 
accomplished by plate washers, bead washers, magnetic wash sta­
tions, or microfluidics.
Antibodies can be attached to solid-phase materials directly or 
indirectly. Antibodies can be passively attached directly to plastic 
surfaces by hydrophobic interactions, and this method is often 
used in the manufacture of enzyme-linked immunosorbent assays 
(ELISA). Automated assays, requiring inline capture and washing 
systems, typically involve

ne capture and washing 
systems, typically involve chemical procedures in which amino acid 
groups or carbohydrate groups on the Fc portion of immunoglobu­
lins are covalently coupled to solid phases such as beads or magnetic 
particles. This can be achieved directly by coupling the antibody 
used in the assay to the solid phase or indirectly by covalently cou­
pling a universal capture to the solid phase. Examples of universal 
capture systems are solid-phase particles with covalently attached 

re solid-phase particles with covalently attached 
streptavidin to capture biotinylated assay antibodies or solid-phase 
particles covalently coated with goat antimouse IgG as attachment 
moiety for mouse monoclonal-based assays. Another novel way of 
accomplishing this separation is to attach high-affinity linkers to 
antiserum, which then can be coupled to a complementary linker 
on the solid phase. 
Quantitation
Fig. 4.3 illustrates the principles of quantitative measurement 
using competitiv

ples of quantitative measurement 
using competitive immunoassay techniques. In the schematic dia­
gram, 8 units of antibody react with 16 units of labeled antigen 
and 4 units of native antigen. At equilibrium (assuming equal 
reactivity), 6 units of labeled antigen and 2 units of native antigen 
are bound to the limited supply of antibody. The antigen bound to 
the antibody is separated from the liquid antigen by any of several 
methods, and the amount of labeled antigen in the bound portion 
i

 amount of labeled antigen in the bound portion 
is quantified (see Fig. 4.3A). The assay is calibrated by measur­
ing standards with known concentrations and cross-plotting the 
signal (i.e., counts of the gamma rays emitted from the radioac­
tive label) versus the concentration of the standard to generate 
a dose-response curve. As the concentration increases, the signal 
decreases exponentially (see Fig. 4.3B).
Statistical data-processing techniques are needed to trans­
late the assay signals

niques are needed to trans­
late the assay signals into concentrations. These dose-response 
curves typically are not linear, and numerous curve-fitting 
algorithms have been developed. Before the introduction of 
microprocessors, tedious, error-prone manual calculations 
were required to mathematically transform the data into linear 
models. Today, curve fitting usually is accomplished electroni­
cally with the use of programs that test the robustness-of-fit of 
multiparameter curves after stat

stness-of-fit of 
multiparameter curves after statistically eliminating discordant 
data points.26,27 However, users of these systems must under­
stand their limitations and should pay attention to any warn­
ings presented by the programs during processing of the data. 
Commercial immunoanalyzers, used by the majority of clinical 
laboratories currently, are closed systems. The manufacturer 
validates not only the method (see “Analytic Validation”) but 
67
Chapter 4  Laboratory Techniques for Re

”) but 
67
also the curve-fitting software, which cannot be altered by the 
user. Thus clinical laboratories and clinicians see only the final 
values for the signal generated and calculated analyte concen­
tration for a given specimen.
In clinical practice today, competitive assays are used primarily 
for the measurement of small molecules, such as steroid hormones 
or bioactive peptides, which present only one antigenic ep

tive peptides, which present only one antigenic epitope. 
For molecules in which multiple epitopes are present, allow­
ing more than one antibody to bind each molecule, two-site or 
immunometric assays are used. Immunometric assays, discussed 
in detail in the following section, are advantageous because they 
do not require the time-consuming establishment of a binding 
steady-state condition and thus can be performed much faster. 
Speed of test performance is an important factor in the clinical

performance is an important factor in the clinical 
laboratory supporting acute care. Speed is also directly related to 
high testing throughput, which is an important cost factor to opti­
mize in modern clinical laboratories.
Immunoassays measure antigen concentrations rather than 
biologic activity. The reactive site for most antibodies is relatively 
small, about 5 to 10 amino acids for linear peptides. Some anti­
body reactions are specific for the tertiary structure that corre­
sponds to un

or the tertiary structure that corre­
sponds to unique molecular configurations. In either case, linear 
or conformational antigenic epitopes, the structural elements of 
the hormone involved in receptor activation and biologic signal­
ing, are not necessarily identical to antigenic epitopes recognized 
in the assay. Clinicians must keep this in mind when interpret­
ing the results of antibody-based assays. When measurements are 
ordered to identify abnormal secretion of hormones, the possible 

ify abnormal secretion of hormones, the possible 
disparity between antigenic and biologic epitopes is not as relevant 
as when measurements are ordered to assess the endocrine stimu­
lus received by the target glands. 
Epitope-Specific Immunometric Assays
As briefly mentioned earlier, for larger analytes that contain 
more than one nonoverlapping antigenic epitope, the develop­
ment of methods to produce monoclonal antibodies facilitates 
a unique assay design in which two antibodies are used. 

ue assay design in which two antibodies are used. This 
format is illustrated in Fig. 4.4. The analyte in this example has 
four nonoverlapping epitopes: A, B, C, and D. A solid-phase 
monoclonal antibody (referred to as the capture antibody) that 
is specific to one site (in this example, A) can be used to bind 
the antigen in calibrators or specimens. Using a second, labeled 
monoclonal antibody (referred to as the detection antibody) that 
is specific to one of the other epitopes (in the exam

specific to one of the other epitopes (in the example, D), the 
captured antigen can be quantified after washing away the unre­
acted reagents. Because there are four distinct antibody-binding 
sites on the analyte, 12 different assays can be configured using 
nonoverlapping epitope-specific monoclonal antibodies to each 
of these epitopes. It is important to realize that each of these 
12 formats is a distinct assay with unique performance charac­
teristics, each requiring validation. The detec

c­
teristics, each requiring validation. The detection systems used 
include all the options discussed earlier for labeling protein anti­
gens in immunoassay formats. Fig. 4.5 illustrates one of the most 
common signaling systems used today in either fully automated 
clinical immunoanalyzers or as multiwell plate assays. The detec­
tion antibody is covalently labeled with ruthenium (tris bipyri­
dine), which can be excited by an electric circuit that draws an 
electron from the molecule, leading

that draws an 
electron from the molecule, leading ultimately to a high-energy 
state that will emit light when it decays; this is an electroche­
miluminescent signaling system.28 The assay buffer contains an 
excess of the electron donor tripropylamine (TPA). The Ru2+ 
(tris bipyridine ruthenium metal cation) complex is used as the 
chemical luminescent label. Ru2+ undergoes an electrochemical 
Non-antigenic blocking protein
Capture antibody
Biomarker [antigen]
B
A
Quantitative
read-out
Detecto

marker [antigen]
B
A
read-out
Detector
Signal
Detection antibody (labeled with signaling molecule)
Solid surface
D
C
• Fig. 4.4  Components and design of an immunometric assay. See discussion in text. (From Sluss PM. 
Methodologies for measurement of cardiac markers. Clin Lab Med. 2014;34:167–185.)
68
oxidation reaction on the electrode surface and transitions to an 
excited state to become Ru3+. When the excited state returns to 
the ground 

3+. When the excited state returns to 
the ground state, light is emitted. The magnetic particles that are 
captured on the electrode are immunocomplexes that consist of 
sample and Ru metal complex (Ru2+) and emit light at a speci­
fied voltage. The amount of light emitted is proportional to the 
amount of the immunocomplex and thus the analyte concen­
tration in the sample. It can therefore be used for quantitative 
measurement. This design is typical of modern detection systems 
in that the s

typical of modern detection systems 
in that the signal generated is controlled in the analyzer (in this 
case, light production initiated by activating the electrode), and 
a regenerating system (in this case, TPA in the assay buffer) is 
used to enhance the signal generated, hence achieving high sen­
sitivity detection.
In contrast to competitive immunoassays, immunometric 
(e.g., two-site, double antibody) assays use a large excess of 
antibody-binding sites relative to the concentration of a

y-binding sites relative to the concentration of anti­
gen. The capture antibody immunoextracts the antigen from 
the sample, and the signal antibody binds to the capture anti­
body-antigen complex to form a tertiary complex. These assays 
are referred to as immunometric because the binding reaction 
is very fast (first-order kinetics due to excess antibody) and 
because it is not necessary to establish a binding steady state 
(a requirement for competitive assays) in the assay before 
quantifyi

competitive assays) in the assay before 
quantifying the amount of label associated with the immune 
complex. Immunometric assays can be performed very quickly 
(5–20 minutes compared to 30 minutes to days for competi­
tive assays) and typically have very broad measuring ranges 
(several log orders).
In contrast to competitive assays, the dose-response curve 
generated in an immunometric assay is directly proportional to 
the analyte concentration (Fig. 4.6). The signal increases pro­
gressively

n (Fig. 4.6). The signal increases pro­
gressively with the analyte concentration because the amount of 
labeled antibody bound increases proportionally to the amount 
of analyte present. Quantitative measurements are achieved in 
the same manner as those used in competitive assays. The signal 
generated by the specimen (the “unknown” in Fig. 4.6) is com­
pared to a calibration curve generated by known concentrations 
of the analyte (plotted on the x-axis in Fig. 4.6).
As with any assay, there i

he x-axis in Fig. 4.6).
As with any assay, there is a minimum detection limit 
(referred to as the limit of detection [LOD]) at which the signal 
generated by the analyte is not statistically different from that 
generated in the absence of analyte (referred to as nonspecific 
signal). Note that for an immunometric assay the LOD is asso­
ciated with a small signal, but in a competitive assay the LOD 
is associated with a large signal (compare the dose-response 
curve in Fig. 4.6 to that illustra

 dose-response 
curve in Fig. 4.6 to that illustrated in Fig. 4.3B). All antibody-
based assays also have an upper limit of measurement associated 
with the maximum signal that can be generated by the assay. 
The working or dynamic range of the assay encompasses only 
analyte concentrations between the LOD and the maximum 
response. Analyte concentrations above the maximum response 
or below the LOD level do not generate signal changes (e.g., 
there is no dose-response relationship). Thus the me

ere is no dose-response relationship). Thus the measurement 
variance across the dynamic range of an antibody-based assay is 
heteroscedastic. The insert in Fig. 4.6 shows the measurement 
variance expressed as the percent coefficient of variance (CV) of 
repeated measurements of the same specimen. The CV is calcu­
lated as the standard deviation (SD) of the repeated measures 
divided by the mean value of the measurements. The highest 
percent CV (e.g., CV × 100) will occur at the extremes of th

stem as employed in an immunometric assay. See text 
for details. byp, bipyridine; TPA, tripropylamine. (From Sluss PM. Methodologies for measurement of car­
diac markers. Clin Lab Med. 2014;34:167–185.)
69
is critical when interpreting assay results or monitoring quality 
control performance. Variance determined in the middle of the 
dynamic range of an assay will always underestimate the variance 
at the extremes.
Although

erestimate the variance 
Although the laboratory can control variance associated 
with high analyte concentrations by determining at what level 
of analyte to dilute and retest the specimen, variance associ­
ated with relatively low concentrations cannot be altered for a 
given assay without changing the kinetics of the system (i.e., 
the concentrations of reagents and/or incubation conditions). 
Changing the kinetics of a commercial clinical assay is not 
possible by the perfor

cial clinical assay is not 
possible by the performing laboratory because the systems are 
“locked” to comply with FDA Good Manufacturing Practice 
regulations.
The combined specificity of two antibodies can produce exqui­
sitely sensitive and specific immunoassays. In the past, a common 
problem with early competitive immunoassays was cross-reactivity 
among the structurally similar glycoprotein hormones: LH, follicle-
stimulating hormone (FSH), thyrotropin (thyroid-stimulating hor­
mone [TSH])

 thyrotropin (thyroid-stimulating hor­
mone [TSH]), and human chorionic gonadotropin. The α-subunits 
of each of these hormones are identical, and the β-subunits, though 
unique, share considerable structural homology. The polyclonal 
antisera used for measuring one of these hormones in many of 
the earlier immunoassays had significant cross-reactivity for the 
other gonadotropins. The cross-reactivity of a pair of antibodies 
is less than the cross-reactivity of each of the individual antibod­

oss-reactivity of each of the individual antibod­
ies because any cross-reacting substance must contain both of the 
binding epitopes to simultaneously bind to both antibodies. For 
example, consider two antibodies for LH, each having 1% cross-
reactivity with hCG. The cross-reactivity of the pair is less than 
the product of the two cross-reactivities or, in this case, less than 
0.01%. Most current immunoassays for LH have a cross-reactivity 
of less than 0.01%. This low cross-reactivity is im

f less than 0.01%. This low cross-reactivity is important because 
pregnant patients or patients with choriocarcinoma can have very 
high hCG concentrations that could interfere with measurements 
of the other gonadotropin hormones. Most hormones circulate 
in the blood in multiple forms. Some hormones (e.g., prolactin, 
growth hormone) circulate with macro forms, which can cause 
difficulty in their analysis if specimens are not pretreated.29,30 For 
hormones composed of subunits (e.g., the gon

 For 
hormones composed of subunits (e.g., the gonadotropins), both 
the intact and the free subunits circulate in blood. Immunometric 
assays can be made specific for intact molecules by pairing an anti­
body specific for the α-β bridge site of the subunits with a second 
antibody specific for the β-subunit. Assays using these antibody 
pairs retain the two-antibody low cross-reactivity needed for mea­
suring gonadotropins and do not react with the free subunit forms 
of the hormones.
The heter

the free subunit forms 
The heterogeneous forms of circulating hormones, and dif­
ferences in specificity characteristics of immunoassays for these 
forms, make calibration and harmonization difficult. Two 
immunoassays calibrated with the same reference preparation 
still can give widely varying measurements on patient speci­
mens. Consider the example of hCG in Table 4.1. The three 
assays are calibrated with a pure preparation of intact hCG, 
Unknown: 
mean concentration
and 

 of intact hCG, 
Unknown: 
and variance
LOD
Maximum
response
Maximum
measurable
100
0
Percent CV
LOD
MR
Unknown:
mean signal and
variance
Nonspecific
background signal
Response: Amount of labeled
detection antibody bound
• Fig. 4.6  Characteristics of the signal generated by an immunometric assay. Signal generated by the 
amount of detection antibody bound to the capture-analyte complex is directly proportional to the con­
centration of analyte in a

roportional to the con­
centration of analyte in an immunometric assay. The concentration can be extrapolated from response 
(signal measured) by measuring known concentrations of “calibrators.” As shown in the inset, the variance 
associated with measurement is heteroscedastic and increases significantly as the upper or lower limits of 
the assay are approached. CV, coefficient of variance; LOD, limit of detection; MR, maximum response. 
(From Sluss PM. Methodologies for measurement of cardiac 

luss PM. Methodologies for measurement of cardiac markers. Clin Lab Med. 2014;34:167–185.)
70
such as the WHO Third International Reference Preparation. 
The three assays differ in their cross-reactivity with free β-hCG 
(0%, 100%, and 200%, respectively). These assays give identi­
cal measurements for a specimen containing only intact hCG 
but progressively disparate values as the percentage of free 
β-hCG in the specimen increases. In reality, the stand

G in the specimen increases. In reality, the standardiza­
tion issue is much more complex because multiple forms of 
hormones (i.e., intact hormone, free subunits, nicked forms, 
glycosylated forms, degradation products) circulate in patients, 
and each assay has different cross-reactivity characteristics with 
respect to these forms.31–34
Because of their speed, specificity, and sensitivity, immu­
nometric assay designs have also been applied successfully to 
point-of-care testing devices. A ty

ccessfully to 
point-of-care testing devices. A typical design is shown in Fig. 
4.7. In this example, a lateral flow assay system is shown with 
two solid-phase monoclonal antibodies affixed to the flow 
device. One antibody is specific for the analyte, and the other, 
located on a different section of the analytic strip, is directed 
at the capture antibody itself. This strip contains a reservoir 
of detection antibody covalently coupled to gold micropar­
ticles. A drop of specimen (e.g., bloo

d micropar­
ticles. A drop of specimen (e.g., blood, serum, plasma, urine) 
is placed on one end of the strip and carried across the ana­
lytic strip by lateral flow, passing first through the detection 
antibody reservoir and then over the capture antibodies in 
sequence. The final state, as illustrated in Fig. 4.7, results in 
a band of gold particles over the capture antibody region that 
is proportional to the amount of analyte in the specimen and 
a positive control band of gold particles o

n and 
a positive control band of gold particles over the antidetec­
tion antibody region. Such tests are generally qualitative but 
with the use of a standardized meter for measuring the gold 
bands and calibrator can be quantitative. Increasingly, these 
systems are being miniaturized and the detection methods 
optimized to enable accurate quantitative measurement at the 
point of care and in other nonlaboratory settings (e.g., field 
testing and low-resource settings). Although still primaril

nd low-resource settings). Although still primarily 
research tools, similar technologies are utilized in developing 
multiarray assays (e.g., “lab on a chip” assays) that will likely 
become part of the clinical laboratory’s repertoire of tools in 
the future.19,24,25,35,36 
Molecular Structure–Based Methods
Extraction Methods
Extraction of hormones from serum and urine specimens before 
measurement is a technique used to enhance both the sensitivity 
and the specificity of immunoassays or mass

ivity 
and the specificity of immunoassays or mass spectrometry–based 
assays. Mass spectrometry–based assays are generally of lim­
ited sensitivity compared with immunoassay. Their application 
in endocrine testing relies on relatively large specimen volumes 
from which analyte can be concentrated by extraction. Extraction, 
especially in combination with chromatography, also results in the 
removal of interfering substances, which can produce inaccurate 
measurements in either immunoassays or 

naccurate 
measurements in either immunoassays or mass spectrometry–
based assays.
Generally, extraction procedures required for the measure­
ment of serum steroids are based on the polarity or water solubil­
ity of the analytes. Extraction methods for proteins/peptides can 
be based on molecular size as well as polarity. It is essential in 
any extraction method that recovery (i.e., the amount of analyte 
extracted) is consistent across all specimens or adjusted quantita­
tively for each specim

imens or adjusted quantita­
tively for each specimen tested. If the extraction recovery is less 
than 100% but consistent, the method will produce biased yet 
usable, albeit method-specific, results. If the recovery is different 
among specimens and cannot be corrected by monitoring, the 
assay is not valid.
Numerous extraction systems have been developed, including 
organic-aqueous partitioning to remove water-soluble interfer­
ences seen with steroids, solid-phase extraction with absorption, 

teroids, solid-phase extraction with absorption, 
and selective elution from resins such as silica gels and immu­
noaffinity chromatography. Early immunoassays for steroids 
relied heavily on extraction prior to assay and provided a basis for 
assessing interference in subsequent direct assays.37,38 However, 
extraction before immunoassay is seldom used in clinical assays 
today. Extraction techniques are difficult to automate, require 
  Effect of Immunoassay Specificity 
on Calibration of Hum

 of Immunoassay Specificity 
on Calibration of Human Chorionic 
Gonadotropin Assays
hCG Sample
Assay 1
Assay 2
Assay 3
Specificity for intact hCG standard (%)
100
100
100
Cross-reactivity with free β-hCG (%)
0
100
200
Measured values (IU/L)
  Specimen with 0% free β-hCG
10.0
10.0
10.0
  Specimen with 10% free β-hCG
9.0
10.0
11.0
  Specimen with 50% free β-hCG
5.0
10.0
15.0
hCG, Human chorionic gonadotropin.
TABLE 4.1
Analyte [specimen]
Positive control captures
unbound detection Ab.
Intense band

ontrol captures
Intense band  reportable result 
Band intensity proportional to
analyte concentration  
• Fig. 4.7  Laminar flow immunometric assay design. See discussion in text. Ab, antibody. (From Sluss PM. 
71
skills and equipment not available in many clinical laboratories, 
and generally require correction based on 

ories, 
and generally require correction based on measuring recovery. 
Monitoring recovery in automated immunoassays is very difficult 
and creates issues with regulatory compliance (i.e., modification 
of manufacturer methods). In contrast, extraction methods are 
a key element in preanalytic processing for mass spectrometry–
based assays in which it is possible to measure recovery using an 
internal standard added to every specimen being tested. Extrac­
tion can also be applied to the measurem

. Extrac­
tion can also be applied to the measurement of proteins/peptides. 
Most current mass spectrometry assays for steroids involve depro­
teinization of the specimen (extraction of the steroids) prior to 
further extraction/purification preanalytically. Similarly, mass 
spectrometry–based assay of proteins/peptides generally utilizes a 
batch extraction based on molecular size or polarity. A good deal 
of progress has been made in developing preanalytic extraction 
methods prior to assay.39

 preanalytic extraction 
methods prior to assay.39–50 
Chromatographic Systems
The second major method of measuring hormone concentrations 
involves chromatographic separation of the various biochemical 
forms and quantitation of specific characteristics of the molecules. 
High-performance liquid chromatography (HPLC) systems use 
multiple forms of detection, including light absorption, fluo­
rescence, and electrochemical properties.51–53 Chromatography 
also is frequently combined with mass spe

ography 
also is frequently combined with mass spectrometry. There are 
two major advantages of these techniques: They can be used to 
simultaneously measure multiple forms of an analyte, and they are 
not dependent on unique immunologic reagents. Therefore har­
monization of measurements made with different assays is more 
feasible. The major disadvantages of these methods are their com­
plexity and their limited availability.
Many chemical separation techniques are based on chromatog­
raphy, b

ration techniques are based on chromatog­
raphy, but the two most commonly used for liquid chromatogra­
phy are normal-phase HPLC and reverse-phase HPLC. In both 
systems, a bonded solid-phase column is made that interacts with 
the analytes as they flow through in a liquid solvent. In normal-
phase HPLC, the functional groups of the stationary phase are 
polar (e.g., amino or nitrile ions) relative to the nonpolar mobile 
phase (e.g., hexane); in reverse-phase HPLC, a nonpolar station­
ary phas

n reverse-phase HPLC, a nonpolar station­
ary phase (e.g., C18 octadecylsilane molecules bonded to silica) is 
used. Polymeric packings made of mixed copolymers have been 
made with C4, C8, and C18 functional groups directly incor­
porated so that they are more stable over a wide pH range. The 
mobile and stationary phases are selected to optimize adherence of 
the analytes to the stationary phase. The adhered molecules can be 
eluted differentially from the solid phase, after washing to separat

lly from the solid phase, after washing to separate 
specific forms of the analyte from interfering substances. The elu­
tion is called an isocratic elution if the composition of the mobile 
phase remains constant throughout the run. If the mobile-phase 
composition is abruptly changed, then it is a step elution. If the 
composition is gradually changed throughout the run, then a gra­
dient elution occurs.
The efficiency of separation in a chromatography system is a 
function of the flow rates o

tography system is a 
function of the flow rates of the different substances. The resolu­
tion of the system is a measure of the separation of the two solute 
bands in terms of their relative retention volumes (Vf) and their 
bandwidths (ω). Resolution (Rs) of solutes A and B is calculated 
as follows:
RS = 2 [Vf (B) −Vf (A)]
ω (A) + ω (B)
Values of Rs less than 0.8 result in inadequate separation, and 
values greater than 1.25 correspond to baseline separation. The 
resolution of a chromatograp

line separation. The 
resolution of a chromatography column is a function of flow rates 
and thermodynamic factors.
HPLC remains the method of choice for clinical measurements 
of catecholamines in biologic fluids.54,55 Simultaneous measure­
ment of the three catecholamines (epinephrine, norepinephrine, 
and dopamine) can be obtained. Prior extraction by absorption 
on activated alumina and acid elution helps to improve specific­
ity. Dihydroxybenzylamine, a molecule similar to endogenous cat­
e

nzylamine, a molecule similar to endogenous cat­
echolamines, can be used as an internal standard. 
Mass Spectrometry
Mass spectrometry depends on the movement of charged particles 
through a magnetic field to separate and quantify them on the 
basis of their mass, or more rigorously their mass-to-charge ratio 
(m/z).56 A mass spectrometer is an instrument designed to ion­
ize analytes, accelerate them into a device (mass analyzer) that 
separates them based on their m/z, and quantifies their re

s them based on their m/z, and quantifies their relative 
abundance. Fig. 4.8 illustrates the components and principles of 
a generic mass spectrometer. The heart of the system is the mass 
analyzer, which utilizes adjustable magnetic fields to accelerate or 
deflect volatile (e.g., in gaseous form) ions, typically in a vacuum 
so that the ion’s flight path is determined only by the magnetic 
field. A source is used to ionize and, if necessary, volatilize and 
fragment analytes to introduce them

olatilize and 
fragment analytes to introduce them into the mass analyzer. Ana­
lytes are introduced into the source via an inlet that can be as 
Specimen entry;
ionizable form
Data
System
Detector
Inlet
Vacuum System
Mass Analyzer
Source
Fragmentation: ionization
• Whole molecule [minimal ionization]
• Components [total ionization]
Separate ionized fragments by size [m/z]:
based upon movement of charged
particles in magnetic field
Computer Software:
• System control
• Data analysis
• Reporting

are:
Count charged
particles: photoplate
and photomultiplier
• Fig. 4.8  Components and principles of a generic mass spectrometer. See discussion in text. m/z, mass-
to-charge ratio.
72
simple as an injection port or as sophisticated as a laser-driven 
matrix desorption system or photoionization chamber. Charged 
particles passing through the mass analyzer are counted by a 
simple Faraday plate detector, which 

ounted by a 
simple Faraday plate detector, which generates an electric cur­
rent proportional in intensity to the frequency (abundance) of 
ions striking the detector. Mass spectrometers used in endocrine 
clinical testing are quite complex, with analytes being delivered 
to the inlet via a chromatography system, subtle selection of ions 
with m/z characteristics unique to the analyte, and measurement 
based on system calibration and recovery of internal standards. 
All of these aspects are con

 internal standards. 
All of these aspects are controlled by the data system (computer 
software), which also generates data outputs that comply with 
clinical reporting requirements and increasingly can be integrated 
into fully electronic laboratory and medical record systems. These 
components are discussed in more detail later.
Analytic mass spectrometry was developed in tandem with 
the discovery and characterization of endocrine steroids during 
the 1930s, 1940s, and 1950s.57 The source fo

ring 
the 1930s, 1940s, and 1950s.57 The source for these instruments 
ionized the analyte by electron impact (i.e., by bombarding gas 
molecules from the sample with electrons emitted from a heated 
filament), creating a full fragmentation of the analyte and multiple 
charged particles of each of the composite atoms. By determin­
ing the relative abundance and mass of each ionized particle, the 
molecular structure of the steroid could be constructed. However, 
the methodology requires that the

ucted. However, 
the methodology requires that the steroids, indeed any analyte, 
be purified and volatilized prior to fragmentation and ionization 
in the source. Most steroid hormones are easily heat damaged 
and must be derivatized with molecules that can be volatilized 
and ionized before mass analysis. This methodology was used in 
strictly research applications in which it was invaluable in delin­
eating the physiology of reproductive steroids. The development 
of gas chromatography, in co

ids. The development 
of gas chromatography, in conjunction with electron impact mass 
analyzers (GC/MS), led to the clinical use of mass spectrometry, 
which was applied first to endocrine steroids and subsequently 
to other small biologically important molecules. GC/MS, using 
quadrupole analyzers in scanning mode (see later), remains a key 
technology in the research laboratory today and arguably is the 
method of choice for the study of steroid hormone metabolites.58 
GC/MS was replaced for 

id hormone metabolites.58 
GC/MS was replaced for clinical endocrine steroid testing by the 
cheaper, higher throughput antibody-based assays, which remain 
the primary method in all but a few very large academic hospi­
tal or reference laboratories. However, dramatic advances in mass 
spectrometry design have led to the availability of instruments 
that are rapidly replacing many antibody-based assays, especially 
competitive immunoassays, in clinical laboratories.
The technologic advances lead

inical laboratories.
The technologic advances leading up to modern mass spec­
trometers involve primarily the source and the mass analyzer com­
ponents. The most dramatic advance in sources, with respect to 
clinical applications of mass spectrometry in endocrine testing, 
was the development of electrospray ionization (ESI).59,60 This 
technology underlies the direct connection of liquid chromatog­
raphy systems to mass spectrometry and is currently the method 
of choice for measuring analytes 

ntly the method 
of choice for measuring analytes relevant to endocrinology, such 
as steroid hormones, in biologic fluids. The principle of ESI is 
illustrated in Fig. 4.9. The specimen to be measured in the effluent 
from a chromatography system is pushed at low speed through a 
capillary column into the source region of the spectrometer to cre­
ate an aerosol when high voltage (positive or negative) is applied 
to the tube in the presence of a nebulizing gas. As the droplets in 
the aerosol d

 nebulizing gas. As the droplets in 
the aerosol dry, often with the aid of a drying gas, the molecules 
in the specimen become charged and volatilized. In gaseous form, 
these molecules then enter the mass analyzer.
ESI has directly resulted in the development of systems that 
are rapidly replacing the antibody-based assays in the clinical 
laboratory. The most obvious advantage is that liquid chroma­
tography systems can be coupled directly to the mass spectrom­
eter. This system allows the ex

id chromatography systems can be supported 
by one mass spectrometer. Thus the technical advantages of the 
mass spectrometer (measurements based directly on the molecular 
composition of the analyte rather than indirect competition of the 
analyte for antibody binding) can be realized in the practical set­
ting of clinical testing services.
The ionization achieved by ESI is also an important technologic 
advance. Although the exact mechanisms are still unclear, ESI is 
characterized by ionizati

e still unclear, ESI is 
characterized by ionization at low temperatures and pressures and 
results in relatively little fragmentation of the analyte so that a 
molecular ion is always generated. This procedure has allowed the 
development of exquisitely specific methods. Most significant with 
respect to current clinical testing is tandem mass spectrometry, espe­
cially triple quadrupole mass analyzers linked together.
The design of a quadrupole mass analyzer is illustrated in Fig. 
4.10. The a

 mass analyzer is illustrated in Fig. 
4.10. The analyzer is composed of four round electrodes. Voltage 
of the same polarity is applied to directly opposite electrodes, and 
opposite voltage polarity is applied to adjacent ones. An oscillating 
electric field is generated within the quadrupole when an alternat­
ing current (voltage V, frequency ω, and time t) is applied with a 
superimposed direct current (voltage U). Thus charged particles 
(ions) moving through the quadrupole follow oscillati

ns) moving through the quadrupole follow oscillating paths 
and only ions with a specific m/z can pass through to the down­
stream detector. Ions with greater or lesser m/z collide with the 
electrodes and are not detected. By controlling the applied volt­
ages, the analyzer can be operated to select ions of specific m/z for 
detection (or transit). Because ions are moving rapidly, and volt­
age can be controlled rapidly, the analysis time is very short. The 
analyzer thus can be operated in thr

y short. The 
analyzer thus can be operated in three distinct modes to (1) filter 
ions for the quantitation of only one m/z, (2) scan to sequentially 
quantify all ions by m/z, or (3) trap ions within the quadrupole.
Combining three quadrupole analyzers results in a very pow­
erful system (Fig. 4.11), often referred to as the triple quad mass 
spectrometer and often in the clinical literature as simply LC/
MS-MS or LC/tandem MS. The molecular ions generated by ESI 
can be filtered by the first 

ns generated by ESI 
can be filtered by the first mass analyzer (quadrupole) to capture, 
in a second mass analyzer, a molecular ion whose m/z is consis­
tent with that of the target analyte. The captured molecular ion 
is fragmented and ionized in the second analyzer, which becomes 
the source for the third analyzer that either analyzes all the frag­
ments or selects one that is unique to the parent ion. By operat­
ing the first and second quadrupoles in various modes, different 
analytic goals

upoles in various modes, different 
analytic goals can be achieved. The primary approaches used for 
endocrine testing are multiple reaction monitoring and product 
ion scanning.
Multiple reaction monitoring mode allows both analytic 
analyzers (quadrupoles 1 and 3) to be fixed, selecting for a spe­
cific m/z. This adjustment increases specificity and sensitivity. 
This mode is used to monitor specific analytes and to confirm 
unambiguously the presence of a compound in a matrix. For 
example, t

resence of a compound in a matrix. For 
example, two unique ions (first/second analyzer) for testoster­
one are 289.221/97.140 and 289.222/109.130. Because steroid 
hormones have well-known and unique elution times from LC 
systems, this mode is also widely used for steroid profiling, as 
illustrated in Fig. 4.12.
Product ion scanning allows a parent or precursor ion to be 
selected in quadrupole 1, and the scan in quadrupole 3 measures 
all the product ions resulting from fragmentation of that 

product ions resulting from fragmentation of that ion. This 
is a particularly useful method of operation for providing struc­
tural information concerning small organic molecules or for gen­
erating peptide sequence information.
Mass analyzer design continues to evolve rapidly. Another system 
that deserves consideration with respect to endocrine clinical testing 
is time-of-flight mass spectrometers (TOFMSs). As illustrated in Fig. 
4.13, TOFMSs are simple, albeit more highly engineered, instr

s are simple, albeit more highly engineered, instru­
ments designed to determine m/z based on the time required to tra­
verse a vacuum tube. The source is designed to align and accelerate 
ions after ionization so that they all enter the vacuum tube at the same 
time. The time required to traverse the vacuum tube is proportional 
to m/z (more precisely, the square root of the m/z); smaller or more 
highly charged ions will move faster to the detector. Most modern 
TOFMSs have electronic reflecto

ange 
of quadrupole analyzers. The range of TOFMS includes DNA frag­
ments and even whole microorganisms. Another particular advantage 
of TOFMSs is their compatibility with pulse ionization sources such 
as matric-assisted laser desorption/ionization (MALDI) or surface-
enhanced laser desorption/ionization (SELDI) systems. Since the 
introduction of TOF spectrometry in the mid-1980s, a wide range 
of combination analyzers such as MALDI-TOF or SELDI-TOF 
Argon Gas
Filter
Filter
Filter
Scan
Produ

LDI-TOF 
Argon Gas
Filter
Filter
Filter
Scan
Product Ion Scanning
Multiple Reaction Monitoring
Quadrupole 1 
Quadrupole 2 
trap/collision cell 
Quadrupole 3 
• Fig. 4.11  Design of a triple quadrupole mass analyzer and modes of operation useful in endocrine testing. 
See text discussion.
7.46
9.56
11.08
11.07
7.48
7.43
13.22
13.98
13.99
16.85
12.67
11.88
DHEAS
DHEA
Cortisol
11-Deoxycortisol
17-Hydroxyprogesterone
Androstenedione
Estradiol
Time (min)
Percent
0
100
0
100

radiol
Time (min)
Percent
0
100
0
100
0
100
0
100
0
100
0
100
0
100
0
100
0
100
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
• Fig. 4.12  Liquid chromatography–tandem mass spectroscopy profiles of nine steroids. DHEA, dehydro­
epiandrosterone; DHEAS, dehydroepiandrosterone 3-sulfate.
75
instruments have been developed and used in genomic, proteomic, 
and metabolomics studies as well as applied to biomarker disco

mics studies as well as applied to biomarker discovery 
and microorganism identification.61–66
Translating the progress achieved by research applications 
of TOFMS into clinical diagnostics is still limited to the iden­
tification of microorganisms, mostly bacteria and fungi based 
on spectral libraries. Improved preanalytic, analytic, and post­
analytic processes, including standardization, improved clinical 
sensitivity without loss of specificity for microorganism iden­
tification, and qualit

ity for microorganism iden­
tification, and qualitative measurement of antibiotic sensitivity 
remain challenges to wider clinical applications.67–71 In the near 
future, TOFMS is likely to find increasing applications in endo­
crine clinical laboratories. For example, current antibody-based 
methods for proteins such as TSH, prolactin, and thyroglob­
ulin are often inaccurate in the presence of endogenous anti­
body. Mass spectrometry, particularly the MALDI-TOF given 
its ability to measure la

rly the MALDI-TOF given 
its ability to measure large proteins, is an attractive approach to 
addressing this issue. 
Free Hormone Methods
The design of assays, either antibody or mass spectrometry 
based, to measure steroid hormones and sterols (such as vitamin 
D) present special issues that warrant discussion. These analytes, 
which for the sake of simplicity are discussed as steroid hor­
mones, are extremely hydrophobic. In aqueous environments, 
particularly blood and blood-derived specimen

ts, 
particularly blood and blood-derived specimens in which mea­
surement is intended, steroid hormones are associated with 
hydrophobic regions of proteins or bound tightly to high-
affinity, specific transport proteins. The former includes albu­
min, prealbumin, transthyretin, and apolipoproteins, among 
others, and the latter includes specific transport proteins listed 
in Table 4.2. Less than 5% to 10% of most steroid hormones 
circulate as free (unbound) analytes, and assay design requires

free (unbound) analytes, and assay design requires 
that the protein-bound analyte be released or does not inter­
fere in the assay in order to have an accurate measure of the 
total hormone present. Although not universally applicable, in 
many cases the physiologic effects of steroid hormones depend 
on the free hormone concentration rather than the total hor­
mone concentration. Of course, under normal conditions the 
free and total hormone concentrations are directly related. This 
concept, 

ncentrations are directly related. This 
concept, known as the free hormone hypothesis, is the basis for 
the design of methods specifically intended to measure only the 
free hormone levels.72–77 The free hormone hypothesis itself is 
controversial, and a critical discussion of it is beyond the scope 
of this chapter; however, the reader is directed to specific appli­
cations in the clinical chapters of this text. Here it is hoped that 
a technology-based discussion will give the reader an appr

logy-based discussion will give the reader an apprecia­
tion of the various methods that have been and are currently 
used to measure free hormones.
There are two basic types of assay designs for measuring free 
hormones: (1) assays based on the physical separation of bound 
and free hormones prior to measurement and (2) antibody-based 
binding assays designed to measure only the free hormones.
Fig. 4.14 illustrates the design of assays based on physically sep­
arating bound from free steroid ho

physically sep­
arating bound from free steroid hormones. Classically, a dialysis 
Source/acceleration voltage
Reflector
Detector
Flight path
Vacuum tubes; no electrical field
Ion speed is inversely
proportional to m/z 
• Fig. 4.13  Design of a time-of-flight mass spectrometer. m/z, mass-to-
charge ratio.
  Circulating High-Affinity Protein Carriers of Steroid Hormones
Protein Carrier
Primary Ligand(s)
Note
Corticosteroid-binding globulin (CBG)
Glucocorticoids, mineralocorticoids
Also binds cel

Also binds cell membranes
Sex hormone–binding globulin
Dihydrotestosterone, testosterone, estradiol
Thyroxine-binding globulin (TBG)
Thyroxine (T4), triiodothyronine (T3)
Vitamin A–binding protein(s)
Vitamin A (retinol)
Vitamin D–binding protein
25(OH) vitamin D2, 25(OH) vitamin D3, 1,25(OH) vitamin D2, 1,25 vitamin D3
TABLE 4.2
Specimen: binding protein and steroid
Dialysis membrane or ultrafiltration filter

teroid
“Free hormone” in physiologic buffer
• Fig. 4.14  Free hormone assay design: Physical separation of free hor­
mone. Dialysis membranes or ultrafiltration allow the separation of free 
hormone from protein-bound hormone prior to measurement of free hor­
mone directly or by determining the percent distribution of labeled hor­
mone added to the specimen before processing.
76
membrane was used to separate two

d Hormone Action
membrane was used to separate two fluid-filled chambers (e.g., 
tubes). The pore size of the dialysis membrane is specific to the 
analyte/binding proteins but in principle allows free movement of 
free steroid hormones while retaining the higher molecular-weight 
binding proteins and conjugated binding protein-steroid hormone 
complexes. Thus by placing the specimen in one chamber (top in 
Fig. 4.14) and matrix-appropriate buffer in the other and allow­
ing the diffusion of fre

r in the other and allow­
ing the diffusion of free hormone to equilibrate, the free hormone 
can be measured directly by this equilibrium dialysis approach. 
Subsequent variations on the method include using an ultrafiltra­
tion membrane to allow faster (e.g., no need to wait for an equi­
librium to be established) separation of bound from free steroid 
hormone (as illustrated in Fig. 4.14) or to chemically separate the 
high-molecular-weight bound hormone from the free hormone 
(e.g., precipit

und hormone from the free hormone 
(e.g., precipitation of sex hormone–binding globulin–bound ste­
roid using ammonium sulfate). The biggest challenge associated 
with this approach, regardless of how separating bound from free 
steroid hormone was achieved, is the measurement of the very low 
concentrations of free steroid hormone after separation (e.g., in 
the dialysate or lower chamber in Fig. 4.14). Thus a variation on 
the equilibrium dialysis design is to add labeled steroid hormone 
to t

sis design is to add labeled steroid hormone 
to the specimen prior to dialysis. High specific activity labels, such 
as radioisotopes, allow the detection of trace amounts of free hor­
mone after dialysis. It is then possible to use the percentage of free 
hormone based on the distribution of labeled hormone to calcu­
late the mass of free hormone from a direct measurement of total 
hormone by traditional methods.
One might easily get the misimpression, especially now that 
LC/MS-MS systems wit

ression, especially now that 
LC/MS-MS systems with sufficiently high sensitivity have been 
combined with it, that equilibrium dialysis is the method of choice 
or a gold standard method for measuring free hormones.57,78,79 
However, it must be emphasized that currently there is no estab­
lished reference method for the measurement of free steroid hor­
mones and that most separation methods, including equilibrium 
dialysis, have not been applied in a fashion that is necessarily valid 
or direct

 in a fashion that is necessarily valid 
or directly applicable to in vivo conditions.73,77,80,81
Antibody-based binding assays designed to measure only the 
free hormone can be divided into two classes: (1) two-step assays 
and (2) one-step assays.
The two-step immunoassay relies on labeled steroid and is illus­
trated in Fig. 4.15. Solid-phase antibody is used to capture the free 
hormone present in the specimen. The amount, if any, of bound 
hormone capture will depend on the relative affinit

ormone capture will depend on the relative affinity of the steroid 
for the antibody versus the binding protein. If the antibody affin­
ity is much higher, the bound steroid will be stripped from the 
binding protein. If the antibody affinity is relatively low compared 
to the binding protein, only free hormone will be bound. In either 
case, after washing the solid-phase antibody, unoccupied anti­
body-binding sites are titrated using labeled steroid, which after 
a second wash step can be quan

eroid, which after 
a second wash step can be quantified. The signal generated by the 
captured labeled steroid is inversely proportional to the amount of 
free hormone in the specimens. It is important to note that free in 
this assay format is defined by the relative affinity of the antibody 
used and the endogenous steroid binding proteins.
One-step immunoassays are designed using either labeled ste­
roid or labeled antibody. The basic formats are illustrated in Fig. 
4.16. These assays are f

 are illustrated in Fig. 
4.16. These assays are fast and easy to perform given their rela­
tively simple format, which is also quite amenable to automation. 
The use of a labeled steroid analogue is summarized in Fig. 4.16A. 
The labeled analogue is not recognized by the binding protein but 
is able to compete with free hormone for antibody-binding sites 
on the solid phase. This type of assay depends on the validity of 
the assumption that the signal generated, which is inversely pro­
portiona

signal generated, which is inversely pro­
portional to the concentration of free steroid in the specimen, is 
solely due to the competition with free hormone. This has been 
shown not to be true for free testosterone assays and is likely valid 
only over a limited range for binding protein concentrations for 
free thyroxine assays.77,82–87 An alternative approach is shown in 
Fig. 4.16B in which a labeled antibody is employed in an assay 
based upon its binding to solid-phase antigen (e.g., the 

pon its binding to solid-phase antigen (e.g., the analyte of 
interest). In this design, the signal generated reflects the amount 
of labeled antibody bound to the solid phase which, after reach­
ing a steady state, is inversely proportional to the concentration 
of free steroid hormone in the specimen. The advantage of this 
newer approach is that a relatively higher signal is measured (e.g., 
improved sensitivity and precision), and it is not necessary to alter 
the structure of the steroid (o

ecessary to alter 
the structure of the steroid (other than that which may be associ­
ated with attachment to the solid phase).
It is important to recognize that for free steroid assay designs, 
the kinetics of competition and binding are very complex given 
the variety of proteins interacting with steroid hormones over a 
wide range of affinities. Specimens with low concentrations of 
binding proteins, including low-affinity but high-capacity bind­
ers such as albumin, are particularly challeng

nd­
ers such as albumin, are particularly challenging. As is true for 
any assay, all of these free hormone measurement methods require 
careful validation and method-specific reference intervals to be 
clinically useful.73,74,80,81,88,89 
Nucleic Acid–Based Methods
Nucleic acid–based assays are designed to identify variations in 
an individual’s DNA or RNA sequence that reflect molecular 
variance (e.g., mutations, rearrangements) that alters gene expres­
sion, regulatory pathways, and bioactiv

ne expres­
sion, regulatory pathways, and bioactive molecules in a fashion 
relevant to human disease (i.e., early diagnosis or increased disease 
Specimen:
 
free steroid [       ]
 
bound steroid [         ]
+ Antibody [       ]
Labeled steroid
Label detected
inversely proportional
to steroid in specimen
• Fig. 4.15  Free hormone assay design: Solid-phase, indirect immunoas­
say. Excess solid-phase antibody binds the free steroid hormone during 
step 1 of this method. After washing, incubation

step 1 of this method. After washing, incubation with labeled steroid hor­
mone (step 2) allows unbound antibody sites to be titrated. After a second 
wash, the amount of labeled steroid bound to the solid-phase antibody 
is inversely proportional to the amount of free hormone in the specimen.
77
susceptibility). Genetic variance results in a range of alterations 
from whole chromosome effects visible by karyotyping/cytoge

e chromosome effects visible by karyotyping/cytogenet­
ics to point mutations leading ultimately to changes in protein 
expression or functionality. As small molecules such as steroid hor­
mones depend on protein enzymes, genetic alterations may affect 
all aspects of endocrine function and hence are important analytic 
targets. A plethora of analytic methods exist for the analysis of 
nucleic acids.
Discussion of the full range of methods is beyond the scope of 
this review, but these methods c

ond the scope of 
this review, but these methods can be grouped into three major 
categories: (1) chromosome visualization methods, with or with­
out application of sequence-selective enzymatic fragmentation; 
(2) assays based on binding of labeled nucleic acid probes, which 
obey Watson and Crick base-pairing rules and thus are sequence 
specific; and (3) direct sequencing of DNA or RNA. Methods 
in categories 2 and 3 are generally combined with methods for 
the amplification or selective enric

 methods for 
the amplification or selective enrichment of target sequences, but 
methods in category 1 generally depend on microscopy (whole 
chromosome analysis), fragmentation, gel electrophoresis, and 
blotting techniques (e.g., Southern blotting for DNA or North­
ern blotting for RNA). Thus the key elements to appreciate are 
hybridization, restriction enzyme fragmentation, electrophoretic 
separation, amplification, and nucleic acid sequencing. Methods 
composed of various combinations of 

ing. Methods 
composed of various combinations of these principles are avail­
able for scanning DNA sequences for new variants, scoring DNA 
sequences for known variants, and expression analysis of DNA or 
RNA target sequences. Direct sequencing effectively scores known 
variants as well as identifies new variants.
Currently, molecular methods being utilized by clinical labo­
ratories are primarily for well-known inherited diseases, cancer 
diagnostics and management, and increasingly in infecti

ostics and management, and increasingly in infectious dis­
ease applications. Based on developments in research laboratories, 
particularly with respect to next generation sequencing (NGS), 
these methods are likely to affect clinical endocrine testing in the 
very near future.90–92 The majority of nucleic acid–based assays 
are laboratory-developed methods applied in research settings or 
highly specialized clinical reference laboratories. However, this 
picture is rapidly changing as devices s

er, this 
picture is rapidly changing as devices suitable for use in hospital 
clinical laboratories are increasingly becoming available.
Hybridization Assays (Mutation Assays, Genotyping)
Nucleic acid molecules have a unique ability to bind with high 
affinity to complementary base-pair sequences. When a fragment 
of a known sequence (probe) is mixed under specific conditions 
with a specimen containing a complementary sequence, hybrid­
ization occurs. This feature is analogous to the antibody-

occurs. This feature is analogous to the antibody-antigen 
binding used in immunoassays. Many of the formats used for 
immunoassay have been adapted to nucleic acid assays, including 
some of the same signal systems (e.g., radioactivity, fluorescence, 
chemiluminescence) and the same solid-phase capture systems 
(e.g., magnetic beads, biotin-streptavidin binding). In situ hybrid­
ization, which involves the binding of probes to intact tissue and 
cells, provides information about morphologic loc

cells, provides information about morphologic localization analo­
gous to that provided by immunohistochemistry. Combining 
hybridization methods with enzymatic procedures to amplify, 
extend, and ligate DNA targets or probes greatly enhances the 
analytic sensitivity and specificity of hybridization-based meth­
ods. Hybridization methods, like other binding assays, are quite 
amenable to automation and incorporation into relatively simple 
devices suitable for clinical laboratory utilization. 

ces suitable for clinical laboratory utilization. 

Labeled Analogue Assay
Labeled Antibody Assay
A
B
• Fig. 4.16  Free hormone assay design: One-step immunoassays. (A) A labeled analogue steroid hor­
mone that binds to antibody but not to binding proteins is used in a classic competitive binding format to 
measure only the free hormone in the specimen. (B) Labeled antibody is used in a single-step competitive 
assay in which free steroid hormone in the specimen competes with solid-phase steroi

e in the specimen competes with solid-phase steroid hormone for 
antibody binding. The amount of labeled antibody bound to the solid phase after washing is inversely pro­
portional to the amount of free steroid hormone in the specimen. Red indicates labeled analogue steroid 
hormone in (A) and labeled antibody in (B).
78
Restriction Fragmentation
DNA restriction enzymes break DNA strands at specific sites 
based on the nucleic acid sequence. Thus digestio

based on the nucleic acid sequence. Thus digestion with a given 
restriction enzyme or combination of restriction enzymes will pro­
duce fragments of different lengths that are directly related to the 
DNA sequence. Mutations that alter the sequence of the enzyme 
cleavage site(s) will result in altered fragment size patterns, referred 
to as restriction fragment length polymorphisms (RFLPs), which 
can be visualized after fragment separation by gel electrophoresis 
or other separation methods.

 gel electrophoresis 
or other separation methods. For known mutations the affected 
DNA sequence can be amplified (see later) prior to RFLP analysis 
(or by single-nucleotide extension if the mutation does not alter a 
restriction enzyme cleavage site). A large number of online tools 
are available to support researchers designing methods involving 
the use of restriction enzymes.93,94 These tools can be useful in 
designing validation studies of commercially available assays for 
molecular var

f commercially available assays for 
molecular variance. 
Electrophoretic Separation
E.M. Southern invented an electrophoretic separation technique 
known as Southern blotting.77 Restriction enzymes are used to 
digest a sample of DNA into fragments, and the product is sub­
jected to electrophoresis. The separated bands of DNA are then 
transferred to a solid support and hybridized. Northern blotting is 
a similar technique in which RNA is used as the starting material. 
Western blotting refers 

s the starting material. 
Western blotting refers to electrophoresis and transfer of proteins. 
Currently a wide range of methods for electrophoretic separation 
and blotting of DNA, RNA, and proteins is available and incor­
porated into clinically relevant methods. All are relatively complex 
laboratory-developed methods. 
Amplification
Nucleic acid assays have an advantage in that low concentrations 
can be amplified in  vitro before quantitation. The best-known 
amplification procedure is the

on. The best-known 
amplification procedure is the polymerase chain reaction (PCR). 
The three steps in the process (denaturation, annealing, and elon­
gation) occur rapidly at different temperatures. Each cycle of 
amplification can occur in less than 90 seconds by cycling the tem­
perature. The target double-stranded DNA is denatured at high 
temperature to make two single-stranded DNA fragments. Oli­
gonucleotide primers, which are specific for the target region, are 
annealed to the DNA when

r the target region, are 
annealed to the DNA when the temperature is lowered. Addition 
of DNA polymerase allows the primer DNA to extend across the 
amplification region, thus doubling the number of DNA copies.
At 85% to 90% efficiency, this process can amplify the DNA 
by about 250,000-fold in 20 cycles. This huge amplification is 
subject to major problems with contamination if special precau­
tions are not taken. 
Sequencing Methods
Traditionally, sequencing was performed using DNA polymera

nally, sequencing was performed using DNA polymerase 
to selectively incorporate dideoxynucleotides (causing chain ter­
mination) during in vitro DNA replication. This method, which 
was developed by Sanger and is now referred to as Sanger sequenc­
ing, remains the gold standard.95,96 Although this method, which 
was used in the first sequencing of the human genome, is straight­
forward and reliable, it is primarily used in directed sequencing of 
relatively small lengths of DNA.
Next generation

Next generation sequencing is a very different approach to 
sequencing and refers to a wide array of applications, including 
whole genome sequencing, exon sequencing, DNA-protein inter­
action assays, and RNA sequencing.90,97–99 NGS methods, also 
referred to as high-throughput sequencing, all involve simultane­
ous identification (i.e., parallel sequencing) of nucleic acids released 
from DNA or RNA fragments rather than identification of nucleic 
acids relea

rather than identification of nucleic 
acids released/identified in a serial fashion as is done in the earlier 
procedures (e.g., Sanger sequencing). NGS approaches hold tre­
mendous clinical diagnostic potential because parallel sequencing is 
faster and orders of magnitude cheaper than serial sequencing.100,101
Methods encompassed in NGS are evolving very rapidly but 
currently include massively parallel signature sequencing, polony 
sequencing, pyrosequencing, dye sequencing (Illumina), and 

, pyrosequencing, dye sequencing (Illumina), and 
sequencing by ligation (Applied Biosystems) as well as a plethora 
of newer methods.102–107 Currently the focus of new sequencing 
approaches is to develop methods capable of sequencing larger, 
more complex (e.g., high redundancy sequence) fragments of 
DNA. In particular, complex, repetitive, megabase DNA regions 
are subject to technical and computational limitations to accu­
rately assessing sequence variance.102,108,109
NGS is still primaril

equence variance.102,108,109
NGS is still primarily a research tool providing unique infor­
mation concerning the mechanisms of disease. NGS has already 
provided valuable insight into endocrine diseases by identifying 
sequence variants associated with (1) endocrine tumors, (2) dis­
orders of bone and mineral metabolism, (3) adrenal disorders, (4) 
gonadal disorders, (5) pituitary and hypothalamic diseases, (6) 
thyroid disorders, (7) sex development disorders, and (8) multiple 
endocrinopathy 

pment disorders, and (8) multiple 
endocrinopathy syndromes.109–119 The clinical value of NGS is 
clear, and its clinical use is a potential game-changer as it allows 
diagnosis of disease at a personal rather than a population level 
and can contribute to prognosis and/or suggest the most effective 
interventions for individual patients.104,120,121 However, to truly 
achieve clinical value, NGS-based diagnostics must be fully vali­
dated, standardized, cost effective, provide for secure data st

rdized, cost effective, provide for secure data storage, 
produce actionable results, and be compliant with ethical stan­
dards of patient care.101,122–130 
Analytic Validation
In this section, the basic elements of method validation are out­
lined and are applicable to any quantitative assay method discussed 
in the methods sections earlier. It is only the degree to which the 
parameters are determined and the frequency with which they are 
verified that vary from method to method or as a funct

fied that vary from method to method or as a function of assay 
class. Clearly, to be clinically valuable an analytic method must be 
valid; that is, the results or measurements generated are accurate 
and reproducible within the context of use (i.e., specified con­
centration limits, specimen types, clinical settings). This is often 
expressed as demonstrating that the method is “fit for use.” In 
more straightforward terms, any given method is valid only within 
specifications of use. In pract

valid only within 
specifications of use. In practical terms, methods are validated, or 
more accurately, their validity is verified, by clinical laboratories 
to the extent required by appropriate regulatory guidelines (see 
“Classes of Assays”).
The validation process begins with the design and develop­
ment of the method, regardless of the technical processes involved. 
Clinical laboratories approach validation differently depending on 
the technologies and reagents used. Commercial systems (

chnologies and reagents used. Commercial systems (instru­
ments and reagents) are validated by the manufacturer, who is 
also responsible for quality control of subsequent reagent lots and 
instrument change. Clinical laboratories conduct limited studies 
to verify the validation. When using instruments and reagents 
made or modified by the clinical laboratory, full validation is nec­
essary. In both settings, the clinical laboratory relies on profes­
sional guidelines specific to the technology

ofes­
sional guidelines specific to the technology.
79
Failure to fully appreciate these subtleties can lead to very erro­
neous perspectives of the results reported by a given laboratory or 
obtained by a given method. For example, the majority of assays 
used to diagnose endocrine diseases are accurate only over specific 
ranges of analytes, only with reference to specific and generally 
nonstandardized calibration materia

and generally 
nonstandardized calibration materials, and only when applied to 
specific specimen types. In many cases, results that are essential 
to patient care are method specific and cannot be extrapolated 
between methods and laboratories.
The basic elements of method validation are listed in Table 
4.3 along with the typical studies conducted to characterize each 
parameter. The parameters that define an assay’s analytic perfor­
mance are dependent on the technology and reagents employed 

dependent on the technology and reagents employed 
and are often referred to as intrinsic characteristics. These char­
acteristics include sensitivity, specificity, precision, and accuracy. 
Validation must also include specification of the assay’s utility 
and provide data to support the clinical interpretation of results 
generated by the assay; these are listed in Table 4.3 as utility and 
interpretation parameters. As illustrated in Fig. 4.17, intrinsic 
parameters are interrelated. For exam

, intrinsic 
parameters are interrelated. For example, as illustrated in Fig. 
4.18, accuracy and precision are related parameters and must 
be optimized and validated in conjunction with each other. In 
the context of method development, assay validation is typi­
cally an iterative process, as shown in Fig. 4.19. It is only after 
an assay is optimized analytically for specificity, sensitivity, and 
precision, and accuracy is applied to clinical testing, that these 
parameters can be fully eval

 testing, that these 
parameters can be fully evaluated and interpretive specifications 
can be established.
Intrinsic Performance Parameters
Analytic Specificity
Analytic specificity can be simply defined as the ability of the 
assay to measure only the intended analyte. In other words, the 
value obtained from a measurement reflects only the concen­
tration of the target analyte. Clearly, then, specificity is closely 
related to accuracy; an assay cannot be accurate if it is not 
specific. On 

say cannot be accurate if it is not 
specific. On the other hand, an assay may be specific but not 
accurate if, for example, the assay measures only the target ana­
lyte but produces a value that overestimates or underestimates 
its concentration due to calibration, recovery, or other techni­
cal issues. From a more practical perspective, specificity is often 
defined based on the signal generated in the assay; that is, the 
signal produced in a specific assay is generated only by the tar­
get 

 specific assay is generated only by the tar­
get analyte. Few assays, regardless of the technology used, are 
truly specific in this sense; typically, the signal measured can be 
generated by components of the specimen or assay systems in 
addition to the target analyte. Thus practical validation of speci­
ficity encompasses not only specificity per se but also interfer­
ences, such as matrix effects or ion suppression that can alter the 
derived concentration resulting in an inaccurate measure

d concentration resulting in an inaccurate measurement. 
It is important to emphasize that interference can be specimen 
specific and is a challenge to assay validation as well as quality 
control and quality assurance.
Cross-Reactivity
Assay cross-reactivity can be generically defined as signal genera­
tion by similar analytes. It is typically a definable and predictable 
assay characteristic (e.g., any specimen containing cross-reacting 
analytes will not be accurately measured).
Cross-reactiv

es will not be accurately measured).
Cross-reactivity in antibody-based assays is due primarily to 
the specificity of the antibody-binding sites used in the assay. 
For example, steroids with a similar structure may bind to the 
antibody and compete with the labeled analyte to produce the 
same signal (decrease in labeled analyte binding) as the target 
analyte. Similarly, proteins containing a binding epitope similar 
to the ones targeted in an immunometric assay can generate 
signal (i.e., in

 immunometric assay can generate 
signal (i.e., increased binding of the detection antibody). Cross-
reactivity is not a term typically used with respect to structural 
or nucleic acid–based assays, but the concept is applicable. For 
example, if the fragment used for quantification can be gener­
ated by more than one analyte, the signal generated is not spe­
cific. Similarly, if the sequence target for a nucleic acid–binding 
assay is large, the detection probe may bind to more than one 
analyt

 detection probe may bind to more than one 
analyte. In all cases, the cross-reactivity is not necessarily com­
plete in that the cross-reacting analyte may generate the same, 
more, or less signal than the target analyte. Thus the degree 
to which cross-reactivity vitiates assay measurements will be 
dependent on the concentration of cross-reacting analyte and 
the degree to which it cross-reacts.
Assays are validated with respect to cross-reactivity primarily 
by two approaches: (1) response c

ivity primarily 
by two approaches: (1) response curve comparison and (2) spiked 
specimen measurement.
Response curve comparisons are done by adding known amounts 
of analytes expected to cross-react (based on the design of the 
assay) to the appropriate matrix to generate a dose-response curve 
for each analyte to be tested. These response curves are compared 
to those used to quantify the target analyte (e.g., the calibration 
curve). When possible, the curves are compared at the half-maximal

sible, the curves are compared at the half-maximal 
response point where precision and sensitivity (see later) are high­
est. The degree of cross-reactivity can then be expressed as a per­
centage. An example of the procedure is shown in Fig. 4.20. The 
half-maximal response (50% Bmax/total labeled antibody bound) is 
generated by a concentration of 200 mass/mL of the target analyte. 
In contrast, 2000 mass/mL of the cross-reacting analyte is required 
to generate a half-maximal response. Thus t

uired 
to generate a half-maximal response. Thus the cross-reactivity of this 
cross-reacting analyte is 10% (i.e., % cross-reactivity = [200/2000] 
× 100). It is important to appreciate that this approach is valid only 
if the response curves are parallel.
  Parameters and Studies for Method 
Validation
Parameter of Performance
Validation Study
Specificity
Cross-reactivity
Interference
Sensitivity
Analytic sensitivity
Limits
Precision
Intra-assay variance
Inter-assay variance
Accuracy
Recovery

ay variance
Accuracy
Recovery
Bias
Linearity
Carryover
Utility (robustness)
Specimen stability
Reagent stability
Assay stability
Interpretation
Reportable range
Reference intervals
Diagnostic power
TABLE 4.3
80
Spiked specimen measurement is often used to determine 
analyte cross-reactivity. This approach involves adding the 
cross-reacting analyte to a specimen that has been measured 
and then performing re-assay to determine if the 

and then performing re-assay to determine if the added ana­
lyte cross-reacted. This approach is often seen in the package 
inserts of commercial assays. An example is shown in Table 
4.4 for a commercial assay for the measurement of cortisol in 
human serum or plasma. The concentration achieved by spik­
ing human serum is indicated for each potential cross-reactant 
listed. The unspiked human serum contained 12 μg/dL of 
endogenous cortisol as measured in the assay. Thus a cross-
reactivity of

measured in the assay. Thus a cross-
reactivity of 36.6% for fludrocortisone means that a cortisol 
value of 16.4 μg/dL was measured after the addition of 100 
μg/dL of fludrocortisone to the specimen. Spiked specimen 
cross-reactivity data must be interpreted carefully as it assumes 
that the percent of cross-reactivity will be the same at all levels 
of cross-reactant and that the concentrations of cross-reactant 
tested are clinically relevant. 
As alluded to previously, interfer

. 
As alluded to previously, interference can be due to the influ­
ence of a specimen component on the signal generated by the 
target analyte or to the generation of signal by the interfering 
substance. In the latter case, what distinguishes interference 
Specificity:
Sensitivity: 
Precision: 
Accuracy (bias):
Utility/Robustness:
Specimen, reagent, and assay stability
Interpretation Parameters
Reportable range, reference intervals, diagnostic power
Limit of detection
Limit of quan

 diagnostic power
Limit of quantitation 
(e.g., functional specifications)
Between assays
Between calibrations
Related molecules
Degradation products
Interferences (e.g., lipemia, hemolysis)
Reference materials
Reference methods
• Fig. 4.17  Relationships and sequencing of method validation parameters.
Accurate and Precise Assay 
Inaccurate and
Imprecise
Accurate but
Imprecise
Precise but Inaccurate 
Possible results of validation testing of new assay
Accuracy 
correct (targe

ion testing of new assay
Accuracy 
correct (target) value 
is measured
Precision 
measurement is reproducible
Systematic error (bias):
useful with reference
intervals, but results are
assay specific 
Random error:
not a useful assay 
• Fig. 4.18  Assay accuracy and precision are closely related parameters that must be optimized and vali­
dated together.
81
from cross-reactivity is the lack of parallelism in the signal gen­

vity is the lack of parallelism in the signal gen­
eration by interfering substances. Often the interfering sub­
stance or the mechanism of interference is something known; 
frequently encountered examples are given in the following 
paragraphs. In other cases, neither the mechanism nor inter­
fering substance is known; in this case, interference is referred 
to as a matrix effect. Matrix effects are typically identified 
only during the validation of accuracy (see later). They can be 
specimen 

on of accuracy (see later). They can be 
specimen specific, in which case they are identified only dur­
ing the investigation of results that are inconsistent with the 
clinical setting or other analytic results. It is critical that the 
clinical laboratory keep in mind that any analytic method can 
be subject to specimen-specific interferences unknown to the 
laboratory or indicated by routine quality control monitoring. 
Thus, despite a numeric value often to several decimal places 
and report

 value often to several decimal places 
and reported from a validated method, an analytic result from 
any single specimen must be interpreted in the overall clinical 
context.
Well-known interference with assays that depend on light or 
fluorescence signaling can be due to hemolyzed, lipemic, and 
icteric specimens. Interference can also be a function of physically 
influencing the system. For example, a severe degree of lipemia 
can result in inaccurate measurement of water-soluble analytes. 

naccurate measurement of water-soluble analytes. 
Interference can also be analyte specific. Some proteins are inac­
curately measured in hemolyzed specimens due to digestion by 
proteases released during hemolysis (rather than due to a nonspe­
cific color interference with light detection).
Two interferences that are well known to affect immunometric 
assays (but an issue to some extent for all antibody-based assays) 
are hook effects and heterophile antibody interferences.
The mechanism of the

phile antibody interferences.
The mechanism of the hook effect is illustrated in Fig. 4.21. As 
the antigen concentration approaches the effective binding capac­
ity of the capture antibody system, the signal no longer increases. 
At this point, laboratory specifications for maximum reportable 
signal are exceeded and the specimen is diluted to obtain an 
accurate measurement. However, analyte concentrations vastly 
exceeding the binding capacity of the capture antibody result in 
also blocking 

 of the capture antibody result in 
also blocking the detection antibody, resulting in a decrease in 
signal back into the reportable range. Extending the principle to 
the extreme, it is theoretically possible to have so much analyte 
present that all the binding sites on both the capture and detection 
New Biomarker(s) Identified
Assay Developed
Analytic Validation 
Clinical Validation
Clinical Utility
Iterative
process
• Fig. 4.19  Iterative nature of the assay development and validation 
pro

ature of the assay development and validation 
process.
Analyte concentration (mass/mL)
1
10
100
1000
10,000
100,000
Response (%Bmax/total labeled analyte bound)
0
25
50
75
100
Target
analyte
Cross-reacting
analyte
• Fig. 4.20  Method for determining analyte cross-reactivity. See discussion 
of cross-reactivity in text.
  Example of Spiked Specimen Cross-
Reactivity Data From a Commercial 
Immunoassay for the Measurement of 
Cortisol
Compound
Concentration 
(μg/dL)
Cross-Reactivity 
(%)
Aldoste

ncentration 
(μg/dL)
(%)
1000
0
Beclomethasone
1000
0
Budesonide
1000
0
Canrenone
1000
0.1
Corticosterone
1000
0.9
Cortisol 21-glucuronide
1000
0.2
Cortisone
1000
2.7
β-Cortol
1000
0
β-Cortolone
1000
0
11-Deoxycorticosterone
100
0
100
1.9
Dexamethasone
1000
0
DHEA
1000
0
DHEAS
1000
0
β-Estradiol
1000
0
Estriol
1000
0
Estrone
1000
0
Fludrocortisone
100
36.6
Fluticasone propionate
1000
0
DHEA, Dehydroepiandrosterone; DHEAS, dehydroepiandrosterone 3-su

piandrosterone; DHEAS, dehydroepiandrosterone 3-sulfate.
   
TABLE 4.4
82
antibodies are occupied; in this case, no detection antibody can be 
bound to the solid phase and the signal is baseline, which would 
be interpreted as no analyte present! Hook effects occur and are 
extremely important to recognize in the context of measuring hor­
mones in patients with tumors that secrete large quantities of the 
hormone. Hook effects have been extensively report

hormone. Hook effects have been extensively reported for prolac­
tin, hCG, thyroglobulin, calcitonin, and α-fetoprotein.29,30,131–139
Heterophile antibody interference is, if not a misnomer, cer­
tainly a process that encompasses more than just the ability of 
endogenous antibodies to animal immunoglobulins to interfere 
in immunometric assays. The mechanism is simple and is illus­
trated in Fig. 4.22. Any analyte-independent process that alters 
the amount of detection antibody bound to the sol

the amount of detection antibody bound to the solid will result in 
inaccurate assay values. As shown in Fig. 4.22, endogenous anti­
bodies (called heterophile antibodies) to animal immunoglobulins 
can, because antibodies are bivalent, link the detection antibody 
to the solid-phase capture antibody in the absence of analyte, 
resulting in a falsely high value reported. Animal immunoglobu­
lins are foreign proteins, and thus most humans are expected to 
have low titers of animal immunoglobulin

ected to 
have low titers of animal immunoglobulin antibodies. Immuno­
metric assays are designed with blockers to eliminate heterophile 
antibody interference. However, some individuals have high titers 
of heterophile antibodies, which overcome the assay blockers and 
result in inaccurate measurements. In some cases, such as patients 
treated with drugs containing animal immunoglobulins (e.g., 
monoclonal-based therapeutics), it is clear why the patient has 
high titers; in others it is diffic

e patient has 
high titers; in others it is difficult to identify a priori individu­
als whose specimens might be subject to this artifact.140–150 Het­
erophiles are not the only form of endogenous antibody that can 
interfere with immunometric assays by this mechanism (e.g., anti­
body-based analyte-independent interference). Fig. 4.22 shows 
that endogenous antibodies to epitopes on the target analyte can 
also result in inaccurate measurements—in this case, falsely low 
results—because the en

—in this case, falsely low 
results—because the endogenous antibodies block the quantita­
tive detection of analyte. Although the protein hormones and bio­
markers relevant to endocrine practice do not elicit the formation 
of endogenous antibodies in healthy individuals, patients with 
various autoimmune conditions or other disease processes may 
have endogenous antibodies that interfere with specific immu­
nometric assays. A classic example of nonheterophile endogenous 
antibody interference i

nonheterophile endogenous 
antibody interference is the interference in thyroglobulin assays by 
thyroglobulin antibodies in cancer patients.151–153
Endogenous antibodies and binding proteins can also interfere 
with the interpretation of values obtained from antibody-based 
assays. For example, endogenous antibodies bound to prolactin 
create what is referred to as macroprolactin. Macroprolactin is not 
biologically active but is measured in many immunometric assays. 
This results in prolactin 

y immunometric assays. 
This results in prolactin levels being reported that are discordant 
Signal
100
80
60
40
20
0
0
1,000,000
100,000
10,000
1000
100
10
Hormone concentration (mIU/L)
• Fig. 4.21  Immunometric high-dose hook effect. The response signal 
reaches a maximum and then decreases when the antigen concentration 
exceeds the limit of the assay.
Autoantibody to capture epitope
Falsely low result 
Autoantibody to detection epitope
Autoantibody to assay 

epitope
Autoantibody to assay antibodies
(human anti-mouse antibodies [“HAMA”])
Falsely high result 
• Fig. 4.22  Heterophile antibody interference. See text for details. (From Sluss PM. Methodologies for 
measurement of cardiac markers. Clin Lab Med. 2014;34:167–185.)
83
with clinical manifestations of hyperprolactinemia.135,154 Another 
example is the ability of competitive assays to measure small mol­

ility of competitive assays to measure small mol­
ecules, such as thyroid or sex hormones, that are inactive when 
bound to high-affinity carrier proteins, such as thyroxine-binding 
globulin or sex hormone–binding globulin. In this case, values 
reported by immunoassay can grossly overestimate the biologic 
signal represented by the hormone measurement.
Antibody-based assays that utilize biotinylated antibod­
ies or analytes are susceptible to interference from high levels 
of biotin in the ass

nterference from high levels 
of biotin in the assay sample, which can occur if the patient 
is taking very large doses of biotin as a health supplement. 
This can result in falsely elevated sample values in competitive 
binding immunoassays and falsely low sample values in immu­
nometric assays. If, for example, free thyroxine and TSH assays 
both use biotin, the reported free thyroxine can be falsely high 
and the reported TSH can be falsely low, a situation that could 
be mistakenly interpret

w, a situation that could 
be mistakenly interpreted as indicative of thyrotoxicosis. Bio­
tin interference can be especially difficult to suspect where 
the artifact causes both a high free thyroxine (FT4) and a low 
TSH.155 There is an increasing awareness that oral biotin can 
interfere with the accuracy of results from not only thyroid 
assays but also a wide range of analytes measured by methods 
involving biotin-streptavidin binding.156–158 As information 
regarding oral biotin use is typi

 As information 
regarding oral biotin use is typically not available to the labo­
ratory, it is important for ordering physicians to be aware of 
this issue. 
Analytic Sensitivity
Strictly, analytic sensitivity is the slope of the response curve. It 
is determined simply as the change in signal as a function of the 
change in analyte concentration and represents the smallest change 
in analyte concentration that can be measured. An example cali­
bration curve is shown in Fig. 4.23. The slope of

bration curve is shown in Fig. 4.23. The slope of this calibration 
curve determined by least squares linear regression analysis was 
1.01 pg/mL with a goodness of fit of r2 = 0.993 and an intercept 
of 1.05. Thus the smallest difference that can be measured over­
all is 1.01 pg/mL with an LOD (intercept) of 1.08 pg/mL. This 
approach is useful only when the calibration curve is linear (or lin­
earized by log transformation of the analyte concentrations) and 
the zero calibrator is accurately d

trations) and 
the zero calibrator is accurately determined (i.e., there is no matrix 
effect on the blank measurement). Dose-response curves for clini­
cal assays, regardless of technology employed, are seldom linear, 
and detector imprecision can be high. Thus limits of detection 
and direct estimates of variance at clinically meaningful analyte 
concentrations are typically more meaningful in describing assay 
performance.
The analytic LOD, often less rigorously referred to as sensitiv­
ity, 

ten less rigorously referred to as sensitiv­
ity, is a statistical definition of the lowest concentration that can be 
measured (i.e., distinguished for zero analyte in the assay system). 
This concentration is mathematically determined as the upper 
95% limit of replicate measurements of the zero standard, calcu­
lated from the average signal plus 2.0 SD. The LOD for the curve 
in Fig. 4.23 is 1.80 pg/mL. This minimal detection limit is valid 
only for the average of multiple replicate measurem

nly for the average of multiple replicate measurements. When 
individual determinations are performed on a specimen having 
a true concentration exactly at the minimal detection limit, the 
probability that the measurement is above the noise level of the 
assay is only about 50%.
A second parameter for the lowest level of reliable measure­
ment for an assay is the functional detection limit, or the limit 
of quantitation. For this value to be determined, multiple pools 
with low concentrations a

ermined, multiple pools 
with low concentrations are made and analyzed in replicate test­
ing. A cross-plot of the coefficient of variation of the measure­
ments versus concentration allows one to generate a precision 
profile. The analyte concentration corresponding to a coefficient 
of variation of 20% is the functional detection limit. This term 
typically applies to across-assay variation, but it also can be cal­
culated for within-assay variation if one uses the tests to evaluate 
results m

the replication of repeated measurements 
of the same specimen; it is a function of the time between repeats 
and the concentration of the analyte. Both short-term precision 
(within a run or within a day) and long-term precision (across 
calibrations and across batches of reagents) should be documented 
at clinically appropriate concentration levels.159 In general, nor­
mal range, abnormally low range, and abnormally high range tar­
gets are chosen for precision studies; however, targets focuse

sen for precision studies; however, targets focused on 
critical medical decision limits may be more appropriate for some 
analytes.
Twenty measurements are generally considered minimal at each 
level for both short-term and long-term precision validations. Pre­
cision usually is expressed as the coefficient of variation, calculated 
as 100 times the SD divided by the average of the replicate mea­
surements.160 There is no universal agreement on the performance 
criteria for analytic precision, 

the performance 
criteria for analytic precision, although numerous recommenda­
tions have been put forth. Two major approaches to defining these 
criteria have been (1) comparison with biologic variation and (2) 
expert opinion of clinicians based on their perceived impact of 
laboratory variation on clinical decisions.
The total variation clinically observed in test measurements is a 
combination of the analytic and biologic variations. For instance, 
if the analytic SD is less than one-fourth

tance, 
if the analytic SD is less than one-fourth of the biologic SD, the 
analytic component increases the SD of the total error by less than 
3%. If the analytic precision is less than one-half of the biologic 
SD, the total error increases by only 12%. These observations have 
led to recommendations for maintaining precision of less than 
one-fourth or one-half of the biologic variation.
The expert opinion precision recommendations are based on 
estimates of the magnitude of change of a test

on 
estimates of the magnitude of change of a test value that would 
cause clinicians to alter their clinical decisions. 
Accuracy
Two methods of assessing the recovery of assays are (1) measur­
ing the proportional changes caused by mixing high-concentra­
tion and low-concentration specimens and (2) measuring the 
increase in test values after the reference analyte is added. Some 
analytes circulate in the blood in multiple forms, and some of 
these forms may be bound to carrier proteins. The r

hese forms may be bound to carrier proteins. The recovery rate 
of pure substances added to a specimen may be low if the assay 
does not measure some of the bound forms. Mixtures of patient 
specimens may not be measured correctly if one of the specimens 
contains cross-reacting substances such as autoantibodies. A thor­
ough understanding of the chemical forms of the analyte and their 
cross-reactivities in the assay is important during assessment of 
recovery data.
Measuring the proportional c

nt of 
Measuring the proportional changes caused by mixing high-
concentration and low-concentration specimens is referred to as a 
linearity validation. An example is shown in Fig. 4.24. A specimen 
containing a relatively high analyte concentration is diluted with a 
specimen containing “no” analyte. Practically, “no” analyte means 
analyte levels less than the detection limit of the assay as specimens 
with no analyte are typically not available. Fig. 4.24A shows the 
measureme

lly not available. Fig. 4.24A shows the 
measurement of diluted specimens as a function of dilution. The 
point at which concentration no longer changes with increasing 
dilution is called the limit of blank (in this example, 10 pg/mL). 
The limit of blank can be significantly higher than the LOD or 
the limit of quantitation in some assays. Dilution linearity data 
(as in Fig. 4.24A) can be replotted (as in Fig. 4.24B) to evaluate 
the accuracy of an assay in terms of the analytic recovery of a

of an assay in terms of the analytic recovery of added 
analyte. Usually data are fitted by a linear regression. Assuming 
that the x-axes and y-axes have identical scaling, the slope*100 is 
the percent analytic recovery. Analytic recoveries less than or more 
than 100 reflect the bias of measurement for a given assay.
A
B
Dilution factor
(diluted with steroid-free serum)
1
10
100
1000 10,000 100,000
Serum estradiol: D1S (pg/mL)
1
10
100
1000
10,000
Expected analyte concentration
(pg/mL)
0
500

added is referred to as a spiked recovery validation. The analytic 
approach is identical to that illustrated in Fig. 4.24B except that 
the expected analytic concentration is based on the addition of 
analyte to specimens rather than calculated based on the dilution 
of specimens. The most appropriate analytes for use in analytic 
recovery studies are certified reference materials, such as those 
from the WHO or the National Institute of Standards and Tech­
nology (NIST), although such well-cha

ds and Tech­
nology (NIST), although such well-characterized materials are 
not available for all analytes. Ideally, a rigorous method valida­
tion would also include comparison to a reference method (i.e., 
a method that has been carefully validated previously). These are 
generally performed by highly specialized laboratories, and refer­
ence methods do not yet exist for many analytes of interest in 
endocrine testing or for novel biomarkers.161–169
At a minimum, the evaluation of accuracy by 

1–169
At a minimum, the evaluation of accuracy by any of the meth­
ods described previously should be conducted using specimens 
from healthy subjects and specimens from patients with the diseases 
being investigated.170 When possible, the assay should be traceable 
to established reference standards or methods. Between 100 and 
200 different specimens distributed over the assay range are recom­
mended for method comparisons.171–175 Although acceptable per­
formance criteria for method compariso

tics Directive, which requires manufacturers marketing in the 
European Union to establish that their products are “traceable to 
reference standards and reference procedures of a higher order” 
when such references exist.176 Hopefully, medically relevant per­
formance characteristics that define the allowable ranges for dif­
ferences between a specific assay’s test values and the traceable 
standards will be linked with this traceability requirement. This 
combination of traceability and allow

ement. This 
combination of traceability and allowable error requirements 
could serve to harmonize many test methods worldwide because 
most diagnostic companies market internationally. Standardiza­
tion and harmonization of hormone assays have become priorities 
for quality health care.164,169,176,177 
Carryover
Many diagnostic systems use automated sample-handling devices. 
If a specimen to be tested is preceded by a specimen with a very 
high concentration, a trace amount remaining from the 

 concentration, a trace amount remaining from the first speci­
men may significantly increase the reported concentration in the 
second specimen. The choice of the concentration that should be 
tested for carryover depends on the pathophysiology of the dis­
ease, but high values may need to be tested because some endo­
crine disorders can produce extremely high values. Validations also 
typically include assessing possible carryover from the sampling 
probe and for plate-based assays assessing d

ling 
probe and for plate-based assays assessing detector carryover from 
nearby wells. 
Utilization Parameters
Once an assay has been analytically validated, it is necessary to val­
idate its utilization. The key aspects of utilization involve defining 
limits associated with specimen and reagent stability and ensuring 
that the assay is stable over time.
Specimen Stability
Validating specimen stability typically involves testing a series of 
aliquots exposed to different handling conditions to

iquots exposed to different handling conditions to determine if 
the analyte measurement changes over time. This evaluation typi­
cally includes specimens representing the full range of specimen 
types to be tested (see later) and encompasses processing times 
and temperatures expected with respect to specimen collection 
and transport to the laboratory as well as stability during labo­
ratory processing and on instrument time and during the assay 
itself. This is a critical aspect of method val

y 
itself. This is a critical aspect of method validation that can be very 
costly and labor intensive. 
Reagent Stability
The stability of reagents used in the assay must also be defined. 
This includes both on-board stability and shelf-life stability for 
commercial, automated systems that are widely used in modern 
clinical laboratories. Although reagent expiration dates are deter­
mined and provided by commercial manufacturers, they must be 
verified under the actual working conditions of th

verified under the actual working conditions of the laboratory 
and take into account workflow processes such as reconstituting 
lyophilized calibrators or refreezing calibrator/control aliquots. 
For laboratory-developed methods such as LC/MS-MS, the labo­
ratory must also determine expiration dates for all reagent compo­
nents and stock materials. 
Robustness (Assay Stability)
Robustness is typically defined as the stability of measurement 
over time, which includes variance associated with re

r time, which includes variance associated with reagent lot 
changes, equipment changes, and technologist performance.178 
Robustness validation provides the specifications for the reliability 
of the method during extended normal usage. These specifications 
become the basis for setting limits on variance, and it is critical 
that laboratories inform clinicians of changes that exceed these 
practice-relevant limits. For example, changes in antisera can cause 
significant changes in immunoassay 

era can cause 
significant changes in immunoassay performance, which in turn 
require revising reference interval and clinical decision points. 
Reportable Range
The measuring range of an assay usually spans from the limit of 
quantitation to the concentration of the highest calibrator. Signal 
measured above the highest standard requires specimen dilution 
and retesting. This is typically done automatically in instrument 
systems but must be done manually in plate assa

t 
systems but must be done manually in plate assay systems. In 
either case, an important aspect of method validation is to assess 
accuracy when a specimen must be diluted. Dilution is often asso­
ciated with alternation of matrix effects and other interferences in 
an assay. Thus an assay that is accurate over the calibration range 
may not be accurate over the full reportable range when dilutions 
are utilized to obtain quantitative measurements. The validity of 
the analytic range is docume

nts. The validity of 
the analytic range is documented by the linearity and recovery 
studies. Most clinical laboratories confirm the reportable range of 
each assay at least twice a year. 
Reference Intervals
Reference intervals, also commonly referred to as normal ranges, 
describe the analyte values expected from a given assay when 
healthy individuals are tested.179–181 This is in contrast to clini­
cal decision points, or cutoff values, determined for identify­
ing patients with specific di

rmined for identify­
ing patients with specific disease conditions (see “Diagnostic 
Power”). The development and validation of reference intervals 
86
for endocrine tests can be very complex tasks as they require defin­
ing the healthy population by clinical evaluation182,183 and testing 
large numbers of healthy individuals,179,181 which often involves 
obtaining informed consent and related costly activities. Manu­
facturers of commercially available a

ities. Manu­
facturers of commercially available assays are required to provide 
reference intervals, but these do not necessarily represent the sub­
population served by a given laboratory.
The normal reference interval for most laboratory tests is based 
on estimates of the central 95 percentile limits of measurements in 
healthy subjects. A minimum of 120 subjects is needed to reliably 
define the 2.5 and 97.5 percentiles. Formal statistic consultation is 
usually required to determine the ap

sultation is 
usually required to determine the appropriate number of subjects 
to test and to develop statistical models for defining multivariate 
reference ranges.184–186
The reference intervals for many endocrine tests depend on 
gender, age, developmental status, and other test values. Fig. 4.25 
illustrates this complexity. Shown are the ranges of values (shaded 
areas) expected on a daily basis for hormone measurements in 
healthy young women across the menstrual cycle. 
Diagnostic Power

men across the menstrual cycle. 
Determining the clinical usefulness of the assay is typically the last 
step in validation. For commercially available assays the manu­
facturer is required to do this. The degree to which the assay is 
validated in this regard depends on specific FDA regulatory require­
ments (see “Classes of Assays”). Clinical laboratories using these 
assays are required to verify clinical utility claims depending on the 
specific requirements of the relevant 

ing on the 
specific requirements of the relevant accrediting organization, but 
generally the approach is to verify the clinical sensitivity and speci­
ficity of the assay using specimens from patients known to have 
or not have a specific clinical condition that the assay is designed 
to address. In contrast, for laboratory-developed methods, which 
include mass spectrometry and many molecular-based assays, the 
clinical laboratory is required to determine clinical decision points 
(cutoff val

to determine clinical decision points 
(cutoff values). In either case, good laboratory practice includes 
verification of clinical utility periodically as part of the laboratory’s 
quality control (QC) and quality assurance (QA) programs. The 
details of clinical utility validation are beyond the scope of this 
chapter, but a high-level understanding of the processes is impor­
tant because clinical sensitivity and specificity and cutoff param­
eters can help clinicians determine the relative we

ters can help clinicians determine the relative weight to give assay 
results in the context of the entire clinical picture. Many excellent 
discussions have been published to provide in-depth details for 
interested readers.186–191
Clinical sensitivity and specificity are not to be confused with 
analytic sensitivity and specificity, which are assay characteristics. 
In contrast, clinical sensitivity and specificity quantify the ability 
of the assay, or any clinical diagnostic procedure, to co

assay, or any clinical diagnostic procedure, to correctly iden­
tify disease states, and these parameters are expressed as percent­
ages. Clinical sensitivity is the percentage of patients with positive 
test results who actually have the target condition (i.e., they have 
the disease the assay is intended to identify); it is the true rate 
of accurate diagnosis. Subtraction of the clinical sensitivity from 
100 provides the rate of false-positive results for an assay (i.e., the 
percentage of p

ired for the interpretation of reproductive hormone mea­
surements across the menstrual cycle. FSH, follicle-stimulating hormone; LH, luteinizing hormone; SD, 
standard deviation.
87
Clinical specificity deals with patients with negative assay 
results. It is the percentage who actually do not have the disease 
(i.e., who are correctly identified as negative by the assay). Sub­
tracting the clinical specificity from 100 give

b­
tracting the clinical specificity from 100 gives the false-negative 
rate of the assay.
Clearly, to determine clinical sensitivity and specificity 
the laboratory must have two key things: (1) specimens from 
patients known to have or not have the disease and (2) either 
reference intervals or clinical cutoff points to provide a positive 
or negative interpretation to quantitative test results. Obtain­
ing specimens from patients with known conditions for the 
purpose of assay validation/veri

nditions for the 
purpose of assay validation/verification is challenging as the 
process requires accurate clinical information, Health Insur­
ance Portability and Accountability Act (HIPAA) compliance, 
and specimens collected and handled in a fashion consistent 
with normal workflow. All these steps can be expensive and 
challenging and are best accomplished with the close collabora­
tion of physicians who utilize the laboratory services.
Clinicians must appreciate that laboratory-derived cli

icians must appreciate that laboratory-derived clinical 
sensitivities and specificities are subject to many potential biases. 
First, in many cases laboratories rely on already established assays 
to provide the disease status information; in other words, they are 
comparing to predicate device results rather than clinical informa­
tion. Even when using clinically characterized individuals to pro­
vide the disease-positive specimens, bias can be incorporated if the 
healthy group is not age or 

 incorporated if the 
healthy group is not age or gender matched. Finally, using either 
clinical cutoffs or reference intervals to define a positive versus 
negative assay result is subject to statistical biases, such as normal 
versus nonnormal distribution of measurement. 
Operational Parameters (Preanalytic 
Considerations)
Specimen Types
Many types of specimens are routinely used for the measurement 
of analytes in bodily fluids. The most common are whole blood, 
serum, plasma, urine, and s

mmon are whole blood, 
serum, plasma, urine, and saliva. Less frequently, fluids or cells 
derived from fine-needle aspirates are sent to the clinical labo­
ratory for analysis. It is critical to understand that each type of 
specimen must be subjected to rigorous validation to ensure accu­
rate measurements. Simply because an assay is valid for a given 
specimen type does not mean that it is valid for any specimen 
type. Even if an assay is capable of reproducible and specific mea­
surement of 

ble of reproducible and specific mea­
surement of an analyte in different specimen types, there may be 
clinically significant bias depending on the specimen type tested. 
Reference intervals and clinical cutoff values must be verified for 
each specimen type utilized.
Whole Blood
Whole blood specimens have both the limitation and the advan­
tage of time dependency. The ability to detect rapid changes to 
a provocative stimulus is a strong advantage, whereas the unsus­
pected changes resulting f

age, whereas the unsus­
pected changes resulting from pulsatile secretions may be a major 
limitation. Whole blood is advantageous when the analyte is very 
labile as the specimen can be collected and tested quickly. The 
ability to test whole blood without processing at the point of care 
can be cost effective and enhance patient management as well as 
effectively address problems of specimen stability with respect to 
very labile analytes.
Blood drops collected on filter paper from punctures o

d drops collected on filter paper from punctures of a finger 
or heel are a convenient system for collecting, transporting, and 
measuring hormones.192,193
If standardized collection conditions and extraction techniques 
are used, these measurements correlate well with serum measure­
ments. Integration of immunochemistry with computer chip 
technology has also led to immunochips that can measure mul­
tiple analytes using a single drop of blood.194
The use of whole blood is severely limited by se

4
The use of whole blood is severely limited by several factors, 
most importantly: (1) Whole blood must be prevented from clot­
ting during analysis, necessitating the use of anticoagulants, which 
often interfere in assays, and (2) whole blood is a very complex 
mixture of components that can directly interfere with analytic 
methods. Solutions to these problems include (1) dilution of the 
whole blood specimen in the assay, which in turn requires that the 
analyte being measured is in relativ

res that the 
analyte being measured is in relatively high concentration or that 
the assay is extremely sensitive, and (2) preanalytic processing to 
remove the cellular fraction or reduce the complexity of the speci­
men. Traditionally specimen types most often used for endocrine 
testing are serum or plasma for these reasons. 
Serum
Serum is obtained from whole blood specimens simply by allow­
ing the blood to clot. Allowing whole blood to clot in glass tubes 
allows the serum to be completel

t in glass tubes 
allows the serum to be completely and easily separated from the 
clot by centrifugation. The resultant serum specimen is free of 
cells, and many of the proteins involved in the clotting process are 
also removed. This has been the method of choice for large protein 
analytes such as immunoglobulins or for very stable analytes such 
as steroid hormones. In most laboratories glass phlebotomy tubes 
have been replaced with safer plastic ones that will not break dur­
ing handling,

lastic ones that will not break dur­
ing handling, especially in fully automated laboratories. Unfor­
tunately, whole blood does not clot cleanly or quickly in plastic 
tubes, so clot activators or enhancers are added to the tubes. These 
factors can interfere with many analytic methods and must be 
carefully validated. 
Plasma
Plasma is obtained by chemically preventing the clotting process 
and then centrifuging to remove the cellular components of whole 
blood. There are many approaches to pr

s of whole 
blood. There are many approaches to preventing clotting; most 
commonly, ethylenediaminetetraacetic acid (EDTA), citrate, or 
heparin is added to prevent clotting. These chemicals, especially 
EDTA, have the additional advantage of inhibiting proteolysis 
and are thus advantageous when testing labile analytes such as 
adrenocorticotropic hormone (ACTH) or PTH. Other additives 
are added to stable plasma for specific tests; for example, sodium 
fluoride is added to EDTA tubes to inhib

, sodium 
fluoride is added to EDTA tubes to inhibit glycolysis when glu­
cose measurements are desired. Of course, all these additives have 
the potential to interfere with specific assay methods.
Separation of the cellular elements in anticoagulated whole 
blood can be enhanced by the use of gel separators. This type of 
tube is often preferred in automated aliquoting systems. Unfor­
tunately, the gel used can interfere directly or indirectly (e.g., by 
trapping analytes) in some analytic meth

e.g., by 
trapping analytes) in some analytic methods.
Phlebotomy tubes with additives also create special consider­
ations when collecting blood in multiple tube types. Tubes must 
be drawn in a specific order to avoid contamination with additives 
that are known to interfere in standard laboratory tests. Failure to 
collect additive tubes in the proper order is not identifiable once 
the tubes are received in the laboratory and can lead to serious 
inaccuracies in test results. The correct ord

ous 
inaccuracies in test results. The correct order for drawing the most 
commonly used phlebotomy tubes is shown in Fig. 4.26.195 The 
stopper color is standardized to indicate what additives each tube 
contains. For example, the tube with a red stopper (3) contains 
88
only clot activators; the yellow stopper tube (2) contains both clot 
activators and a separation gel. The lavender-topped tube (6) con­
tains EDTA and must be drawn after a clot tube fo

tains EDTA and must be drawn after a clot tube for serum (2 or 3) 
or tubes containing heparin (4 and 5). 
Urine
Urine often contains not only the original hormone but also 
key metabolites that may or may not have biologic activity. The 
24-hour urine specimen is used for many endocrine tests. Such 
urine specimens represent a time average that integrates over 
the multiple pulsatile spikes of hormone secretion occurring 
throughout the day. The 24-hour urine specimen also has the 
advantage o

e 24-hour urine specimen also has the 
advantage of better analytic sensitivity for some hormones and 
metabolites.55,196–203
Drawbacks include the inconvenience of collecting the 24-hour 
specimen and delays in collection. Another limitation of urine 
specimens is uncertainty regarding the completeness of the col­
lection. Measurement of urinary creatinine concentrations helps 
in monitoring collection completeness, especially when this value 
is compared with the patient’s muscle mass. Many ur

s compared with the patient’s muscle mass. Many urinary hor­
mones are conjugated to carrier proteins before excretion. There­
fore, both hepatic function and, to a lesser degree, renal function 
may alter urinary hormone values. 
Saliva
Saliva is an attractive alternative specimen for measuring non–
protein-bound hormones and small molecules.204–207 Small ana­
lytes in blood pass into oral fluid by crossing capillary walls and 
basement membranes and by passing through lipophilic mem­
branes of

s and by passing through lipophilic mem­
branes of epithelial cells.208 This transport involves passive dif­
fusion, ultrafiltration, active transport, or some combination of 
these processes. The concentration in saliva depends on the con­
centration of the non–protein-bound analyte in blood, the sali­
vary pH, the acid dissociation constant (pKa) of the analyte, and 
the size of the analyte. Analytes entering saliva by passive diffu­
sion usually are less than 500 Da in size, non–protein bound

ly are less than 500 Da in size, non–protein bound, 
and nonionized. As with any specimen type, it is essential to 
fully validate the use of saliva in each analytic method and to 
establish the data needed for interpretation of test results, such 
as reference intervals.
Saliva measurements reportedly correlate with blood measure­
ments for some hormones such as cortisol, progesterone, estradiol, 
and testosterone, but they do not correlate well for others (e.g., 
thyroid and pituitary hormones

 for others (e.g., 
thyroid and pituitary hormones).209–217 Multiple preanalytic vari­
ables can affect the salivary measurement. Stimulation of oral fluid 
production by chewing or by sucking candy or drops that contain 
stimulants such as citric acid can increase oral fluid volume and 
stabilize pH but may alter some analyte concentrations. Several 
commercial devices are available for collection of oral fluid; how­
ever, these devices need to be validated for each analyte and each 
assay syst

be validated for each analyte and each 
assay system to ensure they adequately recover each of the analytes.
Saliva is also an effective specimen type for obtaining genomic 
DNA and other nucleic acid assay applications. Its use in this 
regard is now well established and rapidly advancing.218–220 
Fluids and Tissue From Fine-Needle Aspiration
Fine-needle aspiration (FNA) involves the insertion of a hollow 
needle into tissue, typically a suspicious lump or inflamed tissue, 
to withdraw fluid or

ous lump or inflamed tissue, 
to withdraw fluid or cells for diagnostic evaluation. The procedure 
is usually performed manually by the surgeon or a cytopathologist 
when a palpable lump is present or the tissue target can be seen 
(e.g., during a surgical procedure). Deep sampling can be achieved 
using, for example, ultrasound guidance. Analysis of the cellular 
components of the FNA is done in the cytopathology laboratory 
and involves the examination of cells by histologic and immuno­
histol

mination of cells by histologic and immuno­
histologic procedures.
 Thus FNA fluids represent a unique specimen type for analy­
sis of biomarkers by traditional immunoassays or by LC/MS-MS 
methods. However, FNA fluids present special considerations 
with respect to handling, stability, validation, and interpretation. 
Aspirated fluids are typically obtained in volumes too small to 
directly assay and often will clot due to contamination with whole 
blood. Analyte stability as well as assay para

le 
blood. Analyte stability as well as assay parameter validation must 
be determined using aspirated fluid and diluents required to pro­
vide the necessary volume and anticoagulation prior to testing. 
Interpretation is often difficult because reference intervals appli­
cable to blood levels are seldom available and often analytes, such 
as thyroglobulin in neck mass aspirates, are high enough to cause 
artifacts in assays (such as hook effects in antibody-based assays).
Aspirated cells can pr

 in antibody-based assays).
Aspirated cells can provide sufficient DNA and RNA for 
genetic analysis. Currently this is an area of intensive investiga­
tion that has only just begun to be applied by clinical laboratories. 
Typical methods being applied include expression arrays, real-
time PCR, and DNA methylation assays as well as the widespread 
and traditional application of immunohistochemical assays for 
specific biomarkers.221 For example, expression arrays can provide 
clinically valuable

expression arrays can provide 
clinically valuable information for the 15% to 30% of thyroid 
nodules subjected to FNA and characterized as indeterminate by 
cytologic tests.222
Other current clinical applications of FNA in conjunction 
with diagnostic assays include chorionic villus sampling223,224 
and measurement of biomarkers in aspirates from pancreatic 
tumors.225–227 
Quality Control
Laboratory quality control procedures are intended to ensure that 
the tests are being performed within de

sure that 
the tests are being performed within defined limits established 
during the validation of the assay.228–232 The goal of these pro­
cedures is to identify circumstances when results obtained may 
not be accurate. They rely heavily on the testing of materials with 
known analyte concentrations. Quality control failures are meant 
1
2
3
4
5
6
7
8
• Fig. 4.26  The order in which blood tubes containing no or various addi­
tives must be filled to avoid contamination and possible interferenc

ed to avoid contamination and possible interference with 
accurate laboratory measurements. Tube types: 1, citrate; 2, clot tube 
(serum) with separator gel; 3, clot tube without separator gel; 4, heparin; 
5, heparin with separator gel; 6 and 7, EDTA; 8, sodium fluoride/EDTA. 
(Courtesy Michael Purugganan.)
89
to detect instrument problems (hardware or instrument failures), 
reagent or calibration failures, and human mistak

reagent or calibration failures, and human mistakes (improper 
handling of reagents or specimens, training problems, or shift 
change communication failures).
Statistically there are two major forms of analytic errors: ran­
dom and systematic. Random error relates to reproducibility; sys­
tematic error relates to the offset or bias of the test values from 
the target or reference value. Performance criteria can be defined 
for each of these parameters, and quality control systems can be 
progra

meters, and quality control systems can be 
programmed to monitor compliance with these criteria. Control 
systems must have low false-positive rates as well as high statis­
tical power to detect assay deviations. The multirule algorithms 
developed by Westgard and colleagues use combinations of con­
trol rules—such as two consecutive controls outside warning 
limits, one control outside action limits, or moving average trend 
analyzers outside limits—to achieve good statistical error detec­
tio

imits—to achieve good statistical error detec­
tion characteristics.229–232 Traditionally, quality control programs 
have focused primarily on precision; however, analytic bias also 
can cause major clinical problems. If fixed decision levels are used 
to trigger clinical actions (e.g., therapy, additional investigations), 
changes in the analytic set-point of an assay can cause major 
changes in the number of follow-up cases. More modern quality 
control systems include using moving averages of

control systems include using moving averages of patient test val­
ues to help monitor changes in analytic bias. Increasing numbers 
of web-based systems are available for laboratories to share qual­
ity performance data, allowing better statistical evaluations (larger 
numbers of values to identify shifts and drifts in quality control 
measurements). 
Quality Assurance
Quality assurance procedures go beyond monitoring test values 
for control materials. Testing of quality control materials on

materials. Testing of quality control materials only 
identifies errors that occur during testing per se (i.e., the analytic 
phase of the overall process from ordering a test until the results 
are reported back to the physician). A look at when errors typically 
occur provides important insight into the issues. As illustrated in 
Fig. 4.27, errors that occur during the analytic process represent 
less than a third of all errors associated with laboratory testing.
Quality assurance procedures a

laboratory testing.
Quality assurance procedures are part of the regulatory require­
ments of a clinical laboratory. All laboratories have procedures 
to monitor things such as specimen transport times and report 
accuracy, which are processes that can be established solely within 
the laboratory. A key element of quality assurance, and one that 
should be emphasized for clinicians who are critical to its suc­
cess, is the identification and investigation of test values that are 
discordant. Som

stigation of test values that are 
discordant. Some of these discordant test values may be analyti­
cally correct, but others may be erroneous. Clinicians must help 
identify and investigate these suspicious test values by requesting 
laboratories to perform a few simple validation procedures.
Repeated testing of the same specimen is a valuable first step. 
The laboratory assumes that the specimen has been stored under 
stable conditions and allows for the propagation or error during 
repeated m

ws for the propagation or error during 
repeated measurements. Typically, a result is considered to be con­
firmed if the absolute value of the difference between the initial 
and the repeated measurements is less than 3 SDs of the analytic 
error associated with the initial measurement.
Linearity and recovery are valuable techniques for evaluating 
test validity in individual specimens. If the initial test value is 
elevated, serial dilution of the specimen in the assay diluent and 
re-assay sh

the specimen in the assay diluent and 
re-assay should be considered. If the initial value is low, one may 
consider adding known quantities of the analyte to part of the 
specimen. Analyzing these spiked or diluted specimens with the 
original specimen allows one to evaluate both reproducibility and 
recovery. It may be helpful to analyze the linearity or recovery of 
the assay standards at the same time, to provide internal controls 
of the dilution or spiking procedures and the appropriatenes

ution or spiking procedures and the appropriateness of 
the diluent and spiking material. If the replication, dilution, or 
recovery experiment appears successful, further analytic trouble­
shooting will vary according to the method used. For example, 
immunoassays may be affected by interference caused by hetero­
phile antibodies or hook effects as described earlier. Addition of 
nonimmune mouse serum or heterophile antibody-blocking solu­
tions may neutralize these effects. Chromatographic ass

 may neutralize these effects. Chromatographic assays are 
usually more robust than immunoassays but often lack the speci­
ficity. Specimens with suspected interference on one type of assay 
can be reanalyzed by means of an alternative methodology. Inter­
ferences with cross-reacting drugs and metabolic products can be 
Preanalytic Phase
Analytic Phase
Postanalytic Phase
10%–55% of
total errors
Test ordered
Patient identification
Tube requirements,
handling specifications
Test tube labeling
Spec

s,
Specimen collection
Specimen processing
Transport to lab
Analysis
Quality control
Report generation
Report retrieval
Report interpretation
30%–75% of
5%–30% of
Specimen identification
Tube labeling/
lab barcoding
• Fig. 4.27  Distribution of errors during the entire process of clinical laboratory testing.
90
minimized with selective extraction or identified by addi

ed with selective extraction or identified by adding a drug 
to nondiscordant specimens. 
Classes of Assays
In the United States the regulations governing clinical laboratories 
fall into several categories based on the manufacture of assay com­
ponents, the intended use of the assay, and how the assay service is 
billed. To be reimbursed by Medicare and other health insurance 
organizations, laboratories must comply with federal legislation 
known as the Clinical Laboratory Improvement Amendmen

wn as the Clinical Laboratory Improvement Amendments 
(CLIA). This legislation requires that laboratories be certified by 
specified organizations (e.g., the College of American Patholo­
gists or the Joint Commission for the Accreditation of Hospital 
Organizations) and is administered by the Centers for Medicare 
& Medicaid Services (CMS) within the US Department of Health 
and Human Services (DHHS). CMS also administers the HIPAA 
of 1996 and other quality standards that laboratories must comp

ther quality standards that laboratories must comply 
with under federal law. CMS/CLIA certification requires quality 
inspections every 2 years to ensure that laboratories are meeting 
the standards outlined in the federal CLIA guidelines and the per­
formance standards specified by the laboratory’s inspecting agency. 
Inspecting agency standards are based on the CLIA guidelines.
In contrast, the manufacture and sale of assay reagents and 
instruments are regulated by guidelines called Current 

uments are regulated by guidelines called Current Good 
Manufacturing Practices (cGMPs), which cover very specific top­
ics and are periodically amended.233–238 Compliance with cGMPs 
is enforced by the FDA under federal law.239 Besides the manu­
facture and sale of instruments and reagents, laboratory-devel­
oped methods used in patient care are encompassed within the 
jurisdiction of the FDA. The FDA does not certify compliance 
and conducts its own quality inspections. Failure to comply with 

s own quality inspections. Failure to comply with 
cGMPs is a violation of federal law (as opposed to reimbursement 
requirements under CLIA) and can result in laboratory closure as 
well as potential fines and legal actions. Sale of assay instruments 
and reagents requires premarket approval by the FDA or FDA 
clearance under Section 510(k), depending on the clinical use of 
the product and its potential impact on patient care.
Common elements of clinical laboratory standards, CLIA 
guidelines,

f clinical laboratory standards, CLIA 
guidelines, and cGMP regulations include laboratory evaluation 
and documentation of validation verification for commercially 
available reagents and instruments, which have been cleared or 
approved by the FDA and whose continued manufacture is over­
seen by the FDA. Requirements for laboratory-developed meth­
ods, which include any modifications of FDA-approved/cleared 
commercial procedures, are more extensive, and validation per se 
is expected. 
Conclu

nsive, and validation per se 
Conclusion
The analytic methods of assessing endocrine problems in patients 
are continually expanding. The newer systems are often based on 
analytic techniques similar to those outlined in this chapter, but 
the configurations are generally more user friendly. These advances 
make the systems more convenient, but they also become more of 
a “black box” that conceals most of the details of the system. The 
methods, their descriptions, and approach to 

The 
methods, their descriptions, and approach to their validation, as 
outlined in this chapter, are intended to provide the clinician with 
insights into the inner workings of these systems and to encourage 
a more detailed level of interaction with the clinical laboratory in 
its ever more challenging endeavors to provide cost-effective yet 
high-quality support for patient care.
Acknowledgment
The authors thank Antony Morrison, Kannan Alpadi DVM, and 
Ajay Jumar PhD for reading the draft man

DVM, and 
Ajay Jumar PhD for reading the draft manuscript and for their 
helpful suggestions.
90.e1
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91
5
The Global Burden of Endocrine 
Diseases
MARIA INÊS SCHMIDT, LAERCIO J. FRANCO, EWERTON COUSIN, 
AND BRUCE B. DUNCAN
Introduction, 91
Obesity and Excess Weight, 92
The Burden of Diabetes, 97
The Burden of Thyroid Dise

s that have produced major demographic, nutri­
tional, and epidemiologic transitions. As seen in Fig. 5.1A, from 
1990 to 2015, the burden of disease (expressed as disability-
adjusted life years [DALYs]) caused by communicable, maternal, 
neonatal, and nutritional disorders decreased remarkably, while 
that due to noncommunicable diseases (NCDs) increased.1 
The increase in NCD burden results, in part, from population 
growth and population aging. Fig. 5.1B shows that DALYs, 
when expressed as 

g. Fig. 5.1B shows that DALYs, 
when expressed as rates per 100,000, which account for popula­
tion growth, are stable, and Fig. 5.1C shows that age-standard­
ized DALYs, which also account for population aging over the 
period, are decreasing. Social progress and advances in scientific 
knowledge translated into clinical and population interventions 
were the main drivers of the declines seen in Fig. 5.1C. How­
ever, since the crude measures (see Fig. 5.1A and B) are the true 
indicators of pop

e Fig. 5.1A and B) are the true 
indicators of population burden, the NCDs, many of which are 
caused by metabolic risk factors and include the principal endo­
crinopathies, have become, at the global level, the major cause 
of disease burden.
The 21st century is now faced with new and complex 
demands for action at both the population and clinical levels. 
Of major concern, progress in alleviating disease burden has 
not resolved longstanding inequities, leaving the less devel­
oped countries o

equities, leaving the less devel­
oped countries often with a double load of disease burden, 
the unresolved old ones and the new ones, thus posing dif­
ficult challenges to policymakers and to global health. We 
will address in this chapter the burden of endocrine diseases, 
namely obesity and excess weight, diabetes, iodine deficiency/
thyroid goiter, and thyroid cancer, selected on the basis of 
their importance and the availability of reliable data. To facili­
tate the interpretation of burd

e data. To facili­
tate the interpretation of burden presented, Table 5.1 summa­
rizes the metrics utilized in its quantification throughout the 
chapter. 
92
Obesity and Excess Weight
Prevalence and Trends
As extensively documented, the prevalence of obesity (body 
mass index [BMI] ≥30 kg/m2) has increased dramatically over 
the past several decades throughout the world. From 1975 
to 2014, as estimated by the NCD Risk Factor Collabora­
tion,2 global age

y the NCD Risk Factor Collabora­
tion,2 global age-standardized mean BMI has increased 3.5 
kg/m2 for men, from 21.7 kg/m2 (95% CI 21.3–22.1 kg/m2) 
to 24.2 kg/m2 (24–24.4 kg/m2), and 2.3 kg/m2 for women, 
from 22.1 kg/m2 (21.7–22.5 kg/m2) to 24.4 kg/m2 (24.2–
24.6 kg/m2). These changes, expressed in the usual nutritional 
status categories, are visualized in Fig. 5.2. Similarly, from 
1990 to 2017, as estimated by the Global Burden of Dis­
ease Study (GBD), the sum of risk across high levels of

Study (GBD), the sum of risk across high levels of BMI 
increased 70%, and over the past decade increased 39%, the 
increase occurring at a constant rate of approximately 2% per 
year.3
Global DALYs (millions)
400
800
1200
1600
1990
1995 2000 2005 2010
2015
1990 1995 2000 2005 2010 2015
Year
Communicable, maternal, neonatal,
and nutritional diseases
Noncommunicable diseases
Injuries
A
B
20 000
20 000
10 000
10 000
5000
15 000
DALY rates per 100000
DALY rates per 100

15
15 000
Year
Year
C
• Fig. 5.1  Trends from 1990 to 2015, by GBD Level 1 disease groups, in (A) global DALYs, (B) crude DALY 
rates, and (C) age-standardized DALY rates. The difference in trends between (A) and (B) is caused by 
population growth, and the difference between (B) and (C) is caused by changes in the age distribution of 
the population. Shaded areas show 95% uncertainty intervals. DALYs, Disability-adjusted life years. (From 
GBD 2015 DA

Disability-adjusted life years. (From 
GBD 2015 DALYs and HALE Collaborators. Global, regional, and national disability-adjusted life-years 
[DALYs] for 315 diseases and injuries and healthy life expectancy [HALE], 1990-2015: a systematic analy­
sis for the Global Burden of Disease Study 2015. Lancet. 2016;388:1603–1658.)
  Epidemiologic Indicators of Disease Presence and Burden
Indicator (Metric)
Abbreviation
Definition
Incidence
—
The number of new cases of a given disease during a given peri

f new cases of a given disease during a given period in a specified population.
Prevalence
—
The total number of cases of a given disease in a specified population at a designated time.
Years lived with disability
YLDs
Morbidity burden: YLDs consider any short-term or long-term health loss. It is calculated by multi­
plying the prevalence of the condition causing health loss (e.g., blindness due to diabetes) by the 
disability weight (0.19) estimated for that condition (i.e., a year lived with b

ated for that condition (i.e., a year lived with blindness is worth 81% 
of a year lived in full health).
Years of life lost due to premature 
mortality
YLLs
Mortality burden: YLLs are calculated by subtracting the age at death from the longest possible life 
expectancy for a person at that age.
Disability-adjusted life years
DALYs
Overall disease burden: The sum of years lost due to premature death (YLLs) and years lived with 
disability (YLDs).
Data from Institute for Health Metrics and Evalua

Data from Institute for Health Metrics and Evaluation.11,12 More detailed information on calculations can be found in the methodology supplements to GBD Capstone papers published in the Lancet in 
201713; and the Institute for Health Metrics and Evaluation (IHME). Terms defined. http://www.healthdata.org/terms-defined; Institute for Health Metrics and Evaluation (IHME). Frequently asked questions. 
http://www.healthdata.org/gbd/faq; Institute for Health Metrics and Evaluation (IHME). GBD public

r Health Metrics and Evaluation (IHME). GBD publications. http://www.healthdata.org/gbd/publications.
   
TABLE 5.1
93
Chapter 5  The Global Burden of Endocrine Diseases
Currently, GBD estimates that 15.2% of all adults—766 
(758–774) million adults—are obese, prevalence being greater in 
women than in men. Additionally, 5.3% of the world’s children 
and adolescents—130 (125–134) million—are obese. Over the 
lifespan, rates of obesity decrease slightly from age 2 to the begin­
ning of adolescenc

ightly from age 2 to the begin­
ning of adolescence, then increase steadily from age 15 and peak 
between the fourth and seventh decades. This peak comes earlier 
and at a lower level in men than in women and in countries of 
lesser versus greater sociodemographic development. At the end 
of life, prevalence declines,4 in large part due to greater premature 
mortality among the obese.
Although obesity rates are increasing in all geographic 
regions, both baseline values and rates of increase var

ns, both baseline values and rates of increase vary tre­
mendously. As a result, obesity prevalence in countries extends 
across a broad range of values, as seen by the color coding in 
world maps (Fig. 5.3). In 2016, Pacific Islanders, followed by 
nations of the Middle East and then several higher income 
countries of Anglo-Saxon origin, had the greatest frequency 
of obesity. Lowest frequencies were seen in Asian countries. 
Examples of extreme country differences in obesity rates for 
women 

e country differences in obesity rates for 
women are seen comparing American Samoa (65.3%), Kuwait 
(47.1%), and the United States (36.5%) with Vietnam (2.7%) 
and Japan (3.9%).5
Part of this variability across nations can be explained by dif­
ferences in socioeconomic development, as described by GBD 
sociodemographic index (SDI) categories, which are based on 
educational attainment, income, and fertility. As seen in Fig. 5.4, 
obesity rates among boys and men in low SDI locations have 
remai

mong boys and men in low SDI locations have 
remained relatively stable over time, whereas rates in all other 
groups have risen steadily, for both children and adolescents, and 
for adults. 
The Burden of Excess Weight
Fig. 5.5 shows the burden due to high BMI relative to that 
caused by other major cardiometabolic risk factors in 1990 and 
2017. Within this comparison, the burden due to high BMI 
showed the greatest increase over this period. Currently, consid­
ering risk factors for all disea

urrently, consid­
ering risk factors for all diseases, high BMI ranks 5th in terms of 
disease burden in the world, having risen from the 13th position 
in 1990.
High BMI causes both premature loss of life (72% of its bur­
den) and disability among those living (the remaining 28%). The 
fraction due to disability has risen from 22% to 28% over recent 
decades and is anticipated to continue to rise in importance. In 
relative terms, considering unstandardized rates for the entire 
world populatio

standardized rates for the entire 
world population, high BMI is estimated to have caused 8.4% 
of deaths, 6.5% of premature mortality (years of life lost [YLLs]), 
4.8% of morbidity (years lived with disability [YLDs]), and 5.9% 
of overall disease burden (DALYs). In relation to 1990, the burden 
it caused had increased 82% for deaths, 155% for YLLs, 90% for 
YLDs, and 133% for DALYs by 2017.
BMI (kg/m2)
<18.5
18.5 to <20
20 to <25
25 to <30
30 to <35
35 to <40
≥40
100
75
50
Prevalence (%)
25
0

to <35
35 to <40
≥40
100
75
50
25
0
1980
1990
2000
Year
100
75
50
25
0
1980
1990
2000
Year
A
B
• Fig. 5.2  Worldwide trend in nutritional status categories. (A) Men and (B) women, 1975 to 2015. BMI, 
Body mass index. (From NCD Risk Factor Collaboration [NCD-RisC]. Trends in adult body-mass index in 
200 countries from 1975 to 2014: a pooled analysis of 1698 population-based measurement studies with 
19·2 million participants. Lancet. 2016;387:1377–1396.)
94
SECTION I    Hormones a

t. 2016;387:1377–1396.)
94
Men
A
B Women
Caribbean
LCA
DMA
ATG
TTO
GRD
VCT
TLS
MDV
BRB
SYC
MUS
COM
Persian Gulf
W Africa
E Med.
MLT
SGP
Balkan Peninsula
TON
WSM
FSM
KIR
FJI
VUT
SLB
MHL
Caribbean
LCA
DMA
ATG
TTO
GRD
VCT
TLS
MDV
BRB
SYC
MUS
COM
W Africa
E Med.
MLT
SGP
TON
WSM
FSM
KIR
FJI
VUT
SLB
MHL
Age-standardized rates of obesity for adults over 20 years of age, 2017
<0.077
0.077  to  0.147
0.147  to  0.217
0.217  to  0.287

.077  to  0.147
0.287  to  0.357
0.357  to  0.427
0.427  to  0.497
0.497  and above
• Fig. 5.3  Age-standardized fraction of the population presenting obesity by country, 2017. (A) Men and (B) 
women. (From Institute for Health Metrics and Evaluation; Global Burden of Disease [GBP] Group. www.
healthdata.org/gbd.)
95
As also seen in Fig. 5.5, the colors of the 2017 high BMI bar 
indicate that 55% of its burden r

17 high BMI bar 
indicate that 55% of its burden resulted from cardiovascular dis­
eases, 2% from chronic respiratory diseases, 28% from diabetes 
and renal diseases, 5% from musculoskeletal disorders, 7% from 
cancer, 3% from neurologic disorders (dementia), and 1% from 
remaining conditions.
Like its prevalence, the burden of excess weight, though rising 
in all regions, is not evenly distributed among nations. Fig. 5.6 
>20 years old
30
25
20
15
10
5
0 1990
1995
Female
Male
Glo

e (%)
30
25
20
15
10
5
0 1990
1995
Female
Male
Global
High–middle SDI
Middle SDI
High SDI
2000
2005
Year
2010
Low–middle SDI
Low SDI
2015
<20 years old
0
1
2
3
4
5
6
7
8
9
10
11
1990
1995
Female
Male
Global
Middle SDI
High SDI
2000
2005
Year
2010
Low SDI
2015
A
B
• Fig. 5.4  Trends in the prevalence of obesity among children (A, aged >20) and adults (B, aged <20) 
from 1990 to 2017 overall and by level of sociodemographic development (SDI), separatel

l of sociodemographic development (SDI), separately for men and 
96
Musculoskeletal disorders
Neoplasms
Respiratory infections and tuberculosis
Cardiovascular diseases
Diabetes and kidney diseases
Chronic respiratory diseases
Neurological disorders
Sense organ diseases
Digestive diseases
0%
1990
High body mass
index
High fasting 
plasm

s
0%
1990
index
plasma glucose
High LDL 
cholesterol
High systolic 
blood pressure
2017
1990
2017
1990
2017
1990
2017
1%
2%
3%
4%
5%
6%
7%
8%
9%
10%
• Fig. 5.5  Percent of total risk-factor–attributable disease burden due to the major risk factors for noncom­
municable diseases, 1990 and 2017. The diseases causing this burden are shown by color codes. LDL, 
Low-density lipoprotein. (From Institute for Health Metrics and Evaluation [IHME]. GBD compare data 
visualizat

nd Evaluation [IHME]. GBD compare data 
visualization. 2017. https://vizhub.healthdata.org/gbd-compare/.)
Fraction of total all-cause DALYS attributable to high body mass index, both sexes, all ages
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
0.22 23%
• Fig. 5.6  Crude fraction of all DALYs due to high BMI, men and women, 2017. (From Institute for Health 
Metrics and Evaluation [IHME]. GBD compare data visualization. 2017. https://vizhub.healthdata.org/gbd-
compare/.)
97
Chapter 5  The Glo

althdata.org/gbd-
97
shows the burden of high BMI, expressed as a fraction of disease 
burden, for the world’s countries in 2017. For countries in dark 
brown greater than 10% of total disease burden was due to high 
BMI. For those in light brown, approximately 8% was due to high 
BMI; in dark yellow, approximately 6%; and in blue, less than 4%.
Fig. 5.7 shows the varying rates of growth of this burden, 
in terms of DALYs, from 2010 t

th of this burden, 
in terms of DALYs, from 2010 to 2017. The few countries 
shown in blue had a decrease in burden over this period and 
the small fraction of countries in yellow, a stable one. For those 
in tan, brown, light chocolate brown, and dark brown, burden 
has increased at annual rates of approximately 1%, 3%, 4% 
and 6%, respectively. The greatest increases are seen across the 
southern countries of Asia and much of Africa, both above and 
below the Sahara. Most of the Americas and m

 and 
below the Sahara. Most of the Americas and much of Africa 
have experienced annual increases ranging from 1.5% to 3%. 
Europe and northern Asia, in general, have experienced the 
lowest increases. 
The Burden of Diabetes
Prevalence, Incidence, and Mortality Trends
From 1980 to 2014, as estimated by the NCD Risk Factor Col­
laboration,6 the global age–standardized diabetes prevalence in 
adults aged 20 years and older more than doubled in men, from 
4.3% (2.4−7%) to 9% (7.2−11.1%), and incr

n, from 
4.3% (2.4−7%) to 9% (7.2−11.1%), and increased almost 60% 
in women, from 5% (2.9−7.9%) to 7.9% (6.4−9.7%). Given 
their large population size, China, India, the United States, 
Brazil, and Indonesia accounted for half of all adults with diabetes 
in 2014.
As seen in Fig. 5.8, from 1990 to 2017, the prevalence of diabe­
tes has increased globally across all levels of development.3 In large 
part this is due to the aging of the world’s populations. As seen in 
Fig. 5.9, the crude inciden

pulations. As seen in 
Fig. 5.9, the crude incidence rates of diabetes generally increased 
over this period as well, driven by the aging of the population and 
the increase in certain risk factors, notably obesity. Also appar­
ent from Fig. 5.9 is the stabilization of incidence in the high-SDI 
group and the decrease in incidence in high-middle SDI popula­
tions. It is noteworthy that the stabilization/decrease in incidence 
in higher SDI groups has not been sufficient to halt their rising 
pre

 has not been sufficient to halt their rising 
prevalence of diabetes. The accentuated recent rise in prevalence in 
the high-SDI group is fueled in large part by the notable decline 
in diabetes mortality observed in this group since 2003, as seen in 
Fig. 5.10. Of note in this figure, all other SDI groups continue to 
show rising mortality due to diabetes.
An issue that has generated great concern in recent years is 
the rising prevalence/incidence of diabetes among the young 
populations. Whi

ence of diabetes among the young 
populations. While incidence of type 1 diabetes has risen world­
wide, the increase also results from the rising incidence of type 2 
diabetes in the young.7 Due to the early disease onset of type 2 
diabetes, those affected will be exposed to metabolic and vascular 
disturbances for longer periods, resulting in an ominous progno­
sis, estimated to be worse than that predicted for type 1 diabetes 
manifesting in a similar period.8 This new burden of young-onset 

a similar period.8 This new burden of young-onset 
diabetes coupled with the aging of the population heralds the 
emergence of growing populations of people with diabetes of 
extremely long duration manifesting premature complications, 
including renal failure, multiple morbidities, and frailty, and thus 
imposing new pressures on individuals, families, health care sys­
tems, and societies. 
8%
High body mass index, both sexes, all ages, annual % change, 2010 to 2017, DALYs per 100,000
6%
–6%
–4

 change, 2010 to 2017, DALYs per 100,000
6%
–6%
–4%
–2%
0%
2%
4%
• Fig. 5.7  Relative annual percent change in the crude rate of DALYs attributable to high BMI, men and 
women, 2010 to 2017. (From Institute for Health Metrics and Evaluation [IHME]. GBD compare data visu­
alization. 2017. https://vizhub.healthdata.org/gbd-compare/.)
98
Burden
To characterize the burden of hyperglycemia along its con­
tinuous spectrum, GBD joins the burden of diabetes toget

s spectrum, GBD joins the burden of diabetes together 
with that of intermediate states of hyperglycemia, labeling it 
as burden due to “high fasting plasma glucose,” defined as 
values above 4.5 to 5.4 mmol/L, their established range of 
minimal risk. High fasting plasma glucose in 2017 was the 
fourth leading risk factor for all-cause disease burden (crude 
DALYs) in the world, having risen from the ninth position in 
1990.
Considering unstandardized rates for the world’s population, 
high fas

rdized rates for the world’s population, 
high fasting plasma glucose is estimated to have caused 11.7% 
of deaths, 7.4% of YLLs, 5.8% of YLDs, and 6.8% of DALYs 
in 2017. In relation to 1990, this burden has increased 57% for 
deaths, 116% for YLL, 50% for YLDs, and 95% for DALYs. Fig. 
5.5 shows that acute or microvascular complications (including 
renal disease) and living with diabetes in general produced 52% of 
the burden; cardiovascular diseases, 39%; cancer, 4%; tuberculo­
sis, 2.5%; neu

seases, 39%; cancer, 4%; tuberculo­
sis, 2.5%; neurologic disorders (dementia), 2%; and cataract and 
glaucoma, <1%. 
The Burden of Thyroid Diseases
GBD 2017 data provide information regarding the two clinically 
important subgroups of thyroid diseases: iodine deficiency/thy­
roid goiter and thyroid cancer.3
Deficiency of iodine, an essential component of the thyroid 
hormones, leads to inadequate thyroid hormone production. 
Thyroid hormones and thus iodine are necessary for many activi­
ties i

 thus iodine are necessary for many activi­
ties in the body. Iodine deficiency produces several consequences 
in humans, collectively called iodine deficiency disorders. It is one 
of the most prevalent preventable nutrient deficiencies in the 
world. Although the number of countries in which iodine intake 
is adequate has increased in recent decades, many nations still 
have a clinically relevant prevalence of deficiency. Most of them 
are low-income and middle-income countries, such as Angola

income and middle-income countries, such as Angola, 
Burkina Faso, Burundi, Haiti, Mozambique, Sudan, and South 
Sudan. However, even countries in transition, such as Russia and 
the Ukraine, and some high-income countries (Finland, Israel, 
and Italy), present problematic levels.9 Fig. 5.11 presents a global 
scorecard of iodine nutrition based on median urinary iodine con­
centration in school-age children and adults, in 2017.
Diabetes mellitus
Both sexes, all ages
8k
High SDI
High-middle SDI

8k
High SDI
Middle SDI
Low-middle SDI
Low SDI
Global
Prevalent cases per 100,000
6k
4k
2k
0
1990 1992 1994 1996 1998 2000 2002 2004
Year
2006 2008 2010 2012 2014 2016
• Fig. 5.8  Trends in the crude prevalence of diabetes globally and across sociodemographic index (SDI) 
quintiles, 1990 to 2017. (From Institute for Health Metrics and Evaluation [IHME]. GBD compare data visu­
99
Chapter 5  The Glob

althdata.org/gbd-compare/.)
99
300
New cases per 100,000
200
100
0
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
High SDI
Middle SDI
Low SDI
Global
Year
• Fig. 5.9  Trends in the crude incidence of diabetes globally and across sociodemographic index (SDI) 
quintiles, 1990 to 2017. (Fr

ographic index (SDI) 
quintiles, 1990 to 2017. (From Institute for Health Metrics and Evaluation [IHME]. GBD compare data 
0
5
10
20
15
Deaths per 100,000
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
Year
2005
2006
2007
2011
2012
2013
2014
2015
2016
2017
2008
2009
2010
High SDI
Middle SDI
Low SDI
Global
• Fig. 5.10  Trends in crude 

e SDI
Low SDI
Global
• Fig. 5.10  Trends in crude diabetes mortality globally and across sociodemographic index (SDI) quintiles, 
1990 to 2017. (From Institute for Health Metrics and Evaluation [IHME]. GBD compare data visualization. 
2017. https://vizhub.healthdata.org/gbd-compare/.)
100
The definition of iodine deficiency in GBD 2017 includes 
estimates of the subset of iodine deficiency associated with 
visible goiter (grade 2) and its associated seque

visible goiter (grade 2) and its associated sequelae because of 
the greater reliability and consistency of the clinical diagno­
sis of grade 2 goiter worldwide. It does not include estimates 
of subclinical iodine deficiency or nonvisible goiter (grade 1) 
induced by iodine deficiency. The global burden of iodine 
deficiency disorders, expressed in all-age DALYs, has shown 
improvement over the 1990–2017 period, declining 43.6%, 
from 61.7/100,000 (37.7–98.7/100,000) to 26.9/100,000 
(16.3–42.

000 (37.7–98.7/100,000) to 26.9/100,000 
(16.3–42.6/100,000).3
Thyroid cancer incidence has increased in all continents, except 
in Africa. In fact, incidence rates are more than twofold higher in 
high-income countries compared with low-middle-income coun­
tries.10 Fig. 5.12 illustrates the differences in age-standardized 
incidence rates for thyroid cancer in the 1990–2017 period, in 
countries categorized by the SDI. In spite of the overall increase 
in incidence in the last three decades, a 

crease 
in incidence in the last three decades, a recent decline has been 
noted in countries with high and high-middle SDI. Fig. 5.12 also 
highlights the discrepancy across the developmental continuum 
in incidence of thyroid cancer. This socioeconomic discrepancy is 
probably due to differences in access to diagnosis, which is more 
available in high-income countries.
Despite this overall increase in incidence, age-standardized 
mortality rates of thyroid cancer have declined in countries wit

s of thyroid cancer have declined in countries with 
high, high-middle, and low SDI (by 18%, 11%, and 5%, respec­
tively), while slightly increasing in countries with middle and 
low-middle SDI (by 6% and 8%, respectively), as shown in Fig. 
5.13. As the decline is more evident in countries with high and 
high-middle SDI, it is likely due to earlier diagnosis with quick 
therapeutic intervention.
According to 2017 GDB data,4 the crude global burden 
of thyroid cancer evaluated by DALYs in the 19

en 
of thyroid cancer evaluated by DALYs in the 1990–2017 
period increased from 12/100,000 (11–13.2) to 14.8/100,000 
(14–16.1). Fig. 5.14 shows age-standardized DALYs due 
to thyroid cancer in countries categorized by SDI in this 
period. The 
global 
age–standardized 
DALY 
declined 
from 14.4/100,000 (13.4–15.8/100,00) to 14.1/100,00 
(13.3–15.3/100,000). Despite the relatively accentuated 
decline in low-SDI countries, the burden of thyroid can­
cer (DALY) continues to be greater in countri

can­
cer (DALY) continues to be greater in countries with low 
or low-middle SDI. This socioeconomic discrepancy is 
reflecting differences in access to diagnosis and adequate 
treatment.
These data highlight the necessity to improve iodination pro­
grams and access to care for patients with thyroid diseases through 
the development of infrastructure and appropriate patient man­
agement in the developing world. These actions could save costs, 
improve quality of life, and produce a healthier wor

prove quality of life, and produce a healthier workforce. These 
GDB results call attention to the need to include thyroid diseases 
in the public health agenda. 
Perspectives for the Next Decade
Recent trends in the prevalence of high BMI and hyperglycemia 
suggest that unless major public health or clinical breakthroughs 
occur, the burden of obesity and diabetes will continue to grow 
and demand commitment of ever-greater resources in the fore­
seeable future. The continuous aging of the worl

­
seeable future. The continuous aging of the world’s population 
will be a major driver in these trends. On the other hand, exten­
sion of policies involving iodine supplementation will likely con­
tinue to decrease the burden of iodine-related thyroid disease 
and complications. Access to early diagnosis and treatment of 
thyroid cancer, especially in countries at low and middle levels 
of development, will hopefully lead to a continuing decrease in 
disease burden.
Insufficient iodine intake 

se in 
Insufficient iodine intake (mUIC <100 µg/L)
Adequate iodine intake (mUIC 100–299 µg/L)
Sub-national data
Survey not in SAC
Excessive iodine intake (mUIC ≥300 µg/L)
No data
• Fig. 5.11  Global scorecard of iodine nutrition based on median urinary iodine concentration in school-age 
children (SAC) and adults, 2017. (From Iodine Global Network [IGN]. http://www.ign.org/index.cfm.)
101
1990
1991
1992
1993
1994
1995
1996
1997
1

Diseases
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
Year
2005
2006
2007
2011
2012
2013
2014
2015
2016
2017
2008
2009
2010
0.0
1.0
2.0
3.0
4.0
5.0
Thyroid cancer
Both sexes, age-standardized
High SDI
Middle SDI
Low SDI
Global
• Fig. 5.12  Trends in age-standardized incidence rates (per 100,000) of thyroid cancer, for both sexes, in 
countries categorized in quintiles by the sociodemographic index (SDI), 1990 to 2

les by the sociodemographic index (SDI), 1990 to 2017. (From Institute for 
Health Metrics and Evaluation [IHME]. GBD compare data visualization. 2017. https://vizhub.healthdata.org/
gbd-compare/.)
0.50
0.30
0.20
0.00
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
Year
2005
2006
2007
2011
2012
2013
2014
2015
2016
2017
2008
2009
2010
0.10
0.40
0.60
High SDI
Middle SDI
Low SDI

Middle SDI
Low SDI
Global
• Fig. 5.13  Trends in age-standardized mortality rates (per 100,000) by thyroid cancer in countries catego­
rized in quintiles by the sociodemographic index (SDI), 1990 to 2017. (From Institute for Health Metrics and 
Evaluation [IHME]. GBD compare data visualization. 2017. https://vizhub.healthdata.org/gbd-compare/.)
102
The complete list of references is available online at ExpertConsu

t of references is available online at ExpertConsult.com.
15
10
DALYs per 100,000
5
0
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
Year
2005
2006
2007
2011
2012
2013
2014
2015
2016
2017
2008
2009
2010
High SDI
Middle SDI
Low SDI
Global
• Fig. 5.14  Trends in DALYs attributable to thyroid cancer in countries categorized in quintiles by the 
sociodemographic index (SDI), 1990 to 2017. (From Ins

iodemographic index (SDI), 1990 to 2017. (From Institute for Health Metrics and Evaluation [IHME]. 
GBD compare data visualization. 2017. https://vizhub.healthdata.org/gbd-compare/.)
102.e1
	 1.	 GBD 2015 DALYs and HALE Collaborators. Global, regional, and 
national disability-adjusted life-years (DALYs) for 315 diseases and 
injuries and healthy life expectancy (HALE), 1990-2015: a system­
atic analysis for the global burden of disease study 2015. Lancet. 
2016;388(10053):1603–1658.

se study 2015. Lancet. 
	 2.	 NCD Risk Factor Collaboration (NCD-RisC). Worldwide trends in 
body-mass index, underweight, overweight, and obesity from 1975 
to 2016: a pooled analysis of 2416 population-based measurement 
studies in 128·9 million children, adolescents, and adults. Lancet. 
2017;390(10113):2627–2642.
	 3.	 Institute for Health Metrics and Evaluation (IHME). GBD Compare 
Data Visualization. 2017. Available at: https://vizhub.healthdata.
org/gbd-compar

Metrics and Evaluation (IHME). Terms defined. 
Available at: http://www.healthdata.org/terms-defined. Accessed 
December 4, 2018.
	12.	 Institute for Health Metrics and Evaluation (IHME). Frequently 
asked questions. Available at: http://www.healthdata.org/gbd/faq. 
Accessed December 4, 2018.
	13.	 Institute for Health Metrics and Evaluation (IHME). GBD pub­
lications. Available at http://www.healthdata.org/gbd/publications. 
103
6
Navigating Through Clinical Practic

4, 2018.
103
6
Navigating Through Clinical Practice 
Guidelines in Endocrinology
NAYKKY SINGH OSPINA, SPYRIDOULA MARAKA, RENE RODRIGUEZ-GUTIERREZ, JUAN 
P. BRITO, AND VICTOR MONTORI
Introduction
Clinical decision making is a complex process in which, ideally, cli­
nicians and patients create plans of care that reflect the best avail­
able research evidence while also considering the patients’ values, 
preferences, and context.1–3 The increasing number of published 
trials and studies, unclear so

umber of published 
trials and studies, unclear sources of evidence, increasing number 
of interventions, patients with more and complex comorbidities, 
and time constraints make clinical decision making and caring 
for patients even more challenging.4–7 Clinical practice guidelines 
are statements that provide actionable recommendations intended 
to support clinicians in the decision-making process and improve 
patient care.8,9 Because of their potential benefits, the production 
of clinical pr

potential benefits, the production 
of clinical practice guidelines has increased during the last few 
years. In fact, the National Guideline Clearinghouse website had 
∼1500 available guidelines with more than 100 guidelines related 
to endocrine diseases.10 The availability of this large number of 
clinical guidelines makes it imperative for clinicians to understand 
the process associated with their development, their applicability, 
and their downsides in clinical practice; at the same time,

 downsides in clinical practice; at the same time, clini­
cians must recognize which guidelines can be trusted and are likely 
to result in improved patient care when implemented. 
Development of Trustworthy Clinical Practice 
Guidelines
To be trustworthy, clinical practice guidelines should be based 
on one or usually multiple systematic reviews of the available 
evidence and include a multidisciplinary guideline panel (free or 
Introduction, 103
Development of Trustworthy Clini

Development of Trustworthy Clinical Practice Guidelines, 103
Clinical Practice Guidelines Recommendations, 106
Clinical Practice Guidelines in Endocrinology, 108
Using Clinical Practice Guidelines in Patient Care, 109
Case Study: Management of Gestational Diabetes Using Clinical 
Practice Guidelines in Patient Care, 109
Shortcomings of Clinical Practice Guidelines, 109
Guidelines as Tools That Support (Not Dictate) Care, 109
Disease-Specific and Context-Blind Guidelines, 110
Gu

ease-Specific and Context-Blind Guidelines, 110
Guideline Focus on Surrogate Outcomes (Not Patient-Important 
Outcomes), 110
Quality and Trustworthiness of Clinical Practice Guidelines, 111
Conclusions, 111
KEY POINTS
 • 
Clinical practice guidelines provide actionable recommenda­
tions for clinicians to provide care.
 • 
The GRADE system provides a systematic framework for an in-
depth assessment of the quality of the evidence and considers 
important factors used in clinical decision making 

mmendation (very 
low, low, moderate, high).
 • 
To provide care supported by clinical guidelines, clinicians 
should engage in meaningful conversations with their patients 
to understand their situation because evidence alone is not suf­
ficient to provide care.
104
transparent regarding financial relationships).8 This panel should 
include members with methodologic and clinical expertise that 
can systematically evaluate the factors that may influence 

matically evaluate the factors that may influence a clinical 
decision and provide recommendations that are clear and action­
able. In addition, patient representatives can be included.8,11–14 
Although the best way to involve patients in the development of 
clinical practice guidelines has not been defined, their integration 
into these panels is feasible. Moreover, patient panels formulate 
similar recommendations when compared to physician panels 
when presented with evidence warranting moder

els 
when presented with evidence warranting moderate to high con­
fidence.14 The Institute of Medicine has suggested a series of cri­
teria that can help different stakeholders evaluate the quality of 
the process used to develop clinical practice guidelines and their 
trustworthiness (Table 6.1).
Many systems have been developed to provide a framework to 
analyze clinical evidence and provide practice recommendations. 
Initially, many clinical practice guidelines were developed based 
on exper

practice guidelines were developed based 
on expert opinion without a systematic evaluation of the available 
literature, making them prone to bias and inclusion of untrust­
worthy recommendations.9 Moreover, many of these approaches 
provided recommendations based on study design alone and did 
not include explicit judgments about the strength of recommen­
dations (i.e., how trade-offs between the expected benefits, harms, 
and costs weigh in during development of the recommendation). 
Furtherm

ring development of the recommendation). 
Furthermore, most systems were found to have low reproducibil­
ity of judgments (suggesting evaluation of different factors when 
making recommendations) and did not fit the needs of stakehold­
ers.15 To overcome the shortcomings of these systems, the Grading 
of Recommendations Assessment, Development, and Evaluation 
(GRADE) system was developed.11 The GRADE system provides 
a systematic framework for an in-depth assessment of the qual­
ity of the evid

n in-depth assessment of the qual­
ity of the evidence that is based on more than study design (ran­
domized control trial vs. observational studies).11–13,16,17 This 
approach also includes additional factors affecting the quality of 
evidence (Table 6.2) as well as other factors to consider when mak­
ing a recommendation, such as the balance between benefits and 
harms, integration of the patient’s values, preference and context, 
resources, and effects on health equity (Table 6.3).12,17 This 

d effects on health equity (Table 6.3).12,17 This frame­
work was first adopted by the Endocrine Society (Table 6.4) and is 
now used by more than 100 other organizations for the develop­
ment of trustworthy guidelines.9,18,19
The GRADE system suggests several steps to develop trustwor­
thy clinical practice guideline recommendations.11–13,16,17,20–27
Identification of the Clinical Question
Guideline panelists should select important clinical questions and 
identify the population to which the r

stions and 
identify the population to which the recommendations will apply, 
the intervention and comparator to be studied, and the health 
outcomes to be evaluated. The selected outcomes should be clas­
sified according to their clinical significance. When possible, 
patient-important outcomes should receive more value in deci­
sion making than surrogate markers (e.g., fracture rate compared 
to bone mineral density changes).11,20,21 
Systematic Review of the Literature
Once the clinical quest

c Review of the Literature
Once the clinical question is identified, a systematic search of the 
literature is performed to identify all the available clinical stud­
ies (i.e., the body of evidence). This systematic and reproducible 
  Suggested Criteria for Developing Trustworthy Guidelines
Criteria
Comment
Include a systematic evaluation 
of the clinical evidence
Allows identification of all the available evidence and prevents recommendations based on a single study or solely expert 
opinion.

based on a single study or solely expert 
opinion.
Allows comparison of effect estimates across different studies (likely including different populations and settings).
Include a multidisciplinary panel 
with clinical and meth­
odologic expertise and other 
important stakeholders
A collaborative and multidisciplinary panel creates a space for evaluation of the evidence and of recommendations from 
different perspectives.
Inclusion of patients is recommended. This can help clarify expected values

recommended. This can help clarify expected values and preferences of patients.
Consider the importance of 
patients’ values and prefer­
ences
Clinical evidence is not sufficient to provide patient-centered care. As such, consideration of how the values and prefer­
ences of patients can affect clinical decision making is required.
This is extremely important when there is expected variability in the values and preferences of patients.
Follow a transparent process
Guidelines can have significant 

ansparent process
Guidelines can have significant impact on patient care and policy. Following a transparent and systematic approach 
increases our confidence that the recommendations are made to improve patient care and are free of conflict of inter­
est.
Guidelines should clearly report their funding source and potential conflict of interest of panel members.
Explain the logical relationship 
between alternative care 
options and health outcomes
Clear explanations of the reasoning behind recom

s
Clear explanations of the reasoning behind recommendations, including harms/benefits, rating of the quality of the evi­
dence, the value of patients’ preferences, and the strength of the recommendation, are required.
Differences in opinion between panel members can also be included.
This transparency allows understanding of how recommendations were derived.
Provide explicit ratings of the 
quality of the supporting 
evidence and the strength of 
the recommendations
A systematic approach should

A systematic approach should be followed when assessing the quality of the evidence supporting different strengths of 
recommendations and clearly presented with each recommendation.
Be revised as new evidence 
warrants modification of the 
recommendations
Clinical practice guidelines should be updated to reflect the best available evidence.
TABLE 6.1
105
Chapter 6  Navigating Through Clinical Practice Guidelines in Endocrinology
evaluation of the literature will allow pane

ology
evaluation of the literature will allow panelists to inform their rec­
ommendations based on the complete spectrum of the available 
literature and not only on the most recent clinical study. In addi­
tion, systematic reviews help generate a best estimate of the effect 
of the clinical intervention based on the body of evidence (and not 
on a single study) and allow the comparison of this estimate across 
different studies, which together often encompass a broad range 
of populations and s

ften encompass a broad range 
of populations and settings. Moreover, having a summary of the 
body of evidence allows assessment for the possibility of publica­
tion bias.11,20,21,23 
Evaluation of the Quality of the Evidence
A pivotal step in the development of clinical practice guidelines 
is the evaluation of the quality of the evidence. The quality of the 
evidence reflects the extent of our confidence that the estimates 
of an effect are adequate to support a recommendation. When 
high-qual

quate to support a recommendation. When 
high-quality clinical evidence is available across outcomes of most 
importance, there is good certainty of the potential effects of an 
intervention, allowing panelists and clinicians to feel confident in 
the expected outcomes of recommendations. When the quality of 
evidence is poor, panelists have less confidence in effect estimates 
and expected outcomes of recommendations.12,13,16,21–26
This evaluation starts by identifying the study design. In gen­

on starts by identifying the study design. In gen­
eral, randomized controlled trials (RCTs) provide a high quality 
of evidence. However, for this to be true, these studies should be 
conducted following the accepted standards for RCTs that aim to 
protect them against bias such as adequate allocation concealment, 
blinding, follow-up, analysis, and complete outcome reporting. 
On the other hand, observational studies are considered to pro­
vide low-quality evidence because the groups are not b

 low-quality evidence because the groups are not balanced in 
terms of known and unknown prognostic factors at the start of the 
study (i.e., confounders). However, similar to the RCTs, specific 
details about how these studies were conducted can improve this 
initial rating (e.g., adequate selection of patients, adjustment of 
unbalanced factors and confounders, and follow-up).12,13,16,21–26
After this initial evaluation of the studies based on their design, 
the body of evidence is systematica

their design, 
the body of evidence is systematically evaluated in eight domains 
(see Table 6.2) that will allow panel members to categorize the 
quality of evidence associated with a specific recommendation 
as high quality, moderate quality, low quality, or very low qual­
ity across each outcome of interest. It is possible that the qual­
ity of the evidence varies among outcomes. In such cases, the 
quality of the evidence associated with the most important out­
come should guide the quality 

most important out­
come should guide the quality of the evidence for a particular 
recommendation.12,13,16,21–26
  Domains Used to Assess the Quality of the Evidence
Domain
Comment
Rate Down the Quality of the Evidence
	1.	 Risk of bias
Randomized clinical trials and observational studies might be at higher risk of bias depending on features 
of their design or conduct that can lead to a systematic deviation of their results from the truth (bias).
The presence of multiple sources of bias decr

as).
The presence of multiple sources of bias decreases the quality of the evidence.
	2.	 Inconsistency
Refers to the difference in the magnitude of the effect across studies. Inconsistency is present when point 
estimates vary widely across studies or if there is no or only minimal overlap between confidence 
intervals.
In addition, statistical tests such as the test for heterogeneity and the I2 can help identify inconsistency.
	3.	 Indirectness
This is present when there are significant diff

ss
This is present when there are significant differences between the populations, interventions, and 
outcomes evaluated in a study compared to the ones we want to study.
In addition, the lack of head-to-head comparison between two alternative management strategies is a 
cause for indirectness.
	4.	 Imprecision
The confidence intervals of an estimate are used to judge for imprecision. If the 95% confidence interval 
is wide, this suggests that the range in which the truth possibly lies is also

the range in which the truth possibly lies is also wide.
In addition, if the number of events and the sample size are small or if the confidence interval crosses the 
line of no effect, the quality of the evidence can also be rated down for imprecision.
	5.	 Publication bias
Studies with statistically significant results are more likely to be published than those with negative 
results; as such, evaluation of the published literature can lead to an inaccurate estimate of the effect 
of an inter

 an inaccurate estimate of the effect 
of an intervention.
Publication bias can be considered when only a small number of studies is available and if most are 
sponsored by industry.
The funnel plot is a commonly used method to assess for publication bias.
Rate Up the Quality of the Evidence
	1.	 Large effect
A magnitude of effect is considered large when the relative risk is between 2 and 5 or 0.5 and 0.2 with­
out possible confounders or very large if the relative risk is above 5 or below 0.2

large if the relative risk is above 5 or below 0.2 without problems of 
serious risk of bias or precision.
	2.	 Dose-response gradient
This gradient is considered when evaluating cause-effect relationships. This is present when the effect 
increases with the dose/exposure.
	3.	 Evaluation of plausible residual confounding
Rigorous observational studies will perform an adjusted analysis based on important prognostic factors. If 
a positive or negative effect is found after this adjustment, care

gative effect is found after this adjustment, careful clinical analysis might help identify 
large effects, despite the presence of uneven groups that are expected to result in no effect.
TABLE 6.2
106
These levels of the quality of evidence refer to the panel’s level of 
confidence that the true effect of an intervention lies within a par­
ticular range or on one side of an important clinical threshold.28 
From Quality of the Evidence to Strength of 
Rec

Recommendation
In contrast to the construct of quality of the evidence, the strength 
of a recommendation refers to the panel’s confidence that the desir­
able effects of an intervention outweigh the undesirable effects. In 
making this assessment, panel members take into consideration 
the quality of the evidence regarding expected benefits and harms 
of an intervention in addition to other factors that are important 
in medical decision making suc

that are important 
in medical decision making such as the values and preferences of 
patients, the setting where the intervention will take place, and 
available resources12,13,17 (Fig. 6.1; also see Table 6.3). 
Clinical Practice Guidelines 
Recommendations
Recommendations have a direction (in favor of or against an 
intervention), a strength (strong or weak), and a judgment of the 
quality of the evidence supporting the recommendation (very low, 
low, moderate, high)12,13,21 (see case study l

w, 
low, moderate, high)12,13,21 (see case study later in the chapter).
Strong Recommendations
Strong recommendations are those in which the panel is confident 
that the desirable effects of following a recommendation clearly 
outweigh the undesirable effects. In general, these recommen­
dations are based on high-quality or moderate-quality evidence. 
Panel members usually frame these recommendations as “We rec­
ommend…”12,13,21 (see case study later in the chapter).
Strong recommendations assum

ater in the chapter).
Strong recommendations assume that most patients faced 
with this health care decision will choose the proposed inter­
vention given the certainty of potential benefits in important 
outcomes.12,13,21 Clinicians are encouraged to follow these rec­
ommendations, policy makers can consider them as part of the 
standard of care, and they usually serve as the basis for quality or 
performance measures.
In some circumstances, strong recommendation can be based 
on low-quality ev

ong recommendation can be based 
on low-quality evidence.12,13,21 For example29:
 • When low-quality evidence suggests benefit in a life-threatening 
situation (e.g., increasing the glucocorticoid dose in patients 
with congenital adrenal hyperplasia during severe illness)
 • When low-quality evidence suggests benefit or equivalence of 
two alternatives and high-quality evidence suggests harm for 
one of the alternatives (e.g., recommendation for unilateral 
laparoscopic adrenalectomy for pati

or unilateral 
laparoscopic adrenalectomy for patients with documented pri­
mary hyperaldosteronism, instead of open adrenalectomy)
 • When high-quality evidence suggests equivalence and low-
quality evidence suggests harm in one treatment alternative 
(e.g., using propylthiouracil instead of methimazole during the 
first trimester of pregnancy) 
Weak Recommendations (Also Called 
Conditional or Discretionary)
Weak recommendations are those in which the panel concludes 
that the desirable effec

hich the panel concludes 
that the desirable effect of following these recommendations prob­
ably outweighs the undesirable effects, but they are not confident 
about this conclusion. Weak recommendations are usually based 
on low-quality or very-low-quality evidence. Panel members usu­
ally frame these recommendations as “We suggest…”12,13,21 (see 
case study later in the chapter).
In addition, in cases when the systematic evaluation of the evi­
dence provides high-quality evidence that the ben

dence provides high-quality evidence that the benefits and risks of 
   Factors Contributing to the Grading of Recommendations
Factors
Effect
Balance between desirable and undesir­
able outcomes
When the gradient between desirable and undesirable effects is large, panels are more likely to make a strong 
recommendation.
When the gradient between desirable and undesirable effects is narrow, panels are more likely to make a weak 
Quality of the evidence (on patient-
important out

important outcomes)
When high-quality evidence for patient-important outcomes is available, panels are more likely to make a strong 
When only very-low-quality evidence for surrogate markers or less important outcomes is available, panels are 
more likely to make a weak recommendation.
Confidence in the expected values and 
preferences of patients
Values and preferences refer to the processes that patients use when considering between alternat

hat patients use when considering between alternative treatment 
options (benefits, harms, cost, and impact on daily life). It can be argued that there is always a degree of 
uncertainty in terms of the values and preferences of each individual patient.
When the patients’ values and preferences are unknown or great variability is expected, panels are more likely to 
make a weak recommendation.
When the variability in patients’ values and preferences is expected to be low, panels are more likely 

ces is expected to be low, panels are more likely to make a 
strong recommendation.
Resources
Reliable evidence regarding resource use should be identified and important differences in resource use between 
alternative management strategies should be considered when grading recommendations.
Acceptability and feasibility of imple­
mentation
Panelists should consider if the proposed recommendations will be accepted and implemented in clinical prac­
tice. For example, they might consider the burden

tice. For example, they might consider the burden of treatment of a particular treatment agent compared to 
the alternative (e.g., multiple injections per day, need to be taken at a specific time of the day).
Health inequity (differences in health 
care that are avoidable and unfair or 
unjust)
Panelists should consider the impact of recommendations on health inequity and groups that are considered 
disadvantaged.
TABLE 6.3
107
Chapter 6  Navigating Through Clinical Practice Guidelines in Endoc

ting Through Clinical Practice Guidelines in Endocrinology
  Endocrine Society Clinical Practice Guidelines
Topic
Year of Publication
Adrenal Disease
The management of primary aldosteronism: case detection, diagnosis, and treatment
2016
Diagnosis and treatment of primary adrenal insufficiency
2016
Treatment of Cushing syndrome
2015
Diagnosis and treatment of pheochromocytoma and paraganglioma
2014
Congenital adrenal hyperplasia due to steroid 21-hydroxylase deficiency
2018
Diagnosis of Cushing 

-hydroxylase deficiency
2018
Diagnosis of Cushing syndrome
2008
Bone Health and Osteoporosis
Pharmacologic management of osteoporosis in postmenopausal women
2019
Paget disease of bone
2014
Osteoporosis in men
2012
Evaluation, treatment, and prevention of vitamin D deficiency
2011
Cardiovascular Endocrinology
Evaluation and treatment of hypertriglyceridemia
2012
Primary prevention of cardiovascular disease and type 2 diabetes in patients at metabolic risk
2008
Diabetes and Glucose Metabolism
Tre

olic risk
2008
Treatment of diabetes in older adults
2019
Diabetes technology—continuous subcutaneous insulin infusion therapy and continuous glucose monitoring in adults
2016
Diabetes and pregnancy
2013
Management of hyperglycemia in hospitalized patients in a noncritical care setting
2012
Continuous glucose monitoring: an Endocrine Society clinical practice guideline
2011
Evaluation and management of adult hypoglycemic disorders
2009
Endocrine Neoplasia and Canc

ycemic disorders
2009
Endocrine Neoplasia and Cancer
Hypothalamic–pituitary and growth disorders in survivors of childhood cancer
2018
2014
Female Reproductive Endocrinology
Evaluation and treatment of hirsutism in premenopausal women
2018
Functional hypothalamic amenorrhea
2017
Treatment of symptoms of menopause
2015
Androgen therapy in women
2014
Diagnosis and treatment of polycystic ovary syndrome
2013
Male Reproductive Endocrinolo

 ovary syndrome
2013
Male Reproductive Endocrinology
Testosterone therapy in men with hypogonadism
2018
Neuroendocrinology
Hormonal replacement in hypopituitarism in adults
2016
Acromegaly
2014
Diagnosis and treatment of hyperprolactinemia
2011
Evaluation and treatment of adult growth hormone deficiency
2011
Pituitary incidentaloma
2011
Obesity
Pharmacologic management of obesity
2015
Endocrine and nutritional management of the post–bariatric surgery patient
2010
TABLE 6.4
Continued
SECTION I   

gery patient
2010
TABLE 6.4
Continued
108
alternative treatment are balanced, a weak recommendation can be 
made (see upcoming case study, question 2).12,13,21 In general, the 
values and preferences of patients will significantly affect the medi­
cal decision when weak recommendations are made. Clinicians are 
encouraged to explore these values and preferences by engaging in 
shared decision making to reach the optimal medical decision based 
on the best

ch the optimal medical decision based 
on the best available evidence and the values of patients.3,12,13,30,31 
In shared decision making, clinicians and patients work together to 
determine the best way to address the patient’s situation.3,30,31 Deci­
sion aids are tools that have shown to increase patients’ knowledge 
and level of satisfaction with their medical decision and to enable 
patient-centered care; they can also help clinicians explore the val­
ues and preferences of patients.30,32 T

e the val­
ues and preferences of patients.30,32 These tools usually present a 
summary of the best available evidence about medical options for 
a particular clinical scenario and should help support the conversa­
tion between patients and clinicians.30,32 
Good Practice Statements
This is a special category of clinical practice statements in which 
a formal evaluation of the literature is not necessary, as there is 
a large body of indirect evidence to support the benefit of the 
recommendatio

dence to support the benefit of the 
recommendation and where the evaluation of alternatives in 
clinical trials would be unproductive and unnecessary. These rec­
ommendations should be clear, necessary, and justified by panel 
members. In addition, guideline panelists should seldom use this 
type of recommendation, as there is the risk of using it to support 
strong recommendations in the absence of clinical evidence.33 The 
following is an example of a Good Practice Statement from the 
Endocri

ple of a Good Practice Statement from the 
Endocrine Society, titled “Endocrine Treatment of Gender-Dys­
phoric/Gender-Incongruent Persons”34:
We advise that the clinician responsible for endocrine treatment 
and the primary care provider ensure appropriate medical clear­
ance of transgender individuals for genital gender-affirming sur­
gery and collaborate with the surgeon regarding hormone use 
during and after surgery.
In this case, it would be unreasonable to conduct a trial in 
which the en

e unreasonable to conduct a trial in 
which the endocrine team, the primary care provider, and the 
surgeon would not collaborate to achieve adequate surgical out­
comes; in addition, there is indirect evidence that clinical collabo­
ration is beneficial for patient care. 
Clinical Practice Guidelines in Endocrinology
An analysis of the Endocrine Society clinical practice guidelines 
from 2008 to 2014, including 25 guidelines, found that up to 
20% of the recommendations were based on very-low-q

0% of the recommendations were based on very-low-quality evi­
dence.35 A similar study, including 17 Endocrine Society clini­
cal practice guidelines published between 2005 and 2011, found 
that 58% of the recommendations were graded as strong (59% 
based on low-quality or very-low-quality evidence) and 42% were 
considered weak (92% based on low-quality or very-low-quality 
evidence).29 These findings should highlight the importance of 
exploring patient values and preferences and engaging in s

g patient values and preferences and engaging in shared 
decision making in endocrinology, given that a large number 
of recommendations are weak (conditional) and based on low-
quality or very-low-quality evidence.1,29,35 In addition, there is an 
urgent need of clinical research aimed at improving the quality of 
evidence supporting clinical practice guidelines in endocrinology. 
In fact, an assessment of current clinical trials in endocrinology 
found that only one in five recommendations tha

y 
found that only one in five recommendations that are based on 
Quality of the
Evidence regarding
Risks and benefits
Values and
Preferences of
Patients
Grading of
Strong
Weak
The panel concludes
that the desirable
effects of following
these recommendations
probably outweigh
the undesirable effects
The panel is confident
that the desirable effects
of following a
recommendation
clearly outweigh the
undesirable effects
Context and
Resources
Burden o

Resources
Burden of Therapy
• Fig. 6.1  Grading of recommendations.
Topic
Pediatric Endocrinology
Pediatric obesity—assessment, treatment, and prevention
2017
Thyroid
Management of thyroid dysfunction during pregnancy and postpartum
2012
Transgender Medicine
Endocrine treatment of gender-dysphoric/gender-incongruent persons
2017
  Endocrine Society Clinical Practice Guidelines—cont’d
TABLE 6.4
109
Chapter 6  Navigating Through Clinical Practic

109
very-low-quality evidence has a clinical protocol in place trying to 
address this knowledge gap.35 
Using Clinical Practice Guidelines in Patient 
Care
In clinical practice, guidelines can help physicians provide care 
that is based on the best available evidence and ideally supports 
patient-centered care.9,12,13,36 Strong recommendations based 
on high-quality evidence can inform clinicians regarding the 
most eff

ence can inform clinicians regarding the 
most effective and ineffective interventions, allowing them to 
adequately discuss the risks/benefits with their patients.9 Weak 
recommendations based on low-quality evidence should lead to 
collaboration between patients and clinicians when deciding the 
next step in management.3,30,31 The integration of decision aids 
and other tools to support shared decision making can help with 
the adequate implementation of clinical practice guidelines.37,38 
Cas

ntation of clinical practice guidelines.37,38 
Case Study: Management of Gestational 
Diabetes Using Clinical Practice Guidelines in 
Patient Care
An otherwise healthy 28-year-old gravida 1, para 0 at 25 weeks’ 
gestation had an initial abnormal 50-g oral glucose challenge test 
(160 mg/dL, 8.8 mmol/L) followed by an abnormal 100-g 3-hour 
glucose tolerance test confirming the diagnosis of gestational diabe­
tes. After 1 week of following medical nutrition therapy and daily 
moderate exercise fo

 nutrition therapy and daily 
moderate exercise for more than 30 minutes, she continues to have 
abnormal fasting glucose (95–109 mg/dL, 5.3–6.0 mmol/L) and 
postprandial hyperglycemia (140–160 mg/dL, 7.8–8.8 mmol/L).
   
Clinical Question 1: What should we do to improve pregnancy 
and offspring outcomes in this pregnant woman with gesta­
tional diabetes?
Answer: As per the Endocrine Society guidelines, “Diabetes and 
Pregnancy,”39 using blood glucose-lowering pharmacologic 
therapy is recommend

ucose-lowering pharmacologic 
therapy is recommended if lifestyle therapy is insufficient to 
maintain normoglycemia in women with gestational diabetes. 
(Strong recommendation, high-quality evidence)
   
The body of evidence is clear that maternal hyperglycemia has 
a deleterious effect on fetal, neonatal, and maternal outcomes.40,41 
Correction of maternal hyperglycemia, however, reduces or pre­
vents adverse outcomes.42 If this is not possible with nonpharma­
cologic management, well-conducte

 with nonpharma­
cologic management, well-conducted randomized clinical trials 
have shown that blood glucose–lowering pharmacologic therapy 
is effective at improving patient-important outcomes in this popu­
lation (e.g., death, shoulder dystocia, large-for-gestational-age off­
spring).42–45 Therefore, as the patient’s clinician, you can strongly 
recommend initiation of a blood glucose–lowering medication, as 
it is expected that most otherwise healthy pregnant patients faced 
with this clinic

 healthy pregnant patients faced 
with this clinical recommendation would like to start a blood glu­
cose–lowering medication.
   
Clinical Question 2: Of the available treatment options, which 
glucose-lowering medication would you recommend?
Answer: Providing evidence-based care for patients with gesta­
tional diabetes remains challenging because of the lack of well-
conducted clinical trials in this population. Even among the 
major medical professional societies, including the Endocrine 
Soc

ofessional societies, including the Endocrine 
Society, the American Diabetes Association, and the American 
College of Obstetricians and Gynecologists (ACOG), consen­
sus on management is often lacking.
   
As per the Endocrine Society guidelines, “Diabetes and Preg­
nancy,”39 long-acting insulin analog detemir may be initiated dur­
ing pregnancy for those women who require basal insulin and for 
whom neutral protamine Hagedorn (NPH) insulin, in appropri­
ate doses, has previously resulted in, 

 appropri­
ate doses, has previously resulted in, or for whom it is thought 
NPH insulin may result in, problematic hypoglycemia. (Weak 
recommendation, high-quality evidence)
It is also suggested that the rapid-acting insulin analogs lispro 
and aspart may be used over regular insulin. (Weak recommenda­
tion, moderate-quality evidence)
In addition, the Endocrine Society guidelines suggest that gly­
buride, an oral agent, is a suitable alternative to insulin therapy 
for glycemic control in wome

e to insulin therapy 
for glycemic control in women with gestational diabetes who 
fail to achieve sufficient glycemic control after a 1-week trial of 
medical nutrition therapy and exercise, except for women with 
a diagnosis of gestational diabetes before 25 weeks’ gestation and 
for women with fasting plasma glucose levels above 110 mg/dL 
(6.1 mmol/L), in which case insulin therapy is preferred. (Weak 
recommendation, low-quality evidence.)
They also suggest that metformin therapy be used fo

hey also suggest that metformin therapy be used for glycemic 
control only for those women with gestational diabetes who do 
not have satisfactory glycemic control despite medical nutrition 
therapy and who prefer not to use insulin or glyburide and are 
not in the first trimester. (Weak recommendation, low-quality 
evidence.)
Therefore, in contrast to the confidence in recommending a 
blood glucose–lowering therapy for women with gestational dia­
betes based on the high-quality evidence support

­
betes based on the high-quality evidence supporting the medi­
cal guidelines, the decision about which agent to choose requires 
further discussion of the clinical evidence and the patient’s situ­
ation. Health care professionals should fully and frankly discuss 
with women diagnosed with gestational diabetes the advantages 
and possible disadvantages of the available blood glucose–lower­
ing pharmacologic therapy options and together reach a decision 
that makes intellectual, practical, and e

ecision 
that makes intellectual, practical, and emotional sense to them. 
Shortcomings of Clinical Practice Guidelines
Although clinical practice guidelines aim to help patients and 
clinicians collaborate in making better health decisions, there are 
still limitations in the development and implementation of clini­
cal guidelines to improve care. 
Guidelines as Tools That Support (Not 
Dictate) Care
There is high-quality evidence that statin therapy in patients with 
type 2 diabetes reduces th

erapy in patients with 
type 2 diabetes reduces the risk of adverse cardiovascular (CV) 
outcomes, including mortality.46,47 Consequently, clinical practice 
diabetes guidelines unanimously advocate, as a strong recommen­
dation, to use statins in this patient population.48 However, not 
every patient will value the use of statins the same way as the pan­
elists do. For instance, consider the case of Mr. Jones, a 54-year-
old white man with a history of type 2 diabetes, obesity, controlled 
hype

tory of type 2 diabetes, obesity, controlled 
hypertension, depression, low back pain, sleep apnea, and new-
onset dyslipidemia. His 10-year American College of Cardiology/
American Heart Association CV risk is 10.0%.49 If 100 patients 
110
like Mr. Jones (Fig. 6.2) were actually not to take a statin for 10 
years, 10 of them would have an adverse CV outcome while 90, 
despite not following the recommendation, would not have an 
adverse CV outcome. Altern

ion, would not have an 
adverse CV outcome. Alternatively, if all 100 patients took a statin 
for a decade, 6 patients would still have a fatal or nonfatal myo­
cardial infarction and 4 patients would avoid a major CV event. 
This event would happen despite taking the statin every day for 
10 straight years, with the risk of harm (i.e., muscle pain), out-of-
pocket cost, and the required monitoring with lipid profile and 
consultation at least once or twice a year.50 While for Mr. Jones 
there i

ce or twice a year.50 While for Mr. Jones 
there is little doubt that using a statin might have some CV bene­
fit, at the moment taking statins does not fit his life. He has a high 
burden of illness (comorbidities), he takes more than 15 medica­
tions with many potential side effects, he has social and financial 
difficulties, and for Mr. Jones a 10-year risk of 10% versus 6% 
is simply not enough to start taking statins. Although guidelines 
recommend prescribing a statin for people like Mr. J

commend prescribing a statin for people like Mr. Jones, statin 
therapy may not be right for Mr. Jones. 
Disease-Specific and Context-Blind 
Although almost a quarter of Americans have multiple chronic 
conditions, clinical guidelines are often disease specific and do not 
consider all aspects of care such as multimorbidity, socio-personal 
context, and patient preferences. A systematic search of clini­
cal practice guidelines for patients with type 2 diabetes mellitus 
published betw

ents with type 2 diabetes mellitus 
published between 2006 and 2012 demonstrated that there was 
a complete absence of incorporating the impact of comorbidities, 
socio-personal context, and patient preferences in 8 (29%), 11 
(39%), and 16 (57%) of the 28 guidelines, respectively.51 In other 
words, many guidelines recommendations are contextually blind. 
This limitation might be secondary to clinical evidence obtained 
from studies excluding patients with comorbidities. Guideline 
panelists sh

tients with comorbidities. Guideline 
panelists should therefore be aware of this limitation and favor 
creating weak or conditional recommendations that promote a 
patient-centered approach such as shared decision making.1 This 
approach allows clinicians to see how a particular recommenda­
tion fits into the life of the patient in front of them.1 
Guideline Focus on Surrogate Outcomes 
(Not Patient-Important Outcomes)
Benefits ideally should be felt, improve quality of life, or reduce 
the ris

 felt, improve quality of life, or reduce 
the risk of an undesired patient-important outcome (i.e., hard 
outcome); if not, they should at least provide the certainty that 
by treating a risk factor (i.e., surrogate outcome)—for instance, 
hemoglobin A1c or bone mineral density—the risk of the patient-
important outcome (e.g., mortality and fracture, respectively) will 
be reduced.8 Although there is no doubt that surrogate outcomes 
may help clinicians and researchers elucidate how and to what

inicians and researchers elucidate how and to what 
extent an intervention can affect health, there are a handful of 
RCTs that have demonstrated a nonlinear relationship between 
a surrogate marker (that physiologically sounds reasonable) and 
patient-important outcomes.8,52,53 Patient-important outcomes 
have become more significant for patients with multiple chronic 
conditions in which the improvement of a surrogate marker 
may not always translate to the improvement of another surro­
gate m

nslate to the improvement of another surro­
gate marker from another disease and may be even harmful for 
the patient.54 However, this problem has been systematically 
• Fig. 6.2  Decision aid for patients considering statin therapy. (From Mayo Foundation for Medical Educa­
tion and Research. Statin choice decision aid [website]. Available at https://statindecisionaid.mayoclinic.
org/index.php/statin/index.)
111
overlook

ical Practice Guidelines in Endocrinology
overlooked. While there is no data on guidelines, in ∼200 diabetes 
trials in high-impact journals only 1 of 5 trials was focused on a 
patient-important outcome as the primary objective.55 
Quality and Trustworthiness of Clinical 
Practice Guidelines
Ideally, clinical practice guidelines offer clinicians an easy and 
accessible way to be up to date with the best available evidence 
regarding a particular disease. However, it is important to acknowl­
edg

 disease. However, it is important to acknowl­
edge that delivering care drawn from “the best available evidence” 
is not an easy and straightforward task, as to do so means every 
recommendation should be linked to the best available evidence, 
preferably based on a systematic review. In addition, as mentioned 
in the previous discussion about how clinical guidelines are devel­
oped, there are strict steps that should be followed and guidelines 
should be systematically developed by authors wit

should be systematically developed by authors with no conflicts of 
interest for them to be trustworthy.
The Appraisal of Guidelines and Research Evaluation II 
(AGREE II) instrument (by the AGREE Collaboration) can help 
clinicians assess the quality of clinical practice guidelines; it cor­
relates with the principles described by the Institute of Medicine 
regarding trustworthy guidelines.56 In addition, other tools that 
can help appraise clinical guidelines are available.57
When assessing t

nical guidelines are available.57
When assessing the quality of clinical practice guidelines, the 
following domains are important to consider56,58:
	1.	 Scope and purpose of the guideline: in which it is important 
to evaluate if the overall objectives, the clinical questions, and 
the patients to whom the guideline will apply are specifically 
described.
	2.	 Stakeholder involvement: assess if the guideline development 
group or task force includes individuals from all pertinent 
professiona

cludes individuals from all pertinent 
professional groups and patients to provide the views and pref­
erences of the targeted population.
	3.	 Rigor in development: observe if systematic methods were used 
to search for the evidence, if methods for formulating recom­
mendations are valid and adequately described, if there is an 
explicit link between recommendations and supporting evi­
dence, and if a clear procedure for guideline updates has been 
provided.
	4.	 Clarity of presentation: asse

da­
tions.
	6.	 Editorial independence: it is critical to recognize the funding 
source and conflicts of interest of the guideline developers. 
Conclusions
Clinical practice guidelines that are developed following rigorous 
and systematic methods by a multidisciplinary panel can provide 
recommendations that can support patient-centered care. In addi­
tion to being able to assess the trustworthiness of clinical practice 
guidelines, clinicians should understand how these recommenda­
tions can s

hould understand how these recommenda­
tions can support care and collaborate with their patients to over­
come their limitations.
         
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112.e1
	 1.	 Rodriguez-Gutierrez R, Gionfriddo MR, Ospina NS, et al. Shared 
decision making in endocrinology: present and future directions. 
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ide­
lines: 15. Going from evidence to recommendation-determinants 
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	16.	 Guyatt GH, Oxman AD, Kunz R, et  al. GRADE guidelines 6. 
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	17.	 Guyatt G, Oxman AD, Sultan S, et  al. GRADE guidelines: 11. 
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ity, consistency, and helpfulness: state-of-the-art clinical practice 
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er O, Conway DL, Berkus MD, et al. A comparison of gly­
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114
7
RONALD M. LECHAN
Historic Perspective
The field of neuroendocrinology has expanded from its original 
focus on the control of pituitary hormone secretion by the hypo­
thalamus to encompass multiple reciprocal interactions between 
the central nervous system (CNS) and endocrine systems in the 
control of homeostasis and physiolo

ystems in the 
control of homeostasis and physiologic responses to environmen­
tal stimuli. Although many of these concepts are relatively recent, 
the relationship between the hypothalamus and the pituitary 
gland was recognized as early as the 2nd century AD by Galen of 
Pergamon in De Usu Partium, the major treatise of Galenic physi­
ology, and Anatomicae Administrationes, in which he described a 
physical connection between the infundibulum of the hypotha­
lamic third ventricle and the pitui

f the hypotha­
lamic third ventricle and the pituitary gland and its association 
with a surrounding vascular network that he called the rete mira­
bilis. However, Galen thought the pituitary gland was a recep­
tacle for brain “waste” in the form of mucus or phlegm (pituita 
in Latin) that is delivered to and “filtered” by the pituitary gland 
before exiting through the nasal cavities. These concepts domi­
nated scientific thought about the hypothalamus and pituitary for 
more than a thousand ye

halamus and pituitary for 
more than a thousand years until the work of Andreas Vesalius in 
De Humani Corporis Fabrica (1543), in which he detailed the ana­
tomic relationships between the hypothalamic infundibulum and 
the pitu­itary gland, and during the 17th century, by Thomas Wil­
lis in Cerebri Anatome, proposing that “humors” from blood per­
fusing the ventral surface of the brain are carried to the pituitary 
gland. The term pituitary stalk was not introduced into the litera­
ture until 

lk was not introduced into the litera­
ture until 1742 by Joseph Lieutaud, and the term hypothalamus 
was introduced in 1893 by the Swiss anatomist Wilhelm His. A 
series of major discoveries in the late 19th and early 20th centuries 
set the groundwork for rapid advancement in the understanding 
of the hypothalamus in the regulation of hormone secretion and 
energy homeostasis and continued to unravel throughout the 20th 
century and into the 21st century (see Anderson and Haymaker1 
CHAPTER OU

st century (see Anderson and Haymaker1 
Historic Perspective, 114
Neural Control of Endocrine Secretion, 115
Hypothalamic-Pituitary Unit, 117
Circumventricular Organs, 122
Pineal Gland, 124
Hypophysiotropic Hormones and Neuroendocrine 
Axes, 126
Thermoregulation and Sleep-Wake Cycle, 169
Neuroendocrine Disease, 173
KEY POINTS
 • 
An underlying principle of neuroendocrinology is that peptide 
and monoamine signaling molecules are secreted from special­
ized neurons directly into 

secreted from special­
ized neurons directly into the circulation.
 • 
Secretion of anterior pituitary hormones and expression of 
genes encoding these hormones are primarily regulated by 
releasing and inhibitory factors produced in hypophysiotropic 
hypothalamic neurons and secreted into the portal vessel sys­
tem located in the median eminence.
 • 
Homeostasis of each respective hypothalamic-pituitary axis is 
maintained by complex integration of positive and negative 
feedback loops involv

on of positive and negative 
feedback loops involving the pituitary hormones themselves, 
downstream signals (including steroid hormones), and synaptic 
input from other brain areas onto the hypophysiotropic neu­
rons.
 • 
Hypothalamic neuropeptides are expressed in neurons 
throughout the brain to modulate activity of neural circuits and 
to coordinate a range of behavioral outputs that complement 
hormonal actions of the hypothalamic-pituitary axes.
 • 
In addition to regulation of the pitui

ani­
festations in addition to alterations in hypothalamic-pituitary 
function.
115
CHAPTER 7  Neuroendocrinology
and Toni2 for expanded historic perspective). Included were the 
discovery of the connection between the hypothalamus and the 
posterior pituitary (supraoptic-hypophysial tract) by Ramón Cajal 
in 1894 and subsequent work on neurosecretion in fish hypothal­
amus by Scharrer3 in 1928, firmly establishing that neurons in 
the hypothalamus were the source of the axons that constitute th

us were the source of the axons that constitute the 
posterior pituitary; evidence by Popa and Fielding4 in 1930 of an 
interconnection between the pituitary and hypothalamus through 
the “hypophyseo-portal vessels”; evidence by Wislocki and King5 
and Harris6 that the blood flow in portal vessels is directed to 
the pituitary gland from the hypothalamic median eminence and 
electric stimulation of the hypothalamus is ineffective in eliciting a 
pituitary response if the pituitary stalk is sever

pituitary response if the pituitary stalk is severed; and the seminal 
studies by Hetherington and Ranson7 in 1940 that destruction of 
the medial basal hypothalamus sparing the pituitary gland results 
in morbid obesity and neuroendocrine derangements recapitulat­
ing the syndrome of obesity, hypogonadotropic hypogonadism, 
and growth retardation described by Alfred Fröhlich8 in 1901.
Subsequently, several important studies, especially those from 
Schally and colleagues and the Guillemin group,

m 
Schally and colleagues and the Guillemin group, established that 
the anterior pituitary is tightly controlled by the hypothalamus.9,10 
Both groups identified several putative peptide hormone–releas­
ing factors (see later sections). These fundamental studies resulted 
in the awarding of the Nobel Prize in Medicine in 1977 to Andrew 
Schally and Roger Guillemin. We now know that these releasing 
factors are the fundamental link between the CNS and the con­
trol of endocrine function. Further

S and the con­
trol of endocrine function. Furthermore, these neuropeptides are 
highly conserved across species and are essential for reproduction, 
growth, and metabolism. The anatomy, physiology, and genetics 
of these factors constitute a major portion of this chapter.
Over the past several decades, work in the field of neuroendo­
crinology has continued to advance across several fronts. Cloning 
and characterization of the specific G protein–coupled recep­
tors (GPCRs) used by the hypothala

–coupled recep­
tors (GPCRs) used by the hypothalamic-releasing factors have 
helped define signaling mechanisms utilized by the releasing fac­
tors. Characterization of the distribution of these receptors has 
universally demonstrated receptor expression in the brain and in 
peripheral tissues other than the pituitary, arguing for multiple 
physiologic roles for the neuropeptide-releasing factors. Finally, 
there have been tremendous advances in our understanding of 
both regulatory neuronal an

 our understanding of 
both regulatory neuronal and humoral inputs to the hypophysio­
tropic neurons.
The adipostatic hormone, leptin, discovered in 1994,11 is an 
example of a humoral factor that has profound effects on mul­
tiple neuroendocrine circuits.12 Reduction in circulating leptin 
is responsible for suppression of the thyroid13 and reproductive 
axes14 during the starvation response. The subsequent discovery of 
ghrelin,15 a stomach peptide that regulates appetite and also acts on 
mul

tide that regulates appetite and also acts on 
multiple neuroendocrine axes, demonstrates that much remains to 
be learned regarding the regulation of the hypothalamic-releasing 
hormones. Traditionally, it has been extremely difficult to study 
releasing-factor gene expression or the specific regulation of the 
releasing-factor neurons because of their small quantities and, 
in some cases, diffuse distribution. Transformative technologies, 
including the development of transgenic mice in which 

uding the development of transgenic mice in which expression 
of fluorescent marker proteins is specifically targeted to hormone-
producing neurons such as gonadotropin-releasing hormone 
(GnRH) neurons16 or arcuate pro-opiomelanocortin (POMC) 
neurons17 (among many others), or the introduction of viral vec­
tors into selective populations of hormone-producing neurons to 
modulate cellular activity and projection profiles, has provided 
new and powerful techniques to study properties of hypothal

owerful techniques to study properties of hypothalamic 
neurons both in vitro in the more native context of slice prepara­
tions or organotypic cultures and in vivo.
Although much of the field of neuroendocrinology has focused 
on hypothalamic-releasing factors and their control of reproduc­
tion, growth, development, fluid balance, and the stress response 
through their control of pituitary hormone production, the term 
neuroendocrinology has come to mean the study of interaction of 
the endocr

me to mean the study of interaction of 
the endocrine and nervous systems in the regulation of homeosta­
sis. The field of neuroendocrinology has been further expanded by 
diverse areas of basic research, fundamental to understanding the 
neuroendocrine system including studies of neuropeptide struc­
ture, function, and mechanism of action; neural secretion; hypo­
thalamic neuroanatomy; GPCR structure, function, and signaling; 
transport of substances into the brain; and the action of hormones 

ances into the brain; and the action of hormones 
on the brain. Moreover, homeostatic systems often involve inte­
grated endocrine, autonomic, and behavioral responses. In many 
of these systems (e.g., energy homeostasis, immune function), the 
classic neuroendocrine axes are important but not autonomous 
pathways, and these subjects are also often studied in the context 
of neuroendocrinology.
In this chapter, the concepts of neural secretion, the neuro­
anatomy of the hypothalamic-pituitary un

he neuro­
anatomy of the hypothalamic-pituitary unit, including CNS 
structures particularly relevant to the control of the neurohypoph­
ysis and adenohypophysis, and other homeostatic mechanisms 
regulated by the hypothalamus are presented. Each classic hypo­
thalamic-pituitary axis is described, including a consideration 
of the immune system and its integration with neuroendocrine 
function. Then, modern concepts in the mechanisms involved in 
thermoregulation and the sleep-wake cycle are dis

thermoregulation and the sleep-wake cycle are discussed. Finally, 
the pathophysiology of disorders of neural regulation of endocrine 
function are reviewed. The neuroendocrinology of energy homeo­
stasis is fully considered in Chapter 39. 
Neural Control of Endocrine Secretion
A fundamental principle of neuroendocrinology encompasses the 
regulated secretion of hormones, neurotransmitters, or neuro­
modulators by specialized cells.3 Endocrine cells and neurons are 
prototypic secretory cells. 

ells and neurons are 
prototypic secretory cells. Both have electrically excitable plasma 
membranes and specific ion conductances that regulate exocytosis 
of their signaling molecules from storage vesicles. Secretory cells 
are broadly classified by their topographic mechanisms of secre­
tion. For example, endocrine cells secrete their contents directly 
into the bloodstream, allowing these substances to act globally as 
hormones. Cells classified as paracrine secrete their contents into 
the 

ied as paracrine secrete their contents into 
the extracellular space and predominantly affect the function of 
closely neighboring cells. Autocrine secretory cells affect their own 
function by the local actions of their secretions. In contrast, secre­
tory cells within exocrine glands secrete proteinaceous substances, 
including enzymes, and lipids into the lumen of ductal systems.
Neurosecretion
Neurons are excitable cells that send their axons throughout the 
nervous system to release their 

s throughout the 
nervous system to release their neurotransmitters and neuro­
modulators predominantly at specialized chemical synapses. Neu­
rohumoral or neurosecretory cells constitute a unique subset of 
neurons whose axon terminals are not associated with classic syn­
apses. Two examples of neurosecretory cells in the hypothalamus 
are neurohypophysial and hypophysiotropic cells. The prototypic 
116
SECTION II    Hypothalamus and Pituitary
neurohypophysial cells are the magnicellular neuron

eurohypophysial cells are the magnicellular neurons of the para­
ventricular hypothalamic nucleus (PVH) and supraoptic nucleus 
(SON). Hypophysiotropic cells encompass all neurons that secrete 
their products into the pituitary portal vessels at the median emi­
nence (Fig. 7.1).
In the most basic sense, neurosecretory cells are neurons that 
secrete substances directly into the bloodstream to act as hor­
mones. The theory of neurosecretion evolved from the seminal 
work of Scharrer,3 who used mo

d from the seminal 
work of Scharrer,3 who used morphologic techniques to identify 
stained secretory granules in the SON and PVH neurons. They 
found that cutting the pituitary stalk led to an accumulation of 
these granules in the hypothalamus, which led them to hypoth­
esize that hypothalamic neurons were the source of substances 
secreted by the neural lobe (posterior pituitary). It is now well 
established that the axon terminals in the neural lobe arise from 
the SON and PVH magnicellular 

al lobe arise from 
the SON and PVH magnicellular neurons that contain oxytocin 
and the antidiuretic hormone, arginine vasopressin (AVP).
The modern definition of neurosecretion has evolved to include 
the release of any neuronal secretory product from a neuron. 
Indeed, a fundamental tenet of neuroscience is that all neurons 
in the CNS, including neurons that secrete AVP and oxytocin in 
the neural lobe, receive multiple synaptic inputs largely onto their 
dendrites and cell bodies. In additi

y onto their 
dendrites and cell bodies. In addition, neurons have the basic abil­
ity to detect and integrate input from multiple neurons through 
specific receptors. They in turn fire action potentials that result 
in the release of neurotransmitters and neuromodulators into 
synapses formed with postsynaptic neurons. The vast majority of 
communications between neurons is accomplished by classic fast-
acting neurotransmitters (e.g., glutamate, γ-aminobutyric acid 
[GABA], acetylcholine) and n

 γ-aminobutyric acid 
[GABA], acetylcholine) and neuromodulators (e.g., dopamine, 
epinephrine, norepinephrine, neuropeptides) acting at chemical 
synapses.18,19 Neurosecretion represents a fundamental concept 
in understanding the mechanisms used by the nervous system to 
control behavior and maintain homeostasis.
Neural
lobe
Location: SON, PVH
Tropic hormones
Releasing
factors
Vasopressin
and oxytocin
Neuronal targets
(e.g., sympathetic
preganglionic neuron
in spinal cord)
Magnice

hetic
Magnicellular Neuron
Parvicellular
Hypophysiotropic Neuron
Projection Neuron
(ACTH, TSH, GH,
  LH, FSH, Prolactin)
(AVP, OXY)
Location: PeVH, PVH
(TRH, CRH,
 Somatostatin)
Arc
POA
(GHRH, GnRH,
  Dopamine)
GnRH
Location: PVH
(AVP, OXY)
Arc
(POMC, AgRP)
LHA
(MCH, ORX)
Kidney, Uterus,
Mammary Gland
Anterior
Pituitary Gland
Neuronal Targets
• Fig. 7.1  Three types of hypothalamic neurosecretory cells. Left, A magnicellular neuron that secretes 

cells. Left, A magnicellular neuron that secretes 
arginine vasopressin (AVP) or oxytocin (OXY). The cell body, which is located in the supraoptic nucleus 
(SON) or paraventricular hypothalamic nucleus (PVH), projects its neuronal process into the neural lobe, 
and neurohormone is released from nerve endings. Center, Parvicellular peptidergic neurons are located 
in the medial basal and dorsal hypothalamus in nuclear groups, including the periventricular hypotha­
lamic nucleus (PeVH), the PVH, i

tricular hypotha­
lamic nucleus (PeVH), the PVH, infundibular or arcuate nucleus of the hypothalamus (Arc), and preoptic 
area (POA). The neuropeptides in this case are released into the specialized blood supply to the pitu­
itary to regulate its secretion. Right, A third category of hypothalamic peptidergic neurons terminates at 
chemical synapses on other neurons. These projection neurons are found in sites that include the PVH, 
Arc, and lateral hypothalamic area (LHA) that innervate multiple

al hypothalamic area (LHA) that innervate multiple central nervous system nuclei, including 
autonomic preganglionic neurons in the brainstem and spinal cord. Such substances act as neurotrans­
mitters or neuromodulators. ACTH, adrenocorticotropin hormone; AgRP, agouti-related peptide; CRH, 
corticotropin-releasing hormone; FSH, follicle-stimulating hormone; GH, growth hormone; GHRH, growth 
hormone–releasing hormone; GnRH, gonadotropin-releasing hormone; LH, luteinizing hormone; MCH, 
melanin-c

 hormone; LH, luteinizing hormone; MCH, 
melanin-concentrating hormone; ORX, orexin/hypocretin; POMC, pro-opiomelanocortin; TRH, thyrotro­
pin-releasing hormone; TSH, thyrotropin.
117
In the era of optogenetics, multidimensional “omics,” and per­
sonalized medicine, the importance of these early observations is 
often not fully appreciated. However, accounts of these early stud­
ies are illuminating, and it is not an overstatement that the con­
firmation of the neur

 overstatement that the con­
firmation of the neurosecretion hypothesis represented one of the 
major advances in the field of neuroscience and neuroendocrinol­
ogy. Indeed, this and other early experiments, including the pio­
neering work of Geoffrey Harris,6 led to the fundamental concept 
that the hypothalamus releases hormones directly into the blood­
stream (neurohypophysial cells). These observations provided the 
principles on which the modern discipline of neuroendocrinology 
is built. 

dern discipline of neuroendocrinology 
is built. 
Contribution of the Autonomic Nervous System 
to Endocrine Control
Another major precept of neuroendocrinology is that the nervous 
system controls or modifies the function of both endocrine and 
exocrine glands. The exquisite control of the anterior pituitary 
gland is accomplished by the action of releasing-factor hormones 
(see “Hypophysiotropic Hormones and Neuroendocrine Axes”). 
Other endocrine and exocrine organs (e.g., pancreas and adre­

ine and exocrine organs (e.g., pancreas and adre­
nal, pineal, and salivary glands) are also regulated through direct 
innervation from the cholinergic and noradrenergic inputs from 
the autonomic nervous system. An appreciation of the functional 
anatomy and pharmacology of the parasympathetic and sympa­
thetic nervous systems is fundamental in understanding the neural 
control of endocrine function.20
The efferent arms of the autonomic nervous system comprise 
the sympathetic and parasympathet

system comprise 
the sympathetic and parasympathetic systems. They have similar 
wiring diagrams characterized by a preganglionic neuron that 
innervates a postganglionic neuron that, in turn, targets an end 
organ.21 Preganglionic and postganglionic parasympathetic neu­
rons are cholinergic. In contrast, preganglionic sympathetic neu­
rons are cholinergic, and postganglionic neurons are noradrenergic 
(except for those innervating sweat glands, which are cholinergic). 
Another basic concept is 

which are cholinergic). 
Another basic concept is that autonomic neurons coexpress several 
neuropeptides. This coexpression is a common feature of neurons 
in the central and peripheral nervous systems.18 For example, 
postganglionic noradrenergic neurons can coexpress somatostatin 
and neuropeptide Y (NPY), whereas postganglionic cholinergic 
neurons can coexpress vasoactive intestinal polypeptide (VIP) and 
calcitonin gene–related peptide (CGRP).
Most sympathetic preganglionic neurons lie in 

P).
Most sympathetic preganglionic neurons lie in the interme­
diolateral cell column in the thoracolumbar regions of the spinal 
cord.21 Most postganglionic neurons are located in sympathetic 
ganglia lying near the vertebral column (e.g., sympathetic chain 
and superior cervical ganglia). Postganglionic fibers innervate tar­
get organs. As a rule, sympathetic preganglionic fibers are rela­
tively short and the postganglionic fibers are long. In contrast, 
the parasympathetic preganglionic neur

 contrast, 
the parasympathetic preganglionic neurons lie in the midbrain 
(perioculomotor area, long misidentified as the Edinger-Westphal 
nucleus22), the medulla oblongata (e.g., dorsal motor nucleus of 
the vagus and nucleus ambiguus), and the sacral spinal cord. Post­
ganglionic neurons that innervate the eye and salivary glands arise 
from the ciliary, pterygopalatine, submandibular, and otic ganglia. 
Postganglionic parasympathetic neurons in the thorax and abdo­
men typically lie within 

 in the thorax and abdo­
men typically lie within the target organs, including the gut wall 
and pancreas.21 Consequently, the parasympathetic preganglionic 
fibers are relatively long and the postganglionic fibers are short.
The dual autonomic innervation of the pancreas illustrates the 
importance of coordinated neural control of endocrine organs. 
The endocrine pancreas receives sympathetic (noradrenergic) and 
parasympathetic (cholinergic) innervation.21,23 The latter activity 
is provided b

nervation.21,23 The latter activity 
is provided by the vagus nerve (dorsal motor nucleus of the vagus) 
and is an excellent example of neural modulation because the 
cholinergic tone of the beta cells affects their secretion of insulin. 
For example, vagal input is thought to modulate insulin secretion 
before (cephalic phase), during, and after ingestion of food.24 In 
addition, noradrenergic stimulation of the endocrine pancreas 
can alter the secretion of glucagon and inhibit insulin release

 secretion of glucagon and inhibit insulin release.23 
Of course, a major regulator of insulin secretion is the extracel­
lular concentration of glucose,25 and glucose can induce insulin 
secretion in the absence of neural input. However, the exquisite 
control by the nervous system is illustrated by the fact that popu­
lations of neurons in the hypothalamus, including ventromedial 
and perifornical hypothalamic neurons, and catecholamine neu­
rons in the rostral ventral lateral medulla of the b

ns in the rostral ventral lateral medulla of the brainstem, like 
the pancreatic beta cell, have the ability to sense glucose levels in 
the bloodstream.26 This information is particularly important to 
initiate counterregulatory responses to prevent hypoglycemia by 
altering the activity of the autonomic nervous system innervat­
ing the pancreas and adrenal glands while simultaneously facili­
tating arousal through projections to histaminergic neurons in 
the tuberomammillary nucleus (TMN) and 

eurons in 
the tuberomammillary nucleus (TMN) and inducing feeding 
responses.27,28
Autonomic regulation of glucose homeostasis is also modulated 
by gut-hypothalamic interactions and adipose tissue–derived sub­
stances such as leptin (see later). Included is hypothalamic sensing 
of bile acids, which after release into the small intestine, stimulates 
FGF-19 secretion that acts on mediobasal hypothalamic neurons 
to induce autonomic responses that lower blood glucose via a 
multisynaptic pathwa

at lower blood glucose via a 
multisynaptic pathway.29 Certainly, an increased understanding of 
the complex interplay between peripheral signals, the CNS, and 
endocrine function is necessary to diagnose and clinically manage 
endocrine disorders and may have particularly important implica­
tions for the treatment of diabetes mellitus. 
Hypothalamic-Pituitary Unit
The hypothalamus is one of the most evolutionarily conserved 
and essential regions of the mammalian brain. Indeed, the hypo­
thalam

s of the mammalian brain. Indeed, the hypo­
thalamus is the ultimate brain structure that allows mammals to 
maintain homeostasis, and its destruction is not compatible with 
life. Hypothalamic control of homeostasis stems from the ability 
of this collection of neurons to orchestrate coordinated endocrine, 
autonomic, and behavioral responses. A key principle is that the 
hypothalamus receives sensory inputs from the external environ­
ment (e.g., light, nociception, temperature, odorants) and i

, light, nociception, temperature, odorants) and infor­
mation regarding the internal environment (e.g., blood pressure, 
blood osmolality, blood glucose levels). Of particular relevance to 
neuroendocrine control, hormones (e.g., glucocorticoids, gonadal 
steroids, thyroid hormone, leptin) exert both negative and positive 
feedback directly on the hypothalamus.
The hypothalamus integrates diverse sensory and hormonal 
inputs and provides coordinated responses through motor outputs 
to key regul

ated responses through motor outputs 
to key regulatory sites. These sites include the anterior pituitary 
gland, posterior pituitary gland, cerebral cortex, premotor and 
motor neurons in the brainstem and spinal cord, limbic system 
structures (including the amygdala, septum, hippocampus, and 
thalamic nuclei), and parasympathetic and sympathetic pregan­
glionic neurons. The patterned hypothalamic outputs to these 
effector sites ultimately result in coordinated endocrine, behav­
ioral, and au

ult in coordinated endocrine, behav­
ioral, and autonomic responses that maintain homeostasis. The 
118
hypothalamic control of the pituitary gland is an elegant system 
that underlies the ability of mammals to coordinate endocrine 
functions that are necessary for survival.
Development and Differentiation of 
Hypothalamic Nuclei
Tremendous advances in knowledge of the molecular and genetic 
bases for embryonic development of the hypothalamic-pituitary 
u

yonic development of the hypothalamic-pituitary 
unit have occurred in the past few decades as a result of the 
genome sequencing projects and use of transgenic model systems. 
Pituitary development is detailed in Chapter 8; only a few key 
points most relevant to the physiology and pathophysiology of the 
neuroendocrine hypothalamus are presented here.
There has been considerable debate concerning the extent to 
which developmental studies in the rodent hypothalamic-pitu­
itary system are appli

e rodent hypothalamic-pitu­
itary system are applicable to the human. However, accumulating 
data suggest that the similarities outweigh the differences. Onto­
genic analyses of the organization of the human hypothalamus 
utilizing a battery of neurochemical markers have reinforced its 
homologies to the better-studied rat brain.30 The cytoarchitec­
tonic boundaries of hypothalamic nuclei are much more easily 
discerned in the fetal human brain than in the adult brain, and 
for the most part cor

han in the adult brain, and 
for the most part correspond to homologous structures in the 
rodent hypothalamus. This finding has important implications 
for the validity of interspecies comparative analyses. Two exam­
ples further illustrate this point. First, the ventromedial nucleus 
of the hypothalamic core (ventromedial hypothalamus [VMH]), 
which plays a role in energy balance and female sexual behav­
ior, differentiates from neuroblasts in both humans and rodents 
at a time-point intermedi

both humans and rodents 
at a time-point intermediate to the earlier differentiation of lat­
eral hypothalamic nuclei and later differentiation of the midline 
nuclei, including the suprachiasmatic nucleus (SCN), the arcuate 
nucleus, and the PVH.30,31 Expression of the transcription factor 
SF1 (steroidogenic factor 1) has been shown to be restricted both 
temporally and spatially to cells in the VMH, and knockout of 
the Sf1 gene in mice alters VMH development by influencing the 
migration of 

 VMH development by influencing the 
migration of cells and hence their ultimate location.31 A second 
example of interspecies homologies in hypothalamic develop­
ment is the migration of GnRH-secreting neurons from their ori­
gins in rostral neuroepithelium to the anterior hypothalamus.32 
As discussed later, spontaneous and inherited mutations in genes 
that affect the migration of these neurons are an important cause 
of Kallmann syndrome or hypogonadotropic hypogonadism 
associated with anos

ypogonadotropic hypogonadism 
associated with anosmia.
In addition to SF1 and the genes associated with Kallmann 
syndrome, there is a growing list of genes primarily encoding tran­
scription factors that have been implicated in human neuroendo­
crine disorders and characterized experimentally in rodent models 
(see Chapter 3).33,34 This list includes the homeobox transcription 
factor OTP and the heterodimeric complex formed by the basic 
helix-loop-helix (bHLH) transcription factors SIM1 and A

loop-helix (bHLH) transcription factors SIM1 and ARNT2 
mediating the expression of the POU III–related homeobox genes 
that include Brn2. These factors are required for the proper devel­
opment of the PVH and SON and for expression of many key 
hypophysiotropic neuropeptide genes. The physiologic impor­
tance of SIM1 is illustrated by the development of an obesity 
phenotype in both mice and humans with a haploinsufficiency of 
SIM1 expression.33 A major breakthrough in the understanding of 
fa

3 A major breakthrough in the understanding of 
factors controlling the development and terminal differentiation 
of human hypothalamic neurons is the ability to generate these 
neurons in vitro from induced pluripotential stem cells.35,36
Two key concepts involved in CNS development, which also 
apply to the hypothalamus, are the balance between neurogenesis 
and cell death in the establishment of nuclei and the role of circu­
lating hormones in providing organizational signals that regulate 
c

 providing organizational signals that regulate 
cell number and synaptic remodeling. The most thoroughly char­
acterized examples are the effects of sex steroid hormones on the 
developing brain that result in key sexual dimorphisms of func­
tional importance in later reproductive behaviors.37 This principle 
has been extended recently to include organizational effects of 
other classes of hormones. For example, leptin plays an important 
role in the development of medial-basal hypothalamic cir

n the development of medial-basal hypothalamic circuits 
important for energy homeostasis.38 
Anatomy of the Hypothalamic-Pituitary Unit
The pituitary gland is regulated by three interacting elements: 
hypothalamic inputs (releasing factors or hypophysiotropic hor­
mones), feedback effects of circulating hormones, and paracrine 
and autocrine secretions of the pituitary itself. In humans, the 
pituitary gland (hypophysis) can be divided into two major parts, 
an anterior, epithelial lobe or aden

major parts, 
an anterior, epithelial lobe or adenohypophysis, and a posterior 
neural lobe or neurohypophysis, which are easily distinguishable 
from each other by T1-weighted magnetic resonance imaging 
(MRI) due to the hyperintense signal denoting the neurohypoph­
ysis (Fig. 7.2).39 Women generally have a pituitary that is ∼20% 
larger than men and increase the size and weight of the adenohy­
pophysis during pregnancy due to lactotroph hyperplasia.40,41 The 
adenohypophysis can be subdivided 

asia.40,41 The 
adenohypophysis can be subdivided into three distinct lobes, the 
pars distalis (anterior lobe), pars intermedia (intermediate lobe), 
and pars tuberalis, with the pars distalis making up the bulk of 
the anterior pituitary and containing all of the hormone-secreting 
cells of the adenohypophysis. Whereas a well-developed interme­
diate lobe is found in most mammals, only rudimentary vestiges 
of the intermediate lobe are detectable in adult humans, with the 
bulk of intermediate

le in adult humans, with the 
bulk of intermediate lobe cells being dispersed in the anterior lobe.
The neurohypophysis is composed of the pars nervosa (also 
known as the neural or posterior lobe), the infundibular stalk, and 
the median eminence, the latter forming the central region of a 
mound, or tuber cinereum, at the base of the hypothalamus (see 
Fig 7.2). The infundibular stalk is surrounded by the pars tubera­
lis, and together they constitute the hypophysial stalk. The pituitary 
glan

stitute the hypophysial stalk. The pituitary 
gland lies in the sella turcica (Turkish saddle) of the sphenoid bone 
and underlies the base of the hypothalamus. Pituitary tissue can 
also be present in the nasopharynx, commonly referred to as a 
pharyngeal pituitary, and is a residual of Rathke pouch during 
its migration from the oral cavity to the middle cranial fossa (see 
next).
The anterior and intermediate lobes of the pituitary derive 
from a dorsal invagination of the pharyngeal epitheli

m a dorsal invagination of the pharyngeal epithelium, called 
Rathke pouch. During development, Rathke pouch forms three 
diverticula, a single medial and two lateral. Precursor cells within 
the pouch undergo steps of organ determination, cell fate com­
mitment to a pituitary phenotype, proliferation, and migration 
with cells in the medial diverticula differentiating into cortico­
trophs and thyrotrophs, lower portions of the lateral diverticulum 
into somatotrophs and gonadotrophs, and all th

um 
into somatotrophs and gonadotrophs, and all three diverticula 
into lactotrophs. The upper part of the lateral diverticula gives 
rise to the pars tuberalis that differentiates into thyrotrophs and 
gonadotrophs. The intermediate lobe is in direct contact with the 
neural lobe and is the least prominent of the three lobes. With 
age, the human intermediate lobe decreases in size to leave a small, 
residual collection of POMC cells. In nonprimate species, these 
119
CHAPTER 7  Neuroendocrinol

ate species, these 
119
cells are responsible for secreting the POMC-derived product, 
α-melanocyte-stimulating hormone (α-MSH).42 Early contact of 
Rathke pouch with the primordial neurohypophysial tissue has 
a key role for histodifferentiation of anterior pituitary cell types 
as indicated by the lack of corticotrophs and gonadotrophs in 
anencephalic fetuses.43 An expanding list of transcription factors 
involved in the development of the anterior pituitary glan

 in the development of the anterior pituitary gland and 
associated with genetic causes for human disorders of pituitary 
development and secretion has been recognized, with mutations 
of PROP1 being the most common genetic cause for combined 
pituitary hormone deficiency.44
The major component of the neural lobe is a collection of axon 
terminals arising from magnicellular secretory neurons located 
in the PVH and SON of the hypothalamus (Fig. 7.3; also see 
Fig. 7.1).45 These axon terminals ar

.3; also see 
Fig. 7.1).45 These axon terminals are in close association with a 
capillary plexus, and they secrete substances, including AVP and 
oxytocin, into the hypophysial veins and into the general circula­
tion (Table 7.1). The blood supply to the neurohypophysis arises 
from the inferior hypophysial artery (a branch of the internal 
carotid artery). Glial-like cells called pituicytes comprise ∼25% 
of the posterior pituitary and have an important role in regu­
lating vasopressin and oxy

important role in regu­
lating vasopressin and oxytocin secretion by at least two major 
mechanisms. These include structural modifications by engulfing 
or retracting from neurosecretory axon endings and the release 
of hormone modulators such as galanin-like peptide (GALP) and 
taurine that stimulate or inhibit the release of vasopressin, respec­
tively.46 As the source of AVP to the general circulation, the PVH 
and SON and their axon terminals in the neural lobe are the main 
effector arms f

s in the neural lobe are the main 
effector arms for the central regulation of blood osmolality, fluid 
balance, and blood pressure (see Chapter 10).
The secretion of oxytocin by magnicellular neurons is critical 
at parturition, resulting in uterine myometrial contraction. In 
A
B
• Fig. 7.2  Normal anatomy of the human hypothalamic-pituitary unit in (A) sagittal and (B) coronal planes. 
Structures that are visible in the T1-weighted magnetic resonance images (left panels) are identified in the

sonance images (left panels) are identified in the 
corresponding diagrams (right panels). The hypothalamus is bounded anteriorly by the optic chiasm, later­
ally by the sulci formed with the temporal lobes, and posteriorly by the mammillary bodies (in which the 
mammillary nuclei are located). Dorsally, the hypothalamus is delineated from the thalamus by the hypo­
thalamic sulcus. The smooth, rounded base of the hypothalamus is the tuber cinereum; the pituitary stalk 
descends from its central 

m; the pituitary stalk 
descends from its central region, which is termed the median eminence. The median eminence stands out 
from the rest of the tuber cinereum because of its dense vascularity, which is formed by the primary plexus 
of the hypothalamic-hypophysial portal system. The long portal veins run along the ventral surface of the 
pituitary stalk. Note the location of the pituitary stalk, the hyperintense signal (white) from the posterior 
pituitary (PP) (A, left), and the anatomic rel

or 
pituitary (PP) (A, left), and the anatomic relationships of the pituitary gland to the optic chiasm (oc) and the 
sphenoidal and cavernous sinuses. ac, anterior commissure; AP, anterior pituitary; cc, corpus callosum; 
MB, mammillary body; pc, posterior commissure. (Magnetic resonance images courtesy Dr. D.M. Cook.)
120
addition, the secretion of oxytocin is regulated by the classic milk 
let-down reflex. Mechanosensory information from the nipple 
re

ex. Mechanosensory information from the nipple 
reaches the magnicellular neurons, directly or indirectly, from the 
dorsal horn of the spinal cord, resulting in a synchronized burst 
of action potentials in the whole population of oxytocin neurons, 
followed by the release of oxytocin into the general circulation.47 
Oxytocin acts on receptors on myoepithelial cells in the mammary 
gland acini leading to the release of milk into the ductal system and, 
ultimately, the release of milk from the m

m and, 
ultimately, the release of milk from the mammary gland. Oxytocin 
also acts on other regions of the brain, such as the medial preoptic 
area, ventral tegmental area, nucleus accumbens, and amygdala, 
to affect social behavior (particularly parental behavior) by reduc­
ing the stress response and increasing empathy, among others, 
whereas vasopressin may mediate various forms of aggression.48 
Vasopressin also synergizes with the corticotropin-releasing hor­
mone (CRH) to release adrenoco

opin-releasing hor­
mone (CRH) to release adrenocorticotropin hormone (ACTH) 
secretion (see upcoming discussion). A potential therapeutic role 
for oxytocin in the treatment of autism has been proposed.49 
The Median Eminence and Hypophysiotropic 
Neuronal System
The median eminence is the functional link between the hypo­
thalamus and the anterior pituitary gland. It lies in the center 
of the tuber cinereum and is composed of an extensive array of 
blood vessels and nerve endings (Fig. 7.4; a

y of 
blood vessels and nerve endings (Fig. 7.4; also see Fig. 7.2).50 Its 
extremely rich blood supply arises from the superior hypophysial 
artery (a branch of the internal carotid artery), which sends off 
many small branches that form capillary loops. The small cap­
illary loops extend into the internal and external zones of the 
median eminence, form anastomoses, and drain into sinusoids 
that become the pituitary portal veins that enter the vascular pool 
of the pituitary gland. The flow o

 vascular pool 
of the pituitary gland. The flow of blood in these short loops is 
thought to be predominantly (if not exclusively) in a hypotha­
lamic-to-pituitary direction. This well-developed plexus results in 
a tremendous increase in the vascular surface area. In addition, 
the vessels are fenestrated, allowing diffusion of the peptide-releas­
ing factors to their site of action in the anterior pituitary gland. 
Because this vascular complex in the base of the hypothalamus 
and its “arteri

x in the base of the hypothalamus 
and its “arteriolized” venous drainage to the pituitary compose a 
circulatory system analogous to the portal vein system of the liver, 
it has been termed the hypothalamic-hypophysial–portal circulation.
Three distinct compartments of the median eminence are rec­
ognized: the innermost ependymal layer, the internal zone, and 
the external zone (see Fig. 7.4).50 Ependymal cells form the floor 
of the third ventricle and are unique in that they have microvilli 

icle and are unique in that they have microvilli 
rather than cilia. Tight junctions at the ventricular pole of the 
ependymal cells prevent the diffusion of high-molecular-weight 
substances between the cerebrospinal fluid (CSF) and the extracel­
lular space within the median eminence. The ependymal layer also 
ot
ot
3v
3v
3v
ME
Arc
SON
PVH
PVH
SON
mp
pm
A
B
C
D
• Fig. 7.3  The tuberoinfundibular system is revealed by retrograde transport of cholera toxin subunit B 
(CtB). The location of hypot

lera toxin subunit B 
(CtB). The location of hypothalamic cell bodies of neurons projecting to the median eminence (ME) and 
the posterior pituitary can be identified by microinjecting a small volume of the retrograde tracer CtB into 
the median eminence of the rat. (A) Retrogradely labeled cells can be seen in the paraventricular (PVH) 
and supraoptic nuclei of the hypothalamus (SON). (B) Magnicellular neurons are observed in the SON. (C) 
Labeled neurons are found in the posterior magnicellula

ed neurons are found in the posterior magnicellular group (pm) as well as the medial parvicellular 
subdivision (mp). The labeled cells in the PVH include those that contain corticotropin-releasing hormone 
(CRH) and thyrotropin-releasing hormone (TRH). (D) Retrogradely labeled cells are also found in the arcu­
ate nucleus of the hypothalamus (Arc). These include neurons that release growth hormone–releasing 
hormone (GHRH) and dopamine. ot, optic tract; 3v, third ventricle.
121
CHAPTER 7  Neuro

c tract; 3v, third ventricle.
121
contains specialized cells, called tanycytes,51 that send processes 
into the other layers of the median eminence and into the medio­
basal hypothalamus. Functionally, tanycytes comprise part of the 
blood-brain and blood-CSF barriers as they express tight junction 
proteins, including occludins, ZO1, and claudins,52 regulating 
access of circulating substances and/or substances secreted into 
the median eminence from entering the h

eted into 
the median eminence from entering the hypothalamus or CSF.53 
However, it is becoming increasingly recognized that tanycytes 
have a number of important neuroendocrine functions. Included 
are hormone transport, regulation of hypophysiotropic hormone 
release, thyroid hormone homeostasis, regulation of reproduction, 
nutrient sensing, changes in energy homeostasis, and to function 
as stem/progenitor cells for neurogenesis and gliogenesis, even 
in adult animals.54–57 Some of these fu

sis, even 
in adult animals.54–57 Some of these functions are discussed in 
greater detail later in the chapter.
The internal zone of the median eminence is composed of 
axons of the SON and PVH magnicellular neurons that contain 
characteristic, large neurosecretory granules or Herring bod­
ies passing en route to the posterior pituitary (see Fig. 7.4C). In 
addition, axons of the hypophysiotropic neurons and processes of 
  Neurotransmitters and Neuromodulators 
in the Paraventricular Nucleus

nd Neuromodulators 
in the Paraventricular Nucleus and the 
Arcuate Nucleus of the Hypothalamus
Paraventricular Nucleus
Arcuate Nucleus
Magnicellular Division
Angiotensin II
Cholecystokinin (CCK)
Corticotropin-releasing hormone 
(CRH)
Dynorphins
Glutamate
Nitric oxide (NO)
Oxytocin
Phoenixin
Spexin
Urocortin 1
Vasopressin (AVP)
Parvicellular Divisions
γ-Aminobutyric acid (GABA)
Atrial natriuretic factor (ANF)
Bombesin-like peptides
Corticotropin-releasing hor

(CRH)
Dopamine
Endocannabinoids
Enkephalins
Galanin
Glutamate
Interleukin 1 (IL-1)
Nesfatin-1
Neuropeptide Y (NPY)
Neurotensin
Peptide histidine isoleucine (PHI)
Pituitary adenylate cyclase 
­activating polypeptide 
(PACAP)
RFamide-related peptides (RFRP)
Somatostatin (SST)
Thyrotropin-releasing hormone 
(TRH)
Vasoactive intestinal peptide 
(VIP)
Acetylcholine
Agouti-related pep

line
Agouti-related peptide (AgRP)
Cocaine-regulated and 
amphetamine-regulated 
transcript (CART)
Dopamine
Dynorphin
Galanin
Galanin-like peptide (GALP)
Ghrelin
Glutamate
Gonadotropin-releasing hormone 
(GnRH)
Growth hormone–releasing hor­
mone (GHRH)
Kisspeptins
Melanocortins (ACTH, α-MSH, β-
MSH, γ-MSH)
Neurokinin B (NKB)
Neuromedin U
Nociceptin/orphanin FQ (OFQ)
Opioids (β-endorphin) peptides

/orphanin FQ (OFQ)
Pancreatic polypeptide
Phoenixin
Pro-opiomelanocortin (POMC)
Pyro-glutamyl-RFamide peptide 
(QRFP)
Substance P
VGF
TABLE 7.1
A
B
C
Arc
Arc
Arc
3v
3v
3v
CRH-IR
TRH-IR
ME (ext)
ME (ext)
AVP-IR
ME (int)
• Fig. 7.4  The median eminence is the functional connection between the 
hypothalamus and the pituitary gland. (A and B) Distribution of corticotro­
pin-releasing hormone and thyrotropin-releasing hormone immunorea

ormone and thyrotropin-releasing hormone immunoreactiv­
ity (CRH-IR and TRH-IR) in the external layer of the median eminence (ME 
ext) of the rat. CRH and TRH cell bodies reside in the medial division of 
the paraventricular hypothalamic nucleus. (C) Arginine vasopressin immu­
noreactivity (AVP-IR) in nerve endings in the internal layer of the median 
eminence (ME int). Arc, arcuate nucleus; 3v, third ventricle.
122
tanycytes destined for the external lay

 Pituitary
tanycytes destined for the external layer of the median eminence 
are seen (see Fig. 7.4A and B).
Finally, the external zone of the median eminence represents 
the exchange point of the hypothalamic releasing factors and the 
pituitary portal vessels.50 In addition to the fenestrated portal 
capillaries and cytoplasmic extensions and end-feet processes of 
tanycytes, it contains numerous fine-caliber, unmyelinated axons 
and axon terminals of hypophysiotropic neurons that contain 
all

nals of hypophysiotropic neurons that contain 
all of the hypothalamic releasing and inhibiting substances. Two 
general types of neurons project to the external zone and com­
prise the so-called hypothalamic tuberoinfundibular system: (1) 
peptide-secreting (peptidergic) neurons, including thyrotropin-
releasing hormone (TRH), CRH, growth hormone–releasing 
hormone (GHRH), somatostatin, and GnRH (see Fig. 7.1), and 
(2) neurons containing monoamines (e.g., dopamine, serotonin). 
Although the se

ines (e.g., dopamine, serotonin). 
Although the secretion of these substances into the portal circu­
lation is an important control mechanism, some peptides and 
neurotransmitters in nerve endings are not released into the hypo­
thalamic-hypophysial–portal circulation but instead function to 
regulate the secretion of other nerve terminals. In addition, some 
nerve terminals, such as GnRH nerve endings, are enveloped by 
tanycytes, which cover or uncover axonal endings in response to 
changes in

 uncover axonal endings in response to 
changes in functional status.58,59 Thus supporting elements, with 
their own sets of receptors, can change the neuroregulatory milieu 
within the hypothalamus, median eminence, and pituitary (for 
further details, see “Physiologic Roles of Melatonin” and “Gonad­
otropin-Releasing Hormone and Control of the Reproductive 
Axis” [specifically “Regulatory Systems”]). As in the case of neu­
rohormone secretion from the neurohypophysis, depolarization of 
hypoth

rom the neurohypophysis, depolarization of 
hypothalamic neurons leads to the release of neuropeptides and 
monoamines at the median eminence.
The site of production, the genetics, and the regulation of 
synthesis and release of individual peptide-releasing factors are 
discussed in detail in later sections. Briefly, there are several cell 
groups in the medial hypothalamus that contain releasing fac­
tors that are secreted into the pituitary portal circulation (Table 
7.2). These cell groups in

tal circulation (Table 
7.2). These cell groups include the infundibular nucleus (called 
the arcuate nucleus in rodents) (see Fig. 7.3D), PVH (see Fig. 
7.3A and C), periventricular nucleus, and a group of cells in the 
medial preoptic area near the organum vasculosum of the lamina 
terminalis (OVLT) (Fig. 7.5). As discussed earlier, magnicellular 
neurons in the SON and PVH send axons that predominantly 
traverse the internal zone of the median eminence to terminate 
in the neural lobe of the 

 eminence to terminate 
in the neural lobe of the pituitary. However, a smaller number of 
magnicellular axons project directly to the external zone of the 
median eminence and may facilitate ACTH secretion.
In addition to axon terminals in the external zone of the median 
eminence, densely packed fibers derived from the sphenopalatine 
ganglion that contain vasoactive intestinal peptide (VIP) and the 
nitric oxide (NO)–synthesizing enzyme, nitric oxide synthase, 
are present on the ventral surf

c oxide synthase, 
are present on the ventral surface of the median eminence, sur­
rounding portal vessels and innervating smooth muscle of precap­
illary arterioles that supply the portal capillary plexus.60 Because 
both VIP and NO are potent vasodilators, these substances may 
have an important role in regulating the rate of blood flow to the 
median eminence and hence to the anterior pituitary.
The third structure often grouped as a component of the median 
eminence is a subdivision of the a

 of the median 
eminence is a subdivision of the adenohypophysis called the pars 
tuberalis. It is a thin sheet of glandular tissue that lies around the 
infundibulum and pituitary stalk. In some animals, the epithelial 
component may make up as much as 10% of the total glandular 
tissue of the anterior pituitary. The pars tuberalis contains cells 
that produce luteinizing hormone (LH) and thyrotropin (thyroid-
stimulating hormone [TSH]). As opposed to thyrotrophs in the 
pars distalis, however,

sed to thyrotrophs in the 
pars distalis, however, TSH-producing cells in the pars tuberalis 
are smaller and lack receptors for TRH.61 The physiologic role of 
the pars tuberalis in humans remains somewhat enigmatic, but 
it is clearly part of the photosensing pathway in the CNS as it 
expresses high concentrations of the melanocortin 1 receptor. Fur­
thermore, in some mammals and birds, the pars tuberalis has an 
established role in the regulation of seasonal reproduction, energy 
balance, and

ion of seasonal reproduction, energy 
balance, and body weight orchestrated through the release of 
melatonin.62–65 The pars tuberalis also secretes a number of other 
substances, including neurokinin A, substance P, and endocan­
nabinoids that are involved in the seasonal release of prolactin in 
some animal species through direct effects on the pars distalis.66 
Circumventricular Organs
A guiding principle of neurophysiology and neuropharmacology 
is that the brain, including the hypothalamus,

gy 
is that the brain, including the hypothalamus, resides in an envi­
ronment that is protected from humoral signals.58,67 The exclusion 
of macromolecules is due to the structural vascular specializations 
that make up the blood-brain barrier.68 These specializations 
include tight and adherens junctions of brain vascular endothelial 
 Structural Formulas of Principal Human 
Hypothalamic Peptides Directly Related to 
Pituitary Hormone Secretiona
Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Arg-Gly

na
Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Arg-Gly-NH2 (MW = 1084.38)
Oxytocin
Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH2 (MW = 1007.35)
Thyrotropin-Releasing Hormone
pGlu-His-Pro-NH2 (MW = 362.42)
Gonadotropin-Releasing Hormone
pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2 (MW = 1182.39)
Corticotropin-Releasing Hormone
Ser-Glu-Glu-Pro-Pro-Ile-Ser-Leu-Asp-Leu-Thr-Phe-His-Leu-Leu-Arg-
Glu-Val-Leu-Glu-Met-Ala-Arg-Ala-Glu-Gln-Leu-Ala-Gln-Gln-Ala-His-
Ser-Asn-Arg-Lys-Leu-Met-Glu-Ile-Ile-NH2 (MW = 4758.1

r-Asn-Arg-Lys-Leu-Met-Glu-Ile-Ile-NH2 (MW = 4758.14)
Growth Hormone–Releasing Hormone
Tyr-Ala-Asp-Ala-Ile-Phe-Thr-Asn-Ser-Tyr-Arg-Lys-Val-Leu-Gly-Gln-Leu-
Ser-Ala-Arg-Lys-Leu-Leu-Gln-Asp-Ile-Met-Ser-Arg-Gln-Gln-Gly-Glu-
Ser-Asn-Gln-Glu-Arg-Gly-Ala-Arg-Ala-Arg-Leu-NH2 (MW = 5040.4)
Somatostatin
Ala-Gly-Cys-Lys-Asn-Phe-Phe-Trp-Lys-Thr-Phe-Thr-Ser-Cys (MW = 
1638.12)
Vasoactive Intestinal Peptide
His-Ser-Asp-Ala-Val-Phe-Thr-Asp-Asn-Tyr-Thr-Arg-Leu-Arg-Lys-Gln-
Met-Ala-Val-Lys-Lys-Tyr-Leu-Asn-Ser-Il

rg-Lys-Gln-
Met-Ala-Val-Lys-Lys-Tyr-Leu-Asn-Ser-Ile-Leu-Asn-NH2 (MW = 
3326.26)
aDisulfide bonds between pairs of cystines that produce cyclization of the peptides are indi­
cated by their italicized cognate Cys residues.
MW, Molecular weight; pGlu, pyroglutamyl.
   
TABLE 7.2
123
cells that preclude the free passage of polarized macromolecules, 
including peptides and hormones. In addition, astrocytic foot 
processes and perivascular microglial cells contribute to 

s and perivascular microglial cells contribute to the integ­
rity of the blood-brain barrier. However, to exert homeostatic 
control, the brain must assess key sensory information from the 
bloodstream, including hormone levels, metabolites, and poten­
tial toxins. For example, to monitor key signals the brain has “win­
dows on the circulation,” or circumventricular organs (CVOs), 
that serve as a conduit of peripheral cues into key neuronal cell 
groups that maintain homeostasis.67
As the name 

As the name implies, CVOs are specialized structures that 
lie on the midline of the brain along the third and fourth ven­
tricles. These structures include the OVLT, subfornical organ 
(SFO), median eminence, neurohypophysis (posterior pituitary), 
subcommissural organ (SCO), and area postrema (see Fig. 7.5). 
Unlike the vasculature in the rest of the brain, the blood vessels 
in CVOs have fenestrated capillaries that allow relatively free pas­
sage of mole

aries that allow relatively free pas­
sage of molecules such as proteins and peptide hormones. Thus 
neurons and glial cells that reside within the CVOs have access 
to these macromolecules. In addition to the distinct nature of 
the vessels themselves, the CVOs have an unusually rich blood 
supply, allowing them to act as integrators at the interface of the 
blood-brain barrier. Several of the CVOs have major projections 
to hypothalamic nuclear groups that regulate homeostasis. There­
fore the

 groups that regulate homeostasis. There­
fore the CVOs serve as a critical link between peripheral meta­
bolic cues, hormones, and potential toxins and cell groups within 
the brain that regulate coordinated endocrine, autonomic, and 
behavioral responses. Detailed discussion of the physiologic roles 
of individual CVOs is beyond the scope of this chapter, but several 
in-depth reviews have assessed the function of each.67,69–71
Median Eminence
The median eminence (described earlier) has an ess

The median eminence (described earlier) has an essential role in 
the regulation of the pars distalis. The anatomic location of the 
median eminence places it in a position to serve as an afferent 
sensory organ as well as a functional link between the hypothala­
mus and the pituitary gland. Specifically, the median eminence is 
located adjacent to several neuroendocrine and autonomic regula­
tory nuclei at the tuberal level of the hypothalamus (see Fig. 7.3). 
These nuclear groups include the i

see Fig. 7.3). 
These nuclear groups include the infundibular or arcuate, ventro­
medial, dorsomedial, and paraventricular nuclei.
The role of hypothalamic nuclei surrounding the median 
eminence as afferent sensory centers is supported by the observa­
tion that it is a portal of entry for hormones circulating in the 
bloodstream such as leptin and several gut-derived circulating hor­
mones, including ghrelin. Both leptin and ghrelin are established 
mediators of body weight and neuroendocrine f

hed 
mediators of body weight and neuroendocrine function that act 
on POMC and/or agouti-related protein (AgRP)/neuropeptide Y 
(NPY) neurons residing in the arcuate nucleus.17,72–74 Thus it is 
likely that the median eminence is involved in conveying informa­
tion from humoral factors to key hypothalamic regulatory neu­
rons in the medial basal hypothalamus.58 
Organum Vasculosum of the Lamina Terminalis 
and the Subfornical Organ
The OVLT and the SFO are located at the anterior wall of the 
t

the SFO are located at the anterior wall of the 
third ventricle, the lamina terminalis. The OVLT and SFO lie, 
respectively, at the ventral and dorsal boundaries of the third ven­
tricle (see Fig. 7.5). Because it lies at the rostral and ventral tip 
of the third ventricle, the OVLT is surrounded by cell groups of 
the preoptic region of the hypothalamus. Like other CVOs, the 
OVLT is composed of neurons, glial cells, and tanycytes. Axon 
terminals containing several neuropeptides and neurotran

als containing several neuropeptides and neurotransmit­
ters, including somatostatin, angiotensin, dopamine, norepineph­
rine, serotonin, acetylcholine, oxytocin, AVP, and TRH, innervate 
the OVLT. Unlike the median eminence, however, blood from the 
OVLT does not drain into a portal plexus, but rather primarily to 
the medial preoptic region. However, in the rodent, neurons that 
contain GnRH and surround the OVLT possess unique projec­
tions with combined properties of dendrites and axons, ter

th combined properties of dendrites and axons, termed 
dendrons, that bridge the distance between the OVLT and the 
median eminence.75 In addition, the OVLT in the rat brain con­
tains estrogen receptors, and the application of estrogen or electric 
stimulation at this site is capable of stimulating ovulation through 
GnRH-containing neurons that project to the median eminence.
Neurons in the OVLT project to many regions of the brain, 
including the preoptic nucleus, subfornical organ, arcuate n

the preoptic nucleus, subfornical organ, arcuate nucleus, 
supraoptic nucleus, medial thalamus, and parts of the limbic 
system. Thus the OVLT is strategically placed to receive blood-
borne information and then transmit this information to specific 
regions of the brain. Therefore, it is not surprising that the OVLT 
is involved in a diverse array of processes. For example, lesions 
of the OVLT and the surrounding preoptic area led to altered 
febrile responses after immunologic stimulation and

ebrile responses after immunologic stimulation and disruptions 
in fluid and electrolyte balance, blood pressure, reproduction, and 
thermoregulation. Large lesions of the OVLT attenuate lipopoly­
saccharide-induced fever.76 Consistent with this finding, it has 
been demonstrated that receptors for prostaglandin E2 are located 
within and immediately surrounding the OVLT.77 Because pros­
taglandin E2 is thought to be an obligate endogenous pyrogen, the 
OVLT may be a critical regulator of febril

n, the 
OVLT may be a critical regulator of febrile responses.
OVLT
SFO
CP
ME
NH
SCO
PI
CP
AP
cc
oc
ac
pc
• Fig. 7.5  Median sagittal section through the human brain to show the 
circumventricular organs (dark brown). Light brown areas are the optic 
chiasm (oc), corpus callosum (cc), anterior (ac) and posterior (pc) commis­
sures. AP, area postrema; CP, choroid plexus; ME, median eminence; NH, 
neurohypophysis; OVLT, organum vasculosum of the lamina terminalis; 
PI, pineal gland; SCO, subcommis

mina terminalis; 
PI, pineal gland; SCO, subcommissural organ; SFO, subfornical organ. 
(Adapted from Weindl A. Neuroendocrine aspects of circumventricular 
organs. In: Ganong WF, Martini L, eds. Frontiers in Neuroendocrinology, 
vol 3. New York: Oxford University Press; 1973:3–32.)
124
The OVLT is also likely to be involved in sensing serum osmo­
lality through osmoreceptor cells that express the transient recep­
tor potential vanilloid (TRPV) subfamilie

t recep­
tor potential vanilloid (TRPV) subfamilies 1 and 4 genes78 and 
respond to circulating levels of angiotensin II and relaxin.79,80 In 
addition, lesions of the OVLT attenuate AVP and oxytocin secre­
tion in response to osmotic stimuli, whereas hypertonic saline 
administration induces activation in OVLT neurons.81
The SFO is located in the roof of the third ventricle below 
the fornix. This CVO critically regulates fluid homeostasis and 
contributes to blood pressure regulation.67 Consis

contributes to blood pressure regulation.67 Consistent with these 
functions, the SFO has receptors for angiotensin II and atrial 
natriuretic peptide.82,83 Through direct projections to the para­
ventricular and supraoptic nuclei, SFO neurons induce the release 
of vasopressin from the posterior pituitary and activate paraven­
tricular nucleus neurons that descend to sympathetic centers of 
the spinal cord that regulate vasoconstriction as well as other neu­
ronal sites involved in fluid and bl

as other neu­
ronal sites involved in fluid and blood pressure balance, including 
the median preoptic nucleus and OVLT.84
The critical importance of the SFO in maintaining fluid bal­
ance was demonstrated by Simpson and Routtenberg, showing 
that substances such as angiotensin II elicited drinking behavior 
when microinjected at low doses directly into the SFO.85 Later 
studies demonstrated that SFO neurons have electrophysiologic 
responses to angiotensin II and that stimulation of the SFO 
el

angiotensin II and that stimulation of the SFO 
elicits AVP secretion.82 Like the OVLT, the SFO is activated by 
hypertonic saline administration.81 Importantly, the use of Cre 
recombinase technology combined with optogenetics has enabled 
researchers to demonstrate that excitation of SFO neurons that 
express neuronal nitric oxide synthase (nNOS) and/or calmod­
ulin kinase II (CaMKII) stimulates thirst, whereas excitation of 
vesicular GABA transporter–expressing neurons suppress thirst.86 
Su

nsporter–expressing neurons suppress thirst.86 
Subsequent studies have also established that chemogenetically 
activated CaMKII neurons in the SFO stimulate NaCl intake, 
indicating that the SFO is also the principal site for monitoring 
sodium levels.87 Sodium sensing is mediated by NaX, the brain 
sodium-level sensor, which is expressed by glial cells in the SFO 
and by the release of lactate, activate GABAergic SFO neurons.88 
While most of the experimental work to establish the role of the 

he experimental work to establish the role of the 
SFO in salt and water balance has been done in rodents, clear 
evidence that the SFO mediates similar functions in humans is 
established by the demonstration in a pediatric patient with adip­
sic hypernatremia due to autoantibodies that target NaX.89 
Area Postrema
The area postrema lies at the caudal end of the fourth ventricle 
adjacent to the nucleus of the tractus solitarius (NTS) (see Fig. 
7.5). In rodents, it is a midline structure lying

7.5). In rodents, it is a midline structure lying above the NTS.67,90 
However, in humans the area postrema is a bilateral structure. 
Because the area postrema overlies the NTS, it also receives direct 
visceral afferent input from the glossopharyngeal nerve and the 
vagus nerves. In addition, the area postrema receives direct input 
from several hypothalamic nuclei. The efferent projections of 
the area postrema include projections to the NTS, ventral lat­
eral medulla, and parabrachial nucle

 ventral lat­
eral medulla, and parabrachial nucleus. Consistent with its role 
as a sensory organ, the area postrema is enriched with receptors 
for several neuropeptides, including glucagon-like peptide-1 and 
amylin.91 It also contains chemosensory neurons that include 
osmoreceptors. The area postrema is thought to be critical in the 
detection of potential toxins and in the induction of vomiting in 
response to foreign substances. In fact, the area postrema is often 
referred to as the chem

he area postrema is often 
referred to as the chemoreceptor trigger zone.91
The best described physiologic role of the area postrema is the 
coordinated control of blood pressure.67 The area postrema con­
tains binding sites for angiotensin II, AVP, and atrial natriuretic 
peptide. Lesions of the area postrema in rats blunt the rise in 
blood pressure induced by angiotensin II.92 Finally, administra­
tion of angiotensin II induces the expression of c-Fos in neurons 
of the area postrema, an indi

of c-Fos in neurons 
of the area postrema, an indication of its activation. The area pos­
trema has also been hypothesized to play a role in responding to 
inflammatory cytokines during the acute febrile response, CNS 
glucose sensing, and the satiating effects of amylin.92 
Subcommissural Organ
The SCO is located near the junction of the third ventricle and 
cerebral aqueduct below the posterior commissure and the pineal 
gland (see Fig. 7.5). It is composed of specialized ependymal cells 
that

t is composed of specialized ependymal cells 
that secrete a highly glycosylated protein of unknown function. 
The secretion of this protein leads to aggregation and formation 
of the so-called Reissner fibers.93 The glycoproteins are extruded 
through the aqueduct, the fourth ventricle, and the spinal cord 
lumen to terminate in the caudal spinal canal. In humans, intra­
cellular secretory granules are identifiable in the SCO, but Reiss­
ner fibers are absent. The SCO secretion in humans is the

ers are absent. The SCO secretion in humans is therefore 
presumed to be more soluble and to be absorbed directly from 
the CSF. Compared with other CVOs, the physiologic role of the 
SCO is largely unknown. Hypothesized roles for the SCO include 
clearance of substances, including monoamines from the CSF.93 
Pineal Gland
Descartes called the pineal gland the “seat of the soul.” A more 
contemporary, although less colorful, viewpoint is that the pineal 
integrates information encoded by light in

pineal 
integrates information encoded by light into coordinated secre­
tions that underlie biologic rhythmicity.94 The pineal is both an 
endocrine gland and a CVO; it is derived from cells located in 
the roof of the third ventricle and lies above the posterior com­
missure near the level of the habenular complex and the sylvian 
aqueduct. The gland is composed of two cell types, pinealocytes 
and interstitial (glial-like) cells. Histologic studies suggest that the 
pineal gland cells are secr

dies suggest that the 
pineal gland cells are secretory in nature, and indeed the pineal is 
the principal source of melatonin in mammals.
The pineal gland is an epithalamic structure and consists of pri­
mordial photoreceptive cells. The gland retains its light sensitivity 
in lower vertebrates such as fish and amphibians but lacks direct 
photosensitivity in mammals and has evolved as a strictly secretory 
organ in higher vertebrates. However, neuroanatomic studies have 
established that light

neuroanatomic studies have 
established that light-encoded information is relayed to the pineal 
by a polysynaptic pathway.95 This series of synapses ultimately 
results in innervation of the gland by noradrenergic sympathetic 
nerve terminals that are critical regulators of melatonin production 
and release. Specifically, retinal ganglion cells directly innervate 
the SCN of the hypothalamus through the retinohypothalamic 
tract. In turn, the SCN provides input to the dorsal parvicellular 
PVH,

N provides input to the dorsal parvicellular 
PVH, a key cell group in neuroendocrine and autonomic con­
trol. This pathway consists of direct and indirect intrahypotha­
lamic projections. The PVH then provides direct innervation to 
sympathetic preganglionic neurons in the intermediolateral cell 
column of the thoracic regions of the spinal cord. Sympathetic 
preganglionic neurons innervate postganglionic neurons in the 
superior cervical ganglion and ultimately supply the noradrener­
gic inner

on and ultimately supply the noradrener­
gic innervation to the pineal gland (see “Hypothalamic-Pituitary 
Unit”). This circuitous pathway represents the anatomic substrate 
125
for light to regulate the secretion of melatonin through the release 
of norepinephrine. In the presence of light, whether morning light 
or a light impulse, norepinephrine release is inhibited, resulting 
in the shutdown of melatonin synthesis. In the absence of light 
input, the pineal gla

is. In the absence of light 
input, the pineal gland rhythms persist but are not entrained to 
the external light-dark cycle.
The Pineal Is the Source of Melatonin
The predominant hormone secreted by the pineal gland is mela­
tonin. However, the pineal contains other biogenic amines, pep­
tides, and GABA. Pineal-derived melatonin is synthesized from 
tryptophan, which is first converted into 5-hydroxytrophan by 
tryptophan hydroxylase and then decarboxylated into serotonin 
(5-HT). Two additiona

carboxylated into serotonin 
(5-HT). Two additional steps transform 5-HT into melatonin, 
including the rate-limiting step catalyzed by the enzyme aralkyl­
amine N-acetyltransferase (AANAT) to yield N-acetylserotonin, 
followed by the transfer of a methyl group from S-adenosyl methi­
onine to the 5-hydroxy group of N-acetylserotonin by hydroxy­
indole-O-methyltransferase (HIOMT) (Fig. 7.6).96 Melatonin 
plays a key role in regulating a myriad of circadian rhythms, and 
a fundamental principle of

circadian rhythms, and 
a fundamental principle of circadian biology is that the synthesis 
of melatonin is exquisitely controlled.97 AANAT-mRNA levels, 
AANAT activity, and melatonin synthesis and release are regulated 
in a circadian fashion and are entrained by the light-dark cycle, 
with darkness thought to be the most important signal.94,96 Mela­
tonin and AANAT levels are highest during the dark and decrease 
sharply with the onset of light. Melatonin is not stored to any 
significant degr

. Melatonin is not stored to any 
significant degree; it is released into the blood or CSF directly 
after its biosynthesis in proportion to AANAT activity but has a 
very rapid half-life (minutes) due to 6-hydroxylation in the liver.
Lack of light ultimately results in the release of norepineph­
rine from postganglionic sympathetic nerve terminals that act on 
β-adrenergic receptors in pinealocytes, resulting in an increase in 
adenylyl cyclase activity and synthesis of cyclic adenosine mono­
p

activity and synthesis of cyclic adenosine mono­
phosphate (cAMP) from adenosine triphosphate. Increased levels 
of intracellular cAMP activate downstream signal transduction 
cascades, including the catalytic subunits of protein kinase A and 
phosphorylation of cAMP response element (CRE) binding pro­
tein. CREs have been identified in the promoter of AANAT.98 
Therefore light (or lack of it) acting through the sympathetic 
nervous system induces an increase in cAMP, representing a fun­
damenta

s an increase in cAMP, representing a fun­
damental regulator of AANAT transcription and melatonin syn­
thesis that ultimately results in a dramatic change of melatonin 
levels across the day.95 Interspecies comparative studies of melato­
nin’s physiologic function must be tempered by knowledge of the 
key differences between rodent and human melatonin regulation, 
however. For example, significantly more light, as much as 4 log 
units, is required in humans to produce an equivalent nocturnal 
s

ed in humans to produce an equivalent nocturnal 
suppression of melatonin,99 and the control of AANAT is largely 
post-transcriptional in humans rather than transcriptional.96 
Physiologic Roles of Melatonin
One of the best-characterized roles of melatonin is the regulation 
of the reproductive axis, including gonadotropin secretion100 and 
the timing and onset of puberty (see “Gonadotropin-Releasing 
Hormone and Control of the Reproductive Axis”). The potent 
regulation of the reproductive axis

). The potent 
regulation of the reproductive axis by melatonin is established 
in rodents and domestic animals such as sheep. It was observed 
experimentally with the demonstration that removal of the pineal 
leads to precocious puberty. In addition, male rats exposed to con­
stant darkness or blinded by enucleation display testicular atro­
phy and decreased levels of testosterone. These profound effects 
of gonadal involution are normalized by removal of the pineal 
gland.95 The physiologic si

removal of the pineal 
gland.95 The physiologic significance of melatonin is probably 
most important in species referred to as seasonal breeders. Indeed, 
the role of melatonin in regulating reproductive capacity in species 
such as the sheep and the horse is now established. This type of 
reproductive strategy probably evolved to synchronize the length 
of day with the gestational period of the species to ensure that the 
offspring are born at favorable times of the year and maximize the 
viab

favorable times of the year and maximize the 
viability of the young. Interestingly, although there is a strong and 
consistent correlation between altered melatonin secretion, day 
5-Hydroxytryptophan
Serotonin
Melatonin
N-Acetylserotonin
Tryptophan
hydroxylase
Aromatic-L-amino acid
decarboxylase
Arylalkylamine 
N-Acetyltransferase
+ Acetyl-CoA
Hydroxyindole-
O-methyltransferase
+ S-Adenosylmethionine
N
H
H
C
H
H
C
C O
OH
NH2
H
C
H
H
C
C O
OH
NH2
N
H
HO 
H
C
H
H
C
H
NH2
N
H
HO 
N
H
HO 
H
C
H
H

2
N
H
HO 
H
C
H
H
C
H
NH2
N
H
HO 
N
H
HO 
H
C
H
H
C
H
N
H O
C
CH3
N
H
CH3O 
H
C
H
H
C
H
N
H O
C
CH3
1
2
3
4
• Fig. 7.6  Biosynthesis of melatonin from tryptophan in the pineal gland. Step 1 is catalyzed by tryptophan 
hydroxylase, step 2 by aromatic-l-amino acid decarboxylase, step 3 by arylalkylamine N-­acetyltransferase, 
and step 4 by hydroxyindole-O-methyltransferase. (From Wurtman RJ, Axelrod J, Kelly DE. Biochemistry 
of the pineal gland. In: Wurtman RJ, Axelrod J, Kelly DE, eds

al gland. In: Wurtman RJ, Axelrod J, Kelly DE, eds. The Pineal. New York: Academic Press; 
1968:47–75.)
126
length, and seasonal breeding in diverse species, the valence of the 
signal can be either positive or negative dependent on the ecologic 
niche for each species.
The signaling pathway by which melatonin regulates repro­
duction and puberty in seasonal species is becoming increasingly 
understood. Changes in the nocturnal duration of melatonin con­

anges in the nocturnal duration of melatonin con­
vey photoperiodic information to cells in the pars tuberalis via the 
MT1 receptor, which is expressed in high density on these cells to 
regulate clockwork proteins such as Cry 1 that are linked to the 
regulation of TSHβ synthesis.101,102 TSHβ acting as a paracrine 
signal from the pars tuberalis binds to TSH receptors on tanycytes 
whose end-feet processes reside immediately dorsal to the pars 
tuberalis. TSHβ regulates the expression of type 

tuberalis. TSHβ regulates the expression of type 2 (D2) and type 
3 (D3) iodothyronine deiodinase in tanycytes, enzymes that either 
activate or inactivate the thyroid hormone, respectively, thus gov­
erning the bioavailabilty of the thyroid hormone in the mediobasal 
hypothalamus.103–105 Under long day conditions (or short melato­
nin duration), D2 is upregulated, thereby increasing T3 availability, 
whereas under short day conditions (or long melatonin duration), 
D3 is upregulated, resulting

melatonin duration), 
D3 is upregulated, resulting in decreased T3 availability in the 
mediobasal hypothalamus. An increase in T3 bioavailabilty results 
in reproductive activation and can be replicated by implanting a 
T3 pellet into the hypothalamus of seasonal breeders maintained 
in short days that otherwise show reproductive inactivation.106 The 
mechanism by which the thyroid hormone affects GnRH secretion 
involves regulation of the encasement of GnRH axon terminals 
in the external zone

ement of GnRH axon terminals 
in the external zone of the median eminence by tanycyte end-feet 
processes. When thyroid hormone is administered into the hypo­
thalamus, it promotes retraction of these processes to allow direct 
access of secreted GnRH to the fenestrated portal vessels for con­
veyance to the pars distalis.107,108 Thyroid hormone receptors are 
also plentiful throughout the hypothalamus and may be involved 
in the regulation of kisspeptin,109 a hormone that has a critical role 
i

sspeptin,109 a hormone that has a critical role 
in the pulsatile release of GnRH (see “Gonadotropin-Releasing 
Hormone and Control of the Reproductive Axis”). 
Melatonin Receptors
Melatonin mediates some of its effects by acting on a family of 
GPCRs, which have been characterized by pharmacologic, neu­
roanatomic, and molecular approaches.94,96,97 The first member 
of the family, MT1 (Mel1a), is a high-affinity receptor that was 
isolated originally from Xenopus melanophores. The second, MT2 

nally from Xenopus melanophores. The second, MT2 
(Mel1b), has approximately 60% homology with MT1. A third 
receptor in mammals, MT3, is not a GPCR but instead a high-
affinity binding site on the cytosolic enzyme quinone reductase 2 
that is involved in cellular detoxification and might explain some 
of melatonin’s effects as an antioxidant.94,96 Melatonin also acts 
directly as a free radical scavenger to detoxify reactive oxygen and 
nitrogen species.95
The mechanisms for melatonin’s effects

 species.95
The mechanisms for melatonin’s effects on regulating and 
entraining circadian rhythms are becoming increasingly under­
stood. For example, melatonin inhibits the activity of neurons in 
the SCN of the hypothalamus, the master circadian pacemaker 
in the mammalian brain.97,110,111 Melatonin can entrain sev­
eral mammalian circadian rhythms, probably by the inhibition 
of neurons in the SCN. Neuroanatomic evidence suggests that 
many of the effects of melatonin on circadian rhythms in

f the effects of melatonin on circadian rhythms involve 
actions on MT1 receptors in that the distribution of MT1 mRNA 
overlaps with radiolabeled melatonin binding sites in the relevant 
brain regions. These sites include the SCN, the retina, and the 
pars tuberalis of the adenohypophysis. The MT2 receptor is also 
expressed in the retina and brain, particularly the SCN, but evi­
dently at much lower levels and more involved with phase-shifting 
effects of melatonin.96,97,110
Genetic studies in

Genetic studies in mice have also helped to illuminate the rela­
tive roles of each melatonin receptor in mediating the effects of 
this hormone. Targeted deletion (knockout) of the MT1, but not 
the MT2 receptor, abolished the ability of melatonin to inhibit 
the activity of SCN neurons.111,112 Several studies have suggested 
that the inhibition of SCN neurons by melatonin is of great physi­
ologic significance. Melatonin may underlie the mechanism by 
which ligh

elatonin may underlie the mechanism by 
which light induces phase shifts. However, it should be noted that 
lack of the MT1 gene does not block the ability of melatonin to 
induce phase shifts. These unexpected and somewhat confusing 
results have resulted in the hypothesis that MT2 is involved in 
melatonin-induced phase shifts, as this receptor may be expressed 
in the SCN in the human brain.96 
Melatonin Therapy in Humans
Melatonin is purported to exert multiple beneficial functions that 
inc

d to exert multiple beneficial functions that 
include slowing or reversing the progression of aging, blood pres­
sure and autonomic cardiovascular regulation, protecting against 
ischemic damage after vascular reperfusion, and enhancing immune 
function.94–96,113,114 However, the most-studied and established 
role of melatonin in humans is that of phase shifting and resetting 
circadian rhythms. In this context, melatonin has been used to treat 
jet lag and may be effective in treating circadia

jet lag and may be effective in treating circadian-based sleep disor­
ders.115 In addition, melatonin administration has been shown to 
regulate sleep in humans, although two meta-analyses of the pub­
lished reports on melatonin for the treatment of either primary or 
secondary sleep disorders concluded that there is limited evidence 
for significant clinical efficacy, but it is safe to use in the short term 
(≤3 months).116,117 Melatonin therapy has also been suggested to 
have oncostatic effe

y has also been suggested to 
have oncostatic effects on several tumors by acting as an antioxi­
dant and to reduce blood pressure in patients with hypertension, 
improve major depressive disorder and anxiety, exert anti-inflam­
matory actions in rheumatoid arthritis and osteoarthritis, and 
because of its protective role against oxidative stress, potentially be 
efficacious for the treatment of Alzheimer disease, Parkinson dis­
ease, Huntington disease, and amyotrophic sclerosis.118,119 
Hypoph

disease, and amyotrophic sclerosis.118,119 
Hypophysiotropic Hormones and 
Neuroendocrine Axes
With the demonstration by the first half of the 1900s that pituitary 
secretion is controlled by hypothalamic hormones released into 
the portal circulation, the race was on to identify hypothalamic 
releasing factors. The search for hypothalamic neurohormones 
with anterior pituitary regulating properties focused on extracts 
of the stalk median eminence, neural lobe, and hypothalamus 
from sheep and 

ce, neural lobe, and hypothalamus 
from sheep and pigs. To give some idea of the herculean nature of 
this effort, approximately 250,000 hypothalamic fragments were 
required to purify and characterize the first such factor, TRH.10 
Such hypophysiotropic substances were initially called releasing 
factors but are now more commonly called releasing hormones.
All of the principal hypothalamic-pituitary regulating hor­
mones are peptides with the notable exception of dopamine, which 
is a biogenic 

table exception of dopamine, which 
is a biogenic amine and the major prolactin-inhibiting factor (PIF) 
(see later discussion and Table 7.2). All are available for clinical 
investigations or diagnostic tests, and therapeutic analogues for 
dopamine, GnRH, and somatostatin are widely prescribed.
127
In addition to regulating hormone release, some hypophysio­
tropic factors control pituitary cell differentiation, proliferation, 
and hormone synthesis. Some act on mo

liferation, 
and hormone synthesis. Some act on more than one pituitary hor­
mone. For example, TRH is a potent releaser of prolactin (PRL) 
in addition to TSH, and under some circumstances, releases cor­
ticotropin (ACTH) and growth hormone (GH). GnRH releases 
both LH and follicle-stimulating hormone (FSH). Somatostatin 
inhibits the secretion of GH, TSH, and a wide variety of nonpi­
tuitary hormones. The principal inhibitor of PRL secretion, dopa­
mine, also inhibits secretion of TSH, gonadot

opa­
mine, also inhibits secretion of TSH, gonadotropin, and under 
certain conditions, GH. Dual control is exerted by the interaction 
of inhibitory and stimulatory hypothalamic hormones. For exam­
ple, somatostatin interacts with growth hormone–releasing hor­
mone and TRH to control secretion of GH and TSH, respectively, 
and dopamine interacts with prolactin-releasing factors (PRFs) to 
regulate PRL secretion. Some hypothalamic hormones act syner­
gistically; for example, CRH and AVP cooperat

ner­
gistically; for example, CRH and AVP cooperatively regulate the 
release of pituitary ACTH.
Secretion of the releasing hormones in turn is regulated by 
neurotransmitters and neuropeptides released by a complex array 
of neurons synapsing with hypophysiotropic neurons. Control of 
secretion is also exerted through feedback control by hormones 
such as glucocorticoids, gonadal steroids, thyroid hormone, 
anterior pituitary hormones (short-loop feedback control), and 
hypophysiotropic factors

p feedback control), and 
hypophysiotropic factors themselves (ultrashort-loop feedback 
control).
The distribution of the hypophysiotropic hormones is not 
limited to the hypothalamus and can be found in many differ­
ent regions of the brain and its periphery, produced by nonhy­
pophysiotropic cells where they mediate functions unrelated to 
pituitary regulation. Most of the peptides, hormones, and neu­
rotransmitters involved in the regulation of hypothalamic-pitu­
itary control transduce thei

of hypothalamic-pitu­
itary control transduce their signals through members of the 
extensive GPCR family.
Feedback Concepts in Neuroendocrinology
To understand the regulation of each hypothalamic-pituitary-
target organ axis, it is important to understand some basic con­
cepts of homeostatic systems. A simplified account of feedback 
control in relation to neuroendocrine regulation is presented here. 
Hormonal systems form part of a feedback loop in which the 
controlled variable (generally the

p in which the 
controlled variable (generally the blood hormone level or some 
biochemical surrogate of the hormone) determines the rate of 
secretion of the hormone. In negative feedback systems the con­
trolled variable inhibits hormone output, and in positive feedback 
control systems it increases hormone secretion. Both negative and 
positive endocrine feedback control systems can be part of a closed 
loop, in which regulation is entirely restricted to the interacting 
regulatory glands, or

stricted to the interacting 
regulatory glands, or an open loop, in which the nervous system 
influences the feedback loop. All pituitary feedback systems have 
nervous system inputs that either alter the set-point of the feed­
back control system or introduce open-loop elements that can 
influence or override the closed-loop control elements.
In engineering formulations of feedback, three controlled vari­
ables can be identified: a sensing element that detects the concen­
tration of the control

nt that detects the concen­
tration of the controlled variable, a reference input that defines 
the proper control levels, and an error signal that determines the 
output of the system. The reference input is the set-point of the 
system.
Hormonal feedback control systems resemble engineering 
systems in that the concentration of the hormone in the blood 
(or some function of the hormone) regulates the output of the 
controlling gland. However, hormonal feedback differs from engi­
neering system

ormonal feedback differs from engi­
neering systems in that the sensor element and the reference input 
element are not readily distinguishable. The set-point of the con­
trolled variable is determined by a complex cascade beginning with 
the kinetics of binding to a receptor and the activities of succes­
sive intermediate messengers. Sophisticated models incorporating 
control elements, compartmental analysis, and hormone produc­
tion and clearance rates exist for many systems. In fact, this so

nce rates exist for many systems. In fact, this sort 
of modeling applied to developmental programming, intracellular 
signaling cascades, and neural circuits in addition to endocrine 
feedback systems is commonly referred to as systems biology.120 
Endocrine Rhythms
Virtually all functions of living animals (regardless of their posi­
tion on the evolutionary scale) are subject to periodic or cyclic 
changes, many of which are influenced primarily by the nervous 
system (Table 7.3).121,122 Most 

y by the nervous 
system (Table 7.3).121,122 Most periodic changes are free running; 
that is, they are intrinsic to the organism, independent of the envi­
ronment, and driven by a biologic clock.
Most free-running rhythms are coordinated (entrained) by 
external signals (cues), such as light-dark changes, meal patterns, 
cycles of the lunar periods, or the ratio of day length to night 
  Terms Used to Describe Cyclic Endocrine 
Phenomena
Term
Period
Length of the cycle
Circadian
Abo

efinition
Period
Circadian
About a day (24 hours)
Diurnal
Exactly a day
Ultradian
Less than a day (i.e., minutes or hours)
Infradian
Longer than a day (i.e., month or year)
Mean
Arithmetic mean of all values within a 
cycle
Range
Difference between the highest and 
lowest values
Nadir
Minimal level (inferred from mathemati­
cal curve-fitting calculations)
Acrophase
Time of maximal levels (inferred from 
curve fitting)
Zeitgeber
Time giver (German); the external cue, 
usually 

r
usually the light-dark cycle that syn­
chronizes endogenous rhythms
Entrainment
The process by which an endogenous 
rhythm is regulated by a zeitgeber
Phase shift
Induced change in an endogenous 
rhythm
Intrinsic clock
Neural structures that possess intrinsic 
capacity for spontaneous rhythms; for 
circadian rhythms, these are located 
in the suprachiasmatic nucleus
Modified from Van Cauter E, Turek FW. Endocrine and other biological rhythms. In: DeGroot

ndocrine and other biological rhythms. In: DeGroot LJ, 
ed. Endocrinology. 3rd ed. Philadelphia: Saunders; 1995:2497–2548.
   
TABLE 7.3
128
length. External signals of this type (zeitgeber, or time giver) do 
not bring about the rhythm but provide the synchronizing time 
cue. Many endogenous rhythms have a period of ∼24 hours (cir­
cadian [around a day] or diurnal rhythms). Circadian changes fol­
low an intrinsic program that is ∼24 hours long, whereas d

ntrinsic program that is ∼24 hours long, whereas diurnal 
rhythms can be either circadian or dependent on shifts in light 
and dark. Rhythms that occur more frequently than once a day 
are ultradian. Infradian rhythms have a period longer than 1 day, 
as in the ∼27-day human menstrual cycle and the yearly breeding 
patterns of some animals.
Most endocrine rhythms are circadian (Fig. 7.7). The secretion 
of GH and PRL in humans is maximal shortly after the onset of 
sleep, and that of cortisol is

after the onset of 
sleep, and that of cortisol is maximal between 2 am and 4 am. 
TSH secretion is lowest between 4 pm and 7 pm and maximal 
between 9 pm and 5 am. Gonadotropin secretion in adolescents 
is increased at night. Superimposed on the circadian cycle are 
ultradian bursts of hormone secretion. LH secretion during ado­
lescence is characterized by rapid, high-amplitude pulsations at 
night, whereas in sexually mature individuals secretory episodes 
are lower in amplitude and occur thr

ory episodes 
are lower in amplitude and occur throughout the 24 hours. GH, 
ACTH, and PRL are also secreted in brief, fairly regular pulses. 
The short-term fluctuations in hormonal secretion have impor­
tant functional significance. In the case of LH, the normal endog­
enous rhythm of pituitary secretion reflects the pulsatile release 
Leptin (% change from 0800 h)
Plasma cortisol (nmol/L)
Normalized CSF IR-hCRH (SD)
Peripheral LH conc.
(ng/mL)
Serum TSH (mU/L)
Plasma melatonin (pmol/L)
24-hou

24-hour clock time
Portal Gn-RH conc.
(pg/mL)
C
140
120
100
80
60
600
400
200
0
2
1
0
−1
−2
2000
1600
1200
0800
0400
2400
2000
1600
1200
0800
F
B
E
A
D
8
30
20
10
0
6
4
2
0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
400
300
200
100
0
2000
1600
1200
0800
0400
2400
2000
1600
1200
0800
• Fig. 7.7  Diurnal rhythms of (A) corticotropin-releasing hormone, (B) cortisol, (C) leptin, (D) melatonin, and 
(E) thyrotropin (TSH) in humans; (F) the relations

(E) thyrotropin (TSH) in humans; (F) the relationship between gonadotropin-releasing hormone (GnRH) and 
luteinizing hormone (LH) secretion in sheep. CSF, cerebrospinal fluid; IR, immunoreactive. (From Kling MA, 
DeBellis MD, O’Rourke DK, et al. Diurnal variation of cerebrospinal fluid immunoreactive corticotropin-
releasing hormone levels in healthy volunteers. J Clin Endocrinol Metab. 1994;79:233–239, Fig. 3; van 
Coevorden A, Mockel J, Laurent E, et al. Neuroendocrine rhythms and sleep in agi

 E, et al. Neuroendocrine rhythms and sleep in aging men. Am J Physiol. 
1991;260:E651–E661, Fig. 1A and C; Sinha MK, Ohannesian JP, Heiman ML, et al. Nocturnal rise of leptin 
in lean, obese, and non-insulin-dependent diabetes mellitus subjects. J Clin Invest. 1996;97:1344–1347, 
Fig. 2; Brabant G, Prank K, Ranft U, et al. Physiological regulation of circadian and pulsatile thyrotropin 
secretion in normal man and woman. J Clin Endocrinol Metab. 1990;70:403–409, Fig. 2B; and Clarke IJ, 
Cummins

 1990;70:403–409, Fig. 2B; and Clarke IJ, 
Cummins JT. The temporal relationship between gonadotropin-releasing hormone (GnRH) and luteinizing 
hormone (LH) secretion in ovariectomized ewes. Endocrinology. 1982;111:1737–1739, Fig. 2A.)
129
of GnRH. The period of ∼90 minutes between LH peaks corre­
sponds to the optimal timing of GnRH pulses to induce maximal 
pituitary stimulation. Episodic secretion of GH also enhances its 
biopotency, but for many rhythms the func

ces its 
biopotency, but for many rhythms the function is not clear. Most 
homeostatic activities are also rhythmic, including body tempera­
ture, water balance, blood volume, sleep, and activity.123,124
Assessment of endocrine function must take into account the 
variability of hormone levels in the blood. Thus appropriately 
obtained samples at different times of the day or night may provide 
useful dynamic indicators of hypothalamic-pituitary function. For 
example, the loss of diurnal rhythm

function. For 
example, the loss of diurnal rhythm of GH and ACTH secretion 
may be an early sign of hypothalamic dysfunction. Furthermore, 
the optimal timing for the administration of glucocorticoids to 
suppress ACTH secretion (as in therapy for congenital adrenal 
hyperplasia) must take into account the varying suppressibility of 
the axis at different times of the day.
The best-understood neural structures responsible for circadian 
rhythms are the SCNs, paired structures in the anterior hy

are the SCNs, paired structures in the anterior hypothala­
mus above the optic chiasm.121,124 Individual cells of the SCN have 
an intrinsic capacity to oscillate in a circadian pattern due to the 
existence of a cell-autonomous transcription-translation feedback 
loop involving the transcription factors CLOCK and BMAL1 that 
interact with the promoters of the period (PER) and cryptochrome 
(CRY) genes.125 If lesioned bilaterally, free-running circadian 
rhythmicity is produced, characterized by

rcadian 
rhythmicity is produced, characterized by disruption of the sleep-
wake cycle and loss of predictable daily oscillations in feeding, 
drinking, melatonin secretion, and the secretion of some anterior 
pituitary hormones.126,127 Normal rhythmicity can be restored if 
the SCN is transplanted back into the lesioned animals.128
In addition to the massive retinohypothalamic projection 
from the retina described earlier carrying pituitary adenyl cyclase-
activating peptide (PACAP) and nitric 

yl cyclase-
activating peptide (PACAP) and nitric oxide, the SCN receives 
neuronal input from many nuclei, including GABA-containing 
and NPY-containing axonal projections from the intergeniculate 
leaflet of the thalamus and serotonin neurons from the midbrain 
raphe, the limbic system (hippocampus, bed nucleus of the stria 
terminalis [BNST], septum), and the hypothalamus itself. These 
inputs have an important role in modulating the endogenous 
rhythms of the individual SCN pacemaker cells a

s 
rhythms of the individual SCN pacemaker cells and contribute to 
phase-shifting effects on SCN pacemaker activity.129 The SCN is 
also organized to permit many reciprocal neuron-neuron interac­
tions mediated by GABA at direct synaptic contacts. It is espe­
cially rich in neuropeptides, including AVP, VIP, gastrin-releasing 
peptide (GRP), and calretinin. The SCN also responds to mela­
tonin through melatonin receptors.94,96 Studies have indicated 
that intrinsic pacemaker function is not uni

ated 
that intrinsic pacemaker function is not unique to neurons of 
the SCN; circadian oscillators are also found in many peripheral 
tissues.124
Metabolic changes in the SCN, such as increased uptake of 
2-deoxyglucose and an increased level of VIP, accompany cir­
cadian rhythms. This nucleus projects to the pineal gland indi­
rectly via the PVH and the autonomic nervous system (see earlier 
discussion) and regulates its activity and to the medial preoptic 
hypothalamus indirectly via the dors

ial preoptic 
hypothalamus indirectly via the dorsal subparaventricular zone to 
regulate body temperature set-point and food-dependent energy 
intake.129,121 However, the bulk of SCN outflow occurs in a trunk 
coursing dorsolaterally through the ventral subparaventricular 
zone and terminating in the dorsal medial hypothalamic nucleus. 
Polysynaptic pathways involving these latter structures are respon­
sible for the actions of the SCN to produce the circadian rhythms 
in thermoregulation, gluc

e the circadian rhythms 
in thermoregulation, glucocorticoid secretion, sleep, arousal, and 
feeding.121,125
Circadian rhythms during fetal life are regulated by mater­
nal circadian rhythms.130 Circadian changes can be detected 2 
to 3 days before birth, and SCN from fetuses of this age dis­
play spontaneous rhythmicity in  vitro. Maternal regulation of 
fetal circadian rhythms may be mediated by circulating melato­
nin or by cyclic changes in the food intake of the mother. The 
timing of the c

he food intake of the mother. The 
timing of the circadian pacemaker can be shifted in humans by 
the administration of triazolam, a short-acting benzodiazepine, 
melatonin (described earlier), or by altered patterns of intense 
illumination.99 
Chemistry and Evolution
TRH, the short peptide hypophysiotropic hormone, is the tri­
peptide pyroGlu-His-Pro-NH2. Six copies of the TRH peptide 
sequence are encoded within the human TRH pre-prohormone 
gene (Fig. 7.8).131 T

he human TRH pre-prohormone 
gene (Fig. 7.8).131 The rat pro-TRH precursor contains five TRH 
peptide repeats flanked by dibasic residues (Lys-Arg or Arg-Arg), 
along with seven or more non-TRH peptides.132 Two prohormone 
convertases, PC1 and PC2, cleave the carboxy-terminal (COOH-
terminal) side of these dibasic residues as the prohormone mol­
ecule transits the regulated secretory pathway. Carboxypeptidase E 
then removes the dibasic residues, leaving the sequence Gln-His-
Pro-Gly. This pepti

 leaving the sequence Gln-His-
Pro-Gly. This peptide is then amidated at the COOH-terminus 
by peptidylglycine α-amidating monooxygenase (PAM), with Gly 
acting as the amide donor. The amino-terminal (NH2-terminal) 
pyroglutamate residue results from cyclization of the Gln.
TRH is a phylogenetically ancient peptide; it has been isolated 
from primitive vertebrates such as the lamprey and even inverte­
brates such as the snail. TRH is widely expressed in both the CNS 
and its periphery in amphibi

ssed in both the CNS 
and its periphery in amphibians, reptiles, and fishes but does not 
stimulate TSH release in these poikilothermic vertebrates. There­
fore TRH has multiple peripheral and central activities and was 
co-opted as a hypophysiotropic factor midway during the evolu­
tion of vertebrates, perhaps specifically as a factor needed for coor­
dinated regulation of temperature homeostasis.
Although the TRH tripeptide is the only established hormone 
encoded within its large prohormone, 

hed hormone 
encoded within its large prohormone, rat pro-TRH yields seven 
additional peptides that have unique tissue distributions. Several 
biologic activities of these peptides have been observed: pro-
TRH(160-169) may be a hypophysiotropic factor because it is 
released from hypothalamic slices and potentiates the TSH-releas­
ing effects of TRH.133 Pro-TRH(178-199) is also released from 
the median eminence and has been reported to stimulate PRL 
release or possibly function as a corticotr

e PRL 
release or possibly function as a corticotropin release-inhibiting 
factor.134 
Effects on the Pituitary Gland and Mechanism of Action
After intravenous injection of TRH in humans, serum TSH lev­
els rise within a few minutes,135 followed by a rise in serum tri­
iodothyronine (T3) levels; there is an increase in thyroxine (T4) 
release as well, but a change in blood levels of T4 is usually not 
demonstrable because the pool of circulating T4 (most of which is 
bound to carrier proteins) i

T4 (most of which is 
bound to carrier proteins) is so large. TRH action on the pituitary 
is blocked by previous treatment with thyroid hormone, which 
is a crucial element in the negative feedback control of pituitary 
TSH secretion.
TRH is also a potent prolactin-releasing factor.135 The time 
course of response of blood PRL levels to TRH, the dose-response 
characteristics, and the suppression by thyroid hormone pretreat­
ment (all of which parallel changes in TSH secretion) suggest that 
13

arallel changes in TSH secretion) suggest that 
130
TRH may be involved in the regulation of PRL secretion. More­
over, TRH is present in the hypothalamic-hypophysial–portal 
blood of lactating rats. However, it is unlikely to be a physiologic 
regulator of PRL secretion because the PRL response to nursing 
in humans is unaccompanied by changes in plasma TSH levels, 
and mice lacking TRH have normal lactotrophs and basal PRL 
secretion.136 Nevertheless, e

rophs and basal PRL 
secretion.136 Nevertheless, elevated levels of TRH associated with 
hypothyroidism may occasionally cause hyperprolactinemia (with 
or without galactorrhea).
In normal individuals, TRH has no influence on the secretion 
of pituitary hormones other than TSH and PRL, but it enhances 
the release of GH in acromegaly and of ACTH in some patients 
with Cushing disease. Furthermore, prolonged stimulation of the 
normal pituitary with GHRH can sensitize it to the GH-releas­
ing eff

th GHRH can sensitize it to the GH-releas­
ing effects of TRH. TRH also causes the release of GH in some 
patients with uremia, hepatic disease, anorexia nervosa, and psy­
chotic depression and in children with hypothyroidism.135 TRH 
inhibits sleep-induced GH release through its actions in the CNS 
(see later discussion).
Stimulatory effects of TRH are initiated by binding of the pep­
tide to its GPCR on the plasma membrane of the thyrotroph.137 
Thyroid hormone and somatostatin antagonize the 

Thyroid hormone and somatostatin antagonize the effects of TRH 
but do not interfere with its binding. TRH action is mediated 
mainly through Gq/11 and hydrolysis of phosphatidylinositol, 
with phosphorylation of key protein kinases and an increase in 
intracellular free calcium (Ca2+) as the crucial steps in postrecep­
tor activation (see Chapter 2).138 TRH effects can be mimicked 
by exposure to a Ca2+ ionophore and are partially abolished by 
a Ca2+-free medium. TRH stimulates the formation

a Ca2+-free medium. TRH stimulates the formation of mRNAs 
coding for TSH and PRL in addition to regulating their secretion 
and stimulates the mitogenesis of thyrotrophs.
TRH is degraded to the dipeptide histidylprolineamide (His-
Pro-NH2) by pyroglutamyl aminopeptidase, following which 
the dipeptide is deaminated to yield His-Pro or cyclizes nonen­
zymatically to histidylproline diketopiperazine (cyclic His-Pro). 
Cyclic His-Pro is reported to act as a PRF and to have other 
neural effects,

to act as a PRF and to have other 
neural effects, including reversal of ethanol-induced sleep (TRH 
is also effective in this system), elevation of brain cyclic guano­
sine monophosphate (cGMP) levels, an increase in stereotypical 
behavior, modification of body temperature, and inhibition of 
eating behavior. Some of the effects of TRH may be mediated 
through cyclic His-Pro, but the fact that cyclic His-Pro is abun­
dant in some areas and is not proportional to the amount of 
TRH suggests tha

ot proportional to the amount of 
TRH suggests that the peptide may not be derived solely from 
TRH. This latter assertion appears to be confirmed by the detec­
tion of substantial amounts of the dipeptide in brains of TRH 
knockout mice.136 
Extrapituitary Function
In addition to the hypophysiotropic TRH neurons, TRH is also 
synthesized by nonhypophysiotropic neurons, including many 
neuronal groups in the hypothalamus and throughout the brain 
(amygdala, hippocampus, thalamus, brainstem, spin

(amygdala, hippocampus, thalamus, brainstem, spinal cord).139 
The extensive distribution of TRH, its localization in nerve end­
ings, and the presence of TRH receptors in brain tissue suggest 
that TRH serves as a neurotransmitter or neuromodulator in addi­
tion to its well-known function to regulate TSH secretion.140,141 
Central administration of TRH reduces appetite; increases loco­
motor activity; reduces anxiety; has an antiepileptic effect; induces 
hyperthermia indicative of a role in c

t; induces 
hyperthermia indicative of a role in central thermoregulation139; 
has broad effects on autonomic regulation, resulting in increased 
blood pressure and heart rate, increased gastrointestinal motility, 
increased gastric acid secretion, and increased insulin secretion; 
and acts as an excitatory modulator of lower motoneuron function 
by directly synapsing on alpha-motoneurons in the ventral spinal 
cord.142 TRH is also found outside the brain, including in ante­
rior pituitary somat

the brain, including in ante­
rior pituitary somatotrophs, where it may contribute to regulation 
of TSH secretion by a paracrine mechanism, retina, parafollicular 
cells of the thyroid, adrenal medulla, epididymis, prostate, Leydig 
cells of the testis, pancreatic islet cells, gastrointestinal tract, heart, 
spleen, lung, ovary, and hair follicles. 
O
O
O
O
C
C
CH
C
N
H
NH
CH2
NH2
N
N
H
N
(pyro) Glu-His-Pro-NH2
CPE
Gln-His-Pro-Gly
PAM
Gln-His-Pro-NH2
Cyclization of Gln
100 bp
Signal peptide
3'-

o-NH2
100 bp
3'-UTR
Intron 1
Intron 2
Exon 3
200 bp
5'-UTR
Poly A
Exon 2
Sp1
Exon 1
CRE
TRE/CRE
TATA
TRE TRE
−225
+50
PC1/PC2
6 x
(Gln-His-Pro-Gly-Lys/Arg-Lys)
Stat
GRE
• Fig. 7.8  Structure of the human thyrotropin-releasing hormone (TRH) 
gene, messenger RNA, and prohormone, showing six repeats of the 
TRH peptide sequence encoded within exon 3. CPE, carboxypeptidase 
E; CRE, cyclic AMP response element; GRE, glucocorticoid response 
element; PAM, peptidylglyc

lucocorticoid response 
element; PAM, peptidylglycine α-amidating monooxygenase; PC1/PC2, 
prohormone convertases 1 and 2; Sp1, specificity protein 1 binding 
sequence; Stat, signal transducer and activator of transcription binding 
sequence; TATA, Goldstein-Hogness box involved in binding RNA poly­
merase; TRE, thyroid hormone response element; UTR, untranslated. 
(Adapted from data in Yamada M, Radovick S, Wondisford FE, et  al. 
Cloning and structure of human genomic DNA and hypothalamic cDNA

ructure of human genomic DNA and hypothalamic cDNA 
encoding human preprothyrotropin-releasing hormone. Mol Endocrinol. 
1990;4:551–556.)
131
Clinical Applications
The use of TRH for the diagnosis of hyperthyroidism or to dis­
criminate between hypothalamic and pituitary causes of TSH 
deficiency is uncommon since the development of ultrasensitive 
assays for TSH (see Chapter 11). TRH testing is also not of value 
in the differential diagnosis of causes of hyperprol

 the differential diagnosis of causes of hyperprolactinemia but is 
useful for the demonstration of residual abnormal somatotropin-
secreting cells in acromegalic patients who release GH in response 
to TRH before treatment.
Studies of the effect of TRH on depression have shown incon­
sistent results, possibly because of poor blood-brain barrier pen­
etration and a short half-life.139,141 Although a role for TRH in 
depression has not established, many depressed patients have a 
blunted TSH resp

, many depressed patients have a 
blunted TSH response to TRH, and changes in TRH responsive­
ness correlate with the clinical course. The mechanism by which 
blunting occurs is unknown.
TRH has been evaluated for the treatment of diverse neurobio­
logic disorders (for review see Gary and colleagues139), including 
spinal muscle atrophy and amyotrophic lateral sclerosis; transient 
improvement in strength was reported in both disorders, but the 
combined experience at many centers using a variet

combined experience at many centers using a variety of treatment 
protocols (including long-term intrathecal administration) failed 
to confirm efficacy. TRH administration also reduces the sever­
ity of experimentally induced spinal and ischemic shock and may 
improve recovery after spinal cord injury and head trauma. TRH 
has been used to treat children with neurologic disorders, includ­
ing West syndrome, Lennox-Gastaut syndrome, early infantile 
epileptic encephalopathy, and intractable epil

le 
epileptic encephalopathy, and intractable epilepsy.143 TRH has 
been proposed to be an analeptic agent. Sleeping or drug-sedated 
animals are awakened by the administration of TRH, and TRH 
reportedly reversed the sedative effects of ethanol in humans and 
awakened a patient with a profound sleep disorder caused by a 
hypothalamic and midbrain eosinophilic granuloma.139 
Regulation of Thyrotropin Release
The secretion of TSH is regulated by two major, interacting ele­
ments: negative feedbac

wo major, interacting ele­
ments: negative feedback by thyroid hormone and open-loop neu­
ral control by hypothalamic hypophysiotropic factors (Fig. 7.9). 
TSH secretion is also modified by tanycyte regulation of TRH 
secretion in the median eminence and other hormones, including 
estrogens, glucocorticoids, and possibly GH, and is inhibited by 
cytokines in the pituitary and hypothalamus.133,144 Aspects of the 
pituitary-thyroid axis are considered further in Chapter 11. 
Feedback Control: Hypo

red further in Chapter 11. 
Feedback Control: Hypothalamic-Pituitary-Thyroid Axis
In the context of a feedback system, the level of thyroid hor­
mone in blood or of its unbound fraction is the controlled 
variable, and the set-point is the normal resting level of plasma 
thyroid hormone. The levels of thyroid hormone inversely regu­
late TSH secretion so that deviations from the set-point lead to 
appropriate changes in the rate of TSH secretion (Fig. 7.10). 
Factors that determine the rate of T

(Fig. 7.10). 
Factors that determine the rate of TSH secretion required to 
maintain a given level of thyroid hormone include the rate at 
which TSH and thyroid hormone disappear from the blood 
(turnover rate) and the rate at which T4 is converted to its more 
active form, T3.
Thyroid hormones act on both the pituitary and the hypothal­
amus.145 Feedback control of the pituitary by thyroid hormone is 
remarkably precise. Administration of small doses of T3 and T4 
inhibit the TSH response to TR

doses of T3 and T4 
inhibit the TSH response to TRH, and barely detectable decreases 
in plasma thyroid hormone levels are sufficient to sensitize the 
pituitary to TRH. TRH stimulates TSH secretion within a few 
minutes through its action on a GPCR, whereas thyroid hormone 
actions, mediated by intranuclear receptors, require several hours 
to take effect.
The secretion of hypophysiotropic TRH is also regulated by 
thyroid hormone feedback. Systemic injections of T3 or implanta­
tions of tiny T

emic injections of T3 or implanta­
tions of tiny T3 pellets in the PVH of hypothyroid rats146 (Fig. 
7.11A and B) reduce the concentration of TRH mRNA and TRH 
prohormone in hypophysiotropic TRH-secreting cells that are 
located in the parvicellular subdivision of the PVH. The molecu­
lar mechanism governing negative regulation of TRH by thyroid 
hormone has not been firmly established but involves binding of 
the T3-thyroid hormone receptor complex to a regulatory element 
in the TRH promoter c

lex to a regulatory element 
in the TRH promoter called site 4 (see Fig. 7.8), the release of 
corepressors such as NCor and SMART, and recruitment of coact­
ivators such as SRC-1.145
Thyroid hormone negative feedback regulation of hypophy­
siotropic TRH neurons is mediated by the beta 2 isoform of 
the thyroid hormone receptor (TRβ2),147 which is expressed in 
hypophysiotropic, PVH TRH neurons. T3 alone is not sufficient 
to normalize TRH expression in the PVH and requires the pres­
ence of T4.

sion in the PVH and requires the pres­
ence of T4.148 The mechanism is believed to involve the conver­
sion of T4 to T3 by D2 within tanycytes, following the uptake of 
T4 circulating in the bloodstream or in the CSF by way of the 
thyroid hormone transporters OATP1 and MCT8 expressed on 
these cells.133 T3 released from tanycytes, can then be taken up by 
axon terminals of hypophysiotropic TRH neurons in the external 
zone of the median eminence that are in intimate contact with 
tanycyte end-f

 that are in intimate contact with 
tanycyte end-foot processes and also express the thyroid hor­
mone transporter MCT8, thereby inhibiting TRH following its 
retrograde axonal transport to cell bodies in the PVH.133,148–150 
Along these lines, it is of particular interest that D3, the thyroid 
hormone degrading enzyme, is rarely expressed in hypophysio­
tropic TRH axons in the median eminence in contrast to axons 
originating from hypophysiotropic CRH, GnRH, and GHRH 
neurons.151 Presumably thi

ic CRH, GnRH, and GHRH 
neurons.151 Presumably this selectivity of D3 expression allows 
hypophysiotropic TRH neurons to remain sensitive to changes in 
peripheral thyroid hormone levels while allowing other hypophy­
siotropic neuronal types to protect intracellular concentrations of 
T3. Other central mechanisms contributing to feedback regula­
tion of hypophysiotropic TRH by thyroid hormone include reg­
ulation of prohormone convertase enzymes in hypophysiotropic 
TRH neurons that are involved

in hypophysiotropic 
TRH neurons that are involved in the post-translational process­
ing of proTRH, and the synthesis of the TRH degrading enzyme, 
pyroglutamyl peptidase II (PPII), by tanycytes that modulate the 
amount of TRH released into the portal circulation for convey­
ance to the anterior pituitary.152,153 
Neural Control
The hypothalamus determines the set-point of feedback control 
around which the usual feedback regulatory responses are elic­
ited.133 Lesions of the thyrotropic area 

re elic­
ited.133 Lesions of the thyrotropic area lower basal thyroid hor­
mone levels and make the pituitary more sensitive to inhibition by 
thyroid hormone, and high doses of TRH raise the levels of TSH 
and thyroid hormone. Synthesis of TRH in the paraventricular 
nuclei is regulated by feedback actions of thyroid hormone.133,145 
The hypothalamus can override normal feedback control through 
an open-loop mechanism involving neuronal inputs to the hypo­
physiotropic TRH neurons (see Fig. 7.9

o the hypo­
physiotropic TRH neurons (see Fig. 7.9). Three main neuronal 
groups are known to project to hypophysiotropic TRH neurons, 
including two groups in the hypothalamus originating in the arcu­
ate nucleus and dorsomedial nucleus and catecholamine-produc­
ing neurons in the brainstem.133
132
Neurons in the arcuate nucleus containing NPY, AgRP, α-
MSH, and cocaine-regulated and amphetamine-regulated tran­
script (CART) have important roles in the r

 tran­
script (CART) have important roles in the regulation of energy 
homeostasis and respond to feeding-related signals in the blood­
stream, including leptin, ghrelin, insulin, and glucose (see Chapter 
39). These neurons establish numerous contacts with hypophy­
siotropic neurons and are involved in resetting the set-point for 
feedback regulation of thyroid hormone during nutritional insuf­
ficiency (see upcoming discussion about fasting and starvation). 
TRH neurons in the PVH of the human

 starvation). 
TRH neurons in the PVH of the human hypothalamus also receive 
inputs from NPY/AgRP neurons in the infundibular nucleus,154 
indicating evolutionary conservation of this important pathway.
The dorsomedial nucleus also has extensive projections to TRH 
neurons in the PVN155 and, if ablated, results in an increase in cir­
culating levels of T3,156 indicative of an inhibitory effect. Little is 
known about how the dorsomedial nucleus regulates hypophysio­
tropic TRH neurons, however,

regulates hypophysio­
tropic TRH neurons, however, but it is hypothesized to be involved 
in circadian regulation of TSH (see “Circadian Rhythm”).
• Fig. 7.9  Regulation of the hypothalamic-pituitary-thyroid axis. Central modulators of the hypothalamic 
tuberoinfundibular system include inputs from brainstem catecholamine neurons (C1–C3, A2, 6) that 
may also coproduce NPY and CART, NPY/AgRP, and α-MSH/CART neurons in the hypothalamic arcu­
ate nucleus and the dorsomedial nucleus. Hypophysiotrop

ucleus and the dorsomedial nucleus. Hypophysiotropic TRH neurons also receive a variety of other 
inputs, including CRH, glutamatergic, GABAergic, TRH, CRH, SRIF, and PACAP inputs. Feedback regula­
tion by circulating thyroid hormone is mediated by the TRβ2 receptor expressed in hypophysiotropic TRH 
neurons. The conversion of T4 to T3 by type 2 iodothyronine 5′-monodeiodinase is required and proposed 
to take place in tanycytes lining the third ventricle. T3 released from tanycytes can be taken

ventricle. T3 released from tanycytes can be taken up by TRH 
axon terminals in the median eminence that are in close contact with tanycyte end-feet processes and then 
transported retrogradely to the cell bodies in the hypothalamic PVH. AGRP, agouti-related peptide; CART, 
cocaine-regulated and amphetamine-regulated transcript; CRH, corticotropin-releasing hormone; D2, type 
2 iodothyronine deiodinase; NPY, neuropeptide Y; POMC, pro-opiomelanocortin; PPII, pyroglutamyl pepti­
dase II; T3, triio

rtin; PPII, pyroglutamyl pepti­
dase II; T3, triiodothyronine; T4, thyroxine; TRH, thyrotropin-releasing hormone; TSH, thyrotropin.
133
Finally, hypophysiotropic TRH neurons receive a particularly 
dense innervation from catecholamine-producing neurons in the 
brainstem, including adrenergic neurons located in regions C1 to 
C3 in the rostroventrolateral medulla and within and adjacent to the 
nucleus tractus solitarius, and noradrenergic neurons located in A2 
and 

ius, and noradrenergic neurons located in A2 
and A6 regions in the ventrolateral medulla and dorsal vagal com­
plex.157 Subpopulations of these catecholaminergic neurons copro­
duce peptides, contributing ∼25% of the NPY innervation and 
∼60% of the CART innervation to TRH neurons in the PVN.158,159 
As will be described later, this projection pathway has a particularly 
important role in the regulation of TRH gene expression by cold.
Hypothalamic regulation of TSH secretion is also influenced 

ic regulation of TSH secretion is also influenced 
by the inhibitory factor somatostatin. Antisomatostatin antibod­
ies increase basal TSH levels and potentiate the response to stimuli 
that normally induce TSH release in the rat, such as cold expo­
sure and TRH administration.160 Thyroid hormone in turn inhib­
its the release of somatostatin, implying coordinated, reciprocal 
regulation of TRH and somatostatin by thyroid hormone. GH 
stimulates hypothalamic somatostatin synthesis and can inhibi

hypothalamic somatostatin synthesis and can inhibit 
TSH secretion. However, the physiologic role of somatostatin in 
the regulation of TSH secretion in humans is uncertain. 
Circadian Rhythm
Plasma TSH in humans is characterized by a circadian periodic­
ity, with a maximum between 9 pm and 5 am and a minimum 
between 4 pm and 7 pm (see Fig. 7.7E).161 Smaller ultradian TSH 
peaks occur every 90 to 180 minutes, probably because of bursts 
of TRH release from the hypothalamus, and are physiologica

elease from the hypothalamus, and are physiologically 
important in controlling the synthesis and glycosylation of TSH. 
Glycosylation is a determinant of TSH potency.162 
Temperature
External cold exposure activates and high ambient temperature 
inhibits the pituitary-thyroid axis in animals, and analogous 
changes occur in humans under certain conditions.163 Exposure of 
infants to cold at the time of delivery causes an increase in blood 
TSH levels, possibly because of alterations in the turn

evels, possibly because of alterations in the turnover and 
degradation of the thyroid hormones. Blood thyroid hormone 
levels are higher in the winter than in the summer among people 
living in cold climates but not in other climates. However, it is dif­
ficult to show that changes in environmental or body temperature 
in adults influence TSH secretion. For example, exposure to cold 
ambient temperature or central hypothalamic cooling does not 
modify TSH levels in young men. Behavioral changes

modify TSH levels in young men. Behavioral changes, activation 
of the sympathetic nervous system, and shivering appear to be 
more important than the thyroid response for temperature regula­
tion in adults.
The autonomic nervous system and the thyroid axis work 
together to maintain temperature homeostasis in mammals, 
and TRH plays a role in both pathways.163 Hypothalamic TRH 
release is rapidly increased (30–45 minutes) in rats exposed to 
cold. Rapid inhibition of somatostatin release in the

d. Rapid inhibition of somatostatin release in the median emi­
nence has also been documented, and both changes appear to 
play important roles in the rise in plasma TSH induced by cold 
exposure. TRH mRNA is elevated within an hour of cold expo­
sure (see Fig. 7.11C and D). The regulation of hypophysiotropic 
Plasma TSH (µg/mL)
PBI (µg%)
PBI (µg%)
Normal
range
A
50
40
30
20
10
0
10
8
6
4
2
0
mU TSH per 100 mL
Standard
error
Normal rats
B
200
180
160
140
120
100
80
60
40
20
0
0
1
2
3
4
5
6
7
8
•

160
140
120
100
80
60
40
20
0
0
1
2
3
4
5
6
7
8
• Fig. 7.10  Relationship between plasma thyrotropin (TSH) levels and thyroid hormone as determined 
by plasma protein-bound iodine (PBI) measurements in humans and rats. These curves illustrate, in the 
human (A) and the rat (B), that plasma TSH levels are a curvilinear function of plasma thyroid hormone 
level. Human studies were carried out by giving myxedematous patients successive increments of thyrox­
ine (T4) at approximately 10-day interva

f thyrox­
ine (T4) at approximately 10-day intervals. Each point represents simultaneous measurements of plasma 
PBI and plasma TSH at various times in the six patients studied. The rat studies were performed by 
treating thyroidectomized animals with various doses of T4 for 2 weeks before assay of plasma TSH and 
plasma PBI. These curves illustrate that the secretion of TSH is regulated over the entire range of thyroid 
hormone levels. At the normal set-point for T4, the small changes above and

rmal set-point for T4, the small changes above and below the control level are 
followed by appropriate increases or decreases in plasma TSH. (A, From Reichlin S, Utiger RD. Regulation 
of the pituitary thyroid axis in man: relationship of TSH concentration to concentration of free and total 
thyroxine in plasma. J Clin Endocrinol Metab. 1967;27:251–255, copyright by The Endocrine Society. B, 
From Reichlin S, Martin JB, Boshans RL, et al. Measurement of TSH in plasma and pituitary of the rat by

ement of TSH in plasma and pituitary of the rat by 
a radioimmunoassay utilizing bovine TSH: effect of thyroidectomy or thyroxine administration on plasma 
TSH levels. Endocrinology. 1970;87:1022–1031, copyright by The Endocrine Society.)
134
A
B
C
D
E
F
G
• Fig. 7.11  (A and B) Direct effects of triiodothyronine (T3) on thyrotropin-releasing hormone (TRH) synthesis 
in the rat hypothalamic paraventricular nucleus (parvicellular division) were shown in th

 nucleus (parvicellular division) were shown in this experiment by 
immunohistochemical detection of pre-pro-TRH(25-50) after implantation of a pellet of either T3 (B) or inac­
tive diiodothyronine (T2) as a control (A). The T2 pellet had no effect on the concentration of pre-pro-TRH 
(A). In contrast, the TRH prohormone concentrations (B) were markedly reduced (black arrow indicates 
the unilateral pellet implantation). These studies indicate that thyroid hormone regulates the hypothalamic 
com

at thyroid hormone regulates the hypothalamic 
component of the pituitary-thyroid axis as well as the pituitary thyrotrope itself. (C and D) Effects of 1 hour 
at 4°C on TRH messenger ribonucleic acid (mRNA). (E to G) Effects on TRH mRNA levels of starvation 
(F) and leptin replacement during starvation (G). White arrows show the location of the paraventricular 
nucleus. III, 3rd ventricle; LH, lateral hypothalamus. (Photomicrographs in panels A and B, from Dyess EM, 
Segerson TP, Liposits Z, et

and B, from Dyess EM, 
Segerson TP, Liposits Z, et al. Triiodothyronine exerts direct cell-specific regulation of thyrotropin-releasing 
hormone gene expression in the hypothalamic paraventricular nucleus. Endocrinology. 1988;123[5]:2291–
2297; photomicrographs in panels E, F, and G, from Légrádi G, Emerson CH, Ahima RS, Flier JS, Lechan 
RM. Leptin prevents fasting-induced suppression of prothyrotropin-releasing hormone messenger ribonu­
cleic acid in neurons of the hypothalamic paraventricular

cid in neurons of the hypothalamic paraventricular nucleus. Endocrinology. 1997;138[6]:2569–2576; 
copyright by The Endocrine Society; photomicrographs in panels C and D, courtesy Dr. P. Joseph-Bravo.)
135
TRH release and expression by cold is largely mediated by cat­
echolamines, and a rapid rise in TRH release is observed after nor­
epinephrine treatment of hypothalamic fragments. As mentioned 
previously, catecholamines arise from neurons originating in the 
brai

amines arise from neurons originating in the 
brainstem and can affect TRH secretion both through contacts 
on TRH perikarya in the PVH and their axon terminals in the 
median eminence (see Fig. 7.9).164 
Stress
Stress is another determinant of TSH secretion.165 In humans, 
physial stress inhibits TSH release, as indicated by the finding that 
low levels of T3 and T4 in patients with the nonthyroidal illness 
syndrome do not cause compensatory increases in TSH secretion 
as would occur in normal

creases in TSH secretion 
as would occur in normal individuals.166
A number of observations demonstrate interactions between the 
thyroid and adrenal axes.165 Physiologically, the bulk of evidence 
suggests that glucocorticoids in humans and rodents act to blunt 
the thyroid axis through actions in the CNS.167 Some actions may 
be direct because the TRH gene (see Fig. 7.8) contains a glucocor­
ticoid response element consensus half-site,132 and hypophysiotro­
pic TRH neurons appear to contain gl

ypophysiotro­
pic TRH neurons appear to contain glucocorticoid receptors.168 
The diurnal rhythm of cortisol is opposite that of TSH (see Fig. 
7.7), and acute administration of glucocorticoids can block the 
nocturnal rise in TSH, but disruption of cortisol synthesis with 
metyrapone only modestly affects the TSH circadian rhythm.
Nevertheless, several lines of evidence identify conditions in 
which elevated glucocorticoids are associated with stimulation 
of the thyroid axis. Human depression 

timulation 
of the thyroid axis. Human depression is often associated with 
hypercortisolism and hyperthyroxinemia, and TRH mRNA levels 
are elevated by glucocorticoids in a number of cell lines as well as 
in cultured fetal hypothalamic TRH neurons from the rat. Thus, 
although glucocorticoids can stimulate TRH production in TRH 
neurons by binding to glucocorticoid receptor binding sites in 
the TRH promoter, their overall inhibitory effect on the thyroid 
axis results from indirect glucocorti

the thyroid 
axis results from indirect glucocorticoid negative feedback on 
structures such as the hippocampus. Disruption of hippocampal 
suppression of the hypothalamic-pituitary-adrenal (HPA) axis is 
proposed to be involved in the hypercortisolemia commonly seen 
in affective illness, and disruption of hippocampal inputs to the 
hypothalamus has been shown to produce a rise in hypophysio­
tropic TRH in the rat.169 
Suckling
Suckling also increases TRH gene expression in hypophysiotro­
pic n

reases TRH gene expression in hypophysiotro­
pic neurons but primarily to increase prolactin secretion from 
the anterior pituitary. TSH and thyroid hormone levels do not 
increase in response to suckling, perhaps due to the simultaneous 
release of CART that is coexpressed with TRH in hypophysiotro­
pic neurons that attenuate TRH-induced TSH secretion.170 
Starvation
The thyroid axis is suppressed during starvation, presumably to 
help conserve energy by depressing metabolism (see Fig. 7.11E–
G

energy by depressing metabolism (see Fig. 7.11E–
G). In humans, reduced T3, T4, and TSH are seen during starva­
tion or fasting.133 There are also changes in the thyroid axis in 
anorexia nervosa, such as low blood levels of T3 and low normal 
levels of T4. Inappropriately low levels of TSH are found, suggest­
ing defective activation of TRH production by low thyroid hor­
mone levels. During starvation in rodents, reduced TRH release 
into hypothalamic-hypophysial portal blood and reduced pro-
T

alamic-hypophysial portal blood and reduced pro-
TRH mRNA levels are seen, despite lowered thyroid hormone 
levels.171 Reduced basal TSH levels are also usually present.
The hypothyroidism seen in fasting or in the leptin-deficient 
ob/ob mouse can be reversed by administration of leptin,14 and 
evidence suggests that the mechanism involves leptin’s ability to 
upregulate TRH gene expression in the PVH (see Fig. 7.11E–
G).13 Leptin appears to act both directly through leptin recep­
tors on hypop

 both directly through leptin recep­
tors on hypophysiotropic TRH neurons and indirectly through 
its actions on other hypothalamic cell groups, such as arcuate 
nucleus POMC and NPY/AgRP neurons.172,173 TRH neurons 
in the PVH receive dense NPY/AgRP and POMC projections 
from the arcuate and express NPY and melanocortin-4 receptors 
(MC4R),174 and α-MSH administration partially prevents the 
fasting-induced drop in thyroid hormone levels.172,173 Indeed, 
the TRH promoter contains a signal trans

 Indeed, 
the TRH promoter contains a signal transducer and activator of 
transcription (STAT) response element and a CRE that have been 
demonstrated to mediate induction of TRH gene expression by 
leptin and α-MSH, respectively, in a heterologous cell system (see 
Fig. 7.8).174 The regulation of TRH by metabolic state is likely 
to be under redundant control, however, because leptin-deficient 
children, unlike rodents, are euthyroid,175 whereas both humans 
and rodents with MC4R deficiency are

 both humans 
and rodents with MC4R deficiency are euthyroid.176 
Dehydration-Induced Anorexia
The addition of 2.5% NaCl to drinking water markedly reduces 
food intake by 80%.177 Despite the very low food intake, TRH 
gene expression in the PVH increases rather than the anticipated 
decrease as described previously. The mechanism for this response 
is uncertain but may involve activating effects of CRH from neu­
rons in the lateral hypothalamus that project to hypophysiotropic 
TRH neurons. Deh

that project to hypophysiotropic 
TRH neurons. Dehydration-induced anorexia (DIA) also increases 
CRH gene expression in lateral hypothalamic neurons, and as 
noted previously, CRH induces TRH release in cell culture.178 
Infection and Inflammation
The molecular basis of infection-induced or inflammation-
induced TSH suppression is partially established. TSH secretion 
is inhibited by sterile abscesses; by the injection of interleukin 1β 
(IL-1β), an endogenous pyrogen and secretory peptide of a

, an endogenous pyrogen and secretory peptide of acti­
vated lymphocytes179; or by tumor necrosis factor-α (TNFα). 
IL-1β stimulates the secretion of somatostatin.180 TNFα inhibits 
TSH secretion directly and induces functional changes in the 
rat characteristic of the sick euthyroid state.181 It is likely that 
the TSH inhibition in animal models of the sick euthyroid syn­
drome is due to cytokine-induced changes in hypothalamic and 
pituitary function.182 IL-6, IL-1, and TNFα contribute to the

unction.182 IL-6, IL-1, and TNFα contribute to the 
suppression of TSH in the nonthyroidal illness syndrome.183 
Other evidence suggests that bacterial lipopolysaccharide can 
directly stimulate tanycytes via their expression of toll-like recep­
tor 4. The stimulated tanycytes express higher levels of type 2 
deiodinase, perhaps directly induced by activation of nuclear 
factor-κB (NFκB), that in turn increases the levels of T3 rela­
tive to T4, causing feedback inhibition on TRH neurons by a 
m

causing feedback inhibition on TRH neurons by a 
mechanism described earlier.133 Endotoxin also increases TSH 
production in the pars tuberalis, which, as described previously 
(see “Physiologic Roles of Melatonin”), can increase tanycyte D2 
expression. 
The HPA axis is the humoral component of an integrated neural 
and endocrine system that has an important role in the regulation 
of energy homeostasis through effects on hepatic glycogeno

y homeostasis through effects on hepatic glycogenolysis 
136
and lipolysis and functions in response to acute and chronic 
internal and external challenges to homeostasis (stressors). The 
system is comprised of neuronal pathways linked to the hypo­
thalamic-pituitary control of ACTH release and hence the release 
of glucocorticoids from the adrenal cortex and can coordinate 
with activation of the autonomic nervous system to induce the 
release of catech

ic nervous system to induce the 
release of catecholamines from the adrenal medulla (fight-or-flight 
response). Pituitary ACTH secretion is stimulated primarily by 
CRH, derived from neurons located in the parvicellular divi­
sion of the PVH, and to a lesser extent by AVP, originating from 
magnicellular neurons in the SON and PVN but also cosecreted 
from hypophysiotropic CRH neurons (see Figs. 7.3 and 7.4). 
In a broader context, the CRH system in the CNS is also vitally 
important for the be

m in the CNS is also vitally 
important for the behavioral responses to stress. This complex 
system also includes nonhypophysiotropic CRH neurons, three 
additional CRH-like peptides, urocortin 1 to 3, at least two cog­
nate GPCRs, CRH receptors (including CRH-R1 and CRH-R2), 
and a high-affinity CRH-binding protein, each with distinct and 
complex distributions in the CNS.
The Schally and Guillemin laboratories demonstrated in 1955 
that extracts from the hypothalamus stimulated ACTH release 

ts from the hypothalamus stimulated ACTH release 
from the pituitary. The primary active principle, CRH, was puri­
fied and characterized from sheep in 1981. Human CRH is an 
amidated 41–amino acid peptide that is cleaved from the COOH-
terminus of a 196–amino acid pre-prohormone precursor by PC1 
and PC2 (Fig. 7.12).184 CRH is highly conserved phylogeneti­
cally; the human peptide is identical in sequence to the mouse 
and rat peptides but differs at seven residues from the ovine 
sequence. Mam

rs at seven residues from the ovine 
sequence. Mammalian CRH, the three urocortin peptides, fish 
urotensin, anuran sauvagine, and the insect diuretic peptides are 
Ser - Glu - Glu - Pro - Pro - Ile - Ser - Leu - Asp - Leu - Thr - Phe - His - Leu - Leu - 
Arg - Glu - Val - Leu - Glu - Met - Ala - Arg - Ala - Glu - Gln - Leu - Ala - Gln - Gln - 
Ala - His - Ser - Asn - Arg - Lys - Leu - Met - Glu - Ile - Ile - Gly - Lys
Ser - Glu - Glu - Pro - Pro - Ile - Ser - Leu - Asp - Leu - Thr - Phe - His -

 - Ile - Ser - Leu - Asp - Leu - Thr - Phe - His - Leu - Leu - 
Ala - His - Ser - Asn - Arg - Lys - Leu - Met - Glu - Ile - Ile - NH2
PC1/PC2
PAM
CRH
200 bp
200 bp
5' UTR
3' UTR
Exon 1
Exon 2
Intron
50 bp
−500
+50
TATA
TATA
GRE
AP-1
ERE
ERE
CRE
RR
RR
GK
proCRH(125-151)
N-terminal proCRH
• Fig. 7.12  Structure of the human corticotropin-releasing hormone (CRH) gene, complementary DNA, 
and pe

ing hormone (CRH) gene, complementary DNA, 
and peptide. The sequence encoding CRH occurs at the carboxy-terminus of the prohormone. Dibasic 
amino acid cleavage sites (RR) and the penultimate Gly and terminal Lys (GK) are shown. AP-1, activator 
protein-1 binding sequence; CRE, cyclic adenosine monophosphate response element; ERE, estrogen 
response element; GRE, glucocorticoid response element; PAM, peptidylglycine α-amidating mono-
oxygenase; PC1/PC2, prohormone convertases 1 and 2; TATA, Gol

PC1/PC2, prohormone convertases 1 and 2; TATA, Goldstein-Hogness box involved in bind­
ing RNA polymerase; UTR, untranslated. (Redrawn from data of Shibahara S, Morimoto Y, Furutani Y, et al. 
Isolation and sequence analysis of the human corticotropin-releasing factor precursor gene. EMBO J. 
1983;2:775–779.)
137
members of an ancient family of peptides that evolved from an 
ancestral precursor early in the evolution of metazoans, ∼500 mil­
lion years ago.185 Compar

 of metazoans, ∼500 mil­
lion years ago.185 Comparison of peptide sequences in vertebrates 
suggests a grouping of the peptides into two subfamilies, CRH-
urotensin-urocortin 1-sauvagine and urocortin 2-urocortin 3 (Fig. 
7.13).186 Urocortin 1 and sauvagine appear to represent tetrapod 
orthologues of fish urotensin. Sauvagine, isolated originally from 
Phyllomedusa sauvagii, is an osmoregulatory peptide produced in 
the skin of certain frogs; urotensin is an osmoregulatory peptide 
produced in 

otensin is an osmoregulatory peptide 
produced in the caudal neurosecretory system of fish. Whereas 
isolation of CRH required 250,000 ovine hypothalami, the vir­
tual cloning of urocortin 2 and 3 was accomplished by computer 
search of the human genome database.186
The CRH peptides signal by binding to CRH-R1187 and 
CRH-R2,188 receptors that couple to the stimulatory G protein 
(Gs) and adenylyl cyclase. Two splice variants of the latter receptor 
that differ in the extracellular NH2-terminal 

or 
that differ in the extracellular NH2-terminal domain, CRH-R2α 
and CRH-R2β, have been found in both rodents and humans,189 
and a third NH2-terminal splice variant, CRH-R2γ, has been 
reported in humans.190 In mammals, CHR-R1 is the predomi­
nant receptor expressed in the brain and anterior pituitary, thereby 
being the primary receptor for CRH-mediated ACTH release,191 
whereas CRH-R2 has a more limited and selective distribution 
in the brain.192 CRH, urotensin, urocortin 1, and sauvagine 

in.192 CRH, urotensin, urocortin 1, and sauvagine are 
potent agonists of CRH-R1; urocortin 1 is also a potent agonist 
of CRH-R2; urocortins 2 and 3 are specific agonists of CRH-
R2.193 Thus CRH activation of the HPA axis is mediated exclu­
sively through CRH-R1, which is expressed in anterior pituitary 
corticotrophs.
While CRH neurons projecting to the median eminence are 
found mostly in the PVH (Fig. 7.14A), some CRH fibers originating 
from the PVH also project to the brainstem. In additio

 the PVH also project to the brainstem. In addition, nonhypo­
physiotropic CRH neurons are abundant elsewhere but primarily 
in limbic structures involved in processing sensory information 
and in regulating the autonomic nervous system. Sites include the 
prefrontal, insular, and cingulate cortices; amygdala; substantia 
nigra; periaqueductal gray; locus coeruleus; NTS; and parabrachial 
nucleus. In the periphery, CRH is found in human placenta, lym­
phocytes, autonomic nerves, heart, lung, ski

 lym­
phocytes, autonomic nerves, heart, lung, skin, kidney, testes, ova­
ries, and the gastrointestinal tract. Urocortin 1 is expressed at the 
highest levels in the nonpreganglionic Edinger-Westphal nucleus, 
the lateral superior olive, and the SON of the rodent brain, with 
additional sites such as the substantia nigra, ventral tegmental area, 
and dorsal raphe (see Fig. 7.14B). In the human, urocortin 1 is 
widely distributed, with highest levels in the frontal cortex, tempo­
ral cortex, and

vels in the frontal cortex, tempo­
ral cortex, and hypothalamus,194 and has also been reported in the 
nonpreganglionic Edinger-Westphal nucleus.22 In the periphery, 
urocortin 1 is seen in placenta, mucosal inflammatory cells of the 
gastrointestinal tract, lymphocytes, and cardiomyocytes. Urocortin 
2 is expressed in hypothalamic neuroendocrine and stress-related 
cell groups in the mouse, including the locus coeruleus, whereas 
urocortin 3 is expressed in the hypothalamus and amygdala and 
pa

expressed in the hypothalamus and amygdala and 
particularly in pancreatic islet beta cells.195,196
In addition to its expression in pituitary corticotrophs, CRH-
R1 is found in the neocortex and cerebellar cortex, subcortical 
limbic structures, and amygdala, with little to no expression in the 
hypothalamus (see Fig. 7.14C). CRH-R1 is also found in a variety 
of peripheral sites in humans, including ovary, endometrium, and 
skin. CRH-R2α is found mainly in the brain in rodents, with high 
leve

nd mainly in the brain in rodents, with high 
levels of expression seen in the ventromedial hypothalamic nucleus 
and lateral septum (see Fig. 7.14C)197; CRH-R2β is found cen­
trally in cerebral arterioles and peripherally in the gastrointestinal 
tract, heart, and muscle.188,198 In humans, CRH-R2α is expressed 
in the brain and periphery, whereas the β and γ subtypes are pri­
marily central.189,190 Little CRH-R2 message is seen in the pitu­
itary. CRH-R1 and CRH-R2 are expressed in the rodent a

y. CRH-R1 and CRH-R2 are expressed in the rodent adrenal 
cortex, suggesting a potential intra-adrenal CRH-ACTH system 
involved in the fine-tuning of adrenocortical glucocorticoid release.
Regulation of signaling at CRH-R1 and CRH-R2 in both the 
brain and periphery also occurs by a 37-kDa high-affinity CRH-
binding protein (CRH-BP), which also binds urocortin 1 with 
equal or greater affinity than to the CRH receptors.199,200 This 
factor was initially postulated from the observation that CRH 

nitially postulated from the observation that CRH 
levels rise dramatically during the second and third trimesters of 
pregnancy without activating the pituitary-adrenal axis. Among 
hypophysiotropic factors, CRH is the only one for which a spe­
cific binding protein (in addition to the receptor) exists in tissue 
or blood.
The functional significance of the CRH-binding protein is not 
fully understood. The CRH-binding protein does not bind to the 
CRH receptor but does inhibit CRH action. For t

e 
CRH receptor but does inhibit CRH action. For this reason, the 
CRH-binding protein probably acts to modulate CRH actions 
at the cellular level. Corticotroph cells in the anterior pituitary 
have membrane CRH receptors and intracellular CRH-binding 
protein.201 Conceivably, the binding protein acts to sequester or 
terminate the action of membrane-bound CRH and has been 
shown to attenuate ACTH-releasing activity of CRH in ante­
rior pituitary cells.202 The CRH-binding protein is also presen

y cells.202 The CRH-binding protein is also present 
in regions of the CNS that synthesize CRH or receive inner­
vation from CRH-containing neurons such as the bed nucleus 
of the stria terminalis and central nucleus of the amygdala.203 
Most importantly, however, CRH-BP may have a regulatory role 
within the CNS to modulate the stress response (reviewed by 
Ketchesin and colleagues200). For example, CRH-BP increases in 
the amygdala following restraint stress, predator stress, and food 
depriva

e comparison of members of the corticotropin-releasing hormone (CRH) peptide fam­
ily. Identical or highly conserved amino acids are indicated in boldface letters. SCP, stresscopin; SRP, 
stresscopin-related peptide.
138
LS/MS
cc
cc
A
LS/MS
OB
HIP
EP/CLA
BNST/AmygPOA
PVH
OVLT
SO
SCN
LHA
HYP
SUM SN
IP MR
PB
MoV SNV
Sp cord
CBL
Dors col
NTS/AP
XII
AMB
IO/LRN
Vll
LSO
PG/TRN
Vest
MB
PP
Retina
Jaw
Cochlea
Face
Pharynx,
Larynx,
Esophagus
Tongue
Skeletal
muscle

Pharynx,
Larynx,
Esophagus
Tongue
Skeletal
muscle
GP
cc
HIP
Mid   thal
Mid thal
Pretect
SC
IC
PAG
DR
RN
EW
PHA
IGL
OB
FrCx
CingCx
ParCx
OccCx
CA1
CA2
CA3
CA4
DG
SC
IC
CBL
PAG
RN
LDTg
LC
PPTg
V
IO
SN
APit
PVH
VMH
MA
BLA
CeA
CoA
AON
DBB
BNST
MS
LS
R
st
BNSTSI
CeA
MePO
ME
PVH
LHA
mfb
PP
POR
CG
DR
MR
LDTg
LC
PB
MVN
DVC
A5
A1
ac
B
C
• Fig. 7.14  Distribution of messenger RNA sequences for (A) corticotropin-releasing hormone (CRH), (B) 
urocortin, and (C) the CRH receptor 1 (CRH-R1, circles) and CRH-R

(C) the CRH receptor 1 (CRH-R1, circles) and CRH-R2, triangles) in the rat brain. A1, nor­
adrenergic cell group 1; A5, noradrenergic cell group 5; ac, anterior commissure; AMB, nucleus ambiguus; 
APit, anterior pituitary; AP, area postrema; BLA, basolateral amygdala; BNST, bed nucleus of the stria 
terminalis; CBL, cerebellum; cc, corpus callosum; CeA, central nucleus amygdala; CG, central gray; DG, 
dentate gyrus; DR, dorsal raphe; DVC, dorsal vagal complex; EW, Edinger-Westphal nucleus, nonch

vagal complex; EW, Edinger-Westphal nucleus, noncholin­
ergic; HIP, hippocampus; IC, inferior colliculus; LC, locus coeruleus; LDTg, laterodorsal tegmental nucleus; 
LHA, lateral hypothalamic area; LS, lateral septum; MA, medial amygdala; ME, median eminence; mfb, 
medial forebrain bundle; Mid Thal, midline thalamic nuclei; MS, medial septum; MePO, medial preoptic 
area; MR, medial raphe; MVN, medial vestibular nucleus; OB, olfactory bulb; PAG, periaqueductal gray; 
PB, parabrachial nucleus; POR

eriaqueductal gray; 
PB, parabrachial nucleus; POR, perioculomotor nucleus; PP, posterior pituitary; PPTg, pedunculopontine 
tegmental nucleus; PVH, paraventricular nucleus hypothalamus; R, raphe; RN, red nucleus; SC, superior 
colliculus; SI, substantia innominata; st, stria terminalis; V/Vest, vestibular nuclei; VMH, ventral medial 
nucleus hypothalamus. (From Swanson LW, Sawchenko PE, Rivier J, et al. Organization of ovine corti­
cotropin-releasing factor immunoreactive cells and fibers in th

asing factor immunoreactive cells and fibers in the rat brain: an immunohistochemical study. 
Neuroendocrinology. 1983;36:165–186; Bittencourt JC, Vaughan J, Arias C, et al. Urocortin expression in 
rat brain: evidence against a pervasive relationship of urocortin-containing projections with targets bearing 
type 2 CRF receptors. J Comp Neurol. 1999;415:285–312, Fig. 17; and Steckler T, Holsboer F. Corti­
cotropin-releasing hormone receptor subtypes and emotion. Biol Psychol. 1999;46:1480–1508, 

pes and emotion. Biol Psychol. 1999;46:1480–1508, Fig. 1.)
139
are both important for biologic activity of CRH. The first CRH 
antagonist described was termed α-helical CRH(9-41).208 A sec­
ond, more potent antagonist, termed astressin, has the structure 
cyclo(30-33)(d-Phe12, Nle21,38, Glu30, Lys33)hCRH(12-41).209 
Both peptides are somewhat nonspecific, antagonizing both CRH-
R1 and CRH-R2. Because of the anxiogenic activity of CRH and 
urocortin 1, several pharma

c activity of CRH and 
urocortin 1, several pharmaceutic companies have developed small 
molecule CRH antagonists; many of these have been the subject 
of clinical trials for anxiety and depression (see later discussion). 
Thus far, this structurally diverse group of small molecule com­
pounds, such as antalarmin, R121919, CP-154,526, NBI27914, 
NBI77860, BMS562086, and a novel series of thiazolol[4,5-d]
pyrimidines-2-imine and 2-hydrazones, are potent antagonists of 
CRH-R1, with little activit

potent antagonists of 
CRH-R1, with little activity at CRH-R2. The efficacy of these 
compounds across the entire behavioral, neuroendocrine, and 
autonomic repertoire of response to stress has been demonstrated 
in a number of laboratory animal studies. For example, oral 
administration of antalarmin in a social stress model in the pri­
mate (introduction of strange males) reduced behavioral measures 
of anxiety such as lack of exploratory behavior, decreased plasma 
ACTH and cortisol, and redu

ior, decreased plasma 
ACTH and cortisol, and reduced plasma epinephrine and norepi­
nephrine.210 Activation of CRH-R2 by urocortin 2 or 3 in mice 
has also been shown to reduce anxiety and depression associated 
with the acute withdrawal of chronic nicotine treatment.211,212 
Thus selective CRH-2 agonists may also find clinical benefit. 
Effects on the Pituitary and Mechanism of Action
Administration of CRH to humans causes prompt release of 
ACTH into the blood, followed by secretion of cortis

TH into the blood, followed by secretion of cortisol (Fig. 7.15) 
and other adrenal steroids, including aldosterone. Most studies 
have used ovine CRH, which is more potent and longer acting 
than human CRH, but human and porcine CRHs appear to have 
equal diagnostic value. The effect of CRH is specific to ACTH 
release and is inhibited by glucocorticoids.
As mentioned, CRH acts on the pituitary corticotroph to regu­
late both basal-induced and stress-induced ACTH release, primar­
ily by binding

tress-induced ACTH release, primar­
ily by binding to CRH-R1 and activating adenylyl cyclase. The 
concentration of cAMP in the tissue is increased in parallel with 
the biologic effects and is reduced by glucocorticoids. The rate of 
transcription of the mRNA that encodes the ACTH prohormone 
POMC is also enhanced by CRH. 
Extrapituitary Functions
CRH and the urocortin peptides have a wide range of biologic 
activities in addition to the hypophysiotropic role of CRH in 
regulating ACTH synthesi

siotropic role of CRH in 
regulating ACTH synthesis and release. Centrally, these peptides 
have behavioral activities in anxiety, mood, arousal, locomotion, 
reward, and feeding213,214 and increase sympathetic activation 
independent of any effects on the HPA axis.215 Many of the nonhy­
pophysiotropic behavioral and autonomic functions of these pep­
tides, however, can be viewed as complementary to activation of 
the HPA axis in the maintenance of homeostasis under exposure to 
stress. In the p

of homeostasis under exposure to 
stress. In the periphery, activities have been reported in immunity, 
cardiac function, gastrointestinal function, and reproduction.216
Hyperactivity of the HPA axis is a common neuroendocrine 
finding in affective disorders (see Fig. 7.15),213,217 and normal­
ization of HPA regulation is highly predictive of successful treat­
ment. Defective dexamethasone suppression of CRH release, 
implying defective corticosteroid receptor signaling, is seen not 
only in dep

eroid receptor signaling, is seen not 
only in depressed patients but also in healthy subjects with a fam­
ily history of depression.218 Depressed patients also show elevated 
levels of CRH in the CSF.219 Extensive behavioral testing in a 
variety of mutant mouse models with genetically altered expres­
sion of either the CRH ligands or receptors generally supports the 
hypothesis that activation of central CRH pathways is a critical 
neurobiologic substrate of anxiety and depressive states.214,2

c substrate of anxiety and depressive states.214,220 Sig­
naling through CRH-R1 also participates in addictive behavior, 
particularly in the central nucleus of the amygdala and bed nucleus 
of the stria terminalis. Acute withdrawal from drugs of abuse or 
withdrawal from ethanol or nicotine results in increases in CRH 
in these regions.221,222 In addition, injections of CRH receptor 
antagonists into the central nucleus of the amygdala reduce self-
administration of addictive substances (review

lf-
administration of addictive substances (reviewed by Roberto and 
colleagues223). Activation of dopaminergic neurons in the ventral 
tegmental area by CRH may also be involved in mediating addic­
tive behaviors by signaling through both CRH-R1 and CRH-R2.
Time (min)
A
B
130
110
90
70
50
30
25
20
15
10
5
0
Controls (n = 34)
Depressed (n = 30)
Cushing (n = 29)
0 1
30 60
−1
120
90
Plasma IR–ACTH (pg/mL)
Time (min)
30
25
20
15
10
5
0
0 1
30 60
−1
120
90
Plasma Cortisol (µg/dL)
• Fig. 7.15  Compar

120
90
• Fig. 7.15  Comparison of plasma immunoreactive adrenocorticotropic 
hormone (IR-ACTH) (A) and plasma cortisol (B) responses to ovine corti­
cotropin-releasing hormone in control subjects, patients with depression, 
and patients with Cushing disease. (From Gold PW, Loriaux DL, Roy A, 
et  al. Responses to corticotropin-releasing hormone in the hypercorti­
solism of depression and Cushing’s disease: pathophysiologic and diag­
nostic implications. N Engl J Med. 1986

 and diag­
nostic implications. N Engl J Med. 1986;314:1329–1335.)
140
Central administration of CRH or urocortin activates the neu­
ronal cell groups involved in cardiovascular control and increases 
blood pressure, heart rate, and cardiac output.224 However, urocor­
tin is expressed in cardiac myocytes, and intravenous administration 
of CRH or urocortin decreases blood pressure and increases heart 
rate in most species, including humans.224 This hypote

 in most species, including humans.224 This hypotensive effect 
is probably mediated peripherally because ganglionic blockade did 
not disrupt the hypotensive effects of intravenous urocortin. Fur­
thermore, high levels of CRH-R2β have been seen in the cardiac 
atria and ventricles,188,198 and knockout of the Crhr2 gene in the 
mouse eliminated the hypotensive effects of intravenous urocortin 
administration.225
Cytokines have an important role in extinguishing inflam­
matory responses through a

n extinguishing inflam­
matory responses through activation of CRH and AVP neurons 
in the PVH and subsequent elevation of anti-inflammatory glu­
cocorticoids. Interestingly, CRH is generally proinflammatory 
in the periphery, where it is found in sympathetic efferents, in 
sensory afferent nerves, in leukocytes, and in macrophages in 
some species.216,226 CRH also functions as a paracrine factor in 
the endometrium, where it may play a role in decidualization and 
implantation and act as a uter

ecidualization and 
implantation and act as a uterine vasodilator.216
The relative contributions of each of the CRH-urocortin pep­
tides and receptors to the different biologic functions reported have 
been the topic of considerable analysis, given the receptor-specific 
antagonists already described as well as the CRH, CRH-R1, and 
CRH-R2 knockout mice available for study (reviewed by Bale and 
Vale220 and Keck and colleagues214). Examination of three potent 
stressors—restraint, ether, and fas

 three potent 
stressors—restraint, ether, and fasting—demonstrated that other 
ACTH secretagogues, such as AVP, oxytocin, and catecholamines, 
could not replace CRH in its role in mounting the stress response. 
In contrast, augmentation of glucocorticoid secretion by a stressor 
after prolonged stress was not defective in CRH knockout mice, 
implicating CRH-independent mechanisms.
CRH and urocortin peptides also have potent anorexigenic 
activity, implicating the CRH system in stress-induced in

y, implicating the CRH system in stress-induced inhibition 
of feeding. However, studies utilizing CRH, CRH-R1, and CRH-
R2 knockout mice have not fully unraveled the complex inter­
actions of these peptides and receptor signaling pathways in the 
acute effects of stress on feeding behavior.
Additional gene knockout studies have suggested that urocor­
tin 2 plays a physiologic role in female mice to dampen basal daily 
rhythms of the HPA axis and reduce behavioral coping mecha­
nisms in response

 reduce behavioral coping mecha­
nisms in response to chronic stress.195 Urocortin 3 may have a 
primary action to augment insulin secretion in response to the 
metabolic stress of excessive calorie intake.196 
No approved therapeutic application of CRH or CRH-like pep­
tides exists, although the peptide has been demonstrated to have 
a number of activities in human and primate studies. For exam­
ple, intravenous administration of CRH was found to stimulate 
energy expendit

ion of CRH was found to stimulate 
energy expenditure but is an unlikely pharmaceutic target for 
inducing weight loss. The development of small molecule, orally 
available, CRH-R1 antagonists such as R121919 produced con­
siderable interest in their potential for treatment of anxiety and 
depression219,227 and even reached a Phase IIa trial.228 However, 
while still of major interest in the treatment of psychiatric condi­
tions, subsequent trials have proved disappointing, perhaps indi­
cating 

s have proved disappointing, perhaps indi­
cating that the neurobiology of signaling through the CRH-R1 
is more complex than currently appreciated.229 Recent studies, 
however, have suggested promising results for the use of the CRH-
R1 antagonist pexacerfont (BMS562086) to reduce craving and 
stress-induced eating, and verucerfont (NBI77860) in the treat­
ment of hyperandrogenism associated with congenital adrenal 
hyperplasia.229,230 
Feedback Control
The administration of glucocorticoids inh

 Control
The administration of glucocorticoids inhibits ACTH secretion; 
conversely, removal of the adrenals (or administration of drugs 
that impair secretion of glucocorticoids) leads to increased ACTH 
release. The set-point of pituitary feedback is determined by the 
hypothalamus acting through hypothalamic-releasing hormones 
CRH and AVP (see Chapter 8).231–234 Glucocorticoids act on 
both the pituitary corticotrophs and the hypothalamic neurons 
that secrete CRH and AVP. These regulatory a

rons 
that secrete CRH and AVP. These regulatory actions are analo­
gous to the control of the pituitary-thyroid axis. However, whereas 
TSH becomes completely unresponsive to TRH when thyroid 
hormone levels are sufficiently high, severe neurogenic stress and 
large amounts of CRH can break through the feedback inhibition 
due to modulation of hypophysiotropic CRH neurons by afferent 
input from other regions of the brain, including brainstem cat­
echolaminergic neurons (primarily the NTS), BNS

t­
echolaminergic neurons (primarily the NTS), BNST, hippocam­
pus, and prefrontal cortex, among others. Comprehensive reviews 
of regulation of the HPA axis have emphasized the complexity of 
this control beyond that of a simple closed-loop feedback.235,236
Glucocorticoids are lipid soluble and freely enter the brain 
through the blood-brain barrier.233 In brain and pituitary they can 
bind to two receptors: Type I (encoded by NR3C1) is called the 
mineralocorticoid receptor because it binds al

he 
mineralocorticoid receptor because it binds aldosterone and gluco­
corticoids with high affinity. Type II (NR3C2), the glucocorticoid 
receptor, has low affinity for mineralocorticoids.232–234 Classic 
glucocorticoid action involves binding of the steroid-receptor 
complex to regulator sequences in the genome. Type I receptors 
are saturated by basal levels of glucocorticoids, whereas type II 
receptors are not saturated under basal conditions but approach 
saturation during peak phases of t

s but approach 
saturation during peak phases of the circadian rhythm and dur­
ing stress. These differences and differences in regional distribu­
tion within the brain suggest that type I receptors determine basal 
activity of the hypothalamic-pituitary axis and that type II recep­
tors mediate stress responses.237
In the pituitary, glucocorticoids inhibit secretion of ACTH by 
inhibiting CRH binding to pituitary cells and reducing cAMP and 
the synthesis of POMC mRNA; in the hypothalamus they 

 synthesis of POMC mRNA; in the hypothalamus they inhibit 
secretion of CRH and AVP and the synthesis of their respective 
mRNAs, although with distinct temporal patterns.233,234,238 The 
latency of the inhibitory effect is so short (<30 minutes) that it is 
likely that gene regulation is not the sole basis of the response.239 
However, long-term suppression (>1 hour) clearly acts through 
genomic mechanisms. Glucocorticoids can also exert additional 
rapid signaling on hypophysiotropic CRH neur

onal 
rapid signaling on hypophysiotropic CRH neurons via an endo­
cannabinoid-mediated suppression of synaptic excitation.240 These 
events involve the action of glucocorticoids on a membrane-asso­
ciated receptor to mobilize endocannabinoids (2-arachidonoyl­
glycerol [2-AG] and N-arachidonoylethanolamine [AEA]) from 
CRH neurons that act through a cannabinoid receptor (CB1R) 
to inhibit the release of glutamate from neurons synapsing on 
hypophysiotropic CRH neurons.241 In addition, glucocorti

ysiotropic CRH neurons.241 In addition, glucocorticoid-
mediated inhibition of hypophysiotropic CRH activity can occur 
at other loci within the brain, such as the hippocampus, prefrontal 
cortex, and NTS, to inhibit CRH neurons directly or through 
multisynaptic pathways (see “Neural Control”), perhaps of great­
est importance in regulation of the HPA stress response (reviewed 
in Keller-Wood235 and Herman and colleagues236). Lesioning or 
selective deletion of glucocorticoid receptors in these

tive deletion of glucocorticoid receptors in these regions is 
141
associated with increased activity of hypophysiotropic CRH neu­
rons and the HPA axis242,243 (see “Neural Control”). 
Significant physiologic or psychologic stressors evoke an adaptive 
response that commonly includes activation of both the HPA axis 
and the sympathoadrenal axis. The end products of these path­
ways then help to mobilize resources to cope with the physiologic 
demands 

e resources to cope with the physiologic 
demands in emergency situations, acutely through the fight-or-
flight response and over the long term through systemic effects of 
glucocorticoids on functions such as gluconeogenesis and energy 
mobilization. The HPA axis also has unique stress-specific homeo­
static roles, the best example being the role of glucocorticoids 
in downregulating immune responses after infection and other 
events that stimulate cytokine production by the immune system.
The 

ate cytokine production by the immune system.
The PVH is the primary hypothalamic nucleus responsible for 
providing the integrated whole-animal response to stress.238,244,245 
This nucleus contains within it three major types of effector neu­
rons that are spatially distinct from one another: magnicellular 
oxytocin and AVP neurons that project to the posterior pitu­
itary and participate in the regulation of blood pressure, fluid 
homeostasis, lactation, and parturition; neurons projecting to 

lactation, and parturition; neurons projecting to the 
brainstem and spinal cord that regulate a variety of autonomic 
responses, including sympathoadrenal activation; and parvicellu­
lar CRH neurons that project to the median eminence and regu­
late ACTH synthesis and release. However, approximately 50% 
of CRH neurons coexpress AVP, which acts synergistically with 
CRH by activating the V1b receptor subtype on corticotrophs. 
AVP is regulated quite differently in parvicellular versus magni­
ce

uite differently in parvicellular versus magni­
cellular neurons but is also regulated somewhat differently from 
CRH by stressors in parvicellular cells that express both pep­
tides.238 Different stressors result in different patterns of activa­
tion of the three major visceromotor cell groups within the PVH, 
as measured by the general neuronal activation marker c-Fos and 
nerve growth factor 1B (NGF1B) (Fig. 7.16). For example, salt 
loading downregulates CRH mRNA in parvicellular CRH cells 

ownregulates CRH mRNA in parvicellular CRH cells 
and upregulates CRH in a small number of magnicellular CRH 
cells, but consistently activates magnicellular cells. Hemorrhage 
activates every division of the PVH, whereas cytokine administra­
tion primarily activates parvicellular CRH cells with some minor 
activation of magnicellular and autonomic divisions.
The synthesis and release of AVP, which regulates renal water 
absorption and vascular smooth muscle, are controlled mainly by 
the volume

mooth muscle, are controlled mainly by 
the volume and tonicity of the blood. This information is relayed 
to the magnicellular AVP cell through the NTS and A1 noradren­
ergic cell group of the ventrolateral medulla and projections from a 
triad of CVOs lining the third ventricle, the SFO, the medial pre­
optic nucleus (MePO), and OVLT. Oxytocin is primarily involved 
in reproductive functions, such as parturition, lactation, and milk 
ejection, although it is cosecreted with AVP in response to 

although it is cosecreted with AVP in response to osmotic 
and volume challenges, and oxytocin cells receive direct projec­
tions from the NTS as well as from the SFO, MePO, and OVLT. 
In contrast to the neurosecretory neurons functionally defined by 
the three peptides, CRH, oxytocin, and AVP, PVH neurons pro­
jecting to the brainstem and spinal cord include neurons express­
ing each of these peptides.
In the rodent, a wide variety of stressors has been determined 
to activate parvicellular CRH

has been determined 
to activate parvicellular CRH neurons, including cytokine injec­
tion, salt loading, hemorrhage, adrenalectomy, restraint, foot 
shock, hypoglycemia, fasting, and ether exposure. In contrast to 
the relative simplicity of inputs to magnicellular cells (Fig. 7.17A), 
as noted previously, parvicellular CRH neurons receive a diverse 
and complex assortment of inputs (Fig. 7.18; also see Fig. 7.17B). 
These inputs are divided into three major categories: brainstem, 
limbic foreb

o three major categories: brainstem, 
limbic forebrain, and hypothalamus. Because the PVH is not 
known to receive any direct projections from the cerebral cortex 
or thalamus, stressors involving emotional or cognitive processing 
must involve indirect relay to the PVH.
Visceral sensory input to the PVH involves primarily two 
pathways. The NTS, the primary recipient of sensory informa­
tion from the thoracic and abdominal viscera via the vagus and 
glossopharyngeal nerves, sends dense catechol

and 
glossopharyngeal nerves, sends dense catecholaminergic projec­
tions to the PVH from A2 noradrenergic and C2 adrenergic 
cell groups, both directly and through relays in the ventrolateral 
medulla. These brainstem projections also account for about 
half of the NPY fibers present in the PVH and contain other 
peptides such as glucagon-like peptide 1 and activin.246,247 
In a similar fashion, increased secretion of glucocorticoids in 
response to infection or inflammation is due to activatio

e to infection or inflammation is due to activation of 
catecholaminergic nerves in the NTS and ventrolateral medulla 
mediated by endotoxin and proinflammatory cytokines.245 If 
the ascending catecholamine pathway to the PVN is transected, 
the ability of IL-1 to increase CRH mRNA in PVH neurons is 
reduced.248 It is proposed that IL-1 exerts its effect on endo­
thelial cells and/or perivascular microglia at the blood-brain 
interface, resulting in activation of cyclooxygenase-2, the release 
o

 in activation of cyclooxygenase-2, the release 
of prostaglandin E2 into surrounding tissue, and ultimately 
activation of catecholaminergic neurons through prostaglandin 
receptors.249 In addition, cytokines may act directly on vas­
cular endothelial cells in the PVH to promote prostaglandin 
synthesis and activation of CRH neurons via EP3 receptors.250 
A second major input responsible for transducing signals from 
blood-borne substances derives from three CVOs adjacent to 
the third ventricl

es from three CVOs adjacent to 
the third ventricle, the SFO, OVLT, and ME. Collectively, these 
pathways account for activation of CRH neurons by what are 
referred to as systemic or physiologic stressors such as systemic 
inflammation, blood loss, hypoxia, or disorders of fluid and 
electrolyte balance.245
By contrast, what are termed neurogenic, emotional, or psycho­
logic stressors involve, in addition, nociceptive or somatosensory 
pathways as well as cognitive and affective brain centers. 

as well as cognitive and affective brain centers. Using 
elevation of c-Fos as an indicator of neuronal activation, detailed 
studies have compared PVH-projecting neurons activated by 
IL-1 treatment (systemic stressor) versus foot shock (neurogenic 
stressor).245 Only catecholaminergic solitary tract nucleus and 
ventrolateral medulla neurons were activated by moderate doses 
of IL-1. In contrast, foot shock activated not only neurons of the 
NTS and ventrolateral medulla but also cell groups i

S and ventrolateral medulla but also cell groups in the limbic 
forebrain and hypothalamus. Notably, pharmacologic or mechan­
ical disruption of the ascending catecholaminergic fibers blocked 
IL-1–mediated activation but not foot shock–mediated activation 
of the HPA axis.244,245 Thus, while the end result to increase cir­
culating levels of glucocorticoids is similar in all stress paradigms, 
depending upon the type of stress, different regions of the brain 
are recruited to allow activation o

 of the brain 
are recruited to allow activation of the hypophysiotropic CRH 
neurons.
Except for the catecholaminergic neurons of the NTS and 
ventrolateral medulla, parts of the BNST, and the dorsomedial 
nucleus of the hypothalamus, many inputs to the PVH, such as 
those deriving from the prefrontal cortex and lateral septum, are 
thought to act indirectly through local hypothalamic glutama­
tergic251 and GABAergic neurons252 with direct synapses to the 
CRH neurons. The BNST is the only limb

ses to the 
CRH neurons. The BNST is the only limbic region with promi­
nent direct projections to the PVH. With substantial projections 
142
from the amygdala, hippocampus, and septal nuclei, it may thus 
serve as a key integrative center for transmission of limbic infor­
mation to the PVH.244 
Inflammation and Cytokines
Stimulation of the immune system by foreign pathogens leads 
to a stereotyped set of responses orchestrated by the CNS. This 
constella

responses orchestrated by the CNS. This 
constellation of stereotyped responses results from the com­
plex interaction of the immune system and the CNS. They are 
mediated in large part by the hypothalamus and include coordi­
nated autonomic, endocrine, and behavioral components with 
adaptive consequences to restore homeostasis. It is now clear that 
cytokines produced by peripheral circulating cells of the immune 
system and central glial cells mediate the CNS responses. Early 
evidence suppor

 mediate the CNS responses. Early 
evidence supporting this hypothesis was provided by the seminal 
observations that cytokines such as IL-1β can activate the HPA 
axis.253–255 The resultant glucocorticoid secretion acts as a classic 
negative feedback to the immune system to dampen its response. 
• Fig. 7.16  Regulation of neurons of the paraventricular nucleus (PVH) by diverse stressors. ADX, adrenal­
ectomy; CRF, corticotropin-releasing factor in situ hybridization (dark-field); dp, dorsal pa

 in situ hybridization (dark-field); dp, dorsal parvicellular; Fos, 
c-Fos immunoreactivity (bright-field); IL-1, interleukin 1; mp, medial parvicellular; NGF1-B, nerve growth 
factor 1-B in situ hybridization (dark-field); pm, posterior magnicellular. (From Sawchenko PE, Brown ER, 
Chan RK, et al. The paraventricular nucleus of the hypothalamus and the functional neuroanatomy of vis­
ceromotor responses to stress. Prog Brain Res. 1996;107:201–222.)
143
In general, 

2.)
143
In general, glucocorticoids inhibit most limbs of the immune 
response, including lymphocyte proliferation, production of 
immunoglobulins, cytokines, and cytotoxicity. These inhibi­
tory reactions form the basis of the anti-inflammatory actions 
of glucocorticoids.
Glucocorticoid feedback on immune responses is regulatory 
and beneficial because loss of this function makes animals with 
adrenal insufficiency vulnerable to inflammation. However, this 
feedba

 vulnerable to inflammation. However, this 
feedback response can have pathophysiologic consequences, as 
chronic activation of the HPA axis can certainly be detrimen­
tal.234,256 Indeed, prolonged exposure to elevated glucocorticoids 
is injurious to pyramidal neurons, and it is well established that 
chronic stress can lead to immunosuppression. The fact that prod­
ucts of inflammation such as IL-1β can activate the HPA axis 
suggests the operation of a negative feedback control loop to regu­

tion of a negative feedback control loop to regu­
late the intensity of inflammation. The role of the hypothalamus 
in regulating pituitary-adrenal function is an excellent example 
of neuroimmunomodulation. Proposed models to explain how 
immune system signals might act upon the CNS to modulate 
homeostatic circuits by the integration of vagal input, peripheral 
cytokine interactions with receptors in the CVOs and cerebral 
blood vessels, and local production of cytokines within the CNS 
are ex

cal production of cytokines within the CNS 
are explored in Chapter 39. 
Other Factors Influencing Secretion of Corticotropin
Circadian Rhythms
In humans, levels of ACTH and cortisol peak in the early morning, 
drop during the day to reach a nadir at about midnight, and begin 
to rise between 1 am and 4 am (see Fig. 7.7). Within the circadian 
cycle approximately 15 to 18 pulses of ACTH can be discerned, 
their height varying with the time of day.257 The set-point of feed­
back control by glucoc

.257 The set-point of feed­
back control by glucocorticoids also varies in a circadian pattern. 
Pituitary-adrenal rhythms are entrained to the light-dark cycle and 
can be changed over several days by exposure to an altered light 
schedule. It has long been assumed that the rhythm of ACTH 
secretion is driven by CRH rhythms, and CRH knockout mice 
were found to exhibit no circadian rhythm in corticosterone pro­
duction. Remarkably, however, a diurnal rhythm in corticosterone 
was restored by a 

urnal rhythm in corticosterone 
was restored by a constant infusion of CRH to CRH knockout 
mice,258 suggesting that CRH is necessary to permit pituitary or 
adrenal responsiveness to another diurnal rhythm generator. 
A
B
SFO
SFO
PVT
PVH
IGL
CG
PB
PP, PIN
HYP
NTS
C1
PPN, LDT
BNST
BNST
OVLT
PVH/SO
NTS
A1
MePO
MePO
OVLT
Limbic
Limbic
Bloodborne
Glucocorticoids
Visceral
Visceral
Somatosensory
Nociceptive
Visual
Auditory
OT
OT, AVP
ACTH
CRH
AVP
• Fig. 7.17  (A) Neuronal inp

OT, AVP
ACTH
CRH
AVP
• Fig. 7.17  (A) Neuronal inputs to magnicellular and (B) parvicellular neurons of the paraventricular nucleus 
(PVH). AVP, arginine vasopressin; BNST, bed nucleus of the stria terminalis; CG, central gray; CRH, 
corticotropin-releasing hormone; HYP, hypothalamus; IGL, intergeniculate leaflet; LDT, laterodorsal teg­
mental nucleus; MePO, medial preoptic nucleus; NTS, nucleus of the tractus solitarius; OT, oxytocin; 
OVLT, organum vasculosum of the lamina terminalis; PB, para

anum vasculosum of the lamina terminalis; PB, parabrachial nucleus; PIN, posterior intralami­
nar nucleus; PP, peripeduncular nucleus; PPN, pedunculopontine nucleus; SFO, subfornical organ; SO, 
supraoptic nucleus. (From Sawchenko PE, Brown ER, Chan RK, et al. The paraventricular nucleus of the 
hypothalamus and the functional neuroanatomy of visceromotor responses to stress. Prog Brain Res. 
1996;107:201–222.)
144
Opiates
It has been long recognized that

Pituitary
Opiates
It has been long recognized that opiate analgesia can suppress the 
HPA axis, with approximately 10% of patients on chronic opi­
ate therapy and 15% of patients receiving opiates intrathecally, 
developing abnormally low basal cortisol levels and flattened or 
reduced circadian rhythms of ACTH.259,260 Profound adrenal 
insufficiency associated with hypotension and low ACTH and 
cortisol levels has also been reported.261 The mechanisms for 
opioid-induced effects on the HPA axis

anisms for 
opioid-induced effects on the HPA axis are likely multifacto­
rial but presumed to be mediated centrally through κ-receptors 
or δ-receptors, as there is relative insensitivity of the HPA axis 
to the potent μ-receptor agonist, naloxone.262,263 Endomorphins 
injected directly into the hypothalamus acutely inhibit CRH 
gene expression,264 but opiates may also act presynaptically on 
Pituitary
Adrenal
Processive
(Neurogenic)
Stress
(fear, restraint)
(cytokines,
hypoxia,
hemorrhage)
Sys

r, restraint)
hypoxia,
Systemic
(Physiologic)
Stress
Cortisol
Immune
cells
IL-1, IL-2,
IL-6, TNFα
(osmotic challenge,
macromolecules)
Systemic
Stress
Lateral
septum
Paraventricular
nucleus
Subfornical org.
OVLT
MePO
CRH neuron
ACTH
CRH
AVP
CRIF?
to
CNS
Brainstem
vasculature
Arcuate,
NPY, and POMC
neurons
Prostaglandins
glutamatergic
inputs
inputs
GABAergic inputs
GABA
Brainstem
catecholaminerg

ic
GABA
Brainstem
catecholaminergic
inputs
Hippocampus
Prefrontal
cortex
Raphe nucleus
NTS
MeA
BNST
5-HT
Glucocorticoid
receptors
Cytokine receptors
Leptin receptors
EP3 receptors
CB1 receptor
CRH-binding 
protein
• Fig. 7.18  Regulation of the hypothalamic-pituitary-adrenal axis. ACTH, adrenocorticotropic hormone; 
AVP, arginine vasopressin; BNST, bed nucleus of the stria terminalis; CNS, central nervous system; CRH, 
corticotropin- releasing hormone; CRIF, cor

 CRH, 
corticotropin- releasing hormone; CRIF, corticotropin release–inhibiting factor; GABA, γ-aminobutyric acid; 
5-HT, 5-hydroxytryptamine; IL-1, interleukin 1; MeA, medial amygdala; MePO, medial preoptic nucleus; 
NPY, neuropeptide Y; NTS, nucleus of the tractus solitarius; OVLT, organum vasculosum of the lamina 
terminalis; POMC, pro-opiomelanocortin; TNFα, tumor necrosis factor-α.
145
noradrenergic terminals in the PVN to reduce CRH release.265 
Evidence that 

 the PVN to reduce CRH release.265 
Evidence that morphine inhibits the rise in ACTH to exogenous 
CRH further suggests an action of opiates on anterior pituitary 
corticotrophs.266 It has been proposed that endogenous opi­
ates may function to diminish the impact of the stress response, 
and during pregnancy, to protect the fetus from glucocorticoid 
excess.267,268 Opiate-induced hyperprolactinemia may also con­
tribute to suppression of the HPA axis given that chronic intra­
cerebroventricular

 axis given that chronic intra­
cerebroventricular (ICV) infusion of antisense oligonucleotides 
against the prolactin receptor increase the stress-induced ACTH 
response, presumably by direct effects on CRH neurons or its 
noradrenergic input.269 
Evidence for neural control of GH secretion originated from stud­
ies of its regulation in animals with lesions of the hypothalamus 
and from the demonstration that hypothalamic extracts stimula

e demonstration that hypothalamic extracts stimulate 
the release of GH from the pituitary. When it was shown that GH 
is released episodically, follows a circadian rhythm, responds rap­
idly to stress, and is blocked by pituitary stalk section, the concept 
of neural control of GH secretion became a certainty. However, 
it was only with the discovery of the paraneoplastic syndrome of 
ectopic GHRH secretion by pancreatic adenomas in humans that 
sufficient starting material became available for

sufficient starting material became available for peptide sequenc­
ing and subsequent cloning of a complementary deoxyribonucleic 
acid (cDNA).270–272
Two principal molecular forms of GHRH occur in the 
human hypothalamus: GHRH(1-44)-NH2 and GHRH(1-
40)-OH (Fig. 7.19).273 As with other neuropeptides, the various 
forms of GHRH arise from post-translational modification of 
a larger prohormone.270 The NH2-terminal tyrosine of GHRH 
(or histidine in rodent GHRHs) is essential for bioactivity, but

in rodent GHRHs) is essential for bioactivity, but a 
COOH-terminal NH2 group is not. A circulating type IV dipep­
tidyl peptidase potently inactivates GHRH to its principal and 
more stable metabolite, GHRH(3-44)-NH2,274 which accounts 
for most of the immunoreactive peptide detected in plasma. 
As in the case of GnRH, there are species differences among 
GHRHs; the peptides from seven species range in sequence 
homology with the human peptide from 93% in the pig to 67% 
in the rat.273 The COOH

rom 93% in the pig to 67% 
in the rat.273 The COOH-terminal end of GHRH exhibits the 
most sequence diversity among species, consistent with the exon 
arrangement of the gene and dispensability of these residues for 
GHRH receptor binding.
Despite its importance for the elucidation of GHRH structure, 
ectopic secretion of the peptide is a rare cause of acromegaly.275 
Fewer than 1% of acromegalic patients have elevated plasma levels 
of GHRH (see Chapters 8 and 9). Approximately 20% of pancre­
a

 Chapters 8 and 9). Approximately 20% of pancre­
atic adenomas and 5% of carcinoid tumors contain immunoreac­
tive GHRH, but most are clinically silent.276
In addition to expression in the hypothalamus, the GHRH 
gene is expressed eutopically in the human ovary, uterus, and 
placenta,277 although its function in these tissues is not known. 
Studies in rat placenta indicate that an alternative transcriptional 
start site 10 kilobases upstream from the hypothalamic promoter 
is utilized together w

 the hypothalamic promoter 
is utilized together with an alternatively spliced exon 1a.278 
Growth Hormone–Releasing Hormone Receptor
The GHRH receptor is a member of a subfamily of GPCRs that 
includes receptors for VIP, pituitary adenylyl cyclase–activating 
peptide, secretin, glucagon, glucagon-like peptide 1, calcitonin, 
parathyroid hormone or parathyroid hormone–related peptide, 
and gastric inhibitory polypeptide.279 GHRH elevates intracellu­
lar cAMP by its receptor coupling to a Gs, whi

lu­
lar cAMP by its receptor coupling to a Gs, which activates adenylyl 
cyclase, increases intracellular free Ca2+, releases preformed GH, 
and stimulates GH mRNA transcription and new GH synthe­
sis.280 GHRH also increases pituitary phosphatidylinositol turn­
over. Nonsense mutations in the human GHRH receptor gene are 
the cause of rare familial forms of GH deficiency281 and indicate 
that no other gene product can fully compensate for the specific 
receptor in the pituitary. 
Effects on the 

cific 
Intravenous administration of GHRH to individuals with normal 
pituitaries causes a prompt, dose-related increase in serum GH 
that peaks after 15 to 45 minutes, followed by a return to basal 
levels by 90 to 120 minutes (Fig. 7.20).282 A maximally stimulat­
ing dose of GHRH is approximately 1 μg/kg, but the response 
differs considerably among individuals and within the same indi­
vidual tested on different occa

hin the same indi­
vidual tested on different occasions, presumably because of endog­
enous cosecretagogue and somatostatin tone that exist at the time 
of GHRH injection. Repeated bolus administration or sustained 
infusions of GHRH over several hours cause a modest decrease in 
the subsequent GH secretory response to acute GHRH admin­
istration. However, unlike the marked desensitization of the 
GnRH receptor and decline in circulating gonadotropins that 
occur in response to continuous GnRH e

opins that 
occur in response to continuous GnRH exposure, pulsatile GH 
secretion and insulin-like growth factor 1 (IGF-1) production are 
maintained by constant GHRH in the human.282 This response 
suggests the involvement of additional factors that mediate the 
intrinsic diurnal rhythm of GH, and these factors are addressed in 
the following sections.
The pituitary effects of a single injection of GHRH are almost 
completely specific for GH secretion, and there is minimal evi­
dence for any i

cretion, and there is minimal evi­
dence for any interaction between GHRH and the other classic 
hypophysiotropic-releasing hormones.282 GHRH has no effect 
on gut peptide hormone secretion. The GH secretory response 
to GHRH is enhanced by estrogen administration, ghrelin, glu­
cocorticoids, and starvation. Major factors known to blunt the 
response to GHRH are somatostatin, obesity, hyperinsulinemia, 
hyperglycemia, and advancing age. (For review see Steyn and 
colleagues283.)
In addition to i

ew see Steyn and 
In addition to its role as a GH secretagogue, GHRH is a physi­
ologically relevant growth factor for somatotrophs. Transgenic 
mice expressing a GHRH cDNA coupled to a suitable promoter 
developed diffuse somatotroph hyperplasia and eventually pitu­
itary macroadenomas.284 The intracellular signal transduction 
pathways mediating the mitogenic action of GHRH are not 
known with certainty but probably involve an elevation of adeny­
lyl cyclase activity. Several l

levation of adeny­
lyl cyclase activity. Several lines of evidence support this conclu­
sion, including the association of activating mutations of the Gsα 
polypeptide in many human somatotroph adenomas.285 
GHRH has few known extrapituitary functions. The most impor­
tant may be its activity as a sleep regulator. The administration of 
nocturnal GHRH boluses to normal men significantly increases 
the density of slow-wave sleep and promotes nonrapid eye move­
ment sleep,

 sleep and promotes nonrapid eye move­
ment sleep,286,287 mediated by GABAergic neurons in the preoptic 
nucleus and independent of GH.286 Furthermore, there is a strik­
ing correlation between the age-related declines in slow-wave sleep 
and daily integrated GH secretion in healthy men.288 These and 
other data suggest that central GHRH secretion is under circadian 
146
entrainment, and nocturnal elevations in GHRH pulse ampli­
tude or frequency directly

ns in GHRH pulse ampli­
tude or frequency directly mediate sleep stage and sleep-induced 
increases in GH secretion.
GHRH has been reported to stimulate food intake in rats and 
sheep, but the effect is dependent on route of administration, time 
of administration, and macronutrient composition of the diet.279 
The neuropeptide’s physiologic relevance to feeding in humans is 
unknown. Evidence suggests that nonpituitary GHRH modulates 
cell proliferation and promotes the healing of skin wounds.2

feration and promotes the healing of skin wounds.289 
Growth Hormone–Releasing Peptides
In studies of the opioid control of GH secretion, several peptide 
analogues of met-enkephalin were found to be potent GH secreta­
gogues (GHS). These include the GH-releasing peptide GHRP-6 
(Fig. 7.21), hexarelin (His-d2MeTrp-Ala-Trp-dPhe-Lys-NH2), 
and other more potent analogues, including cyclic peptides and 
modified pentapeptides.279,290 Subsequently, a series of non­
peptidyl GHRP mimetics were synthe

 series of non­
peptidyl GHRP mimetics were synthesized with greater oral 
bioavailability, including the spiropiperidine MK-0677 and the 
shorter acting benzylpiperidine L-163,540 (see Fig. 7.21). Com­
mon to all these compounds, and the basis of their differentiation 
from GHRH analogues in pharmacologic activity screens, is their 
activation of phospholipase C and inositol 1,4,5-trisphosphate. 
This property was exploited in a cloning strategy that led to the 
identification of a novel GPCR G

 that led to the 
identification of a novel GPCR GHS receptor that is highly selec­
tive for the GHS class of ligands.291 The GHS-R is unrelated to 
the GHRH receptor and is highly expressed in the anterior pitu­
itary gland and multiple brain areas, including GHRH neurons in 
the medial basal hypothalamus, suggesting both direct and indi­
rect effects on GH secretion.292,293
Peptidyl and nonpeptidyl GHSs are active when administered 
by intranasal and oral routes, are more potent on a weight ba

al and oral routes, are more potent on a weight basis 
than GHRH itself, are more effective in vivo than in vitro, syner­
gize with coadministered GHRH, and are almost ineffective in the 
absence of GHRH.279,282 Prolonged infusions of GHRP amplify 
pulsatile GH secretion in normal men. GHRP administration, 
PC1/PC2
CPE
PAM
Type IV
dipeptidylpeptidase
GHRH (3-44) – NH2
Placental Promoter
Intron
5'UTR
3'UTR
Signal
Peptide
GHRH
[GHRH (1-44)] – Gly – Lys
and   GHRH (1-40)
Intron
Intron
Poly A
1500 b

 Lys
Intron
Intron
Poly A
1500 bp
100 bp
100 bp
2 3
4
5
Hypothalamic Promoter
TATA
Exon 1
Alternative
GHRH (1-44) – NH2
Tyr
Gln
Gln
Ala
Leu
Gly
Asp
Ser
Glu
Ala
Ala
Ser
Ile
Arg
Asn
Phe
Lys
Gln
Thr
Leu
Glu
Asn
Leu
Arg
Ser
Gln
Gly
Tyr
Asp
Ala
Arg
Ile
Arg
Lys
Met
Ala
Val
Ser
Arg
Leu
Arg
Leu
Gly
Gln
NH2
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
GSH-1 x 5
Exon 1
• Fig. 7.19  Diagram illustrating the genomic organization, messenger RNA str

rating the genomic organization, messenger RNA structure, and post-translational 
processing of the human growth hormone–releasing hormone (GHRH) prohormone. Five GSH-1 home­
odomain transcription factor–binding sites in the proximal promoter have been characterized in the rat 
gene. All of the amino acid residues required for bioactive GHRH peptides are encoded by exon 3. An 
amino-terminal exopeptidase that cleaves the Tyr-Ala dipeptide is primarily responsible for the inactiva­
tion of GHRH p

arily responsible for the inactiva­
tion of GHRH peptides in extracellular compartments. CPE, carboxypeptidase E; PAM, peptidylglycine 
α-amidating monooxygenase; PC1/PC2, prohormone convertases 1 and 2; TATA, Goldstein-Hogness 
box involved in binding RNA polymerase; UTR, untranslated region. (Compiled from data of Mayo KE, 
Cerelli GM, Lebo RV, et al. Gene encoding human growth hormone-releasing factor precursor: structure, 
sequence, and chromosomal assignment. Proc Natl Acad Sci USA. 1985;82

osomal assignment. Proc Natl Acad Sci USA. 1985;82:63–67; Frohman LA, Downs 
TR, Chomczynski P, et al. Growth hormone-releasing hormone: structure, gene expression and molecular 
heterogeneity. Acta Paediatr Scand Suppl. 1990;367:81–86; González-Crespo S, Boronat A. Expression 
of the rat growth hormone-releasing hormone gene in placenta is directed by an alternative promoter. Proc 
Natl Acad Sci USA. 1991;88:8749–8753; and Mutsuga N, Iwasaki Y, Morishita M, et al. Homeobox protein 
Gsh-1-depend

Morishita M, et al. Homeobox protein 
Gsh-1-dependent regulation of the rat GHRH gene promoter. Mol Endocrinol. 2001;15:2149–2156.)
147
like that of GHRH, facilitates slow-wave sleep. Patients with 
hypothalamic disease leading to GHRH deficiency have low or no 
response to hexarelin; similarly, pediatric patients with complete 
absence of the pituitary stalk have no GH secretory response to 
hexarelin.294
The potent biologic effects of GHRPs and the identification 

 biologic effects of GHRPs and the identification 
of the GHS-R suggested the existence of a natural ligand for the 
receptor that is involved in the physiologic regulation of GH 
secretion. The 28–amino acid peptide, ghrelin, produced and 
secreted into the circulation from the stomach, is this ligand (Fig. 
7.22).15 The effects of ghrelin on GH secretion in humans are 
identical to or more potent than those of the nonnatural GHRPs 
(see Fig. 7.20).295 In addition, ghrelin acutely increases cir

20).295 In addition, ghrelin acutely increases circulating 
PRL, ACTH, cortisol, and aldosterone levels.295 There is debate 
concerning the extent and localization of ghrelin expression in 
the brain that must be resolved before the implications of gastric-
derived ghrelin in the regulation of pituitary hormone secretion 
are fully understood. Furthermore, post-translational process­
ing of pro-ghrelin gives rise to a second neuropeptide, obestatin, 
which may also have functional roles in activ

in, 
which may also have functional roles in activity of the GH/IGF-1 
axis and metabolism.296 A proposed role for ghrelin and obestatin 
peptides in appetite and the regulation of food intake is discussed 
in Chapter 39. It is of particular interest that although GH secre­
tory bursts are smaller in ghrelin knockout mice, they show lit­
tle differences in weight and body length compared to normal 
controls, raising the possibility that ghrelin’s main function on 
GH secretion is to maintain blo

 main function on 
GH secretion is to maintain blood glucose levels during caloric 
restriction.283 
160
120
80
40
0
180
150
120
90
60
30
0
−15
Time (min)
GH (µg/L)
GHRH + ghrelin
Ghrelin
GHRH
• Fig. 7.20  Response of normal men to growth hormone–releasing hor­
mone (GHRH)(1-29) (1 μg/kg), ghrelin (1 μg/kg), or the combination of 
GHRH(1-29) and ghrelin administered by intravenous injection. Note the 
prompt release of GH, followed by a rather prolonged fall in hormone level 
in response to both

olonged fall in hormone level 
in response to both secretagogues. Ghrelin alone was more efficacious 
than GHRH(1-29), and there was an additive effect from the two peptides 
administered simultaneously. (From Arvat E, Macario M, Di Vito L, et al. 
Endocrine activities of ghrelin, a natural growth hormone secretagogue 
[GHS], in humans: comparison and interactions with hexarelin, a nonnatu­
ral peptidyl GHS, and GH-releasing hormone. J Clin Endocrinol Metab. 
2001;86:1169–1174.)
–
–
–
–
–
–
–
–

rinol Metab. 
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
=
GHRP– 6: His–DTrp–Ala–Trp–DPhe–Lys–NH2
O
O
C
C
H
N
N
H
N
CH3
CH3
C
NH2
O
H
C
C
H
N
CH3
CH3
CO2CH2CH3
C
NH2
O
O
H
N
Ghrelin:
Gly
Arg
Ser
Lys
Ser
Glu
Phe
Ser
Leu
Lys
Ser
Lys
Pro
Pro
Glu
Pro
His
Ala
Gln
Lys
Arg
Leu
Val
Gln
Gln
Pro
Gln
Arg
C
O
(CH2)6
CH3
O
MK-0677
L-163,540
CH3SO2
N
• Fig. 7.21  Structure of a synthetic peptidyl growth hormone (GH) secretagogue (GH-releasing peptide 
6 [GHRP-6]) and nonpept

gue (GH-releasing peptide 
6 [GHRP-6]) and nonpeptidyl growth hormone secretagogues (MK-0677 and L-163,540) and a natural 
ligand (ghrelin) that all bind and activate the growth hormone secretagogue (GHS) receptor. Ghrelin is 
an acylated 28–amino acid peptide. The O-n-octanoylation at Ser3 is essential for biologic activity and 
is a unique post-translational modification mediated enzymatically by ghrelin-O-acyltransferase (GOAT). 
(Adapted from Smith RG, Feighner S, Prendergast K, et al. A new

 Smith RG, Feighner S, Prendergast K, et al. A new orphan receptor involved in pulsatile 
growth hormone release. Trends Endocrinol Metab. 1999;10:128–135; Kojima M, Hosoda H, Date Y, et al. 
Ghrelin is a growth hormone-releasing acylated peptide from stomach. Nature. 1999;402:656–660.)
148
GHRH stimulates growth in children with intact pituitaries, 
but the optimal dosage, route, and frequency of administration, 
as well as possible

 frequency of administration, 
as well as possible usefulness by the nasal route, have not been 
determined. The availability of recombinant hGH (which requires 
less frequent injections than GHRH) and the development of 
the more potent GHSs with improved oral bioavailability have 
reduced enthusiasm for the clinical use of GHRH or its analogues. 
GHRH can be used to test for GH deficiency in adults, particu­
larly when combined with arginine to improve diagnostic accu­
racy of the test; is not

 improve diagnostic accu­
racy of the test; is not influenced by age, sex, or body mass index; 
and has a wider margin of safety than an insulin tolerance test.297
The potential clinical applications of GHSs are still being 
explored.279,290 An area of intense interest is the normal, progres­
sive decline in GH secretion with aging and its association with 
frailty, decreased muscle mass, central adiposity, cardiovascular 
morbidities, and increased fractures associated with somatopause. 
GH adm

sed fractures associated with somatopause. 
GH administration in healthy older individuals has been associ­
ated with increased lean body mass, increased muscle strength, 
and decreased fat mass, although there is a high incidence of 
adverse side effects, including edema, hypertension, carpal tunnel 
syndrome, and hyperglycemia. One must also take into consid­
eration that animal models with growth hormone or IGF-1 defi­
ciency tend to be associated with longevity and reduced cancer 
risk,298–3

ated with longevity and reduced cancer 
risk,298–300 and this may also hold true for human longevity and 
slowing of the aging process.301,302 Both GHRH and GHSs are 
also being investigated for the treatment of sleep disorders com­
monly associated with aging. 
Neuroendocrine Regulation of Growth Hormone Secretion
GH secretion is regulated by two major hypothalamic peptides, 
GHRH that stimulates the release of GH and somatostatin that 
inhibits GH secretion, functioning in concert to give rise

 GH secretion, functioning in concert to give rise to the 
rhythmic 10 to 20 pulses of GH per 24 hours, depending upon 
the animal species. GHRH neurons are located in the arcuate 
nucleus or infundibular nucleus, whereas perikarya of the soma­
tostatin neurons involved in GH regulation are located in the 
hypothalamic periventricular nucleus.303 It is presumed that GH 
pulses are primarily driven by somatostatin, whereas the ampli­
tude of the GH pulses is driven by GHRH.304,305 However, other 

H pulses is driven by GHRH.304,305 However, other 
circulating hormones and additional modulatory peptides at the 
level of both the pituitary and the hypothalamus also contribute 
to the fine-tuning of GH secretion (see Fig. 7.22).279,282,296,306–308 
Additional background on somatostatin and its functions other 
than control of GH secretion are presented in “Somatostatin.” 
Feedback control of GH secretion is regulated at both the pitu­
itary and hypothalamus. GH release is me

the pitu­
itary and hypothalamus. GH release is mediated by negative feed­
back of GH, itself, and by IGF-1, the latter being synthesized in 
the liver and other tissues under the control of GH. Somatotrophs 
express GH receptors, and in cell culture their secretion is inhib­
ited by the exogenous administration of GH.309,310 IGF-1 also 
has a direct, major, inhibitory action on GH secretion as demon­
strated by a rise in circulating GH levels when the IGF-1 recep­
tor is selectively deleted fro

en the IGF-1 recep­
tor is selectively deleted from somatotrophs.311 In addition, gene 
expression of both GH and the pituitary-specific transcription fac­
tor PIT1 is inhibited by IGF-1.279
GH also crosses the blood-brain barrier to feedback on the 
hypothalamus to regulate GH secretion, primarily by increasing 
somatostatin secretion into the portal capillary system. Indeed, the 
inhibitory effects of GH are abolished in knockout mice lacking 
somatostatin receptors (Fig. 7.23). As noted earli

somatostatin receptors (Fig. 7.23). As noted earlier, tuberoinfun­
dibular somatostatin neurons are located in the hypothalamic 
periventricular nucleus and express GH receptors. GH receptors 
are also expressed by the majority of NPY neurons in the hypo­
thalamic arcuate nucleus that show c-Fos expression in response 
to GH administration and are known to innervate somatostatin 
neurons in the periventricular nucleus.312,313 In contrast to soma­
tostatin neurons, however, only a small percentag

tostatin neurons, however, only a small percentage of GHRH-
producing neurons in the hypothalamus express GH receptors.313 
Thus, together with data from many other experiments, these 
results strongly support a model of GH negative feedback regula­
tion that involves the activation of periventricular somatostatin 
neurons by GH using both direct and indirect (NPY) pathways. 
Similarly, in normal men, GH pretreatment blocks the subse­
quent GH secretory response to GHRH by a mechanism that is 

ecretory response to GHRH by a mechanism that is 
dependent on somatostatin.314
IGF-1 also has central effects to inhibit GH secretion that 
involves both an increase in somatostatin and a reduction in 
GHRH.315 The feedback effects of IGF-1 account for the fact that 
serum GH levels are elevated in conditions in which circulating 
levels of IGF-1 are low, such as anorexia nervosa, protein-calorie 
starvation,316 and Laron dwarfism (the result of a defect in the 
GH receptor).
GHRH and somatosta

 a defect in the 
GHRH and somatostatin also contribute to the pulsatile 
release of GH by exerting short-loop feedback effects on each 
other through bidirectional interactions.317 Treatment of hypo­
thalamic cultures with somatostatin inhibits GHRH, whereas 
treatment with GHRH induces somatostatin release.318,319 In 
the human brain, however, there are very few GHRH contacts 
on somatostatin neurons, although the vast majority of GHRH 
neurons receive somatostatin contacts.320 T

 GHRH 
neurons receive somatostatin contacts.320 The feedback effects 
of GHRH on somatostatin neurons, however, may be indirect 
via arcuate nucleus NPY neurons (as described previously) that 
receive projections from GHRH neurons.321 Evidence for exten­
sive interactions among GHRH neurons in the human brain have 
also been observed and may be responsible for synchronizing their 
activity.322
As mentioned previously, ghrelin receptors are highly expressed 
on somatotrophs in the anterior pitui

y expressed 
on somatotrophs in the anterior pituitary and in the hypothala­
mus, indicating both direct and indirect effects on GH secretion. 
When acylated with an eight-carbon fatty acid, ghrelin stimulates 
GHRH from hypothalamic explants, increases the firing rate of 
GHRH neurons, and when administered continuously, amplifies 
GH pulsatility.323–325 The effect of ghrelin to increase GH secre­
tion seems unaffected by the inhibitory effects of somatostatin, 
and in sheep, ghrelin also preve

of somatostatin, 
and in sheep, ghrelin also prevents the release of somatostatin 
into the portal capillary system, contributing to enhanced GH 
release.326 Evidence for a gastro-hypophysial feedback loop has 
also been given as ghrelin levels fall in response to rising levels of 
GH.327 
In addition to the regulatory mechanisms already described, mul­
tiple extrahypothalamic brain regions provide efferent connections 
to the hypothalamus and regulate GHRH and somatostatin neu­
r

thalamus and regulate GHRH and somatostatin neu­
ronal activity (Fig. 7.24; also see Fig. 7.22). Somatosensory and 
affective information is integrated and filtered through the amyg­
daloid complex. The basolateral amygdala provides an excitatory 
input to the hypothalamus, and the central extended amygdala, 
which includes the central and medial nuclei of the amygdala 
together with the BNST, provides a GABAergic inhibitory input. 
Many intrinsic neurons of the hypothalamus also release GABA, 

c neurons of the hypothalamus also release GABA, 
149
Pituitary
GH receptors
GHS receptors
GHRH receptors
SST receptors
Periventricular
nucleus
Arcuate
nucleus
Raphe
nucleus
Basal
forebrain
Liver
GHRH
Ghrelin
Leptin
Ghrelin
Galanin
to CNS
IGF-1
SST
DA
NPY
neuron
CRH
neuron
SST
neuron
Brainstem
inputs
5-HT
ACh
GH
SST
neuron
GHRH
neuron
Fat
FFA
Stomach
• Fig. 7.22  Regulation of the hypothalamic-pituitary-growth hormone (GH) axis. GH sec

halamic-pituitary-growth hormone (GH) axis. GH secretion by the pitu­
itary is stimulated by GH-releasing hormone (GHRH) and is inhibited by somatostatin (SST). Negative 
feedback control of GH secretion is exerted at the pituitary level by insulin-like growth factor 1 (IGF-1) and 
by free fatty acids (FFA). GH itself exerts a short-loop negative feedback through activation of SST neurons 
in the hypothalamic periventricular nucleus. These SST neurons directly synapse on arcuate GHRH neu­
rons a

urons directly synapse on arcuate GHRH neu­
rons and project axon collaterals to the median eminence. Neuropeptide Y (NPY) neurons in the arcuate 
nucleus also indirectly modulate GH secretion by integrating peripheral GH, leptin, and ghrelin signals and 
projecting to periventricular SST neurons. Ghrelin is secreted from the stomach and is a natural ligand for 
the GH secretagogue (GHS) receptor that stimulates GH secretion at both the hypothalamic and pituitary 
levels. On the basis of indirec

mic and pituitary 
levels. On the basis of indirect pharmacologic data, it appears that release of GHRH is stimulated by 
galanin, γ-aminobutyric acid (GABA), α2-adrenergic, and dopaminergic inputs and inhibited by SST. Secre­
tion of SST is inhibited by muscarinic acetylcholine (ACh) and 5-HT-1D receptor ligands, and increased 
by β2-adrenergic stimuli and corticotropin-releasing hormone (CRH). CNS, central nervous system; DA, 
dopamine; 5-HT, serotonin (5-hydroxytryptamine).
150
SECTION II    

erotonin (5-hydroxytryptamine).
150
50
45
40
35
30
25
20
15
10
5
0
Sstr2+/+
Sstr2–/–
Octreo/
MK
GH/
MK
Saline/
saline
Saline/
MK
Octreo/
MK
GH/
MK
Saline/
saline
Saline/
MK
Fos-positive cells/section
• Fig. 7.23  Somatostatin and the somatostatin receptor 2 subtype are involved in the short-loop inhibitory 
feedback of growth hormone (GH) on arcuate neurons. Activation of neurons in the arcuate nucleus was 
determined by the quantification of immunoreacti

determined by the quantification of immunoreactive Fos-positive cells after administration of the growth 
hormone secretagogue MK-0677 (MK). Preliminary treatment of wild-type mice (Sstr2+/+) with either GH or 
the somatostatin analogue octreotide (Octreo) significantly attenuated the neuronal activation by MK-0677. 
In contrast, GH and octreotide had no effect on MK-0677 neuronal activation in somatostatin receptor 2–
deficient mice (Sstr2−/−). (Adapted from Zheng H, Bailey A, Jian M-H, et al

. (Adapted from Zheng H, Bailey A, Jian M-H, et al. Somatostatin receptor subtype 
2 knockout mice are refractory to growth hormone-negative feedback on arcuate neurons. Mol Endocrinol. 
1997;11:1709–1717.)
BNST/
Amyg
NTS
VLM
Raphe
SST
GHRH
SCN
Limbic
HYP
GH
Hormonal signals
Cytokines
Metabolic signals
Psychological
stress
Sleep stage
• Fig. 7.24  Neural pathways involved in growth hormone (GH) regulation. This diagram illustrates the varied 
pathways by which impulses from the limbic system and

hways by which impulses from the limbic system and brainstem ultimately impinge on the hypotha­
lamic periventricular and arcuate nuclei to regulate GH release by opposing effects of somatostatin (SST) 
and growth hormone–releasing hormone (GHRH). Psychologic stress modulates hypothalamic function 
indirectly through the bed nucleus of the stria terminalis (BNST) and amygdalar complex (Amyg). Circa­
dian rhythms are entrained in part by projections from the suprachiasmatic nucleus (SCN). Cortex 

ns from the suprachiasmatic nucleus (SCN). Cortex and 
subcortical nuclei are involved in complex reciprocal interactions between sleep stage and GHRH release, 
but the detailed mechanisms are not known. Dopaminergic and histaminergic afferents originate from 
neurons located in the arcuate and mammillary nuclei, respectively, of the hypothalamus (HYP). Ascending 
catecholaminergic projections arise in both the nucleus of the tractus solitarius (NTS) and ventral lateral 
medulla (VLM). Serotonin

NTS) and ventral lateral 
medulla (VLM). Serotoninergic (5-HT) afferents are from the raphe nuclei. In addition to these neural path­
ways, a variety of peripheral hormonal and metabolic signals and cytokines influence GH secretion by 
actions within the medial basal hypothalamus and pituitary gland.
151
often with a peptide cotransmitter. Excitatory cholinergic fibers 
arise to a small extent from forebrain projection nuclei but mostly 
from hypothalamic cholinergi

on nuclei but mostly 
from hypothalamic cholinergic interneurons, which densely inner­
vate the external zone of the median eminence. Similarly, the ori­
gin of dopaminergic and histaminergic neurons is local with their 
cell bodies located in the hypothalamic arcuate and tuberomam­
millary bodies, respectively. Two important ascending pathways to 
the medial basal hypothalamus regulate GH secretion and origi­
nate from serotonergic neurons in the raphe nuclei and adrenergic 
neurons in the NTS 

e raphe nuclei and adrenergic 
neurons in the NTS and ventral lateral nucleus of the medulla.
Both GHRH and somatostatin neurons express presynaptic 
and postsynaptic receptors for multiple neurotransmitters and 
peptides (Table 7.4). The α2-adrenoreceptor agonist clonidine 
reliably stimulates GH release; for this reason, a clonidine test was 
a standard diagnostic tool in pediatric endocrinology. The stimula­
tory effect is blocked by the specific α2-antagonist yohimbine and 
appears to involv

fic α2-antagonist yohimbine and 
appears to involve a dual mechanism of action: inhibition of soma­
tostatin neurons and activation of GHRH neurons. In addition, 
partial attenuation of the effects of clonidine by mixed serotonin 
5-HT1 and 5-HT2 antagonists suggests that some of the relevant 
α2-receptors are located presynaptically on serotonergic nerve ter­
minals and increase serotonin release. Both norepinephrine and 
epinephrine play physiologic roles in the adrenergic stimulation of 
GH s

logic roles in the adrenergic stimulation of 
GH secretion. The α1-agonists have no effect on GH secretion in 
humans, but β2-agonists such as salbutamol inhibit GH secretion 
by stimulating the release of somatostatin from nerve terminals in 
the median eminence. These effects are blocked by propranolol, a 
nonspecific β-receptor antagonist. Dopamine generally has a net 
effect to stimulate GH secretion and may be exerted directly on 
the pituitary gland328 and/or GHRH neurons, as in the human 

ary gland328 and/or GHRH neurons, as in the human 
brain. Dopamine-containing axonal varicosities heavily abut the 
majority of GHRH neurons.329
Serotonin’s effect on GH release in humans is difficult to deci­
pher because of the large number of receptor subtypes. However, 
clinical studies with the receptor-selective agonist sumatriptan 
clearly implicates the 5-HT1D receptor subtype in the stimula­
tion of basal GH levels.330 The drug also potentiates the effect of 
a maximal dose of GHRH, sug

ntiates the effect of 
a maximal dose of GHRH, suggesting the recurring theme of GH 
disinhibition by inhibition of hypothalamic somatostatin neu­
rons in its mechanism of action. Histaminergic pathways acting 
through H1 receptors play only a minor, conditional stimulatory 
role in GH secretion in humans.
Acetylcholine appears to be an important physiologic regula­
tor of GH secretion and may contribute to the rise in GH during 
sleep.331 Blockade of muscarinic acetylcholine receptors reduces 

de of muscarinic acetylcholine receptors reduces 
or abolishes GH secretory responses to GHRH, glucagon and 
arginine, morphine, and exercise. In contrast, drugs that potenti­
ate cholinergic transmission increase basal GH levels and enhance 
the GH response to GHRH. In vitro acetylcholine inhibits soma­
tostatin release from hypothalamic fragments, and acetylcholine 
can act directly on the pituitary to inhibit GH release. There may 
even be a paracrine cholinergic control system within the pit

aracrine cholinergic control system within the pitu­
itary. However, the sum of evidence suggests that the primary 
mechanism of action of M1 agonists is inhibition of somatostatin 
neuronal activity or the release of peptide from somatostatinergic 
terminals. Short-term cholinergic blockade with the M1 musca­
rinic receptor antagonist pirenzepine reduced the GH excess of 
patients with poorly controlled diabetes mellitus.332
Many neuropeptides in addition to GHRH and somatosta­
tin are involved

n addition to GHRH and somatosta­
tin are involved in the modulation of GH secretion in humans 
(see Table 7.4).279,282 Among these, the evidence is most com­
pelling for a stimulatory role of galanin acting in the human 
hypothalamus by a GHRH-dependent mechanism.333 Many 
GHRH neurons are immunopositive for galanin as well as neu­
rotensin and tyrosine hydroxylase. Galanin’s actions may be 
explained, in part, by presynaptic facilitation of catecholamine 
release from nerve terminals and subse

echolamine 
release from nerve terminals and subsequent direct adrenergic 
  Factors That Change Growth Hormone 
Secretion in Humans
Physiologic Factors
Hormones and 
Neurotransmitters
Pathologic 
Factors
Stimulatory Factors
Episodic, spontane­
ous release
Exercise
Stress
Physical
Psychologic
Slow-wave sleep
Postprandial glucose 
decline
Fasting
Insulin hypoglycemia
2-Deoxyglucose
Amino acid infusions
Arginine, lysine
Neuropeptides
GHRH
Ghrelin
Galanin
Opioids (μ-receptors)
Melatonin
Classic ne

Galanin
Melatonin
Classic neurotransmit­
ters
α2-Adrenergic agonists
β-Adrenergic antago­
nists
M1 cholinergic agonists
5-HT1D receptor agonists
H1 histamine agonists
GABA (basal levels)
Dopamine (? D2 receptor)
Estrogen
Glucocorticoids (acute)
TRH
GnRH
Glucose
Arginine
Interleukins 1, 2, 6
Protein depletion
Anorexia nervosa
Renal failure
Liver cirrhosis
Type 1 diabetes 
mellitus
Inhibitory Factorsa
Postprandial hyper­
glycemia
Elevated fr

 Factorsa
glycemia
Elevated free fatty 
acids
Elevated GH levels
Elevated IGF-1 
(pituitary)
REM sleep
Senescence, aging
Glucose infusion
Neuropeptide Y (NPYb)
CRHb
ters
α1/2-Adrenergic antago­
nists
β2-Adrenergic agonists
H1 histamine antago­
nists
Serotonin antagonist
Nicotinic cholinergic 
agonists
Glucocorticoids 
(chronic)
Acromegaly l-Dopa
D2 receptor DA 
agonists
Phentolamine
Galanin
Obesity
Hy

sts
Galanin
Obesity
aIn many instances, the inhibition can be demonstrated only as a suppression of GH release 
induced by a pharmacologic stimulus.
bThe inhibitory actions of NPY and CRH on GH secretion are firmly established in the rodent 
and are secondary to increased somatostatin tone. Contradictory evidence exists in the 
human for both peptides, and further studies are required.
CRH, Corticotropin-releasing hormone; DA, dopamine; GABA, γ-aminobu

n-releasing hormone; DA, dopamine; GABA, γ-aminobutyric acid; GH, 
growth hormone; GHRH, growth hormone–releasing hormone; GnRH, gonadotropin-releasing 
hormone; IGF-1, insulin-like growth factor type 1; REM, rapid eye movement; TRH, thyrotro­
pin-releasing hormone.
   
TABLE 7.4
152
stimulation of GHRH release.334 Opioid peptides also stimu­
late GH release, probably by disinhibition of GHRH neurons, 
but under normal circumstances, endogenous opioid ton

 under normal circumstances, endogenous opioid tone in the 
hypothalamus is presumed to be low because opioid antagonists 
have little acute effect on GH secretion. Studies on the human 
hypothalamus have indicated that enkephalin, but not endor­
phin or dynorphin, is the endogenous mediator of opiate action 
on GHRH neurons.335 
Other Factors Influencing Secretion of Growth Hormone
Human Growth Hormone Rhythms
The deciphering of rhythmic GH secretion has relied on a com­
bination of technical i

etion has relied on a com­
bination of technical innovations in sampling and GH assay, and 
sophisticated mathematical modeling, including deconvolution 
analysis and the calculation of approximate entropy as a measure 
of orderliness or regularity in minute-to-minute secretory pat­
terns.282 At least three distinct categories of GH rhythms, which 
differ markedly in their time scales, can be considered here.
The daily GH secretion rate varies over two orders of mag­
nitude from a maximum of nea

er two orders of mag­
nitude from a maximum of nearly 2.0 mg/day in late puberty 
to a minimum of 20 μg/day in older or obese adults. The neo­
natal period is characterized by markedly amplified GH secre­
tory bursts followed by a prepubertal decade of stable, moderate 
GH secretion of 200 to 600 μg/day. There is a marked increase 
in daily GH secretion during puberty that is accompanied by a 
commensurate rise in plasma IGF-1 to levels that constitute a 
state of physiologic hypersomatotropism.

titute a 
state of physiologic hypersomatotropism. This pubertal increase 
in GH secretion is due to increased GH pulse amplitude rather 
than increased pulse frequency. Although the changes are clearly 
related to the increases in gonadal steroid hormones and can be 
mimicked by administration of estrogen or testosterone to hypo­
gonadal children, the underlying neuroendocrine mechanisms are 
not fully understood. One hypothesis is that decreased sensitivity 
of the hypothalamic-pituitary axis 

d sensitivity 
of the hypothalamic-pituitary axis to negative feedback of GH and 
IGF-1 leads to increased GHRH release and action, particularly 
given that IGF-1 levels also increase during puberty.336 However, 
young adults have a return of daily GH secretion to prepubertal 
levels despite continued gonadal steroid elevation, perhaps asso­
ciated with increased weight gain. The so-called somatopause is 
defined by an exponential decline in GH secretory rate with a 
half-life of 7 years, starti

ecretory rate with a 
half-life of 7 years, starting in the third decade of life, although 
more pronounced in men than women until women go through 
menopause.
GH secretion in young adults exhibits a true circadian rhythm 
over a 24-hour period, characterized by a greater nocturnal secre­
tory mass that is independent of sleep onset.337 However, as dis­
cussed earlier, GH release is further facilitated when slow-wave 
sleep coincides with the normal circadian peak. Under basal con­
ditions, GH 

rmal circadian peak. Under basal con­
ditions, GH levels are low most of the time, with an ultradian 
rhythm of about 10 secretory pulses per 24 hours in men (20 
in women) as calculated by deconvolution analysis.338 Both sexes 
have an increased pulse frequency during the nighttime hours, but 
the fraction of total daily GH secretion associated with the noc­
turnal pulses is much greater in men. Overall, women have more 
continuous GH secretion and more frequent GH pulses that are 
of more unif

and more frequent GH pulses that are 
of more uniform size than men.307,338 A complementary study 
using approximate entropy analysis concluded that the nonpulsa­
tile regularity of GH secretion is also significantly different in men 
and women.339 These sexually dimorphic patterns in the human 
are actually quite similar to those in the rat, although the sex dif­
ferences are not as extreme in humans.282,339
The neuroendocrine basis for sex differences in the ultradian 
rhythm of GH secretion i

erences in the ultradian 
rhythm of GH secretion is not fully understood. Gonadal sex 
steroids play both an organizational role during development of 
the hypothalamus and an activational role in the adult, regulating 
expression of the genes for many of the peptides and receptors 
central to GH regulation.279,282 In the human, unlike the rat, the 
hypothalamic actions of testosterone appear to result predomi­
nantly from its aromatization to 17β-estradiol and interaction 
with estrogen recepto

β-estradiol and interaction 
with estrogen receptors. Hypothalamic somatostatin appears to 
play a more prominent role in men than in women in the regula­
tion of pulsatile GH secretion, and this difference is postulated to 
be a key factor in producing the sexual dimorphism.338,340 
External and Metabolic Signals
The various peripheral signals that modulate GH secretion in 
humans are summarized in Table 7.4 (also see Figs. 7.22 and 
7.24). Of particular importance are factors related to energy

articular importance are factors related to energy 
intake and metabolism because they provide a common signal 
between the peripheral tissues and hypothalamic centers regulat­
ing nonendocrine homeostatic pathways in addition to the classic 
hypophysiotropic neurons. It is also in this complex arena that 
species-specific regulatory responses are particularly prominent, 
making extrapolations between rodent experimental models and 
human GH regulation less reliable.279,282
Important triggers of

lation less reliable.279,282
Important triggers of GH release include the normal decrease 
in blood glucose level after intake of a carbohydrate-rich meal, 
absolute hypoglycemia, exercise, physical and emotional stress, 
and high intake of protein (mediated by amino acids). Some of 
the pathologic causes of elevated GH represent extremes of these 
physiologic signals and include protein-calorie starvation, anorexia 
nervosa, liver failure, and type 1 diabetes mellitus.283 A critical 
concept is

ype 1 diabetes mellitus.283 A critical 
concept is that many of these GH triggers work through the same 
final common mechanism of somatostatin withdrawal and con­
sequent disinhibition of GH secretion. In contrast, postprandial 
hyperglycemia, glucose infusion, elevated plasma-free fatty acids, 
type 2 diabetes mellitus (with obesity and insulin resistance), and 
obesity result in inhibition of GH secretion.283 The majority of 
these stimuli are associated with increased insulin secretion, whic

 associated with increased insulin secretion, which 
can exert direct, inhibitory action on somatotrophs through their 
expression of insulin receptors.341 The specific role of leptin in 
modulating GH release is complicated by its multiple sites of 
action, coexistent secretory environment, and the animal spe­
cies. For example, while leptin prevents the fasting-induced fall 
in GH in rodents, it does not affect GH levels in fasting human 
subjects.342,343 Similarly, other members of the cytoki

cts.342,343 Similarly, other members of the cytokine family, 
including IL-1, IL-2, IL-6, and endotoxin, have been inconsis­
tently shown to stimulate GH in humans.
The actions of steroid hormones on GH secretion are complex 
because of their multiple loci of action within the proximal hypo­
thalamic-pituitary components in addition to secondary effects 
on other neural and endocrine systems.344 The acute adminis­
tration of glucocorticoids results in an increase in GH through 
direct actions on

ts in an increase in GH through 
direct actions on somatotrophs and by increasing their sensitivity 
to GHRH.345,346 High doses or long-term administration of glu­
cocorticoids and individuals with Cushing disease have reduced 
GH secretion because of increased hypothalamic somatostatin 
tone and decreased GHRH.347,348 CRH also has an inhibitory 
effect on GH secretion both by stimulating somatostatin release 
and through direct effects on GHRH neurons, contributing to 
impaired GH secretion ass

eurons, contributing to 
impaired GH secretion associated with stress.349,350 Similarly, 
physiologic levels of thyroid hormones are necessary to maintain 
GH secretion and promote GH gene expression. Excessive thyroid 
153
hormone is also inhibitory to the GH axis, and the mechanism is 
speculated to be a combination of increased hypothalamic soma­
tostatin tone, GHRH deficiency, and suppressed pituitary GH 
production. 
A facto

ion. 
A factor that potently inhibited GH release from pituitary in vitro 
was unexpectedly identified during early efforts to isolate GHRH 
from hypothalamic extracts. Somatostatin, the peptide respon­
sible for this inhibition of GH secretion and the inhibition of 
insulin secretion by a pancreatic islet extract, was eventually iso­
lated from the hypothalamus and sequenced by Brazeau and col­
leagues in 1973.351 The term somatostatin was originally applied

3.351 The term somatostatin was originally applied 
to a cyclic peptide containing 14 amino acids, also called soma­
tostatin-14 (SST-14) (Fig. 7.25). Subsequently, a second form, 
NH2-terminal extended somatostatin-28 (SST-28), was identified 
as a secretory product. Both forms of somatostatin are derived by 
independent cleavage of a common prohormone by prohormone 
DPhe
DTrp
Cys
S
S
Cys
Octreotide
Phe
Lys
OL
Thr
Thr
SST-14
Ala
Gly
Lys
Asn
Phe
Phe
Trp
Lys
Ser
Thr
Thr
Phe
Cys
S
S
Cys
SST-28(1-1

Phe
Trp
Lys
Ser
Thr
Thr
Phe
Cys
S
S
Cys
SST-28(1-12)
Ser
Ala
Asn
Ser
Asn
Pro
Ala
Met
Ala
Pro
Arg
Glu
–
–
–
–
–
–
–
–
–
–
–
SST-28
Ser
Ala
Ala
Gly
Asn
Cys
Ser
Lys
Asn
Asn
Pro
Phe
Ala
Phe
Met
Trp
Ala
Lys
Pro
Thr
Arg
Phe
Glu
Thr
Arg
Ser
Lys
Cys
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
Exon 2
Intron
5'UTR
3'UTR
Signal
Peptide
SST-14
SST-28
Poly A
200 bp
−300
+50
50 bp
100 bp
CRE
UE-A
TATA
Exon 1
TSEII
TSEI
PC1/PC2
CPE
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
• Fig. 7.25  Diagram illustrating the g

–
–
–
–
–
• Fig. 7.25  Diagram illustrating the genomic organization, messenger ribonucleic acid (mRNA) structure, 
and post-translational processing of the human somatostatin (SST) prohormone. Transcriptional regulation 
of the somatostatin gene, including the identification of tissue-specific elements (TSE), upstream elements 
(UE), and the cyclic adenosine monophosphate (cAMP) response element (CRE) that are binding sites 
for specific factors, has been studied extensively in pancreatic isle

s, has been studied extensively in pancreatic islet cell lines. It is not known whether all 
or some of these factors are also involved in the neural-specific expression of somatostatin. SST-28 and 
SST-14 are cyclic peptides containing a single covalent disulfide bond between a pair of cystine (Cys) resi­
dues. A β-turn containing the tetrapeptide Phe-Trp-Lys-Thr is stabilized by hydrogen bonds to produce the 
core receptor binding epitope. This minimal structure has been the model for conforma

 minimal structure has been the model for conformationally restrained 
analogues of somatostatin, including octreotide. CPE, carboxypeptidase E; PC1/PC2, prohormone con­
vertases 1 and 2; TATA, Goldstein-Hogness box involved in binding RNA polymerase; UTR, untranslated 
region. (Compiled from data by Shen LP, Rutter WJ. Sequence of the human somatostatin 1 gene. Sci­
ence. 1984;224:168–171; Goudet G, Delhalle S, Biemar F, et al. Functional and cooperative interactions 
between the homeodomain PD

operative interactions 
between the homeodomain PDX1, Pbx, and Prep1 factors on the somatostatin promoter. J Biol Chem. 
1999;274:4067–4073; and Milner-White EJ. Predicting the biologically active conformations of short poly­
peptides. Trends Pharmacol Sci. 1989;10:70–74.)
154
convertases.352 In addition, the isolation of SST-28(1-12) in some 
tissues suggests that SST-14 can be secondarily processed from 
SST-28. SST-14 is the predominant form in the bra

SST-28. SST-14 is the predominant form in the brain, including 
the hypothalamus, whereas SST-28 is the major form in the gas­
trointestinal tract, especially the duodenum and jejunum.
The name somatostatin is descriptively inadequate because the 
molecule also inhibits TSH secretion from the pituitary and has 
nonpituitary roles, including activity as a neurotransmitter or 
neuromodulator in the central and peripheral nervous systems 
and as a regulatory peptide in gut and pancreas. As a pitui

regulatory peptide in gut and pancreas. As a pituitary 
regulator, somatostatin is a true neurohormone (i.e., a neuronal 
secretory product that enters the blood [hypothalamic-hypophysial–
portal circulation] to affect cell function at remote sites). In the 
gut, somatostatin is present in the myenteric plexus, where it acts 
as a neurotransmitter, and in epithelial cells, where it influences 
the function of adjacent cells as a paracrine secretion. Somatosta­
tin can influence its own secretion

on. Somatosta­
tin can influence its own secretion from delta cells (an autocrine 
function) in addition to acting as a paracrine factor in pancreatic 
islets. Gut exocrine secretion can be modulated by intraluminal 
action, so it is also a lumone. Because of its wide distribution, 
broad spectrum of regulatory effects, and evolutionary history, this 
peptide can be regarded as an archetypical pansystem modulator.
The genes that encode somatostatin in humans353 (see Fig. 
7.25) and a number of o

in in humans353 (see Fig. 
7.25) and a number of other species exhibit striking sequence 
homology, even in primitive fish such as the anglerfish. Further­
more, the amino acid sequence of SST-14 is identical in all ver­
tebrates. Formerly, it was accepted that all tetrapods have a single 
gene encoding both SST-14 and SST-28, whereas teleost fish 
have two nonallelic preprosomatostatin genes (PPSI and PPSII), 
each of which encodes only one form of the mature somatosta­
tin peptides. This situa

 of the mature somatosta­
tin peptides. This situation implied that a common ancestral gene 
underwent a duplication event after the split of teleosts from the 
descendants of tetrapods.
However, both lampreys and amphibians, which predate and 
postdate the teleost evolutionary divergence, respectively, have 
now been shown to have at least two PPS genes.354 A more dis­
tantly related gene has been identified in mammals that encodes 
cortistatin, a somatostatin-like peptide that is highly expres

 a somatostatin-like peptide that is highly expressed in 
cortex and hippocampus.355 Cortistatin-14 differs from SST-14 by 
three amino acid residues but has high affinity for all known sub­
types of somatostatin receptors (see later discussion). The human 
gene sequence predicts a tripeptide-extended cortistatin-17 and a 
further NH2-terminal extended cortistatin-29.356 A revised evo­
lutionary concept of the somatostatin gene family is that a pri­
mordial gene underwent duplication at or befor

ri­
mordial gene underwent duplication at or before the advent of 
chordates, and the two resulting genes underwent mutation at dif­
ferent rates to produce the distinct preprosomatostatin and pre­
procortistatin genes in mammals.354 A second gene duplication 
probably occurred in teleosts to generate PPSI and PPSII from the 
ancestral somatostatin gene.
Apart from its expression in neurons of the periventricular and 
arcuate hypothalamic nuclei and involvement in GH secretion 
(discussed earlie

and involvement in GH secretion 
(discussed earlier), somatostatin is highly expressed in the cortex, 
lateral septum, extended amygdala, reticular nucleus of the thala­
mus, hippocampus, and many brainstem nuclei. Cortistatin is 
present in the brain at a small fraction of the levels of somatosta­
tin and in a more limited distribution primarily confined to the 
cortex and hippocampus. The molecular mechanisms underly­
ing the developmental and hormonal regulation of somatostatin 
gene transcri

hormonal regulation of somatostatin 
gene transcription have been most extensively studied in pancre­
atic islet cells.357,358 Less is known concerning the regulation of 
somatostatin gene expression in neurons except that activation is 
strongly controlled by binding of the phosphorylated transcrip­
tion factor CRE-binding protein to its cognate CRE contained 
in the promoter sequence.359,360 Enhancer elements in the soma­
tostatin gene promoter that bind complexes of homeodomain-
containing tr

 that bind complexes of homeodomain-
containing transcription factors (PAX6, PBX, PREP1) and 
upregulate gene expression in pancreatic islets may actually rep­
resent gene silencer elements in neurons (see Fig. 7.25, promoter 
elements TSEII and UE-A). Conversely, another related cis ele­
ment in the somatostatin gene (see Fig. 7.25, promoter element 
TSEI) apparently binds a homeodomain transcription factor 
PDX1 (also called STF1/IDX1/IPF1) that is common to devel­
oping brain, pancreas, and f

t is common to devel­
oping brain, pancreas, and foregut and regulates gene expression 
in both the CNS and gut.361
The function of somatostatin in GH and TSH regulation is 
considered earlier in this chapter. Its actions in the extrahypotha­
lamic brain and diagnostic and therapeutic roles are considered 
in the remainder of this section and in Chapter 8. An additional 
function of somatostatin in pancreatic islet cell regulation is 
described in Chapter 33, and the manifestations of somatostat

n Chapter 33, and the manifestations of somatostatin 
excess as in somatostatinoma are described in Chapter 45. 
Somatostatin Receptors
Five somatostatin receptor subtypes (SSTR1 to SSTR5) have 
been identified by gene cloning techniques, and one of these 
(SSTR2) is expressed in two alternatively spliced forms.362 These 
subtypes are encoded by separate genes located on different 
chromosomes; they are expressed in unique or partially overlap­
ping distributions in multiple target organs and di

ing distributions in multiple target organs and differ in their 
coupling to second-messenger signaling molecules and in their 
range and mechanism of intracellular actions.362 The subtypes 
also differ in their binding affinity to specific somatostatin ana­
logues. Certain of these differences have important implications 
for the use of somatostatin analogues in therapy and in diagnos­
tic imaging.
All SSTR subtypes are coupled to pertussis toxin–sensitive G 
proteins and bind SST-14 and SST-28

n–sensitive G 
proteins and bind SST-14 and SST-28 with high affinity in the 
low nanomolar range, although SST-28 has a uniquely high affin­
ity for SSTR5. SSTR1 and SSTR2 are the two most abundant 
subtypes in the brain; they probably function as presynaptic auto­
receptors in the hypothalamus and limbic forebrain, respectively, 
in addition to their postsynaptic actions. SSTR4 is most promi­
nent in the hippocampus. All the subtypes are expressed in the 
pituitary, but SSTR2 and SSTR5 are the

sed in the 
pituitary, but SSTR2 and SSTR5 are the most abundant subtypes 
on somatotrophs. They are also the most physiologically impor­
tant in pancreatic islets, with SSTR5 responsible for inhibition of 
insulin secretion from beta cells and SSTR2 responsible for inhi­
bition of glucagon from alpha cells in mice.363
Binding of somatostatin to its receptor leads to activation of 
one or more plasma membrane–bound inhibitory G proteins 
(Gi/o), which in turn inhibit adenylyl cyclase activity an

which in turn inhibit adenylyl cyclase activity and lower 
intracellular cAMP. Other G protein–mediated actions common 
to all SSTRs are activation of a vanadate-sensitive phosphotyrosine 
phosphatase and modulation of mitogen-activated protein kinase 
(MAPK). Different subsets of SSTRs are also coupled to inwardly 
rectifying K+ channels, voltage-dependent Ca2+ channels, an Na+/
H+ exchanger, α-amino-3-hydroxy-5-methyl-4-isoxazole propi­
onic acid (AMPA)-kainate glutamate receptors, phospholipa

id (AMPA)-kainate glutamate receptors, phospholipase 
C, and phospholipase A2.362 The lowering of intracellular cAMP 
and Ca2+ is the most important mechanism for inhibition of hor­
mone secretion, and actions on phosphotyrosine phosphatase and 
MAPK are postulated to play a role in somatostatin’s antiprolifera­
tive effect on tumor cells. 
155
Effects on Target Tissues and Mechanism of Action
In the pituitary, somatostatin inhibits secretion of GH, TSH, and 
under 

ostatin inhibits secretion of GH, TSH, and 
under certain conditions, PRL and ACTH. It exerts inhibitory 
effects on virtually all endocrine and exocrine secretions of the 
pancreas, gut, and gallbladder (Table 7.5). Somatostatin inhib­
its secretion by the salivary glands and, under some conditions, 
the secretion of parathyroid hormone and calcitonin. Somatosta­
tin blocks hormone release in many endocrine-secreting tumors, 
including insulinomas, glucagonomas, VIPomas, carcinoid 
tumors, and 

as, glucagonomas, VIPomas, carcinoid 
tumors, and some gastrinomas.
The physiologic actions of somatostatin in the extrahypotha­
lamic brain remain the subject of investigation.364 In the stria­
tum, somatostatin increases the release of dopamine from nerve 
terminals by a glutamate-dependent mechanism. It is widely 
expressed in GABAergic interneurons of the limbic cortex and 
hippocampus, where it modulates the excitability of pyramidal 
neurons. Temporal lobe epilepsy is associated with a mar

s. Temporal lobe epilepsy is associated with a marked 
reduction in somatostatin-expressing neurons in the hippocam­
pus consistent with a putative inhibitory action on seizures.365 
In the amygdala, somatostatin has been associated with learn­
ing and expression of defensive responses.366 A wealth of correl­
ative data has linked reduced forebrain and CSF concentrations 
of somatostatin with Alzheimer disease, major depression, and 
other neuropsychiatric disorders, raising speculation about th

sychiatric disorders, raising speculation about the 
role of somatostatin in modulating neural circuits underlying 
cognitive and affective behaviors.367 A study using both genetic 
and pharmacologic methods to induce somatostatin deficiency 
in mice bolsters the hypothesis that the neuropeptide plays a 
physiologic role in the acquisition of contextual fear memory, 
possibly by altering long-term potentiation in hippocampal 
circuits.368 
Clinical Applications of Somatostatin Analogues
An exten

al Applications of Somatostatin Analogues
An extensive pharmaceutic discovery program has produced 
somatostatin analogues with receptor subtype selectivity and 
improved pharmacokinetics and oral bioavailability compared 
with the native peptide. Initial efforts focused on the rational 
design of constrained cyclic peptides that incorporated d-amino 
acid residues and included the Trp9-Lys10 dipeptide of somatosta­
tin, which was shown by structure-function studies to be necessary 
for high-aff

ure-function studies to be necessary 
for high-affinity binding to somatostatin receptors (see Fig. 7.25). 
Many such analogues have been studied in clinical trials, includ­
ing octreotide, lanreotide, vapreotide, seglitide, and pasireotide.362 
These compounds are agonists with similarly high-affinity binding 
to SSTR2 and SSTR5, moderate binding to SSTR3, and no (or 
low) binding to SSTR1 (except for pasireotide) and SSTR4. A 
combinatorial chemistry approach has now led to a new genera­
tion 

mistry approach has now led to a new genera­
tion of nonpeptidyl somatostatin agonists that bind selectively and 
with subnanomolar affinity to each of the five SSTR subtypes.369 
In contrast to the marked success in development of potent and 
selective somatostatin agonists, there is a relative paucity of useful 
antagonists.370
The somatostatin analogues have been effective in controlling 
excess secretion of GH in acromegaly in most patients and in 
shrinking the tumor size in about one-third

nd in 
shrinking the tumor size in about one-third of patients. They are 
also indicated for the treatment of recurrent TSH-secreting ade­
nomas after surgery and a variety of functioning metastatic neu­
roendocrine tumors, including carcinoid, VIPoma, glucagonoma, 
and insulinoma, but they are seldom of use for the treatment of 
gastrinoma.371 Somatostatin analogues are also useful in the man­
agement of many forms of diarrhea (acting on salt and water excre­
tion mechanisms in the gut) and in 

d water excre­
tion mechanisms in the gut) and in reducing external secretions in 
pancreatic fistulas (thus permitting healing). A decrease in blood 
flow to the gastrointestinal tract is the basis for their use in bleed­
ing esophageal varices, but they are not effective in the treatment 
of bleeding from a peptic ulcer.
Somatostatin analogues labeled with a radioactive tracer have 
been used as external imaging agents for a wide range of disor­
ders,370,371 including indium-111 (111In)-labele

­
ders,370,371 including indium-111 (111In)-labeled and gallium-68 
(68Ga)-labeled somatostatin analogues, octreotate, DOTATATE, 
DOTATOC, and DOTANOC (Fig. 7.26). The majority of gas­
troenteropancreatic neuroendocrine tumors (GEP-NETs) and 
bronchopulmonary NETs, pheochromocytomas/paragangliomas, 
medullary thyroid carcinoma, many pituitary tumors, and a vari­
ety of nonendocrine tumors that express somatostatin receptors 
such as breast cancer, renal cell carcinoma, meningioma, and 
astrocyto

, renal cell carcinoma, meningioma, and 
astrocytoma can be visualized by external imaging techniques, 
using these agents by either scintigraphy or positron emission 
tomography (PET). Because activated T cells of the immune sys­
tem display somatostatin receptors, inflammatory lesions such as 
sarcoidosis, Wegener granulomatosis, and tuberculosis, and many 
cases of Hodgkin disease and non-Hodgkin lymphoma also take 
up these radionucleotides.372
The ability of somatostatin to inhibit the grow

72
The ability of somatostatin to inhibit the growth of normal and 
some neoplastic cell lines and to reduce the growth of experimen­
tally induced tumors in animal models has stimulated interest in 
somatostatin analogues for the treatment of cancer. Somatostatin’s 
tumoristatic effects may be a combination of direct actions on 
tumor cells related to inhibition of growth factor receptor expres­
sion, inhibition of MAPK, and stimulation of phosphotyrosine 
phosphatase. Activation of SSTR1, SSTR

hotyrosine 
phosphatase. Activation of SSTR1, SSTR2, SSTR4, and SSTR5 
can all promote cell cycle arrest associated with induction of the 
tumor suppressor retinoblastoma (Rb) and p21 (CDKN1A), and 
SSTR3 activation can trigger apoptosis accompanied by induc­
tion of the tumor suppressor p53 and the proapoptotic protein 
Bax.362 In addition, somatostatin has indirect effects on tumor 
growth through its inhibition of circulating, paracrine, and auto­
crine tumor growth–promoting factors, and it 

uto­
crine tumor growth–promoting factors, and it can modulate the 
  Biologic Actions of Somatostatin Outside 
the Central Nervous System
Hormone Secretion Inhibited 
(by Tissue)
Other Gastrointestinal Actions 
Inhibited
Pituitary gland
GH, thyrotropin, ACTH, 
prolactin
Gastrointestinal tract
Gastrin
Secretin
Motilin
Glucagon-like peptide 1
Glucose-dependent insuli­
notropic polypeptide
Vasoactive intestinal peptide
Pancreas
Insulin
Glucagon
Genitourinary tract
Renin
Gastric acid 

matostatin
Renin
Gastric acid secretion
Gastric and jejunal fluid secretion
Gastric emptying
Pancreatic bicarbonate secretion
Pancreatic enzyme secretion
Secretory diarrhea (stimulates 
intestinal absorption of water 
and electrolytes)
Gastrointestinal blood flow
AVP-stimulated water transport
Bile flow
Extragastrointestinal 
Actions Inhibited
Inhibits the function of activated 
immune cells
Inhibition of tumor growth
ACTH, Adrenocorticotropic hormone; AVP, arginine vasopress

renocorticotropic hormone; AVP, arginine vasopressin; GH, growth hormone.
TABLE 7.5
156
activity of immune cells and influence tumor blood supply. Two 
major studies, PROMID and CLARINET trials, demonstrated 
that treated patients with neuroendocrine tumors have signifi­
cantly longer progression-free survival than patients treated with 
placebo.373,374 Pasireotide, a novel multireceptor-targeted soma­
tostatin analogue with 40-fold increased binding effi

tatin analogue with 40-fold increased binding efficacy to 
SSTR5, has been effective in approximately one-third of patients 
with neuroendocrine tumors resistant to octreotide or lanreotide.
A treatment approach using radiolabeled somatostatin ana­
logues has also been used to achieve the arrest of cancer cells.370,371 
The first is receptor-targeted radionuclide therapy using octreotide 
chelated to a variety of γ-emitting or β-emitting radioisotopes, 
including [90Y-DOTA0,Ty3]octreotide or DOT

topes, 
including [90Y-DOTA0,Ty3]octreotide or DOTATATE chelated 
to lutetium 177. Theoretic calculations and empiric data suggest 
that radiolabeled somatostatin analogues can deliver a tumoricidal 
radiotherapeutic dose to tumors expressing somatostatin recep­
tors either by simply binding to the surface receptors or following 
receptor-mediated endocytosis. Improved progression-free sur­
vival with these radionuclides in patients with metastatic GEP-
NETs compared to the use of somatostatin a

ic GEP-
NETs compared to the use of somatostatin analogues alone has 
been established in the NETTER-1 trial and in a recent, large 
Dutch cohort.375,376 A variation on this theme is the chelation 
of a cytotoxic chemotherapeutic agent, such as doxorubicin, to a 
somatostatin analogue. An alternative approach involves somatic 
cell gene therapy to transfect SSTR-negative pancreatic cancer 
cells with an SSTR gene.377 
A
C
B
• Fig. 7.26  The use of 111In-labeled diethylenetriaminepenta-acetic aci

f 111In-labeled diethylenetriaminepenta-acetic acid (DTPA)-octreotide (A and B) and 
68Ga-DOTTATE (C) (radioactive somatostatin analogues) and external imaging techniques to localize a 
carcinoid tumor expressing somatostatin receptors. (A) Anterior view of the abdomen showing nodular 
metastases in an enlarged liver and the primary carcinoid tumor (arrow) in the wall of the jejunum of a 
patient with severe flushing and diarrhea. (B) Posterior view of the chest and neck showing a metastasis in 

iew of the chest and neck showing a metastasis in 
a lymph node on the left side of the neck (arrow) and multiple metastases in the ribs and pleura. (C) Anterior 
view showing presence of a bronchopulmonary carcinoid tumor (arrow) causing ectopic ACTH syndrome 
and Cushing syndrome. (A and B, Modified from Lamberts SWJ, Krenning EP, Reubi J-C. The role of 
somatostatin and its analogs in the diagnosis and treatment of tumors. Endocrine Rev. 1991;12:450–482. 
Copyright 1991, The Endocrine Society

12:450–482. 
Copyright 1991, The Endocrine Society.)
157
Prolactin-Regulating Factors
Dopamine
It is well known that PRL secretion, unlike the secretion of other 
pituitary hormones, is primarily under tonic inhibitory control by 
the hypothalamus (Fig. 7.27).378 Destruction of the stalk median 
eminence or transplantation of the pituitary gland to ectopic sites 
causes a marked constitutive increase in PRL secretion, in con­
trast to a decrease in the release of GH

, in con­
trast to a decrease in the release of GH, TSH, ACTH, and the 
gonadotropins. Many lines of evidence indicate that dopamine is 
the principal physiologic PIF released from the hypothalamus.379 
Dopamine is present in hypothalamic-hypophysial–portal vessel 
Pituitary
Dopamine
GABA
TRH
Oxytocin
VIP
Breast
Suckling
stimulus
Spinal
afferent
Multiple
target organs
PRL
Estrogen
to CNS
GABA
PRF
neurons
opioid neurons
5-HT
Histamine
inputs
AC

-HT
Histamine
inputs
ACh
Dopamine
neuron
nucleus
Arcuate
nucleus
Raphe
nucleus
Mammillary
nuclei
Basal
forebrain
PRL receptors
Estrogen receptors
• Fig. 7.27  Regulation of the hypothalamic-pituitary-prolactin (PRL) axis. The predominant effect of the 
hypothalamus is inhibitory, mediated principally by the tuberohypophyseal dopaminergic neuron system 
and dopamine D2 receptors on lactotrophs. The dopamine neurons are stimulated by acetylcholine (ACh) 

ne neurons are stimulated by acetylcholine (ACh) 
and glutamate and inhibited by histamine and opioid peptides. One or more prolactin-releasing factors 
(PRFs) mediate acute release of PRL (e.g., in suckling, during stress). There are several candidate PRFs, 
including thyrotropin-releasing hormone (TRH), vasoactive intestinal polypeptide (VIP), and oxytocin. PRF 
neurons are activated by serotonin (5-HT). Estrogen sensitizes the pituitary to release PRL, which feeds 
back on the pituitary to re

ease PRL, which feeds 
back on the pituitary to regulate its own secretion (ultrashort-loop feedback) and also influences gonado­
tropin secretion by suppressing the release of gonadotropin-releasing hormone (GnRH). Short-loop feed­
back is also mediated indirectly by prolactin receptor regulation of hypothalamic dopamine synthesis, 
secretion, and turnover. CNS, central nervous system; GABA, γ-aminobutyric acid.
158
blood in sufficient concentration to i

d Pituitary
blood in sufficient concentration to inhibit PRL release, dopamine 
inhibits PRL secretion from lactotrophs both in vivo and in vitro, 
and dopamine D2 receptors are expressed on the plasma membrane 
of lactotrophs. Mutant mice with a targeted disruption of the D2 
receptor gene uniformly develop lactotroph hyperplasia, hyperp­
rolactinemia, and eventually lactotroph adenomas, further empha­
sizing the importance of dopamine in the physiologic regulation 
of lactotroph proliferation 

ysiologic regulation 
of lactotroph proliferation in addition to hormone secretion.380
The intrinsic dopamine neurons of the medial basal hypo­
thalamus constitute a dopaminergic population with regulatory 
properties that are distinct from those in other areas of the brain. 
Notably, they lack D2 autoreceptors but express PRL receptors, 
which are essential for positive feedback control (discussed in 
detail later). In rats, these neurons are subdivided by location 
into the A12 group of Dahlst

bdivided by location 
into the A12 group of Dahlstrom and Fuxe within the arcuate 
nucleus and the A14 group in the hypothalamic periventricu­
lar nucleus. The dorsomedially located A12 dopamine neurons 
are further classified as tuberoinfundibular dopamine neurons 
(TIDA) because they project to the external zone of the median 
eminence and the main group of dopamine neurons responsible 
for prolactin regulation. The more rostral A12 group is classified as 
tuberohypophysial dopamine neurons (T

assified as 
tuberohypophysial dopamine neurons (THDA) and project to the 
neural lobe and intermediate lobe. Finally, the A14 periventricu­
lar hypophysial dopaminergic neurons (PHDA) send their axons 
only to the intermediate lobe of the pituitary gland where they 
are primarily involved in the release of α-melanocyte–stimulating 
hormone.
Although the TIDA neurons are generally considered to be the 
major source of dopamine to the anterior lobe through the long 
portal vessels originating in 

e through the long 
portal vessels originating in the median eminence, dopamine can 
also reach the anterior lobe from the neural and intermediate lobes 
by the interconnecting short portal veins.381 In addition to direct 
actions of dopamine on lactotrophs, central dopamine can indi­
rectly affect PRL secretion by altering the activity of inhibitory 
interneurons that in turn synapse on the TIDA neurons. These 
effects are complicated by opposing intracellular signaling path­
ways linked to D1 

g intracellular signaling path­
ways linked to D1 and D2 receptors located on different popula­
tions of interneurons.382
The binding of dopamine or selective agonists such as bro­
mocriptine to the D2 receptor has multiple effects on lactotroph 
function. The D2 receptor is a member of the cytokine superfam­
ily and couples to pertussis toxin–sensitive G proteins to inhibit 
adenylyl cyclase and decrease intracellular cAMP levels. Other 
effects include activation of an inwardly rectifying K+ c

 include activation of an inwardly rectifying K+ channel, 
increase of voltage-activated K+ currents, decrease of voltage-
activated Ca2+ currents, and inhibition of inositol phosphate pro­
duction. Together, this spectrum of intracellular signaling events 
decreases free Ca2+ concentrations and inhibits exocytosis of PRL 
secretory granules.383
There is continuing debate concerning the mechanism by 
which D2 receptor activation inhibits transcription of the PRL 
gene. Likely pathways involve th

ption of the PRL 
gene. Likely pathways involve the inhibition of MAPK or protein 
kinase C, with a resultant reduction in the phosphorylation of 
Ets family transcription factors. Ets factors are important for the 
stimulatory responses of TRH, insulin, and epidermal growth fac­
tor on PRL expression, and they interact cooperatively with the 
pituitary-specific POU protein Pit1, which is essential for cAMP-
mediated PRL gene expression.384
The second-messenger pathways used by the D2 receptor t

econd-messenger pathways used by the D2 receptor to 
inhibit lactotroph cell division are also unsettled.385 A study using 
primary pituitary cultures from rats demonstrated that forskolin 
treatment, which activates protein kinase A and elevates intracellu­
lar cAMP, and insulin treatment, which activates a potent receptor 
tyrosine kinase, were both effective mitogenic stimuli for lacto­
trophs. Bromocriptine competitively antagonized the proliferative 
response caused by elevated cAMP. Furthe

ferative 
response caused by elevated cAMP. Furthermore, inhibition of 
MAPK signaling by PD98059 markedly suppressed the mitogenic 
action of both insulin and forskolin, suggesting an interaction of 
MAPK and protein kinase A signaling.386
Another line of study has implicated the stimulation of phos­
pholipase D activity by a Rho A–dependent, pertussis toxin–
insensitive pathway in the antiproliferative effects of D2 receptor 
activation in both GH4C1 pituitary cells and NCI-H69 small cell 
lun

 GH4C1 pituitary cells and NCI-H69 small cell 
lung cancer cells.387 Activation of the extracellular signal-regulated 
kinase 1/2 pathway and inhibition of the AKT/protein kinase B 
pathway have also been implicated in the action of the D2 recep­
tor to reduce lactotroph mitogenesis.388 Therefore it is clear that 
dopamine actions on lactotrophs involve multiple different intra­
cellular signaling pathways linked to activation of the D2 receptor, 
but different combinations of these pathways are

but different combinations of these pathways are relevant for the 
inhibitory effects on PRL secretion, PRL gene transcription, and 
lactotroph proliferation.
The other major action of dopamine in the pituitary is the 
inhibition of hormone secretion from the POMC-expressing 
cells of the intermediate lobe389—although, as noted earlier, the 
adult human differs from most other mammals in the rudimen­
tary nature of this lobe. THDA and PHDA axon terminals pro­
vide a dense plexus of synaptic-li

 terminals pro­
vide a dense plexus of synaptic-like contacts on melanotrophs. 
Dopamine release from these terminals is inversely correlated with 
serum MSH levels and also regulates POMC gene expression and 
melanotroph proliferation.
Other hypothalamic factors likely play a role secondary to that 
of dopamine as additional PIFs.378 The primary reason to con­
jecture the existence of these PIFs is the frequent inconsistency 
between portal dopamine levels and circulating PRL in different 
rat 

mine levels and circulating PRL in different 
rat models. GABA is the strongest candidate and most likely acts 
through GABAA inotropic receptors in the anterior pituitary. Mel­
anotrophs, like lactotrophs, are inhibited by both dopamine and 
GABA but with the principal involvement of G protein–coupled, 
metabotropic GABAB receptors.390 Because basal dopamine tone 
is high, the measurable inhibitory effects of GABA on PRL release 
are generally small under normal circumstances. Other putative 
P

mall under normal circumstances. Other putative 
PIFs include somatostatin and calcitonin. 
Prolactin-Releasing Factors
Although tonic suppression of dopamine by prolactin is the domi­
nant effect of the hypothalamus on PRL secretion, other mecha­
nisms may also contribute by either contributing to dopamine 
suppression or directly promoting PRL release from the anterior 
pituitary (see Fig. 7.27). The most important of the putative PRFs 
are TRH, oxytocin, and VIP, but vasopressin, angiotensin 

, oxytocin, and VIP, but vasopressin, angiotensin II, NPY, 
galanin, substance P, bombesin-like peptides, and neurotensin 
can also trigger PRL release under different physiologic circum­
stances.378 TRH has already been discussed. In humans there is 
an imperfect correlation between pulsatile PRL and TSH release, 
suggesting that TRH cannot be the sole physiologic PRF under 
basal conditions.391
Like TRH, oxytocin, vasopressin, and VIP fulfill all the basic 
criteria for a PRF. They are produce

ll the basic 
criteria for a PRF. They are produced in PVH neurons that proj­
ect to the median eminence. Concentrations of the hormones in 
portal blood are much higher than in the peripheral circulation 
and are sufficient to stimulate PRL secretion in vitro. Moreover, 
there are functional receptors for each of the neurohormones in 
the anterior pituitary gland, and either pharmacologic antagonism 
or passive immunization against each hormone can decrease PRL 
secretion, at least under certai

can decrease PRL 
secretion, at least under certain circumstances.392
159
AVP is released during stress and hypovolemic shock, as is 
PRL, suggesting a specific role for vasopressin as a PRF in these 
contexts. Similarly, another candidate PRF, peptide histidine iso­
leucine, may be specifically involved in the secretion of PRL in 
response to stress. Peptide histidine isoleucine and the human 
homologue, peptide histidine methionine, are structurally related 
to VI

tidine methionine, are structurally related 
to VIP and are synthesized from the same prohormone precur­
sor in their respective species.393 Both peptides are coexpressed 
with CRH in parvicellular PVH neurons, and presumably they 
are released by the same stimuli that cause release of CRH into the 
hypothalamic-hypophysial portal vessels.394
Serotonin arising from the brainstem dorsal raphe also appears 
to have a major role in prolactin secretion, particularly during lac­
tation.395,396 These 

on, particularly during lac­
tation.395,396 These neurons innervate TIDA neurons to inhibit 
their secretion of dopamine and hence increase prolactin secre­
tion but may also have an activating effect on PRF through direct 
innervation of TRH in the PVH. Endogenous opiates also may 
have a role during lactation by inhibiting TIDA secretion medi­
ated by dynorphin-producing or beta endorphin–producing neu­
rons in the arcuate nucleus. 
Intrapituitary Regulation of Prolactin Secretion
Probably mor

ary Regulation of Prolactin Secretion
Probably more than that of any other pituitary hormone, the 
secretion of PRL is regulated by autocrine-paracrine factors within 
the anterior lobe and by neurointermediate lobe factors that gain 
access to venous sinusoids of the anterior lobe by way of the short 
portal vessels. The wealth of local regulatory mechanisms within 
the anterior lobe has been reviewed extensively378,397 and is also 
discussed in Chapter 8. In brief, that some gonadotrophs are 

 Chapter 8. In brief, that some gonadotrophs are 
closely apposed to lactotrophs and secrete substances in response 
to GnRH that stimulate prolactin secretion is well known.398,399 
Candidate substances derived from gonadotrophs abound and 
include pituitary glycoprotein hormone alpha subunit (αGSU), 
an N-terminal fragment of POMC (POMC 1-74), neurotensin, 
angiotensin II, PACAP, calcitonin, and CART, among a number 
of other substances.400 Galanin, VIP, endothelin-like peptides, 
epidermal gr

anin, VIP, endothelin-like peptides, 
epidermal growth factor, basic fibroblast growth factor, and the 
cytokine IL-6 are also potent local stimulators of PRL secretion. 
Locally produced inhibitors include PRL itself, acetylcholine, 
transforming growth factor β, and calcitonin. Although none of 
these stimulatory or inhibitory factors have a dominant role in 
the regulation of lactotroph function and much of the research 
in this area has not been directly confirmed in human pituitary, 
it see

een directly confirmed in human pituitary, 
it seems apparent that the local milieu of autocrine and paracrine 
factors plays an essential, modulatory role in determining the 
responsiveness of lactotrophs to hypothalamic factors in different 
physiologic states.
Cell clustering has also been observed in the anterior pituitary 
during lactation,401 in which there is proliferation and reorganiza­
tion of lactotrophs into a honeycomb-like structure with increased 
contacts between the cells. Presu

 with increased 
contacts between the cells. Presumably these changes are a way to 
integrate or amplify signals from the hypothalamus and coordinate 
increased output of prolactin. Recent advances in two-photon imag­
ing of the pituitary and three-dimensional analyses of pituitary cell 
networks reinforce the importance of these local connections.402 
Neuroendocrine Regulation of Prolactin Secretion
Secretion of PRL, like that of other anterior pituitary hormones, 
is regulated by hormonal feed

pituitary hormones, 
is regulated by hormonal feedback and neural influences from the 
hypothalamus.378,379,403 Feedback is exerted by PRL itself at the 
level of the hypothalamus. PRL secretion is regulated by many 
physiologic states, including the estrous and menstrual cycles, 
pregnancy, and lactation. Furthermore, PRL is stimulated by 
several exteroceptive stimuli, including light, ultrasonic vocaliza­
tion of pups (in rodents), olfactory cues, and various modalities of 
stress. Expression

ues, and various modalities of 
stress. Expression and secretion of PRL are also influenced strongly 
by estrogens at the level of both the lactotrophs and TIDA neu­
rons404 (see Fig. 7.27) and by paracrine regulators within the pitu­
itary such as galanin and VIP. 
Negative feedback control of PRL secretion is mediated by a 
unique short-loop mechanism within the hypothalamus. PRL 
activates PRL receptors, which are expressed on all three subpopu­
lations of A12 and A14 dopamin

 all three subpopu­
lations of A12 and A14 dopamine neurons, leading to increased 
tyrosine hydroxylase expression and increased dopamine synthe­
sis and release.404,405 Ames dwarf mice that secrete virtually no 
PRL, GH, or TSH have decreased numbers of arcuate dopamine 
neurons, and this hypoplasia can be reversed by neonatal admin­
istration of PRL, suggesting a trophic action on these neurons.406 
However, another mouse model of isolated PRL deficiency gener­
ated by gene targeting appears t

deficiency gener­
ated by gene targeting appears to have normal numbers of hypo­
functioning dopamine neurons.407 Ultrashort feedback control 
via autocrine or paracrine regulation of prolactin at the level of 
the pituitary gland also occurs as indicated by an increased sen­
sitivity of lactotrophs to dopamine inhibition in transgenic mice 
with conditional deletion of the prolactin receptor, selectively in 
lactotrophs.408 
Lactotrophs have spontaneously high secretory activity;

trophs have spontaneously high secretory activity; therefore 
the predominant effect of the hypothalamus on PRL secretion is 
tonic suppression, which is mediated by regulatory hormones syn­
thesized by THDA neurons. Secretory bursts of PRL are caused 
by the acute withdrawal of dopamine inhibition, stimulation by 
PRFs, or combinations of both events. At any given moment, 
locally produced autocrine and paracrine regulators further mod­
ulate the responsiveness of individual lactotrophs to neur

e responsiveness of individual lactotrophs to neurohor­
monal PIFs and PRFs.
Multiple neurotransmitter systems impinge on the hypotha­
lamic dopamine and PRF neurons to regulate their neurosecre­
tion378 (see Fig. 7.27). Nicotinic cholinergic and glutamatergic 
afferents activate TIDA neurons, whereas histamine, acting pre­
dominantly through H2 receptors, inhibits these neurons. An 
inhibitory peptidergic input to TIDA neurons of major physi­
ologic significance is that associated with the endo

ogic significance is that associated with the endogenous opioid 
peptides enkephalin and dynorphin and their cognate μ-receptor 
and κ-receptor subtypes.409 Opioid inhibition of dopamine release 
has been associated with increased PRL secretion under virtually 
all physiologic conditions, including the basal state, different 
phases of the estrous cycle, lactation, and stress. Ascending seroto­
nergic inputs from the dorsal raphe nucleus are the major activator 
of PRF neurons in the PVH. There 

major activator 
of PRF neurons in the PVH. There is still debate concerning the 
identity of the specific 5-HT receptors involved in this activation.
The PRL regulatory system and its monoaminergic control 
have been scrutinized in detail because of the frequent occurrence 
of syndromes of PRL hypersecretion (see Chapter 8). Both the 
pituitary and the hypothalamus have dopamine receptors, and the 
response to dopamine receptor stimulation and blockade does not 
distinguish between central and 

lockade does not 
distinguish between central and peripheral actions of the drug. 
Many commonly used neuroleptic drugs influence PRL secre­
tion. Reserpine (a catecholamine depleter) and phenothiazines 
such as chlorpromazine and haloperidol enhance PRL release by 
disinhibition of dopamine action on the pituitary, and the PRL 
response is an excellent predictor of the antipsychotic effects of 
phenothiazines because of its correlation with D2 receptor binding 
160
SECTION II    Hypothalamus an

eceptor binding 
160
and activation.410 The major antipsychotic neuroleptic agents act 
on brain dopamine receptors in the mesolimbic system and in 
the pituitary-regulating TIDA system. Consequently, treatment of 
such patients with dopamine agonists such as bromocriptine can 
reverse the psychiatric benefits of such drugs. A report of three 
patients with psychosis and concomitant prolactinomas recom­
mended the combination of clozapine and quinagolide 

nded the combination of clozapine and quinagolide as the 
treatment of choice to manage both diseases simultaneously.411 
Factors Influencing Secretion
PRL is detectable in plasma at all times during the day but is secreted 
in discrete pulses superimposed on basal secretion and exhibits a 
diurnal rhythm with peak values in the early morning hours.412 In 
humans, this is a true circadian rhythm because it is maintained 
in a constant environment independently of the sleep rhyth

stant environment independently of the sleep rhythm.413 
The combined body of data examining TIDA neuronal activity, 
dopamine concentrations in the median eminence, and manipula­
tions of the SCN suggests that endogenous diurnal alterations in 
dopamine tone that are entrained by light constitute the major neu­
roendocrine mechanism underlying the circadian rhythm of PRL 
secretion. The nocturnal rise in PRL secretion in nursing and non­
nursing women may have evolved as a mechanism of milk mai

 women may have evolved as a mechanism of milk mainte­
nance during prolonged nonsuckling periods at night. 
External Stimuli
The suckling stimulus is the most important physiologic regula­
tor of PRL secretion. PRL levels rise within 1 to 3 minutes of 
nipple stimulation and remain elevated for 10 to 20 minutes.414 
This reflex is distinct from the milk let-down, which occurs nearly 
simultaneously with prolactin secretion during suckling and 
involves oxytocin release from the neurohypophysis 

nvolves oxytocin release from the neurohypophysis and con­
traction of mammary alveolar myoepithelial cells. These reflexes 
provide a mechanism by which the infant regulates both the pro­
duction and the delivery of milk. Of interest, in nonlactating rats, 
prolactin secretion inhibits oxytocin neurons and stimulates dopa­
mine release from TIDA neurons, whereas during lactation, pro­
lactin enhances oxytocin release and does not increase dopamine 
release. Mechanisms involved may include prola

ne 
release. Mechanisms involved may include prolactin-induced 
sensitization of TRPV1 channels that are upregulated in oxytocin 
neurons during lactation, an indirect effect mediated by GABA 
release from TIDA neurons,415 and inhibition of the prolactin 
feedback loop by the dissociation of electric activity and dopa­
mine release to allow for sustained elevations in prolactin dur­
ing suckling.416 Signaling of prolactin to TIDA neurons may also 
be diminished as a result of upregulation of SOC

be diminished as a result of upregulation of SOCS proteins that 
inhibit JAK-STAT signaling, induced by the binding of prolactin 
to the prolactin receptor.417
After stimulation of mechanoreceptors, pathways involved in 
the suckling reflex are carried to the spinal cord by way of spinal 
afferent neurons, ascend the spinal cord through spinothalamic 
tracts to the midbrain, and enter the hypothalamus by way of the 
median forebrain bundle (see Fig. 7.27). Neurons regulating the 
oxytocin-depe

 Fig. 7.27). Neurons regulating the 
oxytocin-dependent milk let-down response accompany those 
involved in PRL regulation throughout most of this pathway and 
then separate at the level of the PVH nuclei, although the intrahy­
pothalamic pathways involved have not been precisely elucidated. 
Using c-Fos as a marker for neuronal activation, several poten­
tial relay centers in the brainstem have been identified, includ­
ing the ventrolateral medulla (A1) and dorsal vagal complex (A2) 
catecholam

lla (A1) and dorsal vagal complex (A2) 
catecholamine cell groups, locus coeruleus, lateral parabrachial 
nucleus, caudal portion of the paralemniscal nucleus, and lateral 
and ventrolateral portions of the caudal part of the periaqueduc­
tal gray through direct or indirect synaptic connections to oxyto­
cin neurons.418 Noradrenergic neurons arising in the A1 and A2 
groups directly innervate oxytocin neurons.419 Synchronization of 
oxytocin neurons, which contributes to the coordinated pattern 

ons, which contributes to the coordinated pattern 
of pulsatile oxytocin release, is regulated by afferent inputs from 
glutamatergic neurons, in the lateral septum and bed nucleus of 
the stria terminalis.47 Retraction of astroglial processes that nor­
mally separate oxytocin neurons, allowing increased interactions 
through somatic appositions, also contributes to the pulsatile pat­
tern of oxytocin secretion and may be regulated by “intranuclear 
release” of oxytocin, in which oxytocin is rel

ar 
release” of oxytocin, in which oxytocin is released locally from 
dendrites and soma.420 The suckling reflex brings about an inhibi­
tion of PIF activity and a release of PRFs, although an undisputed 
suckling-induced PRF has not been identified.
Suckling-induced prolactin secretion not only has an important 
role in lactation but coordinates a number of other physiologic 
responses that complement lactation, including the develop­
ment of maternal behavior, increased appetite, and inhibitio

ternal behavior, increased appetite, and inhibition of 
reproductive function. These effects are facilitated by upregula­
tion of prolactin receptors in the choroid plexus during lactation, 
allowing increased entry of prolactin from the circulation into the 
brain through a carrier-mediated transport system.421 Binding of 
prolactin to prolactin receptors in the MePO may be important 
for maternal behavior as antagonizing prolactin receptors with a 
specific prolactin receptor antagonist inject

h a 
specific prolactin receptor antagonist injected into the preoptic 
area delays the onset of maternal behavior.422 Tuberoinfundibular 
peptide of 39 residues (TIP39) may also be an important com­
ponent of the mechanism involved in maternal motivation by 
lactation.423 This peptide, produced by neurons in the thalamus, 
has extensive projections to galanin neurons in the preoptic area, 
which are known to be activated by suckling.424 Suckling also 
results in a marked increase in NPY and AgR

 also 
results in a marked increase in NPY and AgRP gene expression 
in the arcuate nucleus mediated by neuronal afferents from the 
brainstem and the fall in circulating levels of leptin, brought about 
by a negative energy balance secondary to the milk production.425 
As NPY and AgRP are powerful orexigenic peptides (see Chapter 
39 for overview of neuroendocrine regulation of appetite and sati­
ety), their increase during suckling may explain the hyperphagia 
associated with lactation to meet

the hyperphagia 
associated with lactation to meet the energy demands imposed by 
milk production. Indeed, total ablation of NPY/AgRP neurons by 
diphtheria toxin administered to transgenic mice expressing the 
diphtheria toxin receptor exclusively in NPY/AGRP neurons leads 
to a significant reduction in food intake in lactating animals.426 
Activation of arcuate nucleus NPY neurons may also contribute 
to the inhibition of reproductive function associated with suckling 
through direct actions o

associated with suckling 
through direct actions on GnRH neurons mediated by NPY Y5 
receptors.427
Although their significance for PRL regulation in humans 
is not certain, environmental stimuli from seasonal changes in 
light duration and auditory and olfactory cues are clearly of great 
importance to many mammalian species.378 Seasonal breeders 
such as sheep exhibit a reduction in PRL secretion in response to 
shortened days. Vocalization of rodent pups are also potent stim­
uli for PRL secre

odent pups are also potent stim­
uli for PRL secretion in lactating and virgin female rats by activa­
tion of the inferior colliculus and lateral geniculate of the posterior 
thalamus.418 Olfactory stimuli from pheromones also have potent 
actions in rodents. A prime example is the Bruce effect, or sponta­
neous abortion, induced by exposure of a pregnant female rat to an 
unfamiliar male. It is mediated by a well-studied neural circuitry 
involving the vomeronasal nerves, the corticomedial amyg

ing the vomeronasal nerves, the corticomedial amygdala, 
and the medial preoptic area of the hypothalamus, which results 
in activation of TIDA neurons and a reduction in circulating PRL 
161
that is essential for maintenance of luteal function in the first half 
of pregnancy.
Stress in many forms dramatically affects PRL secretion, 
although the teleologic significance is uncertain. It may be related 
to actions of PRL on cells of the immune system or some other 
a

PRL on cells of the immune system or some other 
aspect of homeostasis. Different stressors are associated with either 
a reduction or an increase in PRL secretion, depending on the 
local regulatory environment at the time of the stress. However, 
whereas well-documented changes in PRL are associated with 
relatively severe forms of stress in laboratory animal models, the 
relevance to human physiology is not well established. 
Gonadotropin-Releasing Hormone and Control 
of the Reproductive Axi

asing Hormone and Control 
of the Reproductive Axis
GnRH is a 10–amino acid hypothalamic neuropeptide that con­
trols the function of the reproductive axis. It is synthesized as part 
of a larger precursor molecule that is enzymatically cleaved to 
remove a signal peptide from the NH2-terminus and GnRH-asso­
ciated peptide (GAP) from the COOH-terminus (Fig. 7.28).428 
All forms of the decapeptide have a pyroGlu at the NH2-terminus 
and Gly-amide at the COOH-terminus, indi

terminus 
and Gly-amide at the COOH-terminus, indicating the functional 
importance of the terminal residues throughout evolution.
Two genes encoding GnRH have been identified within mam­
mals.429,430 The first, GNRH1, encodes a 92–amino acid precursor 
protein. This form of GnRH is found in hypothalamic neurons 
and serves as a releasing factor to regulate pituitary gonadotroph 
function.431 The second GnRH gene, GNRH2, encodes a deca­
peptide that differs from the first by three amino acids.43

hat differs from the first by three amino acids.432 This 
form of GnRH is found in the midbrain region and serves as a 
neurotransmitter rather than as a pituitary releasing factor. Both 
GnRH-I and GnRH-II are found in phylogenetically diverse spe­
cies, from fish to mammals, suggesting that these multiple forms 
of GnRH diverged from one another early in vertebrate evolu­
tion.431 A third form of GnRH, GnRH-III, has been identified in 
neurons of the telencephalon in teleost fish. GnRH is also

of the telencephalon in teleost fish. GnRH is also found 
in cells outside the brain but is not well understood and is an area 
under current investigation.
All GnRH genes have the same basic structure, with the pre-
prohormone mRNA encoded in four exons. Exon 1 contains the 
5′ untranslated region of the gene; exon 2 contains the signal 
peptide, GnRH, and the NH2-terminus of GAP; exon 3 contains 
Prohormone convertase
Gln - His - Trp - Ser - Tyr - Gly - Leu - Arg - Pro - Gly - Gly
(pyro) Glu -

r - Gly - Leu - Arg - Pro - Gly - Gly
(pyro) Glu - His - Trp - Ser - Tyr - Gly - Leu - Arg - Pro - Gly - NH2
PAM
cyclization of Gln
Signal
peptide
GnRH
GAP
5' UTR
5'
3'
3' UTR
Exon 1
Exon 2
Exon 3
Exon 4
Intron A
Intron B
Intron C
50 bp
100 bp
−992
−795
−150
+20
TATA
BRN2 BRN2
BRN2 BRN2
Neural-Specific Enhancer
Promoter
GnRH
• Fig. 7.28  Schematic diagram of the human gene for gonadotropin-releasing hormone-1 (GNRH1), the 
hypothalamic complementary deoxyribonucleic acid (cDNA), and post-transla

ary deoxyribonucleic acid (cDNA), and post-translational processing of the 
GnRH prohormone. A cluster of binding sites for the homeodomain transcription factor BRN2 is present in 
both the proximal promoter and a distal enhancer region and is important for neuron-specific expression of 
the gene. Phylogenetically conserved homologous regions have been identified in the rat Gnrh1 gene, but 
in that species the Oct1 transcription factor has been implicated in neuron-specific expression. The cDNA 

mplicated in neuron-specific expression. The cDNA 
for GnRH-I isolated from human placenta has a longer 5′ untranslated region (UTR) because of differential 
splicing of the heterogeneous nuclear RNA (hnRNA) and inclusion of intron A sequences. GAP, GnRH-
associated peptide; PAM, peptidylglycine α-amidating mono-oxygenase; TATA, Goldstein-Hogness box 
involved in binding RNA polymerase. (Compiled from data of Cheng CK, Leung PCK. Molecular biol­
ogy of gonadotropin-releasing hormone [GnRH]-I, Gn

ogy of gonadotropin-releasing hormone [GnRH]-I, GnRH-II, and their receptors in humans. Endocr Rev. 
2005;26:283–306; Wolfe A, Kim HH, Tobet S, et al. Identification of a discrete promoter region of the 
human GnRH gene that is sufficient for directing neuron-specific expression: a role for POU homeodomain 
transcription factors. Mol Endocrinol. 2002;16:435–449.)
162
the central portion of GAP; and exon 4 contains the COOH-
terminus of GAP and the 3′ untr

contains the COOH-
terminus of GAP and the 3′ untranslated region (see Fig. 7.28).431 
Among species, the nucleotide sequences encoding the GnRH 
decapeptide are highly homologous. This section focuses on the 
hypothalamic GnRH that is derived from GNRH1 mRNA and 
plays an important role in the regulation of the hypothalamic-
pituitary-gonadal axis.
Two transcriptional start sites have been identified in the rat 
GNRH1 gene, at the +1 and –579, with the +1 promoter being 
active in hypothalamic 

ith the +1 promoter being 
active in hypothalamic neurons and the other promoter active 
in placenta. The first 173 base pairs of the promoter are highly 
conserved among species. In the rat, this promoter region has 
been shown to contain two Oct1 binding sites, three regions 
that bind the POU domain family of transcription factors (SCIP, 
Oct-6, and Tst-1), and three regions that can bind the progester­
one receptor.433 In addition, a variety of hormones and second 
messengers have been shown

of hormones and second 
messengers have been shown to regulate GnRH gene expres­
sion, and a majority of the cis-acting elements, thus far charac­
terized for hormonal control of GnRH transcription, are located 
in the proximal promoter region.434,435 The 5′ flanking region 
of the rodent and human GNRH1 genes also contains a distal 
300–base pair enhancer region that is 1.8 or 0.9 kb, respectively, 
upstream of the transcription start site.435,436 Studies have impli­
cated the homeodomain trans

36 Studies have impli­
cated the homeodomain transcription factors OCT1, MSX, and 
DLX in the specification of neuron expression and developmental 
activation.436,437 
Anatomic Distribution
GnRH neurons are small, diffusely located cells that are not con­
centrated in a discrete nucleus. They are generally bipolar and 
fusiform in shape, with slender axons projecting predominantly 
to the median eminence and infundibular stalk. The location of 
hypothalamic GnRH neurons is species-dependent. In 

ypothalamic GnRH neurons is species-dependent. In the rat, 
hypothalamic GnRH neurons are concentrated in rostral areas, 
including the medial preoptic area, the diagonal band of Broca, 
the septal areas, and the anterior hypothalamus. In humans and 
nonhuman primates, the majority of hypothalamic GnRH neu­
rons are located more dorsally in the medial basal hypothalamus, 
the infundibulum, and the periventricular region. Throughout the 
hypothalamus, neurohypophysial GnRH neurons are intersperse

mus, neurohypophysial GnRH neurons are interspersed 
with non-neuroendocrine GnRH neurons that extend their axons 
to other areas of the brain, including other hypothalamic regions 
and various regions of the cortex. GnRH secreted from non-neu­
roendocrine neurons has been implicated in the control of sexual 
behavior in rodents but not in higher primates.438 
Embryonic Development
GnRH neuroendocrine neurons are an unusual neuronal popula­
tion in that they originate outside the CNS, from the e

in that they originate outside the CNS, from the epithelial 
tissue of the nasal placode.439 During embryonic development, 
GnRH neurons migrate across the surface of the brain and into 
the hypothalamus, with the final hypothalamic location differing 
somewhat among species. Migration is dependent on a scaffolding 
of neurons and glial cells along which the GnRH neurons move, 
with neural cell adhesion molecules playing a critical role in guid­
ing the migration process.
Failure of GnRH neurons

Failure of GnRH neurons to migrate properly leads to the 
clinical condition Kallmann syndrome, in which GnRH neuroen­
docrine neurons do not reach their final destination and therefore 
do not stimulate pituitary gonadotropin secretion.439,440 Patients 
with Kallmann syndrome do not enter puberty spontaneously. The 
X-linked form of Kallmann syndrome results from a deficiency 
of the KAL1 gene, which encodes the extracellular glycoprotein 
termed anosmin-1. Mutations 

ellular glycoprotein 
termed anosmin-1. Mutations in the fibroblast growth factor 
receptor type 1 (FGFR1), FGF8, prokineticin 2 and its recep­
tors (PROK2 and PROKR2), and chromodomain helicase DNA-
binding protein 7 (CDH7) genes produce an autosomal dominant 
form of Kallmann syndrome, and autosomal recessive forms of 
PROK2 and PROKR2 also occur. Other genes associated with 
Kallman syndrome include CCDC141, DUSP6, FEZF1, FGF17, 
FLRT3, IL17RD, NELF, SEMA3A, SPRY4, and WDR11.441 
These mutati

, NELF, SEMA3A, SPRY4, and WDR11.441 
These mutations, however, still account for only the minority of 
Kallmann cases, and other causes are yet to be characterized.442 
Administration of exogenous GnRH effectively treats this form of 
hypothalamic hypogonadism. Patients with Kallmann syndrome 
due to mutations in the KAL1 gene often have other congenital 
midline defects, including anosmia, which results from hypoplasia 
of the olfactory bulb and tracts. 
Action at the Pituitary
Receptors
GnRH 

d tracts. 
Receptors
GnRH binds to a membrane receptor on pituitary gonadotrophs 
and stimulates both LH and FSH synthesis and secretion. The 
GnRH receptor is a seven-transmembrane-domain GPCR, 
but it lacks a typical intracellular COOH-terminal cytoplasmic 
domain.435 Under physiologic conditions, GnRH receptor num­
ber varies and is usually directly correlated with the gonadotropin 
secretory capacity of pituitary gonadotrophs. For example, across 
the rat estrous cycl

otrophs. For example, across 
the rat estrous cycle, a rise in GnRH receptors is seen just before 
the surge of gonadotropins that occurs on the afternoon of pro­
estrus. GnRH receptor message levels are regulated by a variety 
of hormones and second messengers, including steroid hormones 
(estradiol can both suppress and stimulate; progesterone sup­
presses), gonadotropins (which suppress), and calcium and pro­
tein kinase C (which stimulate).433,435
Gq/11 is the primary guanosine triphosphate–

3,435
Gq/11 is the primary guanosine triphosphate–binding pro­
tein mediating GnRH responses; however, there is evidence that 
GnRH receptors can couple to other G proteins, including Gs 
and Gi.435 With activation, the GnRH receptor couples to a phos­
phoinositide-specific phospholipase C, which leads to increases 
in calcium transport into gonadotrophs and calcium release from 
internal stores through a diacylglycerol-protein kinase C pathway. 
Increased calcium entry is a critical step in GnR

Increased calcium entry is a critical step in GnRH-stimulated 
release of gonadotropin secretion. However, GnRH also stimu­
lates the MAPK cascade.
When there is a decline in GnRH stimulation to the pituitary, 
as occurs in a variety of physiologic conditions (including states 
of lactation, undernutrition, or seasonal periods of reproductive 
quiescence), the number of GnRH receptors on pituitary gonado­
trophs declines dramatically. Subsequent exposure of the pituitary 
to pulses of GnRH rest

 exposure of the pituitary 
to pulses of GnRH restores receptor number by a Ca2+-dependent 
mechanism that requires protein synthesis.443 The effect of GnRH 
to induce its own receptor is termed upregulation or self-priming. 
Only certain physiologic frequencies of pulsatile GnRH can aug­
ment GnRH receptor production, and these frequencies appear to 
differ among species.444 Upregulation of GnRH receptors after a 
period of low GnRH stimulation to the pituitary can take hours 
to days of exposu

to the pituitary can take hours 
to days of exposure to pulsatile GnRH, depending on the duration 
and extent of the prior decrease in GnRH. The self-priming effect 
of GnRH to upregulate its own receptors also plays a crucial role 
in the production of the gonadotropin surge that occurs at mid­
cycle in females of spontaneously ovulating species and triggers 
ovulation. Just before the gonadotropin surge, two factors—the 
163
increased frequency of pulsatile GnRH r

ocrinology
increased frequency of pulsatile GnRH release and a sensitization 
of the pituitary gonadotrophs by rising levels of estradiol—make 
the pituitary exquisitely sensitive to GnRH and allow an output 
of LH that is an order of magnitude greater than the release seen 
during the rest of the female reproductive cycle. This surge of LH 
triggers the ovulatory process at the ovary.
In contrast to upregulation of GnRH receptors by pulsatile reg­
imens of GnRH, continuous exposure to GnRH lead

g­
imens of GnRH, continuous exposure to GnRH leads to down­
regulation of GnRH receptors and an accompanying decrease in 
LH and FSH synthesis and secretion, termed desensitization.445 
Downregulation does not require calcium mobilization or gonad­
otropin secretion. It involves a rapid uncoupling of receptors from 
G proteins and sequestration of the receptors from the plasma 
membrane, followed by internalization and proteolytic degrada­
tion of the receptors.
The concept of downregulation ha

of the receptors.
The concept of downregulation has a number of clinical appli­
cations. For example, the most common current therapy for pre­
cocious puberty of hypothalamic origin (i.e., precocious GnRH 
secretion) is to treat it with a long-acting GnRH superagonist that 
downregulates pituitary GnRH receptors and effectively turns off 
the reproductive axis.444,446 Children with precocious puberty 
can be maintained with long-acting GnRH agonists for years to 
suppress the premature activatio

sts for years to 
suppress the premature activation of the reproductive axis, and at 
the normal age of puberty, agonist treatment can be withdrawn, 
allowing reactivation of pituitary gonadotrophs and a downstream 
increase in gonadal steroid hormone production (also see Chapter 
17). Long-acting GnRH agonists are also used in the treatment of 
forms of breast cancer that are estrogen-dependent and of other 
gonadal steroid-dependent cancers.444 Long-acting antagonists of 
GnRH have been develo

 Long-acting antagonists of 
GnRH have been developed that can also be used for these thera­
pies.447 Antagonists have the advantage of not having a flare effect 
(i.e., an acute stimulation of gonadotropin secretion that is seen 
during the initial treatment of individuals with superagonists). 
Pulsatile Gonadotropin-Releasing Hormone Stimulation
Because a single pulse of GnRH stimulates the release of both 
LH and FSH, and chronic exposure of the pituitary to pulsatile 
GnRH supports the synth

he pituitary to pulsatile 
GnRH supports the synthesis of both LH and FSH, it is gener­
ally believed that there is only one releasing factor regulating the 
synthesis and secretion of LH and FSH. However, in a number 
of physiologic conditions, there are divergent patterns of LH and 
FSH secretion; thus a second FSH-releasing peptide has been pro­
posed, but such a peptide has not been isolated to date. Other 
mechanisms, discussed in more detail later, are likely to account 
for the differenti

l later, are likely to account 
for the differential regulation of LH and FSH release.
The ensemble of GnRH neurons in the hypothalamus that 
send axons to the portal blood system in the median eminence fire 
in a coordinated, repetitive, episodic manner, producing distinct 
pulses of GnRH in the portal bloodstream. The pulsatile nature 
of GnRH stimulation to the pituitary leads to the release of dis­
tinct pulses of LH into the peripheral circulation. In experimental 
animals, in which it is p

lation. In experimental 
animals, in which it is possible to collect blood samples simultane­
ously from the portal and peripheral blood, GnRH and LH pulses 
have been found to correspond in about a 1:1 ratio at most physi­
ologic rates of secretion.448 Because the portal bloodstream is gen­
erally inaccessible in humans, the collection of frequent peripheral 
venous blood samples is used to define the pulsatile nature of LH 
secretion (i.e., frequency and amplitude of LH pulses), and pulsa­
til

quency and amplitude of LH pulses), and pulsa­
tile LH is used as an indirect measure of the activity of the GnRH 
secretory system. Indirect assessment of GnRH secretion by moni­
toring the rate of pulsatile LH secretion is also used in many ani­
mal studies examining the factors that govern the regulation of the 
pulsatile activity of the reproductive neuroendocrine axis. Unlike 
LH secretion, FSH secretion is not always pulsatile; even when 
it is pulsatile, there is only partial concordance 

t is pulsatile, there is only partial concordance between LH and 
FSH pulses.
It is possible to place multiple-unit recording electrodes in the 
medial basal hypothalamus of monkeys and other species and 
detect spikes of electrical activity that are concordant with the 
pulsatile secretion of LH secretion.449 However, it is unknown 
whether these bursts of electrical activity reflect the activity of 
GnRH neurons themselves or the activity of neurons that impinge 
on GnRH neurons and govern the

urons that impinge 
on GnRH neurons and govern their firing. With the develop­
ment of mice in which the gene for green fluorescent protein 
has been put under the regulation of the GnRH promoter, it has 
been possible to identify GnRH neurons in hypothalamic tissue 
slices using fluorescence microscopy, to record from them intra­
cellularly,16 and to simultaneously measure GnRH release from 
the median eminence by fast-scan cyclic voltammetry.450 These 
studies have shown that many, but not all

0 These 
studies have shown that many, but not all, GnRH neurons show 
a bursting pattern of electrical activity. A central question in the 
field of reproductive neuroendocrinology is what causes GnRH 
neurons to pulse in a coordinated manner. Embryonic GnRH 
neurons from rhesus monkeys have shown intrinsic oscillatory 
changes in intracellular calcium concentration and synchronized 
calcium peaks among tens of neurons associated with GnRH 
release. A mathematical network model has been develop

ase. A mathematical network model has been developed to fur­
ther characterize this synchronization process.451 The term GnRH 
pulse generator is often used to acknowledge the fact that GnRH 
secretion occurs in pulses and to refer to the central mechanisms 
responsible for pulsatile GnRH release.
A critical factor governing LH and FSH secretion is the rate of 
pulsatile GnRH stimulation of the gonadotrophs. Experimental 
studies in which the hypothalamus was lesioned and GnRH was 
replaced by p

othalamus was lesioned and GnRH was 
replaced by pulsatile administration of exogenous GnRH showed 
that different frequencies of GnRH can lead to different ratios of 
LH to FSH secretion from the pituitary. Fig. 7.29 shows that in 
a monkey with a hypothalamic lesion, replacement of one pulse 
of GnRH per hour led to a relatively low ratio of FSH to LH 
secretion. Subsequent institution of a slower pulse frequency (one 
pulse of GnRH every 3 hours) led to a decrease in LH secretion 
but an incr

rs) led to a decrease in LH secretion 
but an increase in FSH secretion so that the ratio of FSH to LH 
secretion was greatly elevated. Thus, during the normal menstrual 
cycle, LH pulse frequency increases during the follicular phase as 
the midcycle ovulatory phase is approached, reflecting increased 
GnRH pulsatility, and then slows during the luteal phase. Abnor­
mally low GnRH pulsatility is associated with delayed puberty, 
amenorrhea, 
opioid-induced 
hypogonadism, 
hyperpro

enorrhea, 
hyperprolactinemia, and obesity-related hypogonadism, whereas 
an inappropriately high GnRH pulsatility is associated with pre­
cocious puberty and polycystic ovarian syndrome (PCOS).452 As 
discussed later, steroid hormones act at both the hypothalamus 
and pituitary to strongly influence the rate of pulsatile GnRH 
release and amount of LH and FSH secreted from the pituitary.
GnRH pulse frequency not only influences the rate of pulsatile 
gonadotropin 

ly influences the rate of pulsatile 
gonadotropin release and the ratio of FSH to LH secretion, but also 
plays an important role in modulating the structural makeup of the 
gonadotropins. LH and FSH are structurally similar glycoprotein 
hormones. Each of these hormones is made up of an α-subunit 
and a β-subunit. LH, FSH, and TSH share a common α-subunit, 
and each has a unique β-subunit that conveys receptor specificity 
to the intact hormone. Before secretion of gonadotropins, terminal 
suga

 Before secretion of gonadotropins, terminal 
sugars are attached to each gonadotropin molecule.162 The sugars 
include sialic acid, galactose, N-acetylglucosamine, and mannose, 
164
but the most important is sialic acid. The extent of glycosylation 
of LH and FSH is important for the physiologic function of these 
hormones.162 Forms of gonadotropin with more sialic acid have a 
longer half-life because they are protected from degradation by the 
liver. F

ey are protected from degradation by the 
liver. Forms of gonadotropin with less sialic acid have more potent 
effects at their biologic receptors. Both the rate of GnRH stimula­
tion and ovarian hormone feedback at the level of the pituitary 
regulate the degree of LH and FSH glycosylation. For example, 
slow frequencies of GnRH, seen during follicular development, 
are associated with greater degrees of FSH glycosylation, which 
would provide sustained FSH support to growing follicles. In con­

ustained FSH support to growing follicles. In con­
trast, faster frequencies of GnRH, seen just before the midcycle 
gonadotropin surge, are associated with lesser degrees of FSH gly­
cosylation, providing a more potent but shorter lasting form of 
FSH at the time of ovulation.453 
Regulatory Systems
Many neurotransmitter systems from the brainstem, limbic sys­
tem, and other areas of the hypothalamus convey information to 
GnRH neurons (Fig. 7.30). These afferent systems include neu­
rons that 

0). These afferent systems include neu­
rons that contain norepinephrine, dopamine, serotonin, GABA, 
glutamate, kisspeptin, endogenous opiate peptides, NPY, galanin, 
and a number of other peptide neurotransmitters. Glutamate and 
norepinephrine play important roles in providing stimulatory 
drive to the reproductive axis, whereas GABA and endogenous 
opioid peptides provide a substantial portion of the inhibitory 
drive to GnRH neurons. However, it is the kisspeptin neuronal 
network that is p

, it is the kisspeptin neuronal 
network that is particularly essential for regulation of the GnRH 
neurons454 and will be discussed in greater detail.
GnRH neuronal cell bodies are surrounded by astrocytes that 
may contribute to GnRH secretion through the release of pros­
taglandin E2 (PGE2) and/or transforming growth factor beta 
(TGFβ).455 Changes in the steroid hormone milieu also influ­
ence the degree of glial sheathing of GnRH axon terminals in the 
external zone of the median eminence a

als in the 
external zone of the median eminence and have important roles 
in regulating delivery to the portal capillary plexus for conveyance 
to the anterior pituitary. Thus, during diestrus in the rat when 
GnRH output to the pituitary is very low, GnRH axon terminals 
in the median eminence are fully ensheathed by tanycyte end-feet 
processes, preventing GnRH from reaching the portal blood. The 
sprouting of tanycyte end-feet processes to engulf GnRH-contain­
ing axon terminals is mediated 

gulf GnRH-contain­
ing axon terminals is mediated by transforming growth factor beta 
1 (TGFβ1) and SEMA7A.59 Conversely, during the preovulatory 
surge of proestrus when there is an increase in GnRH, estrogen 
binds to alpha-type estrogen receptors on tanycytes and induces 
retraction of tanycyte foot processes through PGE2-dependent 
production of TGFβ1, allowing GnRH axon terminals to have 
direct contact with the fenestrated portal capillary system.456 
Estrogen may also affect the synthesis

system.456 
Estrogen may also affect the synthesis of adhesion molecules such 
as polysialylated neuronal cell adhesion molecule (PSA-N-CAM) 
and synaptic cell adhesion molecule (SynCAM) that facilitate 
glial-neuronal interactions and remodeling.457 Endothelial cells of 
the portal capillary system may also contribute to tanycyte plastic­
ity via estrogen-induced regulation of nitric oxide production that 
affects actin cytoskeleton remodeling.458 
Feedback Regulation
Steroid hormone receptors 

58 
Steroid hormone receptors are abundant in the hypothalamus and 
in many neural systems that impinge on GnRH neurons, includ­
ing noradrenergic, serotonergic, kisspeptin, β-endorphin–con­
taining, and NPY neurons. Early studies identifying regions of 
the brain that bound labeled estrogens showed that in rodents the 
preoptic area and VMH had the highest concentrations of estro­
gen receptors in the brain. Further localization studies, identifying 
estrogen receptors by im

ion studies, identifying 
estrogen receptors by immunocytochemistry or in situ hybridiza­
tion, confirmed the strong presence of estrogen receptors in the 
hypothalamus and in brain areas with abundant connections to 
the hypothalamus, including the amygdala, septal nuclei, BNST, 
medial part of the NTS, and lateral portion of the parabrachial 
nucleus.459 In 1986, a new member of the steroid hormone recep­
tor superfamily with high-sequence homology to the classic estro­
gen receptor (now refer

logy to the classic estro­
gen receptor (now referred to as estrogen receptor-α) was isolated 
0
50
40
30
20
10
0
500
400
300
200
100
40
35
30
25
20
15
10
5
0
5
10
15
20
1 GnRH pulse/hr
1 GnRH
pulse/3 hr
LH (ng/mL)
FSH (ng/mL)
Days
• Fig. 7.29  The influence of gonadotropin-releasing hormone (GnRH) pulse frequency on luteinizing hor­
mone (LH) and follicle-stimulating hormone (FSH) secretion in a female rhesus monkey with an arcuate 
nucleus lesion ablating endogenous GnRH suppor

te 
nucleus lesion ablating endogenous GnRH support of the pituitary. Decreasing the GnRH pulse frequency 
from 1 pulse every hour to 1 pulse every 3 hours led to a decrease in plasma LH concentrations but an 
increase in plasma FSH concentrations. (Redrawn from Wildt L, Haulser A, Marshall G, et al. Frequency 
and amplitude of gonadotropin-releasing hormone stimulation and gonadotropin secretion in the rhesus 
monkey. Endocrinology. 1981;109:376–385.)
165
from rat 

–385.)
165
from rat prostate and named estrogen receptor-β. This novel estro­
gen receptor was shown to bind estradiol and to activate transcrip­
tion by binding to estrogen response elements.460
In situ hybridization studies examining the localization of 
estrogen receptor-β mRNA have shown that these receptors are 
present throughout the rostral-caudal extent of the brain, with a 
high level of expression in the preoptic area, BNST, PVH and 
SON, amygdala, and lam

eoptic area, BNST, PVH and 
SON, amygdala, and laminae II to VI of the cerebral cortex.461 
Specific receptors for progesterone are induced by estrogen in 
hypothalamic regions of the brain, including the preoptic area, 
the ventromedial and ventrolateral nuclei, and the infundibular-
arcuate nucleus, although there is also evidence for constitutive 
expression of progesterone receptors in some regions.462 Andro­
gen receptor mapping studies have shown considerable overlap in 
the distribution o

 shown considerable overlap in 
the distribution of androgen and estrogen receptors throughout 
the brain. The highest density of androgen receptors was found 
in hypothalamic nuclei known to participate in the control of 
reproduction and sexual behaviors, including the arcuate nucleus, 
PVH, MePO, ventromedial nucleus, and brain regions with 
strong connections to the hypothalamus (including the amygdala, 
nuclei of the septal region, BNST, NTS, and lateral division of 
the parabrachial nucleu

, and lateral division of 
the parabrachial nucleus).459 The anterior pituitary also contains 
receptors for all of the gonadal steroid hormones.
Steroid hormones can dramatically alter the pattern of pulsa­
tile release of GnRH and of the gonadotropins through actions at 
both the hypothalamus and the pituitary. At the hypothalamus, 
estradiol, progesterone, and testosterone can all act to slow the 
frequency of GnRH release into the portal bloodstream as part 
Pituitary
Stress,
Nu

dstream as part 
Pituitary
Stress,
Nutrition,
Exercise,
Seasonal cues
Glutamate
NPY
CRH
NE
GABA
GALP
β-endorphins
GnRH
neuron
Testis
Ovary
LH and
FSH
Progesterone receptors
Androgen receptors
Inhibin receptors
Inhibin
Inhibin
Estradiol
Kisspeptin
neuron
neuron
Arcuate
nucleus
Anteroventral
nucleus
to CNS
to CNS
• Fig. 7.30  Regulation of the hypothalamic-pituitary-gonadal axis. Schematic diagram

thalamic-pituitary-gonadal axis. Schematic diagram of the hypothalamic-
pituitary-gonadal axis showing neural systems that regulate gonadotropin-releasing hormone (GnRH) 
secretion and feedback of gonadal steroid hormones at the level of the hypothalamus and pituitary. CNS, 
central nervous system; CRH, corticotropin-releasing hormone; FSH, follicle-stimulating hormone; GABA, 
γ-aminobutyric acid; GALP, galanin-like peptide; LH, luteinizing hormone; NE, norepinephrine; NPY, neu­
ropeptide Y.
166

ne; NE, norepinephrine; NPY, neu­
166
of a closed negative feedback loop.463 However, GnRH neurons 
lack estrogen receptors, indicating that the effects of estrogen on 
GnRH secretion is mediated by other neural systems that pro­
vide afferent input to GnRH neurons. Most important is the kis­
speptin signaling pathway, which is now well established to be 
involved in the central regulation of puberty and reproduction by 
stimulating GnRH from

puberty and reproduction by 
stimulating GnRH from the hypothalamus.
Kisspeptin, a RFamide-like peptide and the natural ligand 
for the former orphan GPCR, GPR54, is a 54–amino acid 
peptide that derives from the KISS1 gene located on chro­
mosome 1q33, although smaller forms that contain 10 to 14 
amino acids have also been identified. Inactivating mutations 
of the KISS1 gene result in hypogonadotropic hypogonad­
ism or pubertal delay,464 whereas activating mutations result 
in precocious pube

as activating mutations result 
in precocious puberty.465 These neurons have been identified 
in two discrete areas in mammals: (1) the preoptic area and 
the infundibular nucleus in humans and primates and (2) the 
anteroventral periventricular nucleus (AVPV)/preoptic peri­
ventricular nucleus (defined together as the rostral periventric­
ular area of the third ventricle [RP3V]) and arcuate nucleus in 
ruminants.466–468
Although GnRH neurons have an endogenous rhythmic activ­
ity in culture, th

e an endogenous rhythmic activ­
ity in culture, they do not correlate well to the pulses of GnRH 
observed in vivo; therefore the pulse generator has largely been 
ascribed to the kisspeptin neurons. Kisspeptin neurons express 
estrogen (both α and β) and progesterone receptors and densely 
innervate GnRH neuronal cell bodies, particularly their axon 
terminals in the median eminence. In addition, GnRH neurons 
express the kisspeptin receptor (KISS1R).469 Pulsatile secretion 
of GnRH is dependen

SS1R).469 Pulsatile secretion 
of GnRH is dependent upon pulsatile secretion of kisspeptin, as 
if kisspeptin is administered continuously, it results in suppres­
sion of LH secretion.470 The mechanism by which pulsatile secre­
tion of kisspeptin occurs is proposed to be mediated by at least 
two additional peptides coexpressed by kisspeptin neurons in the 
infundibular/arcuate nucleus with opposing actions, namely the 
tachykinin neurokinin B, which stimulates kisspeptin secretion, 
and dynorph

hich stimulates kisspeptin secretion, 
and dynorphin (or possibly enkephalin in humans471), which 
inhibits kisspeptin secretion.454,472 For this reason, this popu­
lation of kisspeptin neurons is commonly referred to as KNDy 
neurons.472 This concept is supported by the presence of both 
tachykinin receptors and κ-opioid receptors on KNDy neurons 
and the extensive axosomatic and axodendritic interconnections 
among these neurons that coordinate their activity.466 In addition, 
inactivating mut

 their activity.466 In addition, 
inactivating mutations of the genes encoding for neurokinin B or 
the neurokinin B receptor, NK3R, results in hypogonadotropic 
hypogonadism.473,474 Substance P has also been shown to have 
an activating effect on KNDy neurons and is either synthesized 
within KNDy neurons themselves or by adjacent populations of 
neurons that innervate KNDy neurons, depending upon the ani­
mal species.454,475 The action of kisspeptin on GnRH secretion, 
however, may occur prima

eptin on GnRH secretion, 
however, may occur primarily at axon terminals, as injection of a 
kisspeptin antagonist into the median eminence of monkeys can 
abolish GnRH pulses.476
Phoenixin-20 amide (PNX), a novel peptide that derives from 
the small, integral membrane protein 20 (SMIM20) and highly 
conserved among species, has also been found to have a role in 
regulation of the gonadal axis both by sensitizing gonadotrophs 
in the anterior pituitary to GnRH and through direct effects on 
GnRH

uitary to GnRH and through direct effects on 
GnRH neurons.477 The observation that PNX is synthesized in 
kisspeptin neurons and increases KISS1 mRNA expression, and 
that kisspeptin neurons express GPR123 receptor for which PNX 
is a ligand, has raised the possibility that the peptide may activate 
kisspeptin neurons through an autocrine mechanism or through 
connections with other kisspeptin/PNS neurons.
Most of the time, the hypothalamic-pituitary axis is under the 
negative feedback influen

itary axis is under the 
negative feedback influence of gonadal steroid hormones. If the 
gonads are removed surgically or their normal secretion of steroid 
hormones is suppressed pharmacologically, there is a dramatic 
increase (10-fold to 20-fold) in circulating levels of LH and FSH 
secretion in both males and females.463 This type of castration 
response occurs normally at menopause in women, when ovarian 
follicular development and therefore ovarian production of large 
quantities of estra

e ovarian production of large 
quantities of estradiol and progesterone decrease and eventually 
cease.
KNDy neurons in the infundibular nucleus are involved in 
mediating negative feedback regulation of GnRH release by ste­
roid hormones, including estradiol (E2), progesterone, and tes­
tosterone.478–480 Estrogen suppresses kisspeptin and neurokinin 
B from infundibular KNDy neurons, and both kisspeptin and 
neurokinin B gene expression are upregulated in ovariectomized 
animals and postmenopau

gulated in ovariectomized 
animals and postmenopausal women.481 Negative feedback of ste­
roid hormones can also occur directly at the level of the pituitary. 
For example, estradiol has been shown to be capable of binding 
to the pituitary, decreasing LH and FSH synthesis and release, 
and decreasing the sensitivity of pituitary gonadotrophs to the 
actions of GnRH so that less LH and FSH are released when a 
pulse of GnRH stimulates the pituitary. Evidence for such a direct 
pituitary action o

ry. Evidence for such a direct 
pituitary action of estradiol came from studies with rhesus mon­
keys that had been rendered deficient in endogenous GnRH by a 
lesion in the arcuate nucleus and showed a decline in endogenous 
gonadotropin secretion. When these monkeys received a pulsatile 
regimen of GnRH treatments, subsequent estradiol infusions dra­
matically suppressed the responsiveness of the pituitary to GnRH 
and suppressed the gonadotropin secretion that was being driven 
by the pulsati

in secretion that was being driven 
by the pulsatile administration of GnRH.482 Similarly, in a com­
pound mutant mouse model on a GnRH-deficient (Hpg) genetic 
background, expression of a human FSH-β transgene was inhib­
ited by testosterone directly at the pituitary level.483 In primate 
species, including humans, there is considerable feedback of estra­
diol at the pituitary, but most of the progesterone and testosterone 
negative feedback occurs at the level of the hypothalamus.463
In additi

urs at the level of the hypothalamus.463
In addition to negative feedback, estradiol can have a positive 
feedback action at the level of the hypothalamus and pituitary to 
lead to a massive release of LH and FSH from the pituitary. This 
release of gonadotropins occurs once each menstrual cycle and is 
referred to as the LH-FSH surge. The positive feedback action of 
estradiol occurs as a response to the rising tide of estradiol that is 
produced during the process of dominant follicle developm

d during the process of dominant follicle development in 
the late follicular phase of the menstrual cycle. In women, elevated 
estradiol levels are generally maintained at about 300 to 500 pg/
mL for about 36 hours before the stimulation of the gonadotropin 
surge.
Experiments have shown that both a critical concentration 
and duration of elevated estradiol are necessary to achieve posi­
tive feedback and a resulting gonadotropin surge. If supraphysi­
ologic doses of estradiol are administered,

physi­
ologic doses of estradiol are administered, the surge can occur as 
early as 18 hours after their administration. Because the ovary is 
responsible for the production of estradiol, and the time course 
and magnitude of estradiol release control the rate of positive 
feedback, the ovary has been referred to as the zeitgeber of the 
menstrual cycle. The dependence of the positive feedback system 
on the magnitude of estradiol production helps explain the fact 
that the portion of the menstr

s explain the fact 
that the portion of the menstrual cycle that varies most in length 
is the follicular phase. Production of higher levels of estradiol by 
167
a dominant follicle in one cycle leads to a more rapid positive 
feedback action, with earlier ovulation and therefore a shorter fol­
licular phase, compared to a cycle in which the dominant follicle 
produced lower levels of estradiol.
As with negative feedback in response to estradiol, the positive 
feedb

back in response to estradiol, the positive 
feedback actions of estradiol occur both at the hypothalamus to 
increase GnRH secretion and at the pituitary to enhance pituitary 
responsiveness to GnRH. Estradiol increases pituitary sensitivity 
to GnRH by increasing the synthesis of new GnRH receptors and 
by enhancing the responsiveness to GnRH at a postreceptor site of 
action. At the level of the hypothalamus, kisspeptin neurons in the 
preoptic and RPV3 nucleus are believed to function as the

c and RPV3 nucleus are believed to function as the surge 
center to induce the ovulatory surge of GnRH. This particular 
subpopulation of kisspeptin neurons is activated in an estrogen-
dependent manner immediately preceding the GnRH surge, as 
detected by Fos expression, and is postulated to play a key role 
in the positive feedback effects of estradiol on GnRH release.484 
In addition, lesions in areas adjacent to the medial preoptic area, 
near the anterior commissure and septal complex, bloc

r the anterior commissure and septal complex, block the abil­
ity of estrogen to induce a surge in these species without blocking 
its negative feedback effects.485 It is also of note that kisspeptin 
expression in the RPV3 neurons is sexually diergic, with a much 
greater number of kisspeptin neurons in the female. The role of 
preoptic kisspeptin neurons in mediating the ovulatory surge in 
humans is controversial, however, as it is possible that KNDy 
neurons in the infundibular nucleus parti

at KNDy 
neurons in the infundibular nucleus participate in the positive 
feedback effects of estrogen.486 Cellular mechanisms that mediate 
the switch from negative to positive feedback of estrogen are not 
fully understood, but there is support for the concept that estro­
gen induction of various transcription factors and receptors may 
play an important role in mediating this switch.487 Alternatively, 
KNDy neurons may not consist of a homogeneous population. 
Regulation of the Ovarian Cycle

eous population. 
Cyclic activity in the ovary is controlled by an interplay between 
steroid hormones produced by the ovary and the hypothalamic-
pituitary neuroendocrine components of the reproductive axis. 
The duration of each phase of the ovarian cycle is species depen­
dent, but the general mechanisms controlling the cycle are similar 
in all species that have spontaneous ovarian cycles. In the human 
menstrual cycle, day 1 is designated as the first day of 

al cycle, day 1 is designated as the first day of menstrual 
bleeding. At this time, small and medium-sized follicles are pres­
ent in the ovaries, and only small amounts of estradiol are pro­
duced by the follicular cells. As a result, there is a low level of 
negative feedback to the hypothalamic-pituitary axis, LH pulse 
frequency is relatively fast (one pulse about every 60 minutes), and 
FSH concentrations are slightly elevated compared to much of the 
rest of the cycle (Fig. 7.31). FSH act

uch of the 
rest of the cycle (Fig. 7.31). FSH acts at the level of the ovarian fol­
licles to stimulate development and causes an increase in follicular 
estradiol production, which in turn provides increased negative 
feedback to the hypothalamic-pituitary unit.
A result of the increased negative feedback is a slowing of pul­
satile LH secretion over the course of the follicular phase to a rate 
of about one pulse every 90 minutes. However, as the growing 
follicle (or follicles, depending on 

the growing 
follicle (or follicles, depending on the species) secretes more estra­
diol, the positive feedback action of estradiol is triggered, which 
leads to an increase in GnRH release and the LH-FSH surge. 
The surge of gonadotropins acts at the fully developed follicle to 
stimulate the dissolution of the follicular wall, leading to ovula­
tion of the matured ovum into the nearby fallopian tube, where 
fertilization takes place if sperm are present. Ovulation results in a 
reorganization 

e present. Ovulation results in a 
reorganization of the cells of the follicular wall; the cells undergo 
hypertrophy and hyperplasia and start to secrete large amounts of 
progesterone and some estradiol. Progesterone and estradiol have 
a negative feedback effect at the level of the hypothalamus and 
pituitary, so the LH pulse frequency becomes very slow during the 
luteal phase of the menstrual cycle. The corpus luteum has a fixed 
life span; without additional stimulation in the form of huma

without additional stimulation in the form of human 
chorionic gonadotropin (hCG) from a developing embryo, the 
corpus luteum regresses spontaneously after ∼14 days and proges­
terone and estradiol secretions diminish. This reduces the negative 
feedback signals to the hypothalamus and pituitary and allows an 
increase in FSH and LH secretion. The fall in progesterone is also 
a withdrawal of steroid hormone support to the endometrial lin­
ing of the uterus; as a result, the endometrium is shed

f the uterus; as a result, the endometrium is shed as menses, 
and a new cycle begins.
In other species, the interplay between the neuroendocrine and 
ovarian hormones is similar, but the timing of events is differ­
ent, and other factors, such as circadian and seasonal regulatory 
factors, play a role in regulating the cycle. The rat has an ovarian 
cycle without menses (the endometrial lining is absorbed rather 
than shed) of 4 to 5 days. The rat also shows strong circadian 
rhythmicity in the

at also shows strong circadian 
rhythmicity in the timing of the LH-FSH surge, with the surge 
always occurring in the afternoon of the day of proestrus. Sheep 
are an example of a species that has a strongly seasonal pattern of 
1
13
28
20
1000
800
600
400
200
10
8
6
4
2
300
200
100
40
60
80
Estradiol
(pg/mL)
(ng/mL)
GnRH (pg/mL)
(Hypophyseal portal)
FSH (mlU/mL)
LH (mlU/mL)
Day of menstrual cycle 
FSH
LH
FSH
LH
20
10
• Fig. 7.31  Diagrammatic representation of changes in plasma le

iagrammatic representation of changes in plasma levels of 
estradiol, progesterone, luteinizing hormone (LH), and follicle-stimulating 
hormone (FSH) and portal levels of gonadotropin-releasing hormone 
(GnRH) over the human menstrual cycle.
168
ovarian cyclicity. During the breeding season, ewes have 15-day 
cycles, with a very short follicular phase and an extended luteal 
phase; during the nonbreeding season, signals relaying informa­
tion about day le

eason, signals relaying informa­
tion about day length through the visual system, pineal, and 
SCN cause a dramatic suppression of GnRH neuronal activity, 
and cyclic ovarian function is prevented by a decrease in trophic 
hormonal support from the pituitary (see “Physiologic Roles of 
Melatonin”). 
Early Development and Puberty
Neuroendocrine stimulation of the reproductive axis is initiated 
during fetal development, and in primates in midgestation, cir­
culating levels of LH and FSH reach val

tion, cir­
culating levels of LH and FSH reach values similar to those in 
castrated adults.488 Later in gestational development, gonadotro­
pin levels decline, restrained by rising levels of circulating gonadal 
steroids. The steroids that have this effect are probably placen­
tal in origin because after parturition there is a rise in circulating 
gonadotropin levels that is apparent for variable periods of the first 
year of life, depending on the species. The decline in reproductive 
hormone 

the species. The decline in reproductive 
hormone secretion in the postnatal period appears to be due to a 
decrease in GnRH stimulation of the reproductive axis because it 
occurs even in the castrated state and because gonadotropin and 
gonadal steroid secretion can be supported by the administration 
of pulses of GnRH.489
Pubertal reawakening of the reproductive axis occurs in late 
childhood and is marked initially by nighttime elevations in 
gonadotropin and gonadal steroid hormone levels.4

gonadotropin and gonadal steroid hormone levels.489,490 The 
mechanisms controlling the pubertal reawakening of the GnRH 
pulse generator have been an area of intense investigation.489,491 
Although the mechanisms are still not fully understood, signifi­
cant progress has been made in identifying central changes in 
the hypothalamus that appear to play a role in this process. At 
puberty, there is both a decrease in transsynaptic inhibition to 
the GnRH neuronal system and an increase in stimul

the GnRH neuronal system and an increase in stimulatory input 
to GnRH neurons.489 One of the major inhibitory inputs to the 
GnRH system is provided by GABAergic neurons. Studies in 
rhesus monkeys have shown that hypothalamic levels of GABA 
decrease during early puberty and that blocking GABAergic input 
before puberty, by intrahypothalamic administration of antisense 
oligodeoxynucleotides against the enzymes responsible for GABA 
synthesis, results in premature activation of the GnRH neuron

results in premature activation of the GnRH neuronal 
system.
It has been suggested on the basis of findings that a subset of 
glutamate receptors (i.e., kainate receptors) increase in the hypo­
thalamus at puberty and that the pubertal decrease in GABA tone 
may be caused by an increase in glutamatergic transmission. Fur­
ther evidence for a role for glutamate comes from studies show­
ing that administration of N-methyl-dl-aspartic acid (NMDA) 
to prepubertal rhesus monkeys can drive the reawak

to prepubertal rhesus monkeys can drive the reawakening of the 
reproductive axis.492 Increased stimulatory drive to the GnRH 
neuronal system also appears to come from increases in norepi­
nephrine and NPY at the time of puberty.489 Furthermore, as dis­
cussed earlier, there is evidence that growth factors act through 
release of prostaglandin from glial cells at puberty to play a role in 
stimulating GnRH neurons.493
Despite an increased understanding of the neural changes 
occurring at pubert

tanding of the neural changes 
occurring at puberty, the question of what signals trigger the 
pubertal awakening of the reproductive axis is unanswered at this 
time.491,494 However, the kisspeptin neuronal system described 
earlier in the section on feedback regulation has become a prime 
integrative candidate for this function, perhaps mediated by the 
activating effects of neurokinin B and substance P or increased 
tachykinin sensitivity.484,454 Also relevant to pubertal onset is 
galanin-li

454 Also relevant to pubertal onset is 
galanin-like peptide (GALP), which is expressed specifically in the 
arcuate nucleus and binds with high affinity to galanin receptors. 
GALP is a potent central stimulator of gonadotropin release and 
sexual behavior in the rat and can reverse the decreased repro­
ductive function associated with diabetes mellitus and hypoin­
sulinemia.495 Whereas GnRH neurons do not express leptin 
receptors, both kisspeptin and GALP neurons are targets of leptin 
and ar

tin and GALP neurons are targets of leptin 
and are hypothesized to be involved in the well-known modu­
lation of puberty and reproductive function by food availabil­
ity and nutritional status (see the following section). Epigenetic 
mechanisms have been proposed to coordinate the changes in 
the expression of multiple genes that accompany the initiation 
of puberty.491 
Reproductive Function and Stress
Many forms of physical stresses, such as fasting, exercise, tem­
perature stress, infection,

asting, exercise, tem­
perature stress, infection, pain, and injury, as well as psychologic 
stresses, such as being subordinate in a dominance hierarchy, 
can suppress the activity of the reproductive axis.496 If the stress 
exposure is brief, there may be acute suppression of circulating 
gonadotropins and gonadal steroid hormones; in females dis­
ruption of normal menstrual cyclicity may occur, but fertility 
is unlikely to be impaired.496 In contrast, prolonged periods 
of significant stress

contrast, prolonged periods 
of significant stress exposure can lead to complete impairment 
of reproductive function, also characterized by low circulating 
levels of gonadotropins and gonadal steroids. Stress appears 
to decrease the activity of the reproductive axis by decreasing 
GnRH drive to the pituitary because in all cases in which it has 
been examined, administration of exogenous GnRH can reverse 
the effects of the stress-induced decline in reproductive hormone 
secretion.
In the cas

ine in reproductive hormone 
In the case of foot shock stress in rats497 or immune stress (i.e., 
injection of IL-1α) in primates,498 the suppression of gonadotro­
pin secretion that occurs was shown to be reversible by admin­
istration of a CRH antagonist, implying that endogenous CRH 
secretion mediates the effects of these stresses on GnRH neurons. 
In other studies, naloxone, a μ-opioid receptor antagonist, was 
shown to be capable of reversing restraint stress–induced suppres­
si

 of reversing restraint stress–induced suppres­
sion of gonadotropin secretion in monkeys; however, naloxone is 
ineffective in reversing the suppression of gonadotropin secretion 
that occurs during insulin-induced hypoglycemia.499,500 In the 
case of metabolic stresses, multiple regulators appear to mediate 
changes in the neural drive to the reproductive axis.
Various metabolic fuels, including glucose and fatty acids, 
can regulate the function of the reproductive axis, and block­
ing cellul

on of the reproductive axis, and block­
ing cellular utilization of these fuels can lead to suppression of 
gonadotropin secretion and decreased gonadal activity. Leptin, 
a hormone produced by fat cells, can also modulate the activity 
of the reproductive axis. Mutant ob/ob mice that are deficient in 
leptin are infertile, but fertility can be restored by administration 
of leptin.501 Moreover, leptin administration has been shown to 
reverse the suppressive effects of undernutrition on the rep

e suppressive effects of undernutrition on the repro­
ductive axis in some situations.502 Leptin receptors are found in 
several populations that are known to have a strong influence on 
the reproductive axis, particularly kisspeptin neurons.484 Fur­
thermore, treatment with a neurokinin B agonist and dynorphin 
antagonist can recover suppressed GnRH and LH secretion.503
In summary, it appears that a number of neural circuits can 
mediate the effects of stress on the GnRH neuronal system and 
th

ects of stress on the GnRH neuronal system and 
that the neural systems involved are at least somewhat specific to 
the type of stress that is experienced. 
169
Thermoregulation and Sleep-Wake Cycle
In addition to regulation of the anterior and posterior pituitary, 
the hypothalamus orchestrates a number of functions that are crit­
ical for normal homeostasis, including the maintenance of nor­
mal body temperature and its resetting under certain physiologic 
conditi

d its resetting under certain physiologic 
conditions and the regulation of sleep and wakefulness. Appetite 
regulation is discussed in Chapter 39.
Preoptic Area Is the Primary Hypothalamic 
Thermoregulatory Center
It has been long recognized that the brain plays an important role 
in the regulation of body temperature mediated by neurons that 
respond to heat, commonly referred to as warm-sensitive neurons, 
and neurons that respond to cold, referred to as cold-sensitive neu­
rons,504 with the 

erred to as cold-sensitive neu­
rons,504 with the preoptic area (POA) being the most important 
thermoregulatory locus (see Siemens and Kamm505 for compre­
hensive review). Neurons in this region serve as a major integrat­
ing center that functions to maintain a constant body temperature 
by receiving information from surface thermoreceptors in the 
skin, spinal cord, and deep structures in the abdominal cavity that 
are relayed to the POA through the lateral parabrachial nucleus 
by multisynapt

h the lateral parabrachial nucleus 
by multisynaptic pathways via the dorsal horn of the spinal cord 
or splanchnic and vagal afferents.504 In addition, some neurons 
in the POA have the intrinsic ability to sense changes in local 
brain temperature. Preoptic warming reduces heat production and 
increases heat-loss responses by vasodilation, sweating, increased 
respiration, inhibition of UCP-1 in brown adipose tissue (BAT), 
and specific behavioral responses such as moving to a cooler area; 
co

ral responses such as moving to a cooler area; 
conversely, preoptic cooling increases heat production and reduces 
heat losses by inducing vasoconstriction, shivering, activation of 
UCP-1 in brown adipose tissue, increasing intermediary metab­
olism in muscle and other parenchymal organs, and by specific 
behavioral responses such as increasing locomotor activity and 
food intake.506 As the number of warm-sensitive neurons is sig­
nificantly greater than the number of cold-sensitive neurons in

eater than the number of cold-sensitive neurons in the 
POA, lesioning this region of the hypothalamus typically results 
in hyperthermia.507 
Mechanisms for Thermoreception
Within the periphery, primary sensory axon terminals express the 
TRP channel, transient receptor potential vanilloid 1 (TRPV1), 
which is activated by heat, and transient receptor potential cat­
ion channel subfamily M member 8 (TRPM8), which is activated 
by cold.508 Animals with TRPM8 deficiency have impairment 
in mainta

s with TRPM8 deficiency have impairment 
in maintaining normal body temperature when placed in a cold 
environment.509 However, these animals can still thermoregulate 
when placed in extreme cold, indicating that other thermorecep­
tors and/or other mechanisms for temperature detection are also 
involved.
Within the POA, warm-sensitive neurons in the MePO and 
lateral preoptic nucleus (LPO) express transient receptor potential 
melastatin type 2 ion channels (TRPM2) that when selectively 
activa

ion channels (TRPM2) that when selectively 
activated or inhibited results in profound hypothermia or hyper­
thermia, respectively.510 In addition, transgenic animals deficient 
in TRPM2 have an exaggerated fever response. However, not all 
neurons in the POA involved in temperature regulation are intrin­
sically temperature sensitive. A subpopulation of neurons resid­
ing in the anterior ventromedial preoptic nucleus (VMPO), for 
example, respond to warming but do not express TRPM2 and 
therefo

d to warming but do not express TRPM2 and 
therefore have been referred to as “warm-activated neurons.”511 
These neurons receive inputs from skin receptors and are largely 
GABAergic but also coexpress the peptides BDNF and PACAP. 
When selectively activated using molecular techniques such as 
optogenetics, there is a decline in body temperature as a result of 
vasodilation and inhibition of heat generation from brown adipose 
tissue, as well as behavioral changes manifested by a preference for

 behavioral changes manifested by a preference for 
a colder environment.511 Thus these warm-activated VMPO are 
capable of coupling three major heat-dissipating responses. Other 
subdivisions of the POA similarly trigger hypothermic responses 
when activated and receive thermal information from skin ther­
moreceptors, including the median preoptic nucleus (MnPO) and 
ventrolateral preoptic nucleus (VLPO).512,513 Thus the POA is the 
primary locus for integrating a variety of thermoregulatory in

s for integrating a variety of thermoregulatory infor­
mation, and through outputs to regions of the brain involved in 
regulation of the autonomic nervous system, behavior and other 
neuroendocrine responses orchestrate a series of responses to 
maintain normal core body temperature. 
Central Circuitries Mediating Thermoregulation
Downstream targets of thermosensitive POA neurons are primar­
ily components of the sympathetic nervous system, of which the 
dorsomedial hypothalamus (DMH) in the hy

hich the 
dorsomedial hypothalamus (DMH) in the hypothalamus, sym­
pathetic premotor neurons in the rostral ventromedial medulla 
that include the rostral raphe pallidus (rRPa), and preganglionic 
autonomic neurons of the intermediolateral cell column in the 
spinal cord, are particularly important (Fig. 7.32).514
On the basis of numerous anatomic and physiologic stud­
ies and more recent molecular approaches (see recent reviews 
by Siemens and Kamm505 and Tan and Knight515), it is now 
believed

amm505 and Tan and Knight515), it is now 
believed that several regions in the POA are involved in relay­
ing thermal information from peripheral thermoreceptors to 
the autonomic nervous system that may work in synchrony, and 
that no one discrete population of neurons in the POA mediates 
any single thermodefensive response (autonomic or behavioral). 
The downstream circuitry involved was initially recognized for 
the MnPO, which in response to skin cooling leads to a number 
of thermoregulato

 skin cooling leads to a number 
of thermoregulatory responses that increase body temperature, 
such as shivering, heat generation from BAT, and vasoconstric­
tion. It is proposed that the MnPO releases GABA to inhibit 
MePO neurons, as antagonizing GABAA receptors in the MePO 
block cooling-induced shivering.516 These neurons have down­
stream projections to the DMH and rRPA, and the DMN also 
extensively innervates the rRPA. If either the DMN or rRPa is 
lesioned, the shivering response is blo

r rRPa is 
lesioned, the shivering response is blocked. In addition, selective 
activation of DMN thermosensitive neurons using DREADDS 
technology increases in temperature, along with an increase in 
energy expenditure and physical activity.512 The rRPA then proj­
ects to α-motor neurons in the ventral horn of the spinal cord to 
mediate the shivering response but also to preganglionic neurons 
in the intermediolateral column of the spinal cord to regulate 
BAT thermogenesis, sweating, and vaso

to regulate 
BAT thermogenesis, sweating, and vasomotor responses.517 The 
MnPO also regulates cutaneous blood flow,518 although it may 
do so by direct projections to the rRPa, bypassing the DMH, 
as cooling-induced cutaneous vasoconstriction is not affected by 
inhibiting DMH neurons.519 The midbrain periaqueductal gray 
(PAG) may also be involved in the circuitry before proceeding 
to sympathetic preganglionic neurons in the spinal cord as PAG 
neurons show strong c-Fos induction following un

neurons show strong c-Fos induction following unilateral pre­
optic region heating520 and induce cutaneous vasodilation when 
stimulated.521
170
Using a technique of molecular profiling of neurons in the 
POA activated by heat, a population of GABAergic neurons in 
the VMPO that coproduce PACAP and BDNF was identified, 
and when activated by optogenetics leads to a rapid decline in 
core body temperature that persists as long as the stimulation con­
tin

 that persists as long as the stimulation con­
tinues.511 Mechanisms for the heat loss included vasodilation and 
suppression of heat production from BAT, as well as behavioral 
responses because the animals move to colder temperatures. Map­
ping of these neurons not only demonstrated a direct projection 
to the DMH, but also the PVN, ventrolateral PAG, and structures 
associated with behavior (including BNST, paraventricular thala­
mus, and medial habenula), indicating that the VMPO mediates 
d

al habenula), indicating that the VMPO mediates 
diverse autonomic and behavioral responses to heat.
Similarly, activation of GABAergic neurons in VLPO triggers 
a rapid reduction in temperature associated with a decrease in 
physical activity; conversely, inhibition raises core temperature.512 
These neurons also project to the DMH, and stimulating VLPO 
terminals locally in the DMN using optogenetics is similarly effec­
tive in reducing temperature by inhibiting DMH neuronal activ­
ity. Becaus

ture by inhibiting DMH neuronal activ­
ity. Because the transsynaptic retrograde marker, pseudorabies 
virus, accumulates in the VLPO following injection into BAT,522 
the VLPO is also presumed to contribute to BAT thermogenesis. 
Modulators of Thermoregulatory Responses
In addition to information relayed to the POA from thermosen­
sory afferent pathways, body temperature can also be affected by 
peptide and hormonal mediators as well through circuitries within 
the CNS that serve to integrate t

cuitries within 
the CNS that serve to integrate thermoregulation with other 
homeostatic functions. Perhaps one of best-studied examples is 
the effect of thyroid hormone on temperature homeostasis.
Thyroid Hormone
Thyroid hormone is essential to sustain the effect of sympa­
thetic activation of BAT, and in its absence, norepinephrine fails 
to increase BAT temperature, resulting in cold intolerance and 
hypothermia commonly observed in hypothyroid individuals.523 
Norepinephrine increases type

oid individuals.523 
Norepinephrine increases type 2 iodothyronine deiodinase (D2) 
in BAT, which by converting T4 to T3 results in the upregulation 
of UCP-1 in mitochondria, allowing conversion of stored energy 
into heat. Thyroid hormone is similarly important for a maximal 
thermogenic response by skeletal muscle in which activation of 
the sympathetic nervous system increases D2, leading to the acti­
vation of UCP-3.524 T3 also has a direct, central action on neurons 
in the hypothalamic ve

 central action on neurons 
in the hypothalamic ventromedial nucleus, which, by decreasing 
Brainstem
thermoreceptors
POA
LPBN
DMH
rRPA
DH
IML
VH
Muscle
BAT, blood vessels
• Fig. 7.32  Simplified diagram of major loci and pathways involved in thermoregulatory responses. BAT, 
brown adipose tissue; DH, dorsal horn; DMH, dorsomedial hypothalamus; IML, intermediolateral cell col­
umn; LPBN, lateral parabrachial nucleus; POA, preoptic area; rRPa, rostral raphe pallidus; VH, v

preoptic area; rRPa, rostral raphe pallidus; VH, ventral 
horn. (Redrawn and modified from Roth J, Blatteis CM. Mechanisms of fever production and lysis: lessons 
from experimental LPS fever. Compr Physiol. 2014;4:1563–1604.)
171
the phosphorylation of AMP-activated kinase (AMPK), leads to 
increased sympathetic activity of nerves innervating BAT.525 T3 
may also modulate feeding-related neurons in the hypothalamic 
arcuate nucleus that produce NPY and AgRP and cont

arcuate nucleus that produce NPY and AgRP and contribute to 
the effect of these peptides on BAT activity. 
Estrogen and Progesterone
The set-point for temperature regulation is sensitive to circulat­
ing levels of sex steroids, with core body temperature falling just 
prior to the midcycle surge in women and rising during the luteal 
phase.526 Estrogen is responsible for the fall in temperature in the 
late follicular phase by activating warm-sensitive neurons in the 
POA,527 whereas progestero

nsitive neurons in the 
POA,527 whereas progesterone increases temperature in the luteal 
phase by decreasing the firing rate of these neurons and, perhaps, 
increasing the firing rate of cold-sensitive neurons.528 Thermo­
regulatory effects of estrogen may also be mediated by KNDy 
neurons in the hypothalamic arcuate nucleus as further elaborated 
in “Neurokinin B.” Estrogen and progesterone exert effects on 
peripheral blood vessels, with estrogen promoting vasodilation 
(and thereby heat diss

gen promoting vasodilation 
(and thereby heat dissipation) by activating nitric oxide synthe­
tase (eNOS) in endothelial cells to produce nitric oxide,529 and 
progesterone promoting vasoconstriction by enhancing adrener­
gic control through a prostaglandin-mediated mechanism.530 It 
remains unclear as to the physiologic importance of these changes 
in body temperature over the menstrual cycle, although it has 
been proposed the increase in temperature during the luteal phase 
may facilitate imp

rature during the luteal phase 
may facilitate implantation.531 
Neurokinin B
Vasomotor symptoms, commonly referred to as hot flashes, are a 
frequent occurrence in menopausal women, can persist for many 
years, and disrupt sleep and daily activities. They are characterized 
by the sudden feeling of heat, leading to vasodilation and sweat­
ing as measures to increase heat loss. While the underlying etiol­
ogy has long been known to be related to estrogen deficiency, the 
thermoregulatory mechani

estrogen deficiency, the 
thermoregulatory mechanisms involved have only recently been 
elucidated with the discovery of kisspeptin and its role in regula­
tion of GnRH (reviewed by Prague and Dhillo532). As mentioned 
previously (see “Gonadotropin-Releasing Hormone and Control 
of the Reproductive Axis”), kisspeptin neurons in the arcuate or 
infundibular nucleus coexpress neurokinin B and in some ani­
mal species, dynorphin, thereby coined KNDy neurons. During 
menopause, KNDy neurons hypertro

 neurons. During 
menopause, KNDy neurons hypertrophy and show increased neu­
ronal excitability, presumably due to feedback effects of estrogen 
deficiency as these changes can be reversed by estrogen administra­
tion.481,533 In addition, if KNDy neurons are selectively ablated, 
altered thermoregulatory responses are observed, suggesting that 
KNDY neurons are involved in the mechanism of heat dissipa­
tion.534 Particularly noteworthy is that a population of thermosen­
sitive neurons in the PO

 population of thermosen­
sitive neurons in the POA receive inputs from KNDy neurons535 
and also express the neurokinin 3 receptor (NK3R).536 Thus, when 
a NK3R agonist is administered into the POA of experimental 
animals or neurokinin B is administered peripherally to human 
subjects, heat dissipation responses are observed.537,538 Con­
versely, administration of an oral NK3R antagonist (MLE4901) 
to postmenopausal women reduces the frequency and severity 
of hot flashes.539,540 As surmised b

and severity 
of hot flashes.539,540 As surmised by Prague and Dhillo,532 the 
improvement in postmenopausal vasomotor symptoms observed 
with NK3R antagonists may provide an alternative to the use of 
estrogens, particularly in women with a history of or at risk of 
developing breast cancer, and for symptomatic men who are being 
treated for prostate cancer with androgen deprivation therapy. 
Orexin-A
Orexin-A (sometimes referred to as hypocretin 1) derives exclu­
sively from neurons in the lat

n 1) derives exclu­
sively from neurons in the lateral hypothalamic area, but it is con­
tained in axons that are widely distributed throughout the central 
nervous system, including projections to thermoregulatory centers 
of the sympathetic nervous system such as the periaqueductal gray, 
rRPA, and intermediolateral cell column of the spinal cord.541–
543 While best known for its effects on arousal (see “Sleep-Wake 
Cycle”) and its deficiency is etiologic for narcolepsy,544 the peptide 
also r

s etiologic for narcolepsy,544 the peptide 
also regulates body temperature. Injection of orexin-A into the 
CSF increases sympathetic activity, including increased c-Fos in 
the rRPA,545 and is associated with a rise in body temperature 
mediated by BAT thermogenesis.546 The importance of this pep­
tide (and/or associated co-neurotransmitters) in the maintenance 
of thermoregulatory homeostasis is indicated by a blunted hyper­
thermic response to cold exposure in mice with ablation of orexin 
n

o cold exposure in mice with ablation of orexin 
neurons547 and may explain dysregulation of thermogenesis in 
individuals with narcolepsy.548 
Under certain circumstances, there is an adaptive advantage to 
elevate body temperature beyond the normal physiologic range 
that is highly conserved among animal species. Such is the situa­
tion during infection, when fever is a necessary response to facil­
itate recovery by improving the efficiency of immune cells and 
impairing replica

 efficiency of immune cells and 
impairing replication of microorganisms.549 A variety of cyto­
kines, including IL-1, IL-6, and TNFα, are secreted in response 
to pathogens, resulting in the production of PGE2 by the brain 
vasculature. GABAergic neurons in the MnPO and MePO 
express the prostaglandin EP3 receptor, which when bound 
by PGE2 alters the thermoregulatory set-point and induces 
fever.550,551 EP3 receptor POA neurons heavily project to DMN 
and rRPA,552,553 disinhibiting these sympa

to DMN 
and rRPA,552,553 disinhibiting these sympathoexcitatory neurons 
to provide excitatory drive for sympathetic outflow to BAT and 
to promote shivering through rRPa projections to ventral horn 
motor cells.554
In addition to inducing fever, endotoxin simultaneously acti­
vates an endogenous, counterregulatory, antipyretic response to 
prevent body temperature from rising too severely. This is largely 
achieved by stimulating the hypothalamic-pituitary-adrenal axis 
that exerts a dampening 

c-pituitary-adrenal axis 
that exerts a dampening effect on the cytokine response, but more 
specifically by the direct antipyretic actions of α-MSH within the 
CNS.555 As the VMPO is heavily innervated by axon terminals 
containing α-MSH, a direct effect on POA thermosensitive neu­
rons is suggested.556 Alpha-MSH is also contained in axons that 
heavily innervate autonomic regulatory neurons in the parvocel­
lular PVN and the hypothalamic DMN, providing an alternative 
route for regulatory cont

roviding an alternative 
route for regulatory control over vasomotor responses and heat 
generation. 
Opioids and Endocannabinoids
In addition to prostaglandins, there is evidence that opioids and 
endocannabinoids participate in febrile responses induced by 
pyrogens. Opioid receptors have been identified in the POA,557 
and activation of μ-opioid receptors causes hyperthermia.558 In 
addition, fever induced by endotoxin administration of TNFα but 
not IL-1β can be reduced by a μ-opioid antagon

ut 
not IL-1β can be reduced by a μ-opioid antagonist, indicating that 
the mechanism for the febrile response can differ among the vari­
ous cytokines.559 Similarly, endocannabinoids can also contribute 
to the febrile response as central administration of the CB1 recep­
tor antagonist, AM251, reduces fever induced by TNFα, IL1β, 
and IL6.560 
172
Sleep-Wake Cycle
Sleep is a natural state of altered consciousness that is divided into 
two distinct states

iousness that is divided into 
two distinct states: non–rapid eye movement (NREM) sleep char­
acterized by the deepest sleep in stage 3 (slow-wave sleep) but with 
variable states of arousal, and rapid eye movement (REM) sleep, 
during which dreams are common and accompanied by autonomic 
activity. The neural organization of the sleep-wake cycle relies on 
multiple regions in the brain that produce either sleep or arousal and 
includes the hypothalamus. The primary sleep centers are located in 

alamus. The primary sleep centers are located in 
the VLPO and MnPO of the hypothalamus and are composed of 
neurons that produce GABA and galanin.561 Melanin-concentrating 
hormone (MCH) and GHRH neurons in the hypothalamus, soma­
tostatin neurons in the basal forebrain, nNOS neurons in the cor­
tex, and GABAergic neurons in the parafacial zone of the brainstem 
also contribute to sleep promotion.562 Wakefulness centers are more 
numerous and include glutaminergic neurons in the parabrachial 
n

clude glutaminergic neurons in the parabrachial 
nucleus and pedunculopontine tegmental nucleus of the brainstem 
and GABAergic, cholinergic, and glutaminergic neurons in the basal 
forebrain.562 Other nuclei involved in wakefulness include norad­
renergic neurons in the locus coeruleus, serotonergic neurons in the 
dorsal raphe, dopaminergic neurons in the ventral periaqueductal 
gray, cholinergic cell groups in the pontine, and mesencephalic retic­
ular formation, including the laterodorsal an

tic­
ular formation, including the laterodorsal and pedunculopontine 
tegmentum, histaminergic neurons in the TMN, orexin (hypocre­
tin)/glutamatergic and possibly GABAergic neurons in the lateral 
hypothalamic area, and neurons in the prefrontal cortex.563 The 
reason for the need of such a large number of wake-promoting cell 
groups remains uncertain; perhaps they serve a redundant purpose, 
as with the exception of the parabrachial nucleus and pedunculo­
pontine tegmentum, lesions of the othe

 pedunculo­
pontine tegmentum, lesions of the other cell groups result in only 
partial loss of wake time (see Saper and colleagues563 for review). 
Central Circuitries Mediating the Sleep-Wake 
Cycle
The central pathways mediating the sleep-wake cycle are com­
plex due to numerous interconnections among many regions of 
the brain, illustrated in Fig. 7.33 and reviewed in detail by Saper 
and colleagues,563 Saper and Fuller,562 Schwartz and Kilduff,564 
and Scammell and colleagues.565 To briefly

ff,564 
and Scammell and colleagues.565 To briefly summarize salient 
points with particular reference to the hypothalamus, wake-
promoting pathways ascend from the brainstem into two major 
divisions, a dorsal pathway that enters the thalamus and a ven­
tral pathway that innervates the hypothalamus, basal forebrain, 
and cortex. Both pathways, however, are required for normal 
wakefulness.566,567 Within the hypothalamus, TMN histamin­
ergic neurons excite neurons in the cortex and thalamus and 

ons excite neurons in the cortex and thalamus and 
suppress VPLO activity. Orexin neurons excite the TMN but 
also project to the locus coeruleus and other arousal centers in 
the CNS.568 NREM sleep is driven by the VLPO and MnPO, 
primarily by inhibiting wakefulness centers in the TMN, locus 
coeruleus, and dorsal raphe, and, if the VLPO is lesioned, there 
is profound reduction in NREM sleep.569 Sleep-promoting 
neurons in the basal forebrain inhibit wake-promoting neurons 
in that same nucleu

nhibit wake-promoting neurons 
in that same nucleus and directly inhibit cortical neurons.565,570 
The hypothalamic VLPO also contributes to REM sleep by 
innervating brainstem regions that promote REM sleep, such 
as the dorsal raphe and locus coeruleus. Hypothalamic neurons 
that synthesize MCH project to the subcoeruleus in the pons, 
producing muscle atonia that is characteristic of REM sleep,571 
and orexin neurons, although part of the wake-promoting sys­
tem also appears to be involved in

-promoting sys­
tem also appears to be involved in REM sleep as inhibiting 
these neurons increases REM sleep, presumably through brain­
stem projections.572 
TMN
VLPO
Cortex/Thalamus
Orexin neurons
SCN
Basal forebrain
Brainstem
wake-activation
centers
• Fig. 7.33  Simplified diagram showing the connectivity of major regions of the brain involved in the sleep-
wake cycle. SCN, suprachiasmatic nucleus; TMN, tuberomammillary nucleus; VLPO, ventrolateral preop­
tic nucleus.
173
CHAPTER 7  Neuroendo

teral preop­
173
Circadian Regulation of Sleep-Wake Cycle
Sleep is coupled to periods of wakefulness, giving rise to a circa­
dian sleep-wake cycle whose timing is orchestrated to a 24-hour 
day-night cycle by photic stimuli transmitted to the hypothalamic 
SCN. As alluded to previously (see “Pineal Gland”), melatonin 
has an essential role in relaying this information to the SCN, ulti­
mately attenuating the wake-promoting effects of the circadian 
clo

g the wake-promoting effects of the circadian 
clock. Thus sleep disorders are commonly observed in the blind 
due to the inability to synchronize with the day-night cycle (see 
Zisapel573 for review), and ablation of the SCN or transgenic mice 
lacking various clock genes shows alterations in sleep time and 
sleep fragmentation.574,575 The SCN is hardwired into the sleep-
wake system, although indirectly. The main output of the SCN 
is to the subparaventricular zone of the hypothalamus that sen

bparaventricular zone of the hypothalamus that sends 
axons to the dorsomedial nucleus of the hypothalamus. The dor­
somedial nucleus then projects to the VLPO to inhibit its activity 
and to orexin neurons in the lateral hypothalamus to activate its 
activity, hence promoting wakefulness.576 
Somnogens
Somnogens are sleep-inducing substances and include adenosine, 
prostaglandin D2, and cytokines, in particular IB-1β and TNF-α. 
Adenosine released from astrocytes is probably the most impor­
tan

ed from astrocytes is probably the most impor­
tant somnogen and tends to increase during prolonged periods of 
wakefulness.577,578 Prostaglandin D2 is produced by the leptomen­
inges, and cytokines can either enter the CNS from the periph­
eral circulation or arise within the brain itself.579 However, the 
somnogenic effect of cytokines may be mediated by prostaglandin 
D2 as the effects of IL-1β on sleep can be suppressed by cyclo-
oxygenase inhibitors, and the somnogenic effect of prostagland

hibitors, and the somnogenic effect of prostaglandin 
D2 can be mediated by an increase in adenosine.580 If adenosine 
A2A receptors are silenced, the somnogenic effect of prostaglandin 
D2 is attenuated.581 Adenosine A2A receptors promote NREM 
sleep by inhibiting histaminergic neurons in the TMN via activa­
tion of the VLPO.582 This mechanism may also partly explain the 
arousing effects of caffeine, an adenosine antagonist that loses its 
wake-promoting effects in A2A knockout mice.583
As all

-promoting effects in A2A knockout mice.583
As alluded to previously, GHRH has somnogenic effects (see 
“Growth Hormone–Releasing Hormone,” then “Extrapituitary 
Functions”), and when administered ICV to experimental animals 
it stimulates NREM sleep that can be antagonized by somatosta­
tin analogues.584 The action of GHRH appears to be indepen­
dent of GH and involves the actions of GHRH on the VLPO and 
MnPO as GABAergic neurons in these regions show increased 
c-Fos expression in response to

ns show increased 
c-Fos expression in response to GHRH. Evidence that GHRH 
also exerts somnogenic effects in humans is suggested by the well-
recognized phenomenon of increased release of GH during sleep 
and individual reports demonstrating a reduction in sleep quality 
in individuals with GHRH resistance.585 
Modifying Factors Influencing Sleep
The brain is also equipped to override the normal mechanisms 
involved in the sleep-wake cycle under situations when remaining 
awake is important fo

r situations when remaining 
awake is important for survival. Such a situation is food depriva­
tion associated with increased wakefulness and locomotor activ­
ity. Orexin-producing neurons in the lateral hypothalamus have a 
major role in this response as transgenic animals in which orexin 
neurons have been ablated fail to respond to increased wakeful­
ness with fasting.586 Orexin neurons serve as an interface between 
wakefulness and energy homeostasis, maintaining arousal during 
periods of 

meostasis, maintaining arousal during 
periods of food unavailability. They are inhibited by glucose and 
leptin and activated by ghrelin, humeral signals that promote 
increased food intake and reduced energy expenditure (see Chap­
ter 39). Orexin neurons project widely throughout the central 
nervous system and excite wakefulness centers (including hista­
minergic neurons in the TMN, monoaminergic neurons in the 
brainstem, and cholinergic neurons in the basal forebrain) and 
inhibit the VLPO.

ons in the basal forebrain) and 
inhibit the VLPO.587
A variety of other factors can contribute to sleep disruption, 
including psychiatric disorders, psychosocial problems, alcohol­
ism, pain, and chronic medical conditions such as diabetes mel­
litus, chronic renal failure, and Alzheimer disease, among many 
others (see Medic and colleagues588 for review). However, the 
pathophysiology is likely multifactorial and not well established 
for most. Activation of the hypothalamic-pituitary-adrenal

. Activation of the hypothalamic-pituitary-adrenal axis is 
likely involved for some conditions such as major depression589 
and due to elevations in cortisol, as similar sleep alterations are 
observed in individuals with Cushing syndrome.590 However, 
CRH-induced activation of wakefulness centers, including orexin 
neurons in the lateral hypothalamus and norepinephrine neurons 
in the locus coeruleus, may also contribute to sleep disruption.591 
Stressed animals also show c-Fos activation in a

Stressed animals also show c-Fos activation in a variety of regions 
of the brain that may contribute to maintaining a waking state, 
including regions of the amygdala and infralimbic cortex.592 
Neuroendocrine Disease
Disease of the hypothalamus can cause pituitary dysfunction, neu­
ropsychiatric and behavioral disorders, and disturbances of auto­
nomic and metabolic regulation. In the diagnosis and treatment 
of suspected hypothalamic or pituitary disease, four issues must 
be considered: th

itary disease, four issues must 
be considered: the extent of the lesion, the physiologic impact, the 
specific cause, and the psychosocial setting. The cause of hypotha­
lamic neuroendocrine disorders categorized by age and syndrome 
is summarized in Tables 7.6 and 7.7.
Manifestations of pituitary insufficiency secondary to hypotha­
lamic or pituitary stalk damage are not identical to those of pri­
mary pituitary insufficiency. Hypothalamic injury causes decreased 
secretion of most pituitary h

ry causes decreased 
secretion of most pituitary hormones but can cause hypersecretion 
of hormones normally under inhibitory control by the hypothala­
mus, as in hypersecretion of PRL after damage to the pituitary 
stalk and precocious puberty caused by loss of the normal restraint 
over gonadotropin maturation. Impairment of inhibitory control 
of the neurohypophysis can lead to the syndrome of inappropri­
ate vasopressin (antidiuretic hormone) secretion (SIADH) (see 
Chapter 10). More subtle 

 secretion (SIADH) (see 
Chapter 10). More subtle abnormalities in secretion can result 
from impairment of the control system. For example, loss of the 
normal circadian rhythm of ACTH secretion may occur before 
loss of pituitary-adrenal secretory reserve, and responses to physi­
ologic stimuli may be paradoxic. Because hypophysiotropic hor­
mone levels cannot be measured directly and pituitary hormone 
secretion is regulated by complex, multilayered controls, assay of 
pituitary hormones in b

ayered controls, assay of 
pituitary hormones in blood does not necessarily give a meaning­
ful picture of events at hypothalamic and higher levels. Rarely, 
tumors secrete excessive amounts of releasing peptides and cause 
hypersecretion of hormones from the pituitary.
Disorders of the hypothalamic-pituitary unit can result from 
lesions at several levels. Defects can arise from destruction of the 
pituitary (as by tumor, infarct, inflammation, or autoimmune dis­
ease) or from a hereditary defi

or autoimmune dis­
ease) or from a hereditary deficiency as with PROP1 mutations 
associated with GH, prolactin, and TSH deficiencies. Selective 
174
loss of thyroid hormone receptors or GnRH receptors in the 
pituitary can give rise to increased TSH secretion and thyrotoxi­
cosis or hypogonadotropic hypogonadism, respectively. Further­
more, disorders can arise through disruption of the contact zone 
between the stalk and the median eminence, the stalk i

een the stalk and the median eminence, the stalk itself, or the 
nerve terminals of the tuberoinfundibular system; such disrup­
tion occurs after surgical stalk section, with tumors involving the 
stalk, including metastatic disease, and in some inflammatory dis­
orders such as sarcoidosis or neurohypophysitis. At a higher level, 
tonic inhibitory and excitatory inputs can be lost as manifested by 
absence of circadian rhythms or the development of precocious 
puberty. Physical stress, cytokine 

of precocious 
puberty. Physical stress, cytokine products of inflammatory cells, 
toxins, and reflex inputs from peripheral homeostatic monitors 
also impinge on the tuberoinfundibular system. At the highest 
  Etiology of Hypothalamic Disease by Age
Premature Infants and Neonates
Intraventricular hemorrhage
Meningitis: bacterial
Tumors: glioma, hemangioma
Trauma
Hydrocephalus, kernicterus
Age 1 Month to 2 Years
Tumors: optic glioma, histiocytosis X, hemangioma
Hydrocephalus
Meningitis
Congeni

sis X, hemangioma
Congenital disorders: Kallmann syndrome, Laurence-Moon-Biedl syn­
drome, Prader-Willi syndrome
Age 2 to 10 Years
Tumors: craniopharyngioma, glioma, dysgerminoma, hamartoma, leuke­
mia, histiocytosis X, ganglioneuroma, ependymoma, medulloblastoma
Meningitis: bacterial, tuberculous
Encephalitis: viral (exanthematous demyelinating)
Congenital diabetes insipidus
Radiation therapy
Diabetic ketoacidosis
Moyamoya disease, circle of Willis
Age 10 to 25 Years
Tu

ya disease, circle of Willis
Tumors: craniopharyngioma, glioma, hamartoma, dysgerminoma, histio­
cytosis X, leukemia, dermoid, lipoma, neuroblastoma
Trauma
Vascular: aneurysm, subarachnoid hemorrhage, arteriovenous malfor­
mation
Inflammatory disease: meningitis, encephalitis, sarcoidosis, tuberculosis
Structural brain defects: chronic hydrocephalus, increased intracranial 
pressure
Age 25 to 50 Years
Nutritional: Wernicke disease
Tumors: glioma, lymphoma, meningioma, cranioph

ase
Tumors: glioma, lymphoma, meningioma, craniopharyngioma, pituitary 
tumors, plasmacytoma, angioma, colloid cysts, sarcoma, ependy­
moma, histiocytosis X
Inflammatory diseases: sarcoidosis, tuberculosis, viral encephalitis
mation
Damage from pituitary radiation therapy
Age 50 Years and Older
Tumors: pituitary tumors, sarcoma, glioblastoma, ependymoma, menin­
gioma, colloid cysts, lymphoma
Vascular

ma, menin­
Vascular disease: infarct, subarachnoid hemorrhage, pituitary apoplexy
Inflammatory disease: encephalitis, sarcoidosis, meningitis
Damage from radiation therapy for ear-nose-throat carcinoma, pituitary 
tumors
Adapted from Plum F, Van Uitert R. Nonendocrine diseases and disorders of the hypothala­
mus. In: Reichlin S, Baldessarini RJ, Martin JB, eds. The Hypothalamus, vol 56. New York: 
Raven Press; 1978:415–473.
   
TABLE 7.6
  Etiology of Endocrine Sy

415–473.
   
TABLE 7.6
  Etiology of Endocrine Syndromes of 
Hypothalamic Origin
Hypophysiotropic Hormone Deficiency
Surgical pituitary stalk section
Inflammatory disease: basilar meningitis and granuloma, sarcoidosis, 
tuberculosis, sphenoid osteomyelitis, eosinophilic granuloma
Craniopharyngioma
Hypothalamic tumor: infundibuloma, teratoma (ectopic pinealoma), 
astrocytoma
Maternal deprivation syndrome, psychosocial dwarfism
Isolated GHRH deficiency
Hypothalamic hypothyroidism
Panhypophysiotro

iency
Panhypophysiotropic failure
Disorders of Regulation of GnRH Secretion
Female
Precocious puberty: GnRH-secreting hamartoma, hCG-secreting germi­
noma
Delayed puberty
Neurogenic amenorrhea
Pseudocyesis
Functional amenorrhea and oligomenorrhea
Drug-induced amenorrhea
Kallman syndrome
GPR54 (KISS1R) mutations
Male
Precocious puberty
Fröhlich syndrome
Drug-induced hypogonadism
Kallmann syndrome
GPR54 (KISSR1) mutations
Disorders of R

n syndrome
Disorders of Regulation of Prolactin-Regulating Factors
Tumor
Sarcoidosis
Drug induced
Reflex
Herpes zoster of chest wall
Post-thoracotomy
Nipple manipulation
Spinal cord tumor
Psychogenic
Carbon dioxide narcosis
Disorders of Regulation of CRH
Paroxysmal corticotropin discharge (Wolff syndrome)
Loss of circadian variation
Depression
CRH-secreting gangliocytoma
CRH, Corticotropin-releasing hormone; hCG, human chorionic gonadotropin; GHRH, growth 

; hCG, human chorionic gonadotropin; GHRH, growth 
hormone–releasing hormone; GnRH, gonadotropin-releasing hormone.
   
TABLE 7.7
175
level of control, emotional stress and psychologic disorders can 
activate the pituitary-adrenal stress response and suppress gonado­
tropin secretion (e.g., psychogenic amenorrhea) or inhibit GH 
secretion (e.g., psychosocial dwarfism) (see Chapter 25). Intrin­
sic disease of the anterior pituitary is reviewed in Chapters 8 and 
9, a

rior pituitary is reviewed in Chapters 8 and 
9, and disturbances in posterior pituitary function are discussed 
in Chapter 10. This chapter primarily considers diseases of the 
hypothalamic-pituitary unit.
Pituitary Isolation Syndrome
Destructive lesions of the pituitary stalk, as occur with head 
injury, surgical transection, tumor, or granuloma, produce a char­
acteristic pattern of pituitary dysfunction.593,594 Central diabetes 
insipidus (DI) develops in a large percentage of patients, depe

) develops in a large percentage of patients, depend­
ing on the level at which the stalk has been sectioned. If the cut is 
close to the hypothalamus, DI is almost always produced, but if 
the section is low on the stalk, the incidence is lower. The extent to 
which nerve terminals in the upper stalk are preserved determines 
the clinical course. The classic triphasic syndrome of initial poly­
uria followed by normal water control and then by AVP deficiency 
over a period of 1 week to 10 days o

P deficiency 
over a period of 1 week to 10 days occurs in fewer than half of the 
patients. The sequence is attributed to an initial loss of neurogenic 
control of the neural lobe, followed by autolysis of the neural lobe 
with release of AVP into the circulation, and finally by complete 
loss of AVP. However, full expression of polyuria requires adequate 
cortisol levels; if cortisol is deficient, AVP deficiency may be pres­
ent with only minimal polyuria. DI can also develop after stalk 
inju

al polyuria. DI can also develop after stalk 
injury without an overt transitional phase. When DI occurs after 
head injury or operative trauma, varying degrees of recovery can 
be seen even after months or years. Sprouting of nerve terminals 
in the stump of the pituitary stalk may give rise to sufficient func­
tioning tissue to maintain water balance. In contrast to the effects 
of stalk section, nondestructive injury to the neurohypophysis or 
stalk, as during surgical resection of sellar tum

stalk, as during surgical resection of sellar tumors, can sometimes 
give rise to transient or delayed SIADH.595
Although head injury, granulomas, and tumors are the most 
common causes of acquired DI, other cases develop in the absence 
of a clear-cut cause.596 Autoimmune disease of the hypothala­
mus may be the cause in some instances, as was suggested by the 
finding of autoantibodies to neurohypophyseal cells in a third of 
cases of idiopathic DI in one series.597 However, autoantibodies 
w

ic DI in one series.597 However, autoantibodies 
were also frequently found in association with histiocytosis X. 
Later reports suggested the importance of continued vigilance in 
cases of idiopathic DI. A definite cause is frequently uncovered in 
time, including a high proportion of occult germinomas, whose 
detection by MRI may be preceded by elevated levels of hCG in 
CSF.598 Congenital DI can be part of a hereditary disease. DI in 
the Brattleboro rat is due to an autosomal recessive geneti

leboro rat is due to an autosomal recessive genetic defect 
that impairs production of AVP but not of oxytocin. In contrast, 
inherited forms of DI in humans have been attributed to muta­
tions in the vasopressin V2 receptor gene or less frequently in the 
aquaporin or the AVP genes.599–601
Menstrual cycles cease after stalk section, although gonadotro­
pins may still be detectable, unlike the situation after hypophysec­
tomy. Plasma glucocorticoid levels and urinary excretion of cortisol 
decli

id levels and urinary excretion of cortisol 
decline after hypophysectomy and stalk section, but the change is 
slower after stalk section. A transient increase in cortisol secretion 
after stalk section is believed to be due to release of ACTH from 
preformed stores. The ACTH response to the lowering of blood 
cortisol is markedly reduced, but ACTH release after stress may 
be normal, possibly because of CRH-independent mechanisms. 
Reduction in thyroid function after stalk section is similar t

 thyroid function after stalk section is similar to that 
seen with hypophysectomy. The fall in GH secretion is said to 
be the most sensitive indication of damage to the stalk, but the 
insidious nature of this endocrinologic change in adults who have 
suffered traumatic brain injuries may cause it to be overlooked and 
therefore contribute to delayed rehabilitation.602
Humans with stalk sections or with tumors of the stalk region 
have widely varying levels of hyperprolactinemia and may have 

arying levels of hyperprolactinemia and may have 
galactorrhea.603 PRL responses to hypoglycemia and to TRH are 
blunted, in part because of loss of neural connections with the 
hypothalamus. PRL responses to dopamine agonists and antago­
nists in patients with pituitary isolation syndrome are similar to 
those in patients with prolactinomas. Interestingly, PRL secre­
tion continues to show a diurnal variation in patients with either 
hypothalamic-pituitary disconnection or microprolactinoma.412

c-pituitary disconnection or microprolactinoma.412 
Both forms of hyperprolactinemia are characterized by a similarly 
increased frequency of PRL pulses and a marked rise in nonpul­
satile or basal PRL secretion, although the disruption is greater in 
the tumoral hyperprolactinemia.
An incomplete pituitary isolation syndrome may occur with 
the empty sella syndrome, intrasellar cysts, or pituitary adeno­
mas.604,605 Anterior pituitary failure after stalk section is in part 
due to loss of specif

er stalk section is in part 
due to loss of specific neural and vascular links to the hypothala­
mus and in part due to pituitary infarction. 
Selective pituitary failure can be due to a deficiency of specific 
pituitary cell types or a deficiency of one or more hypothalamic 
hormones. Isolated GnRH deficiency is the most common 
hypophysiotropic hormone deficiency. In X-linked Kallmann 
syndrome (gonadotropin deficiency commonly associated with 
hyposmia),440

deficiency commonly associated with 
hyposmia),440 hereditary agenesis of the olfactory lobe may be 
demonstrable by MRI.606 Abnormal development of the GnRH 
system is a result of defective migration of the GnRH-containing 
neurons from the olfactory nasal epithelium in early embryologic 
life (see earlier discussion). Other malformations of the cranial 
midline structures, such as absence of the septum pellucidum in 
septo-optic dysplasia (De Morsier syndrome), can cause hypogo­
nadotropic hyp

orsier syndrome), can cause hypogo­
nadotropic hypogonadism or, less commonly, precocious puberty. 
A surprisingly large percentage of children with septo-optic dys­
plasia who otherwise have multiple hypothalamic-pituitary abnor­
malities actually retain normal gonadotropin function and enter 
puberty spontaneously.607 The genetic basis of hypogonadotropic 
hypogonadism include X-linked mutations in KAL1, the Kall­
mann syndrome gene, and in NROB1 (formerly AHC or DAX1), 
the gene that causes a

B1 (formerly AHC or DAX1), 
the gene that causes adrenal hypoplasia congenital with hypogo­
nadotropic hypogonadism.442,608 Autosomal recessive hypogo­
nadotropic hypogonadism has been associated with mutations in 
the genes encoding the GnRH receptor, KISS1 receptor, leptin, 
leptin receptor, FSH, LH, PROP1 (combined pituitary defi­
ciency), and HESX (septo-optic dysplasia) genes, and deficient 
FGFR1 function causes an autosomal dominant form of hypogo­
nadotropic hypogonadism. Mutations in PR

f hypogo­
nadotropic hypogonadism. Mutations in PROK2 and PROKR2, 
which encode prokinectin-2 and its receptor, have been associated 
with heterozygous, homozygous, compound heterozygous, and 
oligogenic patterns of genetic penetrance.
The GnRH stimulation test is of little value in the differen­
tial diagnosis of hypogonadotropic hypogonadism. Most patients 
with GnRH deficiency show little or no response to an initial 
test dose but normal responses after repeated injection. This 
slow respons

onses after repeated injection. This 
slow response has been attributed to downregulation of GnRH 
176
receptors in response to prolonged GnRH deficiency. In patients 
with intrinsic pituitary disease, the response to GnRH may be 
absent or normal. Consequently, it is not possible to distinguish 
between hypothalamic and pituitary disease with a single injec­
tion of GnRH. Prolonged infusions or repeated administration of 
GnRH agonists after hormone repl

dministration of 
GnRH agonists after hormone replacement therapy priming may 
aid in the diagnosis or provide therapeutic options for women 
with Kallmann syndrome who wish to become pregnant.609,610
Deficiency of TRH secretion gives rise to hypothalamic hypo­
thyroidism, also called tertiary hypothyroidism, which can occur 
in hypothalamic disease or more rarely as an isolated defect.611 
Molecular genetic analyses have revealed infrequent autosomal 
recessive mutations in the TRH and TRH rece

somal 
recessive mutations in the TRH and TRH receptor genes in the 
cause of central hypothyroidism.612 Hypothalamic and pituitary 
causes of TSH deficiency are most readily distinguished by imag­
ing methods. Although it is theoretically reasonable to use the 
TRH stimulation test for the differentiation of hypothalamic dis­
ease from pituitary disease, the test is of limited value. The typical 
pituitary response to TRH administration in patients with TRH 
deficiency is an enhanced and somewh

nts with TRH 
deficiency is an enhanced and somewhat delayed peak, whereas the 
response with pituitary failure is subnormal or absent. The hypo­
thalamic type of response has been attributed to an associated GH 
deficiency that sensitizes the pituitary to TRH (possibly through 
suppression of somatostatin secretion), but GH also affects T4 
metabolism and may alter pituitary responses.613 In practice, the 
responses to TRH in hypothalamic and pituitary disease overlap 
so much that they cannot 

tuitary disease overlap 
so much that they cannot be used reliably for a differential diag­
nosis. Persistent failure to demonstrate responses to TRH is good 
evidence for the presence of intrinsic pituitary disease, but the 
presence of a response does not mean that the pituitary is normal. 
Deficient TRH secretion leads to altered TSH biosynthesis by 
the pituitary, including impaired glycosylation. Poorly glycosyl­
ated TSH has low biologic activity, and dissociation of bioactive 
and immunor

tivity, and dissociation of bioactive 
and immunoreactive TSH can lead to the paradox of normal or 
elevated levels of TSH in hypothalamic hypothyroidism.611,614
GHRH deficiency appears to be the principal cause of hGH 
deficiency in children with idiopathic dwarfism.615 This condition 
is frequently associated with abnormal electroencephalograms 
(EEGs), a history of birth trauma, and breech delivery, although 
a cause-and-effect relationship has not been established. MRI 
scans show that most 

s not been established. MRI 
scans show that most children with isolated, idiopathic hGH 
deficiency have a normal-sized or only slightly reduced anterior 
pituitary; less common findings are ectopic posterior pituitary, 
anterior pituitary hypoplasia, or empty sella.616 In contrast, chil­
dren with idiopathic combined pituitary hormone deficiency are 
significantly more likely to have evidence of moderate to severe 
anterior pituitary hypoplasia, ectopic posterior pituitary, com­
plete agenesis

, ectopic posterior pituitary, com­
plete agenesis of the pituitary stalk (both nervous and vascular 
components), and a variety of associated midline cerebral mal­
formations.616 Human GH is the most vulnerable of the anterior 
pituitary hormones when the pituitary stalk is damaged. It can 
be difficult to differentiate between primary pituitary disease and 
GHRH deficiency by standard tests of GH reserve. However, a 
substantial GH secretory response to a single administration of 
hexarelin oc

sponse to a single administration of 
hexarelin occurs only in the presence of at least a partially intact 
vascular stalk (Fig. 7.34).294
In many children with dwarfism, the anatomic abnormalities 
of the intrasellar contents and pituitary stalk together with the fre­
quent occurrence of other midline defects, such as those in septo-
optic dysplasia, are consistent with the alternative hypothesis of 
a developmental defect occurring in embryogenesis.616 There has 
been a remarkable advance in o

esis.616 There has 
been a remarkable advance in our understanding of the molecular 
ontogeny of the hypothalamic-pituitary unit, much of it based 
on mutant mouse models.617 Parallel genetic analyses have been 
conducted in children with isolated GH deficiency or combined 
pituitary hormone deficiencies. These studies have identified 
autosomal recessive mutations in both structural and regulatory 
genes, including the genes encoding the GHRH receptor, PIT1, 
PROP1, HESX1, LHX3, and LHX4 that a

ceptor, PIT1, 
PROP1, HESX1, LHX3, and LHX4 that are responsible for a 
120
105
90
75
60
45
30
15
0
−15
50
40
30
20
10
0
A
B
C
10
8
6
4
2
0
10
8
6
4
2
0
Time (min)
GH (µg/L)
GH (µg/L)
GH (µg/L)
GH Deficiency
Intact Stalk
Normal GH
Disrupted Stalk
Hexarelin
GHRH
Hexarelin
GHRH
Hexarelin
GHRH
• Fig. 7.34  Effect of hypothalamic-pituitary disconnection on the growth 
hormone (GH) secretory responses to GH-releasing hormone (GHRH) (1 
μg/kg) and the GH-secretagogue (GHS) 

e (GHRH) (1 
μg/kg) and the GH-secretagogue (GHS) receptor agonist, hexarelin (2 μg/
kg), administered intravenously to children with GH deficiency. (A) Mean 
responses in a group of 24 prepubertal children with short stature sec­
ondary to familial short stature or constitutional growth delay are shown. 
(B) Children with GH deficiency and an intact vascular pituitary stalk as 
visualized by dynamic magnetic resonance imaging exhibited a clear, 
but blunted, GH response to both secretagogues. (

but blunted, GH response to both secretagogues. (C) In contrast, chil­
dren with pituitary stalk agenesis (both vascular and neural components) 
had no response or a markedly attenuated response to both peptides. 
(From Maghnie M, Spica-Russotto V, Cappa M, et al. The growth hormone 
response to hexarelin in patients with different hypothalamic-pituitary 
abnormalities. J Clin Endocrinol Metab. 1998;83:3886–3889.)
177
sizable proportion of congenital hypothalamic-p

gy
sizable proportion of congenital hypothalamic-pituitary disorders 
once considered idiopathic.281,615,616,618 Other mutations that are 
associated with variable GH deficiencies include ARNT2, GLI2, 
OTX2, PAX6, and SOX2.44
Adrenal insufficiency is another manifestation of hypothalamic 
disease and rarely is caused by CRH deficiency.619 Isolated ACTH 
deficiency is uncommon, but there is suggestive evidence in at 
least one family of genetic linkage to the CRH gene locus.620 More 
recent inves

nkage to the CRH gene locus.620 More 
recent investigations have revealed mutations in TBX19, the gene 
encoding TPIT, a T-box transcription factor expressed only in 
pituitary corticotrophs and melanotrophs, which is associated with 
the majority of cases of isolated ACTH deficiency in neonates.621 
Mutations in ARNT2, a basic helix-loop-helix transcription factor 
and important in development of hypophysiotropic neurons in 
the PVH, can also result in ACTH and TSH deficiencies.622 The 
CRH sti

sult in ACTH and TSH deficiencies.622 The 
CRH stimulation test does not reliably distinguish hypothalamic 
from pituitary failure as a cause of ACTH deficiency.623
Apart from intrinsic diseases of the hypothalamus such as 
tumors and granulomas, two environmental causes of central 
hypophysiotropic deficiencies are of increasing clinical impor­
tance: trauma to the brain,594,602 particularly from motor vehicle 
accidents, and the sequelae of chemotherapy and radiation therapy 
for intracranial 

motherapy and radiation therapy 
for intracranial lesions in children and adults.614,624,625 Improved 
short-term survival from head injuries associated with coma and 
CNS malignancies has greatly increased the prevalence of long-
term neuroendocrine consequences. 
Craniopharyngioma is the most common pediatric tumor occur­
ring in the sellar and parasellar area (see Table 7.6) and accounts 
for 5% to 15% of all intracranial tumors in this age group.626 
Because of their locati

ors in this age group.626 
Because of their location, these benign neoplasms frequently 
cause significant neuroendocrine dysfunction. Adamantinoma­
tous tumors occur at all ages but are most common in children 
and usually contain both a cystic component filled with a tur­
bid, cholesterol-rich fluid and a solid component characterized by 
organized epithelial cells.627 Roughly 25% of craniopharyngiomas 
are diagnosed in patients over the age of 25 and include a pap­
illary variety that is almo

 25 and include a pap­
illary variety that is almost exclusively identified in adults. This 
subset of tumors tends to be solid and less likely to be calcified 
or cystic.627 Both forms of craniopharyngiomas probably result 
from metaplastic changes in vestigial epithelial cell rests that origi­
nate in Rathke pouch and the craniopharyngeal duct during fetal 
development. The majority of adamantinomatous craniopharyn­
giomas have mutations of genes encoding β-catenin (CTNNB1 
and APC), whereas p

es encoding β-catenin (CTNNB1 
and APC), whereas papillary craniopharyngiomas harbor BRAF 
V600E mutations.628,629
Common presenting symptoms are those due to a mass intra­
cranial lesion and increased intracranial pressure. Visual field 
defects, papilledema, and optic atrophy can occur from compres­
sion of the optic chiasm or nerves. Between 80% and 90% of 
affected children have signs and symptoms of endocrine dysfunc­
tion, although these are not usually the chief complaint. The most 
frequ

e not usually the chief complaint. The most 
frequent hormone deficiencies are GH and gonadotropins. The 
latter is almost universal in adolescents and adults and likely also 
present, but undetected, in prepubertal children with craniopha­
ryngiomas. TSH and ACTH deficiency are also common, and 
approximately one-third of these patients develop diabetes insipi­
dus. Even if not present at initial diagnosis, endocrine dysfunc­
tion often occurs subsequent to treatment that may also include 
obes

ubsequent to treatment that may also include 
obesity, imbalances in body temperature, autonomic dysfunction, 
sleep disturbances, and behavioral changes and necessitates long-
term follow-up and retesting.630,631
MRI is the imaging modality of choice in cases of suspected 
craniopharyngioma.632 A recommended examination includes 
T1-weighted thin sagittal and coronal sections through the sella 
and suprasellar regions, obtained before and after contrast admin­
istration. T2-weighted and fluid a

contrast admin­
istration. T2-weighted and fluid attenuation inversion recovery 
(FLAIR) images are useful to further delineate cysts and are hyper­
intense. Computed tomography scans can be useful to determine 
the presence of calcification. 
Hypophysiotropic Hormone Hypersecretion
Pituitary hypersecretion is occasionally caused by tumors of the 
hypothalamus. GnRH-secreting hamartomas can cause preco­
cious puberty.633 CRH-secreting gangliocytomas can cause Cush­
ing syndrome,634 and GHRH-secr

mas can cause Cush­
ing syndrome,634 and GHRH-secreting gangliocytomas of the 
hypothalamus can cause acromegaly.635 Although they do not 
arise from the hypothalamus, paraneoplastic syndromes can also 
cause pituitary hypersecretion, as with bronchial and pancreatic 
islet cell carcinoid tumors secreting CRH or GHRH. 
Neuroendocrine Disorders of Gonadotropin 
Regulation
Precocious Puberty
The term precocious puberty is used when physiologically normal 
pituitary-gonadal function appears at an e

normal 
pituitary-gonadal function appears at an early age (see also Chapter 
26).636 By convention, it is defined as the onset of androgen secre­
tion, and spermatogenesis must occur before the age of 9 or 10 
years in boys and the onset of estrogen secretion and cyclic ovarian 
activity before age 7 or 8 in girls.637,638 Central precocious puberty 
is due to disturbed CNS function, which may or may not have 
an identifiable structural basis. Pseudoprecocious puberty refers to 
premature sexual

eudoprecocious puberty refers to 
premature sexual development resulting from excessive secretion of 
androgens, estrogens, or hCG; it is caused by tumors (both gonadal 
and extragonadal), administration of exogenous gonadal steroids, 
or genetically determined activation of gonadotropin receptors (see 
Chapter 25). Central precocious puberty with neurogenic causes 
and pineal gland disease are discussed in this chapter.
Idiopathic Sexual Precocity
Familial occurrence of idiopathic sexual precoc

ty
Familial occurrence of idiopathic sexual precocity is uncommon, 
but there is a hereditary form of idiopathic sexual precocity that 
is largely confined to boys. Abnormal EEGs and behavioral dis­
turbances, suggesting the presence of brain damage, have been 
reported occasionally in girls with idiopathic precocious puberty. 
The pathogenesis may be related to the rate of hypothalamic 
development or other as yet undetermined nutritional, environ­
mental, or psychosocial factors. Many cases pr

on­
mental, or psychosocial factors. Many cases previously thought 
to be idiopathic are caused by small hypothalamic hamartomas 
(see later discussion). It has been argued that localized activation 
of discrete cellular subsets connected to GnRH neurons may be 
sufficient to initiate puberty.639 
Neurogenic Precocious Puberty
Approximately two-thirds of hypothalamic lesions that influence 
the timing of human puberty are located in the posterior hypothal­
amus, but in the subset of patients who

 hypothal­
amus, but in the subset of patients who come to autopsy, damage 
is extensive. Specific lesions known to cause precocity include cra­
niopharyngioma (although delayed puberty is far more common 
with these lesions), astrocytoma, pineal tumors, subarachnoid 
178
cysts, encephalitis, miliary tuberculosis, tuberous sclerosis or neu­
rofibromatosis type 1, the Sturge-Weber syndrome, porencephaly, 
craniosynostosis, microcephaly, hydrocephalus, empt

raniosynostosis, microcephaly, hydrocephalus, empty sella syn­
drome, and Tay-Sachs disease.640,641
Hamartoma of the hypothalamus, a tumor-like collection of 
normal-appearing nerve tissue lodged in an abnormal location, 
can result in precocious puberty.642,643 The parahypothalamic 
type consists of an encapsulated nodule of nerve tissue attached 
to the floor of the third ventricle or suspended from the floor by a 
peduncle, typically less than 1 cm in diameter. The intrahypotha­
lamic or sess

 1 cm in diameter. The intrahypotha­
lamic or sessile type is enveloped by the posterior hypothalamus, 
which can distort the third ventricle. These hamartomas tend to 
be larger than the pedunculated variety, grow in the interpedun­
cular cistern, and are frequently accompanied by seizures, mental 
retardation, and developmental delays. Before the development of 
high-resolution scanning techniques, harmatoma was considered 
rare, but small ones can now be visualized. Miniature hamarto­
mas of 

 can now be visualized. Miniature hamarto­
mas of the tuber cinereum are common at autopsy. Precocious 
puberty occurs when the hamartoma makes connections with the 
median eminence and thus serves as an accessory hypothalamus. 
Peptidergic nerve terminals containing GnRH have been found in 
the tumors.633 Early pubertal development is presumably due to 
unrestrained GnRH secretion, although the hamartomas almost 
certainly have an intrinsic pulse generator of GnRH secretion 
because pulsatility

e generator of GnRH secretion 
because pulsatility is required for stimulation of gonadotropin 
secretion (see earlier discussion).
Manifestations of premature puberty in patients with hamar­
tomas are similar to those associated with other central causes of 
precocity. Hamartomas occur in both sexes and may be present 
as early as age 3 months. In the past, most cases were thought to 
be fatal by age 20 years, but many hamartomas cause no brain 
damage and need not be excised.642 The interpedun

damage and need not be excised.642 The interpeduncular fossa of 
the brain is difficult to approach, and surgical experience is some­
what limited. Early in the course of illness, epilepsy manifested as 
“brief, repetitive, stereotyped attacks of laughter”644 may provide 
a clue to the disease. Late in the course, hypothalamic damage can 
cause severe neurologic defects and intractable seizures. 
Hypothyroidism can cause precocious puberty in girls that is 
reversible with thyroi

s puberty in girls that is 
reversible with thyroid therapy. Hyperprolactinemia and galactor­
rhea may be present. One possibility is that elevated TSH levels 
(in children with thyroid failure) cross-react with the FSH recep­
tor.645 Alternatively, low levels of thyroid hormone might simulta­
neously activate release of LH, FSH, and TSH. A third possibility 
is that hypothyroidism causes hypothalamic encephalopathy that 
impairs the normal tonic suppression of gonadotropin release by 
the hypot

 suppression of gonadotropin release by 
the hypothalamus. The high PRL levels that sometimes accom­
pany this disorder may be due to a deficiency in PIF secretion, 
increased secretion of TRH, or increased sensitivity of the lacto­
trophs to TRH secretion. 
Tumors of the Pineal Gland
Pineal gland tumors account for only a small percentage of intra­
cranial neoplasms. They occur as a central midline mass with an 
enhancing lesion on MRI frequently accompanied by hydro­
cephalus. Pinealomas cause

y accompanied by hydro­
cephalus. Pinealomas cause a variety of neurologic abnormali­
ties. Parinaud syndrome, which consists of paralysis of upward 
gaze, pupillary areflexia (to light), paralysis of convergence, and 
a wide-based gait, occurs with about half of patients with pine­
alomas. Gait disturbances can also occur because of brainstem 
or cerebellar compression. Additional neurologic signs occurring 
with moderate frequency include spasticity, ataxia, nystagmus, 
syncope, vertigo, crani

icity, ataxia, nystagmus, 
syncope, vertigo, cranial nerve palsies other than VI and VIII, 
intention tremor, scotoma, and tinnitus.
Several discrete cytopathologic entities account for mass lesions 
in the pineal region (Table 7.8).646 The most common nonneo­
plastic conditions are degenerative pineal cysts, arachnoid cysts, 
and cavernous hemangioma. Pinealocytes give rise to primitive 
neuroectodermal tumors, the so-called small blue-cell tumors that 
are immunopositive for the neuronal marke

rs that 
are immunopositive for the neuronal marker synaptophysin and 
negative for the lymphocyte marker CD45. True pinealomas can 
be relatively well-differentiated pineocytomas, intermediate mixed 
forms, or the less-differentiated pineoblastomas,646,647 which are 
essentially identical to medulloblastomas, neuroblastomas, and 
oat cell carcinomas of the lung.
The most common tumors of the pineal gland are actually 
germinomas (a form of teratoma), so designated because of their 
presumed ori

oma), so designated because of their 
presumed origin in germ cells. Germinomas may also occur in the 
anterior hypothalamus or the floor of the third ventricle, where 
they are often associated with the clinical triad of DI, pituitary 
insufficiency, and visual abnormalities.641 Identical tumors can 
be found in the testis and anterior mediastinum. Intracranial 
  Classification of Tumors of the Pineal 
Region
Germ Cell Tumors
Germinoma
Posterior third ventricle and pineal lesions
Anterior thi

or third ventricle and pineal lesions
Anterior third ventricle, suprasellar, or intrasellar lesions
Combined lesions in anterior and posterior third ventricle, apparently 
noncontiguous, with or without foci of cystic or solid teratoma
Teratoma
Evidencing growth along two or three germ lines in varying degrees 
of differentiation
Dermoid and epidermoid cysts with or without solid foci of teratoma
Histologically malignant forms with or without differentiated foci of 
benign, solid, or cystic tera

ferentiated foci of 
benign, solid, or cystic teratoma-teratocarcinoma, chorioepi­
thelioma, embryonal carcinoma (endodermal–sinus tumor or 
yolk-sac carcinoma); combinations of these with or without foci 
of germinoma, chemodectoma
Pineal Parenchymal Tumors
Pinealocytes
Pineocytoma
Pineoblastoma
Ganglioglioma and chemodectoma
Mixed forms exhibiting transitions among these types
Glia
Astrocytoma
Ependymoma
Mixed forms and other less frequent gliomas (e.g., glioblastoma, 
oligodendroglioma)
Tumor

mas (e.g., glioblastoma, 
Tumors of Supporting or Adjacent Structures
Meningioma
Hemangiopericytoma
Nonneoplastic Conditions of Neurosurgical Importance
Degenerative cysts of pineal gland lined by fibrillary astrocytes
Arachnoid cysts
Cavernous hemangioma
From DeGirolami U. Pathology of tumors of the pineal region. In: Schmidek HH, ed. Pineal 
Tumors. New York: Masson; 1977:1–19.
   
TABLE 7.8
179
germinomas have a tendency to spread locally, infi

germinomas have a tendency to spread locally, infiltrate the hypo­
thalamus, and metastasize to the spinal cord and CSF. Extracra­
nial metastases (to skin, lung, or liver) are rare. Teratomas derived 
from two or more germ cell layers also occur in the pineal region. 
Chorionic tissue in teratomas and germinomas may secrete hCG 
in sufficient amounts to cause gonadal maturation, and some of 
these tumors have the histologic and functional characteristics of 
choriocarcinomas. Diagnosis is confi

teristics of 
choriocarcinomas. Diagnosis is confirmed by the combination of 
a mass lesion, cytologic analysis of CSF, and radioimmunoassay 
detection of hCG in the CSF.
Precocious puberty is a relatively unusual manifestation of 
pineal gland disease. When it occurs, neuroanatomic studies sug­
gest that the cause is secondary to pressure or destructive effects of 
the pineal tumor on the function of the adjacent hypothalamus 
or to the secretion of hCG. Most patients have other evidence of 
hy

n of hCG. Most patients have other evidence of 
hypothalamic involvement such as DI, polyphagia, somnolence, 
obesity, or behavioral disturbance. Choriocarcinoma of the pineal 
gland is associated with high plasma levels of hCG. The hCG can 
stimulate testosterone secretion from the testis but not estrogen 
secretion by the ovary; it therefore causes premature puberty almost 
exclusively in boys. The prevalence of elevated hCG levels in chil­
dren with premature puberty related to tumors in the 

n with premature puberty related to tumors in the pineal region 
is unknown, but the fact that this phenomenon occurs further 
challenges the theory that nonparenchymal tumors cause preco­
cious puberty by damaging the normal pineal gland. Rarely, pine­
alomas cause delayed puberty, raising speculation about the role 
of melatonin in inhibiting gonadotropin secretion in these cases.
Management of tumors in the pineal region is not straightfor­
ward.646,648 Operative mortality rates can be high, 

rd.646,648 Operative mortality rates can be high, but the ratio­
nale for an aggressive approach to the pineal region is based on the 
need to make a histologic diagnosis, the variety of lesions found 
in this region, the possibility of cure of an encapsulated lesion, 
and the effectiveness of chemotherapeutic agents for germinomas 
and choriocarcinoma. Stereotaxic biopsy of the pineal region pro­
vided diagnosis in 33 of 34 cases in one series, suggesting that this 
is a useful alternative to o

suggesting that this 
is a useful alternative to open surgical exploration for diagnostic 
purposes.649 Long-term palliation or cure of many pineal region 
tumors is possible by combinations of surgery, radiation, gamma 
knife radiosurgery, or chemotherapy, depending on the nature of 
the lesion.650 
Approach to the Patient with Precocious Puberty
Several groups have reviewed the diagnostic approach to suspected 
central precocious puberty (see Chapter 26).651,652 Although guide­
lines differ, t

hapter 26).651,652 Although guide­
lines differ, the index of suspicion is clearly inversely proportional to 
the age of the patient. A GnRH stimulation test to assess gonado­
tropin release and thereby differentiate between primed and inactive 
gonadotrophs is probably the single most important endocrinologic 
measure. If LH and FSH levels are not stimulated and there is no 
evidence of gonadal germ cell maturation, the cause of precocious 
puberty lies outside the hypothalamic-pituitary axis, 

rty lies outside the hypothalamic-pituitary axis, and the diagnos­
tic process should focus on the adrenal glands and gonads (see Chap­
ters 15 and 17). MRI studies are central to the workup for exclusion 
or characterization of organic lesions in the areas of the sella, optic 
chiasm, suprasellar hypothalamus, and interpeduncular cistern.653 
Management of Sexual Precocity
Structural lesions of the hypothalamus are treated by surgery, 
radiation, chemotherapy, or combinations of these as indica

, chemotherapy, or combinations of these as indicated 
by the pathologic diagnosis and extent of disease. Endocrinologic 
manifestations of precocious puberty are best treated by GnRH 
agonists with the therapeutic goals of delaying sexual maturation 
to a more appropriate age and achieving optimal linear growth 
and bone mass, possibly with the combined use of GH treat­
ment.654,655 Other approaches include the use of cyproterone 
acetate, testolactone, or spironolactone to antagonize or inhibi

lactone, or spironolactone to antagonize or inhibit 
gonadal steroid biosynthesis.656,657 Precocious puberty is stressful 
to both the child and the parents, and it is essential that psycho­
logic support be provided. 
Psychogenic Amenorrhea
Menstrual cycles can cease in young nonpregnant women with 
no demonstrable abnormalities of the brain, pituitary, or ovary in 
several situations,658,659 including pseudocyesis (false pregnancy), 
anorexia nervosa, excessive exercise, psychogenic disorders,

ervosa, excessive exercise, psychogenic disorders, and 
hyperprolactinemic states (see Chapter 17). Psychogenic amenor­
rhea, the most common cause of secondary amenorrhea except 
for pregnancy, can occur with major psychopathology or minor 
psychic stress and is often temporary.
Exercise-induced amenorrhea may be a variant of psychogenic 
amenorrhea or may result from loss of body fat.658,660 The syn­
drome is associated with intense and prolonged physical exertion 
such as running, swimming, o

ed physical exertion 
such as running, swimming, or ballet dancing. Affected women 
are always below ideal body weight and have low stores of fat. If 
the activity is begun before puberty, normal sexual maturation can 
be delayed for many years. Fat mass may be a regulator of gonad­
otropin secretion with adipocyte-derived leptin as the principal 
mediator between peripheral energy stores and hypothalamic reg­
ulatory centers.661 Studies in nonhuman primates showed a direct 
role of caloric inta

man primates showed a direct 
role of caloric intake in the pathogenesis of amenorrhea associated 
with long-distance running.662 Exercise and psychogenic amenor­
rhea can have adverse effects because of the associated estrogen 
deficiency and accompanying infertility, osteopenia, and increased 
risk of stress fractures.663,664 Nevertheless, because of a lack of 
clear benefit from oral estrogen administration on bone density, 
perhaps secondary to its suppression of IGF-1, short-term treat­
men

to its suppression of IGF-1, short-term treat­
ment with transdermal estrogen is recommended in women who 
have not had resumption in menses after a trial of nutritional, 
psychologic, and/or modified exercise interventions.665 
Neurogenic Hypogonadism in Males
A discussion of neurogenic hypogonadism in males should begin 
with an account of Fröhlich syndrome (adiposogenital dystrophy), 
originally characterized as delayed puberty, hypogonadism, and 
obesity associated with a tumor that impinges

and 
obesity associated with a tumor that impinges on the hypothala­
mus.8 It was subsequently recognized that either hypothalamic 
or pituitary dysfunction can induce hypogonadism and that the 
presence of obesity indicates that the appetite-regulating regions 
of the hypothalamus have been damaged. Several organic lesions 
of the hypothalamus can cause this syndrome, including tumors, 
encephalitis, microcephaly, Friedreich ataxia, and demyelinating 
diseases. Other important causes of hypogon

ating 
diseases. Other important causes of hypogonadotropic hypogo­
nadism are Kallmann syndrome and Prader-Willi syndrome.666
However, most males with delayed sexual development do not 
have serious neurologic conditions. Furthermore, most obese boys 
with delayed sexual development have no structural damage to the 
hypothalamus but have constitutional delayed puberty, which is 
commonly associated with obesity. It is not known whether there 
is a functional disorder of the hypothalamus in this

 a functional disorder of the hypothalamus in this condition. 
It is thought that psychosexual development of brain maturation 
depends on the presence of androgens within a critical develop­
mental window corresponding to puberty; therefore hypogonad­
ism in boys (regardless of cause) should be treated by the middle 
teen years (15 years of age at the latest).
180
In adult men, hypogonadism (including reduced spermato­
genesis) can be induced by emotiona

uced spermato­
genesis) can be induced by emotional stress or severe exercise,667 
but this abnormality is seldom diagnosed because the symptoms 
are more subtle than menstrual cycle changes in similarly stressed 
women. Prolonged physical stress and sleep and energy deficiency 
can also decrease testosterone and gonadotropin levels.668 Chronic 
intrathecal administration of opiates for the control of intractable 
pain syndromes is strongly associated with hypogonadotropic 
hypogonadism and to a

iated with hypogonadotropic 
hypogonadism and to a lesser extent with hypocorticism and GH 
deficiency in both men and women.259 Finally, critical illness with 
multiple causes is well known to be associated with hypogonadism 
and ineffectual altered pulsation of GnRH.669 
Neurogenic Disorders of Prolactin Regulation
Neurogenic causes of hyperprolactinemia include irritative lesions 
of the chest wall (e.g., herpes zoster, thoracotomy), excessive tac­
tile stimulation of the nipple, and lesions 

 tac­
tile stimulation of the nipple, and lesions within the spinal cord 
(e.g., ependymoma).670 Prolonged mechanical stimulation of the 
nipples by suckling or the use of a breast pump can initiate lacta­
tion in some women who are not pregnant, and neurologic lesions 
that interrupt the hypothalamic-pituitary connection can cause 
hyperprolactinemia, as discussed earlier. Hyperprolactinemia also 
occurs after certain forms of epileptic seizures. In one series, six of 
eight patients with tempo

. In one series, six of 
eight patients with temporal lobe seizures had a marked increase 
in PRL, whereas only one of eight patients with frontal lobe sei­
zures developed hyperprolactinemia.671 Agents that block D2-like 
dopamine receptors (e.g., phenothiazines, later generation atypi­
cal antipsychotics) or prevent dopamine release (e.g., reserpine, 
methyldopa, opiates) must be excluded in all cases.
Because the nervous system exerts such profound effects on 
PRL secretion, patients with hyp

found effects on 
PRL secretion, patients with hyperprolactinemia (including those 
with adenomas) may have a deficit of PIF or an excess of PRF 
activity. In studies of PRL secretion in patients apparently cured 
of hyperprolactinemia by removal of a pituitary microadenoma, 
regulatory abnormalities persisted in some but not all patients. 
Persistence of regulatory abnormalities may be due to incomplete 
removal of a tumor, abnormal function of the remaining part of 
the gland, or underlying hy

the remaining part of 
the gland, or underlying hypothalamic abnormalities.672 
Neurogenic Disorders of Growth Hormone 
Secretion
Hypothalamic Growth Failure
Loss of the normal nocturnal increase in GH secretion and loss 
of GH secretory responses to provocative stimuli occur early in 
the course of hypothalamic disease and may be the most sensitive 
endocrine indicator of hypothalamic dysfunction. As described 
earlier, anatomic malformations of midline cerebral structures are 
associated with 

 midline cerebral structures are 
associated with abnormal GH secretion, presumably related to 
failure of the development of normal GH regulatory mechanisms. 
Such disorders include optic nerve dysplasia and midline pros­
encephalic malformations (absence of the septum pellucidum, 
abnormal third ventricle, and abnormal lamina terminalis). Cer­
tain complex genetic disorders, including Prader-Willi syndrome, 
also commonly involve reduced GH secretory capacity.673 Idio­
pathic hypopituitarism w

retory capacity.673 Idio­
pathic hypopituitarism with GH deficiency was considered earlier. 
Maternal Deprivation Syndrome and 
Psychosocial Dwarfism
Infant neglect or abuse can impair growth and cause failure to thrive 
(the maternal deprivation syndrome). Malnutrition interacts with 
psychologic factors to cause growth failure in children with the 
maternal deprivation syndrome, and each case should be carefully 
evaluated from this point of view. Older children with growth failure 
in a setti

ew. Older children with growth failure 
in a setting of abuse or severe emotional disturbance (termed psycho­
social dwarfism) may also have abnormal circadian rhythms and defi­
cient hGH release after insulin-induced hypoglycemia or arginine 
infusion (see Chapter 25).674 Deficient release of ACTH and gonad­
otropins may also be present. A variant termed hyperphagic short stat­
ure has been identified.675 These disorders can be reversed by placing 
the child in a supportive milieu; growth and n

ng 
the child in a supportive milieu; growth and neuroendocrine hGH 
responses rapidly return to normal.676 The pathogenesis of altered 
GH secretion in children in response to deprivation is unknown. 
Furthermore, in the adult human, physical or emotional stress usu­
ally causes an increase in hGH secretion (see earlier discussion). 
Neuroregulatory Growth Hormone Deficiency
The availability of biosynthetic hGH for treatment of short stature 
has brought into focus a group of patients who grow 

s brought into focus a group of patients who grow at low rates 
(<3rd percentile) and have low levels of serum IGF-1 but a normal 
hGH secretory reserve. Studies of 24-hour hGH secretion profiles 
indicate that many of these children do not have normal spon­
taneous hGH secretion (i.e., abnormal ultradian and circadian 
rhythms or decreased number or amplitude of secretory bursts, 
or both). These children with idiopathic short stature may have a 
functional regulatory disturbance of the hypotha

functional regulatory disturbance of the hypothalamus and appear 
to grow normally when given exogenous hGH.677
There is considerable uncertainty about the criteria for the 
diagnosis of neuroregulatory hGH deficiency. Many normally 
growing children have profiles of hGH secretion that are indistin­
guishable from those in children with the postulated syndrome.678 
Patterns of hGH secretion do not predict which child will ben­
efit from therapy, and there is a poor correlation between hGH 
sec

, and there is a poor correlation between hGH 
secretion and growth. Furthermore, the results of repeated tests 
in children show considerable variability. It has been suggested 
that specific genetic defects may underlie the pathogenesis of a 
subset of children with this heterogeneous syndrome of growth 
failure.679 The prevalence of an hGH neuroregulatory deficiency 
syndrome is thus unclear, and the decision to treat short children 
with hGH should be made cautiously.680,681 
Neurogenic Hype

should be made cautiously.680,681 
Neurogenic Hypersecretion of Growth Hormone
Diencephalic Syndrome
Children and infants with tumors in and around the third ven­
tricle frequently become cachectic, which is often associated with 
elevated hGH levels and paradoxical GH secretory responses 
to glucose and insulin.682 GH hypersecretion may be due to a 
hypothalamic abnormality or to malnutrition. Deficits of pitu­
itary-adrenal regulation are less common. A striking feature is 
an alert appearance

common. A striking feature is 
an alert appearance and seeming euphoria despite the profound 
emaciation. A variety of associated neurologic abnormalities may 
be present, including nystagmus, irritability, hydrocephalus, 
optic atrophy, tremor, and excessive sweating. CSF abnormali­
ties include increased protein and the presence of abnormal cells. 
Most cases are due to chiasmatic-hypothalamic gliomas, with the 
majority classified as astrocytomas.682 Treatment options include 
surgical resect

mas.682 Treatment options include 
surgical resection, radiation therapy, and chemotherapy.683 
Growth Hormone Hypersecretion Associated With Metabolic 
Disturbances
Apparently inappropriate hGH hypersecretion occurs with uncon­
trolled diabetes mellitus, hepatic failure, uremia, anorexia nervosa, 
and protein-calorie malnutrition. Nutritional factors are probably 
181
important in this response, because in normal persons obesity 
inhibits and fasting stimulates epi

rsons obesity 
inhibits and fasting stimulates episodic GH hypersecretion.684 In 
diabetes mellitus, cholinergic blockers reverse the abnormality,332 
possibly by inhibiting hypothalamic somatostatin secretion (see 
earlier discussion). Peripheral GH resistance leading to reduced 
IGF-1 synthesis and release may also contribute to increased levels 
of GH. 
X-Linked Gigantism
Excess growth hormone secretion, resulting in acromegaly/gigan­
tism, is most often associated with tumors of the anterior

 most often associated with tumors of the anterior pituitary 
and, less commonly, ectopic secretion of GHRH from carcinoid 
tumors, pancreatic islet cell tumors, small cell lung carcinoma, 
adrenal adenomas, and pheochromocytomas (see Chapter 9). 
Genetic abnormalities resulting in gigantism have been associated 
with MEN1, MEN4, Carney complex, McCune-Albright syn­
drome, and AIP mutations. X-linked acrogigantism (XLAG) is a 
more recently identified disorder of early-onset GH excess associ­
at

ified disorder of early-onset GH excess associ­
ated with gigantism due to microduplications of the GPR101 gene 
on the X chromosome. While the majority of affected individuals 
have pituitary adenomas, elevated circulating levels of GHRH, 
and the presence of diffuse hyperplasia of somatotrophs in some 
patients, expression of GPR101 in the arcuate nucleus has raised 
the possibility that increased hypothalamic secretion of GHRH 
underlies the pathogenesis for X-linked acrogigantism (see Triv­

athogenesis for X-linked acrogigantism (see Triv­
ellin and colleagues685 for review). The presence of GPR101 in 
POMC neurons and increased food-seeking behavior in some 
affected individuals also raises the possibility of involvement of 
this gene in the regulation of energy homeostasis. 
Neurogenic Disorders of Corticotropin 
Hypothalamic CRH hypersecretion is the likely cause of sustained 
pituitary-adrenal hyperfunction in at least two situations: Cushing 
syndrome caused by the 

t two situations: Cushing 
syndrome caused by the rare CRH-secreting gangliocytomas of the 
hypothalamus686 and severe depression. Severe depression is asso­
ciated with pituitary-adrenal abnormalities, including inappropri­
ately elevated ACTH levels, abnormal cortisol circadian rhythms, 
and resistance to dexamethasone suppression.213,218,219,687 The 
dexamethasone suppression test has in fact been used as an aid 
in the diagnosis of depressive illness. Another possible example of 
disordered 

 illness. Another possible example of 
disordered neurogenic control of CRH associated with stress is the 
dysmetabolic syndrome.688,689 This syndrome is characterized by 
mild hypercortisolism, blunted dexamethasone suppression of the 
HPA axis, visceral obesity, and hypertension and may be strongly 
associated with greater risks for cardiovascular disease and stroke.
A unique syndrome of ACTH hypersecretion termed periodic 
hypothalamic discharge (Wolff syndrome) has been described in 
one you

ge (Wolff syndrome) has been described in 
one young man. The patient had a recurring cyclic disorder char­
acterized by high fever, paroxysms of glucocorticoid hypersecre­
tion, and electroencephalographic abnormalities.690 
Nonendocrine Manifestations of Hypothalamic 
Disease
The hypothalamus is involved in the regulation of diverse func­
tions and behaviors (Table 7.9). Psychologic abnormalities in 
hypothalamic disease include antisocial behavior; attacks of rage, 
laughing, and crying; dist

vior; attacks of rage, 
laughing, and crying; disturbed sleep patterns; excessive sexual­
ity; and hallucinations. Both somnolence (with posterior lesions) 
and pathologic wakefulness (with anterior lesions) occur, as do 
bulimia and profound anorexia. The abnormal eating patterns are 
analogous to the syndromes of hyperphagia produced in rats by 
destruction of the VMH or of connections to the PVH. Lateral 
hypothalamic damage causes profound anorexia. A more com­
plete discussion of imbalance 

orexia. A more com­
plete discussion of imbalance in energy homeostasis (both obesity 
and cachexia) associated with hypothalamic dysfunction and neu­
ropeptides is presented in Chapter 39.
Patients with hypothalamic damage may experience hyperther­
mia, hypothermia, unexplained fluctuations in body temperature, 
and poikilothermy. Disturbances of sweating, acrocyanosis, loss 
of sphincter control, and diencephalic epilepsy are occasional 
manifestations. Hypothalamic damage also causes loss of 

stations. Hypothalamic damage also causes loss of recent 
memory, believed to be due to damage of the mammillothalamic 
pathways. Severe memory loss, obesity, and personality changes 
(e.g., apathy, loss of ability to concentrate, aggressive antisocial 
  Neurologic Manifestations of 
Nonendocrine Hypothalamic Disease
Disorders of Temperature Regulation
Hyperthermia
Hypothermia
Poikilothermia
Disorders of Food Intake
Hyperphagia (bulimia)
Anorexia nervosa, aphagia
Cachexia
Disorders of Water In

ia nervosa, aphagia
Cachexia
Disorders of Water Intake
Compulsive water drinking
Adipsia
Essential hypernatremia
Disorders of Sleep and Consciousness
Narcolepsy/cataplexy
Somnolence
Sleep rhythm reversal
Akinetic mutism
Coma
Delirium
Periodic Disease of Hypothalamic Origin
Paroxysmal sympathetic storms (diencephalic epilepsy)
Kleine-Levin syndrome
Periodic discharge syndrome of Wolff
Disorders of Psychic Function
Rage behavior
Hallucinations
Hypersexuality
Disorders of the Autonomic Nervous Syst

rsexuality
Disorders of the Autonomic Nervous System
Pulmonary edema
Cardiac arrhythmias
Sphincter disturbance
Congenital Hypothalamic Disease
Prader-Willi syndrome
Laurence-Moon-Biedl syndrome
Miscellaneous
Diencephalic syndrome of infancy
Cerebral gigantism
TABLE 7.9
182
behavior, severe food craving, inability to work or attend school) 
may occur with suprasellar extension of pituitary tumors, hypo­
thalamic radiation, or damage incurred from surgical 

lamic radiation, or damage incurred from surgical removal of 
parasellar tumors. Hypothalamic tumors grow slowly and may 
reach a large size while producing minimal disturbance of behav­
ior or visceral homeostasis, whereas surgery of limited extent can 
produce striking functional abnormalities. Presumably this is 
because slowly growing lesions permit compensatory responses to 
develop. These potential consequences should be weighed care­
fully with the neurosurgeon, patient, and patient’s fam

 with the neurosurgeon, patient, and patient’s family in plan­
ning the therapeutic approach. Adverse effects of treatment have 
led to more conservative surgical guidelines for the treatment of 
craniopharyngioma. A review from the University of Pittsburgh 
summarizes their individualized treatment program that includes 
microsurgical tumor resection, intracavitary 32P radiotherapy, and 
gamma knife stereotactic radiosurgery to produce maximal benefit 
with minimal morbidity.691 
Paroxysmal Sym

enefit 
Paroxysmal Sympathetic Storms (Diencephalic 
Epilepsy)
Diencephalic epilepsy is characterized by episodic, autonomic 
discharge that can include tachycardia, bradycardia, diaphoresis, 
pupillary dilation, flushing, elevations in blood pressure, tachy­
pnea, hypothermia and hyperthermia, and abnormal extensor pos­
turing. Although thought due to seizures in the earlier literature, 
EEGs do not generally show evidence for epileptiform activity, 
and affected ind

dence for epileptiform activity, 
and affected individuals do not respond well to anticonvulsants. 
Therefore it has been proposed to better describe these episodes 
as paroxysmal sympathetic storms.692 The underlying etiology for 
this disorder is unknown, but most often it has been associated 
with acute increases in intracranial pressure and traumatic brain 
injury, although individual case reports have also reported an asso­
ciation with intracranial tumors and congenital malformations. 
Nar

acranial tumors and congenital malformations. 
Narcolepsy
A convergence of functional genomics from two animal species, 
the dog and mouse, has dramatically refocused attention on neu­
ropeptide circuits of the hypothalamus in the control of sleep and 
wakefulness. Positional cloning was used to identify mutations 
in the hypocretin-orexin receptor 2 as a cause of canine narco­
lepsy.693 Subsequently, knockout of the gene encoding the orexin-
hypocretin peptide precursor produced an equivalent n

ocretin peptide precursor produced an equivalent narcoleptic 
syndrome in mice,694 further establishing this neuropeptide sys­
tem as a major component of sleep-modulating neural circuits. 
Targeted ablation of orexin neurons in the lateral hypothalamus 
of rats by means of a hypocretin receptor 2–saporin conjugate 
produced narcoleptic-like sleep behavior,695 closely paralleling the 
clinical findings and profound reduction in numbers of orexin-
hypocretin neurons in the lateral hypothalamus of

hypocretin neurons in the lateral hypothalamus of humans with 
narcolepsy.544 The additional role of orexin-hypocretin in coor­
dinating arousal states and feeding behavior is discussed earlier 
in the chapter (see “Sleep-Wake Cycle” and “Modifying Factors 
Influencing Sleep”) and in Chapter 39.
Most cases of spontaneous narcolepsy with cataplexy result 
from a degenerative hypothalamic disorder, most likely autoim­
mune in pathogenesis, that produces a selective destruction of 
neuropeptidergi

oduces a selective destruction of 
neuropeptidergic neurons. The absence of immunoreactive orexin-
hypocretin in CSF is a sensitive diagnostic test for the disease. 
Future development of bioavailable, orexin-hypocretin receptor-
selective compounds may provide a specific treatment alternative 
or adjunct to the stimulant and antidepressant drugs currently 
used for management of symptoms. More generally, these recent 
discoveries suggest the possibility that other cryptic hypothalamic 
disorder

sibility that other cryptic hypothalamic 
disorders could be caused by selective disturbances in other neu­
ropeptidergic circuits.
Kleine-Levin Syndrome
Kleine-Levin syndrome is a periodic sleep disorder, generally of 
male adolescents, characterized by relapsing episodes of hyper­
somnia that can last hours to days associated with cognitive and 
behavioral abnormalities, including hyperphagia and hypersexual­
ity, but normal behavior between episodes.696 In contrast to nar­
colepsy, CSF orexin

isodes.696 In contrast to nar­
colepsy, CSF orexin concentrations are usually normal, and the 
disorder is associated with a favorable prognosis as it tends to be 
self-limiting and tends to spontaneously resolve. The etiology is 
unknown, although several hypotheses have been put forward and 
reviewed by Al Shareef and colleagues697 Evidence for abnormal 
perfusion by functional imaging studies in several brain regions, 
including the hypothalamus, thalamus, and cortex, has raised the 
possibil

us, thalamus, and cortex, has raised the 
possibility that Kleine-Levin syndrome may be an inflammatory 
disorder either due to viral or autoimmune encephalitis.698 The 
latter possibility is of particular interest because of the tendency for 
the disorder to run in families and its higher prevalence in Ashke­
nazi Jews. Along these lines, linkage analysis and exome sequenc­
ing have identified chromosome 3 and a heterozygous missense 
variant in LMOD3 in a large family with seven affected membe

 LMOD3 in a large family with seven affected members 
with Kleine-Levin syndrome and in three other sporadic cases.699 
Lmod is a member of the tropomodulin gene family (Tmod), 
which has structural proteins involved in the sequestration of 
actin monomers and stimulation of ATP hydrolysis to accelerate 
actin nucleation and polymerization. While initially thought to 
be present exclusively in heart and skeletal muscle, Lmod pro­
teins have been identified in several regions of the brain, includ

identified in several regions of the brain, including 
nuclear groups involved in the regulation of the sleep-wake cycle 
such as orexin neurons in the lateral hypothalamus, TMN, locus 
coeruleus, dorsal raphe, and ventral tegmental area.699 
Hypothalamus and Neuropsychiatric Disorders
The role of the hypothalamus in the pathophysiology of eating 
disorders such as anorexia nervosa, Prader-Willi syndrome, and 
lesions destroying the base of the hypothalamus is well recognized 
and discussed in d

ypothalamus is well recognized 
and discussed in detail in Chapter 39. However, there is evidence 
to suggest that the hypothalamus is also involved in the patho­
physiology of a number of common neuropsychiatric disorders. 
Genome-wide pathway and functional analysis of susceptibility 
genes associated with schizophrenia, for example, identify the 
hypothalamus as one of five major regions in the brain where these 
genes are predominantly expressed.700 Included are glutaminergic, 
serotonergic,

sed.700 Included are glutaminergic, 
serotonergic, GABAergic and dopamine receptors, calcium and 
potassium ion channels, solute transporters, and neurodevelop­
ment genes. Enlargement of the third ventricle is also known to 
be associated with schizophrenia and major depressive illness and 
thought secondary to loss of periventricular gray matter (see Ber­
nstein and colleagues701 for review). Along these lines, a decreased 
number of neurophysin-containing and nNOS-containing cells in 
the PVN

n-containing and nNOS-containing cells in 
the PVN and reduction in its innervation by beta-endorphin have 
been observed in schizophrenics.702–704 A 50% reduction in total 
neuron number but increased numbers of oxytocin and vasopressin 
neurons in the PVN occur, and a similar reduction in enkephalin 
innervation has been observed in major depressive disorder.704–706 
Alterations in the SCN have also been described, including a 
reduced number of vasopressin neurons and vasopressin mRNA 
183
CH

f vasopressin neurons and vasopressin mRNA 
183
in schizophrenics707,708 and increased VIP in major depressive 
illness.709 Furthermore, analysis of panels of susceptibility genes 
common to several psychiatric disorders identified genes involved 
in serotonin and dopamine homeostasis, including the serotonin 
receptor 2A (HTR2A), tryptophan hydroxylase 2 (TPH2), and 
the dopamine receptor D2 (DRD2), supporting the potential 
involvement of the SCN.710
Aberrant HPA 

otential 
Aberrant HPA activity and/or excessive CRHR-1 recep­
tor stimulation are perhaps the best-recognized abnormalities 
associated with neuropsychiatric disorders, including schizo­
phrenia, depression, anxiety disorders, autism, and obsessive-
compulsive disorder.711,712 Mechanisms proposed include 
impaired feedback inhibition of cortisol due to reduction in 
glucocorticoid and mineralocorticoid receptors, polymor­
phisms in glucocorticoid receptors and 11β-hyd

or­
phisms in glucocorticoid receptors and 11β-hydroxysteroid 
dehydrogenase type 1, increased CRH and elevated CRH 
mRNA expression in the hypothalamic PVH, and polymor­
phisms in the genes for CRH and the CRH receptor, CRH-
R1. Nevertheless, with the exception of very few studies, trials 
employing a variety of CRH-R1 receptor antagonists have not 
been particularly successful,229,712 perhaps signifying the com­
plexities involved in the pathophysiology for these disorders 
and the need for mo

hysiology for these disorders 
and the need for more refined methods to establish adequate 
receptor occupancy and/or identify individuals who are most 
likely to respond.
Dysregulation of the oxytocin and orexin systems has also 
been implicated in the pathophysiology of neuropsychiatric 
disorders, although the data have been somewhat inconsis­
tent.713,714 Oxytocin receptor gene polymorphisms have been 
identified in schizophrenia and major depressive disorder, and 
some patients have shown i

pressive disorder, and 
some patients have shown improvement following administra­
tion of nasal oxytocin (including cognition in schizophrenic 
patients) and a decrease in depressive and anxiety symptoms in 
patients with major depressive disorder.715–717 Similarly, oxyto­
cin has been proposed to be involved in the pathophysiology 
of autism, as indicated by a reduction in plasma oxytocin levels 
and an increase in a biologically inactive extended form of oxy­
tocin, as well as improved social

ded form of oxy­
tocin, as well as improved social interactions and reduced repeti­
tive behaviors following intranasal oxytocin administration.713 
Orexin deficiency and downregulation of orexin receptors have 
been observed in depression and may contribute to the disrupted 
sleep pattern associated with this disorder.714,718 In addition, 
the orexin receptor-2 (OxR2) knockout mouse demonstrates 
increased depressive-like behaviors, and in other studies, ICV 
administration of orexin improved s

 studies, ICV 
administration of orexin improved some of the manifestations 
of depression.719,720 Antidepressant actions of kisspetin,721 
the potential antidepressant action of somatostatin mediated 
through heterodimerization of somatostatin with dopamine D2 
receptors722 and elevated levels of nesfatin-1 in major depressive 
disorder,723 further indicate that there is much yet to be learned 
about the role of peptides and the hypothalamus in neuropsy­
chiatric disease. 
Acknowledgments
The a

neuropsy­
The author greatly appreciates the important contributions made 
to previous versions of this chapter by Drs. Harry Friedgood, John 
D. Green, Monte Greer, Seymour Reichlin, Roger D. Cone, Joel 
Elmquist, Judy L. Cameron, and Malcolm Low. Special thanks to 
Dr. Seymour Reichlin for his teaching, guidance, and important 
work he has done to shape the direction of neuroendocrinology.
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184
8
Pituitary Physiology and 
Diagnostic Evaluation
URSULA KAISER AND KEN HO
Anatomy, Development, and Overview of 
Control of Hormone S

Development, and Overview of 
The pituitary gland, situated within the sella turcica, derives its name 
from the Greek ptuo and Latin pituita, meaning “phlegm,” reflecting 
its nasopharyngeal origin. Galen hypothesized that nasal phlegm orig­
inated from the brain and drained via the pituitary gland. It is now 
clear that, together with the hypothalamus, the pituitary orchestrates 
the structural integrity and function of endocrine glands, including 
the thyroid, adr

n of endocrine glands, including 
the thyroid, adrenal, and gonads, in addition to target tissues, includ­
ing liver, cartilage, and breast. The pituitary stalk serves as an anatomic 
and functional connection to the hypothalamus. Preservation of the 
hypothalamo-pituitary unit is critical for integration of systemic and 
central nervous system (CNS) inputs for anterior pituitary control of 
sexual function and fertility, linear and organ growth, lactation, stress 
responses, energy, appetite, a

 lactation, stress 
responses, energy, appetite, and temperature regulation, and second­
arily for carbohydrate and mineral metabolism.
Integration of vital body functions by the brain was first 
proposed by Descartes in the 17th century. In 1733, Morgagni 
recorded the absence of adrenal glands in an anencephalic neo­
nate, providing early evidence for a developmental and func­
tional connection between the brain and the adrenal glands. 
In 1849, Claude Bernard set the stage for the subsequent 

, Claude Bernard set the stage for the subsequent 
advances in neuroendocrinology by demonstrating that central 
lesions to the area of the fourth ventricle resulted in polyuria.1 
Subsequent studies led to the identification and chemical isola­
tion of pituitary hormones, and astute clinical observations led 
to the realization that pituitary tumors were associated with 
functional hypersecretory syndromes, including acromegaly 
and Cushing disease.2–4 In 1948, Geoffrey Harris, the father of 
m

ase.2–4 In 1948, Geoffrey Harris, the father of 
modern neuroendocrinology, reviewed the control of anterior 
pituitary gland hormones and proposed their hypothalamic 
regulation, predicting the subsequent discovery of specific 
hypothalamic regulating hormones.5
KEY POINTS
•  The pituitary gland orchestrates endocrine system integrity 
through central hypothalamic and peripheral hormonal signals.
•  Pituitary gland cells are organized into structural and functional 
networks formed during embry

tural and functional 
networks formed during embryonic development but modified 
throughout life.
•  Differentiated lactotrophs, somatotrophs, gonadotrophs, cortico­
trophs, and thyrotrophs are each regulated by system-specific 
factors.
•  The growth hormone axis controls growth in childhood while 
regulating energy and substrate metabolism throughout life.
•  Prolactin, responsible primarily for milk production during preg­
nancy and lactation, is uniquely under tonic inhibitory hypotha­
lamic

 is uniquely under tonic inhibitory hypotha­
lamic control by dopamine.
•  The hypothalamic-pituitary-adrenal axis is the pivotal system 
subserving stress and survival, elaborated through effects on 
energy supply, fuel metabolism, immunity, and cardiovascular 
function.
•  The hypothalamic-pituitary-gonadal axis plays a pivotal role in 
puberty, reproductive function, and fertility, controlling both 
gametogenesis and sex steroid hormone production.
•  The hypothalamic-pituitary-thyroid axis p

tion.
•  The hypothalamic-pituitary-thyroid axis plays a critical role in 
development, growth, and cellular metabolism, mediated by 
thyroid hormones.
•  Pituitary gland failure can arise from developmental, heritable, 
and acquired disorders and is diagnosed by baseline and pro­
vocative pituitary and target gland hormone testing.
Anatomy, Development, and Overview of Control of Hormone 
Secretion, 184
Physiology and Disorders of Pituitary Hormone Axes, 190
 Developmental, Gene

f Pituitary Hormone Axes, 190
 Developmental, Genetic, and Acquired Causes of Pituitary 
Failure, 228
185
Chapter 8  Pituitary Physiology and  Diagnostic Evaluation
Anatomy
The pituitary gland comprises the predominant anterior lobe, the 
posterior lobe, and a vestigial intermediate lobe (Fig. 8.1). The 
gland is situated within the bony sella turcica and is overlain by 
the dural diaphragma sellae through which the stalk connects to 
the median eminence of the hypothalamus. The adult pituitary 

eminence of the hypothalamus. The adult pituitary 
weighs approximately 600 mg (range 400–900 mg) and measures 
approximately 13 mm in the longest transverse diameter, 6 to 9 
mm in vertical height, and about 9 mm anteroposteriorly. Struc­
tural variation may occur in multiparous women, and gland vol­
ume also changes during the menstrual cycle. During pregnancy, 
these measurements may be increased in either dimension, with 
pituitary weight increasing up to 1 g. Pituitary hypertrophy with­
out

reasing up to 1 g. Pituitary hypertrophy with­
out evidence for the presence of an adenoma was described in 
seven eugonadal women with pituitary height greater than 9 mm 
and a convex upper gland boundary observed on magnetic reso­
nance imaging (MRI).6
The sella turcica, located at the base of the skull, forms the 
thin bony roof of the sphenoid sinus. The lateral walls comprise 
either bony or dural tissue about the cavernous sinuses, which are 
traversed by the third, fourth, and six cranial

e 
traversed by the third, fourth, and six cranial nerves and internal 
carotid arteries. Thus the cavernous sinus contents are vulnerable 
to intrasellar expansion. The dural roofing protects the gland from 
compression by fluctuant cerebrospinal fluid (CSF) pressure. The 
optic chiasm, located anterior to the pituitary stalk, is directly 
above the diaphragma sellae. The optic tracts and central struc­
tures are therefore vulnerable to pressure effects by an expand­
ing pituitary mass, which t

 effects by an expand­
ing pituitary mass, which typically follows the path of least tissue 
resistance. The posterior pituitary gland, in contrast to the anterior 
pituitary, is directly innervated by the supraopticohypophyseal 
and tuberohypophyseal nerve tracts of the posterior stalk. Hypo­
thalamic neuronal lesions, stalk disruption, or systemically derived 
metastases to the hypothalamus are therefore often associated with 
attenuated vasopressin (diabetes insipidus) or oxytocin secretion.

essin (diabetes insipidus) or oxytocin secretion.
The hypothalamus contains, among other neuronal popula­
tions, nerve cell bodies that synthesize hypophysiotropic releasing 
and inhibiting hormones, as well as the neurohypophyseal hor­
mones of the posterior pituitary (arginine vasopressin [AVP] and 
oxytocin). Five distinct hormone-secreting cell types are present in 
the mature anterior pituitary gland:
	1.	 Corticotroph cells express pro-opiomelanocortin (POMC) 
peptides, including adrenoco

n (PRL).
Optic chiasm
Supraoptico-hypophyseal tract
Paraventriculo-hypophyseal tract
Hypothalamo-hypophyseal tract
Long stalk arteries
Artery of the fibrous core
Short portal vessels
Communicating artery
Ascending branch
of inferior
hypophyseal
artery  
Posterior (neural) lobe
Descending branch of
inferior hypophyseal artery
Capsular
artery
Inferior hypophyseal artery
(branch of cavernous portion
of interior carotid artery)
Tubero-hypophyseal tract
Tuber cinereum,
median eminence,
and infundibul

ct
and infundibulum
Anterior branches of
hypophyseal artery
Posterior branch of
superior hypophyseal
artery
Superior hypophyseal artery
(branch intracranial portion
“Gomitoll”
Short stalk arteries
Trabecular artery
(loral artery)
Pars tuberalis
Long portal vessels
Vein
Vein
Vein
Anterior
lobe
Pituitary
sinusoids
• Fig. 8.1  Schematic representation of the blood supply of the hypothalamus and pituitary. (From 
Scheithauer BW. The hypoth

s and pituitary. (From 
Scheithauer BW. The hypothalamus and neurohypophysis. In: Kovacs K, Asa SL, eds. Functional Endo­
crine Pathology. Oxford, UK: Blackwell Scientific; 1991.)
186
Each cell type is under highly specific signal controls that regu­
late their respective differentiated gene expression. The pituitary 
gland also contains support cells, known as pituicytes or follicu­
lostellate cells. 
Pituitary Blood Supply
The pituitary gland enjoys an 

uitary Blood Supply
The pituitary gland enjoys an abundant blood supply derived 
from several sources (see Fig. 8.1). The superior hypophyseal 
arteries branch from the internal carotid arteries to supply the 
hypothalamus, where they form a capillary network in the median 
eminence, which is external to the blood-brain barrier. Both long 
and short hypophyseal portal vessels originate from infundibular 
plexuses and the stalk, respectively. These vessels form the hypo­
thalamic-portal circulati

e vessels form the hypo­
thalamic-portal circulation, the predominant blood supply to 
the anterior pituitary gland. They deliver hypothalamic releasing 
and inhibiting hormones to the trophic hormone-producing cells 
of the adenohypophysis, without significant systemic dilution, 
allowing the pituitary cells to be sensitively regulated by timed 
hypothalamic hormone secretion. Vascular transport of hypotha­
lamic hormones is also locally regulated by a contractile internal 
capillary plexus (go

ed by a contractile internal 
capillary plexus (gomitoli), derived from stalk branches of the 
superior hypophyseal arteries.7 Retrograde blood flow toward the 
median eminence also occurs, facilitating bidirectional functional 
hypothalamic-pituitary interactions.8 Systemic arterial blood sup­
ply is maintained by inferior hypophyseal arterial branches, which 
predominantly supply the posterior pituitary. Disruption of stalk 
integrity may lead to compromised pituitary portal blood flow, 
depri

to compromised pituitary portal blood flow, 
depriving the anterior pituitary cells of hypothalamic hormone 
input. 
Pituitary Development
The pituitary gland arises from within the rostral neural plate. 
Rathke pouch, a primitive ectodermal invagination anterior to 
the roof of the oral cavity, is formed by the fourth or fifth 
week of gestation9 and gives rise to the anterior pituitary 
gland10,11 (Fig. 8.2). The pouch is directly connected to the 
stalk and hypothalamic infundibulum and ultim

the 
stalk and hypothalamic infundibulum and ultimately becomes 
distinct from the oral cavity and nasopharynx. Rathke pouch 
proliferates toward the third ventricle, where it fuses with the 
diverticulum and subsequently obliterates its lumen, which 
may persist as Rathke cleft. The anterior lobe is formed from 
Rathke pouch, and the diverticulum gives rise to the adjacent 
posterior lobe. Remnants of pituitary tissue may persist in the 
nasopharyngeal midline and rarely give rise to functional

ryngeal midline and rarely give rise to functional ecto­
pic hormone-secreting tumors in the nasopharynx. The neuro­
hypophysis arises from neural ectoderm associated with third 
ventricle development.12
Functional development of the anterior pituitary cell types 
involves complex spatiotemporal regulation of cell lineage–spe­
cific transcription factors expressed in pluripotential pituitary 
stem cells, as well as dynamic gradients of locally acting soluble 
factors.13–16 Critical neuroectoderm

ting soluble 
factors.13–16 Critical neuroectodermal signals for organizing the 
dorsal gradient of pituitary morphogenesis include infundibular 
bone morphogenetic protein 4 (BMP4) required for the initial 
pouch invagination,11 fibroblast growth factor 8 (FGF8), FGF10, 
Wnt5, and Wnt4.17 Subsequent ventral developmental patterning 
and transcription factor expression are determined by spatial and 
graded expression of factors, including BMP2 and sonic hedgehog 
(SHH) protein, which appear crit

d sonic hedgehog 
(SHH) protein, which appear critical for directing early patterns 
of cell proliferation.18
The human fetal Rathke pouch is evident at 3 weeks, and the 
pituitary grows rapidly in utero. By 7 weeks, the anterior pituitary 
vasculature begins to develop; by 20 weeks, the entire hypophy­
seal portal system is established. The anterior pituitary undergoes 
major cellular differentiation during the first 12 weeks, by which 
time all the major secretory cell compartments are structu

 the major secretory cell compartments are structurally and 
functionally intact, except for lactotrophs. Totipotential pituitary 
stem cells give rise to acidophilic (mammosomatotroph, somato­
troph, and lactotroph) and basophilic (corticotroph, thyrotroph, 
and gonadotroph) differentiated pituitary cell types, which appear 
at clearly demarcated developmental stages. At 6 weeks, cortico­
troph cells are morphologically identifiable, and immunoreac­
tive ACTH is detectable by 7 weeks. At 8 week

eac­
tive ACTH is detectable by 7 weeks. At 8 weeks, somatotroph 
cells are evident with abundant immunoreactive cytoplasmic GH 
expression. Glycoprotein hormone–secreting cells express a com­
mon α-subunit, and at 12 weeks, differentiated thyrotrophs and 
gonadotrophs express immunoreactive β-subunits for TSH, LH, 
and FSH, respectively.19 Fully differentiated PRL-expressing lac­
totrophs are only evident late in gestation (after 24 weeks). Prior 
to that time, immunoreactive PRL is only detect

r 
to that time, immunoreactive PRL is only detectable in mixed 
mammosomatotrophs, also expressing GH, reflecting the com­
mon genetic origin of these two hormones.20 
Pituitary Transcription Factors
Determination of anterior pituitary cell type lineages results from 
a temporally regulated cascade of homeodomain transcription fac­
tors.13 Although most pituitary developmental information has 
been acquired from murine models,21 histologic and pathogenetic 
observations in human subjects largel

athogenetic 
observations in human subjects largely corroborate these devel­
opmental mechanisms (see Fig. 8.2; Tables 8.1 and 8.2). Early 
cell differentiation requires intracellular HESX1 and PITX expres­
sion. Rathke pouch expresses several transcription factors of the 
LIM homeodomain family, including LHX3, LHX4, and ISL1,21 
which are early determinants of functional pituitary development 
and are required for progenitor cell survival and proliferation. In 
contrast, activated Notch2 delay

roliferation. In 
contrast, activated Notch2 delays murine gonadotroph differen­
tiation,22 underscoring the role of NOTCH signaling pathways 
in the developmental cascade. Diversity of pituitary cell type 
determination is mediated by binary WNT/β-catenin signaling, 
leading to suppression of HESX1 and induction of Prophet of 
PIT 1 (PROP1).23 These specific anterior pituitary transcription 
factors participate in a highly orchestrated cascade, ultimately 
leading to the commitment of the five 

ultimately 
leading to the commitment of the five differentiated cell types 
(see Fig. 8.2). The major proximal determinant of pituitary cell 
lineage is Prop1,24 which determines subsequent development of 
POU1F1(PIT1)-dependent and gonadotroph cell lineages, while 
corticotroph cell commitment is directed by TBX19 protein.25,26
The bicoid homeodomain proteins, PITX1 and PITX2, behave 
as universal pituitary regulators and activate transcription of all the 
major anterior pituitary hormones.27,

 of all the 
major anterior pituitary hormones.27,28 PITX1 is expressed in the 
oral ectoderm and subsequently in all pituitary cell types, particu­
larly those arising ventrally.29 Rieger syndrome, characterized by 
defective eye, tooth, umbilical cord, and pituitary development, 
is caused by mutations in PITX2.30 LHX3 determines GH, PRL, 
and TSH cell differentiation, and Prop1, a member of the paired-
like family of homeodomain transcription factors, is expressed 
early in the development of

factors, is expressed 
early in the development of Rathke pouch and behaves as a pre­
requisite for PIT1. PIT1 is a POU homeodomain transcription 
factor that determines development and appropriate temporal 
and spatial expression of GH, PRL, and TSH binding to specific 
DNA motifs to activate and regulate somatotroph, lactotroph, 
187
and thyrotroph development and mature secretory function. 
Signal-dependent coactivating factors cooper

ion. 
Signal-dependent coactivating factors cooperate with PIT1 to 
determine specific hormone expression. Thus in POU1F1-con­
taining cells, high estrogen receptor levels induce a commitment 
to express PRL, and thyrotroph embryonic factor (TEF) favors 
TSH expression. Selective pituitary cell type specificity is also per­
petuated by binding of PIT1 to its own DNA regulatory elements 
as well as those contained within the GH, PRL, and TSH genes. 
Steroidogenic factor 1 (SF1) and DAX1 determine

. 
Steroidogenic factor 1 (SF1) and DAX1 determine gonadotroph 
development.31,32 TSH and gonadotropin-expressing cells share 
G1i1  Notch  Lhx3,4
Pitx1,2  Hesx1 
IsI1 Pax6 
Six3,6  
Neural
epithelium
Oral
ectoderm
Rathke’s
pouch
Primordial
cell
Pituitary
commitment
Proliferation
Appearance
of cell
lineages
and cell
determination
Cell
differentiation 
FGF8
BMP4
Otx2
Wnt5a
Nkx2.1
Lineage
precursors
Gata-2
Prop-1
Pit-1
ER
Zn15
     Otx1 
            Egr1
NeuroD1
     Tpit
          LIF
SF-1,
DAX-1

  Egr1
NeuroD1
     Tpit
          LIF
SF-1,
DAX-1
Fetal
appearance
12 weeks
12 weeks
12 weeks
8 weeks
8 weeks
FSH, LH
TSH
PRL
GH
ACTH
Gonadotroph
Thyrotroph
Lactotroph
Somatotroph
Corticotroph
Hormone
FSH    LH
TSH
PRL
GH
POMC
Chromosomal
gene locus
β-11p; β-19q
α-6q; β-1p
6
17q
2p
Protein
Glycoprotein
α, β
subunits
α, β
subunits
Polypeptide
Amino acids
210    204
211
199
191
266 (ACTH 1-39)
Inhibitors
Sex steroids,
inhibition
T3,T4,
Dopamine,
somatostatin
g

eroids,
T3,T4,
Dopamine,
glucocorticoids
Dopamine
Somatostatin, IGF
activins
Stimulators
GnRH, estrogen
TRH
Estrogen, TRH
GHRH
GHS
CRH, AVP
gp-130 cytokines
Ovary, testis
Thyroid
Breast, other
tissues
Liver, bones, other
tissues
Adrenal
Trophic effect
Sex steroid
Follicle growth
Germ cell maturation
M, 5-20 IU/L
F (basal) 5-20 IU/L
T4
Synthesis and 
secretion
Milk production
IGF-I production,
growth induction,
insulin antagonism
Steroid produc

rowth induction,
Steroid production
Normal range
0.1-5 mU/L
M <15; F <20 µg/L
<0.5 µg/L
ACTH, 4-22 pg/L
• Fig. 8.2  Model for development of the human anterior pituitary gland and cell lineage determination by a 
cascade of transcription factors. Trophic cells are depicted with transcription factors known to determine 
cell-specific human or murine gene expression. ACTH, adrenocorticotropic hormone; AVP, arginine vaso­
pressin; CRH, corticotrop

one; AVP, arginine vaso­
pressin; CRH, corticotropin-releasing hormone; ER, estrogen receptor; F, female; FSH, follicle-stimulating 
hormone; GH, growth hormone; GHRH, growth hormone–releasing hormone; GHS, growth hormone 
secretagogue; GnRH, gonadotropin-releasing hormone; IGF, insulin-like growth factor; LH, luteinizing hor­
mone; M, male; POMC, pro-opiomelanocortin; PRL, prolactin; T3, triiodothyronine; T4, thyroxine; TRH, 
thyrotropin-releasing hormone; TSH, thyrotropin. (Adapted from Shimon

ng hormone; TSH, thyrotropin. (Adapted from Shimon I, Melmed S. Anterior pituitary 
hormones. In: Conn P, Melmed S, eds. Scientific Basis of Endocrinology. Totowa, NJ: Humana Press; 
1996; Amselem S. Perspectives on the molecular basis of developmental defects in the human pitu­
itary region. In: Rappaport R, Amselem S, eds. Hypothalamic-Pituitary Development. Basel, Switzerland: 
Karger; 2001; Dasen JS, O’Connell SM, Flynn SE, et al. Reciprocal interactions of PIT1 and GATA2 medi­
ate signaling

interactions of PIT1 and GATA2 medi­
ate signaling gradient-induced determination of pituitary cell types. Cell. 1999;97:587–598.)
188
a common α-subunit (αGSU) expression under developmental 
control of GATA2.33 FOXL2, a forkhead transcription factor, 
regulates differentiation of cell types expressing αGSU, including 
gonadotrophs and thyrotrophs, and transcription of αGSU and 
FSHβ.17,34–37 Corticotroph cell commitment, occurring earliest 
during fetal

 cell commitment, occurring earliest 
during fetal development, is independent of PROP1-determined 
lineages, and TBX19 protein is a prerequisite for POMC expres­
sion.38 Hereditary mutations arising within these transcription 
factors may result in isolated or combined pituitary hormone fail­
ure syndromes (see later).39 
Pituitary Stem Cells
The adult pituitary gland exhibits plastic and regenerative trophic 
properties, which allow maintenance of homeostatic functions.40 This 
characteristic 

 of homeostatic functions.40 This 
characteristic is exemplified by pituitary lactotroph expansion during 
pregnancy and pituitary trophic hormone cell hyperplasia occurring 
after target organ ablation. Mechanisms underlying adult pituitary 
cell renewal and expansion are as yet unclear and may include intrin­
sic pituitary transdifferentiation, differentiation of previously uncom­
mitted “null” cells, or expansion of already differentiated cells.
Several lines of evidence support the existence

s.
Several lines of evidence support the existence of cells with stem 
or progenitor characteristics in the adult pituitary gland.41–45 Pitu­
itary progenitor cells exhibit several characteristics of a stem cell 
phenotype, including an undifferentiated gene profile, clonality, the 
ability to form colonies, and expression of known stem cell markers, 
including Scal and CD133. Other markers, including Notch, Wnt, 
and SHH, are essential transcription factors for cell type determi­
nation and exp

tion factors for cell type determi­
nation and expansion of pituitary cell lineages.46,47 Evidence has 
emerged that SOX2, SOX9, and OCT4 are markers of pituitary 
progenitors with properties for multipotent pituitary cell differen­
tiation,48 and a population of nestin-expressing murine pituitary 
cells fulfill criteria consistent with organ-specific multipotent stem 
cells.45 These cells form differentiated pituitary-expressing progeny 
and contribute to an adult pituitary stem cell population

tribute to an adult pituitary stem cell population distinct 
from embryonic precursor cells. Expression of potential stem cell 
markers such as nestin in the marginal zone around Rathke cleft has 
suggested that the stem cell population may exist in this marginal 
zone.49 Cell-lineage tracing analysis has demonstrated that SOX2-
expressing and SOX9-expressing progenitors can self-renew and 
give rise to pituitary endocrine cells in vivo, supporting the model 
of these as tissue stem cells.43,50,

ng the model 
of these as tissue stem cells.43,50,51 Moreover, these cells can become 
mobilized and differentiate toward a specific cell fate in response to 
physiologic stress.50,51 Advances have also been made toward reca­
pitulation of pituitary differentiation in vitro: Three-dimensional 
(3D) cultures of embryonic stem cells have been stimulated to dif­
ferentiate into Rathke pouch–like 3D structures, and various endo­
crine cells, including functional corticotrophs and somatotrophs, 
were

g functional corticotrophs and somatotrophs, 
were subsequently produced, opening new avenues for the applica­
tion of pluripotent stem cells to treat hypopituitarism.52,53 Interest­
ingly, adamantinomatous craniopharyngiomas share expression of 
stem cell markers with pituitary progenitor/stem cells, suggesting a 
common origin.54 
Pituitary Control
The endocrine and nonendocrine cells of the pituitary gland are 
organized into structural and functional networks formed during 
embryonic develop

nctional networks formed during 
embryonic development but modified throughout life.55 The various 
endocrine cell types form distinct networks in spatial relationships 
with the vasculature, which can explain the marked secretory capacity 
not evident from those observed from dispersed cells in culture.56 The 
functional characterization of the network activity of GH, PRL, and 
gonadotropin has provided firm evidence for cell organization in gene 
regulation, in magnitude, and in temporal facet

e 
regulation, in magnitude, and in temporal facets of hormone secre­
tion. As such, the existence of these endocrine cell networks confers 
the pituitary as more than a gland that simply responds to external 
regulation; rather, it acts as an oscillator that can imprint memory and 
adapt to coordinated network responses to hypothalamic inputs.56
Three levels of control subserve the regulation of anterior 
pituitary hormone secretion (Fig. 8.3). Hypothalamic control is 
mediated by adenohypophys

Hypothalamic control is 
mediated by adenohypophysiotropic hormones secreted into the 
hypothalamic portal system to impinge directly upon anterior 
  Etiology of Inherited Pituitary Deficiency
Mutation
Hormone Deficit
Receptor
GHRH receptor
GH
CRH receptor
ACTH
GnRH receptor
FSH, LH
TRH receptor
TSH
Structural
Pituitary aplasia
Any
Pituitary hypoplasia
Any
CNS masses; encephalocele
Any
Transcription Factor Defect
HESX1
GH, PRL, TSH, LH, FSH, ACTH
SOX2/3
LHX3/4
GH, P

TH
SOX2/3
LHX3/4
GH, PRL, TSH, LH, FSH
PITX2
GH
PROP1
POU1F1
PRL, GH, TSH
IGSF1
TBX19
ACTH
NR5A1
LH, FSH
NR0B1
LH, FSH
Hormone Mutation
GH1
GH
Bioinactive GH
GH
FSHβ
FSH
LHβ
LH
TSHβ
TSH
POMC
ACTH
POMC processing defect
ACTH
PC1
ACTH, FSH, LH
ACTH, Adrenocorticotropic hormone; CNS, central nervous system; CRH, corticotropin-
releasing hormone; FSH, follicle-stimulating hormone; GH, growth hormone; GHRH, growth 
hormone–releasing

H, growth hormone; GHRH, growth 
hormone–releasing hormone; GnRH, gonadotropin-releasing hormone; GPR, orphan G pro­
tein–coupled receptor; LH, luteinizing hormone; POMC, pro-opiomelanocortin; PRL, prolactin; 
TSH, thyroid-stimulating hormone; TRH, thyrotropin-releasing hormone.
   
TABLE 8.1
189
pituitary cell surface receptors. G protein–coupled cell surface 
membrane binding sites are highly selective and specific for each 
of the hyp

highly selective and specific for each 
of the hypothalamic hormones and elicit positive or negative sig­
nals mediating pituitary hormone gene transcription and secre­
tion. Peripheral hormones also participate in mediating pituitary 
cell function, predominantly by negative feedback regulation of 
trophic hormones by their respective target hormones. Intrapitu­
itary paracrine and autocrine soluble growth factors and cytokines 
act to locally regulate neighboring cell development and function.

egulate neighboring cell development and function. 
The net result of these three tiers of complex intracellular signals 
is the controlled pulsatile secretion of the six pituitary trophic 
hormones—ACTH, GH, PRL, TSH, FSH, and LH (Fig. 8.4). 
Temporal and quantitative control of pituitary hormone secretion 
is critical for physiologic integration of peripheral hormonal sys­
tems, as exemplified by the menstrual cycle, which relies on com­
plex and precisely regulated hormonal pulse control. 
  
nd precisely regulated hormonal pulse control. 
  Hereditary Pituitary Deficiency Caused by Transcription Factor Mutations
Gene
Chromosome
Pituitary Deficiency
MRI
Associated 
Malformations
Inheritance Mode
POU1F1
3p11
GH, PRL, ± TSH
Normal or hypoplastic anterior 
pituitary
Recessive, dominant
PROP1
5q35
GH, PRL, TSH, LH, FSH, 
± ACTH
Normal, hypoplastic, hyperplas­
tic, or cystic anterior pituitary
Recessive
HESX1
3p21
ACTH
Posterior defects
Hypoplastic or hyperplastic

ACTH
Hypoplastic or hyperplastic 
anterior pituitary; normal or 
ectopic posterior pituitary
Septo-optic dysplasia
Recessive
PITX2
4q25
GH, PRL, TSH, FSH, LH
Rieger syndrome
Dominant
LHX3
9q34
anterior pituitary
Stubby neck with rigid 
cervical spine
Recessive
LHX4
1q25
GH, TSH, ACTH
Hypoplastic anterior pituitary
Ectopic posterior pituitary
Dominant
TBX19
1q23
ACTH
Normal
Recessive
OTX2
ACTH
Hypoplastic

sive
OTX2
ACTH
Eye malformations
Dominant/negative
SIX6
14q22
Hypoplastic pituitary
Absent chiasm
Brachio-otorenal and 
oculoauriculo-verte­
bral syndromes
Haplo-insufficiency
SOX2
3q26
GH, FSH, LH
Anterior pituitary hypoplasia 
Midbrain defects
Anophthalmia
Esophageal atresia
SOX3
Xq27
GH, TSH, ACTH, FSH, LH
Anterior pituitary hypoplasia
X-linked recessive
IGSF1
Xq25
GH, PRL, TSH
Testi

y
IGSF1
Xq25
Testicular enlargement
NR5A1
9q33
FSH, LH
Adrenal insufficiency, 
gonadal defects, XY 
sex reversal
Dominant, recessive
NR0B1
Xp21.3
FSH, LH
Adrenal hypoplasia 
congenital, gonadal 
defects, XY sex 
reversal
X-linked dominant
GPR161
1q24.2
GH, TSH
Pituitary stalk interruption 
syndrome
Alopecia, ptosis, short 
fifth finger
Recessive
ARNT2
15q25.1
ACTH, TSH, GH, LH, FSH
Hypoplastic anterior pituitary 
and pituitary stalk inter­
rupti

terior pituitary 
ruption
Microcephaly, dysmor­
phic facies, visual 
and renal abnormali­
ties
Recessive
NFKB2
10q24
ACTH, GH, TSH
Variable immune 
deficiency
Dominant
Genes involved in pituitary development or in maintaining integrity of the hypothalamic-pituitary axis. Functional defects include missense or frameshifts leading to truncated or deleted protein, DNA 
binding abnormality.
   
TABLE 8.2
190
Physiology and Disorders

ypothalamus and Pituitary
Physiology and Disorders of Pituitary 
Hormone Axes
Prolactin
PRL is produced primarily by pituitary lactotrophs and is tonically 
inhibited by hypothalamic dopamine. The identification of PRL in 
humans was elusive until 1970 because human GH is highly lac­
togenic and active in bioassays used to isolate and measure PRL.57 
Furthermore, GH is present in human pituitary glands in much 
higher concentrations (5–10 mg) than PRL (∼100 μg).58 To dis­
tinguish hum

–10 mg) than PRL (∼100 μg).58 To dis­
tinguish human PRL from GH, lactogenic activity derived from 
GH effects was neutralized with GH antiserum; using this assay, 
sera from postpartum women and patients with galactorrhea still 
had high lactogenic activity in the presence of GH antibodies.59,60 
Purification and isolation of PRL by Friesen and development of 
a specific radioimmunoassay underscored the utility of PRL mea­
surements in understanding human disease.61,62
Lactotroph Cells
About 15

ding human disease.61,62
About 15% to 25% of functioning anterior pituitary cells are 
lactotroph cells. Most PRL-expressing cells appear to arise from 
GH-producing cells. Ablation of somatotrophs by expression of 
GH-diphtheria toxin and GH-thymidine kinase fusion genes 
inserted into the germline of transgenic mice eliminates most 
lactotrophs, suggesting that the majority of PRL-producing 
cells arise from postmitotic somatotrophs in mice.63 Two cell 
forms expressing the PR

rophs in mice.63 Two cell 
forms expressing the PRL gene include large polyhedral cells 
found throughout the gland and smaller angulated or elon­
gated cells clustered in the lateral wings and median wedge. 
Large PRL secretory granules (250–800 nm) are present in the 
evenly distributed cells, while the laterally localized cells are 
sparsely populated by smaller (200–350 nm) granules. Occa­
sional mammosomatotroph cells cosecrete both PRL and GH, 
often stored within the same granule. In anim

GH, 
often stored within the same granule. In animal models, lacto­
troph cell function is heterogeneous. Thus dopamine or thyro­
tropin-releasing hormone (TRH) responsiveness, and shifting 
proportions of PRL versus GH secreting cells, may depend on 
cell localization within the pituitary, as well as the surround­
ing hormonal milieu, especially that of estrogen.64 Although 
their absolute number is similar in men and women and does 
not change with age, lactotroph hyperplasia does occur during

with age, lactotroph hyperplasia does occur during 
pregnancy and lactation65 and resolves within several months 
of delivery. 
CNS Inputs
hormone
Pituitary
Tier II
Tier III
hormones
Tier I
hormones
• Fig. 8.3  Model for regulation of anterior pituitary hormone secretion by 
three tiers of control. Hypothalamic hormones traverse the portal system 
and impinge directly upon their respective 

ystem 
and impinge directly upon their respective target cells. Intrapituitary cyto­
kines and growth factors regulate tropic cell function by paracrine (and 
autocrine) control. Peripheral hormones exert negative feedback inhibi­
tion of respective pituitary trophic hormone synthesis and secretion. CNS, 
central nervous sys­tem. (From Ray D, Melmed S. Pituitary cytokine and 
growth factor expression and action. Endocr Rev. 1997;18:206–228.)
CRH
Cortico-
troph
ACTH
Adrenal
glands
Co

thalamus
CRH
Cortico-
troph
ACTH
Adrenal
glands
Cortisol
Cell homeostasis
and function
TRH
Thyro-
troph
TSH
Thyroid
gland
T4/T3
Thermogenesis
Protein synthesis
GnRH
Gonado-
troph
LH
FSH
Ovaries
Estradiol
Testes
Ovulation
Spermatogenesis
inhibin
GHRH
Somato-
statin
Somato-
troph
GH
Liver
IGF-I
Chondrocytes
Linear and
organ growth
Dopa-
mine
Lacto-
troph
PRL
Lactation
Pituitary cell
+
+
+
+
+
+
+
+
+
+
+
+
+
+
–
–
–
–
–
–
–
–
–
–
–
–
–
–
Trophic
hormone
Targe

–
–
–
–
–
–
–
–
–
–
–
–
–
–
Trophic
hormone
Target organ
Target
hormone
Action
• Fig. 8.4  Control of hypothalamic-pituitary target organ axes. ACTH, adrenocorticotropic hormone; CRH, 
hormone–releasing hormone; GnRH, gonadotropin-releasing hormone; IGF, insulin-like growth factor; LH, 
luteinizing hormone; PRL, prolactin; T3, triiodothyronine; T4, thyroxine; TRH, thyrotropin-releasing hor­
mon

T4, thyroxine; TRH, thyrotropin-releasing hor­
mone; TSH, thyroid-stimulating hormone. (Adapted from Melmed S. Mechanisms for pituitary tumorigen­
esis: the plastic pituitary. J Clin Invest. 2003;112:1603–1618.)
191
Prolactin Structure
The human PRL gene, located on chromosome 6,66 arose from 
a single common ancestral gene, which gave rise to the relatively 
homologous PRL, GH, and placental lactogen-related proteins.67 
Several factors

ntal lactogen-related proteins.67 
Several factors influence PRL gene expression, including estrogen, 
dopamine, and TRH.68 The PRL gene is transcribed from two 
alternative promoter regions, with the more proximal directing 
pituitary-specific expression and the more upstream promoter 
region directing extrapituitary expression (Fig. 8.5; see also Fig. 
8.2).69 PRL is a 199–amino acid polypeptide containing three 
intramolecular disulfide bonds (see Fig. 8.5). It circulates in blood 
in various

(see Fig. 8.5). It circulates in blood 
in various sizes—monomeric PRL (“little prolactin”; 23 kDa), 
dimeric PRL (“big prolactin”; 48–56 kDa), and macroprolactin 
(“big, big prolactin,” comprised of prolactin bound to immu­
noglobulins; >100 kDa).69a–72 The monomeric form is the most 
bioactive PRL. In response to TRH, the proportion of the mono­
meric form increases. A glycosylated form of PRL, identified in 
pituitary extracts, is less biologically active than the nonglycosyl­
ated form.73 Mo

cally active than the nonglycosyl­
ated form.73 Monomeric PRL is cleaved into 8-kDa and 16-kDa 
forms,74 and the 16-kDa variant is antiangiogenic.75,76 Indeed, 
this 16-kDa PRL cleavage product has been implicated in peri­
partum cardiomyopathy.77 The mechanism underlying this asso­
ciation was recently shown to result from actions of the 16-kDa 
PRL, also referred to as vasoinhibin,78 on capillary endothelial 
cells, stimulating the production of miR-146a, which in turn 
acts through a paracrin

f miR-146a, which in turn 
acts through a paracrine mechanism on cardiomyocytes to inhibit 
metabolism by blocking the activity of ERBB4, a tyrosine kinase 
receptor, to impair cardiomyocyte function. Cardiac dysfunction 
could be improved by treatment with the dopamine agonist bro­
mocriptine, which results in a fall in miR-146a levels in parallel 
with an improvement in cardiac function.79–81 
PRL secretion is under inhibitory control by dopamine, pro­
duced by the tuberoinfundibula

l by dopamine, pro­
duced by the tuberoinfundibular (TIDA) cells and the hypotha­
lamic tuberohypophyseal dopaminergic system.82,83 Dopamine 
reaches the lactotrophs via the hypothalamic-pituitary portal 
system and binds to type 2 dopamine (D2) receptors on pituitary 
lactotrophs to inhibit PRL secretion.84 PRL, in turn, participates 
in negative feedback to control its release by increasing tyrosine 
hydroxylase activity, and thereby dopamine synthesis in the 
TIDA neurons.83 In PRL-deficient 

ynthesis in the 
TIDA neurons.83 In PRL-deficient mice, dopamine is decreased 
in the median eminence.85 Mice lacking the D2 receptor develop 
hyperprolactinemia and lactotroph proliferation.84 Many other 
factors modulate PRL secretion, although their physiologic or 
clinical relevance remains unclear. Factors other than dopamine 
inhibit PRL secretion, including endothelin-1 and TGFβ1, 
which act as paracrine PRL inhibitors,86,87 and calcitonin, which 
may be derived from the hypothalamus.88 S

, which 
may be derived from the hypothalamus.88 Several substances act 
as PRL-releasing factors. Growth factors such as basic FGF and 
epidermal growth factor (EGF) induce PRL synthesis and secre­
tion. Neuropeptides acting through G protein-coupled recep­
tors to stimulate PRL include vasoactive intestinal polypeptide 
(VIP), prolactin-releasing peptide (PrRP), opioid peptides, and 
TRH.88a–88c,89 Estrogen stimulates PRL gene transcription and 
PRL secretion,90 explaining why women have highe

PRL secretion,90 explaining why women have higher PRL lev­
els, particularly during the periovulatory menstrual phase and 
in pregnancy.91,92 The physiologic roles of  other neuropeptides 
(e.g., neurotensin, substance P, bombesin, cholecystokinin, sero­
tonin, and histamine) in regulating human PRL secretion are 
unresolved.93 Evidence also suggests a short feedback loop with 
PRL negatively regulating its own secretion either via augmen­
tation of hypothalamic TIDA turnover or through direct

on of hypothalamic TIDA turnover or through direct auto­
crine actions.93–95 
Prolactin Secretion
The calculated production rate of PRL ranges from 200 to 536 
μg/day/m2, and the metabolic clearance rate ranges from 40 to 
71 mL/minute per m2.96 PRL is cleared rapidly, with a calculated 
half-life in the circulation ranging from 26 to 47 minutes. PRL 
secretion occurs episodically, with 4 to 14 secretory pulses over 
24 hours,97,98 with the highest levels achieved during sleep and 
the lowest oc

st levels achieved during sleep and 
the lowest occurring between 10 am and 12 noon.99 The noc­
turnal elevation is sleep entrained, and a temporal relationship 
exists between rapid eye movement (REM) and non-REM sleep 
cycles.100 PRL levels decline with age in both men and women. In 
older men, less PRL is produced with each secretory burst than in 
younger men.101 Postmenopausal women have lower mean serum 
PRL levels and PRL pulse frequency than do premenopausal 
women, suggesting a stimulat

han do premenopausal 
women, suggesting a stimulatory effect of estrogen on both of 
these parameters.92 
Prolactin Action
The PRL receptor gene is a member of the cytokine receptor 
superfamily,102 localized to chromosome 5p13, with an extracellu­
lar domain, a hydrophobic transmembrane domain, and an intra­
cytoplasmic region homologous to the GH receptor103 (Fig. 8.6). 
PRL receptor dimerization occurs in both ligand-dependent and 
ligand-independent manners, with a single PRL molecule bindin

pendent manners, with a single PRL molecule binding 
to both components of the receptor dimer, with phosphorylation 
of intracellular Janus kinase/signal transducers and activators of 
transcription (JAK-STAT) molecules subsequent to PRL binding. 
Two ligand-receptor binding sites are critical for formation of the 
Proximal
promoter 
Pituitary PRL mRNA
Extra-pituitary PRL mRNA
COOH
A
B
   Distal
promoter
E1a
E1b
NH2
E2
E3
E4
E5
• Fig. 8.5  Schematic representation of the human PRL gene and prote

tic representation of the human PRL gene and protein. 
(A) The PRL gene consists of five exons (E1–E5). PRL gene transcrip­
tion is regulated by two independent promoter regions, upstream of two 
alternative first exons (1a and 1b). The proximal promoter regions directs 
pituitary-specific expression, whereas the distal promoter region directs 
extrapituitary expression. Exons 1a and 1b both encode 5′-untranslated 
sequences, so the protein-coding sequences of both transcripts are iden­
tical. (

g sequences of both transcripts are iden­
tical. (B) PRL is a 23-kDa protein comprised of 199 amino acids with three 
disulfide bonds (hatched lines).
192
trimeric ligand-receptor complex and subsequent signaling.104–106 
PRL receptor induces protein tyrosine phosphorylation and activa­
tion of JAK2 and STAT1, STAT3, STAT5A, and STAT5B.107,108 
STAT5A phosphorylation is particularly important for mammary 
gland development and lactogenesis.109
PRL recepto

PRL receptors are ubiquitously expressed in breast tissue as 
well as pituitary, liver, adrenal cortex, kidneys, prostate, ovary, 
testes, intestine, epidermis, pancreatic islets, lung, myocardium, 
brain, and lymphocytes. Regulation of milk production occurs via 
a cascade of intracellular events. Homozygous mice in which the 
PRL receptor has been inactivated are infertile.104 A gain-of-func­
tion mutation conferring constitutive activity of the PRL recep

 conferring constitutive activity of the PRL receptor 
is present in a subset of patients with multiple breast fibroadeno­
mas.110 PRL receptor antagonists have been developed for target­
ing the receptor in PRL-sensitive disturbances, including resistant 
prolactinomas, breast tumors, and prostate tumors.106,111
PRL is essential for human species survival because it is respon­
sible for milk production during pregnancy and lactation. Addi­
tional biologic functions ascribed to PRL include repro

l biologic functions ascribed to PRL include reproductive, 
metabolic, and immune effects.69,93 Although PRL and its recep­
tor are clearly crucial in lower animals,112 the impact of PRL on 
maternal behavior in humans has not been fully delineated.
Mammary Gland Development
PRL is not essential for pubertal mammary development, which 
instead requires GH, the action of which is mediated by insulin-
like growth factor 1 (IGF1).113–115 At birth, the rodent mammary 
gland consists of a fat pad wit

he rodent mammary 
gland consists of a fat pad with small areas of ductal anlagen, which 
differentiate into pubertal mammary glandular elements under the 
influence of estrogen, GH, and IGF1. At puberty, a surge of estro­
gen begins the developmental process. Terminal end buds form 
and lead the process of mammary development by branching and 
extending into the substance of the mammary fat pad, leaving in 
its wake a network of ducts that virtually fill the mouse mammary 
fat pad.116,117 GH ac

ally fill the mouse mammary 
fat pad.116,117 GH acts on the mammary stoma compartment to 
produce IGF1, which in turn stimulates formation of terminal 
end buds and ducts in synergy with estrogen.115,118 Parathyroid 
hormone–related protein is essential for fetal mammary develop­
ment,119 and epidermal growth is essential for pubertal mammary 
development.120 Progesterone, possibly in association with GH 
and PRL, causes formation of lobular “decorations” along ducts, 
which are precursors to tr

orations” along ducts, 
which are precursors to true glands.121 In humans, pubertal mam­
mary development begins in girls between the ages of 8 and 13 (see 
developmental scale of Tanner, Chapter 24). Once fully developed, 
the pubertal mammary gland remains quiescent until pregnancy, 
although cyclic changes occur during the menstrual cycle.
In human pregnancy, mammary gland alveolar elements prolif­
erate and begin to produce milk proteins and colostrum. At 3 to 4 
weeks of gestation, terminal

colostrum. At 3 to 4 
weeks of gestation, terminal ductal sprouting occurs, followed by 
lobular-alveolar formation, and true alveoli form at the end of the 
first trimester. Glandular elements proliferate further and secretory 
products appear in the alveolar lumina. During the third trimester, 
fat droplets are seen within alveolar cells, and the glands fill with 
colostrum.122 A combination of estrogen, PRL, progesterone, and 
PRL
PRLr
PRLr
PRLr
PRLr
PRL
Inactive
Active
I

Lr
PRLr
PRL
Inactive
Active
Stat5a
P
Stat5a
P
P
P
P
P
A Ligand-dependent
dimerization
B Ligand-independent
Jak2
Jak2
Jak2
Jak2
PRL
Active
Jak2
Jak2
PRL
Stat5a
P
Stat5a
P
P
P
P
P
PRL
Active
Inactive
Jak2
Jak2
Jak2
Jak2
• Fig. 8.6  Ligand-dependent and ligand-independent dimerization of the prolactin (PRL) receptor (PRLr). 
(A) Ligand-dependent dimerization model. PRLr is in monomeric form at the cell 

tion model. PRLr is in monomeric form at the cell membrane. One molecule 
of PRL first binds to one PRLr monomer via binding site 1, and this 1:1 complex then recruits the sec­
ond PRLr via binding site 2. Dimerization of the two PRLrs leads to activating changes in the intracellular 
domain, leading to PRL signal transduction, such as phosphorylation (P) of Janus kinase 2 (Jak2), phos­
phorylation of the PRLr, and the recruitment and phosphorylation of the signal transducer and activator 
of tr

tion of the signal transducer and activator 
of transcription (Stat5). (B) Ligand-independent model. PRLr exists in dimeric form at the cell membrane 
in the absence of ligand. The receptors are held in an inactive form until binding of PRL to this preformed 
complex induces activating changes in the intracellular domain, leading to phosphorylation of Jak2, phos­
phorylation of the PRLr, and recruitment and phosphorylation of Stat5a. (From Clevenger C, Gadd SL, 
Zheng J. New mechanisms for PRLr 

ger C, Gadd SL, 
Zheng J. New mechanisms for PRLr action in breast cancer. Trends Endocrinol Metab. 2009;20:223–229.)
193
possibly IGF1 and placental hormones are largely responsible for 
this phase of mammary development.123 In mice with targeted 
disruption of the PRL gene, formation of alveolar structures is 
impaired.84 Likewise, women with isolated PRL deficiency are 
unable to lactate.124 Similarly, in mice lacking the progesterone

te.124 Similarly, in mice lacking the progesterone 
receptor, lobular-alveolar formation does not occur.125 Interest­
ingly, only a minority of women have expressible milk during 
pregnancy, most likely due to the inhibitory effects of estradiol126 
and progesterone127 on PRL-induced milk production. 
Lactation
Active lactation is due in part to attenuation of estrogen and 
progesterone levels and elevation of PRL levels after delivery. 
Suckling also increases milk production after parturition 

 also increases milk production after parturition and is 
essential for continued lactation because of its effect on pitu­
itary hormone production and because it empties the mammary 
gland of milk.128 Milk accumulation inhibits milk synthesis, 
explaining why a certain level of nursing activity is necessary 
for successful breastfeeding. In the absence of suckling, PRL 
concentrations, which rise throughout gestation, return to nor­
mal by 7 days postpartum.129 Although PRL is essential for 
mi

s postpartum.129 Although PRL is essential for 
milk production, the milk yield does not correlate closely with 
serum PRL levels.130 Suckling also stimulates posterior pituitary 
oxytocin release; unlike PRL, oxytocin responses to suckling 
do not decline for up to 6 months if nursing continues. Oxy­
tocin induces myoepithelial cell contraction, thereby causing 
milk ejection.131 Oxytocin also has important effects on alveolar 
proliferation. 
Reproductive Function
Lactation results in amenorrh

eproductive Function
Lactation results in amenorrhea and secondary infertility, and 
this natural form of contraception depends upon the frequency 
and duration of breastfeeding. The Kung hunter-gatherer 
women suckle their infants approximately four times an hour 
and at will during the night, and women bear a mean of 4.7 
children during their reproductive years.132 In contrast, the 
Hutterites of North America bear a mean of 10.6 children dur­
ing their lifetimes, presumably because they nurs

ing their lifetimes, presumably because they nurse according 
to a rigid schedule, use supplemental feedings, and wean at 
1 year. Amenorrhea and infertility result from PRL-mediated 
inhibitory effects on hypothalamic gonadotropin-releasing 
hormone (GnRH) and pituitary gonadotropin secretion of the 
gonadotropins, resulting in a reduction in both amplitude and 
frequency of LH pulses (Fig. 8.7).133 In a hyperprolactinemic 
mouse model, hypothalamic kisspeptin immunoreactivity was 
reduced, an

lamic kisspeptin immunoreactivity was 
reduced, and administration of kisspeptin restored estrous 
cyclicity, ovulation, and circulating LH and FSH levels. In 
addition, kisspeptin neurons have been shown to express PRL 
receptors.134 Taken together, these data suggest that kisspeptin 
may be the missing link between hyperprolactinemia and the 
associated hypogonadotropic hypogonadism, anovulation, and 
infertility.135,136 Indeed, kisspeptin administration to women 
with hyperprolactinemia-induc

nistration to women 
with hyperprolactinemia-induced hypothalamic amenorrhea 
was shown to reactivate gonadotropin secretion.136a During 
lactation, additional metabolic factors induced by negative 
energy balance likely also contribute to disruption of pulsatile 
GnRH and LH secretion. 
Other Actions
Early evidence indicated that PRL regulates immune function. 
However, some evidence indicates that PRL may not be impor­
tant for immune function,137 as innate immunity was not altered 
in transge

137 as innate immunity was not altered 
in transgenic mice devoid of either the PRL receptor (PRLR−/−) 
or PRL (PRL−/−).84,138 Prolactin has been shown to have effects 
on maternal behavior in rodents, but its role in humans remains 
to be determined. Other roles that have been proposed for 
prolactin include effects on neurogenesis, metabolism and glu­
cose homeostasis, appetite regulation, and bone and calcium 
homeostasis.69,93 
Prolactin Measurements
Modern PRL measurements are based on immu

urements
Modern PRL measurements are based on immunoradiometric 
or chemiluminescent assays and are highly specific in distin­
guishing PRL from GH. The serum concentration of PRL can 
be given in mass concentration (μg/L or ng/mL), molar concen­
tration (nmol/L or pmol/L), or international units (mIU/L). As 
these samples are usually assayed at a single dilution, extremely 
high PRL concentrations may saturate the capture and signal 
antibodies, preventing detection of very high PRL levels and 

 preventing detection of very high PRL levels and 
resulting in a falsely low value being reported.139 This “hook 
effect” may result in PRL-secreting macroadenomas diagnosed 
as clinically nonfunctioning adenomas. In patients harboring 
macroadenomas with clear-cut clinical features of hyperprolac­
tinemia, serum samples should be subjected to at least 1:100 
dilution prior to assay.
Conversely, macroprolactin, a physiologically inactive form 
of prolactin bound to IgG, can lead to a falsely el

f prolactin bound to IgG, can lead to a falsely elevated pro­
lactin result, thus resulting in misdiagnosis of hyperprolac­
tinemia. Misdiagnosis can be avoided by asking the laboratory 
to pretreat the serum with polyethylene glycol to precipitate 
macroprolactin before the immunoassay for prolactin in the 
supernatant.140 
10
5
90
70
50
30
10
0
mIU/mL
µg/L
Time (hour)
PRL
PRL
FSH
FSH
LH
LH
Hyperprolactinemia
25
20
15
10
5
0
20
15
0
mIU/mL
µg/L
Normal hormone profile
8
7
6
5
4
3
2
1
0
• Fig. 8.

8
7
6
5
4
3
2
1
0
• Fig. 8.7  Effect of hyperprolactinemia on suppressing follicle-stimulating 
hormone (FSH) and luteinizing hormone (LH) secretory patterns leading to 
hypogonadotrophism in a female patient. PRL, prolactin. (Adapted from 
Tolis G. Prolactin: physiology and pathology. Hosp Pract. 1980;15:85–95.)
194
Prolactin Deficiency
Causes
Congenital PRL deficiency can occur in association with mutations 
in the transcription f

association with mutations 
in the transcription factors involved in lactotroph lineage develop­
ment, including POU1F1, PROP1, LHX3, LHX4, and HESX1 (see 
Tables 8.1 and 8.2). In these cases, deficiencies in other anterior pitu­
itary hormones occur in conjunction with PRL deficiency, with the 
spectrum of pituitary hormone deficiencies depending on the gene 
affected.48 PRL deficiency has also been reported in conjunction with 
central hypothyroidism in a subset of patients with the immunoglob

oidism in a subset of patients with the immunoglob­
ulin superfamily member 1 (IGSF1) deficiency syndrome.167 
Manifestations
The only known clinical manifestation of PRL deficiency is the 
inability to lactate after delivery. Isolated PRL deficiency is rare but 
has been reported in association with autoantibodies directed spe­
cifically against PRL-secreting pituitary cells.168 Most patients with 
acquired PRL deficiency have evidence of other pituitary hormone 
Physiologic
Pregnancy
Sucking
S

pituitary hormone 
Pregnancy
Sucking
Stress
Sleep
Coitus
Exercise
Pathologic
Hypothalamic-Pituitary Stalk Damage
Tumors
Suprasellar pituitary mass extension
Dysgerminoma
Hypothalamic metastases
Granulomas
Infiltrations
Rathke cyst
Irradiation
Trauma: pituitary stalk section, sellar surgery, head trauma
Pituitary
Prolactinoma
Macroadenoma (compressive)
Idiopathic
Plurihormonal adenoma
Lymphocytic hypophysitis
Parasellar mass
Macroprolactinemia
S

 hypophysitis
Systemic Disorders
Chronic renal failure
Polycystic ovary syndrome
Cirrhosis
Epileptic seizures
Cranial irradiation
Chest: neurogenic, chest wall trauma, surgery, herpes zoster
Genetic
Inactivating prolactin receptor mutation
Pharmacologic
Thyrotropin-releasing hormone
Drug-Induced Hypersecretion
Dopamine Receptor Blockers
Phenothiazines: chlorpromazine, perphenazine
Butyrophenones: haloperidol
Thioxanthenes
Metocloprami

rophenones: haloperidol
Metoclopramide
Dopamine Synthesis Inhibitors
α-Methyldopa
Catecholamine Depleters
Reserpine
Cholinergic Agonists
Physostigmine
Antihypertensives
Labetalol
Reserpine
Verapamil
H2 Antihistamines
Cimetidine
Ranitidine
Estrogens
Oral contraceptives
Oral contraceptive withdrawal
Anticonvulsants
Phenytoin
Neuroleptics
Chlorpromazine
Risperidone
Promazine
Promethazine
Trifluoperazine
Fluphenazine
Butaperazine
Perphenazine
Thiethylperazine
Thioridazine
Haloperidol
P

nazine
Pimozide
Thiothixene
Molindone
Opiates and Opiate Antagonists
Heroin
Methadone
Apomorphine
Morphine
Antidepressants
Tricyclic antidepressants: clomipramine, amitriptyline
Selective serotonin reuptake inhibitors: fluoxetine
  Etiology of Hyperprolactinemia
TABLE 8.3
195
deficiencies in association with pituitary injury.169 Infarction of the 
pituitary gland after postpartum hemorrhage, kno

pituitary gland after postpartum hemorrhage, known as Sheehan 
syndrome, has long been recognized as a cause of hypopituitarism.170 
In developed countries, postpartum hemorrhage now less often 
results in Sheehan syndrome than previously, largely due to improve­
ments in obstetric care. No commercially available PRL preparation 
is available for these women, although studies with recombinant 
human PRL administration have been shown to increase milk vol­
ume in women with PRL deficiency or la

e milk vol­
ume in women with PRL deficiency or lactation insufficiency.171 
Causes
The causes of hyperprolactinemia may be physiologic, pathologic, 
or drug induced141,142 (Table 8.3).
Physiologic Causes
Pregnancy. During pregnancy, the normal pituitary gland 
may double or more in size,65 the result of a marked increase in 
the number of PRL-producing cells. Serum PRL concentrations 
rise to 10 times the normal concentration during pregnancy,129 
and amniotic fluid PRL conce

during pregnancy,129 
and amniotic fluid PRL concentrations are 100 times those of 
maternal or fetal blood.129 
Suckling. Suckling increases serum PRL levels approximately 
8.5-fold in actively nursing mothers.143,144 As nursing continues, 
PRL concentrations fall, but each suckling episode continues to 
cause a subsequent episodic rise in serum PRL. Mean serum con­
centrations were 162 μg/L at 2 to 4 weeks postpartum, 130 μg/L 
at 5 to 14 weeks, and 77 μg/L at 15 to 24 weeks.145 It is unclear 

, and 77 μg/L at 15 to 24 weeks.145 It is unclear 
why active milk production continues despite progressively lower 
PRL levels following parturition. 
Idiopathic Hyperprolactinemia. An elevated circulating PRL 
level in patients in whom no cause is identified is considered idio­
pathic, and these patients are relatively resistant to dopamine ago­
nist therapy. Mean serum PRL levels in patients with idiopathic 
hyperprolactinemia are usually less than 100 μg/L.146 
Macroprolactinemia. PRL is a 2

 than 100 μg/L.146 
Macroprolactinemia. PRL is a 23-kDa single-chain polypeptide 
but may also circulate in high-molecular-weight forms. High-molec­
ular-weight PRL variants may, in some situations, represent 85% or 
more of total PRL, but under usual circumstances the 23-kDa vari­
ety predominates. Macroprolactinemia refers to these larger circulat­
ing PRL molecules, comprised of PRL bound to immunoglobulins, 
which exhibit markedly reduced bioactivity. Few of the expected 
clinical abnormalit

activity. Few of the expected 
clinical abnormalities usually associated with hyperprolactinemia 
(sexual dysfunction, hypogonadism, galactorrhea, osteoporosis) 
occur in patients with macroprolactinemia.140,147 Screening for mac­
roprolactinemia can be accomplished by polyethylene glycol precipi­
tation of serum samples. In a 2005 survey, macroprolactinemia was 
detected in 22% of 2089 hyperprolactinemia samples.148 
Pathologic Causes. Pathologic hyperprolactinemia may be 
caused by a prolactin

c hyperprolactinemia may be 
caused by a prolactinoma, pituitary or sellar tumors that inhibit 
dopamine because of pressure on the pituitary stalk, or interrup­
tion of the vascular connections between the pituitary and hypo­
thalamus. In a large series of histologically confirmed cases, a 
serum PRL level greater than 2000 mUL (∼100 μg/L) was almost 
never encountered from stalk dysfunction.149 However, prolacti­
nomas can present with any level of PRL elevation.
Breast stimulation has only a 

l of PRL elevation.
Breast stimulation has only a minimal effect on serum PRL 
levels. In 18 normal women, serum PRL levels rose from a mean 
of 10 to 15 μg/L during breast pump stimulation.143 Chest wall 
lesions (e.g., in association with herpes zoster [shingles]) can also be 
associated with mild hyperprolactinemia as a result of activation of 
neurogenic pathways that inhibit dopamine. Up to 20% of patients 
with hypothyroidism have elevated PRL levels.150 Treatment of 
hypothyroidism with t

PRL levels.150 Treatment of 
hypothyroidism with thyroid hormone normalizes serum PRL if 
the hyperprolactinemia is due to thyroid hormone deprivation.
PRL is moderately elevated (mean 28 μg/L) in patients with 
chronic renal failure and those on dialysis.151 This increase is 
largely a result of an increase in PRL cleavage products, due in 
part to decreased glomerular filtration rate. Sexual dysfunction is 
common in men on dialysis, and reducing PRL with dopamine 
agonists improves sexual fun

ng PRL with dopamine 
agonists improves sexual function152 but does not normalize men­
ses in women.153 Side effects of dopamine agonists in patients 
with renal failure may be exacerbated because of fluid shifts and 
multiple medication interactions. PRL levels rise in response to 
stress, correlate with the degree of stress, and generally return to 
normal as stress abates. The mean peak serum PRL level in 19 
women undergoing general anesthesia was 39 μg/L immediately 
prior to surgery, 173 μ

a was 39 μg/L immediately 
prior to surgery, 173 μg/L at surgery, and still elevated at 47 μg/L 
24 hours after surgery.154 Traumatic brain injury can also result in 
hyperprolactinemia, often accompanied by diabetes insipidus or 
syndrome of inappropriate antidiuretic hormone (SIADH) secre­
tion and other anterior pituitary hormone deficiencies. A meta-
analysis found that 34% of patients develop hyperprolactinemia 
after cranial and hypothalamic irradiation, thought to be the result 
of decrea

c irradiation, thought to be the result 
of decreased hypothalamic dopamine secretion.154a
Hyperprolactinemia was reported in association with an inac­
tivating prolactin receptor mutation in three sisters with oli­
gomenorrhea.155 The mutation disrupted ligand binding and 
downstream signaling, demonstrated in  vitro. The presence of 
hyperprolactinemia in these affected women suggested central 
negative feedback by prolactin on its secretion in humans, as pre­
viously demonstrated in animal st

 humans, as pre­
viously demonstrated in animal studies.84,85 Affected women were 
all heterozygous for the mutation; the presence of hyperprolac­
tinemia suggests that the mutant receptor may interfere with sig­
naling by the normal receptor, encoded by the wild-type allele, to 
result in PRL insensitivity. As the occurrence of oligomenorrhea 
and the ability to lactate is contrary to what might be predicted 
from a state of prolactin insensitivity, the function of PRL and its 
receptor in huma

ity, the function of PRL and its 
receptor in humans requires further study.80 
Drug-Induced Causes. A variety of medications cause minimal 
or moderate prolactin elevations. Neuroleptic drugs elevate PRL 
because of their dopamine receptor antagonist properties, as do 
atypical antipsychotics that act by antagonizing both serotonin 
and dopamine receptors. Clozapine and olanzapine weakly induce 
PRL, and risperidone is a potent PRL stimulator.156,157
Unless patients exhibit hypogonadism, relate

6,157
Unless patients exhibit hypogonadism, related osteoporosis, 
or troublesome galactorrhea, treatment of drug-induced hyper­
prolactinemia may not be necessary.158 It should not always be 
assumed that hyperprolactinemia in patients on drugs known 
to elevate PRL is in fact due to those medications. Prolacti­
noma, other sellar lesions,159 hypothyroidism, and renal failure 
should be considered as possible causes of hyperprolactinemia 
requiring active management. In patients taking neurolep

ing active management. In patients taking neuroleptic 
medications, if the clinical situation permits, temporary drug 
withdrawal might be considered to determine if PRL levels nor­
malize.158 If PRL levels do not normalize or medication with­
drawal is not possible, a pituitary MRI should be performed. 
When neuroleptics elevate PRL levels, switching to olanzap­
ine may be attempted because the medication has minimal 
effects on PRL levels. In determining whether to discontinue 
a drug or wheth

etermining whether to discontinue 
a drug or whether to use alternative medication, the benefits 
should be weighed against the risks of drug replacement or 
cessation.160 Although combined use of dopamine antagonists 
and dopamine agonists is not usually advised because of an 
increased risk of side effects, such as postural hypotension or 
196
exacerbation of the underlying psychosis, some advocate the 
use of both formulations simultaneously.161 
Clini

Clinical Features
Galactorrhea and reproductive dysfunction are the hallmarks of 
pathologic hyperprolactinemia. Women present with galactor­
rhea associated with a range of menstrual disturbances, includ­
ing infertility, oligomenorrhea, and amenorrhea; men present 
with symptoms of hypogonadism and of tumor mass effects, with 
galactorrhea occurring infrequently.
Galactorrhea and amenorrhea were reported in the 19th cen­
tury by Chiari and Frommel.1

rted in the 19th cen­
tury by Chiari and Frommel.162 The Chiari-Frommel syndrome 
comprises postpartum galactorrhea, amenorrhea, and “utero-
ovarian atrophy” in patients not nursing. This disorder is usually 
self-limiting, and fertility eventually returns after normalization of 
PRL levels, sometimes without an intervening menstrual period. 
Patients with postpartum amenorrhea, hyperprolactinemia, and 
galactorrhea have sometimes subsequently been found to harbor 
prolactinomas. In the 1950s, A

en found to harbor 
prolactinomas. In the 1950s, Argonz and Del Castillo163 and 
Forbes et al164 associated galactorrhea and amenorrhea with pitu­
itary tumors. In a report of 18 such patients, galactorrhea and 
amenorrhea were reported for up to 11 years after parturition with 
a mean PRL level of 45 μg/L.150
Inappropriate nipple secretion of milk-like substances may per­
sist after childbirth or discontinuation of nursing for as long as 
6 months. Thereafter, continued milk production is consi

hs. Thereafter, continued milk production is considered 
abnormal, and other causes for galactorrhea should be investi­
gated. Galactorrhea can occur in either women or men and may 
be unilateral or bilateral, can be profuse or sparse, and can vary in 
color and thickness. If blood is present in the galactorrhea fluid, 
it could be the harbinger of an underlying pathologic process, 
such as a ductal papilloma or carcinoma, and mammography or 
sonography is indicated. Twenty-nine of 48 patients w

ography is indicated. Twenty-nine of 48 patients with pituitary 
tumors and galactorrhea had PRL concentrations of less than 200 
μg/L, likely on the basis of stalk compression, suggesting that they 
harbored pituitary tumors other than prolactinomas.150 It is likely 
that most patients with so-called idiopathic galactorrhea with 
amenorrhea harbor microprolactinomas. Fifty percent of patients 
with acromegaly also have galactorrhea, even in the absence of 
hyperprolactinemia, because human GH i

absence of 
hyperprolactinemia, because human GH is a potent lactogen and 
can cause galactorrhea when elevated.165 Normoprolactinemic 
galactorrhea with regular menses represents the most frequent 
cause of galactorrhea. In two-thirds of these patients, galactor­
rhea persists after parturition, despite the resumption of menses, 
and likely does not represent a pathologic entity. Normal PRL 
levels may still permit milk production because treatment of such 
patients with dopamine agonists allev

ent of such 
patients with dopamine agonists alleviates galactorrhea. Galactor­
rhea may also develop transiently after surgical procedures to the 
chest wall, including mammoplasty, arising from neural reflexes 
due to intercostal nerve stimulation.166 The optimal manage­
ment of galactorrhea should be determined by identifying and 
treating the underlying cause. Regardless of the cause, galactor­
rhea associated with hyperprolactinemia responds to correction of 
hyperprolactinemia. 
Growth Hor

 to correction of 
Growth Hormone
Somatotroph Cells
Mammosomatotroph cells expressing both PRL and GH arise 
from the acidophilic stem cell and immunostain mainly for 
PRL. Somatotrophs are located predominantly in the lateral 
wings of the anterior pituitary gland and comprise 35% to 45% 
of pituitary cells (see Chapter 9, Fig. 9.1). These ovoid cells con­
tain prominent secretory granules up to 700 μm in diameter. 
Juxtanuclear Golgi structures are particularly 

. 
Juxtanuclear Golgi structures are particularly prominent with 
secretory granules in formation. The gland contains a total of 5 
to 15 mg of GH.172 
Structure
The human growth hormone (hGH) genome locus spans approxi­
mately 66 kb and contains a cluster of five highly conserved genes 
located on the long arm of human chromosome 17q22-24.173 
It encodes the following forms of hGH and human chorionic 
somatomammotropin (hCS): hGH-N, hCS-L, hCS-A, hGH-V, 
and hCS-B,174 all of which consist of fi

, hGH-V, 
and hCS-B,174 all of which consist of five exons separated by four 
introns (Fig. 8.8).
The GH-N gene is expressed in pituitary somatotrope cells 
and, to a minor extent, in lymphocytes, whereas GH-V and CS 
genes are expressed in the placenta. The level of GH gene expres­
sion in lymphocytes is sufficient to play a local paracrine/autocrine 
immunoregulatory role but is insufficient to fulfill a hormonal 
role at distant sites.
The hGH-N gene codes for a 22-kDa (191–amino acid) pro­
t

-N gene codes for a 22-kDa (191–amino acid) pro­
tein.175 The hCS-A and hCS-B genes are expressed in placental 
trophoblasts.176 hGH-V, expressed exclusively in placental syn­
cytiotrophoblasts, encodes a 22-kDa protein that emerges from 
midgestation as hGH-V2. When hGH-V levels are elevated, 
hGH-N declines progressively, indicating feedback regulation of 
the maternal hypothalamic-pituitary axis. Postpartum circulat­
ing hGH-V levels drop rapidly and are undetectable 1 hour after 
delivery.17

dly and are undetectable 1 hour after 
delivery.177 hCS-L is found in placental villi but has been consid­
ered a pseudogene.
The hGH promoter region contains cis elements that medi­
ate both pituitary-specific and hormone-specific signaling. 
The POU1F1 transcription factor confers tissue-specific GH 
expression; a second, ubiquitous factor binds to a distal PIT1 
site containing a consensus sequence for the Sp1 transcription 
factor. PIT1 and Sp1 both contribute to GH promoter activa­
tion, as

p1 both contribute to GH promoter activa­
tion, as mutation of the Sp1 binding site attenuates promoter 
activity.178 DNase hypersensitive sites of a locus control region 
(LCR) located 14.5 kb upstream of the hGH-N promoter con­
fers and restricts the expression of GH only to somatotrophs and 
mammosomatotrophs among the PIT1-positive cell population 
during development.179 This distant locus also plays a major role 
in sustaining the level of hGH gene expression in somatotrophs. 
Growth hormon

GH gene expression in somatotrophs. 
Growth hormone–releasing hormone (GHRH) stimulates GH 
synthesis and release mediated by cAMP. cAMP-response ele­
ment binding (CREB) protein (CBP) is phosphorylated by pro­
tein kinase A and is a cofactor for PIT1-dependent human GH 
activation.
The GH molecule, a single-chain polypeptide hormone con­
sisting of 191 amino acids, is synthesized, stored, and secreted 
by somatotroph cells. The crystal structure of hGH reveals four 
α-helixes.180 Circulating GH

 of hGH reveals four 
α-helixes.180 Circulating GH molecules comprise several hetero­
geneous forms: 22-kDa and 20-kDa monomers, the latter lacking 
amino acids 32 through 46 deleted from alternative splicing of the 
GH gene (Fig. 8.8). The 22-kDa peptide is the major physiologic 
component, accounting for 75% of pituitary GH secretion, and 
the 20-kDa peptide accounting for about 10%. The 20-kDa GH 
has a slower metabolic clearance.181 The pituitary gland also pro­
duces a number of other varia

tary gland also pro­
duces a number of other variants formed from post-translational 
modification, including two deamidated forms, acylated and 
glycosylated.182 
197
Neuropeptides, neurotransmitters, and opiates impinge on the 
hypothalamus and modulate release of GHRH and somatosta­
tin (somatotropin release–inhibiting factor [SRIF]). Integrated 
effects of these complex neurogenic influences determine the final 
secretory 

rogenic influences determine the final 
secretory pattern of GH (Fig. 8.9). Apomorphine, a central dopa­
mine receptor agonist, stimulates GH secretion,183 as does levodopa 
(l-dopa). Oral l-dopa administration evokes a brisk serum GH 
response within an hour in healthy young subjects. Norepinephrine 
increases GH secretion via α-adrenergic pathways and inhibits GH 
Placenta
Pituitary
GH-N
CS-L
CS-A
GH-V
GH gene cluster
GH-N
A
B
5’
1
2
3
4
5
3’
C
22 KDa GH
CS-A
Phe
COOH
190
170
165
160
Gly
Cys
S

22 KDa GH
CS-A
Phe
COOH
190
170
165
160
Gly
Cys
Ser
Gly
Glu
Val
Ser
Arg
Gln
Val
IIe
Arg
Leu
Phe
Thr
Glu
Val
Lys
Asp
Met
Asp
Lys
Phe
Tyr
Leu
Leu
Gly
Tyr
Asn
Lys
Leu
Leu
Ala
Asp
Asp
Asn
His
Ser
Asn
Thr
Asp
Phe
Lys
Ser
Tyr
Thr
Gln
Lys
Phe
IIe
Gln
Gly
Thr
135
130
125
120
115
Trp
175
180
Arg
Pro
Ser
Gly
Asp
Glu
Leu
Arg
Gly
Met
Leu
Thr
Gln
IIe
Gly
Glu
Glu Leu
Asp
Lys
Leu
Leu His
Leu
Asp
Tyr
Val
Asp
Ser
Asn
Ser
Ala
105
Gly
Tyr
Val
Leu
Ser
100
Asn
Ala
Phe
Val
Ser
Arg
Leu
Phe
Leu
Gln
Val
90
Pro
Glu
Leu

he
Val
Ser
Arg
Leu
Phe
Leu
Gln
Val
90
Pro
Glu
Leu
Trp
Ser
IIe
Leu
Leu
Leu
Ser
IIe
Arg
Leu
Leu
75
145
80
85
10
1
GIn
Leu
Asn
Ser
Lys
Gln
Gln
Thr
Glu
Glu
Arg
Asn
Ser
Pro
Pro
Ile
Ser
Glu
Ser
Phe
Leu
Ser
Thr
Gln
Pro
Asn
Gln
Leu
Phe
Ser
Tyr
Lys
Gln
Glu
Lys
Pro
Ile
Tyr
Ala
Glu
Glu
Glu
30
Gln
Tyr
Thr
Asp
Phe
Ala
Leu
Gln
His
Leu
Arg
His
Arg
Ala
Arg
Leu
Met
Ala
Asn
Asp
Phe
Leu
Arg
Ser
Leu
Pro
5
Ile
Thr
Pro
Phe
NH2
15
Tyr
25
Tyr
Phe
35
40
45
50
55
Thr
60
65
155
70
150
185
Lys
*
Lys
Val
Gln
95
20
Arg
110
A

5
155
70
150
185
Lys
*
Lys
Val
Gln
95
20
Arg
110
Asn
*
140
Ser
Arg
Cys
Cys
Cys
• Fig. 8.8  Schematic presentation of the human growth hormone gene cluster (A) on chromosome 17 con­
taining GH-N, CS-L, CS-A, GH-V and CS-A genes. GH-N is expressed in the pituitary gland while the rest 
are expressed in the placenta. CS-L and GH-V are the predominant forms expressed during pregnancy. 
The GH-N gene (B) encoding human growth hormone contains five exons and four introns, and generates 
a full-length 

ns and four introns, and generates 
a full-length 22-kDa protein (C). Exon 3 contains a splice acceptor site (arrow) that generates a 20-kDa 
isoform that lacks amino acids 32 to 46, shown with thickened lines in the lower panel within the 22-kDa 
hormone. (C, Courtesy Dr. G. Baumann.)
198
release via β-adrenergic pathways. Insulin-induced hypoglycemia, 
clonidine, arginine administration, exercise, and l-dopa facilitate 
GH secretion by α-adrenergic effe

dopa facilitate 
GH secretion by α-adrenergic effects.184 β-Adrenergic blockade 
increases GHRH-induced GH release, possibly due to a direct pitu­
itary action or by decreasing hypothalamic somatostatin release. 
Endorphins and enkephalins stimulate GH and may account for 
GH release during severe physical stress and extreme exercise184 (see 
Fig. 8.9). Galanin, a 29–amino acid neuropeptide, induces GH 
release and enhances responses to GHRH. Cholinergic and seroto­
nergic neurons and several ne

linergic and seroto­
nergic neurons and several neuropeptides stimulate GH, including 
neurotensin, VIP, motilin, cholecystokinin, and glucagon.
The somatotroph cell expresses specific receptors for GHRH,185 
GH secretagogues, and SRIF receptor subtypes 2 and 5, which 
mediate GH secretion.186,187 GHRH selectively induces GH gene 
transcription and hormone release.188 GHRH elicits a prompt rise 
in serum GH levels, with higher levels occurring in females.189 
SRIF suppresses secretion but not GH

 females.189 
SRIF suppresses secretion but not GH biosynthesis.
Ghrelin
The isolation of ghrelin from the stomach has revealed an additional 
control system for GH (see Chapter 39). Ghrelin is a 28–amino acid 
peptide that binds the growth hormone secretagogue (GHS) recep­
tor to induce hypothalamic GHRH and pituitary GH release.197 
A unique n-octanoylated serine 3 residue confers GH-releasing 
activity to the molecule. Ghrelin is synthesized in peripheral tis­
sues, especially gastric mucosal

n peripheral tis­
sues, especially gastric mucosal neuroendocrine cells, as well as in 
the hypothalamus. Control of GH secretion thus requires hypotha­
lamic GHRH/SRIF as well as ghrelin.198 Ghrelin dose-dependently 
evokes GH release. It requires the presence of GHRH to evoke GH 
release as evidence in patients with intact pituitary but disordered 
hypothalamic function, when GHS does not induce GH.199 This 
is because GHRH acts as an allosteric coagonist for the ghrelin 
receptor.200 GHS agon

c coagonist for the ghrelin 
receptor.200 GHS agonists also potentiate GH release in response 
to a maximal stimulating dose of exogenous GHRH. After a satu­
rating dose of GHRH, subsequent GHRH administration is inef­
fective, although GHS agonists remain effective.192 Hypothalamic 
ghrelin likely controls GH secretion interacting with GHRH and 
SRIF.198,201,202 Ghrelin and synthetic hexapeptidergic analogues of 
the GHS receptor induce potent and reproducible GH release asso­
ciated with modes

nd reproducible GH release asso­
ciated with modest increases in PRL and ACTH/cortisol that occur 
with some GHSs (Fig. 8.10). The GHS receptor uses intracellular 
Pituitary
IGF1
SRIF
Suppression
Obesity
FFA
Glucose
IGF1
Stimulation
Deep sleep
α-Adrenergic
Fasting
Sex steroids
Stress
Hypoglycemia
Ghrelin
Inhibition
Undernutrition
Acute illness
Chronic illness
GH receptor
    deficiency
GHR antibodies
IGF1 receptor

ceptor
GHRH
Bone
and
muscle
Growth
Fat
Catabolism
IGF1
(auto/paracrine)
Liver
GH
GH
GHBP
IGF1
IGFBP
IGF1
IGFR
GH
GHR
GH
GHR
GH
GHR
GH-IGF axis
• Fig. 8.9  The growth hormone/insulin-like growth factor (GH-IGF) axis. 
Simplified diagram of GH axis involving hypophysiotropic hormones con­
trolling pituitary GH release, circulating GH-binding protein (GHBP) and its 
GH receptor (GHR) source, IGF1 and its largely GH-dependent binding 
protei

 IGF1 and its largely GH-dependent binding 
proteins (IGFBP), and cellular responsiveness to GH and IGF1 interacting 
with their specific receptors. GHRH, growth hormone–releasing hormone; 
IGFR, IGF1 receptor; FFA, free fatty acids; SRIF, somatotropin release–
inhibiting factor (somatostatin). (From Rosenbloom A. Growth hormone 
insensitivity: physiologic and genetic basis, phenotype and treatment. J 
Pediatr. 1999;135:280–289.)
Ghrelin
HEX
GHRH
Placebo
PRL (µg/L)
20
10
0
180
150
120
90
Time (m

Placebo
PRL (µg/L)
20
10
0
180
150
120
90
Time (minutes)
60
30
0
−15
ACTH (pmol/L)
30
10
20
0
GH (µg/L)
120
80
30
0
• Fig. 8.10  Effect of GH secretagogues on secretion of GH, adrenocortico­
tropic hormone (ACTH), and prolactin (PRL) in healthy subjects. Mean (± 
standard error of the mean [SEM]) curve responses after administration of 
ghrelin (1.0 μg/kg), hexarelin (HEX, 1.0 μg/kg), growth hormone–releasing 
hormone (GHRH) (1.0 μg/kg), or placebo. (Adapted from Arvat E, Mac­
cario M, Di Vito 

ebo. (Adapted from Arvat E, Mac­
cario M, Di Vito L, et al. Endocrine activities of ghrelin, a natural growth 
hormone secretagogue [GHS], in humans: comparison and interactions 
with hexarelin, a nonnatural peptidyl GHS, and GH-releasing hormone. J 
Clin Endocrinol Metab. 2001;86:1169–1174.)
199
signaling pathways distinct from the GHRH receptor.199,200 In 
addition to its GH-releasing action, there is strong evidence that 
ghrelin is a

ction, there is strong evidence that 
ghrelin is a pleotropic hormone regulating appetite, energy bal­
ance, sleep-wake rhythm, gastric motility, glucose homeostasis, cell 
growth, and cardiac function.203,204 
Extrapituitary GH
GH is expressed outside the pituitary gland in the brain, immune 
cells, reproductive tract (breast, ovary, testis, prostate), gastroin­
testinal system (liver, pancreas, gut), and lungs.205 The regulation 
of GH in these tissues is poorly understood. Evidence suggests 

 tissues is poorly understood. Evidence suggests 
that GH subserves developmental and proliferative function, act­
ing in an autocrine and paracrine manner. Autocrine GH binds 
GHRs that are translocated to the nucleus, inducing cell cycle 
proliferation.206 Extrapituitary GH expression is strongly impli­
cated in neoplastic transformation of breast, prostate, and colonic 
neoplasms.207,208 
Secretion
GH secretion is episodic and exhibits a diurnal rhythm with 
approximately two-thirds of the to

al rhythm with 
approximately two-thirds of the total daily GH secretion pro­
duced at night triggered by the onset of slow-wave sleep. Major 
GH secretory pulses accounting for up to 70% of daily GH secre­
tion occur with the first episode of slow-wave sleep.209 Normal 
GH secretion is characterized by secretary episodes separated by 
troughs of minimal basal secretion, during which GH is undetect­
able for more than 50% of a 24-hour period. GH concentration 
is high in the fetal circulation, p

is high in the fetal circulation, peaking at approximately 150 μg/L 
during midgestation. Neonatal levels are lower (∼30 μg/L), pos­
sibly reflecting the negative feedback control by rising levels of cir­
culating IGF1. GH output falls to a stable level during childhood, 
rising at the onset of puberty by a twofold to threefold level at late 
puberty. GH secretion starts to decline exponentially during the 
third decade of life, progressively declining with advancing age to 
about

ogressively declining with advancing age to 
about 50% of that observed in the third decade.184 The decline 
in GH status occurs by a change in pulse amplitude rather than 
frequency. On average, the daily production of GH in the prepu­
bertal state is 200 to 600 μg/day, rising to 1000 to 1800 μg/day at 
the peak of puberty.184 In adulthood, production rates range from 
approximately 200 to 600 μg/day with rates higher in women 
than in men.184,210 Adiposity accounts for a significant compo­
nen

10 Adiposity accounts for a significant compo­
nent of declining GH output with increasing age211 (Table 8.4). 
Exercise and physical stress, including trauma with hypovolemic 
shock and sepsis, increase GH levels.184 Emotional deprivation 
and endogenous depression suppress GH secretion.
The levels of IGF1 in childhood, adolescence, and adulthood 
reflect the status of GH output during the corresponding phases 
of life. IGF1 levels are stable postnatally and during childhood, 
rising at the sta

stnatally and during childhood, 
rising at the start of puberty and peaking in late adolescence (Fig. 
8.11).583 This close correspondence throughout life renders age-
stratified IGF1 level a valuable serum marker of GH status in both 
sexes.
Nutrition plays a major role in GH regulation, in part medi­
ated by the inhibitory actions of insulin.212 Malnutrition increases 
GH secretion, whereas obesity has the opposite effect. These 
nutritional effects occur acutely, as exemplified by fasting, wh

fects occur acutely, as exemplified by fasting, which 
amplifies GH secretion within 12 hours213 (Fig. 8.12), and glu­
cose ingestion, which suppresses GH secretion. Central glucore­
ceptors appear to sense glucose fluctuations rather than absolute 
levels. Intravenous (IV) administration of single amino acids, such 
as arginine and leucine, stimulate GH secretion. Free fatty acids 
blunt the effects of arginine infusion, sleep, l-dopa, exercise, and 
GHRH on GH release.214 Leptin plays a key ro

 and 
GHRH on GH release.214 Leptin plays a key role in regulation of 
food intake and energy expenditure215 and may act as a metabolic 
signal in stimulating GH secretion216 through interactions with 
somatostatin, GHRH, and the neuropeptide Y (NPY) system.
GHRH and SRIF Interaction
Hypothalamic SRIF and GHRH are secreted in independent 
waves and interact together to generate pulsatile GH release. The 
rat hypothalamus releases GHRH and SRIF 180 degrees out of 
phase every 3 to 4 hours, result

0 degrees out of 
phase every 3 to 4 hours, resulting in pulsatile GH levels. SRIF 
antibody administration elevates GH levels, with intact interven­
ing GH pulses,190 implying that hypothalamic SRIF secretion 
generates GH troughs. Similarly, GHRH antibodies eliminate 
spontaneous GH surges. In humans, GH pulsatility persists when 
GHRH is tonically elevated, as in ectopic tumor GHRH produc­
tion or during GHRH infusion,191 suggesting that hypothalamic 
SRIF is largely responsible for GH pulsat

halamic 
SRIF is largely responsible for GH pulsatility. However, GHRH 
antagonists inhibit GH pulses, indicating an important role in 
the generation of pulsatile secretion in humans.192 The periodic, 
pulsatile GH release is the result of coordinated SRIF and GHRH 
secretion and action.
Chronic GHRH stimulation, either by continuous infusion or 
repeated bolus administration, eventually desensitizes GH release 
in vitro and in vivo, possibly due to depletion of a GHRH-sensi­
tive pool of GH. G

e to depletion of a GHRH-sensi­
tive pool of GH. GHRH pretreatment also decreases somatotroph 
GHRH binding sites.193 GH stimulates hypothalamic SRIF, 
GHRH and SRIF autoregulate their own respective secretion, 
and GHRH stimulates SRIF release.194 GH secretion is further 
regulated by its target growth factor, IGF1, which participates 
in a hypothalamic-pituitary peripheral regulatory feedback sys­
tem.195,196 IGF1 stimulates hypothalamic SRIF release and inhib­
its pituitary GH gene transcript

elease and inhib­
its pituitary GH gene transcription and secretion. 
Interaction With Other Hormone Axes
Glucocorticoids acutely stimulate GH secretion, but chronic ste­
roid treatment inhibits GH. Three hours after acute glucocorticoid 
administration, GH levels rise and remain elevated for 2 hours.217 
Glucocorticoids administered to normal subjects dose-dependently 
inhibit GHRH-simulated GH secretion.217 Thyroid hormones are 
required for the function of the GH system. Gonadal steroids regu

e function of the GH system. Gonadal steroids regu­
late GH secretion and GH action in men and women. Testoster­
one stimulates GH secretion, an effect that is mediated centrally 
and dependent on prior aromatization to estrogen.218 In women, 
  Adult Growth Hormone Secretiona
Interval
Young 
Adult
Fasting
Obesity
Middle 
Age
24-h secretion 
(μg/24 h)
540 ± 44
2171 ± 
333
77 ± 20
196 ± 65
Secretory 
bursts 
(number in 
24 h)
12 ± 1
32 ± 2
3 ± 0.5
10 ± 1
GH burst (μg)
45 ± 4
64 ± 9
24 ± 5
10 ± 6

5
10 ± 1
45 ± 4
64 ± 9
24 ± 5
10 ± 6
aDeconvolution analysis of growth hormone (GH) secretion in adult males.
From Thorner MO, Vance ML, Horvath E, et al. The anterior pituitary. In: Wilson JD, Foster D, 
eds. Williams Textbook of Endocrinology. 8th ed. Philadelphia: WB Saunders; 1992:221–310.
   
TABLE 8.4
200
estrogen stimulates GH secretion, evident only with oral but not 
parenteral administration because of a first-pass effect hepatic, 

istration because of a first-pass effect hepatic, 
effect reducing IGF1 production219 and enhancing GH secretion 
through reduced feedback inhibition.219 In women, endogenous 
GH secretion is driven centrally by aromatization of androgens. 
Thus in women, estrogens act centrally in a paracrine manner in 
stimulating but act peripherally in an endocrine manner in enhanc­
ing GH secretion through reduced IGF1 feedback inhibition.220 
Action
Growth hormone exerts its pleiotropic effects directly an

hormone exerts its pleiotropic effects directly and indi­
rectly through the actions of IGF1. These mediated actions stimu­
late growth and a host of functions in various tissues, organs, and 
systems (Fig. 8.13).
GHR Signaling
GH elicits intracellular signaling through a peripheral receptor, 
initiating a phosphorylation cascade involving the JAK-STAT 
pathway.221 The GHR is a 620–amino acid 70-kDa protein of the 
class I cytokine/hematopoietin receptor superfamily that consists 
of an extracel

receptor superfamily that consists 
of an extracellular ligand-binding domain, a single membrane-
spanning domain, and a cytoplasmic component.222 The GHR 
superfamily is homologous with receptors for PRL, interleukin 2 
(IL2) through IL7, erythropoietin, interferon, and colony-stim­
ulating factor. These receptors exist as constitutive dimers. GHR 
activation is triggered by the binding of GH at two distinct sites 
to extracellular domains of each of the two receptor dimers (Fig. 
8.14). The bi

ch of the two receptor dimers (Fig. 
8.14). The binding reorients and rotates the receptor separating 
the transmembrane domains, sliding the pseudokinase inhibi­
tory domain of one JAK kinase away from the kinase domain of 
the other JAK within the receptor dimer-JAK complex (see Fig. 
8.14).223,224 This results in phosphorylation of intracellular sig­
naling molecules,223,224 including the signal transducing activa­
tors of transcription proteins (STAT1, STAT3, and STAT5).225 
Phosphorylated S

ns (STAT1, STAT3, and STAT5).225 
Phosphorylated STAT proteins are directly translocated to the cell 
nucleus, where they elicit GH-specific target gene expression by 
binding to nuclear DNA. STAT1 and STAT5 may also interact 
directly with the GHR molecule.225 GH also induces c-Fos induc­
tion, insulin receptor substrate 1 phosphorylation, and insulin 
synthesis. Additional intracellular signaling pathways induced by 
GH include mitogen-activated protein kinase (MAPK), protein 
kinase C, SH2β, 

d protein kinase (MAPK), protein 
kinase C, SH2β, SHP2, SIRPA, SHC, FAK, CrKll, C-SRC, pax­
illin, and tensin. Intracellular GH signaling is abrogated by sup­
pression of cytokine signaling (SOCS) proteins, which disrupt the 
JAK-STAT pathway and thus disrupt GH action.226 In transgenic 
mice with deletion of SOCS2, gigantism develops presumably due 
to unrestrained GH action. As SOCS proteins are also induced 
by proinflammatory cytokines, critically ill patients or those with 
renal failure ma

cally ill patients or those with 
renal failure may develop GH resistance due to cytokine-induced 
SOCS.227 Unraveling STAT/SOCS regulation in syndromes asso­
ciated with disordered GH signaling will likely yield mechanistic 
insights for dysregulated GH action. The GHR is expressed in vir­
tually all tissues with liver, fat, muscle, and kidney showing high­
est levels of expression.228 A polymorphism of the GHR, resulting 
in the deletion of exon 3, has attracted widespread interest since 
an o

n 3, has attracted widespread interest since 
an observation in 2004 that signaling was 30% higher than that 
of the full-length receptor.229 A recent meta-analysis found no 
Age (years)
Males
IGFI (ng/mL)
0
100
200
300
400
500
600
700
800
A
B
0
10
20
30
40
50
2.5%
60
70
80
25%
50%
75%
97.5%
Females
0
100
200
300
400
500
600
700
800
0
10
20
30
40
50
2.5%
60
70
80
25%
50%
75%
97.5%
• Fig. 8.11  Normal ranges for insulin-like growth factor 1 (IGF1). Individual points and f

ke growth factor 1 (IGF1). Individual points and fitted percentiles for IGF1 
in males (A) and females (B). Displayed are the 2.5%, 25%, 50%, 75%, and 97.5% centiles. (Redrawn from 
Bidlingmaier M, Friedrich N, Emeny RT, et al. Reference intervals for insulin-like growth factor-1 (IGF-I) from birth 
to senescence: results from a multicenter study using a new automated chemiluminescence IGF-I immunoas­
say conforming to recent international recommendations. J Clin Endocrinol Metab. 2014;99:1712–1

endations. J Clin Endocrinol Metab. 2014;99:1712–1721.)
Time (hours)
Control day
GH (µg/L)
20
15
10
5
0
0800
2000
0800
Meals
Fasting day
0800
2000
0800
• Fig. 8.12  Effect of fasting on growth hormone (GH) secretion patterns 
in a healthy male subject. (From Hartman ML, Veldhuis JD, Johnson ML, 
et  al. Augmented growth hormone [GH] secretory burst frequency and 
amplitude mediate enhanced GH secretion during a two-day fast in normal 
men. J Clin Endocrinol Metab. 1992;74:757–765.)

201
convincing influence of the exon 3–deleted GHR on stature, on 
GH-induced growth responsiveness in children, on metabolic 
benefits of GH replacement in adults with growth hormone defi­
ciency (GHD), or on clinical manifestations and response to treat­
ment in acromegaly.230
The pattern of GH secretion also determines tissue responses to 
GH in addition to the absolute amount secreted

to 
GH in addition to the absolute amount secreted. Gender-specific 
patterns of GH secretion profiles determine sex-specific expres­
sion of cytochrome P450 enzymes. In turn, circulating steroids 
regulate neuroendocrine release of GH. SRIF, by suppressing 
interpulse GH levels, serves to masculinize the ultradian GH 
rhythm. In mice harboring a disrupted SRIF gene, plasma GH 
secretory patterns are elevated and liver enzyme induction loses 
its gender-specific dimorphism, but these animals ret

 gender-specific dimorphism, but these animals retain sexually 
dimorphic growth patterns.231 Linear growth patterns and liver 
enzyme induction are phenotypically gender specific, driven by 
higher GH pulse frequency rates involving STAT5B mediation. 
STAT5B is sensitive to repeated pulses of injected GH,232 unlike 
other GH-induced responses, which are desensitized by repeated 
GH administration. Disruption of STAT5B in transgenic mice 
IGF1
−
+
Immune
function
Skeletal muscle
Renal function
C

+
Immune
function
Chondrocyte
proliferation
hypertrophy
Gastrointestinal
Reproduction
Neurogenesis
Bone, tooth
Lipolysis
Insulin resistance
Insulin
synthesis
Hepatic
metabolism
Cardiovascular
endothelium
GH
• Fig. 8.13  Major physiologic actions of growth hormone and insulin-like growth factor 1 (IGF1) action. 
Given that growth hormone uses signal transducer and activator of transcription 5 (STAT5) to induce both 
IGF1 and for many of its direct actions, it is dif

IGF1 and for many of its direct actions, it is difficult to attribute specific roles to growth hormone or IGF1 for 
many biologic effects. However, those actions that involve more direct growth hormone action are placed 
to the right, whereas those involving induced IGF1 to the left, acknowledging that these assignments may 
change. Note that the growth hormone–IGF1 system is regulated by the tight negative feedback loop 
between hepatic IGF1 production and pituitary growth hormone secretion. GH

duction and pituitary growth hormone secretion. GH, growth hormone; IGF1, 
insulin-like growth factor 1. (Redrawn from Brooks AJ, Waters MJ. The growth hormone receptor: mecha­
nism of activation and clinical implications. Nat Rev Endocrinol. 2010;6:515–525.)
GH
GHR
Associated
JAK2
Pseudokinase
domain
• Fig. 8.14  Schematic representation of the GH receptor in the basal state (left) and the active state (right). 
Binding of GH at two distinct sites to predimerized recept

of GH at two distinct sites to predimerized receptors leads to a crossover rotation and realignment, 
producing a separation of the lower transmembrane domains. This results in the separation of two associ­
ated JAK2s and the removal of the inhibitory pseudokinase domain from the kinase domain of the other 
JAK2, bringing the two kinase domains into position for transactivation, initiating tyrosine phosphorylation 
of the receptor cytoplasmic domain and signaling proteins such as STAT5. (Redrawn

ain and signaling proteins such as STAT5. (Redrawn from Brooks AJ, Dai 
W, O’Mara ML, et al. Mechanism of activation of protein kinase JAK2 by the growth hormone receptor. 
Science. 2014;344:1249783.)
202
impairs male pattern body growth233 associated with female 
pattern IGF1 and testosterone levels. Appropriate GH pulsa­
tility drives body growth mediated by STAT5B234,235 but not 
metabolic GH effects. In humans, GH secretion is also sexually 
dimorphic

n humans, GH secretion is also sexually 
dimorphic and regulates the P450 liver enzymes.235 A flatter pat­
tern appears to evoke a higher IGF1 response and a lesser degree 
of lipolysis.236
IGF1 mediates growth-promoting activities of GH in an endo­
crine or paracrine manner.237 In mice, paracrine IGF1 produced 
in extrahepatic tissues is critical for growth, which persists even 
when hepatic IGF1 is deleted.238 GHR mutations are associated 
with partial or complete GH insensitivity and growth f

 partial or complete GH insensitivity and growth failure. 
These syndromes are associated with normal or high circulating 
GH levels, decreased circulating GHBP levels, and low levels of 
circulating IGF1. Multiple homozygous or heterozygous exonic 
and intronic GHR mutations have been described. These occur 
mostly in the extracellular ligand-binding receptor domain (see 
Chapter 25). 
Growth Hormone–Binding Proteins
Two high-affinity and low-affinity circulating GH-binding pro­
teins (GHBPs) i

finity circulating GH-binding pro­
teins (GHBPs) include a 20-kDa low-affinity GHBP and a 
60-kDa high-affinity GHBP, which corresponds to the extracel­
lular domain of the GHR and binds half of the circulating 22-kDa 
GH form.239,240 The high-affinity GHBP in humans is generated 
by proteolytic cleavage through the action of tumor necrosis 
factor-α–converting enzyme, a metalloprotease.241 The 20-kDa 
GH binds preferentially to the low-affinity binding protein, which 
is unrelated to the GHR. I

binding protein, which 
is unrelated to the GHR. It has been proposed that the circulat­
ing level of GHBP reflects global GHR expression in the body. 
The GHBPs dampen the acute oscillations in serum GH levels 
associated with pulsatile pituitary GH secretion, and plasma GH 
half-life is prolonged by decreased renal GH clearance of bound 
GH. The high-affinity binding protein also competes with GH for 
binding to surface GHRs and as such alters GH pharmacokinetics 
and distribution.
GHBP concen

GH pharmacokinetics 
GHBP concentrations are unaffected by GH status such as 
in GHD or acromegaly.242 Some patients with Laron dwarfism 
have absent or reduced levels of GHBP, reflecting mutations that 
result in absent translation of the GHR or of the extracellular 
domain.243 Serum concentrations of GHBP are low in some chil­
dren with idiopathic short stature244 and in African pygmies, sug­
gesting abnormalities in the gene for the GHR.245 GHBP levels 
are increased in obes

for the GHR.245 GHBP levels 
are increased in obesity, pregnancy, and in subjects undergoing 
refeeding245 and by oral estrogen administration.219 Levels are 
reduced in malnutrition, cirrhosis, and hypothyroidism and by 
glucocorticoids and androgens.246 
Metabolic Action
GH functions as a major metabolic hormone in the adult, opti­
mizing body composition and physical function as well as regulat­
ing energy and substrate metabolism. Metabolic actions of GH 
also closely interact with those of 

ctions of GH 
also closely interact with those of insulin in the control of fat, 
glucose, and protein metabolism during fasted and fed states.
GH promotes fat metabolism by enhancing lipolysis and fatty 
acid oxidation. This function is particularly important during the 
fasted state when GH secretion is enhanced, resulting in the par­
titioning of fuel utilization toward fat with the sparing of protein. 
Stimulation of lipolysis occurs indirectly through potentiating the 
activity of hormone-s

ly through potentiating the 
activity of hormone-sensitive lipase by β-adrenergic stimulation. 
GH also regulates lipoprotein metabolism by enhancing low-den­
sity lipoprotein (LDL) clearance and by activating expression of 
hepatic LDL receptors.247,248 The atherogenic profile of lipopro­
teins is increased in GHD and reduced by GH therapy.249
GH exerts profound effects on glucose metabolism either 
directly or by antagonizing insulin action. GH enhances glu­
cose uptake and utilization in cell

 enhances glu­
cose uptake and utilization in cells, referred to as its insulin-like 
effects.250 At the whole-body level, GH suppresses glucose oxi­
dation and utilization while enhancing hepatic glucose produc­
tion subserving the nonoxidative250 use of glucose. As GH is an 
important counterregulatory hormone, it is conceivable that this 
function protects against hypoglycemia.
Protein anabolism is a signature property of GH mediated by 
IGF1. However, GH directly stimulates amino acid uptake

 However, GH directly stimulates amino acid uptake and 
incorporation into protein in  vitro.251 Arteriovenous measure­
ments in the forearm report an acute increase in protein synthesis 
over a few hours of GH infusion, suggesting a direct non-IGF1 
mediated effect.252 Whole-body studies in humans using isotopes 
have consistently shown that GH reduces protein oxidation and 
stimulates protein synthesis.148 The protein-sparing effect of GH 
is coupled to the availability and increased parallel 

oupled to the availability and increased parallel utilization of 
free fatty acids, whereas pharmacologic reduction of free fatty 
acids during fasting augments the rate of protein breakdown.250 
Growth Hormone Assays
Plasma GH is measured by radioimmunoassay (RIA; polyclonal 
or monoclonal) or by immunoradiometric assay (IRMA; dual 
monoclonal), but considerable differences exist in the results of 
GH measurements between assays by as much as threefold.253,254 
These differences are due mainly 

reefold.253,254 
These differences are due mainly to heterogeneity in assay com­
ponents and characteristics. An important contributor is the use 
of different calibrator materials255 and international reference 
preparations, shifting from purified pituitary extracts to recom­
binant GH.255 Not all assays are calibrated to the recombinant 
international GH reference preparation (98/574). Heterogene­
ity of secreted GH poses an additional problem as it consists 
of monomers, dimers, and other po

 as it consists 
of monomers, dimers, and other post-translational modified 
products. Their detection varies, with different antibodies hav­
ing different specificity for the different molecular forms of GH. 
The reporting of assay results also varies. GH assay results are 
expressed in mass units but also in international units, which 
have been arbitrarily defined and do not have a clear relation­
ship to mass.255 Many factors interfere with GH measurements, 
including GHBP, which binds appro

H measurements, 
including GHBP, which binds approximately 50% of circulating 
GH. Modern two-site monoclonal assays are engineered to detect 
solely 22-kDa GH and return lower values to polyclonal RIA, 
thus detecting various circulating forms. The inhomogeneity of 
GH immunoassay results poses a challenge in the definition of 
accepted standards for diagnosis of GHD where criteria are based 
on cutoffs. Clinicians should be aware of the nature of the GH 
assay and how values compare to those p

of the GH 
assay and how values compare to those previously obtained by 
polyclonal RIA.
Similar issues apply to assays for IGF1. A significant issue arises 
from interference of IGF binding proteins, which bind over 95% 
of IGF1. This necessitates dissociating IGF1 from its binding 
proteins as a step before immunologic detection. Two-site assays 
have now overcome the need for prior dissociation. IGF1 mea­
surements are particularly valuable in the management of GH-
related disorders because G

 the management of GH-
related disorders because GH status can only be estimated from 
repeated GH measurements over an extended period. Like GH, 
IGF1 status changes over the human lifespan, rising to a peak 
in late puberty and declining in early adulthood and gradually 
thereafter the fourth decade. A recent study employing a two-site 
chemiluminescent assay calibrated against a recombinant human 
203
IGF1 standard (02/254) in over 15

ostic Evaluation
IGF1 standard (02/254) in over 15,000 subjects has provided 
timely age-stratified and gender-stratified reference ranges from 
childhood to senescence (see Fig. 8.11). 
Growth Hormone Deficiency
GH is the most abundant hormone in the adult pituitary gland, 
and it plays an important role in maintaining the metabolic pro­
cess and the integrity and function of many tissues and systems 
after the cessation of growth. Life expectancy is reduced in hypo­
pituitary patients with GHD

cy is reduced in hypo­
pituitary patients with GHD. In a meta-analysis of six studies 
totaling over 19,000 patients, the mortality is twice that of the 
control population (Fig. 8.15) with the risk greater in women 
than men and greater in those who received radiotherapy.256–260
Pathophysiology
Adult GHD may be acquired or congenital261 (Table 8.5). Surgi­
cal or radiation treatment of pituitary, parasellar, and brain tumors 
is the most common cause of GHD, accounting for nearly two-
thirds of

cause of GHD, accounting for nearly two-
thirds of cases. The frequency of causes differs between patients 
with childhood-onset and adult-onset GHD (Fig. 8.16).262 Idio­
pathic causes, representing the most commonly encountered 
group in childhood-onset GHD, likely represent a heterogeneous 
collection of congenital developmental abnormalities, including 
mutations of PROP1 or POU1F1 genes, causing GHD with other 
pituitary hormone deficiencies.263 Isolated GHD may be com­
plete or partial, and

263 Isolated GHD may be com­
plete or partial, and up to 67% of children initially diagnosed 
with idiopathic GHD had normal GH responses when subse­
quently retested as adults for GHD following cessation of GH 
treatment.264 Children with GHD should be retested before GH 
treatment is continued into adulthood unless they have clearly 
documented panhypopituitarism or a defined genetic or develop­
mental abnormality that causes complete and irreversible GHD. 
Mutations in the GH265 and GHRH rece

ersible GHD. 
Mutations in the GH265 and GHRH receptor genes,266 and GH 
insensitivity as a result of primary GHR dysfunction,267 result in 
a selective lack of GH action. 
Presentation
The clinical consequences of GHD in adults are shown in Table 
8.6. Symptoms of GHD are nonspecific and include fatigue, 
lack of energy, social isolation, low mood, poor concentration, 
and reduced physical function, contributing to a poor quality 
of life.268 The signs are also nonspecific and include general 

e signs are also nonspecific and include general 
and central adiposity and reduced lean tissue and bone mineral 
density (BMD), along with unfavorable biochemical changes 
such as hyperlipidemia269,270 and glucose intolerance.268 Some 
patients have established evidence of macrovascular disease such 
as increased carotid intimal thickness.271,272 GH deficiency 
impairs cardiac function.273,274 Cardiovascular risk abnormali­
ties are less pronounced in adults with childhood-onset GHD 
than in th

ced in adults with childhood-onset GHD 
than in those acquiring the deficiency during adulthood who 
have more pronounced disordered quality of life, lipids, and 
body composition.270
Subjects with isolated GHD or inactivating mutation of the 
GHR manifest some features that contrast with those of GHD as 
a component of multiple pituitary hormone deficits. Patients with 
selective absence of GH action display enhanced insulin sensitiv­
ity, lower prevalence of diabetes, absence of premature athe

 prevalence of diabetes, absence of premature atheroscle­
rosis, and normal life expectancy.275,276 These findings suggest that 
the observed increased mortality rate in hypopituitary adults with 
GHD arises in part from suboptimal replacement regimens and 
coexisting comorbid conditions. 
Evaluation
GHD is diagnosed biochemically within an appropriate clinical 
context. Biochemical testing for GHD occurs with a high prob­
ability in patients with a history of organic hypothalamic-pituitary 
dys

h a history of organic hypothalamic-pituitary 
dysfunction, cranial irradiation, known childhood-onset GHD, 
and traumatic brain injury (TBI).277,278 The features of adult 
GHD are not particularly distinct and mimic body compositional 
and biochemical changes of the aging process. Thus clinical sus­
picion must be confirmed by accurate biochemical diagnosis to 
ensure that GH-deficient patients are accurately identified and 
treated.
Provocative Testing. The diagnosis of adult GHD is establishe

 Testing. The diagnosis of adult GHD is established 
by provocative testing of GH secretion (Table 8.7). Other hor­
mone deficits should be corrected before testing. Several provoca­
tive tests include insulin tolerance test (ITT), arginine, glucagon, 
clonidine, growth hormone–releasing peptides (GHRPs), and 
(n = 172)
Bates et al 1996
Bulow et al 1997
Tomlinson et al 2001
Svensson et al 2004
Van Bunderen 2011
Gaillard et al 2012
RE Model
Heterogeneity
1.99 [ 1.21 , 2.76 ]
(p < 0.0001)
I2 = 97.

geneity
I2 = 97.85%
1.00
2.00
3.00
SMR with 95% CI
4.00
5.00
1.13 [ 1.03 , 1.23 ]
1.27 [ 1.01 , 1.53 ]
3.80 [ 3.42 , 4.18 ]
1.87 [ 1.37 , 2.37 ]
2.17 [ 1.85 , 2.49 ]
1.73 [ 1.23 , 2.23 ]
(n = 344)
(n = 1044)
(n = 1411)
(n = 13983)
(n = 2229)
• Fig. 8.15  Forest plot of standardized mortality ratios with 95% confidence intervals from six studies in 
patients with hypopituitarism. (Redrawn from Pappachan JM, Raskauskiene D, Kutty VR, et al. Excess 
mortality assoc

skiene D, Kutty VR, et al. Excess 
mortality associated with hypopituitarism in adults: a meta-analysis of observational studies. J Clin Endo­
crinol Metab. 2015;100:1405–1411.)
204
GHRH, alone or in combination with arginine or pyridostig­
mine. GHRPs are synthetic analogues of ghrelin. Because pro­
vocative tests vary in the ability to evoke GH release, a single 
value cannot be applied as a diagnostic threshold across different 
tests.279 ITT is a more

hreshold across different 
tests.279 ITT is a more potent stimulator of GH release than argi­
nine, clonidine, or l-dopa, whereas combinations such as arginine 
plus GHRH, or GHRP plus GHRH, are more potent than ITT 
alone.280–282
The ITT is the gold standard test for GHD. Normal subjects 
respond to insulin-induced hypoglycemia with peak GH con­
centrations of more than 5 μg/L283 (Fig. 8.17). Severe GHD is 
defined by a peak GH response to hypoglycemia of less than 3 
μg/L.284 These cutoff valu

lycemia of less than 3 
μg/L.284 These cutoff values were defined using polyclonal GH 
RIAs.284 The test is contraindicated in patients with electrocar­
diographic evidence or history of ischemic heart disease and in 
patients with seizure disorders.
Glucagon,285 GHRH, and ghrelin mimetics (GHRP2, 
GHRP6, and macimorelin286), alone287 or in combination,282,288 
are validated diagnostic tests.281,282,288 The diagnostic thresh­
olds for these tests are shown in Table 8.7. The unavailability 
of GH

 are shown in Table 8.7. The unavailability 
of GHRH has brought glucagon greater attention as a simple 
diagnostic test for GHD when the ITT is not desirable.289 The 
ITT evaluates the integrity of the hypothalamic-pituitary axis 
and has the added advantage of also stimulating ACTH secre­
tion. Diagnostic tests employing GHRH or GHRPs, both of 
which directly stimulate GH release from the pituitary gland, 
may not identify GHD caused by hypothalamic disease.290 This 
is exemplified by studies 

lamic disease.290 This 
is exemplified by studies in patients treated with cranial irradia­
tion, when the ITT shows the greatest sensitivity and specificity 
within the first 5 years after irradiation.291 If peak GH levels 
are normal during a GHRH plus arginine test in patients who 
have received irradiation, then an ITT should also be performed. 
In irradiated patients as well as those with inflammatory and 
infiltrative parasellar lesions, GHD may develop many years 
after the initial insult

D may develop many years 
after the initial insult. Therefore this group of patients should 
be followed in the long term with repeat testing. Obesity con­
founds the diagnostic testing of GHD as it blunts the peak GH 
response.292 Except for the GHRH plus arginine test, body mass 
index (BMI)–stratified normal ranges have not yet been estab­
lished for any of the provocative tests. 
Growth Hormone–Responsive Markers. These markers 
include IGF1, IGF binding protein 3 (IGFBP3), and the acid-
lab

 IGF binding protein 3 (IGFBP3), and the acid-
labile subunit of the IGFBP complex. IGF1 is useful for diag­
nosis only when age-adjusted normal ranges are used. Although 
IGF1 levels are reduced in adult GHD, a normal concentra­
tion does not exclude the diagnosis283 (see Fig. 8.17). A sub­
normal IGF1 level in an adult patient with coexisting pituitary 
hormone deficits is strongly suggestive of GHD, particularly in 
the absence of conditions known to reduce IGF1 levels, such as 
malnutrition,

nown to reduce IGF1 levels, such as 
malnutrition, liver disease, poorly controlled diabetes mellitus, 
and hypothyroidism. The separation of IGF1 values between 
GH-deficient and normal subjects is greatest in the young. As 
IGF1 levels decline with aging in normal subjects, IGF1 mea­
surements become less reliable as a biochemical marker of GHD 
in patients older than 50 years when the values merge with those 
of normal age-matched subjects.293 Measurement of IGFBP3 or 
the acid-labile subunit

 Measurement of IGFBP3 or 
the acid-labile subunit does not offer any diagnostic advantage 
over IGF1.277,278
In patients with organic hypothalamic-pituitary disease, the 
prevalence of GHD is strongly linked to the number of pituitary 
hormone deficits, ranging from approximately 25% to 40% in 
those with no other deficit to virtually 95% to 100% when more 
than three pituitary hormone deficiencies are present.294 Patients 
with three or more pituitary hormone deficiencies and an IGF1 
level be

tuitary hormone deficiencies and an IGF1 
level below the reference range have a greater than 97% chance of 
being GH deficient (see Table 8.7)295 and therefore do not require 
GH stimulation testing.277,278 
Spontaneous GH Secretion. As pituitary GH secretion occurs 
episodically, accurate quantification of integrated GH secretion 
requires continuous measurement of secretion over 24 hours. This 
procedure requires insertion of a continuous withdrawal pump 
or patent indwelling catheter for fre

drawal pump 
or patent indwelling catheter for frequent sampling. Continuous 
24-hour GH measurement in the diagnosis of GHD is inferior 
to provocative testing283 (see Fig. 8.17) and is cumbersome and 
expensive. 
  Causes of Acquired Pituitary Insufficiency
Traumatic
Surgical resection
Radiation damage
Traumatic brain injury
Infiltrative/Inflammatory
Primary hypophysitis
Lymphocytic
Granulomatous
Xanthomatous
Secondary hypophysitis
Histiocytosis X
Infections
Wegener granulomatosis

Takayasu disease
Hemochromatosis
Tuberculosis
Pneumocystis jirovecii infection
Fungal (histoplasmosis, aspergillosis)
Parasites (toxoplasmosis)
Viral (cytomegalovirus)
Vascular
Pregnancy related
Aneurysm
Apoplexy
Diabetes
Hypotension
Arteritis
Sickle cell disease
Neoplastic
Pituitary adenoma
Dermoid cyst
Germinoma
Glioma
Hypothalamic harmatoma, gangliocytoma
Pituitary

ma
Pituitary metastatic deposits
Hematologic malignancy
Leukemia
Lymphoma
TABLE 8.5
205
Gene Expression Analysis
With the advent of high throughput and computational technol­
ogy, there has been much interest in applying genomics and tran­
scriptomics to identify dysfunction of the GH system and predict 
responsiveness to GH therapy. A recent study of well-character­
ized children with GH deficiency 

 well-character­
ized children with GH deficiency combining genotyping and 
gene expression analysis of peripheral blood mononuclear cells 
has identified a cluster of probe sets that strongly associate with 
the biochemical severity of GHD.296 The results provide tantaliz­
ing evidence that gene expression analysis from a blood sample 
may aid in the diagnosis of GH deficiency. However, evidence 
that blood transcriptome changes are useful in predicting growth 
in children297 or identifying GH 

edicting growth 
in children297 or identifying GH abuse is poor.298 
Growth Hormone Replacement Therapy
The effects of GH replacement were first reported in 1989. 
GH replacement induces profound effects on protein, fat, and 
energy metabolism, which result in increased lean body mass and 
decreased fat mass without a significant change in body weight 
within months299 (Fig. 8.18). The greatest reduction of body fat 
occurs in abdominal and visceral adipose tissue300 (Fig. 8.19). 
Significant in

ral adipose tissue300 (Fig. 8.19). 
Significant increases in extracellular water occur as a consequence 
of the dose-dependent antinatriuretic properties of GH.301 GH-
induced reduction in abdominal and visceral fat is accompanied by 
a significant shift in lipoprotein metabolism to a less atherogenic 
profile with improvements in the cholesterol and in HDL-choles­
terol ratio but little or no change in triglycerides.249 Long-term 
experience of more than 10 years indicates that treatment provid

more than 10 years indicates that treatment provides 
sustained benefits in body composition and metabolic risk mark­
ers302 (Fig. 8.20). GH treatment reduces intimal media thickness 
of the carotid arteries.271 Proinflammatory factors such as C-reac­
tive protein and IL6, strongly implicated in the pathogenesis of 
vascular disease, fall significantly with GH treatment.269
GH replacement activates bone formation and bone resorp­
tion, resulting in a transient decline before a net increase in bo

in a transient decline before a net increase in bone 
mass by 12 months.303–305 Markers of bone turnover increase over 
the first 12 months but return to baseline after 3 to 4 years.306 The 
increase is greater in men and in younger subjects.304 BMD may 
increase for up to 10 years in the lumbar spine, whereas a decline 
that occurs in the femoral neck may start sooner307 (Fig. 8.21). 
In an observational study, the fracture incidence over a mean fol­
low-up period of 4.9 years was 30% lower in 

 fol­
low-up period of 4.9 years was 30% lower in patients replaced 
with GH compared to an unreplaced group providing evidence of 
long-term protective benefit.308
GH replacement improves physical function through gains in 
anaerobic309 and aerobic capacity,310,311 the latter mediated in part 
by an increase in red cell mass, plasma volume, and cardiac out­
put.312,313 Most studies assessing GH effects beyond 12 months 
have reported a significant improvement in muscle strength 
through gains i

nt improvement in muscle strength 
through gains in muscle mass,314–316 without affecting contractile 
force or fiber composition.317
Most trials with GH replacement report improved quality of 
life.318 Discrepant results likely arise from varying study tools, 
with some being generic health questionnaires and others being 
disease specific. Domains of energy and emotional reaction tend 
to show the greatest improvements. Disease-specific tools have 
reported unequivocal improvement in measures 

ave 
reported unequivocal improvement in measures of life satisfac­
tion after GH treatment.319 A large survey of 304 patients showed 
improved quality of life and significant reduction in the num­
bers of sick leave days and doctor visits during 12 months of GH 
therapy.320,321
Growth Hormone Administration. GH secretion is greater 
in the young and greater in women than in men. It is rec­
ommended that the starting dose of GH in young men and 
women be 0.2 and 0.3 mg/day, respectively, and in 

women be 0.2 and 0.3 mg/day, respectively, and in older indi­
viduals 0.1 mg/day,277,278 which is then titrated according to 
serum IGF1 concentrations and at a rate that minimizes side 
effects322 (Fig. 8.22). If side effects occur, the dose should 
be reduced; if no side effects are reported, the therapeutic 
goal is to maintain IGF1 levels in the normal age-matched 
and gender-matched range while avoiding levels in the upper 
quintile or above. Dose determination based on body weight 
Unknown

. Dose determination based on body weight 
Unknown
5%
Adult-Onset
n = 1329
Childhood-Onset
N = 494
Other
9%
Unknown
20%
Other
21%
18%
Inflammatory
1%
Benign tumors
17%
Malignant
tumors
8%
Craniopharyn
gioma
10%
Pituitary
adenoma
5%
Pituitary
adenoma
61%
Apoplexy
2%
3%
Benign
tumors
7%
Malignant
tumors
5%
Cranio
pharyngioma
8%
• Fig. 8.16  Frequency of the causes of childhood-onset and adult-onset growth hormone deficiency in 
Denmark. “Other” includes cysts, hypophysitis,

in 
Denmark. “Other” includes cysts, hypophysitis, granulomas, trauma, and empty sella syndrome. (Data 
from Stochholm K, Gravholt CH, Laursen T, et al. Incidence of GH deficiency—a nationwide study. Eur J 
Endocrinol. 2006;155:61–71.)
206
is not recommended because GH secretion is reduced in the 
obese.277,278 GH is administered by nightly subcutaneous 
injection to mimic the greater secretion of GH at night. Side 
effects of GH in children are considera

ght. Side 
effects of GH in children are considerably fewer than those 
observed in adults.
Women with GHD require higher doses of hGH when also 
receiving oral rather than transdermal estrogen323,324 because of a 
first-pass hepatic effect. Fifty percent more GH was required dur­
ing oral estrogen treatment to maintain an IGF1 level equivalent 
to that achieved during transdermal administration; the waste is 
even greater when contraceptive instead of replacement estrogen 
doses are prescribed 

ead of replacement estrogen 
doses are prescribed (Fig. 8.23).325 In contrast, androgens enhance 
metabolic effects of GH.148 The divergent effects of estrogens and 
androgens on GH action largely explain why women are less 
responsive than men to GH.302 
Transition-Age Patients. GH treatment of the GH-deficient 
child normally is terminated when final height and epiphysis 
closure are reached.326 These GHD patients who transition from 
the time of cessation of linear growth do not attain the so

of cessation of linear growth do not attain the somatic 
and structural skeletal maturity that continues to occur in the 
early years of normal adulthood. GH-deficient children should 
continue GH treatment after puberty to complete somatic matu­
ration of muscle and bone.326 A significant proportion of patients 
with childhood-onset GHD exhibit normal GH response when 
retested at the end of GH treatment as young adults. Therefore 
patients considered for continuation of GH replacement in the 

idered for continuation of GH replacement in the 
transition years must be retested. GH retesting is not required 
for those patients with a transcription factor mutation (e.g., 
POU1F1, PROP1) or those with more than three demonstrated 
pituitary hormone deficits.277,278 
Precautions and Caveats of Treating With Human Growth 
Hormone. The most common side effects of hGH replacement 
include edema, arthralgias, and myalgias (Table 8.8). However, 
these symptoms are mild, dose related, and resolv

these symptoms are mild, dose related, and resolve in the major­
ity of patients either spontaneously or with dosage reduction.322 
Although GH antagonizes insulin action, the risk of developing 
hyperglycemia is very low (see Fig. 8.20). A meta-analysis of 13 
placebo-controlled trials involving 511 patients found a mean 
elevation of fasting blood glucose of 0.22 mmol/L compared to 
placebo levels.249 However, the propensity for developing diabetes 
is increased up to eightfold in GHD adults 

betes 
is increased up to eightfold in GHD adults who are obese.327
Patients with active malignancies should not be treated with GH. 
The possibility that hGH might initiate new cancers or stimulate 
growth of preexisting benign tumors is an important theoretic issue. 
The incidence of cancer is markedly reduced among subjects with 
inactivating mutations of the GHR.275 A number of epidemiologic 
studies have reported association between higher, albeit normal, 
IGF1 levels and later risk of deve

albeit normal, 
IGF1 levels and later risk of developing prostate cancer,328 breast 
cancer in premenopausal women,329 and colon and lung cancer.330 
  Adult Somatotropin Deficiency
Clinical Consequence
Effect of GH Replacement
Body Composition
General and central adiposity
Decrease
Reduced lean mass
Increase
Reduced bone mass
Increase
Function
Reduced exercise capacity
Improve
Impaired anaerobic capacity
Improve
Muscle weakness
Increase
Impaired cardiac function
Improve
Hypohydrosis
Increase
Q

d cardiac function
Improve
Increase
Quality of Life
Low mood
Improve
Fatigue
Improve
Low motivation
Improve
Reduced satisfaction
Improve
Cardiovascular Risk Profile
Abnormal lipid profile
Improve
No change
Increased inflammatory markers
Decrease
Intimal media thickening
Decrease
Blunted peak GH to stimulation 
(see Table 8.7)
Low IGF1 (in 50–60%)
Increase
Hyperinsulinemia
Improve
High LDL and low HDL cho­
lesterol
Improve
GH, Growth hormone; HDL, high-d

­
lesterol
Improve
GH, Growth hormone; HDL, high-density lipoprotein; IGF1, insulin-like growth factor 1; LDL, 
low-density lipoprotein.
   
TABLE 8.6
  Validated Diagnostic Tests for the Diagnosis 
of Growth Hormone Deficiency in Adults
Test
Subject 
Numbers 
Normal/GHD
GH Threshold 
(μg/L)
Reference
Insulin-
induceda
35/23
<5
Hoffman et al283
Arginine-GHRHa
74/49
<9
Aimaretti 
et al281
Glucagona
46/73
<3
Gomez et al285
GHRP 6-GHRHa
125/125
<15
Popovic et al282
GHRP 2-GHRHa
30/36

a
125/125
<15
30/36
<17
Mahajan et al288
GHRP 2
77/58
<15
Chihara et al287
Macimorelin
25/114
<5.1
Gomez et al285; 
Garcia 
et al286
Low IGF1 and 
≥3 PHDsa
785
N/A
Hartman et al295
aRecommended by the Growth Hormone Research Society and the Endocrine Society.
GH, Growth hormone; GHD, growth hormone deficiency; GHRH, growth hormone–releasing 
hormone; GHRP, growth hormone–releasing peptide; IGF1, insulin-like growth factor 1; N/A, 
not applicable; PHD, pituitary horm

actor 1; N/A, 
not applicable; PHD, pituitary hormone deficiency.
   
TABLE 8.7
207
In contrast, patients with acromegaly do not have an increased 
incidence of either breast or prostate cancer or cancer in general. 
The overall risk for cancer in acromegaly is lower than expected. 
However, these patients have a significantly increased mortality risk 
from colon cancer.331,332 Pediatric experience shows no convincing 
evidence for a cau

experience shows no convincing 
evidence for a causal link between GH treatment and tumor recur­
rence or the development of neoplasia, including leukemia.333,334 
When comparing the relative risk of brain tumor recurrence in 180 
children treated with hGH versus 891 who did not receive hGH, 
the risk of recurrence after a mean of 6.4 years was lower in the 
treated group than in those not receiving hGH.335 GH treatment 
does, however, increase the risk of radiation-induced second tumor 
formati

he risk of radiation-induced second tumor 
formation, especially meningiomas.336
From observational studies, the incidence of malignancies in 
hypopituitary patients not receiving GH is higher than the general 
population but not in those replaced with GH.337 The recurrence 
rate of craniopharyngiomas338 and the recurrence rate and life 
expectancy in patients with nonfunctioning pituitary adenomas339 
were not different between patients replaced and unreplaced with 
GH over a follow-up period i

 and unreplaced with 
GH over a follow-up period in excess of 10 years. Although obser­
vational studies are subject to selection bias, these findings provide 
some evidence supporting the long-term safety of GH replace­
ment therapy. Nevertheless, long-term surveillance with adequate 
control groups in adults being treated for adult GHD is required 
to ensure that adult GH replacement does not increase the inci­
dence of new cancers or growth of existing benign tumors. 
Investigational Uses of 

 existing benign tumors. 
Investigational Uses of Growth Hormone
Catabolic States. The anabolic actions of GH have prompted 
investigational use of GH in catabolic states, including surgery, 
trauma, burns, parenteral nutrition, and organ failure. The nega­
tive nitrogen balance in critically ill patients is partly attributable 
to GH resistance as well as to decreased IGF-1 production and 
action.340 Beneficial effects of GH have been reported in patients 
with extensive burns, in those receivi

n patients 
with extensive burns, in those receiving high-dose glucocorticoid 
treatment, and in those with chronic obstructive pulmonary dis­
ease. There is some evidence that GH treatment in people with 
large burns results in more rapid healing of the burn wound and 
donor sites, and in reduced length of hospital stay, but there is an 
increased risk of hyperglycemia.341 A study in which critically ill 
patients received very high doses of GH (up to 7 mg/day) approxi­
mating 15 to 20 times no

(up to 7 mg/day) approxi­
mating 15 to 20 times normal daily production rates was prema­
turely terminated due to increased fatality.342 It has been suggested 
that GH may have had an adverse effect on acute-phase protein 
100
10
1.0
0.1
Peak GH (ng/mL)
Hypopituitary
Normal
A
3.5
3.0
2.5
2.0
1.5
1.0
0.5
IGHC (ng/mL)
B
400
300
200
100
0
IGF1 (ng/mL)
C
• Fig. 8.17  Comparison of peak growth hormone (GH) concentration 
obtained during an insulin-tolerance test (A), integrated GH concentration 
(IGH

erance test (A), integrated GH concentration 
(IGHC) obtained from blood samples withdrawn every 20 minutes over 
24 hours (B), and insulin-like growth factor 1 (IGF1) concentrations (C) in 
patients with organic hypopituitarism and sex-matched normal subjects. 
Horizontal lines represent the limit of reading. (Modified from Hoffman DM, 
O’Sullivan AJ, Baxter RC, et al. Diagnosis of growth hormone deficiency in 
adults. Lancet. 1994;343:1064–1068.)
Body weight (kg)
70
Months
rhGH
Placebo
11
12
1

.)
70
Months
rhGH
Placebo
11
12
11
12
11
12
12
12
95
90
85
80
75
Lean body mass (kg)
40
rhGH
Placebo
11
12
10
11
12
12
65
60
55
50
45
Fat mass (kg)
20
6
0
1
2
3
4
5
rhGH
Placebo
11
12
10
11
12
12
35
30
25
• Fig. 8.18  Effects of growth hormone (GH) replacement on lean body 
mass and fat mass in adults with GH deficiency. Effect of GH replacement 
on lean body mass and fat mass in 24 adults with GH deficiency. rhGH, 
recombinant human growth hormone. (From Salomon F, Cuneo RC, He

uman growth hormone. (From Salomon F, Cuneo RC, Hesp 
R, et al. The effects of treatment with recombinant human growth hormone 
on body composition and metabolism in adults with growth hormone defi­
ciency. N Engl J Med. 1989;321:1797–1803.)
208
in these patients.343 Caution is advised for nonapproved uses of 
GH in adults.344 
Osteoporosis. There is strong evidence that GH administered to 
otherwise healthy subjects with idiopathic osteoporosis improves 

hy subjects with idiopathic osteoporosis improves 
bone mineral mass.345 A double-blind, placebo-controlled treat­
ment study for 18 months observed improvements in bone min­
eral content at the lumbar spine, in the femoral neck, and of the 
whole skeleton by up to 14%, which was sustained at 4 years.346 
Unclear long-term side effects, cost, and lack of comparative stud­
ies with other therapies for osteoporosis limit the potential use of 
GH in osteoporosis. 
Human Immunodeficiency Virus Infec

 osteoporosis. 
Human Immunodeficiency Virus Infection. GH is Food and 
Drug Administration (FDA) approved for administration to adult 
patients with human immunodeficiency virus (HIV)–associated 
cachexia and results in positive nitrogen balance, increased lean 
body mass, decreased body fat, and improved work output.347 
One large study using supraphysiologic GH doses of 4 mg/day 
reported impairment in glucose tolerance.348 A study using a 
physiologic dosing to maintain IGF1 in the high-norm

ysiologic dosing to maintain IGF1 in the high-normal range 
reported beneficial body compositional changes of lesser magni­
tude, reduced triglycerides, and lowered diastolic blood pressure 
but impaired glucose tolerance.349 Treatment with a GH-releasing 
factor analogue for 26 weeks improved visceral fat and lipid pro­
files.350 However, long-term beneficial effects of GH on survival 
and quality of life in those with HIV infection have not yet been 
reported. 
Sports. The public policy issues

 been 
reported. 
Sports. The public policy issues of GH abuse in competitive 
sports have received much attention. GH has been widely abused 
by athletes to enhance performance.351 A systematic review con­
cluded that claims that GH enhances physical performance are 
not supported by the scientific literature evaluating effects on 
aerobic capacity, strength, and power352 and that more research 
is needed to conclusively determine the effects of GH on athletic 
performance. A double-blind, plac

 GH on athletic 
performance. A double-blind, placebo-controlled study reported 
that GH enhances anaerobic sprint capacity but not aerobic 
capacity, strength, or power in recreational athletes.353 The benefit 
to sprinting provides the first evidence justifying the prohibition 
of GH as a performance-enhancing drug. 
Aging. Disease mongering of declining GH status is rampant 
and spawned by unsubstantiated claims that GH is an antiaging 
A
B
• Fig. 8.19  Computed tomography scan through the ab

Fig. 8.19  Computed tomography scan through the abdomen before (A) 
and after treatment with human growth hormone (GH) (hGH) (B) in a GH-
deficient patient. (Courtesy B.A. Bengtsson.)
Years
% Change (± 95% CI)
0 1
3
5
7
10
10
7.5
5.0
2.0
0
Men (n = 52)
Women (n = 35)
p < 0.001
Baseline
GH Dose (mg/d)
TC
HDL-C
LDL-C
Glucose
5.9
1.21
3.89
4.00
0.98
0.65
0.53
0.50
0.48
5.37
1.36
3.22
4.96
0.47
Metabolic (mmol/L)
1 yr
3 yr
5 yr
7 yr
10 yr
Lean body mass
Years
0 1
3
5
7
10
10
5
0

ass
Years
0 1
3
5
7
10
10
5
0
−5
−10
−15
p < 0.05
HbA1c
• Fig. 8.20  Ten-year growth hormone (GH) therapy in 87 GH-deficient adults.302 CI, confidence interval; 
HbA1c, hemoglobin A1c; HDL-C, high-density lipoprotein-cholesterol; LDL-C, low-density lipoprotein-
cholesterol; TC, total cholesterol. (Modified from Melmed S. Update in pituitary disease. J Clin Endocrinol 
Metab. 2008;93:331–338.)
209
hormone.354 The claim

y and  Diagnostic Evaluation
hormone.354 The claims are based on observations that body 
composition changes that accompany the physical decline of the 
aging process superficially recapitulate the features of adults with 
organic GHD for which beneficial effects of GH replacement are 
established. A systematic review of literature published on ran­
domized controlled trials in the healthy elderly indicates that GH 
supplementation is associated with small changes in body com­
position, no funct

femur neck (B) in adult men and women with GH deficiency. The results are shown 
as percent change from baseline. The vertical bars indicate the SEM (standard error of the mean) values 
shown. *p < 0.05; ***p < 0.001 versus baseline. p < 0.01 for men versus women. (From Elbornsson M, 
Gotherstrom G, Bosaeus I, et al. Fifteen years of GH replacement increases bone mineral density in hypo­
pituitary patients with adult-onset GH deficiency. Eur J Endocrinol. 2012;166:787–795.)
None
Transdermal Conj

docrinol. 2012;166:787–795.)
None
Transdermal Conjugated
(oral)
17-
Estradiol
(oral)
Ethinyl
estradiol
(oral)
Type of estrogen therapy
40
35
30
25
20
15
10
5
0
IGF1/GH dose (nmol/mg)
• Fig. 8.23  The effect of the route and type of estrogen therapy on the 
efficacy of growth hormone (GH) therapy in women with hypopituitarism. 
The GH sensitivity index was calculated as change in insulin-like growth 
factor 1 (IGF1) level (nmol/L) by dose of GH (mg). Created using data 
from several studies. (F

(mg). Created using data 
from several studies. (From Birznieve V, Ho KKY. Growth and develop­
ment: patching up a better pill for GH-deficient women. Nat Rev Endo. 
2012;8:197–198.)
Pituitary pathology
Clinical features of
hyposomatotropism
Evoked GH <3 µg/L
IGF1 low (not invariable)
Exclude contraindications
for GH replacement
•  No active tumor
•  No retinopathy
•  Diabetes controlled
Assess unique
dose requirements:
women on estrogen;
elderly patients
Initiate GH replacement
0.15-0.3 mg/day

 patients
Measure IGF1 levels
after 1 month
No clinical response
after 6 months
Clinical response
Discontinue Rx
Continue treatment
Monitor IGF1,
lipids, bone density
Titrate GH dose to normalize IGF1 levels,
based on age- and gender-matched controls,
or until symptom tolerance
• Fig. 8.22  Management of somatotropin deficiency in adults. Patients older 
than 60 years require lower maintenance doses. Women receiving oral estro­
gen require higher doses tha

receiving oral estro­
gen require higher doses than those receiving transdermal estrogen prepara­
tions. GH, growth hormone; IGF1, insulin-like growth factor 1; Rx, treatment.
210
adverse events.355 GH cannot be recommended as an antiaging 
therapy.354 
GH Excess
GH secretion is stimulated by fasting and by inadequate nutri­
tion. This arises from the removal of substrates such as glucose 
and fatty acids, and of insulin, all of which suppress GH secre­
t

 and of insulin, all of which suppress GH secre­
tion.213 The liver, being the major source of circulating IGF1, 
plays an important role in modulating feedback control of GH 
secretion. Thus liver disease and systemic illness affecting hepatic 
function and nutritional intake cause reversible hypersecretion 
of GH.356,357 GH secretion is enhanced in type1 diabetes and 
falls when glycemic control improves.358 Oral estrogen therapy 
enhances GH secretion by reducing blood IGF1 levels.219 Under 

ecretion by reducing blood IGF1 levels.219 Under 
these circumstances episodic GH secretion is amplified, retain­
ing a diurnal profile associated with subnormal IGF1 concentra­
tions. This stands in contrast to the hypersecretion in acromegaly 
where secretion is autonomous and associated with elevated IGF1 
concentration. 
Adrenocorticotropic Hormone
The hypothalamic-pituitary-adrenal (HPA) axis is the major sys­
tem subserving stress and survival. Key components of the stress 
resp

s and survival. Key components of the stress 
response are aimed at providing adequate amounts of glucocorti­
coids, which exert vital pleotrophic effects on energy supply, fuel 
metabolism, immunity, and cardiovascular function.
Corticotroph Cells
Corticotroph cells comprise about 20% of functional anterior 
pituitary cells and are the earliest detectable human fetal pituitary 
cell type, appearing by the eighth week of gestation. Corticotrophs 
are clustered mainly in the central median pituit

are clustered mainly in the central median pituitary wedge. They 
are large, irregular cells, and their ultrastructural features include 
prominent neurosecretory granules (150–400 nm), endoplasmic 
reticulum, and Golgi bodies.359 These cells produce the POMC 
gene products, including ACTH(1-39), β-lipotropin (βLPH), 
and endorphins. Because of the rich carbohydrate moiety of these 
molecules, the cells are strongly positive for periodic acid–Schiff 
(PAS). In the presence of excess glucocortic

hiff 
(PAS). In the presence of excess glucocorticoid, characteristic hya­
line deposits develop. In the normal human pituitary, POMC is 
expressed only in corticotroph cells. Most mammals possess an 
intermediate lobe comprising POMC-expressing melanotroph 
cells; however, this lobe is not developed in adult humans. Tpit, 
now referred to as Tbx19,38 is a critical transcriptional factor for 
corticotroph cell differentiation during pituitary development and 
for transcription of the POMC gene.3

elopment and 
for transcription of the POMC gene.38 
Structure
The primary translation product of POMC is a 266–amino acid 
pre-prohormone molecule encoding corticotrophic, opioid, and 
melanotropic peptides. The peptide contains a leader sequence 
and multiple dibasic proteolytic cleavage sites for glycosylation, 
acetylation, and amidation. Products of this processing include 
ACTH(1-39) and βLPH, which in turn give rise to γLPH and 
β-endorphin, also containing metenkephalin. ACTH itself may 

n, also containing metenkephalin. ACTH itself may 
also be cleaved to give rise to α-melanocyte–stimulating hormone 
(MSH)(1-13) and corticotropin-like intermediate lobe peptide 
(CLIP)(18-39).
The 8-kb human POMC gene, located on chromosome 
2p23,38,360 consists of three exons interspersed with two inter­
vening introns (Fig. 8.24). The first exon encodes a leader 
sequence, the second encodes the signal initiation sequence and 
the amino-terminal (N-terminal) portion of the POMC pep­
tide, and

al (N-terminal) portion of the POMC pep­
tide, and the third exon encodes most of the mature peptide 
sequences, including ACTH and βLPH.361 The POMC gene is 
expressed in pituitary and nonpituitary tissues, including brain, 
skin, placenta, gonads, gastrointestinal tissues, liver, kidney, 
adrenal medulla, lung, and lymphocytes. The pattern of POMC 
processing differs between pituitary and extrapituitary issues and 
is determined by tissue-specific expression and activity of cleav­
age enzymes.

fic expression and activity of cleav­
In corticotroph cells, a pituitary-selective promoter region for 
POMC generates a POMC messenger RNA (mRNA) transcript 
of approximately 1200 nucleotides. The 800 nucleotides of the 
coding region are translated into a pre-POMC molecule that 
includes a 26–amino acid signal peptide, which is rapidly cleaved. 
The parent POMC protein of 241 amino acids enters the secre­
tory pathway for subsequent processing.
In extrapituitary tissues, POMC gene

t processing.
In extrapituitary tissues, POMC gene expression is regulated 
differently from that in the pituitary. An upstream promoter gen­
erates a longer transcript of approximately 1350 nucleotides. A 
downstream promoter generates short truncated transcripts of 
approximately 800 nucleotides arising from the 5′ end of exon 
3. In the brain, however, neurons in the arcuate nucleus express 
  Side Effects of Adult Growth Hormone 
Treatment
Edema
Arthralgias
Myalgias
Muscle stiffness
Paresth

dema
Myalgias
Paresthesias
Carpal tunnel syndrome
Atrial fibrillation
Headache
Benign intracranial hypertension
Increase in melanocytic nevi
Hyperglycemia
Sleep apnea
Iatrogenic acromegaly
TABLE 8.8
Exon 1
Exon 2
ATG
ATG
ATG
TGA
Exon 3
(peripheral)
promoter
Pituitary
promoter
ACTH
• Fig. 8.24  Structure of POMC gene. Exon 1 encodes the RNA leader 
sequence, and exon 2 encodes the initiator methionine (ATG), the signal 
peptide, and several N-terminal resid

 the signal 
peptide, and several N-terminal residues of the precursor peptide, the 
remainder of which is encoded by exon 3. Corticotroph expression is deter­
mined by the upstream pituitary promoter (longer arrowhead), whereas 
peripheral expression of the short POMC mRNA is determined by the 
downstream promoter (shorter arrowhead). Translation of these shorter 
transcripts initiates from the initiator methionines (ATG) indicated in exon 3. 
The precursor peptide coding region is lightly shad

he precursor peptide coding region is lightly shaded, and the ACTH coding 
region is darkly shaded. (From Adrian JL, Clark AJL, Swords FM. Molecu­
lar pathology of corticotroph function. In: Rappaport R, Amselem S, eds. 
Hypothalamic-Pituitary Development. Basel, Switzerland: Karger; 2001.)
211
POMC mRNA that is identical to that of the pituitary.362 In 
these neurons, POMC serves as a precursor to brain β-endorphin, 
αMSH, and other pep

ecursor to brain β-endorphin, 
αMSH, and other peptides, which have important functions on 
food intake and energy homeostasis. 
Pituitary POMC expression is primarily under the positive regula­
tion of corticotropin-releasing factor (CRF) and negative regula­
tion of glucocorticoids. However, multiple other signals regulate 
pituitary POMC gene expression: AVP, cytokines, catecholamines, 
and VIP activate while somatostatin and ANP inhibit.363
By contrast, POMC expression in the hypo

hibit.363
By contrast, POMC expression in the hypothalamus is 
stimulated by glucocorticoids, in turn enhancing the produc­
tion of α-MSH, which mediates the suppression of appetite.363 
Leptin and insulin also enhance hypothalamic POMC gene 
expression.364,365
The corticotropin-releasing hormone (CRH) type 1 receptor 
is predominantly expressed on the corticotroph,366 and receptor 
activation increases cAMP, protein kinase A, and CREB induction 
of CRH-binding protein (CRHBP) to the promoter, l

of CRH-binding protein (CRHBP) to the promoter, leading to 
POMC transcription.367 CRH also activates an AP1 site within 
the first exon by a MAPK-mediated pathway. This receptor also 
appears critical for fear and anxiety responses, possibly by a related 
ligand, urocortin.368 The type 2 CRH receptor predominantly 
regulates cardiovascular function.369
The corticotroph is regulated by inflammatory and nutritional 
signals that potentiate or inhibit CRF.370 CRH action is poten­
tiated by vasopr

ibit CRF.370 CRH action is poten­
tiated by vasopressin and β-adrenergic catecholamines, enhanc­
ing POMC transcription and synthesis and increasing ACTH 
secretion.363 Glucocorticoid inhibition of POMC transcription 
involves GR binding 5′-regulatory elements via two cooperative 
binding sites.362 Normal pituitary corticotrophs express soma­
tostatin receptors. Of the five subtypes, subtypes 2 and 5 are pre­
dominantly expressed. Somatostatin inhibits ACTH secretion, 
but the sensitivity is str

hibits ACTH secretion, 
but the sensitivity is strongly regulated by glucocorticoids, which 
nullify the inhibition via downregulating somatostatin receptor 
expression.371 Dopamine receptors have not been characterized in 
normal human corticotrophs, although they are highly expressed 
in the intermediate lobe of the rat pituitary.
POMC Processing
Several post-translational POMC modification steps are required 
for ultimate polypeptide hormone secretion (Fig. 8.25). First, the 
N-terminal signa

ecretion (Fig. 8.25). First, the 
N-terminal signal sequence is removed, followed by glycosylation 
via an O-linkage to Thr45 and N-linkage to Asn65.372 Serine-
phosphorylation then occurs within the Golgi apparatus. After 
being transported to secretory vesicles, the constituent peptides 
are cleaved at dibasic amino acid residues, and ACTH-related pep­
tides are stored in dense secretory granules for ultimate regulated 
release. Some POMC products also undergo carboxy-terminal 
(C-terminal) am

cts also undergo carboxy-terminal 
(C-terminal) amidation mediated by peptidylglycine-amidating 
mono-oxygenase (PAM), peptidylhydroxyglycine-amidating lyase 
(PAL),373 and N-terminal acetylation.
POMC proteolytic processing occurs at Lys-Arg or Arg-
Arg residues by enzymes called prohormone convertases (PCs), 
mediated by either or two called PC1 or PC2. They are part 
of a superfamily of subtilisin/kexin proteinases. Tissue distribu­
tion of the two PCs are distinct and determine the function 

e two PCs are distinct and determine the function of 
the cleaved peptides at the tissue level. PC1 is most abundant 
in the anterior pituitary and in the hypothalamus. PC2 is pres­
ent in the neurointermediate lobe, hypothalamus, various brain 
regions, skin, and pancreatic islets but is absent in the pituitary. 
In corticotrophs, PC1 expression results in cleavage limited 
to four sites, with ACTH and ß-lipotrophin being major end 
products (see Fig. 8.25). In the hypothalamus and CNS, both 
P

e Fig. 8.25). In the hypothalamus and CNS, both 
PC1 and PC2 allow coordinated proteolysis, resulting in the 
generation of αMSH, CLIP, γLPH, and ß-endorphin. Hetero­
zygous mutations of the PC1 gene are associated with childhood 
obesity, adrenal insufficiency, hyperproinsulinemia, and post­
prandial hypoglycemia374 with elevated levels of plasma ACTH 
precursors. 
Extrapituitary and CNS Expression of POMC
POMC is also expressed in the gonads, lung, skin, gastrointesti­
nal and adrenal medullar

ung, skin, gastrointesti­
nal and adrenal medullary neuroendocrine cells, and white blood 
cells. The biology of extrapituitary and extra-CNS POMC is 
poorly understood. There is little evidence that these tissues con­
tain enzymes required for the production and secretion of the pro­
cessed peptides. 
Melanocortin Receptors
The major secreted POMC products from the pituitary are 
ACTH, ßLPH, and ß-endorphin. ACTH-derived and POMC-
derived peptides bind to specific melanocortin receptors (MCR). 

es bind to specific melanocortin receptors (MCR). 
MCRs are members of the rhodopsin-like receptors, a branch of 
the seven transmembrane-spanning domain G protein–coupled 
receptor (GPCR) superfamily. There are five MCRs, each show­
ing different tissue distribution. All MCRs except for MC2R, 
the ACTH receptor, bind melanocortin peptides containing the 
conserved heptapeptide core MEHFRWG, found in αMSH, 
while the ACTH receptor further requires a peptide motif 
C-terminal to the 13 amino acid

s a peptide motif 
C-terminal to the 13 amino acids found in αMSH. Each MCR 
mediates diverse physiologic responses to αMSH, or ACTH, in 
the case of MC2R.375 The distribution of MCR types is shown 
in Table 8.9.
The MC1Rs are found primarily in melanocytes that regu­
late pigmentation of skin and hair follicles. ACTH, βLPH, and 
γLPH produced from the corticotroph share a common hepta­
peptide sequence (Met-Glu-His-Phe-Arg-Trp-Gly) required to 
POMC
N-POC
Pro-γ-MSH
γ3-MSH
α-MSH
γ-MSH
CLIP
 = PC

POMC
N-POC
Pro-γ-MSH
γ3-MSH
α-MSH
γ-MSH
CLIP
 = PC1
 = PC2
JP
ACTH
γ-LPH
β-EP
β-LPH
• Fig. 8.25  Processing and cleavage of pro-opiomelanocortin (POMC). The 
mature POMC precursor peptide is sequentially cleaved by prohormone 
convertase 1 (PC1) in the anterior pituitary corticotroph. In the neuroin­
termediate lobe and other cell types, cleavage by PC2 allows release of 
βMSH or β-endorphin, or both. Carboxypeptidase H (not shown) removes 
residual basic amino acids at cleavage sites. ACTH, adr

ual basic amino acids at cleavage sites. ACTH, adrenocorticotropic 
hormone; CLIP, corticotropin-like intermediate lobe peptide; EP, endor­
phin; JP, joining peptide; LPH, lipotropin; MSH, melanocyte-stimulating 
hormone; N-POC, N-terminal pro-POMC fragment. (From Clark AJL, 
Swords FM. Molecular pathology of corticotroph function. In: Rappaport 
R, Amselem S, eds. Hypothalamic-Pituitary Development. Basel, Switzer­
land: Karger; 2001.)
212
activate MC1R.

ON II    Hypothalamus and Pituitary
activate MC1R. These peptides are responsible for inducing skin 
pigmentation in Addison disease because the other melanostimu­
lating peptides, αMSH and βMSH, are not produced in the pitu­
itary. Some MC1R variations show impaired signaling pathways 
strongly associated with red hair color, fair skin, and poor tan­
ning ability.376 POMC is also expressed in skin melanocytes, the 
expression of which is simulated by ultraviolet light, increasing 
local α-MSH a

ed by ultraviolet light, increasing 
local α-MSH and ß-endorphin production in parallel with PC1 
and PC2 expression. ß-endorphin production alleviates pain and 
is thought to explain UV-seeking addictive behavior.377
MC3R and MCR4 are expressed almost exclusively in the 
CNS, especially the hypothalamus where they exert pivotal roles 
in satiety and energy homeostasis. Genetic and pharmacologic 
abrogation of the central melanocortin system causes profound 
obesity. POMC-deficient mice and huma

es profound 
obesity. POMC-deficient mice and humans are hyperphagic; 
MC4Rs are present in the L cells of the gut where they stimulate 
the production of anorexigenic peptides. The role of pituitary-
secreted POMC peptides on gut MC4Rs is unknown.378 MC5Rs 
play a major role in the regulation of exocrine gland function, 
especially of the sebaceous glands.379 Recent findings provide evi­
dence that MC5Rs in skeletal muscle enhance glucose uptake by 
αMSH independent of insulin.380 
Adrenal Acti

 by 
Adrenal Action
ACTH is a polypeptide of 39 amino acids with a molecular weight 
of 4.5 kDa. It is the only POMC-derived peptide with adreno­
corticotrophic function, and it is the ligand of the MC2R. The 
function of MCR2 is dependent on a protein, melanocortin 2 
receptor assessor protein (MRAP) required to traffic the receptor 
to the cell surface.381 MRAP mutations cause familial glucocorti­
coid deficiency. MC2R activation results in production of adrena

y. MC2R activation results in production of adrenal 
glucocorticoids, androgenic steroids, and to a lesser extent, min­
eralocorticoids. The highly conserved 12 N-terminal amino acid 
residues are critical for adrenal gland steroid synthesis. N-termi­
nal peptide POMC(1-28) exerts an independent mitogenic and 
growth-sustaining effect on the adrenal gland.382
ACTH signals via adenyl cyclase to regulate P450 enzyme tran­
scription of cortisol, aldosterone, 17-hydroxyprogesterone, and to 
a lesser

osterone, 17-hydroxyprogesterone, and to 
a lesser extent, adrenal androgens.383 ACTH stimulates mitochon­
drial cholesterol transport and regulates the rate-limiting, side-
chain cleavage of cholesterol to pregnenolone.384 Adrenal cortisol 
response to ACTH is sensitive to the background ambient ACTH 
milieu. In states of chronic ACTH deficiency, adrenal reserve is 
compromised, although during ongoing ACTH hypersecretion 
the gland is primed such that a given ACTH bolus elicits a higher 
corti

ch that a given ACTH bolus elicits a higher 
cortisol response. Both basal and stimulated (e.g., by CRH) 
ACTH secretion is blunted by glucocorticoids.
ACTH Secretion. The complex control of ACTH secre­
tion reflects the integrated neuroendocrine control of the stress 
response. Similar to other anterior pituitary hormones, ACTH 
regulation is subserved by at least three tiers of control (see Fig. 
8.3). First, the brain and hypothalamus release regulatory mol­
ecules (including CRH, vasopressin

regulatory mol­
ecules (including CRH, vasopressin, and dopamine) that traverse 
the portal system and directly regulate corticotrophic function. 
Second, intrapituitary cytokines and growth factors act locally to 
regulate ACTH, either in concert with hypothalamic factors or 
independently. These paracrine controls often overlap and induce 
sensitive intracellular molecules that limit the ACTH response, 
preventing chronic ACTH hypersecretion. Third, glucocorticoids 
maintain regulatory feedbac

hird, glucocorticoids 
maintain regulatory feedback control of corticotroph secretion by 
rapidly inhibiting hypothalamic CRH and pituitary ACTH secre­
tion. In a short feedback loop, pituitary ACTH inhibits hypotha­
lamic CRH, and in an ultrashort loop, it may also suppress the 
corticotroph itself. 
Stress Response
The HPA axes respond to a range of stressors, including pain, 
infection, inflammation, hemorrhage, hypovolemia, trauma, psy­
chologic stress, hypoglycemia, and critical illness. Th

gic stress, hypoglycemia, and critical illness. The adaptation 
to stress involves vasovagal and catecholamine activation, cytokine 
secretion, and action. A tightly controlled immunoneuroendo­
crine interface regulates the ACTH response to peripheral stress­
ors, which include pain, infection, inflammation, hemorrhage, 
hypovolemia, trauma, psychologic stress, and hypoglycemia. 
These signals vary in their ability to generate ACTH secretion and 
to sensitize the glucocorticoid response to ACTH.

to sensitize the glucocorticoid response to ACTH. In addition to 
CRH, proinflammatory cytokines potently induce POMC tran­
scription and ACTH secretion.363 Sensitive intracellular signals 
within the corticotroph also serve to override the ACTH response 
to stress, thus preventing persistent and chronic hypercortisolemia.
Cytokines such as IL6 and leukemia inhibitory factor (LIF) 
activate the HPA axis and enhance glucocorticoid production, 
protecting the organism against lethality by constra

otecting the organism against lethality by constraining the 
inflammatory response.370 Thus mice with inactivated CRH 
or LIF genes mount an inadequate neuroendocrine response to 
stress, inflammation, or endotoxins. During stress, glucocorticoid 
inhibition of ACTH is also prevented by nuclear factor κB activa­
tion, which interferes with pituitary glucocorticoid receptor func­
tion, thus further exaggerating enhanced ACTH secretion.385
Hypoglycemia evokes acute release of ACTH secretion. Dur­

emia evokes acute release of ACTH secretion. Dur­
ing the ITT, ACTH levels increase fivefold to sixfold, peaking by 
about 45 minutes, which is followed by peak cortisol at 60 to 
90 minutes. Acute nutrient deprivation activates the secretion of 
ACTH. Exercise is a physiologic stimulus of ACTH release. Exer­
cising to 90% of maximum oxygen capacity causes a significant 
elevation of ACTH, similar to levels observed during surgery or 
hypoglycemia.386,387 
Circadian Periodicity
ACTH is secreted 

a.386,387 
ACTH is secreted with both circadian periodicity and ultradian 
pulsatility under the control of the suprachiasmatic nucleus. The 
secretion of cortisol is tightly linked to and follows that of cor­
tisol by 5 to 10 minutes (Fig. 8.26).388 β-endorphin, coreleased 
with ACTH, also displays pulsatile, circadian, and ultradian 
rhythmicity with a close temporal coupling to cortisol.389 The 
circadian pattern of ACTH secretion typically begins at about 4 
  Tissue D

secretion typically begins at about 4 
  Tissue Distribution of Melanocortin 
Receptor (MCR) Types
MC Receptor Types
Tissue
MCR1
Skin, hair follicles
MCR2
Adrenal cortex
MCR3
Hypothalamus, limbic system
MCR4
Hypothalamus, various brain areas,a 
intestines
MCR5
Exocrine glands, skeletal muscle, 
adrenal cortex
aCortex, thalamus, brainstem, and spinal cord.
   
TABLE 8.9
213
am, peaking before 7 am, with both ACTH and adrenal steroid 
lev

fore 7 am, with both ACTH and adrenal steroid 
levels reaching their nadir between 11 pm and 3 am. Within this 
overall 24-hour diurnal cycle, periodic ACTH secretory bursts 
occur at a frequency of 40 pulses per 24 hours; amplitude rather 
than frequency modulation contributes to diurnal changes in 
ACTH profile.390 ACTH circadian rhythm is entrained by 
visual cues, and the light-dark cycle is centrally controlled by 
CRH and other factors391 and is lost in Cushing disease (see 
Fig. 8.26).388

nd is lost in Cushing disease (see 
The HPA axis is a critical component of a circadian system 
regulating substrate metabolism, immunity, cognition, and stress 
adaptation through the actions of glucocorticoids. This 24-hour 
rhythm partly reflects the activity of a master circadian pacemaker 
located in the suprachiasmatic nucleus, that is regulated by an 
oscillatory system governed by clock genes cued by light-dark and 
feeding cycles. The same molecular circuitry of core cloc

 cycles. The same molecular circuitry of core clock ele­
ments is present in multiple, if not all, peripheral tissues, including 
the heart, kidney, muscle, liver, pancreas, blood, fat, and adrenal 
gland.392 The central clock also entrains peripheral clocks via neu­
ral and hormonal systems, the latter through the HPA axis. The 
physiologic effects of glucocorticoids are optimized when the cen­
tral signal that controls the rhythm of glucocorticoid release and 
the peripheral rhythms in tissues

oid release and 
the peripheral rhythms in tissues expressing glucocorticoid recep­
tors are aligned. There is strong evidence that a regular 24-hour 
temporal organization underpins good health. The early-morning 
peak subserves the metabolic and CNS preparation of physical 
activity and stressors at the start with declining levels favoring the 
optimization of insulin sensitivity for the rest of the day. The dis­
ruption of the circadian system brought by the lifestyles and social 
demands of 

 brought by the lifestyles and social 
demands of modern civilization such as shift work and jet lag 
brings adverse health outcomes.392 The 24-hour profile of cortisol 
concentrations does not adapt rapidly to acute shifts in light-dark, 
activity-rest, and/or feeding cycles. Under these circumstances the 
asynchrony between meal times and ultradian rhythm of cortisol 
secretion impairs substrate metabolism and worsens glucose toler­
ance (Fig. 8.27).393 There is mounting evidence that the stan

8.27).393 There is mounting evidence that the standard 
regimen of glucocorticoid replacement therapy does not restore 
physical, metabolic, and psychologic well-being among patients 
with adrenal insufficiency, from the failure to replicate the 24-hour 
rhythm of cortisol concentration.394 
Measurement of ACTH
Measurements of plasma ACTH are extremely useful in the 
diagnosis of both Cushing syndrome and adrenal insufficiency. 
ACTH assays have evolved considerably since the first radio­
immuno

evolved considerably since the first radio­
immunoassay. Current commercially available two-site immu­
nometric assays display high specificity and analytic sensitivity 
of less than 0.5 ng/L.395 However, significant variability in pre­
cision and performance exists between commercial assays.396 
ACTH and other POMC-derived peptides such as αMSH, 
βLPH, or β-endorphin can also be measured with precision with 
current assays. Awareness of the assay peptide specificity may 
be especially critical 

y peptide specificity may 
be especially critical when evaluating ectopic POMC products 
secreted by lung tumors. Ideally, nonstressed resting subjects 
should have venous blood withdrawn between 6 am and 9 am. 
As ACTH is relatively unstable at room temperature and has a 
propensity to adhere to glass, plasma samples should immedi­
ately be separated in iced siliconized glass tubes containing eth­
ylenediaminetetra-acetic acid (EDTA) and stored below –20°C 
for transport. Morning (8 am) plasma 

below –20°C 
for transport. Morning (8 am) plasma ACTH levels range from 
8 to 25 ng/L. Episodic secretion and short plasma half-life result 
in wide and rapid fluctuation of plasma measurements. Cortisol 
values at 4 pm are about half those of morning levels, and 11 
pm levels are usually less than 5 μg/dL. Plasma ACTH levels 
fluctuate broadly within the same individual and are highly sen­
sitive to stress, time of collection, and gender. Pregnant females 
have higher ambient ACTH levels, poss

ant females 
have higher ambient ACTH levels, possibly because of placental 
CRH secretion.397 
ACTH Deficiency
Causes
Congenital ACTH deficiency may occur as an isolated pituitary 
defect or as a component of a wider spectrum of multiple pitu­
itary hormone deficiencies. A mutation of TBX19, encoding Tpit, 
a transcription factor involved in corticotroph differentiation, is 
a cause of isolated ACTH deficiency (see Tables 8.1 and 8.2).38 
Mutations of transcription factors involved in early sta

ons of transcription factors involved in early stages of 
pituitary cell differentiation or midline brain development may 
also give rise to ACTH deficiency as a component of multiple 
hormone deficiencies. These genes with mutations include LHX4 
and HESX1 (see Table 8.2). Secondary causes include pituitary 
tumors, sellar mass lesions, trauma, irradiation, and lymphocytic 
0
9
12
15
18
Cortisol nmol/L
21
24
3
6
9
0
600
400
200
800
1000
20
40
60
80
100
120
140
160
0
9
12
15
18
Time (clock hours

80
100
120
140
160
0
9
12
15
18
Time (clock hours)
ACTH ng/L
21
24
3
6
9
0
100
200
300
400
500
20
80
60
40
100
A
B
• Fig. 8.26  Plasma concentrations of ACTH (dotted line) and cortisol (con­
tinuous line) in a patient with Cushing disease (A) and a normal subject (B), 
showing the concordance between episodic ACTH and cortisol concen­
trations in blood. The ultradian pattern of ACTH and cortisol present in the 
normal subject is lost in the patient with Cushing disease. (Redrawn from 
Roelfsema

ent with Cushing disease. (Redrawn from 
Roelfsema F, Pincus SM, Veldhuis JD. Patients with Cushing’s disease 
secrete adrenocorticotropin and cortisol jointly more asynchronously than 
healthy subjects. J Clin Endocrinol Metab. 1998;83:688–692.)
214
hypophysitis, which may be associated with other autoimmune 
manifestations (see Table 8.5). 
The manifestations of ACTH deficiency are clinically indis­
tinguishable from glucocorticoid def

ically indis­
tinguishable from glucocorticoid deficiency of any cause. The 
clinical features are dependent on severity, the time of onset, 
and clinical context. In the newborn, ACTH deficiency may 
present as hypoglycemia and failure to thrive. In the adult, 
there is slowly progressive weight and appetite loss, anorexia, 
and generalized fatigue, mimicking a wasting syndrome. As 
adrenal mineralocorticoid secretion is largely unimpaired, salt 
wasting, volume contraction, and hyperkalemia, c

wasting, volume contraction, and hyperkalemia, commonly 
encountered features in Addison disease, are not manifest. 
Furthermore, hyperpigmentation, usually associated with exu­
berant ACTH-related peptide secretion in the face of adrenal 
damage, does not occur. 
Diagnostic evaluation of adrenal insufficiency requires concurrent 
measurement of cortisol and ACTH levels. Morning serum corti­
sol levels lower than 3 μg/dL suggest ACTH deficiency, but basal 
morning cortisol levels hi

 deficiency, but basal 
morning cortisol levels higher than 18 μg/dL usually indicate 
normal ACTH reserve. Provocative tests are required to diagnose 
ACTH deficiency. As cortisol is highly bound to cortisol-binding 
globulin (CBG), the level of CBG may confound interpretation 
of cortisol values. Cirrhosis and hyperthyroidism lower CBG con­
centrations, whereas estrogens elevate them. 
Dynamic Testing for ACTH Deficiency
Hypothalamic Testing. Insulin hypoglycemia is a potent 
endogenous stress

nsulin hypoglycemia is a potent 
endogenous stressor that evokes ACTH secretion as well as GH 
release. Insulin (0.1–0.15 U/kg) is injected intravenously after an 
overnight fast to achieve symptomatic hypoglycemia and a blood 
glucose level of less than 40 mg/dL. This test must be performed 
under supervision. Normal HPA response to this stressor evokes 
cortisol levels higher than 20 μg/dL. As hypoglycemia acts cen­
trally, a normal response implies integrity of all three tiers of HPA 
axis co

plies integrity of all three tiers of HPA 
axis control.398 Venous samples are collected at –15, 0, 15, 30, 45, 
60, 90, and 120 minutes for measurement of glucose, ACTH, and 
cortisol levels. After the test, oral glucose should be administered 
to ensure euglycemia. Both intraindividual variations in blood glu­
cose levels attained by a given dose of insulin and fluctuations in 
central sensitivity to glucose and activation of catecholamines may 
lead to difficulties in reproducibility. Contrai

lead to difficulties in reproducibility. Contraindications are similar 
to those for the diagnosis of GH deficiency described earlier. If 
pronounced adrenal insufficiency is likely, insulin injection may 
provoke an adrenal crisis as a result of inadequate adrenal reserve, 
and hydrocortisone (100 mg) should be available for urgent IV 
use, if required, and oral glucocorticoid replacement therapy 
should be initiated pending confirmation of the diagnosis.
The metyrapone test is an alternative

e diagnosis.
The metyrapone test is an alternative to the ITT for hypo­
thalamic evaluation of the HPA axis. In blocking cortisol syn­
thesis by inhibiting adrenal 11β-hydroxylase, it releases the 
HPA axis from negative feedback by cortisol, normally result­
ing in an ACTH surge and elevated levels of 11-deoxycortisol. 
A single oral dose (2–3 g) is given at midnight and serum levels 
of ACTH, 11-deoxycortisol, and cortisol are measured at 8 am 
the following day. The test is only valid when co

the following day. The test is only valid when cortisol levels fall 
to less than 10 μg/dL. In normal subjects, peak ACTH values 
higher than 200 ng/L are achieved. Side effects include nausea, 
gastrointestinal upset, and insomnia.399 False-positive results 
n.s.
P<0.0001
p < 0.006 
Aligned
Misaligned
-100
-20
-100
0
100
200
300
20
0
40
60
0
0
0
4
8
12
16
20
24
28
4
8
12
16
24
28
20
B
L
D
S
B
L
D
S
B
L
Time since wake (hours)
D
S
8
4
12
16
20
24
28
0
80
0
20
40
60
100
120
140
160
50
100
150
20

4
28
0
80
0
20
40
60
100
120
140
160
50
100
150
200
250
Cortisol (% from mean)
Glucose (% from mean)
Insulin (% from mean)
Glucose (mg/dL)
Cortisol (µg/dL)
Insulin (µlU/mL)
-50
0
50
100
150
• Fig. 8.27  Consequences of circadian misalignment on metabolic, auto­
nomic, and endocrine function. Data are plotted according to “time since 
wake,” during normal circadian alignment (open green symbols; sched­
uled awakening at habitual wake time) and during circadian misalignment 
(filled red symbols; s

ing circadian misalignment 
(filled red symbols; scheduled awakening 12 hours out of phase from 
habitual wake time). P-values, statistical significance for effect of misalign­
ment (based on 24-hour cycle for variable mainly driven by circadian cycle 
[cortisol] and 28-hour cycle for variables mainly driven by behavior cycle 
[others]). Gray area, scheduled sleep episode; short vertical gray bars, 
meal times: B, breakfast; L, lunch; D, dinner; S, snack. (Modified from 
Scheer FA, Hilton MF, Ma

S, snack. (Modified from 
Scheer FA, Hilton MF, Mantzoros CS, et al. Adverse metabolic and car­
diovascular consequences of circadian misalignment. Proc Natl Acad Sci 
USA. 2009;106:4453–4458.)
215
may be obtained with phenytoin, which prevents adequate 
enzymatic blockade. This test should be performed under 
observation in a hospital because acute adrenal insufficiency 
may ensue. 
Pituitary Stimulation. Pituitary ACTH secretion is evo

itary Stimulation. Pituitary ACTH secretion is evoked by 
injecting either CRH or AVP. Ovine or human CRH (100 μg or 1 
μg/kg) is administered intravenously, and cortisol and ACTH are 
measured at –5, −1, 0, 15, 30, 60, 90, and 120 minutes. Normally, 
maximal ACTH responses (twofold to fourfold above baseline) 
are evoked at 30 minutes,400 and cortisol levels peak (>20 μg/dL) 
at 60 minutes or increase more than 10 μg/dL above baseline. 
Although CRH readily induces ACTH secretion and may demon­

 CRH readily induces ACTH secretion and may demon­
strate corticotrophic ACTH deficiency or ACTH excess, the wide 
variation of responses observed has limited the utility of this test. 
A useful application of the CRH test is in making the diagnosis of 
Cushing disease (see Chapter 15). 
Adrenal Stimulation. The acute response of the adrenal gland 
to a bolus ACTH injection reflects ambient ACTH concentra­
tions to which the gland has been exposed. Thus the cortisol 
response to an acute ACTH in

d. Thus the cortisol 
response to an acute ACTH injection will be blunted if the 
subject has experienced chronic pituitary ACTH hyposecre­
tion, with resultant adrenal atrophy and diminished cortisol 
reserve.401 This time-honored test involves intramuscular or 
intravenous injection of 250 μg ACTH(1-24) (Cortrosyn or 
Synacthen) and cortisol levels measured before injection and 
then 30 minutes and 60 minutes after injection. Cortisol values 
greater than 20 μg/dL reflect a normal adrenal rese

reater than 20 μg/dL reflect a normal adrenal reserve response. 
The utility of this test in diagnosing diminished pituitary ACTH 
reserve has been challenged as a 250-μg dose may evoke a “nor­
mal” cortisol response in hypopituitary subjects. An unaccept­
ably high false-negative rate (∼65%) has been determined in a 
large series,402 although peak cortisol levels at 30 minutes cor­
relate well with peak responses to ITT.403,404 Low-dose stimula­
tion with 1 μg Synacthen evokes maximal serum cor

tion with 1 μg Synacthen evokes maximal serum cortisol levels 
at 30 minutes, and these correlate well with values observed after 
insulin or high-dose ACTH administration.403 A cutoff value of 
more than 500 nmol/L provides almost 100% sensitivity and a 
specificity of 80% to 100%.405 Failure to respond to low-dose 
ACTH should be corroborated by a standard dose of insulin or 
ACTH stimulation. 
Adrenal Steroid Replacement
Hydrocortisone is widely used for glucocorticoid replacement. 
The norm

ely used for glucocorticoid replacement. 
The normal secretory rate of cortisol is 15 to 20 mg/day, which 
is the recommended total daily dose. As plasma circulating 
half-life of cortisol is less than 2 hours, three times daily dos­
ing of a total daily requirement of 10 to 20 mg (5–10 mg in 
the morning, 2.5–5 mg at noon, and 2.5–5 mg in the evening) 
is recommended.406 Other synthetic glucocorticoids, including 
prednisolone (2.5–5 mg/day) and dexamethasone (0.25–0.5 
mg/day), are suitable al

 dexamethasone (0.25–0.5 
mg/day), are suitable alternatives. Having longer half-lives, they 
can be administered once daily, but they are difficult to moni­
tor biochemically. There is no consensus for monitoring treat­
ment. Central diabetes insipidus may be unmasked after initial 
glucocorticoid replacement. Mineralocorticoid replacement is 
not required for treating secondary hypoadrenalism. Adrenal 
androgen replacement with dehydroepiandrosterone (DHEA) at 
doses of 25 mg/day may improve t

terone (DHEA) at 
doses of 25 mg/day may improve the sense of well-being, relieve 
fatigue, and improve sexual function in patients with primary 
and secondary adrenal insufficiency.407,408
However, conventional regimens do not recapitulate the 
physiologic pattern of cortisol release, and this may explain 
the high prevalence of poor quality of life and osteoporosis 
in these patients.409 Two modified release formulations aimed 
at mimicking diurnal profiles have been evaluated in clinical 
tri

rnal profiles have been evaluated in clinical 
trials.410,411 One of these, a dual-release hydrocortisone tab­
let (DuoCort) containing an immediate-release coating and an 
extended-release core, was approved by the European Medi­
cines Agency for once-daily therapy of adrenal insufficiency in 
2012. In a 12-week randomized crossover study of 64 patients 
with adrenal insufficiency, DuoCort exhibited improved 
weight, glucose, and blood pressure control compared to con­
ventional thrice-daily hy

control compared to con­
ventional thrice-daily hydrocortisone dosing at the same total 
daily dose.410 In a 26-week randomized controlled trial of 
89 patients with adrenal insufficiency, significant benefits in 
weight, metabolic, immune, and quality-of-life measures were 
observed in the group randomized to once-daily, dual-release 
hydrocortisone (DuoCort) treatment compared to the group 
who continued with the conventional regimen of same daily 
dose administered thrice daily (Fig. 8.28).39

ily 
dose administered thrice daily (Fig. 8.28).394 Thus modified-
release preparations mimicking cortisol diurnal profiles are 
a significant advance in the management of glucocorticoid 
insufficiency. 
ACTH Excess
Causes
Ectopic ACTH production occurs in tumors capable of generating 
high amounts of the pituitary-like 1072-nucleotide mRNA. How­
ever, the clinical manifestation depends on whether PC enzymes 
are appropriately expressed. Small cell lung cancers preferentially 
release intact POM

ll lung cancers preferentially 
release intact POMC, but carcinoid tumors tend to process the 
precursor, releasing ACTH and smaller peptides.412 Extrapituitary 
neuroendocrine tumors associated with ectopic ACTH secretion 
do not process the prohormone efficiently because of a general 
defect in PC expression. As ACTH is synthesized in nontumorous 
neuroendocrine cells, ectopic tumor hormone production may in 
fact reflect inappropriate ACTH processing. These patients also 
exhibit a higher rat

cessing. These patients also 
exhibit a higher ratio of circulating ACTH precursors, as well 
as smaller peptides, including CLIP. Where sufficient ACTH 
or ACTH precursors that are biologically active are secreted, 
these tumors can cause florid morbidity from excessive cortisol 
-4
-2
0
Mean change
from baseline
Standard
Weight (kg) Waist (cm)
HbA1c (%) QOL score
Modified release
2
4
6
8
• Fig. 8.28  Changes in body weight, waist circumference, HbA1c, and 
quality of life (QOL) in

ircumference, HbA1c, and 
quality of life (QOL) in patients with hypoadrenalism randomized to stan­
dard treatment or modified-release hydrocortisone formulation for 26 
weeks. Differences between treatments are all statistically significant 
(p < 0.05). (Data from Isidori AM, Venneri MA, Graziadio C, et al. Effect of 
once-daily, modified-release hydrocortisone versus standard glucocorti­
coid therapy on metabolism and innate immunity in patients with adrenal 
insufficiency [DREAM]: a single-bl

s with adrenal 
insufficiency [DREAM]: a single-blind, randomised controlled trial. Lancet 
Diabetes Endocrinol. 2018;6:173–185.)
216
ACTH-induced adrenal hyperfunction causes a syndrome of 
hypercortisolism and of androgen excess in women. Manifesta­
tions arise from appetite stimulation (weight gain), altered fat dis­
tribution (moon facies, buffalo hump, central obesity), catabolism 
(skin thinning, muscle wasting), mood disturbance (

skin thinning, muscle wasting), mood disturbance (depression, 
anxiety), sodium retention (hypertension), and androgen excess 
(menstrual irregularity, hirsutism, acne, oily skin). The evalua­
tion and management of patients with Cushing disease is fully 
described in Chapter 15. 
Gonadotropins
The gonadotropins, LH and FSH, are produced by pituitary 
gonadotrophs. These hormones play an essential role in the repro­
ductive process. LH and FSH act on the ovaries and testes to direct 

 and FSH act on the ovaries and testes to direct 
gametogenesis and sex steroid hormone synthesis. Reproduction 
is a tightly regulated function, influenced by genetic, nutritional, 
environmental, and socioeconomic factors. Befitting their impor­
tant roles, the synthesis and secretion of LH and FSH are under 
complex regulation by hypothalamic input (e.g., GnRH), by posi­
tive and negative feedback from gonadal sex steroid and peptide 
hormones, and by paracrine modulation from local factors p

, and by paracrine modulation from local factors pro­
duced within the pituitary gland itself (e.g., activins, inhibins, 
follistatin).34
Gonadotroph Cells
Gonadotroph cells comprise about 10% to 15% of the func­
tional anterior pituitary cells. They are a heterogeneous cell 
population, with large round cell bodies with prominent rough 
endoplasmic reticulum and Golgi apparatus. Immunocytochem­
ical studies have demonstrated the presence of both bihormonal 
and monohormonal groups of gonadotrop

 bihormonal 
and monohormonal groups of gonadotrophs. Cells with LH 
secretory granules often accumulate peripherally, and their Golgi 
structures may be less prominent. Gonadotrophs are character­
ized also by GnRH receptor expression and by SF1 and DAX1 
nuclear receptors, which contribute to gonadotroph-specific 
gene expression. 
Gonadotropin Structure
FSH and LH function to regulate gonadal steroid hormone 
biosynthesis and initiate and maintain germ cell development 
in concert with periph

tain germ cell development 
in concert with peripheral hormones and paracrine soluble fac­
tors. The four heterodimeric glycoprotein hormones (LH, FSH, 
TSH, and human chorionic gonadotropin [hCG]) share struc­
tural homology, having evolved from a common ancestral gene. 
Although both the homologous LH and FSH molecules are cose­
creted by gonadotrophs, their regulatory mechanisms are not 
uniformly concordant. The αGSU, LHβ, and FSHβ subunits are 
encoded by different genes, located on chromos

re 
encoded by different genes, located on chromosomes 6, 11, and 
19, respectively (Figs. 8.29 and 8.30; see also Fig. 8.2).413 The 
heterodimeric structure of the common—and unique—subunits 
of LH and FSH is essential for biologic activity. Disulfide link­
ages within each subunit result in a tertiary structure that enables 
and maintains noncovalent heterodimerization, which also 
determines the ultrastructure of the mature folded molecule to 
facilitate specific ligand-receptor interaction.4

facilitate specific ligand-receptor interaction.414 Glycosylation of 
the subunits occurs by the transfer of oligosaccharide complexes 
to asparagine residues.415 Post-translational processing of carbo­
hydrate side chains is critical for hormone signaling, distinct for 
LH and FSH, and may even vary physiologically to influence 
biologic activity and metabolic clearance rates.415,416 The human 
LHβ/CGβ gene cluster comprises seven genes, arising from gene 
duplication, of which one gene encode

g from gene 
duplication, of which one gene encodes LHβ, one encodes CGβ, 
and the remainder are pseudogenes. Unlike LH, hCG is present 
only in primate and equine species and is expressed primarily 
in the placenta. The LHβ and CGβ genes have different pro­
moters and transcriptional start sites, accounting for their differ­
ent tissue distribution patterns of expression.417 LHβ includes 
a 24–amino acid signal peptide followed by a 121–amino acid 
mature protein. In contrast, the mature hCGβ p

id 
mature protein. In contrast, the mature hCGβ protein is 145 
amino acids in length and does not include a leader peptide, but 
it contains a 24–amino acid C-terminal extension important for 
the longer biologic half-life of hCG.413,418 The FSHβ gene, on 
chromosome 11, is organized similarly to the other glycopro­
tein hormone β genes, encoding a mature peptide of 111 amino 
acids, with two glycosylation sites, and like LHβ, it is expressed 
only in gonadotrophs. 
1 kb
10 aa
-24
A
B
C
-20
1

1 kb
10 aa
-24
A
B
C
-20
1
-18
1
111
1
2
3
121
1
52
92
Exon
Exon
Exon
123
1
2
3 4
CGA
gene
LHB
gene
FSHB
gene
LHβ protein
FSHβ
protein
Common
α-subunit
protein
30
7
24
78
• Fig. 8.29  Schematic representation of the human gonadotropin sub­
unit genes. (A) Common α-subunit. (B) LHβ. (C) FSHβ. The top part of 
each scheme depicts the gene structure. Open bars indicate noncod­
ing sequences; solid bars indicate coding sequences. The bottom bar of 
each scheme shows the protei

s. The bottom bar of 
each scheme shows the protein structure. Signal peptides are shaded; 
the mature peptide is depicted by an open bar. The positions of N-linked 
glycosylation sites are depicted by triangles. Amino acid positions are 
depicted by numbers. (Redrawn and modified from Themmen APN, 
Huhtaniemi IT. Mutations of gonadotropins and gonadotropin receptors: 
elucidating the physiology and pathophysiology of pituitary-gonadal func­
tion. Endocr Rev. 2000;21:551–583.)
217
Chapter 8  Pit

. Endocr Rev. 2000;21:551–583.)
217
LH and FSH secretion patterns reflect the integration of sensitive 
complex hypothalamic signals (mediated primarily via GnRH), 
paracrine intrapituitary factors (primarily activin and follistatin), 
and peripheral feedback (both gonadal sex steroids and gonadal 
peptide hormones) (Fig. 8.31).
Hypothalamic control of gonadotropin secretion occurs primarily 
thr

ol of gonadotropin secretion occurs primarily 
through actions of GnRH. Hypothalamic GnRH neurons repre­
sent the pivotal integrators of peripheral signals in regulation of 
the pituitary-gonadal axis. Insights into the mechanisms that regu­
late GnRH secretion have been provided by the identification and 
study of genetic abnormalities in patients with pubertal disorders or 
infertility and by the use of animal models. Molecular defects that 
manifest as GnRH deficiency, or hypogonadotropic hyp

nifest as GnRH deficiency, or hypogonadotropic hypogonadism 
(HH), can be classified as defects in GnRH neuronal development, 
defects in control of GnRH secretion, and defects in GnRH action 
(Fig. 8.32; see Table 8.10). Conversely, genetic defects in MKRN3 
and DLK1 were found to cause premature reactivation of GnRH 
secretion and central precocious puberty. MKRN3 encodes a puta­
tive ubiquitin ligase and appears to act as a repressor or inhibitor of 
GnRH secretion during childhood.419 DLK1, 

tor of 
GnRH secretion during childhood.419 DLK1, also known as pre­
adipocyte factor 1, encodes a transmembrane noncanonical ligand 
in the Delta-Notch signaling pathway.420
Many neurotransmitters directly or indirectly modulate 
GnRH secretion, including norepinephrine, dopamine, sero­
tonin, γ-aminobutyric acid (GABA), glutamate, opiates, NPY, 
and galanin, among others. Glutamate and norepinephrine pro­
vide stimulatory drive, whereas GABA and opioid peptides are 
inhibitory. Kisspeptins, en

d opioid peptides are 
inhibitory. Kisspeptins, encoded by the KISS1 gene, and their 
cognate receptor, KISS1R, were identified as key GnRH secre­
tagogues.421,422 Administration of kisspeptin to normal men 
increases FSH, LH, and testosterone concentrations, and in nor­
mal women the peptide stimulates gonadotropin release most 
potently during the preovulatory surge.423 Neurokinin B (NKB), 
a member of the substance P–related tachykinin family, is coex­
pressed with kisspeptin in the hypothala

 is coex­
pressed with kisspeptin in the hypothalamus and appears to 
act through control of kisspeptin secretion to modulate GnRH 
release.424,425 Substance P, another tachykinin family member, has 
also been shown to modulate GnRH secretion. Leptin, a product 
of peripheral adipose tissue, is a positive regulator of the hypo­
thalamic-pituitary-gonadal (HPG) axis. This adipokine enables a 
pivotal link between body fat and reproduction, signaling energy 
availability centrally.426 Nutritional,

ng energy 
availability centrally.426 Nutritional, metabolic, stress, and circa­
dian inputs all appear to act through these peptides to modulate 
GnRH, gonadotropin secretion, and activity of the HPG axis.419
The hallmark of hypothalamic GnRH secretion is the pulsatile 
rather than continuous release into the hypophyseal portal circu­
lation, resulting in episodic stimulation of the gonadotroph.427 
In patients with GnRH deficiency, restoration of gonadotropin 
secretion can be achieved after e

of gonadotropin 
secretion can be achieved after exogenous pulsatile GnRH treat­
ment, whereas continuous GnRH exposure suppresses gonado­
tropin secretion.428 GnRH signaling initiates with recognition by 
its cognate receptor, GnRHR, which belongs to the rhodopsin G 
protein–coupled receptor family. GnRHR activation increases cal­
cium mobilization and stimulates influx of extracellular calcium to 
induce pituitary LH and FSH secretion.429 The pattern of GnRH 
signaling is important in determin

attern of GnRH 
signaling is important in determining the quantity and quality of 
gonadotropins secreted.430,431 The amplitude, frequency, and con­
tour of GnRH pulses can all vary, and each of these characteristics 
can influence gonadotroph responses, providing a mechanism for 
the differential synthesis and secretion of the two gonadotropins, 
• Fig. 8.30  The subunit structure and glycosylation sites of the four glyco­
protein hormone heterodimers (α-subunit, blue; β-subunit, green). (From 

dimers (α-subunit, blue; β-subunit, green). (From 
the University of Glasgow protein crystallography website. Available at 
http://www.chem.gla.ac.uk/protein/glyco/GPH.html.)
Stimulatory
Inhibitory
Central nervous system
Activin/
Follistatin
Inhibin
Gonad
LH/FSH
GnRH
E2/P/T
• Fig. 8.31  The hypothalamic-pituitary-gonadal axis. See text for discus­
sion. E2/P/T, estrogen/progesterone/testosterone; FSH, follicle-stimulating 
hormone; GnRH, gonadotropin-releasing hor

ulating 
hormone; GnRH, gonadotropin-releasing hormone; LH, luteinizing hor­
mone. (From Kaiser UB. Gonadotrophin hormones. In: Melmed S, ed. The 
Pituitary. 3rd ed. San Diego, CA: Elsevier; 2011:205–260.)
218
LH and FSH. These alterations in GnRH pulse pattern are one 
mechanism by which two functionally distinct gonadotropins 
can be differentially regulated by a single hypothalamic-releasing 
hormone. 
Inhibins and Activins
The hypothesis that a peptid

The hypothesis that a peptide of gonadal origin selectively regu­
lates FSH secretion dates back to at least 1932.432 It took over 
50 years to isolate and characterize the structure of inhibin and 
its related peptides.433 Inhibin-related peptides, members of the 
transforming growth factor-β family, are dimeric proteins cova­
lently linked by a disulfide bridge and consisting of a common 
α-subunit and one of two highly homologous β-subunits, βA or 
βB (Fig. 8.33). In add

mologous β-subunits, βA or 
βB (Fig. 8.33). In addition, the β-subunits can form dimers, called 
activins, to stimulate FSH synthesis and secretion.434 A structur­
ally unrelated, monomeric polypeptide, follistatin, was also identi­
fied, based on its ability to inhibit FSH.434,435 These three peptides 
(inhibins, activins, and follistatin) are considered to be relatively 
selective for FSH in terms of their effects on gonadotropins and 
serve as an additional mechanism for the differential cont

 an additional mechanism for the differential control of 
FSH and LH. Although inhibins act primarily as classic circulat­
ing endocrine hormones, originating in the gonads and acting on 
the pituitary to regulate FSH, activins play an important role as 
regulators of growth and differentiation in diverse tissues and are 
produced and act locally in the pituitary as autocrine/paracrine 
factors. In the human male, inhibin B is produced in the testes in 
response to FSH stimulation and circulates

tes in 
response to FSH stimulation and circulates systemically to provide 
feedback inhibition of FSH. In women, inhibin A is secreted by 
dominant ovarian follicles and corpora lutea, contributing to the 
high circulating levels during the late follicular and luteal phases. 
Inhibin B is reciprocally elevated during the late luteal and early 
follicular phases of the menstrual cycle.
Activin receptors are heteromeric complexes comprising type 
I (ActRI) and type II (ActRII) serine-threonine ki

I (ActRI) and type II (ActRII) serine-threonine kinase receptors. 
Activin binds to the type II receptors, thereby increasing associa­
tion with the type I receptor and stimulating its phosphorylation, 
which in turn results in the activation by phosphorylation of 
intracellular signaling Smad proteins, resulting in translocation of 
the Smad complex to the nucleus, where it binds to gene regula­
tory elements and interacts with other transcription factors (such 
as FoxL2) to regulate gene trans

on factors (such 
as FoxL2) to regulate gene transcription, thereby influencing cell 
fate and function.34 Follistatin and inhibin act as extracellular 
modulators of activin through distinct mechanisms.436 Follistatin, 
an activin-binding protein, inhibits activin action by interfering 
with activin binding to its receptor. Inhibins compete for bind­
ing to type II activin receptors, preventing recruitment of type I 
receptors and thereby blocking activin signaling. Additional extra­
cellular a

ng activin signaling. Additional extra­
cellular and intracellular proteins and mechanisms also serve to 
modulate the local activin signal. 
Sex Steroids
Gonadal steroid hormones include estrogens, progesterones, and 
androgens. Effects on gonadotropins occur both directly at the 
level of the gonadotroph and indirectly via effects at the hypo­
thalamus that modulate GnRH secretion. Estrogen, androgen, 
and progesterone receptors have been identified in gonadotrophs, 
consistent with direct act

ified in gonadotrophs, 
consistent with direct actions of these peripheral sex steroid hor­
mones. Within the hypothalamus, these receptors have been iden­
tified in multiple neuronal cell types, suggesting that alterations 
in GnRH release largely occur indirectly through modulation of 
neuronal systems that impinge on GnRH neurons, particularly 
through kisspeptin neurons.437,438
In women, estrogens can exert dual feedback effects on gonado­
tropin secretion, depending on the reproductive stat

opin secretion, depending on the reproductive state. The negative 
GnRH Secretion:
Nasal Placode
Pituitary
Gonads:
Steroid Hormones
GnRH Action:
Development/Migration
of GnRH Neurons: 
GnRH neurons
LEP/LEPR
TAC3/TACR3
KISS1/KISS1R
GNRH1
PCSK1 
GNRHR
LH , FSH 
NR5A1, NR0B1 
SRA1
KAL1
FGF8/FGFR1
FGF17/IL17RD/DUSP6/SPRY4/FLRT3
KLB
PROK2/PROKR2
SEMA3A/SEMA3E
NELF
CHD7
WDR11
HS6ST1
SOX10
CCDC141
FEZF1
GnRH
Hy
Gn
LEP/LE
TAC3/
KISS1/
TAC3/
GNRH
PCSK
PRY4/FLRT3
• Fig. 8.32  Genet

SS1/
TAC3/
GNRH
PCSK
• Fig. 8.32  Genetic and molecular basis of gonadotropin-releasing hormone (GnRH) neuronal develop­
ment and migration, GnRH secretion, and GnRH action. (From Bianco SD, Kaiser UB. The genetic and 
molecular basis of idiopathic hypogonadotrophic hypogonadism. Nat Rev Endocrinol. 2009;5:569–576.)
219
feedback effects of estrogens are clearly demonstrated by the ele­
vated LH and FSH levels that follow ovari

the ele­
vated LH and FSH levels that follow ovariectomy or menopause 
but reverse with estrogen replacement. Negative feedback effects of 
estrogens are observed at the level of α-subunit and at LHβ and 
FSHβ mRNA levels through effects on gene transcription in addi­
tion to effects on LH and FSH secretion, mediated in part directly 
at the level of the pituitary gland. Estrogen also has negative feed­
back effects at the levels of the hypothalamus, mediated in large 
part through effects on ki

mus, mediated in large 
part through effects on kisspeptin neurons.439 On the other hand, 
during the late follicular phase of the menstrual cycle, the feedback 
effects of estrogens shift from negative to positive, triggering the 
midcycle ovulatory surge of LH and FSH secretion. Positive feed­
back effects of estrogen are mediated in large part through effects on 
sexually dimorphic kisspeptin neurons in the anteroventral periven­
tricular region of the hypothalamus in female mice.440,441 Estr

on of the hypothalamus in female mice.440,441 Estrogens 
may also elicit direct positive effects at the pituitary level.
The principal effect of progesterone is to decrease the frequency 
of gonadotropin pulses, mediated by effects on GnRH pulse fre­
quency. During the luteal phase of the human menstrual cycle, 
when progesterone concentrations are the highest, LH pulse fre­
quency is markedly slowed.
Testosterone and its aromatized derivative estradiol are the two 
steroid hormones that exert n

tradiol are the two 
steroid hormones that exert negative feedback effects on gonado­
tropin secretion in the male. The net in vivo effect of testoster­
one administration to normal men is inhibition of serum LH and 
FSH levels. The available evidence suggests that 5α-reduction of 
testosterone is not essential for the inhibitory effects of testoster­
one on LH. Administration of a potent 5α-reductase inhibitor, 
finasteride, to normal men did not result in elevated LH and FSH 
levels.442 Much l

t result in elevated LH and FSH 
levels.442 Much like the effects of estrogen, these inhibitory effects 
are felt to occur largely at the hypothalamic level, by kisspeptin 
neurons, mediated by both androgen and estrogen receptors (AR 
Key:
Inhibin -subunit
Activin type I receptor
Activin type II receptor
TGFBR3
Receptor-regulated Smad
Common Smad
DNA binding partner
Activin target gene
Activin A
A
B
C
Activin B
Inhibin A
Inhibin B
Activin AB
Smad 4
R-Smad
R-Sm

ts. (B) Activin signaling pathway. Activins 
bind to specific sets of serine–threonine kinase type I and type II receptors on the cell surface. Upon ligand 
binding, the type II receptor phosphorylates and thereby activates the type I receptor, which in turn phos­
phorylates downstream signaling molecules, the receptor-regulated S-Smads (R-Smads). Once phos­
phorylated, the R-Smads associate with the common co-Smad (Smad 4) and translocate to the nucleus 
where, in combination with cell type–spe

 nucleus 
where, in combination with cell type–specific binding partners, they bind to the promoter sequences of 
target genes to regulate gene transcription and cellular function. (C) Inhibins bind to activin type II receptors 
and block the recruitment of type I receptors, thereby blocking R-Smad activation and activin signaling. 
The presence of TGFBR3, a TGFβ superfamily accessory receptor also known as betaglycan, enhances 
the binding of inhibins to type II receptors, thereby enhancing the

hibins to type II receptors, thereby enhancing the antagonistic actions of inhibins. (From 
Stenvers KL, Findlay JK. Inhibins: from reproductive hormones to tumor suppressors. Trends Endocrinol 
Metab. 2010;21:174–180.)
220
and ERα). Mechanisms for testosterone feedback are complex, 
as testosterone also exerts a stimulatory effect on FSHβ mRNA 
levels.443 
Secretion
In light of the episodic, pulsatile nature of LH and FSH secretion, 
there was a need to 

ure of LH and FSH secretion, 
there was a need to develop discrete pulse-detection algorithms. 
Santen and Bardin developed an algorithm that defined a peak as 
a 20% increase in the hormone concentration in a single sample 
over the preceding sample, which was subsequently modified to 
define the pulse based on a chosen multiple of the assay coefficient 
of variation. Because of its simplicity of use and apparent freedom 
from assumptions, this program is still widely used, although a 
number o

program is still widely used, although a 
number of refinements and alternative algorithms have also been 
developed. Application of deconvolution to pulse analysis made 
it possible to determine the instantaneous real-time secretory 
rates. The characteristic secretory episodes characterized for LH 
and FSH indicate daily production rates of 1000 IU and 200 IU, 
respectively, and a disappearance half-life of 90 minutes and 500 
minutes for each respective β-subunit.444,445 The longer circulat­

respective β-subunit.444,445 The longer circulat­
ing half-life of FSH makes the pulsatile secretory pattern of this 
gonadotropin less clear than that of LH, although differences in 
secretory pathways for FSH and LH also contribute.446,446a 
Action
The primary targets of FSH and LH are the gonads, and thus targets 
and effects differ in the male and female (Fig. 8.34). The actions of 
FSH and LH in the male and female are presented briefly here but 
are discussed in greater detail in Chapters 

 but 
are discussed in greater detail in Chapters 17 and 19.
Female
FSH acts on FSH receptors in granulosa cells to facilitate follicular 
growth and estradiol biosynthesis (see Fig. 8.34).447 The initia­
tion of follicular growth can occur independently of gonadotro­
pin stimulation, after which further maturation requires FSH. At 
these more advanced stages of development, follicles convert theca 
cell–derived androstenedione to estradiol (E2) by the induction 
of aromatase activity in respons

by the induction 
of aromatase activity in response to FSH.448 FSH also controls 
granulosa cell production of inhibin during the follicular phase 
and induces LH receptor expression in granulosa cells of large 
preovulatory follicles. At the same time that FSH promotes the 
development of the dominant follicle, it also initiates the recruit­
ment of the next generation of follicles that will enlarge during 
subsequent cycles.
LH, acting on LH receptors in ovarian theca cells, is a major 
regula

ceptors in ovarian theca cells, is a major 
regulator of ovarian steroid synthesis. LH stimulates estrogen 
production by promoting synthesis of androgen precursors in 
theca cells, which then diffuse into neighboring granulosa cells, 
where they are aromatized into estrogens under the control of 
FSH.413 LH increases cholesterol availability for ovarian ste­
roidogenesis by inducing the steroidogenic acute regulatory 
(StAR) protein,449 which mediates cholesterol transfer from 
the outer to the

diates cholesterol transfer from 
the outer to the inner membrane, where it becomes available 
for steroidogenesis. LH also enhances cytochrome P450–linked 
enzyme activity to synthesize pregnenolone and induces synthe­
sis of 3β-hydroxysteroid dehydrogenase, 17α-hydroxylase, and 
17,20-lyase. The midcycle LH surge stimulates resumption of 
oocyte meiosis and maturation in the preovulatory follicle, ini­
tiates the rupture of the ovulatory follicle and ovulation, and 
induces conversion of the f

le and ovulation, and 
induces conversion of the follicle wall into the corpus luteum 
(luteinization).447,448 LH stimulates the expression of proges­
terone receptors in the granulosa cells of the dominant follicle, 
which promotes luteinization. In addition, LH helps to sustain 
luteinization by stimulating progesterone synthesis. 
Male
LH acts on LH receptors in Leydig cells to induce intratesticular 
testosterone synthesis, mediated by enhanced cAMP production. 
FSH in the male is involved i

ed cAMP production. 
FSH in the male is involved in spermatogenesis (see Fig. 8.34), 
although the precise role in the spermatogenic process remains 
unclear.450 FSH binds to FSH receptors on Sertoli cells and 
stimulates the production of inhibins, androgen-binding protein, 
androgen receptor, and other proteins. FSH mediates the mat­
uration of spermatids into mature spermatozoa in concert with 
testosterone.451 
Gonadotropin Measurements
Because of the high homology of the glycoprotein hormon

se of the high homology of the glycoprotein hormones, 
development of highly specific assays, especially those distinguish­
ing free α-subunit from intact hormones, was necessary. Hetero­
geneity of circulating LH and FSH molecules, insufficient assay 
sensitivity to distinguish normal from low levels, and lack of rig­
orously pure reference preparations hampered assay development. 
Two-site–directed immunofluorometric and immunochemilumi­
nescent LH and FSH assays have much improved sensitivity

t LH and FSH assays have much improved sensitivity, are 
able to detect LH with a sensitivity of 0.1 mIU/mL, and have 
resolved prior challenges with cross-reactivity.452 Differences in 
carbohydrate moieties result in isoelectric charge heterogeneity 
for LH, accounting for some observed disparities in biologic and 
immunoreactive LH ratios observed after GnRH agonist treat­
ment, acute critical illness, or aging.
α-Subunit Assays
Both GnRH and TRH increase circulating levels of free α-subunit 

TRH increase circulating levels of free α-subunit 
derived from either gonadotrophs or thyrotrophs, especially 
in patients with hypothyroidism, after castration, and during 
menopause. GnRH agonist treatment, TSH-secreting tumors, or 
nonfunctioning pituitary adenomas may result in discordant cir­
culating ratios of free α-subunit from intact LH dimers. 
Male
FSH
• Sertoli cell
  function
• Sperm
• Testosterone
LH
+?
Female
FSH
• Estradiol
• Follicle
  growth
• Androgens
• Oocyte
  maturation
L

licle
  growth
• Oocyte
LH
+?
• Fig. 8.34  Actions of follicle-stimulating hormone (FSH) and luteinizing 
hormone (LH) in the male and female. The plus signs and horizontal arrows 
indicate potentially unrecognized new functions of FSH since the discovery 
of FSHβ gene mutations. (Modified from Layman LC. Genetics of human 
hypogonadotropic hypogonadism. Am J Med Genet [Semin Med Genet]. 
1999;89:240–248; Richards JS, Pangas SA. The ovary: basic biology and 
clinical imp

gas SA. The ovary: basic biology and 
clinical implications. J Clin Invest. 2010;120:963–972; Bhasin S, Fisher 
CE, Sverdloff RS. Follicle-stimulating hormone and luteinizing hormone. 
In: Melmed S, ed. The Pituitary. 2nd ed. Malden, MA: Blackwell Science; 
2002:216–278.)
221
GnRH Stimulation Test
A single bolus of GnRH (25–100 μg) dose-dependently evokes 
serum LH and FSH levels within 20 and 30 minutes. LH rises 
more abundantly than F

0 and 30 minutes. LH rises 
more abundantly than FSH, and peak values range from 8 to 34 
mIU/mL. In contrast, patients with hypogonadotropic hypogo­
nadism and no demonstrable hypothalamic-pituitary lesion have 
blunted LH responses and reversal of the LH:FSH ratio. The 
test, however, cannot adequately distinguish hypothalamic from 
pituitary lesions, and similar patterns are observed in patients 
with anorexia nervosa and hypothalamic amenorrhea. Repeti­
tive GnRH pulses may in fact normalize

ea. Repeti­
tive GnRH pulses may in fact normalize responses, as would 
be expected from an intact hypothalamic-pituitary unit. GnRH 
responses may vary during the stages of puberty, reflecting altered 
pituitary sensitivity. This test is most frequently used in the assess­
ment of pubertal status, for the diagnosis of hypogonadotropic 
hypogonadism or for evaluation of central precocious puberty.453 
Clomiphene Stimulation Test
Clomiphene (100 mg), administered daily for up to 4 weeks, usu­
all

g), administered daily for up to 4 weeks, usu­
ally doubles LH levels, and FSH increases by about 50% over 
baseline. Because an abnormal or absent response does not distin­
guish hypothalamic from pituitary lesions, the utility of this test 
is limited. 
Gonadotropin Deficiency
Causes
Gonadotropin deficiency may be congenital or acquired and may 
arise from hypothalamic or pituitary disorders. Congenital causes 
include gene mutations governing the processes of development 
and migration of GnR

the processes of development 
and migration of GnRH neurons, the control of GnRH secre­
tion, the development of the gonadotroph, or the regulation of 
LH and FSH secretion. Acquired HH may arise from functional 
or organic disorders. Functional causes are frequently encoun­
tered and include stress, malnutrition, chronic illness, depres­
sion, excessive exercise, and low body weight. Several centrally 
acting drugs, including opiates, glucocorticoids, sex steroids, 
GnRH agonists and antagonist

coids, sex steroids, 
GnRH agonists and antagonists, tranquilizers, antidepressants, 
and antipsychotic medications, suppress gonadotropin secretion 
either directly or indirectly via induction of hyperprolactinemia. 
Organic causes include malignant disease, developmental disor­
ders, tumors, infiltrative disease, trauma, and hypothalamic or 
pituitary damage from surgery or radiotherapy. Congenital and 
acquired causes of central hypogonadism are considered more 
fully in Chapter 26.
Hypogonad

re considered more 
Hypogonadotropic Hypogonadism. The genetic basis of HH 
was recognized over 60 years ago with the description by Kall­
mann of hypogonadism and anosmia in two families. It was not 
until 1991 that anosmin, a glycoprotein encoded by the KAL1 
gene, was identified as a cause of X-linked Kallmann syndrome.454 
The realization that KAL1 gene defects only accounted for a small 
proportion of patients with classic Kallmann syndrome has led 
to genotype-phenotyp

ic Kallmann syndrome has led 
to genotype-phenotype studies that have provided important 
insights into the molecular genetics underlying HH. Gene muta­
tions affecting the olfactory bulb and GnRH neuronal migration 
tend to give rise to the Kallmann phenotype. Multiple genetic 
defects have been associated with Kallmann syndrome, which in 
many cases can be distinguished clinically based on well-estab­
lished nonreproductive phenotypes. On the other hand, defects in 
genes involved in regulatin

ther hand, defects in 
genes involved in regulating GnRH secretion or GnRH action are 
usually associated with normosmia, and in some cases the clini­
cal phenotype is solely that of isolated HH (Table 8.10 provides 
details; see also Fig. 8.32). 
Kallmann Syndrome. Kallmann syndrome consists of defective 
GnRH neuronal development, with olfactory nerve agenesis or 
hypoplasia and variable anosmia. Associated developmental dis­
orders include optic atrophy, color blindness, sensorineural deaf­
n

ic atrophy, color blindness, sensorineural deaf­
ness, cleft palate, renal agenesis, cryptorchidism, and movement 
disorders.455 This X-linked recessive disorder has been ascribed to 
a defective KAL1 gene located on chromosome Xp22.3.456 The 
encoded anosmin protein mediates hypothalamic migration of 
GnRH cells from the primitive olfactory placode, and its absence 
leads to defective GnRH synthesis and anosmia.457,458 Both auto­
somal recessive and dominant forms of the disorder have since 
be

 and dominant forms of the disorder have since 
been described, with multiple additional genetic causes identified 
(see Table 8.10 and Fig. 8.32).
These patients are exposed to low or absent sex steroids from 
birth. Consequently, females are tall and present with primary 
amenorrhea and absent secondary sexual development, and males 
have delayed puberty and micropenis.459 Absent GnRH secretory 
pulses result in characteristically low LH and FSH levels in the face 
of very low concentrations o

H levels in the face 
of very low concentrations of estradiol or testosterone. Because the 
nonprimed normal pituitary may not respond initially to GnRH 
stimulation, this test is of little value in distinguishing a hypotha­
lamic from a pituitary defect. In some patients, repetitive GnRH 
priming may elicit pituitary LH and FSH responses, indicating a 
hypothalamic defect in GnRH secretion. 
Gonadotropin deficiency causes hypogonadism with decreased 
sex steroid production of var

dism with decreased 
sex steroid production of varying degrees, depending upon the 
severity of the insult (Table 8.11). This disorder may occur at any 
stage of life. In its congenital form, primary amenorrhea and total 
absence of development of secondary sexual characteristics may 
occur. Onset later in life may present with a varying spectrum of 
reproductive dysfunction, ranging from luteal phase abnormali­
ties with subfertility to oligomenorrhea or amenorrhea in women. 
Women exhibit seco

orrhea or amenorrhea in women. 
Women exhibit secondary amenorrhea, infertility, decreased vagi­
nal secretion, dyspareunia, hot flashes, decreased bone density, 
and breast tissue atrophy.
Males with congenital HH have small testes (<4 mL) and 
absence of secondary sexual characteristics. Tall stature with 
eunuchoid habitus may be present as a result of delayed or absent 
epiphyseal closure. Cryptorchidism and microphallus may be 
present, reflecting the absence of activity of the HPG axis dur

ecting the absence of activity of the HPG axis during 
fetal development.460 Men with gonadotropin deficiency later in 
life present with loss of libido, potency, and fertility. They may 
have impotence, testicular atrophy, decreased libido, low energy, 
infertility, loss of secondary sexual characteristics, decreased mus­
cle strength and mass, decreased bone mass, decreased facial and 
body hair, and fine facial wrinkling.461
In both men and women, serum gonadotropin levels are inap­
propriate

men, serum gonadotropin levels are inap­
propriately low in the face of decreased sex steroid levels. In women 
with amenorrhea or oligomenorrhea, serum LH, FSH, and estra­
diol levels should be measured. Endogenous estrogen sufficiency 
can also be assessed by the response to a progesterone challenge 
(100 mg intramuscularly or 10 mg medroxyprogesterone acetate 
orally daily for 5–10 days). Men should have serum LH, FSH, and 
testosterone levels measured in a morning sample.
Hypothalamic and pi

 measured in a morning sample.
Hypothalamic and pituitary disorders are the most common 
endocrine cause of male subfertility. Because normal secretion 
of both FSH and LH is required for quantitatively and qualita­
tively normal spermatogenesis, any disease that affects hypotha­
lamic secretion of GnRH or pituitary secretion of FSH or LH 
will impair spermatogenesis.462 As FSH is required for normal 
222
  Genetics of Hypogonadotropic Hypogonadism
Pheno

Phenotype/
Mechanism
Genea
Function
Kallmann syn­
drome/neuronal 
development and 
migration
KAL1454
KAL1454
X-linked
Anosmin required for cell surface signaling, adhesion, and migration
FGFR1561
FGFR1561
AD
Role in axonal development and guidance
FGF8562
FGF8562
AD
Endogenous ligand of FGFR1
PROK2563
PROK2563
AD, AR
Development of olfactory bulb and migration of GnRH neurons
PROKR2563
PROKR2563
NELF564
NELF564
?
Encoding nasal embryonic L

R2563
NELF564
NELF564
?
Encoding nasal embryonic LHRH factor
SEMA3A, SEMA3E460,565
SEMA3A460
AD
Axonal pathfinding
HS6ST1566
HS6ST1566
AD
Modifications of extracellular sugars
CHD7567
CHD7567
AD
Chromodomain helicase DNA binding protein 7, chromatin remodeling
WDR11568
WDR11568
AD
WD repeat domain protein interacts with transcription factor EMX1
FGF17, IL17RD, DUSP6, SPRY4, 
FLRT3569
Components of fibroblast growth factor pathway
KLB570
AD
FGF21/FGFR1 coreceptor
SOX10571
AD
Transcription factor 

FGFR1 coreceptor
SOX10571
AD
Transcription factor in olfactory ensheathing cells
CCDC141572
AD
Coiled coil protein in GnRH neurons
FEZF1573
AR
Protease
Normosmic hypo­
gonadotropic 
hypogonadism/
GnRH synthesis, 
secretion, or 
action
KISS1574
KISS1574
AR
Hypothalamic neuropeptide, stimulates GnRH secretion
KISS1R421,422
AR
Receptor for kisspeptin
TAC3425
TAC3425
AR
Encodes neurokinin B, a neuropeptide that stimulates kisspeptin 
release
TACR3425
TACR3425
AR
Encodes neurokinin B re

lease
TACR3425
TACR3425
AR
Encodes neurokinin B receptor
LEP575
LEP575
AR
Encodes leptin, derived from adipose tissue to signal adequacy of 
nutritional status
LEPR575
LEPR575
AR
Encodes leptin receptor
PCSK1374
PCSK1374
AR
Processes GnRH
GNRH1535,576
AR
Encodes GnRH
GNRHR421
GNRHR421
AR
Receptor for GnRH, stimulation of gonadotropin secretion
TABLE 8.10
223
spermatogenesis, isolated FSH deficiency is associated with oli­
go

isolated FSH deficiency is associated with oli­
gospermia or azoospermia. Men with inactivating FSHβ muta­
tions are azoospermic, but some have normal puberty associated 
with normal to low-normal testosterone levels and high LH 
levels, whereas another presented with a low testosterone con­
centration and absent puberty.463–465 Inactivating FSHβ muta­
tions have also been characterized in women and have resulted 
in a phenotype of delayed puberty, absent or incomplete breast 
development, prima

y, absent or incomplete breast 
development, primary amenorrhea, and infertility, with low lev­
els of estradiol and progesterone, undetectable FSH, and high 
LH.463,464
Mutations that abolish the activity of LH, resulting in iso­
lated LH deficiency, have also been reported.466,467 Isolated LH 
deficiency in males may manifest with delayed or absent puberty, 
with eunuchoidal body proportions as a result of low testosterone 
levels. Low LH levels in these patients lead to low intratesticular 
t

s in these patients lead to low intratesticular 
testosterone concentrations with resultant decreased spermatogen­
esis. A female with isolated LH deficiency due to an LH mutation 
had normal pubertal development and menarche but secondary 
amenorrhea and infertility. 
Management
The goals of therapy are to replace or restore sex steroid hormones 
and to induce and maintain normal reproductive function. If fer­
tility is not an immediate objective, sex steroid hormone replace­
ment is usually su

e, sex steroid hormone replace­
ment is usually sufficient. For induction of gametogenesis in those 
desiring fertility, therapy with gonadotropins or GnRH is usually 
required.
Evaluation. In evaluating hypogonadal patients in the absence 
of an obvious pituitary or gonadal disorder, the primary diagnostic 
challenge is to distinguish constitutional pubertal delay from other 
causes of hypogonadotropism.341,468 When puberty is delayed 
after 14 years of age, a primary developmental disorder, HH

years of age, a primary developmental disorder, HH, 
should be considered in the absence of acquired causes. Crypt­
orchidism or micropenis is suggestive of congenital HH, whereas 
low patient height relative to the parents’ height suggests consti­
tutional delay of puberty (CDP) rather than congenital HH, in 
which height tends to be normal or even increased, possibly with 
eunuchoidal proportions. No single test clearly distinguishes con­
stitutional delayed puberty and true HH, and expectant 

tional delayed puberty and true HH, and expectant follow-
up is often helpful as many patients enter puberty spontaneously. 
To enable androgenization, testosterone replacement should be 
provided intermittently until age 18, with periodic interruptions 
to unmask physiologic pubertal advance. 
Sex Steroid Replacement Therapy. Estrogen or testosterone 
replacement is required for inducing and maintaining primary 
and secondary sexual characteristics and to maintain normal 
body composition and i

ics and to maintain normal 
body composition and integrity of BMD and muscle mass. For 
patients not seeking fertility, sex steroid therapy is warranted to 
correct central hypogonadism. However, monitoring of LH and 
FSH responses does not accurately reflect adequate steroid hor­
monal replacement, as basal gonadotropin levels are already low 
or undetectable.
Mechanism
Genea
Function
NR5A1
AD
Transcription factor for hypothalamic, pituitary, gonadal, and adrenal 
develop

thalamic, pituitary, gonadal, and adrenal 
development
NR0B1
X-linked
Transcriptional repressor; hypothalamic, pituitary, gonadal, and adrenal 
SRA1577
AD
SF1 and nuclear receptor coregulator
LHB
AR
Encodes luteinizing hormone β-subunit
FSHB
AR
Encodes follicle-stimulating hormone β-subunit
aSuperscript numbers refer to references at the end of this chapter.
AD, Autosomal dominant; AR, autosomal recessive; FGF, fibroblast growth factor; GNRH, gonadotropin-releasing hormone; GPR, orph

r; GNRH, gonadotropin-releasing hormone; GPR, orphan G protein–coupled receptor; IHH, isolated hypogonadotropic hypo­
gonadism; KAL, Kallmann syndrome; KISS, KiSS metastasis suppressor; LHRH, luteinizing hormone–releasing hormone; NELF, nasal embryonic LHRH factor; PROK, prokineticin.
   
  Genetics of Hypogonadotropic Hypogonadism­—cont’d
TABLE 8.10
  Clinical Features of Hypogonadotropism
Prepubertal Onset
High-pitched voice
Absent terminal facial hair
Decreased or absent body hair
Eunuchoid

acial hair
Eunuchoidal body proportions
Female escutcheon
Testicular volume <6 cm3
Testicular length <2.5 cm
Cryptorchidism may be present
Penile length <5 cm
Hypopigmented scrotum with absent rugae
Small prostate
Decreased libido
Decreased muscle and bone mass
Postpubertal Onset
Decreased spontaneous erections
Slow beard growth
Decreased body hair
Testicular atrophy if long standing
Normal
Voice pitch
Skeletal proport

 and bone mass
Normal
Skeletal proportions
Penis length
Scrotal rugae
Prostate size
TABLE 8.11
224
Estrogens are available as a tablet, patch, gel, or implant. Ini­
tiation of puberty can begin with any type or route of exogenous 
estrogen, oral or transdermal. Initial therapy should consist of 
estrogen alone to maximize breast growth and to induce uterine 
and endometrial proliferation. A progestin eventually needs to be 
added to prevent en

gestin eventually needs to be 
added to prevent endometrial hyperplasia but should be avoided 
before completion of breast development, because it is likely to 
reduce ultimate breast size. In premenopausal women, oral estro­
gens or transdermal estradiol delivering 50 to 100 mg daily can be 
used, with concomitant cyclic progesterone therapy for women 
with an intact uterus to prevent unopposed endometrial prolif­
eration. Although early sex steroid replacement lessens the risk of 
developing o

roid replacement lessens the risk of 
developing osteoporosis, effects of estrogen replacement on car­
diovascular function are unresolved. In patients with hypopituita­
rism, estrogen replacement should be maintained until the age of 
50, after which continuation should be determined on an individ­
ual basis by assessing risks and benefits, especially in terms of bone 
mineral integrity, cardiovascular function, and cancer risk. Estro­
gen treatment may be associated with thromboembolic disease

ment may be associated with thromboembolic disease, 
breast tenderness, and possibly enhanced risk for breast cancer.
For men, androgen replacement is available as intramuscular, 
gel, patch, implants, nasal, or oral preparations. Intramuscu­
lar injection of testosterone enanthate or testosterone cypionate 
is usually administered at doses of 200 to 300 mg every 2 or 3 
weeks.461 Administration of lower doses on a more frequent basis 
(e.g., 100 mg weekly) may stabilize fluctuations of hormone 

 mg weekly) may stabilize fluctuations of hormone levels. 
Elderly males require lower doses, as do boys with delayed puberty. 
Testosterone undecanoate provides long-term replacement for 
3 to 4 months after each injection with improved pharmacoki­
netic profiles, but rare cases of pulmonary oil microembolism and 
anaphylaxis have been reported; it must be administered in an 
office or hospital setting by a trained health care provider, and 
the patient should be monitored for 30 minutes afterw

 patient should be monitored for 30 minutes afterward for 
adverse reactions. Transdermal testosterone patch and gel systems 
deliver 4 to 6 mg and sustained testosterone profiles. Patch sites 
may develop skin irritation, blisters, and vesicles in approximately 
25% of patients.469 Oral androgen replacement therapy is gen­
erally not recommended because of nonuniform absorption and 
hepatotoxicity. Nasal testosterone is a recently approved alterna­
tive with a shorter half-life, administered th

na­
tive with a shorter half-life, administered three times daily470; at 
the other end of the spectrum, longer acting implantable testos­
terone pellets are available. Testosterone may cause acne, gyneco­
mastia, prostatic hypertrophy, and polycythemia. Although there 
is no compelling evidence that testosterone replacement causes 
prostate cancer, benign prostatic hypertrophy can be exacerbated, 
especially in elderly patients. Testosterone replacement should not 
be administered to men with d

lacement should not 
be administered to men with diagnosed prostate cancer. Target 
serum hormone levels, which benefit lean mass, muscle strength, 
and sexual function without inducing undesirable adverse conse­
quences, vary, and in the future, individual targets may guide the 
treatment of hypogonadism in men.471 
Fertility. In patients with HH, fertility may be achieved with 
gonadotropin or GnRH therapy. In males, even relatively low 
sperm counts may be adequate for impregnation when ferti

counts may be adequate for impregnation when fertility 
is induced by gonadotropins or GnRH. As testosterone therapy 
may suppress spermatogenesis, the steroid should be discontinued 
prior to initiating treatment. Partial rather than complete hypogo­
nadism predicts a more favorable response to treatment, whereas 
persistence of cryptorchidism beyond the age of 1 year reduces the 
likelihood of successful fertility induction. hCG is administered 
subcutaneously or intramuscularly (1000–2000 IU 

subcutaneously or intramuscularly (1000–2000 IU two to three 
times weekly) to induce spermatogenesis, with the dose titrated 
according to testosterone levels.472 If necessary, after 6 months, 
human menopausal gonadotropin (hMG) or purified or recombi­
nant FSH should be added to improve sperm quantity, and doses 
may be doubled after a further 6 months. An increase in testicular 
volume correlates well with induction of spermatogenesis. If tes­
tosterone levels are increased, subsequent con

es­
tosterone levels are increased, subsequent conversion to estradiol 
may result. Therefore both testosterone and estradiol levels should 
be monitored.
Pulsatile GnRH therapy is an alternative treatment for patients 
with normal pituitary function (i.e., those with idiopathic HH or 
Kallmann syndrome). GnRH is infused subcutaneously by con­
tinuous minipump (5 mg every 2 hours), with the dose titrated 
to maintain normal gonadotropin and testosterone levels. This 
approach may be marginally m

sterone levels. This 
approach may be marginally more effective than treatment with 
gonadotropins and may cause less gynecomastia. These approaches 
require strong patient commitment, as adequate spermatogenesis 
may not be attained for 2 years or longer despite normalized tes­
tosterone levels. Aliquots of successfully generated sperm samples 
should be frozen for future impregnation.
Clomiphene citrate is a weak estrogen receptor antagonist that 
stimulates gonadotropin secretion in normal wo

at 
stimulates gonadotropin secretion in normal women and men. 
Clomiphene has been used to increase spermatogenesis in men 
with partial hypogonadotropism with oligospermia or azoosper­
mia and normal to mildly low serum testosterone concentrations, 
particularly in men with functional hypogonadism, with variable 
results.
In women with HH, fertility may be effectively achieved by 
pulsatile GnRH administration or by gonadotropin therapy (fully 
discussed in Chapters 17 and 26). Although ovulat

discussed in Chapters 17 and 26). Although ovulation is often 
induced and pregnancy achieved by gonadotropin treatment, a 
high rate of multiple follicle development remains a concern.473 
If residual pituitary gonadotroph reserve is sufficiently robust, 
GnRH therapy is more likely to result in ovulation of a single 
rather than multiple follicles, thereby reducing the chances of 
multiple gestation.474 Therapy with kisspeptin or kisspeptin ana­
logues is an area of current investigation and 

a­
logues is an area of current investigation and holds promise for 
further reducing the chances of multiple gestation or the risks of 
ovarian hyperstimulation475,475a–475c (see Chapter 17). 
Thyroid-Stimulating Hormone
The hypothalamic-pituitary-thyroid system plays a critical role in 
development, growth, and cellular metabolism through the action 
of thyroid hormones.
Thyrotroph Cells
Thyrotroph cells comprise approximately 5% of the functional 
anterior pituitary cells and are s

the functional 
anterior pituitary cells and are situated predominantly in the 
anteromedial areas of the gland. They are smaller than the other 
cell types, irregularly shaped with flattened nuclei and relatively 
small secretory granules, ranging from 120 to 150 μm, and they 
contain TSH that is released under TRH stimulation. Immuno­
reactive TSH cells are present in the fetal pituitary by 12 weeks. 
The differentiation of thyrotrophs is critically dependent on the 
expression of transcriptio

cally dependent on the 
expression of transcription factors GATA2, POU1F1, and PROP1 
during development. 
Structure
TSH is a glycoprotein hormone comprising a 28-kDa heterodi­
mer of two noncovalently linked subunits (α and β).476,477 The 
α-subunit and β-subunit are encoded by two separate genes located 
225
on different chromosomes (Fig. 8.35). The tertiary TSH structure 
comprises three hairpin loops separated by central disulfide bo

ee hairpin loops separated by central disulfide bonds, 
with the longer loop straddling one side.477–479 The α-subunit is 
common to TSH, LH, FSH, and hCG, whereas the β-subunit is 
unique and confers specificity of action.477 The α-subunit is the 
earliest hormone gene expressed embryonically, but activation of 
the β-subunit gene occurs later under the influence of GATA2 
and POUF1F1.480 The 13.5-kb α-subunit gene is located on 
chromosome 6 and comprises four exons and three introns (see 
Fig

d comprises four exons and three introns (see 
Fig. 8.35).481 Although the α-subunit gene is expressed in thyro­
troph, gonadotroph, and placental cells, its regulation is uniquely 
cell specific. The downstream promoter region (−200 and below) 
is required for placental expression, intermediate sequences are 
required for gonadotroph expression, and upstream promoter 
elements are required for thyrotroph-specific expression.482 The 
4.9-kb TSH β-subunit gene located on chromosome 1 comprises 
t

-subunit gene located on chromosome 1 comprises 
three exons and two introns (see Fig. 8.35).483 PIT1 binds directly 
to the gene promoter to confer tissue-specific expression.484
Production of the mature heterodimeric TSH molecule 
requires complex cotranslational glycosylation and folding of 
nascent α-subunit and β-subunit.477 After subunit translation and 
signal peptide cleavage, glycosylation occurs at asparagine 23 on 
the β-subunit and at two asparagine residues, 52 and 78, on the 
α-sub

 two asparagine residues, 52 and 78, on the 
α-subunit.485 Appropriate glycosylation is required for accurate 
molecular folding and subsequent combination of α-subunit and 
β-subunit within the rough endoplasmic reticulum and Golgi 
apparatus. 
α-Subunit transcription is inhibited by the thyroid hormone 
receptor (TR) in concert with other nuclear corepressors.486 
TSHβ gene transcription is also suppressed by the TR acting 
directly on exon 1.487 This potent suppression is evident w

on exon 1.487 This potent suppression is evident within 
30 minutes of T3 exposure and is a critical determinant of ultimate 
TSH secretion. Both αTSH and βTSH subunit gene transcription 
are induced by TRH and suppressed by dopamine.488 Both TRH 
and T3 regulate TSH glycosylation, albeit in opposite directions. 
1st intron
3900 bp
ATG
2nd intron
450 bp
TATA
box
Promoter
Exon 1
37 bp
Exon 2
163 bp
Exon 3
326 bp
5′
A
B
3′
5′
3′
polyA
1st intron
6400 bp
2nd intron
1700 bp
3rd intron
400 bp
ATG
TAT

00 bp
2nd intron
1700 bp
3rd intron
400 bp
ATG
TATA
box
Promoter
Exon 1
94 bp
Exon 2
95 bp
Exon 3
185 bp
Exon 4
295 bp
polyA
TSHβ-Subunit Gene
TSHβ mRNA
α-Subunit Gene
α-Subunit mRNA
• Fig. 8.35  Schematic representation of the TSHβ (A) and the α-subunit (B) genes. The relative locations 
and sizes of the exons and introns are shown within translated regions shown as open boxes and protein 
coding regions as black boxes. The TATA box position in the transcriptional start site is located in the 

the transcriptional start site is located in the 
promoter close to exon 1. The introns are spliced out during transcription and exons joined and a polyA 
tail added at the 3′ end. (From Gordon DF, Sarapura VD, Samuels MH, et al. Thyroid stimulating hormone: 
physiology and secretion. In Jameson JL, De Groot LJ, eds. Endocrinology: Adult and Pediatric. 6th ed. 
Philadelphia: Elsevier; 2010:1362–1383.)
226
TRH exposure or T3 deprivation enhances oligosacch

TRH exposure or T3 deprivation enhances oligosaccharide addi­
tion to the TSH molecule.488,489
The TRH neuron plays a central role in determining the 
set-point of the hypothalamic-pituitary-thyroid axis by regulat­
ing pituitary TSH release.490 Hypothalamic TRH synthesis is in 
turn regulated by thyroid hormones. Three main neuronal groups 
mediate the effects of other physiologic stimuli on hypothalamic 
TRH neurons, which are located in the paraventricular nucleus. 
The first is adrenergic in

raventricular nucleus. 
The first is adrenergic input from the medulla that mediates the 
stimulatory effects of cold exposure on the TRH neuron (Fig. 
8.36). Second, TRH neurons receive projections from the arcu­
ate nucleus that contain two leptin-responsive groups regulating 
energy homeostasis: The POMC system that promotes weight loss 
activates while the NPY/agouti-related protein (AGRP) system 
that promotes weight gain inhibits TRH neurons.491 Fasting 
results in reduction of TRH express

s.491 Fasting 
results in reduction of TRH expression, which is mediated by sup­
pression of the POMC system and stimulation of the NPY/AGRP 
system. Third, the hypothalamic dorsomedial nucleus projects to 
the paraventricular nucleus and represents alternative pathways by 
which leptin acts to regulate TRH neurons.491,492
Feedback regulation by thyroid hormones on TRH and TSH 
are determined primarily by circulating thyroxine (T4) entry into 
the blood-brain barrier elaborated through a complex

e blood-brain barrier elaborated through a complex system of 
paracrine control in the hypothalamus (see Fig. 8.36). The effects 
of thyroid hormones are mediated by thyroid hormone receptors, 
which are members of a superfamily of nuclear hormone recep­
tors. TRs exist as two major isoforms, TRα and TRβ. TRα is the 
key TR isoform responsible for T3-mediated negative-feedback 
T3
T3
T3
T3
T4
Thyroid
T4, T3
Blood
brain
barrier
CSF
TRH
D2 mRNA
ME
TRH
PVN
DMN
ARC
NPY/AGRP
Adrenaline/NPV/CART
α-MSH

TRH
PVN
DMN
ARC
NPY/AGRP
α-MSH/CART
C1-3
NTS
sp5
py
TSH
• Fig. 8.36  Schematic illustration of the feedback system regulating the hypothalamic-pituitary-thyroid 
axis. Thyroid hormones exert negative feedback effect at the level of the pituitary and hypophysiotropic 
TRH neurons. The central feedback effect of thyroid hormones primarily depends on the circulating T4 lev­
els. In the hypothalamus, T4 is converted to T3 by deiodinase 2 (D2) in tanycytes. By volume transmission,

inase 2 (D2) in tanycytes. By volume transmission, 
T3 secreted from tanycytes reaches the hypophysiotropic TRH neurons, where T3 inhibits the proTRH gene 
expression via TRβ2 receptors. The set-point of the feedback regulation can be altered by two mecha­
nisms: (1) regulation of D2 activity in tanycytes and (2) neuronal afferents altering CREB concentration in 
the hypophysiotropic TRH neurons. ARC, hypothalamic arcuate nucleus; C1-3, C1-3 adrenergic area of 
the brainstem; CSF, cerebrospinal 

nergic area of 
the brainstem; CSF, cerebrospinal fluid; DMN, hypothalamic dorsomedial nucleus; ME, median eminence; 
NTS, nucleus tractus solitarius; PVN, hypothalamic paraventricular nucleus; py, pyramidal tract; sp5, spinal 
trigeminal tract. (Redrawn from Fekete C, Lechan RM. Negative feedback regulation of hypophysiotropic 
thyrotropin-releasing hormone (TRH) synthesizing neurons: role of neuronal afferents and type 2 deiodin­
ase. Front Neuroendocrinol. 2007;28:97–114.)
227
Chapter 8  Pitu

roendocrinol. 2007;28:97–114.)
227
regulation by hypophysiotropic TRH neurons.478 The local avail­
ability of T3 is determined by deiodinase 2 and deiodinase 3, 
respectively, which provides and deactivates T3.490
Deiodinase 2 is expressed in surrounding glial cells of the 
hypothalamus491,493 and in tanycytes (lining the third ventricles), 
which generate T3 from circulating T4. Tanycytes appear to be 
the main contributor to the negati

s appear to be 
the main contributor to the negative feedback regulation of the 
hypothalamic-pituitary-thyroid axis.490 The expression by the 
TRH neuron of deiodinase 3, which inactivates T3, points to the 
existence of an important local level of TRH regulation. T3 sup­
presses hypothalamic TRH synthesis and decreases pituitary TRH 
receptor number.
In the anterior pituitary gland, deiodinase 2 is found in fol­
liculostellate cells, whereas TRs and deiodinase 3 are expressed in 
thyrotrophs.4

s and deiodinase 3 are expressed in 
thyrotrophs.491,493 These findings indicate an important role for 
folliculostellate cells in processing and activating T4. Production 
and action of local T3 occur in separate cell types of the hypothala­
mus and anterior pituitary gland resulting in setting the level of 
TSH output.
Transporters of thyroid hormone play a key role in regulat­
ing the TRH neuron. The two most important transporter fami­
lies that are involved in thyroid hormone transport in t

hat are involved in thyroid hormone transport in the brain 
are the organic anion transporting polypeptide (OATP) and the 
monocarboxylate transporter (MCT).490 Among these, OATP14 
is highly expressed in the paraventricular nucleus and OATP8 
in brain neurons.494 In humans, mutations in the MCT8 gene, 
located on the X chromosome, result in males with neurologic 
abnormalities with elevated T3 levels and decreased T4 levels in the 
presence of a normal TSH secretion.495 
Secretion
Daily TSH pro

 normal TSH secretion.495 
Secretion
Daily TSH production is approximately 100 to 400 mU496 with a 
calculated circulating half-life of approximately 50 minutes. Secre­
tion rates are enhanced up to 15-fold in hypothyroidism and are 
suppressed in hyperthyroidism. The degree of TSH glycosylation 
determines both metabolic clearance rate and bioactivity; in hypo­
thyroidism, the molecule appears highly sialylated, enhancing bio­
activity.485 Immunoreactive fetal pituitary TSH is detectable by 12 

oreactive fetal pituitary TSH is detectable by 12 
weeks. Immediately after full-term birth, there is a brisk rise in TSH, 
which remains elevated for up to 5 days before stabilizing at adult lev­
els.497 TSH secretion is pulsatile; however, the low pulse amplitudes 
and long TSH half-life result in modest circulating variances that 
are amplified in hypothyroidism and abrogated in critical illness.498 
Secretory pulses every 2 to 3 hours are interspersed with periods of 
tonic, nonpulsatile TSH

erspersed with periods of 
tonic, nonpulsatile TSH secretion.499 Circadian TSH secretion peaks 
between 11 pm and 5 am, mainly due to increased pulse amplitude 
that is not sleep entrained.500 The 24-hour TSH secretion is stable 
and robust and not influenced by sex, body mass index, or age.501 
Thyroid hormones suppress tonic TSH secretion and pulse ampli­
tude but not frequency. Free TSHα is secreted but not TSHβ.
Regulatory Factors
Several factors and drugs affect the secretion of TSH (Table 

tors and drugs affect the secretion of TSH (Table 8.12). 
Somatostatin infusion inhibits TSH pulse amplitude and blocks 
the nocturnal TSH surge502 directly at the pituitary level. Although 
SRIF analogues are used to treat TSH-secreting pituitary adeno­
mas (see later), SRIF analogue treatment for acromegaly does not 
lead to hypothyroidism, although T4 is lowered within the normal 
range. Dopamine infusions suppress TSH secretion and abrogate 
the nocturnal TSH surge.502 Prolonged use of dopam

the nocturnal TSH surge.502 Prolonged use of dopamine agonists, 
however, does not result in hypothyroidism. Glucocorticoids sup­
press TSH secretion; however, the effect does not result in hypo­
thyroidism. Proinflammatory cytokines such as TNFα and IL1 
inhibit TSH secretion.503 Leptin increases TSH secretion from 
central stimulation of TRH. Acute opiate administration causes a 
mild transient increase of TSH levels.
A number of drugs such as nonsteroidal anti-inflammatory 
agents (e.g., mecl

nonsteroidal anti-inflammatory 
agents (e.g., meclofenamate, fenclofenac) displace thyroid hor­
mone from thyroxine-binding globulin (TBG), increasing free 
hormone concentrations and thus causing transient decrease of 
TSH.504 
Action
TSH induces thyroid hormone synthesis and releases and main­
tains trophic thyroid cell integrity.505 The TSH G protein–cou­
pled receptor is located on the thyrocyte plasma membrane and is 
encoded by a gene on chromosome 11q31. Its regulation is com­
prehensivel

romosome 11q31. Its regulation is com­
prehensively described in Chapters 11 and 12. 
TSH Assays
The availability of ultrasensitive TSH measurements has revo­
lutionized the biochemical and diagnostic evaluation of thyroid 
disorders. This is due to the widespread availability of immuno­
radiometric assays. The assay principle is based on TSH serving 
as a link between an immobilized antibody binding TSH at one 
epitope and a labeled (radioactive, chemiluminescent, or colo­
rimetric) monoclonal 

, chemiluminescent, or colo­
rimetric) monoclonal directed against a second portion of the 
peptide. This technology substantially increases sensitivity and 
specificity with technical improvements leading to sequential 
generations of assays of higher sensitivity. Current commercial 
assays are mainly “third generation” with detection limits to 0.01 
to 0.02 mU/L performed on automated platforms. They pro­
vide ready discrimination between hyperthyroid, euthyroid, and 
hyperthyroid states.506,5

thyroid, euthyroid, and 
hyperthyroid states.506,507 A major consequence of ultrasensitive 
TSH assay is the obsolescence of the TRH test for patients sus­
pected of thyrotoxicosis. TSH measurement alone is not help­
ful in diagnosing central hypothyroidism, which is identified by 
concurrent measurement of thyroid hormone levels. Basal TSH 
levels are subnormal in only about one-third of patients with 
secondary hypothyroidism.508 TSH deficiency is thus associated 
with low T4 levels concomitan

 is thus associated 
with low T4 levels concomitant with low, normal, or even mini­
mally elevated TSH levels. Importantly, this biochemical profile 
may also be encountered in critically ill patients with low TSH 
and T4 levels without evidence of pituitary disease. The TRH 
test does not add to the diagnosis of central hypothyroidism or 
the sick-euthyroid state.509 In a stimulation test, TRH (200–500 
μg) is administered intravenously, and TSH levels are measured 
at –15, 0, 15, 30, 60, and 1

 levels are measured 
at –15, 0, 15, 30, 60, and 120 minutes. In euthyroid subjects, 
  Factors Affecting the Secretion of TSH
Effect
Factors
TRH
Opioids
Dopamine receptor antagonists
Leptin
Somatostatin analogues
Dopamine agonists
Proinflammatory cytokines
TABLE 8.12
228
peak TSH levels (up to 22-fold higher than basal) are observed 
after 30 minutes.507 Because feedback suppression by elevated 
thy

.507 Because feedback suppression by elevated 
thyroid hormone levels on TSH overrides stimulation by the 
hypothalamus, basal TSH levels are undetectable and do rise to 
TRH in hyperthyroidism. The TRH test has a useful role in dif­
ferentiating hyperthyroidism due to a TSH-secreting adenoma 
and elevated thyroid hormone levels due to thyroid hormone 
resistance. The TSH levels do not increase in response to TRH 
in most cases of TSH adenomas, but they do in thyroid hor­
mone resistance.509 
TS

ut they do in thyroid hor­
TSH Deficiency
Causes
Congenital isolated TSH deficiency may arise from mutational 
defects of the TSH and TRH receptor genes. Genetic disorders 
of pituitary gland development involved in cell differentiation 
give rise to TSH deficiency as a component of multiple pituitary 
hormone deficiencies. The developmental genes involved include 
LHX3, PROP1, and POU1F1 (see Table 8.2). Pituitary damage 
may result in functional TSH deficiency. 
Manifestat

y result in functional TSH deficiency. 
The consequences of TSH deficiency are those of thyroid hor­
mone deficiency, which causes childhood mental and growth 
retardation; in adults, it results in clinical features similar to pri­
mary hypothyroidism. 
Treatment
l-Thyroxine is used for replacement therapy, in doses similar to 
those required for treating primary hypothyroidism ranging from 
50 to 200 μg/day. Thyroxine is converted peripherally into the 
active T3 and has a 7-day 

 peripherally into the 
active T3 and has a 7-day half-life with stable blood levels. There 
is strong evidence that patients with central hypothyroidism are 
frequently undertreated; it has been recommended that the dose 
be titrated to achieve midnormal free T4 levels.510 TSH cannot be 
used to guide dose titration in patients with secondary hypothy­
roidism because the damaged thyrotroph is unlikely to adequately 
reflect appropriate feedback suppression. As GH stimulates the 
conversion of T

suppression. As GH stimulates the 
conversion of T4 to T3, blood T4 levels may fall into the subnormal 
range, unmasking central hypothyroidism during GH therapy for 
GH deficiency.277,278 The principles of thyroid hormone replace­
ment apply in this situation and T4 supplementation is initiated. 
As many women with pituitary failure also likely receive estrogen 
replacement, free instead of total T4 level is measured to avoid the 
confounding effects of increased TBG levels. Thyroid hormone 
re

fects of increased TBG levels. Thyroid hormone 
replacement accelerates cortisol metabolism and requirements and 
may therefore exacerbate primary hypoadrenalism or precipitate 
adrenal crisis in patients with coexisting perturbed adrenal func­
tion. Therefore, in pituitary patients suspected of having ACTH 
deficiency, thyroid hormone replacement should only be initiated 
after adrenal status has been evaluated and treated. 
TSH Excess
Other than primary hypothyroidism, TSH excess can rarely 
a

n primary hypothyroidism, TSH excess can rarely 
arise from TSH-secreting pituitary adenomas, thyroid hor­
mone resistance, and TSH receptor resistance. The biochemi­
cal profiles of TSH adenoma and thyroid hormone resistance 
are similar, both conditions manifesting high TSH and thyroid 
hormone levels; however, the two are usually distinguished 
from familial history, imaging, clinical examination, and TRH 
testing.508 Thyroid hormone resistance can arise from muta­
tions of TRα and TRβ, with 

e can arise from muta­
tions of TRα and TRβ, with the latter being more common. 
TSH receptor mutations were first described in 1995 as a cause 
of congenital hypothyroidism.511 However, mutations causing 
partial resistance may manifest as nonautoimmune subclinical 
hypothyroidism.512
 Developmental, Genetic, and Acquired 
Causes of Pituitary Failure
Developmental Disorders
Congenital pituitary gland absence (aplasia), partial hypopla­
sia, or ectopic tissue rudiments are rarely encountered. Pi

ctopic tissue rudiments are rarely encountered. Pituitary 
development follows midline cell migration from Rathke pouch, 
and impaired midline anomalies, including failed forebrain 
cleavage and anterior commissure and corpus collosum defects, 
lead to structural pituitary anomalies. Craniofacial develop­
mental anomalies, including anencephaly, result in cleft lip 
and palate, basal encephalocele, hypertelorism, and optic nerve 
hypoplasia with varying degrees of pituitary dysplasia and apla­
s

varying degrees of pituitary dysplasia and apla­
sia. If these infants survive, lifelong appropriate pituitary hor­
mone replacement is required. With sensitive MRI techniques 
for pituitary visualization, several anatomic features characteris­
tic of hypopituitarism are now apparent. Evidence for acquired 
pituitary damage or destruction is often clearly visible on MRI, 
and patients presenting with hypopituitarism of undetermined 
cause may exhibit decreased gland volume, partial or complete 

ibit decreased gland volume, partial or complete 
empty sella, disturbed sella turcica architecture, absent or tran­
sected pituitary stalk, and an absent or ectopic posterior pitu­
itary bright intensity signal.513 Lesions of the pituitary stalk can 
arise from congenital maldevelopment causing stalk interruption 
(see later) or from acquired diseases involving the infundibu­
lum. Among 92 patients with pituitary stalk lesions, 32% were 
found incidentally. About 15% were due to congenital caus

ncidentally. About 15% were due to congenital causes 
(ectopic posterior pituitary, Rathke cyst), 33% to inflammatory 
causes (sarcoidosis, histiocytosis, hypophysitis), and over 50% 
to neoplastic causes (craniopharyngioma, pituitary adenoma, 
metastatic disease).514 An absent infundibulum noted on MRI 
is associated with pituitary hormone deficits, and approximately 
40% of patients with GHD of unclear cause show imaging evi­
dence of mild stalk defects, reflecting a midline developmental 
ano

k defects, reflecting a midline developmental 
anomaly.
Congenital basal encephalocele may result in the pituitary her­
niating through the sphenoid sinus roof, resulting in pituitary fail­
ure and diabetes insipidus. 
Heritable Disorders
Mutations at each level of pituitary function, including hormones, 
receptors, and transcription factors that determine anterior pitu­
itary development, may lead to pituitary deficiency syndromes515 
(see Table 8.1). Furthermore, mutations in specific pituitar

 8.1). Furthermore, mutations in specific pituitary genes, 
including those for GH, POMC, TSH, LH, and FSH, all lead to 
single hormone deficiencies. Patients heretofore diagnosed with 
idiopathic isolated or polyhormonal pituitary failure may in fact 
harbor a mutation, and as the transcriptional control of pituitary 
development is clarified, increasing numbers of mutant genes have 
become apparent (see Table 8.2).
HESX1, SOX2, SOX3, and OTX2
HESX1 (also known as RPX), one of the earliest tran

ESX1 (also known as RPX), one of the earliest transcriptional 
markers of the primitive pituitary, with expression restricted 
229
to Rathke pouch, is a paired-like homeodomain transcription 
factor that acts as a transcriptional repressor.516 Coinciden­
tally, with appearance of specific pituitary cell types, HESX1 
expression declines and is extinguished in the mature anterior 
pituitary,517 leading to PROP1 activation. The heterogeneo

y,517 leading to PROP1 activation. The heterogeneous 
syndrome of septo-optic dysplasia (hypoplastic optic nerves, 
absent corpus callosum and septum pellucidum, and hypopi­
tuitarism) is associated with mutations in HESX1.518 Hypo­
pituitarism ranges from panhypopituitarism to isolated GHD. 
Although the mutant molecule exhibits reduced DNA binding, 
panhypopituitarism may also occur secondary to profound ana­
tomic defects in midline development. Mutations in HESX1 
have also been associated w

t. Mutations in HESX1 
have also been associated with pituitary stalk interruption syn­
drome (PSIS), a congenital defect of the pituitary gland charac­
terized by a very thin, interrupted pituitary stalk, an ectopic or 
absent posterior pituitary gland, and hypoplasia or aplasia of the 
anterior pituitary gland, visible on MRI.519 Mutated SOX2, a 
member of the SRY-related high mobility group box (Sox) genes, 
is also associated with septo-optic dysplasia, anophthalmia, or 
microphthalmia, and 

 dysplasia, anophthalmia, or 
microphthalmia, and anterior pituitary hypoplasia, frequently 
associated with HH and GHD, and other forebrain defects.520 
SOX3 mutations are associated with X-linked hypopituitarism 
and mental retardation. Affected males have GHD and may 
also have deficient gonadotropins, TSH, or ACTH. Both SOX3 
duplications and loss of function mutations show similar pheno­
types, suggesting that SOX3 dosage is critical for normal pitu­
itary development.521,522
Mutations in O

mal pitu­
Mutations in OTX2 have also been implicated in anophthal­
mia/microphthalmia syndromes in humans. These mutations are 
associated with severe ocular and neurologic phenotypes, includ­
ing developmental delay and seizures. Panhypopituitarism occurs, 
perhaps as a result of failure to activate transcription of HESX1 
and POU1F1.522 
LHX3 and LHX4
LHX3 is expressed early during anterior pituitary development, 
with strong uniform expression within Rathke pouch. E

h strong uniform expression within Rathke pouch. Expression 
persists into adulthood. Missense and deletion mutations of LHX3, 
a LIM-type homeodomain transcription factor essential for pituitary 
development, are associated with failure of pituitary gland morpho­
genesis with reduced numbers of all cell types and multiple anterior 
pituitary hormone deficits, affecting all axes except for largely intact 
ACTH reserve.436 Most missense mutations identified in patients 
have diminished capacity t

identified in patients 
have diminished capacity to activate transcription of the promoters 
of several potential LHX3 target genes, including those encoding 
αGSU, PRL, FSHβ, TSHβ, and POU1F1.523 These patients also 
exhibit defective neck rotation ability due to a rigid cervical spine 
and variable sensorineural hearing loss.524
LHX4 is closely related to LHX3 and is also expressed through­
out the invaginating Rathke pouch, but unlike LHX3, its expres­
sion is transient and not maintained in 

s expres­
sion is transient and not maintained in the adult pituitary. Patients 
with LHX4 mutations exhibit GHD and associated short stature, 
with variable additional endocrine deficits, particularly TSH and 
ACTH deficiency, and extrapituitary abnormalities. LHX4 muta­
tions associated with anterior pituitary hypoplasia, an ectopic pos­
terior pituitary, and with PSIS are unable to activate both PROP1 
and POU1F1, and these result in pituitary failure.522,525 
PITX1 and PITX2
PITX1 and PITX2,

 failure.522,525 
PITX1 and PITX2, members of the class of bicoid homeodomain 
proteins, show a high degree of homology and are expressed in 
an overlapping pattern during pituitary development. PITX1 is 
expressed in all five anterior pituitary lineages in both the fetal and 
adult pituitary gland and is able to activate the expression of all six 
of the major anterior pituitary hormones, including LH and FSH, 
frequently acting in synergy with other pituitary transcription fac­

in synergy with other pituitary transcription fac­
tors. Mutations in PITX2 cause Rieger syndrome, characterized 
by defects in the eyes and teeth and a protuberant umbilicus as 
well as pituitary hormone deficiencies.27,28,526 
PROP1
Mutations in PROP1 are the most common genetic cause of 
combined pituitary hormone deficiency (CPHD) (Fig. 8.37).527 
The role of this gene was first uncovered from studies of the Ames 
dwarf mouse, which harbors a missense PROP1 mutation and 
exhibits a hypoplast

 missense PROP1 mutation and 
exhibits a hypoplastic pituitary gland with combined GH, PRL, 
and TSH deficiency. This mutation abrogates POU1F1 activa­
tion and results in failed development of POU1F1-dependent 
cell lineages.528 Similarly, human PROP1 mutations are asso­
ciated with deficiencies in POU1F1-dependent lineages (GH, 
PRL, and TSH). Impaired FSH and LH secretion, associated 
with delayed or absent puberty, HH, and infertility in females 
and in some males, is also frequently present

les 
and in some males, is also frequently present.529 ACTH defi­
ciency can also occur, frequently with a later onset, suggesting 
a role in maintenance of corticotroph function and emphasiz­
ing the necessity for complete and continued clinical assessment 
of patients with PROP1 mutations.263 Inheritance modes of 
PROP1 mutations usually reflect autosomal recessive patterns. 
Thus patients are usually homozygous for either deletion or mis­
sense frameshift mutations, leading to truncated PROP1

e frameshift mutations, leading to truncated PROP1 pro­
tein products devoid of functional activity.
The clinical spectra of CPHD associated with PROP1 mutations 
are variable and temporal. The phenotype varies with both the type 
of mutation and the age of the patient.522 The onset of clinically 
evident pituitary failure is usually first manifest with slowing of lin­
ear growth (GHD, ∼80%), then thyroid failure (TSH deficiency, 
∼20%), followed by hypogonadism, and later subclinical or overt 

 by hypogonadism, and later subclinical or overt 
adrenal insufficiency.530 The pituitary gland size is usually small or 
normal. Combined hypothalamic hormone stimulation (GnRH, 
TRH, CRH, and GHRH) or insulin-evoked hypoglycemia reveals 
blunted responses consistent with varying degrees of pituitary hor­
mone deficiencies. Serum IGF1 and IGFBP3 levels are usually low, 
but peripheral thyroid hormone levels are low or at the lower limits 
of normal ranges. In the face of low or absent TSH respo

mal ranges. In the face of low or absent TSH responses, these 
findings are consistent with secondary hypothyroidism. Most older 
patients also exhibit blunted cortisol responses to CRH and ACTH 
or insulin stimulation.529 
POU1F1
The POU1F1 gene encodes a POU homeobox protein, PIT1, which 
activates transcription of the GH, PRL, TSHβ, and GHRH recep­
tor genes.531 PIT1 also partners with coactivators, including thy­
roid hormone, estrogen, and retinoic acid receptors, as well as other 
transcri

etinoic acid receptors, as well as other 
transcription factors, including CREB, LHX3, PITX1, HESX1, 
and ZN15. PIT1 autoregulates its own expression. Because of the 
absolute requirement of PIT1 for somatotroph, lactotroph, and 
thyrotroph development and specific gene expression, inactivating 
mutations of the gene result in a spectrum of pituitary hormone 
deficiencies.532 The Snell and Jackson dwarf mouse strains both 
harbor POU1F1 gene mutations.533 Both autosomal recessive (loss 
of funct

tions.533 Both autosomal recessive (loss 
of function) and autosomal dominant (dominant negative action) 
POU1F1 mutations have been identified.
Some POU1F1 mutations exhibit characteristic clinical phe­
notypes, depending on the spectrum of loss of DNA binding, 
230
transcriptional activation, or interaction with partner proteins 
affected.320,534 Both CBP/p300 protein recruitment as well as 
PIT1 dimerization are required for appropriate PIT1 activation

ation are required for appropriate PIT1 activation of 
target hormone genes.535 LHX4 appears to activate POU1F1, and 
mutations of LHX4 also lead to growth retardation.525 Interest­
ingly, adult-onset combined GH, PRL, and TSH deficiencies have 
also been reported in association with circulating autoantibodies 
directed against the PIT1 protein.536 
IGSF1
Loss-of-function mutations have been identified in IGSF1 in asso­
ciation with X-linked congenital central hypothyroidism, often in 
associati

enital central hypothyroidism, often in 
association with variable PRL deficiency and GHD and with tes­
ticular enlargement.167 IGSF1 is a membrane glycoprotein highly 
expressed in the anterior pituitary gland, and the patients with 
mutations appear to have impaired pituitary TRH signaling. 
TBX19
TBX19 (also referred to as TPIT) mutations result in early-onset 
isolated ACTH deficiency and hypocortisolism38 (see Table 8.2). 
Associated phenotypes, including those for POMC deficiency, 
may inc

pes, including those for POMC deficiency, 
may include obesity, red hair pigmentation, and other associated 
pituitary deficiencies. Patients are homozygous or compound het­
erozygous for TBX19 mutations, indicating an autosomal reces­
sive mode of inheritance. Mutations in this gene appear to be the 
principal molecular cause of congenital neonatal isolated ACTH 
deficiency.537 Interestingly, among 22 patients with isolated 
ACTH deficiency, diagnosed between ages 5 and 15 years, and 
no identi

gnosed between ages 5 and 15 years, and 
no identified mutation of TBX19, three had common variable 
immunodeficiency (CVID), characterized by defective immuno­
globulin production and recurrent infections, diagnosed at age 2 
to 8 years. This has led to the proposal of a new syndrome link­
ing these two rare disorders: deficient anterior pituitary function 
and variable immune deficiency (DAVID).538 NFKB2, encoding 
a subunit of the transcription factor complex of nuclear factor-
kappa-B, has r

n factor complex of nuclear factor-
kappa-B, has recently been implicated as the transcription factor 
responsible for this syndrome.539 
NR5A1 and NR0B1
NR5A1 encodes SF1, a member of the nuclear receptor fam­
ily expressed throughout the reproductive axis (hypothalamus, 
pituitary, and gonads) and in the adrenal gland. It is a key 
transcriptional regulator of many genes involved in sexual dif­
ferentiation, steroidogenesis, and reproduction, including the 
pituitary genes encoding αGSU, LHβ, 

ncluding the 
pituitary genes encoding αGSU, LHβ, FSHβ, and GnRHR. 
Patients with mutations in SF1 have been described with vary­
ing degrees of XY sex reversal, testicular dysgenesis, ovarian 
insufficiency, adrenal failure, and impaired pubertal maturation 
with HH.61,540,541
NR0B1 encodes DAX1 (dosage-sensitive sex-reversal adrenal 
hypoplasia critical region on the X chromosome protein 1), a 
nuclear receptor transcription factor related to SF1, with a simi­
lar distribution pattern of expre

F1, with a simi­
lar distribution pattern of expression. Mutations in NR0B1 are 
associated with X-linked HH and adrenal hypoplasia congenita,31 
with the majority of mutations causing truncations or frameshifts 
rendering the protein nonfunctional. HH is often mild, reveal­
ing itself as failure to undergo puberty or as incomplete puberty. 
The hypogonadism appears to be due to variable and combined 
hypothalamic and pituitary dysfunction. DAX1 is a transcrip­
tional repressor and has been show

is a transcrip­
tional repressor and has been shown to inhibit SF1-mediated 
transcription of an array of target genes, including LHβ. How 
the loss of function in these two opposing genes, SF1 and DAX1, 
results in similar phenotypes is still not well understood. The HH 
is likely due to a developmental defect of the hypothalamus and 
pituitary, suggesting a role for DAX1 in proper development of 
these organs.542 
TSH + ACTH +
GHD + FSH/LHD
32 patients
7 PROP1 mutations
ACTH +
FSH/LH + GHD
5 p

patients
ACTH +
5 patients
2 PROP1 mutations
TSH +
22 patients
8 PROP1 mutations
TSH +
ACTH + GHD
8 patients (8 ppa)
GH + TSHD
17 patients (5 ppa)
1 POU1F1 mutations
14 patients
1 PROP1 mutation
GHD + ACTHD
1 patient (1 ppa)
GHD
5 patients (5 ppa)
Non-GHD
6 patients
TSH + ACTH: 1 (1 ppa)
TSH + ACTH +
FSH/LH: 2
TSH + FSH/LH: 1
ACTH + FSH/LH: 2
• Fig. 8.37  Pituitary hormone deficiencies repartition in 110 unrelated patien

e deficiencies repartition in 110 unrelated patients affected by congenital 
pituitary deficiency without stalk pituitary interruption or septo-optic dysplasia. Patients were studied for 
PROP1, POUF1, or LHX3 according to hormonal deficit phenotype. Twenty mutations of PROP1 and 
one mutation of POU1F1 were found. Gonadotroph function was unavailable for prepubertal age (ppa) 
patients. ACTH, adrenocorticotropic hormone; ACTHD, ACTH deficiency; FSH, follicle-stimulating hor­
mone; GH, growth ho

FSH, follicle-stimulating hor­
mone; GH, growth hormone; GHD, growth hormone deficiency; LH, luteinizing hormone; LHD, luteinizing 
hormone deficiency; TSH, thyroid-stimulating hormone; TSHD, thyroid-stimulating hormone deficiency. 
(From Reynaud R, Gueydan M, Saveanu A, et al. Genetic screening of combined pituitary hormone defi­
ciency: experience in 195 patients. J Clin Endocrinol Metab. 2006;91:3326–3329.)
231
Pituitary Stalk Interru

and  Diagnostic Evaluation
Pituitary Stalk Interruption Syndrome
PSIS is a congenital defect of the pituitary gland characterized by a thin 
or interrupted pituitary stalk, anterior pituitary hypoplasia, and an 
ectopic posterior pituitary. Patients may present with an isolated pitu­
itary hormone deficiency or with combined hypothalamic-pituitary 
hormone deficiencies. Accompanying midline defects and eye abnor­
malities suggest involvement of developmental processes. Mutations 
or single nucle

evelopmental processes. Mutations 
or single nucleotide variants in HESX1, LHX4, OTX2, SOX3, and 
PROKR2 have been associated with PSIS. A homozygous missense 
mutation was identified in a family with PSIS in GPR161, an orphan 
G protein–coupled receptor expressed in the hypothalamus and pitu­
itary and implicated in the SHH signaling pathway.543 A homozygous 
frameshift mutation in ARNT2, or aryl hydrocarbon receptor nuclear 
translocator 2, a helix-loop-helix transcription factor, has also bee

elix-loop-helix transcription factor, has also been 
associated with anterior pituitary hypoplasia and pituitary stalk inter­
ruption, together with other CNS defects.544 Mutations have been 
identified in less than 5% of patients with PSIS, suggesting that there 
is much more to be learned about the pathogenesis of this disorder.
Pituitary deficiencies have also been reported recently in 
patients with other syndromic disorders, including patients with 
CHARGE syndrome and CHD7 mutations; patie

ts with 
CHARGE syndrome and CHD7 mutations; patients with PAX6 
mutations, a well-known regulator of eye development; patients 
with septo-optic dysplasia or Kallmann syndrome and PROKR2, 
FGF8, and FGFR1 variants; and patients with holoprosencephaly 
and variants in SHH, GLI2, or TGIF.539 
Acquired Disorders
In the absence of demonstrable hypothalamic-pituitary anatomic 
damage, and after excluding genetic and syndromic causes of pitu­
itary insufficiencies, acquired, often transient, causes o

sufficiencies, acquired, often transient, causes of pituitary 
failure should be considered (see Table 8.5). Causes of pituitary 
insufficiency, including pituitary tumors, parasellar masses, hypoph­
ysitis, aneurysms, and pituitary apoplexy, were discussed earlier. 
Hypothalamic damage reflected by the presence of a large parasel­
lar mass, leading to decreased GnRH production, results in muscle 
wasting, obesity, and central hypogonadism with low levels of FSH 
and LH (Fig. 8.38). Marked calor

ow levels of FSH 
and LH (Fig. 8.38). Marked caloric restriction, anorexia,545 weight 
loss of other causes, and strenuous exercise may attenuate GnRH 
secretion and action. HH may occur in both men and women 
A
B
C
• Fig. 8.38  Features of hypopituitarism and hypogonadism, including central adiposity, proximal muscle 
wasting, loss of body hair, and gynecomastia. Note the contrast between the hypogonadal patient (A, right 
side) and his unaffected identical twin (left side). Laboratory tests co

ed identical twin (left side). Laboratory tests confirmed secondary hypogonadism, 
with testosterone level of 0.4 ng/mL (normal range, 2.9–8.0 ng/mL), follicle-stimulating hormone level of 
2.8 IU/L (normal range, 1.5–12.4 IU/L), and luteinizing hormone level of 1.5 IU/L (normal range, 1.7–8.6 
IU/L). Magnetic resonance imaging showed a lobulated, contrast-enhancing suprasellar mass (coronal 
view in B [arrow] and sagittal view in C [arrow]). Pathologic analysis confirmed the diagnosis of pituic

hologic analysis confirmed the diagnosis of pituicytoma. 
(From Newnham HH, Rivera-Woll LM. Images in clinical medicine: hypogonadism due to pituicytoma in an 
identical twin. N Engl J Med. 2008;359:2824.)
232
(see Chapters 17 and 19). Exogenous anabolic steroid therapy and 
glucocorticoid therapy suppress the reproductive and adrenal axes, 
respectively. Patients with severe critical illnesses or chronic debili­
tating disease (including cirrhosis) may h

debili­
tating disease (including cirrhosis) may have impaired GH/IGF1, 
adrenal, and gonadal axes. Hypothyroidism, hypoadrenalism, or 
hypogonadism cause hyperplasia of specific trophic cells due to lack 
of negative feedback and sometimes actual pituitary tumor forma­
tion.546 Acquired immunodeficiency syndrome (AIDS) is associ­
ated with suppressed pituitary function that is independent of other 
associated infections.547 Drugs such as estrogens, which suppress 
FSH and LH, and GnRH analogues

ns, which suppress 
FSH and LH, and GnRH analogues used for treating prostate can­
cer inhibit gonadotropin action. In addition to pituitary apoplexy, 
other vascular accidents such as aneurysms, strokes, cavernous sinus 
thrombosis, and arteritis can cause pituitary hormone insufficiency. 
Isolated hormone pituitary hormone deficiencies may also occur as 
a manifestation of vascular abnormalities, including arteritis or sub­
arachnoid hemorrhage (Table 8.13).
Head Trauma
The pituitary may be pa

 (Table 8.13).
The pituitary may be partially or totally damaged by birth trauma, 
cranial hemorrhage, fetal asphyxia, or breech delivery. Head trauma 
may lead to direct pituitary damage by a sella turcica fracture, pitu­
itary stalk section, trauma-induced vasospasm, or ischemic infarc­
tion following blunt trauma.548 The most common traumatic cause 
of compromised pituitary function in the adult is iatrogenic neu­
rosurgical trauma. Pituitary manipulation or damage during sur­
ger

. Pituitary manipulation or damage during sur­
gery leads to transient or permanent diabetes insipidus and varying 
degrees of anterior pituitary dysfunction. Hypopituitarism follow­
ing head trauma usually manifests within a year after the insult. 
Seventy-five percent of patients with posttraumatic pituitary failure 
are young men under age 40 years involved in a motor vehicle acci­
dent. Virtually all patients with subsequent pituitary failure have 
a history of loss of consciousness followin

 have 
a history of loss of consciousness following trauma, and half of 
all such patients have documented skull fracture.548 One-third of 
these patients have demonstrable signs of hypothalamic or posterior 
pituitary hemorrhage or anterior lobe infarction on MRI. Diabetes 
insipidus is the most common endocrine disorder, encountered in 
about 30% of these patients.549 As pituitary function can recover 
after trauma, the prevalence and extent of dysfunction vary and 
depend on the time of evalu

 dysfunction vary and 
depend on the time of evaluation. A meta-analysis of more than 700 
adult patients in 2007 reported that up to 35% of subjects acquired 
some impairment to pituitary function evaluated at least 5 months 
after major TBI.550 GHD was found in 11%, gonadotropin defi­
ciency in 13%, ACTH deficiency in 11%, TSH deficiency in 6%, 
and multiple deficits in 9%. There are sparse data of the pituitary 
consequences of TBI in children. A large prospective French study 
of 87 children

. A large prospective French study 
of 87 children, mean age of 6.7 years, reported a prevalence of 7% 
for GHD, 2% for thyroid, and 1% adrenal insufficiency evaluated 
5 months after TBI.551
Klose and associates have questioned the accuracy of the fre­
quency of adult GHD from previous studies because of methodo­
logic issues arising from the use of different diagnostic tests, assays, 
and criteria based on published guidelines.552 Using normative data 
drawn from over 100 healthy control subje

ve data 
drawn from over 100 healthy control subjects, the authors observed 
that the prevalence of GHD among 439 patients with TBI was 
highly dependent on choice of local or guideline-derived cutoffs, 
on diagnostic tests, and on the use of either single or confirma­
tory testing. They reported a prevalence of GHD of 4% with the 
ITT and 12% with the combined pyridostigmine-GHRH test and 
of isolated GHD in just 1% confirmed from these two tests. The 
nonspecific symptomology of hypopituitaris

s. The 
nonspecific symptomology of hypopituitarism and the usually pro­
found and variable CNS consequences of mixed brain and physi­
cal injuries pose a major challenge to the identification of pituitary 
dysfunction and its management. The level of evidence supporting 
who and when to investigate is low because endocrinopathy is one 
component of a much larger complex clinical problem. In the acute 
situation, assessment should focus on the HPA axis because of the 
vital role glucocorticoids 

A axis because of the 
vital role glucocorticoids play in the stress response. As pituitary 
impairment may recover, there is merit in deferring the assessment 
of the other axes to a time when the vital systems are stable. 
Radiation
The progression and degree of hypopituitarism are dose and 
time dependent with pituitary exposure. Susceptibility is also 
  Hypopituitarism After Subarachnoid Hemorrhage
Reference
Number of 
Patients
Any Degree of 
Hypopituitarism (n)
Multiple 
Deficiencies (n)

f 
Multiple 
GH (n)
LH/FSH (n)
ACTH (n)
TSH (n)
Remarks
Kelly et al, 
2000578
2
2
0
2
0
0
0
Brandt et al, 
2004579
10
5
0
1
4
0
0
Aimaretti et al, 
2004580
40
15
4
10
5
1
3
No stimulation test 
for ACTH
Kreitschmann-
Andermahr 
et al, 2004581
40
22
3
8
0
16
1
Dimopoulou et al, 
2004582
30
14
4
11
4
3
2
No stimulation 
test for GH (11 
patients low 
IGF1)
Total number (%)
122 (100)
58 (48)
11 (9)
32 (26)
13 (11)
20 (16)
6 (5)
ACTH, Adrenocorticotropic hormone;

)
20 (16)
6 (5)
ACTH, Adrenocorticotropic hormone; FSH, follicle-stimulating hormone; GH, growth hormone; IGF1, insulin-like growth factor 1; LH, luteinizing hormone; TSH, thyroid-stimulating hormone.
Modified from Schneider H, Aimaretti G, Kreitschmann-Andermahr I, et al. Hypopituitarism. Lancet. 2007;369:1461–1470.
   
TABLE 8.13
233
dependent on the age of irradiation.553,554 The GH axis is the 
most radiosensitive, followed by the go

is is the 
most radiosensitive, followed by the gonadotropin, adrenocorti­
cotropic hormone (ACTH), and TSH axes555,556 (Fig. 8.39). GH 
deficiency can develop with doses as low as 18 Gy with higher 
doses (30–50 Gy) that can reach 50% to 100% within 3 to 5 
years. Gonadotrophin deficiency occurs with doses in excess of 30 
Gy; however, lower doses in children cause precocious puberty. 
Pituitary irradiation induces hyperprolactinemia, a result of hypo­
thalamic damage. This develops with exposu

f hypo­
thalamic damage. This develops with exposure to 40 Gy or higher 
and occurs in both sexes in all age groups. Table 8.14 summarizes 
the conditions for which radiation is given, the exposure to the 
pituitary gland, and the impact on pituitary function.557 Pituitary 
irradiation indicated for pituitary adenoma therapy will directly 
cause atrophy of the gland, in addition to damaging hypotha­
lamic neurons.554 Early anterior-pituitary dysfunction seems less 
likely after stereotactic radi

unction seems less 
likely after stereotactic radiosurgery for pituitary adenomas than 
after fractionated radiotherapy; however, the long-term, cumu­
lative incidence of anterior-pituitary dysfunction is similar.553 
Previously irradiated patients should therefore undergo lifelong 
periodic anterior pituitary hormone testing to unmask incipient 
pituitary failure prior to onset of morbidity.558 
Empty Sella Syndrome
Damage to the sellar diaphragm may lead to arachnoid hernia­
tion into the sell

m may lead to arachnoid hernia­
tion into the sellar space. An empty sella may develop as a con­
sequence of a primary congenital weakness of the diaphragm 
in those patients in whom no secondary cause is evident. Up 
to 50% of patients with primary empty sella have associated 
benign intracranial hypertension.559 A secondary empty sella may 
develop subsequent to infarction of a pituitary adenoma or surgi­
cal or radiation-induced damage to the sellar diaphragm. These 
patients usually exhibit 

sellar diaphragm. These 
patients usually exhibit demonstrable pituitary tissue compressed 
against the sellar floor, with lateral stalk deviation visible on MRI. 
Although an empty sella is usually an incidental finding, if more 
than 90% of pituitary tissue is compressed or atrophied, pituitary 
failure usually occurs. About 10% of patients may develop small 
GH-secreting or PRL-secreting adenomas within the narrow rim 
of compressed pituitary tissue. 
Clinical Features of Hypopituitarism
Pati

tissue. 
Patients with pituitary failure, regardless of cause, have exces­
sive mortality rates, primarily due to vascular disease.256 Age 
at diagnosis, female gender, and history of craniopharyngi­
oma were the most striking determinants of increased fatal­
ity. The spectrum of clinical features of pituitary insufficiency 
depends on several factors. In acquired pituitary insufficiency 
the clinical spectrum depends upon the degree of hormone 
deficiency, t

 depends upon the degree of hormone 
deficiency, the number of hormones impaired, and the rapid­
ity of onset. In congenital forms, the earlier the age of onset, 
the greater the severity of thyroid, gonadal, adrenal, growth, 
TSH
LH/FSH
ACTH
GH
1.0
0.5
0
Probability of normal axis
Years
10
8
6
4
2
0
• Fig. 8.39  Life-table analysis indicating probabilities of initially normal 
hypothalamic-pituitary-target gland axes remaining normal after radio­
therapy (3750–4250 cGy). Growth hormone (GH) sec

­
therapy (3750–4250 cGy). Growth hormone (GH) secretion is the most 
sensitive of the anterior pituitary hormones to the effects of external 
radiotherapy, and thyroid-stimulating hormone (TSH) secretion is the 
most resistant. In two-thirds of patients, gonadotropin deficiency devel­
ops before adrenocorticotropic hormone (ACTH) deficiency. The reverse 
occurs in the remaining third. FSH, follicle-stimulating hormone, LH, lutein­
izing hormone. (From Littley MD, Shalet SM, Beardwell CG, et al.

 (From Littley MD, Shalet SM, Beardwell CG, et al. Hypo­
pituitarism following external radiotherapy for pituitary tumors in adults. 
Q J Med. 1989;70:145–160.)
  Pituitary Dysfunction After Cranial Irradiation in Various Conditions
Condition Treated
Schedule
Dose (Gy)
Pituitary Dysfunction
Leukemia and lymphoma
Fractionated TBI
Fractionated cranial
7–16
18–24
Isolated GHD, mostly pubertal children
Precocious puberty girls only
Nonpituitary brain tumors
Co

us puberty girls only
Conventional fractionated cranial
30–50
GHD (30–100%)
Gonadotrophin, TSH, ACTH, Prl (3–20%)
Precocious puberty both sexes
Nasopharyngeal carcinoma and 
skull-base tumors
50–70
GHD (100% within 5 yr)
Gonadotrophin, TSH, ACTH, Prl (20–50%)
Pituitary tumors
30–50
Gonadotrophin, TSH, ACTH, Prl (20–60%)
Prl, Prolactin; TBI, Total body irradiation.
Modified from Da

tin; TBI, Total body irradiation.
Modified from Darzy KH. Radiation-induced hypopituitarism after cancer therapy: who, how and when to test. Nat Clin Pract Endocrinol Metab. 2009;5:88–99.
   
TABLE 8.14
234
or water disturbances. Heritable genetic disorders invari­
ably exhibit the most severe phenotypic changes, although 
later changes may also occur in these disorders, as seen with 
PROP1 mutations. The resilience of the individual pituitary 
cell linea

resilience of the individual pituitary 
cell lineages to compressive, inflammatory, vascular, radia­
tion, and invasive insults also differs. The lactotroph cell is 
often hyperfunctional as a result of decreased tonic inhibi­
tory signals. PRL deficiency is thus exceedingly rare, except 
for complete pituitary destruction or genetic syndromes. The 
order of diminished trophic hormone reserve function by 
pituitary compression usually is as follows: GH > FSH > LH 
> TSH > ACTH. The corticotroph 

ws: GH > FSH > LH 
> TSH > ACTH. The corticotroph cell appears particularly 
resistant to hypothalamic or pituitary destruction and is usu­
ally the last cell to lose function. The qualitative phenotypic 
manifestations of pituitary failure are determined by which 
specific trophic hormones are lost (see earlier for description 
of individual hormone deficiencies). 
Screening for Pituitary Failure
As the onset of hypopituitarism may be extremely slow, subclini­
cal pituitary failure is often not

slow, subclini­
cal pituitary failure is often not apparent to the patient or physi­
cian. Screening for pituitary dysfunction should be undertaken in 
patients with hypothalamic or pituitary mass lesions, developmen­
tal craniofacial abnormalities, inflammatory disorders, brain gran­
ulomatous disease, prior head or neck irradiation, head trauma, 
prior skull base surgery, those with newly discovered empty sella, 
and those who have previously experienced pregnancy-associated 
hemorrhage or blo

xperienced pregnancy-associated 
hemorrhage or blood pressure changes.560
As hypopituitarism may develop insidiously and is often 
not readily clinically apparent, screening of appropriate 
patients is important to prevent long-term morbidity. There­
fore all patients harboring hypothalamic or pituitary masses 
should be screened for hypopituitarism (Table 8.15). PRL 
should be measured because many patients with hypopituita­
rism may also present with secondary hyperprolactinemia. Up 
  Assess

t with secondary hyperprolactinemia. Up 
  Assessment of Anterior Pituitary Function
Test
Dose
Normal Response
Side Effects
ACTH
Insulin tolerance
0.1–0.15 U/kg IV
Peak cortisol response >18 μg/dL, or 
increase by 7 μg/dL
Sweating, palpitation, tremor
Metyrapone
Oral administration of 30 mg/kg at 
11 pm
Peak 11-DOC ≥7 μg/dL
Peak cortisol ≤7 μg/dL
Peak ACTH >75 pg/mL
Nausea, insomnia, adrenal crisis
CRH stimulation
100 μg IV
Peak ACTH ≥ twofold to fourfold
Peak cortisol ≥20 μg/dL or ↑ ≥7 μg/dL
F

o fourfold
Flushing
ACTH stimulation
250 μg IV or IM or 1 μg IV
Peak cortisol ≥20 μg/dL
Rare
TSH
Serum T4 (free T4)
Total T3
TSH—third generation
TRH stimulation
200–500 μg IV
Peak TSH ≥2.5-fold or ↑ ≥5–6 mU/L 
(females), ↑ ≥2–3 mU/L (males)
Flushing, nausea, urge to micturate
PRL
Serum PRL
PRL ≥2.5-fold
LH/FSH
Serum LH and FSH
Serum testosterone
GnRH stimulation
100 μg IV
Elevated in menopaus

ne
100 μg IV
Elevated in menopause and in men with 
primary testicular failure
300–900 ng/mL (age-adjusted normal 
ranges)
LH ≥ twofold to threefold, or by 10 IU/L
FSH 1.5–2-fold, or by 2 IU/L
Rare
GH
0.1–0.15 U/kg
GH peak >5 μg/L
Glucagon
1–1.5 mg IM
GH peak >3 μg/L
Nausea, headaches
l-Arginine plus GHRH
l-Arginine
0.5 g/kg (max 30 g)
IV over 30 min
Peak GH >9 μg/L
Nausea
GHRH
1 μg/kg
Flushing
ACTH, Adrenocorticotropic hormone; CR

/kg
Flushing
ACTH, Adrenocorticotropic hormone; CRH, corticotropin-releasing hormone; 11-DOC, 11-deoxycorticosterone; FSH, follicle-stimulating hormone; GH, growth hormone; GHRH, growth hormone–releasing 
hormone; GnRH, gonadotropin-releasing hormone; IM, intramuscular; IV, intravenous; LH, luteinizing hormone; PRL, prolactin; T3, triiodothyronine; T4, thyroxine; TSH, thyroid-stimulating hormone; TRH, 
thyrotropin-releasing hormone.
   
TABLE 8.15
235
Chapter 8  Pituitary Physiology and  Diagnos

5
235
  Replacement Therapy for Adult 
Hypopituitarisma
Deficient 
Hormone
Treatment
ACTH
Hydrocortisone, 10–20 mg daily in divided doses
Cortisone acetate, 15–25 mg/day in divided doses
TSH
l-Thyroxine, 0.05–0.2 mg daily according to T4 
levels
FSH/LH
Males:
Testosterone enanthate, 200 mg IM every 2–3 
weeks
Testosterone undecanoate 1000 mg IM every 3–6 
months
Testosterone skin patch, 2.5–5.0 mg/day—can 
increase dose up to 7.5 mg/day

2.5–5.0 mg/day—can 
Testosterone gel, 5–10 gm/day (delivering 
50–100 mg/day)
For fertility: hCG three times weekly, or hCG + FSH 
or menopausal gonadotropin or GnRH
Females (nonoral route is recommended):
Estradiol skin patch, 4–8 mg, twice weekly
Estradiol gel
Conjugated estrogen, 0.65 mg daily
Micronized estradiol, 1 mg daily
Estradiol valerate, 1–2 mg
Piperazine estrone sulfate, 1.25 mg
All of the estrogens are administered with 
progesterone or progestin seque

administered with 
progesterone or progestin sequentially or in 
combination if uterus present
For fertility: menopausal gonadotropin and hCG 
or GnRH
GH
Adults: Somatotropin, 0.2–1.0 mg SC daily
Children: Somatotropin, 0.02–0.05 mg/kg/day
Intranasal desmopressin, via rhinal tube, 5–20 μg 
twice daily
Oral DDAVP, 300–600 μg daily, usually in divided 
doses
aDoses shown should be individualized and reassessed during stress, surgery, or pregnancy. 
Male and female fertility management 

 pregnancy. 
Male and female fertility management is fully discussed in Chapter 26.
ACTH, Adrenocorticotropic hormone; DDAVP, desmopressin acetate; FSH, follicle-stimulating 
hormone; GH, growth hormone; GnRH, gonadotropin-releasing hormone; hCG, human chori­
onic gonadotropin; IM, intramuscularly; SC, subcutaneously; T4, thyroxine.
   
TABLE 8.16
to two-thirds of patients harboring pituitary macroadenomas, 
craniopharyngiomas, and other parasellar lesions have compro­
mised pituitary reserve fu

ar lesions have compro­
mised pituitary reserve function. Less commonly, patients with 
intrasellar aneurysms, pituitary metastases, parasellar meningi­
omas, optic gliomas, and hypothalamic astrocytomas may also 
have pituitary failure. Although about a third of patients with 
hypopituitarism undergoing pituitary surgery recover function 
after decompression, about 25% of patients experience further 
loss of pituitary function after surgery and therefore should be 
screened annually. Treatment 

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tary Masses and Tumors
SHLOMO MELMED
Pituitary Masses
Pituitary Mass Effects
An expanding intrasellar mass may compress normal pituitary tis­
sue and inexorably alter sellar size and shape by bony erosion and 
remodeling (Fig. 9.1). Although the time course for this process is 
unclear, it appears to be slowly progressive over years or decades. 
The tumor may invade soft tissue, and the dorsal sellar roof pres­
ents the least resistance to expansion from within the confines of 
the bony sella. B

ion from within the confines of 
the bony sella. Both suprasellar and parasellar compression and 
invasion may occur with an enlarging mass, with resultant clini­
cal manifestations (Table 9.1). As sellar masses impinge upon the 
optic chiasm, they may interfere with vision. Because of the anat­
omy of the chiasm, pressure from below affects temporal visual 
fields, starting superiorly and ultimately extending to the entire 
temporal field. Continued growth and pressure on the optic appa­
ratus 

nued growth and pressure on the optic appa­
ratus can extend visual loss to the nasal field and may ultimately 
result in blindness. Long-standing optic chiasmal pressure results 
in optic disc atrophy. Extension of pituitary lesions laterally may 
also impinge on or invade the dural wall of the cavernous sinus. 
Despite invasion, these lesions only rarely compromise the third, 
fourth, and sixth cranial nerves, as well as the ophthalmic and 
maxillary branches of the fifth cranial nerve. Althou

illary branches of the fifth cranial nerve. Although tumors 
in the cavernous sinus often surround the internal carotid artery, 
clinical vascular sequelae are rarely encountered. Varying degrees 
of diplopia, ptosis, ophthalmoplegia, and decreased facial sensa­
tion may occur infrequently, depending on the extent of neural 
involvement by the cavernous sinus mass. In contrast to cavernous 
invasion by slow tumor progression, sudden insults to the cavern­
ous sinus by hemorrhage or infarction of

e cavern­
ous sinus by hemorrhage or infarction of a pituitary tumor occur 
more frequently and may affect nerves coursing through the sinus. 
Downward extension into the sphenoid sinus indicates that the 
parasellar mass has eroded the bony sellar floor. Aggressive tumors 
may also invade the roof of the palate and cause nasopharyngeal 
obstruction, infection, and cerebrospinal fluid (CSF) leakage. 
Infrequently, temporal or frontal lobes may be invaded, causing 
uncinate seizures, personality 

 invaded, causing 
uncinate seizures, personality disorders, and anosmia. In addi­
tion to the anatomic lesions caused by the expanding mass, direct 
hypothalamic involvement of the encroaching mass may lead to 
important metabolic sequelae discussed in Chapter 7.
Intrasellar tumors commonly present with headaches, even in 
the absence of demonstrable suprasellar extension. Small changes 
in intrasellar pressure caused by a microadenoma within the 
Pituitary Masses, 236
Parasella

e 
Parasellar Masses, 250
Classification of Pituitary Tumors, 265
Prolactin-Secreting Adenomas, 268
Nonfunctioning Pituitary Tumors, 274
Acromegaly, 279
ACTH-Secreting Tumors (Cushing Disease), 298
Thyrotrophic Hormone-Secreting Tumors, 300
KEY POINTS
 • 
Intrapituitary or parasellar masses are mostly benign pituitary 
adenomas.
 • 
Nonpituitary sellar masses are rare and are usually inflammatory 
processes, infiltrations, or metastatic deposits, or may ar

, infiltrations, or metastatic deposits, or may arise from 
adjacent structures, including aneurysms, meningiomas, or 
chordomas.
 • 
Nonsecreting pituitary adenomas are usually of gonadotroph or 
null cell origin and usually present with compressive features or 
are diagnosed as incidentalomas.
 • 
Secreting pituitary adenomas exhibit unique syndromes due 
to excess production of prolactin (prolactinomas), growth hor­
mone (acromegaly/gigantism), adrenocorticotrophic hormone 
(Cushing disease

m), adrenocorticotrophic hormone 
(Cushing disease), or very rarely, thyrotrophic hormone or 
gonadotrophins.
 • 
Management of pituitary masses includes surgery, radiation, 
and specific targeted medical therapies.
237
Chapter 9  Pituitary Masses and Tumors
confined sella are sufficient to stretch the dural plate with resul­
tant headache. Headache severity does not correlate with the size 
of the adenoma or the presence of suprasellar extension.1 Rela­
tively minor diaphragmatic distortions o

n.1 Rela­
tively minor diaphragmatic distortions or dural impingement 
may be associated with persistent headache. Successful medical 
management of small functional pituitary tumors with dopa­
mine agonists or somatostatin analogues is often accompanied 
by remarkable headache improvement. In a retrospective assess­
ment of transsphenoidal surgery for microadenomas, headaches 
resolved or disappeared in 90% of patients with nonfunctioning 
tumors and in 56% of those with functioning tumors.2 Re

s and in 56% of those with functioning tumors.2 Regard­
less of cause or size, pituitary masses, including adenomas, may 
be associated with compression of surrounding healthy pituitary 
tissue and resultant hypopituitarism. In 49 patients undergoing 
transsphenoidal resection of pituitary adenomas, mean intrasel­
lar pressure was elevated twofold to threefold in patients with 
associated pituitary failure. Furthermore, prevalence of head­
ache and elevated prolactin (PRL) levels correlate posit

nd elevated prolactin (PRL) levels correlate positively with 
intrasellar pressure levels,3 suggesting interrupted portal deliv­
ery of hypothalamic hormones. Thus, surgical decompression 
of a sellar mass may lead to recovery of compromised anterior 
pituitary function. In patients who do not recover pituitary 
function postoperatively, irreversible compromised pituitary 
reserve or ischemic necrosis of residual pituitary tissue is likely 
to have occurred. Stalk compression may result in pitui

ve occurred. Stalk compression may result in pituitary fail­
ure caused by encroachment of the portal vessels that normally 
provide pituitary access to hypothalamic trophic hormones. 
Stalk compression also usually leads to hyperprolactinemia due 
to compromised dopamine access, with concomitant failure of 
other pituitary trophic hormones. 
Evaluation of Pituitary Masses
Approach to the Patient Harboring a Pituitary Mass
Most pituitary masses are adenomas. Ninety-one percent of 1120 
patients 

re adenomas. Ninety-one percent of 1120 
patients undergoing transsphenoidal surgery for sellar masses were 
diagnosed as harboring pituitary adenomas.4 In a series of 2598 
patients undergoing pituitary magnetic resonance imaging (MRI), 
pituitary adenomas accounted for 82% of visible lesions. Most 
commonly encountered nonadenomatous lesions include Rathke 
cleft cyst, craniopharyngioma, and meningioma,5 with Rathke 
cysts accounting for up to 40% of all such masses.6 Thus, the 
Stalk
Optic ch

40% of all such masses.6 Thus, the 
Stalk
Internal
carotid artery
Pituitary
Sphenoid sinus
Frontal
lobe
Adenoma
Adenoma
A
B
C
• Fig. 9.1  Magnetic resonance images of the pituitary. (A) Coronal section of a normal pituitary gland. (B) 
Sagittal view of a large pituitary adenoma lifting and distorting the optic chiasm and invading the sphenoid 
sinus and impinging the frontal lobe. (C) Coronal view of a large macroadenoma elevating the optic chiasm 
and invading the righ

 elevating the optic chiasm 
and invading the right cavernous sinus.
238
differential diagnosis of a pituitary mass should be aimed at exclud­
ing the diagnosis of a pituitary adenoma before considering the 
presence of other rare sellar lesions. Pituitary adenomas arise from 
differentiated cells secreting trophic hormones, including growth 
hormone (GH), PRL, adrenocorticotrophic hormone (ACTH), 
thyroid-stimulating hormone (TSH, thyrotrophic hormone), 

-stimulating hormone (TSH, thyrotrophic hormone), or 
gonadotrophins. These tumors may hypersecrete respective hor­
mones or may be clinically nonsecreting (Fig. 9.2). The manage­
ment and prognosis of anterior pituitary adenomas differ markedly 
from those for other nonpituitary masses, and an important diag­
nostic challenge is to effectively distinguish a pituitary adenoma 
from other parasellar masses. Several physiologic states are asso­
ciated with pituitary enlargement. Lactotroph hyperpl

ted with pituitary enlargement. Lactotroph hyperplasia occurs 
during pregnancy, and thyrotroph, gonadotroph, or rarely cortico­
troph hyperplasias occur in the presence of long-standing primary 
thyroid, gonadal, or adrenal failure, respectively.7 Pituitary enlarge­
ment may also occur as a result of ectopic GH-releasing hormone 
(GHRH) or corticotrophin-releasing hormone (CRH) secretion, 
from pulmonary and pancreatic neuroendocrine tumors or hypo­
thalamic gangliocytomas, with resultant hyper

ypo­
thalamic gangliocytomas, with resultant hyperplasia of somato­
troph or corticotroph cells. Autopsy series show that up to 20% 
of subjects harbor an incidental clinically silent pituitary adenoma 
(incidentaloma). With the widespread use of sensitive imaging 
techniques for nonpituitary indications, including head trauma, 
chronic sinusitis, or headaches, previously inapparent pituitary 
lesions are being identified with increasing frequency. Incidental 
pituitary cysts, hemorrhages, and i

y. Incidental 
pituitary cysts, hemorrhages, and infarctions are also discovered at 
autopsy. Pituitary abnormalities compatible with the diagnosis of 
microadenoma are detectable in about 10% of the normal adult 
population undergoing MRI studies.8 Recognizing that approxi­
mately 90% of observed pituitary lesions represent pituitary ade­
nomas, initial assessment should determine whether the mass is 
hormonally functional and whether local mass effects are apparent 
at the time of diagnosis or

 effects are apparent 
at the time of diagnosis or likely to develop in the future.
As the onset of clinical features associated with disordered hor­
mone secretion is insidious and may be unnoticed for years or 
decades, endocrine function should always be tested at presenta­
tion (Table 9.2). Clinical evaluation for changes compatible with 
hypersecretion or hyposecretion of GH, gonadotrophins, PRL, or 
ACTH may reveal unique long-term sequelae requiring distinct 
therapies. In the absence of 

 requiring distinct 
therapies. In the absence of clinical features of a humoral hyper­
secretory syndrome, cost-effective laboratory screening should be 
performed. Serum PRL levels greater than 200 ng/mL strongly 
suggest the presence of a microprolactinoma or macroprolacti­
noma. Any elevation in serum PRL from minimal to high can 
occur when a microadenoma is present. A minimal to moderate 
elevation can also indicate secondary stalk interruption by a pitu­
itary mass (usually a nonfunctioni

tion by a pitu­
itary mass (usually a nonfunctioning macroadenoma). A PRL level 
greater than 500 ng/mL in a nonpregnant individual is considered 
pathognomonic of a prolactinoma, as significant PRL elevations 
can be caused by drugs such as risperidone and other second-
generation antipsychotics.9 Elevated age-matched and gender-
matched insulin-like growth factor 1 (IGF1) levels indicate the 
presence of GH-secreting adenoma, and a high 24-hour urinary 
free cortisol level or elevated nighttim

 urinary 
free cortisol level or elevated nighttime salivary cortisol10 is an 
effective screen for most patients with Cushing disease. Neverthe­
less, the overall incidence of functional hormone-secreting tumors 
in asymptomatic subjects with incidentally discovered pituitary 
masses is low. The presence of, or the potential for, local compres­
sive effects must also be considered. Because the risk for micro­
adenoma progression to a compressive macroadenoma is low, 
no direct intervention may 

 macroadenoma is low, 
no direct intervention may be warranted. For parasellar masses 
of uncertain origin, histologic examination of surgically excised 
tissue may be the best approach to yield an accurate diagnosis. 
Clearly, the benefits versus risks of biopsy or surgery should be 
considered in such cases, especially for lesions that are not growing 
or not causing a functional deficit. Although imaging features may 
  Local Effects of an Expanding Pituitary, 
Parasellar, or Hypothalamic Ma

panding Pituitary, 
Parasellar, or Hypothalamic Mass
Impacted 
Structure
Clinical Effect
Pituitary
Growth failure, adult hyposomatotropism, hypogo­
nadism, hypothyroidism, hypoadrenalism
Optic tract
Loss of red perception, bitemporal hemianopia, supe­
rior or bitemporal field defect, scotoma, blindness
Temperature dysregulation, obesity, diabetes 
insipidus; thirst, sleep; appetite, behavioral, and 
autonomic nervous system dysfunctions
Cavernous sinus
Ptosis, diplopia, ophthalmople

ons
Ptosis, diplopia, ophthalmoplegia, facial numbness
Temporal lobe
Uncinate seizures
Frontal lobe
Personality disorder, anosmia
Central
Headache, hydrocephalus, psychosis, dementia, 
laughing seizures
Neuro-
ophthalmologic 
tract
Field defects:
Bitemporal hemianopia (50%), amaurosis with 
hemianopia (12%), contralateral or monocular 
hemianopia (7%)
Scotomas—junctional; monocular central, arcu­
ate, altitudinal; hemianopic
Homonymous hemianopia
Acuity loss:
Snellen
Contrast sen

ymous hemianopia
Snellen
Contrast sensitivity
Color vision
Visual evoked potential
Pupillary abnormality:
Impaired light reactivity
Afferent defect
Optic atrophy:
Papilledema
Cranial nerve palsy—oculomotor, trochlear, abdu­
cens, sensory trigeminal
Nystagmus
Visual hallucinations
Postfixation blindness
Adapted from Snyder PJ, Melmed S. Clinically nonfunctioning sellar masses. In: Jameson JL, 
DeGroot L, eds. Endocrinology: Adult and Pediatric, 7th ed. Philadelphia: Elsevier; 2016:25

Pediatric, 7th ed. Philadelphia: Elsevier; 2016:256–
265; Arnold A. Neuroophthalmologic evaluation of pituitary disorders. In: Melmed S, ed. The 
Pituitary, 2nd ed. Boston, MA: Blackwell; 2002.
   
TABLE 9.1
239
• Fig. 9.2  Distribution of normal adenohypophyseal cells is reflected in pituitary adenomas. Nonfunctioning 
tumors, on the other hand, are typically macroadenomas that efface pituitary landmarks. The localization and 
frequency of functioning micr

he localization and 
frequency of functioning microadenomas reflect the maximal concentration of their corresponding normal 
pituitary cells. Left panels show normal pituitary with GH cells in lateral wings (lower left) compared with 
diseased pituitary (lower right). Right panels depict adenoma distribution with multiple incidental adenomas 
shown by arrows. ACTH, adrenocorticotrophic hormone; GH, growth hormone; N, neurohypophysis; PRL, 
prolactin; TSH, thyroid-stimulating hormone. (From Schei

tin; TSH, thyroid-stimulating hormone. (From Scheithauer BW, Horvath E, Lloyd RV, et al. Pathology of 
pituitary adenomas and pituitary hyperplasia. In: Thapar K, Kovacs K, Scheithauer BW, et al, eds. Diagnosis 
and Management of Pituitary Tumors. Totowa, NJ: Humana Press; 2001.)
  Screening Tests for Functional Pituitary Adenomas
Disorder
Test
Comments
IGF1
OGTT with GH obtained at 0, 30, and 60 minutes
Interpret IGF1 relative to age-matched and gender-matched control 
subjects.
Nor

-matched and gender-matched control 
subjects.
Normal subjects should suppress GH to <1 μg/L.
Serum PRL level
A level >500 ng/mL is pathognomonic for macroprolactinoma.
If >200 ng/mL, prolactinoma is likely.a
Cushing disease
24-hour UFC
Nighttime salivary cortisol
Dexamethasone (1 mg) at 11 pm and fasting 
plasma cortisol measured at 8 am
ACTH assay
Ensure that urine collection is total and accurate by measuring urinary 
creatinine. Free salivary cortisol reflects circadian rhythm, 

Free salivary cortisol reflects circadian rhythm, and elevated 
levels may indicate Cushing disease.
Normal subjects suppress to <1.8 μg/dL.
Distinguishes adrenal adenoma from ectopic ACTH secretion or Cushing 
disease.
TSH-secreting tumor
TSH measurement
Free T4 by dialysis
Total T3
If T4 or T3 is elevated and TSH is measurable or elevated, a TSH-secreting 
tumor may be present.
aRisperidol may result in prolactin levels >200 ng/mL.
ACTH, Adrenocorticotrophic hormone; GH, growth hormone; IGF1, 

corticotrophic hormone; GH, growth hormone; IGF1, insulin-like growth factor type 1; OGTT, oral glucose tolerance test; PRL, prolactin; T3, triiodothyronine; T4, thyroxine; TSH, thyroid-
stimulating hormone; UFC, urinary free cortisol.
   
TABLE 9.2
240
be helpful in diagnosing the cause of a nonpituitary sellar mass, 
the final diagnosis may remain elusive until pathologic confirma­
tion is obtained.
Parasellar masses include neoplastic and nonneoplastic

sellar masses include neoplastic and nonneoplastic lesions 
and manifest clinically by local compression of surrounding vital 
structures or as a result of metabolic or hormonal derangements. 
Rarely, sellar masses or infiltrative processes may be the presenting 
feature of a previously undiagnosed systemic disorder such as lym­
phoma, tuberculosis,11 sarcoidosis, or histiocytosis.12 Fever with 
or without associated sterile or septic meningitis may rarely be 
caused by fluid leakage into the su

may rarely be 
caused by fluid leakage into the subarachnoid space from Rathke 
cleft, dermoid, and epidermoid cysts, craniopharyngioma, and 
apoplexy.13 Pituitary masses may present with hemorrhage and 
infarction, especially during pregnancy (see earlier discussion), 
when there is a pituitary tumor or when elderly individuals with 
unsuspected pituitary tumors become hypotensive because of 
another illness. Rarely, these adenomas present with CSF leak, 
which may predispose to meningitis. Pit

CSF leak, 
which may predispose to meningitis. Pituitary masses may also 
undergo silent infarction leading to development of a partial or 
totally empty pituitary sella, with normal pituitary reserve, imply­
ing that the surrounding rim of pituitary tissue is fully functional. 
Rarely, functional pituitary adenomas may arise within the rem­
nant pituitary tissue, and these tumors may not be visible by 
sensitive MRI (i.e., <2 mm in diameter), despite their endocrine 
hyperactivity. More than on

spite their endocrine 
hyperactivity. More than one kind of tumor may be found coin­
cidentally in the same patient, such as a pituitary tumor and a 
meningioma14 or pituitary adenoma with a craniopharyngioma 
component.15 Acute or chronic infection with abscess formation 
may rarely occur within the mass. Compromised pituitary hor­
mone hyposecretion may be due to direct pressure effects of the 
expanding mass on hormone-secreting cells or parasellar pressure 
effects that attenuate synthesis o

ellar pressure 
effects that attenuate synthesis or secretion of hypothalamic hor­
mones, with resultant pituitary failure. 
Imaging
Tumors of the pituitary gland are best diagnosed with MRI 
because the technique has better resolution than other radiologic 
modalities for identifying soft tissue changes (see Fig. 9.1). When 
a pituitary tumor or other parasellar mass is suspected, an MRI 
specifically focused on the pituitary should be performed, as more 
widely spaced cuts during a routine bra

, as more 
widely spaced cuts during a routine brain MRI are often inad­
equate to visualize relatively small pituitary tumors.16 This tech­
nique permits high-contrast detailed visualization of tumor mass 
effects on neighboring soft tissue structures, including the cavern­
ous sinus or optic chiasm. A pituitary MRI includes images of the 
optic chiasm, hypothalamus, pituitary stalk, and cavernous and 
sphenoid sinuses.17 High-resolution T1-weighted sections in the 
coronal and sagittal plane b

hted sections in the 
coronal and sagittal plane both before and after gadolinium-based 
contrast administration will distinguish most pituitary masses. 
Slice thickness should be less than 3 mm to obtain a pixel of 1 mm. 
Contiguous sections are therefore required to diagnose lesions of 
1 to 3 mm. If necessary, especially for diagnosing high-signaling 
hemorrhage, T2-weighted images will provide additional diagnos­
tic information. MRI thus clearly delineates the pituitary gland, 
stalk, optic

arly delineates the pituitary gland, 
stalk, optic tracts, and surrounding soft tissues. The gland may be 
concave, convex, or flat. The posterior pituitary lobe exhibits a dis­
crete bright spot of high signal intensity on T1-weighted images, 
which declines with age and is absent in diabetes insipidus and 
most posterior pituitary lesions. This T1 shortening may reflect 
the presence of antidiuretic hormone (ADH) localized within neu­
rosecretory vesicles.18 The pituitary gland may transiently

ry vesicles.18 The pituitary gland may transiently enlarge 
during adolescence, pregnancy, and postpartum, with teenage girls 
exhibiting increasing gland convexity during the menstrual cycle. 
During pregnancy, the gland should normally not exceed 10 to 12 
mm, and the stalk should not exceed 4 mm in diameter. Pregnant 
women may rarely develop visual field deficits due to an enlarging 
pituitary gland even in the absence of a pituitary tumor. A thick­
ened stalk may indicate the presence of hy

 thick­
ened stalk may indicate the presence of hypophysitis, granuloma, 
germinoma, or chordoma. After gadolinium injection, microad­
enomas usually appear hypodense compared to the normal gland, 
especially when multiple thin section echo sequences are examined 
in the first few minutes after contrast agent injection. It has been 
suggested that intrasellar hypointensity may reflect compromised 
microadenoma vasculature.19 Microadenomas may also cause 
gland asymmetry or stalk deviation. In co

o cause 
gland asymmetry or stalk deviation. In contrast, macroadenomas, 
which are significantly more vascular than microadenomas, have a 
higher uptake of gadolinium. They often enlarge the sella turcica 
by remodeling the bony fossa, suggesting a gradual long-term pro­
cess. These tumors can grow upward toward the optic apparatus 
and cause draping of the nerves over the tumor, often accompa­
nied by visual field abnormalities. Tumors can also extend into 
the sphenoid sinus and not infrequen

 extend into 
the sphenoid sinus and not infrequently invade connective tissue 
separating the pituitary from the cavernous sinus. Radiologically, 
visible tumor tissue surrounding the carotid artery confirms cav­
ernous sinus invasion. Infrequently, these patients may develop 
palsies of the third, fourth, or sixth cranial nerves. MRI may read­
ily distinguish pituitary adenomas from other masses, including 
hyperplasias, craniopharyngiomas, meningiomas, chordomas, cysts, 
and metastatic lesion

ingiomas, chordomas, cysts, 
and metastatic lesions. Visualization of a distinct pituitary gland 
adjacent to a parasellar mass (Fig. 9.3) suggests that the mass is 
not of pituitary origin. Secondary distinguishing features such as 
visualization of noninvolved pituitary tissue, mass consistency, 
calcification, hemorrhage, and suprasellar involvement usually 
allow an imaging diagnosis of these masses, which can often only 
be confirmed by direct tissue histologic diagnosis. Preoperative 
• Fi

ct tissue histologic diagnosis. Preoperative 
• Fig. 9.3  Sagittal magnetic resonance image of a craniopharyngioma, 
with cystic and solid components. The tumor is in the suprasellar area 
located above a normal pituitary gland. The presence of a separate pitu­
itary gland indicates that the suprasellar tumor is not of pituitary origin. 
(Courtesy N. Karavitaki.)
241
localization of carotid artery aneurysms can also be confirmed by 
MRI or MR angiography. A

can also be confirmed by 
MRI or MR angiography. Administration of gadolinium may be 
contraindicated in patients with impaired renal function because 
it may cause acute renal failure or be associated with nephrogenic 
systemic fibrosis.20
Pituitary computed tomography (CT) allows visualization of 
bony structures, including the sellar floor and clinoid bones, and 
identifies bony invasion. CT also recognizes calcifications that 
characterize craniopharyngiomas, meningiomas, and rarely aneu­
ry

aniopharyngiomas, meningiomas, and rarely aneu­
rysms that are not evident on MRI. Rarely, pituitary adenomas 
may calcify. Pituitary CT scan is indicated for discovery of hemor­
rhagic lesions, metastatic deposits, chordomas, and evidence of 
calcification.
Receptor Imaging
As prolactinomas express dopamine 2 (D2) receptors, they can 
be imaged with a radiolabeled D2 receptor antagonist by using 
123I-iodobenzamide single-photon emission scanning. Failure to 
visualize nonfunctioning tumors by 

g. Failure to 
visualize nonfunctioning tumors by this technique has led some to 
advocate its use to distinguish the two tumor types.21 Radiolabeled 
indium-pentetreotide has been used for in vivo tumor imaging. 
Most pituitary adenomas express somatostatin receptor subtypes 
to a varying degree, thus limiting the specificity of the procedure. 
As the sensitivity of single-photon emission CT (SPECT) is about 
1 cm, and also detects normal pituitary tissue receptor expression, 
its use is limite

ary tissue receptor expression, 
its use is limited for pituitary tumor detection, but it may be help­
ful for imaging ectopic ACTH-secreting tumors. 
Neuro-Ophthalmologic Assessment of Pituitary Masses
The optic tracts are particularly vulnerable to compression by 
expanding pituitary masses. Accurate neuro-ophthalmologic eval­
uation is helpful for tumor diagnosis, determining pretreatment 
baseline visual status for posttreatment monitoring, or detection 
of mass recurrence.22 The relationshi

r detection 
of mass recurrence.22 The relationship of the optic chiasm and 
the intracranial components of the optic nerves with the pituitary 
gland and surrounding vessels are depicted in Fig. 9.4. A 10-mm 
posteriorly angled gap separates the optic chiasm and diaphragma 
sellae (Fig. 9.5). Therefore extensive suprasellar mass extension is 
required before visual function is compromised. Decussation of 
neural fibers originating from the nasal half of each retina occurs 
at the chiasm, and th

 half of each retina occurs 
at the chiasm, and those originating from the temporal retinal 
halves are situated ipsilaterally.23 Fibers from the superior and 
inferior retinal aspect are segregated in the corresponding chiasmal 
regions. Local vascular compromise and chiasmal stretching con­
tribute to the pathogenesis of selective visual compromise. Revers­
ibility of visual effects may correlate inversely with acuteness of 
the compressive insult, and long-standing visual tract compression 
f

ult, and long-standing visual tract compression 
from larger tumors invariably leads to adverse outcomes.
Visual Symptoms
An abnormal visual examination may unmask the presence of 
a pituitary mass in an asymptomatic patient. Prior to the avail­
ability of sophisticated assay and imaging techniques, virtually all 
pituitary masses presented with visual loss. Currently, fewer than 
10% of patients present with visual loss and most harbor clinically 
nonfunctioning pituitary adenomas often detecte

y 
nonfunctioning pituitary adenomas often detected by incidental 
imaging. Unilateral or bilateral temporal or central visual loss is 
often asymmetric and may be quite insidious, remitting, or recur­
ring. Rarely, sudden visual loss occurs in a previously asymptom­
atic patient. Other symptoms include diplopia, impaired depth 
perception, and very rarely visual hallucinations.24 The retinal 
nerve fiber layer may be decreased as a reflection of optic atrophy.25 
Clinical Signs and Approach
Imp

 optic atrophy.25 
Impingement of the inferior crossing chiasmal fibers leads to 
bitemporal visual loss, especially in the superior field portions, 
accounting for most pituitary-related visual defects (Fig. 9.6). 
Rarely, tumors compress the optic chiasm from above and cause 
inferior temporal compromise. As damage to the optic chiasm 
becomes more extreme, field cuts can extend into the nasal field 
and also cause optic atrophy. Pituitary-related defects preferen­

ptic atrophy. Pituitary-related defects preferen­
tially marginate at the vertical field midline in contrast to other 
causes of bitemporal defects, which tend to occur away from 
the midline. Despite prominent field defects, many of which 
can be directly correlated with defined tumor location by MRI, 
visual acuity in the remaining fields is invariably normal in most 
patients. Anterior tumor extension may damage central visual acu­
ity.26 Rarely, pupillary abnormalities, optic atrophy, papill

ly, pupillary abnormalities, optic atrophy, papilledema, 
cranial nerve palsies, and nystagmus may be encountered. Preop­
eratively, asymptomatic visual field deficits should be identified by 
visual field and acuity testing by automated quantitative perim­
etry, assessment of visual evoked potentials, and afferent pupillary 
defects, as well as disc appearance. Employing optic coherence 
tomography to assess retinal fiber layer thickness or damaged gan­
glion cell layer may provide an indicatio

ged gan­
glion cell layer may provide an indication of potential for postop­
erative vision recovery.22 
Management of Pituitary Masses
Although pituitary masses are usually benign, they may compress 
local structures or invade brain tissue (Fig. 9.7). The goals of 
therapy are to alleviate local compressive mass effects and to sup­
press hormone hypersecretion or relieve hormone hyposecretion, 
while maintaining intact pituitary trophic function. Three modes 
of available therapy include surgic

. Three modes 
of available therapy include surgical, radiotherapeutic, and medi­
cal approaches. In general, the benefits of each type of therapy 
should be weighed against their respective risks, and comprehen­
sive physician and patient awareness is required to individualize 
treatment approaches. Appropriate consolidated approaches from 
expert endocrinologists, neurosurgeons, radiologists, pathologists, 
and ophthalmologists have been advocated as ideal management 
teams in pituitary center

ted as ideal management 
teams in pituitary centers of excellence.27
Surgical Management of Pituitary Tumors and 
Sellar Masses
Pituitary surgery is indicated for excision of mass lesions caus­
ing central pressure effects, including visual compromise, primary 
correction of hormonal hypersecretion, or functional tumor resec­
tion in patients resistant or not immediately responsive to medi­
cal treatment. Unusual sellar lesions may require diagnostic tissue 
evaluation, and rarely, primary or se

stic tissue 
evaluation, and rarely, primary or secondary parasellar malignan­
cies require wide excision.
In 1904, Horsley reported the surgical resection of a pituitary 
tumor by a lateral middle fossa approach.28 The first success­
ful transsphenoidal approach for pituitary tumor resection was 
reported by Schloffer in 190729 and subsequently refined by Cush­
ing, who between 1910 and 1925 operated on 231 patients harbor­
ing pituitary tumors with a remarkably low 5.6% mortality rate.28 
Cush

with a remarkably low 5.6% mortality rate.28 
Cushing used a sublabial incision to enable an endonasal approach 
for removing the septum and improved visualization using the 
Kanavel headlight. Hardy later improved the technique by using the 
operating microscope and intraoperative fluoroscopy, resulting in 
markedly reduced morbidity and mortality rates compared to those 
usually encountered with craniotomy, and his approach became the 
mainstay surgical technique for resecting these tumors.
24

 surgical technique for resecting these tumors.
242
The transsphenoidal approach avoids invasion of the cranial cav­
ity and precludes the need for brain tissue manipulation required 
during a subfrontal surgical approach (Fig. 9.8). A ventral sphenoid 
approach for resection of pituitary masses likewise does not violate 
the cranial fossa. Thus, transsphenoidal surgery is associated with 
minimal morbidity and mortality rates, most patients are ambula­
t

y and mortality rates, most patients are ambula­
tory within 6 to 9 hours, and the hospital stay is generally about 
3 days. Furthermore, the transsphenoidal approach allows for a 
clearly visible operative field with high magnification and internal 
illumination. Normal pituitary can be clearly distinguished from 
tumor tissue, facilitating microdissection and small tumor resec­
tion (Fig. 9.9). The use of the transsphenoidal approach has been 
greatly enhanced by several technologic advances, 

greatly enhanced by several technologic advances, including head 
immobilization techniques, microinstrumentation development, 
Intracranial
Oculomotor nerve (III)
Trochlear nerve (IV)
Abducent nerve (VI)
Ophthalmic nerve (V1)
Maxillary nerve (V2)
Intracavernous
Microadenoma (diameter <1 cm)
A
B
Macroadenoma (diameter ≥1 cm to <4 cm)
Giant adenoma (diameter ≥4 cm)
• Fig. 9.4  Coronal section o

oma (diameter ≥4 cm)
• Fig. 9.4  Coronal section of sellar structures and cavernous sinus. (A) Illustration showing the relationship 
of the oculomotor (III) and trochlear (IV) cranial nerves to the pituitary gland. (B) MRI of pituitary microad­
enoma (left), macroadenoma (right), and giant adenoma (bottom). Arrows indicate location of adenoma. 
(A, from Silver SI, Sharpe JA. Neuro-ophthalmologic evaluation of pituitary tumors. In: Thapar K, Kovacs K, 
Scheihauer BW, et al, eds. Diagnosis and Ma

cs K, 
Scheihauer BW, et al, eds. Diagnosis and Management of Pituitary Tumors. Totowa, NJ: Humana Press; 
2001:173–200; B, from Molitch ME. Diagnosis and treatment of pituitary adenomas: a review. JAMA. 
2017;317:516–524.)
243
and novel angled endoscopes. Enhanced MRI sensitivity and 
precision, as well as intraoperative use of MRI, allow for clear 
delineation of tumor location, size, and invasiveness—all critical 
determinants of surgical success.
The en

critical 
The endoscopic surgical technique has enabled an approach to 
both intrapituitary and some extrasellar masses.30 Most approaches 
are endonasal31 but some traverse the cranial base, and suprasel­
lar lesions are reached by transposing the pituitary to access the 
lesion. In experienced hands, the endoscopic technique results in 
similar complication rates and outcomes, as compared to the tra­
ditional transsphenoidal approach.32,33
Craniotomy is indica

ranssphenoidal approach.32,33
Craniotomy is indicated for the rare invasive suprasellar masses 
extending into the frontal or middle cranial fossa, optic nerves, or 
extensive posterior clival invasion. Suprasellar extension contained 
by a small diaphragmatic aperture (“hour-glass configuration”) 
may also require a transcranial approach. Very rarely, tumors that 
are too solid to be removed transsphenoidally may require a com­
bination of transsphenoidal and intracranial surgery.
Goals of Surg

sphenoidal and intracranial surgery.
Goals of Surgery
The goal of pituitary surgery is for total resection limited to the 
lesion, without compromising postoperative endogenous pitu­
itary function.34 Careful selective mass resection may be difficult 
for poorly encapsulated lesions, those embedded deeply within 
the gland body, and those extending into the wall or the body of 
the cavernous sinuses or suprasellar lesions. However, suprasel­
lar tumors (e.g., craniopharyngiomas) may also be succ

tumors (e.g., craniopharyngiomas) may also be successfully 
removed via a transnasal approach. Poor operative field visibility 
also limits precise resection. Excision of normal pituitary tissue 
and intraoperative gland manipulation should be avoided unless 
critical for effective tumor dissection. Occasionally, hemihy­
pophysectomy or even nonselective total gland resection may be 
indicated for multifocal tumors if the surrounding normal gland 
is necrotic or if no mass lesion is discernible 

is necrotic or if no mass lesion is discernible despite an accurate 
clinical and biochemical diagnosis (especially for ACTH-cell 
tumors). Successful surgery should decompress central visual 
defects and compromised trophic hormone secretion. For chil­
dren and young adults, the consideration of adequate normal 
tissue for subsequent growth patterns and reproductive function 
is an important determinant for intraoperative decision mak­
ing. Nevertheless, especially for functional tumors, smal

vertheless, especially for functional tumors, small residual 
remnants attached to the dura are difficult to access but remain 
hypersecretory with persistent clinical progression. Thus the 
skilled neurosurgeon will carefully balance maximally effective 
tumor removal with the requirement to preserve nontumorous 
pituitary trophic function.
Recent advances have enabled improved surgical results, 
and long-term outcomes using new techniques have been rig­
orously compared to standard operations 

been rig­
orously compared to standard operations performed by skilled 
surgeons.34,35 Image-guided approaches enable intraoperative 
surgical navigation by three-dimensional (3D) imaging. Intra­
operative ultrasound and MRI technologies allow for real-time 
assessment of the dimensions and extent of the pituitary mass 
and the progress of surgery. Intraoperative MRI is performed 
while the surgical field is still open, thus allowing the surgeon 
to directly assess the need for further dissectio

to directly assess the need for further dissection, and provides an 
excellent baseline for postoperative follow-up. If there is suspicion 
of a vascular lesion, carotid and intracranial angiography is indi­
cated prior to surgery. In contrast, postoperative image stabiliza­
tion may not be evident for months after surgery, and MRI may 
be useful only after 1 year or longer, especially after resection of 
secretory tumors with measurable serum biomarkers.36 Endona­
sal transsphenoidal endoscopy

iomarkers.36 Endona­
sal transsphenoidal endoscopy (Fig. 9.10) avoids use of a retractor 
or speculum, does not require nasal packing, and sometimes leads 
to a shorter operating time, allowing for reduced postoperative 
morbidity and a shorter hospital stay. The advantages of the tech­
nique include a clear panoramic view of bony landmarks and the 
ability to access suprasellar and parasellar tumor extensions into 
the cavernous sinuses.32 Disadvantages of this approach include 
the management 

vantages of this approach include 
the management of perioperative intrasellar bleeding and CSF 
leaks. Combining both techniques may allow the advantages of 
both approaches. 
Indications for Transsphenoidal Surgery
A pituitary mass that may or may not be compressing local vital 
structures should be evaluated for surgical resection (Table 9.3). 
Although surgical resection offers a rapid resolution of hormone 
hypersecretion and many of the resultant clinical features of func­
tioning adenomas

ultant clinical features of func­
tioning adenomas, indications for the procedure differ depend­
ing on tumor type (see upcoming discussion). In general, patients 
who are intolerant or resistant to medical therapy require surgery. 
Surgery is primarily indicated for patients with well-circum­
scribed GH-secreting adenomas, TSH-secreting adenomas, all 
ACTH-secreting tumors, and nonfunctioning macroadenomas 
that require surgery. Surgery may also be indicated when tissue 
histologic confirmation

 be indicated when tissue 
histologic confirmation is required for diagnosing the nature of an 
enigmatic sellar mass. Progressive compressive features, including 
visual field loss, compromised pituitary function, or other cen­
tral nervous system (CNS) functional change, are indications for 
surgical debulking and sellar decompression. Hemorrhage into 
the encased bony sella turcica, usually occurring within a known 
or previously unknown adenoma, may require immediate surgi­
cal decompression

ma, may require immediate surgi­
cal decompression. Urgent surgical decompression is required 
for acute pituitary hemorrhage, especially in patients who have 
developed sudden visual field compromise. Hypopituitarism 
due to increased portal vessel pressure may recover shortly after 
III
15 mm
8 mm
4 mm
10 mm 
45°
C
D
• Fig. 9.5  Relationship of the pituitary gland to the optic chiasm. The intra­
cranial optic nerve/chiasmal complex lies up to 10 mm above the dia­
phragma sellae. C

 lies up to 10 mm above the dia­
phragma sellae. C, anterior clinoid process; D, dorsum of the sella turcica. 
(From Miller NR. Walsh and Hoyt’s Clinical Neuro-Ophthalmology, 4th ed, 
vol 1. Baltimore, MD: Williams & Wilkins; 1985:60–69.)
E
F
Nasal
visual field
Temporal
A
B
Normal
D
C
Hemifield slide
Lesion
Gap
Vertical
slide
Overlap
Fusion
failure
Bitemporal
hemianopic
defect
Fusion
Temporal
retinal fibers
Nasal
Nasal fibers
compressed
by tumor
Tumor
Optic nerve
Opti

 fibers
by tumor
Tumor
Optic
chiasm
Pituitary
gland
• Fig. 9.6  (A–D) Local effects of an expanding pituitary tumor causing visual field defects. (A) Normal vision; (C) bitemporal hemianopia. (B–D) Hemifield slide 
phenomena arising in the setting of bitemporal hemianopia and from fusion instability. The nasal and temporal fields lose their linkage, resulting in overlap 
of the preserved visual fields. (E and F) Threshold fie

e preserved visual fields. (E and F) Threshold field test showing superior bitemporal hemianopia in a patient with pituitary tumor compressing the optic 
chiasma (E) which later advanced to bitemporal hemianopia (F). (A and C, from Newell-Price J. Endocrine assessment. In: Sheaves R, Jenkins PJ, Wass JAH, 
eds. Clinical Endocrinology Oncology. Boston, MA: Blackwell Science; 1977; B and D, from Stiver SI, Sharpe JA. Neuro-ophthalmologic evaluation of pituitary 
tumors. In: Thapar K, Kovacs K, Sch

 of pituitary 
tumors. In: Thapar K, Kovacs K, Scheithauer BW, et al, eds. Diagnosis and Management of Pituitary Tumors. Totowa, NJ: Humana Press; 2001.)
A
B
• Fig. 9.7  (A) Pituitary macroadenoma invading laterally and elevating the optic chiasm dorsally. (B) Large 
invasive macroadenoma invading brain tissue. (B, from Li-Ng M, Sharma M. Invasive pituitary adenoma. J 
Clin Endocrinol Metab. 2008;93:3284–3285.)
Tumor
Adenoma
Adenoma
Carotid artery
Sellar
floor
Sellar
floor
Clivus

us
Sellar
floor
Sellar
floor
Clivus
Sphenoid
sinus
Sphenoid
sinus
Right
Anterior
Posterior
Left
Bony nasal
septum
Midline
Sphenoid
sinus
Mucosa
Speculum
Cartilaginous
septum
A
B
C
D
a
b
e
f
c
d
• Fig. 9.8  Transsphenoidal pituitary surgery. (A) Route of the transsphenoidal approach (lateral view) and 
surgical corridor of the transsphenoidal approach and positioning of the retractor. (B) The extent of removal 
of bone structures is indicated (shaded areas). (C) Parasellar extensio

 indicated (shaded areas). (C) Parasellar extensions of pituitary adenomas (coronal 
sections) are shown: a, intrasellar adenoma; b, displacement of the cavernous sinus; c, focal invasion 
of the cavernous sinus; d, diffuse invasion of the cavernous sinus by the adenoma. (D) Extensions of a 
pituitary adenoma (sagittal sections): e, suprasellar extension; f, invasion of the sphenoid sinus and of the 
clivus. (Adapted from Honegger J, Buchfelder M, Fahlbusch R. Surgery for pituitary tumors. In: S

, Fahlbusch R. Surgery for pituitary tumors. In: Sheaves 
R, Jenkins PJ, Wass JAH, eds. Clinical Endocrinology Oncology. Boston, MA: Blackwell Science; 1977.)
246
decompressive surgery. When pituitary function after surgery 
was assessed in 234 patients, 52 patients developed new trophic 
hormone dysfunction, and 45 of 93 patients with preoperative 
evidence for hypopituitarism recovered between one and three 
previously suppressed axes. Significant facto

ree 
previously suppressed axes. Significant factors determining resto­
ration of postoperative pituitary function were no visible tumor 
remnants as assessed by MRI and no tumor invasion as deter­
mined both by the neurosurgeon and by pathologic examination 
of surrounding tissue.37 Therefore, as some patients with preop­
erative pituitary failure recover function, depending on the clini­
cal circumstance, patients should be considered for retesting of 
pituitary reserve prior to initiating pos

ting of 
pituitary reserve prior to initiating postoperative substitution 
therapy, except for adrenal steroid replacement, which requires 
Optic
chiasm
Pituitary stalk
Diaphragma sellae
Dura
Compressed
normal pituitary
Adenoma
• Fig. 9.9  Transsphenoidal resection of pituitary adenoma.
30° Endoscope within
the pituitary fossa
Diaphragm
Sella floor
Speculum
Endoscope
Sphenoid
sinus
Cavernous
sinus
• Fig. 9.10  Endoscope-assisted microsurgery provides a panoramic view 
of the sphen

crosurgery provides a panoramic view 
of the sphenoid sinus. Using a 30-degree endoscope, a view “around the 
corner” is possible. Parasellar structures can be visualized and residual 
tumor detected and resected. (From Fahlbush R, Buchfelder M, Kreutzer 
J, et al. Surgical management of acromegaly. In: Wass J, ed. Handbook of 
Acromegaly. Bristol, UK: BioScientifica; 2001.)
  Transsphenoidal Pituitary Surgery
Primary Indications
General
Visual tract or central nervous compression arising from 

tract or central nervous compression arising from within sella
Relief of compressive hypopituitarism by presenting, residual, or recur­
rent tumor tissue
Tumor recurrence after surgery or irradiation
Pituitary hemorrhage
Cerebrospinal fluid leak
Resistance to medical therapy
Intolerance of medical therapy
Personal choice
Desire for immediate pregnancy with macroadenoma
Requirement for diagnostic tissue histology
Specific
Clinically nonfunctioning macroadenoma
Prolactin

e
Nelson syndrome
TSH-secreting adenoma
Transient
Diabetes insipidus
Cerebrospinal fluid leak and rhinorrhea
Inappropriate ADH secretion
Arachnoiditis
Postoperative psychosis
Local hematoma
Arterial wall damage
Epistaxis
Local abscess
Pulmonary embolism
Permanent (up to 10%)
Total or partial hypopituitarism
Visual loss
Vascular occlusion
CNS damage—oculomotor pals

tion
CNS damage—oculomotor palsy, hemiparesis, encephalopathy
Nasal septum perforation
Surgery-Related Mortality (up to 1%)
Brain, hypothalamic injury
Vascular damage
Postoperative meningitis
Pneumocephalus
Acute cardiopulmonary disease
Anesthesia-related
Seizure
ADH, Antidiuretic hormone; CNS, central nervous system; TSH, thyroid-stimulating hormone.
   
TABLE 9.3
247
greater caution. Indications for second surge

mors
greater caution. Indications for second surgery in the same patient 
include tumor recurrence, persistent hormonal hypersecretion by 
tumor remnants, and repair of a CSF leak.
After surgery, patients should be kept at bedrest at an angle 
of 30 to 45 degrees, and urine and serum osmolality and serum 
electrolytes are measured every 6 hours. Indications for postop­
erative vasopressin replacement include polyuria, especially with 
elevated serum sodium and osmolality and inappropriately low 

rum sodium and osmolality and inappropriately low 
urine osmolality. Postoperative polyuria alone is not an indication 
for vasopressin replacement, unless it is a reflection of compro­
mised posterior pituitary function. Excess fluid given intraopera­
tively may also result in postoperative polyuria. Requirements for 
fluid replacement should take into consideration both fluid intake 
and urine output. 
The success of surgery is largely determined by the skill and expe­
rience of t

gely determined by the skill and expe­
rience of the neurosurgeon. Higher volume pituitary centers and 
experienced surgeons report superior postoperative outcomes and 
shorter hospital stays.38,39 Pituitary surgery complications include 
sinus damage, disrupted nerve function, postoperative infec­
tions, vascular complications, or endocrine dysfunction. Impor­
tantly, tumor growth persistence or recurrence is a reflection of 
subsequent adverse outcome. Tumor size, degree of invasiveness, 
preo

outcome. Tumor size, degree of invasiveness, 
preoperative hormone levels, and previous pituitary surgery are 
determinants of surgical outcome.40 Overall, the most signifi­
cant predictors of postoperative recurrence of hormone-secreting 
tumors is the postoperative basal hormone level.41,42 CSF leakage, 
transient diabetes insipidus, and inappropriate ADH secretion are 
the most commonly encountered transient side effects, occurring 
in up to 20% of patients (see Table 9.3). Local damage may r

0% of patients (see Table 9.3). Local damage may result 
in arachnoiditis, vascular bleeding, hematoma formation, and epi­
staxis. Rarely, pulmonary embolism, narcolepsy, and local abscess 
have been reported. Iatrogenic hypopituitarism, diabetes insipi­
dus, or syndrome of inappropriate antidiuretic hormone secretion 
(SIADH) is reported in up to 10% of patients. Rarely, the CNS 
may be permanently damaged with hemiparesis, cranial nerve pal­
sies, or encephalopathy.
Three phases of postoperati

es, or encephalopathy.
Three phases of postoperative diabetes insipidus have been 
described. The initial transient disorder is followed by an inter­
phase on days 6 to 11 with no polydipsia or polyuria. During 
this second phase, hyponatremia with features of inappropriate 
ADH secretion have also been reported, even in patients with no 
signs and symptoms of diabetes insipidus after surgery.43 The third 
phase is return of polyuria, polydipsia, and reduced ability to con­
centrate urine. Fatal

 and reduced ability to con­
centrate urine. Fatality has been reported in up to 1% of patients 
undergoing pituitary surgery and may be related to direct hypo­
thalamic or cerebrovascular damage, meningitis, pneumocephalus 
formation, or anesthetic complications. Surgical failure may result 
from a non–pituitary-related event, including anesthesia-related 
complication or bleeding disorder. Incomplete tumor removal 
may also be due to inaccurate preoperative MRI localization or 
identification.

 preoperative MRI localization or 
identification. Rarely, a previously undiagnosed functioning pitu­
itary tumor or ectopic source of ACTH may be unmasked after 
initially unsuccessful pituitary surgery. Readmission rates follow­
ing pituitary surgery are low. In a series of 466 consecutive cases, 
29 were readmitted within 30 days, mainly for epistaxis, hypona­
tremia, or CSF leak.44 
Pituitary Radiation
Principles
High-energy ionizing radiation can be delivered to deep tis­
sues by megavoltag

n can be delivered to deep tis­
sues by megavoltage techniques.45 The challenge is to provide 
maximal localized necrotizing radiation to the pituitary lesion 
while minimally exposing surrounding normal structures to 
radiation damage. Several advances have improved both efficacy 
and safety, including highly precise tumor localization and a 
high-voltage (6–15 MeV) linear accelerator. If required, accu­
rate simulation models with isocentric rotational arcing allow 
repeat head positioning at 

tational arcing allow 
repeat head positioning at the same exact points for recurrent 
patient visits. For fractionated approaches, up to a maximum 
of 5000 rads (= 50 Gy) are administered as 180-rad daily frac­
tions for about 5 to 6 weeks. High-precision techniques such as 
stereotactic radiosurgery may be administered as either single or 
multiple fractions and may be delivered by cobalt-60 Gamma 
Knife or CyberKnife robotic surgery or linear accelerator. These 
allow delivery of high energy 

accelerator. These 
allow delivery of high energy directly targeting the pituitary 
lesion, while minimizing radiation exposure to surrounding tis­
sues46 (Fig. 9.11). Radiosurgery is best suited for intrasellar and 
cavernous lesions distant from the optic nerves (Tables 9.4 and 
9.5). In a long-term study with a mean 96-month follow-up 
of 76 patients, about half were in remission, 23% developed 
new-onset hypopituitarism, and 3 patients developed oculomo­
tor palsies.47 Whether or not stereot

ped oculomo­
tor palsies.47 Whether or not stereotactic radiosurgery exhibits 
superior long-term efficacy or safety over fractionated treat­
ments remains unresolved.48 
B
A
C
• Fig. 9.11  MR images showing radiation target delineation for a pituitary adenoma, axial (A), coronal (B), 
and sagittal (C). Gross tumor and planning target volumes are highlighted in red. Organs at risk include 
optic chiasm, left optic nerve (orange), right optic nerve (cyan), left optic lens (light yellow), right op

e (cyan), left optic lens (light yellow), right optic lens 
(light blue), brainstem (green), pituitary stalk (pink), right hippocampus (purple), and left hippocampus 
(golden yellow). (From Minniti G, Osti MF, Niyazi M. Target delineation and optimal radiosurgical dose for 
pituitary tumors. Radiat Oncol. 2016;11:135.)
248
  Pituitary Irradiation
INDICATIONS
Pituitary adenoma—adjuvant therapy for acromegaly, Cushing disease, nonfunctioning adenoma, prola

ly, Cushing disease, nonfunctioning adenoma, prolactinoma
Nonadenomatous invasive sellar mass
Tumor recurrence
Hormone hypersecretion recurrence
Resistance to surgery and medical treatments
SIDE EFFECTS
Hypopituitarism—deficient growth hormone, gonadotrophin, TSH, and ACTH reserve
Eye—visual loss, optic neuritis
Brain—brain necrosis, temporal lobe deficits, cognitive dysfunction
MALIGNANT BRAIN TUMORS AND MENINGIOMAS AFTER PITUITARY RADIATION
Treatment Period 
(

OMAS AFTER PITUITARY RADIATION
(calendar year)
PATIENTS
CASES
PATIENT-YEARS
INCIDENCE RATE PER 
100,000 PATIENT-YEARS
RR: RT vs No RT
RT
No RT
RT
No RT
RT
No RT
RT
No RT
<1990
1497
1216
26
5
11,751
8227
221.0
60.8
3.64
1990–1999
1363
2090
8
3
9031
13,338
88.6
22.5
3.94
≥2000
376
1621
1
1
1561
6176
64.1
16.5
3.96
ACTH, Adrenocorticotrophic hormone; RR, risk ratio; RT, radiation therapy; TSH, thyroid-stimulating hormone.
Modified from Burman P, van Beek AP, Biller BMK, et al. Rad

from Burman P, van Beek AP, Biller BMK, et al. Radiotherapy, especially ay young age, increases the risk for de novo brain tumors in patients treated for pituitary/sellar lesions. J Clin Endocrinol 
Metab. 2017;102:1051–1058.
   
TABLE 9.4
  Effects of Stereotactic Radiosurgery in Patients With Hormone-Secreting and Nonsecreting Pituitary 
Adenomas
Study
Number of 
Patients
Median Marginal 
Dose (Gy)
Mean Follow-Up 
(months)
Remission 
Rate (%)
Time to Remis­
sion (months)
Predictive Factors 
o

of Remission
Hypopituitarism 
(%)
Acromegalya
Jezkova (2006)
96
32
54
50
NA
Initial hormone 
levels
27
Pollack (2007)
46
20
63
50
36
levels, SRL
33
Vik-Mo (2007)
53
26.5
66
17
NA
None
18
Jagannathan 
(2008)
95
22
57
53
29.8
SRL treatment
34
Losa (2008)
83
21.5
69
60
NA
levels
8.5
Pollock (2008)
27
20
47
67
NA
NA
27
Ronchi459 (2009)
35
20
114
43
144
levels
8.5
Castinetti (2009)b
43
26
96
42
50
Init

levels
8.5
43
26
96
42
50
levels
21
Poon (2010)
40
Range, 20–35
74
75
NA
Cavernous sinus 
invasion
12
Sheehan (2011)
130
Range, 18–30
31
53
30
34
Li (2012)
40
21
72
47.5
45
levels, cav­
ernous sinus 
invasion
40
TABLE 9.5
249
Ding (2018)
371
Range, 8.8–40
79
59
38
Cessation of 
IGF1 lowering 
therapy
22
Cushing Disease
Hoybye (2001)
18
NA
204
83
NA
NA
66
Kobayashi (2002)
20
40
60
35
NA
NA
N

4
83
NA
NA
66
20
40
60
35
NA
NA
NA
(2007)
90
23
42
53
13
Tumor volume
22
18
28.5
96
46
24
levels
28
Prolactinomas
Pouratian (2006)
23
18.6
58
26
24.5
Use of dopamine 
agonists 
at time of 
surgery, tumor 
volume
29
Jezkova (2008)
35
34
75
37
96
None
14.3
15
26
86
43
28
levels
13.3
Nonfunctioning Adenomas
Study
Number of 
Patients
Marginal Dose 
(Gy)
(months)
Tumor Control 
Rate (

Gy)
(months)
Rate (%)
Rate of Visual 
Deficit (%)
(%)
Mingione 
(2006)
100
18.5
44.9
92.2
0
19.7
Liscak (2007)
140
20
60
100
0
2
62
16
64
96.8 (95 at 5 years)
0
27
Kobayashi 
(2009)
71
14.1
50.2
96.7
2.8
8.2
Hayashi (2010)
43
18.2
36
100
0
0
Gopalan (2011)
48
18.4
95
83.3
0
39
Iwata (2011)
100
3×7/5×5
33
98
1
3
Park (2011)
125
13
62
90 (84 at 5 years)
0.8
24
Starke (2012)
140
18
50
89.6 (97 at 5 years)
0
30.3
Sheehan (2013)
512
16
36

.6 (97 at 5 years)
0
30.3
512
16
36
93.4 (95 at 5 years)
7.9
21
Lee (2014)
41
12
48
92.7 (85 at 10 
years)
2.4
24.4
Bir (2015)
57
15
45.5
93 (90 at 10 years)
0
8.8
aRemission after withdrawal of SRL.
bOnly patients with a follow-up >60 months reported.
NA, Not available.
Data from Castinetti F, Regis J, Dufour H, et al. Role of stereotactic radiosurgery in the management of pituitary adenomas. Nat Rev Endocrinol. 2010;6:214–223; Minniti G, Flickinger J, Tolu B, et al. 
Management 

nniti G, Flickinger J, Tolu B, et al. 
Management of nonfunctioning pituitary tumors: radiotherapy. Pituitary. 2018;21:154–161; Abu Dabrh AM, Asi N, Farah WH, et al. Radiotherapy versus radiosurgery in treating patients with acromegaly: a 
systematic review and meta-analysis. Endocr Pract. 2015;21:943–956; Ding D, Mehta GU, Patibandla MR, et al. Stereotactic radiosurgery for acromegaly: an international multicenter retrospective cohort 
study. Neurosurgery. 2019;84:717–725.
   
  Effects of Ste

urosurgery. 2019;84:717–725.
   
Adenomas—cont’d
TABLE 9.5
250
Indications
The use of radiation for treating pituitary tumors is highly indi­
vidualized and depends on the expertise of the treating center, 
the conviction of the treating physician in weighing the potential 
benefits and risks of the procedure, and patient preference based 
upon informed 

dure, and patient preference based 
upon informed choice (see Tables 9.4 and 9.5).49 In general, 
radiation techniques are indicated for persistent hormone hyper­
secretion or residual mass effects after surgery or when surgery 
of a compressive mass is contraindicated. Overall, fractionated 
radiotherapy (45–50 Gy) halts growth in over 90% of nonse­
creting tumors by 10 years, while secretory tumors are usually 
more resistant.50 As GH-secreting and PRL-secreting tumors are 
generally amenable 

 and PRL-secreting tumors are 
generally amenable to medical therapy, indications for irradia­
tion are rare for these adenomas. Most indications for radiation 
are adjuvant to either surgical or medical treatment. Radiation 
may be indicated after resection of a potentially recurring or 
inadequately resected pituitary mass, such as nonfunctioning 
pituitary adenoma, craniopharyngioma, or chordoma. In acro­
megaly, use of radiation as primary treatment is generally not 
recommended and is usual

eatment is generally not 
recommended and is usually reserved as adjuvant to surgery or 
medical treatments,51 but for aggressively growing prolactinomas 
that are resistant to medical therapy, the procedure may prevent 
further local invasion. Recurrent pituitary-dependent Cushing 
disease appears to be particularly suited for radiation, especially 
in younger patients. 
Hypopituitarism. Pituitary failure occurs commonly in 
patients who have received pituitary irradiation (Table 9

s who have received pituitary irradiation (Table 9.6). 
Within 10 years after radiation, up to 80% of patients may have 
gonadotroph, somatotroph, thyrotroph, or corticotroph deficits.47 
The median time to develop pituitary failure after pituitary radio­
surgery is about 5 years.52 The mechanism for hypopituitarism 
appears to involve damage to hypothalamic hormone–releasing 
cells as well as direct pituitary damage. These patients require 
lifelong endocrine follow-up for pituitary reserve tes

long endocrine follow-up for pituitary reserve testing and hor­
mone replacement when appropriate. 
Second Brain Tumors. Glioma has been reported after conven­
tional pituitary radiation for adenomas and craniopharyngioma 
with a mean latency period of 11.5 years53 (see Table 9.4). In 
patients irradiated for pituitary tumors, it appears that the stan­
dardized incidence ratio (SIR) for second brain tumors is approxi­
mately 6 (confidence interval, 3.16–10.69), with a latency of 6 to 
24 years i

l, 3.16–10.69), with a latency of 6 to 
24 years in separate cohorts.54,55 During 53,786 patient-years of 
surveillance following both stereotactic and conventional radio­
therapy, the risk of developing a brain tumor following pituitary-
directed radiotherapy increased 2.4-fold and meningioma risk 
increased 1.6-fold with every decade of younger age at therapy.56 
As patients harboring pituitary tumors are more likely to undergo 
routine brain imaging during follow-up, it is not entirely clear 

maging during follow-up, it is not entirely clear 
whether observed meningiomas are coincidental findings. This 
very rare complication appears to be dose and age related, and frac­
tionated doses should not exceed 50 Gy. Use of confocal radiation 
techniques to irradiate a smaller tissue volume may minimize this 
adverse effect, but long-term, prospective controlled surveillance 
studies are required to further evaluate this important question. 
Cerebrovascular Disease. The incidence of cerebro

Cerebrovascular Disease. The incidence of cerebrovascular 
events, but not mortality, was increased threefold, mainly in men 
irradiated for nonfunctioning adenomas.57 However, in another 
study, mortality rate from cerebrovascular disease appeared higher 
in previously irradiated pituitary-deficient patients.58,59 The 
direct causality of this relationship is unclear, but direct effects on 
cerebral vasculature, including atherosclerotic occlusive lesions, 
have been reported.60 
Visual Damage

ive lesions, 
Visual Damage. The risk of visual damage (and very rarely 
blindness) is minimized by fractionating dosages to less than 200 
rads per treatment session for conventional radiotherapy. The 
incidence of reported new visual damage in patients undergoing 
radiosurgery is approximately 4%.61 
Brain Necrosis. Dose-related radiation-induced brain necrosis 
was documented by MRI in 14 of 45 patients, with temporal lobe 
atrophy and cystic and diffuse cerebral atroph

be 
atrophy and cystic and diffuse cerebral atrophy reported. Cogni­
tive dysfunction, especially memory loss, has also been reported.62 
Medical Management
Pituitary tumors often express receptors mediating hypothalamic 
control of hormone secretion, and appropriate therapeutic ligands 
for dopamine D2 receptor and the somatostatin receptor subtype 
2 (SST2) are employed to effectively suppress PRL, GH, and 
TSH hypersecretion; to block tumor growth; and often to shrink 
tumor size. Medical abl

owth; and often to shrink 
tumor size. Medical ablation of target gland function, including 
thyroid and adrenal, may also be useful in mitigating the dele­
terious impact of pituitary tumor trophic hormone hypersecre­
tion. Peripheral antagonists block GH or cortisol action without 
targeting the respective pituitary tumor source. These medical 
approaches are considered later in this chapter. 
Parasellar Masses
Hypothalamic masses are described in Chapter 7, and causes of 
parasellar masses ar

 in Chapter 7, and causes of 
parasellar masses are depicted in Table 9.7.
  Complications After Stereotactic 
Radiosurgery for Nonfunctioning 
Pituitary Adenomas.
Complication
No. of Patients (%)
Patients w/ new cranial nerve (CN) dysfunctiona
41 of 422 (9.3)
  CN II
29 (6.6)
  CN III
6 (1.36)
  CN IV
1 (0.23)
  CN V
4 (0.90)
  CN VI
2 (0.45)
  CN VII
1 (0.23)
New-onset or worsened hypopituitarism
92 of 435 (21.1)
Cortisol
29 of 293 (9.9)
Thyroid
40 of 246 (16.3)
Gonadotrophin
24 of 288 (8.3)

id
31 of 269 (8.4)
6 of 422 (1.4)
Further tumor growth
31 of 469 (6.6)
Further surgery or radiation therapy
34 of 444 (7.7)
aForty-one patients had 43 deficits.
From Sheehan JP, Starke RM, Mathieu D, et al. Gamma Knife radiosurgery for the man­
agement of nonfunctioning pituitary adenomas: a multicenter study. J Neurosurg. 
2013;119(2):446–456.
   
TABLE 9.6
251
Diagnosis
Total

ter 9  Pituitary Masses and Tumors
Diagnosis
Total
Anterior Pituitary Tumors
395
Nonfunctioning adenoma
364
GH adenoma
127
ACTH adenoma
84
GH/prolactin mixed adenoma
4
2
Pituitary carcinoma
2
LH/FSH functioning adenoma
1
TSH adenoma
1
GH/TSH mixed adenoma
1
Cysts
Rathke cleft cyst
42
33
Arachnoid
2
Epidermoid
1
Pineal cyst
1
Nonadenomatous Neoplasms
32
Chordoma
3
Pituitary lymphoma
2
Chondrosarcoma
1
Embryonal rhabdomyosarcoma
1
Germinoma

rosarcoma
1
1
Germinoma
1
Granular cell tumor
1
Hemangiopericytoma, malignant
1
Leiomyosarcoma
1
Mucoepidermoid carcinoma
1
Pituicytoma
1
Xanthogranuloma
1
Inflammatory and Vasculitides
3
Hypophysitis, unspecified type
2
Lymphocytic infundibulitis
1
Amyloidosis, primary
1
1
1
Diagnosis
Total
Infectious
Pseudomonas aeruginosa
1
Syphilis
1
Metastases
Breast
3
CNS lymphoma, to pituitary stalk
1
Nasopharyngeal lym

 lymphoma, to pituitary stalk
1
Nasopharyngeal lymphoma
1
Liver epithelioid hemangioendothelioma
1
Lung, adenocarcinoma
1
Pineal germinoma/dysgerminoma
1
Plasmacytoma
1
Prostate, adenocarcinoma
1
Sinusoidal squamous cell carcinoma
1
Vascular
Apoplexy with masses
16
Carotid aneurysm
4
Hypothalamic cavernous angioma
1
Hypothalamic interpeduncular hematoma
1
Empty sella
21
Hyperplasia
14
Ectopic pituitary gland
4
Fibrous dysplasia
3
Lipoma
1
2
Germinoma
1
Hama

poma
1
2
Germinoma
1
Hamartoma
1
Undiagnosed Masses
159
Normal Pituitary
1242
aDiagnosis in 2598 patients undergoing pituitary magnetic resonance imaging.
ACTH, Adrenocorticotrophic hormone; CNS, central nervous system; FSH, follicle-stimulating 
hormone; GH, growth hormone; LH, luteinizing hormone; TSH, thyroid-stimulating hormone.
Modified from Famini P, Maya MM, Melmed S. Pituitary magnetic resonance imaging for 
sellar and parasellar masses: ten-year experience in 25

r and parasellar masses: ten-year experience in 2598 patients. J Clin Endocrinol Metab. 
2011;96:1633–1641.
   
  Diagnostic Pituitary Magnetic Resonance Imaging of Parasellar Masses.a
TABLE 9.7
Types of Parasellar Masses
Parasellar masses account for about 20% of all brain tumors, with 
a reported incidence of 13,340 new cases of pituitary tumors and 
craniopharyngiomas in the United States in 2016.63
Rathke Cysts
The anterior and intermediate lobes of the pituitary gland arise 
embryologicall

lobes of the pituitary gland arise 
embryologically from Rathke pouch. Inadequate pouch obliter­
ation results in cysts or cystic remnants at the interface between 
252
the anterior and posterior pituitary lobes found in about 20% 
of pituitary glands at autopsy64 (Fig. 9.12). Pituitary adeno­
mas may also occasionally contain small cleft cysts. They are 
lined by cuboidal or columnar ciliated epithelium surrounding 
mucoid cyst fluid and arise from midli

urrounding 
mucoid cyst fluid and arise from midline rudiments of failed 
Rathke cyst invagination and account for approximately 3% 
of pituitary mass lesions.65 In contrast, pituitary epidermoid 
cysts are lined by squamous epithelium and rarely become 
malignant. Rathke cysts vary in size and may also extend to 
the suprasellar region. These lesions have heterogeneous MRI 
characteristics and may rarely present with panhypopituitarism 
with or without diabetes insipidus.66 Most, however, are n

without diabetes insipidus.66 Most, however, are not 
symptomatic and in 43 of 75 patients followed for up to 126 
months, cyst size did not change. Accordingly, most of these 
patients can be followed expectantly.67 The extent of headache 
or visual disturbance is determined by the size and location 
of the cyst. Cyst formation is associated with sellar enlarge­
ment. MRI reveals hyperdense or hypodense masses on either 
T1-weighted or T2-weighted images, and CT scan shows 
homogeneous hypodens

ed images, and CT scan shows 
homogeneous hypodense areas that may be distinguished from 
pituitary adenomas.66 These patients should all be evaluated 
for hypopituitarism. After surgical resection or drainage, which 
may be required to alleviate severe headache,68 MRI should 
be performed during long-term follow-up for signs of cyst 
recurrence.64,65
Arachnoid, epidermoid, and dermoid cysts develop mainly 
in the cerebellopontine angle but may also arise in the suprasel­
lar region. Dermoid cys

lso arise in the suprasel­
lar region. Dermoid cysts containing greasy sebaceous products 
or hair follicles are rarely encountered in the pituitary, and the 
cyst lining may be calcified. Acquired pituitary cysts may arise 
secondarily to intrapituitary hemorrhage, usually associated 
with an underlying adenoma, and these rarely cause pituitary 
failure. Cyst compression causes internal hydrocephalus, visual 
disturbances, GH or ACTH deficiency, hyperprolactinemia, 
and diabetes insipidus. Rare

 hyperprolactinemia, 
and diabetes insipidus. Rarely, squamous cell carcinoma may 
arise in the cyst.69 
Granular Cell Tumors
Pituitary choristomas, or schwannomas, usually present only after 
the age of 20. Their abundant cytoplasmic granules do not contain 
pituitary hormones, but these lesions may present with diabetes 
insipidus. Pituitary adenomas are occasionally coincidentally asso­
ciated with these tumors.70 
Chordomas
These slow-growing cartilaginous tumors arise from midline 
notochor

 cartilaginous tumors arise from midline 
notochord remnants, are locally invasive, and may metastasize.71 
Most arise from the vertebrae and about one-third involve the cli­
vus region. Chordomas contain a mucin-rich matrix that allows 
diagnosis by fine-needle aspiration. They present with headaches, 
asymmetric visual disturbances, hormone deficiency, and occa­
sional nasopharyngeal obstruction. The tumor mass is associated 
with osteolytic bony erosion and calcification, and MRI may 
allow t

ny erosion and calcification, and MRI may 
allow the normal pituitary gland to be distinguished from the 
very heterogeneous and often flocculent tumor mass. At surgery, 
the tumors are rough, heterogeneous, and lobular. Markers for 
epithelial cells, including cytokeratin and vimentin, are present. 
Recurrences commonly occur after surgical excision, with mean 
patient survival time of about 5 years. Rarely, chordomas undergo 
sarcomatous transformation with an aggressive natural history and 
r

ormation with an aggressive natural history and 
require extensive surgical dissection.72 Because of their anatomic 
location, the endoscopic endonasal approach may be preferable 
for chordoma surgical resection.73 
Craniopharyngiomas
This parasellar tumor constitutes about 3% of all intracranial 
tumors and up to 10% of childhood brain tumors. The tumors 
are commonly diagnosed during childhood and adolescence.74 
However, they show a bimodal age distribution, occurring in 
children between 5 a

e distribution, occurring in 
children between 5 and 14 years and adults from 50 to 74 years 
of age.75 Tumors arise from embryonic squamous remnants of 
Rathke pouch extending dorsally toward the diencephalon and 
may be large (>10 cm in diameter) and invade the third ventricle 
and associated brain structures. Over 60% arise from within the 
sella, and others arise from parasellar cell rests.76,77 When intra­
sellar, they can often be distinguished from pituitary adenomas 
by separate visible 

shed from pituitary adenomas 
by separate visible rim of normal pituitary tissue seen on MRI 
(see Fig. 9.1A). The cystic mass is usually filled with cholesterol-
rich viscous fluid, which may leak into the CSF, causing aseptic 
meningitis. These slow-growing tumors are composed of embry­
onic precursor cells and may also contain calcifications, teeth, 
and immunoreactive human chorionic gonadotropin (hCG) (Fig. 
9.13).78 Histologic appearance shows these tumors comprising 
two cell populations:

ows these tumors comprising 
two cell populations: Cysts are lined with a squamous epithelium 
containing islands characterized by columnar cells, and a mixed 
inflammatory reaction may also occur with calcification. Ada­
mantinomatous craniopharyngiomas have a greater propensity to 
relapse than the less aggressive papillary variant.79 Although large 
craniopharyngiomas may obstruct CSF flow, they rarely undergo 
malignant transformation. Increased intracranial pressure results 
in headache, pr

sed intracranial pressure results 
in headache, projectile vomiting, papilledema, and somnolence, 
especially in children. Only about one-third of patients are over 
40 years of age, and they commonly present with asymmetric 
visual disturbances, including papilledema, optic atrophy, and 
field deficits. If cavernous sinus invasion is present, other cranial 
nerves may also be involved. On CT imaging, most children and 
about half of all adults exhibit characteristic flocculent or convex 
calcif

xhibit characteristic flocculent or convex 
calcifications. Rarely, however, pituitary adenomas, other parasel­
lar tumors, and vascular lesions within the sella are also calcified. 
Rathke’s pouch
Pharyngohypophyseal
stalk
Sphenoid bone
Stomodeum
• Fig. 9.12  Pathogenesis of Rathke cysts. Schematic of the embryologic 
progenitors of sellar and parasellar structures. Rathke pouch arises from 
an outpocketing of stomodeum (ectoderm) and gives rise to the adeno­
hypophysis. The pha

) and gives rise to the adeno­
hypophysis. The pharyngohypophyseal stalk, which connects the stomo­
deum and Rathke pouch, is divided by the sphenoid bone as it grows 
together (arrows), isolating Rathke pouch and the neurohypophysis within 
the sella. (From Harrison MJ, Morgello S, Post KD. Epithelial cystic lesions 
of the sellular and parasellular region: a continuum of ectodermal deri­
vates? J Neurosurg. 1994;80:1018–1025.)
253
In contrast to pituitary

tuitary Masses and Tumors
In contrast to pituitary adenomas, where it is rarely encountered, 
diabetes insipidus is often the earliest feature of craniopharyngi­
oma. These patients may also develop partial or complete pituitary 
deficiency. GH deficiency with short stature, diabetes insipidus, 
and gonadal failure are common. Pituitary stalk compression or 
damage to hypothalamic dopaminergic neurons results in hyper­
prolactinemia. Thus, craniopharyngioma may mimic a prolacti­
noma by intrapit

pharyngioma may mimic a prolacti­
noma by intrapituitary imaging, presence of hyperprolactinemia, 
and favorable biochemical response to dopamine agonists.
Treatment of primary or recurrent craniopharyngiomas may 
involve radical surgery, radiotherapy, or a combination of these 
modalities.75 A major side effect associated with such surgery is 
severe postoperative obesity, which can be mitigated by surgery 
that spares the hypothalamus.80 The more complicated the surgi­
cal operation, the more 

re complicated the surgi­
cal operation, the more visual problems are encountered. Patients 
with diabetes insipidus also have higher rates of anterior pituitary 
hormone deficits and subsequent obesity.81 Treatment outcome 
appears to be related to hypothalamic involvement of the tumor, 
and careful hypothalamus-sparing surgery followed by local 
radiation therapy is recommended. Postoperative recurrences 
and tumor growth progressions are frequent.74 ­Trans-sphenoidal 
microscopic surgery is s

ent.74 ­Trans-sphenoidal 
microscopic surgery is successfully used for intrasellar craniopha­
ryngiomas, and the expanded endoscopic transnasal approach 
has been successfully crafted to approach suprasellar tumors.74 
Adjuvant stereotactic irradiation is also used. Postoperative 
recurrence may occur in about 20% of patients undergoing 
radical surgical excision, and the degree of hypothalamic impair­
ment is a major determinant of outcome (Fig. 9.14).82 As several 
oncogenic mutations have bee

 9.14).82 As several 
oncogenic mutations have been discovered in adamantinoma­
tous (CTNNB1) and papillary (BRAF V600E) craniopharyngio­
mas, therapeutic approaches with BRAF/MEK targeting appear 
promising.83
Life-complicating obesity that occurs after craniopharyngioma 
resection is associated with increased appetite (often insatiable), 
nonoptimal social development, as well as altered food intake–
regulating hormones leptin and ghrelin.82 Preoperative treatment 
with glucagon-like peptide-1

eoperative treatment 
with glucagon-like peptide-1 (GLP1) analogues led to weight 
loss.84 These patients are at high risk for developing diabetes and 
cerebrovascular disease with increased mortality (standardized 
mortality ratio [SMR] 2.7; 95% confidence interval [CI]: 2.0–3.8) 
observed in 224 patients with 3153 years of follow-up.85 Mortal­
ity from cerebrovascular disease is increased fivefold in those with 
childhood-onset craniopharyngiomas.86 
Meningiomas
Meningiomas arise from arachnoi

as.86 
Meningiomas arise from arachnoid and meningioendothelial 
cells; those occurring in the sellar and parasellar region account 
for about one-fifth of all meningiomas.87 Sellar meningiomas are 
usually well circumscribed and do not attain the size of cranio­
pharyngiomas. Suprasellar meningiomas may invade the pituitary 
ventrally, and intrasellar tumor origins are rare.88 Coexisting func­
tional pituitary adenomas have been described in patients with 
parasellar meningiomas. Se

ribed in patients with 
parasellar meningiomas. Secondary hyperprolactinemia occurs 
in up to half of these patients, who usually present with local 
mass effects, including headache and progressive visual distur­
bances accompanied by optic atrophy. The differential distinction 
of a suprasellar meningioma with downward extension from an 
upwardly extending pituitary adenoma may be difficult. On MRI, 
A
B
• Fig. 9.13  Adamantinomatous craniopharyngioma containing teeth. (A) MRI showing tooth-li

yngioma containing teeth. (A) MRI showing tooth-like structures 
in the parasellar mass and mandible (arrows). (B) Fully formed teeth in the resected tumor. (From Beaty NB, 
Ahn E. Adamantinomatous craniopharyngioma containing teeth. N Engl J Med. 2014;370:860.)
1.0
0.8
0.6
0.4
0.2
0.0
0
10
20
95% CI:
No HI: 0.87–1.0
HI: 0.76–0.92
HI, n = 132 (0.84 ± 0.04)
No HI, n = 82 (0.95 ± 0.04)
Follow-up time (years)
Overall survival
30
40
50
p = 0.006
• Fig. 9.14  Overall survival for craniopharyngioma pa

g. 9.14  Overall survival for craniopharyngioma patients after surgical 
resection with and without hypothalamic involvement (HI). (Redrawn from 
Sterkenburg AS, Hoffmann A, Gebhardt U, et al. Survival, hypothalamic 
obesity, and neuropsychological/psychosocial status after childhood-
onset craniopharyngioma: newly reported long-term outcomes. Neuro 
Oncol. 2015;17:1029–1038.)
254
meningiomas are isodense on both T1-weighted and T2-weighted 
imaging, in c

on both T1-weighted and T2-weighted 
imaging, in contrast to other parasellar lesions, which are usu­
ally hyperdense on T2-weighted imaging. Dural calcification may 
be evident on CT scanning. Because of their rich vascularization, 
these tumors pose an intraoperative risk for hemorrhage and a 
resultant higher surgical mortality rate than is usually encountered 
for pituitary adenoma resection. 
Gliomas
Optic gliomas and low-grade astrocytomas arise from within the 
optic chiasm or optic tract

arise from within the 
optic chiasm or optic tract, they often infiltrate the optic nerve, 
and less than one-third are intraorbital. Von Recklinghausen dis­
ease is the underlying cause in about one-third of these patients, 
and occasionally these tumors may be associated with growth 
retardation and delayed or precocious puberty and mass effects 
that include visual disturbances, diencephalic syndrome, diabe­
tes insipidus, and hydrocephalus. Rarely, gliomas arise within the 
sella associated 

arely, gliomas arise within the 
sella associated with hyperprolactinemia and should be consid­
ered in the uncommon differential diagnosis of a PRL-secreting 
pituitary adenoma.89 Important distinguishing features include 
the young age of these patients (80% are <10 years old), relatively 
intact pituitary function, gross visual disturbances, and localiza­
tion of the mass as visualized on MRI. Gliomas, unlike hamarto­
mas, usually enhance after contrast injection. 
Mucocele
Mucoceles are expa

r contrast injection. 
Mucocele
Mucoceles are expanding accumulations of fluid within the sphe­
noid sinus and may compress parasellar structures. Headaches, 
visual disturbances (usually unilateral), and exophthalmos are 
characteristic features. On MRI, the homogeneous sphenoid mass 
may be quite prominent but may be distinguished from the pitu­
itary gland dorsally. 
Parasellar Aneurysms
A parasellar aneurysm may mimic a pituitary adenoma, and 
intraoperative rupture may be catastrophic, unde

intraoperative rupture may be catastrophic, underlying the abso­
lute need for preoperative diagnosis. Differentiating features of 
aneurysms from other pituitary masses may be subtle, including 
eye pain, very intense headaches, and relatively sudden onset of 
cranial nerve palsies (Fig. 9.15). Although imaging techniques 
usually distinguish blood and hemorrhage from solid tumor or 
tissue, a highly vascular meningioma may be confused with an 
aneurysm. Rarely encountered internal carotid ar

aneurysm. Rarely encountered internal carotid artery aneurysms 
may cause hyperprolactinemia.90 
Pituitary Infections
Acute pituitary abscesses and perisellar arachnoiditis are encoun­
tered with sinus infections, especially after transsphenoidal sur­
gery. Pituitary abscess may develop from hematogenous or direct 
local spread of infectious agents or may arise within a preexisting 
pituitary adenoma and may be difficult to distinguish from an 
adenoma, as these patients may not have fever or 

adenoma, as these patients may not have fever or signs of meningi­
tis. They often present with diabetes insipidus and headache, and 
over 80% of a series of 66 patients presented with pituitary insuf­
ficiency.91 On MRI, an isointense central cavity with surround­
ing ring enhancement is characteristic for an abscess.4,91 In 33 
consecutive patients with pituitary abscess, most presented with a 
mass associated with pituitary failure, and 5 had undergone pre­
vious surgery. Typical MRI feature

 undergone pre­
vious surgery. Typical MRI features included a sellar cystic mass 
with an enhanced rim.92 Gram-positive streptococci or staphylo­
cocci may originate from nasopharyngeal passages. Disseminated 
Entamoeba histolytica, Pneumocystis jirovecii, or Klebsiella may 
also seed to the pituitary.93,94 Immunosuppressed patients may 
develop pituitary infections, including cytomegalovirus, toxo­
plasmosis, aspergillosis, histoplasmosis, and coccidiosis. Syphilitic 
gumma may also lead to pi

coccidiosis. Syphilitic 
gumma may also lead to pituitary damage, insufficiency, and pain­
ful hypophysitis.95 Common viral infections, including influenza, 
measles, mumps, and herpes, are rarely associated with pituitary 
damage and insufficiency. Although tuberculosis is rarely confined 
to the pituitary gland, most of the fewer than 20 reported patients 
exhibited suprasellar extension of the pituitary mass, compro­
mised pituitary function, and visual defects. Although systemic 
tuberculosi

and visual defects. Although systemic 
tuberculosis is usually present, isolated sellar tuberculomas have 
been described.96 
A
B
C
• Fig. 9.15  Giant cavernous aneurysm of the left internal carotid artery. (A) Coronal T1-weighted MRI with­
out intravenous contrast. A mildly hypointense mass is seen within the sella and left cavernous sinus. 
(B) Coronal T1-weighted MRI after gadolinium contrast. The mass enhances heterogeneously. (C) CT 
angiogram with maximum intensity reconstruction. Arrows i

am with maximum intensity reconstruction. Arrows indicate origin of the left internal carotid artery 
giant cavernous aneurysm. (From Lawson EA, Buchbinder BR, Daniels GH. Image in endocrinology: 
hypopituitarism associated with a giant aneurysm of the internal carotid artery. J Clin Endocrinol Metab. 
2008;93:4616.)
255
Hematologic Malignancies
Primary CNS lymphomas are usually B-cell non-Hodgkin types, 
and to date, 33 such patients with pituitary lymphom

d to date, 33 such patients with pituitary lymphoma have been 
described.97–99 The pituitary mass may be an isolated presentation 
of the underlying disease. The disorder is usually diagnosed by his­
tologic finding of tissue obtained by excision biopsy. Presentation 
includes headache and cranial nerve abnormalities with varying 
degrees of hypopituitarism. MRI reveals cavernous sinus inva­
sion and isotense T1-weighted and T2-weighted images, which 
enhance after gadolinium. About 38% of patie

hich 
enhance after gadolinium. About 38% of patients with solitary 
pituitary plasmacytomas may develop classic multiple myeloma.100 
Acute lymphoblastic leukemia may be associated with periglandu­
lar pituitary infiltrates with minimal pituitary dysfunction. 
Pituicytoma is a rare benign central noninvasive suprasellar glial 
cell tumor that presents with mass effects or hypopituitarism.101 
The tumor, which is isointense or hypointense on T1-weighted 
MR and hyperintense on T2, ar

ense on T1-weighted 
MR and hyperintense on T2, arises from cells in the neurohy­
pophysis and stains for vimentin, S100 protein, and glial fibril­
lar acidic protein.102 The preferred effective resection is by an 
expanded endoscopic endonasal transsphenoidal and transplanum 
approach.103 
Hypothalamic granulomatous involvement is commonly encoun­
tered in patients with CNS sarcoidosis and may be the sole mani­
festation of the disease.104 The hypothalamus, pituitary stalk, and 
pos

se.104 The hypothalamus, pituitary stalk, and 
posterior pituitary are diffusely invaded by noncaseating granulo­
mas, consisting of giant cells, macrophages, and lymphocytes. These 
patients may present with varying degrees of anterior pituitary fail­
ure with or without diabetes insipidus.105 Onset of diabetes insipi­
dus with no obvious features of a pituitary disorder should alert 
the physician to exclude hypothalamic sarcoid deposits especially in 
the face of a thickened stalk on MRI.106 

ally in 
the face of a thickened stalk on MRI.106 In 24 patients with hypo­
thalamic-pituitary sarcoid, all but 2 had anterior pituitary dysfunc­
tion (gonadotrophin deficiency 21/24, TSH deficiency 15/24, and 
hyperprolactinemia in 12/24 cases), and 12 had diabetes insipidus. 
Imaging studies showed pituitary stalk thickness and involvement 
of the infundibulum and pituitary gland, which improved or dis­
appeared in 50% of patients. After glucocorticoid treatment, two 
patients experienced reve

orticoid treatment, two 
patients experienced reversal of hypopituitarism.107 
Langerhans Cell Histiocytosis
The disorder may be associated with granulomatous damage to 
the hypothalamus and/or posterior pituitary, with characteris­
tic diabetes insipidus occurring in about 25% of children with 
this disorder.108 Sleep disorders, adipsia, morbid obesity, axillary 
skin rash, history of recurrent pneumothorax, and classic bony 
lesions can occur.109 Pituitary lesions comprise dendritic Lang­
erha

09 Pituitary lesions comprise dendritic Lang­
erhans cells, and pituitary MRI may reveal stalk thickening or a 
diminished posterior pituitary bright spot. Adults with the disor­
der should be carefully evaluated for anterior pituitary hormone 
deficits and appropriately replaced. Multisystem Langerhans cell 
histiocytosis causes long-term morbidity extending into adult­
hood.110 Although surgery and radiation were, for many years, the 
mainstay treatments, a chemotherapeutic approach using clad

treatments, a chemotherapeutic approach using cladrib­
ine has been successful in some patients.111 
Hereditary Iron Storage Diseases
Hemochromatosis and hemosiderosis result in predominantly 
gonadotroph cell damage. 
Idiopathic Retroperitoneal Fibrosis
Idiopathic retroperitoneal fibrosis may also be associated with a 
suprasellar mass and hypothalamic panhypopituitarism.112 
Metastases to the Pituitary Region
Pituitary metastases are found in up to 3.5% of cancer patients,113 
especially in ol

p to 3.5% of cancer patients,113 
especially in older patients with diffuse malignant disease. As 
the vascular supply to the posterior pituitary is derived directly 
from the systemic circulation via the internal carotid arteries, the 
posterior pituitary is the preferred site for bloodborne metastatic 
spread.114 Over one-third of carcinomas that metastasize to the 
pituitary are breast metastases (37.2% of 425 reported cases), fol­
lowed by lung (24.2%), prostate (5.2%), renal (4.9%), and 28 

ng (24.2%), prostate (5.2%), renal (4.9%), and 28 
other sites.115 Diabetes insipidus is a common presenting sign, 
and cranial nerve palsies and hypopituitarism may occur as 
well.116 If extensive bony erosion is present and disease onset is 
rapid, the diagnosis is more readily apparent. However, pituitary 
imaging may not clearly distinguish metastatic deposits from a 
pituitary adenoma; these lesions may masquerade as an adenoma 
and the diagnosis only made by histologic study of the resecte

nosis only made by histologic study of the resected 
specimen.117 When the diagnosis is clear cut in the presence of a 
primary cancer, relatively low-dose pituitary radiation may be suf­
ficient to shrink the metastasis and improve morbidity. 
Evaluation of Parasellar Lesions
Because the differential diagnosis of parasellar lesions covers a 
wide spectrum of neoplastic, vascular, inflammatory, and infec­
tious processes, patient age and sex, relevant clinical history and 
symptoms, and other co

evant clinical history and 
symptoms, and other comorbid conditions can often help narrow 
down the differential diagnosis. MRI with gadolinium is essen­
tial in defining the exact location of the lesion, the gadolinium 
enhancement pattern, the presence of vascular voids or cystic 
regions, and associated vasogenic edema in surrounding brain tis­
sue. Other imaging studies are often used in select cases: CT scan 
identifies calcification in craniopharyngiomas; CT angiogram, 
MR angiogram, or ot

iopharyngiomas; CT angiogram, 
MR angiogram, or other angiography may be used when vascu­
lar lesions are suspected; and positron emission tomography may 
occasionally be used to identify metabolically active and rapidly 
growing lesions.
In a subset of lesions, serum molecular markers may be helpful 
in the diagnostic workup. For example, nongerminomatous germ 
cell tumors, which often localize to the suprasellar space, can be 
diagnosed by elevation of characteristic tumor markers, including 

ation of characteristic tumor markers, including 
β-hCG or α-fetoprotein. Furthermore, if the lesion is suspected to 
cause pituitary or hypothalamic dysfunction, pituitary function 
testing is useful in characterizing potential endocrine dysfunction. 
As parasellar lesions often impinge on the optic apparatus, due 
to their anatomic proximity, neuro-ophthalmologic evaluation, 
including Humphrey visual fields test, is required. The decision to 
resect a parasellar lesion depends on factors rela

resect a parasellar lesion depends on factors related to the patient 
(age, neurologic status, medical comorbid conditions) and the 
lesion itself (size, anatomic location, vascular pattern, benign or 
malignant growth pattern, sensitivity to radiotherapy or chemo­
therapy, sensitivity to medical therapy).
If resection is pursued, a number of surgical approaches can 
be used, including craniotomies (pterional, supraorbital, sub­
frontal), as well as endoscopic transnasal approaches. Minimally 
i

 as endoscopic transnasal approaches. Minimally 
invasive transnasal endoscopy is useful for resection of select 
parasellar lesions, with excellent surgical outcomes and poten­
tially fewer complications than those observed after conventional 
craniotomy.30 
256
Primary Hypophysitis
Pituitary mass lesions composed of inflammatory cells may arise as 
primary disorders of the anterior and posterior pituitary glands or 
the neurohypophysis.118,119 At least 

y glands or 
the neurohypophysis.118,119 At least five clinicopathologic forms 
have been described, and the definitive diagnosis may require 
pituitary biopsy.
Lymphocytic Hypophysitis
This autoimmune inflammatory disorder may occur during or 
shortly after parturition120 and has also been reported after meno­
pause119; approximately 15% of reported cases occur in males. Of 
the 57% of patients developing the disorder in association with 
pregnancy, most often the disorder occurs during the las

ncy, most often the disorder occurs during the last month 
of pregnancy or during the first 2 months postpartum.119 It is 
characterized by a lymphocytic and plasma cell pituitary infiltrate, 
which may be isolated or associated with other recognized endo­
crinopathies. Circulating antipituitary antibodies have occasion­
ally been reported, and the presence of isolated pituitary hormone 
deficiency may imply a selectively targeted autoimmune process 
to pituitary cell types. Although the natural

ess 
to pituitary cell types. Although the natural history is often short 
lived, the few comprehensive pathologic evaluations suggest that 
secondary adenohypophyseal cell atrophy, with a resultant empty 
sella, is a frequent outcome. Pathologic criteria for diagnosis 
include islands of anterior pituitary cells surrounded by diffuse 
lymphocytic (T-cell and B-cell) infiltrates. The defining feature is 
lymphocytic infiltration comprising T and B lymphocytes; plasma 
cells were found in 53%, eo

B lymphocytes; plasma 
cells were found in 53%, eosinophils in 12%, and macrophage 
histiocytes and neutrophils in 6% of cases,119 and mast cells have 
also been identified.
Over half of patients with lymphocytic hypophysitis present with 
headache, visual field impairment, and hyperprolactinemia,119 
and pituitary deficiency accounts for the remaining cases. Fifty-
six percent of patients have secondary hypoadrenalism, followed 
in frequency by hypothyroidism, hypogonadism, an

in frequency by hypothyroidism, hypogonadism, and GH or 
PRL deficiency. Hypothyroidism may occur later, even after 9 
months. In 76 patients reported by the German Pituitary Work­
ing Group, headache and obesity were prominent, and only 11% 
were associated with pregnancy. Hypogonadism was the most 
frequent feature of pituitary failure.121 MRI reveals a pituitary 
mass, often indistinguishable from an adenoma. Both intrasellar 
and suprasellar pituitary enlargement occur, and the pituitary 

r pituitary enlargement occur, and the pituitary 
stalk may be thickened, especially when diabetes insipidus is 
present. The inflammatory process often resolves with time, and 
initially abnormal pituitary function may be restored as well or 
may remain chronically compromised. Diabetes insipidus, which 
is encountered in up to 20% of patients, has been attributed to 
posterior pituitary or stalk infiltration of the inflammatory pro­
cess.122 In a study of 95 patients with autoimmune hypopitu­

 a study of 95 patients with autoimmune hypopitu­
itarism and positive antipituitary antibodies, those with central 
diabetes insipidus had antihypothalamic antibodies, suggest­
ing autoimmune involvement of the hypothalamus rather than 
expansion of the pituitary inflammatory process.123 In one-third 
of patients, other autoimmune conditions, including thyroiditis, 
hypoadrenalism, parathyroid failure, atrophic gastritis, systemic 
lupus erythematosus, or Sjögren syndrome, may also be present. 

atosus, or Sjögren syndrome, may also be present. 
The differential diagnosis includes prolactinoma and other sellar 
masses, and careful history and demonstrated loss of the poste­
rior pituitary “bright spot” on MRI are useful for supporting the 
diagnosis. 
Laboratory Findings
The erythrocyte sedimentation rate is often elevated; antibodies to 
a 49-kDa cytosolic protein were detected in 70% of patients with 
histologically confirmed lymphocytic hypophysitis and in 10% of 
control subjects.12

ic hypophysitis and in 10% of 
control subjects.124 PRL levels are usually elevated in both female 
and male patients. Hyperprolactinemia is expected during preg­
nancy and during the early postpartum period, and the mass effect 
may contribute to stalk compression and secondary hyperprolac­
tinemia in the others. GH and ACTH responses to hypothalamic 
hormone challenges may be blunted. Rarely, the disorder may be 
associated with isolated ACTH or TSH deficiencies. 
Treatment
If the diagnosis is

r TSH deficiencies. 
Treatment
If the diagnosis is convincingly supported, then in the absence 
of compressive visual field disturbances, surgical therapy should 
be withheld, pituitary hormone deficits appropriately replaced, 
and spontaneous resolution of the inflammatory mass expec­
tantly followed. Treatment with high-dose glucocorticoids is the 
mainstay, often resolving the sellar mass and improving endocrine 
dysfunction, although recurrences are reported in over 40% of 
those responding 

ces are reported in over 40% of 
those responding to glucocorticoids.125 Glucocorticoids are also 
indicated if adrenal reserve is compromised. Transsphenoidal or 
endoscopic surgical resection may be required to confirm a tissue 
diagnosis and may also relieve compression symptoms, but the 
degree of surgical resection should be constrained by the need to 
conserve viable pituitary tissue, particularly in view of frequent 
spontaneous resolution.125 
Granulomatous Hypophysitis
Granulomatous hyp

.125 
Granulomatous hypophysitis is not usually associated with preg­
nancy, but in a systematic review comprising 82 patients, females 
predominated.126 Rarely, the condition may coexist with lympho­
cytic hypophysitis in the same gland. Pituitary histologic appear­
ance shows features of chronic inflammation and granuloma with 
histiocytes and multinucleated giant cells. Patients present with 
headache and may have aseptic meningitis. Fever, nausea, or vom­
iting at 

eptic meningitis. Fever, nausea, or vom­
iting at presentation and histologic evidence of necrosis correlate 
with reduced time to presentation. Panhypopituitarism at presen­
tation is predictive of the requirement for long-term replacement 
therapy.126 MRI shows pituitary enlargement. Suprasellar exten­
sion occurs in about 60% of patients, often with extension to or 
compression of the optic chiasm (25.7%). The condition may 
reflect an underlying systemic disorder such as sarcoidosis127 or 
T

ing systemic disorder such as sarcoidosis127 or 
Takayasu disease.128 
Xanthomatous Hypophysitis
This rare primary pituitary inflammatory process occurs at equal 
frequency in both sexes and comprises lipid-laden macrophages. 
MRI often reveals a highly cystic lesion, possibly reflecting an 
inflammatory response to a ruptured pituitary cyst. 
Necrotizing Infundibulo-Hypophysitis
This rare form of hypophysitis has been reported in patients with 
an enlarged sellar mass. Patients present with dia

an enlarged sellar mass. Patients present with diabetes insipidus, 
hypopituitarism, and severe headache129 (Fig. 9.16). 
IgG4-Related Hypophysitis
Patients with these rarely encountered pituitary lesions may also 
have retroperitoneal fibrosis, pancreatitis, and thyroid, lung, and 
meningeal involvement.130 Only 34 such patients have been 
reported,131 and they may present with pituitary failure, diabetes 
insipidus, and elevated IgG4 levels. 
257
A
D
B
C

57
A
D
B
C
• Fig. 9.16  Infundibulo-hypophysitis depicted by MRI. The 22 × 20 × 16-mm pituitary mass is isointense to 
hyperintense on T2-weighted image with suprasellar extension (A) and hypointense in T1-weighted, non­
enhanced image (B). After intravenous contrast (C), only slight to moderate and inhomogeneous enhance­
ment occurs, most pronounced at the periphery of the mass. The pituitary stalk and infundibulum are 
slightly thickened, showing avid enh

ndibulum are 
slightly thickened, showing avid enhancement (D). (From Gutenberg A, Caturegli P, Metz I, et al. Necrotiz­
ing infundibulo-hypophysitis: an entity too rare to be true? Pituitary. 2012;15:202–208.)
Immune Checkpoint Inhibitor-Induced Hypophysitis
This form of hypophysitis is caused by exposure to an antibody, 
ipilimumab, used to treat metastatic cancer. The drug blocks 
the cytotoxic T-lymphocyte antigen 4 (CTLA4), which is also 
expressed on pituitary tissue, leading to local comp

pressed on pituitary tissue, leading to local complement acti­
vation132 (Table 9.8). In a single-center analysis of 211 patients 
with advanced melanoma, an 8% overall incidence of hypophy­
sitis was reported. Presenting symptoms include headache, nau­
sea, vomiting, extreme fatigue, diarrhea, arthralgias, and mental 
status changes.133 The median time to onset following drug 
administration was 4 months, but a delay in up to 19 months 
after treatment was also observed. The most common endocri

eatment was also observed. The most common endocrine 
deficit encountered was secondary adrenal insufficiency (84%). 
In contrast to other forms of hypophysitis, no patients receiving 
ipilimumab developed diabetes insipidus.134 Many also had evi­
dence of hypothyroidism/thyroiditis (6%). Eleven of 19 patients 
had biochemical evidence consistent with central hypothyroidism 
(low free T4 [thyroxine] and normal or low TSH). Diabetes has 
also been observed in these patients. Pituitary MRIs were c

 observed in these patients. Pituitary MRIs were clearly 
abnormal in most patients with diffuse enlargement. Treatment 
with high-dose steroids initially, followed by replacement doses in 
those with sustained secondary adrenal insufficiency, is effective. 
However, few patients exhibit return of normal endocrine func­
tion.135 Azathioprine has been found effective in some patients.136 
Less commonly, the antibodies pembrolizumab, nivolumab, and 
their respective combinations may also induce hy

their respective combinations may also induce hypophysitis. 
Hemorrhage and Infarction
Intrapituitary hemorrhage and infarction are usually caused by 
ischemic damage to the hypophyseal-portal system and may be 
catastrophic. These acute events cause significant damage to the 
258
pituitary gland, and small clinically silent microinfarcts are found 
in up to 5% of unselected autopsies. Pituitary cells are relatively 
resilient to vascular insult, and pi

e relatively 
resilient to vascular insult, and pituitary insufficiency is only clini­
cally apparent when approximately 75% of the gland is ischemi­
cally damaged. Ten percent residual functional pituitary cell mass 
appears sufficient to mask complete pituitary failure. Ischemic 
damage is limited to the anterior lobe, and posterior pituitary 
function usually remains intact, reflecting the predominant neu­
ral control of oxytocin and ADH secretion. Acute intrapituitary 
hemorrhage can cause s

tion. Acute intrapituitary 
hemorrhage can cause significant life-threatening damage to the 
pituitary and its surrounding vital structures.137
Postpartum Pituitary Infarction
During pregnancy, the pituitary gland normally enlarges in 
response to estrogen stimulation. The hypervascular gland is thus 
particularly vulnerable to arterial pressure changes and prone to 
hemorrhage. Sheehan syndrome classically described after severe 
postpartum hemorrhage is now less commonly encountered with 
the 

rrhage is now less commonly encountered with 
the advent of modern obstetric care,138 but it occurs much more 
frequently in developing countries.139,140 The presentation varies 
from development of hypovolemic shock resulting in adenohy­
pophyseal vessel vasospasm and pituitary necrosis to the gradual 
onset of partial to complete pituitary insufficiency over months to 
years. Initial presentations also include hyponatremia, asthenia, 
and weight loss. Most prominent among symptoms are inabil­

t loss. Most prominent among symptoms are inabil­
ity to nurse and postpartum amenorrhea.139 Pituitary autoim­
munity has been implicated in gland failure after postpartum 
hemorrhage.141 
Pituitary Apoplexy
Pituitary apoplexy may result from spontaneous hemorrhage into 
a pituitary adenoma (pituitary tumor apoplexy) or may occur 
after head trauma, with skull base fracture, or in association with 
hypertension and diabetes mellitus, sickle cell anemia, or acute 
hypovolemic shock142 (Table 9.9)

anemia, or acute 
hypovolemic shock142 (Table 9.9). Precipitating factors include 
major surgery, pregnancy, stereotactic irradiation, anticoagulant 
therapy,143 coagulopathy secondary to liver failure,144 and admin­
istration of thyrotrophin-releasing hormone (TRH), gonadotro­
phin-releasing hormone (GnRH) agonists, bromocriptine, and 
cabergoline.145,146
Pituitary apoplexy is often an endocrine emergency.147 The condi­
tion may evolve over 1 to 2 days, usually presenting with

y evolve over 1 to 2 days, usually presenting with severe 
headache and ocular palsies or visual field defects. Cardiovascular 
collapse, change in consciousness, neck stiffness, and sometimes 
hypoglycemia may occur. Bilateral cerebral infarction may also 
occur.148 Acute adrenal insufficiency is a frequent occurrence due 
to loss of ACTH. It may also be superimposed owing to disor­
dered intravascular clotting disorders, heparin administration, 
or acute effects of CNS hemorrhage. Pituitary im

or acute effects of CNS hemorrhage. Pituitary imaging without 
contrast (CT or MRI) usually reveals signs of intrapituitary or 
intra-adenoma hemorrhage, stalk deviation, compression of nor­
mal pituitary tissue, and in severe cases signs of parasellar hemor­
rhage149 (Fig. 9.17). In a study of 13 consecutive patients with 
pituitary apoplexy, baseline serum cortisol was below 5 μg/dL in 
7, between 5 and 15 μg/dL in 4, and above 15 μg/dL in 2 patients. 
Five patients also had low T4 levels, a

patients. 
Five patients also had low T4 levels, and all 13 had evidence of 
gonadal dysfunction. Thus, these patients with underlying pitu­
itary tumors likely had preexisting pituitary insufficiency.150 Apo­
plexy, like Sheehan syndrome, is one of the few pituitary tumor 
presentations in which hyperprolactinemia is not a feature unless 
the infarction occurs within a prolactinoma. Patient characteris­
tics, signs and symptoms, and outcomes in 207 patients in five 
series are portrayed in Tabl

207 patients in five 
series are portrayed in Table 9.9.13 
Patients with visual field compromise require emergency trans­
sphenoidal surgery. Others may recover spontaneously but may 
develop long-term pituitary insufficiency. Patients who are fully 
alert and conscious with no visual symptoms may be observed. 
The decision to initiate therapy with high-dose glucocorticoids 
depends on the clinical status,137 but the high incidence of adrenal 
dysfunction either before or after treat

 adrenal 
dysfunction either before or after treatment indicates a need for 
replacement or stress doses of cortisone in most. Ophthalmople­
gia, which is common, may resolve spontaneously over time.137 
Postoperative recovery of visual function correlates inversely with 
the time elapsed subsequent to the acute hemorrhage.151 Cranial 
nerve palsies, however, often improve whether or not surgery is 
undertaken. Pituitary function does not commonly recover after 
resolution of the acute hemorrhag

y recover after 
resolution of the acute hemorrhage, and adrenal, thyroid, and 
gonadal steroid hormone replacement may be required. The sub­
sequent atrophy of infarcted pituitary tissue often results in the 
development of a complete or partially empty sella evident on 
MRI. 
Pituitary Adenomas
Pituitary Trophic Activity
Benign Adenomas
Transgenic animal models have been developed in which pitu­
itary growth factors or genes have been overexpressed or deleted 
to recapitulate both functional a

ssed or deleted 
to recapitulate both functional and nonfunctional pituitary 
adenomas. For example, a transgenic zebrafish expressing corti­
cotroph-targeted pituitary tumor-transforming gene (PTTG) 
recapitulates a Cushing phenotype with pituitary adenoma growth 
  Features of Ipilimumab-Induced 
Hypophysitis
Measurement
Result
Incidence
10%
Ipilimumab dose
3 or 10 mg/kg
M:F ratio
2:1
Time until hypophysitis diagnosis
7–16 weeks
Anterior hypopituitarism
All affected patients
Central hypothyro

ituitarism
Central hypothyroidism
60–100%
Adrenal insufficiency
50–84%
Thyroiditis
Up to 25%
Hyponatremia
Up to 50%
Prolactin
Usually low
Data from retrospective analyses of 365 patients treated for melanoma in Faje AT, Sullivan R, 
Lawrence D, et al. Ipilimumab-induced hypophysitis: a detailed longitudinal analysis in a large 
cohort of patients with metastatic melanoma. J Clin Endocrinol Metab. 2014;99:4078–4085; 
Ryder M, Callahan M, Postow MA, e

4;99:4078–4085; 
Ryder M, Callahan M, Postow MA, et al. Endocrine-related adverse events following ipilim­
umab in patients with advanced melanoma: a comprehensive retrospective review from a 
single institution. Endocr Relat Cancer. 2014;21:371–381.
   
TABLE 9.8
259
  Presentation and Outcomes After Conservative or Surgical Management of Pituitary Apoplexy
AYUK (2004)
GRUBER (2006)
SIBAL (2004)
LEYER (2011)
BUJAWANSA (2014)
Conservative
Surgery
Conservat

1)
Surgery
Surgery
Surgery
Surgery
Surgery
N
18
15
20
10
18
27
25
19
22
33
Decreased visual acuity, n (%)
NA
NA
11 (55)
7 (70)
4/15 (26)
14/24 (58)a
8 (32)
16 (84)
NA
NA
Visual field defect, n (%)
6 (33)
7 (46)
4 (20)
6 (60)
4/17 (24)
16/25 (64)a
5 (20)
14 (74)
10 (45)
13 (39)
Ocular palsy, n (%)
7 (39)
8 (53)
12 (60)
3 (37)
8/17 (47)
14/26 (54)
12 (48)
10 (53)
15 (68)
18 (54)
Hypopituitarism, n (%)
13

10 (53)
15 (68)
18 (54)
13 (87)
15 (83)
15 (75)
9 (90)
13/18 (72)
21/24 (87)
20/23 (87)
15/17 (88)
NA
NA
Outcome
Decreased visual acuity
Complete resolution, n (%)
NA
NA
5/11 (45)
4/7 (57)
3/4 (75)
8/14 (57)
6/8 (75)
7/16 (44)
NA
NA
Partial resolution, n (%)
NA
NA
4/11 (36)
2/7 (28)
1/4 (25)
5/14 (36)
1/8 (12)
1/16 (6)
NA
NA
No improvement, n (%)
NA
NA
2/11 (19)
1/7 (15)
0
1/14 (7)
1/8 (12)
6/16 (37)
NA
NA
Visual field defect
6/6 (100)
4/7 (57)

ect
6/6 (100)
4/7 (57)
2/4 (50)
2/6 (33)
3/4 (75)
7/16 (43)
4/5 (80)
8/14 (57)
6/10 (60)
4/13 (31)
0
NA
1/4 (25)
3/6 (50)
1/4 (25)
8/16 (50)
NA
1/14 (7)
NA
NA
0
NA
1/4 (25)
1/6 (17)
0
1/16 (7)
NA
4/14 (29)
NA
NA
Ocular palsy
7/7 (100)
5/8 (63)
10/12 (83)
2/3 (66)
6/8 (75)
9/14 (64)
11/12 (92)
6/10 (60)
15/15 (100)
15/18 (83)
0
NA
2/12 (17)
1/3 (33)
2/8 (25)
4/14 (29)
1/

 (%)
0
NA
2/12 (17)
1/3 (33)
2/8 (25)
4/14 (29)
1/12 (9)
1/10 (10)
0
3/18 (17)
0
NA
0
0
0
1/14 (7)
0
2/10 (20)
0
0
Endocrine function
Normal
NA
NA
1 (5)
2 (20)
2 (11)
5 (19)
9 (37)
3 (16)
2/22 (9)
3/33 (9)
ACTH deficiency, n (%)
13/18 (72)
13/15 (87)
(68)
(60)
NA
NA
NA
NA
NA
NA
TSH deficiency, n (%)
9/15 (60)
13/15 (87)
(70)
(68)
NA
NA
NA
NA
NA
NA
LH/FSH deficiency, n (%)
15/18 (83)
10/15 (67)
(80)
(86)
NA
NA
NA
NA
NA
NA
aP = 0.01.
ACTH, Adrenocorticotrophic hormone FSH, fo

= 0.01.
ACTH, Adrenocorticotrophic hormone FSH, follicle-stimulating hormone; LH, luteinizing hormone; NA, not available; TSH, thyrotrophic hormone.
Modified from Briet C, Salneve S, Bonneville JF, et al. Pituitary apoplexy. Endocr Rev. 2015;36:622–645.
   
TABLE 9.9
260
and hypercortisolism.152 In humans, benign monoclonal pituitary 
adenomas arise from differentiated pituitary cells (see Fig. 9.2). 
Pituitary trophic signals may enhance or restrain expa

itary trophic signals may enhance or restrain expansion of a 
monoclonal tumor cell population by regulating the intrapituitary 
milieu.153,154
Normal and hyperplastic pituitary tissues are polyclonal, whereas 
pituitary adenomas arise as the result of monoclonal pituitary cell 
proliferation. Using X-chromosomal inactivation analysis, the 
monoclonal origin of GH-secreting, PRL-secreting,155 and ACTH-
secreting adenomas156,157 and nonfunctioning pituitary tumors was 
confirmed in female patient

 pituitary tumors was 
confirmed in female patients heterozygous for variant alleles of the 
X-linked genes hypoxanthine phosphoribosyltransferase (HPRT) 
and phosphoglycerate kinase (PGK). Thus an intrinsic somatic 
pituitary cell genetic alteration likely gives rise to clonal expansion 
of a single cell, resulting in adenoma formation (Table 9.10).
The pituitary gland responds to central and peripheral signals 
that regulate both hormone production and cell proliferation 
ranging from microade

tion and cell proliferation 
ranging from microadenoma to aggressive adenoma (Fig. 9.18). 
For example, during pregnancy hypothalamic and peripheral 
hormones act to regulate pituitary trophic activity resulting in 
increased pituitary volume, and prolonged target gland failure 
(e.g., hypothyroidism) can cause pituitary hyperplasia by releas­
ing the gland from negative feedback inhibition. There is, how­
ever, no direct evidence in humans that pituitary hyperplasia is 
a prerequisite for tumor

pituitary hyperplasia is 
a prerequisite for tumor development. Thus lactotroph hyper­
plasia occurring with pregnancy and lactation does not lead to 
increased frequency of prolactinomas. Oral contraception use 
is also not associated with pituitary adenoma development, and 
somatotroph hyperplasia caused by ectopic GHRH production158 
is not commonly associated with true adenoma formation. Ade­
nohypophyseal tissue surrounding pituitary tumors is usually not 
hyperplastic, supporting the notio

is usually not 
hyperplastic, supporting the notion that hypothalamic hormones, 
pituitary growth factors, and sex steroid hormones enable a per­
missive environment, which potentiates cell mutation and subse­
quent tumor growth. 
Hormonal Factors
Hypothalamic factors may have a specific role in the pathogenesis 
of pituitary cell proliferation, in addition to regulating pituitary 
hormone gene expression and secretion (see Table 9.10). For 
example, ectopic GHRH-secreting tumors (bronchial neur

ple, ectopic GHRH-secreting tumors (bronchial neuro­
endocrine tumors, pancreatic islet cell tumors, or small cell lung 
carcinomas) result in GH hypersecretion and acromegaly with 
somatotroph hyperplasia.158,159 In transgenic mice overexpressing 
a GHRH transgene, the pituitary size increases dramatically due 
to somatotroph hyperplasia, and older mice develop GH-secreting 
adenomas.160 However, adenomatous hormonal secretion is usually 
independent of physiologic hypothalamic control, and the

ndent of physiologic hypothalamic control, and the surgical 
resection of small well-defined adenomas usually results in defini­
tive cure of hormonal hypersecretion. These observations imply 
that these tumors do not arise because of excessive polyclonal pitu­
itary cell proliferation due to generalized hypothalamic stimulation. 
However, hypothalamic factors may promote and maintain growth 
of already transformed adenomatous pituitary cells (Fig. 9.19). 
Genetic Factors
Activating mutations of

g. 9.19). 
Activating mutations of the stimulatory G protein (Gs) are pres­
ent in up to 40% of human GH-secreting adenomas (Table 9.11). 
These somatic heterozygous activating point mutations of the 
G protein α-subunit (Gαs) gene involving either arginine 201 
(replaced by cysteine or histidine) or glutamine 227 (replaced 
by arginine or leucine) constitutively activate the Gαs protein 
and convert it into an oncogene (GSP). This G protein activa­
tion increases cyclic adenosin

s G protein activa­
tion increases cyclic adenosine monophosphate (cAMP) levels; 
activates protein kinase A, which in turn phosphorylates the 
cAMP response element-binding protein (CREB); and leads to 
sustained constitutive GH hypersecretion and cell proliferation. 
GSP-bearing adenomas are smaller, have mildly lower GH lev­
els and enhanced intratumoral cAMP, do not respond briskly to 
GHRH, and are sensitive to the inhibitory effect of somatosta­
tin.161 GSP-activating mutations do not occu

osta­
tin.161 GSP-activating mutations do not occur in PRL-secreting 
and in TSH-producing adenomas and are very rarely present in 
nonfunctioning pituitary tumors or ACTH-secreting tumors 
(<10%). Similar early postzygotic somatic mutations in codon 
201 of the Gαs were identified in tissues derived from patients 
with McCune-Albright syndrome.162 As transgenic mice overex­
pressing an inactive pituitary CREB mutant exhibit a dwarf phe­
notype and somatotroph hypoplasia,163 cAMP likely stimulat

nd somatotroph hypoplasia,163 cAMP likely stimulates 
A
B
• Fig. 9.17  Pituitary apoplexy depicted by MRI showing intrapituitary fluid levels. T1-weighted sagittal (A) 
and (B) axial images show hyperintensity in the upper compartment and isointensity in the lower compart­
ment. (From Briet C, Salenave S, Bonneville JF, et al. Pituitary apoplexy. Endocr Rev. 2015;36:622–645.)
261
somatotroph proliferation mediated by CREB phosphorylation. 
This was borne ou

diated by CREB phosphorylation. 
This was borne out by the observation that 15 human GH-secret­
ing pituitary adenomas were shown to express elevated levels of 
phosphorylated CREB.164 However, only four of these tumors 
also contained the mutant GSP oncogene, and CREB phosphory­
lation was also demonstrated in adenomas overexpressing wild-
type Gαs protein, suggesting a trophic role of CREB independent 
of G protein actions. Other signaling pathways overexpressed in 
pituitary tumors include th

ways overexpressed in 
pituitary tumors include those for AKT and mitogen-activated 
protein kinase (MAPK).165
Mice with heterozygous Rb1 inactivation develop pituitary 
tumors with high penetrance, but mice with deregulated pituitary 
E2F activity develop tissue hyperplasia without progression to 
tumor formation, likely because sustained E2F activity ultimately 
triggers premature senescence in a pRB-dependent, p16-depen­
dent, and p19-dependent fashion.166 About 15% of spontane­
ous pituitary

t fashion.166 About 15% of spontane­
ous pituitary adenomas exhibit loss of heterozygosity (LOH) for 
chromosomes 11q13, 13, and 9, often correlating with tumor size 
and invasiveness. Although highly invasive pituitary tumors and 
pituitary metastases exhibit LOH of region 13q14 (RB1 locus), 
no distinct tumor suppressor gene has yet been identified for spo­
radic pituitary tumors.167 Nevertheless approximately 25% of 
GH-secreting adenomas do exhibit loss of pRB expression, likely 
associated 

xhibit loss of pRB expression, likely 
associated with promoter hypermethylation.168 TP53 gene muta­
tions (encoding the p53 protein) have not been detected in pitu­
itary adenomas or in pituitary carcinomas and their metastases.169
PTTG, isolated from experimental pituitary tumors, is abun­
dant in all pituitary tumor types, especially prolactinomas.170,171 
PTTG, a mammalian securin homolog, also induces fibroblast 
growth factor (FGF) production and angiogenesis and is upregu­
lated by estrog

on and angiogenesis and is upregu­
lated by estrogen.172 PTTG overexpression may lead to dysregu­
lated chromatid separation and cell aneuploidy,173,174 and pituitary 
targeted PTTG transgene expression in mice leads to hormone-
secreting adenomas.175 PTTG mutations have not been identified 
in pituitary tumors, but tumor PTTG abundance is induced by 
the RWD-containing sumoylation enhancer (RSUME).176 In a 
meta-analysis of 1464 adenomas in 24 studies, PTTG abundance 
was shown to correlate wit

tudies, PTTG abundance 
was shown to correlate with pituitary tumor invasiveness.177
Cyclin D1, D2, and D3 are upregulated when quiescent cells 
enter the cell cycle and cyclin-dependent kinase (CDK) complexes 
lead to pRB phosphorylation, releasing E2F to promote cell cycle 
progression. Allelic imbalance at the CCND1 locus encoding 
cyclin D1 is frequently observed in invasive, nonfunctioning pitu­
itary adenomas.178 The CDK4 and CDK6 inhibitor p16INK4a, 
encoded by the CDKN2A gene, maintains 

 p16INK4a, 
encoded by the CDKN2A gene, maintains RB in the unphosphor­
ylated state. In nonfunctioning pituitary tumors, the CDKN2A 
promoter is hypermethylated and consequently not expressed; 
less frequently the CDKN2B gene that encodes p15INK4b is also 
silenced. Mice deficient in p18INK4c develop features of gigan­
tism with intermediate lobe pituitary hyperplasia and tumors.179 
When the CDK1 and CDK2 inhibitor p27Kip1 is knocked out, 
multiorgan hyperplasia occurs with development of inte

iorgan hyperplasia occurs with development of intermediate 
lobe pro-opiomelanocortin (POMC) cell pituitary tumors.180
Basic FGF (FGF2) is abundantly expressed in the pituitary, 
and pituitary PTTG and FGF2 expression is increased in a time-
dependent and dose-dependent manner in estrogen-treated 
rats.172 Human prolactinomas express FGF4, and transfected 
FGF4 enhances PRL secretion and tumor vascularity.181
Epidermal growth factor (EGF) exhibits potent mitogenic 
activity in pituitary cells, a

s potent mitogenic 
activity in pituitary cells, and both EGF and its receptor EGFR are 
overexpressed in pituitary tumors, particularly in ACTH-secreting 
and nonfunctioning adenomas. EGFR abundance in a subset of 
ACTH-secreting tumors is induced by USP8 mutations leading 
to deubiquitination.182 Both ErbB2 and ErbB3 are expressed 
in aggressive, recurrent prolactinomas.183 Gefitinib, the EGFR 
antagonist, decreases experimental prolactinoma cell proliferation 
and PRL secretion in vitro and i

ll proliferation 
and PRL secretion in vitro and in vivo, associated with abrogated 
EGFR/ERK signaling.184 Two patients with dopamine-resistant 
aggressive prolactinomas showed attenuation of PRL levels and 
stabilization of tumor growth after treatment with lapatinib, a 
tyrosine kinase inhibitor.185
The growth arrest and DNA damage–inducible protein γ 
(GADD45γ), a tumor growth suppressor, is silenced in most 
pituitary adenomas, likely by methylation of CpG islands in 
  Factors Involved in

hylation of CpG islands in 
  Factors Involved in Pituitary Tumor 
Pathogenesis
Hereditary
MEN1
Carney complex
AIP mutations
Excess GHRH or CRH production
Receptor activation
Dopamine deprivation
Pituitary
Signal transduction mutations or constitutive activation (GSP, USP8, 
GPR101, CREB, McCune-Albright syndrome)
Disrupted paracrine growth factor or cytokine action (EGFR, FGF2, FGF4, 
LIF, BMP, STAT3)
Activated oncogene or cell cycle disruption (PTTG, RAS, P27, HMG)
Intrapituitary

le disruption (PTTG, RAS, P27, HMG)
Intrapituitary paracrine hypothalamic hormone action (GHRH, TRH)
Loss of tumor suppressor gene function with LOH (11q13, GADD45γ)
Environmental
Estrogens
Target failure (ovary, thyroid, adrenal)
Ectopic hypothalamic hormone secretion
Evidence for an Intrinsic Pituitary Defect in the 
Pathogenesis of Pituitary Tumors
Pituitary adenomas are monoclonal.
There is no hyperplasia surrounding the adenomas.
Surgical resection of well-circumscrib

e adenomas.
Surgical resection of well-circumscribed small adenomas leads to 
biochemical control in ∼75% of patients.
Unrestrained pituitary hormonal hypersecretion persists independent of 
feedback suppression by elevated target hormones.
Hormonal pulsatility pattern is often restored after adenoma resection.
AIP, Aryl hydrocarbon receptor–interacting protein; BMP, bone morphogenetic protein; CREB, 
cyclic adenosine monophosphate response element–binding protein; CRH, corticotrophin-
releasing

nt–binding protein; CRH, corticotrophin-
releasing hormone; EGF, epidermal growth factor; FGF, fibroblast growth factor; GADD45γ, 
growth arrest and DNA damage–inducible gamma gene; GHRH, growth hormone–releasing 
hormone; GSP, stimulatory G protein α-subunit oncogene; HMG, high mobility group; LIF, 
leukemia inhibitory factor; LOH, loss of heterozygosity; MEN1, multiple endocrine neoplasia 
type 1; P27, cyclin-dependent kinase inhibitor 1B; PROP1, prophet of Pit1 (paired-like home­
odomain tran

1, prophet of Pit1 (paired-like home­
odomain transcription factor); PTTG, pituitary tumor-transforming gene; RAS, RAS family of 
oncogenes; TRH, thyrotrophin-releasing hormone.
Data from Melmed S. Acromegaly pathogenesis and treatment. J Clin Invest. 2009;119:3189–
3202; Melmed S. Pathogenesis of pituitary tumors. Nat Rev Endocrinol. 2011;7:257–266.
   
TABLE 9.10
Oncogene activation
Tumor suppressor gene inactivation
Cell cycle dysregulation
Normal pituitary
Microadenoma
Macroadenoma
Aggressiv

rmal pituitary
Aggressive and/or
malignant
Additional mutations
Stromal events
Epigenetic events
Unknown changes
Proliferative
restraint
Senescence
Humoral factors
• Fig. 9.18  Cascade of pituitary tumorigenesis. Pituitary hyperplasia is usually reversible, as exemplified by 
the situation that occurs during pregnancy. (Modified from Melmed S. Pathogenesis of pituitary tumors. 
Nat Rev Endocrinol. 2011;7:257–266; Di Ieva A, Rotondo F, Sy

docrinol. 2011;7:257–266; Di Ieva A, Rotondo F, Syro LV, et al. Aggressive pituitary adenomas: 
diagnosis and emerging treatments. Nat Rev Endocrinol. 2014;10:423–435, used with permission.)
constraints
Chromosomal instability
DNA damage 
CDK
inhibitors
p15, p16, p18, p19
CDK4
CDK6
CDK2
Cyclin E
Cyclin D
p21, p27, p57
p53
Cell cycle disruptors
Cell
Stimulating
receptors:
GHRH, CRH, GnRH
Suppressing
Dopamine
PRL, GH, ACTH,
TSH, LH, FS

:
Dopamine
TSH, LH, FSH
Mitogenic hormones
P
P
P
P
P
P
Rb
E2Fs
Hormone gene
Cytoplasm
Nucleus
CDK2
Cyclin A
CDK1
Cyclin B 
M
G0
S
G1
G2
P
P
P
P
P
P
• Fig. 9.19  Pituitary tumorigenesis. Transcription of hormone genes in a differentiated cell and cell prolifera­
tion are mostly induced by pituitary mitogenic factors, including hypothalamic hormones and transcription 
factors, as well as endocrine hormones. Pituitary proliferative constraints include 

ones. Pituitary proliferative constraints include somatotrophin release-
inhibiting factor and tumor suppressor genes. Cell cycle progression through G1 to S phase is mediated 
by CDK-cyclin complexes that phosphorylate Rb and cause it to release E2F, which drives cell proliferation. 
CDK inhibitors block kinase phosphorylation, thereby restraining the cell cycle. Chromosomal instabil­
ity, DNA damage, and senescence may act to constrain malignant transformation of pituitary tumors. 
ACTH, adren

t transformation of pituitary tumors. 
ACTH, adrenocorticotrophic hormone; CDK, cyclin-dependent kinase; CRH, corticotrophin-releasing hor­
mone; FSH, follicle-stimulating hormone; GHRH, gonadotrophin-releasing hormone; GH, growth hormone; 
GnRH, growth hormone–releasing hormone; LH, luteinizing hormone; PRL, prolactin; TSH, thyroid-stim­
ulating hormone. (Modified from the American Society for Clinical Investigation © Melmed S. Acromegaly 
pathogenesis and treatment. J Clin Invest. 2009;119:318

genesis and treatment. J Clin Invest. 2009;119:3189–3202, used with permission.)
263
the gene promoter.186 An isoform of MEG3 containing an 
extra exon (MEG3a) is undetectable in both nonfunctioning 
and GH-secreting pituitary adenomas, conferring a tumor 
growth advantage, likely due to hypermethylation of the pro­
moter region.187
Transgenic HMGA1 and HMGA2 overexpression results in 
murine GH-secreting adenomas and prolactinomas, and chro­
mosome 12 tris

enomas and prolactinomas, and chro­
mosome 12 trisomy, the locus for HMGA2, is frequently encoun­
tered in human PRL-secreting pituitary adenomas. HMGA2 
was overexpressed in 38 of 98 (39%) pituitary adenomas, being 
found in 15 of 22 FSH/LH-secreting adenomas (68%), 5 of 15 
prolactinomas (31%), and 12 of 18 ACTH-secreting adenomas 
(18%).188 HMGA2 levels correlate with tumor size, invasiveness, 
and proliferation markers. HMGA2 tumorigenic effects may also 
be mediated by inducing cyclin B2 ex

cts may also 
be mediated by inducing cyclin B2 expression189 and activation 
of the E2F pathway. HMGA2 is suppressed by Let-7 microRNA, 
and HMGA2 and Let-7 expression correlates inversely in human 
pituitary adenoma samples.188 Pituitary adenomas exhibit a high 
degree of aneuploidy, and when 127 adenomas were subjected 
to multiplexed next-generation sequencing, copy number varia­
tions were found to be more prominent in hormone-secreting 
adenomas.190
Frequent epigenetic changes are encounte

nomas.190
Frequent epigenetic changes are encountered in all pituitary 
tumor types, and novel targeted therapies to reverse disrupted 
epigenomic events have been proposed.191 
Pituitary Senescence
Cellular senescence, or cell growth arrest, is induced by age-linked 
telomere shortening, DNA damage, oxidative stress, and onco­
gene activation. Oncogene-induced premature cell cycle arrest is 
protective of the cellular response to oncogenic events, is largely 
irreversible, and is mediated throu

s, is largely 
irreversible, and is mediated through upregulation of cell cycle 
inhibitors, including p16INK4A, p15INK4B, p21CIP1, and p53. 
p21CIP1 induction and senescence markers were elevated in all of 
the 38 GH-secreting adenomas tested.192 In contrast, p21CIP1 
was undetectable in pituitary carcinomas, nonsecreting pituitary 
oncocytomas, and null cell adenomas. Senescence-associated 
β-galactosidase activity, a marker of senescence, is also strongly 
positive in GH-secreting adenomas. A

lso strongly 
positive in GH-secreting adenomas. As cellular senescence is pro­
tective for malignant transformation,193 this process may underlie 
the invariably benign nature of pituitary adenomas.166 
Familial Syndromes
These rare syndromes are summarized in Table 9.12 and include 
the following types.
Multiple Endocrine Neoplasia Type 1
Multiple endocrine neoplasia type 1 (MEN1) is an autosomal 
dominant hereditary disorder characterized by tumor forma­
tion or hyperfunction of parathyroid, 

umor forma­
tion or hyperfunction of parathyroid, pancreatic islets, anterior 
  Selected Genes Associated With Molecular Pathogenesis of Pituitary Adenomas
Gene
Function
Mode of Activation or Inactivation
Clinical Context
GNAS
Oncogene
Activating, imprinting
Nonfamilial, syndromic or sporadic
CREB
Transcription factor
Constitutive phosphorylation
Sporadic
AIP
Tumor suppressor
Inactivating
Familial, syndromic
MEN1
PRKAR1A
Inacti

amilial, syndromic
PRKAR1A
H-Ras
Oncogene
Activating
Invasive or malignant
CCNB2
Cyclin
Induced by HMGA
Sporadic
CCND1
Oncogene
Overexpression
Sporadic
CDKNIB
Cyclin-dependent kinase inhibitor
Sporadic, syndromic
HMGA2
Oncogene
Sporadic
FGFR4
Oncogene
Alternative transcription
Sporadic
PTTG
Securin
Sporadic
Rb
Epigenetic silencing
Sporadic
CDKN2A
Epigene

c
CDKN2A
Sporadic
BRG1
Glucocorticoid receptor function
Sporadic
GADD45G
Proliferation inhibitor
Sporadic
MEG3
Sporadic
USP8
Pro-proliferative (corticotrophs)
Ubiquitination
Sporadic
GPR101
Hypersecretion (GH)
Duplication
Sporadic
STAT3
Transcriptional
Sporadic
Modified from Melmed S. Acromegaly pathogenesis and treatment. J Clin Invest. 20

galy pathogenesis and treatment. J Clin Invest. 2009;119:3189–3202; used with permission of the American Society for Clinical Investigation.
   
TABLE 9.11
264
pituitary, and less commonly, other neuroendocrine, thyroid, 
and adrenal tumors.194 Pituitary tumors are nonfunctioning 
or prolactin-secreting or GH-secreting microadenomas.195 The 
MEN1 syndrome (fully described in Chapter 42), is associ­
ated with germ cell inactivation of the MEN1 gene (encodi

th germ cell inactivation of the MEN1 gene (encoding 
the menin protein) located on chromosome 11q13.196 Unlike 
pituitary tumors in the MEN1 syndrome, MEN1 gene muta­
tions were not identified in non-MEN1 familial pituitary ade­
nomas.197 Mutations of other genes may also confer a MEN1 
clinical syndrome. Approximately 20% of patients with a clini­
cal diagnosis of MEN1 do not exhibit identifiable MEN1 muta­
tions, and rarely the gene for p27Kip1 (CDKN1B) is mutated 
in patients with clinical f

1 (CDKN1B) is mutated 
in patients with clinical features of MEN1 but with no MEN1 
mutations.198 
Familial Isolated Pituitary Adenomas
Less than 5% of prolactinomas and GH-secreting tumors are 
inherited on a familial basis.199 In familial acromegaly, about 
25% of afflicted individuals are diagnosed as teenagers or young 
adults, usually with gigantism. These patients have been linked 
to LOH at the 11q13.1-11q13.3 locus.200 Germline mutations 
of the aryl hydrocarbon receptor–interacting prot

of the aryl hydrocarbon receptor–interacting protein (AIP) were 
found to predispose to familial pituitary tumors with autosomal 
dominant inheritance.199,201 Eleven of 73 families with familial 
isolated pituitary adenomas were found to have 10 germline AIP 
gene mutations associated mainly with GH-secreting and PRL-
secreting tumors.202,203 AIP mutations are rarely encountered in 
patients with sporadic tumors and have been reported mainly in 
the youngest patients with acromegaly or gigantis

the youngest patients with acromegaly or gigantism.204,205 When 
patients under the age of 30 with sporadic macroadenomas were 
tested, 11.7% were found to harbor germline AIP mutations. In 
young acromegaly patients, the frequency of AIP mutations ranged 
from 2.3% to 5.5% with no apparent differences in patients with 
or without mutations.206 
Carney Complex
Carney complex is an autosomal dominant disorder compris­
ing benign mesenchymal tumors, including cardiac myxomas, 
schwannomas, thyroi

s, including cardiac myxomas, 
schwannomas, thyroid adenomas, and pituitary adenomas asso­
ciated with spotty skin pigmentation.207 The disorder has been 
mapped to chromosome 17q24 and results from a mutated 
type 1α regulatory subunit (R1α) of the cAMP-dependent 
protein kinase A (PRKAR1A).208 These patients may have 
elevated levels of GH, IGF1, or PRL, and 10% exhibit clini­
cal acromegaly with GH-secreting tumor formation associated 
with inactivating mutations of PRKAR1A, leading to consti

activating mutations of PRKAR1A, leading to constitu­
tive protein kinase A catalytic subunit activation. Excess GH 
levels in those patients with concomitant acromegaly may, in 
fact, be a growth factor in development of cardiac myxomas.209 
In some patients, the wild-type PRKAR1A allele is retained 
in tumor tissue, and haploinsufficiency may be sufficient for 
tumorigenesis. A 17-miRNA signature for pituitary tumors 
was identified on the background of pituitary hyperplasia 
and PRKARIA mutat

ground of pituitary hyperplasia 
and PRKARIA mutations. In GH-secreting tumors, miR-26b 
and miR-128 appear to regulate the PTEN-AKT pathway.210 
These defects provide attractive potential subcellular thera­
peutic targets.
In summary, multifactorial mechanisms subserve the mul­
tistep pathogenetic process of pituitary adenoma formation, 
including early initiating chromosomal mutations that result 
in mutated pituitary stem or progenitor cells. The transformed 
pituitary cell is subjected to si

The transformed 
pituitary cell is subjected to signals facilitating clonal expansion, 
and permissive factors, including hypothalamic hormone recep­
tor signals, intrapituitary growth factors, and disordered cell 
  Genes Associated With Familial Pituitary Tumor Syndromes
Syndrome
Gene (Locus)
Most Frequent Mutation(s)
Pituitary Features
Other Features
MEN1
MEN1 (11q13)
c.249-252delGTCT, an exon 
2 predicted frameshift, 
in 4.5%
Pituitary adenoma in 
30–40% (PRL 60%, NFA 
15%, GH 10%, ACTH 5%,

ma in 
15%, GH 10%, ACTH 5%, 
TSH rare)
Primary hyperparathyroidism, pancreatic 
tumors, foregut carcinoid tumors, 
adrenocortical tumors (usually 
nonfunctional), rarely pheochromocy­
tomas, skin lesions (facial angiomas, 
collagenomas, and lipomas)
MEN1-like (MEN4)
CDKN1B (12p13)
Only two reported cases
Pituitary adenomaa
Primary hyperparathyroidism and single 
cases reported of renal angiomyo­
lipoma, neuroendocrine cervical 
carcinoma
PRKAR1A (17q23-24)
c

cal 
carcinoma
c.491-492delTG in exon 5
Pituitary hyperplasia in most 
patients
Adenoma in ∼10% (GH and 
PRL)
Atrial myxomas, lentigines, Schwann cell 
tumors, adrenal hyperplasia
Familial, isolated 
pituitary adeno­
mas
AIP b (11q13.3)
Gln14X nonsense mutationc
Pituitary adenoma (majority 
GH, PRL, or mixed GH 
and PRL)
Young patients, often macroadenomas 
with gigantism
aOnly two reported cases to date: one GH-secreting adenoma and one ACTH-secreting adenoma.

secreting adenoma and one ACTH-secreting adenoma.
bAIP mutations reported in 15% of individuals with familial isolated pituitary adenoma and 50% of those with isolated familial somatotropinomas.
cThis is the most commonly identified mutation but is likely to be overrepresented secondary to a Finnish founder effect.
ACTH, Adrenocorticotrophic hormone; GH, growth hormone; MEN1, multiple endocrine neoplasia type 1; NFA, nonfunctioning adenoma; NR, not reported; PRL, prolactin.
From Elston MS, McDon

ot reported; PRL, prolactin.
From Elston MS, McDonald KL, Clifton-Bligh RJ, et al. Familial pituitary tumor syndromes. Nat Rev Endocrinol. 2009;5:453–461.
   
TABLE 9.12
265
cycle regulation, may determine the ultimate biologic fate of the 
tumor. Autonomous anterior pituitary hormone production and 
secretion and cell proliferation, which are the hallmarks of pitu­
itary adenomas, result. However, proximal subcellular events ini­
tiating the formation of m

subcellular events ini­
tiating the formation of most sporadic pituitary adenomas have 
yet to be elucidated. 
Pituitary tumors account for about 15% of all intracranial neo­
plasms and are commonly encountered at autopsy211 (Table 
9.13). The Brain Tumor Registry of Japan reports that 15.8% 
of 28,424 cases were histologically confirmed pituitary adeno­
mas.212 The prevalence of pituitary tumors in Belgium is 1 in 
1064 inhabitants,213 and in Banbury, United Kingdom, 63 pitu­
itary

213 and in Banbury, United Kingdom, 63 pitu­
itary tumors were noted in 89,334 inhabitants, a population 
prevalence of approximately 77 cases per 100,000. Of these, 
57% were prolactinomas, 28% were nonfunctioning adenomas, 
11% were GH-secreting adenomas, and 2% were Cushing ade­
nomas. The median age of onset was 37 years, but nonfunction­
ing tumors were most commonly encountered in patients older 
than 60 years.214 A recent population-based study in Iceland 
showed a rising prevalence over 

study in Iceland 
showed a rising prevalence over time. The total prevalence of 
adenomas from 2003 to 2012 was 115 per 100,000, up from 
72 per 100,000 in the period from 1993 to 2002. Prolactino­
mas accounted for 54/100,000 and nonfunctioning adenomas 
accounted for 43/100,000.215 Using large and comprehensive 
population-based cancer registries, the annual pituitary tumor 
incidence rate in the United States was reported to increase from 
2.52 per 100,000 population in 2004 to 3.13 in 2009 (

2 per 100,000 population in 2004 to 3.13 in 2009 (Table 
9.14).216 An increase in incidence was similarly seen in a study 
in Sweden.217 Whether or not this increase is due to an inher­
ent actual tumor increase or to enhanced reporting, awareness, 
and diagnosis is yet unclear. These benign monoclonal adeno­
mas may express and secrete hormones autonomously, lead­
ing to hyperprolactinemia, acromegaly, Cushing disease, and 
hyperthyroidism, or they may be functionally silent and initially 
diag

hey may be functionally silent and initially 
diagnosed as a sellar mass. Although almost invariably benign, 
the neoplastic features of these adenomas represent a unique 
tumor biology, which is reflected in their important local and 
systemic manifestations. These neoplasms have a slow doubling 
time and rarely resolve spontaneously. Nevertheless, they can 
be aggressive and locally invasive or compressive to vital central 
structures. Two classifications, Hardy and Knosp, are used to 
charact

sifications, Hardy and Knosp, are used to 
characterize adenoma mass characteristics (Fig. 9.20). Although 
pituitary adenomas usually express a single differentiated gene 
product, polyhormonal expression may reflect a primitive stem 
cell or mature bimorphous cellular origin. In children, ACTH-
secreting and PRL-secreting adenomas account for about 40% 
of surgical series, with a 65% surgical remission rate.218 These 
tumors are particularly challenging to manage due to high 
recurrence rates 

allenging to manage due to high 
recurrence rates and the need for rigorous pituitary hormone 
replacement. 
Classification of Pituitary Tumors
Pituitary tumors arise from hormone-secreting adenohypophy­
seal cells, and their secretory products depend upon the cell of 
origin (Table 9.15). Pathologic classification enables accurate 
clinical correlates and identifies cell-type origin.219–221 Previ­
ously, clinically inapparent pituitary adenomas were found in 
about 11% of autopsies. They locali

were found in 
about 11% of autopsies. They localize to unique areas of the 
gland, reflecting relative cell-type abundance and intragland 
distribution (see Fig. 9.2). Although 46% of a subset of these 
immunostain for PRL,222 expectant management may still be 
indicated.223 In a study on 100 normal volunteers, 10 were 
found to have focal abnormalities on MRI consistent with 
microadenomas; they measured from 3 to 6 mm in diameter.8 
Such tumors have been termed incidentalomas. In a survey of 

s have been termed incidentalomas. In a survey of 
506 patients harboring incidentalomas, 20% were nonfunction­
ing; of these, 20% increased in size during a mean follow-up of 
50 months.222 When larger, particularly nonfunctioning tumors 
are encountered inadvertently, pituitary function should be 
assessed, including measuring PRL, IGF1, luteinizing hormone 
(LH), follicle-stimulating hormone (FSH), and sex steroids. A 
24-hour urinary free cortisol or salivary cortisol measurement 
may exclud

tisol or salivary cortisol measurement 
may exclude Cushing disease. In a study of 52 patients with 
macroadenomas that were incidentally discovered by CT or 
MRI, 22 were gonadotroph cell adenomas, 21 were null cell 
adenomas, and 9 were clinically nonfunctioning but immu­
nostained for various pituitary hormones.224 Radiologic and 
surgical classifications are based upon tumor localization, size, 
and degree of invasiveness (see Fig. 9.20). Microadenomas are 
  Pituitary Adenoma Prevalencea
B

roadenomas are 
Belgium 
(n = 68)
Finland 
(n = 164)
Iceland 
(n = 471)
Malta 
(n = 316)
Sweden 
(n = 592)
Switzerland 
(n = 44)
United 
Kingdom 
(n = 63)
Population prevalence
1/1064
1/1471
1/865
1/1321
1/2688
1/1241
1/1289
ACTH secreting, %
6
3
6
2
4
4
2
GH secreting, %
13
8
11
16
9
9
11
PRL secreting, %
66
51
40
46
32
56
57
Nonfunctioning, %
15
37
43
34
54
30
28
aIncludes three adenomas not classified, nine TSH-secreting adenomas, and one gonadotrophin-secreti

-secreting adenomas, and one gonadotrophin-secreting adenoma.
Data from Molitch ME. Diagnosis and treatment of pituitary adenomas: a review. JAMA. 2017;317:516–524.
   
TABLE 9.13
266
Grade 0
Grade I 
Grade II
Grade III
Grade IV
Noninvasive (enclosed)
A
Invasive
A
B
C
HARDY CLASSIFICATION SYSTEM
D
E
Symmetrical
Asymmetrical
B
Grade 0
Grade 1
Grade 2
KNOSP CLASSIFICATION SYSTEM
Grade 3
Grade 4
• Fig. 9.20  Classification systems characterizing pituitary ad

Classification systems characterizing pituitary adenomas. (A) Hardy classification system. Sella 
turcica tumors can be noninvasive (grade 0, intact with normal contour; grade I, intact with bulging floor; 
or grade II, intact, enlarged fossa) or invasive (grade III, localized sellar destruction; or grade IV, diffuse 
destruction). Suprasellar tumors can be symmetrical (grade A, suprasellar cistern only; grade B, recess of 
the third ventricle; or grade C, whole anterior third ventricle) or asym

r grade C, whole anterior third ventricle) or asymmetrical (grade D, intracranial extradu­
ral; or grade E, extracranial extradural [cavernous sinus]). (B) Knosp classification system used to quantify 
invasion of the cavernous sinus, in which only grades 3 and 4 define true invasion of the tumor into the cav­
ernous sinus. Grade 0, no cavernous sinus involvement; grades 1 and 2, the tumor pushes into the medial 
wall of the cavernous sinus but does not go beyond a hypothetical line extending be

oes not go beyond a hypothetical line extending between the centers of 
the two segments of the internal carotid artery (grade 1) or it goes beyond such a line but without passing 
a line tangent to the lateral margins of the artery itself (grade 2); grade 3, the tumor extends laterally to the 
internal carotid artery within the cavernous sinus; grade 4, total encasement of the intracavernous carotid 
artery. (Modified from Di Ieva A, Rotondo F, Syro LV, et al. Aggressive pituitary adenomas: dia

Syro LV, et al. Aggressive pituitary adenomas: diagnosis and 
emerging treatments. Nat Rev Endocrinol. 2014;10:423–435, used with permission.)
  Pituitary Tumor Age-Adjusted Incidence 
and Annual Percent Change in Incidence 
in the United States
Year
No. of 
Cases
Age-Adjusted 
Incidence Rate 
(95% CI)
Annual Percent 
Change (95% 
CI)
2004–2009
51.125
2.87 (2.85–2.90)
4.25 (2.91–5.61)
2004
7243
2.52 (2.46–2.58)
2005
7811
2.69 (2.63–2.75)
2006
8485
2.88 (2.82–2.94)
2007
8722
2.92 (2.86–2.98)
200

85
2007
8722
2008
9266
3.07 (3.01–3.14)
2009
9598
3.13 (3.07–3.20)
Modified from Gittleman H, Ostrom QT, Farah PD, et al. Descriptive epidemiology of pituitary 
tumors in the United States, 2004-2009. J Neurosurg. 2014;121:527–535.
   
TABLE 9.14
intrasellar and generally smaller than 10 mm in widest diam­
eter. Macroadenomas are 10 mm or larger and usually impinge 
upon adjacent sellar structures. Specific tumor types are consid­
ered later for each respective 

 types are consid­
ered later for each respective cell type. Immunocytochemistry 
detects pituitary cell gene products at both the light and electron 
microscopic levels and allows classification of pituitary tumors 
based on their function. Unlike the corticotroph, somatotroph, 
lactotroph, and thyrotroph cell tumors, which hypersecrete 
their respective hormones, gonadotroph cell tumors are usually 
clinically silent and do not efficiently secrete their gene prod­
ucts. Double immunostaining i

ete their gene prod­
ucts. Double immunostaining identifies mixed tumors express­
ing combinations of hormones; they are often macroadenomas 
secreting GH concomitantly with PRL, TSH, or ACTH. Gener­
ally, immunohistochemical identification of pituitary hormones 
correlates with tumor-specific messenger RNA (mRNA) hor­
mone transcription factor markers measured either in whole tis­
sue extracts or at the single cell level.225 With the exception of 
the glycoprotein α-subunit, immunohistochemical

f 
the glycoprotein α-subunit, immunohistochemical positivity of 
greater than 5% of cells composing the tumor is usually reflec­
tive of peripheral circulating hormone levels. Quantification of 
immunostaining intensity is subjective, and a scale of intensity 
267
should also include a description of the extent of staining, such 
as whether occasional, scattered, or most tumor cells express 
the immunodetectable protein. Electron microscopy is useful 
for 

table protein. Electron microscopy is useful 
for assessing the ultrastructure of hormone secretory granules, 
their size, and distribution. Other subcellular features impor­
tant for diagnosis include visualizing large mitochondria in non­
functioning oncocytomas and the secretory nature of Golgi and 
endoplasmic reticulum, especially for prolactinomas. Peroxidase 
or colloid gold particles of different diameters are also sensi­
tive electron microscopic markers for identifying and localizing 

croscopic markers for identifying and localizing 
intracellular hormone signals. Because of the cost, other mark­
ers have superseded routine electron microscope usage. Invasive 
pituitary tumors are slow growing, and mitotic markers, includ­
ing proliferating cell nuclear antigen (PCNA) and Ki-67, are 
useful.226,227
Aggressive Adenomas
True pituitary carcinoma with demonstrable extrapituitary 
metastases is exceedingly rare.228,229 However, about 30% 
of patients undergoing adenoma resection e

ut 30% 
of patients undergoing adenoma resection exhibit persistent 
tumor growth or progressive growth for up to 37 years after sur­
gery.230 Tumors are regarded as aggressive if they are resistant to 
medical therapy and exhibit multiple recurrences despite rig­
orous surgical, medical, and radiation approaches.231 Aggres­
sive adenomas, previously referred to as atypical adenomas, are 
invasive, with elevated Ki-67 proliferative index and increased 
mitotic activity231,232 (Fig. 9.21). In a s

eased 
mitotic activity231,232 (Fig. 9.21). In a series of 121 consecutive 
patients undergoing transsphenoidal pituitary tumor resection, 
18 were classified as atypical.233 These tumors grow aggres­
sively and often persist or recur after surgery. In a study of 
50 aggressive adenomas, more than 80% were macroadenomas 
and 42% had invaded the cavernous sinus.234 They may arise 
from any of the pituitary cell types, and about 20% actively 
secrete GH or PRL; they rarely secrete ACTH. Several hi

te GH or PRL; they rarely secrete ACTH. Several histo­
logic types are particularly prone to aggressive growth, such 
as silent corticotroph adenomas.186 No controlled prospective 
studies have rigorously assessed the postoperative recurrence 
rates of these tumors, and the utility of p53 as a prognostic 
marker has been questioned. There is yet no compelling evi­
dence that these tumors undergo true malignant transforma­
tion. Nevertheless, frequently recurring or persistent invasive 
adenomas 

uently recurring or persistent invasive 
adenomas with high Ki-67 indices should raise awareness to 
consider the rare diagnosis of malignancy by excluding the 
presence of metastasis.235
Malignant Pituitary Tumors
Very rarely, pituitary tumors may metastasize either outside the 
central nervous system or as a separate focus within the brain; 
they account for 0.2% of pituitary adenomas.236 Because no 
cell markers clearly distinguish aggressive invasiveness from 
malignancy, demonstration of ex

invasiveness from 
malignancy, demonstration of extracranial metastasis is a pre­
requisite for diagnosis of pituitary malignancy.237 When they 
occur, these cancers most often secrete either ACTH or PRL. 
  The 2017 WHO Pathologic Classification of Pituitary Adenomas
Adenoma Type
Morphologic Variants
Pituitary Hormones by 
Immunohistochemistry
Transcription Factors 
and Other Cofactors
Clinical Syndrome
Densely granulated
GH, α-subunit
Pit1
Acromegaly, gigantism
Sparsely granulated

nit
Pit1
GH
Pit1
Mammosomatotroph
GH + PRL (in same cells) ± α-subunit
Pit1, ERα
Mixed somatotroph-lactotroph
GH + PRL (in different cells) ± 
α-subunit
Pit1, ERα
PRL
Pit1, ERα
Hypogonadism, galactorrhea
PRL
Pit1, ERα
Acidophil stem cell
PRL, GH (focal and variable)
Pit1, ERα
βTSH, α-subunit
Pit1, GATA2
ACTH
Tpit
Sparsely granulate

lated
ACTH
Tpit
ACTH
Tpit
Silenta
ACTH
Tpit
Crooke cell
ACTH
Tpit
βFSH, βLH, α-subunit (various com­
binations)
SF1, GATA2, ERα
Hypogonadism, local signs
Null cella
None
None
None or pituitary failure
Plurihormonal
Pit1 positivea
GH, PRL, βTSH ± α-subunit
Pit1
Mixed, local signs
Unusual immunohistochemical 
combinations
Various combinations
aUsually nonsecreting and clinically silent.
Modified from Mete O, Lopes MB. Overview of the 2017 WHO classif

Mete O, Lopes MB. Overview of the 2017 WHO classification of pituitary tumors. Endocr Pathol. 2017;28:228–243.
   
TABLE 9.15
268
As HRAS mutations are rarely encountered in distant meta­
static pituitary carcinomas, but not in their respective primary 
pituitary tumors or in noninvasive adenomas,169,238 these 
mutations may be important in the very rare progression to 
malignancy.
Temozolomide, an alkylating agent that induces DNA dam­
age to disrupt gen

ing agent that induces DNA dam­
age to disrupt gene transcription, has been used to treat aggres­
sive pituitary tumors in patients who have failed to respond 
to other therapies or who have evidence of pituitary carci­
noma. Results of tumor shrinkage or hormone suppression 
have been highly variable, with about half of patients show­
ing initial responses.239 It has been recommended that drug 
therapy be reevaluated after three treatment cycles. Although 
O6-methylguanine-DNA methyltransferase

. Although 
O6-methylguanine-DNA methyltransferase (MGMT) may 
interfere with drug efficacy, assessing MGMT expression in 
pituitary tumor samples has been reported to have variable 
use.240–243 
Prolactin-Secreting Adenomas
Prolactinomas are the most frequently encountered secre­
tory pituitary tumors, occurring with an annual incidence of 
approximately 30 per 100,000 population.214 This incidence 
would be much higher if the estimate included microadenomas 
discovered in approximately 11% of 

microadenomas 
discovered in approximately 11% of pituitaries at autopsy, 46% 
of which immunostain positively for PRL.222 The female:male 
ratio for microprolactinomas is 20:1, and for macroadenomas 
the sex ratio is roughly equivalent. Both PRL levels and tumor 
size generally remain stable, although in some patients, PRL 
levels fall over time. Microadenomas may disappear after dis­
continuing dopamine agonist therapy, although 7% to 14% 
of microadenomas continue to grow.244 Additionally, sm

icroadenomas continue to grow.244 Additionally, smaller 
prolactinomas may sometimes regress after pregnancy and 
lactation.245
Macroprolactinomas have a greater propensity to grow and 
tumor size correlates with serum PRL levels (Fig. 9.22), so a PRL 
level higher than 200 ng/mL is strongly indicative of a PRL-
secreting pituitary tumor. Although prolactinomas account for 
more than 75% of all female pituitary adenomas,214 men harbor 
larger tumors. In 45 men and 51 women with prolactinomas, 
m

ors. In 45 men and 51 women with prolactinomas, 
mean serum PRL was 2789 ± 572 ng/mL versus 292 ± 74 ng/mL, 
respectively. Prolactinomas are larger in men than in women (26 
± 2 mm vs. 10 ± 1 mm), are more invasive, and show histologic 
evidence of more rapid growth.
PRL levels higher than 200 ng/mL may reflect use of a 
drug such as risperidone, but levels higher than 500 ng/mL are 
ACTH
Ki-67
p53
Invasion of neighboring soft tissue and bone
Rapid growth
Recurrence
Resistance to conventional th

id growth
Resistance to conventional therapy
Elevated mitotic count/Ki-67 index
•
•
•
•
•
WHO 2017 Features
• Fig. 9.21  Aggressive pituitary adenomas. Criteria for diagnosis as proposed by World Health Organiza­
tion (WHO). Histologic panels depict atypical adrenocorticotrophic hormone (ACTH)–expressing adenoma. 
(From Zada G, Woodmansee WW, Ramkissoon S, et al. Atypical pituitary adenomas: incidence, clinical 
characteristics, and implications. J Neurosurg. 2011;114:336–344; Lopes M

plications. J Neurosurg. 2011;114:336–344; Lopes MBS. The 2017 World Health 
Organization classification of tumors of the pituitary gland: a summary. Acta Neuropathol. 2017;134:
521–535.)
269
exclusively observed in patients with prolactinomas.246 In con­
trast, a lower PRL concentration of less than 200 ng/mL in a 
patient harboring a macroadenoma indicates that the tumor is 
likely not producing PRL, but hyperprolactinemia may occur 
as a result of mass p

yperprolactinemia may occur 
as a result of mass pressure on the pituitary stalk or portal cir­
culation, likely interrupting inhibitory control by dopamine.247 
Importantly, microprolactinomas can be associated with PRL 
levels ranging from minimal elevations to hundreds of nano­
grams per milliliter. However, when a patient with a small mac­
roadenoma and a PRL level approximately 200 ng/mL is first 
encountered, it is prudent to first treat medically. If the tumor 
is indeed a prolactinoma, d

dically. If the tumor 
is indeed a prolactinoma, dopamine agonist treatment should 
lower PRL levels and shrink the tumor. If the tumor does not 
shrink, the mass is likely not secretory, and the hyperprolacti­
noma is caused by compressive stalk effect.
Pathology and Pathogenesis
Although more than 99% of prolactinomas are not malignant, 
and often sharply demarcated without evidence of invasion, 
about half may invade local structures248 (Fig. 9.23). Invasive 
tumors may have higher mitotic ac

9.23). Invasive 
tumors may have higher mitotic activity and are more cellular 
and pleomorphic. Invasion into adjacent dura, bone, or venous 
structures may represent an intermediate form of prolactinoma 
between the sharply demarcated benign variety and the exceed­
ingly rare malignant tumor. Invasive tumors that do not metas­
tasize are considered benign. Immunostaining for PRL confirms 
the diagnosis of prolactinoma, which is usually distinct from the 
adjacent normal pituitary but is not tr

 from the 
adjacent normal pituitary but is not truly encapsulated. These 
tumors have a “pseudocapsule” composed of compressed ade­
nohypophyseal cells and a reticulin fiber network. About 20% 
of macroprolactinomas contain areas of hemorrhage not usu­
ally associated with features of apoplexy, and these areas may 
resolve.249
Prolactinomas are mostly slow growing, arise sporadically, 
usually occur singly, and are the most common pituitary tumors 
associated with MEN1, occurring in approximate

rs 
associated with MEN1, occurring in approximately 20% of a 
large kindred.250,251 Familial prolactinomas have been described 
with no other features of MEN1,252 and rarely prolactinomas 
also occur in patients with germline AIP mutations.253 Hyper­
prolactinemia derived from ectopic secretion from a perivascu­
lar epithelioid tumor resolved after resection of the abdominal 
mass.254
The entity of giant prolactinomas has been ascribed to 
those aggressive tumors representing <5% of prolactinom

 aggressive tumors representing <5% of prolactinomas 
that exhibit very high PRL levels (>1000 ng/mL) with a diam­
eter greater than 40 mm, and with a high male preponderance 
(9:1).255 Although these invasive tumors usually respond well 
to dopamine agonists, a subset is highly invasive and surgery is 
required, especially if mass effects do not resolve.251,256 PRL lev­
els may drop with cabergoline therapy or surgery, but recovery 
of gonadal function is only seen in about 30% of these patient

unction is only seen in about 30% of these patients. 
Recurrence rates are high and repeat surgery and/or radiation in 
addition to high-dose cabergoline are required for these aggres­
sive tumors. 
Prolactinomas usually come to attention because of symptoms or 
signs associated with hyperprolactinemia or with tumor size or 
invasiveness (Table 9.16).
Tumor diameter (mm)
Males
Females
r = 0.88
p = 0.0001
PRL level (ng/mL)
100,000
10,000
1,000
100
10
100
10
0
• Fig. 9.22  Prolac

0
10,000
1,000
100
10
100
10
0
• Fig. 9.22  Prolactin (PRL)–secreting tumors are more often macroad­
enomas in men (n = 31) versus women (n = 45). Serum prolactin levels 
highly correlate with tumor size. (Adapted from Danila DC, Kliban­
ski A. Prolactin secreting pituitary tumors in men. Endocrinologist. 
2001;11:105–111.)
A
B
• Fig. 9.23  (A) Densely granulated prolactin-secreting adenoma. (B) Pro­
lactin-producing pituitary adenoma removed by surgery from a patient 
treated with dopamine agon

surgery from a patient 
treated with dopamine agonist in the preoperative period. The adenoma 
cells are small, possessing dark nuclei and a narrow rim of cytoplasm. 
Mild accumulation of interstitial connective tissue is apparent. (Hematoxy­
lin-eosin stain; original magnification ×400.) (Photomicrograph kindly pro­
vided by Dr. Kalman Kovacs. University of Toronto, Toronto, Canada.)
270
Both large and small PRL-secreting tumors can pr

a
Both large and small PRL-secreting tumors can present with signs 
and symptoms of hyperprolactinemia. Menstrual irregularities, 
sexual dysfunction, galactorrhea,257 osteopenia,258 and impaired 
quality of life259 are attributable to elevated PRL levels and pres­
ence of a tumor. Elevated PRL causes sexual dysfunction via a 
short loop feedback effect on gonadotrophin pulsatility, presum­
ably inhibiting GnRH260 and LH pulse frequency and amplitude. 
High PRL also directly inhibits ovarian and

tude. 
High PRL also directly inhibits ovarian and testicular function. 
Women with prolactinomas may present with primary or second­
ary amenorrhea, oligomenorrhea, menorrhagia, delayed men­
arche, or regular menses with a short luteal phase that may cause 
infertility. Patients may also report changes in libido and vaginal 
dryness. Sexual dysfunction in men usually manifests as loss or 
decrease in libido, impotence, premature ejaculation or intracoital 
erection loss, oligospermia, or azospe

ntracoital 
erection loss, oligospermia, or azospermia.261
Up to 50% of women and 35% of men with prolactinomas 
have galactorrhea; this difference may occur because male mam­
mary tissue is less susceptible to lactogenic effects of hyperprolac­
tinemia. Galactorrhea can be overlooked unless actively elicited. 
Bone density may decrease in both men and women as a result of 
hyperprolactinemia-induced sex steroid deficiency, and an increase 
in vertebral fractures detected radiologically has been

rtebral fractures detected radiologically has been reported in 
women.262 
Tumor Mass Effects
Prolactinomas may present as a result of tumor size or invasiveness. 
Microadenomas range from entirely asymptomatic tumors found 
at autopsy as small as 2 to 3 mm in diameter to larger ones that 
are still less than 10 mm in diameter. These tumors can be invasive 
despite their small size. In contrast, macroadenomas range in size 
from noninvasive or diffuse tumors approximately 1 cm in diam­
eter to h

ffuse tumors approximately 1 cm in diam­
eter to huge tumors that may impinge upon parasellar structures. 
Signs and symptoms caused by large or invasive tumors are often 
related to compressive effects on visual structures. Objective find­
ings are superior bitemporal defects, bitemporal hemianopsia, and 
decreased visual acuity. Headaches are common, but seizures (a 
result of extension into the temporal lobe) and hydrocephalus263 
are rarely encountered, as is unilateral exophthalmos. Tumors 

ncountered, as is unilateral exophthalmos. Tumors may 
invade the cavernous sinuses and yet cranial nerve palsies are rare. 
A sudden insult, such as pituitary apoplexy, is the more common 
cause of such palsies and may be a presenting symptom. Prolacti­
nomas can also be discovered incidentally on an MRI or CT scan 
performed for another clinically unrelated purpose. 
Patients with pituitary tumors should all have serum PRL 
levels measured. Conversely, patients with elevated serum 

asured. Conversely, patients with elevated serum 
PRL levels not fully explicable by an obvious cause (such as 
pregnancy or exposure to neuroleptic medications) should be 
evaluated for the presence of a pituitary tumor. Prolactinomas 
may also coexist with another cause of hyperprolactinemia, 
such as neuroleptic drug administration (see Chapter 8). Even 
minimal to moderate PRL elevations are important to inves­
tigate because they may indicate the presence of a large pitu­
itary tumor that d

e the presence of a large pitu­
itary tumor that does not secrete PRL. PRL levels correlate 
with tumor size and are usually higher in male patients. Occa­
sionally, a patient with a very high serum PRL level might 
have a “normal” result reported if serum dilutions are not 
assayed, a phenomenon called the “high-dose hook effect.”264 
In contrast, serum PRL may be elevated by the presence of 
high-molecular-weight PRL, which is a weaker lactogen 
than the monomeric PRL molecule. Although usuall

than the monomeric PRL molecule. Although usually clini­
cally inactive, macroprolactinemia can also occur in patients 
with pituitary tumors. Pituitary adenomas are diagnosed in 
approximately 20% of patients with macroprolactinemia, 
some of which are associated with galactorrhea, oligomen­
orrhea or amenorrhea, or erectile dysfunction and decreased 
libido. Thus, assessment of macroprolactinemia by polyethyl­
ene glycol precipitation should be performed in patients with 
reported high level

be performed in patients with 
reported high levels of PRL and few or absent clinical features 
of hyperprolactinemia.265
A careful history will often unmask symptoms or signs of a 
space-related mass such as visual field abnormalities, impaired 
visual acuity, blurred or double vision, CSF rhinorrhea, head­
aches, diabetes insipidus, and hypopituitarism. Patients should 
also be questioned carefully about sexual history, including onset 
of menarche, regularity of menses, fertility, libido, pot

rche, regularity of menses, fertility, libido, potency, and 
ability to maintain an erection. A history of galactorrhea should 
also be ascertained. The coexistence of galactorrhea and amenor­
rhea suggests a diagnosis of pituitary adenoma until otherwise 
proven.
PRL is also elevated in up to 50% of patients with acromeg­
aly.266 Patients in the early stages of acromegaly or with mild dis­
ease or patients harboring acidophilic stem cell adenomas may 
have few obvious signs of GH excess. Becaus

s may 
have few obvious signs of GH excess. Because the human GH 
molecule has lactogenic properties similar to those of PRL,267 
signs and symptoms of a prolactinoma may be mimicked by a 
purely GH-secreting tumor, and serum IGF1 should be mea­
sured. Elevated PRL levels are occasionally encountered in 
patients with TSH-secreting tumors. Other pituitary hormone 
functions should be ascertained to determine the presence of 
hypopituitarism. An MRI is required to establish a definitive 
diagnosi

RI is required to establish a definitive 
diagnosis of prolactinoma. 
Treatment
Optimal treatment outcomes for a prolactinoma include normal­
ization of PRL levels (and associated signs and symptoms) and 
complete tumor removal or shrinkage with a reversal of tumor 
mass effects. Specifically, previously abnormal sexual function and 
fertility should be restored, galactorrhea stopped, impaired bone 
density improved, tumor eliminated or reduced in size without 
impairing pituitary or hypothalami

n size without 
impairing pituitary or hypothalamic function, and vision normal­
ized, if impaired.
  Signs and Symptoms of Prolactinomas
Associated With Tumor Mass
Associated With 
Visual field abnormalities
Blurred vision or decreased 
visual acuity
Symptoms of hypopituitarism
Headaches
Cranial nerve palsies
Pituitary apoplexy
Seizures (temporal lobe)
Hydrocephalus (rare)
Unilateral exophthalmos (rare)
Amenorrhea, oligomenorrhea, 
infertility
Decreased libido, impotence, pr

rhea, 
Decreased libido, impotence, pre­
mature ejaculation, oligospermia
Galactorrhea
TABLE 9.16
271
Medical Treatment
Medical management of prolactinomas with dopamine agonist 
drugs has been widely recommended as the treatment of choice268 
(Table 9.17).
Bromocriptine
Bromocriptine, a semisynthetic ergot alkaloid dopamine agonist, 
lowers elevated PRL levels, restores abnormal menstrual function in 
80% to 90% of patients, shrink

strual function in 
80% to 90% of patients, shrinks prolactinomas, restores impaired 
sexual function, and resolves galactorrhea.269 Improvement in 
visual field abnormalities occurs in approximately 90% of affected 
patients.270 Drug withdrawal can result in rapid tumor expansion.271 
Occasionally, tumors that shrink during bromocriptine therapy do 
not enlarge following drug withdrawal. In a subset of patients, hyper­
prolactinemia disappears spontaneously after long-term observation. 
Very oc

pontaneously after long-term observation. 
Very occasionally, bromocriptine lowers PRL levels despite contin­
ued tumor expansion, although when tumors grow during dopa­
mine agonist therapy there is usually a simultaneous PRL elevation.
Despite high doses of bromocriptine, some patients are entirely 
or partially resistant to its effects and to those of cabergoline as 
well.272 Not infrequently, it is difficult to completely normalize 
PRL levels in patients with initially very high levels, alt

s in patients with initially very high levels, although these 
patients respond to treatment with impressive tumor shrinkage 
and sometimes improved sexual function. Although higher doses 
or a change in the form of dopamine agonist has been reported 
to further normalize PRL in some cases,272 many such patients 
remain with elevated PRL levels regardless of treatment used. 
Resistance to dopamine agonists may reflect reduced D2 receptor 
binding sites or receptor gene polymorphisms.273
Bromocri

 sites or receptor gene polymorphisms.273
Bromocriptine shrinks prolactinomas by shrinking tumor cell 
size, including cytoplasmic, nuclear, and nucleolar areas.274 His­
tologic sections appear very dense as a result of small cell size and 
clumping of nuclei (see Fig. 9.23). PRL mRNA and PRL synthe­
sis are inhibited, exocytoses are reduced, PRL secretory granules 
decrease, and rough endoplasmic reticulum and Golgi apparatus 
involute. The net effect is reduced cell volume. Tumor necrosis 
may

effect is reduced cell volume. Tumor necrosis 
may also occur.275
Perivascular fibrosis observed in prolactinomas derived from 
patients treated with bromocriptine has been attributed to dif­
ficulty in tumor resection. However, there is no effect of prior 
treatment with bromocriptine on surgical success rates,276 and 
bromocriptine was a helpful adjunct to transsphenoidal microsur­
gery for macroprolactinomas.277 Even the largest tumors or those 
with the highest PRL levels respond well to tre

e 
with the highest PRL levels respond well to treatment with up to 
2.5 mg bromocriptine three times daily. Higher doses are often not 
more effective. Once positive effects on tumor size and amenorrhea 
and galactorrhea are established, some patients can be satisfactorily 
maintained with lower doses but rarely without medication. 
Cabergoline
Because cabergoline has a longer duration of action than bro­
mocriptine and is usually administered once or twice weekly, it 
has surpassed bromocripti

nce or twice weekly, it 
has surpassed bromocriptine as the first-line therapeutic choice for 
most patients, unless pregnancy is desired.270 The long half-life of 
cabergoline is a result of its high affinity for lactotroph D2 recep­
tors and a greater propensity of the drug to remain in pituitary 
tissue. In pharmacokinetic studies, cabergoline lowered PRL levels 
in a dose-related manner.278 PRL levels were normalized in 83% 
of 459 women with hyperprolactinemia treated with cabergoline 
(0.5

 hyperprolactinemia treated with cabergoline 
(0.5–1 mg twice weekly) and in 52% of women on bromocriptine 
(2.5–5 mg twice daily). Cabergoline was also more effective than 
bromocriptine in restoring ovulatory cycles and fertility (72% 
vs. 52%; p < 0.001), was better tolerated than bromocriptine, 
and caused fewer but similar side effects (Fig. 9.24). Tumor size 
decreased in 11 of 15 patients with macroadenomas, and menses 
resumed in 3 of 4 premenopausal women.279 In 85 patients with 
  Dop

remenopausal women.279 In 85 patients with 
  Dopamine Agonist Treatment of 
Prolactinomasa
Bromocriptineb 
(2.5–7.5 mg/day)
Cabergolineb 
(0.5–1 mg twice 
weekly)
PRL Normalized (range, 40–100%)
Macroadenomas
70
80
Microadenomas
65
70
Menses Resumed (range, 40–100%)
70
80
85
80
Tumor Shrinkage (range, 20–100%)
None
20
20
Up to 50%
40
55
50% or more
40
25
Visual Field Improve­
ment (range, 
33–100%)
90
70
Drug Intolerance
15
5
aLong-acting cabergoline has improved pa

ance
15
5
aLong-acting cabergoline has improved patient compliance and has fewer gastrointestinal 
side effects. For fertility, bromocriptine is preferred as it is short acting and can be discontin­
ued immediately on pregnancy confirmation.
bValues = % of patients.
Data from Webster J, Piscatelli G, Polli A, et al. A comparison of cabergoline and bromocrip­
tine in the treatment of hyperprolactinemic amenorrhea. Cabergoline Comparative Study 
Group. N Engl J Med. 1994;331:904–909; Verhelst J, A

oup. N Engl J Med. 1994;331:904–909; Verhelst J, Abs R, Maiter D, et al. Cabergoline 
in the treatment of hyperprolactinemia: a study in 455 patients. J Clin Endocrinol Metab. 
1999;84:2518–2522; Melmed S, Casanueva FF, Hoffman AR, et al. Diagnosis and treat­
ment of hyperprolactinemia: an Endocrine Society clinical practice guideline. J Clin Endocrinol 
Metab. 2011;96:273–288.
   
TABLE 9.17
Weeks of treatment
Serum prolactin (µg/L)
120
100
80
60
40
20
0
24
22
20
18
16

lactin (µg/L)
120
100
80
60
40
20
0
24
22
20
18
16
14
12
10
8
6
4
2
0
• Fig. 9.24  Comparison of bromocriptine and cabergoline in suppress­
ing prolactin levels in women with hyperprolactinemia. (From Webster J, 
Piscitelli G, Polli A, et  al. A comparison of cabergoline and bromocrip­
tine in the treatment of hyperprolactinemic amenorrhea. N Engl J Med. 
1996;331:904–909.)
272
macroprolactinomas treated with cabergoline (0.25–10.5 mg/
week), PRL concentr

ith cabergoline (0.25–10.5 mg/
week), PRL concentrations were normalized in 61% of patients 
and decreased by at least 75% in an additional 24 patients, and 
tumor size decreased in 66% of patients. Nine patients were 
resistant to cabergoline despite doses of up to 7 mg per week.272 
Cabergoline also may result in dramatic improvement of prolacti­
noma-associated headache.280
Prolactinomas completely or substantially resistant to medi­
cation are infrequently encountered. Up to 15% of patients 

e infrequently encountered. Up to 15% of patients 
receiving optimal cabergoline doses fail to normalize PRL levels 
or exhibit less than 50% tumor mass shrinkage. Many of these 
patients respond to higher cabergoline doses (Fig. 9.25).281 Most 
“resistant” patients may have partial resistance (i.e., tumors shrink 
and PRL levels are lowered but do not normalize). These patients 
may require surgery or radiation therapy. With tumor growth 
controlled on treatment, persistently elevated PRL level

lled on treatment, persistently elevated PRL levels should 
be addressed by evaluating and treating specific reproductive and 
bone disorders caused by hyperprolactinemia. 
Administration
Attention to the mode of dopamine agonist administration may 
avoid or minimize potential adverse effects (Fig. 9.26). Usual start­
ing doses are 1.25 mg bromocriptine (daily) or 0.25 mg cabergo­
line (weekly). Doses of medication are either increased gradually, 
as tolerated, or decreased depending on tolerabi

as tolerated, or decreased depending on tolerability and should 
be initiated with a small dose with food before bedtime. Patients 
should initially avoid activities that cause peripheral vasodilatation 
(e.g., hot baths), thereby decreasing the risk of postural hypoten­
sion. If side effects are troublesome, the subsequent dose should be 
halved, then doses increased gradually thereafter to achieve effec­
tive levels. Switching from one medication to another may be ben­
eficial. Application o

tion to another may be ben­
eficial. Application of intravaginal bromocriptine administration 
has been advocated to alleviate adverse gastrointestinal events.282 
Dopamine agonists are also effective in persistently shrinking cys­
tic prolactinoma size.283
Remission of hyperprolactinemia may occur in about 20% of 
patients in whom dopamine agonists are tapered and then dis­
continued. Generally, withdrawal should only be attempted after 
at least 2 years of therapy and in those patients with no

t 2 years of therapy and in those patients with no evidence 
of tumor invasion.268,284 Tapering of cabergoline doses prior to 
complete therapy withdrawal has been recommended. Predictors 
of achieving normoprolactinemia after withdrawal include lower 
cabergoline doses required with evident tumor mass shrinkage and 
mass resolution.285 Recurrences after withdrawal are more likely 
to occur for macroadenomas and are reported in about 30% of 
patients.244,284,286,287
In contrast to adults, about 

ents.244,284,286,287
In contrast to adults, about 75% of childhood prolactinomas 
are large macroadenomas.288 In a cohort of 77 children and ado­
lescents with macroprolactinomas, factors associated with dopa­
mine agonist resistance encountered in 25% of patients included 
younger age of onset, higher PRL levels, and larger tumors.289 
Adverse Effects of Dopamine Agonists
Side effects of dopamine agonists are common. Nausea occurs 
in up to 50% of patients; nasal stuffiness, depression, and dig

of patients; nasal stuffiness, depression, and digi­
tal vasospasm occur, the latter more frequently with higher doses, 
as seen in patients with Parkinson disease. Postural hypotension 
can cause loss of consciousness, occurs infrequently, and is usually 
avoided by careful dosing. Signs and symptoms of psychosis or 
exacerbation of preexisting psychosis can be encountered in up 
to 1.3% of patients receiving bromocriptine.290 A history of psy­
chotic symptoms should raise concerns about using 

chotic symptoms should raise concerns about using these medica­
tions. If psychosis occurs in a patient in whom dopamine agonists 
are clearly the treatment of choice, the judicious combination of 
this agent and antipsychotic medication can be effective. A neu­
roleptic that is not a potent PRL stimulator, such as olanzapine, is 
preferred. CSF rhinorrhea occurs during dopamine agonist treat­
ment in up to 6.1% of patients with macroadenomas, some of 
which are more resistant to dopamine agonis

me of 
which are more resistant to dopamine agonists.291 Other rarely 
reported serious side effects include hepatic dysfunction and car­
diac arrhythmias. Retroperitoneal fibrosis, pleural effusions and 
thickening, and restrictive mitral regurgitation have been reported 
in patients taking high doses of bromocriptine.292,293 Hypersexu­
ality with disordered impulse control may occur with restoration 
of eugonadism or may be a direct dopamine effect.294
In patients with Parkinson disease high d

fect.294
In patients with Parkinson disease high doses of ergot-derived 
dopamine agonists may lead to increase of moderate to severe 
regurgitation in at least one valve.295 These observations with high 
ergot doses raise concern for patients with pituitary tumors who 
mostly require far lower drug doses. Several reports have shown 
no evidence that low doses of cabergoline place patients at risk 
for significant valve disease,296 while some have raised the pos­
sibility that heart valve diseas

e raised the pos­
sibility that heart valve disease might be associated with standard 
endocrine doses of cabergoline used for patients with prolactino­
mas. One hundred prospectively studied patients with hyperpro­
lactinemia who were exposed to dopamine agonists for a median 
of 124 months and received a median cumulative dose of 277 mg 
cabergoline showed no altered cardiac valve function or calcifi­
cation.297 In a cross-sectional study of 747 patients with hyper­
prolactinemia, there were n

7 patients with hyper­
prolactinemia, there were no associations of cumulative doses of 
dopamine agonists and cardiac valvulopathy.298 These results were 
affirmed in a follow-up of 192 of these patients.299 
Radiation Therapy
Linear accelerator radiotherapy is effective in controlling or reduc­
ing the size of prolactinomas. However, this therapy takes years to 
achieve maximal effect. The usual recommended radiation dose is 
4500 to 4600 cGy, and normalization of PRL was achieved in 18 
of 36

and normalization of PRL was achieved in 18 
of 36 patients at a mean of 7.3 years after treatment.300 Hypopi­
tuitarism occurs as a side effect of radiation. In 36 patients with 
prolactinomas, of whom 83% had normal GH responses to insu­
lin-induced hypoglycemia before therapy, 34 were GH deficient 
25
20
15
Cabergoline (mg/wk)
Prolactin (ng/mL)
10
5
0
May-
97
Jun-
97
Aug-
97
Oct-
97
Dec-
97
Feb-
98
Jun-
98
600
500
400
300
200
100
0
• Fig. 9.25  Stepwise decrease in PRL levels (diamonds) conco

  Stepwise decrease in PRL levels (diamonds) concordant with 
stepwise increase in cabergoline doses in a patient with a prolactin-secret­
ing macoadenoma (bars). (Redrawn from Molitch ME. Pharmacologic 
resistance in prolactinoma patients. Pituitary. 2005;8:43–52.)
273
at 9 to 12 years after radiotherapy.300 Stereotactic radiosurgery is 
often effective in treating prolactinomas resistant to or intolerant 
of dopamine agonists.301 
Surgery
Endoscopic endon

Surgery
Endoscopic endonasal transsphenoidal surgery has been used to 
resect prolactinomas. Most patients with microprolactinomas 
experience normalization of PRL levels, and overall about 50% of 
patients with macroprolactinoma are in remission after surgery.302 
The success rate of pituitary surgery is determined by experience 
of the surgeon and correlates inversely with tumor size and serum 
PRL concentrations.276 In a compilation of results in 31 pub­
lished surgi

n a compilation of results in 31 pub­
lished surgical series, serum PRL was normalized in 71% of 1224 
patients with microprolactinomas. More recent studies report up 
to 100% cure rates for microprolactinomas achieved by high-vol­
ume surgeons.302 Nevertheless, a high rate of hyperprolactinemia 
recurrence is commonly encountered postoperatively. Complete 
resection of macroprolactinomas, especially large invasive tumors, 
is difficult to achieve, and postoperative serum PRL levels were 
initia

e, and postoperative serum PRL levels were 
initially normalized in only 32% of patients, with a 19% to 45% 
recurrence rate.303
Although results of medical therapy are superior to those of 
surgery, there remains a role for surgery, especially in those resis­
tant to dopamine agonist therapy who are particularly well suited 
for surgery. If tumor removal is only partial, adjunctive radiation 
therapy could be considered, as second surgery is usually associ­
ated with a higher complication rate.

ally associ­
ated with a higher complication rate. Prophylactic transsphenoi­
dal surgery should be considered in women whose prolactinomas 
are large enough to potentially threaten vision during pregnancy. 
A subset of patients cannot tolerate dopamine agonists, and others 
prefer surgery and refuse medication. 
Chemotherapy
For aggressive PRL-secreting tumors unresponsive to other thera­
pies, temozolomide, an alkylating compound that readily crosses 
the blood-brain barrier, may control tumor

rosses 
the blood-brain barrier, may control tumor growth.304–306 The 
Exclude secondary causes of hyperprolactinemia
Dopamine agonist
Titration of drug dose
Reduction in PRL but
    still elevated
Sexual function restored
Tumor shrinkage
Normal PRL
Drug intolerance
Sexual function not restored
Continue medication
May increase dose
Lower dose
Change medication
Intravaginal applica

Lower dose
Intravaginal application
Consider surgery
Replace sex steroids
Monitor PRL levels
Repeat MRI annually
PRL reduced
Tumor not smaller
PRL not reduced
No tumor shrinkage
No tumor shrinkage or
visual field not improved
Repeat MRI 
in 4 months
Increase dose or
switch medication
Surgery or radiotherapy
rarely required for
residual tumor

r radiotherapy
Surgery
Desires pregnancy
(See Table 9.18)
Assess pitiuitary
reserve function
Test visual fields
Macroadenoma,
inadequate response
to medication
• Fig. 9.26  Prolactinoma management. After secondary causes of hyperprolactinemia have been excluded, 
subsequent management decisions are based on clinical imaging and biochemical criteria. MRI, magnetic 
resonance imaging; PRL, prolactin.
274
SECTION II    Hypothalamus and Pi

, prolactin.
274
response to temozolomide may sometimes be predicted by low 
tumor staining for MGMT.307,308 
Pregnancy
The normal pituitary enlarges during pregnancy and prolactino­
mas may also increase in size during pregnancy. The incidence of 
pregnancy-associated tumor enlargement, as determined by devel­
opment of abnormal visual fields, has been estimated to occur in 
1.4% of women with microadenomas and 16% of women with 
macroadenomas.308 In oth

as and 16% of women with 
macroadenomas.308 In other reports, the risk for macroadenoma 
enlargement has been estimated to be as high as 36%. In a pro­
spective analysis in which 57 patients with microprolactinomas 
were followed by formal visual field examinations during preg­
nancy, none developed visual disturbances.309
Although dopamine agonists have been used during pregnancy 
to prevent tumor growth (Fig. 9.27),310 it seems prudent to reduce 
fetal exposure to medication if possible.311 It

e 
fetal exposure to medication if possible.311 It is recommended 
that menstrual periods be allowed to occur naturally for a suffi­
cient period of time (3–4 months) to predict that a missed period 
might be a result of pregnancy (Table 9.18). Barrier contraception 
is recommended during this period. Within several days to a 
week of obtaining a positive hCG test, medication should be dis­
continued. Of 6239 pregnancies managed in this manner, bro­
mocriptine therapy was not associated with inc

ro­
mocriptine therapy was not associated with increased abortions 
or terminations, prematurity, multiple births, or infant malforma­
tions above that expected in the control population. There is no 
compelling evidence that other dopamine agonists are less safe, 
but pregnancy exposure to the other agonist forms is less compre­
hensively documented. Treatment options for patients harboring 
prolactinomas whose vision becomes impaired during pregnancy 
include administering bromocriptine during

gnancy 
include administering bromocriptine during pregnancy, high-
dose steroids, and surgical resection.308 Of 53 pregnant women 
receiving bromocriptine, mean offspring birth weight was normal, 
congenital abnormalities occurred in four babies, and physical 
and intellectual development of children was normal for up to 9 
years. In an observational report on 380 pregnancies in women 
treated with cabergoline, early fetal exposure did not increase the 
risk of miscarriage or fetal malformation

ase the 
risk of miscarriage or fetal malformation.312 In 91 patients with 
hyperprolactinemia treated with cabergoline prior to pregnancy, 
the medication was discontinued at 6 weeks of gestation, and no 
increase in miscarriages or fetal malformations was observed when 
monitored for up to 60 months.313
To avoid neurologic complications of tumor enlargement dur­
ing pregnancy, it is recommended that women with prolactinomas 
be tested for prolactin sensitivity to dopamine agonists, as well as 

ctin sensitivity to dopamine agonists, as well as 
tolerance of these drugs, before proceeding with a pregnancy. If 
tumors are insensitive to dopamine agonist–related tumor shrink­
age, prophylactic surgery could be appropriate. If the tumor is 
a macroadenoma approximating the optic chiasm, the likelihood 
of visual difficulties is greater and therefore undertaking surgery 
prior to pregnancy could be prudent.314 
Nonfunctioning Pituitary Tumors
Nonfunctioning pituitary tumors comprise approxi

s
Nonfunctioning pituitary tumors comprise approximately 25% 
to 35% of pituitary tumors.315 These tumors are clinically silent 
(i.e., do not actively secrete hormones), but they arise from pitu­
itary cells capable of expressing hormone genes, including LH 
and FSH, and more rarely, ACTH and TSH. Diagnostic markers 
useful for determining pituitary tumor classification and behav­
ior include hormone immunohistochemistry and cell-specific 
transcription factors to define differentiated cell typ

cription factors to define differentiated cell types, as well as 
B
C
A
• Fig. 9.27  Shrinkage of a macroadenoma by cabergoline in a woman harboring a macroadenoma at 22 
weeks of gestation (A), when prolactin was 488 μg/L (B), and further reduction at 3 weeks postpartum 
(C). (Reproduced from Liu C, Tyrrell JB. Successful treatment of large macroprolactinoma with cabergoline 
during pregnancy. Pituitary. 2001;4:179–185.)
  Management of Patients with 
Prolactinomas Planning Pregnancy
Microaden

s with 
Discontinue dopamine agonist 
when pregnancy test is 
positive
Periodic visual field examina­
tions during pregnancy
Postpartum magnetic resonance 
imaging (MRI) after 6 weeksa
Consider surgery before pregnancy
Ensure bromocriptine sensitivity 
before pregnancy
Monitor visual fields expectantly 
and frequently
Administer bromocriptine if vision 
becomes compromised
Or continue bromocriptine through­
out pregnancy if tumor previou

mocriptine through­
out pregnancy if tumor previously 
affected vision
Consider high-dose steroids or 
surgery during pregnancy if 
vision is threatened or adenoma 
hemorrhage occurs
Postpartum MRI after 6 weeks
aPituitary MRI may be performed during pregnancy if deemed necessary.
   
TABLE 9.18
275
proliferative indices (Fig. 9.28). Gonadotroph and corticotroph 
cell tumors predominate amongst the nonfunctioning tumors.316
Gonadotroph Cell Tumors
Most nonf

oning tumors.316
Most nonfunctioning or hormonally silent tumors arise from 
gonadotroph cells, and they most frequently present as clini­
cally nonfunctioning masses not associated with elevated serum 
gonadotrophins. Yet, they usually express gonadotrophin sub­
units detectable by immunohistochemistry. In a series of non­
functioning adenomas, 42% of tumors immunostained for TSH 
β-subunit, 83% for LH β-subunit, 75% for FSH β-subunit, and 
92% for α-subunit.317 Although

FSH β-subunit, and 
92% for α-subunit.317 Although LH, FSH, and α-subunit are 
released from these nonfunctioning tumors when maintained in 
culture, production is usually not sufficient to elevate blood levels. 
A small subset of gonadotroph tumors do secrete sufficient hor­
mone to elevate serum gonadotrophin or α-subunit levels, which 
occasionally cause clinical syndromes.
Clinically nonfunctioning tumors generally come to attention 
because of their large size, or they are dete

ion 
because of their large size, or they are detected incidentally (inci­
dentaloma) (Table 9.19). Of 506 incidentally discovered pitu­
itary masses, 324 were clinically nonfunctioning tumors, and the 
remainder were cystic or parasellar masses.318 A gradual visual 
deficit arising from optic chiasmal compression is common, and 
patients are often unaware of the disturbance. Recognition of 
visual field deficits is often delayed because formal visual fields 
are not routinely evaluated unless a

isual fields 
are not routinely evaluated unless a defect is suspected clinically. 
In the absence of associated space-occupying or hormonal disor­
ders these large tumors may go unrecognized for many years and 
be inadvertently detected on MRI performed for other purposes. 
Sinusitis evaluation, pituitary apoplexy, or performance of a brain 
MRI for an unrelated indication (e.g., head trauma) may bring 
these tumors to clinical attention. Although not often the initial 
presenting complaint, th

gh not often the initial 
presenting complaint, these patients are commonly deficient in 
one or more pituitary hormones, as noted in two-thirds of 56 
patients with nonfunctioning macroadenomas.37 Although the 
most commonly encountered endocrine symptoms are related to 
gonadotrophin deficiency, quality of life may be decreased, and 
daytime somnolence has also been reported.319
A very small subset of gonadotroph adenomas producing 
elevated serum FSH, LH, and α-subunit concentrations are 
con

rum FSH, LH, and α-subunit concentrations are 
considered functioning gonadotroph adenomas and may be 
associated with specific endocrine syndromes. Most of these 
are macroadenomas, and imaging may also reveal ovarian cysts 
or increased testicular volumes.320 High serum FSH level, usu­
ally with a low LH level, is usually the only sign that a pituitary 
Nonfunctioning gonadotroph 
adenoma
Silent corticotroph adenoma
Nonfunctioning lactotroph 
adenoma
2a
3a
1b
1a
2b
3b
1c
2c
3c
HE
Pituitary
hor

denoma
2a
3a
1b
1a
2b
3b
1c
2c
3c
HE
Pituitary
hormones
factors
1c
• Fig. 9.28  Transcription factors determine cell-specific diagnosis of pituitary adenoma subtype when 
hormone expression is sparse or absent. Nonfunctioning gonadotroph adenoma showing hematoxylin-
eosin staining (1a), negative FSH and LH immunoreactivity (1b); and nuclear expression of SF1 (1c). Silent 
corticotroph adenoma showing hematoxylin-eosin staining (2a), sparse ACTH immunoreactivity (2b); and 
nuclear e

 sparse ACTH immunoreactivity (2b); and 
nuclear expression of Tpit (2c). Nonfunctioning lactotroph adenoma showing hematoxylin-eosin staining 
(3a), sparse prolactin immunoreactivity (3b); and nuclear expression of Pit1 (3c). (Magnification ×400.) 
(From Manojlovic-Gacic, Engstrom BE, Casar-Borota O. Histopathological classification of nonfunctioning 
pituitary neuroendocrine tumors. Pituitary. 2018;21:119–129.)
276
tumor secretes FSH. Female patients wi

tumor secretes FSH. Female patients with such tumors may 
present with pelvic pain due to ovarian hyperstimulation.321 
High gonadotrophin levels associated with menopause or tes­
ticular failure may complicate interpretation of gonadotrophin 
levels, but both LH and FSH are high in primary gonadal fail­
ure (Table 9.20). LH-producing tumors are exceedingly rare 
and in males may cause elevations in serum testosterone with 
acne and skin oiliness. Very rarely, isosexual precocious pu

kin oiliness. Very rarely, isosexual precocious puberty 
may be a presenting feature in children. Paradoxically, these 
patients may sometimes present with hypogonadism due to 
gonadal downregulation. 
Patients with incidentally discovered adenomas (incidentalomas) 
should be comprehensively evaluated for hormone hypersecretion, 
pituitary failure, and local mass effects.322 Subsequent decisions 
on whether to undertake surgery are determined by the size (>10 
mm) of the mass, evidenc

ermined by the size (>10 
mm) of the mass, evidence of continued growth, visual effects, 
central compressive features, or signs of hemorrhage. If hormone 
hypersecretion is detected, specific therapeutic approaches are 
required.
MRI, visual field examination, and pituitary hormones should 
be evaluated, the latter not only to detect hypopituitarism but 
also to exclude hormone overproduction that may not be clini­
cally apparent. This distinction of nonfunctioning versus secretory 
tumors is i

on of nonfunctioning versus secretory 
tumors is important for determining subsequent management.323 
LH, FSH, α-subunit, PRL, T4, triiodothyronine (T3), TSH, cor­
tisol, and IGF1 levels should be measured. A serum cortisol at 8 
am, cortisol response to cosyntropin, or an insulin tolerance test 
is used to exclude secondary adrenal insufficiency. The extent of 
hormonal evaluation requires clinical judgment. Elevated LH or 
FSH levels should be interpreted in light of the patient’s physi­
ologi

interpreted in light of the patient’s physi­
ologic state. Elevated serum FSH in a woman with regular men­
strual cycles would be interpreted differently from those detected 
in a menopausal patient; gonadotrophin elevations in patients 
with primary gonadal failure are not generally limited to one 
hormone, and circulating α-subunit elevation is consistent with 
a pituitary tumor but not gonadal failure. TRH stimulation may 
differentiate elevated gonadotrophin levels ascribed to end-organ 
fai

ed gonadotrophin levels ascribed to end-organ 
failure or to independent tumor production. In patients harboring 
gonadotroph cell adenomas, increased FSH, LH, LH β-subunit, 
or α-subunit are evoked in response to TRH.324 
Treatment
Clinical judgment should be used in determining appropriate 
therapy, including surgery, surgery followed by radiotherapy, 
radiotherapy alone, or expectant observation (Fig. 9.29). No reli­
able tumor marker is predictive of mass growth or recurrence.
  Clinical Pr

ctive of mass growth or recurrence.
  Clinical Presentation of Silent 
(Nonsecreting) Pituitary Adenomas
Frequency (%)
Incidental finding on MRI
8–38
Neurologic symptoms
Visual field deficits
61
Extraocular muscle palsy
14
Headache
10–61
Endocrine symptoms
Amenorrhea
10
26
Apoplexy
2–12
Hormonal deficiencies
GH
36–61
LH/FSH
40
TSH
36
ACTH
33
2
4–10
Immunostaining
Gonadotrophin subunits
44
POMC/ACTH
5–19
GH
2–4
PRL
2
TSH
1
Data from Mayson S

MC/ACTH
5–19
GH
2–4
PRL
2
TSH
1
Data from Mayson SE, Snyder PJ. Silent pituitary adenomas. Endocrinol Metab Clin North Am. 
2015;44:79–87; Saeger W, Ludecke DK, Buchfelder M, et al. Pathohistological classification 
of pituitary tumors: 10 years of experience with the German Pituitary Tumor Registry. Eur 
J Endocrinol. 2007;156:203–216; Brochier S, Galland F, Kujas M, et al. Factors predict­
ing relapse of nonfunctioning pituitary macroadenomas after neurosurgery: a study of 142 
patients. Eur J

fter neurosurgery: a study of 142 
patients. Eur J Endocrinol. 2010;163:193–200; Yamada S, Ohyama K, Taguchi M, et al. A 
study of the correlation between morphological findings and biological activities in clinically 
nonfunctioning pituitary adenomas. Neurosurgery. 2007;61:580–584; Nishioka H, Inoshita 
N, Mete O, et al. The complementary role of transcription factors in the accurate diagnosis of 
clinically nonfunctioning pituitary adenomas. Endocr Pathol. 2015;26:349–355.
   
TABLE 9.19
  C

ndocr Pathol. 2015;26:349–355.
   
TABLE 9.19
  Characteristics of Gonadotroph Adenomas 
and Primary Hypogonadism in Men
Gonadotroph 
Adenoma
Primary 
Hypogonadism
Puberty
Normal
Often incomplete
Fertility history
Normal
Decreased
Testicular size
Normal
Small
Serum testosterone level
Low to high
Low to normal
Testosterone response to 
hCG (when basal value 
is subnormal)
Brisk to well within 
normal range
Subnormal
Serum FSH level
High
High
Serum LH level
Usually normal or 
slightly high
High i

m LH level
High if testoster­
one is low
α-Subunit level
High to very high
High
FSH response to TRH
Common
Absent
LHβ response to TRH
Common
Absent
FSH, Follicle-stimulating hormone; hCG, human chorionic gonadotropin; LH, luteinizing hor­
mone; TRH, thyrotrophin-releasing hormone.
Modified from Wass JAH, Karavitaki N. Nonfunctioning and gonadotrophin-secreting adeno­
mas. In Melmed S, ed. The Pituitary, 4th ed. Philadelphia, PA: Elsevier; 2017:589–603.
   
TABLE 

iladelphia, PA: Elsevier; 2017:589–603.
   
TABLE 9.20
277
Surgery
Surgical resection of nonfunctioning adenomas is recommended 
for symptomatic patients, if vision is threatened, or for macroad­
enomas whose size threatens vital structures.325 Transsphenoidal 
microscopic or endoscopic endonasal transsphenoidal surgery 
is the recommended approach326 (Table 9.21). Gross total mass 
removal was reported after surgery in 137 of 173 patients with 
nonfunction

r surgery in 137 of 173 patients with 
nonfunctioning pituitary adenomas,326 and in another study 
gross total resection was achieved in 65.3% of 359 patients with 
improved vision or normalization of visual loss in 80% of affected 
patients.327 In a large prospective study, visual function was 
restored in 75% to 90%, while hypopituitarism was rescued in 
up to 50% of patients.328 Progression-free survival after surgery in 
365 patients was strongly determined by extent of tumor invasion 
and g

ngly determined by extent of tumor invasion 
and grade.329 
Postoperative Radiotherapy
To lower the postoperative risk of tumor growth progression, 
postoperative radiotherapy may be recommended.330 An expect­
ant follow-up of 65 patients after pituitary surgery for nonfunc­
tioning adenomas showed that 32% of patients not receiving 
postoperative radiotherapy exhibited tumor regrowth during a 
mean follow-up period of 76 months.331 Similar recurrence rates 
were observed in a retrospective foll

rence rates 
were observed in a retrospective follow-up of 212 patients.332 
In another study, tumor recurrence or regrowth occurred in 
6% to 46% of patients not receiving radiotherapy after trans­
sphenoidal surgery, and patients having received radiotherapy 
had a recurrence rate of 0% to 36%.333 Nevertheless, despite 
the relatively high incidence of postoperative tumor regrowth, 
even after apparently complete resection, many neurosurgeons 
avoid routine postoperative radiation therapy. Thi

avoid routine postoperative radiation therapy. This approach 
requires advising careful follow-up, with periodic annual MRIs 
and visual evaluations, and encouraging patients to maintain 
medical follow-up.
Radiation can be offered if the tumor mass reexpands. In a 
retrospective study of 62 patients with nonfunctioning pitu­
itary tumors treated with stereotactic radiosurgery, 60% expe­
rienced decreased tumor size and 37% of tumors remained 
unchanged. However, the risk of developing new anter

changed. However, the risk of developing new anterior pitu­
itary hormone deficits at 5 years was 32%.334 In 140 consecu­
tive patients undergoing stereotactic radiosurgery, the tumor 
mass was stabilized or decreased in 90% and median time to 
tumor progression was 14.5 years, with delayed hypopituita­
rism observed in 30% of patients.335 In a large multicenter 
Nonfunctioning pituitary adenoma
MRI differential diagnosis
Evaluate and
manage as
nonadenomatous
sellar mass
Assess pituitary functio

onadenomatous
Assess pituitary function
Follow-up:
MRI at 1, 2, and 5 years;
reassess if symptomatic
MRI
Trophic hormone testing every
6 months for 2 years,
annually thereafter;
hormone replacement
if required
Observe
Transsphenoidal
surgery
• Fig. 9.29  Management of nonfunctioning pituitary adenomas. Skilled 
magnetic resonance imaging (MRI) interpretation is crucial to diagnose 
nonadenomatous mass (e.g., meningioma, aneurysm, or ot

denomatous mass (e.g., meningioma, aneurysm, or other sellar 
lesion).
  Outcomes of Transsphenoidal Surgery for 
Outcomes
Risk (95% CI)
No. of Patients
I2 (%)
Complete removal
0.20 (0.09–0.38)
1207
95
Surgical death
0.01 (0.01–0.02)
1232
0
CSF leakage/
fistula
0.03 (0.02–0.06)
868
44
0.01 (0.01–0.03)
547
0
Transient diabetes 
insipidus
0.11 (0.04–0.27)
774
95
Persistent diabe­
tes insipidus
0.05 (0.03–0.07)
622
14
New anterior pitu­
itary deficits
0.0

0.07)
622
14
0.09 (0.03–0.23)
850
87
Pituitary function 
improvement
0.30 (0.12–0.57)
714
89
New visual field 
defects
0.03 (0.02–0.04)
1032
0
Visual field defect 
0.78 (0.62–0.89)
795
93
Recurrence after 
surgery alone
0.18 (0.12–0.26)
734
79
ACTH improve­
ment
0.37 (0.22–0.54)
145
64
ACTH worsening
0.39 (0.26–0.53)
49
0
TSH improvement
0.22 (0.07–0.51)
46
58
TSH worsening
0.17 (0.09–0.28)
160
31
LH/FSH improve­
ment
0.23 (0.13–0.28)
190
71
LH/FSH w

FSH improve­
ment
190
71
LH/FSH worsening
0.10 (0.01–0.71)
143
89
Median follow-up was 4.29 years after surgery. ACTH, Adrenocorticotrophic hormone; CSF, 
cerebrospinal fluid; FSH, follicle-stimulating hormone; LH, luteinizing hormone; TSH, thyroid-
stimulating hormone.
From Murad MH, Fernández-Balsells MM, Barwise A, et al. Outcomes of surgical treatment for 
nonfunctioning pituitary adenomas: a systematic review and meta-analysis. Clin Endocrinol. 
2010;73:777–791.
   
TABLE 9

is. Clin Endocrinol. 
   
TABLE 9.21
278
outcomes analysis of 512 patients undergoing stereotactic 
radiosurgery for nonfunctioning pituitary adenomas, median 
follow-up was 36 months. Progression-free tumor survival was 
associated with smaller tumor size and absence of suprasellar 
extension. New or aggravated pituitary failure was encountered 
in 21% of patients.336 Because patients may experience tumor 
regrowth even after radiation t

 experience tumor 
regrowth even after radiation therapy, they too should undergo 
periodic posttreatment MRIs, albeit less frequently. In a ret­
rospective study of 237 patients followed for a median of 5.9 
years, the 5-year second regrowth rate was 36% after surgery, 
12.5% after radiotherapy, and 12.7% after surgery with adju­
vant radiotherapy. 
Expectant Observation for Incidentalomas
Given the slow growth rate for nonfunctioning microadenomas 
or small macroadenomas, patients may be follo

mas 
or small macroadenomas, patients may be followed expectantly.337 
Some tumors do not grow over years or even decades. Overall, 
only 10% of incidentally discovered microadenomas will con­
tinue to enlarge. However, over 20% of incidental macroadeno­
mas are reported to continue growing (Table 9.22). After 5 years, 
the rate of growth appears to increase.337 Overall, about 10% 
of incidentalomas may decrease in size when followed for up to 
8 years. Serial MRIs should be undertaken at 1, 2, 

 years. Serial MRIs should be undertaken at 1, 2, and 5 years. 
Nonfunctioning adenomas may grow insidiously postoperatively 
and are usually asymptomatic until such time that they are large 
enough to affect vision (Fig. 9.30). As 20% of tumor regrowth 
occurs more than 10 years after surgery, performing serial MRIs, 
perhaps indefinitely, is suggested.338 Periodic, but less frequent, 
endocrine evaluation is also suggested because hypopituitarism 
occurs frequently and may be challenging to de

sm 
occurs frequently and may be challenging to detect. Over half 
of these patients exhibit pituitary hormone deficits (especially 
GH and gonadotrophins) prior to surgery, and postoperatively 
the condition may persist, recover, or become newly manifest. In 
  Changes in Pituitary Incidentaloma Size During Observation
MICROADENOMAS
MACROADENOMAS
Total
Increased
Decreased
No change
Total
Enlarged
Decreased
No change
Years 
Followed
Reincke
7
1
1
5
7
2
0
5
8
Donovan
15
0
0
15
16
4a
0
12
6-7
Nis

1
5
7
2
0
5
8
Donovan
15
0
0
15
16
4a
0
12
6-7
Nishizawa
28
2a
0
26
5.6
Feldkamp
31
1
1
29
19
5
1
13
2.7
Igarashi
1
0
0
1
22
6
10
6
5.1
Sanno
74
10
7
57
165
20a
22
123
2.3
Day
11
1
0
10
7
1
0
6
3.2
Arita
5
2
0
3
37
19a
0
18
5.2
Karavitaki
16
2
1
13
24
12
4
8
3.6
Dekkers
28
14
8
6
7.1
Anagnostis
6
0
1
5
3
1
0
2
4.0
Lendersb
27
2a
3
22
23
9
2
12
3.0
Esteves
14
0
2
12
12
1
3
8
3.2
Iglesiasb
22
4
1
17
28
0
1
27
1.2
Total (n = 648)
229
23 (10%)
17 (7%)
189 (83%)
419
96 (23%)
51 (12%)
272 (65%)
aEight

)
189 (83%)
419
96 (23%)
51 (12%)
272 (65%)
aEight cases had tumor enlargement due to apoplexy.
bPersonal communication.
From Huang W, Molitch ME. Management of nonfunctioning pituitary adenomas (NFAs): observation. Pituitary. 2018;21:162–167.
   
TABLE 9.22
Months
Residual – No RT
Residual – RT
No Residual
Recurrence-free survival (%)
100
80
60
40
20
0
120
100
80
60
40
20
0
• Fig. 9.30  Recurrence-free survival by Kaplan-Meier plots in 436 patients 
with silent (clinically nonfunctioning) pitui

nts 
with silent (clinically nonfunctioning) pituitary adenomas according to 
the presence or absence of residual adenoma after surgery and with or 
without radiation therapy (RT). (Adapted from Losa M, Mortini P, Barza­
ghi R, et al. Early results of surgery in patients with nonfunctioning pitu­
itary adenoma and analysis of the risk of tumor recurrence. J Neurosurg. 
2008;108(3):525–532. As adapted in Mayson SE, Snyder PJ. Silent (clini­
cally nonfunctioning) pituitary adenomas. J Neurooncol. 

nonfunctioning) pituitary adenomas. J Neurooncol. 2014;117:429–
436.)
279
a nationwide follow-up of 2795 patients with nonfunctioning 
tumors in Sweden, overall excess mortality was reported, espe­
cially for patients younger than 40 years of age and women with 
hypopituitarism.339 As ACTH and gonadotrophin deficiency are 
important determinants of mortality in patients with nonfunc­
tioning adenomas (about a twofold to fivefold increase), careful 
hormone 

a twofold to fivefold increase), careful 
hormone replacement regimens are required.340 
Pregnancy
Microadenomas only very rarely lead to impaired vision during 
pregnancy, as opposed to macroadenomas, which do so with 
greater frequency.341 Because macroadenomas do not usually 
shrink on medical therapy, the risks of visual impairment arising 
during pregnancy must be weighed carefully, and tumor resection 
may be indicated prior to pregnancy. 
Medications
Medications are not usually effective 

Medications are not usually effective in reducing tumor size and 
visual compromise. As most nonfunctioning adenomas express 
D2 receptors, treatment with cabergoline has been advocated 
to shrink some nonfunctioning tumors or to prevent regrowth 
of postoperative remnants.342 Treatment of 79 patients with 
cabergoline (mean weekly dose 1.5 mg) for a mean of 8.8 
years resulted in overall control of tumor growth in 87%, with 
38% experiencing tumor shrinkage. Overall, 13% of treated 

riencing tumor shrinkage. Overall, 13% of treated 
versus 42% of untreated patients required further surgery or 
radiotherapy.343 
Silent Corticotroph Tumors
These tumors are usually resected with the clinical diagnosis 
of nonfunctioning macroadenomas and the diagnosis is subse­
quently made by pathologic assessment. They immunostain for 
ACTH and POMC but are clinically silent and more aggressive 
than nonsecreting tumors of pure gonadotroph cell origin.344 
ACTH secretion is unaltered, with n

ll origin.344 
ACTH secretion is unaltered, with no associated clinical or bio­
chemical features of hypercortisolism, although these tumors 
are morphologically indistinguishable from adenomas associ­
ated with Cushing disease. A mixed corticotroph-gonadotroph 
cell origin may underlie the primitive tumor cell type, with 
associated aggressive growth.345 Silent corticotroph tumors 
may represent up to 7% of all surgically removed adenomas 
and are usually hemorrhagic and invariably macroadenoma

re usually hemorrhagic and invariably macroadenomas. 
Unlike Cushing disease, they have a 2:1 male preponderance, 
often present with mass effects, and about one-third have pre­
operative evidence for pituitary insufficiency. About half exhibit 
cavernous sinus or bony invasion, hemorrhage, necrosis, and 
cyst formation. They often recur and postoperative radiation 
and reoperation are required to eradicate tumor regrowth or 
residual mass.346
A review of 20 cases of silent ACTH-producing macroa

review of 20 cases of silent ACTH-producing macroadeno­
mas reported visual dysfunction as the most common presenting 
complaint (38%), and 13% presented with pituitary apoplexy.346 
In a study of 50 patients undergoing surgical resection and ste­
reotactic radiosurgery for silent ACTH-secreting tumors and 
followed for a median of 40 months, 82% exhibited tumor 
growth control versus 91% control achieved for nonfunction­
ing adenomas.347 Regular MRI follow-up is required because of 
the high ri

 MRI follow-up is required because of 
the high risk for recurrence. Unless appropriate immunostain­
ing is performed, many of these tumors remain undiagnosed 
and are inadvertently classified as recurrent nonfunctioning 
macroadenomas. 
Silent Subtype 3 Tumors
This entity usually expresses GH, PRL, and/or TSH (i.e., cells of 
the Pit1 lineage). They are clinically aggressive, nonsecreting ade­
nomas that exhibit cytologic features of nuclear atypia.348 
Silent GH-Expressing Tumors
Silent GH-exp

pia.348 
Silent GH-expressing tumors occur with a prevalence of about 
2% of all pituitary adenomas. These patients should be followed 
prospectively for recurrences and for potential progression to 
acromegaly.349 
In 1886, Pierre Marie published the first clinical description of 
disordered somatic growth and proportion and proposed the 
name “acromegaly.”350 When relation of this syndrome to a pitu­
itary tumor was later recognized, Benda showed in 1900 

 tumor was later recognized, Benda showed in 1900 that 
these tumors comprise mainly adenohypophyseal eosinophilic 
cells, which he proposed to be hyperfunctioning.351 Cushing, 
Davidoff, and Bailey documented the clinicopathologic features 
of acromegaly and demonstrated clinical remission of soft tis­
sue signs after adenoma resection.352 Evans and Long induced 
gigantism in rats injected with anterior pituitary extracts, 
confirming the association of a pituitary factor with somatic 
growth.3

ation of a pituitary factor with somatic 
growth.353 Establishment of the unequivocal pathophysiologic 
link between hyperfunctioning adenoma and acromegaly was 
the earliest example of a pituitary disorder to be clinically and 
pathologically recognized and appropriately managed by surgical 
excision of a hypersecreting source.
Incidence
The prevalence of acromegaly is estimated to range from 28 to 137 
cases per million. Recent surveys indicate an increase in the annual 
incidence to about 10 

 an increase in the annual 
incidence to about 10 cases per million213,215,217,354–363 (Table 
9.23). In the United States, over 3000 new cases of acromegaly 
are diagnosed annually, with an estimated population prevalence 
of 25,000 patients.363 
GH and IGF1 act both independently and dependently to induce 
features of hypersomatotropism. Acromegaly is caused by pituitary 
tumors secreting GH or very rarely by extrapituitary disorders364 
(Fig. 9.31). Regardless of the cause, the d

rs364 
(Fig. 9.31). Regardless of the cause, the disease is characterized by 
elevated levels of GH and IGF1 with resultant signs and symp­
toms of hypersomatotropism.
Pituitary Acromegaly
Over 95% of patients with acromegaly harbor a GH-secreting 
pituitary adenoma (Fig. 9.32 and Table 9.24). Pure GH-cell 
adenomas contain either densely or sparsely staining cytoplas­
mic GH granules, and these two variants are either slow (densely 
granulated) or rapidly growing (sparsely granulated).365 The f

or rapidly growing (sparsely granulated).365 The for­
mer type arises insidiously and presents during or after middle 
age, but the latter type arises in younger subjects with more 
aggressive tumor growth and florid disease. Mixed GH cell 
and PRL cell adenomas are composed of distinct somatotrophs 
expressing GH and lactotrophs expressing PRL. Monomor­
phous acidophilic stem cell adenomas arise from the common 
280
GH and PRL stem cell and often contain

GH and PRL stem cell and often contain giant mitochondria 
and misplaced GH granule exocytosis. They grow rapidly, are 
invasive, and present with predominant features of hyperprolac­
tinemia. Monomorphous mammosomatotroph cell adenomas 
express both GH and PRL from a single cell, but plurihormonal 
tumors may express GH with any combination of PRL, TSH, 
ACTH, and α-subunit. These patients present with clinical fea­
tures of acromegaly as well as hyperprolactinemia, Cushing dis­
eas

ly as well as hyperprolactinemia, Cushing dis­
ease, or very rarely hyperthyroxinemia. Somatotroph hyperplasia 
is difficult to distinguish from a GH cell adenoma, and silver 
staining displays a well-preserved reticulin network without a 
surrounding pseudocapsule. The rigorous morphologic diagno­
sis of GH cell hyperplasia is usually associated with stimulation 
by ectopic GHRH derived from an extrapituitary tumor causing 
acromegaly.
Both pituitary and hypothalamic factors influence pituitary

itary and hypothalamic factors influence pituitary 
tumor pathogenesis. Even when exhibiting marked nuclear pleo­
morphism, mitotic activity, and invasiveness, these tumors are 
invariably benign.
Disordered GHRH Secretion or Action
Adenomas express receptors for GHRH, ghrelin,366 and soma­
tostatin, but functional mutations of either the GHRH or 
somatostatin receptor have not been reported. GHRH directly 
stimulates GH gene expression and also induces somatotroph 
mitotic activity. Clinically,

induces somatotroph 
mitotic activity. Clinically, GHRH production by hypothalamic, 
abdominal, or chest neuroendocrine tumors causes somatotroph 
hyperplasia, and rarely adenoma, with resultant unrestrained GH 
secretion and acromegaly.159 However, histologic examination 
of most pituitary GH cell adenoma tissue specimens does not 
show hyperplastic somatotroph tissue surrounding the adenoma, 
implying no generalized hypothalamic overstimulation. Failure to 
downregulate GH secretion during pro

. Failure to 
downregulate GH secretion during prolonged GHRH stimula­
tion also points to a role for GHRH in maintaining persistent 
GH hypersecretion. Furthermore, a GHRH antagonist reduced 
human growth hormone production in 50 patients with acromeg­
aly, suggesting a role for endogenous GHRH.367 Expression of 
intra-adenomatous GHRH abundance correlates with tumor size 
and activity, implying a paracrine role for GHRH in mediating 
adenoma growth.368 However, complete surgical resection of w

owth.368 However, complete surgical resection of well-
defined GH-secreting microadenomas usually results in a defini­
tive cure of excess hormone secretion with very low postoperative 
tumor recurrence rates, strongly suggestive of intact hypothalamic 
function in these patients. 
Disordered Somatotroph Cell Function
The sequence of events leading to somatotroph clonal expansions 
appears multifactorial (see earlier discussion). An activated onco­
gene may be required for initiating tumorigenes

o­
gene may be required for initiating tumorigenesis, and promotion 
of tumor growth may require GHRH and other growth factor 
stimulation. The cellular mutation may not be sufficient to pro­
vide a growth advantage for a GH-secreting adenoma without 
additional disordered hypothalamic or paracrine growth factor 
Monoclonal origin of somatotroph adenomas was determined 
by X-chromosome inactivation analysis of somatotroph tumor 
DNA.155 An altered Gαs protein identified in a subset of

5 An altered Gαs protein identified in a subset of GH-
secreting pituitary adenomas leads to high levels of intracellular 
cAMP and GH hypersecretion.161 Point mutations in two criti­
cal sites, Arg201, the site for adenosine diphosphate ribosylation, 
and Gly227, the guanosine triphosphate (GTP)–binding domain 
of Gαs proteins, prevent GTPase activity and result in constitu­
tive adenyl cyclase activation. This dominant GSP mutant mimics 
GHRH effects, results in elevated cAMP levels, and is pr

ffects, results in elevated cAMP levels, and is present in 
about 30% of GH-secreting tumors. Germline inactivating muta­
tions of AIP have been found in a subset of familial somatotropi­
nomas,203,253 especially in younger patients with acromegaly or 
gigantism. 
McCune-Albright Syndrome
This rare hypersecretory syndrome consists of polyostotic fibrous 
dysplasia, cutaneous pigmentation, sexual precocity, hyperthy­
roidism, hypercortisolism, hyperprolactinemia, and acromegaly 
due to somatotrop

erprolactinemia, and acromegaly 
due to somatotroph hyperplasia. In a comprehensive review of 
112 patients, acromegaly was reported in up to 30% of patients 
with the syndrome and was invariably associated with skull base 
fibrous dysplasia.369 About half of these patients have definitive 
imaging evidence for a pituitary adenoma. Gαs mutations have 
been detected in both endocrine and nonendocrine tissues.162 GH 
hypersecretion is rarely controlled by surgery, and these patients 
require somat

lled by surgery, and these patients 
require somatostatin receptor ligands, GH receptor antagonist, or 
pituitary irradiation. 
  Epidemiology of Acromegaly
Study
Population 
Covered
Prevalence 
(per 100,000)
Annual 
Incidence 
Mestron 
(2004)
Population of 
Spain in 2001
3.4
0.2
Daly (2006)
72,792
12.5
NA
Bex (2007)
10,850,000
4
0.2
Fernandez 
(2010)
81,449
8.6
NA
Raappana 
(2010)
722,000–733,000
NA
0.3
Gruppetta 
(2013)
417,608
12.4
0.3
Kwon 
(2013)
48,456,369
2.8
0.4
Tjornstran

2.4
0.3
Kwon 
(2013)
2.8
0.4
Tjornstrand 
(2014)
1,590,640
3.3
0.4
Agustsson 
(2015)
321,857
13.7
NA
Hoskulds­
dottir 
(2015)
316,075
13.3
0.8
Broder 
(2016)
24,508,019 
(prevalence)
14,785,312 
(incidence)
8.78
1.17
Burton 
(2016)
50,170,946
7.8
1.1
Dal (2016)
5,534,738
8.5
0.4
Modified from Lavrentaki A, Paluzzi A, Wass JAH, et al. Epidemiology of acromegaly: review 
of population studies. Pituitary. 2017;20:4–9; Broder MS, Change E, Cherepanov D, et al. 
Incidence and prevalence of

 Cherepanov D, et al. 
Incidence and prevalence of acromegaly in the United States: a claims-based analysis. Endocr 
Pract. 2016;22:1327–1335.
   
TABLE 9.23
281
Extrapituitary Acromegaly
The source of excess GH secretion in acromegaly may not nec­
essarily be of pituitary origin. Because management of ectopic 
acromegaly differs from that for pituitary-dependent GH hyper­
secretion, rigorous clinical and biochemical criteria should be 
fulfilled to confirm

ochemical criteria should be 
fulfilled to confirm the diagnosis of ectopic acromegaly.370 These 
include demonstration of elevated circulating GHRH or GH 
levels in the absence of a primary pituitary lesion, a significant 
arteriovenous hormone gradient across the ectopic tumor source, 
biochemical and clinical cure of acromegaly after resection of the 
ectopic hormone-producing tumor, as well as normalization of 
the GHRH/GH/IGF1 axis. Finally, GHRH or GH gene product 
expression should be sho

 GHRH or GH gene product 
expression should be shown. Patients with nonconclusive imaging 
or biochemical or clinical features of pituitary acromegaly may 
inadvertently be diagnosed as harboring a nonpituitary source of 
excess GH secretion and hence be inappropriately treated.
GHRH Hypersecretion
Hypothalamic tumors, including hamartomas, choristomas, 
gliomas, and gangliocytomas, may produce GHRH with sub­
sequent somatotroph hyperplasia and resultant acromegaly159 
(see Fig. 9.31). Primary m

esultant acromegaly159 
(see Fig. 9.31). Primary mammosomatotroph hyperplasia with 
no evidence for pituitary adenoma or an extrapituitary tumor 
source of GHRH has been described in gigantism.371 The struc­
ture of hypothalamic GHRH was in fact elucidated from mate­
rial extracted from pancreatic GHRH-secreting neuroendocrine 
tumors in patients with acromegaly.158 GHRH immunoreactivity 
is detectable in about 25% of neuroendocrine tumor samples, and 
those of bronchial (foregut) origin compris

, and 
those of bronchial (foregut) origin comprise most tumors associ­
ated with ectopic GHRH secretion.372 Acromegaly in neuroendo­
crine tumor patients, however, is uncommon. In a retrospective 
survey of 177 patients with acromegaly, only a single patient was 
identified with elevated plasma GHRH levels.373 Rare pancre­
atic cell tumors, small cell lung cancers, adrenal adenoma, pheo­
chromocytoma, and medullary thyroid, endometrial, and breast 
cancer express GHRH and may cause acromegaly.3

st 
cancer express GHRH and may cause acromegaly.374,375 Surgical 
resection of the tumor secreting ectopic GHRH should reverse 
GH hypersecretion, and pituitary surgery is not required in these 
patients. Carcinoid syndrome with ectopic GHRH secretion can 
also be managed with somatostatin receptor ligands (SRL), which 
lower GH and IGF1 levels and suppress ectopic tumor elabora­
tion of GHRH.376 
Ectopic Pituitary Adenomas
GH-secreting adenomas may arise from ectopic pituitary remnants 
in the

 may arise from ectopic pituitary remnants 
in the sphenoid sinus, petrous temporal bone, or nasopharyngeal 
Primary GH excess
GHRH excess
Central
Hypothalamic tumor
Bronchial carcinoid
Pancreatic islet cell tumor
Small cell lung cancer
Adrenal adenoma
Medullary thyroid carcinoma
Pheochromocytoma
Lymphoma
Iatrogenic
Densely granulated GH cell
Mixed GH cell and PRL
Mammosomatotroph cell

omatotroph cell
Silent somatotroph
Ectopic
Familial syndromes
Multiple endocrine neoplasia-type I
McCune-Albright syndrome
Familial acromegaly
+
+
–
+
–
+
–
+
GHRH
GH
GH
GH
GH
Somatic growth
metabolic
dysfunction
+
+
metabolic
+
+
metabolic
+
IGF1
+
IGF1
+
IGF1
Extrapituitary GH excess
GHRH
SRIF
GHRH
GHS
GHS
SRIF
GHRH
GHS
SRIF
• Fig. 9.31  Pathogenesis of acromegaly. GH, gro

F
• Fig. 9.31  Pathogenesis of acromegaly. GH, growth hormone; GHRH, growth hormone–releasing hor­
mone; GHS, growth hormone secretagogue; IGF1, insulin-like growth factor type 1; PRL, prolactin; SRIF, 
somatostatin (somatotropin release–inhibiting factor). (From Melmed S. Medical progress: acromegaly. N 
Engl J Med. 2006;355:2558–2573.)
282
cavity.377 Very rarely, pituitary carcinoma may spread to the 
meninges, CSF, or cervical lymph nodes, resulting in

ninges, CSF, or cervical lymph nodes, resulting in functional 
GH-secreting metastases, which may be diagnosed by radiola­
beled octreotide imaging (Octreoscan).378 
Peripheral Growth Hormone–Secreting Tumors
Lung adenocarcinoma, breast cancer, and ovarian tissues con­
tain immunoreactive GH without clinical evidence of acromeg­
aly. Rarely, GH-secreting intramesenteric pancreatic islet cell 
tumor370 or a non-Hodgkin lymphoma379 may cause acromegaly. 
These patients have a normal-sized or small

galy. 
These patients have a normal-sized or small pituitary gland on 
MRI, absent GH response to TRH injection, and normal levels of 
circulating plasma GHRH. 
Acromegaloidism
Rarely, patients exhibit soft tissue and skin changes usually associ­
ated with acromegaly but normal baseline and dynamic GH and 
IGF1 with no demonstrable pituitary or extrapituitary tumor, 
termed acromegaloid. Pachydermoperiostosis should be considered 
in the differential diagnosis. Insulin resistance and defective I

tial diagnosis. Insulin resistance and defective IGF1 
binding demonstrated in cells derived from some patients with 
acanthosis nigricans, and treatment is symptomatic. 
Gigantism
Tall stature may be caused by a GH-secreting pituitary tumor or 
hyperplasia associated with several specific syndromes, includ­
ing the McCune-Albright syndrome, with somatotroph hyper­
plasia or rarely pituitary adenomas.380 AIP mutations have 
been traced in the lineage of some patients with gigantism.381 
Somatotr

eage of some patients with gigantism.381 
Somatotroph hyperplasia and acidophilic stem cell adenomas 
(with hyperprolactinemia) may rarely cause gigantism during 
infancy or early childhood, suggesting early hypersecretion of 
GHRH or disordered pituitary cell differentiation.371 Eighteen 
patients were reported with X-linked gigantism (X-LAG syn­
drome), with Xq26.3 chromosomal microduplications (Fig. 
9.33). These patients exhibit features of very early accelerated 
gigantism. Tumors derived f

ery early accelerated 
gigantism. Tumors derived from these patients overexpress 
GPR101, a putative regulator of GH.382 Pituitary gigantism 
Control
GH µg/L
0
5
10
15
9 AM
10 PM
9 AM
GH µg/L
0
20
40
60
80
9 AM
10 PM
9 AM
SRIF
SRIF
GH
Ghrelin
Ghrelin
Ghrelin
receptor
GHRH 
receptor
cAMP
Protein kinase C
Ion channels
MAPK
GH synthesis
GH secretion
Adenyl cyclase
Phospholipase C
Na+ proton antiporter
PTP
MAPK
SRIF receptor subtypes
SSTR1, SSTR2,
SSTR3, SSTR5
G

RIF receptor subtypes
GH inhibition
POU1F1
GHRH
Pituitary
GHRH
–
+
+
• Fig. 9.32  Normal and disrupted GHRH-GH-IGF1 axis and molecular targets for therapy. Normal and 
disrupted GHRH-GH-IGF1 axis in GH-secreting somatotroph adenomas, pituitary somatotroph cell devel­
opment, and gene expression are determined by the POU1F1 transcription factor. Net GH secretion is 
determined by integration of hypothalamic, nutritional, h

ned by integration of hypothalamic, nutritional, hormonal, and intrapituitary signals. GH synthesis 
and secretion are induced by hypothalamic GHRH and gut-derived ghrelin. Hypothalamic somatostatin 
(SRIF) suppresses GH secretion mainly by high-affinity binding to SSTR2 and SSTR5 receptor subtypes 
expressed on somatotrophs. SRIF or analogs signal through SSTR2 and SSTR5 to control GH hypersecre­
tion and shrink tumor mass. GH secretion patterns in a normal subject and in acromegaly are depicte

 in a normal subject and in acromegaly are depicted in 
the insets showing secretory bursts (mainly at night) and daytime troughs. cAMP, cyclic adenosine mono­
phosphate; GH, growth hormone; GHRH, growth hormone–releasing hormone; IGF1, insulin-like growth 
factor type 1; MAPK, mitogen-activated protein kinase; PTP, protein tyrosine phosphatase; SRIF, soma­
tostatin (somatotropin release–inhibiting factor); SSTR, SRIF receptor subtypes. (Modified from Melmed 
S. Acromegaly pathogenesis and treat

 from Melmed 
S. Acromegaly pathogenesis and treatment. J Clin Invest. 2009;119:3189–3202, used with permission.)
283
  Causes of Acromegaly
Cause
(%)
Hormonal Products
Pathologic Characteristics
Excess GH Secretion
Pituitary
98
Densely granulated GH cell 
adenoma
30
GH
Slow growing, clinically insidious
Resemble normal somatotrophs, 
numerous large secretory granules
Sparsely granulated adenoma
30
GH
Rapidly growing, often in

granulated adenoma
30
GH
Rapidly growing, often invasive
Cellular pleomorphism, characteristic 
ultrastructure
Mixed GH cell and PRL cell 
adenoma
25
GH and PRL
Variable
Densely granulated somatotrophs, 
sparsely granulated lactotrophs
Mammosomatotroph cell 
adenoma
10
GH and PRL
Common in children; gigantism, 
mild hyperprolactinemia
Both GH and PRL in same cell, often 
same secretory granule
Acidophil stem cell adenoma
PRL and GH
Rapidly growing, invasive, 
hyperprolactinemia dominant
Distinct

g, invasive, 
Distinctive ultrastructure, giant 
mitochondria
GH (PRL with αGSU, 
FSH/LH, TSH, or 
ACTH)
Often secondary hormonal 
products are clinically silent
Variable; either monomorphous or 
plurimorphous
GH cell carcinoma or metastases
GH
Usually aggressive
Documented metastasis
MEN1 (adenoma)
GH or PRL
Pancreatic, parathyroid, or 
pituitary tumors
Adenoma
GH, PRL
Classic triad
Ectopic sphenoid or paraph

assic triad
Ectopic sphenoid or parapharyn­
geal sinus pituitary adenoma
GH
Ectopic mass
Adenoma
GH
Young patients
Large adenomas
Carney syndrome
GH
Classic syndrome
Adenoma
Extrapituitary Tumor
<1
Small pituitary
Excess GHRH Secretion
Central—hypothalamic hamar­
toma, choristoma, ganglio­
neuroma
<1
Hypothalamic mass
Somatotroph hyperplasia
Peripheral—bronchial carcinoid, 
pancreatic islet cell tumor, 
small cell lung cancer, adrenal 

let cell tumor, 
adenoma, medullary thyroid 
carcinoma, pheochromocy­
toma
1
GH, PRL
Systemic features
Somatotroph hyperplasia, rarely 
adenoma
ACTH, Adrenocorticotrophic hormone; FSH, follicle-stimulating hormone; GH, growth hormone; GHRH, growth hormone–releasing hormone; αGSU, glycoprotein α-subunit; LH, luteinizing hormone; MEN1, 
multiple endocrine neoplasia type 1; PRL, prolactin; TSH, thyroid-stimulating hormone.
Adapted from Melmed S. Medical progress: Ac

rmone.
Adapted from Melmed S. Medical progress: Acromegaly. N Engl J Med. 2006;355:2558–2573; Melmed S, Braunstein GD, Horvath E, et al. Pathophysiology of acromegaly. Endocr Rev. 1983;4:271–290.
   
TABLE 9.24
should be considered in children who are more than 3 standard 
deviations above normal mean height for age or more than 2 
standard deviations over their adjusted mean parental height. 
The biochemical diagnosis is similar to that for acromegaly (i.e., 
GH levels >1 ng/mL after a glucose 

megaly (i.e., 
GH levels >1 ng/mL after a glucose load, elevated concentra­
tions of serum IGF1). In children undergoing pubertal growth 
spurts, GH responses to glucose may be paradoxical, and serum 
IGF1 concentrations are often physiologically elevated. Thus, 
the diagnosis requires clear-cut MRI evidence for a pituitary 
lesion. The differential diagnosis includes familial tall stature, 
redundancy of Y chromosomes, Marfan syndrome, and homo­
cystinuria. Aggressive control of the tumor mass 

cystinuria. Aggressive control of the tumor mass as well as hor­
mone hypersecretion is important to mitigate long-term tissue 
damage from excess GH and IGF1. Surgery is recommended as 
primary therapy, and postoperative SRLs and the GH receptor 
antagonist have been used effectively.383,384 Radiation therapy 
should be followed by lifelong serial evaluation of pituitary 
function. 
284
Clinical Features of Acromegaly
Manifestations of acromegaly are ca

 of Acromegaly
Manifestations of acromegaly are caused by either central pres­
sure effects of the pituitary mass or peripheral actions of excess 
GH and IGF1. Headache is often severe and debilitating. Local 
signs are especially important presenting features because a higher 
preponderance of macroadenomas (>65%) is encountered in acro­
megaly as compared to mostly microadenomas for PRL-secreting 
tumors.385
Effects of hypersomatotropism on acral and soft tissue 
growth and metabolic function 

al and soft tissue 
growth and metabolic function occur insidiously over sev­
eral years386(Table 9.25) (Figs. 9.34 and 9.35). As a result, 
acromegaly remains underrecognized; the slow onset and elu­
sive symptomatology often result in delayed diagnosis, with a 
mean delay of almost 10 years, especially in females.387 In a 
comparison of acromegaly features observed between 1981 and 
1994, and between 1995 and 2006, the delay in diagnosis was 
quite similar (5.9 vs. 5.2 years). Although biochem

uite similar (5.9 vs. 5.2 years). Although biochemical features 
were also remarkably similar, sleep apnea and colon polyps 
were more frequently encountered in the latter time frames, 
likely reflecting enhanced awareness.388 Patients may seek care 
for dental, orthopedic, rheumatologic, or cardiac disorders. 
Only 13% of 256 patients diagnosed during a 20-year period 
A
B
C
• Fig. 9.33  Growth patterns due to Xq26.3 microduplication. (A) Patient age 3 years and growth chart up 
to 24 months of

t age 3 years and growth chart up 
to 24 months of age depicting early rapid acceleration in weight; acceleration in height did not begin until 
after age 2. (B) Patient age 3 years at a height of 120 cm and her unaffected mother. (C) Growth chart for 
another female patient depicting early increase in height and weight starting at 6 months. (From Trivellin G, 
Daly AF, Faucz FR, et al. Gigantism and acromegaly due to Xq26 microduplications and GPR101 mutation. 
N Engl J Med. 2014; 371:2363–2374

PR101 mutation. 
N Engl J Med. 2014; 371:2363–2374.)
285
  Clinical Features of Acromegalya
Local Tumor Effects
Pituitary enlargement
Visual field defects
Cranial nerve palsy
Headache
Somatic Effects
Acral Enlargement
Thickening of soft tissues in hands and feet
Musculoskeletal
Gigantism
Prognathism
Jaw malocclusion
Arthralgias and arthritis
Acroparesthesia
Proximal myopathy
Hypertrophy of frontal bones
Skin
Hyperhidrosis
Oily
Skin t

hy of frontal bones
Skin
Oily
Skin tags
Colon
Polyps
Left ventricular hypertrophy
Asymmetric septal hypertrophy
Cardiomyopathy
Congestive heart failure
Pulmonary
Sleep disturbances
Sleep apnea—central and obstructive
Visceromegaly
Tongue
Thyroid
Salivary gland
Liver
Spleen
Kidney
Prostate
Endocrine-Metabolic Effects
Reproductive
Menstrual abnormalities
Decreased libido, impotence, low sex hormone–blinding globulin
Multiple Endocri

low sex hormone–blinding globulin
Multiple Endocrine Neoplasia Type 1 (MEN1)
Hyperparathyroidism
Pancreatic islet cell tumors
TABLE 9.25
Carbohydrates
Impaired glucose tolerance
Insulin resistance and hyperinsulinemia
Lipids
Hypertriglyceridemia
Minerals
Hypercalciuria, increased 1,25-hydroxyvitamin D3
Urinary hydroxyproline
Electrolytes
Low renin
Increased aldosterone
Thyroid
Low thyroxine-binding globulin
Goiter
aMost soft tissue and metabolic changes are reversible by tight 

sue and metabolic changes are reversible by tight hormonal control. Bony 
changes, hypertension, and central sleep apnea are generally not reversible.
Modified from Bonert V, Melmed S. Acromegaly. In: Bar S, ed. Contemporary Endocrinology. 
Totowa, NJ: Humana Press; 2002:201–228.
   
  Clinical Features of Acromegalya—cont’d
TABLE 9.25
presented with primary symptoms of altered facial appear­
ance or enlarged extremities.389 In a review of several hundred 
patients presenting with acromegaly wo

al hundred 
patients presenting with acromegaly worldwide, 98% had acral 
enlargement, and hyperhidrosis was prominent in 70%.386 
When presenting early, facial and peripheral features are usually 
not obvious, and serial review of old photographs often accen­
tuates the progress of subtle physical changes. Characteristic 
features include large fleshy lips and nose, spade-like hands, 
frontal skull bossing, and cranial ridges. Enlarged tongue, 
bones, salivary glands, thyroid, heart, liver, and

bones, salivary glands, thyroid, heart, liver, and spleen are the 
effects of generalized visceromegaly. Increase in shoe, ring, or 
hat size is commonly reported. Progressive acral changes may 
lead to facial coarsening and skeletal disfigurement especially 
if excess GH secretion begins prior to epiphyseal closure.390 
These changes include mandibular overgrowth with progna­
thism, maxillary widening, teeth separation, jaw malocclusion 
overbite, large nose, and coarse oily skin. Sonorous voic

e, large nose, and coarse oily skin. Sonorous voice deep­
ening occurs in association with laryngeal hypertrophy and 
enlarged paranasal sinuses. Up to half of patients may experi­
ence joint symptoms severe enough to limit daily activities. 
Arthropathy occurs in about 70% of patients, most of whom 
exhibit joint swelling, hypermobility, and cartilaginous thick­
ening.391 Cartilage T2 relaxation times on MRI scans are high, 
reflecting increased water content.392 These signs may often 
persist 

 water content.392 These signs may often 
persist after complete remission.393 Local periarticular fibrous 
tissue thickening may cause joint stiffening or deformities and 
nerve entrapment. Knees, hips, shoulders, lumbosacral joints, 
elbows, and ankles are affected as monoarticular or polyar­
ticular arthritides, but joint effusions rarely develop. Spinal 
involvement includes osteophytosis, disk space widening, and 
increased anteroposterior vertebral length, which may result in 
dorsal kypho

ertebral length, which may result in 
dorsal kyphosis. Neural enlargement and wrist tissue swelling 
may lead to carpal tunnel syndrome in up to half of all patients. 
286
Both median and ulnar nerve cross-sectional areas increase, 
and nerve conduction is abnormal.394 Chondrocyte prolifera­
tion with increased joint space occurs early, and ulcerations 
and fissures of weight-bearing cartilage areas are often accom­
panied by new bone formation. Debilitat

ten accom­
panied by new bone formation. Debilitating osteoarthritis may 
result in bone remodeling, osteophyte formation, subchondral 
cysts, narrowed joint spaces, and lax periarticular ligaments. 
Vertebral fractures occur with increasing frequency, and osteo­
phytes commonly occur at the phalangeal tufts and over the 
anterior aspects of spinal vertebrae.395–397 Synovial edema leads 
to hyperplastic wrist ligaments and tendons that contribute to 
painful median nerve compression. Peripheral 

 to 
painful median nerve compression. Peripheral acroparesthesias 
and symmetric peripheral neuropathy should be distinguished 
from diabetic neuropathy, which may occur secondarily to 
acromegaly.398 Proximal myopathy may also be accompanied 
by myalgias and cramps. Ligaments may ossify, and periarticu­
lar calcium pyrophosphate deposition occurs. Hyperhidrosis 
and malodorous oily skin are common early signs, occurring 
in up to 70% of patients. Facial wrinkles, nasolabial folds, heel 
pad th

s. Facial wrinkles, nasolabial folds, heel 
pad thickening, and body hair coarsening399 are attributed to 
glycosaminoglycan deposition and increased connective tissue 
collagen production. Skin tags are common and may be mark­
ers for the adenomatous colonic polyps. Raynaud phenomenon 
is reported in up to one-third of patients.
Symptomatic cardiac disease is present in about 20% of 
patients at diagnosis and is a major cause of morbidity and 
mortality.400 Hypertension is present in about 50% 

ortality.400 Hypertension is present in about 50% of patients 
with active acromegaly, and half of these patients have evi­
dence of left ventricular (LV) dysfunction with or without 
regurgitant valve disease.401 Asymmetric septal hypertrophy 
and cardiac failure with increased ventricular ejection fraction 
may occur, and subclinical LV diastolic dysfunction is due to 
myocardial hypertrophy, interstitial fibrosis, and lymphocytic 
myocardial infiltrates. Increased aortic root diameter and aor

nfiltrates. Increased aortic root diameter and aor­
tic ectasia were reported in 26% of patients.402 Resting elec­
trocardiograms are abnormal in about 50% of patients, with 
ST-segment depression, T-wave abnormalities, conduction 
defects, and arrhythmias. Plasma renin levels are suppressed, 
and renal sodium channel activity is induced by GH at the 
aldosterone-sensitive distal nephron.403 In a prospective study 
of 30 patients, cardiovascular responses to SRL therapy were 
shown to be highly 

responses to SRL therapy were 
shown to be highly variable, despite attainment of biochemi­
cal control.404 Despite increased cardiovascular risk fac­
tors,405 there is no reported increased myocardial infarction in 
acromegaly.406
Prognathism, thick lips, macroglossia, and hypertrophied 
nasal structures may obstruct airways. Irregular laryngeal 
mucosa, cartilage hypertrophy, tracheal calcification, and crico­
arytenoid joint arthropathy lead to unilateral or bilateral vocal 
cord fixation or 

o unilateral or bilateral vocal 
cord fixation or laryngeal stenosis with voice changes. Tracheal 
intubation may be particularly difficult in patients undergoing 
anesthesia, and tracheostomy may be required. Both central 
respiratory depression and airway obstruction lead to paroxys­
mal daytime sleep (narcolepsy), sleep apnea, and habitual exces­
sive snoring. Obstructive sleep apnea with daytime somnolence 
occurs especially in men with acromegaly, who also may have a 
ventilation-perfusion 

egaly, who also may have a 
ventilation-perfusion defect with hypoxemia. Sleep apnea may 
also be central in origin and associated with higher GH and 
IGF1 levels.407
Hypertrophied tissue surrounding the canal of Schlemm may 
impede aqueous filtration, leading to open-angle glaucoma. Pro­
gressive facial and bodily disfigurement often leads to lowered 
B
A
• Fig. 9.34  Harvey Cushing’s first acromegaly patient. (A) Some years before presentation and (B) at 
admission. (From Jane JA, Laws ER. His

and (B) at 
admission. (From Jane JA, Laws ER. History of acromegaly. In: Wass J, ed. Handbook of Acromegaly. 
Bristol, UK: BioScientifica; 2001:3–15.)
287
self-esteem. Depression and mood swings may occur secondarily 
to physical deformity, with impaired quality of life.408 
Growth Hormone and Tumor Formation
GH and IGF1 as well as insulin may exhibit direct or indirect 
mitogenic effects on mammalian cells and act as permissive cell 
growth stimulators.40

 and act as permissive cell 
growth stimulators.409 In a meta-analysis, the risks of adenoma­
tous and hyperplastic polyps and colorectal cancer were increased 
2.5-fold, 3.6-fold, and 4.4-fold, respectively.410 The prevalence of 
polyps was 32% in 165 patients in a case control study, with an 
estimated relative risk of 6.21 (95% CI, 4.08–9.48). Higher IGF1 
levels at diagnosis were more likely associated with distal colon 
lesions.411 Hypertrophic mucosal folds and colonic hypertrophy as 
well

hic mucosal folds and colonic hypertrophy as 
well as diverticulae412 are commonly present, and dolicomegaco­
lon may be visualized by CT colonography.413 Increased incidence 
of gallbladder polyps414 and benign prostate hypertrophy415 have 
been reported. Although the German Acromegaly Registry does 
not report higher cancer incidence,416 a survey of 1512 patients 
in Italy revealed a moderately increased cancer incidence.417 Simi­
larly, a nationwide study in Denmark of 529 acromegaly patients

onwide study in Denmark of 529 acromegaly patients 
revealed an overall cancer SIR of 1.5 (95% CI, 1.2–1.8). Incidence 
of colorectal, kidney, and thyroid cancers appear to be the most 
significantly elevated.418–420 Thus, although disordered cell pro­
liferation and increased risk for growth promotion of coexisting 
neoplasms as well as mortality could be anticipated from elevated 
GH and IGF1 levels, a significantly enhanced cancer incidence 
has not been consistently observed. Colon cancer ap

as not been consistently observed. Colon cancer appears to be 
of particular concern, and screening colonoscopy should be per­
formed at diagnosis in all patients. Thereafter, screening should 
follow established guidelines. As acromegaly patients are now liv­
ing longer as a result of improved biochemical and cardiac control, 
long-term prospective studies are required to resolve this question 
in an aging population.
Endocrine Complications
About 30% of patients exhibit elevated serum PRL leve

ut 30% of patients exhibit elevated serum PRL levels (up 
to 100 ng/mL or more), with or without galactorrhea. Func­
tional pituitary stalk compression by a pituitary mass prevents 
lactotroph access of hypothalamic dopamine, releasing the cell 
from tonic hypothalamic inhibition. GH-secreting adenoma 
A
D
C
F
B
E
• Fig. 9.35  Clinical features of acromegaly. (A–C) Features of acromegaly/gigantism in identical twins. A 
22-year-old man with gigantism due to excess growth hormone is shown to the 

tism due to excess growth hormone is shown to the left of his identical twin. 
The increased height and prognathism (A) and enlarged hand (B) and foot (C) of the affected twin are 
apparent. Their clinical features began to diverge at the age of approximately 13 years. (D) Increased inci­
sor spacing and prognathism in patient with acromegaly. (E) Macroglossia (left) and a normal tongue (right). 
(F) Dolicomegacolon in acromegaly as visualized by computed tomography colonography. (A–C, from 
Gag

 computed tomography colonography. (A–C, from 
Gagel R, McCutcheon IE. Images in clinical medicine: pituitary gigantism. N Engl J Med. 1999;324:524, 
used with permission; D and E, from Turner HE. Clinical features, investigation and complications of acro­
megaly. In: Wass J, ed. Handbook of Acromegaly. Bristol, UK: BioScientifica; 2001, used with permission; 
F, from Resmini E, Tagliafico A, Bacigalupo L, et al. Computed tomography colonography in acromegaly. J 
Clin Endocrinol Metab. 2009;94:2

in acromegaly. J 
Clin Endocrinol Metab. 2009;94:218–222, used with permission.)
288
subtypes may also concomitantly secrete PRL. As GH behaves 
as an agonist for breast PRL-binding sites, the tumor may 
cause galactorrhea in the face of normal PRL levels. Tumor 
mass compressing surrounding normal pituitary tissue may 
also cause hypopituitarism. Over half of all patients have amen­
orrhea or impotence,385–421 and secondary thyroid or adrenal 
failure is

5–421 and secondary thyroid or adrenal 
failure is present in about 20% of patients. Gonadal dysfunc­
tion may exacerbate bone loss and vertebral fractures.396,397 In 
a prospective controlled study of 22 patients, active acromegaly 
was associated with PTH-independent calcium-phosphate bal­
ance, with low PTH and high phosphorus levels associated 
with increased coupled bone formation and resorption.422 
Skeletal fragility may persist despite long-term biochemical 
control.423,424
GH suppresses

ng-term biochemical 
GH suppresses beta cell function and antagonizes insulin 
action, and acromegaly is associated with increased insulin resis­
tance, lipolysis, and gluconeogenesis,425,426 despite increased lean 
body mass, decreased fat mass, and elevated IGF1 concentrations 
that would be expected to enhance insulin sensitivity. Accordingly, 
fasting glucose intolerance and diabetes mellitus occur in about 
50% of patients.426,427 Increased diabetes risk with a hazard ratio 

6,427 Increased diabetes risk with a hazard ratio 
of 4.0 (95% CI: 2.7–5.8) was reported in a nationwide cohort 
study in Denmark.361 Unique cardiomyopathy features are also 
associated with diabetes and acromegaly.425 Carbohydrate intoler­
ance and insulin requirements improve rapidly on lowering GH 
after surgery or somatostatin analogue therapy; however, diabe­
tes may persist in some patients after biochemical remission.428 
Hypertriglyceridemia (type IV), hypercalciuria, and hypercalce­
mia

mia (type IV), hypercalciuria, and hypercalce­
mia also occur. Thyroid dysfunction may be associated with diffuse 
or nodular toxic or nontoxic goiter or Graves disease, especially as 
IGF1 is a major determinant of thyroid cell growth.429 Associated 
MEN1 features may be present in affected individuals, including 
hypercalcemia with hyperparathyroidism or pancreatic tumors. 
Morbidity and Mortality
In a meta-analysis of 16 studies, overall mortality rate was 
reported to be increased in acromeg

lity rate was 
reported to be increased in acromegaly with a standardized mor­
tality ratio of 1.72. Lower recent mortality rates reported likely 
reflect the positive impact of introduction of somatostatin ana­
logues, improved surgical technique, and enhanced cardiac thera­
pies430 (Fig. 9.36). Cardiovascular disease is the leading cause of 
death followed by respiratory disease (18%) and cerebrovascular 
disease (14%). Diabetes mellitus, occurring in 20% of patients, is 
associated with 2.5 t

ring in 20% of patients, is 
associated with 2.5 times the predicted mortality rate, and hyper­
tension is present in about half of all patients.431 The most signifi­
cant mortality determinants are GH levels greater than 2.5 μg/L, 
elevated IGF1 levels, the presence of coexisting hypertension and 
cardiac disease, older age, a history of pituitary irradiation, and 
overreplaced ACTH-dependent adrenal insufficiency.59 Over­
treatment of adrenal insufficiency with doses of hydrocortisone 
more th

nsufficiency with doses of hydrocortisone 
more than 25 mg daily also is predictive of mortality.59 In the large 
UK West Midlands acromegaly registry comprising 501 patients, 
a last available recorded GH level of 1 ng/mL or less versus 1 
ng/mL or more was highly predictive of mortality rate. However, 
when summated over time in an unbiased fashion, the relative 
risk of fatality appeared to be associated with a GH cutoff level 
of 5 ng/mL.432 Moreover, control of GH levels to less than 2.5 
μ

Moreover, control of GH levels to less than 2.5 
μg/L and normal IGF1 levels after surgery or medical treatments 
is associated with significant reductions of both morbidity and 
mortality.431,433 However, in a 20-year follow-up controlled study, 
all-cause mortality rates remained elevated (odds ratio [OR] 1.6; 
95% CI, 1.2–2.2) and was more evident in women (Table 9.26). 
Causes of death shifted with time from predominantly cardiovas­
cular to cancer diagnoses.434 
Diagnosis
Measurement of Gro

cancer diagnoses.434 
Diagnosis
Measurement of Growth Hormone and IGF1 Levels
The diagnosis of acromegaly requires measurement of a GH nadir 
during a 75-g glucose load of less than 0.4 μg/L using ultrasensi­
tive GH assays or GH less than 1.0 μg/L using standard assays 
with an accompanying elevated IGF1 level435,436 (Fig. 9.37). 
Follow-up (years)
Matched New Zealand
population
p = 0.001
Survival probability
1.0
0.8
0.6
0.4
0.2
40
35
30
25
20
15
10
5
0
• Fig. 9.36  Mortality rate ou

30
25
20
15
10
5
0
• Fig. 9.36  Mortality rate outcome in retrospective studies of acromegaly. 
(Data integrated from Holdaway IM, Rajasoorya RC, Wong J, et al. The 
natural history of treated functional pituitary adenomas. In: Webb S, ed. 
Pituitary Tumors. Bristol, UK: BioScientifica; 1998:31–42.)
  Factors Associated With All-Cause 
Acromegaly Mortality on Multivariate 
Analysisa
Variable
HR
95% CI
p Value
Age at diagnosis (years)
1.1
1.08–1.13
<0.001
Gender (M/F)
  Up to 20 years from 
dia

.13
<0.001
diagnosis
2.5
1.46–4.17
<0.001
  After 20 years from 
diagnosis
0.9
0.42–1.91
0.78
Serum basal GH at 
diagnosis (μg/L)
1.0
0.99–1.01
0.91
Primary operation (no 
vs yes)
1.5
0.86–2.61
0.15
Radiotherapy (no vs 
yes)
0.5
0.34–0.88
0.012
Last known basal 
serum GH <2.5 μg/L 
after primary treat­
ment (no vs yes)b
1.8
1.21–2.83
0.048
aPiecewise Cox model.
bMeasured a median of 5 years after diagnosis.
CI, Confidence interval; F, female; GH, growth hormon

 Confidence interval; F, female; GH, growth hormone; HR, hazard ratio; M, male.
Modified from Ritvonen E, Löyttyniemi E, Jaatinen P, et al. Mortality in acromegaly: a 20-year 
follow-up study. Endocr Relat Cancer. 2016;23:469–480.
   
TABLE 9.26
289
In healthy subjects, serum GH levels initially fall after oral glu­
cose administration and subsequently increase as plasma glucose 
declines. In patients with acromegaly, oral glucose fails to sup­
press GH; GH

cromegaly, oral glucose fails to sup­
press GH; GH levels may increase, remain unchanged, or fall 
modestly in approximately one-third of patients. Basal morning 
(am) and random GH levels are usually elevated in acromegaly. 
Because of the episodic nature of GH secretion, however, serum 
concentrations may normally fluctuate from “undetectable” up 
to 30 μg/L. Unlike the largely undetectable nadir GH levels in 
normal subjects, those with acromegaly sampled over 24 hours 
have mean GH levels ab

galy sampled over 24 hours 
have mean GH levels above 2 μg/L.436 Evoked GH responses to 
GHRH administration are not of diagnostic use. Random GH 
levels measured with sensitive assays in acromegaly may be as low 
as 0.37 μg/L with persistently elevated postoperative IGF1 lev­
els.437 Serum IGF1 levels are high438,439 and correlate with clinical 
features (Fig. 9.38). Basal GH levels determine ambient circulat­
ing IGF1 levels in a log-linear fashion.440 Age-matched and gen­
der-matched IGF1 ele

hion.440 Age-matched and gen­
der-matched IGF1 elevations may persist for several months after 
GH levels are biochemically controlled after surgery. Although 
elevated IGF1 levels may be encountered during pregnancy and 
late puberty, an appropriately corrected high IGF1 level is highly 
specific for acromegaly and correlates with clinical indices of 
disease activity. IGF-binding protein 3 (IGFBP3) levels are also 
elevated but provide little added diagnostic value. GH-secreting 
adenomas exhi

dded diagnostic value. GH-secreting 
adenomas exhibit discordant GH responses to TRH and GnRH 
administration in up to 50% of patients, but these adjunctive tests 
are rarely required to confirm the diagnosis. 
Differential Diagnosis
The overwhelming majority of patients with acromegaly harbor a 
GH cell pituitary adenoma; rarely extrapituitary acromegaly should 
be considered. Regardless of the cause of unrestrained GH secre­
tion, IGF1 levels are invariably elevated, and GH levels fail to sup­

re invariably elevated, and GH levels fail to sup­
press after an oral glucose load. When clinical features of acromegaly 
are associated with normal GH and IGF1 levels, “burned out” or 
“silent” acromegaly associated with an infarcted pituitary adenoma, 
often with a secondary empty sella, should be considered.441 About 
5% of consecutive patients with proven GH cell adenomas have 
“normal” GH and elevated IGF1 levels. Plasma GHRH levels are 
invariably elevated in patients with peripheral GHRH

variably elevated in patients with peripheral GHRH-secreting 
tumors but are normal or low in those with hypothalamic GHRH-
secreting tumors.373 Presumably, excess eutopic hypothalamic 
GHRH secretion into the hypophyseal portal system does not 
appreciably enter the systemic circulation.
MRI and CT scanning are used to localize pituitary or extrapitu­
itary tumor. Routine abdominal or chest imaging of all acromegaly 
patients will yield a very low incidence of ectopic tumors and is not 
recomme

ow incidence of ectopic tumors and is not 
recommended. A normal-sized or small pituitary gland, or clinical 
and biochemical features of other tumors known to be associated 
with extrapituitary acromegaly and elevated circulating GHRH lev­
els, are indications for extrapituitary imaging. An enlarged hyper­
plastic pituitary gland, however, is often present in patients with 
peripheral GHRH-secreting tumors, and the radiologic diagnosis 
of a pituitary adenoma may be difficult to exclude. The ve

uitary adenoma may be difficult to exclude. The very rare 
McCune-Albright syndrome should be considered after definitive 
exclusion of pituitary and extrapituitary tumors. 
Treatment
Aims
A comprehensive strategy for treating patients with acromegaly 
should aim to manage the pituitary mass, suppress GH and 
IGF1 hypersecretion, and prevent long-term clinical sequelae of 
hypersomatotropism while maintaining normal anterior pitu­
itary function.442,443 Serum GH levels should be suppressed 
to a

442,443 Serum GH levels should be suppressed 
to at least 1 μg/L or less after an oral glucose load, and age-
matched and gender-matched serum IGF1 levels should be 
normalized. Ideally, a controlled patient should also have a “nor­
mal” 24-hour integrated secretion of GH (<2.5 μg/L), but these 
measurements are impractical. As current therapeutic modes for 
acromegaly management, including surgery, irradiation, and 
medical treatment, do not uniformly fulfill these goals,444 per­
sonalized appr

formly fulfill these goals,444 per­
sonalized approaches have been proposed (discussion to come). 
Surgical Management
Well-circumscribed somatotroph cell adenomas should prefer­
ably be resected by transsphenoidal surgery using either micro­
scopic or laparoscopic techniques.445–447 Successful resection 
alleviates preoperative compression effects and compromised 
trophic hormone secretion, and the skilled surgeon balances the 
extent of maximal tumor tissue removal while preserving ante­
rior 

 tumor tissue removal while preserving ante­
rior pituitary function. Within 2 hours of successful resection, 
metabolic dysfunction and soft tissue swelling start improving, 
and GH levels are sometimes controlled within an hour. Over­
all, 38% to 83% of patients achieve normalized IGF1 levels. 
Fewer patients with macroadenomas are controlled. Of 1018 
patients reported in 13 studies, 73% with microadenomas and 
61% harboring macroadenomas achieved GH levels lower than 
1.0 μg/L during glucose

eved GH levels lower than 
1.0 μg/L during glucose loading and normal serum IGF1 levels 
(Table 9.27).448–450 When remission was rigorously defined as 
a normal IGF1 and either a nadir glucose-suppressed GH level 
less than 0.4 μg/L or a random GH level less than 1 μg/L, all 14 
of 14 patients with microadenomas and 28 of 46 patients with 
macroadenomas achieved remission.445 As over one-third of 
premenopausal women exhibit cavernous sinus tumor invasion, 
remission rates are lower than in men 

 invasion, 
remission rates are lower than in men under 50 years of age.451 
Experience of the neurosurgeon,38 smaller tumor size, lower 
Knosp score, and lower preoperative IGF1 and GH levels are 
predictive of postsurgical remission (Table 9.28). Postoperative 
GH level measured within 24 hours of surgery is a significant 
outcome predictor (Fig. 9.39).42 Difficulties in endotracheal 
intubation due to macroglossia or severe kyphosis may rarely 
necessitate tracheostomy for anesthesia.
Side Ef

Although often transient, surgical complications may require life­
long pituitary hormone replacement. New hypopituitarism devel­
ops in up to approximately 20% of patients, reflecting operative 
damage to the surrounding normal pituitary tissue. Permanent 
diabetes insipidus, CSF leaks, hemorrhage, and meningitis occur 
in up to 10% of patients (see Table 9.3). Postoperative pituitary 
deficits assessed in a meta-analysis occur in ∼15% of 

cits assessed in a meta-analysis occur in ∼15% of patients over­
all. GH deficiency may require careful replacement if clinical fea­
tures of GH deficiency become apparent.452 Secondary adrenal 
insufficiency is an important determinant of mortality,453 and the 
incidence of postoperative adrenal failure is 2/1000 person-years 
in acromegaly patients in remission.454 The extent and prevalence 
of local complications depend upon tumor size and invasiveness. 
Experienced pituitary surgeons report 

asiveness. 
Experienced pituitary surgeons report more favorable postopera­
tive complication rates.38,39 Biochemical or anatomic acromegaly 
recurrence (∼7% over 10 years) or postoperative tumor persistence 
may indicate incomplete resection of adenomatous tissue, surgi­
cally inaccessible cavernous sinus tissue, or nesting of functional 
tumor tissue within the dura. 
290
Clinical features of acromegaly
Measure
IGF1 level
Perform oral
glucose tolerance

Measure
IGF1 level
test and measure
GH levels
Active acromegaly
excluded
Normal for age 
and gender
Elevated
GH suppression
Inadequate GH
suppression
Pituitary
MRI
Pituitary mass
Normal or 
hyperplastic
or small 
pituitary gland
Extra-
pituitary
acromegaly
Resect primary 
tumor
Chest and
abdominal CT
GHRH
measurement
GH-
secreting
pituitary
adenoma
Assess
likelihood of
surgical
success
Surgery
SRL
Disease
persistence
Control
Inadequate response
or drug intolerance

e
Control
GH receptor antagonist
with or without SRL;
repeat surgery or
radiation
SRL
• Fig. 9.37  Diagnosis and treatment of acromegaly. Oral glucose tolerance test (OGTT) is performed with 
75 g glucose and growth hormone (GH) measured during 2 hours. Disease control implies nadir GH level 
of less than 1 μg/L after OGTT using standard assays and less than 0.4 μg/L using ultrasensitive assays, 
and age-matched and gender-matched normal insulin-like growt

tched and gender-matched normal insulin-like growth factor type 1 (IGF1) level. CT, computed 
tomography; GHRH, growth hormone–releasing hormone; MRI, magnetic resonance imaging; SRL, soma­
tostatin receptor ligand. Insets depict pituitary adenoma (left) and extrapituitary acromegaly (right): L, liver; 
P, pancreas; T, tumorsecreting GH. (From Melmed S. Acromegaly. N Engl J Med. 2006;355:2558–2573. 
Clinical features figure from Minkowski O. Ueber einen Fal von Akromegalie Berliner. Klin Wochens

ear regres­
sion analysis between fasting somatomedin C (IGF1) and growth hormone levels 1 hour after oral glucose 
administration. (Adapted from Clemmons DR, Van Wyk JJ, Ridgway EC, et al. Evaluation of acromegaly 
by radioimmunoassay of somatomedin-C. N Engl J Med. 1979;301:1138–1142.)
  Remission After Transsphenoidal Surgery for GH-Secreting Pituitary Adenomas Published Since 2010
REMISSION
Study
n
Overall
Remission Criteria (GH Nadir), (μg/L)
Technique
Hofstette

on Criteria (GH Nadir), (μg/L)
Technique
Hofstetter (2010)
24
38
NA
NA
<1.0
Endoscope
Gondim (2010)
67
75
86
72
<1.0
Endoscope
Campbell (2010)
26
58
75
55
<1.0
Endoscope
Jane (2011)
60
70
100
61
<1.0
Endoscope
Wang (2012)
43
67
77
63
<1.0
Endoscope
Starke (2013)
43
70
82
66
<1.0
Microscope
72
71
88
66
<1.0
Endoscope
Shin (2013)
53
49
83
46
<1.0
Endoscope
Hazer (2013)
214
63
63
63
<1.0
Endoscope
Yilidirim (2014)
56
66
80
67
<1.0
Endoscope
Fathalla (2015)
41
45
NA
NA
<1.0
Endoscope
F

scope
41
45
NA
NA
<1.0
Endoscope
23
34
NA
NA
<1.0
Netuka (2016)
105
61
75
58
<1.0
Endoscope
Babu (2017)
55
78
35
65
<1.0
Endoscope
Sarkar (2017)
66
29
13
44
<1.0
Endoscope
Kim (2017)
134
73
87
72
<0.4
Endoscope
Data from Buchfelder M, Schlaffer SM. The surgical treatment of acromegaly. Pituitary. 2017;20:76–83; Babu H, Ortega A, Nuno M, et al. Long-term endocrine outcomes following endoscopic endonasal 
transsphenoidal surgery for acromegaly and associa

transsphenoidal surgery for acromegaly and associated prognostic factors. Neurosurgery. 2017;81:357–366; Kim JH, Hur KY, Lee JH, et al. Outcome of endoscopic transsphenoidal surgery for acromegaly. 
World Neurosurg. 2017;104:272–278.
   
TABLE 9.27
Primary or adjuvant radiation of GH-secreting tumors may be 
achieved by conventional external deep x-ray therapy as well as 
heavy-particle (proton-beam) or stereotactic radiosurgery. Maxi­
mal tumor radiation should ideally be atta

. Maxi­
mal tumor radiation should ideally be attained with minimal soft 
tissue damage. Radiation is a highly individualized choice, depend­
ing on the expertise and experience of the treating radiotherapist, 
and includes careful physician and patient consideration of the 
benefits of therapy weighed against potential risks. For conven­
tional therapy, up to 5000 rads are administered in split doses of 
180-rad fractions divided over 6 weeks. Radiation arrests tumor 
growth, and most adenomas 

adiation arrests tumor 
growth, and most adenomas ultimately shrink. As GH levels fall 
gradually during the first year after treatment, most patients are 
still exposed to unacceptably high levels of circulating GH and 
292
IGF1 during the initial years if not receiving concomitant medical 
therapy. Stereotactic radiosurgery effectively focuses on the ade­
noma, thereby sparing normal surrounding structures.51,455–457 Of 
136 patients followed for a medi

es.51,455–457 Of 
136 patients followed for a median 61.5 months, 65% achieved 
normalized IGF1 levels or nadir GH level less than 1 ng/mL after 
an oral glucose load.51 Higher radiation doses to the tumor margin 
as well as maximal doses and lower initial IGF1 levels portend a 
significantly more favorable outcome.51 In a comprehensive meta-
analysis, stereotactic radiosurgery was shown to be modestly more 
effective than conventional radiotherapy, with significantly lower 
IGF1 levels achieved

py, with significantly lower 
IGF1 levels achieved.458 In a multicenter study comprising 371 
patients receiving stereotactic radiosurgery, endocrine remission 
was achieved in 59% of patients, with a mean time to remission of 
38 months. Biochemical relapse was observed in 9%, with a mean 
time to recurrence of 17 months.459
After 10 years, about half of all patients receiving radiotherapy 
have signs of pituitary trophic hormone disruption, and this prev­
alence increases annually

sruption, and this prev­
alence increases annually thereafter, requiring gonadal steroids, 
thyroid hormone, or corticosteroid replacement. Side effects of 
conventional radiation, including hair loss, cranial nerve palsies, 
tumor necrosis with hemorrhage, and rarely loss of vision or pitu­
itary apoplexy, have been documented in up to 2% of patients. 
Lethargy, impaired memory, and personality changes may also 
occur. In a 10-year follow-up study of 35 patients treated with 
Gamma Knife radios

dy of 35 patients treated with 
Gamma Knife radiosurgery, half the patients developed pituitary 
hormone deficiencies (40% hypoadrenalism, 11% hypothyroid­
ism, 13% hypogonadism, and 6% GH deficiency).458 In another 
study, new pituitary hormone deficits were reported in 31% of 
patients within 61.5 months; however, visual deterioration and 
new cranial nerve palsies were rarely observed.51 Because of side 
effects and slow onset of efficacy, radiation therapy should be used 
as an adjuvant for 

iation therapy should be used 
as an adjuvant for patients not controlled by surgery or medical 
management or for those who do not consent to these therapies. 
Prior radiation has been associated with increased acromegaly 
mortality, especially with a 4.42 (95% CI, 2.71–7.22) standard­
ized mortality ratio for cerebrovascular deaths.453 
Dopamine Agonists
D2 receptor agonists have been used as either primary or adju­
vant therapy for acromegaly, especially in patients with mi

apy for acromegaly, especially in patients with mildly 
elevated GH and IGF1 levels.460 In open nonrandomized stud­
ies, cabergoline is reported to suppress GH to less than 2 μg/L 
and normalize IGF1 in up to a third of patients with relatively 
mild IGF1 elevations.460 In patients resistant to SRLs, the 
addition of cabergoline may normalize IGF1 levels in about 
50% of patients.461 Dopamine agonist efficacy appears to be 
  Significant Predictors of Postoperative 
Biochemical Remission in Acr

ors of Postoperative 
Biochemical Remission in Acromegaly 
Patients
Older age
Smaller tumor size
Lower Knosp grade
Lower preoperative GH level
Lower preoperative IGF1 level
GH, Growth hormone; IGF1, insulin-like growth factor 1.
Modified from Sun H, Brzana J, Yedinak CG, et al. Factors associated with biochemical remis­
sion after microscopic transsphenoidal surgery for acromegaly. J Neurol Surg B Skull Base. 
2014;75:47–52.
   
TABLE 9.28
Nadir growth hormone (g/L)
Probability of remission (%)

growth hormone (g/L)
10
0
20
30
40
50
60
70
80
90
100
Nadir GH level (g/L)
Remission (%)
0.1
0.3
0.5
0.7
0.9
1
1.2
1.4
1.6
1.8
2
2.1
2.3
2.5
2.7
2.9
3
4
5
98
97.1
95.7
93.7
90.8
88.9
84.2
78
70.2
61.1
51.1
46
36.2
27.4
20
14.3
12
1.7
0.2
0
2
4
6
• Fig. 9.39  Postoperative GH monitoring. The probability of surgical remission according to the nadir 
growth hormone (GH) level in a 1-week postoperative oral glucose tolerance test (OGTT). When the nadir 
GH level on the

rance test (OGTT). When the nadir 
GH level on the 1-week postoperative OGTT is around 1 μg/L, an 88.9% probability of surgical remission 
is expected. However, if it is greater than 4 μg/L, it is unlikely that the patients will achieve delayed surgical 
remission. (Derived from Kim, Oh MC, Lee EJ, et al. Predicting long-term remission by measuring immedi­
ate postoperative growth hormone levels and oral glucose tolerance test in acromegaly. Neurosurgery. 
2012;70:1106–1113.)
293
Chapter 9  Pitu

osurgery. 
293
independent of PRL concentrations.462 Side effects of cabergo­
line include gastrointestinal symptoms, dizziness, headache, and 
mood disorders. 
SRLs
Of the five somatostatin receptor subtypes, SST2 and SST5 are 
preferentially expressed on somatotroph and thyrotroph cell sur­
faces and mediate suppression of GH and TSH secretion.463,464 
Several SRLs have been safely used as approved or investigational 
drugs for acromeg

 as approved or investigational 
drugs for acromegaly (Fig. 9.40). Octreotide (d-Phe-Cys-Phe-
d-Trp-Lys-Thr-Cys-Thr-OH), an octapeptide somatostatin ana­
logue, binds predominantly to SST2 and less avidly to SST5465 
and inhibits GH secretion with a potency 45 times greater than 
native somatostatin, but its potency for inhibiting insulin release 
is only 1.3-fold that of somatostatin. The in vivo half-life of the 
analogue is prolonged (up to 2 hours) because of its relative 
resistance to enzy

hours) because of its relative 
resistance to enzymatic degradation. These properties are highly 
advantageous for long-term use in acromegaly.466 GH respon­
siveness directly correlates with the abundance of pituitary SST2 
receptors.
Subcutaneous octreotide (50 or 100 μg) suppresses GH secre­
tion for up to 5 hours. In patients harboring microadenomas, 
integrated GH and IGF1 levels are almost invariably normalized, 
but the response in larger tumors is less pronounced.
Long-acting SRL formula

tumors is less pronounced.
Long-acting SRL formulations are convenient and allow for 
enhanced compliance and sustained biochemical control. Serum 
levels of long-acting release (LAR) octreotide (20–30 mg/intra­
muscularly), a sustained-release octreotide depot preparation,467 
peak at 28 days, with integrated GH levels effectively suppressed 
for up to 49 days. Monthly injections for 9 years reduced inte­
grated serum GH levels to less than 2 μg/L in over 75% of 
patients.468
B
A
D
C
Treatment 

/L in over 75% of 
B
A
D
C
Treatment duration (years) 
GH (µg/L)
IGF1 (× 103 U/L)
35
25
15
5
6
5
4
3
2
1
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
Days after injection 
GH <2 µg/L 
GH <2.5 ng/mL 
IGF1 normalized 
Octreotide (ng/L) 
GH (µg/L)
% of patients 
1200
900
42
8
6
4
2
0
600
300
0
100
80
60
40
20
0
63
56
49
42
35
28
21
14
7
1
Injection time (months) 
GH (mU/L) 
30
25
20
15
10
5
0
31
28
25
22
18
15
12
9
6
3
1
0
• Fig. 9.40  (A) Growth hormone (GH) and insulin-like growth factor 1 (IGF1) c

one (GH) and insulin-like growth factor 1 (IGF1) concentrations with long-
term octreotide treatment. Comparison of primary octreotide treatment in 25 previously untreated patients 
and in 80 patients who had previously undergone surgical resection or irradiation. (B) Pharmacodynam­
ics of octreotide long-acting release (LAR) 12-hour mean serum octreotide and GH concentrations in a 
representative patient treated with a single 30-mg injection of octreotide LAR and followed for 60 days. 
After in

octreotide LAR and followed for 60 days. 
After injection, drug levels peak at 28 days, and nadir GH levels are sustained for 4 weeks. (C) Mean GH 
concentration with octreotide (long-acting release) long-term treatment. Serum GH levels in acromegaly 
following monthly LAR octreotide injections in 12 patients for 1 year and 8 patients for 31 months. (D) Clini­
cal impact of octreotide in reducing soft tissue swelling. Acromegaly in a patient suffering from obstructive 
sleep apnea before octreot

ering from obstructive 
sleep apnea before octreotide. Note the macroglossia, tracheotomy for airway obstruction, and intranasal 
feeding tube. After 6 months’ treatment with octreotide, tongue size was reduced by half. Tracheotomy 
and nasal tube have been removed and sleep apnea has resolved. (A, from Newman C, Melmed S, 
George A, et al. Octreotide as primary therapy for acromegaly. J Clin Endocrinol Metab. 1998;83:3034–
3040; B, adapted from Lancranjan I, Bruns C, Grass P, et al. Sandostatin

Lancranjan I, Bruns C, Grass P, et al. Sandostatin LAR: a promising therapeutic 
tool in the management of acromegalic patients. Metabolism. 1996;45:S67–S71; C, from Davies PH, 
Stewart SE, Lancranjan I, et al: Long-term therapy with long-acting octreotide [Sandostatin-LAR] for the 
management of acromegaly. Clin Endocrinol. 1998;48:311–316; D, courtesy Seymour Reichlin, University 
of Arizona, Treson.)
294
A long-acting preparation of lanreotide (autogel

y
A long-acting preparation of lanreotide (autogel) is admin­
istered by monthly deep subcutaneous injection. In a random­
ized, placebo-controlled, multicenter study with a 52-week 
open extension, serum GH levels decreased by more than 50% 
from baseline in 63% of patients receiving lanreotide auto­
gel.469 When administered (60, 90, or 120 mg) every 28 to 42 
days, GH was suppressed to less than 2.5 μg/L in 130 patients 
at 1 year.470
Pasireotide, a multireceptor-targeting SRL, exhibits highe

ide, a multireceptor-targeting SRL, exhibits highest 
affinity for SST5 more than SST2471 and exhibits superior effi­
cacy over octreotide.472 In a randomized 12-month study in 358 
patients, pasireotide LAR (40 mg) or octreotide (20 mg) resulted in 
biochemical control in 31% and 19% of patients, respectively.472 
Of those patients resistant to maximal doses of octreotide or lan­
reotide, 20% subsequently achieved control when placed on 60 
mg pasireotide.473 Reported side effects included thos

asireotide.473 Reported side effects included those expected 
for SRLs as well as hyperglycemia noted in 57% of patients.
Oral octreotide formulated with a transient permeability 
enhancer attenuates basal and GHRH-elicited GH levels in 
healthy volunteers.474 When administered to acromegaly patients, 
GH and IGF1 levels were controlled in 65% after being switched 
from injectable SRLs, and about 90% maintained responsiveness 
for up to 13 months. Although not yet approved, this formulation 
may

. Although not yet approved, this formulation 
may offer an alternative future treatment option.475
Effects of SRLs on Pituitary Adenoma. Tumors rarely grow 
while patients receive depot SRL preparations, and a significant 
decrease in tumor size has been reported. In a comprehensive 
meta-analysis of 64 studies, 59% of patients were reported to 
experience a 50% reduction of tumor mass; this effect generally 
correlates with controlled GH and IGF1 levels476 and may be 
observed within 6 months.

F1 levels476 and may be 
observed within 6 months.477 As the magnitude of shrinkage 
is variable, the use of preoperative SRL to improve subsequent 
surgical outcomes has been debated478–480; however, enthusiasm 
for this approach is dampened by the short prospective follow-
up duration. 
Effects on Clinical Features. Over 70% of patients experience 
improved general well-being, and soft tissue swelling dissipates 
within several days of treatment.481 Headache, a common symp­
tom, usually resolv

t.481 Headache, a common symp­
tom, usually resolves within minutes of octreotide injection,482 
likely reflecting a specific central analgesic effect. Decreased heart 
rate and diminished LV wall thickness, reduced systemic arte­
rial resistance and fluid volume, and restored functional activity 
are reported. Control of IGF1 and GH levels is associated with 
improved LV ejection function.483 Joint function improves and 
crepitus is reduced, ultrasound shows evidence of bone or cartilage 
repai

rasound shows evidence of bone or cartilage 
repair, and after several months sleep apnea resolves.415 Although 
most soft tissue disease comorbidities are ameliorated once bio­
chemical control has been achieved, low quality-of-life measures, 
including anxiety and depression, may persist.484 Hypertension, 
joint space narrowing, and new-onset vertebral fractures do 
not appear to be ameliorated despite controlled GH and IGF1 
levels.424 
Determinants of SRL Responsiveness. In a meta-analysis o

inants of SRL Responsiveness. In a meta-analysis of 
4464 patients treated with an SRL, average GH control rates and 
IGF1 normalization rates were 56% and 55%, respectively (Fig. 
9.41).485 Lanreotide autogel and octreotide LAR exhibit simi­
lar clinical efficacy and side effect profiles, and they were shown 
equivalent in control of acromegaly symptoms and biochemical 
markers.486 In 166 patients with acromegaly, results of oral glu­
cose tolerance tests were concordant for appropriate diagnos

ance tests were concordant for appropriate diagnosis 
and establishing efficacy of surgery and radiotherapy but were 
not helpful in evaluating effectiveness of medical SRL therapy. 
Measurement of serum IGF1 levels was the most rigorous marker 
to assess effectiveness of SRL therapy.487 Results of controlled pro­
spective studies show IGF1 levels normalized in up to 35% of 
patients.469,472,488,489
Understanding mechanisms driving SRL responsiveness and 
resistance, including both individual pa

ness and 
resistance, including both individual patient and tumor character­
istics, has enabled a personalized approach to acromegaly classifica­
tion and management.365 Negative predictors of SRL therapeutic 
responses as well as markers of aggressive disease correlate with 
more adverse long-term outcomes. These determinants include 
patient age, treatment duration, frequency of drug administra­
tion, total daily dose, tumor size, degree of tumor GH granular­
ity, and pretreatment GH and IGF1

mor GH granular­
ity, and pretreatment GH and IGF1 levels. More favorable results 
reported in earlier studies may reflect clinical trial design, sub­
ject heterogeneity, and possibly selection of responsive patients. 
Increasing the monthly dose of octreotide LAR to 40 to 60 mg or 
the monthly lanreotide autogel dose to 180 mg, or, alternatively, 
increasing the dose frequency of lanreotide to 120 mg every 21 
days, may improve efficacy.490,491 Control rates also improve with 
time.492 The most

Control rates also improve with 
time.492 The most important determinant of therapeutic respon­
siveness is tumor SST2 expression.493 Tumors immunopositive 
for SST2 expression are more likely to respond to octreotide and 
lanreotide,494 and those with a higher SST2 to SST5 ratio show 
IGF1–normalized rates (%)
Study number
0
20
40
60
80
0
A
25
50
75
100
GH response rates (%)
0
20
40
60
80
0
B
25
50
75
100
• Fig. 9.41  Meta-analysis of somatostatin receptor ligand (SRL) respon­
sive

of somatostatin receptor ligand (SRL) respon­
siveness in acromegaly. (A) Insulin-like growth factor 1 (IGF1) response 
rates and 95% confidence intervals for 90 analyzed cohorts. (B) Growth 
hormone (GH) response rates and 95% confidence intervals for the 90 
analyzed cohorts. Median response rates for GH and IGF1 noted by the 
vertical lines. Figures are sorted from least to greatest percent response 
rate. (From Carmichael J, Bonert VS, Nuno M, et al. Acromegaly clinical 
trial methodology im

, et al. Acromegaly clinical 
trial methodology impact on reported biochemical efficacy rates of soma­
tostatin receptor ligand treatments: a meta-analysis. J Clin Endocrinol 
Metab. 2014;99:1825–1833.)
295
improved therapeutic outcomes.495 Tumors lacking SST5 immu­
noreactivity are resistant to pasireotide, while those immunoreac­
tive for SST5 had superior biochemical responses.496 Truncated 
SST5 variants may correlate inversely with GH responses to 
SRL

 may correlate inversely with GH responses to 
SRLs.497
Tumors containing large, dense intratumoral GH granules 
diffusely distributed throughout the cytosol are more respon­
sive to SRLs than are those with sparse GH granules498 (Fig. 
9.42). Sparsely granulated adenomas express SST2 less abun­
dantly and more SST5, and are more resistant to SRLs.495,499–
501 Sparse granules are also associated with larger and more 
invasive tumors, especially in younger patients,502 which dis­
play MRI feature

n younger patients,502 which dis­
play MRI features of T2-weighted hyperintensity. Hypoin­
tense adenomas are smaller and less invasive than hyperintense 
and isointense adenomas and exhibit higher IGF1 levels. 
These MRI and histologic features are helpful in predicting 
both biochemical responses and tumor shrinkage in response 
to SRLs.503,504
Molecular tumor markers associated with SRL responsive­
ness include p21 expression, low AIP expression,505–507 high 
β-arrestin,508 and presence of th

on,505–507 high 
β-arrestin,508 and presence of the gsp oncogene mutation.509 
Side Effects. SRLs are generally safe and well tolerated. Gas­
trointestinal side effects predominate, occur early, and include 
transient loose stools, nausea, cramps, mild malabsorption, and 
flatulence, reported in about one-third of patients. Hypogly­
cemia or hyperglycemia is not commonly encountered, and 
insulin requirements in diabetic patients with acromegaly are 
reduced within hours of receiving octreotide,

are 
reduced within hours of receiving octreotide, concomitant 
with GH lowering. Overall, SRLs do not cause major effects 
on glucose homeostasis, and hyperglycemia may also be associ­
ated with uncontrolled underlying disease.510 Pasireotide, how­
ever, is associated with hyperglycemia and new-onset diabetes 
in about 50% of patients.511 The drugs attenuate gallbladder 
contractility, and delayed emptying leads to reversible sludge 
formation evidenced by ultrasonography in up to 25% of 
patie

videnced by ultrasonography in up to 25% of 
patients.512 Cholecystitis is very rarely reported.467 Octreotide 
may interact with cyclosporine, enhancing transplant rejec­
tion risk. SRL dose adjustments should be carefully titrated in 
patients requiring insulin or oral hypoglycemic agents, calcium 
channel blockers, and beta blockers. Asymptomatic sinus brady­
cardia has also been recognized. 
Growth Hormone Receptor Antagonist
GH action through the surface membrane GH receptor is mediated 
by

h the surface membrane GH receptor is mediated 
by ligand-induced receptor signaling513 (Fig. 9.43). The postrecep­
tor GH signal is not elicited if the receptor is bound by pegvisomant, 
a GH-receptor antagonist, which blocks subsequent IGF1 genera­
tion.514 The drug blocks peripheral GH action and does not target 
the pituitary tumor. IGF1 measurement is the appropriate marker 
of patient responsiveness, and measuring GH is therefore not an 
efficacy marker. Earlier studies showed that daily i

ficacy marker. Earlier studies showed that daily injections (up to 
40 mg daily) normalize IGF1 levels in about 90% of patients, and 
they dose-dependently improve fatigue, decrease soft tissue swelling 
as assessed by ring size, and diminish perspiration.515 In a subse­
quent drug surveillance report of 1288 patients, control of IGF1 was 
reported in 63% of patients,516 likely reflecting submaximal dose 
titration as compared to controlled dosing of a clinical trial. Accord­
ingly, in a 91-mont

g of a clinical trial. Accord­
ingly, in a 91-month follow-up (median 18 months), IGF1 levels 
were controlled in 95% of patients treated in two centers.517 Similar 
results were obtained in a 9-year therapeutic follow-up.518 The drug 
may be used as primary monotherapy519 and is particularly useful 
in patients resistant to SRL therapy because it effectively normalizes 
IGF1 levels when added in these patients.520,521 Significant drug 
response determinants include age, body mass index, and bas

determinants include age, body mass index, and baseline 
IGF1 levels.522 There is a concern that loss of negative IGF1 feed­
back control on somatotroph tumor proliferation may theoretically 
A
B
C
D
A
B
D
GH densely granulated
GH sparsely granulated
• Fig. 9.42  Growth hormone (GH) granulation pattern and clinical characteristics of GH-secreting adeno­
mas. (A) Small noninvasive microadenomas usually contain (B) densely packed cytoplasmic GH granules 
(type 1 acromegaly).369 (C) Larger and inva

nules 
(type 1 acromegaly).369 (C) Larger and invasive macroadenomas are usually (D) very sparsely granulated, 
and these features portend a more adverse outcome (type 3 acromegaly). (A, from author’s personal col­
lection; B through D, courtesy Luis V. Syro, MD.)
296
lead to rebound mass expansion. However, rebound tumor enlarge­
ment rarely occurs and may reflect discontinuing SRL, yet tumor 
growth while receiving pegvisomant should be monitored by MRI

e receiving pegvisomant should be monitored by MRI, 
especially if the residual tumor mass abuts visual tracts.523–526 The 
GH receptor antagonist enhances insulin sensitivity and is therefore 
particularly suited for patients with coexisting diabetes.517,527
Side Effects. Because elevated (threefold or higher) hepatic 
transaminases have been reported,528 liver enzymes should be 
–
+
GH
D2 agonist
GHR
antagonist
SRL
Controlled somatic
growth and
metabolic dysfunction
Endogenous
SRIF14
SRIF28
SS

SRIF14
SRIF28
SST1
SST2
Human SST affinity (IC50 nmol/L)
Drug
Dose
ns = affinity >1 µmol/L
SST3
0.2-1.3
0.2-4.1
0.1-2.3
0.1-2.2
0.3-1.6
0.3-6.1
SST4
0.3-1.8
0.3-7.9
SST5
0.2-0.09
0.05-0.4
Clinically
approved
Lanreotide
0.6
0.8
ns
ns
35
98
ns
ns
7
4.2
Pasireotide
1.0
9.3
1.5
>100
0.2
GHR
Site 1
8 a.a. substitutions
Improve binding
Site 2
a.a. substitution
Impairs binding
Binding
site 1
Binding
site 2
P
STAT
P
P
P
P
P
P
P
SHC
No signaling
JAK

g
site 2
P
STAT
P
P
P
P
P
P
P
SHC
JAK2 JAK2
SRL
Glα
γ
β
SSTs
GH
secretion
PL-C
Ion channels (Ca+, K+)
PTP
MAPK
N
C
P13K
MAPK
transcription
Cell proliferation
and differentiation
Increased
IGFI
IGFBP3
ALS
Glucose
Inactive receptors
GH
SRL
GH antagonist
Octreotide LAR
Lanreotide autogel
Pegvisomant
50-400 µg SC every 8 h
10-40 mg IM every 4 wks
30 mg IM every 10-14 days
60-129 mg deep SC ever

s
60-129 mg deep SC every 4-6 wks
10-40 mg SC daily
1-4 mg orally every wk
Pasireotide LAR
40-60 mg IM every 4 wks
IGF1
GHRH
SRIF
GHS
• Fig. 9.43  Action of growth hormone receptor (GHR) antagonist, somatostatin receptor ligands (SRLs), 
and dopamine (D2) agonists. Normally a single molecule of GH binds two GH receptors through sites 1 and 
2, and the GH signal transduction pathway is activated. Pegvisomant increases binding of GHR to site 1 
and blocks

ant increases binding of GHR to site 1 
and blocks binding at site 2 to prevent functional GHR dimerization, initiation of GH action, and induction 
of insulin-like growth factor type 1 (IGF1) synthesis and secretion. The peripheral effects of excess GH are 
antagonized at the cellular level, independent of the presence of somatostatin (SST) or dopamine receptors 
on the pituitary tumor. SRLs inhibit GH secretion and IGF1 synthesis and suppress pituitary tumor growth. 
GH-secreting adenomas expr

ituitary tumor growth. 
GH-secreting adenomas express predominantly SST2 and SST5. Figure depicts affinities for SST recep­
tor subtypes and for D2 receptor. a.a., amino acid; ALS, acid-labile subunit; C, carboxyl terminal; GHRH, 
gonadotrophin hormone–releasing hormone; GHS, gonadotrophin hormone secretagogue; IC50, 50% 
inhibitory concentration; IGFBP3, insulin-like growth factor–binding protein 3; JAK2, Janus kinase 2 tyrosine 
kinase; LAR, long-acting release; MAPK, mitogen-activated protein

ng-acting release; MAPK, mitogen-activated protein kinase; N, amino terminal; ns, not sig­
nificant; P, elemental phosphorus; PI3K, phosphoinositide 3 kinase; PL-C, phospholipase C; PTP, protein 
tyrosine phosphatase; SHC, Src homology–containing protein; SRIF, somatostatin; STAT, signal transducer 
and activator or transcription. (Adapted from Melmed S. Acromegaly. N Engl J Med. 2006;355:2558–2573; 
Heaney AP, Melmed S. Molecular targets in pituitary tumours. Nat Rev Cancer. 2004;4:285–295.)
29

itary tumours. Nat Rev Cancer. 2004;4:285–295.)
297
measured every 6 months. Local injection site inflammation and 
lipodystrophy have been reported.529 Levels of GH rise as IGF1 
negative feedback on the pituitary is lost. 
SRLs and GH Receptor Antagonist Combination
Combination treatment for acromegaly is most effective for 
patients in whom there has been tumor shrinkage on somatostatin 
analogue with reduction, albeit inadequate, in GH or IGF1 levels 
(

uction, albeit inadequate, in GH or IGF1 levels 
(Fig. 9.44). In 11 such uncontrolled patients, dual blockade of the 
GH axis with pegvisomant and SRLs has been shown to exhibit 
greater efficacy than either drug alone.530 Monthly doses of long-
acting somatostatin analogue have been successfully combined with 
weekly doses of pegvisomant.531 A study in 63 patients showed that 
4 years of combination treatment was safe, but 23 patients devel­
oped elevated liver enzymes, especially if they were 

d elevated liver enzymes, especially if they were diabetic.532 
Management Approach
Employing an integrated classification base on clinical, bio­
chemical, pathologic, and imaging features, acromegaly may be 
classified to enable evidence-based personalized management 
(Table 9.29).365 Type I acromegaly accounts for over 50% of 
patients. These are usually older patients with smaller tumors 
containing dense GH granularity and abundant SST2 and p21 
expression. IGF1 levels are lower and therapeu

21 
expression. IGF1 levels are lower and therapeutic outcomes are 
most favorable. In contrast, type 3 acromegaly is encountered 
in younger patients with smaller, sparsely granulated tumors 
and low p21 and SST2 expression. IGF1 levels are higher, and 
both surgical and medical outcomes are less favorable. Type 2 
patients exhibit intermediate features.365 Given the challenge 
of choosing the most appropriate therapy, use of classification 
systems, as well as clinical outcome instruments, may

tems, as well as clinical outcome instruments, may enable a 
rigorous personalized approach for evidence-based care.365,533 
Using these parameters prospectively will enable a personal­
ized approach to determining optimal management for these 
patients.
Ideally, the goal of therapy should be tight control of GH secre­
tion and normalization of IGF1 levels, as adverse comorbidities 
and mortality rates correlate with GH levels. Discordant GH 
and IGF1 results after treatment should necessitate r

 IGF1 results after treatment should necessitate repeating the 
respective assay(s) in a reputable laboratory using rigorous assay 
standardizations and appropriate sensitivity cutoffs.439 Normal or 
low IGF1 levels may be encountered with systemic disease or mal­
nutrition and GH elevations reflecting persistent disease activity. 
Each treatment modality has respective advantages and disadvan­
tages that should be assessed to individualize patient care (Fig. 
9.45 and Tables 9.30 and 9.31).534 

nt care (Fig. 
9.45 and Tables 9.30 and 9.31).534 Counterintuitively, features of 
GH deficiency may be encountered after effective therapy, and 
judicious GH replacement may be required to reverse adverse 
quality of life and lipid disorders.535
Selective surgical excision of a well-defined pituitary mass is 
recommended for most patients with microadenomas.536 As 
remission rates are lower for macroadenomas and locally invasive 
tumors, attempted medical debulking of the sellar mass prior to 

ed medical debulking of the sellar mass prior to 
surgery would be intuitively desirable, and limited controlled pro­
spective studies appear to confirm the validity of this approach 
to improve outcomes, especially for patients with surgically inac­
cessible tumor tissue and cavernous sinus invasion.537 Surgical 
debulking may also improve subsequent responsiveness to SRL 
therapy.538,539 For uncontrolled patients after surgery, a long-
acting SRL should be administered.443 Patients with mild G

RL should be administered.443 Patients with mild GH 
100
Percentage of
IGF1 normalisation
Median pegvisomant dose
80
60
40
20
0
100
90
80
70
60
50
40
1
2
3
4
Years of treatment
5
6
7
8
9
N =
112
91
78
70
56
33
28
23
17
• Fig. 9.44  Percentages of patients with insulin-like growth factor type 1 
(IGF1) < 1.2 × ULN (left axis) and median pegvisomant doses (dotted line, 
right axis) each year during 9 years of combined somatostatin receptor 
ligand (SRL) and pegvisomant treatment. Number of patient

(SRL) and pegvisomant treatment. Number of patients each treat­
ment year is depicted at the bottom of the bar. (Redrawn from Neggers SJ, 
Franck SE, de Rooij FW, et al. Long-term efficacy and safety of pegviso­
mant in combination with long-acting somatostatin analogs in acromegaly. 
J Clin Endocrinol Metab. 2014;99:3644–3652.)
  Acromegaly Subclassification for Precise, 
Personalized Approach to Therapy
ACROMEGALY TYPES
Type 1
Type 2
Type 3
Frequency
50%
19%
31%
Tumor
Tumor shape 
and CSI
Con

equency
50%
19%
31%
Tumor
and CSI
Concave
Flat
Peanut
Size
Micro or 
macro
Macro
Macro
Invasiveness 
by MRI
Interme­
diate
Rarely
Always
GH 
granulation
Dense
Both
Sparse
Immunore­
activity
α-Subunit
Positive
Positive or 
negative
Negative
Ki-67 index 
<3%
90%
33%
42%
SST2
58%
30%
22%
p21
38%
15%
4%
Biochem­
istry
IGF1 at 
diagnosis
Lower
Higher
Manage­
ment and 
outcomes
No. of medi­
cations
≤2
≤2
≥2
No. of sur­
geries
1
1 or 2
≥2
Disease 
control
Frequent

1
1 or 2
≥2
Disease 
control
Frequent
Rare
CSI, Cavernous sinus invasion; GH, growth hormone; IGF1, insulin-like growth factor type 1; 
MRI, magnetic resonance imaging.
Modified from Cuevas-Ramos D, Carmichael JD, Cooper O, et al. A structural and functional 
acromegaly classification. J Clin Endocrinol Metab. 2015;100:122–131.
   
TABLE 9.29
298
elevations can also be treated with cabergoline; although efficacy 
of this drug is low, it is re

; although efficacy 
of this drug is low, it is relatively inexpensive and free of major 
side effects.
Primary therapy with SRLs may be offered to those patients 
in whom complete tumor removal is not likely or to those who 
refuse surgery or in whom the risks of surgery or anesthesia are 
unacceptable (Fig. 9.46). As invasive macroadenomas invariably 
hypersecrete GH postoperatively in patients whose pituitary 
lesion does not compress vital structures, primary medical man­
agement may be an a

ructures, primary medical man­
agement may be an appropriate therapeutic option.489,540 Pegvi­
somant, either alone or in combination with an SRL, should be 
offered to resistant patients. Radiation should be administered to 
patients who are resistant to or cannot tolerate medications, who 
prefer not to receive long-term injections, or those who cannot 
afford medication. After irradiation, medications are required for 
several years until GH levels are effectively controlled. Tumors 
recurrin

evels are effectively controlled. Tumors 
recurring despite medical therapy or irradiation may rarely require 
reoperation.
Acromegaly patients also require counseling for anxiety 
engendered by disfigurements and interpretation of laboratory 
test results. Patients should be followed until biochemical con­
trol is achieved; thereafter, hormone evaluation is performed 
semiannually. In those who are biochemically in remission and in 
whom no residual tumor tissue is present, MRI should be repeat

dual tumor tissue is present, MRI should be repeated 
every 1 to 2 years. Follow-up evaluation includes treating new soft 
tissue overgrowth, nerve entrapments, and jaw overbites; rheuma­
tologic, dental, and cardiac evaluations; and metabolic assessment. 
Maximal and sustained long-term GH and IGF1 control should 
ameliorate most deleterious comorbidities by judicious use of 
available treatment modalities. 
ACTH-Secreting Tumors (Cushing Disease)
The evaluation and management of patients with 

e)
The evaluation and management of patients with ACTH-secreting 
pituitary tumor (Cushing disease) is fully described in Chapter 15. 
Briefly, the diagnosis of an ACTH-secreting pituitary tumor is sug­
gested by features of hypercortisolism, elevated 24-hour urinary free 
cortisol levels, and elevated late night salivary cortisol values, together 
with nonsuppressed serum ACTH levels. Failure to suppress morn­
ing cortisol levels to less than 1.8 μg/dL after 1 mg dexamethasone 
administered at 

8 μg/dL after 1 mg dexamethasone 
administered at 11 pm supports the diagnosis.541 In healthy subjects, 
glucocorticoid feedback suppresses CRH and ACTH, attenuating 
cortisol secretion. Surgical resection of an ACTH-secreting ade­
noma is the treatment of choice. As these tumors are usually small, 
sometimes less than 2 mm diameter, they may not be visible by sen­
sitive MRI and venous sampling for ACTH. Therefore these tumors 
pose a significant challenge even for the experienced surgeon. Bila

t challenge even for the experienced surgeon. Bilat­
eral petrosal venous sampling for ACTH levels and cavernous sinus 
venography should ideally be performed prior to surgery. However, 
if sellar venous sinus drainage is predominantly unilateral, left-right 
ACTH gradients may not reliably lateralize the lesion. If an ACTH 
gradient is indeed detected with normal venous drainage patterns, 
hemihypophysectomy may in fact be curative in most patients with 
clearly defined biochemical features of 

nts with 
clearly defined biochemical features of ACTH-dependent Cushing 
disease. Meticulous surgical exploration of both anterior and poste­
rior lobes is required for these tiny tumors, which are often off-white 
and speckled by petechiae, and may be inadvertently suctioned. 
Even carefully performed preoperative lateralization is not infallible, 
and the “normal” side should also be carefully explored.
Assessment of Surgical Outcome
Transsphenoidal adenoma resection is the preferred treatmen

noidal adenoma resection is the preferred treatment for 
these adenomas.542 After selective adenomectomy of a clearly iden­
tifiable adenoma, remission was achieved in 75% of 295 patients. 
However, partial hypophysectomy performed in patients in whom 
an adenoma cannot be identified may result in biochemical remis­
sion in some patients.543 On the third postoperative day, 1 mg dexa­
methasone can be given at 10 pm, and cortisol levels are measured 
the following morning prior to initiating hydr

ed 
the following morning prior to initiating hydrocortisone therapy. 
If the immediate postoperative cortisol level is less than 3 μg/dL, 
a 95% 5-year remission rate can be expected. In 21 of 27 patients 
tested prior to glucocorticoid administration, postoperative cortisol 
levels lower than those obtained from preoperative midnight sam­
pling and values less than 10 μg/dL were predictive of remission.544 
Pasireotide, an SRL, has been approved for treating ACTH 
hypersecret

, has been approved for treating ACTH 
hypersecretion associated with Cushing disease.545 About 20% 
of patients exhibit normalized urinary free cortisol levels, associ­
ated with improved clinical features. Up to 50% of patients may 
develop hyperglycemia, and blood sugars should be aggressively 
monitored. Mifepristone, a glucocorticoid receptor antagonist, is 
aOnly in partial responders (>50% decrease in GH/IGF1); 
bIf IGF1 remains modestly elevated during SRL administration.
Inadequate cont

levated during SRL administration.
Inadequate control with first-generation SRL
Monitor IGF1
SRS or surgical
(re)intervention
First-generation 
SRL + PegV
Increase SRL dose/
increase dose frequencya
Add cabergoline to SRLb
Not controlled
Impaired glucose
Well controlled
Tumor concern and 
impaired glucose 
Tumor concern
• Fig. 9.45  A proposed algorithm for the treatment of acromegaly in 
patients inadequa

 the treatment of acromegaly in 
patients inadequately controlled with first-generation SRLs lanreotide 
autogel and octreotide LAR. Tumor concern is defined as clinically rel­
evant residual tumor on imaging and/or clinical concern for tumor growth. 
GH, growth hormone; IGF1, insulin-like growth factor 1; PegV, pegviso­
mant; SRL, somatostatin receptor ligand; SRS, stereotactic radiosurgery. 
(Redrawn from Melmed S, Bronstein MD, Chanson P, et al. A consen­
sus statement on acromegaly therapeut

l. A consen­
sus statement on acromegaly therapeutic outcomes. Nat Rev Endocrinol. 
2018;14:552–561.)
  Management of Acromegaly
Goals
Control GH and IGF1 secretion
Control tumor growth
Relieve central compressive effects, if present
Preserve or restore pituitary trophic hormone function
Treat comorbidities (hypertension, cardiac failure, hyperglycemia, sleep apnea, arthritis)
Normalize mortality rates
Prevent biochemical recurrence
TREATMENTS
Surgery
Radiotherapy
SRL
GHR Antagon

haracteristic
Surgery
SRL
GHR Antagonist
Dopamine Agonist
Advantages
mode
Transsphenoidal resection
Noninvasive
Monthly injection
Daily injection
Oral
Biochemical control
GH <2.5 μg/L
Macroadenomas, <50%
Microadenomas, >80%
∼35% in 10 years
∼55–65%
Increases
<15%
IGF1 normalized
<30%
∼55–65%
>65%
<15%
Onset
Rapid
Slow (years)
Rapid
Rapid
Slow (weeks)
Patient compliance
One-time consent
Good
Must be sustained
Good
Tumor mass
Debulked or resected
Ablated
Growth const

mor mass
Ablated
Growth constrained 
or shrinks ∼50%
Unknown
Unchanged
Disadvantages
Cost
One time
One time
Ongoing
Ongoing
Ongoing
Hypopituitarism
∼10%
>50%
None
Very low IGF1 if 
overtreated
None
Other
Tumor persistence or 
recurrence, 6%
Diabetes insipidus, 3%
Local complications, 5%
Local nerve damage
Second brain tumor
Visual and CNS disorders, ∼2%
Cerebrovascular risk
Gallstones, 20%
Nausea, diarrhea
Elevated liver 
enzymes (rare)
Nausea, ∼30%
Sinusitis
High dose requi

ymes (rare)
Sinusitis
High dose required
OUTCOMES
Feature
Treatment
Safe Biochemical Activity
Nadir GH <0.4 μg/L
Assess GH/IGF1 axis
Evaluate adrenal, thyroid, and gonadal axes
Periodic but less frequent MRI
None or no change in current treatment
Age-matched normal IGF1
Asymptomatic
No comorbidities
Unsafe Biochemical Activity
Nadir GH >0.4 μg/L
Evaluate pituitary function
Periodic MRI
Weigh treatment benefit vs. risks
Consider new treatment if being t

enefit vs. risks
Consider new treatment if being treated
Elevated IGF1
Discordant GH and IGF1
Unsafe Biochemical and Clinical Activity
Nadir GH >1 μg/L
Assess cardiovascular, metabolic, and tumoral comorbidity
Actively treat or change treatment
Clinically active tumor growing
CNS, Central nervous system; GH, growth hormone; GHR, growth hormone receptor; IGF1, insulin-like growth factor type 1

 receptor; IGF1, insulin-like growth factor type 1; MRI, magnetic resonance imaging; SRL, somatostatin receptor ligand.
Modified from Melmed S. Medical progress: acromegaly. N Engl J Med. 2006;355:2558–2573.
   
TABLE 9.30
300
  Medical Therapy of Acromegaly
Therapy
Receptor Target
Route of 
Dose
Frequency
Efficacy (GH/IGF1 
Normalization)
D2 receptor
Oral
1–4 mg
Biweekly up to daily
Nausea, dizziness, ortho­
stati

weekly up to daily
static hypotension
30–40%
SST2, SST5
SC
50–400 μg/
day
One to three times 
daily
Nausea, vomiting, diarrhea, 
constipation, abdominal 
pain, cholelithiasis/biliary 
sludge, bloating, brady­
cardia, fatigue, headache, 
alopecia, dysglycemia
50–60%
SST2, SST5
IM
20–40 mg
Monthly
Lanreotide 
Autogel
SST2, SST5
Deep SC
6–120 mg
Every 4–6 weeks
SST1, SST2, 
SST3, SST5
IM
40–60 mg
Monthly
Same as above, with more 
h

T5
IM
40–60 mg
Monthly
hyperglycemia
Up to 80%
Oral octreotidea
SST2, SST5
Oral
40–80 mg
Twice daily
dyspepsia, cholelithiasis, 
headache, dizziness, 
dysglycemia
65%
SC
10–40 mg
Daily to once 
weekly (less fre­
quent when used 
in combination)
Transaminase elevation, 
lipodystrophy, arthralgias
60–90%
aInvestigational
D2, Dopamine type 2; GH, growth hormone; IGF1, insulin-like growth factor type 1; IM, intramuscular;

ulin-like growth factor type 1; IM, intramuscular; LAR, long-acting release; SC, subcutaneous; SSTR, somatostatin receptor.
Modified from Langlois F, McCartney S, Fleseriu M. Recent progress in the medical therapy of pituitary tumors. Endocrinol Metab (Seoul). 2017;32:162–170; Melmed S. New therapeutic agents for 
acromegaly. Nat Rev Endocrinol. 2016;12:90–98.
   
TABLE 9.31
approved to ameliorate hyperglycemia associated with Cushing 
disease.546 Side effects include adrenal dysfunction due to 

6 Side effects include adrenal dysfunction due to cortisol 
withdrawal, hypokalemia, and excessive vaginal bleeding. 
Thyrotrophic Hormone-Secreting Tumors
TSH-producing pituitary tumors are rare. During 1979 to 1992, 
Mindermann and Wilson analyzed tumor type by immunohisto­
chemistry and found that the overall prevalence of TSH-secreting 
tumors was 19/2225 (0.85%). Between 1989 and 1991, the same 
group found a prevalence of 2.8% among pituitary adenomas.547 
More recent studies have confirme

ry adenomas.547 
More recent studies have confirmed a progressive increased preva­
lence, accounting for 4% of pituitary tumor resections.548 It is not 
clear whether newer, more sensitive TSH assays account for this 
or whether the tumor type frequency is increasing. TSH-secreting 
tumors can also cosecrete other hormones, including GH, PRL, and 
rarely ACTH, and can cause elevated serum IGF1 or PRL levels.549
The hallmark of these rare tumors is an elevation of thyroid 
hormone levels in the f

s an elevation of thyroid 
hormone levels in the face of inappropriately suppressed TSH lev­
els. These biochemical features are reflective of autonomous tumor 
TSH production that is not suppressed by negative thyroid hor­
mone feedback.
Pathology
These tumors are invasive but for the most part benign, and dis­
tant metastases are extremely rare. The expression pattern is often 
multihormonal, with immunoreactivity to TSHβ, α-subunit, GH, 
PRL, and sometimes ACTH.550 Twenty-four-hour sampling i

and sometimes ACTH.550 Twenty-four-hour sampling indi­
cates that pulse frequency of TSH is increased, yet diurnal rhythm 
was preserved at a higher mean hormone level.551 TSH-secreting 
tumors exhibit positive immunostaining for α-subunit and TSHβ 
in up to 75% of cells and for the pituitary-specific transcription 
factor Pit1. These tumors express SST2 mRNA and, in some 
cases, SST3 and SST5 mRNA.552 
Patients may present with thyroid abnormalities, including goiter, 
nodules, and

oid abnormalities, including goiter, 
nodules, and features of hyperthyroidism. As most TSH-secreting 
tumors are already macroadenomas at presentation,553 they may 
present with symptoms due to tumor growth (including visual 
field abnormalities, cranial nerve palsies, or headache). Signs and 
symptoms of hyperthyroidism, including palpitations, arrhyth­
mias, weight loss, tremor and nervousness, or goiter, are common. 
Very rarely, periodic paralysis and postoperative thyroid storm have 
been 

alysis and postoperative thyroid storm have 
been reported.554,555 Serum TSH is often but not invariably ele­
vated. In those cases, the combination of abnormally high thyroid 
hormone levels and a TSH value within the normal range points 
to a TSH-producing pituitary tumor. A relatively long period of 
hyperthyroidism, initially thought to be Graves disease and treated 
accordingly, often predates the realization that the hyperthyroidism 
is indeed a result of a TSH-secreting pituitary tumor. A

eed a result of a TSH-secreting pituitary tumor. Alternatively, 
thyroid hormone insensitivity can also present with similar labora­
tory profiles.556 TSH-secreting tumors are usually large, and 70% 
to 90% are macroadenomas, with most locally invasive.557 From 
analysis of 10 reports on a total of 153 patients, it appears that TSH 
is frankly elevated in 58% of patients, with the remainder having 
normal albeit inappropriately elevated TSH levels. Patients previ­
ously treated with radioactive 

s. Patients previ­
ously treated with radioactive iodine for presumed Graves disease 
present with significantly higher TSH levels than patients not previ­
ously radioablated (mean: 56 mU/L vs. 9 mU/L, respectively). An 
301
ectopic TSH-producing tumor has also been reported.558 Serum T4 
is high in most patients, as is the glycoprotein hormone α-subunit. 
Approximately two-thirds of patients with TSH-producing pitu­
itary tumors have goiters with elevated 

ing pitu­
itary tumors have goiters with elevated radioactive iodine uptake. 
Rarely, patients with TSHomas have been reported with concurrent 
differentiated thyroid cancer.559 About 30% of TSHomas cosecrete 
GH or PRL.560 Consequently, features of acromegaly or hyperpro­
lactinemia may also be present. 
Serum T4, T3, TSH (by high sensitivity assay), and α-subunit 
should be measured. The combination of high T4, T3, and 
α-subunit; high or inappropriately normal TSH; and a pituitary 

gh or inappropriately normal TSH; and a pituitary 
tumor strongly confirms the diagnosis of a TSH-producing pitu­
itary adenoma. TRH stimulation can differentiate between TSH 
overproduction by a TSH-secreting tumor and thyroid hormone 
insensitivity. In TSH-secreting tumors, the TSH response elicited 
by TRH is blunted. In contrast, TSH usually rises in response 
to TRH in patients with thyroid hormone insensitivity and in 
normal subjects. Concomitant measurement of α-subunit at each 
point du

omitant measurement of α-subunit at each 
point during the TRH test is helpful because the molar ratio of 
α-subunit to TRH is high (>1) in almost 85% of patients with 
TSH-secreting tumors. A T3 suppression test is helpful in that 
complete inhibition of TSH does not occur in patients with TSH-
secreting tumors. This test can also differentiate subclinical hypo­
thyroidism in patients previously treated with radioactive iodine 
for hyperthyroidism but found to have an incidental pituitary 
tumo

sm but found to have an incidental pituitary 
tumor. TSH elevation may also result from inadequate thyroid 
hormone replacement. A pituitary MRI should be performed, and 
IGF1 and PRL levels determined, to exclude acromegaly or hyper­
prolactinemia. Expression of other pituitary hormones in immu­
nostained histologic sections does not necessarily imply elevated 
serum levels.
Importantly, the degree of hyperthyroidism should be assessed 
to determine whether control of these signs and symptoms s

mine whether control of these signs and symptoms should 
be undertaken prior to further evaluation or treatment of the 
pituitary tumor. Hyperthyroidism may be clinically severe and 
present in most patients for years before the diagnosis is made. 
Surgery
The European Thyroid Association guidelines suggest surgery 
as first-line therapy. Cures are achieved for most patients with 
microadenomas, but remission is achieved in about 60% of 
patients with macroadenomas.560 After a mean fo

f 
patients with macroadenomas.560 After a mean follow-up of 
64.4 months, 75% of 70 patients achieved biochemical con­
trol and 58% normalized both pituitary imaging and thyroid 
function, but complete surgical cures are achieved in fewer 
than 40% of patients557 (Table 9.32 and Fig. 9.47). However, 
the rarity of this tumor type has precluded large controlled 
studies. Most patients have cavernous or sphenoid sinus inva­
sion, and tumors are often fibrous and unusually hard. About 
one-third o

ten fibrous and unusually hard. About 
one-third of patients require adjuvant radiotherapy to achieve 
biochemical normalization. Nevertheless, 9% developed pitu­
itary hormone deficiencies, and 3% had recurrence of tumor or 
hyperthyroidism within the first 2 years of radiotherapy. After 
18 to 96 months of follow-up, 32% of patients had developed 
new-onset pituitary deficits.557 
There are no large series reporting treatment of TSH-secreting 
tumors with radiotherapy alone. 

of TSH-secreting 
tumors with radiotherapy alone. Radiation has mostly been used 
as adjunctive therapy to surgery, especially when the latter was not 
curative. 
SRLs
Octreotide, used as either primary or adjunctive treatment, 
normalizes T4 and T3 and reduces TSH levels by half.561 Over­
all, tumors shrink in about a third of patients. In 18 patients 
with TSH-secreting adenomas, lanreotide (30 mg every 10 or 
14 days) significantly decreased TSH levels from 2.72 to 1.89 
mU/L, decreased T4 le

SH levels from 2.72 to 1.89 
mU/L, decreased T4 levels, but did not shrink tumors. Octreo­
tide LAR (up to 30 mg monthly) responsiveness appeared simi­
lar to that observed for the subcutaneous preparation in seven 
patients.562 In another report, octreotide suppressed TSH in 
90% of patients with TSH-secreting tumors and reduced tumor 
size in 50% of these patients.563 SRLs are effective in more than 
90% of patients with TSHomas,560 but tachyphylaxis may occa­
sionally occur. 
Preoperative Man

ylaxis may occa­
Preoperative Management
Unless vision is threatened, patients should be evaluated to deter­
mine whether clinical signs of hyperthyroidism warrant immedi­
ate treatment. Propranolol, radioactive iodine thyroid ablation, 
thyroidectomy, antithyroid medications (including methimazole), 
and SRLs are used. Both radioactive iodine and antithyroid medi­
cations are targeted to the thyroid gland rather than the pituitary 
seat of the disorder. This approach also inhib

ry 
seat of the disorder. This approach also inhibits the negative feed­
back of T3 on TSH and leads to increased tumor TSH produc­
tion. Surgery and SRLs simultaneously treat hyperthyroidism and 
tumor TSH hypersecretion. SRLs lower TSH, α-subunit, and T4 
and are recommended as first-line drugs in the initial control of 
hyperthyroidism due to TSH-secreting tumors because their onset 
of action is faster than other therapeutic approaches and tumor 
shrinkage occurs in up to 40% of patients. Wh

mor 
shrinkage occurs in up to 40% of patients. When invasive tumor 
tissue persists, patients continue to have abnormal TSH responses 
to TRH and require SRL therapy. 
GH (mU/L)
Before Rx
(24)
(71)
(23)
6 months
12 months
24 months
60
20
100
Irradiated
Operated
Primary Rx
• Fig. 9.46  Primary medical therapy in acromegaly patients. Con­
trol with medical therapy compared to surgery or radiation. Plasma 
growth hormone (GH) levels (mean ± SEM) in the group of patients 
(n = 118) before and durin

 the group of patients 
(n = 118) before and during long-term treatment with lanreotide. Rx, 
treatment; SEM, standard error of the mean. (Data from Baldelli R, 
Colao A, Razzore P, et al. Two-year follow-up of acromegalic patients 
treated with slow release lanreotide [30 mg]. J Clin Endocrinol Metab. 
2000;85[11]:4099–4103.)
302
  TSH-Secreting Adenomas: Results of Surgical Treatment
Study
Country
Study 
Period
Prevalence Among 
All Resected Pituitary 

Period
Adenomas (%)
N
Macroadenoma 
(%)
Postoperative 
Euthyroidism (%)
Median 
Follow-Up 
(Months)
Kirkman (2014)
United Kingdom
2002–2012
3.5
32
88
69
80 (10–204)
Malchiodi (2014)
Italy
1982–2012
NA
68
NA
58
NA
Yamada (2014)
Japan
1991–2013
90
82
84
33.6
Azzalin (2015)
United States
1993–2013
1.2
20
95
66a
10.4 (1.2–150)
aIn patients with active tumors and central hyperthyroidism.
Modified from Azzalin A, Appin CL, Schniederjan MJ, et al. Comprehensi

n A, Appin CL, Schniederjan MJ, et al. Comprehensive evaluation of thyrotropinomas: single-center 20-year experience. Pituitary. 2016;19:183–193.
   
TABLE 9.32
Assess thyroid function
T4, T3, TSH, TRH test
and clinical evaluation
Euthyroid
Surgery
Tumor removed
Biochemically cured
Incomplete tumor removal
No hyperthyroidism
and hyperthyroidism
If tumor enlarges or
hyperthyroidism persists
Follow-up with MRIs and
thyroid function studies rou

llow-up with MRIs and
thyroid function studies routinely
Patients who elect to
remain on medication
Add PTU if not controlled
on octreotide
Treat with octreotide
Hyperthyroid
or radiate, or both
Radiation
Watchful follow-up
Treat with
octreotide
• Fig. 9.47  Management of thyrotrophic hormone (TSH)–secreting pituitary tumors. MRI, magnetic reso­
nance imaging; PTU, propylthiouracil; T3, triiodothyronine; T4, thyroxine; TRH, thyrotrophin-releasing hor­
mon

4, thyroxine; TRH, thyrotrophin-releasing hor­
mone.
Silent TSH-Secreting Tumors
A subset of TSH-secreting tumors immunostain positively for but 
do not hypersecrete TSH or cause thyrotoxicosis. Of 29 tumors 
that immunostained positively for TSH, 9 were not associated 
with hyperthyroidism.564
302.e1
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sporadic 

lpha 2 proteins on clinical features of 
sporadic GH-secreting pituitary adenomas. Eur J Endocrinol. 
2017;176:243–252.
	506.	 Kasuki L, Vieira Neto L, Wildemberg LE, et al. AIP expression in 
sporadic somatotropinomas is a predictor of the response to octreo­
tide LAR therapy independent of SSTR2 expression. Endocr Relat 
Cancer. 2012;19:L25–29.
	507.	 Chahal HS, Trivellin G, Leontiou CA, et al. Somatostatin analogs 
modulate AIP in somatotroph adenomas: the role of the ZAC1 
pathway. J Clin 

h adenomas: the role of the ZAC1 
pathway. J Clin Endocrinol Metab. 2012;97:E1411–E1420.
	508.	 Gatto F, Feelders R, van der Pas R, et al. β-Arrestin 1 and 2 and G 
protein-coupled receptor kinase 2 expression in pituitary adenomas: 
role in the regulation of response to somatostatin analogue treat­
ment in patients with acromegaly. Endocrinology. 2013;154:4715–
4725.
	509.	 Efstathiadou ZA, Bargiota A, Chrisoulidou A, et al. Impact of gsp 
mutations in somatotroph pituitary adenomas on growth

ations in somatotroph pituitary adenomas on growth hor­
mone response to somatostatin analogs: a meta-analysis. Pituitary. 
2015;18:861–867.
	510.	 Mazziotti G, Floriani I, Bonadonna S, Torri V, Chanson P, Gius­
tina A. Effects of somatostatin analogs on glucose homeostasis: 
a metaanalysis of acromegaly studies. J Clin Endocrinol Metab. 
2009;94:1500–1508.
	511.	 Schmid HA, Brue T, Colao A, et al. Effect of pasireotide on glu­
cose- and growth hormone-related biomarkers in patients with 
inad

ith acromegaly: comprehensive review 
of 1288 subjects in ACROSTUDY. J Clin Endocrinol Metab. 
2012;97:1589–1597.
	517.	 Higham CE, Rowles S, Russell-Jones D, Umpleby AM, Trainer PJ. 
Pegvisomant improves insulin sensitivity and reduces overnight free 
fatty acid concentrations in patients with acromegaly. J Clin Endo­
crinol Metab. 2009;94:2459–2463.
	518.	 Ramos-Levi AM, Bernabeu I, Alvarez-Escola C, et al. Long-term 
treatment with pegvisomant for acromegaly: a 10-year experience. 
Clin End

  al. Glucose homeo­
stasis and safety in patients with acromegaly converted from 
long-acting octreotide to pegvisomant. J Clin Endocrinol Metab. 
2005;90:5684–5691.
	528.	 Biering H, Saller B, Bauditz J, et al. Elevated transaminases during 
medical treatment of acromegaly: a review of the German pegvi­
somant surveillance experience and a report of a patient with his­
tologically proven chronic mild active hepatitis. Eur J Endocrinol. 
2006;154:213–220.
	529.	 Bonert VS, Kennedy L, Petersen

al. Combined therapy 
with somatostatin analogues and weekly pegvisomant in active 
acromegaly. Lancet. 2005;365:1644–1646.
	532.	 Neggers SJ, de Herder WW, Janssen JA, Feelders RA, van der Lely 
AJ. Combined treatment for acromegaly with long-acting soma­
tostatin analogs and pegvisomant: long-term safety for up to 4.5 
years (median 2.2 years) of follow-up in 86 patients. Eur J Endocri­
nol. 2009;160:529–533.
302.e14
	533.	 Giustina A, Bevan JS, Bronstein MD, et al. SAGIT(R): clin

a A, Bevan JS, Bronstein MD, et al. SAGIT(R): clinician-
reported outcome instrument for managing acromegaly in clinical 
practice—development and results from a pilot study. Pituitary. 
2016;19:39–49.
	534.	 Marko NF, LaSota E, Hamrahian AH, Weil RJ. Comparative effec­
tiveness review of treatment options for pituitary microadenomas 
in acromegaly. J Neurosurg. 2012;117:522–538.
	535.	 Tritos NA, Johannsson G, Korbonits M, et al. Effects of long-term 
growth hormone replacement in adults with

ng-term 
growth hormone replacement in adults with growth hormone defi­
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crinol Metab. 2014;99:2018–2029.
	536.	 Katznelson L, Laws Jr ER, Melmed S, et al. Acromegaly: an Endo­
2014;99:3933–3951.
	537.	 Pita-Gutierrez F, Pertega-Diaz S, Pita-Fernandez S, et al. Place of 
preoperative treatment of acromegaly with somatostatin analog on 
surgical outcome: a syste

 somatostatin analog on 
surgical outcome: a systematic review and meta-analysis. PLoS One. 
2013;8:e61523.
	538.	 Jallad RS, Musolino NR, Kodaira S, Cescato VA, Bronstein MD. 
Does partial surgical tumour removal influence the response to 
octreotide-LAR in acromegalic patients previously resistant to the 
somatostatin analogue? Clin Endocrinol (Oxf). 2007;67:310–315.
	539.	 Fahlbusch R, Kleinberg D, Biller B, et al. Surgical debulking of 
pituitary adenomas improves responsiveness to octreot

iller BM, Grossman AB, Stewart PM, et al. Treatment of adre­
nocorticotropin-dependent Cushing’s syndrome: a consensus state­
ment. J Clin Endocrinol Metab. 2008;93:2454–2462.
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cal treatment of craniopharyngiomas: experience with 168 patients. 
J Neurosurg. 1999;90:237–250.
	544.	 Simmons NE, Alden TD, Thorner MO, Laws Jr ER. Serum corti­
sol response to transsphenoidal surgery for Cushing disease. J Neu­
rosurg. 2001;95:1–8.

52–4057.
	552.	 Yoshihara A, Isozaki O, Hizuka N, et al. Expression of type 5 soma­
tostatin receptor in TSH-secreting pituitary adenomas: a possible 
marker for predicting long-term response to octreotide therapy. 
Endocr J. 2007;54:133–138.
	553.	 Kirkman MA, Jaunmuktane Z, Brandner S, Khan AA, Powell 
M, Baldeweg SE. Active and silent thyroid-stimulating hormone-
expressing pituitary adenomas: presenting symptoms, treatment, 
outcomes, and recurrence. World Neurosurg. 2014;82:1224–1231.
	55

apradang C, Erdogan MF, et al. Management of 
differentiated thyroid cancer in the presence of resistance to thyroid 
hormone and TSH-secreting adenomas: a report of four cases and 
review of the literature. J Clin Endocrinol Metab. 2013;98:2210–
2217.
	560.	 Beck-Peccoz P, Lania A, Beckers A, Chatterjee K, Wemeau JL. 
2013 European Thyroid Association guidelines for the diagnosis 
and treatment of thyrotropin-secreting pituitary tumors. Eur Thy­
roid J. 2013;2:76–82.
	561.	 Beck-Peccoz P, Mar

1;86:2849–2853.
	563.	 Colao A, Cappabianca P, Caron P, et al. Octreotide LAR vs. surgery 
in newly diagnosed patients with acromegaly: a randomized, open-
label, multicentre study. Clin Endocrinol (Oxf). 2009;70:757–768.
	564.	 Wang EL, Qian ZR, Yamada S, et al. Clinicopathological charac­
terization of TSH-producing adenomas: special reference to TSH-
immunoreactive but clinically non-functioning adenomas. Endocr 
Pathol. 2009;20:209–220.
303
10
Posterior Pituitary
CHRISTOPHER J. THOMPSON AN

10
CHRISTOPHER J. THOMPSON AND JOSEPH G. VERBALIS
Anatomy
Normal
The posterior pituitary is neural tissue and consists only of the 
distal axons of the hypothalamic magnocellular neurons that make 
up the neurohypophysis. The perikarya (cell bodies) of these axons 
are located in paired paraventricular and supraoptic nuclei of the 
hypothalamus. During embryogenesis neuroepithelial cells of the 
lining of the third ventricle mature into magnocellular neurons 
while migrating

mature into magnocellular neurons 
while migrating laterally to and above the optic chiasm to form 
the supraoptic nuclei and to the walls of the third ventricle to form 
the paraventricular nuclei.1 In the posterior pituitary the axon 
terminals of the magnocellular neurons contain neurosecretory 
granules, membrane-bound packets of hormones stored for subse­
quent release. The blood supply for the anterior pituitary is via the 
hypothalamic/pituitary portal system, but the posterior pituitary 

uitary portal system, but the posterior pituitary 
is supplied directly from the inferior hypophyseal arteries, which 
are branches of the posterior communicating and internal carotid 
arteries. The drainage is into the cavernous sinus and internal 
jugular vein.
The hormones of the posterior pituitary, oxytocin and vaso­
pressin, are for the most part synthesized in individual hormone-
specific magnocellular neurons although a small number of 
neurons (∼3%) express both peptides.2 The supraopti

eurons (∼3%) express both peptides.2 The supraoptic nucleus 
(SON) is relatively simple with 80% to 90% of the neurons pro­
ducing vasopressin3 and virtually all axons projecting to the pos­
terior pituitary.1 The organization of the paraventricular nucleus 
Anatomy, 303
Synthesis and Release of Neurohypophyseal Hormones, 304
Physiology of Secretion of Vasopressin and Thirst, 305
Diabetes Insipidus, 308
The Syndrome of Inappropriate Antidiuresis, 315
Oxytocin, 328
KEY POINTS
	•	

te Antidiuresis, 315
KEY POINTS
	•	
The posterior pituitary is composed of neural tissue and consists 
of distal axon terminals of the hypothalamic magnocellular 
vasopressin and oxytocin neurons that constitute the neurohy­
pophysis.
	•	
Regulation of neurohypophyseal hormone synthesis occurs at 
the level of transcription. Stimuli for the secretion of vasopressin 
or oxytocin also induce transcription and increase the hormone 
messenger ribonucleic acid (mRNA) content in the ma

essenger ribonucleic acid (mRNA) content in the magnocel­
lular neurons.
	•	
Physiologic regulation of vasopressin synthesis and secretion 
involves two systems: osmotic and pressure/volume. Each are 
controlled by separate neural inputs to the neurohypophysis 
and have different thresholds for vasopressin secretion.
	•	
Once released, vasopressin elicits end-organ effects via activa­
tion of vasopressin receptors. Renal vasopressin V2 receptors 
induce antidiuresis via a signal transduction c

s 
induce antidiuresis via a signal transduction cascade that results 
in insertion of aquaporin 2 water channels into the apical mem­
brane of principal cells in the renal collecting duct.
	•	
Diabetes insipidus is a disorder of a large volume of urine (dia­
betes) that is hypotonic (dilute) and tasteless (insipid), leading 
to polyuria and polydipsia. It can be caused by deficient vaso­
pressin section, increased vasopressin catabolism, or decreased 
vasopressin effect in the kidneys.
	•	
Th

reased 
	•	
The syndrome of inappropriate antidiuresis (SIAD) occurs when 
plasma levels of vasopressin are elevated at times when physi­
ologic vasopressin secretion from the posterior pituitary would 
normally be osmotically suppressed, leading to water retention 
and hypoosmolality. It can be caused by several diseases and 
drugs.
304
(PVN), however, is much more complex and varies among spe­
cies. In the human there

lex and varies among spe­
cies. In the human there are five subnuclei,3 and parvocellular 
(smaller cell) divisions synthesize other peptides such as cortico­
tropin-releasing hormone (CRH), thyrotropin-releasing hormone 
(TRH), and somatostatin4 as well as opioids.5 The parvocellular 
neurons project to the median eminence, brainstem, and spinal 
cord6 where they play a role in a variety of neuroendocrine auto­
nomic functions. The suprachiasmatic nucleus, which is located in 
the midline at th

ic nucleus, which is located in 
the midline at the base of and anterior to the third ventricle, also 
synthesizes vasopressin and controls both circadian and seasonal 
rhythms.3
The major stimulatory neurotransmitter in the neurohypophy­
sis is glutamate with noradrenergic stimulatory inputs acting by 
stimulation of glutamate.7,8 Glutamate receptors account for 25% 
of synapses on magnocellular neurons.7 The major inhibitory 
input is γ-aminobutyric acid (GABA), which accounts for 20% to 
40% 

tyric acid (GABA), which accounts for 20% to 
40% of the synaptic input to the magnocellular neurons.9 Phasic 
firing of vasopressin neurons is the most efficient activity pattern 
for release of vasopressin from axon terminals. Phasic activity is 
controlled by glutamate stimulation and opioid inhibition. Dyn­
orphin is synthesized in vasopressin neurons and co-released with 
vasopressin from dendrites at the somatic level where it acts in an 
autocrine fashion to inhibit the activity of the va

utocrine fashion to inhibit the activity of the vasopressin neu­
rons, contributing to the phasic firing pattern.10,11 One of the 
most remarkable aspects of the magnocellular system is the plastic­
ity of the system in response to prolonged stimulation. Plasticity 
is of most import in humans during parturition and lactation.9 
Ectopic Posterior Pituitary
With the development of magnetic resonance imaging (MRI) 
scans of the brain it was discovered that T1-weighted images with 
MRI produced a b

red that T1-weighted images with 
MRI produced a bright signal in the posterior pituitary.12 This 
allowed the identification of children in whom there was abnor­
mal anatomy of the posterior pituitary when the “bright spot” 
was recognized in the base of the hypothalamus. These cases are 
referred to as ectopic posterior pituitary or pituitary stalk interrup­
tion. Etiologies include traumatic delivery (these patients have a 
higher incidence of breech delivery and perinatal injuries) and 
gene

 breech delivery and perinatal injuries) and 
genetic abnormalities of the transcription factors that regulate 
pituitary embryogenesis.13 The latter is supported in cases where 
abnormalities of the posterior pituitary and/or stalk are associated 
with extrapituitary malformations such as septal optic dysplasia. 
Cases with malformations are more likely to have diabetes insipi­
dus or other osmotic dysfunction than simple ectopic posterior 
pituitary.14,15 Cases are recognized in children with 

itary.14,15 Cases are recognized in children with growth retar­
dation and anterior pituitary deficiency rather than posterior pitu­
itary deficiency. The degree of anterior pituitary deficit depends on 
the persistence of a pituitary stalk and a retained portal vasculature 
from the hypothalamus to the anterior pituitary.16–18 Deficiency 
of adrenocorticotropic hormone (ACTH) is common and should 
be investigated as the patients may not respond appropriately to 
stress.13 
Synthesis and Release

appropriately to 
stress.13 
Synthesis and Release of Neurohypophyseal 
Hormones
Vasopressin and oxytocin are nonapeptides consisting of a six–
amino acid ring with a cysteine-to-cysteine bridge and a three–
amino acid tail. All mammals have arginine vasopressin and 
oxytocin, as illustrated in Fig. 10.1, with the exception of the pig 
in which lysine is substituted for arginine in position 8 produc­
ing lysine vasopressin. Both genes are found on chromosome 2019 
although they are situated in a

n chromosome 2019 
although they are situated in a tail-to-tail position and transcribed 
in opposite directions.20 The hormones are synthesized as part of 
a precursor molecule consisting of the nonapeptide and a hor­
mone-specific neurophysin and, for vasopressin, a 39–amino acid 
peptide named copeptin.21 The precursor is packaged in neurose­
cretory granules and cleaved to the products during transport to 
the posterior pituitary.
When a stimulus for secretion of vasopressin or oxytocin acts

ulus for secretion of vasopressin or oxytocin acts 
on the appropriate magnocellular cell body, an action potential 
is generated and propagates down the long axon to the posterior 
pituitary. The action potential causes an influx of calcium, which 
induces neurosecretory granules to fuse with the cell membrane 
and extrude the entire contents of the neurosecretory granule into 
the perivascular space and subsequently into the capillary system 
of the posterior pituitary. At physiologic pH of pl

f the posterior pituitary. At physiologic pH of plasma there is 
no binding of hormone (vasopressin or oxytocin) to their respec­
tive neurophysins, so each peptide circulates independently in the 
bloodstream.
The control of hormone synthesis is at the level of tran­
scription. Stimuli for secretion of vasopressin or oxytocin also 
stimulate transcription and increase the mRNA content in the 
magnocellular neurons. This has been studied in most detail in 
rats where dehydration22 accelerates tr

detail in 
rats where dehydration22 accelerates transcription and increases 
the levels of vasopressin (and oxytocin) mRNA23–25 and where 
hypoosmolality produces a decrease in the content of vasopressin 
mRNA.26
The transport of neurosecretory vesicles from the site of syn­
thesis to the posterior pituitary along microtubule tracks27 is 
also regulated. When synthesis is turned off, transport stops; 
when synthesis is increased, transport is upregulated.27 Thus 
A  Arginine vasopressin
Phe3
Gln

egulated.27 Thus 
Phe3
Gln4
Tyr2
Cys1
Cys6
Pro7
Gly9
NH2
L Arg8
S S
S S
S S
H2N
Asn5
B  Oxytocin
Ile3
Gln4
Tyr2
Cys1
Cys6
Pro7
Gly9
NH2
Leu8
H2N
Asn5
C  Desmopressin
Phe3
Gln4
Tyr2
Cys1
Cys6
Pro7
Gly9
NH2
D Arg8
H
Asn5
• Fig. 10.1  Comparison of the chemical structures of arginine vasopres­
sin (A), oxytocin (B), and desmopressin (C). The differences are illustrated 
by the shaded areas. Oxytocin differs from vasopressin in position 3 (Ile 
for Phe) and position 8 (Leu fo

n position 3 (Ile 
for Phe) and position 8 (Leu for Arg). Desmopressin differs from arginine 
vasopressin in that the terminal cystine is deaminated and the arginine 
in position 8 is a d-isomer rather than an l-isomer. (From A.G. Robinson, 
University of California at Los Angeles, used with permission.)
305
Chapter 10  Posterior Pituitary
there is coordination of stimulated release of hormone, trans­
port of hormone, and synthesis of new hormone. There is, how­
ever, asynchrony in the timing of

. There is, how­
ever, asynchrony in the timing of these events. The asynchrony is 
demonstrated by changes in the content of vasopressin stored in 
the posterior pituitary. The absolute content varies considerably 
among species but is a remarkable store, generally equivalent to 
the amount of hormone required to sustain basal release for 30 to 
50 days or maximum release for 5 to 10 days.28 In animals, pro­
longed and intense stimulation of vasopressin release such as dehy­
dration or salt loa

opressin release such as dehy­
dration or salt loading produces a depletion of stored hormone in 
the posterior pituitary.25,29,30 Then when animals are returned to 
normal water intake, there is in 7 to 14 days a gradual recovery of 
pituitary content back to baseline. Modeling of this phenomenon 
has provided experimental evidence that a long half-life of the 
vasopressin message, approximately 2 days, is (from a minimal­
ist point of view) a plausible explanation.31 When a strong and/
or sust

lausible explanation.31 When a strong and/
or sustained stimulus releases vasopressin there is an immediate 
stimulus to transcription of new mRNA, but it requires several 
days for the peak level of mRNA to be reached; thus while release 
of hormone is rapid, translation increases slowly. When the stimu­
lus is removed, the elevated mRNA slowly declines while continu­
ing to synthesize hormone that repletes the store in the posterior 
pituitary. 
Physiology of Secretion of Vasopressin and 
Thir

Thirst
The physiologic regulation of vasopressin synthesis and secretion 
involves two systems: osmotic and pressure/volume (Fig. 10.2). 
Functions of these two systems are so distinct that historically it 
was thought there were two hormones, an antidiuretic hormone 
and a vasopressor hormone. Hence the two names that are used 
interchangeably for (8-arginine) vasopressin. There are separate 
systems at the level of the receptors on the end organs o

 at the level of the receptors on the end organs of response. 
The V1a receptors on blood vessels are distinct from V2 receptors 
on renal collecting duct epithelia. These two vasopressin recep­
tor subtypes are responsible for the main physiologic actions of 
vasopressin. A third receptor, V1b, is responsible for the nontra­
ditional biologic action of vasopressin to stimulate ACTH secre­
tion from the anterior pituitary and has been found in numerous 
peripheral tissues and areas of the brain.

merous 
peripheral tissues and areas of the brain.32 V2 receptors also regu­
late the nontraditional action of vasopressin to stimulate factor 
VIII and von Willebrand factor production. Vasopressin is the 
main hormone involved in the regulation of water homeostasis 
and osmolality, while the renin-angiotensin-aldosterone system 
(RAAS) is mainly responsible for regulation of blood pressure 
and volume. Pathologic disorders of the neurohypophysis are 
primarily expressed as abnormalities of osm

s are 
primarily expressed as abnormalities of osmolality produced by 
abnormal excretion or retention of water. In the case of osmo­
regulation vasopressin secretion is relatively uncomplicated with 
small increases in osmolality producing a parallel increase in 
vasopressin secretion and small decreases in osmolality causing a 
parallel decrease in vasopressin secretion. The regulation of vol­
ume and blood pressure is significantly more complicated (see 
review by Thrasher33) and experimental

cated (see 
review by Thrasher33) and experimental models of vasopressin 
and baroreceptor regulation in animals often involve inhibiting 
and/or measuring other concurrent sympathetic inputs to the 
system to ascertain direct effects of any stimulus on secretion 
of vasopressin (see Fig. 10.2). Other influences on secretion of 
vasopressin such as the inhibiting influence of glucocorticoids 
and the potent stimulus of nausea and vomiting are less impor­
tant as physiologic regulators of vasopre

s impor­
tant as physiologic regulators of vasopressin but might contrib­
ute during pathologic situations.
Volume and Pressure Regulation
High-pressure arterial baroreceptors are located in the carotid 
sinus and aortic arch; low-pressure volume receptors are located 
in the atria and pulmonary venous system.33 The afferent signals 
from these receptors are carried from the chest to the brainstem 
through cranial nerves 9 and 10. Interruption of the vagal input 
by vagal cold block in dogs34,35

 the vagal input 
by vagal cold block in dogs34,35 and destruction in rabbits of the 
A1 area of the medulla, which receives input from cranial nerves 
9 and 10,36–38 leads to an increase in vasopressin secretion. These 
and other data led to the concept that baroreceptors and volume 
receptors normally inhibit the magnocellular neurons and that 
decreases in this tonic inhibition result in release of vasopressin. 
Arterial and venous constriction induced by vasopressin action on 
V1a receptors 

n induced by vasopressin action on 
V1a receptors will contract the vessels around the existing plasma 
volume to effectively “increase” plasma volume and reestablish the 
inhibition of secretion of vasopressin. Vasopressin’s action at the 
kidney to retain water will help replace volume but, in fact, the 
major hormonal regulation to control volume is the RAAS, which 
stimulates sodium reabsorption in the kidney (see Chapter 15). 
The concept of tonic inhibition of vasopressin secretion by baro

 tonic inhibition of vasopressin secretion by baro­
receptors has been questioned,33,39 but there is agreement that the 
volume/baroreceptor responses are much less sensitive than are 
the osmoreceptors (see Fig. 10.2). The lesser response has been 
attributed to the fact that changes in blood volume and central 
venous pressure have little effect to increase vasopressin in humans 
as long as arterial pressure can be maintained by alternative regula­
tory mechanisms such as RAAS and sympathetic 

ula­
tory mechanisms such as RAAS and sympathetic reflexes.33 When 
the hypovolemia is sufficient to cause a decrease in blood pres­
sure there is an exponential increase in the level of vasopressin in 
plasma33,40 (see Fig. 10.2). There is also agreement that changes in 
Percent change
Pressure
Volume
Osmolality
Plasma vasopressin (pg/mL)
25
20
15
10
5
0
(Osmolality)
(Pressure or volume)
+30
−30
+20
−20
+10
−10
0
0
• Fig. 10.2  Comparison in humans of the release of vasopressin in 
response to 

ans of the release of vasopressin in 
response to increased osmolality (open triangles) or to decreased blood 
pressure (filled circles) or blood volume (open circles). Plasma vasopressin 
is much more sensitive to change in osmolality, responding to as little as 
a 1% increase, whereas a change of 10% to 15% or greater in volume or 
pressure is required to stimulate release of vasopressin. (Redrawn from 
Robertson GL, Berl T. Water metabolism. In: Brenner B, Rector F Jr, eds. 
The Kidney. 3rd e

n: Brenner B, Rector F Jr, eds. 
The Kidney. 3rd ed, vol 1. Philadelphia, PA: Elsevier; 1986:385. Figure by 
A.G. Robinson, University of California at Los Angeles, used with permis­
sion.)
306
volume or pressure that are insufficient to cause direct increases in 
vasopressin can nonetheless modify the response of the vasopres­
sin system to osmoregulatory inputs.40,41 Increases in pressure and 
central volume will decrease the secretion of vasopressin,42

lume will decrease the secretion of vasopressin,42 but the 
response of the RAAS to cause sodium excretion is much more 
sensitive to increases of pressure and volume than is the response 
to decrease secretion of vasopressin.33 Consequently, changes in 
blood pressure and volume involve both excitatory and inhibitory 
influences from the brainstem to magnocellular neurons, with the 
dominant effect depending on the physiologic circumstances. 
Osmotic Regulation
The primary receptors for sensing

motic Regulation
The primary receptors for sensing changes in osmolality are located 
in the brain. Most of the brain is within the blood-brain barrier, 
which is generally impermeable to polar solutes. The osmostat is 
insensitive to urea and glucose, which readily cross cellular mem­
branes but not the blood-brain barrier; this provides strong evi­
dence that the osmoreceptors must be outside the blood-brain 
barrier. Experimental brain lesions in animals have implicated 
cells in the organum 

 in animals have implicated 
cells in the organum vasculosum of the lamina terminalis (OVLT) 
and in areas of the anterior hypothalamus near the anterior wall of 
the third cerebral ventricle as the primary osmoreceptors. Because 
these and other circumventricular organs are perfused by fenes­
trated capillaries, they are outside the blood-brain barrier. Surgical 
destruction of the OVLT abolishes vasopressin secretion and thirst 
responses to hyperosmolality but not their responses to hypovole­

perosmolality but not their responses to hypovole­
mia.43 Patients with brain damage that destroys the region around 
the OVLT cannot maintain normal plasma osmolalities even 
under basal conditions.44 In contrast, destruction of the magno­
cellular neurons of the SON and PVN eliminates dehydration-
induced secretion of vasopressin but does not alter thirst, clearly 
indicating that osmotically stimulated thirst must be generated at 
a site proximal to the magnocellular cells. Because the osmore

mal to the magnocellular cells. Because the osmorecep­
tors are relatively solute specific, they are most sensitive to altera­
tions in plasma sodium concentrations but respond less well to 
elevations in blood urea and seem insensitive to changes in plasma 
glucose concentration.45
Extracellular fluid osmolality (predominantly determined by 
sodium concentration) varies from 280 to 295 mOsm/kg H2O 
in normal subjects, but in any individual it is maintained within 
a more narrow range. The abili

 maintained within 
a more narrow range. The ability to maintain this narrow range 
is dependent upon the sensitive response of plasma vasopressin 
to changes in plasma osmolality, the sensitive response of urine 
osmolality to changes in plasma vasopressin, and the gain in the 
system by the response of urine volume to change in plasma vaso­
pressin (Fig. 10.3). Basal plasma vasopressin is in the range of 
0.5 to 2 pg/mL. As little as a 1% increase or decrease in plasma 
osmolality will cause a

ase or decrease in plasma 
osmolality will cause a rapid increase or decrease of vasopressin 
released from the store of hormone in the posterior pituitary.40 
Rapid metabolism of vasopressin is also characteristic of the hor­
mone that circulates in plasma with a half-life of approximately 
15 minutes, which allows rapid changes in levels of vasopressin in 
plasma. Thus small increases in plasma osmolality produce a 
concentrated urine and small decreases produce a water diuresis. 
Fig. 10.3A i

 decreases produce a water diuresis. 
Fig. 10.3A illustrates the linear relationship between plasma osmolal­
ity and plasma vasopressin that has been described in humans.40 This 
linear relationship for osmolalities persists well above the normal 
excursion of osmolalities as demonstrated when the increase is 
induced by infusion of hypertonic saline or is observed during 
dehydration of patients with nephrogenic diabetes insipidus.46 
Similarly, Fig. 10.3B illustrates that there is a sensitive 

 Fig. 10.3B illustrates that there is a sensitive and lin­
ear relationship between the level of vasopressin in plasma and 
C
B
PAVP
U volume/24 h
30
25
20
15
10
5
0
1400
1050
700
350
0
12
9
6
3
0
UOsm
UOsm
1400
1050
700
350
0
12
9
6
3
0
A
PAVP
pOsm
320
310
300
290
280
270
12
9
6
3
0
PAVP
• Fig. 10.3  Effect of change in plasma osmolality (pOsm, in mOsm/kg of 
H2O) on plasma arginine vasopressin (PAVP, in pg/mL) and consequent 
effects on urine osmolality (UOsm, in mOsm/kg of H2O) and urine volu

smolality (UOsm, in mOsm/kg of H2O) and urine volume 
(L/day). The shaded area represents the normal range. (A) Small changes 
in pOsm induce changes in PAVP, typically between less than 0.5 and 5 
to 6 pg/mL. (B) Changes in PVAP induce changes in UOsm through the 
full range, from maximally dilute to maximally concentrated urine. Although 
PAVP can rise to higher levels than 6 pg/mL, this does not translate into 
increased UOsm, which has a maximum determined by the osmolality 
of an inner medu

mum determined by the osmolality 
of an inner medulla of the kidney. (C) The relationship of urine volume to 
UOsm is logarithmic, assuming a constant osmolar load and the urine 
volume that would excrete that osmolar load at the UOsm indicated. As a 
result, urine volume changes relatively little with small changes in the other 
parameters until there is almost complete absence of PAVP, after which the 
urine volume increases dramatically. (Calculated from a formula presented 
in Robertson G, S

ulated from a formula presented 
in Robertson G, Shelton R, Athar S. The osmoregulation of vasopressin. 
Kidney Int. 1976;10:25–37. Figure by A.G. Robinson, University of Cali­
fornia at Los Angeles, used with permission of Macmillan Publishers, Ltd.)
307
the induced osmolality of the urine. Although plasma vasopres­
sin might increase above the normal physiologic range, the urine 
osmolality plateaus at approximately 800 to 1200 mOsm/kg H2O 
because the maximum c

ely 800 to 1200 mOsm/kg H2O 
because the maximum concentration of fluid in the renal collect­
ing duct is the osmolality of the inner medulla. Fig. 10.3C shows 
the relationship of plasma vasopressin to urine volume. This is 
a calculated relationship based on the urine volume necessary to 
excrete a fixed quantity of osmolytes (800 mOsm) at the urine 
osmolality produced by the change in plasma vasopressin. These 
graphs demonstrate the gain in the system when considering the 
changes in urine 

the system when considering the 
changes in urine volume relative to plasma vasopressin. When 
vasopressin is absent, 18 to 20 L of urine are excreted daily; with 
an increase of vasopressin by as little as 0.5 to 1 pg/mL, how­
ever, urine volume is reduced to less than 4 L/day. This illustrates 
the important point that at low plasma levels of vasopressin small 
changes of vasopressin are much larger determinants of polyuria 
than are greater changes at higher plasma levels.
In the kidney, wate

anges at higher plasma levels.
In the kidney, water is conserved by the combined functions 
of the loop of Henle and the collecting duct. The loop of Henle 
generates a high osmolality in the renal medulla via the coun­
tercurrent multiplier system. Vasopressin acts in the collecting 
duct to increase water (and urea) permeability, thereby allow­
ing osmotic equilibration between the urine and the hypertonic 
medullary interstitium. The net effect of this process is to extract 
water from the ur

t of this process is to extract 
water from the urine (which is removed from the medulla by 
interstitial blood vessels, vasa recta), resulting in increased urine 
concentration and decreased urine volume (antidiuresis). Vaso­
pressin produces antidiuresis by binding to V2 receptors on the 
epithelial principal cells of the renal collecting tubule. Binding 
activates adenylate cyclase, increasing cyclic adenosine mono­
phosphate (cAMP), which then stimulates protein kinase A. 
This leads to phos

n stimulates protein kinase A. 
This leads to phosphorylation and activation of aquaporin 2 and 
movement of the water channels into the luminal membrane.47 
Aquaporin 2 is one of the widely expressed family of water chan­
nels that mediate rapid water transport across cell membranes.48 
In the kidney, water moves from the collecting duct into the 
hypertonic inner medulla producing concentrated urine.49 In 
addition to moving constitutively synthesized aquaporin 2 from 
the cytoplasm to the lum

hesized aquaporin 2 from 
the cytoplasm to the luminal membrane, activation of the V2 
receptor also increases the synthesis of aquaporin 2 and the 
permeability of aquaporin 2 to water.50 Aquaporin 3 and 4 are 
constitutively synthesized and are expressed at high levels in the 
basolateral plasma membranes of principal cells, where they are 
responsible for the high water permeability of the basolateral 
plasma membrane.48,49 Dissociation of vasopressin from the V2 
receptor allows intracellula

opressin from the V2 
receptor allows intracellular cAMP levels to decrease; the water 
channels are then reinternalized, terminating the increased water 
permeability. The aquaporin-containing vesicles remain just 
below the apical membrane and can be quickly “shuttled” into 
and out of the membrane in response to changes in intracellular 
cAMP levels. This mechanism allows minute-to-minute regula­
tion of renal water excretion in response to changes in ambient 
levels of vasopressin in plasma.

anges in ambient 
levels of vasopressin in plasma. There is also long-term regulation 
of collecting duct water permeability in response to prolonged 
high levels of circulating vasopressin. Chronically high levels of 
vasopressin induce increased synthesis of aquaporin 2 and 3 water 
channels in the collecting duct principal cells and hence high lev­
els of these proteins. This response requires at least 24 hours and 
is not rapidly reversible. Increased numbers of aquaporin 2 and 3 
water chan

Increased numbers of aquaporin 2 and 3 
water channels combined with the effect of vasopressin to insert 
aquaporin 2 into the apical plasma membrane allows the col­
lecting ducts to achieve extremely high water permeabilities and 
water conservation during prolonged dehydration.48,49 
Thirst
Renal free water clearance can be reduced to a minimum by the 
antidiuretic actions of vasopressin, but water loss is not completely 
eliminated, and insensible water loss from respiration and sweat­
ing is

ible water loss from respiration and sweat­
ing is a continuous process. To maintain water homeostasis, water 
must also be consumed to replace the obligate urinary and insen­
sible fluid losses. This is regulated by thirst. The sensation of thirst 
can be stimulated by increases in plasma osmolality or by decreases 
in intravascular volume, in a manner identical to vasopressin. 
Furthermore, there is evidence that the receptors are similar (i.e., 
osmoreceptors in the anterior hypothalamus and 

, 
osmoreceptors in the anterior hypothalamus and low-pressure 
and/or high-pressure baroreceptors in the chest [with a likely con­
tribution from circulating angiotensin II to stimulate thirst during 
more severe degrees of intravascular hypovolemia and hypoten­
sion]).51 Careful physiologic studies in healthy humans, using 
quantitative estimates of subjective symptoms of thirst, have con­
firmed that the characteristics of osmotically stimulated thirst are 
similar to those of vasopressin sec

ed thirst are 
similar to those of vasopressin secretion; the osmotic thresholds 
are similar, and there is a close linear relationship between thirst 
scores and plasma osmolality throughout a wide range of plasma 
tonicity.52 The threshold for producing thirst by hypovolemia is 
significantly higher than that for osmotic stimulation.
Although osmotic changes clearly are effective stimulants of 
thirst, most humans consume the bulk of their ingested water as a 
result of the relatively unregula

sted water as a 
result of the relatively unregulated components of fluid intake—
that is, beverages are consumed with food for reasons of palatabil­
ity, taken for desired secondary effects (e.g., caffeine), or taken for 
social or habitual reasons (e.g., sodas or alcoholic beverages). As a 
result, humans generally ingest volumes in excess of what can be 
considered an actual need for fluid. In humans, plasma osmolality 
does not vary by more than 1% to 2% of basal levels. However, in 
adipsic

han 1% to 2% of basal levels. However, in 
adipsic patients the absence of thirst is associated with profound 
alterations of plasma water homeostasis, which emphasizes that 
even in humans who mostly drink for pleasure, thirst still plays a 
fundamental role in physiologic osmoregulation. 
Clinical Consequences of Osmotic and Volume 
Under physiologic situations and during most pathophysiologic 
conditions, the integration of osmotic and baroregulated vasopres­
sin release and thirst

and baroregulated vasopres­
sin release and thirst are closely integrated and maintain plasma 
osmolality and sodium concentrations within normal ranges. 
During dehydration, for example, plasma osmolality increases 
and blood volume decreases, and vasopressin secretion and thirst 
sensation occur as a composite result of the two separate stimuli. 
There is good evidence that a decrease in blood volume shifts the 
plasma vasopressin/plasma osmolality response curve to the left, 
resulting in a g

lity response curve to the left, 
resulting in a greater release of vasopressin per unit rise osmolality; 
in other words, hypovolemia enhances the vasopressin response 
to an osmotic stimulus.33,53 Similarly, an excess of fluid produces 
a decrease in osmolality and an increase in volume, and both will 
cause a decrease in vasopressin secretion and thirst.
The clinical manifestations of osmoregulatory diseases are 
largely explainable on the basis of the physiology of vasopressin 
secretion and

is of the physiology of vasopressin 
secretion and its antidiuretic actions. Fig. 10.3 shows that loss of 
vasopressin neurons sufficient to decrease the vasopressin secretory 
capacity from that able to produce plasma concentrations of 10 to 
20 pg/mL down to a secretory capacity able to generate plasma 
concentrations of only 5 pg/mL would not compromise the abil­
ity to maximally concentrate urine. Further losses of vasopressin 
neurons such that maximal plasma vasopressin concentrations are 

hat maximal plasma vasopressin concentrations are 
308
in the 1 to 5 pg/mL range are associated with a linear decrease in 
the ability to maximally concentrate the urine; however, from the 
urine volume curve it can be seen that this results in only modest 
increases in urine volume. But when loss of vasopressin neurons to 
produce maximal plasma vasopressin concentrations below 1 pg/
mL occurs, there is a large increase in urine volume, as urine con­
cen

 large increase in urine volume, as urine con­
centration is severely compromised. Water conservation is there­
fore protected despite minimal ability to secrete vasopressin. In 
addition, the phenomenon of recovery of diabetes insipidus after 
transsphenoidal surgery or traumatic brain injury, even after pro­
longed symptoms, might reflect a comparatively minor recovery 
of vasopressin neurons sufficient to abolish polyuric symptoms.
In contrast, in the syndrome of inappropriate antidiuresis, a

, in the syndrome of inappropriate antidiuresis, a 
patient who is unable to suppress plasma vasopressin levels to less 
than 1 pg/mL might be unable to excrete more than 2 to 4 L of 
urine a day at a standard osmolar load. However, because osmo­
regulated thirst is not suppressed despite hyponatremia in SIAD,54 
if fluid intake increases above these levels the antidiuretic effects 
of nonsuppressible vasopressin secretion lead to dilutional hypo­
natremia. Thus, although the regulation of vasop

­
natremia. Thus, although the regulation of vasopressin secretion 
and thirst demonstrates a simple but elegant human system to 
maintain water balance, abnormal functioning of these complex 
systems leads to the development of the pathophysiologic states 
that will be described in this chapter. 
Reset Osmostat During Pregnancy
Major shifts of fluid during normal pregnancy produce a fall in 
plasma osmolality of about 10 mmol/kg and an increase in plasma 
volume55; this is the best physiologic 

in plasma 
volume55; this is the best physiologic example of a true resetting 
of the osmostat. The shift in osmotic threshold for vasopressin 
appears at about 5 to 8 weeks of gestation and persists throughout 
pregnancy, returning to normal by 2 weeks after delivery.55 The 
physiology of the reset osmostat has been considered in relation 
to the expanded plasma volume. Total body water in pregnant 
women is increased by 7 to 8 L due to profound vasodilatation.56 
This volume is sensed as norma

 vasodilatation.56 
This volume is sensed as normal and vasopressin responds appro­
priately to decreases and increases of the expanded volume.55,57 
Both the changes in volume and the changes in osmolality have 
been reproduced by infusion of relaxin (a normal hormone of 
pregnancy that is a member of the insulin-like growth factor 
[IGF] family) into virgin female and normal rats,58,59 and reversed 
in pregnant rats by immunoneutralization of relaxin.60 Increased 
nitric oxide (NO) by relaxin 

elaxin.60 Increased 
nitric oxide (NO) by relaxin is reported to increase vasodilatation, 
and estrogens also increase NO synthesis.61
In women the placenta produces an enzyme, cysteine ami­
nopeptidase, which is released into the plasma and is known as 
oxytocinase.55,62 This enzyme is equally potent in degrading oxy­
tocin and vasopressin. The activity of oxytocinase (vasopressinase) 
increases markedly around 20 weeks of gestation and increases 
further to 40 weeks, returning slowly to normal

s 
further to 40 weeks, returning slowly to normal over a few weeks 
after delivery.63 The potential pathology produced by oxytocinase 
is described in “Diabetes Insipidus.” 
Osmotic Regulation in Aging
Numerous studies have reported that elderly humans are at risk 
for both hypernatremia and hyponatremia.64,65 In older subjects 
there is a decrease in glomerular filtration rate,64 and the collecting 
duct in the aged kidney is less responsive to vasopressin-stimulated 
increases in aquaporin 2 

 vasopressin-stimulated 
increases in aquaporin 2 water channels, thus limiting the ability 
to excrete free water.66 Elderly subjects are reported to have lower 
nocturnal plasma levels of arginine vasopressin (AVP) and a pro­
longed effect of administered desmopressin.67 Many other abnor­
malities of fluid and electrolyte balance in the elderly are due to 
comorbid conditions and/or the numerous pharmacologic agents 
these patients are often taking.68 Studies of responses to dehydra­
tion, osm

king.68 Studies of responses to dehydra­
tion, osmolar stimulation, or volume stimulation in the elderly 
are complicated by the fact that by age 75 to 80 years total body 
water declines to 50% of the level of normal young adults.69 The 
elderly have a decreased thirst with dehydration and a lesser fluid 
intake during recovery from dehydration.70,71 At the other end of 
the spectrum, elderly patients have been found to excrete a water 
load less well than younger subjects, and at least part of

s well than younger subjects, and at least part of this is 
due to decreased suppression of vasopressin.72 In summary, there 
are age-related changes in body volumes and renal function that 
predispose the elderly to abnormalities in water and electrolyte 
balance.73 Diseases that are more common in the elderly aggravate 
this; in addition, the therapy for these diseases affects water bal­
ance. Healthy elderly humans probably have at least a normal (or 
increased) ability to secrete vasopressin

mal (or 
increased) ability to secrete vasopressin but a decreased apprecia­
tion for thirst and a decreased ability to achieve either a maximum 
concentration of urine to retain water or a maximum dilution of 
urine to excrete water. This demonstrates the necessity of paying 
attention to fluid balance problems in the elderly as undetected 
hypernatremia or hyponatremia can lead to increased morbidity 
and mortality.74 
Diabetes Insipidus
Diabetes insipidus is a clinical disorder that is charac

es insipidus is a clinical disorder that is characterized by 
the excretion of large volumes of urine (diabetes) that is hypo­
tonic, dilute, and tasteless (i.e., insipid). This is in contrast to the 
hypertonic and sweet urine of diabetes mellitus (i.e., honey). Dia­
betes insipidus can result from four abnormal pathophysiologic 
processes:
	1.	 Primary polydipsia, caused by excess fluid intake
	2.	 Hypothalamic or central diabetes insipidus, caused by dimin­
ished synthesis or secretion of v

caused by dimin­
ished synthesis or secretion of vasopressin
	3.	 Diabetes insipidus of pregnancy, caused by increased enzy­
matic metabolism of vasopressin
	4.	 Nephrogenic diabetes insipidus, caused by renal resistance to 
vasopressin
Causes of Diabetes Insipidus
Diabetes Insipidus Due to Excess Fluid Intake (Primary 
Polydipsia)
Primary polydipsia is associated with a wide variety of organic 
structural brain lesions, including sarcoidosis of the hypothala­
mus75 and craniopharyngioma.76 It

f the hypothala­
mus75 and craniopharyngioma.76 It can also be produced by drugs 
that cause a dry mouth or by any peripheral disorder causing an 
elevation of renin and/or angiotensin.77 However, there is often 
no identifiable pathologic etiology; in this circumstance the dis­
order can be associated with psychiatric syndromes or be habitual 
throughout a lifetime. Series of patients in psychiatric hospitals 
have shown that as many as 42% have some form of polydipsia, 
and for over half of th

 some form of polydipsia, 
and for over half of those there was no obvious explanation for the 
polydipsia.78,79 
Hypothalamic/Central Diabetes Insipidus
Hypothalamic or central diabetes insipidus (CDI) can occur sec­
ondary to a large variety of genetic, immunologic, and structural 
conditions. The relative frequency of the causes of CDI depends on 
309
the nature of the unit where the endocrinologist works. However, 
as most pituitary endocrinologists work in un

er, 
as most pituitary endocrinologists work in units where their work 
is integrated with that of a neurosurgical unit, neurosurgical condi­
tions are usually the commonest encountered causes of CDI.80
Pituitary adenomas almost never cause CDI to such an extent 
that if a patient presents with a pituitary mass and polyuria and 
polydipsia, it is safe to assume that he or she does not have a pitu­
itary adenoma. However, neurosurgical intervention with trans­
sphenoidal or transcranial surgery c

n with trans­
sphenoidal or transcranial surgery causes CDI in as many as 50% 
to 60% of patients, most of whom will recover, with only a small 
number having permanent diabetes insipidus.81 Permanent CDI 
is more common after craniotomy for large tumors. If there is 
complete stalk section, patients can exhibit a pattern known as 
triphasic diabetes insipidus (Fig. 10.4). The first phase is diabetes 
insipidus with onset within the first 24 hours of surgery and is 
thought to be due to axon sho

s of surgery and is 
thought to be due to axon shock and inability of action potentials 
to be propagated from the cell body to the neurons terminating in 
the posterior pituitary. The second phase is an antidiuretic phase, 
which is due to unregulated release of vasopressin from damaged 
neurohypophyseal neurons; this phase, which occurs typically 5 
to 7 days following surgery, is often marked by hyponatremia, 
particularly if there is injudicious administration of intravenous 
hypotonic fluid

ous administration of intravenous 
hypotonic fluids. This phase terminates in permanent CDI, as 
the damaged neurons undergo gliosis, and loss of secretory func­
tion. An important observation is that the second phase of the 
triphasic response (i.e., uncontrolled release of vasopressin due to 
axon trauma) can occur without preceding or subsequent diabetes 
insipidus.82,83 This has been reported clinically and has been pro­
duced experimentally in the rat by unilateral lesion of the supra­
opti

in the rat by unilateral lesion of the supra­
opticohypophyseal track.83 The interpretation is that if the trauma 
is to only some of the axons coursing to the posterior pituitary 
then the remaining intact axons will have sufficient vasopressin 
function to avoid clinically apparent diabetes insipidus character­
istic of the first and third phases of the triphasic response. How­
ever, the store of hormone in the posterior pituitary is sufficiently 
large that necrosis of even a fraction of thes

ly 
large that necrosis of even a fraction of these vasopressin neurons 
will cause enough uncontrolled release of vasopressin to produce 
hyponatremia if excess fluid is administered. The hyponatremia 
is often symptomatic and patients present with headache, nau­
sea and emesis, or seizure.84 When all the vasopressin from the 
damaged neurons has been secreted, the stimulus for water reten­
tion resolves and the retained water is excreted producing recovery 
from the hyponatremia. Thus the clin

ing recovery 
from the hyponatremia. Thus the clinical picture is one of hypo­
natremia occurring around 7 to 10 days after pituitary surgery, 
persisting for a few days, and then returning to normal. This syn­
drome of transient hyponatremia has been referred to as “isolated 
second phase”83 to emphasize the pathophysiologic etiology. Iso­
lated hyponatremia has been reported in 10% to 25% of patients 
after pituitary surgery.85–87
A patient presenting with a sellar mass is more likely to have 

senting with a sellar mass is more likely to have 
another tumor in the pituitary area or, alternatively, a granulo­
matous disorder. Craniopharyngioma is particularly associated 
with CDI, particularly after extensive suprasellar surgery; it is 
hoped that the new recommendations for a more limited surgi­
cal approach in conjunction with postoperative radiotherapy for 
craniopharyngioma will result in fewer hypothalamic complica­
tions such as CDI, but there is a paucity of randomized controlle

DI, but there is a paucity of randomized controlled 
data to address this possibility. Other tumors, such as germinoma, 
pinealoma, or a metastasis from a distant solid organ tumor, can 
also present with CDI. Metastatic disease involving the pituitary 
is usually found in association with widespread metastatic disease 
and may be asymptomatic and only reported at autopsy. Metas­
tases are twice as likely to involve the posterior pituitary as the 
anterior pituitary,88,89 which is thought to be 

anterior pituitary,88,89 which is thought to be due to a more direct 
arterial blood supply to the posterior pituitary.90 Most primary 
tumors in the hypothalamic/pituitary area that cause diabetes 
insipidus are relatively slow growing, and any tumor in this area 
that shows rapid growth in a short period of time should be con­
sidered as a possible metastatic tumor.91,92 Pituitary abscess is a 
rare cause of a pituitary mass and diabetes insipidus.93,94 Diabetes 
insipidus has been reported 

pidus.93,94 Diabetes 
insipidus has been reported with lymphomas and leukemia in the 
hypothalamic/pituitary area.95–97 Diabetes insipidus is distinctly 
more common in nonlymphocytic leukemia.98–100 MRI studies 
in leukemia can show infiltration or an infundibular mass98 but 
often are normal even when leukemic cells are found in the cere­
brospinal fluid (CSF).100
A variety of granulomatous diseases, such as sarcoidosis and 
histiocytosis, have been associated with CDI; usually there is evi­
d

 been associated with CDI; usually there is evi­
dence of characteristic disease elsewhere in the body.101–103 The 
MRI often shows involvement of the hypothalamus and absence 
of the posterior pituitary bright spot on T1-weighted images with 
widening of the pituitary stalk (Table 10.1). Although there are 
occasional reports of resolution of the diabetes insipidus with 
Urine output/24 h
10
8
6
4
2
0
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
Days post surgery
Diabetes
insipidus
Diabetes
insipidus
A

st surgery
Diabetes
insipidus
Diabetes
insipidus
Antidiuretic
interphase
1
• Fig. 10.4  Typical triphasic response of urine volume after sectioning of 
the pituitary stalk induced by surgery or head trauma. The first phase of 
diabetes insipidus occurs immediately postoperatively and continues to 
day 6. The second phase of antidiuresis occurs from day 7 and continues 
to day 12. The third stage is the recurrence of diabetes insipidus on day 
13. Durations vary; see text for detailed discussion.

res­
ents commonly with CDI. Lymphocytic adenohypophysitis pres­
ents classically in females around the time of a pregnancy, whereas 
infundibuloneurohypophysitis occurs in either sex. Infundibulo­
neurohypophysitis occurring in middle-aged to elderly males in 
association with IgG4-related systemic disease has recently been 
reported. IgG hypophysitis is often associated with other organ 
involvement, including the pancreas, and other endocrine glands. 
The diagnosis can be established by eleva

glands. 
The diagnosis can be established by elevated serum IgG4 and 
characteristic histology of biopsies. Response to steroids or other 
immunosuppressive drugs is characteristic.107,108
CDI with no obvious underlying cause has traditionally been 
considered to be idiopathic, but evidence suggests that a signifi­
cant proportion of idiopathic CDI cases are in fact autoimmune 
in origin.109,110 Several groups have reported the presence of anti­
bodies to vasopressin cells in the plasma of patie

bodies to vasopressin cells in the plasma of patients with “idio­
pathic” CDI, and one series reported an incidence of associated 
autoimmune endocrine disease in 30% of patients with idiopathic 
CDI,111 which is higher than the rate reported in, for example, 
type 1 diabetes mellitus.
CDI has been reported in a number of vascular and traumatic 
brain disorders. Approximately 15% of patients who have sus­
tained subarachnoid hemorrhage develop acute CDI,112 which 
nearly always resolves if the p

ute CDI,112 which 
nearly always resolves if the patient survives the initial insult. 
Very few survivors of subarachnoid hemorrhage have permanent 
CDI; however, some patients who have surgical clipping of ante­
rior communicating artery aneurysms can develop permanent 
adipsic CDI due to vascular damage to the perforating arteries 
of the anterior communicating artery, which provide vascular 
input to the anterior hypothalamus where the osmoreceptors are 
situated (see upcoming discussion). Pa

eptors are 
situated (see upcoming discussion). Patients who have moderate 
or severe traumatic brain injury (TBI) also develop CDI quite 
commonly in the immediate aftermath of injury: 15% to 20% 
develop CDI, and if close attention is not paid to fluid balance 
when the patient is obtunded or has cognitive decline, hyper­
natremia can develop. Hypernatremia and persistent CDI have 
been shown to be a poor prognostic indicator in acute TBI, often 
heralding a rise in intracerebral pressure as a

n 
heralding a rise in intracerebral pressure as a preterminal event. 
Patients who develop CDI secondary to TBI also occasionally 
go through the triphasic response of early polyuria, followed by 
transient hyponatremia due to unregulated AVP release, before 
gliosis produces permanent CDI. There may be a risk in TBI 
patients during the second phase of SIAD, as hyponatremia will 
produce cerebral edema, which leads to raised intracranial pres­
sure. It is important therefore to treat early CDI

sure. It is important therefore to treat early CDI with intermit­
tent dosing of parenteral desmopressin; the effect of one dose 
should be allowed to wane before administering another dose to 
ensure that the patient has not developed second phase SIAD. 
Patients with moderate/severe TBI can also develop anterior 
pituitary deficiency, including ACTH and cortisol deficiency, 
which can affect the ability to excrete water. Cortisol deficiency 
should therefore be considered if diabetes insipidus

ould therefore be considered if diabetes insipidus appears to 
resolve spontaneously with reversal of polyuria. Cortisol defi­
ciency alone decreases the ability to excrete water even in the 
absence of vasopressin.113 Lastly, in a long-term follow-up of 
these patients partial diabetes insipidus can occur,114,115 but 
there may be return of sufficient vasopressin function that under 
basal conditions the patient is no longer symptomatic from 
polyuria.113,114
Most patients with CDI have normal 

lyuria.113,114
Most patients with CDI have normal osmoregulated thirst; as 
a result, they do not develop hypernatremia unless they develop 
diminished consciousness or cognitive impairment or are in an 
environment where water supply is insufficient.116 However, 
damage to the osmoreceptors in the anterior hypothalamus117 
may lead to the dangerous combination of CDI and adipsia. 
Adipsic DI can be seen where surgical damage is so severe that 
the damage is not only to the neurohypophysis but a

he damage is not only to the neurohypophysis but also to the 
central anteriorly placed osmostat,117 such as following extensive 
surgery for large craniopharyngioma.76 Adipsic DI can also be 
caused by isolated damage to the hypothalamic osmoreceptors, 
with intact baroreceptors, as seen after surgery to an anterior 
communicating artery aneurysm.118 In the former case, there is 
no release of vasopressin in response to either osmotic or barore­
ceptor stimulation; in the latter case, however, 

ceptor stimulation; in the latter case, however, there is adequate 
synthesis of vasopressin with normal baroregulated but attenu­
ated osmotic release of AVP.119 Adipsic DI is sometimes associ­
ated with widespread damage to other parts of the hypothalamus 
and can be associated with other manifestations of hypothalamic 
syndrome, such as hyperphagia, sleep apnea, thermoregulation, 
seizures, and high mortality.117,120
Although rare, genetic abnormalities of the vasopressin gene 
are a recogni

normalities of the vasopressin gene 
are a recognized cause of CDI in childhood. Familial CDI is 
characterized by the onset of classic diabetes insipidus with poly­
dipsia and polyuria in childhood or as a young adult, but dur­
ing infancy they may be asymptomatic.121,122 This is in contrast 
to cases of familial nephrogenic diabetes insipidus in which the 
defect is expressed as a polyuric disease at birth (see later descrip­
tion). A rare type of familial CDI that can be present at birth 
is 

 of familial CDI that can be present at birth 
is in infants with homozygotes mutation of the AVP hormone 
region of the pre-prohormone. This can produce excretion of an 
inactive vasopressin but no difficulty in folding of the pre-pro­
hormone.123 In the usual familial CDI, MRI findings are vari­
able even within affected family members, but the most constant 
finding is the presence of a posterior pituitary bright spot in chil­
dren, which progressively disappears with time.124 The genetic 
de

ressively disappears with time.124 The genetic 
defect is usually in the biologically inactive neurophysin or in 
the signal peptide of the pre-prohormone. Although genetically 
heterozygous with the defect expressed in only one allele, the 
clinical phenotype is autosomal dominant. Lack of normal cleav­
age of the signal peptide from the prohormone and abnormal 
folding of the vasopressin/neurophysin precursor are thought 
to produce fibrillar aggregations in the endoplasmic reticulum, 
which i

ggregations in the endoplasmic reticulum, 
which is cytotoxic to the neuron, explaining the dominant phe­
notype.125,126 Autopsy studies have confirmed neuronal cell 
death.127 Genetic testing of asymptomatic children in affected 
families will negate the need for repeated dehydration testing 
and allow early treatment.128 Wolfram syndrome is a rare auto­
somal recessive disease with diabetes insipidus, diabetes mellitus, 
optic atrophy, and deafness (DIDMOAD). The genetic defect 
is for the pro

ness (DIDMOAD). The genetic defect 
is for the protein wolframin that is found in the endoplasmic 
reticulum and is important for folding proteins.129 Wolframin is 
localized to chromosome 4. It is involved in beta-cell prolifera­
tion and intracellular protein processing and calcium homeo­
stasis, producing a wide spectrum of endocrine and central 
nervous system (CNS) disorders. Diabetes insipidus is usually 
a late manifestation and is associated with decreased magnocel­
lular neurons in the 

ted with decreased magnocel­
lular neurons in the paraventricular and supraoptic nuclei.124,130
Diabetes insipidus is reported as a premorbid development 
in 50% to 90% of patients with brain death.131,132 This is most 
likely to represent a reaction to rising intracranial pressure. 
Although some aspects of hormonal treatment of organ donors 
311
are controversial, a consensus has emerged that treatment of dia­
betes insipidus should be standard in donors who dev

tes insipidus should be standard in donors who develop CDI.133 
Diabetes Insipidus Due to Accelerated Metabolism of 
Vasopressin (Diabetes Insipidus of Pregnancy)
The physiologic adaptations associated with normal pregnancy 
include expansion of blood volume and decreased plasma osmo­
lality and serum sodium. Although thirst and increased fluid 
intake are commonly reported in pregnancy, in some patients 
the increased thirst is driven by marked polyuria, which could 
be indicative of diabetes i

polyuria, which could 
be indicative of diabetes insipidus. There are two types of tran­
sient diabetes insipidus in pregnancy, both caused by the placental 
enzyme cysteine aminopeptidase, named oxytocinase, which enzy­
matically degrades oxytocin, which protects the fetal unit from 
early labor.134 Because of the close structural homology between 
AVP and oxytocin, this enzyme also metabolizes AVP. In the first 
type of pregnancy-associated DI, the activity of oxytocinase is 
abnormally elevat

 the activity of oxytocinase is 
abnormally elevated. This syndrome has been referred to as “vaso­
pressin resistant diabetes insipidus of pregnancy,”135 and has been 
reported with preeclampsia, acute fatty liver, and coagulopathies. 
Affected patients have decreased metabolism of vasopressinase by 
the liver.56,136–138 Characteristically, subsequent pregnancies are 
not complicated by diabetes insipidus or acute fatty liver. In the 
second type of pregnancy-associated DI, the accelerated metab

 of pregnancy-associated DI, the accelerated metabolic 
clearance of vasopressin produces diabetes insipidus in a patient 
with borderline vasopressin function from a specific disease (e.g., 
mild nephrogenic diabetes insipidus or partial hypothalamic/neu­
rohypophyseal diabetes insipidus).62,139,140 Vasopressin is rapidly 
destroyed and the neurohypophysis is unable to keep up with 
the increased demand. Labor and parturition usually proceed 
normally and patients have no trouble with lactation

rmally and patients have no trouble with lactation.141 There 
is the threat of chronic and severe dehydration when diabetes 
insipidus is unrecognized, and this can pose a threat to a pregnant 
woman.142 Patients with Sheehan syndrome have been reported to 
have asymptomatic partial diabetes insipidus143 but rarely develop 
overt diabetes insipidus.144 
Nephrogenic Diabetes Insipidus
Genetic variants of nephrogenic diabetes insipidus (NDI) usu­
ally present in infancy, with vomiting, failure to 

lly present in infancy, with vomiting, failure to thrive, and 
polyuria.145,146 NDI can occur as a result of mutations in the V2 
receptor and mutations of the aquaporin 2 water channels.145–147 
Over 90% of cases are X-linked recessive in males, and over 200 
different mutations of the V2 receptor have been reported.148 
Three general categories of V2 receptor mutations have been 
described148:
	1.	 Type 1, characterized by impaired AVP binding
	2.	 Type 2, typified by defective transport
	3.

	3.	 Type 3, where unstable receptors are rapidly degraded
Most of the reported cases are of type 2.149 Ten percent of 
the V2 receptor defects causing congenital nephrogenic diabetes 
insipidus occur de novo. The majority of female carriers of the 
X-linked V2 receptor mutation are asymptomatic, though careful 
physiologic testing reveals that some might have attenuated maxi­
mum urine osmolality in response to vasopressin.150 In rare cases 
where

n response to vasopressin.150 In rare cases 
where heterozygous females have a defect as severe as males, it is 
thought that there is coexisting inactivation of the function of the 
X chromosome.151,152
When the proband is a female it is likely the defect is a muta­
tion of the aquaporin 2 water channel gene due to an autosomal 
recessive disease.153 This should be especially considered where 
consanguinity is known in the family and the disease is expressed 
in males and females. The patients 

 is expressed 
in males and females. The patients can be heterozygous for two 
different recessive mutations154 or be homozygous for the same 
abnormality from both parents.155 Mutations of the aquaporin 2 
protein can produce an autosomal dominant nephrogenic diabe­
tes insipidus when the mutant aquaporin 2 protein associates with 
the wild-type normal protein to inhibit normal intracellular rout­
ing and function of the wild type.156
The development of NDI in an adult is less likely to reflect

pment of NDI in an adult is less likely to reflect 
a genetic cause. The commonest cause of acquired NDI in 
clinical practice is lithium therapy, with other causes (includ­
ing hypokalemia, hypercalcemia, and release of bilateral urinary 
tract obstruction) associated with downregulation of aquaporin 
2 and decreased function of vasopressin.153,156 Lithium produces 
a decrease in urea transporters, reducing vasopressin-stimulated 
urea uptake and decreasing urea recycling, which reduces inter­

d decreasing urea recycling, which reduces inter­
medullary osmolality.153,157 Even more dramatic is the reduction 
in aquaporin 2 levels to decrease water transport in the collecting 
duct.156 There is as much as a 95% decrease in aquaporin 2 con­
tent and even the 5% of aquaporin 2 that persists is not normally 
transported to the renal principal cell membrane.158 The defect of 
aquaporins with lithium is slow to correct both in experimental 
animals and in humans and can be permanent.156,159 

nimals and in humans and can be permanent.156,159 Demeclocy­
cline is another commonly recognized drug to cause nephrogenic 
diabetes insipidus and is sometimes used clinically to treat SIAD. 
A comprehensive review160 has documented the large number of 
drugs that can cause NDI. 
Approach to the Differential Diagnosis of 
Polyuric States
The first diagnostic step is to confirm polyuria, because up to 15% 
of patients referred for investigation of polyuria have urinary fre­
quency due to bladder

f polyuria have urinary fre­
quency due to bladder wall defects, infection, or prostate disease, 
with normal urine volume. A 24-hour urine volume greater than 
50 mL/kg body weight is worthy of further investigation. Diabe­
tes mellitus, hypercalcemia, hypokalemia, and chronic renal fail­
ure are excluded by biochemical tests. Urine osmolality should 
be low in all polyuric states; however, a random urine osmolality 
above 700 mOsm/kg excludes diabetes insipidus and makes the 
diagnosis of prim

iabetes insipidus and makes the 
diagnosis of primary polydipsia certain. Presenting serum sodium 
concentration is almost always normal in diabetes insipidus, but 
results at the higher part of the reference range are more suggestive 
of DI than primary polydipsia. However, it is difficult to confi­
dently distinguish between CDI, NDI, and thirst disorders solely 
on baseline laboratory measurements, so osmotic stimulation of 
the posterior pituitary is needed to secure a firm diagnosis.
The wa

itary is needed to secure a firm diagnosis.
The water deprivation test, which incorporates a dehydration 
step followed by a desmopressin challenge, is usually the first line 
of investigation. The interpretation of the initial period of water 
deprivation is based on the understanding that a functioning 
osmoregulatory system will respond to elevation of plasma osmo­
lality with the secretion of vasopressin and subsequent concentra­
tion of the urine. As the test progresses, urine volume decrea

urine. As the test progresses, urine volume decreases 
and urine osmolality rises usually to over 750 mOsm/kg H2O. 
Importantly, an adequate osmotic stimulus to AVP secretion is 
necessary; a plasma osmolality of over 295 mOsm/kg H2O is 
needed to stimulate sufficient AVP to maximally concentrate the 
urine. In primary polydipsia urine should concentrate relatively 
normally, as AVP secretion is normal, whereas in patients with 
either CDI or NDI, urine remains dilute at the end of dehydra­
tion

, urine remains dilute at the end of dehydra­
tion. The second part of the test, the administration of desmopres­
sin, is designed to differentiate CDI from NDI. As patients with 
312
CDI are deficient in AVP, exogenous desmopressin concentrates 
urine osmolality, whereas patients with NDI do not respond to 
the antidiuretic effects of desmopressin and do not concentrate 
urine appropriately. The water deprivation test is best performed 
in a specialized 

rivation test is best performed 
in a specialized center. Careful patient supervision to monitor for 
surreptitious drinking is important, and body weight is checked as 
patients with severe CDI can develop hypernatremia when fluid is 
withheld due to excess urination. In addition, there are subtleties 
of interpretation of results that must be considered. Although the 
water deprivation test differentiates between primary polydipsia, 
CDI, and NDI in classic and complete cases, published data h

DI in classic and complete cases, published data have 
shown that accurate diagnosis was made in only 70% of polyuric 
patients, and the correct diagnosis in primary polydipsia was con­
cluded in only 41% of cases.161 A number of confounding factors 
can produce misleading or uninterpretable results:
	1.	 Patients with prolonged severe polyuria may not concentrate 
urine in response to endogenous or exogenous vasopressin. 
Chronic hypoosmolality suppresses AVP secretion, and in the 
absence of 

 suppresses AVP secretion, and in the 
absence of V2 receptor stimulation intracellular aquaporin 2 is 
not generated. Patients with thirst disorders may therefore be 
classified as partial DI on the basis of a subnormal rise in urine 
osmolality during the water deprivation step, despite normal 
AVP responses. For the same reason, receptor stimulation with 
desmopressin in step 2 might not stimulate increased urine 
concentration in patients with CDI, leading to an erroneous 
diagnosis of NDI.

h CDI, leading to an erroneous 
	2.	 In patients with partial CDI, upregulation of V2 receptors can 
result in urine concentration with relatively low plasma AVP 
concentrations.
	3.	 High plasma AVP concentrations achieved by fluid deprivation 
in NDI can partially overcome renal resistance; urine osmolal­
ity can rise to over 300 mOsm/kg H2O, leading to diagnostic 
confusion with partial CDI.162
Several studies have demonstrated that the diagnostic accu­
racy of the water d

ated that the diagnostic accu­
racy of the water deprivation test was enhanced by the inclusion 
of measurement of plasma AVP concentration using a sensitive 
radioimmunoassay.163,164 However, AVP is unstable after veni­
puncture and requires careful sample handling, immediate centrif­
ugation, and storage at -70°C. In addition, commercially available 
AVP antibodies for use in clinical radioimmunoassays are poorly 
sensitive and have lower limits of detection for the hormone that 
exceeds low p

s of detection for the hormone that 
exceeds low physiologic plasma concentrations. Good-quality, 
sensitive and specific assays are available in a few specialized cen­
ters, but results are often only available weeks after testing has 
been performed. A recent analysis claimed a diagnostic accuracy of 
only 46% when direct AVP measurement was prospectively tested 
in 50 patients with polyuria,161 but the results may have reflected 
suboptimal performance of the AVP assay that was used. Alterna­

rformance of the AVP assay that was used. Alterna­
tively, measurement of the AVP responses during the intravenous 
infusion of hypertonic (3–5%) sodium chloride solution at the 
end of a water deprivation test enables more confident distinction 
between CDI and NDI or primary polydipsia.
Because the logistic difficulty of accurate and sensitive plasma 
AVP measurements make routine use of the assay impractical for 
most clinical laboratories, there has been much interest in the mea­
surement of

ere has been much interest in the mea­
surement of plasma copeptin, which offers a biologic surrogate for 
the measurement of plasma AVP. Copeptin is the C-terminal locus 
of the AVP precursor provasopressin, and it is co-secreted from 
the neurohypophysis in equimolar amounts with AVP in response 
to the same stimuli. It is much more stable ex vivo, so sample 
handling is straightforward. A sandwich immunoluminomet­
ric assay is used rather than a radioimmunoassay, which enables 
quick, reliabl

 a radioimmunoassay, which enables 
quick, reliable turnover of results. Careful studies have revealed 
that the copeptin responses to changes in plasma osmolality are 
similar to those seen with plasma AVP levels when measured dur­
ing water deprivation.165,166 In a prospective multicenter study, 
a postoperative copeptin concentration of less than 2.5 pmol/L 
had a post-transsphenoidal surgery predictive value of 81% for 
CDI, with a specificity of 97%. Conversely, high plasma copeptin 
concen

y of 97%. Conversely, high plasma copeptin 
concentrations effectively excluded the diagnosis of central DI; a 
plasma copeptin concentration over 30 pmol/L on day 1 follow­
ing transsphenoidal surgery was associated with a negative pre­
dictive value of 95%, with a sensitivity of 94%.167 In addition, 
a single copeptin concentration of over 21.4 pmol/L has been 
shown to differentiate NDI from other causes of polyuria, with a 
sensitivity and specificity of 100%, thus mitigating the need for 
w

ecificity of 100%, thus mitigating the need for 
water deprivation testing.168 Conversely, a baseline copeptin less 
than 2.6 pmol/L diagnoses complete CDI with 95% sensitivity 
and 100% specificity. Copeptin is therefore a promising marker 
of plasma AVP secretion, which offers a convenient, rapid, and 
sensitive diagnostic tool for polyuric states. The assay should be 
manageable in most clinical laboratories, and it is likely to replace 
measurement of plasma AVP in routine diagnostic practic

rement of plasma AVP in routine diagnostic practice.
Measurement of thirst, using a simple visual analogue scale, in 
osmotic studies has demonstrated that thirst onset occurs at the 
same osmotic threshold as AVP secretion52 and is rapidly switched 
off by drinking.169 The characteristics of osmoregulated thirst are 
surprisingly reproducible.170 When patients with CDI are stud­
ied during water deprivation testing or hypertonic saline infusion, 
they show a similar pattern of linear elevation 

they show a similar pattern of linear elevation during osmotic 
stimulation, and they show suppression after drinking.116
Measurement of thirst is valuable diagnostically in two clini­
cal situations. In adipsic DI, absent osmoregulated thirst during 
water deprivation or hypertonic saline infusion is the gold stan­
dard for diagnosis of the condition.116,117 Patients with primary 
polydipsia have three clearly defined abnormalities of thirst: (1) 
a low osmotic threshold for thirst, which is 

(1) 
a low osmotic threshold for thirst, which is disconnected from 
that of AVP release; (2) exaggerated thirst responses to elevation of 
plasma osmolality; and (3) failure to suppress thirst during drink­
ing after osmotic stimulation.171 Failure to suppress thirst after 
drinking by more than 50% of stimulated levels is the most use­
ful of these abnormalities, as it is a strong diagnostic indicator of 
primary polydipsia. 
Further Investigations of Diabetes Insipidus
Once a diagnosis of CDI

ions of Diabetes Insipidus
Once a diagnosis of CDI is established, MRI of the hypothalamo-
neurohypophyseal region is indicated. T1-weighted MRI images 
show a classic bright spot in the posterior pituitary12 caused by 
AVP stored in neurosecretory granules.29,172–175 The bright spot 
is present in over 80% of normal subjects176,177 and it is absent 
in most patients with central diabetes insipidus.178 Some patients 
with familial hypothalamic/neurohypophyseal diabetes insipidus 
can have a post

urohypophyseal diabetes insipidus 
can have a posterior pituitary bright spot early in the disease, when 
diabetes insipidus is partial; however, the bright spot disappears as 
AVP-secreting neurons diminish in numbers and AVP deficiency 
gets worse.179
The posterior pituitary bright spot decreases with a prolonged 
stimulus to vasopressin secretion180 and has been variably reported 
in other polyuric disorders. In primary polydipsia the bright spot 
usually is seen.174,181 In nephrogenic diabet

pot 
usually is seen.174,181 In nephrogenic diabetes insipidus the bright 
spot has been reported to be absent in some patients174 but pres­
ent in others.174,182 Patients with nephrogenic diabetes insipi­
dus have high levels of vasopressin in plasma and are chronically 
313
dehydrated, so the posterior pituitary might be depleted of vaso­
pressin stores. Similarly, with the osmotic stress of untreated dia­
betes mellitus or the transient diabetes insipidus of pr

mellitus or the transient diabetes insipidus of pregnancy the 
posterior pituitary can be depleted of vasopressin, and the bright 
spot is transiently lost only to reappear with resolution of the 
underlying condition.180,183
Imaging of the hypothalamus is also important to establish 
whether structural abnormalities of the neurohypophysis are 
responsible for CDI. Because 90% of the vasopressin neurons 
must be destroyed to produce symptomatic CDI,113,184 a mass 
lesion must either destroy a la

DI,113,184 a mass 
lesion must either destroy a large area of the hypothalamus or 
be located where the tracks of the supraoptic and paraventricu­
lar nuclei converge at the base of the hypothalamus, superior to 
the pituitary stalk. Pituitary adenomas confined within the sella 
do not cause diabetes insipidus.113 Diseases of the pituitary stalk 
commonly cause CDI and are listed in Table 10.1. When there is 
a diagnosis of CDI, thickening of the stalk is usually associated 
with absence of the 

 stalk is usually associated 
with absence of the posterior pituitary bright spot, and a search for 
systemic diseases is indicated.101 A thickened stalk with coexistent 
anterior pituitary deficiency is especially suggestive of etiologic 
systemic disease.185,186 A recent retrospective study has suggested 
that T2 DRIVE sequence images of the pituitary stalk are so reli­
able that the need for gadolinium enhancement is unnecessary.187
In the absence of structural causes of CDI on MRI, it is imp

ence of structural causes of CDI on MRI, it is impor­
tant to recognize that in some cases of germinoma the tumor may 
not be detectable on the initial scans. Therefore, particularly in 
children and young adults who are more vulnerable to germ cell 
tumors, markers such as beta human chorionic gonadotropin 
(βhCG) and alpha-fetoprotein (AFP) should be measured in the 
blood and in the cerebrospinal fluid if the index of clinical suspi­
cion is high enough. In addition, repeat MRI should be obta

igh enough. In addition, repeat MRI should be obtained 
every 3 to 6 months for the first 2 years of follow-up.185,186,188,189 
If follow-up imaging shows a decrease in size of the stalk, a likely 
diagnosis is infundibulo-neurohypophysitis.190 Increases in size 
often occur initially with infundibuloneurohypophysitis, but 
sustained increases over a 2-year period necessitate biopsy of the 
pituitary stalk.
Other investigations are dictated by clinical circumstances; if 
histiocytosis X is suspe

inical circumstances; if 
histiocytosis X is suspected, a radiologic skeletal survey should 
be performed. Sarcoidosis can be revealed by classic radiographic 
or computed tomography (CT) thorax changes or elevation of 
serum angiotensin-converting enzyme activity. When MRI of 
the pituitary is normal, autoimmune variants of CDI should be 
considered; studies of patients classified as “idiopathic” CDI have 
revealed the presence of antibodies to AVP-neurons and a high 
incidence of related autoi

AVP-neurons and a high 
incidence of related autoimmune conditions, most commonly 
thyroid disease.111 
Treatment of Polyuric Conditions
Central Diabetes Insipidus in Ambulatory Patients
Treatment of CDI depends on regulation of water intake and 
excretion. Because thirst is normal in CDI, water intake self-reg­
ulates. Replacement of absent AVP is generally done with des­
mopressin,191,192 a synthetic analog of vasopressin in which the 
substitution of d-arginine markedly reduces pressor activi

tion of d-arginine markedly reduces pressor activity and 
removing the terminal amine increases the half-life (see Fig. 10.1). 
These two changes produce an agent nearly 2000 times more spe­
cific for antidiuresis than naturally occurring l-arginine vasopres­
sin.193 Desmopressin is available as tablets for oral administration, 
a lyophilate for sublingual administration (oral melt), a solution 
for intranasal administration, and a solution for parenteral use.194 
Most patients prefer desmopress

arenteral use.194 
Most patients prefer desmopressin tablets (0.1 and 0.2 mg); the 
oral preparations are effective, and unlike nasal sprays absorption 
is not impaired with nasal infections. However, intestinal pepti­
dase activity can result in increased degradation before absorption, 
so the tablets should be taken 1 hour before or 2 hours after meals. 
Desmopressin melt (60, 120, and 240 μg) is reported to be more 
acceptable in some children.195 As the degree of AVP deficiency is 
variable 

.195 As the degree of AVP deficiency is 
variable among patients, the dosage and frequency of desmopres­
sin are also highly variable.196–198 Some patients with partial CDI 
need a single dose before bed to prevent nocturia, whereas some 
patients with complete AVP deficiency need desmopressin two to 
four times daily. The total duration of action of desmopressin will 
usually be 6 to 18 hours depending on the route of administration.
When a dose of desmopressin is sufficient to elicit a stable 

 of desmopressin is sufficient to elicit a stable ther­
apeutic response, further increasing (e.g., doubling) the dose pro­
duces a moderate increase in duration of only a few hours,196,197 
consistent with the half-life of desmopressin in plasma.196 The 
maximum dose rarely exceeds 0.2 mg orally or 20 μg intrana­
sally (two sprays) given two or three times a day (usually three 
for tablets and two for intranasal).197 For greater flexibility with 
intranasal administration, the patient can be ta

intranasal administration, the patient can be taught to use the 
rhinal catheter.192 Parenterally administered desmopressin (2-mL 
vials of 4 μg/mL) is only needed for ambulatory patients during 
an intercurrent illness.197 Parenterally administered desmopres­
sin gives virtually identical therapeutic response when given as an 
intravenous bolus, intramuscularly or subcutaneously,197 and the 
parenteral administration is 5 to 20 times as potent as an intrana­
sally administrated dose.192,197 P

 as an intrana­
sally administrated dose.192,197 Published studies have reported an 
increased antidiuretic response to desmopressin in women and in 
the elderly.199
Hyponatremia is a common complication of desmopressin 
therapy. Mild hyponatremia (plasma sodium 131–134 mmol/L) 
occurs in 27% of ambulatory blood samples in patients with 
intact thirst, and in 15% of blood samples serum sodium is less 
than 130 mmol/L.80 This reflects the tendency in humans to drink 
for pleasure or to drink soci

 in humans to drink 
for pleasure or to drink socially; as desmopressin produces a con­
tinuous antidiuretic effect, the usual physiologic suppression of 
endogenous AVP that occurs during drinking does not occur, with 
consequent dilutional hyponatremia. The hyponatremia seen due 
to desmopressin is severe enough to precipitate hospital admis­
sion in 6% of patients.80 Hyponatremia is particularly an issue 
in infants, who consume a large part of their calories as liquid 
formula or breast milk

f their calories as liquid 
formula or breast milk, and in whom treatment with desmopres­
sin can cause erratic serum sodium consequences and the risk of 
symptomatic hyponatremia.200 The risk of dilutional hypona­
tremia can be reduced by any one of three desmopressin dosing 
schedules:
	1.	 Omitting a full dose once a week to allow an aquaresis to occur; 
this is effective, but unpleasant for the patient
	2.	 Delaying a desmopressin dose once or twice weekly until the 
patient urinates two o

 or twice weekly until the 
patient urinates two or three times
	3.	 Delaying each dose of desmopressin until the patient begins to 
urinate
In practice, individual patients vary as to which of these meth­
ods they prefer.
Hypernatremia is much less common in ambulant patients, 
as the intact thirst mechanism ensures that fluid intake is 
sufficient to supply physiologic needs; only 1% of ambulant 
plasma sodium concentrations are above the normal reference 
range.80 In patients with adipsic DI

al reference 
range.80 In patients with adipsic DI, ambulatory hypernatre­
mia is much more common, occurring in up to 20% of blood 
samples. 
314
Central Diabetes Insipidus in Hospitalized Patients
Although hypernatremia is rare in ambulatory patients with nor­
mal thirst, the incidence is much higher during intercurrent ill­
ness, particularly if the patient is vomiting, and unable to absorb 
oral desmopressin. As a result, close attention must be paid 

ressin. As a result, close attention must be paid to fluid 
balance when patients are admitted as emergencies. If the patient 
develops severe hypernatremic dehydration, consideration should 
be given to anticoagulation to prevent thrombotic complications. 
Equally, if a patient is on intravenous fluids, regular electrolyte 
estimations are necessary to ensure that dilutional hyponatremia 
does not develop, particularly if hypotonic fluids such as dextrose 
solutions are used. In patients with a

s dextrose 
solutions are used. In patients with abnormal thirst, hypernatre­
mia and hyponatremia are both much more common, and the 
need for careful monitoring of electrolytes and fluid balance is 
more crucial.80 
Central Diabetes Insipidus in Neurosurgical Patients
Transient polyuria due to CDI is not uncommon after pituitary 
surgery; it is more common after extensive surgery for larger pitu­
itary adenomas or for craniopharyngioma. The diagnosis is made 
by the presence of the classic tri

gnosis is made 
by the presence of the classic triad of polyuria, hypernatremia, and 
dilute urine after exclusion of other possibilities such as diabetes 
mellitus and mannitol therapy. Sometimes diuresis after surgery 
is the result of water retention during the procedure. Vasopressin 
is released during surgical procedures and administered fluid is 
retained. When the stress of surgery abates, the vasopressin level 
falls and retained fluid is excreted. If an attempt is made to match 
the uri

 excreted. If an attempt is made to match 
the urine output with further fluid infusion, persistent polyuria 
might be mistaken for diabetes insipidus. If in doubt, fluid should 
be withheld until there is a modest increase in serum sodium. If 
the urine output decreases and the serum sodium remains nor­
mal, the polyuria was due to excretion of physiologically retained 
fluid. If the serum sodium begins to rise while urine osmolality is 
low and there is a positive response to administered ­des

 there is a positive response to administered ­desmopressin, 
the diagnosis of diabetes insipidus can be established.201 If the 
duration of diabetes insipidus is quite transient, it is acceptable 
to treat this simply with fluid replacement, parenterally or orally. 
However, most patients who develop diabetes insipidus require 
desmopressin 0.5 to 2 μg subcutaneously, intramuscularly, or 
intravenously. Urine output will be reduced in 1 to 2 hours and 
the duration of effect is 6 to 24 hours. C

rs and 
the duration of effect is 6 to 24 hours. Care should be taken that 
hypotonic intravenous fluids are not given excessively after admin­
istering desmopressin, as the combination can lead to profound 
hyponatremia.202 Because there is always the possibility of devel­
oping the triphasic response due to pituitary stalk damage, it is 
recommended that polyuria should be recurrent before a decision 
to administer subsequent doses of desmopressin is made.203
Acute diabetes insipidus after tra

sin is made.203
Acute diabetes insipidus after traumatic brain injury can be 
treated similarly to that recommended for patients following pitu­
itary surgery; as the patient with head injury is more likely to be 
comatose or cognitively impaired, and thus unable to depend on 
thirst, hypernatremia is more likely to develop. Because a coma­
tose patient must be given fluids parenterally, it is important to 
monitor serum sodium concentration regularly to check for dilu­
tional hyponatremia. Pers

larly to check for dilu­
tional hyponatremia. Persistent diabetes insipidus has been dem­
onstrated in prospective studies to be a predictor of fatal outcome 
after traumatic brain injury, perhaps because it is a manifestation 
of rising intracranial pressure and imminent herniation.204 
Adipsic Diabetes Insipidus
The combination of attenuated thirst and diabetes insipidus con­
fers high risk of severe hypernatremia. As well, when treated with 
desmopressin they are at risk for developing hypona

esmopressin they are at risk for developing hyponatremia. A care­
fully monitored regimen of a fixed dose of desmopressin to main­
tain chronic antidiuresis and a prescribed volume of fluid intake 
is preferable.205,206 Daily weight can be used to guide intake, and 
regular follow-up with measurement of serum sodium is essential 
to ensure that these patients do not develop water intoxication 
with hyponatremia or recurrent dehydration with hypernatremia. 
Patients with adipsic DI have been repo

natremia. 
Patients with adipsic DI have been reported to develop throm­
botic complications when severely dehydrated, and they should 
be prophylactically anticoagulated when admitted with severe 
intercurrent illness. Although management of water balance is the 
clinical priority for endocrinologists, it is important to holistically 
approach the hypothalamic complication associated with adipsic 
DI. Sleep apnea can respond to treatment with continuous positive 
airway pressure (CPAP) or modaf

ntinuous positive 
airway pressure (CPAP) or modafinil, and treatment for associated 
hypothalamic obesity is also important.117 This group of patients 
has high mortality rates and requires very careful management. 
Treatment of Diabetes Insipidus in Pregnancy
Desmopressin is the only therapy recommended for treatment 
of diabetes insipidus during pregnancy. Desmopressin has 2% to 
25% the oxytocic activity of lysine vasopressin or arginine vaso­
pressin193 and can be used with minimal stimulat

­
pressin193 and can be used with minimal stimulation of the oxy­
tocin receptors in the uterus.141,207 Desmopressin is not destroyed 
by the cysteine aminopeptidase (oxytocinase) of pregnancy141,208 
and is reported to be safe for both the mother and the child.209,210 
During delivery, patients can maintain adequate oral intake and 
are therefore safe to continue administration of desmopressin. 
Physicians should be cautious about overadministration of fluid 
parenterally during delivery becaus

tion of fluid 
parenterally during delivery because these patients will not be 
able to excrete the fluid and can develop water intoxication and 
hyponatremia. After delivery, plasma oxytocinase decreases and 
patients can recover completely or be asymptomatic with regard 
to fluid intake and urine excretion. 
Treatment of Nephrogenic Diabetes Insipidus
Adequate water intake is the mainstay of treatment of NDI as phar­
macologic reversal of the renal resistance to AVP is rarely possible. 
Nephro

enal resistance to AVP is rarely possible. 
Nephrogenic diabetes insipidus does not respond to desmopressin 
therapy, although occasionally partial defects have limited response 
to high doses of desmopressin.211,212 In congenital NDI, therapy 
aimed at reducing symptomatic polyuria is addressed primarily by 
inducing plasma volume contraction via a low sodium diet and 
a thiazide diuretic. It has been hypothesized that contraction of 
extracellular fluid volume, decreased glomerular filtration 

lar fluid volume, decreased glomerular filtration rate, 
proximal tubular sodium and water reabsorption, and decreased 
delivery of fluid to the collecting duct result in a decreased volume 
of urine.213 Studies have also demonstrated that thiazide diuret­
ics can increase aquaporin 2 independent of vasopressin.214 All the 
thiazide diuretics appear to have similar effects. Potassium replace­
ment and/or coadministration of a potassium-sparing antidiuretic 
may be necessary, as hypokalemia can w

tidiuretic 
may be necessary, as hypokalemia can worsen renal resistance to 
AVP. The effect of thiazides can be augmented by coadministration 
of nonsteroidal anti-inflammatory drugs (NSAIDs), but this com­
bination is nephrotoxic, and careful monitoring of renal function 
is mandatory. Selective cyclooxygenase 2 (COX2) inhibitors with 
less gastrointestinal (GI) effect have been reported to decrease water 
loss, but long-term safety has not been documented.156
Drug-induced nephrogenic diabetes

n documented.156
Drug-induced nephrogenic diabetes insipidus should be 
treated by stopping the offending agent if possible. Persistence 
315
of nephrogenic diabetes insipidus can be treated by hydrochloro­
thiazide and amiloride. Plasma volume contraction produced by 
thiazide diuretics can decrease lithium excretion and predispose 
to lithium toxicity.160,215 The diuretic amiloride blocks sodium 
channels in the luminal membrane of the collecting duct cells 
and

luminal membrane of the collecting duct cells 
and inhibits lithium reabsorption, a unique advantage in treat­
ing lithium-induced nephrogenic diabetes insipidus.216 In animal 
studies of lithium-induced nephrogenic diabetes insipidus, treat­
ment with amiloride increased levels of both aquaporin 2 and urea 
transporters.157
Studies have reported the possibility of rescuing mutant recep­
tors in NDI. In autosomal dominant type 2 NDI, the misfolded 
receptor protein is trapped in the quality cont

d 
receptor protein is trapped in the quality control system of the 
endoplasmic reticulum, but in some cases the defect involves 
transport of the receptor, and were the receptor to reach the cell 
membrane it would respond to vasopressin. V2 receptor antago­
nists (vaptans) have been reported to act as pharmacologic chap­
erones that combine with the misfolded receptor, changing the 
confirmation to allow maturation and transport to the plasma 
membrane where vasopressin (in excess of the vapt

membrane where vasopressin (in excess of the vaptans) would 
cause the receptor to be activated.217–219 Similar studies of rescue 
have recently been reported with nonpeptide V2 receptor agonists. 
These agonists combine with the mutant receptor trapped in the 
endoplasmic reticulum and allow maturation of the mutant recep­
tor. The rescued receptor is then inserted into the cell membrane 
and when stimulated by vasopressin or desmopressin generates 
sufficient cAMP to move aquaporin 2 from the

ates 
sufficient cAMP to move aquaporin 2 from the cytoplasm to the 
cell membrane to enhance water transport.218,219 Nonpeptide 
antagonists working as chaperones is a potential new treatment of 
nephrogenic diabetes insipidus, especially in patients with a partial 
disorder.220 Sequencing of genes in all families with NDI is rec­
ommended because of the small size of the genes to be sequenced 
and because of the value of the information.149 In X-linked disor­
ders, carrier females can be disti

X-linked disor­
ders, carrier females can be distinguished from noncarrier females, 
so it will be known which sibling’s children are at risk and require 
special observation at birth. Molecular testing of newborns will 
confirm the need for long-term treatment to avoid complications 
in the affected children and obviate the need for water deprivation 
or other testing in unaffected children.152,221
Treatment of Primary Polydipsia
Treatment of primary polydipsia entails reduction of excessive 
f

imary polydipsia entails reduction of excessive 
fluid intakes, best done in a graded fashion to allow patients to 
slowly achieve a level of intake that reduced urine volume below 
polyuric levels (50 mL/kg BW). Measures to reduce mouth dry­
ness (e.g., ice chips, hard candy to stimulate salivary flow) are use­
ful adjuncts to reduce thirst. Pharmacologic therapies have been 
tried but without consistent evidence of success. 
Diabetes Insipidus in Association With Other 
Therapeutic Decisions
R

in Association With Other 
Routine Surgical Procedures
In all cases there should be preoperative consultation of the sur­
geon, the anesthesiologist, and the endocrinologist/nephrologist. 
For most routine surgical procedures the patient is not uncon­
scious for a sufficiently long period of time to require anything 
more that administration of the usual dose of desmopressin and 
careful monitoring of fluids during the surgery to prevent overhy­
dration. If the patient is t

ry to prevent overhy­
dration. If the patient is taking desmopressin orally and is NPO, 
a parenteral dose can be administered before the procedure. Close 
monitoring of serum sodium is essential. In NDI there should be 
a greater emphasis on fluid replacement to avoid dehydration and 
hypernatremia.222 
Panhypopituitarism
Patients with coexistent diabetes and anterior pituitary deficiency 
are at risk of developing hyponatremia, as cortisol deficiency is 
associated with impaired water excretio

ciency is 
associated with impaired water excretion. Patients who miss their 
hydrocortisone doses are vulnerable to desmopressin-induced 
hyponatremia. 
Hypertonic Encephalopathy
Hypertonic encephalopathy is uncommon in diabetes insipidus 
and is only seen when there is inadequate fluid intake in an adip­
sic patient or in a patient who is unconscious and not receiving 
adequate fluid supplementation. Conditions other than diabetes 
insipidus are more common causes of hypernatremic encephalop­

e more common causes of hypernatremic encephalop­
athy. The condition is associated with loss of hypotonic fluids by 
the kidney or the intestines, or from insensible losses; or, it might 
occur following administration of hypertonic sodium containing 
fluids or hyperalimentation.223 Hypernatremia leads to osmotic 
movement of water out of cells and cellular dehydration. Stud­
ies indicate that in the brain idiogenic osmoles, mainly polyols, 
trimethylamines, and amino acids, are generated intra

methylamines, and amino acids, are generated intracellularly so 
the degree of cell shrinkage is less than would occur based on the 
degree of hypernatremia.224 Loss of water from the brain occurs 
in minutes and electrolytes enter the brain in a few hours, but 
the increase in organic osmoles occurs over several days.225 When 
fluid is replaced, intracellular organic osmoles dissipate more 
slowly than the decrease in osmolality of extracellular fluid. This 
asynchrony increases the potential f

 fluid. This 
asynchrony increases the potential for cerebral edema and wors­
ening of the neurologic condition with overzealous treatment of 
hypernatremia.225 In most cases of diabetes insipidus seen follow­
ing neurosurgery or head injury the diagnosis will be made within 
a few hours and therapy should be instituted promptly. In cases 
where the duration of the hypernatremia is not known, the degree 
of correction of hypernatremia should not exceed 0.5 mEq/L/
hour to prevent cerebral edema a

exceed 0.5 mEq/L/
hour to prevent cerebral edema and convulsions.223,225 
The Syndrome of Inappropriate Antidiuresis
The SIAD is produced when plasma levels of vasopressin are ele­
vated at times when the physiologic secretion of vasopressin from 
the posterior pituitary would normally be osmotically suppressed. 
The clinical abnormality is a decrease in the osmotic pressure of 
body fluids, so the hallmark of SIAD is hypoosmolality. This led 
to the identification of the first well-described ca

 the identification of the first well-described cases of this dis­
order in 1957226 and the subsequent clinical investigations that 
resulted in delineation of the essential characteristics of the syn­
drome.227 Although initially called the syndrome of inappropriate 
antidiuretic hormone (SIADH) secretion, the term SIAD is more 
appropriate in view of the inability to measure elevated vasopres­
sin levels in some patients with this disorder. It is necessary to first 
review hypoosmolality and h

is necessary to first 
review hypoosmolality and hyponatremia before discussing details 
that are specific to SIAD.
Hypoosmolality and Hyponatremia
Incidence
Hypoosmolality is the most common disorder of fluid and 
electrolyte balance encountered in hospitalized patients. The 
incidence and prevalence of hypoosmolar disorders depend on 
the nature of the patient population studied as well as on the 
316
laboratory methods and criteria used to diagnose hyp

boratory methods and criteria used to diagnose hyponatremia. 
Most investigators have used the serum sodium concentration 
([Na+]) to determine the clinical incidence of hypoosmolality. 
When hyponatremia is defined as a serum [Na+] of less than 
135 mEq/L, prevalences as high as 15% to 38% have been 
observed in studies of both acutely and chronically hospital­
ized patients.228,229 However, incidences decrease to the range 
of 1% to 4% when only patients with serum [Na+] under 130 to 
131 mEq/

y patients with serum [Na+] under 130 to 
131 mEq/L are included, which represents a more appropriate 
level at which to define the occurrence of clinically significant 
cases of this disorder.230 Even using these more stringent criteria, 
incidences from 7% to 53% have been reported in institution­
alized geriatric patients.231 Although hyponatremia and hypoos­
molality are quite common, the majority of cases are relatively 
mild and most are acquired during the course of hospitaliza­
tion. Non

quired during the course of hospitaliza­
tion. Nonetheless, hyponatremia is important clinically because 
(1) severe hypoosmolality (serum [Na+] levels <120 mEq/L) is 
associated with substantial morbidity and mortality,232 (2) even 
relatively mild hypoosmolality can quickly progress to more 
dangerous levels during the course of therapeutic management 
of other disorders, (3) overly rapid correction of hyponatremia 
can itself cause severe neurologic morbidity and mortality,233 
and (4) it has

ologic morbidity and mortality,233 
and (4) it has been observed that mortality rates are from 3-fold 
to 60-fold higher in patients with even asymptomatic degrees 
of hypoosmolality compared to normonatremic patients.234,235 
Osmolality, Tonicity, and Serum [Na+]
As discussed, the osmolality of body fluid normally is maintained 
within narrow limits for each individual by osmotically regulated 
vasopressin secretion and thirst. Plasma osmolality can be deter­
mined directly by measuring the fre

 can be deter­
mined directly by measuring the freezing-point depression or 
the vapor pressure of plasma. Alternatively, it can be calculated 
indirectly from the concentrations of the three major solutes in 
plasma:
pOsm
mOsm/kg H2O = 2 × [Na + ] (mEq/L)
+ glucose (mg/dL) / 18 + blood urea
nitrogen (mg/dL) / 2.8
Direct measure and calculation produce comparable results 
under most conditions. However, while either of these meth­
ods produce valid measures of total osmolality, this is not alway

id measures of total osmolality, this is not always 
equivalent to the effective osmolality, which is commonly referred 
to as the “tonicity” of the plasma. Only solutes such as Na+ and Cl- 
that are impermeable to the cell membrane and remain relatively 
compartmentalized within the extracellular fluid (ECF) space are 
effective solutes, because these solutes create osmotic gradients 
across cell membranes and regulate the osmotic movement of 
water between the intracellular fluid (ICF) compart

ater between the intracellular fluid (ICF) compartment and the 
ECF compartment. Solutes that readily permeate cell membranes 
(e.g., urea, ethanol, and methanol) are not effective solutes. There­
fore only the concentrations of effective solutes in plasma should 
be used to ascertain whether clinically significant hyperosmolality 
or hypoosmolality is present.
Sodium and its accompanying anions represent the major 
effective plasma solutes, so hyponatremia and hypoosmolality 
are usually synony

yponatremia and hypoosmolality 
are usually synonymous. However, there are two situations in 
which hyponatremia will not reflect true hypoosmolality. The 
first is pseudohyponatremia, which is produced by marked eleva­
tions of either lipids or proteins in plasma. If serum [Na+] is 
measured by flame photometry, the concentration of sodium 
per liter of plasma is artifactually decreased because of the larger 
relative proportion of plasma volume that is occupied by the 
excess lipids or protein

 that is occupied by the 
excess lipids or proteins.236 However, the increased protein or 
lipid will not appreciably change the total number of solute par­
ticles in solution, so the directly measured plasma osmolality will 
not be significantly affected. Measurement of serum [Na+] by 
ion-specific electrodes, which is now commonly used by most 
clinical laboratories, is less influenced by high concentrations 
of lipids or proteins than is measurement of serum [Na+] by 
flame photometry. Howeve

rement of serum [Na+] by 
flame photometry. However, this can still occur if the electrode 
measurement is done using a diluted sample of the serum. The 
second situation in which hyponatremia does not reflect true 
plasma hypoosmolality occurs when high concentrations of 
effective solutes other than Na+ are present in the plasma. The 
initial hyperosmolality produced by the additional solute causes 
an osmotic shift of water from the ICF to the ECF, which in turn 
produces a dilutional decreas

 ECF, which in turn 
produces a dilutional decrease in serum [Na+]. Once equilibrium 
between both fluid compartments is achieved, the total effective 
osmolality remains relatively unchanged. This situation most 
commonly occurs with hyperglycemia and represents a frequent 
cause of hyponatremia in hospitalized patients, accounting for 
up to 10% to 20% of all cases.234 Misdiagnosis of true hypoos­
molality in such cases can be avoided by measuring plasma 
osmolality directly or, alternatively,

ing plasma 
osmolality directly or, alternatively, by correcting the measured 
serum [Na+] for the glucose elevation. Traditionally this correc­
tion factor has been 1.6 mEq/L for each 100-mg/dL increase in 
serum glucose concentration above normal levels,237 but some 
studies have shown a more complex relation between hyper­
glycemia and serum [Na+] and reported that a more accurate 
correction factor is closer to 2.4 mEq/L.238 When the plasma 
contains significant amounts of unmeasured solutes

contains significant amounts of unmeasured solutes, such as 
osmotic diuretics, radiographic contrast agents, and some toxins 
(ethanol, methanol, and ethylene glycol), plasma osmolality can­
not be calculated accurately, and in these situations osmolality 
must be ascertained by direct measurement. 
Pathogenesis of Hypoosmolality
Water moves freely between the ICF and ECF, consequently 
osmolality will always be equivalent in both of these fluid com­
partments. Since the bulk of body solute is 

 com­
partments. Since the bulk of body solute is comprised of elec­
trolytes, namely the exchangeable Na+ (Na+E) in the ECF and 
the exchangeable K+ (K+E) in the ICF along with their associated 
anions, total body osmolality (OSMT) will largely be a function of 
these parameters239:
OSMT = OSMECF = OSMICF
OSMT = (ECF solute + ICF solute) / body water
OSMT (2 × Na +
E
+ 2 × K +
E
+ nonelectrolyte solute) / body water
According to this definition, the spresence of plasma hypoos­
molality indicate

 the spresence of plasma hypoos­
molality indicates a relative excess of water to solute in the ECF. 
This can be produced either by an excess of body water, resulting 
in a dilution of remaining body solute, or by a depletion of body 
solute, either Na+ or K+, relative to body water. This classification 
is an oversimplification, because most hypoosmolar states involve 
significant components of both solute depletion and water reten­
tion. Nonetheless, it is conceptually useful for understandin

heless, it is conceptually useful for understanding the 
mechanisms underlying the pathogenesis of hypoosmolality and 
as a framework for therapy of hypoosmolar disorders.
317
Solute Depletion
Depletion of body solute can result from any significant losses of 
ECF. Body fluid losses by themselves rarely cause hypoosmolality 
because excreted or secreted body fluids are usually isotonic or 
hypotonic relative to plasma and therefore tend to increase plasma 
osmolal

sma and therefore tend to increase plasma 
osmolality. When hypoosmolality accompanies ECF losses, it is 
the result of replacement of body fluid losses by more hypotonic 
solutions either by drinking or by infusion, thereby diluting the 
remaining body solutes. If the solute losses are marked, these 
patients show signs of volume depletion (e.g., addisonian crisis). 
However, such patients often have a more deceptive clinical pre­
sentation because the volume deficits have been partially replac

use the volume deficits have been partially replaced. 
Moreover, they may not manifest signs or symptoms of cellular 
dehydration because osmotic gradients will draw water into the 
ICF, which is relatively hypertonic to the solute-depleted ECF. 
Therefore clinical evidence of hypovolemia strongly supports sol­
ute depletion as the cause of plasma hypoosmolality, but absence 
of clinically evident hypovolemia never completely eliminates this 
as a possibility. Although ECF solute losses are resp

a possibility. Although ECF solute losses are responsible for 
most cases of depletion-induced hypoosmolality, ICF solute loss 
can also cause hypoosmolality as a result of osmotic water shifts 
from the ICF into the ECF. This mechanism contributes to some 
cases of diuretic-induced hypoosmolality in which depletion of 
total body K+ often occurs.240 
Water Retention
Despite the importance of solute depletion in some patients, 
most cases of clinically significant hypoosmolality are caused by 
i

ically significant hypoosmolality are caused by 
increases in total body water rather than by primary losses of 
extracellular solute. This can occur because of either impaired 
renal free water excretion or excessive free water intake. The for­
mer accounts for most hypoosmolar disorders because normal 
kidneys have sufficient diluting capacity to allow excretion of 18 
to 24 L/day of free water. Intakes of this magnitude are occasion­
ally seen in some psychiatric patients but not in most pati

 in some psychiatric patients but not in most patients 
with SIAD in whom fluid intakes average 2 to 3 L/day.241 Con­
sequently, dilutional hypoosmolality usually is the result of an 
abnormality of renal free water excretion. The renal mechanisms 
responsible for impairments in free water excretion can be sub­
grouped according to whether the major impairment in free 
water excretion occurs in proximal or distal parts of the nephron, 
or both. Any disorder that leads to a decrease in glomerular

ny disorder that leads to a decrease in glomerular filtra­
tion rate causes increased reabsorption of both Na+ and water in 
the proximal tubule. As a result, the ability to excrete free water 
is limited because of decreased delivery of tubular fluid to the 
distal nephron. Disorders that cause a decreased glomerular fil­
tration rate in the absence of significant ECF fluid losses are, for 
the most part, edema-forming states associated with decreased 
effective arterial blood volume (EABV) and

reased 
effective arterial blood volume (EABV) and secondary hyperal­
dosteronism.242 Even though these conditions are characterized 
by increased proximal reabsorption of both Na+ and fluid, water 
retention also results from increased distal reabsorption caused 
by nonosmotic baroreceptor-mediated stimulated increases in 
plasma vasopressin levels. Distal nephron impairments in free 
water excretion are characterized by an inability to dilute tubu­
lar fluid maximally. These disorders are usua

ubu­
lar fluid maximally. These disorders are usually associated with 
abnormalities in the secretion of vasopressin. Just as depletion-
induced hypoosmolar disorders usually include an important 
component of secondary impairments of free water excretion, 
most dilution-induced hypoosmolar disorders also involve sig­
nificant degrees of secondary solute depletion. This is described 
later with SIAD.
Some dilutional disorders do not fit well into either category, 
specifically the hyponatremia t

 either category, 
specifically the hyponatremia that sometimes occurs in patients 
who ingest large volumes of beer with little food intake for pro­
longed periods, called “beer potomania.”243 Even though the 
volume of fluid ingested may not seem sufficiently excessive to 
overwhelm renal diluting mechanisms, free water excretion is lim­
ited by very low urinary solute excretion, thereby causing water 
retention and dilutional hyponatremia. 
Adaptation to Hyponatremia: ICF and ECF Volume 
Regu

aptation to Hyponatremia: ICF and ECF Volume 
Many past studies have suggested that the combined effects of water 
retention plus urinary solute excretion cannot adequately explain 
the degree of plasma hypoosmolality observed in patients.227,244 
This observation led to the theory of cellular inactivation of solute, 
which suggested that as ECF osmolality falls, water moves into 
cells along osmotic gradients, thereby causing the cells to swell; 
at some point during this volume expa

s to swell; 
at some point during this volume expansion, the cells theoreti­
cally osmotically “inactivate” some of their intracellular solutes as 
a defense mechanism to prevent continued cellular swelling with 
subsequent detrimental effects on cell function and survival. This 
effect would decrease the intracellular osmolality allowing water to 
shift back out of the ICF into the ECF, thereby further worsening 
the dilution-induced hypoosmolality. Despite the appeal of this 
theory, its valid

ity. Despite the appeal of this 
theory, its validity has never been demonstrated conclusively in 
either human or animal studies. An alternative theory is that cell 
volume is maintained under hypoosmolar conditions by extrusion 
of intracellular solutes such as potassium.245 Whole brain volume 
regulation via electrolyte losses was first described by Yannet246 
and has long been recognized as the mechanism by which the 
brain is able to adapt to hyponatremia and limit brain edema to 
sublethal

o hyponatremia and limit brain edema to 
sublethal levels.247 Following the recognition that low molecular 
weight organic compounds, called organic osmolytes, also consti­
tuted a significant osmotic component of a wide variety of cells, 
studies demonstrated the accumulation of these compounds in 
response to hyperosmolality in both kidney248 and brain249 tis­
sue and conversely that the brain also loses organic osmolytes in 
addition to electrolytes during volume regulation to hypoosmolar 
co

olytes during volume regulation to hypoosmolar 
conditions in experimental animals250,251 and human patients.252 
These losses occur relatively quickly (within 24–48 hours in rats) 
and can account for as much as one-third of the brain solute losses 
during hyponatremia.253 Such coordinate losses of both electro­
lytes and organic osmolytes from brain cells allow effective regula­
tion of brain volume during chronic hyponatremia.
Although recent studies of volume regulation during hypo­
natremia

studies of volume regulation during hypo­
natremia have focused on the brain, all cells regulate volume by 
cellular losses of both electrolyte and organic solutes to varying 
degrees. However, volume regulatory processes are not limited to 
cells. In most cases of hyponatremia induced by stimulated antidi­
uresis and water retention, natriuresis also regulates the volumes of 
the ECF and intravascular spaces. Both experimental and clinical 
observations are consistent with ECF volume regulation

rvations are consistent with ECF volume regulation via sec­
ondary solute losses. First, the concentrations of most blood con­
stituents other than Na+ and Cl- are not decreased in patients with 
SIAD,254 suggesting that plasma volume is not nearly as expanded 
as would be predicted simply by the measured decreases in serum 
[Na+]. Second, an increased incidence of hypertension has never 
been observed in patients with SIAD, again evidence against sig­
nificant expansion of the arterial blood vo

t sig­
nificant expansion of the arterial blood volume. Third, results of 
animal studies in both dogs255 and rats256 have indicated that a 
significant component of chronic hyponatremia is attributable 
to secondary Na+ losses rather than water retention; the relative 
318
contributions from water retention versus sodium loss vary with 
the duration and severity of the hyponatremia: Water retention 
was found to be the major cause of decreased serum [Na+

ound to be the major cause of decreased serum [Na+] in the 
first 24 hours of induced hyponatremia in rats, but Na+ depletion 
then became the predominant etiologic factor after longer periods 
(7–14 days) of sustained hyponatremia, particularly at very low 
(<115 mEq/L) serum [Na+] levels.256 Finally, multiple studies of 
body fluid compartment volumes in hyponatremic patients have 
not demonstrated either plasma or ECF expansion. For example, 
a report of body fluid space measurements using is

a report of body fluid space measurements using isotope dilution 
techniques in hyponatremic and normonatremic patients with 
small cell lung carcinoma showed no differences between the two 
groups with regard to exchangeable sodium space, ECF volume by 
35SO4 distribution, or total body water.257 Fig. 10.5 schematically 
illustrates the volume regulatory processes that occur in response 
to water retention induced by inappropriate antidiuresis. 
Differential Diagnosis of Hyponatremia and 
Hypoo

Hypoosmolality
Because of the multiplicity of disorders causing hypoosmolality 
and the fact that many involve more than one pathologic mecha­
nism, a definitive diagnosis is not always possible at the time of 
initial presentation. Nonetheless, an approach based on clinical 
parameters of ECF volume status and urine sodium concentration 
generally allows a sufficient categorization for appropriate deci­
sions regarding initial therapy and further eva

i­
sions regarding initial therapy and further evaluation.
Decreased Extracellular Fluid Volume
Clinically detectable hypovolemia always signifies total body sol­
ute depletion. A low urine [Na+] indicates a nonrenal cause and 
an appropriate renal response. A high urine [Na+] indicates renal 
causes of solute depletion are more likely. Therapy with thiazide 
diuretics is the most common cause of renal solute losses,240 par­
ticularly in the elderly,258 but mineralocorticoid deficiency as a 
res

rly,258 but mineralocorticoid deficiency as a 
result of adrenal insufficiency or mineralocorticoid resistance must 
be considered as well as (less commonly) renal solute losses due to 
salt-wasting nephropathy (e.g., polycystic kidney disease, intersti­
tial nephritis, or chemotherapy). 
Increased Extracellular Fluid Volume
Clinically detectable hypervolemia always signifies total body 
Na+ excess. In these patients hypoosmolality results from an even 
greater expansion of total body water caus

n even 
greater expansion of total body water caused by a marked reduc­
tion in the rate of water excretion (and sometimes an increased 
rate of water ingestion). The impairment in water excretion is sec­
ondary to a decreased EABV,242 which increases the reabsorption 
of glomerular filtrate not only in the proximal nephron but also in 
the distal and collecting tubules by stimulated secretion of vaso­
pressin. These patients generally have a low urine [Na+] because 
of secondary hyperaldosteron

 urine [Na+] because 
of secondary hyperaldosteronism. However, under certain condi­
tions urine [Na+] may be elevated if there is concurrent diuretic 
therapy, a solute diuresis (e.g., glucosuria in diabetics), or after 
successful treatment of the underlying disease (e.g., improved car­
diac output in patients with congestive heart failure). 
Normal Extracellular Fluid Volume
Many different hypoosmolar disorders present with euvolemia, 
and measure of urinary [Na+] is an especially important f

sure of urinary [Na+] is an especially important first 
step.259 A high urine [Na+] usually implies a distally mediated, 
dilution-induced hypoosmolality such as SIAD. However, gluco­
corticoid deficiency can mimic SIAD so closely that these two 
disorders are often indistinguishable in terms of water balance. 
Hyponatremia from diuretic use also can present without clini­
cally evident hypovolemia, and the urine [Na+] will usually be 
elevated.240 A low urine [Na+] suggests a depletion-induced 

40 A low urine [Na+] suggests a depletion-induced 
hypoosmolality from ECF losses with subsequent volume replace­
ment by water or other hypotonic fluids. The solute loss often 
[S]ICF
[S]ICF
ICF
VRD
ECF
VRD
K+
osmolytes
Na+, Cl–
[S]ECF
[S]ICF
[S]ECF
[S]ICF
[S]ECF
H2O
H2O
H2O
[S]ECF
A
B
C
D
E
• Fig. 10.5  Schematic illustration of potential changes in whole-body fluid 
compartment volumes at various times during adaptation to hyponatre­
mia. (A) Under basal conditions, the concentrations of effe

Under basal conditions, the concentrations of effective solutes 
in the extracellular fluid ([S]ECF) and in the intracellular fluid ([S]ICF) are in 
osmotic balance. (B) During the first phase of water retention resulting from 
inappropriate antidiuresis, the excess water distributes across total body 
water, causing expansion of both ECF and ICF volumes (dotted lines), with 
equivalent dilutional decreases in both [S]ICF and [S]ECF. (C) In response to 
the volume expansion, compensatory volume 

nse to 
the volume expansion, compensatory volume regulatory decreases (VRD) 
occur to reduce the effective solute content of the ECF (via pressure diure­
sis and natriuretic factors) and the ICF (via increased electrolyte and osmo­
lyte extrusion mediated by stretch-activated channels and downregulation 
of synthesis of osmolytes and osmolyte uptake transporters). (D and E) If 
both processes go to completion, such as under conditions of fluid restric­
tion, a final steady state can be reached 

estric­
tion, a final steady state can be reached in which ICF and ECF volumes 
have returned to normal levels but [S]ICF and [S]ECF remain low. In most 
cases, this final steady state is not reached, and moderate degrees of ECF 
and ICF expansion persist, although they are significantly less than would 
be predicted from the decrease in body osmolality (D). Consequently, the 
degree to which hyponatremia is the result of dilution due to water reten­
tion versus solute depletion from volume regu

ten­
tion versus solute depletion from volume regulatory processes can vary 
markedly, depending on which phase of adaptation the patient is in and 
the relative rates at which the different compensatory processes occur. For 
example, delayed ICF VRD can worsen hyponatremia because of shifts 
of intracellular water into the ECF as intracellular organic osmolytes are 
extruded and subsequently metabolized; this likely accounts for some 
component of the hyponatremia that was unexplained by the co

of the hyponatremia that was unexplained by the combina­
tion of water retention and sodium excretion in early clinical studies. (From 
Verbalis JG. Hyponatremia: epidemiology, pathophysiology, and therapy. 
Curr Opin Nephrol Hypertens. 1993;2:626–652.)
319
is nonrenal, but an important exception is recent cessation of 
diuretic therapy, because urine [Na+] can decrease to low values 
within 12 to 24 hours after discontinuation of the drug. A low 
urine [Na+] also

scontinuation of the drug. A low 
urine [Na+] also can also be seen during the recovery phase from 
SIAD. 
Syndrome of Inappropriate Antidiuresis
SIAD is the most common cause of euvolemic hypoosmolality 
and it is also the single most common cause of hypoosmolality 
of all etiologies encountered in clinical practice with prevalence 
rates from 20% to 40% among all hypoosmolar patients.234,241 
The clinical criteria necessary to diagnose SIAD remain basically 
as set forth by Schwartz and Bartte

ain basically 
as set forth by Schwartz and Bartter in 1967:227
	1.	 Decreased effective osmolality of the extracellular fluid (Posm 
<275 mOsm/kg H2O). Pseudohyponatremia or hyperglycemia 
alone must be excluded.
	2.	 Inappropriate urinary concentration at some level of hypoos­
molality. This does not mean that urine osmolality is greater 
than plasma osmolality, only less than maximally dilute (i.e., 
urine osmolality >100 mOsm/kg H2O). Also, urine osmolal­
ity need not be elevated inappropr

 urine osmolal­
ity need not be elevated inappropriately at all levels of plasma 
osmolality, because in the reset osmostat variant form of SIAD, 
vasopressin secretion can be suppressed with resultant maximal 
urinary dilution if plasma osmolality is decreased to sufficiently 
low levels.260
	3.	 Clinical euvolemia, as defined by the absence of signs of hypo­
volemia (orthostasis, tachycardia, decreased skin turgor, dry 
mucous membranes) or hypervolemia (subcutaneous edema, 
ascites). Hypovol

pervolemia (subcutaneous edema, 
ascites). Hypovolemia or hypervolemia strongly suggests differ­
ent causes of hypoosmolality. Patients with SIAD can become 
hypovolemic or hypervolemic for other reasons, but in such 
cases it is impossible to diagnose the underlying inappropri­
ate antidiuresis until the patient is rendered euvolemic and is 
found to have persistent hypoosmolality.
	4.	 Elevated urinary sodium excretion while on a normal salt and 
water intake. This criterion is included becau

nd 
water intake. This criterion is included because of its utility in 
differentiating between hypoosmolality caused by a decreased 
EABV, in which case renal Na+ conservation occurs, and dis­
tal dilution-induced disorders, in which urine Na+ excretion 
is normal or increased secondary to ECF volume expansion. 
Patients with SIAD can have low urine Na+ excretion if they 
subsequently become hypovolemic or solute depleted, condi­
tions that sometimes follow severe salt and water restriction. 
C

times follow severe salt and water restriction. 
Consequently, a high urine Na+ excretion is the rule in most 
patients with SIAD; its presence does not guarantee this diag­
nosis, and its absence does not rule out the diagnosis.
	5.	 Absence of other potential causes of euvolemic hypoosmolality. 
Most notably are hypothyroidism, hypocortisolism (Addison 
disease or pituitary ACTH insufficiency), and diuretic use.
Several other criteria support but are not essential for a diag­
nosis of SIAD. V

but are not essential for a diag­
nosis of SIAD. Volume expansion and vasopressin acting on V1 
receptors in the kidney increase the clearance of uric acid, so hyp­
ouricemia is found with SIAD. When patients are hyponatremic, 
values of uric acid are generally <4 mg/dL (<0.24 mmol/L).261 A 
water-loading test is of value when there is uncertainty regarding 
the etiology of modest degrees of hypoosmolality in euvolemic 
patients, but it does not add useful information if the plasma osmo­
lality 

add useful information if the plasma osmo­
lality is already under 275 mOsm/kg H2O. Inability to excrete a 
standard water load normally (with normal excretion defined as a 
cumulative urine output of at least 90% of the administered water 
load within 4 hours and suppression of urine osmolality to <100 
mOsm/kg H2O) confirms the presence of an underlying defect 
in free water excretion. However, water excretion is abnormal 
in almost all disorders that cause hypoosmolality, whether dilu­
tional

rs that cause hypoosmolality, whether dilu­
tional or depletion induced with secondary impairments in free 
water excretion. Two exceptions are primary polydipsia, in which 
hypoosmolality can rarely be secondary to excessive water intake 
alone, and the reset osmostat variant of SIAD, in which normal 
excretion of a water load can occur once plasma osmolality falls 
below the new set-point for vasopressin secretion.
Another supportive criterion is an inappropriately elevated 
plasma vasopressin

is an inappropriately elevated 
plasma vasopressin level in relation to plasma osmolality. How­
ever, several factors limit the utility of vasopressin measurements 
to diagnose SIAD. First, although plasma vasopressin levels are 
elevated in most patients with this syndrome, the elevations 
generally remain within the normal physiologic range and are 
abnormal only in relation to plasma osmolality (Fig. 10.6). Sec­
ond, 10% to 20% of patients with SIAD do not have measurably 
elevated plasma vas

h SIAD do not have measurably 
elevated plasma vasopressin levels and are at the limits of detec­
tion by radioimmunoassay (see Fig. 10.6).262 Third, and perhaps 
most importantly, most disorders causing solute and volume 
depletion or decreased EABV are associated with elevations of 
plasma vasopressin levels secondary to nonosmotic hemody­
namic stimuli. 
Etiology
Although the list of disorders associated with SIAD is long (Table 
10.2), they can be divided into several major etiologic groups.

an be divided into several major etiologic groups.
Tumors
The most common association of SIAD is with tumors. Many 
different types of tumors have been associated with SIAD, but 
bronchogenic carcinoma of the lung has been uniquely asso­
ciated with SIAD since the first description of this disorder in 
1957.226 In virtually all cases, the bronchogenic carcinomas caus­
ing this syndrome have been of the small cell variety. Incidence 
11
10
9
8
7
6
5
4
3
2
1
0
310
300
29

essin (pg/mL)
11
10
9
8
7
6
5
4
3
2
1
0
310
300
290
280
270
Plasma osmolality (mOsm/kg)
Normal
range
260
250
240
230
• Fig. 10.6  Plasma arginine vasopressin (AVP) levels in patients with the 
syndrome of inappropriate antidiuretic hormone (SIADH) secretion as a 
function of plasma osmolality. Each point depicts one patient at a single 
point in time. The shaded area represents AVP levels in normal subjects 
over physiologic ranges of plasma osmolality. The lowest measurable 
plasma AVP level th

lality. The lowest measurable 
plasma AVP level that could be detected with this radioimmunoassay was 
0.5 pg/mL. (From Robertson GL, Aycinena P, Zerbe RL. Neurogenic disor­
ders of osmoregulation. Am J Med. 1982;2:339–353.)
320
of hyponatremia is as high as 11% of all patients with small cell 
carcinoma,263 or 33% of cases with more extensive disease,264 
have been reported. The high incidence of small cell carcinoma 
of the lung makes it imperative that

ll carcinoma 
of the lung makes it imperative that all adult patients presenting 
with an otherwise unexplained SIAD be investigated thoroughly 
and aggressively for a possible lung tumor. Head and neck can­
cers account for another group of malignancies associated with 
relatively higher incidences of SIAD,265 and some of these tumors 
have clearly been shown to synthesize vasopressin.266 A report 
from a large cancer hospital showed an incidence of hyponatre­
mia for all malignancies of 3.7%, 

e of hyponatre­
mia for all malignancies of 3.7%, with approximately one-third of 
these due to SIAD.267 
Central Nervous System Disorders
A large number of different CNS disorders are associated with 
SIAD, but without a common denominator linking them. This 
is not surprising when one considers the neuroanatomy described 
earlier. Magnocellular vasopressin neurons receive excitatory inputs 
from osmoreceptive cells located in the anterior hypothalamus, 
but also a major innervation from brains

halamus, 
but also a major innervation from brainstem cardiovascular regu­
latory and emetic centers. Although various components of these 
pathways have yet to be elucidated fully, many of them appear to 
have inhibitory as well as excitatory components. Consequently, 
any CNS disorder can potentially cause vasopressin hypersecre­
tion either by nonspecifically exciting these pathways via irritative 
foci or alternatively by disrupting them and thereby decreasing the 
level of inhibition. 
Drug

thereby decreasing the 
Drugs
Drug-induced hyponatremia is a common cause of hypoosmolal­
ity.268 Table 10.2 lists some of the agents that have been associated 
with SIAD, but new drugs are added continually. Pharmacologic 
agents may stimulate secretion of vasopressin, activate renal V2 
receptors, or potentiate the antidiuretic effect of vasopressin. Not 
all of the drug effects are fully understood, and many appear to 
work through a combination of mechanisms. A particul

rk through a combination of mechanisms. A particularly inter­
esting and clinically important class of agents is the selective sero­
tonin reuptake inhibitors (SSRIs). Hyponatremia following SSRI 
administration has been reported almost exclusively in the elderly, 
with rates as high as 22% to 28%, although in larger series the 
incidence was closer to 1 in 200.269 A similar effect is likely also 
responsible for the recent reports of severe fatal hyponatremia 
caused by use of the recreational 

l hyponatremia 
caused by use of the recreational drug 3,4-methylenedioxymeth­
amphetamine, “ecstasy,” which possesses substantial serotonergic 
activity.270 
Pulmonary Disorders
A variety of pulmonary disorders have been associated with 
this syndrome, but other than tuberculosis, acute pneumonia, 
and advanced chronic obstructive lung disease, the occur­
rence of hypoosmolality has been noted only sporadically. 
Hypoxia stimulates secretion of vasopressin in animals,271 
but in humans hypercar

vasopressin in animals,271 
but in humans hypercarbia is more associated with abnormal 
water retention. Elevated vasopressin may be limited to the 
initial days of hospitalization, when respiratory failure is most 
marked. Therefore, with SIAD in nontumor pulmonary dis­
ease, the pulmonary disease is obvious with severe dyspnea 
or extensive radiographically evident infiltrates, and the inap­
propriate antidiuresis will usually be limited to the period of 
respiratory failure. Mechanical ventil

 period of 
respiratory failure. Mechanical ventilation can cause SIAD via 
inappropriate secretion of vasopressin via decreased venous 
return. 
Other Causes
In acquired immunodeficiency syndrome (AIDS) or AIDS-related 
complex (ARC) and in patients with human immunodeficiency 
virus (HIV) infection, hyponatremia has been reported as high 
as 30% to 38% in adults and children.272 Although there are 
many potential etiologies, including dehydration, adrenal insuf­
ficiency, and pneumonitis, from

on, adrenal insuf­
ficiency, and pneumonitis, from 12% to 68% of AIDS patients 
who develop hyponatremia appear to meet criteria for a diagnosis 
of SIAD.272 Not unexpectedly, some of the medications used to 
treat these patients may cause the hyponatremia, either via direct 
renal tubular toxicity or induced SIAD.273
Elderly patients often develop SIAD without any apparent 
underlying etiology, and the high incidence of hyponatremia in 
geriatric patients231,274 suggests that the normal aging p

c patients231,274 suggests that the normal aging process may 
be accompanied by abnormalities of regulation of water balance 
and of secretion of vasopressin as noted earlier. Such an effect 
could potentially account for the fact that drug-induced hypo­
natremia occurs much more frequently in elderly patients. In a 
  Common Causes of the Syndrome of 
Inappropriate Antidiuretic Hormone 
(SIADH) Secretion
Tumors
Pulmonary/mediastinal (bronchogenic carcinoma, mesothelioma, 
thymoma)
Nonchest (du

ic carcinoma, mesothelioma, 
thymoma)
Nonchest (duodenal carcinoma, pancreatic carcinoma, ureteral/prostate 
carcinoma, uterine carcinoma, nasopharyngeal carcinoma, leukemia)
Mass lesions (tumors, brain abscesses, subdural hematoma)
Inflammatory diseases (encephalitis, meningitis, systemic lupus erythe­
matosus, acute intermittent porphyria, multiple sclerosis)
Degenerative/demyelinative diseases (Guillain-Barré syndrome, spinal 
cord lesions)
Miscellaneous (suba

yndrome, spinal 
Miscellaneous (subarachnoid hemorrhage, head trauma, acute psy­
chosis, delirium tremens, pituitary stalk section, transsphenoidal 
adenomectomy, hydrocephalus)
Drug Related
Stimulated release of AVP (nicotine, phenothiazines, tricyclics)
Direct renal effects or potentiation of AVP antidiuretic effects (dDAVP, 
oxytocin, prostaglandin synthesis inhibitors)
Mixed or uncertain actions (ACE inhibitors, carbamazepine and oxcar­
bazepine, chlorpropamide, clofibrate, clo

d oxcar­
bazepine, chlorpropamide, clofibrate, clozapine, cyclophosphamide, 
3,4-methylenedioxymethamphetamine [ecstasy], omeprazole; 
serotonin reuptake inhibitors, vincristine)
Pulmonary
Infections (tuberculosis, acute bacterial and viral pneumonia, aspergil­
losis, empyema)
Mechanical/ventilatory causes (acute respiratory failure, COPD, positive-
pressure ventilation)
Acquired immunodeficiency syndrome (AIDS) and AIDS-related complex
Prolonged strenuous exercise (marathon, triath

lex
Prolonged strenuous exercise (marathon, triathlon, ultramarathon, hot-
weather hiking)
Senile atrophy
ACE, Angiotensin-converting enzyme; AVP, arginine vasopressin; COPD, chronic obstructive 
pulmonary disease; dDAVP, desmopressin.
   
TABLE 10.2
321
series of 50 consecutive elderly patients meeting criteria for SIAD, 
60% remained idiopathic despite rigorous evaluation, leading the 
authors to conclude that extensive diagnostic procedures were 
not

ude that extensive diagnostic procedures were 
not warranted in such elderly patients if routine history, physi­
cal examination, and laboratory evaluation failed to suggest an 
underlying etiology.275 
Sources of Vasopressin Secretion
Elevated plasma levels of vasopressin can be broadly divided 
into those associated with paraneoplastic (“ectopic”) secretion 
of vasopressin or pituitary hypersecretion of vasopressin. There 
is substantial cumulative evidence that tumor tissue ca

bstantial cumulative evidence that tumor tissue can in fact 
synthesize vasopressin,276 but it is not certain that all tumors 
associated with SIAD do so because only about half of small cell 
carcinomas have been found to contain vasopressin immunore­
activity, and many of the tumors listed in Table 10.2 have not 
been carefully studied. 
Pituitary Vasopressin Secretion—Inappropriate Versus 
Appropriate
In the majority of cases of SIAD, the vasopressin secretion origi­
nates from the posterior 

pressin secretion origi­
nates from the posterior pituitary. This is also true of over 90% of 
all cases of hyponatremia, including patients with hypovolemic 
and hypervolemic hyponatremia.234 This raises the question of 
what constitutes “inappropriate” secretion of vasopressin. Secre­
tion of vasopressin in response to a hypovolemic stimulus is clearly 
physiologically “appropriate,” but when it leads to symptomatic 
hyponatremia it could be considered inappropriate for the ECF 
osmolality. De

nsidered inappropriate for the ECF 
osmolality. Despite these semantic conundrums, the diagnosis of 
SIAD should be based upon the original Schwartz-Bartter criteria 
and specifically exclude other clinical conditions that cause known 
impairments in free water excretion even when these are mediated 
by a secondary nonosmotic physiologic stimulation of vasopres­
sin secretion. Without maintaining these distinctions, arguable as 
some may be, the definition of SIAD becomes too broad to retain 
an

definition of SIAD becomes too broad to retain 
any practical clinical utility. 
Patterns of Vasopressin Secretion
Studies of plasma vasopressin levels in patients with SIAD dur­
ing graded increases in plasma osmolality produced by hypertonic 
saline administration have defined four patterns of secretion: (1) 
random hypersecretion of vasopressin; (2) inappropriate non­
suppressible basal vasopressin release, but normal secretion in 
response to osmolar changes above basal plasma osmolality; (3

 osmolar changes above basal plasma osmolality; (3) 
a reset osmostat system, whereby vasopressin is secreted at an 
abnormally low threshold of plasma osmolality but otherwise dis­
plays a normal response to relative changes in osmolality; and (4) 
low or even undetectable plasma vasopressin levels despite clas­
sic clinical characteristics of SIAD.262 The pattern of SIAD that 
occurs without measurable vasopressin secretion is not yet well 
understood, but the positive response of one such pat

erstood, but the positive response of one such patient to a 
vasopressin V2-receptor antagonist would suggest that this may 
represent increased renal sensitivity to low circulating levels of 
vasopressin.277 It is surprising that no correlation has been found 
between any of these patterns of secretion of vasopressin and the 
various etiologies of SIAD.262 Recent studies of pediatric patients 
with hyponatremia and unmeasurable plasma vasopressin levels 
led to the discovery of an activating mu

n levels 
led to the discovery of an activating mutation of the vasopressin 
V2 receptor as the cause of their inappropriate antidiuresis.278 It 
is more appropriate to call these cases the nephrogenic syndrome 
of inappropriate antidiuresis, reserving SIAD only for those cases 
where measured plasma vasopressin levels are really inappropriate. 
Although the incidence of nephrogenic syndrome of inappropri­
ate antidiuresis in the general population is unknown, the descrip­
tion of Belgian kindre

on is unknown, the descrip­
tion of Belgian kindred with this mutation suggests that it can 
present later in life as well as in childhood. 
Contribution of Natriuresis to the Hyponatremia of SIAD
Since the original cases studied by Schwartz and Bartter, increased 
renal Na+ excretion has been one of the cardinal manifestations of 
SIAD, indeed one which later became embedded in the require­
ments for its diagnosis.227 Demonstration that the natriuresis 
accompanying administration of antidiuret

iuresis 
accompanying administration of antidiuretic hormone is not due 
to vasopressin itself but to the volume expansion produced as a 
result of water retention was unequivocally shown by Leaf and 
associates even before the description of the disorder.279 Although 
a negative Na+ balance occurs during the development of hypo­
natremia in patients with SIAD, eventually urinary sodium excre­
tion simply reflects daily sodium intake.226 Thus the term renal 
sodium wasting is used to describe co

 term renal 
sodium wasting is used to describe continued excretion of sodium 
despite being hyponatremic, but in reality there is a new steady 
state in which patients are in neutral sodium balance. Studies of 
long-term antidiuretic-induced hyponatremia in both dogs and 
rats have indicated that a large proportion of the hyponatremia 
was attributable to secondary Na+ losses rather than to water 
retention,255,256 but the natriuresis did not actually worsen the 
hyponatremia; rather, it allowe

tually worsen the 
hyponatremia; rather, it allowed volume regulation of ECF.280 
Secondary natriuresis in patients with SIAD likely explains the 
failure to find expanded plasma or ECF volumes using tracer dilu­
tion techniques (see Fig. 10.5).257 
Cerebral Salt Wasting
The degree to which hyponatremia might occur primarily as a 
result of primary natriuresis is controversial. Cerebral salt wast­
ing syndrome (CSWS) was proposed by Peters and associates in 
1950281 as an explanation for the nat

sociates in 
1950281 as an explanation for the natriuresis and hyponatremia 
that sometimes accompanies intracranial disease, particularly sub­
arachnoid hemorrhage in which up to one-third of patients often 
develop hyponatremia. After the description of SIAD in 1957, 
such patients were generally assumed to have hyponatremia sec­
ondary to vasopressin hypersecretion with a secondary natriuresis. 
However, over the last decade clinical and experimental data have 
been interpreted to indicate th

imental data have 
been interpreted to indicate that some patients with subarachnoid 
hemorrhage and other intracranial diseases indeed have a primary 
natriuresis leading to volume contraction rather than SIAD,282,283 
and the elevated plasma vasopressin levels may be physiologically 
appropriate for the degree of volume contraction. With regard to 
the potential mechanisms of natriuresis, both plasma and CSF lev­
els of atrial natriuretic peptide are elevated in many patients with 
subarachnoi

de are elevated in many patients with 
subarachnoid hemorrhage and have been found to correlate vari­
ably with hyponatremia in patients with intracranial diseases.284 
However, clearly documented SIAD also is frequently associated 
with elevated plasma levels of atrial natriuretic peptide, so this 
finding does not prove causality. In other disorders of hypona­
tremia due to Na+ wasting (e.g., Addison disease) and diuretic-
induced hyponatremia, infusion of saline restores normal ECF 
volume an

 infusion of saline restores normal ECF 
volume and plasma tonicity by shutting off the secondary vaso­
pressin secretion. In subarachnoid hemorrhage, however, large 
volumes of isotonic saline sufficient to maintain plasma volume 
did not change the incidence of hyponatremia.285
Those authors who have distinguished CSWS from SIAD 
have emphasized that in CSWS the primary disorder, salt 
322
wasting, produces convincing evidence of decreased extracellular

ces convincing evidence of decreased extracellular 
fluid volume.286,287 There are only a few case reports of patients 
after traumatic brain injury or neurosurgery who while being 
observed in the hospital have acute onset of massive diuresis and 
natriuresis with clear evidence of volume contraction by weight 
loss, decreased central venous pressure, increased blood urea 
nitrogen (BUN), or increased hematocrit. Most of these cases 
have been in children288,289 and have responded to replacemen

n children288,289 and have responded to replacement 
with normal or hypertonic saline, but concurrent treatment 
with fludrocortisone has also been advocated.288,290 However, a 
recent study of 100 consecutive adult patients with acute non­
traumatic aneurysmal subarachnoid hemorrhage found that the 
cause of the hyponatremia was attributable to SIAD in 71.4% 
and acute glucocorticoid deficiency in 8.2%, with the remain­
ing cases caused by incorrect intravenous fluid administration or 
hypovole

rect intravenous fluid administration or 
hypovolemia. Most significantly, no cases were found that met 
historically accepted criteria for a diagnosis of CSWS.112 This 
suggests that CSWS is an exceedingly rare cause of the hypona­
tremia with intracranial disorders. 
Renal Escape From Antidiuresis
In addition to excreting osmoles to bring volumes back toward 
normal, there are intrarenal adaptations that allow excretion of 
more water. Chronic stimulation by vasopressin in SIAD produces 
drama

stimulation by vasopressin in SIAD produces 
dramatic increases of aquaporin 2 content and insertion into the 
collecting duct principal cell membranes, which increases the effi­
ciency of water retention and aggravates the pathology. However, 
the induced volume expansion and hypotonicity act on the tubu­
lar cells of the collecting duct to decrease the content and action 
of aquaporin 2 substantially, thus decreasing the amount of water 
resorbed in spite of high vasopressin levels. Experiment

ed in spite of high vasopressin levels. Experimental studies 
have suggested this may be due to downregulation of vasopres­
sin V2-receptor expression in the kidney.291 This renal “escape” 
therefore represents another (in addition to natriuresis) adapta­
tion that allows patients with persistent SIAD to come into a new 
steady state of Na+ and water balance despite low serum sodium 
concentrations.292 
Hypoosmolar Symptoms, Morbidity, and 
Mortality
Regardless of the etiology of hypoosmolality,

lity
Regardless of the etiology of hypoosmolality, most clinical 
manifestations are similar. Nonneurologic symptoms are rela­
tively uncommon, although a number of cases of rhabdomy­
olysis have been reported presumably secondary to osmotically 
induced swelling of muscle fibers. Hypoosmolality is primarily 
associated with a broad spectrum of neurologic manifestations 
ranging from mild nonspecific symptoms (e.g., headache, nau­
sea) to more significant disorders (e.g., disorientation, confu­

gnificant disorders (e.g., disorientation, confu­
sion, obtundation, focal neurologic deficits, and seizures).293 
This neurologic symptom complex has been termed “hypona­
tremic encephalopathy”294 and primarily reflects brain edema 
resulting from osmotic water shifts into the brain because of 
decreased effective plasma osmolality. Significant neurologic 
symptoms generally do not occur until serum [Na+] falls below 
125 mmol/L, and the severity of symptoms are roughly corre­
lated with the de

y of symptoms are roughly corre­
lated with the degree of hypoosmolality.293,295 However, indi­
vidual variability is marked, and for any single patient the level 
of serum [Na+] at which symptoms appear cannot be predicted. 
The rate of fall of serum [Na+] is often more strongly correlated 
with morbidity and mortality than is the actual magnitude of the 
decrease.293,295 This is due to the fact that the volume-adaptation 
process takes a finite period of time to complete, and the more 
rapid t

 period of time to complete, and the more 
rapid the fall in serum [Na+] the more brain edema will be accu­
mulated before the brain is able to volume regulate. Thus there 
is a much higher incidence of neurologic symptoms as well as 
a higher mortality in patients with acute hyponatremia than in 
those with chronic hyponatremia.293 For example, the most dra­
matic cases of death due to hyponatremic encephalopathy have 
generally been reported in postoperative patients in whom hypo­
natremia dev

 postoperative patients in whom hypo­
natremia develops rapidly as a result of intravenous infusion of 
hypotonic fluids.293,296 In such cases nausea and vomiting are 
frequently overlooked as potential early signs of increased intra­
cranial pressure. Critically ill patients with unexplained seizures 
also should be immediately evaluated for possible hyponatremia, 
since as many as one-third of such patients have a serum [Na+] 
below 125 mEq/L as the cause of the seizure activity.297 Under­
lyi

s the cause of the seizure activity.297 Under­
lying neurologic disease and nonneurologic metabolic disorders 
(e.g., hypoxia,298 acidosis, hypercalcemia) can raise the level of 
plasma osmolality at which CNS symptoms occur.
In the most severe cases of hyponatremic encephalopathy, 
death results from respiratory failure after tentorial cerebral her­
niation and brainstem compression. One-quarter of patients with 
severe postoperative hyponatremic encephalopathy manifested 
hypercapneic respirat

c encephalopathy manifested 
hypercapneic respiratory failure, the expected result of brainstem 
compression, but three-quarters had pulmonary edema as the 
apparent cause of the hypoxia.299 Studies of acute hyponatremia 
after marathon races have shown hypoxia and pulmonary edema 
in association with brain edema.300 These results suggest the pos­
sibility that hypoxia from noncardiogenic pulmonary edema 
may represent an early sign of developing cerebral edema even 
before brainstem compression

 cerebral edema even 
before brainstem compression and tentorial herniation. Clinical 
studies have suggested that menstruating women296 and young 
children301 may be particularly susceptible to the development of 
neurologic morbidity and mortality during hyponatremia, espe­
cially in the acute postoperative setting.294 However, other studies 
have failed to corroborate these findings.302,303
Although some studies have suggested that hyponatremia 
is an indicator of severe underlying disease an

a 
is an indicator of severe underlying disease and poor prognosis 
rather than a cause of the increased mortality in such patients, a 
meta-analysis of studies in which some patients had correction of 
their hyponatremia indicated that improvement in serum [Na+] 
was associated with a 50% reduction in mortality of the cor­
rected groups compared to patients in whom the hyponatremia 
remained uncorrected, suggesting that hyponatremia may in fact 
be causally related to increased mortality.304 Re

be causally related to increased mortality.304 Recent studies have 
further indicated that even mild hyponatremia is an independent 
predictor of higher mortality across a wide variety of disorders, 
including patients with acute ST-elevation myocardial infarction, 
heart failure, and liver disease.228,235A large study of over 55,000 
electronic heath records from a single Boston hospital showed 
that the association of hyponatremia with inpatient mortality was 
significant across all levels of

nt mortality was 
significant across all levels of hyponatremia and even began at 
serum [Na+] levels in the lower part of the normal range.229 These 
findings were corroborated in studies of 249,000 Danish patients 
hospitalized over a 5-year period that showed increased 30-day 
and 1-year mortality associated with all levels of hyponatremia, 
including the range of 130 to 134.9 mmol/L,305 and analyses of 
2.3 million hospitalized patients enrolled in the Cerner Health 
Facts database.306 The m

led in the Cerner Health 
Facts database.306 The mortality associated with chronic hypo­
natremia is much less well studied, but results of the Rotterdam 
Longitudinal Aging Study noted significantly decreased survival 
of elderly patients with hyponatremia over a 12-year period of 
observation.307
323
Once the brain has volume regulated via solute losses, thereby 
reducing brain edema, neurologic symptoms are not as promi­
nent and may even be virtually absent. T

as promi­
nent and may even be virtually absent. This accounts for the fairly 
common finding of relatively asymptomatic patients even with 
severe levels of hyponatremia.295,308 Despite this powerful adapta­
tion process, chronic hyponatremia is frequently associated with 
neurocognitive symptomatology, albeit milder and more subtle 
in nature, such as headaches, nausea, mood disturbances, depres­
sion, difficulty concentrating, slowed reaction times, unstable 
gait, increased falls, confusion,

times, unstable 
gait, increased falls, confusion, and disorientation.309,310 Even 
in patients adjudged to be “asymptomatic” by virtue of a normal 
neurologic examination, accumulating evidence suggests that 
there may be previously unrecognized adverse effects as a result 
of chronic hyponatremia. In one study, 16 patients with hypona­
tremia secondary to SIAD, in the range of 124 to 130 mmol/L, 
demonstrated a significant gait instability that normalized after 
correction of the hyponatremia 

 normalized after 
correction of the hyponatremia to a normal range.309 The func­
tional significance of the gait instability was illustrated in a study 
of 122 Belgian patients with a variety of levels of hyponatremia all 
judged to be asymptomatic at the time of visit to an emergency 
department (ED). These patients were compared with 244 age-
matched, gender-matched, and disease-matched controls also pre­
senting to the ED during the same time period. Researchers found 
that 21% of the hypona

 period. Researchers found 
that 21% of the hyponatremic patients came to the ED because of 
a recent fall, compared to only 5% of the controls; this difference 
was highly significant and remained so after multivariable adjust­
ment.309 Analogous results were found in a study of admissions 
to a US hospital geriatric trauma unit over a 3-year period; when 
patients admitted because of a fall (n = 1841) were analyzed for 
risk factors associated with fall, the odds ratio for serum [Na+] 
below 1

with fall, the odds ratio for serum [Na+] 
below 135 mmol/L was 1.81 (p <0.001), which was greater than 
all other risk factors except age over 85 years.311 Consequently, 
these studies clearly document an increased incidence of falls in 
so-called asymptomatic hyponatremic patients. Recent studies in 
both experimental animals312 and humans313 have demonstrated 
decreases in nerve conduction associated with hyponatremia as a 
potential cause of the gait disturbances.
The clinical significance o

the gait disturbances.
The clinical significance of the gait instability and fall data have 
been indicated by multiple independent international studies that 
have demonstrated increased rates of bone fractures in patients 
with hyponatremia.307,314–316 Other studies have shown that 
hyponatremia is associated with increased bone loss in experimen­
tal animals and a significantly increased odds ratio for osteoporo­
sis of the femoral neck in those older than 50 years in the Third 
National Heal

se older than 50 years in the Third 
National Health and Nutrition Examination Survey (NHANES 
III) database.317 These findings have been corroborated by mul­
tiple epidemiologic studies that have demonstrated decreased 
bone mineral density in human subjects.318,319 In the largest epi­
demiologic analysis to date (2.9 million independent electronic 
health records), the odds ratios for osteoporosis and fractures were 
significantly greater for hyponatremia (3.99 and 3.05, respec­
tively) than f

yponatremia (3.99 and 3.05, respec­
tively) than for any other diseases or medications associated with 
increased bone loss and fracture risk.320 Of particular note, the 
odds ratios for both osteoporosis and fractures were the highest 
in patients with chronic persistent hyponatremia, indicating that 
duration of hyponatremia represents an important risk factor for 
bone disease and fractures.320 These findings were supported by a 
subsequent clinical study of hip fractures in Argentina.321
Rec

inical study of hip fractures in Argentina.321
Recent studies of a cohort of 5435 community dwelling 
men over the age of 65 in the United States have found that 
hyponatremia is independently associated with greater cogni­
tive impairment and cognitive decline in this population,322 
and a retrospective study of 4900 hyponatremic patients in 
Taiwan found that the hyponatremic patients had a 2.36-
fold higher hazard ratio of developing dementias compared to 
matched controls, which increased to

 compared to 
matched controls, which increased to a hazard ratio of 4.29 
for patients with more severe hyponatremia. Thus the major 
clinical significance of chronic hyponatremia may lie in the 
increased morbidity and mortality associated with falls, frac­
tures, neurocognitive impairments, and dementias in our older 
population as well as potential adverse effects not yet studied 
in humans.323 
Therapy of SIAD and Other Hypoosmolar 
Disorders
General Principles
Correction of hyponatremia is

s
Correction of hyponatremia is associated with markedly improved 
neurologic outcomes in patients with severely symptomatic hypo­
natremia. In a retrospective review of patients who presented with 
severe neurologic symptoms and serum [Na+] below 125 mmol/L, 
prompt therapy with isotonic or hypertonic saline resulted in a 
correction in the range of 20 mEq/L over several days and neu­
rologic recovery in almost all cases; in contrast, in patients who 
were treated with fluid 

ontrast, in patients who 
were treated with fluid restriction alone, there was very little cor­
rection over the study period (<5 mmol/L over 72 hours), and 
the neurologic outcomes were much worse, with most of these 
patients either dying or entering a persistently vegetative state.324 
Based on this and similar retrospective analyses, prompt therapy 
to rapidly increase the serum [Na+] represents the standard of 
care for treatment of patients presenting with severe symptoms of 
hyponatremia.

 presenting with severe symptoms of 
Brain herniation, the most dreaded complication of hyponatre­
mia, is seen almost exclusively in patients with acute hyponatremia 
(usually <24 hours) or in patients with intracranial pathology.325 
In postoperative patients and in patients with self-induced water 
intoxication associated with marathon running, psychosis, or 
use 
of 
“ecstasy” 
(3,4-methylenedioxy-N-methamphetamine 
[MDMA]), nonspecific symptoms such as headache, nausea and 
vo

specific symptoms such as headache, nausea and 
vomiting, or confusion can rapidly progress to seizures, respira­
tory arrest, and ultimately death or a permanent vegetative state 
as a complication of cerebral edema.232 Hypoxia from noncardio­
genic pulmonary edema and/or hypoventilation can exacerbate 
brain swelling caused by the low serum [Na+].299,300 Although 
usually self-limited, hyponatremic seizures may be refractory to 
anticonvulsants.
As discussed earlier, chronic hyponatremia is mu

.
As discussed earlier, chronic hyponatremia is much less 
symptomatic as a result of the process of brain volume regula­
tion. Because of this adaptation process, chronic hyponatremia 
is arguably a condition that clinicians feel less concerned about, 
which has been reinforced by the common usage of the descrip­
tor asymptomatic hyponatremia for many such patients. However, 
as discussed, it is clear that many such patients very often have 
neurologic symptoms, even if milder and more subtle i

rologic symptoms, even if milder and more subtle in nature. 
Consequently, all patients with hyponatremia who manifest any 
neurologic symptoms that could possibly be related to the hypo­
natremia should be considered candidates for treatment of the 
hyponatremia, regardless of the chronicity of the hyponatremia 
or the level of serum [Na+]. An additional reason to treat even 
asymptomatic hyponatremia effectively is to prevent a lowering 
of the serum [Na+] to more symptomatic and dangerous lev

 serum [Na+] to more symptomatic and dangerous levels 
during treatment of underlying conditions (e.g., increased fluid 
administration via parenteral nutrition, treatment of heart failure 
with diuretics). 
324
Therapies for Treatment of Hyponatremia
Conventional management strategies for hyponatremia range 
from saline infusion and fluid restriction to pharmacologic mea­
sures to adjust fluid balance. Although the number of available 
treatments for hyp

though the number of available 
treatments for hyponatremia is large, some are not appropriate 
for correction of symptomatic hyponatremia because they work 
too slowly or inconsistently to be effective in hospitalized patients 
(e.g., demeclocycline, mineralocorticoids). Consideration of treat­
ment options should always include an evaluation of the benefits 
as well as the potential toxicities of any therapy and must be indi­
vidualized for each patient.310,326 For all therapies, careful atten

h patient.310,326 For all therapies, careful atten­
tion should be paid to recommendations for goals and limits of 
correction of the serum [Na+] to reduce the risk of the osmotic 
demyelination syndrome (ODS)310 (Fig. 10.7). It should also be 
remembered that sometimes simply stopping treatment with an 
agent that is associated with causing hyponatremia is sufficient to 
correct a low serum [Na+].
Hypertonic Saline
Acute hyponatremia presenting with severe neurologic symp­
toms is life threaten

with severe neurologic symp­
toms is life threatening and should be treated promptly with 
hypertonic solutions, typically 3% NaCl ([Na+] = 513 mmol/L), 
as this represents the most reliable method to quickly raise the 
serum [Na+]. A continuous infusion of hypertonic NaCl is usu­
ally utilized in inpatient settings. Various formulae have been 
suggested for calculating the initial rate of infusion of hypertonic 
solutions,325 but until now there has been no consensus regard­
ing optimal infusio

 has been no consensus regard­
ing optimal infusion rates of 3% NaCl. One of the simplest 
methods to estimate an initial 3% NaCl infusion rate utilizes the 
following relationship310:
Patient’s weight (kg) × desired correction rate (mEq/L/hr)
= infusion rate of 3% NaCl (mL/hr)
This may not achieve the desired correction rate, but frequent 
monitoring of the serum [Na+] will inform the clinician whether 
the rate should be increased or decreased, similar to using mea­
surement of serum glucose t

 similar to using mea­
surement of serum glucose to guide the infusion rate of insulin 
drips. Depending on individual hospital policies, the administra­
tion of hypertonic solutions may require special considerations 
(e.g., placement in the intensive care unit [ICU], sign-off by a 
consultant), which each clinician needs to take into account to 
optimize patient care. One barrier to the use of hypertonic saline 
that appears to be overstated and unfounded is the frequent 
requirement for a cen

d unfounded is the frequent 
requirement for a central intravenous catheter for chronic infu­
sion. A recent study demonstrated a low rate of complications 
using peripheral infusions of 3% NaCl (6% infiltration and 3% 
thrombophlebitis) and concluded that peripheral administration 
of 3% NaCl carries a low risk of minor nonlimb or life-threaten­
ing complications.327
An alternative option for more emergent situations is admin­
istration of a 100-mL bolus of 3% NaCl, repeated twice if there 
is 

-mL bolus of 3% NaCl, repeated twice if there 
is no clinical improvement in 30 minutes, which has been recom­
mended by a consensus conference organized to develop guidelines 
for the prevention and treatment of exercise-induced hyponatre­
mia328 and adopted as a general recommendation by several expert 
panels.310,329 Injecting this amount of hypertonic saline intrave­
nously raises the serum [Na+] by an average of 2 to 4 mmol/L, 
which is well below the recommended maximal daily rate of 
chan

 below the recommended maximal daily rate of 
change of 10 to 12 mmol/L/24 hours or 8 mmol/L/24 hours for 
patients with increased risk factors for ODS (serum [Na+] ≤105 
mmol/L, hypokalemia, advanced liver disease, malnutrition, or a 
history of alcoholism)310,330 (see Fig. 10.7). Because the adult brain 
can only accommodate an average increase of approximately 8% 
in brain volume before herniation occurs, quickly increasing the 
serum [Na+] by as little as 2 to 4 mmol/L in acute hyponatremia 

 as little as 2 to 4 mmol/L in acute hyponatremia 
can effectively reduce brain swelling and intracranial pressure.331 
Isotonic Saline
The treatment of choice for depletional hyponatremia (i.e., hypo­
volemic hyponatremia) is isotonic saline ([Na+] = 154 mmol/L) 
to restore ECF volume and ensure adequate organ perfusion. This 
initial therapy is appropriate for patients who either have clinical 
signs of hypovolemia or in whom a spot urine [Na+] concentra­
tion is below 20 to 30 mEq/L.310 Such 

 concentra­
tion is below 20 to 30 mEq/L.310 Such patients often develop a 
free water diuresis (aquaresis) as their ECF volume is corrected, 
potentially leading to an overly rapid correction with increased 
risk of ODS, so the serum [Na+] and urine output should be fol­
lowed carefully during the first 24 to 48 hours of therapy. How­
ever, isotonic saline is ineffective for dilutional hyponatremias 
such as SIAD,226 and continued administration of isotonic saline 
to a euvolemic patient may wo

 of isotonic saline 
to a euvolemic patient may worsen their hyponatremia332 and/
or cause fluid overload. Although saline may improve the serum 
[Na+] in some patients with hypervolemic hyponatremia, their 
volume status will generally worsen with this therapy, so unless 
the hyponatremia is profound both hypertonic and isotonic saline 
should be avoided. 
Fluid Restriction
For patients with chronic hyponatremia, fluid restriction has been 
the most popular and most widely accepted treatment. W

most popular and most widely accepted treatment. When 
SIAD is present, fluids should generally be limited to 500 to 1000 
mL/24 hours. Because fluid restriction increases the serum [Na+] 
by underreplacing the excretion of fluid by the kidneys, some have 
advocated an initial restriction to 500 mL less than the 24-hour 
urine output.333 When instituting a fluid restriction, it is impor­
tant for the nursing staff and the patient to understand that this 
includes all fluids that are consumed, no

at this 
includes all fluids that are consumed, not just water (Table 10.3). 
Generally the water content of ingested food is not included in 
the restriction because this is balanced by insensible water losses 
Change in serum sodium concentration
in first 24 hours (mmol/L)
Acute water
intoxication
High
risk of ODS
Low to moderate
risk of ODS
Relowering
unnecessary
optional
Goal
Goal
recommended
20
18
16
14
10
8
12
6
4
2
0
• Fig. 10.7  Recommended goals (green) and limits 

• Fig. 10.7  Recommended goals (green) and limits (red) for correction of 
hyponatremia based on risk of producing ODS and recommendations 
for relowering of serum sodium concentration ([Na+]) to goals for patients 
presenting with serum [Na+] lower than 120 mmol/L who exceed the rec­
ommended limits of correction in the first 24 hours. ODS, osmotic demy­
elination syndrome. (From Verbalis JG, Goldsmith SR, Greenberg A, et al. 
Diagnosis, evaluation, and treatment of hyponatremia: expert panel r

ion, and treatment of hyponatremia: expert panel rec­
ommendations. Am J Med. 2013;126:S1–S42.)
325
(perspiration, exhaled air, feces, etc.), but caution should be exer­
cised with foods that have high fluid concentrations (such as fruits 
and soups). Restricting fluid intake can be effective when prop­
erly applied and managed in selected patients, but serum [Na+] 
is generally increased only slowly (1–2 mmol/L/day) even with 
severe fluid restriction.226 In addi

y) even with 
severe fluid restriction.226 In addition, this therapy is often poorly 
tolerated because of an associated increase in thirst leading to poor 
compliance with long-term therapy.
Fluid restriction should not be used with hypovolemic patients 
and is particularly difficult to maintain in hospitalized patients 
with very elevated urine osmolalities secondary to high vasopres­
sin levels; similarly, if the sum of urine [Na+] and [K+] exceeds 
the serum [Na+], most patients will not res

ceeds 
the serum [Na+], most patients will not respond to a fluid restric­
tion since an electrolyte-free water clearance will be difficult to 
achieve.334,335 These and other known predictors of failure of 
fluid restriction are summarized in Table 10.3; the presence of 
any of these factors in hospitalized patients with symptomatic 
hyponatremia make this therapy less than ideal. In addition, fluid 
restriction is not practical for some patients, particularly including 
patients in intensive c

s, particularly including 
patients in intensive care settings who often require administra­
tion of significant volumes of fluids as part of their therapies. Such 
patients are candidates for more effective pharmacologic or saline 
treatment strategies. 
Arginine Vasopressin Receptor Antagonists
Conventional therapies for hyponatremia, although effective in 
specific circumstances, are suboptimal for many different rea­
sons, including variable efficacy, slow responses, intolerable side 
effect

efficacy, slow responses, intolerable side 
effects, and serious toxicities. But perhaps the greatest deficiency 
of most conventional therapies is that they do not directly target 
the underlying cause of most dilutional hyponatremias, namely 
inappropriately elevated plasma vasopressin levels. A new class of 
pharmacologic agents (vasopressin receptor antagonists [vaptans]) 
that directly block vasopressin-mediated receptor activation have 
been approved for the treatment of euvolemic hyponatr

n approved for the treatment of euvolemic hyponatremia and 
hypervolemic hyponatremia in many countries.326,336
Conivaptan has been approved by the US Food and Drug 
Administration (FDA) for euvolemic and hypervolemic hypona­
tremia in hospitalized patients. It is available only as an intrave­
nous preparation and is given as a 20-mg loading dose over 30 
minutes, followed by a continuous infusion of 20 or 40 mg/day. 
Generally, the 20-mg continuous infusion is used for the first 24 
hours to ga

ous infusion is used for the first 24 
hours to gauge the initial response. If the correction of serum [Na+] 
is thought to be inadequate (e.g., <5 mmol/L), the infusion rate 
can be increased to 40 mg/day. Clinical studies have supported the 
efficacy of bolus infusions of conivaptan rather than continuous 
infusions.337 Therapy is limited to a maximum duration of 4 days 
because of drug interaction effects with other agents metabolized 
by the CYP3A4 hepatic isoenzyme. Importantly, for conivap

CYP3A4 hepatic isoenzyme. Importantly, for conivaptan 
and all other vaptans, it is critical that the serum [Na+] be mea­
sured frequently during the active phase of correction of the hypo­
natremia—a minimum of every 6 to 8 hours for conivaptan but 
more frequently in patients with risk factors for the ODS.310 If the 
correction exceeds 10 to 12 mmol/L in the first 24 hours, the infu­
sion should be stopped and the patient monitored closely. Consid­
eration should be given to administering suff

sid­
eration should be given to administering sufficient water, orally or 
as intravenous 5% dextrose in water, to avoid a correction of more 
than 12 mmol/L/day. The maximum correction limit should be 
reduced to 8 mmol/L over the first 24 hours in patients with risk 
factors for ODS310 (see Fig. 10.7). The most common side effects 
of conivaptan include headache, thirst, and hypokalemia.338
Tolvaptan, an oral vasopressin receptor antagonist, is also FDA 
approved for the treatment of euvolemic

 also FDA 
approved for the treatment of euvolemic and hypervolemic hypo­
natremia. In contrast to conivaptan, the availability of tolvaptan 
in tablet form allows short-term and long-term use.339 Similar to 
conivaptan, tolvaptan treatment must be initiated in the hospi­
tal so that the rate of correction can be monitored carefully. In 
the United States, patients with a serum [Na+] lower than 125 
mmol/L are eligible for therapy with tolvaptan as primary therapy; 
if the serum [Na+] is 125 mmo

as primary therapy; 
if the serum [Na+] is 125 mmol/L or higher, tolvaptan therapy is 
only indicated if the patient has symptoms that could be attrib­
utable to the hyponatremia and is resistant to attempts at fluid 
restriction.340 In the European Union, tolvaptan is approved only 
for the treatment of euvolemic hyponatremia due to SIAD, but 
any symptomatic euvolemic patient is eligible for tolvaptan ther­
apy regardless of the level of hyponatremia or response to previ­
ous fluid restriction

tremia or response to previ­
ous fluid restriction. The starting dose of tolvaptan is 15 mg on 
the first day, although in clinical practice some clinicians recom­
mend starting with a lower dose of 7.5 mg,341 and the dose can be 
titrated to 30 and 60 mg at 24-hour intervals if the serum [Na+] 
remains lower than 135 mmol/L or the increase in serum [Na+] 
has been less than 5 mmol/L in the previous 24 hours. As with 
conivaptan, it is essential that the serum [Na+] be measured fre­
quently duri

that the serum [Na+] be measured fre­
quently during the active phase of correction of the hyponatremia 
at a minimum of every 6 to 8 hours, particularly in patients with 
risk factors for ODS. Goals and limits for the safe correction of 
hyponatremia and methods to compensate for overly rapid correc­
tions are the same as described previously for conivaptan (see Fig. 
10.7). One additional factor that helps avoid overly rapid correc­
tion with tolvaptan is the recommendation that fluid restrict

olvaptan is the recommendation that fluid restriction 
should not be used during the active phase of correction, thereby 
allowing the patient’s thirst to compensate for an overly vigorous 
aquaresis. Common side effects of tolvaptan include dry mouth, 
thirst, increased urinary frequency, dizziness, nausea, and ortho­
static hypotension.340
Vaptans are not needed in the treatment of hypovolemic hypo­
natremia because simple volume expansion would be expected to 
abolish the nonosmotic stimulus 

d be expected to 
abolish the nonosmotic stimulus to AVP secretion and lead to a 
prompt aquaresis. Furthermore, inducing increased renal fluid 
excretion via diuresis or aquaresis can cause or worsen hypoten­
sion in such patients. This possibility has resulted in the labeling 
of these drugs as contraindicated for hypovolemic hyponatremia. 
Importantly, clinically significant hypotension was not observed 
in the conivaptan or tolvaptan clinical trials in euvolemic and 
  General Recommendatio

 function, these agents gener­
ally will not be effective if the serum creatinine level is more than 
3 mg/dL.
The FDA has issued a caution about hepatic injury that was 
noted in patients who received tolvaptan in a 3-year clinical 
trial examining the effect of tolvaptan on autosomal dominant 
polycystic kidney disease, the Tolvaptan Efficacy and Safety in 
Management of Autosomal Dominant Polycystic Kidney Dis­
ease and Its Outcomes (TEMPO) study.342 Although the doses 
used in the TEMPO stud

udy.342 Although the doses 
used in the TEMPO study were up to twice the maximum dose 
approved for hyponatremia (e.g., tolvaptan, 120 mg/day) and 
in clinical trials of tolvaptan at doses approved by the FDA for 
treatment of clinically significant euvolemic or hypervolemic 
hyponatremia liver damage was not reported, the FDA recom­
mended that “Samsca treatment should be stopped if the patient 
develops signs of liver disease. Treatment duration should be 
limited to 30 days or less, and use s

n should be 
limited to 30 days or less, and use should be avoided in patients 
with underlying liver disease, including cirrhosis.” The European 
Medicines Agency (EMA) has approved the use of tolvaptan for 
SIAD but not for hyponatremia due to heart failure or cirrhosis. 
Based on the TEMPO trial results, the EMA also issued a warn­
ing about the possible occurrence of hepatic injury in patients 
treated with tolvaptan but did not recommend any restriction 
on the duration of treatment of SIAD

 restriction 
on the duration of treatment of SIAD patients with tolvaptan. 
Accordingly, appropriate caution should be exercised in patients 
treated with tolvaptan for hyponatremia for extended periods 
(e.g., >30 days), but this decision should be based on the clinical 
judgment of the treating physician. Patients who are refractory 
to or unable to tolerate or obtain other therapies for hypona­
tremia, and for whom the benefit of tolvaptan treatment out­
weighs the risks, remain candidates f

eatment out­
weighs the risks, remain candidates for long-term therapy with 
tolvaptan.
An additional barrier to the use of vasopressin antagonists for 
treatment of hyponatremia is the high cost of the drug. This is true 
in the United States and the European Union but interestingly not 
in Asian countries. Despite this pronounced geographic disparity, 
many economic analyses have confirmed the increased economic 
burden of hyponatremia, which is largely driven by longer hospi­
tal and intensiv

s largely driven by longer hospi­
tal and intensive care stays.343,344 An analysis of use of tolvaptan 
compared with fluid restriction showed a favorable cost savings 
that offset the high cost of tolvaptan, suggesting that selective use 
of these agents in appropriate inpatients may in fact be cost effec­
tive in hospitalized patients in the United States and the European 
Union.345 
Urea
Urea has been described as an alternative oral treatment for SIAD 
and other hyponatremic disorders. The m

 for SIAD 
and other hyponatremic disorders. The mode of action is to cor­
rect hypoosmolality not only by increasing solute-free water excre­
tion but also by decreasing urinary sodium excretion. Doses of 
15 to 60 g/day are generally effective; the dose can be titrated in 
increments of 15 g/day at weekly intervals as necessary to achieve 
normalization of the serum [Na+]. It is advisable to dissolve the 
urea in orange juice or some other strongly flavored liquid to cam­
ouflage the bitter ta

ngly flavored liquid to cam­
ouflage the bitter taste. Even if completely normal water balance 
is not achieved, it is often possible to allow the patient to main­
tain a less strict regimen of fluid restriction while receiving urea. 
The disadvantages associated with the use of urea include poor 
palatability, the development of azotemia at higher doses, and 
the unavailability of a convenient or FDA-approved form of the 
agent. Data suggest that blood urea concentrations may double 
during tre

t blood urea concentrations may double 
during treatment,346 but it is important to remember that this 
does not represent renal impairment.
Reports of retrospective, uncontrolled studies suggest that 
the use of urea has been effective in treating SIAD in patients 
with hyponatremia due to subarachnoid hemorrhage and in criti­
cal care patients,347 and case reports have documented success in 
infants with chronic SIAD348 and the nephrogenic syndrome of 
inappropriate antidiuresis.349 More recen

rome of 
inappropriate antidiuresis.349 More recent evidence from a short 
study in a small cohort of SIAD patients suggests that urea may 
have a comparable efficacy to vaptans in reversing hyponatremia 
due to chronic SIAD.350 Although these reports suggest that urea 
might be an acceptable alternative for treatment of chronic hypo­
natremia, data regarding efficacy and safety of long-term urea 
treatment of hyponatremia are lacking.351 
Furosemide and NaCl
The use of furosemide (20–40 mg/day)

mide and NaCl
The use of furosemide (20–40 mg/day) coupled with a high sodium 
intake (200 mEq/day) represents an extension of the treatment of 
acute symptomatic hyponatremia352 in selected cases.329,353 How­
ever, similar to urea, the efficacy of this approach to correct symp­
tomatic hyponatremia promptly and within accepted goals limits 
is unknown. 
Efficacy of Hyponatremia Treatment
There have been no adequately powered randomized controlled 
trials to compare the efficacy and safety of di

d 
trials to compare the efficacy and safety of different treatments 
utilized to correct hyponatremia. However, results of a prospective 
observational study in a large number of hospitalized patients in 
the United States and the European Union provide useful data 
about the success rates of different therapies in euvolemic hypona­
tremic patients.354,355 In this study, success was defined by three 
different criteria, from least to most stringent: (1) an increase 
in serum [Na+] of at least 5

ent: (1) an increase 
in serum [Na+] of at least 5 mmol/L, (2) correction to a serum 
[Na+] of 130 mmol/L or above, and (3) correction to a normal 
serum [Na+] of 135 mmol/L or above. Only 3% NaCl and tolvap­
tan had success rates significantly greater than 50% for the least 
stringent criterion, and only tolvaptan achieved this level for the 
next most stringent criteria and a significantly higher rate for the 
most stringent criteria of normalization of the serum [Na+]. Of 
particular note, fl

zation of the serum [Na+]. Of 
particular note, fluid restriction, the most frequently prescribed 
therapy in the Hyponatremia Registry patients, achieved a cor­
rection of serum [Na+] in only 44% of patients treated with this 
therapy and isotonic saline in only 36% of patients. This is con­
sistent with a prospective study of 183 hyponatremic patients that 
demonstrated that up to 60% of patients with SIAD had one or 
more criteria predicting nonresponse to fluid restriction (see Table 
10.3).

onresponse to fluid restriction (see Table 
10.3).356 These data underscore the importance of carefully select­
ing therapy for individual patients to meet predefined goals for 
correction of serum [Na+]. 
Hyponatremia Treatment Guidelines Based on Symptom 
Severity
Although many authors have published recommendations on the 
treatment of hyponatremia,310,329,357–361 no standardized treat­
ment algorithms have yet been universally accepted, and some 
major differences between the various guideli

ome 
major differences between the various guidelines and expert rec­
ommendations exist.326,362 For almost all treatment recommenda­
tions, the initial evaluation includes an assessment of the ECF 
volume status of the patient because treatment recommendations 
differ for hypovolemic, euvolemic, and hypervolemic hypona­
tremic patients.310 Euvolemic patients, mainly including those 
with SIAD, represent a unique challenge because of the multiplicity 
327
of cause

city 
327
of causes and presentations of patients with SIAD. Recent expert 
opinion recommendations are based primarily on the neurologic 
symptomatology of hyponatremic patients rather than the serum 
[Na+] or the chronicity of the hyponatremia, because the latter 
is often difficult to ascertain accurately.310 A careful neurologic 
history and assessment should always be conducted to identify 
potential causes for the patient’s symptoms other than hyponatre­
mia

r the patient’s symptoms other than hyponatre­
mia, although it will not always be possible to exclude an additive 
contribution from the hyponatremia to an underlying neurologic 
condition. In this algorithm, patients are divided into three major 
groups based on their presenting symptoms.
Severe Symptoms. Coma, obtundation, seizures, respiratory 
distress or arrest, and unexplained vomiting usually imply a more 
acute onset or worsening of hyponatremia that requires immedi­
ate active treatmen

natremia that requires immedi­
ate active treatment. Therapies that will quickly raise serum [Na+] 
are necessary to reduce cerebral edema and decrease the risk of 
potentially fatal brain herniation. 
Moderate Symptoms. Altered mental status, disorientation, 
confusion, unexplained nausea, gait instability, and falls gener­
ally indicate some degree of brain volume regulation and absence 
of clinically significant cerebral edema. These symptoms can be 
chronic or acute, but allow more time to e

can be 
chronic or acute, but allow more time to elaborate a deliberate 
approach to the choice of therapies. 
Mild or Absent Symptoms. Minimal symptoms, such as dif­
ficulty concentrating, irritability, altered mood, depression, or 
unexplained headache, or a virtual absence of discernible symp­
toms, indicate that the patient may have chronic or slowly 
evolving hyponatremia. These symptoms necessitate a cautious 
approach, especially when patients have underlying comorbidi­
ties, to prevent b

ents have underlying comorbidi­
ties, to prevent both worsening of the hyponatremia and overly 
rapid correction with production of ODS.
Patients with severe neurologic symptoms should be treated 
with hypertonic (3%) NaCl as first-line therapy, followed after 
24 to 48 hours by fluid restriction and/or vaptan therapy. Because 
overly rapid correction of serum [Na+] occurs in more than 10% 
of patients treated with hypertonic NaCl,363 such patients are at 
risk for ODS unless carefully monitored

ts are at 
risk for ODS unless carefully monitored. For this reason, some 
authors have proposed simultaneous treatment with desmopres­
sin to reduce the rate of correction to only that produced by the 
hypertonic NaCl infusion itself.364 Whether sufficient clinical 
data eventually prove that this approach is effective and safe in 
larger numbers of patients remains to be determined.365,366 Only 
one case of ODS has been reported in a patient receiving vap­
tan monotherapy,367 and two abstracts

ceiving vap­
tan monotherapy,367 and two abstracts have reported ODS when 
vaptans were used directly following hypertonic saline administra­
tion within the same 24-hour period. Consequently, no additional 
active hyponatremia therapy should be administered until at least 
24 hours following successful increases in serum [Na+] using 
hypertonic NaCl.
The choice of treatment for patients with moderate symptoms 
will depend on their ECF volume status. Hypovolemic patients 
should be treated with 

tus. Hypovolemic patients 
should be treated with solute repletion via isotonic NaCl infusion 
or oral sodium replacement. Euvolemic patients, typically with 
SIAD, will benefit from vaptan therapy, limited hypertonic saline 
administration, or in some cases urea (where available). This can 
be followed by fluid restriction or long-term vaptan therapy when 
the cause of the SIAD is expected to be chronic. In hypervolemic 
patients with heart failure, vaptans are usually the best choice 
because 

ure, vaptans are usually the best choice 
because fluid restriction is rarely successful in this group,368 saline 
administration can cause fluid retention with increased edema, 
and urea can lead to ammonia buildup in the GI tract if hepatic 
function is impaired. Although moderate neurologic symptoms 
can indicate that a patient is in an early stage of acute hyponatre­
mia, they more often indicate a chronically hyponatremic state 
with sufficient brain volume adaptation to prevent marked symp

ent brain volume adaptation to prevent marked symp­
tomatology from cerebral edema. Most patients with moderate 
hyponatremic symptoms have a more chronic form of hypona­
tremia, so guidelines for goals and limits of correction should be 
followed closely (see Fig. 10.7), and close monitoring of these 
patients in a hospital setting is warranted until the symptoms 
improve or stabilize.
Patients with no or minimal symptoms should be managed 
initially with fluid restriction, although treatment w

ially with fluid restriction, although treatment with pharma­
cologic therapy, such as vaptans or urea, may be appropriate for 
a wide range of specific clinical conditions. Foremost of these is a 
failure to improve the serum [Na+], despite reasonable attempts at 
fluid restriction, or the presence of clinical characteristics associ­
ated with poor responses to fluid restriction (see Table 10.3).
A special case is when spontaneous correction of hyponatre­
mia occurs at an undesirably rapid rate

hyponatre­
mia occurs at an undesirably rapid rate because of the onset of 
water diuresis. This can occur following cessation of desmopressin 
therapy in a patient who has become hyponatremic, replacement 
of glucocorticoids in a patient with adrenal insufficiency, replace­
ment of solutes in a patient with diuretic-induced hyponatremia, 
or spontaneous resolution of transient SIAD. Brain damage from 
ODS can clearly ensue in this setting if the preceding period of 
hyponatremia has been long e

 preceding period of 
hyponatremia has been long enough (usually ≥48 hours) to allow 
brain volume regulation to occur. If the correction parameters dis­
cussed earlier have been exceeded and the correction is proceeding 
more rapidly than planned (usually because of continued excretion 
of hypotonic urine), the pathologic events leading to demyelin­
ation can be reversed by administration of hypotonic fluids, with 
or without desmopressin. The efficacy of this approach has been 
suggested from 

fficacy of this approach has been 
suggested from animal studies369 and case reports in humans,370 
even when patients are overtly symptomatic.371 However, relow­
ering the serum [Na+] after an initial, overly rapid correction is 
only strongly recommended for patients at high risk of ODS; it 
is considered optional for patients with a low-to-moderate risk of 
ODS, and unnecessary for patients with acute water intoxication 
(see Fig. 10.7). 
Monitoring the Serum [Na+] in Hyponatremic Patients
Th

toring the Serum [Na+] in Hyponatremic Patients
The frequency of serum [Na+] monitoring is dependent on both 
the severity of the hyponatremia and the therapy chosen. All 
patients undergoing active treatment with hypertonic saline for 
symptomatic hyponatremia should have frequent monitoring of 
serum [Na+], urine output, and ECF volume status (every 2–4 
hours) to ensure that the serum [Na+] does not exceed the limits of 
safe correction during the active phase of correction310 since overly 
r

 the active phase of correction310 since overly 
rapid correction of serum [Na+] will increase the risk of ODS.233 
Patients treated with vaptans for moderate or mild symptoms 
should have serum [Na+] monitored every 6 to 8 hours during the 
active phase of correction, which will generally be the first 24 to 
48 hours of therapy. Active treatment with any therapy should be 
stopped when the patient’s symptoms are no longer present, a safe 
serum [Na+] (usually >125 mmol/L) has been achieved, or 

[Na+] (usually >125 mmol/L) has been achieved, or the rate 
of correction has reached maximum limits of 10 to 12 mmol/L 
within 24 hours, 18 mmol/L within 48 hours,310,330 or 8 mmol/L 
over any 24-hour period in patients at high risk of ODS (see Fig. 
10.7). In patients with a stable level of serum [Na+] treated with 
fluid restriction or therapies other than hypertonic saline, mea­
surement of serum [Na+] daily is generally sufficient since levels 
will not change that quickly in the absence of

ls 
will not change that quickly in the absence of active therapy or 
large changes in fluid intake or administration. 
328
Long-Term Treatment of Chronic Hyponatremia
Some patients will benefit from continued treatment of hypona­
tremia following discharge from the hospital. In many cases, this 
will consist of a continued fluid restriction, but long-term com­
pliance with this therapy is poor because of the increased thirst 
that occurs with more severe

the increased thirst 
that occurs with more severe degrees of fluid restriction. Thus for 
select patients who have responded to tolvaptan in the hospital, 
consideration should be given to continuing the treatment as an 
outpatient after discharge. In patients with established chronic 
hyponatremia, tolvaptan has shown to be effective for maintaining 
a normal [Na+] for as long as 3 years of continued daily therapy.372 
However, many patients with inpatient hyponatremia have a tran­
sient form 

th inpatient hyponatremia have a tran­
sient form of SIAD without the need for long-term therapy. Selec­
tion of which patients with inpatient hyponatremia are candidates 
for long-term therapy should be based on the cause of the SIAD. 
In all cases, consideration should be given to a trial of stopping the 
drug 2 to 4 weeks after discharge to determine if hyponatremia is 
still present. A reasonable period of tolvaptan cessation to evaluate 
the presence of continued SIAD is 7 days because this

 presence of continued SIAD is 7 days because this period was 
found to be sufficient for demonstration of a recurrence of hypo­
natremia in the tolvaptan SALT339 and SALT-WATER trials.372
Findings of hepatotoxicity in a small number of patients on 
high doses of tolvaptan in a clinical trial of polycystic kidney dis­
ease led to a recent FDA recommendation that tolvaptan not be 
used for longer than 30 days. If tolvaptan is used for longer than 
30 days, liver function should be assessed at reg

30 days, liver function should be assessed at regular intervals (e.g., 
every 3 months) at least for the first year of therapy. As is always 
the case, decisions about appropriate treatment of hyponatremia 
should be based on clinical judgment and risk/benefit analysis that 
is individualized for specific patients, because there is no single 
treatment that represents the “best” therapy for all patients with 
SIAD.326 
Oxytocin
Study of the normal physiologic regulation of oxytocin secretion 
a

al physiologic regulation of oxytocin secretion 
and action is complicated by the fact that secretion and func­
tion of oxytocin varies markedly among different experimental 
mammals. There are varying sites of synthesis in the ovary and 
in tissues of the uterus that are different among species. It is dif­
ficult to study pregnant women and human tissue, so physiologic 
regulation of oxytocin secretion and function is less well known 
in humans than other species. The classic roles of oxytocin 

than other species. The classic roles of oxytocin are 
smooth muscle activation promoting milk letdown with nursing 
and uterine myometrial contraction at parturition.
Lactation
A characteristic of all mammals is lactation, and all mammals 
secrete oxytocin to stimulate milk letdown associated with nurs­
ing.373 The other hormone critical to lactation is prolactin. Each of 
these pituitary/hypothalamic hormones is importantly influenced 
and regulated by gonadal steroid hormones. The milk-produc

lated by gonadal steroid hormones. The milk-producing 
unit of the breast is the alveolar system with multiple clusters of 
milk-producing cells surrounded by specialized myoepithelial 
cells. The alveoli are directly connected to ductules, and then 
ducts converge and lead to the nipple. Milk is synthesized in the 
glandular cells of the alveoli.374 Oxytocin receptors are localized 
on glandular cells, and oxytocin in the systemic circulation acts on 
these receptors to cause myoepithelial cont

ts on 
these receptors to cause myoepithelial contraction. Oxytocin also 
acts on myoepithelial cells along the duct to shorten and widen 
the ducts to enhance milk flow through the ducts to the nipple.375
When an infant begins sucking at the breast, an afferent signal 
is transmitted from the mechanoreceptors or tactile receptors in 
the breast to the spinal cord and eventually ascend to the oxytocin 
magnocellular neurons in the supraoptic nucleus and the paraven­
tricular nucleus.375,376 Puls

eus and the paraven­
tricular nucleus.375,376 Pulsatile release of oxytocin produces a pul­
satile pumping action on the alveoli, which promotes maximum 
emptying of milk from the alveoli.374 The importance of oxytocin 
in maintaining milk secretion is demonstrated by transgenic mice 
with a knockout that inhibits oxytocin synthesis. These animals 
deliver their young normally and have normal milk production, 
but there is no milk release in spite of normal suckling. The pups 
die of dehydration

e of normal suckling. The pups 
die of dehydration with no milk in the stomach.377 Adminis­
tration of oxytocin to these oxytocin-deficient mice rescues the 
ability to secrete milk and allows the pups to survive. Similarly 
oxytocin may promote successful lactation in women who have 
lactational insufficiency.378
As breastfeeding continues in humans, the basal levels of oxy­
tocin decrease but pulses of oxytocin in response to suckling con­
tinue and may increase.379 Humans with diabetes insipi

e and may increase.379 Humans with diabetes insipidus have 
been able to successfully breastfeed infants, and this has caused 
some to question the importance of oxytocin in humans.380 How­
ever, oxytocin secretion may be preserved in the absence of vaso­
pressin in patients with diabetes insipidus. 
Parturition
The isolation of oxytocin was followed quickly by the description 
of oxytocin to stimulate uterine contractions, and this was fol­
lowed shortly by clinical use of oxytocin as a uteroto

d shortly by clinical use of oxytocin as a uterotonic agent.381 
Parturition in humans is much more complex than just the role of 
oxytocin.382 In all species the uterus must grow during pregnancy, 
and estrogen is a promoter of this growth. Levels of oxytocin in 
humans are not well defined in pregnancy but are not reported to 
increase until the expulsive stage at term.382–384 The uterine myo­
metrial cells have intrinsic contractile activity, but during preg­
nancy the uterus is maintained in

but during preg­
nancy the uterus is maintained in a quiescent state by the actions 
of progesterone and relaxin (produced by the corpus luteum and 
decidual tissue).385–387 The initiation of labor is accomplished by 
a relative increase in estrogen activation and a decrease in proges­
terone activation. Changes in oxytocin receptors and oxytocin 
produced by the placenta may be more important than levels of 
oxytocin in the circulation. During early labor there is an upregu­
lation in the uteru

arly labor there is an upregu­
lation in the uterus of oxytocin receptor mRNA, and oxytocin 
receptor numbers increase.384,388 Oxytocin receptors are promi­
nent in the fundus of the uterus where they stimulate myometrial 
contraction and in decidual cells where they stimulate the produc­
tion of prostaglandins. At parturition increased oxytocin activity 
in the fundus will push the fetus toward the cervix that is thinned 
and relaxed by the effects of prostaglandins.383,389 Prostaglandins 
play

cts of prostaglandins.383,389 Prostaglandins 
play a key role in an inflammatory process that is important in the 
uterus at parturition. Cytokines induce enzymes that digest extra­
cellular matrix to soften and ripen the cervix.383 The role of pro­
gesterone in maintaining uterine quiescence is not only the action 
on oxytocin receptors but also the antagonizing of the inflamma­
tory response that softens the lower uterus and cervix.384
There are three situations in pregnancy where a pharmacolo

e three situations in pregnancy where a pharmacologic 
role of oxytocin is of interest. The first situation and most widely 
used role of oxytocin is to induce and augment labor.390 This has 
received increased interest in an effort to decrease the number of 
and morbidity of cesarean sections.391 Oxytocin may be delivered 
alone or in combination with another pharmacologic agent such 
as propranolol or prostaglandins.391,392 The second area of interest 
329
is pr

nterest 
329
is preterm labor with an effort toward prevention by decreasing 
contractile activity of the uterus and/or inhibiting the inflamma­
tory response.393 Peptide and nonpeptide oxytocin antagonists 
have been of special interest to inhibit myometrial contractions, 
but widespread clinical use awaits the development of antagonist 
with better risk/benefit activity.394 The third pharmacologic inter­
est in oxytocin is as a uterotonic to decrease postpartum 

xytocin is as a uterotonic to decrease postpartum hemor­
rhage associated with uterine atony.395 Postpartum hemorrhage is 
the major cause of maternal deaths worldwide and ranks second 
to embolism as a cause of maternal death in the United States.396 
Mechanical options in the active management of the third stage of 
labor include cord traction to reduce the risk of retained placenta 
and uterine massage, which has been augmented by pharmacologic 
agents, most commonly oxytocin and/or ergotamin

c 
agents, most commonly oxytocin and/or ergotamine.395 Maternal 
death by postpartum hemorrhage is most significant in develop­
ing countries. Oxytocin is heat labile and requires a trained staff 
for appropriate administration,397 prompting a search for other 
agents. Promising results have been reported with prostaglandin 
analogues (e.g., misoprostol).394,395,398 
Behavior
This chapter is about functions of vasopressin and oxytocin as tra­
ditional endocrine hormones secreted by the posterio

tional endocrine hormones secreted by the posterior pituitary. 
For further discussion related to these hormones in purported 
functions as neurotransmitters, especially with regard to influenc­
ing social and ingestive behaviors, the reader is referred to Chap­
ters 9 and 20.
         
330.e1
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al. Characterization 
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ct of chronic 
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332
11
Thyroid Pathophysiology and 
DOMENICO SALVATORE, RONALD COHEN, PETER A. KOPP, AND P

NICO SALVATORE, RONALD COHEN, PETER A. KOPP, AND P. REED LARSEN
D
ysfunction and anatomic abnormalities of the thyroid 
are among the most common diseases of the endocrine 
glands. This chapter provides the physiologic and bio­
chemical background and describes the various tests for evaluating 
patients with suspected thyroid dysfunction based on the patho­
physiology of these conditions.
Phylogeny, Embryology, and Ontogeny
Phylogeny
The phylogeny, embryogenesis, and certain aspects of thyroid 

y, embryogenesis, and certain aspects of thyroid 
function are closely interlinked with the gastrointestinal tract. 
The capacity of the thyroid to metabolize iodine and incorporate 
it into a variety of organic compounds occurs widely through­
out the animal and plant kingdoms. However, the anatomy of the 
thyroid gland differs considerably among the vertebrate classes. 
Monoiodotyrosine (3′-monoiodo-l-tyrosine [MIT]) and diiodo­
tyrosine (3,5-diiodo-l-tyrosine [DIT]) are present in a variety o

iiodo-l-tyrosine [DIT]) are present in a variety of 
invertebrate species, including mollusks, crustaceans, coelenter­
ates, annelids, insects, and certain marine algae. In these lower 
forms, however, no recognizable thyroid tissue is present. Thyroid 
tissue is confined to, and is present in, all vertebrates. A close link 
to the thyroid of higher vertebrates is evident in the ammocoete, 
the larval form of the lamprey, where the ventral part of the phar­
ynx is the origin of a structure prese

f the phar­
ynx is the origin of a structure present only during larval life, the 
endostyle. The epithelium of the endostyle is capable of carrying 
out iodinations, and these cells are fated to become follicular cells 
only after metamorphosis, when they will form classic thyroid 
follicles.1
The phylogenetic association of the thyroid gland and the 
gastrointestinal tract is evident in several functions. The salivary 
and gastric glands, like the thyroid, are capable of concentrating 
iodide 

the thyroid, are capable of concentrating 
iodide in their secretions, although iodide transport in these sites 
is not responsive to stimulation by thyrotropin (TSH, thyroid-
stimulating hormone). The salivary gland contains enzymes that 
are capable of iodinating tyrosine in the presence of hydrogen per­
oxide, although it forms insignificant quantities of iodoproteins 
under normal circumstances. 
Structural Embryology
The morphogenesis of the thyroid gland, the anterior-most 
organ that buds

 thyroid gland, the anterior-most 
organ that buds from the gut tube, begins with a thickening of 
the endodermal epithelium in the foregut, which is referred to as 
the thyroid anlage. The human thyroid anlage is first recognizable 
Phylogeny, Embryology, and Ontogeny, 332
Anatomy and Histology, 333
Iodine and the Synthesis and Secretion of Thyroid Hormones, 334
Thyroid Hormones in Peripheral Tissues, 338
Regulation of Thyroid Function, 347
Physical Evaluation of the Thyroid Gla

tion of patients with clinical thyroid 
dysfunction or disease.
333
Chapter 11  Thyroid Pathophysiology and  Diagnostic Evaluation
at embryonic day 16 or 17. This median thickening deepens and 
forms first a small pit and then an outpouching of the endoderm 
adjacent to the developing myocardial cells. With continuing devel­
opment, the median diverticulum is migrating caudally, following 
the myocardial cells in their descent. The primitive stalk connect­
ing the primordium with the pharyngeal 

k connect­
ing the primordium with the pharyngeal floor elongates into the 
thyroglossal duct. During its caudal migration, the primordium 
assumes a bilobate shape, coming into contact and fusing with the 
ventral aspect of the fourth pharyngeal pouch when it reaches its 
final position at about embryonic day 50. Normally the thyroglossal 
duct undergoes dissolution and fragmentation by about the second 
month after conception, leaving at its point of origin a small dimple 
at the junction of t

int of origin a small dimple 
at the junction of the middle and posterior thirds of the tongue, the 
foramen caecum. Cells of the lower portion of the duct differentiate 
into thyroid tissue, forming the pyramidal lobe of the gland. At this 
time, the lobes contact the ultimobranchial glands, leading to the 
incorporation of C cells into the thyroid. Concomitantly, histologic 
alterations occur throughout the gland. Complex interconnecting 
cordlike arrangements of cells interspersed with vascul

ike arrangements of cells interspersed with vascular connec­
tive tissue replace the solid epithelial mass and become tubule-like 
structures at about the third month of fetal life; shortly thereafter, 
follicular arrangements devoid of colloid appear, and by 13 to 
14 weeks the follicles begin to fill with colloid. Investigations of thy­
roid gland development in mice using gene-targeting techniques are 
beginning to identify the critical factors that are required for normal 
thyroid gland deve

s that are required for normal 
thyroid gland development.2,3 The role of these various proteins is 
currently being evaluated with respect to the potential for defects in 
the synthesis or formation of the thyroid gland (see Chapter 13). 
Functional Ontogeny
The ontogeny of thyroid function and its regulation in the human 
fetus are fairly well defined.4,5 Future follicular cells acquire the 
capacity to form thyroglobulin (Tg) as early as the 29th day of ges­
tation, whereas the capacities to 

9th day of ges­
tation, whereas the capacities to concentrate iodide and synthesize 
thyroxine (T4) are delayed until about the 11th week. Radioactive 
iodine inadvertently given to the mother would be accumulated 
by the fetal thyroid soon thereafter. Early growth and develop­
ment of the thyroid do not seem to be TSH dependent because 
the capacity of the pituitary to synthesize and secrete TSH is not 
apparent until the 14th week. Subsequently, rapid changes in pitu­
itary and thyroid functio

, rapid changes in pitu­
itary and thyroid function take place. Probably as a consequence 
of hypothalamic maturation and increasing secretion of thyro­
tropin-releasing hormone (TRH), the serum TSH concentration 
increases between 18 and 26 weeks of gestation, after which levels 
remain higher than those in the mother.4 The higher levels may 
reflect a higher set-point of the negative feedback control of TSH 
secretion during fetal life than at maturity. Thyroxine-binding 
globulin (TBG), the m

maturity. Thyroxine-binding 
globulin (TBG), the major thyroid hormone–binding protein in 
plasma, is detectable in the serum by the 10th gestational week 
and increases in concentration progressively to term. This increase 
in TBG concentration accounts in part for the progressive increase 
in the serum T4 concentration during the second and third trimes­
ters, but increased secretion of T4 must also play a role because the 
concentration of free T4 also rises.
Several aspects of thyroid develo

e T4 also rises.
Several aspects of thyroid development are of note from a 
clinical standpoint. Rarely, thyroid tissue may develop from 
remnants of the thyroglossal duct near the base of the tongue. 
Such lingual thyroid tissue may be the sole functioning thyroid 
present, and thus its surgical removal will lead to hypothyroid­
ism. More commonly, elements of the thyroglossal duct may 
persist and later give rise to thyroglossal duct cysts, or ectopic 
thyroid tissue may be present at any loca

ectopic 
thyroid tissue may be present at any location in the mediasti­
num or, rarely, even in the heart. 
Anatomy and Histology
The thyroid is one of the largest of the endocrine organs, weighing 
approximately 15 to 20 g in North American adults. Moreover, the 
potential of the thyroid for growth is tremendous. The enlarged 
thyroid, commonly termed goiter, can weigh many hundreds of 
grams. The normal thyroid is made up of two lobes joined by a 
thin band of tissue, the isthmus, which is app

 a 
thin band of tissue, the isthmus, which is approximately 0.5 cm 
thick, 2 cm wide, and 1 to 2 cm high. The individual lobes nor­
mally have a pointed superior pole and a poorly defined blunt 
inferior pole that merges medially with the isthmus. Each lobe 
is approximately 2 to 2.5 cm in thickness and width at its larg­
est diameter and is approximately 4 cm in length. Occasionally, 
especially when the remainder of the gland is enlarged, a pyrami­
dal lobe is discernible as a finger-like pro

rami­
dal lobe is discernible as a finger-like projection directed upward 
from the isthmus, generally just lateral to the midline, usually on 
the left. The right lobe is normally more vascular than the left, is 
often the larger of the two, and tends to enlarge more in disorders 
associated with a diffuse increase in gland size. Two pairs of vessels 
constitute the major arterial blood supply: the superior thyroid 
artery, arising from the external carotid artery, and the inferior 
thyroid art

rnal carotid artery, and the inferior 
thyroid artery, arising from the subclavian artery. Estimates of thy­
roid blood flow range from 4 to 6 mL/minute/g, well in excess of 
the blood flow to the kidney (3 mL/minute/g). In a diffuse toxic 
goiter due to Graves disease, blood flow may exceed 1 L/minute 
and be associated with an audible bruit or even a palpable thrill.
The gland is composed of closely packed spherical units termed 
follicles, which are invested with a rich capillary network. The

ch are invested with a rich capillary network. The inte­
rior of the follicle is filled with the clear proteinaceous colloid that 
normally is the major constituent of the total thyroid mass. On 
cross section, thyroid tissue appears as closely packed, ring-shaped 
structures consisting of a single layer of thyroid cells surrounding 
a lumen. The diameter of the follicles varies considerably, even 
within a single gland, but averages about 200 nm. The follicu­
lar cells vary in height with the d

. The follicu­
lar cells vary in height with the degree of glandular stimulation, 
becoming columnar when active and cuboidal when inactive. The 
epithelium rests on a basement membrane that is rich with glyco­
proteins separating the follicular cells from the surrounding capil­
laries. From 20 to 40 follicles are demarcated by connective tissue 
septa to form a lobule supplied by a single artery. The function of 
a given lobule may differ from that of its neighbors.
On electron microscopy, the 

hat of its neighbors.
On electron microscopy, the thyroid follicular epithelium has 
many features in common with other secretory cells and some 
peculiar to the thyroid. From the apex of the follicular cell, numer­
ous microvilli extend into the colloid. It is at or near this surface 
of the cell that iodination, exocytosis, and the initial phase of hor­
mone secretion, colloid resorption, occur (Fig. 11.1). The nucleus 
has no distinctive features, and the cytoplasm contains an extensive 
endo

res, and the cytoplasm contains an extensive 
endoplasmic reticulum (ER) laden with microsomes. The ER is 
composed of a network of wide irregular tubules that contain the 
precursor of Tg. The carbohydrate component of Tg is added to 
this precursor in the Golgi apparatus, which is located apically. 
Lysosomes and mitochondria are scattered throughout the cyto­
plasm. Stimulation by TSH results in enlargement of the Golgi 
apparatus, formation of pseudopodia at the apical surface, and the 
appe

f pseudopodia at the apical surface, and the 
appearance in the apical portion of the cell of many droplets that 
contain colloid taken up from the follicular lumen (see Fig. 11.1).
The thyroid also contains parafollicular cells, or C cells, which 
are the source of calcitonin. The traditional view was that these 
334
SECTION III    Thyroid
cells derive from the neural crest, but newer lineage tracing data 
suggest that they arise also in the endoderm.6 Ultimately they 
come to rest either among

oderm.6 Ultimately they 
come to rest either among the cells of the follicular epithelium or 
in the thyroid interstitium. They differ from the cells of the fol­
licular epithelium in never bordering on the follicular lumen and 
in being rich in mitochondria. The C cells, detected also in human 
lingual thyroids,7 undergo hyperplasia early in the syndrome of 
familial medullary carcinoma of the thyroid (multiple endocrine 
neoplasia type 2 [MEN2]) and give rise to this tumor in both its 
familia

) and give rise to this tumor in both its 
familial and its sporadic forms (see Chapter 42). 
Iodine and the Synthesis and Secretion of 
Thyroid Hormones
The function of the thyroid is to generate the quantity of thyroid 
hormone necessary to meet the demands of the peripheral tissues. 
This action requires iodide uptake by the thyroidal sodium-iodide 
symporter (NIS), its transfer to the colloid, and its oxidation by 
thyroid peroxidase (TPO) to allow the synthesis of approximately 
110 nmol/L 

 allow the synthesis of approximately 
110 nmol/L (85 μg) of T4, which is 65% iodine by weight. This 
requires the synthesis of an approximately 330-kDa glycoprotein, 
Tg. Specific tyrosine residues of Tg homodimers are then iodin­
ated at the apical border of the thyroid cell to form MIT and 
DIT (see Fig. 11.1). This requires formation of hydrogen perox­
ide (H2O2) by dual oxidases (DUOX1 and 2) and TPO, which 
catalyzes the oxidation of iodide and its transfer to tyrosine. TPO 
also catalyzes

 and its transfer to tyrosine. TPO 
also catalyzes the coupling of two molecules of DIT or one of DIT 
and one of MIT, leading to formation of T4 and triiodothyronine 
(T3), respectively, which are then stored within the colloid, still as 
part of the Tg molecule. Pinocytosis of stored colloid leads to the 
formation of phagolysosomes, the colloid droplets in which Tg is 
digested by specific proteases to release T4, T3, DIT, and MIT as 
the droplet is translocated toward the basal portion of th

let is translocated toward the basal portion of the cell. T4 
and T3 are transported out of the phagolysosomes and across the 
basolateral cell membrane, at least in part by MCT8, to exit the 
cell and enter circulation, whereas DIT and MIT are deiodinated 
by iodotyrosine dehalogenase (DEHAL1) to allow recycling of the 
iodide. The synthesis of thyroid hormones requires the expression 
of a number of thyroid cell–specific proteins. In addition to Tg 
and TPO, the TSH receptor (TSHR) is also req

o Tg 
and TPO, the TSH receptor (TSHR) is also required to transduce 
the effects of extracellular TSH for efficient hormone synthesis. A 
number of transcription factors, including thyroid transcription 
factors TTF1 (NKX2-1) and TTF2 (FOXE1), PAX8, and hepato­
cyte nuclear factor 3 (HNF3 [FOXE2]), as well as TSH, are neces­
sary to achieve functional differentiation of the thyroid follicular 
cells and the onset of hormonogenesis.1,3 Transient overexpres­
sion of the transcription factors NKX2

overexpres­
sion of the transcription factors NKX2-1 and PAX8 is sufficient 
to direct mouse embryonic stem cell (mESC) differentiation into 
thyroid follicular cells that organize into three-dimensional follic­
ular structures when treated with TSH, and follicles show sig­
nificant iodide organification activity.8 Although the biochemical 
K+
Na+
2 K+
3 Na+
H2O2
I-
Colloid
Capillary
PDS 
ANO 
T4 
T3 
DEHAL1 
MCT8 
MIT 
DIT 
Hydrolysis
NIS 
Na/K-ATPase 
D2 
Tg 
TPO 
DUOX2A 
H2O2
I- 
I- 
I-
1 I-

ATPase 
D2 
Tg 
TPO 
DUOX2A 
H2O2
I- 
I- 
I-
1 I-
Ca2+ 
NADP+ 
Na+ 
+ 
+ 
2 Na+ 
NADPH
I- 
DUOX2 
IP3 
TSHR 
TSH 
AC 
cAMP
PLC 
DAG
Iodinated
Tg
(T4,T3,MIT,DIT)
D1 
Tg 
MIT 
Tg 
T3 
Tg 
T4 
Tg 
DIT
Org
anif
ica
tio
n
Co
up
li
n
g
• Fig. 11.1  Schematic illustration of a follicular cell showing the key aspects of thyroid iodine transport and 
thyroid hormone synthesis. AC, adenyl cyclase; ATPase, adenosine triphosphatase; cAMP, cyclic adenos­
ine monophosphate; D1, thyroidal deiodinase type 1; D2

monophosphate; D1, thyroidal deiodinase type 1; D2, thyroidal deiodinase type 2; DAG, diacylglycerol; 
DEHAL1, iodotyrosine dehalogenase 1 (IYD); DIT, diiodotyrosine; DUOX, dual oxidase; IP3, inositol triphos­
phate; MIT, monoiodotyrosine; NADP+, oxidized form of nicotinamide adenosine dinucleotide phosphate; 
NADPH, reduced nicotinamide adenosine dinucleotide phosphate; NIS, sodium-iodide symporter; PDS, 
pendrin (SLC26A4); PLC, phospholipase C; T3, triiodothyronine; T4, thyroxine; Tg, thyroglo

 T3, triiodothyronine; T4, thyroxine; Tg, thyroglobulin; TPO, 
thyroid peroxidase; TSH, thyrotropin; TSHR, thyrotropin receptor.
335
details of these processes are beyond the scope of this discussion, 
those aspects with clinical relevance are reviewed in greater detail 
in the following section.
Dietary Iodine
Formation of normal quantities of thyroid hormone requires the 
availability of adequate quantities of exogenous iodine to a

ty of adequate quantities of exogenous iodine to allow 
thyroidal uptake of approximately 60 to 75 μg daily, taking into 
account the fecal losses of about 10 to 20 μg iodine of iodothyro­
nines as glucuronides and about 100 to 150 μg as urinary iodine 
in iodine-sufficient populations.9 Plasma iodide (I−), the form of 
the element in biologic solutions, is completely filterable with 
about 60% to 70% of the filtered load reabsorbed. At least 100 μg 
of iodine per day is required to eliminate al

 μg 
of iodine per day is required to eliminate all signs of iodine defi­
ciency (Table 11.1). In healthy adults, the absorption of iodide 
is greater than 90%. In North America, the daily dietary iodine 
intake is in the range of 150 to 300 μg, largely owing to the iodin­
ation of salt; in Japan, however, where large quantities of foods 
rich in iodine are consumed, intakes may be as high as several 
milligrams per day. Notably, iodine intake in the United States is 
decreasing due to a reducti

 the United States is 
decreasing due to a reduction in salt consumption, with median 
urinary iodine of 160 μg/L but a low urinary iodine (<5 μg/dL) 
in close to 10% of the population.10 The daily dietary intake of iodine 
varies widely throughout the world, depending on the iodine con­
tent of soil and water and on dietary practice (see Table 11.1) 
(Iodine Global Network; http://www.ign.org/). Even in a single 
area, iodine intake varies among different individuals and in the 
same individual

 different individuals and in the 
same individual from day to day. Iodine may also enter the body 
via medications, diagnostic agents, dietary supplements, and food 
additives. As discussed more extensively under “Regulation of 
Thyroid Function,” iodine deficiency is common, especially in 
mountainous and formerly glaciated regions of the earth.11 An 
estimated 1 billion individuals live in iodine-deficient areas of 
the world, and these people often develop TSH-induced com­
pensatory enlargem

 often develop TSH-induced com­
pensatory enlargement of the thyroid (endemic goiter). If iodine 
deficiency is severe during pregnancy, fetal thyroid hormone 
production falls with irreparable damage to the developing central 
nervous system (CNS). This damage is manifested by varying 
degrees of mental retardation and is termed endemic cretinism. 
Thus iodine-deficiency disorders (IDDs), including endemic goi­
ter and cretinism, are the most common thyroid-related human 
illnesses; indeed, the

mmon thyroid-related human 
illnesses; indeed, they are the most common endocrine disorders 
worldwide. Iodine deficiency is also the most prevalent prevent­
able cause of mental impairment.
Plasma iodide is partly replenished by that lost from the 
thyroid into the blood and by iodide liberated through deiodin­
ation of iodothyronines in peripheral tissues. Ultimately, however, 
the diet is its most important source. Iodine is ingested in both 
inorganic and organically bound forms. Iodide per 

inorganic and organically bound forms. Iodide per se is rapidly 
and efficiently absorbed from the gastrointestinal tract (within 
30 minutes), and little is lost in the stool. In the body, iodide is 
confined largely to the extracellular fluid. It is also found, how­
ever, in red blood cells and is concentrated in the intraluminal 
fluids of the gastrointestinal tract, notably the saliva and gastric 
juice, from which it is reabsorbed, thus reentering the extracel­
lular fluid. Iodide is also c

tering the extracel­
lular fluid. Iodide is also concentrated in milk. Until oxidized and 
organified on tyrosyl residues in Tg, iodide entering the thyroid by 
active transport is in rapid equilibrium with the main iodide pool. 
The concentration of iodide in the extracellular fluid is normally 
10 to 15 μg/L (∼10−7 mol/L), and the content of the peripheral 
pool is approximately 250 μg. The thyroid contains the largest 
pool of body iodine, under normal circumstances approximately 
8000 μg, mo

er normal circumstances approximately 
8000 μg, most of which is in the form of DIT and MIT. Normally 
this pool of iodine turns over slowly (about 1% per day). 
Iodide Metabolism by the Thyroid Cell
The spherical thyroid follicle, which is formed by a single layer epi­
thelium of thyroid cells surrounding the lumen, is the functional 
unit of the thyroid and its integrity is essential for the synthesis of 
thyroid hormones. Because the concentration of iodide in plasma 
is so low, a mechanism i

tion of iodide in plasma 
is so low, a mechanism is required for the thyroid cell to concen­
trate the required amounts of this element. This process, iodide 
trapping, is accomplished by a membrane protein, the sodium-
iodide symporter NIS, encoded by the SLC5A gene. Human NIS 
is a 643–amino acid glycoprotein with 13 membrane-spanning 
domains. The transport of iodide is an active process dependent 
on the presence of a sodium gradient across the basal membrane 
of the thyroid cell such that d

he basal membrane 
of the thyroid cell such that downhill transport of two Na+ ions 
results in the entry of one iodide atom against an electrochemi­
cal gradient (see Fig. 11.1). In addition to being expressed in the 
basolateral membrane of the thyroid cell, NIS has also been iden­
tified in other iodide-concentrating cells, including salivary and 
lactating mammary glands, choroid plexus, and gastric mucosa, 
and in the cytotrophoblast and syncytiotrophoblast.12,13 NIS is 
also expressed in t

cytiotrophoblast.12,13 NIS is 
also expressed in the ovary and testis and in ovarian cancer and the 
majority of seminomas and embryonal testicular carcinomas.14 In 
the lactating mammary gland, NIS plays an important role by con­
centrating iodide in the milk, thereby supplying newborns with 
iodide for thyroid hormone synthesis. The iodide transport sys­
tem generates an iodide gradient of 20 to 40 across the cell mem­
brane, and NIS also transports pertechnetate (TcO4−), perchlorate 
(ClO4−),

ports pertechnetate (TcO4−), perchlorate 
(ClO4−), and thiocyanate (SCN−), accounting for the utility of 
radioactive TcO4− as a thyroid scanning tool and the capacity of 
KClO4− to block iodide uptake as a competitive inhibitor.15,16 The 
affinity of NIS for iodide is much higher than it is for the other 
inorganic halide anions, such as bromide and chloride, accounting 
for the selectivity of the thyroid transport mechanism.17
  Recommended and Typical Values for 
Dietary Iodine Intake
Recomm

d Typical Values for 
Recommended Daily Intake
Adults
150 μg
During pregnancya
220 μg
Children
90–120 μg
Median Urinary Iodine Concentrationsb
United States (2010)
213 μg
China (2017)
239 μg
Belgium (2011)
113 μg
Switzerland (2015)
137 μg
Russia (2004)
78 μg
aRecommendations by the Institute of Medicine, Food and Nutrition Board.
The World Health Organization (WHO), United Nations Children’s Fund (UNICEF), and the Inter­
national Council for the Control of Iodine Deficiency

ional Council for the Control of Iodine Deficiency Disorders (ICCIDD) recommend a slightly 
higher iodine intake for pregnant women of 250 μg/day.
bData from Iodine Global Network, http://www.ign.org/.
   
TABLE 11.1
336
Transcription of the NIS gene is increased by TSH, and 
TSH also prolongs NIS protein half-life and targets the protein to 
the cell membrane. Iodide uptake by NIS and the organification 
process are inversely regulated by high intracellular iodide concen­

ely regulated by high intracellular iodide concen­
trations (Wolff-Chaikoff effect; see later).18
That the iodide-concentrating mechanism is required for 
normal thyroid function has been known for decades in that its 
absence is associated with congenital hypothyroidism and goiter 
unless large quantities of inorganic iodide are provided.19 A number 
of families have now been identified in which biallelic mutations in 
the NIS gene are associated with an iodide transport defect, result­
ing in 

d with an iodide transport defect, result­
ing in congenital hypothyroidism. Importantly, several studies have 
documented decreases in NIS expression in human thyroid adeno­
mas and carcinomas that contribute to the loss of iodine uptake in 
neoplastic thyroid cells, which therefore present as “cold” nodules 
on radioisotopic imaging.19 However, changes in the subcellular 
location of NIS may also explain this phenomenon. At the apical 
membrane, iodide must enter the follicular lumen. This pro

, iodide must enter the follicular lumen. This process is 
thought to involve pendrin, a highly hydrophobic membrane 
glycoprotein and multianion exchanger located at the apical mem­
brane of thyrocytes.20,21 In addition to the thyroid, pendrin is also 
expressed in the kidney and in the inner ear.22 In the kidney, pen­
drin plays an important role in acid-base metabolism as a chloride/
bicarbonate exchanger.23 In the inner ear, pendrin is important for 
generation of the endocochlear potential.

ant for 
generation of the endocochlear potential.19 Pendrin belongs to the 
SLC26A family and is encoded by the SLC26A4 gene. Mutations 
in the SLC26A4 gene lead to Pendred syndrome, an autosomal 
recessive disorder characterized by sensorineural deafness, goiter, 
and a partial defect in iodide organification.24,25 Deafness or hear­
ing impairment is the major phenotypic manifestation in Pendred 
syndrome. Goiter usually develops during childhood, and under 
conditions of scarce iodine intake,

od, and under 
conditions of scarce iodine intake, some individuals with biallelic 
mutations in pendrin present with congenital hypothyroidism. 
There is, however, a substantial variation within and between fam­
ilies and different geographic regions. This observation along with 
the absence of thyroid dysfunction in mice with targeted inactiva­
tion of the Slc26A4 gene suggest that iodide efflux can also occur 
in the absence of pendrin.21 Anoctamin 1 (ANO1/TMEM16A), 
a calcium-activated anion

tamin 1 (ANO1/TMEM16A), 
a calcium-activated anion channel, which is also expressed at the 
apical membrane of thyrocytes, may also be involved in mediating 
apical efflux.26,27 
Iodide Oxidation and Organification
After reaching the follicular lumen, iodide participates in a series of 
reactions that lead to the synthesis of the active thyroid hormones. 
The first of these involves oxidation of iodide and incorporation of 
the resulting intermediate into the hormonally inactive iodotyrosines 
M

iate into the hormonally inactive iodotyrosines 
MIT and DIT, a process termed organification. Iodide is normally oxi­
dized rapidly, immediately appearing in organic combination with Tg. 
The iodinations that lead to formation of iodotyrosines occur within 
Tg, rather than on the free amino acids. Oxidation of thyroidal iodide 
is mediated by the heme-containing protein TPO and requires the 
H2O2 generated by the calcium-dependent DUOX1 and DUOX2 
enzymes. The protein contains a membrane-spanni

2 
enzymes. The protein contains a membrane-spanning region near the 
carboxy-terminus, and it is oriented in the apical membrane of the 
thyroid cell with residues 1-844 in the follicular lumen in which iodin­
ation occurs (see Fig. 11.1). TPO is the major thyroid microsomal 
antigen, and recombinant human TPO is now used for the detection 
of antithyroid microsomal antibodies commonly present in the serum 
of patients with Hashimoto thyroiditis. The evanescent product of 
the peroxidation of i

. The evanescent product of 
the peroxidation of iodide (i.e., the active iodinating form) remains 
unclear and hypoiodite (OI-), hypoiodous acid (HOI), or iodonium 
(I+) have been proposed.28 The DUOX1 and DUOX2 genes encode 
glycoflavoproteins predominantly expressed at the apical thyrocyte 
membrane where they constitute the catalytic core of the H2O2 gen­
erator required for thyroid hormone synthesis (see Fig. 11.1).29 They 
are Ca2+, NADPH-dependent oxidases that catalyze the formation of 

ependent oxidases that catalyze the formation of 
the H2O2 required for TPO-catalyzed Tg iodination. The maturation 
factor DUOXA2, a resident ER protein, is required for the mat­
uration, plasma membrane localization of DUOX2, and H2O2 
generation.30 Iodide excess inhibits DUOX2 glycosylation, which 
may be an additional mechanism contributing to the Wolff-
Chaikoff effect.31
The rate of organic iodination is dependent on the degree of 
thyroid stimulation by TSH (see later). Congenital defects

stimulation by TSH (see later). Congenital defects in the 
organification process cause goitrous congenital hypothyroidism 
or, if less severe, goiter without hypothyroidism (complete or par­
tial iodide organification defect).32 They can be caused by usu­
ally biallelic mutations in the TG, SLC26A4, TPO, DUOX2, and 
DUOX2A genes. Complete or partial TPO defects are among the 
most frequent causes of abnormal thyroid hormone biosynthe­
sis.33 Mutations in the DUOX2 and DUOXA2 genes have been 
id

ations in the DUOX2 and DUOXA2 genes have been 
identified in patients with permanent and transient congenital 
hypothyroidism, and digenic mutations in DUOX1 and DUOX2 
have been found in a family with very severe congenital hypothy­
roidism (see Chapter 13).34,35 
Iodothyronine Synthesis
The MIT and DIT are precursors of the hormonally active 
iodothyronines T4 and T3. Synthesis of T4 from DIT requires 
the TPO-catalyzed fusion of two DIT molecules to yield a 
structure with two diiodinated ri

ules to yield a 
structure with two diiodinated rings linked by an ether bridge 
(the coupling reaction). Concomitantly, a residual dehydroala­
nine is formed at the site of the DIT residue contributing to 
the phenolic hydroxyl group.
Efficient synthesis of T4 and T3 in the thyroid requires Tg.36 
The Tg messenger RNA (mRNA) is approximately 8.5 kb in length 
and encodes a 330-kDa (12S) subunit that is 10% carbohydrate 
by weight. There are 134 tyrosyl residues in the 660-kDa homodi­
mer. Only 

tyrosyl residues in the 660-kDa homodi­
mer. Only 25 to 30 of these are iodinated, but only residues 5, 
1290, and 2553 form T4 and residue 2746, T3.37 The T4-forming, 
readily iodinated, and iodothyronine-forming acceptor residues of 
Tg from different species are in a Glu/AspTyr or a Thr/SerTyrSer 
sequence, suggesting an important role of primary sequence in 
these reactions. There are three to four T4 molecules in each molecule 
of human Tg under conditions of normal iodination (25 atoms 
pe

nder conditions of normal iodination (25 atoms 
per Tg molecule, approximately 0.5% iodine by weight), but only 
about one in five molecules of human Tg contains a T3 residue. In 
Tg from patients with untreated Graves disease, the content of T4 
residues remains approximately the same, but the number of T3 
residues doubles to an average of 0.4 per molecule. This difference 
is independent of the iodination state of the Tg and is a conse­
quence of thyroidal stimulation. Because the coupling re

 of thyroidal stimulation. Because the coupling reaction is 
catalyzed by TPO, virtually all agents that inhibit organic binding 
(e.g., the thiourea drugs) also inhibit coupling. 
Storage and Release of Thyroid Hormone
The thyroid is unique among the endocrine glands by virtue of the 
large store of hormone it contains and the low rate at which the 
337
hormone turns over (1% per day). This aspect of thyroid hormone 
economy has hom

). This aspect of thyroid hormone 
economy has homeostatic value in that the reservoir provides pro­
longed protection against depletion of circulating hormone in 
case synthesis ceases. In normal humans, the administration of 
antithyroid agents for as long as 2 weeks has little effect on serum 
T4 concentrations. There are approximately 250 μg T4 per gram 
of wet weight in normal human thyroid, or 5000 μg of T4 in 
a 20-g gland.38 This amount is sufficient to maintain a euthyroid 
state for at

s sufficient to maintain a euthyroid 
state for at least 50 days. When released rapidly in an uncontrolled 
fashion during subacute or painless thyroiditis, this quantity of T4 
will cause significant transient thyrotoxicosis. Tg is present in the 
plasma of normal individuals at concentrations up to 50 ng/mL, 
probably leaving the thyroid through the lymphatics. However, 
peripheral hydrolysis of Tg does not contribute significantly to 
the thyroid hormones in the circulation, even during thyro

oid hormones in the circulation, even during thyroiditis, 
when large quantities of this protein are present.
The first step in thyroid hormone release is the endocytosis 
of colloid from the follicular lumen by two processes: macropi­
nocytosis by pseudopods formed at the apical membrane and 
micropinocytosis by small coated vesicles that form at the apical sur­
face (see Fig. 11.1). Both processes are stimulated by TSH, but 
the relative importance of the two pathways varies among species, 
wi

ance of the two pathways varies among species, 
with micropinocytosis thought to predominate in humans. Follow­
ing endocytosis, endocytotic vesicles fuse with lysosomes, and 
proteolysis is catalyzed by cathepsin D and D-like thiol proteases, 
all of which are active at the acidic pH of the lysosome. The iodo­
tyrosines MIT and DIT released from Tg are rapidly deiodinated 
by the NADPH-dependent iodotyrosine deiodinase DEHAL1/
IYD, and the released iodine is recycled.39 Thyroid hormones are 
re

sed iodine is recycled.39 Thyroid hormones are 
released from Tg in the lysosome, but it is not entirely clear 
how their transfer into the cytosol and subsequently the plasma 
is affected. Exit at the basolateral membrane involves, at least 
in part, the thyroid hormone transporter MCT8.40 It has been 
shown that T4 can be released from Tg within the thyroid cell 
with minimal disruption of its molecular weight. This presum­
ably is a consequence of selective proteolysis, which is facil­
itated

e of selective proteolysis, which is facil­
itated by the fact that the major hormonogenic peptides of 
the Tg molecule are located at the amino-terminus and the 
carboxy-terminus of the Tg monomer.
Presumably the T4 becomes accessible to the thyroidal type 
1 and 2 deiodinases (D1 and D2) because basal and TSH-
stimulated conversion of T4 to T3 is readily demonstrated in 
the perfused dog thyroid. Because this conversion is inhibited 
by propylthiouracil (PTU), it is catalyzed by D1 (Table 11.2

hiouracil (PTU), it is catalyzed by D1 (Table 11.2). 
The contribution of thyroidal T4 deiodination to T3 secretion in 
humans under physiologic conditions is not known. The fact that 
the ratio of T4 to T3 in human Tg is 15:1, although estimates of 
the molar ratio of T4 to T3 in thyroid secretion is approximately 
10:1, suggests that this does occur. Stimulation of D1-catalyzed 
and D2-catalyzed 5′-deiodination of T4 in the thyroid gland of 
patients with Graves disease may enhance that pathwa

tients with Graves disease may enhance that pathway and con­
tribute to the marked increase of the ratio of T3 to T4 production 
in that condition.41 An inhibition of the D1-catalyzed T4 to T3 
conversion may contribute to the rapid effect of PTU to reduce 
circulating T3 in patients with Graves disease (see Chapter 
12).41,42 That deiodinases in thyroid-derived cells can modulate 
the systemic conversion of T4 to T3 has been shown in several 
patients with metastatic thyroid carcinoma. The high

tients with metastatic thyroid carcinoma. The high expression 
of D2 in one large mediastinal tumor mass was associated with a 
high normal T3 and reduced T4 with a normal TSH. Removal of 
the tumor reversed these abnormalities.43
T4 release from the thyroid cells is inhibited by several agents, 
the most important of which is iodide. Inhibition of hormone 
release is responsible for the rapid improvement that iodide causes 
in hyperthyroid patients. The mechanism by which this effect is 
mediat

nts. The mechanism by which this effect is 
mediated is uncertain, but iodide inhibits the stimulation of thyroid 
adenylate cyclase by TSH and by the stimulatory immunoglobu­
lins of Graves disease. Increasing iodination of Tg also increases its 
resistance to hydrolysis by acid proteases in the lysosomes. Lithium 
inhibits thyroid hormone release, although its mechanism of action 
is poorly understood and may differ from that of iodide.44
Deiodination of Iodotyrosines
In addition to the active

ination of Iodotyrosines
In addition to the active transport of iodide from the extracellular 
fluid, intracellular iodide is also generated by the action of the 
DEHAL1 or iodotyrosine deiodinase (IYD) enzyme. IYD cata­
lyzes nicotinamide adenosine dinucleotide phosphate (NADPH)–
dependent deiodination of MIT and DIT, with greater activity 
against MIT.45 Dhal1 transcription is stimulated by cyclic adenos­
ine monophosphate (cAMP) and encodes a membrane protein 
concentrated at the apical cell 

membrane protein 
concentrated at the apical cell surface, which catalyzes NADPH-
dependent deiodination of MIT and DIT and recycles iodide. The 
iodide thereby released is immediately reconjugated to newly syn­
thesized Tg after exiting the apical membrane of the cell. This pro­
cess is interrupted by the thiourea class of antithyroid drugs that 
inhibit TPO, such as methimazole (MMI), carbimazole (CBZ), 
and PTU, thus causing intrathyroidal iodine deficiency in patients 
receiving these agents

ine deficiency in patients 
receiving these agents.38 Biallelic mutations in the IYD gene have 
been identified in patients with hypothyroidism, goiter, and an 
elevated DIT level.39 Functional studies revealed that the muta­
tions abolished the capacity of IYD to deiodinate MIT and DIT. 
Role and Mechanism of Thyrotropin Effects
All steps in the formation and release of thyroid hormones are 
stimulated by TSH secreted by the pituitary thyrotrophs (see 
Chapter 8). Thyroid cells express the TSHR

s (see 
Chapter 8). Thyroid cells express the TSHR, a member of the 
glycoprotein G protein–coupled receptor family. This protein 
contains a large extracellular amino-terminal domain, seven 
membrane-spanning domains, and an intracellular domain 
that transduces the signal by promoting exchange of guanosine 
diphosphate (GDP) for guanosine triphosphate (GTP) on the 
α-subunit of G proteins.46 In fact, the TSHR has been reported 
to couple to 11 different G protein α-subunits in  vitro, and 
the

different G protein α-subunits in  vitro, and 
therefore much remains to be learned about signaling through 
it. Although the TSHR mainly couples to Gs, when activated 
by high concentrations of TSH (100 times physiologic levels), 
it couples also to Gq/G11, activating the inositol-phosphate dia­
cylglycerol cascade. The induction of signal via the phospholi­
pase C (PLC) and intracellular Ca2+ pathways regulates iodide 
efflux, H2O2 production, and Tg iodination, whereas the signal 
via the pro

and Tg iodination, whereas the signal 
via the protein kinase A (PKA) pathways mediated by cAMP 
stimulates growth and regulates iodine uptake and transcription 
of Tg, TPO, and the NIS mRNAs leading to thyroid hormone 
production (Table 11.3).47 Although the discovery that different 
mutations in various regions of the TSHR molecule result in 
intrinsic activation and the identification of important domains 
for intramolecular TSHR signal transduction (see Chapter 12), 
the precise mechanisms o

uction (see Chapter 12), 
the precise mechanisms of receptor activation and the early 
events of TSHR signal transduction are not fully understood.46 
Remarkably, the wild-type TSHR itself displays constitutive 
activity, a phenomenon that is not shared by the closely related 
338
receptors for luteinizing hormone/chorionic gonadotropin (LH/
CG) and follicle-stimulating hormone (FSH). This suggests that 
the unliganded TSHR might be less constrained than other G 
protein–c

 might be less constrained than other G 
protein–coupled seven-transmembrane receptors.46
The TSHR, in addition to the TSH, also binds TSHR-
stimulating antibody (TRAb), thyroid-blocking antibodies 
(TBAb), and neutral antibodies to the TSHR (see Chapter 12). 
The closely related luteinizing hormone and chorionic gonado­
tropin also bind to and activate TSHR signaling when present 
at high levels.48 The latter accounts for the physiologic hyperthy­
roidism of early pregnancy. Besides the thyrocy

y­
roidism of early pregnancy. Besides the thyrocyte, the TSHR is 
also expressed in a variety of tissues such as osteoclasts, fibroblasts, 
and adipocytes, as well as retroorbital adipocytes and skin.48,49 
Activating mutations in the TSHR in the germline cause congeni­
tal nonautoimmune hyperthyroidism; somatic gain-of-function 
mutations result in toxic adenomas.50 In contrast, biallelic inacti­
vating mutations in the TSHR result in congenital hypothyroid­
ism with thyroid hypoplasia or, in 

l hypothyroid­
ism with thyroid hypoplasia or, in the case of partial inactivation, 
euthyroid hyperthyrotropinemia.46 
Thyroid Hormones in Peripheral Tissues
Plasma Transport
The metabolic transformations of thyroid hormones in periph­
eral tissues determine their biologic potency and regulate their 
biologic effects. Consequently, an understanding of thyroid 
physiopathology requires knowledge of the pathways of thy­
roid hormone metabolism. A wide variety of iodothyronines 
and their metaboli

wide variety of iodothyronines 
and their metabolic derivatives exist in plasma. Of these, T4 
is highest in concentration and the only one that arises solely 
from direct secretion by the thyroid gland. In normal humans, 
T3 is also released from the thyroid, but approximately 80% is 
derived from the peripheral tissues by the enzymatic removal 
of a single 5′ iodine atom (outer ring or 5′ monodeiodination) 
from T4.51 The remaining iodothyronines and their derivatives 
are generated in the per

es and their derivatives 
are generated in the peripheral tissues from T4 and T3. Prin­
cipal among them are 3,3′,5′-triiodothyronine (reverse T3, or 
rT3) and 3,3′-diiodo-l-thyronine (3,3′-T2) (Fig. 11.2). Trace 
concentrations of other diiodothyronines, monoiodothyro­
nines, and conjugates thereof with glucuronic or sulfuric acid 
are also present.52 Deaminated derivatives of T4 and T3 that 
bear an acetic acid rather than an alanine side chain (tetrac and 
triac) are also present in low conce

 (tetrac and 
triac) are also present in low concentrations. 3-Iodothyronamine 
(T1AM) is an endogenous thyroid hormone derivative with 
unknown biosynthetic origins.53 Structural similarities have 
led to the hypothesis that T1AM is an extrathyroidal metabo­
lite of T4.
  Human Iodothyronine Selenodeiodinases
Parameter
Type 1 (Outer and Inner Ring)
Type 2 (Outer Ring)
Type 3 (Inner Ring)
Physiologic role
rT3 and T3S degradation, the source 
of plasma T3 in thyrotoxic patients
Provide intracell

plasma T3 in thyrotoxic patients
Provide intracellular T3 in specific 
tissues, a source of plasma T3
Inactivate T3 and T4
Tissue location
Liver, kidney, thyroid, pituitary (?) 
(not CNS)
CNS, pituitary, BAT, placenta 
thyroid, skeletal muscle, heart
Placenta, CNS, fetal or adult 
liver, skeletal muscle
Subcellular location
Endoplasmic reticulum
Preferred substrates (position deiodinated)
rT3 (5′), T3S (5)
T4, rT3 (5′)
T3, T4 (5)
Km
rT3, 10−7; T4, 10−6
10−9
10−9
S

(5′)
T3, T4 (5)
Km
10−9
10−9
Susceptibility to PTU
High
Absent
Absent
Response to increased T4
↑
↓
↑
BAT, Brown adipose tissue; CNS, central nervous system; Km, Michaelis-Menten constant; PTU, 6-N-propylthiouracil; rT3, reverse triiodothyronine; T3, triiodothyronine; T3S, T3SO4; T4, thyroxine.
   
TABLE 11.2
  Thyroid Cell Functions Stimulated by 
Thyrotropin
Function Affected
General Mechanism
Iodide Metabolism
Increase I− in follicular lumen
PLC
Delayed increase in NIS exp

n follicular lumen
PLC
Delayed increase in NIS expression
cAMP
Increase in thyroid blood flow
↑ Nitric oxide synthesis 
(↓ cellular iodide)
Increase in I− efflux from thyroid cell
?
Thyroid Hormone Synthesis
Hydrogen peroxide
PLC
Thyroglobulin and TPO synthesis
cAMP
NADPH via pentose-phosphate
?
Pathway
Thyroid Hormone Secretion
Pinocytosis of thyroglobulin
cAMP
Release of thyroglobulin into plasma via 
basolateral membrane
cAMP (?)
Thyrocyte Proliferation and Differentiation
Mitogenesis, differ

liferation and Differentiation
Mitogenesis, differentiation
cAMP, PLC, and IGFI– and 
FGF-mediated kinase 
cAMP, Cyclic adenosine monophosphate; FGF, follicular growth factor; IGF, insulin-like growth 
factor; I−, plasma iodide; NADPH, nicotinamide adenosine dinucleotide phosphate; NIS, 
sodium-iodide symporter; PLC, phospholipase C; TPO, thyroid peroxidase.
   
TABLE 11.3
339
The major iodothyronines are poorly soluble in

ion
The major iodothyronines are poorly soluble in water and thus 
bind reversibly to plasma proteins. The plasma proteins with 
which T4 is mainly associated are TBG and transthyretin (TTR; 
formerly termed T4-binding prealbumin [TBPA]) and albumin 
(Table 11.4). About 75% to 80% of T3 is bound by TBG and the 
remainder by TTR and albumin.
Thyroxine-Binding Globulin
TBG is a glycoprotein with a molecular mass of about 54 kDa, 
about 20% of which is carbohydrate. It is encoded by a 3.8-kb tran­

 is carbohydrate. It is encoded by a 3.8-kb tran­
script generated by the X chromosomal TBG (SERPINA7) gene.54 
The protein sequence of TBG resembles that of the serpin family 
of serine antiproteases. Because there is one iodothyronine binding 
site per TBG molecule, the T4 or T3 binding capacity of TBG in 
normal human serum is equivalent to its concentration, which is 
approximately 270 nmol/L (21 μg/dL). The half-life of the protein 
in plasma is about 5 days.
Congenital deficiency of TBG is

a is about 5 days.
Congenital deficiency of TBG is common, occurring in 1 
in 5000 newborns, and is associated with the complete absence 
of the protein in males. l-Asparaginase blocks the synthesis of 
TBG, which accounts for the low T4 concentrations in patients 
receiving this agent.
The glycosylation of TBG influences its clearance from the 
plasma and its behavior during isoelectric focusing. In estrogen-
treated patients, there is an increase in the prevalence of the more 
acidic bands of 

se in the prevalence of the more 
acidic bands of TBG. The more highly sialylated TBG is cleared 
more slowly from plasma than is the more positively charged 
TBG, because sialylation inhibits the hepatic uptake of glycopro­
teins. Sera from pregnant patients, women receiving oral con­
traceptives, and patients with acute hepatitis have increased 
fractions of acidic TBG. Patients with inherited TBG excess have 
normal amounts of highly sialylated TBG, as do men and non­
pregnant women. Because 

d TBG, as do men and non­
pregnant women. Because TBG is the principal T4-binding and 
T3-binding protein, changes in TBG or its binding are paralleled 
by changes in total plasma T4 and T3 even though T4 and T3 
production is little changed.
Another post-translational modification affecting TBG occurs 
in septic patients or following cardiopulmonary bypass surgery.55 
TBG is subjected to cleavage by a serine protease released from 
polymorphonuclear leukocytes, resulting in the release of a 5-k

lear leukocytes, resulting in the release of a 5-kDa 
carboxy-terminal loop with a consequent decrease in affinity for 
T4. An analogous reaction has been described for cortisol-binding 
globulin, which releases cortisol at the site of inflammation.56 It 
has been postulated that the released T4 might play a critical role 
Glucuronidation (T4G)
HO
OH
D3
D1
D2
D3
D1
D2
3,5,3´,5´-Tetraiodo-L-thyronine
(Thyroxine, T4)
NH2
O
O
H
I
I
I
I
HO
OH
3,3´-Diiodo-L-thyronine (T3)
NH2
O
O
H
I
I
HO
OH
3,5,3´-T

iodo-L-thyronine (T3)
NH2
O
O
H
I
I
HO
OH
3,5,3´-Triiodo-L-thyronine (T3)
NH2
O
O
H
I
I
I
HO
OH
3,3´,5´-Triiodo-L-thyronine
(reverse T3)
NH2
O
O
H
I
I
I
Sulfation (T4S)
Inactivation via D1
Biliary excretion
I
OH
NH2
HO
O
H
I
OH
NH2
HO
O
H
I
MIT
3-Monoiodotyrosine
DIT
3,5-Diiodotyrosine
Precursors
• Fig. 11.2  Major deiodinative and nondeiodinative pathways of thyroid hormone metabolism. The iodothyronine 
deiodinases are abbreviated D1, D2, and D3 for type 1, 2, and 3 deiodinases, respectively. 

3 for type 1, 2, and 3 deiodinases, respectively. Arrows refer to 
monodeiodination of the outer or inner ring of the iodothyronine nucleus, which are termed 5′ or 5 by conven­
tion. T4 is activated by monodeiodination of the phenolic thyronine ring by D1 or D2 to form T3. Deiodination 
of the tyrosyl ring by D1 or D3 inactivates T4 and T3. This inactivation pathway is markedly favored by sulfation 
of the phenolic hydroxyl to form T4SO4 (T4S) or T3SO4 (T3S). Glucuronidated T4 and T3 (T4G and T3

 T3SO4 (T3S). Glucuronidated T4 and T3 (T4G and T3G) are 
excreted into the bile but may be partially reabsorbed after deglucuronidation in the intestine.
340
in the response to injury, perhaps by providing a supply of iodine 
for antibacterial purposes.55 The cleaved TBG of approximately 
49 kDa circulates, and because it binds T4 with lower avidity, it 
may explain the increased ratio of free to bound T4 in acute ill­
ness even when TBG saturation studies or immunoassays

s even when TBG saturation studies or immunoassays indicate 
TBG concentration is normal (see “Thyroid Function During 
Fasting or Illness”). 
Transthyretin
TTR is a transport protein for T4 and retinol-binding protein 
bound to retinol (vitamin A), hence its name. It consists of 
four identical polypeptide chains with a total molecular mass 
of approximately 55 kDa and is not glycosylated. Its concen­
tration in plasma is approximately 4 mmol/L (250 μg/mL). 
Each mole of TTR binds 1 mole of T4 

(250 μg/mL). 
Each mole of TTR binds 1 mole of T4 with high affinity, and 
a second T4 molecule is bound with lower affinity at high con­
centrations of T4. Its half-life in plasma is normally about 2 
days, but this decreases during illness. TTR is expressed in the 
choroid plexus, and it is the major thyroid hormone–binding 
protein in the cerebrospinal fluid (CSF).57 High levels of TTR 
have been detected in fetal serum, probably directly produced 
by placental cells.58 Targeted TTR gene disr

uced 
by placental cells.58 Targeted TTR gene disruption in mice 
shows that there is no impairment of uptake of T4 into the 
brain, leaving the role of TTR in CSF undefined in regard to 
thyroid physiology.59,60
Variant forms of TTR are associated with familial amyloidotic 
polyneuropathy.61 In affected families, the TTR monomer has one 
of several different point mutations, and TTR accumulates in the 
amyloid tissue deposits. Neither thyroid dysfunction nor altered 
vitamin A metabolism has be

sfunction nor altered 
vitamin A metabolism has been reported, although there is altered 
affinity of some of the mutant proteins for T4. Families with both 
high-affinity TTR and a few with increased TTR levels have been 
reported. 
Competition for T4 and T3 Binding to TBG and TTR 
by ­Therapeutic Agents
The TBG binding site has an affinity for T3 that is about 20-fold 
less than that for T4 (see Table 11.4). Binding of T4 and T3 by 
TBG is inhibited by phenytoin,62 salicylate,63 salsalate, fur

bited by phenytoin,62 salicylate,63 salsalate, furose­
mide, fenclofenac, and mitotane. The affinity of these compounds 
for TBG is much weaker than is that of the iodothyronines, but 
their concentration in plasma is sufficiently high to compete with 
T4 and T3 binding and reduce total hormone levels, although free 
T4 remains normal. Because all methods used for estimating the 
free fractions of T4 and T3 in human serum except ultrafiltration 
dilute the serum, euthyroid patients receiving the

dilute the serum, euthyroid patients receiving these drugs may 
appear to have low total and free T4 or T3, whereas in vivo the free 
fraction is normal. 
Albumin
The affinity of albumin for T4 and T3 binding is much lower 
than that of either TBG or TTR, but the high concentration 
of this protein results in the binding of 10% of the plasma thy­
roid hormones (see Table 11.4). Changes in albumin concentra­
tion per se have little influence on total hormone levels, unless 
accompanied by alterat

tal hormone levels, unless 
accompanied by alterations in TBG and TTR, all three of 
which are synthesized in the liver. Hepatic failure or nephrotic 
syndrome leads to decreases in the plasma concentration of all 
three, and the serum albumin concentration in patients with 
these illnesses may serve as a surrogate for estimating TBG 
The role of albumin in thyroid physiology becomes clinically 
important in patients with familial dysalbuminemic hyperthyrox­
inemia (FDHT4) and hy

 dysalbuminemic hyperthyrox­
inemia (FDHT4) and hypertriiodothyroninemia (FDHT3).64,65 
Both are dominantly inherited and characterized by a high con­
centration of total T4 or T3 (FDH). However, the free hormone 
concentrations remain normal and the patients are euthyroid. 
Recognition of these variants is important because affected 
individuals are at risk of erroneous treatment. Such patients 
may have a confusing pattern of test results, especially when 
analogue methods or labeled T3 are us

cially when 
analogue methods or labeled T3 are used to estimate the free T4 
or T3 (see Chapter 4). 
Other Plasma Thyroid Hormone–Binding Proteins
Between 3% and 6% of plasma T4 and T3 are bound to lipoproteins. 
The T4-binding lipoprotein is a 27-kDa homodimer with an affinity 
for T4 that is lower than that of TBG. This binding is of uncertain 
physiologic significance but could play a role in targeting T4 delivery 
to specific tissues. 
Free Thyroid Hormones
Because most of the circulating T

Because most of the circulating T4 and T3 is bound to TBG, its 
concentration and degree of saturation are the major determi­
nants of the free fraction of T4. Binding of the thyroid hormones 
to the plasma proteins alters their metabolism. The negligible 
urinary excretion of T3 and T4 is due to the limited filterability 
of the hormone-binding protein complexes at the glomerulus. 
In vitro, the interaction between the thyroid hormones and their 
binding proteins conforms to a 

ormones and their 
binding proteins conforms to a reversible binding equilibrium 
that can be expressed by conventional equilibrium equations. 
For the formulations that follow, T4 is used as the prototype, 
  Comparison of the Major Human Thyroid 
Hormone–Binding Proteins
Parameter
Thyroxine-
Binding 
Globulin
Albumin
Molecular weight of 
holoprotein (kDa)
54,000
54,000 
(4 subunits)
66,000
Plasma concentrations 
(μmol/L)
0.27
4.6
640
T4 binding capacity as 
μg T4/dL
21
350
50,00

640
μg T4/dL
21
350
50,000
Association constants of the major binding site (L/mol)
  T4
1 × 1010
7 × 107
7 × 105
  T3
5 × 108
1.4 × 107
1 × 105
Fraction of sites occupied 
by T4 in euthyroid 
plasma
0.31
0.02
<0.001
Distribution volume (L)
7
5.7
7.8
Turnover rate (% day)
13
59
5
Distribution of iodothyronines (% protein)
  T4
68
11
20
  T3
80
9
11
T3, Triiodothyronine; T4, thyroxine.
   
TABLE 11.4
341
with t

 Pathophysiology and  Diagnostic Evaluation
with the understanding that similar interactions apply in the case 
of T3. The interaction between T4 and TBG can be expressed as 
follows:
T4
K2
+ TBG →T4 • TBG
where TBG represents the unoccupied binding protein, ka the equi­
librium association constant for the interaction, and T4 the concen­
tration of free T4; T4 • TBG is T4 bound to TBG (∼68% of total T4 
is bound to TBG).
Rearranging, we can express this as:
T4 • ΤΒG
(T4) (TBG)
= ka
T4
T4 • TBG

ess this as:
T4 • ΤΒG
(T4) (TBG)
= ka
T4
T4 • TBG
=
1
(TBG) k2
Thus the free fraction of T4 (T4/T4 • TBG) is inversely propor­
tional to the concentration of unoccupied TBG binding sites. Esti­
mates of the free T4 concentration in serum can be generated by 
direct or indirect assays. In normal serum, the free T4 is approxi­
mately 0.02% of the total (about 20 pmol/L, 1.5 ng/dL). The 
approximately 20-fold lower affinity of TBG for T3 results in a 
higher proportion of unbound T3 (0.30%) (Table 

a 
higher proportion of unbound T3 (0.30%) (Table 11.5; also see 
Table 11.4).
It is the free hormone that is available to the tissues for cellular 
uptake and feedback regulation that induces its metabolic effects 
and that undergoes deiodination or degradation. The bound hor­
mone acts merely as a reservoir. It follows that the concentration 
of the free hormone is the determinant of the metabolic state, and 
it is this concentration that is defended by homeostatic mecha­
nisms. If a change in

fended by homeostatic mecha­
nisms. If a change in TBG occurs, the free T4 concentrations and 
T3 concentrations can be maintained at normal levels only if the 
bound hormone changes in the same direction. For example, when 
TBG concentrations are increased by administration of estrogen, 
the free T4 reduction reduces T4 clearance, allowing an increase in 
the plasma total T4 concentration. This is an iterative process that 
eventually would normalize the free T4 at a new equilibrium with­
out a

alize the free T4 at a new equilibrium with­
out a change in T4 secretion rate. The transient decrease in free 
thyroid hormones also slightly reduces the negative feedback on 
the hypothalamic-pituitary-thyroid axis, which causes an increase 
in thyroid hormone production as an additional compensation.66
The preceding formulation is termed the free thyroid hormone 
hypothesis.67,68 If it is free hormone that is available for cellular 
entry, what is the role, if any, of the hormone-binding prot

t is the role, if any, of the hormone-binding proteins? 
Protein binding facilitates the distribution of the hydrophobic 
thyroid hormones throughout the vascular system. For exam­
ple, if a protein-free solution containing tracer T3 is perfused 
through rat liver via the portal vein, there is a steep concentra­
tion gradient with a decreasing quantity of T3 in the solution 
as the distance from the center of the portal lobule increases. 
In fact, virtually all of the T3 is taken up by the first

, virtually all of the T3 is taken up by the first cells to 
be contacted by the bolus. In contrast, if albumin is added to 
the perfusate, the distribution of tracer is uniform throughout 
the lobule. Both influx and efflux of thyroid hormone from tis­
sues are rapid. Thus intracellular free T3 and T4 are in equilib­
rium with the free hormone pool in plasma although transporter 
activity and metabolism will influence the magnitude of the 
ratio. In the steady state, the rate of T3 and T4 metab

. In the steady state, the rate of T3 and T4 metabolism, not 
the dissociation rate from plasma proteins, is rate limiting in the 
exit of hormones from the plasma. 
T4 and T3 Transport Across Cell Membranes and 
Intracellular T3 Binding
It has been assumed for a long time that transport of iodothyro­
nines across the plasma membrane occurs by passive diffusion, but 
it has become increasingly clear that cellular uptake and efflux of 
thyroid hormone are mediated by transporter proteins.69,70 Se

mone are mediated by transporter proteins.69,70 Several 
specific thyroid hormone transporters have been identified, includ­
ing monocarboxylate transporter 8 (MCT8), MCT10, organic 
anion transporting polypeptide 1C1 (OATP1C1), and L-type 
amino acid transporters LAT1 and LAT2. MCT8 and MCT10 are 
expressed in multiple tissues, where they facilitate transport of T3, 
T4, rT3, and T2 across cell membranes; OATP1C1 is expressed pre­
dominantly in the brain and transports preferentially T4, wherei

the brain and transports preferentially T4, wherein 
it may mediate the entry of T4 into the astrocytes. A defect in a 
single thyroid hormone transporter molecule, MCT8, has been 
shown to cause a severe developmental neurologic phenotype.71 
The Allan-Herndon-Dudley syndrome (AHDS) is an X-linked 
condition characterized by severe mental retardation, dysarthria, 
athetoid movements, muscle hypoplasia, and spastic paraplegia 
associated with an elevated serum T3.72 All patients tested with 
thi

elevated serum T3.72 All patients tested with 
this syndrome have mutations in the MCT8 gene. More than 200 
individuals belonging to some 100 families of all races and diverse 
ethnic origins harboring more than 70 different mutations have 
been identified.73 Although most mutations result in a complete 
functional inactivation of the MCT8 protein, significant residual 
activity was observed with a number of MCT8 mutations, some of 
which are associated with a milder clinical phenotype.74 MCT8-

sociated with a milder clinical phenotype.74 MCT8-
null mice, despite the presence of marked increased T3 levels, lack 
any overt neurologic abnormalities, a rather unexpected finding in 
light of the severe human phenotype.75,76 The discrepancy with the 
human phenotype is now explained by the fact that mice express 
OATP1C1 at the blood-brain barrier, which compensates for the 
lack of transport by MCT8.77 Coexistence of thyroid hormone 
excess and deprivation in different tissues is a distinc

 and deprivation in different tissues is a distinct character­
istic of this syndrome. Tissues expressing transporters other than 
  Comparison of Triiodothyronine (T3) and 
Thyroxine (T4) in Humans
Parameter
T3
T4
Production rate (nmol/day)
50
110
  Fraction from thyroid
0.2
1.0
Relative metabolic potency
1.0
0.3
Serum concentration
  Total (nmol/L)
1.8
100
  Free (pmol/L)
5
20
Fraction of total hormone in free form (×10−2)
0.3
0.02
40
10
Fraction intracellular
0.64
0.1

n volume (L)
40
10
0.64
0.15
Half-life (days)
0.75
6.7
To convert T4 from nmol/L to μg/dL (total) or pmol/L to ng/dL (free), divide by 12.87. To con­
vert T3 from nmol/L to ng/dL (total) or pmol/L to pg/dL (free), multiply by 65.1.
   
TABLE 11.5
342
MCT8, such as liver and kidney, respond to the high circulating 
T3 levels resulting in a local hyperthyroid state, whereas tissues 
depending on MCT8 for thyroid hormone entry into cells, such 
as the b

r thyroid hormone entry into cells, such 
as the brain, are hypothyroid.73,78 Two therapeutic options, PTU 
combined with levothyroxine T4 (L-T4)73 and a thyromimetic com­
pound, diiodothyropropionic acid (DITPA), which is not depen­
dent on MCT8 for cellular entry, have been used to treat several 
patients ­harboring MCT8 gene mutations.79,80
Another transporter specific for T4, OATP1C1 (a member of 
the organic anion transporting polypeptide family), is expressed 
in capillaries throughout the

mily), is expressed 
in capillaries throughout the brain, suggesting it may be involved 
in the transport of T4 across the blood-brain barrier.71 Taken 
together, these results suggest that the supply of T3 to neurons 
may occur according to the schema shown in Fig. 11.3.81 T4 is 
transferred into the choroid plexus or into tanycytes via the action 
of OATP1C1, which is negatively regulated in brain capillaries by 
thyroid hormone. In the tanycyte or astrocyte, T4 is converted to 
T3 by the type

e or astrocyte, T4 is converted to 
T3 by the type 2 iodothyronine deiodinase (D2) and exits the cell, 
possibly via the MCT8/MCT10 transporters, where it becomes 
available for neuronal uptake, also via MCT8.82,83 Neurons 
express the type 3 deiodinase (D3), which prevents activation of 
T4 and catalyzes degradation of T3 (see “Iodothyronine Deiodin­
ation”). This would provide a logical explanation of the association 
of the mutations in MCT8 with attention-deficit/hyperactivity 
disorder (ADH

ith attention-deficit/hyperactivity 
disorder (ADHD), although it remains puzzling why the neuro­
logic manifestations of this condition are so different from those 
seen in patients with untreated congenital hypothyroidism or 
severe iodine deficiency (see Chapter 13).
The transport field has become more complex as evidence 
accumulates of tissue-specific as well as generalized iodothyro­
nine transporters belonging to a number of different transporter 
protein families. Each of these has many 

sporter 
protein families. Each of these has many members with small 
variations in structure, which alter the specificity of the target 
substance. A thorough review of this topic is beyond the scope 
of this chapter, and the interested reader is referred to excellent 
reviews for further information.69,70
In most cells, about 90% of the intracellular T3 is located in the 
cytosol. The known exception is in the pituitary, where approxi­
mately 50% of the intracellular T3 is present in the nucle

0% of the intracellular T3 is present in the nucleus. The 
mechanisms determining this distribution are still unknown, but 
it would not be surprising if there were active transport of thyroid 
hormones in and out of the nucleus and between other intracel­
lular compartments. An intracellular T3-binding protein (mu-
crystallin) has been identified, which is expressed at high levels 
in the human brain, cochlea, and heart but is widely distributed. 
Disruption of the Crym gene encoding mu-crystal

. 
Disruption of the Crym gene encoding mu-crystallin leads to con­
genital deafness.84 This or similar proteins may also play a role in 
the subcellular localization of the active hormone. 
T2 
T3 
D3 
OATP1C1
MCT8 
Blood-brain
barrier
Neuron
TR 
T3 responsive
gene 
T3 
T4 
T4 
D2 
T3 
? 
Astrocyte
• Fig. 11.3  Potential pathways for entry of T3 into the central nervous system. Thyroid hormones are trans­
ported through the blood-brain barrier (organic anion transporting polypeptide [OATP]) or

(organic anion transporting polypeptide [OATP]) or the blood-CSF 
barrier (OATP and MCT8). In the astrocytes and tanycytes, T4 is converted to T3, which then enters the 
neurons, possibly through MCT8. In the neurons, both T4 and T3 are degraded by D3. T3 from the tany­
cytes may reach the portal vessels in the median eminence. Other transporters may be present on the 
astrocyte or tanycyte membranes. In most cases, the transport could be bidirectional, although only one 
direction is shown. The

tional, although only one 
direction is shown. The interaction of T3 with the thyroid hormone receptor (TR) bound as a heterodi­
mer with retinoid X receptor to the thyroid hormone–response element, often in the 5′ flanking region 
of a T3-responsive gene, causes either an increase or a decrease in the transcription of that gene. This 
leads to parallel changes in the concentrations of critical proteins, thus producing the thyroid hormone 
response characteristic of a given cell. D2 and D3, type

se characteristic of a given cell. D2 and D3, type 2 and type 3 iodothyronine deiodinases; MCT8, 
the ­monocarboxylate transporter 8; T2, diiodothyronine; T3, triiodothyronine; T4, thyroxine.
343
Iodothyronine Deiodination
The most important pathway for T4 metabolism is its outer ring 
(5′) monodeiodination to the active thyroid hormone, T3. This 
reaction is catalyzed by the type 1 and 2 deiodinases (D1 and D2) and 
is the source of

nd 2 deiodinases (D1 and D2) and 
is the source of more than 80% of the circulating T3 in humans 
(see Fig. 11.2). Inner ring deiodination, an inactivating step, is 
catalyzed primarily by D3, which inactivates T3 and prevents 
activation of T4 by converting it to rT3 (see Fig. 11.2).51,85 The 
structures of the three human deiodinases are similar, all being 
homodimers and integral membrane proteins and all requiring 
a thiol cofactor for successful catalysis (Fig. 11.4). They contain 
the rare

sful catalysis (Fig. 11.4). They contain 
the rare amino acid selenocysteine in the active catalytic center 
(see Table 11.2). Selenocysteine has nucleophilic properties that 
make it ideal for catalysis of oxidoreductive reactions such as iodo­
thyronine deiodination and the reduction of H2O2 by another 
family of selenoenzymes, the glutathione peroxidases.86,87 The 
crystal structure of the catalytic domain of mouse deiodinase 3 
(Dio3) revealed a close structural similarity to atypical 2-cyst

d a close structural similarity to atypical 2-cysteine 
peroxiredoxin(s).88 Selenium is thought to be the iodine acceptor 
during deiodination reactions. Mutagenesis of selenocysteine in 
D1 to cysteine, that is, replacing selenium with sulfur, reduces 
the enzyme velocity by approximately 200-fold. The presence of 
selenocysteine has implications beyond catalytic activity, consider­
ing that the cellular processes for synthesizing selenoproteins are 
complex and inefficient.51 This is accomplis

 are 
complex and inefficient.51 This is accomplished by a combina­
tion of a specific structural feature, the selenocysteine insertion 
sequence (SECIS) element, in the 3′ untranslated region of the 
mRNAs, encoding these proteins together with a specific group of 
selenocysteine incorporating gene products. All these elements are 
required for the complex cellular function by which the normal 
STOP codon UGA is recognized as the specific codon for the inser­
tion of the selenocysteine residue 

for the inser­
tion of the selenocysteine residue during protein translation.89,90
Biallelic mutations in SECIS-binding protein 2 (SBP2) lead 
to abnormal thyroid function tests with elevated TSH, T4, FT4, 
and reverse T3 but low total and free T3 concentrations and can be 
associated with short stature and delayed bone age.91
Enzymology and Regulation of the Selenodeiodinases
Although both D1 and D2 activate T4, they have several 
important differences (see Table 11.2). D1 catalyzes both 5′ 
an

erences (see Table 11.2). D1 catalyzes both 5′ 
and 5 deiodination of T4 to form T3 and rT3, respectively, 
although the Michaelis-Menten constant (Km) for these reac­
tions is approximately 3 orders of magnitude greater than that 
of D2 and D3 for this substrate. The preferred substrates of 
D1 are rT3 (5′ deiodination) and T3SO4 (5 deiodination). 
D1 is inhibited by PTU, unlike D2 or D3. D1 also differs from 
D2 in being markedly increased by excess thyroid hormone 
through increased gene tran

xcess thyroid hormone 
through increased gene transcription, whereas D2 mRNA and 
protein are reduced by thyroid hormones. D2 has a half-life of 
only 20 to 30 minutes but that of D1 and D3 is more than 12 
hours. This is due to the rapid ubiquitination of D2, a process 
that is accelerated by interaction with its substrates T4 or rT3. 
D1 and D3 are not thought to be ubiquitinated.
The intracellular location of D2 close to the nucleus gives the 
T3 formed by its catalytic action better access t

T3 formed by its catalytic action better access to the nucleus than 
that formed by D1.92 The T3 produced by D2 is especially effective 
in entering the nucleus and binding to thyroid hormone receptors 
(TRs), a property explained by its location in the endoplasmic 
reticulum (ER) (see Fig. 11.4). D1, on the other hand, is located 
in the plasma membrane, and the T3 produced by this enzyme 
preferentially enters the plasma pool.51 Studies with deiodinase 
inhibitors and cofactors differentially

eiodinase 
inhibitors and cofactors differentially able to cross the cell mem­
brane indicate that the D3 active center is outside the cell and that 
of D2 and D1 are intracellular (see Fig. 11.4).93 This makes D2 
especially important for regulating the hypothalamic-pituitary-
thyroid axis, where its activity increases in response to a decrease 
in serum T4 concentrations, such as occurs in iodine deficiency or 
early autoimmune thyroid disease, well before the serum T3 falls. 
If the decrease 

 well before the serum T3 falls. 
If the decrease in plasma T4 is too great to be compensated for by 
the increase in D2 activity in the hypothalamus and thyrotrophs, 
an increase in TRH and TSH will occur to stimulate the thyroid. 
For this reason, D2 has principally been thought of as an enzyme 
that provides intracellular T3, but there is increasing evidence that 
D1
D2
Catalytic center
Nucleus
GSH
GSH
GRX
TRX
D3
• Fig. 11.4  Predicted topologies of the three iodothyroni

1.4  Predicted topologies of the three iodothyronine deiodinases. The deiodinases are integral mem­
brane proteins that require a thiol cofactor for catalytic activity. The type 1 deiodinase (D1) is in the plasma 
membrane and type 2 (D2) is localized in the endoplasmic reticulum. The active centers of D1 and D2 are 
in the cytosol and depend on intracellular thiols such as reduced glutathione (GSH), thioredoxin (TRX), and 
glutaredoxin (GRX) for catalytic activity. The type 3 deiodinase (D3) is

 catalytic activity. The type 3 deiodinase (D3) is also anchored in the plasma mem­
brane but has access to extracellular thiols.
344
D2 could also contribute to plasma T3. Recent studies in rodents 
suggest that D2 ubiquitination in the hypothalamus is relatively 
less than in other D2-expressing tissues. As a result, the hypo­
thalamus is wired to have increased sensitivity to T4 and allows 
the physiologic TSH response to minimal variation in serum T4.94 
On the other h

o minimal variation in serum T4.94 
On the other hand, in thyrotoxicosis, the threefold to fourfold 
increase in D1, particularly in the thyroid, and the reduced D2 
make D1 the major extrathyroidal source of T3. This explains why 
the D1-inhibitor PTU causes a much more rapid fall in circulating 
T3 than does methimazole in the patient with Graves disease.41,42
A single-nucleotide polymorphism, A/G, in the DIO2 
gene, predicts a threonine (Thr) to alanine (Ala) substitution 
at codon 92 (D2 Thr

to alanine (Ala) substitution 
at codon 92 (D2 Thr92Ala).95 This is present in about 20% 
of the Caucasian population. This polymorphism has been 
found, in some studies, to be associated with insulin resistance 
in obese patients, bipolar mood disorder, psychologic well-
being, mental retardation, hypertension, and risk for osteoar­
thritis, although other studies failed to find such association. 
It is not yet clear whether this D2 polymorphism impairs its 
catalytic efficiency in vivo.96,97 A

 impairs its 
catalytic efficiency in vivo.96,97 A recent study indicates that the 
D2 Thr92Ala polymorphism reduces D2 enzyme activity and 
serum T3 levels in thyroid-deficient patients.98
In the opposite direction, D2 has been found overexpressed 
in a few follicular carcinomas. In these patients, a reduced T4:T3 
ratio has been found and is likely due to the increased D2-mediated 
T4 to T3 conversion.43
D3 is the most important thyroid hormone–inactivating 
enzyme catalyzing deiodination of t

–inactivating 
enzyme catalyzing deiodination of the inner ring of both T3 and 
T4.99,100 D3 activity has been identified in only a limited number 
of postnatal tissues, including placenta and uterine endometrium, 
the CNS (in which it is primarily in neurons), and skin.
Much higher D3 expression has been demonstrated in vari­
ous fetal tissues such as liver, brain, placenta, uterus, and umbilical 
arteries and veins. However, in adult tissues, D3 expression 
may be reinduced under conditions in

3 expression 
may be reinduced under conditions in which cellular prolifera­
tion is required.101 In the adult, D3 has been identified in some 
malignant cell lines and in a number of human tumors, includ­
ing astrocytomas, oligodendromas, gliosarcomas, glioblastomas, 
TSH-secreting pituitary adenomas, colon cancers,102 and basal cell 
carcinomas.103 Tumoral D3 activity can be robust, and the high­
est D3 activity reported in any human tissue to date has been in 
infantile hemangiomas. In infant

date has been in 
infantile hemangiomas. In infants with extensive hepatic heman­
giomas, D3 may overwhelm the secretory capacity of the infant’s 
thyroid, causing hypothyroidism, a syndrome termed consump­
tive hypothyroidism.104 Although most patients with consumptive 
hypothyroidism have hemangiomas, it now has become evident 
that it can occur in other types of tumors, including gastrointesti­
nal stromal tumors.105 Patients with consumptive hypothyroidism 
may only represent the extreme of 

hypothyroidism 
may only represent the extreme of a clinical spectrum of hypo­
thyroidism due to dysregulated thyroid hormone metabolism in 
malignant tissues. DIO3 gene expression is increased by thyroid 
hormone at a transcriptional level.106
Gene-targeting studies have begun to provide further insights 
into the physiologic roles of the deiodinases in mammals.107 Inac­
tivation of the Dio2 gene results in a phenotypically normal mouse 
with an elevated serum T4, normal serum T3, and elevated 

 elevated serum T4, normal serum T3, and elevated serum 
TSH.85,108 These animals have hypothalamic-pituitary resistance 
to T4, impaired auditory function, impaired thermogenesis in 
response to cold stress, impaired muscle regeneration,109 and rela­
tively subtle defects in neurologic function. These findings are all 
consistent with the expectations based on earlier studies indicat­
ing an important role for D2 in brown fat cell function, cochlear 
maturation, and neurologic development. The 

lear 
maturation, and neurologic development. The intracellular bal­
ance between D2 and D3 is dynamically regulated and plays a 
central role in controlling muscle homeostasis and regenerative 
potential.110 Mice with targeted inactivation of Dio1 are pheno­
typically normal but also have an elevated serum T4 and a normal 
serum T3, but TSH is normal.111 The most striking finding in the 
D1-deficient mouse is a marked shift in the T4 clearance pathway 
from deiodinative to biliary/fecal clearan

athway 
from deiodinative to biliary/fecal clearance. Interestingly, mice 
lacking both of the activating deiodinases D1 and D2 are still 
capable (by increasing TSH and thyroidal T3 secretion) of main­
taining normal T3 concentrations in serum and do not suffer from 
systemic hypothyroidism, indicating that thyroidal T3 production 
can guarantee T3 homeostasis at least in rodents.85,108 Mice with 
targeted inactivation of the Dio3 gene show profound abnormali­
ties. They have impaired fertility

ound abnormali­
ties. They have impaired fertility and develop central hypothyroid­
ism in adult life, presumably due to hypothalamic thyrotoxicosis 
during developmental programming.112 
Quantitative and Qualitative Aspects of 
Thyroid Hormone Metabolism
Thyroid Hormone Turnover
In the normal adult, T4 has a distribution volume of approximately 
10 L (see Table 11.5). Because the concentration of total T4 in 
plasma is approximately 100 nmol/L (∼8 μg/dL), the extrathyroi­
dal T4 pool is approxi

∼8 μg/dL), the extrathyroi­
dal T4 pool is approximately 1 μmol (800 μg). In the adult, the 
fractional rate of turnover of T4 in the periphery is about 10% per 
day (half-life, 6.7 days). Thus about 1.1 L of the peripheral T4 dis­
tribution space is cleared of hormone daily, a volume containing 
approximately 110 nmol (85 μg) of T4.
The kinetics of T3 metabolism differ from those of T4, partly 
because of its 10-fold to 15-fold lower affinity for TBG. The vol­
ume of distribution of T3 in the n

r TBG. The vol­
ume of distribution of T3 in the normal adult is about 40 L, about 
four times that of T4, and its fractional turnover rate is approxi­
mately 60% per day. At a mean normal serum T3 concentration of 
1.8 nmol/L (120 ng/dL), 50-fold lower than T4, the daily produc­
tion of T3 is approximately 50 nmol (33 μg) or about 46% that 
of T4 (see Table 11.5). The rapid metabolic clearance rate of the 
product of inner ring T4 deiodination, rT3, and the low concen­
tration in plasma (0.25 n

rT3, and the low concen­
tration in plasma (0.25 nmol/L, 15 ng/dL) combine to yield daily 
production rates for rT3 of about 45 nmol. Thus about 80% of 
T3 and all of rT3 production in humans can be accounted for by 
peripheral deiodination of T4, findings consonant with the high 
ratio of T4 to T3 (15:1) and rT3 (100:1) in human Tg. Of the 
T3 generated via T4 5′ deiodination in euthyroid humans, only 
about 70% is inhibited by PTU, consistent with a significant 
contribution of D2-dependent T3

ith a significant 
contribution of D2-dependent T3 production.113 Although much 
of the T3 and rT3 produced from T4 in peripheral tissues exits 
those tissues and enters the blood, an uncertain fraction of both 
are degraded intracellularly before their exit. As discussed later, 
in some D2-containing tissues, such as the pituitary, a significant 
fraction of T3 in the cell nucleus is derived from intracellular T4 
deiodination to T3, rather than from the plasma. This is particu­
larly true in t

 from the plasma. This is particu­
larly true in the thyrotroph.114
Other pathways are also involved in T4 and T3 metabolism. In 
humans, T4 undergoes glucuronidation of the phenolic hydroxyl 
by the uridine diphosphate glucuronyl transferases (UDPGTs), but 
only minimal amounts of T3 undergo this process (see Fig. 11.2). 
This pathway is clinically significant because certain pharma­
cotherapeutic agents may enhance glucuronide conjugation 
through induction of UDPGT, leading to biliary excreti

ugh induction of UDPGT, leading to biliary excretion of 
T4-glucuronide (T4-G) into the intestine.115 These agents include 
345
phenobarbital, phenytoin, rifampin, and possibly certain of the 
synaptosomal serotonin reuptake inhibitors, such as sertraline. 
Because T4-glucuronide may not be easily reabsorbed from 
intestinal contents, the clinical significance of this pathway is 
that therapy with such agents will generally increase 

 therapy with such agents will generally increase levothyroxine 
requirements. In patients with an intact thyroid, this will not 
be apparent because internal adjustments will increase the T4 
production rate to compensate for the accelerated biliary excretion. 
In patients with hypothyroidism, however, an increase in levothy­
roxine dosage will often be required. 
Sources of Intracellular T3
In view of the differential tissue distribution of the various 
deiodinases, their different Km values, 

 various 
deiodinases, their different Km values, and differential regula­
tion, it is not surprising that tissues may derive intracellular T3 
via different pathways (Fig. 11.5). In several rat tissues, includ­
ing tissues expressing D1 such as kidney and liver, most of 
the nuclear T3 is derived from plasma T3. In D2-containing 
tissues, such as the rat cerebral cortex, pituitary, brown fat, 
and skeletal muscle, D2 functions as an additional intracellular 
source of T3, such that the nuclear 

ntracellular 
source of T3, such that the nuclear T3 concentration will be 
higher given the combination of T3 from the plasma and the 
T3 that is locally converted from T4. In these tissues, half or 
more of intracellular T3 is generated locally from T4 within the 
tissue. In the CNS, the D2-generated T3 in neurons is likely 
to derive from paracrine sources in tanycytes and astrocytes 
(see Fig. 11.3). In the rat, the tissues that depend on D2 for 
nuclear T3 are those in which a constant supp

for 
nuclear T3 are those in which a constant supply of thyroid 
hormone is critical for either normal development (cerebral 
cortex), thyroid gland regulation (pituitary), or survival dur­
ing cold stress (brown adipose tissue). These tissues are also 
characterized by a high degree of saturation of the nuclear T3 
receptors in comparison to tissues such as the liver and kidney 
in which nuclear T3 receptor sites are only about 50% occupied 
at normal serum T3 concentrations (see Fig. 11.5).
In

normal serum T3 concentrations (see Fig. 11.5).
Intracellular D2-catalyzed T3 production has important 
implications for thyroid hormone physiology. First, because 
the T3 produced from T4 occupies a significant fraction of the 
receptors in those tissues, changes in either serum T4 or serum 
T3 can change receptor occupancy. However, because a fall in 
T4 will also increase D2 protein half-life by decreasing the rate 
of ubiquitination and its proteasomal degradation, a rise in 
D2 activity mit

roteasomal degradation, a rise in 
D2 activity mitigates the impact of a reduction of serum T4 in 
D2-expressing tissues, helping to maintain T3 homeostasis.51 The 
requirement for both T3 and T4 for normal saturation of pitu­
itary and CNS T3 receptors permits a response of the hypotha­
lamic-pituitary axis to a reduction in plasma T4, which is the 
earliest manifestation of iodine deficiency or primary hypothy­
roidism (see “Regulation of Thyroid Function”). Because the 
Dio2 gene is positivel

id Function”). Because the 
Dio2 gene is positively regulated by cAMP, D2 activity and 
T3 production increase rapidly in brown adipose tissue under 
stimulation by the sympathetic nervous system. This response 
is critical to adaptive thermogenesis during cold exposure in 
the human neonate and lifelong in the rodent.116
At the same time, tissues expressing D3 have lower T3 con­
centrations than would be expected from the plasma contribu­
tion (Fig. 11.6); thus D3-expressing tissues have a gene

Fig. 11.6); thus D3-expressing tissues have a gene expression 
profile similar to hypothyroid cells. This is explained by the inac­
tivation of T3 and T4 that takes place immediately after these 
hormones enter the cell. The D3-mediated reduction in T3 levels 
likely occurs in several physiologic (development, regeneration) or 
pathologic settings (cancer cells, inflammation, myocardial infarc­
tion) in which D3 is upregulated.117 
Pharmacologic Agents Inhibiting Thyroid 
Hormone Deiodination
A 

Agents Inhibiting Thyroid 
A number of commonly used pharmacologic agents have signifi­
cant effects on thyroid hormone deiodination. PTU inhibition 
of D1 has been mentioned earlier. The antiarrhythmic drug 
amiodarone shares sufficient structural similarity with T4 that 
it can inhibit deiodination of T4 and rT3 by D1 and possibly by 
D2 (Fig. 11.7). This causes an increase in plasma T4 to main­
tain serum T3 in the normal range. Also, levels of TSH increase 
within the fi

range. Also, levels of TSH increase 
within the first weeks of therapy but gradually return to normal 
as the thyroid axis reequilibrates.118 The T4 and rT3 metabolic 
clearance rates are reduced by 20% to 25%, with a reduction in the 
fractional T4 to T3 conversion rate of about 50%. Amiodarone 
also inhibits the active transport of T4 and T3 into hepatocytes, 
and the drug or one of its degradation products may interfere 
with T3 binding to TRs.119
The effects of amiodarone resemble those obse

.119
The effects of amiodarone resemble those observed with the 
iodoaniline derivatives formerly used for gallbladder visualiza­
tion (see Fig. 11.7). Iopanoic and iopodipic acid inhibit the 
deiodinases by competing with the iodothyronine substrates.51 
These agents are useful in the acute treatment of patients with 
severe hyperthyroidism, but they are no longer available for 
clinical use in the United States.
High dosages of glucocorticoids (10 times replacement) 
will acutely reduce the ra

(10 times replacement) 
will acutely reduce the ratio of T3 to T4 in plasma, suggest­
ing that conversion of T4 to T3 is blocked. The ratio of rT3 
to T4 increases, raising the possibility that D3 action is also 
increased.120 These effects resolve during long-term therapy 
such that thyroid function is little affected and thyroid hor­
mone requirements are not increased by chronic glucocorticoid 
therapy.
Recombinant growth hormone increases the circulating T3:T4 
ratio. Growth hormone deficien

 circulating T3:T4 
ratio. Growth hormone deficiency is associated with a decrease in 
the ratio of T3 to T4 in serum, possibly associated with a decrease 
in outer ring deiodination. As expected, dietary selenium deficiency 
also inhibits the synthesis of D1 in humans.121 
Kidney Liver Cortex BAT
Unoccupied receptor
PIT
Total sites occupied
T3 receptors/nucleus (×103)
Local T3 (D2)
T3 from plasma
(D1 + D2)
Total T3 receptor sites
8
6
4
2
0
• Fig. 11.5  Schematic diagram of the origin of the spe

. 11.5  Schematic diagram of the origin of the specifically bound 
nuclear T3 in various rat tissues. Data are derived from studies in which 
the sources of specifically bound nuclear T3 in rat tissues were estimated 
using double-isotope labeling techniques. In tissues in which the receptor 
saturation is significantly greater than 50%, the additional T3 is provided by 
D2-catalyzed conversion of thyroxine (T4) to T3. T3 in rat plasma is derived 
from thyroid secretion (∼40%) with the remainder

from thyroid secretion (∼40%) with the remainder from D1-catalyzed and 
D2-catalyzed T4 to T3 conversion. BAT, brown adipose tissue; D1 and D2, 
type 1 and 2 iodothyronine deiodinases; PIT, pituitary; T3, triiodothyronine.
346
Mechanism of Thyroid Hormone Action
Thyroid hormone acts by binding to specific nuclear TRs, which 
in turn bind to DNA, usually as a heterodimer with retinoid X 
receptor (RXR) at specific sequences (thyroid hormone response 
elements [TREs]) dict

s (thyroid hormone response 
elements [TREs]) dictated by the DNA binding-site preferences of 
the RXR-TR (or TR-TR) complex. In humans, there are two TR 
genes, α and β, found on different chromosomes (TRα, chromo­
some 17; TRβ, chromosome 3). Several alternatively spliced gene 
products from each of these genes form both active and inactive 
gene products. The active proteins are TRα1 and TRs β1, β2, and 
β3.122 The protein structure of TRs includes three major functional 
domains, one binding

ludes three major functional 
domains, one binding DNA, one binding ligand, and a major tran­
scriptional activation domain in the carboxy-terminus.
The general mechanism by which nuclear receptor–activating 
ligands such as T3 produce their effects is discussed in Chapter 2. 
T3 has a 15-fold higher binding affinity for TRs than does T4, 
explaining its function as the active thyroid hormone.
There are tissue-specific preferences in expression of the various 
TRs, suggesting that they subserve 

f the various 
TRs, suggesting that they subserve different functions in differ­
ent tissues.123 TRα1 mRNA is expressed in the brain and brown 
adipose tissue and also in skeletal muscle, the ­gastrointestinal 
tract, lungs, and heart. In general, TRβ, particularly TRβ2, is 
thought to be important in the hypothalamus and pituitary, in 
which regulation of thyroid function occurs.124 TRβ1 is expressed 
in all tissues, although its mRNA is especially highly expressed 
in the kidney and liver. TRβ

lly highly expressed 
in the kidney and liver. TRβ2 is also expressed in the cochlea and 
the retina. TRβ3 mRNA is expressed at very low levels but is more 
abundant in the liver, kidneys, and lungs in comparison with other 
tissues. In addition to differences in the amino-terminus between 
TRβ1 and TRα1, the two proteins are encoded by distinct genes 
and hence under the regulation of different promoters, which can 
function in tissue-specific patterns. TRβ2 is downregulated by T3, 
whereas TRα

atterns. TRβ2 is downregulated by T3, 
whereas TRα1 mRNA expression is not affected.125
Experiments in which TRα and TRβ have been inactivated 
illuminate their different physiologic roles. Disruption of the 
Thrb gene (encoding both TRβ1 and TRβ2) in mice causes deaf­
ness, a marked reduction in feedback sensitivity of the hypotha­
lamic-pituitary-thyroid axis, and a decrease in hepatic D1. Thus 
these mice have significant elevations in both TSH and thyroid 
hormones similar to those in famili

H and thyroid 
hormones similar to those in families with resistance to thyroid 
hormone (RTH), in which TRβ mutations markedly reduce its 
binding affinity for T3. RTH is dominantly inherited and charac­
terized by resistance to thyroid hormone in tissues predominantly 
expressing TRβ (e.g., the liver and the pituitary) but signs of thy­
rotoxicosis in tissues with predominant expression of TRα (e.g., 
the heart).126 Peripheral thyroid hormones are elevated and TSH 
remains inadequately normal 

are elevated and TSH 
remains inadequately normal or elevated. Clinical signs include 
goiter, tachycardia, and hyperactivity. The missense mutations 
in TRβ1 or TRβ2 disrupt binding of T3, and the mutated allele 
acts as a dominant negative inhibitor of the intact TRβ proteins 
encoded by the wild-type allele (see Chapters 2 and 13).
The effect of a TRα1 disruption in the mouse is quite differ­
ent. The predominant phenotypic effects are modest bradycar­
dia and hypothermia. Several patients wi

bradycar­
dia and hypothermia. Several patients with mutations in THRA 
have now been identified.127,128 All patients are heterozygous, 
suggesting dominant negative effects of the mutant receptors 
on wild-type TRα. They presented with low serum T4 and high 
serum T3 levels, growth retardation, delayed mental and bone 
development, bradycardia, and decreased colonic motility with 
severe constipation.127–129
Capillary
T3 
T4 
rT3 
D2 
D3
Target tissue 
T3 
T3 
T4 
T2 
Plasma T3 
Local T3 
Nucle

tissue 
T3 
T3 
T4 
T2 
Plasma T3 
Local T3 
Nucleus 
D3
D3
D3
• Fig. 11.6  Schematic diagram of thyroid hormone activation and inactivation in a cell expressing the iodo­
thyronine deiodinases D2 and D3. The triiodothyronine (T3) that enters the cell can either be deiodinated 
to 3,3′-diiodothyronine (T2) or enter the nucleus and bind to the thyroid hormone receptor. An additional 
source of T3 is that generated by outer ring deiodination of thyroxine (T4) within the cell. rT3, reverse T3.
347

roxine (T4) within the cell. rT3, reverse T3.
347
It is likely that small differences in the ligand-binding domains 
of TRα and TRβ will allow design of thyroid hormone analogues 
selective for one or the other of these receptors. This may result 
in agents that could, for example, suppress TSH in patients with 
thyroid cancer, without inducing tachycardia, such as GC1, a 
potential treatment for obesity via stimulation of metabolic 

reatment for obesity via stimulation of metabolic rate 
and oxygen consumption.130
The ability of thyroid hormone to cause rapid effects, at least 
under certain experimental situations, has led to the investiga­
tion of other potential mechanisms for thyroid hormone action 
that are referred to as extranuclear, noncanonic, or nongenomic 
actions. Several examples are provided here. Studies of mice in 
which the endogenous thyroid hormone receptor genes have been 
mutated to prevent thyroid horm

r genes have been 
mutated to prevent thyroid hormone receptor–DNA binding 
have provided strong evidence that heart rate, body temperature, 
blood glucose, and serum triglyceride levels can be regulated by 
nongenomic actions of TRs, whereas negative regulation of the 
hypothalamic-pituitary-thyroid axis requires DNA binding of 
TRβ.131 T3 has been shown to rapidly activate phosphatidylino­
sitol 3-kinase at the cell membrane via interaction with TRβ, and 
this action enhances maturation of syn

h TRβ, and 
this action enhances maturation of synapses in the mouse hippo­
campus in vivo.132 In addition, integrin αVβ3 has been identified 
as a putative plasma membrane thyroid hormone–binding site 
and has been proposed as a mediator of rapid effects of T4.133 The 
effect of T4 per se to initiate the ubiquitination of D2 is perhaps 
the most important nongenomic effect of physiologic concentra­
tions of free T4.51 
Regulation of Thyroid Function
The Hypothalamic-Pituitary-Thyroid Axis
The t

tion
The thyroid participates with the hypothalamus and pituitary in 
a classic feedback control loop (Fig. 11.8). In addition, there is an 
inverse relationship between the iodine level in the thyroid and 
the fractional rate of hormone formation. Such autoregulatory 
mechanisms stabilize the rate of hormone synthesis despite fluc­
tuations in the availability of iodine. Stability in hormone produc­
tion is achieved in part because the large intraglandula

s achieved in part because the large intraglandular store of 
hormone buffers the effect of acute increases or decreases in hor­
mone synthesis. Autoregulatory mechanisms within the gland in 
turn tend to maintain a constant thyroid hormone pool. Finally, 
the hypothalamic-pituitary feedback mechanism senses variations 
in the availability of free thyroid hormones, however small, and 
acts to correct them. Hypothalamic tanycytes play a role in the 
homeostatic regulation of the HPT axis to maint

e 
homeostatic regulation of the HPT axis to maintain circulating 
thyroid hormone levels in a narrow physiologic range.134
There is a close relationship between the hypothalamus, the 
anterior pituitary, the thyroid gland, and still higher centers in 
the brain, the function of the entire complex being modified in a 
typical negative-feedback manner by the availability of the thyroid 
hormones. In addition, other hormones and neuropeptides also 
influence this axis (see Chapters 7 and 8).
Thyro

Thyrotropin-Releasing Hormone Synthesis and Secretion
TRH, a modified tripeptide (pyroglutamyl-histidyl-proline amide), 
is derived from a large prepro-TRH molecule of 29 kDa that con­
tains five progenitor sequences. The TRH peptides are released 
from the prepro molecule by a peptidase that acts at flanking 
lysine/arginine residues. TRH is expressed in the hypothalamus, 
the brain, the C cells of the thyroid gland, the beta cells of 
the pancreas, 

e thyroid gland, the beta cells of 
the pancreas, the myocardium, the reproductive organs (including 
prostate and testis), and the spinal cord. The parvocellular region 
of the paraventricular nuclei (PVN) of the hypothalamus is the 
source of the TRH that regulates TSH secretion. The 5′ flanking 
region of the gene encoding TRH has sequences for mediating 
Thyroxine
Amiodarone
Iopanoic Acid
OH
O
CH2
NH2
CH
COOH
I
I
I
I
C
O
O
CH2 CH2 N
I
I
C2H5
C2H5
I
CH2
C2H5
CH
COOH
I
I
H2N
O
C4H9
• Fig. 11.7

C2H5
I
CH2
C2H5
CH
COOH
I
I
H2N
O
C4H9
• Fig. 11.7  Comparison of the chemical structure of thyroxine (T4) with that 
of two agents that block the deiodination of the iodothyronines. The inhibition 
of T4-to-T3 conversion, which occurs in patients receiving amiodarone, 
may be due to the drug itself or to a metabolic product. Iopanoic acid 
and related iodoanilines are competitive inhibitors of all three iodothyronine 
deiodinases.
Pituitary
Thyroid
D1 + D2
T4 + T3(−)
T4 + T3
T4 (T3

ituitary
Thyroid
D1 + D2
T4 + T3(−)
T4 + T3
T4 (T3)
TSH (+)
TRH (+)
SRIH (−)
(D2)
T4     T3(−)
T3(−)
(D2)
T4     T3
T4     T3
T, L, K (D1)
T, SM, CM (D2)
• Fig. 11.8  Roles of thyroxine (T4) and triiodothyronine (T3) in the feedback 
regulation of secretion of thyrotropin-releasing hormone (TRH) and thyroid-
stimulating hormone (TSH). Secreted T4 must be converted to T3 to pro­
duce its effects. This conversion may take place in tissues such as the 
liver (L), kidney (K), and thyroid (T) catalyz

he 
liver (L), kidney (K), and thyroid (T) catalyzed by the type 1 iodothyronine 
deiodinase, D1. Type 2 (D2) is present in human thyroid (T), skeletal muscle 
(SM), possibly cardiac muscle (CM), and the pituitary and hypothalamus. 
SRIH, somatotropin release-inhibiting factor (somatostatin hormone).
348
responses to glucocorticoids and cAMP. In addition, at least two 
elements in this region are responsible for the negative regulation 
of this gene by thyroid hormone.135 

e regulation 
of this gene by thyroid hormone.135 TRH travels in the axons 
of the peptidergic neurons through the median eminence and is 
released close to the hypothalamic-pituitary portal plexus. The 
neuron bodies producing TRH are innervated by catecholamine, 
leptin, neuropeptide Y (NPY), agouti-related protein (AgRP) 
or melanocyte-stimulating hormone (MSH), and somatostatin-
containing axons, all of which potentially influence the rate of syn­
thesis of the prepro-TRH molecule (see Chapt

 syn­
thesis of the prepro-TRH molecule (see Chapter 7). T3 suppresses 
the levels of prepro-TRH mRNA in the hypothalamus,136,137 
but normal feedback regulation of prepro-TRH mRNA synthe­
sis by thyroid hormone requires a combination of T3 and T4 in 
the circulation, the latter giving rise to T3 via T4 5′ deiodination 
in the CNS in astrocytes and tanycytes (see Fig. 11.3). Another 
event in this feedback regulation may be the thyroid hormone–
mediated induction of the TRH-inactivating pyroglut

ediated induction of the TRH-inactivating pyroglutamyl pep­
tidase II (PPII) in the hypothalamic tanycytes. This regulation is 
observed in vivo exclusively in the parvocellular division of the 
PVN, but in tissues outside the CNS expressing the TRH gene, 
negative regulation by thyroid hormone is absent. Thus part of the 
negative feedback induced by T4 may be generated at the median 
eminence/arcuate nucleus at a point where neuropeptides and T3 
enter the pituitary portal system.138 Although 

3 
enter the pituitary portal system.138 Although D2 is also present 
in astrocytes in the median eminence and arcuate nucleus region, 
selective ablation of D2 from astrocytes in transgenic mice has 
no significant effect on feedback regulation of the hypophy­
siotropic TRH neurons, indicating that astrocytes have little 
or no role in the regulation of this response.139 In addition to 
inhibiting the synthesis of prepro-TRH mRNA, thyroid hor­
mone also blocks the capacity of TRH to stimulate T

one also blocks the capacity of TRH to stimulate TSH release 
from the thyrotroph.
TRH is rapidly inactivated within the CNS by a cell-surface 
peptidase called TRH-degrading ectoenzyme (TRH-DE), also 
termed protein peptidase II. TRH-DE is very specific: There is no 
other ectopeptidase known capable of degrading TRH, and TRH 
is the only known substrate of this unique enzyme.140 
Thyrotropin Synthesis and Secretion
TSH is the major regulator of the morphologic and functional 
states of the thy

 the morphologic and functional 
states of the thyroid. It is a glycoprotein secreted by the thy­
rotrophs in the anteromedial portion of the adenohypophysis 
(see Chapter 8). TSH is composed of an α-subunit of 14 kDa 
(92 amino acids) that is common to luteinizing hormone, 
follicle-stimulating hormone, human chorionic gonadotropin 
(hCG), and a specific β-subunit synthesized only in thyrotrophs, 
which is a 112–amino acid protein. In normal thyrotrophs and 
in thyrotroph tumors, synthesis of α

rotrophs and 
in thyrotroph tumors, synthesis of α-subunit is in excess, indi­
cating that the quantity of β-subunit is rate limiting for TSH 
secretion. Levels of the α-subunit in serum range from 0.5 to 
5 μg/L but are increased in postmenopausal women and patients 
with pituitary tumors. TRH increases and thyroid hormone 
suppresses the transcription of both subunits; these are the two 
most important influences on TSH synthesis.
The pretranslational regulation of TSH synthesis and secre­
tio

ational regulation of TSH synthesis and secre­
tion is a complex process. The physiologic glycosylation of TSH 
involves several post-translational steps, including the excision of 
signal peptides from both subunits and cotranslational glycosyl­
ation with high mannose oligosaccharides.141 The glycosylation of 
the subunits protects them from intracellular degradation and per­
mits normal folding of the protein chains so that internal disulfide 
linkages are correctly formed. Glycosylation is r

linkages are correctly formed. Glycosylation is required for full 
biologic activity.142,143 TRH is required for this process as illus­
trated by the inappropriately low biologic activity of the TSH in 
the serum of patients with pituitary tumors or hypothalamic dis­
orders. In animals that breed seasonally, alternatively glycosylated 
TSH secreted from the pars tuberalis (PT) of the anterior pituitary 
gland activates thyroid hormone within the hypothalamus, which 
in turn induces gonadotropin

 hypothalamus, which 
in turn induces gonadotropin-releasing hormone (GnRH), leading 
to gonadal growth in a seasonal fashion.144
In normal serum, TSH is present at concentrations between 0.4 
and 4.2 mU/L. The level is increased in primary hypothyroidism 
and reduced in thyrotoxicosis. The plasma TSH half-life is about 
30 minutes, and production rates in humans are 40 to 150 mU/day. 
Circulating TSH displays both pulsatile and circadian variations. 
The former are characterized by fluctuations

ons. 
The former are characterized by fluctuations at intervals of 1 to 
2 hours. The magnitude of TSH pulsations is decreased during 
fasting, during illness, or after surgery. There is an acute reduction 
of TSH in fasting humans, associated with a fall in leptin levels. 
This is due to a decrease in the amplitude of the TSH pulses.145 
The circadian variation is characterized by a nocturnal surge that 
precedes the onset of sleep and appears to be independent of the 
cortisol rhythm and fluct

o be independent of the 
cortisol rhythm and fluctuations in the serum and T4 and T3 con­
centrations. The circadian rhythm shows parallelism with changes 
in T3 levels.146 When the onset of sleep is delayed, the nocturnal 
TSH surge is enhanced and prolonged, and the early onset of sleep 
results in a surge of lesser magnitude and shorter duration. TSH 
levels also show seasonal changes in humans with a decrease during 
the summer and an increase during winter, changes that correlate 
with dail

e during winter, changes that correlate 
with daily temperatures.147
The degree of thyroid hypofunction after destruction of the 
hypothalamus is less severe than that which follows hypophysec­
tomy, and residual thyroid function in the former circumstance can 
be altered by raising or lowering the concentration of thyroid hor­
mones in the blood. Thus both T4 and T3 mediate the feedback 
regulation of TSH secretion, and TRH determines its set-point (see 
Fig. 11.8). There is a linear inverse re

int (see 
Fig. 11.8). There is a linear inverse relationship between the serum 
free T4 concentration and the log of the TSH (Fig. 11.9), making 
the serum TSH concentration an exquisitely sensitive indicator of 
the thyroid state of patients with an intact hypothalamic-pituitary 
axis. Gene-targeting studies show that TRH secretion is likely to be 
the dominant factor mediating the thyroid hormone feedback regu­
lation of TSH secretion because the markedly elevated TSH secre­
tion of mice with 

he markedly elevated TSH secre­
tion of mice with inactivation of TRβ cannot be sustained in mice 
lacking the TRH gene.148 This is somewhat surprising given the less 
severe hypothyroidism associated with hypothalamic (as opposed 
to primary) hypothyroidism but may be explained by the absolute 
nature of the TRH deficiency achieved by the genetic manipulation 
as opposed to the clinical situation in humans with central hypothy­
roidism, wherein the TRH deficiency is not likely to be complete.
S

the TRH deficiency is not likely to be complete.
Somatostatin (somatotropin release–inhibiting hormone [SRIH]), 
acting through inhibitory G protein (Gi), decreases TSH secretion 
in vitro and in vivo, but prolonged treatment with a somatosta­
tin analogue does not cause hypothyroidism.149,150 Similar acute 
effects occur during dopamine infusion and the administration of 
bromocriptine, a dopamine agonist. Both of these agents inhibit 
adenylate cyclase.
A number of drugs or hormones may suppre

 cyclase.
A number of drugs or hormones may suppress or stimulate 
TSH secretion (Table 11.6). Glucocorticoids given in high doses 
transiently suppress TSH secretion, although prolonged therapy 
is not associated with central hypothyroidism.149 Patients with 
Cushing disease have subnormal TSH production but with mini­
mal effects on T4 production.149 Bexarotene, a RXR agonist used 
for treatment of T-cell lymphoma, suppresses TSH sufficiently 
349
Chapter 11  Thyroid Pathophysiology and  Diagn

349
to cause central hypothyroidism, presumably by reducing TSHβ 
gene transcription.151,152
Neurotransmitters are important direct and indirect modu­
lators in TSH synthesis and secretion. A complex network of 
neurotransmitter neurons terminates on cell bodies of hypo­
physiotropic neurons, and several neurotransmitters (such as 
dopamine) are directly released into hypophyseal portal blood, 
exerting direct effects on anterior pit

al blood, 
exerting direct effects on anterior pituitary cells. Furthermore, 
many dopaminergic, serotoninergic, histaminergic, catechol­
aminergic, opioidergic, and GABAergic systems project from 
other hypothalamic/brain regions to the hypophysiotropic 
neurons involved in TSH regulation. These projections are 
important for a normal TSH circadian rhythm and response 
to stress and cold exposure, but basal TSH secretion is mainly 
regulated by intrinsic hypothalamic activity. 
Iodine Deficienc

intrinsic hypothalamic activity. 
Iodine Deficiency
The response of vertebrates to iodine deficiency is designed 
to conserve this limited resource and improve the efficiency 
of its utilization. These adjustments occur at the hypothalamic, 
pituitary, thyroid, and peripheral tissue levels. Removal of iodine 
from the diet causes a rapid decrease in serum T4 concentrations 
and a simultaneous increase in serum TSH (Fig. 11.10).153 Inter­
estingly, no detectable decrease in T3 occurs, suggesting 

, no detectable decrease in T3 occurs, suggesting that the 
signal to increase TSH must derive from a decrease in the T3 gen­
erated intracellularly from T4 in the pituitary, the hypothalamus, 
or both. TSH increases NIS, Tg, and TPO synthesis and iodine 
organification and Tg turnover (see Fig. 11.1). Because of the 
decrease in iodide supply and the ratio of DIT/MIT, the ratio of 
T4 to T3 in Tg decreases and the rate of thyroidal T3 secretion 
may increase despite a fall in T4 secretion. TSH 

may increase despite a fall in T4 secretion. TSH also stimulates 
cell division, leading to goiter. In the rat model, the fall in plasma 
T4 increases D2 from 5-fold to 20-fold in the CNS, hypothalamus, 
and pituitary, increasing the efficiency of T4 conversion to T3. 
With moderately severe iodine deficiency, D3 in the CNS is 
also reduced, prolonging the mean residence time of T3 in that 
organ.154 This permits serum T3 to remain normal and the CNS 
T3 to be only moderately reduced despite up

e CNS 
T3 to be only moderately reduced despite up to a 10-fold decrease 
in circulating T4.
Despite the TSH elevation and nearly undetectable serum 
T4 in acutely iodine-deficient rodents, growth, O2 consump­
tion, and thermal homeostasis can be maintained.155 However, 
if iodine deficiency is prolonged and severe, hypothyroidism will 
supervene. In humans, these compensatory alterations in thyroid 
function come into operation when total iodine intake falls below 
Undetectable
values
4.0
3.6
3

 intake falls below 
values
4.0
3.6
3.2
2.8
2.4
2.0
1.2
0.8
0.4
Normal TSH
1000
100
10
1.0
0.1
0.01
TSH (mU/L)
Hypo-
thyroid
Euthyroid
FT4 (ng/dL)
• Fig. 11.9  The log/linear relationship between thyroid-stimulating hormone (TSH) (on the vertical axis) and 
the free T4 concentrations (FT4). Typical free T4 concentrations in hypothyroid, euthyroid, and hyperthyroid 
patients are shown.
  Endogenous and Exogenous Agents That 
May Stimulate or Inhibit Thyrotropin 
Secreti

hat 
Secretion
Stimulatory Agents
Inhibitory Agents
(TRH)
Prostaglandins (?)
α-Adrenergic agonist (? via 
TRH)
Opioids (humans)
Arginine vasopressin (AVP)
Glucagon-like peptide 1 (GLP1)
Galanin
Leptin
Glucocorticoids (in vitro)
Thyroid hormones and analogues
Dopamine and dopamine agonists
Biotin
Opioids (rat)
Glucocorticoids (in vivo, high dose)
Serotonin
Gastrin or gastrin-releasing peptide 
(GRP)
Arginin

astrin or gastrin-releasing peptide 
(GRP)
Interleukin 1β and 6
Tumor necrosis factor α (TNFα)
Bexarotene (retinoid receptor 
agonist)
Phenytoin
Somatostatin and somatostatin 
analogues
Mitotane
TABLE 11.6
350
75 μg/day (see Table 11.1). This situation can occur even after 
the implementation of iodine supplementation programs in the 
absence of adequate longitudinal monitoring.156
Changes in serum hormones seen in experiment

g.156
Changes in serum hormones seen in experimental animals 
have been well documented in humans in areas of iodine defi­
ciency and in patients with NIS mutations.157 However, they 
may not be seen in older members of the population when thyroid 
autonomy often develops. The physiologic response to iodine 
deficiency is similar to that which occurs during the develop­
ment of primary hypothyroidism in humans. It is also repro­
duced when the efficiency of iodide trapping and organification 
is

ficiency of iodide trapping and organification 
is reduced in Hashimoto disease or in the patient with Graves 
disease receiving thiourea drugs.38 The physiologic effects of 
this series of events are clear. T3 has approximately 10 times the 
potency of the prohormone T4 and contains only three iodine 
atoms. This results in a more efficient use of the iodine atom. 
Maintenance of normal circulating T3 independent of serum 
T4 concentrations should provide hormone for those tissues in 
which the

ld provide hormone for those tissues in 
which the nuclear T3 is completely derived from the plasma such 
as liver and kidney (see Fig. 11.5). 
Iodine Excess
The thyroid is also protected against an excess of iodide that 
might otherwise lead to hyperthyroidism. As with the response 
to iodine deficiency, there are multiple levels of defense against 
this eventuality. The usual source of excess iodine is pharmaceutic, 
with radiographic dyes, amiodarone, and povidone-iodine being 
the most commo

odarone, and povidone-iodine being 
the most common sources (Table 11.7).
Effects of Increased Iodine Intake on Thyroid 
Hormone Synthesis
The quantity of iodine organified in Tg, which includes T4 and T3, 
displays a biphasic response to increasing doses of iodide, at first 
increasing and then decreasing as a result of a relative blockade 
of organic binding. This decreasing yield of organic iodine from 
increasing doses of iodide, the Wolff-Chaikoff effect, results from 
a high concentration 

aikoff effect, results from 
a high concentration of inorganic iodide within the thyroid cell.18 
The susceptibility to the Wolff-Chaikoff effect can be increased 
either by stimulation of iodide trapping, as occurs in patients with 
Graves disease, or during persistent TSH stimulation by impair­
ment of iodine organification in the human fetus, in patients 
with Hashimoto disease, or in thyroids previously irradiated by 
either 131I or external beam therapy. In such situations, goiter and 
hypo

beam therapy. In such situations, goiter and 
hypothyroidism (iodide myxedema) can develop if excess iodide is 
given for long periods. The mechanism for organification inhibi­
tion may involve inhibitory effects of high iodide concentrations 
on TPO and DUOX2.
In normal subjects given iodide, the inhibition of iodothyro­
nine formation is reversible over time. This escape or adaptation 
phenomenon occurs because iodide transport activity decreases 
probably through a decrease in NIS expression.

es 
probably through a decrease in NIS expression. Consequently, 
thyroidal iodide falls to levels insufficient to maintain the full 
Wolff-Chaikoff effect.19,158 Importantly, it does not occur in 
the third-trimester fetus, so chronic high iodine intake during 
pregnancy must be avoided because it will cause fetal hypothy­
roidism and compensatory potentially obstructive goiter in the 
newborn (Fig. 11.11). 
Effects on Thyroid Hormone Release
An important practical effect of pharmacologic doses

 important practical effect of pharmacologic doses of iodide is 
the prompt inhibition of thyroid hormone release. This occurs to 
Serum TSH
LID
LID + KI
30
20
10
0
2000
1000
0
ng/mL
Serum T4
LID
LID + KI
4
2
0
µg/100 mL
Serum T3
LID
LID + KI
80
60
40
20
0
ng/100 mL
Days on LID
• Fig. 11.10  Effects of acute depletion of dietary iodine on serum triiodothy­
ronine (T3), thyroxine (T4), and thyroid-stimulating hormone (TSH) in rats. 
Animals received a low iodine diet (LID) without or with supplem

ed a low iodine diet (LID) without or with supplementa­
tion of potassium iodide (KI) in drinking water. (From Riesco G, Taurog A, 
Larsen PR, et al. Acute and chronic responses to iodine deficiency in rats. 
Endocrinology 1977;100:303–313.)
  Iodine Content of Various Iodinated 
Pharmaceuticals a
Agent
Iodine Content
Saturated solution of potassium iodide
38 mg/drop
Lugol solution
6 mg/drop
Iodized salt (1 part KI/10,000 NaCl)
760 μg/10 g
Amiodarone (200-mg tablet)
75 mg organic iodine, 8–17% 

arone (200-mg tablet)
is released as iodide
Iopanoate, ipodate
350 mg/tablet
Angiographic and CT dyes
400–4000 mg/dose
Povidone-iodine
10 mg/mL
Kelp tablets
150 μg/tablet
Prenatal vitamins
Iodinated glycerol
25 mg/mL
Quantity of iodine required to suppress 
radioactive iodine uptake to <2%
>30 mg/day
aTypical iodide intake in the United States is 100–400 μg/day.
CT, Computed tomography; KI, potassium iodide; NaCl, sodium chloride.
   
TABLE 11.7
351
Cha

ide; NaCl, sodium chloride.
   
TABLE 11.7
351
some extent normally but is especially apparent in patients with 
Graves disease or toxic nodules (see Chapter 12). The mechanism 
is unknown, but the effect is mediated at the thyroid cell level 
rather than through an action on TSH. Iodine also diminishes 
the hypervascularity and hyperplasia that characterize the diffuse 
toxic goiter of Graves disease. The pharmacologic use of iodide

of Graves disease. The pharmacologic use of iodide is 
therefore useful in patients with severe hyperthyroidsm (thyroid 
storm) or in the preparation of thyrotoxic patients for surgery.159 
Thyroid Function in Pregnancy and in the Fetus 
and Newborn
Pregnancy affects virtually all aspects of thyroid hormone econ­
omy (Table 11.8).160 The total serum T4 and T3 concentrations 
rise to levels about 1.5-fold those of nonpregnant women, owing to 
the increase in TBG concentration in the first trimest

increase in TBG concentration in the first trimester (Fig. 11.12). 
The markedly increased TBG extracellular pool must be filled 
steadily with increasing amounts of T4 until a new equilibrium 
is reached. Human chorionic gonadotropin (hCG) cross-reacts 
with the TSHR, which results in a small and transient increase in 
free T4 levels near the end of the first trimester (peak circulating 
hCG), resulting in a partial TSH suppression. In some women, 
hCG can induce transient gestational thyrotoxi

n, 
hCG can induce transient gestational thyrotoxicosis, and tropho­
blast tumors (hydatidiform moles, choriocarcinomas) can rarely 
lead to severe hyperthyroidism. A mutation in the TSHR result­
ing in increased affinity for hCG has been found in familial ges­
tational hyperthyroidism.161 When tested in bioassays, normal 
hCG is only about 1/100 as potent as TSH. This weak thyrotropic 
activity explains why, in normal conditions, the effects of hCG 
remain largely unnoticed.162 In addition to t

hCG 
remain largely unnoticed.162 In addition to the increase in serum 
TBG, there is also an increased plasma volume as well as accel­
erated inactivation of T3 and T4 by D3 expression in the fetal-
placental-uterine unit.163 Based on the changes in requirements 
for levothyroxine during gestation in women with primary hypo­
thyroidism, the estimated increase in T4 production required dur­
ing this period is 20% to 40%.
The requirement for increased T4 secretion increases iodine 
requirements d

ased T4 secretion increases iodine 
requirements during pregnancy.164 This need is compounded by 
the fact that the higher glomerular filtration rate during gesta­
tion enhances renal iodide clearance, leading to higher fractional 
• Fig. 11.11  Newborn infant with iodide-induced goiter due to Lugol solu­
tion treatment of the mother during the third trimester. This illustrates the 
danger of chronic excess iodide administration during pregnancy.
  Effects of Pregnancy on Thyroid Physiology
Phy

Physiologic Change
Thyroid-Related Consequences
↑ Serum thyroxine-binding 
globulin
↑ Total T4 and T3; ↑ T4 production
↑ Plasma volume
↑ T4 and T3 pool size; ↑ T4 
production; ↑ cardiac output
D3 expression in placenta and 
(?) uterus
↑ T4 production
First trimester ↑ in hCG
↑Free T4; ↓ basal thyrotropin; ↑ T4 
production
↑ Renal I− clearance
↑ Iodine requirements
↑ T4 production; fetal T4 synthesis 
during second and third 
trimesters
↑ Oxygen cons

↑ Oxygen consumption by 
fetoplacental unit, gravid 
uterus, and mother
↑ Basal metabolic rate; ↑ cardiac 
output
D3, Type 3 iodothyronine deiodinase; I−, plasma iodide; hCG, human chorionic gonadotropin; 
T3, triiodothyronine; T4, thyroxine.
   
TABLE 11.8
Mother
Free T4
Total T4
hCG
TBG
40
30
20
10
0
Week of pregnancy
• Fig. 11.12  Changes in various critical components of the thyroid-pituitary 
axis during pregnancy. Note the early increase in 

axis during pregnancy. Note the early increase in free thyroxine (T4), prob­
ably due to thyroidal stimulation by human chorionic gonadotropin (hCG), 
which causes a reciprocal modest suppression of serum thyrotropin (TSH) 
during the late first trimester. (From Burrow GN, Fisher DA, Larsen PR. 
Mechanisms of disease: maternal and fetal thyroid function. N Engl J Med 
1994;331:1072–1078.)
352
urinary excretion of circulating iodide. In addition, maternal 
iodine intake mus

g iodide. In addition, maternal 
iodine intake must be increased to supply the requirements 
of the fetal thyroid during the second and third trimesters (see 
Table 11.8). If these increased requirements for iodide are not 
met, serum T4 falls and TSH rises. This series of events is well 
documented in areas of endemic iodine deficiency or borderline 
iodine supply, such as Brussels, Belgium.9 In that city, 70% of 
pregnant women carefully followed throughout pregnancy had a 
20% or greater incr

ed throughout pregnancy had a 
20% or greater increase in thyroid volume during gestation due 
to increased TSH. After delivery, the changes in thyroid function 
gradually return to normal, and serum TBG values reach normal 
levels 6 to 8 weeks postpartum.
During pregnancy, autoimmunity is suppressed, affecting 
patients with Graves and Hashimoto diseases (see Chapters 12 and 
13). In general, TSHR antibody (TRAb)–mediated thyroid stimu­
lation in the Graves disease patient is exacerbated during

n the Graves disease patient is exacerbated during the first 
trimester and is attenuated during the second and third trimesters 
only to exacerbate in the first several months postpartum. Thyroid 
autoantibody titers fall during gestation in patients with Hashimoto 
disease only to rise sharply postpartum in association with a phase 
of acute T-cell–mediated thyroid cell destruction—postpartum thy­
roid disease (PPTD)—which occurs in about 30% of patients with 
Hashimoto disease and significant

f patients with 
Hashimoto disease and significant residual thyroid tissue.165
The basal metabolic rate (BMR) increases during the second 
trimester owing to the increase in the total mass of body tis­
sue consequent to the pregnancy. The changes of pregnancy, 
together with the decreased peripheral vascular resistance, vaso­
dilatation, and modest tachycardia, may suggest thyrotoxicosis 
(see Table 11.8). It is important to appreciate that such changes 
are physiologic in pregnancy, especially 

changes 
are physiologic in pregnancy, especially when managing the 
hyperthyroid pregnant patient.
Fetal Thyroid Function
The peripheral metabolism of T4 in the human fetus differs mark­
edly from that in the adult, both quantitatively and qualitatively. 
Overall, rates of production and degradation of T4 in terms of 
units per body mass exceed those in the adult by 10-fold. In addi­
tion, D1 catalysis is reduced and D3 is enhanced, favoring the 
formation of the inactive rT3 at the expense of 

formation of the inactive rT3 at the expense of T3. D3 is highly 
expressed in fetal tissues, including the liver, skin, tracheobron­
chial, urothelial, and gastrointestinal epithelia.163 This condition 
results in a persistently low serum T3 concentration and an ele­
vated serum rT3. This change permits the highly regulatable con­
version of T4 to T3 by D2 to be the major pathway for generating 
tissue T3.166
Fetal thyroid function begins at about the end of the first 
trimester.5 Thereafter,

bout the end of the first 
trimester.5 Thereafter, there are steady increases in fetal TBG 
and total T4 and T3.9,167 Throughout gestation, the serum TSH 
values are greater than are present in maternal circulation and 
higher than would be expected in adults with normal thyroid 
function. This indicates that there is increasing hypothalamic-
pituitary resistance to T4 during fetal development, which is 
speculated to be a consequence of increased TRH secretion.168 
Despite the low circulating T

d TRH secretion.168 
Despite the low circulating T3, the fetal free T4 concentra­
tions approximate those in the maternal circulation from the 
­gestational age of 28 weeks and onward. 
Maternal-Fetal Interactions
The fetal pituitary-thyroid axis functions as a unit that is essen­
tially independent from the mother.4,168 Transplacental passage of 
TSH from mother to fetus is negligible, but the same is not true of 
maternal T4. In infants with congenital hypothyroidism caused by 
either genetic 

ngenital hypothyroidism caused by 
either genetic TPO deficiency or athyreosis, serum concentrations 
of T4 in cord blood are usually one-third to one-half of normal.169 
Thus at least when the maternal-fetal concentration gradient is 
high, significant transfer of maternal T4 to the fetal circulation 
can occur. This transfer may be significant, given the capacity 
of the fetal brain to increase the efficiency of T4-to-T3 conversion. 
Furthermore, T4 can be found in coelomic and amniotic fluids

e, T4 can be found in coelomic and amniotic fluids 
before the onset of thyroid function.170 The major factor limiting 
T4 and T3 transfer from mother to fetus is the D3 expressed in the 
uterus, placenta, and fetal epithelium. 
Thyroid Function in the Newborn
Mean total T4 level in cord sera is 150 nmol/L (12 μg/dL). Serum 
TBG concentrations are elevated, but are not as high as in the 
maternal serum. At term, free T4 concentrations are slightly lower 
than those in the mother. Cord serum T3 c

y lower 
than those in the mother. Cord serum T3 concentrations are low 
(0.8 nmol/L, 50 ng/dL), and rT3 and T3SO4 are elevated.4,171,172 
After delivery, the serum TSH level in the neonate increases rap­
idly to a peak at about 2 to 4 hours after birth, returning to its 
initial value within 48 hours.171 Levels above 60 mU/L are typical. 
This neonatal TSH surge is thought to occur in response to 
the rapid reduction in environmental temperature after deliv­
ery. In response, the serum T4, T3, 

 after deliv­
ery. In response, the serum T4, T3, and Tg concentrations increase 
rapidly during the first few hours after delivery and are in the 
hyperthyroid range by 24 hours of life.173 The TSH surge doubt­
less contributes to the increase in serum T3 concentration, but 
enhancement of extrathyroidal conversion of T4 to T3 by D1 
or D2 is thought to be a major factor as well.51 The adrenergic 
stimulation of the Dio2 gene and the reactivation of D2 by its 
deubiquitination in brown adipose 

n of D2 by its 
deubiquitination in brown adipose tissue are likely to be major 
contributors to this increase.174
Premature infants have an immature hypothalamic-pituitary-
thyroid axis with low T4, T3, and TSH.171,175 Serum T4, TBG, 
and free T4 all tend to correlate with gestational age. Preterm 
infants also have an attenuated TSH surge after delivery. In addi­
tion, when prematurity is accompanied by complications, such 
as respiratory distress syndrome or nutritional problems, serum 
T4, a

ess syndrome or nutritional problems, serum 
T4, and especially T3, may fall to low levels as a result of a com­
bination of reduced TBG production, immaturity of the thyroid 
gland, suppression of the hypothalamic-pituitary axis due to 
illness, impairment of T4-to-T3 conversion, and increases in D3 
activity.176,177 These changes are in many respects similar to those 
in adults with severe illness. All of these issues need to be taken 
into account when evaluating the thyroid status of the pre

ount when evaluating the thyroid status of the preterm 
infant, particularly given the increased prevalence of congenital 
hypothyroidism in this age group.175
Thyroid hormone production rates are higher per unit of body 
weight in neonatal infants and children than in adults. The daily 
levothyroxine requirement is about 10 μg/kg in the newborn, 
decreasing progressively to about 1.6 μg/kg in the adult.175 
Aging and the Thyroid
The thyroid gland undergoes several anatomic changes with age. 
Th

d undergoes several anatomic changes with age. 
There is a reduction in weight of the gland, in the size of follicles, and 
in the content of colloid, and there is increased fibrosis, often with 
marked lymphocytic infiltration. However, these changes do not 
correlate with thyroid function.176 In the healthy elderly patient, 
there is a normal level of free T4, but serum T3 levels appear to be 
lower, although studies of selected healthy people indicated that 
T3 levels are unaffected by aging.

indicated that 
T3 levels are unaffected by aging.179 TSH may increase or decrease 
with age in relation to the iodine intake.180 Population studies in 
353
humans and animal models show negative correlations between 
thyroid hormone levels and longevity, a finding that led to the 
hypothesis that constraints on thyroid hormone signaling at cer­
tain life stages, notably during maturity, are advantageous for opti­
mal aging.181 Howev

ty, are advantageous for opti­
mal aging.181 However, some other studies have suggested that 
both subclinical hypothyroidism and subclinical hyperthyroidism 
might be associated with increased mortality in the elderly.182 
Thyroid Function During Fasting or Illness
A number of changes take place in thyroid function during nutri­
tional deprivation or illness. These changes consist of a central 
reduction in TSH secretion and a decrease in plasma T3 levels 
and T4 and T3 binding in serum. This c

 T3 levels 
and T4 and T3 binding in serum. This constellation of findings is 
termed the low T3 syndrome, the euthyroid sick syndrome, or non­
thyroidal illness.183 The patterns of changes in circulating thyroid 
hormones and TSH during fasting and illness are quite similar. 
During fasting, there is a reduction of 50% or more in serum T3 
and an increase in serum rT3 without initial changes in serum 
total or free T4 (Table 11.9).145,184 Although the role of specific 
deiodinases in causing th

gh the role of specific 
deiodinases in causing these changes has not been documented at 
a tissue level in humans during fasting, several lines of evidence 
suggest that decreases in peripheral T4-to-T3 conversion by both 
D1 and D2 and a reduced clearance of rT3 by D1 play a role 
in this process. The finding of normal T3 plasma levels in mice 
with the genetic absence of both D1 and D2 enzymes suggests 
that, under normal conditions, the thyroid gland by itself is able 
to compensate for impa

id gland by itself is able 
to compensate for impaired peripheral conversion to normalize 
serum T3. This observation suggests that very powerful mecha­
nisms are in place to maintain serum T3 levels within the normal 
range, with the notable exceptions of when it is not meant to be in 
that range (i.e., during fasting or illness). In these circumstances, 
by a mechanism probably regulated by the hypothalamus, all 
compensatory mechanisms are reduced and serum T3 may drop 
to almost undetectable

uced and serum T3 may drop 
to almost undetectable levels.92 Deiodination by D3 increases the 
generation of rT3 from T4 and converts T3 to 3,3′-diiodothyronine 
(see Fig. 11.2), which exaggerates the changes resulting from the 
decrease in D1 and D2. The finding that D3-null mice can still 
develop the low T3 syndrome suggests that D3 upregulation is not 
the only event occurring in this clinical state. It is not yet known if 
such an increase in D3 also occurs during caloric restriction. The 

n D3 also occurs during caloric restriction. The 
attenuation of TSH secretion despite a fall in serum T3 levels dur­
ing fasting is discussed earlier (see “The Hypothalamic-Pituitary-
Thyroid Axis”).
During fasting, basal oxygen consumption and heart rate 
decline, and nitrogen balance, initially negative, returns toward 
normal.185 In some studies, these changes in overall metabolism 
are partially reversed by replacement of exogenous T3 while fast­
ing continues. Thus the decrease in T3 durin

fast­
ing continues. Thus the decrease in T3 during fasting (and pre­
sumably illness) can be viewed as a beneficial energy-sparing and 
nitrogen-sparing adaptation. Chronic malnutrition such as occurs 
in anorexia nervosa is also associated with a reduction in serum T3 
and rarely in free T4.186 TSH concentrations remain in the refer­
ence range, although again, they are inappropriately low in the 
context of the reductions in circulating T3. In contrast, overfeed­
ing, particularly with carboh

 contrast, overfeed­
ing, particularly with carbohydrates, increases T3 production rates 
and the serum T3 concentration, reduces serum rT3, and increases 
basal thermogenesis.187
During illness, decreases in T3 and pulsatile TSH release and 
increases in rT3 also occur.183 If illness progresses, the hypotha­
lamic-pituitary-thyroid axis is even further suppressed, with a con­
sequent reduction in the free T4. Severe decreases in serum T4 are 
associated with a high probability of death. This sy

sociated with a high probability of death. This syndrome is asso­
ciated with a decrease in TRH mRNA in the human PVN.188 
An increase in T3 production by D2-catalyzed T4-to-T3 conver­
sion in the tanycytes lining the third ventricle during illness may 
contribute to the blunted response of TSH to the reduced serum 
T3, particularly during infections.189 Cytokines, such as interleu­
kin 6 (IL6), also increase during illness and are coincident with 
the decrease in circulating T3, although it is 

h 
the decrease in circulating T3, although it is not clear whether this 
is the cause of the hypothalamic changes.190 An in vitro study 
indicated that IL6 causes an increase in intracellular and extracel­
lular reactive oxygen species (ROS).93 As mentioned, the active 
centers of both D1 and D2 are intracellular and that of D3 has 
access to extracellular thiols, which do not pass easily through 
the cell membrane (see Fig. 11.4). The increase in ROS reduces 
intracellular thiols such as GSH (

in ROS reduces 
intracellular thiols such as GSH (glutathione) and presumably the 
GSH/NADPH-dependent thioredoxin and glutaredoxin. Thus 
T4-to-T3 conversion by D1 and D2 is reduced while D3-mediated 
T3 and T4 inactivation continues. In addition to these cofactor 
changes, the increase in ROS and the activation of MAPK-dependent 
pathways due to IL6 and ROS induce increases in the tran­
scription of the deiodinases, especially D3.93 In vitro, all these 
changes are reversed by addition of N-ac

ll these 
changes are reversed by addition of N-acetylcysteine (NAC) to the 
media, which rescues intracellular GSH synthesis. Interestingly, 
follow-up studies in patients with the low T3 syndrome due 
to acute myocardial infarction show that the reduction in T3 and 
the increase in rT3 can be prevented by NAC infusions,191 sug­
gesting these in vitro results are relevant to sick patients. These 
endogenous changes may be further exaggerated by agents such as 
dopamine or glucocorticoids, which

agents such as 
dopamine or glucocorticoids, which will also, at least transiently, 
suppress the TRH-TSH axis.192 The changes in thyroid function 
are a continuum, with the abnormalities becoming progressively 
more severe in parallel with the patient’s clinical condition (see 
Table 11.9). Postmortem studies show that hepatic D1 activity is 
reduced by about 50%, skeletal muscle D2 is absent, and D3 is 
present in liver and skeletal muscle.193 No differences have been 
found in the T3 transpor

No differences have been 
found in the T3 transporter MCT8 in skeletal muscle or liver, 
and possible abnormalities in other thyroid hormone transport­
ers have not been evaluated. Interestingly, the same global pat­
tern of changes during acute medical illness has been described in 
patients with primary hypothyroidism receiving levothyroxine.194 
In such patients, serum T4, T3, and TSH concentrations all fell 
about 50% over the first 3 days, presumably due to a disruption 
of T4 binding due t

resumably due to a disruption 
of T4 binding due to a decrease in TBG, TTR, and albumin, as 
well as a blockade of T4-protein interactions due to circulating 
  Changes in Thyroid Hormone Levels 
During Illness
Severity 
of Illness
Free 
T3
Free 
T4
Reverse 
T3
TSH
Probable 
Cause
Mild
↓
N
↑
N
↓ D2, D1
Moderate
↓↓
N, ↑↓
↑↑
N, ↓
↓↓ D2, D1, ? 
↑ D3
Severe
↓↓↓
↓
↑
↓↓
↓↓ D2, D1, ↑ D3
Recovery
↓
↓
↑
↑
?
D1 through D3, Iodothyronine deiodinases; N, no change; T3, triiodothyronine; T4, thyroxine; 
TSH

 change; T3, triiodothyronine; T4, thyroxine; 
TSH, thyroid-stimulating hormone (thyrotropin).
   
TABLE 11.9
354
interfering substances. Contributing to this may be the trans­
lational modification of TBG due to a serpin-catalyzed release of 
a carboxy-terminal fragment of TBG in inflamed tissues discussed 
earlier (see “Thyroxine-Binding Globulin”).
Therapies have been introduced in an attempt to ameliorate 
certain of the illness-related central abnormalities in the hyp

e illness-related central abnormalities in the hypo­
thalamic-pituitary axis (including decreases in growth hor­
mone and gonadotropins). One of these, infusions of growth 
hormone–releasing peptide 2 (GHRP2) combined with TRH, 
has resulted in increases in TSH, T3, and T4, as well as insulin-
like growth factor 1 (IGF1), insulin, and the IGF-binding pro­
teins 1, 3, and 5.195 Although the biochemical improvements 
were significant, the clinical state did not change, suggesting 
that the thyroid

state did not change, suggesting 
that the thyroid dysfunction is a marker of the severity of the 
illness rather than its cause.
Although serum TSH concentrations in severely ill patients are 
reduced, an increase in TSH above the normal range may appear 
during recovery, with the elevation in TSH concentration persisting 
until circulating free T4 and T3 levels return to normal.196 This pat­
tern can be confusing if the elevated TSH concentration is associ­
ated with the still-reduced concentr

on is associ­
ated with the still-reduced concentrations of free T4. Such patients 
meet all laboratory criteria for primary hypothyroidism with the 
exception of the clinical context. Follow-up generally reveals a nor­
malization of TSH and T4 within 1 to 2 months (see Table 11.9).
Despite the severity of the abnormalities, particularly in serum 
T3, there is still disagreement as to whether therapeutic intervention 
should be initiated even in the most severely ill patients because 
most contr

the most severely ill patients because 
most controlled studies have not shown beneficial effects of T4 or 
T3 supplementation in such individuals.183 The one exception is 
a possible beneficial effect of T3 therapy in patients after coronary 
artery bypass grafting, with one study showing a positive effect but 
a second study showing none.197,198 Therefore the cautious use of 
thyroid hormonal therapy in patients with low T3 syndrome or a 
stunned myocardium was proposed.199 
The Thyroid Axis a

ed myocardium was proposed.199 
The Thyroid Axis and Neuropsychiatric Illness
Patients with neuropsychiatric disease can present with any of 
a number of abnormalities in thyroid function. Patients with 
bipolar disorders may show slight elevations in serum TSH and 
reductions in free T4, whereas patients with severe depression 
have slight elevation in serum T4 and reduced serum TSH. 
Other acutely psychotic patients may have either high or low 
serum TSH concentrations and tend to have elevati

serum TSH concentrations and tend to have elevations in free 
T4.200 The cause of these minor abnormalities is not clear, but 
such patients may have thyroid function test results resembling 
those of patients with primary thyroid disease from whom they 
must be differentiated. 
Effects of Hormones on Thyroid Function
The acute administration of pharmacologic doses of glucocor­
ticoid eliminates pulsatile release of serum TSH concentrations 
in normal patients, presumably by red

centrations 
in normal patients, presumably by reducing TRH release. 
With continued administration, there is an escape from this sup­
pression (Table 11.10). Pharmacologic doses of glucocorticoid 
decrease the serum T3 concentration in normal and hyperthy­
roid patients as well as in hypothyroid patients maintained on 
levothyroxine. The latter finding and the accompanying increase 
in rT3 production suggest that glucocorticoids may increase D3 
activity.120
Primary adrenal insufficiency may be

Primary adrenal insufficiency may be associated with reduced 
serum T4 and elevated serum TSH concentrations, suggesting the 
coexistence of primary hypothyroidism. However, treatment of 
the adrenal insufficiency can lead to complete resolution of these 
abnormalities, suggesting that in some patients they are a con­
sequence of glucocorticoid deficiency rather than primary thyroid 
disease.201 Nevertheless, the prevalence of primary hypothyroid­
ism is increased in patients with 

ry hypothyroid­
ism is increased in patients with autoimmune hypoadrenalism, so 
the two causes must be differentiated (see Chapter 15). Likewise, 
patients successfully treated for Cushing disease can develop thy­
roid autoimmunity. 
Gonadal Steroids
Estrogen increases TBG by mechanisms already mentioned.202 
Presumably this increases T4 secretion, in that total T4 increases 
and free T4 is unchanged. Estrogen also increases the levothy­
roxine requirement in patients with primary hypothyroidis

requirement in patients with primary hypothyroidism.66 
In contrast, administration of androgens to women decreases 
TBG and decreases T4 turnover and levothyroxine requirements 
in patients with primary hypothyroidism.203 
Growth hormone increases the serum free T3 and decreases free 
T4 in both levothyroxine-treated and normal individuals, sug­
gesting either suppression of D3 activity or increased T4-to-T3 
conversion. 
Physical Evaluation of the Thyroid Gland
Manifestations of

 Evaluation of the Thyroid Gland
Manifestations of thyroid disease are usually due to excessive or 
insufficient production of thyroid hormone, goitrous enlarge­
ment of the thyroid or nodules resulting in local symptoms 
in the neck through compression of adjacent structures, or, in 
the case of Graves disease, ophthalmopathy or dermopathy. 
  Effects of Hormones on Thyroid Function
Excess
Decrease TSH, TBG, TTR (high dose)
Decrease serum T3/T4 and increase rT3/T4 ratios
Increa

ease serum T3/T4 and increase rT3/T4 ratios
Increase rT3 production (? ↑ D3)
Decrease T4 and T3 secretion in Graves disease
Deficiency
Increase TSH
Estrogen
Increase TBG sialylation and half-life in serum
Increase TSH in postmenopausal women
Increase T4 requirement in hypothyroid patients
Androgen
Decrease TBG
Decrease T4 turnover in women and reduce T4 requirements in hypothy­
roid patients
Decrease D3 activity
D3, Type 3 deiodinase; rT3, reverse T3; T3, triiodothyronine; T4, thy

se; rT3, reverse T3; T3, triiodothyronine; T4, thyroxine; TBG, thyroxine-
binding globulin; TSH, thyrotropin; TTR, transthyretin.
   
TABLE 11.10
355
A functional diagnosis of thyroid disease is based on a carefully 
taken history, a thorough search for the physical signs of hypothy­
roidism or thyrotoxicosis, and an appraisal of the results of labora­
tory tests. Although conditioned by the functional diagnosis, the 
anatomic diagno

 by the functional diagnosis, the 
anatomic diagnosis depends largely on the physical examination 
of the thyroid gland itself. The typical symptoms of an excess or a 
deficiency of thyroid hormone are discussed in Chapters 12 and 13.
Physical Examination
Examination of the neck is best accomplished with the patient 
seated in good light with the neck relaxed. The patient should 
be provided with a cup of water to facilitate swallowing. The 
physician should first inspect the neck, especially wh

ician should first inspect the neck, especially while the 
patient swallows, with the neck slightly extended. The presence 
of old surgical scars, distended veins, and redness or fixation of 
the overlying skin should be noted. The position of the trachea 
should be noted. If a mass is present, a determination should be 
made as to whether it moves with swallowing. A midline mass 
high in the neck, which rises further when the patient extends the 
tongue, is typical of a thyroglossal duct remnan

tongue, is typical of a thyroglossal duct remnant or cyst. Movement 
on swallowing is a characteristic of the thyroid gland because it is 
ensheathed in the pretracheal fascia; this feature distinguishes a 
goiter from most other neck masses. However, if the thyroid is so 
large that it occupies all the available space in the neck, movement 
on swallowing may be lost. The physician should also inspect the 
posterior dorsum of the tongue, which is the origin of the thyro­
glossal duct and the l

is the origin of the thyro­
glossal duct and the location of lingual thyroid tissue.
Except when the thyroid enlargement is extreme, the thyroid 
examination can be readily performed with the physician facing 
the seated patient. The thyroid may also be examined with the 
physician standing behind the seated patient, palpating with the 
fingertips of both hands. The physician should use gentle thumb 
pressure to locate the thyroid isthmus just caudal to the cricoid 
cartilage. This provides a co

udal to the cricoid 
cartilage. This provides a convenient starting point for the palpa­
tion of the lobes of the gland, but an increase in the thickness of 
the isthmus or a firm texture will already suggest the presence of 
some generalized thyroid enlargement. To examine the right lobe, 
the right thumb is then moved laterally, without release of gentle 
pressure, to locate the lobe of the thyroid by pressing it against the 
trachea as the patient swallows sips of water. This strategy allows 

ient swallows sips of water. This strategy allows 
the palpating thumb to laterally displace the medial border of the 
sternocleidomastoid muscle, allowing direct access to the entire 
thyroid lobe. As the patient swallows with the thumb pressing 
the lobe against the trachea with sufficient tension to displace it 
slightly over the midline, it will slide up and down under the ball 
of the thumb. This permits an appreciation of the size and texture 
of the gland as well as the presence or absenc

re 
of the gland as well as the presence or absence of nodules. A simi­
lar strategy with the left thumb is used for the left lobe.
The examiner should note the shape of the gland, its size in 
relation to normal, and its consistency, which is usually slightly 
greater than adipose tissue but less than muscle. The normal thy­
roid lobe has approximately the same size in frontal projection 
as the terminal phalanx of the patient’s thumb. Whereas a diffuse 
goiter and the hyperplastic gland of the

 diffuse 
goiter and the hyperplastic gland of the hyperthyroid patient 
with Graves disease may be softer than normal, the gland of Hashi­
moto disease is usually firm. Irregularities of the surface, variations 
in consistency, and tender areas should be noted. If nodules are 
palpated, their shape, size, position, translucency, and consistency 
in relation to the surrounding tissue should be determined. A firm 
mass may reflect a cyst, more rarely a malignancy. A search should 
be made for the

ely a malignancy. A search should 
be made for the pyramidal lobe, which is a thin band of tissue 
extending upward from the isthmus to the thyroid cartilage to the 
right or left of the midline. A hypertrophied pyramidal lobe may 
be mistaken for a pretracheal lymph node that sometimes accom­
panies thyroid carcinoma or thyroiditis. It is usually palpable in 
patients with generalized thyroid disease, such as Hashimoto or 
Graves disease. During palpation, a vascular thrill may be felt that, 
i

 palpation, a vascular thrill may be felt that, 
in the absence of cardiac disease, is suggestive of hyperthyroid­
ism. Finally, palpation should always include examination of the 
regional lymph nodes along the jugular vein, posterior to the ster­
nocleidomastoids and in the supraclavicular region.
Auscultation of the neck may confirm the increased vascularity 
of an enlarged, hyperactive gland, suggesting Graves disease. A 
systolic or continuous bruit is sometimes heard over a hyperplastic 
g

us bruit is sometimes heard over a hyperplastic 
gland. Care should be taken to distinguish a thyroid bruit from a 
murmur transmitted from the base of the heart or from a venous 
hum that can be obliterated by gentle compression of the exter­
nal jugular vein or by turning the head. A venous hum is gener­
ally found in younger patients with high cardiac output, such as 
occurs in Graves disease or with severe anemia.
An arm-raising test is useful when a retrosternal goiter is sus­
pected. The b

l when a retrosternal goiter is sus­
pected. The basis for this maneuver is that if the size of the thoracic 
inlet is already reduced by such a goiter, raising both arms until they 
touch the sides of the head further narrows the thoracic inlet and 
causes congestion and venous engorgement of the face and some­
times respiratory distress (Pemberton sign) or even (rarely) syncope.
In addition to examination of the thyroid gland and regional 
lymph nodes, evidence of compression or displacement o

h nodes, evidence of compression or displacement of adjacent 
structures should be sought. Hoarseness may indicate compression 
or infiltration of the recurrent laryngeal nerve, usually by a malignant 
thyroid neoplasm, and this should be confirmed by laryngoscopy. 
Displacement of the trachea may be evident, usually associated with a 
large nodule(s), and inspiratory stridor may indicate its compression.
Ultrasound is now widely available and has a superior sensitivity 
for the detection and ch

s a superior sensitivity 
for the detection and characterization of thyroid nodules. Qualita­
tive systems and Thyroid Imaging Reporting and Data Systems 
(TI-RADS) allow categorization of cystic and solid lesions and 
their relative risk for malignancy. It is also useful in the evaluation 
of patients with thyroiditis.204,205 The use of ultrasound should 
enhance, not replace, the physical examination of the thyroid. 
Laboratory Assessment of Thyroid Status
In considering the laboratory assessm

yroid Status
In considering the laboratory assessment of the patient with known 
or suspected thyroid disease, the physician should seek to arrive at 
both a functional and, when appropriate, an anatomic diagnosis. 
Laboratory determinations will confirm whether there is an excess, 
normal, or insufficient supply of thyroid hormone to verify the infer­
ences from the clinical history and physical examination. Laboratory 
tests can be divided into five major categories: (1) those that assess 
the

 five major categories: (1) those that assess 
the state of the hypothalamic-pituitary-thyroid axis, (2) estimates of 
the T4 and T3 concentrations in the serum, (3) tests that reflect the 
impact of thyroid hormone on tissues, (4) tests for the presence of 
autoimmune thyroid disease, and (5) tests that provide information 
about thyroidal iodine metabolism. The use of iodine and other iso­
topes for thyroid scanning is discussed in Chapter 14.
Tests of the Hypothalamic-Pituitary-Thyroid Axis
T

Although an inherently indirect reflection of thyroid hormone 
supply, tests that assess the state of the hypothalamic-pituitary-
thyroid axis play a critical role in the diagnosis of thyroid disease. 
356
This is because the rate of TSH secretion is exquisitely sensitive to 
the plasma concentrations of free thyroid hormones, thus provid­
ing a precise and specific indicator of the thyroid status

ecise and specific indicator of the thyroid status of the 
patient (see Figs. 11.8 and 11.9). The rare exceptions to this rule 
are discussed later. Immunometric assay technology now makes it 
possible to define the normal range for serum TSH and hence to 
ascertain both when thyroid function is inadequate and when the 
hormone supply is excessive (see Chapter 4). This assay uses the 
TSH molecule to link a TSH antibody bound to an inert surface 
(e.g., particles, the side of a test tube) to a s

(e.g., particles, the side of a test tube) to a second antibody directed 
against a different TSH epitope that is labeled with a detectable 
marker (125I, an enzyme, or a chemiluminescent reagent). Thus 
the signal generated is proportional to the concentration of TSH 
in the serum. This technique is more specific, sensitive, and rapid 
than radioimmunoassay.
Many of these assays take advantage of the biotin/streptavidin 
binding for high affinity for complex formation. This interaction 
can be

ty for complex formation. This interaction 
can be affected by circulating biotin (vitamin B7) in the serum of 
patients taking large amounts of this substance commonly avail­
able over the counter or prescribed for various skin conditions or 
even for type 2 diabetes. This is especially worrisome since high 
levels of serum biotin can both result in very low TSH levels and 
artifactually raise the free T4 measured in robotic assays based on 
biotin/streptavidin chemistry (see Chapter 4).206
The

tin/streptavidin chemistry (see Chapter 4).206
The reference range of the serum TSH concentration by immu­
nometric assay is 0.4 to 4.2 mU/L. The lower limit of 0.4 is too 
high for pregnancy due to hCG-induced hyperthyroidism as dis­
cussed earlier.162 It should be kept in mind that there is a diurnal 
variation of TSH secretion with peak values in the late evening 
and a nadir in the afternoon. A borderline abnormal value should 
always be repeated within a week or so to be certain that it is 

ated within a week or so to be certain that it is 
representative. A minimally suitable TSH assay should be able to 
quantitate concentrations of TSH of 0.1 mU/L with a coefficient 
of variation of less than 20%. Potential artifacts of these assays are 
discussed in Chapter 4.
The free α-subunit common to TSH, FSH, LH, and hCG is 
generally detectable in serum with a reference range of 1 to 5 μg/L, 
but the TSH β-subunit is not. When FSH and LH production are 
increased, as in postmenopausal wom

roduction are 
increased, as in postmenopausal women, or when TSH produc­
tion is increased, as in primary hypothyroidism, the free α-subunit 
level is also increased. The α-subunit level may also be increased 
in patients with glycoprotein-producing tumors of the anterior 
pituitary (see Chapter 9). Its measurement may be useful in the 
rare patient with hyperthyroidism and a normal or elevated TSH 
to differentiate between neoplastic and nonneoplastic causes of 
TSH excess.207,208 
TSH in Pati

plastic causes of 
TSH in Patients with Thyroid Dysfunction
Patients with primary hyperthyroidism (excess thyroid hormone 
secretion) or thyrotoxicosis (excess thyroid hormone from any 
cause) will virtually always have a subnormal TSH. The values fall 
into two general categories: (1) those between the lower limit of 
normal and 0.1 mU/L and (2) those less than 0.1 mU/L. Indi­
viduals in the former category may be asymptomatic (subclinical 
hyperthyroidism), whereas those in

c (subclinical 
hyperthyroidism), whereas those in the latter group usually have 
symptomatic thyrotoxicosis and a significant elevation in free T4. 
Patients with hypothalamic or pituitary hypothyroidism often 
have normal or possibly even slightly elevated serum TSH. The 
circulating TSH generally has reduced biologic activity due to 
abnormal glycosylation, reflecting the impaired access of TRH to 
the thyrotroph.142,143 Patients with primary hypothyroidism have 
serum TSH concentrations that

hypothyroidism have 
serum TSH concentrations that range from minimally elevated to 
1000 mU/L. In general, the degree of TSH elevation correlates 
with the clinical severity of the hypothyroidism. Patients with 
serum TSH values in the range of 5 to 15 mU/L have few if any 
symptoms, and the serum free T4 or free T4 index (FT4I) is typi­
cally low-normal, whereas the serum free T3 concentration is often 
low-normal. Such individuals with modest TSH elevation are said 
to have subclinical hypoth

TSH elevation are said 
to have subclinical hypothyroidism if the serum free T4 is in the nor­
mal range. These findings may indicate early thyroidal failure with 
a compensatory increase in TSH secretion. A detailed discussion 
of the various conditions associated with abnormal serum TSH 
concentrations follows that describing the quantitation of serum 
thyroid hormones (see upcoming discussion).
An elevation in both serum TSH and free T4 is unusual and 
indicates either autonomous TSH producti

sual and 
indicates either autonomous TSH production (due to a TSH-
secreting pituitary tumor, resistance to thyroid hormone) or 
hyperthyroidism with an artifactual elevation in TSH. Differen­
tiating between these diagnoses may require magnetic resonance 
imaging (MRI) of the hypothalamic-pituitary region, markers of 
thyroid hormone action (e.g., ferritin or sex hormone–binding 
globulin), or consultation with the clinical chemistry laboratory 
to rule out an assay artifact (see Chapter 4). 

Quantitation of Serum Thyroid Hormone 
Concentrations
Total T4 and T3
Quantitation of the circulating thyroid hormone concentrations 
is essential to confirm that the thyroid status abnormality sug­
gested by an abnormal TSH result is accurate and to document its 
severity. Sensitive and specific radioimmunoassays are available for 
measuring the total concentrations of T4 and T3 and some of their 
metabolic byproducts (see Chapter 4). Because th

metabolic byproducts (see Chapter 4). Because the thyroid status 
correlates with the free, rather than with the total, hormone con­
centration, the physician usually must also obtain an estimate of 
that (see following discussion). The degree of abnormality in the 
free T4 generally correlates with the severity of the hormone excess 
or deficiency, whereas the serum TSH concentration is an indica­
tion of the impact of this abnormality in that specific patient. The 
reference range for total 

 specific patient. The 
reference range for total T4 in healthy, euthyroid adults with a 
normal circulating TBG concentration is 64 to 142 nmol/L (5–11 
μg/dL). Normal serum T3 concentrations are 1.1 to 2.9 nmol/L 
(70–190 ng/dL). At birth (cord serum), T3 concentrations are 
about 50% of those in normal adults, but within a few hours T3 
rises abruptly, peaking at about 24 hours at concentrations in the 
low thyrotoxic range for adults.
Radioimmunoassays for reverse T3 (rT3), T3SO4, triac, tet

munoassays for reverse T3 (rT3), T3SO4, triac, tetrac, 
and the diiodothyronines are of primary interest in the research 
setting because these iodothyronines are derived from the circulat­
ing T4 or T3, both of which can be easily quantitated. 
Concentrations of Free T4 and Free T3
The most accurate and direct measurements of the concentrations 
of free T4 and free T3 in serum are performed by assay of these 
hormones in a dialysate or ultrafiltrate of serum. Such assays are 
not practical for 

rate of serum. Such assays are 
not practical for clinical purposes, so alternative strategies have 
been developed to estimate free thyroid hormone concentrations. 
The absolute concentration of free hormone is the product of the 
total hormone concentration and the fraction that is dialyzable 
or ultrafiltrable. About 0.02% of T4 and 0.3% of T3 is free or 
357
unbound (see Table 11.5). The reference range for free T4 is 9 to 
30 pm

5). The reference range for free T4 is 9 to 
30 pmol/L (0.7–2.5 ng/dL), and for free T3 the range is 3.5 to 
6.5 pmol/L (0.22–0.43 ng/dL).
So-called direct measurements of free T4 are now available 
by robotic assays in virtually all laboratories. They have largely 
replaced the free T4 index discussed in more detail in past editions 
of this textbook. These automated tests imply that they quantitate 
free T4 directly, but they do not, and rarely results in sera with 
abnormal binding proteins o

 results in sera with 
abnormal binding proteins or in very sick patients may not 
be accurate.209 In most situations, especially in outpatients, this 
automated free T4 estimate is all that is required to ascertain the 
state of thyroid secretion or supply.
These assays are subject to artifacts from endogenous anti­
bodies to T4, abnormal binding proteins, or severe illness.208,210 
Thus the clinician must be wary if the free hormone estimate 
by any method does not agree with the clinical stat

y any method does not agree with the clinical state and the 
TSH. In such situations, the patient should be questioned as to 
potential excessive biotin (vitamin B7) ingestion, and another 
method should be used to estimate the free T4. A total T4 
and thyroid hormone–binding ratio (THBR) should be mea­
sured and the FT4 I calculated or the result should be ignored. 
For pregnant or severely ill patients, the automated methods 
typically give falsely low results. A reasonable alternative for 
pr

sely low results. A reasonable alternative for 
pregnancy is to use the normal range for the serum total T4 
concentration multiplied by 1.5 in lieu of an automated free 
T4 assay.211,212 
The Free T4 Index
Although rarely necessary with the widespread availability of 
“direct” free T4 assays, particularly useful in estimating the free T4 
in severely ill patients is the determination of the THBR, multi­
plying this result by the total T4 (or T3) to obtain a free hormone 
index (FT4I or FT3I). I

 to obtain a free hormone 
index (FT4I or FT3I). In this test, a tracer quantity of labeled T4 
(or T3) is added to serum, which is then exposed to a solid phase 
matrix coated with T4 or T3 antibody or to an inert matrix that 
binds the iodothyronine irreversibly. The proportion of labeled T4 
or T3 bound by the solid phase is then quantitated. This value 
varies inversely with the concentration of unoccupied TBG sites 
in the serum.
The results of such assays are normalized by comparing them 

 of such assays are normalized by comparing them 
with those obtained simultaneously for standard control sera with 
normal TBG and T4 concentrations. This step can be performed 
by dividing the result for the unknown sample by that obtained 
for control sera in the same assay; in this case, the quotient is called 
the THBR (thyroid hormone–binding ratio), which typically has 
a normal range of 0.85 to 1.10. Because the THBR is proportional 
to the free fraction of the endogenous thyroid hormone

he free fraction of the endogenous thyroid hormones in the 
serum, it can be multiplied by the total T4 (or T3) concentration 
to estimate the free thyroid hormone concentration (i.e., the free 
T4 or T3 index [FT4I or FT3I]). Because the midnormal THBR 
is 1.0, the FT4I has a normal range in units that are identical to 
that of the total T4 (or T3)—for example, 64 to 142 nmol/L (SI 
units) and 5 to 11 μg/dL (gravimetric terms). A schematic demon­
stration of the relationships between total and 

­
stration of the relationships between total and free T4, occupied 
and unoccupied TBG binding sites, and the THBR is shown in 
Fig. 11.13 for euthyroid individuals with variations in TBG con­
centrations and in Fig. 11.14 for subjects with a constant TBG 
and alterations in serum thyroid hormone production rates.
Estrogen, pregnancy, and severe illness are more common 
causes of changes in total T4 concentrations than are hyper­
thyroidism and hypothyroidism (Table 11.11). In the euthyroid 
pe

hypothyroidism (Table 11.11). In the euthyroid 
person, only about one-third of the available binding sites on 
TBG are occupied by T4, and the free T4 fraction is 2 × 10−4 of the 
total. During pregnancy, the TBG-binding capacity, the serum T4, 
and the number of unoccupied TBG-binding sites approximately 
double, leading to an approximately 50% reduction of the free 
T4 fraction. If the reduced THBR (or free fraction) is multiplied 
by an increased total T4, the FT4I estimate is normal, an acc

ased total T4, the FT4I estimate is normal, an accurate 
reflection of the free T4 concentration. In patients in whom the 
serum T4 concentration is reduced owing to a low TBG, the 
concentration of unoccupied binding sites is reduced to an even 
greater extent. This reduction leads to an increase in the free T4 
(and T3) fractions and the THBR, and both the free T4 and the 
FT4I remain in the normal range. These concepts and expected 
results in patients with abnormalities in total T4 or TBG ar

 patients with abnormalities in total T4 or TBG are 
shown in Figs. 11.13 and 11.14. When concentrations of TBG are 
altered, the deviation of the total T4 measurements from normal is 
in the opposite direction to that of the THBR (see central panels of 
Fig. 11.13). On the other hand, when the T4 level is elevated due 
to increased T4 secretion or overreplacement, the concentration of 
Serum TBG-
binding capacity
Serum hormone
concentration
Unoccupied TBG-
binding sites
Fraction of
hormone free

cupied TBG-
THBR
Normal
Increased TBG
Decreased TBG
Free hormone
520
260
0
nmol T4/L
260
195
130
65
0
nmol T4/L
520
260
0
nmol T4/L
0.04
0.02
0
Fraction free (×102)
2
1
0
Normal = 1.0
39
26
13
0
pmol T4/L
• Fig. 11.13  Pattern of changes in total serum thyroxine (T4) concentrations 
and the thyroid hormone–binding ratio (THBR) in euthyroid patients with alter­
ations in the circulating concentrations of thyroxine-binding globulin (TBG). To 
co

ations of thyroxine-binding globulin (TBG). To 
convert T4 from nmol/L to μg/dL (total) or pmol/L (free), divide by 12.87.
Euthyroid
Hypothyroid
Serum TBG-
THBR
520
260
0
nmol T4/L
260
195
130
65
0
nmol T4/L
520
260
0
nmol T4/L
0.04
0.02
0
2
1
0
104
26
13
0
pmol T4/L
• Fig. 11.14  Pattern of changes in total serum thyroxine 

1.14  Pattern of changes in total serum thyroxine (T4) concentration 
and thyroid hormone–binding ratio (THBR) in patients with hyperthyroid­
ism or hypothyroidism with normal serum thyroxine-binding globulin (TBG) 
concentration.
358
unoccupied TBG binding sites is reduced, and both the free fraction 
and the total T4 are altered in the same direction (see Fig. 11.14).
Several caveats should be kept in mind in the interpretation 
of these results. The use of labeled T3 in

tation 
of these results. The use of labeled T3 in some assays can produce 
difficulties in three situations: (1) in cases of familial dysalbumin­
emic hyperthyroxinemia (FDH), (2) in the presence of endogenous 
antibodies directed against T3, and (3) in sick patients, as already 
discussed. In FDH, the abnormal albumin binds T4, but not T3, 
with increased avidity. Therefore these patients have an elevated 
total T4 and reduced free fraction of T4 (when measured by the 
FT4I) but not T3.213 It 

T4 (when measured by the 
FT4I) but not T3.213 It is also possible to measure TBG either by 
saturation analysis or radioimmunoassay. Normal concentrations 
of TBG by radioimmunoassay are about 270 nmol/L (1–1.5 mg/dL) 
and are only slightly higher in women than in men. 
Causes of Abnormal TSH or Thyroid Hormone 
Several causes of an abnormal TSH should be considered by 
the clinician (Table 11.12). The clinical status and free T4 results 
(and sometimes T3 levels) allow evaluatio

results 
(and sometimes T3 levels) allow evaluation of the cause for 
abnormal TSH levels.
Causes of a Suppressed TSH
The most common cause of a reduction in serum TSH is an excess 
supply of thyroid hormone due to either increased endogenous 
thyroid hormone production or excessive exogenous thyroid 
hormone. Because the concentration of TSH is inversely propor­
tional to the degree of thyroid hormone excess, patients with clini­
cal symptoms almost invariably have serum TSH concentrations 
bel

most invariably have serum TSH concentrations 
below 0.1 mU/L. Such patients nearly always have an increase 
in the free T4. When thyroid hormone supply is only slightly in 
excess of the requirement for that patient, serum TSH is sup­
pressed, but clinical manifestations are subtle or absent, and the 
free T4 is in the high-normal range. Such minimal changes can 
sometimes occur with mild Graves disease, autonomous thyroid 
hormone–producing adenomas, multinodular goiters, subacute or 
painless

nomas, multinodular goiters, subacute or 
painless thyroiditis, and the ingestion of an amount of exogenous 
thyroid hormone slightly higher than that required for metabolic 
needs. This condition is termed subclinical hyperthyroidism.
The hypothalamic-pituitary axis may remain suppressed for 
several months after complete resolution of the thyrotoxic state.214 
The best test for assessing the physiologic state in such patients 
is the free T4 (or FT4I). A common scenario for this pattern is 
du

r FT4I). A common scenario for this pattern is 
during follow-up of patients receiving antithyroid drugs or 131I 
for Graves disease. With time, the TSH feedback regulatory loop 
will normalize, and TSH secretion will return and become appro­
priate for the circulating free thyroid hormone concentration. As 
mentioned, patients with few or no symptoms of thyrotoxicosis 
and subnormal TSH and high free T4 values should be questioned 
regarding biotin ingestion.
In severe illnesses, with or withou

tin ingestion.
In severe illnesses, with or without dopamine infusion or excess 
glucocorticoid, TSH is suppressed, making assessment of thyroid 
functional status difficult (see earlier discussion). Because the free 
T4 may also be reduced in such patients, astute clinical judgment 
is required to assign the thyroid status.
Because hCG can activate the TSHR, conditions in which 
hCG is particularly high, such as in the first trimester of preg­
nancy, with twin pregnancies, and in patients with 

ancy, with twin pregnancies, and in patients with hydatidiform 
mole or choriocarcinoma, the TSH concentration is often sup­
pressed.162 Interestingly, there is a correlation with hCG levels and 
the occurrence of hyperemesis gravidarum. TSH returns to normal 
in the second and third trimesters in the euthyroid patient. A per­
sistently suppressed TSH (<0.1 mU/L) in the pregnant patient 
after the first trimester suggests that the hyperthyroidism is due to 
autonomous thyroid function.
Changes i

 is due to 
Changes in thyroid test results in patients with psychosis or 
depression, in the geriatric population, and with the use of long-
term glucocorticoids have been discussed earlier. If the serum 
TSH is suppressed and the serum free T4 is low, one should be 
suspicious that liothyronine (triiodothyronine) is being ingested 
before assuming pituitary or hypothalamic dysfunction. Desic­
cated thyroid also has a high T3:T4 ratio and if given in excess 
may cau

a high T3:T4 ratio and if given in excess 
may cause a similar abnormality.215 
Causes of an Elevated TSH
Elevations in TSH nearly always imply a reduction in the supply 
of T4 or T3, which may be permanent or transient. Primary 
hypothyroidism is far and away the usual explanation. Other 
causes include acutely ill patients, such as those with renal 
insufficiency,216 or the asynchronous return of the hypotha­
lamic-pituitary-thyroid axis to normal as critically ill patients 
recover.217 Iodine

mal as critically ill patients 
recover.217 Iodine deficiency is the most common cause of an 
elevation in TSH worldwide, but this does not occur in North 
America. The rare patient with RTH due to a mutation in the 
THRB gene (RTHβ) may have a complex phenotype with signs 
of hypothyroidism in some tissues and hyperthyroidism in 
others. The most common laboratory pattern is a serum TSH 
level that is normal or mildly elevated in absolute terms but 
inappropriately high for the elevated free T4

but 
inappropriately high for the elevated free T4.126 They must be 
differentiated from the patient with a thyrotroph tumor in 
whom the persistent secretion of TSH causes hyperthyroid­
ism (see Chapters 9 and 12).207 Patients with mutations in the 
THRA gene (RTHα) have a normal TSH, free T3, but a low 
free and total T4.127
Patients with hypothalamic-pituitary dysfunction may have 
clinical and chemical hypothyroidism but low, normal, or even 
elevated serum TSH concentrations. The explanatio

elevated serum TSH concentrations. The explanation for this 
  Circumstances Associated with Altered 
Binding of Thyroxine by Thyroxine-
Binding Globulin
Increased Binding
Decreased Binding
Pregnancy
Androgens
Neonatal state
Large doses of glucocorticoids
Estrogens and hyperestrogenemic 
states
Nephrotic syndrome
Tamoxifen
Major systemic illness
Genetic factors
Acute intermittent porphyria
Asparaginase
Infectious and chronic active hepatitis
Biliary cirrho

ctious and chronic active hepatitis
Biliary cirrhosis
Human immunodeficiency virus 
infection
TABLE 11.11
359
  Thyroid Status and Free Thyroid Hormone Levels in Clinical States Associated With Abnormal Serum 
Thyrotropin (TSH) Concentrationsa
Expected TSH (mU/L)
Clinical Thyroid Status
Free T4
Free T3
Thyrotropin Reduceda
Hyperthyroidism of any cause
<0.1
↑
↑
↑
Euthyroid Graves disease
0.2–0.5
N, (↑)
N

1
↑
↑
↑
0.2–0.5
N, (↑)
N
N, (↑)
Autonomous nodule or multinodular goiter
0.2–0.5
N, (↑)
N
↑
Exogenous thyroid hormone excess
<0.1–0.5
N, ↑
N, ↑
↑
Thyroiditis (subacute or painless)
<0.1–0.5
N, ↑
N, ↑
↑, (N)
Recent thyrotoxicosis due to any cause
<0.1–0.5
↑, N, ↓
N, ↓
N, ↓
Illness with or without dopamine infusion
<0.1–5.0
N
↑, N, ↓
↓
First trimester of pregnancy
0.2–0.5
N, (↑)
N, (↑)
↑
Hyperemesis gravidarum
0.2–0.5
N, (↑)
↑, (N)
↑
Hydatidiform mole
0.1–0.4
↑
↑
↑
Acute p

)
↑, (N)
↑
0.1–0.4
↑
↑
↑
Acute psychosis or depression (rare)
0.4–10
N
N, (↑)
N, (↓ or↑)
Elderly (small fraction)
0.2–0.5
N
N
N
Glucocorticoids (acute, high dose)
0.1–0.5
N
N
↓
Biotin
↓
N
N
N
Congenital TSH deficiency
a.  Combined pituitary hormone deficiency 
(POU1F1/PIT1, PROP1, LHX3, HESX1)
0 - ↓
↓
↓
↓
b.  TSH beta gene mutations
0 - ↓
↓
↓
↓
Thyrotropin Elevated
Primary hypothyroidism
6–500
↓
↓
N, ↓
Recovery from severe illness
5–30
N, (?)
N, ↓
N, ↓
Iodine deficiency
6–150
N

ss
5–30
N, (?)
N, ↓
N, ↓
6–150
N, ↓
↓
N
Thyroid hormone resistance
1–20
↑, N, ↓
↑
↑
Thyrotroph tumor
0.5–50
↑
↑
↑
Hypothalamic-pituitary disease
1–20
↓
↓
N, ↓
Psychiatric illnesses
0.4–10
N
N
N, ↓
5–30
N
N
N, ↓
Artifact (endogenous antimouse γ-globulin antibodies)
10–500
N
N
N
aArrows indicate the nature of the abnormality in free T4 or T3. Parentheses indicate that such a result is unusual but may occur. Excess serum biotin may suppress TSH and elevate fr

xcess serum biotin may suppress TSH and elevate free T4.
N, No change; T3, triiodothyronine; T4, thyroxine.
   
TABLE 11.12
paradox is that the biologic effectiveness of the circulating TSH 
is impaired as a result of abnormal glycosylation secondary 
to reduced TRH stimulation of the thyrotrophs. Nonetheless, 
the abnormal TSH is a suitable antigen in the immunometric 
assay. In adrenal insufficiency, TSH may be modestly elevated 
but returns to normal with glucocorticoid replacement.201 
This 

 normal with glucocorticoid replacement.201 
This may reflect glucocorticoid-mediated amelioration of Hashi­
moto thyroiditis.
Despite the utility and general efficacy of the serum TSH 
measurement alone as a screening tool for identifying patients 
with thyroid dysfunction, a patient should not receive treat­
ment solely on the basis of an abnormal TSH. The TSH assay 
is an indirect reflection of thyroid hormone supply and does 
not, by itself, permit a conclusive diagnosis of a specific dis­
o

permit a conclusive diagnosis of a specific dis­
order of thyroid hormone production. Accordingly, the TSH 
abnormality must be confirmed and the expected alteration in 
thyroid hormone concentrations documented before initiating 
treatment. 
Tests That Assess the Metabolic Impact of 
Abnormalities in the supply of thyroid hormone to the peripheral 
tissues are associated with alterations in a number of metabolic 
360
processes that can be quantitated. Som

    Thyroid
processes that can be quantitated. Some of these may be useful in 
the rare patient in whom serum TSH is not an accurate barometer 
of thyroid status, such as those with RTHβ. These tests may be 
the sole means of evaluating the metabolic response of the periph­
eral tissues to thyroid hormones in such patients but are rarely 
required otherwise.
Basal Metabolic Rate
Thyroid hormones increase energy expenditure and heat produc­
tion, as manifested by weight loss, increased caloric re

as manifested by weight loss, increased caloric requirement, 
and heat intolerance. Because it is impractical to measure heat 
production directly, the basal metabolic rate (BMR) measures 
oxygen consumption under specified conditions of fasting, rest, 
and tranquil surroundings. Under these conditions, the energy 
equivalent of 1 L of oxygen is 4.83 kcal.
Under basal conditions, approximately 25% of oxygen con­
sumption is due to energy expenditure in visceral organs, includ­
ing the liver, kid

ure in visceral organs, includ­
ing the liver, kidneys, and heart; 10% occurs in the brain, 10% in 
respiratory activity, and the remainder in skeletal muscle. Because 
energy expenditure is related to functioning tissue mass, oxygen 
consumption is related to some index thereof, most often body 
surface area. Calculated in this way, basal oxygen consumption 
(resting energy expenditure) is higher in men than in women and 
declines rapidly from infancy to the third decade and more slowly 
therea

nfancy to the third decade and more slowly 
thereafter. Values in patients, calculated as a percentage of estab­
lished normal means for sex and age, normally range from −15% 
to +5%. In severely hypothyroid patients, values may be as low as 
−40%, and in thyrotoxic patients, these values may reach +25% to 
+50%. Abnormal, usually elevated, values are seen during recov­
ery in burn patients and in those with systemic disorders, such as 
febrile illnesses, pheochromocytoma, myeloproliferative dis

llnesses, pheochromocytoma, myeloproliferative disorders, 
anxiety, and disorders associated with involuntary muscular activ­
ity. Resting energy expenditure correlates very well with the 
free T4 and TSH in hypothyroid patients given varying doses of 
exogenous levothyroxine.218 
Biochemical Markers of Altered Thyroid Status
Occasionally a diagnosis of thyroid dysfunction is first suspected 
due to an abnormality in a laboratory result performed during an 
evaluation for an unrelated medical pr

 during an 
evaluation for an unrelated medical problem. Classic examples are 
a markedly elevated creatine kinase MM isoenzyme or low-density 
lipoprotein (LDL) cholesterol reading, leading to the recognition 
of hypothyroidism.219 Other similar markers are listed in Table 
11.13. These tests are not useful in the diagnosis of thyroid dis­
ease, but some, such as sex hormone–binding globulin (SHBG), 
ferritin, or LDL cholesterol, have been used as end points in clini­
cal studies of the respons

as end points in clini­
cal studies of the responsivity of the liver to thyroid hormone in 
patients with thyroid hormone resistance. 
Serum Thyroglobulin
The functional sensitivity of most Tg assays is 1 ng/mL or less.220 
The results can be artifactually altered by serum anti-Tg antibodies, 
and serum should be screened for Tg antibodies with a sensitive Tg-
antibody immunoassay. In immunoradiometric assays, interferences 
lead to underestimations of Tg or false-negative values, whereas val­
u

ons of Tg or false-negative values, whereas val­
ues when measured by radioimmunoassay may be falsely elevated.
Tg is normally present in the serum, the concentration rang­
ing up to 50 ng/mL; mean normal values vary with the assay 
used but are on the order of 20 ng/mL.221 Concentrations are 
somewhat higher in women than in men and are elevated several 
fold in pregnant women and in the newborn. Levels are elevated 
in three types of thyroid disorders: goiter and thyroid hyperfunc­
tion, infla

sorders: goiter and thyroid hyperfunc­
tion, inflammatory or physical injury to the thyroid, and dif­
ferentiated follicular cell–derived thyroid tumors and especially 
in consumptive hypothyroidism.104 Values are elevated in both 
endemic and sporadic nontoxic goiter, and the degree of eleva­
tion correlates with the thyroid size. Transient elevations occur 
in patients with subacute thyroiditis and as a result of trauma to 
the gland during thyroid surgery or after 131I therapy. Subnormal 
or 

roid surgery or after 131I therapy. Subnormal 
or undetectable concentrations are found in patients with thy­
rotoxicosis factitia and aid in differentiating this disorder from 
other causes of thyrotoxicosis with a low thyroid radioiodine 
uptake (RAIU).
A major clinical value of measuring the level of serum Tg is 
in the management, but not in the diagnosis, of differentiated thy­
roid carcinoma.220,222 Serum Tg concentrations are increased 
in patients with both benign and differentiated mali

 patients with both benign and differentiated malignant fol­
licular cell–derived tumors of the thyroid and do not serve 
to distinguish between the two. After total thyroid ablation 
for papillary or follicular thyroid carcinoma, Tg should not 
be detectable, and its subsequent appearance typically signifies 
the presence of persistent or recurrent disease.222 The serum 
Tg level is related to the mass of neoplastic tissue and may be 
undetectable in patients with small lymph node micrometasta­

le in patients with small lymph node micrometasta­
ses (see Chapter 14). Secretion of Tg is TSH dependent. There­
fore the serum Tg level may rise when suppressive therapy is 
withdrawn or after injections of recombinant human TSH 
(rhTSH), which will increase the sensitivity of the marker for 
the detection of persistent or recurrent thyroid carcinoma, even 
when 131I scans are negative (see Chapter 14). Supersensitive assays 
of Tg with a functional sensitivity less than 0.1 ng/mL improve 
the

ional sensitivity less than 0.1 ng/mL improve 
the sensitivity during thyroid hormone treatment, but at the 
expense of a decreased specificity.223,224 This measurement is 
useful for the follow-up of patients treated with total thyroid­
ectomy with or without radioactive iodine ablation.225 Unfortu­
nately there is a major potential artifact in some patients due to 
the presence of autoantibodies binding to Tg. In this situation, 
  Biochemical Markers of Thyroid Status
Thyrotoxicosis
Increase

 Markers of Thyroid Status
Increased
Osteocalcin
Urine pyridinium collagen cross-links
Alkaline phosphatase (bone or liver)
Atrial natriuretic hormone
Ferritin 
von Willebrand factor
Decreased
Low-density lipoprotein cholesterol
Lipoprotein(a)
Increased
Creatine kinase (MM isoform)
Plasma norepinephrine
Decreased
TABLE 11.13
361
Chapter 11  Thyroid Pathophysiology and  Diagno

61
measurement of Tg by radioimmunoassay or liquid chroma­
tography-tandem mass spectrometry (LC-MS/MS) may be more 
informative.226
In the hypothyroid newborn, serum Tg is undetectable in 
patients with thyroid agenesis and is usually elevated in those with 
ectopic thyroid tissue or goiter and in consumptive hypothyroid­
ism due to infantile hemangioma. 
Tests for Thyroid Autoantibodies
Graves disease and Hashimoto thyroiditis are 

dies
Graves disease and Hashimoto thyroiditis are well characterized and 
interrelated autoimmune thyroid disorders with a variety of clinical 
manifestations. The diagnostic hallmark of the autoimmune thyroid 
disorders is the presence, in most patients, of circulating autoanti­
bodies and reactive T cells against one or more thyroid antigens.227 
Three varieties of thyroid autoantibodies are in common use and 
widely available in clinical diagnostic laboratories (Table 11.14). 
In this section

ostic laboratories (Table 11.14). 
In this section, autoantibodies to Tg and TPO are discussed. Anti­
bodies directed against the TSHR, the cause of hyperthyroidism in 
patients with Graves disease, are detailed in Chapter 12.
Autoantibodies to Thyroid Peroxidase and Thyroglobulin
Current automated assay techniques for thyroid autoantibodies 
have good precision because they depend on direct measurement 
of the interaction between autoantibody and autoantigen (i.e., the 
interaction between thyr

d autoantigen (i.e., the 
interaction between thyroid antigen and the patient’s serum). In 
general, the more sensitive an assay, the more precise and antigen 
specific it is. However, many euthyroid individuals exhibit low levels 
of autoantibodies, and in this situation, the absolute concentration 
becomes important. The higher the concentration of autoantibody, 
the greater the clinical specificity (see Table 11.14).228
So that comparisons of thyroid antibody concentrations can be 
made from 

thyroid antibody concentrations can be 
made from one office visit to the next, among different patients 
and among laboratories, assays for thyroid autoantibodies have 
been standardized with results expressed as standard units per mil­
liliter. Of course, the actual standard serum preparation cannot be 
included in every assay. Instead, a serum pool is usually compared 
and normalized to the original standard. Autoantibodies differ 
considerably in their affinity and epitope recognition of ant

y in their affinity and epitope recognition of antigen, 
and so the slopes of different standard curves may vary. As a result, 
despite this attempt at standardization, assay results from differ­
ent commercial assays may still vary considerably. Hence when 
monitoring antibody titers (e.g., measuring Tg-Ab after the treat­
ment of thyroid cancer), it is always best to consistently use the 
same autoantibody assay. 
Do Thyroglobulin and Thyroid Peroxidase Antibodies 
Have a Pathogenic Role?
Tg-A

eroxidase Antibodies 
Tg-Ab and TPO autoantibodies (TPO-Ab) appear to be a second­
ary response to thyroid injury and are not thought to cause disease 
themselves, although they may contribute to its development and 
chronicity. Both types of antibodies are polyclonal, and although 
they are of the immunoglobulin G class, they are not restricted to 
a particular immunoglobulin G subclass. These thyroid antibodies 
cannot transfer disease from mother to fetus or between an

ransfer disease from mother to fetus or between animals 
even though they can pass across the placenta.229 However, both 
antibodies may contribute to disease mechanisms. For example, 
TPO-Ab on the surface of B cells may be involved in antigen pre­
sentation, thus activating thyroid-specific T cells.230 Such autoan­
tibodies may have complement-fixing cytotoxic activity; TPO-Ab, 
in particular, correlates well with thyroid damage and lymphocytic 
infiltration. 
Thyroid Autoantibodies in Hashimo

Thyroid Autoantibodies in Hashimoto Thyroiditis 
and Graves Disease
The disease most widely associated with Tg-Ab and TPO-Ab 
is autoimmune thyroiditis, or Hashimoto disease (terms that 
embrace both goitrous thyroiditis, as first described by Hashi­
moto, and atrophic thyroid failure, previously referred to as 
primary myxedema). Both Tg-Ab and TPO-Ab are found 
in almost 100% of such patients, but TPO-Ab has a higher 
­affinity and occurs in higher concentrations, so testing f

 and occurs in higher concentrations, so testing for 
TPO-Ab is more helpful.
Tg-Ab and TPO-Ab are also detectable in 50% to 90% of 
patients with Graves disease, indicative of the associated thy­
roiditis that is evident histologically as a heterogeneous lym­
phocytic infiltration. Hence Graves disease tends to develop on 
a background of autoimmune thyroiditis. Although the presence 
of such autoantibodies favors an autoimmune cause for hyper­
thyroidism over other causes, the tests are neithe

thyroidism over other causes, the tests are neither sensitive nor 
specific in this setting and are interpretable only as part of the 
clinical scenario. Testing for TSHR antibodies is important in the 
evaluation of hyperthyroid patients, along with consideration of a 
radioiodine uptake and scan. 
Thyroid Autoantibodies in Nonautoimmune 
Thyroid Disorders
Tg-Ab and TPO-Ab are more common in patients with sporadic 
goiter, multinodular goiter, or isolated thyroid nodules and cancer 
than in the

r isolated thyroid nodules and cancer 
than in the general population. This finding usually represents 
an associated thyroiditis on histologic examination. Low levels of 
thyroid autoantibodies may occur transiently in patients with sub­
acute (de Quervain) thyroiditis but correlate poorly with disease 
course and are probably a nonspecific response to thyroid injury. 
There is also a higher prevalence of thyroid autoantibodies in 
many other autoimmune diseases, particularly insulin-dependent 

toimmune diseases, particularly insulin-dependent 
diabetes mellitus. 
Thyroid Autoantibodies in Pregnancy
Euthyroid women with thyroid autoantibodies have been shown 
to suffer from increased early pregnancy loss. The miscarriage rate in 
the presence of TPO-Ab is almost doubled in many studies,231–233 
and the cause is uncertain. It is unclear if this is a reflection of an 
immune diathesis or due to subtle alterations in thyroid function. 
  Prevalence of Thyroid Autoantibodies
Group
TSHR-Ab

Prevalence of Thyroid Autoantibodies
Group
TSHR-Ab 
(%)
hTg-Ab 
(%)
hTPO-Ab 
(%)
General population
0
5–20
8–27
Patients with Graves disease
80–95
50–70
50–80
Patients with autoimmune 
thyroiditis
10–20
80–90
90–100
Relatives of patients
0
40–50
40–50
Patients with IDDM
0
40
40
Pregnant women
0
14
14
IDDM, Insulin-dependent diabetes mellitus; hTg-Ab, human thyroglobulin antibody; hTPO-
Ab, human thyroid peroxidase antibody; TSHR-Ab, thyroid-stimulating hormone receptor 
antibody.
   
TABLE 11.14

lating hormone receptor 
antibody.
   
362
The screening of pregnant women has been advocated by some 
groups but is controversial.234 
The Normal Population
Although the prevalence of thyroid autoantibodies depends on 
the technique used for detection, Tg-Ab and TPO-Ab are com­
mon in the general population (see Table 11.14). At all ages, 
these antibodies are almost five times more common in women 
than in men. Selected groups at risk include younger women 
a

. Selected groups at risk include younger women 
and relatives of patients with an autoimmune thyroid disorder, 
in whom the incidence is higher. The low levels of TPO-Ab and 
Tg-Ab found in many individuals are of uncertain significance in 
the presence of normal thyroid function; however, they remain 
a significant risk factor in families with autoimmune thyroid 
disorders.235 
Radioiodine Uptake
A direct test of thyroid function uses a radioactive isotope of 
iodine as a marker for the body’s

tive isotope of 
iodine as a marker for the body’s stable form of iodine, 127I. Most 
often the test involves the measurement of the fractional uptake by 
the thyroid of a tracer (chemically inconsequential) dose of radio­
iodine. However, several factors make this test less frequently used 
than in the past. The first is the improvement in indirect methods 
for assessing thyroid status. The second is the decrease in normal 
values for thyroid RAIU consequent to the widespread increase in 
daily

IU consequent to the widespread increase in 
daily dietary iodine intake, reducing the utility of the test in the 
diagnosis of thyroid disorders. However, the test is still useful in 
defining the etiology of thyrotoxicosis. It should not be performed 
in pregnant and breastfeeding women.
131I (half-life 8.1 days) and 123I (half-life 0.55 day) both emit 
gamma radiation, which permits their external detection and 
quantitation at sites of accumulation, such as the thyroid. These 
isotopes (abbr

lation, such as the thyroid. These 
isotopes (abbreviated I* hereafter) are physiologically indistin­
guishable, not only from one another but also from the naturally 
occurring I, which permits their use as valid tracers. The shorter 
half-life of 123I is preferable because the radiation delivered to the 
thyroid per amount of administered 123I is only about 1% of that 
delivered by 131I. 131I also emits beta radiation, hence its utility 
in the therapy of patients with Graves disease, hyperfun

 therapy of patients with Graves disease, hyperfunctioning 
nodules, and well-differentiated thyroid cancer.
Physiologic Basis
When tracer quantities of inorganic radioiodine are adminis­
tered orally or intravenously, the isotope quickly mixes with the 
endogenous stable iodide in the extracellular fluid and begins to 
be removed by the two major sites of clearance, the thyroid and 
the kidneys. As this process continues, the plasma level of tracer 
iodide I* decreases exponentially. Low levels

cer 
iodide I* decreases exponentially. Low levels are reached by 24 
hours, and inorganic I* is virtually undetectable in the plasma 72 
hours after its administration. The thyroid content of I* increases 
rapidly during the early hours and then at a decreasing rate until 
a plateau is approached. The proportion of administered I* ulti­
mately accumulated by the thyroid is a function of the clearance 
of iodide by the thyroid and kidneys. The relationship is simply 
expressed by the following e

lationship is simply 
expressed by the following equation:
RAIU at plateau =
CT
CT + CK
where CT represents the thyroid iodide clearance rate and CK the 
renal iodide clearance rate. The normal thyroid iodide clearance 
rate is approximately 0.4 L/hour, and the renal iodide clearance 
rate is 2.0 L/hour, so the uptake of I* normally approximates 20% 
of the administered dose.
Measurements of RAIU are generally made at 24 hours, both 
as a matter of convenience and because the value at 24 hours i

of convenience and because the value at 24 hours is 
usually near the plateau, but RAIU can be measured at 6 hours 
with appropriate determination of a reference range. 
Radioactive Iodine Uptake
Little difference will be noted if the uptake is measured at any 
time during the day following that on which the isotope was 
administered, and for the calculation of therapeutic radioiodine 
doses in treating thyrotoxic Graves disease an early uptake at 3 to 
6 hours may produce results comparable to 

t 3 to 
6 hours may produce results comparable to those found at 20 to 
28 hours.236 With the use of this modified early RAIU measure­
ment, diagnosis and treatment of thyrotoxic Graves disease can 
be accomplished on the same day. In general, the reference range 
in North America is approximately 5% to 25%. Higher values 
are found in iodine-deficient regions or in patients with thyroid 
hyperfunction, but as with other procedures, patients with mild 
hyperthyroidism may display values at or ju

 mild 
hyperthyroidism may display values at or just above the upper 
limit of the reference range (Table 11.15). 
The Perchlorate Discharge Test
In normal individuals, more than 90% of thyroidal radioiodine 
is organified and present as iodotyrosine and iodothyronine 
within minutes of its entry into the thyroid. It is then no lon­
ger in the intracellular iodide pool. In patients with Pendred 
syndrome or with other disorders that inhibit the iodination 
of tyrosine, such as Hashimoto thyroidi

odination 
of tyrosine, such as Hashimoto thyroiditis, or those receiving 
thiourea drugs, this process is delayed, as shown by the exit (dis­
charge) of more than 10% of the thyroidal radioiodine within 
2 hours of administration of 500 mg of KClO4.12 Perchlorate 
inhibits NIS function by competing with iodide for NIS, elimi­
nating the iodide gradient that is required for maintaining the 
radioiodide in the gland. This illustrates that both iodide trans­
port by NIS at the basal pole of the th

ide trans­
port by NIS at the basal pole of the thyrocyte and its efflux 
across the apical membrane by pendrin are required for thyroid 
hormone synthesis. 
States Associated With Increased RAIU
Hyperthyroidism (sustained excess endogenous thyroid hormone 
production) as distinct from thyrotoxicosis, which refers to the 
manifestations of excess thyroid hormone levels due to any cause, 
is accompanied by an elevated RAIU at 24 hours unless body 
iodide stores are increased. The 

urs unless body 
iodide stores are increased. The increased uptake is always evident 
unless the release of labeled hormone is so rapid that the thyroidal 
content of I* has decreased to the normal range by the time the 
measurement is made. This condition is rare, is typically associated 
with severe thyrotoxicosis, and can be recognized by checking the 
thyroid uptake at 6 hours. 
Aberrant Hormone Synthesis
RAIU can be increased in the absence of hyperthyroidism 
in disorders in which iodine a

of hyperthyroidism 
in disorders in which iodine accumulation is normal but the 
secretion of hormone is impaired, such as in patients with 
abnormal Tg synthesis.237 The magnitude of the increase in 
uptake and the time at which the plateau is achieved vary with 
the nature and severity of the disorder. Differentiation of the 
foregoing states from hyperthyroidism is generally not difficult 
because in the former, clinical findings and laboratory evidence 
of hyperthyroidism are lacking, and in

y evidence 
of hyperthyroidism are lacking, and indeed hypothyroidism 
may be present. 
363
RAIU is increased in acute or chronic iodine deficiency, as dem­
onstrated by measurement of urinary iodine excretion, with uri­
nary iodine values lower than 100 μg/day, indicating deficiency. 
Chronic iodine deficiency is usually the result of an inadequate 
amount of iodine in food and water (endemic iodine deficiency). 
P

 in food and water (endemic iodine deficiency). 
Patients with cardiac, renal, or hepatic disease may develop 
iodine deficiency if given diets severely restricted in salt, espe­
cially if diuretic agents are administered. 
Response to Thyroid Hormone Depletion
Rebound increases in RAIU are seen after withdrawal of antithy­
roid therapy, after subsidence of transient or subacute thyroiditis, 
and after recovery from prolonged suppression of thyroid function 
by exogenous hormone. A striking incr

id function 
by exogenous hormone. A striking increase in uptake occurs in 
patients with iodide-induced myxedema after cessation of iodide 
administration. The duration of the rebound depends on the time 
required to replenish thyroid hormone stores. 
Excessive Hormone Losses
In nephrotic syndrome, excessive losses of hormone in the urine 
occurring in association with urinary loss of binding protein cause 
a compensatory increase in hormone synthesis and RAIU. A simi­
lar sequence may occur wh

thesis and RAIU. A simi­
lar sequence may occur when losses of hormone via the gastroin­
testinal tract are abnormal, as in chronic diarrheal states or during 
ingestion of agents such as soybean protein and cholestyramine, 
which bind T4 in the gut. 
States Associated With Decreased RAIU
A general increase in iodine intake has made values of the RAIU in 
hypothyroidism indistinguishable from those at the lower end of 
the reference range. Therefore the major indication for measuring 
the RAIU i

ore the major indication for measuring 
the RAIU is to establish whether thyrotoxicosis is due to hyper­
thyroidism (high RAIU) or thyroiditis (low RAIU).
Exogenous Thyroid Hormone: Thyrotoxicosis Factitia
Except in disorders in which homeostatic control is disrupted 
or overridden (e.g., Graves disease or autonomously functioning 
thyroid nodules), administration of exogenous thyroid hormone 
suppresses TSH secretion and reduces the RAIU, usually to values 
below 5%.
Low values of the RAIU in a

y to values 
below 5%.
Low values of the RAIU in a patient who is clinically thyro­
toxic may also indicate the presence of thyrotoxicosis factitia, 
the syndrome produced by the ingestion of excess thyroid hor­
mone. The unmeasurably low level of Tg in serum differenti­
ates thyrotoxicosis factitia from other causes of thyrotoxicosis 
with decreased RAIU.238 
Disorders of Hormone Storage
The RAIU is usually low in the early phase of subacute thyroiditis 
and in chronic thyroiditis with transien

roiditis 
and in chronic thyroiditis with transient hyperthyroidism. Here, 
inflammatory follicular disruption leads to loss of the normal stor­
age function of the gland and leakage of hormone into the blood. 
In the early stage of subacute thyroiditis, leakage of hormone is usu­
ally sufficient to suppress TSH secretion and the RAIU. Transient 
hypothyroidism often occurs late in both diseases, when stores of 
preformed hormone are depleted; the RAIU may return to normal 
or increased values a

e RAIU may return to normal 
or increased values at that time. 
Exposure to Excessive Iodine
Exposure to excessive iodine is a common cause of a subnormal 
RAIU. Such decreases are spurious in the clinical sense because 
they do not indicate decreased absolute iodine uptake or decreased 
hormone production but can be produced by the introduction 
of excessive iodine in any form: inorganic, organic, or elemental. 
Common offenders are organic iodinated dyes used as radiograph 
contrast media and 

nated dyes used as radiograph 
contrast media and amiodarone (see Table 11.7). The duration 
of suppression of the uptake varies among individuals and with 
the compound administered. In general, dyes used for pyelog­
raphy or computed tomography scanning are cleared within a 
month, whereas amiodarone may influence the uptake for up 
to 12 months because of its storage in fat. A single large dose of 
inorganic iodide can decrease uptake for several days, and chronic 
ingestion of iodide may dep

ral days, and chronic 
ingestion of iodide may depress the uptake for many weeks. Exces­
sive quantities of iodine may also be present in vitamin and min­
eral preparations, vaginal or rectal suppositories, and iodinated 
antiseptics such as povidone (see Table 11.7).
The measurement of urinary iodine excretion is an invalu­
able means of establishing or excluding the existence of excessive 
body iodide stores; the 24-hour iodine excretion can be roughly 
extrapolated from the iodide-to-creatini

 roughly 
extrapolated from the iodide-to-creatinine ratio in a random urine 
sample. Values in excess of 2 mg/day can account for a low RAIU 
value, whereas values less than 1 mg/day suggest that a low RAIU 
is due to one of the other disorders discussed in this section.
  Factors That Influence 24-Hour Thyroid 
Iodide Uptake
Factors That Increase Uptake
Increased Hormone Synthesis
Response to 

sed Hormone Synthesis
Response to glandular hormone depletion
Recovery from thyroid suppression
Recovery from subacute thyroiditis
Antithyroid agents
Excessive hormone losses
Chronic diarrheal states
Soybean ingestion
Normal Hormone Synthesis
Dietary insufficiency
Excessive loss (dehalogenase defect, pregnancy)
Hormone biosynthetic defects
Factors That Decrease Uptake
Decreased Hormone Synthesis
Primary hypofunction
Anti

s
Hashimoto disease
Subacute thyroiditis
Secondary hypofunction
Exogenous thyroid hormones
Not Reflecting Decreased Hormone Synthesis
Increased availability of iodine
Diet or drugs
Cardiac or renal insufficiency
Increased hormone release
Very severe hyperthyroidism (rare)
TABLE 11.15
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364
12
Hyperthyroid Disorders
ANTHONY HOLLENBERG AND WILMAR M. WIERSINGA
T
hese days the terms thyrotoxicosis and hyperthyroidism are 
used interchangeably and refer to the classic or subtle phys­
iologic manifestations of excessive quantities of the thyroid 
hormones, which are the characteristics of this condition (Table 
12.1). In addition to ove

f this condition (Table 
12.1). In addition to overstimulation of the thyroid via the TSH 
receptor and true TSH receptor mutations, other common condi­
tions causing hyperthyroidism include the passive release of thy­
roid hormones from damaged thyroid follicles; inflammation of 
the thyroid gland (called thyroiditis), which may be autoimmune, 
postviral, or drug induced; and extrathyroidal sources of thyroid 
hormone, most often iatrogenic or self-administered. For most 
patients with thyrotox

elf-administered. For most 
patients with thyrotoxicosis, the symptoms and signs caused by an 
excess of thyroid hormone, whatever the source, lead to medical 
attention. Others may have surprisingly few symptoms and are 
referred because of a suppressed TSH. This chapter begins with a 
brief review of the symptoms and signs of thyrotoxicosis and their 
Clinical Manifestations of Thyrotoxicosis, 365
Laboratory Diagnosis, 368
Graves Hyperthyroidism, 369
Graves Orbitopathy, 380
Gra

s Hyperthyroidism, 369
Graves Dermopathy, 387
Pregnancy and the Thyroid, 387
Inherited Nonimmune Hyperthyroidism, 392
Toxic Multinodular Goiter, 392
Toxic Adenoma, 394
Subclinical Hyperthyroidism, 395
Induced Hyperthyroidism, 396
Amiodarone-Induced Thyrotoxicosis, 396
Hyperthyroidism Due to Thyrotropin Secretion, 398
Tumor Chorionic Gonadotropin-Induced Hyperthyroidism, 398
Transient Thyrotoxicosis, 399
Other Causes of Thyrotoxicosis With a Low Radioiodine Uptake, 402
KEY

tis seen in the postpartum period.
 • 
The treatment of hyperthyroidism is best initiated with the 
antithyroid drug methimazole; propylthiouracil (PTU) is no 
longer recommended as first-line therapy because of its rare, 
but occasionally severe, hepatic toxicity. PTU may be useful in 
treating severe hyperthyroidism because of its capacity to block 
conversion of thyroxine (T4) to triiodothyronine (T3) by the type 
1 deiodinase (D1) in liver, kidney, and Graves thyroid.
 • 
Methimazole embry

kidney, and Graves thyroid.
 • 
Methimazole embryopathy is also rare. It may be avoided by the 
use of PTU in the first trimester as well as for women planning a 
pregnancy while under treatment for hyperthyroidism.
 • 
Overtreatment of the pregnant hyperthyroid patient must be 
avoided because of the transplacental passage of both PTU 
and methimazole. Fetal hypothyroidism can impair cognitive 
development. Typically the TSH should remain suppressed and 
the free T4 slightly above normal thro

ressed and 
the free T4 slightly above normal throughout pregnancy. If pos­
sible, the patient should be followed in concert with a high-risk 
obstetrician.
365
Chapter 12  Hyperthyroid Disorders
pathophysiologic basis. The appropriate use of the laboratory tests 
already described in Chapter 11 is then presented to show how 
these results can focus the search for a diagnosis.
Clinical Manifestations of Thyrotoxicosis
One very important clinical clue to the cause of the patient’s 
thyrotoxicosis

clue to the cause of the patient’s 
thyrotoxicosis is the duration of symptoms. Patients with hyper­
thyroidism have generally had manifestations for months before 
presentation, but because the week-to-week increases in thyroid 
hormones are small, the effects of the disorder may become rather 
extreme, while going unnoticed by the patient. In addition, 
patients will often attribute the symptoms to other causes; for 
example, they may ascribe their fatigue to family or work respon­
sibilities,

heir fatigue to family or work respon­
sibilities, heat intolerance to the weather, weight loss to an effec­
tive diet, and dyspnea and palpitations to a lack of regular exercise. 
On the other hand, patients with thyrotoxicosis due to thyroiditis 
can often date the onset of their symptoms precisely, usually to 
within a month or so of their seeking medical attention, as might 
be expected from the effects of the release of the equivalent of 30 
to 60 days’ supply of thyroid hormone into the ci

to 60 days’ supply of thyroid hormone into the circulation over a 
few days to weeks. Thus ascertaining the chronology as well as the 
spectrum of symptoms is a critical goal of the interview process.
Another general characteristic is that the symptoms and signs 
of thyrotoxicosis are more readily recognized in the younger than 
in the older patient. The term masked or apathetic thyrotoxicosis 
is used to describe the syndrome sometimes seen in the elderly, 
which may present as congestive hear

the elderly, 
which may present as congestive heart failure with arrhythmia 
or as unexplained weight loss without the increased appetite and 
other typical symptoms and signs of the younger patient.
At present, the ready availability of sensitive serum TSH assays, 
a reliable indicator of excess thyroid hormone in the ambulatory 
patient (see Chapter 11), has made the more classic and severe 
manifestations of long-standing thyrotoxicosis less prevalent. In 
fact, a continued area of some contr

revalent. In 
fact, a continued area of some controversy is how aggressively to 
treat the condition termed subclinical hyperthyroidism, a bio­
chemical diagnosis in which a subnormal serum TSH level is 
accompanied by normal free thyroid hormone concentrations. 
Nonetheless, the classic presentation is still common, serves to 
illustrate the pleiotropic physiologic effects of excess thyroid hor­
mones, and, if not recognized, can progress to life-threatening 
severity despite the fact that hype

e-threatening 
severity despite the fact that hyperthyroidism is a benign condi­
tion (accelerated hyperthyroidism). The next sections review the 
pathophysiology of the most important manifestations of excess 
thyroid hormone.
Cardiovascular System
Alterations in cardiovascular function in the thyrotoxic patient 
are in part due to increased circulatory demands that result from 
the hypermetabolism and the need to dissipate the excess heat 
produced.1 At rest, peripheral vascular resistance is 

uced.1 At rest, peripheral vascular resistance is decreased, and 
cardiac output is increased as a result of an increase first in heart 
rate and, with more severe disease, in stroke volume. Thyroid 
hormones in excess also have a direct ionotropic effect on cardiac 
contraction mediated by an increase in the ratio of α-myosin to 
β-myosin heavy chain expression. Tachycardia is virtually always 
present and is due to a combination of increased sympathetic and 
decreased vagal tone.2 Widening of 

mpathetic and 
decreased vagal tone.2 Widening of the pulse pressure is due to 
the increase in systolic pressure and decrease in diastolic pressure 
caused by reduced resistance.3 The decreased resistance is due to 
increased nitric oxide production via the PI3K/AKT signaling 
pathway.4 The increased systolic force is often felt by the patient 
as a palpitation and is evident on inspection or palpation of the 
precordium. Because of the diffuse and forceful nature of the 
apex beat, the heart m

and forceful nature of the 
apex beat, the heart may seem enlarged, and echocardiography 
may show an increased ventricular mass. In addition, the pre­
ejection period is shortened and the ratio of preejection period 
to left ventricular ejection time is decreased.3,5 The heart sounds 
are enhanced, particularly S1, and a scratchy systolic sound along 
the left sternal border, resembling a pleuropericardial friction 
rub (Means-Lerman scratch), may also be heard. These manifes­
tations abate whe

ay also be heard. These manifes­
tations abate when a normal metabolic state is restored. Mitral 
valve prolapse occurs more frequently in Graves or Hashimoto 
disease than in the normal population6 and has been suggested as 
autoimmune in origin.7 Cardiac arrhythmias are almost invari­
ably supraventricular, especially in younger patients. Between 
2% and 20% of patients with thyrotoxicosis have atrial fibril­
lation, and about 15% of patients with otherwise unexplained 
atrial fibrillation are

ith otherwise unexplained 
atrial fibrillation are thyrotoxic,1 which may be caused directly 
by the thyroid hormone excess or by activating autoantibodies to 
the β1-adrenergic receptors.8,9 In the Framingham cohort, indi­
viduals over age 60 with a suppressed TSH level had a 2.8-fold 
increased risk of developing atrial fibrillation compared to those 
with normal serum TSH values,10 and such a finding has been 
widely confirmed.11
The increased cardiovascular cost of a standard workload or 
me

 cardiovascular cost of a standard workload or 
metabolic challenge is adequately met if the thyrotoxic patient is 
not, or has not previously been, in heart failure. Thus in most 
patients without underlying heart disease, cardiac competence is 
maintained. Mild peripheral edema may occur in the absence of 
heart failure. Heart failure per se usually, but not always, occurs in 
patients with preexisting heart disease and therefore is more typi­
cally seen in the elderly, but it may not be possi

cally seen in the elderly, but it may not be possible to determine 
  Causes of Hyperthyroidism
I. Excessive TSH-Receptor Stimulation
Graves disease (TRAb)
Pregnancy-associated transient hyperthyroidism (hCG)
Trophoblastic disease (hCG)
Familial gestational hyperthyroidism (mutant TSH receptor)
TSH-producing pituitary adenoma
II. Autonomous Thyroid Hormone Secretion
Multinodular toxic goiter (somatic mutations)
Solitary toxic thyroid adenoma (somatic mutation)
Congenital activating TSH-receptor

matic mutation)
Congenital activating TSH-receptor mutation (genomic mutation)
III. Destruction of Follicles With Release of Hormone
Subacute de Quervain thyroiditis (virus infection)
Painless thyroiditis/postpartum thyroiditis (hashitoxicosis—autoimmune)
Acute thyroiditis (bacterial infection)
Drug-induced thyroiditis (amiodarone, interferon-γ)
IV. Extrathyroidal Sources of Thyroid Hormone
Iatrogenic overreplacement with thyroid hormone
Excessive self-administered thyroid medication
Food and su

e self-administered thyroid medication
Food and supplements containing excessive thyroid hormone
Functional thyroid cancer metastases
Struma ovarii
hCG, Human chorionic gonadotropin; TRAb, thyrotropin receptor antibodies; TSH, thyroid-
stimulating hormone (thyrotropin).
   
TABLE 12.1
366
whether underlying heart disease is present until after thyrotoxico­
sis is relieved. Atrial fibrillation decreases the efficiency of the car­
diac response to any increased circulatory d

 car­
diac response to any increased circulatory demand and may play a 
role in causing cardiac failure.5 Attempts to convert or abate atrial 
fibrillation to sinus rhythm are not indicated while thyrotoxicosis 
is present, and about 60% of patients revert spontaneously to sinus 
rhythm after treatment, most within 4 months. While thrombo­
embolism is not frequent in patients under 50 with thyrotoxico­
sis, the decision to anticoagulate should be potentially considered 
in such patients taking i

 potentially considered 
in such patients taking into account additional risks for embolic 
events and risks of therapy.12 Medical or electrical cardioversion 
of patients with thyrotoxicosis-induced atrial fibrillation is often 
successful even after a year has passed.13 
Protein, Carbohydrate, and Lipid Metabolism
The stimulation of metabolism and heat production is reflected 
in increased appetite and heat intolerance but only rarely by 
elevated basal body temperature.14 Despite an increased

ted basal body temperature.14 Despite an increased food 
intake, a state of chronic caloric and nutritional inadequacy 
often ensues, depending on the degree of increased metabolism. 
Both synthesis and degradation rates of proteins are increased, 
the latter to a greater extent than the former, with the result that 
in severe thyrotoxicosis there is a net decrease in tissue protein 
as indicated by loss of weight, muscle wasting, proximal muscle 
weakness, and even mild hypoalbuminemia. Preexis

weakness, and even mild hypoalbuminemia. Preexisting diabetes 
mellitus may be aggravated, one cause being accelerated turnover 
of insulin. Both lipogenesis and lipolysis are increased in thyro­
toxicosis, but the net effect is lipolysis as reflected by an increase 
in the plasma concentration of free fatty acids and glycerol and 
a decrease in serum cholesterol level; triglyceride levels are usu­
ally slightly decreased. The enhanced mobilization and oxidation 
of free fatty acids in respons

tion and oxidation 
of free fatty acids in response to fasting or catecholamines are 
due to enhancement of lipolytic pathways by thyroid hormones, 
including their effects on mitochondrial beta oxidation in the 
liver.14,15 
Sympathetic Nervous System and 
Catecholamines
Many of the manifestations of thyrotoxicosis and of sympa­
thetic nervous system activation are similar. Nonetheless, the 
plasma concentrations of epinephrine and norepinephrine, as 
well as their urinary excretion and that of

e, as 
well as their urinary excretion and that of their metabolites, are 
not increased in patients with thyrotoxicosis, and thyroid hor­
mones exert effects separate from, but similar and additive to, 
those of the catecholamines. The improvement in cardiac func­
tion in patients with hyperthyroidism by β-adrenergic blockade 
has led to the concept that there is increased sympathetic tone or 
increased cardiac sensitivity to the sympathetic nervous system.16 
Support for the latter are the res

vous system.16 
Support for the latter are the results in the transgenic mouse in 
which overexpression of type 2 deiodinase in the heart increases 
myocardial T3 and the cyclic adenosine monophosphate (cAMP) 
response to norepinephrine in the cardiac myocytes due to altera­
tions in G proteins.17,18 However, stimulation of the heart by 
thyroid hormones does not require β-adrenergic receptors in 
mouse models such that it is likely that β-adrenergic blockade 
in hyperthyroidism is only influenc

rgic blockade 
in hyperthyroidism is only influencing part of the disorder. Still 
intact thyroid hormone signaling pathways are required for 
adrenergic signaling in white adipocytes leading to lipolysis and 
for the induction of UCP1 in brown adipocytes by adrenergic 
signaling pathways.19,20 
Nervous System
Alterations in the function of the nervous system in thyrotoxicosis 
are manifested by nervousness, emotional lability, and hyperkine­
sia. Fatigue may be due both to muscle weakness and t

. Fatigue may be due both to muscle weakness and to the insom­
nia that is commonly present. Emotional lability is common, and 
in rare cases mental disturbance may be severe; manic depressive, 
schizoid, or paranoid reactions may emerge. The hyperkinesia of 
the thyrotoxic patient is characteristic and may manifest to such 
a point that the patient is almost levitating. During the interview 
the patient shifts positions frequently and movements are quick, 
jerky, exaggerated, and often purposel

are quick, 
jerky, exaggerated, and often purposeless. In children, in whom 
such manifestations tend to be more severe, inability to focus may 
lead to deterioration in school performance suggesting attention 
deficit hyperactivity disorder. There may be a fine tremor of the 
hands, tongue, or lightly closed eyelids. The electroencephalogram 
reveals an increase in fast wave activity, and in patients with con­
vulsive disorders the frequency of seizures is increased. 
Muscle
Weakness and fatiga

seizures is increased. 
Muscle
Weakness and fatigability are usually not accompanied by objec­
tive evidence of muscle disease save for the generalized wasting 
associated with weight loss. The weakness is most prominent in 
the proximal muscles of the limbs, causing difficulty in climb­
ing stairs or fatigue from minimal exertion such as using a blow 
dryer or lifting an infant. Proximal muscle wasting may be out 
of proportion to the overall loss of weight (often referred to as 
thyrotoxic myo

ss of weight (often referred to as 
thyrotoxic myopathy). In the most severe forms, the myopathy may 
involve the more distal muscles of the extremities and the muscles 
of the trunk and face. Although myopathy of ocular muscles is 
unusual, the disorder may mimic myasthenia gravis or ophthal­
mic myasthenia.21–25 Muscular strength returns to normal when 
a normal metabolic state is restored, but muscle mass takes longer 
to recover.
Graves disease occurs in about 3% to 5% of patients with myas­

se occurs in about 3% to 5% of patients with myas­
thenia gravis, and about 1% of the patients with Graves disease 
develop myasthenia gravis. Antibodies and T cells specific for the 
TSH and acetylcholine receptors are involved in the pathogenesis 
of the two diseases.26 Unlike thyrotoxic myopathy, the association 
of myasthenia gravis with Graves disease has a distinct female pre­
ponderance. The effect of both thyrotoxicosis and its alleviation 
on the course of myasthenia gravis is variable,

n 
on the course of myasthenia gravis is variable, but in the majority 
of instances, myasthenia is accentuated during the thyrotoxic state 
and improves when a normal metabolic state is restored. A form 
of myasthenia affecting mainly the orbital muscles may also occur 
more commonly in patients with Graves disease and needs to be 
distinguished from GO by the prominence of bilateral ptosis of a 
variable degree.22
Periodic paralysis of the hypokalemic type may occur together 
with thyrotoxicos

kalemic type may occur together 
with thyrotoxicosis, and its severity is accentuated by the latter 
disorder. The coincidence of the two disorders is particularly com­
mon in Asian and Latino males.27,28 
Eyes
Some retraction of the upper or lower eyelids, or both, evident as 
the presence of a rim of sclera between either lid and the limbus, 
may be seen in all forms of thyrotoxicosis regardless of the under­
lying cause and is responsible for the typical stare of the patient. 
Also common is 

the typical stare of the patient. 
Also common is either lid lag, a phenomenon in which the upper 
lid lags behind the globe when the patient is asked to shift the gaze 
367
slowly downward, or globe lag, which becomes evident when the 
eye lags behind the upper lid when the patient looks up. These 
ocular manifestations appear to be the result of increased adren­
ergic tone. It is important to differentiate these signs, which may 
occur in all forms of thyroto

se signs, which may 
occur in all forms of thyrotoxicosis, from those of infiltrative auto­
immune orbitopathy, which are associated with Graves disease 
and are described later. 
Skin and Hair
The most characteristic change in the patient with long-standing 
thyrotoxicosis is the warm, moist feel of the skin that results from 
cutaneous vasodilation and excessive sweating. The elbows may be 
smooth and pink, the complexion is rosy, and the patient blushes 
readily. Palmar erythema may resemble 

nt blushes 
readily. Palmar erythema may resemble liver palms (palmar ery­
thema), and telangiectasia may be present. The hair is fine and 
friable, and hair loss may increase. The nails are often soft and 
friable. A characteristic but uncommon finding is Plummer nails, 
onycholysis, typically involving the fourth and fifth fingers. Vit­
iligo, another autoimmune disease, is more common in patients 
with autoimmune thyroid disease. 
Respiratory System
Dyspnea is common in severe thyrotoxicosis,

System
Dyspnea is common in severe thyrotoxicosis, and several factors may 
contribute to this condition. Vital capacity is commonly reduced, 
mainly from weakness of the respiratory muscles. During exercise, 
ventilation is increased out of proportion to the increase in oxygen 
uptake, but the diffusing capacity of the lung is normal. Because 
of the general increase in oxygen consumption associated with thy­
rotoxicosis, patients with chronic lung diseases may experience a 
rather severe worse

ung diseases may experience a 
rather severe worsening of the condition if they become thyrotoxic. 
Alimentary System
An increase in appetite is common but is usually not seen in 
patients with mild disease. In more severe disease the increased 
intake of food is inadequate to meet the increased caloric require­
ments, and weight is lost at a variable rate. More often, the patient 
reports a gratifying success with a previously frustrated attempt at 
weight control. The frequency of bowel moveme

 at 
weight control. The frequency of bowel movements is increased; 
diarrhea, although rare, can also be a problem. The increased gas­
tric emptying and intestinal motility in thyrotoxicosis appear to 
be responsible for slight malabsorption of fat, and these functions 
return to normal when a normal metabolic state has been restored. 
Celiac and Graves diseases coexist more commonly than once 
thought, and there is an increased prevalence of pernicious anemia.
Hepatic dysfunction occurs, parti

rnicious anemia.
Hepatic dysfunction occurs, particularly when thyrotoxicosis 
is severe; hypoproteinemia and increases in serum alanine ami­
notransferase (ALT) and bone or liver alkaline phosphatase lev­
els may be elevated. Progressive hepatomegaly and jaundice was 
a cause of death prior to the development of successful treatment 
for Graves patients likely exacerbated by congestive heart failure. 
Skeletal System: Calcium and Phosphorus 
Thyrotoxicosis is generally associated wit

abolism
Thyrotoxicosis is generally associated with increased excretion 
of calcium and phosphorus in urine and stool; with an increase 
in bone turnover and a net demineralization of bone, as dem­
onstrated by routine bone densitometry; and occasionally with 
pathologic fractures, especially in elderly women.21,23,29–32
In such instances the pathologic changes are variable and 
may include osteitis fibrosa, osteomalacia, or osteoporosis, 
most likely varying with vitamin D status. Urinary excre

ikely varying with vitamin D status. Urinary excretion 
of collagen breakdown products (teleopeptides) is increased 
in thyrotoxicosis. Kinetic studies indicate an increase in the 
exchangeable calcium pool and acceleration of both bone resorp­
tion and accretion, particularly the former. Thyroid hormone 
(T3) has been shown to accelerate activity of the osteoclasts 
via its nuclear receptors and helps explain these widespread 
changes.21,24,32 Some data indicate that TSH itself may have 
a loca

ome data indicate that TSH itself may have 
a local action, which may normally balance thyroid hormone 
action on osteoclasts and enhance osteoblast activity.25,33,34 Such 
an action by TSH would be absent in hyperthyroidism, allow­
ing accentuation of the thyroid hormone effects. However, not 
all models recapitulate these findings, and taken together it is 
likely that excess levels of thyroid hormones have a more pro­
found effect on bone mineral density.35 As the thyrotoxicosis is 
treated, 

ral density.35 As the thyrotoxicosis is 
treated, bone density may normalize in many younger patients 
but not all.36 Postmenopausal women, however, may have an 
accelerated reduction in bone density that requires treatment 
(see Chapter 30). Much controversy has existed over the induc­
tion of decreased bone density by TSH-suppression therapy in 
patients with thyroid cancer. Suffice it to say that postmeno­
pausal, but not premenopausal, women given a TSH-suppressive 
dosage of thyroid hormone

given a TSH-suppressive 
dosage of thyroid hormones are at risk of osteopenia and require 
prophylaxis with calcium and vitamin D or more aggressive 
approaches.37,38 The decision to relax TSH suppression in low-
risk patients may be influenced by their bone status.
For all the same reasons, hypercalcemia may occur in patients 
with severe thyrotoxicosis. The total serum calcium concentration 
is increased in as many as 27% of patients, and the ionized serum 
calcium is elevated in 47%. The conc

onized serum 
calcium is elevated in 47%. The concentrations of heat labile 
serum alkaline phosphatase and osteocalcin are also frequently 
elevated. These findings resemble those of primary hyperparathy­
roidism, but the concentration of parathyroid hormone in serum 
is low-normal in most. True primary hyperparathyroidism and 
thyrotoxicosis sometimes coexist. Plasma 25-hydroxycholecalcif­
erol levels are decreased in thyrotoxic patients, and this alteration 
could contribute to the decreased 

his alteration 
could contribute to the decreased intestinal absorption of calcium 
and osteomalacia noted in some. 
Renal Function: Water and Electrolyte 
Thyrotoxicosis produces no symptoms referable to the urinary 
tract save for mild polyuria, which may lead to nocturia. Never­
theless renal blood flow, glomerular filtration, and tubular reab­
sorptive and secretory maxima are increased. Total exchangeable 
potassium is decreased, possibly due to a decrease in lean body 
mass, but

possibly due to a decrease in lean body 
mass, but electrolytes are normal except when hypokalemic peri­
odic paralysis occurs. 
Hematopoietic System
The red blood cells are usually normal, as judged by the usual 
indices, but red blood cell mass is increased. The increase in eryth­
ropoiesis is due to the direct effect of thyroid hormones on the 
erythroid marrow, as patients that harbor mutations in the thyroid 
hormone receptor alpha (TRα) have mild anemia secondary to 
defects in erythropoie

e mild anemia secondary to 
defects in erythropoiesis.39 A parallel increase in plasma volume 
also occurs, with the result that the hematocrit is normal in hyper­
thyroid patients.
368
Approximately 3% of patients with Graves disease have perni­
cious anemia, and a further 3% have antibodies to intrinsic fac­
tor but normal absorption of vitamin B12. Autoantibodies against 
gastric parietal cells may also be present in patients with Graves 
disease, and the requirements f

ients with Graves 
disease, and the requirements for vitamin B12 and folic acid 
appear to be increased. The total white blood cell count is often 
low because of a decrease in the number of neutrophils. The abso­
lute lymphocyte count is normal or increased, leading to a relative 
lymphocytosis. The numbers of monocytes and eosinophils may 
also be increased. Splenic enlargement occurs in about 10% of the 
patients, and thymic enlargement is common in Graves disease.40 
The latter may present a

mon in Graves disease.40 
The latter may present as a mediastinal mass. Thymic hyperpla­
sia is also due to thyrotoxicosis because it is sometimes seen in 
patients receiving excess exogenous T4 for suppression.41
Platelet levels and the intrinsic clotting mechanism are nor­
mal, but the concentration of factor VIII is often increased and 
returns to normal when the thyrotoxicosis is treated. Despite this 
increase, there is an enhanced sensitivity to warfarin because of the 
accelerated clearan

ty to warfarin because of the 
accelerated clearance of the vitamin K–dependent clotting factors. 
Therefore the dosage of warfarin needs to be reduced in thyrotoxic 
patients.42 This must be kept in mind if initiating anticoagulant 
treatment for atrial fibrillation in older patients.43 Coincidental 
autoimmune thrombocytopenia may also occur. 
Pituitary and Adrenocortical Function
The thyrotoxic state imposes several challenges on pituitary and 
adrenocortical function. The hepatic inactivatio

adrenocortical function. The hepatic inactivation of cortisol is accel­
erated, including enhanced 5α/5β-reductases and 11β hydroxys­
teroid dehydrogenase. As a result of these changes, the disposal of 
cortisol is accelerated, but its rate of secretion is also increased, so the 
plasma cortisol concentration remains normal. The concentration 
of corticosteroid-binding globulin in plasma is also normal. The 
urinary excretion of free cortisol is normal or slightly increased44 
(see Chapter 15)

s normal or slightly increased44 
(see Chapter 15). Interestingly, thyroid hormone receptors have 
now been identified in adrenal cortical cells in the mouse. It is not 
clear if they are also present similarly in humans.45 
Thyrotoxicosis in early life may cause delayed sexual maturation, 
although physical development is normal and skeletal growth may 
be accelerated. Thyrotoxicosis after puberty influences reproduc­
tive function, especially in women. The intermenstrual 

function, especially in women. The intermenstrual interval 
may be prolonged or shortened, and menstrual flow is initially 
diminished and ultimately ceases. Fertility may be reduced, and 
if conception takes place, there is an increased risk of miscarriage 
and other complications.46–48 In some patients, menstrual cycles 
are predominantly anovulatory with oligomenorrhea, but in most, 
ovulation occurs as indicated by a secretory endometrium. In the 
former, a subnormal midcycle surge of lutein

 the 
former, a subnormal midcycle surge of luteinizing hormone (LH) 
may be responsible. In premenopausal women with thyrotoxico­
sis, basal plasma concentrations of LH and follicle-stimulating 
hormone (FSH) are reportedly normal but may display enhanced 
responsiveness to gonadotropin-releasing hormone (GnRH).
Thyrotoxicosis, whether spontaneous or induced by exogenous 
hormone, is accompanied by an increase in the concentration of 
sex hormone–binding globulin (SHBG) in plasma.49 As a result

e–binding globulin (SHBG) in plasma.49 As a result, 
the plasma concentrations of total testosterone, dihydrotestoster­
one, and estradiol are increased, but their unbound fractions are 
normal or transiently decreased. The increased binding in plasma 
may be responsible for the decreased metabolic clearance rate of 
testosterone and dihydrotestosterone. In the case of estradiol, 
however, the metabolic clearance rate is normal, suggesting that 
tissue metabolism of the hormone is increased. Con

tissue metabolism of the hormone is increased. Conversion rates 
of androstenedione to testosterone, estrone, and estradiol and of 
testosterone to dihydrotestosterone are increased.50 The increased 
rate of conversion of androgens to estrogenic byproducts may 
be the mechanism for gynecomastia and erectile dysfunction in 
some 10% of thyrotoxic men and one mechanism for menstrual 
irregularities in women. Another likely mechanism for menstrual 
changes is the disruption in amplitude and frequen

changes is the disruption in amplitude and frequency of LH/FSH 
pulses due to thyroid hormone influences on GnRH signaling. 
Laboratory Diagnosis
The effects of thyrotoxicosis on the major organ systems are the 
same regardless of the underlying cause. Their frequency and 
intensity and the other findings with which they are associated 
are influenced by the cause of the excess thyroid hormone. To a 
large extent, the same is true of laboratory test results. However, 
the patient with thyrotoxic

est results. However, 
the patient with thyrotoxic symptoms will virtually always have 
a serum TSH concentration less than 0.1 mU/L and an elevated 
serum free T4. In general, serum free T3 is more elevated than is 
the free T4, but free T4 is relatively high if thyrotoxicosis is caused 
by thyroiditis or intake of levothyroxine.
If the possibility of exogenous thyroid hormone can be elimi­
nated, the primary differential is between excess thyroid hormone 
production and excess thyroid hormone 

id hormone 
production and excess thyroid hormone release from sick cells, 
as in thyroiditis. Often this differentiation can be made on the 
basis of the history and physical. Laboratory tests, including an 
increased sedimentation rate and a high serum thyroglobulin 
(Tg), may favor thyroiditis, but the most critical differentiating 
test is the radioactive iodine uptake (RAIU), which is elevated or 
inappropriately high-normal given the suppressed serum TSH 
level with excess thyroid hormone 

ssed serum TSH 
level with excess thyroid hormone production and very low (<5%) 
in patients with thyroiditis (Fig. 12.1). However, the RAIU may 
also be low in a hyperthyroid patient who has recently received an 
iodine load, usually iodinated contrast for a computed tomogra­
phy (CT) scan or for angiography. A 24-hour urine iodine mea­
surement can confirm this. Also helpful is measurement of TSH 
TSH<0.01
Graves Disease
hCG
Hot Nodule
Multinodular Goiter
Iodine
Exogenous
Struma Ov

ular Goiter
Iodine
Exogenous
Struma Ovarii
Metastatic Thyroid Carcinoma
(RAIU)
Normal or Increased
Absent
• Fig. 12.1  Determination of the cause of hyperthyroidism based on the 123I 
uptake in the gland. In the setting of a suppressed thyroid-stimulating hor­
mone (TSH) a normal or increased uptake is indicative of something else 
driving uptake rather than endogenous TSH. In the absence of uptake the 
gland has either been damaged or an external factor suc

 has either been damaged or an external factor such as exogenous 
hormone or iodine is playing a role. Rarely, ectopic thyroid hormone pro­
duction may occur. hCG, human chorionic gonadotropin.
369
receptor antibodies (TSHRAb), which when associated with a 
suppressed TSH is consistent with Graves disease.
If the physical examination or thyroid ultrasonography indi­
cates the presence of a nodular thyroid, thyroid scanning may 
confirm which nodules are hyperfu

id scanning may 
confirm which nodules are hyperfunctioning. The association 
of thyrotoxicosis with an elevated TSH is rare and suggests a 
TSH-producing pituitary tumor. The possibility of an artifac­
tually elevated TSH in a patient with Graves disease should 
be ruled out by repeating the assay by a different method in 
another laboratory (see Chapters 4 and 8). Exceptions to these 
general guidelines are discussed later within the appropriate 
subsection. 
GRAVES DISEASE
Graves disease is a

e 
Graves disease is a multisystem autoimmune disorder character­
ized by TSHRAb. The disease has been described by von Base­
dow in Germany, Graves in Ireland, and Parry in the United 
Kingdom, all in the first half of the 19th century and inde­
pendent of each other.51 Hallmarks of the disease were palpita­
tions (hyperthyroidism), goiter, and exophthalmos, called the 
Merseburg triad after the hometown of von Basedow (Merse­
burg, Germany). The nature of the diseas

w (Merse­
burg, Germany). The nature of the disease remained unclear for 
many years, and cardiac, neurologic, and pituitary origins were 
proposed. The autoimmune nature was not recognized before 
the late 1950s when Adams reported on the presence of a long-
acting thyroid stimulator (LATS) in serum, different from 
TSH, which turned out to be an antibody capable of stimulat­
ing the TSH receptor.52,53 Self-infusion of sera from patients 
with Graves disease caused thyroid stimulation and was t

raves disease caused thyroid stimulation and was the 
first demonstration of the role of TSHRAb in the induction 
of human hyperthyroidism.54 Another example of the in vivo 
effects of TSHRAb is the transplacental passage of maternal 
TSHRAb, which may stimulate the fetal thyroid gland and 
cause fetal/neonatal thyrotoxicosis.55 It is now recognized that 
TSH receptors are not only expressed on thyrocytes but are also 
outside the thyroid gland on fibroblasts, fibrocytes, adipocytes, 
osteoblast

n fibroblasts, fibrocytes, adipocytes, 
osteoblasts, osteoclasts, and pituitary folliculostellate cells. It 
is likely that immune responses directed against TSH recep­
tors on orbital fibroblasts and subcutaneous connective tissue 
cells are involved in other expressions of Graves disease, namely 
Graves orbitopathy and Graves dermopathy. Besides stimulat­
ing TSHRAb there also exist blocking TSHRAb, which are 
found in a small minority of patients with autoimmune hypo­
thyroidism.56 The clinic

nts with autoimmune hypo­
thyroidism.56 The clinical phenotypes of Graves disease are thus 
manifold (Table 12.2). 
Graves Hyperthyroidism
Clinical Presentation
Graves hyperthyroidism (GH) is at least four times more common 
in women than in men. GH is relatively rare in children. Its inci­
dence starts to increase at the age of 13 years and remains rather 
stable after the age of 30 years. Average age is 47 years. The typi­
cal patient complains about weight loss despite increased appe­
tite, h

 about weight loss despite increased appe­
tite, heat intolerance, sweating, palpitations, fine finger tremor, 
nervousness, and loose stools. Duration of symptoms until diag­
nosis is less than 6 months in 61% of patients.57 A positive fam­
ily history of autoimmune thyroid disease (AITD) is present in 
about 50%.57,58 There is some evidence for genetic anticipation, 
which is a younger age of onset in patients with a family history 
of AITD.57–59
Goiter is present in about half of patients. Ty

59
Goiter is present in about half of patients. Typically the goiter 
is diffuse in nature, with a soft to firm consistency and a smooth 
surface. In severe cases, a thrill may be felt, usually over the upper 
or lower poles where the superior and inferior thyroid arteries, 
respectively, enter the thyroid gland; a thrill is accompanied by an 
audible bruit in systole. The presence of a nodule in the setting of 
Graves disease should raise suspicion of thyroid cancer. Advanc­
ing age is associat

ion of thyroid cancer. Advanc­
ing age is associated with less severe Graves hyperthyroidism.58,60 
Remarkably symptom-poor hyperthyroidism may occur in elderly 
people, called apathetic thyrotoxicosis. Graves orbitopathy is 
present in 26% of patients when Graves hyperthyroidism is diag­
nosed. Thereafter new cases of GO develop in 9%, rendering a 
total GO frequency of 35% in Graves hyperthyroidism.61 Taking 
into account the severity of the Merseburg triad (as judged from 
serum free thyroxin

rseburg triad (as judged from 
serum free thyroxine [FT4], thyroid size, and GO signs), 44% of 
patients have severe Graves disease, 34% moderate disease, and 
22% mild disease.62 The phenotypic appearance of Graves disease 
is apparently milder than in the past; this may be due to earlier 
diagnosis and treatment, improved iodine nutrition, and a secular 
trend to less smoking. 
Pathology
The thyroid gland in Graves hyperthyroidism is typically both 
enlarged and uniformly affected, hence often

both 
enlarged and uniformly affected, hence often referred to as dif­
fuse toxic goiter. It is extremely rare that the disease process 
remains restricted to one lobe.63 Such unilateral Graves hyper­
thyroidism is difficult to reconcile with the systemic nature of 
thyroid-stimulating immunoglobulins. The follicles are small 
and lined with hyperplastic columnar epithelium and contain 
scant colloid that displays much marginal scalloping and vacu­
olization (Fig. 12.2). Papillary projections of

u­
olization (Fig. 12.2). Papillary projections of the hyperplastic 
epithelium extend into the lumina of the follicles. Vascularity 
is increased. There is varying heterogeneous infiltration by lym­
phocytes and plasma cells that collect in aggregates and may 
form infrequent B-cell germinal centers (in contrast to their 
abundance in Hashimoto disease). In such regions, thyroid 
epithelial cells express human leukocyte antigen (HLA) class 
II antigens (a phenomenon not seen in normal thyroid g

ntigens (a phenomenon not seen in normal thyroid glands) 
and are large (perhaps due to local stimulation by TSHRAb). 
The intrathyroidal lymphocytic population is mixed, consisting 
  Phenotypes of Graves Disease and Their 
Estimated Incidence Rates
Phenotype
% Affected
Cases/Million/Year
All
100
350
Graves hyperthyroidism
90–95
325
Goiter
50
175
Graves orbitopathy
30
105
Severe orbitopathy
5
17
Hypo + orbitopathy
5
17
Graves dermopathy
0.5
4
Neonatal hyperthyroidism
0.2
1.0
Fetal hyperthyroid

eonatal hyperthyroidism
0.2
1.0
Fetal hyperthyroidism
0.1
0.5
Acropachy
0.1
0.5
TABLE 12.2
370
besides B cells mostly of T helper 1 (Th1) and Th2 lymphocytes 
and less often Th17 and regulatory T cells. When the patient 
is given iodine or antithyroid drugs (ATDs), the thyroid gland 
may undergo involution as TSHRAb decrease. Then hyperplasia 
and vascularity regress, papillary projections recede, and follicles 
enlarge and become filled with colloid again. 
Immunopathogen

 become filled with colloid again. 
Immunopathogenesis
Autoimmune thyroid diseases can be defined as complex or multi­
factorial diseases in which autoimmune responses to thyroid anti­
gens develop against a particular genetic background, influenced 
by exposure to environmental factors. Breakdown of self-tolerance 
to thyroid antigens results in thyroid autoimmunity. A number 
of complex regulatory mechanisms serve to protect an immune 
response directed against self-antigens.64 During developm

 directed against self-antigens.64 During development, 
immature T cells enter the thymus where they undergo a process 
of selection.65 T cells that recognize with high affinity self-pep­
tides expressed on thymic medullary epithelial cells are deleted by 
apoptosis, and T cells with moderate affinities leave the thymus 
as mature T cells. The full-length thyroid-stimulating hormone 
receptor (TSHR) is indeed expressed in the hyperplastic thymus 
of Graves hyperthyroid patients.66 Genetic-epigen

of Graves hyperthyroid patients.66 Genetic-epigenetic interactions 
involving a noncoding single-nucleotide polymorphism (SNP) in 
the TSHR gene that regulates thymic TSHR gene expression may 
facilitate escape of TSHR-reactive T cells from central tolerance 
in the thymus, thereby triggering Graves disease.67 Central toler­
ance may not eliminate all self-reactive T cells. Control of auto­
reactive T cells in the periphery (e.g., by engagement of cytotoxic 
T-lymphocyte–associated protein 4 [C

of cytotoxic 
T-lymphocyte–associated protein 4 [CTLA4] resulting in T-cell 
anergy) is considered as a secondary or fail-safe mechanism to 
prevent autoimmune reactions (peripheral tolerance). Self-anti­
gen presentation in the thymus also generates regulatory T cells 
(Treg) that can inhibit in peripheral tissues those self-reactive T 
cells that escaped negative selection in the thymus. Besides these 
natural Treg there are inducible Treg generated in the periphery 
after antigenic stimulatio

rated in the periphery 
after antigenic stimulation. Treg are characterized by the expression 
of CD4, CD25 (the interleukin 2 [IL2] receptor α-chain), and 
FOXP3 (the transcription factor forkhead box P3 protein); they 
may act on effector CD4+ and CD8+ lymphocytes by cell-to-cell 
contact hampering their activation and proliferation or indirectly 
via secretion of IL10 and transforming growth factor-β (TGFβ). 
Treg are crucial in maintaining tolerance through the active sup­
pression of self-r

lerance through the active sup­
pression of self-reactive T-cell activation and expansion. A fourth 
subset of T helper cells, induced by IL6, are highly proinflamma­
tory and designated Th17 cells. They generate IL17, exacerbating 
autoimmune responses. Thus a balance between Th17 and Treg is 
crucial for immune homeostasis. A number of studies now report 
a decrease in the number of Treg cells and an increase in Th17 cells 
in peripheral blood of Graves patients, in relationship with stimu­
la

of Graves patients, in relationship with stimu­
lating TSHRAb and with improvement upon methimazole treat­
ment.65,68–70 Low Treg and high IL17 levels are also found in an 
animal model of Graves disease.71 Recent studies have identified 
regulatory beta cells (Breg) that contribute to peripheral tolerance 
by inhibiting immune reactions to specific self-antigens.72,73
Traditionally Graves disease is viewed as the result of humoral 
immunity (hallmark TSHR antibodies) and Hashimoto thyroid­
itis

lmark TSHR antibodies) and Hashimoto thyroid­
itis as the result of cell-mediated immunity (hallmark thyroid per­
oxidase [TPO] antibodies [TPOAb]). However, TPOAb are also 
present in about 70% of Graves patients, and TSHR antibodies 
may occur in a minority of Hashimoto patients. Actually, humoral 
and cellular immune mechanisms are closely connected, and Th1 
(interferon gamma [IFNγ]) and Th2 (IL4) subtypes of helper T 
cell responses are involved in both Graves and Hashimoto dis­
ease. IgG1 

lved in both Graves and Hashimoto dis­
ease. IgG1 antibodies arise early in the immune response, whereas 
IgG4 antibodies (typically Th2 related) arise after prolonged 
immune stimulation. TPOAb and TgAb may comprise IgG1 
as well as IgG4 classes indicating participation of Th1 and Th2 
cytokines, respectively. Stimulating TSHRAb are mostly found 
B
C
A
• Fig. 12.2  Histopathologic features of the thyroid gland in Graves hyper­
thyroidism (B and C) as compared to normal tissue (A). (Courtesy Dr.

C) as compared to normal tissue (A). (Courtesy Dr. 
Pamela Unger, Mount Sinai Scholl of Medicine, New York.)
371
in the IgG1 subclass, which is selectively induced by Th1 cells; 
Graves hyperthyroidism is a predominantly Th1-type cytokine 
disease.74 Oligoclonality and light chain restriction of stimulat­
ing TSHRAb support their primary role in disease causation.75,76 
Th1 cells may also induce antibody production through secretion 
of IL10, which in turn acti

ion through secretion 
of IL10, which in turn activates B cells. B cells may turn into 
plasma cells secreting antibodies. The thyroidal lymphocytic infil­
trate is a major production site of thyroid autoantibodies. Trans­
plantation of Graves thyroid tissue into mice deficient in both T 
cells and B cells with severe combined immunodeficiency (SCID 
mice) results in the appearance of human thyroid autoantibodies 
in serum, including TSHRAb.77 Thyroid antibodies can also be 
produced outside the

yroid antibodies can also be 
produced outside the thyroid gland as they may persist after total 
thyroidectomy. 
TSH Receptor, the Major Autoantigen in Graves 
Disease
A mice animal model of Graves hyperthyroidism is provided by 
genetic immunization against the TSH receptor, which induces 
stimulating TSHRAb and hyperthyroidism.78 The gene encoding 
the TSH receptor is located on chromosome 14q31. The TSHR 
belongs to the family of G protein–coupled receptors and has 
seven transmembrane domai

upled receptors and has 
seven transmembrane domains, a large extracellular domain, and 
a small intracellular domain (Fig. 12.3). The TSH holoreceptor 
consists of a 100-kDa glycosylated 744–amino acid sequence and 
a 20–amino acid signal peptide. The holoreceptor is cleaved into 
two subunits, alpha and beta, which are linked by disulfide bonds. 
The 50-kDa α-subunit is water soluble and heavily glycosylated. 
TSH and TSHRAb bind to the leucine-rich repeat regions of the 
α-subunit. The 30-kDa

-rich repeat regions of the 
α-subunit. The 30-kDa β-subunit is water insoluble, contains the 
membrane-spanning domain with its three extracellular loops and 
three cytoplasmic loops, and is 70% to 75% homologous with the 
LH/human chorionic gonadotropin (hCG) receptor. The TSHR 
forms dimers and multimeric complexes on the thyrocyte surface, 
probably enhancing the stability of the receptor. The holoreceptor 
undergoes cleavage in the hinge region due to breakage of disul­
fide bonds. As a res

ion due to breakage of disul­
fide bonds. As a result of this post-translational modification, the 
extracellular α-subunit is shed. The shed TSHR α-subunit, rather 
than the holoreceptor, is apparently the autoantigen in Graves 
disease.79 Enhanced α-subunit shedding from damaged follicular 
cells may be the cause of the increase of serum TSHRAb concen­
trations after radioiodine therapy.80
Stimulating TSHRAb isolated from patients with Graves 
hyperthyroidism bind to the TSHR and activate Gαs 

hyperthyroidism bind to the TSHR and activate Gαs and Gq 
signaling pathways, thereby inducing thyroid growth, increased 
vascularity, and increased production and secretion of thyroid 
hormones.81,82 In contrast, some TSHRAb act as TSH antagonists 
and are referred to as blocking TSHRAb; they may occur in some 
patients with autoimmune hypothyroidism but also after treat­
ment of Graves hyperthyroidism.56,81 So-called neutral or cleav­
age region TSHRAb neither block TSH binding nor block TSH 

n TSHRAb neither block TSH binding nor block TSH 
action; they do not induce cAMP but have the potential to induce 
apoptosis by binding to the hinge region.81 
Etiology
A complex interaction between existential factors, genetic vari­
ants, and environmental insults determines susceptibility of sub­
jects to develop Graves disease83 (Fig. 12.4).
Existential Factors
The strong female preponderance in Graves patients remains 
incompletely understood. Among all women with Graves hyper­
thyroidism, 

d. Among all women with Graves hyper­
thyroidism, the rate of postpartum onset is 7.2%,84 and parity 
as a risk factor has received much attention. Women with chil­
dren, compared to childless women, have a relative risk for Graves 
Hinge
α
β
LRD
ECLs
ICLs
TMD
• Fig. 12.3  Structure of the human thyroid-stimulating hormone receptor 
(TSHR). ECLs, extracellular loops; ICLs, small intracellular loops; LRD, leu­
cine-rich domain; TMD, transmembrane domain. The receptor is cleaved, 
probably after a

domain. The receptor is cleaved, 
probably after activation, into α (or A) and β (or B) subunits. The α-subunit 
is thought to be shed from the cell surface.
GRAVES  HYPERTHYROIDISM
ENVIRONMENT
• iodine
• smoking
• alcohol
• selenium
• stress
• infections
• drugs
GENES
• TSHR
• Tg
• HLA
• IL2A
• CD25
• CD40
• CTLA4
• PTPN22
EXISTENTIAL FACTORS
• female sex
• parity
• Fig. 12.4  Venn diagram illustrating how the combination of existential fac­
tors, genetic variants, and environmental insults may

s, genetic variants, and environmental insults may result in susceptibility 
to Graves hyperthyroidism.
372
disease of 1.19 (CI 1.14–1.24).85 Fetal microchimerism (persis­
tence of fetal cells in maternal tissues) may trigger autoimmunity 
through maternal immune responses against foreign fetal anti­
gens, but its contribution to the female preponderance appears 
limited.86 At odds with this hypothesis is a recent report that 
fetal microchimerism is more frequent in healt

at 
fetal microchimerism is more frequent in healthy controls than in 
Graves disease (64% vs. 33%, p = 0.0004), suggesting a possible 
protective role.87 More relevant might be the epigenetic phenom­
enon of X-chromosome inactivation (XCI): In female cells, one 
of the two X chromosomes is inactivated in early embryonic life. 
Female tissues are thus mosaics of two cell lines, one with the 
paternal X and the other with the maternal X chromosome, usu­
ally in a 50:50 ratio. The consequence of s

, usu­
ally in a 50:50 ratio. The consequence of skewed XCI (arbitrarily 
defined as inactivation of the same X chromosome in ≥80% of 
cells) might be that self-antigens on one X chromosome are not 
expressed at sufficiently high levels to induce tolerance.88 A meta-
analysis confirmed significant skewing of XCI in women with 
Graves disease (odds ratio 2.54, CI 1.58–4.10).89 FOXP3 is a key 
gene in the development of Treg, located on the X chromosome. 
Polymorphisms in FOXP3 have been linked to

osome. 
Polymorphisms in FOXP3 have been linked to Graves disease in 
some but not all studies. 
Genetic Variants
Twin studies suggest that 79% of the liability to develop Graves 
disease is attributable to genetic factors.90 Identified susceptibility 
genes for Graves disease include thyroid-specific genes (TSHR, Tg) 
and genes involved in the regulation of immune responses (HLA, 
CD25, CD40, CTLA4, and PTPN22).91 These genes together 
probably do not explain more than about 10% of the heritabi

do not explain more than about 10% of the heritabil­
ity of Graves disease92; it follows there must be many more still 
undetected genetic loci, each contributing a little. The TSHR gene 
is most tightly associated with Graves disease, but the functional 
consequences of SNPs in the unusual large intron 1 are not clear.93 
They could give rise to RNA splice variants, increasing the level 
of autoantigenic TSHR α-subunits. Alternatively, SNP carriers 
may have fewer thymic TSHR transcripts, decre

ers 
may have fewer thymic TSHR transcripts, decreasing central tol­
erance to TSHR. Multiple SNPs in the thyroglobulin gene are 
linked to both Graves and Hashimoto disease.91 Possible interac­
tions between HLA-DRβ1 and thyroglobulin variants have been 
described in Graves disease, which would result in more effec­
tive presentation of disease-associated thyroglobulin-SNP allelles 
to T cells.94 The immunoregulatory genes HLA, CD25, CD40, 
CTLA4, and PTPN22 are all involved in the immunologic 

4, and PTPN22 are all involved in the immunologic syn­
apse, in which antigenic peptides complexed in HLA molecules are 
presented by antigen-presenting cells (APCs; macrophages, den­
dritic cells, but also B cells) to T-cell receptors (TCRs) on T cells. 
Formation of the trimolecular complex (HLA, antigenic peptide, 
TCR) activates CD4+ T cells via expression of the interleukin 2 
receptor (IL2Rα; CD25 is a marker for the IL2R α-chain) and by 
inducing co-stimulation via induction of CD40 ligan

nducing co-stimulation via induction of CD40 ligand on T cells, 
which binds to constitutively expressed CD40 on APCs. CTLA4 
is finally induced, which should terminate the immune response. 
Polymorphisms in these genes may functionally hinder the proper 
development of central and peripheral tolerance and alter T-cell 
interactions with APCs in the immunologic synapse.91 They 
confer susceptibility for AITD but also for other autoimmune 
diseases, explaining the co-occurrence of various autoimm

s, explaining the co-occurrence of various autoimmune dis­
eases.83 Odds ratios of individual loci for AITD are rather low 
(<2.0), but slightly higher for HLA (2.0–4.0). HLA-C, an HLA 
class I molecule, is stronger associated with Graves disease than 
HLA class II molecules (HLA-DRB1, DQA1, and DQB1), at 
least in Caucasians.95 This is interesting as HLA class I molecules 
present endogenous antigens to immune cells, including those 
derived from viruses, which are possible triggers for AITD. N

m viruses, which are possible triggers for AITD. Note­
worthy is the lack of an association between Graves disease and 
mutations in the autoimmune regulator gene (AIRE ) expressed 
in thymic medullary epithelial cells, resulting in failure to present 
self-antigens correctly in the thymus; AIRE mutations cause auto­
immune polyglandular syndrome type 1.96 Polymorphisms in the 
PTPN22 gene encoding lymphoid protein tyrosine phosphatase is 
another susceptibility gene, with a gene dose-dependent 

r susceptibility gene, with a gene dose-dependent effect on 
the age of onset of Graves disease.97 
Environmental Insults
Iodine
Iodine-induced thyroid autoimmunity is related to TgAb, and the 
unmasking of a cryptic epitope on Tg contributes to this rela­
tionship in humans.98 Longitudinal epidemiologic studies dem­
onstrate that iodine fortification in iodine-deficient areas initially 
lead to a transient increase in the incidence of toxic multinodular 
goiter and Graves hyperthyroidism. In th

inodular 
goiter and Graves hyperthyroidism. In the long term, however, 
there is a decline in the incidence rate of thyrotoxicosis, mainly 
due to fewer cases of toxic multinodular goiter but also—although 
to a lesser extent—to fewer cases of Graves disease.99,100 
Smoking
Smoking is a well-established risk factor for Graves disease.101 The 
odds ratio for Graves hyperthyroidism is 3.30 (CI 2.09–5.22) in 
current smokers when compared with never smokers. The risk dis­
appears a few years after

r smokers. The risk dis­
appears a few years after cessation of smoking. 
Alcohol
Moderate alcohol consumption is associated with a considerable 
reduction in the risk of Graves hyperthyroidism. Odds ratios 
are 1.73 (CI 1.17–2.56) for 0 units/week, 1.00 for 1 to 2 units/
week (reference), 0.56 (0.39–0.79) for 3 to 10 units/week, 0.37 
(0.21–0.65) for 11 to 20 units/week, and 0.22 (0.08–0.60) for 
21+ units/week.83,102 No interaction was found with type of alco­
hol (wine vs beer), smoking habit

th type of alco­
hol (wine vs beer), smoking habits, gender, or iodine intake. 
Selenium
As evident from population-based studies in China, low selenium 
intake might be a risk factor for autoimmune thyroiditis and 
hypothyroidism, but not for Graves hyperthyroidism.103 
Stress
Stressful life events have already been implicated in the early 
descriptions of Graves hyperthyroidism.51 Numerous reports on 
exposure to severe emotional stress prior to the onset of Graves 
disease support a causal re

o the onset of Graves 
disease support a causal relationship,104 but prospective studies on 
this issue are lacking.83 
Similarity between different antigens can lead to specificity cross­
over (molecular mimicry). Antigenic similarity between bacteria/
viruses and human proteins is common. The best studied example 
is infection with Yersinia enterocolitica (YE). IgG from Graves 
patients inhibit TSH binding to outer membranes of YE, and 
conversely IgG from patients with YE infection

nd 
conversely IgG from patients with YE infection inhibit binding of 
TSH to thyroid membranes. There is cross-reactivity between YE 
outer membrane proteins and epitopes of TSHR antibodies.105 
Despite this clear evidence of molecular mimicry, epidemiologic 
studies have not detected an association between YE infection and 
AITD.106 A local insult (whether trauma or infection) may cause 
373
an inflammatory infiltrate and production of cytokines in the thy­
r

filtrate and production of cytokines in the thy­
roid gland, which may induce HLA class II expression; this may 
facilitate presentation of thyroid antigens and activation of local 
autoreactive thyroid-specific T cells in susceptible individuals.107 
A number of infections (e.g., with enteroviruses, Helicobacter 
pylori, reoviruses) have been implicated in Graves disease, but 
conclusive evidence for their involvement has not been obtained. 
The exception is hepatitis C virus (HCV), which seems

 exception is hepatitis C virus (HCV), which seems the only 
infectious agent that is clearly associated with an increased risk 
for autoimmune thyroiditis.108 HCV can infect human thyrocytes 
resulting in production of proinflammatory cytokines, which may 
enhance the autoimmune response.109 
Drugs
Interferon-α has direct cytotoxic effects on thyrocytes but can also 
provoke destructive bystander immune responses. Treatment of 
hepatitis C with interferon-α and ribavirin combination therapy 
is

interferon-α and ribavirin combination therapy 
is associated with development of a suppressed TSH in 10.4%, 
due to Graves disease in 0.8%, and to destructive thyroiditis in 
9.6%.110 Exogenous estrogens such as oral contraceptives are asso­
ciated with a lower risk of Graves hyperthyroidism.111,112
Graves hyperthyroidism may occur during periods of lymphocyte 
recovery after induced lymphopenia, known as the immune recon­
stitution syndrome. It is observed after bone marrow or hematopoi­
etic 

 is observed after bone marrow or hematopoi­
etic stem cell transplantation,113 during highly active antiretroviral 
therapy (HAART) for human immunodeficiency virus (HIV) infec­
tion in 3%,114 and during alemtuzumab therapy for multiple sclero­
sis in 30%.115 Alemtuzumab is a humanized anti-CD52 monoclonal 
antibody, inducing a rapid and prolonged depletion of lymphocytes 
from the circulation, resulting in a profound immunosuppression 
followed by an immune reconstitution phase. Median onset o

 by an immune reconstitution phase. Median onset of 
Graves hyperthyroidism is 17 months (range 2–107 months) fol­
lowing last dose of alemtuzumab.116 Graves hyperthyroidism in 
this context frequently requires definitive or prolonged ATD treat­
ment. Furthermore, fluctuating thyroid status and high frequency of 
TSHRAb-positive hypothyroidism suggest switches between block­
ing and stimulating TSHRAb in these patients.
Immune checkpoint inhibitors are emerging as a new class of 
anticancer drug

rs are emerging as a new class of 
anticancer drugs associated with frequent thyroidal side effects. Ipi­
limumab is a monoclonal antibody against CTLA4 that blocks an 
inhibitory signal to T cells, thereby prolonging a stimulated immune 
response against tumor cells.117 Nivolumab and pembrolizumab are 
antibodies against programmed death protein 1 (PD1), which is 
upregulated on activated T cells. By interrupting PD1 binding to its 
ligand PDL1, effector T-cell activity increases in the tumor m

 effector T-cell activity increases in the tumor micro­
environment.117 Treatment with these checkpoint inhibitors may 
result in subclinical hyperthyroidism due to destructive thyroiditis 
and, rarely, in Graves hyperthyroidism.117–119 
Diagnosis and Differential Diagnosis
When the syndromal diagnosis of thyrotoxicosis has been established 
by the finding of a suppressed serum TSH and an increased serum 
FT4 and/or FT3, a nosologic diagnosis should be reached: Which 
disease entity causes hyper

uld be reached: Which 
disease entity causes hyperthyroidism? If Graves orbitopathy is pres­
ent, it is quite evident Graves hyperthyroidism is present. Otherwise 
the presence of TSH receptor antibodies (TSHRAb) in serum con­
firms Graves disease. Most current assays use a competitive binding 
methodology, measuring TSH-binding inhibitory immunoglobu­
lins (TBII) (Table 12.3). These binding assays do not discriminate 
between stimulating and blocking TSHRAb. Nevertheless, their sen­
sitivity an

cking TSHRAb. Nevertheless, their sen­
sitivity and specificity for the diagnosis of Graves hyperthyroidism 
are very high (97% and 98%, respectively).120 In contrast, cell-based 
bioassays are able to differentiate between stimulating TSHRAb 
(TSAb or TSI) and blocking TSHRAb (TBAb).121,122
Large variation exists in the use of diagnostic imaging. A recent 
study in France among hyperthyroid patients indicated thyroid 
ultrasonography was done in 94% and isotope scanning in 40%.123 
European Thy

 94% and isotope scanning in 40%.123 
European Thyroid Association (ETA) guidelines recommend 
ultrasonography (comprising grey scale analysis and color flow or 
power Doppler examination) as the preferred imaging procedure 
for the diagnosis of Graves hyperthyroidism.124 Graves hyperthy­
roidism is typically characterized by diffuse thyroid enlargement, 
hypoechogenicity, and increased vascularity. The latter can be used 
to differentiate between thyrotoxicosis due to Graves disease and 
destru

n thyrotoxicosis due to Graves disease and 
destructive thyroiditis (as in subacute thyroiditis, amiodarone-
induced thyrotoxicosis type 2). Typical ultrasound patterns com­
bined with positive TSHRAb may obviate the need for thyroid 
scintigraphy. ETA guidelines suggest scintigraphy when thyroid 
nodularity coexists with hyperthyroidism and prior to 131I ther­
apy. The few patients in whom no TSHRAb are detected (TBII 
<2 U/L) have biochemically less severe Graves hyperthyroidism 
compared to T

 less severe Graves hyperthyroidism 
compared to TSHRAb-positive patients125; however, familial 
nonautoimmune hyperthyroidism due to germline TSHR activat­
ing mutations is present in 4.5% of such patients.126 
Natural History and Prognosis
Knowledge about the natural history of Graves hyperthyroidism 
is limited because nowadays almost no patient escapes therapeutic 
intervention. Case histories from the older literature and cumu­
lative experience with the outcome of ATDs, however, allow us 

ence with the outcome of ATDs, however, allow us 
to hypothesize that patients with Graves hyperthyroidism can be 
  Assays of Thyroid-Stimulating Hormone 
Receptor Antibodies: Nomenclature and 
Nomenclature of TSHRAb Assays
TBII (TSH binding inhibitory 
immunoglobulins)
Measurement of inhibition of labelled 
TSH (or labelled thyroid-stimulating 
monoclonal antibody) binding to 
recombinant TSHR by serum 
antibodies
TSAb or TSI (thyroid-
stimulating 
antibodies)
Measurement of cAMP 

oid-
Measurement of cAMP production by 
thyroid cell lines transfected with 
TSHR
TBAb (thyroid-blocking 
Measurement of inhibition of cAMP 
production after TSH-mediated 
stimulation of thyroid cells or 
TSHR-transfected cells
Indications for Assay of TSHRAb
Diagnosis
Graves orbitopathy and Graves 
dermopathy
Fetal and neonatal thyrotoxicosis
Treatment
Chance of remission of hyperthyroidism 
at baseline, and during treatment 
with anti

dism 
with antithyroid drugs.
cAMP, Cyclic adenosine monophosphate; TSH, thyroid-stimulating hormone; TSHR, thyroid-
stimulating hormone receptor; TSHRAb, thyroid-stimulating hormone receptor antibodies.
   
TABLE 12.3
374
divided into three groups, each with a different natural history: (1) 
patients who have a prolonged continuous episode of hyperthy­
roidism that never goes into remission (∼10%), (2) patients who 
follow a relapsing an

on (∼10%), (2) patients who 
follow a relapsing and remitting course over many years (∼50%), 
and (3) patients who have a single episode of hyperthyroidism fol­
lowed by permanent remission (∼40%). Patients who are in remis­
sion after a course of ATDs have a low prevalence of TSHRAb 
(<30%) and a much higher prevalence of TPOAb (∼80%). Long-
term follow-up studies of patients in remission indicate devel­
opment of subclinical hypothyroidism in about 20% and overt 
hypothyroidism in about 6%, as

bout 20% and overt 
hypothyroidism in about 6%, associated with either blocking 
TSHRAb or TPOAb.127–129
A series of population-based studies in Denmark have provided 
evidence that Graves hyperthyroidism (but also toxic nodular goi­
ter) is associated with increased all-cause mortality (hazard ratio 
1.42, CI 1.25–1.60) due to cardiovascular and lung diseases.130 
Excess mortality is related to cumulative periods of low serum 
TSH.131 Hyperthyroid patients treated with radioactive iodine 
remai

id patients treated with radioactive iodine 
remain at a higher risk of cardiovascular diseases compared to 
patients treated with thyroidectomy, but hypothyroidism during 
follow-up predicts better cardiovascular outcome.132
Treatment
Recent guidelines and reviews give detailed information on 
the management of Graves hyperthyroidism.124,133,134 Treat­
ment of hyperthyroidism in the presence of Graves orbitopa­
thy is discussed in the section on Graves orbitopathy. There are 
three treatment op

 Graves orbitopathy. There are 
three treatment options (ATDs, thyroidectomy, and radioactive 
iodine) that are effective in restoring euthyroidism. It should be 
realized, however, that a causal therapy of Graves disease directed 
against the underlying abnormal immune responses is not yet 
available. 
Antithyroid Drugs: Thionamides
Mechanism of Action
The major agents for treating thyrotoxicosis are drugs of the thion­
amide class: carbimazole (CBZ), methimazole (MMI), and PTU. 
CBZ is rapidly

(CBZ), methimazole (MMI), and PTU. 
CBZ is rapidly decarboxylated in the liver to the active substance 
MMI. Equivalent doses are 40 mg CBZ, 30 mg MMI, and 400 
mg PTU. Each of these agents inhibits the function of thyroid per­
oxidase, thereby reducing oxidation and organification of thyroid 
iodide, iodotyrosine coupling, and thyroid hormone biosynthesis. 
In addition, large doses of PTU, but not methimazole, impair the 
conversion of T4 to T3 by D1 in the thyroid and peripheral tis­
sues.135 

by D1 in the thyroid and peripheral tis­
sues.135 Consequently large doses of PTU may provide more rapid 
alleviation of very severe thyrotoxicosis.
The plasma half-life of MMI is about 6 hours, whereas that 
of PTU is about 1.5 hours. Both drugs are accumulated by the 
thyroid gland. Whereas the daily amount of MMI can be given 
in one dose, the daily amount of PTU should be divided in three 
doses with each given every 8 hours. In patients with severe hyper­
thyroidism, splitting the daily dos

 severe hyper­
thyroidism, splitting the daily dose of MMI might sometimes be 
required. Single daily dosing improves compliance and should be 
used whenever possible. Thionamides cross the placenta and can 
inhibit fetal thyroid function.
MMI and PTU may also have immunosuppressive effects.136 
In vitro MMI reduces HLA-DR expression on thyroid cells, either 
directly or indirectly via inhibiting IFNγ. In vivo, MMI decreases 
the number of intrathyroidal activated T cells and the concentra­
tion

hyroidal activated T cells and the concentra­
tion of serum TSHRAb (but not of nonthyroid antibodies such as 
parietal cell antibodies). MMI, but not glucocorticoids, given prior 
to radioiodine therapy blunts the increase in TSHRAb usually seen 
after 131I.137 The unresolved question, however, is whether the 
effects are caused directly by the action of the drug on the immune 
system or indirectly by restoration of the euthyroid state. 
Adverse Effects (Table 12.4)
Minor cutaneous reactions suc

Effects (Table 12.4)
Minor cutaneous reactions such as skin rash and urticaria usually 
can be managed by antihistamine therapy without stopping the 
drug. The lesions may resolve spontaneously or after switching 
to another ATD. Prescribing the alternative drug is not recom­
mended in the case of serious allergic reactions.124,138
Agranulocytosis (<500 neutrophils/mm3) is a serious side effect, 
with an incidence of 0.28% in the first 3 months of therapy.139 
Risk factors are older age, higher 

f therapy.139 
Risk factors are older age, higher doses of ATD, and the pres­
ence of particular HLA-B and HLA-DRB1 alleles or rare NOX3 
genetic variants.140–142 The onset of agranulocytosis is rather 
abrupt, accompanied by fever and sore throat.138 When therapy 
with ATD is begun, the patient should be instructed to discon­
tinue the drug and to notify the physician immediately should 
these symptoms develop. This precaution is more important than 
the frequent measurement of white blood cell

than 
the frequent measurement of white blood cell counts because 
agranulocytosis may develop within 1 to 2 days. Neither the 
American Thyroid Association (ATA) nor the ETA recommend 
routine monitoring of white blood cells during ATD therapy. If 
agranulocytosis occurs, the drug should be discontinued immedi­
ately and the patient treated with antibiotics as appropriate. Gran­
ulocyte colony-stimulating factor may speed the recovery that 
invariably takes place. Lymphocytes of patients who ha

riably takes place. Lymphocytes of patients who have devel­
oped agranulocytosis while taking PTU undergo blast transforma­
tion when exposed in vitro to PTU or methimazole; consequently, 
thionamides should not be given again. Granulocytopenia occurs 
during ATD therapy and is sometimes a forerunner of agranulo­
cytosis, but it can also be a manifestation of thyrotoxicosis itself.
Although rare, PTU has been associated with fulminant 
hepatic necrosis, and it is the third most common cause of d

crosis, and it is the third most common cause of drug-
related liver failure, accounting for 10% of all drug-related liver 
transplants. Children are at a higher risk than adults.143 Fortu­
nately, stopping PTU results in recovery in most cases. This 
  Adverse Events of Antithyroid Drugs
Common (1–5%)
Skin rash
Urticaria
Arthralgia, polyarthritis
Transient mild leukopenia
Rare (0.2–1%)
Abnormal smell and taste
Agranulocytosis
Very rare (<0.1%)
Aplastic anemia (PTU, CBZ)
Thromb

ery rare (<0.1%)
Thrombocytopenia (PTU, CBZ)
Vasculitis, lupus-like, ANCA+ve (PTU)
Hepatitis (PTU)
Hypoglycemia (anti-insulin antibod­
ies) (PTU)
Cholestatic jaundice (CBZ, MMI)
ANCA+ve, Antineutrophil cytoplasmic antibody positive; CBZ, carbimazole;  PTU, propylthio­
uracil; MMI, methimazole.
Adapted from Strieder TG, Prummel MF, Tijssen JG, et al. Risk factors for and prevalence of 
thyroid disorders in a cross-sectional study among healthy female relatives of patien

nal study among healthy female relatives of patients with 
autoimmune thyroid disease. Clin Endocrinol. 2003;59:396–401.
   
TABLE 12.4
375
PTU-associated liver failure may occur at any time during therapy, 
so routine monitoring of liver function may not be helpful.138 
Because of this well-known rare but serious PTU side effect of 
hepatic failure, sometimes requiring liver transplantation, in June 
2009 the Food and Drug Administration (FDA) issued an adviso

ood and Drug Administration (FDA) issued an advisory 
that PTU should not be used as a first-line agent in hyperthyroid­
ism.144 The use of PTU should be restricted to the first trimester 
of pregnancy (see later), to thyroid storm, and to patients who 
experience minor side effects of MMI and are unable or unwilling 
to undergo 131I therapy or thyroidectomy. MMI is associated in 
a dose-dependent manner with an increased risk for hepatitis and 
cholestasis.145 There are no reported cases of liv

cholestasis.145 There are no reported cases of liver transplantation 
attributed to MMI toxicity. 
Practical Use
Guidelines agree MMI (CBZ) should be used in virtually every 
nonpregnant patient who chooses ATD therapy.124,134 Guide­
lines also recommend that patients should be informed of side 
effects of ATD and the necessity of informing the physician 
promptly if they should develop jaundice, light-colored stools, 
dark urine, fever, or pharyngitis. Preferably this information 
should be in 

ngitis. Preferably this information 
should be in writing. Prior to ATD therapy it is suggested to 
obtain a baseline complete blood count, including white blood 
cell count with differential, and a liver profile, including bili­
rubin and transaminases. Restoration of the euthyroid state 
by ATD can be done according to the titration method or the 
block-and-replace regimen. In the titration method one starts 
with a relatively high single daily dose of 20 to 30 mg MMI 
(or 30–40 mg CBZ). Norma

 dose of 20 to 30 mg MMI 
(or 30–40 mg CBZ). Normal FT4 and T3 concentrations are 
usually reached after 4 to 6 weeks, which should be followed 
by tapering the MMI dose toward a maintenance dose of about 
5 to 10 mg MMI, which keeps the patient euthyroid. In the 
block-and-replace regimen one starts with the same high ini­
tial dose of MMI, then adds LT4 when the patient has become 
euthyroid under continuation of the high initial MMI dose. 
In the rare patient given PTU, the high starting PTU 

the rare patient given PTU, the high starting PTU dose is 
300 to 400 mg daily divided in three to four doses, whereas 
the maintenance dose is about 50 to 100 mg daily. The advan­
tage of the titration method is the use of lower doses of MMI 
and thereby slightly fewer side effects; the disadvantage is the 
need for more frequent blood sampling to adjust the MMI dose 
and larger fluctuations between hypothyroidism and hyperthy­
roidism. The advantages of the block-and-replace regimen are 
fewer

ntages of the block-and-replace regimen are 
fewer blood samples and more stable thyroid function during 
ATD therapy; the disadvantages are the higher MMI dose and 
slightly more side effects. The two methods have never been 
compared in a randomized clinical trial. The recurrence rate of 
Graves hyperthyroidism is similar between both regimens. In 
many patients, serum TSH may remain suppressed for a long 
time despite already normalized serum FT4 and T3 concentra­
tions.146 This phenomenon ha

FT4 and T3 concentra­
tions.146 This phenomenon has been linked to the presence of 
still high serum concentrations of stimulating TSHRAb, which 
via binding to TSH receptors in pituitary folliculostellate cells 
downregulate TSH release.147
MMI is administered for 12 to 18 months and then discon­
tinued to see if the disease has gone into remission. Remission 
  Predictive Scores for the Risk of Recurrent Graves Hyperthyroidism After 1-Year Treatment With 
Antithyroid Drugs
Items (Assessed Bef

atment With 
Items (Assessed Before 
Starting Therapy)
GREAT Score 
Range 0–6
GREAT SCORE
Risk Class	
Recurrences
Age (yr)        ≥40
      <40
0
+1
Class I (score 0–1)	
	
16%
Class II (score 2–3) 	 	
44%
Class III (score 4–6) 	 	
68%
FT4 (pmol/L)  <40
 ≥40
0
+1
TBII (U/L) 
 
<6
 
6–19.9
 
≥20
0
+1
+2
Goiter sizea gr. 0–I
 
 gr. II–III
0
+2
Items Added
GREAT+ Score 
Range 0–10
GREAT+ SCORE
Risk Class 	
PTPN22 C/C wild type
C/T
0
+1
Class I+ (score 0–2) 	 	
4%
Class 

d type
C/T
0
+1
4%
Class II+ (score 3–4) 		
21%
Class III+ (score 5–6) 		
49%
Class IV+ (score 7–10) 	
84%
HLA nr.b 0
1–2
3 (LD)
0
+2
+3
aGoiter size: grade 0, thyroid not or distinctly palpable but usually not visible; grade I, thyroid easily palpable and visible with head in normal or raised position; grade II, thyroid easily visible with the head 
in a normal position; grade III, goiter visible at a distance.
bNumber of HLA subtypes (DQB1-02, DQA1-05, DRB1-03) present

f HLA subtypes (DQB1-02, DQA1-05, DRB1-03) present.
FT4, Free thyroxine; HLA, human leukocyte antigen; LD, linkage disequilibrium; TBII, thyroid-stimulating hormone–binding inhibi­tory immunoglobulins. 
Modified from Vos XG, Endert E, Zwinderman AH, et al. Predicting the risk of recurrence before the start of antithyroid drug therapy in patients with Graves’ hyperthyroidism. J Clin Endocrinol Metab. 
2016;101:1381–1389.
   
TABLE 12.5
376
rate is between 40% and 60%; it is

 III    Thyroid
rate is between 40% and 60%; it is not influenced very much by 
dose or duration of ATD therapy. Most recurrences occur in the 
first year after stopping ATD. Exposure to stressful life events 
and daily hassles after withdrawal of ATD increase the risk of 
recurrent hyperthyroidism.104,148 It is recommended to check 
thyroid function annually to detect late recurrences or develop­
ment of hypothyroidism. Recurrences should be treated with 
radioiodine or surgery. About one-third

ated with 
radioiodine or surgery. About one-third of patients experience 
a lasting remission.
Chances of remission increase if during ATD therapy goiter 
size and/or TSHRAb decrease and TSH has become normal. It 
has been recommended to measure TSHRAb just prior to stop­
ping ATD. If TSHRAb have disappeared, chances of remission are 
high. If TSHRAb can still be detected, one may consider prolong­
ing ATD therapy because chances of remission are low. A major 
advantage of a more prolonged ther

e low. A major 
advantage of a more prolonged therapy is that nearly all patients 
remain euthyroid as long as therapy is given, even if the dose of 
the drug is low.
Predicting who will experience relapsing hyperthyroidism 
after stopping ATD has proven to be difficult. A change from 
stimulating to blocking TSHRAb might play a role, as well as 
many other factors.149 A systematic review and meta-analysis of 
7595 patients, of whom 48.7% relapsed, identified a significant 
association with rela

d, identified a significant 
association with relapse of the following items, all assessed before 
starting treatment with ATD: smoking, thyroid size (either by 
sonography or by inspection and palpation), orbitopathy, FT4, 
FT3, and TBII.150 These risk factors by themselves, however, 
have not sufficient power for accurate prediction in individual 
patients. Combining a number of independent risk factors led 
to a predictive score composed of age, FT4, TBII, and goiter 
size (by inspection and 

e, FT4, TBII, and goiter 
size (by inspection and palpation), called the GREAT score 
(Graves Recurrent Events After Therapy) (Table 12.5).151 The 
clinical relevance is that the GREAT score provides a reasonable 
prediction of recurrence risk after a course of ATDs based on 
just four items, readily available before the start of treatment 
(Fig. 12.5). It might allow for discussions with the patient of 
whether the treatment option of ATDs is optimal for them. If 
the recurrence risk is low, AT

timal for them. If 
the recurrence risk is low, ATD might be a good choice; if the 
recurrence risk is rather high, surgery or radioiodine could be 
a better option. The GREAT score has been validated in an 
independent study.152 Adding the results of specific genotypes 
provides the GREAT score with even better prediction. It can 
be foreseen that such predictive scores will be of great value in 
personalized medicine. 
Other Drugs Used in Hyperthyroidism
Beta-Adrenoceptor Blocking Agents. Thyr

Beta-Adrenoceptor Blocking Agents. Thyroid hormone excess 
increases sensitivity of the sympathetic nervous system to catechol­
amines. Drugs that block the response to catecholamines at the 
receptor site (e.g., propranolol) ameliorate some of the manifesta­
tions of thyrotoxicosis and are often used as adjuncts in manage­
ment. Beta blockers rapidly improve complaints of tremulousness, 
palpitations, excessive sweating, and eyelid retraction. Beta block­
ers are recommended in all p

traction. Beta block­
ers are recommended in all patients with symptomatic thyrotoxi­
cosis, especially in elderly patients and patients with resting heart 
rates in excess of 90 beats per minute or coexistent cardiovascular 
100
A
B
Recurrence rate (%)
80
60
40
20
0
0
6
12
18
24
68% Class III
44% Class II
16% Class I
100
Time of follow-up after withdrawal of antithyroid drugs (months)
80
60
40
20
0
0
6
12
18
24
84% Class IV+
49% Class III+
21% Class II+
4% Class I+
• Fig. 12

4% Class I+
• Fig. 12.5  Kaplan-Meier curves for recurrent Graves hyperthyroidism after a 1-year course of antithyroid 
drugs according to risk classes I to III of the GREAT score (A) and risk classes I+ to IV+ of the GREAT+ 
score (B). (Redrawn from Vos XG, Endert E, Zwinderman AH, et al. Predicting the risk of recurrence before 
the start of antithyroid drug therapy in patients with Graves’ hyperthyroidism. J Clin Endocrinol Metab. 
2016;101:1381–1389.)
377
Chapter

ndocrinol Metab. 
377
disease.124,134 Beta blockers are most useful in the interval before 
the response to thionamides or radioiodine therapy occurs. Pro­
pranolol, in addition to its action as a β-adrenoreceptor antago­
nist, also inhibits 5′-deiodination resulting in a decrease of plasma 
T3 and an increase of plasma reverse T3.153 Plasma T3 decreases 
by 20% at a daily dose of 80 mg propranolol (four 20-mg doses) 
and by 30% at 160 mg p

ranolol (four 20-mg doses) 
and by 30% at 160 mg propranolol (four 40-mg doses). Beta 
blockers without membrane-stabilizing activity do not decrease 
plasma T3, but the clinical response to β-blockers is independent 
of the decrease in T3. The largest experience is with propranolol, 
but the drug can be contraindicated (e.g., in asthma and chronic 
obstructive pulmonary disease). Longer acting drugs with relative 
β1 selectivity might then be preferred, such as atenolol (25–100 
mg once or twic

eferred, such as atenolol (25–100 
mg once or twice daily) and metoprolol (25–50 mg twice or three 
times daily).124 
Perchlorate. Perchlorate inhibits thyroid iodide transport 
and has been used in the past to treat Graves hyperthyroidism. 
Although it was effective in restoring euthyroidism and decreasing 
TSHRAb, it is no longer used in view of its side effects (except in 
amiodarone-induced thyrotoxicosis type 1).154,155 
Iodine. Iodine is now rarely used as a sole therapy. The mecha­
nism o

w rarely used as a sole therapy. The mecha­
nism of action of iodine in relieving thyrotoxicosis differs from 
that of the thionamides. Although quantities of iodine in excess 
of several milligrams can acutely inhibit organic binding (acute 
Wolff-Chaikoff effect), this transient phenomenon probably does 
not contribute to the therapeutic effect. Instead, the major action 
of iodine is to inhibit hormone release. Administration of iodine 
increases glandular stores of organic iodine, but the be

ses glandular stores of organic iodine, but the beneficial 
effect of iodine is evident more quickly than the effects of even 
large doses of agents that inhibit hormone synthesis. In patients 
with Graves disease, iodine acutely retards the rate of secretion 
of T4, an effect that is rapidly lost when iodine is withdrawn. 
These features of iodine action provide both disadvantages and 
advantages. The enrichment of glandular organic iodine stores 
that occurs when this agent is given alone may 

s 
that occurs when this agent is given alone may retard the clini­
cal response to subsequently administered thionamides, and the 
decrease in RAIU produced by iodine prevents the use of radio­
iodine as treatment for several weeks. Furthermore, if iodine is 
withdrawn, resumption of accelerated release of hormone from 
an enriched glandular hormone pool may exacerbate the disorder. 
Another reason for not using iodine alone is that the therapeu­
tic response on occasion is either incomplete or

­
tic response on occasion is either incomplete or absent. Even if 
initially effective, iodine treatment may lose its effect with time. 
This phenomenon, termed iodine escape, should not be confused 
with the escape from the acute Wolff-Chaikoff effect.156 Never­
theless, the rapid slowing of hormone release by iodine makes 
it more effective than the thionamide drugs when prompt relief 
of thyrotoxicosis is mandatory. Therefore, aside from its use in 
preparation for thyroid surgery, iodine is

use in 
preparation for thyroid surgery, iodine is useful mainly in patients 
with actual or impending thyrotoxic crisis, severe thyrocardiac 
disease, or acute surgical emergencies. If iodine is used in these 
circumstances, it should be administered with large doses of a 
thionamide.
Recent studies suggest a potential role for iodine in patients 
who have adverse reactions to ATD or contraindications for radio­
iodine or surgery.157,158 Sometimes iodine is used as a beneficial 
adjunct to ATD:

es iodine is used as a beneficial 
adjunct to ATD: 38 mg potassium iodide (KI) + 15 mg MMI 
resulted in better control of hyperthyroidism and fewer adverse 
reactions than 30 mg MMI alone.158
The lowest dose of iodine required for control of thyrotoxi­
cosis is approximately 6 mg daily. Six milligrams of iodine are 
present in one-eighth of a drop of saturated solution of potassium 
iodide (SSKI) or approximately 1 drop of Lugol solution; many 
physicians, however, prescribe 5 to 10 drops of one

hysicians, however, prescribe 5 to 10 drops of one of these 
agents three times daily. Although it is advisable to administer 
amounts larger than the suggested minimal effective dose, huge 
quantities of iodine are more likely to produce adverse reactions. 
We recommend the use of a maximum 2 to 3 drops of SSKI 
twice daily. Adverse reactions to iodine are unusual and in general 
not serious. They include rash, which may be acneiform; drug 
fever; sialadenitis; conjunctivitis and rhinitis; vasc

r; sialadenitis; conjunctivitis and rhinitis; vasculitis; and a leu­
kemoid eosinophilic granulocytosis. Sialadenitis may respond to 
reduction of dosage and the addition of lemon/lime candies to 
increase salivary flow; in the case of the other reactions, iodine 
should be stopped. 
Lithium. Lithium carbonate inhibits thyroid hormone secre­
tion, but unlike iodine it does not interfere with the accumu­
lation of radioiodine. Lithium, 300 to 450 mg every 8 hours, 
is used only to provide tempora

mg every 8 hours, 
is used only to provide temporary control of thyrotoxicosis in 
patients who are allergic to both thionamides and iodide.159 
Another short-term use for lithium has been as an adjunct to 
radioiodine therapy because the drug slows the release of iodine 
from the thyroid.160 
Selenium. Selenium levels were higher in patients in remission 
and correlated inversely to TSHRAb.161 Therefore it was hypoth­
esized that selenium supplementation might increase remission 
rate. Addition

mentation might increase remission 
rate. Addition of 300 μg sodium selenite daily to methimazole 
did not, however, increase remission rate in a placebo-controlled 
trial.162 
Cholecystographic Agents. The oral iodine-containing cho­
lecystographic contrast agent sodium ipodate causes a prompt 
decrease in serum T4 and serum T3 in Graves hyperthyroidism. 
After a course of 500 mg sodium ipodate daily for 5 days, plasma 
T3 is normalized in all patients, allowing uneventful thyroidec­
tomy on da

atients, allowing uneventful thyroidec­
tomy on day 5.163 However, supplies of such agents are generally 
no longer available. 
Cholestyramine. Thyroxine is metabolized in the liver to gluc­
uronides and sulfates that enter the enterohepatic circulation. 
Cholestyramine interferes with the enterohepatic cycle, thereby 
acting as an effective and well-tolerated adjunctive therapy in 
patients with resistant Graves hyperthyroidism.164,165 It can pro­
duce a rapid and complete decline in thyroid ho

o­
duce a rapid and complete decline in thyroid hormone levels. 
Immunosuppressive Drugs. Because of the autoimmune 
nature of Graves disease, additional use of immunosuppressive 
agents might improve outcomes. A systematic review identi­
fied seven randomized or controlled trials in which the effect of 
addition of glucocorticoids or rituximab to standard treatment 
was compared to standard treatment alone.166 Relapse rate was 
much lower in the intervention group with immunosuppressive 
drugs 

 intervention group with immunosuppressive 
drugs (24%) than in the control group (59%), with a risk ratio 
of 0.55 (CI 0.41–0.75, p < 0.001). However, the study has several 
limitations, such as small sample sizes and moderate to high risk 
of bias. 
Future Developments. It is foreseen that novel treatment 
modalities for Graves hyperthyroidism will become available in the 
next decade. There may be causal therapies directed against stimu­
lating TSHRAb, the immediate cause of Graves disease. E

g TSHRAb, the immediate cause of Graves disease. Examples 
are antigen-specific immunotherapy with tolerogenic peptides of 
the TSH receptor, monoclonal TSH receptor–blocking antibod­
ies, and low-molecular-weight TSH receptor antagonists.167,168 
Thyroidectomy. Thyroidectomy has a high cure rate for Graves 
hyperthyroidism. Total thyroidectomy has a nearly 0% risk of 
recurrence, whereas subtotal thyroidectomy may have an 8% 
chance of persistent or recurrent hyperthyroidism at 5 years.124,134 

t or recurrent hyperthyroidism at 5 years.124,134 
378
Near-total or total thyroidectomy is therefore the procedure of 
choice. It is recommended to refer the patient to a high-volume 
thyroid surgeon. Average complication rates, length of hospital 
stay, and cost are reduced when the operation is performed by 
a surgeon who conducts many thyroidectomies. Surgeons who 
perform more than 25 thyroid surgeries per year have superior 
outcomes compared to surgeons who perform 

perior 
outcomes compared to surgeons who perform fewer.169 Compli­
cation rates are on average 51% higher when surgery is performed 
by low-volume surgeons.134 
Complications. In the hands of high-volume thyroid surgeons 
the rate of permanent hypoparathyroidism is less than 2% and 
that of permanent recurrent laryngeal nerve injury is less than 1%. 
Bleeding into the operative site necessitating reoperation occurs 
in 0.3% to 0.7%; it is the most serious postoperative complica­
tion, which can

st serious postoperative complica­
tion, which can rapidly produce death by asphyxia and requires 
immediate evacuation of the blood and ligation of the bleeding 
vessel. Even with subtotal surgery, the recurrent laryngeal nerve 
can be damaged. If such damage is unilateral, it causes dysphonia 
that usually improves in a few weeks but may leave the patient 
slightly hoarse. Intraoperative recurrent laryngeal nerve moni­
toring does not necessarily improve long-term outcomes. Hypo­
parathyroidis

ly improve long-term outcomes. Hypo­
parathyroidism can be either transient or permanent. Transient 
hypoparathyroidism results from inadvertent removal of some 
parathyroids and/or impairment of blood supply to those that 
remain. Depending on the severity of these insults, symptoms 
and signs of hypocalcemia appear, usually within 1 to 7 days after 
surgery. Severe hypoparathyroidism should be treated with intra­
venous calcium gluconate. Milder cases can be treated with oral 
calcium carbonat

r cases can be treated with oral 
calcium carbonate and cholecalciferol, although the active form of 
vitamin D (calcitriol) is more effective and preferred in most cases. 
However, the hypocalcemia that occurs immediately after surgery 
for thyrotoxicosis may not be due to transient hypoparathyroid­
ism because it occurs more frequently in the Graves patient than 
after surgery for other thyroid disorders. Instead it may be due 
to “hungry bones” because of the demineralization of bone that 
oc

” because of the demineralization of bone that 
occurs in hyperthyroidism. This begins to be reversed after cure of 
the hyperthyroid state and may contribute to the modest elevation 
in alkaline phosphatase during recovery unless the patients have 
been rendered euthyroid for some time prior to surgery. Many 
surgeons who fear that they have caused damage to the parathy­
roid glands at total thyroidectomy may reimplant the apparent 
parathyroid tissue into local muscles. 
Preparation for Surger

tissue into local muscles. 
Preparation for Surgery. If surgery is chosen, patients should 
be rendered euthyroid prior to the procedure with ATD pretreat­
ment, with or without β-adrenergic blockade. These agents do 
not improve the hyperplasia and hypervascularity of the gland in 
the short term. Iodine, however, is reported to cause a decrease 
in height of the follicular cells, enlargement of follicles with 
retention of colloid, and reduction of hypervascularity. Hence 
the recommendation t

n of hypervascularity. Hence 
the recommendation to give a KI-containing preparation in the 
immediate preoperative period should be made.124,134 SSKI 2 to 3 
drops twice daily might be initiated 7 to 10 days before surgery to 
decrease thyroid blood flow and vascularity and hence intraopera­
tive blood loss during thyroidectomy. During this period a preex­
isting bruit or thrill may decrease in intensity or disappear entirely, 
and the gland may become firm. However, little clinical evidence 
e

 become firm. However, little clinical evidence 
exists that postoperative outcomes are any different following a 
course of iodine.170 A recent prospective controlled trial observed 
that preoperative use of KI decreased gland vascularity but not 
the overall difficulty of thyroidectomy; however, KI was associated 
with less transient hypoparathyroidism and transient hoarseness, 
suggesting KI improves the safety of thyroidectomy.171 KI or Lugol 
solution is used by less than 40% of thyroidolog

solution is used by less than 40% of thyroidologists in Europe.172 
In exceptional circumstances, when it is not possible to render a 
patient euthyroid prior to thyroidectomy, the patient should be 
adequately treated with KI and β-blockers (eventually also with 
glucocorticoids and cholestyramine) in the immediate preopera­
tive period.134 Lastly, checking for calcium and 25-hydroxy vita­
min D preoperatively is recommended. Preoperative vitamin D 
deficiency is a risk factor for postoperati

min D 
deficiency is a risk factor for postoperative hypocalcemia. Supple­
menting calcitriol for a brief period preoperatively helps to reduce 
postoperative hypocalcemia.124,134
After thyroidectomy, ATDs are stopped immediately and 
β-blockers should be weaned. Serum calcium with or without 
parathyroid hormone (PTH) levels can be measured and oral 
calcium and calcitriol supplementation be given based on these 
results; alternatively, prophylactic calcium with or without cal­
citriol is presc

ctic calcium with or without cal­
citriol is prescribed empirically. l-Thyroxine should be started at 
a daily dose appropriate for the patient’s weight (1.6 μg/kg) and 
serum TSH measured 6 weeks postoperatively.134 
Radioactive Iodine
Radioactive iodine (RAI) has been used since 1941. Prospective 
trials in this area are few, leaving many questions.173 The cellular 
effect of the ionizing radiation leads to cell death and thereby to a 
decrease in thyroid function and a reduction in thyroid si

 in thyroid function and a reduction in thyroid size. One 
year after 131I therapy, thyroid size is mostly normalized, hyper­
thyroidism has disappeared in 50% to 90% of patients, and hypo­
thyroidism has developed in up to 50% of patients directly related 
to the thyroid RAI dose.173 This is followed by a yearly hypothy­
roidism rate of 3% to 5%, largely independent of RAI dose.124 A 
seminal question thus arises: What is the goal of 131I therapy in 
Graves hyperthyroidism? Is it to get rid of 

y in 
Graves hyperthyroidism? Is it to get rid of hyperthyroidism and 
restore euthyroidism? That has proven to be an elusive goal, as even 
meticulous calculation of a seemingly appropriate 131I dose does 
not prevent a high incidence of post-radioiodine hypothyroidism. 
The ATA therefore recommends the following: “Sufficient activ­
ity of RAI should be administered in a single application, typi­
cally a mean dose of 10–15 mCi (370–555 MBq), to render the 
patient with Graves disease hypothyroi

render the 
patient with Graves disease hypothyroid.”134 It means that cure 
of one disease (hyperthyroidism) is exchanged for the creation 
of another disease (hypothyroidism). ETA guidelines agree: “No 
dose calculation can secure long-term euthyroidism and it is fully 
acceptable to offer a fixed dose of RAI.”124 Therefore many have 
given up meticulous dose calculation and offer fixed activities of, 
for example, 185, 370, or 555 MBq, depending on thyroid size. 
An increased mortality rate h

ng on thyroid size. 
An increased mortality rate has been reported in hyperthyroid 
patients treated with RAI.174 This was not observed in patients 
rendered hypothyroid, and these findings support the practice of 
treating hyperthyroidism with doses of radioiodine sufficient to 
induce overt hypothyroidism.
There has been concern that RAI therapy might produce can­
cer. A hypothetical 0.8% lifetime cancer risk attributable to a 
15-mCi 131I dose at the age of 20 years has been calculated. This 

 at the age of 20 years has been calculated. This 
is only a small increase in baseline cancer risk, and most studies 
have found no significant increase in the prevalence of thyroid 
cancer or secondary malignancies in adult patients treated with 
RAI. Also the frequency of genetic damage in the offspring of 
patients treated earlier with radioiodine does not appear to be 
increased. In view of the lack of evidence of serious toxicity from 
RAI in doses generally used for treating adults with h

in doses generally used for treating adults with hyperthyroid­
ism, the age limit for the use of RAI has been lowered progres­
sively from the initial lower limit of 40 years to age 10 or younger. 
However, in a 5-year-old child the theoretical lifetime cancer risk 
379
after 15 mCi 131I is 4%, so in very young children ATDs should 
be considered. The use of RAI in reproductive-age women also 
remains unpopular, and RAI is contraindicated during pregnancy. 
Fet

 and RAI is contraindicated during pregnancy. 
Fetuses exposed to 131I after 10 weeks of gestation may be born 
athyreotic. In addition, 131I should not be administered for at 
least 8 weeks after cessation of lactation because it is concentrated 
in breast milk.
Complications of RAI Therapy. The early induction of euthy­
roidism and later the development of hypothyroidism are both 
consequences of radiation-induced destruction of thyroid paren­
chyma. With large doses of RAI a tender radiation 

chyma. With large doses of RAI a tender radiation thyroiditis may 
develop within the first week of treatment, as evidenced by epi­
thelial swelling and necrosis, disruption of follicular architecture, 
edema, and infiltration with mononuclear cells. Resolution of the 
acute phase is followed by fibrosis, vascular narrowing, and fur­
ther lymphocytic infiltration. Radiation thyroiditis may lead to an 
exacerbation of thyrotoxicosis 10 to 14 days after RAI is admin­
istered. The effect of RAI on 

after RAI is admin­
istered. The effect of RAI on other tissues that concentrate iodide 
(e.g., the salivary glands, the gastric glands, and the breasts) is not 
likely to be a problem with the relatively low doses prescribed for 
Graves hyperthyroidism.
RAI therapy is associated with development or worsening of 
Graves orbitopathy in about 15% of cases compared to ATDs 
or thyroidectomy, as evident from three randomized clinical 
trials.175–177 This effect is presumably related to the substan­

This effect is presumably related to the substan­
tial increase in serum TSHRAb after RAI therapy, whereas in 
contrast TSHRAb decrease after thyroidectomy or upon treat­
ment with ATD.80 Five years later the level of TSHRAb is still 
higher after RAI than after ATD or surgery (Fig. 12.6). Thus the 
presence of Graves orbitopathy may influence which treatment 
modality is chosen for hyperthyroidism (see section on Graves 
orbitopathy). 
Preparation for 131I Therapy. Because 131I therapy can caus

on for 131I Therapy. Because 131I therapy can cause a 
transient exacerbation of hyperthyroidism, β-blockers should be 
considered even in asymptomatic patients who are at increased 
risk for complications due to worsening of hyperthyroidism (i.e., 
elderly patients and patients with comorbidities). In addition to 
β-adrenergic blockade, pretreatment with MMI should also be 
considered in patients at increased risk for complications. ATD 
should be paused around 1 week before and after 131I ther

be paused around 1 week before and after 131I therapy to 
avoid decreasing RAI efficacy.124,134 A pregnancy test should be 
obtained within 48 hours prior to RAI treatment in any woman 
with childbearing potential. Pregnancy and breastfeeding consti­
tute absolute contraindications to RAI therapy.
The physician administering RAI should provide written 
advice concerning radiation safety precautions following treat­
ment.124,134 National differences in radiation regulations have 
considerable imp

es in radiation regulations have 
considerable impact on the way RAI therapy is given in different 
countries (e.g., in the criteria for hospitalization). Attempts have 
been made to standardize the radiation delivered to the thyroid 
gland by varying the dose of radioiodine according to the size of 
the gland, the uptake of 131I, and its subsequent rate of release 
(dosimetry). However, no data support an advantage of dosimetry 
over a fixed-dose regimen. Most clinics have redefined the goal 
o

e regimen. Most clinics have redefined the goal 
of RAI therapy from making patients euthyroid to ablating the 
thyroid with a permanent need for thyroid hormone replacement. 
A dose of 20 mCi alleviates hyperthyroidism in almost all patients 
and results in approximately 90% hypothyroidism.
After RAI therapy, patients are seen at intervals of 4 to 6 weeks 
for 6 months, and FT4 and TSH levels are monitored. However, 
TSH is not a reliable guide as to when to begin levothyroxine 
replacement the

as to when to begin levothyroxine 
replacement therapy, as it can remain suppressed for a consid­
erable period of time even after the FT4 falls below normal. To 
avoid overt hypothyroidism, levothyroxine replacement therapy 
may be initiated when the FT4 reaches the lower portion of the 
reference range. The starting dose should be relatively low to avoid 
overtreatment given that the residual thyroid function will not 
be subject to feedback regulation. When hyperthyroidism persists 
after 6 m

gulation. When hyperthyroidism persists 
after 6 months, retreatment with RAI is suggested, generally with 
about 1.5 times the initial dose of 131I. Conception should be 
postponed until at least 6 months after RAI therapy, both in males 
and females.124 
Choice of Therapy
Advantages and disadvantages of each of the three treatment modal­
ities are listed in Table 12.6. Weighing the pros and cons of each 
treatment by patients varies greatly. Clinical conditions leading to 
a preferred therapy 

inical conditions leading to 
a preferred therapy are summarized in Table 12.7. Patients with 
high likelihood of remission are ideal candidates for ATD therapy; 
application of the GREAT score might identify such patients.151 
70
s-TRAb (% inhibition of 125-I TSH binding)
60
50
40
30
20
10
0
0
Medication
Surgery
Radioiodine
1
2
3
Years
4
5
• Fig. 12.6  Course of TSH receptor antibodies after treatment of Graves 
hyperthyroidism with antithyroid drugs, thyroidectomy, or radioactive 
iodine. (Red

drugs, thyroidectomy, or radioactive 
iodine. (Redrawn from Leimberg P, Wallen G, Tallstedt L, et  al. TSH-
receptor autoimmunity in Graves’ disease after therapy with anti-thyroid 
drugs, surgery, or radioiodine: a 5-year prospective randomized study. Eur 
J Endocrinol. 2008;158:69–75.)
  Advantages and Disadvantages 
of Treatment Options for Graves 
Treatment Option
Antithyroid drugs
Chance of permanent 
remission (∼35%)
Side effects of ATDs
Long durat

Long duration (12–18 
months)
High recurrence risk
Radioactive iodine
Simplicity
Low recurrence risk
Risk of orbitopathy
Lifelong LT4 needed
Possible small increase 
in cancer risk
Thyroidectomy
Rapidity
Almost no 
recurrences
Low but unavoidable 
morbidity
ATDs, Antithyroid drugs; LT4, l-thyroxine.
   
TABLE 12.6
380
ATD might also be preferred in patients with active Graves orbi­
topathy. Contraindications are m

tive Graves orbi­
topathy. Contraindications are major adverse reactions to ATD. 
RAI therapy is preferred in patients with contraindications for 
ATD or surgery, in case of recurrences after ATD, and in patients 
with hypokalemic periodic paralysis, pulmonary hypertension, or 
congestive heart failure. Contraindications for RAI are pregnancy 
or planning pregnancy within 6 months, lactation, and suspicion 
of thyroid cancer. Thyroidectomy might be preferred in women 
planning pregnancy within 6

be preferred in women 
planning pregnancy within 6 months, in large or compressive goi­
ters (>80 g), when thyroid malignancy is suspected, and in case 
of coexisting hyperparathyroidism. Thyroidectomy is best avoided 
in the first and third trimesters of pregnancy and in patients with 
increased surgical risk and substantial comorbidity. Long-term 
follow-up of patients randomized to receive one of the three treat­
ment options did not disclose differences in perceived quality-of-
life between 

differences in perceived quality-of-
life between treatments.178
The choice of the most appropriate treatment in a particular 
patient thus depends on a multitude of factors, including per­
sonal ones such as emotional attitudes, economic considerations, 
and family issues. It is obvious that a satisfactory choice can only 
be made in close consultation with the patient. One approach is 
to start with ATD in all patients. This would be useful also in 
patients who opt later for RAI or surgery. T

o in 
patients who opt later for RAI or surgery. The patient will become 
euthyroid in 4 to 6 weeks, and this period can be used to discuss 
with the patient what the optimal treatment option would be. 
This approach allows the establishment of a workable physician-
patient relationship, which is especially important in addressing 
anxieties about the use of radioiodine.
First-line treatment of uncomplicated Graves hyperthyroid­
ism in Europe is with ATD in 84%, with RAI in 14%, and with 
surger

with ATD in 84%, with RAI in 14%, and with 
surgery in 2%.172 In contrast, the first-line treatment in North 
America had been RAI in the vast majority of cases in the 20th 
century. Recently this has changed dramatically; as of 2014, ATD 
also is the preferred treatment option in the United States, fol­
lowed by RAI and then surgery179 (Fig. 12.7). 
Graves Orbitopathy
Graves orbitopathy is also known as Graves ophthalmopathy, 
thyroid-associated ophthalmopathy (TAO), and thyroid eye 
disease (T

 ophthalmopathy (TAO), and thyroid eye 
disease (TED). It is one of the phenotypes of Graves disease. 
Graves hyperthyroidism is present in about 90% of GO patients, 
whereas 5% to 10% of GO patients are euthyroid or hypothyroid 
(Fig. 12.8).180
Most patients initially notice a change in appearance. There is 
redness in the eyes or lids and swelling or feeling of fullness in 
the upper eyelids and/or bags under the eyes.181 The most com­
mon presenting sign is eyelid swelli

The most com­
mon presenting sign is eyelid swelling, followed by eyelid lagging 
behind eyeball movement on downgaze (von Graefe sign). Early 
symptoms are a gritty sensation in the eyes, light sensitivity (pho­
tophobia), and excess tearing. With ongoing disease, upper eyelid 
retraction occurs in most patients. Exophthalmos (also referred to 
as proptosis) develops in about 60% of patients. These patients are 
more likely to show incomplete eyelid closure (lagophthalmos); 
many such patients,

elid closure (lagophthalmos); 
many such patients, especially those with a wide palpebral fis­
sure, will show punctate inferior corneal staining with fluorescein. 
Patients may complain about retrobulbar pressure and blurred 
vision. Extraocular muscle involvement may result in aberrant 
position of the globe or, in extreme cases, fixation of the globe. 
More common is limitation of eye muscle movements in cer­
tain directions of gaze, especially in upward gaze. This may cause 
diplopia (double

y in upward gaze. This may cause 
diplopia (double vision), occurring in about 50% of patients. 
  Clinical Conditions That Favor a 
Particular Treatment Modality for 
Condition
ATD
RAI
Surgery
High chance of remission
+
Active Graves orbitopathy
+
Elderly with comorbidities
+
+
Increased surgical risk
+
+
Liver disease
+
Major adverse reactions ATD
+
Hypokalemic periodic paralysis
+
Pulmonary hypertension or 
congestive heart failure
+
Previous neck surgery or irradiatio

eart failure
+
Previous neck surgery or irradiation
+
Recurrent hyperthyroidism
+
+
Malignancy suspected
+
Large thyroid nodules
+
Coexistent hyperparathyroidism
+
ATD, Antithyroid drug; RAI, radioactive iodine
Modified from Ross DS, Burch HB, Cooper DS, et al. 2016 American Thyroid Association 
guidelines for diagnosis and management of hyperthyroidism and other causes of thyrotoxi­
cosis. Thyroid. 2016;26:1343–1421.
   
TABLE 12.7
80
Percentage of patients treated (%)
70
60
50
40
30
20
10
0
20

 of patients treated (%)
70
60
50
40
30
20
10
0
2005 2006 2007 2008 2009 2010 2011 2012 2013 2014
Thionamides
Surgery
Graves’ disease:
Treatment choice by year
• Fig. 12.7  Antithyroid drugs have become the most common treatment 
modality for Graves hyperthyroidism in the last decade in the United 
States. (Redrawn from Brito JP, Schilz S, Singh Ospina N, et al. Antithyroid 
drugs—the most common treatment for Graves’ disease in the United 
States: a nationwide population-based study

nited 
States: a nationwide population-based study. Thyroid. 2016;26:1144–
1145.)
381
Diplopia will not occur if the vision of one eye is very low (such 
as in amblyopia) or if the impairment of muscle motility is strictly 
symmetrical. Impairment of elevation and of abduction are most 
common, related to swollen inferior and medial rectus muscles, 
respectively, whereas impairment of depression and adduction are 
less frequent. Corneal ulceration nowadays is r

e 
less frequent. Corneal ulceration nowadays is rare. It develops only 
when normal corneal protection is lost.181 This may happen in 
patients with lagophthalmos in whom the cornea remains visible 
when the eyelids are closed. Sight loss due to optic nerve involve­
ment (referred to as dysthyroid optic neuropathy [DON]) occurs 
in about 5%. Patients with DON may complain about decrease of 
visual acuity, loss of color vision, visual field defects, and blurred 
vision.182 Visual blurring may di

ts, and blurred 
vision.182 Visual blurring may disappear after blinking (caused by 
alteration of the tear film on the surface of the cornea due to lacri­
mation or dry eyes) or after closing one eye (caused by eye muscle 
imbalance). Visual blurring that persists is of concern as it may 
indicate DON.
The average age of presentation is 49 years, a few years after the 
average age of presenting with Graves hyperthyroidism. Female 
preponderance is obvious, but GO is more severe in males and 
th

is obvious, but GO is more severe in males and 
the elderly. GO is a typical bilateral and rather symmetrical eye 
disease. Unilateral GO, however, occurs in about 10% of all GO 
patients. There is good evidence that the GO presentation is 
changing. Upon referral, GO patients in 2012 had less severe and 
less active disease than in 2000.183 
Epidemiology
In the general Swedish population the incidence rate of Graves 
hyperthyroidism is 210 per million per year, and the incidence 
rate of GO is 

illion per year, and the incidence 
rate of GO is 42 per million per year.184 It means 20% of patients 
with Graves hyperthyroidism also develop GO, which is mild in 
15% and more severe in 5%. The data are in good agreement 
with studies from Denmark and Italy, reporting that moderate-
to-severe GO occurs in about 5% of all patients with Graves 
hyperthyroidism.61,185 When Graves hyperthyroidism is diag­
nosed, 74% have no GO, but 13% in this group will develop 
GO (mild in 10% and moderate to 

oup will develop 
GO (mild in 10% and moderate to severe in 3%) during subse­
quent ATD therapy (Fig. 12.9). Mild GO at the time of diagnos­
ing hyperthyroidism disappears spontaneously in 58% of these 
cases.61 Thus GO is diagnosed at the same time as Graves hyper­
thyroidism in about 75% of GO patients, whereas GO is diag­
nosed later in about 25%.186 In a few patients GO onset is before 
that of Graves hyperthyroidism.
There seems to be a secular trend to a lower incidence of 
GO.187 Earlier 

lar trend to a lower incidence of 
GO.187 Earlier diagnosis and treatment of hyperthyroidism, iden­
tification of risks conferred by 131I therapy and postradioiodine 
hypothyroidism, and focus on the detrimental effects of smoking 
have likely contributed to this decline.183,186 
Pathology
The pathologic anatomy of GO orbits is characterized by enlarge­
ment of the extraocular muscles and the retrobulbar fat/connec­
tive tissue compartment. The increased volumes of muscles and 
fat are attribute

ncreased volumes of muscles and 
fat are attributed to an increase of the ground substance consisting 
of collagen and glycosaminoglycans (GAGs). GAGs (mainly hyal­
uronate) are very hydrophilic and thus attract much water, result­
ing in edematous swelling. The ground substance accumulates in 
the endomysial space between muscle fibers. The number of mus­
cle fibers is not increased, and there is no damage to the muscle 
cells except in very advanced cases. The number of fibroblasts is 
increas

anced cases. The number of fibroblasts is 
increased in the endomysial space and in the fat/connective tissue 
compartment. The orbital fibroblasts (OFs) are responsible for the 
excessive production of GAGs. A subset of OFs may differentiate 
into mature adipocytes thereby adding to the volume expansion. 
OFs have been recognized as the target of the autoimmune attack 
in GO.
The increased muscle and fat volumes within the confined 
space of the bony orbit allow a mechanistic explanation of the

 bony orbit allow a mechanistic explanation of the 
A
B
• Fig. 12.8  Patient with Graves orbitopathy. Note the typical bilateral rather symmetrical eye disease with 
periorbital swelling, stare, and exophthalmos (A), subsequently corrected by orbital decompression sur­
gery (B). (Courtesy Dr. Jack Rootman, University of British Columbia, Vancouver, BC, Canada.)
382
eye changes. The swollen retrobulbar tissues will impair venous 
drainage of eyelids and conjunctiva, resulti

nous 
drainage of eyelids and conjunctiva, resulting in eyelid edema 
and chemosis. Eyelid swelling can also be caused by herniation of 
retrobulbar fat through openings in the orbital septum. Increased 
retrobulbar pressure will push the globe forward, resulting in 
exophthalmos. Upper eyelid retraction and proptosis contrib­
ute to overexposure of the cornea, which may become dry and 
inflamed. The enlargement of extraocular muscles impairs muscle 
relaxation, not the ability for muscle contra

cle 
relaxation, not the ability for muscle contraction. For instance, 
impairment of elevation is caused by insufficient relaxation of the 
rectus inferior muscle, which may cause diplopia when gazing 
up. Marked swelling of rectus muscles in the apex of the orbit 
(known as apical crowding), close to the entrance of the optic 
nerve in the optic canal, may compress the optic nerve, resulting 
in DON. 
Microscopy reveals orbital lymphocytic infiltration, edema, and 
fibrosis.

al lymphocytic infiltration, edema, and 
fibrosis. The lymphocytic infiltration is often focal and consists of T 
helper cells, cytotoxic T cells, many macrophages, and a few B cells. 
The infiltrating immunocompetent cells produce cytokines capable 
of remodeling orbital tissues. The cytokine profile in the early stages of 
GO is predominantly that of Th1 cells, whereas cytokines are mostly 
100
Percent of Patients
80
60
40
20
0
100
B
C
A
80
60
40
20
0
10

t of Patients
80
60
40
20
0
100
80
60
40
20
0
Absent
Mild
Prevalence of GO
Moderate-
to-severe
Absent
Mild
GO after 18 months
No GO at baseline (n = 194)
Mild GO at baseline (n = 43)
Moderate-
to-severe
Absent
Mild
Moderate-
to-severe
Sight-
threatening
87%
10%
3%
58%
74%
20%
5.8%
0.3%
40%
2%
• Fig. 12.9  (A) Prevalence of Graves orbitopathy (GO) in patients with newly diagnosed Graves hyperthy­
roidism and during subsequent treatment with antithyroid drugs

during subsequent treatment with antithyroid drugs in those with initially absent GO (B) or mild 
GO (C). (Modified from Tanda ML, Piantanida E, Liparulo L, et al. Prevalence of natural history of Graves’ 
orbitopathy in a large series of patients with newly diagnosed Graves’ hyperthyroidism seen in a single 
center. J Clin Endocrinol Metab. 2013;98:1443–1449.)
383
derived from Th2 cells in patients with a duration of GO for more 
than 2 years.188 The data sugg

ion of GO for more 
than 2 years.188 The data suggest GO is primarily a T-cell–mediated 
disease. The cytokines induce expression of immunomodulatory pro­
teins on endothelial cells and fibroblasts, including HLA-DR, heat 
shock protein 72, and several adhesion molecules. Cytokine-activated 
OF synthesize IL16 and RANTES (regulated on activation, normal 
T cell expressed and secreted, also known as CCL5) attracting more 
T cells to the orbit. Macrophages present antigen to T cells; activated 
T 

rophages present antigen to T cells; activated 
T cells may bind to OF, inducing hyaluronan synthesis, cytokines, 
COX2, and PGE2. OF are considered the target (and effector) cells 
of the autoimmune attack in GO. Retrobulbar T cells of GO patients 
recognize autologous OF (but not eye muscle extract) and proliferate 
in response to proteins from autologous OF (but not from orbital 
myoblasts); conversely, OF proliferate in response to autologous T 
cells dependent on MHC class II and CD40/CD40L

 T 
cells dependent on MHC class II and CD40/CD40L signaling.189
The TSH receptor is presently viewed as the major autoanti­
gen in GO. OF express full-length functional TSH receptors; the 
expression is more abundant in active than in inactive GO and is 
directly related to IL1β levels.190 Graves immunoglobulins as well as 
monoclonal TSHR-stimulating antibodies recognize TSHR on OF 
as evident from increased cAMP and hyaluronan production in cell 
cultures of OF exposed to these agents. OF not

ll 
cultures of OF exposed to these agents. OF not expressing THY1 
(present in orbital fat but not in extraocular muscles) may under 
the influence of IL1 or PPARγ agonists differentiate into mature 
adipocytes, associated with increased TSHR expression. The role of 
TSHR is underscored by a direct relationship between TSHRAb 
and both activity and severity of GO.191 Genetic immunization 
against the TSH receptor (but not against the IGF1 receptor) also 
gives rise to a fair although not perfec

or) also 
gives rise to a fair although not perfect animal model of GO.192,193
The IGF1 receptor (IGF1R) has been proposed as another 
major autoantigen in GO. IGF1Rs are indeed overexpressed in 
OF of GO patients. Graves IgG can induce hyaluronan produc­
tion in OF, effects that can be attenuated by IGF1R-blocking anti­
body.194 Stimulating IGF1R autoantibodies (IGF1RAb) thus have 
been postulated and have been found in 10% of GO samples and 
in 10% of control samples; however, the IGF1RAb fail

 10% of control samples; however, the IGF1RAb failed to stim­
ulate IGF1R autophosphorylation but instead inhibited IGF1-
induced signaling.195 These data do not support the hypothesis 
that IGF1RAb contribute to GO pathogenesis. Immunoglobulins 
of GO patients stimulate Akt in OF as well as in cells expressing 
TSHR and IGF1R; knockdown of IGF1R causes a 65% decrease 
in IGF1-stimulated Akt but has no effect on GO-Ig stimulation 
of Akt.196 GO immunoglobulins thus do not activate the IGF1R, 
an

mmunoglobulins thus do not activate the IGF1R, 
and there is no evidence of stimulating IGF1R antibodies (Fig. 
12.10).197 The implication is that TSHRAb stimulate Akt via 
the PI3K pathway. The therapeutic effect of teprotumumab, an 
IGF1R-blocking monoclonal antibody, in GO thus could be 
explained at least in part by decreasing Akt, thereby also impairing 
the effects of TSHR signaling via the PI3K pathway.198
Whereas postreceptor signaling pathways of TSHR and IGF1R 
partly overlap, it has f

thways of TSHR and IGF1R 
partly overlap, it has further been suggested that TSHR and 
IGF1R form a physical and functional complex in OF199,200 and 
TSH receptor
Orbital fibroblast
AC
Gαs
Gαq
cAMP
PLC
P13K
Akt
mTOR
IRS-1/2
IGF-1
IGF-1
Adipogenesis
Orbital adipocytes
Expanded orbital muscles
and adipose tissue
IGF 1 receptor
Hyaluronic acid (HA)
synthesis
HA
• Fig. 12.10  Role of the thyroid-stimulating hormone (TSH) receptor in the immunopathogenesis of Graves 
orbitopat

tor in the immunopathogenesis of Graves 
orbitopathy. Ligation of the TSH receptor on orbital fibroblasts with stimulating TSH receptor antibodies results 
in activation of the adenylyl/cyclic adenosine monophosphate (cAMP) pathway and the phosphoinositide 
3-kinase (PI3K)/Akt signaling cascade. It induces hyaluronic acid production by the orbital fibroblasts, with a 
subset exhibiting enhanced adipogenesis. (From Iyer S, Bahn RS. Immunopathogenesis of Graves’ ophthal­
mopathy: the role of the T

sis of Graves’ ophthal­
mopathy: the role of the TSH receptor. Best Pract Res Clin Endocrinol Metab. 2012;26:281–289.)
384
that GO immunoglobulins exert their action via TSHR/IGF1R 
crosstalk rather than direct binding to IGF1R.201 A subset of OF 
are CD34+, and they might be derived from circulating CD34+ 
fibrocytes, which unexpectedly express relatively high levels of 
TSH receptors.202 The relevance of these findings is subject of 
active research. 
Genetics and Enviro

 subject of 
Genetics and Environment
No differences are found in the frequency of known susceptibil­
ity genes for Graves disease between patients with Graves hyper­
thyroidism and Graves orbitopathy. The genetic makeup in both 
phenotypes seems largely the same.203 Smoking is a risk factor for 
GO, as in Graves hyperthyroidism, but the odds ratio is much 
higher (OR 4.40, CI 2.88–6.73).101 The risk is dose dependent 
and decreases in ex-smokers. Exposure of OF in vitro to cig

ases in ex-smokers. Exposure of OF in vitro to cigarette 
smoke extract dose-dependently increases hyaluronan production 
and adipogenesis.204
Exposure to ionizing radiation in the form of RAI therapy 
is associated with a relative risk for progression of GO of 4.23 
(CI 2.04–8.77) when compared to ATD.205 The immediate and 
substantial rise of TSHRAb after 131I therapy might be causally 
connected to the worsening of GO.80 Risk factors for develop­
ing GO in Graves hyperthyroidism were evaluate

op­
ing GO in Graves hyperthyroidism were evaluated in a large 
longitudinal cohort study from Australia.206 The odds of GO 
increased by 17% for each decade increase in the age of onset 
of Graves hyperthyroidism and by 7% for each year increase 
in the duration of Graves hyperthyroidism. Smoking increased 
the odds for GO to 2.22 for current smokers and 2.07 for ex-
smokers compared with never smokers. The odds for GO were 
86% less in patients using ATD than those not (OR 0.14, CI 
0.06–0.34)

 using ATD than those not (OR 0.14, CI 
0.06–0.34). Risk factors for the development of GO in Graves 
hyperthyroid patients treated for 18 months with ATD were 
current smoking, mild conjunctival redness or mild eyelid swell­
ing, duration of hyperthyroid symptoms, and TBII.186 These 
items, assessed before starting ATD, can be combined in a quan­
titative manner in a predictive score (called PREDIGO), which 
is better in predicting those patients who will not develop GO 
than who will. Another 

s who will not develop GO 
than who will. Another longitudinal study used a large data­
base of a US managed care network.207 Of Graves hyperthyroid 
patients, 8.8% developed GO; surgical thyroidectomy alone or 
in combination with medical therapy was associated with a 74% 
decreased hazard for GO (HR 0.26, CI 0.12–0.51) compared 
with RAI therapy alone.207 Statin use also decreased the risk 
of GO (HR 0.60, CI 0.37–0.93), an unexpected and hard-to-
understand finding. 
Natural History and Progn

to-
GO has a tendency to spontaneous improvement. Early studies 
from Rundle in the 1940s and 1950s about the natural history 
of GO describe the initial development of upper eyelid retrac­
tion, exophthalmos, and restricted eye muscle motility, followed 
by spontaneous but incomplete recovery.208 Thus in about 60% 
of patients some eye changes persist. In GO patients who did 
not require immediate treatment, a 1-year follow-up revealed 
substan

te treatment, a 1-year follow-up revealed 
substantial improvement in 22%, slight improvement in 42%, 
no change in 22%, and worsening in 14%.209 The available data 
suggest there is initially an active stage of the disease character­
ized by inflammatory edema and lymphocytic infiltration; immu­
nosuppression given in this stage is likely to improve GO (Fig. 
12.11).210 The early active stage is followed by a late inactive 
stage of the disease characterized by fibrosis, with immunosup­
pressio

characterized by fibrosis, with immunosup­
pression less likely to be effective. The time interval between onset 
of GO and reaching the late inactive stage of the disease varies 
considerably between patients, from several months to several 
years. Assessment of GO activity may thus influence the treat­
ment plan. The prognosis of GO has improved substantially in 
the last few decades, thanks to earlier diagnosis and treatment, 
more attention to risk factors, and better organization of care. 

o risk factors, and better organization of care. 
Nowadays it is the rare patient who becomes blind as a result of 
GO. But the sequelae of GO are substantial: 45% of patients are 
restricted in their daily activities, 36% are on sick leave, 28% are 
disabled, 5% went into early retirement, and 3% lost their jobs, 
all because of GO.211 
Diagnosis of GO can be very easy in the case of a bilateral sym­
metric ophthalmopathy in a patient with Graves hyperthyroi

phthalmopathy in a patient with Graves hyperthyroidism. 
Diagnosis can be difficult in euthyroid or hypothyroid patients 
presenting with eye changes; in euthyroid or hypothyroid GO 
patients, GO is milder and more often asymmetric.180 Unilateral 
GO occurs in about 10% of all GO patients, whereas GO is the 
most prevalent cause of unilateral exophthalmos. Schematically, 
GO is diagnosed along three avenues: ocular symptoms and signs, 
thyroid autoimmunity, and orbital imaging.
Ocular Symptoms a

toimmunity, and orbital imaging.
Ocular Symptoms and Signs
It should be realized that none of the symptoms and signs is spe­
cific for GO. Guidelines recommend to assess severity and activity 
of the disease in every patient.134,212 GO severity is assessed using 
red = severity
blue = activity
100
A
75
50
25
0
%
Immunosuppression
Time
100
B
75
50
25
0
%
Time
• Fig. 12.11  Natural history of Graves orbitopathy, as reflected by a curve 
describing disease severity (red continuous

curve 
describing disease severity (red continuous line) and a curve describ­
ing disease activity (blue discontinuous line) over time. Intervention with 
immunosuppressive agents at peak activity is likely to result in modi­
fication of the natural course (gain reflected by the red stippled area) 
(A), whereas late intervention when the disease has become inactive, is 
unlikely to modify the natural course (B). (Modified from Wiersinga WM. 
Advances in medical therapy of thyroid-associated opht

nces in medical therapy of thyroid-associated ophthalmopathy. 
Orbit. 1996;15:177–186.)
385
the mnemonic NOSPECS (Table 12.8). Three degrees of severity 
are distinguished: mild GO (characterized by lid retraction <2 mm, 
mild soft tissue involvement, proptosis <3 mm above upper normal 
limit, absent or intermittent diplopia, absent corneal exposure, and 
normal optic nerve status), moderate-to-severe GO (lid retraction 
≥2 mm, moderate to severe soft tissue in

traction 
≥2 mm, moderate to severe soft tissue involvement, proptosis ≥3 
mm above upper normal limit, inconstant or constant diplopia, 
mild punctuate keratopathy, and normal optic nerve status), and 
very severe sight-threatening GO (corneal breakdown, or DON). 
Upper normal limits of proptosis are 23/24 mm in African Ameri­
cans, 19/21 mm in whites, and 16/17 mm in Asians (female/male 
values, respectively). Disease activity is best assessed by the clinical 
activity score (CAS), based on th

by the clinical 
activity score (CAS), based on the classic signs of inflammation (see 
Table 12.8).213 GO is likely to be inactive at CAS values below 3 
and active at CAS values of 3 or greater. 
Thyroid Autoimmunity
Besides assay of TSH and FT4 to determine thyroid function, assay 
of TPOAb and TSHRAb are useful to assess whether thyroid auto­
immunity is present. This is especially relevant in euthyroid and 
hypothyroid cases. Knowledge of the level of TSHRAb would any­
way be useful in view

e level of TSHRAb would any­
way be useful in view of their direct relationship with disease activ­
ity and severity and their prognostic value for the course of GO.214 
Orbital Imaging
Orbital imaging is not always necessary. It is indicated in patients 
with suspected DON (to look for apical crowding of enlarged 
muscles), in euthyroid or hypothyroid GO, and in unilateral GO 
to exclude alternative diagnoses. Orbital imaging is performed by 
CT or magnetic resonance imaging (MRI). Orbital CT s

 or magnetic resonance imaging (MRI). Orbital CT scanning 
is indicated before surgical orbital decompression to have a close 
look at the bony orbit (Fig. 12.12). GO activity might be assessed 
by MRI provided sophisticated programs are installed measuring 
T2-weighted relaxation time and/or short tau inversion recovery 
sequence. The most frequent diseases mimicking GO are orbital 
myositis, caroticocavernous fistula, non-Hodgkin lymphoma, 
orbital meningioma, and IgG4-related orbital disease.

ital meningioma, and IgG4-related orbital disease.215 
Treatment
Specific guidelines for the management of Graves orbitopathy are 
available.212 Recommendations are divided into three groups, deal­
ing with general measures, thyroid treatment, and eye treatment.
General Measures
It is recommended that GO patients—except the mildest cases—
should be referred to combined thyroid-eye clinics or specialized 
centers providing endocrinologic and ophthalmologic expertise. 
A multidisciplinary approach

almologic expertise. 
A multidisciplinary approach is likely to improve outcomes.216 A 
patient-focused approach to the treatment is also advocated, which 
encompasses the effects of the disease and its treatment on quality 
of life (QoL) and psychosocial well-being. Use of the GO-QoL, a 
A
B
• Fig. 12.12  Axial (A) and coronal (B) CT scans of a patient with Graves 
orbitopathy demonstrating generalized enlargement of all extraocular 
muscles, expansion of the orbital fat, and marked bilateral p

up gaze, side gaze, or down 
gaze
1
1
Redness
Redness of the eyelids
Redness of the conjunctiva
1
1
Swelling
Swelling of the eyelids
Swelling of the caruncle and/or plica
Chemosis
1
1
1
Maximum CAS score (assessed momently)
7
Impaired func­
tion
Increase in proptosis ≥2 mm in 1–3 
months
Decrease of ≥8⁰ in eye muscle motil­
ity in any direction in 1–3 months
Decrease in visual acuity of more than 
one line on the Snellen chart (using 
pinhole) in 1–3 months
1
1
1
Maximum CAS score (assessed over

 1–3 months
1
1
1
Maximum CAS score (assessed over time)
10
aColor atlas in Dickinson AJ, Perros P. Controversies in the clinical evaluation of active thyroid-
associated orbitopathy: use of a detailed protocol with comparative photographs for objective 
assessment. Clin Endocrinol (Oxf). 2001;55:283–303.
bIntermittent diplopia = at awakening or when tired; inconstant diplopia = at extremes of 
gaze; constant diplopia = in primary or reading position.
   
TABLE 12.8
386
di

tion.
   
TABLE 12.8
386
disease-specific QoL questionnaire and well-validated tool, is rec­
ommended in routine clinical practice and is available in several 
languages (www.eugogo.eu).212,217 It consists of eight questions 
about visual functioning and eight questions about appearance. 
It may serve as the primary outcome measure in clinical trials. 
Treatment should not only work from the physician’s perspective 
but also help from the patient’s perspective. It can also

o help from the patient’s perspective. It can also be useful in 
daily clinical practice to facilitate discussion with the patient on 
the most disturbing features and to identify patients in need of 
further counseling. Counseling may reduce anxiety, provide reas­
surance, and help in developing better coping strategies.
Smokers should be urged to refrain from smoking, and specialized 
smoking cessation programs or clinics should be offered. The success 
rate of quitting smoking is rather low, 

 success 
rate of quitting smoking is rather low, but success rate could be some­
what higher in GO because many patients are very much concerned 
about their appearance. Patients should be confronted with the evi­
dence that (a) smokers have more severe GO, (b) smokers are more 
likely to have progression of GO after 131I therapy, and (c) smoking 
delays or worsens the outcome of immunosuppressive treatment.
Simple measures that may be helpful in any stage of the dis­
ease are (1) artificial te

n any stage of the dis­
ease are (1) artificial tears to reduce surface symptoms and protect 
the epithelium, (2) sunglasses to reduce photophobia but also to 
comfort patients who are self-conscious of their appearance, (3) 
lubricant ointments to protect from exposure keratopathy dur­
ing sleep, (4) prisms to improve diplopia, and (5) botulinum 
toxin A injections to possibly provide temporary control of eyelid 
retraction. 
Thyroid Treatment
Euthyroidism should be promptly restored and mainta

uthyroidism should be promptly restored and maintained. 
Antithyroid drugs or thyroidectomy is preferred as neither mod­
ifies the natural history of GO, whereas ample evidence indi­
cates 131I therapy carries a risk of developing or worsening of 
GO. The risk can be reduced by concomitant oral prednisone 
in a daily dose of 0.3 to 0.5 mg/kg for 3 months. This steroid 
prophylaxis should be considered in high-risk patients (i.e., in 
smokers, in active GO, and when TSHRAb are high). Lower 
doses

active GO, and when TSHRAb are high). Lower 
doses (0.2 mg/kg prednisone for 6 weeks) can be used in low-
risk patients. Patients with inactive GO can safely receive RAI 
without steroid cover as long as post-radioiodine hypothyroidism 
is avoided.218,219 Some experts prefer very long-term ATD treat­
ment until GO has become inactive and no further treatment 
of GO is required; if a relapse of Graves hyperthyroidism occurs 
after discontinuation of ATD, it can be safely managed with RAI 
without

of ATD, it can be safely managed with RAI 
without flare-up of GO.218 
Eye Treatment
Management depends on severity and activity of GO (Fig. 
12.13).212 Mild GO is best managed by a wait-and-see policy or 
selenium. Spontaneous improvement is to be expected in about 
All patients with GO
Moderate to severe
Sight threatening (DON)
• Restore euthyroidism
• Urge smoking cessation
• Local measures
• Refer to specialists except for 
mildest cases
Wait and see
Selenium
Stable and 
inactive
Progression

 and see
Selenium
Stable and 
inactive
i.v. GCs 
(1st choice)
Prompt 
decompression
Stable and 
inactive
Rehabilitative surgery
(if needed)
i.v. GCs if QoL is 
severely impaired
Poor response 
(2 weeks)
Active
Inactive
Inactive
Mild
i.v. GCs
Still active
i.v. GCs
• Fig. 12.13  Algorithm for the management of patients with Graves orbitopathy. (Redrawn from Bartalena 
L, Baldeschi L, Boboridis K, et al. The 2016 European Thyroid Association/European Group on Graves 
Orbitopathy guideli

tion/European Group on Graves 
Orbitopathy guidelines for the management of Graves’ orbitopathy. Eur Thyroid J. 2016;5:9–26.)
387
one-third of cases. Intervention with a 6-month course of sele­
nium (100 μg sodium selenite twice daily) improves QoL and eye 
manifestations (in 61% vs 36% in placebo) and prevents progres­
sion to more severe GO (which occurred in 7% vs 26% in the 
placebo group).220 These results were obtained in European coun­
tries in which sel

were obtained in European coun­
tries in which selenium intake is relatively low. It is not known 
if selenium supplementation is also effective in areas with suffi­
cient selenium intake. Steroids could be considered if the QoL is 
severely impaired.
Active moderate-to-severe GO qualifies for immunosuppres­
sion. Steroids are considered as first-line treatment; they are rather 
effective in reducing swelling and redness of eyelids and conjunc­
tiva and in improving diplopia, less so in reducing

iva and in improving diplopia, less so in reducing exophthalmos. 
Intravenous methylprednisolone pulses (IVMP) rather than oral 
prednisone are recommended because IVMP have greater effi­
cacy than oral prednisone (74% vs 51%) and fewer side effects 
(56% vs 81%).212 A dose-finding study indicated a cumulative 
dose of 4.5 g IVMP is appropriate for most patients, administered 
as 500 mg IV once weekly for 6 weeks followed by 250 mg IV 
once weekly for another 6 weeks.221 Higher doses (750 mg onc

y for another 6 weeks.221 Higher doses (750 mg once 
weekly for 6 weeks followed by 500 mg once weekly for another 6 
weeks for a cumulative dose of 7.5 g) are slightly more effective at 
the expense of more side effects and should be reserved for worst 
cases. IVMP have been associated with significant cardiovascular 
or cerebrovascular morbidity and hepatic toxicity if administered 
in high single doses of 1000 mg or more, in cumulative doses of 
more than 8 g, and/or as repeat infusions on co

f 
more than 8 g, and/or as repeat infusions on consecutive days. It 
is therefore recommended that cumulative doses of IVMP should 
not exceed 8 g and that IVMP should not be given in patients 
with recent viral hepatitis, significant hepatic dysfunction, and 
severe cardiovascular morbidity or psychiatric disorders.212 Severe 
hypertension, inadequately managed diabetes, and glaucoma are 
other contraindications. IVMP therapy should be monitored 
by regular measurements of blood pressure, bloo

d 
by regular measurements of blood pressure, blood glucose, and 
liver function tests. Efficacy of IVMP can be slightly enhanced 
by coadministration of mycophenolate (one 360-mg tablet twice 
daily for 24 weeks).222
Patients who do not or partially respond to IVMP and patients 
in whom GO flares up after discontinuation of IVMP may require 
a second-line therapy. Shared decision making is recommended 
to select one of the available treatment options: low-dose oral 
prednisone + either cyclospo

tions: low-dose oral 
prednisone + either cyclosporine or retrobulbar irradiation (usu­
ally 20 Gy fractionated in 10 daily doses of 2 Gy over a 2-week 
period), repeat IVMP, or rituximab (RTX; 1 g twice at a 2-week 
interval). RTX is an anti-CD20 monoclonal antibody that effec­
tively causes B-cell depletion. Open studies report a remarkable 
improvement in CAS and GO severity.223 Side effects occur in 
about 30%; notable is the risk of the cytokine-release syndrome 
with the development of DON

kine-release syndrome 
with the development of DON. RTX has been tested in two ran­
domized clinical trials. One compared RTX with placebo: CAS 
decreased similarly in both treatment arms, and RTX was not 
better than placebo.224 The other compared RTX with IVMP 
(cumulative dose 7.5 g). The decrease in CAS was greater in the 
RTX group than in the placebo group, and RTX was judged to be 
slightly better than IVMP.225 Sample size in both trials was very 
modest, and some differences in patient c

as very 
modest, and some differences in patient characteristics and trial 
execution have been noted.226 Nevertheless, it remains difficult to 
reconcile the difference in outcomes. It seems too early to accept 
RTX as an alternative for IVMP but also too early to dismiss RTX 
as a disease-modifying drug. The same holds true for tocilizumab, 
a monoclonal antibody directed against the IL6 receptor. It had 
good efficacy in GO patients refractory to IVMP in an open study, 
but the drug has not b

 to IVMP in an open study, 
but the drug has not been tested in a RCT.227 Teprotumumab is 
a monoclonal antibody inhibitor of IGF1 receptors. In a placebo-
controlled RCT it had great efficacy in reducing CAS, reducing 
exophthalmos, and improving diplopia and GO-QoL scores.228 
Its potential to reduce exophthalmos is remarkable, and in this 
respect the drug could be superior to IVMP. However, to displace 
IVMP as first-line treatment, teprotumumab should be compared 
to IVMP in a RCT.
Very sev

mab should be compared 
Very severe GO, DON requires urgent intervention. It is recom­
mended to start with IVMP: 1 g IV on 3 consecutive days in the 
first week, followed by 1 g IV on 3 consecutive days in the second 
week. If visual functions improve by the end of the second week, 
continue with oral prednisone; if not, then urgent surgical decom­
pression is indicated.229
Inactive GO indicates rehabilitative surgery can be done when 
GO has reached this state. If surgery is 

ne when 
GO has reached this state. If surgery is performed while the dis­
ease is still active, results might be lost because of ongoing dis­
ease. Most orbital surgeons require stable eye disease for 6 months 
before surgery. Rehabilitative surgery includes orbital decompres­
sion, eye muscle surgery, and eyelid surgery and should be carried 
out in this sequence if several procedures are needed. 
Graves Dermopathy
Graves dermopathy is also known as pretibial myxedema or local 
myxedema. It is

wn as pretibial myxedema or local 
myxedema. It is a rather rare phenotype of Graves disease occur­
ring almost always in the presence of Graves orbitopathy and asso­
ciated with very high levels of TSHRAb. Thus the typical patient 
with Graves dermopathy has also Graves orbitopathy and Graves 
hyperthyroidism (sometimes also thyroid acropachy), constitut­
ing the most severe expression of Graves disease. The skin lesion 
consists usually of nonpitting edema with violet discoloration, 
induratio

itting edema with violet discoloration, 
induration, and prominent hair follicles, giving the appearance 
and texture of an orange peel (peau d’orange).230 Other forms are 
plaques, nodules, and elephantiasis (Fig. 12.14). Its predilection 
site is the pretibial area (hence the name pretibial myxedema), but 
it has been observed at other sites also exposed to local mechani­
cal pressure.231 Local trauma may provoke Graves dermopathy as 
well. Its pathogenesis looks similar to that of Graves orbi

 pathogenesis looks similar to that of Graves orbitopathy, 
which likewise develops in a confined space with increased local 
pressure. Dermal fibroblasts seem the target of the autoimmune 
attack. There is an upregulated expression of TSH receptors on 
dermal fibroblasts, which also produce cytokine-induced gly­
cosaminoglycans. Treatment is not always required. Spontane­
ous regression occurs in the long term. If treatment is necessary 
because of functional or cosmetic complaints, nighttime o

 of functional or cosmetic complaints, nighttime occlusive 
dressings with 0.05% to 0.1% triamcinolone acetonide in a cream 
base might be helpful as well as compressive bandages or stock­
ings. Treatment of GO with glucocorticoids or biologics may also 
cause regression of Graves dermopathy. 
Pregnancy and the Thyroid
Human Chorionic Gonadotropin
hCG is a glycoprotein heterodimer composed of an α-subunit that 
is common to all glycoprotein hormones (TSH, LH, and FSH) and 
a specific β-subunit t

nes (TSH, LH, and FSH) and 
a specific β-subunit that allows for biologic specificity. Despite this 
separate structure hCG can bind to and stimulate the hTSHR,232–
234 with an in vitro potency of about 1 U hCG = 0.7 μU of human 
TSH, depending on its carbohydrate content. In high concentra­
tions as found in hyperemesis gravidarum, multiple gestation, or 
molar pregnancy hCG will cause hyperthyroidism characterized by 
a diffuse goiter, elevated free T4, and a suppressed TSH.
388
SECTION III   

 free T4, and a suppressed TSH.
388
Transient Gestational Thyrotoxicosis
In the late first trimester of normal pregnancy in humans there 
is often a physiologic mild transient gestational thyrotoxicosis 
or hyperthyroidism.235,236 An exaggeration of this physiologic 
increase in thyroid stimulation in the first trimester may also be 
seen in some women and is associated with high levels of hCG 
(100,000–200,000 U/L), such as those found in twin pregnan­
cies, and is often 

s those found in twin pregnan­
cies, and is often accompanied by hyperemesis.237–239 In most 
patients, the condition is self-limited and the risk of birth defects 
warrants against the use of antithyroid drugs in early pregnancy. 
It may be difficult to separate this syndrome from early Graves 
disease, and a TSHRAb test may be helpful.240 
Abnormal Responses to Human Chorionic Gonadotropin
A few patients have been reported with an inherited variant of 
gestational thyrotoxicosis in which a mut

iant of 
gestational thyrotoxicosis in which a mutation in the TSHR gene 
resulted in a receptor protein with an increase in its responsive­
ness to hCG.241 Such patients develop hyperthyroidism with each 
pregnancy due to even physiologic serum hCG concentrations. 
Similarly, the use of gonadotropins in in vitro fertilization and 
indirectly the use of GnRH agonists have been associated with 
cases of thyroid dysfunction.242 
Graves Disease During Pregnancy and the 
Postpartum Period
Although s

ng Pregnancy and the 
Although seen more regularly in clinical practice, a truly overac­
tive thyroid gland is uncommon in established pregnancy, affect­
ing approximately 0.2% of women. This low rate is because 
pregnancy tends to suppress autoimmune responses during preg­
nancy, and Graves disease, an autoimmune disorder, is the most 
common cause of thyrotoxicosis in young women. Furthermore, 
although thyrotoxicosis has a variety of negative influences on fer­
tility itself

riety of negative influences on fer­
tility itself, it is also associated with increased pregnancy loss and 
serious medical complications for both the mother and the infant 
if it should persist.47,48,243–245 More commonly, a woman under 
treatment for hyperthyroidism becomes pregnant. Whatever the 
sequence, pregnancy complicates the diagnosis and treatment of 
hyperthyroidism in Graves disease and influences its severity and 
course.
Influence of Pregnancy on the Immune System
The development

 of Pregnancy on the Immune System
The development of pregnancy and the growth of the placenta 
have profound influences on the immune system, as discussed 
earlier. The overall suppression of autoimmune responses, which 
occurs in pregnancy and is mediated by a variety of placental fac­
tors, is designed to allow the fetus with its 50% paternal antigens 
to survive immune assault.246,247 These changes promote mater­
nal-fetal tolerance, but the increased role of the regulatory T cells 
and thei

increased role of the regulatory T cells 
and their suppression of maternal responses to the fetus appears 
to be predominant and long lived.248,249 It has been shown that 
a major shift in such T-cell control reduces the effectiveness of all 
inflammatory T cells. 
Thyroid Antibodies in Pregnant Patients With Graves 
Disease
The hallmark of the immune effects initiated by the placenta 
is the fall in thyroid autoantibody secretion—TPOAb, TgAb, 
and TSHRAbs—that is seen in almost all patients as

and TSHRAbs—that is seen in almost all patients as pregnancy 
progresses.250,251 This is now considered secondary to enhanced 
regulatory T-cell activity252 and precedes a rapid increase in 
autoantibody levels after the immunosuppression is lost in 
the postpartum period. Assays for TSHRAbs in the serum of 
pregnant women with Graves disease may be of clinical value 
in selected cases because a failure of this immunosuppression 
may indicate potential fetal problems.55,253 Because maternal 
ant

ential fetal problems.55,253 Because maternal 
antibodies cross the placenta, there is a correlation between the 
maternal level of stimulatory TSHRAbs and the development 
A
B
• Fig. 12.14  (A) Chronic pretibial myxedema in a patient with Graves disease and orbitopathy. The lesions 
are firm and nonpitting, with a clear edge to feel. (B) Here the chronic myxedema has continued to spread 
to the foot, causing severe disfiguration and immobility. (A, Courtesy Dr. Andrew Werner, Mount Sinai 
Schoo

(A, Courtesy Dr. Andrew Werner, Mount Sinai 
School of Medicine, New York, NY.)
389
of fetal thyrotoxicosis. Fortunately fetal and neonatal thyrotoxi­
cosis occurs in only 1% of infants of mothers with Graves dis­
ease, and high levels of TSHRAbs, usually greater than three 
times the upper normal limit, are correlated with fetal thyroid 
stimulation.254,255 Pregnant women at risk for failure to sup­
press thyroid autoantibodies include those with more severe 

id autoantibodies include those with more severe 
hyperthyroidism and those with significant GO or infiltrative 
dermopathy. In addition, the prior treatment of the mother, 
especially with radioiodine, may not always be accompanied 
by a sufficient reduction in TRAbs. Thus the fetus of a treated 
patient with Graves disease may still be at risk for development 
of fetal or neonatal thyrotoxicosis, and the mother may need 
antithyroid drug treatment and the fetus monitored by umbilic 
cord blood

ent and the fetus monitored by umbilic 
cord blood testing and ultrasonography.256 
When mild thyrotoxicosis is present during early pregnancy, it 
may be due to gestational thyrotoxicosis secondary to hCG stimu­
lation of the thyroid gland (see later).257,258 When it is more severe 
it is usually due to Graves disease because toxic multinodular goi­
ters and hot nodules are uncommon in this age group. 
Diagnosis
Pregnancy and hyperthyroidism are both accompanied by thyroi

and hyperthyroidism are both accompanied by thyroid 
stimulation, a hyperdynamic circulation, and hypermetabolism. 
In pregnancy, serum TBG levels are increased by estrogen-induced 
changes in glycosylation, which increases TBG production and 
lengthens its half-life, and thus in both conditions the total serum 
T4 and T3 levels are elevated so that the upper limit of the normal 
range during second and third trimesters of gestation is about 1.5 
times the upper nonpregnant reference limit.254 H

times the upper nonpregnant reference limit.254 However, serum 
free T4 levels, as measured by both analog and equilibrium dialy­
sis methodologies, may actually decrease as pregnancy progresses 
and the normal third trimester reference range for a given assay is 
significantly less than its nonpregnant reference range.236 Serum 
TSH levels also tend to decrease in early pregnancy and up to 15% 
of women can have TSH levels below the normal limit.235 The 
95th percentile confidence interval low

t.235 The 
95th percentile confidence interval lower limits for serum TSH 
levels are 0.06 mU/L, 0.3 mU/L, and 0.3 mU/L, respectively, for 
the first, second, and third trimesters. Nevertheless, each labora­
tory needs to establish its own normal ranges for thyroid testing 
in pregnancy.259,260
Biochemically, the diagnosis of thyrotoxicosis is confirmed 
when the serum TSH level is below the trimester-specific lower 
limit and the total or free T4 levels are above the normative range 
for pregna

4 levels are above the normative range 
for pregnancy (Fig. 12.15). Detection of TSHRAbs can confirm 
the diagnosis of Graves disease, which may or may not be obvious 
from the clinical history and examination. 
Treatment During Pregnancy
Hyperthyroidism in pregnancy is associated with a variety of com­
plications for mother and child (Table 12.9). Although mild hyper­
thyroidism in pregnancy does not lead to a significant increase in 
risk for mother or fetus, severe thyrotoxicosis can lead to 

other or fetus, severe thyrotoxicosis can lead to many 
complications and endanger the life of the mother and fetus. Fur­
thermore, the management of hyperthyroidism during pregnancy 
can be an even greater problem than its diagnosis. Graves disease 
can worsen in the first trimester, but the subsequent trimesters 
have an attenuating influence on the hyperthyroid state because of 
the immunosuppression associated with pregnancy. Pregnancy is 
also one of the few clinical situations in which an 

so one of the few clinical situations in which an assay of the bio­
logic activity of the TSHRAbs is helpful in predicting its potential 
effect on the newborn. This assay is especially useful in pregnant 
women who previously received ablative therapy for Graves hyper­
thyroidism and who still have high levels of TSHRAb.
Antithyroid Drugs in Pregnancy
Medical therapy is the usual method of choice in pregnancy. 
Because of the usual improvement in the disease, the dosage of 
antithyroid drug req

n the disease, the dosage of 
antithyroid drug required to control the disease in the later phases 
of pregnancy is generally much less than that required in the same 
patient were she not pregnant. Overtreatment of the hyperthyroid 
pregnant woman remains a common but avoidable clinical prob­
lem with potentially severe consequences for the fetus. Thus the 
clinician should prefer mild undertreatment to the risk of hypo­
thyroidism.256,261–263 Therefore it is imperative for the clinician to 
un

herefore it is imperative for the clinician to 
understand the therapeutic targets for titrating antithyroid drug 
dosages during pregnancy.264
Certain aspects of placental physiology are relevant to the use of 
antithyroid drugs. PTU and methimazole readily and rapidly cross 
the placenta equally well and are concentrated in the fetal thyroid. 
In excess quantity these agents can cause goitrous hypothyroidism 
in the fetus.265 Maternal T4 crosses the placenta (as evidenced by 
infants born with

s the placenta (as evidenced by 
infants born with significant circulating serum T4 concentrations 
despite congenital hypothyroidism) and is the major source of fetal 
thyroid hormone prior to the complete functional development of 
0
0.5
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 39 40
Weeks
TSH mu/mL
97.5th
90th
50th
10th
2.5th
• Fig. 12.15  Gestation age-specific thyroid-stimulating hormone nomo­
gram derived from 13,599 singleton and 132 twin pregn

m derived from 13,599 singleton and 132 twin pregnancies. Different 
percentiles are shown by colored lines. (From Dashe JS, Casey BM, Wells 
CE, et al. Thyroid-stimulating hormone in singleton and twin pregnancy: 
importance of gestational age-specific reference ranges. Obstet Gynecol. 
2005;106:753–757.)
  Complications of Hyperthyroidism in 
Pregnancy
Increased and recurrent pregnancy loss
Preterm delivery
Preeclampsia
Fetal growth restriction
Fetal thyroid hyperfunction or hypofunction caus

n
Fetal thyroid hyperfunction or hypofunction caused by TRAb
Fetal goiter from excessive antithyroid drug treatment
Neonatal thyrotoxicosis
Increased perinatal and maternal mortality risk
Potential for decreased IQ of offspring because of excessive use of 
antithyroid drugs
IQ, Intelligence quotient; TRAb, thyrotropin receptor antibodies.
   
TABLE 12.9
390
the hypothalamic-pituitary axis in the fetus at around 20 weeks 
of gestation. Furthermore, the transplacental passag

 gestation. Furthermore, the transplacental passage of maternal 
TSHRAbs in the latter half of pregnancy can result in fetal thyroid 
stimulation. Therefore the fetal thyroid is subject to the same fac­
tors that influence maternal thyroid hormone production.
Previously, the antithyroid drug of choice throughout pregnancy 
in the United States was PTU, but because of the rare yet serious 
side effect of PTU-induced hepatic failure, in June 2009 the FDA 
issued an advisory that PTU should be rese

he FDA 
issued an advisory that PTU should be reserved for the first trimester 
of pregnancy while organogenesis is occurring144 (see later). Subse­
quently, methimazole could be prescribed. One must keep in mind 
that the therapeutic antithyroid potency ratio of methimazole to 
PTU is around 20:1. Thus a patient who requires only 50 mg of 
PTU in the first trimester may be given 2.5 mg methimazole, and 
she may not even require a thionamide during the latter part of preg­
nancy. Of course, the 

ng the latter part of preg­
nancy. Of course, the best practice is to ensure definitive treatment 
prior to pregnancy so that antithyroid drugs are not needed at all. 
Danger of Antithyroid Drugs in Pregnancy
The first report of birth defects after maternal use of antithyroid 
drugs was a brief letter reporting on congenital scalp skin defects 
after the use of methimazole.266,267 Subsequently, several case 
reports confirmed the association, and other types of defects were 
described as well.26

 other types of defects were 
described as well.268 Such defects included a specific combina­
tion of facial features, and the condition was termed methimazole/
carbimazole embryopathy. Because PTU was not at first associated 
with such birth defects, guidelines have been proposed that recom­
mend the use of PTU in the first trimester and to consider shifting 
from methimazole to PTU in women planning a pregnancy.269,270 
Recent larger studies have since expanded our knowledge on 
methimazole-as

ve since expanded our knowledge on 
methimazole-associated birth defects.271,272 Apart from aplasia 
cutis, defects are seen in the abdominal wall (gastroschisis, ompha­
locele), gut (esophageal atresia), upper airways (choanal atresia), 
urinary system, heart (ventricular septum defect), and eye (Fig. 
12.16). Around 1 in 30 women exposed to methimazole or car­
bimazole in early pregnancy will give birth to children with defects 
associated with this therapy. This is in addition to the 5% gener

 this therapy. This is in addition to the 5% general 
population risk of giving birth to a child who has a birth defect 
diagnosed before the age of 2 years.273 However, PTU also is terato­
genic,274,275 and the use of PTU is also associated with birth defects 
(estimated to be around 1 in 40 exposed).272 These abnormalities 
tend to be milder than with methimazole-associated defects and 
include preauricular sinuses and cysts and urinary abnormalities.276
The period of risk when these drugs may

lities.276
The period of risk when these drugs may be teratogenic is 
especially weeks 6 to 10 of pregnancy.277 Accordingly, to reduce 
the risk of birth defects, withdrawal of antithyroid medication 
in patients who are considered in remission of Graves disease or 
an unavoidable shift from methimazole to PTU has to take place 
very early in pregnancy. Additionally, the lowest possible dose 
of an antithyroid drug must be used.278 The risk of birth defects 
has to be balanced against the risk o

rth defects 
has to be balanced against the risk of abnormal maternal thyroid 
function in early pregnancy. It is therefore wise to recommend to 
young women who receive antithyroid drugs for Graves disease 
that they test for pregnancy just a few days after a missing men­
struation and immediately contact their physician to plan future 
therapy or withdrawal of medication. No firm international con­
sensus has yet been reached on how to minimize the risk of birth 
defects from antithyroid drug 

 the risk of birth 
defects from antithyroid drug use in early pregnancy. 
Caring for the Pregnant Patient
The therapeutic target for a pregnant Graves patient is a healthy 
infant. In the second half of pregnancy both the untreated preg­
nant woman with hyperthyroidism caused by Graves disease and 
her fetus will be thyrotoxic because TSHRAb passes the placenta 
and stimulates the fetal thyroid. Antithyroid drugs given to the 
mother also pass through the placenta and thus treat both the 
mater

hrough the placenta and thus treat both the 
maternal and the fetal hyperthyroidism. However, the drugs may 
overtreat the fetus compared with the mother, and accordingly 
the aim of therapy is to keep the mother in a state of subclinical 
hyperthyroidism analogous to the normal gestational physiol­
ogy of the first trimester.264 This is associated with the lowest 
impact on fetal thyroid function and the highest rate of normal 
neonatal thyroid hormone levels.279 The maternal serum free T4 
lev

hormone levels.279 The maternal serum free T4 
level should be maintained at or just above the upper normal 
nonpregnant range, and no attempt should be made to normal­
ize the serum TSH concentration. Indeed, a normal TSH during 
drug therapy is an indication that the dose of drug should be 
reduced.280 The clinical status of the patient is an important indi­
cation for treatment or increases in dosage. A modest tachycardia 
is a physiologic response to the increased metabolic demands of 
pregn

ponse to the increased metabolic demands of 
pregnancy, and pulse rates of 90 to 100 beats/minute are well 
tolerated without evidence of myocardial decompensation dur­
ing delivery. The natural amelioration of Graves disease in the 
third trimester should be kept in mind, and repeated attempts 
should be made to reduce or discontinue the thionamide as the 
delivery date approaches to avoid TSH-induced fetal/newborn 
goiter, which may cause asphyxia (Fig. 12.17). The serum TSH 
concentration sho

xia (Fig. 12.17). The serum TSH 
concentration should be monitored monthly—more to avoid 
inadvertent overtreatment rather than as the target for normal­
ization. Because thionamides, but not thyroid hormone given to 
the mother, pass the placenta rather freely, a block-and-replace 
strategy may induce severe fetal hypothyroidism and goiter, and 
it is in general not appropriate in the pregnant patient. The rare 
exception is a pregnant woman who was previously given ablative 
therapy for Graves

 was previously given ablative 
therapy for Graves disease but who still produces TSHRAb that 
leads to isolated fetal hyperthyroidism.
All pregnant patients with significant Graves disease should 
be managed in close cooperation with obstetricians experienced 
with modern techniques for monitoring the fetus for intrauterine 
A
B
• Fig. 12.16  Examples of methimazole embryopathy. (A) Dysmorphic astig­
matism. (B) Aplasia cutis. (From Bowman P, Osborne NJ, Sturley R, et al. 
Carbimazole embryopat

borne NJ, Sturley R, et al. 
Carbimazole embryopathy: implications for the choice of antithyroid drugs 
in pregnancy. QJM. 2012;105:189–193.)
391
thyroid dysfunction. These techniques normally include fetal 
heart rate monitoring and ultrasonographic assessment of fetal 
growth rate. With advanced ultrasonography it is usually possible 
to examine the fetus for the presence of goiter. Fetal goiter can 
develop both from the stimulatory effect of maternal TSHRAb

oth from the stimulatory effect of maternal TSHRAb 
passing through the placenta and secondary to antithyroid drugs 
given to the mother. Occasionally, cordocentesis with fetal thyroid 
function testing may be appropriate. The amount of fetal monitor­
ing necessary can be judged based on the degree of hyperthyroid­
ism present, which is indicative of the potency of TRAb present 
and also by the increase in circulating TRAb levels in mothers 
previously treated for Graves. A recent systematic rev

iously treated for Graves. A recent systematic review dem­
onstrated that values of TRAb of over 3.7 times the upper limit 
of normal could be associated with fetal thyrotoxicosis.281 This 
is consistent with the American Thyroid Association guidelines 
where fetal monitoring is recommended in mothers who have 
TSHRAb levels greater than three times the upper limit of normal 
at any time during pregnancy.235 
Iodide and Beta Blockers
Obviously therapeutic radioiodine is contraindicated in preg­

erapeutic radioiodine is contraindicated in preg­
nancy, although no harm has been found after diagnostic doses of 
123I.282 Iodide itself should also not be used as therapy for more 
than 2 to 3 weeks in the pregnant woman because it readily crosses 
the placenta and can induce a large goiter that may cause airway 
obstruction in the newborn. Large amounts of iodide are contra­
indicated in the last month of pregnancy but can be used at earlier 
times in emergent situations. Whether propranolol

times in emergent situations. Whether propranolol or other beta 
blockers should be used in the pregnant woman with hyperthy­
roidism has been a matter of debate. In the experience of some, it 
can cause intrauterine growth retardation, delayed lung develop­
ment, and neonatal hypoglycemia or depression,283 but large stud­
ies have suggested that it can be used with safety for short periods 
or at very low doses.284,285 
Surgery
Surgery during the first and third trimesters is not desirable 
be

he first and third trimesters is not desirable 
because of the possible induction of early pregnancy loss and later 
premature labor, respectively. Surgery may be successful during 
the second trimester, but it is best to avoid major surgery during 
pregnancy if possible. Nevertheless, if antithyroid drug require­
ments are very high or cannot be used, surgery may be indicated. 
Iodide can be given for 7 to 10 days to aid in patient prepara­
tion with large and highly vascular thyroid glands. Im

 with large and highly vascular thyroid glands. Importantly, 
thyroid surgery may cure the hyperthyroidism of the mother, but 
TRAb will not disappear immediately. Thus the fetal thyroid may 
still be stimulated, and withdrawal of antithyroid drugs from the 
pregnant woman may lead to isolated fetal hyperthyroidism, espe­
cially in mothers with high levels of TSHRAb.286 
Consequences of Overtreatment
The influence of maternal hypothyroidism on fetal brain develop­
ment and the subsequently reduc

tal brain develop­
ment and the subsequently reduced IQ of the children of hypo­
thyroid mothers are discussed in Chapter 13. Needless to say, the 
overuse of antithyroid drugs in pregnancy may lead to the same 
consequences. There is considerable evidence that many pregnant 
patients with Graves disease were overtreated in the past as far as 
the fetus is concerned, as evidenced by transiently elevated serum 
TSH levels on newborn screening tests.287 This is another reason 
why one should accep

s.287 This is another reason 
why one should accept patients being slightly hyperthyroid rather 
than slightly hypothyroid. 
Graves Disease in the Postpartum Period
Changes in the Immune Response in the Postpartum Period
As discussed earlier, pregnancy induces a variety of immune 
changes that are responses to placental influences and the paternal 
foreign antigens and that are designed to prevent rejection of the 
foreign fetus. These changes include enhanced regulatory T-cell 
influences and a

clude enhanced regulatory T-cell 
influences and a T-cell shift from Th1 to Th2, resulting in an over­
all decrease in all autoimmune responses as evidenced by marked 
decreases in thyroid autoantibodies.251 Following delivery, these 
immune changes are slowly lost and a return to normal is observed 
but only after a period of exacerbated autoimmune reactivity in 
which large increases in T-cell and autoantibody activity occur. It 
is at this time—4 to 12 months postpartum—that new-onset or 
rec

e—4 to 12 months postpartum—that new-onset or 
recurrent thyrotoxicosis is seen. Such thyroid dysfunction may be 
transient or permanent. 
Transient Postpartum Thyroiditis
Transient postpartum thyroiditis remains the most common form 
of hyperthyroidism in the postpartum period and usually precedes 
a period of hypothyroidism.288 The transient thyrotoxicosis is due 
to thyroid cell destruction and may occur in approximately 5% to 
10% of patients during the 4 to 12 months postpartum.289 This 
ca

 during the 4 to 12 months postpartum.289 This 
can be double in those with other autoimmune diseases. How­
ever, the rapid return of true Graves hyperthyroidism is less com­
mon but is similarly dependent on the subsequent changes in the 
immune response. 
Presentation of Postpartum Graves Disease
A high percentage of women with Graves disease age 20 to 35 
years give a history of pregnancy in the 12 months before the 
onset of Graves disease.290,291 Pregnancy and the postpartum 
state also app

0,291 Pregnancy and the postpartum 
state also apparently influence the course of preexisting Graves 
disease. Patients in clinical remission during pregnancy are prone 
to postpartum relapse. In 41 pregnancies in 35 patients in remis­
sion, 78% were followed by development of thyrotoxicosis dur­
ing the postpartum period. The patients with Graves disease and 
postpartum thyrotoxicosis were classified into three categories: (1) 
Some had persistent recurrent hyperthyroidism with an elevated 
RAI

nt recurrent hyperthyroidism with an elevated 
RAIU (classic Graves disease). (2) Some had a transient disorder 
associated with a normal or an elevated RAIU (transient Graves 
• Fig. 12.17  Sagittal view of a fetus at 23.9 weeks showing a large goiter 
(arrow) preventing neck flexion. (From Mayor-Lynn KA, Rohrs JH III, Cruz 
AC, et al. Antenatal diagnosis and treatment of a dyshormonogenetic fetal 
goiter. J Ultrasound Med. 2009;28:67–71.)
392
disease). (3) Some patients,

ECTION III    Thyroid
disease). (3) Some patients, especially those with the highest titers 
of TPOAb, experienced transient thyrotoxicosis with a decreased 
RAIU (the thyrotoxic phase of postpartum thyroiditis referred 
to earlier). This phase in turn may be followed by a hypothyroid 
phase.292 
Preconception Counseling
A special problem related to hyperthyroidism and pregnancy is 
presented by the patient who wishes to conceive in the near future 
and is either in early remission after a cours

re 
and is either in early remission after a course of antithyroid drug 
treatment or is being treated with antithyroid agents for active 
Graves disease.293 For the first scenario, antithyroid drugs can be 
reluctantly reintroduced if required during pregnancy if symp­
tomatic thyrotoxicosis recurs. In the second situation, definitive 
therapy (radioiodine therapy or surgery) should be considered to 
forestall the complexities of managing hyperthyroidism during 
pregnancy. As with the therapy o

hyroidism during 
pregnancy. As with the therapy of Graves disease in general, such 
decisions must involve education of the patient so that the risks 
and benefits of the various alternatives are clearly appreciated. 
Fertile women who receive antithyroid drugs should be educated 
to perform pregnancy testing already within the first days after a 
missed menstrual period if pregnancy is possible, and if the test is 
positive they should immediately contact the physician for further 
planning of

ely contact the physician for further 
planning of therapy. The 1-year surge in TSHRAb after radioio­
dine therapy80,294 may increase the risk of fetal exposure to high 
TSHRAb levels if the woman is treated with radioiodine shortly 
before she becomes pregnant. In all cases where radioiodine ther­
apy is used, pregnancy should be delayed for at least 6 months 
and until normal thyroid function has returned on replacement 
therapy. 
Nursing and Antithyroid Drugs
Older studies suggested that rela

ntithyroid Drugs
Older studies suggested that relatively more methimazole than 
PTU appeared in breast milk of women receiving these drugs 
during lactation, but more recent evidence shows little differ­
ence between them.265,295,296 It is occasionally recommended that 
women who take high doses of antithyroid drugs not nurse their 
infants because of the difficulty in monitoring thyroid function in 
infants. The drug doses transferred via breast milk are very small, 
and no drug side effects ha

 milk are very small, 
and no drug side effects have been reported in neonates whose 
mothers were taking antithyroid drugs, including neurologic 
function.297 
Inherited Nonimmune Hyperthyroidism
Toxic diffuse thyroid hyperplasia without the pathologic char­
acteristics of autoimmune disease has been reported in families 
and appears to be inherited as an autosomal dominant condi­
tion.298–300 Polymorphic genomic mutations in the TSHR gene 
have been reported to cause constitutively activated T

 been reported to cause constitutively activated TSHRs 
differing from family to family.301 Recessive mutations on 
both chromosomes have also been described as causing hyper­
thyroidism while the parents remained euthyroid. These gain 
of function mutations, mostly in the transmembrane regions 
of the TSHR, are similar to those somatic mutations seen in 
toxic adenomas but are in the germline.302 Treatment is by 
radioiodine ablation or thyroidectomy, depending on the age 
of the patient. 
Toxi

ctomy, depending on the age 
Toxic Multinodular Goiter
Toxic multinodular goiter is a disorder in which hyperthyroidism 
arises in a multinodular goiter, usually of long standing, and is 
the result of one of several pathogenetic factors.303 Its incidence is 
highly dependent on the iodine intake of the population.
The pathogenesis of toxic multinodular goiter cannot be con­
sidered apart from that of its invariable forerunner, nontoxic 
multinodular goiter, from wh

forerunner, nontoxic 
multinodular goiter, from which it may slowly emerge. Two 
hallmarks of the disorder, structural and functional heterogene­
ity and functional autonomy, evolve over time; the increase in 
the extent of autonomous function causes the disease to move 
from the nontoxic to the toxic phase. Somatic mutations in the 
TSHR gene, first demonstrated in toxic adenomas,298 have been 
demonstrated in toxic multinodular goiter, and the individual 
mutations appear to differ from nodule

individual 
mutations appear to differ from nodule to nodule. However, 
only about 60% of toxic nodules have reported TSHR muta­
tions, and only a very few have G protein mutations. Hence 
there are many nodules with autonomy of undetermined 
cause304 and that presumably involve mutations in additional 
parts of the signaling pathways.
Radioiodine scans show localization of isotope in one or more 
discrete nodules, whereas iodine accumulation in the remainder 
of the gland is usually suppressed 

the remainder 
of the gland is usually suppressed because TSH is suppressed by 
the hyperthyroidism. However, the degree of hyperthyroidism 
can be variable and TSH may not be totally inhibited, so the 
background uptake of radioisotope may also be variable. His­
topathologically the functioning areas may resemble adenomas 
in being reasonably well demarcated from surrounding tissue. 
They generally consist of large follicles, sometimes with hyper­
plastic epithelium, but here, too, architecture

r­
plastic epithelium, but here, too, architecture correlates poorly 
with functional state. The remaining tissue appears inactive, 
and zones of degeneration are present in both functioning and 
nonfunctioning areas. Hence, from the pathophysiologic stand­
point, these thyroids harbor multiple solitary hyperfunction­
ing and hypofunctioning adenomas interspersed by suppressed 
normal thyroid tissue. 
The overproduction of thyroid hormone in toxic multinodular 
goiter is us

hyroid hormone in toxic multinodular 
goiter is usually less than that in Graves disease, and the disease 
presentation is milder (Fig. 12.18); in addition, toxic multinodu­
lar goiter usually occurs after the age of 50 in patients who have 
had nontoxic multinodular goiter for many years (Fig. 12.19). 
Like its forerunner, toxic multinodular goiter is more common 
in women than in men (6:1).305 Sometimes hyperthyroidism 
develops abruptly, often after exposure to increased quantities of 
iodine

 after exposure to increased quantities of 
iodine such as the contrast media for CT scanning, which permits 
autonomous foci to increase hormone secretion to excessive levels 
and which may simply exacerbate already established mild hyper­
thyroidism (see “Iodide-Induced Hyperthyroidism”). The serum 
T4 and T3 concentrations may be only marginally increased, and a 
suppressed TSH may be the major abnormality. The total RAIU is 
only slightly increased or within the normal range unless follow­
i

reased or within the normal range unless follow­
ing iodine exposure.
A toxic multinodular goiter may also be found as part of 
Graves disease as confirmed by the presence of TSHRAb of the 
stimulating variety.306 Presumably this represents two separate 
diseases, although TSHRAbs have growth-stimulating activity 
but this should apply to all cells. Toxic multinodular goiter 
alone is not accompanied by infiltrative ophthalmopathy, and 
when the two coexist it represents the emergence of Graves 

two coexist it represents the emergence of Graves 
disease.
393
Cardiovascular manifestations often predominate, possibly 
because of patient age, and include atrial fibrillation or tachycardia 
with or without heart failure. Weakness and wasting of muscles are 
common, the so-called apathetic or masked thyrotoxicosis. The ner­
vous manifestations are less prominent than in younger patients 
with thyrotoxicosis, but emotional lability may be pronounced, 
and ev

 but emotional lability may be pronounced, 
and even osteoporosis may be the factor leading to thyroid func­
tion testing and diagnosis. Because of the physical characteris­
tics of the thyroid gland and its frequent retrosternal extension, 
obstructive symptoms are more common than in Graves disease.
On palpation, the characteristics of the goiter are the same as 
those of the more common nontoxic multinodular goiter. In as 
many as 20% of elderly patients with thyrotoxicosis, the thyroid 
glan

ly patients with thyrotoxicosis, the thyroid 
gland is firm and irregular but not distinctly enlarged. A thyroid 
scan and ultrasound examination will confirm the diagnosis as 
toxic multinodular goiter rather than a single toxic adenoma or 
Graves disease. 
Laboratory Tests and Differential Diagnosis
All patients with a multinodular goiter should be screened annu­
ally with a serum TSH. If suppressed, the free T4 should be deter­
mined. Serum TSH levels intermediate between 0.1 and 0.4 
mU/L ar

H levels intermediate between 0.1 and 0.4 
mU/L are not usually associated with significant clinical symp­
toms. Such patients have thyroid autonomy but are not thyrotoxic 
(see “Subclinical Hyperthyroidism”). For patients with established 
thyrotoxicosis, an RAIU with scan will help in gauging the dose of 
131I to be administered as well as identify the autonomously func­
tioning nodules. The latter can then be followed by 131I therapy. 
Treatment
The treatment of toxic multinodular goiter with

nt
The treatment of toxic multinodular goiter with overt hyperthy­
roidism can be approached with surgery, radioactive iodine ther­
apy, or in some cases antithyroid drug therapy. The correct choice 
depends upon a combination of patient preference with associated 
risk factors. For example, a gland with significantly high radioac­
tive iodine uptake in select nodules would be ideal for radioiodine 
therapy. In contrast, a large gland with compressive symptoms 
could be better addressed with sur

ssive symptoms 
could be better addressed with surgery.134
Radioiodine Therapy
Radioiodine may be the treatment of choice for patients with 
toxic multinodular goiter despite disagreement about the size and 
number of doses required to achieve a therapeutic response.303,307 
In the United States, iodine intake is higher than in many regions 
2.5
A
B
C
3.5
3.0
4.0
4.5
5.0
5.5
Graves disease
150
Serum T4 (mmol/L)
Serum T3 (mmol/L)
Total T3 : T4 ratio (1000)
17

)
175
200
225
250
275
14
16
18
20
22
24
Manipulation
PPTD
PPTD
PPTD
STA
STA
STA
SAT
SAT
SAT
MNTG
MNTG
MNTG
Painless
Painless
Painless
Lithium
Lithium
Lithium
• Fig. 12.18  Serum T3 (A), T4 (B), and T3:T4 ratio (C) in 10 types of hyper­
thyroidism. Means (±SEM) are shown. MNTG, multinodular toxic goiter; 
PPTD, postpartum thyroid disease; SAT, subacute thyroiditis;

partum thyroid disease; SAT, subacute thyroiditis; SEM, stan­
dard error of the mean; STA, solitary toxic adenoma; T3, triiodothyronine; 
T4, thyroxine. (From Carlé A, Knudsen N, Pedersen IB, et al. Determinants 
of serum T4 and T3 at the time of diagnosis in nosological types of thyro­
toxicosis: a population-based study. Eur J Endocrinol. 2013;169:537–545.)
0
50
IR, per 100,000 py
100
150
Multinodular toxic goiter
Solitary toxic adenoma
200
250
0–9
10–19
20–29
30–39
40–49
50–59

adenoma
200
250
0–9
10–19
20–29
30–39
40–49
50–59
60–69
70–79
80+
• Fig. 12.19  An example of age-specific incidence rates (IR) of the three 
most common types of hyperthyroidism. py, person-years. (From Carlé 
A, Pedersen IB, Knudsen N, et al. Epidemiology of subtypes of hyper­
thyroidism in Denmark: a population-based study. Eur J Endocrinol. 
2011;164:801–809.)
394
in Europe so that 24-hour RAIU values of 20% to 30% are not 
unusual. Such patients will requi

 to 30% are not 
unusual. Such patients will require significant doses of radioiodine 
to restore a euthyroid state and may even require a second treatment.
Because a number of patients with this disorder have underly­
ing or the potential for heart disease, the administration of radio­
iodine should be preceded by a course of antithyroid therapy with 
methimazole until a near eumetabolic state is achieved, but still with 
a suppressed TSH to avoid iodine uptake in normal thyroid tissue. 
Medica

id iodine uptake in normal thyroid tissue. 
Medication is then discontinued for at least 4 to 7 days before radio­
iodine is administered. A week later the antithyroid drug may be 
reinstituted so that the thyrotoxicosis is controlled until radioiodine 
takes effect, which typically requires 3 to 4 months. A decrease in 
size of the hyperfunctioning nodules is a positive sign. At that time, 
the antithyroid drug can be tapered, but if the TSH level remains 
below 0.1 mU/L after 6 months, a secon

el remains 
below 0.1 mU/L after 6 months, a second dose may be required. 
Surgery
Surgical therapy is often recommended after adequate preopera­
tive preparation in patients with large goiters or obstructive mani­
festations. In these patients, a CT scan or MRI is recommended 
to define the extent of the goiter and the adequacy of the tracheal 
walls. Respiratory function studies may also be helpful in assess­
ing the need for surgery. Patients with fixed, especially partially 
retrosternal, go

with fixed, especially partially 
retrosternal, goiter should be considered for surgery because of 
the risk of more complete obstruction should hemorrhage into 
a nodule occur. However, when surgery is contraindicated, even 
significant obstructive symptoms can be relieved by adequate 
radioiodine therapy.308 In elderly patients who are not candidates 
for either radioiodine therapy or surgery, lifelong low-dose anti­
thyroid drug therapy remains an option. 
Additional Treatment Options
Outside

s an option. 
Outside of the United States there is greater experience with newer 
treatment options, including ethanol ablation and radiofrequency 
ablation. Success with these techniques in context of diminished 
nodule size and function is reported. However, their utility has not 
been broadly studied and they should likely only be used when the 
more standard modalities of radioactive iodine therapy, surgery, or 
antithyroid drugs are not possible or contraindica

antithyroid drugs are not possible or contraindicated.309,310 
Toxic Adenoma
A third, less common form of hyperthyroidism (∼5% of cases)305 
is caused by one or more autonomous adenomas of the thyroid 
gland. As herein used, the term toxic adenoma refers to a tumor 
in a thyroid that is otherwise intrinsically normal. The disorder is 
usually caused by a single adenoma that is either palpable or seen 
on ultrasound as a solitary nodule and hence is sometimes referred 
to as a hyperfunctioning so

is sometimes referred 
to as a hyperfunctioning solitary nodule or toxic nodule. Occasion­
ally two or three adenomas of similar character are present.
Toxic adenomas are true follicular adenomas (for histopathologic 
characteristics, see Chapter 14). The basic pathogenesis of many 
toxic adenomas (up to 70%) is one of several somatic point muta­
tions in the TSHR gene, commonly in the third transmembrane 
loop.301,311 These single nucleotide substitutions cause amino 
acid changes 

ucleotide substitutions cause amino 
acid changes that lead to constitutive activation of the TSHR 
in the absence of TSH.312 It appears therefore that the TSHR is 
allosterically switched from an off state to an on state. Similarly, 
loss-of-function rather than gain-of-function mutations may also 
occur in the TSHR gene and cause hypothyroidism (see later). A 
small number of autonomous adenomas have mutations in the G 
stimulatory protein genes downstream of the TSH receptor that 
lead to a s

s downstream of the TSH receptor that 
lead to a similar state of constitutive activation.304 More recently, 
mutations have also been identified in the EZH1 genes in toxic 
adenomas from patients who possess a TSH receptor activating 
mutation implying that a second hit may be necessary for full acti­
vation of the toxic adenoma phenotype.313 
The toxic adenoma often presents as a nodule in a patient with a 
suppressed TSH; on ultrasound it appears as a single hypoecho­
ge

 on ultrasound it appears as a single hypoecho­
genic nodule. A radioiodine thyroid scan shows a localized area of 
increased radioiodine accumulation (Fig. 12.20). This condition 
may occur at a younger age than toxic multinodular goiter and 
may be seen in patients in their 30s and 40s.
Frequently there is a history of a long-standing, slowly grow­
ing lump in the neck. It is unusual for adenomas to produce thy­
rotoxicosis until they have achieved a diameter of greater than 
3 cm303; up to th

eved a diameter of greater than 
3 cm303; up to that point, patients have subclinical hyperthyroid­
ism. The adenoma can undergo central necrosis and hemorrhage 
spontaneously, relieving the thyrotoxicosis, and the remainder of 
the thyroid may then resume its function. Calcification in the 
area of hemorrhage may take place and may be evident on sono­
gram examination. Such calcification is usually macroscopic and 
irregular and does not resemble the finely stippled calcification 
suggestive of

e the finely stippled calcification 
suggestive of papillary cancers. The peripheral clinical manifesta­
tions of a toxic adenoma are generally milder than those of Graves 
disease and are notable for the absence of infiltrative orbitopa­
thy and myopathy, although cardiovascular manifestations may 
occur. 
• Fig. 12.20  Radioiodine (123I) thyroid scan shows a hyperfunctioning hot 
nodule corresponding to physical examination findings with a faint outline 
of the remaining suppressed gland. In t

t outline 
of the remaining suppressed gland. In this unusual case, Graves disease 
developed a few months later after an oral contrast agent load. (From 
Soule J, Mayfield R. Graves’ disease after 131I therapy for toxic nodule. 
Thyroid. 2001;11:91–92.)
395
Laboratory Tests
The results of laboratory tests depend on the stage and function 
of the adenoma. At first, serum thyroid hormone concentrations 
are normal except for borderline suppression of the serum T

l except for borderline suppression of the serum TSH. 
This, together with ultrasound examination to exclude multiple 
nodules, confirms the diagnosis. Later a thyroid scan may show 
localization of radioisotope in the palpated nodule, but this is 
not obvious until TSH secretion is suppressed. If the nodule con­
tinues to grow, frank hyperthyroidism is accompanied by eleva­
tion of serum thyroid hormone levels. Occasionally the serum 
free T4 concentration is normal, and only the serum T3 level

ncentration is normal, and only the serum T3 level 
is increased (T3 thyrotoxicosis). Incidental thyroid carcinoma 
may rarely coexist within a gland exhibiting a hyperfunctioning 
adenoma. The incidence of carcinoma in these nodules is rare, 
but there are few clinical indicators of malignancy should it be 
present. Thus if the nodule is not treated surgically it is reason­
able to follow closely if any clinical or radiographic features of 
malignancy are seen.314 
Treatment
Although hyperfunct

gnancy are seen.314 
Treatment
Although hyperfunctioning adenomas may eventually cause clini­
cal hyperthyroidism, many do so slowly and others not at all.303 
Therefore treatment of asymptomatic patients with functional 
adenomas is decided on an individual basis. Clinically euthyroid 
subjects who wish to avoid both surgery and radioiodine can in 
theory be followed with annual assessments. However, suppres­
sion of TSH below normal (particularly to <0.1 mU/L) indicates 
hyperthyroidism, and t

ly to <0.1 mU/L) indicates 
hyperthyroidism, and therapy may be warranted. Two definitive 
therapies are available: radioiodine and surgery.
In terms of the specificity of treatment, functioning thyroid nod­
ules are candidates for radioiodine therapy. The radiation should 
in theory be directed almost exclusively to the diseased tissue. 
This is because TSH is suppressed and the normal thyroid tis­
sue surrounding the nodule does not take up excess radioiodine. 
However, thi

does not take up excess radioiodine. 
However, this suppression may be incomplete, and a significant 
fraction of patients receiving radioiodine develop thyroid failure. 
For the patient over age 18 with a nodule 5 cm in diameter or 
smaller, 131I is an appropriate treatment if the risk of eventual 
hypothyroidism is acceptable to the patient. For such lesions, 
doses of radioiodine are given sufficient to result in the presence 
of 300 to 370 MBq (8–10 mCi) in the nodule at 24 hours based 
on t

q (8–10 mCi) in the nodule at 24 hours based 
on the uptake.303 Clearly higher doses of RAI result in enhanced 
success in treatment of the hyperthyroidism but also lead to an 
increased incidence of hypothyroidism.316 Recombinant TSH, 
used to enhance the RAIU to avoid a large treatment dose in 
nontoxic multinodular goiters, is not appropriate here because it 
will direct radioiodine into surrounding normal thyroid tissue. 
The potential for hypothyroidism also indicates the need for pro­
long

pothyroidism also indicates the need for pro­
longed follow-up. 
Surgery
Toxic nodules are readily treated by surgical excision. A hemithy­
roidectomy may avoid the long-term development of hypothy­
roidism and with modern surgical procedures can be performed 
on an outpatient basis or even under local anesthesia. Surgical 
excision is always preferable in patients younger than 18 years of 
age to avoid the long-term consequences of irradiation, including 
effects on perinodular tissue. The toxi

including 
effects on perinodular tissue. The toxic adenoma is not diffusely 
hypervascular, and consequently preoperative preparation with 
iodine is not required. In the patient with overt thyrotoxicosis, 
however, a normal metabolic state should be restored with an anti­
thyroid drug before surgery. 
Subclinical Hyperthyroidism
The availability of sensitive assays for TSH allowed the recog­
nition of mild hyperthyroidism, in which there are no signs or 
symptoms of thyrotoxicosis b

here are no signs or 
symptoms of thyrotoxicosis but the serum TSH is subnormal 
despite normal serum free thyroid hormone concentrations. The 
term subclinical is somewhat of a misnomer because the condi­
tion is defined by biochemical characteristics, and it is preferable 
to use the term mild thyroid dysfunction.317 Nonetheless, it is still 
not clear whether a patient is classified as having mild hyperthy­
roidism simply because our ability to detect physiologic evidence 
of excess thyroid h

o detect physiologic evidence 
of excess thyroid hormone is less sensitive than our capacity to 
measure changes in TSH. This is further complicated by the fact 
that the hypothalamic-pituitary axis is sensitive to both serum 
free T4 and T3, whereas the peripheral tissues such as the heart 
primarily sense the free T3 (see Chapter 11).318,319 It is easy to 
assume, given the wide normal range for free thyroid hormone 
concentrations, that an individual with a low-normal free T4 set-
point for T

ividual with a low-normal free T4 set-
point for TSH secretion would have a reduced TSH if that con­
centration were increased by 50% but could still remain within 
the normal range. In fact, in patients with primary hypothyroid­
ism, small additional quantities of levothyroxine given to patients 
with normal TSH will decrease TSH below normal without a 
supranormal free T4.318 Thus the true lack of additional sensi­
tive biomarkers in the periphery makes it difficult to determine 
whether all t

ery makes it difficult to determine 
whether all tissues are in fact hyperthyroid when the TSH is 
suppressed.
On the other hand, in the now classic studies in the Framing­
ham population over 60 years of age, the cumulative incidence of 
atrial fibrillation over 10 years was 28% in patients with a serum 
TSH concentration of 0.1 mU/L or less, whereas it was only 11% 
in those with serum TSH concentrations falling between 0.1 and 
0.4 mU/L. The latter was only slightly higher than that in the 
n

atter was only slightly higher than that in the 
normal population.10,11 Similar results have been obtained in one 
other prospective study.11 Furthermore, heart failure is the leading 
cause of increased cardiovascular mortality rate in both overt and 
mild hyperthyroidism.320 Taken together these data support the 
notion that a suppressed TSH is indicative of hyperthyroidism in 
the heart.
Bone density is another end point for such studies because it is 
well known that thyroid hormone causes 

use it is 
well known that thyroid hormone causes a net resorption of corti­
cal bone321 and that lack of TSH may contribute to this phenom­
enon.34 Several studies demonstrate lower bone density in patients 
with mild thyrotoxicosis, although others do not.38,322 This is a 
subject of considerable interest because the condition is much 
more common than overt thyrotoxicosis (0.7% of the population 
in NHANES III) and has broad implications with respect to the 
cost of diagnosis, treatment, and 

respect to the 
cost of diagnosis, treatment, and follow-up.323 In general, nor­
malization of thyroid function in postmenopausal women with 
subclinical hyperthyroidism seems to improve bone density and 
certain aspects of cardiac function.324
Overall these data would generally favor treatment in the older 
population, but unfortunately there are still no large, long-term 
randomized studies to allow evidence-based conclusions as to the 
risk-benefit ratio.325,326 
396
Di

nefit ratio.325,326 
396
Diagnosis
The diagnosis of subclinical hyperthyroidism requires tests reveal­
ing several subnormal TSH concentration results spaced months 
apart in the presence of normal free T3 and T4 concentrations. 
Several studies show that suppressed TSH can normalize sponta­
neously over several years, particularly in patients without nodular 
goiter.327,328 As with overt thyrotoxicosis, there are two sources of 
excess thyroid hormones, endogenous and exo

es of 
excess thyroid hormones, endogenous and exogenous. In a study 
of over 25,000 individuals attending health fairs in Colorado, 58% 
of those with a TSH less than 0.3 mU/L were receiving thyroid 
hormones.329 When this is not being done intentionally for the 
treatment of persistent thyroid carcinoma, it is easily treated by 
more careful monitoring of the levothyroxine dosage using serum 
TSH concentrations. Endogenous subclinical thyrotoxicosis has 
the same causes as overt thyrotoxicosis

cosis has 
the same causes as overt thyrotoxicosis. In the population over 60, 
multinodular goiter is a more likely cause of hyperthyroidism than 
it is in younger individuals especially in the United States. 
Treatment
There are insufficient data to conclude that individuals with serum 
TSH concentrations greater than 0.1 mU/L due to hyperthyroid­
ism will benefit from treatment.134,325 However, treatment should 
be considered in those over age 65 with cardiac disease or osteopo­
rosis. There 

 65 with cardiac disease or osteopo­
rosis. There is increasing support for treatment of individuals with 
persistently subnormal TSH concentrations less than 0.1 mU/L 
(with normal free thyroid hormones) who are over age 65 and for 
those younger than age 65 with cardiac disease or risk factors for 
cardiac disease or significant risk for osteoporosis.134
Identifying the cause of the hyperthyroidism allows assessment 
of the potential risks of treatment. At one extreme, the treatment of 
mild G

reatment. At one extreme, the treatment of 
mild Graves disease with radioiodine usually causes hypothyroid­
ism, whereas this typically does not occur in patients with multi­
nodular toxic goiter. Thus, in an asymptomatic patient with mild 
Graves disease, watchful waiting for several years, awaiting a pos­
sible spontaneous remission, may be the best course of action330; 
if therapy is considered necessary, the chance of prolonged remis­
sion after a course of antithyroid drug therapy is good.

fter a course of antithyroid drug therapy is good. On the 
other hand, patients with subclinical hyperthyroidism due to toxic 
nodular goiter or a solitary hyperfunctioning adenoma can often 
be treated with a single dose of radioactive iodine with a relatively 
low risk of subsequent hypothyroidism. Thus the threshold for 
treatment of such patients is lower. As always, the rationale for 
treatment, its risks, and its benefits should be carefully discussed 
with the patient, and one should be g

y discussed 
with the patient, and one should be guided by common sense and 
not by the principle of simply treating an abnormal test result.331 
Induced Hyperthyroidism
The incidence of hyperthyroidism corresponds in general to popu­
lation iodine nutrition. Rates are lower in iodine-sufficient coun­
tries and higher in iodine-deficient countries mostly due to an 
excess of nodular thyroid disease in the elderly.332–334 In China, 
the prevalence of overt and subclinical hyperthyroidism was like

 of overt and subclinical hyperthyroidism was like­
wise higher in iodine-deficient areas compared with iodine-suffi­
cient areas.335 Iodine fortification in previously iodine-deficient 
regions will result in an adequate iodine intake but is associated 
with a transient increase in the incidence of hyperthyroidism 
(mainly due to toxic nodular goiter); in the long run, however, 
the incidence of hyperthyroidism may be lower than the initial 
rate due to a decrease in the prevalence of thyroid n

e due to a decrease in the prevalence of thyroid nodules in the 
population.99,333 Exposure to pharmacologic quantities of iodine 
may cause occasionally iodide-induced thyrotoxicosis (IIT), also 
called Jod-Basedow, more so in iodine-deficient areas than in 
iodine-sufficient areas.336,337 Elderly subjects with nodular thyroid 
disease or latent Graves disease (i.e., with autonomous thyroid 
function and low TSH) are at risk. Such patients with underlying 
thyroid disease may have normal or eve

underlying 
thyroid disease may have normal or even high thyroidal radioio­
dine uptake. In contrast, radioiodine uptake is low in IIT patients 
without preexistent thyroid disease, obviously caused by the cyto­
toxic effect of iodine excess on thyrocytes resulting in destructive 
thyroiditis. IIT usually resolves spontaneously in 6 months.
Many drugs contain huge quantities of iodine, such as expec­
torants, kelp tablets, and iodinated contrast agents. The risk for 
developing IIT is 0.3% in un

agents. The risk for 
developing IIT is 0.3% in unselected people after coronary angi­
ography in iodine-deficient areas.338 Treatment of patients who 
had thyroid autonomy before coronary angiography (with 20 mg 
thiamazole and/or 900 mg sodium perchlorate, starting the day 
before angiography and continued for 2 weeks) was not effective 
in preventing IIT.339,340 Close monitoring of high-risk patients 
rather than prophylaxis is thus recommended, with beta blockers 
if thyrotoxicosis occurs.34

d, with beta blockers 
if thyrotoxicosis occurs.341 
Amiodarone-Induced Thyrotoxicosis
Amiodarone is a potent class III antiarrhythmic drug. It has hypo­
thyroid-like effects such as bradycardia, reduction of myocardial 
oxygen consumption, lengthening of the cardiac action potential, 
and hypercholesterolemia342; these effects can be reversed by thy­
roid hormone. Its main metabolite desethylamiodarone acts as a 
competitive inhibitor of T3 binding to thyroid hormone recep­
tor α (TRα) and as a

ing to thyroid hormone recep­
tor α (TRα) and as a noncompetitive inhibitor of T3 binding to 
TRβ.343 One of the mechanisms of action of amiodarone might 
thus be induction of a hypothyroid-like condition in extrathyroi­
dal tissues. The drug contains 37% iodine by weight and generates 
iodine excess; one 200-mg tablet/day releases 6 mg of iodide daily. 
Amiodarone has obligatory and facultative effects on the thyroid. 
Obligatory effects occur in everyone taking the drug. They consist 
of a tra

n everyone taking the drug. They consist 
of a transient rise of serum TSH up to 5 to 10 mU/L, caused by 
the Wolff-Chaikoff effect induced by the iodine overload, usually 
followed by an escape of the Wolff-Chaikoff effect and sponta­
neous normalization of TSH in 3 months. Serum total and free 
T4 and reverse T3 increase, and total and free T3 decrease due to 
decreased metabolic clearance rate of T4 and inhibition of liver 
deiodinase type 1. Facultative effects of amiodarone develop in a 
su

Facultative effects of amiodarone develop in a 
subset of patients. Amiodarone-induced hypothyroidisim (AIH) 
is caused by a failure to escape from the Wolff-Chaikoff effect. 
It occurs especially in women with TPO antibodies and is more 
prevalent in iodine-sufficient regions. Amiodarone-induced thyro­
toxicosis (AIT), in contrast, is more prevalent in iodine-deficient 
regions. AIT type 1 is caused by iodine excess superimposed on 
preexistent thyroid disease such as Graves disease or nontoxic

thyroid disease such as Graves disease or nontoxic goi­
ter. AIT type 2 is due to the cytotoxic effect of amiodarone on 
thyrocytes resulting in transient destructive thyroiditis.344
Epidemiology and Screening
In a prospective study with a median follow-up of 3.3 years in 
an area with an average daily iodine intake of 150 μg, 8% of 
patients taking amiodarone developed AIT and 6% AIH.345 Inci­
dence rates of AIT were 1.9 and of AIH 1.6 per 100 person-years 
(Fig. 12.21). It is striking that no 

erson-years 
(Fig. 12.21). It is striking that no new cases of AIH developed 
after 2 years on amiodarone therapy, whereas new cases of AIT 
397
continued to occur with continuation of amiodarone.345,346 Thus 
one of every five to six patients taking amiodarone will develop 
overt thyroid dysfunction. It provides the rationale for the rec­
ommendation to measure TSH before starting amiodarone and 
at regular intervals during amiodarone.347 Limitations of regula

ervals during amiodarone.347 Limitations of regular 
tests are, however, twofold. First, AIT type 2 often has a sudden, 
unpredictable onset. It may happen shortly after a previous blood 
test found a perfectly normal TSH, which may thus give rise to a 
false sense of security. Second, a suppressed TSH might just indi­
cate subclinical hyperthyroidism, which spontaneously reverts to 
a normal TSH in 30% to 50% of cases despite continuation of 
amiodarone.346,348 AIT occurs without preceding subc

iodarone.346,348 AIT occurs without preceding subclinical AIT 
in 64% of patients, and only 25% of patients with subclinical AIT 
progress to overt AIT.348 AIT type 1 develops rather shortly after 
the start of amiodarone (median onset time is 3.5 months with 
just a single case occurring after 2.5 years), whereas the median 
onset time of AIT type 2 is 30 months.349 Nowadays most AIT 
patients have AIT type 2. An iodine-free analog of amiodarone has 
been developed, called dronedarone, to get r

 has 
been developed, called dronedarone, to get rid of the side effects 
caused by iodine excess. The pharmacologic properties of drone­
darone, however, do not match those of amiodarone, and drone­
darone has not displaced amiodarone in clinical practice. 
Diagnosis
About half of AIT patients experience complaints, mainly palpita­
tions but also weight loss and agitation.345 Biochemical diagnosis 
of AIT is by suppressed TSH and elevated FT4. FT3 can be nor­
mal, however, due to the drug’s inh

T3 can be nor­
mal, however, due to the drug’s inhibition of type I deiodinase, 
and cases of T4 toxicosis occur. Although AIT type 1 currently 
appears to be much less common than type 2, identifying the type 
may still be of value because the treatments of types 1 and 2 differ. 
Diagnostic features are listed in Table 12.10. Type 2 patients usu­
ally have no goiter and no thyroid antibodies. Most useful is color 
flow Doppler sonography that provides a noninvasive, real-time 
assessment of thy

ovides a noninvasive, real-time 
assessment of thyroid vascularity. 
Specific ETA guidelines recommend treatment with antithyroid 
drugs in AIT type 1344 (Fig. 12.22). The iodine-replete thyroid 
gland of AIT patients is less sensitive to methimazole, requiring 
rather high daily doses of 40 to 60 mg for longer periods than 
usual. To increase the response to methimazole, one may opt to 
coadminister sodium perchlorate (not available in the United 
States), which acutely inhibits furt

n the United 
States), which acutely inhibits further thyroidal iodine uptake. To 
restrict its toxic side effects, NaClO4 should be used no longer 
than 4 to 6 weeks in a daily dose not exceeding 1 g (e.g., 500 
mg twice daily). Amiodarone itself should be stopped, if feasi­
ble. Management is completely different in AIT type 2. Here the 
preferred drug is oral prednisone, in a daily dose of 30 mg.344 
Tapering down prednisone can be started when TSH has become 
normal, which takes about 3 mont

n TSH has become 
normal, which takes about 3 months. In view of the self-limiting 
character of AIT type 2, it is possible to continue amiodarone 
in these patients.350 Time to restoration of euthyroidism is little 
affected by continuation of amiodarone in type 2 patients.
Occasionally it is difficult to decide whether type 1 or type 2 
AIT exists, and mixed forms do occur. One may opt for triple ther­
apy (prednisone + methimazole + NaClO4) under such circum­
stances, also when the response t

der such circum­
stances, also when the response to instituted treatment is too slow. 
Some patients do not fare well, and guidelines recommend total 
thyroidectomy to be performed without delay in AIT patients with 
deterioration of cardiac function (reduced left ventricular ejection 
fraction) or severe underlying cardiac disease and in those patients 
whose thyrotoxicosis is unresponsive to medical therapies.344 
Prognosis
Patients with AIT have more major adverse cardiovascular 
events than 

ve more major adverse cardiovascular 
events than patients who remain euthyroid (31.6% vs. 10.7%, p 
< 0.01).351 Mortality is also higher in AIT patients with severe left 
ventricle dysfunction (31% at ejection fraction <50% vs 14% at 
ejection fraction ≥50%).352 Total thyroidectomy improves cardiac 
function and mortality, especially in those with ejection fraction 
below 40% preoperatively.353
Thyroid ablation is preferred before reintroduction of amioda­
rone after successful treatment of AIT

 of amioda­
rone after successful treatment of AIT type 1.354 Continuation of 
Amiodarone-
associated
hypothyroidism
1.00
0.95
0.90
0.85
0.80
Thyroid dysfunction-free survival
0
1
2
3
Time (years)
4
5
• Fig. 12.21  Kaplan-Meier curve depicting the incidence of amiodarone-
associated thyrotoxicosis and hypothyroidism. (Redrawn from Ahmed S, 
Van Gelder IC, Wiesfeld AC, et al. Determinants of outcome of amioda­
rone-associated dysfunction. Clin Endocrinol. 201

rone-associated dysfunction. Clin Endocrinol. 2011;75:388–394.)
  Differences Between Amiodarone-
Induced Thyrotoxicosis Type 1 and Type 2
AIT Type 1
AIT Type 2
Underlying thyroid 
abnormality
Yes
No
Onset after start amio­
darone
Short (3 months)
Long (30 months)
Thyroid antibodies
Present in Graves
Usually absent
Color flow Doppler 
sonography
High vascularity
Low/absent vascu­
larity
Thyroid radioiodine 
uptake
Low, normal, high
Suppressed
Preferred treatment
Oral predniso

referred treatment
Oral prednisone
Amiodarone continu­
ation
No
Possible
Spontaneous remission
No
Frequent
Subsequent hypothy­
roidism
No
Possible (17%)
Subsequent definitive 
treatment
Generally yes
No
AIT, Amiodarone-induced thyrotoxicosis.
   
TABLE 12.10
398
amiodarone after cure of AIT type 2 is feasible; some patients will 
experience recurrent AIT (6–18%), which apparently is less severe 
and more easy to handle.354,355 Permanent hypothyroidism dev

asy to handle.354,355 Permanent hypothyroidism devel­
ops in 17% of cured AIT type 2 patients, occurring at 10 months 
(range 6–24 months) after reaching euthyroidism.350,356 
Hyperthyroidism Due to Thyrotropin 
Secretion
Pituitary Tumor
Excess TSH is an exceedingly rare cause of hyperthyroidism. 
However, pituitary thyrotroph tumors cause this condition and 
may present as a Graves-like syndrome with diffuse goiter and 
substantial thyrotoxicosis. Only 1% of pituitary adenomas are 
TSH producin

s. Only 1% of pituitary adenomas are 
TSH producing and 25% may co-secrete growth hormone or pro­
lactin.357 Guidelines for the management of such patients have 
recently been released.358 Laboratory studies demonstrating an 
inappropriately detectable or somewhat elevated TSH in the pres­
ence of elevated thyroid hormone levels must first be confirmed 
by eliminating assay artifacts. This condition is discussed in depth 
in Chapter 9 and must be differentiated from the rare patient who 
has res

 differentiated from the rare patient who 
has resistance to thyroid hormone (RTH).359–361 
Thyroid Hormone Resistance
The syndromes of RTH are now known to be caused by inher­
ited mutations in both of the thyroid hormone receptor isoforms. 
RTHβ (due to mutations in the beta isoform) leads to a syndrome 
of inappropriate TSH secretion in the presence of elevated cir­
culating levels of thyroid hormones because the beta isoform is 
responsible for the regulation of TSH. Similarly, TRβ is highly

or the regulation of TSH. Similarly, TRβ is highly 
expressed in the liver, which is also functionally hypothyroid in 
RTHβ patients. However, in tissues that express primarily the 
TRα isoform, such as the heart and bone, tissue-specific hyper­
thyroidism may be present because these TRα expressing tissues 
sense the higher circulating thyroid hormone levels appropriately 
as high.359–361 These patients may therefore present with a hyper­
thyroid appearance with tachycardia, nervousness, and go

d appearance with tachycardia, nervousness, and goiter 
associated with an elevated free T4 and may require tissue-specific 
treatment such as with β-adrenergic receptor blocking agents 
rather than antithyroid drugs (see Chapter 13 for a more extensive 
discussion of RTH). RTHβ should be suspected in patients who 
present with hyperthyroidism and inappropriate TSH secretion 
without an apparent pituitary adenoma. Furthermore, a careful 
family history should be taken as the disorder is inherite

istory should be taken as the disorder is inherited in an 
autosomal dominant fashion. In contrast to RTHβ, RTHα due to 
mutations in TRα isoform leads to a syndrome of tissue-specific 
hypothyroidism because the TRα isoform is not involved in the 
regulation of the HPT axis and thus thyroid hormone levels are 
not elevated.362 
Tumor Chorionic Gonadotropin-Induced 
hCG exhibits specificity crossover with the TSHR (see ear­
lier discussion under “Pregnancy and the Thyroid”). Thyr

iscussion under “Pregnancy and the Thyroid”). Thyroid 
hyperfunction may therefore accompany hydatidiform mole, 
Suspected mixed/indefinite AIT
AIT 2
AIT 1
Stop amiodarone, if feasible
Amiodarone can be continued
± sodium perchlorate
Oral
± oral glucocorticoids
Add oral glucocorticoids
(if not given initially)
Poor response
Follow-up
Remission
Definitive thyroid treatment
with thyroidectomy or radioiodine
Euthyroidism
Emergency thyroid

tomy or radioiodine
Emergency thyroidectomy in selected cases
Amiodarone-induced thyrotoxicosis
(AIT)
• Fig. 12.22  Algorithm for the management of amiodarone-induced thyrotoxicosis. (Redrawn from ­Bartalena 
L, Bogazzi F, Chiovato L, et al. 2018 European Thyroid Association (ETA) guidelines for the management of 
amiodarone-associated thyroid dysfunction. Eur Thyroid J. 2018;7:55–66.)
399
choriocarcinoma, or metastatic embryonal carcinoma of the t

cinoma, or metastatic embryonal carcinoma of the tes­
tis.258 Such neoplasms, particularly hydatidiform mole, elaborate 
differentially glycosylated hCG molecules that exhibit crossover 
specificity for binding to the TSHR and can induce variable 
degrees of thyroid overactivity.232,363 Some patients have clinically 
overt thyrotoxicosis; however, clinical manifestations are usually 
not prominent, and goiter is absent or minimal. The free T4 and 
free T3 levels are increased, and TSH values are

free T3 levels are increased, and TSH values are suppressed. The 
possibility of a molar pregnancy should be considered in a young 
woman with hyperthyroidism and amenorrhea because the appro­
priate therapy is evacuation of the uterus. 
Transient Thyrotoxicosis
Overview
As mentioned at the outset of this chapter, transient thyrotoxi­
cosis must be differentiated from the sustained hyperthyroidism 
of Graves disease and other causes of hyperthyroidism. Transient 
thyrotoxicosis is caused by th

roidism. Transient 
thyrotoxicosis is caused by thyroid cell breakdown, and the hyper­
thyroid symptoms are of abrupt onset and shorter duration. This 
process may be followed by recovery of thyroid function or the 
development of transient or permanent thyroid failure. The discus­
sion in this chapter focuses on thyroiditis as the most common 
cause of transient thyrotoxicosis, and this disorder is covered more 
completely in Chapter 13 because Hashimoto disease most com­
monly causes hypothyro

Hashimoto disease most com­
monly causes hypothyroidism after the initial phase of transient 
hyperthyroidism. Unfortunately transient thyrotoxicosis continues 
to have a confusing nomenclature, which can be clarified as follows:
   
Autoimmune thyroiditis: In the autoimmune forms (Hashimoto 
thyroiditis) there are typically no local symptoms of thyroid 
inflammation, leading to the terms silent or painless thyroiditis, 
also referred to as lymphocytic thyroiditis or hashitoxicosis. This 
condit

ocytic thyroiditis or hashitoxicosis. This 
condition may uncommonly present with thyroid tenderness if 
the thyroid has expanded rapidly, stretching the capsule.
Viral thyroiditis: In what is thought to be postviral thyroiditis 
(also termed subacute, de Quervain, or granulomatous thyroid­
itis), thyroid tenderness may be the most prominent symptom, 
and thyrotoxicosis is rare and typically self-limited, although 
this form may rarely also be painless.
Acute thyroiditis: Acute thyroiditis due t

inless.
Acute thyroiditis: Acute thyroiditis due to bacterial or fungal in­
fections is only rarely accompanied by thyrotoxicosis, and the 
local symptoms predominate (see Chapter 13).
Drug-induced thyroiditis: Thyroiditis may also be drug induced, 
the principal offenders being amiodarone and lithium. Some of 
the new small molecule kinase inhibitors (such as sunitinib) may 
also cause this form of thyroiditis, resulting eventually in hypo­
thyroidism.364,365 In addition, the introduction of im

oidism.364,365 In addition, the introduction of immunother­
apy for cancer treatment, including CTLA4 inhibitors and PD1 
inhibitors, has dramatically increased the incidence of several 
endocrine toxicities, including transient hyperthyroidism.366 
Transient Thyrotoxicosis Due to Autoimmune 
(Hashimoto) Thyroiditis
As described earlier, Hashimoto disease causes two different thyro­
toxicosis-associated transient syndromes. The most common is the 
painless form in which the symptoms of thyrotoxi

painless form in which the symptoms of thyrotoxicosis are usually 
mild and predominate; the much more uncommon form has a 
painful presentation probably secondary to a more acute onset. 
Histopathologic examination in such patients with thyroiditis 
shows diffuse or local lymphocytic infiltration, varying degrees of 
fibrosis, and disruption of the follicular architecture (Fig. 12.23).
Transient Thyrotoxicosis From Painless Autoimmune 
Painless autoimmune thyroiditis may occur pos

itis
Painless autoimmune thyroiditis may occur postpartum or spon­
taneously. Postpartum thyroiditis is the most common example; 
its pathophysiology, postpartum enhancement of thyroid-directed 
autoimmunity (Hashimoto disease), is analogous to the postpar­
tum exacerbation of Graves disease (see “Graves Disease in the 
Postpartum Period”). The incidence of postpartum thyroiditis var­
ies but may occur in as many as 10% of women and in more than 
30% of those with positive TPOAb and even a large

30% of those with positive TPOAb and even a larger fraction in 
patients with type 1 diabetes mellitus.245,289 In women found to 
be TPOAb positive prenatally, postpartum assessment of thyroid 
function is recommended at 3, 6, and 12 months. Thyrotoxicosis 
from spontaneous autoimmune thyroiditis has all the same char­
acteristics as postpartum thyroiditis and is seen in patients early 
in their development of classic Hashimoto disease and before the 
onset of hypothyroidism. 
Transient Thyroto

e the 
Transient Thyrotoxicosis from Painful Autoimmune 
Although some patients may present with local thyroid tender­
ness, this occurrence is uncommon. Such tender episodes, which 
may be unilateral, may recur until the thyroid gland is completely 
destroyed by the disease process. Only rarely does the pain persist, 
sometimes requiring surgical intervention. 
Clinical Presentation of Transient Autoimmune 
More than 75% of patients are women

More than 75% of patients are women who present with the 
acute onset of symptoms of thyrotoxicosis, usually nervousness, 
palpitations, and irritability; they can often pinpoint the time of 
recent onset. In the postpartum syndrome, symptoms present 4 to 
12 months after delivery but may be mild and overlooked in the 
myriad of events involved in the care of the newborn.288 After 1 to 
• Fig. 12.23  Lymphocytic thyroiditis in a patient with transient thyrotoxico­
sis (painless th

tient with transient thyrotoxico­
sis (painless thyroiditis) secondary to autoimmune (Hashimoto) thyroiditis. 
Notice the diffuse lymphocytic invasion of the tissue, including the follicular 
epithelium, and the loss of follicles. Multinucleated giant cells may also 
be seen in the follicular lumen. (Courtesy Dr. Vania Nosé, Brigham and 
Women’s Hospital, Boston, MA.)
400
2 months the thyrotoxic symptoms fade but are often replaced by 
those suggesting hypothyroidism (Fig.

replaced by 
those suggesting hypothyroidism (Fig. 12.24).
In a significant number of postpartum patients the thyrotoxic 
phase is too mild to be noticed, and the patient presents somewhat 
later after delivery with hypothyroid symptoms. The physical 
examination shows mild signs of thyrotoxicosis, tachycardia being 
the most prominent, without the specific eye signs or dermopathy 
associated with Graves disease. The thyroid gland is normal in size 
but may be firm if the Hashimoto disease is ch

ze 
but may be firm if the Hashimoto disease is chronic. 
Diagnosis
Thyrotoxicosis is usually mild and is reflected in the degree of 
suppression of the serum TSH level and elevation of the serum 
free T4. Significant elevation of the TPOAb is typical. Systemic 
manifestations of inflammation are lacking, and the erythrocyte 
sedimentation rate is normal or nearly normal, but the ultra­
sound may indicate the heterogeneity of an inflamed gland. If 
true hyperthyroidism cannot be eliminated as a 

f 
true hyperthyroidism cannot be eliminated as a diagnosis on clini­
cal grounds, TRAb should be measured or an RAIU test should 
be performed unless the patient is nursing. The classic decreased 
RAIU is due partly to feedback suppression of TSH secretion but 
also to thyroid follicular cell destruction. The tendency of the dis­
order to pass through a hypothyroid phase is not surprising in 
view of the extensive depletion of Tg, which is processed to T4 and 
not replaced by the dysfunctional 

ssed to T4 and 
not replaced by the dysfunctional cells. 
Natural History
The duration of the thyrotoxic phase, typically not severe enough 
to require treatment, averages about 1 to 2 months. About one-
half of the patients return to a euthyroid phase and remain well in 
the short term. In the remaining half, a hypothyroid phase may 
follow and may last from 2 to 9 months. In most, there is even­
tual restoration of euthyroidism, but some develop permanent 
hypothyroidism years later.367,368 Ab

p permanent 
hypothyroidism years later.367,368 About one-third retain a goiter, 
usually with persistence of thyroid autoantibodies in the serum. 
The opposite sequela, recurrence of thyrotoxicosis, may also occur 
months or years after restoration of a euthyroid state or particu­
larly after pregnancy. 
Treatment
The thyrotoxic phase may require alleviation of the peripheral 
manifestations through the use of beta blockers. Prednisone 
(20–40 mg/day) may decrease the duration of the thyrotoxic

g/day) may decrease the duration of the thyrotoxic 
phase but is typically not needed except when the painful form 
of the disease is present. If mild and brief, the hypothyroid phase 
may also not require treatment. When treatment with levothyrox­
ine is required, it should be withdrawn slowly approximately 6 
months later because the hypothyroidism is often not permanent. 
Subacute Thyroiditis
Subacute thyroiditis (also termed granulomatous, giant cell, or de 
Quervain thyroiditis) is thought 

ant cell, or de 
Quervain thyroiditis) is thought to be caused directly or indirectly 
by a viral infection of the thyroid gland and often follows an upper 
respiratory illness. A tendency to appear in the spring in the northern 
latitudes has been noted, and again it predominates in the female. 
The mumps virus has been implicated in some cases; coxsackievirus, 
influenza virus, echovirus, and adenoviruses may also be etiologic 
agents. Positive TPOAb are present transiently during the active 

 TPOAb are present transiently during the active 
phase of the disease, although some patients may retain evidence of 
thyroid autoimmunity for many years. A small number of patients 
eventually develop AITD.369 Subacute thyroiditis is uncommon, 
but mild cases may be mistakenly diagnosed as pharyngitis.
Pathology
The histopathologic changes are different from those in Hashi­
moto disease. The lesions are patchy in distribution and vary in 
their stage of development from area to area. Affected 

 stage of development from area to area. Affected follicles 
are infiltrated predominantly with mononuclear cells and show 
disruption of epithelium, partial or complete loss of colloid, and 
fragmentation and duplication of the basement membrane (Fig. 
12.25). To this extent, the histopathologic appearance may resem­
ble that in Hashimoto disease. A characteristic feature is the well-
developed follicular lesion that consists of a central core of colloid 
surrounded by the multinucleated giant 

f colloid 
surrounded by the multinucleated giant cells (MNGCs), from 
which stems the designation giant cell thyroiditis. Colloid may be 
found in the interstitium or within the giant cells. The follicu­
lar changes progress to form granulomas. Interfollicular fibrosis 
and an interstitial inflammatory reaction are present to varying 
degrees. When the disease subsides, an essentially normal histo­
logic appearance is restored. 
Delivery
2
0
4
6
8
Time after delivery (months)
Thyroid function
G

8
Transient
Hashimoto thyroiditis
Transient
• Fig. 12.24  The postpartum thyroid syndromes. These potential patterns 
of thyroid dysfunction may be seen in the postpartum period.
• Fig. 12.25  Subacute (viral or postviral) thyroiditis. Diffuse neutrophilic 
invasion with active destruction of follicles and a multinucleated giant cell. 
Fibrosis and near-complete loss of follicles have occurred. (Courtesy Dr. 
Vania Nose

follicles have occurred. (Courtesy Dr. 
Vania Nose, Brigham and Women’s Hospital, Boston, MA.)
401
Apoptosis of follicular epithelium and loss of follicular integ­
rity are the primary events in the pathophysiology. Tg, T4, and 
iodinated Tg fragments are released into the circulation, often in 
quantities sufficient to elevate not just the serum Tg level but also 
the serum free T4 level, producing clinical thyrotoxicosis and sup­
pressing TSH 

ing clinical thyrotoxicosis and sup­
pressing TSH secretion. As a result, the RAIU decreases to low lev­
els, and hormone synthesis ceases. Later in the disease, when stores 
of preformed hormone are depleted, serum T4 and T3 concen­
trations decline, sometimes into the hypothyroid range, and the 
serum TSH level rises, often to elevated values exactly as occurs in 
silent thyroiditis (Fig. 12.26). As the disease becomes inactive, the 
RAIU may be greater than normal for a time as hormone stores

e greater than normal for a time as hormone stores 
are repleted. Ultimately, when hormone secretion resumes, serum 
T4 and T3 concentrations rise, and serum TSH concentration 
decreases to normal values. 
Clinical Picture
The characteristic feature is the gradual or sudden appearance of 
pain in the region of the thyroid gland with or without fever. 
The pain, which is aggravated by turning the head or swallow­
ing, characteristically radiates to the ear, jaw, or occiput and 
may mimic disorder

o the ear, jaw, or occiput and 
may mimic disorders arising in these areas. The absence of pain 
does not exclude the diagnosis, because biopsy-proven painless 
subacute thyroiditis occurs, but it must be distinguished from 
acute autoimmune thyroiditis. Hoarseness and dysphagia may 
be present; patients may complain of palpitation, nervousness, 
and lassitude. The latter symptoms can be extreme, considering 
the local nature of the disease, and suggest a systemic compo­
nent. Although acute man

suggest a systemic compo­
nent. Although acute manifestations are present in severe cases, 
in milder disease symptoms may be present for months yet are 
often overlooked.
On palpation at least part of the thyroid is slightly to moder­
ately enlarged, firm, often nodular, and usually exquisitely tender, 
one lobe frequently being more severely affected than the other. 
Indeed the symptoms may be truly unilateral. The overlying 
skin may be warm and erythematous. Occasionally the locus of 
maxima

nd erythematous. Occasionally the locus of 
maximal involvement migrates over the course of a few weeks to 
other parts of the gland. The disease usually subsides within a few 
months, leaving no residual deficiency of thyroid function in 90% 
of patients. In rare patients the disease smolders, with repeated 
exacerbations over many months, hypothyroidism sometimes 
being the final result. 
Diagnosis
The laboratory findings vary with the phase of the disease. During 
the active phase, the erythr

 the disease. During 
the active phase, the erythrocyte sedimentation rate is increased, 
often to a remarkable extent (>100 mm/hour). Indeed a diagnosis 
of active subacute thyroiditis is hardly tenable when the sedimen­
tation rate is normal. The white blood cell count is normal or, at 
most, moderately increased. The serum Tg level is characteristi­
cally high, in keeping with the degree of thyroid destruction.
Subacute thyroiditis must be differentiated from acute hemor­
rhagic degeneration 

ferentiated from acute hemor­
rhagic degeneration in a preexisting thyroid nodule, Hashimoto 
disease with painful recurrence (see earlier), acute pyogenic thy­
roiditis or fungal infection, and rarely thyroid malignancy with 
painful nodules. Acute painful exacerbations of Hashimoto thy­
roiditis may be difficult to distinguish from subacute thyroiditis. 
Lack of elevation of the erythrocyte sedimentation rate and high 
titers of thyroid autoantibodies strongly suggest the former. Acute 
pyogen

ibodies strongly suggest the former. Acute 
pyogenic thyroiditis is distinguished by the presence of a septic 
focus elsewhere, by a greater inflammatory reaction in the tissues 
adjacent to the thyroid, and by much greater leukocytic and febrile 
responses (see Chapter 13). The RAIU and thyroid function are 
16
14
12
10
8
6
4
2
FTI
600
500
400
300
200
100
50
40
30
20
10
0
40
20
10
8
6
4
2
0
10
20
30
40
50
60
70
150
Tg (ng/mL)
Normal
range
Days from the onset of symptoms
TSH (µU/mL)
• Fig. 12.26

rom the onset of symptoms
• Fig. 12.26  Thyroid function in a patient during the course of subacute (viral or postviral) thyroiditis. Dur­
ing the thyrotoxic phase (days 10–20), the serum thyroglobulin (Tg) concentration was greatly elevated, 
the free thyroxine index (FTI) was high, and the thyroid-stimulating hormone (TSH) was suppressed; the 
erythrocyte sedimentation rate was 86 mm/hour, and the thyroidal radioactive iodine uptake (RAIU) was 
2%. The Tg level and the FTI declined

 (RAIU) was 
2%. The Tg level and the FTI declined in parallel. During the phase of hypothyroidism (days 30–63), when 
the FTI was below normal, a modest transient increase in serum Tg occurred in parallel with the increase 
in serum TSH. All parameters of thyroid function were normal by day 150, 5 months after the onset of 
symptoms. (From DeGroot LJ, Larsen PR, Hennemann G. Acute and subacute thyroiditis. In: DeGroot LJ, 
Larsen PR, Hennemann G, eds. The Thyroid and Its Diseases. 6th ed. New Y

, eds. The Thyroid and Its Diseases. 6th ed. New York: Churchill Livingstone; 
1996:705.)
402
usually preserved in acute pyogenic thyroiditis. Rarely, widespread 
infiltrating cancer of the thyroid can present with a clinical and 
laboratory picture almost indistinguishable from that of subacute 
thyroiditis. Ultrasound and fine-needle aspiration should be per­
formed if this is a consideration. 
Treatment
In mild cases, aspirin or nonsteroidal antiinflammatory drugs or 
c

pirin or nonsteroidal antiinflammatory drugs or 
cyclooxygenase 2 (COX2) inhibitors may control the symptoms. 
With more severe pain, glucocorticoids (e.g., prednisone up to 40 
mg/day) are the only solution for the extreme discomfort. This 
drug may be required for several months and should then be with­
drawn gradually. If the TSH is not suppressed, TSH-suppressive 
therapy with levothyroxine may decrease the size of the gland, 
relieving the pressure on the thyroid capsule. TSH is needed for 

ressure on the thyroid capsule. TSH is needed for 
thyroid cell regeneration, so such therapy should be decreased as 
the symptoms subside. 
Drug-Associated Thyroiditis
Thyroiditis is an uncommon complication of pharmacotherapy. 
Amiodarone is an important exception and was discussed earlier. 
Most of the thyroiditis associated with various therapeutic agents 
appears to be due to drug-induced exacerbation of underlying 
autoimmune disease. This effect is understandable with agents 
that are spe

effect is understandable with agents 
that are specifically administered to modify the immune system. 
They include IL2, interferon alpha, granulocyte/macrophage 
colony-stimulating factor (GM-CSF), and the newer immune 
therapy modulators, all of which can precipitate silent thyroid­
itis.108,366,370,371 This has also been described with lithium and the 
GnRH agonist leuprolide, but the pathophysiologic mechanism 
is obscure.372–374
Thyroiditis has been found in association with multitargeting 

has been found in association with multitargeting 
kinase inhibitors such as sunitinib and sorafenib given for a variety 
of tumors, including gastrointestinal stromal tumors, hepatocel­
lular carcinoma, and renal cell carcinoma.365,375 This may present 
as subacute thyroiditis with a suppressed TSH as the major mani­
festation of the early phase but then progress to destruction of 
the gland through an unclear mechanism. Although imatinib has 
been associated with an increase in levothyroxine r

een associated with an increase in levothyroxine requirements 
in hypothyroid patients (analogous to the effects of phenytoin, 
carbamazepine, and rifampin), those changes are independent of 
thyroid function.376 
Other Causes of Thyrotoxicosis With a Low 
In addition to silent and subacute thyroiditis, several other entities 
should be considered in the patient with thyrotoxicosis in whom 
the thyroid gland is either not palpable or not enlarged and who 
has biochemical findi

ble or not enlarged and who 
has biochemical findings of thyrotoxicosis accompanied by a low 
RAIU.
Thyrotoxicosis Factitia
Thyrotoxicosis that arises from the ingestion, usually chronic, of 
excessive quantities of thyroid hormone usually occurs in individu­
als with a background of underlying psychiatric disease, especially 
in paramedical personnel who have access to thyroid hormone 
or in patients for whom thyroid hormone medication has been 
prescribed in the past. Generally the patient is 

prescribed in the past. Generally the patient is aware of taking 
thyroid hormone but may adamantly deny it. In other instances, 
large doses of thyroid hormone or other thyroactive material may 
be given without the knowledge of the patient, usually as part of a 
regimen for weight reduction. Some “natural” products for weight 
reduction stated not to contain thyroid hormone nonetheless do. 
Symptoms are typical of thyrotoxicosis and may be severe.
In the absence of preexisting disease of the 

ere.
In the absence of preexisting disease of the thyroid, the diagno­
sis is made from the combination of typical thyrotoxic manifesta­
tions, together with thyroid atrophy and hypofunction. Infiltrative 
ophthalmopathy never occurs, but lid lag, stare, and other thyro­
toxic eye signs may be present; TSH levels are suppressed. Serum 
T4 concentrations are increased unless the patient is taking T3, 
in which case they will be subnormal. Serum T3 concentrations 
are increased in either case. Hyp

 concentrations 
are increased in either case. Hypofunction of the thyroid gland is 
evidenced by the subnormal values of RAIU. The presence of low, 
rather than elevated, values of serum Tg is a clear indication that 
the thyrotoxicosis results from exogenous hormone rather than 
thyroid hyperfunction.
This disorder may be confused with other varieties of thyro­
toxicosis associated with a subnormal RAIU and absence of goiter, 
including silent thyroiditis, ectopic thyroid tissue, and hyperfunc

thyroiditis, ectopic thyroid tissue, and hyperfunc­
tioning metastatic follicular carcinoma. Evidence for the two lat­
ter disorders can be obtained by demonstration of the ectopic 
focus or foci by external radioiodine scanning or the presence 
of normal to elevated serum Tg concentrations. Differentiation 
from silent thyroiditis may be difficult. The presence of TPOAb 
points to painless chronic autoimmune thyroiditis, whereas a firm 
thyroid and brief history suggest the painless variant of 

and brief history suggest the painless variant of subacute 
thyroiditis. Treatment of thyrotoxicosis factitia consists of with­
drawing the offending medication. Psychiatric consultation is 
often required. 
Hamburger Thyrotoxicosis
An unusual form of exogenous thyrotoxicosis occurred in the 
midwestern portion of the United States in 1984 and 1985. The 
source was the inclusion of large quantities of bovine thyroid in 
ground beef preparations.377 When the slaughtering practices were 
changed, 

77 When the slaughtering practices were 
changed, this condition disappeared. Such a possibility, although 
remote, should be considered, especially if one is confronted with 
epidemic exogenous thyrotoxicosis. 
Thyrotoxicosis Due to Extrathyroidal Tissue
Thyroid tissue may be present in 5% to 10% of ovarian terato­
mas, and occasionally such foci are hyperfunctional.378,379 About 
5% to 10% of these tumors are bilateral. Although thyrotoxicosis 
is unusual, it may occur in as many

hyrotoxicosis 
is unusual, it may occur in as many as 8% to 10% of patients. 
Rarely, males with germ cell tumors may also develop hCG-
induced hyperthyroidism.380 
Patients present with variable degrees of thyrotoxicosis but with­
out goiter and generally have lower abdominal symptoms such 
as pain or a mass. Rarely ascites is present. Laboratory studies 
show reduced TSH and increased free T4 of a variable degree, 
but the RAIU is low. The Tg may be elevated, particularly

 RAIU is low. The Tg may be elevated, particularly if the 
teratoma is malignant and has metastasized to the peritoneum. 
Abdominal CT scan or MRI shows a multilocular ovarian mass 
or masses.381 Rarely a struma ovarii is accompanied by Graves 
disease.382 
403
Treatment
The patient should be rendered euthyroid if thyrotoxicosis is sig­
nificant, followed by removal of the involved ovary or ovaries. 
Therapeutic radioiodine will be required for metastatic disea

 radioiodine will be required for metastatic disease 
after ablation of the normal thyroid gland. 
Thyrotoxicosis Due to Metastatic Thyroid 
Carcinoma
In general, thyroid carcinomas are made up of poorly function­
ing tissue. On occasion, follicular thyroid carcinomas will have 
sufficient function when combined with the total mass of the 
metastases to result in an elevation in serum free T4 or T3 and 
may even be seen in Graves disease with TRAbs activating the 
tissue.383 Typically such a cou

bs activating the 
tissue.383 Typically such a course is a complication of a previ­
ously diagnosed lesion.384 The symptoms of thyrotoxicosis will 
vary and the metastatic disease is usually obvious from radio­
logic studies. On occasion, the presentation may be confusing 
if the patient is receiving TSH-suppressive therapy, and diag­
nosis will require its discontinuation. In spite of that, TSH will 
remain suppressed and the serum free T4 is elevated. Treatment 
of this condition is typical fo

evated. Treatment 
of this condition is typical for that of thyroid carcinoma and 
is described in Chapter 14. In patients with thyrotoxicosis due 
to metastatic tumor, serum Tg is quite elevated, indicating 
that the thyrotoxicosis is caused by thyroidal tissue that is not 
located in the neck. An RAIU during the thyrotoxic phase will 
show no neck uptake due to TSH suppression even if the thy­
roid is still present.
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 to thyroid replacement using levothyrox­
ine alone, although not a perfect replication of normal physiol­
ogy, is satisfactory for virtually all patients.
 • 
Levothyroxine requirements are increased in malabsorption due 
to bowel diseases and impaired gastric acid secretion or adsorp­
tion of levothyroxine to coadministered medications.
 • 
Athyreotic patients who are planning a pregnancy should be 
advised to increase the dose of levothyroxine by around 30% as 
soon as the diagnosis is conf

ne by around 30% as 
soon as the diagnosis is confirmed; the increased dose require­
ment persists throughout pregnancy, but dosage can return to 
normal within a few weeks after delivery.
13
Hypothyroidism and Thyroiditis
GREGORY A. BRENT AND ANTHONY P. WEETMAN
Reduced production of thyroid hormone is the central feature 
of the clinical state termed hypothyroidism.1,2 Permanent loss or 
destruction of the thyroid through processes such as autoim­
mune destruction, referred to as

s such as autoim­
mune destruction, referred to as Hashimoto disease,3 or irradia­
tion injury is described as primary hypothyroidism (Table 13.1). 
Hypothyroidism due to transient or progressive impairment of 
hormone biosynthesis is typically associated with compensatory 
thyroid enlargement. Central or secondary hypothyroidism due to 
insufficient stimulation of a normal gland is the result of hypotha­
lamic or pituitary disease or defects in the TSH molecule.4 Tran­
sient or temporary hypoth

the TSH molecule.4 Tran­
sient or temporary hypothyroidism can be observed as a phase 
of subacute thyroiditis.5 Primary hypothyroidism is the cause in 
approximately 99% of cases of hypothyroidism, with less than 1% 
being due to TSH deficiency or other causes. Central hypothy­
roidism is discussed in Chapter 11.
Reduced action of thyroid hormone at the tissue level, despite 
normal or increased thyroid hormone production from the thy­
roid gland, can also be associated with clinical hypothyroi

d, can also be associated with clinical hypothyroidism. 
Conditions associated with reduced thyroid hormone action are 
rare and include abnormalities of thyroid hormone metabolism 
and defects in nuclear signaling.6 Consumptive hypothyroid­
ism, identified in an increasing number of clinical settings, is the 
result of accelerated inactivation of thyroid hormone by the type 3 
iodothyronine deiodinase (D3).7 Defects of activation of the pro­
hormone, T4, to the active form, triiodothyronine (T3

mone, T4, to the active form, triiodothyronine (T3), have also 
been identified.8 Polymorphisms in genes regulating thyroid hor­
mone production and activation may influence thyroid hormone 
action in some tissues.9,10 Resistance to thyroid hormone (RTH), 
the result of defects in the thyroid hormone nuclear receptor (TR) 
or nuclear cofactors, is associated with elevated circulating lev­
els of thyroid hormone. Some tissues, depending on the level of 
expression of the mutant receptor and other

el of 
expression of the mutant receptor and other forms of local com­
pensation, have evidence of reduced thyroid hormone action.11
Estimates of the incidence of hypothyroidism vary depend­
ing on the population studied.12,13 In the United States, 0.3% 
have overt hypothyroidism, defined as an elevated serum TSH 
concentration and reduced free thyroxine concentration (FT4), 
and 4.3% have what has been described as subclinical or mild 
Hypothyroidism, 404
Thyroiditis, 431
405
Ch

UTLINE
405
Chapter 13  Hypothyroidism and Thyroiditis
hypothyroidism.13 Although a number of clinical manifestations 
have been associated with this early or mild phase of hypothy­
roidism, we will use the term subclinical to describe this group, 
as is used in most clinical studies. Subclinical hypothyroidism is 
defined as an elevated serum TSH level with a normal serum FT4 
concentration.14,15 Subclinical hypothyroidism can progress to 
overt hypothyroidis

ypothyroidism can progress to 
overt hypothyroidism and be associated with manifestations that, 
in some patients, may benefit from treatment.16 The incidence 
of hypothyroidism is higher among women, in the elderly, and 
in some racial and ethnic groups.17 Neonatal screening programs 
for congenital hypothyroidism identify hypothyroidism (almost 
all primary) in almost 1 in 3000 newborns.18
Hypothyroidism can affect all organ systems, and these manifesta­
tions are largely

an systems, and these manifesta­
tions are largely independent of the underlying disorder but are a 
function of the degree of hormone deficiency. The following sec­
tions discuss the pathophysiology of each organ system at various 
levels of thyroid hormone deficiency, from mild to severe. The 
term myxedema, formerly used as a synonym for hypothyroidism, 
refers to the appearance of the skin and subcutaneous tissues in 
the patient in a severely hypothyroid state (Fig. 13.1). Hypothy­
roidism 

y hypothyroid state (Fig. 13.1). Hypothy­
roidism of this severity is rarely seen today, and the term should be 
reserved to describe the physical signs.
Skin and Appendages
Hypothyroidism causes an accumulation of hyaluronic acid that 
alters the composition of the ground substance in the dermis and 
other tissues.19,20 This material is hygroscopic, producing the 
mucinous edema that is responsible for the thickened features and 
puffy appearance (myxedema) with full-blown hypothyroidism. 
Myxe

e (myxedema) with full-blown hypothyroidism. 
Myxedematous tissue is characteristically boggy and nonpitting 
and is apparent around the eyes, on the dorsa of the hands and 
feet, and in the supraclavicular fossae (see Fig. 13.1). It causes 
enlargement of the tongue and thickening of the pharyngeal and 
laryngeal mucous membranes.
A clinically similar deposit may occur in patients with Graves 
disease, usually over the pretibial area (infiltrative dermopathy or 
pretibial myxedema), but it can 

ve dermopathy or 
pretibial myxedema), but it can be differentiated histologically.21 
In addition to having a puffy appearance, the skin in hypothyroid­
ism is pale and cool as a result of cutaneous vasoconstriction. Ane­
mia may contribute to the pallor; hypercarotenemia gives the skin 
a yellow tint but does not cause scleral icterus (see Fig. 13.1). The 
secretions of the sweat glands and sebaceous glands are reduced, 
leading to dryness and coarseness of the skin, which in extreme 
cases ma

coarseness of the skin, which in extreme 
cases may resemble that seen in patients with ichthyosis.
Wounds of the skin tend to heal slowly. Easy bruising is due 
to an increase in capillary fragility. Head and body hair is dry 
and brittle, lacks luster, and tends to fall out. Hair may be lost 
from the temporal aspects of the eyebrows, although this is not 
specific for hypothyroidism (see Fig. 13.1B). Growth of hair is 
retarded so that haircuts and shaves are required less often. The 
nails a

s and shaves are required less often. The 
nails are brittle and grow slowly. Topical T3 has been shown to 
accelerate wound healing and stimulate hair growth in a euthy­
roid mouse model, demonstrating a role for thyroid hormone in 
these processes.20
Histopathologic examination of the skin reveals hyperkerato­
sis with plugging of hair follicles and sweat glands. The dermis is 
edematous, and the connective tissue fibers are separated by an 
increased amount of metachromatically staining, peri

creased amount of metachromatically staining, periodic acid–
Schiff (PAS)–positive mucinous material. This material consists 
of protein complexed with two mucopolysaccharides: hyaluronic 
acid and chondroitin sulfate B. The hygroscopic glycosaminogly­
cans are mobilized early during treatment with thyroid hormone, 
leading to an increase in urinary excretion of nitrogen and hexos­
amine as well as tissue water.19
Patients with hypothyroidism due to Hashimoto thyroiditis 
may also have skin lesi

 to Hashimoto thyroiditis 
may also have skin lesions with loss of pigmentation characteristic 
of the autoimmune skin condition vitiligo. This feature is not a 
  Causes of Hypothyroidism
Primary Hypothyroidism
Acquired
Iodine deficiency (endemic goiter)
Drugs blocking synthesis or release of T4 (e.g., lithium, ethionamide, 
sulfonamides, iodide)
Drug-induced thyroid destruction (e.g., interferon alpha, interleukin 2, 
tyrosine kinase inhibitors, blockers of CTLA4 or PD1)

osine kinase inhibitors, blockers of CTLA4 or PD1)
Amiodarone (reversible or permanent)
Goitrogens in foodstuffs or as endemic substances or pollutants
Thyroid infiltration (amyloidosis, hemochromatosis, sarcoidosis, Riedel 
struma, cystinosis, scleroderma)
Postablative thyroiditis due to 131I, surgery, or therapeutic irradiation for 
nonthyroidal malignancy
Transient hypothyroidism following painless thyroiditis (including post­
partum) or painful subacute thyroiditis
Congenital
Iodide transpor

ul subacute thyroiditis
Iodide transport or utilization defect (NIS or pendrin mutations)
Iodotyrosine dehalogenase deficiency
Organification disorders (TPO deficiency or dysfunction)
Defects in thyroglobulin synthesis or processing
Thyroid agenesis or dysplasia
TSH receptor defects
Thyroidal Gs protein abnormalities (pseudohypoparathyroidism type 1a)
Idiopathic TSH unresponsiveness
Consumptive Hypothyroidism
Rapid destruction of thyroid hormone due to D3 expression in large 
hemangio

id hormone due to D3 expression in large 
hemangiomas or hemangioendotheliomas
Defects of Thyroxine to Triiodothyronine Conversion
Selenocysteine insertion sequence–binding protein 2 (SECISBP2) defect
Central Hypothyroidism
Acquired
Pituitary origin (secondary)
Hypothalamic disorders (tertiary)
Bexarotene (retinoid X receptor agonist)
Dopamine or severe illness
TSH deficiency or structural abnormality
TSH receptor defect
Resistance to Thyroid Hormone
Generalized
“Pituitary” dominant
N

NIS, Sodium-iodide symporter; TPO, thyroid peroxidase; TSH, thyroid-stimulating hormone 
(thyrotropin).
   
TABLE 13.1
406
manifestation of reduced thyroid hormone action but reflects the 
common association of autoimmune endocrine disease and this 
skin condition, which is recognized as a component of autoim­
mune polyendocrine syndromes.22,23 
The cardiac output at rest is decreased because of reducti

iac output at rest is decreased because of reduction in 
both stroke volume and heart rate, reflecting loss of the inotropic 
and chronotropic effects of thyroid hormones. Peripheral vascular 
resistance at rest is increased, and blood volume is reduced. These 
hemodynamic alterations cause narrowing of pulse pressure, pro­
longation of circulation time, and decrease in blood flow to the 
tissues.24,25 The reduction in cutaneous circulation is responsible 
for the coolness and pallor of the skin

sponsible 
for the coolness and pallor of the skin and the sensitivity to cold. 
In most tissues, the decrease in blood flow is proportional to the 
decrease in oxygen consumption, so the arteriovenous oxygen 
difference remains normal. The hemodynamic alterations at rest 
resemble those of congestive heart failure. However, in hypothy­
roidism, cardiac output increases and peripheral vascular resistance 
decreases normally in response to exercise unless the hypothyroid 
state is severe and of l

e unless the hypothyroid 
state is severe and of long standing.
In severe primary hypothyroidism the cardiac silhouette 
is enlarged (Fig. 13.2), and the heart sounds are diminished in 
intensity. These findings are the result largely of effusion into the 
pericardial sac of fluid rich in protein and glycosaminoglycans, but 
the myocardium may also be dilated. Pericardial effusion is rarely 
of sufficient magnitude to cause tamponade.
Angina pectoris may first appear or worsen during treatment 

oris may first appear or worsen during treatment 
of the hypothyroid state with thyroid hormone, although most 
patients with hypothyroidism and coronary artery disease have 
no change, or improvement, in anginal symptoms with T4 treat­
ment.26 Electrocardiographic changes include sinus bradycardia, 
prolongation of the PR interval, low amplitude of the P wave and 
QRS complex, alterations of the ST segment, and flattened or 
inverted T waves. Pericardial effusion is probably responsible for 
th

ricardial effusion is probably responsible for 
the low amplitude in severe hypothyroidism. Systolic time inter­
vals are altered; the preejection period is prolonged, and the ratio 
of preejection period to left ventricular ejection time is increased. 
Echocardiographic studies have revealed resting left ventricular 
diastolic dysfunction in overt and, in some studies, subclinical 
hypothyroidism.27 These findings normalize when the hypothy­
roidism is treated.
Serum levels of homocysteine, cre

dism is treated.
Serum levels of homocysteine, creatine kinase, aspartate ami­
notransferase, and lactate dehydrogenase may be increased in 
hypothyroidism.25,28 Typically the isoenzyme patterns suggest 
that the source of the increased creatine kinase and lactate dehy­
drogenase is skeletal, not cardiac, muscle. All levels return to nor­
mal with therapy. Sequential cardiac biopsies in a hypothyroid 
patient with heart failure showed that messenger RNA (mRNA) 
levels from genes regulated by thy

ger RNA (mRNA) 
levels from genes regulated by thyroid hormone and important 
for the strength of myocardial contraction were normalized after 
T4 treatment.29
The combination of large heart, hemodynamic and electro­
cardiographic alterations, and serum enzyme changes has been 
termed myxedema heart. In the absence of coexisting organic heart 
disease, treatment with thyroid hormone corrects the hemody­
namic, electrocardiographic, and serum enzyme alterations of 
myxedema heart and restores hea

me alterations of 
myxedema heart and restores heart size to normal (see Fig. 13.2).
Hypothyroidism is consistently associated with elevations 
of total and low-density lipoprotein (LDL) cholesterol, which 
A
B
• Fig. 13.1  (A and B) Typical appearance with moderately severe primary hypothyroidism or myxedema. 
Note dry skin and sallow complexion; the absence of scleral pigmentation differentiates the carotenemia 
from jaundice. Both individuals demonstrate periorbital myxedema. (B) This patient

demonstrate periorbital myxedema. (B) This patient illustrates the loss of 
the lateral aspect of the eyebrow, sometimes termed Queen Anne sign. That finding is not unusual in the 
age group that is commonly affected by severe hypothyroidism and should not be considered to be a 
specific sign of the condition.
407
improve with T4 replacement.30 The higher the original serum 
TSH concentration and elevation of serum LDL, the greater the 
magnitude of red

on of serum LDL, the greater the 
magnitude of reduction in LDL cholesterol after T4 therapy. Lipo­
protein fractionation has shown that the cholesterol elevation is 
predominantly due to the less atherogenic large LDL particles. 
A subset of younger (<50 years) male hypothyroid patients had 
elevated serum triglycerides and C-reactive protein that improved 
with T4 treatment.31 Most studies have shown that serum high-
density lipoprotein (HDL) levels are not influenced by thyroid 
status.
Hypot

evels are not influenced by thyroid 
status.
Hypothyroidism has been shown to be a risk factor for athero­
sclerosis and cardiovascular disease by several studies, although 
others have not shown this association.32 The Whickham Study 
showed no increase in cardiovascular mortality rate in patients 
with subclinical hypothyroidism followed for more than 20 
years.12 A prospective study in the United States, following men 
and women age 65 or older for more than 10 years, showed no 
influence of 

r for more than 10 years, showed no 
influence of hypothyroidism (overt or subclinical) on cardio­
vascular outcome or mortality rate.33 Cardiovascular outcome 
studies suggest that improvement from treatment of hypothy­
roidism, especially subclinical hypothyroidism, is primarily in 
those who are middle age and not older individuals (older than 
65 years of age).15,34,35 
Hypothyroidism affects breathing by actions on the central 
regulation of respiration as well as the inn

tral 
regulation of respiration as well as the innervation and func­
tion of the respiratory muscles, upper airways, and tongue.36 
Pleural effusions usually are evident only on radiologic exami­
nation but in rare instances may cause dyspnea. Lung volumes 
are usually normal, but maximal breathing capacity and diffus­
ing capacity are reduced. In severe hypothyroidism, myxedema­
tous involvement of respiratory muscles and depression of both 
the hypoxic and the hypercapnic ventilatory drives ma

 hypoxic and the hypercapnic ventilatory drives may cause 
alveolar hypoventilation and carbon dioxide retention, which in 
turn can contribute to the development of myxedema coma. An 
increased prevalence of obstructive sleep apnea is seen in hypo­
thyroid patients, and it is usually reversed with restoration of a 
euthyroid state.37 
Although most patients experience a modest gain in weight, appe­
tite is usually reduced. The weight gain that occurs is caused partly 
by reten

weight gain that occurs is caused partly 
by retention of fluid by the hydrophilic glycoprotein deposits in 
the tissues but generally does not exceed 10% of body weight. 
Peristaltic activity is decreased and, together with the decreased 
food intake, is responsible for the frequent complaint of constipa­
tion. The latter may lead to fecal impaction (myxedema mega­
colon). Gaseous distention of the abdomen (myxedema ileus), if 
accompanied by colicky pain and vomiting, may mimic mechani­
cal il

licky pain and vomiting, may mimic mechani­
cal ileus.38
Elevations in the serum levels of carcinoembryonic antigen, 
which may occur on the basis of hypothyroidism alone, add to the 
impression that an obstruction is present. Ascites in the absence 
of another cause is unusual in hypothyroidism, but it can occur, 
usually in association with pleural and pericardial effusions. Like 
pericardial and pleural effusions, the ascitic fluid is rich in protein 
and glycosaminoglycans.
Achlorhydria afte

protein 
Achlorhydria after maximal histamine stimulation may be 
present in patients with primary hypothyroidism. Circulating 
antibodies against gastric parietal cells have been found in about 
one-third of patients with primary hypothyroidism and may be 
secondary to atrophy of the gastric mucosa. Hypothyroid patients 
with positive parietal cell antibodies have a higher T4 requirement 
compared with antibody-negative patients.39 Among Swedish 
celiac disease patients 

atients.39 Among Swedish 
celiac disease patients there was a 4.4-fold increased risk for hypo­
thyroidism compared with the general population.40 Overt perni­
cious anemia is reported in about 12% of patients with primary 
hypothyroidism. The coexistence of pernicious anemia and other 
autoimmune diseases with primary hypothyroidism reflects the 
fact that autoimmunity plays the central role in the pathogenesis 
of these diseases (see Chapter 43).22
Hypothyroidism has complex effects on intesti

).22
Hypothyroidism has complex effects on intestinal absorp­
tion. Although the rates of absorption for many substances are 
decreased, the total amount absorbed may be normal or even 
increased because the decreased bowel motility may allow more 
time for absorption. Malabsorption is occasionally overt.
Liver function test results are usually normal, but levels of ami­
notransaminases may be elevated, probably because of impaired 
A
B
• Fig. 13.2  (A and B) Chest roentgenograms in a patient wi

.2  (A and B) Chest roentgenograms in a patient with myxedema heart disease. The patient had 
signs of severe congestive heart failure and was given thyroid hormone alone. Within 4 months, the heart 
had returned to normal size (B) and there was no evidence of underlying heart disease.
408
clearance.41 The gallbladder contracts sluggishly and may be dis­
tended. In a population study of those without diagnosed thy­
roid disease, men (but not women) with an elevated TSH had

ease, men (but not women) with an elevated TSH had 
a 3.8-fold increased risk of cholelithiasis.42 Hypothyroidism is 
being recognized as a predisposing factor for nonalcoholic fatty 
liver disease.43
Atrophy of the gastric and intestinal mucosa and myxedema­
tous infiltration of the bowel wall may be demonstrated on histo­
logic examination. The colon may be greatly distended, and the 
volume of fluid in the peritoneal cavity is usually increased. The 
liver and pancreas are normal. 
Central an

d. The 
Central and Peripheral Nervous Systems
Thyroid hormone is essential for the development of the central 
nervous system.18,44,45 Deficiency in fetal life or at birth impairs 
neurologic development, including hypoplasia of cortical neurons 
with poor development of cellular processes, retarded myelina­
tion, and reduced vascularity. If the deficiency is not corrected 
in early postnatal life, the damage is irreversible. Deficiency of 
thyroid hormone beginn

rreversible. Deficiency of 
thyroid hormone beginning in adult life causes less severe mani­
festations that usually respond to treatment with the hormone. 
Cerebral blood flow is reduced, but cerebral oxygen consumption 
is usually normal; this finding is in accord with the conclusion 
that the oxygen consumption of isolated brain tissue in  vitro, 
unlike that of most other tissues, is not stimulated by administra­
tion of thyroid hormones. In severe cases, decreased cerebral blood 
flow may l

severe cases, decreased cerebral blood 
flow may lead to cerebral hypoxia.
All intellectual functions, including speech, are slowed in 
thyroid hormone deficiency.46 Loss of initiative is present and 
memory defects are common, lethargy and somnolence are prom­
inent, and dementia in elderly patients may be mistaken for senile 
dementia.47 Positron emission tomography (PET) brain scans of 
hypothyroid patients before and after T4 therapy demonstrate 
reversible reduced glucose uptake in specific

ate 
reversible reduced glucose uptake in specific brain areas, such as the 
limbic system, which also correlates with behavioral and psychiat­
ric symptoms.48 Psychiatric disorders are common and are usually 
of the paranoid or depressive type and may induce agitation (myx­
edema madness).47 Headaches are frequent. Cerebral hypoxia due 
to circulatory alterations may predispose to confusional attacks 
and syncope, which may be prolonged and lead to stupor or coma. 
Other factors predisposing to

 to stupor or coma. 
Other factors predisposing to coma in hypothyroidism include 
exposure to severe cold, infection, trauma, hypoventilation with 
carbon dioxide retention, and depressant drugs.
Epileptic seizures have been reported and tend to occur in 
myxedema coma. Night blindness is due to deficient synthesis 
of the pigment required for dark adaptation. Hearing loss of the 
perceptive type is frequent due to myxedema of the eighth cranial 
nerve and serous otitis media. Perceptive deafne

nerve and serous otitis media. Perceptive deafness may also occur 
in association with a defect in the organic binding of thyroidal 
iodide (Pendred syndrome; see Chapter 11), but in these instances 
it is not due to hypothyroidism per se.
Thick, slurred speech and hoarseness are due to myxedema­
tous infiltration of the tongue and larynx, respectively.45 Body 
movements are slow and clumsy, and cerebellar ataxia may occur. 
Numbness and tingling of the extremities are frequent; in the fin­
ge

ng of the extremities are frequent; in the fin­
gers these symptoms may be due to compression by glycosami­
noglycan deposits in and around the median nerve in the carpal 
tunnel (carpal tunnel syndrome).49 The tendon reflexes are slow, 
especially during the relaxation phase, producing the characteris­
tic “hung-up reflexes”; this phenomenon is due to a decrease in 
the rate of muscle contraction and relaxation rather than a delay 
in nerve conduction.
The presence of extensor plantar responses

uction.
The presence of extensor plantar responses or diminished vibra­
tion sense should alert the physician to the possibility of coexisting 
pernicious anemia with combined system disease. Electroencepha­
lographic changes include slow alpha-wave activity and general loss 
of amplitude. The concentration of protein in the cerebrospinal 
fluid is often increased, but cerebrospinal fluid pressure is normal.
Histopathologic examination of the brain in patients with 
untreated hypothyroidism reve

in in patients with 
untreated hypothyroidism reveals that the nervous system is 
edematous with mucinous deposits in and around nerve fibers. 
In patients with cerebellar ataxia, neural myxedematous infiltrates 
of glycogen and mucinous material are present in the cerebellum. 
There may be foci of degeneration and an increase in glial tissue. 
The cerebral vessels show atherosclerosis, but this finding is much 
more common if the patient has had coexistent hypertension.
Hypothyroidism has been 

 coexistent hypertension.
Hypothyroidism has been associated with several neurologic 
conditions, although a strong etiologic link has not been estab­
lished. Epidemiologic studies have shown an association between 
Alzheimer disease and hypothyroidism.50 It is difficult to convinc­
ingly demonstrate this association because the incidence of thy­
roid disease in the elderly population is high and, like dementia, 
increases with age. A mechanistic link is suggested by the obser­
vation of amyloid

 link is suggested by the obser­
vation of amyloid deposition in Down syndrome, a condition 
associated with an increased incidence of Hashimoto disease, and 
thyroid hormone regulates amyloid gene processing in a number 
of cellular and animal models. Subclinical hyperthyroidism, how­
ever, has also been associated with Alzheimer disease.51 There is an 
increase in cerebrospinal fluid reverse T3 levels in Alzheimer dis­
ease patients, all with normal circulating thyroid hormone levels, 
suggest

ormal circulating thyroid hormone levels, 
suggesting the potential for altered thyroid hormone metabolism 
in the brain.52 The impact of normalizing T3 levels in the brain, 
however, is not known. A corticosteroid-responsive encephalopa­
thy is associated with chronic Hashimoto thyroiditis but may be 
linked to autoimmunity rather than a process mediated specifi­
cally by low thyroid hormone levels or thyroid autoantibodies.53 
Muscular System
Stiffness and aching of muscles are common in hypot

tiffness and aching of muscles are common in hypothyroidism 
and are worsened by cold temperatures.49 Delayed muscle con­
traction and relaxation cause slowness of movement and delayed 
tendon jerks.45 Muscle mass may be reduced or enlarged due to 
interstitial myxedema. Muscle mass may be slightly increased, and 
the muscles tend to be firm. Rarely, a profound increase in muscle 
mass with slowness of muscular activity may be the predominant 
manifestation (the Kocher-Debré-Sémélaigne, or Hoffm

nifestation (the Kocher-Debré-Sémélaigne, or Hoffmann, syn­
drome). Myoclonus may be present. The electromyogram may be 
normal or may exhibit disordered discharge, hyperirritability, and 
polyphasic action potentials.
On histopathologic examination, the muscles appear pale and 
swollen. The muscle fibers may show swelling, loss of normal stria­
tions, and separation by mucinous deposits. Type I muscle fibers 
tend to predominate. 
Skeletal System: Calcium and Phosphorus Metabolism
Thyroid hormo

m: Calcium and Phosphorus Metabolism
Thyroid hormone is essential for normal growth and matura­
tion of the skeleton, and growth failure is due both to impaired 
general protein synthesis and to a reduction in growth hormone, 
but especially of insulin-like growth factor 1 (Fig. 13.3).54 The 
thyroid hormone receptor isoforms α and β have specific roles in 
bone maturation. Before puberty, thyroid hormone plays a major 
role in the maturation of bone. Deficiency of thyroid hormone 
in early life

bone. Deficiency of thyroid hormone 
in early life leads to both a delay in development and an abnor­
mal, stippled appearance of the epiphyseal centers of ossification 
409
(epiphyseal dysgenesis) (Fig. 13.4). Impairment of linear growth 
leads to dwarfism, in which the limbs are disproportionately short 
in relation to the trunk, but cartilage growth is unaffected (see 
Fig. 13.3). Children with prolonged hypothyroidism, even after 
adequate treatment

ged hypothyroidism, even after 
adequate treatment, do not reach predicted height based on mid­
parental height calculations.55
Urinary excretion of calcium is decreased, as is the glomerular 
filtration rate, whereas fecal excretion of calcium and both urinary 
and fecal excretion of phosphorus are variable. Calcium balance is 
also variable, and any changes are slight. The exchangeable pool of 
calcium and its rate of turnover are reduced, reflecting decreased 
bone formation and resorption. B

ecting decreased 
bone formation and resorption. Because levels of parathyroid 
hormone are often slightly increased, some degree of resistance 
to its action may be present; levels of 1,25-dihydroxyvitamin D 
(1,25[OH]2D) are also increased.
Levels of calcium and phosphorus in serum are usually normal, 
but calcium may be slightly elevated. The alkaline phosphatase 
level is usually below normal in infantile and juvenile hypothy­
roidism. Bone density may be increased. The radiologic appear­
an

ensity may be increased. The radiologic appear­
ance of the skeleton in cretinism and juvenile hypothyroidism is 
discussed subsequently. 
Renal Function: Water and Electrolyte Metabolism
Reversible reductions in renal blood flow, glomerular filtration rate, 
and tubular reabsorptive and secretory maxima are seen in hypothy­
roidism. Blood urea nitrogen and serum creatinine levels are normal, 
but uric acid levels may be increased. Urine flow is reduced, and 
delay in the excretion of a water lo

reduced, and 
delay in the excretion of a water load may result in reversal of the 
normal diurnal pattern of urine excretion. The delay in water excre­
tion appears to be due to decreased volume delivery to the distal 
diluting segment of the nephron as a result of the diminished renal 
perfusion; evidence supporting inappropriate secretion of vasopres­
sin (syndrome of inappropriate antidiuretic hormone [SIADH] 
secretion) is less compelling.56 There is a high prevalence of hypo­
thyroidism in

 There is a high prevalence of hypo­
thyroidism in patients with chronic kidney disease, and improve­
ment in renal function has been demonstrated with T4 treatment.57
The impaired renal excretion of water and the retention 
of water by the hydrophilic deposits in the tissues result in an 
• Fig. 13.3  The consequences of untreated congenital hypothyroidism are 
demonstrated in this 17-year-old girl. Her condition had been diagnosed 
at birth but, through a series of misunderstandings, was not t

, through a series of misunderstandings, was not treated with 
thyroid hormone. Note her size, the poorly developed nasal bridge, the 
wide-set eyes, and the ears, which are larger than are appropriate for head 
size. Her tongue is enlarged, and her extremities are inappropriately short 
in relation to her trunk. (Courtesy Dr. Ronald B. Stein.)
A
B
• Fig. 13.4  X-ray films of the skull and hand of the 17-year-old patient illustrated in Fig. 13.3. (A) Skull film 
shows that the posterior and ante

 (A) Skull film 
shows that the posterior and anterior fontanels are open and that the sutures are not fused. The deciduous 
and permanent teeth are present. (B) Radiograph of the wrist and hand shows the delayed appearance 
of the epiphyseal centers of the bones of the hand and the absence of the distal radial epiphysis. The 
estimated bone age is 9 months. (Courtesy Dr. Ronald B. Stein.)
410
increase in total body water, even though plasma volume is 
reduced. This increa

even though plasma volume is 
reduced. This increase accounts for the hyponatremia occasion­
ally noted because the level of exchangeable sodium is increased. 
The amount of exchangeable potassium is usually normal in rela­
tion to lean body mass. Serum magnesium concentration may be 
increased, but exchangeable magnesium levels and urinary magne­
sium excretion are decreased. 
In response to the diminished oxygen requirements and decreased 
production of erythropoietin, the

s and decreased 
production of erythropoietin, the red blood cell mass is decreased; 
this is evident in the mild normocytic, normochromic anemia 
that often occurs. Less commonly, the anemia is macrocytic, 
sometimes from deficiency of vitamin B12. Reference has already 
been made to the high incidence of pernicious anemia (and of 
achlorhydria and vitamin B12 deficiency without overt anemia) 
in primary hypothyroidism (see Chapter 43). Conversely, overt 
and subclinical hypothyroidism is prese

ly, overt 
and subclinical hypothyroidism is present in 12% and 15% of 
patients, respectively, with pernicious anemia. Folate deficiency 
from malabsorption or dietary inadequacy may also cause mac­
rocytic anemia. The frequent menorrhagia and the defective 
absorption of iron resulting from achlorhydria may contribute to 
a microcytic, hypochromic anemia.
The total and differential white blood cell counts are usually 
normal, and platelets are adequate, although platelet adhesive­
ness may be 

adequate, although platelet adhesive­
ness may be impaired. If pernicious anemia or significant folate 
deficiency is present, the characteristic changes in peripheral 
blood and bone marrow will be found. The intrinsic clotting 
mechanism may be defective because of decreased concentra­
tions in plasma of factors VIII and IX, which, together with an 
increase in capillary fragility and the decrease in platelet adhe­
siveness, may account for the bleeding tendency that some­
times occurs.38,58,5

 bleeding tendency that some­
times occurs.38,58,59 
In long-standing primary hypothyroidism, hyperplasia of the thy­
rotropes may cause the pituitary gland to be enlarged. This feature 
can be detected radiologically as an increase in the volume of the 
pituitary gland.60 Rarely, the pituitary enlargement compromises 
the function of other pituitary cells and causes pituitary insuffi­
ciency or visual field defects. Patients with severe hypothyroidism 
may 

defects. Patients with severe hypothyroidism 
may have increased serum prolactin levels, stimulated by the eleva­
tion in thyrotropin-releasing hormone (TRH) and proportional 
to the level of serum TSH elevation, and galactorrhea may develop 
in some patients. Treatment with thyroid hormone normalizes the 
serum prolactin and TSH levels and causes disappearance of galac­
torrhea, if present.
In rodents, thyroid hormone directly regulates growth hor­
mone synthesis. Growth hormone is not directly

or­
mone synthesis. Growth hormone is not directly regulated by thy­
roid hormone in humans, but thyroid status influences the growth 
hormone axis.61 Hypothyroid children have delayed growth and 
the response of growth hormone to provocative stimuli may be 
subnormal. Individuals affected with RTHα have delayed growth 
and short stature, indicating the consequences of interference with 
thyroid hormone signaling through thyroid hormone receptor 
alpha.62
As a result of the decreased rate of tur

alpha.62
As a result of the decreased rate of turnover of cortisol due 
to decreased hepatic 11β-hydroxysteroid dehydrogenase type 
1 (11βHSD1), the 24-hour urinary excretion of cortisol and 
17-hydroxycorticosteroids is decreased, but the plasma cortisol 
level is usually normal (see Chapter 15). The response of uri­
nary 17-hydroxycorticosteroid to exogenous adrenocorticotropic 
hormone is usually normal but may be decreased. The response of 
plasma cortisol to insulin-induced hypoglycemia ma

plasma cortisol to insulin-induced hypoglycemia may be impaired.
In severe, long-standing primary hypothyroidism, pituitary 
and adrenal function may be secondarily decreased, and adrenal 
insufficiency may be precipitated by stress or by rapid replacement 
therapy with thyroid hormone.61 The rate of turnover of aldoste­
rone is decreased, but the plasma level is normal. Plasma renin 
activity is decreased, and sensitivity to angiotensin II is increased, 
which may contribute to the association 

creased, 
which may contribute to the association of hypertension with 
hypothyroidism (see Chapter 16).63,64 
In both sexes, thyroid hormones influence sexual development and 
reproductive function.65 Infantile hypothyroidism, if untreated, 
leads to sexual immaturity, and juvenile hypothyroidism causes 
a delay in the onset of puberty followed by anovulatory cycles in 
girls. Paradoxically, primary hypothyroidism may also rarely cause 
precocious sexual development and ga

rarely cause 
precocious sexual development and galactorrhea, presumably due 
to “spillover” of elevated TSH stimulating the luteinizing hor­
mone (LH) receptor and elevated TRH initiating excess prolactin 
release.
In adult women, severe hypothyroidism may be associated 
with diminished libido and failure of ovulation. Secretion of pro­
gesterone is inadequate, and endometrial proliferation persists, 
resulting in excessive and irregular breakthrough menstrual bleed­
ing. These changes may be d

rough menstrual bleed­
ing. These changes may be due to deficient secretion of LH and 
pulse frequency and amplitude. Rarely, in primary hypothyroid­
ism, secondary depression of pituitary function may lead to ovar­
ian atrophy and amenorrhea. Fertility is reduced, and there is an 
increase in spontaneous abortion and preterm delivery, although 
many pregnancies are successful.66 Pregnancy complications are 
associated with overt and subclinical hypothyroidism, although 
the impact has varied am

hypothyroidism, although 
the impact has varied among different studies.67 A randomized 
prospective study of levothyroxine treatment in pregnant women 
with thyroid peroxidase antibody (TPOAb) positivity but nor­
mal range TSH has shown that the increased incidence of pre­
term delivery and spontaneous abortions is reversed by treatment, 
although this result remains to be confirmed.66,68 Primary ovarian 
failure can also be seen in patients with Hashimoto thyroiditis 
as part of an autoimmune 

h Hashimoto thyroiditis 
as part of an autoimmune polyendocrine syndrome.22,23 Hypo­
thyroidism in men may cause diminished libido, erectile dysfunc­
tion, and oligospermia. A significant fraction of men with both 
hypothyroidism and hyperthyroidism have moderate to severe 
erectile dysfunction, which improves with treatment of the thy­
roid disease.69
Values for plasma gonadotropins are usually in the normal 
range in primary hypothyroidism; in postmenopausal women, 
levels are usually somewhat

postmenopausal women, 
levels are usually somewhat lower than in euthyroid women of the 
same age but are nevertheless within the menopausal range. This 
feature provides a valuable means of differentiating primary from 
secondary hypothyroidism.
The metabolism of both androgens and estrogens is altered 
in hypothyroidism. Secretion of androgens is decreased, and 
the metabolism of testosterone is shifted toward etiochola­
nolone rather than androsterone. With respect to estradiol and 
estrone, 

osterone. With respect to estradiol and 
estrone, hypothyroidism favors metabolism of these steroids via 
16α-hydroxylation over that via 2-oxygenation, with the result 
that formation of estriol is increased and that of 2-hydroxyestrone 
and its derivative, 2-methoxyestrone, is decreased. The sex hor­
mone–binding globulin in plasma is decreased, with the result 
that the plasma concentrations of both testosterone and estra­
diol are decreased, but the unbound fractions are increased. The 
411

but the unbound fractions are increased. The 
411
alterations in steroid metabolism are corrected by restoration of 
the euthyroid state.70 
The plasma cyclic adenosine monophosphate (cAMP) response to 
epinephrine is decreased in hypothyroidism, suggesting a state of 
decreased adrenergic responsiveness. The fact that the responses 
of plasma cAMP to glucagon and parathyroid hormone are also 
decreased suggests that thyroid hormones have

lso 
decreased suggests that thyroid hormones have a general modu­
lating influence on cAMP generation.71 The reduced adrenergic 
responsiveness associated with hypothyroidism has been linked to 
all steps of catecholamine signaling, including receptor and post­
receptor actions, resulting in an impaired cAMP response. Direct 
measurement of norepinephrine in abdominal fat of hypothyroid 
patients shows reduced levels, and there is reduced production of 
glycerol in response to adrenergic agonis

tion of 
glycerol in response to adrenergic agonist stimulation.72 Augmen­
tation of α2-receptor signaling has also been proposed as a factor 
reducing catecholamine responsiveness. 
Energy Metabolism: Protein, Carbohydrate, and Lipid 
The decrease in energy metabolism and heat production is reflected 
in a low basal metabolic rate, decreased appetite, cold intolerance, 
and slightly low basal body temperature.73–75 Both the synthesis 
and the degradation of protein are decreased, the

and the degradation of protein are decreased, the latter especially 
so, with the result that nitrogen balance is usually slightly positive. 
The decrease in protein synthesis is reflected in retardation of both 
skeletal and soft tissue growth.
Permeability of capillaries to protein is increased, accounting 
for the high levels of protein in effusions and in cerebrospinal 
fluid. In addition, the albumin pool is increased because of the 
greater decrease in albumin degradation compared to albu

r decrease in albumin degradation compared to albumin 
synthesis. A greater than normal fraction of exchangeable albumin 
is in the extravascular space. The total concentration of serum pro­
teins may be increased.
Hypothyroidism is associated with a reduction in glucose dis­
posal to skeletal muscle and adipose tissue.76 Thyroid hormone has 
been shown to stimulate expression of the insulin-sensitive glucose 
transporter (GLUT4), and the levels of this transporter are reduced 
in hypothyroidism

of this transporter are reduced 
in hypothyroidism. Hypothyroidism is also, however, associated 
with reduced gluconeogenesis. The net effect of these influences is 
usually a minimal effect of hypothyroidism on serum glucose levels. 
Thyroid hormone downregulates expression of prohormone pro­
cessing enzymes, which therefore have increased activity in hypo­
thyroidism. Degradation of insulin therefore is slowed, and the 
sensitivity to exogenous insulin may be increased. In a patient with 
pree

 insulin may be increased. In a patient with 
preexisting diabetes mellitus who develops hypothyroidism, insulin 
requirements may be reduced. A further influence on glucose uptake 
may occur at the tissue level. Polymorphisms in the 5′-deiodinase 
type 2 (D2) gene, which may affect local T3 production, have been 
shown to be associated with impaired glucose disposal.77
Both the synthesis and the degradation of lipid are depressed 
in hypothyroidism. Degradation, however, is reduced to a greater

ism. Degradation, however, is reduced to a greater 
extent, with a net effect of accumulation of LDL and triglycer­
ides.30,73 The decrease in the lipid degradation rate may reflect 
the decrease in postheparin lipolytic activity as well as reduced 
LDL receptors. Plasma free fatty acid levels are decreased, and the 
mobilization of free fatty acids in response to fasting, catechol­
amines, and growth hormone is impaired. Impaired lipolysis of 
white fat in hypothyroid patients at baseline and i

hite fat in hypothyroid patients at baseline and in response to 
catecholamine reflects impaired free fatty acid mobilization.71,72 
All of these abnormalities are relieved by treatment.
An elevation in serum LDL cholesterol has been associated, in 
most studies, with both overt and subclinical hypothyroidism.30 
According to most studies, serum HDL and triglyceride levels are 
not influenced by hypothyroidism.73 The reduction in LDL with 
T4 therapy is generally related to the original magnitud

rapy is generally related to the original magnitude of LDL 
and TSH elevation; the higher the initial levels, the greater the 
reduction in LDL that is observed. A typical reduction in LDL is 
5% to 10% of the original level.
The role of adipocytokines, such as leptin, adiponectin, and 
resistin, in metabolic regulation has been increasingly recognized 
as well as the potential for interaction with thyroid hormone.75 
Rodent studies have shown that leptin regulates central adaptation 
between th

at leptin regulates central adaptation 
between the starved and fed state and that falling leptin levels, 
associated with starvation, lead to a suppression of the thyroid 
axis. Hypothyroidism in rodents is associated with reduced leptin 
and increased resistin levels. Leptin infusion into the cerebral ven­
tricles reverses some of the metabolic changes seen with hypothy­
roidism, including improved glucose disposal and reduced skeletal 
muscle fat.78 Human studies, however, have not shown cons

fat.78 Human studies, however, have not shown consistent 
changes in adipocytokines in hypothyroidism.79 
Current Clinical Picture
In the adult, the onset of hypothyroidism is usually so insidious 
that the typical manifestations may take months or years to appear 
and go unnoticed by family and friends. The gradual development 
of the hypothyroid state is due to slow progression both of thy­
roid hypofunction and of the clinical manifestations after thyroid 
failure is complete. This course is 

fter thyroid 
failure is complete. This course is in contrast with the more rapid 
development of the hypothyroid state when replacement therapy is 
discontinued in a patient with treated primary hypothyroidism or 
when the thyroid gland of a normal subject is surgically removed. 
In such patients, manifestations of frank hypothyroidism are usu­
ally present by 6 weeks and myxedema appears by 3 months.
Hypothyroidism continues to be diagnosed at earlier stages.1,2,80 
Based on the most recent da

earlier stages.1,2,80 
Based on the most recent data, subclinical or early hypothyroid­
ism is seen approximately 14 times more commonly than overt 
hypothyroidism. Early symptoms are variable and relatively non­
specific. The reason for the increased prevalence of hypothyroid 
patients presenting with minimal symptoms is largely the avail­
ability of sensitive and specific laboratory tests that allow rec­
ognition of the primary form of the disease long before severe 
symptoms have developed. T

ase long before severe 
symptoms have developed. There should thus be a low thresh­
old to test patients for suspected primary hypothyroidism with a 
serum TSH determination. Patients with significant biochemical 
abnormalities of hypothyroidism may not score high on indices of 
symptoms and signs.81
With respect to physical signs of hypothyroidism, the presence 
of coarse skin, periorbital puffiness that obscures the curve of the 
malar bone (see Fig. 13.1), cold skin, and delayed ankle reflex 

e Fig. 13.1), cold skin, and delayed ankle reflex 
relaxation phase are all signs that should lead to appropriate diag­
nostic tests.
Acute hypothyroidism in the previously hyperthyroid patient 
seen after radioiodine therapy may also be characterized by pain­
ful cramping of large muscle groups, as discussed in Chapter 12 
regarding Graves disease. 
Hypothyroidism in Infants and Children
Severe hypothyroidism is seldom apparent at birth, perhaps due 
to the partial protection afforded by transp

 due 
to the partial protection afforded by transplacental transfer of 
maternal thyroid hormones, hence the requirement for systematic 
screening for congenital hypothyroidism.18 Congenital hypothy­
roidism can be due to complete thyroid agenesis, ectopic thyroid, 
412
or incomplete thyroid development. Mutations in genes impor­
tant for thyroid development have been identified in a number of 
patients and in some cases may explain associated abnormalities 
in development

y explain associated abnormalities 
in development of other structures, such as the heart, because of 
their spatial association during development. The age at which 
symptoms appear depends on the degree of impairment of thy­
roid function (see Figs. 13.3 and 13.4). Severe hypothyroidism in 
infancy is termed cretinism. As the age at onset increases, the clini­
cal picture of cretinism merges imperceptibly with that of juvenile 
hypothyroidism. Retardation of mental development and growth, 
the

Retardation of mental development and growth, 
the hallmark of cretinism, becomes manifest only in later infancy, 
and the former is largely irreversible. Consequently, early recog­
nition is crucial and has been achieved by universal population 
screening in the developed world by measuring serum T4 or TSH 
concentrations routinely in filter paper blood spots from neo­
nates. During the first few months of life, symptoms and signs of 
hypothyroidism include feeding problems, failure to thrive, 

dism include feeding problems, failure to thrive, con­
stipation, a hoarse cry, somnolence, and jaundice. In succeeding 
months, especially in severe cases, protuberance of the abdomen, 
dry skin, poor growth of hair and nails, and delayed eruption of 
the deciduous teeth become evident. Retardation of mental and 
physical development is manifested by delay in reaching the nor­
mal milestones of development, such as holding up the head, sit­
ting, walking, and talking.
Thyroid hormone plays a ma

, walking, and talking.
Thyroid hormone plays a major role in bone development, 
and thyroid hormone receptors are expressed in osteoclasts and 
osteoblasts.54 The primary targets of thyroid hormone have been 
identified in the epiphyseal plates. Impairment of linear growth 
in congenital hypothyroidism results in dwarfism, with the limbs 
disproportionately short in relation to the trunk (see Fig. 13.3). 
Delayed closure of the fontanels causes the head to be large in rela­
tion to the body. Th

the head to be large in rela­
tion to the body. The naso-orbital configuration remains infantile. 
Maldevelopment of the femoral epiphyses results in a waddling 
gait. The teeth are malformed and susceptible to caries. The char­
acteristic appearance includes a broad and flat nose, widely set 
eyes, periorbital puffiness, large protruding tongue, sparse hair, 
rough skin, short neck, and protuberant abdomen with an umbili­
cal hernia. Mental deficiency is usually severe.
Radiologic examination o

ciency is usually severe.
Radiologic examination of the skeleton is diagnostic. The skull 
shows a poorly developed base, delayed closure of the fontanels, 
widely set orbits, and a short and flat nasal bone. The pituitary 
fossa may be enlarged. Shedding of deciduous teeth and eruption 
of permanent teeth are delayed (see Fig. 13.4).
The radiologic picture of epiphyseal dysgenesis is virtually 
pathognomonic of hypothyroidism in infancy and childhood and 
may involve any center of endochondral 

dhood and 
may involve any center of endochondral ossification, depending on 
the age at onset of the hypothyroid state; it is usually best seen in 
the femoral and humeral heads and the navicular bone of the foot. 
The centers of ossification appear late, so bone age is retarded in 
relation to chronologic age; when they eventually appear, instead 
of a single center, multiple small centers are scattered throughout a 
misshapen epiphysis (see Fig. 13.4). These small centers of ossifica­
tion ev

g. 13.4). These small centers of ossifica­
tion eventually coalesce and form a single center with an irregular 
outline and a stippled appearance (stippled epiphysis). Epiphyseal 
dysgenesis is evident only in centers that normally ossify at a time 
after the onset of the hypothyroidism. After a normal metabolic 
state is restored by treatment, centers destined to ossify at a later 
age develop normally.
Hypothyroidism that begins in childhood is usually Hashi­
moto disease and can be transient 

 usually Hashi­
moto disease and can be transient in this age group. Subclinical 
hypothyroidism is also seen in children and adolescents, and in 
one study those affected were more likely to be obese and have 
a family history of thyroid disease.82 The clinical manifestations 
of hypothyroidism in children are intermediate, between those of 
infantile and those of adult hypothyroidism, in that the devel­
opmental retardation is not as severe as that of cretinism, and 
the manifestations of full

that of cretinism, and 
the manifestations of full-blown adult myxedema are rarely seen. 
Growth and sexual development are affected predominantly. If left 
untreated, linear growth is severely retarded and sexual maturation 
and the onset of puberty are delayed.55,83 On radiologic examina­
tion, epiphyseal dysgenesis may be present, and epiphyseal union 
is always delayed, resulting in a bone age that is younger relative 
to chronologic age. 
Laboratory Evaluation
Primary and Central Hypothyroi

boratory Evaluation
Primary and Central Hypothyroidism
A decrease in secretion of the thyroid hormones is common to all 
varieties of hypothyroidism, except for disorders of thyroid hor­
mone metabolism or action, such as consumptive hypothyroidism 
and resistance to thyroid hormone (see later). In patients with pri­
mary thyroid disease—the cause of hypothyroidism in more than 
99% of the patients—there is a significant increase in basal serum 
TSH concentration. A strategy for evaluating the p

TSH concentration. A strategy for evaluating the patient suspected 
of hypothyroidism involves a TSH determination (Table 13.2). If 
the suspicion of hypothyroidism is strong, if a goiter is present, 
or if central hypothyroidism is part of the differential diagnosis, 
an FT4 assay should be included (see Chapter 11). If hypothy­
roidism is thought to be unlikely but must be excluded, only a 
TSH determination is required because primary hypothyroidism 
is almost always the cause. If TSH is elev

oidism 
is almost always the cause. If TSH is elevated, an FT4 assay can be 
added to the same determination (Fig. 13.5). As hypothyroidism 
progresses, the serum TSH increases further, the serum FT4 falls, 
and finally at the most severe stage serum T3 concentrations may 
become subnormal (see Table 13.2). The persistence of a normal 
serum T3 is in part due to preferential synthesis and secretion of 
T3 by residual functioning thyroid tissue under the influence of 
the increased plasma TSH. In

der the influence of 
the increased plasma TSH. In addition, the efficiency of conver­
sion of T4 to T3 by D2 is increased as the serum T4 level falls.84 
Consequently, the serum T3 concentration may remain within the 
normal range.
The principal differential diagnosis is between primary and 
central hypothyroidism (see Chapter 9).85 The serum TSH con­
centration is the critical laboratory determination that in general 
allows recognition of the cause of the disease when the serum 
FT4 is reduce

cause of the disease when the serum 
FT4 is reduced. An exception is the individual with a recent 
history of thyrotoxicosis (and suppressed TSH) in whom a low 
FT4 level may be associated with a reduced TSH level for several 
months after treatment of the thyrotoxicosis. In patients with 
primary hypothyroidism, the absence of TPOAb raises a pos­
sible diagnosis of transient hypothyroidism following an undi­
agnosed episode of painful subacute thyroiditis, also referred to 
as postviral, de Que

hyroiditis, also referred to 
as postviral, de Quervain, granulomatous, or pseudotuberculous 
thyroiditis.
The differentiation of hypothyroidism due to intrinsic thy­
roid failure from hypothyroidism due to diminished TSH secre­
tion from hypothalamic or pituitary disease (central or secondary 
hypothyroidism) is the most critical decision point in this path­
way (see Fig. 13.5).4 A low thyroid hormone level with a normal 
or low TSH level should lead to an evaluation for the possibility 
of fai

 lead to an evaluation for the possibility 
of failure of other endocrine systems that require trophic pituitary 
hormones for normal function (see Table 13.1) (see Chapters 8 
and 9). In some patients with central hypothyroidism, the basal 
serum TSH concentration (and the response to TRH) may even 
413
be somewhat elevated, but the TSH has reduced biologic potency 
even though it is immunologically reactive.4
In patients with an elevated TSH level and

ctive.4
In patients with an elevated TSH level and a reduced FT4, 
the presence of TPOAb generally points to autoimmune thyroid 
disease (Hashimoto disease) as the cause of the hypothyroidism 
(see Fig. 13.5). On the other hand, the absence of TPOAb raises 
the possibility of less common causes of hypothyroidism such as 
transient hypothyroidism, infiltrative thyroid disorders, and exter­
nal irradiation, as discussed later (see Table 13.1), although rarely 
patients with Hashimoto disease will 

ough rarely 
patients with Hashimoto disease will not have detectable thyro­
globulin or TPOAb.
Measurement of radioactive iodine uptake (RAIU) is rarely 
required in the evaluation of hypothyroidism. Tests that use radio­
iodine to assess the function of the thyroid gland display a variable 
pattern, depending on the underlying thyroid disorder. The diag­
nostic value of a low RAIU is limited because of the relatively high 
dietary iodine intake in North America, reducing uptake of the 
tracer 

 in North America, reducing uptake of the 
tracer dose of radioiodine, and variation in iodine intake from day 
to day in the same individual. National surveys of dietary iodine 
intake had shown a progressive reduction in iodine intake over 
the past several decades, but the intake has now stabilized.86 The 
RAIU may be normal or even increased when hypothyroidism 
results primarily from a biochemical defect in thyroid hormone 
synthesis rather than thyroid cell destruction leading to compensa­

than thyroid cell destruction leading to compensa­
tory thyroid enlargement. Specific functional patterns in relation 
to the causes of hypothyroidism are discussed later. Nonetheless, 
measurement of RAIU is almost never required in the diagnostic 
evaluation of the hypothyroid patient. 
The clinical picture of fully developed hypothyroidism is quite 
characteristic, but the abnormalities can be overlooked, even by 
experienced clinicians, if the diagnosis is not consider

enced clinicians, if the diagnosis is not considered. Despite 
the availability of inexpensive and specific tests, it is still surprising 
how often what is retrospectively obvious, severe, primary hypo­
thyroidism is not recognized. A high index of suspicion is required 
to avoid this oversight.
For the milder forms of hypothyroidism, the clinical presenta­
tion overlaps to a significant extent with other conditions. The fact 
that these disorders often occur in older patients is partly respon­

rs often occur in older patients is partly respon­
sible for the diagnostic uncertainty.2 In some cases, slowing of 
mental and physical activity, dry skin, and loss of hair may mimic 
similar findings in hypothyroidism. Furthermore, older people 
often become hypothermic with cold exposure. In patients with 
chronic renal insufficiency, anorexia, torpor, periorbital puffiness, 
sallow complexion, and anemia (e.g., see Fig. 13.1) may suggest 
hypothyroidism and may call for specific testing. Dis

othyroidism and may call for specific testing. Distinguishing 
nephrotic states from hypothyroidism by clinical examination 
alone may be even more difficult. In this disorder, waxy pallor, 
edema, hypercholesterolemia, and hypometabolism may suggest 
hypothyroidism. In addition, the total serum T4 concentration 
may be decreased if significant thyroid-binding globulin is lost in 
the urine but the FT4 and TSH would be normal. In patients with 
pernicious anemia, psychiatric abnormalities, pallo

ernicious anemia, psychiatric abnormalities, pallor, and numb­
ness and tingling of the extremities may mimic similar findings 
in hypothyroidism. Although there is a clinical and immunologic 
overlap between primary hypothyroidism and pernicious anemia, 
this association is not invariable (see Chapter 43). The presence 
of hypothyroidism is often suspected in patients who are severely 
ill, especially in the elderly.34,87 In such patients, the total T4 con­
centration may be decreased, often ma

otal T4 con­
centration may be decreased, often markedly so, but the FT4 is 
generally normal unless the patient is severely ill (see Chapter 11). 
  Laboratory Evaluation of Patients With 
Suspected Hypothyroidism or Thyroid 
Enlargementa
TSH, Free T4
TPOAb
Diagnosis
TSH >10 mU/L
Low
+
Primary hypothyroidism due to 
autoimmune thyroid disease
Low-normal
+
Primary “subclinical” hypothyroidism 
(autoimmune)
Low or 
low-normal
−
Recovery from systemic illness
External irradiation, drug-induced, 

emic illness
congenital hypothyroidism
Seronegative autoimmune thyroid 
disease
Rare thyroid disorders (amyloidosis, 
sarcoidosis, etc.)
Recovery from subacute granulomatous 
Normal
+, −
Consider TSH or T4 assay artifacts
Elevated
−
Blockade of T4 to T3 conversion (amio­
darone) or a congenital 5′-deiodinase 
Consider assay artifacts
TSH 5–10 mU/L
Low, 
+
Early primary autoimmune h

mU/L
Low, 
+
Early primary autoimmune hypothyroid­
ism
Low, 
−
Milder forms of nonautoimmune 
hypothyroidism (see earlier)
Central hypothyroidism with impaired 
TSH bioactivity
Elevated
− (+)
Consider thyroid hormone resistance
T4 to T3 conversion blockade (e.g., 
amiodarone)
TSH 0.5–5 mU/L
Low, 
− (+)
Salicylate or phenytoin therapy
Desiccated thyroid or T3 replacement
TSH <0.5 μU/L
Low, 
− (+)
“Post-hyperthyroid” hypothyroidism

ow-normal
− (+)
“Post-hyperthyroid” hypothyroidism (131I 
or surgery)
T3 or desiccated thyroid excess
Following excess levothyroxine with­
drawal
aInitial tests: serum TSH, serum free T4, TPO, or TgAb.
TgAb, Anti-thyroglobulin antibody; TPOAb, thyroid peroxidase autoantibody; TSH, thyroid-
stimulating hormone (thyrotropin); +, present; −, not present.
   
TABLE 13.2
414
These features, together with the absence of an elevation of serum 
TSH, usually 

he absence of an elevation of serum 
TSH, usually serve to differentiate the ill euthyroid patient from 
one with primary hypothyroidism. The serum TSH, however, can 
be transiently increased (up to 20 mU/L) during recovery from 
severe illness.88
Hypothyroidism may develop either because of some extrin­
sic factor or acquired condition or because of a congenital defect 
impairing thyroid hormone biosynthesis (see Table 13.1). Inad­
equate synthesis of hormone leads to hypersecretion of TSH, 
wh

sis of hormone leads to hypersecretion of TSH, 
which in turn produces both goiter and stimulation of all steps 
in hormone biosynthesis capable of response. In some instances, 
however, the compensatory TSH response overcomes the impair­
ment in hormone biosynthesis, and the patient is euthyroid with 
a goiter. The latter condition is discussed in Chapter 14 regard­
ing simple or nontoxic goiter. Less commonly, hypothyroidism 
is associated with an atrophic gland or, in the case of a congenital

 an atrophic gland or, in the case of a congenital 
abnormality, one that never developed properly. Hypothyroidism 
occurs in about 20% of patients after surgical lobectomy, with an 
increased risk in areas of iodine insufficiency or in patients with 
anti-TPOAb.89 
Symptoms and signs suggesting hypothyroidism
TSH normal or low
Free T4 low or low normal
No phenytoin
No salicylates
No recent thyrotoxicosis
TSH increased
Free T4 low or
low normal
MRI
T

entral hypothyroidism
MRI
TPO antibody
Serum TSH and
free T4 or free T4I
Permanent
T4
Increased
Normal
Normal
thyroid
Reduce T4 by
50% for 6 weeks
Check TSH
Check
adrenal, prolactin,
gonads
Treat with surgery
or drugs after
correction of
adrenal, thyroid,
other deficiencies
? Transient
hypothyroidism?
Postviral
Absent
Present
Hashimoto
disease
T4
~4 months
T4
Congenital TRH,
TSH deficiency
infiltrative disease
of pituitary,
hypothalamus
Normal
Ab

ative disease
Normal
Abnormal
Pituitary or
hypothalamic
lesion
• Fig. 13.5  Strategy for the laboratory evaluation of patients with suspected hypothyroidism. The principal 
differential diagnosis is between primary and central hypothyroidism (see Chapter 9). The serum thyro­
tropin (TSH) concentration is the critical laboratory determination that in general allows recognition of the 
cause of the disease. An exception is the individual with a recent history of thyrotox

s the individual with a recent history of thyrotoxicosis (and suppressed 
TSH) in whom a low free thyroxine (T4) level may be associated with a reduced TSH level for several 
months after relief of the thyrotoxicosis. In patients with primary hypothyroidism, the absence of thyroid 
peroxidase (TPO) antibodies raises a possible diagnosis of transient hypothyroidism following an undi­
agnosed episode of subacute or postviral thyroiditis. In such patients, a trial of levothyroxine in reduced 
dosag

tients, a trial of levothyroxine in reduced 
dosage after 4 months may reveal recovery of thyroid function, thus avoiding permanent levothyroxine 
replacement. MRI, magnetic resonance imaging; TRH, thyrotropin-releasing hormone; T4I, thyroxine index.
415
Classification
Immune-Mediated
Autoimmune Hypothyroidism
Autoimmunity is responsible for over 90% of noniatrogenic hypo­
thyroidism in countries with iodine sufficiency. The annual inci­
dence of autoim

dine sufficiency. The annual inci­
dence of autoimmune hypothyroidism is around 80 per 100,000 
men and 350 per 100,000 women.90 All ages may be affected, 
although the average age of onset is between 40 and 60 years. 
The disorder is more frequent in whites and Asians than in Afri­
can Americans. The initial presentation depends on the stage of 
disease. Juvenile and adolescent autoimmune thyroiditis may be 
self-limiting. Hashimoto thyroiditis is the commonest cause of 
goiter in iodine-suffic

is the commonest cause of 
goiter in iodine-sufficient regions; atrophic thyroiditis (primary 
myxedema) presents as hypothyroidism without a goiter.
Circulating autoantibodies against thyroglobulin and TPO 
are present in almost all patients with autoimmune hypothyroid­
ism. Up to 20% of patients with autoimmune hypothyroidism 
have TSH receptor antibodies that block the receptor, rather than 
stimulating it, as in Graves disease; in rare patients there may be 
switching from one type of antibo

ts there may be 
switching from one type of antibody to the other, resulting in 
alternating hypothyroidism and hyperthyroidism.91,92 Less com­
monly, patients produce autoantibodies against the sodium-iodide 
symporter (NIS), pendrin, and T4 and T3, but the functional rel­
evance of these antibodies is not known.
Around 15% of women and 3% of men have positive thy­
roid autoantibodies but no other clinical features of thyroid 
disease; most of them, however, will have histologic evidence of 
fo

hem, however, will have histologic evidence of 
focal thyroiditis. Longitudinal studies have shown that euthyroid 
women with high initial levels of autoantibodies against thyro­
globulin or TPO and those whose TSH is within the upper half 
of the reference interval are the most likely to progress to overt 
hypothyroidism.93
Autoimmune hypothyroidism is commonly found in associa­
tion with a range of autoimmune disorders, including pernicious 
anemia, systemic lupus erythematosus, Addison diseas

emia, systemic lupus erythematosus, Addison disease, celiac 
disease, and vitiligo.23,94 A steroid-responsive encephalopathy 
(referred to as Hashimoto encephalopathy) has been reported in 
individuals with positive TPOAb, irrespective of thyroid dysfunc­
tion, but it is unclear whether there is a true causal relationship, 
for instance, through immunologic cross-reactivity with brain 
tissue.53,95
Pathophysiology. The current understanding of autoimmune 
mechanisms has been discussed in Chapter

toimmune 
mechanisms has been discussed in Chapter 12, and the main 
features associated with autoimmune hypothyroidism are sum­
marized in Fig. 13.6. T-cell–mediated tissue injury is believed to 
be the most important cause of autoimmune thyroid follicular 
cell destruction. Perforin-containing cytotoxic CD8+ T cells are 
abundant in the intrathyroidal lymphocytic infiltrate in Hashi­
moto thyroiditis. These T cells increase during the evolution of 
disease and recognize both thyroglobulin and 

 of 
disease and recognize both thyroglobulin and TPO.96 Apoptosis is 
an additional pathway for thyroid cell destruction. In Hashimoto 
thyroiditis, thyroid follicular cells express both Fas (CD95) and 
Fas ligand (CD95L) and may thus self-destruct when these mol­
ecules interact; it is now clear that other decoy death receptors and 
regulators of apoptosis signaling play an additional role.97 There 
is also an increase in the number of intrathyroidal Th17 lympho­
cytes in Hashimoto thyroiditis

roidal Th17 lympho­
cytes in Hashimoto thyroiditis, implying a pathogenic role for 
this proinflammatory T-cell subset.98 The differentiation of Th17 
cells may be enhanced by iodine. Cytokines released by T cells 
and other inflammatory cells cause Hürthle cell formation and 
thyroid dysfunction. The thyroid cells also respond to cytokines 
by expressing a number of proinflammatory molecules, such as 
chemokines and adhesion molecules, which increase the potential 
for T-cell binding and cytoto

rease the potential 
for T-cell binding and cytotoxicity.
Apart from the striking activity of TSH receptor blocking anti­
bodies, which can induce temporary neonatal hypothyroidism fol­
lowing their transfer across the placenta,99,100 the pathogenic role 
of antibodies in autoimmune hypothyroidism is unclear.101 No 
neonatal disorders have been associated with the presence of high 
thyroglobulin or TPO autoantibodies in mothers, indicating that 
any role in tissue injury is likely to be secondar

any role in tissue injury is likely to be secondary to an initial phase 
of T-cell–mediated damage, which allows the autoantibodies to 
access their target antigens. Such injury may be mediated through 
antibody-dependent cell-mediated cytotoxicity, involving natural 
killer (NK) cells, or through complement fixation in the case of 
TPO antibodies.102 
Histopathology. The pathologic features of autoimmune 
hypothyroidism vary from mild focal thyroiditis to extensive 
lymphocytic infiltration and

oiditis to extensive 
lymphocytic infiltration and fibrosis. In classical Hashimoto 
thyroiditis (originally termed struma lymphomatosa), the thyroid 
gland may be diffusely enlarged or nodular; the tissue is pale 
and firm and has a rubbery texture (Fig. 13.7A). Typically there 
is a diffuse lymphocytic infiltration with germinal center for­
mation and obliteration of thyroid follicles, accompanied by a 
variable degree of fibrosis (see Fig. 13.7B). Destruction of thy­
roid epithelial cells occ

7B). Destruction of thy­
roid epithelial cells occurs as disease progresses from euthyroid­
ism to hypothyroidism; in some patients there is follicular cell 
metaplasia and the formation of Hürthle cells. Rarely, there are 
concurrent histologic changes of Graves disease, so-called hashi­
toxicosis. In the other broad type of autoimmune hypothyroid­
ism, termed atrophic thyroiditis or primary myxedema, the gland 
is atrophied and consists of extensive fibrotic tissue, moderate 
lymphocytic infil

nsive fibrotic tissue, moderate 
lymphocytic infiltration, and widespread loss of thyroid follicles, 
but the fibrosis is not as extensive as in Riedel thyroiditis (see 
Fig. 13.7C). The histopathologic changes in painless thyroiditis 
resemble Hashimoto thyroiditis.
Although it is now generally thought that these variations rep­
resent a spectrum of disease arising from a common underlying 
autoimmune process, a distinct subset of patients with Hashimoto 
thyroiditis has been delineated recentl

Hashimoto 
thyroiditis has been delineated recently in whom there are high 
circulating levels of IgG4 and increased numbers of IgG4-posi­
tive plasma cells in the thyroid. Such IgG4-related thyroiditis is 
characterized pathologically by a greater degree of stromal fibrosis, 
lymphoplasmacytic infiltration, and hypothyroidism.103 
Risk Factors
Genetic Susceptibility. The importance of genetic factors in the 
cause of autoimmune hypothyroidism is indicated by the frequent 
presence of thyroid au

 indicated by the frequent 
presence of thyroid autoantibodies, thyroid disease, and other 
autoimmune disorders in family members and by twin studies, 
which show a high concordance rate (0.55) in monozygotic but 
not dizygotic twins.104 As with all autoimmune endocrinopathies, 
human leukocyte antigen (HLA)–D region polymorphisms play 
a role in susceptibility, and Hashimoto thyroiditis is associated 
with HLA-DR3 and to a lesser extent HLA-DR4.105 Polymor­
phisms in the PD1 gene also confer s

.105 Polymor­
phisms in the PD1 gene also confer susceptibility, with lesser con­
tributions from polymorphisms in the CD40 gene and the gene 
encoding thyroglobulin, as well as other unconfirmed candidate 
genes.106,107 It is clear that new analytic approaches are likely to 
reveal many other genes, which make small etiologic contributions 
that account for the diversity of clinical presentation. For example, 
a combination of novel genetic markers has been described and 
is associated with an 

ers has been described and 
is associated with an increased risk of progression from TPOAb 
416
positivity to hypothyroidism, including polymorphism in the 
MAGI3 gene.108
Shared genetic susceptibility accounts for the frequent occur­
rence of other autoimmune disorders in patients with autoimmune 
hypothyroidism. Around half of women with Turner syndrome 
are positive for thyroid autoantibodies and a third develop hypo­
thyroidism.109 There is also an increase in autoimmu

yroidism.109 There is also an increase in autoimmune hypothy­
roidism in children with Down syndrome, which may evolve into 
Graves disease in some cases.110 
Nongenetic Risk Factors. Many of the factors that have been 
identified as increasing the risk for Graves disease (pregnancy, 
drugs, age, sex, iodine, and irradiation) apply equally to autoim­
mune thyroiditis. These factors are detailed in Chapter 12 but 
briefly considered here. Epidemiologic data suggest a strong influ­
ence of environ

logic data suggest a strong influ­
ence of environmental factors in Hashimoto thyroiditis, as this 
was a rare disease before the 1950s but it is now one of the most 
common autoimmune disorders.111 
Sex and Pregnancy. The female preponderance of autoimmune 
hypothyroidism may be due to sex hormones; skewed X-chromo­
some inactivation has also been proposed as an additional explana­
tion. During pregnancy, fetal tolerance is maintained by changes 
in immunoregulation that have the coincidental e

in immunoregulation that have the coincidental effect of improv­
ing thyroid autoimmunity but then lead to postpartum exacer­
bation of the autoimmune process.112 This phenomenon results 
in transient postpartum thyroiditis, a form of painless subacute 
thyroiditis (see Chapter 12), and in 10% to 50% of these cases 
permanent hypothyroidism may appear over the next decade.113 
Those women with hypothyroidism and positive TPO antibodies 
during the phase of postpartum thyroiditis are most at ri

the phase of postpartum thyroiditis are most at risk of such 
an outcome. 
Iodine and Selenium. An excessive intake of iodine can pre­
cipitate autoimmune thyroiditis in susceptible populations.114 
This form of hypothyroidism should be distinguished from 
direct blockade of a thyroid gland by iodine (the Wolff-Chaikoff 
effect).115 Evidence accumulated in animal models suggests that 
increased iodination of thyroglobulin enhances its immunoreac­
tivity, and iodine may also cause thyroid injury 

tivity, and iodine may also cause thyroid injury through the gen­
eration of reactive oxygen metabolites.116 There are epidemiologic 
data to suggest that selenium deficiency exacerbates autoimmune 
thyroiditis, but trials of selenium supplementation have been 
inconclusive with regard to clinical benefit.117,118 
Drugs and Smoking. Treatment of patients with cytokines may 
precipitate the appearance of autoimmune thyroid disease in the 
form of Hashimoto thyroiditis or Graves disease (see late

 Hashimoto thyroiditis or Graves disease (see later).119 
A number of novel anticancer treatments, including TKIs (such 
as sunitinib), cytotoxic T-lymphocyte–associated protein 4 
(CTLA4) blockers (such as ipilimumab), and PD1 blockers (such 
as pembrolizumab and nivolumab), can also induce autoimmune 
thyroiditis.120,121 There is a higher than expected prevalence of 
autoimmune hypothyroidism in patients treated with lithium. 
Anthracene derivatives and other chemicals produce autoim­
mune thy

tives and other chemicals produce autoim­
mune thyroiditis in animals, but the role of environmental toxins 
in human disease is poorly studied. Smoking is associated with a 
decreased risk of autoimmune thyroiditis, but the risk rises tem­
porarily when smoking is stopped.122 Moderate alcohol consump­
tion is also protective.123 
Irradiation. Radiation exposure has been shown to induce 
thyroid autoantibodies and autoimmune thyroid disease in a 
number of studies. These exposures include radiat

number of studies. These exposures include radiation from the 
atomic bomb detonation in Japan124 and radioactive fallout from 
the Chernobyl disaster, which was followed by an increase in the 
prevalence of thyroid autoantibodies in exposed children, with a 
small overall increase in the prevalence of hypothyroidism 12 to 
14 years later.125 Hodgkin disease survivors have a 17-fold relative 
risk of developing hypothyroidism, but this may in part be a direct 
effect of irradiation.126 
Age. Au

rt be a direct 
Age. Autoimmune hypothyroidism continues to occur through­
out adult life (except in the very elderly whose longevity may be 
Th1 and Th17 cells and
macrophages releasing
cytokines, which cause
tissue injury
Immune complexes fix
complement, resulting
in tissue injury
NK cell
mediating ADCC
Loss of tight junction after injury
allows thyroid autoantibodies to
access autoantigens, leading to
complement fixation and ADCC
Apoptosis triggered
by Fas-Fas ligan

tion and ADCC
by Fas-Fas ligand
binding and other
death receptors
Th1
CD8+
Th17
Macrophage
CD8+ cytotoxic
T cell
mediating
cytolysis
NK
• Fig. 13.6  Summary of the main mechanisms involved in the pathogenesis of autoimmune hypothyroid­
ism. ADCC, antibody-dependent cell-mediated cytotoxicity; NK, natural killer.
417
associated with superior immunoregulation) so that the preva­
lence of the disease increases markedly with age.12,17,87

f the disease increases markedly with age.12,17,87 This fea­
ture is similar to other types of autoimmunity and may reflect an 
increasing loss of tolerance to self. 
Infection. There is no direct evidence that infection causes 
autoimmune thyroiditis in humans, although there is some evi­
dence that the hepatitis C virus may precipitate thyroid disease 
in susceptible patients.118 In addition, there is follow-up evi­
dence from patients with painful subacute thyroiditis, following 
a viral infe

nful subacute thyroiditis, following 
a viral infection (see later), that long-term hypothyroidism occurs 
in around 15% of patients and some of these cases may have an 
autoimmune basis. 
Clinical Picture. Goiter, the hallmark of classic Hashimoto 
disease, usually develops gradually and may be found during rou­
tine examination or by ultrasonography. On occasion, the thyroid 
gland enlarges rapidly and, when accompanied by pain and ten­
derness, may mimic painful subacute thyroiditis (see Chap

, may mimic painful subacute thyroiditis (see Chapter 12). 
Some patients are hypothyroid when first seen. The goiter is gener­
ally painless, moderate in size, and firm in consistency and moves 
freely on swallowing. The surface can be either smooth or nodular. 
Both lobes are enlarged, but the gland may be asymmetric. The 
pyramidal lobe may also be enlarged; rarely, adjacent structures, 
such as the trachea, esophagus, and recurrent laryngeal nerves, 
may be compressed. Enlargement of regiona

nerves, 
may be compressed. Enlargement of regional lymph nodes is 
unusual.
Other patients with hypothyroidism present without a 
goiter (atrophic thyroiditis), which is thought to be the end 
result of autoimmune destruction of the thyroid, although 
the progression of goitrous Hashimoto thyroiditis to the atro­
phied state is not commonly seen in the individual patient. 
The atrophic thyroid is likely the reflection of rapid destruc­
tion early in the onset of autoimmune thyroiditis, combined

y in the onset of autoimmune thyroiditis, combined 
in some patients with TSH receptor antibodies of the blocking 
variety, although such antibodies may also occur in those with 
a goiter. Generally the disease tends to progress slowly with an 
increase in fibrous tissue and loss of thyroid follicular cells. A 
study of thyroid volume by ultrasound in patients with newly 
diagnosed autoimmune hypothyroidism found that there was 
a continuum of thyroid size, with atrophy and goiter repre­
senting

yroid size, with atrophy and goiter repre­
senting extremes of the distribution, supporting the idea that 
these are not distinct entities but part of the same underlying 
autoimmune process.127
A
B
C
• Fig. 13.7  Hashimoto thyroiditis. (A) Gross appearance of a cut section of the thyroid lobe demonstrating 
the pale color of the tissue due to lymphocytic infiltration, fibrosis, and loss of follicles. (B) Typical histologic 
appearance illustrating a germinal center, heavy lymphocytic infiltrati

ng a germinal center, heavy lymphocytic infiltration, and a partially disrupted thyroid fol­
licle. (C) Fibrous variant showing extensive fibrosis and loss of follicles. (A and C, Courtesy Dr. Vania Nosé, 
Brigham and Women’s Hospital, Boston, MA; B, From Nosé V, Asa SL, Erickson LA, et al. Diagnostic 
Pathology: Endocrine. Salt Lake City: Amirsys; 2012.)
418
Clinically the untreated goiter remains unchanged or enlarges 
gradually over many years. The manifestations of hyp

adually over many years. The manifestations of hypothyroid­
ism vary and often develop over many years in patients who are 
initially euthyroid. Thyroid lymphoma occurs almost exclusively 
in patients with underlying Hashimoto thyroiditis and should be 
suspected if there is rapid and sometimes painful enlargement of 
the thyroid gland.128 As mentioned earlier, the presence of coex­
istent Hashimoto thyroiditis may be a favorable prognostic fac­
tor in patients with papillary carcinoma, but the 

tor in patients with papillary carcinoma, but the risk of papillary 
carcinoma is probably not increased in Hashimoto thyroiditis.129
Occasionally hyperthyroidism due to Graves disease develops 
in patients with Hashimoto thyroiditis. In other patients with 
early autoimmune thyroiditis, transitory thyrotoxicosis (pain­
less thyroiditis with thyrotoxicosis) occurs as the result of thy­
roid cell destruction. In such cases, evidence of ongoing thyroid 
hyperfunction is lacking because the thyroid

roid 
hyperfunction is lacking because the thyroid RAIU is depressed. 
As described earlier, 50% of women with thyroid autoantibod­
ies who are euthyroid in the first trimester of pregnancy develop 
postpartum thyroiditis, accompanied by transient thyrotoxicosis, 
hypothyroidism, or fluctuation from one state to the other.130 
Laboratory Tests. The results of the common tests of thyroid 
function depend on the stage of the disease (see Table 13.2). 
Rarely, the tests may suggest thyrotoxicosis w

). 
Rarely, the tests may suggest thyrotoxicosis with a suppressed TSH 
and elevated serum T4 and T3 levels, due to either release of stored 
thyroid hormone as a result of rapid tissue destruction or the 
relative overproduction of autoantibodies, which stimulate rather 
than block the TSH receptor (sometimes called hashitoxicosis). In 
the latter case, the RAIU may be increased, whereas it is decreased 
if there is tissue destruction. Typically, patients with Hashimoto 
thyroiditis present wit

, patients with Hashimoto 
thyroiditis present with a goiter, and blood tests show a normal or 
slightly raised TSH, with normal serum T4 and T3 levels. As tissue 
destruction continues, the TSH level rises further, but the abil­
ity of the thyroid to respond to TSH diminishes, and the RAIU 
and serum T4 level decline to subnormal values, resulting in overt 
hypothyroidism. This is the typical biochemical finding in atro­
phic thyroiditis, in which there is no goiter to alert the patient or 
phy

ch there is no goiter to alert the patient or 
physician to the underlying disorder. The serum T3 level remains 
normal until late in the disease process, reflecting maximal stimu­
lation of the failing thyroid by the increased serum TSH. The early 
phase of the foregoing sequence, when the serum TSH is increased 
but T4 and T3 are still normal, is termed subclinical hypothyroidism 
(see Table 13.2).
The diagnosis of autoimmune hypothyroidism is confirmed 
by the presence of thyroid autoantibodi

confirmed 
by the presence of thyroid autoantibodies in the serum, usually 
in high levels. TPO and thyroglobulin autoantibodies occur in 
roughly similar frequencies; the presence of both autoantibodies 
is twice as frequent as the isolated individual autoantibodies.131 
Thyroid autoantibodies may be absent in rare patients due to assay 
insensitivity or the occurrence of an entirely intrathyroidal auto­
immune process. Sometimes part of a gland with autoimmune 
thyroiditis may look and feel li

 with autoimmune 
thyroiditis may look and feel like a firm thyroid nodule, and ultra­
sonography or even aspiration biopsy should be performed to con­
firm the diagnosis. 
Differential Diagnosis. Differentiation of autoimmune hypo­
thyroidism from other forms of hypothyroidism is facilitated 
by the demonstration that high levels of thyroid autoantibodies 
occur more commonly than in other thyroid disorders. The fre­
quent coexistence of hypothyroidism and Hashimoto thyroiditis 
serves to disti

roidism and Hashimoto thyroiditis 
serves to distinguish this disease from nontoxic goiter and thyroid 
neoplasia.
Differentiation of a euthyroid Hashimoto goiter from a mul­
tinodular goiter is often difficult without ultrasonography, and 
diffuse nontoxic goiter tends to be softer than that of Hashimoto 
thyroiditis. Ultrasound examination typically reveals a diffuse and 
patchy heterogeneous echotexture or hypoechoic micronodules 
with echogenic septations in Hashimoto thyroiditis. In adoles­

ic septations in Hashimoto thyroiditis. In adoles­
cents, differentiation of Hashimoto goiter from diffuse nontoxic 
goiter is even more difficult because in this age group Hashimoto 
thyroiditis may not be accompanied by such high levels of thy­
roid autoantibodies. The presence of well-defined nodules usu­
ally distinguishes nontoxic multinodular goiter from Hashimoto 
Differentiation between euthyroid Hashimoto thyroiditis and 
thyroid carcinoma can sometimes be made on clinical 

yroid carcinoma can sometimes be made on clinical grounds, 
but an ultrasound examination and aspiration biopsy are neces­
sary in any case in which there is uncertainty. Lymphoma must 
always be excluded if there is a sudden change in a known Hashi­
moto goiter; core needle biopsy or open surgical biopsy may be 
required for final diagnosis. Thyroid carcinoma usually occurs as a 
solitary nodule that is firm or hard, and the gland may be fixed to 
adjacent structures. Compression of the recurre

o 
adjacent structures. Compression of the recurrent laryngeal nerve 
with hoarseness is virtually pathognomonic of thyroid carcinoma 
but occurs late in the cancer progression. A history of a recent 
enlargement of the goiter is more common in thyroid malignan­
cies (either carcinoma or lymphoma) than in Hashimoto thyroid­
itis. Enlargement of regional lymph nodes also suggests thyroid 
malignancy but can rarely occur in autoimmune thyroiditis. 
Treatment. In euthyroid patients with Hashimoto t

Treatment. In euthyroid patients with Hashimoto thyroiditis, 
no treatment is required because the goiter is usually asymptom­
atic. Levothyroxine treatment may be indicated in patients when 
the goiter presses on adjacent structures or is unsightly, and it is 
most effective in goiters of recent onset. The aim is to keep the 
TSH level in the lower half of the reference interval. In long-stand­
ing goiter, treatment with thyroid hormone is usually ineffective, 
possibly because of fibrosis. Ra

lly ineffective, 
possibly because of fibrosis. Rarely, the goiter may be painful, and 
this symptom may respond to levothyroxine therapy. Glucocorti­
coids, such as prednisolone, are usually ineffective. Surgery may be 
justified if symptoms or unsightly enlargement persists after a trial 
of levothyroxine therapy.
Replacement doses of thyroid hormone should be given when 
hypothyroidism is present, appropriate to the degree of hor­
mone deficiency (see later). There may be a spontaneous return

ncy (see later). There may be a spontaneous return 
to euthyroidism in up to 10% of patients after starting levothy­
roxine, associated in some cases with the disappearance of TSH 
receptor blocking antibodies. However, it has not been established 
that such remissions are durable and there is no need to routinely 
stop levothyroxine once it has been started. It has recently been 
suggested that prednisolone treatment can reverse the hypothy­
roidism of IgG4-related thyroiditis, but the long-ter

dism of IgG4-related thyroiditis, but the long-term outcome in 
such cases is not yet known.132 
Iodine Deficiency (Endemic Goiter)
The term endemic goiter denotes any goiter occurring in a region 
where goiter is prevalent.132 As mentioned, endemic goiter almost 
always occurs in areas of environmental iodine deficiency.133 
Although this condition is estimated to affect more than 200 mil­
lion people throughout the world and is of major public health 
significance, it is most common in mountai

health 
significance, it is most common in mountainous areas, such as the 
Alps, Himalayas, and Andes, or in the Great Lakes and Missis­
sippi Valley regions of the United States, owing to the depletion of 
iodine consequent to the persistent glacial run-off in these regions.
The causative role of iodine deficiency in the genesis of endemic 
goiter is supported by the inverse correlation between the iodine 
content of soil and water and the incidence of goiter, the kinetics 
419
Chapter 13  Hypo

ence of goiter, the kinetics 
419
of iodine metabolism in patients with the disorder, and a decrease 
in incidence after iodine prophylaxis. The latter accounts for its 
absence in the population residing in the Great Plains region of 
the United States.
The occurrence of endemic goiter can vary, even within an 
area of known iodine deficiency; the roles of dietary minerals or 
naturally occurring goitrogens and of pollution of water supplies 
have been

gens and of pollution of water supplies 
have been suggested in instances of this type.134 For example, in 
the Cauca Valley of Colombia, waterborne goitrogens have been 
implicated, and in many areas of endemic iodine deficiency, 
consumption of cassava meal, which gives rise to thiocyanate, 
aggravates the iodine-deficient state by inhibiting thyroid iodide 
transport. Familial clustering of goiters within iodine-insufficient 
areas, usually with an autosomal dominant inheritance, suggests 
an

th an autosomal dominant inheritance, suggests 
an important genetic component.135
Most abnormalities in iodine metabolism in patients with 
endemic goiter are consistent with the expected effects of iodine 
deficiency (see Chapter 11 regarding iodine metabolism). Thyroid 
iodide clearance rates and RAIU are increased in proportion to 
the decrease in the urinary excretion of stable iodine. The abso­
lute iodine uptake is normal or low. In areas of moderate iodine 
deficiency, the serum T4 conce

of moderate iodine 
deficiency, the serum T4 concentration is usually in the lower 
range of normal; in areas of severe deficiency, however, values are 
decreased. Nevertheless, most patients in these areas do not appear 
to be in a hypothyroid state because of an increase in the synthesis 
of T3 at the expense of T4 and because of an increase in the activity 
of thyroidal D1 and D2.84 TSH levels are typically in the upper 
range of normal.
The incidence and severity of endemic goiter and the me

ncidence and severity of endemic goiter and the meta­
bolic state of the goitrous patient depend mainly on the degree of 
iodine deficiency. In the absence of hypothyroidism, the effects of 
the goiter are mainly cosmetic. When the goiter becomes nodu­
lar, however, hemorrhage into a nodule may cause acute pain and 
swelling, mimicking painful subacute thyroiditis or neoplasia. 
The goiter may also compress adjacent structures, such as the tra­
chea, esophagus, and recurrent laryngeal nerves. Th

hea, esophagus, and recurrent laryngeal nerves. The borderline 
nature of the iodine supply in many countries of Western Europe 
is exemplified by the development of compensatory maternal and 
fetal goiter during pregnancy due to the increased requirement for 
thyroid hormone during gestation.133
The incidence of endemic goiter has been greatly reduced in 
many areas by the introduction of iodized salt.133 In the United 
States, table salt is enriched with potassium iodide to a concen­
tration o

iched with potassium iodide to a concen­
tration of 0.01%, which, if the intake of salt is average, would 
provide an iodine intake of approximately 150 to 300 μg/day, the 
desired amount in an adult (see Table 11.1). The use of iodine-
containing flour in bread products and iodized salt in commer­
cially produced food has been markedly reduced.136 The iodine 
content of bread and infant formula is variable within a given 
product and often does not match the measured content.136 As 
mentioned, 

not match the measured content.136 As 
mentioned, iodine intake in the United States has been decreas­
ing in recent decades, likely due to reduced iodine in commercial 
food products, although iodine intake has now stabilized. Preg­
nant women, however, remain a susceptible population because 
of their increased iodine requirements.134 Most prescription pre­
natal vitamins do not contain iodine.137 An annual injection of 
iodized oil is another effective means of administering iodine, and 
ende

effective means of administering iodine, and 
endemic goiter can be treated by the addition of iodine to com­
munal drinking water.
Administration of iodine has little if any effect on a long-stand­
ing endemic goiter, but it causes the early endemic hyperplastic 
goiter of iodine deficiency to regress. Similarly, thyroid hormone 
usually has no effect on long-standing goiter or on established 
mental or skeletal changes, but it should be given in full-replace­
ment doses if there is evidence of

n full-replace­
ment doses if there is evidence of hypothyroidism. This is of 
paramount importance in pregnant women. Surgical treatment is 
indicated if the adjacent structures are compressed or if the goiter 
is either very large or enlarging rapidly. 
Endemic Cretinism
Endemic cretinism is a developmental disorder that occurs in 
regions of severe endemic goiter.133 Both parents of an endemic 
cretin are usually goitrous, and in addition to the features of spo­
radic cretinism described earl

he features of spo­
radic cretinism described earlier, endemic cretins often have deaf-
mutism, spasticity, motor dysfunction, and abnormalities in the 
basal ganglia demonstrable by magnetic resonance imaging.
Three types of cretins can be discerned: (1) hypothyroid cretins, 
(2) neurologic cretins, and (3) cretins with combined features of 
the two. The pathogenesis of neurologic cretinism is obscure but 
may be due to severe thyroid hormone deficiency during a criti­
cal early phase of centra

ficiency during a criti­
cal early phase of central nervous system development in utero.44 
Some cretins are goitrous, but the thyroid may also be atrophic, 
possibly as a consequence of exhaustion atrophy from continuous 
overstimulation or the lack of iodine. 
Iodide Excess
Goiter and hypothyroidism, either alone or in combination, are 
sometimes induced by chronic administration of large doses of 
iodine in either organic or inorganic form (see Table 11.7).133,138 
Iodide-induced goiter was f

e Table 11.7).133,138 
Iodide-induced goiter was formerly seen in patients with chronic 
respiratory disease who were given potassium iodide as an expec­
torant. The development of iodide goiter has also been reported 
after a single administration of radiographic contrast medium 
from which iodide is released slowly over a long period and may 
also occur during amiodarone administration. Iodide goiter with­
out hypothyroidism may occur endemically, such as on the island 
of Hokkaido, Japan, whe

ly, such as on the island 
of Hokkaido, Japan, where seaweed products are consumed in 
large quantities.
From an analysis of reported cases and from the fact that 
only a small percentage of patients who receive iodides chroni­
cally develop goiter, it appears that the disorder evolves on a 
background of underlying thyroid dysfunction.138 Categories of 
susceptible individuals include the following: patients with Hashi­
moto disease, patients with Graves disease especially after treat­
ment wit

th Graves disease especially after treat­
ment with radioiodine, and patients with cystic fibrosis.
Among these groups, many individuals display a positive 
iodide-perchlorate discharge test, indicating a defect in the thy­
roidal organic iodine-binding mechanism (see Chapter 11 regard­
ing iodine metabolism). However, intrinsic thyroid disease need 
not be present because a propensity to develop iodide goiter and 
hypothyroidism has also been demonstrated in patients who have 
undergone hemithy

monstrated in patients who have 
undergone hemithyroidectomy for a solitary thyroid nodule in 
whom the remaining lobe was histologically normal.89 In these 
patients, as in those with Hashimoto disease or Graves disease 
studied prospectively, individuals with the highest basal serum 
TSH concentrations, even within the normal range, were those 
who developed iodide goiter. Iodinated contrast material, amioda­
rone, and povidone-iodine are common sources.138
Goiter and hypothyroidism commonly o

n sources.138
Goiter and hypothyroidism commonly occur in newborns of 
women given large quantities of iodine during pregnancy, and 
death from neonatal asphyxia has been reported (see Fig. 11.11). 
In such cases, the mother is usually free from goiter. Pregnant 
women should not receive large doses of iodine (>1 mg/day) over 
prolonged periods (>10 days), especially near term. Maternal 
420
amiodarone therapy causes thyroidal dysfunction in up to 20% of 
newborns.133 It i

dal dysfunction in up to 20% of 
newborns.133 It is not known whether iodide goiter in newborns 
results from an inherent hypersensitivity of the fetal thyroid or 
from the fact that the placenta concentrates iodide several-fold, 
or both.
As discussed earlier (see Chapter 11, “Regulation of Thy­
roid Function”), large doses of iodine cause an acute inhibition 
of organic binding that abates in the normal individual, despite 
continued iodine administration (acute Wolff-Chaikoff effect and 
esca

inistration (acute Wolff-Chaikoff effect and 
escape).139 Iodide goiter appears to result from a more pronounced 
inhibition of organic binding and the failure of the escape phe­
nomenon. As a consequence of decreased hormone synthesis and 
the consequent increase in TSH, iodide transport is enhanced. 
Because inhibition of organic binding is a function of the intra­
thyroidal concentration of iodide, a vicious circle, augmented by 
this increase in serum TSH, is set in motion.
The disorder usua

 in serum TSH, is set in motion.
The disorder usually appears as a goiter with or without hypo­
thyroidism, although in rare instances iodine may produce hypo­
thyroidism unaccompanied by goiter. Usually the thyroid gland 
is firm and diffusely enlarged, often greatly so. Histopathologic 
examination reveals intense hyperplasia. The FT4 concentration 
is low, TSH concentration is increased, and the 24-hour urinary 
iodine excretion and the serum inorganic iodide concentration are 
increased. The

inorganic iodide concentration are 
increased. The disorder regresses after iodine is withdrawn. Thy­
roid hormone may also be given to relieve severe symptoms. 
Drugs Blocking Thyroid Hormone Synthesis or Release, 
Causing Goiter Formation
Ingestion of compounds that block thyroid hormone synthesis or 
release may cause goiter with or without hypothyroidism. Apart 
from the agents used in the treatment of hyperthyroidism, anti­
thyroid agents may be encountered either as drugs for the treat­
me

y be encountered either as drugs for the treat­
ment of disorders unrelated to the thyroid gland or as natural 
agents in foodstuffs.
Goiter with or without hypothyroidism can occur in patients 
given lithium, usually for bipolar manic-depressive psychosis.140 
Like iodide, lithium inhibits thyroid hormone release, and in high 
concentrations it can inhibit organic binding reactions. At least 
acutely, iodide and lithium act synergistically in the latter respect. 
The mechanisms underlying the s

e latter respect. 
The mechanisms underlying the several effects of lithium are 
uncertain; what differentiates patients who develop thyroid dis­
ease during lithium therapy from those who do not is also unclear. 
A promoting effect on underlying autoimmune thyroiditis may be 
at least one factor because many patients with this combination 
have autoimmune thyroid disease.
Other drugs that occasionally produce goitrous hypothyroid­
ism include para-aminosalicylic acid, phenylbutazone, aminoglu­

ra-aminosalicylic acid, phenylbutazone, aminoglu­
tethimide, and ethionamide. Like the thionamides, these drugs 
interfere with both the organic binding of iodine and perhaps in 
later steps in hormone biosynthesis. Although soybean flour is 
not an antithyroid agent, soybean products in feeding formulas 
formerly resulted in goiter in infants by enhancing fecal loss of 
hormone, which, together with the low iodine content of soybean 
products, produced a state of iodine deficiency. Feeding form

roduced a state of iodine deficiency. Feeding formulas 
containing soybean products are now enriched with iodine.
Cigarette smoking reduces the hypothyroidism in patients 
with underlying autoimmune thyroid disease, although the risk 
is transiently increased if smoking is stopped.122 Thiocyanate, 
hydroxypyridine, and benzopyrene derivatives found in cigarette 
smoke may also interfere with thyroid hormone action.141
Both the goiter and the hypothyroidism usually subside after 
the antithyroid 

thyroidism usually subside after 
the antithyroid agent is withdrawn. If continued administration 
of pharmacologic goitrogens is required, however, replacement 
therapy with thyroid hormone causes the goiter to regress. 
Goitrogens in Foodstuffs or as Endemic Substances or 
Pollutants
Antithyroid agents also occur naturally in foods. They are widely 
distributed in the family Cruciferae or Brassicaceae, particularly 
in the genus Brassica, including cabbages, turnips, kale, kohl­
rabi, rutabaga

ding cabbages, turnips, kale, kohl­
rabi, rutabaga, mustard, and various plants that are not eaten by 
humans but that serve as animal fodder.142 It is likely that some 
thiocyanate is present in such plants (particularly cabbage).143 
Cassava meal, a dietary staple in many regions of the world, con­
tains linamarin, a cyanogenic glycoside, the preparation of which 
leads to the formation of thiocyanate. Ingestion of cassava can 
accentuate goiter formation in areas of endemic iodine deficiency.

r formation in areas of endemic iodine deficiency. 
Except for thiocyanate, dietary goitrogens influence thyroid iodine 
metabolism in the same manner as do the thionamides, which 
they resemble chemically; their role in the induction of disease in 
humans is uncertain. Waterborne, sulfur-containing goitrogens of 
mineral origin are believed to contribute to the development of 
endemic goiter in certain areas of Colombia.
A number of synthetic chemical pollutants have been impli­
cated as a caus

emical pollutants have been impli­
cated as a cause of goitrous hypothyroidism, including polychlo­
rinated biphenyls and resorcinol derivatives.144 Perchlorate has 
also been noted in high concentrations in geographic regions in 
which explosives and rocket fuel were made. Perchlorate has been 
detected in water, food, and breast milk, although the amount 
does not appear to be sufficient to disrupt thyroid function. In 
an area of Chile with a high level of natural perchlorate contam­
ination 

high level of natural perchlorate contam­
ination in the water, thyroid function in pregnant women was 
not different from that in a region with no perchlorate, although 
iodine intake is quite high in this area.145 
Cytokines
Patients with chronic hepatitis C or various malignancies may 
be given interferon-α or interleukin 2.119,146 Such patients may 
experience hypothyroidism, which is often a transient destructive 
thyroiditis and associated with an initial thyrotoxic phase but in 
other cas

with an initial thyrotoxic phase but in 
other cases may persist. Graves disease with hyperthyroidism may 
also develop, and ablative therapy may be required to treat this 
condition. Women and those who have positive TPOAb prior to 
treatment are at higher risk for these complications and should be 
monitored especially carefully during and after a course of treat­
ment with either of these cytokines. 
Congenital Causes of Goiter
Inherited defects in hormone biosynthesis are rare causes of goi­

ts in hormone biosynthesis are rare causes of goi­
trous hypothyroidism and account for only about 10% to 15% of 
the 1 in 3000 newborns with congenital hypothyroidism.18,147,148 
In most instances, the defect appears to be transmitted as an auto­
somal recessive trait. Individuals with goitrous hypothyroidism 
are believed to be homozygous for the abnormal gene, whereas 
euthyroid relatives with slightly enlarged thyroids are presumably 
heterozygous. In the latter group, appropriate functional

ygous. In the latter group, appropriate functional testing 
may disclose a mild abnormality of the same biosynthetic step 
that is defective in the homozygous individual. In contrast with 
nontoxic goiter, which is more common in females than in males, 
these defects, as a group, affect females only slightly more com­
monly than males.
Although goiter may be present at birth, it usually does not 
appear until several years later. Therefore the absence of goi­
ter in a child with functioning thyr

sence of goi­
ter in a child with functioning thyroid tissue does not exclude 
421
the presence of hypothyroidism. The goiter is initially diffusely 
hyperplastic, often intensely so, suggesting papillary carcinoma, 
but eventually it becomes nodular. In general, the more severe the 
biosynthetic defect, the earlier the goiter appears, the larger it is, 
and the greater the likelihood of early development of hypothy­
roidism or even cretinism.147 Five s

t of hypothy­
roidism or even cretinism.147 Five specific defects in the pathways 
of hormone synthesis have been identified.
Iodide Transport Defect. An iodide transport defect, a result of 
impaired iodide transport by the NIS protein mechanism, is rare 
and is reflected in defective iodide transport in the thyroid, salivary 
gland, and gastric mucosa.149,150 Some mutations in such patients 
produce reduced activity, and others completely inactivate NIS by 
preventing the protein from being tr

ivate NIS by 
preventing the protein from being transported and inserted into 
the membrane. With the milder NIS mutations, administration of 
iodide raises the plasma and intrathyroidal iodide concentration, 
permitting the synthesis of normal quantities of hormone. 
Defects in Expression or Function of Thyroid Peroxidase. 
TPO is a protein that is required for normal synthesis of iodo­
thyronines. Quantitative or qualitative abnormalities of TPO 
have been identified in 1 in 66,000 infants in 

O 
have been identified in 1 in 66,000 infants in the Netherlands. 
The most common of the 16 mutations identified in 35 families 
was a GGCC insertion in exon 8, leading to a premature stop 
codon.151 
Pendred Syndrome. The most common presentation in 
patients with Pendred syndrome is a defect in iodine organifica­
tion accompanied by sensory nerve deafness.152 The abnormality 
is in the PDS gene encoding pendrin, which is involved in the 
apical secretion of iodide into the follicular lumen (

al secretion of iodide into the follicular lumen (see Fig. 11.1 
and the discussion regarding iodine metabolism in Chapter 11). 
Thyroid function is only mildly impaired in this disorder. 
Defects in Thyroglobulin Synthesis. Defects in the synthesis of 
thyroglobulin due to genetic causes are rare, having been identified 
only in a small number of families with congenital hypothyroid­
ism.147 Some defects lead to premature termination of translation, 
whereas another defect causes deficiency in 

ion, 
whereas another defect causes deficiency in endoplasmic reticu­
lum processing of the thyroglobulin molecule. The complex regu­
lation and the huge size of this gene make screening for mutations 
a difficult task, and considerable work is still required to unravel 
the extent of the defects in this gene. 
Iodotyrosine Dehalogenase Defect. The pathogenesis of goiter 
and hypothyroidism in the iodotyrosine dehalogenase defect is 
complex. The major abnormality is an impairment of both intra­

 major abnormality is an impairment of both intra­
thyroidal and peripheral deiodination of iodotyrosines, presum­
ably because of the dysfunction of the iodotyrosine DEHAL1B 
gene (see discussion regarding iodide metabolism in Chapter 
11).153,154
As a consequence of intense thyroid stimulation and lack of 
intrathyroidal recycling of iodide derived from dehalogenation, 
iodide is rapidly accumulated by the thyroid gland and is rapidly 
released; monoiodotyrosine (MIT) and diiodotyrosine (DIT) 

; monoiodotyrosine (MIT) and diiodotyrosine (DIT) are 
elevated in plasma and, together with their deaminated deriva­
tives, in the urine. Hypothyroidism is presumed to result from 
the loss of large quantities of MIT and DIT in the urine and to 
secondary iodine deficiency. The goiter and hypothyroidism are 
relieved by administration of high doses of iodine. 
Thyroid Infiltration Causing Hypothyroidism and Goiter
A number of infiltrative or fibrosing conditions may cause 
hypothyroidism. Some 

rosing conditions may cause 
hypothyroidism. Some are often associated with goiter, such 
as Riedel struma (see later).155 Others, such as amyloidosis,156 
hemochromatosis,157 or scleroderma,158 may not be. Although 
the other manifestations of these conditions are usually obvious 
and hypothyroidism is only a complication, the presence of sig­
nificant hypothyroidism without evidence of autoimmune thy­
roiditis should lead to a consideration of these rare causes of this 
condition. 
Postablativ

these rare causes of this 
Postablative Hypothyroidism
Postablative hypothyroidism is a common cause of thyroid fail­
ure in adults. One type follows total thyroidectomy usually per­
formed for thyroid carcinoma. Although functioning remnants 
may be present, as indicated by foci of radioiodine accumulation, 
hypothyroidism invariably develops. Another etiologic mecha­
nism is subtotal resection of the diffuse goiter of Graves disease 
or multinodular goiter. Its frequency depends on

or multinodular goiter. Its frequency depends on the amount of 
tissue remaining, but continued autoimmune destruction of the 
thyroid remnant in patients with Graves disease may be a factor 
because some studies suggest a correlation between the presence 
of circulating thyroid autoantibodies in thyrotoxicosis and the 
development of hypothyroidism after surgery. Hypothyroidism 
can be manifested during the first year after surgery, but as with 
postradioiodine hypothyroidism, the incidence i

h 
postradioiodine hypothyroidism, the incidence increases with 
time. In some patients, mild hypothyroidism appears during the 
early postoperative period and then may occasionally remit, as also 
occurs after radioiodine treatment.
Hypothyroidism after destruction of thyroid tissue with radio­
iodine is common and is the one established disadvantage of this 
form of treatment for hyperthyroidism in adults. Its frequency is 
determined in large part by the dose of radioiodine and radioio­
dine 

part by the dose of radioiodine and radioio­
dine uptake, but it is also influenced by other factors, including 
age, thyroid gland size, magnitude of thyroid hormone elevations, 
and use of antithyroid drugs.159 The incidence of postradioio­
dine hypothyroidism increases with time, approaching 100%. 
Although the FT4 is low in patients with postablative hypothy­
roidism, serum TSH levels may be anomalously low for several 
months after either surgical or 132I-induced hypothyroidism if 
TSH synt

rgical or 132I-induced hypothyroidism if 
TSH synthesis has been suppressed for a long period prior to 
Primary atrophic thyroid failure may also develop in patients 
with Hodgkin disease after treatment with mantle irradiation or 
after high-dose neck irradiation for other forms of lymphoma 
or carcinoma.126 Surgical, radioiodine, or external beam therapy 
may also lead to a state of subclinical hypothyroidism (see Table 
13.2). 
Thyroid Agenesis or Dysplasia
Developmental defects of

oid Agenesis or Dysplasia
Developmental defects of the thyroid are often responsible for 
the hypothyroidism that occurs in 1 in 3000 newborns.18 These 
defects may take the form of complete absence of thyroid tissue 
or failure of the thyroid to descend properly during embryo­
logic development. Thyroid tissue may then be found anywhere 
along its normal route of descent from the foramen cecum at 
the junction of the anterior two-thirds and posterior third of the 
tongue (lingual thyroid) to th

erior third of the 
tongue (lingual thyroid) to the normal site or below. Absence 
of thyroid tissue or its ectopic location can be ascertained by 
scintiscanning.
As indicated, a number of proteins are known to be crucial for 
normal thyroid gland development.18 These proteins include the 
thyroid-specific transcription factor PAX8 as well as thyroid tran­
scription factors 1 and 2 (TTF1 and TTF2, respectively). It might 
be anticipated that defects in one or more of these proteins may 
explain

ects in one or more of these proteins may 
explain abnormalities in thyroidal development. These abnormali­
ties have been identified in several patients with PAX8 mutations, 
and a mutation in the human TTF2 gene was associated with 
422
thyroid agenesis, cleft palate, and choanal atresia. Despite a spe­
cific search, no mutations have been found in the TTF1 gene in 
infants with congenital hypothyroidism. 
Thyroid Aplasia Due to Thyrotropin Receptor 
Unresponsiveness
Sev

 Due to Thyrotropin Receptor 
Several families exist in which thyroid hypoplasia, high TSH con­
centrations, and a low FT4 level are associated with loss-of-func­
tion mutations in the TSH receptor.160 The thyroid gland of these 
patients was in the normal location but did not trap pertechnetate 
(TcO4−). Somewhat surprisingly, thyroglobulin levels were still 
detectable. The molecular details of these patients are still under 
study.
A second type of abnormality that may cause 

tudy.
A second type of abnormality that may cause TSH unrespon­
siveness is a mutation in the Gs protein that occurs in pseudo­
hypoparathyroidism type 1A. These patients have inactivating 
mutations in the α-subunit of the Gs protein and, consequently, 
mild hypothyroidism.161 Other as yet unexplained patients with 
elevated TSH levels and hypothyroidism in which the molecular 
nature of the defect has not been defined have been reported.162 
Transient Hypothyroidism
Transient hypothyroidism is

ansient Hypothyroidism
Transient hypothyroidism is defined as a period of reduced FT4 
with suppressed, normal, or elevated TSH levels that are eventu­
ally followed by a euthyroid state. This form of hypothyroidism 
usually occurs in the clinical context of a patient with painful or 
painless subacute thyroiditis.130 These conditions are reviewed in 
detail in Chapter 12.
The patient reports mild to moderate symptoms of hypothy­
roidism of short duration, and serum TSH concentrations are 
typic

 duration, and serum TSH concentrations are 
typically elevated, although not greatly so. The patient often has 
a preceding episode of symptoms consistent with mild or moder­
ate thyrotoxicosis. If these symptoms cannot be elucidated from 
the history, it may be difficult to distinguish such patients from 
those with a permanent form of hypothyroidism. In the early 
phases of post-thyroiditis hypothyroidism, TSH concentrations 
may still be suppressed even though the FT4 is low because of the 

ressed even though the FT4 is low because of the 
delayed recovery of pituitary TSH synthesis, such as in patients 
with Graves disease or with toxic nodules who have undergone 
surgery and who have experienced rapid relief of hypothyroidism 
(see Table 13.2). In that situation, the TSH response to hypothy­
roidism may be suppressed for many months; in post-thyroiditis 
hypothyroidism, this period is rarely longer than 3 to 4 weeks.
A significant fraction (50%) of women with autoimmune 
thyroidi

 fraction (50%) of women with autoimmune 
thyroiditis but normal thyroid function have episodes of hypo­
thyroidism during the postpartum period.130 In some, the pre­
ceding thyrotoxicosis is relatively asymptomatic, which can make 
an accurate clinical diagnosis difficult. Patients who have had an 
episode of typical painful subacute thyroiditis, with pain, tender­
ness, and thyrotoxicosis, are not difficult to recognize.
Diagnostic evaluation should include a determination of TSH, 
FT4, and TP

hould include a determination of TSH, 
FT4, and TPOAb. Negative or low antibodies argue strongly for a 
nonautoimmune cause. This is significant, in that it may be pos­
sible for the patient to be treated only temporarily for hypothy­
roidism. In such patients, a trial of a lower levothyroxine dosage 
after 3 to 6 months may reveal that thyroid function has recovered 
(see Fig. 13.5). This may also occur in patients with hypothyroid­
ism that follows painless subacute thyroiditis (e.g., in the p

lows painless subacute thyroiditis (e.g., in the postpar­
tum period), but it is somewhat less likely to occur because of the 
underlying progressive nature of the autoimmune thyroiditis.
In patients with hypothyroidism due to painful subacute thy­
roiditis, the thyroid gland is usually relatively small and atrophic. 
In patients with hypothyroidism that follows an episode of pain­
less subacute thyroiditis, the gland is usually slightly enlarged and 
somewhat firm, reflecting the underlying sca

 and 
somewhat firm, reflecting the underlying scarring and infiltration 
associated with that condition. 
Consumptive hypothyroidism is the term given to an unusual cause 
of hypothyroidism that has been identified in infants with visceral 
hemangiomas or related tumors.7,163 The first patient reported 
with this syndrome presented with abdominal distention caused 
by a large hepatic hemangioma with respiratory compromise 
secondary to upward displacement of the diaph

ise 
secondary to upward displacement of the diaphragm. However, 
clinical signs suggested hypothyroidism, which was confirmed 
by finding a markedly elevated TSH level and undetectable T4 
and T3 levels. The infant’s response to an initial intravenous infu­
sion of liothyronine (T3) was transient, leading to the decision to 
use parenteral thyroid hormone replacement to relieve the severe 
hypothyroidism. The accelerated degradation of thyroid hormone 
was apparent from the fact that 96 μg of l

ormone 
was apparent from the fact that 96 μg of liothyronine plus 50 μg 
of levothyroxine were required to normalize the TSH level. The 
equivalent dosage of levothyroxine alone is roughly nine times that 
ordinarily required for treatment of infants with congenital hypo­
thyroidism. The infant succumbed to complications of the hem­
angioma, and a postmortem tumor biopsy showed D3 activity in 
the tumor at levels eightfold higher than those normally present in 
term placenta. The serum reverse 

ally present in 
term placenta. The serum reverse T3 was extremely elevated (400 
ng/dL), and the serum thyroglobulin was greater than 1000 ng/
mL, indicating the presence of a highly stimulated thyroid gland. 
Retrospective search revealed two other patients with similar 
pathophysiology in whom the cause of the hypothyroidism had 
not been recognized. Significant D3 expression has subsequently 
been noted in all proliferating cutaneous hemangiomas studied 
to date. The cutaneous hemangiomas of

mas studied 
to date. The cutaneous hemangiomas of infancy, although they 
express D3, are not associated with hypothyroidism owing to their 
small size. Most infantile hemangiomas involute with proprano­
lol therapy, but such patients must also receive adequate doses of 
thyroid hormone to prevent the permanent neurologic complica­
tions associated with untreated hypothyroidism during this criti­
cal phase of neurologic development.164 Subsequent reports have 
identified a similar syndrome in a

t reports have 
identified a similar syndrome in adults, including a patient with 
an epithelioid hemangioendothelioma and an individual with a 
fibrous tumor as well as extensive gastrointestinal stromal tumors 
(GISTs).165 Some of these tumors express D3, or this deiodinase 
may be induced during treatment with TKIs (see later).166–169 
Defects in Conversion of Thyroxine to Triiodothyronine
The enzymes that convert the precursor T4 to the active form, 
T3, are 5′-deiodinase 1 (D1) and D2, both

e form, 
T3, are 5′-deiodinase 1 (D1) and D2, both of which contain 
selenocysteine in their active site.84 A stem loop structure in the 
3′-untranslated region of the mRNA, termed a SECIS element, 
directs insertion of selenocysteine at the UGA codon, rather 
than allowing it to function as a stop codon. Defects in a SECIS-
binding protein (selenocysteine insertion sequence–binding pro­
tein 2 [SECISBP2]) were found in two families with an elevated 
FT4, reduced T3, and elevated TSH.170 Affecte

ted 
FT4, reduced T3, and elevated TSH.170 Affected individuals have 
growth retardation, compared with unaffected family members.
Polymorphisms in genes associated with thyroid hormone 
metabolism have been associated with patterns of thyroid func­
tion studies as well as obesity. The D2 polymorphism result­
ing in a change from threonine (Thr) to alanine (Ala) at codon 
92 (Thr92Ala) has been associated with obesity, reduced glu­
cose disposal, and lower D2 activity in skeletal muscle.171 This

 and lower D2 activity in skeletal muscle.171 This 
423
polymorphism also has a higher frequency in groups with a high 
incidence of obesity and type 2 diabetes such as Mexican Ameri­
cans and Pima Indians.171 The D2 polymorphism has also been 
associated with a distinct pattern of gene expression in the cortex, 
associated with inflammation and chronic disease, linked to pro­
longed half-life of the D2 protein in the Golgi.172 
Hypothyroidism Due to Dr

protein in the Golgi.172 
Hypothyroidism Due to Drug-Induced Thyroid Destruction
Thyroid inflammation or activation of autoimmune thyroid 
destruction has been associated with a number of drugs.146,173 
The TKIs, such as sunitinib, have been associated with a high inci­
dence of hypothyroidism due to thyroid destruction.166,167 Suni­
tinib is used to treat renal cell carcinoma and GISTs and inhibits 
multiple tyrosine protein kinases, including KIT, PDGF, VEGF, 
and RET. An abnormal TSH was foun

IT, PDGF, VEGF, 
and RET. An abnormal TSH was found in 62% of patients receiv­
ing sunitinib who were followed for 37 weeks.168 Patients studied 
by ultrasound demonstrated no thyroid tissue. Although 40% of 
hypothyroid patients initially had a suppressed TSH, suggesting 
thyroiditis, the long-term course was most consistent with suni­
tinib-induced follicular cell apoptosis. Thus such patients require 
repeated thyroid function testing. These agents have now been 
shown to slow disease progres

gents have now been 
shown to slow disease progression in advanced thyroid cancer 
unresponsive to radioiodine but may increase thyroid hormone 
requirements due to increased D3 expression.174 
Central hypothyroidism is due to TSH deficiency caused by either 
acquired or congenital hypothalamic or pituitary gland disorders 
(see Chapters 8 and 9). The causes of TSH deficiency may be clas­
sified as those of pituitary (secondary hypothyroidism) and hypo­
thalamic (tertiary 

dary hypothyroidism) and hypo­
thalamic (tertiary hypothyroidism) origins, but this distinction 
is not necessary in the initial separation of primary from central 
hypothyroidism.
In many cases, hyposecretion of TSH is accompanied by 
decreased secretion of other pituitary hormones, with the result 
that evidence of somatotroph, gonadotroph, and corticotroph 
failure is also present. Hyposecretion of TSH as the sole demon­
strable abnormality (monotropic deficiency) is less common but 
does occ

onotropic deficiency) is less common but 
does occur in both acquired and congenital forms. Hypothyroid­
ism due to pituitary insufficiency varies in severity from instances 
in which it is mild and overshadowed by features of gonadal 
and adrenocortical failure to those in which the features of the 
hypothyroid state are predominant. Because a small but signifi­
cant fraction of thyroid gland function is independent of TSH 
(∼10–15%), hypothyroidism due to central causes is less severe 
than pr

dism due to central causes is less severe 
than primary hypothyroidism.
The causes of central hypothyroidism are both acquired and 
congenital. The general subject is discussed in Chapters 8 and 9, 
and those causes with relatively specific thyroid-related deficien­
cies are mentioned here for completeness. In addition to pituitary 
tumors, hypothalamic disorders, and the like, an unusual cause of 
secondary hypothyroidism occurs in individuals given bexarotene 
(a retinoid X receptor [RXR] agon

iven bexarotene 
(a retinoid X receptor [RXR] agonist) for T-cell lymphoma.175,176 
This drug suppresses the activity of the human TSH β-subunit pro­
moter in vitro. Serum T4 concentrations are reduced about 50%, 
and patients experience clinical benefit from thyroid hormone 
replacement. Dopamine, dobutamine, high-dose glucocorticoids, 
or severe illness may suppress TSH release transiently, leading to 
a pattern of thyroid hormone abnormalities suggesting central 
hypothyroidism.176 As discuss

 suggesting central 
hypothyroidism.176 As discussed earlier (see Chapter 11 discus­
sion regarding changes in thyroid function during severe illness), 
this severe state of hypothalamic-pituitary-thyroid suppression 
is a manifestation of stage 3 illness (see Table 11.10). Although 
these agents may be expected to have similar effects when given 
long term, they do not, nor does somatostatin have a similar effect 
when given for acromegaly, although it does block the response 
of TSH to TRH and

ough it does block the response 
of TSH to TRH and it has been administered to patients with 
thyrotropin-secreting pituitary adenomas.177
Congenital defects in either the stimulation or the synthesis of 
TSH or in its structure have been identified as rare causes of con­
genital hypothyroidism18 and include the consequences of defects 
in several of the homeobox genes, including POU1F1 (formerly 
termed Pit1), PROP1, and HESX1. The last gene encodes a factor 
necessary for the development of th

odes a factor 
necessary for the development of the hypothalamus, pituitary, and 
olfactory portions of the brain. Defects in POU1F1 and PROP1 
cause hereditary hypothyroidism usually accompanied by defi­
ciencies in growth hormone and prolactin.178 One patient has 
been identified with a familial defect in the TRH receptor gene.179 
All of these conditions are associated with the typical pattern of 
central hypothyroidism, a reduced FT4, and inappropriately low 
TSH.
Structural defects in TSH h

ppropriately low 
TSH.
Structural defects in TSH have also been described. They 
include those with a mutation in the CAGYC peptide sequence of 
the β-subunit, thought to be necessary for its association with the 
α-subunit,180 and defects that produce premature termination of 
the TSH β-subunit gene.181 As mentioned, some of these abnor­
malities may be associated with elevations in TSH, suggesting the 
diagnosis of primary hypothyroidism, but the TSH molecule is 
immunologically, but not biolo

he TSH molecule is 
immunologically, but not biologically, intact. 
The clinical manifestations of resistance to thyroid hormone 
depend on the nature of the mutation.6,11,182 The majority of 
patients with RTH have a mutation in the gene encoding the thy­
roid hormone receptor β (TRβ gene, THRB) that interferes with 
the capacity of that receptor to respond normally to T3, usually 
by reducing its T3-binding affinity (see Chapter 2 ). A small num­
ber of individual

y (see Chapter 2 ). A small num­
ber of individuals have been identified with TRα gene (THRA) 
mutations, referred to as RTHα1,183 who differ in significant ways 
from patients with RTH due to THRB mutations.184–186
RTH is probably produced by the heterodimerization of the 
mutant TRβ with RXR or homodimerization with a normal TRβ 
or TRα. These mutant TRβ-containing dimers compete with 
wild-type TR-containing dimers for binding to the thyroid hor­
mone response elements (TREs) of thyroid hormo

or­
mone response elements (TREs) of thyroid hormone–dependent 
genes.182 Because these complexes bind corepressor molecules that 
cannot be released in the absence of T3 binding, genes containing 
these TREs are more repressed than they would be normally at 
the prevailing concentrations of circulating thyroid hormones (see 
Chapter 2).6 Receptors that contain mutations in the activation 
domain may have a combination of both decreased affinity for T3 
and impaired activating potential.
Thus th

for T3 
Thus the mutant TRβ complex can interfere with the func­
tion of the wild-type TRs, producing a pattern termed dominant 
negative inhibition with an autosomal dominant pattern of inheri­
tance. At least 400 families have been identified with this condi­
tion, and there are probably many more unreported cases. The 
gene frequency estimate is about 1:50,000 for mutations in TRβ 
and the study of the function of the mutant receptors in this disor­
der has 

on of the mutant receptors in this disor­
der has provided valuable insights into the mechanism of thyroid 
hormone action.182 The frequency of TRα mutations is probably 
much lower but undefined.
Patients with RTH usually are recognized because of thy­
roid enlargement, which is present in about two-thirds of these 
424
individuals. Patients usually report a mixture of symptoms of 
hyperthyroidism and hypothyroidism. With respect to the heart, 
palpitations and tachycardi

respect to the heart, 
palpitations and tachycardia are more common than a reduced 
heart rate; however, patients may also demonstrate growth retar­
dation and retarded skeletal maturation.187 This has been attrib­
uted to the fact that thyroid hormone effects in the heart and 
bone appear to be primarily dependent on TRα rather than TRβ, 
whereas the hypothalamic-pituitary axis is primarily regulated 
through TRβ.
Abnormalities in neuropsychologic development exist, with 
an increased prevalenc

ic development exist, with 
an increased prevalence of attention-deficit/hyperactivity disor­
der, which is found in approximately 10% of such individuals.187 
Other neuropsychologic abnormalities have also been described. 
Deafness in patients with RTH reflects the important role of TRβ 
and thyroid hormone in the normal development of auditory 
function. The mixture of symptoms, some suggesting hypothy­
roidism and others suggesting hyperthyroidism, may even differ in 
individuals within the s

dism, may even differ in 
individuals within the same family, despite the identical mutation, 
thus confusing the clinical picture.
Because patients may present with symptoms suggesting 
hyperthyroidism, it is important to keep this diagnosis in mind 
in a patient with tachycardia, goiter, and elevated thyroid hor­
mones. RTH is discussed here because a reduced response to 
thyroid hormone is the biochemical basis for the condition. 
However, the laboratory results may be the first clear evidenc

 laboratory results may be the first clear evidence 
that a patient, otherwise thought to have hyperthyroidism, has 
RTH. These tests show the unusual combination of an increased 
FT4 accompanied by normal or slightly increased TSH levels (see 
Table 13.2). Thus the principal differential diagnosis is between 
a TSH-secreting pituitary tumor causing hyperthyroidism and 
RTH.188
Factors that may assist in the differential diagnosis are as fol­
lows: absence of a family history in patients with TS

s: absence of a family history in patients with TSH-producing 
tumors, normal thyroid hormone levels in family members of 
individuals with TSH-induced hyperthyroidism due to pituitary 
tumor, and the presence of an elevated glycoprotein α-subunit in 
patients with pituitary tumor but not in those with RTH.
A definitive diagnosis requires sequencing of the TRβ gene 
demonstrating the abnormality. Mutations in the TRβ gene are 
found in about 90% of individuals with a clinical diagnosis. In a 
fe

of individuals with a clinical diagnosis. In a 
few individuals this is not the case, suggesting that there may be 
mutations in coactivator proteins or one of the RXR receptors, 
which can also present in a similar fashion.189
Treatment is difficult because thyroid hormone analogues 
designed to suppress TSH, thereby relieving the hyperthyroxin­
emia, may lead to worsening of the cardiovascular manifestations 
of the condition.190 Therapy with 3,5,3′-triiodothyroacetic acid 
(TRIAC) has been us

,5,3′-triiodothyroacetic acid 
(TRIAC) has been used in several patients. The development of 
analogues of thyroid hormone with TRβ, as opposed to mixed or 
TRα preferential effects, as well as analogues that selectively bind 
mutant TRs, may eventually prove useful in treatment.
Individuals with RTHα1 have more subtle changes in thyroid 
function studies, compared with RTH due to TRβ gene muta­
tions, and a clinical phenotype consistent with hypothyroidism 
in tissue that predominantly expresse

othyroidism 
in tissue that predominantly expresses the TRα isoform.184,185 
Because regulation of TSH is predominantly mediated by TRβ, 
these individuals have a normal serum TSH but a reduced T4/T3 
ratio and low concentration of serum reverse T3, presumably due 
to reduced activity of the D3 enzyme. Common phenotypic fea­
tures include growth failure, developmental delays, constipation, 
and delayed bone maturation, with some improvement associated 
with T4 treatment.186 
Treatment
Hypothyroi

iated 
Treatment
Hypothyroidism, either primary or central, is gratifying to treat 
because of the ease and completeness with which it responds to 
thyroid hormone.81,191 Treatment is nearly always with levothy­
roxine, and the proper use of this medication has been reviewed 
extensively.80 A primary advantage of levothyroxine therapy is that 
the peripheral deiodination mechanisms can continue to produce 
the amount of  T3 required in tissues under the normal physiologic 

 required in tissues under the normal physiologic 
control.84 If one accepts the principle that replicating the natural 
state is the goal of hormone replacement, it is logical to provide 
the “prohormone” and allow the peripheral tissues to activate it by 
physiologically regulated mechanisms. There is, however, signifi­
cant interest in combined T4 and T3 therapy.192,193
Pharmacologic and Physiologic Considerations
Levothyroxine has a 7-day half-life; about 80% of the hormone is 
absorbed rela

f-life; about 80% of the hormone is 
absorbed relatively slowly (over hours) and it equilibrates rapidly 
in its extracellular distribution volume, therefore avoiding large 
postabsorptive perturbations in FT4 levels. With its long half-life, 
omission of a single day’s tablet has no significant effect and the 
patient may safely take an omitted tablet the following day. In fact, 
the levothyroxine dosage can be calculated almost as satisfactorily 
on a weekly, as on a daily, basis. Although T4 

y 
on a weekly, as on a daily, basis. Although T4 is well absorbed 
and does not require fasting, regular ingestion of levothyroxine 
on an empty stomach results in the least variation in serum TSH 
concentration.194
The Food and Drug Administration (FDA) has issued stan­
dards for single-dose bioequivalence studies in normal volunteers 
to assess and compare T4 products in the United States.195 The 
area under the curve (AUC) confidence interval must fall within 
80% to 125% of the comparison p

 must fall within 
80% to 125% of the comparison product for a preparation to be 
considered equivalent. The desirability of a pharmacotherapeutic 
measurement, such as TSH level as an end point, has been sug­
gested by many professional organizations.195 The guidelines for 
measured T4 content have narrowed from 90% to 110% to 95% 
to 105% of the stated tablet dose and require that content to be 
maintained for the entire shelf life.196 The availability in many 
countries of a multiplicity of t

lability in many 
countries of a multiplicity of tablet strengths with content ranging 
from 25 to 300 μg allows precise titration of the daily levothyrox­
ine dosage for most patients with a single daily tablet, improving 
compliance significantly.
The typical dose of levothyroxine, approximately 1.6 to 1.8 
μg/kg ideal body weight per day (0.7 to 0.8 μg/lb), generally 
results in the prescription of between 75 and 125 μg/day for 
women and 125 to 200 μg/day for men. Replacement doses 
need not

to 200 μg/day for men. Replacement doses 
need not be adjusted upward in obese patients and should be 
based on lean body mass.197 Levothyroxine dose reduction is 
often required after bariatric surgery, but the dose reduction is 
proportional to the reduction in lean body mass.198 This dos­
age is about 20% greater than the T4 production rate owing 
to incomplete absorption of the levothyroxine. In patients 
with primary hypothyroidism, these amounts usually result 
in serum TSH concentrations 

ounts usually result 
in serum TSH concentrations that are within the normal 
range. Because of the 7-day half-life, approximately 6 weeks 
are required before there is complete equilibration of the FT4 
and the biologic effects of levothyroxine. Accordingly, assess­
ments of the adequacy of a given dose or the effects of a change 
in dosage, with rare exceptions such as pregnancy, should not 
be made until this interval has passed. This long half-life also 
means that it is safe for a patient t

f-life also 
means that it is safe for a patient to take any missed doses of T4 
for up to 1 week after missing tablets.
425
By and large, levothyroxine products are clinically equivalent, 
although problems do occur.199 However, the variation permit­
ted by the FDA in tablet content can result in slight variations 
in serum TSH in patients with primary hypothyroidism even 
when the same brand is used. Using levothyroxine from a single 
manufacturer red

sing levothyroxine from a single 
manufacturer reduces variability that may be relevant for patients, 
such as elderly, pregnant, and thyroid cancer patients, when close 
titration is required. Although the serum TSH level is an indi­
rect reflection of the levothyroxine effect in patients with primary 
hypothyroidism, it is far superior to any other readily available 
method of assessing the adequacy of therapy. Return of the serum 
TSH level to normal is therefore the goal of levothyroxine the

 normal is therefore the goal of levothyroxine therapy 
in the patient with primary hypothyroidism. Some patients may 
require slightly higher or lower doses than generally used, owing 
to individual variations in absorption, and a number of conditions 
or associated medications may change levothyroxine requirements 
in patients with established hypothyroidism (see later). Liquid 
preparations of levothyroxine, including preparations in capsules, 
may be better absorbed in select patients.200
In

In decades past, desiccated thyroid was successfully used for the 
treatment of hypothyroidism and still accounts for a small fraction 
of the prescriptions written for thyroid replacement in the United 
States. Although this approach was successful, desiccated thyroid 
preparations contain thyroid hormone derived from animal thy­
roids that have twofold to threefold higher ratios of T3 to T4 than 
the 1:15 value in normal human thyroglobulin.201 A

the 1:15 value in normal human thyroglobulin.201 Accordingly, 
these preparations may lead to superphysiologic levels of T3 in the 
immediate postabsorptive period (2–4 hours) owing to the rapid 
release of T3 from thyroglobulin, its immediate and nearly com­
plete absorption, and the 1-day period required for T3 to equili­
brate with its 40-L volume of distribution (see Table 11.5).202 A 
prospective double-blind randomized crossover study compared 
4 months of levothyroxine monotherapy with de

red 
4 months of levothyroxine monotherapy with desiccated thyroid 
preparations in the same hypothyroid patients.203 There were no 
significant differences in outcomes, although among patients who 
had a preference, more preferred desiccated thyroid extract, and 
these patients lost a modest amount of weight.
Mixtures of liothyronine and levothyroxine (liotrix) contain 
in a 1-grain (64-μg) equivalent tablet (Thyrolar-1 in the United 
States) the amounts of T3 (12.5 μg) and T4 (50 μg) present i

e amounts of T3 (12.5 μg) and T4 (50 μg) present in 
the most popular desiccated thyroid tablet.204 The levothyroxine 
equivalency of a 1-grain desiccated thyroid tablet or its liotrix 
equivalent can be estimated as follows: The 12.5 μg of liothyronine 
(T3) is completely absorbed from desiccated thyroid or from liotrix 
tablets.202 Levothyroxine is approximately 80% absorbed,205 and 
about 36% of the 40 μg of levothyroxine absorbed is converted to 
T3, with the molecular weight of T3 (651) bei

 to 
T3, with the molecular weight of T3 (651) being 84% that of T4 
(777). Accordingly, a 1-grain tablet should provide about 25 μg 
of T3, which would be approximately equivalent to that obtained 
from 100 μg of levothyroxine. This equivalency ratio can be used 
as an initial guide in switching patients from desiccated thyroid 
or liotrix to levothyroxine. Although levothyroxine is absorbed in 
the stomach and small intestine, normal gastric acid secretion is 
required for complete absorption.

id secretion is 
required for complete absorption.206 Patients with impaired acid 
secretion on levothyroxine therapy require a 22% to 34% higher 
dose of levothyroxine to maintain the desired serum TSH. In 
those patients in whom acid secretion was normalized therapeuti­
cally, the levothyroxine dose returned to baseline.206
As indicated earlier, the use of levothyroxine as thyroid hor­
mone replacement is a compromise with the normal pathway 
of T3 production, in which about 80% of T3 is deriv

f T3 production, in which about 80% of T3 is derived from T4 
5′-monodeiodination and approximately 20% (∼6 μg) is secreted 
directly from the thyroid gland.84 Studies in thyroidectomized 
rats, for example, show that it is not possible to normalize T3 
simultaneously in all tissues by an intravenous infusion of T4.207 
However, it should be recalled from the earlier discussion of T4 
deiodination that the ratio of  T3 to T4 in the human thyroid 
gland is about 0.09 but is 0.17 in the rat thyroi

gland is about 0.09 but is 0.17 in the rat thyroid gland.84 Thus 
about 40% of the rat’s daily T3 production is derived from the 
thyroid versus about 20% in humans.84 Accordingly, the demon­
stration that T4 alone cannot provide normal levels of T3 in all tis­
sues in the rat is of interest but is not strictly applicable to thyroid 
hormone replacement in humans. Nonetheless, the ratio of T3 to 
T4 in the serum of a patient receiving levothyroxine as the only 
source of T3 is about 20% lower t

ine as the only 
source of T3 is about 20% lower than that in a normal individual.
Similarly, the quantity of levothyroxine required to normalize 
TSH in an athyreotic patient results in a slightly higher serum 
FT4 concentration than is present in normal individuals. This 
has been shown in a comparison of thyroid function in the same 
patient before and after thyroidectomy.208 Although serum T3 was 
the same level in patients before or after thyroidectomy, a higher 
serum T4 concentration was 

yroidectomy, a higher 
serum T4 concentration was necessary when on T4 replacement 
to maintain the same serum T3.208 T4 has an independent mecha­
nism for TSH suppression, owing to the intracellular generation 
of T3 in the hypothalamic-pituitary-thyroid axis, resulting in a 
portion of the feedback TSH regulation being independent of the 
plasma T3. A retrospective, cross-sectional study compared thy­
roid function studies in 1800 athyreotic thyroid cancer patients 
on levothyroxine monotherap

yroid cancer patients 
on levothyroxine monotherapy compared to control subjects.209 
In these patients, the serum FT4 levels were significantly higher, 
and free T3 levels significantly lower, than euthyroid control sub­
jects. However, these patients came from a region of variable iodine 
intake, which makes interpretation of the control data difficult.
Although the concept of combined T4/T3 therapy has been 
recognized for many years, a positive study generated a great deal 
of interest in th

ve study generated a great deal 
of interest in this approach.210 Patients received 12.5 μg of T3 as 
a substitution for 50 μg of their levothyroxine preparation and 
scored, on average, somewhat higher on tests of mood than when 
they were taking levothyroxine alone. The dosage of thyroid hor­
mone used in these studies was excessive, as judged by the fact 
that 20% of the group had serum TSH values below normal on 
either regimen and the test period was only a few months. A large 
number of su

eriod was only a few months. A large 
number of subsequent studies using a wide range of replacement 
strategies and relative T4/T3 content were performed in different 
populations, and none has shown an advantage of combination 
therapy over T4 alone.211 A study that compared T4 monotherapy 
to T4/T3 combined therapy in hypothyroid patients evaluated 
the results based on the presence or absence of D2 gene poly­
morphisms.212 The hypothyroid patients homozygous for the 
D2 polymorphism showed g

ients homozygous for the 
D2 polymorphism showed greater improvement in measures of 
well-being while on T4/T3 combination therapy compared to T4 
monotherapy. This study requires replication in a separate popu­
lation but may indicate that T4/T3 combination therapy can be 
targeted to specific hypothyroid patients who will benefit based on 
a genetic profile of genes important for thyroid hormone metabo­
lism and action.9,193,213
On the other hand, the FT4 index correlated as closely with the 

nd, the FT4 index correlated as closely with the 
resting energy expenditure as TSH levels in a group of patients in 
whom small adjustments above or below their ideal replacement 
levothyroxine dosage were made.214 The correlation with serum 
T3 was not statistically significant, suggesting that in humans, 
perhaps as a result of differences in the peripheral metabolism of 
T4 from that in rodents, the FT4 concentration may be as accu­
rate as the TSH value as an index of satisfactory thyroid h

he TSH value as an index of satisfactory thyroid hormone 
426
replacement. The practical difficulty with the design of tablets 
providing combinations of T3 and T4 is that the approximate dose 
of 6 μg of T3 provided would need to be released in a sustained 
fashion over 24 hours, as well as replicating the diurnal rhythm 
of T3,215 which is quite different from the rapid absorption of T3 
with a peak at 2 to 4 hours when given in its conventional form. 
Thus, for the pres

iven in its conventional form. 
Thus, for the present, it appears that the current approach to thy­
roid replacement using levothyroxine alone, although not a per­
fect replication of normal physiology, is satisfactory for virtually 
all patients. A sustained-release T3 preparation has been developed 
and produces more stable levels of serum T3.216 The clinical effects 
of this more “physiologic” replacement are not known. 
Institution of Replacement Therapy
The initial dose of levothyroxine pre

ment Therapy
The initial dose of levothyroxine prescribed depends on the degree 
of hypothyroidism and the age and general health of the patient. 
Patients who are young or middle aged and otherwise healthy with 
no associated cardiovascular or other abnormalities and mild to 
moderate hypothyroidism (TSH concentrations of 5–50 mU/L) 
can be given an initial complete replacement dose of about 1.7 μg/
kg of ideal body weight. The resulting increase in serum T4 con­
centration to normal requires 5

e in serum T4 con­
centration to normal requires 5 to 6 weeks, and the biologic effects 
of T3 are sufficiently delayed that these patients do not experience 
adverse effects. At the other extreme, the elderly patient with heart 
disease, particularly angina pectoris, without reversible coronary 
lesions, should be given a small initial dose of levothyroxine (25 
μg/day), and the dosage should be increased in 12.5-μg incre­
ments at intervals of 2 to 3 months with careful clinical and labo­
rato

2 to 3 months with careful clinical and labo­
ratory evaluation.
The goal in the patient with primary hypothyroidism is to 
return serum TSH concentrations to normal, reflecting normal­
ization of that patient’s thyroid hormone supply. This usually 
results in a mid-normal to high-normal serum FT4. The serum 
TSH should be evaluated 6 weeks after a theoretically complete 
replacement dose has been instituted to allow minor adjustments 
to optimize the individual dose.217 In patients with central

e the individual dose.217 In patients with central hypo­
thyroidism, serum TSH is not a reliable index of adequate replace­
ment, and the serum FT4 should be restored to a concentration 
in the upper half of the normal range. T4 dosing based on body 
weight and a serum FT4 in the upper reference range improved 
markers of thyroid hormone action and was superior to replace­
ment with a combination of T4/T3.218 Patients with central hypo­
thyroidism should also be evaluated and treated for glucoco

m should also be evaluated and treated for glucocorticoid 
deficiency, if necessary, before institution of thyroid replacement 
(see Chapter 9).
Although the adverse effects of the rapid institution of therapy 
are unusual, pseudotumor cerebri has been reported in profoundly 
hypothyroid juveniles between ages 8 and 12 years who were given 
even modest initial levothyroxine replacement.219 This compli­
cation appears 1 to 10 months after initiation of treatment and 
responds to acetazolamide and

on of treatment and 
responds to acetazolamide and dexamethasone.
The interval between the initiation of treatment and the first 
evidence of improvement depends on the strength of dose given 
and the degree of the deficit. An early clinical response in moder­
ate to severe hypothyroidism is a diuresis of 2 to 4 kg. The serum 
sodium (Na+) level increases even sooner if hyponatremia was 
present initially. Thereafter, pulse rate and pulse pressure increase, 
appetite improves, and constipation m

e increase, 
appetite improves, and constipation may disappear. Later, psycho­
motor activity increases and the delay in the deep tendon reflex 
disappears. Hoarseness abates slowly, and changes in skin and hair 
do not disappear for several months. In individuals started on a 
complete replacement dose, the serum FT4 level should normalize 
after 6 weeks; a somewhat longer period may be necessary for 
serum TSH levels to return to normal, perhaps up to 3 months.
In some cases (e.g., myxedema co

s up to 3 months.
In some cases (e.g., myxedema coma [see later]), it is clinically 
appropriate to alleviate hypothyroidism rapidly. For example, 
patients with severe hypothyroidism withstand acute infections or 
other serious illnesses poorly, and myxedema coma may develop as 
a complication. In such circumstances, rapid near repletion of the 
peripheral hormone pool in the average adult can be accomplished 
by a single intravenous dose of 500 μg of levothyroxine. Alter­
natively, by virtue o

 μg of levothyroxine. Alter­
natively, by virtue of its rapid onset of action, liothyronine (25 
μg orally every 12 hours) can be administered if the patient can 
take medication by mouth. With both approaches, an initial bio­
logic effect is achieved within 24 hours. Parenteral therapy with 
levothyroxine is then continued with a dose that is 80% of the 
appropriate oral dose but not in excess of 1.4 μg/kg of ideal body 
weight. Because of the possibility that rapid increases in meta­
bolic rat

ossibility that rapid increases in meta­
bolic rate will overtax the existing pituitary-adrenocortical reserve, 
supplemental glucocorticoid (intravenous hydrocortisone 5 mg/
hour) should also be given to patients with severe hypothyroid­
ism receiving high initial doses of thyroid hormones. Finally, in 
view of the tendency of hypothyroid patients to retain free water, 
intravenous fluids containing only dextrose should not be given.
When replacement therapy is withdrawn for short periods 
(4–6

ement therapy is withdrawn for short periods 
(4–6 weeks) for purposes of evaluating therapy for thyroid cancer, 
rapid reinstitution of levothyroxine using a loading dose of three 
times the daily replacement dose for 3 days can usually be given 
unless there are other complicating medical illnesses.
When hypothyroidism results from administration of iodine-
containing or antithyroid drugs, withdrawal of the offending 
agent usually relieves both the hypothyroidism and the accompa­
nying goiter

h the hypothyroidism and the accompa­
nying goiter, although it is appropriate to provide interim replace­
ment until the gland recovers its function.138,220 This is especially 
true for amiodarone, which may remain in tissues for up to 1 year.
Infants and Children
In infants with congenital hypothyroidism, the determining factor 
for eventual intellectual attainment is the age at which adequate 
treatment with thyroid hormone is begun.18,221 The therapy for 
infants with congenital hypothyroidi

 therapy for 
infants with congenital hypothyroidism should consist initially of 
raising the serum T4 level to more than 130 nmol/L (10 μg/dL) 
as rapidly as possible and maintaining it at that level for the first 
3 to 4 years of life. This is usually accomplished by administering 
an initial levothyroxine dose of 50 μg/day, which is higher than 
the adult dose on a weight basis and in keeping with the higher 
metabolic clearance of the hormone in the infant. The serum TSH 
concentration may n

 in the infant. The serum TSH 
concentration may not return to normal even with this high dose 
because of residual reset of the pituitary feedback mechanism. 
After 2 years of age, however, a TSH level in the normal range is 
an index of optimal therapy as it is in adults.222 
Monitoring Replacement Therapy
Monitoring the adequacy of, and compliance with, thyroid hor­
mone therapy in patients with primary hypothyroidism is easily 
done by measurement of serum TSH.9 This value should be within 

ement of serum TSH.9 This value should be within 
the normal range for an assay sufficiently sensitive to measure, 
with confidence, the lower limit of the normal range. The normal 
serum TSH concentration varies between 0.5 and 4 mU/L in most 
second-generation and third-generation assays, and results within 
this range are associated with the elimination of all clinical and 
biochemical manifestations of primary hypothyroidism, except in 
patients with RTH. Based on analysis of the National He

nts with RTH. Based on analysis of the National Health and 
Nutrition Examination Survey (NHANES III) reference group,13 
427
a reference TSH range with an upper limit of 2.5 mU/L has been 
suggested. This adjustment, however, would identify a large num­
ber of individuals as having abnormal thyroid function, without 
a clear indication of the clinical significance of TSH levels in this 
range. A more recent analysis, based on age-specific reference 
ra

cent analysis, based on age-specific reference 
ranges, indicates that older adults, without thyroid autoantibod­
ies, have an increased upper limit TSH (>4.5 mU/L) that is not 
associated with thyroid disease.223 In related studies, this progres­
sive shift with age to higher levels of TSH has been associated with 
extreme longevity in several populations.224
After the first 6 months of therapy, the dose should be reas­
sessed because restoration of euthyroidism increases the metabolic 
clearan

n of euthyroidism increases the metabolic 
clearance of T4. A dose that was adequate during the early phases 
of therapy may not be so when the same patient is euthyroid 
owing to an acceleration in the clearance of thyroid hormone.
Under normal circumstances, the finding of a normal serum 
TSH level on an annual basis is adequate to ensure that the proper 
levothyroxine dose is being taken by the patient. If the serum 
TSH level is above the normal range and noncompliance is not 
the explanatio

mal range and noncompliance is not 
the explanation, small adjustments, usually in 12-μg increments, 
can be made with reassessment of TSH concentrations after the 
6 weeks required for full equilibration have passed. In North 
America, this strategy is simplified by the availability of multiple 
tablet strengths, many of which differ by only 12 μg. Most patients 
can receive the same dose until they reach their seventh or eighth 
decade, at which point a downward adjustment of 20% to 30% 
may b

h point a downward adjustment of 20% to 30% 
may be indicated because thyroid hormone clearance decreases in 
the elderly.
Thyroid hormone requirements may be altered in several situ­
ations (Table 13.3). Most conditions or medications increase the 
levothyroxine requirement in patients receiving maintenance ther­
apy. During pregnancy, the levothyroxine requirement is increased 
by 25% to 50% in most hypothyroid women, and a prospective 
study demonstrated that the increased requirement occurs 

emonstrated that the increased requirement occurs early 
in the first trimester.225 The required increment is higher in 
athyreotic patients compared to those with autoimmune hypo­
thyroidism.226 Athyreotic patients who are planning a pregnancy 
should be advised to increase the dose by around 30% as soon 
as the diagnosis is confirmed because the change in requirement 
appears soon after implantation.9,113 The increased requirement 
is probably due to a combination of factors, including increas

due to a combination of factors, including increases 
in T4-binding globulin and the volume of distribution of T4, an 
increase in body mass, and an increase in D3 in the placenta and 
uterus probably due to estradiol-induced increases in Dio3 gene 
transcription.84 The increased requirement persists throughout 
pregnancy but returns to normal within a few weeks after delivery. 
Therefore the dose should be reduced to the original prepregnancy 
level at the time of delivery. Maternal T4 is criti

evel at the time of delivery. Maternal T4 is critically important to 
the athyreotic fetus and in the normal fetus in the first trimester 
before fetal thyroid function and feedback regulation mature.227 
Maternal hypothyroidism has been associated with fetal loss, pre­
term delivery, and intellectual deficit in the offspring.67,113 These 
findings are not seen in hypothyroid women on T4 replacement 
sufficient to normalize their TSH, suggesting that these associa­
tions are directly related to 

that these associa­
tions are directly related to maternal thyroid hormone status. A 
randomized prospective study in pregnant women with anti-
TPOAb and normal range TSH demonstrated the benefit of levo­
thyroxine treatment to prevent these complications.228
Other conditions in which levothyroxine requirements are 
increased (see Table 13.3)9,80,146 include malabsorption due to 
bowel diseases, impaired gastric acid secretion,206 and adsorp­
tion of levothyroxine to coadministered medications s

n of levothyroxine to coadministered medications such as 
sucralfate, aluminum hydroxide, calcium carbonate, ferrous sul­
fate, lovastatin, or various resins. Certain medications, notably 
rifampin, carbamazepine, phenytoin, and sertraline, increase the 
clearance of levothyroxine by inducing CYP3A4 in the liver. 
Estrogen given to postmenopausal women may act in the same 
way, although the increases in D3 also play a role.229 Soy pro­
tein and soybean isoflavones have been proposed to interfere

oybean isoflavones have been proposed to interfere 
directly with thyroid hormone action as well as synthetic T4 
absorption.230 There is no evidence that soy interferes with thy­
roid function in euthyroid individuals who are iodine sufficient, 
and the effect of soy on T4 absorption in hypothyroid patients is 
modest.230 Amiodarone increases levothyroxine requirements by 
blocking conversion of T4 to T3 and perhaps by interfering with 
T3–thyroid hormone receptor binding.231 Selenium deficienc

id hormone receptor binding.231 Selenium deficiency 
is rare, but because it is rate limiting in the synthesis of D1 (see 
Fig. 11.2),84 any significant deficiency, such as may occur in 
patients receiving diets restricted in protein, may increase levo­
thyroxine requirements.
Occasionally in patients who have been treated with radioac­
tive iodine for Graves disease or toxic nodular goiter, some degree 
of thyroid hormone secretion persists and, although insufficient 
to sustain normal thyroid 

 although insufficient 
to sustain normal thyroid hormone levels, is autonomous. Such 
patients may have a suppressed TSH on what otherwise would 
  Conditions That Alter Levothyroxine 
Requirements
Increased Levothyroxine Requirements
Pregnancy
Gastrointestinal Disorders
Mucosal diseases of the small bowel (e.g., sprue)
After jejunoileal bypass and small bowel resection
Impaired gastric acid secretion (e.g., atrophic gastritis)
Diabetic diarrhea
Therapy With Certain Pharmacologic Agents
Drugs 

a
Drugs That Interfere With Levothyroxine Absorption
Cholestyramine
Sucralfate
Aluminum hydroxide
Calcium carbonate
Ferrous sulfate
Drugs That Increase the Cytochrome P450 Enzyme (CYP3A4)
Rifampin
Carbamazepine
Estrogen
Phenytoin
Sertraline
? Statins
Drugs That Block T4 to T3 Conversion
Conditions That May Block Deiodinase Synthesis
Selenium deficiency
Cirrhosis
Decreased Levothyroxine Requirements
Aging (≥65 years)
T

ents
T4, Thyroxine; T3, triiodothyronine.
   
TABLE 13.3
428
be considered a replacement dose of levothyroxine. The levothy­
roxine dose in these individuals should be reduced until TSH 
levels rise to normal, keeping in mind that several months may 
be required before TSH secretion recovers after its prolonged sup­
pression. Because of either the delayed effects of radioiodine or the 
natural history of Graves disease per se, th

 the 
natural history of Graves disease per se, this autonomous T4 secre­
tion may decrease with time, leading to an increase in levothyrox­
ine requirements in subsequent years. Rarely, the opposite occurs; 
that is, a patient treated with radioiodine develops an increased 
TSH level, but after several months of therapy the requirement for 
such replacement is either reduced or eliminated. This response 
may reflect transient impairment of thyroid function by a combi­
nation of preirradiation a

id function by a combi­
nation of preirradiation antithyroid drug therapy and immediate 
but transient effects of irradiation on the thyroid. In such patients, 
frequent monitoring of levothyroxine replacement is required to 
avoid overreplacement.
In North America, based on the recent previously discussed 
changes in assessment of levothyroxine bioequivalence, the pos­
sibility of a difference in tablet levothyroxine content should be 
considered if a new preparation changes the biologic or bio

d if a new preparation changes the biologic or biochemi­
cal effects of the same dosage. Although the difference in prepara­
tion is unlikely to cause a significant difference in most patients, 
the change in manufacturer introduces another potential source 
of variability. 
Adverse Effects of Levothyroxine Therapy
Although the administration of excessive doses of levothyroxine 
causes accelerated bone loss in postmenopausal patients, most 
authorities believe that returning thyroid status to no

rities believe that returning thyroid status to normal does not 
have adverse effects on bone density.232 Administration of exces­
sive doses also increases cardiac wall thickness and contractility 
and, in elderly patients, increases the risk of atrial fibrillation.16,25
In some patients, TSH levels remain elevated despite the pre­
scription of adequate replacement doses.233 This response is most 
often a consequence of poor adherence. The combination of nor­
mal or even elevated serum FT4 valu

nation of nor­
mal or even elevated serum FT4 values and elevated TSH levels 
can occur if the patient does not take levothyroxine regularly but 
ingests several pills the day before testing. The integrated dose 
of levothyroxine over prior weeks is best reflected in the serum 
TSH level, and nonadherent patients require careful education 
as to the rationale for treatment. Subtle changes in dietary hab­
its, such as increasing the ingestion of bran-containing products, 
soy, or calcium or proto

ran-containing products, 
soy, or calcium or proton pump inhibitors, may decrease levo­
thyroxine absorption, and their recognition requires a careful 
history.80,146,233 In patients on T4 replacement with erratic TSH 
levels and consideration of inconsistent adherence with therapy 
or malabsorption, a protocol has been developed to evaluate such 
patients with weekly supervised dosing of LT4 and monitoring 
of serum TSH and FT4 concentrations.1 Use of weekly dosing of 
LT4 should be done very c

Use of weekly dosing of 
LT4 should be done very cautiously in older patients or those with 
known cardiac disease. 
Patients With Hypothyroid Symptoms Despite Restitution 
of Normal Thyroid Function
In patients taking levothyroxine replacement with a normal serum 
TSH concentration, symptoms consistent with hypothyroidism 
may persist.234 A survey of hypothyroid patients on levothyrox­
ine with normal TSH and control patients included questions 
about symptoms that might be associated with thyr

about symptoms that might be associated with thyroid hormone 
deficiency.235 Although a significant fraction of both groups 
reported such symptoms, a greater fraction of patients on levo­
thyroxine replacement had these symptoms. Patients on thyroxine 
replacement post-thyroidectomy consistently require a higher 
serum T4 than preoperatively to maintain the same serum T3 
concentration.208 The addition of thyroid replacement contain­
ing triiodothyronine continues to be a focus in these patien

othyronine continues to be a focus in these patients but 
has not been shown to confer long-term relief of symptoms.234 
Additionally, such patients should be educated as to the relation­
ship between symptoms of hypothyroidism and the role of thyroid 
hormone in relieving them, and other causes should be sought for 
the symptoms. 
Special Aspects of Hypothyroidism
Subclinical Hypothyroidism
The term subclinical hypothyroidism was originally used to describe 
the patient with a low-normal FT4 bu

 to describe 
the patient with a low-normal FT4 but a slightly elevated serum 
TSH level. Other terms for this condition are mild hypothyroidism, 
early thyroid failure, preclinical hypothyroidism, and decreased thy­
roid reserve (see Table 13.2). The TSH elevation in such patients is 
modest, with values typically between 5 and 15 mU/L, although 
patients with a TSH above 10 mU/L more often have a reduced 
FT4 and may have true hypothyroid symptoms. The definition of 
this syndrome depends sign

oms. The definition of 
this syndrome depends significantly on the reference range for a 
normal TSH concentration. This syndrome is most often seen in 
patients with early Hashimoto disease and is a common phenom­
enon, occurring in 7% to 10% of older women.14–16
A number of studies on the effects of thyroid hormone treat­
ment in such patients have used physiologic end points (e.g., 
measurements of various serum enzymes, systolic time intervals, 
serum lipids, psychometric testing), and resul

s, 
serum lipids, psychometric testing), and results have been vari­
able.14–16 In the most carefully controlled studies, one or another 
of the parameters has returned to normal in about 25% to 50% 
of patients. A prospective randomized trial of subclinical hypothy­
roidism in elderly males showed no benefit of therapy.236 Among 
those men originally enrolled based on an elevated TSH, 62% 
had a normal TSH on subsequent measurement, indicating the 
importance of repeating a TSH measurement in s

he 
importance of repeating a TSH measurement in suspected sub­
clinical hypothyroidism. In general, FT4 and TSH levels normal­
ize, but free T3, usually normal at the outset, does not change. 
Modest improvements in cardiac indices and lipid profiles have 
been noted in most but not all studies, although the benefit on 
cardiovascular risk is seen in middle-aged patients.30,34 The asso­
ciation of mild hypothyroidism with an increase in risk for ath­
erosclerotic heart disease has been shown by

 ath­
erosclerotic heart disease has been shown by some studies but not 
others.14,16 The impact of treatment to reduce the risk of athero­
sclerotic heart disease, other than reduction in risk factors such 
as cholesterol and C-reactive protein, have not yet been studied.
One factor favoring a decision to recommend levothyroxine 
therapy is the likelihood of developing overt hypothyroidism. 
The risk of progression from subclinical to overt hypothyroidism 
(elevated serum TSH and reduced FT4) i

thyroidism 
(elevated serum TSH and reduced FT4) is most closely related to 
the magnitude of serum TSH elevation and the presence of anti-
TPOAb. Prospective studies of women with subclinical hypo­
thyroidism have shown rates of progression from approximately 
3% to 8% per year, with the higher rates seen in individuals with 
initial TSH concentration greater than 10 and those with posi­
tive anti-TPOAb.237 Although most individuals progress slowly 
to overt hypothyroidism, rapid progression ov

wly 
to overt hypothyroidism, rapid progression over weeks to months 
has been reported.238 Factors that may predispose to rapid pro­
gression include increased age, high levels of TPOAb, intercur­
rent systemic infection or inflammation, iodine contrast agents, 
and medications such as amiodarone and lithium. The decision to 
treat with levothyroxine must also take into account the expense 
429
and inconvenience of a daily medication, which is not acce

nvenience of a daily medication, which is not acceptable 
to some patients, and the possibility that unintended overdosage 
may exacerbate osteoporosis or cause cardiac arrhythmias. Ulti­
mately the decision to treat must depend on a careful consider­
ation of the individual clinical situation and patient preference. If 
a therapeutic trial is performed, the TSH concentration should be 
monitored carefully and should not be reduced below normal. If 
no therapy is given, such patients should be m

If 
no therapy is given, such patients should be monitored at intervals 
of 6 to 12 months both clinically and biochemically. 
Metabolic Insufficiency
Nonspecific symptoms of true hypothyroidism include mild las­
situde, fatigue, slight anemia, constipation, apathy, cold intoler­
ance, menstrual irregularities, loss of hair, and weight gain. For 
this reason, some patients with such complaints but with nor­
mal laboratory results for thyroid function have been considered 
candidates for levothyr

tion have been considered 
candidates for levothyroxine therapy. The response to thyroid 
hormone therapy is sometimes gratifying, at least initially, but 
symptomatic improvement usually disappears after a time unless 
the dose is increased. Eventually, even larger doses fail to alleviate 
the symptoms, confirming that they do not arise from a deficiency 
of thyroid hormone.
Thus thyroid hormone therapy should be avoided in patients 
with no biochemical documentation of impaired thyroid functio

chemical documentation of impaired thyroid function. 
Furthermore, even in patients with subclinical hypothyroidism, 
symptoms may be out of proportion to abnormalities in the FT4. 
It is unwise to raise a patient’s expectations that such symptoms 
will be relieved by correction of mild biochemical abnormalities. 
Thyroid Function Testing in Patients Receiving Replace­
ment Therapy for Unclear Reasons
Physicians are frequently confronted with patients receiving levo­
thyroxine in whom the basis 

ients receiving levo­
thyroxine in whom the basis for the diagnosis cannot be estab­
lished. It may be difficult to document previous clinical findings 
or laboratory data to determine whether thyroid hormone replace­
ment is indicated. If serum TSH is in the normal range and pri­
mary hypothyroidism is suspected, a simple way of assessing the 
need for levothyroxine therapy is to switch levothyroxine to an 
every-other-day dosage or to reduce the daily dose by 50% and 
to reevaluate TSH and FT4

e daily dose by 50% and 
to reevaluate TSH and FT4 after 4 weeks. If there is no significant 
increase in TSH concentration and FT4 remains constant during 
that period, residual thyroid function is present, although it may 
still not be completely normal. To answer this question, levothy­
roxine can then be withdrawn and blood tests repeated 4 to 8 
weeks later.
If the initial TSH level is suppressed, indicating overreplace­
ment, the levothyroxine dose should be reduced until TSH 
becomes dete

ine dose should be reduced until TSH 
becomes detectable before this trial is instituted. If central hypo­
thyroidism is suspected, the FT4 must be monitored. 
Emergent Surgery in the Hypothyroid Patient
The perioperative course of patients with untreated hypothyroid­
ism has been evaluated in several studies. In general, such patients 
were not recognized to be hypothyroid or did not require sur­
gery despite the presence of significant hypothyroidism. Com­
plications were uncommon. Perioperati

oidism. Com­
plications were uncommon. Perioperative hypotension, ileus, 
and central nervous system disturbances were more common 
in hypothyroid patients, and patients with major infections had 
fewer episodes of fever than did euthyroid control subjects.239 
Other complications were delayed recovery from anesthesia and 
abnormal hemostasis, possibly owing to an acquired form of von 
Willebrand disease.59
From these studies one may conclude that emergent surgery 
should not be postponed in hyp

t emergent surgery 
should not be postponed in hypothyroid patients but that such 
patients should be rigorously monitored for evidence of carbon 
dioxide retention, bleeding, infection, and hyponatremia. These 
findings are also relevant to the treatment of hypothyroid individ­
uals with symptomatic coronary artery disease. Considering the 
lack of significant increase in perioperative complications in the 
hypothyroid patient, the option of surgery for remediable coro­
nary artery lesions is o

gery for remediable coro­
nary artery lesions is open to hypothyroid individuals without the 
risk of a myocardial infarction in association with restitution of 
the euthyroid state (see later).240 
Heart Disease and Thyroid Hormone Therapy
Coexisting Coronary Artery Disease and Hypothyroidism
In many patients with coronary artery disease and primary hypo­
thyroidism, cardiac function is improved in response to levo­
thyroxine therapy because of a decrease in peripheral vascular 
resistance and 

a decrease in peripheral vascular 
resistance and improvement in myocardial function. However, 
patients with preexisting angina pectoris should be evaluated for 
correctable lesions of the coronary arteries and treated appropri­
ately before levothyroxine is administered.240,241 Retrospective 
studies indicate that this approach is safer than the institution of 
replacement therapy prior to angiography and angioplasty or even 
coronary artery bypass grafting (CABG).240
In a few patients, lesion

pass grafting (CABG).240
In a few patients, lesions may not be remediable, or small-­vessel 
disease is severe even after stenting or bypass grafting, so that com­
plete replacement cannot be instituted. Such patients must receive 
optimal antianginal therapy combined with β-adrenergic receptor 
blockers in judicious quantities, and complete restitution of the 
euthyroid state may not be possible. 
Thyroid Hormone for Compromised Cardiovascular 
Function
In addition to the issues raised in patie

Function
In addition to the issues raised in patients with combined hypo­
thyroidism and coronary artery disease, there is interest in the 
potential therapeutic use of thyroid hormone in the treatment 
of patients with cardiomyopathy or those who have undergone 
CABG or other cardiac procedures.25 As expected, T3 levels 
are reduced in patients with advanced congestive heart failure, 
as with any illness. In one report, 23 patients with advanced 
heart failure (mean ejection fraction, 22%) were

heart failure (mean ejection fraction, 22%) were given up to 
2.7 μg/kg of liothyronine over 6 hours with an increase in car­
diac output and decrease in systemic vascular resistance but 
without increase in heart or metabolic rate.242 Similar effects 
were seen with a dose of 110 μg liothyronine over 6 hours after 
CABG.243
Liothyronine has also been given postoperatively for congeni­
tal heart disease and, again, an improvement in cardiac output 
and decrease in vascular resistance occurred 

put 
and decrease in vascular resistance occurred without adverse 
side effects.209 These results suggest that, in certain selected cir­
cumstances, liothyronine may be useful as adjunctive therapy in 
patients with congestive heart failure because of its effect of relax­
ing vascular smooth muscle.
Although most therapeutic trials of thyroid hormone treat­
ment have used T3, thyroid hormone analogues have also been 
used.25 The most extensively studied is 3,5-diiodothyropropi­
onic acid (DITPA)

studied is 3,5-diiodothyropropi­
onic acid (DITPA), an analogue that binds both TRα and TRβ 
with low affinity. A randomized study of DITPA in heart failure 
showed some improved cardiac performance,244 but the study was 
terminated because of significant metabolic side effects, including 
weight loss.245 
430
Screening for Primary Hypothyroidism
The use of screening for hypothyroidism has been addressed by a 
number of studies but remains controversial.191 The conclusions

dies but remains controversial.191 The conclusions 
depend, to a great extent, on assumptions regarding the effective­
ness and economic value of identifying and treating patients with 
subclinical hypothyroidism. An evidenced-based medicine review 
of the literature by an expert panel concluded that there was insuf­
ficient evidence to support population-based screening.246 Aggres­
sive “case finding,” based on identification of risk factors such as 
family history, was advocated for pregnant w

h as 
family history, was advocated for pregnant women, women older 
than 60 years, and others at high risk. The fraction of patients with 
hypothyroidism missed when a “case finding” strategy is used, 
however, is not known. A updated report from the US Preven­
tive Services Task Force concluded that there is still insufficient 
data to recommend population screening for hypothyroidism in 
nonpregnant adults.247 Large, randomized, prospective studies of 
levothyroxine treatment in patients with

udies of 
levothyroxine treatment in patients with subclinical hypothyroid­
ism to establish benefit, however, have not yet been performed. 
Given the very high incidence of hypothyroidism in older women 
and the absence of robust clinical symptoms, an assessment of 
TSH levels at 5-year intervals in women older than age 50 years 
seems justified until more extensive studies have been performed.
A second complex issue involves whether women planning preg­
nancy should be screened for the presenc

ing preg­
nancy should be screened for the presence of hypothyroidism as a 
routine part of a prenatal visit.113 This question is raised because of 
increasing association of adverse outcomes in pregnancy, even with 
subclinical hypothyroidism, including impairment of mental devel­
opment in infants, fetal loss, and preterm delivery.66,67 The preva­
lence of overt hypothyroidism during pregnancy is approximately 
2%, and screening of all patients has been advocated by several pro­
fessional orga

 has been advocated by several pro­
fessional organizations. Thyroid testing in high-risk patients, “case 
finding,” has been advocated, although a prospective study showed 
that approximately a third of pregnant women with underlying 
thyroid disease are missed by this testing approach.248
Isolated maternal hypothyroxinemia in pregnancy, generally 
defined as a FT4 in the lowest 2.5 to 5th percentile, with a normal 
reference range TSH, has been associated with a range of neu­
rologic deficits,

 associated with a range of neu­
rologic deficits, including reduced IQ, reduced cortical volume, 
language delay, and autism.67 Treatment of hypothyroxinemic 
women during pregnancy has not been associated with improved 
cognitive outcome of the offspring,249,250 and routine treatment is 
not currently recommended.113
A number of questions are raised regarding the appropriate tim­
ing of testing, whether thyroid autoantibodies should be measured, 
the relative importance of TSH and FT4, the inf

, 
the relative importance of TSH and FT4, the influence of trimes­
ter on the normal ranges, and the threshold for intervention. The 
association of maternal subclinical hypothyroidism and preterm 
delivery is a much more proximal and defined end point to study 
compared to intellectual performance in offspring. The morbidity 
and mortality rates from preterm delivery are significant for the 
newborn, and these findings are likely to allow for more focused 
intervention studies to determine the

ore focused 
intervention studies to determine the response to T4 treatment.66
For the moment, it appears that any patient with a family his­
tory of autoimmune thyroid disease, with symptoms suggesting 
hypothyroidism, or with thyroid enlargement should be tested for 
thyroid dysfunction prior to pregnancy or as soon after conception 
as is feasible. Optimization of levothyroxine therapy for women 
known to have hypothyroidism prior to conception, when pos­
sible, may be the most effective inte

n, when pos­
sible, may be the most effective intervention to prevent hypothy­
roid-related complications of pregnancy. Although the data do not 
yet reach the threshold to mandate universal screening, the ease of 
testing, associated adverse outcomes, and demonstrated benefit of 
intervention make thyroid testing of all pregnant women a reason­
able choice. 
Myxedema Coma
Myxedema coma is the ultimate stage of severe long-standing hypo­
thyroidism.251,252 This state, which almost invariably aff

sm.251,252 This state, which almost invariably affects older 
patients, occurs most commonly during the winter months and is 
associated with a high mortality rate. It is usually accompanied by a 
subnormal temperature. Values as low as 23°C have been recorded. 
The external manifestations of severe myxedema, bradycardia, and 
severe hypotension are invariably present. The characteristic delay 
in deep tendon reflexes may be lacking if the patient is areflexic. 
Seizures may accompany the comato

t is areflexic. 
Seizures may accompany the comatose state. Although the patho­
genesis of myxedema coma is not clear, factors that predispose to its 
development include exposure to cold, infection, trauma, and cen­
tral nervous system depressants or anesthetics. Alveolar hypoven­
tilation, leading to carbon dioxide retention and narcosis, and 
dilutional hyponatremia resembling that seen with inappropriate 
ADH secretion may also contribute to the clinical state.
From the foregoing, it appears

the clinical state.
From the foregoing, it appears that myxedema coma should 
be readily recognized from its clinical signs, but this is not the 
case. Hypothermia of any cause (e.g., to cold exposure) may cause 
changes suggestive of myxedema, including delayed relaxation of 
deep tendon reflexes. The importance of diagnosing myxedema 
coma is that a delay in therapy worsens the prognosis. Conse­
quently, a rapid serum FT4 and TSH should be obtained when­
ever this diagnosis is being considered

ined when­
ever this diagnosis is being considered. Otherwise the diagnosis 
should be made on clinical grounds; after serum has been sent for 
thyroid function tests, therapy should be initiated without await­
ing the results of delayed confirmatory tests because the mortality 
rate may be 20% or higher.
Treatment consists of administration of thyroid hormone 
and correction of the associated physiologic disturbances.251,252 
Because of the sluggish circulation and severe hypometabolism, 
absor

gish circulation and severe hypometabolism, 
absorption of therapeutic agents from the gut or from subcuta­
neous or intramuscular sites is unpredictable, and medications 
should be administered intravenously if possible. Administration 
of levothyroxine as a single intravenous dose of 500 to 800 μg 
repletes the peripheral hormone pool and may cause improvement 
within hours. Daily doses of intravenous levothyroxine, 100 μg, 
are given thereafter. Hydrocortisone (5–10 mg/hour) should also 
be g

r. Hydrocortisone (5–10 mg/hour) should also 
be given because of the possibility of relative adrenocortical insuf­
ficiency as the metabolic rate increases.
Alternatively, intravenous liothyronine may be given at a dose 
of 25 μg every 12 hours. Others have used a combination of 200 
to 300 μg T4 and 25 μg T3 intravenously as a single dose, followed 
by 25 μg T3 and 100 μg T4 24 hours later, and then 50 μg T4 daily 
until the patient regains consciousness. Hypotonic fluids should 
not be given 

sciousness. Hypotonic fluids should 
not be given because of the danger of water intoxication owing 
to the reduced free water clearance of the hypothyroid patient. 
Hypertonic saline and glucose may be required to alleviate severe 
dilutional hyponatremia and the occasional hypoglycemia.
A critical element in therapy is support of respiratory function 
by means of assisted ventilation and controlled oxygen adminis­
tration. Internal warming by gastric perfusion may be useful, but 
external warm

astric perfusion may be useful, but 
external warming should be avoided because it may lead to vascu­
lar collapse due to peripheral vasodilatation. Further heat loss can 
be prevented with blankets. An increase in temperature may be 
seen within 24 hours in response to levothyroxine. General mea­
sures applicable to the comatose patient should be undertaken, 
such as frequent turning, prevention of aspiration, and attention 
to fecal impaction and urinary retention.
431
Chapter 13  Hypothyroidi

nd urinary retention.
431
Finally, the physician should assess the patient for the presence 
of coexisting disease, especially infection, cardiac disease, or cere­
brovascular disease. The myxedematous patient may be afebrile 
despite a significant infection. As soon as the patient is able to take 
medication by mouth, treatment with oral levothyroxine should 
be instituted. 
Thyroiditis is a term indicating the presence of thyroid inflamma­

 term indicating the presence of thyroid inflamma­
tion and thus comprises a large group of diverse inflammatory 
conditions. These conditions include the following: autoimmune 
or quasi-autoimmune causes and viral or postviral conditions 
and infections, including those of bacterial and fungal origins; 
a chronic sclerosing form of thyroiditis, termed Riedel thyroiditis 
(or struma); and miscellaneous causes of various types, including 
radiation-induced and granulomatous causes, such as sarcoi

n-induced and granulomatous causes, such as sarcoidosis, 
as well as lithium.5
Not only are the causes of thyroiditis extremely varied, their 
clinical presentations may also be diverse and are difficult to cate­
gorize in a simple fashion (Table 13.4). Thus, as already discussed, 
autoimmune thyroiditis may present with hypothyroidism but 
often patients remain euthyroid for long periods after the disease 
begins. On the other hand, in a euthyroid patient with Hashi­
moto disease who becomes pr

id patient with Hashi­
moto disease who becomes pregnant, the postpartum period is 
often complicated by an acute form of thyrotoxicosis due to the 
transient exacerbation of thyroiditis, often followed by a period of 
hypothyroidism (see Chapter 12).66
A similar syndrome has been observed in nonpregnant patients, 
called painless subacute thyroiditis. It is manifested primarily as thy­
rotoxicosis of sudden onset without localized pain and often with­
out evidence of autoimmune disease. This co

 with­
out evidence of autoimmune disease. This condition may be viral in 
origin in some patients; however, the classic presentation of postvi­
ral thyroiditis, a condition referred to as painful subacute thyroiditis, 
is characterized by extreme thyroid tenderness, with pain radiating 
to the oropharynx and ears, and must be differentiated from acute 
infectious thyroiditis caused by bacterial or fungal infection.253
Thus inflammatory conditions of the thyroid present a didactic 
dilemma becau

s of the thyroid present a didactic 
dilemma because one must decide whether to discuss these entities 
as a group with the common denominator of inflammation or to 
categorize them according to their principal clinical effects, namely, 
thyrotoxicosis or thyroid hormone deficiency. We have chosen 
the latter approach and have already discussed autoimmune thy­
roiditis, the major cause of thyroid gland failure (see Table 13.1). 
However, patients with acute autoimmune thyroiditis may also 
devel

 with acute autoimmune thyroiditis may also 
develop thyrotoxicosis, such as in postpartum painless thyroiditis 
(see Chapter 12 discussion regarding autoimmune thyroiditis).66 
These patients must be differentiated from those with Graves dis­
ease. In addition, some patients with painful subacute thyroiditis 
have thyrotoxicosis as a major manifestation with varying degrees 
of neck discomfort. For that reason, this thyroiditis syndrome is 
also discussed in Chapter 12, even though the pain ass

 discussed in Chapter 12, even though the pain associated with 
the typical form of this condition makes the principal differential 
diagnosis lie between that and infectious thyroiditis. In that con­
text, subacute thyroiditis is also mentioned later.
Acute Infectious Thyroiditis
Although the thyroid gland is remarkably resistant to infec­
tion, congenital abnormalities of the piriform sinus, underlying 
autoimmune disease, or immunocompromise of the host may 
lead to the development of an infe

f the host may 
lead to the development of an infectious disease of the thyroid 
gland, acute infectious thyroiditis.253,254 The cause may be any 
bacterium, including Staphylococcus, Pneumococcus, Salmonella, 
or Mycobacterium tuberculosis. In addition, infections with certain 
fungi, including Coccidioides immitis, Candida, Aspergillus, and 
Histoplasma, have been reported.
The most common cause of repeated childhood infectious thy­
roiditis, particularly in the left lobe, is a consequence of 

articularly in the left lobe, is a consequence of an inter­
nal fistula extending from the piriform sinus to the thyroid.254 
This sinus is the residual connection following the path of migra­
tion of the ultimobranchial body from the fifth pharyngeal pouch 
to the thyroid gland. The predominance of thyroiditis of the left 
lobe is explained by the fact that the right ultimobranchial body is 
often atrophic, whereas this is not the case for the left side. None­
theless, a patient with a complete

eft side. None­
theless, a patient with a completely normal thyroid gland may 
develop bacterial thyroiditis. This is an extremely rare disease even 
as a complication of direct puncture of the thyroid gland, such as 
in fine-needle aspiration. In individuals with midline infections, 
persistence of the thyroglossal duct should be considered.
Incidence
Infectious thyroiditis is extremely rare, with no more than a few 
cases being seen in large tertiary care centers. 
Clinical Manifestations
The 

rtiary care centers. 
The clinical manifestations of infectious thyroiditis are domi­
nated by local pain and tenderness in the affected lobe or entire 
gland. This is accompanied by painful and difficult swallowing. 
Because of the tendency for referral of pain to the pharynx or ear, 
the patient may not recognize the tenderness in the anterior neck. 
Depending on the virulence of the organism and the presence of 
septicemia, symptoms such as fever and chills may also ac

mia, symptoms such as fever and chills may also accom­
pany the condition.
The major differential diagnosis lies between an infectious form 
of thyroiditis and painful subacute thyroiditis. It is instructive to 
compare the principal features of these two diseases to arrive at an 
accurate diagnosis (Table 13.5). By and large, patients with acute 
infectious thyroiditis caused by a bacterium are much sicker than 
patients with painful subacute thyroiditis; they have more severe and 
localized te

yroiditis; they have more severe and 
localized tenderness and are less likely to have laboratory evidence of 
thyrotoxicosis, which is present in approximately 60% of patients 
with painful subacute thyroiditis. Ultrasonographic examination 
often reveals the abscess in the thyroid gland or evidence of swelling, 
and needle aspiration may help pinpoint the responsible organism. 
A gallium scan will be positive as a result of the diffuseness of the 
inflammation and, particularly in children wit

he 
inflammation and, particularly in children with infectious thyroid­
itis of the left lobe, a barium swallow showing a fistula connecting 
the piriform sinus and left lobe of the thyroid is diagnostic.254
Occasionally, pertechnetate scanning is useful in showing nor­
mal function of one lobe of the thyroid gland, which is much 
less common in painful subacute thyroiditis (which more often 
affects the entire gland). Needle aspiration should be used to drain 
the affected lobe, although occasi

 used to drain 
the affected lobe, although occasionally surgical drainage may be 
  Causes of Thyroiditis
Autoimmune thyroiditis
Painless subacute thyroiditis, including postpartum thyroiditis (see 
Chapter 12)
Painful subacute thyroiditis (see Chapter 12)
Acute infectious thyroiditis
Riedel thyroiditis
Postirradiation (131I or external-beam therapy)
TABLE 13.4
432
required. If a piriform sinus fistula can be demonstrated, it must 
be removed to prevent recur

demonstrated, it must 
be removed to prevent recurrence of the problem.
Antibiotics should be administered appropriate to the offend­
ing organism. Fungal infections should be treated appropriately, 
especially because many of these individuals are immunocompro­
mised. Endemic organisms should be kept in mind as a cause, 
in that both Echinococcus and Trypanosomiasis infections of the 
thyroid gland have been reported.
The prognosis is excellent with preservation of thyroid func­
tion in general

with preservation of thyroid func­
tion in general, although post-thyroiditis thyroid function tests 
should be monitored to ascertain that thyroid failure has not 
occurred. 
Riedel Thyroiditis
Riedel chronic sclerosing thyroiditis is rare and occurs primar­
ily in middle-age women.155,255,256 The etiologic mechanism 
is uncertain, and any association with autoimmune thyroid 
disease is probably coincidental.257 The morphologic similari­
ties between the fibrosis of Riedel thyroiditis and IgG4-

tween the fibrosis of Riedel thyroiditis and IgG4-related 
sclerosing disease suggest that these entities are closely related, 
with thyroiditis representing an initial manifestation of a more 
generalized process.258 Retroperitoneal, orbital, and mediastinal 
fibrosis, as well as rarer fibrotic syndromes, are associated with 
Riedel thyroiditis.259
Symptoms develop insidiously and are related chiefly to com­
pression of adjacent structures, including the trachea, esophagus, 
and recurrent laryn

uding the trachea, esophagus, 
and recurrent laryngeal nerves. Systemic evidence of inflamma­
tion is uncommon. The thyroid gland is moderately enlarged, 
stony hard, and usually asymmetric. The consistency of the gland 
and the invasion of adjacent structures suggest carcinoma, but 
there is no enlargement of regional lymph nodes. Temperature, 
pulse, and leukocyte count are normal. Severe hypothyroidism is 
unusual but does occur, as does loss of parathyroid function. The 
RAIU may be normal o

of parathyroid function. The 
RAIU may be normal or low. Elevated circulating thyroid autoan­
tibodies are much less common and are found in lower titers than 
in Hashimoto disease.
Tamoxifen, 10 to 20 mg/day (with or without corticosteroids), 
has been successful in many of these patients and is thought to 
suppress transforming growth factor beta (TGFβ).155 Surgery may 
be required to preserve tracheal and esophageal function, although 
the response to tamoxifen will often preclude the necessi

ponse to tamoxifen will often preclude the necessity for this. 
Treatment with thyroid hormone relieves the hypothyroidism but 
has no effect on the primary process. 
Miscellaneous Causes
Only a few causes of generalized inflammation of the thyroid gland 
have been reported. These include inflammation arising after 131I 
treatment for Graves disease, a residual thyroid lobe in a patient 
with thyroid cancer of the contralateral lobe, and thyroiditis aris­
ing from external beam therapy for condi

tis aris­
ing from external beam therapy for conditions such as Hodgkin 
or non-Hodgkin lymphoma, breast carcinoma, or other lesions 
of the oropharynx. Anaplastic thyroid carcinoma may rarely be 
associated with a diffuse thyroiditis and elevation of thyroid hor­
mone levels.260 In general, only radioiodine-induced thyroiditis is 
associated with pain, and glucocorticoid treatment may be useful 
in symptomatic therapy.
The complete list of references is available online at ExpertCons

st of references is available online at ExpertConsult.com.
  Features Useful in Differentiating 
Between Acute Infectious Thyroiditis and 
Acute 
Thyroiditis 
(% with 
Feature)
Subacute 
(% with 
Feature)
History
Preceding upper respiratory infection
88
17
Fever
100
54
Symptoms of thyrotoxicosis
Uncommon
47
Sore throat
90
36
Physical Examination of the Thyroid
Painful thyroid swelling
100
77
Left side affected
85
Not specific
Migrating thyroid te

side affected
85
Migrating thyroid tenderness
Possible
27
Erythema of overlying skin
83
Not usually
Elevated white blood cell count
57
25–50
Elevated ESR (>30 mm/hr)
100
85
Abnormal thyroid hormone levels 
(elevated or depressed)
5–10
60
Alkaline phosphatase, transaminases 
increased
Rare
Common
Results of Needle Aspiration
Purulent, bacteria or fungi present
∼100
0
Lymphocytes, macrophages, some 
polyps, giant cells
0
∼100
123I uptake low
Uncommon
∼100
Radiologi

lls
0
∼100
Uncommon
∼100
Radiologic Findings
Abnormal thyroid scan
92
—
Thyroid scan or ultrasound helpful in 
diagnosis
75
—
Gallium scan positive
∼100
∼100
Barium swallow showing fistula
Common
0
CT scan useful
Rarely
Not indi­
cated
Clinical Course
Clinical response to glucocorticoid 
treatment
Transient
100
Incision and drainage required
85
No
Recurrence following operative 
­drainage
16
No
Piriform sinus fistula discovered
96
No
CT, Computed tomography; ESR, erythrocyte sedi

auses 
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arathyroidism. Endocr Pract. 
433
14
Nontoxic Diffuse Goiter, Nodular Thyroid 
Disorders, and Thyroid Malignancies
SEBASTIANO FILETTI, R. MICHAEL TUTTLE, SOPHIE LEBOULLEUX, AND ERIK K. 
ALEXANDER
T
his chapter reviews the imaging techniques available for 
evaluating thyroid structural abnormalities. Moreover, it 
deals with the management of nontoxic diffuse goiter as 
well as benign and malignant thyroid neoplasia. Goiter resulting 
in thyrotoxicosis and other thyroid condit

ulting 
in thyrotoxicosis and other thyroid conditions arising from auto­
immune thyroid disease are considered in Chapters 11–13. The 
section on management of thyroid nodules and thyroid malig­
nancies is largely based on recently available guidelines from the 
American Thyroid Association (ATA).1
Structural and Functional Imaging 
of the Thyroid
Ultrasonography
Sonography is a noninvasive technique that has become an inte­
gral part of the clinical evaluation of a thyroid patient.2 High-
freq

nical evaluation of a thyroid patient.2 High-
frequency sound waves are emitted by a transducer and reflected 
as they pass through the body, whereupon the returning echoes 
are received by the transducer, which also acts as a receiver. The 
amplitude of the reflected sound waves is influenced by differences 
in the acoustic impedance of tissues encountered by the sound; 
for example, fluid-filled structures reflect few echoes and therefore 
have no or few internal echoes and well-defined margin

 no or few internal echoes and well-defined margins, solid 
structures reflect varying amounts of sound and thus have vary­
ing degrees of internal echoes and less well-defined margins, and 
calcified structures reflect virtually all incoming sound and yield 
pronounced echoes with an acoustic shadow posteriorly. Thyroid 
parenchyma, surrounding anatomic structures, and thyroid nod­
ules as small as 2 mm in diameter can be readily detected. Color 
flow Doppler ultrasonography allows visualizatio

flow Doppler ultrasonography allows visualization of vessels as 
well as assessment of nodular vascularity. The thyroid gland must 
be examined thoroughly in transverse and longitudinal planes. 
Imaging of patients with thyroid nodules and during follow-up of 
thyroid cancer should also include evaluation of the regional neck 
lymph node compartments, with the goal of identifying enlarged 
and pathologic nodes.3 The normal thyroid parenchyma has a 
characteristic homogeneous medium-level echog

s a 
characteristic homogeneous medium-level echogenicity, with 
little identifiable internal architecture (Fig. 14.1). The surround­
ing muscles typically have a hypoechoic appearance. The air-filled 
Structural and Functional Imaging of the Thyroid, 433
Nontoxic Goiter and Thyroid Nodular Disease, 437
Management Options for Patients With Nontoxic Diffuse Goiter 
and Nodular Thyroid Disease, 444
Malignant Thyroid Disorders, 445
The Surgical Treatment of Thyroid Malignancy, 463
P

or high-risk thyroid cancer.
 • 
Targeted therapy has changed the approach to patients with 
metastatic differentiated and medullary thyroid cancer. 
434
trachea in the midline gives a characteristic curvilinear reflecting 
surface with an associated reverberation artifact.
A diagrammatic representation of the neck showing the 
location(s) of any abnormal finding and their characteristics is a use­
ful supplement to the routine film images recorded during an ultra­
sound 

utine film images recorded during an ultra­
sound examination. Such a cervical map with compartments4 (Fig. 
14.2) can help communicate the anatomic relationships of the find­
ings (and/or pathology) more clearly to the referring clinician and 
serves as a reference for the sonographer on follow-up examinations.
Neck ultrasonography is clinically useful at each step of thyroid 
evaluation (Table 14.1). It confirms the presence or absence of a thy­
roid nodule when the findings on physical examin

­
roid nodule when the findings on physical examination are equiv­
ocal and may reveal the presence of other nonpalpable nodules. 
Gray-scale and color Doppler ultrasound are used to evaluate the 
nodule’s sonographic features, including size, shape, echogenicity 
(markedly or mildly hypoechoic, isoechoic, or hyperechoic), mar­
gins (irregular or smooth), composition (cystic, solid, or mixed), 
the presence of coarse or fine (microscopic) calcifications, and inter­
nal blood flow. This assessmen

cations, and inter­
nal blood flow. This assessment can be highly useful for estimating 
the cancer risk in a given nodule. However, descriptions of single 
ultrasonographic features display wide interobserver variability. In 
part because of this, the American Thyroid Association1 and other 
scientific bodies5–8 have proposed tiered systems that can be used 
to estimate a nodule’s risk of malignancy (based on its sonographic 
features) and determine whether it requires fine-needle aspiration 
b

mine whether it requires fine-needle aspiration 
biopsy (FNAB) based on size. In 2017, the American College of 
Radiology (ACR)6 published a scoring system that promotes sys­
tematic assessment of the imaging features of thyroid nodules, the 
Thyroid Imaging Reporting and Data System (TIRADS), which 
is inspired by the ACR’s recommended approaches to the imaging 
of the breast (BIRADS) and other organs. Five ultrasound features 
are assessed and numerically scored, and based on the sum of the 
f

numerically scored, and based on the sum of the 
five scores, the nodule is assigned to one of five TIRADS classes, 
each reflecting an estimated risk of cancer and recommendations 
for management (often including FNAB or surveillance). These 
classification systems have been shown to improve interobserver 
agreement.9 Ultrasound also permits a comprehensive evaluation 
of regional lymph node compartments.
Elastography assesses tissue stiffness within an isolated, solid thy­
roid nodule and may 

ithin an isolated, solid thy­
roid nodule and may prove useful as an indicator of malignancy risk 
(particularly in nodules that are cytologically indeterminate).10 Early 
reports suggested very high specificity and sensitivity, independent 
of the nodule size. More recent reports suggest that elastographic 
cancer risk assessment may be inferior to gray-scale ultrasound,11 
with positive predictive values of only 30% to 40%.12 These contra­
dictory data suggest that elastographic evaluation of 

ory data suggest that elastographic evaluation of thyroid nodules 
is highly user dependent. Furthermore, it requires special software 
packages. Elastography cannot be applied to partly or mostly cystic 
nodules and can prove difficult in a multinodular goiter.
In patients with known thyroid cancer, sonography can be 
useful in evaluating the extent of disease, both preoperatively 
and postoperatively.13 Thus in patients who present with cervical 
lymphadenopathy caused by papillary thyroid car

l 
lymphadenopathy caused by papillary thyroid carcinoma (PTC) 
but in whom the gland is palpably normal, sonography may be 
used preoperatively to detect an occult primary intrathyroid focus. 
A preoperative sonogram should be obtained in all patients with 
differentiated thyroid carcinoma or medullary thyroid carcinoma 
(MTC) to preoperatively identify the anatomic locations of any 
sonographically suspicious regional lymph nodes and thereby per­
mit planning of nodal dissection.1,14 Occasiona

r­
mit planning of nodal dissection.1,14 Occasionally impalpable resid­
ual cancer that had been identified by preoperative ultrasonography 
and found cytologically positive by ultrasound-guided FNAB can 
be identified intraoperatively by the use of a handheld ultrasound 
probe or by preoperative ultrasound-guided charcoal tattooing.15
After initial therapy for follicular cell–derived thyroid cancer 
(FCTC), sonography (together with measurement of serum thy­
roglobulin [Tg]) represents the most

of serum thy­
roglobulin [Tg]) represents the most useful method for detect­
ing residual, recurrent, or metastatic disease in the neck.3,16,17 In 
patients who have undergone less than a total thyroidectomy, the 
sonographic appearance of the remaining thyroid tissue may be an 
important factor in the decision whether to recommend comple­
tion thyroidectomy. Also, it is more sensitive than neck palpation 
in detecting recurrent disease within the thyroid bed and meta­
static disease in cervical

e thyroid bed and meta­
static disease in cervical lymph nodes.
Sonography is also the standard modality for guiding FNAB 
of most thyroid nodules and cervical lymph nodes, demonstrating 
improved accuracy and a reduction in nondiagnostic specimens.1 
External Scintiscanning
Localization of functioning or nonfunctioning thyroid tissue in 
the area of the thyroid gland or elsewhere is made possible by 
techniques of external scintiscanning. The underlying principle 
is that isotopes that are sele

derlying principle 
is that isotopes that are selectively accumulated by thyroid tissue 
can be detected by a gamma camera and the data transformed 
into a visual display. Radioactivity in specific areas can be quan­
tified.18–20 The units of measurement used in evaluation of the 
radiation dose and radioactivity are defined in Table 14.2.
Several radioisotopes are used in thyroid imaging. Techne­
tium-99m (99mTc) pertechnetate is a monovalent anion that is 
actively concentrated by the thyroid 

ion that is 
actively concentrated by the thyroid gland but undergoes negli­
gible organic binding and diffuses out of the thyroid gland as its 
concentration in the blood decreases. The short physical half-life 
of 99mTc (6 hours), its low fractional uptake, and its transient stay 
Trachea
Thyroid
Strap muscles
Sternocleidomastoid
muscle
Common
Esophagus
Longus colli
muscle
C7
Internal
jugular vein
B
A
• Fig. 14.1  Transverse composite sonogram (A) and corresponding anatomic 
map

osite sonogram (A) and corresponding anatomic 
map (B) of the normal thyroid gland. C, common carotid artery; C7, seventh 
cervical vertebra; J, internal jugular vein; LC, longus colli muscle; SM, strap 
muscles; SCM, sternocleidomastoid muscle; T, thyroid; TR, trachea. (From 
Rifkin MD, Charboneau JW, Laing FC. Special course: ultrasound 1991. In: 
Reading CC, ed. Syllabus: Thyroid, Parathyroid, and Cervical Lymph Nodes. 
Oak Brook, IL: Radiological Society of North America; 1991:363–377.)
435

ical Society of North America; 1991:363–377.)
435
Chapter 14  Nontoxic Diffuse Goiter, Nodular Thyroid Disorders, and Thyroid Malignancies
within the thyroid make the radiation delivered to the thyroid 
gland by standard activity very low. Consequently, the intravenous 
administration of radioactivities greater than 37 MBq (1 mCi) 
permits adequate imaging of the thyroid about 30 minutes later.
Two radioactive isotopes of iodine have been used in thyroid 
imaging. Iodine-131 (131I) was commonly 

 thyroid 
imaging. Iodine-131 (131I) was commonly used in the past. Con­
trary to 99mTc, iodine isotopes undergo organic binding. How­
ever, 131I is a beta emitter, its physical half-life is 8.1 days, and 
the energy of its main gamma ray is high. Because of this, 131I is 
poorly adapted for its detection.20 In many respects, 123I is an ideal 
isotope for thyroid imaging because of its short half-life (0.55 day) 
and the absence of beta radiation, but it is unfortunately much 
more expensive.21

, but it is unfortunately much 
Submandibular
gland
IB
IA
VI
IV
III
IIA
IIB
VA
VB
Anterior digastric
Hyoid bone
Cricoid cartilage
Jugular vein
Spinal accessory
nerve
A
1
2
3
6
4
5
Hyoid bone
Posterior margin of
submandibular gland
sternocleidomastoid
Carotid bifurcation
Anterior margin of trapezius
Anterior margin of
cricoid cartilage
Lateral margin of CCA
B
• Fig. 14.2  (A) Anatomic scheme of the neck with compartments. (B

 Anatomic scheme of the neck with compartments. (B) Cervical map, derived from sono­
graphic images, helps communicate anatomic relationships of disease to clinicians and serves as a refer­
ence for follow-up examinations; standard colors are used to characterize any finding. CCA, common 
carotid artery. (A, from Cooper DS, Doherty GM, Haugen BR, et al. Revised management guidelines for 
patients with thyroid nodules and differentiated thyroid cancer. Thyroid. 2009;19:1167–1214, used with 
permi

ncer. Thyroid. 2009;19:1167–1214, used with 
permission; B, courtesy J.W. Charboneau, Mayo Clinic, used with permission.)
  Clinical Utility of Neck Ultrasound
Map of the neck (thyroid and lymph node areas)
Thyroid gland: size, volume, characteristics
Nodules: number and characteristics of each nodule: diameter, shape, 
echogenicity, composition, limits, presence of calcifications, vascu­
larization
Lymph node compartments
Follow-up: numbers and diameters of nodules
Guidance for fine-needle asp

 diameters of nodules
Guidance for fine-needle aspiration biopsy
Follow-up of thyroid cancer: thyroid bed and regional lymph nodes
Guidance for thermal and ethanol ablation
TABLE 14.1
436
The most important use of scintigraphic imaging of thyroid tis­
sue is in defining areas of increased or decreased function (hot or 
cold areas, respectively) relative to the function of the remainder of 
the gland. Almost all malignant nodules are hypofunctioning, but 
more than 80% to 8

dules are hypofunctioning, but 
more than 80% to 85% of benign nodules are also nonfunction­
ing. Conversely, functioning nodules (hot nodules), particularly if 
the function of the surrounding tissue is decreased or absent, are 
rarely malignant.
Scintiscanning with radioactive iodine can also be used to 
demonstrate that intrathoracic masses represent thyroid tissue and 
to detect ectopic thyroid tissue in the neck. In FCTC patients, 
whole-body scanning (WBS) is used to detect functioning met

y scanning (WBS) is used to detect functioning metas­
tases.22,23 This scan is performed after the administration of larger 
activities of radioiodine, for diagnosis (1–5 mCi 131I or 1–5 mCi 
123I), or more commonly for therapy (≥30 mCi 131I). Such dos­
ing must follow intense stimulation with endogenous or exog­
enous thyroid-stimulating hormone (TSH, thyrotropin) and be 
performed in the absence of iodine contamination. Superimposi­
tion of computed tomography (CT) and gamma camera images 
(si

puted tomography (CT) and gamma camera images 
(single-photon emission computed tomography [SPECT]/CT) 
greatly improves both the sensitivity and specificity of the tech­
nique and the anatomic localization of any focus of uptake.24–27 
Computed Tomography
The CT appearance of the anatomic structures depends on the 
attenuation of the tissue examined. The thyroid gland, because 
of its high concentration of iodine, has a higher attenuation than 
the surrounding soft tissues. Recent advances with

the surrounding soft tissues. Recent advances with spiral CT and 
reconstruction algorithms have improved the performance of this 
method.28,29
CT scanning cannot distinguish benign from malignant nod­
ules or provide information on a nodule’s functional status. How­
ever, it can define the anatomic extent of large goiters with great 
clarity. CT scanning can provide useful information regarding the 
presence and extent of intrathoracic (substernal) goiters. The CT 
findings of an intrathoracic 

al) goiters. The CT 
findings of an intrathoracic mass in continuity with the thyroid 
gland, with high attenuation on non–contrast-enhanced images 
and marked enhancement after intravenous contrast material 
injection, all suggest intrathoracic goiter. Radioiodine scanning 
can also be performed in this clinical setting, but false-negative 
results can occur when little or no functional tissue is present in 
the intrathoracic goiter.
In aggressive pathologic processes, such as anaplastic thyroi

ve pathologic processes, such as anaplastic thyroid 
carcinoma (ATC), CT with infusion of contrast medium is the 
most frequently recommended first-line technique to identify 
lymph node metastases and to define the tumor’s relationships 
to surrounding structures, including vessels and the aerodiges­
tive tract.28,29 CT imaging is less sensitive than neck ultraso­
nography for the detection of lymph node metastases. However, 
contrast-enhanced CT can be a useful complement to ultrasound 
for ex

T can be a useful complement to ultrasound 
for exploring the mediastinum and for preoperative assessments 
of vascular invasion and involvement of the retrotracheal space. 
High-resolution CT without contrast enhancement is the most 
sensitive method for the detection of micrometastases in the 
lungs. Because of the necessity of infusing iodine-containing con­
trast agents for CT scanning of the neck and mediastinum, CT 
should be performed at least 4 weeks before any administration 
of radioio

east 4 weeks before any administration 
of radioiodine.30 
Magnetic Resonance Imaging
Because the hydrogen atoms of different tissues have different 
relaxation times (termed T1 and T2), a computer-assisted analysis 
of T1-weighted and T2-weighted signals is used to differentiate 
the thyroid gland from skeletal muscles, blood vessels, or regional 
lymph nodes. Normal thyroid tissue tends to be slightly more 
intense than muscles on a T1-weighted image, and tumors often 
appear more intense than

 image, and tumors often 
appear more intense than normal thyroid tissue.
Magnetic resonance imaging (MRI) does not distinguish 
benign from malignant nodules and does not assess functional 
status. Recurrent neoplasms in the thyroid bed or regional lymph 
nodes can be detected with MRI. Recurrence is characterized by 
a mass with low to medium intensity on T1-weighted images and 
medium to high signal intensity on T2-weighted images. Con­
versely, scar tissue or fibrous tissue has low signal in

y, scar tissue or fibrous tissue has low signal intensity on both 
T1-weighted and T2-weighted images.28,29 Tumor invasion of 
adjacent skeletal muscle has high signal intensity on T2-weighted 
images. Edema or inflammation in the muscle can cause a simi­
lar appearance and can be difficult to differentiate from recurrent 
tumor.
Compared with CT scan, MRI may better delineate any 
involvement of the aerodigestive axis. It is often used as a second-
line imaging technique in patients with demons

nd-
line imaging technique in patients with demonstrated or suspi­
cious lesions in the upper part of the neck on CT scan to better 
delineate these lesions from soft tissues. In the lower part of the 
neck, movements of the aerodigestive axis during the procedure 
that may last several minutes will decrease the quality of images. 
Endoscopy of the trachea and/or esophagus, with or without 
  Radiation Nomenclature: Traditional and 
International System (SI) Units
Absorbed Radiation Dose
Units

l System (SI) Units
Units
Gy (gray) and rad (radiation absorbed dose)
Conversions
1 Gy = 100 rad = absorption of 1 J/kg
1 rad = 0.01 Gy = 1 cGy
Dose Equivalent Radiation
Units
Sv (sievert) and rem (roentgen equivalent in man)
Conversion
1 Sv = 100 rem
Radioactivity (or Activity)
Units
Bq (becquerel) and Ci (curie)
1 Bq = 1 disintegration per second = 27 pCi
1 mCi = 37 MBq
1 GBq = 103 MBq = 106 kBq = 109 Bq
Notes
Because the becquerel is extremely small, common

s
Because the becquerel is extremely small, commonly used multiples of 
the Bq unit are kBq (kilobecquerel), MBq (megabecquerel), and GBq 
(gigabecquerel).
A curie, however, is extremely large, so commonly used subunits are 
mCi (millicurie), μCi (microcurie), nCi (nanocurie), and pCi (picocurie).
Gy, Sv, and Bq are SI units; the rad, rem, and Ci are non-SI units.
TABLE 14.2
437
ultrasonography, looking for 

Thyroid Malignancies
ultrasonography, looking for evidence of intraluminal exten­
sion, can also be helpful in cases of suspected aerodigestive tract 
invasion.
MRI is superior to CT for detecting brain metastases from 
solid tumors because it offers higher soft tissue contrast with no 
bone artifacts and relatively few partial volume effects. The para­
magnetic contrast agents used with MRI also produce significantly 
stronger enhancement than those used with CT. For these reasons, 
in patients

hose used with CT. For these reasons, 
in patients with multiple brain lesions, CT scanning will miss 
approximately one in five of those seen on MRI.31 MRI is useful 
for assessment of the extent of bone involvement in cases of axial 
bone metastases from follicular cell–derived carcinomas and MTC, 
which are poorly visualized on bone scintigraphy.32,33 For MTC 
patients, contrast-enhanced MRI is more sensitive than three-phase 
contrast-enhanced CT for the detection of liver metastases.33 
Pos

d CT for the detection of liver metastases.33 
Positron Emission Tomography (PET)
PET is both quantitative and tomographic. The radionuclide used 
emits a positron that is converted into a pair of photons after a 
short path of a few millimeters in the tissue. The coincident detec­
tion of the two photons, which travel on a line in opposite direc­
tions, permits the localization of the site of the radionuclide decay.
PET is being used with increasing frequency for the assess­
ment of all types o

sing frequency for the assess­
ment of all types of cancers. The tracer used most often in clinical 
settings is the glucose analog [18F]fluorodeoxyglucose (18FDG). 
It is taken up by both neoplastic and benign cells and remains 
metabolically trapped inside tumor cells because of its inability 
to undergo glycolysis. PET scanners permit in vivo images related 
to regional glucose metabolism, with high sensitivity and spa­
tial resolution. Superimposition of CT and PET images greatly 
improves b

mposition of CT and PET images greatly 
improves both the sensitivity and specificity of the technique and 
the anatomic localization of any focus of abnormal uptake. The 
uptake in any focus can be quantified, and the most frequently 
used parameter is the maximum standardized uptake value (SUV­
max). The sensitivity of 18FDG-PET scanning may be improved 
with TSH stimulation.34,35
PET scanning should be performed in only selected patients 
with thyroid carcinoma. Low-risk patients are very unl

 thyroid carcinoma. Low-risk patients are very unlikely to 
require 18FDG-PET scanning as part of initial staging or follow-
up. 18FDG-PET scanning in thyroid cancer23,34 may be used as 
follows:
 • To localize disease in Tg-positive patients (having serum Tg 
levels >10 ng/mL) with no other abnormality on diagnostic 
imaging; it is mostly useful for the detection of lymph node 
metastases in the posterior neck and mediastinum or distant 
metastases. In two meta-analyses, the patient-based sens

ases. In two meta-analyses, the patient-based sensitivity 
of 18FDG-PET/CT for detecting persistent or recurrent dif­
ferentiated thyroid cancer (DTC) was found to be appreciably 
higher than that of PET alone (93–94% vs 83–84%, respec­
tively).36,37
 • For the initial staging and follow-up of patients with anaplas­
tic, poorly differentiated, or Hürthle cell thyroid cancers to 
identify sites of disease that may be missed with conventional 
imaging; in these cancers, FDG uptake is usually high

ging; in these cancers, FDG uptake is usually high and 131I 
uptake is either low or absent.
 • In patients with known distant metastases in whom high FDG 
uptake in large metastases indicates a high risk for disease-spe­
cific fatality and poor response to 131I therapy.38,39
 • As a measurement of posttreatment response following local 
(external beam irradiation, surgical resection, thermal ablation, 
embolization) or systemic therapy.40
Inflammatory lymph nodes, suture granulomas, and incre

ammatory lymph nodes, suture granulomas, and increased 
muscle activity are common causes of false-positive 18FDG-
PET findings. Also, asymmetric laryngeal uptake is frequently 
observed in patients with vocal cord paralysis. Therefore cytologic 
or histologic confirmation is required before one can be certain 
that an 18FDG-positive lesion represents metastatic disease. High 
uptake has also been observed in several thyroid diseases, such 
as Hashimoto thyroiditis, and PET cannot be used to dif

shimoto thyroiditis, and PET cannot be used to differen­
tiate benign from malignant thyroid nodules. The discovery of 
focal thyroid uptake (thyroid incidentaloma) on a FDG-PET scan 
performed for other reasons should lead to a complete workup, 
including ultrasound and fine-needle aspiration (FNA) cyto­
logic testing, because one-third of these nodules may prove to be 
malignant.41,42 Delineation between lymph node metastases or 
local extension of the tumor and vessels or the aerodigestive ax

n of the tumor and vessels or the aerodigestive axis 
is often not well visualized on 18FDG-PET/CT in the absence 
of contrast injection; if necessary, other imaging techniques (CT 
and MRI with contrast medium) may be performed, especially 
for a preoperative workup.
PET scans can also be done with iodine-124 (124I) (although 
this approach is currently used almost exclusively in research set­
tings).13 Thanks to its tomographic capacity and superior resolu­
tion, 124I PET is more sensitive tha

erior resolu­
tion, 124I PET is more sensitive than diagnostic and therapeutic 
WBS performed with 131I. PET scanning with 18F-labeled dihy­
droxyphenylalanine (18F-DOPA) can be gainfully used to visualize 
neoplastic foci of MTC43,44 because the 18FDG uptake is usually 
low in MTC patients, and 18FDG-PET scan in that clinical set­
ting is therefore rarely informative.33,45 
Nontoxic Goiter and Thyroid Nodular Disease
Definitions
Nontoxic goiter is defined as any thyroid enlargement that is char

is defined as any thyroid enlargement that is char­
acterized by uniform or selective (i.e., restricted to one or more 
areas) expansion of thyroid tissue separate from a nodule or neo­
plastic growth. A goiter may or may not be associated with overt 
hyperthyroidism or hypothyroidism. A thyroid nodule is defined 
as a discrete lesion within the thyroid gland due to an abnormal 
focal growth of thyroid cells. 
Epidemiology of Goiter
The prevalence of goiter varies widely by global location and c

e of goiter varies widely by global location and can 
depend on the iodine intake of a given population. Thus goiter 
may occur endemically, due mainly to iodine deficiency, or spo­
radically, depending on whether the goiter prevalence in chil­
dren is more or less than 5%, respectively. In pregnancy, goitrous 
enlargement is physiologic and usually regresses postpartum. In an 
adult nonpregnant population, the Framingham survey revealed 
a 4.6% prevalence of goiter, with a strong female predomi

prevalence of goiter, with a strong female predominance 
(6.4% in women and 1.5% in men), whereas the Wickham 
study displayed a 3.2% prevalence (6.6:1 hazard ratio of women 
to men).46,47 However, different variables (regional variation in 
the iodine intake, smoking habits, age and sex distribution, and 
primarily the methodology [palpation vs. sonography] used to 
determine thyroid volume) may have influenced these data. Using 
sonography as the screening method, the prevalence of goiter in 

he screening method, the prevalence of goiter in 
an unselected adult population has been reported to be as high as 
30% to 50%. This prevalence is even higher in iodine-deficient 
areas and in older populations. Similarly, a prevalence of thyroid 
nodules nearing 50% has been described in adult and geriatric 
438
autopsy series48 and up to 65% in healthy adults screened with 
sonography. 49 
Etiology and Pathophysiology of Diffuse Goiter
Diffuse goiter has been traditiona

 Diffuse Goiter
Diffuse goiter has been traditionally regarded as the adaptive 
response of the thyroid follicular cell to any factor that impairs 
thyroid hormone synthesis. This classic concept, however, no 
longer appears to encompass the many aspects of goiters. Indeed, 
goiter is characterized by a variety of clinical, functional, and mor­
phologic presentations, and whether this heterogeneity represents 
different entities remains to be clarified. Also, iodine deficiency 
as the sole facto

rified. Also, iodine deficiency 
as the sole factor responsible for goiter appears to be an oversim­
plification. Thus not all inhabitants in an iodine-deficient region 
develop goiter; moreover, endemic goiter has been observed in 
countries with no iodine deficiency, and even in some regions with 
iodine excess, and has not been observed in some regions with 
severe iodine deficiency. These findings suggest that other factors 
(genetic, demographic, and environmental) may play a role in the 
g

phic, and environmental) may play a role in the 
genesis of diffuse and nodular goiter, and some of these factors 
may act synergistically. Multiple nodules causing a goiter are often 
caused by somatic mutations leading to neoplastic growth.
The role of genetic factors in goiter formation is suggested 
by several lines of evidence,50 such as (1) the clustering of goi­
ters within families, (2) the higher concordance rate for goiters 
in monozygotic than in dizygotic twins, (3) the female-to-mal

tic than in dizygotic twins, (3) the female-to-male 
ratio (1:1 in endemic vs. 7:1 to 9:1 in sporadic goiters), and (4) 
the persistence of goiters in areas where a widespread iodine pro­
phylaxis program has been properly implemented. By studying 
families affected by diffuse goiter, researchers have been able to 
detect several gene abnormalities involving proteins related to thy­
roid hormone synthesis, such as mutations in genes encoding TG, 
sodium/iodide symporter (NIS), thyroid peroxidase

sodium/iodide symporter (NIS), thyroid peroxidase (TPO), dual 
oxidase 2 (DUOX2), pendrin (Pendred syndrome [PDS]), and 
TSH receptor (TSHR). In addition, three loci for this disorder 
have been identified that map to chromosomes 14q, Xp22, and 
3q26, respectively.51,52 Although an autosomal dominant inheri­
tance has been demonstrated in several families, multiple genes 
may be involved in other families, indicating a marked genetic 
background underlying the cause of the goiter. This complica

 underlying the cause of the goiter. This complicated 
genetic pattern may explain why predisposing gene alterations 
remain unidentified in most patients with nontoxic goiter.
In addition to iodine deficiency and genetic susceptibility, the 
exposure to a variety of environmental factors has been linked to 
goiter generation.53,54 Thus certain endocrine disrupters have been 
suggested to be involved in goiter development, including phthal­
ates,55,56 perchlorate, thiocyanate, and nitrate; isofl

55,56 perchlorate, thiocyanate, and nitrate; isoflavones; and 
organochlorines as well as drugs, smoking, selenium deficiency, 
insulin resistance, oral contraceptives, parity, and alcohol.57,58
TSH has long been considered the major stimulus for thy­
roid growth in response to any factor impairing thyroid hormone 
synthesis. Indeed, in the rare clinical setting of a functioning 
TSH-secreting pituitary adenoma, the increased serum TSH con­
centrations typically cause enlargement of the thyroid 

ations typically cause enlargement of the thyroid gland.59 
Similarly, goiter is also a typical feature of Graves disease, in which 
a stimulatory growth effect on thyroid tissue is induced by thy­
roid-stimulating immunoglobulin (TSI) antibody through TSHR 
activation. Moreover, thyroid enlargement may appear during the 
course of Graves disease when increased TSH levels result from 
overtreatment with antithyroid drugs. In addition, toxic thyroid 
hyperplasia is usually present in nonautoimmun

id 
hyperplasia is usually present in nonautoimmune autosomal dom­
inant hyperthyroidism, a disorder related to germline-activating 
mutations of the TSHR gene.60 This clinical condition further 
emphasizes the role of TSH-TSHR system activation in the genesis 
of thyroid hyperplasia.50 Serum TSH concentration is normal in 
most patients with nontoxic goiter.53 Experimentally, it has been 
demonstrated that in rats iodine depletion enhances the promo­
tion of thyroid growth by normal levels of T

romo­
tion of thyroid growth by normal levels of TSH. Hence, any factor 
that impairs intrathyroidal iodine levels may lead to gradual devel­
opment of goiter in response to normal concentrations of TSH.
More intriguing is the relationship between TSH levels and 
iodine supply. Indeed, even small differences in the level of iodine 
intake are correlated with significant differences in TSH levels; 
this change has been demonstrated after the 11-year follow-up 
in the longitudinal population-based

ar follow-up 
in the longitudinal population-based Danish program monitoring 
the nationwide iodine fortification (DanThyr study).61 A complex 
network of both TSH-dependent and TSH-independent path­
ways directs thyroid follicular cell growth and function and plays 
a role in the goitrogenic process. In particular, a variety of growth 
factors, derived either from the bloodstream or through autocrine 
or paracrine secretion, may serve to regulate thyroid cell prolifera­
tion and differentiation

e thyroid cell prolifera­
tion and differentiation processes.50 Typically, early in the course 
of goiter formation, areas of microheterogeneity of structure and 
function are intermixed and include areas of functional autonomy 
and areas of focal hemorrhage. Analysis of hyperplastic nodules by 
rigid criteria also indicated that morphologically indistinguishable 
hyperplastic thyroid nodules may be either monoclonal or poly­
clonal. Monoclonal adenomas within hyperplastic thyroid glands 
may re

denomas within hyperplastic thyroid glands 
may reflect a stage in progression along the hyperplasia-neoplasia 
spectrum; accumulation of multiple somatic mutations may sub­
sequently confer a selective growth advantage to this single-cell 
clone. 62
Histologically, thyroid nodules contain irregularly enlarged, 
involuted follicles distended with colloid or clusters of smaller 
follicles lined by taller epithelium and containing small colloid 
droplets. The nodules tend to be incompletely encaps

oplets. The nodules tend to be incompletely encapsulated and 
are poorly demarcated from and merge with the internodular tis­
sue, which also has an altered architecture. However, the nodules 
in some glands appear to be localized, with areas of apparently 
normal architecture elsewhere. Here, the distinction from a fol­
licular adenoma may be difficult, and some pathologists apply 
terms such as colloid or adenomatous nodules to such lesions. 
Adenomatoid nodules show distinct gene expression p

denomatoid nodules show distinct gene expression patterns 
separate from those found in malignant lesions. Recent data have 
confirmed that distinct somatic mutations in SPOP, ZNF148, 
and EZH1 are responsible for the formation and growth of most 
benign nodules.63 
Natural History of Goiter and Thyroid Nodules
Nontoxic diffuse goiter has a female preponderance. There appears 
to be no physiologic increase in thyroid volume during normal 
adolescence. Development of a diffuse goiter during adole

ence. Development of a diffuse goiter during adolescence 
therefore is a pathologic rather than a physiologic process.64 A 
notable exception in women of childbearing age is pregnancy, 
which causes a diffuse enlargement of the gland as hormone 
demand and production increase.
Iodine intake influences the natural history of nodular goiter 
disease. In the DanThyr follow-up study58 it was demonstrated 
that 11 years after the iodization program, one-third of solitary 
thyroid nodules identified a

ne-third of solitary 
thyroid nodules identified at baseline had disappeared; interest­
ingly, one-fifth of previous multinodular goiters turned out to be 
diffuse. This finding suggested that the iodine intake is the main 
factor in determining the nodular thyroid disease appearance in 
439
a given area. Furthermore, the survey demonstrated that thyroid 
nodularity is a dynamic and not necessarily an irreve

ularity is a dynamic and not necessarily an irreversible pro­
cess. Therefore dissimilar iodine intake may account for the epide­
miologic thyroid nodular disease differences between the United 
States, which carries an adequate iodine supply, and European as 
well other countries, which previously displayed a mild to moder­
ate iodine deficiency.
The natural history of benign thyroid nodules, once detected, 
is to grow slowly if followed long enough, though there is exten­
sive heterogeneity wi

ough, though there is exten­
sive heterogeneity within any population of nodules. While on 
average nodules will slowly grow,65,66 many will stay dormant for 
years or decades.67 Rarely, benign nodules will shrink. When this 
happens, it is most often associated with the resorption of cystic 
fluid in a nodule.68 
Clinical Presentation of Goiter and Nodules
For most patients, the finding of a palpable abnormality in such a 
superficial location as the thyroid gland is disconcerting, and the 
aff

s the thyroid gland is disconcerting, and the 
affected patient is likely to seek medical evaluation. At the end 
of an appropriate investigation, the clinician can usually reassure 
the patient that the goiter or the nodule is benign. Autonomous 
nodules or autonomous functional areas in the context of a multi­
nodular goiter may result in an increased thyroid hormone secre­
tion and subsequently a subclinical or overt thyrotoxicosis. This 
feature is, however, a rare event, especially in the U

ure is, however, a rare event, especially in the United States, 
being mainly linked to the iodine deficiency. However, in general, 
thyroid nodules are usually not associated with abnormal thyroid 
hormone secretion. Therefore affected patients do not exhibit clin­
ical signs of thyroid dysfunction and are often asymptomatic. The 
only clinical features of nontoxic goiter may be those of thyroid 
enlargement. In a health care system in which the extent of cross-
sectional imaging has increased,

 extent of cross-
sectional imaging has increased, a large proportion of clinically 
relevant nodules are incidentally detected during carotid ultraso­
nography or CT and MRI studies of the chest, neck, or head. Such 
incidentally detected nodules carry the same risk of malignancy as 
do nodules identified on clinical examination. However, an inci­
dentally discovered thyroid nodule with increased metabolism on 
18FDG-PET is different, as discrete uptake of 18FDG in a thyroid 
nodule is associat

e uptake of 18FDG in a thyroid 
nodule is associated with an increased risk of thyroid cancer.69
Most thyroid nodules are asymptomatic. However, large nod­
ules, which may displace or compress the trachea, esophagus, and 
neck vessels, can be rarely associated with symptoms and signs, 
including neck tightness, dysphagia, and a choking sensation. 
These obstructive symptoms may be accentuated by the so-called 
Pemberton maneuver (see Chapter 11). Invasion or compression 
of the recurrent larynge

 Invasion or compression 
of the recurrent laryngeal nerve, causing hoarseness, rarely occurs, 
though when present, it often suggests advanced thyroid carci­
noma. More commonly, acute hemorrhage into a cystic nodule 
may produce acute, painful asymmetric enlargement of the neck 
and can enhance or induce obstructive symptoms.53 
The Approach to Thyroid Nodular Disease
Thyroid nodules are generally benign hyperplastic (or colloid) 
nodules or benign follicular adenomas. However, multiple ret­
r

nign follicular adenomas. However, multiple ret­
rospective studies confirm that about 5% to 15% of clinically 
relevant nodules prove cancerous.70,71 The prevalence of thyroid 
cancer in the United States, as well as in most industrialized coun­
tries, has been steadily increasing.72 There is debate if this increase 
is mostly due to increased detection (and increased reporting) 
of small, indolent malignancies or represents a true increase in 
thyroid cancer incidence.73–75 An increase in more

thyroid cancer incidence.73–75 An increase in more advanced thy­
roid cancer has been detected in some studies, raising questions 
that other factors beyond simply sampling bias may impact this 
finding. Regardless, the mortality rate attributable to thyroid can­
cer remains very low.74
In general, thyroid nodules larger than 1 to 1.5 cm in diameter 
are generally considered clinically relevant. Nodules smaller than 
this size, even if malignant, rarely cause harm and therefore are 
typically fo

 rarely cause harm and therefore are 
typically followed conservatively. Recent guidelines of the Ameri­
can Thyroid Association provide guidance for how ultrasound 
assessment of a nodule can be used to determine the size cutoff at 
which evaluation is recommended.1
In the evaluation of a clinically relevant thyroid nodule (Table 
14.3), a thorough history and careful physical examination should 
be supplemented with laboratory testing, imaging procedures 
(including neck ultrasonography), and 

procedures 
(including neck ultrasonography), and most importantly consid­
eration of fine-needle aspiration for cytologic and/or molecular 
assessment. With this approach, an individualized assessment of 
malignancy risk and the specific morbidity and mortality risks 
attributable to such malignancy can be made. This evaluation 
allows the health care worker to advise appropriate treatment in 
relation to the patient’s other illnesses and desires. Historic fea­
tures that suggest malignancy inc

s. Historic fea­
tures that suggest malignancy include young age under approxi­
mately 30 years,76 male sex, a history of external neck radiation 
during childhood or adolescence, total-body radiation for bone 
marrow transplant, and rapid nodule growth or persistent changes 
in speaking, breathing, or swallowing. Rarely, a family history of 
multiple endocrine neoplasia (MEN) type 2, PTEN hamartoma 
tumor syndrome (Cowden disease), familial adenomatous polypo­
sis, or Carney complex is detected

omatous polypo­
sis, or Carney complex is detected. When identified, this should 
prompt thyroid evaluation in the family members.53
On physical examination, a large, fixed, and firm nodule is 
worrisome for malignancy, especially when suspicious regional 
lymphadenopathy is detected.3,4 It should be noted, however, 
that most patients are asymptomatic at presentation, and physi­
cal examination simply detects a nodule 1 to 3 cm in size that is 
nontender and mobile with swallowing.
Many studies

Many studies have shown that nodule size minimally impacts 
the risk of malignancy77 and that the incidence of cancer in inci­
dentally identified nodules is the same as in those with palpable 
nodules. However, in nodules larger than 4 cm in diameter, the 
incidence of carcinoma may be higher.78 The presence of mul­
tiple nodules does not decrease the likelihood of thyroid cancer. 
In patients with multiple clinically relevant nodules, the rate of 
malignan

clinically relevant nodules, the rate of 
malignancy per nodule decreases, but the decrease is approxi­
mately proportional to the number of detected nodules. Therefore 
  Clinical Findings Associated With 
Malignant Thyroid Nodules
Historic Features
Young age (<20–30 years old)
Male sex
Neck irradiation during childhood or adolescence
Recent, persistent changes in speaking, breathing, or swallowing
Family history of multiple endocrine neoplasia type 2
Firm, fi

ine neoplasia type 2
Firm, fixed, and irregular consistency of nodule
Vocal cord paralysis or hoarseness
Persistent regional lymph adenopathy
TABLE 14.3
440
the overall cancer rate per patient is the same in those with mul­
tiple nodules as in those with a solitary nodule. Importantly, when 
multiple nodules are present, each must be separately evaluated 
because the dominant (largest) nodule is not solely representative 
of thyroid cancer risk.65
The 

ely representative 
The Evaluation of Patients With Nodular Disease
In all patients with suspected or known thyroid nodules, mea­
surement of serum TSH is recommended. A low or undetectable 
serum TSH, even if associated with normal free thyroid hormone 
levels, should suggest the possibility of toxic, autonomously func­
tioning nodules and prompt thyroid scintigraphy. Higher serum 
TSH concentrations, even within the normal reference range, may 
increase the risk that 

rmal reference range, may 
increase the risk that a thyroid nodule is cancerous.79,80
Measurement of serum anti-TPO antibody (TPOAb) con­
centration may assist with the diagnosis of chronic lymphocytic 
thyroiditis (Hashimoto thyroiditis) when the serum TSH level 
is elevated. Hashimoto thyroiditis causes a heterogeneous paren­
chymal appearance on sonography that at times can mimic a 
pseudonodule. When an elevated TPOAb and heterogeneous 
sonographic pattern are detected, a thyroid nodule must

raphic pattern are detected, a thyroid nodule must be 
sonographically discrete in three separate dimensions to warrant 
evaluation. Hashimoto disease may also be associated with the 
presence of bilateral, enlarged but benign-appearing lymphade­
nopathy. This feature is due to the immune nature of this disease 
and should not necessarily cause alarm. In some patients, an FNA 
(described later) will be required to help distinguish benign from 
suspicious disease.
FCTCs may release increased amou

spicious disease.
FCTCs may release increased amounts of Tg into the blood­
stream. Unfortunately, there is overlap of serum Tg levels in 
FCTCs and in most benign conditions. Therefore the measure­
ment of serum Tg levels is not useful in the initial workup of 
nodular thyroid disease. Some investigators recommend routine 
measurement of serum calcitonin levels in all patients with nodu­
lar thyroid disease to screen for MTC.81,82 However, because of 
the rarity of unsuspected MTC, the high fre

se of 
the rarity of unsuspected MTC, the high frequency of false-posi­
tive results that often prompts further workup or thyroidectomy, 
and the unknown clinical relevance of medullary microcarcino­
mas (<1 cm), it is neither cost-effective nor necessary to measure 
serum calcitonin levels in the initial evaluation of patients with 
nodular thyroid disease. In circumstances of greater suspicion 
(e.g., the presence of microcalcifications in the nodule), however, 
the measurement of serum calcit

nodule), however, 
the measurement of serum calcitonin may prove useful.3,53 If the 
unstimulated serum calcitonin determination is greater than 100 
pg/mL, MTC is likely present.82
Ultrasonographic evaluation is the optimal means of evaluating 
the anatomic structure of the thyroid. Ultrasonography allows the 
health care provider to assess both the morphologic appearance 
and the size of the gland, while also assessing cancer risk in thy­
roid nodules.1,3,5,53 Ultrasonography is capable of det

nodules.1,3,5,53 Ultrasonography is capable of detecting even 
minute thyroid nodules. In fact, of 1000 normal control subjects, 
65% had detectable nodularity on high-resolution sonography.49 
Numerous studies demonstrate that ultrasound can effectively 
stratify malignancy risk in thyroid nodules (Figs. 14.3 and 14.4). 
Such risk assessment then guides diagnostic and evaluative strate­
gies for any given patient. For example, FNA of higher risk nodules 
is generally recommended when equal to o

 nodules 
is generally recommended when equal to or larger than 1 cm. In 
contrast, very low-risk nodules may not require FNA until growth 
beyond 2 cm is detected.1 Features with the highest specificity 
for thyroid cancer include the presence of microcalcifications, 
hypoechoic parenchyma, and infiltrative or irregular margins.83–87 
Such features are most predictive when present in combination. 
The presence of abnormal adenopathy, especially when unilateral 
and in the lower neck, also incre

when unilateral 
and in the lower neck, also increases the risk of cancer when a 
thyroid nodule is confirmed. Macrocalcifications, however, do not 
predict malignancy, unless seen in combination with microcalci­
fications.83,87 A taller-than-wider shape (i.e., the anteroposterior 
dimension is larger than the transverse dimension on a transverse 
image) has been associated with increased malignancy risk in some 
studies, though this remains controversial, especially given the 
lack of a clear h

roversial, especially given the 
lack of a clear hypothesis suggesting why such a growth pattern 
would prove more malignant. In contrast, purely cystic nodules, a 
spongiform parenchyma, and homogeneously hyperechoic lesions 
carry the lowest risk of malignancy.5,49,83,84,88
Extensive published research confirming the utility of sono­
graphic risk assessment, combined with substantial advances in 
ultrasound technology, has led experts to now routinely recom­
mend a sonographic risk classificat

utinely recom­
mend a sonographic risk classification for all thyroid nodules 
(ATA guidelines, European Thyroid Association [ETA], Ameri­
can Association of Clinical Endocrinologists [AACE], TIRADS). 
Nodules should be classified into suspicion categories (high, inter­
mediate, low, and very low), as this allows an evidenced-based 
strategy to support future intervention such as FNA versus con­
servative follow-up.
High-risk nodules are solid and hypoechoic with additional 
findings of microcal

d hypoechoic with additional 
findings of microcalcifications or an irregular border. Cancer risk 
is estimated at 70% to 90% in such lesions. Intermediate-risk and 
low-risk nodules constitute the majority of nodules seen in clini­
cal practice. Intermediate-risk nodules are solid and hypoechoic 
but without the additional concerning features listed for high-risk 
nodules. Low-risk nodules are solid, isoechoic or hyperechoic, or 
partially cystic, yet they also lack concerning features of micro

c, yet they also lack concerning features of microcal­
cifications, irregular margins, and abnormal adenopathy. Cancer 
risk in these two groups is approximately 25% and 10%, respec­
tively. Nodules at high or indeterminate risk are generally recom­
mended for FNA if their maximal diameter exceeds 1 cm, whereas 
low-risk nodules can be followed until growth exceeds 1.5 cm. 
Very low-risk nodules are mostly cystic or spongiform, and risk of 
malignancy is very low. For this reason, growing consen

nancy is very low. For this reason, growing consensus sug­
gests that FNA should not be performed in such nodules unless 
the maximal nodule diameter exceeds 2 cm.88–91 Importantly, 
purely cystic nodules are so rarely malignant that FNA is not indi­
cated for diagnostic purposes.
Such guidelines can provide a roadmap for clinicians to con­
sider, though individual assessment of every patient is nonetheless 
required. Certain clinical factors, patient or physician concerns, 
or other findings ma

tient or physician concerns, 
or other findings may appropriately sway a practitioner to biopsy 
a low-risk nodule even when it is less than 1 cm, or conversely to 
choose to follow a high-risk nodule even without FNA. These are 
reasonable decisions, as the overall risk of thyroid cancer is consid­
ered in conjunction with the patient’s comorbid illnesses, desires, 
and risks of intervention.
Ultrasound elastography (USE) is a technique that seeks to use 
pressure and ultrasound as a measuremen

ks to use 
pressure and ultrasound as a measurement of tissue stiffness. In 
general, the stiffer the nodule, the higher the risk of cancer. USE 
was initially reported as highly predictive of benign or malignant 
disease.92 However, more recent trials indicate inferior perfor­
mance of USE in comparison to ultrasound assessment.
CT and MRI studies of the neck have also been used. Although 
such tests are highly useful to assess surrounding neck structures in 
preparation for surgery, their perf

structures in 
preparation for surgery, their performance is generally inferior to 
that of thyroid ultrasound. Furthermore, cancer risk characteris­
tics cannot be as readily defined (such as hypoechoic parenchyma 
or irregular margins) as with ultrasound.
441
Before the advent of ultrasound-guided FNA, thyroid scintig­
raphy with 131I, 123I, or 99mTc was used to image the gland. Most 
thyroid carcinomas ar

ed to image the gland. Most 
thyroid carcinomas are inefficient in trapping and organifying 
iodine and appear on scans as areas of diminished isotope uptake, 
referred to as a cold nodule. This feature reflects the early decrease 
of NIS expression during tumorigenesis.93 Unfortunately, most 
benign nodules also do not concentrate iodine. Furthermore, not 
all nodules with normal or slightly increased 99mTc uptake are 
benign and some may appear cold on a thyroid scan with radioac­
tive iodine.

 cold on a thyroid scan with radioac­
tive iodine.15,16 This confirms the limited utility of thyroid scin­
tigraphy. The only situation in which an iodine scan can exclude 
malignancy with reasonable certainty is in the case of a toxic (hot) 
adenoma. Such a nodule demonstrates focal 123I uptake though 
markedly suppressed or absent uptake in the remainder of the 
gland. These lesions are typically associated with a suppressed 
serum TSH level. They account for fewer than 5% to 10% of 
thyroid n

hey account for fewer than 5% to 10% of 
thyroid nodules and are almost invariably benign.53 Thyroid scin­
tigraphy is used much less often than previously, though it can 
still prove valuable in the assessment of a patient with multiple 
thyroid nodules or a borderline low serum TSH concentration. 
Scintigraphy in such cases allows the practitioner to initially tar­
get aspiration of the nonfunctional nodules. Finally, 18FDG-PET 
is increasingly performed during the evaluation of patients with 

 performed during the evaluation of patients with 
various illnesses. Although not recommended for the routine 
evaluation of thyroid nodules, incidental PET-positive nodules 
have a cancer risk of 30% to 40%.33,34,69,94–96 In such patients, 
FNA is warranted. Importantly, diffuse FDG-PET uptake most 
often is found in the setting of Hashimoto disease and should not 
be considered pathologic or malignant if ultrasound confirms the 
absence of any nodularity. 
Thyroid Nodule Fine-Needle Aspiratio

 nodularity. 
Thyroid Nodule Fine-Needle Aspiration
FNA of thyroid nodules has eclipsed all other techniques for diag­
nosing thyroid cancer, with reported overall rates of sensitivity 
and specificity exceeding 90% in iodine-sufficient areas.3,5,53,87 
The technique is easy to perform and safe, with only a handful of 
complications having been reported in the literature, and causes 
only mild discomfort. However, care must be taken to obtain an 
A
B
D
E
F
C
• Fig. 14.3  Suspicious nodule ultras

A
B
D
E
F
C
• Fig. 14.3  Suspicious nodule ultrasound features. (A) Markedly hypoechoic nodule (similar echogenicity as 
the surrounding strap muscles) with irregular margins. (B) Taller-than-wide hypoechoic nodule. (C) Mark­
edly hypoechoic nodule with regular margins. (D) Hypoechoic nodule with infiltrative margins and suspi­
cious extrathyroidal extension. (E) Multiple interruptions in calcific rim with evidence of extrusive tissue 
(echogenicity is difficult to interpret because of acoustic

city is difficult to interpret because of acoustic shadowing of the calcific rim). (F) Hypoechoic 
solid nodule with microcalcifications and irregular margins. The arrows indicate the thyroid nodule. The 
gray scale graphically represents the shades of gray that can be provided by the ultrasound equipment. 
(Modified from Durante C, Grani G, Lamartina L, et al. The diagnosis and management of thyroid nodules: 
a review. JAMA. 2018;319:914–924.)
442
adequate specimen; most 

42
adequate specimen; most authors recommend two to four aspira­
tions per nodule. Routine use of ultrasound-guided biopsy even 
for clinically palpable solid nodules combined with onsite cyto­
logic examination decreases the risk of inadequate sampling.97,98 
A satisfactory specimen must contain at least five groups of 10 
to 15 well-preserved cells. FNA samples are routinely evaluated 
microscopically for cytologic features of benign or malignant dis­
ease and classified

es of benign or malignant dis­
ease and classified using the Bethesda system for reporting thyroid 
cytopathology.99 Separately, FNA samples can be sent for molecu­
lar analysis using RNA expression–based tests or single gene muta­
tion panels.
Not all thyroid nodules require FNA, and many can be safely 
followed without intervention over time and pose minimal risk. 
A decision to consider thyroid nodule FNA should initially rest 
upon an assessment of whether cytologic interpretation would 
mod

ent of whether cytologic interpretation would 
modify the clinical care ahead. If, for example, advancing patient 
age and comorbid conditions would render further surgical 
intervention unlikely, FNA may be unnecessary. For those in 
whom evaluation is warranted, however, recommendation for 
FNA is then based on nodule size and sonographic features. As 
mentioned, for those with high-risk or intermediate-risk features, 
most nodules should generally be considered for FNA when larger 
than 1 cm.

ally be considered for FNA when larger 
than 1 cm. In contrast, low-suspicion and very low-suspicion nod­
ules should be considered for FNA when larger than 1.5 cm and 
2 cm, respectively. These guidelines were developed with the hope 
of identifying clinically relevant thyroid cancer that benefits from 
therapeutic intervention while avoiding excessive diagnostic inter­
vention, but prospective investigations of this approach are not 
yet available. Smaller nodules (generally <1 cm) can most of

ble. Smaller nodules (generally <1 cm) can most often be 
conservatively followed with repeat sonographic assessment in 1 
to 2 years unless unique circumstances or symptoms raise concern.
Thyroid nodule FNA cytologic findings should be reported 
using diagnostic categories outlined in the Bethesda System for 
Reporting Thyroid Cytopathology (Table 14.4).99 The diagnosis 
A
B
D
E
F
C
• Fig. 14.4  Indeterminate ultrasound features. (A) Elevated stiffness at elastography (red indicates soft 
tissu

iffness at elastography (red indicates soft 
tissues, blue hard tissues, and green intermediate values of stiffness). (B) Complete rim calcification. (C) 
Slightly hypoechoic nodule with intranodular vascularization (the mean flow velocity is converted into a 
color scale: flow toward the transducer is represented in red, while away from the transducer is depicted in 
blue). Low-or very-low suspicion US features: (D) Pure cyst. (E) Mixed composition nodule with isoechoic 
solid component, withou

ion nodule with isoechoic 
solid component, without any suspicious features. (F) Solid hyperechoic nodule. The arrows indicate the 
thyroid nodule. The gray scale graphically represents the shades of gray that can be provided by the ultra­
sound equipment. (Modified from Durante C, Grani G, Lamartina L, et al. The diagnosis and management 
of thyroid nodules: a review. JAMA. 2018;319:914–924.)
443
of PTC (Be

oid Disorders, and Thyroid Malignancies
of PTC (Bethesda category: malignant) by FNA on the basis of 
characteristic nuclear changes is both reliable and accurate, with 
sensitivity and specificity both approaching 100% provided that 
these changes are evaluated by an experienced cytopathologist. 
Similarly, a benign result should be viewed as highly accurate, 
as data confirm a low risk (∼1–5%) of false-negative results and 
negligible mortality risk from false-negative aspirates during an 
8.5

 risk from false-negative aspirates during an 
8.5-year follow-up.77 However, cytologically indeterminate nod­
ules harbor malignant risk. Bethesda classification allows malig­
nancy risk stratification within this category, ranging from those 
with highest indeterminate risk (suspicious for papillary carci­
noma [SUSP]) to those with lower risk (suspicious for a follicular 
neoplasm [SFN/FN] or atypia/follicular lesion of undetermined 
significance [AUS/FLUS]). Regardless, an indeterminate FNA 

nce [AUS/FLUS]). Regardless, an indeterminate FNA 
cytologic finding implies concern that such a nodule may be a 
thyroid malignancy. Cytologic results should be combined with 
clinical and ultrasound characteristics, allowing further individual 
assessment. At times, such indeterminate cytologic findings—
especially if SUSP, or combined with other clinical factors such 
as large nodule size, cosmetic concerns, or difficulty swallowing—
may prove concerning enough to warrant a recommendation for

 concerning enough to warrant a recommendation for 
surgical removal. This approach is reasonable.
Recently, a new diagnostic term was applied to thyroid lesions 
formerly classified as low-risk malignancy but felt to harbor a 
very indolent course. Such lesions have been labeled noninva­
sive follicular thyroid neoplasm with papillary-like nuclear features 
(NIFTP).100 Such lesions cannot be reliably diagnosed preop­
eratively and typically harbor RAS mutations. FNA cytology of 
NIFTP lesions o

or RAS mutations. FNA cytology of 
NIFTP lesions often reveals cytology classified as atypia of undeter­
mined significance101 or suspicious for malignancy.102 Increasingly, 
certain microscopic features can be detected in FNA cytology 
specimens that allow preoperative concern for an NIFTP diagno­
sis to be conveyed.101–103
Low-risk indeterminate nodules (SFN/FN or AUS/FLUS) 
often harbor a relatively lower risk of malignancy, and if malignant, 
often harbor less aggressive variants of cancer.1

often harbor less aggressive variants of cancer.104 However, the 
interobserver reproducibility of these diagnoses is poor.105 His­
torically, surgical intervention was commonly recommended for 
nodules with SFN/FN or AUS/FLUS cytologic findings, though 
the majority of patients would prove to have benign disease. For 
such patients, surgery was unnecessary yet exposed them to sub­
stantial morbidity, time lost for recovery, and excess health care 
cost. To address these issues, the field has w

re 
cost. To address these issues, the field has witnessed a rapid expan­
sion in the discovery, development, and validation of thyroid-spe­
cific molecular diagnostic tests.
Historically, immunostaining for galectin 3 either alone 
or combined with TPO was suggested as a valuable adjunct to 
indeterminate nodules.106 A series of 17 single-gene oncogenic 
mutations or translocations in BRAF, RAS, RET/PTC, and PAX8/
PPARγ first demonstrated promise as an effective diagnostic 
marker for cytologic

e as an effective diagnostic 
marker for cytologically indeterminate thyroid nodules.107–111 
When detected, such mutations were initially felt to convey a 
very high positive predictive value and therefore hold promise 
as a “rule in” test. However, a blinded, multicenter prospective 
trial confirmed worse test performance for the full 17-gene muta­
tion panel than previously reported.110 These data raise questions 
about the overall transferability of initial data into clinical prac­
tice, esp

lity of initial data into clinical prac­
tice, especially when applied to nodules with AUS/FLUS cyto­
logic findings.110
Newer versions of this DNA-based mutation panel have been 
created that also identify genetic rearrangements and copy num­
ber derangements.112,113 With these updated versions, both test 
sensitivity and specificity have improved, though real-world use 
of this test has demonstrated lower than expected performance 
in both blinded and nonblinded analyses.114–116 Most recently,

ded and nonblinded analyses.114–116 Most recently, 
analytic performance of a third version of this test has been 
reported.117 In general, data increasingly suggest that such a test 
is not robust for ruling in malignancy when applied to low-risk, 
cytologically indeterminate nodules. However, the test’s negative 
predictive value appears high.107,111 The most complex output 
from such DNA-based mutation panels is the finding of a muta­
tion in the RAS gene. Many have associated RAS mutations w

the RAS gene. Many have associated RAS mutations with 
papillary carcinoma.118 However, RAS gene mutations have fre­
quently been identified in benign thyroid nodules that show no 
evidence of malignant transformation.119,120
A separate diagnostic molecular test has investigated the util­
ity of an RNA gene expression classifier (GEC) using microarray 
technology. Via initial analysis of the expression patterns from 162 
genes, a first-generation test was developed for use with nodules 
with SFN

 test was developed for use with nodules 
with SFN/FN and AUS/FLUS cytologic findings, with the goal of 
maximizing sensitivity and negative predictive value. A prospec­
tive, blinded, multicenter validation trial was performed enrolling 
nearly 4000 thyroid nodules with SFN/FN or AUS/FLUS cyto­
logic findings.121 A benign GEC test resulted in negative predictive 
values of 94% and 95%, respectively, which is similar to the find­
ings of a benign FNA cytologic result itself. The positive predict

 FNA cytologic result itself. The positive predictive 
values were 37% and 38%, respectively. Follow-up analyses of real-
world use confirmed variability from site to site, largely influenced 
by patient population and variation in the distribution of Bethesda 
cytologic classification.122 A recent study compared the long-term 
follow-up of cytologically indeterminate but GEC benign nodules 
with the follow-up of cytologically benign nodules using high-
resolution ultrasound. Over a mean of abou

g high-
resolution ultrasound. Over a mean of about 14 months (dura­
tion up to 40 months), no difference between these groups was 
identified, confirming that GEC benign nodules behave like true 
  Probability of Malignancy Based on 
Fine-Needle Aspiration Cytologic 
Categorization
Cytologic 
% of Results
% of Malignancy, for 
Each Category
Insufficient/non­
diagnostic
∼5–10
<5% if cystic nodule; 
10–20% in solid 
nodules
Benign
70 (53–90)
1–5%
Indeterminate
20 (5–23)
Suspicious for

53–90)
1–5%
20 (5–23)
Suspicious for 
papillary 
carcinoma
60–70%
a follicular 
neoplasm 
(SFN/FN)
15–30%
Atypia (follicular 
lesion) of 
undetermined 
significance 
(AUS/FLUS)
10–25%
Malignant
5 (1–10)
>97%
TABLE 14.4
444
benign lesions.123 More recently, some data have suggested that 
GEC performance may be lower when applied to nodules contain­
ing abundant Hürthle cells.124 A newer version of this RNA-based 
expression classifier has been 

of this RNA-based 
expression classifier has been developed, called the gene sequencing 
classifier (GSC), demonstrating improved performance.
Increasingly, molecular testing of cytologically indeterminate 
thyroid nodules is endorsed because of its ability to substantially 
improve preoperative cancer risk assessment and modify clinical 
care. Specifically, most molecular tests are demonstrating high 
sensitivity and thus high negative predicative value. Until recently, 
use of the GSC is favor

ve value. Until recently, 
use of the GSC is favored because of the strength of its clinical val­
idation. Furthermore, initial cost-effectiveness analyses have dem­
onstrated, in the United States, a cost savings via this approach.125 
The gene mutation panels may prove to have superior specificity 
and positive predictive values, though the utility of this metric is 
of limited use as the decision for hemithyroidectomy versus near-
total thyroidectomy takes into account variables such as patie

dectomy takes into account variables such as patient 
demographic and preference, ultrasound findings, and molecular 
analysis. To date, there has been no head-to-head prospective or 
blinded investigations comparing the various molecular tests.
MicroRNA testing has been proposed as a separate molecu­
lar test for use in cytologically indeterminate nodules. Initial data 
suggest potential for this approach, though strong validation stud­
ies performed in the United States are still pending.126,1

ormed in the United States are still pending.126,127 Thus 
further prospective validations of these tests are required. The use 
of large-needle core biopsy in addition to standard FNAB may 
improve diagnostic accuracy in difficult FNA cases, but the tech­
nique is associated with increased morbidity.128 Particularly for 
cystic thyroid nodules, sampling from the margin of the nodule 
under ultrasound guidance, rather than from the cystic fluid and 
debris in the center, increases accuracy.
Init

nd 
Initial nondiagnostic cytologic testing should prompt repeat 
ultrasound-guided FNA. If available, onsite cytologic evaluation 
will assist in ensuring an adequate specimen for evaluation.98,129 
When the sonographic pattern of a nondiagnostic nodule is con­
cerning, close observation or consideration for surgical excision 
should be given.130 Although most nondiagnostic aspirates are 
due to cystic content, solid nodules with persistent nondiagnosti

ontent, solid nodules with persistent nondiagnostic 
aspirates are associated with higher malignant risk. Repeat FNA of 
an initially nondiagnostic thyroid nodule yields adequate results 
in 60% to 80% of specimens.68,131 
Management Options for Patients With 
Nontoxic Diffuse Goiter and Nodular Thyroid 
Disease
Patients with small, asymptomatic, nontoxic goiters can be moni­
tored by clinical examination and evaluated periodically with ultra­
sound measurements. In fact, goiter growth can be va

und measurements. In fact, goiter growth can be variable, and 
some patients have stable goiters for many years. For more than a 
century, thyroid hormone supplementation was used to reduce the 
size of nontoxic goiters. A 1953 report of Greer and Astwood, in 
which two-thirds of patients’ goiters regressed with thyroid therapy, 
led to widespread acceptance of suppressive therapy132 despite some 
doubts about its value.133 An overview of studies performed from 
1960 to 1992 suggested that 60% o

 performed from 
1960 to 1992 suggested that 60% or more of sporadic nontoxic 
goiters responded to suppressive therapy. In a prospective placebo-
controlled, double-blind randomized clinical trial, 58% of the 
thyroxine-treated group had a significant response at 9 months, as 
measured by ultrasonography, in contrast with 5% after placebo.134
Patients with nodular thyroid disease appear to be less respon­
sive to suppressive therapy than those with diffuse nontoxic goi­
ters. A meta-analysis fa

ith diffuse nontoxic goi­
ters. A meta-analysis failed to demonstrate a significant benefit 
of thyroxine therapy, which was found to carry a relative risk 
of nodule shrinkage of only 1.9 (95% confidence interval [CI], 
0.95–3.81).135 Statistical significance emerged from a multi­
center, randomized, double-blind, placebo-controlled trial: After 
18 months of follow-up the nodule shrinkage was significantly 
greater in the levothyroxine group than in the placebo group (p 
= 0.01), as well as th

an in the placebo group (p 
= 0.01), as well as the proportion of responders (p = 0.04).136 It 
is likely that a subset of patients respond to thyroxine suppres­
sive therapy, particularly younger patients with small or recently 
diagnosed nodules.135 However, thyroid nodules rapidly return to 
the pretreatment size after discontinuation of therapy. Therefore 
maintenance of the size reduction may require continuous treat­
ment, which carries long-term risk.
A major concern in relation to long-t

g-term risk.
A major concern in relation to long-term thyroxine suppres­
sion therapy is the possibility of detrimental effects on the skel­
eton and heart. TSH suppression therapy is generally associated 
with variable degrees of bone loss, particularly in postmenopausal 
women.137 Furthermore, there is evidence that levothyroxine sup­
pressive therapy is detrimental to the heart, especially in older 
patients.137
Surgery for nontoxic goiter is physiologically unsound 
because it further restri

physiologically unsound 
because it further restricts the ability of the thyroid to meet hor­
mone requirements. Nevertheless, surgery may become neces­
sary because of persistence of obstructive manifestations despite 
a trial of levothyroxine. Surgery should consist of a near-total 
or total thyroidectomy, but recurrence is seen in about 10% 
to 20% within 10 years.138 Surgical complications have been 
reported in 7% to 10% of cases and are more common with 
large goiters and with reoperation.

e common with 
large goiters and with reoperation. Prophylactic treatment with 
levothyroxine after goiter resection probably does not prevent 
goiter recurrence.139
Traditionally, the role of 131I therapy for nontoxic goiter was 
to reduce the size of a massive goiter in elderly patients who were 
poor candidates for surgery or to treat goiter that recurs after 
resection. However, several studies have demonstrated that pri­
mary treatment of nontoxic goiter with 131I is followed by a reduc­
ti

ntoxic goiter with 131I is followed by a reduc­
tion in thyroid volume. In one study, thyroid volume (assessed by 
ultrasonography) was reduced by 40% after 1 year and 55% after 
2 years with no further reduction thereafter, and 60% of the total 
reduction occurred within the first 3 months.140
Considering its effectiveness in reducing the size of the thy­
roid gland, 131I therapy has also been used for the treatment of 
nonautonomous thyroid nodules where significant shrinkage has 
been observe

ules where significant shrinkage has 
been observed, ranging from 31% to 60%141 (Fig. 14.5). It was 
formerly argued that treatment of large goiters or goiters with sub­
sternal extension with 131I should be avoided because of the risks 
of acute swelling of the gland and consequent tracheal compres­
sion. Ultrasonographic studies of thyroid volume after 131I have 
failed to demonstrate significant early volume increase. Moreover, 
decreased tracheal deviation and increased tracheal lumen size 

heal deviation and increased tracheal lumen size 
were demonstrable by MRI in patients who had compression by 
nontoxic goiters with substernal extension.140
Therefore it appears that 131I treatment of nontoxic diffuse goi­
ter or multinodular thyroid disease is effective and safe. Hypothy­
roidism has been reported in 20% to 40%; transient thyrotoxicosis 
and mild pain can occur.140 Regular follow-up, preferably by a sys­
tematic annual recall scheme, is necessary. The activities used are in 
t

cheme, is necessary. The activities used are in 
the range of those used for 131I treatment of hyperthyroidism, and 
thus radiation doses are comparable, and long-term thyroid and 
445
nonthyroidal cancer risk after 131I treatment for hyperthyroidism 
is reassuring. Stimulation with low doses of recombinant human 
TSH (rhTSH) (0.01–0.03 mg) increases thyroid 131I uptake and 
therefore may allow the administr

131I uptake and 
therefore may allow the administration of a lower dosage of 131I, 
but rhTSH also increases thyroid hormone production, and over­
production of thyroid hormones should be excluded before its 
use.142 Long-term randomized studies comparing the effects, side 
effects, and costs and benefits of surgery and 131I treatment need 
to be performed.
Percutaneous ethanol injection (PEI) should be used only for 
recurrent symptomatic cystic nodules.143 Laser ablation, cryoabla­
tion, and r

 nodules.143 Laser ablation, cryoabla­
tion, and radiofrequency ablation are still generally experimen­
tal procedures and can be proposed, in experienced centers, for 
selected patients with symptomatic nodular goiters when surgery 
is not possible.144 
Malignant Thyroid Disorders
Thyroid tumors are the most common endocrine neoplasms. The 
management of a patient with typical well-differentiated, follicu­
lar cell–derived thyroid cancer is effective and usually consists of 
surgical resection,

ctive and usually consists of 
surgical resection, followed by medical therapy and regular sur­
veillance.1,14,22,145 Some degree of consensus has been achieved 
regarding the initial management of differentiated thyroid cancer, 
but many important clinical and biologic questions remain unan­
swered. In the following discussion, a widely used scheme for clas­
sifying and staging tumors of the thyroid gland is presented. The 
distinguishing features of the principal types of benign and malig­
nan

es of the principal types of benign and malig­
nant thyroid neoplasms and the controversies in the management 
of differentiated thyroid carcinoma, based on recent consensus 
and guidelines, are also reviewed.1,14,22
Classification and Staging of Thyroid Cancer
Two monographs have had a major impact on the histologic clas­
sification of thyroid tumors. One is from the World Health Orga­
nization (WHO); the other was developed by the Armed Forces 
Institute of Pathology (AFIP).146 The WHO classif

Institute of Pathology (AFIP).146 The WHO classification was 
updated in 2017147 and is described in Table 14.5.
Lesions of follicular cell origin constitute more than 95% of the 
cases, and the remainder are largely made up of tumors exhibiting 
C-cell differentiation. Mixed medullary and follicular carcinomas, 
made up of cells with both C-cell and follicular differentiation, 
are rare and of uncertain histogenesis. Nonepithelial thyroid 
tumors mainly include malignant lymphomas, which may i

rs mainly include malignant lymphomas, which may involve 
the thyroid gland as the only manifestation of the disease or as 
part of a systemic disease. True sarcomas and malignant heman­
gioendotheliomas are exceptionally rare. Blood-borne metastases 
from separate malignancies to the thyroid are not uncommon at 
autopsy in patients with widespread malignancy, but rarely cause 
clinically detectable thyroid enlargement.
In addition to the histologic classification of thyroid tumors 
developed by

gic classification of thyroid tumors 
developed by the WHO and AFIP groups, the Union for Interna­
tional Cancer Control (UICC) and the American Joint Commit­
tee on Cancer (AJCC) have agreed on a staging system for thyroid 
cancer.148–151 As stated by the AJCC, “The principal purpose 
served by international agreement on the classification of cancer 
cases by extent of disease was to provide a method of conveying 
clinical experience to others without ambiguity.”149
The main goal of the AJCC-IU

ithout ambiguity.”149
The main goal of the AJCC-IUCC staging system is to pre­
dict disease-specific survival. It is based on the TNM (tumor-
node-metastasis) classification system, which focuses on (1) the 
extent of the primary tumor (T), (2) the absence or presence of 
regional lymph node metastases (N), and (3) the absence or pres­
ence of distant metastases (M). The classification may be either 
clinical (cTNM), based on evidence (often from FNA cytol­
ogy) acquired before treatment, or pat

 FNA cytol­
ogy) acquired before treatment, or pathologic (pTNM) when 
intraoperative and surgical pathologic data are available. The 
pTNM classification is preferable as this allows a precise size to 
be assigned to the primary tumor, the type of cancer to be clari­
fied histologically, and the presence of extrathyroid invasion to 
be demonstrated unequivocally. Whereas head and neck cancers 
are most often staged entirely on the basis of anatomic extent, 
for well-differentiated thyroid cance

mic extent, 
for well-differentiated thyroid cancer staging, both the histo­
logic diagnosis and the age of the patient are included because 
of their importance in predicting the behavior and prognosis 
ahead. It is noteworthy that in children the risk of recurrence is 
Months after treatment
Change in thyroid volume (mL)
120
100
80
60
40
20
0
60
48
36
24
12
6
3
1
0
• Fig. 14.5  Median changes in thyroid volume alterations after 131I treat­
ment in 39 patients with nontoxic multinodular thyroid

ges introduced since the 2010 edition. The main changes 
are (1) an increase in the age threshold (>55 years versus >45 
years) for defining cases at high risk for thyroid cancer–related 
death, and (2) attenuation of the previous unfavorable prognos­
tic significance attributed to small cervical lymph node metasta­
sis and microscopic extrathyroidal extension. The age criterion 
for defining high-risk disease has been raised from 45 years to 55 
years. This change increases the proportion of re

years. This change increases the proportion of relatively young 
patients whose mortality risk can be defined solely on the basis 
of the absence or presence of distant metastases (stages I and II, 
respectively). As for microscopic extension of the tumor to the 
perithyroidal soft tissues, this finding is no longer considered an 
absolute indication for assigning the tumor to the T3 category. 
The ability to recognize this minimal extrathyroidal invasion can 
vary widely, even between skilled 

l invasion can 
vary widely, even between skilled pathologists. The T3 category 
now includes tumors whose largest diameter exceeds 4 cm and 
those that invade the perithyroidal muscles (sternohyoid, ster­
nothyroid, thyroidhyoid, omohyoid). Upper mediastinal lymph 
node metastases (level VII) are now considered features of N1a 
disease, which was formerly used solely in the presence of cen­
tral neck node lesions. The expanded definition reflects the well-
known difficulties in distinguishing l

s the well-
known difficulties in distinguishing level VI and VII nodes from 
an anatomic point of view. Stage I thyroid cancer now refers 
exclusively to T1 or T2 tumors with no lymph node metastases, 
whereas stage II includes T3, N0, M0 tumors and any T1-T3 
tumor with metastatic lymph node disease (either N1a or N1b).
  The Tumor-Node-Metastasis (TNM) Scoring System
DEFINITION OF TNM
Category
2010 Version (AJCC 7th Edition)
2017 Version (AJCC 8th Edition)
Primary Tumor (T)
T0
No evidence of

C 8th Edition)
T0
No evidence of primary tumor
T1
Tumor ≤2 cm limited to the thyroid
T1a: ≤1 cm
T1a: ≤1 cm
T1b: >1 cm to 2 cm
T2
Tumor >2 to ≤4 cm limited to the thyroid
T3
Tumor >4 cm limited to the thyroid or any tumor 
with minimal extrathyroid extension (e.g., exten­
sion to sternothyroid muscle or perithyroid soft 
tissues)
Tumor >4 cm limited to the 

thyroid soft 
tissues)
Tumor >4 cm limited to the thyroid or gross extra­
thyroidal extension invading only strap muscles 
(sternohyoid, sternothyroid, thyroidhyoid, 
omohyoid) from a tumor of any size
T4
T4a
Gross extrathyroidal extension invading subcutane­
ous soft tissues, larynx, trachea, esophagus, or 
recurrent laryngeal nerve from a tumor of any 
size or intrathyroidal anaplastic thyroid cancer 
of any size
Gross extrathyroidal ex

hyroid cancer 
of any size
size
T4b
Gross extrathyroidal extension invading prever­
tebral fascia or encasing the carotid artery or 
mediastinal vessels from a tumor of any size 
or anaplastic thyroid cancer of any size with 
extrathyroidal extension
medi

ral fascia or encasing the carotid artery or 
mediastinal vessels from a tumor of any size
Regional Lymph Node (N)
N0
No regional lymph node metastasis
No evidence of locoregional lymph node metastasis
N0a: one or more cytologically or histologically 
confirmed benign lymph nodes
N0b: no radiologic or clinical evidence of locore­
gional lymph node metastasis
N1
Regional lymph node metastasis
N1a
Metastases in pretracheal and paratracheal lymph 
nodes, including pre

cheal and paratracheal lymph 
nodes, including prelaryngeal and delphian 
lymph nodes, unilateral or bilateral
Metastasis in pretracheal, paratracheal, prelar­
yngeal/delphian, or upper mediastinal lymph 
nodes, unilateral or bilateral
N1b
Metastases in lateral neck lymph nodes or upper 
mediastinal lymph nodes, unilateral or bilateral
Metastases in lateral neck lymph nodes, unilateral 
or bilateral disease
TABLE 14.6
447
Chapter 14  Nontoxic Diffuse Goiter, Nodular Thyroid Disorders, and Thyroi

fuse Goiter, Nodular Thyroid Disorders, and Thyroid Malignancies
To date, prognostic differences and, especially, the differing risk 
of cause-specific mortality between central lymph node metastases 
(N1a) and other regional metastases (N1b) have yet to be widely 
validated. Indeed, the risk of persistent/recurrent disease appears 
to be more closely related to the number and size of involved 
lymph nodes as well as the number of lymph nodes with extracap­
sular extension. These characteristics

h extracap­
sular extension. These characteristics, however, are not taken into 
account in the TNM AJCC staging system.153,154 Stages III and 
IVa include T4a and T4b tumors, respectively, regardless of their 
lymph node status. Stage IVb includes any tumor with distant 
Category
Distant Metastases (M)
Category
2010 Version
2017 Version
M0
No distant metastasis
M1
Distant metastasis
Distant m

distant metastasis
M1
TNM STAGING FOR PAPILLARY, FOLLICULAR, AND POORLY DIFFERENTIATED THYROID CANCER
Age Cutoff
Age <45 Years
Age <55 Years
Stage I
Any T, any N, M0
Stage II
Any T, any N, M1
Stage III
None
Stage IV
None
Age Cutoff
Age ≥45 Years
Age ≥55 Years
Stage I
T1, N0, M0
T1-T2, N0, M0
Stage II
T2, N0, M0
T1-T2, N1a-N1b, M0 or
T3, any N, M0
Stage III
T3, N0, M0 or any T1-3, N1a, M0
T4a, any N, M0
Stage IV
Stage IVA
T1-3, N1b, M0 or T4a

 any N, M0
Stage IV
Stage IVA
T1-3, N1b, M0 or T4a, any N, M0
T4b, any N, M0
Stage IVB
Stage IVC
-
TNM STAGING FOR MEDULLARY THYROID CANCER
Category
Stage I
T1, N0, M0
T1, N0, M0
Stage II
T2-T3, N0, M0
Stage III
T1-3, N1a, M0
Stage IVA
T1-3, N1b, M0 or T4b, any N, M0
Stage IVB
Stage IVC
TNM STAGING F

VC
TNM STAGING FOR ANAPLASTIC THYROID CANCER
Stage IVA
T4a, N0, M0
T1-T3a, N0, M0
Stage IVB
T1-T3a, N1, M0 or T3b-T4, any N, M0
Stage IVC
AJCC, American Joint Committee on Cancer.
   
  The Tumor-Node-Metastasis (TNM) Scoring System—cont’d
TABLE 14.6
448
metastases, and stage IVc has been eliminated entirely in the new 
eighth edition system.
For MTC, the scheme is similar, in that T1 N0 M0

.
For MTC, the scheme is similar, in that T1 N0 M0 carcinoma 
is stage I, but T2-T3 N0 M0 is stage II and a T1-T3 N1a M0 
tumor is stage III, underlying the prognostic impact of lymph 
node involvement. There is no age distinction for MTC, although 
age is a significant independent prognostic indicator in most mul­
tivariate analyses.155,156
All anaplastic carcinomas were considered T4 tumors in the 
1992, 2002, and 2010 editions of the AJCC/UICC classification. 
With the eighth edition (2017), 

 classification. 
With the eighth edition (2017), the T categories for anaplastic 
carcinomas are the same as those used for differentiated carcino­
mas. All anaplastic carcinomas are considered stage IV tumors, 
with stage IVA referring to intrathyroidal tumors, whereas stage 
IVB tumors are cases with gross extrathyroidal extension or lymph 
node metastases. Stage IVC is assigned to anaplastic malignancies 
with distant metastases.
Although the AJCC staging system has been designed to pre­
dic

 AJCC staging system has been designed to pre­
dict disease mortality risk, it has also been used in practice to 
predict the risk of persistent/recurrent disease. Other systems, 
designed specifically to predict postoperative tumor recurrence, 
are available and are discussed later in this chapter. 
Follicular Adenomas and Borderline Thyroid 
Lesions With Malignant Potential
A follicular adenoma is a benign, encapsulated tumor with evi­
dence of follicular cell differentiation.146 It is the mos

 follicular cell differentiation.146 It is the most common 
thyroid neoplasm and may be found in 4% to 20% of glands 
examined at autopsy.157 The tumor has a well-defined fibrous 
capsule that is grossly and microscopically complete. There is a 
sharp demarcation and distinct structural difference from the sur­
rounding parenchyma. These adenomas vary in size, but most 
have a diameter of 1 to 3 cm at the time of excision. Degenera­
tive changes, including necrosis, hemorrhage, edema, fibrosis, 

 including necrosis, hemorrhage, edema, fibrosis, or 
calcification, are common features, particularly in larger tumors.
Follicular adenomas can be classified into subtypes according 
to the size or presence of follicles and degree of cellularity. Each 
adenoma tends to have a consistent architectural pattern. Micro­
follicular, normofollicular, and macrofollicular adenomas owe 
their names to the size of their follicles compared with follicles 
in the neighboring, nonneoplastic areas of the gla

in the neighboring, nonneoplastic areas of the gland. Trabecular 
adenomas are cellular and consist of columns of cells arranged in 
compact cords. They show little follicle formation and rarely con­
tain colloid. Hyalinizing trabecular tumor, considered a variant 
of adenoma, has unusually elongated cells and prominent hyaline 
changes in the extracellular space.158 The histologic differences 
among these subtypes are striking but of no clinical importance. 
The only practical value of the clas

 importance. 
The only practical value of the classification is that the more cellu­
lar a follicular nodule is, the more one should search for evidence 
of malignancy in the form of invasion of blood vessels and capsule, 
either singly or in combination.146
Atypical adenomas are hypercellular or heterogeneous, or both, 
with gross and histologic appearances that suggest the possibility 
of malignancy but without invasion. Classification of these tumors 
is difficult and poorly reproducible amon

 tumors 
is difficult and poorly reproducible among pathologists. They 
account for fewer than 3% of all follicular adenomas. Follow-up 
indicates that this lesion behaves in a benign fashion. However, 
the fact that the tumor does not recur or produce metastases after 
removal does not prove that it is actually benign. Removal may 
have interrupted a natural history in such nodules that would have 
culminated in invasion and metastases. For this reason, they are 
classified as tumors of uncerta

 reason, they are 
classified as tumors of uncertain malignancy. Increasingly, molec­
ular analysis of such lesions helps to better define their prognosis 
and associated risk.
The most important cytologic variant is the oxyphilic or onco­
cytic (Hürthle cell) adenoma, which is composed predominantly 
(at least 75%) or entirely of large cells with granular, eosinophilic 
cytoplasm. Ultrastructurally, the cells are rich in mitochondria 
and may exhibit nuclear pleomorphism with distinct nucleoli.

hibit nuclear pleomorphism with distinct nucleoli. 
Although all such neoplasms are thought by some to be poten­
tially malignant, the biologic behavior and clinical course of onco­
cytic tumors correlate closely with the histologic appearance and 
the size of the initial lesion. The absence of invasion predicts a 
benign outcome, but larger tumors may rarely be associated with 
later recurrence or metastases, even in the absence of obvious 
microscopic evidence of invasion; fortunately, such an

oscopic evidence of invasion; fortunately, such an occurrence 
is rare, and generally a diagnosis of benign Hürthle cell adenoma 
is reliable.159
Some follicular adenomas may contain pseudopapillary struc­
tures that can be confused with the papillae of papillary carcinoma. 
These structures are probably an expression of localized hyperac­
tivity and are most common in adenomas that show autonomous 
function.
In the majority of hyperfunctioning follicular adenomas, acti­
vating point mutations h

ollicular adenomas, acti­
vating point mutations have been identified in the TSHR or in the 
α-subunit of the stimulatory guanyl nucleotide protein (Gαs) (Fig. 
14.6).60,160 Such mutations result in a constitutive hyperstimula­
tion of the cells. Genetic abnormalities found in hypofunctioning 
adenomas are detailed later.
In 2017, the WHO introduced a new entity, included in 
the group of thyroid tumors of follicular origin with borderline 
histologic features. Such lesions, as mentioned previou

logic features. Such lesions, as mentioned previously, are 
NIFTP.100,147,161 These neoplasms are encapsulated follicular vari­
ants of papillary thyroid carcinoma (FVPTC) without evidence of 
either capsular or vascular invasion. Proper pathologic processing 
is essential for confident exclusion of capsular or vascular spread. 
NIFTPs are thought to represent up to 18.6% of tumors previously 
classified as low-risk PTCs. Adverse events, such as cancer-related 
death, distant or regional metasta

cancer-related 
death, distant or regional metastases, and structural or biochemical 
recurrence do not occur in patients with NIFTPs that are properly 
diagnosed. Because of this, the word cancer has been eliminated 
from the definition of these tumors to underscore their outstand­
ing prognosis, thereby discouraging overly aggressive treatment 
and follow-up. This borderline thyroid entity was first proposed 
by an international panel of experts and subsequently endorsed 
by the ATA. However, 

s and subsequently endorsed 
by the ATA. However, the presumably excellent outcomes associ­
ated with NIFTPs have yet to be independently confirmed with 
long-term prospective studies. Importantly, the current body of 
evidence supporting our approach to NIFTPs comes from retro­
spective studies and is thus of only moderate quality. 
Papillary Thyroid Carcinoma
PTC is defined as “a malignant epithelial tumor showing evidence 
of follicular cell differentiation and a set of distinctive nuclear fe

ifferentiation and a set of distinctive nuclear fea­
ture. Papillae, invasion or cytological features of papillary thyroid 
carcinoma are required” (WHO 2017).146,162 The most common 
thyroid malignancy, PTC constitutes 50% to 90% of differenti­
ated FCTCs worldwide.72,163 The 2017 WHO classification rec­
ognizes 15 PTC subtypes (or variants), but the vast majority of 
PTCs belong to one of the following five subtypes (or variants): 
classic variant, follicular variant, diffuse sclerosing varian

ant, follicular variant, diffuse sclerosing variant, encap­
sulated variant, and tall cell variant.147
449
TA
BRAF V600E-like
RAS-like
TSH-R
Gas
Well
differentiated
Poorly
Undifferentiated
ATC, all histotypes
RAS
BRAF K601E
RAS
BRAF K601E
BRAF V600E
RET/PTC and
other gene fusions
Some TERT
Quiet genome
TP53
TERT promoter
Accumulation of other mutations
TP53
Accumulation of other 

utations
TP53
Aneuploidy
RAS
CNA
RAS
mtDNA
CNA
RAS
mtDNA
CNA
RAS
PAX8-PPARG
CNA
Increased
mutational burden
Thyroid
follicular cell
HCA
FTA
NIFTP
PTC, CT
PTC, TCV
PTC, FV
FTC
HCC
PDCA, BRAF V600E-like
PDCA, RAS-like
BRAF V600E
Some TERT
• Fig. 14.6  Model of thyroid carcinoma (TC) initiation and progression with primary genetic alterations. 
Benign thyroid tumors (FTA and HCA) and differentiated TCs (PTC, FTC, and HCC) develop from folli

ntiated TCs (PTC, FTC, and HCC) develop from follicular 
cells and follow two distinct pathways, BRAF V600E-like and RAS-like, depending on the specific driver 
mutations present. Some differentiated TCs accumulate additional mutations and progress to more clini­
cally aggressive and less differentiated types (PDCA and ATC). ATC represents the most aggressive form 
of TC with a high mutational burden, genomic instability, aneuploidy, and high rates of TP53 and TERT 
promoter mutations. ATC, anap

es of TP53 and TERT 
promoter mutations. ATC, anaplastic thyroid (undifferentiated) carcinoma; CNA, copy number alterations; 
CT, classical type; FTA, follicular thyroid adenoma; FTC, follicular thyroid carcinoma; FV, follicular vari­
ant; HCA, Hürthle cell/oncocytic adenoma; HCC, Hürthle cell/oncocytic carcinoma; mtDNA, mitochondrial 
deoxyribonucleic acid; NIFTP, noninvasive follicular neoplasm with papillary-like nuclear features; PDCA, 
poorly differentiated carcinoma; PTC, papillary thyroid

y differentiated carcinoma; PTC, papillary thyroid cancer; TA, toxic adenoma; TC, thyroid carcinoma; 
TCV, tall cell variant; TSH-R, thyrotropin receptor. (Modified from Giordano TJ. Genomic hallmarks of thy­
roid neoplasia. Annu Rev Pathol. 2018;13:141–162.)
450
Papillary thyroid microcarcinomas (PTMs), or pT1a lesions 
are defined by WHO as PTCs whose largest diameter measures 
1 cm or less.164,165 In the United States, the reported incidence 
of clinically diagnosed PTM

he reported incidence 
of clinically diagnosed PTMs (approximately 1 per 100,000 per­
sons) is considerably lower than that reported for larger PTCs 
(approximately 5 per 100,000). It is also lower than the reported 
incidence of PTMs found at autopsy in various parts of the world 
(4–36%).72,146 The increasing detection of small PTCs through­
out the industrialized world can be attributed in large part to 
screening programs.72–74,166 The incidence of larger tumors has 
remained essentially sta

nce of larger tumors has 
remained essentially stable since the 1980s. Therefore most thy­
roid cancers being diagnosed today are small tumors with limited 
extension and excellent prognoses,167 and protocols for their ini­
tial treatment and follow-up must therefore be revised to avoid 
overzealous management.
Classic PTCs appear as firm, unencapsulated or partially 
encapsulated tumors. PTCs may be partly necrotic, and some 
are cystic. The presence of necrosis and a high mitotic rate are 
ass

sence of necrosis and a high mitotic rate are 
associated with aggressive behavior similar to that of poorly dif­
ferentiated thyroid cancer (PDTC).39,168 Typically PTC shows a 
predominance of papillary structures, consisting of a fibrovascu­
lar core lined by a single layer of epithelial cells, but the papillae 
are usually admixed with neoplastic follicles having characteristic 
nuclear features. The nuclei of PTC cells have a distinctive appear­
ance, which has a diagnostic significance comp

ar­
ance, which has a diagnostic significance comparable to that of 
the papillae. Indeed, the preoperative diagnosis of PTC can often 
be made on the basis of the characteristic nuclear changes seen 
in FNA material: Nuclei are larger than in normal follicular cells 
and overlap; they may be fissured like coffee beans; chromatin 
is hypodense (ground-glass nuclei); limits are irregular; and they 
frequently contain an inclusion corresponding to a cytoplasmic 
invagination. Psammoma bodies are o

a cytoplasmic 
invagination. Psammoma bodies are often present in the core of 
papillae or in the tumor stroma; they are microscopic structures 
of calcified layers.
The term encapsulated variant of PTC is used to describe a clas­
sic PTC surrounded by a fibrous capsule, regardless of whether the 
capsule shows signs of tumor infiltration. The tumor is designated 
a follicular variant of PTC when the lining cells of the neoplas­
tic follicles have the same nuclear features as seen in typical PTC

e the same nuclear features as seen in typical PTC 
and the follicular predominance over the papillae is complete. 
The encapsulated FVPTC is associated with a favorable outcome, 
unless extensive vascular invasion is present.146 The diffuse scleros­
ing variant of PTC is characterized by diffuse involvement of one or 
both thyroid lobes, widespread lymphatic permeation, prominent 
fibrosis, and lymphoid infiltration. The tall cell variant of PTC is 
characterized by well-formed papillae that ar

 is 
characterized by well-formed papillae that are covered by cells that 
are twice to three times tall as they are wide. Other rare variants 
of PTC include the columnar cell variant, which differs from other 
forms of PTC because of the presence of prominent nuclear strati­
fication of elongated cells. The hobnail variant of PTC is a recently 
recognized variant with poor prognostic findings.169 The tall cell 
and columnar cell variants are more aggressive, but controversy 
exists regarding o

re aggressive, but controversy 
exists regarding outcome for the diffuse sclerosing variant.
In children, PTCs represent the large majority of newly diag­
nosed cases, and histologic subtypes include the classic PTC, the 
presence of the solid/trabecular variant features (of unknown prog­
nostic significance), the follicular variant, and the diffuse scleros­
ing variant. Tumor extension is usually substantial at diagnosis: 
tumors are large, multifocal, unencapsulated, and invasive. Exten­
sion 

ifocal, unencapsulated, and invasive. Exten­
sion beyond the thyroid capsule, lymph node metastases, and lung 
metastases are frequently observed.152,170,171
Molecular Pathogenesis of Papillary Thyroid Carcinoma
Nearly all PTCs arise as the result of alterations in genes encoding 
components of the mitogen-activated protein kinase (MAPK) and 
PI3K/PTEN/AKT/mTOR signaling pathways. The gene most 
frequently mutated in PTCs is the proto-oncogene BRAF, which 
encodes a serine-threonine protein kina

AF, which 
encodes a serine-threonine protein kinase that plays a central role 
in regulating signaling through the MAPK pathway. In most 
cases, BRAF is activated as a result of a point mutation that causes 
a glutamate-for-valine substitution at residue 600 (p.V600E). This 
alteration is found in up to 40% to 60% of all PTCs,118,172,173 
primarily the classic and tall cell variants.118 The BRAFV600E 
mutation occurs early in tumorigenesis,174 inducing loss of dif­
ferentiation175 and promoting

nducing loss of dif­
ferentiation175 and promoting tumor invasion and progression.176
The second most prevalent mutations found in PTCs involve 
the Ras family of genes, which encode proteins that act as sig­
nal transduction switches to regulate diverse cytoplasmic signal­
ing pathways.177 Although RAS is a classic activator of both the 
MAPK and PI3K-AKT pathways, the RAS mutations associated 
with thyroid tumorigenesis seem to preferentially activate the 
PI3K-AKT pathway. Missense mutations 

ctivate the 
PI3K-AKT pathway. Missense mutations at codons 12, 13, and 
61 result in constitutive RAS signaling, and they are found in 10% 
to 30% of PTCs118,178 (FVPTCs and NIFTPs in particular).100,179
Of all PTCs, 5% to 10% are characterized by genetic recom­
bination of the RET proto-oncogene, which encodes a trans­
membrane receptor with a tyrosine kinase domain.118,180 These 
alterations are characteristic of PTCs linked to radiation exposure 
(environmental or therapeutic).180 There are 

ure 
(environmental or therapeutic).180 There are at least 10 different 
types of RET/PTC, all resulting from the fusion of the tyrosine 
kinase domain of RET to the 5′ portion of different genes. RET/
PTC1 and RET/PTC3 are the most common types, followed by 
RET/PTC2. RET/PTC1 is formed by an intrachromosomal rear­
rangement fusing the RET tyrosine kinase domain to a gene 
designated H4 (CCDC6). RET/PTC2 is formed by an interchro­
mosomal rearrangement fusing the RET tyrosine kinase domain 
to 

ngement fusing the RET tyrosine kinase domain 
to a gene located on chromosome 17 encoding the RIα regula­
tory subunit of protein kinase A. RET/PTC3 is formed by an 
intrachromosomal rearrangement fusing the RET tyrosine kinase 
domain to a gene designated RFG (NCOA4). RET/PTC3 was 
more frequently found in aggressive tumors that occurred early 
after the Chernobyl disaster, and RET/PTC1 was found in classic 
variant tumors that are less aggressive and occurred later.
In addition to RET/PTC rea

ive and occurred later.
In addition to RET/PTC rearrangements, PTCs have been 
found to harbor fusion genes involving NRTK1,181 NTRK3,182 
BRAF,183 ALK,184 FGFR2, THADA, MET, LTK,118 and ROS1.185 
PTCs with oncogenic gene fusions are closely associated with 
radiation exposure, and these tumors usually have distinctive 
clinicopathologic features.182 Fusions (especially those involv­
ing NRTK1/3) are particularly common in PTCs that develop 
in pediatric patients, where they are associated with 

ediatric patients, where they are associated with more aggres­
sive disease.186,187 On the whole, adult PTCs with gene fusions 
are usually associated with an intermediate risk of recurrence.1 
Fusions involving the PAX8 and PPARG genes, which are fre­
quently found in FTCs, have also been reported in a small subset 
of PTCs, particularly FVPTCs.118,188
Telomerase reverse transcriptase (TERT) promoter mutations 
are identified in 8% to 27% of PTCs, particularly those that also 
harbor mutations 

s, particularly those that also 
harbor mutations affecting the MAPK pathways (e.g., BRAFV600E, 
activating RAS mutations).189–192 They upregulate the expres­
sion of TERT, thereby reactivating telomerase to promote cellular 
immortality. The C228T substitution is the most common (7%), 
and the C228A and C250T substitutions are less common (0.3% 
451
and 2.1%, respectively).118 TERT mutations are associated 

, respectively).118 TERT mutations are associated with 
aggressive clinicopathologic features and a high risk of recurrence, 
particularly when coexisting with BRAF, suggesting a synergistic 
interaction between BRAF and TERT.193
In 2014 there was a major advance in defining the genomic 
landscape of PTCs: the comprehensive multiplatform analysis of 
nearly 500 adult tumors carried out as part of The Cancer Genome 
Atlas (TCGA).118 Characterization of these tumors in terms of 
genomic variants, 

on of these tumors in terms of 
genomic variants, gene expression, microRNA (miR) expression, 
alterations in methylation, and proteomic profiles shed new light 
on the genomic hallmarks of PTC, which have greatly enhanced 
our understanding of its pathogenesis. The overarching conclusion 
is that PTCs are relatively simple cancers with fairly low muta­
tional burdens (0.41 nonsynonymous mutations per megabase on 
the average) and with a few copy number variations.
The TCGA study has also identi

 number variations.
The TCGA study has also identified EIF1AX as a novel driver 
oncogene, whose mutations are nearly mutually exclusive with 
other MAPK pathway mutations and occur in 1% of PTCs, mostly 
in FVPTC.118 An increased prevalence of EIF1AX mutations has 
also been reported in PDTC and ATC (∼10% in both), particu­
larly in those that also harbor RAS mutations.194 This intriguing 
association between RAS and EIF1AX mutations suggests that the 
ribosomal protein might also contribute si

at the 
ribosomal protein might also contribute significantly to tumor 
progression, making it an attractive potential target for further 
investigation.
Mutations in the DNA repair genes CHEK2 and PPMID have 
also been found in PTCs, each affecting approximately 1% of all 
tumors. These mutations occurred concomitantly with MAPK-
pathway driver mutations, suggesting that they are late genetic 
events in PTC tumorigenesis and that the development of particu­
larly aggressive phenotypes may be tr

 of particu­
larly aggressive phenotypes may be triggered by an acquired defect 
in DNA repair.118
TCGA data have demonstrated that BRAF and RAS mutations 
are almost always mutually exclusive in PTCs. This knowledge 
has been leveraged to develop the BRAFV600E-RAS gene expres­
sion score (BRS), which has been used to highlight the remarkable 
biologic differences between RAS-driven and BRAFV600E-driven 
PTCs. Using the BRS scheme, tumors with the biologic properties 
of RAS-mutated tumors are d

e biologic properties 
of RAS-mutated tumors are designated RAS-like, in contrast to 
tumors with the biologic properties of BRAFV600E, which are des­
ignated BRAFV600E-like. BRAFV600E-mutated PTCs are strongly 
BRAFV600E-like with the strongest activation of the MAPK path­
way. One tumor harboring a BRAF K601E mutation paradoxically 
displayed RAS-like features consisting of more limited MAPK 
signaling than that [typically] observed in BRAFV600E-driven 
cancers. Interestingly, PTCs with BRAF K

E-driven 
cancers. Interestingly, PTCs with BRAF K601E (like those bearing 
RAS mutations) are enriched among PTCs with follicular pat­
terns.195–197 PTCs with BRAF indels have also been found to have 
gene expression patterns that are surprisingly RAS-like.
The thyroid differentiation score (TDS) was developed as a 
measure of the thyroid-specific differentiation of PTCs, based on 
the expression levels of 16 thyroid-related genes. TDS classifica­
tion of PTCs characterizes BRAFV600E-like PTCs 

a­
tion of PTCs characterizes BRAFV600E-like PTCs as a whole as less 
differentiated than RAS-like tumors, whose differentiation scores 
are closer to those of normal thyroid tissue. The distinctive intra­
cellular signaling profiles of BRAFV600E-like and RAS-like PTCs 
have also been documented at the transcriptional and protein 
expression levels.118
The pan-genomic differences between BRAFV600E-like and 
RAS-like tumors documented by TCGA data are striking, and 
a revised classification of th

 are striking, and 
a revised classification of thyroid carcinomas is needed to reflect 
these findings. One possible solution would entail the creation of 
a new entity—separate from PTC—that includes the follicular-
patterned tumors (FTC, FVPTC) and highlights the typically 
RAS-like nature of their underlying biology.
TCGA data have also been used to identify subgroups of 
BRAFV600E-like tumors with distinct biologic and pathologic fea­
tures. This molecular heterogeneity of BRAFV600E-like PT

 This molecular heterogeneity of BRAFV600E-like PTCs thus 
appears to be more substantial than previously believed, and it 
might at least partially explain the conflicting findings published 
on the prognostic significance of the BRAFV600E mutation in 
PTCs.198 The heterogeneity was particularly evident when the 
miRNA expression profiles of these PTCs were analyzed: six 
distinct clusters were identified. Cluster 1 was characterized by 
relatively high expression of miR-182-5p and miR-183-5p a

ely high expression of miR-182-5p and miR-183-5p and 
was enriched for RAS-mutated tumors and the follicular variant. 
BRAF-like tumors comprised five miRNA-defined clusters. Three 
clusters were associated with highly differentiated, relatively non­
aggressive PTCs. The other two were associated with less differen­
tiated tumors and a higher likelihood of recurrence. Cluster 5 was 
characterized by higher levels of miR-146b (3p and 5p isoforms) 
and miR-375, relatively low levels of miR-204-5p,

and miR-375, relatively low levels of miR-204-5p, and overrep­
resentation of classic PTCs harboring the BRAFV600E mutation. 
Cluster 6 presented high levels of miR-21-5p, low levels of miR-
204-5p, and enrichment for tall cell variant and classic PTCs. This 
cluster had the highest frequency of BRAFV600E mutation, the low­
est differentiation scores, and a very high risk of recurrence.
It remains to be seen whether miRNA profiles can help differ­
entiate low-risk from high-risk BRAF-like PTCs,

r­
entiate low-risk from high-risk BRAF-like PTCs, but recent stud­
ies have yielded promising findings.199,200 In addition to clinically 
informative miRNA expression patterns, several specific miRNAs 
have roles in thyroid carcinogenesis that have been experimen­
tally demonstrated. MiR-146b-5p, which is currently one of the 
most thoroughly studied miRNAs in PTC, has been linked to 
aggressive clinicopathologic features and poor outcomes.199,201 
Both isoforms of this miRNA specifically repre

01 
Both isoforms of this miRNA specifically repress the expression of 
PAX8 and NIS, which are essential determinants of the differenti­
ated thyroid cancer phenotype.202 Antagonism of miR-146b in 
human thyroid cancer cells has been shown to restore NIS-medi­
ated iodide uptake.202 This miRNA is also predicted to repress the 
expression of other iodide-metabolizing proteins, such as DEHAL 
and DIO2.202 In addition, the transcription of miR-146b is regu­
lated by PAX8. As a result, a negative f

 is regu­
lated by PAX8. As a result, a negative feedback loop is created 
whereby PAX8 limits its own activity by inducing the expression 
of its repressor, miR-146b. This repression also extends to NIS, 
DEHAL, and DIO2, which are downstream targets of PAX8.203
NIS expression is also repressed by miR-21-5p,202 a miRNA 
that has been associated with poor differentiation and high recur­
rence rates in PTC.118,199 MiR-182, which is predominant in 
RAS-mutated PTCs, is predicted to repress PAX8 an

RAS-mutated PTCs, is predicted to repress PAX8 and DEHAL, 
whereas miR-375, which is more prominent in BRAF tumors, is 
predicted to repress DEHAL and NKX2.1.202 Therapies targeting 
miRNAs in thyroid cancer can thus disrupt regulatory circuits 
that help maintain tumor cells in a poorly differentiated state, 
and this approach might be an effective new strategy for inducing 
redifferentiation and increased iodide uptake in these cells.
A high incidence of PTC has been reported in patients with

ncidence of PTC has been reported in patients with 
adenomatous polyposis coli who have a peculiar histologic appear­
ance, with solid areas and elongated cells, and Cowden disease 
(the multiple hamartoma syndrome), suggesting that the predis­
posing genes may play a role in the occurrence of papillary carci­
noma. The familial risk of thyroid cancer is higher than for other 
452
cancers, and about 3% to 10% of cases of PTC are familial204; 
their behavior is similar to o

C are familial204; 
their behavior is similar to or slightly more aggressive than that of 
nonfamilial cases.205 At least five loci of predisposition have been 
individualized but they do not explain all hereditary cases.206–210 
The gene predisposing to familial thyroid tumors with cellular oxy­
philia has been mapped to chromosome 19q13.2, and in a family 
with PTC and renal carcinoma, a separate gene was mapped to 
chromosome 1p13.2.q22. Variants of two genes, FOXE1 (TTF2) 
located at chromos

nts of two genes, FOXE1 (TTF2) 
located at chromosomal locus at 9q22, and NKX2-1 (TTF1) on 
14q, both coding for thyroid-specific transcription factors, confer 
an increased risk of thyroid cancer. 
Presenting Features of Papillary Thyroid Carcinoma
Although PTCs can occur at any age, most occur in patients 
between 30 and 50 years of age (mean age, 45 years). Women are 
affected more frequently (female predominance, 60–80%). Most 
primary tumors are 1 to 3 cm, and in recent years the proportion

 are 1 to 3 cm, and in recent years the proportion 
of small tumors has increased, largely due to detection of small 
PTCs by screening.72,73,163,211–213 PTC is frequently multifocal 
when it occurs in a single lobe and is bilateral in 20% to 80% of 
cases, depending on whether the thyroid is meticulously exam­
ined. Some studies have suggested that contralateral PTCs may 
have independent clonal origins, but this idea remains controver­
sial.214 Extrathyroidal invasion of adjacent soft tissues 

 Extrathyroidal invasion of adjacent soft tissues is present in 
about 15% (range 5–34%) at the time of primary diagnosis.145,163 
About 35% to 50% of excised neck lymph nodes have histologic 
evidence of involvement (microscopic foci of metastases in most 
of the cases), though in patients 17 years of age or younger, nodal 
involvement may be present in up to 90%.152,170,171,215 Only 1% 
to 7% of patients with PTC have distant metastases at diagno­
sis.163 Spread to superior mediastinal nodes i

no­
sis.163 Spread to superior mediastinal nodes is usually associated 
with extensive neck nodal involvement.
At presentation, most PTCs were classified as TNM stage I 
or II tumors (60% and 22%, respectively) using the seventh edi­
tion of the AJCC staging system. With the implementation of the 
eighth edition in 2018, stage I and II tumors account for an even 
larger proportion of PTCs.216,217 Fewer than 10% of all PTCs 
will be classified as stage III or IV using this newer system; these 
ar

stage III or IV using this newer system; these 
are reserved for cases in which the risk of cancer-related death is 
substantial. 
Risk of Papillary Thyroid Carcinoma Recurrence and 
Mortality
Three types of tumor recurrences may occur following the initial 
therapy for PTC, including local recurrence (LR), nodal metasta­
ses (NM), and distant metastases (DM). Local recurrence may be 
defined as “histologically confirmed tumor occurring in the resected 
thyroid bed, thyroid remnant, or other adj

sected 
thyroid bed, thyroid remnant, or other adjacent tissues of the neck 
(excluding lymph nodes)” after complete surgical removal of the 
primary tumor.218 Fig. 14.7 illustrates rates of PTC recurrence at 
local, nodal, and distant sites in 2150 patients with PTC treated 
at one institution from 1940 to 1997. After 20 years of follow-up, 
postoperative NM had been discovered in 9%, and LR and DM 
occurred in 5% and 4%, respectively. Both local recurrence and dis­
tant metastases are less com

l recurrence and dis­
tant metastases are less common in PTC than in FTC (Fig. 14.8), 
though postoperative cases of nodal metastases are more frequent in 
PTC. Another series of 1020 patients diagnosed between 1990 and 
2000 demonstrated a rate of recurrence of only 1.4%, and most 
such relapses (80%) occurred within 5 years of diagnosis.219
Cause-specific mortality (CSM) rates for differentiated thy­
roid cancer are shown in Fig. 14.9. CSM rates for PTC were 
2% at 5 years, 4% at 10 years, and

s for PTC were 
2% at 5 years, 4% at 10 years, and 5% at 20 years. Among 
Years after initial treatment
Papillary
Medullary
n = 2150
n = 194
Nodes
Local recurrences
Distant metastases
Occurrence of postop
events (cumulative %)
30
20
10
0
20
15
10
5
0
20
15
10
5
0
• Fig. 14.7  Development of neck nodal metastases, local recurrences, and 
distant metastases in the first 20 years after definitive surgery for papillary 
thyroid cancer (PTC) or medullary thyroid cancer (MTC) performed at the 
Mayo Cl

ary thyroid cancer (MTC) performed at the 
Mayo Clinic from 1940 to 1997. Based on 2150 consecutive PTC (left) and 
194 MTC (right) patients who had complete surgical resection (i.e., had 
no gross residual disease) and were without distant metastases on initial 
examination. Postop, postoperative.
Follicular
Hürthle cell
n = 115
n = 110
LR
DM
NM
30
20
10
0
20
15
10
5
0
20
15
10
5
0
• Fig. 14.8  Development of neck nodal me

5
10
5
0
• Fig. 14.8  Development of neck nodal metastases (NM), local recurrences 
(LR), and distant metastases (DM) in the first 20 years after definitive sur­
gery for follicular thyroid cancer (FTC) or Hürthle cell cancer (HCC) per­
formed at the Mayo Clinic from 1940 to 1997. Based on 110 consecutive 
FTC patients (left) and 115 HCC patients (right) who had complete surgi­
cal resection and were without distant metastases on initial examination. 
Postop, postoperative.
Years after initial t

ion. 
n = 2768
p = 0.0001
FTC (141)
HCC (125)
MTC (218)
PTC (2284)
Dying of thyroid carcinoma
(cumulative %)
40
30
20
10
0
25
20
15
10
5
0
• Fig. 14.9  Cumulative cause-specific mortality rates for patients with dif­
ferentiated thyroid carcinoma in the first 25 years after treatment with ini­
tial surgery performed at the Mayo Clinic from 1940 to 1997. Based on 
2768 consecutively treated patients (2284 with papillary thyroid carcinoma 
[PTC]

ents (2284 with papillary thyroid carcinoma 
[PTC], 141 with follicular thyroid cancer [FTC], 125 with Hürthle cell cancer 
[HCC], and 218 with medullary thyroid cancer [MTC]).
453
those with lethal PTC, 20% of deaths occurred in the first year 
after diagnosis, and 80% of the deaths occurred within 10 years. 
The 25-year cause-specific survival rate of 95% for PTC was sig­
nificantly higher than the 79%, 71

or PTC was sig­
nificantly higher than the 79%, 71%, and 66% rates seen with 
MTC, Hürthle cell cancer (HCC), and FTC, respectively.163 
Survival outcomes according to the 2017 TNM eighth edition 
staging system were retrospectively evaluated in two large US 
registry cohorts (Surveillance, Epidemiology, and End Results 
[SEER] and National Cancer Data Base [NCDB]).220 Five-year 
disease-specific survival (DSS) was 99.7% to 96.6% for stage I, 
96.7% to 88% for stage II, 85.2% to 74.3% for stage 

.7% to 88% for stage II, 85.2% to 74.3% for stage III, and 
66.9% to 49.5% for stage IV patients. Fig. 14.10 shows the dis­
tribution of SEER and NCDB papillary thyroid cancer popula­
tions according to both the TNM seventh and eighth editions. 
Fig. 14.11 illustrates the rates of DSS according to the TNM 
eighth edition staging system. 
Prediction of Papillary Thyroid Carcinoma Recurrence
Only a fraction (∼15–25%) of patients with PTC are likely to 
experience relapse of disease, and even fewer

 to 
experience relapse of disease, and even fewer (∼5%) will prove 
to have a lethal outcome. Exceptional patients who have an 
aggressive course tend to experience relapse early, and the rare 
fatalities usually occur within 5 to 10 years of diagnosis.163 
Multivariate analyses have been used to identify variables pre­
dictive of CSM.Multivariate analyses have been used to iden­
tify variables predictive of CSM.221–223 Increasing age of the 
patient and the presence of extrathyroidal invasion 

tient and the presence of extrathyroidal invasion (especially 
gross extrathyroidal invasion) are independent prognostic fac­
tors in all studies.
The presence of initial distant metastases and large size of the pri­
mary tumor are also significant variables in most studies,163,221,223 
and some groups163,221,222,224 have reported that histopathologic 
grade (degree of differentiation) is an independent variable. The 
completeness of initial tumor resection (postoperative status) is 
also a pred

r resection (postoperative status) is 
also a predictor of fatality.221,223 The presence of initial neck nodal 
metastases, although relevant to future nodal recurrence, surpris­
ingly does not influence CSM rate (Fig. 14.12).163,223
Several scoring systems based on these significant prognostic 
indicators have been devised. Each system allows one to assign the 
majority of PTC patients (≥80%) to a low-risk group, in which 
the CSM rate at 25 years is less than 2%, and the others (a small 
minor

rs is less than 2%, and the others (a small 
minority) to a high-risk group, in which almost all cancer-related 
deaths are observed. In general, these systems provide prediction 
of postoperative events comparable to that of the internationally 
accepted TNM staging system.225
A scoring index devised to assign PTC patients to prognostic 
risk groups226 was named the AGES scheme after the four inde­
pendent variables: patient’s age, tumor grade, tumor extent (local 
invasion, distant metastases)

tumor extent (local 
invasion, distant metastases), and tumor size. With the use of such 
a scoring system, 86% of patients were in the minimal risk group 
(AGES score <4) and they experienced a 20-year CSM rate of only 
1%. By contrast, patients with AGES scores of 4 or above (high 
risk; 14% of the total) had a 20-year CSM rate of 36%. Such 
prognostic scoring systems make it possible to counsel patients 
and to aid in the planning of individualized postoperative man­
agement programs in PTC.2

lized postoperative man­
agement programs in PTC.223,226
Although the AGES scheme had the potential for universal 
application, some centers could not include the differentiation 
(G) variable because their surgical pathologists did not recognize 
higher-grade PTC tumors.225 Accordingly, a prognostic scoring 
system for predicting PTC mortality rates was devised with the 
use of candidate variables that included completeness of primary 
tumor resection but excluded histologic grade.223 Cox model

ection but excluded histologic grade.223 Cox model 
analysis and stepwise variable selection led to a final prognostic 
model that included five variables: metastasis, age, completeness of 
resection, invasion, and size. This model was termed the MACIS 
prognostic scoring system. The final score utilized a calculation of 
(3.1 [age ≤39 years] or 0.08 × age [age ≥40 years]) + (0.3 × tumor 
size [in centimeters]) + (1 [if tumor not completely resected]) + (1 
[if locally invasive]) + (3 [if distan

ted]) + (1 
[if locally invasive]) + (3 [if distant metastases present]).
Stage I: 48,815
A
Stage II: 4,355
Stage III: 7,530
Stage IV: 3,642
7th Edition
SEER
8th Edition
Stage I: 57,769
Stage II: 5,245
Stage III: 668
Stage IV: 660
Stage I: 135,574
B
Stage II: 12,927
Stage III: 21,302
Stage IV: 9,895
NCDB
Stage I: 160,538
Stage II: 15,744
Stage III: 2,367
Stage IV: 1,049
• Fig. 14.10  Alluvial flow diagram representing the restaging of patient 
cohorts from the seventh to 

restaging of patient 
cohorts from the seventh to the eighth edition of the American Joint Com­
mission on Cancer/Union for International Cancer Control (AJCC/UICC) 
tumor, node, metastasis (TNM) staging system in (A) the Surveillance, Epi­
demiology, and End Results (SEER) program and (B) the National Cancer 
Database (NCDB). Numbers represent the absolute number of patients 
within each stage, with flow line width proportional to the number of 
patients moving to a new stage classification. (R

patients moving to a new stage classification. (Redrawn from Pontius LN, 
Oyekunle TO, Thomas SM, et  al. Projecting survival in papillary thyroid 
cancer: a comparison of the seventh and eighth editions of the Ameri­
can Joint Commission on Cancer/Union for International Cancer Control 
Staging Systems in two contemporary national patient cohorts. Thyroid. 
2017;27:1408–1416.)
454
As illustrated by Fig. 14.13, the MACIS scoring system per­
mits identification of groups o

coring system per­
mits identification of groups of patients with a broad range of 
risk of death from PTC. Twenty-year cause-specific survival 
rates for patients with MACIS scores of less than 6, 6 to 6.99, 
7 to 7.99, and 8+ were 99%, 89%, 56%, and 27%, respec­
tively (p <0.0001). When the cumulative mortality rate from 
all causes of death was considered, approximately 85% of PTC 
patients with AGES scores below 4 or MACIS scores below 6 
had no excess mortality rate over rates predicted for

 no excess mortality rate over rates predicted for control 
subjects.223,226
It should be emphasized that the five variables in MACIS scor­
ing are easy to define after primary operation; consequently, the 
system can be applied in any clinical setting. The MACIS system 
can be used for counseling individual PTC patients and can help 
guide decision making concerning the intensity of the postop­
erative tumor surveillance and the appropriateness of adjunctive 
radioiodine therapy. Because the CI

of adjunctive 
radioiodine therapy. Because the CIS (completeness of resection, 
invasion, and size) variables require information obtained at sur­
gery, the system should not be used to decide the extent of pri­
mary surgery.227,228
An important consideration for patients with well-differenti­
ated thyroid cancer is that the risk of thyroid cancer recurrence is 
higher than the risk of thyroid cancer–related death. To address 
this important shortcoming impacting clinical care, the ATA 
propose

rtcoming impacting clinical care, the ATA 
proposed a stratification system predicting the risk of recurrence 
after initial treatment,1 which should be taken into account for 
the indication of postoperative administration of radioiodine 
and for the subsequent follow-up strategy. The stratification is 
based on individual factors from various studies and reports a 
continuous increasing risk of recurrence according to each factor 
(Fig. 14.14). No multivariate analysis is thus far available. O

 No multivariate analysis is thus far available. Of 
note, the age of the patient at the time of initial treatment is not 
taken into account for the risk of recurrence, but many other 
prognostic factors for thyroid cancer death are also prognostic 
for the risk of recurrence, such as some histologic characteristics, 
the size of the thyroid tumor, the extension of the tumor beyond 
the thyroid capsule, the presence of lymph node metastases, and 
the presence of residual disease due to either i

the presence of residual disease due to either incomplete sur­
gical resection or presence of distant metastases. The encapsu­
lated follicular variant is associated with a low risk of recurrence. 
Aggressive histologic subtypes, the presence of necrosis, a high 
mitotic count, and vascular invasion are, in contrast, associated 
with higher risk. Indeed, the risk of recurrence is minimal (<2%) 
for unifocal micropapillary carcinoma, but higher for multifo­
cal micropapillary carcinoma (∼4%), a

for multifo­
cal micropapillary carcinoma (∼4%), and increases with the size 
of the thyroid tumor but is still low (∼5%) in patients with an 
intrathyroid tumor of 3 to 4 cm without extension beyond the 
thyroid capsule.229
The prognostic impact of extension beyond the thyroid cap­
sule is low in micropapillary carcinomas that are classified as N0. 
This risk increases with the size of the thyroid tumor and with 
the extent of the extension.230–232 Minimal lymph node involve­
ment (i.e., less t

232 Minimal lymph node involve­
ment (i.e., less than three lymph node metastases each <2 mm) is 
0.6
p < 0.001
stage=I
stage=II
stage=III
stage=IV
0.8
1.0
Disease-specific survival probability 
0.4
0.2
0.0
0
12
24
36
48
60
Time from diagnosis (months) 
72
84
96
108 120 132
• Fig. 14.11  Unadjusted disease-specific survival (DSS) curves for patients 
with papillary thyroid cancer (PTC) in the SEER program using the AJCC/
UICC TNM staging eighth edition models. (Redrawn from Pontius LN, 
Oyekunle

dition models. (Redrawn from Pontius LN, 
n = 2150
Extrathyroid, without DM
Intrathyroid, without DM
p = 0.56
p = 0.31
T4N0 (88)
T4N1 (121)
T1-T3N0 (1,230)
T1-T3N1

 0.31
T4N0 (88)
T4N1 (121)
T1-T3N1 (711)
Dying of papillary carcinoma
40
30
20
10
0
30
20
10
0
• Fig. 14.12  Lack of influence of nodal metastases at initial operation on 
cumulative mortality from papillary thyroid carcinoma in 1941 patients with 
pathologic T1 to T3 intrathyroidal tumors (completely confined to the thy­
roid gland) and 209 pathologic T4 patients with extrathyroidal (locally inva­
sive) tumors. All patients had initial surgical treatment at the Ma

 patients had initial surgical treatment at the Mayo Clinic 
from 1940 to 1997. DM, distant metastases.
n = 2284
p = 0.0001
MACIS score
<6 (1900:83%)
6-6.99 (201:9%)
7-7.99 (75:3%)
≥8 (108:5%)
Surviving papillary thyroid
carcinoma (%)
100
80
60
40
20
0
25
20
15
10
5
0
• Fig. 14.13  Cause-specific survival according to MACIS (metastases, age, 
completeness of resection, invasion, and size) scores of less than 6, 6 
to 6.99, 7 to 7.99, and 8+ in a cohort of 2284 conse

 6.99, 7 to 7.99, and 8+ in a cohort of 2284 consecutive patients with 
papillary thyroid carcinoma (PTC) undergoing initial treatment at the Mayo 
Clinic from 1940 to 1997. The numbers in parentheses represent the num­
bers and percentages of PTC patients in each of the four risk groups.
455
associated with a low risk of recurrence. This risk again increases 
with the number of lymph node metastases, with t

with the number of lymph node metastases, with the size of the 
lymph node metastases, and with the presence of extranodal exten­
sion.153,154,233 Therefore minimal lymph node involvement that is 
usually found at prophylactic lymph node dissection may have a 
minimal prognostic impact and may not change the indication for 
postoperative treatment with radioiodine. However, large lymph 
node metastases (>3 cm) that are usually palpable and easily visual­
ized on neck ultrasonography and are fr

ly visual­
ized on neck ultrasonography and are frequently multiple and asso­
ciated with extracapsular nodal extension have a major prognostic 
impact on recurrence that may occur in up to 40% of patients.
The risk of PTC recurrence is also associated with the molec­
ular profile and with the presence of BRAF mutation associated 
with a higher risk of recurrence for most tumor stages.111,234,235 
This risk is even higher when both BRAF and TERT mutations 
are present in the tumor.111,193 Each o

mutations 
are present in the tumor.111,193 Each of these characteristics is 
associated with a risk of tumor recurrence that may range from 
less than 2% up to more than 40%. For practical purposes, 
cases can be divided into three discrete groups based on their 
estimated risk of recurrence (low, <5%; intermediate, 5–20%; 
high, >20%), and the group to which the case is assigned can be 
used as a guide for planning subsequent treatment and follow-up 
(Table 14.7). (See also “Outcome Prediction

ow-up 
(Table 14.7). (See also “Outcome Prediction for Follicular Thy­
roid Carcinoma.”) 
Follicular Thyroid Carcinoma
WHO defines follicular thyroid cancer as a malignant epithelial 
tumor characterized by follicular cell differentiation and absence 
of the nuclear features typical of PTC.147 This definition excludes 
the follicular variant of PTC, and it is also customary to exclude 
both the poorly differentiated carcinoma236 and the rare mixed 
medullary and follicular carcinoma.237
Thus FTC

d 
Thus FTC is a relatively rare neoplasm whose identification 
requires invasion of the capsule, blood vessel, or adjacent thyroid. 
In initial epidemiologic surveys, FTC constituted 5% to 50% of 
differentiated thyroid cancers and tended to be more common in 
areas with iodine deficiency.238 Changing diagnostic criteria and 
increases in the incidence of PTC associated with dietary iodine 
supplementation have reduced the frequency of FTC diagnoses. 
Data

have reduced the frequency of FTC diagnoses. 
Data from the SEER program from 1980 to 2009 show incidence 
rates for FTCs (1.19 per 100,000 woman-years, 0.55 per 100,000 
man-years) that are markedly lower than those for PTCs (9.21 per 
100,000 woman-years, 3.10 per 100,000 man-years).239
Risk of Structural Disease Recurrence
(In patients without structurally identifiable disease after initial therapy)
FTC, extensive vascular invasion (≈30-55%)
High Risk
Gross extrathyroidal extension,
incomplet

igh Risk
incomplete tumor resection, distant metastases,
or lymph node >3 cm
Aggressive histology, minor extrathyroidal
extension, vascular invasion,
or >5 involved lymph nodes (0.2-3 cm)
Intermediate Risk
Low Risk
pT4a gross ETE (≈30-40%)
pN1 with extranodal extension, >3 LN involved (≈ 40%)
PTC >1cm, TERT mutated ± BRAF mutated* (>40%)
pN1, any LN >3 cm (≈30%)
PTC, extrathyroidal, BRAF mutated*(≈10-40%)
PTC, vascular invasion (≈15-30%)
Clinical N1 (≈20%)
pN1, >5

ular invasion (≈15-30%)
pN1, >5 LN involved (≈20%)
Intrathyroidal PTC, <4 cm, BRAF mutated* (≈10%)
pT3 minor ETE (≈3-8%)
pN1, all LN <0.2 cm (≈5%)
pN1, ≤5 LN involved (≈5%)
Intrathyroidal PTC, 2-4 cm (≈5%)
Multifocal PTMC (≈4-6%)
pN1 without extranodal extension, ≤3  LN lnvolved (2%)
Minimally invasive FTC (≈2-3%)
lntrathyroidal, <4 cm, BRAF wild type* (≈1-2%)
Intrathyroidal unifocal PTMC, BRAF mutated*, (≈1-2%)
Intrathyroidal, encapsulated, FVPTC (≈1-2%)
Unifocal PTMC (≈1-2%)

 encapsulated, FVPTC (≈1-2%)
lntrathyroidal DTC
≤5 LN micrometastases (<0.2 cm)
• Fig. 14.14  Differentiated thyroid cancer risk of recurrence. DTC, dif­ferentiated thyroid cancer; ETE, extra­
thyroidal extension; FTC, follicular thyroid cancer; FVPTC, follicular variants of papillary thyroid carcinoma; 
LN, lymph node; PTC, papillary thyroid carcinoma; PTMC, papillary thyroid microcarcinoma. (From Hau­
gen BR, Alexander EK, Bible KC, et al. 2015 American Thyroid Associatio

 Bible KC, et al. 2015 American Thyroid Association management guidelines for 
adult patients with thyroid nodules and differentiated thyroid cancer: the American Thyroid Association 
guidelines task force on thyroid nodules and differentiated thyroid cancer. Thyroid. 2016;26:1–133.)
456
The microscopic appearance of FTC varies from well-formed 
follicles to a predominantly solid growth pattern.146 Poorly 
formed follicles and atypical patterns (e.g., cribriform) may 
occu

and atypical patterns (e.g., cribriform) may 
occur, and multiple architectural types may coexist. FTC is best 
divided into three categories on the basis of degree of invasive­
ness: (1) minimally invasive or encapsulated, (2) encapsulated 
angioinvasive, and (3) widely invasive. Minimally invasive FTC is 
an encapsulated tumor whose growth pattern resembles that of a 
trabecular or solid microfollicular or atypical adenoma. The diag­
nosis of malignancy depends on the demonstration of capsular

alignancy depends on the demonstration of capsular 
invasion. The criteria for invasion must therefore be strict.146,147 
Interruption of the capsule must involve the full thickness to 
qualify as capsular invasion. Penetration of only the inner half 
or the presence of tumor cells embedded in the capsule does not 
qualify for the diagnosis of FTC. Foci of capsular invasion must 
be distinguished from the capsular rupture that can result from 
FNA. The acronym WHAFFT (worrisome histologic altera

A. The acronym WHAFFT (worrisome histologic alterations 
following FNA of the thyroid) is applied to such changes.240 
Extensive histologic sampling may be required to distinguish 
minimally invasive FTCs from follicular adenomas, and the abil­
ity to discriminate between the two varies substantially from one 
pathologist to another.241 The diagnosis of malignancy of these 
tumors may be difficult and not reproducible among pathologists, 
and immunohistochemistry with markers such as TPO, galect

unohistochemistry with markers such as TPO, galectin 
3, or HMBE1 may help for this purpose,106 but these techniques 
did not reliably improve the accuracy in case of suspicious find­
ings. Global gene expression studies with the microarray tech­
nology and more recently the gene classifier or a panel of gene 
mutations may help to differentiate malignant from benign fol­
licular tumors.242
Minimally invasive FTC with vascular invasion (encapsulated 
angioinvasive, defined by some authors as mod

ted 
angioinvasive, defined by some authors as moderately invasive) 
must also be distinguished from minimally invasive FTCs with 
capsular invasion alone because vascular invasion increases the 
risks of recurrence and metastasis. Vascular spread should be diag­
nosed even when there is only a single focus of angioinvasion. 
Grossly evident blood vessel invasion is almost never seen.
In contrast, the rare, widely invasive form of FTC can be 
distinguished easily from benign lesions. Although th

tinguished easily from benign lesions. Although the tumor 
may be partially encapsulated, the margins are infiltrative even 
on gross examination, and vascular invasion is often extensive. 
The structural features are variable, but a follicular element 
is always present. When follicular differentiation is poor or 
absent, or in the presence of trabecular, insular, or solid com­
ponents, the tumor may be classified as a poorly differentiated 
carcinoma (see later).146,236 Focal or extensive clea

cinoma (see later).146,236 Focal or extensive clear cell changes 
can also be seen on histologic analysis. A rare clear cell variant 
of FTC has been described in which glycogen accumulation or 
dilatation of the granular endoplasmic reticulum is responsible 
for the clear cells.243
Molecular Pathogenesis of Follicular Thyroid Carcinoma
There is still no accepted paradigm for the pathogenesis of FTC. 
A multistep adenoma-to-carcinoma pathogenesis, similar to that 
for colon cancer and other aden

, similar to that 
for colon cancer and other adenocarcinomas, is not universally 
accepted because pathologists do not recognize follicular carci­
noma in situ, and documentation of the evolution of adenoma 
to carcinoma is rare. Nevertheless, several facts about the patho­
genesis of FTC are firmly established.111,160,244,245 First, most 
follicular adenomas and all FTCs are probably of monoclonal 
origin. Second, oncogene activation, particularly by point muta­
tion of the RAS oncogene, is co

rly by point muta­
tion of the RAS oncogene, is common both in follicular adeno­
mas (∼20%) and in FTCs (∼40%), supporting a role for these 
mutations in early tumorigenesis.245,246 This commonality sug­
gests that FTA and FTC are etiologically related or, more specifi­
cally, that FTAs are premalignant neoplasms that can (at least in 
some cases, but likely not all) progress to become FTC.245 More­
over, the fact that RAS mutations are common in FTA, FTC, 
and FVPTC implies that all these folli

mension)
Intrathyroidal, encapsulated follicular variant of 
papillary thyroid cancer
Intrathyroidal, well-differentiated follicular thyroid 
cancer with capsular invasion and no or mini­
mal (<4 foci) vascular invasion
Intrathyroidal, papillary microcarcinoma, unifocal 
or multifocal, including BRAF V600E mutated (if 
known)
ATA Intermediate 
Risk
Microscopic invasion of tumor into the perithyroidal 
soft tissues
RAI-avid metastatic foci in the neck on the first 
posttreatment whole-body RAI sc

neck on the first 
posttreatment whole-body RAI scan
Aggressive histology (e.g., tall cell, hobnail variant, 
columnar cell carcinoma)
Papillary thyroid cancer with vascular invasion
Clinical N1 or >5 pathologic N1 with all involved 
lymph nodes <3 cm in largest dimension
Multifocal papillary microcarcinoma with extra­
thyroidal extension and BRAFV600E mutated (if 
known)
ATA High Risk
Macroscopic invasion of tumor into the perithyroi­
dal soft tissues (gross extrathyroidal extension)
Incomplete

issues (gross extrathyroidal extension)
Incomplete tumor resection
Postoperative serum thyroglobulin suggestive of 
distant metastases
Pathologic N1 with any metastatic lymph node ≥3 
cm in largest dimension
Follicular thyroid cancer with extensive vascular 
invasion (>4 foci of vascular invasion)
ATA, American Thyroid Association; RAI, radioactive iodine
From Haugen BR, Alexander EK, Bible KC, et al. 2015 American Thyroid Association manage­
ment guidelines for adult patients

ciation manage­
ment guidelines for adult patients with thyroid nodules and differentiated thyroid cancer: 
the American Thyroid Association guidelines task force on thyroid nodules and differentiated 
thyroid cancer. Thyroid. 2016;26:1–133.
   
TABLE 14.7
457
more common in FTC than in PTC and also occur in follicular 
adenomas.244,248
Of the cytogenetic abnormalities described in FTC, the most 
common are 

ormalities described in FTC, the most 
common are deletions, partial deletions, and deletion rearrange­
ments involving the p arm of chromosome 3. Loss of heterozygosity 
(LOH) on chromosome 3p appears to be limited to FTC because 
no evidence for 3p LOH has been found in follicular adenomas or 
PTC. A translocation, t(2;3)(q13;p25), resulting in the fusion of the 
DNA binding domains of the thyroid transcription factor PAX8 to 
domains of the peroxisome proliferator–activated receptor gamma 
1 

roxisome proliferator–activated receptor gamma 
1 (PPARγ1) was detected in 30% (range 11–63%) of FTCs, 10% 
of follicular adenomas, and a smaller percentage of FVPTCs.249–253 
PAX8 encodes a paired-box transcription factor that is highly 
expressed in thyroid follicular cells, where it plays a key role in 
their development.254,255 The fusion gene mentioned earlier there­
fore results in increased expression of PPARγ.256 CREB3L2-PPARγ 
fusions have also been described in a small number of FTCs,2

ve also been described in a small number of FTCs,257 
suggesting that PPARγ may play a role in the development of these 
tumors. Compared with classic FTCs, PAX8-PPARγ FTCs are 
more frequently diagnosed in younger patients, and they are more 
likely to be associated with vascular invasion, although their clinical 
behavior is relatively indolent.251 The PAX8-PPARγ tumors’ gene 
expression profiles are also distinctive, reflecting as they do the tran­
scriptional output of the PAX8-PPARγ fusion 

tran­
scriptional output of the PAX8-PPARγ fusion protein.258
Genes encoding components of the PI3K/PTEN/AKT sig­
naling pathway are also likely to be genetically altered or epige­
netically silenced in FTCs.259 Somatic mutations involving the 
tumor suppressor gene PTEN are found in up to 1 of 10 sporadic 
FTCs,260 and germline PTEN mutations are associated with the 
Cowden syndrome (CS). Up to 10% of all FTCs also harbor acti­
vating PIK3CA mutations.261
Mutations of the TERT promoter are also

ations.261
Mutations of the TERT promoter are also found in some 
FTCs, and they are associated with aggressive disease and poor 
outcomes.192 Interestingly, TERT promoter mutations have also 
been reportedly found in a small subset of FTAs and atypical 
adenomas. Some of these tumors ultimately displayed cancer-like 
behavior, raising the possibility that TERT mutations may be a 
potential biomarker of FTC for use in cases with nondiagnostic 
histologic features.262
In a recent genomic study of

stologic features.262
In a recent genomic study of FTA, FTC, and PTC,263 the 
transcriptomes of FTA, minimally invasive FTC, and noninvasive 
FVPTC proved to be indistinguishable, while that of the infiltra­
tive FVPTC was more similar to the classic PTC transcriptome.263 
These findings add support to the view that follicular-patterned 
tumors are characterized by a common RAS-like pathogenesis. 
The strong biologic relation between FVPTC and FTC has also 
been confirmed by miRNA expression pro

C has also 
been confirmed by miRNA expression profiling, which revealed 
upregulated expression of miR-182-5p and miR-183-5p exclu­
sively in these thyroid carcinoma histotypes.118,264,265 Nearly 90% 
of the miRNAs found to be dysregulated in FTC were similarly 
dysregulated in follicular adenomas (FA), including miR-182-5p, 
miR-183-5p, and miR-96, which were upregulated, and miR-
1247, which was downregulated with respect to levels found in 
normal thyroid tissues.264 In contrast, downregulat

ormal thyroid tissues.264 In contrast, downregulation of miR-
150 seemed to be an FTC-specific change264 and may function 
as a suppressor gene in tumor cells by inhibiting the RAB11A/
WNT/β-catenin pathway.266 Three well-characterized miRNAs—
miR-146b-5p, miR-221-3p, and miR-222-3p—are commonly 
upregulated in FTCs, PTCs, and ATC. Their dysregulation thus 
appears to be an early event in the development of follicular cell–
derived thyroid cancers that is essential for sustaining the onco­
genic

s that is essential for sustaining the onco­
genic process.118,265,267 
Hürthle Cell Carcinoma
When more than 75% of cells in an FTC exhibit Hürthle cell 
(or oncocytic) features, the tumor is classified as a Hürthle cell 
(oncocytic) carcinoma.146,238 The current version of the WHO 
classification includes this tumor as a separate entity,147 although 
in the past it was considered an oxyphilic variant of FTC.162 The 
AFIP monograph already stated that “the tumors made up of this 
cell type have

d that “the tumors made up of this 
cell type have gross, microscopic, behavioral, cytogenetic features 
that set them apart from all others and justify discussing them in 
a separate section.”146 The mutational, transcriptional, and copy 
number profiles of HCC were distinct from those of PTC and 
FTC, indicating HCC to be a unique type of thyroid malignancy. 
Molecular pathways that differentiate Hürthle cell adenoma 
from widely invasive HCC included the PIK3CA-Akt-mTOR 
(mammalian target of 

ncluded the PIK3CA-Akt-mTOR 
(mammalian target of rapamycin) and Wnt/β-catenin pathways, 
potentially providing a rationale for new targets for this type of 
malignancy. Recent data have confirmed that molecular abnor­
malities are different from those found in follicular cancer.268 
Presenting Features of Follicular Thyroid Carcinoma
FTC tends to occur in older individuals.238 The mean age at diag­
nosis of FTC (49 years) is slightly higher than those for both PTC 
and follicular variant PTCs (

 those for both PTC 
and follicular variant PTCs (44 and 46 years, respectively).269 
The median age of patients with HCC is about 60 years. As in 
most thyroid malignancies, women outnumber men (>2:1). Most 
patients with FTC present with a painless thyroid nodule, with or 
without background thyroid nodularity, and they rarely (2–8%) 
have clinically evident lymphadenopathy at presentation.238 
Lymph node metastases to the neck in FTC are so exceptional 
that “wherever they are observed, the a

ceptional 
that “wherever they are observed, the alternative possibilities of 
follicular variant papillary carcinoma, oncocytic carcinoma, and 
poorly differentiated carcinoma should be considered.”146 HCC 
are the exception where up to one-third of these FTCs have cervi­
cal lymph node metastases at the time of diagnosis.238
In most series in which tumor sizes were reported, the average 
tumor in FTC and HCC was larger than those seen with PTC.270 
Direct extrathyroidal extension, by definitio

270 
Direct extrathyroidal extension, by definition, does not occur with 
minimally invasive FTC but is common in the rare patients with 
widely invasive FTC. Between 15% and 27% of patients may have 
distant metastases at presentation.238 The most common sites for 
distant metastases in FTC are lung and bone.238 The bones most 
often involved are long bones (e.g., femur), flat bones (particu­
larly the pelvis, sternum, and skull), and vertebrae. When a distant 
metastasis is the first manifesta

 When a distant 
metastasis is the first manifestation of the disease, definitive proof 
of its thyroid origin should be obtained, usually by biopsy of a 
metastasis, before performing any thyroid surgery. It is unusual 
but possible for patients with FTC to have thyrotoxicosis caused 
by massive tumor burden producing thyroid hormone.271 
Risk of Follicular Thyroid Carcinoma Recurrence and 
Mortality
Recurrence rates reported in patients with FTCs vary widely, from 
3% to 43.5%, and recurrence 

Cs vary widely, from 
3% to 43.5%, and recurrence is most frequent in widely invasive 
FTC. Similar recurrence rates are seen in HCC (14–44%). Over 
half of all FTC recurrences are detected within the 3 years fol­
lowing diagnosis, and 80% are found within the first 6 years. The 
vast majority of recurrences (up to 85%) occur at distant sites, but 
lesions can also be found in the thyroid bed and regional lymph 
nodes.
Nodal metastases are rare in typical FTC, and the nodal recur­
rence rate at 

n typical FTC, and the nodal recur­
rence rate at 20 postoperative years is the lowest among differ­
entiated thyroid carcinoma, being around 2% (see Fig. 14.8), 
458
although it is higher (about 17%) in HCC patients.272 When 
recurrences at either the neck or distant sites are taken into consid­
eration, patients with HCC (Fig. 14.15) have the highest numbers 
of tumor recurrences after 10 to 20 years. As illustrated by Fig. 
14.8, local recurrences at 20 years have occur

g. 
14.8, local recurrences at 20 years have occurred in 20% of FTCs 
and 30% of HCCs. Comparable distant metastasis rates are 23% 
and 28%, respectively.
The mortality rate from FTC exceeds that of PTC (Fig. 14.16). 
CSMs vary with the presenting TNM stage in both FTC and 
HCC (Fig. 14.17). The death rates tend to parallel the curves for 
development of distant metastases (see Fig. 14.8). In more than 
50 years of experience at the Mayo Clinic, the mortality rate for 
FTC initially exceeds that

the mortality rate for 
FTC initially exceeds that of HCC, but by 20 to 30 postoperative 
years, there are no significant differences in cause-specific survival 
rates between FTC and HCC, both being around 80% at 20 years 
and 70% at 30 postoperative years.270 Curves representing death 
from all causes differ in FTC and HCC. On average, patients with 
FTC are about 5 years younger, tend to die within the first 10 
postoperative years, and have a high all-cause mortality rate for 10 
to 30 posto

 high all-cause mortality rate for 10 
to 30 postoperative years. Deaths related to HCC occur gradually 
over the first 15 years. However, by 25 years, the average survivor 
of HCC is 84 years old, and by that time, almost 50% of the 
treated cohort would be predicted by the actuarial curve to have 
died from all causes. 
Outcome Prediction for Follicular Thyroid Carcinoma
The risk factors that predict outcome in FTC are similar to those 
recognized in PTC273–276 and include distant metastases a

zed in PTC273–276 and include distant metastases at pre­
sentation, advanced age at diagnosis (>45 years for most studies), 
large primary tumors (>4 cm), the presence and extent of local 
(extrathyroidal) invasion, and the completeness of excision. To a 
lesser degree, increased mortality risk is associated with male sex. 
The number of vessels involved (<4 or ≥4) has also prognostic 
value for both cancer-specific survival and disease-free survival 
(Fig. 14.18).238,275
Inconsistencies between

ival 
Inconsistencies between the results of different studies in 
predicting the outcome of FTC patients may reflect the use 
of different classification systems over long follow-up periods, 
the quality of the pathologic reviews, and/or the small sample 
sizes. Several systems have been proposed for staging thyroid 
cancer. Some were developed solely for use with PTC, others 
were derived from data on PTC and FTC (differentiated thyroid 
cancer), and still others were deve

iated thyroid 
cancer), and still others were developed for use with all histologic 
types of thyroid cancers (including medullary and anaplastic 
forms). The AIM (age, invasion, metastases) system is the only 
one developed specifically for staging FTC. In some of these 
systems, FTC patients are classified in a higher risk category 
(GAMES [grade, age, metastases, extent, size], National Thyroid 
Cancer Treatment Cooperative Study Group [NTCTCSG]). On 
the contrary, the AMES (age, metastases, 

G]). On 
the contrary, the AMES (age, metastases, extent, size), Clinical 
Class, MACIS, and AGES prognostic scoring systems and vari­
ous versions of the UICC/AJCC pTNM system do not include 
FTC histology as an independent risk factor.274 The UICC/
AJCC pTNM (tumor, nodes, metastasis) system is still the most 
widely used staging system for thyroid carcinoma and provides 
fairly accurate prognostic information on the tumors (see Fig. 
14.17). One of the most hotly debated staging criteria is t

ne of the most hotly debated staging criteria is the age 
threshold for defining higher risk disease. In the recently imple­
mented eighth edition of the AJCC/IUCC staging system, the 
threshold was raised from 45 years to 55 years to avoid encour­
aging overtreatment. Moreover, a recent reassessment of the 
NTCTCSG thyroid cancer staging system showed that a new 
model for FTC, with a threshold of 50 years, outperformed the 
current system.277 
n = 2569
p = 0.0001

n = 2569
p = 0.0001
Papillary
Medullary
(2150)
(110)
(194)
(115)
Histologic subtype
(45 yr)
(52 yr)
(40 yr)
(58 yr)
Surviving neck or distant
recurrence (%)
100
80
60
40
20
0
25
20
15
10
5
0
• Fig. 14.15  Postoperative recurrence (any site) in the first 20 years after 
definitive surgery for differentiated thyroid carcinoma performed at the 
Mayo Clinic from 1940 to 1997. Based on 2569 consecutive patients 
(2150 papillary thyroid carcinoma, 

tive patients 
(2150 papillary thyroid carcinoma, 110 follicular thyroid carcinoma, 115 
Hürthle cell carcinoma, and 194 medullary thyroid carcinoma) who had 
complete tumor resection and had no distant metastases at presentation. 
The ages in parentheses represent the median age at diagnosis for each 
of the four histologic subtypes.
1.00
0.75
0.50
PTC
FTC
p < 0.0001
0.25
0
A
B
0
2
4
6
8
10
Survival time (years)
Survival rate
12
14
16
18
20
1.00
0.75
0.50
PTC
FTC
p < 0.0001
0.25
0
0
2
4
6
8
10

0.75
0.50
PTC
FTC
p < 0.0001
0.25
0
0
2
4
6
8
10
12
14
16
18
20
• Fig. 14.16  Comparison of disease-specific survival by carcinoma type (papillary [PTC] vs follicular thyroid 
cancer [FTC]) in patients less than 45 years (A) or at least 45 years old (B). (Redrawn from Oyer SL, Fritsch 
VA, Lentsch EJ. Comparison of survival rates between papillary and follicular thyroid carcinomas among 
36,725 patients. Ann Otol Rhinol Laryngol. 2014;123:94–100.)
459
Chapter

tol Rhinol Laryngol. 2014;123:94–100.)
459
Poorly Differentiated Carcinoma
Poorly differentiated thyroid carcinoma is rare, representing less 
than 5% of all thyroid cancers. It has been defined as “a tumor of 
follicular cell origin with morphological and biologic attributes 
intermediate between differentiated and anaplastic carcinomas of 
the thyroid.”146 A consensus conference proposed the following 
dia

A consensus conference proposed the following 
diagnostic criteria (the Turin criteria) for poorly differentiated car­
cinoma: (1) solid, trabecular, and insular pattern of growth; (2) 
absence of the conventional nuclear features of papillary carcino­
mas; and (3) presence of at least convoluted nuclei, mitotic activ­
ity greater than 3 × 10 high-power fields, and tumor necrosis.236 
Similarly, in well-differentiated cancer, necrosis, mitosis, and cel­
lular atypia associated with vascular inva

nd cel­
lular atypia associated with vascular invasion are considered as 
features of poor prognosis and aggressiveness.39,274 Most poorly 
differentiated tumors are larger than 5 cm in diameter at diagno­
sis, with extrathyroidal extension and blood vessel invasion.
RAS mutations appear to be a common molecular signature 
in poorly differentiated tumors, although they were found at a 
highly variable frequency and wide variation in specific types 
of RAS mutations. The occurrence of β-catenin m

s 
of RAS mutations. The occurrence of β-catenin mutations in 
PDTC is controversial, ranging from 0% to 32% of tumors. 
Finally, TERT promoter mutations, which have been described 
in a subset of PDTC, have been proposed as a molecular marker 
of thyroid tumor dedifferentiation and progression, and they 
are considered to be a molecular signature of aggressive tumors. 
TERT promoter mutations are found in around 40% of PDTCs, 
particularly those that also harbor an additional BRAF or RAS 
mutat

 that also harbor an additional BRAF or RAS 
mutation. TP53 mutations are reported in 10% of the cases, but 
are more common in anaplastic thyroid cancer (Fig. 14.19).278 
Recently, a prognostic role of specific microRNAs has been dem­
onstrated in PDTC.
The mean age at diagnosis is about 55 years, and the female-
to-male ratio is about 2:1. Poorly differentiated carcinoma is 
aggressive and can be lethal. Radioiodine uptake is rarely pres­
ent, and FDG uptake on PET is frequently high; producti

and FDG uptake on PET is frequently high; production 
of Tg in blood may be lower than in differentiated carcinomas. 
Metastases are common in regional nodes and distant sites (lung, 
bone, brain). In one series, 56% of patients died from their tumor 
within 8 years of initial therapy, though not all have replicated 
these findings.236,279,280 
Undifferentiated (Anaplastic) Carcinoma
Anaplastic carcinoma constitutes about 1% to 2% of all thyroid 
carcinomas, usually occurs after the age of 60 ye

carcinomas, usually occurs after the age of 60 years, and is slightly 
more common in women than men (1.3:1 to 1.5:1).281 This carci­
noma is highly malignant, nonencapsulated, and extends widely. 
Evidence of invasion of adjacent structures, such as the skin, 
muscles, nerves, blood vessels, larynx, and esophagus, is common. 
Distant metastases occur early in the course of the disease in lungs, 
liver, bones, and brain.
On histopathologic examination, the lesion is composed of 
atypical cells 

nation, the lesion is composed of 
atypical cells that exhibit numerous mitoses and form a variety of 
patterns. Spindled, pleomorphic, squamoid, and even rhabdoid 
cells have been described. Areas of necrosis and polymorphonu­
clear infiltration are common, and the presence of PTC or FTC 
suggests that they may be the precursors of anaplastic carcinoma. 
Immunohistochemistry revealed that a significant proportion of 
cells are tumor-associated macrophages.282,283
Mutations of the TP53 gene are 

acrophages.282,283
Mutations of the TP53 gene are present in many (60–80%) 
undifferentiated carcinomas but may not be found in the residual 
well-differentiated component, suggesting that these mutations 
occurred after the development of the original tumor and may 
have played a key role in tumor progression.163,284 BRAF muta­
tion is found mostly in anaplastic thyroid carcinoma with a papil­
lary component, and RAS mutation is found in 20% or more of 
anaplas­tic thyroid carcinomas; PI3KCA mu

more of 
anaplas­tic thyroid carcinomas; PI3KCA mutation is rare in differ­
entiated thyroid cancer and was found in 23% of anaplastic thy­
roid carcinoma; finally, ALK rearrangement was found in about 
10% of anaplastic thyroid carcinomas.184 BRAF and RAS muta­
tions are still the principal genetic drivers of thyroid carcinomas, 
but additional mutations accumulate in ATCs, including those 
affecting the TERT promoter (see Fig. 14.19), mismatch repair 
genes, and genes encoding components of th

repair 
genes, and genes encoding components of the PIK3CA-PTEN-
AKT-mTOR pathway, the SWI-SNF complex, or histomethyl­
transferases. Several alterations may serve as adverse prognostic 
markers in ATC, including EIF1AX mutations, loss of chromo­
some 13q, and gain of chromosome 20q.278
n = 141
p = 0.0001
n = 125
p = 0.0001
I
II
III
IV
TNM stage
(31)
(75)
(11)
(24)
Surviving thyroid carcinoma (%)
100
80
60
40
20
0
10
8
6
4
2
0
I
II
III
IV
TNM

(%)
100
80
60
40
20
0
10
8
6
4
2
0
I
II
III
IV
TNM stage
(21)
(86)
(12)
(6)
10
8
6
4
2
0
• Fig. 14.17  Cause-specific survival according to pathologic tumor-node-
metastasis (pTNM) stages in a cohort of 141 patients with follicular thyroid 
carcinoma (left panel) and 125 patients with Hürthle cell carcinoma (right 
panel) treated at the Mayo Clinic from 1940 to 1997. Numbers in paren­
theses represent the number of patients in each pTNM stage grouping.
1.0
0.8
0.6
0.4
0.2
0
2
4
6
Survival time (

uping.
1.0
0.8
0.6
0.4
0.2
0
2
4
6
Log-rank test: p < 0.001
8
10
MIFC
WIFC
• Fig. 14.18  Disease-specific survival of minimally invasive follicular thyroid 
cancer (MIFC) versus widely invasive follicular thyroid cancer (WIFC). (From 
Goffredo P, Cheung K, Roman SA, et al. Can minimally invasive follicular 
thyroid cancer be approached as a benign lesion? A population-level anal­
ysis of survival among 1200 patients. Ann Surg Oncol. 2013;20:767–772.)
46

200 patients. Ann Surg Oncol. 2013;20:767–772.)
460
The usual clinical complaint is of rapid, often painful enlarge­
ment of a mass that may have been present in the thyroid gland 
for many years. The tumor invades adjacent structures, causing 
hoarseness, inspiratory stridor, and difficulty in swallowing. On 
examination, the overlying skin is often warm and discolored. 
The mass is tender and is often fixed to adjacent structures. The 
regional lymph nodes are enlarged, 

ructures. The 
regional lymph nodes are enlarged, and there may be evidence 
of distant metastases. Anaplastic carcinomas do not accumulate 
iodine and do not typically produce Tg; high FDG uptake is usu­
ally found on PET, which is the best tool for tumor staging and for 
control of treatment efficacy.285,286
Treatment should be initiated rapidly to avoid death from 
locally infiltrative disease and possible suffocation. It consists of 
surgical resection of the tumor tissue present in the neck

 resection of the tumor tissue present in the neck, when 
this is feasible, followed by a combination of external irradia­
tion and chemotherapy. The overall prognosis for ATC remains 
poor despite recent advances (Fig. 14.20). The median overall 
survival remains around 3 to 5 months with a 1-year survival 
of approximately 20%. Patient factors associated with poorer 
prognosis include advanced age (>60–70 years), male gender, 
presence of leukocytosis (>10,000), and symptoms (such as 
rapidly 

cytosis (>10,000), and symptoms (such as 
rapidly growing tumor, pain in the neck, dyspnea, dysphagia, 
and hoarseness).
However, treatment paradigms for ATC are evolving rapidly. 
Patients with BRAF-mutant ATCs can now be treated with a 
combination of selective BRAF and MEK inhibitors. For patients 
whose disease was initially regarded as unresectable, response to 
this combination therapy can render them eligible for surgery. 
For patients interested in taking part in a clinical trial, the tu

erested in taking part in a clinical trial, the tumor 
should be assessed for a broad range of mutations and fusions. 
This helps determine whether the patient is suitable for enroll­
ment in trials testing selective inhibitors, such as the selective 
NTRK or RET inhibitor trials. Other driver mutations that are 
identified may help identify a clinical trial that is appropriate 
for the patient, particularly as more targeted therapies become 
available.287 
Medullary Thyroid Carcinoma
Medullary 

ilable.287 
Medullary thyroid carcinoma accounts for approximately 2% of 
all thyroid cancers. MTCs are rare neuroendocrine cancers that 
arise from the parafollicular C cells of the thyroid gland. Com­
pared with differentiated thyroid cancers, MTC is associated 
with less favorable outcomes (Fig. 14.21), including persistent or 
recurrent disease, need for reoperation, and mortality.288 How­
ever, the prognosis and associated risk are highly variable, in large 
part

ssociated risk are highly variable, in large 
part determined by the specific molecular mutation detected in 
the MTC. On histologic assessment, MTC readily invades the 
intraglandular lymphatics and spreads to other parts of the gland, 
in addition to the pericapsular and regional lymph nodes. It also 
regularly spreads through the bloodstream to the lungs, bones, 
and liver.14,155,156,289–291
Histologic Diagnosis of Medullary Thyroid Carcinoma
MTC tumors are firm and usually unencapsulated. On

MTC tumors are firm and usually unencapsulated. On histo­
pathologic examination, the tumor is composed of cells that vary 
in morphologic features and arrangement. Round, polyhedral, 
and spindle-shaped cells form a variety of patterns, which may 
vary from solid and trabecular to endocrine or glandular-like 
structures. An amyloid stroma is commonly present.146 Gross or 
microscopic foci of carcinoma may be present in other parts of the 
gland, and blood vessels may be invaded. In all cases, t

 and blood vessels may be invaded. In all cases, the diagnosis 
can be confirmed by positive immunostaining of tumor tissue for 
calcitonin and carcinoembryonic antigen (CEA). 
TERT 9%
Papillary thyroid tumors (TCGA, n = 381)
PDTC tumors (n = 84)
ATC tumors (n = 33)
TERT promoter:
BRAF 59%
NRAS 8%
HRAS 4%
KRAS 1%
TERT 40%
NRAS 21%
HRAS 5%
KRAS 2%
BRAF 33%
TERT 73%
C228T (–124)
C228A (–124)
BRAFIRAS:
C250T (–146)
CC242–243TT
Missense
NRAS 18%
HRAS 6%
KRAS 0%
BRAF 45%
• Fig. 14.19  Oncoprints of T

 6%
KRAS 0%
BRAF 45%
• Fig. 14.19  Oncoprints of TERT promoter mutations, BRAF, and RAS mutations (with reported rate) in 
PTCs from TCGA (n = 381), PDTCs (n = 84), and ATCs (n = 33). (From Landa I, Ibrahimpasic T, Boucai L, 
et al. Genomic and transcriptomic hallmarks of poorly differentiated and anaplastic thyroid cancers. J Clin 
Invest. 2016;126:1052–1066.)
461
Clinical Presentation of Medullary Thyroid 

ancies
Clinical Presentation of Medullary Thyroid Carcinoma
MTC first appears either as a hard nodule or mass in the thyroid 
gland or as an enlargement of the regional lymph nodes. Occa­
sionally a metastatic lesion in a distant site is found first. The neck 
masses are frequently painful; they are sometimes bilateral and are 
often localized to the upper two-thirds of each lobe of the gland, 
which reflects the anatomic location of the parafollicular cells.
Differentiation of sporadic MTC from

icular cells.
Differentiation of sporadic MTC from other types of thy­
roid nodules on clinical grounds alone may be difficult. FNAB 
has made it possible to diagnose MTC before surgery. In some 
1.0
0.8
0.6
0.4
0.2
0.0
0
10
20
30
Time (months)
40
Surgery
 No Surgery
50
Log rank test: p <0.001 
Stage IV A
1.0
0.8
0.6
0.4
0.2
0.0
0
10
20
30
40
Surgery
 No Surgery
50
Log rank test: p =0.074
Stage IV B
1.0
0.8
0.6
0.4
0.2

age IV B
1.0
0.8
0.6
0.4
0.2
0.0
0
10
20
30
40
Surgery
50
Log rank test: p =0.009
Stage IV C
 No Surgery
• Fig. 14.20  Overall survival of patients with ATC who did and did not undergo surgery, grouped by AJCC/
TNM stage. (From Goffredo P, Thomas SM, Adam MA, et al. Impact of timeliness of resection and thyroid­
ectomy margin status on survival for patients with anaplastic thyroid cancer: an analysis of 335 cases. Ann 
Surg Oncol. 2015;22:4166–4174.)
8th Editio

s. Ann 
1.0
0.8
0.6
p < 0.001
I
II
III
IV
0.4
0.2
0.0
0
12 24 36 48 60
Time from diagnosis (months)
72 84 96 108120132
stage=I
stage=II
stage=III
stage=IV
1.0
0.8
0.6
p < 0.001
I
II
III
IV
0.4
0.2
0.0
0
Disease-specific survival
stage=I
stage=II
stage=III
stage=IV
• Fig. 14.21  Overall survival according to pathologic tumor-node-metastasis 

val according to pathologic tumor-node-metastasis (pTNM) in the National Cancer 
Database Cohort. (Redrawn from Pontius LN, Oyekunle TO, Thomas SM, et al. Projecting survival in papil­
lary thyroid cancer: a comparison of the seventh and eighth editions of the American Joint Commission 
on Cancer/Union for International Cancer Control staging systems in two contemporary national patient 
cohorts. Thyroid. 2017;27:1408–1416.)
462
patients, however, cytologic findings may be

yroid
patients, however, cytologic findings may be misleading because 
the type of carcinoma is difficult to determine and HCC may 
occasionally be confused with MTC.146,162 Positive immunocyto­
chemical staining for calcitonin allows confirmation of the diag­
nosis. Basal plasma calcitonin levels are elevated in virtually all 
patients with clinical MTC, but it is still controversial whether it 
should be performed in all patients with thyroid nodules or only 
in those with suspicious or malign

odules or only 
in those with suspicious or malignant cytologic findings.81,82
When the diagnosis of MTC is made from calcitonin measure­
ments or FNAB, patients should be evaluated for hyperparathy­
roidism and for pheochromocytoma, unless a hereditary form of 
the disease has been excluded. If these diagnoses are satisfactorily 
excluded, total thyroidectomy with removal of regional nodes can 
safely be performed.14,292–295
The tumor occurs in both sporadic and hereditary forms, the 
latter ma

both sporadic and hereditary forms, the 
latter making up about 20% to 30% of the total. The hereditary 
variety arises as part of MEN syndrome type 2A or 2B. A germline-
activating RET point mutation is found in almost all hereditary 
cases, and RET proto-oncogene testing should be performed in 
all MTC patients. The finding of a germline RET mutation indi­
cates a hereditary disease; the mutation should then be sought in 
all first-degree family members. The hereditary form is typically 
bilat

y members. The hereditary form is typically 
bilateral and is usually preceded by a premalignant C-cell hyper­
plasia. Total thyroidectomy at this premalignant stage can cure the 
disease in more than 95% of cases.14,81,82,291–293
As mentioned, there is a strong relationship in MTC between 
genotype and phenotype: most MEN2B are due to a codon 918 
mutation in exon 16; the most frequent mutation found in patients 
with MEN2A is a codon 634 mutation in exon 11; the other muta­
tions are located i

on in exon 11; the other muta­
tions are located in exons 10, 13, 14, and 15 and are usually associated 
with less aggressive phenotypes. Somatic RET mutations are found 
in 40% of sporadic MTCs, with the codon 918 mutation being the 
most frequent and associated with a more aggressive course.296 In up 
to two-thirds of tumors with no RET mutation, a RAS point muta­
tion was found in most studies297–299 but not in all.300
Cushing syndrome may occur at an advanced stage of the dis­
ease because o

ur at an advanced stage of the dis­
ease because of secretion of corticotropin by the tumor. Prosta­
glandins, serotonin, kinins, and vasoactive intestinal peptide may 
also be secreted and are variously responsible for flushing and for 
the attacks of watery diarrhea that about one-third of patients 
experience, usually at an advanced stage of the disease.155,156,290,291 
In MEN2A, hyperparathyroidism occurs late and is usually mild. 
Pheochromocytomas invariably occur later than MTC; they are 

cytomas invariably occur later than MTC; they are 
often bilateral and may be clinically silent, and patients at risk 
should be screened with measurements of urinary metaneph­
rine excretion. Familial MTC is a variant of MEN2A. MTC is 
transmitted as a single entity without any associated abnormality 
in the family and usually occurs later in life and is less aggres­
sive than MTC occurring in the context of other subgroups of 
MEN2A; pheochromocytoma should be screened even if its risk 
is low

cytoma should be screened even if its risk 
is low because it cannot be totally excluded in any hereditary 
form. In MEN2B, MTC and pheochromocytomas are associated 
with multiple mucosal neuromas (bumpy lip syndrome), a mar­
fanoid habitus, and typical facies, but such patients do not have 
hyperparathyroidism.14,291
In patients with a family history of thyroid cancer associated 
with hypertension or hyperparathyroidism, the MEN2A syn­
drome should be suspected. In patients with MEN2, surgery 

uld be suspected. In patients with MEN2, surgery 
should be performed for pheochromocytomas before surgery for 
MTC is performed.
First-degree relatives of patients with MEN should undergo 
DNA testing for the presence of the mutant RET gene (see 
Chapter 42). Gene carriers should undergo a prophylactic total 
thyroidectomy at an age that depends on the mutation: within the 
first year of life for those with MEN2B and before age 5 years for 
those with the 634 RET mutation (the most frequent).14

e with the 634 RET mutation (the most frequent).14,292,293 
For carriers of other mutations, prophylactic total thyroidectomy 
may be delayed beyond age 5 years in the setting of a normal 
annual basal serum calcitonin, normal annual neck ultrasonog­
raphy, less aggressive MTC family history, and family preference. 
Surgery is indicated if all of these features are not present and 
consists of a total thyroidectomy with lymph node dissection; 
lymph node dissection may be obviated when the thyro

mph node dissection may be obviated when the thyroid nodule 
is smaller than 5 mm, when there is no lymph node abnormality 
on neck ultrasonography, and when the plasma calcitonin level is 
less than 40 pg/mL.14 
Prognosis for Medullary Thyroid Carcinoma
Early series of MTC mainly described sporadic cases in which 
80% of patients presented with TNM stage II or III. Patients 
with MEN2A are diagnosed earlier and have curable (stage I) 
disease.291,292 Patients with MTC now have outcomes similar 

1,292 Patients with MTC now have outcomes similar 
to or better than those of patients with nonpapillary FCTC (see 
Fig. 14.7). In a study of patients undergoing surgery for MTC 
in California, independent risk factors for disease-specific mortal­
ity were older age (HR, 1.36 per decade; 95% CI, 1.17–1.59), 
tumor size exceeding 2 cm (HR, 2.83; 95% CI, 1.08–7.44 for 
>2 to 4 cm and HR, 2.89; 95% CI, 1.09–7.71 for >4 cm), and 
the administration of external beam radiotherapy (HR, 2.14; 95% 
CI, 1

f external beam radiotherapy (HR, 2.14; 95% 
CI, 1.23–3.71). Disease stage was the strongest predictor of mor­
tality from regional (HR, 4.77; 95% CI, 2.29–9.94) or metastatic 
(HR, 21.08; 95% CI, 9.90–44.89) disease.288 
Primary Malignant Lymphoma of the Thyroid
Primary thyroid lymphomas are rare tumors, accounting for less 
than 2% of all extranodal lymphomas and less than 3% of all 
malignant thyroid tumors. The peak incidence is during the sev­
enth decade of life and the male-to-female rati

v­
enth decade of life and the male-to-female ratio is 1:2–8.301
Primary thyroid lymphoma almost always has a B-cell lin­
eage.301 The majority of thyroid malignant lymphomas are 
mucosa-associated lymphoid tissue (MALT) lymphomas that usu­
ally arise in a background of Hashimoto thyroiditis. These small 
cell lymphomas are characterized by a low grade of malignancy, a 
slow growth rate, and a tendency for recurrence at other MALT 
sites such as the gastrointestinal or respiratory tract, the thy

the gastrointestinal or respiratory tract, the thymus, 
or the salivary glands. At diagnosis, diffuse large cell lymphomas 
account for about 70% to 80% of tumors, and a substantial pro­
portion of clinical cases arise from the transformation of low-
grade MALT lymphoma to high-grade B-cell lymphoma. Other 
histologic findings are rare.
Clinical thyroid lymphomas almost invariably present as a rap­
idly enlarging, painless neck mass. One-third of the patients have 
compressive symptoms. The mass

 the patients have 
compressive symptoms. The mass is often fixed to surrounding 
tissues, and half of the patients have unilateral or bilateral cervi­
cal lymph node enlargement. Clinically evident distant disease is 
uncommon. About 20% of patients already have a long-standing 
goiter, and hypothyroidism is reported in up to 40% of cases. The 
palpated mass is solid and hypoechoic on ultrasonography, which 
often depicts a characteristic asymmetric pseudocystic pattern. 
Most patients have ser

tric pseudocystic pattern. 
Most patients have serum antiperoxidase and anti-Tg antibodies.
The diagnosis of lymphoma can often be established by FNA 
cytologic and flow cytometric tests, particularly in diffuse large 
B-cell type. The need for core-needle or surgical biopsies has 
463
decreased, but they can still be helpful for distinguishing thyroid­
itis from low-grade MALT lymphoma and for reliably excl

from low-grade MALT lymphoma and for reliably excluding 
aggressive histologies because histology, immunostaining, and 
testing for gene rearrangements may be needed.
Accurate staging is critical for treatment planning. Stag­
ing includes a physical examination; complete blood count; 
serum lactate dehydrogenase; liver function tests; bone marrow 
biopsy; CT scan or MRI of the neck; CT scan of the thorax, 
abdomen, and pelvis; and appropriate biopsies at other sites 
where tumor is suspected. 18

psies at other sites 
where tumor is suspected. 18FDG-PET scanning may be useful 
for the initial diagnosis and for monitoring responses to treat­
ment. Involvement of the Waldeyer ring and the gastrointes­
tinal tract has been associated with thyroid lymphomas, and 
therefore upper gastrointestinal tract radiographs or endoscopy 
should be performed.
Treatment is guided by the histologic subtype, the extent of the 
disease, and (in case of diffuse large B-cell lymphoma) the age-
adjusted intern

se large B-cell lymphoma) the age-
adjusted international prognostic index.302,303 Surgical debulk­
ing of thyroid lymphomas is neither feasible nor necessary. Small 
tumors are often treated initially as primary thyroid carcinomas 
with surgery, and additional radiotherapy may be necessary in case 
of indolent lymphoma.
For high-grade B-cell lymphoma, chemotherapy combined 
with rituximab (chimeric human-mouse anti-CD20 monoclonal 
antibody) has become the standard treatment.304 The chemo­
ther

 become the standard treatment.304 The chemo­
therapy prescribed should be an anthracycline-based regimen. It 
usually consists of four to six cycles of the CHOP regimen (cyclo­
phosphamide 750 mg/m2 on day 1, doxorubicin 50 mg/m2 on 
day 1, vincristine 1.4 mg/m2 on day 1, and prednisone 40 mg/m2 
per day on days 1–5) every 3 weeks. For localized aggressive lym­
phoma, the combination of chemotherapy and radiotherapy used 
before the era of rituximab reduced distant recurrence compared 
with rad

imab reduced distant recurrence compared 
with radiotherapy alone, which should be used only for elderly 
patients who cannot receive medical treatment because a recur­
rence at distant sites will occur in one third of the patients and 
generally within the first year of treatment.
For MALT lymphomas, if disease is localized after accurate 
staging, total thyroidectomy (predicted overall survival rate and 
disease-free survival, 100% at 5 years) or involved field radiation 
therapy alone, 2 Gy p

or involved field radiation 
therapy alone, 2 Gy per fraction for 5 days per week up to a total 
dose of 30 to 40 Gy (5-year overall survival rate, 90%) may be 
adequate.305 For disseminated MALT lymphoma, chemotherapy 
alone with a single agent such as chlorambucil or combined with 
local radiation therapy can be proposed. 
The Surgical Treatment of Thyroid 
Malignancy
Surgical resection of the primary tumor, gross disease that has 
extended beyond the thyroid capsule, and clinically significan

ond the thyroid capsule, and clinically significant 
lymph node metastases remains the mainstay of initial therapy 
in thyroid cancer.1 Effective initial surgery minimizes the risk of 
disease recurrence, improves disease-specific survival, prevents 
complications associated with local growth of tumor into impor­
tant structures in the neck, facilitates postoperative radioactive 
iodine therapy if indicated, and facilitates accurate staging and 
risk stratification.
Until recently, a one-size-fi

Until recently, a one-size-fits-all approach to the management 
of thyroid cancer meant that the vast majority of thyroid cancer 
patients were subjected to high-intensity management approaches 
(total thyroidectomy with or without prophylactic central neck 
dissection, radioactive iodine therapy, prolonged TSH suppression, 
and highly sensitive follow-up evaluations).23,306 However, the 
increasing incidence of very low-risk thyroid cancers (which usu­
ally display an indol

 thyroid cancers (which usu­
ally display an indolent disease course) coupled with a much more 
selective use of radioactive iodine (RAI) therapy has resulted in a 
reevaluation of the need for immediate high-intensity treatment 
options in low-risk differentiated thyroid cancer patients.307–310 
The current thyroid cancer guidelines now endorse consider­
ation of low-intensity treatment options (observation or thyroid 
lobectomy without the need for RAI therapy or prolonged TSH 
suppressive the

 for RAI therapy or prolonged TSH 
suppressive therapy) in properly selected patients.1,311 Nonethe­
less, it is important to emphasize that some combination of high-
intensity treatment options is still recommended for most patients 
with intermediate-risk or high-risk thyroid cancers. Because the 
evidence evaluating the comparative effectiveness of high-inten­
sity versus low-intensity treatment options is largely retrospective, 
observational and devoid of high-quality prospective randomized

 and devoid of high-quality prospective randomized 
clinical trials, the recommendations in these major guidelines are 
largely based on expert opinion in which the committees are trying 
to balance the risk and benefits of immediate therapy in low-risk 
thyroid cancer patients. Reevaluation of the intensity of thyroid 
cancer therapies is imperative because thyroid cancer mortality has 
remained stable or slightly increased over the last several decades75 
despite widespread use of highly sensi

 decades75 
despite widespread use of highly sensitive disease detection tools 
(such as high-resolution neck ultrasonography, FNA of small thy­
roid nodules, and ultrasensitive thyroglobulin assays) and increas­
ing use of aggressive therapies (such as prophylactic and therapeutic 
neck dissections for small volume disease and repeated administra­
tions of RAI for biochemical evidence of persistent disease).
Active Surveillance as an Alternative to 
Immediate Surgery in Low-Risk Differentiated 

 to 
Thyroid Cancer
When thyroid nodule evaluations are done according to the rec­
ommendations of the American Thyroid Association, FNA cytol­
ogy that is diagnostic for thyroid cancer will almost always lead 
to thyroid surgery.1 However, an active surveillance management 
approach (also known as deferred surgical intervention) can be 
considered as an alternative to immediate surgery in (a) healthy 
patients with very low-risk intrathyroidal papil

patients with very low-risk intrathyroidal papillary thyroid can­
cers, (b) patients with comorbid conditions that would increase 
the risk of surgery or be associated with relatively short life expec­
tancy, or (c) patients with more pressing concurrent medical or 
surgical issues that take precedence over treatment of the thyroid 
cancer.1
Building on the experience of our Japanese colleagues, several 
publications now support the effectiveness of an observational, 
active surveillance manag

ss of an observational, 
active surveillance management approach to low-risk papillary 
thyroid cancers.1,312–315 In these studies, serial neck ultrasound 
evaluations of the thyroid and cervical lymph nodes at intervals 
of 6 to 12 months are offered to properly selected patients as an 
alternative to immediate surgery. While the majority of patients 
demonstrate little if any disease progression during observation, 
salvage therapy at the time of disease progression is very effective 
when nee

of disease progression is very effective 
when needed.312
Obviously the key to safe and effective implementation of an 
active surveillance management program is proper patient selec­
tion. A recent clinical framework describes how consideration of 
(a) imaging and clinical findings, (b) medical team characteristics, 
and (c) patient characteristics can be used to classify patients as 
ideal, appropriate, or inappropriate for low-risk treatment options 
464
(active surveil

ptions 
464
(active surveillance or thyroid lobectomy) (Fig. 14.22).316,317 The 
definitions associated with the ideal, appropriate, and inappropri­
ate patients with regard to the selection for active surveillance are 
given in Table 14.8. Although there are multiple patient character­
istics that influence decision making, the construct that describes 
patients as medical maximalists or medical minimalists appears to 
be particularly relevant.318 Given the choice, a medi

particularly relevant.318 Given the choice, a medical maximalist 
often chooses high-intensity treatment options while a medical 
minimalist will often select low-intensity treatment options.318
Patients classified as ideal or appropriate for active surveillance 
are offered observation as an alternative to immediate surgery. 
Patients choosing active surveillance are followed with neck ultra­
sound evaluations every 6 to 12 months for several years (then 
less frequently over time) unless disea

ars (then 
less frequently over time) unless disease progression occurs or the 
patient opts for surgery even in the absence of disease progres­
sion. Interestingly, it appears that only about 50% of patients 
eligible for active surveillance will choose an initial observational 
management program while the remainder elect to proceed with 
immediate surgery.312 Patients classified as being inappropriate for 
active surveillance are recommended to have appropriate surgical 
resection.316,317 
Se

o have appropriate surgical 
Selecting Total Thyroidectomy or Thyroid 
Lobectomy in Differentiated Thyroid Cancer: A 
Risk-Adapted Approach
There continues to be uniform agreement that total thyroidec­
tomy is the preferred treatment option for patients with (a) dif­
ferentiated thyroid cancers with primary tumors larger than 4 cm, 
(b) gross extrathyroidal extension, (c) clinically apparent disease 
Imaging and
Clinical Findings
Medical Team
Characteristics
Patient
Characteri

astases.1,311 
Likewise, there is a general consensus that thyroid lobectomy is 
an adequate oncologic procedure for papillary thyroid cancers 
smaller than 1 cm without extrathyroidal extension or clinically 
apparent lymph node metastases.
Thus preoperative evaluations designed to carefully determine 
the size of the primary tumor, the presence of extrathyroidal 
extension, and/or metastatic disease assume primary importance 
in medical decision making. These evaluations include neck 
ultrason

n making. These evaluations include neck 
ultrasonography (evaluation of the thyroid gland, surrounding 
structures, and cervical lymph nodes) and voice evaluation in all 
patients being considered for surgery for differentiated thyroid 
cancer.319 Additional functional and imaging studies are usu­
ally required for surgical planning and adequate initial staging in 
patients demonstrating bulky lymph node metastases, evidence of 
local invasion, or distant metastases. Endoscopic evaluation of th

or distant metastases. Endoscopic evaluation of the 
airway or gastrointestinal tract may be necessary to determine the 
extent of involvement in locally invasive disease.
Decision making in patients with intrathyroidal differenti­
ated thyroid cancers with primary tumors larger than 1 cm and 
smaller than 4 cm without evidence of extrathyroidal extension 
or clinically apparent lymph node metastases has become com­
plicated since both the National Comprehensive Cancer Network 
(NCCN) and the AT

al Comprehensive Cancer Network 
(NCCN) and the ATA guidelines allow for either thyroid lobec­
tomy or total thyroidectomy in this setting.1,311 Largely based on 
the assumption that nearly all of these patients would require RAI 
therapy and further strengthened by the Bilimoria report demon­
strating 10-year survival of 98.4% for total thyroidectomy versus 
97.1% for thyroid lobectomy (p <0.05),320 the ATA guidelines 
had previously strongly recommended total thyroidectomy for all 
tumors over

commended total thyroidectomy for all 
tumors over 1 cm.23,306,311 However, multiple publications have 
failed to convincingly demonstrate a statistically significant sur­
vival benefit for total thyroidectomy versus lobectomy in differen­
tiated thyroid cancers less than 4 cm when patients are properly 
selected and the statistical analysis controls for important con­
founding variables.321–327 Decision making must also consider 
the risk of surgical complications and recognize that the risks o

gical complications and recognize that the risks of 
recurrent laryngeal nerve injury, transient and permanent hypo­
parathyroidism, and hemorrhage/hematoma are increased follow­
ing total thyroidectomy compared with thyroid lobectomy.328–330 
However, total thyroidectomy is associated with a slightly lower 
risk of recurrence than a thyroid lobectomy.327 Nonetheless, expe­
rienced centers report recurrence rates of less than 1% to 4% by 
combining high-quality preoperative ultrasound with appro

ng high-quality preoperative ultrasound with appropriate 
clinical judgment.325–327,331 These same studies show that salvage 
therapy is very effective in the few patients who have disease recur­
rence after low-intensity initial therapy.
Building on the same basic clinical framework described for 
active surveillance (see Fig. 14.22), patients can be classified as 
ideal, appropriate, or inappropriate for thyroid lobectomy (or 
isthmusectomy) (Table 14.9).317 However, unlike active sur­
veillan

able 14.9).317 However, unlike active sur­
veillance, patients selected for lobectomy must understand that 
information may become available either intraoperatively or post­
operatively (primarily the final pathology report) that could result 
in a recommendation for complete removal of the thyroid gland. 
Thus the final classification of the patient as ideal, appropriate, 
  Preoperative Classification System to Aid in Proper Patient Selection for Lobectomy/Isthmusectomy in 
Differentiated Thy

idisciplinary team; RAI, radioactive iodine; TSH, thyroid-stimulating hormone; US, ultrasound.
   
TABLE 14.9
466
or inappropriate requires consideration of additional factors that 
can only be known postoperatively (Table 14.10). In centers that 
practice a very risk-adapted management approach and a selective 
approach to RAI therapy, immediate completion thyroidectomy 
after lobectomy is only required in 5% to 6% of properly selected 
patients.325–327,331 However, other

erly selected 
patients.325–327,331 However, other centers report that the findings 
on the lobectomy final histology report result in recommenda­
tions for immediate completion thyroidectomy in up to 20% of 
the patients selected for thyroid lobectomy.332–334 
Determining the Extent of Initial Cervical Lymph 
Node Dissection in Differentiated Thyroid Cancer
Consistent with the goal of primary therapy to remove all vis­
ible gross disease, compartment-oriented resection of clinically 
apparent m

tment-oriented resection of clinically 
apparent metastatic cervical lymphadenopathy is routinely recom­
mended.1,311 This would include removal of all visible metastatic 
lymph nodes in the central neck (level VI/VII) and/or lateral neck 
(levels II, III, IV, and V). Rather than “berry picking” abnormal 
lymph nodes, a compartment-oriented neck dissection is designed 
to clear all metastatic lymph nodes by systematically removing 
the fibroadipose tissue in the compartment (which often contains

se tissue in the compartment (which often contains 
additional subclinical metastatic lymph nodes) with the clinically 
apparent lymph node metastases. This compartment-oriented 
approach to the removal of clinically apparent lymph node metasta­
ses is associated with decreased rates of recurrent/persistent disease.
Since very small-volume, subclinical lymph node micrometas­
tases are present in up to 70% to 80% of patients with papillary 
microcarcinomas, it is not surprising that meticulous di

arcinomas, it is not surprising that meticulous dissection of 
central neck lymph nodes would often yield metastatic disease.335 
However, preoperative neck ultrasonography identifies suspicious 
cervical lymphadenopathy (clinical N1a or N1b disease) in only 
20% to 30% of cases.336–341 It remains controversial whether rou­
tine removal of subclinical central neck lymph nodes (prophylactic 
dissection) has a clinically significant impact on the risk of recur­
rence or disease-specific survival i

isk of recur­
rence or disease-specific survival in papillary thyroid cancer.342 
Prophylactic neck dissections are not routinely recommended for 
follicular thyroid cancer because they are associated with a much 
lower rate of subclinical lymph node metastases. While prophylactic 
neck dissection can be done safely with a low risk of complications 
in experienced hands, the increased risk of hypoparathyroidism 
and recurrent laryngeal nerve injury may outweigh the potential 
benefit when perfor

ry may outweigh the potential 
benefit when performed by less experienced surgeons.342 
Surgical Approach to Medullary Thyroid Cancer
Medullary thyroid cancer is usually treated with total thyroidec­
tomy and surgical resection of clinically apparent central or lateral 
neck lymph nodes in a standard compartment-oriented fash­
ion.14,311 Depending on the results of preoperative calcitonin and 
neck ultrasound, prophylactic central or lateral neck dissections 
may also be recommended. 
Surgical A

 dissections 
Surgical Approach to Anaplastic Thyroid Cancer
Ideally, patients with anaplastic carcinoma should be treated with 
total thyroidectomy and therapeutic lymph node dissection, but 
these tumors usually present with locally invasive disease, which is 
seldom amenable to effective surgical resection.343 In the setting 
of unresectable disease, patients desiring high-intensity therapies 
are usually treated with a combination of systemic therapy and 
external b

 a combination of systemic therapy and 
external beam irradiation. 
Postoperative Management of Thyroid 
In view of disease-specific uncertainties and the unique needs of 
individual patients, the postoperative treatment of thyroid carci­
noma does not occur with a rigid algorithm.1 Indeed, physicians 
must consider the extent of disease at surgery, the histopathology 
and differentiation of the tumor, the age of the patient and his or 
her risk group category for tumor-related death 

her risk group category for tumor-related death and recurrence, 
and the results of postoperative serum Tg determination with 
neck ultrasonography.
131I Administration
131I is an effective agent for delivering high radiation doses to the 
thyroid tissue with low spillover to other portions of the body. 
  Postoperative Histologic Confirmation 
of Proper Patient Selection Following 
Lobectomy/Isthmusectomy in 
Differentiated Thyroid Cancer
Ideal
 • Intrathyroidal unifocal or multifocal papil

thyroidal extension
FTC, Follicular thyroid cancer; FVPTC, follicular variant of papillary thyroid carcinoma; HCC, 
Hürthle cell cancer; NIFTP, noninvasive follicular thyroid neoplasm with papillary-like nuclear 
features; PTC, papillary thyroid carcinoma.
   
TABLE 14.10
467
The radiation dose to the thyroid tissue is related to the tissue 
concentration (the ratio between the total tissue uptake and the 
v

e ratio between the total tissue uptake and the 
volume of functional tissue) and the effective half-life of 131I in the 
tissue.93,344 Thyroid tissue is able to concentrate iodine only after 
TSH stimulation, but even after optimal TSH stimulation, iodine 
uptake in neoplastic tissue is always lower than in normal thyroid 
tissue and may not be detectable in about one-third of cases.93
131I therapy is given postoperatively for three reasons.1 First, 
it destroys normal thyroid remnants (ablatio

rst, 
it destroys normal thyroid remnants (ablation), thereby increas­
ing the sensitivity and the specificity of measurements of serum 
Tg for the detection of persistent or recurrent disease. Ablation 
is mandatory for interpreting TSH-stimulated Tg determination 
because Tg may be produced by normal thyroid remnants and 
by neoplastic foci. During levothyroxine treatment, the produc­
tion of Tg by normal thyroid cells may be low or suppressed, and 
serum Tg can be used for the follow-up of th

 and 
serum Tg can be used for the follow-up of these patients.345 Sec­
ond, 131I therapy may destroy occult or known microscopic car­
cinoma, thereby potentially decreasing the long-term recurrence 
rate. Third, 131I therapy makes it possible to perform a postabla­
tive 131I total-body scan (TBS), which is a sensitive tool for detect­
ing persistent carcinoma.
It cannot be emphasized enough that postoperative 131I ther­
apy should be used selectively and that not all patients with a 
diagnosis 

ively and that not all patients with a 
diagnosis of FCTC benefit from routine postoperative radioio­
dine ablative therapy.1,22,23,224,346 The classification of the risk of 
recurrence (low, intermediate, or high) from the 2009 ATA guide­
lines was redefined in the 2015 guidelines, taking into account 
(among other criteria) the extent of lymph node involvement and 
pathologic features.1,23 In low-risk thyroid cancer, the long-term 
prognosis after surgery alone is so favorable that 131I ablati

ter surgery alone is so favorable that 131I ablation is 
not routinely recommended.1,22,23,347,348 This is widely applied to 
patients with microcarcinoma, but unfortunately routine treat­
ment is still given routinely to low-risk patients in some coun­
tries. In high-risk patients with a risk of recurrence above 40%, 
postoperative radioactive iodine administration (see Table 14.7) is 
routinely performed because such therapy can potentially decrease 
both recurrence and death rates. Also, radi

rease 
both recurrence and death rates. Also, radioiodine is administered 
postoperatively when surgery has not been complete or its success 
is doubtful. Young children have traditionally been considered as 
candidates for postoperative radioiodine therapy because they may 
have extensive neck lymph node involvement and frequently har­
bor pulmonary metastases that may not be detectable even with 
CT imaging of the chest.152,171,215 Finally, in the other patients 
(intermediate risk of recurren

the other patients 
(intermediate risk of recurrence, and some low risk), there is cur­
rently no evidence that radioiodine remnant ablation (RRA) may 
improve the long-term disease-specific mortality, and prospective 
randomized trials are needed to validate its current indications.
When the risk of persistent disease is low and postoperative 
serum Tg is undetectable, 131I administration may not be justified. 
This is especially true in patients with a low risk of recurrence.349–353 
This reco

s with a low risk of recurrence.349–353 
This recommendation is particularly relevant to patients with N0 
disease.349 On the contrary, the likelihood of identifying 131I avid 
metastatic disease on the post-therapy 131I TBS increases when 
postoperative Tg values are greater than 5 to 10 ng/mL, suggesting 
that 131I should be administered to such patients. Postoperative 
neck ultrasonography may also provide reassuring data or show 
abnormalities that should lead to 131I administration.354
When

s that should lead to 131I administration.354
When considering 131I therapy, levothyroxine treatment is 
most often administered following surgery, and both the indica­
tion and the protocol of stimulation and the 131I activity to be 
administered are decided according to the ATA risk stratification. 
Data from postoperative serum Tg determination and neck ultra­
sonography may also be taken into account because the risk of 
persistent/recurrent disease in case of undetectable Tg levels (<0.2 
n

disease in case of undetectable Tg levels (<0.2 
ng/mL under thyroid hormones or <1 ng/mL after TSH stimula­
tion) in the absence of Tg antibodies with normal neck ultraso­
nography is very low (<3%).
To administer 131I therapy, levothyroxine treatment is typi­
cally withheld for 3 to 4 weeks. Another approach allows liothy­
ronine to be substituted for 3 to 4 weeks and then discontinued 
for 1 to 2 weeks before radioiodine studies. At the time of treat­
ment, the serum TSH level should be great

f treat­
ment, the serum TSH level should be greater than an empirically 
determined level of 25 to 30 mU/L. Intramuscular injections of 
rhTSH (0.9 mg for 2 consecutive days, with 131I administered 
1 day after the second injection) given while on levothyroxine 
treatment may achieve an equally effective stimulation of radio­
iodine uptake by normal thyroid remnant, with ablation rates 
similar to those obtained with withdrawal using either a high 
(100 mCi) or a low (30 mCi) activity.352,355,3

gh 
(100 mCi) or a low (30 mCi) activity.352,355,356 The use of rhTSH 
prevents hypothyroidism (as the patient remains on levothyrox­
ine) and induces lower radiation exposure to the body, permit­
ting an earlier discharge from the hospital.352,355–359 However, in 
the United States, most 131I therapy is now administered in the 
outpatient setting, making this latter issue less of a concern. Sim­
ilar outcomes were reported in intermediate-risk and high-risk 
patients prepared for 131I therapy u

nd high-risk 
patients prepared for 131I therapy using rhTSH versus thyroid 
hormone withdrawal.360–362 In addition, short-term recurrence 
rates have been found to be similar in patients prepared with thy­
roid hormone withdrawal or rhTSH, even in those with initial 
lymph node involvement.363,364 A retrospective study reported a 
similar outcome at 10 years after ablation with a low activity in 
patients prepared with either withdrawal or rhTSH.365 rhTSH is 
approved for radioactive iodine adm

.365 rhTSH is 
approved for radioactive iodine administration with 100 mCi (or 
more) or 30 mCi in the United States, Europe, and many other 
countries around the world. The cost of rhTSH, however, must 
be factored into care, as thyroid hormone withdrawal is much 
less costly.
In a patient who has undergone an incomplete thyroidectomy, 
neck uptake may be measured with a tracer activity of 123I (or 
rarely, 131I). The activity used should be small enough to avoid 
stunning (i.e., a decrease of 

ll enough to avoid 
stunning (i.e., a decrease of thyroid uptake with the subsequent 
high activity of therapeutic radioiodine).366,367 High uptake 
(>10%) and high risk of persistent disease should lead to com­
pletion surgery. Following administration, a total body scan is 
performed and is highly informative in patients with low uptake 
(<1%) in the thyroid bed. However, additional metastatic foci 
have been reported in 10% to 26% of patients scanned follow­
ing high-dose radioiodine treatmen

scanned follow­
ing high-dose radioiodine treatment compared with the diagnos­
tic scan.368 131I SPECT/CT fusion imaging may provide superior 
lesion localization.24–27
After radioiodine therapy, levothyroxine therapy is then 
resumed and maintained. Total ablation (defined as no visible 
uptake) was previously verified by 131I TBS 6 to 12 months fol­
lowing initial therapy, typically with 2 to 5 mCi (74–185 MBq). 
However, a follow-up 131I TBS is no longer routinely performed 
when postablation

s no longer routinely performed 
when postablation scans have been informative because such 
repeat scanning does not afford any further information.369,370 
Moreover, total ablation is now currently defined by an undetect­
able serum Tg level following rhTSH stimulation in the absence 
of anti-Tg antibody (or a serum Tg level <0.2–0.3 ng/mL on levo­
thyroxine treatment when using a sensitive assay) and unremark­
able neck ultrasonography.1
468
Total ablation (eradication 

Total ablation (eradication of normal thyroid remnants) is 
achieved after administration of either 100 mCi (3700 MBq) 
or 30 mCi (1100 MBq) in more than 80% of patients who 
had at least a near-total thyroidectomy. This is similar whether 
preparation is with withdrawal or rhTSH.352,355,356,371,372 After 
less extensive surgery, ablation is achieved in only two thirds of 
patients with 30 mCi (1100 MBq). Therefore a total thyroidec­
tomy should be performed in all patients

hyroidec­
tomy should be performed in all patients who are to be treated 
with 131I. Also, in low-risk or intermediate-risk patients, levo­
thyroxine treatment is initiated soon after surgery and 30 mCi 
(1100 MBq) is administered following injections of rhTSH, 
with the aim of irradiating normal thyroid remnants. In high-
risk patients, higher activity (≥100 mCi) is administered with 
the aims of both ablating normal thyroid remnants and irradi­
ating residual neoplastic tissue. In patients wit

ating residual neoplastic tissue. In patients with known distant 
metastases, 131I is administered following thyroid hormone 
withdrawal, but in low-risk or intermediate-risk patients, 
rhTSH injections represent a valid alternative. High activities 
should be administered with caution to elderly patients.373
131I ablation therapy has no role in the management of patients 
with anaplastic thyroid cancer, MTC, or thyroid lymphoma. 
External Radiotherapy
External radiotherapy to the neck and medi

therapy
External radiotherapy to the neck and mediastinum is indicated 
only for older patients (>55 years) with extensive and aggressive 
pathology subtypes in whom complete surgical excision is impos­
sible, invasion of critical neck structures is likely, and tumor tissue 
does not take up 131I. Retrospective studies have shown that in 
these selected patients, external radiotherapy decreases the risk of 
neck recurrence.374,375 The target volume encompasses the thy­
roid bed, bilateral neck l

me encompasses the thy­
roid bed, bilateral neck lymph node areas, and the upper part 
of the mediastinum. Typically, 50 Gy (5000 rad) is delivered in 
25 fractions over 5 weeks, with a boost of 5 to 10 Gy on any 
residual macroscopic focus. The current approach with intensity-
modulated radiation therapy permits the delivery of 63 to 66 Gy 
to gross disease and high-risk areas and 54 to 56 Gy in 30 to 33 
fractions to cervical and mediastinal nodal regions. Its use may 
decrease late morbidity.

dal regions. Its use may 
In patients with anaplastic thyroid carcinoma, when the extent 
of disease is limited and surgery is feasible, accelerated external 
radiotherapy in combination with chemotherapy permits local 
control of the disease in two-thirds of the patients and long-term 
survival in about 20%.281,376,377 
TSH Suppressive Therapy
The growth of thyroid tumor cells is controlled by TSH, and 
inhibition of TSH secretion with levothyroxine is thought to 
impro

 secretion with levothyroxine is thought to 
improve the recurrence and survival rates.22,23,137 Therefore levo­
thyroxine should be given to all patients with FCTC, whatever the 
extent of thyroid surgery and other treatment. The initial effec­
tive dose is about 1.6 to 2 μg/kg body weight in adults, though 
children require a higher dose and elderly patients a lower dose. 
The adequacy of therapy is monitored by measuring serum TSH 
approximately 3 months after initiation. The initial goal is 

ly 3 months after initiation. The initial goal is a serum 
TSH concentration below about 0.1 mU/L for high-risk thyroid 
cancer patients and maintenance of the TSH at or slightly below 
the lower limit of normal (0.1–1.5 mU/L) for low-risk patients. 
Similar recommendations apply to low-risk patients who have not 
undergone remnant ablation (i.e., serum TSH 0.1–1.5 mU/L). All 
TSH targets must also weigh the individual risks associated with 
TSH suppressive therapy. The dose of levothyroxine is 

suppressive therapy. The dose of levothyroxine is then modi­
fied according to the ATA initial response to therapy. In patients 
with no evidence of disease, the dose of levothyroxine is decreased 
to maintain the serum TSH level within the normal range. Thus 
the dosing of levothyroxine (and target TSH concentration) must 
be continually reevaluated based on disease risk and potential side 
effects of long-term TSH suppression.
In patients with MTC or thyroid lymphoma, a replacement 
dose of le

MTC or thyroid lymphoma, a replacement 
dose of levothyroxine is given with the aim of obtaining a serum 
TSH level in the normal range. 
Long-Term Follow-Up of Patients With 
Thyroid Cancer Following Initial Therapy
After completion of initial therapy, the risk-adapted approach 
to the management of thyroid cancer continues by tailoring the 
types, extent, and timing of follow-up evaluations to the pre­
dicted risks of recurrence and disease-specific survival.307 Ini­
tial treatment and early f

cific survival.307 Ini­
tial treatment and early follow-up recommendations are based 
on the initial static risk assessments provided by AJCC staging 
(stage I, II, III, or IV) and ATA risk stratification (low, interme­
diate, or high).1,217 These initial risk estimates are then continu­
ally modified over time as new data become available so that the 
management plan can be appropriately altered to become either 
more or less intensive, depending on the biologic course of the 
disease and the r

g on the biologic course of the 
disease and the response to therapy.1 Over the last several years, 
a nomenclature has been developed to describe the response to 
therapy and clinical status of patients during follow-up (Table 
14.11).1,378,379 Unlike the AJCC stage and the ATA risk cat­
egory, which do not change over the lifetime of the patient, the 
response to therapy definitions may change during follow-up as 
new data become available (Fig. 14.23). This approach allows 
for characterizati

. 14.23). This approach allows 
for characterization of the clinical status of each patient based 
on initial AJCC stage, ATA risk, and response to therapy at 
each follow-up visit. This information can then be used to guide 
management recommendations both with regard to the selec­
tion of follow-up test and also possible indications for addi­
tional therapies or ongoing observational management.
  Response to Therapy Definitions
Excellent response
No clinical, biochemical, or structural 
evid

onse
evidence of disease
Biochemical incomplete 
response
Persistent abnormal thyroglobulin values or 
rising antithyroglobulin antibody levels in 
the absence of localizable disease
Structural incomplete 
response
Persistent or newly identified locoregional or 
Indeterminate response
Nonspecific biochemical or structural find­
ings that cannot be confidently classified 
as either benign or malignant
Regardless of the extent of initial 

 or malignant
Regardless of the extent of initial therapy, this provides the general description of each of the 
responses to therapy categories that can be used to describe the clinical status of a patient 
with either differentiated or medullary thyroid cancer at any point during follow-up. See text 
for precise definitions of excellent, indeterminate, and biochemical incomplete responses, 
which will vary by tumor type and extent of initial therapy.
   
TABLE 14.11
469
Chapter 14  Nontoxic Di

erapy.
   
469
Initial Follow-Up Recommendations for 
Differentiated Thyroid Cancer (First Year After 
Initial Therapy)
Regardless of ATA risk, most patients are seen at intervals of 3 to 6 
months with biochemical testing during the first year of follow-up 
(Table 14.12). ATA low-risk patients are expected to do exception­
ally well with very low recurrence rates that probably would not 
become 

 recurrence rates that probably would not 
become apparent for at least 3 to 5 years.167,219 Thus the primary 
follow-up tools for these patients will be physical examination, 
serum thyroglobulin, thyroglobulin antibody, and thyroid func­
tion test.1 In these low-risk patients, the primary goal of follow-
up is to confirm the absence of disease in early follow-up so that 
they can rapidly be transitioned to less intensive management 
approaches. They usually return for follow-up visits every 6 

 They usually return for follow-up visits every 6 to 12 
months for the first year then less frequently after that if they dem­
onstrate the expected excellent response to therapy. Neck ultraso­
nography is usually done about 1 year after surgery, although the 
utility of this test is unproven and has the potential to identify 
more false-positive findings than real disease given the high likeli­
hood of nonspecific findings on surveillance neck ultrasonography 
coupled with a very low recurrenc

ultrasonography 
coupled with a very low recurrence risk.380,381 Diagnostic whole-
body scans, additional cross-sectional imaging, or FDG-PET scans 
are not routinely used for follow-up surveillance. Stimulated thy­
roglobulin values are only done if the available thyroglobulin assay 
does not reliably read down to less than 0.2 ng/mL.
ATA intermediate-risk patients are initially followed at 6-month 
intervals for the first year with the same biochemical testing as the 
low-risk patients but wit

chemical testing as the 
low-risk patients but with more intensive ultrasound follow-up, 
given their higher risk of disease.1 Intermediate-risk patients usually 
have follow-up ultrasound in 6 months to establish the baseline and 
verify the completeness of initial resection. Additional ultrasounds 
during the first year may be required if this initial evaluation identi­
fies worrisome nonspecific findings or persistent disease. In patients 
who had extensive bulky metastatic lymphadenopathy, a

 had extensive bulky metastatic lymphadenopathy, a CT scan 
of the neck and chest with contrast is usually considered at 6 to 12 
months to definitively establish the presence or absence of clinically 
significant persistent disease. Radioactive iodine scans are not rou­
tinely obtained for surveillance in these patients but can be impor­
tant tools to characterize the functional status of suspicious lesions 
identified during follow-up and can be considered if thyroglobulin 
antibodies are risi

e considered if thyroglobulin 
antibodies are rising during follow-up. Additional testing is not rou­
tinely planned but may be required, depending on the response to 
therapy, as described later in the chapter.
ATA high-risk patients require a much more individualized 
management approach.1 While still utilizing the same biochemi­
cal evaluations as the ATA low-risk and intermediate-risk patients, 
ATA high-risk patients may also require a wide variety of cross-
sectional and imaging studies on

variety of cross-
sectional and imaging studies on the basis of either a structural 
incomplete response to initial therapy or their high risk of local 
recurrence and distant metastases. In general, high-risk patients 
are evaluated every 2 to 3 months with appropriate cross-sectional 
and functional imaging. Because of the aggressive nature of their 
disease, their response to therapy can often be characterized within 
the first 6 to 12 months, leading to early alterations in the initial 
mana

 leading to early alterations in the initial 
management plans. 
  Overview of Plans for First Year of 
Follow-Up Following Initial Therapy
Initial Plan Based on 
ATA Risk for the First 
Year of Follow-Up
ATA Low 
Risk
ATA 
Intermediate 
Risk
ATA High 
Risk
Tg, TgAb, TFTs, every 
3–6 months
√
√
√
Neck US in 3–6 months
–
√
√
Neck/chest CT with con­
trast in 6–12 months
–
Considera
√b
Cross-sectional imaging 
of other sites (brain, 
abdomen, pelvis)
–
–
Considerc
Routine surveillance 
diagnostic 

s)
–
–
Considerc
diagnostic RAI scan
–
–
Consider
18FDG-PET scan
–
–
Consider
Dynamic risk assessment 
at each visit
√
√
√
While most patients will return for physical examination and biochemical testing every 3–6 
months for the first year, consideration for additional testing is based on ATA risk and on the 
dynamic risk assessment done at each follow-up visit.
aConsidered for intermediate-risk patient status postresection clinical N1a or N1b disease.
bDepending on presen

 clinical N1a or N1b disease.
bDepending on presenting features, CT of the neck/chest may need to be done as early as 
2–3 months after initial therapy.
cDepending on presenting features, functional imaging results, and serum Tg levels.
ATA, American Thyroid Association; CT, computed tomography; 18FDG-PET, fluorodeoxyglu­
cose positron emission tomography; RAI, radioactive iodine; Tg, thyroglobulin; TgAb, antithy­
roglobulin antibodies; TFTs, thyroid function tests; US, ultrasound;
   
TABLE 14.

roid function tests; US, ultrasound;
   
TABLE 14.12
Risk Assessment: Dynamic, Active Process
Initial Assessment of Risk of Recurrence/Death
AJCC & ATA Risk Estimates
Ongoing Risk Re-assessment
(Re-assessed at each follow-up visit)
Excellent
Response
Response
Response
Selection of Follow-Up Tests
Indications for additional therapies
Biochemical
Response
• Fig. 14.23  Risk-adapted approach to follow-up. Management plans for the 
first year of follow-

p. Management plans for the 
first year of follow-up are based on initial assessment of the risk of recurrence 
and death provided by American Joint Commission on Cancer (AJCC) and 
American Thyroid Association (ATA) risk staging. These plans are continu­
ally modified during follow-up as new data become available, with ongoing 
risk stratification defined by response to therapy assessments (excellent, 
biochemical incomplete, structural incomplete, or indeterminate response). 
Unlike the AJCC a

te, or indeterminate response). 
Unlike the AJCC and ATA risk staging, the response to therapy status may 
change over time, depending on disease behavior and response to therapy.
470
Modifying Initial Follow-Up Recommendations 
for Differentiated Thyroid Cancer Using 
Response to Therapy Definitions
While originally conceived to be used as a restaging system to be 
utilized after 2 years of follow-up, the dynamic risk assessment 
approach has evolved such that the respons

ssment 
approach has evolved such that the response to therapy definitions 
can be used at any point during follow-up to describe the clinical 
status of a patient.378,382,383 Thus it is appropriate even within the 
first year of follow-up to describe the clinical status of a patient 
using the response to therapy nomenclature (excellent response, 
biochemical incomplete response, structural incomplete response, inde­
terminate) (see Table 14.11). Even at these early time points, the 
response t

. Even at these early time points, the 
response to therapy can be used to alter the initial management 
plans. This is most apparent in patients who have ATA interme­
diate-risk disease who manifest an excellent response within the 
first year of initial therapy and can therefore have their manage­
ment intensity decreased. Or alternatively, ATA low-risk or inter­
mediate-risk patients demonstrating a biochemical or structural 
incomplete response may require more aggressive follow-up, dis­
eas

se may require more aggressive follow-up, dis­
ease detection studies, or therapy.
In oncologic terms, excellent response can be thought of as 
“remission” as it describes patients who have no biochemical, clin­
ical, or structural evidence of disease (see Table 14.11). Patients 
with abnormal thyroglobulin values or rising antithyroglobulin 
antibodies in the absence of localizable disease are described as 
having a biochemical incomplete response. Structural incomplete 
response is used to des

se. Structural incomplete 
response is used to describe patients who have persistent or recur­
rent imaging evidence of disease. The indeterminate response cat­
egory describes patients who have nonspecific either low-level 
thyroglobulin values or nonspecific findings on imaging that 
cannot be confidently classified as either benign or malignant. 
Over time, most patients can be moved out of the indeterminate 
response category as follow-up studies help to define whether the 
nonspecific findi

dies help to define whether the 
nonspecific findings are actually benign or malignant.
The precise definitions for each of the response to therapy cate­
gories and the management implications are given in Table 14.13.1 
An excellent response to therapy should lead to an early decrease 
in the intensity and frequency of follow-up and the degree of TSH 
suppression as these patients do exceptionally well with essen­
tially no mortality and very low recurrence rates. Interestingly, at 
least 30% o

w recurrence rates. Interestingly, at 
least 30% of the biochemical incomplete response patients will 
eventually evolve to having no evidence of disease with observa­
tion alone. Unfortunately, patients with a structural incomplete 
response usually maintained persistent disease (either biochemical 
or structural incomplete response) despite additional therapies. 
Fortunately, the majority of patients classified as having an inde­
terminate response remain disease-free with only 15% to 20% of 

onse remain disease-free with only 15% to 20% of 
these patients eventually being classified as having either biochem­
ical or structural incomplete response.
  Clinical Implications of Response to Therapy Reclassification in Differentiated Thyroid Cancer Patients 
Treated With Total Thyroidectomy and RAI Remnant Ablation
Category
Clinical Outcomes
Management Implications
Excellent Response
Negative imaging
and either suppressed Tg <0.2 ng/mLa 
or TSH-stimulated Tg <1 ng/mLa
1–4% re

0.2 ng/mLa 
1–4% recurrence <1% disease-
specific death
An excellent response to therapy should 
lead to an early decrease in the intensity 
and frequency of follow-up and the 
degree of TSH suppression
Biochemical Incomplete 
Response
and suppressed Tg >1 ng/mLa or
stimulated Tg >10 ng/mLa or rising 
anti-TgAb levels
At least 30% spontaneously evolve 
to NED; 20% achieve NED after 
additional therapy; 20% develop 
structural disease; <1% disease-

py; 20% develop 
If associated with stable or declining serum 
Tg values, a biochemical incomplete 
response should lead to continued 
observation with ongoing TSH suppres­
sion in most patients
Rising Tg or Tg antibody values should 
prompt additional investigations and 
potentially additional therapies
Structural Incomplete 
Response
Structural or functional evidence of 
disease with any Tg level +/- TgAb
50–85% continue to have persistent 
disea

+/- TgAb
disease despite additional therapy
Disease-specific death rates as high 
as 11% with locoregional metas­
tases and 50% with structural 
A structural incomplete response may lead 
to additional treatments or ongoing 
observation, depending on multiple 
clinicopathologic factors, including the 
size, location, rate of growth, RAI avidity, 
FDG avidity, and specific pathology of 
the structural lesions
Indeterminate Response
Nonspecifi

ructural lesions
Nonspecific findings on imaging studies 
Faint uptake in thyroid bed on RAI 
scanning 
Nonstimulated Tg detectable, but <1 
ng/mL Stimulated Tg detectable, 
but <10 ng/mL or TgAb stable or 
declining in the absence of struc­
tural or functional disease
15–20% will have structural disease 
identified during follow-up
In the remainder, the nonspecific 
changes are either stable or 
resolve
<1% disease-specific death
An indeterminate response should lead to 

c death
continued observation with appropriate 
serial imaging of the nonspecific lesions 
and serum Tg monitoring
Nonspecific findings that become suspi­
cious over time can be further evaluated 
with additional imaging or biopsy
aIn the absence of anti-TgAb.
NED denotes a patient as having no evidence of disease at final follow-up.
FDG, Fluorodeoxyglucose; NED, no evidence of disease; RAI, radioactive iodine; Tg, thyroglobulin; TgAb, antithyroglobulin 

odine; Tg, thyroglobulin; TgAb, antithyroglobulin antibodies; TSH, thyroid-stimulating hormone.
From Haugen BR, Alexander EK, Bible KC, et al. 2015 American Thyroid Association management guidelines for adult patients with thyroid nodules and differentiated thyroid cancer: the American Thyroid 
Association guidelines task force on thyroid nodules and differentiated thyroid cancer. Thyroid. 2016;26:1–133.
   
TABLE 14.13
471
Chapter 14  Nontoxic Diffuse Goiter, Nodular Thyroid Disorders, and Thyr

iffuse Goiter, Nodular Thyroid Disorders, and Thyroid Malignancies
In the past, stimulated thyroglobulin values were a key compo­
nent of follow-up testing. However, with the development of new 
ultrasensitive assays, the definition of excellent response to therapy 
can be established with the suppressed thyroglobulin value of less 
than 0.2 ng/mL.1 However, if reliable ultrasensitive thyroglobulin 
values are not available, then stimulated thyroglobulin values will 
be needed to establish an ex

globulin values will 
be needed to establish an excellent response to therapy.
Because the response to therapy definitions rely heavily on 
serum thyroglobulin measurements, patients with antithyroglob­
ulin antibodies present a challenge. In these cases, the serum thy­
roglobulin measurement may or may not be reliable and therefore 
cannot be trusted. Antithyroglobulin antibodies that decline by at 
least 50% over the first 6 months usually indicate the absence of 
persistent disease.384 Conver

cate the absence of 
persistent disease.384 Conversely, rising thyroglobulin values over 
time are often an early indicator of subsequent structural incom­
plete response and would lead to additional structural or func­
tional imaging.
Additionally, the response to therapy definitions were origi­
nally developed for patients who had total thyroidectomy and 
radioactive iodine ablation. Therefore the definitions need to be 
modified slightly to be able to accommodate patients who were 
treated wi

 able to accommodate patients who were 
treated with lobectomy alone or total thyroidectomy without 
radioactive iodine.385,386 In patients who underwent total thy­
roidectomy without radioactive iodine, an excellent response to 
therapy requires a nonstimulated thyroglobulin less than 0.2 ng/
mL, an indeterminate response is defined as a serum thyroglobu­
lin of 0.2 to 5 ng/mL, and a biochemical incomplete response 
requires a serum thyroglobulin greater than 5 ng/mL. In patients 
treated with 

n greater than 5 ng/mL. In patients 
treated with lobectomy, any thyroglobulin value less than 30 ng/
mL is classified as excellent, whereas a value greater than 30 ng/
mL is classified as biochemical incomplete. 
Follow-Up Recommendations Based on 
Response to Therapy Status
ATA low-risk and intermediate-risk patients who demonstrate 
an excellent response to therapy can transition to low-intensity 
follow-up. In the long term, patients with an excellent response 
to therapy are followed primar

excellent response 
to therapy are followed primarily with serum thyroglobulin, 
thyroglobulin antibody, and physical examination every 1 to 2 
years. Routine use of neck ultrasonography in the follow-up of 
these excellent response to therapy patients is likely to yield more 
false-positive findings than true structural disease and therefore 
is not routinely recommended.380,381 While an excellent response 
to therapy almost always leads to less aggressive follow-up, it is 
important to note th

 aggressive follow-up, it is 
important to note that ATA high-risk patients can still have a risk 
of recurrence in the 5% to 10% range, despite having been classi­
fied as an excellent response to therapy, because they often mani­
fest distant sites of disease that are not apparent on early imaging 
or they are poor thyroglobulin producers, leading to them being 
inappropriately classified as excellent response to therapy.378,382 
As a result, we consider cross-sectional imaging in ATA high-ris

e consider cross-sectional imaging in ATA high-risk 
patients every 3 to 5 years, depending on the specifics of indi­
vidual cases.
The evaluation of the patient with biochemical incomplete 
response begins first with an assessment of whether appropri­
ate imaging studies have been done. The level of serum thyro­
globulin roughly correlates with the location of disease, with 
nonstimulated Tg values less than 10 ng/mL usually identifying 
persistent/recurrent disease in the neck, values in the 1

ent/recurrent disease in the neck, values in the 10 to 100 
range suggesting pulmonary metastasis, and values in the thou­
sands suggesting bone metastases.387 In addition to meticulous 
cross-sectional imaging, functional imaging with radioactive 
iodine or 18FDG-PET may identify the source of the abnor­
mal thyroglobulin. Once appropriate imaging has been done 
to definitively rule out structurally identifiable disease, then 
the follow-up of patients with biochemical incomplete response 
depe

atients with biochemical incomplete response 
depends on the trend over time in their thyroglobulin and thy­
roglobulin antibodies. Patients with stable or declining thyro­
globulin and thyroglobulin antibodies are usually followed with 
observation. Patients with rising thyroglobulin or thyroglobulin 
antibodies are followed with additional cross-sectional imaging 
with the type, intensity, and frequency of testing dependent on 
the rate of rise of the biochemical markers. Studies have now 
ver

 of the biochemical markers. Studies have now 
verified correlation between thyroglobulin doubling time and 
clinical outcomes.388
Patients with a structural incomplete response are particularly 
problematic as the majority will maintain persistent biochemical 
or structural evidence of disease despite additional therapies.1 Pos­
sible treatment options for structural incomplete response include 
observation, surgical intervention, radioactive iodine therapy, 
external beam radiation therapy, fo

dine therapy, 
external beam radiation therapy, focal therapies, and systemic 
therapies. Decision making in this setting is best done in a multi­
disciplinary team setting. Indications for intervention in patients 
with a structural incomplete response should be guided by fac­
tors such as the size, location, rate of growth, radioactive iodine 
avidity, FDG avidity, and underlying histology of the metastatic 
disease.
Patients with an indeterminate response are usually followed 
with additional

ate response are usually followed 
with additional observation with serial evaluations of the indeter­
minate biochemical or structural area of interest.1 In many cases, 
the nonspecific biochemical markers or structural findings resolve 
spontaneously, in which case the patients can be reclassified as 
having an excellent response to therapy. In other cases, the inde­
terminate findings become more clearly abnormal and the patient 
is then reclassified as biochemical or structural incomplete.
W

ssified as biochemical or structural incomplete.
While these response to therapy recommendations have 
been primarily based on patients who underwent total thyroid­
ectomy and radioactive iodine ablation, follow-up recommen­
dations are essentially the same in patients treated either with 
lobectomy or with total thyroidectomy without radioactive 
iodine.385,386 By definition, these patients are low risk and thus 
are not expected to have early recurrences or distant metastasis. 
While the signi

ecurrences or distant metastasis. 
While the significance of a single serum thyroglobulin determi­
nation is uncertain in the majority of these patients, the trend 
and thyroglobulin over time can be informative despite being 
less sensitive and more nonspecific. Because of the lack of sen­
sitivity in the thyroglobulin measurements or persistent thy­
roglobulin antibodies, follow-up in these patients relies more 
heavily on neck ultrasonography done every 3 to 5 years at least 
for the first de

done every 3 to 5 years at least 
for the first decade. 
Risk-Adapted Follow-Up Recommendations for 
Medullary Thyroid Cancer
Recommendations regarding the extent, intensity, and timing of 
follow-up in medullary thyroid cancer will largely be dependent 
on postoperative calcitonin and CEA values. As with thyroglobu­
lin values, the calcitonin and CEA doubling-time values provide 
important clinical insights into both the rate of disease progres­
sion and disease-specific survival.14 Similar to 

sion and disease-specific survival.14 Similar to the approach used 
in differentiated thyroid cancer, ongoing follow-up recommenda­
tions are based on response to therapy reevaluations in which the 
definitions of excellent, biochemical incomplete, and structural 
472
incomplete response are modified using calcitonin and CEA val­
ues rather than thyroglobulin values.389–391 For medullary cancer, 
an excellent response is defined as an undetectable serum cal­
citonin and n

efined as an undetectable serum cal­
citonin and normal range CEA value. Biochemical incomplete 
response is defined as a measurable postsurgical serum calcitonin 
or elevated CEA value. Just as with differentiated thyroid cancer, 
the follow-up plans are adjusted based on response to therapy. 
Treatment of Thyroid Cancer Recurrence and 
Distant Disease
Locoregional Recurrences of PTC and FTC
Locoregional recurrences occur in 5% to 20% of patients with 
PTC and FTC. However, more than a third of

s with 
PTC and FTC. However, more than a third of reoperations for 
persistent or recurrent disease are related to inadequate initial 
thyroid surgery.392 Small lymph node metastases may be treated 
with radioiodine, but their persistence or growth should lead to 
consideration of surgery.393 A recurrence larger than 8 to 10 mm 
in diameter should be excised.393,394 Compartmental dissection of 
previously unexplored compartments with clinically significant per­
sistent/recurrent disease while s

significant per­
sistent/recurrent disease while sparing vital structures is performed 
because microscopic lymph node metastases are commonly more 
extensive than would appear from imaging studies alone. This dis­
section provides long-term local control of the disease in the major­
ity of patients.395 Conversely, compartmental surgical dissections 
may not be feasible in the setting of compartments that have been 
previously explored because of extensive scarring, and only a more 
limited or t

 extensive scarring, and only a more 
limited or targeted lymph node resection may be possible.
Total excision can be facilitated by TBS 3 to 5 days after admin­
istration of 100 mCi (3700 MBq) of 131I because additional tissue 
that should be excised may be identified. In rare cases, surgery has 
been performed 1 day after the total body scan is done, typically 
using an intraoperative probe. The completeness of resection is 
verified 1 to 2 days after surgery by another total-body scan, and 
i

s after surgery by another total-body scan, and 
in one series total excision was achieved in 92% of cases.394 Other 
methods may be used to facilitate the excision of small neoplas­
tic foci located in scar tissue or in sites that are difficult to redis­
sect, primarily using intraoperative ultrasound or preoperative 
ultrasound-guided charcoal tattooing.15 Considering both normal 
neck ultrasonography and undetectable Tg levels (<1 ng/mL after 
TSH stimulation or <0.2 ng/mL under thyroid hormo

TSH stimulation or <0.2 ng/mL under thyroid hormones) to 
define excellent response after a reoperation for persistent/recur­
rent disease as defined by the ATA guidelines, the rate of excellent 
response reaches 63%.396 In general, external radiotherapy is indi­
cated in FCTC patients with soft tissue recurrences that cannot be 
completely excised and that do not take up 131I.
In rare cases, patients with DTC who are not eligible for fur­
ther surgery or 131I therapy have been treated for regi

surgery or 131I therapy have been treated for regional nodal 
recurrence with ultrasound-guided radiofrequency ablation or 
percutaneous ethanol injection.397–403 No studies prospectively 
and respectively compared the two techniques. Both should be 
avoided, however, for the treatment of central lesions close to the 
trachea and/or the laryngeal nerve and/or the esophagus, as well as 
locations where surgery is also challenging. These procedures are 
especially safe for nodes not adjacent to cr

 are 
especially safe for nodes not adjacent to critical structures such as 
lateral neck recurrences. For tumors that invade the upper aerodi­
gestive tract, patient outcome is related to complete resection of all 
gross disease, with techniques ranging from shaving tumor off the 
trachea or esophagus for superficial invasion with the preservation 
of function to more aggressive techniques in case of direct intra­
luminal invasion, including tracheal resection and anastomosis or 
laryngopharyng

cheal resection and anastomosis or 
laryngopharyngoesophagectomy.404,405 In these cases, surgery is 
usually combined with 131I and external beam radiation. 
Management of Distant Metastatic Disease
Among patients with differentiated carcinoma (PTC, FTC, and 
HCC), only 9% develop distant metastases.406 Mortality rates at 
5 and 10 years after the diagnosis of metastasis were 65% and 
75% for all patients with distant metastases, and nearly 80% of 
the deaths were due to thyroid cancer.406–408 O

f 
the deaths were due to thyroid cancer.406–408 Of note, up to 20% 
of deaths were due to locoregional disease.409 Thus the develop­
ment of distant metastases in FCTC has traditionally foreseen an 
ominous prognosis. Lung metastases are more frequent in young 
patients with PTC, and the lung is almost always the only site of 
distant spread in children. Bone metastases are more common in 
older patients and in those with FTC. Other less common sites are 
the brain, liver, and skin.406–408
Clin

sites are 
Clinical symptoms of lung involvement are uncommon. By 
contrast, pain, swelling, or fracture occurs in more than 80% of 
patients with bone metastases. They are a poor prognostic indica­
tor.410 The pattern of lung involvement may vary from macronod­
ular to diffuse infiltrates. The latter are usually diagnosed with 131I 
TBS and may be confirmed by spiral CT; enlarged mediastinal 
lymph nodes are often present in patients with PTC, especially 
chil

ten present in patients with PTC, especially 
children. Bone metastases are osteolytic and are better visual­
ized by CT scan, MRI, or 18FDG-PET. 18FDG-PET scanning 
is useful in these patients for determining the extent of disease 
and for prognostic assessment.23,34,38,39 Nearly all patients with 
distant metastases have high serum Tg concentrations unless the 
lung metastases are not visible on CT scan, and two-thirds of such 
patients have 131I uptake in their sites of metastasis.407
Focal t

1I uptake in their sites of metastasis.407
Focal treatment of bone metastases includes surgery, external 
beam radiation therapy, or thermal ablation (radiofrequency abla­
tion or cryoablation) and cement injection.411 Focal treatment is 
indicated when there are neurologic or orthopedic complications 
or a high risk of such complications and when bone metastases are 
visible on CT scan or MRI, even in the presence of 131I uptake, 
because radioiodine alone will not control the disease. In patie

odine alone will not control the disease. In patients 
with a single or a few bone metastases, focal treatment may also be 
performed with curative intent.412 Surgery and stereotaxic radia­
tion therapy may be indicated in patients with brain metastases. 
Thermal ablation or stereotaxic radiation therapy may be used in 
case of a few lung metastases. Completeness of thermal ablation 
and outcome of the treated lesions can be assessed with 18FDG-
PET scanning.40
Patients with distant metastases t

Patients with distant metastases that take up 131I are treated 
with 100 to 200 mCi (3700–7400 MBq) every 4 to 6 months 
during the first 2 years and then at longer intervals. Between 131I 
treatments, suppressive doses of levothyroxine are given to main­
tain the serum TSH level below 0.1 mUI/L. In one study, the 
radiation dose to the tumor tissue and outcome of 131I therapy 
are correlated.344 A radiation dose higher than 80 Gy (8000 rads) 
should be delivered to obtain cure; 

 (8000 rads) 
should be delivered to obtain cure; with radiation doses less than 
35 Gy (3500 rads), there is little chance for success. This is the 
rationale for using higher activities of radioiodine either as stan­
dard activity or based on individual dosimetry. However, the com­
parison between repeated administrations of a standard activity 
of 100 mCi (3.7 GBq) to the administration of higher activities 
after dosimetric study did not show a benefit of dosimetry over 
standard activities 

 a benefit of dosimetry over 
standard activities in terms of overall survival.413 In patients with 
functioning metastases, PET scanning with 124I showed that in a 
given patient uptake may vary between metastases and also within 
a given metastasis.414,415 Finally, uptake may be heterogeneous 
at the cellular level.93,414 This heterogeneity in the dose distri­
bution in neoplastic foci may explain the ineffectiveness of 131I 
treatment, despite significant mean uptake on total-body scan. 
For 

 significant mean uptake on total-body scan. 
For treatment to be effective in this clinical setting, appropriate 
473
levels of TSH stimulation and absence of iodine contamination 
are essential. Excess iodine is eliminated 1 month after administra­
tion of an iodinated CT scan.30 The urinary iodine excretion can 
be obtained to confirm clearance. Similar short-term survival rates 
were observed in patients

ort-term survival rates 
were observed in patients with distant metastases after 131I treat­
ment prepared with either withdrawal or rhTSH.416 However, 
most patients with 131I uptake in their metastases are alive at 5 
years, and there are no available data on long-term outcome after 
preparation with rhTSH. Prolonged withdrawal usually induces 
higher uptake in neoplastic foci than rhTSH and should be the 
preferred method of TSH stimulation in patients with metastatic 
disease.417 rhTSH-media

 patients with metastatic 
disease.417 rhTSH-mediated therapy may be indicated in selected 
patients with underlying comorbid conditions, making iatrogenic 
hypothyroidism potentially risky, and in patients with pituitary 
disease who are unable to raise their serum TSH.358 Such patients 
should be given the same or higher activity than would have been 
given had they been prepared with hypothyroidism or a dosi­
metrically determined activity. Lower activities (1–2 mCi [37–74 
MBq]/kg body weigh

wer activities (1–2 mCi [37–74 
MBq]/kg body weight) are given to children. There is no absolute 
limit to the cumulative activity of 131I that can be given to patients 
with distant metastases as long as treatment is efficient. There is, 
however, an increased risk of leukemia and of solid cancer, espe­
cially when the cumulative activity rises above 600 mCi (22,000 
MBq)418–420; also, above this activity, further 131I therapy may 
rarely provide a cure.407
Disappearance of imaging abnormalitie

e a cure.407
Disappearance of imaging abnormalities has been obtained 
overall in about 45% of patients with distant metastases show­
ing avidity for 131I, and responses are more frequent in younger 
patients, those with small pulmonary metastases, those who had a 
well-differentiated cancer, and those who have no 18FDG uptake 
on PET scan; complete responses may be obtained several years 
after initiation of therapy.406–408 When response was judged to 
have been complete after 131I therapy, sub

ged to 
have been complete after 131I therapy, subsequent relapse rarely 
(<10%) occurred even though serum Tg levels were persistently 
detectable in some patients.407 
Complications of Treatment With 131I
Acute side effects (nausea, sialadenitis, loss of taste) after treatment 
with 131I are common but are typically mild and resolve rapidly.421 
Radiation thyroiditis is usually trivial, but if the thyroid remnant 
is large, the patient may have enough pain to warrant corticoste­
roid therapy f

 enough pain to warrant corticoste­
roid therapy for a few days. Tumor in certain locations, such as the 
brain, spinal cord, and paratracheal region, may swell in response 
to TSH stimulation or after 131I therapy, causing compressive 
symptoms, and this problem should be prevented with cortico­
steroid therapy. Xerostomia and obstruction of lacrimal ducts may 
occur in 5% to 10% of patients treated with 131I.422,423 Radiation 
fibrosis may rarely develop in patients with diffuse lung metastase

ly develop in patients with diffuse lung metastases 
and can eventually prove fatal if high activities (>150 mCi [5550 
MBq]) are administered at short intervals (<3 months).248
Particular attention must be paid to avoid administration of 
131I to pregnant women. After 131I treatment, spermatogenesis 
may be transiently depressed,424 and women may have transient 
ovarian failure. Genetic damage induced by exposure to 131I 
before conception has been a major subject of concern. How­
ever, no anom

een a major subject of concern. How­
ever, no anomaly has been reported to date. Therefore it is rec­
ommended that conception be postponed for at least 6 months 
after treatment with 131I.425 There is no clear evidence that preg­
nancy affects tumor growth in women receiving adequate levo­
thyroxine therapy. In case of pregnancy in a patient treated with 
a replacement dose of levothyroxine, the dose of levothyroxine 
is increased by 30% as soon as the pregnancy is confirmed and 
serum TSH, T3,

 as the pregnancy is confirmed and 
serum TSH, T3, and T4 levels are measured every month during 
the first half of pregnancy.426 In a patient treated with a sup­
pressive dose of levothyroxine, serum TSH, T3, and T4 levels 
are controlled every month, and the daily dose of levothyroxine 
is increased when serum TSH is increased or when T3 and T4 
decrease.
Mild pancytopenia may occur after repeated or high-dose 131I 
therapy, especially in patients with bone metastases who are also 
treated wit

nts with bone metastases who are also 
treated with external radiotherapy. The overall relative risk of 
leukemia and solid tumors was found to be increased in patients 
treated with radioactive iodine, especially in case of high cumula­
tive activity of 131I (>600 mCi [22,000 MBq]) or in association 
with external radiotherapy.418–420 
Refractoriness to Radioactive Iodine
Two-thirds of distant metastases will become refractory to radio­
iodine, and treatment with 131I will not provide any benef

and treatment with 131I will not provide any benefit. This 
group includes patients with metastatic disease who do not take 
up 131I at the time of initial treatment or who lose the ability to 
take up 131I after previous evidence of uptake, patients with 131I 
uptake retained in some lesions but not in others, and patients 
with metastatic disease that progresses despite significant uptake 
of 131I in the metastases.427 Less clear is the situation for patients 
with persistent 131I uptake in al

on for patients 
with persistent 131I uptake in all lesions who are not cured despite 
several treatment courses (particularly after receiving >22,000 
MBq [600 mCi] of 131I) and whose disease does not prog­
ress according to Response Evaluation Criteria in Solid Tumors 
(RECIST) criteria.407 The decision to continue 131I treatment in 
such patients is generally based on their response to previous treat­
ment courses, persistence of a significant level of 131I uptake on 
the previous post-therap

 level of 131I uptake on 
the previous post-therapy TBS, low FDG uptake in tumor foci, 
and absence of side effects.427 Large distant metastases with high 
FDG uptake on PET scanning almost never respond to 131I ther­
apy and usually rapidly progress, confirming the clinical prognos­
tic classification.38,39
In patients with refractory disease, 131I treatment should be 
abandoned, and follow-up is performed with imaging every 4 
to 6 months on levothyroxine treatment that should maintain 
serum 

vothyroxine treatment that should maintain 
serum TSH below 0.1 mUI/L.427–429 Optimal TSH goals for 
individual patients must, however, balance the potential benefit 
of TSH suppression with the possible harm from subclinical thy­
rotoxicosis, especially in patients with medical conditions that 
can be exacerbated with aggressive TSH suppression. Focal treat­
ment modalities are used as previously described. In patients 
with multiple lesions greater than 1 to 2 cm in diameter with 
documented p

ater than 1 to 2 cm in diameter with 
documented progression on imaging within 12 months, a sys­
temic treatment may be indicated.427 Of note, within a given 
patient, the intensity of FDG uptake cannot be used to deter­
mine the lesions at higher risk of progression.430 Molecular tar­
geted therapies with antiangiogenic effects are used as first-line 
treatment. 1 
Molecular Targeted Systemic Therapies
An initiating carcinogenic event can be found in most DTC and 
thus molecular targeted therap

nd in most DTC and 
thus molecular targeted therapy is based upon a sound scientific 
rationale.118 The MAP kinase pathway is activated in the majority 
of papillary thyroid cancers, mainly by either gene rearrangements 
(RET-PTC and NTRK) or point mutations of the RAS and BRAF 
genes. RAS point mutations are frequently found in follicular and 
poorly differentiated carcinomas. Additional genetic abnormali­
ties may be found in poorly differentiated thyroid carcinomas. 
Angiogenesis is also acti

ted thyroid carcinomas. 
Angiogenesis is also activated in thyroid cancers,431 by activation 
of the VEGFR pathway. Other pathways may also be activated, 
including the FGFR and PDGFR pathways.
474
Up to now, most drugs used in refractory DTC had an anti­
angiogenic action, and some also target from the MAP kinase 
pathway. With these agents, the rate of partial response observed 
ranges from 0% to 65% (Table 14.14), with three drugs show­
ing response rates close to or hi

hree drugs show­
ing response rates close to or higher than 50% (lenvatinib, cabo­
zantinib, and pazopanib).432–444 Improvement of progression-free 
survival (PFS) has also been demonstrated in two phase III trials: 
sorafenib in the DECISION trial and lenvatinib in the SELECT 
trial.437,440,443 
Vandetanib
The ZACTHYF phase II randomized trial with vandetanib 
produced a significant prolongation of the PFS compared with 
placebo (HR, 0.63, p = 0.008; median, 11.1 vs 5.9 months, 
respectively), 

.008; median, 11.1 vs 5.9 months, 
respectively), and an objective partial response rate of 8%.443 
The subsequent VERIFY phase III trial with vandetanib ver­
sus placebo has been conducted, and the results are pending 
(NCT01876784). 
Sorafenib
The DECISION phase III trial with sorafenib showed a pro­
longation of the PFS compared with placebo (HR, 0.587; 95% 
CI, 0.454–0.758; p <0.0001; median PFS, 10.8 vs 5.8 months, 
respectively). The improvement in PFS was seen in all clinical sub­
groups.

ement in PFS was seen in all clinical sub­
groups. The partial response rate was 12%, and stable disease for 
6 months or longer was achieved in 41.8 % of patients. PFS was 
improved in all biomarker subgroups, irrespective of BRAF and 
RAS mutation status. 
Lenvatinib
The SELECT phase III trial with lenvatinib significantly 
improved the median PFS compared with placebo (HR, 0.21; 
99% CI, 0.14–0.31, p <0.001; median PFS, 18.3 vs 3.6 months, 
respectively) and the objective response rate was 65

spectively) and the objective response rate was 65% with com­
plete responses in 2%. The improvement in PFS was seen in 
all clinical subgroups, including in the 20% of patients who 
had received prior vascular endothelial growth factor (VEGF)–
targeted therapy and irrespective of BRAF and RAS mutation 
status.
None of the studies demonstrated an improvement in overall 
survival, which might have been related to the crossover design of 
the studies with treatment in an open phase in case of prog

es with treatment in an open phase in case of progression 
in the placebo arm and the long survival of some patients after their 
participation in the trial during which other treatment modalities 
  Drugs Used in Refractory Differentiated Thyroid Cancer
Drugs
N
Targets(s)
Partial Response 
(RECIST) (%)
Median Progression-Free 
Survival (Months)
Axitinib
Cohen432
Locati433
Capdevilla434
45
45
47
VEGF, RET, PDGFR, KIT
31
38
28
18.1
16.1
8
Cabozantinib
Cabanillas436
Cabanillas435
15
25
VEGFR, RET

tinib
15
25
VEGFR, RET, CMET
53
40
>12.2, not reached
12.7
Schlumberger437
(phase III trial)
392
VEGFR RET, FGFR, PDGFR, C-KIT
65
18.3 vs 3.6
Pazopanib
Bible439
37
VEGFR, PDGFR, KIT
49
11.7
Motesanib
Sherman438
93
VEGFR, PDGFR, KIT, RET
14
10
Sorafenib
Brose440
(phase III vs placebo)
417
VEGFR, RET, BRAF, PDGFR
12
10.8 vs 5.8
Sunitinib
Carr441
28
VEGFR, RET, PDGFR, KIT
11
NA
Leboulleux443
(phase II vs placebo)
145
VEGFR, RET, EGFR
0
11.1 vs 5.9
V

I vs placebo)
145
0
11.1 vs 5.9
Vemurafenib
Brose442
51
BRAF
31
18 TKI naïve, 9 prior TKI
Dabrafenib
Falchook444
13
BRAF
29
11.3
NA, Not applicable; RECIST, Response Evaluation Criteria in Solid Tumors; TKI, tyrosine kinase inhibitor.
   
TABLE 14.14
475
were used. However, in a subgroup analysis of the SELECT study, 
a significant benefit in overall survival was observed in patients 
aged a

 overall survival was observed in patients 
aged above 65 years, related to the beneficial effects of lenvatinib in 
patients with more aggressive disease.445 In a subsequent analysis, a 
rapid decline in tumor size by 25% was reported at the 8-week eval­
uation; the duration of PFS was related to the magnitude of the ini­
tial tumor size decrease, and a multivariate analysis identified two 
prognostic indicators, performance Eastern Cooperative Oncology 
Group (ECOG) status of 0 or 1 and small 

Oncology 
Group (ECOG) status of 0 or 1 and small size of metastases.446
The safety profile of sorafenib was as expected, but with a 
higher incidence of adverse effects than in patients with other 
cancer types. Most adverse effects were grade 1 and 2, the most 
common being hand-foot skin reaction (76%), diarrhea (69%), 
alopecia (67%), and rash/desquamation (50%). Toxicities led to 
dose reduction in 64% of patients and to drug withdrawal in 19%.
The safety profile of the lenvatinib group inc

9%.
The safety profile of the lenvatinib group includes hyperten­
sion (68%), fatigue (64%), diarrhea (59%), and decreased appe­
tite (50%). Proteinuria occurred in 32% and thromboembolic 
events in 11%. A total of 68% of the patients required dose reduc­
tion, 82% required dose interruption, and 14% of patients were 
taken off the drug.
Prevention and early diagnosis of toxicity are the best ways to 
decrease the frequency and gravity of the secondary effects. Prior 
to starting treatment, pati

ondary effects. Prior 
to starting treatment, patients should have normal blood pressure 
and should be educated for self–blood pressure measurements. 
They should be educated to use moisturizing lotion and to pro­
tect themselves from the sun. They should be checked for normal 
heart function, normal electrolytes, normal renal function, and 
the absence of proteinuria. Interactions with concomitant drugs 
should be checked to avoid drugs interacting with the cytochrome 
P450 and drugs prolongin

ting with the cytochrome 
P450 and drugs prolonging QTc.
It is highly recommended after initiation of treatment that cli­
nicians follow up patients at 2-week intervals for the first 2 to 3 
months and then once a month, to proactively manage adverse 
events in accordance with the tolerance of each individual patient.
Tyrosine kinase inhibitors interacting in the MAP kinase path­
way have been used to redifferentiate tumor cells and reinduce 
RAI uptake. A MEK inhibitor (selumetinib) and BRAF in

 uptake. A MEK inhibitor (selumetinib) and BRAF inhibitor 
(dabrafenib) have been given prior to radioactive iodine admin­
istration with partial response in 20% to 25% of the cases.447,448 
The concept is appealing, but these results have to be confirmed in 
larger prospective studies.
In conclusion, results of DECISION and SELECT led to the 
approval of sorafenib and lenvatinib for advanced, refractory, and 
progressive DTC by the US Food and Drug Administration (FDA) 
and the European Medicin

rug Administration (FDA) 
and the European Medicines Agency (EMA). In the SELECT trial 
and in real life, maximal benefits were achieved in patients with an 
ECOG performance status of 0 to 1 who were not heavily pretreated 
and who had a limited tumor burden. There was no unexpected 
toxicity, but toxicities were significant and led to dose reduction and 
to drug withdrawal in a significant proportion of patients. This sug­
gests that these treatments should be initiated only in patients with 

tments should be initiated only in patients with 
significant tumor burden and with documented progression of the 
disease, and they should be managed by experienced teams. 
Therapy for Recurrence of Medullary Thyroid Carcinoma
Patients with locoregional recurrence of MTC are at high risk of 
distant metastases, particularly when calcitonin levels exceed 150 
pg/mL.14 Distant metastases frequently involve multiple organs 
and are usually multifocal in each involved organ. The most fre­
quently i

al in each involved organ. The most fre­
quently invaded organs are liver, lungs, and bones. For a complete 
workup, the best imaging combination includes a neck ultraso­
nography, a liver MRI, and a neck-chest and abdominopelvic CT 
scan, together with a spine MRI or a bone scintigraphy.33 18FDG-
PET/CT is useful for initial workup, especially when morphologic 
images are normal or doubtful.
Patients with distant metastases may progress slowly and may be 
compatible with decades of survival. Ca

nd may be 
compatible with decades of survival. Calcitonin and CEA doubling 
time are prognostic of survival, with short doubling time being in 
favor of a shorter survival, and are correlated to RECIST progres­
sion.449,450 Symptomatic treatments may be necessary, in particular 
against pain and diarrhea. Cytotoxic chemotherapy is poorly efficient 
and may be indicated only in cases of rapid tumor progression.451 
Chemoembolization with doxorubicin (Adriamycin) of liver metas­
tases provides a 

cin (Adriamycin) of liver metas­
tases provides a high response rate for both symptoms and tumor 
masses.452 Kinase inhibitors directed against tumor cells (RET and 
other kinases) and endothelial cells (VEGF receptors) provide a high 
response rate and should be used as first-line treatment. 
Vandetanib and Cabozantinib (XL184)
Vandetanib inhibits kinases of VEGFR1, VEGFR2, RET, and 
EGFR. In a phase II trial, including 30 patients with hereditary 
MTC, vandetanib given at the dose of 300 mg/da

ry 
MTC, vandetanib given at the dose of 300 mg/day showed a par­
tial RECIST response in 10 patients and stable disease lasting lon­
ger than 24 weeks in 16 other patients.456 In another phase II trial, 
including 19 patients with hereditary MTC, vandetanib given at 
the dose of 100 mg/day showed a partial response in 3 patients 
and prolonged stabilization in 10 cases. There were, however, no 
differences in the rate of toxicity.467 Vandetanib also proved its 
efficacy in children with advance

also proved its 
efficacy in children with advanced MTC in the context of mul­
tiple endocrine neoplasia type 2B.468
The phase II trial with vandetanib randomized the drug at the 
dose of 300 mg against placebo in patients with metastatic and 
aggressive MTC.466 Patients with MTC-related symptoms were 
included even in the absence of RECIST progression. The PFS 
under vandetanib was longer (>30.5 months) than under pla­
cebo (19.3 months; HR, 0.46; p <0.001). Partial responses were 
observed in 

6; p <0.001). Partial responses were 
observed in 45% of the cases, with a median length of 22 months. 
The subgroup analyses showed a benefit of vandetanib in patients 
regardless of their WHO and RET status. The study did not 
show an improvement in the overall survival, but a crossover was 
allowed in the study, and patients under placebo were treated with 
vandetanib after unblinding of the study. Vandetanib use in real 
life has demonstrated a response rate in 22% of the patients.469
Caboza

 a response rate in 22% of the patients.469
Cabozantinib inhibits kinases of RET, VEGFR2, and c-MET. 
In a phase I trial of 34 patients with MTC, partial responses were 
observed in 17 patients and stable disease in 15 other patients.460 
The phase III trial with cabozantinib randomized the drug at the 
dose of 140 mg against placebo in patients with metastatic and 
RECIST progressive MTC. The study did not allow crossover.465 
Median PFS was 11.2 months for cabozantinib versus 4 months 
for pla

2 months for cabozantinib versus 4 months 
for placebo (HR, 0.28; p <0.001). The benefits of cabozantinib 
on PFS were observed in all groups of patients regardless of age, 
sex, WHO performance status, tumor location, and previous 
treatment with TKI. The benefits of cabozantinib on PFS were 
observed in all patients except those without RET mutation.470 
The response rate was 28% with a median length of response of 
14.7 months. Response rates ranged from 20% to 34% depend­
ing on the RET and 

ranged from 20% to 34% depend­
ing on the RET and RAS status.470 The median overall survival 
was 26.6 months in cabozantinib-treated patients versus 21.1 
months for placebo (HR, 0.85, p = 0.24). However, in the group 
of 126 patients with the RET M918T mutation, patient overall 
476
survival was 44.3 months in cabozantinib-treated patients com­
pared with 18.9 months for placebo (HR, 0.60, p = 0.3) in favor 
of RET M918T being a predictive factor of treatment efficacy of

being a predictive factor of treatment efficacy of 
cabozantinib.471
The most frequent secondary effects of vandetanib are diar­
rhea, fatigue, cutaneous manifestations (folliculitis, photosen­
sitivity, rash), hypertension, and QTc segment prolongation 
on electrocardiogram (EKG). In the phase III trial, 12% of the 
patients stopped the drug due to toxicity and 35% decreased the 
doses.466 The most frequent secondary effects of cabozantinib are 
diarrhea, abdominal pain, hypertension, hand-foot

diarrhea, abdominal pain, hypertension, hand-foot syndrome, 
mucositis, loss of weight, nausea, and fatigue. In the phase III 
trial, 16% of the patients stopped the drug due to toxicity and 
79% decreased the doses.465 In general, prevention and early 
diagnosis are the best ways to decrease the frequency and grav­
ity of the secondary effects. Prior to starting treatment, patients 
should have normal blood pressure and should be educated on 
self–blood pressure measurements. They should be ed

elf–blood pressure measurements. They should be educated 
to use moisturizing lotion and to protect themselves from the 
sun. They should be checked for normal heart function, normal 
electrolytes, normal renal function, and the absence of protein­
uria. Diarrhea should be controlled if possible. Interactions with 
concomitant drugs should be checked to avoid drugs interact­
ing with the cytochrome P450 and drugs prolonging QTc. After 
starting the drug, patients should be monitored closely to d

he drug, patients should be monitored closely to detect 
prolongation of QTc and to monitor calcium, vitamin D, and 
thyroid hormone levels.472 It is highly recommended after initia­
tion of treatment that clinicians follow up with patients at 2-week 
intervals for the first 2 to 3 months and then once a month to 
proactively manage adverse events in accordance with the toler­
ance of each individual patient.
Vandetanib (Caprelsa) and cabozantinib (Cometriq) are both 
FDA and EMA approved. Vande

b (Cometriq) are both 
FDA and EMA approved. Vandetanib is approved for the treat­
ment of symptomatic or progressive MTC with unresectable 
locally advanced or metastatic disease. The use of vandetanib in 
patients with indolent asymptomatic or slowly progressive disease 
should be considered carefully because of the treatment-related 
risks of vandetanib. Cabozantinib is approved for the treatment 
of progressive, unresectable, locally advanced or metastatic MTC. 
These drugs should not be use

 or metastatic MTC. 
These drugs should not be used for patients with isolated elevated 
calcitonin levels with normal morphologic imaging or in patients 
with a small tumor burden and stable disease. They should be pro­
posed only to MTC patients with large tumor foci and progressive 
disease on imaging.
Targeted Molecular Therapy
Activating mutations of the proto-oncogene RET are involved in 
the oncogenesis of MTC. They are found in all hereditary MTC 
and in 50% to 60% of sporadic MTC.297–29

tary MTC 
and in 50% to 60% of sporadic MTC.297–299,453 In selected 
patients with distant and progressive metastases, they are found 
  Phase II Prospective Trials in Medullary Thyroid Cancer
Drugs
n
Progression-Free Survival 
(Months)
Wells456
30
30
27.9
Sorafenib
Lam457
19
VEGFR, BRAF
11
17.9
Motesanib
Schlumberger458
83
VEGFR, PDGFR, C-KIT
2
12.0
Axitinib
Cohen432
12
3
VEGFR1,2,3
22
23
NA
9.4
Sunitinib
Carr4

la434
12
3
22
23
NA
9.4
Sunitinib
Carr441
6
VEGFR, RET
50
NA
Schlumberger459
59
RET, VEGFR, FGFR, PDGFR, C-KIT
36
9.0
Kurzrock460
35
VEGFR, RET, C-MET
49
NA
Pazopanib
Bible461
35
VEGFR, PDGFR, RET, C-KIT
14
9.4
Gefitinib
Pennell462
4
EGFR
0
NA
Imatinib
De Groot463
Frank-Raue464
15
9
C-KIT, PDGFR
0
0
NA
NA
NA, Not applicable; RECIST, Response Evaluation Criteria in Solid Tumors.
   
TABLE 14.15
477
Chapter 14  Nontoxic Diffuse Goiter, Nodular Thyroid Disorders, 

ntoxic Diffuse Goiter, Nodular Thyroid Disorders, and Thyroid Malignancies
in 91% of the cases.297–299,453 The most frequent RET mutation 
found is the M918T mutation. In the absence of RET mutation, 
RAS mutations are found in 10% to 45% of the cases, with the 
HRAS mutation being more frequent than KRAS, which is more 
frequent than NRAS.297–299,453 ALK rearrangement and MET 
mutation have also been found, but in less than 5% of the speci­
mens.454 VEGF receptors 1 and 2 also often are overexp

.454 VEGF receptors 1 and 2 also often are overexpressed in 
MTC, which together with the fibroblast growth factor (FGF) 
and platelet-derived growth factor (PDGF) play a role in tumor 
angiogenesis.455 In general, tyrosine kinase inhibitors (TKIs) have 
shown significant results in patients with advanced MTC.
Many TKIs targeting RET and VEGFR2, among other targets, 
have been used in phase II trials, with significant tumor response 
rates ranging from none to 50%432,434,441,456–464 (Table 14.15

g from none to 50%432,434,441,456–464 (Table 14.15). 
Two drugs outlined next have been evaluated in phase III trials: 
vandetanib and cabozantinib in prospective randomized double-
blind studies.465,466 Their main objective was to show a benefit in 
PFS (Table 14.16).
  Phase III Trials in Metastatic or Locally 
Advanced MTC
Vandetanib: ZETA 
Study Wells466
Cabozantinib: 
EXAM Study 
Elisei465
Patient Characte

ozantinib: 
EXAM Study 
Elisei465
Patient Characteristics
Number of patients included
331
330
Frequency of hereditary MTC
10%
6%
RET mutation: present 
unknown
RETM918T mutation
38%
41%
31%
45%
39%
35%
94%
95%
Previous treatment with TKI
Unknown
20%
RECIST progression prior 
enrollment
Not mandatory
Yes, within 14 
months
Results
Median follow-up
24 months
14 months
Median PFS
>30.5 months 
(vandetanib) vs 
19.3 months 
(placebo)
11,2 (cabo­
zantinib) vs 
4 months 
(placebo)
C

o)
4 months 
(placebo)
Complete response
0%
0%
Partial RECIST response
45%
28%
Not available
26.6 months 
(cabo­
zantinib) 
vs 21.1 
months 
(placebo)
Toxicity
Toxicity any grade (≥ grade 3)
55% (24%)
69% (33%)
Decrease in dose due to 
toxicity
35%
65%
Stop for toxicity
12%
16%
Deaths
2% (vandetanib) vs 
2% (placebo)
5.6% (cabo­
zantinib) 
vs 2.8% 
(placebo)
EXAM, Efficacy of XL184 in Advanced Medullary Thyroid Cancer; MTC, medullary thyroid 
carcinoma;

Thyroid Cancer; MTC, medullary thyroid 
carcinoma; PFS, progression-free survival; RECIST, Response Evaluation Criteria in Solid 
Tumors; TKI, tyrosine kinase inhibitor.
   
TABLE 14.16
         
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empiric radioiodine therapy in patients with structurally identifi­
able metastatic follicular cell-derived thyroid carcinoma with nega­
tive diagnostic but positive post-therapy 131I whole-body scans. 
Thyr

positive post-therapy 131I whole-body scans. 
Thyroid. 2012;22:877–883.
	429.	 Carhill AA, Litofsky DR, Ross DS, et  al. Long-term out­
comes following therapy in differentiated thyroid carcinoma: 
NTCTCS registry analysis 1987-2012. J Clin Endocrinol Metab. 
2015;100:3270–3279.
	430.	 Terroir M, Borget I, Bidault F, et  al. The intensity of 18FDG 
uptake does not predict tumor growth in patients with meta­
static differentiated thyroid cancer. Eur J Nucl Med Mol Imaging. 
2017;44:638–646.
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ed thy­
roid cancers: results of a phase 2 consortium study. Lancet Oncol. 
2010;11:962–972.
	440.	 Brose MS, Nutting CM, Jarzab B, et al. Sorafenib in radioactive 
iodine-refractory, locally advanced or metastatic differentiated 
thyroid cancer: a randomised, double-blind, phase 3 trial. Lancet. 
2014;384:319–328.
	441.	 Carr LL, Mankoff DA, Goulart BH, et al. Phase II study of daily 
sunitinib in FDG-PET-positive, iodine-refractory differenti­
ated thyroid cancer and metastatic medullary car

­
ated thyroid cancer and metastatic medullary carcinoma of the 
thyroid with functional imaging correlation. Clin Cancer Res. 
2010;16:5260–5268.
	442.	 Brose MS, Cabanillas ME, Cohen EE, et  al. Vemurafenib in 
patients with BRAF(V600E)-positive metastatic or unresectable 
papillary thyroid cancer refractory to radioactive iodine: a non-
randomised, multicentre, open-label, phase 2 trial. Lancet Oncol. 
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double-blind, phase 2 trial. Lancet Oncol. 2012;13:897–905.
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Thyroid. 2015;25:71–77.
	445.	 Brose MS, Worden FP, Newbold KL, et al. Effect of age on the 
efficacy and safety of lenvatinib in radioiodine-refractory differenti­
ated thyroid cancer

 al. Identification of driving ALK fusion 
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ment of patients with locally advanced or metastatic hereditary 
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phase II trial of imatinib therapy for metastatic medullary thyroid 
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480
15
The Adrenal Cortex
JOHN D.C. NEWELL-PRICE AND RICHARD J. AUCHUS
The Adrenal Cortex—Historical Milestones
The anatomy of the adrenal glands was described almost 450 years 
ago by Bartolomeo Eustachius,1 and the zonation of the gland 
and its distinction from the medulla were elucidated shortly there­
after. However, a functional role for the adrenal glands was not 
accurately defined until the pioneering work of Thomas Addison, 
who d

ntil the pioneering work of Thomas Addison, 
who described the clinical and autopsy findings in 11 cases of 
Addison disease in his classic monograph in 1855.2 Just a year 
later, Brown-Séquard demonstrated that the adrenal glands were 
“organs essential for life” by performing adrenalectomies in dogs, 
cats, and guinea pigs.3 In 1896, William Osler first administered 
adrenal extract to a patient with Addison disease, a feat that was 
repeated by others in animal and human studies over the next

y others in animal and human studies over the next 
40 years. Between 1937 and 1955, the adrenocorticosteroid hor­
mones were isolated, and their structures were defined and synthe­
sized.4 Notable breakthroughs included the discovery of cortisone 
and clinical evaluation of its anti-inflammatory effect in patients 
with rheumatoid arthritis5 and the isolation of aldosterone.6
The control of adrenocortical function by a pituitary factor was 
demonstrated in the 1920s, and this led to the isolati

nstrated in the 1920s, and this led to the isolation of sheep 
adrenocorticotropic hormone (ACTH) by Li, Evans, and Simpson 
in 1943.7 Such a concept was supported through clinical studies, 
notably by Harvey Cushing in 1932, who associated his original 
clinical observations of 1912 (a “polyglandular syndrome” caused 
by pituitary basophilism) with adrenal hyperactivity.8 The neural 
control of pituitary ACTH secretion by corticotropin-releasing 
factor (later renamed corticotropin-releasing ho

factor (later renamed corticotropin-releasing hormone [CRH]) 
was defined by Harris and other workers in the 1940s, but CRH 
was not characterized and synthesized until 1981 in the laboratory 
of Wylie Vale.9 Jerome Conn described primary aldosteronism in 
1955,10 and the control of adrenal aldosterone secretion by angio­
tensin II was confirmed shortly afterward. Advances in radioim­
munoassay, and particularly molecular biology, have facilitated an 
exponential increase in the understanding 

ted an 
exponential increase in the understanding of adrenal physiology 
and pathophysiology (Table 15.1). 
Anatomy and Development
The cells forming the adrenal cortex originate from the intermedi­
ate mesoderm. These cells derive from the urogenital ridge and 
have a common embryologic origin with the gonad and the kid­
ney. Early differentiation of the adrenogonadal primordium from 
the urogenital ridge requires signaling cascades and transcription 
factors GLI3, SALL1, FOXD2, WT1, PBX1, and 

ption 
factors GLI3, SALL1, FOXD2, WT1, PBX1, and WNT4, and 
the regulator of telomerase activity, ACD (Fig. 15.1). The adreno­
gonadal primordium can be seen as the medial part of the urogeni­
tal ridge at 4 weeks. Separation of the adrenogonadal primordium 
and formation of the adrenal primordium seem to depend on 
the actions of transcription factors SF1 (steroidogenic factor 1), 
The Adrenal Cortex—Historical Milestones, 480
Anatomy and Development, 480
Adrenal Steroids and S

natomy and Development, 480
Adrenal Steroids and Steroidogenesis, 482
Corticosteroid Hormone Action, 490
Classification and Pathophysiology of Cushing Syndrome, 502
Glucocorticoid Deficiency, 517
Congenital Adrenal Hyperplasia, 527
Adrenal Adenomas, Incidentalomas, and Carcinomas, 539
KEY POINTS
 • 
This chapter discusses mechanisms and regulation of adrenal 
steroid production, function of the hypothalamic-
pituitary-adrenal axis, and negative regulation.
 • 
The chapter goes on to describe t

apter 15  The Adrenal Cortex
DAX1, WNT4, and CITED2. The adrenocortical primordium 
develops at approximately 8 weeks of gestation and can be dif­
ferentiated into two distinct layers, the inner fetal zone (FZ) and 
the outer definitive zone (DZ). At approximately 9 weeks, the 
adrenal blastema encapsulates and the adrenal medulla develops 
when neural crest cells migrate into the adrenal gland.11 During 
the second trimester, the FZ enlarges, becomes larger than the 
fetal kidney, and secretes 

comes larger than the 
fetal kidney, and secretes abundant amounts of dehydroepiandros­
terone (DHEA) and dehydroepiandrosterone sulfate (DHEAS). 
Concentrations of these hormones abruptly decline postnatally, in 
parallel with the postnatal involution of the FZ. The neocortex 
develops over the subsequent years into the adult adrenal gland.
   History of the Adrenal Cortex: Important Milestones
Year
Event
1563
Eustachius describes the adrenals (published by Lancisi in 1714).
1849
Thomas Addiso

(published by Lancisi in 1714).
1849
Thomas Addison, while searching for the cause of pernicious anemia, stumbles on a bronzed appearance associated with the adrenal 
glands—melasma suprarenale.
1855
Thomas Addison describes the clinical features and autopsy findings in 11 cases of diseases of the suprarenal capsules, at least 6 of which 
were tuberculous in origin.
1856
In adrenalectomy experiments, Brown-Séquard demonstrates that the adrenal glands are essential for life.
1896
William Osler pr

ands are essential for life.
1896
William Osler prepares an oral glycerin extract derived from pig adrenals and demonstrates that it has clinical benefit in patients with Addison 
disease.
1905
Bulloch and Sequeira describe patients with congenital adrenal hyperplasia.
1929
Liquid extracts of cortical tissue are used to keep adrenalectomized cats alive indefinitely (Swingle and Pfiffner); subsequently, this extract 
was used successfully to treat a patient with Addison disease (Rowntree and Gree

 a patient with Addison disease (Rowntree and Greene).
1932
Harvey Cushing associates the polyglandular syndrome of pituitary basophilism, which he first described in 1912, with hyperactivity of the 
pituitary-adrenal glands.
1936
The concept of stress and its effect on pituitary-adrenal function are described by Selye.
1937–1952
Isolation and structural characterization of adrenocortical hormones are reported by Kendall and Reichstein.
1943
Li and colleagues isolate pure adrenocorticotropic hor

nd colleagues isolate pure adrenocorticotropic hormone from sheep pituitary.
1950
Hench, Kendall, and Reichstein share the Nobel Prize in Medicine for describing the anti-inflammatory effects of cortisone in patients with 
rheumatoid arthritis.
1953
Isolation and analysis of the structure of aldosterone are reported by Simpson and Tait.
1956
Conn describes primary aldosteronism.
1981
Characterization and synthesis of corticotropin-releasing hormone are reported by Vale.
1980–
present
The molecul

ne are reported by Vale.
1980–
present
The molecular era: cloning and functional characterization of steroid receptors, steroidogenic enzymes, and adrenal transcription factors are 
reported, and the molecular basis for human adrenal diseases is defined.
TABLE 15.1
Birth
Medulla
DZ
FZ
DZ
FZ
ZG
ZF
ZG
ZF
ZR
Islets
ZG
ZF
ZR
4 Weeks
6 Months
2-3 Years
6-8 Years
8 Weeks
9 Weeks
24-28 Weeks
Urogenital
ridge
Adreno-
gonadal
primordium
Adrenal
Fetal adrenal
Adrenal
Adrenal
from
2-3 yrs
Adrena

Adrenal
Adrenal
from
2-3 yrs
Adrenal
after
adrenache
Hedgehog
signaling,
GLI3, SALL1,
FOXD2, WT1,
PBX1, WNT4, ACD
SF1, DAX1,
CITED2,
WNT4
NGFIB, POMC-peptides,
growth factors, midkine,
SPARC, neural feedback
POMC-peptides,
neural feedback
Prenatal life
Postnatal life
• Fig. 15.1  Schematic diagram of the development of the human adrenal cortex during prenatal and post­
natal life showing transcription factors that are active at each stage (see text for details). DZ, definitive zon

h stage (see text for details). DZ, definitive zone; 
FZ, fetal zone; POMC, pro-opiomelanocortin; SPARC, secreted protein, acidic, cysteine-rich (osteonectin); 
ZF, zona fasciculata; ZG, zona glomerulosa; ZR, zona reticularis.
482
SECTION  IV    Adrenal Cortex and Endocrine Hypertension
In fetal life and up to 12 months of age, two distinct zones are 
evident, an inner prominent FZ and an outer DZ that differen­
tiates into the adult adrenal gland. After birth, the FZ regresses 
and the DZ, whic

d. After birth, the FZ regresses 
and the DZ, which contains an inner zona fasciculata (ZF) and 
an outer zona glomerulosa (ZG), proliferates.12,13 The innermost 
zone, the zona reticularis (ZR), is evident after 2 years of life. The 
differentiation of the adrenal cortex into distinct zones has impor­
tant functional consequences and is thought to depend on the tem­
poral expression of transcription factors, including Pref1/ZOG, 
inner zone antigen, and SF1.14,15 In preadrenarchal children, foc

gen, and SF1.14,15 In preadrenarchal children, focal 
reticular zone islets can be found, but the ZG and ZF are clearly 
differentiated.16 The occurrence of these ZR islets is consistent 
with the observation that DHEA and DHEAS synthesis gradually 
begins to rise from about 3 years of age.17 At adrenarche, the inner 
zone (ZR) thickens, corresponding with increased production 
of DHEA and DHEAS. Concurrently, changes in zone-specific 
enzyme expression patterns, such as decreased 3β-hydroxyster

ression patterns, such as decreased 3β-hydroxysteroid 
dehydrogenase type 2 (3βHSD2) and increased cytochrome b5 
and sulfotransferase (SULT2A1) in the ZR, lead to increased flux 
toward DHEA. Clinically, adrenarche becomes apparent at 6 to 
8 years of age. Adrenal androgen production peaks in the third 
decade and then declines at a variable rate. Mineralocorticoids and 
glucocorticoids show a less age-specific variation.
The adult adrenal gland is a pyramidal structure, approxi­
mately 4 g in 

 is a pyramidal structure, approxi­
mately 4 g in weight, 2 cm wide, 5 cm long, and 1 cm thick, that 
lies immediately above the kidney on its posteromedial surface. 
Beneath the capsule, the ZG makes up approximately 15% of the 
cortex (depending on sodium intake) (Fig. 15.2). Cells are clus­
tered in spherical nests and are small, with smaller nuclei in com­
parison with cells in other zones. The ZF makes up 75% of the 
cortex; cells are large and lipid laden and form radial cords within 
the 

and lipid laden and form radial cords within 
the fibrovascular radial network. The innermost ZR is sharply 
demarcated from both the ZF and the adrenal medulla. Cells 
there are irregular with little lipid content. The maintenance of 
normal adrenal size appears to involve a progenitor cell population 
lying between the ZG and ZF; cell migration and differentiation 
occur within the ZF, and senescence occurs within the ZR, but 
the factors regulating this important aspect of adrenal regenera­
t

ating this important aspect of adrenal regenera­
tion are unknown. Fetal cells give rise to a subcapsular stem cell 
population that differentiates in a centripetal direction.18 ACTH 
administration results in glomerulosa cells adopting a fasciculata 
phenotype, and in turn, the innermost fasciculata cells adopt a 
reticularis phenotype that is reversible on withdrawal of ACTH.
The vasculature of the adrenal cortex is complex. Arterial sup­
ply is conveyed by up to 12 small arteries from the aor

s conveyed by up to 12 small arteries from the aorta and the 
inferior phrenic, renal, and intercostal arteries. These arteries 
branch to form a subcapsular arteriolar plexus from which radial 
capillaries penetrate deeper into the cortex. In the ZR, a dense 
sinusoidal plexus is created, which empties into a central vein. The 
right adrenal vein is short, draining directly into the inferior vena 
cava, whereas the longer left adrenal vein usually drains into the 
left renal vein. 
Adrenal Ster

ly drains into the 
Adrenal Steroids and Steroidogenesis
Three main types of hormones are produced by the adrenal cor­
tex—glucocorticoids (cortisol, corticosterone), mineralocorticoids 
(aldosterone, deoxycorticosterone [DOC]), androgen precur­
sors (DHEA, DHEAS, androstenedione), and a small amount of 
androgens (testosterone and 11-oxygenated 19-carbon androgens/
precursors). All steroid hormones are derived from the cyclopen­
tanoperhydrophenanthrene structure, that is, thr

­
tanoperhydrophenanthrene structure, that is, three cyclohexane 
rings and a single cyclopentane ring (Fig. 15.3). Steroid nomen­
clature is defined in one of two ways: by trivial names (e.g., cor­
tisol, aldosterone) or by the chemical structure as defined by the 
International Union of Pure and Applied Chemistry (IUPAC).19 
The IUPAC classification is inappropriate for clinical use but does 
provide an invaluable insight into steroid structure. The basic 
structure, trivial name, and IUPAC na

. The basic 
structure, trivial name, and IUPAC name of some common ste­
roids are given in Fig. 15.3 and Table 15.2. Estrogens have 18 
carbon atoms (C18 steroids) and androgens have 19 carbon atoms 
(C19), whereas glucocorticoids, mineralocorticoids, and progesto­
gens are C21-steroid derivatives.
Cholesterol is the precursor for adrenal steroidogenesis. It is 
provided principally from the circulation, in the form of low-
density lipoprotein (LDL) cholesterol.20 Uptake is by specific cell-
su

LDL) cholesterol.20 Uptake is by specific cell-
surface LDL receptors present on adrenal tissue21; LDL is then 
internalized via receptor-mediated endocytosis,22 the resulting 
vesicles fuse with lysozymes, and free cholesterol is produced after 
hydrolysis. However, it is clear that this cannot be the sole source 
Zona glomerulosa
Zona fasciculata
Zona reticularis
Capsule
Cortex
Medulla
• Fig. 15.2  Schematic diagram of the structure of the human adrenal cor­
tex, depicting the outer zona glome

n adrenal cor­
tex, depicting the outer zona glomerulosa and inner zona fasciculata and 
zona reticularis.
483
Chapter 15  The Adrenal Cortex
A
1
HO
HO
O
Dehydroepiandrosterone
β
5-Androsten-3   -ol-17-one
CH3
O
C
CH2OH
O
C
O
C
Pregnenolone
β
5-Pregnen-3   -ol-20-one
CH3
O
C
CH3
O
O
O
OH
17-OH-Progesterone
α
4-Pregnen-17   -ol-3,20, dione
Deoxycorticosterone
4-Pregnen-21-ol-3,20-dione
O
O
O
O
O
O
O
4-Androsten-3,17-dione
OH
CH2OH
O
C
CH2OH
O
C
CH2OH
O
C
OH
Cortisone
4-Pregnen-17a

O
C
CH2OH
O
C
CH2OH
O
C
OH
Cortisone
4-Pregnen-17a,21-diol-3,11,20-trione
HO
HO
Cortisol
4-Pregnen-11b,17a,21-triol-3,20-dione
CH
4-Pregnen-11b,21-diol-3,18,20-trione
4-Pregnen-3,20-dione
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
B
C
D
• Fig. 15.3  The cyclopentanoperhydrophenanthrene structure of corticosteroid hormones, highlighting the 
structure of some endogenous steroid hormones together with their nomenclature.
   IUPAC and Trivial Names of Natural and 

ature.
   IUPAC and Trivial Names of Natural and Synthetic Steroids
Trivial Name
IUPAC Name
4-Pregnen-11β,21-diol-3,18,20-trione
Cortisol
4-Pregnen-11β,17α,21-triol-3,20-dione
Cortisone
4-Pregnen-17α,21-diol-3,11,20-trione
5-Androsten-3β-ol-17-one
1,4-Pregnadien-9α-fluoro-16α-methyl-11β,17α,21-triol-3,20-dione
Dihydrotestosterone
5α-Androstan-17β-ol-3-one
Estrad

hydrotestosterone
Estradiol
1,3,5(10)-Estratrien-3,17β-diol
4-Pregnen-9α-fluoro-11β,17α,21-triol-3,20-dione
4-Pregnen-17α-ol-3,20-dione
Methylprednisolone
1,4-Pregnadien-6α-methyl-11β,17α,21-triol-3,20-dione
Prednisolone
1,4-Pregnadien-11β,17α,21-triol-3,20-dione
TABLE 15.2
Continued
484
of adrenal cholesterol, because patients with abetalipoproteinemia 
who have undetectable

s with abetalipoproteinemia 
who have undetectable circulating LDL and patients with defec­
tive LDL receptors in the setting of familial hypercholesterolemia 
still have normal basal adrenal steroidogenesis. Cholesterol can be 
generated de novo within the adrenal cortex from acetyl coenzyme 
A (CoA). In addition, there is evidence that the adrenal gland can 
utilize high-density lipoprotein (HDL) cholesterol after uptake 
through the putative HDL receptor, SR-B1.23
The biochemical pathways inv

DL receptor, SR-B1.23
The biochemical pathways involved in adrenal steroidogenesis 
are shown in Fig. 15.4. The initial hormone-dependent, rate-
limiting step is the transport of intracellular cholesterol from the 
outer to inner mitochondrial membrane for conversion to preg­
nenolone by cytochrome P450 side-chain cleavage enzyme (P450 
11A1). Naturally occurring human mutations have confirmed the 
importance of a 30-kDa protein, steroidogenic acute regulatory 
protein (StAR), in mediating this 

ute regulatory 
protein (StAR), in mediating this effect. StAR is induced by an 
increase in intracellular cyclic adenosine monophosphate (cAMP) 
after binding of ACTH to its cognate receptor, providing the first 
important rate-limiting step in adrenal steroidogenesis.24 Other 
transporters, including the peripheral benzodiazepine-like recep­
tor, may be involved.25
Steroidogenesis involves the concerted action of several 
enzymes, including a series of cytochrome P450 enzymes, all of 
which ha

eries of cytochrome P450 enzymes, all of 
which have been cloned and characterized (Table 15.3). Cyto­
chrome P450 enzymes are classified into two types, according to 
their subcellular localization and their specific electron shuttle 
system. Mitochondrial (type I) cytochrome P450 enzymes such 
as CYP11A1 (P450 11A1), 11α-hydroxylase (CYP11B1, or 
P45011B1), and aldosterone synthase (CYP11B2, or P450 11B2) 
rely on electron transfer facilitated by adrenodoxin and adreno­
doxin reductase.26,27 M

by adrenodoxin and adreno­
doxin reductase.26,27 Micrososomal (type II) cytochrome P450 
enzymes localized to the endoplasmic reticulum include the ste­
roidogenic enzymes 17α-hydroxylase (CYP17A1, or P450 17A1), 
21-hydroxylase (CYP21A2, or P450 21A2), and P450 aromatase 
(CYP19A1, or P450 19A1). The functions of cytochrome P450 
type II enzymes crucially depend on P450 oxidoreductase (POR), 
which provides electrons required for monooxygenase reactions 
catalyzed by the P450 enzyme.27,28 This 

eactions 
catalyzed by the P450 enzyme.27,28 This category also includes 
hepatic P450 enzymes involved in drug metabolism and enzymes 
involved in sterol and bile acid synthesis.27,28 In addition, the 
17,20-lyase activity of P450 17A1 is dependent on a hemoprotein 
cytochrome b5, which αβ functions as an allosteric facilitator of 
P450 17A1 with POR (Fig. 15.5; see also Fig. 15.4).29
Mutations in the genes encoding these enzymes result in human 
disease, so some understanding of the underlying

disease, so some understanding of the underlying pathways and 
steroid precursors is required.30 After uptake of cholesterol to the 
mitochondrion, cholesterol is cleaved by the P450 11A1 enzyme 
to form pregnenolone.31 In the cytoplasm, pregnenolone is con­
verted to progesterone by the type II isozyme 3βHSD through 
a reaction involving dehydrogenation of the 3-hydroxyl group 
and isomerization of the double bond at C5.32 Progesterone is 
hydroxylated to 17-hydroxyprogesterone (17OHP) throug

roxylated to 17-hydroxyprogesterone (17OHP) through the 
17α-hydroxylase activity of P450 17A1. 17α-Hydroxylation is 
an essential prerequisite for cortisol synthesis, and the ZG does 
not express 17α-hydroxylase. P450 17A1 also possesses 17,20-
lyase activity, which results in production of the C19 adrenal 
androgens DHEA and androstenedione.33 In humans, however, 
17OHP is not an efficient substrate for P450 17A1, and there 
is negligible conversion of 17OHP to androstenedione. Adrenal 
andros

rsion of 17OHP to androstenedione. Adrenal 
androstenedione secretion is dependent on the conversion of 
DHEA to androstenedione by 3βHSD. This enzyme also converts 
17-hydroxypregnenolone to 17OHP, but the preferred substrate is 
pregnenolone. The human adrenal gland is capable of synthesis of 
small but significant amounts of testosterone, which increases in 
clinical conditions associated with androgen excess. This conver­
sion is facilitated by the enzyme 17βHSD type 5 (17βHSD5), also 
calle

by the enzyme 17βHSD type 5 (17βHSD5), also 
called aldo-keto reductase 1C3 (AKR1C3).34 21-Hydroxylation 
of either progesterone (in the ZG) or 17OHP (in the ZF) is car­
ried out by the product of the CYP21A2 gene, 21-hydroxylase, to 
yield DOC or 11-deoxycortisol, respectively.35 The final step in 
cortisol biosynthesis takes place in the mitochondria and involves 
the conversion of 11-deoxycortisol to cortisol by the enzyme P450 
11B1 (11β-hydroxylase).36 In the ZG, 11β-hydroxylase may also 
c

oxylase).36 In the ZG, 11β-hydroxylase may also 
convert DOC to corticosterone. The enzyme P450 11B2 (aldo­
sterone synthase) may also carry out this reaction; in addition, 
P450 11B2 is required for conversion of corticosterone to aldoste­
rone via the intermediate 18OH corticosterone; CYP11B1 lacks 
these two enzymatic activities.37,38 Therefore P450 11B2 can carry 
out 11β-hydroxylation, 18-hydroxylation, and 18-methyl oxida­
tion to yield the characteristic C11-18 hemiacetyl structure of 
al

 characteristic C11-18 hemiacetyl structure of 
aldosterone.
Regulation of Adrenal Steroidogenesis: 
Functional Zonation of the Adrenal Cortex
Glucocorticoids are secreted in relatively high amounts (cortisol, 
10–20 mg/day) from the ZF under the control of ACTH; miner­
alocorticoids are secreted in low amounts (aldosterone, 100–150 
μg/day) from the ZG under the principal control of angiotensin 
II. As a class, adrenal androgen precursors (DHEA, DHEAS, 
androstenedione, 11β-hydroxyandrostenedio

 DHEAS, 
androstenedione, 11β-hydroxyandrostenedione) are the most 
abundant steroids secreted from the adult adrenal gland (>20 mg/
day). In each case, secretion is facilitated through the expression of 
steroidogenic enzymes in a specific zonal manner. The ZG cannot 
IUPAC Name
Prednisone
1,4-Pregnadien-17α,21-diol-3,11,20-trione
5-Pregnen-3β-ol-20-one
4-Androsten-17β-ol-3-one
Triamcinolone
1,4-Pregnadien-9α-fluoro-11β,16

-one
1,4-Pregnadien-9α-fluoro-11β,16α,17α,21-tetrol-3,20-dione
IUPAC, International Union of Pure and Applied Chemistry.
   
   IUPAC and Trivial Names of Natural and Synthetic Steroids—cont’d
TABLE 15.2
485
Cholesterol
StAR
CYP11A1
Outer
Inner
Mitochondrial membrane
ADR/Adx
ADR/Adx
ADR/Adx
H6PDH
POR
HSD3B2
HSD3B2
HSD3B2
Pregnenediol
Pregnenetriol
CYP17A1
*
POR
b5
17OH-Pregnenolone
DHEAS
CYP17A1
DHEA, 16αOH-DHEA
Androsterone,

olone
DHEAS
CYP17A1
Androsterone, Etiocholanolone
Dehydroepi-
androsterone
SULT2A1
PAPSS2
POR
POR
CYP21A2
POR
CYP21A2
HSD17B
Pregnanediol
Pregnanetriol
17-OH-Pregnenolone
Androsterone
Etiocholanolone
CYP17A1
POR
b5
17OH-Progesterone
CYP17A1
CYP11B1
THDOC
THS
THF, 5αTHF
ADR/Adx
CYP11B2
THA, THB,
5αTHA, 5αTHB
Cortisol
THE
ADR/Adx
18OH-Corticosterone
CYP11B2
18OH-THA
Ald

E
ADR/Adx
CYP11B2
18OH-THA
THALDO
Cortisone
Pregnanetriolone
Androgens
Mineralocorticoid
21-Deoxycortisol
HSD11B1
HSD11B2
CYP11B2
CYP11B1
• Fig. 15.4  Adrenal steroidogenesis. After the steroidogenic acute regulatory (StAR) protein-mediated 
uptake of cholesterol into mitochondria within adrenocortical cells, aldosterone, cortisol, and adrenal 
androgens are synthesized through the coordinated action of a series of steroidogenic enzymes

inated action of a series of steroidogenic enzymes in a 
zone-specific fashion. The mitochondrial cytochrome P450 (CYP) type I enzymes (CYP11A1, CYP11B1, 
CYP11B2) requiring electron transfer via adrenodoxin reductase (ADR) and adrenodoxin (Adx) are marked 
with a box labeled ADR/Adx. The microsomal CYP type II enzymes (CYP17A1, CYP21A2) receive electrons 
from P450 oxidoreductase (circle labeled POR). The 17,20-lyase reaction catalyzed by CYP17A1 requires, 
in addition to POR, cytochrome b5, in

1 requires, 
in addition to POR, cytochrome b5, indicated by a circle labeled b5. Urinary steroid hormone metabolites 
are given in italics below the plasma hormones. The asterisk (*) indicates 11-hydroxylation of 17OH-
progesterone to 21-deoxycortisol in cases of 21-hydroxylase deficiency. The adrenal conversion of andro­
stenedione to testosterone is catalyzed by the aldo-keto reductase AKR1C3 (HSD17B5). CYP11A1, P450 
side-chain cleavage enzyme; CYP11B1, 11β-hydroxylase; CYP11B2, aldosterone 

e; CYP11B1, 11β-hydroxylase; CYP11B2, aldosterone synthase; CYP17A1, 
17α-hydroxylase; CYP21A2, 21-hydroxylase; DHEA, dehydroepiandrosterone; DHEAS, dehydroepiandros­
terone sulfate; H6PDH, hexose-6-phosphate dehydrogenase; HSD11B1, 11β-hydroxysteroid dehydroge­
nase 1; HSD11B2, 11β-hydroxysteroid dehydrogenase 2; HSD17B, 17β-hydroxysteroid dehydrogenase; 
HSD3B2, 3β-hydroxysteroid dehydrogenase type 2; 17OH-progesterone, 17α-hydroxyprogesterone; 
PAPSS2, 3′-phosphoadenosine, 5′-phosphosulfate s

PAPSS2, 3′-phosphoadenosine, 5′-phosphosulfate synthase 2; SULT2A1, sulfotransferase 2A1; THA, 
tetrahydro-11-dehydrocorticosterone; THB, tetrahydro-corticosterone; THALDO, tetrahydro-aldosterone; 
THDOC, tetrahydro-11-deoxycorticosterone; THF, tetrahydrocortisol; THS, tetrahydro-11-deoxycortisol.
486
synthesize cortisol because it does not express 17α-hydroxylase. In 
contrast, aldosterone secretion is confined to the outer ZG because 

ne secretion is confined to the outer ZG because 
of the restricted expression of P450 11B2. Although P450 11B1 
and P450 11B2 share 95% homology, the 5′ promoter sequences 
differ, permitting regulation of the final steps in glucocorticoid 
and mineralocorticoid biosynthesis by ACTH and angiotensin II, 
respectively. In the ZR, high levels of cytochrome b5 confer 17,20-
lyase activity on P450 17A1 and androgen precursor production. 
DHEA is sulfated in the ZR by the DHEA SULT2A1 to form 
DHEAS.

ated in the ZR by the DHEA SULT2A1 to form 
DHEAS. This sulfonation reaction facilitated by SULT2A1 relies 
on the donor 3′-phosphoadenosine 5′-phosphosulfate (PAPS) 
to transfer a sulfonate group to an acceptor molecule. PAPS is 
synthesized by PAPS synthase, of which two isoenzymes exist 
(PAPSS1 and PAPSS2).39
In the fetal adrenal, steroidogenesis occurs primarily within the 
inner FZ. The FZ is a characteristic feature of higher primates, but 
the biologic role of fetal androgen production r

the biologic role of fetal androgen production remains unclear. 
Because of a relative lack of 3βHSD and high SULT2A1 activ­
ity, the principal steroidogenic products are DHEA and DHEAS, 
which are then aromatized by placental trophoblast to estrogens. 
Therefore the majority of maternal estrogen across pregnancy is, 
indirectly, fetally derived.40
Classic endocrine feedback loops are in place to control the 
secretion of both hormones. Cortisol inhibits the secretion of 
CRH from the hypothal

l inhibits the secretion of 
CRH from the hypothalamus and ACTH from the pituitary, 
and aldosterone-induced sodium retention inhibits renal renin 
secretion. 
Glucocorticoid Secretion: The Hypothalamic-
Pituitary-Adrenal Axis
Pro-opiomelanocortin and ACTH
ACTH is the principal hormone stimulating adrenal glucocorti­
coid biosynthesis and secretion. ACTH has 39 amino acids but is 
synthesized within the anterior pituitary as part of a much larger, 
241–amino acid precursor called pro-opiomelanoc

, 
241–amino acid precursor called pro-opiomelanocortin (POMC). 
A transcription factor, TPIT, appears to be essential for differen­
tiation of POMC-expressing cells within the anterior pituitary.41 
POMC is cleaved in a tissue-specific fashion by prohormone con­
vertases to yield smaller peptide hormones. In the anterior pitu­
itary, this results in the secretion of β-lipoprotein (βLPH) and 
pro-ACTH, the latter being further cleaved to an amino-terminal 
peptide, joining peptide, and ACTH itse

-terminal 
peptide, joining peptide, and ACTH itself (Fig. 15.6).42,43 Post­
secretion cleavage of the precursor to γ-melanocyte-stimulating 
hormone (pro-γMSH) by a serine protease (AsP) expressed in the 
outer adrenal cortex is thought to mediate the trophic action of 
ACTH on the adrenal cortex.44 The first 24 amino acids of ACTH 
are common to all species, and a synthetic ACTH(1-24), tetraco­
sactide or cosyntropin (Synacthen, Cortrosyn), is available com­
mercially for clinical testing of t

available com­
mercially for clinical testing of the hypothalamic-pituitary-adrenal 
(HPA) axis and assessing adrenal glucocorticoid reserve. The hor­
mones αMSH, βMSH, and γMSH are also cleaved products from 
POMC, but the increased pigmentation characteristic of Addison 
disease is thought to arise directly from increased ACTH concen­
trations binding to the melanocortin-1 receptor (MC1R) rather 
than from αMSH secretion.45
POMC is also transcribed in many extrapituitary tissues, 
notably brai

ibed in many extrapituitary tissues, 
notably brain, liver, kidney, gonad, and placenta.42,46,47 In these 
   Nomenclature for Adrenal Steroidogenic Enzymes and Their Genes
Enzyme Name
Enzyme Family
Gene
P450 11A1, Cholesterol side-chain cleavage (SCC) 
(desmolase)
Cytochrome P450 type I
CYP11A1
15q23-q24
3β-Hydroxysteroid dehydrogenase (3βHSD) (type II 
isozyme)
Short-chain alcohol dehydrogenase reductase 
superfamily
HSD3B2
1p13.1
P450 17A1, 17α-Hydroxylase/17,20-lyase
Cytochrome P

450 17A1, 17α-Hydroxylase/17,20-lyase
Cytochrome P450 type II
CYP17A1
10q24.3
P450 21A2, 21-Hydroxylase
CYP21A2
6p21.3
P450 11B1, 11β-Hydroxylase
CYP11B1
8q24.3
P450 11B2, Aldosterone synthase
CYP11B2
8q24.3
TABLE 15.3
NADPH
NADP+
A
Fp
Fp.
Andrenodoxin
reductase
Fe2+
Fe3+
Adrenodoxin
Fe3+
Fe2+
CYP11A1,
CYP11B1, or
CYP11B2
HOCH2-
Steroid
CH3-Steroid
H2O
O2
CYP.O
NADPH
NADP+
Fp
Fp.
Fe2+
Fe3+
HOCH2-Steroid
H2O
O2
CYP.

p.
Fe2+
Fe3+
H2O
O2
CYP.O
B
P450 oxidoreductase
or
cytochrome b5
CYP17A1
or
CYP21A2
• Fig. 15.5  (A) Electron shuttle system for the mitochondrial enzymes 
CYP11A1, CYP11B1, and CYP11B2. Adrenodoxin reductase receives 
electrons from reduced nicotinamide adenine dinucleotide phosphate 
(NADPH) and reduces adrenodoxin, which transfers reducing equivalents 
to the cytochrome P450 (CYP) enzyme. The enzyme then uses these elec­
trons, plus molecular oxygen, to oxygenate the

ec­
trons, plus molecular oxygen, to oxygenate the steroid. (B) Electron shuttle 
system for the microsomal enzymes CYP17A1 and CYP21A2. P450 oxi­
doreductase, a flavoprotein, accepts electrons from NADPH and transfers 
them to the NADPH-P450 enzyme. The enzyme then uses these elec­
trons, plus molecular oxygen, to oxygenate the steroid. A second reducing 
equivalent may be supplied to CYP17A1 by NADPH-P450 oxidoreduc­
tase or cytochrome b5. Fp, flavoprotein; Fp•, reduced form of flavoprotein; 

flavoprotein; Fp•, reduced form of flavoprotein; 
NADP+, nicotinamide adenine dinucleotide phosphate.
487
normal tissues, POMC messenger RNA (mRNA) is usually 
shorter (800nt) than the pituitary 1200nt transcript because of 
lack of exons 1 and 2 and the 5′ region of exon 3.48 Because the 
POMC-like peptide product from this shorter transcript lacks a 
signal sequence needed to cross the endoplasmic reticulum, it is 
probable that it is neither secreted nor active 

probable that it is neither secreted nor active in normal circum­
stances. However, in ectopic ACTH syndrome, additional POMC 
mRNA species are described that are longer than the normal pitu­
itary POMC species (typically 1450nt) as a result of the use of 
alternative promoters in the 5′ region of the gene.49,50 This may, 
in part, explain the resistance of POMC expression to glucocor­
ticoid feedback in these tumors. Other factors, including interac­
tion with tissue-specific transcription fa

nterac­
tion with tissue-specific transcription factors51 and lack of POMC 
promoter methylation,52 may explain the ectopic expression of 
ACTH in some malignant tissues. The cleavage of POMC is also 
tissue specific,53 and it is possible, at least in some cases of ecto­
pic ACTH syndrome, that circulating ACTH precursors (notably 
pro-ACTH) may cross-react in current ACTH radioimmunoas­
says.54,55 The biologic activity of POMC itself on adrenal function 
is thought to be negligible.
POMC expres

function 
POMC expression and processing within neurons in the hypo­
thalamus, specifically the generation of αMSH that interacts 
with melanocortin-4 receptors (MC4R), appears to be of crucial 
importance in appetite control and energy homeostasis (see later 
discussion).56 
Corticotropin-Releasing Hormone and Arginine 
POMC secretion is tightly controlled by numerous factors, notably 
CRH and arginine vasopressin (AVP) (Fig. 15.7).57,58 Additional 
cont

sopressin (AVP) (Fig. 15.7).57,58 Additional 
control is provided through an endogenous circadian rhythm 
and by stress and feedback inhibition by cortisol itself. CRH is a 
41–amino acid peptide that is synthesized in neurons within the 
paraventricular nucleus of the hypothalamus.9,59,60 Human and 
rat CRH are identical, but ovine CRH differs by seven amino 
acids61,62; ovine-sequence CRH is slightly more potent than 
human-sequence CRH in stimulating ACTH secretion and has a 
longer half-life

ulating ACTH secretion and has a 
longer half-life, but both are used diagnostically.
CRH is secreted into the hypophyseal portal blood, where it 
binds to specific type I CRH receptors on anterior pituitary cor­
ticotrophs63 to stimulate POMC transcription through a process 
that includes activation of adenylyl cyclase. It is unclear whether 
hypothalamic CRH contributes in any way to circulating levels; 
CRH is also synthesized in other tissues, and it is likely that cir­
culating CRH reflects

, and it is likely that cir­
culating CRH reflects synthesis from testis, gastrointestinal tract, 
adrenal medulla, and particularly the placenta,64 in which the 
increased secretion across pregnancy results in a threefold increase 
in circulating CRH levels.65 In the circulation, CRH is bound 
to CRH-binding protein (CRH-BP); levels of CRH-BP also 
increase during pregnancy so that cortisol secretion is not mark­
edly elevated.66 CRH is the principal stimulus for ACTH secre­
tion,67 but AVP is 

cipal stimulus for ACTH secre­
tion,67 but AVP is able to potentiate CRH-mediated secretion.68 
In this case, AVP acts through the V1b receptor to activate protein 
kinase C. The peak response of ACTH to CRH does not differ 
across the day, but it is affected by endogenous function of the 
HPA axis in that responsiveness is reduced in subjects treated with 
corticosteroids but increased in subjects with Cushing disease. 
Other reported ACTH secretagogues, including angiotensin II, 
cholecystokin

tagogues, including angiotensin II, 
cholecystokinin, atrial natriuretic factor, and vasoactive peptides, 
probably act to modulate the CRH control of ACTH secretion.69 
Intron A
Intron B
N-POC (1-76)
Joining peptide
Exon 1
Exon 2
Exon 3
Gppp
Pre-pro-opiomelanocortin
Pro-opiomelanocortin (31K)
Pro-ACTH (22K)
βLPH
N-POC (1-48)
γMSH
Pro-γMSH
ACTH
γLPH
β-endorphin
5′ UT
3′ UT
AAAAAAAAAA
• Fig. 15.6  Synthesis and cleavage of pro-opiomelanocortin (POMC) within the human anterior pitu­

elanocortin (POMC) within the human anterior pitu­
itary gland. Prohormone convertase enzymes sequentially cleave POMC to adrenocorticotropic hor­
mone (ACTH). Shaded areas represent melanocyte-stimulating hormone (MSH) structural units. βLPH, 
β-lipoprotein; γLPH, γ-lipoprotein; N-POC, amino-terminal pro-opiomelanocortin.
488
The Stress Response and Immune-Endocrine Axis
The proinflammatory cytokines, notably interleukin 1 (IL1), 
IL6, an

y cytokines, notably interleukin 1 (IL1), 
IL6, and tumor necrosis factor-α, also increase ACTH secretion, 
either directly or by augmenting the effect of CRH.70,71 Leukemia 
inhibitory factor (LIF), a cytokine of the IL6 family, is a further 
activator of the HPA axis.72 This explains the response of the HPA 
axis to an inflammatory stimulus and is an important immune-
endocrine interaction (see Chapter 7). Physical stresses increase 
ACTH and cortisol secretion, again through central actions 

ortisol secretion, again through central actions 
mediated via CRH and AVP. Cortisol secretion rises in response 
to fever, surgery,73 burn injury,74 hypoglycemia,75 hypotension, 
and exercise.76 In all of these cases, this increased secretion can be 
viewed as a normal counterregulatory response to the insult. Acute 
psychologic stress raises cortisol levels,77 but secretion rates appear 
to be normal in patients with chronic anxiety states and underly­
ing psychotic illness. However, depressio

underly­
ing psychotic illness. However, depression is associated with high 
circulating cortisol concentrations, and this is an important con­
sideration in the differential diagnosis of Cushing syndrome (see 
later discussion). 
ACTH, and hence cortisol, is secreted in a pulsatile fashion 
with a circadian rhythm; levels are highest on awakening and 
decline throughout the day, reaching nadir values in the evening 
(Fig. 15.8).78 The average ACTH pulse frequency is higher in 

78 The average ACTH pulse frequency is higher in 
normal adult men compared with women (18 vs. 10 pulses/24 
hours, respectively). The circadian ACTH rhythm appears to be 
mediated principally by an increased ACTH pulse amplitude 
occurring between 5 and 9 am but also by a reduction in ACTH 
pulse frequency occurring between 6 pm and midnight.79,80 Food 
ingestion is a further stimulus to ACTH secretion. An ultradian 
rhythm overlies the circadian and appears to be driven by an oscil­
lator crea

n and appears to be driven by an oscil­
lator created between the secretion of ACTH, the short delay in 
response at the adrenal, and the subsequent negative feedback by 
cortisol at the hypothalamus and pituitary.81
Circadian rhythm is dependent on both day-night82 and sleep-
wake83 patterns and is disrupted by alternating day-night shift 
work and by long-distance travel across time zones.84 It can take 
up to 2 weeks for the circadian rhythm to reset to an altered day-
night cycle. 
Negative 

o reset to an altered day-
Negative Feedback
An important aspect of CRH and ACTH secretion is the nega­
tive feedback control exerted by glucocorticoids themselves. 
Glucocorticoids inhibit POMC transcription in the anterior 
pituitary57 and CRH and AVP mRNA synthesis and secretion in 
the hypothalamus.85,86 Annexin 1 (formerly called lipocortin 1) 
may also play a critical role in effecting the negative feedback of 
Cortisol
ACTH
CRH
Renin
Liver
Lungs
Kidney

isol
ACTH
CRH
Renin
Liver
Lungs
Kidneys
ANP
Dopamine
Angiotensin I
Angiotensinogen
Diurnal rhythm
Pituitary
AVP
Cytokines
Adrenals
ECF [K+]
–
–
–
–
Stressors
(hypoglycemia,
hypotension,
fever, trauma,
surgery)
Regulation of cortisol secretion
Gluconeogenesis
Glycogenolysis
Proteolysis
Lipolysis
   volume
Renal arterial
   pressure
Na+ (+ water)
   retention
K+ excretion
Cardiovascular system
Myocardial contractility
Cardiac output
Catecholami

yocardial contractility
Catecholamine pressor
effect
Renal arterial pressure
β-Adrenergic action
A
Regulation of aldosterone secretion
B
• Fig. 15.7  Normal negative feedback regulation of cortisol and aldosterone secretion. (A) Hypothalamic-
pituitary-adrenal axis. Adrenocorticotropic hormone (ACTH) is secreted from the anterior pituitary under 
the influence of two principal secretagogues, corticotropin-releasing hormone (CRH) and arginine vaso­
pressin; other fac

ormone (CRH) and arginine vaso­
pressin; other factors, including cytokines, also play a role. CRH secretion is regulated by an inbuilt circa­
dian rhythm and by additional stressors operating through the hypothalamus. Secretion of CRH and ACTH 
is inhibited by cortisol, highlighting the importance of negative feedback control. (B) Renin-angiotensin-
aldosterone system (RAAS). Renin is secreted from the juxtaglomerular cells in the kidney dependent on 
renal arterial blood pressure. Renin conver

nt on 
renal arterial blood pressure. Renin converts angiotensinogen to angiotensin I, which is converted in the 
lungs by angiotensin-converting enzyme (ACE) into angiotensin II. Angiotensin stimulates adrenal aldo­
sterone synthesis. Extracellular fraction (ECF) of potassium has an important direct inhibitory influence 
on aldosterone secretion. AVP, arginine vasopressin (antidiuretic hormone); ANP, atrial natriuretic peptide.
489
glucocorticoids on ACTH and CRH 

he Adrenal Cortex
glucocorticoids on ACTH and CRH release.87 The negative feed­
back effect depends on the dose, potency, half-life, and duration 
of administration of the glucocorticoid and has important physi­
ologic and diagnostic consequences. Suppression of the HPA axis 
by pharmacologic corticosteroids may persist for many months 
after cessation of therapy, and adrenocortical insufficiency should 
be anticipated. Diagnostically, the feedback mechanism explains 
ACTH hypersecretion in Addi

ck mechanism explains 
ACTH hypersecretion in Addison disease, as well as undetect­
able ACTH levels in patients with a cortisol-secreting adrenal 
adenoma. Feedback inhibition is principally mediated via the glu­
cocorticoid receptor (GR); patients with glucocorticoid resistance 
resulting from mutations in the GR88 and mice lacking the GR 
gene (Nr3c1)89 have ACTH and cortisol hypersecretion due to 
perceived lack of negative feedback. 
The ACTH Receptor and ACTH Effects on the Adrenal 
Gland

H Receptor and ACTH Effects on the Adrenal 
Gland
ACTH binds to a G protein–coupled, melanocortin-2 recep­
tor (MC2R),90 of which there are approximately 3500 on each 
adrenocortical cell. Melanocortin-2 receptor accessory pro­
tein (MRAP) is required for correct localization and signaling 
of MC2R.91 Current data suggest that MRAP might promote 
three different activities: as a chaperone assisting correct folding 
of MC2R in the endoplasmic reticulum, as an accessory pro­
tein essential for tra

culum, as an accessory pro­
tein essential for trafficking of MC2R to the plasma membrane, 
and as a coreceptor enabling MC2R to bind or to signal ACTH 
response.92 Downstream signal transduction is mediated princi­
pally through the stimulation of adenylyl cyclase and intracellular 
cAMP,93 although both extracellular and intracellular Ca2+ play a 
role.94 Other factors synergize with or inhibit the effects of ACTH 
on the adrenal cortex, including angiotensin II, activin, inhibin, 
and cytokin

ing angiotensin II, activin, inhibin, 
and cytokines (tumor necrosis factor-α and leptin).95 Cell-to-cell 
communication via gap junctions is also important in mediating 
the effects of ACTH.96
ACTH produces both immediate and chronic effects on the 
adrenal gland; the end result is the stimulation of adrenal steroido­
genesis and growth. Acutely, steroidogenesis is stimulated through 
a StAR-mediated increase in cholesterol delivery to the P450 
11A1 enzyme in the inner mitochondrial membrane.2

11A1 enzyme in the inner mitochondrial membrane.24 Chroni­
cally (within 24–26 hours of exposure), ACTH acts to increase 
the synthesis of all steroidogenic CYP enzymes (P450 11A1, P450 
17A1, P450 21A2, P450 11B1) in addition to adrenodoxin,97,98 
the effects of which are mediated at the transcriptional level. 
ACTH increases synthesis of the LDL and HDL receptors and 
possibly also synthesis of 3-hydroxy-3-methylglutaryl (HMG)–
CoA reductase, the rate-limiting step in cholesterol biosynthesis

the rate-limiting step in cholesterol biosynthesis. 
ACTH increases adrenal weight by inducing both hyperplasia and 
hypertrophy. Adrenal atrophy is a feature of ACTH deficiency. 
Mineralocorticoid Secretion: The Renin-
Angiotensin-Aldosterone Axis
Aldosterone is secreted from the ZG under the control of three 
principal secretagogues: angiotensin II, potassium, and, to a lesser 
extent, ACTH (see Fig. 15.7). Other factors, notably somatosta­
tin, heparin, atrial natriuretic factor, and dopamine

, heparin, atrial natriuretic factor, and dopamine, can directly 
inhibit aldosterone synthesis. The secretion of aldosterone and its 
intermediary 18-hydroxylated metabolites is restricted to the ZG 
because of the zone-specific expression of P450 11B2 (aldosterone 
synthase).99 Corticosterone and DOC, synthesized in both the ZF 
and ZG, can act as mineralocorticoids, which becomes significant 
in some clinical diseases, notably some forms of congenital adrenal 
hyperplasia (CAH) and adrenal tu

ngenital adrenal 
hyperplasia (CAH) and adrenal tumors. Similarly, it is now estab­
lished that cortisol can act as a mineralocorticoid in the setting 
of impaired metabolism of cortisol to cortisone by the enzyme 
11β-hydroxysteroid dehydrogenase type 2 (11βHSD2); this is 
important in patients with hypertension, ectopic ACTH syn­
drome, or renal disease. The renin-angiotensin system is described 
in detail in Chapter 16.
Normal
Normal
nmol/L
nmol/L
ng/L
ng/L
24

 Disease
Normal
Normal
nmol/L
nmol/L
ng/L
ng/L
24
2
4
6
8
10
12
Time (24-hour clock)
14
16
18
20
22
24
1400
1200
1000
800
600
400
200
0
Cortisol
600
Cortisol
500
400
300
200
0
100
25
ACTH
20
15
10
5
0
ACTH
80
60
40
20
0
• Fig. 15.8  Circadian and pulsatile secretion of adrenocorticotropic hor­
mone (ACTH) and cortisol in a normal subject (top two panels) and in a 
patient with Cushing disease. In a normal subject, secretion of ACTH and 
cortisol is highest in early morning and falls to a nadir a

is highest in early morning and falls to a nadir at midnight. ACTH 
pulse frequency and pulse amplitude are increased in Cushing disease, 
and circadian rhythmic secretion is lost.
490
Angiotensin II and potassium stimulate aldosterone secretion 
principally by increasing the transcription of CYP11B2 through 
common intracellular signaling pathways. cAMP response ele­
ments in the 5′ region of the CYP11B2 gene are activated after an 
incre

 of the CYP11B2 gene are activated after an 
increase in intracellular Ca2+ and activation of calmodulin kinases. 
The potassium effect is mediated through membrane depolariza­
tion and opening of calcium channels and the angiotensin II effect 
after binding of angiotensin II to the surface AT1 receptor and 
activation of phospholipase C.99
The effect of ACTH on aldosterone secretion is modest and 
differs in the acute and chronic situation (see Chapter 16). An 
acute bolus of ACTH will increase

Chapter 16). An 
acute bolus of ACTH will increase aldosterone secretion, princi­
pally by stimulating the early pathways of adrenal steroidogenesis 
(see earlier discussion), but circulating levels increase by no more 
than 10% to 20% above baseline values. ACTH has no effect on 
CYP11B2 gene transcription or enzyme activity. Chronic contin­
ual ACTH stimulation has either no effect or an inhibitory effect 
on aldosterone production, possibly because of receptor down­
regulation or suppression 

cause of receptor down­
regulation or suppression of angiotensin II–stimulated secretion 
because of a mineralocorticoid effect of cortisol, DOC, or corti­
costerone. Dopamine and atrial natriuretic peptide inhibit aldo­
sterone secretion, as does heparin.
These separate lines of control—through the HPA axis for 
glucocorticoid biosynthesis and via the renin-angiotensin system 
for mineralocorticoid synthesis—have important clinical con­
sequences. Patients with primary adrenal failure invariabl

s. Patients with primary adrenal failure invariably have 
both cortisol and aldosterone deficiency, whereas patients with 
ACTH deficiency due to pituitary disease have glucocorticoid 
deficiency but normal aldosterone concentrations because the 
renin-angiotensin system is intact. 
Adrenal Androgen Secretion
Adrenal-derived androgens represent an important component 
(>50%) of circulating androgens in premenopausal women.100 
In men, this contribution is much smaller because of the tes­
ticular

bution is much smaller because of the tes­
ticular production of androgens, but adrenal androgen excess 
even in men may be of clinical significance, notably in patients 
with CAH, which results in a suppression of the hypothalamic-
pituitary-gonadal axis. The adult adrenal secretes approximately 
4 mg per day of DHEA, 7 to 15 mg per day of DHEAS, 1.5 
mg of androstenedione (AD), and 0.05 mg per day of testoster­
one. More recently it has been recognized that the 11-oxygen­
ated 19-carbon androg

cognized that the 11-oxygen­
ated 19-carbon androgens are important adrenal androgens, with 
11β-hydroxyandrostenedione (11OHAD) being the most highly 
secreted, being derived from AD via the actions of P450 11B1.101 
11-ketotestosterone (11KT) is derived from 11OHAD following 
oxidation and reduction, and has equimolar affinity at the andro­
gen receptor as testosterone. These androgens, and 11-ketoandros­
tendione and 11β-hydroxytestosterone, are significantly increased 
in patients with CAH d

re significantly increased 
in patients with CAH due to 21-hydroxylase deficiency and are 
adrenal-specific biomarkers of androgen excess.102
DHEA is a crucial precursor of human sex steroid biosynthesis 
and exerts androgenic or estrogenic activity after conversion by 
the activities of 3βHSD, 17βHSD, and aromatase; these enzymes 
are expressed in peripheral target tissues, a fact that is of clini­
cal importance in many diseases.103 Some studies have postulated 
direct effects of DHEA acting a

s have postulated 
direct effects of DHEA acting as a classic hormone in peripheral 
tissues. Specific plasma membrane receptors have been identi­
fied but await full characterization.104 Conventionally, desulfated 
DHEA is thought to be converted downstream to a biologi­
cally active hormone. Serum DHEAS was previously thought to 
represent a circulating storage pool for DHEA regeneration, but 
it was later suggested that conversion of DHEAS to DHEA by 
steroid sulfatase plays a minor role in a

DHEA by 
steroid sulfatase plays a minor role in adult physiology and that 
the equilibrium between serum DHEA and DHEAS is mainly 
regulated by SULT2A1 activity. This implies that serum DHEAS 
may not always appropriately reflect the active DHEA pool, par­
ticularly if SULT2A1 activity is impaired, as in the inflammatory 
stress response.105
ACTH stimulates androgen secretion; DHEA (but not DHEAS 
because of its increased plasma half-life) and androstenedione dem­
onstrate a circadian rhythm si

ndrostenedione dem­
onstrate a circadian rhythm similar to that of cortisol.106 However, 
there are many discrepancies between adrenal androgen and glu­
cocorticoid secretion, leading to the suggestion of an additional 
cortical androgen-stimulating hormone (CASH). Many putative 
CASHs have been proposed, including POMC derivatives such as 
joining peptide, prolactin, and insulin-like growth factor type 1 
(IGF1), but conclusive proof is lacking. Efficient adrenal steroido­
genesis toward androg

 Efficient adrenal steroido­
genesis toward androgen synthesis is crucially dependent on the 
relative activities of 3βHSD and 17α-hydroxylase and, in particu­
lar, on the 17,20-lyase activity of 17α-hydroxylase. Factors that 
determine whether the 17-hydroxylated substrates will undergo 
21-hydroxylation to form glucocorticoid or side-chain cleavage by 
17α-hydroxylase to form DHEA and androstenedione are unre­
solved and seem likely to be important in defining the activity of 
any putative CAS

tant in defining the activity of 
any putative CASH (Table 15.4). 
Corticosteroid Hormone Action
Receptors and Gene Transcription
Both cortisol and aldosterone exert their effects after uptake of 
free hormone from the circulation and binding to intracellular 
receptors; these are termed, respectively, the glucocorticoid recep­
tor (GR, encoded by NR3C1) and the mineralocorticoid receptor 
(MR, encoded by NR3C2).107–109 These are members of the thy­
roid/steroid hormone receptor superfamily of t

hy­
roid/steroid hormone receptor superfamily of transcription fac­
tors; they consist of a carboxy-terminal ligand-binding domain, 
a central DNA-binding domain that interacts with specific DNA 
sequences on target genes, and an amino-terminal hypervariable 
region. Although only single genes encode the GR and MR, splice 
variants (i.e., GRα and GRβ) have been described in both recep­
tor types; this, together with tissue-specific post-translational 
modification (phosphorylation, sumoylation, 

onal 
modification (phosphorylation, sumoylation, and ubiquitina­
tion), is thought to account for many of the diverse actions of 
corticosteroids (Fig. 15.9).110,111
  Dissociation of Adrenal Androgen and 
Glucocorticoid Secretion: Evidence for an 
Adrenal-Stimulating Hormone
Dexamethasone studies: Complete cortisol suppression with chronic 
high-dose dexamethasone; DHEA falls by only 20% (lower sensitivity 
of DHEA to acute low-dose dexamethasone administration causing 
ACTH suppression).
Adr

one administration causing 
Adrenarche: Clinically significant rise in circulating DHEA at 6–8 years of 
age; cortisol production unaltered.
Aging: Reduction in DHEA production; no change in cortisol.
Anorexia nervosa and illness: Fall in DHEA, no change (or increase) in 
cortisol.
DHEA, Dehydroepiandrosterone.
   
TABLE 15.4
491
Glucocorticoid hormone action has been studied in more depth 
than mineralocorticoid action. The binding of steroid to

ineralocorticoid action. The binding of steroid to the GRα 
in the cytosol results in activation of the steroid-receptor com­
plex through a process that involves the dissociation of heat shock 
proteins (HSP90 and HSP70).112 Following translocation to the 
nucleus, gene transcription is stimulated or repressed after binding 
of the dimerized GR-ligand complex to a specific DNA sequence 
in the promoter regions of target genes.113 This sequence, known 
as the glucocorticoid-response element (GRE

known 
as the glucocorticoid-response element (GRE), is invariably a palin­
dromic CGTACAnnnTGTACT sequence that binds with high 
affinity to two loops of DNA within the DNA-binding domain 
of the GR (zinc fingers). This stabilizes the RNA polymerase II 
complex, facilitating gene transcription. The GRβ variant may act 
as a dominant negative regulator of GRα transactivation.110
Naturally occurring mutations in the GR (as seen in patients 
with glucocorticoid resistance) and GR mutants generated

lucocorticoid resistance) and GR mutants generated in vitro 
have highlighted critical regions of the receptor that are respon­
sible for binding and transactivation,114 but numerous other 
factors are required (e.g., coactivators, corepressors115), and this 
may make responses tissue specific. This is a rapidly evolving field 
and beyond the scope of this chapter. However, the interaction 
between GR and two particular transcription factors are impor­
tant in mediating the anti-inflammatory eff

impor­
tant in mediating the anti-inflammatory effects of glucocorticoids 
and explain the effect of glucocorticoids on genes that do not con­
tain obvious GREs in their promoter regions.116 Activator protein 
1 (AP1) comprises Fos and Jun subunits and is a proinflammatory 
transcription factor induced by a series of cytokines and phor­
bol ester. The GR-ligand complex can bind to c-Jun and prevent 
interaction with the AP1 site, thereby mediating the so-called 
transrepressive effects of glucoc

g the so-called 
transrepressive effects of glucocorticoids.117 Similarly, functional 
antagonism exists between the GR and nuclear factor-κB (NF-
κB), a ubiquitously expressed transcription factor that activates 
a series of genes involved in lymphocyte development, inflam­
matory response, host defense, and apoptosis (Fig. 15.10).118 In 
keeping with the diverse array of actions of cortisol, many hun­
dreds of glucocorticoid-responsive genes have been identified. 
Some glucocorticoid-induced g

ve been identified. 
Some glucocorticoid-induced genes and repressed genes are listed 
in Table 15.5.
In contrast to the diverse actions of glucocorticoids, miner­
alocorticoids have a more restricted role, principally stimulation 
of epithelial sodium transport in the distal nephron, distal colon, 
and salivary glands.119 This action is mediated through induction 
of the apical sodium channel (comprising three subunits—α, β, 
and γ)120 and the α1 and β1 subunits of the basolateral sodium-
potas

1 and β1 subunits of the basolateral sodium-
potassium adenosine triphosphatase pump (Na+/K+-ATPase)121 
through transcriptional regulation of serum-induced and gluco­
corticoid-induced kinase (SGK).122 Aldosterone binds to the MR, 
principally in the cytosol (although there is evidence for expres­
sion of the unliganded MR in the nucleus), and the hormone-
receptor complex is then translocated to the nucleus (Fig. 15.11).
The MR and GR share considerable homology—57% in the 
steroid-binding dom

siderable homology—57% in the 
steroid-binding domain and 94% in the DNA-binding domain. 
It is perhaps not surprising, therefore, that there is promiscuity of 
ligand binding, with aldosterone (and the synthetic mineralocor­
ticoid, fludrocortisone) binding to the GR and cortisol binding to 
the MR. For the MR, this is particularly impressive: in vitro, the 
MR has the same inherent affinity for aldosterone, corticosterone, 
or cortisol.108 Specificity on the MR is conferred through the 
“prere

ificity on the MR is conferred through the 
“prereceptor” metabolism of cortisol via the enzyme 11βHSD2, 
which converts cortisol and corticosterone to inactive 11-keto 
metabolites, enabling aldosterone to bind to the MR.123,124 Min­
eralocorticoid hormone action was extended beyond this classic 
action in sodium-transporting epithelia with the demonstration 
that aldosterone can induce cardiac fibrosis and inflammatory 
changes in renal vasculature. The underlying signaling pathways 
remain to

ture. The underlying signaling pathways 
remain to be fully clarified, but the effects are reversible with MR 
antagonists.125
Finally, for both glucocorticoids and mineralocorticoids, there 
is accumulating evidence for so-called nongenomic effects involv­
ing hormone response obviating the genomic GR or MR. A series 
of responses have been reported to occur within seconds or min­
utes after exposure to corticosteroids and are thought to be medi­
ated by as yet uncharacterized membrane-coupled 

­
ated by as yet uncharacterized membrane-coupled receptors.126–128 
Corticosteroid-Binding Globulin and 
Corticosteroid Hormone Metabolism
More than 90% of circulating cortisol is bound predominantly 
to the α2-globulin, corticosteroid-binding globulin (CBG).129 
This 383–amino acid protein is synthesized in the liver and binds 
Glucocorticoid receptor
Mineralocorticoid receptor
GRα mRNA
9α
GRβ mRNA
GRα protein
GRβ protein
GRα elicits
specific biological
responses
GRβ functions
as a dominant
ne

iological
responses
negative inhibitor
of GRα receptor
9β
1α
αMR mRNA
1β
βMR mRNA
MR protein
1
2
3 4
5
6 7 8
9α
9β
1β
1α
2
3
4
5
6
7
8
9
• Fig. 15.9  Schematic structure of the human genes encoding the glucocorticoid receptor (GR) and min­
eralocorticoid receptor (MR). In both cases, splice variants have been described. In the case of the GR, 
there is evidence that the GRβ isoform can act as a dominant negative inhibitor of GRα action. mRNA, 
messenger ribonucleic ac

tor of GRα action. mRNA, 
messenger ribonucleic acid.
492
complex
Cortisol
GR
GR
GR
GR
HSP
HSP
NF-κB
IκB
IκB
IκB
Inflammatory response
(TNFα, IL1β, IL6,
IL8, MCSF)
C-fos C-jun
complex
Cell
membrane
3
+
+
–
2
1
Nucleus
Cytokines
• Fig. 15.10  The anti-inflammatory action of glucocorticoids. Cortisol binds to the cytoplasmic glucocor­
ticoid receptor (GR). Conformational changes in the receptor-ligand complex result i

al changes in the receptor-ligand complex result in dissociation from 
heat shock proteins (HSP70 and HSP90) and migration to the nucleus. Binding occurs to specific DNA 
motifs—glucocorticoid response elements—in association with the activator protein 1 (AP1) comprising 
C-fos and C-jun. Glucocorticoids mediate their anti-inflammatory effects through several mechanisms: (1) 
the inhibitory protein IκB, which binds and inactivates nuclear factor-κB (NF-κB), is induced; (2) the GR-
cortisol compl

κB (NF-κB), is induced; (2) the GR-
cortisol complex is able to bind NF-κB and thereby prevent initiation of an inflammatory process; (3) GR 
and NF-κB compete for the limited availability of coactivators, which include cyclic adenosine monophos­
phate response element–binding protein (CREB) and steroid receptor coactivator-1. IL, interleukin; MCSF, 
macrophage colony-stimulating factor; TNFα, tumor necrosis factor-α.
   Some of the Genes Regulated by Glucocorticoids or Glucocorticoid Receptors

ted by Glucocorticoids or Glucocorticoid Receptors
Site of Action
Induced Genes
Repressed Genes
Immune system
IκB (nuclear factor-κB inhibitor)
Interleukins
Haptoglobin
Tumor necrosis factor-α (TNFα)
T-cell receptor (TCR)–ζ
Interferon-γ
p21, p27, and p57
E-selectin
Lipocortin
Intercellular adhesion molecule-1
Cyclooxygenase 2
Inducible nitric oxide synthase (iNOS)
Metabolic
PPAR-γ
Tryptophan hydroxylase
Tyrosine aminotransferase
Metalloprotease
Glutamine synthase
Glycogen synthase
Glucose-6-phos

lutamine synthase
Glucose-6-phosphatase
PEPCK
Leptin
γ-Fibrinogen
Cholesterol 7α-hydroxylase
C/EBP/β
Bone
Androgen receptor
Calcitonin receptor
Collagenase
Alkaline phosphatase
IGFBP6
Channels and transporters
ENaCα, ENaCβ, and ENaCγ
SGK
Aquaporin 1
TABLE 15.5
493
cortisol with high affinity. Affinity for synthetic corticosteroids 
is negligible except for prednisolone, which has an affinity for 
CBG approximately half that of cortisol

inity for 
CBG approximately half that of cortisol. Circulating CBG con­
centrations are approximately 700 nmol/L. Levels are increased 
by estrogens and in some patients with chronic active hepati­
tis; they are reduced by glucocorticoids and in patients with cir­
rhosis, nephrosis, and hyperthyroidism. The estrogen effect can 
be marked, with levels increasing twofold to threefold across 
pregnancy, a fact that should be taken into account when mea­
suring plasma total cortisol in pregnancy an

 mea­
suring plasma total cortisol in pregnancy and in women taking 
estrogens.
CBG plays a key role in determining circulating cortisol lev­
els.130 Inherited abnormalities in CBG synthesis are much rarer 
than those described for thyroxine-binding globulin but include 
cases of elevated CBG, partial or complete deficiency of CBG, 
and CBG variants with reduced affinity for cortisol.131,132 In each 
case, alterations in CBG concentrations change the total circulat­
ing cortisol concentrations a

 the total circulat­
ing cortisol concentrations accordingly, but free cortisol concen­
trations are normal. Only this free circulating fraction is available 
for transport into tissues for biologic activity. The excretion of free 
cortisol through the kidneys results in urinary free cortisol, which 
represents less than 1% of the total cortisol secretion.
The circulating half-life of cortisol varies between 70 and 120 
minutes. The major steps in cortisol metabolism are depicted in 
Fig. 15.121

n cortisol metabolism are depicted in 
Fig. 15.12133 and can be summarized as follows:
 • Interconversion of the 11-hydroxyl group (cortisol, Kendall 
compound F) to the 11-oxo group (cortisone, compound E) 
through activity of the 11βHSD system (EC 1.1.1.146).134,135 
The metabolism of cortisol and that of cortisone then follow 
similar pathways.
 • Reduction of the C4-5 double bond to form dihydrocortisol or 
dihydrocortisone, followed by reduction of the 3-oxo group to 
form tetrahydrocorti

uction of the 3-oxo group to 
form tetrahydrocortisol (THF) or tetrahydrocortisone (THE). 
The reduction of the C4-5 double bond can be carried out 
by either 5β-reductase or 5α-reductase, yielding, respectively, 
5β-tetrahydrocortisol (THF) and 5α-THF (allo-THF). In nor­
mal subjects, the ratio of THF to allo-THF is 2:1. THF, allo-
THF, and THE are rapidly conjugated with glucuronic acid 
and excreted in the urine.
 • Further reduction of the 20-oxo group by either 20αHSD or 
20βHSD to yield α

anolone.
 • Oxidation of the C21 position or cortols and cortolones to form 
the extremely polar metabolites, cortolic and cortolonic acids.
Endocrine
Basic fibroblast growth factor (bFGF)
Prolactin
Endothelin
POMC/CRH
Retinoid X receptor
PTHrP
Natriuretic peptide receptors
Growth and development
Surfactant proteins A, B, and C
Fibronectin
α-Fetoprotein
Nerve growth facto

and C
Nerve growth factor
Erythropoietin
G1 cyclins
Cyclin-dependent kinases
CRH, Corticotropin-releasing hormone; C/EBP/β, CAAT-enhancer binding protein-β; ENaC, epithelial sodium channel; GHRH, growth hormone–releasing hormone; IGFBP6, insulin-like growth factor–bind­
ing protein 6; PEPCK, phosphoenolpyruvate carboxykinase; POMC, pro-opiomelanocortin; PPAR, peroxisome proliferator-activated receptor; PTHrP, parathyroid hormone-related protein; SGK, serum- and 
glucoco

 hormone-related protein; SGK, serum- and 
glucocorticoid-induced kinase.
Modified from McKay LI, Cidlowski JA. Molecular control of immune/inflammatory responses: interactions between nuclear factor-κB and steroid receptor-signalling pathways. Endocr Rev. 1999;20:435–
459.
   
   Some of the Genes Regulated by Glucocorticoids or Glucocorticoid Receptors—cont’d
TABLE 15.5
ENaC
Na+
Na+
SGK
proteins
protein
MR
MR
Cortisol
Cortisone
K+
Na+
K+
11βHSD2
K-Ras
• Fig. 

tisone
K+
Na+
K+
11βHSD2
K-Ras
• Fig. 15.11  Mineralocorticoid hormone action. An epithelial cell in the distal 
nephron or distal colon is depicted. The much higher concentrations of 
cortisol are inactivated by the type 2 isozyme of 11β-hydroxysteroid dehy­
drogenase (11βHSD2) to cortisone, permitting the endogenous ligand, 
aldosterone, to bind to the mineralocorticoid receptor (MR). Relatively 
few mineralocorticoid target genes have been identified, but they include 
serum-induc

ave been identified, but they include 
serum-induced and glucocorticoid-induced kinase (SGK), subunits of the 
epithelial sodium channel (ENaC), and basolateral Na+/K+-adenosine tri­
phosphatase.
494
Approximately 50% of secreted cortisol appears in the urine 
as THF, allo-THF, and THE; 25% as cortols/cortolones; 10% as 
C19 steroids; and 10% as cortolic/cortolonic acids. The remaining 
metabolites are free unconjugated steroids (cortisol,

abolites are free unconjugated steroids (cortisol, cortisone, and 
their 6β-metabolites, and 20α/20β-metabolites).
The principal site of cortisol metabolism has been considered 
to be the liver, but many of the enzymes listed have been described 
in mammalian kidney, notably the interconversion of cortisol to 
cortisone by 11βHSD2. Quantitatively, this is the most impor­
tant pathway. Furthermore, the bioactivity of glucocorticoids 
is in part related to the hydroxyl group at C11; because corti­

lated to the hydroxyl group at C11; because corti­
sone with a C11-oxo group is an inactive steroid, expression of 
11βHSD in peripheral tissues plays a crucial role in regulating 
corticosteroid hormone action. Two distinct 11βHSD isozymes 
have been reported: type 1, reduced nicotinamide adenine 
dinucleotide phosphate (NADPH)–dependent oxo-reductase 
expressed principally in the liver, which confers bioactivity on 
orally administered cortisone by converting it to cortisol,135 and 
a type 2, 

e by converting it to cortisol,135 and 
a type 2, nicotinamide adenine dinucleotide (NAD)–dependent 
dehydrogenase. It is the 11βHSD2, coexpressed with the MR in 
Cortisone
Cortisol
tetrahydrocortisone
(THE)
20β-dihydrocortisone
20β-dihydrocortisol
5β-dihydrocortisol
5α-dihydrocortisol
6   -hydroxylase
6β-hydroxycortisone
6β-hydroxycortisol
5β-reductase
3   -hydroxysteroid
dehydrogenase
11β-hydroxysteroid
5β-tetrahydrocortisol
(THF)
5α-tetrahydrocortisol
(allo-THF)
O
O
OH
OH
H
CH2O

O
O
OH
OH
H
CH2OH
C
O
O
OH
OH
O
CH2OH
C
O
O
OH
O
CH2OH
C
O
OH
O
CH2OH
C
HO
HO
O
OH
OH
O
CH2OH
C
HO
O
OH
O
CH2OH
C
HO
OH
O
O
CH2OH
C
HO
OH
O
CH2OH
C
HO
HO
OH
O
CH2OH
C
HO
HO
O
OH
O
CH2OH
C
HO
O
OH
OH
H
CH2OH
C
β
α
α
5   -reductase
α
β
20β-oxoreductase
• Fig. 15.12  The principal pathways of cortisol metabolism. Interconversion of hormonally active cortisol 
to inactive cortisone is catalyzed by t

 cortisol 
to inactive cortisone is catalyzed by two isozymes of 11β-hydroxysteroid dehydrogenase (11βHSD), with 
11βHSD1 principally converting cortisone to cortisol and 11βHSD2 doing the reverse. Cortisol can be 
hydroxylated at the C6 and C20 positions. A ring reduction is undertaken by 5α-reductase or 5β-reductase 
and 3αHSD.
495
the kidney, colon, and salivary gland, that inactivates cortisol to 
cortisone and permits aldosterone to bind to the MR in vivo. If 

permits aldosterone to bind to the MR in vivo. If 
this enzyme-protective mechanism is impaired, cortisol is able to 
act as a mineralocorticoid; this explains some forms of endocrine 
hypertension (apparent mineralocorticoid excess, licorice inges­
tion) and the mineralocorticoid excess state that characterizes the 
ectopic ACTH syndrome.132,136
Hyperthyroidism results in increased cortisol metabolism and 
clearance, and hypothyroidism produces the converse, principally 
because of an effect of

the converse, principally 
because of an effect of thyroid hormone on hepatic 11βHSD1 and 
5α/5β-reductases.135 IGF1 increases cortisol clearance by inhib­
iting hepatic 11βHSD1 (conversion of cortisone to cortisol).137 
6β-Hydroxylation is normally a minor pathway, but cortisol itself 
induces 6β-hydroxylase activity, so that 6β-hydroxycortisol excre­
tion is markedly increased in patients with Cushing syndrome.138 
Some drugs, notably rifampicin and phenytoin, induce P450 
3A4 expression and i

n and phenytoin, induce P450 
3A4 expression and increase cortisol clearance through this path­
way.139 Patients with renal disease have impaired cortisol clearance 
because of reduced conversion of renal cortisol to cortisone.140 
These observations have clinical implications for patients with 
thyroid disease, acromegaly, or renal disease and for patients tak­
ing cortisol replacement therapy. Adrenal crisis has been reported 
in steroid-replaced addisonian patients given rifampicin,141 and 
h

ed addisonian patients given rifampicin,141 and 
hydrocortisone replacement therapy may need to be increased 
in treated patients who develop hyperthyroidism or reduced in 
patients with untreated growth hormone (GH) deficiency.
Aldosterone is also metabolized in the liver and kidneys. In 
the liver, it undergoes tetrahydro reduction and is excreted in the 
urine as a tetrahydroaldosterone 3-glucuronide derivative. How­
ever, glucuronide conjugation at the 18 position occurs directly 
in the kid

ion at the 18 position occurs directly 
in the kidney, as does 3α and 5α/5β metabolism of the free ste­
roid.142 Because of the aldehyde group at the C18 position, aldo­
sterone is not metabolized by 11βHSD2.143 Hepatic aldosterone 
clearance is reduced in patients with cirrhosis, ascites, or severe 
congestive heart failure. 
Effects of Glucocorticoids
The principal sites of action of glucocorticoids and some of the 
consequences of glucocorticoid excess are shown in Fig. 15.13.
Brain/CNS:
   D

id excess are shown in Fig. 15.13.
   Depression
   Psychosis
Eye:
   Glaucoma
Endocrine system:
     LH, FSH release
     TSH release
     GH secretion
GI tract:
   Peptic ulcerations
Adipose tissue distribution:
   Promotes visceral obesity
Cardiovascular/renal:
   Salt and water retention
   Hypertension
Skin/muscle/connective tissue:
   Protein catabolism/collagen breakdown
   Skin thinning
   Muscular atrophy
Bone and calcium metabolism:
     Bone formation
     Bone mass and ost

bolism:
     Bone mass and osteoporosis
Growth and development:
     Linear growth
Immune system:
   Anti-inflammatory action
   Immunosuppression
Carbohydrate/lipid metabolism:
     Hepatic glycogen deposition
     Peripheral insulin resistance
     Gluconeogenesis
     Free fatty acid production
   Overall diabetogenic effect
• Fig. 15.13  The principal sites of action of glucocorticoids in humans, highlighting some of the conse­
quences of glucocorticoid excess. CNS, centr

onse­
quences of glucocorticoid excess. CNS, central nervous system; FSH, follicle-stimulating hormone; GH, 
growth hormone; GI, gastrointestinal; LH, luteinizing hormone; TSH, thyroid-stimulating hormone.
496
Carbohydrate, Protein, and Lipid Metabolism
Glucocorticoids increase blood glucose concentrations through 
their action on glycogen, protein, and lipid metabolism. In 
the liver, cortisol stimulates glycogen deposition by increas­
in

isol stimulates glycogen deposition by increas­
ing glycogen synthase and inhibiting the glycogen-mobilizing 
enzyme, glycogen phosphorylase.144 Hepatic glucose output 
increases through the activation of key enzymes involved in 
gluconeogenesis, principally glucose-6-phosphatase and phos­
phoenolpyruvate carboxykinase (PEPCK).145,146 In peripheral 
tissues (e.g., muscle, fat), cortisol inhibits glucose uptake and 
utilization.147 In adipose tissue, lipolysis is activated, resulting 
in the rele

ue, lipolysis is activated, resulting 
in the release of free fatty acids into the circulation. An increase 
in total circulating cholesterol and triglycerides is observed, but 
HDL cholesterol levels fall. Glucocorticoids also have a per­
missive effect on other hormones, including catecholamines 
and glucagon. The result is insulin resistance and an increase 
in blood glucose concentrations, at the expense of protein and 
lipid catabolism.
Glucocorticoids stimulate adipocyte differentiation, p

ocorticoids stimulate adipocyte differentiation, promoting 
adipogenesis through the transcriptional activation of key differ­
entiation genes, including lipoprotein lipase, glycerol-3-phosphate 
dehydrogenase, and leptin.148 Long-term effects of glucocorticoid 
excess on adipose tissue are more complex, at least in humans, in 
whom the deposition of visceral or central adipose tissue is stimu­
lated,149 providing a useful discriminatory sign for the diagnosis 
of Cushing syndrome. The predilect

 the diagnosis 
of Cushing syndrome. The predilection for visceral obesity may 
relate to the increased expression of the GR150 and 11βHSD1 in 
omental compared with subcutaneous adipose tissue.151 
Skin, Muscle, and Connective Tissue
In addition to inducing insulin resistance in muscle tissue, gluco­
corticoids also cause catabolic changes in muscle, skin, and con­
nective tissue. In the skin and connective tissue, glucocorticoids 
inhibit epidermal cell division and DNA synthesis and reduce 
s

rmal cell division and DNA synthesis and reduce 
synthesis and production of collagen.152 In muscle, glucocorti­
coids cause atrophy (but not necrosis), which seems to be spe­
cific for type II (phasic) muscle fibers. Muscle protein synthesis 
is reduced. 
Bone and Calcium Metabolism
Glucocorticoids inhibit osteoblast function, which is thought to 
account for the osteopenia and osteoporosis that especially affect 
the axial skeleton and characterize glucocorticoid excess.153 Up 
to 1% of Wester

rize glucocorticoid excess.153 Up 
to 1% of Western populations are taking long-term glucocor­
ticoid therapy,154 and glucocorticoid-induced osteoporosis is 
becoming a prevalent health concern, affecting 50% of patients 
treated with corticosteroids for longer than 12 months. How­
ever, the complication perhaps most feared by physicians is 
osteonecrosis. Osteonecrosis (also termed avascular necrosis) pro­
duces rapid and focal deterioration of bone quality and primar­
ily affects the femoral h

bone quality and primar­
ily affects the femoral head, leading to pain and ultimately to 
collapse of the bone, often necessitating hip replacement. It can 
affect individuals of any age and may occur with relatively low 
doses of glucocorticoids (e.g., during corticosteroid replacement 
therapy for adrenal failure).155 Importantly, defects may not be 
detectable on conventional radiographs but are readily seen on 
magnetic resonance imaging (MRI). Glucocorticoid-induced 
osteocyte apoptosis has

). Glucocorticoid-induced 
osteocyte apoptosis has been implicated in the pathogenesis of 
the condition,156 and the lack of a direct role for an interrupted 
blood supply suggests that the term osteonecrosis is preferable to 
avascular femoral necrosis. However, there is still no explanation 
for individual susceptibility.
Glucocorticoids also induce negative calcium balance by inhib­
iting intestinal calcium absorption and increasing renal calcium 
excretion. As a consequence, parathyroid secr

ium 
excretion. As a consequence, parathyroid secretion is usually 
increased. In children, glucocorticoids suppress growth, but the 
increases in body mass index (BMI) are thought to offset a delete­
rious effect on bone mineral density.157 
Salt and Water Homeostasis and Blood Pressure Control
Glucocorticoids increase blood pressure by a variety of mecha­
nisms involving actions on the kidney and vasculature.158 In vas­
cular smooth muscle, they increase sensitivity to pressor agents 
such as 

y increase sensitivity to pressor agents 
such as catecholamines and angiotensin II while reducing nitric 
oxide–mediated endothelial dilatation. Angiotensinogen synthesis 
is increased by glucocorticoids.159 In the kidney, depending on 
the activity of 11βHSD2, cortisol can act on the distal nephron 
to cause sodium retention and potassium loss (mediated via the 
MR).136 Elsewhere across the nephron, glucocorticoids increase 
the glomerular filtration rate, proximal tubular epithelial sodium 
t

ration rate, proximal tubular epithelial sodium 
transport, and free water clearance.160 This last effect involves 
antagonism of the action of vasopressin and explains the dilutional 
hyponatremia seen in patients with glucocorticoid deficiency.161 
Anti-inflammatory Actions and the Immune System
Glucocorticoids suppress immunologic responses, and this action 
has been the stimulus to develop a series of highly potent phar­
macologic glucocorticoids to treat a variety of autoimmune and 
inflamm

oids to treat a variety of autoimmune and 
inflammatory conditions. The inhibitory effects are mediated at 
many levels. In the peripheral blood, glucocorticoids reduce lym­
phocyte counts acutely (T lymphocytes > B lymphocytes) by redis­
tributing lymphocytes from the intravascular compartment to the 
spleen, lymph nodes, and bone marrow. Conversely, neutrophil 
counts increase after glucocorticoid administration. Eosinophil 
counts rapidly fall, an effect that was historically used as a bioass

, an effect that was historically used as a bioassay 
for glucocorticoids. The immunologic actions of glucocorticoids 
involve direct actions on both T and B lymphocytes, including 
inhibition of immunoglobulin synthesis and stimulation of lym­
phocyte apoptosis. Inhibition of cytokine production from lym­
phocytes is mediated through inhibition of the action of NF-κB. 
NF-κB plays a crucial and generalized role in inducing cytokine 
gene transcription; glucocorticoids can bind directly to NF-κB

iption; glucocorticoids can bind directly to NF-κB to 
prevent nuclear translocation, and they induce NF-κB inhibitor, 
which sequesters NF-κB in the cytoplasm, thereby inactivating its 
effect.118
Additional anti-inflammatory effects involve the inhibition 
of monocyte differentiation into macrophages and macrophage 
phagocytosis and cytotoxic activity. Glucocorticoids reduce the 
local inflammatory response by preventing the actions of his­
tamine and plasminogen activators. Prostaglandin synt

ine and plasminogen activators. Prostaglandin synthesis is 
impaired through the induction of lipocortins, which inhibit 
phospholipase A2 activity.162 
Central Nervous System and Mood
Clinical observations of patients with glucocorticoid excess and 
deficiency reveal that the brain is an important target tissue for 
glucocorticoids, with depression, euphoria, psychosis, apathy, and 
lethargy being important manifestations. Both GRs and MRs are 
expressed in discrete regions of the rodent brain,

expressed in discrete regions of the rodent brain, including hip­
pocampus, hypothalamus, cerebellum, and cortex.163 Glucocorti­
coids cause neuronal death, notably in the hippocampus164; this 
effect may underlie the interest in glucocorticoids in relation to 
cognitive function, memory, and neurodegenerative diseases such 
as Alzheimer disease.165 Local blockade of cortisol generation by 
497
   Therapeutic Use of Corticosteroids
Endocrine: Replacement therapy (

f Corticosteroids
Endocrine: Replacement therapy (Addison disease, pituitary disease, 
congenital adrenal hyperplasia), Graves orbitopathy
Skin: Dermatitis, pemphigus
Hematology: Leukemia, lymphoma, hemolytic anemia, idiopathic throm­
bocytopenic purpura
Gastrointestinal: Inflammatory bowel disease (ulcerative colitis, Crohn 
disease)
Liver: Chronic active hepatitis, transplantation, organ rejection
Renal: Nephrotic syndrome, vasculitides, transplantation, rejection
Central nervous system: Cereb

lantation, rejection
Central nervous system: Cerebral edema, raised intracranial pressure
Respiratory: Angioedema, anaphylaxis, asthma, sarcoidosis, tuberculo­
sis, obstructive airway disease
Rheumatology: Systemic lupus erythematosus, polyarteritis, temporal 
arteritis, rheumatoid arthritis
Muscle: Polymyalgia rheumatica, myasthenia gravis
TABLE 15.6
11βHSD1 has been shown to improve cognitive function.166 
DHEA has been shown to have neuroprotective effects in the 
hippocampus region.167 P450 

ctive effects in the 
hippocampus region.167 P450 7B1, an enzyme that metabolizes 
DHEA to its 7α-hydroxylated metabolite, is highly expressed in 
brain, but expression was decreased in dentate neurons in the 
hippocampus.168 
Eye
In the eye, glucocorticoids act to raise intraocular pressure 
through an increase in aqueous humor production and deposition 
of matrix within the trabecular meshwork, which inhibits aque­
ous drainage. Steroid-induced glaucoma appears to have a genetic 
predispositio

 glaucoma appears to have a genetic 
predisposition, but the underlying mechanisms are unknown.169 
Gut
Long-term but not acute administration of glucocorticoids 
increases the risk of developing peptic ulcer disease.170 Pancreatitis 
with fat necrosis is reported in patients with glucocorticoid excess. 
The GR is expressed throughout the gastrointestinal tract, and the 
MR is expressed in the distal colon; they mediate the corticoste­
roid control of epithelial ion transport. 
Growth and Develo

ol of epithelial ion transport. 
Growth and Development
Although glucocorticoids stimulate transcription of the gene 
encoding GH in  vitro, glucocorticoids in excess inhibit linear 
skeletal growth,157,171 probably as a result of catabolic effects on 
connective tissue, muscle, and bone and through inhibition of the 
effects of IGF1. The results of experiments on mice lacking the GR 
gene89 have emphasized the role of glucocorticoids in normal fetal 
development. In particular, glucocorticoids 

etal 
development. In particular, glucocorticoids stimulate lung matura­
tion through the synthesis of surfactant proteins (SP-A, SP-B, and 
SP-C),172 and mice lacking the GR die shortly after birth due to 
hypoxia from lung atelectasis. Glucocorticoids also stimulate the 
enzyme phenylethanolamine N-methyltransferase (PNMT), which 
converts norepinephrine to epinephrine in adrenal medulla and 
chromaffin tissue. Mice lacking the GR do not develop an adrenal 
medulla.89 There is also adrenomedul

p an adrenal 
medulla.89 There is also adrenomedullary dysplasia and hypofunc­
tion in patients with classic 21-hydroxylase deficiency.173,174 
Endocrine Effects
Glucocorticoids suppress the thyroid axis, probably through a direct 
action on the secretion of thyroid-stimulating hormone (TSH, 
thyrotropin). In addition, they inhibit 5′ deiodinase activity that 
mediates the conversion of thyroxine to active triiodothyronine.
Glucocorticoids also act centrally to inhibit gonadotropin-
releasing ho

ct centrally to inhibit gonadotropin-
releasing hormone (GnRH) pulsatility and release of luteinizing 
hormone (LH) and follicle-stimulating hormone (FSH). 
Therapeutic Corticosteroids
Since the dramatic anti-inflammatory effect of cortisone was first 
demonstrated in the 1950s, a series of synthetic corticosteroids have 
been developed for therapeutic purposes. These agents are used to 
treat a diverse variety of human diseases, principally relying on their 
anti-inflammatory and immunologic ac

ing on their 
anti-inflammatory and immunologic actions (Table 15.6). The main 
corticosteroids used in clinical practice, together with their relative glu­
cocorticoid and mineralocorticoid potencies, are listed in Table 15.7.
The structures of common synthetic steroids are depicted in 
Fig. 15.14. The biologic activity of a corticosteroid depends on 
a delta-4, 3-keto, 11β,17α,21-trihydroxyl configuration.175 Con­
version of the C11 hydroxyl group to a C11 keto group (i.e., cor­
tisol to corti

oup to a C11 keto group (i.e., cor­
tisol to cortisone) inactivates the steroid. The addition of a 1,2 
unsaturated bond to cortisol results in prednisolone, which is four 
times more potent than cortisol in classic glucocorticoid bioassays 
such as hepatic glycogen deposition, suppression of eosinophils, 
and anti-inflammatory actions. Prednisone, widely prescribed in 
the United States, is the cortisone equivalent of prednisolone and 
relies on conversion by 11βHSD1 in the liver for bioactivit

 conversion by 11βHSD1 in the liver for bioactivity.176 
Potency is further increased by the addition of a 6α-methyl group 
to prednisolone (methylprednisolone).
Fludrocortisone is a synthetic mineralocorticoid having 125-
fold greater potency than cortisol in stimulating sodium reabsorp­
tion. This effect is achieved through the addition of a 9α-fluoro 
group to cortisol. Fludrocortisone also has glucocorticoid potency 
(12-fold greater than cortisol). The addition of a 16α-methyl group 
and 1,

tisol). The addition of a 16α-methyl group 
and 1,2 saturated bond to fludrocortisone results in dexametha­
sone, a highly potent glucocorticoid (25-fold greater potency than 
cortisol) that has negligible mineralocorticoid activity.175,177
Widely used synthetic glucocorticoids in respiratory and nasal 
aerosol sprays are betamethasone, beclomethasone, and flutica­
sone. Betamethasone has the same structure as dexamethasone but 
with a 16α-methyl group. Beclomethasone has the same

th a 16α-methyl group. Beclomethasone has the same structure 
as betamethasone apart from the replacement of the 9α-fluoro 
group with a 9α-chloro group. Fluticasone has the same structure 
  Relative Biologic Potencies of Synthetic 
Steroids in Bioassay Systems
Steroid
Anti-
Inflammatory 
Action
Hypothalamic-
Pituitary-Adrenal 
Salt 
Retention
Cortisol
1
1
1
3
4
0.75
3
4
0.75
6.2
4
0.5
12
12
125
5
4
0
Dexameth

rocortisone
12
12
125
5
4
0
26
17
0
TABLE 15.7
498
Cortisone
Cortisol
CH3
CH3
O
O
OH
O
CH2OH
C
O
OH
O
CH2OH
C
HO
O
OH
O
CH2OH
C
HO
O
OH
O
CH2OH
C
O
O
OH
O
CH2OH
C
HO
HO
O
OH
O
CH2OH
C
F
OH
O
OH
O
CH2OH
C
O
H
O
H
O
OH
O
CH2OH
C
F
F
Fluticasone
O
OH
O
SCH2F
C
HO
F
F
O
OH
O
CH2OH
C
HO
Cl
CH3
CH3
• Fig. 15.14  Struc

F
O
OH
O
CH2OH
C
HO
Cl
CH3
CH3
• Fig. 15.14  Structures of the natural glucocorticoid cortisol, some of the more 
commonly prescribed synthetic glucocorticoids, and the mineralocorticoid 
fludrocortisone. Triamcinolone is identical to dexamethasone except that a 
16α-hydroxyl group is substituted for the 16α-methyl group. Betametha­
sone, another widely used glucocorticoid, has a 16β-methyl group. Beclo­
methasone is derived from betamethasone by replacement of the 9α-fluoro 
group with a chloro

 replacement of the 9α-fluoro 
group with a chloro group. Fluticasone is identical to dexamethasone except 
that an additional 6α-fluoro group has been added, and the hydroxymethyl 
group at position 21 has been exchanged by a thiofluoromethyl group.
as dexamethasone with an additional 6α-fluoro group and a 5-flu­
oromethyl group replacing the hydroxymethyl group.
Corticosteroids are given orally, parenterally, and by numer­
ous topical routes (e.g., eyes, skin, nose, inhalation, rectal sup­
pos

.g., eyes, skin, nose, inhalation, rectal sup­
positories).177 Unlike hydrocortisone, which has a high affinity for 
CBG, most synthetic steroids have low affinity for this binding 
protein and circulate as free steroid (∼30%) or bound to albu­
min (∼70%). Circulating half-lives vary depending on individual 
variability and underlying disease, particularly renal and hepatic 
impairment. Cortisone acetate should not be used parenterally 
because it requires metabolism by the liver to active corti

t requires metabolism by the liver to active cortisol.
It is beyond the scope of this chapter to describe which steroid 
should be given and by which route for the nonendocrine con­
ditions listed in Table 15.6. Acute and long-term corticosteroid 
therapy in patients with hypoadrenalism or CAH is discussed in 
later sections. 
Long-Term Therapy
In addition to the undoubted benefit that corticosteroids provide, 
there is an increasing incidence of overuse, particularly in patients 
with respirato

 overuse, particularly in patients 
with respiratory or rheumatologic disease, to such an extent that 
up to 1% of the population is now prescribed long-term corti­
costeroid therapy.154 Because of their established euphoric effect, 
corticosteroids often make patients feel better but without any 
objective improvements in underlying disease parameters. In view 
of the long-term harm of chronic glucocorticoid excess,178 deci­
sions regarding treatment should be evidence based and subject 
to reg

tment should be evidence based and subject 
to regular review based on efficacy and side effects. The endocri­
nologic consequences of chronic glucocorticoid excess, notably 
suppression of the HPA axis, are an important aspect of modern 
clinical practice and are described later (see “Primary and Cen­
tral Hypoadrenalism”). Endocrinologists need to be aware of the 
effects of long-term therapy and of steroid withdrawal. Selective 
glucocorticoid receptor agonists (SEGRAs) are being developed 
w

 receptor agonists (SEGRAs) are being developed 
with the aim of dissociating the transrepressive, anti-inflammatory 
actions of glucocorticoids from the transactivating effects that, by 
and large, mediate deleterious side effects.179 
Adrenocortical Diseases
With the exception of common adrenocortical incidentalomas (see 
upcoming discussion) adrenocortical diseases with a clear clinical 
phenotype are relatively rare. Their importance lies in their high 
rates of morbidity and mortality if un

their high 
rates of morbidity and mortality if untreated, coupled with the 
relative ease of diagnosis and the availability of effective therapy. 
The diseases are most readily classified on the basis of hormone 
excess or deficiency (Table 15.8). 
Glucocorticoid Excess
Cushing Syndrome
In 1912, Harvey Cushing first described a 23-year-old female 
with obesity, hirsutism, and amenorrhea, and 20 years later, he 
postulated that this “polyglandular syndrome” was due to a pri­
mary pituitary abnor

r syndrome” was due to a pri­
mary pituitary abnormality causing adrenal hyperplasia.8 Adre­
nal tumors were shown to cause the syndrome in some cases,180 
but ectopic ACTH production was not characterized until much 
later, in 1962.181 The term Cushing syndrome is used to describe all 
causes, whereas Cushing disease is reserved for pituitary-dependent 
Cushing syndrome.
Cushing syndrome comprises the symptoms and signs associ­
ated with prolonged exposure to inappropriately elevated levels 
of

ed exposure to inappropriately elevated levels 
of free plasma glucocorticoids. The use of the term glucocorticoid 
in the definition covers excess from both endogenous (cortisol) 
and exogenous (e.g., prednisolone, dexamethasone) sources. Iat­
rogenic Cushing syndrome is common,177,182 occurring to some 
degree in most patients taking long-term corticosteroid therapy. 
499
Endogenous causes of Cushing syndrome are rare and result in 
loss of the normal feedback me

rare and result in 
loss of the normal feedback mechanism of the HPA axis and the 
normal circadian rhythm of cortisol secretion.
The incidence of Cushing disease is estimated to be 2 to 3 cases 
per 1 million population per year. The incidence of ectopic ACTH 
syndrome parallels that of bronchogenic carcinoma, and although 
0.5% of lung cancer patients have ectopic ACTH syndrome, rapid 
progression of the underlying disease often precludes an early diag­
nosis. Cushing disease and adrenal adeno

rly diag­
nosis. Cushing disease and adrenal adenomas are four times more 
common in women, whereas ectopic ACTH syndrome is more com­
mon in men. Neuroendocrine tumors have an incidence of 7 to 8 per 
100,000 and may be a rare cause of the ectopic ACTH syndrome. 
Clinical Features of Cushing Syndrome
The classic features of Cushing syndrome—centripetal obesity, 
moon face, hirsutism, and plethora—have been well known since 
Cushing’s initial descriptions in 1912 and 1932 (Figs. 15.15, 
15.16, a

criptions in 1912 and 1932 (Figs. 15.15, 
15.16, and 15.17). However, this gross clinical picture is not 
always present, and a high index of suspicion is required in many 
cases. Once the normal physiologic effects of glucocorticoids are 
appreciated (see Fig. 15.13), the clinical features of glucocorticoid 
excess are easier to define. They are summarized in Table 15.9, 
together with the most discriminatory features that will assist in 
distinguishing Cushing syndrome from simple obesity.183,

guishing Cushing syndrome from simple obesity.183,184
Obesity and Weight Gain
Weight gain and obesity are the most common signs of Cushing 
syndrome. At least in adults, this weight gain is invariably cen­
tripetal in nature.149,185 In fact, generalized obesity is more com­
mon in the general population than it is in patients with Cushing 
syndrome. One exception is seen in pediatric patients, in whom 
glucocorticoid excess may result in generalized obesity. In addition 
to centripetal obesity, 

zed obesity. In addition 
to centripetal obesity, patients develop fat depots over the thoraco­
cervical spine (buffalo hump), in the supraclavicular region, and 
over the cheeks and temporal regions, giving rise to the rounded, 
moon-like facies. The epidural space, another site of abnormal fat 
deposition, may lead to neurologic deficits. 
Reproductive Organs
Gonadal dysfunction is common, with menstrual irregularity in 
females and loss of libido in both sexes. Hirsutism is frequently 
found 

ido in both sexes. Hirsutism is frequently 
found in female patients, as is acne. The most common form of 
hirsutism is vellus hypertrichosis on the face; this type should 
be distinguished from the darker, terminal differentiated hirsut­
ism that may occur because of ACTH-mediated adrenal andro­
gen excess. Hypogonadotropic hypogonadism occurs because 
of a direct inhibitory effect of cortisol on GnRH pulsatility 
and LH/FSH secretion, and it is reversible on correction of the 
hypercortisolism

 reversible on correction of the 
hypercortisolism.186,187 
Psychiatric Features
Psychiatric abnormalities occur in approximately 50% of patients 
with Cushing syndrome, regardless of cause.188,189 Agitated depres­
sion and lethargy are among the most common problems, but 
paranoia and overt psychosis are also well recognized. Memory and 
cognitive function may also be affected, and increased irritability 
may be an early feature. Insomnia is common, and both rapid eye 
movement and delta-wave s

mon, and both rapid eye 
movement and delta-wave sleep patterns are reduced.190 Lowering 
of plasma cortisol by medical or surgical therapy usually results in 
a rapid improvement in the psychiatric state. Overall quality of life 
is significantly reduced in patients with Cushing syndrome, par­
ticularly affecting physical health and functioning. Quality-of-life 
scores improve after treatment but do not return to normal.191 
   Adrenocortical Diseases
Cushing syndrome (pa

iseases
Cushing syndrome (pathologic/neoplastic hypercortisolism)
Pseudo-Cushing syndromes (physiologic/nonneoplastic hypercorti­
solism)
Glucocorticoid Resistance
Glucocorticoid Deficiency
Primary hypoadrenalism
Secondary hypoadrenalism
Postchronic corticosteroid replacement therapy
Congenital Adrenal Hyperplasia
Deficiencies of 21-hydroxylase, 3βHSD, 17α-hydroxylase, 11β-
hydroxylase, P450 oxidoreductase, P450 side-chain cleavage, and 
StAR
Mineralocorticoid Excess
Minera

leavage, and 
StAR
Mineralocorticoid Deficiency
Defects in aldosterone synthesis
Defects in aldosterone action
Hyporeninemic hypoaldosteronism
Adrenal Incidentalomas, Adenomas, and Carcinomas
HSD, Hydroxysteroid dehydrogenase; StAR, steroidogenic acute regulatory (protein).
   
TABLE 15.8
• Fig. 15.15  Minnie G., Cushing’s index patient, at age 23 years. (From 
Cushing H. The basophil adenomas of the pituitary body and their clini­
cal manifestations [pituitary basophili

eir clini­
cal manifestations [pituitary basophilism]. Bull Johns Hopkins Hosp. 
1932;50:137–195.)
500
Bone
In childhood, the most common presentation is poor linear growth 
and weight gain155; as discussed earlier, glucocorticoids have pro­
found effects on growth and development.171 Many patients with 
long-standing Cushing syndrome have lost height because of 
osteoporotic vertebral collapse. This can be assessed by measuring 
the patie

apse. This can be assessed by measuring 
the patient’s sitting height or comparing the height with arm span; 
in normal subjects, height and arm span should be equal. Patho­
logic fractures, occurring spontaneously or after minor trauma, 
are not uncommon. Rib fractures, in contrast to those of the ver­
tebrae, are often painless. The radiographic appearance is typical, 
with exuberant callus formation at the site of the healing fracture. 
In addition, osteonecrosis of the femoral and humeral he

ition, osteonecrosis of the femoral and humeral heads is a 
recognized feature of endogenous (although less commonly seen 
than in exogenous) Cushing syndrome (see Fig. 15.17). Hypercal­
ciuria may lead to renal calculi, but hypercalcemia is not a feature. 
Skin
Hypercortisolism results in thinning of the skin and separation 
and exposure of the subcutaneous vascular tissue. On examina­
tion, wrinkling of the skin on the dorsum of the hand may be 
seen, resulting in a “cigarette paper” appearanc

seen, resulting in a “cigarette paper” appearance (Liddle sign). 
Minimal trauma may result in bruising, which frequently resem­
bles the appearance of senile purpura. The plethoric appearance of 
the patient with Cushing syndrome is secondary to the thinning 
of the skin192 combined with loss of facial subcutaneous fat and 
is not caused by true polycythemia. Acne and papular lesions may 
occur over the face, chest, and back.
The typical, almost pathognomonic, red-purple, nonblanching, 
livid

st pathognomonic, red-purple, nonblanching, 
livid striae greater than 1 cm in width are most frequently found on 
the abdomen but may also be present on the upper thighs, breasts, 
and arms. They are very common in younger patients and less so in 
those older than 50 years of age. They must be differentiated from 
the paler, less pigmented striae that occur as a result of pregnancy 
(striae gravidarum) or in association with rapid weight loss.
Skin pigmentation is rare in Cushing disease but co

kin pigmentation is rare in Cushing disease but common in 
the ectopic ACTH syndrome. It arises because of overstimulation 
of melanocyte receptors by ACTH and possibly POMC-derived 
peptides. 
Muscle
Myopathy and bruising are two of the most discriminatory fea­
tures of the syndrome.183 The myopathy of Cushing syndrome 
involves the proximal muscles of the lower limbs and the shoulder 
girdle.193 Complaints of weakness, such as inability to climb stairs 
or get up from a deep chair, are relativ

b stairs 
or get up from a deep chair, are relatively uncommon, but test­
ing for proximal myopathy by asking the patient to rise from a 
crouching position often reveals the problem. 
• Fig. 15.16  Clinical features of Cushing syndrome. (A) Centripetal and some generalized obesity and dorsal 
kyphosis in a 30-year-old woman with Cushing disease. (B) Same patient as in (A), showing moon facies, 
plethora, hirsutism, and enlarged supraclavicular fat pads. (C) Facial rounding, hirsutism, and acne 

at pads. (C) Facial rounding, hirsutism, and acne in a 
14-year-old girl with Cushing disease. (D) Central and generalized obesity and moon facies in a 14-year-old 
boy with Cushing disease. (E, F) Typical centripetal obesity with livid abdominal striae seen in a 41-year-
old woman (E) and a 40-year-old man (F) with Cushing syndrome. (G) Striae in a 24-year-old patient with 
congenital adrenal hyperplasia treated with excessive doses of dexamethasone as replacement therapy. 
(H) Typical bruising

sone as replacement therapy. 
(H) Typical bruising and thin skin of a patient with Cushing syndrome. In this case, the bruising occurred 
without obvious injury.
501
Cardiovascular Features
Hypertension is another prominent feature, occurring in up to 
75% of cases. Even though epidemiologic data show a strong 
association between high blood pressure and obesity, hyper­
tension is much more common in patients with Cushing syn­
drome than in those with simple obesit

ushing syn­
drome than in those with simple obesity.158 This, together with 
the established metabolic consequences of the disease (diabetes, 
hyperlipidemia), is thought to explain the increased cardiovas­
cular mortality rate in untreated cases.194–196 Cardiovascular 
events are also more common in patients with presumed iatro­
genic Cushing syndrome, which results from prescribed corti­
costeroids.176 In addition, thromboembolic events may be more 
common in Cushing patients, but this develop

more 
common in Cushing patients, but this development appears to 
be limited to those with ACTH-dependent disease.197 
Infections are more common in patients with Cushing syn­
drome.198,199 In many instances, infections are asymptomatic and 
occur because the normal inflammatory response is suppressed. 
Reactivation of tuberculosis has been reported200 and has even been 
the presenting feature in some cases. Fungal infections of the skin 
(notably tinea versicolor) and nails may occu

kin 
(notably tinea versicolor) and nails may occur, as may opportunistic 
fungal infections. Bowel perforation is more common in patients 
with extreme hypercortisolism, and the hypercortisolism may mask 
• Fig. 15.17  Bone abnormalities in Cushing disease. (A) Aseptic necrosis of the right humeral head in a 
43-year-old woman with Cushing disease of about 8 months of duration. (B) Aseptic necrosis of the right 
femoral head in a 24-year-old woman with Cushing disease of about 4.5 years of dura

an with Cushing disease of about 4.5 years of duration. The arrows 
indicate the crescent subchondral radiolucency, best seen in this lateral view. (C) Diffuse osteoporosis, 
vertebral collapse, and subchondral sclerosis in the same patient shown in (A). (D) Rib fracture in a 38-year-
old man with Cushing disease. (A–C, from Phillips KA, Nance Jr EP, Rodriguez RM, et al. Avascular necro­
sis of bone: a manifestation of Cushing’s disease. South Med J. 1986;79:825–829.)
502
SECTION  IV    Adrenal 

d J. 1986;79:825–829.)
502
the usual symptoms and signs of the condition. Wound infections 
are more common and contribute to poor wound healing. 
Metabolic and Endocrine Features
Glucose intolerance occurs, and overt diabetes mellitus is present 
in up to one-third of patients in some series. Hepatic lipoprotein 
synthesis is stimulated, and increases in circulating cholesterol and 
triglycerides may be found.201 Hypokalemic alkalosis is 

cerides may be found.201 Hypokalemic alkalosis is found in 
10% to 15% of patients with Cushing disease but in more than 
95% of patients with ectopic ACTH syndrome. Several factors 
may contribute to this mineralocorticoid excess state, includ­
ing corticosterone and DOC excess, but the principal culprit is 
thought to be cortisol itself. Depending on the prevailing cortisol 
production rate, cortisol swamps 11βHSD2 in the kidney and acts 
as a mineralocorticoid. Hypokalemic alkalosis is more c

mineralocorticoid. Hypokalemic alkalosis is more common 
in ectopic ACTH syndrome because cortisol production rates are 
higher than in patients with Cushing disease.136 This can be diag­
nosed by documenting an increase in the ratio of urinary cortisol 
to cortisone metabolites. In addition, hepatic 5α-reductase activ­
ity is inhibited, resulting in disproportionately greater excretion of 
5β-cortisol metabolites.202
The functions of the pituitary-thyroid axis and the pituitary-
gonadal axis ar

ry-thyroid axis and the pituitary-
gonadal axis are suppressed in patients with Cushing syndrome 
because of a direct effect of cortisol on TSH and gonadotropin 
secretion.203,204 Cortisol causes a reversible form of hypogonado­
tropic hypogonadism but also directly inhibits Leydig cell func­
tion. GH secretion is reduced, possibly mediated through an 
increase in somatostatinergic tone. 
Eye
Ocular effects include raised intraocular pressure205 and exoph­
thalmos206 (in up to one-third of patie

and exoph­
thalmos206 (in up to one-third of patients in Cushing’s original 
series), the latter occurring because of increased retroorbital fat 
deposition. Cataracts, a well-recognized complication of cortico­
steroid therapy, seem to be uncommon,207 except as a complica­
tion of diabetes. In our experience, chemosis is a sensitive and 
underreported feature of Cushing syndrome. 
Classification and Pathophysiology of 
Cushing syndrome is most readily classified into ACTH-depen

yndrome is most readily classified into ACTH-depen­
dent and ACTH-independent causes (Table 15.10).
ACTH-Dependent Causes
When iatrogenic causes are excluded, the most common cause of 
Cushing syndrome is Cushing disease, accounting for approximately 
70% of cases. The adrenal glands in these patients show bilateral adre­
nocortical hyperplasia with widening of the ZF and ZR.184
Cushing himself raised the question as to whether this disease 
was a primary pituitary condition or s

is disease 
was a primary pituitary condition or secondary to an abnormality 
in the hypothalamus. The hypothalamus may have an initiating 
role, but the overwhelming evidence is that, at presentation, the 
condition is pituitary dependent (Table 15.11). In 85% to 90% of 
cases, the disease is caused by a pituitary adenoma of monoclonal 
origin208,209; basophil hyperplasia alone is found in 9% to 33% 
of pathologic series.210 The majority of tumors are small microad­
enomas (<1 cm), but larger m

rs are small microad­
enomas (<1 cm), but larger macroadenomas occur in up to 10% 
of cases and usually signify a more invasive tumor.211 Selective 
surgical removal of a causative corticotroph adenoma results in 
remission, but on very long-term follow-up, relapse may occur 
in up to 20% to 30% of microadenoma and macroadenoma 
patients.212 Moreover, recent data demonstrate that around 40% 
of patients with Cushing disease have somatic missense mutations 
in ubiquitin-specific protease 8 (USP8)

asia
Iatrogenic (treatment with [1-24]ACTH)
ACTH-Independent Causes
Adrenal adenoma and carcinoma
Primary pigmented nodular adrenal hyperplasia and Carney syndrome
Aberrant receptor expression (gastric inhibitory polypeptide, interleukin-
1β)
Iatrogenic (e.g., pharmacologic doses of prednisolone, dexamethasone)
Other Causes of Hypercortisolism (non-neoplastic)
Alcoholism
Obesity
Pregnancy
ACTH, Adrenocorticotropic hormone; CRH, corticotropin-releasing hormone.

pic hormone; CRH, corticotropin-releasing hormone.
   
TABLE 15.10
503
pituitary tumors. USP8 codes for a deubiquitinating enzyme, and 
the mutations affect the binding of an inhibitory 14-3-3 protein 
that keeps the deubiquitinating activity in check. Ubiquitination 
of cell surface receptors usually marks them for degradation by 
the endosome-lysosome system, downregulating expression. As 
a result of the USP8 mutations, deubiquitination is enhanced, 
and the epi

ations, deubiquitination is enhanced, 
and the epidermal growth factor receptor (EGFR) is recycled 
with enhanced signaling. EGF213,214 increases POMC mRNA 
transcription, cell proliferation, and ACTH secretion. Cortico­
troph215 adenomas with the USP8 mutant thus express higher 
levels of EGFR and produce increased amounts of POMC mRNA 
and ACTH compared to tumors with wild-type USP8.213,214 It is 
possible that targeting EGFR may have therapeutic potential in 
Cushing disease.
A key biochemica

ic potential in 
A key biochemical hallmark of the disease is a relative resis­
tance of ACTH secretion to normal glucocorticoid feedback 
inhibition.216 ACTH-secreting pituitary adenomas function at a 
higher than normal set-point for cortisol feedback. The predomi­
nant finding in Cushing disease is an increase in ACTH pulse 
amplitude with loss of normal circadian rhythm, but ACTH pulse 
frequency is also increased in some cases (see Fig. 15.8).217 
Ectopic ACTH Syndrome
In 1

es (see Fig. 15.8).217 
In 15% of cases, Cushing syndrome is associated with nonpitu­
itary tumors secreting ACTH—the ectopic ACTH syndrome.218 
On clinical grounds, these tumors can be divided into two entities: 
highly malignant tumors such as small cell carcinoma of bronchus 
(Table 15.12) and highly proliferative neuroendocrine carcinoma 
(e.g., pancreas), and more indolent tumors occurring in patients 
with underlying neuroendocrine tumors such as bronchial neuro­
endo

euroendocrine tumors such as bronchial neuro­
endocrine tumors (carcinoids) with a low proliferative rate. In the 
former group, the clinical presentation is often that of a wasting 
syndrome with weakness and pigmentation. Circulating ACTH 
concentrations and cortisol secretion rates can be extremely high. 
As a result, duration of symptoms from onset to presentation is 
short (<3 months); patients are commonly pigmented, and the 
metabolic manifestations of glucocorticoid excess are often rapi

festations of glucocorticoid excess are often rapid 
and progressive. Weight loss, myopathy, and glucose intolerance 
are prominent symptoms and signs. The association of these fea­
tures with hypokalemic alkalosis and peripheral edema should 
alert the clinician to the diagnosis.
Depending on local referral practice, approximately 20% 
of cases of ectopic ACTH syndrome are explained by indolent 
tumors, such as benign bronchial neuroendocrine tumors, that 
produce ACTH.218,219 In these cases, s

mors, that 
produce ACTH.218,219 In these cases, symptoms and signs are 
commonly present for 18 months before clinical presentation. 
Such patients present with the typical features of Cushing syn­
drome and may be biochemically similar to patients with Cushing 
disease. Therefore, once a diagnosis of Cushing syndrome is estab­
lished, the principal diagnostic dilemma is in the distinction of 
pituitary-dependent Cushing disease from these indolent causes 
of ectopic ACTH syndrome.
Tumors most 

ent causes 
Tumors most commonly associated with ectopic ACTH syn­
drome arise from neuroendocrine tissues, the cells of which possess 
the ability to uptake and decarboxylate amine precursors (APUD 
cells). Although POMC transcripts of 1200-1450nt are frequently 
found in small cell lung cancer, only 0.5% to 1% of tumors are 
associated with ectopic ACTH syndrome, and the explanation 
for the development of ectopic ACTH secretion remains unclear. 
In contrast, POMC mRN

 secretion remains unclear. 
In contrast, POMC mRNA transcripts of the short 800nt length 
may be found in tumors not associated with ectopic ACTH syn­
drome. In addition to aberrant transcriptional regulation of the 
POMC, interaction with tissue-specific transcription factors or the 
promoter methylation status of POMC may be involved. Once 
translated, POMC is cleaved in the pituitary by specific serine 
endoproteases to produce ACTH precursors; in ectopic ACTH 
syndrome, aberrant peripheral 

s; in ectopic ACTH 
syndrome, aberrant peripheral processing of POMC may lead to 
increased concentrations of circulating ACTH precursors (pro-
ACTH, N-POC) (see Fig. 15.6). In contrast to ACTH secretion 
from pituitary adenomas, ectopic POMC/ACTH production is 
not responsive to normal glucocorticoid feedback220 because of 
the defective GR or GR-signaling mechanism.221 However, this 
  Etiology of Cushing Disease: 
Hypothalamic Theory Versus Pituitary 
Theory
Hypothalamic Theory
Pituitary The

ituitary 
Theory
Pituitary Theory
Neuroendocrine abnormali­
ties290,291
Lack of cure after pituitary stalk 
section
Loss of circadian rhythm, 
sleep disturbance, other 
hypothalamic defects (TSH, 
LH/FSH secretion)
Circulating and CSF CRH levels are 
suppressed292
Reversal of hypothalamic defects on 
correction of hypercortisolism
Efficacy of centrally acting 
drugs293,294 (bromocriptine, 
cyproheptadine, sodium 
valproate)
High surgical cure rate (recur­
rences resulting fro

gh surgical cure rate (recur­
rences resulting from regrowth of 
inadequately resected tumor rather 
than real recurrence)295,300
Secondary hypoadrenalism after 
successful pituitary surgery (may 
be prolonged and associated 
with reduced ACTH expression 
in surrounding adjacent normal 
corticotrophs)514
Recurrences after pituitary 
surgery
Ectopic CRH-secreting tumors 
cause Cushing disease,289 
but pathologic examination 
shows basophil hyperpla­
sia, not adenomas
Pituitary ACTH-secreting aden

a­
Pituitary ACTH-secreting adenoma in 
almost 90% of cases are monoclo­
nal in origin301,515
Note: Superscript numbers indicate references listed at the end of the chapter.
ACTH, Adrenocorticotropic hormone; CRH, corticotropin-releasing hormone; CSF, cerebrospi­
nal fluid; FSH, follicle-stimulating hormone; LH, luteinizing hormone; TSH, thyroid-stimulating 
hormone.
   
TABLE 15.11
  Tumors Associated With the Ectopic 
Adrenocorticotropic Hormone Syndrome
Tumor Type
Approxima

orticotropic Hormone Syndrome
Tumor Type
Approximate Incidence (%)
Small cell lung carcinoma
50
Non–small cell lung carcinoma
5
Pancreatic neuroendocrine tumors
10
Thymic neuroendocrine tumors
5
Lung neuroendocrine tumors
10
Other neuroendocrine tumors
2
Medullary carcinoma of thyroid
5
Pheochromocytoma and related 
tumors
3
Rare carcinomas of prostate, breast, 
ovary, gallbladder, colon
10
TABLE 15.12
504
sensitivity to glucocorticoid fee

ine Hypertension
sensitivity to glucocorticoid feedback is far from absolute, which 
is one reason why the differential diagnosis of ACTH-dependent 
Cushing syndrome can be challenging.218 
Ectopic Corticotropin-Releasing Hormone Syndrome
Ectopic production of CRH is a very rare cause of pituitary-depen­
dent Cushing disease. A number of cases have now been described 
in which a tumor (usually bronchial carcinoid, medullary thyroid, 
or prostate carcinoma) has been shown to secrete CRH alone or 

carcinoma) has been shown to secrete CRH alone or 
in combination with ACTH.222–224 When available, pituitary his­
tologic examination has revealed corticotroph hyperplasia but not 
adenoma formation. Biochemically, these patients, like those with 
ectopic ACTH syndrome, lose the normal negative glucocorti­
coid feedback mechanism—50% have resistance to high-dose 
dexamethasone therapy. Ectopic CRH production may explain 
the suppression of cortisol secretion after high-dose dexametha­
sone that

sol secretion after high-dose dexametha­
sone that is observed in some patients with the ectopic ACTH 
syndrome. 
Macronodular Adrenal Hyperplasia
In 10% to 40% of patients with Cushing disease, there is bilat­
eral adrenocortical hyperplasia associated with one or more nod­
ules, which may be up to several centimeters in diameter.225–228 
Patients tend to be older and to have had symptoms for a longer 
time, but they otherwise present with the classic clinical features of 
Cushing syndrome. Pat

lassic clinical features of 
Cushing syndrome. Pathologically, the nodules are lobulated and 
can be markedly enlarged, but internodular hyperplasia is invari­
ably found. Macronodular adrenal hyperplasia (MAH) is thought 
to result from long-standing adrenal ACTH stimulation, which 
leads to autonomous adrenal adenoma formation. Therefore, 
as the adrenals in a patient with Cushing disease become more 
hyperplastic, they secrete more cortisol for a given ACTH level, 
which ultimately can lead t

r a given ACTH level, 
which ultimately can lead to autosuppression. Individual clinical 
cases support this hypothesis, and MAH should be regarded as 
an ACTH-dependent form of Cushing syndrome, even though 
ACTH levels may be relatively low and dexamethasone suppress­
ibility less marked than in other cases of Cushing disease.229 These 
features can be a trap for the unwary because they may be mistaken 
for primary adrenal tumors, especially as up to 30% of patients 
with Cushing disease have 

 up to 30% of patients 
with Cushing disease have asymmetric adrenal hyperplasia. 
Cortisol-Secreting Adrenal Adenoma and Carcinoma
Excluding iatrogenic cases, adrenal adenomas are responsible for 
about 10% to 15% of Cushing syndrome cases, and carcinomas 
for less than 5%. By contrast, 65% of cases of Cushing syndrome 
in children have an adrenal cause (15% adenomas, 50% carci­
nomas).227–229 Onset of clinical features is gradual in patients 
with adenomas, but it is of

s gradual in patients 
with adenomas, but it is often rapid in adrenal carcinoma. Muta­
tions of PRKACA, which encodes the catalytic subunit of cAMP-
dependent protein kinase A (PKA), at the hotspot L205R have 
been shown by several independent groups to be the cause of 
approximately 50% of adrenal adenomas causing overt Cushing 
syndrome.230–233
In addition to the features of hypercortisolism, patients may 
complain of loin or abdominal pain, and a tumor may be palpa­
ble. Adrenal carcinoma ma

nd a tumor may be palpa­
ble. Adrenal carcinoma may secrete other steroids, such as andro­
gens or mineralocorticoids, but this is very unusual in adenomas. 
Therefore, in females, there may be features of virilization, with 
hirsutism, clitoromegaly, breast atrophy, deepening of the voice, 
temporal recession, and severe acne. In pure cortisol-secreting 
adenomas, hirsutism is uncommon. Subclinical Cushing syn­
drome has been reported in up to 10% of patients with adrenal 
incidentalomas (see l

0% of patients with adrenal 
incidentalomas (see later discussion). 
Primary Pigmented Nodular Adrenal Hyperplasia and 
Carney Syndrome
Less than 2% of all Cushing syndrome is caused by ACTH-inde­
pendent bilateral, small, often pigmented adrenal nodules. Patho­
logically, these nodules are usually 2 to 4 mm in diameter (although 
they can be larger) and black or brown on cut section. Adjacent 
adrenal tissue is atrophic, distinguishing this primary pigmented 
nodular adrenocortical disease (PPN

ary pigmented 
nodular adrenocortical disease (PPNAD) from MAH. Presentation 
is with typical features of Cushing syndrome in persons younger 
than 30 years of age and, in 50% of cases, in persons younger than 
15 years of age.234 Cases of PPNAD have been reported without 
Cushing syndrome. Bilateral adrenalectomy is curative.
A familial autosomal dominant variant, called Carney complex 
(Table 15.13), comprises mesenchymal tumors (especially atrial 
myxomas), spotty skin pigmentation, periphera

ial 
myxomas), spotty skin pigmentation, peripheral nerve tumors, 
and various other tumors, including breast lesions, testicular 
tumors, and GH-secreting pituitary tumors.235 Mutations of the 
gene encoding the PKA regulatory subunit type IA (PRKAR1A) 
lead to abnormal PKA signaling and explain the phenotype in 
some cases.236 Other cases have been mapped to chromosome 
2p16, but the underlying genetic mutation is unknown. 
In McCune-Albright syndrome, fibrous dysplasi

rome
In McCune-Albright syndrome, fibrous dysplasia and cutaneous 
pigmentation may be associated with pituitary, thyroid, adrenal, 
and gonadal hyperfunction. The most common manifestation is 
with sexual precocity and GH excess, but Cushing syndrome has 
been reported.237 The underlying abnormality is a somatic muta­
tion in the α-subunit of the stimulatory G protein, which is linked 
to adenylyl cyclase, with a mosaic tissue distribution. The muta­
tion results in constitutive activation of t

muta­
tion results in constitutive activation of the G protein, mimicking 
constant ACTH stimulation at the level of the adrenal. ACTH 
levels are suppressed, and adrenal adenomas may occur. 
   Clinical Features of Carney Complex
Feature
Skin lesions
80
  Pigmented lesions
  Blue nevi
  Cutaneous myxomas
Cardiac myxomas
72
Pigmented nodular adrenal hyperplasia
45
Breast lesions
  Bilateral fibroadenomas
45 (females only)
Testicular tumors
56 (males only)
Pituitary lesions, usual

ar tumors
Pituitary lesions, usually growth hormone 
secreting
10
Neural lesions (gastric schwannomas)
<5
  Thyroid cancers
Rare
  Acoustic neuromas
Rare
  Hepatomas
Rare
TABLE 15.13
505
Macronodular Hyperplasia
Although MAH commonly occurs in patients with ACTH-­
dependent Cushing syndrome, truly ACTH-independent mac­
ronodular adrenal hyperplasia (AIMAH) is also recognized as 
a distinct entity.238 The nodules are nonpigmented and gr

nct entity.238 The nodules are nonpigmented and greater 
than 5 mm in diameter; occasionally, the adrenals are massively 
enlarged. Most cases are explained on the basis of aberrant receptor 
expression within the adrenal cortex.239 Food-induced hypercor­
tisolism due to enhanced adrenal responsiveness to gastric inhibi­
tory polypeptide (GIP) was the first cause of AIMAH described, 
due to expression of GIP receptors within the adrenal cortex, but 
aberrant expression of the vasopressin V1, β-a

ut 
aberrant expression of the vasopressin V1, β-adrenergic, LH, sero­
tonin, and angiotensin (AT1) receptors have also been linked to 
AIMAH. Protocols have been suggested for the further investiga­
tion of AIMAH.239
Familial cases suggest a genetic cause for this condition in some 
patients, and inactivating mutations Armadillo repeat 5 (ARMC5) 
have been demonstrated as a cause.240 
Iatrogenic Cushing Syndrome
A careful drug history is required to exclude iatrogenic Cushing 
syndrome. Develop

d to exclude iatrogenic Cushing 
syndrome. Development of the features of Cushing syndrome 
depends on the dose, duration, and potency of the corticosteroids 
used in clinical practice. ACTH is rarely prescribed, but long-term 
administration will also result in cushingoid features. Some fea­
tures, such as an increase in intraocular pressure, cataracts, benign 
intracranial hypertension, aseptic necrosis of the femoral head, 
osteoporosis, and pancreatitis, are more common in iatrogenic 
than e

ancreatitis, are more common in iatrogenic 
than endogenous Cushing syndrome, whereas other features, 
notably hypertension, hirsutism, and oligomenorrhea/amenor­
rhea, are less prevalent. 
Special Features of Cushing Syndrome
Cyclic Cushing Syndrome
Of particular clinical interest has been a group of patients with 
cyclic Cushing syndrome, characterized by periods of excess 
cortisol production interspersed with intervals of normal corti­
sol production (Fig. 15.18). Some of these patients demo

oduction (Fig. 15.18). Some of these patients demonstrate 
a paradoxic rise in plasma ACTH and cortisol when treated 
with dexamethasone, and occasionally, a patient is benefited by 
dopamine agonist (bromocriptine or cabergoline) therapy. Most 
patients have been thought to have pituitary-dependent disease, 
and in many of these patients, basophil adenomas have been 
removed, with long-term cure in some cases. However, cortisol 
secretion may show some evidence of cyclicity in patients with an 

ow some evidence of cyclicity in patients with an 
ectopic source of ACTH and in PPNAD.241,242 
Cushing Syndrome in Children
Cushing syndrome can occur at any age, but the causes differ 
across age groups (Fig. 15.19). In children, adrenal causes account 
for 65% of all cases, and in addition to the previously mentioned 
features, growth arrest is almost invariable.243 The dissociation 
between height and weight is obvious, with the height most com­
monly below the mean, whereas the BMI is almos

om­
monly below the mean, whereas the BMI is almost always above 
the mean. If the height and weight are increasing along the same 
percentile line, then the diagnosis of Cushing syndrome is highly 
unlikely. Obesity in childhood Cushing syndrome tends to be gen­
eralized. Most patients have a delayed bone age that is negatively 
correlated with height standard deviation score (SDS), duration 
of symptoms, and age at diagnosis. The observed growth failure 
often precedes other manifestations suc

h failure 
often precedes other manifestations such as weight gain, pubertal 
arrest, fatigue, depression, hypertension, and acne. Pubertal devel­
opment can be advanced in patients with virilizing tumors caus­
ing precocious pseudopuberty. However, in patients with true 
puberty, glucocorticoid-mediated suppression of gonadotropins 
may occur.244
Glucocorticoid excess influences not only the hypothalamic-
pituitary-gonadal axis but also the GH/IGF1 axis, leading to 
both reduction of spontaneou

GF1 axis, leading to 
both reduction of spontaneous GH secretion and pharmacologic 
GH response. Furthermore, direct effects of glucocorticoids on 
epiphyseal chondrocytes, probably together with disturbance of 
microvascularization of the growth plate, result in a negative effect 
on growth. Poor catch-up has been reported in children after 
cure of Cushing disease, and evidence exists for GH inhibition 
by hypercortisolemia for 1 to 2 years after cure of Cushing dis­
ease. GH secretion should 

er cure of Cushing dis­
ease. GH secretion should be assessed 3 months after treatment. 
If GH deficiency is demonstrated, GH in a replacement dose of 
600
500
400
300
200
100
120
100
80
60
40
20
600
500
400
300
200
100
0
Urinary cortisol/creatinine ratio
0
20
40
60
80
100
Day of collection
0
0
• Fig. 15.18  Patterns of cortisol secretion in three patients with cyclic Cush­
ing syndrome. In each case, the ratio of early-morning urinary cortisol (in 
nanomoles per liter) to creatinine (in millimo

in 
nanomoles per liter) to creatinine (in millimoles per liter) are plotted against 
time. Variable periodicity in cortisol hypersecretion is shown. (From Atkin­
son AB, McCance DR, Kennedy L, et al. Cyclical Cushing’s syndrome first 
diagnosed after pituitary surgery: a trap for the unwary. Clin Endocrinol. 
1992;36:297–299.)
506
25 μg/kg per day should be given. Catch-up growth is observed in 
most patients, and target adult height is a

ed in 
most patients, and target adult height is achieved. However, many 
patients remain obese. 
Pregnancy
Pregnancy is rare in women with Cushing syndrome because of 
associated amenorrhea due to androgen excess or hypercortisolism, 
but when present is due to an adrenal adenoma in 50% of cases.245 
A few cases of true pregnancy-induced Cushing syndrome have 
been described, with regression after delivery.246 In these cases, 
the cause is unknown. Establishing a diagnosis and a cause can be 
d

wn. Establishing a diagnosis and a cause can be 
difficult. Clinically, striae, hypertension, and gestational diabetes 
are common features in normal pregnancies, yet hypertension and 
diabetes are also the most common signs of Cushing syndrome 
in a pregnant woman (70% and 30% of all cases, respectively). 
Furthermore, biochemically normal pregnancy is associated with 
a threefold increase in plasma cortisol due to increased production 
of cortisol and CBG. Urinary free cortisol also rises, and

sol and CBG. Urinary free cortisol also rises, and dexa­
methasone does not suppress plasma cortisol to the same degree as 
the nonpregnant state. Left untreated, the condition is associated 
with high rates of maternal and fetal morbidity and mortality. Any 
adrenal or pituitary adenomas should be excised. Metyrapone, 
which is not teratogenic, has been effective in many cases in con­
trolling the hypercortisolism. 
Other Syndromes of Hypercortisolemia
Other states occur when there are clinical

solemia
Other states occur when there are clinical and biochemical features 
of Cushing syndrome, but when hypercortisolemia is secondary to 
other factors it is often referred to as pseudo-Cushing syndrome. 
Resolution of the underlying cause results in disappearance of the 
cushingoid state. Several causes are described. 
Alcohol
In the original description of alcohol-related pseudo-Cushing syn­
drome, urinary and plasma cortisol levels were elevated and failed 
to suppress with dexamethasone.

evated and failed 
to suppress with dexamethasone. Plasma ACTH has been found 
to be normal or suppressed. The condition is rare but should be 
suspected in a patient with an ongoing history of heavy alco­
hol intake and biochemical or clinical evidence of chronic liver 
disease.247 The pathogenesis of this condition remains unknown, 
but a two-hit hypothesis has been put forward. Chronic liver dis­
ease of any cause is associated with impaired cortisol metabolism, 
but in alcoholic patients the

ortisol metabolism, 
but in alcoholic patients there is an increased cortisol secretion 
rate, rather than concomitant suppression in the face of impaired 
metabolism.248 In some studies, alcohol has directly stimulated 
cortisol secretion; alternatively, AVP levels are elevated in patients 
with decompensated liver disease and may stimulate the HPA 
axis. With abstinence from alcohol, the biochemical abnormali­
ties revert to normal within days. 
Although the cause is unknown, it is 

Although the cause is unknown, it is recognized that patients with 
depression may exhibit the hormonal abnormalities of patients 
with Cushing syndrome.249 These abnormalities are reversible on 
correction of the psychiatric condition. Conversely, patients with 
Cushing syndrome are frequently depressed, and a careful clinical 
and endocrinologic assessment is required. 
Obesity
Although one of the most common referrals to a clinical endocrinol­
ogist is for exclusion of an underly

l endocrinol­
ogist is for exclusion of an underlying endocrine cause in a patient 
with obesity, the diagnosis of Cushing syndrome in such patients 
should not cause difficulties. Patients with obesity have mildly 
increased cortisol secretion rates, and the data suggest that this is 
due to activation of the HPA axis.250,251 However, circulating cor­
tisol concentrations are invariably normal, and urinary free cortisol 
excretion rates are either normal or only slightly elevated. The stim­
ulu

er normal or only slightly elevated. The stim­
ulus for the increased secretion of cortisol appears to be increased 
peripheral metabolism and clearance of cortisol—principally, 
reduced hepatic conversion of cortisone to cortisol by 11βHSD1 
and increased conversion of cortisol to 5α-reduced derivatives.251 
Investigation of Patients With Suspected 
There are two stages in the investigation of suspected Cushing syn­
drome: (1) Does this patient have Cushing syndrome? (2) If so,

oes this patient have Cushing syndrome? (2) If so, 
what is the cause? Unfortunately, many investigators fail to make 
this distinction and ill advisedly use tests that are relevant to the 
second question in trying to answer the first question. In particu­
lar, it is essential that radiologic investigations not be undertaken 
until Cushing syndrome has been confirmed biochemically. The 
starting point should be to investigate patients in whom there is 
a high clinical index of suspicion for the

ere is 
a high clinical index of suspicion for the diagnosis of Cushing 
syndrome, focusing on the features that are most discriminating 
for the condition (see Table 15.9). Widespread indiscriminate bio­
chemical screening in obese, hypertensive, and diabetic popula­
tions is not recommended. Very few laboratories have developed 
methods for the measurement of free serum cortisol.252 Because 
more than 90% of serum cortisol is protein bound, the results 
of the conventional assay are affected b

 results 
of the conventional assay are affected by drugs and by conditions 
that alter CBG levels. Oral estrogen therapy or pregnancy may 
elevate CBG and total serum cortisol, and should be stopped for 6 
weeks prior to assessment by tests using serum cortisol. The major 
tests are listed in Table 15.14.184,249,253,254
Question 1: Does This Patient Have Cushing Syndrome?
Circadian Rhythm of Plasma Cortisol
In normal subjects, plasma cortisol levels are at their highest early 
in the morning an

vels are at their highest early 
in the morning and reach a nadir (<50 nmol/L [<2 μg/dL] in a 
nonstressed subject) at about midnight.255 This circadian rhythm 
14.1 yr
13.0 yr
10.1 yr
4.5 yr
Cushing disease (n = 182)
Nodular adrenal hyperplasia (n = 25)
Ectopic ACTH syndrome (n = 11)
1.2 yr
Age (yrs)
30
20
15
10
5
0
Adrenal hyperplasia secondary to
McCune-Albright syndrome (n = 16)
Adrenocortical tumors (n = 164)
• Fig. 15.19  Etiology and age-dependency of pediatric Cushing syn­
drome. Note ho

ependency of pediatric Cushing syn­
drome. Note how the causes differ across age groups. (From Storr HL, 
Chan LF, Grossman AB, et al. Paediatric Cushing’s syndrome: epidemiol­
ogy, investigation and therapeutic advances. Trends Endocrinol Metab. 
2007;18:167–174.)
507
is lost in patients with Cushing syndrome; in the majority, the 9 
am plasma cortisol is normal but nocturnal levels are raised. Ran­
dom morning plasma cortisol levels are therefore of little value 

sma cortisol levels are therefore of little value in 
making the diagnosis, whereas a midnight cortisol level greater 
than 200 nmol/L (>7.5 μg/dL) indicates Cushing syndrome. 
However, various factors such as stress of venipuncture, intercur­
rent illness, and admission to hospital may lead to false-positive 
results. Conversely, if a serum cortisol value is less than 50 nmol/L 
at midnight, Cushing syndrome is excluded at that time. Ideally, 
patients should be hospitalized for 24 to 48 hours 

atients should be hospitalized for 24 to 48 hours before the mid­
night cortisol level is measured, but some centers have reported 
discriminant results for midnight levels measured in outpatients. 
Nevertheless, this is a cumbersome test and has been largely sup­
planted by measurement of salivary cortisol (see next). 
Salivary Cortisol
CBG is absent from saliva, and the use of salivary cortisol measure­
ments offers a sensible alternative in that the test does not require 
hospitalization. The

at the test does not require 
hospitalization. The diagnostic accuracy of a single midnight sali­
vary cortisol level has been established in several studies. The cutoff 
points that define disease vary depending on the assay used. In one 
study, a cortisol value greater than 2.0 ng/mL (>5.5 nmol/L) had 
a 100% sensitivity and a 96% specificity for diagnosis of Cush­
ing syndrome.253,256,257 It is important to note, however, that late-
night salivary cortisol tends to increase with age and cardi

vary cortisol tends to increase with age and cardiovascular 
comorbid conditions such as hypertension and diabetes, and thus 
the discriminating power diminishes in the elderly population.258 
Urinary Free Cortisol Excretion
For many years, the diagnosis of Cushing syndrome was based 
on the measurement of urinary metabolites of cortisol (24-hour 
urinary 17-hydroxycorticosteroid or 17-oxogenic steroid excre­
tion, depending on the method used). However, the sensitivity 
and specificity of these

However, the sensitivity 
and specificity of these methods are poor, and most centers have 
replaced these assays with the more sensitive measurement of uri­
nary free cortisol. This is an integrated measure of plasma free cor­
tisol: As cortisol secretion increases, the binding capacity of CBG is 
exceeded, resulting in a disproportionate rise in urinary free cortisol. 
Normal values depend on the assay used and tend to be lower when 
analyzed by liquid chromatography and tandem mass spectromet

y liquid chromatography and tandem mass spectrometry 
(LC-MS/MS). Although in widespread use, urinary free cortisol is 
less sensitive than salivary cortisol and dexamethasone suppression 
testing. Patients should make two or three complete consecutive 
collections to account for patient error in collecting samples and 
for episodic cortisol secretion, notably from adrenal adenomas. 
Simultaneous creatinine excretion (which differs by no more than 
10% from day to day) may be used to ensure adeq

n 
10% from day to day) may be used to ensure adequacy of col­
lection. Urinary free cortisol is a useful screening test, although it 
is accepted that the value can be normal in up to 8% to 15% of 
patients with Cushing syndrome.253,254,259 Conversely, moderately 
elevated results should always be verified by further testing before 
a diagnosis of Cushing syndrome is made. In addition, variations 
in levels of up to 50% have been observed in patients with proven 
Cushing disease, reinforcing th

ients with proven 
Cushing disease, reinforcing the need for multiple collections.260
Measurement of the cortisol-to-creatinine ratio in the first 
urine specimen passed on waking obviates the need for a timed 
collection and has been used as a screening test, particularly when 
cyclic Cushing syndrome is suspected.261 Urine aliquots may be 
sent to the local endocrinology laboratory, with cortisol-to-creati­
nine ratios greater than 25 nmol/mmol on repeated measurement 
being indicative of hype

 on repeated measurement 
being indicative of hypercortisolism. 
Low-Dose Overnight Dexamethasone Suppression Tests
In normal subjects, the administration of a supraphysiologic dose 
of glucocorticoid results in suppression of ACTH and cortisol 
secretion. In Cushing syndrome of whatever cause, there is a fail­
ure of this suppression when low doses of the synthetic glucocor­
ticoid dexamethasone are given.216
The overnight test is a useful outpatient screening test,
249,253,262 in which 1 mg of

tient screening test,
249,253,262 in which 1 mg of dexamethasone is given at 11 pm. A 
normal response is a plasma cortisol level of less than 50 nmol/L 
(<1.8 μg/dL) between 8 and 9 am the following morning. The 
outpatient overnight test has high sensitivity (95%) but lower 
specificity, and further investigation is often required.263,264
In the 48-hour low-dose dexamethasone test, plasma corti­
sol is measured at 9 am on day 0 and again 48 hours later, after 
administration of dexamethasone 0

rs later, after 
administration of dexamethasone 0.5 mg every 6 hours for 48 
hours. Using a postdexamethasone plasma cortisol concentration 
of less than 50 nmol/L (<1.8 μg/dL) as the cutoff point, this test 
is reported to have a 97% to 100% true-positive rate and a false-
positive rate of less than 1%.249,263
Certain drugs (e.g., phenytoin, rifampicin) may increase the 
metabolic clearance rate of dexamethasone, leading to false-posi­
tive results. Simultaneous measurement of plasma dexametha

ults. Simultaneous measurement of plasma dexamethasone 
may be useful in such cases and will also detect whether patients 
failed to take the drug, and a value of greater than 3.3 nmo/L as 
measure by LC-MS/MS appears to improve the accuracy of the 
test.265 
Other Causes of Hypercortisolemia: Pseudo-Cushing or True 
Cushing Syndrome?
In patients with depression, urinary free cortisol excretion may 
be elevated and may overlap with those seen in patients with 
true Cushing syndrome. Compared wit

patients with 
true Cushing syndrome. Compared with patients with Cush­
ing disease, depressed patients have greater suppressibility after 
dexamethasone and reduced response to CRH, but neither test 
is diagnostic.249,266 The dexamethasone-suppressed CRH test 
has been proposed as a tool to discriminate between true Cush­
ing syndrome and other states but has been shown to have no 
advantage over the standard low-dose dexamethasone suppression 
   Tests Used in the Diagnosis and 
Differential 

Differential Diagnosis of Cushing 
Syndrome
Diagnosis—Does the Patient Have Cushing Syndrome?
Late night salivary cortisol/circadian rhythm of plasma cortisol
Urinary free cortisol excretiona
Low-dose dexamethasone suppression testa
Differential Diagnosis—What Is the Cause of the Cushing 
Syndrome?
Plasma ACTH
Plasma potassium, bicarbonate
High-dose dexamethasone suppression test
Corticotropin-releasing hormone
Inferior petrosal sinus sampling
CT, MRI scannin

e
CT, MRI scanning of pituitary, adrenals
Scintigraphy
Tumor markers
aValuable outpatient screening tests (see text discussion).
ACTH, Adrenocorticotropic hormone; CT, computed tomography; MRI, magnetic resonance 
imaging.
   
TABLE 15.14
508
test. In normal subjects and in patients with endogenous depres­
sion, insulin-induced hypoglycemia results in a rise in ACTH and 
cortisol levels, a response that usu

 in ACTH and 
cortisol levels, a response that usually is not seen in patients with 
Cushing syndrome, but this test has largely been abandoned for 
this purpose.249 
Diagnostic Guidelines
The Endocrine Society, in collaboration with the European Soci­
ety for Endocrinology, has issued evidence-based guidelines for 
the diagnosis of Cushing syndrome.267 Recommendations are to 
proceed initially with one of four highly sensitive screening tests: 
urinary free cortisol, late-night salivary cortiso

urinary free cortisol, late-night salivary cortisol, overnight dexa­
methasone, or the 2-mg/48-hour dexamethasone suppression test. 
Abnormality detected by any of these tests in a patient with clini­
cally suspected Cushing syndrome should be confirmed with one 
of the additional tests; if both test results are abnormal, patients 
should then undergo testing for the cause of the Cushing syn­
drome (Fig. 15.20). 
Question 2: What Is the Cause of Cushing Syndrome in 
This Patient?
Having confirme

Cushing Syndrome in 
Having confirmed Cushing syndrome clinically and bio­
chemically, the clinician’s next step is to determine the cause 
(Fig. 15.21).
Morning Plasma ACTH
Ideally, ACTH should be measured with the use of a modern, two-
site immunoradiometric assay. Such a test differentiates ACTH-
dependent from ACTH-independent causes. The samples should 
be taken in ice cold tubes and immediately separated ahead of 
storage at −40°C ahead of analysis to prevent inadvertent degr

40°C ahead of analysis to prevent inadvertent degrada­
tion. In Cushing disease, 50% of patients have a 9 am ACTH level 
within the normal reference range (2–11 pmol/L [9–52 pg/mL]); 
in the remainder, it is modestly elevated. Occasionally, due to epi­
sodic secretion, levels may be very low, and thus measurement of 
at least two values is recommended to avoid misclassification of 
mild Cushing disease as ACTH independent. ACTH levels in the 
ectopic ACTH syndrome are high (usually >20 pmol/L [>

opic ACTH syndrome are high (usually >20 pmol/L [>90 pg/
mL]); nevertheless, overlap values are seen in Cushing disease in 
30% of cases.257 Therefore this test cannot be used to differenti­
ate the two conditions (Fig. 15.22). The measurement of ACTH 
precursors (pro-ACTH, POMC) has been suggested, but it is not 
routinely available and has not been proven to detect an ectopic 
source of ACTH.
In patients with adrenal tumors and clear clinical features, 
plasma ACTH is invariably undetectable (

eatures, 
plasma ACTH is invariably undetectable (<1 pmol/L) unless there 
is assay interference. The presence of plasma ACTH levels that 
are low-normal or intermittently detectable, which may occur in 
MAH, is problematic. The danger is that in some patients the 
asymmetry of the nodular hyperplasia may lead to a diagnosis of 
adrenal adenoma, the plasma ACTH is ignored, and an inappro­
priate adrenalectomy is performed. Conversely, in some patients 
with this syndrome, an autonomous adrenal t

ients 
with this syndrome, an autonomous adrenal tumor develops, and 
unilateral adrenalectomy is required despite the detectable ACTH. 
Plasma Potassium
Hypokalemic alkalosis is present in more than 95% of patients 
with the ectopic ACTH syndrome but in fewer than 10% of those 
with Cushing disease. The cause of this mineralocorticoid excess 
state is now established. Patients with the ectopic syndrome usu­
ally have higher rates of cortisol secretion. Cortisol saturates the 
renal-protective 1

retion. Cortisol saturates the 
renal-protective 11βHSD2 enzyme, resulting in cortisol-induced 
mineralocorticoid hypertension (see Chapter 16).136 In addition, 
these patients have higher levels of the ACTH-dependent miner­
alocorticoid, DOC. 
High-Dose Dexamethasone Suppression Test
The rationale for the high-dose dexamethasone suppression test is 
that in Cushing disease the negative feedback control of ACTH is 
reset to a higher level than normal. Therefore cortisol levels do not 
suppress w

rmal. Therefore cortisol levels do not 
suppress with low-dose dexamethasone but do so after high doses. 
Cushing syndrome suspected
(consider endocrinologist consultation)
Exclude exogenous glucocorticoid exposure
Perform one of the following tests
Exclude physiologic causes of hypercortisolism
Consult endocrinologist
Cushing syndrome
ANY ABNORMAL
RESULT
Normal
(CS unlikely)
ABNORMAL
Discrepant
(Suggest additional
evaluation)
Normal
24-h UFC
(≥2 tests)
Overnight
1-mg DST
Perform 1

)
24-h UFC
(≥2 tests)
Overnight
1-mg DST
Perform 1 or 2 other studies shown above
Suggest repeating the abnormal study
Suggest Dex-CRH or midnight serum cortisol in
certain populations (see text)
Late night salivary
cortisol (≥2 tests)
Consider caveats for each test (see text)
Use 48-h, 2-mg DST in certain populations (see text)
• Fig. 15.20  Algorithm for testing patients with suspected Cushing syn­
drome (CS) according to the 2008 Endocrine Society clinical practice 
guideline. All statements 

iety clinical practice 
guideline. All statements are recommendations except for those prefaced 
by the word Suggest. Diagnostic criteria that point to Cushing syndrome 
are a urinary free cortisol (UFC) value greater than the normal range for 
the assay, a serum cortisol level greater than 1.8 μg/dL (>50 nmol/L) after 
administration of 1 mg dexamethasone (1-mg DST), and a late-night 
salivary cortisol concentration greater than 145 ng/dL (>4 nmol/L). CRH, 
corticotropin-releasing hormone; Dex,

ol/L). CRH, 
corticotropin-releasing hormone; Dex, dexamethasone; DST, dexametha­
sone suppression test. (From Nieman LK, Biller BM, Findling JW, et al. The 
diagnosis of Cushing’s syndrome: an Endocrine Society Clinical Practice 
Guideline. J Clin Endocrinol Metab. 2008;93:1526–1540.)
509
The original test introduced by Liddle was based on giving 2 mg 
dexamethasone every 6 hours for 48 hours and demonstrating a 
fall of greater than 50% in urinary 17-hydroxycorti

all of greater than 50% in urinary 17-hydroxycorticosteroids.216 
In the modern test, the plasma or urinary free cortisol (or both) is 
measured at 0 and +48 hours, and a greater than 50% suppression 
of plasma cortisol from the basal value has been used to define a 
positive response. In all cases, the response is graded and is depen­
dent on the original cortisol secretion rate: Greater suppression 
is often observed in patients with lower basal cortisol values. In 
women with ACTH-dependent C

l cortisol values. In 
women with ACTH-dependent Cushing syndrome, in whom the 
a priori likelihood of Cushing disease is 90%, the sensitivity of this 
test for the Cushing disease is 80%, lower than the pretest prob­
ability. Because of this, there is little logic to the continued use of 
this test when inferior petrosal sinus sampling (IPSS) is available 
(see later). Moreover, if the low-dose dexamethasone suppression 
test has been used in the diagnosis of Cushing syndrome and a 
greater tha

e diagnosis of Cushing syndrome and a 
greater than 50% fall in cortisol is observed, there is no added 
value in the high-dose dexamethasone suppression test.263 
Corticotropin-Releasing Hormone Test
CRH is a 41–amino acid peptide that was identified by Vale in 
1981 from ovine hypothalami. The ovine sequence differs by 
seven amino acid residues from that of the human hormone but 
is slightly more effective in stimulating the release of ACTH in 
humans.268 The test involves the intravenous inj

humans.268 The test involves the intravenous injection of either 
ovine or human sequence CRH in a dose of 1 μg/kg body weight 
or a single dose of 100 μg (Fig. 15.23). The test can be performed 
in the morning or afternoon. After basal sampling, CRH is admin­
istered, and blood samples for ACTH and cortisol are then taken 
every 15 minutes for 1 to 2 hours.261,264,269,270
In normal subjects, CRH produces a rise in ACTH and cortisol 
of 15% to 20%. This response is exaggerated in Cushing disea

20%. This response is exaggerated in Cushing disease, 
in which typically an ACTH increase greater than 50% and a cor­
tisol rise greater than 20% over baseline values are seen. Responses 
are seldom seen in the ectopic ACTH syndrome, but false-positive 
results have been reported. In distinguishing pituitary-dependent 
Cushing disease from the ectopic ACTH syndrome, the response 
clinically/biochemically confirmed
Differential diagnosis
If tests inconclusive (i.e., failure of a

iagnosis
If tests inconclusive (i.e., failure of any above criteria)
Inferior petrosal sinus sampling with CRH
Positive ACTH gradient
Transsphenoidal hypophysectomy
No ACTH gradient
Ectopic ACTH syndrome
Appropriate radiology ± venous sampling
Then proceed to
transsphenoidal hypophysectomy
ACTH
CT of adrenals
(consider macronodular hyperplasia)
ACTH-dependent
disease
Detectable
• 50% cortisol suppression following high- or low-dose 
  dexamethasone suppression test (2 

 or low-dose 
  dexamethasone suppression test (2 mg 6 hourly for 48 hours)
                                         AND
• >50% increased serum cortisol post CRH stimulation test
                                         AND
• Positive MRI scan of pituitary
• Fig. 15.21  The tests to uncover the cause of Cushing syndrome are debatable and differ in any given 
center depending on many factors, including familiarity and turnaround time of hormone assays and local 
expertise in techniques such as in

says and local 
expertise in techniques such as inferior petrosal sinus sampling. Depicted here is an algorithm in use within 
many endocrine units based on the reported sensitivity and specificity of each endocrine test. ACTH, 
adrenocorticotropic hormone; CRH, corticotropin-releasing hormone; CT, computed tomography; MRI, 
magnetic resonance imaging.
510
of ACTH and cortisol to CRH has a specificity and a sensitivity 
of approximately 90

specificity and a sensitivity 
of approximately 90%. However, a positive response defined as an 
ACTH increase of 100% or a cortisol rise of 50% over baseline 
values effectively eliminates a diagnosis of ectopic ACTH syn­
drome, which is the real benefit of this test. Up to 10% of patients 
with Cushing disease do not respond to CRH. 
Inferior Petrosal Sinus Sampling and Selective Venous 
Catheterization
The most robust test to distinguish Cushing disease from the ecto­
pic ACTH syndrome is IPS

ng disease from the ecto­
pic ACTH syndrome is IPSS.184 Because blood from each half 
of the pituitary drains into the ipsilateral inferior petrosal sinus, 
catheterization and venous sampling of both sinuses simultane­
ously can distinguish a pituitary from an ectopic source of ACTH 
(Fig. 15.24).271,272 In virtually all patients with the ectopic ACTH 
syndrome, the ratio of the ACTH concentration in the inferior 
petrosal sinus and that in simultaneously drawn peripheral venous 
blood is less 

ltaneously drawn peripheral venous 
blood is less than 1.4:1. In contrast, the ratio is elevated to greater 
than 2 in Cushing disease. However, because of the problem of 
intermittent ACTH secretion, it is useful to take measurements 
before and at intervals (e.g., 2, 5, and 15 minutes) after intrave­
nous injection of 100 μg synthetic ovine or human CRH.273,274 
Using this approach, an ACTH petrosal sinus/peripheral ratio 
greater than 3 after CRH administration has a sensitivity of 95% 
and s

CRH administration has a sensitivity of 95% 
and specificity of nearly 100% for diagnosing Cushing disease.274 
When CRH is not available, some centers use desmopressin as 
a secretagogue, but central to peripheral gradients have been 
observed in some patients with ectopic ACTH when using this 
peptide. When no gradient is found and venograms confirm cor­
rect catheter placement, measurement of prolactin in the samples 
and correction of the ACTH values can reduce the false-negative 
rate in Cu

H values can reduce the false-negative 
rate in Cushing disease.275
IPSS may also be of value in lateralizing a pituitary tumor in a 
patient in whom imaging techniques have failed to demonstrate a 
microadenoma; however, some centers have found this procedure 
to be of little value in predicting tumor location. Because many 
tumors are central and drain into both sinuses, current evidence 
suggests that it is unwise to base the surgical procedure on the 
results of IPSS studies alone.
IPSS is t

e on the 
IPSS is technically demanding; it has been associated with 
complications (referred aural pain, thrombosis) and should be 
performed only in an experienced tertiary referral center. In some 
centers, a clinical diagnostic algorithm is used (see Fig. 15.21), and 
IPSS is performed if the differential diagnosis remains in doubt 
(i.e., lack of adequate suppression after high-dose dexamethasone 
or CRH response or the absence of a macroadenoma on pituitary 

se or the absence of a macroadenoma on pituitary 
MRI scanning). 
Imaging
CT/MRI Scanning of Pituitary and Adrenal Glands
High-resolution, thin-section, contrast-enhanced computed 
tomography (CT) or MRI imaging has revolutionized the inves­
tigation of Cushing syndrome.271,272 However, it is essential that 
the results of any imaging technique be interpreted alongside 
the biochemical results if mistakes are to be avoided. When the 
adrenals are imaged, asymmetric nodular hyperplasia may lead t

 imaged, asymmetric nodular hyperplasia may lead to 
a false diagnosis of adrenal adenoma. In the general population 
there is an approximately 10% rate of small abnormalities on pitu­
itary MRI, so-called pituitary incidentalomas, which may mimic 
a corticotrope adenoma to the unwary, thus emphasizing the need 
for careful biochemical assessment.276
Pituitary MRI is the investigation of choice once the biochemi­
cal tests have suggested Cushing disease, with a sensitivity of 60% 
and specificit

disease, with a sensitivity of 60% 
and specificity of 87%. About 90% of ACTH-secreting pituitary 
tumors are microadenomas (i.e., <10 mm in diameter). The clas­
sic features of a pituitary microadenoma are a hypointense lesion 
on T1-weighted images after gadolinium contrast enhancement, 
which may be associated with deviation of the pituitary stalk, and 
a convex upper surface of the pituitary gland (Fig. 15.25).
For adrenal imaging, CT offers better spatial resolution277 (Fig. 
15.26) and is 

better spatial resolution277 (Fig. 
15.26) and is the procedure of choice, but MRI may provide diag­
nostic information in patients with suspected adrenal carcinoma. 
Once again, so-called adrenal incidentalomas are present in up 
to 5% of normal subjects, so adrenal imaging should not be per­
formed unless biochemical investigation has suggested a primary 
adrenal cause (i.e., undetectable ACTH concentrations). Adrenal 
carcinomas are usually large and are often associated with meta­
static spr

rge and are often associated with meta­
static spread at presentation (Fig. 15.27).
In patients with occult ectopic ACTH syndrome, high-defi­
nition CT/MRI scanning of thorax, abdomen, and pelvis with 
images obtained every 0.1 to 0.5 cm may be required to detect 
small ACTH-secreting neuroendocrine tumors (Fig. 15.28). 
Nuclear Medicine and Molecular Imaging Studies
Scintigraphy is used in some centers for patients with primary 
adrenal disease. The most commonly used agent is 136I-labeled 
6β-

 The most commonly used agent is 136I-labeled 
6β-iodomethyl-19-norcholesterol,278 a marker of adrenocorti­
cal cholesterol uptake. In patients with adrenal adenomas, the 
Cushing
disease
Adrenal
tumor
Ectopic
ACTH
Plasma immunoreactive ACTH (pg/mL)
0
50
100
150
200
250
300
500
700
900
4000-12,000
2000-4000
1000-2000
Normal
range
8-10 AM
• Fig. 15.22  Plasma adrenocorticotropic hormone (ACTH) concentrations 
in patients with Cushing disease, Cushing syndrome associated with adre­
nocortical tumo

ing syndrome associated with adre­
nocortical tumors, or the ectopic ACTH syndrome. To convert values to 
picomoles per liter, multiply by 0.2202. (From Besser GM, Edwards CRW. 
Cushing’s syndrome. Clin Endocrinol Metab. 1972;1:451–490.)
511
isotope is taken up by the adenoma but not by the contralateral 
suppressed adrenal gland. Adrenal scintigraphy may be useful in 
patients with suspected adrenocortical macronodular hyperplasia, 
although it is not in widesprea

ular hyperplasia, 
although it is not in widespread use; CT scanning may mislead­
ingly suggest unilateral disease, whereas isotope scanning identi­
fies bilateral adrenal involvement.
Many neuroendocrine tumors giving rise to the ectopic ACTH 
syndrome express somatostatin receptors (sst) and can be imaged by 
administering radiolabeled analogues of somatostatin, most com­
monly [114In]-labeled octreotide, or with more sensitive [69Ga]-
DOTATATE PET-CT. This latter technique can detect tumors a

 PET-CT. This latter technique can detect tumors as 
small as a few millimeters in diameter and should be considered 
for patients with ACTH-dependent Cushing syndrome in whom 
pituitary disease has been excluded, although discovery of a lesion 
that has not been seen on axial imaging is rare.279 Because the sst2 
receptor on the causative tumor to which these analogues bind 
may be downregulated in Cushing syndrome, medical therapy to 
either lower or offset the actions of cortisol may allow an

wer or offset the actions of cortisol may allow an initially 
negative scan to be repeated after some months and detect an 
occult source.280 
Treatment of Cushing Syndrome
The Endocrine Society has produced evidence-based clinical prac­
tice guidelines for the treatment of Cushing syndrome.281
Adrenal Causes
Unilateral adrenal adenomas should be removed by adrenalec­
tomy; the cure rate approaches 100%, with success reflected by 
postoperative serum cortisol values that are less than 50 nmol/L 

erum cortisol values that are less than 50 nmol/L 
at 9 am, or even lower in many modern assays.281,282 Laparo­
scopic adrenalectomy by an expert surgeon is the standard of 
care for unilateral tumors, offering reduced surgical morbidity 
and postoperative hospital stay compared with traditional open 
approaches.281,283 After surgery, it may take many months or 
even years for the contralateral suppressed adrenal to recover. 
Glucocorticoid replacement regimens vary, but many centers 
use low do

cement regimens vary, but many centers 
use low doses (15–20 mg) of hydrocortisone, and withdrawal 
regimens differ. One practical approach is to measure the morn­
ing plasma cortisol having omitted the dose of hydrocortisone 
in the morning at 3-month intervals. Patients with serum 
Plasma cortisol (mg/dL)
Plasma IR-ACTH (pg/mL)
(n = 6)
Controls (n = 10)
Ectopic ACTH (n = 6)
−15 0 15 30 60
90
120 150 180
Time (min)
Time (min)
−15 0 15 30
60
90
120
150
180
200
1

in)
Time (min)
−15 0 15 30
60
90
120
150
180
200
190
180
170
160
150
140
130
120
110
100
90
80
70
60
50
40
30
20
10
0
55
50
45
40
35
30
25
20
15
10
5
0
  120
110
100
90
80
70
60
50
40
30
20
10
0
55
50
45
40
35
30
25
20
15
10
5
0
• Fig. 15.23  Comparison of cortisol and adrenocorticotropic hormone (ACTH) responses to an intrave­
nous injection of ovine corticotropin-releasing hormone (1 μg/kg) in normal subjects, patients with Cush­
ing disease, and patients with the ectopic ACTH syndrome. IR, im

nd patients with the ectopic ACTH syndrome. IR, immunoreactive. (From Chrousos GP, 
Schulte HM, Oldfield EH, et al. The corticotropin-releasing factor stimulation test: an aid in the evaluation 
of patients with Cushing’s syndrome. N Engl J Med. 1984;310:622–626.)
512
cortisol values less than 200 nmol/L (<7.5 μg/dL) should con­
tinue glucocorticoid replacement, whereas in patients with val­
ues above 500 nmol/L (>18.3 μg/dL), this replace

­
ues above 500 nmol/L (>18.3 μg/dL), this replacement can be 
stopped. In patients with values between 200 and 500 nmol/L, 
ACTH(1-24) testing can be used to assess whether there is suf­
ficient response to stress, although insulin tolerance testing 
may be used in some centers. In the interim, all patients should 
carry a steroid alert card and increase their dose of replacement 
therapy in the event of an intercurrent illness.
Adrenal carcinomas have had a very poor prognosis, and most 
patie

as have had a very poor prognosis, and most 
patients have died within 2 years of diagnosis.284 It is usual prac­
tice to try to remove the primary tumor, even though metastases 
may be present, so as to enhance the response to the adrenolytic 
agent o,p′-DDD285 (mitotane). Radiotherapy to the tumor bed 
and to some metastases, such as those in the spine, may be of lim­
ited value. However, in recent years, significant progress has been 
made by implementing collaborative multicenter studies. Ph

implementing collaborative multicenter studies. Phase 
III trials of mitotane therapy achieving therapeutic plasma mito­
tane levels have shown significant benefit286; drug combinations 
include etoposide, doxorubicin, and cisplatin plus mitotane or 
streptozotocin plus mitotane. Several targeted therapies, includ­
ing IGF1 inhibitors, sunitinib, and sorafenib, have been tried in 
cases of mitotane failure, but there are no consistent responses. 
The 10-year survival rate for patients with T1 N0

The 10-year survival rate for patients with T1 N0 M0 disease is 
about 80% but is significantly impaired with increased tumor 
mass, positive lymph nodes, and distant metastases, reaching less 
than 20% for patients with T1-4 N0-1 M1.287 
Pituitary-Dependent Cushing Syndrome
The treatment of Cushing disease has been significantly enhanced 
with transsphenoidal surgery conducted by an experienced 
surgeon.281,288
The surgical outcome for transsphenoidal hypophysectomy 
varies from center to cent

enoidal hypophysectomy 
varies from center to center and with surgical expertise.289 
Because of the hazards of untreated Cushing disease and the 
potential complications of surgery, the endocrinologist should 
refer patients only to a recognized surgical specialist at a center 
in which outcome data have been established. In optimal centers, 
remission rates are 70% to 90% for microadenomas and 50% 
for macroadenomas.290 Rates for postoperative hypopituitarism 
and permanent diabetes insipidus 

hypopituitarism 
and permanent diabetes insipidus depend on how aggressive the 
surgeon was in removing pituitary tissue. The ideal outcome is a 
cured patient with intact pituitary function, but this result may 
Cavernous
sinus
Inferior
petrosal sinus
Confluent
pituitary veins
Jugular
vein
• Fig. 15.24  Anatomy of the venous drainage of the pituitary gland through 
the inferior petrosal venous sinuses. (From Oldfield EH, Chrousos GP, 
Schulte HM, et al. Preoperative lateralization of ACTH-secre

, et al. Preoperative lateralization of ACTH-secreting pituitary 
microadenomas by bilateral and simultaneous inferior petrosal sinus sam­
pling. N Engl J Med. 1985;312:100–103.)
A
B
• Fig. 15.25  (A) Magnetic resonance imaging (MRI) scan of pituitary demonstrates the typical appear­
ance of a pituitary microadenoma. A hypodense lesion is seen in the right side of the gland (arrow), with 
deviation of the pituitary stalk away from the lesion. After a biochemical diagnosis of Cushing disease, thi

er a biochemical diagnosis of Cushing disease, this 
patient was cured by transsphenoidal hypophysectomy. (B) MRI scan of the pituitary gland demonstrates 
a large macroadenoma (arrow) in a patient with Cushing disease. In contrast to smaller tumors, large 
macroadenomas are usually invasive and recur after surgery.
513
A
B
C
D
• Fig. 15.26  (A) Adrenal computed tomographic (CT) scan demonstrates bilateral adrenal hyperplasia 
(arrows) in a patient with Cushing dis

yperplasia 
(arrows) in a patient with Cushing disease. (B) CT scan of a typical solitary left adrenal adenoma (arrow) 
causing Cushing syndrome. (C) Cushing syndrome caused by massive macronodular hyperplasia. Adre­
nal glands are replaced by multiple nodules (arrows). The combined weight of the adrenal glands was 
more than 100 g. (D) Cushing syndrome caused by surgically proven primary pigmented nodular adrenal 
disease in a 21-year-old patient. Notice the multiple small nodules with relative

t. Notice the multiple small nodules with relatively atrophic internodular 
adrenocortical tissue involving the medial limb of the right adrenal gland (arrow). (C, D, from Findling JW, 
Doppman JL. Biochemical and radiologic diagnosis of Cushing’s syndrome. Endocrinol Metab Clin North 
Am. 1994;23:511–537.)
A
B
• Fig. 15.27  Computed tomographic scan of a patient with rapidly progressing Cushing syndrome caused 
by an adrenal carcinoma. An irregular right adrenal mass is shown in (A) and a large

lar right adrenal mass is shown in (A) and a large liver metastasis is 
seen in (B).
514
not be possible for a patient with Cushing disease in whom a 
pituitary adenoma was not identified preoperatively or during 
the operation itself.
In centers that lack facilities for frequent monitoring of cor­
tisol levels, perioperative and postoperative hydrocortisone cover 
is advised; this can be reduced to maintenance replacement doses 
usually w

educed to maintenance replacement doses 
usually within 3 to 7 days. On days 2 to 5, postoperatively, a 9 
am plasma cortisol level should be measured with the patient hav­
ing omitted hydrocortisone for 24 hours. After selective removal 
of a microadenoma, the surrounding corticotrophs are usually 
suppressed (Fig. 15.29). As a result, plasma cortisol levels are less 
than 30 nmol/L (<1 μg/dL) postoperatively, and ongoing gluco­
corticoid replacement therapy is required. The HPA axis usually 
(

ement therapy is required. The HPA axis usually 
(but not invariably) exhibits gradual recovery, and glucocorti­
coid therapy is needed until this occurs (Fig. 15.30). A nonsup­
pressed plasma cortisol postoperatively suggests that the patient 
is not in remission even though cortisol secretion may have fallen 
to normal or subnormal values.291,292 The overall recurrence rate 
on long-term follow-up even for patients in remission after pitu­
itary surgery is up to 30%, but this value is higher i

y surgery is up to 30%, but this value is higher in children 
(up to 40%).281,293,294 This emphasizes the need for long-term 
monitoring. A detailed assessment of residual pituitary function is 
required in each patient, and close follow-up of such individuals 
is warranted.
In the past, pituitary irradiation was often used in the treat­
ment of Cushing disease. However, because of the improvements 
in pituitary surgery, far fewer patients are so treated. In children, 
pituitary irradiation appe

 treated. In children, 
pituitary irradiation appears to be more effective.295 Radiotherapy 
A
B
C
D
• Fig. 15.28  Imaging of the thorax in a patient with the ectopic adrenocorticotropic hormone (ACTH) syn­
drome. (A) Plain chest radiograph demonstrates a suspicious lesion behind the left heart border (arrow). 
(B, C) Axial and sagittal computed tomographic images demonstrate a bronchial carcinoid tumor (arrow) 
abutting the diaphragm. (D) Three-dimensional reconstruction illustrates adherence o

dimensional reconstruction illustrates adherence of the tumor to the 
diaphragm (arrow), which was confirmed at surgery. (From Newell-Prince J, Trainer P, Besser M, et al. The 
diagnosis and differential diagnosis of Cushing’s syndrome and pseudo-Cushing’s states. Endocr Rev. 
1998;19:647–672.)
A
B
C
ACTH –ve
ACTH –ve
ACTH –ve
ACTH
ACTH +++
ACTH +
Cortisol +++
Cortisol +
+++
+
–ve
Secretion above normal
Secretion is detectable
Secretion suppressed
ACTH secreting tumor
Cortisol ND
• Fig. 15.29  S

d
• Fig. 15.29  Selective removal of a microadenoma and its effect on the 
hypothalamic-pituitary-adrenal axis. (A) Before treatment. (B) After total 
removal of adenoma. (C) After incomplete excision. Because the sur­
rounding normal pituitary corticotrophs are suppressed in a patient with 
an adrenocorticotropic hormone (ACTH)-secreting pituitary adenoma, 
successful removal of the tumor results in ACTH, and hence adreno­
cortical, deficiency, with an undetecta

ce adreno­
cortical, deficiency, with an undetectable (<50 nmol/L [2 μg/dL]) plasma 
cortisol level. A postoperative plasma cortisol level higher than 50 nmol/L 
(2 μg/dL) implies that the patient is not cured. (Courtesy Professor Peter 
Trainer.)
515
is not recommended as a primary treatment but is reserved for 
patients who are not responding to pituitary microsurgery, patients 
who have undergone bilateral adrenalectomy, and patients with 
established Nelson syn

lectomy, and patients with 
established Nelson syndrome (see upcoming discussion).
The management of recurrent Cushing disease involves a con­
sideration of repeat pituitary surgery, Gamma Knife radiosurgery, 
medical therapies, and adrenal surgery.281,288,296 Bilateral adre­
nalectomy carries a cure rate approaching 100%. The major risk 
is the subsequent development of Nelson syndrome (postadre­
nalectomy hyperpigmentation with a locally aggressive pituitary 
tumor) (Fig. 15.31), which was att

sive pituitary 
tumor) (Fig. 15.31), which was attributed to loss of any negative 
feedback after adrenalectomy but is more likely to be due to an 
aggressive pituitary tumor from the outset.297 In an attempt to 
avoid this complication, pituitary irradiation was often carried 
out at the time of bilateral adrenalectomy, but this is not widely 
practiced today.298 If a pituitary corticotroph tumor is not visible 
at the time of surgery, the risk of Nelson syndrome is reduced; 
monitoring is by p

of Nelson syndrome is reduced; 
monitoring is by plasma ACTH and MRI of the pituitary.297 In 
addition, these patients require lifelong replacement therapy with 
hydrocortisone and fludrocortisone. Currently, bilateral adrenal­
ectomy is indicated for patients with Cushing disease if pituitary 
surgery has failed or if the condition has recurred. 
Treatment of the ectopic ACTH syndrome depends on the cause. 
If the tumor can be found and has not spread, then its removal 
ca

 be found and has not spread, then its removal 
can lead to cure (e.g., neuroendocrine tumor of the bronchus or 
thymus). However, the prognosis for small cell lung cancer associ­
ated with the ectopic ACTH syndrome is poor. The cortisol excess 
and associated hypokalemic alkalosis and diabetes mellitus can be 
ameliorated by medical therapy. Treatment of the small cell tumor 
itself will also, at least initially, produce improvement. Sometimes, 
if the ectopic source of ACTH cannot be found, it

if the ectopic source of ACTH cannot be found, it may be nec­
essary to perform bilateral adrenalectomy and then monitor the 
patient carefully (sometimes for several years) before the primary 
tumor becomes apparent. 
0900 Plasma cortisol (µmol/L)
Weeks
IHT
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
5
10
15
20
25
Plasma cortisol response following transsphenoidal
microsurgery for Cushing disease
microadenectomy 
Cushing disease confirmed
Remission
Tumor
resection
Resection not
possible

Remission
Tumor
resection
possible
Monitor for
recurrence
Failed surgery, no surgery, or recurrence
Control hypercortisolism
Repeat
localization
studies
Consider:
Repeat transsphenoidal surgery
Steroidogenesis inhibitors
Radiotherapy and steroidogenesis inhibitors
Pituitary-directed medical treatment
Combinations of the above
Glucocorticoid receptor antagonist
Bilateral adrenalectomy
.......
A
B
• Fig. 15.30  (A) Suggested approach for management of Cushing disease. (B) Gradual re

 for management of Cushing disease. (B) Gradual recovery of function 
of the hypothalamic-pituitary-adrenal axis in a patient after removal of a pituitary adrenocorticotropic hor­
mone-secreting microadenoma. Morning (9 am) plasma cortisol levels were measured. The insulin hypo­
glycemia test (IHT) eventually demonstrated the return of a normal stress response. (A, reproduced from 
Nieman LK, Biller BM, Findling JW, et al. Treatment of Cushing’s syndrome: an Endocrine Society clinical 
practice 

syndrome: an Endocrine Society clinical 
practice guideline. J Clin Endocrinol Metab. 2015;100:2807–2831.)
516
Medical Treatment of Cushing Syndrome
Several drugs have been used in the treatment of Cushing syn­
drome.288 Metyrapone inhibits 11β-hydroxylase, with a goal of 
lowering cortisol concentrations, often before definitive therapy 
or while awaiting benefit from pituitary irradiation. The daily dose 
must be determined by measuremen

. The daily dose 
must be determined by measurements of plasma or urinary free 
cortisol. The aim should be to achieve a mean plasma cortisol con­
centration of about 300 nmol/L (11 μg/dL) during the day or a 
normal urinary free cortisol level. Since levels of 11-deoxycortisol 
are increased, use of LC-MS/MS assays are encouraged for moni­
toring due the potential cross-reactivity with cortisol in immuno­
assays.281 The drug is usually given in doses ranging from 250 mg 
A
C
D
E
B
• Fig. 15.31 

doses ranging from 250 mg 
A
C
D
E
B
• Fig. 15.31  A young woman with Cushing disease, photographed initially beside her identical twin sister 
(A). In this case, treatment with bilateral adrenalectomy was undertaken. Several years later, the patient 
presented with Nelson syndrome and a right third cranial nerve palsy (B, C) related to cavernous sinus 
infiltration from a locally invasive corticotropinoma (D). Hypophysectomy and radiotherapy were performed 
with reversal of the third cranial ne

e performed 
with reversal of the third cranial nerve palsy (E). Note the advancing skin pigmentation of Nelson syndrome.
517
twice daily to 1.5 g every 6 hours, with lower doses for adrenal 
adenoma and higher for ectopic ACTH, and overall is effective in 
around 50% of all patients.299 Nausea is a side effect that can be 
helped (if it is not caused by adrenal insufficiency) by giving the 
drug with milk.300
Ketoconazole is an imidazole that has been widely used 

onazole is an imidazole that has been widely used as 
an antifungal agent but causes abnormal liver function tests in 
about 15% of patients. Ketoconazole blocks a variety of ste­
roidogenic cytochrome P450–dependent enzymes and thus 
lowers plasma cortisol levels. For effective control of Cushing 
syndrome, 400 to 1600 mg daily has been required, and an 
acidic stomach is needed for absorption; the drug is effective in 
around 50% of patients.301,302 Ketoconazole frequently causes 
an increase 

1,302 Ketoconazole frequently causes 
an increase in hepatic enzyme values, and if this level remains 
less than three times the upper limit of normal, the drug may 
be continued, but hepatic failure has been described, and the 
drug has been withdrawn from the market for the treatment of 
fungal infections.
Mitotane (o,p′-DDD) is an adrenolytic drug that is taken up by 
both normal and malignant adrenal tissue, causing adrenal atrophy 
and necrosis.285 Because of its toxicity, it has been used 

sis.285 Because of its toxicity, it has been used mainly in 
the management of adrenocortical carcinoma (ACC). Doses of up 
to 5 g/day are required to control glucocorticoid excess, although 
evidence that the drug causes tumor shrinkage or improves long-
term survival is lacking. This agent also produces mineralocor­
ticoid deficiency, and concomitant high-dose glucocorticoid 
therapy is required, often with mineralocorticoid replacement if 
there is no prior glucocorticoid excess due to the AC

re is no prior glucocorticoid excess due to the ACC. High-
dose hydrocortisone replacement is needed in patients with ACC 
causing Cushing syndrome when mitotane is used in a “block and 
replace” regime due to the increases in CBG, and induction of 
CYP3A4 due to the mitotane, which then causes less bioavail­
ability and increased metabolism of hydrocortisone.303 Side effects 
are common and include fatigue, skin rashes, neurotoxicity, and 
gastrointestinal disturbance, and therapeutic drug moni

ointestinal disturbance, and therapeutic drug monitoring is 
essential during use. It has also been used at lower doses in Cush­
ing disease.304
Somatostatin analogues such as octreotide and lanreotide are 
generally ineffective in Cushing disease. However, the multirecep­
tor somatostatin analogue, pasireotide, which demonstrates high-
affinity binding to somatostatin receptor subtypes 1, 2, 3, and 5, 
normalizes urinary free cortisol in 17% to 40% of patients with 
Cushing disease depending on

40% of patients with 
Cushing disease depending on the severity of disease, with hyper­
glycemia being a common side effect.305,306 The GR antagonist 
mifepristone has been shown to improve diabetes in patients with 
Cushing syndrome, but biochemical monitoring is not possible 
when using this drug.307 
Prognosis of Cushing Syndrome
Studies performed before the introduction of effective therapy 
revealed that 50% of patients with untreated Cushing syndrome 
died within 5 years, principally from 

g syndrome 
died within 5 years, principally from vascular disease.194 Even 
with modern management, an increased prevalence of cardiovas­
cular risk factors persists for many years after an apparent remis­
sion.195,196 If remission is achieved at the first pituitary surgery 
the standardized mortality rate approaches the normal popula­
tion, emphasizing the need for expert surgeons.308 Paradoxically, 
on correction of the hypercortisolism, patients often feel worse. 
Skin desquamation, steroid-

nts often feel worse. 
Skin desquamation, steroid-withdrawal arthropathy, profound 
lethargy, and mood changes may occur and can take several weeks 
or months to resolve.309 They can usually be ameliorated by a 
transient increase in glucocorticoid replacement therapy. Patients 
are invariably GH deficient, and GH replacement therapy may 
produce clinical benefit.
Features of Cushing syndrome disappear over a period of 2 to 
12 months after treatment. Hypertension and diabetes mellitus 
improve,

ment. Hypertension and diabetes mellitus 
improve, but as with other secondary causes, they may not resolve 
completely. The osteopenia of Cushing syndrome improves rap­
idly during the first 2 years after treatment but resolves more 
slowly thereafter.310 Vertebral fractures and osteonecrosis are irre­
versible, and permanent deformity results. Visceral obesity and 
myopathy are both reversible features. Reproductive and sexual 
function usually return to normal within 6 months, provided 
that 

 return to normal within 6 months, provided 
that anterior pituitary function was not compromised. Long-term 
health-related quality of life in adults significantly improves after 
treatment, but quality-of-life scores do not return to normal lev­
els.191 Similar observations have been made in pediatric patients, 
in whom significant improvement was noted before and after 
treatment but residual impairment of health-related quality of life 
persisted 1 year after cure.311
Glucocorticoid Resistan

sted 1 year after cure.311
A small number of patients have been described as having 
increased cortisol secretion but without the stigmata of Cush­
ing syndrome.88,312 These patients are resistant to suppression of 
cortisol with low-dose dexamethasone but respond to high doses. 
ACTH levels are elevated and lead to increased adrenal produc­
tion of androgens and DOC. Therefore these patients may pres­
ent with the features of androgen or mineralocorticoid excess, 
or b

res of androgen or mineralocorticoid excess, 
or both. Treatment with a dose of dexamethasone (usually >3 
mg/day) adequate to suppress ACTH results in a fall in adrenal 
androgens and often returns plasma potassium and blood pres­
sure to normal levels. Many of these patients have been found to 
have point mutations in the steroid-binding domain of the GR, 
with consequent reduction of glucocorticoid-binding affinity, 
but this finding is not invariable. A useful clinical discriminatory 
test t

variable. A useful clinical discriminatory 
test to differentiate this condition from Cushing syndrome is to 
measure bone mineral density: It is preserved in patients with 
glucocorticoid resistance or even increased in females because 
of the androgen excess. In addition, the circadian rhythm for 
ACTH and cortisol is preserved in patients with glucocorticoid 
resistance. 
Primary and Central Hypoadrenalism
Primary hypoadrenalism refers to glucocorticoid deficiency oc

oadrenalism refers to glucocorticoid deficiency occur­
ring in the setting of adrenal disease, whereas central hypoadrenal­
ism arises because of deficiency of ACTH (Table 15.15). A major 
distinction between these forms of hypoadrenalism is that miner­
alocorticoid deficiency invariably accompanies primary hypoadre­
nalism, but this does not occur in central hypoadrenalism: Here, 
only ACTH is deficient, and the renin-angiotensin-aldosterone 
(RAA) axis is intact. A further important cause of a

AA) axis is intact. A further important cause of adrenal insuf­
ficiency in which there may be dissociation of glucocorticoid and 
mineralocorticoid secretion is CAH. 
Primary Hypoadrenalism
Addison Disease
Thomas Addison described the condition now known as primary 
hypoadrenalism in his classic monograph published in 1855.2 
518
Addison disease is a rare condition with an estimated incidence in 
the developed world of 0.8 cases per 100,0

nce in 
the developed world of 0.8 cases per 100,000 and a prevalence of 
4 to 11 cases per 100,000 population. Nevertheless, it is associated 
with significant morbidity and a twofold excess mortality rate,313 
but once the diagnosis is made it can be easily treated.314,315 
Despite treatment, however, patients carry significant burden of 
metabolic and psychologic comorbidities.316 Causes of Addison 
disease are listed in Table 15.15.
Autoimmune Adrenalitis
In the Western world, autoimmune adr

e Adrenalitis
In the Western world, autoimmune adrenalitis accounts for more 
than 70% of all cases of primary hypoadrenalism.317 Pathologi­
cally, the adrenal glands are atrophic with loss of most of the 
cortical cells, but the medulla is usually intact. In 75% of cases, 
adrenal autoantibodies—specifically anti-21-hydroxylase anti­
bodies—can be detected.318 Fifty percent of patients with this 
form of Addison disease have an associated autoimmune disease 
(Table 15.16), thyroid disease being

mune disease 
(Table 15.16), thyroid disease being the most common. Con­
versely, only 1% to 2% of patients with more common auto­
immune diseases such as insulin-dependent diabetes mellitus 
or thyrotoxicosis have antiadrenal autoantibodies and develop 
adrenal disease, although the figure is higher in patients with 
autoimmune hypoparathyroidism (16%). These autoimmune 
polyglandular syndromes (APSs) have been classified into two 
distinct variants.318 APS type I, or autoimmune polyendocrinop­

ants.318 APS type I, or autoimmune polyendocrinop­
athy-candidiasis-ectodermal dysplasia (APECED), is a rare auto­
somal recessive condition comprising Addison disease, chronic 
mucocutaneous candidiasis, and hypoparathyroidism. The more 
common APS type II comprises Addison disease, autoimmune 
thyroid disease, diabetes mellitus, and hypogonadism. Here, 
autoantibodies to 21-hydroxylase are usually present and are pre­
dictive for the development of adrenal destruction, particularly 
for APS ty

t of adrenal destruction, particularly 
for APS type I.318 Polyglandular autoimmune syndromes are dis­
cussed in greater detail in Chapter 43. 
Worldwide, infectious diseases are the most common cause of 
primary adrenal insufficiency. These diseases include tuberculosis, 
fungal infections (histoplasmosis, cryptococcosis), and cytomega­
lovirus infection. Adrenal failure may also occur in the acquired 
immunodeficiency syndrome (AIDS).319
Tuberculous Addison disease results from hema

.319
Tuberculous Addison disease results from hematogenous 
spread of the infection from elsewhere in the body; extra-adrenal 
disease is usually evident. The adrenals are initially enlarged, with 
extensive epithelioid granulomas and caseation, and both the 
   Etiology of Adrenocortical Insufficiency 
(Excluding Congenital Adrenal 
Hyperplasia)
Primary Causes: Addison Disease
Autoimmune
Sporadic
Autoimmune polyendocrine syndrome type I (Addison disease, 
chronic mucocutaneous candidiasis, hyp

n disease, 
chronic mucocutaneous candidiasis, hypoparathyroidism, dental 
enamel hypoplasia, alopecia, primary gonadal failure—see 
Chapter 43)
Autoimmune polyendocrine syndrome type II (Schmidt syndrome) 
(Addison disease, primary hypothyroidism, primary hypogonad­
ism, insulin-dependent diabetes, pernicious anemia, vitiligo— 
see Chapter 43)
Fungal infections
Cytomegalovirus
HIV
Metastatic tumor
Amyloid
Intra-adrenal hemorrhage (Waterhouse

mochromatosis
Intra-adrenal hemorrhage (Waterhouse-Friderichsen syndrome) after 
meningococcal septicemia
Adrenoleukodystrophies
Congenital adrenal hypoplasia
DAX1 (NR0B1) mutations
SF1 mutations
ACTH resistance syndromes
MC2R gene mutations
MRAP gene mutations
AAAS (ALADIN) gene mutations (triple-A syndrome)
Secondary Causes: Central Hypoadrenalism
Exogenous glucocorticoid therapy
Selective removal of ACTH-secreting pituitary adenoma
Pituitary tumors and 

-secreting pituitary adenoma
Pituitary tumors and pituitary surgery, craniopharyngiomas
Granulomatous disease (tuberculosis, sarcoid, eosinophilic granuloma)
Secondary tumor deposits (breast, bronchus)
Postpartum pituitary infarction (Sheehan syndrome)
Pituitary irradiation (effect usually delayed for several years)
Isolated ACTH deficiency
TPIT (TBX19) gene mutations
PCSK1 gene mutation (POMC processing defect)
POMC gene mutations
Multiple 

C processing defect)
Multiple pituitary hormone deficiencies
HESX1 gene mutations
LHX4 gene mutations
SOX3 gene mutations
PROP1 gene mutations
ACTH, Adrenocorticotropic hormone; HIV, human immunodeficiency virus; POMC, pro-opi­
omelanocortin.
   
TABLE 15.15
   Incidence of Other Endocrine and 
Autoimmune Diseases in Patients With 
Autoimmune Adrenal Insufficiency
Disease
Incidence (%)
Thyroid disease
  Hypothyroidism
8
  Nontoxic goiter
7
  Thyrotoxicosis
7
Gonadal failure

toxic goiter
7
7
  Ovarian
20
  Testicular
2
Insulin-dependent diabetes mellitus
11
Hypoparathyroidism
10
Pernicious anemia
5
None
53
TABLE 15.16
519
cortex and the medulla are affected. Fibrosis ensues, and the adre­
nals become normal in size or smaller, with calcification evident 
in 50% of cases.
The adrenals are frequently involved in patients with 
AIDS319,320; adrenalitis may occur after infection with cyto­
megalovirus or at

occur after infection with cyto­
megalovirus or atypical mycobacteria, and Kaposi sarcoma may 
result in adrenal replacement. Onset is often insidious, but if 
tested, more than 10% of patients with AIDS will demonstrate 
a subnormal cortisol response to a short synacthen test (SST). 
Adrenal insufficiency may be precipitated through the concomi­
tant administration of appropriate anti-infectives such as keto­
conazole (which inhibits cortisol synthesis) or rifampicin (which 
increases cortisol 

nthesis) or rifampicin (which 
increases cortisol metabolism). Rarely, patients with AIDS and 
features of adrenal insufficiency are found to have elevated cir­
culating ACTH and cortisol concentrations that fail to suppress 
normally after low-dose dexamethasone administration. This is 
thought to reflect an acquired form of glucocorticoid resistance 
resulting from reduced GR affinity, but the underlying cause 
remains unknown.321
Acquired Primary Adrenal Insufficiency
With the exception of tu

ary Adrenal Insufficiency
With the exception of tuberculosis and autoimmune adrenal fail­
ure, other causes of Addison disease are rare (see Table 15.15). 
With the exception of lymphoma, adrenal metastases (most 
commonly from primary tumors in the lung or breast) uncom­
monly cause adrenal insufficiency,322 perhaps because more than 
90% of the adrenal cortex must be compromised before symp­
toms and signs become apparent. Necrosis of the adrenals due 
to intra-adrenal hemorrhage should be con

als due 
to intra-adrenal hemorrhage should be considered in any severely 
sick patient, particularly a patient with underlying infection, 
trauma, or coagulopathy.323 Intra-adrenal bleeding may be found 
in patients with severe septicemia of any cause, particularly in chil­
dren, in whom a common cause is infection with Pseudomonas 
aeruginosa. When caused by meningococci, the association with 
adrenal insufficiency is known as the Waterhouse-Friderichsen 
syndrome. Adrenal replacement may also

iderichsen 
syndrome. Adrenal replacement may also occur with amyloidosis 
and hemochromatosis. 
Inherited Primary Adrenal Insufficiency
Adrenal hypoplasia congenita (AHC) is an X-linked disorder com­
prising congenital adrenal insufficiency and combined primary 
and central hypogonadotropic hypogonadism. The condition is 
caused by mutations in the DAX1 (NR0B1) gene, a member of 
a nuclear receptor family that is expressed in the adrenal cortex, 
gonads, and hypothalamus.324,325 Depending on th

gonads, and hypothalamus.324,325 Depending on the molecular 
defect, the clinical presentation can be highly variable. Severe cases 
often manifest with mineralocorticoid deficiency and gradually 
develop glucocorticoid deficiency. Hypogonadism is combined 
with primary testicular abnormalities and low gonadotropin lev­
els. However, the so-called minipuberty of infancy can be nor­
mal.326,327 Patients presenting with late-onset adrenal failure have 
also been described.328
Mutations in anothe

 have 
Mutations in another transcription factor, SF1, may also 
result in adrenal insufficiency due to lack of development of a 
functional adrenal cortex. The transcriptional regulation of many 
P450 steroidogenic enzymes is dependent on SF1.15 When it was 
first described, SF1 mutation was associated with complete sex 
reversal causing 46,XY disorder of sex development (DSD).329 
However, novel clinical phenotypes in SF1-deficient patients 
are now emerging; they range

1-deficient patients 
are now emerging; they range from isolated adrenal failure330 
to isolated gonadal failure331 and ovarian insufficiency.332 AHC 
may also occur in association with glycerol kinase deficiency 
and Duchenne muscular dystrophy caused by a contiguous gene 
deletion, including the DAX1 gene.333
Adrenoleukodystrophy has a prevalence rate of 1:20,000 and 
is a cause of adrenal insufficiency in association with demyelin­
ation within the nervous system; demyelination results from a

n the nervous system; demyelination results from a 
failure of β-oxidation of fatty acids within peroxisomes. Increased 
accumulation of very long-chain fatty acids (VLCFAs) occurs in 
many tissues, and serum assays can be used diagnostically. Only 
males have the fully expressed condition, and carrier females are 
usually normal. Several forms are recognized: a childhood cere­
bral form (30–40% of cases), adult adrenomyeloneuropathy 
(40%), and Addison disease (7%). The childhood-onset form 
ma

Addison disease (7%). The childhood-onset form 
manifests at 5 to 10 years of age with progression eventually to a 
blind, mute, and severely spastic tetraplegic state. Adrenal insuffi­
ciency is usually present but does not appear to correlate with the 
neurologic deficit. Nevertheless, this is the most common form 
of adrenal insufficiency in a child younger than age 7 years.334 
Adrenomyeloneuropathy, by contrast, manifests later in life with 
the gradual development of spastic paresis and pe

the gradual development of spastic paresis and peripheral neu­
ropathy. Both the childhood and the adult condition result from 
mutations in the ABCD1 gene on chromosome Xq28, which 
encodes for an ABC peroxisomal membrane protein involved in 
the import of VLCFA into the peroxisome.335 So far, more than 
400 mutations have been reported in the ABCD1 gene with no 
relationship between genotype and phenotype.336,337 Treatment 
options are few. Monounsaturated fatty acids, which block the 
synthe

nounsaturated fatty acids, which block the 
synthesis of the saturated VLCFA, have been used; a combina­
tion of erucic acid and oleic acid (Lorenzo oil) has led to normal 
levels of VLCFA. Treatment does not alter the rate of neuro­
logic deterioration but may prevent new neurologic damage in 
asymptomatic cases.337 Bone marrow transplantation is a further 
possibility.
Familial glucocorticoid deficiency (FGD), or inherited unre­
sponsiveness to ACTH, is a rare autosomal recessive cause of 
hyp

 ACTH, is a rare autosomal recessive cause of 
hypoadrenalism that usually manifests in childhood. Most 
patients present with neonatal hypoglycemia or later in life with 
increasing pigmentation, and they often have enhanced growth 
velocity. Primary adrenal failure in a child with normal activity 
of the RAA system is highly suggestive of FGD. The diagnosis 
can be confirmed by demonstrating low cortisol in combination 
with increased ACTH concentrations and normal plasma renin 
and aldosteron

entrations and normal plasma renin 
and aldosterone measurements.92 The type 1 variant accounts 
for approximately 25% of all cases and is explained by inacti­
vating mutations in the ACTH-binding receptor, MC2R.338–340 
The FGD type 2 variant caused by mutations in the MRAP 
gene, which is thought to mediate intracellular trafficking of 
the MC2R, has been reported in some families.91 However, 
50% of patients with FGD do not have mutations in either 
MC2R or MRAP; other loci that have been def

ither 
MC2R or MRAP; other loci that have been defined include 
MCM4, TXNRD2, NNT, and partial mutations in STAR and 
CYP11A1.341,342
A variant called the triple A syndrome or Allgrove syndrome refers 
to the triad of adrenal insufficiency due to ACTH resistance, acha­
lasia, and alacrima. It is caused by mutations in the AAAS gene, 
which encodes ALADIN, a tryptophan/aspartate WD-repeat–
containing protein of the nuclear pore complex.343,344 The exact 
function of ALADIN is unknown, but its int

 exact 
function of ALADIN is unknown, but its interaction with other 
proteins of the nuclear pore complex suggests that it is part of a 
structural scaffold.
Several syndromic disorders are associated with adrenal insuffi­
ciency for which the underlying molecular genetic defect remains 
to be elucidated.11 
520
Secondary Hypoadrenalism
Inherited Central Hypoadrenalism
Central hypoadrenalism may be defined as hypocortisolemia sec­
ondary

ism may be defined as hypocortisolemia sec­
ondary to a deficiency in ACTH. The prevalence of central hypo­
adrenalism is 125 to 280 per million,1,345 which is likely to be an 
underestimate considering the use of therapeutic corticosteroids 
in the general population. It occurs in up to one-third of patients 
with pituitary disease.3,346 ACTH deficiency is an important diag­
nosis to make; in subjects with tumoral and posttraumatic hypo­
pituitarism, central hypoadrenalism is associated with in

rism, central hypoadrenalism is associated with increased 
mortality rates.4–6,347–349 The causes of central hypoadrenalism are 
outlined in Table 15.15; the most common reason is ACTH sup­
pression by exogenous glucocorticoid treatment.7,185
When caused by pituitary disease, other pituitary hormones 
are often deficient, so the patient presents with partial or com­
plete hypopituitarism. The clinical features of hypopituitarism 
make this a relatively easy diagnosis. By contrast, isolated ACTH 

tively easy diagnosis. By contrast, isolated ACTH 
deficiency is rare, and the diagnosis is difficult to make.350 It may 
occur in patients with lymphocytic hypophysitis. Mutations in 
the TBX19 gene, the product of which (Tpit) regulates POMC 
expression, have been reported in a few cases of isolated ACTH 
deficiency occurring in neonatal life.351 A rare but fascinating 
cause relates to a defect in the normal post-translational process­
ing of POMC to ACTH by the prohormone convertase enzymes 

POMC to ACTH by the prohormone convertase enzymes 
(PC1/3 and PC2).352 Such patients may have more generalized 
defects in peptide processing (e.g., cleavage of proinsulin to insu­
lin) giving rise to diabetes mellitus.
Some patients have mutations in the POMC gene that inter­
rupt the synthesis of ACTH and cause ACTH deficiency. Elucida­
tion of the phenotype of these patients has uncovered a novel role 
for POMC peptides in regulating appetite and hair color: In addi­
tion to adrenal insuffici

and hair color: In addi­
tion to adrenal insufficiency, POMC mutations result in severe 
obesity and often red hair pigmentation.353 A central role for 
αMSH in regulating food intake via the hypothalamic MC4R has 
been established,56 and in recombinant mice lacking the POMC 
gene, the obese phenotype can be reversed by giving an αMSH 
agonist peripherally.354
Other rare inborn causes of secondary insufficiency are the 
result of mutations in genes involved in pituitary development, 
such as HES

es involved in pituitary development, 
such as HESX1,355 LHX4,356 SOX3,357 and PROP1.358 These 
defects result in congenital hypopituitarism with multiple pitu­
itary hormone deficiencies: ACTH deficiency may not be present 
at the time of diagnosis, but it develops progressively over time.
Secondary hypoadrenalism is also observed in patients with 
Cushing disease after successful and selective removal of the 
ACTH-secreting pituitary adenoma. The function of adjacent 
normal pituitary corticot

he function of adjacent 
normal pituitary corticotrophs is suppressed and may remain so 
for many months after curative surgery.291–293 
ACTH Suppression by Exogenous Glucocorticoids
The ability of exogenously administered corticosteroids to cause 
adrenal atrophy has been appreciated since their discovery in 
the 1940s. HPA axis suppression by exogenous glucocorticoid 
is a serious medical problem and has been described with intra-
articular, topical, ocular, rectal, and inhaled, as well as sys

pical, ocular, rectal, and inhaled, as well as systemic, 
therapy.359–361
There is considerable interindividual variability in response to 
glucocorticoids; there are no absolute cutoff values for the type of 
steroid taken, dose, route of administration, duration of treatment, 
or time since steroid withdrawal that predict adrenal suppression. 
However, there are some generic issues that can guide diagnosis 
and therapy. Relative steroid potencies in their affinity/transac­
tivation of the GR h

es in their affinity/transac­
tivation of the GR have been described based on suppression of 
corticosterone production, in vitro binding to the glucocorticoid 
(GR), and functional changes in GC target tissues by individual 
steroids362; these suggest that oral doses of 20 mg of hydrocorti­
sone, 5 mg of prednisolone, and 0.75 mg of dexamethasone are 
bioequivalent. Dexamethasone has a longer half-life and higher 
affinity for the GR than hydrocortisone and exerts a more sus­
tained suppressive

ortisone and exerts a more sus­
tained suppressive effect on the HPA axis. Similarly, it is unclear 
how the potency of oral hydrocortisone compares to that of glu­
cocorticoids taken by other routes. Budesonide has more potent 
action at the GR than dexamethasone or prednisolone,363 but its 
effect on adrenal suppression is dependent on systemic absorption 
of the inhaled compound. Comparison of inhaled fluticasone with 
inhaled steroids such as budesonide or beclomethasone indicated 
that flut

 budesonide or beclomethasone indicated 
that fluticasone was more frequently associated with suppression 
of the HPA axis and that adults using over 1000 μg of inhaled 
fluticasone for over a year were at risk of adrenal suppression.364
Concomitant therapies can augment potency and adrenal sup­
pression. The coadministration of inhaled fluticasone with one of 
the many medications that suppress its clearance by inhibition of 
CYP3A4 is associated with adrenal suppression (e.g., ritonavir).365 

d with adrenal suppression (e.g., ritonavir).365 
Co-prescriptions of agents that do not affect glucocorticoid clear­
ance but have affinity for the GR are also associated with adre­
nal suppression, an example being progesterone derivatives such 
as medroxyprogesterone acetate given in high dose to oncology 
patients.179
In terms of dose and duration, adrenal atrophy and subsequent 
deficiency should be anticipated in any subject who has taken 
more than the equivalent of 30 mg hydrocortisone p

more than the equivalent of 30 mg hydrocortisone per day orally 
(>7.5 mg/day prednisolone or >0.75 mg/day dexamethasone) for 
longer than 3 weeks. In addition to the magnitude of the dose of 
glucocorticoid, the timing of administration may affect the degree 
of adrenal suppression. If prednisolone is given as 5 mg at night 
and 2.5 mg in the morning, there will be more marked suppres­
sion of the HPA axis compared with 2.5 mg at night and 5 mg 
in the morning, because the larger evening dose b

in the morning, because the larger evening dose blocks the early-
morning surge of ACTH. LaRochelle and colleagues reported 
that adrenal function recovered if patients’ steroid doses could be 
tapered to 5 mg of prednisone daily.366 This report has formed 
the basis of the practice of assessing HPA axis function in patients 
who have been on more than 5 mg of prednisolone or equiva­
lent for more than 3 months. Lower doses of glucocorticoid have 
been shown to suppress cortisol production; in 

e 
been shown to suppress cortisol production; in one study, more 
than 60% of subjects on a glucocorticoid dose equivalent of less 
than 5 mg of prednisolone per day had a subnormal ACTH or 
cortisol response to CRH.367 In patients on long-term tapering 
of prednisolone who were considered for steroid withdrawal once 
they had reached a dose of 7 mg/day, 48% had adrenal insuffi­
ciency defined by basal cortisol (<100 nmol/L) or response to 250 
μg synthetic ACTH (<550 nmol/L); a longer duration

μg synthetic ACTH (<550 nmol/L); a longer duration of pred­
nisolone therapy (13.7 years) was seen in the hypoadrenal group 
defined by a low baseline cortisol but not in the group defined 
by cortisol response to 250 μg synthetic ACTH (6.1 years).368 
Dynamic testing of adrenal function, such as the SST and CRH 
tests, suggests that cortisol production recovers after withdrawal 
of long-term glucocorticoids; in a meta-analysis of patients receiv­
ing glucocorticoid therapy, 46% to 100% had insu

­
ing glucocorticoid therapy, 46% to 100% had insufficient cortisol 
response 1 day after withdrawal, which improved to 26% to 49% 
in patients at assessment 1 week later.37 Up to 10% of patients still 
521
have biochemical evidence of hypoadrenalism between 6 and 20 
months after withdrawal of glucocorticoids.369
All patients receiving long-term therapy with corticosteroids 
should be treated in a similar fashion to patients with chronic 
ACTH deficiency; they sho

o patients with chronic 
ACTH deficiency; they should carry steroid cards and be offered 
steroid alert bracelets or necklaces. In the event of an intercurrent 
stress (e.g., infection, surgery), supplemental steroid cover should 
be given. If the patient is unable to take drugs orally, parenteral 
therapy is required.
During recovery from suppression and without replace­
ment therapy, patients may experience symptoms of gluco­
corticoid deficiency, including anorexia, nausea, weight loss, 
arth

cy, including anorexia, nausea, weight loss, 
arthralgia, lethargy, skin desquamation, and postural dizziness 
(see the later discussions of adrenal insufficiency).370 To avoid 
these symptoms, steroids should be cautiously withdrawn over 
a period of months.182 Assuming that the underlying disease 
permits steroid reduction, doses should be reduced from phar­
macologic levels to physiologic levels (equivalent to 7.5 mg/
day prednisolone) over a few weeks. Thereafter doses should be 
reduced by 

few weeks. Thereafter doses should be 
reduced by 1 mg per day of prednisolone every 2 to 4 weeks 
depending on patient well-being. An alternative approach is to 
switch the patient to hydrocortisone 20 mg/day and reduce the 
daily dose by 2.5 mg/day every week to a level of 10 mg/day. 
After 2 to 3 months on reduced doses, endogenous function of 
the HPA axis can be assessed by basal 9:00 am serum cortisol 
values and an SST or an insulin-induced hypoglycemia test, 
as needed. A “pass” response

d hypoglycemia test, 
as needed. A “pass” response to these tests indicates adequate 
function of the HPA axis, and corticosteroid therapy can be 
safely withdrawn. The level of basal 9:00 am serum cortisol 
that predicts a pass on an SST varies by the assay used from 
336 to 506 nmol/L, indicating the importance of knowledge of 
assay characteristics.371 In those patients who are taking physi­
ologic doses of prednisolone (<5–7.5 mg/day) or equivalent 
corticosteroid, an SST given 12 to 24 hour

valent 
corticosteroid, an SST given 12 to 24 hours after omitted ste­
roid therapy will provide an immediate answer as to whether 
sudden or gradual withdrawal of steroid therapy is indicated 
(Table 15.17).361
Iatrogenic-induced Cushing syndrome occurs in patients 
who take suppressive doses of corticosteroids for longer than 3 
weeks.182 The rapidity of onset of clinical features depends on the 
administered dose but can occur within 1 month of therapy. 
Hypoadrenalism During Critical Illness

f therapy. 
Hypoadrenalism may also complicate critical illness, even in 
individuals with a previously intact HPA axis.372 This has been 
termed functional adrenal insufficiency to reflect the notion that 
hypoadrenalism is transient and is not caused by a structural 
lesion. Functional adrenal insufficiency has been difficult to 
define biochemically and is of uncertain cause. Moreover, in 
patients with hypoproteinemia, with serum albumin less than 
2.5 

ypoproteinemia, with serum albumin less than 
2.5 g/dL, total serum cortisol may be low, but free cortisol is 
normal.252 Nevertheless, an inability to mount an adequate and 
appropriate cortisol response to overwhelming stress or sepsis 
is frequently encountered in intensive care units and substan­
tially increases the risk of death during acute illness.373 This has 
stimulated attempts to define functional adrenal insufficiency 
quantitatively and to treat it with supplemental corticosteroids

 and to treat it with supplemental corticosteroids. 
Although this diagnosis remains highly contentious, if a subopti­
mal cortisol response is suspected, the current recommendations 
suggest (1) treatment with hydrocortisone, 200 mg/day in four 
divided doses or, preferably, 10 mg/hour as a continuous infu­
sion, for patients with septic shock and (2) treatment with meth­
ylprednisolone, 1 mg/kg per day, for patients with severe early 
acute respiratory distress syndrome. Glucocorticoid treatme

piratory distress syndrome. Glucocorticoid treatment 
should be tapered off rather than stopped abruptly. Treatment of 
critical illness–related adrenal insufficiency with dexamethasone 
is not recommended.374 
Clinical Features of Adrenal Insufficiency
Patients with primary adrenal failure usually have both gluco­
corticoid and mineralocorticoid deficiencies. In contrast, those 
with secondary adrenal insufficiency have an intact RAA system. 
This accounts for differences in salt and water bala

is accounts for differences in salt and water balance in the two 
groups of patients, which in turn result in different clinical pre­
sentations. The most obvious feature that differentiates primary 
from secondary hypoadrenalism is skin pigmentation (Table 
15.18), which is almost always present in cases of primary adre­
nal insufficiency (unless of short duration) and absent in second­
ary insufficiency. The pigmentation is seen in sun-exposed areas, 
recent rather than old scars, axillae, nip

areas, 
recent rather than old scars, axillae, nipples, palmar creases, pres­
sure points, and mucous membranes (buccal, vaginal, vulval, 
anal). The cause of the pigmentation has long been debated but is 
   Suggested Plan for Steroid Replacement in Patients Withdrawing From Chronic Corticosteroid Therapy
Pred Dose (mg/day)
DURATION OF GLUCOCORTICOID TREATMENT
≤3 wka
>3  wk
≥7.5
Can stop
↓ rapidly (e.g., 2.5 mg q3-4d)
THEN
5–7.5
Can stop
↓ 1 mg q2-4 wk
OR
Convert 5 mg pred to 20 mg HC, then ↓ 

 q2-4 wk
OR
Convert 5 mg pred to 20 mg HC, then ↓ 2.5 mg/wk to 10 mg/day
THEN
THEN
<5
Can stop
After 2-3 mo HC 10 mg/day, administer SST/ITT:
Pass → Withdraw
Fail → Continue
aBeware of frequent steroid courses (e.g., in asthma).
HC, Hydrocortisone; ITT, insulin tolerance test; pred, prednisolone; SST, short synacthen test. Basal 0900h ACTH can be used to monitor for recovery of HPA axis and levels may be above the normal 
range ahead of a “pass” on an SST.
   
TABLE 15.17
522
SECT

ad of a “pass” on an SST.
   
522
thought to reflect increased stimulation of the MC1R by ACTH 
itself. In autoimmune Addison disease, there may be associated 
vitiligo (Fig. 15.32).
The clinical features relate to the rate of onset and the sever­
ity of adrenal deficiency.314 In many cases, the disease has an 
insidious onset, and a diagnosis is made only when the patient 
presents with an acute crisis during an intercurrent i

ents with an acute crisis during an intercurrent illness. 
Acute adrenal insufficiency, termed an adrenal crisis or addiso­
nian crisis, is a medical emergency manifesting as hypotension 
and acute circulatory failure (Table 15.19). Anorexia may be an 
early feature; it progresses to nausea, vomiting, diarrhea, and 
sometimes abdominal pain. Fever may be present, and hypo­
glycemia may occur. Patients presenting acutely with adrenal 
hemorrhage have hypotension; abdominal, flank, or lower 
chest

ave hypotension; abdominal, flank, or lower 
chest pain; anorexia; and vomiting. The condition is difficult 
to diagnose, but evidence of occult hemorrhage (rapidly falling 
hemoglobin), progressive hyperkalemia, and shock should alert 
the clinician to the diagnosis.
Alternatively, the patient may present with vague features of 
chronic adrenal insufficiency—weakness, tiredness, weight loss, 
anorexia or nausea, intermittent vomiting, abdominal pain, diar­
rhea or constipation, general malaise,

pain, diar­
rhea or constipation, general malaise, muscle cramps, arthralgia, 
and symptoms suggestive of postural hypotension (see Table 
15.18). Salt craving may be a feature, and a low-grade fever may 
be present. Supine blood pressure is usually normal, but almost 
invariably there is a fall in blood pressure on standing. Adre­
nal androgen secretion is lost; this is clinically more apparent 
in women, who may notice loss of axillary and pubic hair and 
frequently have dry and itchy skin. Ps

c hair and 
frequently have dry and itchy skin. Psychiatric symptoms may 
occur in long-standing cases and include memory impairment, 
depression, and psychosis. Formal quality-of-life measures indi­
cate significant impairment in patients with primary or secondary 
adrenal insufficiency.375 Tiredness is often profound, and patients 
may be inappropriately diagnosed with chronic fatigue syndrome 
or anorexia nervosa.376
In secondary adrenal insufficiency due to hypopituitarism, 
the presentation

ficiency due to hypopituitarism, 
the presentation may relate to deficiency of hormones other than 
ACTH, notably LH/FSH (infertility, oligomenorrhea/amenorrhea, 
poor libido) and TSH (weight gain, cold intolerance). Fasting 
hypoglycemia occurs because of loss of the gluconeogenic effects 
of cortisol. It is rare in adults unless there is concomitant alcohol 
abuse or additional GH deficiency. However, hypoglycemia is a 
common presenting feature of ACTH/adrenal insufficiency in 
childhood.377 

e of ACTH/adrenal insufficiency in 
childhood.377 In addition, patients with ACTH deficiency pres­
ent with malaise, weight loss, and other features of chronic adre­
nal insufficiency. Rarely, the presentation is more acute in patients 
with pituitary apoplexy. 
Investigation of Hypoadrenalism
Routine Biochemical Profile
Among patients with established primary adrenal insufficiency, 
hyponatremia is present in about 90% and hyperkalemia in 65%. 
The blood urea concentration is usually elevated. 

The blood urea concentration is usually elevated. Hyperkalemia 
occurs because of aldosterone deficiency, so it is usually absent 
in patients with secondary adrenal failure. Hyponatremia may 
be depletional in an addisonian crisis, but vasopressin levels are 
elevated, resulting in increased free water retention.378 Therefore, 
in secondary adrenal insufficiency, there may be a dilutional hypo­
natremia with normal or low blood urea.
Reversible abnormalities in liver transaminases frequently 
o

abnormalities in liver transaminases frequently 
occur. Hypercalcemia occurs in 6% of all cases379 and may be 
particularly marked in patients with coexisting thyrotoxicosis. 
Free thyroxine concentrations are usually low or normal, but 
TSH values are frequently moderately elevated.380 This is a direct 
effect of glucocorticoid deficiency and reverses with replacement 
therapy. Persistent elevation of TSH in association with positive 
thyroid autoantibodies suggests concomitant autoimmune thy­

toantibodies suggests concomitant autoimmune thy­
roid disease. 
Mineralocorticoid Status
In primary hypoadrenalism, mineralocorticoid deficiency usually 
occurs, manifested by elevated plasma renin activity and either 
low or low-normal plasma aldosterone. The investigation of ZG 
activity is frequently neglected in Addison disease, compared with 
assessment of ZF function. In secondary adrenal insufficiency, the 
RAA system is intact. 
   Clinical Features of Primary Adrenal 
Insufficiency
Fe

ical Features of Primary Adrenal 
Feature
Symptoms
Weakness, tiredness, fatigue
100
Anorexia
100
Gastrointestinal symptoms
92
  Nausea
86
  Vomiting
75
  Constipation
33
  Abdominal pain
31
  Diarrhea
16
Salt craving
16
Postural dizziness
12
Muscle or joint pains
13
Signs
Weight loss
100
Hyperpigmentation
94
Hypotension (<110 mm Hg systolic)
88-94
Vitiligo
10-20
Auricular calcification
5
Electrolyte disturbances
92
  Hyponatremia
88
  Hyperkalemia

 disturbances
92
88
64
  Hypercalcemia
6
Azotemia
55
Anemia
40
Eosinophilia
17
TABLE 15.18
523
Assessing Adequacy of Function of the HPA Axis
Clinical suspicion of the diagnosis should be confirmed with 
definitive diagnostic tests. Basal plasma cortisol and urinary free 
cortisol levels are often in the low-normal range and cannot be 
used to exclude the diagnosis. However, a basal cortisol value 
greater than 400 nmol/L (>14.5 μg/dL)

tisol value 
greater than 400 nmol/L (>14.5 μg/dL) indicates an intact HPA 
axis. In patients with a suspected addisonian crisis, however, treat­
ment should be instigated immediately and prior to the result of 
biochemical testing being available. The SST involves intramus­
cular or intravenous administration of 250 μg tetracosactrin, 
synthetic ACTH(1-24).381 Plasma cortisol levels are measured 
at 0 and 30 to 60 minutes after ACTH administration, and a 
normal response has previously been def

on, and a 
normal response has previously been defined by a peak plasma 
cortisol level greater than 550 nmol/L (>20 μg/dL).382 This value 
equates to the 5th percentile response in normal subjects but is 
very much assay dependent, with different cortisol radioimmu­
noassays giving different results. With more recent immunoassays 
and LC-MS/MS assays that read lower, levels above 430 to 450 
nmol/L are usually consistent with normal adrenal function.383 
Incremental responses (i.e., the differe

tion.383 
Incremental responses (i.e., the difference between peak and basal 
values) are of no value in defining a “pass” response, with the pos­
sible exception of diagnosing functional adrenal insufficiency in 
patients with critical illness. Response is unaffected by the time 
of day of the test, and the test can be performed in patients who 
A
B
D
C
E
• Fig. 15.32  Pigmentation in Addison disease. (A) Hands of an 18-year-old woman with autoimmune poly­
endocrine syndrome and Addison disease

mmune poly­
endocrine syndrome and Addison disease. Pigmentation in a patient with Addison disease before (B) and 
after (C) treatment with hydrocortisone and fludrocortisone. Notice the additional presence of vitiligo. (D) 
Similar changes in a 60-year-old man with tuberculous Addison disease before (left) and after (right) corti­
costeroid therapy. (E) Buccal pigmentation in the same patient as in (D). (B, C, courtesy Professor C.R.W. 
Edwards.)
524
SECTION  IV    Adrenal Cortex and Endocrine 

)
524
have commenced corticosteroid replacement therapy, as long as 
this therapy is of short duration and does not include hydrocor­
tisone (which would be detected in the cortisol assay). Depot or 
prolonged ACTH stimulation tests to discriminate between pri­
mary and secondary adrenal insufficiency are no longer indicated 
if plasma ACTH has been appropriately measured at baseline.
For investigation of secondary adrenal insufficiency th

nvestigation of secondary adrenal insufficiency the insu­
lin-induced hypoglycemia test or insulin tolerance test (ITT) 
remains the gold standard test of the integrity of the HPA axis.384 
It should not be performed in patients with ischemic heart disease 
(always check an electrocardiogram before the test), epilepsy, or 
severe hypopituitarism (i.e., 9 am plasma cortisol <180 nmol/L 
[<6.5 μg/dL]). The test involves the intravenous administration 
of soluble insulin in a dose of 0.1 to 0.15 U/

on 
of soluble insulin in a dose of 0.1 to 0.15 U/kg body weight, with 
measurement of plasma cortisol at 0, 30, 45, 60, 90, and 120 min­
utes. Adequate hypoglycemia (blood glucose <2.2 mmol/L with 
signs of neuroglycopenia—sweating and tachycardia) is required 
for a fail result. In normal subjects, the peak plasma cortisol con­
centration exceeds 500 nmol/L (18 μg/dL). However, the cortisol 
response to hypoglycemia can be reliably predicted by the SST—a 
safer, cheaper, and quicker test.381,3

 the SST—a 
safer, cheaper, and quicker test.381,385
The SST relies on the principle that the cortisol response to 
an exogenous bolus of ACTH is determined by the endogenous 
ACTH trophic drive to the adrenal cortex; impaired ACTH secre­
tion from the anterior pituitary results in an impaired cortisol 
response after synacthen administration. However, the ACTH 
test should not be used to diagnose central hypoadrenalism in 
patients with a recent pituitary insult (e.g., surgery, apoplexy). 
Tota

 pituitary insult (e.g., surgery, apoplexy). 
Total hypophysectomy results in a failed cortisol response to ITT 
immediately thereafter, but it takes 6 weeks for the adrenal cortex 
to readjust to the reduced level of ACTH secretion; in the interim, 
a false-positive (normal) cortisol response is seen. The SST should 
also be avoided in patients with a primary diagnosis of Cushing 
disease, in whom an exaggerated cortisol response to ACTH may 
persist following surgical removal of the adenoma.
I

rsist following surgical removal of the adenoma.
In clinical practice, if the SST is normal, insulin hypoglycemia 
testing is not necessary in most cases unless there is also a need 
to document endogenous GH reserve in a patient with pituitary 
disease. Some patients have an inadequate SST response but then 
respond normally to hypoglycemia385; they do not require corti­
costeroid replacement therapy. Conversely, falsely normal results 
have been reported for the SST.386 Although these are rare

n reported for the SST.386 Although these are rare (<2%), 
the possibility should be noted, particularly in patients with ongo­
ing symptoms and signs indicative of hypoadrenalism.
A low-dose SST giving only 1 μg ACTH(1-24) has been 
proposed as a screen for adequacy of function of the HPA axis, 
with the suggestion that it may be more sensitive than the con­
ventional 250-μg test.387–389 Other researchers dispute this 
suggestion,390,391 and although popular with pediatric endocri­
nologists, w

hough popular with pediatric endocri­
nologists, wide variations in the dose administered using differing 
dilutional methods to prepare the 1-μg dose preclude reliance on 
this test.392
Two other tests have been advocated to assess adequacy of func­
tion of the HPA axis, but their use in modern clinical practice 
should be restricted to difficult diagnostic cases. In the overnight 
metyrapone test, 30 mg/kg (maximum, 3 g) metyrapone is given 
at midnight, and plasma cortisol and 11-deoxycortiso

 midnight, and plasma cortisol and 11-deoxycortisol are mea­
sured at 8 am the following morning. In patients with an intact 
HPA axis, ACTH levels rise after the blockade of cortisol syn­
thesis by metyrapone (documented with a serum cortisol <5 μg/
dL), and a normal result is signified by a peak 11-deoxycortisol 
value greater than 7 μg/dL.393 The CRH stimulation test has been 
used to diagnose adrenal insufficiency; unlike the metyrapone 
test, it differentiates primary from secondary causes.

, it differentiates primary from secondary causes. Patients with 
primary adrenal failure have high ACTH levels that rise further 
after CRH stimulation. Patients with secondary adrenal failure 
have low ACTH levels that fail to respond to CRH. Patients with 
hypothalamic disease show a steady rise in ACTH levels after 
CRH administration.394
Testing the HPA Axis During Critical Illness
Many factors complicate investigation of the HPA axis during 
critical illness. Cortisol levels vary broadly w

critical illness. Cortisol levels vary broadly with disease severity, 
making it difficult to define appropriate responses. Addition­
ally, CBG levels decrease substantially, leading to increases in 
the ratio of free to bound serum cortisol; for this reason, tests 
that assess the whole axis (e.g., ITT) are not appropriate in the 
critical care setting. Investigations are therefore limited mainly 
to basal cortisol levels, with the caveat of low total but preserved 
free serum cortisol.252 Me

ow total but preserved 
free serum cortisol.252 Methylprednisolone is recommended for 
patients with severe early acute respiratory distress syndrome, 
particularly those with poor response to fluid resuscitation and 
vasopressor agents.395 The role of glucocorticoids in the man­
agement of critically ill patients with other conditions requires 
further research. 
Other Tests
Autoantibodies to 21-hydroxylase should be analyzed in patients 
with primary adrenal failure. In autoimmune Addison dise

rimary adrenal failure. In autoimmune Addison disease, 
it is also important to look for evidence of other organ-specific 
autoimmune disease. A CT scan may reveal enlarged or calci­
fied adrenals, suggesting an infective, hemorrhagic, or malignant 
diagnosis (Fig. 15.33). Chest radiography, tuberculin testing, and 
early-morning urine samples cultured for Mycobacterium tubercu­
losis should be performed if tuberculosis is suspected. CT-guided 
adrenal biopsy may reveal an underlying diagnosis i

drenal biopsy may reveal an underlying diagnosis in patients with 
suspected malignant deposits in the adrenal gland. Adrenoleuko­
dystrophy can be diagnosed by measuring circulating levels of 
VLCFA, which should be obtained in males with primary adre­
nal insufficiency and negative 21-hydroxylase antibodies. Finally, 
appropriate investigations, including pituitary MRI scans and an 
assessment of anterior function, are required for patients with sus­
pected secondary hypoadrenalism who are not

h sus­
pected secondary hypoadrenalism who are not taking corticoste­
roid therapy. 
Treatment of Acute Adrenal Insufficiency
Acute adrenal insufficiency is a life-threatening emergency, and 
treatment should not be delayed while waiting for definitive proof 
of diagnosis (Table 15.20). However, in addition to measurement 
   Clinical and Laboratory Features of an 
Adrenal Crisis
Dehydration, hypotension, or shock out of proportion to severity of cur­
rent illness
Nausea and vomiting with a his

f cur­
Nausea and vomiting with a history of weight loss and anorexia
Abdominal pain, so-called acute abdomen
Unexplained hypoglycemia
Unexplained fever
Hyponatremia, hyperkalemia, azotemia, hypercalcemia, or eosinophilia
Hyperpigmentation or vitiligo
Other autoimmune endocrine deficiencies, such as hypothyroidism or 
gonadal failure
TABLE 15.19
525
of plasma electrolytes and blood glucose, appropriate samples 
for ACTH and cortisol should be taken bef

samples 
for ACTH and cortisol should be taken before corticosteroid 
therapy is given. If the patient is not critically ill, an SST can be 
performed.
In adults, intravenous hydrocortisone should be given in a dose 
of 100 mg every 6 to 8 hours. If venous access is not possible, 
then the intramuscular route should be used. In the patient with 
shock, 1 L of normal saline should be given intravenously over the 
first hour. Because of possible hypoglycemia, it is normal to give 
5% dextrose in s

poglycemia, it is normal to give 
5% dextrose in saline. Subsequent saline and dextrose therapy will 
depend on biochemical monitoring and the patient’s condition. 
Clinical improvement, especially in the blood pressure, should be 
seen within 4 to 6 hours if the diagnosis is correct. It is important 
to recognize and treat any associated condition (e.g., infection) 
that may have precipitated the acute adrenal crisis.
After the first 24 hours, the dose of hydrocortisone can be 
reduced, usually

he dose of hydrocortisone can be 
reduced, usually to 50 mg intramuscularly every 6 hours and then 
to oral hydrocortisone, 40 mg in the morning and 20 mg at 3 to 
6 pm. This dose can then be rapidly reduced to a more standard 
replacement dose of 10 to 20 mg on awakening and 5 to 10 mg 
at 3 to 6 pm. 
Long-Term Replacement Therapy
The aim of long-term therapy is to give replacement doses of 
hydrocortisone to mimic the normal cortisol secretion rate (Table 
15.21). In the past, this rate was th

rate (Table 
15.21). In the past, this rate was thought to be approximately 25 
to 30 mg/day, but stable isotope studies have indicated lower 
normal cortisol production rates of 8 to 15 mg/day.396 Most 
patients are adequately treated with less than 30 mg/day (usu­
ally 15–25 mg/day in divided doses). Doses are usually given on 
awakening, with a smaller dose at 3 to 6 pm, but some patients 
feel better with three-times-a-day dosing. In cases of primary 
adrenal failure, some clinicians advocat

 primary 
adrenal failure, some clinicians advocate cortisol day curves with 
simultaneous ACTH measurements to provide some insight 
into the adequacy of replacement therapy.397 A simple weight-
adjusted regime significantly reduces intraindividual variation in 
circulating levels.398 There are no good biomarkers of glucocor­
ticoid adequacy in patients with central hypoadrenalism. Deci­
sions regarding doses of replacement therapy are largely based on 
crude, yet important, end points such as 

ased on 
crude, yet important, end points such as weight, well-being, and 
blood pressure.399 Bone mineral density is moderately reduced 
in a dose-dependent manner in patients treated with more than 
A
B
C
D
• Fig. 15.33  Computed tomographic (CT) scans of patients with primary adrenal insufficiency. The affected 
adrenal glands are indicated by arrows. (A) CT scan of a 59-year-old man with histoplasmosis. Notice the 
subcapsular calcium in both glands. (B) CT scan of a 59-year-old man with met

 glands. (B) CT scan of a 59-year-old man with metastatic melanoma. (C) CT 
scan of an 80-year-old man with bilateral adrenal hemorrhage resulting from anticoagulation for pulmonary 
emboli. (D) Bilateral adrenal tuberculomas in a 79-year-old man with tuberculosis affecting the urogenital 
tract. (A, B, courtesy Dr. William D. Salmon, Jr.; C, courtesy Dr. Craig R. Sussman.)
526
25 mg/day of hydrocortisone,400 highlighting the need to striv

 hydrocortisone,400 highlighting the need to strive 
for minimally effective but safe doses.401,402 Possibly because 
of the known action of IGF1 to increase cortisol clearance,137 
glucocorticoid requirements are slightly lower in hypopituitary, 
GH-deficient subjects than in patients with primary adrenal 
insufficiency.
In primary adrenal failure, mineralocorticoid replacement is usu­
ally also required in the form of fludrocortisone (or 9α-fluorinated 
hydrocortisone), 0.05 to 0.2 mg/day. The

orinated 
hydrocortisone), 0.05 to 0.2 mg/day. The mineralocorticoid activ­
ity of this drug is about 125 times that of hydrocortisone. After 
the acute phase has passed, the adequacy of mineralocorticoid 
replacement should be assessed by measuring electrolytes, supine 
and erect blood pressures, and plasma renin activity or mass.403 Too 
little fludrocortisone may cause postural hypotension with elevated 
plasma renin activity, whereas too much causes the converse. Min­
eralocorticoid replacem

 causes the converse. Min­
eralocorticoid replacement therapy is all too frequently neglected in 
patients with adrenal failure.404
Patients on glucocorticoid replacement therapy should be 
advised to double their daily dose in the event of intercurrent 
febrile illness or accidents. If the patient is vomiting and cannot 
take medication by mouth, parenteral hydrocortisone must be 
given urgently. For minor surgery, 50 to 100 mg hydrocortisone 
hemisuccinate is given with the premedication. For 

emisuccinate is given with the premedication. For major opera­
tions, this pretreatment is followed by the same regimen as for 
acute adrenal insufficiency (see Table 15.21). Pregnancy proceeds 
normally in patients taking replacement therapy, but daily doses of 
hydrocortisone are usually increased modestly (5–10 mg/day) in 
the last trimester. Progesterone is a mineralocorticoid antagonist, 
and the rising levels across pregnancy may necessitate an increased 
dose of fludrocortisone. During la

e an increased 
dose of fludrocortisone. During labor, patients should be well 
hydrated with a saline drip and should receive hydrocortisone 50 
mg intramuscularly every 6 hours until delivery. Thereafter, doses 
can be rapidly tapered to prepregnancy levels.
Every patient on glucocorticoid therapy should be advised 
to register for a medical alert bracelet or necklace and to carry 
information describing their condition, treatment, and doctors. 
Patients should receive regular education regard

us access with a large-gauge needle.
	2.	 Draw blood for immediate serum electrolytes and glucose and 
routine measurement of plasma cortisol and ACTH. Do not wait for 
laboratory results.
	3.	 Infuse 2–3 L of 154 mmol/L NaCl (0.9% saline) solution, or 50 g/L 
(5%) dextrose in 154 mmol/L NaCl (0.9% saline) solution, as quickly 
as possible. Monitor for signs of fluid overload by measuring central 
or peripheral venous pressure and listening for pulmonary rales. 
Reduce infusion rate if indicat

 local anesthesia and most radiologic 
studies, no extra supplementation is needed.
 • For moderately stressful procedures such as barium enema, endos­
copy, or arteriography, give a single 100-mg IV dose of hydrocorti­
sone just before the procedure.
 • For major surgery, give hydrocortisone 100 mg IV just before induc­
tion of anesthesia and continue q8h for first 24 hours. Taper dose 
rapidly, decreasing by half per day, to maintenance level.
ACTH, Adrenocorticotropic hormone; bid, twice a 

.
ACTH, Adrenocorticotropic hormone; bid, twice a day; IV, intravenous; PO, orally; q, every.
   
TABLE 15.21
527
For women with both primary and secondary adrenal failure, 
beneficial effects of adrenal androgen replacement therapy with 
25 to 50 mg/day of DHEA have been reported. To date, reported 
benefit is principally confined to female patients and includes 
improvement in sexual function and well-being.405 However, 
patients with adrenal insufficiency on cur

wever, 
patients with adrenal insufficiency on current steroid replacement 
regimens have significantly impaired health-related subjective 
health status irrespective of the origin of disease or concomitant 
disease.375 Delayed-release hydrocortisone preparations, such as 
Plenadren, that more closely replicate normal circadian cortisol 
concentrations have recently been licensed and approved; early 
clinical trials show improved quality of life in both primary and 
central hypoadrenalism compar

in both primary and 
central hypoadrenalism compared to conventional twice-daily or 
thrice-daily hydrocortisone administration.406 
CAH comprises a group of autosomal recessive disorders caused 
by deficient adrenal corticosteroid biosynthesis.407,408 It results 
from defects in one of the steroidogenic enzymes involved in 
cortisol biosynthesis or in the electron-providing factor, POR. 
Congenital lipoid adrenal hyperplasia, caused by StAR deficiency 
affecting m

yperplasia, caused by StAR deficiency 
affecting mitochondrial cholesterol uptake, is a subform of this 
disease complex with the unique feature of lipid accumulation 
leading to cell destruction. In each case, there is reduced negative 
feedback inhibition of cortisol and, depending on the steroido­
genic pathway involved, alteration in adrenal mineralocorticoid 
and androgen secretion (Table 15.22).
Aldosterone synthase deficiency does not affect glucocorticoid 
biosynthesis and does not lead 

ct glucocorticoid 
biosynthesis and does not lead to adrenal hyperplasia, but it has 
been historically grouped into this disease complex. All forms of 
CAH together represent a disease continuum, ranging from severe 
forms caused by complete loss-of-function defects to milder forms 
in which the defective proteins have partial residual activity.
21-Hydroxylase Deficiency
Between 90% and 95% of cases of CAH are caused by 21-hydrox­
ylase deficiency.407,408 In Western societies, the incidence of 

cy.407,408 In Western societies, the incidence of classic 
21-hydroxylase deficiency (defined by cortisol deficiency) var­
ies from 1 in 10,000 to 1 in 15,000 live births, but in isolated 
communities the incidence may be much higher (e.g., 1:300 in 
Alaskan Yupik populations). Nonclassic 21-hydroxylase deficiency 
is more common, with an incidence of about 1 in 500 to 1000 
live births. The condition arises because of defective conversion 
of 17OHP to 11-deoxycortisol. Reduced cortisol biosynth

OHP to 11-deoxycortisol. Reduced cortisol biosynthesis 
results in reduced negative feedback drive and increased ACTH 
secretion; as a consequence, adrenal androgens are produced in 
excess (Fig. 15.34). Seventy-five percent of patients with classic 
21-hydroxylase deficiency have clinically manifest mineralocor­
ticoid deficiency because of failure to convert sufficient proges­
terone to DOC in the ZG. Clinically, several distinct variants of 
21-hydroxylase deficiency have been recognized (Tab

1-hydroxylase deficiency have been recognized (Table 15.23).
Simple Virilizing Form
In the simple virilizing form of 21-hydroxylase deficiency, the 
enhanced ACTH drive to adrenal androgen secretion in utero 
leads to virilization of an affected female fetus. Depending on the 
severity, clitoral enlargement, labial fusion, and development of a 
urogenital sinus may occur, leading to sexual ambiguity at birth 
and even inappropriate sex assignment. Males are phenotypically 
normal at birth and ar

. Males are phenotypically 
normal at birth and are at risk of not being diagnosed; this explains 
the skewed female-to-male ratio of simple virilizing CAH diag­
nosed in the preneonatal screening era. Such patients may present 
in early childhood with signs of precocious pseudopuberty such 
as sexual precocity, pubic hair development, or growth accelera­
tion due to premature androgen excess. If left untreated, this sex 
steroid production stimulates premature epiphyseal closure, and 
final adu

lates premature epiphyseal closure, and 
final adult height is invariably diminished.409,410 
Salt-Wasting Form
Seventy-five percent of patients of both sexes with classic 
21-hydroxylase deficiency have the salt-wasting form and also 
have concomitant, clinically manifested aldosterone deficiency. 
In addition to the described features, neonates commonly present 
after the first 2 weeks of life with a salt-wasting crisis and hypo­
tension if not diagnosed from screening. The clinical signs and 

 diagnosed from screening. The clinical signs and 
symptoms of salt wasting include poor feeding, vomiting, failure 
to thrive, lethargy, and sepsis-like symptoms. These features may 
alert the clinician to the diagnosis in a male baby, but the diag­
nosis is still delayed in many cases, and the condition carries a 
significant neonatal mortality rate. 
Nonclassic or Late-Onset 21-Hydroxylase Deficiency
Patients with nonclassic 21-hydroxylase (NCAH) deficiency pres­
ent in childhood or early adu

AH) deficiency pres­
ent in childhood or early adulthood with premature pubarche or 
with a phenotype that may masquerade as polycystic ovary syn­
drome (PCOS).407,411,412 Indeed, nonclassic 21-hydroxylase defi­
ciency is a recognized secondary cause of PCOS and is far more 
common than the classic variant. Some evidence suggests that at 
least 30% of adult patients have an impaired cortisol response 
to ACTH(1-24)413 and may be prone to stress-induced adrenal 
insufficiency, but the rates of ad

nduced adrenal 
insufficiency, but the rates of adrenal crises in untreated patients 
appear to be very low. There is overlap in the effects of mutations 
that cause NCAH and classic 21-hydroxylase deficiency, and if 
patients fail an SST they are better characterised as occult clas­
sic 21-hydroxylase deficiency. In some series from tertiary referral 
centers, nonclassic 21-hydroxylase deficiency accounts for up to 
12% of all PCOS patients, but more realistic prevalence rates are 
probably 1% 

 more realistic prevalence rates are 
probably 1% to 3%.414 Females present with hirsutism, primary 
or secondary amenorrhea, or anovulatory infertility.411 Andro­
genic alopecia and acne may be other presenting features. 
Heterozygote 21-Hydroxylase Deficiency
Salt wasting, simple virilizing, and late-onset 21-hydroxylase defi­
ciency are all caused by homozygous or compound heterozygote 
mutations in the human 21-hydroxylase gene (CYP21A2). In 
the carrier or heterozygote state, only one allel

the carrier or heterozygote state, only one allele is mutated. The 
clinical significance of the heterozygote state is uncertain; it does 
not appear to disadvantage reproductive capability but may cause 
signs of hyperandrogenism in adult women.407 
Molecular Genetics
21-Hydroxylase deficiency is inherited as an autosomal recessive 
trait, and the higher incidence of the condition in some ethnic com­
munities almost certainly relates to consanguinity. The CYP21A2 
gene and its highly homologou

uinity. The CYP21A2 
gene and its highly homologous pseudogene (CYP21A1P) are 
located on the short arm of chromosome 6 (6p21.3). Because 
of the genomic localization within the human leukocyte antigen 
(HLA) locus, a region with a high frequency of genomic recombi­
nations, most of the mutations causing 21-hydroxylase deficiency 
are generated by gene conversion events from the CYP21A1P pseu­
dogene. Complete gene deletions or conversions of the CYP21A2 
gene, eight pseudogene-derived point mut

 CYP21A2 
gene, eight pseudogene-derived point mutations, and an 8–base 
528
   Congenital Adrenal Hyperplasia: Features for Each Enzyme Defect
21-Hydroxylase
11β- Hydroxylase
17α- Hydroxylase
3βHSD Type 2
P450 Oxidoreductase
Lipoid Adrenal 
P450 Side-
Chain Cleavage
Aldosterone 
Synthase
Apparent Cortisone 
Reductase
OMIM No.
+201910
#202010
#202110
+201810
#201750
a600617
+118485
a124080
a138090
Gene/Protein
CYP21

a600617
+118485
a124080
a138090
CYP21A2
CYP11B1
CYP17A1
HSD3B2
POR
StAR
CYP11A1
CYP11B2
H6PDH
Alias
P450 21A2
P450 11B1
P450 17A1
3β-HSD
CPR, CYPOR
P450 11A1
P450 11B2
Incidence
Classic: 1:10,000 
to 1:15,000
Nonclassic: 1:500 
to 1:1000
1:100,000 to 
1:200,000
Rare
Rare
Unknown
Rare
Rare
Rare
Rare
DSD
Classic: 46,XX
Nonclassic: No
46,XX
46,XY
46,XYa
46,XX + 46,XYb
46,XY
46,XY
No
No
Primary affected 
organ
Adrenal
Adrenal
Adrenal, gonads
Adrenal, gonads, liver, all 

nads
CYP type 2–express­
ing tissues
Adrenal, gonads Adrenal, gonads
Adrenal
Liver, adrenal, all 
H6PDH/11βHSD1-
expressing tissues
Classic: Reduced
Nonclassic: 
Normal
Reduced
Reduced
Reduced
Reduced to normal, 
impaired stress 
response
Reduced
Reduced
Normal
Normal, but reduced 
tissue levels due to 
increased cortisol 
clearance
Mineralocorti­
coids
Classic: Reduced 
in SW
Normal
Increased, mainly 
Increased

c: 
Normal
Increased
Reduced often
Reduced to increased
Reduced
Reduced
Reduced
Normal
Sex hormones
Increased
Increased
Reduced
Reduced in males, 
increased in 
femalesc
Reduced
Reduced
Reduced
Normal
Increased
Increased 
marker 
metabolites in 
plasma
17OHP, 21DOF
DOC, S
Pregnenolone, 
progesterone, 
DOC, S
17OH preg­
nenolone, DHEA
Pregnenolone, proges­
terone, 17OHP
DOC, B, 18OHB
Increased 
marker 
urine
Pregnane­
triol, 17OH 
pregn

urine
Pregnane­
pregnenolone, 
pregnanetrio­
lone
THDOC, THS
THDOC, THB, 
pregnenediol, 
pregnanediol
Pregnenediol, pregnane­
diol, pregnanetriol, 
17OH pregnanolone
TABLE 15.22
529
PRA
Classic: Increased
Nonclassic: Nor­
mal to mildly 
increased
Reduced
Reduced
Increased
Increased
Increased
Increased
Normal
No
Yes
Yes
No
No or mild
No
No
No
No
Plasma sodium
in SW
Normal
Increase

assic: Reduced 
in SW
Normal
Increased
Increased
Reduced in SW
Normal
Reduced
Reduced
Reduced
Normal
Plasma potas­
sium
Classic: Increased 
in SW
Normal
Reduced
Reduced
Increased in SW
Normal
Increased
Increased
Increased
Normal
Urinary salt loss
Classic: Yes
No
No
Yes
No
Yes
Yes
Yes
No
Skeletal malfor­
mation
No
No
No
No
Yesd
No
No
No
No
aMasculinization of the external genitalia in females at birth is rare and usually mild; signs of increased androgens 

re and usually mild; signs of increased androgens usually manifest later.
bDSD is observed in both sexes, and normal sex-specific sexual development is also reported.
cSteroid hormone conversion by 3βHSD1 in peripheral tissues.
dIn most cases published thus far; however, absence of skeletal malformations does not rule out POR deficiency.
B, Corticosterone; CYP, cytochrome P450; DHEA, dehydroepiandrosterone; DOC, 11-deoxycorticosterone; 21DOF, 21-deoxycortisol; DSD, disorder of sex development; H

deoxycortisol; DSD, disorder of sex development; H6PDH, hexose-6-phosphate dehydrogenase; HSD, hydroxysteroid dehydrogenase; OMIM, Online 
Mendelian Inheritance in Man; 18OHB, 18-hydroxycorticosterone; 17OHP, 17-hydroxyprogesterone; POR, P450 oxidoreductase; PRA, plasma renin activity; S, 11-deoxycortisol; StAR, steroidogenic acute regulatory protein; SW, salt wasting; THB, tetrahydrocortico­
sterone; THS, tetrahydro-11-deoxycortisol; THDOC, tetrahydro-11-deoxycorticosterone.
   
Cho

ahydro-11-deoxycorticosterone.
   
StAR
CYP11A1
HSD3B2
CYP17A1
17OH Pregnenolone
CYP17A1
17OH Progesterone
DHEA
(Blocked at)
CYP21A2
CYP21A2
CYP11B1
DOC
CYP11B2
CYP11B1
Cortisol
18OH Corticosterone
CYP11B2
CYP11B2
Androgens
• Fig. 15.34  Congenital adrenal hyperplasia related to 21-hydroxylase deficiency. The normal synthesis of 
cortisol is 

 deficiency. The normal synthesis of 
cortisol is impaired, and adrenocorticotropic hormone (ACTH) levels increase because of loss of normal 
negative feedback inhibition, resulting in an increase in adrenal steroid precursors proximal to the block. 
The results are cortisol deficiency, variable mineralocorticoid deficiency, and excessive secretion of adrenal 
androgens. CYP, cytochrome P450; DHEA, dehydroepiandrosterone; DOC, deoxycorticosterone; HSD, 
hydroxysteroid dehydrogenase; StAR, steroi

; HSD, 
hydroxysteroid dehydrogenase; StAR, steroidogenic acute regulatory protein.
   Forms of 21-Hydroxylase Deficiency
Phenotype
Classic Salt Wasting
Simple Virilizing
Nonclassic
Age at diagnosis
Newborn to 6 mo
Female: Newborn to 2 yr
Male: 2–4 yr
Child to adult
Genitalia
Female: Ambiguous
Male: Normal
Female: Virilized
Incidence
1:20,000
1:60,000
1:1000
Hormones
  Aldosterone
Reduced
Normal
Normal
  Renin
Increased
Normal or increased
Normal
  Co

  Renin
Increased
Normal
  Cortisol
Reduced
Reduced
Normal
  17OHP
>50,000 ng/dL
10,000-50,000 ng/dL
1000-10,000 ng/dL (ACTH stimulation)
  Testosterone
Increased
Increased
Variable, increased
Growth
−2 to −3 SD
−1 to −2 SD
Probably normal
21-Hydroxylase activity 
(% of wild type)
0
1–5
5–20
Typical CYP21A2 mutations
Deletions, conversions, nt656g
I172N
V281L
G110Δ8nt, R356W
Intron 2 splice site (nt656g)
P30L
I236N, V237E, M239K, Q318X
17OHP, 17-Hydroxyprogesterone; ACTH, ad

39K, Q318X
17OHP, 17-Hydroxyprogesterone; ACTH, adrenocorticotropic hormone; SD, standard deviation.
   
TABLE 15.23
531
pair deletion are found in more than 95% of cases. Other rare 
pseudogene-independent CYP21A2-inactivating mutations have 
been reported in single families or small populations. Approxi­
mately 65% to 75% of CAH patients are compound heterozygous 
for the disease-causing mutations.415
The genotype-phenotype correlation in CAH due to the 
21-hydro

-phenotype correlation in CAH due to the 
21-hydroxylase deficiency is well established. The clinical phenotype 
correlates with the less severely mutated allele and, consequently, 
with the residual 21-hydroxylase activity (Fig. 15.35).416,417 This 
correlation appears to be high in populations, although divergence 
between genotype and phenotype has been observed for individu­
als.418 The 21-hydroxylase activity measured by in vitro analysis 
provides a possibility for estimating disease sever

rovides a possibility for estimating disease severity, although some 
phenotypic variability (e.g., salt wasting, age at onset) seems likely 
to depend on other interacting genes and maturation processes 
rather than CYP21A2 itself. One such factor might be the length 
of the CAG repeats in the androgen receptor modulating andro­
gen action.419 Potential variations in the degree of recovery from 
Gene del/conversion
∆8 bp
E6 cluster
F306 +t
Q318X
R356W
Intron 2 splice site
P453S
CYP21A1P
CYP21A1

R356W
P453S
CYP21A1P
CYP21A1P
RP2
RP1
C4A
TNXA
TNXB
CREBL1
Centromere
Telomere
C4B
LTA
CYP21A2
CYP21A2
P30L
in2 splice
I172N
E6 cluster
V281L
F306 +t
Q318X
R356W
∆8 bp
5′
5′
3′
3′
Speiser et al., 1992
II-IV (IV)
III-V (IV)
I-IV (III)
0-IV (0)
Wedell et al., 1994
III-IV
II-V
I-IV
Jääskeläinen et al., 1997
II-V
0-V
II-V
0
Krone et al., 2000
III-V (IV)
II-V (IV)
II-V (III)
0-IV (0)
Mutation group
Null
A
1
2
3
4
5
6
7
8
9
10
1
2
3
4
5
6
7
8
9
10
B
C
CAH form
Salt wasting
Prader

3
4
5
6
7
8
9
10
B
C
CAH form
Prader genital stages
96%
85%
73%
63%
65%
100%
90%
74%
Stikkelbroeck et al., 2003
97%
96%
53%
100%
Positive predictive values
Simple virilizing
0-1%
0%
Disease severity
21-hydroxylase deficiency
1-5%
20-30%
30-50%
I172N
P30L
V281L
P453S
C
B
A
• Fig. 15.35  Genetics of 21-hydroxylase deficiency. (A) Genomic organization of the functional CYP21A2 
gene and its nonfunctional CYP21A1P pseudogene. (B) Nine 

d its nonfunctional CYP21A1P pseudogene. (B) Nine of 10 common mutations are transferred by 
microconversions from the CYP21A1P pseudogene into the CYP21A2 gene. (C) Genotype-phenotype 
correlation in 21-hydroxylase deficiency is well established. Based on the in  vitro enzyme activity, the 
CYP21A2 gene-inactivating mutations can be categorized into four major mutation groups. Although varia­
tion has been reported for the milder mutations, the overall correlation is high regarding expression o

overall correlation is high regarding expression of the 
adrenal phenotype. Considerable variability exists for the correlation with genital virilization. CAH, congeni­
tal adrenal hyperplasia.
532
glucocorticoid and mineralocorticoid deficiency during later life 
might be explained by significant 21-hydroxylase activity of the 
enzymes cytochrome P450 2C19 and P450 3A4.420 
Diagnostic Criteria
A diagnosis of 21-hydroxylase deficiency shou

eria
A diagnosis of 21-hydroxylase deficiency should be considered 
in any newborn infant with genital ambiguity and salt wasting, 
hypotension, or hypoglycemia. Hyponatremia and hyperkalemia 
with raised plasma renin activity are found in salt-wasters. In later 
life, adrenal androgen excess (DHEAS, androstenedione) is found 
in patients presenting with sexual precocity or a PCOS-like phe­
notype. Randomly timed measurements of the plasma 17OHP 
concentration are significantly increased in clas

concentration are significantly increased in classic 21-hydroxylase 
deficiency. Commonly, 17OHP concentrations in patients with 
salt-wasting CAH are higher than in non–salt-losing patients.
In nonclassic 21-hydroxylase deficiency, an SST is required to 
establish normal adrenal glucocorticoid reserve. Clinically use­
ful nomograms have been developed that compare circulating 
concentrations of 17OHP before and 60 minutes after stimula­
tion to investigate borderline cases and to differentiate

 investigate borderline cases and to differentiate between 
nonclassic 21-hydroxylase deficiency and heterozygous carriers 
(Fig. 15.36).421 This test separates patients with classic and non­
classic 21-hydroxylase deficiency from heterozygote carriers and 
normal subjects, but there is some overlap between values seen 
in heterozygotes and in normal subjects. 17OHP and cortisol are 
measured basally and then 60 minutes after administration of 250 
μg synacthen. Stimulated values are invariably 

0 
μg synacthen. Stimulated values are invariably grossly elevated 
(>35 nmol/L [>1100 ng/dL]) in patients with classic and non­
classic forms of the disorder. Heterozygote patients usually have 
stimulated values between 10 and 30 nmol/L (330 and 1000 ng/
dL) (see Fig. 15.36). Stimulation tests are not always required to 
make a diagnosis; for example, a basal 17OHP concentration of 
less than 5 nmol/L (<150 ng/dL) in the follicular phase of the 
menstrual cycle effectively excludes nonclassic 

menstrual cycle effectively excludes nonclassic 21-hydroxylase 
deficiency.411 CYP21A2 genotyping to confirm the clinical and 
biochemical diagnosis is a useful adjunct to hormonal measure­
ments.422,423 Androgen excess in 21-hydroxylase deficiency is 
readily suppressed after glucocorticoid administration.
Prenatal diagnosis of 21-hydroxylase deficiency has been advo­
cated, because treatment of an affected female may prevent mas­
culinization in utero.424 17OHP can be assayed in amniotic flu

 in utero.424 17OHP can be assayed in amniotic fluid, 
but the most robust approach is the rapid genotyping of fetal 
cells obtained by chorionic villous sampling in early gestation. In 
patients with known 21-hydroxylase deficiency (male or female) 
seeking fertility, genotyping the partner before conception will 
uncover nonclassic or heterozygote cases and provide the endocri­
nologist/geneticist with some assignment of risk before pregnancy. 
Treatment
The objectives for treatment of 21-hydr

Treatment
The objectives for treatment of 21-hydroxylase deficiency differ 
with age, but at all ages treatment and overall patient management 
can be fraught with difficulties. In childhood, the overall goal is to 
replace glucocorticoid and mineralocorticoid, thereby preventing 
further salt-wasting crises, but also to normalize adrenal andro­
gen secretion so that normal growth and skeletal maturation can 
proceed. Accurate replacement is essential; in excess, glucocorti­
coids will suppress

ential; in excess, glucocorti­
coids will suppress growth, whereas inadequate replacement will 
result initially in accelerated linear growth and ultimately in short 
stature due to premature epiphyseal closure.407 Response is best 
monitored through growth velocity and bone age, with biochemi­
cal markers from blood (17OHP, androstenedione, testosterone), 
urine, and saliva (17OHP, androstenedione, testosterone) being 
useful adjuncts. In difficult cases, a day curve study, as described 
for pa

ult cases, a day curve study, as described 
for patients with primary adrenal failure but measuring the ACTH 
and 17OHP response before and after corticosteroid replacement, 
may confirm overreplacement or underreplacement. The optimal 
glucocorticoid dose fails to suppress 17OHP and its metabolites 
and maintains sex hormone concentrations in the middle of the 
age-specific and sex-specific normal range. Ideally, the biochemi­
cal investigations will indicate the need for dose adjustments 
befo

 will indicate the need for dose adjustments 
before physical changes, growth, and skeletal maturation indicate 
inadequate or excessive glucocorticoid treatment.424
Corrective surgery (e.g., clitoral reduction, vaginoplasty) is 
frequently addressed during childhood. The method of choice 
should be a one-stage complete repair using the newest techniques 
of vaginoplasty, clitoral, and labial surgery.425
In late childhood and adolescence, appropriate replacement 
therapy is equally important. Ov

iate replacement 
therapy is equally important. Overtreatment may result in obesity 
and delayed menarche/puberty with sexual infantilism, whereas 
underreplacement will result in sexual precocity. Compliance with 
regular medication is often an issue throughout adolescence.
Although much has been written about adequate control in 
childhood, adults with 21-hydroxylase deficiency often provide 
an ongoing dilemma for the endocrinologist. The follow-up of 
such patients should involve multidiscip

ow-up of 
such patients should involve multidisciplinary clinics, initially 
     General population
     Genetically unaffected
Patients with:
     Congenital adrenal
         hyperplasia
     Aquired adrenal
     Cryptic 21OH
         deficiency
Heterozygotes for:
17OHP 60 min
Baseline
17OHP progesterone
ng/dL
10
102
103
104
105
1
2
3
4
5
17OHP proge

one
ng/dL
10
102
103
104
105
1
2
3
4
5
Stimulated
102
5
4
3
2
103
104
105
ng/dL
Log10
Log10
c
v
h
p
u
• Fig. 15.36  Basal and stimulated plasma 17α-hydroxyprogesterone 
(17OHP) concentrations in patients with 21-hydroxylase (21OH, CYP21A2) 
deficiency. To convert values to nmol/L, multiply by 0.0303. The mean 
for each group is indicated by a large cross and an adjacent letter: c, 
patients with classic CYP21A2 deficiency; h, heterozygotes for all forms 
of CYP21A2 deficiency;

eterozygotes for all forms 
of CYP21A2 deficiency; p, general population; u, known unaffected per­
sons (e.g., siblings of patients with CYP21A2 deficiency who carry neither 
affected parental haplotype as determined by human leukocyte antigen 
typing); v, patients with nonclassic (acquired and cryptic) CYP21A2 defi­
ciency. (From White PC, New MI, Dupont B. Congenital adrenal hyperpla­
sia: part 1. N Engl J Med. 1987;316:1519–1524.)
533
with transition adolescence

15  The Adrenal Cortex
with transition adolescence clinics to facilitate transfer from pedi­
atric to adult care. Problems in adulthood relate to fertility con­
cerns, hirsutism, and menstrual irregularity in women; obesity, 
metabolic consequences, and impact of short stature; probable 
increased cardiovascular risk; sexual dysfunction; and psychologic 
problems.407,426,427 Counseling is often required in addition to 
endocrine support. Males may develop enlargement of the testes 
due to so-cal

y develop enlargement of the testes 
due to so-called testicular adrenal rest tumors (TART)—that is, 
ectopic adrenal tissue, which may regress after glucocorticoid sup­
pression, and if untreated can lead to infertility and affect around 
one in three male patients. These patients need adequate endo­
crine therapy rather than urologic referral with ensuing risk of 
removal of testis mistaken for a tumor.428
In the absence of any evidence-based data, there are no pre­
scriptive steroid regimens 

ata, there are no pre­
scriptive steroid regimens to treat patients with 21-hydroxy­
lase deficiency at any age, and as a result, many individualized 
regimens are used in clinical practice. Hydrocortisone is recom­
mended for replacement therapy from the newborn period to ado­
lescence.425 Usual starting doses of hydrocortisone in childhood 
are 10 to 15 mg/m2 per day in three divided doses, with up to 25 
mg/m2 in infancy only seldom required. These doses are higher 
than those used for replac

These doses are higher 
than those used for replacement of adrenal insufficiency because 
treatment also aims at normalization of ACTH-driven adrenal 
androgen excess. The optimal timing for providing the highest 
dose of hydrocortisone remains an ongoing matter of debate, with 
no data supporting either circadian replacement (giving the high­
est dose in the morning) or reverse-phase therapy (giving the larg­
est dose of hydrocortisone at night). Long-acting steroids such as 
prednisone, predni

. Long-acting steroids such as 
prednisone, prednisolone, and dexamethasone are more effective 
in this regard but should not be given before the end of puberty to 
avoid oversuppression and reduction in linear growth.
Fludrocortisone is required for infants with classic 21-hydroxy­
lase deficiency, although this need may spontaneously change with 
age. Fludrocortisone doses during the first year of life are com­
monly 150 μg/m2 per day. Sodium needs to be supplemented, 
as milk feeds provide on

eeds to be supplemented, 
as milk feeds provide only minimal sodium amounts. Adequate 
mineralocorticoid replacement usually allows hydrocortisone dose 
reduction. The relative dose in relation to body surface decreases 
throughout life. Fludrocortisone doses of 100 μg/m2 per day after 
the first 2 years of life are commonly sufficient. This requirement 
drops further with adolescence and adulthood to a daily dose of 
100 to 200 μg (50–100 μg/m2 per day). Mineralocorticoid sub­
stitution is moni

per day). Mineralocorticoid sub­
stitution is monitored by measurements of plasma renin activity 
(low or suppressed levels indicating overtreatment), serum potas­
sium, and standing blood pressure.429
Adrenomedullary dysfunction has been reported in the 
21-hydroxylase–deficient adrenal gland, probably because of 
relative glucocorticoid deficiency, which results in epinephrine 
deficiency.173,430 In clinical practice, sufficient supplementation 
of glucose during exercise and illness should be

of glucose during exercise and illness should be guaranteed, to 
prevent hypoglycemic episodes. Bilateral adrenalectomy might 
appear effective in the short term but should be regarded as a last 
resort174; adrenal rest tumor has appeared after adrenalectomy 
even in women.431 In addition, because of the requirement for 
lifelong corticosteroid replacement therapy, patients could also 
develop ACTH-secreting pituitary tumors.432 The adrenalectomy 
procedure also bears a number of risks, includ

my 
procedure also bears a number of risks, including surgical and 
anesthetic complications, and the patient is left completely adre­
nal insufficient.
Prenatal dexamethasone treatment has been proposed to avoid 
virilization of the external genitalia in the female fetus.407 Unlike 
hydrocortisone, which is inactivated by placental 11βHSD2, 
maternally administered dexamethasone can cross the placenta to 
suppress the fetal HPA axis. However, due to concerns over the 
long-term effects of dexam

e to concerns over the 
long-term effects of dexamethasone on the mother and fetus, pre­
natal therapy is not recommended outside research studies.433,434 
Fetal sex and CYP21A2 genotype can be determined as early as 
week 6 of gestation with the use of novel molecular diagnostic 
methods that analyze cell-free fetal DNA from maternal blood 
using real-time polymerase chain reaction.435 In this way, the 
number of unnecessarily treated cases can be substantially reduced. 
Dexamethasone can lead 

be substantially reduced. 
Dexamethasone can lead to maternal cushingoid effects in preg­
nancy434,436 and may in turn have long-term, deleterious effects 
on the fetus, including metabolic, psychologic, and intellectual 
consequences. Prenatal treatment is controversial and has to be 
regarded as experimental; patients treated should be included in 
ongoing multicenter studies.437
In adult women with hyperandrogenism and untreated non­
classic 21-hydroxylase deficiency, there is no evidence tha

1-hydroxylase deficiency, there is no evidence that final 
height is affected. In this setting, glucocorticoid therapy in isola­
tion rarely controls hirsutism, and additional antiandrogen ther­
apy is often required (e.g., cyproterone acetate, spironolactone, 
flutamide together with an oral estrogen-containing contracep­
tive pill). Most studies have found increased risk of miscarriage 
in women with nonclassic 21-hydroxylase deficiency, and some 
retrospective series suggest that the risk is 

me 
retrospective series suggest that the risk is lower in women treated 
with glucocorticoid replacement during pregnancy.438,439 Hypo­
gonadotropic hypogonadism in male patients is a consequence of 
increased aromatization of adrenal androgens, in particular andro­
stenedione to estrone, resulting in suppression of pituitary LH 
and FSH secretion.440 The condition is reversible after optimiza­
tion of glucocorticoid therapy. However, overreplacement in men 
or women may also lead to hypogonado

ement in men 
or women may also lead to hypogonadotropic hypogonadism due 
to glucocorticoid-mediated suppression of GnRH secretion. 
Long-Term Complications and Comorbid Conditions
Outcome assessed by final height is not optimal in many patients 
treated for 21-hydroxylase deficiency. In a large UK cohort the 
mean adult height of patients with classic 21-hydroxylase defi­
ciency was 14 cm and 8 cm below the population mean for 
men and women, respectively.441 Similar data from the United 
Stat

espectively.441 Similar data from the United 
States shows that final height is -1.1 SDS compared to the con­
trol population.442 The pubertal growth spurt occurs earlier and 
is less pronounced than normal. An often overlooked problem is 
glucocorticoid overtreatment during the first 2 years of life; over­
treatment suppresses the infant growth spurt, which is character­
ized by the highest postnatal growth velocity. Therefore the lowest 
optimal dose of glucocorticoid replacement should be est

l dose of glucocorticoid replacement should be established 
as early in life as possible.
Increased fat mass and obesity are common among children 
and adolescents with 21-hydroxylase deficiency.443–445 Glucocor­
ticoid dose, chronologic age, advanced bone age maturation, and 
parental obesity all contribute to elevated BMI SDS.444
Increased fat mass and higher insulin levels have been described 
in women older than 30 years of age with 21-hydroxylase defi­
ciency. However, clear evidence of car

ylase defi­
ciency. However, clear evidence of cardiovascular risk factors had 
not been shown. Women with classic 21-hydroxylase deficiency do 
have a significantly higher rate of gestational diabetes, a possible 
forerunner for the development of type 2 diabetes,446 and women 
with nonclassic447 and young adults with classic 21-hydroxylase 
deficiency448 have reduced insulin sensitivity. Increased intima 
media thickness as a marker of atherosclerosis has been detected.448
Daytime systolic blo

lerosis has been detected.448
Daytime systolic blood pressure in children and adolescents 
with 21-hydroxylase deficiency is elevated, and the physiologic 
534
nocturnal dip in blood pressure is absent.449 Elevated systolic 
blood pressure correlates with the degree of overweight and obe­
sity.450 In adults, hypertension was found to be more common in 
men and in patients with elevated 17OHP,442 in data from the 
United States, but in the 

P,442 in data from the 
United States, but in the large UK Congenital adrenal Hyperpla­
sia Adult Study Executive (CaHASE) cohort men with CAH had 
blood pressure significantly lower than controls. The reasons for 
these discrepancies are not established. 
11β-Hydroxylase Deficiency
11β-Hydroxylase deficiency accounts for up to 7% of all cases 
of CAH, with an incidence of 1 in 100,000 live births.451 The 
condition arises because of mutations in the 11β-hydroxylase 
(CYP11B1) gene that result i

 the 11β-hydroxylase 
(CYP11B1) gene that result in loss of enzyme activity and a block 
in the conversion of 11-deoxycortisol to cortisol. The CYP11B1 
gene is located on chromosome 8q24.3, approximately 40 kilo­
bases from the highly homologous aldosterone synthase gene 
(CYP11B2).451 CYP11B1-inactivating mutations have been 
shown to be distributed over the entire coding region consisting 
of nine exons. Although mutation clusters are reported in exons 
2, 6, 7, and 8,415,451 real hot spots, 

 in exons 
2, 6, 7, and 8,415,451 real hot spots, as seen in 21-hydroxylase 
deficiency, do not exist. Most of the reported mutations lead to 
absent or almost absent 11β-hydroxylase enzyme activity, with 
only some cases of mild or nonclassic 11β-hydroxylase deficiency 
reported.452–454
Loss of negative cortisol feedback and enhanced ACTH-medi­
ated adrenal androgen excess occur in 11β-hydroxylase deficiency 
(Fig. 15.37). Clinical features therefore are very similar to those 
reported in the s

efore are very similar to those 
reported in the simple virilizing form of 21-hydroxylase deficiency 
(46,XX DSD, including virilization of the external genitalia and 
sexual ambiguity); again, milder cases can manifest later in child­
hood or even young adulthood. The principal difference from 
21-hydroxylase deficiency is hypertension, which is thought to be 
secondary to the mineralocorticoid effect of DOC excess. How­
ever, there is a poor correlation between DOC secretion and the 
presence 

rrelation between DOC secretion and the 
presence of hypertension, and unexplained salt wasting has been 
reported in few patients during early life. On this clinical back­
ground, the diagnosis can be made by measuring a plasma syn­
acthen–stimulated 11-deoxycortisol value, which will be greater 
than 52 nmol/L (>1800 ng/dL) and often much higher.455 Basal 
concentrations of 17OHP are commonly increased but may be 
normal even during the first weeks of life.455
Treatment is with replacement glu

eeks of life.455
Treatment is with replacement glucocorticoid therapy; with 
suppression of DOC secretion, the plasma renin activity, which 
is suppressed at baseline, increases into the normal range. In gen­
eral, higher glucocorticoid doses are needed to suppress hyper­
androgenism compared with the situation in 21-hydroxylase 
StAR
CYP11A1
HSD3B21
CYP21A2
CYP11B2
CYP21A2
CYP17A1
CYP17A1
DHEA

ogesterone
DHEA
CYP11B1
CYP11B1
DOC
Cortisol
CYP11B2
CYP11B2
Androgens
Mineralocorticoids
• Fig. 15.37  Congenital adrenal hyperplasia related to 11β-hydroxylase deficiency. The normal synthesis of 
cortisol is impaired, and adrenocorticotropic hormone (ACTH) levels increase because of the loss of nor­
mal negative feedback inhibition, which results in an increase in adrenal steroid 

, which results in an increase in adrenal steroid precursors proximal to the 
block. The results are cortisol deficiency, mineralocorticoid excess related to excessive deoxycorticosterone 
(DOC) secretion, and excessive secretion of adrenal androgens. CYP, cytochrome P450; DHEA, dehydro­
epiandrosterone; HSD, hydroxysteroid dehydrogenase; StAR, steroidogenic acute regulatory protein.
535
deficiency, and add-on antihypertensive therapy may be nec­
essary in some cas

ypertensive therapy may be nec­
essary in some cases. Antihypertensive treatment should be 
commenced at an early stage to avoid excessive glucocorticoid 
exposure. 
17α-Hydroxylase Deficiency
17α-hydroxylase deficiency is less common than 11β-hydroxylase 
deficiency, except in Brazil.456–459 Mutations within the CYP17A1 
gene result in failure to synthesize cortisol (17α-hydroxylase activ­
ity), adrenal androgens (17,20-lyase activity), and gonadal ste­
roids (Fig. 15.38). Therefore, in contras

dal ste­
roids (Fig. 15.38). Therefore, in contrast to 21-hydroxylase and 
11β-hydroxylase deficiencies, 17α-hydroxylase deficiency results 
in adrenal and gonadal insufficiency and causes 46,XY DSD. 
A single enzyme is expressed in the adrenal and the gonads and 
possesses both 17α-hydroxylation and 17,20-lyase activities, but 
rare patients with isolated deficiency of 17,20-lyase deficiency 
have been reported.457,460,461 Loss of negative feedback results in 
increased secretion of steroids pr

ack results in 
increased secretion of steroids proximal to the block, and min­
eralocorticoid synthesis is enhanced. Corticosterone has weaker 
glucocorticoid activity than cortisol, but corticosterone excess 
generally prevents adrenal crises. Accumulation of corticosterone 
and DOC results in severe hypokalemic hypertension. Sex ste­
roid deficiency caused by loss of 17,20-lyase activity results in 
46,XY DSD; this manifests as undervirilization in male newborns 
and primary amenorrhea in 46,

on in male newborns 
and primary amenorrhea in 46,XX individuals. There is lack of 
pubertal development due to hypergonadotropic hypogonadism 
in both sexes.458
The 17α-hydroxylase enzyme is a microsomal cytochrome 
P450 type II enzyme that requires electron transfer from NADPH 
via POR for catalytic activity.27 For efficient catalysis of the 17,20-
lyase reaction, the P450 17A1-POR complex requires additional 
allosteric interaction with cytochrome b5.460,462 The CYP17A1 
gene consists of eigh

rome b5.460,462 The CYP17A1 
gene consists of eight exons and is located on chromosome 
10q24.3. A variety of different mutations have been described, 
without evidence of a hot spot.415,463 Relative hydroxylase/lyase 
activities of P450 17A1 mutants have been shown to vary in 
in vitro functional assays, but correlations with clinical phenotype 
are lacking. Patients with clinically pure 17,20-lyase deficiency 
have P450 17A1 mutations that selectively compromise 17,20-
lyase activity.460,461,4

ctively compromise 17,20-
lyase activity.460,461,464 Mutations underlying isolated 17,20-lyase 
deficiency are located within the area of the P450 17A1 mol­
ecule that is thought to interact with the cofactor cytochrome 
b5, thereby disrupting the electron transfer from POR to P450 
17A1, specifically for the conversion of 17-hydroxypregnenolone 
to DHEA.460,464
The diagnosis is usually made at the time of puberty when 
patients present with hypertension, hypokalemia, and hypergo­

rtension, hypokalemia, and hypergo­
nadotropic hypogonadism, the last occurring because of lack of 
P450 17A1 activities within the gonad and impaired gonadal ste­
roidogenesis. As a result, LH and FSH levels are elevated. Female 
patients (XX) have primary amenorrhea with absent sexual 
StAR
CYP11A1
HSD3B21
CYP21A2
CYP21A2
CYP17A1
CYP17A1
CYP17A1
CYP17A1
DHEA
CYP11B2
CYP11B1
CYP11B1
11-Deoxycor

tenedione
DHEA
CYP11B2
CYP11B1
CYP11B1
DOC
Cortisol
CYP11B2
CYP11B2
Androgens
• Fig. 15.38  Congenital adrenal hyperplasia related to 17α-hydroxylase deficiency. The normal synthe­
sis of cortisol is impaired, and adrenocorticotropic hormone (ACTH) levels increase because of loss of 
normal negative feedback inhibition resulting in an increase in adrenal steroid precursors proximal to the 
block. T

renal steroid precursors proximal to the 
block. The result is cortisol deficiency and mineralocorticoid excess usually related to deoxycorticosterone 
(DOC) excess. Because gonadal 17α-hydroxylase activity is also absent, sex steroid secretion in addition 
to adrenal androgen secretion is severely impaired, resulting in hypogonadism. CYP, cytochrome P450; 
DHEA, dehydroepiandrosterone; HSD, hydroxysteroid dehydrogenase; StAR, steroidogenic acute regula­
tory protein.
536

 regula­
536
characteristics, whereas 46,XY individuals present with 46,XY 
DSD with female external genitalia but absent uterus and fallo­
pian tubes. The intra-abdominal testes should be removed due to 
the risk of malignancies, and such patients are usually reared as 
females.
Glucocorticoid replacement reverses the DOC-induced sup­
pression of the renin-angiotensin system and lowers blood pres­
sure. Additional sex steroi

and lowers blood pres­
sure. Additional sex steroid replacement is required from puberty 
onward. 
P450 Oxidoreductase Deficiency: Apparent 
Combined 17α-Hydroxylase and 21-Hydroxylase 
Deficiencies
Very few patients have been described with biochemical evidence 
of apparent combined 17α-hydroxylase and 21-hydroxylase 
deficiencies.465 Urinary gas chromatography/mass spectrometry 
analysis reveals a typical pattern comprising increased pregneno­
lone and progesterone metabolites, slightly increa

lone and progesterone metabolites, slightly increased corticoste­
rone metabolites, increased pregnanetriolone excretion, and low 
androgen metabolites (see Fig. 15.4). Mothers pregnant with an 
affected child present with low serum estriol and a characteristic 
urinary steroid profile, allowing for prenatal biochemical diag­
nosis.466,467 Limited analysis of serum steroids only may lead to 
misdiagnosis.468 Cortisol baseline secretion may be normal, but 
most if not all patients show an insuffi

mal, but 
most if not all patients show an insufficient cortisol response 
to synacthen stimulation and therefore require glucocorticoid 
replacement. Impaired 17,20-lyase activity results in deficient 
androgen synthesis, and affected boys are often born underviril­
ized. Most of the affected girls are born with virilized genitalia. 
Therefore patients can present with 46,XY or 46,XX DSD or 
with appropriate development of the external genitalia in both 
sexes. After birth, virilization does no

 in both 
sexes. After birth, virilization does not progress, and circulating 
androgen concentrations are typically low. Some mothers develop 
signs of virilization during midpregnancy with an affected child; 
this commonly resolves soon after birth, further indicating intra­
uterine androgen excess.466 In addition to these features of CAH, 
affected children may also present with bone malformations, 
including midface hypoplasia, craniosynostosis, and radiohumeral 
synostosis, in some cases me

is, and radiohumeral 
synostosis, in some cases meeting criteria for the Antley-Bixler 
congenital malformation syndrome.469,470 The bone phenotype in 
affected patients with POR deficiency is most likely caused by an 
impairment of sterol biosynthesis, specifically of POR-dependent 
14α-lanosterol demethylase (P450 51A1).
The paradox of fetal virilization but sex hormone deficiency 
in postnatal life might be mediated by a newly discovered “back­
door” pathway of androgen synthesis in fetal lif

­
door” pathway of androgen synthesis in fetal life that relies on nei­
ther androstenedione nor testosterone as an intermediate.470–472 
Pubertal development in POR deficiency appears to be dominated 
by the consequences of sex steroid deficiency,472 and most patients 
require sex hormone substitution. The overall incidence of POR 
deficiency has not been established. However, a relatively large 
number of patients with POR deficiency were reported within a 
short period after the initial descr

ted within a 
short period after the initial description of the molecular cause of 
the disease.472,473
The POR gene is located on chromosome 7q11.2 and consists 
of 15 translated exons spanning 32.9 kilobytes and encoding a 
protein of 680 amino acids. A variety of POR-inactivating muta­
tions have been reported, including missense, frameshift, and 
splice site mutations. A287P is the most common mutation in 
Caucasians, whereas R457H is the most frequent founder muta­
tion in the Japanese popu

t frequent founder muta­
tion in the Japanese population. All patients carry POR mutations 
that are either partially inactivating or, in case of major loss-of-
function mutations, manifest only in the compound heterozygous 
state. Homozygous mutations with total loss of function are most 
likely not viable—a view supported by the nonviability of com­
plete POR gene deletion in the murine model.474 
3β-Hydroxysteroid Dehydrogenase Deficiency
In this rare form of CAH, the secretion of all classes

his rare form of CAH, the secretion of all classes of adrenal 
and ovarian steroids is impaired due to mutations within the 
HSD3B2 gene encoding 3βHSD2.475,476 There are two isoforms 
of 3βHSD, encoded by HSD3B1 and HSD3B2, respectively. 
The HSD3B2 gene is located on chromosome 1p13.1 and con­
sists of four exons. 3βHSD2 is expressed mainly in the adrenal 
and the gonad, whereas 3βHSD1 is expressed in the placenta and 
almost ubiquitously in peripheral target tissues.28,476 The enzyme 
3βHSD2 

ripheral target tissues.28,476 The enzyme 
3βHSD2 catalyzes three key reactions in adrenal steroidogenesis: 
the conversion of the Δ5 steroids pregnenolone, 17-hydroxypreg­
nenolone, and DHEA to the Δ4 steroids progesterone, 17OHP, 
and androstenedione, respectively. 3βHSD2 deficiency affects all 
three steroid hormone pathways (i.e., mineralocorticoids, gluco­
corticoids, and sex steroids).
The clinical spectrum shows a wide variety of disease expres­
sion. Patients usually present in early inf

xpres­
sion. Patients usually present in early infancy with adrenal insuf­
ficiency. Loss of mineralocorticoid secretion results in salt wasting, 
although this is absent in 30% to 40% of cases (Fig. 15.39). As 
with 21-hydroxylase deficiency, absence of salt wasting may delay 
the presentation into childhood or puberty, ranging from a severe 
salt-wasting form with or without ambiguous genitalia in affected 
male neonates to isolated premature pubarche in infants and chil­
dren of both sexes an

ubarche in infants and chil­
dren of both sexes and a rare nonclassic variant manifesting with 
hirsutism and menstrual irregularities. In general, the functional 
and biochemical data are in close agreement with the expressed 
phenotype in patients with the non–salt-wasting form of 3βHSD2 
deficiency. However, some variability exists, and identical muta­
tions have been found in the HSD3B2 gene in both salt-wasters 
and non–salt-wasters.475,476 The correlation between the impair­
ment in male s

The correlation between the impair­
ment in male sexual differentiation and salt wasting is poor. The 
spectrum of genital development is variable in both sexes. In males, 
because the 3βHSD2 enzyme is also expressed within the gonad, 
46,XY DSD may occur, resulting in female external genitalia. 
However, most patients present with hypospadias, and even nor­
mal male genitalia may be found. In females, genital development 
can be normal, but usually there is evidence of mild virilization, 
presu

lly there is evidence of mild virilization, 
presumably because of enhanced adrenal DHEA secretion, which 
is converted peripherally to testosterone. A nonclassic form has 
been described in patients with premature pubarche and a PCOS-
like phenotype (i.e., hirsutism, oligomenorrhea/amenorrhea).477
Because activity of the 3βHSD1 enzyme, present in peripheral 
tissues, is intact, levels of circulating Δ4 steroids (progesterone, 
17OHP, androstenedione) may be normal (or even increased). 
However,

ione) may be normal (or even increased). 
However, a diagnosis is established by demonstration of an 
increased ratio of Δ5 steroids (pregnenolone, 17-hydroxypreg­
nenolone, DHEA) to Δ4 steroids in plasma or urine. Hormonal 
criteria have been refined for the diagnosis of 3βHSD2 deficiency 
based on genotyping of the HSD3B2 gene. The 17-hydroxypreg­
nenolone concentrations and the ratios of 17-hydroxypregneno­
lone to cortisol at baseline and after ACTH stimulation are of the 
highest discrimina

er ACTH stimulation are of the 
highest discriminatory value in differentiating between patients 
affected by 3βHSD2 deficiency and those with milder biochemi­
cal abnormalities, who are commonly negative for HSD3B2 
mutations.478,479 
537
StAR Deficiency: Congenital Lipoid Adrenal 
Mutations in the gene encoding StAR result in a failure of trans­
port of cholesterol from the outer to the inner mitochondrial 
membrane in steroidogenic tissues. StAR-inde

rial 
membrane in steroidogenic tissues. StAR-independent cholesterol 
transport occurs only at a low rate. As a result, there is deficiency of 
all adrenal and gonadal steroid hormones.24,480 The adrenal glands 
are often massively enlarged and full of lipid; before the character­
ization of StAR, the condition was termed congenital lipoid adrenal 
hyperplasia.480 StAR deficiency severely but incompletely abolishes 
pregnenolone synthesis. Cholesterol esters accumulate under the 
increased tone

sterol esters accumulate under the 
increased tone of ACTH stimulation. Consequently, the lipid 
accumulation worsens the dysfunction and leads to adrenal cell 
destruction. Presentation is with acute adrenal insufficiency in 
the neonatal period, and males exhibit 46,XY DSD due to absent 
gonadal steroids.
The most severe form of this disorder manifests with 46,XY 
DSD and combined adrenal insufficiency. Salt wasting typically 
develops in the neonatal period or after a few weeks of life, but 

eonatal period or after a few weeks of life, but 
later onset may also occur. Females can show spontaneous puber­
tal development. A milder form of StAR deficiency has also been 
described in normally virilized 46,XY individuals who present 
with adrenal failure during early childhood.481 Treatment consists 
of glucocorticoid and mineralocorticoid replacement and substi­
tution of sex hormones in later life. 
P450 Side-Chain Cleavage Deficiency
Deficiency of P450scc (P450 11A1) enzyme is a rare 

eficiency of P450scc (P450 11A1) enzyme is a rare inborn error 
of steroidogenesis. It was previously thought that such mutations 
would not be viable, because the maintenance of human preg­
nancy relies on progesterone that is produced by the fetal part 
of the placenta from the second trimester onward. P450 11A1 
deficiency manifests clinically and biochemically with similar 
signs and symptoms as StAR deficiency, but patients do not have 
enlarged adrenals.482–484 Depending on the impairment 

rged adrenals.482–484 Depending on the impairment of P450 
11A1 function, a spectrum of clinical presentation ranges from 
46,XY DSD with severe adrenal insufficiency in the newborn 
period to hypospadias and cryptorchidism and later manifestation 
of adrenal insufficiency during childhood.485 Concentrations of 
all steroid hormones are decreased, in keeping with impaired con­
version of cholesterol to pregnenolone. Treatment with glucocor­
ticoid, mineralocorticoid, and sex steroid replacement 

d, mineralocorticoid, and sex steroid replacement is required. 
Cortisone Reductase Deficiency
In cortisone reductase deficiency, adrenal glands become hyper­
plastic because of ACTH stimulation due to a defect in corti­
sol metabolism rather than an inherent defect within the gland 
itself.135,486,487 Patients with this condition have a defect in the 
conversion of cortisone to cortisol, suggesting inhibition of 11-oxo-
reductase activity and, by implication, inhibition of 11βHSD1 
18OH Cortico

y implication, inhibition of 11βHSD1 
CYP11B2
CYP11B2
StAR
CYP11A1
CYP21A2
CYP21A2
CYP17A1
CYP17A1
DHEA
3βHSD2
3βHSD2
DOC
Cortisol
CYP11B2
CYP11B1
CYP11B1
Androgens
• Fig. 15.39  Congenital adrenal hyperplasia related to 3β-hydroxysteroid dehydrogenase type 2 (3βHSD2) 
deficiency resulting in corti

ase type 2 (3βHSD2) 
deficiency resulting in cortisol deficiency and variable mineralocorticoid deficiency. Gonadal 3βHSD2 activ­
ity is also absent, resulting in 46,XY DSD and hypogonadism or primary amenorrhea in females. Virilization 
in females can occur due to 3β-hydroxysteroid dehydrogenase type 1 activity. DOC, deoxycorticosterone; 
DHEA, dehydroepiandrosterone; DSD, disorder of sex development; StAR, steroidogenic acute regulatory 
protein.
538
SECTION  IV    Adrenal Cortex and Endocrine

n.
538
(see Fig. 15.12). Cortisol clearance is increased; consequently, 
ACTH secretion is elevated to maintain normal circulating corti­
sol concentrations but at the expense of adrenal androgen excess. 
Female patients present with hirsutism, menstrual irregularity, 
androgenic alopecia, or some combination of these features. Males 
may present with premature pubarche. Dexamethasone treatment 
to suppress ACTH has been used with some suc

ment 
to suppress ACTH has been used with some success to control the 
hyperandrogenism in these cases. Urinary tetrahydrometabolites 
of cortisol and cortisone show almost exclusively THE with little 
or no detectable THF or allo-THF; the ratio of THF+allo-THF 
to THE is less than 0.05 (reference range, 0.8–1.3). The molecular 
bases for cortisone reductase deficiency are inactivating mutations 
in hexose-6-phosphate dehydrogenase (H6PDH)487 and domi­
nant negative mutations of 11βHSD1.488 H6PD

 domi­
nant negative mutations of 11βHSD1.488 H6PDH, located in 
the endoplasmic reticulum, catalyzes the conversion of glucose-
6-phosphate to glucose-6-phosphogluconate, thereby generating 
NADPH, which is crucial in conveying oxo-reductase activity on 
11βHSD1.
Patients with PCOS share many of the same clinical charac­
teristics as those with cortisone reductase deficiency. Whereas 
there is evidence to support increased cortisol secretion rates in 
PCOS, perhaps indicating a defect in the co

es in 
PCOS, perhaps indicating a defect in the conversion of cortisone 
to cortisol, a consensus with respect to THF+allo-THF to THE 
ratios is still lacking.489 Association studies using single-nucleotide 
polymorphic markers in the HSD11B1 and H6PDH genes have 
largely been negative. 
The mineralocorticoid deficiency syndromes are listed in Table 
15.24. They can be divided into congenital and acquired 
syndromes. Mineralocorticoid deficiency may occur in some 
fo

Mineralocorticoid deficiency may occur in some 
forms of CAH and with other causes of adrenal insufficiency 
(e.g., Addison disease, CAH).
Primary Defects in Aldosterone Biosynthesis: Aldosterone 
Synthase Deficiency
Before the characterization of the CYP11B2 gene, two diseases 
were recognized: corticosterone methyl oxidase type I (CMO I) 
deficiency and corticosterone methyl oxidase type II (CMO II) defi­
ciency.490 Subsequently, both variants were shown to be second­
ary to mutations in aldos

were shown to be second­
ary to mutations in aldosterone synthase (P450 11B2), and they 
are now termed aldosterone synthase deficiency, types I and II.491 
Aldosterone synthase catalyzes the three terminal steps of aldoste­
rone biosynthesis, 11β-hydroxylation of DOC to corticosterone, 
18-hydroxylation to 18-hydroxycorticosterone, and 18-oxidation 
to aldosterone. Patients with type I aldosterone synthase defi­
ciency have low to normal levels of 18-hydroxycorticosterone but 
undetectable leve

of 18-hydroxycorticosterone but 
undetectable levels of aldosterone (or urinary tetrahydroaldoste­
rone), whereas patients with the type II variant have high levels 
of 18-hydroxycorticosterone and only subnormal or even normal 
levels of aldosterone. This suggests blockade of only the terminal 
18-oxidation step, with some residual aldosterone synthase activ­
ity remaining. The explanation for the variable biochemical phe­
notype is unknown, particularly now that the same mutation in 
aldostero

ticularly now that the same mutation in 
aldosterone synthase has been uncovered in both variants. It is 
possible that the phenotypic variation may reflect polymorphic 
variants in the residual and normal product of the CYP11B1 gene, 
11β-hydroxylase.
Both variants are rare and are inherited as autosomal recessive 
traits.491 Patients usually present in neonatal life with a salt-wast­
ing crisis involving severe dehydration, vomiting, and failure to 
grow and thrive. Hyperkalemia, metabolic aci

e to 
grow and thrive. Hyperkalemia, metabolic acidosis, dehydration, 
and hyponatremia are found. The plasma renin activity is elevated, 
and plasma aldosterone levels are low. Plasma 18-hydroxycorti­
costerone levels, the ratio of plasma 18-hydroxycorticosterone to 
aldosterone, and the levels of their urinary metabolites are used to 
differentiate the type I and II variants. In most infants, the disor­
ders become less severe as the child ages; indeed, in older children, 
adolescents, and adu

; indeed, in older children, 
adolescents, and adults, the abnormal steroid pattern described 
may be present and may persist throughout life without clinical 
manifestations.
Patients with P450 11B2 deficiency typically respond well to 
9α-fludrocortisone (starting dose, 150 μg/m2 per day in neonates 
and infants) and may also benefit from salt supplementation. 
Patients with failure to grow and thrive usually show a good catch-
up growth. Electrolytes often tend to normalize spontaneously 
bet

rolytes often tend to normalize spontaneously 
between 3 and 4 years of age. However, untreated patients are at 
significant risk for being growth retarded, although spontaneous 
normalization of growth can occur. Adults are usually asymptom­
atic but are more susceptible to salt loss. Rarely, presentation is in 
adulthood.492 Mineralocorticoid treatment in later life has to be 
established on an individual basis. 
Postadrenalectomy Hypoaldosteronism
In a patient with a unilateral aldosteronoma 

nism
In a patient with a unilateral aldosteronoma (Conn syndrome), the 
contralateral ZG is frequently suppressed. Without reversal of the 
chronic volume expansion preoperatively, patients may develop 
severe hyperkalemia and hypotension lasting several days to sev­
eral weeks after adrenalectomy. This effect may be exacerbated by 
the use of spironolactone preoperatively, and is more likely with 
older age, renal impairment, and proteinuria.493 Spironolactone 
has a long half-life and should b

 Spironolactone 
has a long half-life and should be discontinued 2 to 3 days before 
surgery to minimize the risk of postoperative mineralocorticoid 
deficiency. 
Defects in Aldosterone Action: Pseudohypoaldosteronism
Pseudohypoaldosteronism (PHA) is a rare, inherited salt-wasting 
disorder that was first described by Cheek and Perry in 1958 as a 
defective renal tubular response to mineralocorticoid in infancy. 
Patients present in the neonatal period with dehydration, hypo­
natremia, hyperkale

period with dehydration, hypo­
natremia, hyperkalemia, metabolic acidosis, and failure to thrive 
despite normal glomerular filtration and normal renal and adrenal 
function.494 Renin levels and plasma aldosterone are grossly ele­
vated. When patients fail to respond to mineralocorticoid therapy, 
PHA is suspected as the underlying disorder.
PHA type I can be divided into two distinct disorders based 
on unique physiologic and genetic characteristics: the renal form 
of PHA, which is inherited a

tics: the renal form 
of PHA, which is inherited as an autosomal dominant trait, and 
a generalized autosomal recessive form of PHA. The autosomal 
dominant form is usually less severe; the patient’s condition 
often improves spontaneously within the first several years of life, 
   Causes of Mineralocorticoid Deficiency
Addison disease
Adrenal hypoplasia
Congenital adrenal hyperplasia (21-hydroxylase and 3β-hydroxysteroid 
dehydrogenase deficiencies)
Pseudohypoaldosteronism types I and II
Hypo

ncies)
Aldosterone biosynthetic defects
TABLE 15.24
539
allowing discontinuation of therapy. By contrast, the autosomal 
recessive form produces a multiorgan disorder, with mineralo­
corticoid resistance seen in the kidney, sweat and salivary glands, 
and the colonic mucosa. The condition does not spontaneously 
improve with age and is generally more severe than the autosomal 
domin

is generally more severe than the autosomal 
dominant form.
The underlying basis for the autosomal dominant form of PHA 
is explained on the basis of inactivating, dominant-negative muta­
tions in the MR (hMR, NR3C2).494,495 By contrast, inactivating 
mutations in the α-subunit and, to a lesser extent, in the β-subunit 
and γ-subunit of the epithelial sodium channel (ENaC) account 
for the generalized autosomal recessive form of mineralocorticoid 
resistance.496,497 (In effect, this represents t

resistance.496,497 (In effect, this represents the opposite of Liddle 
syndrome—see Chapter 16.) Generalized loss of ENaC activity 
leads to renal salt wasting (as seen in the renal form) in addition to 
recurrent respiratory infections and neonatal respiratory distress, 
cholelithiasis, and polyhydramnios.
PHA type I (PHA-I) is resistant to mineralocorticoid therapy, 
so standard treatment involves supplementation with salt (2–8 g/
day) in the form of sodium chloride and sodium bicarbonate as

 form of sodium chloride and sodium bicarbonate as 
well as cation exchange resins. This supplementation usually cor­
rects the patient’s biochemical imbalance. However, if a patient 
shows signs of severe hyperkalemia, peritoneal dialysis may be nec­
essary. Hypercalciuria has been reported in some cases of PHA-I. 
The recommended course of treatment for these patients usu­
ally involves indomethacin or hydrochlorothiazide. Indometha­
cin is thought to act by causing a reduction in the glomerul

ught to act by causing a reduction in the glomerular 
filtration rate or an inhibition of the effect of prostaglandin E2 
on renal tubules. Indomethacin has been shown to reduce poly­
uria, sodium loss, and hypercalciuria. Hydrochlorothiazide has 
been used to diminish hyperkalemia and reduce hypercalciuria in 
patients with PHA-I.
In patients with the autosomal dominant or renal form of 
PHA-I, the signs and symptoms of PHA decrease with age; never­
theless, these patients usually require salt 

ver­
theless, these patients usually require salt supplementation for the 
first 2 to 3 years of life. In patients with the autosomal recessive 
or multiorgan type of PHA-I, resistance to therapy with sodium 
chloride or drugs that decrease serum potassium concentrations 
often occurs and may even lead to death in infancy from hyperka­
lemia. PHA-I patients with multiorgan involvement often require 
very high amounts of salt in their diet (up to 45 g NaCl per day). 
Carbenoxolone, a derivative o

45 g NaCl per day). 
Carbenoxolone, a derivative of glycyrrhetinic acid in licorice, has 
been used with moderate success in helping to reduce the high lev­
els of dietary salt needed by renal PHA-I patients. Carbenoxolone 
acts by inhibiting 11βHSD2 activity and allows unmetabolized 
cortisol to bind to and activate MRs in a manner similar to that 
of aldosterone.498 Carbenoxolone was found to be ineffective in 
patients with multiorgan PHA-I.
Two other variants of PHA have been described—types

wo other variants of PHA have been described—types II and 
III. Type II PHA, or Gordon syndrome, is in retrospect a misno­
mer. Patients with Gordon syndrome share some of the features 
of patients with PHA-I, notably hyperkalemia and metabolic 
acidosis, but they exhibit salt retention with mild hypertension 
and suppressed plasma renin activity rather than salt wasting. The 
condition is explained by mutations in a serine threonine kinase 
family (WNK1 and WNK4) that result in increased expres

ly (WNK1 and WNK4) that result in increased expression 
of these proteins with activation of the thiazide-sensitive sodium 
chloride cotransporter in the cortical and medullary collecting 
ducts.499 The condition represents the exact opposite of Gitelman 
syndrome but is not a true form of PHA.
Type III PHA is an acquired and usually transient form of min­
eralocorticoid resistance seen in patients with underlying renal 
diseases, including obstruction and infection and in patients with 
excessi

uction and infection and in patients with 
excessive loss of salt through the gut or skin. Reduced glomerular 
filtration rate is a hallmark of the condition. The cause is unknown, 
although increased transforming growth factor-β–mediated aldo­
sterone resistance has been suggested to be an underlying factor. 
Hyporeninemic Hypoaldosteronism
Angiotensin II is a key stimulus to aldosterone secretion, and 
damage or blockade of the renin-angiotensin system may result 
in mineralocorticoid deficien

n system may result 
in mineralocorticoid deficiency. Various renal diseases have been 
associated with damage to the juxtaglomerular apparatus and 
subsequent renin deficiency. These include systemic lupus erythe­
matosus, myeloma, amyloid, AIDS, and damage related to use 
of nonsteroidal anti-inflammatory drugs, but the most common 
(>75% of cases) is diabetic nephropathy.500,501
The usual picture is that of an elderly patient with hyperkale­
mia, acidosis, and mild to moderate impairment of r

ia, acidosis, and mild to moderate impairment of renal function. 
Plasma renin activity and aldosterone levels are low and fail to 
respond to sodium depletion, erect posture, or furosemide admin­
istration. In contrast to adrenal insufficiency, patients have normal 
or elevated blood pressure and no postural hypotension. Muscle 
weakness and cardiac arrhythmias may also occur. Other factors 
may contribute to the hyperkalemia, including the use of potas­
sium-sparing diuretics, potassium supple

of potas­
sium-sparing diuretics, potassium supplementation, insulin defi­
ciency, and use of β-adrenoceptor blocking drugs or prostaglandin 
synthetase inhibitors, which inhibit renin release.
Treatment of primary renin deficiency is with fludrocortisone 
in the first instance together with dietary potassium restriction. 
However, these patients are not salt depleted and may become 
hypertensive with fludrocortisone. In such a scenario, the addition 
of a loop-acting diuretic, such as furosemid

tion 
of a loop-acting diuretic, such as furosemide, is appropriate. This 
will increase acid excretion and improve the metabolic acidosis. 
Adrenal Adenomas, Incidentalomas, and 
Carcinomas
Adenomas
Cortisol-secreting adrenal adenomas were discussed earlier, and 
aldosterone-secreting adenomas (Conn syndrome) are discussed 
in Chapter 16. Pure virilizing benign adrenal adenomas are very 
rare. Most cases occur in women; in males, the disorder is gen­
erally recognized in childhood, when present

 gen­
erally recognized in childhood, when presentation is with sexual 
precocity and accelerated bone age. Such tumors have to be con­
sidered in the differential diagnosis of CAH in patients present­
ing during childhood. In females, most patients present before 
menopause with marked hirsutism, deepening of the voice, and 
amenorrhea. Clitoromegaly is found in 80% of cases. Testosterone 
is usually strikingly elevated, but gonadotropin levels may not be 
suppressed. By definition, urinary fre

may not be 
suppressed. By definition, urinary free cortisol is normal. Tumors 
vary in size and should be treated surgically. Postoperatively, clini­
cal features invariably improve, and normal menses return.502 
Incidentalomas
Incidentally discovered adrenal masses have become a common 
clinical problem. An adrenal mass is uncovered in up to 4% of 
patients imaged for nonadrenal disease.503 Incidentalomas are 
uncommon in patients younger than 30 years of age but increase 
in frequency with ag

30 years of age but increase 
in frequency with age; they occur more frequent in females, most 
commonly in the sixth and seventh decades. Clinically, two issues 
540
arise: whether the lesion is functional (i.e., secreting hormones) 
and whether it is malignant. Most incidentalomas are adrenocor­
tical adenomas, but occasionally they represent myelolipomas, 
hamartomas, or granulomatous infiltrations of the adrenal gland 
and result in a 

filtrations of the adrenal gland 
and result in a characteristic CT/MRI appearance (Fig. 15.40). 
Functioning tumors with a clear clinical phenotype (pheochromo­
cytomas and those secreting cortisol, aldosterone, or sex steroids) 
and carcinomas account for around 4% of all incidentalomas. In 
addition, it is established that some incidentalomas cause abnor­
mal hormone secretion without obvious clinical manifestations 
of a hormone excess state. The best example is so-called subclini­
cal Cushi

 The best example is so-called subclini­
cal Cushing syndrome, which occurs in up to 20% to 30% of 
all patients with adrenocortical incidentalomas.503–505 There 
is debate as to the best means to biochemically define this phe­
nomenon, but serum cortisol after dexamethasone testing has 
the widest acceptance, although cutoff values vary. The European 
Society of Endocrinology and European Network for the Study of 
Adrenal Tumors (ENSAT) have published guidelines for diagnosis 
and management.50

lished guidelines for diagnosis 
and management.506 When there is evidence of low-grade excess 
biochemical hypercortisolism, there is an associated increase in the 
prevalence of diabetes, obesity, hypertension, new cardiovascular 
events, osteoporosis, and fatality. However, no prospective study 
has proven that the adrenal adenoma is the cause of the observed 
complications, as these are highly prevalent in the population at 
this age. Intervention by adrenalectomy has shown some benefit, 
pa

ntion by adrenalectomy has shown some benefit, 
particularly lower blood pressure,507 but the studies performed 
have been largely retrospective and highly selected, and hence the 
approach to each patient needs individualization with most being 
observed in current clinical practice.
As a result, all patients with incidentally discovered adrenal 
masses should undergo appropriate endocrine screening tests. 
This testing should comprise 24-hour urinary metanephrine col­
lection or measurement of

rinary metanephrine col­
lection or measurement of plasma metanephrines, an overnight 
dexamethasone suppression test, and for those with hypertension, 
measurement of plasma renin and aldosterone. DHEAS should 
be measured as a marker of adrenal androgen secretion. Low lev­
els may occur in patients with suppressed ACTH concentrations 
due to autonomous cortisol secretion from the adenoma.508 Some 
A
C
B
T1 MRI in phase
T1 MRI out of phase
• Fig. 15.40  (A) Adrenal incidentaloma (arrow) discove

. 15.40  (A) Adrenal incidentaloma (arrow) discovered in a woman undergoing investigation for abdomi­
nal pain. (B) Incidentally discovered right adrenal myelolipoma (arrow). (C) Chemical shift MRI of adre­
nal incidentaloma demonstrating signal dropout (arrow) for out-of-phase image, consistent with benign 
disease.
541
studies have also documented high levels of 17OHP after ACTH 
stimulation tests, suggesting partial defects in 21-hydroxylase in 
some tumors, and

ial defects in 21-hydroxylase in 
some tumors, and this should be measured in cases of bilateral 
disease.
The possibility of malignancy should be considered in each 
case. In patients with a known extra-adrenal primary tumor, the 
incidence of malignancy is obviously much higher; for example, 
up to 20% of patients with lung cancer have adrenal metastases 
on CT scanning. In those with no evidence of malignancy, adre­
nal carcinoma is rare.509 In true incidentalomas, size appears to 
be predict

n true incidentalomas, size appears to 
be predictive of malignancy: Fewer than 2% of incidentalomas 
smaller than 4 cm but 25% of those larger than 6 cm in diameter 
are malignant (Fig. 15.41).510 Smooth, homogeneous adenomas 
on an enhanced adrenal scan with a Hounsfield unit (HU) score (a 
marker of radiodensity) less than 10 HU or signal dropout on out-
of-phase MRI sequences (see Fig. 15.40) are invariably benign; 
malignancy is suspected in irregular, inhomogeneous adenomas 
with a score g

 irregular, inhomogeneous adenomas 
with a score greater than 20 HU. On this background, adrenal­
ectomy is considered for functional tumors and for tumors larger 
than 4 cm in diameter, although if the imaging characteristics are 
unequivocally benign tumors greater than this size may be left.506 
Repeat CT scanning in patients with smaller tumors can be used 
to guide management, but development of functional autonomy 
is very rare, and patients may be discharged if tumors are static. If 
surg

s may be discharged if tumors are static. If 
surgery is indicated, laparoscopic adrenalectomy is the treatment 
of choice, offering shorter hospital stays and reduced operative 
complications (e.g., blood loss, morbidity) compared with open 
adrenalectomy. The exception is the patient with highly suggestive 
invasive adrenal carcinoma, because breach of the tumor capsule 
is associated with a poorer outcome. Adequate preparation and 
close endocrine supervision perioperatively and postoperative

rine supervision perioperatively and postoperatively 
are required for functional tumors. 
Primary adrenal carcinoma is very rare, with an annual incidence of 
1 per 1 million population. Women are more commonly affected 
than men, with a ratio of 2.5:1. The mean age at onset is between 
40 and 50 years, although men tend to be older at presentation. 
Eighty percent of tumors are functional, most commonly secret­
ing glucocorticoids alone (45%), glucocorticoids and androgens 
(45%), o

one (45%), glucocorticoids and androgens 
(45%), or androgens alone (10%). Fewer than 1% of all tumors 
secrete aldosterone, and production of hormone precursors is 
common.511 Patients present with features of the hormone excess 
state (glucocorticoid, androgen, or both), but abdominal pain, 
weight loss, anorexia, and fever occur in 25% of cases. An abdomi­
nal mass may be palpable. Current treatment choices for what is 
often an aggressive tumor are poor. Surgery offers the only chance 
of cu

or are poor. Surgery offers the only chance 
of cure for patients with local disease, but metastatic spread is evi­
dent in 75% of cases at presentation. Radiotherapy is ineffective, 
as are most chemotherapeutic regimens. Mitotane in high dose, 
in conjunction with therapeutic drug monitoring, offers benefit 
in reducing tumor growth286 and in controlling hormonal hyper­
secretion in 75% of cases.285 Overall, the prognosis is poor, with 
5-year survival rates of less than 20%. Newer chemotherap

survival rates of less than 20%. Newer chemotherapies are 
being evaluated.287 
Etiology of Adrenal Tumors
Other than the factors discussed under adrenal Cushing syn­
drome, the underlying basis for adrenal tumorigenesis is 
unknown. Clonal analysis suggests progression from a normal 
to an adenomatous to a carcinomatous lesion, but the molecu­
lar pathways involved remain obscure. Several factors have been 
associated with malignant transformation, including the genes 
encoding p53, p57, cyclin

on, including the genes 
encoding p53, p57, cyclin-dependent kinase, menin, IGF2, 
MC2R, and inhibin-α.512 Mice lacking the inhibin-α gene 
develop adrenal tumors through a process that is also gonadotro­
pin dependent.513
Acknowledgment is given to Professor Paul Stewart, the previous 
coauthor of this chapter, and to others who have assisted in the 
past: Professors W. Arlt, M.S. Cooper, J.W. Tomlinson, and W. 
Young.
The complete list of references is available on l

The complete list of references is available on line at ExpertConsult.com.
Adenoma
Adrenal carcinoma
Adrenal cyst
Ganglioneuroma
Myelolipoma
Other
Other
Adenoma
100
90
80
70
60
50
40
30
20
10
0
<2-4
cm
4.1-6
cm
>6
cm
Total
52%
2%
6%
8%
4%
5%
11%
12%
Percent
A
B
• Fig. 15.41  Distribution of diagnosis of adrenal incidentalomas. (A) Data 
from eight studies with histologically determi

ta 
from eight studies with histologically determined diagnoses (n = 103) relat­
ing to tumor size. (B) Distribution of 380 incidental adrenal masses by 
histologic diagnosis. (From Mansmann G, Lau L, Balk E, et al. The clinically 
inapparent adrenal mass: update in diagnosis and management. Endocr 
Rev. 2004;25:309–340.)
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21-hydroxylase deficiency and/or aromatase deficiency: diagnos­
tic value of urine steroid hormone analysis. Pediatr Res. 2006;59:
276–280.
	469.	 Fluck CE, Tajima T, Pandey AV, et al. Mutant P450 oxidoreduc­
tase causes disordered steroidogenes

0 oxidoreduc­
tase causes disordered steroidogenesis with and without Antley-
Bixler syndrome. Nat Genet. 2004;36:228–230.
	470.	 Arlt W, Walker EA, Draper N, et al. Congenital adrenal hyperpla­
sia caused by mutant P450 oxidoreductase and human androgen 
synthesis: analytical study. Lancet. 2004;363:2128–2135.
	471.	 Homma K, Hasegawa T, Nagai T, et al. Urine steroid hormone 
profile analysis in cytochrome P450 oxidoreductase deficiency: 
implication for the backdoor pathway to dihydrotestost

ication for the backdoor pathway to dihydrotestosterone. J 
Clin Endocrinol Metab. 2006;91:2643–2649.
	472.	 Fukami M, Nishimura G, Homma K, et  al. Cytochrome P450 
oxidoreductase deficiency: identification and characterization of 
biallelic mutations and genotype-phenotype correlations in 35 
Japanese patients. J Clin Endocrinol Metab. 2009;94:1723–1731.
	473.	 Huang N, Pandey AV, Agrawal V, et al. Diversity and function 
of mutations in p450 oxidoreductase in patients with Antley-
Bixler sy

 oxidoreductase in patients with Antley-
Bixler syndrome and disordered steroidogenesis. Am J Hum Genet. 
2005;76:729–749.
	474.	 Otto DM, Henderson CJ, Carrie D, et al. Identification of novel 
roles of the cytochrome p450 system in early embryogenesis: effects 
on vasculogenesis and retinoic acid homeostasis. Mol Cell Biol. 
2003;23:6103–6116.
	475.	 Rheaume E, Simard J, Morel Y, et al. Congenital adrenal hyperpla­
sia due to point mutations in the type II 3β-hydroxysteroid dehy­
drogenase g

 2005;90:1287–1293.
	480.	 Bose HS, Sugawara T, Strauss JF 3rd, et al. The pathophysiology 
and genetics of congenital lipoid adrenal hyperplasia. International 
Congenital Lipoid Adrenal Hyperplasia Consortium. N Engl J 
Med. 1996;335:1870–1878.
	481.	 Baker BY, Lin L, Kim CJ, et al. Nonclassic congenital lipoid adre­
nal hyperplasia: a new disorder of the steroidogenic acute regula­
tory protein with very late presentation and normal male genitalia. 
J Clin Endocrinol Metab. 2006;91:4781–478

italia. 
J Clin Endocrinol Metab. 2006;91:4781–4785.
	482.	 Tajima T, Fujieda K, Kouda N, et al. Heterozygous mutation in the 
cholesterol side chain cleavage enzyme (p450scc) gene in a patient 
with 46,XY sex reversal and adrenal insufficiency. J Clin Endocrinol 
Metab. 2001;86:3820–3825.
	483.	 Hiort O, Holterhus PM, Werner R, et  al. Heterozygous muta­
tion in the cholesterol side chain cleavage enzyme (p450scc) gene 
in a patient with 46,XY sex. J Clin Endocrinol Metab. 2005;90:
538–541.
	
Clin Endocrinol 
Metab. 2009;94:936–939.
	486.	 Jamieson A, Wallace AM, Andrew R, et  al. Apparent cortisone 
reductase deficiency: a functional defect in 11β-hydroxysteroid 
dehydrogenase type 1. J Clin Endocrinol Metab. 1999;84:
3570–3574.
	487.	 Lavery GG, Walker EA, Tiganescu A, et al. Steroid biomarkers and 
genetic studies reveal inactivating mutations in hexose-6-phosphate 
dehydrogenase in patients with cortisone reductase deficiency. J 
Clin Endocrinol Metab. 2008;93:3827–3832.
	488.	

cause autosomal dominant 
pseudohypoaldosteronism type I. Nat Genet. 1998;19:279–281.
	496.	 Chang SS, Grunder S, Hanukoglu A, et al. Mutations in subunits 
of the epithelial sodium channel cause salt wasting with hyper­
kalaemic acidosis, pseudohypoaldosteronism type 1. Nat Genet. 
1996;12:248–253.
	497.	 Strautnieks SS, Thompson RJ, Gardiner RM, et al. A novel splice-
site mutation in the gamma subunit of the epithelial sodium chan­
nel gene in three pseudohypoaldosteronism type 1 families. 

ssnacht M, Arlt W, Bancos I, et  al. Management of adrenal 
incidentalomas: European society of endocrinology clinical prac­
tice guideline in collaboration with the European Network for the 
study of adrenal tumors. Eur J Endocrinol. 2016;175(2):G1–G34.
	507.	 Chiodini I, Morelli V, Salcuni AS, et al. Beneficial metabolic effects 
of prompt surgical treatment in patients with an adrenal inciden­
taloma causing biochemical hypercortisolism. J Clin Endocrinol 
Metab. 2010;95(6):2736–2745.
	508.	

in Endocrinol 
	508.	 Flecchia D, Mazza E, Carlini M, et al. Reduced serum levels of dehy­
droepiandrosterone sulphate in adrenal incidentalomas: a marker of 
adrenocortical tumour. Clin Endocrinol. 1995;42:129–134.
	509.	 Cawood TJ, Hunt PJ, O’Shea D, et al. Recommended evaluation 
of adrenal incidentalomas is costly, has high false-positive rates and 
confers a risk of fatal cancer that is similar to the risk of the adrenal 
lesion becoming malignant: time for a 

e Bouc Y, Luton JP, et  al. Pathogenesis and treat­
ment of adrenocortical carcinoma. Curr Opin Endocrinol Diab. 
1998;5:189–196.
	513.	 Matzuk MM, Finegold MJ, Mather JP, et al. Development of can­
cer cachexia-like syndrome and adrenal tumors in inhibin-deficient 
mice. Proc Natl Acad Sci U S A. 1994;91:8817–8821.
542
16
Endocrine Hypertension
WILLIAM F. YOUNG, JR.
KEY POINTS
 • 
There are at least 14 endocrine disorders for which hypertension 
may be the initial clinical presentation. An ac

n 
may be the initial clinical presentation. An accurate diagnosis 
of endocrine hypertension provides the clinician with a unique 
treatment opportunity: to render a surgical cure or to achieve a 
dramatic response with pharmacologic therapy.
 • 
Catecholamines affect many cardiovascular and metabolic 
processes—they increase heart rate, blood pressure, myo­
cardial contractility, and cardiac conduction velocity. The 
identification of three types of adrenergic receptors (α, β, and 
dopaminerg

pes of adrenergic receptors (α, β, and 
dopaminergic receptors) and their subtypes (α1, α2, β1, β2, 
β3, D1, and D2) has led to understanding of the physiologic 
responses to endogenous and exogenous administration of 
catecholamines.
 • 
Catecholamine-secreting tumors are rare, with an annual 
incidence of 2 to 8 cases per 1 million people. Neverthe­
less, it is important to suspect, confirm, localize, and resect 
these tumors because (1) the associated hypertension is 
curable with surgical r

ssociated hypertension is 
curable with surgical removal of the tumor, (2) a risk of lethal 
paroxysm exists, (3) at least 10% of the tumors are malignant, 
and (4) 40% of these tumors are familial and their detection 
in the proband may result in early diagnosis in other family 
members.
 • 
Germline mutations are responsible for approximately 40% of 
all catecholamine-secreting tumors. Mutations contributing 
to pheochromocytoma and paraganglioma have two general 
transcription signatures: cl

oma have two general 
transcription signatures: cluster 1—genes encoding proteins 
that function in the cellular response to hypoxia, and cluster 2—
genes encoding proteins that activate kinase signaling.
 • 
Pheochromocytoma/paraganglioma must be confirmed 
biochemically by the presence of increased concentrations of 
fractionated metanephrines and catecholamines in urine or 
plasma. Then the tumor should be localized with computed 
tomography (CT) of the abdomen and pelvis. Approximately 
85%

(CT) of the abdomen and pelvis. Approximately 
85% of these tumors are found in the adrenal glands, and 95% 
are found in the abdomen and pelvis.
 • 
Some form of preoperative pharmacologic preparation is indi­
cated for all patients with catecholamine-secreting neoplasms, 
including those who are asymptomatic and normotensive. Sur­
gical resection of a catecholamine-secreting tumor is a high-risk 
surgical procedure, and an experienced surgeon-anesthesiolo­
gist team is required.
 • 
Hyperten

anesthesiolo­
 • 
Hypertension, suppressed plasma renin activity, and increased 
aldosterone excretion characterize the syndrome of primary 
aldosteronism, which was first described in 1955.
 • 
Aldosterone-producing adenoma and bilateral idiopathic 
­hyperaldosteronism are the most common subtypes of primary 
aldosteronism. Somatic mutations account for the majority of 
aldosterone-producing adenomas and include mutations in 
genes encoding components of the potassium c

s in 
genes encoding components of the potassium channel (KCNJ5), 
the ­sodium/potassium and calcium adenosine triphosphatases 
­(ATPases) (ATP1A1 and ATP2B3), a voltage-dependent C-type 
calcium channel (CACNA1D), and β-catenin–activating CTNNB1 
mutations.
 • 
Use of the plasma aldosterone to plasma renin activity ratio 
as a case-detection test, followed by aldosterone suppression 
for confirmatory testing, has resulted in prevalence estimates 
for primary aldosteronism of 5% to 10% of all p

s 
for primary aldosteronism of 5% to 10% of all patients with 
­hypertension.
 • 
The treatment goal in patients with primary aldosteronism is 
to prevent the morbidity and fatality associated with hyperten­
sion, hypokalemia, nephrotoxicity, and cardiovascular dam­
age. Knowing the cause of the primary aldosteronism helps to 
determine the appropriate treatment. Normalization of blood 
pressure should not be the only goal—normalization of circulat­
ing aldosterone or mineralocorticoid recepto

ulat­
ing aldosterone or mineralocorticoid receptor blockade should 
be part of the management plan for all patients with primary 
aldosteronism.
Adrenal Medulla and Catecholamines, 543
Pheochromocytoma and Paraganglioma, 545
Renin-Angiotensin-Aldosterone System, 559
Primary Aldosteronism, 561
Other Forms of Mineralocorticoid Excess or Effect, 570
Other Endocrine Disorders Associated With Hypertension, 571
543
Chapter 16  Endocrine Hypertension
A
n estimated 68 million people in 

e Hypertension
A
n estimated 68 million people in the United States are 
hypertensive.1,2 In most, hypertension is primary (i.e., 
essential or idiopathic), but a subgroup of approximately 
15% have secondary ­hypertension. The secondary causes of hyper­
tension can be divided into renal causes, such as renal parenchy­
mal or renovascular disease, and endocrine causes. There are at 
least 14 endocrine disorders for which hypertension may be the 
initial clinical presentation (Table 16.1). An acc

initial clinical presentation (Table 16.1). An accurate diagnosis of 
endocrine hypertension provides the clinician with a unique treat­
ment opportunity: to render a surgical cure or to achieve a dra­
matic response with pharmacologic therapy. The diagnostic and 
therapeutic approaches to endocrine hypertension—ranging from 
the classic adrenal causes of hypertension (e.g., pheochromocy­
toma, primary aldosteronism) to pituitary-dependent hyperten­
sion (e.g., Cushing syndrome, acromegaly)—are 

en­
sion (e.g., Cushing syndrome, acromegaly)—are reviewed in this 
chapter.
Adrenal Medulla and Catecholamines
The adrenal medulla occupies the central portion of the adrenal 
gland and accounts for 10% of total adrenal gland volume. There 
is no clear demarcation between the adrenal cortex and the adrenal 
medulla. The adrenal glands derive blood supply from the supe­
rior, middle, and inferior branches of the inferior phrenic artery; 
from the renal arteries; and directly from the aorta. The 

 renal arteries; and directly from the aorta. The adrenal 
arteries branch and form a plexus under the capsule. This plexus 
supplies the cortex. Some of the plexus arteries penetrate the cor­
tex and supply the medulla, as do capillaries draining the corti­
cal cells, forming the corticomedullary portal system. The right 
adrenal vein is short and drains directly into the inferior vena cava 
(IVC). The left adrenal vein merges with the inferior phrenic vein, 
and this larger vein (the common ph

phrenic vein, 
and this larger vein (the common phrenic vein) drains into the 
left renal vein.
Adrenomedullary cells are called chromaffin cells (stain brown 
with chromium salts) or pheochromocytes. Cytoplasmic granules 
turn dark when stained with chromic acid because of the oxida­
tion of epinephrine and norepinephrine to melanin. Chromaffin 
cells differentiate in the center of the adrenal gland in response to 
cortisol; some chromaffin cells also migrate to form paraganglia, 
collections o

s also migrate to form paraganglia, 
collections of chromaffin cells located on both sides of the aorta. 
The largest cluster of chromaffin cells outside the adrenal medulla 
is located near the level of the inferior mesenteric artery and is 
referred to as the organ of Zuckerkandl; it is quite prominent in 
the fetus and is a major source of catecholamines during the first 
year of life. The preganglionic sympathetic neurons receive synap­
tic input from neurons within the pons, medulla, and hy

nput from neurons within the pons, medulla, and hypothala­
mus, providing regulation of sympathetic activity by the brain. 
Axons from the lower thoracic and lumbar preganglionic neurons, 
via splanchnic nerves, directly innervate the cells of the adrenal 
medulla.
The term catecholamine refers to substances that contain cat­
echol (ortho-dihydroxybenzene) and a side chain with an amino 
group—the catechol nucleus (Fig. 16.1).3 Epinephrine is syn­
thesized and stored in the adrenal medulla and r

­
thesized and stored in the adrenal medulla and released into the 
systemic circulation. Norepinephrine is synthesized and stored 
not only in the adrenal medulla but also in the peripheral sym­
pathetic nerves. Dopamine, the precursor of norepinephrine, is 
found in the adrenal medulla and peripheral sympathetic nerves 
and acts primarily as a neurotransmitter in the central nervous 
system.
Catecholamines affect many cardiovascular and metabolic pro­
cesses. They increase the heart rate, bloo

ic pro­
cesses. They increase the heart rate, blood pressure, myocardial 
contractility, and cardiac conduction velocity. Activation of G 
protein–coupled receptors mediates the biologic actions of cat­
echolamines. The identification of three types of adrenergic recep­
tors (α, β, and dopaminergic receptors) and their subtypes (α1, α2, 
β1, β2, β3, D1, and D2) has led to understanding of the physiologic 
responses to endogenous and exogenous administration of cate­
cholamines.4 The 2012 Nobel P

inistration of cate­
cholamines.4 The 2012 Nobel Prize in Chemistry was awarded 
to Brian K. Kobilka and Robert J. Lefkowitz for their studies of 
G protein–coupled receptors. The α1 subtype is a postsynaptic 
receptor that mediates vascular and smooth muscle contraction; 
stimulation causes vasoconstriction and increases blood pressure. 
The α2 receptors are located on presynaptic sympathetic nerve 
endings; when activated, they inhibit release of norepinephrine. 
Stimulation causes suppression

of norepinephrine. 
Stimulation causes suppression of central sympathetic outflow 
and decreased blood pressure.
There are three major β-receptor subtypes. The β1 receptor 
mediates cardiac effects and is more responsive to isoproterenol 
than to epinephrine or norepinephrine. Stimulation causes posi­
tive inotropic and chronotropic effects on the heart, increased 
renin secretion in the kidney, and lipolysis in adipocytes. The β2 
receptor mediates bronchial, vascular, and uterine smooth muscle

tes bronchial, vascular, and uterine smooth muscle 
relaxation. Stimulation causes bronchodilation, vasodilatation in 
skeletal muscle, glycogenolysis, and increased release of norepi­
nephrine from sympathetic nerve terminals. The β3 receptor regu­
lates energy expenditure and lipolysis.
D1 receptors are localized to the cerebral, renal, mesenteric, 
and coronary vasculatures; stimulation causes vasodilation in these 
vascular beds. D2 receptors are presynaptic; they are localized to 
sympathet

 are presynaptic; they are localized to 
sympathetic nerve endings, sympathetic ganglia, and the brain. 
  Endocrine Causes of Hypertension
Adrenal-Dependent Causes
Primary aldosteronism
Hyperdeoxycorticosteronism
Congenital adrenal hyperplasia
11β-Hydroxylase deficiency
17α-Hydroxylase deficiency
Deoxycorticosterone-producing tumor
Primary cortisol resistance
AME/11βHSD Deficiency
Genetic
Type 1 AME
Type 2 AME
Acquired
Licorice or carbenoxolone ingestion (type

Acquired
Licorice or carbenoxolone ingestion (type 1 AME)
Cushing syndrome (type 2 AME)
Thyroid-Dependent Causes
Renin-Secreting Tumor
Pituitary-Dependent Causes
AME, Apparent mineralocorticoid excess; HSD, hydroxysteroid dehydrogenase.
   
TABLE 16.1
544
SECTION IV    Adrenal Cortex and Endocrine Hypertension
Stimulation of D2 receptors in these locations inhibits the release 
of norepinephrine, inhibits ganglionic transmission, and inh

ephrine, inhibits ganglionic transmission, and inhibits 
prolactin release, respectively.
Most cells in the body have adrenergic receptors. The phar­
macologic development of selective α-adrenergic and β-adrenergic 
agonists and antagonists has advanced pharmacotherapy for vari­
ous clinical disorders. For example, β1 antagonists (e.g., ateno­
lol, metoprolol) are now considered standard therapies for angina 
pectoris, hypertension, and cardiac arrhythmias.5 Administra­
tion of β2 agonists (e.g.

rrhythmias.5 Administra­
tion of β2 agonists (e.g., terbutaline, albuterol) causes bronchial 
smooth muscle relaxation; these agents are commonly prescribed 
in inhaled formulations for the treatment of asthma.6
Catecholamine Synthesis
Catecholamines are synthesized from tyrosine by a process of 
hydroxylation and decarboxylation (see Fig. 16.1). Tyrosine 
is derived from ingested food or synthesized from phenylala­
nine in the liver, and it enters neurons and chromaffin cells 
by active transpo

rs neurons and chromaffin cells 
by active transport. Tyrosine is converted to 3,4-dihydroxyphe­
nylalanine (DOPA) by tyrosine hydroxylase, the rate-limiting 
step in catecholamine synthesis. Increased intracellular levels of 
catechols downregulate the activity of tyrosine hydroxylase; as 
catecholamines are released from secretory granules in response 
to a stimulus, cytoplasmic catecholamines are depleted and the 
feedback inhibition of tyrosine hydroxylase is released. Tran­
scription of tyr

ne hydroxylase is released. Tran­
scription of tyrosine hydroxylase is stimulated by glucocorti­
coids, cyclic adenosine monophosphate (cAMP)–dependent 
protein kinases, calcium/phospholipid-dependent protein 
kinase, and calcium/calmodulin-dependent protein kinase. 
α-Methyl-paratyrosine (metyrosine) is a tyrosine hydroxylase 
inhibitor that may be used therapeutically in patients with cat­
echolamine-secreting neoplasms to decrease tumoral synthesis 
of catecholamines.7
Aromatic l-amino acid d

hesis 
Aromatic l-amino acid decarboxylase catalyzes the decar­
boxylation of DOPA to dopamine (see Fig. 16.1). Dopamine is 
actively transported into granulated vesicles to be hydroxylated 
to norepinephrine by the copper-containing enzyme dopamine 
β-hydroxylase. Ascorbic acid is a cofactor and electron donor. 
The enzyme is structurally similar to tyrosine hydroxylase and 
may share similar transcriptional regulatory elements, and both 
are stimulated by glucocorticoids an

ts, and both 
are stimulated by glucocorticoids and cAMP-dependent kinases. 
These reactions occur in the synaptic vesicle of adrenergic neurons 
in the central nervous system, the peripheral nervous system, and 
the chromaffin cells of the adrenal medulla. The major constitu­
ents of the granulated vesicle are dopamine β-hydroxylase, ascor­
bic acid, chromogranin A, and adenosine triphosphate (ATP).
In the adrenal medulla, norepinephrine is released from the 
granule into the cytoplasm, where t

ased from the 
granule into the cytoplasm, where the cytosolic enzyme phenyl­
ethanolamine N-methyltransferase (PNMT) converts it to epi­
nephrine (see Fig. 16.1). Epinephrine is then transported back 
into another storage vesicle. The N-methylation reaction by 
PNMT involves S-adenosylmethionine as the methyl donor as 
well as oxygen and magnesium. PNMT expression is regulated by 
the presence of glucocorticoids, which are in high concentration in 
the adrenal medulla through the corticomedulla

in 
the adrenal medulla through the corticomedullary portal system. 
Therefore, catecholamine-secreting tumors that secrete primarily 
epinephrine are localized to the adrenal medulla. In normal adre­
nal medullary tissue, approximately 80% of the catecholamines 
released are epinephrine. 
Catecholamine Storage and Secretion
Catecholamines are found in the adrenal medulla and in sympa­
thetically innervated organs. Catecholamines are stored in elec­
tron-dense granules that also contain ATP, neu

ec­
tron-dense granules that also contain ATP, neuropeptides (e.g., 
adrenomedullin, corticotropin [i.e., adrenocorticotropic hormone 
(ACTH)], vasoactive intestinal polypeptide), calcium, magne­
sium, and chromogranins. Uptake into the storage vesicles is 
facilitated by active transport by vesicular monoamine transport­
ers (VMAT).8 The VMAT ATP-driven pump maintains a steep 
electrical gradient. For every monoamine transported, ATP is 
hydrolyzed, and two hydrogen ions are transported from th

zed, and two hydrogen ions are transported from the ves­
icle into the cytosol. Iodine-123 (123I) and 131I-labeled metaiodo­
benzylguanidine (MIBG) are imported by VMAT into the storage 
vesicles in the adrenal medulla, which makes 123I-MIBG useful 
for imaging localization of catecholamine-secreting tumors and 
131I-MIBG potentially useful in treating malignant catechol­
amine-secreting tumors.9–11 Catecholamine uptake, as well as that 
of MIBG, is inhibited by reserpine. The catecholamine stor

 is inhibited by reserpine. The catecholamine stores are 
dynamic, with constant leakage and reuptake.8
Stressful stimuli (e.g., myocardial infarction, anesthesia, hypo­
glycemia) trigger adrenal medullary catecholamine secretion. 
Acetylcholine from preganglionic sympathetic fibers stimulates 
Tyrosine
DOPA
Dopamine
Catechol
OH
TH
DBH
AADC
PNMT
HCH
HC – COOH
OH
OH
HO
NH2
OH
HCH
HC – COOH
NH2
HO
OH
HCH
HCH
NH2
HO
OH
H
OH
C
H
OH
C
HCH
NH2
HO
OH
H
H
C
HN – CH3
• Fig. 16.

H
C
H
OH
C
HCH
NH2
HO
OH
H
H
C
HN – CH3
• Fig. 16.1  Biosynthetic pathway for catecholamines. The term catecholamine comes from the catechol 
(ortho-dihydroxybenzene) structure and a side chain with an amino group—the catechol nucleus (shown 
on left). Tyrosine is converted to 3,4-dihydroxyphenylalanine (DOPA) by tyrosine hydroxylase (TH); this 
rate-limiting step provides the clinician with the option to treat pheochromocytoma with a TH inhibitor, 
α-methyl-paratyrosine (metyrosine). Aromatic l

r, 
α-methyl-paratyrosine (metyrosine). Aromatic l-amino acid decarboxylase (AADC) converts DOPA to 
dopamine. Dopamine is hydroxylated to norepinephrine by dopamine β-hydroxylase (DBH). Norepineph­
rine is converted to epinephrine by phenylethanolamine N-methyltransferase (PNMT). Cortisol induces the 
expression of PNMT, which explains why epinephrine-secreting neoplasms are almost exclusively localized 
to the adrenal medulla.
545
nicotinic cholinergic recept

ndocrine Hypertension
nicotinic cholinergic receptors and causes depolarization of adre­
nomedullary chromaffin cells. Depolarization leads to activation 
of voltage-gated calcium channels, which results in exocytosis of 
secretory vesicle contents. A calcium-sensing receptor appears to 
be involved in the process of exocytosis. During exocytosis, all the 
granular contents are released into the extracellular space. Nor­
epinephrine modulates its own release by activating the α2 recep­
tors on t

 own release by activating the α2 recep­
tors on the presynaptic membrane. Stimulation of the presynaptic 
α2 receptors inhibits norepinephrine release (the mechanism of 
action of some antihypertensive medications such as clonidine 
and guanfacine). Catecholamines are among the shortest lived 
signaling molecules in plasma; the initial biologic half-life of cir­
culating catecholamines is between 10 and 100 seconds. Approxi­
mately one-half of the catecholamines circulate in plasma in loose 
as

he catecholamines circulate in plasma in loose 
association with albumin. Therefore plasma concentrations of cat­
echolamines fluctuate widely. 
Catecholamine Metabolism and Inactivation
Catecholamines are removed from the circulation either by reup­
take in sympathetic nerve terminals or by metabolism through 
two enzyme pathways (Fig. 16.2), followed by sulfate conjugation 
and renal excretion. Most of the metabolism of catecholamines 
occurs in the same cell in which they are synthesized.8 Al

n the same cell in which they are synthesized.8 Almost 
90% of catecholamines released at sympathetic synapses are taken 
up locally by the nerve endings, termed uptake 1. Uptake 1 can 
be blocked by cocaine, tricyclic antidepressants, and phenothi­
azines. Extraneuronal tissues also take up catecholamines, and this 
is termed uptake 2. Most of these catecholamines are metabolized 
by catechol-O-methyltransferase (COMT).
Although COMT is found primarily outside neural tissue, 
O-methylation in t

imarily outside neural tissue, 
O-methylation in the adrenal medulla is the predominant source 
of metanephrine (COMT converts epinephrine to metaneph­
rine) and a major source of normetanephrine (COMT converts 
norepinephrine to normetanephrine) through methylation of the 
3-hydroxy group.8 S-Adenosylmethionine is used as the methyl 
donor, and calcium is required for this enzymatic step. Metaneph­
rine and normetanephrine are oxidized by monoamine oxidase 
(MAO) to vanillylmandelic acid (VMA) 

ine oxidase 
(MAO) to vanillylmandelic acid (VMA) by oxidative deamina­
tion. MAO may also oxidize epinephrine and norepinephrine to 
3,4-dihydroxymandelic acid, which is then converted by COMT 
to VMA. MAO is located on the outer membrane of mitochon­
dria. In the storage vesicle, norepinephrine is protected from 
metabolism by MAO. MAO and COMT metabolize dopamine 
to homovanillic acid (see Fig. 16.2). 
Pheochromocytoma and Paraganglioma
Catecholamine-secreting tumors that arise from chromaffi

olamine-secreting tumors that arise from chromaffin cells 
of the adrenal medulla and the sympathetic ganglia are referred to 
as pheochromocytomas and catecholamine-secreting paragangliomas, 
respectively.12 Because the tumors have similar clinical presenta­
tions and are treated with similar approaches, many clinicians use 
the term pheochromocytoma to refer to both adrenal pheochro­
mocytomas and extra-adrenal catecholamine-secreting paragan­
gliomas. However, the distinction between pheochro

gliomas. However, the distinction between pheochromocytoma 
and paraganglioma is an important one because of implications 
for associated neoplasms, risk for malignancy, and genetic testing. 
Catecholamine-secreting tumors are rare, with an annual inci­
dence of 2 to 8 cases per 1 million people.13 Based on screening 
Dopamine
Dihydroxymandelic
acid
HO
MAO
MAO
MAO
MAO
COMT
MAO
CH
CH2
NHCH3
OH
OH
OH
HO
HO
CH
C
O
OH
C
O
OH
OH
HO
Dihydroxyphenyl-
acetic acid
HO
CH2
C
O
OH

H
OH
HO
HO
CH2
C
O
OH
HO
3-Methoxytyramine
CH3O
CH2
CH2
NH2
HO
COMT
COMT
COMT
HO
CH
CH2
NH2
OH
HO
HO
CH2
CH2
NH2
HO
Homovanillic acid (HVA)
COMT
MAO
CH3O
CH2
HO
CH2
NH2
Normetanephrine
CH3O
CH
HO
OH
CH2
NHCH3
Metanephrine
CH3O
CH
HO
C
O
OH
Vanillylmandelic acid (VMA)
CH3O
CH
HO
• Fig. 16.2  Catecholamine metabolism. Metabolism of catecholamines occurs through two enzymatic 
pathways. Catechol-O-methyltransferase (COMT) converts epinephrine to metanephrine and convert

) converts epinephrine to metanephrine and converts 
norepinephrine to normetanephrine through meta-O-methylation. Metanephrine and normetanephrine are 
oxidized by monoamine oxidase (MAO) to vanillylmandelic acid (VMA) by oxidative deamination. MAO also 
may oxidize epinephrine and norepinephrine to dihydroxymandelic acid, which is then converted by COMT 
to VMA. Dopamine is also metabolized by MAO and COMT to the final metabolite, homovanillic acid (HVA).
546
SECTION IV    Adrenal Cortex and E

acid (HVA).
546
studies for secondary causes of hypertension in outpatients, the 
prevalence of pheochromocytoma has been estimated at 0.1% to 
0.6%.14,15 Nevertheless, it is important to suspect, confirm, local­
ize, and resect these tumors because (1) the associated hypertension 
is curable with surgical removal of the tumor, (2) a risk of lethal 
paroxysm exists, (3) at least 10% of the tumors are malignant, and 
(4) 40% of these tumors 

umors are malignant, and 
(4) 40% of these tumors are familial, and their detection in the 
proband may result in early diagnosis in other family members.
History
The association between adrenal medullary tumors and symptoms 
was first recognized by Fränkel in 1886.16 He described Fraulein 
Minna Roll, age 18 years, who had intermittent attacks of palpita­
tion, anxiety, vertigo, headache, chest pain, cold sweats, and vom­
iting. She had a hard, noncompressible pulse and retinitis. Despite 
cham

noncompressible pulse and retinitis. Despite 
champagne therapy and injections of ether, she died. At autopsy, 
bilateral adrenal tumors were initially thought to be angiosarco­
mas, but later a positive chromaffin reaction confirmed pheochro­
mocytoma. A subsequent study published in 2007 documented 
the presence of a germline RET proto-oncogene mutation in four 
living relatives of Fraulein Roll—proving that the original patient 
and her family had multiple endocrine neoplasia type 2 (MEN2).17

 had multiple endocrine neoplasia type 2 (MEN2).17
The term pheochromocytoma, proposed by Pick in 1912,18 comes 
from the Greek words phaios (dusky), chroma (color), and cytoma 
(tumor)—words that describe the dark staining reaction that is 
caused by the oxidation of intracellular catecholamines when they 
are exposed to dichromate salts. In 1926, César Roux in Lausanne, 
Switzerland, and Charles Mayo in Rochester, Minnesota, suc­
cessfully surgically removed abdominal catecholamine-secreting 

ically removed abdominal catecholamine-secreting 
tumors.19,20 In 1929, it was discovered that a pheochromocytoma 
contained an excess amount of a pressor agent. Subsequently, 
epinephrine (in 1936) and norepinephrine (in 1949) were iso­
lated from pheochromocytoma tissue. In 1950, it was found that 
patients with pheochromocytoma excreted increased amounts of 
epinephrine, norepinephrine, and dopamine in the urine.21 
Catecholamine-secreting tumors occur with equal frequen

holamine-secreting tumors occur with equal frequency in 
men and women, primarily in the third, fourth, and fifth decades. 
These tumors are rare in children, and when discovered, they may 
be multifocal and associated with a hereditary syndrome. The 
symptoms, listed in Table 16.2, are caused by the pharmacologic 
effects of excess concentrations of circulating catecholamines. The 
associated hypertension may be sustained or paroxysmal, and 
patients whose pheochromocytoma is diagnosed in the p

ients whose pheochromocytoma is diagnosed in the presymp­
tomatic stage may have normal blood pressure. The lability in 
blood pressure can be attributed to episodic release of catechol­
amines, chronic volume depletion, and impaired sympathetic 
reflexes. In addition to volume depletion, altered sympathetic 
vascular regulation may have a role in orthostasis, which may 
be observed in patients with pheochromocytoma. Symptoms of 
orthostatic hypotension (e.g., lightheadedness, presyncope, syn­
c

tension (e.g., lightheadedness, presyncope, syn­
cope) may dominate the presentation, especially in patients with 
epinephrine-predominant or dopamine-predominant tumors.
Episodic symptoms may occur in spells, or paroxysms, that can 
be extremely variable in presentation but typically include forceful 
heartbeat, pallor, tremor, headache, and diaphoresis.22 The spell may 
start with a sensation of a “rush” in the chest and a sense of shortness 
of breath, followed by a forceful heartbeat and a t

f breath, followed by a forceful heartbeat and a throbbing head­
ache. Peripheral vasoconstriction associated with a spell results in 
cool or cold hands and feet and facial pallor. Increased sense of body 
heat and sweating are common symptoms that occur toward the end 
of the spell. Spells may be either spontaneous or precipitated by pos­
tural change, anxiety, medications (e.g., β-adrenergic antagonists, 
metoclopramide, anesthetic agents), exercise, or maneuvers that 
increase intra-abdomina

ercise, or maneuvers that 
increase intra-abdominal pressure (e.g., change in position, lifting, 
defecation, exercise, colonoscopy, pregnancy, trauma). Although the 
types of spells experienced across the patient population are highly 
variable, spells tend to be stereotypical for each patient. Spells may 
occur multiple times daily or as infrequently as once monthly. The 
typical duration of a pheochromocytoma spell is 15 to 20 minutes, 
but it may be much shorter or last several hours. Howeve

 may be much shorter or last several hours. However, the cli­
nician must recognize that most patients with spells do not have a 
pheochromocytoma (Table 16.3).23
Additional clinical signs of pheochromocytoma include hyper­
tensive retinopathy, orthostatic hypotension, angina, nausea, 
  Signs and Symptoms Associated With 
Catecholamine-Secreting Tumors
Spell-Related Signs and Symptoms
Anxiety and fear of impending death
Diaphoresis
Dyspnea
Epigastric and chest pain
Headache
Nausea

astric and chest pain
Headache
Nausea and vomiting
Pallor
Palpitation (forceful heartbeat)
Tremor
Chronic Signs and Symptoms
Cold hands and feet
Congestive heart failure—dilated or hypertrophic cardiomyopathy
Constipation
Dyspnea
Ectopic hormone secretion–dependent symptoms (e.g., CRH/ACTH, 
GHRH, PTHrP, VIP)
Fatigue
Fever
General increase in sweating
Grade II to IV hypertensive retinopathy
Headache

che
Orthostatic hypotension
Painless hematuria (associated with urinary bladder paraganglioma)
Pallor
Tremor
Not Typical of Pheochromocytoma
Flushing
ACTH, Adrenocorticotropic hormone; CRH, corticotropin-releasing hormone; GHRH, growth 
hormone–releasing hormone; PTHrP, parathyroid hormone–related peptide; VIP, vasoactive 
intestinal polypeptide.
Adapted from Young WF Jr. Pheochromocytoma, 1926-1993. Tren

rom Young WF Jr. Pheochromocytoma, 1926-1993. Trends Endocrinol Metab. 
1993;4:122–127.
   
TABLE 16.2
547
constipation (megacolon may be the presenting symptom), hyper­
glycemia, diabetes mellitus, hypercalcemia, Raynaud phenom­
enon, livedo reticularis, erythrocytosis, and mass effects from the 
tumor. Although hypercalcemia may be a sign of primary hyper­
parathyroidism in patients with MEN2A, in most patients with 
pheochromocytoma, it is an isolated findin

s with 
pheochromocytoma, it is an isolated finding and resolves with 
resection of the catecholamine-secreting tumor. In addition, cal­
citonin secretion is in part a catecholamine-dependent process; 
serum calcitonin concentrations are frequently mildly elevated in 
patients with pheochromocytoma, usually unrelated to MEN2. 
Fasting hyperglycemia and diabetes mellitus are caused in part by 
the α-adrenergic inhibition of insulin release. Painless hematuria 
and paroxysmal attacks induced by mi

ss hematuria 
and paroxysmal attacks induced by micturition and defecation are 
associated with urinary bladder paragangliomas.
Some of the cosecreted hormones that may dominate the 
clinical presentation include ACTH (Cushing syndrome), para­
thyroid hormone–related peptide (hypercalcemia), vasopressin 
(syndrome of inappropriate antidiuretic hormone secretion), 
vasoactive intestinal peptide (watery diarrhea), and growth hor­
mone–releasing hormone (acromegaly). Cardiomyopathy and 
congestive 

mone (acromegaly). Cardiomyopathy and 
congestive heart failure are the symptomatic presentations caused 
by pheochromocytoma that are most frequently unrecognized 
by clinicians.24 The cardiomyopathy, whether dilated or hyper­
trophic, may be totally reversible with tumor resection.24,25 In 
addition, Takotsubo cardiomyopathy with ventricular apical 
ballooning can also be a presentation of pheochromocytoma.26 
Myocarditis and myocardial infarction with normal coronary 
arteries seen on angiogr

ion with normal coronary 
arteries seen on angiography are also cardiac-based presenta­
tions of pheochromocytoma.24 The myocarditis is characterized 
by infiltration of inflammatory cells and focal contraction-band 
necrosis.24,27 Many physical examination findings can be associ­
ated with genetic syndromes that predispose to pheochromocy­
toma; these findings include retinal angiomas, iris hamartomas, 
marfanoid body habitus, café au lait spots, axillary freckling, 
subcutaneous neurofibromas,

, axillary freckling, 
subcutaneous neurofibromas, and mucosal neuromas on the 
eyelids and tongue. Some patients with pheochromocytoma are 
asymptomatic despite high circulating levels of catecholamines; 
this type most likely reflects adrenergic receptor desensitization 
related to chronic stimulation.
Because of the increased and widespread use of computed 
tomography (CT) and magnetic resonance imaging (MRI) in 
patients with abdominal symptoms, pheochromocytoma and 
abdominal paraganglioma 

ms, pheochromocytoma and 
abdominal paraganglioma may be detected as an incidental 
adrenal mass in many patients before any symptoms develop.28 
In the era of widespread use of computed cross-sectional imag­
ing, approximately 60% of adrenal pheochromocytoma patients 
had their adrenal tumors discovered incidentally on imaging per­
formed for other reasons.29–31 Although typically these inciden­
tally discovered tumors in asymptomatic patients are small (<3 
cm), they may be up to 10 cm in larg

re small (<3 
cm), they may be up to 10 cm in largest diameter.
At the time of symptom-based detection, pheochromocytomas 
have an average diameter of 4.5 cm (Fig. 16.3).32 Paragangliomas 
are found where there is chromaffin tissue: along the para-aortic 
sympathetic chain, within the organ of Zuckerkandl (at the ori­
gin of the inferior mesenteric artery), in the wall of the urinary 
bladder, and along the sympathetic chain in the neck or medi­
astinum.33 During early postnatal life, the extra-

astinum.33 During early postnatal life, the extra-adrenal sympa­
thetic paraganglionic tissues are prominent; later they degenerate, 
leaving residual foci associated with the vagus nerves, carotid ves­
sels, aortic arch, pulmonary vessels, and mesenteric arteries. Odd 
locations for paragangliomas include the neck, intra-atrial cardiac 
septum, spermatic cord, vagina, scrotum, and sacrococcygeal 
region. Paragangliomas in the head and neck region (e.g., carotid 
body tumors, glomus tumors, chem

n (e.g., carotid 
body tumors, glomus tumors, chemodectomas) usually arise from 
parasympathetic tissue and typically do not hypersecrete catechol­
amines and metanephrines. Paragangliomas in the mediastinum, 
abdomen, and pelvis usually arise from sympathetic chromaffin 
tissue and usually hypersecrete catecholamines and metanephrines. 
Syndromic Forms of Pheochromocytoma and 
Paraganglioma
Multiple Endocrine Neoplasia Type 2A
MEN2A (previously known as Sipple syndrome) is an autoso­
mal domina

known as Sipple syndrome) is an autoso­
mal dominant disorder with age-related penetrance.34 MEN2A is 
characterized by medullary thyroid cancer (MTC) in all patients, 
  Differential Diagnosis of 
Pheochromocytoma-Type Spells
Endocrine Causes
Carbohydrate intolerance
Hyperadrenergic spells
Pancreatic tumors (e.g., insulinoma)
Primary hypogonadism (menopausal syndrome)
Cardiovascular Causes
Angina
Cardiovascular deconditioning
Labile essential hypert

diovascular deconditioning
Labile essential hypertension
Paroxysmal cardiac arrhythmia
Renovascular disease
Syncope (e.g., vasovagal reaction)
Psychologic Causes
Factitious (e.g., drugs, Valsalva maneuver)
Hyperventilation
Severe anxiety and panic disorders
Somatization disorder
Pharmacologic Causes
Chlorpropamide-alcohol flush
Combination of a monoamine oxidase inhibitor and a decongestant
Illegal drug ingestion (cocaine, phencyclidine, lysergic acid diet

estion (cocaine, phencyclidine, lysergic acid diethylamide)
Sympathomimetic drug ingestion
Vancomycin (red man syndrome)
Withdrawal of adrenergic-inhibitor
Neurologic Causes
Autonomic neuropathy
Cerebrovascular insufficiency
Diencephalic epilepsy (autonomic seizures)
Migraine headache
Postural orthostatic tachycardia syndrome
Stroke
Carcinoid syndrome
Mast cell disease
Recurrent idiopathic anaphylaxis
Unexplained flushing spells
TABLE 16.3
548
SECTION IV    Adrenal Cortex and Endocr

E 16.3
548
adrenergic (epinephrine and metanephrines are predominant) 
pheochromocytoma in 50% (usually bilateral and frequently 
asynchronous), primary hyperparathyroidism in 20%, cutaneous 
lichen amyloidosis in 5%, and very rarely Hirschsprung disease. 
Cutaneous lichen amyloidosis is a pruritic, papular, scaly, and pig­
mented skin lesion that is typically located in the interscapular 
region or on the extensor surfaces of the extremiti

egion or on the extensor surfaces of the extremities.
MTC is usually detected before the pheochromocytoma 
is diagnosed. The prevalence of MEN2A is approximately 1 in 
200,000 live births. Numerous activating mutations throughout 
the RET (rearranged during transfection) proto-oncogene have 
been documented in persons with MEN2A. RET, located on 
chromosome 10q11.2, encodes a transmembrane receptor tyro­
sine kinase that is involved in the regulation of cell proliferation 
and apoptosis by activ

tion of cell proliferation 
and apoptosis by activation of the PI3K/AKT (phosphoinositide 
3-kinase) and MAPK/ERK (mitogen-activated protein kinase/
extracellular signal-regulated kinase) signaling pathways. RET can 
be constitutively activated by gain-of-function mutations in seven 
specific exons of the RET gene (exons 8, 10, 11, 13, 14, 15, or 16). 
Most mutations in MEN2A kindreds (>90%) involve RET exons 
10 (codons 609, 611, 618, and 620) or 11 (codons 630 or 634). 
Eighty-five percent of 

r 11 (codons 630 or 634). 
Eighty-five percent of individuals with MEN2A have a mutation 
in codon 634, particularly p.Cys634Arg (c.1900C>T).
Hirschsprung disease is characterized by the absence of auto­
nomic ganglion cells within the distal colon parasympathetic 
plexus, which results in constipation, megacolon, or obstipation. 
Hirschsprung disease may occur in patients with MEN2A who 
have a Janus mutation—a mutation that acts simultaneously as 
both a gain-in-function and a loss-of-function

as 
both a gain-in-function and a loss-of-function mutation—in the 
RET proto-oncogene (exon 10: codons 609, 611, 618, 620).35 
It is important to distinguish the constipation/obstipation due 
to Hirschsprung disease in patients with MEN2A from that due 
to ganglioneuromatosis of MEN2B or from the colonic paralytic 
effects seen in patients with massive elevations in catecholamines 
(most commonly seen in patients with widespread metastatic 
paraganglioma or pheochromocytoma). 
Multiple Endocrin

ganglioma or pheochromocytoma). 
Multiple Endocrine Neoplasia Type 2B
MEN2B (previously known as Gorlin syndrome) is also an auto­
somal dominant disorder with age-related penetrance, and it 
represents approximately 5% of all MEN2 cases.34 MEN2B is 
characterized by MTC in all patients, adrenergic (epinephrine 
and metanephrine predominant) pheochromocytoma in 50%, 
mucocutaneous neuromas (typically involving the tongue, lips, 
and eyelids) in most patients, and by skeletal deformities (e.g., 

ost patients, and by skeletal deformities (e.g., 
kyphoscoliosis, lordosis), joint laxity, myelinated corneal nerves, 
and intestinal ganglioneuromas.
MEN2B-associated tumors are caused by mutations in the 
RET protein’s intracellular domain. A single methionine-to-threo­
nine missense mutation in exon 16 (p.Met918Thr; c.2753T>C) is 
responsible for more than 95% of MEN2B cases. Another muta­
tion, alanine to phenylalanine at codon 883 in exon 15, has been 
found in 4% of MEN2B kindreds.
More th

, has been 
More than 95% of patients with MEN2A and more than 98% 
of those with MEN2B have an identifiable mutation in the RET 
proto-oncogene. Pheochromocytoma in MEN2 has an adrenergic 
(epinephrine and metanephrine are predominant) biochemical 
phenotype, and this knowledge directs genetic testing.36 Genetic 
testing for mutations in the RET proto-oncogene should be con­
sidered for patients with co-phenotype disorders (e.g., MTC) or 
adrenergic biochemical ph

isorders (e.g., MTC) or 
adrenergic biochemical phenotype pheochromocytoma (unilateral 
or bilateral). In a family with MEN2, a family member with a 
clinical diagnosis of MEN2 should be the first to have genetic test­
ing. If a RET mutation is found, all family members of unknown 
status should be offered genotyping. Genetic consultation should 
be considered before genetic testing is performed. In families 
with known MEN2, genetic testing shortly after birth facilitates 
prompt surgical manag

tly after birth facilitates 
prompt surgical management of the thyroid gland (see Chapter 42 
for further discussion of MEN2). 
von Hippel-Lindau Disease
von Hippel-Lindau (VHL) syndrome is an autosomal dominant 
disorder that may manifest with a variety of benign and malignant 
B
A
• Fig. 16.3  A computed tomographic (CT) scan of the abdomen with intravenous contrast in a 71-year-old 
man with an incidentally discovered right adrenal mass. The concentrations of plasma fractionated free 
metanep

oncentrations of plasma fractionated free 
metanephrines were abnormal: metanephrine, 0.34 nmol/L (normal, <0.5 nmol/L) and normetanephrine, 
8.59 nmol/L (normal, <0.9 nmol/L). The 24-hour urine studies were abnormal: norepinephrine, 455 μg 
(normal, <170 μg); epinephrine, 7.2 μg (normal, <35 μg); dopamine, 160 μg (normal, <700 μg); metaneph­
rine, 173 μg (normal, <400 μg); and normetanephrine, 3147 μg (normal, <900 μg). (A) The axial CT image 
shows a typical 3.8-cm heterogeneously enhancing ri

hows a typical 3.8-cm heterogeneously enhancing right adrenal mass just lateral to the inferior vena cava 
and consistent with pheochromocytoma (arrow). (B) Coronal view shows the location (arrow) of the mass 
superior to the right kidney and inferior and medial to the liver. After α-adrenergic and β-adrenergic block­
ade, a 20-g (2.5 × 1.5 × 1.5 cm) pheochromocytoma was removed laparoscopically.
549
neoplasms: noradrenergic (norepinephrine and normetanephrine 

noradrenergic (norepinephrine and normetanephrine 
are predominant) pheochromocytoma or paraganglioma (medi­
astinal, abdominal, pelvic), hemangioblastoma (involving the 
cerebellum, spinal cord, or brainstem), retinal angioma, clear cell 
renal cell carcinoma, pancreatic neuroendocrine tumors, endo­
lymphatic sac tumors of the middle ear, serous cystadenomas of 
the pancreas, and papillary cystadenomas of the epididymis and 
broad ligament.37 The appearance of these VHL-related neo­
plasms in m

e appearance of these VHL-related neo­
plasms in mutation-positive individuals is close to 100% by age 
65 years. The average age of detection of pheochromocytoma is 20 
to 29 years of age.37
The prevalence of VHL syndrome is between 1 in 35,000 and 1 
in 91,000 people.38,39 The VHL tumor suppressor gene, located on 
chromosome 3p25-26, encodes a protein that regulates ubiquiti­
nation and proteosomal degradation of hypoxia-inducible factors 
(HIFs). Loss-of-function mutations in VHL lead to the

Fs). Loss-of-function mutations in VHL lead to the inappro­
priate activation of the hypoxic response—promoting glycolysis, 
angiogenesis, and proliferation. Genotype-phenotype correlations 
may be used to divide patients into two groups: type 1 and type 2. 
Patients from kindreds with the type 1 syndrome have mutations 
that lead to total loss of biologic activity of the VHL protein and 
are at very low risk to develop pheochromocytoma; those from 
kindreds with the type 2 syndrome have missens

om 
kindreds with the type 2 syndrome have missense mutations that 
allow residual activity of the VHL protein and are at high risk for 
developing pheochromocytoma.
Genetic testing for VHL syndrome should be considered 
for patients with bilateral noradrenergic (norepinephrine and 
normetanephrine predominant) pheochromocytoma, diagnosis of 
unilateral noradrenergic pheochromocytoma at a young age (e.g., 
≤45 years), or pheochromocytoma/paraganglioma patients with 
co-phenotype disorders (e.g.,

lioma patients with 
co-phenotype disorders (e.g., retinal angioma).
Pheochromocytomas occurring in patients with MEN2 pro­
duce predominantly epinephrine and its major metabolite, 
metanephrine, whereas those occurring in patients with VHL 
syndrome produce predominantly norepinephrine and its major 
metabolite, normetanephrine.36 These biochemical phenotypes 
result from mutation-specific differential gene expression. PNMT 
is overexpressed in MEN2-associated tumors (epinephrine and 
metanephr

MEN2-associated tumors (epinephrine and 
metanephrine profile) and underexpressed in VHL-associated 
tumors (norepinephrine and normetanephrine profile).40 In addi­
tion, pheochromocytomas occurring in patients with MEN2 have 
increased tyrosine hydroxylase activity compared with those occur­
ring in patients with VHL; this difference accounts for higher lev­
els of catecholamines and metabolites in patients with MEN2. 
Neurofibromatosis Type 1
Neurofibromatosis 1 (NF1), previously known as von 

eurofibromatosis 1 (NF1), previously known as von Reckling­
hausen disease, is one of the most common genetic syndromes, 
with a prevalence of 1 in 2000 to 1 in 5000 persons.41 NF1 is an 
autosomal dominant disorder with 100% penetrance and is char­
acterized by neurofibromas, multiple café au lait spots, axillary and 
inguinal freckling, iris hamartomas (Lisch nodules), bony abnor­
malities, central nervous system gliomas, pheochromocytoma and 
paraganglioma, macrocephaly, and cognitive deficit

paraganglioma, macrocephaly, and cognitive deficits. Although 
penetrance is 100%, the expression of NF1 features is variable. 
Approximately 3% of patients with NF1 develop catecholamine-
secreting tumors.42 In these patients, the catecholamine-secreting 
tumor is usually a solitary, benign adrenal pheochromocytoma, 
occasionally a bilateral adrenal pheochromocytoma, and rarely an 
abdominal periadrenal paraganglioma.42 Frequently the adrenal 
pheochromocytoma is detected as an incidental adren

heochromocytoma is detected as an incidental adrenal mass on 
imaging performed for other reasons.42
The NF1 tumor suppressor gene, located on chromosome 
17q11.2, encodes neurofibromin, which downregulates RAS pro­
teins and the downstream RAS-RAF-MAPK signaling cascade. 
Inactivating NF1 mutations cause the disorder. Unless a patient 
with pheochromocytoma presents with additional clinical charac­
teristics consistent with an NF1 diagnosis, genetic testing of the 
NF1 gene is not recommended. 

etic testing of the 
Congenital Polycythemia
The most common congenital polycythemia-associated somatic 
mutations that also predispose to pheochromocytoma and 
paraganglioma occur in HIF2α, which lead to stabilization of 
HIF2α, resulting in the upregulation of HIF2α-related genes 
(e.g., erythropoietin gene). Patients with HIF2α mutations are at 
risk for congenital polycythemia, multiple paragangliomas, and 
somatostatinomas.43,44 These HIF2α somatic mutations oc

ostatinomas.43,44 These HIF2α somatic mutations occur 
exclusively in females in the postzygotic neural crest precursor 
cells.45 Two other congenital polycythemia-associated mutations 
with potential links to pheochromocytoma and paraganglioma 
include VHL (Chuvash polycythemia; however, these patients 
have not been reported to develop pheochromocytoma or para­
ganglioma) and prolyl hydroxylase domain 2 (PHD2)–containing 
protein (with only one patient reported to date).46 
Carney Triad
The Ca

 patient reported to date).46 
The Carney triad (gastrointestinal stromal tumor, pulmonary chon­
droma, and catecholamine-secreting paraganglioma; less frequent 
neoplasms include esophageal leiomyoma and adrenal adenoma) 
is another syndrome associated with catecholamine-secreting 
tumors.47 This syndrome is a rare (∼150 patients have been reported) 
disorder of unknown cause that primarily affects young women.48 
The gastric stromal tumors are frequently multicentric and associ­
a

l tumors are frequently multicentric and associ­
ated with early liver metastases; however, most affected patients have 
a very indolent course.49 The pulmonary chondromas are benign 
and, if asymptomatic, require no specific therapy. The paraganglio­
mas secrete catecholamines and should be resected when discovered. 
Additional features of the Carney triad include esophageal leiomyo­
mas and adrenocortical adenomas. The esophageal leiomyomas are 
benign and usually asymptomatic. The adrenocorti

benign and usually asymptomatic. The adrenocortical adenomas 
may be nonfunctioning or may secrete cortisol autonomously.50 
The molecular pathogenesis of Carney triad is related to the down­
regulation of the succinate dehydrogenase (SDH) enzyme complex 
through site-specific hypermethylation of the SDHC gene.51,52 
Congenital Heart Disease
The association between cyanotic congenital heart disease and 
pheochromocytoma/paraganglioma has been recognized for more 
than 50 years.53–56 The underl

ecognized for more 
than 50 years.53–56 The underlying pathogenesis appears to be 
gain-of-function somatic mutations of EPAS1, which encodes 
HIF2α.57 Mutations in residues 530 and 531 of EPAS1 result in 
constitutive HIF2α activation, leading to development of pheo­
chromocytoma and paraganglioma in patients with cyanotic con­
genital heart disease. EPAS1 mutations in the setting of chronic 
hypoxia amplify the oncogenic properties of HIF2α.57 
Other Genetic Forms of Pheochromocytoma 
and Para

and Paraganglioma
Mutations contributing to pheochromocytoma and paragan­
glioma have two general transcription signatures: cluster 1—
genes encoding proteins that function in the cellular response 
550
to hypoxia, and cluster 2—genes encoding proteins that activate 
kinase signaling (Table 16.4). Cluster 1 tumors are mostly extra-
adrenal paragangliomas (except in VHL, in which most tumors 
are loc

omas (except in VHL, in which most tumors 
are localized to the adrenal gland), and nearly all have a noradren­
ergic biochemical phenotype. Cluster 2 tumors are usually adrenal 
pheochromocytomas with an adrenergic biochemical phenotype 
(see Table 16.4). Since 1990, 18 pheochromocytoma/paragan­
glioma susceptibility genes have been reported: NF1, RET, VHL, 
SDHD, SDHC, SDHB, EGLN1 (PHD2), EGLN2 (PDH1), 
KIF1B, SDHAF2, IDH1, TMEM127, SDHA, MAX, HIF2A, 
MDH2, FH, and DNMT3A.58,59
Succinate Dehyd

HIF2A, 
Succinate Dehydrogenase Gene Mutations
Most cases of familial paraganglioma are caused by mutations in 
the SDH (succinate:ubiquinone oxidoreductase) subunit genes 
(SDHB, SDHC, SDHD, SHDA, and SDHAF2), which make 
up portions of mitochondrial complex II.60 The SDHx genes are 
considered tumor suppressor genes and encode the proteins that 
form the mitochondrial complex II, a crucial link between the 
Krebs cycle and mitochondrial electron transport chain. SDH 

e and mitochondrial electron transport chain. SDH 
is a heterotetramer protein complex consisting of four subunits 
encoded by nuclear genes. SDHA and SDHB form the catalytic 
domain, and SDHC and SDHD anchor the complex to the inner 
mitochondrial membrane. The assembly factors, SDHAF1 and 
SDHAF2, are needed for functional and structural integrity of 
the complex. Defects in the SDH genes result in succinate accu­
mulation, which is a competitive inhibitor of the 2-oxogluta­
rate-dependent dio

ve inhibitor of the 2-oxogluta­
rate-dependent dioxygenases (e.g., HIF prolyl hydroxylases and 
histone or DNA demethylases), which leads to the stabilization 
of HIF isoforms and the activation of hypoxic signaling and to 
epigenetic modifications.
In patients with SDHD or SDHAF2 mutations, penetrance 
depends on the mutation’s parent of origin. With rare excep­
tions,61,62 the disease is not manifested when the mutation is inher­
ited from the mother but is highly penetrant when inherited from

mother but is highly penetrant when inherited from 
the father.63 This phenomenon is known as maternal imprinting.64 
  Germline Mutations Associated With Pheochromocytoma and Paraganglioma
Syndrome/Name
Gene
Typical Tumor Location and Other Associations
Hypoxic Pathway: Cluster 1a
SDHD mutation (familial paraganglioma type 1)b
SDHD
Primarily skull base and neck; occasionally adrenal medulla, mediastinum, abdo­
men, pelvis; GIST; possible pituitary adenoma
SDHAF2 mutation (familial paragangliom

ary adenoma
SDHAF2 mutation (familial paraganglioma type 2)b
SDHAF2
Primarily skull base and neck; occasionally abdomen and pelvis
SDHC mutation (familial paraganglioma type 3)
SDHC
Primarily skull base and neck; occasionally abdomen, pelvis, or chest; GIST; pos­
sible pituitary adenoma
SDHB mutation (familial paraganglioma type 4)
SDHB
Abdomen, pelvis, and mediastinum; rarely adrenal medulla, skull base, and neck; 
GIST; renal cell carcinoma; possible pituitary adenoma
SDHA mutation
SDHA
Primar

ssible pituitary adenoma
SDHA
Primarily skull base and neck; occasionally abdomen and pelvis; GIST; possible 
pituitary adenoma
von Hippel-Lindau (VHL) disease
VHL
Adrenal medulla, frequently bilateral; occasionally paraganglioma that may be 
localized from skull base to pelvis; see text for VHL-associated findings
Hereditary leiomyomatosis and renal cell carcinoma 
(Reed syndrome)—fumarate hydratase mutation
FH
Multifocal and metastatic; associated with hereditary leiomyomatosis, 

tatic; associated with hereditary leiomyomatosis, uterine 
fibroids, and renal cell cancer
Hypoxia-inducible factor (HIF) 2α
HIF2A
Paraganglioma, polycythemia, and rarely somatostatinoma
Familial erythrocytosis associated with mutation in 
prolyl hydroxylase isoform 1 (PDH1)
EGLN2
Polycythemia associated with pheochromocytoma and paraganglioma
prolyl hydroxylase isoform 2 (PDH2)
EGLN1
Polycythemia associated with pheochromocytoma and paragangl

mia associated with pheochromocytoma and paraganglioma
KIF1B
KIF1B
Neuroblastoma
Kinase Signaling Pathway: Cluster 2c
MEN2A and MEN2B
RET
Adrenal medulla, frequently bilateral; see text for MEN2A and MEN2B associated 
findings
Neurofibromatosis type 1 (NF1)
NF1
Adrenal or periadrenal; see text for NF1 associated findings
MAXb
MAX
Adrenal medulla
Familial pheochromocytoma
TMEM127
Adrenal medulla; possible renal cell carcinoma
aCluster 1 tumors are mostly extra-adrenal paragangliomas (except in VH

 mostly extra-adrenal paragangliomas (except in VHL where most tumors are localized to the adrenal) and nearly all have a noradrenergic biochemical phenotype.
bAssociated with maternal imprinting—see text.
cCluster 2 tumors are usually adrenal pheochromocytomas with an adrenergic biochemical phenotype.
GIST, Gastrointestinal stromal tumor; MEN, multiple endocrine neoplasia; SDH, succinate dehydrogenase.
   
TABLE 16.4
551
TMEM127 Mutations
TMEM127 is a negative

pertension
TMEM127 is a negative regulator of mammalian target of rapamy­
cin (mTOR) effector proteins. In a study of 990 individuals with 
pheochromocytoma or paraganglioma, germline mutations in 
TMEM127 were identified in 20 individuals with adrenal tumors 
(7 bilateral), 5 of whom had a family history of pheochromocy­
toma.65 Among 547 patients who presented with sporadic pheo­
chromocytoma (unilateral adrenal tumor with negative family 
history), 11 (2%) had TMEM127 mutati

ative family 
history), 11 (2%) had TMEM127 mutations. 
MAX Mutations
Loss-of-function mutations in the MYC-associated factor X 
(MAX) gene are associated with familial pheochromocytoma.66 In 
an initial study of three individuals with familial pheochromocy­
toma (who did not have mutations in any of the nine previously 
described susceptibility genes), MAX germline mutations were 
found. MAX is a component of the MYC-MAX-MXD1 tran­
scription factors that regulate cell proliferation, differentia

tors that regulate cell proliferation, differentiation, 
and apoptosis. In an extension of this study, MAX mutations were 
found in 5 of 59 patients (8.5%) with suspected familial pheo­
chromocytoma (based on age of onset <30 years, bilateral pheo­
chromocytoma, or positive family history).66 
FH Mutations
Germline mutations in the FH gene encoding fumarate hydratase 
were identified in 5 of 598 patients (1%) with pheochromocy­
toma/paraganglioma without mutations in known susceptibility 
gene m

 without mutations in known susceptibility 
gene mutations.67 Clinically, a metastatic phenotype and multi­
ple tumors were significantly more frequent in patients with FH 
mutations than those without such mutations. 
Genetic Testing
Genetic testing should be considered if a patient has one or more of 
the following: (1) paraganglioma, (2) bilateral adrenal pheochromo­
cytoma, (3) unilateral adrenal pheochromocytoma and a family his­
tory of pheochromocytoma/paraganglioma, (4) unilateral adrena

ochromocytoma/paraganglioma, (4) unilateral adrenal 
pheochromocytoma with onset at a young age (<45 years), or (5) 
other clinical findings suggestive of one of the previously discussed 
syndromic disorders. An asymptomatic person at risk for disease on 
the basis of family history of pheochromocytoma/paraganglioma 
should have genetic testing only if an affected family member has 
a known mutation. Genetic testing can be complex, and testing 
of one family member has implications for related i

f one family member has implications for related individuals. 
Genetic counseling is recommended to help families understand 
the implications of genetic test results, to coordinate testing of at-
risk individuals, and to help families work through the psychosocial 
issues that may arise before, during, or after the testing process.
A sequential genetic testing algorithm based on biochemical 
phenotype, age, and tumor has been proposed.22 However, the 
field of genetic testing is rapidly evolvin

, the 
field of genetic testing is rapidly evolving, and at many clinical 
laboratories, sequential genetic testing is no longer done as it is less 
expensive to utilize next-generation sequencing technology for all 
clinically available mutations as a package.68 
Evaluation and Monitoring of Carriers of 
Succinate Dehydrogenase Mutations
If an SDHx mutation is identified in a relative of a proband, 
annual clinical assessment (including blood pressure check and 
biochemical testing) is indicate

essure check and 
biochemical testing) is indicated for early detection of pheochro­
mocytoma/paraganglioma. Prospective studies to guide the clini­
cian in the frequency, age to start, and type of testing are lacking. 
Biochemical testing for fractionated metanephrines in plasma or 
in a 24-hour urine collection for fractionated metanephrines and 
catecholamines should be performed annually in all carriers of an 
SDHx mutation starting around age 10 years. Because paragangli­
omas may be nonfun

e 10 years. Because paragangli­
omas may be nonfunctioning or may be detected before catechol­
amine-secretory autonomy is evident, periodic imaging studies 
are advised. For example, at-risk SDHx (e.g., paternally inherited 
for SDHD and SDHAF2) mutation carriers should have MRI 
of the abdomen, pelvis, chest, and neck every 2 to 3 years; total 
body imaging with either 123I-MIBG scintigraphy or Gallium 68 
(68-Ga) 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid 
(DOTA)-octreotate (DOT

-1,4,7,10-tetraacetic acid 
(DOTA)-octreotate (DOTATATE) positron emission tomogra­
phy (PET) CT should be performed every 5 years. Patients with 
SDHx mutations are at risk (albeit low) to develop renal cell car­
cinoma, and the periodic abdominal cross-section imaging also 
screens for this possibility.69 In addition, as new tumor associa­
tions are identified, additional surveillance testing will be indi­
cated. For example, several reports have identified an association 
between SDHx mutatio

ve identified an association 
between SDHx mutations and pituitary tumor risk.70,71 If this risk 
proves to be real and clinically important, then pituitary-directed 
MRI may become part of the surveillance program. 
Diagnostic Investigation
Numerous disorders can cause signs and symptoms that may 
prompt the clinician to test for pheochromocytoma (see Table 16.3). 
The disorders span much of medicine and include endocrine dis­
orders (e.g., primary hypogonadism), cardiova

dis­
orders (e.g., primary hypogonadism), cardiovascular disorders (e.g., 
idiopathic orthostatic hypotension), psychologic disorders (e.g., 
panic disorder), pharmacologic causes (e.g., withdrawal from an 
adrenergic inhibitor), neurologic disorders (e.g., postural orthostatic 
tachycardia syndrome), and miscellaneous disorders (e.g., mast cell 
disease). Indeed, most patients tested for pheochromocytoma do not 
have it. In addition, levels of fractionated catecholamines and meta­
nephrines may

ractionated catecholamines and meta­
nephrines may be elevated in several clinical scenarios, including 
withdrawal from medications or drugs (e.g., clonidine, alcohol), any 
acute illness (e.g., subarachnoid hemorrhage, migraine headache, 
preeclampsia), and administration of many drugs and medications 
(e.g., tricyclic antidepressants, levodopa, buspirone, antipsychotic 
agents, cocaine, phencyclidine, amphetamines, ephedrine, pseudo­
ephedrine, phenylpropanolamine, isoproterenol) (Table 16.5)

, phenylpropanolamine, isoproterenol) (Table 16.5).22 
Case Detection
Pheochromocytoma should be suspected in patients who have one 
or more of the following:
 • Hyperadrenergic spells (e.g., self-limited episodes of nonexertional 
forceful palpitations, diaphoresis, headache, tremor, or pallor)
  Medications that May Increase Measured 
Levels of Fractionated Catecholamines 
and Metanephrines
Tricyclic antidepressants (including cyclobenzaprine)
Levodopa
Drugs containing adrenergic receptor ag

must be confirmed biochemically by the presence 
of increased concentrations of fractionated metanephrines and 
catecholamines in urine or plasma (Fig. 16.4).22,72,73 The metabo­
lism of catecholamines is primarily intratumoral, with formation 
of metanephrine from epinephrine and normetanephrine from 
norepinephrine.8 Most laboratories now measure fractionated cat­
echolamines (dopamine, norepinephrine, and epinephrine) and 
fractionated metanephrines (metanephrine and normetanephrine) 
by high

hrines (metanephrine and normetanephrine) 
by high-performance liquid chromatography with electrochemical 
detection or tandem mass spectrometry. These techniques have 
overcome the problems with fluorometric analysis, which include 
false-positive results caused by α-methyldopa, labetalol, sotalol, 
and imaging contrast agents.
At Mayo Clinic, the most reliable case-detection strategy is mea­
surement of fractionated metanephrines and catecholamines in a 
24-hour urine collection (sensitivity, 

ines in a 
24-hour urine collection (sensitivity, 98%; specificity, 98%).72,74 
Because of the higher false-positive rate with plasma fractionated 
metanephrines, they should be reserved for high clinical suspi­
cion cases. The index of suspicion for pheochromocytoma should 
be high in the following scenarios: resistant hypertension, spells 
with associated pallor, a family history of pheochromocytoma, a 
genetic syndrome that predisposes to pheochromocytoma (e.g., 
MEN2), a past history of rese

chromocytoma (e.g., 
MEN2), a past history of resected pheochromocytoma and pres­
ent history of recurrent hypertension or spells, and an incidentally 
discovered adrenal mass that has imaging characteristics consis­
tent with pheochromocytoma (Table 16.6).28 In addition, mea­
surement of plasma fractionated metanephrines is a good first-line 
• Fig. 16.4  Evaluation and treatment of catecholamine-secreting tumors. Clinical suspicion is triggered by 
paroxysmal symptoms (especially hypertension)

 by 
paroxysmal symptoms (especially hypertension); hypertension that is intermittent, unusually labile, or resis­
tant to treatment; a family history of pheochromocytoma or associated conditions; or an incidentally dis­
covered adrenal mass (see text for details). CT, computed tomography; 18FDG, 18F-fluorodeoxyglucose; 
68Ga-DOTATATE, Gallium-68 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-octreotate; 
123I-MIBG, iodine-123–labeled metaiodobenzylguanidine; Mets, metanephrines

beled metaiodobenzylguanidine; Mets, metanephrines; MRI, magnetic resonance 
imaging; nl, normal; Nmet, normetanephrine; PET, positron emission tomography. (Modified from Young Jr 
WF. Pheochromocytoma, 1926-1993. Trends Endocrinol Metab. 1993;4:122–127, used with permission.)
24-h urine:
   Fractionated Mets
   Fractionated catecholamines
Plasma:
123I-MIBG scan if:
  • >10-cm adrenal mass
  • Paraganglioma
Reassess the diagnosis
Consider:
  
 Paraganglioma
Consider:
  • 123I-MIBG scan
  • 68Ga DOTATATE PET
  • Whole body MRI scan
  • 18FDG PET
  •  PET  scan 
Discontinue interfering medications (Table 16.5)
Recheck
during a spell
Normal
Typical adrenal or
para-aortic mass
Negative abdominal imaging
Tumor found
Clinical suspicion (pretest probabilty)
Low
High
Normal
Normal:
Investigate other
causes of spells
(Table 16.3)
Localization: adrenal/abdominal MRI or CT scan
Consider genetic testing
Preoperative α- & β

 scan
Preoperative α- & β-adrenergic blockade
>2-fold elevation above upper limit of nl in urine
catecholamines or   urine Mets
(Nmet >900 µg or Met >400 µg) or
“significant increase” in fractionated plasma mets  
553
test for children because obtaining a complete 24-hour urine 
collection is difficult in pediatric patients. Measurement of uri­
nary dopamine or plasma methoxytyramine can be very useful in 
detecting t

methoxytyramine can be very useful in 
detecting the rare tumor with selective dopamine hypersecretion 
because plasma metanephrine fractions are not direct metabolites 
of dopamine and may be normal in the setting of a dopamine-
secreting tumor.74–76
The 24-hour urine collection for fractionated metanephrines 
and catecholamines should include measurement of urinary cre­
atinine to verify an adequate collection. The diagnostic cutoffs for 
most 24-hour urinary fractionated metanephrine assays a

24-hour urinary fractionated metanephrine assays are based 
on normal ranges derived from normotensive volunteer reference 
groups, and this can result in excessive false-positive test results. 
For example, in normotensive laboratory volunteers, the 95th 
percentiles are 428 μg for normetanephrine and 200 μg for meta­
nephrine, whereas the corresponding values in individuals who 
are being tested for pheochromocytoma as part of routine clinical 
practice but who do not have the neoplasm are, re

practice but who do not have the neoplasm are, respectively, 71% 
and 51% higher than those of the normal volunteers (<900 μg for 
normetanephrine and <400 μg for metanephrines).73
Although it is preferred that patients not receive any medica­
tion during the diagnostic evaluation, treatment with most medi­
cations may be continued. Tricyclic antidepressants are the drugs 
that interfere most frequently with the interpretation of 24-hour 
urinary catecholamines and metabolites. To effectively s

y catecholamines and metabolites. To effectively screen for 
catecholamine-secreting tumors, treatment with tricyclic antide­
pressants and other psychoactive agents listed in Table 16.5 should 
be tapered and discontinued at least 2 weeks before any hormonal 
assessments. In some clinical situations it is contraindicated to 
discontinue certain medications (e.g., antipsychotics), and if case-
detection testing is positive, then CT or MRI of the abdomen 
and pelvis would be needed to exclude a c

abdomen 
and pelvis would be needed to exclude a catecholamine-secreting 
tumor. Furthermore, catecholamine secretion may be appro­
priately increased in situations of physical stress or illness (e.g., 
stroke, myocardial infarction, congestive heart failure, obstructive 
sleep apnea). There are no reliable reference ranges for fraction­
ated metanephrines or catecholamines in patients requiring inten­
sive care unit hospitalization. Therefore the clinical circumstances 
under which catecholamin

e clinical circumstances 
under which catecholamines and metanephrines are measured 
must be assessed in each case. 
Other Tests That Have Been Used to Assess for 
Because of poor overall accuracy in testing for pheochromocy­
toma, measurement of plasma catecholamines no longer has a role 
except to detect dopamine-secreting paragangliomas. Chromo­
granin A is stored and released from dense-core secretory granules 
of neuroendocrine cells and is increased in 80% of patients with

ine cells and is increased in 80% of patients with 
pheochromocytoma. Chromogranin A is not specific for pheo­
chromocytoma, and elevations may be seen with other neuroen­
docrine tumors. The 24-hour urinary VMA excretion has poor 
diagnostic sensitivity and specificity compared with fractionated 
24-hour urinary metanephrines.
The high false-positive rate for plasma fractionated catechol­
amines and fractionated metanephrines triggered the develop­
ment of a confirmatory test, the clonidine sup

op­
ment of a confirmatory test, the clonidine suppression test.77 
This test is intended to distinguish between pheochromocytoma 
and false-positive increases in plasma fractionated catecholamines 
and metanephrines. Clonidine is a centrally acting α2-adrenergic 
receptor agonist that normally suppresses the release of catechol­
amines from neurons but does not affect the catecholamine secre­
tion from a pheochromocytoma.78
  Typical Imaging Phenotypes of Adrenal Masses
Tumor Type
Size 
(cm)
S

enotypes of Adrenal Masses
Tumor Type
Size 
(cm)
Shape
Texture
Laterality
Contrast 
Enhancement
CTa
MRIb
Necrosis, 
Hemorrhage, 
or 
Calcifications
Growth
Cortical adenoma
≤3
Round to 
oval with 
smooth 
margins
Homogeneous
Usually 
unilateral
Limited
<10 HU; 
>50% 
washout
Isointense
Rare
Slow
Cortical 
carcinoma
>4
Irregular 
with 
unclear 
margins
Inhomogeneous
Usually 
Marked
>10 HU; 
<50% 
washout
Hyperintense
Common
Rapid
>3
Round to 
oval with 
smooth 
margins

omocytoma
>3
Round to 
oval with 
smooth 
margins
Inhomogeneous 
with areas 
of cystic 
degeneration
Usually 
solitary 
and 
Marked
>10 HU; 
<50% 
washout
Common
0.5–1 
cm/yr
Metastasis
Variable
Oval to 
irregular 
with 
unclear 
margins
Often 
bilateral
Marked
>10 HU; 
<50% 
washout
Common
Variable
aPrecontrast CT attenuation (measured in HU) and percentage of contrast medium washout at 10 min.
bRelative intensity compared with liver on T2-weig

bRelative intensity compared with liver on T2-weighted images.
CT, Computed tomography; HU, Hounsfield unit; MRI, magnetic resonance imaging.
   
TABLE 16.6
554
Because of advances in the methodology for measuring cat­
echolamines and metanephrines, phentolamine, glucagon, his­
tamine, metoclopramide, and tyramine tests are rarely needed. 
From 1975 to 1994 at Mayo Clinic, we performed histamine 
and glucagon stimulation testing in 542 pati

mine 
and glucagon stimulation testing in 542 patients in whom pheo­
chromocytoma was highly suspected despite normal 24-hour uri­
nary excretion of total metanephrines or catecholamines; not one 
patient had a positive stimulation test in this setting.79 
Renal Failure
Measurements of urinary catecholamines and metabolites may be 
invalid if the patient has advanced renal insufficiency.80 Serum 
chromogranin A levels have poor diagnostic specificity in these 
patients.81 In patients without phe

city in these 
patients.81 In patients without pheochromocytoma who are 
receiving hemodialysis, plasma norepinephrine and dopamine 
concentrations are increased, respectively, threefold and twofold 
above the upper limit of normal.82 However, standard normal 
ranges can be used for interpreting plasma epinephrine concentra­
tions.83 Therefore when patients with renal failure have plasma 
norepinephrine concentrations more than three times above the 
upper normal limit or epinephrine concentrati

the 
upper normal limit or epinephrine concentrations greater than 
the upper normal limit, pheochromocytoma should be suspected. 
The findings of one study suggested that plasma concentrations of 
fractionated metanephrines are increased approximately twofold 
in patients with renal failure and may be useful in the biochemi­
cal evaluation of patients with marked renal insufficiency or renal 
failure.84 However, the results from other studies suggested that 
concentrations of plasma fractionate

ggested that 
concentrations of plasma fractionated metanephrines cannot dis­
tinguish between patients with pheochromocytoma and end-stage 
renal disease.85,86 
Factitious Pheochromocytoma
As with other similar disorders, factitious pheochromocytoma can 
be very difficult to confirm.87 The patient usually has a medical 
background. The patient may “spike” the 24-hour urine container, 
or the catecholamines may be administered systemically.88 Other 
patients take drugs that mimic the symptoms of

er 
patients take drugs that mimic the symptoms of pheochromocy­
toma and can cause false-positive biochemical testing. 
Localization
Localization studies should not be initiated until biochemical 
studies have confirmed the diagnosis of a catecholamine-secreting 
tumor (see Fig. 16.4). CT or MRI of the abdomen and pelvis 
should be the first localization test (sensitivity, >95%; specificity, 
>65%).22 Approximately 85% of these tumors are found in the 
adrenal glands, and 95% are found in the a

in the 
adrenal glands, and 95% are found in the abdomen and pelvis. 
The most common locations of catecholamine-secreting paragan­
gliomas include superior abdominal para-aortic region, 46%; infe­
rior abdominal para-aortic region, 29%; urinary bladder, 10%; 
mediastinum, 10%; head and neck, 3%; and pelvis, 2%.33
Imaging Phenotype
The term imaging phenotype refers to the characteristics of the mass 
on CT or MRI (see Table 16.6).28 The lipid-rich nature of cortical 
adenomas is helpful in disti

h nature of cortical 
adenomas is helpful in distinguishing these benign neoplasms from 
pheochromocytoma. On CT scans, the density of the image (with 
darker tissues being less dense) is attributed to x-ray attenuation. 
At the extremes of the CT density spectrum are air (black) and 
bone (white). The Hounsfield scale is a semiquantitative method 
of measuring x-ray attenuation. Typical Hounsfield unit (HU) 
values are -20 to -150 HU for adipose tissue and 20 to 50 HU for 
kidney. If an adrenal

 tissue and 20 to 50 HU for 
kidney. If an adrenal mass is less than 10 HU on unenhanced CT, 
it cannot be a pheochromocytoma.89 Adrenal cortical adenomas 
show a much earlier washout of contrast enhancement than do 
nonadenomas.90 For example, Korobkin and colleagues90 found 
that the mean percentage washout for adenomas was 51% at 5 
minutes and 70% at 15 minutes, compared with 8% and 20%, 
respectively, for nonadenomas.
Although CT is still the primary adrenal imaging modality, 
MRI has advan

e primary adrenal imaging modality, 
MRI has advantages in certain clinical situations.91 Several dif­
ferent MRI techniques have been used to characterize adrenal 
masses. Conventional spin-echo MRI was the first and is still the 
most frequently used technique. Early in the history of abdominal 
MRI, it became clear that with low-strength or midfield-strength 
magnets, T1-weighted and T2-weighted imaging could be used to 
differentiate pheochromocytoma and malignancies from benign 
adenomas. O

mocytoma and malignancies from benign 
adenomas. On gadolinium-diethylenetriamine penta-acetic acid 
(DPTA)–enhanced MRI, pheochromocytomas and malignant 
lesions show rapid and marked enhancement and a slower wash­
out pattern, whereas adenomas demonstrate mild enhancement 
and a rapid washout of contrast agent.90 Similar findings are made 
with CT.
Chemical shift MRI is a form of lipid-sensitive imaging. 
Chemical shift MRI is based on the principle that the hydrogen 
protons in water and lipi

ciple that the hydrogen 
protons in water and lipid molecules resonate at different fre­
quencies. Benign cortical adenomas contain approximately equal 
amounts of lipid and water, whereas the lipid content of pheo­
chromocytomas is usually low. When the protons of water and 
lipid are aligned, they are said to be in phase, and when opposite 
each other, they are out of phase. When fat and water are in phase 
on MRI, the signal intensity is maximized; when they are out 
of phase, the signal inte

ized; when they are out 
of phase, the signal intensity is reduced. This in-phase and out-
of-phase process is the chemical shift technique. Benign adrenal 
cortical adenomas lose signal on out-of-phase images due to high 
lipid content but appear relatively bright on in-phase images. A 
modification of the chemical shift MRI technique uses gradient 
echo pulse sequences to produce a similar effect.
Imaging characteristics consistent with a benign cortical adenoma 
include round and homogeneous 

n cortical adenoma 
include round and homogeneous density, smooth contour with sharp 
margination, diameter usually less than 3 cm, unilateral location, low 
unenhanced CT attenuation values (e.g., <10 HU) with rapid con­
trast medium washout at 10 minutes after administration of con­
trast medium,90,92 isointensity with liver on both T1-weighted and 
T2-weighted MRI sequences, and chemical shift evidence of lipid on 
MRI (see Table 16.6). The imaging phenotype consistent with pheo­
chromocytoma

aging phenotype consistent with pheo­
chromocytoma includes noncontrast CT attenuation above 20 HU, 
marked enhancement with intravenous contrast medium on CT (see 
Fig. 16.3), high signal intensity on T2-weighted MRI and no sig­
nal loss on out-of-phase images (Fig. 16.5), cystic and hemorrhagic 
changes, and variable size with potential to be bilateral. Although it 
has been suggested that patients with apparent simple adrenal cysts 
do not require hormonal evaluation, pheochromocytoma can mim

uire hormonal evaluation, pheochromocytoma can mimic 
an adrenal cyst.93 
68Ga-DOTATATE PET/CT and 123I-MIBG Scintigraphy
In patients with a biochemically confirmed catecholamine-secret­
ing tumor where the results of abdominal and pelvic imaging are 
negative, additional localization studies are indicated with either 
68Ga-DOTATATE PET/CT or scintigraphy with 123I-MIBG 
(Fig. 16.6). These agents accumulate preferentially in catechol­
amine-producing tumors. 123I-MIBG scintigraphy is not as sens

cing tumors. 123I-MIBG scintigraphy is not as sensi­
tive as was initially hoped (sensitivity, 80%; specificity, 99%).10 In 
a study of 282 patients with catecholamine-secreting tumors that 
were surgically confirmed, the overall sensitivity was 89% for CT, 
555
98% for MRI, and 81% for 131I-MIBG.10 If a typical (<10 cm) 
unilateral adrenal pheochromocytoma is found on CT or MRI, 
123I-MIBG scintigraphy is superfluous, and the results may even 
confuse the clin

fluous, and the results may even 
confuse the clinician.94,95 On the other hand, if the adrenal pheo­
chromocytoma is more than 10 cm in diameter or if a paragan­
glioma is identified on CT or MRI, then 68Ga-DOTATATE PET/
CT or 123I-MIBG scintigraphy is indicated because the patient 
has increased risk of metastatic disease and additional paraganglio­
mas. It is important for the clinician to recognize the medications 
that may interfere with 123I-MIBG uptake and have the patient 
discontinue th

I-MIBG uptake and have the patient 
discontinue them before imaging is performed (Table 16.7).96
68Ga-DOTATATE PET/CT is proving to be more sensitive 
in some patients than 123I-MIBG, CT/MRI, or 18F-fluorode­
oxyglucose (FDG) PET/CT for detection of metastatic disease 
(see Fig. 16.6).97 In addition, 68Ga-DOTATATE PET/CT offers 
higher spatial resolution than conventional 111In-pentetreotide 
scintigraphy. 68Ga-DOTATATE injection as a radioactive diag­
nostic agent for PET imaging was approved b

 diag­
nostic agent for PET imaging was approved by the US Food and 
Drug Administration (FDA) in June 2016. 
Other Localizing Procedures
Other localizing procedures that may be used include computer-
assisted cross-sectional imaging of the chest, neck, and skull 
base. Because of activation of aerobic glycolysis in patients with 
pheochromocytoma or paraganglioma associated with SDHx 
mutations, 18FDG-PET/CT is an ideal imaging technique for 
localization of primary and metastatic tumors.98 Sel

calization of primary and metastatic tumors.98 Selective adrenal 
venous sampling (AVS) for catecholamines is usually misleading 
and should be avoided.99 
Treatment
The treatment of choice for pheochromocytoma is complete sur­
gical resection. Surgical survival rates are 98% to 100% and are 
highly dependent on the skill of the endocrinologist, endocrine 
surgeon, and anesthesiologist team.22,32 The most common com­
plications are intraoperative blood pressure lability and postopera­
tive hypot

 blood pressure lability and postopera­
tive hypotension. Careful preoperative pharmacologic preparation 
is crucial for successful treatment.22,100 Most catecholamine-
secreting tumors are benign and can be totally excised. Tumor 
excision usually cures hypertension.
T-1
T-2
• Fig. 16.5  Magnetic resonance images of the abdomen of a 34-year-
old woman with a recent onset of palpitations and hypertension. She 
presented with acute left ventricular failure after a single dose of a 
β-adrenergic b

r failure after a single dose of a 
β-adrenergic blocker. The 24-hour urine test for total metanephrines and 
catecholamines showed the following: total metanephrines, 3800 μg (nor­
mal, <1000 μg); norepinephrine, 37 μg (normal, <170 μg); epinephrine, 
7.7 μg (normal, <35 μg); and dopamine, 147 μg (normal, <700 μg). The 
images show a slightly heterogeneous, right adrenal mass (3.3 × 3.5 × 
4.5 cm) consistent with pheochromocytoma (arrows) that has increased 
signal intensity on T2-weighted imag

as increased 
signal intensity on T2-weighted images (lower panel). After α-adrenergic 
blockade and restoration of normal left ventricular function, the patient 
had a laparoscopic adrenalectomy to remove a 5 cm × 4 cm × 3 cm, 33-g 
pheochromocytoma. Postoperatively, the 24-hour urinary excretion of total 
metanephrines normalized.
A
13
Ι
B
• Fig. 16.6  Gallium-68 (68Ga) 1,4,7,10-tetraazacyclododecane-1,4,7,10-
tetraacetic acid (DOTA)-octreotate (DOTATATE) positron emission tomog­
raphy (PET) a

 (DOTATATE) positron emission tomog­
raphy (PET) and 123I-metaiodobenzylguanidine (123I-MIBG) imaging from a 
57-year-old woman who had an 8.7-cm right adrenal pheochromocytoma 
resected 7 years previously. The results from a 24-hour urine for fraction­
ated metanephrines and catecholamines suggested recurrent disease. 
Contemporaneous 123I-MIBG and 68Ga-DOTATATE PET scans demon­
strate the superiority of 68Ga-DOTATATE PET. (A) 68Ga-DOTATATE PET scan 
shows widespread metastatic lesions that wer

scan 
shows widespread metastatic lesions that were seen throughout the axial 
and appendicular skeleton and retroperitoneal lymph node involvement. 
(B) 123I-MIBG scintigraphy shows very faint increased radiotracer uptake in 
three vertebral bodies (T3, T8, and T10) and medial right iliac bone.
556
cated for all patients with catecholamine-secreting neopla

r all patients with catecholamine-secreting neoplasms, 
including those who are asymptomatic and normotensive.22,100 
However, no randomized controlled trials have compared the dif­
ferent approaches.101 Combined α-adrenergic and β-adrenergic 
blockade is one approach to control blood pressure and prevent 
intraoperative hypertensive crises.100 α-Adrenergic blockade 
should be started 7 to 10 days preoperatively to normalize blood 
pressure and expand the contracted blood volume. A longer 
durat

xpand the contracted blood volume. A longer 
duration of preoperative α-adrenergic blockade is indicated for 
patients with recent myocardial infarction, catecholamine cardio­
myopathy, or catecholamine-induced vasculitis. Blood pressure 
should be monitored with the patient in the seated and standing 
positions twice daily. Target blood pressure is low-normal blood 
pressure for age (e.g., <120/80 mm Hg in the seated position), 
with systolic blood pressure greater than 90 mm Hg (standing); 
bo

ood pressure greater than 90 mm Hg (standing); 
both targets should be modified on the basis of the patient’s age 
and comorbid disease. Orthostasis is not a goal of treatment 
but rather a side effect. Therefore, on the second or third day 
of α-adrenergic blockade, patients are encouraged to start a diet 
high in sodium content (≥5000 mg/day) because of the catechol­
amine-induced volume contraction and the orthostasis associated 
with α-adrenergic blockade. This degree of volume expansion 
ma

rgic blockade. This degree of volume expansion 
may be contraindicated in patients with congestive heart failure 
or renal insufficiency. After adequate α-adrenergic blockade has 
been achieved, β-adrenergic blockade is initiated, typically 2 to 3 
days preoperatively.
α-Adrenergic Blockade
Phenoxybenzamine is the preferred drug for preoperative prepa­
ration to control blood pressure and arrhythmia. It is an irre­
versible, long-acting, nonspecific α-adrenergic blocking agent. 
The initial dosa

fic α-adrenergic blocking agent. 
The initial dosage is 10 mg once or twice daily, and the dose is 
increased by 10 to 20 mg in divided doses every 2 to 3 days as 
needed to control blood pressure and spells (Table 16.8). The 
final dosage of phenoxybenzamine is typically between 20 and 
100 mg daily. The patient should be warned about the orthosta­
sis, nasal congestion, retrograde ejaculation in men, and marked 
fatigue that occur in almost all patients. With their more favor­
able side effect

 patients. With their more favor­
able side effect profiles, selective α1-adrenergic blocking agents 
(e.g., prazosin, terazosin, doxazosin) are preferable to phenoxy­
benzamine when long-term pharmacologic treatment is indi­
cated (e.g., for metastatic pheochromocytoma). However, due 
to the high cost of phenoxybenzamine, selective α1-adrenergic 
blocking agents are being used more commonly for preoperative 
preparation. 
β-Adrenergic Blockade
The β-adrenergic antagonist should be administered 

he β-adrenergic antagonist should be administered only after 
α-adrenergic blockade is effective because with β-adrenergic 
blockade alone, severe hypertension or cardiopulmonary decom­
pensation may occur as a result of the unopposed α-adrenergic 
  Drugs That May Interfere With 
Metaiodobenzylguanidine (MIBG) Uptake
Uptake-1 Inhibitiona
Antiemetics (e.g., prochlorperazine)
Antipsychotics (e.g., chlorpromazine, haloperidol)
Cocaine
Labetalol
Phenylpropanolamine
Tricyclic antidepressants (e.g.,

enylpropanolamine
Tricyclic antidepressants (e.g., amitriptyline, amoxapine, desipramine, 
doxepin, imipramine, nortriptyline)
Depletion of Storage Vesicle Contentsb
Amphetamines (e.g., dextroamphetamine, fenfluramine, phentermine)
Dopamine
Labetalol
Reserpine
Sympathomimetics (e.g., ephedrine, phenylephrine, pseudoephedrine, 
salbutamol, terbutaline)
Inhibition of Vesicular Monoamine Transportersb
Reserpine
Unknown Mechanisma
Calcium channel blockers (e.g., diltiazem, nicardipine, nifedipine, 

ckers (e.g., diltiazem, nicardipine, nifedipine, 
nimodipine, verapamil)
aShould be stopped at least 48 hours before MIBG administration.
bShould be stopped at least 72 hours before MIBG administration.
   
TABLE 16.7
  Orally Administered Drugs Used to Treat 
Drug
Initial Dosage, 
mg/daya 
(Maximum)
α-Adrenergic Blocking Agents
Phenoxybenzamine
10b (100)b
Postural hypotension, tachycar­
dia, miosis, nasal congestion, 
diarrhea, retrograde ejacula­
tion, fatigue
Pr

n, 
Prazosin
1 (20)c
First-dose effect, dizziness, 
drowsiness, headache, 
fatigue, palpitations, nausea
Terazosin
1 (20)b
First-dose effect, asthenia, 
blurred vision, dizziness, 
nasal congestion, nausea, 
peripheral edema, palpita­
tions, somnolence
Doxazosin
1 (20)
First-dose effect, orthostasis, 
peripheral edema, fatigue, 
somnolence
Combined α-Adrenergic and β-Adrenergic Blocking Agent
Labetalol
200b (1200)b
Dizziness, fatigue, nausea, nasal 
co

congestion, impotence
Calcium Channel Blocker
Nicardipine 
sustained-
release
30b (120)b
Edema, dizziness, headache, 
flushing, nausea, dyspepsia
Catecholamine Synthesis Inhibitor
α-Methyl-p-l-
tyrosine 
(metyrosine)
1000c (4000)c
Sedation, diarrhea, anxiety, 
nightmares, crystalluria, 
galactorrhea, extrapyramidal 
symptoms
aGiven once daily unless otherwise indicated.
bGiven in two doses daily.
cGiven in three or four doses daily.
   
TABLE 16.8

iven in three or four doses daily.
   
TABLE 16.8
557
stimulation. Preoperative β-adrenergic blockade is indicated 
to control the tachycardia associated with both the high con­
centrations of circulating catecholamines and the α-adrenergic 
blockade. The clinician should exercise caution if the patient is 
asthmatic or has congestive heart failure. Chronic catecholamine 
excess can produce a myocardiopathy that may become evident 
with the initiation of β-adre

 may become evident 
with the initiation of β-adrenergic blockade, resulting in acute 
pulmonary edema. Therefore, when the β-adrenergic blocker is 
administered, it should be used cautiously and at a low dose. For 
example, a patient is usually given 10 mg of propranolol every 6 
hours to start. On the second day of treatment, the β-adrenergic 
blockade (assuming the patient tolerates the drug) is converted to 
a single long-acting dose. The dose is then increased as necessary 
to control the t

e is then increased as necessary 
to control the tachycardia (goal heart rate is 60–80 beats/minute). 
Metyrosine should be used with caution and only after other agents 
have been ineffective or in patients in whom tumor manipulation 
or destruction (e.g., radiofrequency ablation of metastatic sites) 
will be marked.102 Most centers reserve metyrosine for patients 
who cannot be treated with the typical combined α-adrenergic 
and β-adrenergic blockade protocol 

 α-adrenergic 
and β-adrenergic blockade protocol for cardiopulmonary rea­
sons.7 Metyrosine inhibits catecholamine synthesis by blocking the 
enzyme tyrosine hydroxylase.103 The side effects of metyrosine can 
be disabling; with long-term therapy, they include sedation, depres­
sion, diarrhea, anxiety, nightmares, crystalluria and urolithiasis, 
galactorrhea, and extrapyramidal signs. Metyrosine may be added 
to α-adrenergic and β-adrenergic blockade if the resection will be 
difficult (e.g., m

ckade if the resection will be 
difficult (e.g., malignant paraganglioma) or if destructive therapy 
is planned (e.g., radiofrequency ablation of hepatic metastases).7 
Our typical protocol with short-term preprocedure preparation is 
to start with metyrosine 250 mg every 6 hours on day 1, 500 mg 
every 6 hours on day 2, 750 mg every 6 hours on day 3, and 1000 
mg every 6 hours on the day before the procedure, with the last 
dose (1000 mg) given on the morning of the procedure.7 With 
this short

on the morning of the procedure.7 With 
this short-course therapy, the main side effect is hypersomnolence. 
Calcium Channel Blockers
Calcium channel blockers, which block norepinephrine-mediated 
calcium transport into vascular smooth muscle, have been used 
successfully at several medical centers to preoperatively prepare 
patients with pheochromocytoma.104 Nicardipine is the most 
commonly used calcium channel blocker in this setting; the start­
ing dose is 30 mg twice daily of the sustained-

t­
ing dose is 30 mg twice daily of the sustained-release prepara­
tion (see Table 16.8).105,106 Nicardipine is given orally to control 
blood pressure preoperatively and if needed is given as an intra­
venous infusion intraoperatively (Table 16.9). Although there is 
less collective experience with calcium channel blockers than with 
α-adrenergic and β-adrenergic blockers, when calcium channel 
blockers are used as the primary mode of antihypertensive ther­
apy, they may be just as effective.10

ensive ther­
apy, they may be just as effective.106,107 Clearly, the exclusive use 
of calcium channel blockers for the perioperative management of 
patients with catecholamine-secreting tumors does not prevent all 
hemodynamic changes; however, its use has been associated with 
low morbidity and mortality rates.107 The main role for this class 
of drugs may be either to supplement the combined α-adrenergic 
and β-adrenergic blockade protocol when blood pressure control 
is inadequate or to repl

n blood pressure control 
is inadequate or to replace the adrenergic blockade protocol in 
patients with intolerable side effects. 
Acute Hypertensive Crises
Acute hypertensive crises may occur before or during an opera­
tion, and they should be treated with intravenously administered 
sodium nitroprusside, phentolamine, or nicardipine (see Table 
16.9). Sodium nitroprusside is an ideal vasodilator for intraop­
erative management of hypertensive episodes because of its rapid 
onset of action and

episodes because of its rapid 
onset of action and short duration of effect. It is administered 
as an intravenous infusion at 0.5 to 5 μg/kg of body weight per 
minute and adjusted every few minutes for target blood pressure 
response; to keep the steady-state thiocyanate concentration below 
1 mmol/L, the rate of a prolonged infusion should be no more than 
3 μg/kg per minute. Phentolamine is a short-acting, nonselective 
α-adrenergic blocker that is available in lyophilized form in 5-mg 
vial

hat is available in lyophilized form in 5-mg 
vials. An initial test dose of 1 mg is administered and is followed, 
if necessary, by repeat 5-mg boluses or continuous infusion. The 
response to phentolamine is maximal 2 to 3 minutes after a bolus 
injection and lasts 10 to 15 minutes. Nicardipine can be started at 
an infusion rate of 5 mg per hour and titrated for blood pressure 
control (the infusion rate may be increased by 2.5 mg/hour every 
15 minutes up to a maximum of 15 mg/hour) (see Tab

15 minutes up to a maximum of 15 mg/hour) (see Table 16.9). 
Anesthesia and Surgery
Surgical resection of a catecholamine-secreting tumor is a high-
risk surgical procedure, and an experienced surgeon-anesthesi­
ologist team is required. The last oral doses of α-adrenergic and 
β-adrenergic blockers can be administered early in the morning on 
the day of the operation. Fentanyl, ketamine, and morphine should 
be avoided because they potentially can stimulate catecholamine 
  Intravenously Admin

an stimulate catecholamine 
  Intravenously Administered Drugs Used 
to Treat Pheochromocytoma
Agent
Dosage Range
For Hypertension
Administer a 1-mg IV test dose, then 2-mg to 5-mg 
IV boluses as needed or continuous infusion.
Nitroprusside
IV infusion rates of 2 μg/kg of body weight per minute 
are suggested as safe. Rates >4 μg/kg per minute 
may lead to cyanide toxicity within 3 hours. Doses 
>10 μg/kg per minute are rarely required, and the 
maximal dose should not exceed 800 μ

red, and the 
maximal dose should not exceed 800 μg/min.
Nicardipine
Initiate therapy at 5 mg/hr; the IV infusion rate 
may be increased by 2.5 mg/hr q15 min up to 
a maximum of 15 mg/hr.
For Cardiac Arrhythmia
Lidocaine
Initiate therapy with an IV bolus of 1–1.5 mg/kg 
(75–100 mg); additional boluses of 0.5–0.75 mg/
kg (25–50 mg) can be given q5–10 min if needed 
up to a maximum of 3 mg/kg. Loading is followed 
by maintenance IV infusion of 2–4 mg/min (30–50 
μg/kg per minute) adjusted for effe

mg/min (30–50 
μg/kg per minute) adjusted for effect and settings 
of altered metabolism (e.g., heart failure, liver con­
gestion) and as guided by blood level monitoring.
Esmolol
An initial IV loading dose of 0.5 mg/kg is infused over 
1 minute, followed by a maintenance infusion of 
0.05 mg/kg per minute for the next 4 minutes. 
Depending on the desired ventricular response, 
the maintenance infusion may then be continued 
at 0.05 mg/kg per minute or increased stepwise 
(e.g., by 0.1 mg/kg per

ute or increased stepwise 
(e.g., by 0.1 mg/kg per minute increments to a 
maximum of 0.2 mg/kg per minute), with each 
step being maintained for ≥4 minutes.
IV, Intravenous; q, every.
   
TABLE 16.9
558
release from a pheochromocytoma.108 Also, parasympathetic ner­
vous system blockade with atropine should be avoided because of 
the associated tachycardia. Anesthesia may be induced with intra­
venous injection of propofol, etomidate, or ba

ra­
venous injection of propofol, etomidate, or barbiturates in com­
bination with synthetic opioids.108 Most anesthetic gases can be 
used, but halothane and desflurane should be avoided. Cardiovas­
cular and hemodynamic variables must be monitored closely. Con­
tinuous measurement of intra-arterial pressure and heart rhythm is 
required. If the patient has congestive heart failure or decreased car­
diac reserve, monitoring of pulmonary capillary wedge pressure is 
indicated. The preoperative a

y wedge pressure is 
indicated. The preoperative and perioperative treatment approach 
outlined here is the same for adults and children.109,110
The laparoscopic approach to the adrenal gland is currently 
the procedure of choice for patients with solitary adrenal pheo­
chromocytomas smaller than 8 cm in diameter.111–113 If the pheo­
chromocytoma is in the adrenal gland, the entire gland should 
be removed. Laparoscopic adrenalectomy for pheochromocytoma 
should be converted to open adrenalectom

mocytoma 
should be converted to open adrenalectomy in cases of difficult 
dissection, invasion, adhesions, or surgeon inexperience. Great 
care should be taken to avoid tumor capsule rupture—this can 
create incurable diffuse peritoneal disease implants from an appar­
ent benign pheochromocytoma.114 If the tumor is malignant, 
as much of the tumor should be removed as possible. If a bilat­
eral adrenalectomy is planned preoperatively, the patient should 
receive glucocorticoid stress coverage w

t should 
receive glucocorticoid stress coverage while awaiting transfer to 
the operating room. In addition, glucocorticoid coverage should 
be initiated in the operating room if unexpected bilateral adre­
nalectomy is necessary. Cortical-sparing bilateral adrenalectomies 
have been used to treat patients with VHL disease.115,116
An anterior midline abdominal surgical approach is indicated 
for abdominal paragangliomas. The midline abdomen should be 
inspected carefully. Paragangliomas of the n

d be 
inspected carefully. Paragangliomas of the neck, chest, and urinary 
bladder require specialized approaches.117 Unresectable cardiac 
pheochromocytomas may require cardiac transplantation.118
Hypotension may occur during and after surgical resection of 
the pheochromocytoma, and it should be treated with fluids and 
colloids and then intravenous pressor agents if necessary. Postop­
erative hypotension occurs less frequently in patients who have had 
adequate preoperative α-adrenergic block

have had 
adequate preoperative α-adrenergic blockade and volume expan­
sion. If both adrenal glands were manipulated during surgery, adre­
nocortical insufficiency should be considered as a potential cause 
of postoperative hypotension. Because hypoglycemia can occur in 
the immediate postoperative period, blood glucose levels should 
be monitored, and fluid given intravenously should contain 5% 
dextrose. Blood pressure is usually normal by the time of hospital 
discharge. Long-standing, persi

 time of hospital 
discharge. Long-standing, persistent hypertension does occur and 
may be due to resection-related renal injury, resetting of barorecep­
tors, hemodynamic changes, structural changes of the blood vessels, 
altered sensitivity of the vessels to pressor substances, functional or 
structural renal changes, or coincident primary hypertension. 
Long-Term Postoperative Follow-Up
Approximately 1 to 2 weeks after surgery, 24-hour urinary fraction­
ated catecholamines and metanephrines 

y fraction­
ated catecholamines and metanephrines should be measured. If the 
levels are normal, the resection of the pheochromocytoma should 
be considered complete. The survival rate after removal of a benign 
pheochromocytoma is almost equal to that of age-matched and sex-
matched normal control subjects. Increased levels of fractionated 
catecholamines and metanephrines detected postoperatively are con­
sistent with residual tumor (i.e., a second primary lesion or occult 
metastases). If bil

cond primary lesion or occult 
metastases). If bilateral adrenalectomy was performed, lifelong glu­
cocorticoid and mineralocorticoid replacement therapy is prescribed.
The risk for recurrent disease (usually metastatic) in patients 
with apparent benign pheochromocytoma or paraganglioma is 
approximately 15% on long-term follow-up.119 The 24-hour uri­
nary excretion of fractionated catecholamines and metanephrines 
or plasma fractionated metanephrines should be checked annu­
ally for life.22 An

hrines should be checked annu­
ally for life.22 Annual biochemical testing assesses for metastatic 
disease (which can occur even 50 years later),120 tumor recurrence 
in the adrenal bed, and delayed appearance of multiple primary 
tumors. Recurrence rates are highest for patients with familial dis­
ease, large tumor size (>5 cm), or paraganglioma.121 Follow-up 
CT or MRI is not needed unless metanephrine or catecholamine 
levels become elevated or the original tumor was associated with 
minimal

or the original tumor was associated with 
minimal catecholamine excess.
Genetic testing should be considered for patients younger than 
45 years of age or those with one or more of the following: a fam­
ily history of pheochromocytoma; paraganglioma; and any sign 
that suggests a genetic cause, such as retinal angiomas, axillary 
freckling, café au lait spots, cerebellar tumor, MTC, or hyperpara­
thyroidism. In addition, all first-degree relatives of a patient with 
pheochromocytoma or paragang

es of a patient with 
pheochromocytoma or paraganglioma should have biochemical 
testing (e.g., 24-hour urine collection for fractionated metaneph­
rines and catecholamines). If mutation testing in a patient is posi­
tive, the patient’s first-degree relatives should be offered genetic 
testing. 
Metastatic Pheochromocytoma and 
Distinguishing between benign and malignant catecholamine-
secreting tumors is difficult on the basis of clinical, biochemical, 
or histopathologic characte

linical, biochemical, 
or histopathologic characteristics.12 Malignancy is rare in patients 
with MEN2 or VHL syndrome, but it is common in those with 
familial paraganglioma caused by mutations in SDHB.122,123 In a 
series of 272 patients with metastatic pheochromocytoma or para­
ganglioma, the median age at initial tumor diagnosis was 39 years 
(range, 7–83 years), and in 65% of patients the metastases devel­
oped at a median of 5.5 years (range, 0.3–53.4 years) from the ini­
tial diagnosis.12

e, 0.3–53.4 years) from the ini­
tial diagnosis.120 The median overall and disease-specific survivals 
were 24.6 and 33.7 years, respectively.120 The clinician should first 
assess the pace of the malignant disease and then target the level of 
therapy to the aggressiveness of tumor behavior. A multimodality, 
multidisciplinary, individualized approach is indicated to control 
catecholamine-dependent symptoms, local mass effect symptoms 
from the tumor, and overall tumor burden. Long-term pharma

 tumor, and overall tumor burden. Long-term pharmaco­
logic therapy for the patient with metastatic pheochromocytoma 
is similar to that outlined for preoperative preparation in a patient 
with a catecholamine-secreting tumor.
Metastatic sites include local tissue invasion, bone, liver, lung, 
omentum, and lymph nodes. Metastatic lesions should be resected, 
if possible, to decrease tumor burden.124 Skeletal metastatic lesions 
that are painful or threaten structural function can be treated 
wit

r threaten structural function can be treated 
with external radiotherapy or thermalablation or approached 
surgically.102,125 Thrombotic therapy for large, unresectable liver 
metastases and radiofrequency ablation for small (<3 cm) liver 
metastases are options to be considered.102 In selected cases, long-
acting octreotide has been beneficial.126 Because of the risk of mas­
sive catecholamine release, ablative therapy should be performed 
with great caution and only at centers with experience

 great caution and only at centers with experience with these 
techniques; in addition to α-adrenergic and β-adrenergic block­
ade, these patients are usually treated with metyrosine before the 
procedure.102 External radiotherapy can also be used to treat unre­
sectable soft tissue lesions.125
559
Local tumor irradiation with therapeutic doses of 131I-MIBG 
has produced partial and temporary responses in approximately 
one-third of patients.9,11,127 If the tum

imately 
one-third of patients.9,11,127 If the tumor is considered aggres­
sive and the patient’s quality of life is affected, combination 
chemotherapy can provide disease stabilization.128,129 In a non­
randomized, single-arm trial, the efficacy of chemotherapy with a 
combination CVD protocol (cyclophosphamide 750 mg/m2 body 
surface area on day 1, vincristine 1.4 mg/m2 on day 1, and dacar­
bazine 600 mg/m2 on days 1 and 2; repeated every 21 days) was 
studied in 14 patients with malignant ph

ays) was 
studied in 14 patients with malignant pheochromocytoma.130,131 
This protocol produced a complete and partial response rate of 
57% (median duration, 21 months; range, 7 to >34 months). 
Complete and partial biochemical responses were seen in 79% of 
patients (median duration, >22 months; range, 6 to >35 months). 
All responding patients had objective improvement in perfor­
mance status and blood pressure.130,131 CVD chemotherapy can 
be continued until the patient develops new lesions

e continued until the patient develops new lesions or there is 
a significant (e.g., >25%) increase in the size of known tumor 
sites. Because CVD chemotherapy may induce massive catechol­
amine release, it is important that the patient be optimally α and 
β blocked, just as for surgery. In addition, the first cycle of CVD 
should be completed in the hospital and with close medical obser­
vation. Management of metastatic pheochromocytoma can be 
frustrating because there are no curative options.

frustrating because there are no curative options. Tyrosine kinase 
inhibitors (e.g., sunitinib) may have a role in the treatment of 
metastatic pheochromocytoma; however, they are not curative.132 
Pheochromocytoma in Pregnancy
Pheochromocytoma in pregnancy can cause the death of both the 
fetus and the mother.133,134 The approach to the biochemical diag­
nosis is the same as that for the nonpregnant patient. MRI (with­
out gadolinium enhancement) is the preferred imaging modality, 
and 123I-MI

t) is the preferred imaging modality, 
and 123I-MIBG scintigraphy and 68Ga-DOTATATE PET/CT are 
contraindicated. The treatment of hypertensive crises is the same as 
for nonpregnant patients except that use of nitroprusside should be 
avoided. Although the most appropriate management is debated, 
adrenal pheochromocytomas should be removed in the second 
trimester if diagnosed during the first or second trimester of preg­
nancy. The preoperative preparation is the same as for a nonpregnant 
pati

preparation is the same as for a nonpregnant 
patient. If the pregnancy is already in the third trimester, medical 
management is indicated and delivery completed by cesarean sec­
tion. Spontaneous labor and delivery should be avoided. The pheo­
chromocytoma can be resected postpartum. The management of 
catecholamine-secreting paragangliomas in pregnancy may require 
modification of these guidelines depending on tumor location. 
Renin-Angiotensin-Aldosterone System
The components of the renin-a

n-Aldosterone System
The components of the renin-angiotensin-aldosterone (RAA) 
system are shown in Fig. 16.7.135 Aldosterone is secreted from 
the zona glomerulosa under the control of three primary factors: 
angiotensin II, potassium, and ACTH. The secretion of aldoste­
rone is restricted to the zona glomerulosa because of zone-specific 
expression of aldosterone synthase (P450 11B2) (see Chapter 15). 
Dopamine, atrial natriuretic peptide, and heparin inhibit aldoste­
rone secretion.
Renin and

heparin inhibit aldoste­
Renin and Angiotensin
Renin is an enzyme that is produced primarily in the juxtaglomer­
ular apparatus of the kidney; it is stored in granules and released 
in response to specific secretagogues. The protein consists of 340 
amino acids, of which the first 43 are a prosegment that is cleaved 
to produce the active enzyme. The release of renin into the cir­
culation is the rate-limiting step in the RAA system. Renal renin 
release is controlled by four fac

ase of 
renin, particularly in response to upright posture
	4.	 Humoral factors, including potassium, angiotensin II, and 
atrial natriuretic peptides
Renin release is maximized in conditions of low renal perfusion 
pressure or low tubular sodium content (e.g., renal artery stenosis, 
hemorrhage, volume depletion). Renin release is suppressed by 
elevated perfusion pressure at the kidney (e.g., hypertension) and 
a high-sodium diet. Renin release is increased directly by hypoka­
lemia and decre

e is increased directly by hypoka­
lemia and decreased by hyperkalemia.
Angiotensinogen, an α2-globulin synthesized in the liver, is 
the only known substrate for renin and is broken down into the 
angiotensin peptides. The protein consists of 485 amino acids, 33 
of which constitute a presegment that is cleaved after secretion. 
The action of renin on angiotensinogen produces angiotensin I. 
Angiotensin I comprises the first 10–amino acid sequence after 
the presegment and does not appear to ha

ce after 
the presegment and does not appear to have biologic activity. 
Angiotensin II, the main biologically active angiotensin, is created 
by cleavage of the two carboxy-terminal peptides of angiotensin I 
by angiotensin-converting enzyme (ACE) (see Fig. 16.7). ACE is 
localized to cell membranes in the lung and intracellular granules 
in certain tissues that produce angiotensin II. Amino peptidase 
A can remove the amino-terminal aspartic acid to produce the 
Prorenin
Renin

id to produce the 
Prorenin
Renin
Angiotensin 1-7
Angiotensin III
Angiotensin IV
Inactive
   fragments
receptors
Aldosterone synthesis and secretion
Constriction of vascular smooth muscle
Release of NE and EPI from adrenal medulla
Central sympathetic outflow and NE release
Release of vasopressin
Angiotensin-
converting enzyme
Endopeptidase
Amino-
peptidases
• Fig. 16.7  Components of the renin-angiotensin system. EPI, epinephrine; 
NE, 

 renin-angiotensin system. EPI, epinephrine; 
NE, norepinephrine. (Adapted and redrawn from Williams GH, Chao J, 
Chao L. Kidney hormones. In: Conn PM, Melmed S, eds. Endocrinology: 
Basic and Clinical Principles. Totowa, NJ: Humana Press; 1997:393–404.)
560
heptapeptide, angiotensin III. Angiotensin II and angiotensin III 
have equivalent efficacy in promoting aldosterone secretion and 
modifying renal blood flow. The half-life in the circ

ifying renal blood flow. The half-life in the circulation of 
angiotensin II is short (<60 seconds). Elements of the RAA system 
are present in the adrenals, kidneys, heart, and brain. For example, 
the adrenal glomerulosa cells contain the proteins needed to pro­
duce and secrete angiotensin II. Other tissues contain one or more 
components of the system but require other cells or circulating 
components, or both, to generate angiotensin II.
Angiotensin II functions through the angiotensin rece

iotensin II functions through the angiotensin receptor to 
maintain normal extracellular volume and blood pressure by (1) 
increasing aldosterone secretion from the zona glomerulosa via 
increased transcription of CYP11B2; (2) constricting vascular 
smooth muscle, thereby increasing blood pressure and reducing 
renal blood flow; (3) releasing norepinephrine and epinephrine 
from the adrenal medulla; (4) enhancing the activity of the sympa­
thetic nervous system by increasing central sympathetic 

 nervous system by increasing central sympathetic outflow, 
thereby increasing norepinephrine discharge from sympathetic 
nerve terminals; and (5) promoting the release of vasopressin. 
Approximately 50% to 70% of aldosterone circulates bound to 
albumin or weakly bound to corticosteroid-binding globulin; 
30% to 50% of total plasma aldosterone is free. The half-life is 
relatively short at 15 to 20 minutes. In the liver, aldosterone is rap­
idly inactivated to tetrahydroaldosterone.

is rap­
idly inactivated to tetrahydroaldosterone. The classic functions of 
aldosterone are regulation of extracellular volume and control of 
potassium homeostasis. These effects are mediated by the binding 
of free aldosterone to the mineralocorticoid receptor in the cytosol 
of epithelial cells, principally in the kidney.
Mineralocorticoid receptors have tissue-specific expression. 
For example, the tissues with the highest concentrations of these 
receptors are the distal nephron, colon, an

these 
receptors are the distal nephron, colon, and hippocampus. Lower 
levels of mineralocorticoid receptors are found in the rest of the 
gastrointestinal tract and heart. Transport to the nucleus and bind­
ing to specific binding domains on targeted genes leads to their 
increased expression. The aldosterone-regulated kinases, primarily 
serum-inducible and glucocorticoid-inducible kinase-1 (SGK1) 
appear to be key intermediaries, and increased SGK1 expression 
leads to modification of the ap

d SGK1 expression 
leads to modification of the apical sodium channel, resulting in 
increased sodium ion transport across the cell membrane (see 
Chapter 15). The increased luminal negativity augments tubular 
secretion of potassium by the tubular cells and of hydrogen ion by 
the interstitial cells.
Endogenous glucocorticoids and mineralocorticoids bind 
equally to the mineralocorticoid receptor. Specificity of action 
is provided in many tissues by the presence of a glucocorticoid-
inactivati

es by the presence of a glucocorticoid-
inactivating enzyme, 11β-hydroxysteroid dehydrogenase 2, which 
prevents cortisol and corticosterone from interacting with the 
receptor (see Chapter 15). Mineralocorticoid escape refers to the 
counterregulatory mechanisms that are manifested after 3 to 5 
days of excessive mineralocorticoid administration. Several mech­
anisms contribute to this escape, including renal hemodynamic 
factors and increased levels of atrial natriuretic peptide.
In addition t

evels of atrial natriuretic peptide.
In addition to the classic genomic actions mediated by aldo­
sterone binding to cytosolic receptors, mineralocorticoids have 
acute, nongenomic actions resulting from activation of an uniden­
tified cell surface receptor. This action involves a G protein–signal­
ing pathway and probably a modification of the sodium-hydrogen 
exchange activity. This effect has been demonstrated in both epi­
thelial and nonepithelial cells.136
Aldosterone has additional, noncla

elial cells.136
Aldosterone has additional, nonclassic effects primarily on 
nonepithelial cells.137 These actions, although probably genomic 
and therefore mediated by activation of the cytosolic mineralocor­
ticoid receptor, do not include modification of sodium-potassium 
balance. Aldosterone-mediated actions include the expression 
of several collagen genes, genes controlling tissue growth factors 
(e.g., transforming growth factor-β, plasminogen activator inhibi­
tor type 1) and genes media

ogen activator inhibi­
tor type 1) and genes mediating inflammation.138 The resultant 
actions lead to microangiopathy, necrosis (acutely), and fibrosis 
in various tissues, such as the heart, the vasculature, and the kid­
ney.137 Increased levels of aldosterone are not necessary to cause 
this damage; an imbalance between the volume or sodium bal­
ance state and the level of aldosterone appears to be the critical 
factors.137
The action of angiotensin II on aldosterone involves a nega­
tive fee

tensin II on aldosterone involves a nega­
tive feedback loop that also includes extracellular fluid volume 
(Fig. 16.8).139 The major function of this feedback loop is to 
modify sodium homeostasis and, secondarily, to regulate blood 
pressure. Sodium restriction activates the RAA axis. The effects 
of angiotensin II on both the adrenal cortex and the renal 
Renal
potassium
excretion
Renal
perfusion
pressure
Potassium
balance
release
Juxtaglomerular
cells
Renal Na+
retention
Angioten

cells
Renal Na+
retention
Angiotensin II and III
Circulating
blood volume
Renin release
Potassium
(liver)
ACE (lung)
Blood pressure
Macula densa
“feedback”
based on
tubular Na+
• Fig. 16.8  Renin-angiotensin-aldosterone and potassium-aldosterone negative feedback loops. Aldo­
sterone production is determined by input from each loop. ACE, angiotensin-converting enzyme; Na+, 
sodium. (Adapted and redrawn from Williams GH, Dluhy RG. Disea

pted and redrawn from Williams GH, Dluhy RG. Diseases of the adrenal cortex. In: Braunwald 
E, Fauci AD, Kasper D, et al, eds. Harrison’s Principles of Internal Medicine. 15th ed. New York, NY: 
McGraw-Hill; 2001:2087.)
561
vasculature promote renal sodium conservation. On the other 
hand, with suppression of renin release and suppression of the 
level of circulating angiotensin, aldosterone secretion is reduced 
and renal blood flow is increased, promoting sod

and renal blood flow is increased, promoting sodium loss. The 
RAA loop is very sensitive to dietary sodium intake. Sodium 
excess enhances the responsiveness of the renal and peripheral 
vasculature and reduces the adrenal responsiveness to angiotensin 
II. Sodium restriction has the opposite effect. Therefore, sodium 
intake modifies target tissue responsiveness to angiotensin II, a 
fine-tuning mechanism that appears to be critical to maintain­
ing normal sodium homeostasis without a chroni

in­
ing normal sodium homeostasis without a chronic effect on 
blood pressure. Excess aldosterone secretion causes hyperten­
sion through two main mechanisms: mineralocorticoid-induced 
expansion of plasma and extracellular fluid volume and increased 
total peripheral vascular resistance. 
Primary Aldosteronism
Hypertension, suppressed plasma renin activity (PRA), and 
increased aldosterone excretion characterize the syndrome of pri­
mary aldosteronism, which was first fully described in 1955.14

ronism, which was first fully described in 1955.140 
Aldosterone-producing adenoma (APA) and bilateral idiopathic 
hyperaldosteronism (IHA) are the most common subtypes of pri­
mary aldosteronism (Table 16.10). Somatic mutations account for 
nearly all of APAs and include mutations in genes encoding com­
ponents of the Kir 3.4 (GIRK4) potassium channel (KCNJ5), the 
sodium/potassium and calcium ATPases (ATP1A1 and ATP2B3), 
a voltage-dependent C-type calcium channel (CACNA1D), and 
β-catenin–act

type calcium channel (CACNA1D), and 
β-catenin–activating mutations (CTNNB1).141,142 A much less 
common form, unilateral hyperplasia or primary adrenal hyper­
plasia (PAH), is caused by micronodular or macronodular hyper­
plasia of the zona glomerulosa of predominantly one adrenal 
gland. Familial hyperaldosteronism (FH) is also rare, and four 
types have been described (see later discussion).141
History
In his presidential address at the annual meeting of the Central 
Society for Clinical Rese

 meeting of the Central 
Society for Clinical Research, Chicago, Illinois, October 29, 
1954, Dr. Jerome W. Conn stated140: “I have prepared no compre­
hensive review of my personal philosophy of clinical investigation. 
Instead, I plan to make a scientific report to you about a clinical 
syndrome, the investigation of which has been most exciting to 
me since I initiated it in April of this year.” Conn, a professor of 
medicine at the University of Michigan, had been active in govern­
ment-fund

 of Michigan, had been active in govern­
ment-funded research on the mechanisms of human acclimatiza­
tion to humid heat. He established that the body’s acclimatization 
response was to rapidly diminish renal salt and water loss and to 
abruptly curtail the salt content of body sweat and saliva. He sug­
gested that these responses were the result of increased adrenocor­
tical function with elaboration of salt-retaining steroids. He also 
showed that intramuscular administration of deoxycorticost

hat intramuscular administration of deoxycorticosterone 
acetate (DOCA) produced similar changes in the electrolyte com­
position of urine, sweat, and saliva.
In April 1954, Professor Conn was asked to see M.W., a 
34-year-old woman with a 7-year history of muscle spasms, tem­
porary paralysis, tetany, and weakness and a 4-year history of 
hypertension. She was found to have a blood pressure of 176/104 
mm Hg, severe hypokalemia (1.6–2.5 mEq/L), mild hypernatre­
mia (146–151 mEq/L), and alkalosi

mild hypernatre­
mia (146–151 mEq/L), and alkalosis (serum pH 7.62). Because 
there were no signs or symptoms of glucocorticoid or androgen 
excess, Conn suspected, based on his past research, that M.W.’s 
clinical presentation could result from excess secretion of the adre­
nal salt-retaining corticoid. Conn studied M.W. in the Metabo­
lism Research Unit for 227 days. The Streeten bioassay technique 
developed to measure sodium retention in adrenalectomized rats 
after intraperitoneal injection

nalectomized rats 
after intraperitoneal injection of human urine was used, and M.W. 
averaged 1333 μg DOCA equivalent per day, compared with 
normotensive control subjects at 61.4 μg/day. In his presidential 
address, Conn stated: “It is believed that these studies delineate a 
new clinical syndrome, which is designated temporarily as primary 
aldosteronism.”140 (Note: The word temporarily was used because 
aldosterone was yet to be measured in any human bodily fluid.)
Conn planned for a bilate

any human bodily fluid.)
Conn planned for a bilateral adrenalectomy for his patient on 
December 10, 1954. In 1995, Gittler and Fajans described the 
surgical scene: “To the immense delight of Conn and those in 
the operating room, the surgeon, Dr. William Baum, encountered 
a right 13-g adrenal tumor, which was removed while leaving 
the contralateral gland intact. The patient’s postoperative studies 
showed an almost total reversal of the preoperative metabolic and 
  Adrenocortical Causes of

erative metabolic and 
  Adrenocortical Causes of Hypertension
Low Renin and High Aldosterone
Aldosterone-producing adenoma (APA)—30% of cases
Bilateral idiopathic hyperplasia (IHA)—60% of cases
Primary (unilateral) adrenal hyperplasia—2% of cases
Aldosterone-producing adrenocortical carcinoma—<1% of cases
Familial hyperaldosteronism (FH)
FH type I (CYP11B1/CYP11B2 germline chimeric gene)—<1% of cases
FH type II (APA or IHA; germline CLCN2 mutations—<6% of cases
FH type II

; germline CLCN2 mutations—<6% of cases
FH type III (germline KCNJ5 mutations)—<1% of cases
FH type IV (germline CACNA1H mutations)—<0.1% of cases
Ectopic aldosterone-producing adenoma or carcinoma—<0.1% 
of cases
Low Renin and Low Aldosterone
Apparent mineralocorticoid excess (AME)/11β-HSD 2 deficiency
Genetic
Type 1 AME
Type 2 AME
Acquired
Licoric

ncy
Genetic
Type 1 AME
Type 2 AME
Acquired
Exogenous glucocorticoid administration—most common cause
ACTH-dependent—85% of cases
Pituitary
Ectopic
ACTH-independent—15% of cases
Unilateral adrenal disease (adenoma or carcinoma)
Bilateral adrenal disease
Bilateral adenoma
Macronodular hyperplasia
Primary pigmented nodular adrenal disease (rare)
ACTH, Corticotropin; HSD, hydroxysteroid dehydro

)
ACTH, Corticotropin; HSD, hydroxysteroid dehydrogenase.
   
TABLE 16.10
562
clinical abnormalities. Conn had achieved irrefutable proof of the 
validity of his investigative conclusions and established for the first 
time the relationship among an adrenal aldosterone-producing 
tumor, hypertension, and hypokalemia. A new era had arrived in 
the study of hypertension and adrenal mineralocorticoids.”143
By 1964, Conn had collected 145 cases

ticoids.”143
By 1964, Conn had collected 145 cases,144 and he suggested 
that up to 20% of patients with essential hypertension might have 
primary aldosteronism.145 This suggestion was downplayed by 
most as a gross overestimate.146,147 Later, Conn decreased his pre­
dicted prevalence of primary aldosteronism to 10% of hyperten­
sives,148 a prediction that was substantiated nearly 40 years later. 
In the past, clinicians would not consider the diagnosis of primary 
aldosteronism unle

sider the diagnosis of primary 
aldosteronism unless the patient presented with spontaneous hypo­
kalemia, and then the diagnostic evaluation would require discon­
tinuation of antihypertensive medications for at least 2 weeks. This 
diagnostic approach resulted in predicted prevalence rates of less 
than 0.5% of hypertensive patients.14,146,147,149–152 However, it is 
now recognized that most patients with primary aldosteronism are 
not hypokalemic153,154 and that screening can be completed whi

mic153,154 and that screening can be completed while 
the patient is taking antihypertensive drugs.153 Measurement of 
plasma aldosterone concentration (PAC) and renin activity or 
concentration (PRA or PRC, respectively) as a case-detection test, 
followed by aldosterone suppression for confirmatory testing, has 
resulted in much higher prevalence estimates for primary aldoste­
ronism—5% to 10% of all patients with hypertension.153–158 
The diagnosis of primary aldosteroni

 Presentation
The diagnosis of primary aldosteronism is usually made in patients 
who are in the third to sixth decades of life. Few symptoms are 
specific to the syndrome. Patients with marked hypokalemia may 
have muscle weakness and cramping, headaches, palpitations, poly­
dipsia, polyuria, nocturia, or a combination of these.159 Periodic 
paralysis is a very rare presentation in Caucasians, but it is not an 
infrequent presentation in patients of Asian descent.160 The poly­
uria and nocturia

s of Asian descent.160 The poly­
uria and nocturia are a result of hypokalemia-induced renal con­
centrating defect, and the presentation is frequently mistaken for 
prostatism in men. There are no specific physical findings. Edema 
is not a common finding because of the phenomenon of miner­
alocorticoid escape, described earlier. The degree of hypertension 
is typically moderate to severe and may be resistant to the usual 
pharmacologic treatments.159,161 In the first 262 cases of primary 
aldo

ts.159,161 In the first 262 cases of primary 
aldosteronism diagnosed at Mayo Clinic (1957–1986), the highest 
blood pressure was 260/155 mm Hg; the mean (± SD [standard 
deviation]) was 184/112 ± 28/16 mm Hg.161 Patients with APA 
tend to have higher blood pressures than those with IHA.
Hypokalemia is frequently absent, so all patients with hyper­
tension are candidates for this disorder. In other patients, the 
hypokalemia becomes evident only with the addition of a potas­
sium-wasting diureti

with the addition of a potas­
sium-wasting diuretic (e.g., hydrochlorothiazide, furosemide). 
Deep-seated renal cysts are found in up to 60% of patients with 
chronic hypokalemia.162 Because of a reset osmostat, the serum 
sodium concentration tends to be high-normal or slightly above 
the upper limit of normal. This clinical clue is very useful in the 
initial assessment for potential primary aldosteronism.
Several studies have shown that patients with primary aldoste­
ronism are at higher risk

ts with primary aldoste­
ronism are at higher risk than other patients with hypertension 
for target-organ damage of the heart and kidney.163,164 Chronic 
kidney disease is common in patients with long-standing primary 
aldosteronism.165 When matched for age, blood pressure, and 
duration of hypertension, patients with primary aldosteronism 
have greater left ventricular mass measurements than patients with 
other types of hypertension (e.g., pheochromocytoma, Cushing 
syndrome, essential hypert

ochromocytoma, Cushing 
syndrome, essential hypertension).166 In patients with APA, the 
left ventricular wall thickness and mass were markedly decreased 
1 year after adrenalectomy.167 A case control study of 124 patients 
with primary aldosteronism and 465 patients with essential 
hypertension (matched for age, sex, and systolic and diastolic 
blood pressure) found that patients presenting with either APA 
or IHA had a significantly higher rate of cardiovascular events 
(e.g., stroke, atrial f

 of cardiovascular events 
(e.g., stroke, atrial fibrillation, myocardial infarction) than the 
matched patients with essential hypertension.163 A negative effect 
of circulating aldosterone on cardiac function was found in young 
nonhypertensive subjects with glucocorticoid-remediable aldoste­
ronism (GRA) who had increased left ventricular wall thickness 
and reduced diastolic function compared with age-matched and 
sex-matched control subjects.168
The risk to develop new onset diabetes mellit

.168
The risk to develop new onset diabetes mellitus was docu­
mented in a study of 2367 patients with primary aldosteronism 
who had no prior diagnosis of diabetes mellitus, where 754 surgi­
cally treated patients with APA were matched with 3016 essential 
hypertension controls.169 The patients with primary aldosteron­
ism who underwent adrenalectomy had statistically significant 
reduced risk for incident diabetes and all-cause of mortality, com­
pared with matched hypertensive controls.169
Se

m­
Several studies have documented the negative impact of pri­
mary aldosteronism on quality of life.170 Untreated patients with 
primary aldosteronism have an impaired physical and mental 
quality of life compared to the general population. Symptoms of 
anxiety, demoralization, stress, depression, and nervousness were 
more frequently reported in untreated patients with primary aldo­
steronism than in the general population and in patients with 
hype

 the general population and in patients with 
hypertension.170 A prospective quality-of-life study compared 
patients with primary aldosteronism who were managed surgically 
to those managed medically.171 After 1 year, almost all quality-
of-life measures had normalized for surgically managed patients; 
whereas for patients on medical treatment, most of the measures 
had improved but not to the level of the general population.171 
Diagnosis
There is no typical clinical phenotype to guide the cli

 is no typical clinical phenotype to guide the clinician for 
when to suspect primary aldosteronism—72% of patients with 
primary aldosteronism are normokalemic.155,157 Most clinical 
practice guidelines recommend case-detection testing in high-risk 
groups for primary aldosteronism (e.g., sustained blood pressure 
>150/100 mm Hg, hypertension resistant to three conventional 
antihypertensive drugs, controlled blood pressure on four or 
more antihypertensive drugs, hypertension and spontaneous o

hypertensive drugs, hypertension and spontaneous or 
diuretic-induced hypokalemia, hypertension and adrenal inciden­
taloma, hypertension and sleep apnea, hypertension and a family 
history of early onset hypertension or cerebrovascular accident at 
a young age).153 However, most primary care physicians do not 
test patients for primary aldosteronism—most patients go undiag­
nosed and untreated.172–174 Due to the excess cardiovascular mor­
bidity and nephrotoxicity associated with primary aldost

 and nephrotoxicity associated with primary aldosteronism, 
a case can be made that all patients with hypertension should be 
tested for this disorder at least once (Fig. 16.9).
The diagnostic approach to primary aldosteronism can be con­
sidered in three phases: case-detection tests, confirmatory tests, 
and subtype evaluation tests.
563
Case-Detection Tests
Case-detection testing can be accomplished by paired measure­
ments of PAC and PRA in a random morning 

measure­
ments of PAC and PRA in a random morning ambulatory blood 
sample (preferably obtained between 8 and 10 am) (see Fig. 
16.9). This test may be performed while the patient is taking 
antihypertensive medications (with some exceptions, discussed 
later) and without posture stimulation.159 Hypokalemia reduces 
the secretion of aldosterone, and it is optimal to restore the 
serum level of potassium to normal before performing diagnos­
tic studies.
It may be difficult to interpret data obtai

udies.
It may be difficult to interpret data obtained from patients 
treated with a mineralocorticoid receptor antagonist (MRA) (spi­
ronolactone and eplerenone). These drugs prevent aldosterone 
from activating the receptor, resulting sequentially in sodium loss, 
a decrease in plasma volume, and an elevation in renin, which can 
lead to false-negative case-detection testing. For this reason, spi­
ronolactone and eplerenone should not be initiated until the eval­
uation is completed and the fin

ed until the eval­
uation is completed and the final decisions about treatment are 
made. However, there are exceptions to this rule. For example, if 
the patient is hypokalemic despite treatment with spironolactone 
or eplerenone, then the mineralocorticoid receptors are not fully 
blocked, and PRA or PRC should be suppressed in such a patient 
with primary aldosteronism. Thus if renin is suppressed, the evalu­
ation for primary aldosteronism (case-detection testing, confirma­
tory testing, and

ase-detection testing, confirma­
tory testing, and AVS) can proceed despite treatment with MRAs. 
Whereas if renin is not suppressed in a patient treated with a min­
eralocorticoid antagonist, therapy should be discontinued for at 
least 6 weeks before retesting. Other potassium-sparing diuretics, 
such as amiloride and triamterene, usually do not interfere with 
testing unless the patient is on high doses.
ACE inhibitors and angiotensin receptor blockers (ARBs) have 
the potential to elevate re

 blockers (ARBs) have 
the potential to elevate renin in patients with primary aldosteron­
ism. Therefore the finding of detectable renin in a patient taking 
one of these drugs does not exclude the diagnosis of primary aldo­
steronism. However, a PRA level less than 1 ng/mL per hour or 
PRC below the reference range in a patient taking an ACE inhibi­
tor or ARB makes primary aldosteronism likely.
The PAC:PRA ratio, first proposed as a case-detection test 
for primary aldosteronism in 1981,175 i

tion test 
for primary aldosteronism in 1981,175 is based on the concept 
of paired hormone measurements. The PAC is measured in 
nanograms per deciliter; the PRA is measured in nanograms per 
milliliter per hour. In a hypertensive hypokalemic patient, sec­
ondary hyperaldosteronism should be considered if both PRA and 
PAC are increased and the PAC:PRA ratio is less than 10 (e.g., 
renovascular disease). An alternative source of mineralocorticoid 
receptor agonism should be considered if both P

d 
receptor agonism should be considered if both PRA and PAC are 
suppressed (e.g., hypercortisolism). Primary aldosteronism should 
be suspected if the PRA is suppressed (e.g., <1.0 ng/mL per hour) 
and the PAC is increased (e.g., >10 ng/dL) (see Fig. 16.9). It is 
critical for the clinician to recognize that the measurement of PAC 
and renin is only a case-detection tool, and most positive results 
should be followed by a confirmatory aldosterone suppression test 
to verify autonomous aldoster

ne suppression test 
to verify autonomous aldosterone production before treatment is 
initiated.153 The exception to the requirement for confirmatory 
testing involves patients with hypertension, spontaneous hypoka­
lemia, and marked primary aldosteronism (e.g., PAC >20 ng/dL 
and PRA <1.0 ng/mL per hour) (see Fig. 16.9).153 
Confirmatory Tests
A positive case detection test with PAC above 10 ng/dL and PRA 
below 1 ng/mL per hour is not diagnostic by itself, and (in the 
absence of spontaneous h

c by itself, and (in the 
absence of spontaneous hypokalemia—see earlier discussion) 
primary aldosteronism must be confirmed by demonstration 
of inappropriate aldosterone secretion.154 The list of drugs and 
hormones capable of affecting the RAA axis is extensive, and a 
medication-contaminated evaluation is frequently unavoidable in 
patients with poorly controlled hypertension despite a three-drug 
program. Calcium channel blockers and α1-adrenergic receptor 
blockers do not affect the diagn

energic receptor 
blockers do not affect the diagnostic accuracy in most cases.153 
However, as long as renin is suppressed and PAC is above 10 ng/
dL, the clinician can proceed with confirmatory testing regard­
less of medications. No medication causes false-positive testing for 
primary aldosteronism when PAC is above 10 ng/dL.
Aldosterone suppression testing can be performed with orally 
administered sodium chloride and measurement of urinary aldo­
sterone or with intravenous sodium chloride 

aldo­
sterone or with intravenous sodium chloride loading and measure­
ment of PAC.153,159
Oral Sodium Loading Test
After hypertension and hypokalemia have been controlled, patients 
should receive a high-sodium diet (supplemented with sodium 
chloride tablets if needed) for 3 days, with a goal sodium intake 
of 5000 mg (equivalent to 218 mEq sodium or 12.8 g sodium 
chloride).161 The risk of increasing dietary sodium in patients with 
severe hypertension must be assessed in each case.176 Becaus

ertension must be assessed in each case.176 Because the 
high-sodium diet can increase kaliuresis and hypokalemia, vigor­
ous replacement of potassium chloride may be needed, and the 
serum level of potassium should be monitored daily. On the third 
day of the high-sodium diet, a 24-hour urine specimen is col­
lected for measurement of aldosterone, sodium, and creatinine. 
To document adequate sodium repletion, the 24-hour urinary 
sodium excretion should exceed 200 mEq. Urinary aldosterone 
exc

on should exceed 200 mEq. Urinary aldosterone 
excretion of more than 12 μg/24 hours in this setting is consis­
tent with autonomous aldosterone secretion.161 The sensitivity 
and specificity of the oral sodium loading test are 96% and 93%, 
respectively.177 
Intravenous Saline Infusion Test
The intravenous saline infusion test has also been used widely 
for the diagnosis of primary aldosteronism.153,154 Normal sub­
jects show suppression of PAC after volume expansion with iso­
tonic saline; sub

after volume expansion with iso­
tonic saline; subjects with primary aldosteronism do not show 
Morning blood sample in seated ambulant patient
• All patients with hypertension should be tested
When to consider testing for primary aldosteronism:
Investigate for primary aldosteronism
• Plasma aldosterone concentration (PAC)
• Plasma renin activity (PRA) or PRC
PAC (≥10 ng/dL)
  PRA (<1 ng/mL/hr) or
   PRC (<lower limit of normal for the assay)
• Fig. 16.9  This algorithm provides guidance on when

ig. 16.9  This algorithm provides guidance on when to consider test­
ing for primary aldosteronism and use of plasma aldosterone concentra­
tion (PAC) and plasma renin activity (PRA) as a case-detection tool. PRC, 
plasma renin concentration.
564
this suppression. The test is done in the seated position after an 
overnight fast.178 Two liters of 0.9% sodium chloride solution is 
infused intravenously with an infusion pump over 4 hours. Bloo

avenously with an infusion pump over 4 hours. Blood 
pressure and heart rate are monitored during the infusion. At the 
completion of the infusion, blood is drawn for measurement of 
PAC. PAC levels in normal subjects decrease to less than 5 ng/dL, 
whereas most patients with primary aldosteronism do not sup­
press to less than 10 ng/dL. Postinfusion PAC values between 5 
and 10 ng/dL are indeterminate and may be seen in patients with 
IHA. 
Fludrocortisone Suppression Test
In the fludrocortison

rocortisone Suppression Test
In the fludrocortisone suppression test, fludrocortisone acetate is 
administered for 4 days (0.1 mg every 6 hours) in combination 
with sodium chloride tablets (2 g three times daily with food). 
Blood pressure and serum potassium levels must be monitored 
daily. In the setting of low PRA, failure to suppress the upright 
10 am PAC to less than 6 ng/dL on day 4 is diagnostic of primary 
aldosteronism.179 Increased QT interval dispersion and deteriora­
tion of left v

 interval dispersion and deteriora­
tion of left ventricular function have been reported during fludro­
cortisone suppression tests.176 Most centers no longer use this test. 
Subtype Studies
After case-detection and confirmatory testing, the third manage­
ment issue guides the therapeutic approach by distinguishing 
APA and PAH from IHA and GRA. Unilateral adrenalectomy in 
patients with APA or PAH results in normalization of hypokale­
mia in all cases; hypertension is improved in all cases and 

 cases; hypertension is improved in all cases and is cured 
in 30% to 60%.88,180–182 In IHA unilateral or bilateral adrenalec­
tomy seldom corrects the hypertension.161 IHA and GRA should 
be treated medically. APA is found in approximately 35% of cases 
and bilateral IHA in approximately 60% (see Table 16.10). APAs 
are usually small hypodense adrenal nodules (<2 cm in diameter) 
on CT and are golden yellow when resected. IHA adrenal glands 
may be normal on CT or may show nodular changes. Aldo

 be normal on CT or may show nodular changes. Aldoste­
rone-producing adrenal carcinomas are almost always larger than 
4 cm in diameter and have an inhomogeneous imaging phenotype 
on CT (see Table 16.6).
Computed Tomography of the Adrenal Glands
Primary aldosteronism subtype evaluation may require one or 
more tests, the first of which is imaging of the adrenal glands 
with CT (Fig. 16.10). If a solitary unilateral low noncontrast CT 
attenuation (<10 HU) macroadenoma (>1 cm) and normal con­
t

on (<10 HU) macroadenoma (>1 cm) and normal con­
tralateral adrenal morphologic appearance are found on CT in a 
young patient (<35 years) with severe primary aldosteronism, uni­
lateral adrenalectomy is a reasonable therapeutic option (see Fig. 
16.10).153,183,184 However, most patients with primary aldosteron­
ism are over age 35 years, and in many cases, CT shows normal-
appearing adrenals, minimal unilateral adrenal limb thickening, 
unilateral microadenomas (≤1 cm), or bilateral macroadenom

al microadenomas (≤1 cm), or bilateral macroadenomas 
(Fig. 16.11). In these cases, additional testing is required to deter­
mine the source of excess aldosterone secretion.
Small APAs may be labeled incorrectly as IHA on the basis of 
CT findings of bilateral nodularity or normal-appearing adrenals. 
Also, apparent adrenal microadenomas may actually represent 
areas of hyperplasia, and unilateral adrenalectomy would be inap­
propriate. In addition, nonfunctioning unilateral adrenal mac­
roadeno

on, nonfunctioning unilateral adrenal mac­
roadenomas are not uncommon, especially in older patients (>40 
years).185 Unilateral PAH may be visible on CT, or the PAH may 
appear normal on CT. In general, patients with APAs have more 
severe hypertension, more frequent hypokalemia, higher levels 
of plasma aldosterone (>30 ng/dL) and urinary aldosterone (>30 
μg/24 hours), and are younger (<50 years), compared with those 
who have IHA.161 Patients fitting these descriptors are considered 
to have

 fitting these descriptors are considered 
to have a high probability of APA regardless of the CT findings 
(see Fig. 16.10), and 41% of patients with a high probability of 
APA and a normal adrenal CT scan prove to have unilateral aldo­
sterone hypersecretion.186
Adrenal CT is not accurate in distinguishing between APA 
and IHA.183,186–188 In a study of 203 patients with primary aldo­
steronism who were evaluated with both CT and AVS, CT was 
accurate in only 53% of patients; based on the CT fi

curate in only 53% of patients; based on the CT findings, 42 
patients (22%) would have been incorrectly excluded as candi­
dates for adrenalectomy and 48 patients (25%) might have had 
unnecessary or inappropriate surgery.186 In another study of 208 
patients with primary aldosteronism, based on CT findings, 19 
patients (12%) would have been bypassed for curative surgery and 
48 patients (30%) would have had noncurative surgery.188 In a 
systematic review of 38 studies involving 950 patients w

atic review of 38 studies involving 950 patients with pri­
mary aldosteronism, adrenal CT/MRI results did not agree with 
the findings from AVS in 359 patients (38%); based on CT/MRI, 
19% of the 950 patients would have undergone noncurative sur­
gery, and 19% would have been offered medical therapy instead of 
curative adrenalectomy.187
A study published in 2016 caused some confusion regarding 
the well-documented superiority of AVS compared to CT.189 The 
reported success rate of CT and failur

CT.189 The 
reported success rate of CT and failure rate of AVS were much 
higher than any other series. In addition, the interpretation of the 
data was complicated by the use of MRAs postoperatively—lead­
ing to blood pressure control in the patients with uncured primary 
aldosteronism from both groups. The authors used an inappropri­
ately low cutoff for the adrenal-to-IVC cortisol gradient of greater 
Confirmed primary aldosteronism
Adrenal CT scan
Unilateral
hypodense
nodule >1 cm
and marke

T scan
hypodense
and marked PA
Normal
Micronodularity
Bilateral masses
>35 y
consider
<35 y
consider
High
probability
APA
Lateralization
with AVS
No
lateralization
with AVS
Low
APA
APA or PAH:
laparoscopic
adrenalectomy
IHA:
therapy
AVS
• Fig. 16.10  Subtype evaluation of primary aldosteronism. For patients who 
want to pursue a surgical treatment for their hypertension, adrenal venous 
sampling is frequently a key diagnostic step (see

sampling is frequently a key diagnostic step (see text for details). APA, 
aldosterone-producing adenoma; AVS, adrenal venous sampling; CT, 
computed tomography; IHA, idiopathic hyperaldosteronism; PA, primary 
aldosteronism; PAH, primary adrenal hyperplasia. (Modified from Young 
Jr WF, Hogan MJ. Renin-independent hypermineralocorticoidism. Trends 
Endocrinol Metab. 1994;5:97–106.)
565
than 3:1.189 When cosyntropin is used during AVS, the minimal 
cortisol gr

ropin is used during AVS, the minimal 
cortisol gradient cutoff to determine successful catheterizations 
is greater than 5:1 (see AVS discussion to come). The persistent 
postoperative primary aldosteronism in 11% of the AVS-directed 
treatment group simply reflected the lack of reliable AVS data—
likely related to multiple factors.189 The key message from this 
study was not that CT and AVS were equivalent in subtype test­
ing; rather, centers that have a suboptimal AVS program will have 
poor

that have a suboptimal AVS program will have 
poor AVS-directed outcomes. Thus centers that have low-volume 
AVS programs or enlist multiple interventional radiologists (and 
dilute the expertise) or simply lack the effort to ensure accurate 
AVS,190 should refer their patients with primary aldosteronism to 
centers that champion expertise in AVS.
Therefore AVS is essential to direct appropriate therapy in 
patients with primary aldosteronism who have a high probability 
of APA and are seeking a

 have a high probability 
of APA and are seeking a potential surgical cure. 
Adrenal Venous Sampling
AVS is the criterion standard test to distinguish between uni­
lateral and bilateral disease in patients with primary aldosteron­
ism.153,183,187,188 AVS is an intricate procedure because the right 
adrenal vein is small and may be difficult to locate and cannulate; 
the success rate depends on the proficiency of the angiographer.191 
A review of 47 reports found that the success rate for cannula

47 reports found that the success rate for cannulation 
of the right adrenal vein in 384 patients was 74%.161 With experi­
ence and focusing the expertise to one or two radiologists at a refer­
ral center, the AVS success rate can be as high as 96%.186,192–194
The five keys to a successful AVS program are (1) appropri­
ate patient selection, (2) careful patient preparation, (3) focused 
technical expertise, (4) defined protocol, and (5) accurate data 
interpretation.191 A center-specific written

data 
interpretation.191 A center-specific written protocol is manda­
tory. The protocol should be developed by an interested group of 
endocrinologists, hypertension specialists, internists, radiologists, 
and laboratory personnel. Safeguards should be in place to prevent 
mislabeling of the blood tubes in the radiology suite and to pre­
vent sample mixup in the laboratory.191
At Mayo Clinic, we use continuous cosyntropin infusion dur­
ing AVS (50 μg/hour starting 30 minutes before sampling and

50 μg/hour starting 30 minutes before sampling and 
continuing throughout the procedure) for the following reasons: 
(1) to minimize stress-induced fluctuations in aldosterone secre­
tion during nonsimultaneous AVS, (2) to maximize the gradient 
in cortisol from adrenal vein to IVC and thus confirm success­
ful sampling of the adrenal veins,188,195 and (3) to maximize the 
secretion of aldosterone from an APA.186,191,195 The adrenal veins 
are catheterized through the percutaneous femoral vein a

theterized through the percutaneous femoral vein approach, 
and the position of the catheter tip is verified by gentle injection 
of a small amount of nonionic contrast medium and radiographic 
documentation. Blood is obtained from both adrenal veins and 
from the IVC below the renal veins and assayed for aldosterone 
and cortisol concentrations. To be sure that there is no cross-con­
tamination, the IVC sample should be obtained from the exter­
nal iliac vein. The venous sample from the left si

nal iliac vein. The venous sample from the left side typically is 
obtained from the common phrenic vein immediately adjacent to 
the entrance of the adrenal vein. Whereas the blood sample from 
the right adrenal vein is obtained just at the orifice of the vein—to 
stabilize the catheter in proper position and avoid the error of 
advancing the catheter tip too deep into the adrenal vein—a guide 
wire may be used.196 The cortisol concentrations from the adrenal 
veins and IVC are used to confirm 

om the adrenal 
veins and IVC are used to confirm successful catheterization—
a minimal adrenal vein-to-IVC cortisol gradient is 5:1 or more. 
At Mayo Clinic, the mean adrenal-to-IVC cortisol gradient was 
33.9:1 on the right side and 23.8:1 on the left.186
Dividing the right and left adrenal vein PAC values by their 
respective cortisol concentrations corrects for the dilutional effect 
of the inferior phrenic vein flow into the left adrenal vein; these 
are termed cortisol-corrected ratios (Fi

n; these 
are termed cortisol-corrected ratios (Fig. 16.12). In patients with 
APA, the mean cortisol-corrected aldosterone ratio (i.e., the ratio 
of PAC/cortisol from the APA side to that from the normal side) 
is 18:1.186 A cutoff point of more than 4:1 for this ratio is used to 
indicate unilateral aldosterone excess (see Fig. 16.12). In patients 
with IHA, the mean cortisol-corrected aldosterone ratio is 1.8:1 
(high side to low side), and a ratio of less than 3:1 suggests bilat­
eral aldos

 ratio of less than 3:1 suggests bilat­
eral aldosterone hypersecretion (see Fig. 16.12).186 Therefore most 
patients with a unilateral source of aldosterone have cortisol-cor­
rected aldosterone lateralization ratios greater than 4, and ratios 
greater than 3 but less than 4 represent a zone of overlap. Ratios no 
higher than 3 are consistent with bilateral aldosterone secretion.
In addition to the AVS aldosterone lateralization ratio, a con­
tralateral suppression index can be calculated: The 

alateral suppression index can be calculated: The contralateral 
(nondominant adrenal) aldosterone-cortisol (A/C) ratio is divided 
by the IVC A/C ratio. In the early Mayo Clinic series, the con­
tralateral suppression index was less than 1 in 93.4% of patients 
with surgically confirmed APA.186 Although a supportive finding, 
if the aldosterone lateralization ratio is 4 or above, a contralateral 
A
Vein
R adrenal
vein
L adrenal
vein
Inferior
vena cava
29,338
363
259
668
540
31
43.9
62.7
0.7
8.4

a cava
29,338
363
259
668
540
31
43.9
62.7
0.7
8.4
*R adrenal vein A/C ratio divided by L adrenal vein A/C ratio.
(A)(ng/dL)
Ratio*
Cortisol
(C)(µg/dL)
A/C
Ratio
Results of bilateral adrenal venous sampling
B
• Fig. 16.11  A 43-year-old woman had a 2-year history of hypertension 
and hypokalemia. The screening test for primary aldosteronism was posi­
tive, with a plasma aldosterone concentration (PAC) of 37 ng/dL and low 
plasma renin activity (PRA) at less than 0.6 ng/mL

plasma renin activity (PRA) at less than 0.6 ng/mL per hour (PAC/PRA ratio 
>61). The confirmatory test for primary aldosteronism was also positive, 
with the 24-hour urinary excretion of aldosterone measured at 53 μg on a 
high-sodium diet (urinary sodium, 196 mEq/24 hours). (A) Adrenal com­
puted tomography shows a 12-mm, low-density mass (arrow, right panel) 
in the medial limb of the left adrenal and two low-density, 10-mm nod­
ules (arrows, left panel) within the right adrenal gland. (B) Ad

left panel) within the right adrenal gland. (B) Adrenal venous 
sampling lateralized aldosterone secretion to the right, and two cortical 
adenomas (1.8 × 1.2 × 0.8 cm and 2.5 × 1.5 × 1.2 cm) were found at 
laparoscopic right adrenalectomy. The postoperative plasma aldosterone 
concentration was less than 1 ng/dL. Hypokalemia was cured, and blood 
pressure was normal without the aid of antihypertensive medications.
566
suppression index les

x and Endocrine Hypertension
suppression index less than 1 is not associated with better postop­
erative blood pressure outcomes and should not be a requirement 
to send patients to surgery.197,198 However, there are several situa­
tions where the contralateral suppression index can be very help­
ful. For example, when the aldosterone lateralization ratio is in the 
gray zone (3–<4), a contralateral suppression index of less than 1 
is predictive of good operative outcomes.199 In addition, if AV

 of good operative outcomes.199 In addition, if AVS 
is not bilaterally successful, a contralateral suppression index of 
less than 0.5 is highly predictive of contralateral disease.200 Also, 
patients with a contralateral suppression index of less than 0.47 
are at increased risk for postoperative hyperkalemia (see treatment 
section).201
The test characteristics of AVS using an aldosterone lateral­
ization ratio of 4 or above for detection of unilateral aldosterone 
hypersecretion (APA or PAH)

nilateral aldosterone 
hypersecretion (APA or PAH) are 95% sensitivity and 98.6% 
specificity.186,202 The use of AVS should be based on patient pref­
erence, patient age, clinical comorbid conditions, and the clinical 
probability of finding an APA. A practical approach is the selective 
use of AVS as outlined in Fig. 16.10.203
At centers with experience with AVS, the complication rate is 
2.5% or less.186,192 Complications can include symptomatic groin 
hematoma, adrenal hemorrhage, and dissect

c groin 
hematoma, adrenal hemorrhage, and dissection of an adrenal vein. 
Noninvasive Alternatives to Adrenal Vein Sampling
Noninvasive alternatives to AVS are being explored. For exam­
ple, 11C-metomidate can be used as a PET radiotracer. In an 
initial study of 25 patients with PA, dexamethasone-suppressed 
11C-metomidate PET/CT had a specificity of 87% and sensitivity 
of 76% for unilateral adrenal disease.204 It was recently reported 
that there is a high expression of CXC chemokine recepto

here is a high expression of CXC chemokine receptor type 
4 (CXCR4) in adrenal glands and APAs, which correlates with 
the expression of P450 11B2 (aldosterone synthase).205 The spe­
cific CXCR4 ligand 68Ga-pentixafor was used as a PET imaging 
molecule in localizing APAs in nine patients with primary aldo­
steronism.205 Finally, in vitro and animal studies have shown a 
high selectivity of a 18F PET imaging molecule (CDP2230) for 
P450 11B2 over P450 11B1 with a favorable biodistribution for 
i

 P450 11B1 with a favorable biodistribution for 
imaging P450 11B2.206 Clinical studies will be needed to see if 
this 18F-based PET imaging molecule or those similar to it will 
prove to be clinically useful. 
Familial Hyperaldosteronism
Familial hyperaldosteronism should be considered when primary 
aldosteronism is diagnosed before age 20 years or when it is diag­
nosed in more than one family member. To date, four forms of FH 
have been described.141 However, it is very likely that many more 

bed.141 However, it is very likely that many more 
disease-causing germline mutations will be described in the com­
ing years, and labeling each mutation with FH type V, VI, VII, 
VIII, IX, etc., will become impractical. A more practical approach 
may be to simply indicate FH due to the specific mutation.
Glucocorticoid-Remediable Aldosteronism: Familial 
Hyperaldosteronism Type I—CYP11B1/CYP11B2 Germline 
Chimeric Gene
GRA (FH type I; Online Mendelian Inheritance in Man [OMIM] 
103900) was firs

delian Inheritance in Man [OMIM] 
103900) was first described in a single family in 1966.207 Twenty-
six years later the causative CYP11B1/CYP11B2 chimeric gene 
was discovered.208 GRA is a form of hyperaldosteronism in which 
the hypersecretion of aldosterone can be reversed with physiologic 
doses of glucocorticoid.209 It is rare, as illustrated by a study of 
300 consecutive patients with primary aldosteronism; only two 
patients were diagnosed with GRA (prevalence = 0.66%) (see 
Table 16.10)

d with GRA (prevalence = 0.66%) (see 
Table 16.10).210 GRA is characterized by early-onset hypertension 
that is usually severe and refractory to conventional antihyper­
tensive therapies, aldosterone excess, suppressed PRA, and excess 
production of 18-hydroxycortisol and 18-oxocortisol. Mineralo­
corticoid production is regulated by ACTH instead of by the nor­
mal secretagogue, angiotensin II. Therefore aldosterone secretion 
can be suppressed by glucocorticoid therapy. In the absence of 
gluc

by glucocorticoid therapy. In the absence of 
glucocorticoid therapy, this mutation results in overproduction 
of aldosterone and the hybrid steroids 18-hydroxycortisol and 
18-oxocortisol, which can be measured in the urine or plasma to 
make the diagnosis.
Genetic testing is a sensitive and specific means of diagnos­
ing GRA and obviates the need to measure the urinary levels of 
18-oxocortisol and 18-hydroxycortisol or to perform dexametha­
sone suppression testing. Genetic testing for GRA sh

ne suppression testing. Genetic testing for GRA should be 
considered for patients with primary aldosteronism who have a 
family history of primary aldosteronism, onset of primary aldo­
steronism at a young age (<20 years), or family history of strokes 
at a young age. 
Familial Hyperaldosteronism Type II—CLCN2 Chloride 
Channel Germline Mutations
FH type II is autosomal dominant in inheritance.211–213 The 
hyperaldosteronism in FH type II does not suppress with dexa­
methasone, and GRA mutation

ot suppress with dexa­
methasone, and GRA mutation testing is negative. FH type II 
is more common than FH type I, but it still accounts for fewer 
than 6% of all patients with primary aldosteronism.210 In a recent 
report of a family with FH type II and 80 additional probands 
with unsolved early-onset primary aldosteronism, a germline 
CLCN2 chloride channel mutation was found in 8 of the pro­
bands.214 All relatives with early-onset primary aldosteronism car­
ried the CLCN2 variant found in t

dosteronism car­
ried the CLCN2 variant found in the proband. CLCN2 encodes 
a voltage-gated chloride channel expressed in adrenal glomerulosa 
cells. Mutations in the CLCN2 chloride channel may be respon­
sible for all or some of the individuals who have been classified at 
FH type II.215 Over time we may learn of a series of mutations 
that contribute to what has been classified as FH type II. 
APA (n = 102)
IHA (n = 84)
PAH (n = 8)
Aldosterone lateralization ratio
100
10
1
• Fig. 16.12  Adren

 lateralization ratio
100
10
1
• Fig. 16.12  Adrenal vein aldosterone lateralization ratios for patients 
with unilateral aldosterone-producing adenomas (APA), bilateral idio­
pathic hyperplasia (IHA), and unilateral primary adrenal hyperplasia (PAH). 
Shaded symbols indicate that the diagnosis was confirmed surgically. The 
sensitivity and specificity of a cortisol-corrected plasma aldosterone con­
centration lateralization ratio greater than 4 for unilateral disease are 95.2% 
and 100%, respec

for unilateral disease are 95.2% 
and 100%, respectively. (From Young Jr WF, Stanson AW, Thompson GB, 
et al. Role for adrenal venous sampling in primary aldosteronism. Surgery. 
2004;136:1227–1235. Used with permission.)
567
Familial Hyperaldosteronism Type III—Germline KCNJ5 
Mutations
FH type III (OMIM 613677) was first described in a 5-year-old 
boy in 1959.216 However, the familial nature of the disorder did 
not become apparent until the detection of prim

d 
not become apparent until the detection of primary aldosteron­
ism in the index patient’s two daughters in 2008; all presented 
with refractory hypertension before age 7 years and were treated 
with bilateral adrenalectomy.217 The adrenal glands showed mas­
sive hyperplasia. Three years later the causative germline mutation 
in this family was discovered: a point mutation in and near the 
selectivity filter of the potassium channel KCNJ5.218 This KCNJ5 
mutation produces increased sodium cond

his KCNJ5 
mutation produces increased sodium conductance and cell depo­
larization, triggering calcium entry into glomerulosa cells, the sig­
nal for aldosterone production and cell proliferation. Thus far, 12 
kindreds carrying 6 different KCNJ5 mutations have been iden­
tified: p.Thr158Ala, p.Gly151Glu, p.Gly151Arg, p. Ile157Ser, 
p.Tyr152Cys, and p.Glu145Gln.218–223 The estimated prevalence 
of germline KCNJ5 mutations is 0.3% in patients with primary 
aldosteronism and 8% among patients wit

h primary 
aldosteronism and 8% among patients with familial primary 
aldosteronism.220 Most patients with germline KCNJ5 muta­
tions present with polyuria, polydipsia, and refractory hyperten­
sion in childhood—investigations show marked hypokalemia and 
marked primary aldosteronism. In most cases the degree of aldo­
sterone hypersecretion is so marked that bilateral adrenalectomy is 
required. However, there is some heterogeneity in age at presenta­
tion (as old as 48 years),221 and the hypert

enta­
tion (as old as 48 years),221 and the hypertension and hypokalemia 
in some patients can be controlled with MRAs. 
Familial Hyperaldosteronism Type IV—Germline CACNA1H 
Gene Mutations
FH type IV (OMIM 617027) is inherited in an autosomal domi­
nant fashion and with incomplete penetrance and is caused by 
mutations in the CACNA1H gene, which encodes the α-subunit 
of an L-type voltage-gated calcium channel (Cav3.2).224,225 A 
novel germline CACNA1H mutation (p.Met1549Val) was identi­
fied i

CACNA1H mutation (p.Met1549Val) was identi­
fied in 5 of 40 unrelated patients affected by hypertension and 
primary aldosteronism in childhood.224 Four additional germline 
CACNA1H mutations have been reported in patients with pri­
mary aldosteronism.225 
Primary Aldosteronism With Seizures and Neurologic 
Abnormalities (PASNA)—Germline CACNA1D Mutations
PASNA is caused by de novo germline mutations in CACNA1D 
(OMIM 615474). CACNA1D encodes for the α1D-subunit of a 
L-type voltage-gated calciu

 the α1D-subunit of a 
L-type voltage-gated calcium channel (Cav 1.3). PASNA has been 
reported in two children with primary aldosteronism, seizures, 
and neurologic abnormalities.226 The severe neurologic abnormal­
ities do not allow these individuals to reproduce; thus, although 
due to a germline mutation, it is not technically a familial form of 
primary aldosteronism. In addition, a missense CACNA1D germ­
line mutation was reported in a patient affected by autism and 
epilepsy.227 
Primary 

ent affected by autism and 
Primary Aldosteronism and ARMC5 Mutations
Heterozygous germline mutations of the armadillo repeat con­
taining 5 (ARMC5) gene are most commonly linked to sub­
clinical glucocorticoid secretory autonomy or Cushing syndrome 
in patients with bilateral macronodular adrenal hyperplasia 
(BMAH).228 However, there are reports of ARMC5 mutations in 
patients with both primary aldosteronism and Cushing syndrome 
due to BMAH, a co-occurrence seen more commonly in

due to BMAH, a co-occurrence seen more commonly in African-
American patients.229 
Somatic Mutations in KCNJ5, ATP1A1, ATP2B3, CACNA1D, and 
CTNNB1 Genes
Somatic mutations account for the majority of APAs and include 
mutations in genes encoding components of the Kir 3.4 (GIRK4) 
potassium channel (KCNJ5), the sodium/potassium and calcium 
ATPases (ATP1A1 and ATP2B3), and a voltage-dependent C-type 
calcium channel (CACNA1D). In a multicenter study of 351 
APAs from patients with primary aldoste

dy of 351 
APAs from patients with primary aldosteronism and 130 other 
adrenocortical lesions, two somatic mutations in KCNJ5 (G151R 
or L168R) were identified in 47% of APAs.230 Somatic KCNJ5 
mutations were absent in adrenal tissue from patients with pri­
mary aldosteronism due to unilateral hyperplasia and in 130 non–
aldosterone-secreting adrenal lesions. KCNJ5 mutations were 
overrepresented in APAs from women compared with men (63% 
vs 24%), and APAs with KCNJ5 mutations had larger lesion

), and APAs with KCNJ5 mutations had larger lesional 
diameters than those without the mutation (2.7 cm vs 1.7 cm, 
respectively).230 The larger size of KCNJ5 APAs is explained in part 
by the zona fasciculata–like cells and lower levels of aldosterone 
synthase expression compared to APAs caused by somatic muta­
tions in ATP1A1, ATP2B3, or CACNA1D.231 In a meta-analysis 
including 13 studies with 1636 patients, the overall prevalence 
of somatic KCNJ5 mutations in patients with APAs was 43%.232

 KCNJ5 mutations in patients with APAs was 43%.232
Somatic APA mutations have been identified in three other 
genes: ATP1A1 encoding Na+/K+-ATPase 1, ATP2B3 encod­
ing Ca2+-ATPase 3, and CACNA1D encoding a voltage-gated 
calcium channel. In a study of 112 APAs, somatic APA muta­
tions in KCNJ5, ATP1A1, and ATP2B3 were present in 39.2%, 
6.3%, and 0.9%, respectively.233 To date, 13 different ATP1A1 
and 9 ATP2B3 somatic mutations have been reported.234 In addi­
tion, 31 different CACNA1D mutation

d.234 In addi­
tion, 31 different CACNA1D mutations have been described and 
account for approximately 9% of sporadic APAs.234 CACNA1D 
APAs are composed of zona glomerulosa–like cells and are smaller 
compared with those with either no somatic mutation or KCNJ5 
APAs.231 Somatic CACNA1D mutations are the most frequent 
somatic mutation in patients with APAs with negative or minimal 
nodularity (<1 cm) on preoperative cross-sectional imaging.235
The CTNNB1 gene encodes β-catenin, and activating 

CTNNB1 mutations have been found in 3% of sporadic APAs.236 
Similar to KCNJ5 APAs, CTNNB1 APAs are associated with 
female sex and larger adenomas. It is likely that nearly 100% of 
APAs will eventually be associated with somatic driver mutations. 
Aldosterone-Producing Cell Clusters
Immunohistochemical staining studies for P450 11B2 and P450 
11B1 have detected focal subcapsular nests of adrenocortical cells 
that extend into the zona fascicul

cortical cells 
that extend into the zona fasciculata and stain strongly for P450 11B2 
and are termed aldosterone-producing cell clusters (APCCs).237 
APCCs have been identified in normal adrenals and in pathology 
specimens from patients with primary aldosteronism, where APCCs 
may occur adjacent to APAs.237 APCCs have been shown to harbor 
somatic mutations in CACNA1D and ATP1A1 and raise the possi­
bility that APCCs may secrete aldosterone autonomously and could 
be precursors to APAs.238 In

tonomously and could 
be precursors to APAs.238 Interestingly, APCCs increase with age 
and may in part simply reflect the aging adrenal gland.239,240 The 
clinical relevance of APCCs is yet to be determined. 
Cortisol Cosecretion
Patients with primary aldosteronism have been shown to be at 
increased risk for insulin resistance and metabolic syndrome,241–243 
depression,244 and osteoporotic fractures.245 These associations are 
more in keeping with glucocorticoid secretory autonomy or excess. 

ith glucocorticoid secretory autonomy or excess. 
A steroid metabolome study found that (a) patients with primary 
568
aldosteronism (APA and IHA) had significantly increased cortisol 
and total glucocorticoid metabolite excretion compared to controls 
and patients with subclinical Cushing syndrome (p <0.001); (b) 
surrogate parameters of metabolic risk correlated with glucocorti­
coid, but not mineralocorticoid excretion; and (c) unilatera

not mineralocorticoid excretion; and (c) unilateral adre­
nalectomy in patients with APA resolved both mineralocorticoid 
and glucocorticoid excess.246 These findings suggest that some 
degree of cortisol cosecretion is common in patients with primary 
aldosteronism and is linked to metabolic risk—a finding that may 
indicate that treatment with mineralocorticoid receptor antago­
nists in patients with IHA and APA may not prevent glucocorti­
coid-dependent metabolic risks.246 However, these data

-dependent metabolic risks.246 However, these data should be 
interpreted with caution because this study did not simply identify 
cortisol cosecretion from APAs but rather an increased corticoste­
roid metabolome in patients with APA and IHA. The hypotha­
lamic-pituitary-adrenal axis is not suppressed in these patients, 
and there is no adrenal crisis or steroid withdrawal in patients with 
APAs who are treated surgically. Thus the etiology of the increased 
glucocorticoid metabolites in patien

he increased 
glucocorticoid metabolites in patients with primary aldosteronism 
is not clear and may be due to a yet to be determined mechanism.
A separate and less common issue is APAs that cosecrete corti­
sol that may be clinically important for perioperative management 
(e.g., perioperative corticosteroid coverage and postoperative 
replacement and taper).247 When should clinicians test for cortisol 
cosecretion in patients with APAs? In general, clinically impor­
tant cortisol secretion fr

eral, clinically impor­
tant cortisol secretion from an adrenal adenoma is correlated with 
tumor size. Unlike aldosterone secretion from an adenoma, clini­
cally important cortisol secretion requires a “large factory”—typi­
cally with cortical adenoma diameters greater than 2 cm.247 Thus 
it is reasonable to test patients with primary aldosteronism for cor­
tisol cosecretion when the adrenal adenoma is greater than 1.5 cm 
in diameter; testing should include baseline dehydroepiandros­
terone-su

hould include baseline dehydroepiandros­
terone-sulfate and an overnight 1-mg dexamethasone suppression 
test.248 When glucocorticoid secretory autonomy is documented 
in a patient with primary aldosteronism who has a single cortical 
adenoma greater than 1.5 cm in diameter, AVS is not needed, 
assuming that the contralateral adrenal appears normal on CT. 
Principles of Treatment
The treatment goal is to prevent the morbidity and fatality associ­
ated with hypertension, hypokalemia, cardiovascul

ated with hypertension, hypokalemia, cardiovascular damage, and 
nephrotoxicity. Knowing the cause of the primary aldosteronism 
helps to determine the appropriate treatment. Normalization of 
blood pressure should not be the only goal. In addition to the kid­
ney and colon, mineralocorticoid receptors are present in the heart, 
brain, and blood vessels. Excessive secretion of aldosterone is asso­
ciated with increased risk of cardiovascular disease and morbidity. 
Therefore normalization of ci

ease and morbidity. 
Therefore normalization of circulating aldosterone or mineralocor­
ticoid receptor blockade should be part of the management plan for 
all patients with primary aldosteronism. However, clinicians must 
understand that most patients with long-standing primary aldoste­
ronism have some degree of renal insufficiency that is masked by the 
glomerular hyperfiltration associated with aldosterone excess.249,250 
The true degree of underlying chronic kidney disease may become 
evide

nderlying chronic kidney disease may become 
evident only after effective pharmacologic or surgical therapy.249,250
Surgical Treatment of Aldosterone-Producing Adenoma 
and Unilateral Hyperplasia
Unilateral laparoscopic adrenalectomy is an excellent treatment 
option for patients with APA or unilateral hyperplasia.111 As 
outlined earlier, in terms of quality-of-life parameters, the benefi­
cial effects of surgery were superior to chronic medical treatment 
with mineralocorticoid receptor antago

 treatment 
with mineralocorticoid receptor antagonists.171 Unilateral adrenal­
ectomy is the optimal treatment for patients with unilateral adrenal 
disease. Surgical treatment with bilateral adrenalectomy is not a 
good treatment option for patients with IHA; instead lifelong treat­
ment with mineralocorticoid antagonists is the treatment of choice.
Laparoscopic adrenalectomy with an expert adrenal surgeon is 
the preferred surgical approach. Because APAs are small and may 
be multiple, the en

ecause APAs are small and may 
be multiple, the entire adrenal gland should be removed.188,251 
To decrease the surgical risk, hypokalemia should be corrected 
with potassium supplements or a mineralocorticoid receptor 
antagonist, or both, preoperatively (but discontinued immedi­
ately postoperatively). PAC should be measured 1 to 2 days after 
surgery to confirm a biochemical cure.183 In general, the number 
and dosages of antihypertensive medications can be decreased by 
50% postoperatively—d

cations can be decreased by 
50% postoperatively—discontinuing any that may predispose to 
hyperkalemia (e.g., ACE-I, ARB). The full impact of surgery on 
hypertension may take up to 3 months. Because of the risk of 
temporary hypoaldosteronism due to chronic suppression of the 
RAA axis, serum potassium levels should be monitored weekly 
for 4 weeks after surgery, and a generous sodium diet should be 
followed.201,252,253 In a multicenter study of 142 surgically treated 
patients, the prevalenc

of 142 surgically treated 
patients, the prevalence of postoperative hyperkalemia was 9.9%; 
the hyperkalemic patients were older and had worse renal func­
tion than the nonhyperkalemic patient group.254 In a study of 192 
patients with primary aldosteronism who were treated surgically, 
12 (6.3%) developed postoperative hyperkalemia (median serum 
potassium 5.5 mmol/L, range 5.2–6.2 mmol/L); median time to 
onset was 13.5 days (range, 7–55 days).201
Although blood pressure control improves in a

.201
Although blood pressure control improves in almost 100% of 
patients postoperatively, average long-term cure rates of hyperten­
sion after unilateral adrenalectomy for APA range from 30% to 
60%.88,182,183 Persistent hypertension after adrenalectomy is directly 
correlated with having more than one first-degree relative with hyper­
tension, use of more than two antihypertensive agents preoperatively, 
older age, increased serum creatinine level, and duration of hyperten­
sion and is most li

vel, and duration of hyperten­
sion and is most likely caused by coexistent primary hypertension.88 
Pharmacologic Treatment
IHA and GRA should be treated medically. In addition, although 
not optimal, APA may be treated medically if therapy includes 
mineralocorticoid receptor blockade. A sodium-restricted diet 
(<100 mEq sodium per day), maintenance of ideal body weight, 
tobacco avoidance, and regular aerobic exercise contribute signifi­
cantly to the success of pharmacologic treatment. No pl

y to the success of pharmacologic treatment. No placebo-
controlled, randomized trials have evaluated the relative efficacy 
of drugs in the treatment of primary aldosteronism.255
In a longitudinal study assessing 602 patients with primary 
aldosteronism treated with mineralocorticoid receptor antago­
nists, the incidence of cardiovascular events was higher in patients 
with primary aldosteronism compared to patients with essential 
hypertension.256 The excess risk for cardiovascular events and 

256 The excess risk for cardiovascular events and mor­
tality was limited to patients with primary aldosteronism who had 
suppressed PRA on medical treatment—suggesting inadequate 
dosing of mineralocorticoid receptor antagonists.256 Due to con­
comitant low-renin essential hypertension in some patients with 
primary aldosteronism, measurement of PRA may not be optimal 
to guide management; rather, a more practical treatment target to 
document effective mineralocorticoid blockade is a high-norm

ffective mineralocorticoid blockade is a high-normal 
serum potassium without the aid of oral potassium supplements.
569
Spironolactone has been the drug of choice to treat primary 
aldosteronism for more than 50 years. It is available as 25-mg, 
50-mg, and 100-mg tablets. The dosage is 12.5 to 25 mg per day 
initially and can be increased to 400 mg per day if necessary to 
achieve a high-normal serum potassium concentration without 
the aid of oral potassium c

concentration without 
the aid of oral potassium chloride supplementation. Hypokalemia 
responds promptly, but hypertension can take as long as 4 to 8 
weeks to be corrected. After several months of therapy, the dosage 
of spironolactone often can be decreased to as little as 25 to 50 mg 
per day; dosage titration is based on a goal serum potassium level 
in the high-normal range. Serum potassium and creatinine should 
be monitored frequently during the first 4 to 6 weeks of therapy 
(especially

ing the first 4 to 6 weeks of therapy 
(especially in patients with renal insufficiency or diabetes mellitus). 
Spironolactone increases the half-life of digoxin, and the digoxin 
dosage may need to be adjusted when treatment with spironolac­
tone is started. Concomitant therapy with salicylates should be 
avoided because they interfere with the tubular secretion of an 
active metabolite and decrease the effectiveness of spironolactone. 
However, spironolactone is not selective for the mineraloc

 spironolactone is not selective for the mineralocorticoid 
receptor. For example, antagonism at the androgen receptor may 
result in painful gynecomastia, erectile dysfunction, and decreased 
libido in men, and agonist activity at the progesterone receptor 
results in menstrual irregularity in women.
Eplerenone is a steroid-based antimineralocorticoid that acts as 
a competitive and selective mineralocorticoid receptor antagonist 
and was approved by the US Food and Drug Administration for 
the

ed by the US Food and Drug Administration for 
the treatment of uncomplicated essential hypertension in 2003. 
The 9,11-epoxide group in eplerenone results in a marked reduc­
tion of the molecule’s progestational and antiandrogenic actions; 
compared with spironolactone, eplerenone has 0.1% of the bind­
ing affinity to androgen receptors and less than 1% of the binding 
affinity to progesterone receptors. In a randomized, double-blind 
trial comparing the efficacy, safety, and tolerability of ep

aring the efficacy, safety, and tolerability of eplerenone 
to that of spironolactone (100–300 mg vs 75–225 mg, respec­
tively) in patients with primary aldosteronism, researchers found 
spironolactone to be superior in terms of lowering blood pres­
sure but associated with higher rates of male gynecomastia (21% 
vs 5% for eplerenone) and female mastodynia (21% vs 0%).257 
Eplerenone is available as 25-mg and 50-mg tablets. For primary 
aldosteronism, it is reasonable to start with a dose of 25 

nism, it is reasonable to start with a dose of 25 mg twice 
daily (twice daily because of the shorter half-life of eplerenone 
compared with spironolactone) and titrated upward; the target 
is a high-normal serum potassium concentration without the aid 
of potassium supplements. The maximum dose approved by the 
FDA for hypertension is 100 mg per day. However, potency stud­
ies with eplerenone show 25% to 50% less milligram-per-milli­
gram potency compared with spironolactone. Thus it is common 

y compared with spironolactone. Thus it is common 
to treat patients with primary aldosteronism with 200 to 300 mg 
of eplerenone per day to reach the target serum potassium concen­
tration. As with spironolactone, it is important to monitor blood 
pressure, serum potassium, and serum creatinine levels closely. 
Side effects include dizziness, headache, fatigue, diarrhea, hyper­
triglyceridemia, and elevated liver enzymes.
Patients with IHA frequently require a second antihypertensive 
agent to 

ently require a second antihypertensive 
agent to achieve good blood pressure control. Hypervolemia is 
a major reason for resistance to drug therapy, and low doses of 
a thiazide (e.g., 12.5–50 mg of hydrochlorothiazide daily) or a 
related sulfonamide diuretic are effective in combination with the 
mineralocorticoid receptor antagonist. Because these agents often 
lead to further hypokalemia, serum potassium levels should be 
monitored.
Before treatment for GRA is initiated, the diagnosis of G

treatment for GRA is initiated, the diagnosis of GRA 
should be confirmed with genetic testing. In the GRA patient, 
chronic treatment with physiologic doses of a glucocorticoid 
normalizes blood pressure and corrects hypokalemia. The clini­
cian should be cautious about iatrogenic Cushing syndrome with 
excessive doses of glucocorticoids, especially when dexamethasone 
is used in children. Shorter-acting agents such as prednisone or 
hydrocortisone should be prescribed, using the smallest effec

one should be prescribed, using the smallest effec­
tive dose in relation to body surface area (e.g., hydrocortisone, 
10–12 mg/m2/day). Target blood pressure in children should be 
guided by age-specific blood pressure percentiles. Children should 
be monitored by pediatricians with expertise in glucocorticoid 
therapy, with careful attention paid to preventing retardation of 
linear growth due to overtreatment. Treatment with mineralocor­
ticoid receptor antagonists in these patients may be ju

d receptor antagonists in these patients may be just as effec­
tive as glucocorticoids and avoids the potential disruption of the 
hypothalamic-pituitary-adrenal axis and risk of iatrogenic side 
effects. In addition, glucocorticoid therapy or mineralocorticoid 
receptor blockade may even have a role in normotensive patients 
with GRA.168 
Primary Aldosteronism in the Setting of 
Pregnancy
Primary aldosteronism is uncommon in pregnancy, and fewer 
than 50 patients are reported in the medical lit

than 50 patients are reported in the medical literature.258–263 Pri­
mary aldosteronism in pregnancy can lead to preterm delivery, 
intrauterine growth retardation, placental abruption, and intra­
uterine fetal demise.263–265 In some women, the high blood con­
centrations of pregnancy-related progesterone are antagonistic at 
the mineralocorticoid receptor and partially block the action of 
aldosterone; these patients actually have an improvement in the 
degree of hypertension and hypokalemia 

nt in the 
degree of hypertension and hypokalemia during pregnancy.266,267 
Whereas in other pregnant women, increased expression of the 
luteinizing hormone/chorionic gonadotropin receptor in APAs 
harboring β-catenin mutations has been documented, and the 
degree of hypertension and hypokalemia can be aggravated by 
the increased pregnancy-related blood levels of human chorionic 
gonadotropin.268,269
Case detection testing is identical to that in nonpregnant 
women; a morning blood sample shou

in nonpregnant 
women; a morning blood sample should be obtained for the mea­
surement of PAC and renin (PRA or PRC). If spontaneous hypo­
kalemia is present in the pregnant woman with suppressed renin 
and the PAC is greater than 20 ng/dL, additional confirmatory 
testing is not needed. However, if the patient is normokalemic, 
confirmatory testing should be pursued. Confirmatory testing in 
the setting of pregnancy can be challenging because the capto­
pril challenge test is contraindicated in

e capto­
pril challenge test is contraindicated in pregnancy, and the saline 
infusion test may not be well tolerated because of edema. The 
optimal confirmatory test in the setting of pregnancy is measure­
ment of aldosterone excretion in a 24-hour urine collection on an 
ambient sodium diet. To avoid exposure to radiation or contrast 
material, subtype testing in the setting of pregnancy should start 
with abdominal MRI without gadolinium. Unilateral APA can be 
diagnosed in the unique clinica

ateral APA can be 
diagnosed in the unique clinical setting of a pregnant woman with 
marked primary aldosteronism (spontaneous hypokalemia and 
PAC >30 ng/dL) and a clear-cut unilateral adrenal adenoma on 
MRI.202
The severity of hypertension and hypokalemia dictates the 
optimal treatment of primary aldosteronism in pregnancy. For 
example, if the patient is in the subset of patients who have a 
570
remission in the degree of primary aldo

remission in the degree of primary aldosteronism, then surgery 
or treatment with a mineralocorticoid receptor antagonist can be 
avoided until after delivery. However, if hypertension and hypo­
kalemia are marked, then surgical and/or medical intervention 
is indicated. Unilateral laparoscopic adrenalectomy during the 
second trimester can be considered in those women with severe 
primary aldosteronism and documented unilateral APA. The US 
FDA lists spironolactone as a pregnancy cat

he US 
FDA lists spironolactone as a pregnancy category C drug because 
feminization of newborn male rats has been documented. How­
ever, only one human case has been reported where treatment 
with spironolactone in pregnancy led to ambiguous genitalia in a 
male infant due to spironolactone treatment for polycystic ovary 
syndrome pre-pregnancy and through the fifth week of gesta­
tion.270 Eplerenone is a FDA pregnancy category B drug. When 
primary aldosteronism is managed medically in pregnan

mary aldosteronism is managed medically in pregnant women, 
the hypertension should be treated with standard antihyperten­
sive drugs that are approved for use during pregnancy. Hypo­
kalemia should be treated with oral potassium supplements. In 
those cases of severe primary aldosteronism in pregnancy where 
surgery is not an option, low-dose eplerenone may be cautiously 
considered.258,271,272 
Other Forms of Mineralocorticoid Excess or 
Effect
The medical disorders associated with excess mine

The medical disorders associated with excess mineralocorticoid 
effect from 11-deoxycorticosterone (DOC) and cortisol are listed 
in Table 16.10. These diagnoses should be considered if PAC and 
PRA are low in a patient with hypertension and hypokalemia.
Congenital adrenal hyperplasia (CAH) is a group of autosomal 
recessive disorders caused by enzymatic defects in adrenal ste­
roidogenesis that result in deficient secretion of cortisol 

is that result in deficient secretion of cortisol (see 
Chapter 15).273 Approximately 90% of CAH cases are caused 
by 21-hydroxylase deficiency, which does not result in hyperten­
sion.274 Deficiencies of 11β-hydroxylase (CYP11B1, P450 11B1) 
or 17α-hydroxylase (CYP17A1, P450 17A1) cause hypertension 
and hypokalemia because of hypersecretion of the mineralocor­
ticoid DOC. The mineralocorticoid effect of increased circulat­
ing levels of DOC also decreases renin and aldosterone secretion. 
Thes

o decreases renin and aldosterone secretion. 
These mutations are autosomal recessive in inheritance and typi­
cally are diagnosed in childhood. However, partial enzymatic 
defects have been shown to cause hypertension in adults.
Approximately 5% of all cases of CAH are caused by 
11β-hydroxylase deficiency; the prevalence in Caucasians is 1 
in 100,000.275 More than 40 mutations have been described in 
CYP11B1, the gene encoding 11β-hydroxylase.276 There is an 
increa

e encoding 11β-hydroxylase.276 There is an 
increased prevalence among Sephardic Jews from Morocco due to 
a founder effect. The impaired conversion of DOC to corticoste­
rone results in high levels of DOC and 11-deoxycortisol; the sub­
strate mass effect results in increased levels of adrenal androgens. 
Girls present in infancy or childhood with hypertension, hypo­
kalemia, acne, hirsutism, and virilization. Boys with CAH due to 
11β-hydroxylase deficiency present with hypertension, hypokale­

e deficiency present with hypertension, hypokale­
mia, and precocious pseudopuberty. Approximately two-thirds of 
patients have mild to moderate hypertension. The initial screening 
tests include measurement of blood levels of DOC, 11-deoxycor­
tisol, androstenedione, testosterone, and dehydroepiandrosterone 
sulfate (DHEAS)—all of which should be increased above the 
upper limit of the respective reference ranges. Confirmatory test­
ing includes germline mutation testing. 
17α-Hydroxylase Defic

 germline mutation testing. 
17α-Hydroxylase deficiency is a very rare cause of CAH; good 
prevalence data are not available, but the prevalence is likely less 
than 1 in 1 million live births.277 17α-Hydroxylase is essential 
for the synthesis of cortisol and gonadal hormones, and defi­
ciency results in decreased production of cortisol and sex steroids. 
Genetic 46,XY males present with either genital ambiguity or as 
phenotypic females, and 46,XX females present wit

phenotypic females, and 46,XX females present with primary 
amenorrhea. Therefore a person with this form of CAH may not 
come to medical attention until puberty. Children, adolescents, 
and young adults present with hypertension and spontaneous 
hypokalemia and low levels of aldosterone and renin. Although 
very rare, there is an increased prevalence of 17α-hydroxylase 
deficiency among Dutch Mennonites. The initial screening tests 
include measurement of blood levels of androstenedione, testo

surement of blood levels of androstenedione, testos­
terone, DHEAS, 17-hydroxyprogesterone, aldosterone, and cor­
tisol—all of which should be either low or at the lower quartile 
of the respective reference ranges. The plasma concentrations of 
DOC and corticosterone should be above the upper limit of the 
respective reference ranges. Confirmatory testing includes germ­
line mutation testing. 
Deoxycorticosterone-Producing Tumor
Pure DOC-producing adrenal tumors are very rare and usually 
large

ng adrenal tumors are very rare and usually 
large and malignant. Some patients have been documented to 
have benign DOC-producing adrenocortical adenomas. Some 
of these adrenal neoplasms cosecrete androgens and estrogens 
in addition to DOC, which may cause virilization in women or 
feminization in men. The typical clinical presentation would be 
that of relatively rapid onset of marked hypertension associated 
with hypokalemia and low blood levels of aldosterone and renin. 
A high level of pl

vels of aldosterone and renin. 
A high level of plasma DOC or urinary tetrahydrodeoxycortico­
sterone and a large adrenal tumor seen on CT confirm the diagno­
sis. Aldosterone secretion in these patients is typically suppressed. 
Primary Cortisol Resistance
Increased cortisol secretion and plasma cortisol concentrations 
without evidence of Cushing syndrome are found in patients 
with primary cortisol resistance (or glucocorticoid resistance), a 
rare familial syndrome.278,279 Primary cortisol r

rare familial syndrome.278,279 Primary cortisol resistance is caused 
by genetic defects in the glucocorticoid receptor and the steroid-
receptor complex. The syndrome is characterized by hypokalemic 
alkalosis, hypertension, increased plasma concentrations of DOC, 
and increased adrenal androgen secretion. The hypertension and 
hypokalemia result from the combined effects of excess DOC 
and increased cortisol access to the mineralocorticoid receptor, 
resulting from high rates of cortisol prod

eptor, 
resulting from high rates of cortisol production that overwhelm 
11β-hydroxysteroid dehydrogenase type 2 (11βHSD2) activity. 
Most affected individuals present in childhood with hypertension 
and spontaneous hypokalemia and low levels of aldosterone and 
renin. The initial screening tests include measurement of blood 
levels of cortisol, DOC, 11-deoxycortisol, androstenedione, tes­
tosterone, and DHEAS—all of which should be increased above 
the upper limit of the respective reference ra

ve 
the upper limit of the respective reference ranges. In addition, 
571
24-hour urinary cortisol excretion is above the upper limit of the 
reference range, and serum ACTH is not suppressed. Confirma­
tory testing includes germline mutation testing. 
Apparent Mineralocorticoid Excess Syndrome
Apparent mineralocorticoid excess is the result of impaired activ­
ity of the microsomal enzyme 11βHSD2, which normally inacti­
vates cortisol in the kidney by convertin

 inacti­
vates cortisol in the kidney by converting it to the inactive 11-keto 
compound, cortisone.280 Cortisol can be a potent mineralocorti­
coid, and when 11βHSD2 is genetically deficient or its activity 
blocked, high levels of cortisol accumulate in the kidney. Decreased 
11βHSD2 activity may be hereditary or it may be secondary to 
pharmacologic inhibition of enzyme activity by metabolites of 
glycyrrhizic acid, the active principle of licorice root (Glycyrrhiza 
glabra).281 The congenita

orice root (Glycyrrhiza 
glabra).281 The congenital forms are rare autosomal recessive dis­
orders; fewer than 50 patients have been identified worldwide.282 
Congenital apparent mineralocorticoid excess typically presents 
in childhood with hypertension, hypokalemia, low birth weight, 
failure to thrive, hypertension, polyuria and polydipsia, and poor 
growth.276 Acquired apparent mineralocorticoid excess due to 
licorice root ingestion presents with hypertension and hypokale­
mia—the cause bec

 with hypertension and hypokale­
mia—the cause becomes evident when a good medical history is 
obtained. In addition, when 11βHSD2 is overwhelmed by massive 
cortisol hypersecretion associated with Cushing syndrome due to 
ectopic ACTH syndrome, hypokalemic hypertension may be one 
of the outcomes. The clinical phenotype of patients with apparent 
mineralocorticoid excess due to congenital deficiency of or inhibi­
tion of 11βHSD2 includes hypertension, hypokalemia, metabolic 
alkalosis, low reni

nsion, hypokalemia, metabolic 
alkalosis, low renin, low aldosterone, and normal plasma cortisol 
levels. The diagnosis of apparent mineralocorticoid excess is con­
firmed by demonstration of an abnormal (high) ratio of cortisol to 
cortisone in a 24-hour urine collection. The characteristic abnor­
mal urinary cortisol-cortisone metabolite profile reflects decreased 
11βHSD2 activity; the ratio of cortisol to cortisone is typically 
increased 10-fold above the normal value.280 DOC levels may als

fold above the normal value.280 DOC levels may also 
be increased in severe ACTH-dependent Cushing syndrome and 
contribute to the hypertension and hypokalemia in this disorder. 
Liddle Syndrome: Abnormal Renal Tubular Ionic 
Transport
In 1963, Grant Liddle described an autosomal dominant renal 
disorder with a presentation similar to primary aldosteronism 
with hypertension, hypokalemia, and inappropriate kaliuresis.283 
However, blood levels of aldosterone and renin were very low, so 
the diso

 aldosterone and renin were very low, so 
the disorder was termed pseudoaldosteronism. Liddle syndrome 
is caused by autosomal dominant mutations in the β-unit or 
γ-subunit of the amiloride-sensitive epithelial sodium channel.276 
It is extremely rare, with fewer than 30 families reported world­
wide.284 This mutation results in enhanced activity of the epithelial 
sodium channel, and patients present with increased renal sodium 
reabsorption, potassium wasting, hypertension, and hypokalemia. 

otassium wasting, hypertension, and hypokalemia. 
However, as mentioned earlier, blood levels of aldosterone and 
renin are low. Affected individuals usually present as children or 
young adults with hypertension and spontaneous hypokalemia 
and low levels of aldosterone and renin. A family history of hyper­
tension associated with hypokalemia makes Liddle syndrome 
more likely. The finding of low aldosterone and renin levels in 
the hypokalemic hypertensive patient should raise the possibility 

hypertensive patient should raise the possibility 
of Liddle syndrome. When the other causes of this presentation 
have been excluded, then a treatment trial with amiloride or tri­
amterene should be considered. Liddle syndrome can easily be 
distinguished from apparent mineralocorticoid excess based on 
a good clinical response to amiloride or triamterene combined 
with a sodium-restricted diet, lack of efficacy of spironolactone 
and dexamethasone, and normal 24-hour urine cortisone/cortisol 

one, and normal 24-hour urine cortisone/cortisol 
ratio. Clinical genetic testing is available. 
Other Endocrine Disorders Associated With 
Iatrogenic Cushing syndrome is relatively common. However, 
endogenous Cushing disease is rare, with an incidence of less than 
1 case per 1 million people per year.285 Hypertension occurs in 
75% to 80% of patients with Cushing syndrome (see Chapter 15). 
The mechanisms of hypertension include increased production of 
DOC (in A

tension include increased production of 
DOC (in ACTH-dependent Cushing syndrome), enhanced pres­
sor sensitivity to endogenous vasoconstrictors (e.g., epinephrine, 
angiotensin II), increased cardiac output, activation of the RAA 
system by increased hepatic production of angiotensinogen, and 
cortisol activation of the mineralocorticoid receptor. 
Thyroid Dysfunction
When excessive amounts of circulating thyroid hormones interact 
with thyroid hormone receptors on peripheral ti

t 
with thyroid hormone receptors on peripheral tissues, both meta­
bolic activity and sensitivity to circulating catecholamines increase. 
Thyrotoxic patients usually have tachycardia, high cardiac output, 
increased stroke volume, decreased peripheral vascular resistance, 
and increased systolic blood pressure. The initial management 
in patients with hypertension and hyperthyroidism includes use 
of a β-adrenergic blocker to treat hypertension, tachycardia, and 
tremor. The definitive treatme

, tachycardia, and 
tremor. The definitive treatment of hyperthyroidism is cause spe­
cific (see Chapter 12). 
The frequency of hypertension (usually diastolic) is increased 
threefold in hypothyroid patients and may account for as many 
as 1% of cases of diastolic hypertension in the general popula­
tion.286,287 The mechanisms for the elevation in blood pressure 
include increased systemic vascular resistance and extracellular 
volume expansion. Treatment of thyroid hormone defic

lume expansion. Treatment of thyroid hormone deficiency 
decreases blood pressure in most patients with hypertension and 
normalizes blood pressure in one-third of them. Synthetic levo­
thyroxine is the treatment of choice for hypothyroidism (see 
Chapter 13). 
Renin-secreting tumors are rare and usually benign mesenchymal 
tumors localized to the kidney. These juxtaglomerular cell tumors 
typically present with secondary hyperaldosteronism (hyper­
tension, hypokalemia, hig

peraldosteronism (hyper­
tension, hypokalemia, high PAC, and elevated renin) in young 
adults.288 The renal neoplasm can usually be localized with con­
trast-enhanced renal CT or MRI. Occasionally renal vein renins 
may be needed to confirm localization. Surgical resection cures the 
secondary hyperaldosteronism. 
572
Hypertension occurs in 20% to 40% of the patients with acro­
megaly and is associated with sodium retention and e

galy and is associated with sodium retention and extracellu­
lar volume expansion (see Chapter 9).289,290 The hypertension 
of acromegaly is treated most effectively by curing the excess of 
growth hormone.290 If a surgical cure is not possible, the hyperten­
sion usually responds well to medications that either block growth 
hormone effect (e.g., pegvisomant) or suppress growth hormone 
secretion (e.g., somatostatin analogs). Residual hypertension can 
be treated with diuretic therapy.
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574
Reproductive Physiology, 574
Reproductive Functions of the Hypothalamus, 576
Reproductive Functions of the Anterior Pituitary, 579
Ovary, 580
Endometrium, 595
Approach to the W

 579
Ovary, 580
Approach to the Woman With Reproductive Dysfunction, 599
Disorders of the Female Reproductive System, 601
Diagnosis and Management of Anovulatory 
Uterine Bleeding, 623
Hormone-Dependent Benign Gynecologic Disorders, 628
Management of Menopause, 631
Postmenopausal Hormone Therapy, 634
17
Physiology and Pathology 
of the Female Reproductive Axis
SERDAR E. BULUN
Reproductive Physiology
Tightly coordinated functions of the hypothalamus, pituitary, 
ovaries, and endo

of the hypothalamus, pituitary, 
ovaries, and endometrium give rise to cyclic, predictable menses 
that indicate regular ovulation. Regular ovulation requires nor­
mal functioning of other endocrine glands, such as the thyroid 
and adrenals, and patients with hypothyroidism, hyperthyroid­
ism, Cushing syndrome, or glucocorticoid resistance may present 
with anovulation. Clinicians need a thorough knowledge of the 
functions and interactions of the hypothalamus, pituitary, ovaries, 
and uterus wi

e hypothalamus, pituitary, ovaries, 
and uterus with other systems to correctly diagnose reproductive 
disorders and design treatment strategies.
A prominent reproductive function of the hypothalamus is pulsa­
tile secretion of gonadotropin-releasing hormone (GnRH). Negative 
feedback effects of several factors, including ovarian steroids, regulate 
hypothalamic GnRH secretion into the portal vessels. Dopamine, 
norepinephrine, serotonin, and opioids produced in the brain may 
mediate the regula

oids produced in the brain may 
mediate the regulation of GnRH secretion by ovarian hormones or 
other stimuli. In response to GnRH, the anterior pituitary cells secrete 
follicle-stimulating hormone (FSH) and luteinizing hormone (LH). 
Steroids (e.g., estradiol, progesterone) and peptides (e.g., inhibin) of 
ovarian origin and activin and follistatin of pituitary origin modify 
secretion of FSH and LH. LH stimulates androstenedione produc­
tion in theca cells of the ovary; FSH regulates estradi

in theca cells of the ovary; FSH regulates estradiol and inhibin 
B production in the granulosa cells and follicular growth. Release of 
an egg from the mature follicle depends on a sudden rise in LH lev­
els in midcycle. After ovulation, the follicle transforms into a corpus 
luteum that secretes estradiol and progesterone under the control of 
FSH and LH. LH also stimulates granulosa-lutein cells of the corpus 
luteum to secrete inhibin A (Fig. 17.1A).
KEY POINTS
 • 
Ovulation and the prepara

 17.1A).
KEY POINTS
 • 
Ovulation and the preparation of the uterus for pregnancy are 
extremely delicate and parallel physiologic processes that are 
tightly regulated by a number of hormones released primarily 
by the hypothalamus, pituitary, and ovary.
 • 
In women, the biologically active steroids—estradiol, testoster­
one, and dihydrotestosterone (DHT)—are formed in the ovary, 
peripheral tissues, and locally in androgen or estrogen target 
tissues. Additionally, the adrenal gland and ova

tissues. Additionally, the adrenal gland and ovary secrete andro­
gen and estrogen precursors that are converted to biologically 
active steroids in the peripheral tissues.
 • 
A premenopausal woman often seeks medical help because of 
disorders that disrupt or complicate ovulation, normal menses, 
or fertility; the most common disorders include hypothalamic 
anovulation, hyperprolactinemia, polycystic ovary syndrome, 
ovarian insufficiency, endometriosis, and uterine fibroids.
 • 
Combinati

ndometriosis, and uterine fibroids.
 • 
Combination oral contraceptives are commonly prescribed to 
suppress ovarian activity for the management of various benign 
causes of anovulatory uterine bleeding or androgen excess, 
such as polycystic ovary syndrome, and for the management of 
cyclic or chronic pelvic pain associated with endometriosis.
 • 
Menopause is the depletion of all ovarian follicles; it effectively 
stops secretion of estradiol and progesterone. The management 
of postmenopaus

 and progesterone. The management 
of postmenopausal ovarian deficiency, characterized thus by 
vasomotor symptoms, bone loss, and vulvovaginal atrophy, is 
challenging and still highly debated in regard to the effective­
ness and the side effects of existing treatment regimens.
575
Chapter 17  Physiology and Pathology  of the Female Reproductive Axis
The endocrine effects of FSH, LH, estradiol, progesterone, 
inhibin A, and inhibin B have been deduced from changes in 
their serum levels through

educed from changes in 
their serum levels throughout the menstrual cycle (see Fig. 
17.1A). The postulated endocrine effects were then demon­
strated in cell-based and in vivo studies (see Fig. 17.1B). Activin 
and follistatin are produced in the ovary and the pituitary. They 
appear to regulate FSH release from the pituitary via autocrine 
or paracrine but not endocrine pathways. Activin stimulates 
FSH production, whereas follistatin suppresses this action 
of activin.
Follicular phase
Ovary

s this action 
Ovary
Endometrium
Menses
Serum hormone levels
P
P
P
P
P
FSH
LH
InhA
InhB
FSH
FSH
FSH
FSH
FSH
LH
LH
LH
LH
LH
LH
LH
LH
E2
E2
E2
E2
E2
E2
Days
A
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28
Ovulation
Luteal phase
Proliferative phase
Secretory phase
InhB
InhB
InhB
InhA
InhA
InhA
B
GnRH
Pituitary
Inhibin
Activin
+
–
+
–
+
–
–
FSH
Ovary
E2
E2
P
P
LH
• Fig. 17.1  (A) Changes in the ovarian fol

m
P
LH
• Fig. 17.1  (A) Changes in the ovarian follicle, endometrial thickness, and serum hormone levels during a 28-day 
menstrual cycle. Menses occur during the first few days of the cycle. (B) Endocrine interactions in the female 
reproductive axis. Some of the well-characterized endocrine interactions among the hypothalamus, pituitary, 
ovary, and endometrium for regulation of the menstrual cycle are depicted. E2, estradiol; FSH, follicle-stimulating 
hormone; GnRH, gonadotropin-releasing ho

mulating 
hormone; GnRH, gonadotropin-releasing hormone; Inh, inhibin; LH, luteinizing hormone; P, progesterone.
576
SECTION V    Sexual Development and Function
Endometrium, the mucosal lining of the uterine cavity, has 
extremely high concentrations of nuclear receptors for estrogen 
and progesterone and is highly sensitive to these hormones. The 
biologically active estrogen, estradiol, induces the growth of endo­
metrium; progesterone limits this estrogenic effect and enhances 
differentiati

this estrogenic effect and enhances 
differentiation. Sloughing off of the functional layer (stratum 
functionalis) of the endometrium follows withdrawal of estrogen 
or progesterone. The remaining basal layer (stratum basalis) is 
capable of full regeneration in response to estrogen.
Ovaries remain quiescent until puberty because the GnRH-
releasing system in the hypothalamus is immature in prepubertal 
children associated with very low circulating FSH and LH. The 
entire reproductive function 

ing FSH and LH. The 
entire reproductive function and most of the endocrine function 
of the ovaries cease after menopause because ovaries are depleted 
of all oocytes and surrounding steroidogenic cells by this time. 
These prepubertal and postmenopausal states, characterized by the 
absence of ovarian function, are associated with the lack of menses.
In summary, the female reproductive function from puberty to 
menopause can be viewed as an extremely delicate ticking clock. 
The normal functio

remely delicate ticking clock. 
The normal function of this apparatus depends on coordinated 
actions of the hypothalamus, pituitary, ovaries, and endometrium. 
The result is regular menses every 24 to 35 days. Any disorder 
of these tissues or dysfunction of other systems that affect these 
reproductive units secondarily may result in anovulation and con­
sequent irregular uterine bleeding. 
Reproductive Functions of the 
GnRH and its analogues are us

in-Releasing Hormone
GnRH and its analogues are used for the treatment of hormone-
dependent disorders and assisted reproductive technologies such 
as in vitro fertilization (IVF).1,2 In a number of vertebrates, three 
GnRHs and three cognate receptors with distinct distributions 
and functions have been identified. In humans, the hypothalamic 
GnRH is primarily encoded by the GnRH type I (GnRH-I) gene 
(GNRH1) and regulates gonadotropin secretion through the pitu­
itary GnRH type I receptor, wh

n through the pitu­
itary GnRH type I receptor, which functions as a G protein–coupled 
receptor. Binding of GnRH-I to its type I receptor leads primarily 
to activation of Gq. A second form of GnRH, called GnRH-II, is 
conserved in all higher vertebrates, including humans.3 In contrast 
to GnRH-I, GnRH-II is expressed at the highest levels outside the 
brain. A cognate receptor for GnRH-II has been cloned from various 
vertebrate species, including primates.3 The human gene homologue 
of this r

ing primates.3 The human gene homologue 
of this receptor has a frameshift and stop codon, and it appears that 
GnRH-II signaling occurs through the type I GnRH receptor. There 
seems to be considerable plasticity in the use of different GnRHs, 
receptors, and signaling pathways for diverse functions.3 For practi­
cal purposes, GnRH-I is referred to as GnRH in this chapter.
GnRH is a 10–amino acid peptide that is synthesized primarily 
in specialized neuronal bodies of the arcuate nucleus of the

ized neuronal bodies of the arcuate nucleus of the medial 
basal hypothalamus.3 Axons from GnRH neurons project to the 
median eminence and terminate in the capillaries that drain into 
the portal vessels.
The portal vein is a low-flow transport system that descends 
along the pituitary stalk and connects the hypothalamus to the 
anterior pituitary. The direction of the blood flow in this hypoph­
yseal portal circulation is from the hypothalamus to the pituitary. 
GnRH originating in the neurons

to the pituitary. 
GnRH originating in the neurons of the arcuate nucleus is secreted 
at the median eminence into the portal circulation, which delivers 
this hormone to the anterior pituitary (Fig. 17.2).
The mature decapeptide GnRH is derived from the post-
translational processing of a large precursor molecule, pre-pro-
GnRH (see Fig. 17.2).4 This precursor peptide is the product of 
the GNRH gene.3 The pre-pro-GnRH consists of 92 amino acids 
and contains four parts (from amino-terminal to 

and contains four parts (from amino-terminal to carboxyl-termi­
nal): a 23–amino acid signal domain, the GnRH decapeptide, a 
3–amino acid proteolytic processing site, and a 56–amino acid 
domain called GnRH-associated peptide.5 The cleavage products of 
this precursor, GnRH and GnRH-associated peptide (GAP), are 
transported to the nerve terminals and secreted into the portal 
Glu
-23
GAP (56 aa)
GnRH (10 aa)
+92
A
B
GnRH gene
Neuronal body
Pre-pro-GnRH
Processing
GnRH decapeptide
GAP
Axon
Po

e-pro-GnRH
GAP
Axon
Portal vessel
2
1
3
4
5
6
His Trp Ser Trp
Gly
Gly
7
8
9
10
Leu Arg Pro
Gly
L
y
s
 
A
r
g
• Fig. 17.2  Gonadotropin-releasing hormone (GnRH) production. (A) The 
GnRH gene encodes a precursor protein, pre-pro-GnRH, in the neuronal 
body. GnRH is released from this protein by proteolytic processing, which 
gives rise to GnRH and GnRH-associated protein (GAP) within the neu­
ronal body. Both GnRH and GAP are transported in an axon to th

Both GnRH and GAP are transported in an axon to the nerve 
terminal and secreted into the portal circulation. (B) Pre-pro-GnRH is a 92–
amino acid (aa) protein. The biologically active decapeptide (amino acids 
1–10) is sandwiched between the 23–amino acid signal peptide and the 
Gly-Lys-Arg sequence. The arrow indicates the site of proteolytic process­
ing. The C-terminal 56–amino acid peptide is cleaved to produce GAP. 
LHRH, luteinizing hormone–releasing hormone (gonadotropin-releasing 
hormo

e–releasing hormone (gonadotropin-releasing 
hormone 1). (From Yen SSC. Endocrine regulation of the reproductive sys­
tem. In: Yen SSC, Jaffe RB, Barbieri RL, eds. Reproductive Endocrinology. 
4th ed. Philadelphia, PA: WB Saunders; 1999:44.)
577
circulation (see Fig. 17.2).4,6 A physiologic role for GAP has not 
been established.6
In humans, GnRH neurons are located primarily in the arcu­
ate nucleus of the medial basal hypoth

n the arcu­
ate nucleus of the medial basal hypothalamus and the preoptic 
area of the anterior hypothalamus.7 The population of GnRH-
producing neurons is relatively limited and is in the range of 
1000 to 2000. The neurons that produce GnRH originate from 
the olfactory area during embryogenesis.7 GnRH and olfactory 
neurons migrate together along cranial nerves connecting the 
nose and forebrain to the hypothalamus during embryologic 
development, and disruption of this process causes idiopat

ent, and disruption of this process causes idiopathic 
hypogonadotropic hypogonadism with anosmia, or Kallmann 
syndrome.7 Individuals with Kallmann syndrome usually have 
lack of pubertal development and subsequent infertility due 
to deficient GnRH and pituitary gonadotropins. The neuronal 
proteins anosmin 1 (encoded by the KAL1 gene) and fibroblast 
growth factor receptor type 1 (encoded by the FGFR1 gene) 
affect olfactory and GnRH neuron migration. Mutations in 
these genes cause Kallmann 

gration. Mutations in 
these genes cause Kallmann syndrome.7 Data suggest that muta­
tions of the genes for nasal embryonic LH-releasing hormone 
factor (NELF) and chromodomain helicase DNA-binding pro­
tein 7 (CHD7) may cause Kallmann syndrome, but this corre­
lation is not as conclusively established as it is for KAL1 and 
FGFR1.7 Selective clinical phenotypes in men and women are 
highly associated with genetic causes of Kallman syndrome. Syn­
kinesia (KAL1), dental agenesis (FGF8/FGFR1), dig

kinesia (KAL1), dental agenesis (FGF8/FGFR1), digital bony 
abnormalities (FGF8/FGFR1), and hearing loss (CHD7) may be 
useful for prioritizing genetic testing.8,9
Knobil demonstrated in a pioneering series of experiments that 
normal gonadotropin secretion requires pulsatile GnRH discharge 
within a critical frequency and amplitude.10 The periodicity and 
amplitude of the pulsatile rhythm of GnRH and gonadotropin 
secretion are crucial in regulating gonadal activity and therefore 
the entire r

ating gonadal activity and therefore 
the entire reproductive axis (Fig. 17.3). The self-priming effect of 
GnRH in upregulating its receptors on pituitary gonadotropin-
producing cells manifests only at the physiologic periodicity of 
60 to 90 minutes.11,12 Slower frequency causes anovulation and 
amenorrhea because of inadequate stimulation. Higher frequency 
or constant exposure to GnRH also gives rise to anovulation by 
downregulating expression of GnRH receptors, thereby abolish­
ing gonado

ion of GnRH receptors, thereby abolish­
ing gonadotropin responses.
The activation of gene expression for gonadotropin subunits, 
including the common α-subunit and specific β-subunits for LH 
and FSH, dimerization of αβ subunits, and glycosylation appear 
to be governed by intermittency of GnRH inputs to pituitary 
gonadotrophs.13 In humans, measurement of LH pulses is com­
monly used as an indication of GnRH pulsatile secretion.14 The 
LH pulse frequency is approximately 90 minutes during the 

 frequency is approximately 90 minutes during the early 
follicular phase, 60 to 70 minutes during the late follicular phase, 
100 minutes during the early luteal phase, and 200 minutes dur­
ing the late luteal phase.15 This variation is accompanied by pre­
dictable changes in FSH and LH levels and ovarian steroid release 
during these phases of the menstrual cycle. More rapid pulse 
frequencies favor LH secretion, whereas slower pulse frequencies 
favor FSH. It appears that variations in GnRH p

s 
favor FSH. It appears that variations in GnRH pulse frequency 
markedly influence both the absolute levels and the ratio of LH 
and FSH release. 
Regulation of Gonadotropin-Releasing 
Hormone Secretion
Cyclic, predictable menses require the pulsatile release of 
GnRH within a critical range of frequencies. Pulsatile, rhyth­
mic activity is an intrinsic property of GnRH neurons, and 
various hormones and neurotransmitters modulate this rhythm 
(Fig. 17.4).
The variations in GnRH pulse frequenc

The variations in GnRH pulse frequency are achieved, in part, 
by gonadal steroid feedback. Estradiol increases GnRH pulse fre­
quency, and elevated progesterone levels decrease GnRH pulsa­
tility.13 Increased progesterone levels may decrease GnRH pulse 
20
999
15
10
5
0
10
5
0
5
10
15
20
25
30
35
200
150
100
50
0
Pulsatile
Pulsatile
LH (ng/mL)
Days
Continuous
• Fig. 17.3  Effect of pulsatile or continuous administration of gonadotro­
pin-releasing hormone (GnRH) to ovar

of gonadotro­
pin-releasing hormone (GnRH) to ovariectomized monkeys previously 
rendered GnRH deficient by placement of a lesion in the hypothalamus. 
Release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) 
was restored by hourly GnRH infusion, inhibited during a continuous infu­
sion, and again restored after reinstitution of pulsatile GnRH administra­
tion. (Adapted from Belchetz PE, Plant TM, Nakai Y, et al. Hypophysial 
responses to continuous and intermittent delivery o

esponses to continuous and intermittent delivery of hypothalamic gonad­
otropin releasing hormone. Science. 1978;202:631–633. Copyright © 
1978 by American Association for the Advancement of Science.)
Dopamine
Serotonin
Opioids
CRH
Ovary
GnRH
FSH
LH
Pituitary
Activin
α
FSHβ
LHβ
E2
Locally
produced
E2
P
E2
P
Inhibin
• Fig. 17.4  Regulation of gonadotropin-releasing hormone (GnRH), lutein­
izing hormone (LH), and follicle-stimulating hormone (FSH) secretion.

 and follicle-stimulating hormone (FSH) secretion. 
Locally synthesized and systemic hormones regulate the pulsatile secre­
tion of GnRH from the hypothalamus into the portal circulation. GnRH 
and a number of steroid and peptide hormones regulate the synthesis 
of gonadotropin subunits, including the common α-subunit and specific 
β-subunits for LH and FSH, and the formation and secretion of FSH and 
LH. CRH, corticotropin-releasing hormone; E2, estradiol; P, progesterone.
578
SECTION V    Sexu

 estradiol; P, progesterone.
578
frequency and thereby lead to preferential biosynthesis and secre­
tion of FSH, as observed in the late luteal phase.13
GnRH pulsatility is also modulated by the actions of locally 
released neurotransmitters. Norepinephrine stimulates GnRH 
release, whereas dopamine exerts an inhibitory effect (see Fig. 
17.4).16 β-Endorphin and other opioids may suppress the hypo­
thalamic release of GnRH.17, 18 It was proposed that 

lamic release of GnRH.17, 18 It was proposed that sex steroids 
enhance the activity of endogenous opioids that exert an inhibi­
tory effect on GnRH secretion.15 The negative effect of opioids 
on GnRH secretion is clinically explicable because the reduced 
GnRH secretion associated with hypothalamic amenorrhea may 
be mediated by an increase in endogenous opioid inhibitory 
tone.19
Estrogen signaling to GnRH neurons appears to be critical 
for suppressing FSH and LH and for coordinating the pre

uppressing FSH and LH and for coordinating the preovula­
tory surge release of LH. The precise roles of estrogen receptors α 
and β (ERα and ERβ, respectively) within the GnRH neurons or 
estradiol-sensitive afferent neurons in these negative and positive 
feedback effects of estrogen are not well understood.20 Stimula­
tion of FSH release after treatment of premenopausal women with 
an aromatase inhibitor and in vitro studies suggest that estrogen 
locally produced by aromatase activity in hypo

en 
locally produced by aromatase activity in hypothalamic neurons 
may regulate gonadotropin secretion.21,22
Binding of the peptide kisspeptin to its G protein–cou­
pled receptor KISS1R (previously known as GPR54), which is 
expressed in GnRH neurons, stimulates GnRH release in the 
hypothalamus.23,24 Kisspeptin neurons contact GnRH neurons 
and act at the cell body and the nerve terminals.25 Kisspeptin 
can act directly on GnRH neurons or indirectly through synaptic 
input from other neurons t

ectly through synaptic 
input from other neurons to inhibit inwardly rectifying potassium 
channels and activate nonspecific cation channels, producing 
long-lasting depolarization and an increased action-potential fir­
ing rate.26 Mutations or knockout of KISS1R produces isolated 
hypogonadotropic hypogonadism in humans and mice, indicating 
that signaling through this receptor is essential for sexual devel­
opment and function.23,24 Moreover, kisspeptin-expressing and 
KISS1R-expressing neuron

isspeptin-expressing and 
KISS1R-expressing neurons may be critical targets for the nega­
tive and positive feedback actions of estrogen and progesterone.27 
Gonadotropin-Releasing Hormone Analogues
The half-life of GnRH is short (2–4 minutes) because it is degraded 
rapidly by peptidases in the hypothalamus and pituitary gland.28 
These peptidases cleave the bonds between amino acids 5 and 6, 6 
and 7, and 9 and 10. Analogues of GnRH with different proper­
ties have been synthesized by alterati

ent proper­
ties have been synthesized by alteration of amino acids at these 
positions. Many agonistic and antagonistic GnRH analogues with 
various biologic effects have been produced.
Peptide Gonadotropin-Releasing Hormone Agonists
Several GnRH agonist peptides are generated by substitution of 
amino acids at the 6 or 10 position (Fig. 17.5). The increased 
biologic activity of agonistic peptides has been attributed to their 
high binding affinity to GnRH receptors and their reduced sus­
cept

nity to GnRH receptors and their reduced sus­
ceptibility to enzymatic degradation. An amino acid substitution 
at position 6 gives rise to metabolic stability, whereas replace­
ment of the carboxy-terminal glycinamide residue by an ethyl­
amide group increases strikingly the affinity for the receptors.28,29 
Leuprolide
acetate
NEt
NH2
GnRH
NH2
NH2
NH2
NH2
NH2
NH2
NH2
Goserelin
acetate
D Ser
(tBu)
D Leu
DNal
D Trp
D His
(ImBzl)
D hArg
(Et)2
D Aph
(Cba)
D Cit
Agonists
Antagonists
Aph
(Hor)
D Pal

(Cba)
D Cit
Agonists
Aph
(Hor)
D Pal
D Ala
D Ala
D Ala
D Cpa
D hArg
(Et)2
Lys
(iPr)
D Nal
D Pal
D Cpa
D Nal
D Pal
D Cpa
D Nal
Histrelin
acetate
Nafarelin
acetate
Triptorelin
acetate
Cetrorelix
acetate
Ganirelix
acetate
Degarelix
acetate
• Fig. 17.5  Gonadotropin-releasing hormone (GnRH) agonist and antagonist analogues in clinical prac­
tice. Purple circles indicate amino acids in the wild-type GnRH decapeptide, and white circles are 
labeled with the changes made to the analogues. 

labeled with the changes made to the analogues. (Modified from Millar RP, Lu Z, Pawson AJ, et al. 
Gonadotropin-releasing hormone receptors. Endocr Rev. 2004;25:235–275. Copyright © 2004 by the 
Endocrine Society.)
579
Peptide GnRH agonists are administered subcutaneously, intra­
nasally, or intramuscularly. An initial agonistic action (i.e., flare 
effect) is associated with an increase in the circulating levels of LH 
and 

 an increase in the circulating levels of LH 
and FSH. The most prominent agonistic response is observed dur­
ing the early follicular phase, when the combined effects of GnRH 
agonist and elevated levels of estradiol create a large reserve pool 
of gonadotropins.30
Administration of a long-acting depot formulation of a GnRH 
agonist gives rise to an initial flare followed by downregulation of 
the gonadotropin-gonadal axis within 1 to 3 weeks. The initial 
downregulation effect is caused by des

he initial 
downregulation effect is caused by desensitization, whereas the 
sustained response results from loss of receptors and the uncou­
pling of the receptor from its effector system.
The US Food and Drug Administration (FDA) approved the 
use of these agonists for the treatment of GnRH-dependent pre­
cocious puberty, endometriosis, and prostate cancer. Another 
indication is preoperative hematologic improvement of patients 
with anemia caused by uterine leiomyomas. Off-label indications 

sed by uterine leiomyomas. Off-label indications 
for GnRH agonists include downregulation of the pituitary dur­
ing ovulation induction, induction of endometrial atrophy before 
endometrial ablation surgery, and prevention of menstrual bleed­
ing in patients with coagulation defects. GnRH agonists have also 
been used to suppress ovarian steroidogenesis in hirsute patients.31
The most prominent side effects of long-term use of depot 
GnRH agonist formulations are caused by estrogen deficiency. 

t formulations are caused by estrogen deficiency. 
Depot GnRH agonists induce a menopause-like state character­
ized by hot flashes, vaginal dryness, bone resorption, and osteo­
penia. Osteopenia is reversible in young women if treatment is 
maintained for no more than 6 months.32, 33 The risk-benefit ratio 
must be considered carefully before GnRH agonist treatment is 
extended for longer periods. Add-back regimens using low-dose 
estrogens or progestins, or both, administered along with GnRH 

rogestins, or both, administered along with GnRH 
agonists have provided a means to overcome these side effects and 
to extend the length of agonist therapy.34 
Peptide Gonadotropin-Releasing Hormone Antagonists
Inhibition of a premature LH rise by GnRH agonists requires at 
least 7 days, because it is accompanied by an initial stimulation of 
GnRH receptors before gonadotroph desensitization is achieved. 
In contrast, GnRH antagonists compete directly with endogenous 
GnRH for receptor binding 

rectly with endogenous 
GnRH for receptor binding and therefore rapidly inhibit secretion 
of gonadotropin and steroid hormones (see Fig. 17.5).35–38 This 
property conveys a potential advantage over GnRH agonists in 
the management of ovarian stimulation. However, because of the 
constant need to block endogenous GnRH, much higher doses 
of antagonists are required. The GnRH antagonists incorporate a 
number of amino acid substitutions in the amino-terminal domain 
(involved in receptor activat

ino-terminal domain 
(involved in receptor activation) combined with a d-amino acid 
substitution for Gly6, which enhances the βII-type bend neces­
sary for receptor binding.3 GnRH antagonists have the advantage 
of inducing an immediate decrease in circulating gonadotropin 
levels with rapid reversal.35–38 GnRH antagonists are alternative 
drugs to GnRH agonists for the prevention of a natural LH surge 
during ovulation induction by injectable FSH.39 Use of GnRH 
antagonists has become popular 

SH.39 Use of GnRH 
antagonists has become popular in ovulation induction protocols 
for IVF (see Fig. 17.5).39 
Nonpeptide Gonadotropin-Releasing Hormone 
Small molecular compound collections have been screened using 
mammalian cells that heterologously expressed human GnRH 
type I receptor. These studies led to the identification of synthetic 
compounds that bind to the GnRH type I receptor and block signal 
transduction. Several companies have manufactured orally admin­
istered GnR

panies have manufactured orally admin­
istered GnRH antagonists intended for various indications, includ­
ing endometriosis.1 The FDA approved the oral GnRH antagonist 
named elagolix to treat endometriosis-associated pelvic pain.2 
Reproductive Functions of the Anterior 
Pituitary
Gonadotrophs
Gonadotrophs are specialized cell types of the anterior pituitary 
that synthesize and secrete LH and FSH. These cells constitute 
7% to 15% of the total number of anterior pituitary cells and 
are detect

number of anterior pituitary cells and 
are detected in this location from early fetal life.40 Most gonado­
trophs are capable of synthesizing both LH and FSH.40, 41 LH 
and FSH are each composed of two distinct, noncovalently asso­
ciated protein α-subunits and β-subunits (see Fig. 17.4). In the 
gonadotroph, the subunit genes encode the subunit precursors. 
Gonadotrophs contain cell-surface GnRH type I receptors that 
mediate the action of GnRH. These receptors belong to the seven-
transmembra

. These receptors belong to the seven-
transmembrane domain and G protein–coupled receptor family. 
Gonadotropin-Releasing Hormone Receptor
In humans, hypothalamic GnRH regulates gonadotropin secre­
tion through the pituitary GnRH type I receptor by activation of 
Gq/11.42 Although the predominant coupling of the type I GnRH 
receptor in the gonadotroph is through Gq/11 stimulation, signal 
transduction can occur through other G proteins and potentially 
by G protein–independent means.3,42 A num

ntially 
by G protein–independent means.3,42 A number of downstream 
signaling cascades include protein kinase C (PKC)–, Ca2+-, and 
tyrosine kinase–dependent pathways.3 In mouse pituitary gonad­
otrophs, the GnRH receptor activates several mitogen-activated 
protein (MAP) kinase cascades, including the ERK1/2, the JUN 
amino-terminal kinase (JNK), the p38 MAP kinase, and the big 
MAP kinase (BMK1/ERK5).3 The cross-talk between these path­
ways remains to be clarified. 
Luteinizing Hormone and F

emains to be clarified. 
Luteinizing Hormone and Follicle-Stimulating 
Hormone
The α-subunits of human LH, FSH, thyroid-stimulating hor­
mone (TSH), and human chorionic gonadotropin (hCG) have 
an identical polypeptide structure. In contrast, the β-subunit of 
each hormone has a unique amino acid sequence and confers the 
specific activity of the αβ-heterodimer. Each subunit is rich in 
cysteine and contains multiple disulfide linkages. Each subunit 
also contains multiple carbohydrate moieties 

nit 
also contains multiple carbohydrate moieties that play important 
roles in the biologic activity and metabolism of these hormones.
The human common α-subunit gene encodes a precur­
sor polypeptide with a 24–amino acid leader sequence that is 
cleaved post-translationally to produce the mature 92–amino 
acid α-subunit. The β-subunits of human FSH, LH, and hCG 
contain 117, 121, and 145 amino acids, respectively.43–46 On 
binding of GnRH to its receptor, the biosynthesis of the gonado­
tropin

s receptor, the biosynthesis of the gonado­
tropins proceeds by transcription of the subunit genes, trans­
lation of the subunit messenger ribonucleic acids (mRNAs), 
post-translational modifications of the precursor subunits and 
subunit folding and combination, mature hormone packaging, 
and secretion (see Fig. 17.4).
580
The human LH and hCG β-subunit genes are located on chro­
mosome 19q13.3, which contains a cluster of seven β-subunit–
like genes

 contains a cluster of seven β-subunit–
like genes.44 Five of these sequences are noncoding pseudogenes 
arranged in groups of tandem and inverted pairs. Only LH and 
hCG β-subunit genes give rise to two distinct and functional 
mRNA species. The LH β-subunit mRNA encodes a 145–amino 
acid precursor protein that is later cleaved to produce a 24–amino 
acid leader peptide and a 121–amino acid, biologically active, 
mature peptide. The hCG β-subunit mRNA also encodes a 
145–amino acid protein. Thi

t mRNA also encodes a 
145–amino acid protein. This protein, however, is not processed 
post-translationally and functions as the biologically active hCG 
β-subunit. The amino acid sequences of the human LH and 
hCG β-subunits are 82% homologous. These two β-subunits 
confer identical biologic activities when associated with the 
α-subunit.44–46
A single gene encodes the FSH β-subunit.47 Complementary 
DNA encoding human FSHβ, LHβ, or hCGβ in combination 
with the complementary DNA of the α-subu

bination 
with the complementary DNA of the α-subunit is expressed in 
mammalian cells in culture. These cells can synthesize these pro­
teins, modify them after translation, glycosylate and combine the 
subunits, and secrete them as intact FSH, LH, or hCG.48 Recom­
binant gonadotropins are used clinically to stimulate gonadal 
function.49 
Regulation of Circulating Levels of Follicle-
Stimulating Hormone and Luteinizing Hormone
The molecular mechanisms responsible for formation and com­
binatio

hanisms responsible for formation and com­
bination of the α-subunits and β-subunits of FSH and LH are 
not completely understood. Production rates of α-subunits and 
β-subunits are regulated in part by negative feedback by estrogen, 
which regulates the pulsatile release of GnRH from the hypothala­
mus.48,50 The pituitary contains more α-subunit than β-subunit 
mRNA, and readily detectable levels of free α-subunit are present 
in serum. The free β-subunit is present at relatively low levels in 

 β-subunit is present at relatively low levels in 
the pituitary and is rarely found in serum or urine. The specific 
β-subunit may be the rate-limiting factor in the synthesis of these 
glycoprotein hormones.
Inhibin, activin, and follistatin were first identified as gonadal 
hormones that exerted selective effects on FSH secretion.51 
Although the primary source of inhibin remains the ovary, 
activin and follistatin are produced in extragonadal tissues and 
can exert effects on FSH through an 

 tissues and 
can exert effects on FSH through an autocrine-paracrine mecha­
nism. Inhibin-B is secreted by ovarian granulosa cells during the 
follicular phase (under the control of FSH) and inhibin-A by 
the corpus luteum in the luteal phase (under the control of LH). 
Inhibins act synergistically with estradiol to inhibit FSH secre­
tion. Activin can directly stimulate FSH biosynthesis and release 
from the gonadotroph cells of the pituitary gland.51 Follistatin 
can negatively regulate biolo

land.51 Follistatin 
can negatively regulate biologic functions of activin via binding 
and prevent it from interacting with the activin receptor at the 
cell membrane.52
Serum levels of gonadotropins are proportional to their secre­
tion rates and serum half-lives, which are regulated by the number 
of carbohydrate residues. The sialic acid content of gonadotropic 
hormones and other glycoproteins has a marked effect on their 
rate of clearance and influences their apparent molecular size.53 
T

and influences their apparent molecular size.53 
The higher content of sialic acid in FSH compared with LH is 
responsible for slower clearance of FSH, which has a half-life of 3 
to 4 hours. LH, which has a half-life of 20 minutes, has the most 
rapid clearance rate. The hCG is highly sialylated and has the lon­
gest half-life (24 hours). 
Ovary
The ovary is essential for periodic release of oocytes and produc­
tion of the steroid hormones, estradiol and progesterone. These 
activities are inte

adiol and progesterone. These 
activities are integrated into the cyclic repetitive process of fol­
licular maturation, ovulation, and formation and regression of the 
corpus luteum. The ovary fulfills two major objectives: genera­
tion of a fertilizable ovum and preparation of the endometrium 
for implantation through the sequential secretion of estradiol and 
progesterone.49 The ovarian follicle comprising the egg and sur­
rounding granulosa and theca cells constitutes the fundamental 
functio

d theca cells constitutes the fundamental 
functional unit of the ovary.
Adult human ovaries are oval bodies with a length of 2 to 5 cm, 
a width of 1.5 to 3 cm, and a thickness of 0.5 to 1.5 cm. The ova­
ries lie near the posterior and lateral pelvic wall and are attached to 
the posterior surface of the broad ligament by the peritoneal fold, 
called the mesovarium.
The ovary consists of three structurally distinct regions: an 
outer cortex containing the surface germinal epithelium and 
the fo

aining the surface germinal epithelium and 
the follicles, a central medulla consisting of stroma, and a hilum 
around the area of attachment of the ovary to the mesovarium 
(Fig. 17.6). The hilum is the point of attachment of the ovary to 
the mesovarium. It contains nerves, blood vessels, and hilus cells, 
which have the potential to become active in steroidogenesis or 
to form androgen-secreting tumors. These cells are similar to the 
testosterone-producing Leydig cells of the testes.
The out

rone-producing Leydig cells of the testes.
The outermost portion of the cortex, called the tunica albu­
ginea, is covered by a single layer of surface cuboidal epithelium 
called the germinal epithelium. The oocytes, enclosed in complexes 
called follicles, are in the inner part of the cortex, embedded in 
stromal tissue. One dominant follicle is recruited for ovulation 
during each cycle (see Fig. 17.6). The preovulatory follicle trans­
forms into a corpus luteum after ovulation. In the absence

to a corpus luteum after ovulation. In the absence of preg­
nancy, the corpus luteum regresses to become the corpus albicans 
(see Fig. 17.6). The stromal tissue is composed of connective tissue 
and interstitial cells, which are derived from mesenchymal cells 
and are presumed to have the ability to respond to LH or hCG 
with the production of androstenedione. The central medullary 
area of the ovary is derived largely from mesonephric cells.
Genetic Determinants of Ovarian Differentiation 
and

netic Determinants of Ovarian Differentiation 
and Folliculogenesis
Nascent components of the human ovary develop long before a 
distinct ovary-like organ can be discerned. The female germ cells 
are formed during embryogenesis when the precursors of primor­
dial germ cells differentiate from somatic lineages of the embryo 
and take a unique route from the base of the yolk sac along the 
hindgut to reach the genital ridge. This starts the differentiation of 
female gonads (ovaries) at the genita

ntiation of 
female gonads (ovaries) at the genital ridge. The originally undif­
ferentiated gonad differentiates along a female pathway, and the 
newly formed oocytes proliferate and subsequently enter meiosis.49
Ovarian differentiation and folliculogenesis depend on coor­
dinated expression and interaction of a multitude of genes.54 Tar­
geted gene disruption or insertion in mice has made it possible to 
inquire about the function of specific genes in ovarian differentia­
tion and folliculogen

enes in ovarian differentia­
tion and folliculogenesis. Fig. 17.7 summarizes the biologic roles 
of some of these genes.54 Genetically altered mice represent a first 
step in attempts to understand in vivo the various gene interac­
tions that result in a functional ovary. Ovarian pathologic con­
ditions in transgenic mice closely resemble disorders observed in 
mutant human homologues, as exemplified in cases involving the 
581
Chapter 17  Physiology and Pathology  of the Female Reproductive Axi

logy and Pathology  of the Female Reproductive Axis
Mesovarium
Primary follicles
Germinal
Developing
follicle
Atretic
follicle
Mature
follicle
Stroma
Hilum
Corpus albicans
Corpus luteum
Ovulation
Ovum
Antrum
Granulosa
Basement
membrane
Theca interna
Theca externa
Collapsed antrum
(fibrin and clot)
Theca lutein cells
Granulosa lutein cells
• Fig. 17.6  Functional anatomy and changes in the adult ovary during an ovarian cycle. (From Carr BR, Wilson 
JD. Dis

g an ovarian cycle. (From Carr BR, Wilson 
JD. Disorders of the ovary and female reproductive tract. In: Braunwald E, Isselbacher KJ, Petersdorf RG, 
et al, eds. Harrison’s Principles of Internal Medicine. 11th ed. New York, NY: McGraw-Hill; 1987:1818–1837.)
Ovulation
Fertilization
Follicle
size (mm):
formation
Gonadotropin-dependent growth
Antral
Gonadotropin-independent growth
Preantral
Secondary
Primary
Antral
Preovulatory
<20
70
20–70
Figla
Bmp15
Amh
Kit
Kitl
Nobox
G

tory
<20
70
20–70
Figla
Bmp15
Amh
Kit
Kitl
Nobox
Gdf9
Kitl
Foxl2
Fshb
Fshr
Igf1
Ccnd2
Smad3
Taf4b
Acvr2
Inha
Inhba
Gja4
Esr1
Esr2
• Fig. 17.7  Developmental stages at which certain murine genes affect oogenesis. Data from transgenic 
mice with disruption of various genes have delineated critical roles of several genes during various phases 
of the follicular development. Preantral follicular growth is thought to be gonadotropin independent, 
whereas antrum formation and follicular maturation req

eas antrum formation and follicular maturation require the action of follicle-stimulating hormone (FSH). 
Acvr2, activin type II receptor; Amh, antimüllerian hormone; Bmp15, bone morphogenetic protein-15; 
Ccnd2, cyclin d2; Esr1, estrogen receptor-α; Esr2, estrogen receptor-β; Figla, factor in the germline-α; 
Foxl2, forkhead box L2; Fshb, FSHβ-subunit; Fshr, FSH receptor; Gdf9, growth differentiation factor 9; 
Gja4, gap junction protein connexin 37; Igf1, insulin-like growth factor 1; Inha, in

n 37; Igf1, insulin-like growth factor 1; Inha, inhibin α-subunit; Inhba, 
inhibin subunit-βA; Kit, kit receptor; Kitl, kit-ligand; Nobox, newborn oogenesis homeobox gene; Smad3, 
Sma mothers against decapentaplegic-3; Taf4b, TATA-box-binding protein-associated factor-4b. (Modified 
from Simpson JL, Rajkovic A. Ovarian differentiation and gonadal failure. Am J Med Genet. 1999;89:186–
2000; Choi Y, Rajkovic A. Genetics of mammalian folliculogenesis. Cell Mol Life Sci. 2006;63:579–590.)
582
SECTIO

s. Cell Mol Life Sci. 2006;63:579–590.)
582
FSH β-subunit and FSH receptor. Many mouse models of ovar­
ian pathologic conditions are available.49,54 They can be divided 
into mice that have prenatal ovarian insufficiency with disordered 
gonad formation and diminished number of germ cells or absent 
germ cells and mice that have postnatal ovarian insufficiency as 
a result of defects at various stages of folliculogenesis (see Fig. 
17.7).54 These mode

of folliculogenesis (see Fig. 
17.7).54 These models should lead to the identification of genetic 
and molecular mechanisms responsible for the development and 
function of the human ovary.
In humans, certain gene defects give rise to specific defects in 
folliculogenesis. This was demonstrated by the discovery of a het­
erozygous mutation in the bone morphogenetic protein 15 gene 
(BMP15) that caused ovarian dysgenesis. BMP15 is a growth and 
differentiation factor that is primarily expressed i

fferentiation factor that is primarily expressed in the oocyte and 
stimulates folliculogenesis and granulosa cell growth. In  vitro, 
mutant BMP15 reduced granulosa cell growth and antagonized 
the stimulatory activity of wild-type protein on granulosa cell pro­
liferation. In vivo, this mutation was associated with familial ovar­
ian dysgenesis, indicating that the action of BMP15 is required 
for progression of human folliculogenesis.55 A comprehensive 
discussion of genes responsible for ova

rehensive 
discussion of genes responsible for ovarian development, folliculo­
genesis, and ovulation is provided in an excellent review article by 
Edson and colleagues.49 
Oocytes
The fertilization of an oocyte by a spermatozoon gives rise to a 
zygote that starts to divide rapidly. An eight-cell embryo is formed 
usually on the third day after fertilization. Up to that point, all 
embryonic cells are morphologically identical, truly totipotential, 
and capable of starting a new individual or 

ial, 
and capable of starting a new individual or any lineage. The for­
mation of a 16-cell morula marks the beginning of the process of 
differentiation, with cells being allocated to the inside or outside 
of the embryo. At the next stage, the blastocyst, three lineages are 
defined: trophectoderm, which is the precursor of the placenta; 
epiblast, which gives rise to the somatic cells of the embryo; and 
primitive endoderm, which eventually forms the yolk sac. After 
the embryo implants, a gr

rms the yolk sac. After 
the embryo implants, a group of cells within the epiblast form 
the precursors of the primordial germ cells, the first cells of the 
future ovary to be defined.49 The extraembryonic trophectoderm 
and primitive endoderm, which surround the epiblast cells of 
the postimplantation egg cylinder, are the sources of signals that 
instruct this small number of epiblast cells to become primordial 
germ cells; the rest of the cells commence differentiation into 
somatic tissues.

ls commence differentiation into 
somatic tissues. The first primordial germ cell precursors express a 
key protein named PRDM1 (PRDI-BF1-RIZ domain containing 
1, formerly called BLIMP1); these precursor cells represent the 
first cells of the mammalian embryo with committed cell fates.49
The primordial germ cells first become recognizable as a cluster 
of cells that stain intensely for alkaline phosphatase activity; these 
epiblast cells are observed at the base of the yolk sac before forma­
t

served at the base of the yolk sac before forma­
tion of the allantois.49 Studies have confirmed that these cells are 
the only primordial germ cells because their ablation results in 
embryos without germ cells, whereas transplantation of these cells 
leads to their proliferation followed by migration to the genital 
ridge.49
With the use of alkaline phosphatase as a marker, migration of 
these primordial germ cells from the yolk sac–epiblast junction to 
the indifferent gonad can be tracked; e

nction to 
the indifferent gonad can be tracked; eventually, the ovary forms 
and permits the primordial germ cells to differentiate into oocytes. 
The oocytes enter meiosis and subsequently arrest. Entry into 
meiosis marks the developmental stage at which any progenitor 
cells that are capable of differentiating to oocytes disappear. The 
meiotically arrested oocytes eventually become surrounded by 
pre-granulosa cells and form individual primordial follicles, the 
resting pool of oocytes that

rdial follicles, the 
resting pool of oocytes that have the potential to be recruited into 
the growing follicle pool during the postpubertal stage to be fertil­
ized and to contribute to the next generation. These phenomena 
have been primarily observed in mice and are thought to be appli­
cable to humans.49
Primordial germ cells of epiblast origin migrate to cross a 
remarkably long distance from the base of the yolk sac to the genital 
ridge in the fetus by ameboid movements with the aid of p

n the fetus by ameboid movements with the aid of pseudopo­
dia.56 This long route of migration along the dorsal mesentery of the 
hindgut is interrupted only by the required lateral crossing of the 
coelomic angle at the level of the genital ridge (Fig. 17.8). The trig­
gers that initiate primordial germ cell migration and the chemoat­
tractants required for directional movement toward the genital ridge 
are beginning to be uncovered. A critical trigger may be expression 
of a key receptor on th

trigger may be expression 
of a key receptor on the primordial germ cell and expression of the 
secreted chemoattractants from the genital ridge. For example, sup­
pression of transforming growth factor-β (TGFβ) signaling leads to 
enhanced migration due to reduction in the levels of TGFβ-induced 
collagen type 1 in the extracellular matrix.57 An extracellular matrix 
gradient along the path of migration is important, and if too much 
matrix is laid down, germ cells show reduced migration. The K

aid down, germ cells show reduced migration. The KIT 
ligand (KITLG) may function as an effective chemoattractant 
for the primordial germ cells. The phosphatidylinositol 3-kinase 
(PI3K)/AKT and SRC kinase pathways are involved downstream of 
KIT in the primordial germ cell.58
Germ cells appear to be unable to persist outside the genital 
ridge, which may be viewed as the only region competent to sus­
tain gonadal development. By the same token, germ cells play an 
indispensable role in the ind

 germ cells play an 
indispensable role in the induction of gonadal development. No 
functional gonad can form in the absence of germ cells.
Wolffian
duct
Aorta
Sympathetic ganglion
Suprarenal medulla
Suprarenal cortex
CC
CC
CC
CC
CC
CC
Origin of
Müllerian
duct
Dorsal
mesentery
Gonadal
ridge
Yolk sac
Hindgut
• Fig. 17.8  Transverse section of the caudal region of a 5-week embryo 
shows the location of gonadal ridges, the primordium of the adrenal 
glands, and the migration path of primordial ger

glands, and the migration path of primordial germ cells. From the third 
week on, germ cells of epiblast origin located at the base of the yolk sac 
cross the dorsal mesentery of the hindgut and migrate to the gonadal 
ridges. By the end of the fifth week, rapid division of primordial germ cells, 
gonadal epithelium, and mesenchyme starts the early gonad that differen­
tiates subsequently into the ovary in a 46,XX fetus. CC, coelomic cavity. 
(Modified from Moore K. The Developing Human. Phila

Modified from Moore K. The Developing Human. Philadelphia, PA: WB 
Saunders; 1983.)
583
On arrival at the genital ridge by the fifth week of gestation, 
the premeiotic germ cells are referred to as oogonia.59 During the 
subsequent 2 weeks of intrauterine life (weeks 5–7 of gestation, or 
the indifferent stage), the primordial gonadal structure constitutes 
no more than a bulge on the medial aspect of the urogenital ridge 
(se

 on the medial aspect of the urogenital ridge 
(see Fig. 17.8). This protuberance is created by proliferation of 
surface (coelomic) germinal epithelium, by growth of the under­
lying mesenchyme, and by oogonial multiplication. The oogonia 
total 10,000 by about 6 to 7 weeks of intrauterine life.59 Because 
meiosis and oogonial atresia are not occurring, the actual number 
of germ cells is dictated by mitotic division at this time.59
During the indifferent phase, the gonadal cortex and medulla 

ndifferent phase, the gonadal cortex and medulla 
are first delineated. However, short of cytogenetic evidence, the 
precise sexual identity of the gonadal ridge cannot be ascertained 
at this point. Nevertheless, the absence of testicular development 
beyond 7 weeks of gestation is considered presumptive evidence 
of formation of the ovary. Additional clues to the sexual identity 
of the gonad can be derived from the detection of oogonial meio­
sis at about 8 weeks of gestation because no compa

sis at about 8 weeks of gestation because no comparable process 
is observed in the testis until puberty. The sexual identity of the 
gonadal ridge is histologically clear by 16 weeks of gestation, when 
the first primordial follicles can be visualized.
By about 8 weeks of intrauterine life, persistent mitosis increases 
the total number of oogonia to 600,000 (Fig. 17.9). From this 
point on, the oogonial endowment is subject to three simulta­
neous processes: mitosis, meiosis, and oogonial atre

ous processes: mitosis, meiosis, and oogonial atresia. Stated 
differently, the onset of oogonial meiosis and oogonial atresia is 
superimposed on oogonial mitosis. As a result of the combined 
impact of these processes, the number of germ cells peaks at 6 
to 7 × 106 by 20 weeks of gestation (see Fig. 17.9). At this time, 
two-thirds of the total germ cells are intrameiotic primary oocytes; 
the remaining third can still be viewed as oogonial. The midges­
tational peak and the postpeak decline 

he midges­
tational peak and the postpeak decline are accounted for in part 
by the progressively decreasing rate of oogonial mitosis, a process 
destined to end entirely by about the seventh month of intrauter­
ine life. Equally relevant is the increasing rate of oogonial atresia, 
which peaks at 5 months of gestation. During this period, regula­
tion of the ovarian developmental process is complex and prob­
ably involves a diverse group of genes (see Fig. 17.7).
From midgestation onward, relen

s (see Fig. 17.7).
From midgestation onward, relentless and irreversible attrition 
progressively diminishes the germ cell endowment of the gonad.60 
About 50 years later, it is exhausted. For the most part, this is 
accomplished through follicular atresia rather than oogonial atre­
sia, begins at about month 6 of gestation, and continues through­
out life (see Fig. 17.9). In contrast, oogonial atresia is destined 
to end at 7 months of intrauterine life as follicular atresia sets in. 
Follicula

ine life as follicular atresia sets in. 
Follicular atresia has a profound effect on germ cell endowment, 
because only 1 to 2 × 106 germ cells are present at birth (see Fig. 
17.9).60 Remarkably, this dramatic depletion of the germ cell mass 
occurs during a period as short as 20 weeks. No similar rate of 
depletion occurs earlier or subsequently. Consequently, newborn 
girls enter life still far from realizing their reproductive potential 
but having lost as much as 80% of their germ cell endo

having lost as much as 80% of their germ cell endowment. 
The germ cell mass decreases further to approximately 300,000 by 
the onset of puberty. Of these follicles, only 400 to 500 (<1% of 
the total) are recruited for ovulation in the course of a reproduc­
tive life span.
Between weeks 8 and 13 of fetal life, some of the oogonia 
depart from the mitotic cycle to enter the prophase of the first 
meiotic division. This change marks the conversion of these cells 
to primary oocytes well before ac

 of these cells 
to primary oocytes well before actual follicle formation. Meiosis 
(beginning at about 8 weeks of gestation) provides temporary pro­
tection from oogonial atresia, allowing the germ cells to invest 
themselves with granulosa cells and to form primordial follicles. 
Oogonia that persist beyond the seventh month of gestation and 
have not entered meiosis are subject to oogonial atresia; conse­
quently, no oogonia are usually present at birth.
Once formed, the primary oocyte persis

t at birth.
Once formed, the primary oocyte persists in prophase of the 
first meiotic division until the time of ovulation, when meiosis 
is resumed and the first polar body is formed and extruded (Fig. 
17.10). Although the exact cellular mechanisms responsible for 
this meiotic arrest remain uncertain, it is presumed that a granu­
losa cell–derived meiosis inhibitor is responsible. This hypothesis 
is based on the observation that denuded (granulosa-free) oocytes 
are capable of spontaneously

nulosa-free) oocytes 
are capable of spontaneously completing meiotic maturation 
in vitro.
The primary oocyte is converted into a secondary oocyte by 
completion of the first meiotic metaphase and formation of the 
first polar body, which occurs before ovulation but after the LH 
surge. At ovulation, the secondary oocyte and the surrounding 
granulosa cells (cumulus oophorus) are extruded and enter the 
fallopian tube. If sperm penetration occurs, the secondary oocyte 
undergoes a second meioti

s, the secondary oocyte 
undergoes a second meiotic division, after which the second polar 
body is eliminated (see Fig. 17.10). 
Granulosa Cell Layer
In the developing ovaries of a human female fetus, oocytes ini­
tially exist as germ cell clusters before an ovarian follicle is formed. 
Number of germ cells (millions)
7.0
5.0
3.0
1.0
0.6
0.3
3
6
9
5 10
30
50
Age: Months
In utero
Birth
Postnatal
Age: Years
• Fig. 17.9  Age-dependent changes in germ cell number in the human 
ovary. The highest nu

rm cell number in the human 
ovary. The highest number of oocytes is found in the ovaries of a human 
fetus at midgestation. This number decreases sharply during the third tri­
mester. After birth, the progressive decline in the number of ovarian folli­
cles containing oocytes continues until complete depletion at menopause. 
(From Baker TG. A quantitative and cytological study of germ cells in the 
human ovaries. Proc R Soc Biol Sci. 1963;158:417–433.)
584
SECTION V    Sexual Development and Fu

7–433.)
584
During the second half of in utero life, these germ cell clusters break 
down, and the surviving oocytes become individually surrounded 
with squamous pregranulosa cells to give rise to primordial follicles. 
The transition from primordial to primary follicle is marked histo­
logically by a morphologic change in granulosa cells from squamous 
to cuboidal. By the secondary stage, there are at least two layers of 
cuboidal granulosa cells an

t least two layers of 
cuboidal granulosa cells and an additional layer of somatic cells, the 
theca, which forms outside the basement membrane of the follicle 
(Fig. 17.11).49 At puberty, FSH secreted by the pituitary promotes 
further granulosa cell proliferation and survival.
A basement lamina separates the oocyte and granulosa cells 
from the surrounding stromal cells.59 The granulosa cells do not 
have direct access to the circulation before ovulation (Fig. 17.12).
The avascular nature of t

 ovulation (Fig. 17.12).
The avascular nature of the granulosa cell compartment neces­
sitates contact between neighboring cells. The granulosa cells are 
interconnected by extensive intercellular gap junctions, which 
result in their coupling to yield an expanded, integrated, and func­
tional syncytium (Fig. 17.13).61 Gap junctions are composed of 
proteins called connexins. Connexin 37 and other connexins have 
been demonstrated in gap junctions in follicles. Gap junction 
Meiosis I
2nd trimes

s in follicles. Gap junction 
Meiosis I
2nd trimester
Entry to
meiosis
DNA
synthesis
Crossing-
over
Pat
Mat
Mat
Pat
2n DNA
oogonium
Birth
Meiotic maturation
Preovulatory LH surge
Puberty
GVBD
Metaphase I
primary oocyte
2n DNA
secondary oocyte
2n DNA
fertilized egg
1st
polar
body
Meiosis II
n
n DNA
2nd
polar
body
Sperm
Sperm
Egg
Egg
Ovulation
2n DNA
metaphase II
(short arrest)
GV
4n DNA
prophase I
(long arrest)
• Fig. 17.10  Meiotic cell division. During meiosis, 

ig. 17.10  Meiotic cell division. During meiosis, the chromosomes that were inherited from the parents of 
the individual and stored in gonads are processed to prepare their genetic material for transmission to the 
offspring. Meiosis occurs exclusively in germ cells and serves two critical purposes: generation of germ 
cells genetically distinct from the somatic cells and generation of a mature egg (or sperm) with a reduc­
tion in the number of chromosomes from 46 to 23. Genetic recombination t

chromosomes from 46 to 23. Genetic recombination through crossover of genes 
between homologous chromosomes and random assortment of (grand-) maternal and (grand-) paternal 
chromosomes into daughter cells during the first meiotic division are responsible for the first function of 
meiosis, maintenance of genetic diversity. The second function is provided by a reduction in the number of 
chromosomes so that each daughter cell, or ovum, receives randomly one chromosome from each of the 
23 pairs.

andomly one chromosome from each of the 
23 pairs. During fertilization, the fusion of ovum and sperm, each of which contributes 23 chromosomes, 
produces a genetically novel individual with 46 chromosomes. The chromosome marked as white in the 
oogonium (upper left corner) originates from the father of the female fetus, whereas the blue chromosome 
comes from the mother of the fetus. The random exchange of genes (alleles) between homologous chro­
mosomes (crossover) takes place before the meiot

­
mosomes (crossover) takes place before the meiotic arrest in the prophase I stage before birth. After birth, 
the oocytes of this child remain in meiotic arrest until puberty. In the developing oocyte in the Graafian 
follicle, meiosis I is resumed immediately after the preovulatory luteinizing hormone (LH) surge during each 
ovulatory cycle. Meiotic maturation is defined as the period from the breakdown of the oocyte’s nucleus 
(germinal vesicle [GV]) until the oocyte reaches metaphase II (i.

le [GV]) until the oocyte reaches metaphase II (i.e., transition from oocyte to egg). A second 
and short meiotic arrest occurs at metaphase II until the oocyte is fertilized by a sperm. DNA, deoxyribo­
nucleic acid; GVBD, germinal vesicle breakdown; Mat, maternal; n, amount of DNA material in the haploid 
number (23) of chromosomes; Pat, paternal.
585
protein connexin 37 (GJA4)–deficient mice lack Graafian follicles, 
fail to

)–deficient mice lack Graafian follicles, 
fail to ovulate, and develop inappropriate corpora lutea.61 These 
specialized cell junctions may be important in metabolic exchange 
and in the transport of small molecules between neighboring 
granulosa cells. Moreover, the granulosa cells extend cytoplasmic 
processes that penetrate the zona pellucida to form gap junctions 
with the plasma membrane of the oocyte (see Fig. 17.13). In 
the GJA4-deficient mice, oocyte development is arrested before 
mei

t mice, oocyte development is arrested before 
meiotic competence.62 Gap junctions represent a crucial commu­
nication system that is needed for the tight control exerted by 
the cumulus granulosa cells on the resumption of meiosis by the 
enclosed primary oocyte.
Several gene products regulate the transition from primordial to 
primary follicle, which is marked by a change in the morphologic 
appearance of granulosa cells from squamous to cuboidal, followed 
by an increase in granulosa cell lay

al, followed 
by an increase in granulosa cell layers in the secondary follicle.49 
These genes are expressed in the oocyte or granulosa cells, empha­
sizing the active role of the oocyte in granulosa cell differentiation. 
Newborn oogenesis homeobox (NOBOX), spermatogenesis and 
oogenesis helix-loop-helix 1 (SOHLH1), and SOHLH2 are critical 
transcription factors during the transition from primordial to primary 
follicles.49 Interactions between KIT ligand expressed in granulosa 
cells and the 

 KIT ligand expressed in granulosa 
cells and the KIT tyrosine kinase receptor expressed in oocytes also 
appear to be critical in early folliculogenesis. The KITLG/KIT path­
way induces the PI3K/AKT pathway, leading to phosphorylation 
and inactivation of forkhead box O3 (FOXO3), an inhibitor of pri­
mordial follicle activation.49 These genetic studies supported a critical 
role of the PI3K/AKT/FOXO3 pathway in early follicle development 
and granulosa cell differentiation. Whereas FOXO3 is the

anulosa cell differentiation. Whereas FOXO3 is the key oocyte 
factor critical for suppressing primordial follicle activation, another 
forkhead domain transcription factor, forkhead box L2 (FOXL2), is 
crucial in the transition from squamous to cuboidal granulosa cells.49
Antimüllerian hormone (AMH) produced by the granulosa 
cells of growing follicles appears to inhibit the growth of primor­
dial follicles; in its absence there is a faster depletion of growing 
follicles, although it is unknow

etion of growing 
follicles, although it is unknown whether this is a direct or indi­
rect effect of AMH.49 Clinically, serum AMH may be a useful 
biomarker of ovarian reserve. In humans and mice, serum AMH 
declines with increasing age. Although it is difficult to establish a 
direct link between serum AMH and the primordial follicle pool 
in humans, antral follicle number is positively correlated with 
AMH levels.49 More detailed information has been reviewed by 
Edson and colleagues.49
The gr

s been reviewed by 
The granulosa cells in the fully developed Graafian follicle 
shortly before ovulation are stratified in a manner that allows the 
distinction of a number of populations of cells.63 Distinct popula­
tions of granulosa cells exhibit specific, specialized functions.63,64 
Mural granulosa cells within the outermost layer adjacent to the 
basement layer contain high levels of gonadotropin hormone 
receptors and steroidogenic enzymes and account for most of

 and steroidogenic enzymes and account for most of the 
steroidogenesis in the follicle.63,64 The cumulus oophorus contains 
the egg and a surrounding mass of granulosa cells that have cell-
cell interactions with the egg and seem to have critical roles in 
oocyte development (Fig. 17.13; see also 17.12).63,64
Mural and cumulus granulosa cells also exhibit distinct patterns 
of gene expression. For example, the tumor suppressor BRCA1 is 
highly expressed in ovarian granulosa cells of developing 

xpressed in ovarian granulosa cells of developing follicles.65 
However, in large antral or preovulatory follicles, BRCA1 expres­
sion significantly decreases in mural granulosa cells and becomes 
restricted to cumulus granulosa cells; these cells, unlike the mural 
ones, do not contain abundant aromatase, so this development gives 
rise to an intriguing inverse correlation of BRCA1 with aromatase 
mRNA and protein levels.65 Stimulation of an FSH-dependent 
Basement membrane
Granulosa cells
Base

-dependent 
Oocyte
Zona pellucida
Oocyte
Theca
Multiple layers of
granulosa cells
Fully grown oocyte
Antrum
Membrana
Cumulus oophorus
Antrum
(follicular fluid)
follicle
Secondary
follicle
Primary
fol

follicle
Secondary
follicle
Primary
follicle
Tertiary
follicle
Graafian
follicle
• Fig. 17.11  Developmental stages of the ovarian follicle. The primordial follicle 
is composed of a single layer of granulosa cells and a single immature oocyte 
arrested in the diplotene stage of the first meiotic division. The primordial fol­
licle is separated from the surrounding stroma by a thin basal lamina (i.e., 
basement membrane). The oocyte and granulosa cells do not have a direct 
blood supp

d granulosa cells do not have a direct 
blood supply. The first sign of follicular recruitment is cuboidal differentiation 
in the spindle-shaped cells inside the basal lamina, which thereafter undergo 
successive mitotic divisions to form a multilayered granulosa cell zone. The 
oocyte enlarges and secretes a glycoprotein-containing mucoid substance 
called the zona pellucida, which surrounds the oocyte and separates the 
granulosa cells from the oocyte. This structure is a primary follicle. Th

e oocyte. This structure is a primary follicle. The sec­
ondary follicle is formed by further proliferation of granulosa cells and by the 
final phase of oocyte growth, in which the oocyte reaches 120 μm in diameter, 
coincident with proliferation of layers of cells immediately outside the basal 
lamina to constitute the theca. The portion of the theca adjacent to the basal 
lamina is called the theca interna. Theca cells that merge with the surrounding 
stroma are designated the theca externa. 

ounding 
stroma are designated the theca externa. The secondary follicle acquires an 
independent blood supply consisting of one or more arterioles that terminate 
in a capillary bed at the basal lamina. Capillaries do not penetrate the base­
ment membrane, and the granulosa and oocyte remain avascular. The tertiary 
follicle is characterized by further hypertrophy of the theca and the appear­
ance of a fluid-filled space among the granulosa cells, called the antrum. The 
fluid in the antrum con

s, called the antrum. The 
fluid in the antrum consists of a plasma transudate and secretory products of 
granulosa cells, some of which (estrogens) are found there in strikingly higher 
concentrations than in peripheral blood. The follicle rapidly increases in size 
under the influence of gonadotropins to form the mature or Graafian follicle. 
In the Graafian follicle, the granulosa and oocyte remain encased by the basal 
lamina and are devoid of direct vascularization. The antral fluid increas

f direct vascularization. The antral fluid increases in 
volume, and the oocyte, surrounded by an accumulation of granulosa cells 
(i.e., cumulus oophorus), occupies a polar, eccentric position within the follicle. 
The mature Graafian follicle is then ready to release the ovum by the process 
of ovulation. (Adapted from Erickson GF, Magoffin DA, Dyer CA. The ovar­
ian androgen producing cells: a review of structure-function relations. Endocr 
Rev. 1985;6:371–379. Copyright © 1985 by The Endocri

v. 1985;6:371–379. Copyright © 1985 by The Endocrine Society.)
586
signaling pathway greatly induces aromatase but suppresses BRCA1 
expression in granulosa cells. Moreover, BRCA1 binds to the aro­
matase promoter and inhibits its activity.66 Therefore BRCA1 may 
exert its tumor suppressor activity, in part, by limiting excessive 
estrogen formation in the ovary. A summary of ovarian follicle 
development is presented in Fig. 17.11. 
Theca Cell Layer

ent is presented in Fig. 17.11. 
After the follicle achieves two layers of granulosa cells, another 
morphologically distinct layer of somatic cells, the theca, differen­
tiates from ovarian stroma (see Figs. 17.11, 17.12, and 17.13).49 
The cells making up the theca-interstitial compartment are hetero­
geneous in nature.67 Cells of the theca interna layer, which forms 
just outside the basement membrane surrounding the granulosa 
cells, show typical steroidogenic features, incl

cells, show typical steroidogenic features, including mitochondria 
with tubular cristae, smooth endoplasmic reticulum, and abun­
dant lipid vesicles (see Figs. 17.11 and 17.12). Theca interna 
cells are responsible for producing the C19 steroids that diffuse 
into the neighboring granulosa cells and serve as substrates for 
estrogen production. Theca externa is the outermost layer of the 
follicle and is composed of fibroblasts, smooth muscle–like cells, 
and macrophages (see Figs. 17.11 and 

like cells, 
and macrophages (see Figs. 17.11 and 17.12). The theca externa 
is thought to have an important function during ovulation. Cells 
that contribute to the theca differentiate from mesenchymal pre­
cursor cells present in the ovarian stroma, adjacent to developing 
follicles. Like preantral folliculogenesis, theca formation is gonad­
otropin independent. Theca precursor cells lack LH receptors, and 
the theca layer still forms in the ovaries of FSH-deficient mice.49 
After a discernibl

aries of FSH-deficient mice.49 
After a discernible theca interna layer has developed, theca cell 
C19 steroid production is regulated primarily by LH.
The differentiated state of theca interna cells is marked by 
expression of a number of steroidogenic genes, including those 
that encode the LH receptor (LHCGR), steroidogenic acute regu­
latory protein (STAR), side-chain cleavage enzyme (CYP11A1), 
3β-hydroxysteroid 
dehydrogenase-Δ5,4 
isomerase 
type 
2 
(HSD3B2), and 17-hydroxylase/17,20-lya

e 
type 
2 
(HSD3B2), and 17-hydroxylase/17,20-lyase (CYP17A1) in the 
human. Granulosa cells of the developing follicles appear to secrete 
factors that regulate theca cell differentiation. Candidate factors 
that may contribute to theca cell differentiation include insulin-
like growth factors (IGFs), KITLG, and growth differentiation 
factor 9 (GDF9). IGF1 induces expression of Lhcgr, Cyp11a1, 
and Hsd3b1 (counterpart of human HDS3B2), whereas KITLG 
stimulates Star and Cyp17a1 expression in 

 KITLG 
stimulates Star and Cyp17a1 expression in rat theca cells.49 In 
mice lacking the Gdf9 gene, a theca layer fails to form in the ovary. 
Whether GDF9 regulates theca cell recruitment or differentiation 
Early antral follicle
Preovulatory (graafian) follicle
Egg
Egg
Cytoplasm
Nucleus
Egg
Mural
granulosa
cells
Theca
cells
Cumulus
granulosa
cells
Granulosa
cells
Theca
cells
Antrum
Basal lamina
Granulosa
cells
A
C
B
Antrum
Theca cells
• Fig. 17.12  Histology of gonadotropin-dependent ovarian 

7.12  Histology of gonadotropin-dependent ovarian follicle development. (A) Development of an 
antrum marks gonadotropin dependency. Multiple layers of granulosa and theca cells are present. (B) The 
follicle destined to ovulate distinguishes itself from the rest of the cohort through accumulation of large 
quantities of antral fluid. The granulosa cells, which accumulate around the oocyte, are called cumulus 
granulosa cells and primarily function to support egg development. The mural granulosa

on to support egg development. The mural granulosa cells in the periph­
ery primarily serve as steroidogenic cells. (C) A membrane called the basal lamina (arrows), which has 
been formed at the primary stage, separates the granulosa cells from the theca component of the follicle.
587
directly or indirectly through regulation of preantral granulosa cell 
development is unknown.49 
Follicles
The follicle represents the key func

49 
Follicles
The follicle represents the key functional unit in the ovary with 
respect to germ cell development and steroid production. The 
follicles are embedded in loose connective tissue of the ovarian 
cortex and can be subdivided into two functional types: nongrow­
ing (primordial) and growing. Between 90% and 95% of follicles 
are nongrowing throughout reproductive life. Recruitment of a 
primordial follicle initiates dramatic changes in growth, structure, 
and function. The growing fol

 growth, structure, 
and function. The growing follicles are divided into four stages: 
primary, secondary, tertiary, and graafian (see Figs. 17.7 and 
17.11). The first three stages of growth can occur in the absence of 
the pituitary and therefore appear to be controlled by intraovarian 
mechanisms (see Figs. 17.7 and 17.11). The follicle destined to 
ovulate is recruited during the first few days of the current cycle.68
The early growth of follicles occurs over several menstrual 
cycles, but 

licles occurs over several menstrual 
cycles, but the ovulatory follicle is one of a cohort recruited at 
the time of transition from the previous cycle’s luteal phase to the 
current cycle’s follicular phase.68 The total time to achieve preovu­
latory status is approximately 85 days.68 Most of this developmen­
tal period is FSH independent. Eventually this cohort of follicles 
reaches a stage at which, unless recruited by FSH, the next step 
is atresia. A cohort of follicles measuring 2 to 5 mm

atresia. A cohort of follicles measuring 2 to 5 mm in diameter 
is continuously available for a response to FSH. The late luteal 
increase in FSH is the critical feature in rescuing this cohort of 
follicles from atresia; it allows a dominant follicle to emerge and 
pursue a path to ovulation. The increase in the FSH level must be 
maintained for a critical period (see Fig. 17.1).69
Recruited primordial follicles either develop into dominant, 
mature Graafian follicles destined to ovulate or deg

ture Graafian follicles destined to ovulate or degenerate as a 
result of atresia.70 The average time for development of a selected 
follicle to the point of ovulation is 10 to 14 days. If a follicle is not 
recruited, it goes through a process called atresia, during which 
the oocyte and granulosa cells within the basal lamina die and are 
replaced by fibrous tissue. There is general agreement that atresia 
of follicles occurs through apoptosis.71 
Ovulation
There is a dramatic rise in circulat

71 
Ovulation
There is a dramatic rise in circulating estradiol level as midcycle 
approaches. This is followed by a striking LH surge and, to a lesser 
extent, an FSH surge, which trigger the dominant follicle to ovu­
late. During each menstrual cycle, usually one follicle ovulates 
and gives rise to a corpus luteum. In women, LH or its surrogate, 
hCG, is essential to stimulate rupture of the mature follicle. It has 
been proposed that increased local prostaglandin biosynthesis in 
the follicl

d local prostaglandin biosynthesis in 
the follicle mediates the ovulatory effect of LH.72,73
Ovulation consists of rapid follicular enlargement followed by 
protrusion of the follicle from the surface of the ovarian cortex. 
This is followed by the rupture of the follicle and extrusion of 
an egg-cumulus complex into the peritoneal cavity (Fig. 17.14). 
Follicular rupture or ovulation occurs predictably 34 to 36 hours 
after the start of the LH surge. Elevation of a conical stigma on 
the surfa

surge. Elevation of a conical stigma on 
the surface of the protruding follicle precedes rupture (see Fig. 
17.14). Rupture of this stigma is accompanied by a gentle rather 
than explosive expulsion of the ovum and antral fluid. A number 
of transcriptional regulators downstream of the LH receptor are 
required for ovulation. After the LH surge, progesterone recep­
tor (PR) levels rapidly increase in the mural granulosa cells of the 
preovulatory follicle.49 LH-dependent or PR-dependent produc­

 follicle.49 LH-dependent or PR-dependent produc­
tion of proteases acting locally on protein substrates in the basal 
lamina may play an important role in stigma formation and fol­
licular rupture.62,74 In particular, levels of plasminogen activator 
increase in the follicle before rupture.75 Plasminogen activator–
mediated conversion of plasminogen to plasmin may contribute 
to the proteolytic digestion of the follicular wall, which is a pre­
requisite for follicular rupture. Gene knockout stu

equisite for follicular rupture. Gene knockout studies in mice 
suggest that other factors important for ovulation or follicular 
rupture include endothelin 2, peroxisome proliferator-activated 
A
B
• Fig. 17.13  Structural relationship between the cumulus granulosa cell and 
the oocyte. (A) Microvilli of an oocyte interdigitate with cytoplasmic exten­
sions of granulosa cells, penetrating the zona pellucida. (B) Notice the 
penetration of the zona pellucida (ZP) by cytoplasmic processes of the 

na pellucida (ZP) by cytoplasmic processes of the 
granulosa cells. Small gap junctions (thin arrows) are observed between 
processes of the granulosa cell and the oocyte membrane. The thick 
arrows indicate a gap junction between granulosa cells. (From Erickson 
GF. An analysis of follicle development and ovum maturation. Semin 
Reprod Endocrinol. 1986;4:233, used with permission of Thieme Medical 
Publishers, New York.)
588
receptor-γ (PPARγ), CCAAT

Development and Function
receptor-γ (PPARγ), CCAAT/enhancer-binding protein-β, liver 
receptor homologue 1 (LRH1), steroidogenic factor 1 (SF1), and 
nuclear receptor–interacting protein 1.49
Corpus Luteum
After ovulation, the dominant follicle reorganizes to become the 
corpus luteum (Fig. 17.15). After rupture of the follicle, capillaries 
and fibroblasts from the surrounding stroma proliferate and pen­
etrate the basal lamina (see Fig. 17.11). This rapid vascularization 
of the corpus luteum 

 This rapid vascularization 
of the corpus luteum may be guided by angiogenic factors, some 
of which are detected in the follicular fluid.76 Vascular endothelial 
growth factor has been isolated from corpora lutea and has been 
postulated, along with basic fibroblast growth factor, to be a poten­
tial angiogenic agent in corpora lutea.77 Concurrently, the granulosa 
and theca cells undergo morphologic changes collectively referred 
to as luteinization. The granulosa cells become granulosa-lutei

zation. The granulosa cells become granulosa-lutein cells 
(large cells), and the theca cells are transformed into theca-lutein 
cells (small cells) (see Fig. 17.15).78 The so-called K cells, scattered 
throughout the corpus luteum, are believed to be macrophages.
The corpus luteum is the major source of sex steroid hormones 
secreted by the postovulatory ovary. The human corpus luteum 
secretes as much as 40 mg of progesterone per day during the 
midluteal phase of the ovarian cycle.79 In view 

midluteal phase of the ovarian cycle.79 In view of the small size 
of the corpus luteum, it is the most active steroidogenic tissue in 
humans. An important aspect of corpus luteum formation is the 
penetration of the follicle basement membrane by blood vessels. 
This vascularization provides the granulosa-lutein cells with low-
density lipoprotein (LDL) cholesterol.76 LDL cholesterol serves as 
the primary substrate for corpus luteum progesterone production.
A key regulator of steroidogenesis

one production.
A key regulator of steroidogenesis in the corpus luteum is LH. 
In humans, the LH receptor is maintained throughout the func­
tional life span of the corpus luteum and is not downregulated 
during the maternal recognition of pregnancy.80 The rate-limiting 
step in LH-mediated progesterone formation in luteinized granu­
losa cells is the entry of cholesterol into the mitochondria, which 
is regulated by the steroidogenic acute regulatory (StAR) pro­
tein.81 The availability of LDL

latory (StAR) pro­
tein.81 The availability of LDL cholesterol and the StAR-mediated 
mitochondrial entry of cholesterol into the mitochondria seem to 
be the two critical factors that account for the production of large 
amounts of progesterone in the corpus luteum.
Unless pregnancy occurs, the functional life span of the cor­
pus luteum is normally 14 ± 2 days, after which it spontaneously 
regresses and is replaced by an avascular scar named the corpus 
albicans. There is little doubt about t

he corpus 
albicans. There is little doubt about the central role of LH or hCG 
in the maintenance of corpus luteum function. Withdrawal of 
LH support in a variety of experimental circumstances has almost 
invariably resulted in luteal regression.82 In pregnancy, however, 
the LH surrogate, hCG, secreted by the gestational trophoblast, 
maintains the ability of the corpus luteum to elaborate proges­
terone; this stimulus helps to maintain the early gestation until 
the luteoplacental shift.82 T

rly gestation until 
the luteoplacental shift.82 The corpus luteum doubles in size 
Stigma
EGG-Cumulus complex
Basal
constrictions
• Fig. 17.14  Ovulation of the cumulus-oocyte complex through the stigma. 
(From Erickson GF. An analysis of follicle development and ovum matu­
ration. Semin Reprod Endocrinol. 1986;4:233, used with permission of 
Thieme Medical Publishers, New York.)
B
CYP17A1 (17-hydroxylase/17,20-lyase)
CYP19A1 (aromatase)
Theca-lutein
cells
Granulosa-lutein
cells
ce

utein
cells
cells
cells
A
cells
cells
Hematoxylin and eosin
cells
• Fig. 17.15  Corpus luteum. (A) Hematoxylin and eosin stain shows the large granulosa-lutein cells occupy­
ing the center and smaller theca-lutein cells in the periphery. (B) Immunoreactive aromatase, the product 
of the CYP19A1 gene (brown stain), is the hallmark of granulosa-lutein cells, whereas immunoreactive 
17-hydroxylase/17,20-lyase, the product of t

tive 
17-hydroxylase/17,20-lyase, the product of the CYP17A1 gene (purple stain), is selectively localized to 
theca-lutein cells. (Courtesy Dr. Hironobu Sasano, Tohoku University, Sendai, Japan.)
589
(compared with the pre-pregnancy size) during the first 6 weeks of 
gestation because of hypertrophy of the luteinized granulosa and 
theca cells (see Fig. 17.15). This early hypertrophy is followed by 
regression. The corpus lut

rtrophy is followed by 
regression. The corpus luteum at term is only one-half of its size 
during the menstrual cycle.
Hormones such as estrogens and prostaglandins have been 
suggested to be important factors in the promotion of luteal 
demise.83 Immune factors may influence luteal life span because 
corpus luteum regression is associated with a progressive infiltra­
tion of lymphocytes and macrophages. In the absence of LH or 
hCG, apoptosis is a critical endpoint mechanism by which human 
co

s a critical endpoint mechanism by which human 
corpora lutea are deleted.84 
Ovarian Follicle-Stimulating Hormone and Luteinizing 
Hormone Receptors
The FSH receptor is expressed exclusively by granulosa cells. The 
LH or hCG receptor (LHCGR) is expressed primarily by the 
theca-interstitial cells of all follicles and by granulosa cells of large 
preovulatory follicles.
Granulosa cells in primary or secondary follicles that are in 
the early developmental stages before antrum formation primaril

elopmental stages before antrum formation primarily 
bind FSH but not LH. In these preantral follicles, the binding 
of LH or hCG is confined to theca-interstitial cells.85 Granulosa 
cells in more mature tertiary follicles with an antrum appear capa­
ble of binding both LH and FSH. The FSH receptors are found 
in granulosa cells from follicles of all sizes, but LH receptors are 
found only in granulosa cells of large preovulatory follicles.86,87 
These observations are consistent with the conce

These observations are consistent with the concept that the acqui­
sition of LH receptors on granulosa cells is under the influence of 
FSH.88
The receptors for the glycoprotein hormones have related 
structures (see Fig. 17.17, later). The receptors belong to the large 
family of G protein–coupled receptors, whose members all have a 
transmembrane domain that consists of seven membrane–travers­
ing α-helices connected by three extracellular and three intracel­
lular loops. The glycoprotein ho

d three intracel­
lular loops. The glycoprotein hormone receptors form a separate 
subgroup within this large family by virtue of their large extra­
cellular hormone-binding domain at the amino-terminus. FSH 
binds to the FSH receptor, and LH and hCG bind to the same 
LH receptor. The LH and FSH receptor genes are located on chro­
mosome 2 in the p21 region.47 The carboxy-terminal half of the 
receptor is encoded by a single last exon and contains the seven 
transmembrane segments and the G prot

s the seven 
transmembrane segments and the G protein–coupling domain. 
The unusually large extracellular domain of the glycoprotein hor­
mone receptors, on the other hand, is encoded by the first 9 or 
10 exons.
Role of Follicle-Stimulating Hormone in Ovarian Function
FSH is the main promoter of follicular maturation. Given that 
FSH receptors have been exclusively localized to granulosa cells, 
it is presumed that FSH action in the ovary involves the granu­
losa cells. The ability of FSH to or

es the granu­
losa cells. The ability of FSH to orchestrate follicular growth and 
differentiation depends on its ability to exert multiple actions 
concurrently.
Phenotypes of women with mutations that disrupt the func­
tion of the FSH β-subunit gene are in good agreement and dem­
onstrate that FSH is necessary for normal follicular development, 
ovulation, and fertility. Pubertal development is hampered in the 
absence of sufficient numbers of later stage follicles with the gran­
ulosa cells n

later stage follicles with the gran­
ulosa cells needed for adequate estrogen production. Treatment 
of affected patients with exogenous FSH has resulted in follicular 
maturation, ovulation, and normal pregnancy.47 The presenting 
phenotype of FSH β-subunit deficiency is practically identical to 
that caused by inactivating mutations of the FSH receptor, except 
that the latter fail to respond to exogenous FSH.47
Women with FSH receptor mutations are clinically similar to 
patients with gonadal

s are clinically similar to 
patients with gonadal dysgenesis; they have absent or poorly devel­
oped secondary sexual characteristics and high serum levels of FSH 
and LH.47 The notable difference is the presence of ovarian follicles 
in women with mutated FSH receptors, consistent with the FSH 
independence of primordial follicle recruitment and early follicu­
lar growth and development. Total absence of any follicles, includ­
ing those in the primordial stage, occurs in women in whom FSH 
rec

primordial stage, occurs in women in whom FSH 
receptor mutations cannot be demonstrated.47 The ovarian pheno­
type of FSH receptor deficiency is distinct from the common form 
of gonadal dysgenesis (Turner syndrome), which is characterized by 
streak gonads and an absence of growing follicles.47
In vivo rodent studies suggest that FSH is capable of increas­
ing the number of its own receptors in the granulosa cell. Whereas 
estradiol by itself may be without effect on the distribution, num­
ber

ay be without effect on the distribution, num­
ber, or affinity of granulosa cell FSH receptors, estrogens synergize 
with FSH to enhance the overall number of granulosa cell FSH 
receptors.89 Changes in the production of estradiol by preantral 
follicles can increase their response to FSH through regulation of 
granulosa cell surface FSH receptors. This interaction between FSH 
and estradiol in follicular development has been well established in 
rodents. It appears that ERα and ERβ mediate the

rodents. It appears that ERα and ERβ mediate the estrogenic effect 
on ovarian development and follicular maturation in mice.90 How­
ever, it is not clear whether a similar relationship exists in the human 
ovary. ERα is not detected in the human ovary in significant quan­
tities. Nevertheless the demonstration of ERβ in the human ovary 
suggests an interaction between FSH and estrogen in the regulation 
of normal follicle development and ovulation in women.91
One of the major actions of FSH i

tion in women.91
One of the major actions of FSH is induction of granulosa 
cell aromatase activity.92 Little or no estrogen can be produced 
by FSH-unprimed granulosa cells even if they are supplied with 
aromatizable androgen precursors. Treatment with FSH enhances 
the aromatization capability of granulosa cells, an effect related to 
enhancement of the granulosa cell aromatase content.92
Treatment with FSH has also been shown to induce LH recep­
tors in granulosa cells. The ability of FSH to

ep­
tors in granulosa cells. The ability of FSH to induce LH receptors 
is augmented by the concomitant presence of estrogens.93 Proges­
tins, androgens, and LH itself may also induce LH receptors. After 
induction, the granulosa cell LH receptor requires the continued 
presence of FSH for its maintenance.
Circumstantial evidence, deduced from studies of women 
with disrupting mutations of the genes that encode FSH and 
LH receptors and aromatase (CYP19A1), indicates that FSH 
action, but not es

 (CYP19A1), indicates that FSH 
action, but not estrogen or LH action, is essential for follicu­
lar growth in humans.47,93 Follicular growth and development 
up to the antral stage were observed in women with deficient 
LH action or estrogen biosynthesis, although these individu­
als were anovulatory.47,93 Women with mutations of the FSH 
β-subunit or FSH receptor have only primordial follicles in 
their ovaries.47 These data indicate that estrogen and LH are 
not critical for follicular develo

gen and LH are 
not critical for follicular development at least until the tertiary 
stage (see Figs. 17.11 and 17.12). However, FSH by itself is 
not sufficient to achieve normal follicular development and 
ovulation. 
Role of Luteinizing Hormone in Ovarian Function
LH is essential for ovulation (follicular rupture) and the suste­
nance of corpus luteum function; in addition, it plays other 
important roles in follicular function.93 First, LH probably plays 
a major role in the promotion of the

obably plays 
a major role in the promotion of theca-interstitial cell androgen 
production. Second, LH may well synergize with FSH in the more 
advanced phases of follicular development. Third, small and sus­
tained increments in the circulating levels of LH are necessary and 
sufficient to cause small antral follicles to grow and develop to the 
preovulatory stage.93
It is presumed that LH acts on the theca-interstitial cells of small 
follicles, where it promotes the biosynthesis of C19 stero

s, where it promotes the biosynthesis of C19 steroids.93 The 
consequent increase in estrogen production is presumed to con­
tribute to the growth and development of the follicles. Treatment 
with small doses of LH also results in an increase in LH receptor 
content and in induction of the key steroidogenic proteins, such 
as StAR, CYP11A1, HSD3B2, and CYP17A1.93
The role of LH action in human ovarian physiology was exem­
plified by the phenotype of a woman with a disrupting mutation of 
the LH 

 of a woman with a disrupting mutation of 
the LH receptor gene.47 She presented with amenorrhea, normally 
developed secondary sexual characteristics, increased circulating 
FSH and LH levels, and low levels of estradiol and progesterone 
that were unresponsive to hCG treatment.47 The ovary contained 
follicles that developed up to antral stage with a well-developed 
theca layer but no preovulatory follicles or corpora lutea. These 
observations collectively support the view that LH is essentia

 collectively support the view that LH is essential for 
ovulation and sufficient estrogen production, whereas follicular 
development is initially autonomous but at later stages depends 
on intact FSH action.47 
Ovarian Steroidogenesis
The steroid hormone contents of the ovarian vein effluents and periph­
eral venous blood were compared to distinguish steroids secreted by 
the ovary from those secreted by the adrenal or produced by periph­
eral conversion of precursors.94 These studies revealed

conversion of precursors.94 These studies revealed that the ovaries 
secrete pregnenolone, progesterone, 17α-hydroxyprogesterone, dehy­
droepiandrosterone (DHEA), androstenedione, testosterone, estrone, 
and estradiol.95,96 Although such measurements provide insights into 
the steroidogenic pathways under study, they do not identify the spe­
cific ovarian cells involved. Studies using microdissected preovulatory 
follicles identified estrone and estradiol as the major steroid products 
(Fig. 17.

estradiol as the major steroid products 
(Fig. 17.16). Progesterone and 17α-hydroxyprogesterone proved to 
be the major products of the corpus luteum (see Fig. 17.16).
HO
HO
StAR
CYP11A1
Mitochondrion
CH3
CYP17A1
CYP17A1
CYP17A1
CYP17A1
CYP19A1
(aromatase)
CYP19A1
17-Hydroxypregnenolone
Estrone
Estradiol
A
B
C
D
1
2
3 4 5
6 7
8
9
10
111213
14
15
16
17
18
19
20
21
22
23
24 25
26
27
17-Hydroxyprog

16
17
18
19
20
21
22
23
24 25
26
27
O
OH
C
O
O
O
CH3
O
C
HO
HO
CH3
O
C
OH
CH3
O
C
OH
OH
HO
O
O
O
O
HO
HSD17B1
HSD17B1
HSD3B2
HSD3B2
HSD3B2
• Fig. 17.16  Steroidogenic pathway in the human ovary. The biologically active steroids progesterone and 
estradiol are produced primarily in the ovary of a woman of reproductive age. Estradiol production requires 
the activity of six steroidogenic proteins, including StAR, and six enzymatic steps. 17-Hydroxylase/17,20-
lyase, the prod

matic steps. 17-Hydroxylase/17,20-
lyase, the product of the CYP17A1 gene, catalyzes two enzymatic reactions. The four rings of the cho­
lesterol molecule and its derivative steroids are identified by the first four letters in the alphabet, and 
the carbons are numbered in the sequence shown in the insert. CYP17A1, 17-hydroxylase/17,20-lyase; 
CYP19A1, aromatase; HSD17B1, 17β-hydroxysteroid dehydrogenase type 1; HSD3B2, 3β-hydroxysteroid 
dehydrogenase-Δ5,4 isomerase type 2; StAR, steroidogenic 

genase-Δ5,4 isomerase type 2; StAR, steroidogenic acute regulatory protein.
590
Granulosa
Theca
Granulosa
Theca
fluid
Cyclase
Cyclase
SF1
LRH1
CYP19A1
HSD17B1
Estradiol
HSD17B1
CYP19A1
HSD3B2
Estrone
A
LDL
LH
LHCGR
LH
LHCGR
LH
LHCGR
Gsα
Gsα
Gsα
FSH
FSHR
CYP11A1
CYP11A1
Circulation
Cytosol
Cytosol
B
SF1

irculation
Cytosol
Cytosol
B
SF1
LRH1
SF1
Estradiol
Estrone
STAR
StAR
ATP
cAMP
ATP
cAMP
Cyclase
HSD3B2
Gsα
CYP11A1
CYP17A1
HSD3B2
CYP17A1
Cytosol
SF1
StAR
ATP
cAMP
Cyclase
Gsα
FSH
FSHR
ATP
cAMP
• Fig. 17.17  Two-cell hypothesis for ovarian steroidogenesis. (A) The preovulatory follicle produces estra­
diol through a paracrine interaction between theca and granulosa cells. In response to stimulati

heca and granulosa cells. In response to stimulation with a 
gonadotropin, steroidogenic factor 1 (SF1, encoded by NR5A1, a member of the nuclear receptor family) 
acts as a master switch to initiate transcription of a series of steroidogenic genes in theca cells. In follicular 
granulosa cells, another nuclear receptor, liver homologue receptor 1 (LRH1, encoded by NR5A2), seems 
to primarily mediate the downstream effects of follicle-stimulating hormone (FSH) in the rodent ovary. In 
humans, th

 hormone (FSH) in the rodent ovary. In 
humans, the roles of SF1 and LRH1 in steroidogenesis in preovulatory granulosa cells are not well under­
stood. Because granulosa cells do not have a direct connection to the circulation, CYP19A1 (aromatase) 
in granulosa cells depends for substrate on androstenedione that diffuses from theca cells. Two critical 
steps in estradiol formation are the entry of cholesterol into mitochondria facilitated by steroidogenic acute 
regulatory protein (StAR) in thec

oidogenic acute 
regulatory protein (StAR) in theca cells and the conversion of androstenedione to estrone catalyzed by 
CYP19A1 in granulosa cells. (B) In the corpus luteum, granulosa-lutein cells are heavily vascularized, a 
condition that is critical for entry of abundant quantities of cholesterol into this cell type through primarily 
low-density lipoprotein (LDL)-cholesterol receptors and for secretion of large amounts of progesterone 
into the circulation. The entry of cholesterol into mit

the circulation. The entry of cholesterol into mitochondria (mediated by StAR) is probably the most 
critical steroidogenic step for progesterone formation in granulosa-lutein cells. Androstenedione produced 
in theca-lutein cells serves as a substrate for estrone, which is further converted to estradiol in granulosa-
lutein cells. Human data suggest that LRH1 may mediate at least a portion of gonadotropin-dependent 
steroidogenesis in the corpus luteum. Gonadotropins, SF1, and possibly LRH1 pla

 luteum. Gonadotropins, SF1, and possibly LRH1 play key roles for important 
steroidogenic steps in the ovary. ATP, adenosine triphosphate; cAMP, cyclic adenosine monophosphate; 
FSHR, FSH receptor; HSD, hydroxysteroid dehydrogenase; LHCGR, LH receptor.
592
The general steroidogenic pathway for the production of 
estrogens and androgens is depicted in Fig. 17.16. The biologi­
cally active ovarian steroids are estradiol and progesterone. The 
major C19

ids are estradiol and progesterone. The 
major C19 steroid product of the ovary, androstenedione, is not 
biologically active. However, it acts as a dual precursor and con­
tributes to circulating levels of estrone and testosterone through 
conversion in extraglandular tissues such as adipose tissue and skin 
(discussed later).97 It is likely that estrogenically weak estrone is 
further converted to the potent estrogen estradiol and that tes­
tosterone is converted to the most potent endogenous 

terone is converted to the most potent endogenous androgen 
dihydrotestosterone (DHT) locally in target tissues such as brain, 
breast, prostate, and genital skin and subsequently exert potent 
biologic effects. This notion is supported by the presence in many 
human tissues of multiple proteins with overlapping enzymatic 
activities that catalyze these conversions (e.g., reductive 17βHSD 
and 5α-reductase).98
The preovulatory follicle secretes estradiol during the first half 
of the menstrual c

stradiol during the first half 
of the menstrual cycle, and the corpus luteum secretes estradiol 
and progesterone during the second half of the cycle. The produc­
tion of these two biologically active steroids is orchestrated in the 
follicle and corpus luteum in a cell-specific manner that is under 
the control of LH and FSH.
Steroids formed by the ovary and other steroid-producing 
organs are derived from cholesterol (see Fig. 17.16). Several 
sources of cholesterol can provide the ovary with

sources of cholesterol can provide the ovary with substrate for 
steroidogenesis, including plasma lipoprotein cholesterol, choles­
terol synthesized de novo within the ovary, and cholesterol from 
intracellular stores of cholesterol esters within lipid droplets. In 
the human ovary, LDL cholesterol is an important source of cho­
lesterol used for steroidogenesis.79 LH stimulates the activity of 
adenylyl cyclase, increasing production of cyclic adenosine mono­
phosphate (cAMP), which serves as

 adenosine mono­
phosphate (cAMP), which serves as a second messenger to increase 
LDL receptor messenger RNA (mRNA), binding and uptake of 
LDL cholesterol, and the formation of cholesterol esters.78,79 
LDL-derived cholesterol is particularly essential for normal lev­
els of progesterone production in the granulosa-lutein cells of the 
corpus luteum.79
Steroidogenic Genes and Their Functions in the Ovary
The ovarian granulosa, theca, and corpus luteum cells pos­
sess StAR plus five distinct pr

 luteum cells pos­
sess StAR plus five distinct proteins with specific enzyme 
activities for steroid hormone formation. These steroidogenic 
enzymes are CYP11A1 (side-chain cleavage of P450), 3βHSD2 
(3β-hydroxysteroid dehydrogenase-Δ5,4 isomerase type 2), 
CYP17A1 (17-hydroxylase/17,20-lyase), CYP19A1 (aroma­
tase), and 17βHSD1 (17β-hydroxysteroid dehydrogenase type 
1).92 These enzymes are responsible for the conversion of choles­
terol to the two major biologically active products: estradiol

 two major biologically active products: estradiol and 
progesterone.92,97
The first and rate-limiting step in the synthesis of all ovarian 
steroid hormones is the movement of cholesterol into the mito­
chondrion, which is regulated by a mitochondrial membrane pro­
tein encoded by the STAR gene (see Fig. 17.16).99 This movement 
is followed by conversion of cholesterol to pregnenolone, which is 
catalyzed by the mitochondrial side-chain cleavage enzyme com­
plex consisting of CYP11A1, adrenodox

 enzyme com­
plex consisting of CYP11A1, adrenodoxin, and flavoprotein. LH 
induces steroidogenesis by increasing the conversion of cholesterol 
to pregnenolone in two distinct ways: acute regulation, which 
occurs over minutes through phosphorylation of preexisting StAR 
and rapid synthesis of new StAR, and chronic stimulation, which 
occurs within hours to days through the induction of CYP11A1 
expression and consequent increased steroidogenesis (Fig. 17.17). 
StAR increases the flow of choles

s (Fig. 17.17). 
StAR increases the flow of cholesterol to mitochondria, regulat­
ing substrate availability to CYP11A1 on the inner mitochondrial 
membrane.99 In the absence of StAR, only 14% of the maximal 
StAR-induced level of steroidogenesis persists.99
StAR expression in the preovulatory Graafian follicle is limited 
primarily to the theca cells (see Fig. 17.17).100 The most impor­
tant product of the theca cell during the follicular phase is the 
estrogen precursor androstenedione, and it

is the 
estrogen precursor androstenedione, and its production is con­
trolled primarily by StAR. The biologically active steroid product 
of the ovary during the follicular phase is estradiol, which arises 
from the granulosa cells located adjacent to theca cells. The rate-
limiting step for granulosa cell estradiol production is regulated by 
the FSH-dependent activity of the aromatase enzyme in a cyclic 
fashion.92 During the luteal phase, cells of the corpus luteum, 
including granulosa-lute

ls of the corpus luteum, 
including granulosa-lutein cells, also show intense StAR immuno­
reactivity with a patchy distribution.81,100 The delivery of choles­
terol to the mitochondrial side-chain cleavage enzyme system in 
the corpus luteum is the rate-limiting step for progesterone bio­
synthesis and is regulated by StAR.81 Thus estradiol production 
seems to be regulated primarily by StAR and aromatase, whereas 
progesterone biosynthesis may be primarily under the control of 
StAR.
Steroidog

be primarily under the control of 
StAR.
Steroidogenesis that depends on LH and FSH in theca and 
granulosa cells is mediated by common signaling molecules, 
including cAMP and the specific transcription factors SF1, prod­
uct of the NR5A1 gene, and LRH1, product of the NR5A2 gene, 
which belong to the nuclear receptor family (see Fig. 17.17).101,102 
SF1 and LRH1 regulate the expression of genes that encode StAR, 
CYP11A1, 3βHSD2, CYP17A1, and CYP19A1 (see Fig. 17.17). 
SF1 and possibly LRH1 ca

YP19A1 (see Fig. 17.17). 
SF1 and possibly LRH1 can be regarded as downstream master 
switches that orchestrate ovarian steroidogenesis.102
All steroid hormones are derived from cholesterol. C27 choles­
terol is converted to the carbon steroid hormones 18, 19, and 21 
that are secreted by the ovary (see Fig. 17.16).
C21 Steroids
The principal progestogens are C21 steroids and include pregneno­
lone, progesterone, and 17- hydroxyprogesterone (see Fig. 17.16). 
Pregnenolone is of primary importanc

Fig. 17.16). 
Pregnenolone is of primary importance in the ovary because of 
its key position as a precursor of all steroid hormones.103 Proges­
terone, the principal secretory product of the corpus luteum, is 
responsible for the progestational effects (i.e., cell differentiation 
and induction of secretory activity in the endometrium of the 
estrogen-primed uterus).103 Progesterone is essential for implan­
tation of the fertilized ovum and maintenance of pregnancy. It 
also induces decidualiza

tenance of pregnancy. It 
also induces decidualization of the endometrium, inhibits uterine 
contractions, increases the viscosity of cervical mucus, promotes 
lateral (alveolar) development of the breast glands, and increases 
basal body temperature.103 
C19 Steroids
The ovary secretes a variety of C19 steroids, including DHEA, 
androstenedione, and testosterone, all of which primarily serve as 
distant or immediate precursors for the potent androgen, DHT, 
or potent estrogen, estradiol (see Fi

rogen, DHT, 
or potent estrogen, estradiol (see Fig. 17.16; also see Fig. 17.26, 
later).97 C19 steroids are produced by the theca cells and, pos­
sibly, to a lesser degree by the ovarian stroma. The major C19 
steroid is androstenedione, part of which is secreted directly into 
plasma, with the remainder converted to estrogen by the granu­
losa cells.92 DHEA and androstenedione do not appear to have 
major androgenic actions. In the ovary and in peripheral tissues, 
DHEA is converted to androst

 peripheral tissues, 
DHEA is converted to androstenedione, which can be converted 
593
to estrone or testosterone.97 Testosterone is converted locally to 
DHT at target tissues for full androgenic action or estradiol for 
estrogenic activity. 
C18 Steroids
Estrogen regulates gonadotropin secretion, development of the 
secondary sexual characteristics of women, uterine growth, thick­
ening of the vaginal mucosa, thinning of th

thick­
ening of the vaginal mucosa, thinning of the cervical mucus, linear 
growth of the ductal system of the breast, growth spurt, epiphyseal 
closure, and bone mineralization.104 The naturally occurring estro­
gens are C18 steroids characterized by the presence of an aromatic 
A ring, a phenolic hydroxyl group at C3, and a hydroxyl group 
(estradiol) or a ketone group (estrone) at C17.92,97 Aromatase 
is the key enzyme for estrogen production in the ovary (see Fig. 
17.16).92 The protein arom

in the ovary (see Fig. 
17.16).92 The protein aromatase, encoded by the CYP19A1 gene, 
confers the specific activity of the aromatase enzyme complex.
Aromatase mRNA and protein expression and its enzyme 
activity in the ovarian granulosa cell are regulated primarily by 
FSH.92,97 The principal and most potent estrogen secreted by the 
ovary is estradiol. Although estrone is also secreted by the ovary, 
another important source of estrone is extraglandular conver­
sion of androstenedione in perip

glandular conver­
sion of androstenedione in peripheral tissues.97 All C18 steroids, 
including estrone, estradiol, and estriol, are commonly referred to 
as estrogens. However, estrone is only weakly estrogenic and must 
be converted to estradiol to show full estrogenic action. At least 
seven enzymes with overlapping activities are capable of convert­
ing estrone to estradiol in the ovary and extraovarian tissues.105
Catechol estrogens are formed by hydroxylation of estrogens at 
the C2 or C4 

ed by hydroxylation of estrogens at 
the C2 or C4 position. The physiologic role of catechol estrogen is 
unclear. Estrone sulfate, formed by peripheral conversion of estra­
diol and estrone, is the most abundant estrogen in blood, but it 
is not physiologically active. Estrone sulfate is presumed to serve 
as a reservoir for estrone and eventually estradiol formation in a 
number of tissues, including those that are targets of estrogen.97 
Two-Cell Theory for Ovarian Steroidogenesis
The classic

ell Theory for Ovarian Steroidogenesis
The classic two-cell theory is supported by molecular findings. 
Ovarian steroidogenesis in the preovulatory follicle takes place 
through LH receptors on theca cells and FSH (possibly also LH) 
receptors on granulosa cells (see Fig. 17.17). cAMP production 
and increased SF1 binding to multiple steroidogenic promot­
ers mediate LH action in theca cells.106 The StAR protein is the 
primary regulator of production of androstenedione, which sub­
sequently dif

ction of androstenedione, which sub­
sequently diffuses into granulosa cells to serve as the estrogen 
precursor.97 In the preovulatory follicle, cholesterol in theca cells 
arises from circulating lipoproteins and de novo biosynthesis. 
FSH is responsible for follicular growth and estrogen formation. 
FSH induces cAMP formation, activation of protein kinase A and 
certain MAP kinases, and increased binding activity of LRH1 or 
SF1 to the CYP191A1 promoter in preovulatory granulosa cells to 
for

1 promoter in preovulatory granulosa cells to 
form estrone and then estradiol primarily through aromatization 
of androstenedione (see Fig. 17.17). The relative roles of SF1 and 
LRH1 in estrogen formation in human ovarian granulosa cells are 
not well understood.106
In the corpus luteum, large stores of cholesterol esters (which 
provide the yellow color) arise primarily from circulating lipopro­
teins to support the production of extremely high quantities of 
progesterone.107 Other key anatom

h quantities of 
progesterone.107 Other key anatomic events in the formation of 
the corpus luteum are the disruption of the basement membrane 
between the granulosa and theca cells and strikingly increased vas­
cularization of granulosa-lutein cells (see Fig. 17.15). Theca-lutein 
cells possess LH receptors and produce androstenedione. cAMP, 
SF1, and StAR induced by LH remain as the key regulators for 
biosynthesis of androstenedione, which serves as the estrogen pre­
cursor in neighboring gra

ves as the estrogen pre­
cursor in neighboring granulosa-lutein cells (see Fig. 17.17).
The granulosa-lutein cell of the corpus luteum is anatomically 
and functionally different from its counterpart in the preovulatory 
follicle in several ways. First, these cells are characterized by large 
and granular cytoplasms, are heavily vascularized, and contain 
large quantities of cholesterol esters. Second, granulosa-lutein cells 
contain high levels of LH receptors in addition to FSH receptors. 
Thi

of LH receptors in addition to FSH receptors. 
Third, they produce large quantities of progesterone; this function 
is regulated primarily by LH and StAR. Granulosa-lutein cells also 
aromatize androstenedione of thecal origin and eventually give 
rise to estradiol formation through FSH action and CYP19A1. 
The known mediators of LH and FSH in human granulosa-lutein 
cells are cAMP and increased LRH1 levels.106,107 The relative roles 
of LRH1 and SF1 for progesterone and estradiol production in 

 SF1 for progesterone and estradiol production in 
granulosa-lutein cells are not clear. Specific functions of the two 
gonadotropins (i.e., differentiation, growth, and progesterone vs 
estradiol formation) are probably determined by numerous modi­
fying factors (see Fig. 17.17). 
Peptide Hormones Produced by the Ovary
The ovary produces a large number of peptides that can act in 
an intracrine, autocrine, paracrine, or endocrine fashion.106,108,109 
They include numerous growth factors (e.g., 

,109 
They include numerous growth factors (e.g., IGFs) and cytokines 
(e.g., interleukin 1β). IGFs cross-talk with the FSH-dependent 
signaling cascade to augment the effects of FSH in granulosa 
cells.108,109
Inhibin, activin, and follistatin are produced in ovarian gran­
ulosa cells under the control of FSH and LH (Fig. 17.18).106 
Production of inhibin and activin is not limited to the ovary; a 
number of other tissues, including adrenal, pituitary, and pla­
centa, synthesize these members o

uitary, and pla­
centa, synthesize these members of the broader TGFβ family of 
paracrine/endocrine factors. Two isoforms of inhibin have been 
isolated: inhibin-A and inhibin-B.106 They contain an identical 
α-subunit but distinct β-subunits (βA and βB), encoded by sepa­
rate genes. The heterodimers of inhibin, αβA and αβB, are called 
inhibin-A and inhibin-B, respectively (see Fig. 17.18). Although 
inhibin is produced by a number of tissues in the body, most of 
it is derived from the gonads.

 the body, most of 
it is derived from the gonads. In the ovary, the source of inhibin 
is granulosa cells. The main role of inhibin is to suppress FSH 
production in the pituitary.106
Although both inhibin isoforms seem to have similar biologic 
properties, their synthesis is regulated differently during the fol­
licular and luteal phases (see Fig. 17.1A). Under the influence 
of FSH, inhibin-B is secreted mainly during the early follicular 
phase, with levels decreasing in midfollicular phase 

se, with levels decreasing in midfollicular phase and becoming 
undetectable after the LH surge.106 LH-induced inhibin-A levels 
are low during the first half of the follicular phase but increase 
gradually during the midfollicular phase and peak during the 
luteal phase. All three subunits are detected in small antral fol­
licles by immunohistochemistry and in situ hybridization.106 The 
α-subunit and βA-subunit are found in the dominant follicle and 
in the corpus luteum. All three subunits ar

e and 
in the corpus luteum. All three subunits are expressed in response 
to gonadotropins or factors that increase intracellular cAMP.106
Activin is structurally related to inhibin but exerts opposite 
actions.106 Activin contains two subunits that are identical to the 
β-subunits of inhibin-A and inhibin-B. The three activin isoforms 
are activin A (βAβA), activin B (βBβB), and activin AB (βAβB). In 
the pituitary, activin stimulates the release of FSH. In the ovarian 
follicle, activin enhan

se of FSH. In the ovarian 
follicle, activin enhances FSH action (see Fig. 17.18). As in the 
case of inhibin, activins are also produced in ovarian granulosa 
594
cells and pituitary gonadotrophs. Unlike inhibin, locally syn­
thesized activin in the pituitary, rather than the ovarian-derived 
activin, is responsible for regulating FSH (see Fig. 17.4).106
Follistatin is a single-unit peptide that is produced in sev­
eral human tissues, including the p

oduced in sev­
eral human tissues, including the pituitary and ovary (see Fig. 
17.18).106 It binds and neutralizes the biologic functions of 
activin. It appears that local follistatin levels in tissues modulate 
the effects of activin. This explains the inhibitory effect of fol­
listatin on pituitary FSH secretion (see Fig. 17.4).106 
Overview of the Hormonal Changes During the 
Ovarian Cycle
FSH secretion is suppressed by negative feedback of the ovar­
ian hormones estradiol, inhibin, and pro

the ovar­
ian hormones estradiol, inhibin, and progesterone during the 
early and midluteal phase. The sharp decline of these hormones 
upon regression of the corpus luteum during the late luteal phase 
abolishes this negative feedback (see Fig. 17.1A). This permits 
increased secretion of FSH just before and during menses. This ini­
tial increase in FSH is essential for follicle recruitment and growth 
and steroidogenesis. With continued growth of the follicle, auto­
crine and paracrine factors

of the follicle, auto­
crine and paracrine factors produced within the follicle maintain 
follicular sensitivity to FSH. Continuing and combined action of 
FSH and activin leads to the appearance of LH receptors on the 
granulosa cells, a prerequisite for ovulation and luteinization.
Ovulation is triggered by the rapid rise in circulating levels of 
estradiol. A positive feedback response at the level of the anterior 
pituitary and possibly at the hypothalamus results in the mid­
cycle surge of 

e hypothalamus results in the mid­
cycle surge of LH that is necessary for expulsion of the egg and 
formation of the corpus luteum (see Fig. 17.1A). A rise in the 
progesterone level follows ovulation, along with a second rise in 
the estradiol level, producing the 14-day luteal phase character­
ized by low FSH and LH levels. Demise of the corpus luteum 
concomitant with a fall in hormone (progesterone, estradiol, and 
inhibin-A) levels allows FSH to increase again toward the end of 
the luteal

SH to increase again toward the end of 
the luteal phase, initiating a new cycle. If pregnancy is established 
by implantation of a blastocyst, the structural integrity and func­
tion (i.e., progesterone and estradiol production) of the corpus 
luteum are maintained by hCG secreted from the trophoblast. 
The hCG acts as a surrogate for LH on the corpus luteum.
In addition to FSH and LH, local factors (e.g., activin, inhibin) 
regulate follicular development and steroidogenesis. In the early 
fol

development and steroidogenesis. In the early 
follicular phase, activin produced by granulosa cells in immature 
follicles enhances the action of FSH on aromatase activity and 
FSH and LH receptor formation while simultaneously suppress­
ing C19 steroid formation in theca cells. In the late follicular 
phase, increased production of inhibin by the granulosa cells and 
decreased activin levels promote the synthesis of C19 steroids in 
the theca layer in response to LH and local growth factors an

ayer in response to LH and local growth factors and 
cytokines; this provides larger amounts of the precursor andro­
stenedione for production of estrone and ultimately of estradiol in 
the granulosa cells.110
LH-mediated androstenedione production in theca cells and 
FSH-mediated estradiol production in granulosa cells are poten­
tiated by IGFs.108 The major endogenous IGF produced in the 
human ovarian follicle is IGF2 (rather than IGF1), which is pro­
duced by granulosa and theca cells. The a

 is pro­
duced by granulosa and theca cells. The actions of IGF1 and IGF2 
are mediated by IGF receptor type I in both cells. IGF receptor 
type I is structurally similar to the insulin receptor. It appears that 
gonadotropin-related IGF action in the ovary is regulated primar­
ily by IGF2 and IGF receptor type I.108,109
In summary, ovulation is under the control of substances 
functioning as classic hormones (i.e., FSH, LH, estradiol, and 
inhibin), which transmit messages between the ovary and

in), which transmit messages between the ovary and the 
hypothalamic-pituitary axis and of paracrine and autocrine factors 
such as IGF2, inhibin, and activin, which coordinate sequential 
activities within the follicle destined to ovulate. The negative feed­
back relationship between corpus luteum products (i.e., estradiol, 
α Inhibin subunit
Precursor
Precursor
105 kDa
95 kDa
75 kDa
55 kDa
31–44k
Activin
Inhibin
Activin A, AB, or B
Pro β
β Monomer
Pro-αN-αC
αN
αN
αC
Pro
6k 23k
20k

Pro β
β Monomer
Pro-αN-αC
αN
αN
αC
Pro
6k 23k
20k
43k
15k
Pre
β
β
β
Pro
Pre
Pro-αC
βA,B Inhibin subunit
βA,B
α
32 kDa
βA,B
α
• Fig. 17.18  Structures of inhibin subunit precursors and processed forms in serum, the activins and the fol­
listatins. The precise contribution of each molecular-weight form of inhibin to the biologic activity in serum 
is not known, but it has been established that the 55-kDa and 32-kDa forms are biologically active. αN, 
amino region of alpha-inhibin; αC, carboxy regi

, 
amino region of alpha-inhibin; αC, carboxy region of alpha-inhibin. (Modified from Burger H. Inhibin, activin 
and neoplasia. In: Yen SC, Jaffe RB, Barbieri RL, eds. Reproductive Endocrinology. 4th ed. Philadelphia, 
PA: Saunders; 1999:669–675.)
595
progesterone, and inhibin) and FSH results in the critical initial 
rise in FSH immediately before and during menses, and the posi­
tive feedback relationship between estradiol 

osi­
tive feedback relationship between estradiol and LH is responsible 
for the ovulatory stimulus (see Fig. 17.1). Within the ovary, IGF2, 
inhibin, and activin modify follicular responses necessary for 
growth and function. These endocrine, paracrine, and autocrine 
factors undoubtedly represent only a portion of the complete pic­
ture. The causes of anovulation are diverse and may be related to 
defects in cell surface receptors, intracellular elements of signal 
transduction, or cell-cell i

r elements of signal 
transduction, or cell-cell interactions.111,112 
Extraovarian Steroidogenesis
Estradiol formation takes place in several tissues in the woman of 
reproductive age, including the ovary, peripheral tissues such as 
subcutaneous fat and skin, and physiologic and pathologic tar­
get sites such as the hypothalamus, breast cancer cells, and the 
cells of endometriosis (Fig. 17.19).97 Fat and skin are particularly 
critical sources of estrogen in anovulatory premenopausal and 
pos

 of estrogen in anovulatory premenopausal and 
postmenopausal women. Although only small quantities of estro­
gen are produced by an individual adipocyte or skin fibroblast 
in a continuous fashion, these cell types contribute to circulating 
estradiol levels because of their relative abundance.97 This effect is 
more pronounced in obese women because of increased mass of 
the adipose tissue and skin.97
Aromatase (CYP19A1) in adipocytes and skin fibroblasts is 
responsible for peripheral aromati

fibroblasts is 
responsible for peripheral aromatization of androstenedione that 
arises from the ovary and the adrenal gland in premenopausal 
women and primarily from the adrenal in postmenopausal women 
(see Fig. 17.19).97 However, the product of this reaction, estrone, 
is only weakly estrogenic. Estrone is further converted to estrone 
sulfate, which serves as a reservoir for estrone in blood and other 
tissues. Estrone (arising from androstenedione and estrone sulfate) 
is further converte

enedione and estrone sulfate) 
is further converted to the biologically active estradiol in target 
tissues such as the endometrium and breast by a number of enzy­
matic proteins with overlapping reductive 17βHSD activity (see 
Fig. 17.19).97,105 It is likely that local CYP19A1 expression in the 
hypothalamus is critical for the regulation of gonadotropin secre­
tion.21,113 Estrogen-dependent pathologic tissues such as those in 
breast cancer and endometriosis contain extremely high levels of 
C

 endometriosis contain extremely high levels of 
CYP19A1 that enhances tissue growth by increasing local estradiol 
concentrations (see Fig. 17.19).97 Circulating androstenedione is 
the major substrate for aromatase activity in these physiologic and 
pathologic target tissues.97
Significant quantities of circulating androstenedione can 
also be converted to testosterone in peripheral tissues (discussed 
later). This is probably accomplished by the presence of multiple 
17βHSDs with overlapping 

he presence of multiple 
17βHSDs with overlapping reductive activities in peripheral tis­
sues.105 Androgenic action of testosterone is strikingly amplified 
by its conversion to DHT in peripheral and target tissues (e.g., 
skin, prostate). At least two distinct proteins encoded by two sepa­
rate genes, 5α-reductase type 1 and type 2, catalyze the conversion 
of testosterone to DHT in liver, prostate, and skin.98 Local pro­
duction of DHT in genital skin fibroblasts is critical for normal 
mascu

tal skin fibroblasts is critical for normal 
masculinization of external genitalia of male fetuses in utero.114 
DHT formation in the skin is an important cause of hirsutism.115 
The endometrium is the mucosal lining of the uterine cavity. 
The decidua is the highly modified and specialized endometrium 
of pregnancy. From an evolutionary perspective, the human 
endometrium is highly developed to accommodate the hemocho­
rioendothelial type of placentation, which requires the presence

 type of placentation, which requires the presence of 
spiral arteries (Fig. 17.20). Trophoblasts of the blastocyst invade 
spiral arteries during implantation and placentation in the estab­
lishment of uteroplacental vessels.
Spiral arteries of the human endometrium confer another 
unique process, menstruation. Menstruation is shedding of 
endometrial tissue with hemorrhage that depends on sex steroid 
hormone–directed changes in blood flow in the spiral arteries. 
Spiral arteries are essential

he spiral arteries. 
Spiral arteries are essential for menstruation; only humans and a 
few other primates that have endometrial spiral arteries experi­
ence menstruation. With nonfertile but ovulatory ovarian cycles, 
menstruation effects desquamation of the endometrium. New 
endometrial growth and development must be initiated with 
each ovarian cycle, so endometrial maturation corresponds with 
the next opportunity for pregnancy. There seems to be a narrow 
window of endometrial receptivity t

o be a narrow 
window of endometrial receptivity to blastocyst implantation, 
comprising the period between days 20 and 24 during a 28-day 
menstrual cycle.116
Functional Anatomy of the Endometrium
The endometrium can be divided morphologically into an upper 
two-thirds functionalis layer and a lower one-third basalis layer (see 
Adrenal
Ovary
Aromatase
Aromatase
Aromatase
A
E1,E2
E1,E2
E1
A
E1
A
E2
E2
Skin
Adipose
tissue
tissues
Estrogen
target tis

pose
tissue
tissues
Estrogen
target tissues
Brain
Breast
Skin
Blood vessel
Breast cancer
Endometriosis
17  HSD
β
17  HSD
β
• Fig. 17.19  Estrogen biosynthesis in women. The biologically active estro­
gen, estradiol (E2), is produced in at least three major sites: (1) by direct 
secretion from the ovary in reproductive-age women; (2) by conversion 
of circulating androstenedione (A), originating from the adrenal or ovary 
or both, to estrone (E1) in peripheral ti

r ovary 
or both, to estrone (E1) in peripheral tissues; and (3) by conversion of A 
to E1 in estrogen target tissues. In the latter two instances, estrogeni­
cally weak estrone (E1) is converted to E2 within the same tissue. The 
expression of genes that encode the enzymes aromatase and reductive 
17β-hydroxysteroid dehydrogenases (17βHSD) is critical for E2 formation 
at these sites. Reductive 17βHSD activity in peripheral tissues may be 
conferred by protein products of several genes with ove

rred by protein products of several genes with overlapping functions. 
HSD17B1 is a distinct reductive 17βHSD enzyme that is encoded by a 
specific gene expressed primarily in the ovary. Aromatase is encoded by 
a single gene (CYP19A1). E2 formation by peripheral and local conver­
sion is particularly important in postmenopausal women and in those with 
estrogen-dependent diseases such as breast cancer, endometriosis, or 
endometrial cancer.
596
Fig. 

Fig. 17.20). The purpose of the functionalis layer is to prepare for 
the implantation of the blastocyst; it is the site of proliferation, 
secretion, and degeneration. The purpose of the basalis layer is to 
provide the regenerative endometrium after menstrual loss of the 
functionalis.117 Major histologic components of the endometrium 
include stromal cells, which constitute the skeleton of the tissue; a 
single layer of epithelial cells, which line

e; a 
single layer of epithelial cells, which lines the lumen of the endo­
metrial cavity and invaginations of the stroma; blood vessels; and 
resident immune cells. The epithelial cells that line the rather deep 
invaginations of the stroma are also referred to as glandular cells. 
However, these deep crypts represent extensions of the intracavi­
tary lumen and are not true glands. These invaginations lined by 
epithelial cells extend from the surface of the functionalis layer 
(i.e., luminal e

urface of the functionalis layer 
(i.e., luminal epithelium) deep into the basalis level (i.e., glan­
dular epithelium). After the functionalis layer is shed at the time 
of menstruation, the tissue stem cells in the basalis layer respond 
rapidly to estrogen and give rise to a new functionalis layer for the 
upcoming cycle (Fig. 17.21).118 In humans and some other pri­
mates, the cellular components of the functionalis layer undergo a 
striking progression during the menstrual cycle, whereas th

progression during the menstrual cycle, whereas the basa­
lis layer shows only modest alterations.117,119,120 
Hormone-Induced Morphologic Changes of the 
The cyclic changes in endometrial histology are faithfully repro­
duced during each ovulatory ovarian cycle (see Fig. 17.21). First, 
during the preovulatory, or follicular, phase of the cycle, estradiol 
is secreted (principally by a single dominant follicle of one ovary) 
in increasing quantities until just before ovulation. Seco

asing quantities until just before ovulation. Second, dur­
ing the postovulatory, or luteal, phase of the cycle, progesterone 
is secreted by the corpus luteum in increasing amounts (up to 
40–50 mg/day) until the midluteal phase. Third, beginning about 
7 to 8 days after ovulation, the rates of progesterone and estradiol 
secretion by the corpus luteum begin to decline and then diminish 
progressively before menstruation (see Fig. 17.1).
In response to these cyclic changes in the rates of ovari

onse to these cyclic changes in the rates of ovarian 
sex steroid hormone secretion, there are five main stages of the 
corresponding endometrial cycle: (1) menstrual-postmenstrual 
reepithelialization; (2) endometrial proliferation in response to 
stimulation by estradiol; (3) abundant epithelial secretion, occur­
ring in response to the combined action of estradiol and proges­
terone; (4) premenstrual ischemia, the result of endometrial tissue 
volume involution, which causes stasis of blood i

volume involution, which causes stasis of blood in the spiral arter­
ies; and (5) menstruation, which is preceded and accompanied by 
severe vasoconstriction of the endometrial spiral arteries and col­
lapse and desquamation of all but the deepest layer of the endo­
metrium. In the final analysis, menstruation is the consequence of 
the withdrawal of factors that maintain endometrial growth and 
differentiation (see Fig. 17.21).
Commonly, the initiation of menstruation is attributed to 
progest

itiation of menstruation is attributed to 
progesterone withdrawal. This concept was developed because 
the administration of estrogen to postmenopausal women and 
treatment with and then withdrawal of a progestin causes men­
struation, even with continued estrogen treatment. Moreover, 
progesterone facilitates decidualization of the endometrium and 
the maintenance of pregnancy, whereas progesterone withdrawal 
favors the initiation of menstruation, lactation, and parturition.
The preovulatory 

ion, lactation, and parturition.
The preovulatory (follicular or proliferative) phase and the 
postovulatory (luteal or secretory) phase of the ovarian-endome­
trial cycles are customarily divided into early and late stages (see 
Fig. 17.21). The normal secretory phase of the menstrual cycle 
can be subdivided almost daily by histologic criteria, from shortly 
after ovulation until the onset of menstruation.121 Some gyne­
cologists use histologic dating of endometrial biopsies obtained 
during t

 dating of endometrial biopsies obtained 
during the luteal phase to evaluate ovulation, progesterone pro­
duction, or the degree of biologic response of the endometrium 
to progesterone.121 
Effects of Ovarian Steroids on Endometrium
The exogenous administration of only estradiol and progesterone 
is sufficient to prepare the endometrium for implantation in the 
absence of ovarian function.122,123 This observation underscores 
the essential roles of these steroids in uterine physiology. Estra­

s of these steroids in uterine physiology. Estra­
diol or synthetic estrogens such as ethinyl estradiol cause a strik­
ing thickening of endometrial tissue. Stromal and epithelial cells 
of the endometrium proliferate rapidly under the influence of 
estradiol. Estradiol greatly increases mitotic activity and deoxy­
ribonucleic acid (DNA) synthesis in both cell types (Fig. 17.22). 
While promoting growth, estradiol also renders endometrial tis­
sue responsive to progesterone by inducing the expre

e responsive to progesterone by inducing the expression of PRs; 
progesterone action depends on previous or concurrent estrogen 
exposure of the endometrium.124
In contrast to the proliferative effects of estrogen, progester­
one action primarily enables differentiation of the endometrium. 
Progesterone can inhibit and even reverse the proliferative action 
of estrogen on epithelial cells in the functionalis layer (see Fig. 
17.22). Moreover, progesterone action prepares the endometrium 
Epithel

gesterone action prepares the endometrium 
Epithelium
Stroma
Functionalis
Basalis
Spiral
artery
Myometrium
Radial branch
Arcuate artery
• Fig. 17.20  Functional anatomy of the endometrium. Endometrium is a 
multilayered mucosa specialized for implantation and support of preg­
nancy. A single, continuous layer of epithelial cells lines the surface of the 
stroma and penetrates the stroma with deep invaginations almost all the 
way down to the myometrium-endometrium junction. The entire thickness 

etrium-endometrium junction. The entire thickness 
of the endometrium is penetrated by the spiral arteries and their capillar­
ies. Spiral arteries originate from the radial branches of arcuate arteries, 
which arise from uterine arteries. The superficial layer (functionalis) is shed 
during menstruation, whereas the permanent bottom layer (basalis) gives 
rise to the regeneration of endometrium after each menstruation. The strik­
ing changes in the spiral arteries (i.e., coiling, stasis, and va

the spiral arteries (i.e., coiling, stasis, and vasodilatation 
followed by intense vasoconstriction) are consistently observed before 
the onset of every menstruation episode. (Courtesy Dr. Kristof Chwalisz, 
AbbVie, Inc., North Chicago, IL.)
597
for implantation of the embryo through differentiation of stro­
mal and epithelial cells. Progesterone induces the production and 
secretion of a glycogen-rich substance from the epi

ecretion of a glycogen-rich substance from the epithelial cells. 
Progesterone also causes an increase in stromal cell cytoplasm, a 
process called pseudodecidualization. The term decidualization is 
reserved for stroma differentiated under the combined influence 
of progesterone and hCG of placental origin during pregnancy.
Blood vessels that carry estrogen or progesterone come in con­
tact with endometrial stromal cells first. These steroid ligands 
interact with their nuclear receptors in end

ands 
interact with their nuclear receptors in endometrial stromal cells, 
which in turn send paracrine signals to neighboring epithelial cells 
to regulate their functions.103,124
Estrogen Action
Estradiol enters endometrial cells from blood by simple diffu­
sion and binds ERs, proteins with a high affinity for estradiol and 
biologically active synthetic estrogens. Although both ERα and 
ERβ are present in the endometrium, ERα is the primary media­
tor of the estrogenic action in that tissue.1

dia­
tor of the estrogenic action in that tissue.125,126 The estradiol-
ERα complex is a transcriptional factor that becomes associated 
Days
0
14
25
3-5
8-9
15-18
28-1
3-5
Phase
Functional
layer
Basal
layer
Menstrual
Menstrual
Luteal ——— Ovary
Secretory ——— Uterus
OR
Follicular —— Ovary
Proliferative —— Uterus
OR
Early
Advanced
Early
Advanced
Approximately 14 days
Late
(pre-
menstrual)
Day of ovulation
• Fig. 17.21  Cyclic changes in thickness and morphologic changes of endometrium and the rela

nd morphologic changes of endometrium and the relationship of 
these changes to those of the ovarian cycle. (From Cunningham FG, MacDonald PC, Gant NF, et al. The 
endometrium and decidua: menstruation and pregnancy. In: Cunningham FG, ed. Williams Obstetrics. 
19th ed. Stamford, CT: Appleton & Lange; 1993:81–109.)
PR
ER
PR
ER
PR
ER
PR
ER
Estradiol
Stroma
Angiogenesis
Differentiation
OH

tion
OH
OH
O
CH3
C
O
+
+
+
• Fig. 17.22  Critical epithelial effects of estrogen (i.e., deoxyribonucleic acid [DNA] synthesis, proliferation, 
and gene expression) are mediated primarily by estrogen receptor-α (ER) in stromal cells in a paracrine 
manner in the endometrium. This was demonstrated in mice. It was also shown in mice and humans that 
the antiestrogenic effects of progesterone on epithelial cells (e.g., decreased proliferation, 

 epithelial cells (e.g., decreased proliferation, enhanced dif­
ferentiation) are mediated primarily by progesterone receptors (PRs) in stromal cells.
598
with chromatin.127 Estradiol-ERα complexes bind thousands of 
DNA sites across the entire genome and regulate the transcrip­
tion of hundreds of genes at one time.127 Most ERα binding sites 
are outside basal promoters and interact with the transcriptional 
start sites through DNA looping.114,127 Th

tional 
start sites through DNA looping.114,127 This interaction brings 
about ER-specific initiation of gene transcription, which pro­
motes the synthesis of specific mRNAs and, thereafter, specific 
proteins.114,127 Estradiol acts in the endometrium and in other 
estrogen-responsive tissues to promote the responsiveness of that 
tissue to progesterone via inducing PR expression.103
The endometrial epithelial cells are estrogen responsive but 
probably do not replicate as a result of direct act

robably do not replicate as a result of direct action of estradiol on 
them. Replication of human endometrial epithelial cells in culture 
is not increased appreciably, if at all, when estrogen is added to the 
medium. Estrogen acts on mouse uterine stromal cells to promote 
the synthesis of growth factors that act on epithelial cells (see Fig. 
17.22).124 These growth factors operate in a paracrine manner to 
cause increased DNA synthesis and replication in the adjacent epi­
thelial cells. 
Pro

plication in the adjacent epi­
Progesterone Action
Progesterone enters cells by diffusion and binds to its nuclear 
receptor (PR). Two PR isoforms, PR-A and PR-B, are present in 
the human endometrium.128 Because PR-B but not PR-A levels 
in the endometrium are tightly regulated during the human men­
strual cycle, PR-B is presumed to play a more important biologic 
role.128 The cellular content of PRs typically depends on previous 
estrogen action.
The progesterone-PR complex reg

The progesterone-PR complex regulates gene transcription, 
but the response to progesterone is strikingly different from that 
evoked by the estradiol-ER complex. Similar to ERα, PR binding 
sites are also widely distributed across the genome, lie frequently 
outside basal promoters, and do not involve classically defined 
progesterone response elements.129 Interaction of PR binding sites 
with basal promoters possibly requires DNA looping.129
Progesterone acts as an antiestro

 DNA looping.129
Progesterone acts as an antiestrogen by reducing ERα expres­
sion, by decreasing the tissue levels of estradiol through its con­
version to estrone via inducing the 17βHSD2 enzyme, and by 
enhancing estrogen inactivation through sulfation.130–132 Tissue 
recombination experiments using uteri of PR knockout and nor­
mal mice have demonstrated that many effects of progesterone 
on epithelial cells are also mediated in a paracrine fashion by 
PRs in stromal cells but not by those i

ashion by 
PRs in stromal cells but not by those in epithelial cells (see Fig. 
17.22).103,132
The most striking consequences of progesterone action, stro­
mal predecidualization and epithelial secretion, parallel increased 
levels of circulating progesterone during the luteal phase. The PR 
content of human endometrial tissue peaks during the late pro­
liferative phase, just before ovulation, and declines sharply before 
circulating progesterone levels increase during the luteal phase.128 
This

 levels increase during the luteal phase.128 
This dyssynchrony between endometrial PR expression and circu­
lating progesterone is not well understood. Molecular correlates 
of progesterone action with respect to differentiation include 
increased production of lactoferrin and glycodelin in epithelial 
cells and of prolactin and IGF-binding protein 1 in stromal cells 
of the endometrium.103 
The Receptive Phase of the Endometrium for 
Implantation
Unless the ovum is fertilized within 24 hours a

on
Unless the ovum is fertilized within 24 hours after ovulation, it 
does not survive. Fertilization takes place in the ampullary, the 
distal one-third of the oviduct. Over the next 2 days, the fertil­
ized ovum remains unattached within the tubal lumen. After this 
stage, the embryo (which consists of a solid ball of cells called the 
morula) leaves the oviduct and enters the uterine cavity. By this 
time, endometrial secretions under the influence of luteal proges­
terone have filled the cav

uence of luteal proges­
terone have filled the cavity and bathe the embryo in nutrients. 
This is the first of many neatly synchronized events that mark the 
conceptus-endometrial relationship. By 6 days after ovulation, the 
embryo (now a blastocyst) is ready to implant. It finds an endome­
trial lining of sufficient depth, vascularity, and nutritional richness 
to sustain the important events of early placentation that are to 
follow. Just below the epithelial lining, a rich capillary plexus h

w the epithelial lining, a rich capillary plexus has 
been formed and is available for creation of the trophoblast-mater­
nal blood interface. Later, the surrounding superficial portion of 
the functionalis zone, now occupying more and more of the endo­
metrial cavity, provides a sturdy splint to retain endometrial archi­
tecture despite the invasive inroads of the burgeoning trophoblast.
Progesterone is essential for the maintenance of pregnancy. 
The blastocyst depends on progesterone produced

. 
The blastocyst depends on progesterone produced by the corpus 
luteum at this time. The hCG secreted by the trophoblast acts as 
a surrogate LH and prevents regression of the corpus luteum that 
provides a continuous supply of progesterone until the placental 
tissue starts to produce sufficient quantities of progesterone (6–7 
weeks after fertilization).
The receptive phase of the endometrium is the temporal win­
dow of endometrial maturation during which the trophectoderm 
of the blastocyst

 during which the trophectoderm 
of the blastocyst can attach to the endometrial epithelial cells and 
proceed to invade the endometrial stroma. The window of uterine 
receptivity can be inferred from what has been learned from trans­
fer of embryos to uteri of women primed with exogenous estro­
gen and progesterone preparations (Fig. 17.23). There is a distinct 
window for embryo transfer leading to implantation, which spans 
endometrial cycle days 16 to 20. The actual window of implan­
tation 

ays 16 to 20. The actual window of implan­
tation follows this window of transfer, because embryos need to 
develop further, from the four-cell to eight-cell stage to the blasto­
cyst stage, before initiation of attachment and frank invasion can 
occur. Based on serial measurements of serum hCG as a marker 
of initial embryonic-maternal interaction, the window of implan­
tation in humans is estimated to be between days 20 and 24 of 
the cycle.133 This relatively wide window agrees with the earli

 This relatively wide window agrees with the earlier 
morphologic data.134 
Control of Endometrial Function With the Use of 
Exogenous Hormones
The fertility potential of a woman is primarily determined by the 
biologic quality of her oocytes, reflected in part by a normal chro­
mosomal complement and the capacity of the fertilized ovum to 
divide at an optimal rate. This biologic quality declines sharply after 
the age of 35 years. However, the biologic potential of the endome­
trium for succes

biologic potential of the endome­
trium for successful implantation remains intact even at advanced 
ages.122 Oocyte donation from a fertile woman and fertilization of 
the donor eggs in vitro with the recipient’s male partner’s sperm, 
followed by embryo transfer into the uterine cavity of the recipient 
woman with nonfunctioning ovaries (e.g., premature ovarian insuf­
ficiency [POI]), have been used successfully as a therapeutic strategy 
to treat infertility (see Fig. 17.23).122,123 This clin

eat infertility (see Fig. 17.23).122,123 This clinical application has 
provided unique opportunities to examine the hormonal therapy 
for endometrial maturation. The success of these procedures (i.e., 
pregnancy rate) has averaged higher than in conventional IVF.123
The follicular phase is mimicked by administration of oral 
micronized estradiol in daily doses of up to 8 mg for about 10 
599
days. Serum estradiol during the r

productive Axis
days. Serum estradiol during the replacement follicular phase 
reaches sufficiently high levels to stimulate endometrial growth. 
This is followed by up to 8 mg per day of oral estradiol combined 
with daily intramuscular (50 mg) or vaginal (200–400 mg) pro­
gesterone to promote the secretory transformation. Progesterone 
supplementation is ordinarily continued until 8 to 10 weeks of 
gestation. 
Mechanism of Menstruation
In the absence of pregnancy, failure of the appearance of 

bsence of pregnancy, failure of the appearance of hCG 
despite otherwise appropriate tissue reactions leads to the vasomo­
tor changes associated with estrogen-progesterone withdrawal and 
menstrual desquamation. A program of endometrial remodeling is 
initiated; alterations in the extracellular matrix and infiltration of 
leukocytes lead to hypoxia-reperfusion injury and sloughing of the 
functionalis, followed by activation of hemostatic and regenerative 
processes. The main histologic feature

generative 
processes. The main histologic features of the premenstrual phase 
are degradation of the stromal reticular network, stromal infiltra­
tion by polymorphonuclear and mononuclear leukocytes, and 
secretory exhaustion of the endometrial glands, whose epithelial 
cells now have basal nuclei. The endometrium shrinks preceding 
menstruation in part as a result of diminished secretory activity 
and the catabolism of extracellular matrix.
An ischemic phase caused by vasoconstriction of the a

ischemic phase caused by vasoconstriction of the arterioles 
and coiled arteries precedes the onset of menstrual bleeding by 4 to 
24 hours.119 Bleeding occurs after the arterioles and arteries relax, 
leading to hypoxia-reperfusion injury. The superficial endometrial 
layers are distended by the formation of hematomas, and fissures 
develop, leading to the detachment of tissue fragments. Lysis and 
fragmentation of cells and apoptosis are evident. The menstrual 
efflux is composed of shed fragm

t. The menstrual 
efflux is composed of shed fragments of endometrium mixed with 
blood and liquefied by the fibrinolytic activity of the cellular debris 
(see Fig. 17.21). Clots of various sizes may be present if blood flow 
is excessive. Myometrium contracts to mechanically stop bleeding 
from the spiral arteries and other endometrial vessels. 
Approach to the Woman With Reproductive 
Dysfunction
Reproductive dysfunction in an adult woman is most often 
manifested by disruption of cyclic, pred

st often 
manifested by disruption of cyclic, predictable menses. Efficient 
diagnosis of the underlying disorder requires a thorough under­
standing of female reproductive physiology and pathologic condi­
tions and an accurate history and physical examination. Without a 
critical analysis of clinical findings based on thorough knowledge 
of normal and abnormal reproductive function, the application 
of predetermined algorithms of laboratory testing causes unneces­
sary use of hormone measuremen

ing causes unneces­
sary use of hormone measurements or imaging studies and delays 
diagnosis.
History
An essential tool for the evaluation of a woman with a reproduc­
tive disorder is a carefully recorded history. The history should be 
obtained from the patient with the aim of assessing the biologic 
effects of each of the various hormones. Recording the details of 
pubertal development as a reference for the onset of particular 
symptoms provides critical clues to the cause of certain reprodu

des critical clues to the cause of certain reproduc­
tive disorders. For example, anovulation manifested by irregular 
uterine bleeding associated with the polycystic ovary syndrome 
(PCOS) most often begins during the pubertal years. The early 
onset of pubic hair and gradually progressing hirsutism at or 
before the time of puberty suggests nonclassic congenital adre­
nal hyperplasia or PCOS. In these cases, measurement of serum 
17-hydroxyprogesterone may help to differentiate nonclassic 
con

esterone may help to differentiate nonclassic 
congenital adrenal hyperplasia from PCOS. The appearance of 
hirsutism before puberty or several years after normal pubertal 
development should alert the clinician to the possibility of ovarian 
or adrenal neoplasms. Sudden onset of hirsutism at any age or the 
presence of virilization should prompt the physician to rule out 
steroid-secreting ovarian or adrenal tumors. Most women with 
symptomatic endometriosis suffer from severe episodes of painf

endometriosis suffer from severe episodes of painful 
menses (i.e., dysmenorrhea) that start during pubertal years.
A
Donor
egg
Embryo
transfer
Cycle days
Estradiol
Estradiol (pg/mL)
250
200
150
100
50
Progesterone (ng/mL)
30
20
10
2400
2100
1800
1500
1200
900
600
300
hCG
FSH
Egg retrieval
2
4
6
8
10
12
14
16
18
20
22
24
26
28
Cycle days
2
4
6
8
10
12
14
16
18
20
22
24
26
28
Egg recipient
B
Egg donor
• Fig. 17.23  Donation of oocytes by a woman undergoing egg retri

onation of oocytes by a woman undergoing egg retrieval to 
a woman with ovarian insufficiency treated with exogenous estrogen and 
progesterone. The window of implantation in both women is synchronized 
by different but comparable hormonal treatments. (A) Woman with ovarian 
insufficiency is initially treated with oral micronized estradiol during days 1 
through 14 of the cycle. Exogenous intramuscular progesterone is added 
to the estradiol treatment on days 15 through 28 and continued if preg­

tment on days 15 through 28 and continued if preg­
nancy is diagnosed. Several donor eggs are fertilized with sperm from the 
recipient’s husband, and one or two embryos are transferred to the uterus 
on day 16 to 19, depending on the stage of embryo development. These 
embryos are expected to implant between days 20 and 24. (B) The egg 
donor is simultaneously treated with human recombinant follicle-stimulat­
ing hormone (FSH) with or without menopausal gonadotropin until cycle 
day 8 to 12, wh

nopausal gonadotropin until cycle 
day 8 to 12, when human chorionic gonadotropin (hCG) is given to induce 
ovulation, and oocytes are harvested 32 to 36 hours later. One or two 
fertilized eggs are transferred to the uterus of the recipient. Progesterone 
supplement to the recipient is started before the embryo transfer. Serum 
levels of estradiol and progesterone in both women are shown. To convert 
estradiol values to picomoles per liter, multiply by 3.671. To convert pro­
gesterone values to

iply by 3.671. To convert pro­
gesterone values to nanomoles per liter, multiply by 3.180.
600
Evaluation of female reproductive function depends on a 
detailed history of the menses. For example, PCOS is unlikely with­
out a long-standing history of irregular periods since the menarche. 
A history of a period of cyclic, predictable menses before the onset 
of menstrual irregularities should draw attention to hypothalamic 
or other causes of anovulati

tion to hypothalamic 
or other causes of anovulation. The current frequency, regular­
ity, length, and quantity of uterine bleeding should be carefully 
recorded for several reasons. First, this information reflects tightly 
regulated interactions of several tissues, including the hypothala­
mus, pituitary, ovaries, and endometrium. Second, regular, predict­
able menses imply ovulation. Third, defining the type of menstrual 
irregularity may help with diagnosis of the underlying cause. For 
exam

 with diagnosis of the underlying cause. For 
example, prolonged amenorrhea in a thin and estrogen-deficient 
woman suggests anovulation of hypothalamic origin. Infrequent 
periods of varying duration and with a varying amount of blood loss 
in a well-estrogenized, overweight woman suggest a primary ovarian 
dysfunction such as PCOS. Anovulation in a thin but well-estro­
genized woman may also be caused by PCOS. Regular but heavy 
and prolonged menses with intermittent spotting may result from 

enses with intermittent spotting may result from 
uterine anatomic disorders such as adenomyosis or leiomyomas. 
Fourth, neoplastic disorders of the endometrium, including endo­
metrial polyps, hyperplasia, or malignancies, may be manifested by 
any pattern of irregular bleeding. The combination of vaginal ultra­
sonography and endometrial biopsy is helpful for the diagnosis of 
endometrial neoplasia.135
Disruption of cyclic and predictable menses is a common and 
alarming symptom that initially

s is a common and 
alarming symptom that initially brings the patient to the clini­
cian. After a careful evaluation of the menstrual symptoms, the 
clinician should identify other obvious symptoms of the endo­
crine disorder underlying the irregular periods. Pregnancy is the 
most common cause of amenorrhea (and other menstrual irregu­
larities) in a woman of reproductive age. In a woman presenting 
with amenorrhea or any other menstrual irregularity, normal preg­
nancy, ectopic pregnancy, or g

arity, normal preg­
nancy, ectopic pregnancy, or gestational trophoblastic disease must 
be excluded at the onset. Careful evaluation of past reproductive 
history and the patient’s sexual activity and contraceptive practices 
can provide useful indications of the likelihood of pregnancy. The 
reproductive history may suggest the possibility of Sheehan syn­
drome of postpartum pituitary necrosis if menses did not resume 
after a delivery complicated by significant hemorrhage.136 In such 
instanc

ted by significant hemorrhage.136 In such 
instances, evidence of adrenal and thyroid insufficiency should be 
sought. A classic symptom of Sheehan syndrome is the absence 
of postpartum lactation, which is related to prolactin deficiency.
Amenorrhea is traditionally categorized as primary (no history 
of menstruation) or secondary (cessation of menses after a variable 
time). The diverse causes of primary amenorrhea are discussed 
extensively in Chapters 23 and 25. Although the distinction 
bet

 Chapters 23 and 25. Although the distinction 
between primary and secondary amenorrhea may be useful for 
identifying the mechanism of disease and the differential diagno­
sis, the clinician should be aware that a disorder can initially mani­
fest with either primary or secondary amenorrhea. For example, 
most women with gonadal dysgenesis have primary amenorrhea, 
but some patients have residual follicles and ovulate, and in these 
women with partial gonadal dysgenesis, some menstruation and 

artial gonadal dysgenesis, some menstruation and 
rare pregnancies may occur before the cessation of ovarian func­
tion.137,138 Patients with PCOS usually have secondary amenor­
rhea but occasionally have primary amenorrhea. Rarely, pregnancy 
may manifest as primary amenorrhea.
After pregnancy is ruled out, secondary amenorrhea is most 
often caused by chronic anovulation, which can be broadly catego­
rized as hypothalamic dysfunction, hyperprolactinemia-associated 
anovulation, ovarian insuffi

actinemia-associated 
anovulation, ovarian insufficiency, androgen excess, or chronic 
illness or primary uterine disease (e.g., intrauterine adhesion for­
mation after a postpartum curettage). Establishing any associa­
tion of secondary amenorrhea with life events is extremely useful. 
Strenuous exercise is often associated with amenorrhea. Weight 
loss often precedes or accompanies secondary amenorrhea and 
has been suggested as evidence of hypothalamic dysfunction. An 
unusual dietary history

othalamic dysfunction. An 
unusual dietary history may suggest bulimia or anorexia nervosa. 
The presence of any signs or symptoms of estrogen deficiency, 
including painful intercourse, atrophic vagina, emotional labil­
ity, and vasomotor instability, suggests anovulation of a central 
nature with low concentrations of circulating gonadotropins (i.e., 
hypogonadotropic hypogonadism) or ovarian insufficiency with 
elevated gonadotropins (i.e., hypergonadotropic hypogonadism).
Galactorrhea in the

pergonadotropic hypogonadism).
Galactorrhea in the absence of a recent history of pregnancy 
suggests a host of diagnostic possibilities and is frequently a mani­
festation of excessive prolactin secretion, although it may result 
from increased sensitivity of breast tissue to the hormones nec­
essary for milk production. This history frequently reveals drug 
ingestion as the cause. Various drugs, including several psychotro­
pic agents, antihypertensive agents, and oral contraceptives, have 
be

rtensive agents, and oral contraceptives, have 
been implicated. Primary hypothyroidism may be associated with 
precocious puberty with galactorrhea in the child and with amen­
orrhea or galactorrhea, or both, in the adult woman. A history of 
excessive nipple manipulation or chest wall disease should be elic­
ited because it may be the cause of galactorrhea. Prolactinomas, 
the prolactin-secreting adenomas of the pituitary, are a common 
cause of galactorrhea related to abnormally high serum le

f galactorrhea related to abnormally high serum levels of 
prolactin. A history of dilatation and curettage, postpartum endo­
metritis, or disseminated tuberculosis with absent to scant menses 
suggests the possibility of intrauterine adhesion.139 
The quantity and distribution of excessive hair growth should 
be considered in light of the familial history. Hypertrichosis—
excessive growth of hair on the extremities, the head, and the 
back—must be distinguished from true hi

, and the 
back—must be distinguished from true hirsutism, which is the 
development of facial hair, chest hair, and a male escutcheon with 
or without signs of virilization in response to increased produc­
tion of or sensitivity to biologically active androgens. Some degree 
of hypertrichosis is not uncommon in women of Mediterranean 
descent, whereas the occurrence of any facial hirsutism in the 
relatively hairless Asian woman may require thorough investiga­
tion. Hirsutism is best documented

ough investiga­
tion. Hirsutism is best documented and quantified with the help 
of photographs. Virilization is characterized as deepening of the 
voice, severe cystic acne, hair loss, increased muscle mass, and cli­
toromegaly and implies a more severe degree of androgen excess 
than that found with hirsutism. The syndrome of complete andro­
gen insensitivity is characterized by sparse to absent pubic and 
axillary hair due to resistance to androgen.
A careful inspection of the breasts is esse

rogen.
A careful inspection of the breasts is essential for a thorough 
physical examination. Classification of the stage of breast devel­
opment according to the method of Marshall and Tanner140 is 
a convenient and valuable adjunct. The physician should assess 
whether the breasts appear to have decreased in size recently (e.g., 
severe androgen excess), whether the areolae are well formed and 
pigmented (as they are in pregnancy), and whether a discharge 
(e.g., galactorrhea) can be expressed

a discharge 
(e.g., galactorrhea) can be expressed.
A woman with PCOS who has never ovulated or taken a 
progestin-containing medication may have Tanner stage 4 breast 
development related to adequate estrogen production, whereas the 
progression to Tanner stage 5 requires exposure to progesterone 
601
through ovulation, ingestion of a progestin (e.g., administration 
of oral contraceptives), or pregnancy. Chapter 25 discusses

ontraceptives), or pregnancy. Chapter 25 discusses Tanner 
staging of breast development.
The vulva, vagina, and cervix also represent sensitive indica­
tors of gonadal steroid action. Because sensitivity of the genital 
skin and mucosa to androgen decreases with time from the early 
stages of fetal development to adulthood, the extent of any vir­
ilization can be helpful in suggesting the timing of androgen 
exposure. The most profound androgenic effects, such as poste­
rior labial fusion with 

c effects, such as poste­
rior labial fusion with or without formation of a penile urethra, 
are usually observed in 46,XX fetuses exposed to androgens 
during the first 8 to 10 weeks of pregnancy. Similar findings 
have been described in patients with virilizing congenital adre­
nal hyperplasia, true hermaphroditism, and drug-induced vir­
ilization. Postnatal development of significant clitoromegaly in 
a 46,XX infant requires marked hormonal stimulation and, in 
the absence of significant exog

imulation and, in 
the absence of significant exogenous steroids, strongly impli­
cates an androgen-secreting tumor. The size of the glans clito­
ris can be quantified by determining the clitoral index, which 
is the product of the sagittal and transverse diameters of the 
glans. Ninety-five percent of normal women have a clitoral 
index less than 35 mm2.
The vagina and uterine cervix are the most sensitive indicators 
of estrogen action. Under the influence of estrogen, the vaginal 
mucosa prog

he influence of estrogen, the vaginal 
mucosa progresses during sexual maturation from a tissue with 
a shiny, bright red appearance with sparse, thin secretions to a 
tissue with a dull, gray-pink, rugated surface with copious, thick 
secretions. Well-estrogenized vaginal mucosa with stretchable 
cervical mucus (i.e., spinnbarkeit) may indicate the proliferative 
phase of the menstrual cycle in an ovulatory woman or extraovar­
ian estrogen formation in an anovulatory woman with PCOS. The 
biolo

tion in an anovulatory woman with PCOS. The 
biologic activity of estrogen can also be quantified by vaginal cyto­
logic examination.
To summarize, irregular uterine bleeding is a common symp­
tom that brings the woman with reproductive dysfunction to the 
physician’s office. Various disorders of the hypothalamus, pitu­
itary, ovaries, or uterus or other tissues that affect reproductive 
function may be responsible for this alarming symptom. After 
pregnancy is ruled out, a detailed history and 

r 
pregnancy is ruled out, a detailed history and physical exami­
nation should be carefully recorded. In particular, the physician 
should pay attention to the salient features in the history and the 
biologic indicators of hormone action at target tissues during the 
physical examination. An analysis of these findings most often 
leads to a tentative diagnosis, which may be confirmed by labora­
tory testing. 
Disorders of the Female Reproductive 
System
Chronic Anovulation
Chronic anovulation 

e 
System
Chronic anovulation is one of the most common gynecologic 
problems encountered by the practitioner. Patients may present 
with secondary amenorrhea, infrequent uterine bleeding (i.e., oli­
gomenorrhea), or irregular episodes of excessive uterine bleeding. 
Infertility is an obvious consequence of chronic anovulation. Preg­
nancy, end-organ defects (e.g., intrauterine adhesions, müllerian 
agenesis), amenorrhea associated with genital ambiguity at birth 
(e.g., 46,X

iated with genital ambiguity at birth 
(e.g., 46,XX, 46,XY, and ovotesticular disorders of sex develop­
ment), or sexual infantilism due to gonadal dysgenesis should ini­
tially be ruled out.
For practical purposes, most of the etiologic factors giving rise 
to chronic anovulation in a woman of reproductive age fall into 
five broad categories: hypothalamic anovulation, hyperprolac­
tinemia, androgen excess, POI, and chronic illness (e.g., hepatic 
or renal insufficiency, acquired immunodeficien

c 
or renal insufficiency, acquired immunodeficiency syndrome 
[AIDS]). Salient features of the history and physical examination 
help to place a woman with anovulation in one or more of these 
categories.
One group of anovulatory patients is estrogen deficient. Com­
mon disorders in this group include hypothalamic anovulation, 
galactorrhea-hyperprolactinemia (e.g., hypothyroidism, prolacti­
noma, nonfunctioning pituitary tumor), and POI in a woman 
of reproductive age. These patients are usual

man 
of reproductive age. These patients are usually amenorrheic. All 
patients present with signs of estrogen deficiency (e.g., vaginal 
atrophy). Patients with hypothalamic anovulation or galactor­
rhea-hyperprolactinemia usually do not complain of hot flashes, 
whereas women with POI present with vasomotor symptoms. 
One serious consequence of estrogen deficiency is bone loss, giv­
ing rise to osteopenia and osteoporosis. If possible, the underlying 
cause should be corrected. Hormone therapy

rlying 
cause should be corrected. Hormone therapy should be provided 
if ovulation cannot be restored.
Women with androgen excess constitute the second major 
group of anovulatory patients. A serious consequence of anovu­
lation in this group is the greater risk for carcinoma of the 
endometrium because of unopposed action of estrogen formed 
continuously in extraovarian tissues. The most common disorder 
of the ovary associated with androgen excess and anovulation is 
PCOS. Insulin resistance 

cess and anovulation is 
PCOS. Insulin resistance plays a significant role in this condition 
and, along with hyperandrogenism, increases the risk of develop­
ing cardiovascular disease (CVD), diabetes mellitus, or both.111 
The clinician must recognize the long-term impact of PCOS 
and undertake therapeutic management of these anovulatory 
patients to avoid unwanted consequences. The clinician should 
also develop a plan with the patient to address long-term compli­
cations of unopposed estroge

ess long-term compli­
cations of unopposed estrogen formation associated with PCOS 
(e.g., endometrial neoplasia). Oral contraceptives or periodic pro­
gestin supplementation may be provided to prevent endometrial 
hyperplasia and cancer.
Many mechanisms may be responsible for anovulation in 
chronic illness. Effective treatment of the primary illness may 
restore normal menses. Alternatively, anovulatory bleeding may 
be managed with the use of exogenous hormones in these chroni­
cally ill pati

exogenous hormones in these chroni­
cally ill patients (discussed later).
Measurements of FSH and prolactin help to categorize anovu­
latory patients. An undetectable or low-normal FSH level is 
consistent with hypothalamic amenorrhea, PCOS, or hyperpro­
lactinemia, whereas high FSH levels suggest ovarian insufficiency. 
High prolactin levels may indicate a pituitary prolactinoma, pitu­
itary disease, or hypothyroidism. The following sections describe 
specific disorders that cause chronic anovu

cribe 
specific disorders that cause chronic anovulation in women of 
reproductive age. 
Hypothalamic Anovulation
Anovulation of hypothalamic origin usually manifests as amenor­
rhea. The terms hypothalamic anovulation and hypothalamic amen­
orrhea are used interchangeably in this chapter. A reduction in 
GnRH pulse frequency from the characteristic 60 to 120 minutes 
to intervals longer than 180 minutes leads to lower levels of LH 
and FSH secretion by the pituitary gland.13,141 This functional

tion by the pituitary gland.13,141 This functional 
gonadotropin deficiency fails to provide adequate stimulation 
602
to the ovarian follicles, and the normal sequence of follicular 
growth, maturation, follicular selection, and ovulation becomes 
attenuated. Downstream ovarian estradiol production remains 
low, and endometrial growth is reduced or arrested, resulting in 
a prolonged interval of amenorrhea. The transition from normal 
menstrual cycli

rrhea. The transition from normal 
menstrual cyclicity to anovulation and amenorrhea can take place 
gradually and may be characterized by inadequate luteal phases, 
irregular menstrual bleeding, and amenorrhea. Patients with 
hypothalamic amenorrhea do not complain of hot flashes, even 
though circulating levels of estradiol are within the menopausal 
range. This suggests a significant role for GnRH or gonadotropins 
in the cause of hot flashes.
Any disorder of the central nervous system that i

Any disorder of the central nervous system that interferes with 
normal GnRH pulse frequency can cause anovulation. Some of 
these disorders may be defined genetic or anatomic disorders such 
as isolated gonadotropin deficiency (with or without anosmia), 
infection, suprasellar tumors (e.g., pituitary adenomas, craniopha­
ryngioma), and head trauma.10 These genetic and anatomic disor­
ders affect the function of the hypothalamus, and some of them 
may be ruled out by the medical history, physic

m 
may be ruled out by the medical history, physical examination, 
and imaging of the head (Table 17.1).
The most commonly observed form of hypothalamic anovu­
lation is not associated with a demonstrable neuroanatomic 
finding.142 This common form is called functional hypothalamic 
amenorrhea because it is presumed to involve aberrant but revers­
ible regulation of otherwise normal neuroendocrine pathways. 
Other causes of hypothalamic anovulation demonstrable by neu­
roanatomic or genetic evid

on demonstrable by neu­
roanatomic or genetic evidence are less common (see Table 17.1).
Functional Hypothalamic Amenorrhea
Anovulation resulting from stress-associated changes is viewed 
as a functional disorder in which no anatomic or organic abnor­
malities of the hypothalamic-pituitary-ovarian axis can be iden­
tified.142 This condition typically manifests as amenorrhea of 
at least a 6-month duration and has also been called functional 
hypothalamic amenorrhea.142 The overall prevalence of 

thalamic amenorrhea.142 The overall prevalence of functional 
hypothalamic amenorrhea among all amenorrhea disorders ranges 
from 15% to 48%.143
Anovulation of hypothalamic origin is characterized by estro­
gen deficiency and low levels of gonadotropins. No genetic or 
anatomic disorders are identified in most patients.142 The concept 
of functional hypothalamic anovulation (i.e., amenorrhea) was 
postulated as the failure of the hypothalamic-pituitary pathways 
to release gonadotropins from the

uitary pathways 
to release gonadotropins from the anterior pituitary.142 Clinical 
studies have supported this common mechanism associated with 
an alteration in the pulsatile secretion of GnRH.14 Diverse etio­
logic factors such as malnutrition or caloric restriction, depres­
sion, psychogenic stress, excessive energy expenditure related to 
exercise, or combinations of these disorders precede the onset of 
functional hypothalamic anovulation.142 Heightened awareness of 
diet or exercise and u

42 Heightened awareness of 
diet or exercise and unrealistic expectations with respect to body 
image most likely contributed to the epidemic of this anovulatory 
disorder.
Diagnosis of Functional Hypothalamic Amenorrhea
Patients most commonly present with secondary amenorrhea 
characterized by absence of menstrual cycles for longer than 6 
months without evidence of an organic disorder. The diagnosis is 
one of exclusion. There are many neuroanatomic or genetic dis­
orders that can mimic functi

tomic or genetic dis­
orders that can mimic functional hypothalamic anovulation (see 
Table 17.1), and a careful and complete evaluation is essential to 
make this diagnosis.
Women with functional hypothalamic amenorrhea usually 
present with a history of regular menses for some period after 
menarche. This period of normal ovulatory function (determined 
by history) is interrupted by anovulation that usually manifests 
as secondary amenorrhea. Women with functional hypothalamic 
anovulation may

omen with functional hypothalamic 
anovulation may occasionally present with primary amenorrhea.
Women with functional hypothalamic amenorrhea typically 
have a normal body weight or are thin. They may be highly driven 
for success and involved in high-stress occupations. The occupa­
tion of the patient (e.g., ballerina, competitive athlete) may be 
an important clue. A detailed interview may also reveal a variety 
of emotional crises or stressful events (e.g., divorce, death of a 
friend) prece

l events (e.g., divorce, death of a 
friend) preceding the onset of amenorrhea. During the interview, 
additional environmental and interpersonal factors may become 
evident, such as academic pressure, social maladjustment, or psy­
chosexual problems.
When evaluating the patient, the physician should take note 
of the current diet regimen, the use of any sedatives or hypnotics, 
and the nature and extent of the patient’s exercise habits. Despite 
  Classification of Anovulation Caused by 
Disor

Disorders of the Hypothalamic-Pituitary 
Unit
Functional Hypothalamic Anovulation (Amenorrhea)
Stress (psychogenic or physical)
Dieting
Vigorous exercise
Chronic illness (e.g., chronic liver or renal insufficiency, AIDS)
Psychiatric-Medical Emergencies
Antipsychotics (e.g., olanzapine, risperidone, amisulpride, clozapine)
Opiates
Anatomically or Genetically Defined Pathologic Conditions 
of the Hypothalamic-

efined Pathologic Conditions 
of the Hypothalamic-Pituitary Unit
Clinically nonfunctioning adenoma
GH-secreting adenoma (acromegaly)
ACTH-secreting adenoma (Cushing disease)
Other pituitary tumors (e.g., metastasis, meningioma)
Pituitary stalk section
Hemorrhagic pituitary destruction, including pituitary apoplexy and 
Sheehan syndrome
Pituitary aneurysm
Infiltrative disease of the pituitary (e.g., lymphocytic hypophysitis, 
sarcoidosis, histiocytosis X, tuberculosi

ysitis, 
sarcoidosis, histiocytosis X, tuberculosis)
Empty sella syndrome
Tumors that affect hypothalamic function (e.g., metastasis, craniopha­
ryngioma)
Infiltrative granulomatous disease of the hypothalamus (e.g., sarcoid­
osis, histiocytosis X, tuberculosis)
Head trauma
Irradiation to the head
CNS infection
Isolated gonadotropin deficiency (including Kallmann syndrome)
Other
ACTH, Adrenocorticotropic hormone; AIDS, acquired immunodeficiency syndrome; CNS, 
central nervous system; GH, growth 

yndrome; CNS, 
central nervous system; GH, growth hormone.
   
TABLE 17.1
603
a careful interview, a history of stress, excessive physical exercise, 
or an eating disorder may not be readily revealed by some women 
with functional hypothalamic anovulation. These women do 
not complain of hot flashes, in contrast to women with ovarian 
These women usually have normal secondary sexual character­
istics. The pelvic

mal secondary sexual character­
istics. The pelvic examination usually shows a thinning vaginal 
mucosa accompanied by scant to absent cervical mucus with a 
normal to small uterus, which are all evidence of estrogen defi­
ciency. Signs of a well-estrogenized vagina and cervix observed 
during the physical examination make the diagnosis of hypo­
thalamic amenorrhea unlikely. The physician should exclude a 
possible hyperprolactinemic cause (e.g., prolactinoma, hypothy­
roidism) and evidence of a

 prolactinoma, hypothy­
roidism) and evidence of androgen excess (e.g., PCOS).
Laboratory tests are obtained to exclude other causes of anovu­
lation and secondary amenorrhea. LH and FSH levels are usually 
lower than the normal values ordinarily found in the early fol­
licular phase. TSH and prolactin levels are obtained to rule out 
hypothyroidism and hyperprolactinemia. The progestin challenge 
test (medroxyprogesterone acetate [MPA] at 5 mg/day for 10 
days) elicits either a small spotting e

ay for 10 
days) elicits either a small spotting episode or an absence of with­
drawal uterine bleeding in most patients. The administration of 
combined estrogen (2 mg/day of oral micronized estradiol) with 
progestin (5 mg/day of MPA for 10 days) will result in endome­
trial growth followed by vaginal bleeding after one or more cycles 
of therapy because the uterine compartment remains functionally 
normal.
These results suggest that there is a scant or absent estrogenic 
effect on the endomet

 scant or absent estrogenic 
effect on the endometrium, because circulating estradiol levels are 
typically in the low or early follicular phase range. Measurement 
of the serum estradiol level is not necessary. Because a suprasellar 
or large pituitary tumor is in the differential diagnosis, magnetic 
resonance imaging (MRI) of the head is necessary to rule out these 
possibilities. Imaging of the head is especially important if amen­
orrhea develops suddenly or is associated with a neurologic 

elops suddenly or is associated with a neurologic sign, 
both of which make the presence of a tumor more likely. 
Pathophysiology of Functional Hypothalamic Anovulation
A critical defect in hypothalamic amenorrhea is the reduction in 
GnRH release from the medial basal hypothalamus, which leads 
to a reduction in GnRH pulse frequency.14,144 LH pulse fre­
quency is used as a surrogate measure to evaluate GnRH secretion 
because each GnRH pulse is accompanied by a concomitant LH 
pulse.143 A key o

accompanied by a concomitant LH 
pulse.143 A key observation in functional hypothalamic anovula­
tion is the absence of increased gonadotropin secretion despite 
the lack of inhibitory factors of ovarian origin, such as estradiol 
and inhibin.
There is considerable variability in the amplitude and fre­
quency of the pulsatile LH secretion in women with functional 
hypothalamic amenorrhea. When the LH secretory patterns are 
compared with those of the follicular phase of the menstrual cycle, 
a c

 the follicular phase of the menstrual cycle, 
a characteristic abnormality in LH pulse frequency and amplitude 
is seen; occasionally, regression to a pronounced variability similar 
to what is seen in the prepubertal pattern is observed.14,143,144 In 
severe cases, the frequency and amplitude of LH pulses are mark­
edly reduced. These LH patterns also suggest that GnRH pulsatile 
secretion is not altered to the same degree in every patient. During 
the recovery phase of hypothalamic amenorrhea

ing 
the recovery phase of hypothalamic amenorrhea, reversal to a pat­
tern of LH secretion seen early in puberty often occurs, and it is 
characterized by a sleep-associated increase in LH amplitude.143
The response of the pituitary gland to GnRH with respect 
to production and release of gonadotropins is not impaired 
in functional hypothalamic anovulation. Intravenous pulsa­
tile GnRH administration can restore normal levels of LH and 
FSH.145
Norepinephrine, dopamine, and serotonin produced 

Norepinephrine, dopamine, and serotonin produced in the 
brain have been shown to modulate GnRH or LH release in 
animal studies.146 Patients receiving medication that alters these 
neurotransmitters (e.g., sedatives, antidepressants, stimulants, 
antipsychotics) have presented with abnormalities in their men­
strual cycles. These responses to medications provide circumstan­
tial evidence that disruptions of neural pathways can alter GnRH 
release in humans. It appears that activation of the no

se in humans. It appears that activation of the noradrenergic 
neurons principally stimulates release of GnRH, whereas dopa­
minergic and serotoninergic neurons can stimulate or inhibit 
GnRH-LH secretion.146
Another group of substances that have inhibitory influences on 
GnRH secretion are endogenous opioid peptides.147,148 Blockade 
of endogenous opiate receptors by the administration of nalox­
one, an opiate antagonist, causes an increase in the frequency and 
amplitude of pulsatile LH releas

he frequency and 
amplitude of pulsatile LH release in the majority of women with 
hypothalamic amenorrhea.144 Gonadotropin secretion resumes if 
the activity of the opiate receptor is blocked by long-term nalox­
one use, and ovulatory function may be regained in some cases.149 
These studies suggest that there is an overall increase in endog­
enous opiate activity, which can reduce pulsatile GnRH secretion 
in some cases of functional hypothalamic amenorrhea.
The hypothalamic-pituitary-adrenal 

ic amenorrhea.
The hypothalamic-pituitary-adrenal axis is dysfunctional in 
many women with functional hypothalamic amenorrhea, with 
increased secretion of corticotropin-releasing hormone (CRH), 
adrenocorticotropic hormone (ACTH), and cortisol. Activation 
of the pituitary-adrenocortical system is a common response in 
patients with chronic stress.150 In functional hypothalamic amen­
orrhea, stressors such as exercise or emotional distress can chroni­
cally activate the hypothalamic-pituitary-

chroni­
cally activate the hypothalamic-pituitary-adrenal axis and disrupt 
reproductive function.
Studies have demonstrated an increase in pulsatile ACTH secre­
tion, increased adrenal sensitivity to ACTH, and increased corti­
sol secretion with a normal diurnal rhythm.143 Daytime cortisol 
levels are markedly elevated, and the pituitary response to CRH is 
blunted.151 In an animal model, CRH seems to be an important 
factor in the inhibition of GnRH pulsatility.152,153 This inhibitory 
effect 

 GnRH pulsatility.152,153 This inhibitory 
effect can be prevented by coadministration of a CRH antago­
nist or reversed by the opiate antagonist naloxone, suggesting that 
cross-talk occurs between the action of CRH and activation of the 
opioidergic system. Moreover, ACTH administration blocks the 
pituitary response to GnRH at the pituitary level.154,155 In sum­
mary, overproduction of CRH and other stress-related hormones 
in the brain and activation of the pituitary-adrenocortical system 
b

tivation of the pituitary-adrenocortical system 
by chronic stress seem to play causative roles in the inhibition of 
gonadotropin secretion in functional hypothalamic anovulation.
The roles of energy balance–regulating peptides such as leptin 
and ghrelin were investigated in the mechanism of hypothalamic 
amenorrhea.143 Leptin is a cytokine produced by the adipocytes 
and is considered to be an appetite-suppressor peptide. Leptin is 
secreted in a pulsatile manner with a diurnal rhythm. A decr

n a pulsatile manner with a diurnal rhythm. A decrease 
in total circulating leptin with loss of the normal diurnal rhythm 
was reported in women with hypothalamic amenorrhea.156 This 
relative hypoleptinemia is a common characteristic of several 
energy-deficient conditions and is associated with slowing of 
the LH pulse frequency.156 Leptin administration to correct the 
relative leptin deficiency in women with hypothalamic amenor­
rhea was shown to improve reproductive, thyroid, and growth 
h

wn to improve reproductive, thyroid, and growth 
hormone axes and markers of bone formation, suggesting that 
604
leptin, a peripheral signal reflecting the adequacy of energy stores, 
is required for normal reproductive and neuroendocrine func­
tion.156 In contrast to leptin, ghrelin is an appetite-inducing pep­
tide secreted from the stomach. During the fasting state, ghrelin 
serves as the hunger signal from the periphery to the hypothalamic 
arcua

gnal from the periphery to the hypothalamic 
arcuate nucleus, a region that is known to control food intake. 
Ghrelin levels are reported to be elevated in women with hypotha­
lamic amenorrhea.157 
Hypothalamic Anovulation and Exercise
Regular vigorous exercise can lead to menstrual disturbances, a 
delay in menarche, luteal phase dysfunction, and secondary amen­
orrhea. Thirty percent of adolescent ballet dancers have problems 
with the progression of puberty. The mean age at menarche is 
delay

ion of puberty. The mean age at menarche is 
delayed until the age of 15 years. Advancement of pubertal stages 
seems to coincide with times of prolonged rest or after recovery 
from an injury.158–161 The intensity, length, and type of the sport 
determine the severity of the disease. Activities associated with an 
increased frequency of reproductive dysfunction are those that 
favor a lower body weight and include middle-distance and long-
distance running, competitive swimming, gymnastics, and

nce running, competitive swimming, gymnastics, and ballet 
dancing.
Competitive athletes show endocrine abnormalities in the 
central nervous system consistent with those in other forms of 
functional hypothalamic anovulation. Abnormalities include ele­
vations of central CRH and β-endorphin levels.
The management of exercise-related anovulation depends on 
the patient’s choices and expectations. Side effects such as osteopo­
rosis and delay of puberty must be discussed thoroughly with the 
pati

uberty must be discussed thoroughly with the 
patient.162 Decrease in exercise level and behavioral modification 
may be sufficient for the return of ovulatory function. Hormone 
therapy should be provided if sufficient results are not achieved. 
A low-dose oral contraceptive is a suitable option for women 
younger than 35 years of age. 
Hypothalamic Anovulation Associated With Eating Disorders
Two common eating disorders associated with hypothalamic dys­
function are anorexia nervosa and bulimi

amic dys­
function are anorexia nervosa and bulimia. Patients with anorexia 
nervosa have extreme weight loss (>25% of original body weight) 
and a distorted body image accompanied by a striking fear of 
obesity. Bulimia is a related disorder characterized by alternating 
episodes of binge eating followed by periods of food restriction, 
self-induced vomiting, or excessive use of laxatives or diuretics. 
Approximately 90% to 95% of these patients are women. The 
incidence of classic anorexia ner

 are women. The 
incidence of classic anorexia nervosa is approximately 1 case per 
100,000 people in the general population.163 Among female high 
school and college students, bulimia is fairly common. The inci­
dence of anorexia nervosa peaks twice during the teen years, at 
ages 13 and 17. Bulimia usually begins at a later age, between 17 
and 25 years. Anorexia nervosa has an extremely high mortality 
rate of 9% and is a true medical emergency. Death may result 
from cardiac arrhythmia, whic

y. Death may result 
from cardiac arrhythmia, which may be precipitated by dimin­
ished heart muscle mass and associated electrolyte abnormali­
ties.164 These patients are also at increased risk for suicide.165
Gonadotropin secretion in anorexic women exhibits a prepu­
bertal pattern that is similar to that observed in other forms of 
hypothalamic anovulation. Transitional patterns of LH secretion 
are seen with moderate degrees of weight recovery, and there is a 
normal or supranormal response 

y, and there is a 
normal or supranormal response to GnRH. Anovulation can per­
sist in up to 50% of anorexic patients, even after normal weight 
is achieved. Anorexic and bulimic patients exhibit hyperactiva­
tion of the hypothalamic-pituitary-adrenal system. Although the 
diurnal variation is maintained, persistent hypersecretion of cor­
tisol occurs throughout the day.166 Cushingoid features are not 
present, in part because of mild hypercortisolemia and a reduc­
tion of peripheral glucocorti

solemia and a reduc­
tion of peripheral glucocorticoid receptors. Levels of CRH and 
β-endorphin are increased in the central nervous system.167,168
In anorexia nervosa, basal metabolism is decreased because 
peripheral conversion of thyroxine (T4) to biologically potent tri­
iodothyronine (T3) is decreased. Instead, T4 is converted to reverse 
T3, an inactive isomer. This alteration is also observed in severely 
ill patients and during starvation.169 Anorexics have partial dia­
betes insipidus 

n.169 Anorexics have partial dia­
betes insipidus and are unable to concentrate urine appropriately 
because of the impaired secretion of vasopressin.170
Anorexia nervosa and bulimia are extremely difficult to treat. 
The most accepted approaches include individual psychotherapy, 
group therapy, and behavior modification. Patients with eating 
disorders should have psychiatric consultation and follow-up. This 
helps with the diagnosis and treatment. For patients who weigh 
less than 75% of their

nt. For patients who weigh 
less than 75% of their ideal body weight, immediate hospitaliza­
tion and aggressive treatment are recommended. Complications 
of anorexia nervosa include osteoporosis, estrogen deficiency, and 
generalized effects of malnutrition.159 Hormone therapy in the 
form of a low-dose oral contraceptive should be provided until 
ovulatory function is achieved. 
Treatment and Management of Functional Hypothalamic 
Anovulation
Treatment of chronic anovulation resulting from cen

reatment of chronic anovulation resulting from central nervous 
system–hypothalamic disorders should be directed at reversal of 
the primary cause (e.g., stress management, reduction of exercise, 
correction of weight loss). The importance of successful treatment 
of this disease state is underscored because these women are prone 
to the development of osteoporosis. For a considerable number of 
patients, spontaneous recovery of menstrual function takes place 
after a modification of lifestyle, 

n takes place 
after a modification of lifestyle, psychologic guidance, or accom­
modation to environmental stress. The initial treatment should 
be directed to a change in lifestyle and tailored to the individual 
patient. For individuals who remain amenorrheic, periodic assess­
ment of reproductive status (every 4–6 months) is prudent.
Modification of the stress response through cognitive-behav­
ioral therapy is a logical approach to lowering the endogenous 
stress levels in women with hypotha

he endogenous 
stress levels in women with hypothalamic amenorrhea. This 
approach was explored in 16 subjects with hypothalamic amenor­
rhea randomized to cognitive-behavioral therapy or observation 
for a 20-week period.171 The therapy design focused on attitudes 
and habits concerning eating, exercise, body image, problem-
solving skills, and stress reduction. The results were encouraging. 
Approximately 88% of those who underwent cognitive-behavioral 
therapy had evidence of ovulation, compa

havioral 
therapy had evidence of ovulation, compared with only 25% of 
those who were observed.171 These results suggest that endoge­
nous stress is a major factor in the development and maintenance 
of hypothalamic amenorrhea and that modification of this stress 
response can restore normal menses.
If anovulation persists for longer than 6 months or if reversal of 
the primary cause is not practical (e.g., professional athletes, balle­
rinas), a major concern is the long-term effect of hypoest

a major concern is the long-term effect of hypoestrogenism, 
especially on bone metabolism. In addition to estrogen deficiency, 
IGF1 deficiency, hypercortisolism, and nutritional factors may 
contribute to bone loss in this disorder.172 However, epidemio­
logic data on the risk of fractures and the benefits of hormone 
therapy are scant.162,172 On the basis of studies of reproductive-
age women who were ovariectomized or underwent treatment 
with GnRH agonist for endometriosis, bone density is 

h GnRH agonist for endometriosis, bone density is expected 
605
to decrease significantly, even within the first 6 months of amen­
orrhea. Because these patients are often reluctant to take medica­
tions, serial bone density studies of the lumbar spine and femur 
may be necessary to convince them of the necessity to begin estro­
gen replacement therapy. If the patient is not at risk for thrombo­
embolism and does not smoke cig

 risk for thrombo­
embolism and does not smoke cigarettes, a low-dose combination 
oral contraceptive is a reasonable replacement option. Alterna­
tively, a combination of conjugated equine estrogens (0.625 mg) 
and MPA (2.5 mg) daily may be administered to provide estro­
genic support. The progestin (MPA) is added solely to prevent 
endometrial hyperplasia.
If the patient desires ovulation to achieve pregnancy, the most 
physiologic approach is ovulation induction with pulsatile GnRH. 
This is 

ovulation induction with pulsatile GnRH. 
This is the best physiologic means of induction because the cause of 
the anovulatory state is decreased endogenous GnRH secretion. Pul­
satile intravenous GnRH (5 μg every 90 minutes) was shown to be 
effective.173 Monitoring of serum estradiol levels or follicular develop­
ment can be minimized because the ovarian follicular response and 
gonadotropin output mimic the natural menstrual cycle. In these 
patients, continuation of pulsatile GnRH or hCG (1

patients, continuation of pulsatile GnRH or hCG (1500 units admin­
istered intramuscularly every 3 days for a total of four doses) can sup­
port the function of the corpus luteum. Intravenous GnRH treatment 
results in ovulation rates of approximately 90%, pregnancy rates of 
up to 30%, and hyperstimulation rates of less than 1% per treatment 
cycle. Because the intravenous GnRH pump is not a practical choice 
for many women, an alternative strategy is the use of subcutaneous 
recombinant FSH fo

egy is the use of subcutaneous 
recombinant FSH for the development of one to three follicles and 
the induction of ovulation with intramuscular hCG followed by luteal 
support using intramuscular hCG or progesterone in oil. 
Chronic Anovulation Associated With Pituitary 
Disorders
The most common pituitary-related causes of anovulation are 
associated with hyperprolactinemia caused by prolactinomas or 
other functional or anatomic disorders of the pituitary. These dis­
orders are frequently ass

he pituitary. These dis­
orders are frequently associated with dysregulation of gonadotro­
pin secretion. Hyperprolactinemia and other pituitary disorders 
and their relation to reproduction are discussed in Chapter 9. 
Chronic Anovulation Associated With Androgen 
Excess
The most common ovary-related disorder of chronic anovulation 
is PCOS. Irregular periods or amenorrhea and androgen excess are 
the most commonly observed features of PCOS. Other causes of 
ovary-related anovulation include st

er causes of 
ovary-related anovulation include steroid-secreting ovarian tumors 
and POI. Androgen excess arising from extraovarian sources (e.g., 
adrenal disorders) is also associated with anovulation.
Approach to the Patient With Androgen Excess
Two natural androgens are testosterone, which is transported to 
target tissue by the circulation, and DHT, which is produced pri­
marily by target tissues. Increased levels of these androgens can 
lead to hirsutism, which is excessive androgenic hai

ad to hirsutism, which is excessive androgenic hair growth, or to 
virilization, a more severe form of androgen excess. Emerging evi­
dence also suggest that the adrenal steroid 11-hydroxyandrostene­
dione may be converted to 11-ketotesosterone and 11-keto-DHT 
in peripheral or target tissues.174,175 These latter steroids may act 
as potent androgens.174,175
Hirsutism is defined as the presence of terminal (coarse) hair 
in locations at which hair is not commonly found in women, 
including facia

r is not commonly found in women, 
including facial hair on the cheek, above the upper lip, and on 
the chin (Fig. 17.24 A and B). The presence of midline chest hair 
is also significant (see Fig. 17.24C). A male escutcheon, hair on 
the inner aspects of the thighs, and midline lower back hair enter­
ing the intergluteal area are hair growth patterns compatible with 
androgen excess. A moderate amount of hair on the forearms 
and lower legs by itself may not be abnormal, although it may 
be view

self may not be abnormal, although it may 
be viewed by the patient as undesirable and may be mistaken for 
hirsutism. Numerous scoring systems are available for quantifying 
hirsutism. One of the most detailed scales was proposed by Fer­
riman and Gallwey.176 A practical and clinically useful means of 
quantifying hirsutism is recording the hair growth in detail using 
simple drawings and photographs. In particular, photographs are 
invaluable for documenting hirsutism accurately.
Compared with

or documenting hirsutism accurately.
Compared with hirsutism, virilization is a more severe form 
of androgen excess and implies significantly higher rates of tes­
tosterone production. Its manifestations include temporal balding, 
deepening of voice, decreased breast size, increased muscle mass, 
loss of female body contours, and clitoral enlargement (Fig. 17.25). 
Even if testosterone levels are moderately increased (<1.5 ng/mL), 
temporal balding and clitoromegaly may be observed after a long

ing and clitoromegaly may be observed after a long 
period (>1 year) in the presence of persistent androgen excess. A 
marked increase in androgen secretion, such as that which may 
occur from production by neoplasms, leads to a more full-blown 
picture of virilization in less than a few months (see Fig. 17.25).
Measurements of an enlarged clitoris may be used for the quan­
tification of virilization. A clitoral length greater than 10 mm is 
considered abnormal (see Fig. 17.25). Clitoral length 

idered abnormal (see Fig. 17.25). Clitoral length is quite vari­
able, however. An increase in clitoral diameter is a much more 
sensitive indicator of androgen action. Normal values for clitoral 
diameter are less than 7 mm at the base of the glans (see Fig. 
17.25). The most accurate definition of clitoromegaly involves 
use of the clitoral index (the product of the sagittal and trans­
verse diameters of the glans clitoris). A clitoral index greater than 
35 mm2 is abnormal and correlates stat

eater than 
35 mm2 is abnormal and correlates statistically with androgen 
excess.177 
Origins of Androgens
Among the natural C19 steroids, DHT is a biologically potent 
androgen that is capable of acting through androgen recep­
tors on target cells. Almost all testosterone target tissues contain 
5α-reductase activity, which converts testosterone to DHT, or 
aromatase activity, which produces estradiol in an intracrine fash­
ion. There is no convincing evidence that the other C19 steroids, 
inc

vincing evidence that the other C19 steroids, 
including androstenedione, DHEA, and dehydroepiandrosterone 
sulfate (DHEAS), are biologically active.
Testosterone in reproductive-age women is produced by two 
major mechanisms: direct secretion by the ovary, which accounts 
for roughly one-third of testosterone production, and conversion 
of the precursor, androstenedione, to testosterone in the periph­
eral (extragonadal) tissues, which accounts for two-thirds of tes­
tosterone production (Fig. 

for two-thirds of tes­
tosterone production (Fig. 17.26).178 These peripheral tissues 
include the skin and adipose tissue. Androstenedione, the direct 
precursor of testosterone, is produced in the ovary and the adrenal 
gland. The C19 steroids DHEAS and DHEA of adrenal origin, 
and DHEA of ovarian origin, indirectly contribute to testosterone 
formation by first being converted to androstenedione, which is 
subsequently converted to testosterone (see Fig. 17.26). Andro­
stenedione is the major

e (see Fig. 17.26). Andro­
stenedione is the major precursor that is converted directly to tes­
tosterone. The conversion fraction of circulating androstenedione 
to testosterone in extragonadal tissues is about 5% in both men 
and women.
606
A
C
B
• Fig. 17.24  Hirsutism. (A) Mild facial hirsutism. (B) Severe facial hirsutism (chin), which requires regular 
shaving. (C) Severe hirsutism on chest. (B and C, from Dunaif A, Hoffman AR, Scully RE, et al.

nd C, from Dunaif A, Hoffman AR, Scully RE, et al. The clinical, 
biochemical and ovarian morphologic features in women with acanthosis nigricans and masculinization. 
Obstet Gynecol. 1985;66:545–552.)
A
B
• Fig. 17.25  Severe clitoromegaly resulting from a testosterone-secreting ovarian tumor. (A) The entire 
length of the clitoris is approximately 4 cm (normal, <1 cm). (B) The transverse diameter of the clitoris 
measures 1.5 cm (normal, <0.7 cm).
607
Chapter 17  Physiology and Pathology  of t

m).
607
Testosterone is converted to a potent steroid, DHT, to exert 
full androgenic effects on certain target tissues such as hair fol­
licles and external genitalia.98,115 This conversion is catalyzed by 
the enzyme 5α-reductase and takes place in the liver for systemic 
DHT production and within androgen target cells such as sex skin 
fibroblasts for an intracrine effect.98
The androgenic effects of testosterone in target 

The androgenic effects of testosterone in target tissues are deter­
mined by the level of local 5α-reductase activity and the androgen 
receptor content (see Fig. 17.26). Androgen receptors medi­
ate androgenic action in critical target tissues.115,179 Other local 
enzymes at target tissues (e.g., aromatase, oxidative 17βHSD) 
also regulate hormone action by metabolizing testosterone to the 
androgenically inactive androstenedione or to estradiol, a potent 
estrogen. There appears to be a balan

l, a potent 
estrogen. There appears to be a balance between potent androgen 
action when DHT is formed and reduction of androgenicity when 
inactive C19 steroids or estradiol is formed from testosterone in 
target tissues and other extragonadal tissues. This may be particu­
larly relevant for androgen-dependent disorders (e.g., hirsutism, 
virilization) and estrogen-dependent disorders such as malignan­
cies of breast and endometrium (see Figs. 17.19 and 17.26). 
Laboratory Evaluation of Androg

17.19 and 17.26). 
Laboratory Evaluation of Androgen Action
Testosterone circulates in three forms: that which is bound to sex 
hormone–binding globulin (SHBG), the portion not bound to 
SHBG but loosely associated with albumin, and the fraction not 
bound by SHBG or albumin (i.e., free or dialyzable testosterone). 
The blood testosterone that is available to diffuse into target tis­
sues includes the free and albumin-bound fractions and is referred 
to as bioavailable or non–SHBG-bound testoste

red 
to as bioavailable or non–SHBG-bound testosterone. The remain­
der is tightly bound to the protein SHBG.
SHBG is one of the primary regulators that determine the 
amounts of circulating bound and bioavailable testosterone avail­
able to act on target tissues. Conditions that decrease SHBG bind­
ing (e.g., androgen excess, obesity, acromegaly, hypothyroidism, 
liver disease) also increase bioavailable testosterone, augmenting 
the effect of testosterone. SHBG also regulates the circulating 

estosterone. SHBG also regulates the circulating 
amounts of bioavailable estradiol by binding a significant fraction 
of circulating estradiol. Conditions that decrease SHBG levels 
give rise to increased bioavailable (non–SHBG-bound) estradiol.
The measurement of non–SHBG-bound (bioavailable) forms 
of testosterone has been advocated for states of androgen excess 
to detect more accurately subtle forms of hirsutism. Although 
the diagnostic yield of this measurement is superior to that of 
tot

ld of this measurement is superior to that of 
total serum testosterone, the correlation between total and non–
SHBG-bound testosterone is excellent, so that bioavailable tes­
tosterone can usually be predicted from the total testosterone and 
SHBG levels.180 The purpose of measuring serum testosterone 
is to establish the presence of circulating androgen excess and to 
detect extremely high values that may originate from an andro­
gen-secreting neoplasm.
The normal serum levels of androgens, es

neoplasm.
The normal serum levels of androgens, especially free testoster­
one determined by radioimmunoassay (RIA) or other direct meth­
ods, vary from laboratory to laboratory. A group of investigators 
compared serum-free testosterone levels measured by equilibrium 
dialysis with those measured by direct RIA and with those calcu­
lated from the free androgen index (100 × testosterone/SHBG), a 
simple index that correlates with the free testosterone level.181 Cal­
culated values for free testo

erone level.181 Cal­
culated values for free testosterone using the free androgen index 
correlated well with those obtained from equilibrium dialysis. 
In contrast, the direct free testosterone measurements had unac­
ceptably high systematic bias and random variability and did not 
correlate as well with equilibrium dialysis values. Moreover, the 
lower limit of detection was higher for the direct RIA than for 
equilibrium dialysis or calculated free testosterone.181 The clini­
cian should be a

 free testosterone.181 The clini­
cian should be aware of the limitations of direct free testosterone 
measurements performed without rigorous quality control.
Measuring the levels of all C19 steroids is not clinically neces­
sary for most patients presenting with androgen excess. The most 
useful initial test is a serum total testosterone level. An abnormal 
level in the presence of hirsutism or virilization may be associated 
with PCOS, hyperthecosis, nonclassic congenital adrenal hyper­
plasi

ecosis, nonclassic congenital adrenal hyper­
plasia, or an androgen-secreting neoplasm. Most androgen-secret­
ing tumors are of ovarian origin. The likelihood of a neoplasm 
correlates roughly with increasing testosterone levels. The follow­
ing tests may be added on the basis of the clinical presentation: 
serum 17-hydroxyprogesterone (i.e., nonclassic congenital adre­
nal hyperplasia), serum prolactin and TSH (i.e., mild androgen 
excess associated with hyperprolactinemia), serum FSH and LH 
(

ated with hyperprolactinemia), serum FSH and LH 
(i.e., elevated LH/FSH ratio in PCOS), serum DHEAS (i.e., adre­
nal tumors), and imaging of ovaries and adrenals (i.e., PCOS and 
steroid-secreting tumors). 
Causes of Androgen Excess
Several disorders give rise to androgen excess. They include unusual 
causes such as iatrogenic or drug-induced androgen excess, congen­
ital genital ambiguity (e.g., excessive in utero androgen formation 
T ,DHT
T
DHT
DHT
Hair follicle
Clitoris
(skin
fibroblasts)
5 

T
DHT
Clitoris
(skin
5  -reductase
α
α
Adrenal gland
Ovary
A
T
T
A
A
tissues
Androgen
HSD17B1
17  HSD
β
17  HSD
β
DHEAS
DHEA
DHEA
• Fig. 17.26  Androgen biosynthesis in women. Depending on the men­
strual cycle phase or postmenopausal status, 20% to 30% of testosterone 
(T) is secreted by the ovary. The rest is accounted for by the conversion of 
circulating androstenedione (A) to T in various 

circulating androstenedione (A) to T in various peripheral tissues. Both the 
adrenal gland and the ovary contribute to circulating A directly or indirectly, 
depending on the cycle phase or postmenopausal status and chrono­
logic age. T may also be formed locally in androgen target tissues. T is 
converted to the biologically potent androgen dihydrotestosterone (DHT) 
within the target tissues and cells. For example, local conversion of T to 
DHT by 5α-reductase activity, which is conferred b

DHT by 5α-reductase activity, which is conferred by products of at least 
two genes, in sex skin fibroblasts and hair follicles plays a key role in clito­
ral enlargement and hirsutism. The enzyme activity of 17βHSD in periph­
eral tissues may be conferred by protein products of several genes with 
overlapping functions; HSD17B1, a distinct reductive 17βHSD enzyme, 
is encoded by a specific gene expressed primarily in the ovary. DHEAS, 
dehydroepiandrosterone sulfate; 17βHSD, reductive 17β-hydro

iandrosterone sulfate; 17βHSD, reductive 17β-hydroxysteroid 
dehydrogenase.
608
in 46,XX disorders of sex development), and conditions unique 
to pregnancy (e.g., luteoma of pregnancy, hyperreactio luteinalis). 
These uncommon causes and relatively more prevalent disorders 
associated with androgen excess are listed in Table 17.2. The term 
extraovarian steroid formation is used synonymously with extrag­
landular, extragonadal, or peripheral steroid f

g­
landular, extragonadal, or peripheral steroid formation in this text.
Overall, the prevalence of androgen-excess disorders was found 
to be as follows: 72.1% for PCOS (anovulatory patients, 56.6%; 
mildly affected ovulatory patients, 15.5%), 15.8% for idiopathic 
hyperandrogenism, 7.6% for idiopathic hirsutism, 4.3% for 
21-hydroxylase-deficient nonclassic congenital adrenal hyperpla­
sia, and 0.2% for androgen-secreting tumors.182
In most hyperandrogenic disorders, androgen originates from 

erandrogenic disorders, androgen originates from 
more than one source (see Fig. 17.26). For example, testosterone 
secretion is somewhat increased from the ovary in PCOS, but the 
bulk of testosterone comes from extraovarian conversion of sig­
nificantly elevated circulating androstenedione of ovarian origin 
to testosterone. Patients with PCOS also show increased adrenal 
output of DHEAS, which (after peripheral conversion to DHEA 
that is further converted to androstenedione) contributes indi

her converted to androstenedione) contributes indi­
rectly to extraovarian testosterone formation.
If androgen excess is associated with primary amenorrhea, 
abnormal in utero sexual differentiation should be strongly sus­
pected. These disorders are discussed in Chapter 23. Before 
embarking on a major workup for hirsutism or virilization, the 
physician is well advised to rule out exogenous androgen use. It is 
best to ask the patient to list all prescriptions and over-the-counter 
medications

ll prescriptions and over-the-counter 
medications that she takes on her own, including injections. This 
is usually more rewarding than asking the patient whether she 
takes any androgens. Medications that can cause hirsutism or vir­
ilization are related to testosterone and include anabolic steroids 
and similar compounds.
The most common identifiable cause of androgen excess is 
PCOS, which is discussed elsewhere in this chapter. In this sec­
tion, we first define some of the other disorders 

tion, we first define some of the other disorders associated with 
hirsutism or virilization. This is followed by a simplified treatment 
strategy that may be applied to most hirsute patients within the 
categories of PCOS, nonclassic congenital adrenal hyperplasia, 
and idiopathic hirsutism. 
Idiopathic Hirsutism
Hirsutism is defined subjectively as the presence in a woman of 
terminal hair growth in a male-distribution pattern that affects 
quality of life sufficiently to prompt her to seek me

lity of life sufficiently to prompt her to seek medical advice. 
Hirsutism should be distinguished from hypertrichosis, in which 
the excessive hair growth is not restricted to androgen-dependent 
areas and comprises vellus or lanugo-type hair. Hypertricosis is 
considered to be a phenotype not associated with male pattern 
hair growth and is unlikely to be modified by the known treat­
ments of hirsutism.
Idiopathic (constitutional) hirsutism is characterized by exces­
sive hair growth in the ab

characterized by exces­
sive hair growth in the absence of elevated circulating androgen 
levels in ovulatory women, and it occurs more frequently in cer­
tain ethnic populations, particularly in women of Mediterranean 
ancestry.115 It is defined as hirsutism in conjunction with regular 
menstrual cycles and normal levels of serum testosterone. Idio­
pathic hirsutism is not associated with any signs of virilization. 
Its cause is not understood completely. It has been proposed that 
women with i

ompletely. It has been proposed that 
women with idiopathic hirsutism have significantly increased 
cutaneous 5α-reductase activity,183 but this association has not 
been confirmed. It is also unclear whether a certain 5α-reductase 
isoenzyme (type 1 or 2) is involved in the development of idio­
pathic hirsutism.115
Idiopathic hirsutism is diagnosed in women who have hirsut­
ism,115 normal ovulatory function, and normal total or free tes­
tosterone levels. Cyclic predictable menses are usually i

ne levels. Cyclic predictable menses are usually indicative 
of regular ovulation. If in doubt, ovulatory function may be veri­
fied by a luteal phase day 7 progesterone level, which should be 
at least 5 ng/mL. Luteal phase day 7 corresponds to cycle day 17 
for 24-day intervals, cycle day 21 for 28-day intervals, and cycle 
day 28 for 35-day intervals. The presence of oligo-ovulation or 
anovulation in hirsute women after exclusion of related disorders 
(e.g., hypothyroidism, hyperprolactinemi

isorders 
(e.g., hypothyroidism, hyperprolactinemia, nonclassic congenital 
adrenal hyperplasia) is consistent with the diagnosis of PCOS.115 
Thyroid dysfunction or hyperprolactinemia should be excluded 
by the measurements of TSH and prolactin. The follicular-phase 
basal 17-hydroxyprogesterone level should be measured to exclude 
21-hydroxylase–deficient, nonclassic congenital adrenal hyperpla­
sia. The use of exogenous androgens should also be excluded. In 
summary, the diagnosis of idiopath

e excluded. In 
summary, the diagnosis of idiopathic hirsutism is one of exclusion 
in which ovulatory dysfunction, elevated circulating testosterone 
levels, and other causes of androgen excess are ruled out. 
Androgen-Secreting Tumors of the Ovary and Adrenal
Most androgen-secreting tumors arise from the ovary and secrete 
large quantities of testosterone or its precursor, androstenedione. 
These include Sertoli-Leydig cell tumors, hilus cell tumors, lipoid 
cell tumors, and (infrequently) gra

umors, lipoid 
cell tumors, and (infrequently) granulosa-theca tumors. Steroido­
genically inert ovarian neoplasms such as epithelial cystadenomas 
or cystadenocarcinomas may produce factors that stimulate ste­
roidogenesis in adjacent nonneoplastic ovarian stroma and induce 
production of sufficient amounts of androgen precursors such 
as androstenedione to give rise to clinically detectable androgen 
excess. Approximately 5% of androstenedione is converted to tes­
tosterone in extraovarian tis

is converted to tes­
tosterone in extraovarian tissues, ultimately producing androgen 
excess (see Fig. 17.26).
Sertoli-Leydig cell tumors, which account for fewer than 1% of 
all solid ovarian tumors, tend to occur during the second to fourth 
  Causes of Androgen Excess in Women of 
Reproductive Age
Ovarian
Polycystic ovary syndrome (PCOS)
Hyperthecosis (a severe PCOS variant)
Ovarian tumor (e.g., Sertoli-Leydig cell tumor)
Adrenal
Nonclassic congenital adrenal hyperplasia
Gl

congenital adrenal hyperplasia
Glucocorticoid resistance
Adrenal tumor (e.g., adenoma, carcinoma)
Specific Conditions of Pregnancy
Luteoma of pregnancy
Hyperreactio luteinalis
Aromatase deficiency in fetus
Hyperprolactinemia, hypothyroidism
Medications (danazol, testosterone, anabolizing agents)
Idiopathic hirsutism (normal serum testosterone in an ovulatory 
woman)
Idiopathic hyperandrogenism (patients who do not fall into any of the 
other categories listed)
TABLE

ll into any of the 
TABLE 17.2
609
decades of life, whereas hilus cell tumors occur more frequently 
in postmenopausal women. By the time the signs and symptoms 
of androgen excess cause the patient to seek medical assistance, 
Sertoli-Leydig cell tumors are usually so large that they are readily 
palpable on pelvic examination, whereas hilus cell tumors are still 
small. In women with either type of t

s are still 
small. In women with either type of tumor, the serum testosterone 
level is markedly elevated. Granulosa-theca tumors primarily pro­
duce estradiol but may occasionally produce testosterone.
Rapidly progressing symptoms of androgen excess suggest the 
presence of an androgen-producing tumor unless proven other­
wise. This rapid progression is typical of both ovarian and adrenal 
androgen-producing tumors. Progression is usually associated with 
defeminizing signs, such as loss of fe

iated with 
defeminizing signs, such as loss of female body contour, increased 
muscle mass, and decreased breast size. As the tumor continues to 
grow, more and more testosterone is produced, resulting in rapidly 
worsening hirsutism and progressive virilization. Elevated serum 
testosterone levels are characteristically associated with ovarian 
tumors. This change may be mediated by production and secre­
tion of testosterone directly by the tumor or by secretion of large 
quantities of androst

or or by secretion of large 
quantities of androstenedione that are converted to testosterone in 
extragonadal tissues. The testosterone levels produced by certain 
ovarian tumors (e.g., Sertoli-Leydig cell tumors) may be suppressed 
by GnRH agonists,184 so use of a GnRH agonist cannot be relied on 
to differentiate a neoplasm from another functional state.
In interpreting testosterone levels, the clinician should be 
familiar with the normal ranges of the clinical laboratory used. A 
value of t

ges of the clinical laboratory used. A 
value of three times the upper-normal range (or >2 ng/mL) sug­
gests a neoplasm, particularly if the clinical history supports this 
diagnosis. Lower serum testosterone levels occasionally may be 
associated with virilizing ovarian tumors. If an androgen-secreting 
tumor is suspected, measurement of androstenedione is clinically 
useful. A severely elevated level of androstenedione is not diag­
nostic but consistent with an ovarian or adrenal tumor. When a

onsistent with an ovarian or adrenal tumor. When an 
elevated level of testosterone is associated with suggestive clinical 
history, meticulously performed transvaginal ultrasonography is 
the most sensitive method to detect an ovarian tumor.
In contrast to testosterone-secreting tumors of the ovary, tes­
tosterone-secreting tumors of the adrenal are rare. The cells of 
some testosterone-producing adrenal tumors may resemble ovar­
ian hilus cells, which are analogous to Leydig cells. These tumor

, which are analogous to Leydig cells. These tumor 
cells produce testosterone and may be stimulated by LH or hCG. 
In patients with testosterone-producing adrenal adenomas, tes­
tosterone secretion usually decreases after LH suppression and 
increases after hCG stimulation. Testosterone-secreting adrenal 
carcinomas also have been reported.185
Virilizing adrenal tumors commonly secrete large quantities of 
DHEAS, DHEA, and androstenedione, and testosterone is usu­
ally produced by extraovarian 

estosterone is usu­
ally produced by extraovarian conversion of these precursors. Lev­
els of serum DHEAS are highly elevated in most patients with 
virilizing adrenal tumors.186 If DHEAS levels exceed 8 μg/mL, 
adrenal imaging by computed tomography (CT) or MRI should 
be ordered. Occasionally, such high levels of DHEAS are asso­
ciated with a functional abnormality such as congenital adrenal 
hyperplasia caused by an enzymatic defect or an unexplained 
hyperfunctional adrenal state that is com

plained 
hyperfunctional adrenal state that is commonly associated with 
PCOS. These circumstances may explain a negative CT or MRI 
result, which warrants further investigation.
Levels of a variety of adrenal steroids, including corticosteroids, 
may be elevated in various combinations in the presence of an 
adrenal tumor. It is not possible to describe a particular pattern 
of hormones that defines an adrenal tumor.186 Very high levels of 
serum DHEAS (>8 μg/mL) suggest an adrenal tumor. Viril

m DHEAS (>8 μg/mL) suggest an adrenal tumor. Virilizing 
ovarian tumors are encountered much more frequently than 
those of adrenal origin. If the presentation is compatible with an 
androgen-secreting tumor and the ovaries are normal by trans­
vaginal ultrasonography, the adrenals should be evaluated next by 
imaging.
Testosterone levels three times the upper-normal range (i.e., 
>2 ng/mL) and DHEAS levels higher than 8 μg/mL have been 
used traditionally as guidelines to investigate further wh

ditionally as guidelines to investigate further whether neo­
plasms of the ovary or adrenal are the sources of androgen excess. 
These numbers are provided only as guidelines, not as rules, and 
there are exceptions. First, because tumors secrete androgens epi­
sodically, more than one measurement may be required to detect 
a significantly elevated level.187 Second, other precursor steroids 
are often elevated as well (particularly androstenedione), and their 
measurement should be considered. T

e), and their 
measurement should be considered. Third, some tumors may give 
rise to milder elevations of DHEAS and testosterone levels. Even 
mild elevations in a postmenopausal woman are highly suspicious 
for an androgen-secreting tumor. By the same token, greatly ele­
vated serum testosterone levels may be observed in women with 
severe ovarian hyperthecosis (a severe variant of PCOS) in the 
absence of a tumor.
Virilization of recent onset and short duration warrants imme­
diate investigat

and short duration warrants imme­
diate investigation, even if testosterone and DHEAS levels are 
mildly elevated. With improvements in imaging techniques—
vaginal ultrasonography for the ovary; abdominal ultrasonogra­
phy, CT, and MRI for the adrenal glands—the diagnosis of even 
a small ovarian or adrenal tumor may be made. If no neoplasm 
can be localized, imaging of the ovary or adrenal after intravenous 
administration of radiolabeled iodomethylnorcholesterol (NP59), 
which detects active s

thylnorcholesterol (NP59), 
which detects active steroid-producing tumors, has proven use­
ful.188 These diagnostic studies should be pursued aggressively 
before surgical exploration of a suspected tumor.
The clinician should question whether an ovarian or adrenal 
tumor detected by imaging is the actual source of androgen excess 
before resorting to its surgical resection. Occasionally, a hemor­
rhagic corpus luteum cyst of the ovary may mimic an androgen-
secreting tumor, or a woman with andr

an androgen-
secreting tumor, or a woman with androgen excess may have an 
adrenal incidentaloma, which does not secrete androgen. Intra­
operative selective ovarian or adrenal vein catheterization may 
be considered as a last resort to demonstrate significant steroid 
gradients before surgical exploration of an adrenal or ovary for a 
small tumor is undertaken, especially if the clinical picture is not 
certain.189 
Nonneoplastic Adrenal Disorders and Androgen Excess
Adrenal disorders such as c

rs and Androgen Excess
Adrenal disorders such as classic congenital adrenal hyperplasia, 
Cushing syndrome, and glucocorticoid resistance give rise to 
androgen excess related to overproduction of testosterone precur­
sors from the adrenal gland. These disorders are discussed else­
where in this text. In this chapter, we discuss nonclassic congenital 
adrenal hyperplasia.
The diagnosis and prevalence of nonclassic congenital adrenal 
hyperplasia continue to be debated, although the disorder clea

continue to be debated, although the disorder clearly 
exists. Other terms that have been used to describe this syndrome 
include late-onset, adult-onset, attenuated, incomplete, and cryptic 
adrenal hyperplasia. This form of adrenal hyperplasia is caused by a 
partial deficiency in 21-hydroxylase activity. Although deficiencies 
in 11β-hydroxylase and 3βHSD may result in the disorder, defects 
in 21-hydroxylase account for more than 90% of cases.190
The clinical presentation is almost identical

.190
The clinical presentation is almost identical to that of patients 
with PCOS. The prevalence of this disorder varies according 
to ethnic background, and the prevalence reported by different 
610
investigators has varied widely. The characteristic presentation 
consists of anovulatory uterine bleeding and progressive hirsut­
ism of pubertal onset. These individuals are born with normal 
genitalia, do not exhibit salt wasting, and many have premat

 do not exhibit salt wasting, and many have premature 
pubarche and accelerated linear growth with advanced bone age. 
Patients of northern European ancestry have a low frequency of 
this disorder, whereas Ashkenazi Jews, Hispanics, and patients of 
central European ancestry have a much higher prevalence.191 The 
patient with androgen excess from a high-risk ethnic group should 
be screened. (See Chapter 15 for more complete discussion.)
Screening may first be carried out by obtaining an 8 am se

g may first be carried out by obtaining an 8 am serum 
level of 17-hydroxyprogesterone in an anovulatory patient on any 
day. Although most women with nonclassic congenital adrenal 
hyperplasia are anovulatory, some women with this disorder pres­
ent with regular periods and hirsutism of pubertal onset or with 
only unexplained infertility and frequent miscarriage.190 If non­
classic congenital adrenal hyperplasia is suspected in an ovulatory 
patient on the basis of clinical presentation, an 8 

tient on the basis of clinical presentation, an 8 am serum level 
of 17-hydroxyprogesterone should be obtained during the follicu­
lar phase, because 17-hydroxyprogesterone levels are higher in the 
luteal phase in ovulatory women.190 A level of less than 2 ng/mL 
effectively rules out this diagnosis.190
The diagnosis of nonclassic congenital adrenal hyperplasia can 
be made if the basal 17-hydroxyprogesterone level is higher than 
10 ng/mL. No further testing is required in these cases. Values 

urther testing is required in these cases. Values 
between 2 and 10 ng/mL are considered increased but not diag­
nostic of nonclassic congenital adrenal hyperplasia. For example, 
disease-free women and patients with PCOS may also have basal 
levels of 17-hydroxyprogesterone in this indeterminate range.190 
In these circumstances, an ACTH stimulation test should be used 
to distinguish nonclassic congenital adrenal hyperplasia from 
PCOS.190 A rise of the 17-hydroxyprogesterone level to at least

se of the 17-hydroxyprogesterone level to at least 10 
ng/mL 60 minutes after intravenous injection of ACTH is con­
sidered diagnostic of nonclassic adrenal hyperplasia.192 A higher 
basal level of 17-hydroxprogesterone within the range of 2 to 10 
ng/mL is associated with a higher likelihood of nonclassic con­
genital adrenal hyperplasia. For example, an 8 am 17-hydroxypro­
gesterone level higher than 4 ng/mL had a sensitivity of 90% for 
the diagnosis of nonclassic congenital adrenal hyperplas

agnosis of nonclassic congenital adrenal hyperplasia.190
In an androgen excess patient from a high-risk ethnic group, a 
baseline level of 17-hydroxyprogesterone should be measured at 8 
am. A screening baseline level of 17-hydroxyprogesterone should 
be obtained for patients with premature pubarche, those with 
androgen excess of early pubertal onset, women with progressive 
hirsutism or virilization, and patients with strong family histories 
of severe androgen excess. 
Laboratory Testing to A

f severe androgen excess. 
Laboratory Testing to Aid the Differential Diagnosis of 
Androgen Excess
Algorithms exist for the differential diagnosis of anovulation asso­
ciated with hirsutism or virilization or both. Salient clinical fea­
tures are of paramount importance to guide laboratory testing. 
The most important features are the onset and severity of the signs 
and the rapidity with which they progress. Rapidly progressing 
severe androgen excess implies an androgen-secreting tumor until 

 excess implies an androgen-secreting tumor until 
proven otherwise. The possibility of a tumor is further underscored 
in a postmenopausal woman or in a reproductive-age woman with 
a recent history of cyclic, predictable periods. Ovarian hyperthe­
cosis, a severe variant of PCOS, also gives rise to severe androgen 
excess that may progress rapidly, especially at the time of expected 
puberty. Androgen excess emerging at the time of puberty may be 
indicative of PCOS or nonclassic congenital ad

be 
indicative of PCOS or nonclassic congenital adrenal hyperplasia.
The most useful initial test to evaluate androgen excess is 
the serum level of total testosterone (Table 17.3). Testosterone 
levels in most normal ovulatory women are lower than 0.6 ng/
mL, although the value may vary from laboratory to laboratory. 
Women with idiopathic hirsutism have cyclic menses and nor­
mal testosterone levels. No further testing for androgen excess is 
required in this group.
If the testosterone level i

equired in this group.
If the testosterone level is elevated in an anovulatory woman, 
serum levels of TSH and prolactin should be obtained to rule out 
anovulation associated with hyperprolactinemia. Ultrasonography of 
the ovaries also can help to identify an ovarian tumor or polycystic 
ovaries. If the ethnic background of the patient (i.e., Ashkenazi Jews, 
Hispanics, and those of central European ancestry), onset of hirsut­
ism (i.e., puberty), or family history suggests nonclassic congenit

y), or family history suggests nonclassic congenital 
adrenal hyperplasia, a baseline serum level of 17-hydroxyprogesterone 
should be obtained at 8 am. Rare causes of androgen excess include 
an adrenal tumor, Cushing syndrome, and glucocorticoid resistance. 
A serum level of DHEAS and adrenal imaging are required to assess 
the presence or absence of an adrenal tumor. CT, MRI, or abdominal 
ultrasonography may be used to assess the adrenals, depending on 
the expertise of the local radiology l

pending on 
the expertise of the local radiology laboratory. A screening test for 
Cushing syndrome and glucocorticoid resistance may be performed 
to explore rare adrenal causes of androgen excess (see Chapter 15).193
Most women with chronic anovulation and mild to moderate 
hirsutism of pubertal onset fall into the category of PCOS. These 
women usually have high-normal or mildly elevated testosterone 
levels and may not have any other laboratory abnormalities. After 
other diagnoses are ruled

ry abnormalities. After 
other diagnoses are ruled out by laboratory testing or on clinical 
grounds, a diagnosis of PCOS can be made. 
Treatment of Hirsutism
Therapy for androgen excess should be directed toward its specific 
cause and suppression of abnormal androgen secretion. Neoplasms 
warrant surgical intervention and are not discussed in greater detail 
here. Suppression with a GnRH analogue may be tried initially 
for ovarian hyperthecosis. However, bilateral oophorectomy may 
become nec

s. However, bilateral oophorectomy may 
become necessary to control androgen excess arising from hyperthe­
cosis (see later discussion). Patients with adrenal disease are treated 
specifically. For Cushing syndrome, treatment correlates with the 
  Laboratory Tests for the Differential 
Diagnosis of Androgen Excess
Initial Testing
Total testosterone
Prolactin
Thyroid-stimulating hormone
Further Testing Based on Clinical Presentationa
17-Hydroxyprogesterone (8 am)
17-Hydroxyprogesterone 60 min a

rogesterone (8 am)
17-Hydroxyprogesterone 60 min after intravenous ACTH
Cortisol (8 am) after 1 mg dexamethasone at midnight
DHEAS
Imaging of ovaries (transvaginal ultrasonography)
Imaging of adrenal glands (abdominal ultrasonography, CT, MRI)
Nuclear imaging after intravenous administration of radiolabeled 
aSee text.
ACTH, Adrenocorticotropic hormone; CT, computed tomography; DHEAS, dehydroepiandros­
terone sulfate; MRI, magnetic resonance imaging.
   
TABLE 17.3
61

TABLE 17.3
611
source of hypercortisolism. When treating androgen excess associ­
ated with nonclassic congenital adrenal hyperplasia, an antiandro­
gen (e.g., spironolactone) in combination with an oral contraceptive 
is used. Although a glucocorticoid may be considered, particularly 
if fertility is desired, the doses of glucocorticoids needed to suppress 
the adrenal can cause symptoms an

ded to suppress 
the adrenal can cause symptoms and signs of glucocorticoid excess 
during long-term treatment. Thus a combination oral contraceptive 
plus spironolactone is favored to treat androgen excess if the patient 
responds to this treatment with decreased hirsutism. Several classes 
of medications are reviewed in detail later in the chapter for the 
treatment of androgen excess and hirsutism.
Oral Contraceptives
Oral contraceptives reduce circulating testosterone and androgen 
precursor

e circulating testosterone and androgen 
precursors by suppression of LH and stimulation of SHBG lev­
els, thereby reducing hirsutism in hyperandrogenic patients.115 
Oral contraceptives decrease circulating androgen in patients with 
PCOS and synergize with the effects of antiandrogens. Oral con­
traceptives may further improve the results of antiandrogen ther­
apy in patients with idiopathic hirsutism or nonclassic congenital 
adrenal hyperplasia. It is advisable to use an oral contraceptive 

ia. It is advisable to use an oral contraceptive 
containing 30 or 35 μg of ethinyl estradiol to achieve effective 
suppression of LH.115 A meta-analysis showed that treatment with 
oral contraceptives for 6 months reduced Ferriman-Gallwey scores 
of hirsutism by an average of 27%.194 
Spironolactone
The most commonly used androgen blocker for the treatment of 
hirsutism in the United States is spironolactone, an aldosterone 
antagonist structurally related to progestins. Spironolactone is 
effe

lly related to progestins. Spironolactone is 
effective for abnormal hair growth associated with PCOS, non­
classic congenital adrenal hyperplasia, or idiopathic hirsutism. 
Treatment with spironolactone for 6 months reduces Ferriman-
Gallwey scores of hirsutism by an average of 38.4%.
Because spironolactone acts through mechanisms different 
from those of oral contraceptives, overall effectiveness is improved 
by combining these two medications in patients with hirsutism, 
including those with 

in patients with hirsutism, 
including those with PCOS, idiopathic hirsutism, or nonclassic 
congenital adrenal hyperplasia. Apart from inhibiting steroido­
genesis and acting as an androgen antagonist, spironolactone has 
a significant effect in inhibiting 5α-reductase activity.115,195 Basic 
experimental and several clinical studies have confirmed the effi­
cacy of spironolactone for hyperandrogenism and suggest that the 
principal effect is related to its ability to block peripheral androgen 

lated to its ability to block peripheral androgen 
production and action.115
Doses of spironolactone used in clinical studies have varied 
from 50 to 400 mg daily. Although doses of 100 mg/day are usu­
ally effective for the treatment of hirsutism, higher doses (200–300 
mg/day) may be preferable in extremely hirsute or markedly obese 
women.115,195 The initial recommended dosage is 100 mg/day, 
which is gradually increased by increments of 25 mg/day every 
3 months up to 200 mg/day on the basis

/day every 
3 months up to 200 mg/day on the basis of the response. This 
approach may be helpful to minimize side effects such as gastritis, 
dry skin, and anovulation. In patients with normal renal function, 
hyperkalemia is almost never seen. Hypotension is rare except in 
older women. Monitoring for electrolytes and blood pressure is 
imperative within the first 2 weeks at each dose level. Adjustments 
in dose should be made only after 3 to 6 months, as with other 
antiandrogens, to account 

 months, as with other 
antiandrogens, to account for the slow changes in the hair cycle.
Patients usually notice an initial transient diuretic effect. 
Some women with normal cycles complain of menstrual irregu­
larity with spironolactone; this is remedied by a downward dose 
adjustment or the addition of an oral contraceptive. The mecha­
nism for abnormal bleeding is unclear. In women with oligomen­
orrhea, such as those with PCOS, resumption of normal menses 
may occur. This change may be cau

f normal menses 
may occur. This change may be caused in part by an alteration in 
levels of circulating androgens; LH levels have only occasionally 
been reported to decrease.196 Another important consideration is 
the potential in utero feminizing effect of this antiandrogen on the 
genitalia of a 46,XY fetus. Effective contraception should always 
be provided in women taking spironolactone. 
Cyproterone Acetate
Cyproterone acetate is a 17-hydroxyprogesterone acetate deriva­
tive with strong p

oxyprogesterone acetate deriva­
tive with strong progestagenic properties. Cyproterone acetate 
acts as an antiandrogen by competing with DHT and testosterone 
for binding to the androgen receptor. There is also some evidence 
that cyproterone acetate and ethinyl estradiol in combination 
can inhibit 5α-reductase activity in skin.197 Cyproterone acetate 
is not available in the United States but has been used in other 
countries. The drug is usually administered daily in doses of 50 
to 100 mg o

lly administered daily in doses of 50 
to 100 mg on days 5 through 15 of the treatment cycle. Because 
of its slow metabolism, it is administered early in the treatment 
cycle; when ethinyl estradiol is added, it is usually administered 
in 50-μg doses on days 5 through 26. This regimen is needed for 
menstrual control and is usually referred to as the reverse sequen­
tial regimen. Cyproterone acetate in doses of 50 to 100 mg/day, 
combined with ethinyl estradiol at 30 to 35 μg/day, is as effect

ethinyl estradiol at 30 to 35 μg/day, is as effective 
as the combination of spironolactone (100 mg/day) and an oral 
contraceptive in the treatment of hirsutism.115 In smaller doses (2 
mg), cyproterone acetate has been administered as an oral contra­
ceptive in daily combination with 50 or 35 μg of ethinyl estradiol. 
This regimen is primarily suited for individuals with a milder form 
of hyperandrogenism.115 
Finasteride
Finasteride inhibits 5α-reductase activity and has been used pri­
marily

α-reductase activity and has been used pri­
marily for the treatment of prostatic hyperplasia. It can also be 
used in the treatment of hirsutism.198,199 At a dose of 5 mg/
day, a significant improvement of hirsutism is observed after 6 
months of therapy, without significant side effects. In hirsute 
women, the decline in circulating DHT levels is small and 
cannot be used to monitor therapy. Although this treatment 
regimen increases testosterone levels, SHBG levels remain unaf­
fected.198 A m

ne levels, SHBG levels remain unaf­
fected.198 A meta-analysis showed that finasteride treatment for 
6 months reduced Ferriman-Gallwey scores of hirsutism by an 
average of 20.3%.194
Finasteride primarily inhibits 5α-reductase type 2. Because 
hirsutism results from the combined effects of type 1 and type 2, 
this agent is only partially effective. Although prolonged experi­
ence with finasteride is lacking, one of the potential advantages 
of this agent is its benign side effect profile. One s

his agent is its benign side effect profile. One study showed 
efficacy with 1 year of hirsutism treatment.200 It was also reported 
that finasteride is less effective than spironolactone with respect to 
the reduction of hirsutism in women.115 Nevertheless, finasteride 
at a dose of 5 mg/day for prolonged periods represents a useful 
option because of its benign side effect profile and good tolerance 
by patients. Like spironolactone, finasteride may cause congenital 
genital ambiguity in a 46,

e may cause congenital 
genital ambiguity in a 46,XY fetus, and effective contraception 
should be provided during its use. 
Flutamide
Flutamide is a potent antiandrogen used in the treatment of pros­
tate cancer. It has been shown to be effective in the treatment of 
612
hirsutism.201,202 The mean Ferriman-Gallwey score is reduced by 
41.3%.194 Nevertheless, occasional severe hepatotoxicity makes 
this drug unsuitable for the indication of hirsutism.

s drug unsuitable for the indication of hirsutism.203 
Metformin and Thiazolidinediones
Because PCOS is often associated with insulin resistance, drugs 
that mitigate insulin resistance have been used in this disorder.110 
Metformin (1500–2700 mg/day) for 6 months significantly 
reduces hirsutism by 19.1% as assessed by the Ferriman-Gallwey 
scoring system.194 In obese adolescent women with PCOS, met­
formin in combination with lifestyle modification (i.e., diet with 
a 500 kcal/day deficit and 

tion (i.e., diet with 
a 500 kcal/day deficit and exercise 30 minutes/day) and oral con­
traceptives reduced the total testosterone level and waist circum­
ference.204 The thiazolidinediones (4 mg/day of rosiglitazone or 
30 mg/day of pioglitazone) also reduced Ferriman-Gallwey scores 
significantly.194 These studies suggested that insulin-sensitizing 
agents may be used in the treatment of hirsutism of PCOS, espe­
cially for women who do not wish to use other oral agents. 
Lifestyle Modificatio

h to use other oral agents. 
Lifestyle Modification
In obese adolescent women with PCOS, lifestyle modification 
(i.e., diet with a 500 kcal/day deficit and exercise 30 minutes/
day) alone resulted in a 59% reduction in the testosterone/SHBG 
ratio, with a 122% increase in SHBG.204 A moderate diet and 
exercise program should be recommended as part of hirsutism 
management, particularly for obese women. 
A Comprehensive Treatment Strategy for Hirsutism
The medications described in the previous p

sutism
The medications described in the previous paragraphs may be 
effective when administered as individual treatments. Patients 
with the most common form of hirsutism (i.e., PCOS, nonclassic 
congenital adrenal hyperplasia, or idiopathic hirsutism) are often 
initially treated with a combination of two agents, one that sup­
presses the ovary (e.g., an oral contraceptive) and another that 
suppresses the extraovarian (peripheral) action of androgens (e.g., 
spironolactone). An oral contracept

rogens (e.g., 
spironolactone). An oral contraceptive containing 30 to 35 μg of 
ethinyl estradiol combined with spironolactone (100 mg/day) is 
the initial treatment of choice. Even in women with idiopathic 
hirsutism, the addition of an oral contraceptive to the antiandro­
gen spironolactone can improve efficacy and prevent abnormal 
bleeding. For women with only minor complaints of hirsutism, 
the use of an oral contraceptive alone may be an appropriate first 
approach. Moderate lifestyle mod

ppropriate first 
approach. Moderate lifestyle modification (i.e., diet with a 500-
kcal/day deficit and 30 minutes/day of exercise) should be a part 
of hirsutism management in obese patients.
Because the growth phase of body hair lasts 3 to 6 months, a 
response should not be expected before 6 months after onset of 
treatment. Objective means should be used to assess changes in 
hair growth. Scoring systems and evaluation of anagen hair shafts 
are difficult; taking photographs is the simplest

are difficult; taking photographs is the simplest and most objec­
tive tool. Patients are often unaware that change is taking place 
unless there is some objective measurement. Pictures of the face 
and selected midline body areas before and during therapy are 
especially useful for the encouragement of the patient and compli­
ance with the treatment.
Suppression of androgen production and action inhibits only 
new hair growth. Existing coarse hair should be removed mechani­
cally. Plucking, wa

air should be removed mechani­
cally. Plucking, waxing, and shaving are ineffective for hair removal 
and cause irritation, folliculitis, and ingrown hairs. Electrolysis and 
laser epilation are more effective and preferred methods.115
Most patients with PCOS, nonclassic congenital adrenal hyper­
plasia, or idiopathic hirsutism respond to this strategy within 1 
year. Patients should be encouraged to continue treatment for at 
least 2 years. Then, depending on the wishes and clinical responses 

, depending on the wishes and clinical responses 
of patients, therapy can be stopped and the patient reevaluated. 
Many patients require continuous treatment for suppression of 
hirsutism. Patients with clitoromegaly may be referred to a urolo­
gist for clitoral reduction surgery after the source of virilization has 
been effectively eliminated. 
Polycystic Ovary Syndrome
PCOS is the most common form of chronic anovulation associ­
ated with androgen excess; it occurs in perhaps 5% to 10% of 
re

ogen excess; it occurs in perhaps 5% to 10% of 
reproductive-age women.205 The diagnosis of PCOS is made by 
excluding other hyperandrogenic disorders (e.g., nonclassic con­
genital adrenal hyperplasia, androgen-secreting tumors, hyperp­
rolactinemia) in women with chronic anovulation and androgen 
excess.205
During the reproductive years, PCOS is associated with impor­
tant reproductive morbidity, including infertility, irregular uter­
ine bleeding, and increased pregnancy loss. The endometrium

ing, and increased pregnancy loss. The endometrium 
of the patient with PCOS must be evaluated by biopsy because 
long-term unopposed estrogen stimulation leaves these patients 
at increased risk for endometrial cancer. PCOS is also associated 
with increased metabolic and cardiovascular risk factors.206 These 
risks are linked to insulin resistance and are compounded by the 
common occurrence of obesity, although insulin resistance also 
occurs in nonobese women with PCOS.111
PCOS is considered

in nonobese women with PCOS.111
PCOS is considered to be a heterogeneous disorder with 
multifactorial causes. PCOS risk is significantly increased with 
a positive family history of chronic anovulation and androgen 
excess, and this complex disorder may be inherited in a polygenic 
fashion.207,208
Historical Perspective
In their pioneering studies, Stein and Leventhal described an asso­
ciation between the presence of bilateral polycystic ovaries and 
signs of amenorrhea, oligomenorrhea, hirsut

s and 
signs of amenorrhea, oligomenorrhea, hirsutism, and obesity (Fig. 
17.27).209 At the time, these signs were strictly adhered to in the 
diagnosis of what was then known as Stein-Leventhal syndrome. 
These investigators also reported the results of bilateral wedge 
resection of the ovaries, in which at least half of each ovary was 
removed as a therapy for PCOS; most of their patients resumed 
menses and achieved pregnancy after ovarian wedge resection. 
The exact mechanism responsible for

ge resection. 
The exact mechanism responsible for the therapeutic effect of 
removal or destruction of part of the ovarian tissue is still not well 
understood.
On the basis of Stein and Leventhal’s original work, a primary 
ovarian defect was inferred. Subsequent clinical, morphologic, 
hormonal, and metabolic studies uncovered multiple underlying 
pathologies, and the term polycystic ovary syndrome was introduced 
to reflect the heterogeneity of this disorder. One of the most sig­
nificant di

of this disorder. One of the most sig­
nificant discoveries regarding the pathophysiology of PCOS was 
the demonstration of a unique form of insulin resistance and asso­
ciated hyperinsulinemia.111,209 
Diagnosis of Polycystic Ovary Syndrome and Laboratory 
Testing
One of the most prominent features of PCOS is the history of 
ovulatory dysfunction (i.e., amenorrhea, oligomenorrhea, or other 
forms of irregular uterine bleeding) of pubertal onset. A clear his­
tory of cyclic predictable menses of

 A clear his­
tory of cyclic predictable menses of menarchal onset makes the 
diagnosis of PCOS unlikely. Acquired insulin resistance associated 
613
with significant weight gain or an unknown cause may induce the 
clinical picture of PCOS in a woman with a history of previously 
normal ovulatory function. Hirsutism may develop prepubertally 
or during adolescence, or it may be absent until the third decade 
of life. Seborrhea

 absent until the third decade 
of life. Seborrhea, acne, and alopecia are other common clinical 
signs of androgen excess. In extreme cases of ovarian hyperthecosis 
(a severe variant of PCOS), clitoromegaly may be observed. None­
theless, a history of rapid progression of androgenic symptoms 
and virilization is unusual. Some women may never have signs of 
androgen excess because of hereditary differences in target tissue 
sensitivity to androgens.115 Infertility related to the anovulation 
ma

ens.115 Infertility related to the anovulation 
may be the only presenting symptom.
During the physical examination, it is essential to search for 
and document signs of androgen excess (hirsutism, virilization, or 
both), insulin resistance (acanthosis nigricans) (Fig. 17.28), and 
the presence of unopposed estrogen action (well-rugated vagina 
and stretchable, clear cervical mucus) to support the diagnosis of 
PCOS. None of these signs is specific for PCOS, and each may 
be associated with any

fic for PCOS, and each may 
be associated with any of the conditions listed in the differential 
diagnosis of PCOS (Table 17.4).
PCOS was previously defined according to the proceedings of 
an expert conference sponsored by the National Institutes of Health 
(NIH) in 1990, which described the disorder as including hyperan­
drogenism or hyperandrogenemia (or both), oligo-ovulation, and 
exclusion of known disorders of androgen excess and anovulation 
(Table 17.5).210,211 Another expert conference

on 
(Table 17.5).210,211 Another expert conference held in Rotterdam 
in 2003 defined PCOS, after the exclusion of related disorders, by 
the presence of two of the following three features: oligo-ovulation 
or anovulation, clinical or biochemical signs of hyperandrogen­
ism (or both), and polycystic ovaries (Fig. 17.29).212 In essence, 
A
C
B
• Fig. 17.27  Polycystic ovaries. (A) Operative findings of classic enlarged polycystic ovaries. The uterus is 
located adjacent to the two enlarged ovari

rus is 
located adjacent to the two enlarged ovaries. (B) Sectioned polycystic ovary with numerous follicles. (C) 
Histologic section of a polycystic ovary with multiple subcapsular follicular cysts and stromal hypertrophy 
at low power (left). At higher power (×100), islands of luteinized theca cells are visible in the stroma (right). 
This morphologic change is called stromal hyperthecosis, and it appears to correlate directly with circu­
lating insulin levels. (C, from Dunaif A. Insulin resis

g insulin levels. (C, from Dunaif A. Insulin resistance and the polycystic ovary syndrome: mechanism 
and implications for pathogenesis. Endocr Rev. 1997;18:774–800. Copyright © 1997 by The Endocrine 
Society.)
614
the Rotterdam 2003 criteria expanded the NIH 1990 definition 
by creating two new phenotypes: ovulatory women with polycys­
tic ovaries plus hyperandrogenism and oligo-anovulatory women 
with polycystic ovaries but without hyperandrogenism.

h polycystic ovaries but without hyperandrogenism. The clinical 
usefulness of including these new groups with respect to increased 
risk of infertility, insulin resistance, and long-term metabolic com­
plications is not clear at this time.213 More recently, the Androgen 
Excess Society published a broad consensus statement that included 
discussions of the merits and disadvantages of the NIH and Rot­
terdam criteria and suggested a practical definition that integrates 
both sets of diagnostic c

inition that integrates 
both sets of diagnostic criteria (see Table 17.5).212
The exclusion of hyperprolactinemia, hypothyroidism, nonclas­
sic congenital adrenal hyperplasia, and tumors requires a careful 
history, physical examination, and laboratory testing, as detailed 
previously (see Table 17.4). Cushing syndrome and glucocorticoid 
resistance may give rise to androgen excess and anovulation after 
a period of normal ovulatory function in teens. An 8 am cortisol 
level after dexamethasone

teens. An 8 am cortisol 
level after dexamethasone (1 mg) administration at midnight is a 
useful screening test for both conditions. Cushing syndrome may 
be recognized by its typical signs, whereas 8 am and 4 pm cortisol 
levels are essential to confirm the diagnosis of glucocorticoid resis­
tance.193 Glucocorticoid resistance is characterized by preserved 
diurnal rhythm despite significantly elevated cortisol, ACTH, and 
adrenal C19 steroid levels and absence of cushingoid symptoms 
and sign

evels and absence of cushingoid symptoms 
and signs.193
Elevated total testosterone is the most direct evidence for 
androgen excess. Various levels of testosterone are found in 
women with PCOS. Rarely, serum testosterone levels higher than 
2 ng/mL may be encountered in association with the most severe 
form of PCOS, ovarian hyperthecosis. Overall, it is much more 
common to observe high-normal levels or borderline elevations of 
testosterone in women with PCOS.
A
B
• Fig. 17.28  Acanthosis ni

n women with PCOS.
A
B
• Fig. 17.28  Acanthosis nigricans. (A) Moderate acanthosis nigricans (i.e., darkening and thickening of 
skin) at the lateral lower fold of the neck. Notice facial hirsutism (sideburns) in the same patient. (B) Severe 
acanthosis nigricans in another patient with severe insulin resistance. (B, courtesy Dr. R. Ann Word, UT 
Southwestern Medical Center, Dallas, TX.)
  Differential Diagnosis of Polycystic Ovary 
Syndrome
Idiopathic hirsutism
Hyperprolactinemia, hypothyroidi

hing syndrome, and glucocorticoid 
resistance. Superscript numbers in table indicate references at the end of the chapter.
AES, Androgen Excess Society; ASRM, American Society for Reproductive Medicine; ESHRE, 
European Society for Human Reproduction and Embryology; NIH, National Institutes of Health.
   
TABLE 17.5
615
Prolactin and TSH concentrations should be measured 
routinely to rule out mild androgen excess and anovulat

nely to rule out mild androgen excess and anovulation 
that may be associated with hyperprolactinemia. If basal LH 
levels are used as a marker for PCOS, a significant number 
of patients will slip through the cracks because they do not 
manifest elevated LH levels or increased LH/FSH ratios. The 
NIH-sponsored consensus conference on diagnostic criteria 
for PCOS in 1990 recommended that LH and the LH/FSH 
ratio are not required for the diagnosis of PCOS.211,214 The 
heterogeneity of LH values 

s of PCOS.211,214 The 
heterogeneity of LH values in PCOS may be caused by the 
pulsatile nature of LH secretion and negative effects of obe­
sity on LH levels. An elevated LH/FSH ratio supports the 
diagnosis of PCOS and may be useful in differentiating mild 
cases of nonobese PCOS without prominent androgen excess 
from hypothalamic anovulation. However, failure to exhibit 
an elevated LH level is of no diagnostic value. By definition, 
nonclassic congenital adrenal hyperplasia does not manife

sic congenital adrenal hyperplasia does not manifest 
as congenital virilization of external genitalia. Hyperandro­
genic symptoms most commonly appear peripubertally or 
postpubertally. The clinical evaluation and laboratory-based 
diagnosis of nonclassic congenital adrenal hyperplasia were 
discussed earlier. Chapter 15 describes the ACTH stimula­
tion test. A screening test for Cushing syndrome or glucocor­
ticoid resistance should be performed as clinically indicated 
(see Chapter 15).
Serum

d as clinically indicated 
Serum DHEAS levels may be increased (up to 8 μg/mL) in 
about 50% of anovulatory women with PCOS. DHEAS origi­
nates almost exclusively from the adrenal gland.215 The cause 
of adrenal hyperactivity in PCOS is unknown. Obtaining a 
DHEAS level routinely in a patient with PCOS is not recom­
mended because it does not change the diagnosis or manage­
ment. If an adrenal tumor is suspected, a DHEAS level should 
be obtained. DHEAS levels higher than 8 μg/

hould 
be obtained. DHEAS levels higher than 8 μg/mL may be associ­
ated with steroidogenically active adrenal tumors, and imaging 
is then indicated.
The Rotterdam 2003 criteria include the use of ultrasound as 
a diagnostic tool. The use of ultrasonography in the diagnosis of 
PCOS must be tempered by an awareness of the broad spectrum 
of women with ultrasonographic findings characteristic of poly­
cystic ovaries. The typical polycystic-appearing ovary may emerge 
in a nonspecific fashion whe

ing ovary may emerge 
in a nonspecific fashion when a state of anovulation of any cause 
persists for any length of time (see Fig. 17.29).216 Thus the poly­
cystic-appearing ovary is the result of a functional derangement 
but not a specific central or local defect.
Biochemical evidence of insulin resistance or glucose intoler­
ance is not necessary for the diagnosis of PCOS. Nonetheless, 
glucose intolerance should be investigated. Plasma glucose levels 
should be measured after a 75-g glucose 

e levels 
should be measured after a 75-g glucose load as a screen for glu­
cose intolerance.
Women with PCOS commonly present with irregular uterine 
bleeding in the form of infrequent periods (i.e., oligomenorrhea) 
or amenorrhea. It is not necessary to document anovulation by 
ultrasonography, progesterone levels, or otherwise, especially if 
menstrual cycles are irregular with periods of amenorrhea. To con­
firm the diagnosis of chronic anovulation and unopposed estrogen 
exposure, most clin

lation and unopposed estrogen 
exposure, most clinicians perform a progestin challenge test after 
a negative urine pregnancy test. Because endometrium is exposed 
to estradiol chronically in PCOS, these women respond to a chal­
lenge with a progestin (e.g., 5 mg/day of MPA given orally for 10 
days) by uterine bleeding within a few days after the last pill of 
progestin. Reasons for lack of uterine bleeding after a progestin 
challenge include pregnancy, insufficient prior estrogen exposure 
of

regnancy, insufficient prior estrogen exposure 
of the endometrium, or an anatomic defect. If uterine bleeding 
does not follow progestin challenge, pregnancy should be ruled 
out again, along with other causes of chronic anovulation, as 
described in this chapter. An anatomic defect such as intrauter­
ine adhesions may be ruled out with a hysterosalpingogram or 
hysteroscopy.
During the initial workup, an endometrial biopsy speci­
men should be obtained with the use of a plastic minisuc­
tion c

obtained with the use of a plastic minisuc­
tion cannula (e.g., Pipelle) in the physician’s office. If chronic 
anovulation persists, endometrial biopsies should be repeated 
periodically. Pregnancy should be ruled out by a urine or 
serum pregnancy test before each biopsy. Response to oral 
contraceptives or periodic progestin treatment with predict­
able withdrawal bleeding episodes is reassuring, and patients 
with predictable bleeding patterns do not need endometrial 
sampling during these t

s do not need endometrial 
sampling during these treatments. In untreated patients, the 
risk of endometrial hyperplasia and malignancy is significantly 
increased even in young women with PCOS because of unop­
posed estrogen exposure. 
Gonadotropin Production in Polycystic Ovary Syndrome
Women with PCOS have higher mean concentrations of LH but 
low or low-normal levels of FSH compared with levels found in 
normal women in the early follicular phase.217 The elevated LH 
levels in PCOS are presu

hase.217 The elevated LH 
levels in PCOS are presumed to be primarily caused by accelerated 
GnRH-LH pulsatile activity (Fig. 17.30).218–220 Central opioid 
tone appears to be suppressed because the pattern of LH secre­
tion does not change in response to naloxone.221 The enhanced 
pulsatile secretion of GnRH has been attributed to a reduction 
in hypothalamic opioid inhibition caused by the chronic absence 
of progesterone.189 An increase in amplitude and frequency of 
LH secretion also correla

litude and frequency of 
LH secretion also correlates with steady-state levels of circulating 
estrogen.
In obese women with PCOS, LH levels may not be increased. 
The increase in LH pulse frequency is characteristic of the anovu­
latory state regardless of body fat content.222 LH pulse amplitude, 
however, is comparatively normal in overweight women with 
PCOS, whereas it is increased in nonobese women with PCOS.223 
The overall LH reduction in obese women with PCOS may result 
from factors oth

obese women with PCOS may result 
from factors other than changes in LH pulse amplitude.224 A low 
• Fig. 17.29  Transvaginal ultrasound image of a polycystic ovary. Notice the 
multiple, midsized follicles in the periphery and the increased solid area in 
the middle.
616
LH value does not rule out the diagnosis of PCOS, whereas a high 
LH-FSH ratio supports this diagnosis in an anovulatory woman.
Insulin has been implicated as a potential regulator o

lin has been implicated as a potential regulator of LH 
secretion in PCOS. Insulin enhances the transcription of the LHβ 
gene (LHB).225,226 This laboratory observation was supported by 
an in vivo human study showing that insulin infusion suppresses 
pituitary response to GnRH in normal women and in women 
with PCOS.227 These studies support the concept that insulin 
resistance or hyperinsulinemia may be responsible for abnormal 
gonadotropin release (see Fig. 17.30). 
Steroid Production in Pol

lease (see Fig. 17.30). 
Steroid Production in Polycystic Ovary Syndrome
Ovulatory cycles are characterized by cyclic fluctuating hor­
mone levels that regulate ovulation and menses (see Fig. 17.1A). 
Anovulation in women with PCOS is associated with steady-state 
levels of gonadotropins and ovarian steroids. In patients with 
persistent anovulation, the average daily production of estrogen 
and androgens is increased and depends on LH stimulation (see 
Fig. 17.30).228 This is reflected in highe

n (see 
Fig. 17.30).228 This is reflected in higher circulating levels of 
testosterone, androstenedione, DHEA, DHEAS, 17-hydroxy­
progesterone, and estrone.229 Testosterone, androstenedione, 
and DHEA are secreted directly by the ovary, whereas DHEAS, 
which is elevated in about 50% of anovulatory women with 
PCOS, is almost exclusively an adrenal contribution.215 Circu­
lating levels of androstenedione, secreted by polycystic ovaries, 
are particularly high.
Local conversion of steroid precurs

icularly high.
Local conversion of steroid precursors to estradiol is an impor­
tant physiologic process for certain estrogen target tissues, such as 
normal breast and genital skin, and can also promote the growth 
of pathologic estrogen-dependent tissues, such as endometrial 
or breast cancer (Fig. 17.31).97,105,229–231 Androstenedione of 
ovarian origin is the most strikingly elevated steroid in PCOS. 
Androstenedione is not biologically active but serves as a dual pre­
cursor for androgen (i

e but serves as a dual pre­
cursor for androgen (i.e., testosterone that is further converted to 
the biologically far stronger androgen DHT) and estrogen (i.e., 
estrone that is further converted to biologically active estradiol in 
target tissues) (see Fig. 17.31). Increased androstenedione of ovar­
ian origin in women with PCOS gives rise to circulating levels of 
estradiol higher than those measured during the first few days of 
an ovulatory cycle.
Estradiol is an extremely potent steroid. B

cycle.
Estradiol is an extremely potent steroid. Biologically effec­
tive circulating levels of estradiol are measured in units of pico­
grams per milliliter (pg/mL) or picomoles per liter (pmol/L); 
biologically effective levels of testosterone are measured in 
units of nanograms per milliliter (ng/mL) or nanomoles per 
liter (nmol/L) and circulate at 10 to 100 times the physiologic 
levels of estradiol. Even small rates of conversion of andro­
stenedione to estrone may have a significant biolo

stenedione to estrone may have a significant biologic impact, 
whereas markedly elevated production of androstenedione is 
required to produce significant amounts of testosterone and 
manifestations of androgen excess (see Fig. 17.31). Because 
elevated production of androstenedione does occur in PCOS, 
SHBG
IGFBP1
Ovary
DHEAS
A
A
A
Skin
Adipose
tissue
IGFs
Insulin
LH
T
E1
E2
DHT
Peripheral and target tissues
Aromatase
Altered sex steroid levels
Abnormal GnRH
secretion

secretion
Pituitary
LH
FSH
FSH
Insulin   
Bioavailable E2
Bioavailable T
Bioavailable IGFI
T
Liver
17  HSD
β
17  HSD
β
5  -red
α
• Fig. 17.30  Pathologic mechanisms in polycystic ovary syndrome (PCOS). A deficient in vivo response of 
the ovarian follicle to physiologic quantities of follicle-stimulating hormone (FSH), possibly because of an 
impaired interaction between signaling pathways associated with FSH and insulin-like growth fac

ys associated with FSH and insulin-like growth factors (IGFs) 
or insulin, may be an important defect responsible for anovulation in PCOS. Insulin resistance associated with 
increased circulating and tissue levels of insulin and bioavailable estradiol (E2), testosterone (T), and IGF1 gives 
rise to abnormal hormone production in a number of tissues. Oversecretion of luteinizing hormone (LH) and 
decreased output of FSH by the pituitary, decreased production of sex hormone–binding globulin (SHBG

d production of sex hormone–binding globulin (SHBG) 
and IGF-binding protein 1 (IGFBP1) in the liver, increased adrenal secretion of dehydroepiandrosterone sul­
fate (DHEAS), and increased ovarian secretion of androstenedione (A) all contribute to the feed-forward cycle 
that maintains anovulation and androgen excess in PCOS. Excessive amounts of E2 and T arise primarily 
from the conversion of A in peripheral and target tissues. T is converted to the potent steroids estradiol or 
DHT (dihydrote

o the potent steroids estradiol or 
DHT (dihydrotestosterone). Reductive 17β-hydroxysteroid dehydrogenase (17βHSD) enzyme activity may be 
conferred by protein products of several genes with overlapping functions; 5α-reductase (5α-red) is encoded 
by at least two genes, and aromatase is encoded by a single gene. GnRH, gonadotropin-releasing hormone.
617
extraovarian production of testosterone is biologically sig­
nificant in t

of testosterone is biologically sig­
nificant in this disease. In postmenopausal women, who have 
much lower levels of androstenedione, extraovarian production 
of testosterone is less important. Relatively small quantities of 
estrone (and estradiol) produced primarily by peripheral aro­
matization of androstenedione have a biologic impact in men 
and postmenopausal women. 
Production of Sex Hormone–Binding Globulin in 
SHBG binds testosterone and estradiol and decreas

SHBG binds testosterone and estradiol and decreases their biologic 
activities. In PCOS, the net production of androgen and estrogen 
is increased. Amplified estrogenic and androgenic effects in PCOS 
also are caused by a decreased SHBG concentration, giving rise to 
increased free or biologically active circulating levels of estradiol 
and testosterone (see Fig. 17.30). The levels of SHBG are con­
trolled by a balance of hormonal influences on its synthesis in the 
liver. Testosterone and insu

its synthesis in the 
liver. Testosterone and insulin inhibit, and estrogen and thyroid 
hormone stimulate, SHBG formation.232 In anovulatory women 
with PCOS, circulating levels of SHBG are reduced by approxi­
mately 50%; this effect may be a hepatic response to increased 
circulating levels of testosterone and insulin (see Fig. 17.30).232
Testosterone decreases serum SHBG levels, giving rise to a 
vicious feedback circle favoring low SHBG and high bioavail­
able testosterone levels (see Fig. 1

igh bioavail­
able testosterone levels (see Fig. 17.30). Insulin directly decreases 
serum SHBG concentrations in women with PCOS independent 
of any action of sex steroids.232 Insulin increases free testosterone 
levels in PCOS by two mechanisms: increasing ovarian secretion 
of testosterone precursors (e.g., androstenedione) and suppressing 
SHBG.232 
Follicular Fate in Polycystic Ovary Syndrome
Under the influence of relatively low but constant levels of FSH, 
follicular growth is continuousl

t levels of FSH, 
follicular growth is continuously stimulated, although not to the 
point of full maturation and ovulation.233 Although full growth 
potential is not realized, the follicular life span may be extended 
by several months in the form of multiple follicular cysts. Most 
of these follicles in polycystic ovaries are 2 to 10 mm in diameter, 
and some can be as large as 20 mm. Hyperplastic theca cells, often 
luteinized in response to the high LH levels, surround these fol­
licles (see

he high LH levels, surround these fol­
licles (see Fig. 17.27). The accumulation of follicles arrested at 
various stages of development allows increased and relatively con­
stant production of steroids in response to steady-state levels of 
gonadotropins.
These follicles are subject to atresia and are replaced by new 
follicles of similar limited growth potential. Steady-state turnover 
of stromal cells contributes to the stromal compartment of the 
ovary, and it is sustained by tissue derived 

the 
ovary, and it is sustained by tissue derived from follicular atresia. 
A degenerating granulosa compartment, leaving the theca cells to 
contribute to the stromal compartment of the ovary, accompanies 
atresia (see Fig. 17.27). This functioning stromal tissue secretes 
significant amounts of androstenedione under the influence of 
increased LH. Androstenedione, by the mechanisms discussed 
previously, leads to increases in free testosterone and free estra­
diol levels and decreases in SHBG 

and free estra­
diol levels and decreases in SHBG concentrations (see Fig. 17.30). 
From the point of view of steroidogenesis and steroid action, 
PCOS is the result of a complex vicious circle that includes a 
number of positive and negative feedback mechanisms.
Fig. 17.30 summarizes the postulated mechanisms underlying 
PCOS. Because FSH, insulin, and IGF pathways can synergize, it 
was postulated that this synergy does not occur in the presence of 
insulin resistance and might lead to relativ

e of 
insulin resistance and might lead to relative resistance of the ovar­
ian follicle to FSH. However, in vitro studies provide mixed results 
with respect to this hypothesis. For example, cultured granulosa 
cells obtained from the small follicles of polycystic ovaries pro­
duce negligible amounts of estradiol but show a dramatic increase 
in estrogen production when FSH or IGF1 is added to the cul­
ture medium. When FSH and IGF1 were added together in vitro, 
they synergized to increase est

ogether in vitro, 
they synergized to increase estrogen biosynthesis in granulosa cells 
from polycystic ovaries.234
Induction of ovulation in PCOS is achieved by increasing FSH 
levels that are hypothesized to overcome this postulated in vivo block 
to FSH at the granulosa cell level. Two popular treatments, oral clomi­
phene citrate and injectable recombinant FSH, can provide increased 
levels of endogenous or exogenous FSH that may lead to ovulation 
at various doses. Some PCOS patients requi

lation 
at various doses. Some PCOS patients require large doses of clomi­
phene citrate or FSH to achieve ovulation. Paradoxically, the polycys­
tic ovary may overreact to pharmacologic levels of FSH by recruiting 
a large number of developing follicles at once; this occasionally gives 
rise to the ovarian hyperstimulation syndrome (OHSS) (discussed 
later).235 The therapeutic window between ovarian nonresponsive­
ness and hyperreactivity is usually narrow. There are significant gaps 
of knowle

ally narrow. There are significant gaps 
of knowledge that do not permit reconciliation of clinical postulates 
derived from in vitro or in vivo studies of PCOS. 
Ovarian Hyperthecosis
Ovarian hyperthecosis is a severe variant of PCOS. The term refers 
to significantly increased stromal tissue with luteinized theca-like 
cells scattered throughout large sheets of fibroblast-like cells. The 
clinical and histologic findings and pathophysiology represent an 
exaggerated version of PCOS.236 This di

resent an 
exaggerated version of PCOS.236 This diagnosis can be made on 
clinical grounds; an ovarian biopsy is not necessary except to rule 
out an ovarian tumor.
(circulating
precursor)
Dihydro-
testosterone
and local
(target tissue)
conversion
and local
Estradiol
Estrone
Ovary
5%
1.3%
• Fig. 17.31  Extraovarian conversion of androstenedione to androgen and 
estrogen. Androstenedione of adrenal or ovar

 and 
estrogen. Androstenedione of adrenal or ovarian origin, or both, acts as 
a dual precursor for androgen and estrogen. Approximately 5% of cir­
culating androstenedione is converted to circulating testosterone, and 
approximately 1.3% of circulating androstenedione is converted to circu­
lating estrone in peripheral tissues. Testosterone and estrone are further 
converted to biologically potent steroids, dihydrotestosterone and estra­
diol, in peripheral and target tissues. Biologically act

in peripheral and target tissues. Biologically active amounts of estra­
diol in serum are measured in picograms or picomoles per milliliter (pg/
mL or pmol/L), whereas biologically active levels of testosterone in serum 
are measured in nanograms or nanomoles per milliliter (ng/mL or nmol/L). 
The 1.3% conversion of normal quantities of androstenedione to estrone 
may have a critical biologic impact in settings such as postmenopausal 
endometrial or breast cancer. Significant androgen excess is 

 or breast cancer. Significant androgen excess is observed 
in conditions with abnormally increased androstenedione formation (e.g., 
polycystic ovary syndrome).
618
Increased androgen production leads to the clinical picture of 
more intense androgenization. The higher testosterone levels may 
also lower LH levels by blocking estrogen action at the hypotha­
lamic-pituitary level.224 The severity of hyperthecosis correlates 
with the degree of insulin

perthecosis correlates 
with the degree of insulin resistance.224 Because insulin and IGF1 
stimulate proliferation of thecal interstitial cells, hyperinsulinemia 
may be an important pathophysiologic factor in the cause of 
hyperthecosis.
It is not uncommon to encounter markedly high levels of tes­
tosterone, even above 2 ng/mL, in patients with ovarian hyper­
thecosis. Virilization is common. These patients do not usually 
ovulate in response to clomiphene or recombinant FSH. It is 
usually di

o clomiphene or recombinant FSH. It is 
usually difficult to suppress testosterone production, even using a 
GnRH analogue. Bilateral oophorectomy should be a last resort, 
but it may be necessary to control testosterone production in some 
of these patients. 
Genetics of Polycystic Ovary Syndrome
The strong trend of PCOS to aggregate in families suggests an 
underlying genetic basis.237,238 Some key clinical features of PCOS 
are genetically transmitted. In particular, there is familial aggrega

nsmitted. In particular, there is familial aggrega­
tion of hyperandrogenemia (with or without oligomenorrhea) in 
PCOS kindreds, suggesting that it is a genetic trait.208 Another 
study showed that hyperinsulinism may be a familial characteristic 
in daughters of women with PCOS.239
The PCOS phenotype is a consequence of genes and environ­
ment. For example, obesity associated with unhealthy lifestyle 
choices aggravates the PCOS phenotype in genetically susceptible 
women. The variable PCOS ph

netically susceptible 
women. The variable PCOS phenotypes add further challenge to 
genetic studies of PCOS. Several genomic loci have been proposed 
to account for the PCOS phenotype. These include CYP11A1, 
the insulin gene, and the follistatin gene; however, no convinc­
ing evidence regarding any of these loci has been published.240 
Although independent studies identified a dinucleotide repeat 
marker near the insulin receptor gene that maps to chromosome 
19p13.2, the particular PCOS gene 

 to chromosome 
19p13.2, the particular PCOS gene in this locus has not been 
isolated.241 In the Han Chinese population, genome-wide PCOS 
association signals showed evidence of enrichment for candidate 
genes related to insulin signaling (INSR), gonadotropin recep­
tors (FSHR, LHCGR), and type 2 diabetes (HMGA2, THADA, 
DENND1A).242 
Insulin Resistance and Polycystic Ovary Syndrome
Insulin resistance is a major factor in the pathogenesis of non–
insulin-dependent diabetes mellitus (NIDDM). The

–
insulin-dependent diabetes mellitus (NIDDM). The term insulin 
resistance can be defined as impaired whole-body insulin-mediated 
glucose disposal, as determined with the use of techniques such as 
the hyperinsulinemic glucose clamp technique.111 Insulin resis­
tance is defined clinically as the inability of a known quantity of 
exogenous or endogenous insulin to increase glucose uptake and 
use in an affected individual as much as it does in a normal per­
son. Insulin resistance is frequently

 normal per­
son. Insulin resistance is frequently observed in lean and obese 
women with PCOS. More severe degrees of insulin resistance or 
impaired glucose tolerance are more common in obese women 
with PCOS.111
Androgen excess and insulin resistance are often associated 
with acanthosis nigricans. Acanthosis nigricans is a gray-brown, 
velvety discoloration and increased thickness of the skin, usually 
seen at the neck, groin, and axillae and under the breasts; it is a 
marker for insulin re

 under the breasts; it is a 
marker for insulin resistance (see Fig. 17.28). Hyperkeratosis and 
papillomatosis are the histologic characteristics of acanthosis nigri­
cans. Acanthosis nigricans in hyperandrogenic women depends 
on the presence and severity of hyperinsulinemia and insulin 
resistance.243 The mechanism responsible for the development of 
acanthosis nigricans is uncertain. This abnormal growth response 
of the skin may be mediated through receptors for various growth 
factors, inc

through receptors for various growth 
factors, including those for insulin and IGF1. Acanthosis nigri­
cans can also be observed in the absence of insulin resistance or 
androgen excess.
Insulin resistance is characterized by an impaired glucose 
response to a specific amount of insulin. In many of these patients, 
normal glucose levels are maintained at the expense of increased 
circulating insulin to overcome the underlying defect. More 
severe forms of insulin resistance in PCOS range from im

 forms of insulin resistance in PCOS range from impaired 
glucose tolerance to frank NIDDM. Resistance to insulin-stimu­
lated glucose uptake is a relatively common phenomenon in the 
general population and is sometimes referred to as syndrome X 
or metabolic syndrome. The fundamental abnormality leading to 
the manifestations that make up the metabolic syndrome is resis­
tance to insulin-mediated glucose uptake in muscle and increased 
lipolysis, which produces elevated levels of circulating fr

, which produces elevated levels of circulating free fatty 
acids.244 These individuals also have dyslipidemia, hypertension, 
and increased risk of developing CVD. Not surprisingly, the 
incidences of dyslipidemia and cardiovascular risk are increased 
significantly in women with PCOS.245,246 The incidence of hyper­
tension increases significantly after menopause in women with a 
history of PCOS.111 There is a significant clinical and pathologic 
overlap between the metabolic syndrome and PCOS.

overlap between the metabolic syndrome and PCOS.247
The clinical presentation of patients with insulin resistance 
depends on the ability of the pancreas to compensate for the target 
tissue resistance to insulin. During the first stages of this condition, 
compensation is effective, and the only metabolic abnormality is 
hyperinsulinemia. In many patients, the beta cells of the pancreas 
eventually fail to meet the challenge, and declining insulin levels 
lead to impaired glucose tolerance an

ulin levels 
lead to impaired glucose tolerance and eventually to frank diabe­
tes mellitus. Beta-cell dysfunction is demonstrable in women with 
PCOS before the onset of glucose intolerance.248
Studies of well-characterized causes of hyperinsulinemia and 
androgen excess have illuminated various mechanisms of insulin 
resistance. Factors such as a decrease in insulin binding related 
to autoantibodies to insulin receptors, postreceptor defects, and a 
decrease in insulin receptor sites in targe

and a 
decrease in insulin receptor sites in target tissues are all involved 
in insulin resistance.249 These rare syndromes are found in an 
extremely small portion of women with anovulation, androgen 
excess, and insulin resistance, leaving most PCOS patients with­
out any demonstrable abnormalities in the number or quality of 
receptors or in antibody formation. The exact nature of insulin 
resistance in most women with PCOS is not well understood.
To understand the molecular defect underlyin

tood.
To understand the molecular defect underlying insulin resis­
tance in PCOS, Dunaif and coworkers studied the differences 
between skin fibroblasts from women with and without PCOS 
with respect to insulin-dependent signal transduction.111 The 
fibroblasts of women with PCOS showed no change in insu­
lin binding or receptor affinity, but a postreceptor defect was 
observed in one-half of the women with PCOS.111 This defect 
is characterized by increased basal insulin receptor serine phos­
p

by increased basal insulin receptor serine phos­
phorylation and decreased insulin-dependent tyrosine phosphory­
lation of the insulin receptor.111 At about the same time, Miller 
and coworkers evaluated whether post-translational modification 
of the product of the CYP17A1 gene alters the ratio of its hydroxy­
lase to lyase activity; they found that serine phosphorylation of 
CYP17A1 dramatically increases the enzyme’s 17,20-lyase but not 
its 17α-hydroxylase activity. These observations led th

7α-hydroxylase activity. These observations led the investiga­
tors to hypothesize that a dominantly inherited aberrant kinase 
619
activity phosphorylates serine residues at the insulin receptor-β 
and CYP17A1 product, leading to insulin resistance and increased 
androgen production, respectively. The cause of this abnormal 
phosphorylation pattern and consequences for insulin action and 
androgen production are important top

 action and 
androgen production are important topics for further study.250
Within the context of a unified hypothesis, insulin resistance 
seems to be a critical defect that explains most of the endocrine 
abnormalities observed in PCOS (see Fig. 17.30). Insulin resis­
tance is associated with abnormal responses of the ovarian fol­
licle to FSH, which lead to anovulation and androgen secretion. 
This results in noncyclic formation of estrogen from androgens in 
peripheral tissues. Estradiol tog

om androgens in 
peripheral tissues. Estradiol together with elevated androgen and 
insulin levels gives rise to abnormal gonadotropin secretion. This 
creates an anovulatory state favoring continuous excess of LH, ste­
roid precursors, androgen, and estrogen (see Fig. 17.30).
Role of Obesity in Insulin Resistance and Anovulation
Increased waist-to-hip ratio compounded by significantly 
increased body mass index is called android obesity because this 
type of adipose tissue distribution is obser

this 
type of adipose tissue distribution is observed more commonly 
in men. Overweight women with anovulatory androgen excess 
commonly have this particular body fat distribution.251 Android 
obesity is the result of fat deposition in the abdominal wall and in 
visceral mesenteric locations. This fat is more sensitive to catechol­
amines, less sensitive to insulin, and more active metabolically. 
Android obesity is associated with insulin resistance, glucose intol­
erance, diabetes mellitus, an

ance, glucose intol­
erance, diabetes mellitus, and an increase in androgen production 
rate and results in decreased levels of SHBG and increased levels 
of free testosterone and estradiol.251 Android obesity is associated 
significantly with cardiovascular risk factors, including hyperten­
sion and dyslipidemia, and it has been tied to a notable increase in 
the risk of poor-prognosis breast cancer.252,253 However, no direct 
association has been reported between PCOS and breast cancer 
risk.2

en reported between PCOS and breast cancer 
risk.254
Although the combination of insulin resistance and andro­
gen excess is often observed in obese women overall, women 
with android-type obesity appear to be at a significantly higher 
risk for insulin resistance and androgen excess. However, insulin 
resistance and androgen excess are not confined to obese anovula­
tory women but also occur in nonobese anovulatory women.222 
Although obesity by itself causes insulin resistance, the combina­
ti

 itself causes insulin resistance, the combina­
tion of insulin resistance and androgen excess is a specific feature 
of PCOS. Not surprisingly, the combination of obesity and PCOS 
is associated with more severe degrees of insulin resistance than 
those found in nonobese women with PCOS.222,255 Android-type 
obesity, in contrast to general obesity, is a more specific risk factor 
for PCOS. 
Laboratory Evaluation of Metabolic Syndrome in PCOS
In everyday clinical practice, the criteria for diagn

everyday clinical practice, the criteria for diagnosing insulin 
resistance in an individual patient have not been standardized and 
present extremely complex issues. One-fourth of the normal pop­
ulation has fasting and glucose-stimulated insulin levels that over­
lap with those of insulin-resistant individuals because of the great 
variability of insulin sensitivity in normal subjects.111 Clinically 
available measures of insulin action, such as fasting or glucose-
stimulated insulin levels, d

s fasting or glucose-
stimulated insulin levels, do not correlate well with more detailed 
measurements of insulin sensitivity in research settings.
In view of these constraints, it is reasonable to consider all 
women with PCOS at risk for insulin resistance and the associated 
abnormalities of the metabolic syndrome—dyslipidemia, hyper­
tension, and CVD.247 A lipid profile should be obtained in all 
cases of PCOS. Especially for obese women with PCOS, fasting 
glucose levels and glucose levels

h PCOS, fasting 
glucose levels and glucose levels 2 hours after a 75-g glucose load 
should be obtained as a screen for glucose intolerance. The clini­
cian should encourage the patient to take every possible measure 
(e.g., weight reduction, exercise) to reduce insulin resistance. 
Use of Antidiabetic Drugs to Treat Anovulation and Androgen 
Excess
A logical approach to the management of PCOS includes using 
medications that improve insulin sensitivity in target tissues, 
achieving reductions 

sitivity in target tissues, 
achieving reductions in insulin secretion, and stabilizing glucose 
tolerance. Antidiabetic medications in the classes of biguanides 
(metformin) and thiazolidinediones (pioglitazone and rosigli­
tazone) have been used to reduce insulin resistance. Although 
metformin appears to influence ovarian steroidogenesis directly, 
this effect does not appear to be primarily responsible for the 
attenuation of ovarian androgen production in women with 
PCOS. Rather, metformin

 production in women with 
PCOS. Rather, metformin inhibits the output of hepatic glucose, 
necessitating a lower insulin concentration and thereby probably 
reducing androgen production by theca cells.256
Metformin at a dose of 500 mg three times daily reduced 
hyperinsulinemia, basal and stimulated levels of LH, and free tes­
tosterone concentrations in overweight women with PCOS.257,258 
Some anovulatory women ovulated and achieved pregnancy; how­
ever, clomiphene is superior to metformin in 

how­
ever, clomiphene is superior to metformin in achieving live births 
in infertile women with PCOS.259–261 Among published studies 
of metformin use in women with PCOS, subject characteristics 
and control measures for effects of weight change, dose of metfor­
min, and outcome vary widely. A meta-analysis of 13 studies in 
which metformin was administered to 543 participants reported 
that patients taking metformin had an odds ratio for ovulation 
of 3.88 (95% confidence interval [CI], 2.25–6

ion 
of 3.88 (95% confidence interval [CI], 2.25–6.69) compared 
with placebo and an odds ratio for ovulation of 4.41 (95% CI, 
2.37–8.22) for metformin plus clomiphene compared with clo­
miphene alone.262 Although the addition of metformin to clomi­
phene seems to increase the ovulation rate, it does not result in 
a higher rate of live births.260 Metformin also improved fasting 
insulin levels, blood pressure, and levels of LDL cholesterol. These 
effects were judged to be independent of any c

se 
effects were judged to be independent of any changes in weight 
that were associated with metformin treatment, but controversy 
persists about whether the beneficial effects of metformin are 
entirely independent of the weight loss that is typically seen early 
in the course of therapy.261,262
The thiazolidinediones are pharmacologic ligands for the 
nuclear receptor PPARγ. They improve the action of insulin in the 
liver, skeletal muscle, and adipose tissue and have only a modest 
effect on

d adipose tissue and have only a modest 
effect on hepatic glucose output. As with metformin, the thiazoli­
dinediones are reported to affect ovarian steroid synthesis directly, 
although most evidence indicates that the reduction in insulin 
levels is responsible for decreased concentrations of circulating 
androgen.256
Women with PCOS who took troglitazone had consistent 
improvements in insulin resistance, pancreatic beta-cell func­
tion, hyperandrogenemia, and glucose tolerance.263,264 In a 

randrogenemia, and glucose tolerance.263,264 In a dou­
ble-blind, randomized, placebo-controlled study, ovulation was 
significantly greater for women with PCOS who received trogli­
tazone than for those who received placebo; free testosterone lev­
els decreased, and levels of SHBG increased in a dose-dependent 
fashion.265 Although troglitazone is no longer available because 
of its hepatotoxicity, subsequent studies using rosiglitazone and 
pioglitazone had similar results.266–268 Because of c

glitazone had similar results.266–268 Because of concern about 
the use of thiazolidinediones in pregnancy and recent evidence 
620
linking these drugs to serious side effects such as heart failure and 
stroke, they have been less readily adopted for routine treatment 
of PCOS. The success of the strategy of reversing insulin resistance 
as a way to correct the critical abnormalities in PCOS argues for 
this defect as central to the pathogenesis of th

this defect as central to the pathogenesis of the disorder. 
Management of Long-Term Deleterious Effects of 
The long-term consequences of PCOS include irregular uterine 
bleeding, anovulatory infertility, androgen excess, chronically 
elevated levels of free estrogen associated with an increased risk 
of endometrial cancer, and insulin resistance associated with an 
increased risk of CVD and diabetes mellitus. Treatment must aid 
in achieving a healthy lifestyle and 

nt must aid 
in achieving a healthy lifestyle and normal body weight, protect 
the endometrium from the effects of unopposed estrogen, and 
reduce testosterone levels.
Any woman with PCOS should be counseled to maintain 
a healthy lifestyle. In obese PCOS women, permanent lifestyle 
modification should be emphasized as the primary preven­
tive measure to minimize short-term and long-term deleterious 
effects. Simple measures such as decreasing daily food intake by 
500 kcal and introducing any t

ily food intake by 
500 kcal and introducing any type of moderate exercise for 30 
minutes daily for 6 months can decrease hyperandrogenemia and 
diastolic blood pressure.204 Because insulin resistance contributes 
to the abnormal lipid profile and increased cardiovascular risk 
in women with PCOS, weight loss is a high priority for patients 
who are overweight.269 Insulin resistance and androgen excess can 
be reduced with a weight reduction of at least 5%.270,271 Signifi­
cant weight loss has 

at least 5%.270,271 Signifi­
cant weight loss has also resulted in ovulation and pregnancy in 
a number of patients with PCOS.272 Nutritional counseling and 
an emphasis on lifestyle changes are essential components of the 
long-term management of PCOS.
If the patient does not wish to become pregnant, medical 
therapy is directed toward the interruption of the effect of unop­
posed estrogen on the endometrium. Nonfluctuating levels of 
unopposed estradiol in the absence of progesterone cause irr

estradiol in the absence of progesterone cause irregular 
uterine bleeding, amenorrhea, and infertility and increase the risk 
of endometrial cancer. Anovulatory women with PCOS may have 
endometrial cancer even in their early 20s.273 Endometrial biopsy 
should be performed periodically in untreated women with PCOS 
regardless of age. Pregnancy should be ruled out before each endo­
metrial biopsy. The uterine bleeding pattern should not influence 
the decision to perform an endometrial biopsy. T

the decision to perform an endometrial biopsy. The presence of 
amenorrhea does not rule out endometrial hyperplasia. The criti­
cal factor that determines the risk of endometrial neoplasia is the 
duration of anovulation and exposure to unopposed estradiol. 
Long-term treatment with a progestin or oral contraceptive sig­
nificantly decreases the risk of endometrial cancer.
One of the simplest and most effective ways to administer a 
progestin in the long term is to use an oral contraceptive. 

in the long term is to use an oral contraceptive. Oral 
contraceptives provide two additional benefits: reduction of 
androgen excess and contraception. Oral contraceptive pills reduce 
circulating androgen levels through suppression of circulating LH 
and stimulation of SHBG levels, and they have been shown to 
reduce hirsutism.152 Oral contraceptive treatment for anovulation 
and hyperinsulinemia in women with androgen excess does not 
increase cardiovascular risk.274
For the patient who does 

 cardiovascular risk.274
For the patient who does not complain of hirsutism but is 
anovulatory and has irregular bleeding, treatment with progestin 
alone may be attempted as an alternative to oral contraceptive use. 
Progestin therapy is directed toward interruption of the chronic 
exposure of endometrium to unopposed effects of estrogen. MPA 
may be administered intermittently (e.g., 5 mg daily for the first 
10 days of every other month) to ensure withdrawal bleeding and 
prevent endometrial

nsure withdrawal bleeding and 
prevent endometrial hyperplasia. This treatment does not decrease 
androgen excess, nor does it provide contraception. Because an 
oral contraceptive with an ethinyl estradiol content of 30 μg or 
less can suppress androgen excess of ovarian origin, provide con­
traception, and protect the endometrium and does not increase 
insulin resistance, a low-dose oral contraceptive is the treatment 
of choice for nonsmokers with PCOS. An oral contraceptive 
together with th

with PCOS. An oral contraceptive 
together with the antiandrogen spironolactone (100 mg/day) is 
the recommended starting treatment for a hirsute woman with 
PCOS. The dose of spironolactone can be increased in increments 
to suppress hair growth, as described earlier.
Treatment with an oral contraceptive, with or without spirono­
lactone, may not be effective in androgen suppression in severe 
cases of PCOS. In patients resistant to oral contraceptives, suppres­
sion of the ovary using a depot 

eptives, suppres­
sion of the ovary using a depot GnRH agonist may be required. 
Spironolactone does not affect insulin sensitivity in anovulatory 
women and can be used safely without causing adverse effects on 
carbohydrate or lipid metabolism.275
The clinician must counsel women with PCOS regarding 
their increased risk of future diabetes mellitus. The age at onset 
of NIDDM is significantly earlier in these women than in the 
general population.206 Women with PCOS are more likely to 
experie

on.206 Women with PCOS are more likely to 
experience gestational diabetes.276 Long-term follow-up studies 
have shown a significantly increased risk for frank diabetes mel­
litus in anovulatory patients with PCOS.111 It is therefore impor­
tant to monitor glucose tolerance with periodic measurements of 
glucose levels after fasting and after a 75-g glucose load. The place 
of metformin in the long-term treatment of PCOS remains to be 
determined.261,277
The physician should alert the patient wi

.261,277
The physician should alert the patient with PCOS that up to 
one-half of first-degree relatives and sisters may be affected by 
PCOS or at least by androgen excess in the presence of regular 
menses.208 These individuals may be at higher than average risk 
for CVD and may benefit from preventive measures that reduce 
this risk. 
Ovulation Induction in Polycystic Ovary Syndrome
When pregnancy is achieved, patients with PCOS appear to have 
an increased risk of spontaneous miscarriage.278

an increased risk of spontaneous miscarriage.278 This increased 
risk may be related to elevated levels of LH that may produce an 
adverse environment for the oocyte and the endometrium. LH 
levels should be suppressed with oral contraceptives before ovula­
tion is induced. Suppression can be achieved in most patients with 
PCOS by the use of an oral contraceptive for 4 to 6 weeks before 
ovulation induction with clomiphene citrate, an aromatase inhibi­
tor, or recombinant FSH.
To induce ovula

e inhibi­
To induce ovulation in women with PCOS, FSH levels may 
be increased by oral administration of an antiestrogen (clomi­
phene citrate) or an aromatase inhibitor (letrozole, anastrozole) 
or by subcutaneous injection of recombinant FSH. Presumably, 
pharmacologic levels of FSH overcome the ovarian defect that is 
responsible for anovulation in PCOS.
Clomiphene Citrate
Clomiphene citrate is a nonsteroidal, ovulation-inducing ER ligand 
with mixed agonistic-antagon

n-inducing ER ligand 
with mixed agonistic-antagonistic properties.279 Acting as an anti­
estrogen, clomiphene citrate is thought to displace endogenous 
estrogen from hypothalamic ERs, thereby removing the negative 
feedback effect exerted by endogenous estrogens. The resultant 
change in pulsatile GnRH release is thought to normalize the release 
621
of pituitary FSH and LH, which is followed by follicular recruit­
ment and 

which is followed by follicular recruit­
ment and selection, assertion of dominance, and ovulation.279
Clomiphene citrate treatment can be started at any time to 
induce ovulation in an amenorrheic and anovulatory patient pro­
vided that a pregnancy test is performed beforehand (Fig. 17.32). 
Alternatively, uterine bleeding may be induced after a 10-day treat­
ment with a combination oral contraceptive or MPA (5 mg/day). 
On cycle day 2 or 3 (day 1 is the first day of uterine bleeding), a 
basel

 1 is the first day of uterine bleeding), a 
baseline ultrasound study is performed to rule out any ovarian fol­
licular cyst of more than 25-mm average diameter. If one or more 
large cysts are seen, ovulation induction should be delayed until 
after gonadotropin suppression by continuous oral contraception 
treatment for 4 to 6 weeks to eliminate these cysts or decrease 
their size. Clomiphene citrate is started at 50 mg/day orally on 
day 3 of the cycle and continued for 5 days. Ultrasonograp

 the cycle and continued for 5 days. Ultrasonography is 
performed on cycle day 13 or 14 to ensure follicular development 
(i.e., at least one new follicle measuring at least 16 mm in diam­
eter). The patient should be encouraged to have intercourse every 
other day during the 10-day period following the last clomiphene 
citrate dose. Alternatively, measurement of urinary LH to detect 
an LH surge can be used to time intercourse. Intercourse is recom­
mended on the day of a positive urinary LH p

ecom­
mended on the day of a positive urinary LH peak and on the next 
day. If an hCG injection is used to induce ovulation, intercourse is 
recommended within 24 to 34 hours after the injection.
If follicular development does not occur after the first course 
of therapy with clomiphene citrate at 50 mg/day, a second course 
of 100 mg daily for 5 days may be started. Lack of response at 
doses of 150 to 200 mg daily for 5 days should be an indica­
tion for a change of treatment. Most patients de

­
tion for a change of treatment. Most patients destined to con­
ceive do so with the starting dose of clomiphene citrate (50 mg/
day for 5 days). Most clomiphene citrate–initiated conceptions 
occur within the first 6 ovulatory cycles.279 The incidence rate for 
multiple gestation in clomiphene citrate–induced pregnancies is 
6% (4% for twins and 2% for triplets).260 Failure to achieve preg­
nancy after three clomiphene cycles despite sonographic evidence 
of follicular development should promp

c evidence 
of follicular development should prompt the clinician to perform 
a comprehensive infertility workup, including a semen analysis 
and evaluation of the uterine cavity and tubal patency. 
Aromatase Inhibitors
Via decreasing aromatization of the estrogen precursor steroids 
in the brain and other body sites, aromatase inhibitors reduce 
hypothalamic-pituitary estrogen feedback, and this leads to 
increased GnRH secretion, concomitant elevations in LH and 
FSH, and increased ovarian fol

evations in LH and 
FSH, and increased ovarian follicular development in premeno­
pausal women.104,280 The gonadotropin-stimulating aromatase 
inhibitors letrozole and anastrozole have been used off-label in the 
treatment of patients with ovulatory dysfunction, such as PCOS, 
and for increasing the number of ovarian follicles recruited for 
ovulation in women who are already ovulatory.281 Oral adminis­
tration of letrozole (2.5 or 5 mg/day) or anastrozole (1 mg/day) on 
days 3 to 7 after uterin

nastrozole (1 mg/day) on 
days 3 to 7 after uterine bleeding is effective for ovulation induc­
tion in women with anovulatory infertility (see Fig. 17.32).281,282 
In a randomized study, as compared with clomiphene (50 mg/
day), letrozole (2.5 mg/day) was associated with higher live-
birth and ovulation rates among infertile women with PCOS.282 
A retrospective study did not show any difference in the overall 
rates of major and minor congenital malformations among new­
borns from mothers who co

malformations among new­
borns from mothers who conceived after letrozole or clomiphene 
treatment.283 
Metformin
Head-to-head randomized trials showed that clomiphene alone 
is clearly superior to metformin only with respect to achieving 
ovulation and live births in women with PCOS.260 However, the 
ovulatory response to clomiphene was increased in obese women 
with PCOS by decreasing insulin secretion with the addition of 
metformin.277 Another randomized study showed that the higher 
rate of

r randomized study showed that the higher 
rate of ovulation in the users of clomiphene plus metformin 
seemed to be offset by a higher rate of pregnancy losses, producing 
similar live-birth rates in clomiphene-only and clomiphene plus 
metformin groups.260 The benefit of the addition of metformin 
to clomiphene in obese clomiphene-nonresponders with PCOS 
needs to be assessed further. 
Low-Dose Gonadotropin Therapy
For women who do not ovulate in response to clomiphene citrate 
or letrozole, F

in response to clomiphene citrate 
or letrozole, FSH injections are started on day 3 of spontane­
ous or progestin-induced uterine bleeding. Recombinant FSH is 
administered subcutaneously starting with a daily dose of 75 IU 
for up to 10 days, if necessary, and using small incremental dose 
increases (12.5–37.5 IU) from then on at intervals of 3 to 7 days 
until serum estradiol concentrations begin to increase. The dose 
is then maintained until follicular rupture, which is induced by 
Cycle da

 follicular rupture, which is induced by 
Cycle days
Ultrasound
Ultrasound Intercourse
Check
pregnancy
(urine hCG)
1
2
3
4
5
6
7
Menses
LH surge or
hCG injection
Clomiphene
or letrozole
once daily
for 5 days
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 1
2
3
• Fig. 17.32  Monitoring of clomiphene citrate or letrozole-initiated ovulation. On cycle day 2 or 3, a baseline 
ultrasound examination is performed to rule out any large ovarian follicular cyst. Oral administration of 
clom

rian follicular cyst. Oral administration of 
clomiphene citrate or letrozole is started on day 3 of the cycle and continued for 5 days. Ultrasonography 
is performed on cycle day 13 or 14 to ensure follicular development. Upon observing at least one mature 
follicle by ultrasound, ovulation may be triggered by a human chorionic gonadotropin (hCG) injection fol­
lowed by intercourse in 24 to 34 hours. Alternatively, the patient may be encouraged to have intercourse 
every other day during the 12

to have intercourse 
every other day during the 12-day period after the last clomiphene citrate or letrozole dose. Urinary hCG 
levels are checked to determine whether pregnancy has occurred. LH, luteinizing hormone.
622
subcutaneous injection of recombinant hCG (250 μg). Follicular 
growth is monitored by transvaginal ultrasonography and blood 
estradiol levels, which serve as biochemical markers for the gran­
ulosa cell mass in the growing follicle.

the gran­
ulosa cell mass in the growing follicle.235 This regimen induces 
development of a single follicle in most cycles and has succeeded 
in reducing the rate of multiple pregnancies to as low as 6% in 
some series.235 The low-dose regimen also practically eliminated 
the complication of severe OHSS.235 Conception rates are com­
parable to those achieved with conventional therapy. The miscar­
riage rate remains somewhat higher than that after spontaneous 
conceptions (20–25%).
Conventional-

r spontaneous 
Conventional-dose gonadotropin therapy (starting FSH dose, 
150 IU daily) should not be used as first-line treatment in patients 
with PCOS because it causes an alarming number of multiple 
pregnancies (14–50% of treatment cycles) and a significantly 
increased risk of OHSS (1.3–9.4% of treatment cycles).284 OHSS 
is a more common complication of conventional-dose compared 
with low-dose gonadotropin treatment. Milder forms are rela­
tively common and are cha

. Milder forms are rela­
tively common and are characterized by weight gain, abdominal 
discomfort, and enlarged ovaries. Home bed rest and increased 
oral intake of fluids are sufficient to manage this form. Severe 
OHSS occurs in 0.1% to 0.2% of stimulation attempts and is 
accompanied by severe ascites, pleural effusion, electrolyte imbal­
ance, and hypovolemia with oliguria. The most dreaded com­
plication is deep venous thrombosis and embolism. Its cause is 
poorly understood. Large numbers

sm. Its cause is 
poorly understood. Large numbers of follicles, peak estradiol lev­
els greater than 2000 pg/mL, and pregnancy are associated with 
a higher likelihood of OHSS. Prevention includes withholding of 
the hCG injection and intrauterine insemination. Treatment of 
severe OHSS includes hospitalization, maintenance of fluid and 
electrolyte balance, prophylaxis for thromboembolism by hepa­
rin, and drainage of severe ascites or pleural effusions. Frequently, 
supportive measures are su

effusions. Frequently, 
supportive measures are sufficient to manage this self-limited 
Premature Ovarian Insufficiency
On average, menopause occurs at the age of 50 years, with 1% 
of women continuing to menstruate beyond the age of 60 years 
and another 1% whose menopause occurs before age 40 years. 
Premature menopause or ovarian insufficiency has been arbitrarily 
defined as the cessation of menses before 40 years of age.285
POI, which is defined as early depletion of ovarian fol

which is defined as early depletion of ovarian follicles 
(before the age of 40 years), is a state of hypergonadotropic hypo­
gonadism. These patients go through a normal puberty and a 
variable period of cyclic menses followed by oligomenorrhea or 
amenorrhea accompanied by hot flashes and urogenital atrophy. 
POI should always be included in the differential diagnosis of 
chronic anovulation. History and physical examination may reveal 
menstrual irregularity or secondary amenorrhea accompanie

al irregularity or secondary amenorrhea accompanied by 
symptoms and signs of estrogen deficiency, such as hot flashes and 
urogenital atrophy.286
The cause or genetic basis of POI is not well understood. Two 
genetic syndromes associated with POI are gonadal dysgenesis 
with primarily mosaic X-chromosome defects and FMR1 gene 
premutation, a variant of fragile X syndrome.287 Syndromes 
resulting from single-gene mutations (e.g., blepharophimosis-pto­
sis-epicanthus inversus syndrome [FOXL2 muta

-pto­
sis-epicanthus inversus syndrome [FOXL2 mutation], galactose­
mia [GALT mutation]) may be associated with POI.287 However, 
the cause of POI remains unknown in most cases.
The underlying ovarian defect may manifest at various ages, 
depending on the number of functional follicles left in the ovaries. 
If loss of follicles occurs rapidly before puberty, primary amenor­
rhea and lack of secondary sexual development ensue. The degree 
to which the adult phenotype develops and the age at which

 the adult phenotype develops and the age at which sec­
ondary amenorrhea actually occurs depend on whether follicle 
loss took place during or after puberty. In cases of primary amen­
orrhea associated with sexual infantilism, the ovarian remnants 
exist as streaks, and transvaginal ultrasonography usually cannot 
detect any ovaries. Many gene defects (e.g., FSHR, CYP17A1, 
CYP19A1) involve ovarian failure at the expected time of puberty, 
and the phenotypes manifest with primary amenorrhea and

he phenotypes manifest with primary amenorrhea and lack 
of secondary sexual development (see Chapter 25).287
POI can also result from an autoimmune process and may be 
associated with an autoimmune polyendocrine syndrome.288 Other 
causes of premature insufficiency can be related to the sudden 
destruction of follicles by chemotherapy, irradiation, or infections 
such as mumps oophoritis. The effect of irradiation depends on the 
patient’s age and the x-ray dose.289 Steroid levels begin to fall

nd the x-ray dose.289 Steroid levels begin to fall and 
gonadotropins rise within 2 weeks after irradiation of the ovaries. 
Younger women exposed to radiation are less likely to have perma­
nent ovarian insufficiency because of the higher number of oocytes 
present at earlier ages. When the radiation field excludes the pel­
vis or the ovaries are transposed out of the pelvis by laparoscopic 
surgery before irradiation, the risk for POI is minimized.290 Most 
chemotherapeutic agents used for era

zed.290 Most 
chemotherapeutic agents used for eradication of malignancies are 
toxic to the ovaries and cause ovarian insufficiency.291 Resumption 
of menses and pregnancy have been reported after radiotherapy or 
chemotherapy,292 but POI may occur years after these therapies.272
Diagnosis and Management of Premature Ovarian 
POI should be suspected in a woman younger than 40 years of age 
who presents with amenorrhea, oligomenorrhea, or another form 
of menstrual irregularity acc

ea, or another form 
of menstrual irregularity accompanied by hot flashes. Menopausal 
serum FSH levels (40 IU/L) on at least two occasions are sufficient 
for the diagnosis of POI.
Case reports of pregnancies in affected women occurring dur­
ing hormone replacement therapy have been published.293,294 In 
particular, young women with POI may experience intermittent 
periods of ovarian function, with antral follicles present at ovarian 
ultrasonography and ovulation described in cases that follow

raphy and ovulation described in cases that followed 
up with regular endocrine assessments. A randomized trial of hor­
mone therapy in this setting showed that folliculogenesis occurred 
often but was less frequently followed by ovulation and even less 
frequently by pregnancy (up to 14%); conventional-dose estro­
gen therapy did not improve the rate of folliculogenesis, ovula­
tion, or pregnancy.286 Later pilot studies or case reports suggested 
that lowering FSH levels to less than 15 IU/L by

that lowering FSH levels to less than 15 IU/L by use of high-dose 
estrogen or a GnRH antagonist in young women with POI may 
trigger ovulation or permit ovulation induction and pregnancy in 
a small number of patients.295,296
The clinician should inform patients diagnosed with POI that 
there is a small but significant likelihood of spontaneous preg­
nancy or pregnancy after ovulation induction. Women who desire 
pregnancy are still best served by assisted reproductive technology 
using donor

d by assisted reproductive technology 
using donor oocytes, because the probability of pregnancy with 
an autologous egg is low. Use of donor oocytes followed by IVF 
with the partner’s sperm and intrauterine embryo transfer after 
synchronization of the recipient patient’s endometrium with the 
donor’s cycle using exogenous estrogen and progesterone is offered 
to the patient who wishes to carry a pregnancy in her uterus (see 
Fig. 17.23). This approach offers an excellent chance of achieving 

approach offers an excellent chance of achieving 
live birth (>50% per donor oocyte IVF cycle).
623
Patients with POI have an increased incidence of an abnormal 
complement of chromosomes.297 The risk of having an abnor­
mal karyotype increases with decreasing age at onset of the POI. 
A chromosomal analysis is recommended for the POI patients 
younger than 30 years of age because of the increased risk of a 
gonadal tumor asso

use of the increased risk of a 
gonadal tumor associated with the presence of a Y chromosome.298 
It is extremely rare to encounter a gonadal tumor in patients with 
POI after the age of 30 years.299
Mosaicism that includes a Y chromosome has been associated 
with a high incidence of gonadal tumors.298 These malignant 
tumors arise from germ cells and include gonadoblastomas, dys­
germinomas, yolk sac tumors, and choriocarcinoma. Secondary 
virilization in patients with karyotypic abnormalities 

ization in patients with karyotypic abnormalities and POI is 
associated with a significantly increased risk of a gonadal tumor. 
The precise risk of a tumor in various subsets of these patients is 
not well known because a significant number of women carry­
ing a Y chromosome do not have symptoms of virilization. The 
frequency of Y chromosomal material determined by polymerase 
chain reaction is high in those with Turner syndrome (12.2%), 
but the occurrence of a gonadal tumor among these Y-po

the occurrence of a gonadal tumor among these Y-positive 
patients is about 7% to 10%.300
Fragile X–associated disorders are caused by a CGG trinucleo­
tide repeat expansion in the promoter region of the FMR1 gene. 
Expansion of the CGG trinucleotide repeats to more than 200 
copies induces methylation of the FMR1 gene, with an outcome 
of transcriptional silencing.301 This so-called full mutation was 
linked to mental retardation or autism. Individuals who carried 
the premutation (defined as >

viduals who carried 
the premutation (defined as >55 but <200 CGG repeats) have 
increased FMR1 mRNA levels with decreased levels of fragile X 
mental retardation protein (FMRP). Convincing evidence relates 
the FMR1 premutation to altered ovarian function and loss of 
fertility.302 The natural history of the altered ovarian function in 
women who carry the FMR1 premutation is still not well under­
stood. Women with POI are at increased risk for an FMR1 pre­
mutation and should be informed of th

an FMR1 pre­
mutation and should be informed of the availability of fragile X 
testing.
POI usually occurs as an isolated autoimmune disorder. Rarely, 
it may be associated with hypothyroidism, diabetes mellitus, 
hypoadrenalism, hypoparathyroidism, or systemic lupus erythe­
matosus (SLE).303 It can be part of an autoimmune polyendo­
crine syndrome.288 Thyroid insufficiency, adrenal insufficiency, 
and diabetes mellitus are the endocrine disorders most frequently 
associated with POI.304 Periodi

s most frequently 
associated with POI.304 Periodic endocrine testing for glucose 
intolerance, adrenal or parathyroid function, and autoimmune 
disease (e.g., SLE) should be considered based on the clinical 
presentation (Table 17.6). Because hypothyroidism is more com­
monly found than other endocrine disorders associated with POI, 
this author prefers to check a TSH level during the initial evalua­
tion (see Table 17.6).
Treatment of POI should be directed toward its specific 
cause. In most 

d be directed toward its specific 
cause. In most cases, however, it is not possible to identify a 
specific cause if there are no karyotypic anomalies. Besides 
infertility, long-term ovarian steroid deficiency has far-reach­
ing health implications. Early menopause has been associated 
with increased cardiovascular mortality and stroke, bone frac­
ture, and colorectal cancer risks.287 Despite reduced risks for 
development of breast cancer, overall quality of life and life 
expectancy decline 

rall quality of life and life 
expectancy decline with early menopause.287 Hormone ther­
apy, using combined estrogen and progestin or a low-dose oral 
contraceptive, is the cornerstone of the management for these 
women. The added value of androgen replacement remains 
uncertain.287 
Uterine Bleeding
Acyclic production of estrogen during anovulatory cycles gives rise 
to irregular shedding of the endometrium. These bleeding mani­
festations of anovulator

ium. These bleeding mani­
festations of anovulatory cycles in the absence of uterine pathol­
ogy or systemic illness are commonly referred to as dysfunctional 
uterine bleeding. Anovulatory uterine bleeding, which is the most 
common cause of chronic menstrual irregularities, is a diagnosis 
of exclusion. Pregnancy, uterine leiomyomas, endometrial pol­
yps, and adenomyosis should be ruled out as anatomic causes of 
irregular or excessive uterine bleeding. Malignancies of the vagina, 
cervix, end

bleeding. Malignancies of the vagina, 
cervix, endometrium, myometrium, fallopian tubes, and ovaries 
or coagulation abnormalities should also be ruled out before a 
diagnosis of anovulatory uterine bleeding is made.
Anovulatory uterine bleeding can be managed without surgi­
cal intervention by either restoring ovulation or mimicking the 
ovulatory hormonal profile by providing exogenous steroids. The 
rationale for use of exogenous steroids is based on the knowledge 
of predictable responses of

ased on the knowledge 
of predictable responses of the endometrium to estrogen and pro­
gesterone. Physiologic responses of the endometrium to natural 
ovarian steroids have been uncovered by observation of the gross 
and microscopic changes occurring in the endometrium dur­
ing thousands of normal ovulatory cycles in humans and other 
primates.119,120,305 The pharmacologic application of exogenous 
estrogens and progestins in women with anovulatory bleeding 
aims to correct the production of lo

ory bleeding 
aims to correct the production of local tissue factors that mediate 
physiologic steroid action and reverse the excessive and prolonged 
flow typical of anovulatory cycles.
Clinical management of irregular uterine bleeding with exog­
enous hormones is a time-honored method, and it has diagnostic 
value. Failure to control vaginal bleeding with hormonal therapy, 
despite appropriate application and use, makes the diagnosis of 
anovulatory uterine bleeding considerably less likely. I

atory uterine bleeding considerably less likely. In such cases, 
attention is directed to an anatomic pathologic entity within the 
reproductive axis as the cause of abnormal bleeding.
Heavy but regular menstrual bleeding (i.e., hypermenorrhea) 
can be encountered in ovulatory women. It may have an anatomic 
cause, such as a leiomyoma impinging on the endometrial cav­
ity or the diffuse and pathologic presence of benign endometrial 
glands in the myometrium (i.e., adenomyosis). In the absence of

 myometrium (i.e., adenomyosis). In the absence of 
a specific pathologic cause, it is presumed that hypermenorrhea 
reflects subtle disturbances in the endometrial tissue mechanism. 
In essentially all cases, evaluation and treatment are identical to 
the approach detailed in this section.
Characteristics of Normal Menses
Normal menstruation takes place about 14 days after each ovulation 
episode as a consequence of postovulatory estrogen-progesterone 
  Laboratory Evaluation of Premature 
Ova

terone 
Ovarian Insufficiency
Follicle-stimulating hormone (to establish the diagnosis of premature 
ovarian insufficiency)
Karyotype (<30 yr of age or sexual infantilism)
Testing for FMR1 gene premutation carrier state
Thyroid-stimulating hormone (hypothyroidism)
FMR1, Fragile X mental retardation 1.
   
TABLE 17.6
624
withdrawal. The quantity and duration of bleeding are quite 
reproducible. This predictability

eding are quite 
reproducible. This predictability leads many women to expect a 
certain characteristic flow pattern. Any slight deviations, such as 
plus or minus 1 day in duration or minor deviation from expected 
tampon use, are causes for major concern in the patient. Most 
women of reproductive age can predict the timing of their flows 
so accurately that even minor variability may require reassurance 
by the clinician. Although variability of menstrual cycles is a com­
mon feature during t

of menstrual cycles is a com­
mon feature during the teenage years and perimenopausal tran­
sition, the characteristics of menstrual bleeding do not undergo 
appreciable change between ages 20 and 40.306
For ovulatory women, the changes in the length of the men­
strual cycle over the period of reproductive age are predictable. 
Between menarche and age 20, the cycle length for most ovula­
tory women is relatively longer. Between 20 and 40 years, there is 
increased regularity as cycles shorten. 

there is 
increased regularity as cycles shorten. In the 40s, cycles begin to 
lengthen again. The highest incidence of anovulatory cycles occurs 
before age 20 and after age 40.307 In these age groups, the average 
length of a cycle is between 25 and 28 days. Among ovulatory 
women, the frequency of a cycle of less than 21 days or more than 
35 days is rare (<2%).308 Overall, most women have cycles that last 
24 to 35 days (Fig. 17.33).306 Between ages 40 and 50, menstrual 
cycle length increas

en ages 40 and 50, menstrual 
cycle length increases and anovulation becomes more prevalent.309
The average postovulatory bleeding lasts from 4 to 6 days. The 
normal volume of menstrual blood loss is 30 mL. More than 80 
mL is considered abnormal. Most of the blood loss occurs during 
the first 3 days of a period, so excessive flow may exist without 
prolongation of flow.310,311
During an ovulatory cycle, the duration from ovulation to 
menses is relatively constant and averages 14 days (see Fi

s relatively constant and averages 14 days (see Fig. 17.1A). 
Greater variability in the length of the proliferative phase, how­
ever, produces a distribution in the duration of the menstrual cycle. 
Menstrual bleeding more often than every 24 days or less often 
than every 35 days requires evaluation.306,309 Flow that lasts 7 or 
more days also requires evaluation. A flow that totals more than 
80 mL/month usually leads to anemia and should be treated.312 In 
clinical practice, however, it is d

reated.312 In 
clinical practice, however, it is difficult to quantify menstrual flow 
because evaluation and treatment are based solely on the patient’s 
perceptions regarding the duration, amount, and timing of her 
menstrual bleeding. Despite this difficulty in quantifying men­
strual blood loss, the clinician should evaluate the cause of excessive 
uterine bleeding. Anemia should be ruled out by a complete blood 
cell count.313 A low hemoglobin value accompanied by microcytic 
and hypochromi

in value accompanied by microcytic 
and hypochromic red blood cells suggests excessive blood loss dur­
ing menses. These patients should be provided with iron supple­
mentation. The likely presence of coagulation defects, uterine 
leiomyomas, or adenomyosis underlying prolonged menses should 
be evaluated in anemic patients through a meticulous history and 
physical examination followed by relevant laboratory tests. 
Terminology Describing Abnormal Uterine 
Bleeding
Oligomenorrhea is defined as 

al Uterine 
Bleeding
Oligomenorrhea is defined as intervals between episodes of uterine 
bleeding longer than 35 days, and the term polymenorrhea is used 
to describe intervals shorter than 24 days. Hypermenorrhea refers 
to regular intervals (24–35 days) but excessive flow or duration 
of bleeding, or both. An alternative term for hypermenorrhea is 
menorrhagia. Hypomenorrhea refers to diminution of the flow or 
shortening of the duration of regular menses, or both. 
Uterine Bleeding in Respons

ular menses, or both. 
Uterine Bleeding in Response to Steroid 
Hormones
Estrogen Withdrawal Bleeding
Uterine bleeding follows acute cessation of estrogen support to the 
endometrium. This type of uterine bleeding can occur after bilat­
eral oophorectomy, irradiation of mature follicles, or administra­
tion of estrogen to a woman who previously underwent removal 
of both of her ovaries, followed by discontinuation of therapy. 
Similarly, the bleeding that occurs after bilateral removal of ova­
r

ding that occurs after bilateral removal of ova­
ries can be delayed by concomitant estrogen therapy. Flow occurs 
on discontinuation of exogenous estrogen. Estrogen withdrawal 
by itself (in the absence of progesterone) almost invariably causes 
uterine bleeding. 
Estrogen Breakthrough Bleeding
Chronic exposure to various quantities of estrogen stimulates 
the growth of endometrium continuously in the absence of pro­
gesterone, as in the case of excessive extragonadal estrogen pro­
duction in P

 excessive extragonadal estrogen pro­
duction in PCOS. After a certain point, the amount of estrogen 
produced in extraovarian tissue remains insufficient to maintain 
structural support for the endometrium. This gives rise to unpre­
dictable episodes of shedding of the surface endometrium. Rela­
tively low doses of estrogen yield intermittent spotting that may 
be prolonged, but the quantity is light. High levels of estrogen and 
sustained availability lead to prolonged periods of amenorrhea fo

ability lead to prolonged periods of amenorrhea fol­
lowed by acute, often profuse episodes of bleeding with excessive 
loss of blood. 
Progesterone Withdrawal Bleeding
Typical progesterone withdrawal bleeding occurs after ovulation 
in the absence of pregnancy. Removal of the corpus luteum also 
leads to endometrial desquamation. Pharmacologically, a similar 
event can be achieved by administration and then discontinuation 
of progesterone or a synthetic progestin. Progesterone withdrawal 
Leng

synthetic progestin. Progesterone withdrawal 
Length of menstrual cycle in days
16
12
8
4
2
40
38
36
34
32
30
28
26
24
22
20
• Fig. 17.33  Variation of the duration of the menstrual cycle in women with 
regular cycles. (From Cunningham FG, MacDonald PC, Gant NF, et al. The 
endometrium and decidua: menstruation and pregnancy. In: Cunningham 
FG, ed. Williams Obstetrics. 19th ed. Stamford, CT: Appleton & Lange; 
1993:81–109.)
625
Chapter 17  Physiology and Pathology  of the Female R

pter 17  Physiology and Pathology  of the Female Reproductive Axis
bleeding occurs only if the endometrium is initially primed by 
endogenous or exogenous estrogen. If estrogen therapy is contin­
ued as progesterone is withdrawn, the progesterone withdrawal 
bleeding still occurs. Only when estrogen levels are increased 
markedly is progesterone withdrawal bleeding delayed.314 Proges­
terone withdrawal bleeding is quite predictable in the presence of 
previous or concomitant estrogen exposure. 

e of 
Progestin Breakthrough Bleeding
The pharmacologic phenomenon of breakthrough bleeding occurs 
in the setting of an unfavorably high ratio of progestin to estro­
gen. In the absence of sufficient estrogen, continuous progestin 
therapy leads to intermittent bleeding of variable duration, similar 
to the low-dose estrogen breakthrough bleeding described previ­
ously. This type of bleeding is associated with combination oral 
contraceptives that con

ted with combination oral 
contraceptives that contain low-dose estrogen and the long-acting, 
progestin-only contraceptive methods such as Norplant and Depo-
Provera.315 Progestin breakthrough bleeding is highly unpredict­
able and is characterized by extensive variability among women. 
Causes of Irregular Uterine Bleeding
The most common cause of disruption of a normal menstrual pat­
tern is pregnancy or a complication of pregnancy. Pregnancy and 
pregnancy-related problems such as ectopic pre

nd 
pregnancy-related problems such as ectopic pregnancy or spon­
taneous miscarriage are extremely common causes of abnormal 
uterine bleeding (Table 17.7). Pregnancy should be ruled out by 
a urine test in any woman of reproductive age who presents with 
irregular bleeding (Table 17.8).
Anovulatory uterine bleeding is a diagnosis of exclusion for 
several reasons. Vulvar, vaginal, or uterine malignancies or an 
estrogen-secreting or androgen-secreting ovarian tumor may cause 
abnormal uterine 

creting ovarian tumor may cause 
abnormal uterine bleeding (see Table 17.7). Anovulatory uterine 
bleeding arising from responses of the endometrium to inappro­
priate production of ovarian steroids has also been called dysfunc­
tional uterine bleeding because treatments that restore ovulatory 
function potentially reverse the irregular bleeding pattern. Com­
mon examples of anovulatory bleeding include those associated 
with exercise-related anovulation, hyperprolactinemia, hypothy­
roidism, or

vulation, hyperprolactinemia, hypothy­
roidism, or PCOS.316 In these cases, restoration of ovulatory men­
ses by correction of the underlying disorder or use of exogenous 
hormones can achieve predictable uterine bleeding.
Another common cause of irregular uterine bleeding is 
observed in oral contraceptive users in the form of progestin break­
through bleeding. Progestin breakthrough bleeding during post­
menopausal hormone therapy is also common. Patients may be 
unknowingly using other hormon

n. Patients may be 
unknowingly using other hormonal medications with an impact 
on the endometrium. For example, the use of ginseng, an herbal 
root, has been associated with estrogenic activity and abnormal 
bleeding.317 Although uterine bleeding is a common benign side 
effect of various long-term hormone treatments, the clinician 
should always be convinced first that no other pathologic condi­
tion is present. Anatomically demonstrable pathologic disorders 
of the menstrual outflow tract in

logic disorders 
of the menstrual outflow tract include endometrial hyperplasia 
and cancer, endometrial polyps, uterine leiomyomas, adenomyo­
sis, and endometritis. Irregular uterine bleeding may be associated 
with chronic illness, such as renal insufficiency, liver insufficiency, 
or AIDS. Careful examination may discover genital injury or a 
foreign object (see Table 17.7).
At puberty, the most common cause of irregular uterine bleeding 
is anovulation. Approximately 20% of adolescents with 

novulation. Approximately 20% of adolescents with excessive 
  Causes of Irregular Uterine Bleeding
Complications of Pregnancy
Threatened miscarriage
Incomplete miscarriage
Ectopic pregnancy
Uncomplicated pregnancy (amenorrhea)
Pubertal (postmenarchal) anovulation
Anovulation immediately before menopause
Medications (e.g., oral contraceptives, GnRH agonists, danazol)
Hypothalamic (frequently presents as amenorrhea)
Functional (e.g., diet, exercise, stress)
Anatomic (e.g.

onal (e.g., diet, exercise, stress)
Anatomic (e.g., tumor, granulomatous disease, infection)
Hyperprolactinemia, other pituitary disorders
Other pituitary tumors, granulomatous disease
Other
Androgen excess
PCOS, hyperthecosis
Medications (e.g., testosterone, danazol)
Other
Premature o

ns (e.g., testosterone, danazol)
Other
Premature ovarian insufficiency (frequently presents as amenorrhea)
Liver insufficiency
Renal insufficiency
AIDS
Other
Anatomic Defects Affecting the Uterus
Uterine leiomyomas
Endometrial polyps
Adenomyosis (usually manifests as hypermenorrhea)
Intrauterine adhesions (usually manifests as amenorrhea)
Endometritis
Endometrial hyperplasia, cancer
Chronic estrogen exposure (e.g., PCOS, medication, liver insufficiency)
Estrogen-secreting ovarian

n, liver insufficiency)
Estrogen-secreting ovarian tumor (e.g., granulosa cell tumor)
Advanced cervical cancer
Other
Coagulation Defects (Usually Manifest as Hypermenorrhea)
Von Willebrand disease
Factor XI deficiency
Other
Extrauterine Genital Bleeding (May Mimic Uterine Bleeding)
Vaginitis
Genital trauma
Foreign body
Vaginal neoplasia
Vulvar neoplasia
Other
AIDS, Acquired immunodeficiency syndrome; GnRH, gonadotropin-releasing hormone; PCOS, 
polycystic ovary syndrome.
   
TABLE 17.7
626
SECTI

olycystic ovary syndrome.
   
TABLE 17.7
626
irregular uterine bleeding have a coagulation defect.318,319 Among 
all women of reproductive age with hypermenorrhea, the preva­
lence of a coagulation disorder is 17%; von Willebrand disease is 
the most common defect, and factor XI deficiency is the second 
most common diagnosis. Bleeding because of a coagulation defect 
usually consists of a heavy flow with regular, cyclic menses (i.e., 
hypermenorrhea)

ith regular, cyclic menses (i.e., 
hypermenorrhea), and the same pattern can be seen in patients 
being treated with anticoagulants.320 Bleeding disorders are usu­
ally associated with hypermenorrhea since menarche and a history 
of bleeding with surgery or trauma. Hypermenorrhea may be the 
only sign of an inherited bleeding disorder.321
Early pregnancy or its complications should always be ruled 
out first by a sensitive urine hCG measurement in any repro­
ductive-age woman presenting with irr

n any repro­
ductive-age woman presenting with irregular bleeding. Other 
tests should be ordered on the basis of the initial clinical evalua­
tion, including tests to evaluate anovulatory disorders of various 
causes (see Table 17.8). In patients with a history of prolonged 
heavy menses (i.e., hypermenorrhea) of pubertal origin, coagu­
lation studies (e.g., prothrombin time, partial thromboplastin 
time, bleeding time) and a complete blood cell count should be 
obtained.
Pelvic ultrasonography

 count should be 
obtained.
Pelvic ultrasonography through a vaginal probe is an extremely 
useful test for the evaluation of normal or abnormal pregnancy, 
uterine leiomyomas, endometrial neoplasia, and ovarian tumors 
(see Table 17.8). Other imaging studies may be used judiciously 
to rule out pathologic involvement of the hypothalamus, pitu­
itary, and adrenal (discussed earlier). Pelvic MRI or ultrasound is 
used to assess adenomyosis, a uterine disorder characterized by the 
abnormal presen

ine disorder characterized by the 
abnormal presence of diffuse endometrial tissue in the myometrial 
layer. Advanced adenomyosis is associated with diffuse enlarge­
ment of the uterus, hypermenorrhea, and anemia.
Endometrial histologic evaluation should be determined by an 
endometrial biopsy performed in the physician’s office in patients 
at risk for endometrial hyperplasia or cancer (e.g., PCOS, liver 
insufficiency, obesity, diabetes mellitus, hormone therapy). A 
benign endometrial polyp o

s, hormone therapy). A 
benign endometrial polyp or a uterine leiomyoma protruding 
into the uterine cavity can be diagnosed by hysterosonography 
using intrauterine saline installation or by hysteroscopy. Hystero­
sonography and hysteroscopy are not appropriate tests to evaluate 
endometrial hyperplasia or cancer because these procedures may 
cause dissemination of malignant cells. If malignancy is suspected, 
it should be ruled out by an office endometrial biopsy (see Table 
17.8). Occasionall

 endometrial biopsy (see Table 
17.8). Occasionally an office endometrial biopsy cannot be per­
formed or is not diagnostic of endometrial neoplasia. In these rare 
instances, endometrial curettage under anesthesia is performed for 
a reliable tissue diagnosis.
A careful history and physical examination may eliminate the 
need for most of these diagnostic tests. Before ordering a certain 
diagnostic study, it is useful to consider whether a particular test 
result would alter the ultimate clinic

cular test 
result would alter the ultimate clinical management. 
Management of Anovulatory Uterine Bleeding
If ovulatory function can be restored, anovulatory bleeding usu­
ally gives way to predictable cyclic periods. Because restoration of 
ovulatory function may not be possible or practical in many of 
these women, exogenous estrogen and progestin are administered 
for several purposes. The indications for hormonal treatment of 
uterine bleeding include the need to stop acute uterine bleedin

ing include the need to stop acute uterine bleeding, 
to maintain predictable bleeding episodes, or to prevent endome­
trial hyperplasia.
Anovulatory uterine bleeding is a diagnosis of exclusion. Vari­
ous anatomically demonstrable pathologic involvements of the 
genital tract (see Table 17.7) should be ruled out before adminis­
tration of estrogen, progestin, or GnRH analogues.
Use of combination oral contraceptives in an acute or chronic 
fashion is the most common treatmen

te or chronic 
fashion is the most common treatment for irregular uterine bleed­
ing. The estrogen component of the combination pill stabilizes the 
endometrial tissue and stops shedding within hours; it decreases 
ovarian secretion of sex steroids by suppression of gonadotro­
pins within several days. The progestin component of the pill 
directly affects endometrial tissue to decrease shedding over days 
and potentiates ovarian suppression induced by estrogen. The 
progestin (in the presence of

ed by estrogen. The 
progestin (in the presence of estrogen) induces differentiation of 
the endometrial tissue into a stable form called pseudodecidua. 
Typically, a monophasic oral contraceptive preparation that con­
tains 30 or 35 μg of ethinyl estradiol is preferred. Triphasic oral 
contraceptives and those with less than 30 μg of ethinyl estra­
diol are not suitable for the treatment of excessive anovulatory 
uterine bleeding. A combination oral contraceptive in high doses 
(two or three pi

oral contraceptive in high doses 
(two or three pills per day) can be used for short intervals (i.e., 
weeks) to treat an acute episode of excessive uterine bleeding. The 
usual dose of one pill per day may be administered for years to 
manage chronic anovulatory bleeding associated with PCOS or 
hyperprolactinemia.
Oral Contraceptives and Acute Excessive Uterine Bleeding 
Associated With Anemia
Unopposed estrogen exposure in women with anovulatory uter­
ine bleeding is commonly associated with 

ry uter­
ine bleeding is commonly associated with chronic endometrial 
buildup and heavy bleeding episodes. Therapy is administered as 
  Diagnostic Tests to Evaluate Irregular 
Commonly Used Tests
Urine hCG test
Serum hCG level (incomplete miscarriage, ectopic pregnancy)
Transvaginal pelvic ultrasonography (intrauterine or ectopic pregnancy, 
uterine leiomyoma, endometrial polyp or neoplasia, ovarian tumor)
Serum FSH, LH (anovulation; ovarian insufficiency)
Serum prolactin, TS

lation; ovarian insufficiency)
Serum prolactin, TSH (anovulation; hyperprolactinemia)
Complete blood count, PT, PTT (evaluation for anemia, coagulation 
defect)
Liver and renal functions, HIV (anovulation; chronic disease)
Endometrial biopsy (endometrial disease; polyp, neoplasia, endome­
tritis)
Less Commonly Used Tests
Evaluation for PCOS, ovarian or adrenal tumor, nonclassic congenital 
adrenal hyperplasia, Cushing syndrome, and glucocorticoid resis­
tance (androgen excess)
Head CT or MRI sca

 resis­
Head CT or MRI scan (hypothalamic anovulation, hyperprolactinemia)
Pelvic MRI scan (adenomyosis, uterine leiomyoma)
Hysterosonography with intrauterine saline instillation (endometrial 
polyp, uterine leiomyoma)
Hysteroscopy (endometrial polyp, uterine leiomyoma)
Dilatation and curettage (endometrial disease not diagnosed by ultra­
sonography or biopsy)
CT, Computed tomography; FSH, follicle-stimulating hormone; hCG, human chorionic 
gonadotropin; HIV, human immun

G, human chorionic 
gonadotropin; HIV, human immunodeficiency virus; LH, luteinizing hormone; MRI, magnetic 
resonance imaging; PCOS, polycystic ovary syndrome; PT, prothrombin time; PTT, partial 
thromboplastin time; TSH, thyroid-stimulating hormone.
   
TABLE 17.8
627
one combination oral contraceptive pill twice daily for 1 week. 
In obese women, the oral contraceptive may be given three times 
daily. This therapy is mainta

e given three times 
daily. This therapy is maintained despite cessation of flow within 
2 days. If flow does not abate, other diagnostic possibilities (e.g., 
previously missed diagnosis of polyps, incomplete abortion, or 
neoplasia) should be reevaluated. In case of anovulatory bleed­
ing, the flow does diminish rapidly within 2 days after the begin­
ning of high-dose oral contraceptive treatment (i.e., one pill two 
or three times daily). Specific causes of anovulation and possible 
coagulati

ific causes of anovulation and possible 
coagulation disorders should be evaluated during the next few 
days. The physician should also consider whether blood replace­
ment or initiation of iron therapy is necessary. The patient should 
also be warned of possible nausea that may be caused by high-dose 
oral contraceptive treatment.
At the end of a week of high-dose oral contraceptive treat­
ment, the pill is stopped temporarily. A heavy flow usually starts 
within a few days. On the third day of

lly starts 
within a few days. On the third day of this withdrawal bleeding, 
a regular dose of combination oral contraceptive medication (one 
pill/day) is started. This is repeated for several 3-week treatments 
interrupted by 1-week withdrawal intervals. A decrease in volume 
with each successive cycle is expected. Oral contraceptives reduce 
menstrual flow by more than one-half in most women.322
Because oral contraceptive use does not treat the underlying 
cause of anovulation but provides s

he underlying 
cause of anovulation but provides symptomatic relief by directly 
affecting the endometrium, its cessation results in the return of 
erratic uterine bleeding. Regardless of the requirement for contra­
ception, use of oral contraceptives represents the best choice for 
hormonal management of heavy anovulatory bleeding and should 
be offered as long-term management. 
Oral Contraceptives and Chronic Irregular Uterine Bleeding
PCOS is a common form of anovulation associated with chron

a common form of anovulation associated with chronic 
steady-state levels of unopposed estrogen that may give rise to 
endometrial hyperplasia and cancer (discussed earlier). Hypotha­
lamic anovulation and hyperprolactinemia are associated with low 
estrogen levels that are insufficient to prevent bone loss. A com­
bination oral contraceptive is a suitable long-term treatment for 
both forms of chronic anovulation.
Before the administration of an oral contraceptive, pregnancy 
should be ruled ou

 oral contraceptive, pregnancy 
should be ruled out. One pill per day is ordinarily administered 
for 3-week periods interrupted by 1-week hormone-free inter­
vals. Withdrawal bleeding is expected during the hormone-free 
interval. The progestin component serves to prevent endometrial 
hyperplasia associated with steady-state unopposed estrogen expo­
sure in PCOS. In cases of anovulation associated with hypoes­
trogenism (e.g., hypothalamic anovulation, hyperprolactinemia), 
the estrogen compone

lation, hyperprolactinemia), 
the estrogen component of the pill provides sufficient replacement 
to prevent bone loss. The risk of thromboembolism, stroke, or 
myocardial infarction associated with long-term administration is 
extremely low in current nonsmokers and in the absence of a his­
tory of thromboembolism. Provided that the oral contraceptive 
controls the abnormal uterine bleeding effectively, a chronically 
anovulatory woman can continue this regimen until menopause. 
Synthetic Proge

nue this regimen until menopause. 
Synthetic Progestins
Synthetic progestins enhance endometrial differentiation and 
antagonize the proliferative effects of estrogen on the endome­
trium (see Fig. 17.22).130,323 The effects of progestins or natural 
progesterone include limitation of estrogen-induced endometrial 
growth and prevention of endometrial hyperplasia. The absence 
of naturally synthesized progesterone in anovulatory states is the 
rationale for administering a progestin.
The most com

ionale for administering a progestin.
The most common indication for long-term cyclic progestin 
administration is to prevent endometrial malignancy in a patient 
with PCOS and unopposed chronic estrogen exposure of the 
endometrium. A combination oral contraceptive is the treatment 
of choice in these cases. If the patient cannot use an oral con­
traceptive for some reason (e.g., history of thromboembolism), 
a progestin may be administered in a cyclic fashion to prevent 
endometrial hyperplasi

 cyclic fashion to prevent 
endometrial hyperplasia. Before the administration of a proges­
tin (or oral contraceptive), pregnancy should be ruled out. In the 
treatment of oligomenorrhea associated with PCOS, orderly, lim­
ited withdrawal bleeding can be accomplished by administration 
of a progestin such as MPA (5 mg/day) for at least 10 days every 
2 months. Alternatively, norethindrone acetate at 5 mg/day or 
megestrol acetate at 20 mg/day may be administered for 10 days 
every 2 months. Abs

y be administered for 10 days 
every 2 months. Absence of withdrawal bleeding requires further 
workup.
In the treatment of excessive uterine bleeding (i.e., hypermen­
orrhea or polymenorrhea), these progestins at higher daily doses 
(20 mg/day of MPA, 10 mg/day of norethindrone acetate, or 40 
mg/day of megestrol acetate) are prescribed for 2 weeks to induce 
predecidual stromal changes in the endometrium. A heavy pro­
gestin withdrawal flow usually follows within 3 days after the last 
dose is

ally follows within 3 days after the last 
dose is administered. Thereafter, repeated progestin treatment (5 
mg/day of MPA, 5 mg/day of norethindrone acetate, or 20 mg/
day of megestrol acetate) is offered cyclically for at least the first 10 
days of every other month to ensure therapeutic effect. Failure of 
progestin to correct irregular bleeding requires diagnostic reevalu­
ation such as endometrial biopsy. Predictable withdrawal bleeding 
within several days after each cycle of progestin a

ithin several days after each cycle of progestin administration 
suggests the absence of endometrial malignancy. 
High-Dose Estrogen for Acute Excessive Uterine Bleeding
An oral contraceptive given two or three times daily is the treat­
ment of choice to stop heavy anovulatory bleeding. A high-dose 
oral contraceptive regimen should be offered to women with heavy 
uterine bleeding with or without asymptomatic anemia after ana­
tomically demonstrable pathology of the genital tract has been 
ruled

ble pathology of the genital tract has been 
ruled out (see Table 17.7). A patient with acute and severe anovu­
latory bleeding accompanied by symptomatic anemia represents a 
medical emergency. These patients should be hospitalized imme­
diately and offered a blood transfusion. After genital tract disease 
has been ruled out by history, physical examination, and pelvic 
ultrasonography, intravenously administered high-dose estrogen is 
the treatment of choice to stop life-threatening bleeding. 

ment of choice to stop life-threatening bleeding. A well-
established regimen is to administer 25 mg of conjugated estrogen 
intravenously every 4 hours until bleeding markedly slows down 
or for at least 24 hours.324 Estrogen most likely acts on the capil­
laries to induce clotting.325 Before intravenous estrogen treatment 
is discontinued, an oral contraceptive pill is started three times 
daily. Oral contraceptive treatment is then continued as described 
previously.
Because high-dose estroge

s described 
Because high-dose estrogen is a risk factor for thromboembo­
lism, taking two or three oral contraceptive pills per day for a week 
or large doses of intravenous conjugated equine estrogens for 24 
hours should be regarded as presenting a significant risk. However, 
no data are available to evaluate any risk associated with this type 
of acute use of hormonal therapy for such short intervals. The 
physician and patient should make a decision regarding high-dose 
hormone 

ould make a decision regarding high-dose 
hormone therapy after considering its risks and benefits. Alter­
native treatment options may be offered to patients with signifi­
cant risk factors. Exposure to high doses of estrogen should be 
avoided in women with a past episode or a strong family history of 
628
idiopathic venous thromboembolism. High-dose hormone treat­
ment should also be avoided in women with severe chronic illness 
such as liver insuf

n with severe chronic illness 
such as liver insufficiency or renal insufficiency. One alternative 
for these patients is dilatation and curettage, followed by treat­
ment with an oral contraceptive (one pill per day) until the uterine 
bleeding is under control. 
Gonadotropin-Releasing Hormone Analogues for 
Excessive Anovulatory Uterine Bleeding
A GnRH analogue may be given to women with excessive anovu­
latory bleeding or hypermenorrhea related to severe chronic illness 
such as liver insuffi

d to severe chronic illness 
such as liver insufficiency or coagulation disorders. Monthly depot 
injections of GnRH agonists are not effective for acute excessive 
uterine bleeding and may increase uterine bleeding for the first 
2 weeks. GnRH antagonists downregulate FSH and LH without 
a delay and achieve amenorrhea more rapidly. The GnRH ago­
nist leuprolide acetate depot (3.75 mg/month intramuscularly) 
may be administered for 6 months or longer to control uterine 
bleeding due to chronic i

nger to control uterine 
bleeding due to chronic illness. GnRH antagonists can probably 
be used to halt acute or chronic anovulatory bleeding; however, 
insufficient published data are available to provide dose recom­
mendations. Long-term side effects of GnRH analogues, includ­
ing osteoporosis, make this an undesirable choice for long-term 
therapy. If long-term treatment with GnRH analogues is chosen, 
norethindrone acetate (2.5 mg daily) should be added back. This 
add-back regimen is usual

uld be added back. This 
add-back regimen is usually sufficient to prevent osteoporosis and 
does not ordinarily worsen the uterine bleeding. 
Hormone-Dependent Benign Gynecologic 
Disorders
Endometriosis is defined as the presence of endometrium-like tis­
sue in ectopic sites outside the uterine cavity, primarily on pelvic 
peritoneum and ovaries, and it is associated with chronic pelvic 
pain, pain during intercourse, and infertility.326 This estrogen-
dependent inflammatory dise

ity.326 This estrogen-
dependent inflammatory disease affects 5% to 10% of US women 
of reproductive age.326 This classic presentation may represent 
a common phenotype resulting from diverse anatomic or bio­
chemical aberrations of uterine function. As cellular and molecu­
lar mechanisms in endometriosis are uncovered, this condition is 
coming to be viewed as a systemic and chronic complex disease, 
much like diabetes mellitus or asthma.327 Endometriosis may be 
inherited in a polygenic manner

ometriosis may be 
inherited in a polygenic manner, because its incidence is increased 
by up to sevenfold in relatives of women with endometriosis.328
Pathology
There are three clinically distinct forms of endometriosis: endome­
triotic implants on the surface of pelvic peritoneum and ovaries 
(i.e., peritoneal endometriosis); ovarian cysts lined by endome­
trioid mucosa (i.e., endometriomas); and a complex, solid mass 
composed of endometriotic tissue blended with adipose and 
fibromuscular ti

 tissue blended with adipose and 
fibromuscular tissue and residing between rectum and vagina (i.e., 
rectovaginal nodule). These three types of lesions may be variant 
phenotypes of the same pathologic process, or they may be caused 
by different mechanisms.329,330 Their common histologic charac­
teristic is the presence of endometrial stromal or epithelial cells, 
along with chronic bleeding and inflammatory changes. These 
lesions may occur singly or in combination and are associated 
with si

ngly or in combination and are associated 
with significantly increased risk of infertility and chronic pelvic 
pain.329,330 The inflammatory process in endometriosis may stim­
ulate nerve endings in the pelvis to cause pain, impair the function 
of the uterine tubes, decrease receptivity of the endometrium, and 
negatively affect development of the oocyte and embryo. Endome­
triosis may also cause infertility by physically blocking the tubes. 
An ovarian endometrioma may decrease the quality of

n ovarian endometrioma may decrease the quality of the eggs or 
become sufficiently large to interfere with the ovulation process.
Clinical evidence points to a deleterious effect of uninterrupted 
ovulatory cycles on the development and persistence of endome­
triosis.327 First, symptoms of endometriosis usually appear after 
menarche and vanish after menopause. Occasionally, a rectovagi­
nal nodule remains symptomatic in a postmenopausal woman, 
suggesting that its persistence is independent of

suggesting that its persistence is independent of ovarian estrogen. 
Second, multiparity is associated with a decreased risk of endome­
triosis. Third, disruption of ovulation by GnRH analogues, oral 
contraceptives, or progestins reduces pelvic disease and associated 
pain. In line with these observations, basic and clinical research 
findings indicate major roles of the ovarian steroids estrogen and 
progesterone in the pathologic development of endometriosis. 
In humans and primate models, e

of endometriosis. 
In humans and primate models, estrogen stimulates the growth 
of endometriotic tissue, whereas aromatase inhibitors that block 
estrogen formation and antiprogestins are therapeutic.327 Lev­
els of nuclear receptors for estrogen and progesterone in endo­
metriotic tissue are strikingly different from those in normal 
endometrium.327 Fourth, biologically significant quantities of 
progesterone and estrogen are produced locally by an abnormally 
active steroidogenic cascade that

y an abnormally 
active steroidogenic cascade that includes aromatase.327 
Mechanism of Disease
A number of hypotheses have been proposed regarding the histo­
logic origin of endometriosis. Sampson suggested that fragments 
of menstrual endometrium pass retrograde through the tubes and 
then implant and persist on peritoneal surfaces.331 This mecha­
nism has been demonstrated in primate models and observed 
naturally in human disease, and it is supported by the observation 
that spontaneous endo

upported by the observation 
that spontaneous endometriosis occurs exclusively in species that 
menstruate. Alternatively, the coelomic-metaplasia hypothesis 
describes the genesis of endometriotic lesions within the perito­
neal cavity by differentiation of mesothelial cells into endome­
trium-like tissue. A third hypothesis argues that menstrual tissue 
from the endometrial cavity reaches other body sites through veins 
or lymphatic vessels.327 Finally, it has been proposed that circulat­
ing 

 Finally, it has been proposed that circulat­
ing blood cells originating from bone marrow differentiate into 
endometriotic tissue at various body sites.332 Sampson’s implanta­
tion hypothesis offers a plausible mechanism for most endometri­
otic lesions but does not explain why only some women develop 
endometriosis. Although most women of reproductive age have 
reflux menstruation into the peritoneal cavity, endometriosis is 
encountered in only 5% to 10% of this population.
Two possible mech

ly 5% to 10% of this population.
Two possible mechanisms may explain the successful implan­
tation of refluxed endometrium on the peritoneal surface or in a 
hemorrhagic corpus luteum cyst of the ovary. First, the eutopic 
endometrium of women with endometriosis exhibits multiple 
subtle but significant molecular abnormalities, including acti­
vation of oncogenic pathways or biosynthetic cascades favor­
ing increased production of estrogen, cytokines, prostaglandins, 
and metalloproteinases.327 

ines, prostaglandins, 
and metalloproteinases.327 When this biologically distinct tissue 
attaches to mesothelial cells, the magnitude of these abnormalities 
is amplified dramatically to enhance implant survival.327 A sec­
ond mechanism suggests that a defective immune system fails to 
clear implants off the peritoneal surface.327 It is possible that both 
mechanisms may contribute to the same phenotype.
629
Clear molecular d

 of the Female Reproductive Axis
Clear molecular distinctions, such as overproduction of estro­
gen, prostaglandins, and cytokines, are observed between endo­
metriotic tissue and endometrium (Fig. 17.34).327 Subtler forms 
of these abnormalities are also observed in endometrium from a 
patient with endometriosis compared with endometrium from a 
disease-free woman. Inflammation is a hallmark of endometriotic 
tissue that overproduces prostaglandins, metalloproteinases, cyto­
kines, and chemokin

ins, metalloproteinases, cyto­
kines, and chemokines.327 Increased levels of acute inflammatory 
cytokines such as interleukin 1β (IL1β), IL6, and tumor necrosis 
factor (TNF) likely enhance adhesion of shed endometrial tis­
sue fragments on peritoneal surfaces, and proteolytic membrane 
metalloproteinases may further promote their implantation.327 
Monocyte chemoattractant protein 1, IL8, and RANTES (i.e., 
regulated on activation, normal T cell expressed and secreted) 
attract the granulocytes

 expressed and secreted) 
attract the granulocytes, natural killer cells, and macrophages typi­
cally observed in endometriosis.333 Autoregulatory positive feed­
back loops ensure further accumulation of these immune cells, 
cytokines, and chemokines in established lesions.
Basic biologic functions such as inflammation, immune 
response, angiogenesis, and apoptosis are altered in favor of sur­
vival and replenishment of endometriotic tissue.327 These func­
tions depend in part on estrogen or pro

hese func­
tions depend in part on estrogen or progesterone action. Excessive 
formation of estrogen and prostaglandin and development of 
progesterone resistance have emerged as clinically useful concepts, 
because targeting of aromatase in the estrogen biosynthetic path­
way, cyclooxygenase 2 (COX2) in the prostaglandin pathway, or 
the PR significantly reduces laparoscopically visible endometriosis 
and pelvic pain (see Fig. 17.34).327 These three critical mecha­
nisms have been linked by spe

hree critical mecha­
nisms have been linked by specific epigenetic (hypomethylation) 
defects that cause overexpression of the nuclear receptors SF1 
and ERβ.327 The genome-wide unique epigenetic fingerprint in 
endometriosis suggests DNA methylation is an integral compo­
nent of the disease and identifies a novel role for the GATA family 
as key regulators of uterine physiology; aberrant DNA methyla­
tion in endometriotic cells correlates with a shift in GATA iso­
form expression that facilitat

 shift in GATA iso­
form expression that facilitates progesterone resistance and disease 
progression.334 
Diagnosis
A history of severely painful menses during the teenage years, 
which eventually progressed into chronic pelvic pain experienced 
both during and between periods, is suggestive of endometrio­
sis. Reliable diagnosis of peritoneal endometriosis can be made 
only by direct visualization of these lesions by laparoscopy or lapa­
rotomy. Ovarian endometriotic cysts filled with a thick,

. Ovarian endometriotic cysts filled with a thick, bloody 
fluid (i.e., endometriomas) can be diagnosed accurately by vaginal 
ultrasonography. 
Treatment
Treatment of infertility caused by endometriosis consists of surgical 
removal with or without assisted reproductive technology, whereas 
pain is usually treated with a combination of medical suppres­
sion of ovulation and surgery. Peritoneal implants are resected or 
vaporized by electric current or laser. Ovarian endometriomas and 
rectovagi

ent or laser. Ovarian endometriomas and 
rectovaginal endometriotic nodules may be effectively removed 
only by full dissection. Epidemiologic and laboratory data suggest 
a link between ovarian endometriosis and distinct types of ovarian 
cancers.327
Although current hormonal therapy for infertility associated 
with endometriosis is not of proven value, it is somewhat success­
ful for pelvic pain associated with endometriosis. Various agents 
used are comparable in terms of efficacy. Most of th

ed are comparable in terms of efficacy. Most of the current 
medical treatments were designed to suppress ovulation (e.g., 
GnRH agonists and antagonists, oral contraceptives, danazol, pro­
gestins). A possible alternative mechanism of action of the andro­
genic steroid danazol or a progestin is a direct growth-suppressive 
effect on endometriotic tissue.
Many patients and physicians do not favor danazol because of 
its anabolic and androgenic side effects of weight gain and muscle 
cramps and o

de effects of weight gain and muscle 
cramps and occasional irreversible virilization (e.g., clitoromegaly, 
voice changes).335 Up to 50% of patients with endometriosis fail 
to complete 6 months of treatment with danazol.336 The rest of 
the hormonal agents—oral contraceptives, progestins, and GnRH 
agonists—show comparable efficacy for control of endometriosis-
associated pain.337–339 A 6-month course using any one of these 
agents results in a significant reduction of pain in more than 50% 
o

 significant reduction of pain in more than 50% 
of patients.337–339 Induction of pain relief with a continuously 
administered oral contraceptive or progestin takes longer than with 
a GnRH agonist. Recently, an oral GnRH antagonist, elagolix, 
has been introduced to treat endometriosis-associated pain.2 Both 
higher and lower doses of elagolix decreased dysmenorrhea and 
nonmenstrual pelvic pain and were approved in 2018 for 6-month 
and 24-month periods, respectively, in women with endometrio

th periods, respectively, in women with endometrio­
sis-associated pain. Greater duration is not recommended due to 
a dose-dependent decrease in bone mineral density. Both doses 
of elagolix were also associated with other hypoestrogenic adverse 
effects.2
There is a high incidence of recurrence or persistence of the 
disease and pain after all of these medical therapies.340 Thus suc­
cessful medical management of pain requires long-term ovarian 
suppression for years. Because it is practical t

suppression for years. Because it is practical to maintain these 
patients on a combination oral contraceptive for years with a rela­
tively benign side effect profile, birth control pills as a class has 
been the most suitable long-term management option.34
We are still far from the cure of endometriosis, and current 
treatments are not satisfactory for effective control of pain. The 
radical treatment is removal of both ovaries, and even this was 
not found to be effective in a number of cas

 was 
not found to be effective in a number of cases of postmenopausal 
endometriosis.341 New alternative strategies are needed to offer 
women with endometriosis a reasonable chance to live without 
suffering from chronic pelvic pain for decades.
There are two important caveats about ovulation suppression–
based treatments. First, large quantities of estrogen can be pro­
duced locally within the endometriotic cells. This represents an 
intracrine mechanism of estrogen action, in contrast to ova

e mechanism of estrogen action, in contrast to ovarian 
secretion, which is an endocrine means of supplying this steroid 
to target tissues (see Fig. 17.19).97,338 Second, estradiol produced 
in peripheral tissue sites (e.g., adipose tissue, skin fibroblasts) may 
give rise to pathologically significant circulating levels of estradiol 
in a subset of women.97 GnRH agonists do not inhibit periph­
eral estrogen formation or local estrogen production within the 
estrogen-responsive lesion. Moreover

n within the 
estrogen-responsive lesion. Moreover, endometriosis is resistant to 
selective effects of progesterone and progestins.327
Aromatase inhibitors and selective progesterone response 
modulators are candidate therapeutic agents for endometrio­
sis refractory to existing treatment options. Aromatase expres­
sion and local estrogen biosynthesis in endometriotic implants 
prompted pilot studies to target aromatase in endometriosis using 
its third-generation inhibitors. Among these inhibi

ts third-generation inhibitors. Among these inhibitors, anastro­
zole and letrozole were used successfully to treat endometriosis 
in postmenopausal and premenopausal women.336,342–345 An aro­
matase inhibitor is the medical treatment of choice for persistent 
postmenopausal endometriosis. Use of aromatase inhibitors in 
premenopausal women with endometriosis requires concomitant 
630
ovarian suppression by the addition of a GnRH analogue, proges­
tin

on by the addition of a GnRH analogue, proges­
tin, or combination oral contraceptive.
For the medical management of pain in premenopausal women 
with endometriosis, this author favors the following simple algo­
rithm. Unless contraindicated, the continuous use of a combi­
nation oral contraceptive is the initial treatment of choice. The 
patient is reassured that the majority of women will have minimal 
or no breakthrough bleeding after 6 months of continuous oral 
contraceptive treatment. If p

 of continuous oral 
contraceptive treatment. If pain relief is adequate, the patient can 
remain on this regimen for years. If adequate pain relief is not 
achieved after 6 months of use, a daily oral aromatase inhibitor 
(anastrozole 1 mg/day or letrozole 2.5 mg/day) is added to the 
Uterus
E p i t h e l i u m
S t r o m a
PGE2
estradiol 
SF1 
COX2 
ER
ER
PR
RA 
HSD17B2  
Apoptosis of
normal cells
Recurrent
menstruation
Cycling steroid
hormones
Inflammation
(caused by
disease, injury,
autoimmun

(caused by
autoimmune
effects)
Epigenomic and/or
geomic defects in
endometrial cells:
• Increased expression:
MMPs, ER, SF1
KRAS, aromatase
• Decreased expression:
TIMPs, ER, PR, HOXA10
Inherited
abnormalities
effects
(endocrine
disruptors,
drugs, diet)
Ectopic
implants in
peritoneum
Red lesion
Uterus
Adhesion
Blue–black
lesion
Adhesion
aromatase 
• Fig. 17.34  Molecular mechanisms in endometriosis. Endometriosis is defined as the presence of endome­

ndometriosis is defined as the presence of endome­
trium-like tissue on the pelvic peritoneum (red and blue-black lesions) or in the ovary (blood-filled cyst, i.e., 
endometrioma). These lesions are thought to originate from abnormal endometrial tissue stem cells (blue), 
which have migrated retrograde during menstruation. Normal endometrial cells (red) without such survival 
capabilities are thought to go through apoptosis in the peritoneal or ovarian environments. It is possible 
that women wi

varian environments. It is possible 
that women with endometriosis have higher numbers of the abnormal cells in their eutopic endometrial 
tissues. Thus recurrent menstruation seems to be a significant risk factor for developing endometriosis. 
These abnormal cells (blue) contain genome-wide epigenetic abnormalities, such as DNA methylation, 
that affect the expression of a wide variety of genes. These epigenetic abnormalities may be inherited or 
caused by environmental influences such as infla

caused by environmental influences such as inflammation and endocrine disruptors. Two nuclear recep­
tors, steroidogenic factor 1 (SF1) and estrogen receptor-β (ERβ), play significant roles in the pathology 
of endometriosis. In normal endometrial stromal cells, cytosine-phosphate-guanine islands located at 
the SF1 and ERβ promoters are robustly methylated and silenced. A lack of promoter methylation is 
associated with promoter activation and the presence of extraordinarily large quantities 

 the presence of extraordinarily large quantities of these nuclear 
receptors in endometriotic stromal cells. Prostaglandin E2 (PGE2) induces multiple steroidogenic genes, 
including aromatase, and formation of estradiol from cholesterol. SF1 mediates this steroidogenic action 
of PGE2. ERβ suppresses ERα and progesterone receptors (PR). This results in defective retinoic acid (RA) 
production and action, leading to 17β-hydroxysteroid dehydrogenase 2 (HSD17B2) deficiency and failure 
to metaboli

se 2 (HSD17B2) deficiency and failure 
to metabolize estradiol. ERβ also induces cyclooxygenase 2 (COX2) and formation of PGE2. Estradiol and 
PGE2 are produced in large quantities and enhance cell survival and inflammation in endometriotic tissue. 
HOXA10, homeobox A10; KRAS, V-Ki-ras2 Kirsten rat sarcoma viral oncogene homologue; MMPs, matrix 
metalloproteinases; TIMPs, tissue inhibitors of metalloproteinases.
631
continuous

hology  of the Female Reproductive Axis
continuous oral contraceptive regimen. This combination may be 
maintained for at least 1 year. If pain relief is still not satisfactory, 
conservative laparoscopic surgery is considered. 
Uterine Leiomyomas
Uterine fibroids (leiomyomas) represent the most common tumor 
in women.346 These lesions disrupt the functions of the uterus and 
cause excessive uterine bleeding, anemia, defective implantation 
of an embryo, recurrent pregnancy loss, preterm labor, 

 embryo, recurrent pregnancy loss, preterm labor, obstruc­
tion of labor, pelvic discomfort, and urinary incontinence and 
may mimic or mask malignant tumors. By the time they reach 50 
years of age, nearly 70% of white women and more than 80% of 
black women will have had at least one fibroid; severe symptoms 
develop in 15% to 30% of these women. Uterine fibroids in black 
women are significantly larger at diagnosis than those in white 
women, are diagnosed at an earlier age, and are character

are diagnosed at an earlier age, and are characterized by 
more severe symptoms and a longer period of sustained growth. 
Approximately 200,000 hysterectomies, 30,000 myomectomies, 
and thousands of selective uterine-artery embolizations and high-
intensity focused ultrasound procedures are performed annually 
in the United States to remove or destroy uterine fibroids.346
Each fibroid seems to originate from the transformation and 
monoclonal expansion of a single somatic stem cell of the myome­

ansion of a single somatic stem cell of the myome­
trium under the influence of ovarian hormones. Human uterine 
fibroid tissue contains fewer stem cells than normal myometrium. 
However, stem cells derived from fibroid tissue—not the myome­
trium—carry MED12 mutations, which suggests that at least one 
genetic hit initially transforms a myometrial stem cell, which sub­
sequently interacts with the surrounding myometrial tissue to give 
rise to a fibroid tumor.346 Each fibroid tumor seems to con

 fibroid tumor.346 Each fibroid tumor seems to contain a 
unique driver mutation. MED12 mutations are found in approxi­
mately 70% of the fibroids.346 Somatic mutations that affect the 
HMGA2 gene represent the second most common genetic disrup­
tion in a fibroid tumor.346
Estrogen stimulates the growth of uterine fibroids through its 
receptor ERα. The primary roles of estrogen and ERα in fibroid 
growth are permissive in that they enable tissue to respond to pro­
gesterone by inducing the expr

 to respond to pro­
gesterone by inducing the expression of PR, which is essential and 
sufficient for tumor growth, as indicated by the stimulation of cell 
proliferation, the accumulation of extracellular matrix, and cellu­
lar hypertrophy.346 Since the stem cell population expresses much 
lower levels of PR than the population of mature cells but serves as 
the key source of tissue growth, a paracrine signal originating from 
PR-rich differentiated cells may mediate the proliferative effects 

iated cells may mediate the proliferative effects of 
progesterone on fibroid stem cells.346
Diagnosis can be made by abdominal or transvaginal ultraso­
nography. Transvaginal ultrasonography is a sensitive method for 
determining the size, number, and location of uterine leiomyomas.
The therapeutic choices depend on the goals of therapy, with 
hysterectomy most often used for definitive treatment and myo­
mectomy used when preservation of childbearing capability is 
desired. Intracavitary and s

earing capability is 
desired. Intracavitary and submucous leiomyomas can be removed 
by hysteroscopic resection. Laparoscopic myomectomy is techni­
cally possible but involves an increased risk of uterine rupture 
during pregnancy. The overall recurrence rate after myomectomy 
varies widely, from 10% to 50%. Other FDA-approved treatment 
options include selective uterine artery embolization and extracor­
poreal ablation of uterine fibroids with MRI-guided, high-inten­
sity, focused ultrasound.3

MRI-guided, high-inten­
sity, focused ultrasound.347
Although GnRH agonist–induced hypogonadism can reduce 
the overall volume of the uterus containing leiomyomas and 
tumor vascularity, the severe side effects and prompt recurrences 
make GnRH agonists useful only for short-term goals such as 
reducing anemia related to uterine bleeding or decreasing tumor 
vascularity before hysteroscopic resection. Trials have consistently 
demonstrated that treatment with an antiprogestin such as mife­
prist

reatment with an antiprogestin such as mife­
pristone or ulipristal acetate reduces fibroid size.346 This observa­
tion underscores the role of progesterone in the cause of uterine 
fibroids and opens a new area of therapeutic investigation.346 
Management of Menopause
Consequences of Menopause
Perimenopause Stage
Menopause is the cessation of ovulatory menses as a result of the 
irreversible loss of a number of ovarian functions, including ovula­
tion and estrogen production. Perimenopause is a

­
tion and estrogen production. Perimenopause is a critical period 
of life during which striking hormonal, somatic, and psychologic 
alterations occur in the transition to menopause. Perimenopause 
encompasses the change from ovulatory cycles to cessation of 
menses and is marked by irregularity of menstrual bleeding.
The most sensitive clinical indication of perimenopause is the 
progressively increasing occurrence of menstrual irregularities. The 
menstrual cycle for most ovulatory women last

The 
menstrual cycle for most ovulatory women lasts 24 to 35 days, 
and approximately 20% of all reproductive-age women experience 
irregular cycles.306 When women are in their 40s, anovulation 
becomes more prevalent and the menstrual cycle length increases, 
beginning several years before menopause.309 The median age at 
the onset of perimenopause is 47.5 years.348 Regardless of the age 
at onset, menopause (i.e., cessation of menses) is consistently pre­
ceded by a period of prolonged cycle i

tently pre­
ceded by a period of prolonged cycle intervals.349 Elevated cir­
culating levels of FSH mark this menstrual cycle change before 
menopause and are accompanied by decreased inhibin levels, nor­
mal levels of LH, and slightly elevated levels of estradiol.350 These 
changes in serum hormone levels reflect a decreasing ovarian fol­
licular reserve and can be detected most reliably on day 2 or 3 of 
the menstrual cycle.
Serum estradiol levels do not begin to decline until less than 
a yea

els do not begin to decline until less than 
a year before menopause. The average circulating estradiol levels 
in perimenopausal women are estimated to be somewhat higher 
than those in younger women because of an increased follicular 
response to elevated FSH levels.351 The decline in inhibin produc­
tion by the follicle, which allows a rise in FSH levels, in the later 
reproductive years reflects diminishing follicular reserve and com­
petence. Ovarian follicular output of inhibin begins to d

. Ovarian follicular output of inhibin begins to decrease 
after 30 years of age, and this decline becomes much more pro­
nounced after age 40. These hormonal changes parallel a sharp 
decline in fecundity, which starts at age 35.
Perimenopause is a transitional period during which postmeno­
pausal levels of FSH can be observed despite continued menses; 
LH levels remain in the normal range. Pregnancy is still possible 
in the perimenopausal woman, because occasional ovulation and 
functional co

n, because occasional ovulation and 
functional corpus luteum formation can occur. Until complete 
cessation of menses is observed or FSH levels higher than 40 IU/L 
are measured on two separate occasions, some form of contracep­
tion should be recommended to prevent unwanted pregnancy.
Perimenopause represents an optimal period in which to evalu­
ate the general health of the mature woman and introduce mea­
sures to prepare her for the striking physiologic changes that come 
with menopause. The

physiologic changes that come 
with menopause. The patient and her clinician should attempt to 
achieve several important aims during perimenopause. The long-
term goal is to maintain an optimal quality of physical and social 
632
life. Another immediate objective is the detection of any major 
chronic disorders that occur with aging. The benefits and risks 
of hormone therapy should be discussed thoroughly at this time. 
Menopause Features
The median

ughly at this time. 
The median age at menopause is approximately 51 years.352 The 
age at menopause is probably determined in part by genetic fac­
tors, because mothers and daughters tend to experience meno­
pause at about the same age.353–355 Environmental factors may 
modify the age at menopause. For example, current smoking is 
associated with an earlier menopause, whereas alcohol consump­
tion delays menopause.352 Oral contraceptive use does not affect 
the age at which m

traceptive use does not affect 
the age at which menopause begins.
The symptoms frequently seen and related to decreased estro­
gen production in menopause include irregular frequency of men­
ses followed by amenorrhea, vasomotor instability manifested 
as hot flashes and sweats, urogenital atrophy giving rise to pain 
during intercourse and a variety of urinary symptoms, and conse­
quences of osteoporosis and CVD. The combination and extent of 
these symptoms vary widely for each patient. Some 

these symptoms vary widely for each patient. Some patients expe­
rience multiple severe symptoms that may be disabling, whereas 
others have no symptoms or only mild discomfort associated with 
perimenopause. 
Biosynthesis of Estrogen and Other Steroids in the 
Postmenopausal Woman
No follicular units can be detected histologically in the ovaries after 
menopause. In reproductive-age women, the granulosa cell of the 
ovulatory follicle is the major source of inhibin and estradiol. In the 
absenc

or source of inhibin and estradiol. In the 
absence of these factors that inhibit gonadotropin secretion, FSH 
and LH levels increase sharply after menopause. These levels peak a 
few years after menopause and decrease gradually and slightly there­
after.356 The postmenopausal serum level of either gonadotropin 
may be more than 100 IU/L. FSH levels are usually higher than LH 
levels because LH is cleared from the blood much more quickly and 
possibly because the low levels of inhibin in menopau

sibly because the low levels of inhibin in menopause selectively 
lead to increased FSH secretion. Nevertheless, increased LH is a 
major factor that maintains significant quantities of androstenedi­
one and testosterone secretion from the ovary, although the total 
production rates of both steroids decline after menopause.
The primary steroid products of the postmenopausal ovary are 
androstenedione and testosterone.97 The average premenopausal 
rate of production of androstenedione of 3 mg/day

rate of production of androstenedione of 3 mg/day is decreased 
by one-half to approximately 1.5 mg/day.97 The decrease primar­
ily results from a substantial reduction in the ovarian contribu­
tion to the circulating androstenedione pool. Adrenal secretion 
accounts for most of the androstenedione production in the post­
menopausal woman, with a smaller amount secreted from the 
ovary.97 DHEA and DHEAS originate almost exclusively from 
the adrenal gland and decline steadily with advancing age

enal gland and decline steadily with advancing age inde­
pendent of menopause. The serum levels of DHEA and DHEAS 
after menopause are about one-fourth of those in young adult 
women.357
Testosterone production is decreased by approximately one-
third after menopause.97 Total testosterone production can be 
approximated by the sum of ovarian secretion and peripheral for­
mation from androstenedione (see Fig. 17.26). In the premeno­
pausal woman, significant amounts of testosterone are produced 

significant amounts of testosterone are produced 
by conversion of androstenedione in extraovarian tissues. Because 
ovarian androstenedione secretion is substantially decreased after 
menopause, the decrease in postmenopausal testosterone produc­
tion is accounted for in large measure by a decrease in the relative 
contribution of extraovarian sources.97 With the disappearance of 
follicles and decreased estrogen, the elevated gonadotropins drive 
the remaining stromal tissue in the ovary to ma

e 
the remaining stromal tissue in the ovary to maintain testosterone 
secretion at levels observed during the premenopausal years. The 
contribution of the postmenopausal ovary to the total testoster­
one production is increased in the presence of seemingly unaltered 
ovarian secretion.
The most dramatic endocrine alteration of perimenopause 
involves the decline in the circulating level and production rate 
of estradiol. The average menopausal level of circulating estradiol 
is less than 20 pg

level of circulating estradiol 
is less than 20 pg/mL. The estradiol and estrone levels in post­
menopausal women are usually slightly less than those in adult 
men. Circulating estradiol in postmenopausal women (and men) 
is derived from the peripheral conversion of androstenedione 
to estrone, which is converted peripherally to estradiol (see Fig. 
17.19).97 The mean circulating level of estrone in postmenopausal 
women (37 pg/mL) is higher than that of estradiol. The average 
postmenopausal p

n that of estradiol. The average 
postmenopausal production rate of estrone is approximately 42 
μg every 24 hours. After menopause, almost all estrone and estra­
diol is derived from the peripheral aromatization of androstene­
dione. There is a dramatic change in the androgen-to-estrogen 
ratio because of the sharp decrease in estradiol level and the only 
slightly reduced testosterone. The frequent onset of mild hirsutism 
after menopause reflects this striking shift in the hormone ratio. 
Dur

cts this striking shift in the hormone ratio. 
During the postmenopausal years, DHEAS and DHEA levels 
continue to decline steadily with advancing age, whereas serum 
androstenedione, testosterone, estrone, and estradiol levels do not 
change significantly.356
The aromatization of androstenedione to estrone in extraovar­
ian tissues correlates positively with weight and advancing age (see 
Figs. 17.19 and 17.31).97 Body weight correlates positively with 
the circulating levels of estrone and est

ly with 
the circulating levels of estrone and estradiol. Because aromatase 
enzyme activity is present in significant quantities in adipose tissue, 
increased aromatization of androstenedione in overweight individu­
als may reflect the increased bulk of tissue containing the enzyme.97 
There is a twofold to fourfold increase in the specific activity of aro­
matase per cell with advancing age.97 An increased overall number 
of adipose fibroblasts with aromatase activity and a decrease in the 
le

 with aromatase activity and a decrease in the 
levels of SHBG increase the free estradiol level and contribute to the 
increased risk of endometrial cancer in obese women.97
In postmenopausal women, estrogen produced from andro­
stenedione peripherally in fat and skin and locally in breast cancer 
tissue promotes the growth of this malignancy (Fig. 17.35).97 The 
clinical relevance of extraovarian estrogen formation is exempli­
fied by the successful use of aromatase inhibitors as the current 

ssful use of aromatase inhibitors as the current 
endocrine treatment for postmenopausal breast cancer.97 
Postmenopausal Uterine Bleeding
Perimenopausal or postmenopausal bleeding can be caused by 
hormone administration or excessive extraovarian estrogen for­
mation. Irregular uterine bleeding is commonly observed dur­
ing the perimenopausal transition as anovulatory cycles alternate 
with ovulatory cycles. Uterine bleeding after menopause is less 
common if the patient is not receiving hormon

ess 
common if the patient is not receiving hormone therapy. Obese 
women are more likely to experience postmenopausal bleeding 
because of increased peripheral aromatization of adrenal andro­
stenedione. Patients receiving a continuous combination regi­
men of hormone therapy may experience unpredictable uterine 
bleeding. The major objective in these circumstances is to rule out 
endometrial malignancy. This can be best achieved by tissue diag­
nosis through an office endometrial biopsy using 

nosis through an office endometrial biopsy using a plastic cannula. 
633
Transvaginal ultrasonographic measurement of endometrial thick­
ness may be used in postmenopausal women to avoid unnecessary 
biopsies.135 A biopsy is required if an endometrial thickness of 5 
mm or greater is observed.
Unpredictable irregular uterine bleeding is observed in approx­
imately 20% of postmenopausal women receiving a long-term (>1 
year) c

enopausal women receiving a long-term (>1 
year) continuous estrogen-progestin combination. Before using 
ultrasonography and endometrial biopsy to explore the cause of 
bleeding that is assumed to arise from the intrauterine cavity, the 
clinician should rule out diseases of the vulva, vagina, and cer­
vix.135 Careful inspection of these organs along with a normal 
cervical Papanicolaou (Pap) smear within the past year is suffi­
cient to rule out the vulva, vagina, and cervix as potential sourc

t the vulva, vagina, and cervix as potential sources 
of bleeding. The causes of postmenopausal uterine bleeding are 
benign most of the time. Endometrial malignancy is encountered 
in patients with bleeding in only about 1% to 2% of postmeno­
pausal endometrial biopsies.358 Approximately three-fourths of 
endometrial biopsies from postmenopausal women reveal no 
pathologic change or an atrophic endometrium. Other histologic 
findings include hyperplasia (15%) and endometrial polyps (3%). 
Persi

erplasia (15%) and endometrial polyps (3%). 
Persistent unexplained uterine bleeding requires repeated evalua­
tion, biopsy, hysteroscopy, or dilatation and curettage. 
Hot Flashes
The most frequent and striking symptom during perimenopause 
is the hot flash. It typically occurs during the transition from 
perimenopause to postmenopause. The hot flash is also a major 
symptom of postmenopause, and it can occur up to 5 years after 
menopause.359 More than four-fifths of postmenopausal women 
expe

ore than four-fifths of postmenopausal women 
experience hot flashes within 3 months after the cessation of 
ovarian function, whether natural or surgical in origin. Of these 
women, more than three-fourths have hot flashes for more than 1 
year, and approximately one-half have them for up to 5 years.359 
Hot flashes lessen in frequency and intensity with advancing age, 
unlike other sequelae of menopause, which progress with time.
A hot flash is a subjective sensation of intense warmth of the 

 a subjective sensation of intense warmth of the 
upper body, which typically lasts for approximately 4 minutes 
but may range in duration from 30 seconds to 5 minutes. It can 
follow a prodrome of palpitations or headache and is frequently 
accompanied by weakness, faintness, or vertigo. The episode usu­
ally ends in profuse sweating and a cold sensation. Hot flashes 
may occur rarely or recur every few minutes. At night, hot flashes 
are more frequent and can be severe enough to awaken a woman

requent and can be severe enough to awaken a woman 
from sleep. They are also more intense during times of stress. In a 
cool environment, hot flashes are fewer, less intense, and shorter 
in duration than in a warm environment.360
The hot flash results from a sudden reduction of estrogen levels 
rather than from hypoestrogenism itself. Regardless of the cause of 
menopause—natural, surgical, or estrogen withdrawal caused by a 
GnRH agonist—hot flashes are associated with an acute and sig­
nific

lashes are associated with an acute and sig­
nificant drop in estrogen level. The consistent association between 
the onset of flashes and acute estrogen withdrawal is supported 
by the effectiveness of estrogen therapy and the absence of flashes 
in prolonged hypoestrogenic states, such as gonadal dysgenesis 
or hypothalamic amenorrhea. Hypogonadal women experience 
Post-
menopausal
ovary
Adipose
fibroblasts
Malignant
epithelial cell
Skin
A
A
Adipose
tissue
E1
E2
A
E1
E2
Periphera

ll
Skin
A
A
Adipose
tissue
E1
E2
A
E1
E2
tissues
Breast tumor tissue
Aromatase
Aromatase
precursor
17  HSD
β
• Fig. 17.35  Tissue sources of estrogen in postmenopausal breast cancer. The important pathologic roles 
of extraovarian (peripheral) and local estrogen biosynthesis are shown for an estrogen-dependent disease 
in postmenopausal women. The estrogen precursor androstenedione (A) originates primarily from the 
adrenal gland in the postmenopausal woman. Aromatase expr

 gland in the postmenopausal woman. Aromatase expression and enzyme activity in extraovarian 
tissues such as fat increase with advancing age. The aromatase activity in skin and subcutaneous adipose 
fibroblasts gives rise to formation of systemically available estrone (E1) and, to a smaller extent, estradiol 
(E2). The conversion of circulating A to E1 in undifferentiated breast adipose fibroblasts compacted around 
malignant epithelial cells and subsequent conversion of E1 to E2 in malignant e

d subsequent conversion of E1 to E2 in malignant epithelial cells provide high 
tissue concentrations of E2 for tumor growth. The clinical relevance of these findings is exemplified by 
the successful use of aromatase inhibitors to treat breast cancer in postmenopausal women. 17βHSD, 
reductive-type 17β-hydroxysteroid dehydrogenase.
634
hot flashes only after estrogen is administered and withdrawn.361 
Higher body mass index, and body fat in particula

Higher body mass index, and body fat in particular, is associ­
ated with greater vasomotor symptom reporting, primarily hot 
flashes.362
Other conditions cause symptoms similar to the menopausal 
hot flash. Sudden episodes of sweating and/or flushing may 
be caused by catecholamine-secreting or histamine-secreting 
tumors (e.g., pheochromocytoma, carcinoid), hyperthyroidism, 
or chronic infection (e.g., tuberculosis). The hot flash may also 
be psychosomatic in origin. In these circumstances, t

psychosomatic in origin. In these circumstances, the clinician 
should obtain a serum FSH level to confirm perimenopause or 
menopause before initiating hormone therapy. 
Urogenital Atrophy
The urogenital sinus gives rise to development of the lower 
vagina, vulva, and urethra during embryonic development, and 
these tissues are estrogen responsive. The decrease in estrogen at 
menopause causes the vaginal walls to become pale because of 
diminished vascularity and to thin down to only three or 

hed vascularity and to thin down to only three or four 
squamous epithelial cell layers. Loss of this protective mechanism 
leaves the thin, friable tissue vulnerable to infection and ulcer­
ation. The vagina also loses its rugae and becomes shorter and 
inelastic. Postmenopausal women may complain of symptoms 
caused by vaginal dryness, such as pain during intercourse, vaginal 
discharge, burning, itching, or bleeding. Genitourinary atrophy 
leads to a variety of symptoms that affect the ease a

ds to a variety of symptoms that affect the ease and quality 
of living. Intravaginal estrogen treatment can effectively alleviate 
these vaginal symptoms in the postmenopausal patient.363 
Postmenopausal Osteoporosis
Osteoporosis is a disorder characterized by low bone mass and 
microarchitectural deterioration of bone tissue, leading to 
enhanced bone fragility and a consequent increase in fracture risk. 
The World Health Organization criteria for defining and diag­
nosing osteoporosis in post

for defining and diag­
nosing osteoporosis in postmenopausal women is the finding of a 
T-score of –2.5 or lower at the lumbar spine, femur neck, or total 
hip by bone mineral density (BMD) testing.364 Osteoporosis is 
a public health concern that is associated with more than 2 mil­
lion fractures per year in the United States.364 The most frequent 
sites of fracture are the vertebral bodies, distal radius, and femoral 
neck. Most osteoporotic patients are postmenopausal women.
Osteoporosis in p

tients are postmenopausal women.
Osteoporosis in postmenopausal women is a function of 
advancing age and estrogen deficiency. Seventy-five percent or 
more of the bone loss in women during the first 15 years after 
menopause is attributed to estrogen deficiency rather than to 
aging.365,366 For the first 20 years after the cessation of ovarian 
estrogen secretion, postmenopausal osteoporosis accounts for 
a 50% reduction in trabecular bone and 30% loss of cortical 
bone.365,366 Vertebral bone i

0% loss of cortical 
bone.365,366 Vertebral bone is especially vulnerable because the tra­
becular portion of the vertebral bodies is metabolically very active 
and decreases dramatically in amount in response to estrogen 
deficiency. Vertebral bone mass is already significantly decreased 
in perimenopausal and early postmenopausal women who have 
rising FSH and decreasing estrogen levels, whereas bone loss from 
the radius is not detected until at least 1 year after menopause.366
The risk of fr

at least 1 year after menopause.366
The risk of fracture depends on two factors: the peak bone mass 
achieved at maturity (at approximately age 30) and the subsequent 
rate of bone loss. An accelerated rate of bone loss after menopause 
strongly predicts an increased risk of fracture. The unfavorable 
effects of low premenopausal bone mass and accelerated loss of 
bone after menopause are additive, and these individuals are at 
the highest risk for fracture. An increased rate of average bone los

or fracture. An increased rate of average bone loss 
during menopause is an indicator of lower endogenous estrogen 
levels, possibly because postmenopausal bone loss is considerably 
slower in women with increased adipose tissue mass and conse­
quent increased peripheral estrogen formation.230
Numerous studies showed that hormone therapy started at 
perimenopause prevents postmenopausal bone loss.367 Hormone 
therapy started at any age in a postmenopausal woman prevents 
additional bone loss. Th

enopausal woman prevents 
additional bone loss. The Women’s Health Initiative (WHI) trial 
results revealed the most conclusive evidence for a decreased num­
ber of vertebral and hip fractures in the group of postmenopausal 
women receiving estrogen plus progestin (E+P) or estrogen (E) 
only.368 The risk reductions were attenuated in both trials after 
the intervention was stopped; however, a significant hip fracture 
benefit persisted over 13 years for women assigned to conjugated 
equine estro

ars for women assigned to conjugated 
equine estrogens (CEEs) plus MPA.368 
Postmenopausal Hormone Therapy
A key and complex decision facing postmenopausal women is 
whether to use menopausal hormone therapy (HT). HT is the 
most effective treatment for vasomotor symptoms and may also 
improve vaginal dryness, sleep, and quality of life.369 Postmeno­
pausal women who have undergone a hysterectomy ordinarily 
receive HT with estrogen only (HT-E). A progestin is added to 
estrogen (HT-EP) in the p

A progestin is added to 
estrogen (HT-EP) in the postmenopausal woman with a uterus 
to prevent endometrial hyperplasia and cancer. Although either 
form of HT was originally prescribed primarily to treat vasomo­
tor symptoms, starting from the 1950s, HT-E and later HT-EP 
had been increasingly viewed as a way to forestall many chronic 
diseases of aging, including CVD, cognitive impairment, and 
osteoporotic fractures. Approximately 40% of postmenopausal 
women in the United States were using H

enopausal 
women in the United States were using HT shortly before the 
publication of the initial findings from the WHI.368,370 A number 
of observational studies had suggested benefits of HT for CVD, 
including coronary heart disease (CHD) and all-cause mortality, 
and an overall favorable benefit-risk profile.371 There had been, 
however, no large-scale randomized prevention trials to address 
the balance of risks and benefits of HT. In these observational 
studies, the apparent benefits of H

observational 
studies, the apparent benefits of HT may result in part from dif­
ferences between women who choose to take postmenopausal HT 
and women who do not; HT users tend to be healthier and have 
better access to medical care.372
Until the late 1990s, the most common practice was to treat all 
women disturbed by the symptoms of hormone deprivation (e.g., 
hot flashes) with HT and to use long-term hormonal prophylaxis 
against osteoporosis. The assumption that HT was cardioprotec­
tive pl

. The assumption that HT was cardioprotec­
tive played an important role in encouraging postmenopausal 
women to stay on this regimen indefinitely.373 This trend changed 
dramatically in the early 2000s, after publication of the principal 
results of two large randomized trials, the Heart and Estrogen/
Progestin Replacement Study (HERS) in 1998 and the WHI 
trial in 2002.374–376 The WHI results constituted the direct cause 
of discontinuation of HT in approximately 30% of postmeno­
pausal women.

HT in approximately 30% of postmeno­
pausal women.376 Debate continues regarding the applicability 
of the WHI results to all or various subsets of postmenopausal 
women.373 Although the use of HT has dramatically decreased, 
the WHI trials raised a number of important issues that require 
further studies to address.373,377
The WHI investigators sought to determine the benefits and 
risks of HT taken for chronic disease prevention in predominantly 
635
Chapter 17  Physiology and Pathology  of th

y 
635
healthy postmenopausal women aged 50 to 79 years at enroll­
ment.368,370 The WHI trials elected to include the most popular 
HT formulations in the United States at that time, which were 
CEEs (0.625 mg/day) plus MPA (2.5 mg/day) and CEEs (0.625 
mg/day) alone.368,370 Starting from 2002, a large number of origi­
nal and review articles came from the WHI trials. A comprehen­
sive integrated overview of findings from the 

en­
sive integrated overview of findings from the two WHI HT trials 
with extended postintervention follow-up for a median of 13 years 
was published.368 Here the WHI investigators reported the key 
outcomes and stratified results by age and by time since meno­
pause onset.368 A discussion of the WHI and other large trials and 
how they changed HT practices is provided next.
The Long-Term Benefits and Side Effects of 
Hormone Therapy
Unless there are contraindications, HT should be offered to ev

 are contraindications, HT should be offered to every 
menopausal woman suffering from vasomotor symptoms, vulvo­
vaginal atrophy, or dyspareunia. For these indications, HT may 
be maintained for up to 5 years based on the preference of the 
woman appropriately informed of the risks and benefits.
Menopause is also associated with increased risks for chronic dis­
orders, including heart disease, osteoporosis, cognitive impairment, 
or certain malignancies. Previously, hormone therapy was used for

gnancies. Previously, hormone therapy was used for 
the primary prevention of these chronic conditions, but results from 
the WHI and other randomized clinical trials provided a clearer 
picture regarding the benefits and risks of HT and have provided 
insights to improve decision making in HT practices.368,370,373–376 
A number of clinical characteristics are useful in identifying post­
menopausal women for whom benefits of HT are likely to outweigh 
the risks.368 Age and time since menopause w

eigh 
the risks.368 Age and time since menopause were considered as pre­
dictors of risks of HT.368 The role of age was particularly promi­
nent in the HT-E (estrogen only, for women without uteri) trial.368 
Younger women (50–59 years) taking HT-E had more favorable 
results for all-cause mortality, myocardial infarction, and the global 
index but not for stroke and venous thrombosis.368 The influence of 
age was less clear for HT-EP (women with intact uteri, taking estro­
gen/progestin), owing

 intact uteri, taking estro­
gen/progestin), owing to increased risks of breast cancer, stroke, and 
venous thrombosis in all age groups. Overall, risks of adverse events 
were lower in younger than in older women among both HT-E and 
HT-EP users.368
A recent benefit versus harm analysis of 18 recent trials, includ­
ing the two WHI trials (HT-EP and HT-E), is summarized in 
Table 17.9, showing intervention-phase findings such as absolute 
risks per 10,000 women per year, rate differences, and re

er 10,000 women per year, rate differences, and relative 
risks for a wide range of chronic disease outcomes.378 HT-E users 
compared with placebo had significantly lower risks, per 10,000 
person-years, for diabetes (–19 cases [95% CI, –34 to –3]) and 
frac­tures (–53 cases [95% CI, –69 to –39]). On the other hand, 
increased risks for HT-E users (per 10,000 person-years) included 
gallbladder disease (30 more cases [95% CI, 16–48]), stroke (11 
more cases [95% CI, 2–23]), venous thromboembolis

more cases [95% CI, 2–23]), venous thromboembolism (11 more 
cases [95% CI, 3–22]), and urinary incontinence (1261 more 
cases [95% CI, 880–1689]) (see Table 17.9).378
HT-EP users compared with placebo had lower risks, per 
10,000 person-years, for colorectal cancer (–6 cases [95% CI, –9 to 
–1]), diabetes (–14 cases [95% CI, –24 to –3), and fractures (–44 
cases [95% CI, –71 to –13).378 Increased risks for HT-EP users, 
per 10,000 person-years, included invasive breast cancer (9 more 
cases [9

 included invasive breast cancer (9 more 
cases [95% CI, 1–19]), probable dementia (22 more cases [95% 
CI, 4–53]), gallbladder disease (21 more cases [95% CI, 10–34]), 
stroke (9 more cases [95% CI, 2–19]), urinary incontinence (876 
more cases [95% CI, 606–1168]), and venous thromboembolism 
(21 more cases [95% CI, 12–33]).378 The authors of this recent 
meta-analysis and review, including 18 trials (including the WHI 
trials), stated “the available evidence regarding benefits and harms 
of ea

lable evidence regarding benefits and harms 
of early initiation of hormone therapy is inconclusive.”378
An analysis of WHI results indicated that neither HT modality 
altered the mortality rate.368,370 Interestingly, breast cancer find­
ings were divergent between the two trials, and for both cancer 
and CVD outcomes, results tended to be more adverse for HT-EP 
than for HT-E.368,370 There was a clear and sizable benefit for 
reducing vasomotor symptoms; the results for other quality of life 
m

symptoms; the results for other quality of life 
measures, however, varied widely.368,370
The age of the woman and time since menopause seemed to 
be the key modifiers of most of the WHI results.368,370 In the 
HT-E trial, younger women 50 to 59 years of age had more favor­
able results for all-cause mortality, myocardial infarction, colorec­
tal cancer, and the global index.368 Age, however, did not affect 
increased risks of stroke, venous thrombosis, gallbladder disease, 
or urinary incontine

mbosis, gallbladder disease, 
or urinary incontinence associated with both HT regimens. 
Among HT-EP users, breast cancer was an additional adverse 
effect. Although risk of myocardial infarction varied by time since 
menopause, the overall risks of chronic disease events outweighed 
benefits across all age groups for HT-EP.368 In general, variations 
in hazard ratios by age were more apparent among the HT-E users 
than in the HT-EP users.368,370 However, risks of unfavorable out­
comes were muc

 However, risks of unfavorable out­
comes were much lower in younger women than in older women 
in both HT-E and HT-EP trials. Absolute risks measured by the 
global index per 10,000 women per year on HT-EP varied from 
12 excess cases for ages 50 to 59 years to 38 excess cases for ages 
70 to 79 years and, for HT-E, from 19 fewer cases for ages 50 to 
59 years to 51 excess cases for the age group 70 to 79 years.370 
Overall, findings from both WHI trials suggest that HT has a 
harmful effect on

HI trials suggest that HT has a 
harmful effect on CHD risk in older women and those at higher 
baseline risk of CVD, whereas the results in younger and low-risk 
women tend to be neutral for HT-EP and in a favorable direction 
for HT-E.370 HT use had a negative impact on cognitive function 
among women older than 65 years, whereas HT effect was neutral 
for women younger than 55 years of age.370,379 
Risks and Contraindications of Hormone Therapy
Coronary Heart Disease
Data suggest that initiat

y
Data suggest that initiation of HT many years after menopause 
is associated with excess CHD risk, whereas HT given for a lim­
ited period soon after menopause is not. HT-EP or HT-E does 
not prevent CHD as was previously proposed. To the contrary, 
there may be a small but significant increase in the rate of CHD 
among HT-EP users; these women with preexisting CHD and 
healthy women are at risk.368,380 HT-E, on the other hand, does 
not increase the risk for CHD in heal

 hand, does 
not increase the risk for CHD in healthy women.368 
Stroke
Another outcome that was consistent across the two WHI trials and 
HERS is the increased risk of stroke among women assigned to HT-E 
or HT-EP. Increased stroke risk may be attributable to the estrogen 
component of the hormone regimen because its increase is statistically 
significant among both HT-E and HT-EP users.368,370,380 
Pulmonary Embolism
A pattern of increased pulmonary embolism was observed in all 
randomized stu

onary embolism was observed in all 
randomized studies, although the risk was attenuated and was not 
statistically significant in the WHI HT-E trial.368,370,380 
636
Breast Cancer
Findings of the WHI HT-E trial were markedly different from 
those of the HERS and WHI HT-EP trials with respect to breast 
cancer risk.370 The WHI HT-E trial indicated a trend, albeit sta­
tistically not significant, toward a lower breast cancer risk during 
the interventi

 a lower breast cancer risk during 
the intervention phase. This protective effect was statistically sig­
nificant during cumulative (intervention plus postintervention) 
follow-up (HR 0.79, CI 0.65–0.97).368 This result was in con­
trast to findings of an observational literature that mostly reports 
a moderate increase in risk with estrogen-alone preparations.381 
  Absolute Risk Reductions or Increases for Women Treated With Estrogen Alone and With Estrogen Plus 
Progestin
ESTROGEN ALONE
Out

d With Estrogen Plus 
Progestin
Outcome
No. of Trials
Strength of Evidence
RR (95% of CI)
Absolute Risk 
Difference, Events per 
10,000 Women (95% CI)
Breast cancer (invasive)
1
Moderate
0.79 (0.61–1.01)
-52 (-97–3)
Colorectal cancer
1
Low
1.15 (0.81–1.63)
16 (-20–73)
Lung cancer
1
Low
1.04 (0.73–1.48)
5 (-30–54)
Coronary heart disease
3
High
0.95 (0.79–1.14)
-20 (-82–55)
Dementia (probable)
1
Low
1.49 (0.84–2.66)
63 (-21–213)
Diabetes
1
Moderate
0.87 (0.77–0.98)
-137 (-242–-21)
F

etes
1
Moderate
Fractures (osteoporotic)
1
High
0.73 (0.65–0.80)
-382 (-495–-283)
Gallbladder disease
1
Moderate
1.63 (1.33–2.00)
213 (111–338)
Stroke
1
Moderate
1.33 (1.06–1.67)
79 (14–160)
Urinary incontinence
1
Moderate
1.53 (1.37–1.71)
1261 (880–1689)
Venous thromboembolism
1
Moderate
1.43 (1.11–1.85)
78 (20–153)
All-cause mortality
3
High
1.01 (0.88–1.17)
6 (-74–96)
ESTROGEN + PROGESTIN
Outcome

1
High
1.27 (1.0–1.56)
52 (6–107)
Cervical cancer
1
Low
1.52 (0.50–4.66)
3 (-3–23)
1
Moderate
0.64 (0.44–0.91)
-33 (-52–-8)
Endometrial cancer
1
Low
0.86 (0.51–1.44)
-5 (-18–6)
1
Moderate
1.06 (0.77–1.46)
5 (-20–40)
Ovarian cancer
1
Low
1.43 (0.76–2.69)
9 (-5–33)
3
High
1.23 (1.00–1.52)
41 (7–93)
1
Moder

 (1.00–1.52)
41 (7–93)
1
Moderate
1.97 (1.16–3.33)
88 (15–213)
Diabetes
1
Moderate
0.84 (0.72–0.97)
-77 (-135–-15)
5
High
0.80 (0.68–0.94)
-221 (-353–-66)
1
Moderate
1.59 (1.29–1.97)
116 (57–190)
Stroke
1
High
1.39 (1.09–1.77)
53 (12–104)
1
Moderate
1.39 (1.27–1.52)
876 (606–1168)
1
Moderate
1.95 (1.54–2.47)
120 (68–185)
3
Moderate
4 (-48–68)
Modified 

y
3
Moderate
4 (-48–68)
Modified from Gartlehner G, Patel SV, Feltner C, et al. Hormone therapy for the primary prevention of chronic conditions in postmenopausal women: evidence report and systematic review for the us preven­
tive services task force. JAMA. 2017;318(22):2234–2249. © 2017 American Medical Association.
   
TABLE 17.9
637
However, after control for prior use of postmenopausal HT and 
additional 

or prior use of postmenopausal HT and 
additional control for time from menopause to first use of post­
menopausal HT, the hazard ratios agreed closely between the 
observational and randomized trial data.381 Nonetheless, the 
higher risk for breast cancer observed in the HT-EP trials prob­
ably represents a harmful additional effect of MPA, the proges­
tin used in these studies.368 The increased breast cancer risk was 
statistically significant in the WHI HT-EP trial, which demon­
strated an at

in the WHI HT-EP trial, which demon­
strated an attributable risk of 9 cases per 10,000 person-years 
during the 5.6-year intervention phase.368 This risk persisted 
and remained significant during the postintervention phase, 
giving rise to an overall risk of 9 cases per 10,000 person-years 
during the 13 years of cumulative follow-up.368 This result was 
matched by a trend of the same magnitude in the HERS study 
and was supported by evidence from large observational studies 
suggesting that t

rom large observational studies 
suggesting that the addition of MPA or another progestin to 
estrogen may significantly increase the risk for breast cancer (see 
Table 17.9).382 
Dementia
In postmenopausal women 65 years of age or older, HT-EP sig­
nificantly increased risk and resulted in an additional 23 cases of 
probable dementia per 10,000 women per year.383 Alzheimer dis­
ease was the most common classification of dementia. A similar 
trend was observed for HT-E but did not reach statisti

d was observed for HT-E but did not reach statistical signifi­
cance.383 When the data were pooled, HT significantly increased 
probable dementia risk.383 
Worsening of hypertriglyceridemia is a rare side effect of HT, par­
ticularly in patients with severe familial hypertriglyceridemia. An oral 
estrogen regimen can hasten development of severe hypertriglyceride­
mia in women with previously elevated triglyceride levels, which can 
precipitate pancreatitis.384 Estrogen repl

ch can 
precipitate pancreatitis.384 Estrogen replacement is relatively contra­
indicated in women with substantially increased triglyceride levels. 
Gallbladder Disease
Both WHI trials (HT-E and HT-EP) showed substantially greater 
risk for any gallbladder disease or surgery with estrogen.385 Both 
trials indicated a higher risk for cholecystitis and for cholelithiasis. 
Women on either HT regimen were more likely to undergo cho­
lecystectomy.368,370 These data suggest an increase in risk of bi

8,370 These data suggest an increase in risk of biliary 
tract disease among postmenopausal women using estrogen ther­
apy. The morbidity and cost associated with these outcomes may 
need to be considered in decisions regarding the use of estrogen 
therapy. Preexisting gallbladder disease is a relative contraindica­
tion for estrogen replacement. 
Urinary Incontinence
Both HT-E and HT-EP users are at strikingly higher risk for uri­
nary incontinence.378 The type of incontinence or the underly­
i

nce.378 The type of incontinence or the underly­
ing mechanisms for this side effect are not well understood. Thus, 
until these issues are resolved, urinary incontinence should be 
considered at least a relative contraindication. 
Indications for Hormone Therapy
HT-E or HT-EP reliably treats hot flashes in most women.386 
Currently, hot flashes constitute the most common indication for 
a short course of HT (<5 years). 
Fractures
HT-EP or HT-E significantly decreased the incidence o

EP or HT-E significantly decreased the incidence of hip, ver­
tebral, and other osteoporotic fractures.368 In this case, the results 
of observational studies of estrogen and fracture risk and of trials 
using a surrogate endpoint (i.e., BMD) agree with the results of 
clinical trials of fracture prevention.378 
Diabetes
Diabetes was significantly less common with both HT-E and 
HT-EP users.368,370,378 Thus diabetes prevention should be con­
sidered in menopausal women starting either form of HT

red in menopausal women starting either form of HT.378 
Post-WHI Recommendations for Hormone 
Therapy
A number of publications with flow charts are available to cli­
nicians for decision making about HT use.370 Here, a number 
of useful principles are offered to guide the clinician and the 
patient for an optimal plan that suits her short-term and long-
term needs and expectations. Decision making for women 
interested in HT involves balancing the potential benefits of 
HT against the potential 

e potential benefits of 
HT against the potential risks.370,387 HT is extremely effective 
for treatment of hot flashes, and for this indication women 
with intact uteri should receive HT-EP, whereas women with­
out uteri should be prescribed HT-E because the only known 
benefit of adding a progestin is to prevent endometrial can­
cer. Overall, HT-E seems to be associated with less risk than 
HT-EP.386 Nonetheless, HT-E also has adverse effects, and it 
is prudent to keep the dose low and the du

and it 
is prudent to keep the dose low and the duration of treatment 
short for all HT regimens.386
Many clinicians and epidemiologists agree that short-term 
estrogen therapy using the lowest effective estrogen dose is a rea­
sonable option for recently menopausal women who have moder­
ate to severe symptoms and no previous history or elevated risk of 
CHD, stroke, breast cancer, or venous thromboembolism.370,387 
HT usually lasts for 2 to 3 years but rarely more than 5 years, 
because menopau

ars but rarely more than 5 years, 
because menopausal symptoms diminish after several years, 
whereas the risk of breast cancer increases with longer duration 
of HT.386
A small group of women may need long-term therapy for 
severe, persistent symptoms after stopping HT. These few women 
may be encouraged to first try nonhormonal options such as selec­
tive serotonin reuptake inhibitors (SSRIs); estrogen treatment 
should be resumed only if these alternatives are not helpful.369,386 
For isolate

 alternatives are not helpful.369,386 
For isolated symptoms of genitourinary atrophy, low-dose vaginal 
estrogen with relatively lesser systemic absorption or endometrial 
effects is highly beneficial.387
In the absence of evidence for an overall net benefit of post­
menopausal treatment with HT-E and with the evidence that 
HT-EP is harmful, neither therapy should be used for preventing 
CHD or improving mental function.378 
Target Groups for Hormone Therapy
In women with gonadal dysgenesis an

ormone Therapy
In women with gonadal dysgenesis and surgical menopause, the 
duration of estrogen deprivation is prolonged. Estrogen replace­
ment is recommended for these patients for reduction of hot 
flashes and for long-term prophylaxis against osteoporosis and 
target organ atrophy. A low-dose contraceptive may be offered to 
nonsmoking women until the age of 45 years. After that point, 
638
doses of estrogen equivalent to 0.625 mg of conjugated 

 of estrogen equivalent to 0.625 mg of conjugated equine 
estrogens may be more appropriate because of a sharp age-related 
increase in risk for thromboembolic events. The physician should 
recommend a continuous or cyclical estrogen-progestin combina­
tion to those women with a uterus and an estrogen-only regimen 
to women without a uterus.
During perimenopause, hot flashes can be suppressed with 
an estrogen-progestin combination. Because bone loss related 
to estrogen deprivation also begins 

loss related 
to estrogen deprivation also begins during this period, a benefit 
for women who take HT to prevent hot flashes is that bone loss 
will not start for the few years of therapy.388 In perimenopausal 
women, unexplained uterine bleeding should be evaluated with 
an endometrial biopsy before the start of HT. 
Estrogen Preparations and Beneficial Dose of 
Estrogen
Oral Estrogens: Combined Conjugated Equine Estrogens
The amount of estrogen that provides effective treatment for 
hot flash

n that provides effective treatment for 
hot flashes varies. For this purpose, it is reasonable to start with 
CEEs at 0.3 mg/day (or micronized estradiol at 0.5 mg/day) and 
gradually increase this dose to CEEs at 0.625 mg/day (equiva­
lent to micronized estradiol at 1 mg/day) and 1.25 mg/day 
(equivalent to micronized estradiol at 2 mg/day). If hot flashes in 
a postmenopausal woman are not alleviated by 1.25 mg/day of 
CEEs or an equivalent transdermal estradiol dose, it is unlikely 
that hig

ansdermal estradiol dose, it is unlikely 
that higher doses will be effective. In this case, alternative diag­
noses (e.g., tuberculosis, depression, thyroid disease) should be 
ruled out.
Low-dose estrogen (0.3 mg/day of CEEs or 0.5 mg/day of 
micronized estradiol) maintains blood estradiol levels between 17 
and 32 pg/mL (average, 22 pg/mL) and may be sufficient for pre­
serving bone density and for alleviating menopausal symptoms 
in early postmenopausal women.389 Osteoporosis and fracture 
r

tmenopausal women.389 Osteoporosis and fracture 
risk should be assessed via periodic dual-energy x-ray absorptiom­
etry (DXA) scans, especially because low-dose estrogen prepara­
tions may not prevent bone loss. The effect of estrogen on arterial 
thrombosis is probably dose related.389 When choosing a dose for 
HT, it is imperative to achieve and maintain the lowest beneficial 
levels of circulating estradiol and to avoid higher levels to mini­
mize the risk of thrombosis.
The addition of a pr

The addition of a progestin, either cyclically or continuously, 
to concomitant estrogen replacement reduces the risk of estrogen-
induced endometrial hyperplasia or carcinoma but poses addi­
tional problems, which include regular withdrawal bleeding in 
up to 90% of women treated with cyclic therapy and irregular 
spotting in 20% of women treated with continuous estrogen plus 
progestin.390 Progestins appear to reduce the beneficial effects of 
estrogen on high-den

ce the beneficial effects of 
estrogen on high-density lipoprotein (HDL) and LDL cholesterol 
and to increase the risks of pulmonary embolism, CHD, and 
breast cancer.368,370,380
A time-honored sequential regimen involves oral adminis­
tration of 0.625 mg of CEEs or the equivalent dose of a variety 
of available products on days 1 through 25 of each month (Fig. 
17.36). A daily dose of 5 mg of MPA is added on days 12 through 
25 or on days 16 through 25. Withdrawal bleeding is expected on 
or af

ough 25. Withdrawal bleeding is expected on 
or after day 26 of each month. Another common cyclic regimen 
involves continuous oral administration of 0.625 mg of CEEs or 
the equivalent daily dose (see Fig. 17.36). A daily dose of 5 mg of 
MPA is added for the first 10 to 14 days of every month. One-
year randomized trial data indicate that the 5-mg dose protects 
the endometrium as well as the 10-mg dose does.391 Progestin 
withdrawal bleeding occurs in 90% of women with a sequential 
or cyclic

ccurs in 90% of women with a sequential 
or cyclic regimen.392,393 These regimens can also cause adverse 
symptoms related to the relatively high daily doses of progestin, 
including breast tenderness, bloating, fluid retention, and depres­
sion. The lowest possible dose of a progestin is recommended.
The continuous combined method of treatment has the 
potential benefit of reduced bleeding and amenorrhea, but 
it is occasionally complicated by breakthrough bleeding (see 
Fig. 17.36).392,393 In 

reakthrough bleeding (see 
Fig. 17.36).392,393 In this regimen, a combination of 0.625 mg 
of CEEs and 2.5 mg of MPA is given orally every day. The 
continuous combination regimen is simple and convenient, 
and it is associated with an incidence of amenorrhea in 80% 
of patients after at least 6 months of use. The other 20% of 
patients continue to experience some degree of unpredictable 
spotting. Overall compliance is much better in users of the 
continuous combination regimen. Moreover, the l

e 
continuous combination regimen. Moreover, the lower daily 
dose of MPA is associated with a lower incidence of breast ten­
derness with this regimen. Other estrogen-progestin combina­
tions are also available for similar continuous use.
Cyclic progestin has also been used at less frequent intervals, such 
as every 3 to 6 months. When added to standard-dosage estrogen, 
10 mg of MPA every 3 months for 14 days produced a 1.5% rate of 
hyperplasia (a rate low enough to be interpreted as endometr

a (a rate low enough to be interpreted as endometrial pro­
tection), and long-term use of MPA at 6-month intervals was associ­
ated with a low rate of endometrial cancer.394,395 However, clinicians 
have not determined the optimal progestin dosage and schedule to use 
with low-dosage estrogen. Low-dosage estrogen use can reasonably be 
assumed to require less progestin for protection of the endometrium.
Day 1
A
B
C
Day 1
1
10/14
1
14
Continuous (daily)
Day 1
Day 1
Continuous (

daily)
Day 1
Day 1
E
E
P
P
1
14
P
E
P
D
Day 1
E
1
10/14
P
1/2 mo
2 1/2 mo
1/2 mo
1
10/14
P
1
E
P
P
25
25
1
25
25
12/16
25
12/16
E
P
25
12/16
• Fig. 17.36  Regimens of hormone therapy. Estrogen (E) is replaced in a 
postmenopausal woman to prevent osteoporosis, urogenital atrophy, and 
hot flashes. In the postmenopausal woman with a uterus, a progestin (P) 
should be added to estrogen to prevent endometrial hyperplasia and can­
cer. E and P

vent endometrial hyperplasia and can­
cer. E and P can be administered in several ways. (A, B) Postmenopausal 
women receiving hormone therapy have predictable withdrawal bleeding 
episodes after each P course. (C) E and P are administered together. After 
a year of continuous combination therapy, the rate of unpredictable break­
through spotting is 20%. (D) This relatively new regimen was introduced to 
minimize the harmful effects of a progestin. Its long-term safety for endo­
metrial hyperpla

n. Its long-term safety for endo­
metrial hyperplasia or cancer risk is unknown. Predictable bleeding after 
each P course every 3 months is presumed to be reassuring.
639
Most postmenopausal women can switch their HT regimen 
from standard-dosage HT to low-dosage estrogen combined with 
a 15-day MPA treatment at 3-month intervals or start HT at this 
low dose.389 Although its long-term safety has not been proven with 
respect

long-term safety has not been proven with 
respect to endometrial hyperplasia, the following regimen appears 
to be a reasonable compromise for treating hot flashes and prevent­
ing osteoporosis while minimizing the harmful effects of progestins 
and high-dose estrogen389: 0.3 mg/day of CEEs or 0.025 mg of 
transdermal estradiol is administered continuously. Every 3 months, 
a 14-day course of 5 mg of MPA is added (see Fig. 17.36D). 
Endometrial biopsy is not required in the presence of withdraw

biopsy is not required in the presence of withdrawal 
bleeding after each periodic progestin intake and in the absence of 
irregular bleeding. This regimen may be continued for up to 5 years. 
After discontinuation of HT, the postmenopausal woman can be 
switched to a bisphosphonate or a selective estrogen receptor modu­
lator (SERM) for bone protection if needed. 
Transdermal Estrogen
Transdermal estrogen preparations appear to be as effective as oral 
estrogens for treating hot flashes and mai

s oral 
estrogens for treating hot flashes and maintaining BMD, but they 
have different metabolic profiles. Oral estrogens seem to have favor­
able effects on lipoprotein profiles. However, oral estrogens are asso­
ciated with several disadvantages, including unfavorable changes in 
serum levels of triglycerides, C-reactive protein, fibrinogen, factor 
VII, and plasminogen activator inhibitor type 1.387 A meta-analysis 
of clinical trials suggested a higher risk of venous thromboembolic 
events

ted a higher risk of venous thromboembolic 
events among oral HT users compared with transdermal estrogen 
users.387 Clinical trial data that address the effect of transdermal 
estrogen on CHD and stroke risk are limited.387
A daily dose of 0.05 mg of transdermal estradiol is equivalent 
to 0.625 mg of oral CEEs or 1 mg of oral micronized estradiol. A 
lower-dose transdermal preparation includes 0.025 mg of estradiol 
(equivalent to 0.3 mg of oral CEEs). An ultralow-dose transdermal 
estradiol p

l CEEs). An ultralow-dose transdermal 
estradiol preparation (0.014 mg) is also available. In women with 
symptoms not responding to smaller doses, high-dose transdermal 
estradiol at 0.1 mg/day (equivalent to CEEs at 1.25 mg/day) may 
be used. For the average menopausal woman with hot flashes, it is 
reasonable to start with a daily dose of 0.025 mg, which should be 
accompanied by a progestin in a woman with a uterus. 
Vaginal Estrogen
Vaginal estrogen formulations are the first choice for the

estrogen formulations are the first choice for the initial 
management of menopause-related vaginal atrophy symptoms. 
Low-dose vaginal tablets, rings, and creams administered via the 
vagina are equally effective when used for relief of vulvovaginal 
symptoms. A usual starting dose is 0.625 mg CEEs (in 1 g cream) 
applied vaginally daily for 1 week; the following maintenance dose 
is twice weekly. A 12-week study designed to elucidate the low­
est effective dose of estradiol cream for relieving

st effective dose of estradiol cream for relieving vaginal symp­
toms reported that 100% of women responded to the lowest dose 
tested (10 μg daily). Circulating estradiol levels remained in the 
postmenopausal range (3–10 pg/mL, 13.6–36.7 pmol/L) using a 
highly sensitive assay, and improvements in vaginal cytologic fea­
tures and decreases in vaginal pH were significant. No endome­
trial hyperplasia was noted during the course of the study.396 
Management of Breakthrough Bleeding During 
Postm

Approximately 90% of women receiving estrogen plus cyclic 
administration of a progestin have monthly progestin with­
drawal bleeding in a predictable fashion. Continuous combined 
estrogen-progestin therapy causes breakthrough bleeding in 
approximately 40% of women during the first 6 months, with 
the remaining 60% being amenorrheic. The pattern of vaginal 
bleeding in women taking the continuous combined regimen is 
u

women taking the continuous combined regimen is 
unpredictable and causes anxiety in most patients, but the inci­
dence of breakthrough bleeding decreases to 20% after 1 year of 
treatment.392,393,397 Nevertheless, breakthrough bleeding remains 
the most important reason for discontinuance of this therapy. A 
significant number of patients find it unacceptable and prefer to 
switch to a cyclic progestin regimen or to discontinue HT alto­
gether. There is no effective pharmacologic method to mana

There is no effective pharmacologic method to manage 
the breakthrough bleeding associated with continuous combined 
estrogen-progestin regimens. The physician can only reassure the 
patient that the bleeding is likely to subside within 1 year from the 
start of HT. If breakthrough bleeding continues beyond 1 year, the 
regimen may be changed to daily estrogen plus cyclic progestin.
HT can be started in the amenorrheic postmenopausal patient 
at any time. Perimenopausal women with oligomenorrhea

any time. Perimenopausal women with oligomenorrhea, hot 
flashes, or other associated symptoms can also be treated with HT. 
In the oligomenorrheic patient, an HT regimen may be initiated 
on day 3 of one of the infrequent menses. If the candidate for HT 
does not have irregular uterine bleeding, it is not essential to per­
form an endometrial biopsy routinely before beginning treatment. 
Studies indicate that asymptomatic postmenopausal women rarely 
have endometrial abnormalities.341,397,398 P

rely 
have endometrial abnormalities.341,397,398 Pretreatment biopsies 
using a thin plastic biopsy cannula in the office may be limited 
to patients who are at higher risk for endometrial hyperplasia 
(e.g., unpredictable uterine bleeding, history of PCOS or chronic 
anovulation, obesity, liver disease, diabetes mellitus).
Prescribing a combined estrogen-progestin regimen does not 
preclude the development of endometrial cancer.399 Therefore it is 
necessary to rule out endometrial malignancy i

is 
necessary to rule out endometrial malignancy in women receiving 
HT who are experiencing irregular uterine bleeding. The impor­
tant task is to differentiate breakthrough bleeding from bleeding 
induced by hyperplasia or cancer. Because breakthrough bleeding 
is extremely common, many biopsies must be performed to detect 
a rare case of endometrial abnormality during HT. To decrease the 
number of endometrial biopsies, a screening method using trans­
vaginal ultrasonography has been introduc

g trans­
vaginal ultrasonography has been introduced.135 The thickness of 
the postmenopausal endometrium as measured by transvaginal 
ultrasonography in postmenopausal women correlates with the 
presence or absence of pathologic changes.135 Patients receiving a 
cyclic or daily combination HT regimen who have an endometrial 
thickness of less than 5 mm can be managed conservatively.400–402 
An endometrial thickness equal to or greater than 5 mm requires 
biopsy. Based on this algorithm, it is e

requires 
biopsy. Based on this algorithm, it is estimated that 50% to 75% 
of bleeding patients receiving HT and evaluated by ultrasonogra­
phy require biopsy.135 
Management of Menopausal Symptoms in 
Breast Cancer Survivors
Vasomotor symptoms constitute a major problem for survivors 
of breast cancer. Approximately 65% of women become symp­
tomatic with hot flashes (mostly severe) after treatment for breast 
cancer.403 Hot flashes are encountered more frequently among 
tamoxifen users and wom

red more frequently among 
tamoxifen users and women treated with chemotherapy. Up to 
90% of premenopausal women who receive chemotherapy and 
tamoxifen have vasomotor symptoms.403
Breast cancer survivors often seek relief from hot flashes.404 HT 
is typically withheld from women with breast cancer because of 
concerns that estrogen may stimulate recurrence. One randomized 
640
study showed that after extended follow-up, there was a statis­
tically s

 extended follow-up, there was a statis­
tically significant increased risk of a new breast cancer event in 
survivors who took HT.405 In this randomized, non–placebo-
controlled study, 442 women assigned to receive HT-EP/HT-E or 
best symptomatic management without hormones were followed 
for a median of 4 years. Thirty-nine of the 221 women in the HT 
arm and 17 of the 221 women in the control arm had a new breast 
cancer diagnosed (hazard ratio, 2.4; 95% CI, 1.3–4.2). Cumula­
tive breast canc

o, 2.4; 95% CI, 1.3–4.2). Cumula­
tive breast cancer incidences at 5 years were 22.2% in the HT arm 
and 8% in the control arm. No difference in mortality rate from 
breast cancer was found.405
Because of higher breast cancer recurrence associated with HT, 
many breast cancer survivors have sought nonhormonal alterna­
tives, including other pharmaceutic agents, herbal or dietary rem­
edies, and mind-body or behavioral therapies.403 Mind-body or 
behavioral treatments for hot flashes are particul

behavioral treatments for hot flashes are particularly attractive to 
survivors of breast cancer because they do not have the side effects 
caused by pharmaceutic agents, but it is not yet known whether 
they are effective.386,403,404
Several nonhormonal pharmaceutic agents have been used off-
label for women who cannot take or elect not to take HT. Among 
the non-HT treatments, SSRIs, serotonin-norepinephrine reup­
take inhibitors (SNRIs), clonidine, and gabapentin were found to 
be more effect

dine, and gabapentin were found to 
be more effective than placebo.387 Although all of these nonsteroi­
dal medications could reduce the number of hot flashes per day, 
their beneficial effects were far less than what has been observed 
with estrogen, and these drugs are all associated with significant 
side effects that can limit their use in some women. Extracts of 
black cohosh or red clover were found to be ineffective, and the 
results for soy isoflavone extracts were mixed.387 The benefici

oy isoflavone extracts were mixed.387 The beneficial 
doses for hot flashes include paroxetine (an SSRI) at 10 to 20 
mg/day, paroxetine controlled release at 12.5 to 25 mg per day, 
venlafaxine (an SNRI) at 75 mg/day, desvenlafaxine (a SNRI) 
at 100 mg/day, and gabapentin at 900 mg/day.386 Gabapentin 
should be started with a smaller dose given at bedtime, and the 
dose should be gradually increased. Paroxetine reduces the metab­
olism of tamoxifen to its most active metabolite, endoxifen, and 

fen to its most active metabolite, endoxifen, and 
should be avoided in women with breast cancer who are receiving 
tamoxifen.387
For long-term prophylaxis against osteoporosis in breast can­
cer survivors, tamoxifen, raloxifene, or a bisphosphonate are viable 
options. However, tamoxifen and raloxifene intensify hot flashes.404 
Selective Estrogen Receptor Modulators and 
Bisphosphonates for Osteoporosis Prevention
Postmenopausal women who are at risk for osteoporosis should 
be screened at lea

t risk for osteoporosis should 
be screened at least once for osteoporosis by DXA scan. Based on 
the initial results and other risk factors, DXA should be repeated 
periodically, preferably annually or every other year, to monitor 
the effectiveness of osteoporosis treatment and prevention.406
SERMs are compounds that act like estrogen in some target 
tissues but antagonize estrogenic effects in others (see Chapter 
29).407 One of the first SERMs was tamoxifen, for which estro­
gen-like agonist

s was tamoxifen, for which estro­
gen-like agonist activity on bone was observed to occur simultane­
ously with estrogen antagonist activity on the breast.407 Tamoxifen 
had been approved initially for treatment and prevention of breast 
cancer. An unwanted effect of tamoxifen is its estrogen-like action 
on the endometrium. Second-generation compounds have since 
been developed, most notably raloxifene, which has estrogen-
like actions on bone, lipids, and the coagulation system; estrogen 
anta

lipids, and the coagulation system; estrogen 
antagonist effects on the breast; and no detectable action in the 
endometrium.408 In 2007 the FDA approved raloxifene for reduc­
ing the risk of invasive breast cancer in postmenopausal women 
with osteoporosis and in postmenopausal women who are at high 
risk for invasive breast cancer. Raloxifene is most often used to 
prevent and treat osteoporosis in postmenopausal women.409
In placebo-controlled trials, raloxifene reduced vertebral 
fractures, 

 trials, raloxifene reduced vertebral 
fractures, whereas tamoxifen reduced nonvertebral fractures.408 
Tamoxifen and raloxifene had similar effects on fractures at mul­
tiple sites in a randomized, head-to-head trial.408 Neither drug has 
been shown to prevent hip fractures, however. Tamoxifen or ral­
oxifene reduced risk for invasive breast cancer compared with pla­
cebo by approximately 7 to 10 cases per 1000 women per year.408 
Tamoxifen and raloxifene reduced ER-positive breast cancer but 

raloxifene reduced ER-positive breast cancer but 
not ER-negative breast cancer, noninvasive breast cancer, or mor­
tality rate.408 Tamoxifen and raloxifene increase thromboembolic 
events by 4 to 7 cases per 1000 women per year; raloxifene causes 
fewer events than tamoxifen. Tamoxifen increases risk for endo­
metrial cancer compared with placebo by 4 cases per 1000 women 
per year and causes cataracts.408 Raloxifene use is not associated 
with endometrial cancer or cataract risk.408 The most c

endometrial cancer or cataract risk.408 The most common 
side effects for tamoxifen are hot flashes and other vasomotor 
symptoms and vaginal discharge, itching, or dryness. For raloxi­
fene, vasomotor symptoms and leg cramps are most common. In a 
head-to-head trial, raloxifene users reported more musculoskeletal 
problems, dyspareunia, and weight gain, whereas tamoxifen users 
had more gynecologic problems, vasomotor symptoms, and blad­
der control symptoms.408 Tamoxifen and raloxifene may be 

trol symptoms.408 Tamoxifen and raloxifene may be used 
to reduce the incidence of vertebral fractures and invasive breast 
cancer in postmenopausal women. The major drawbacks include 
hot flashes and increased thromboembolic events for both drugs 
and endometrial cancer for tamoxifen.
SERMs are effective for managing certain aspects of estrogen 
deficiency in postmenopausal women, but recent data suggest 
that pairing a SERM with estrogens may provide a more optimal 
therapeutic profile for wom

rovide a more optimal 
therapeutic profile for women with a uterus. The drug combina­
tion bazedoxifene/CEE is a medication approved by the FDA in 
2013 for the treatment of menopause symptoms and the preven­
tion of postmenopausal osteoporosis but not for the treatment of 
osteoporosis.409 It is a fixed-dose combination drug containing the 
SERM bazedoxifene and CEE.410 This new agent demonstrated 
efficacy in postmenopausal women with a uterus, while allow­
ing these women to avoid progestins 

 while allow­
ing these women to avoid progestins and their possible adverse 
effects.410
The major treatment goal in postmenopausal osteoporosis is to 
prevent fractures by maintaining or increasing BMD and reduc­
ing excessive bone turnover.411 Bisphosphonates suppress resorp­
tion by inhibiting the attachment of osteoclasts to bone matrix 
and enhancing programmed cell death in osteoclasts. They have 
increased BMD and reduced the risk for osteoporotic fractures in 
numerous clinical trials.4

eoporotic fractures in 
numerous clinical trials.411 The FDA has approved a number of 
bisphosphonates for treatment. Oral alendronate and oral risedro­
nate were approved in 1995 and 2000, respectively. In 2003, oral 
ibandronate was approved, followed by intravenous ibandronate 
in 2006. Intravenous zoledronic acid was approved in 2007. A 
5-mg dose of zoledronic acid is infused over 15 minutes once each 
year.409 Alendronate is given as a once-weekly 35-mg or 70-mg 
tablet or a once-daily 5-m

-weekly 35-mg or 70-mg 
tablet or a once-daily 5-mg or 10-mg tablet; risedronate is given as 
a once-daily 5-mg tablet, a once-weekly 35-mg tablet, or a once-
monthly 150-mg tablet; ibandronate is given as a once-monthly 
150-mg tablet or as 3 mg intravenously every 3 months.412
Compared with placebo control subjects, all approved bisphos­
phonates reduce the relative risk of new vertebral fractures by on 
641
average 50% in w

  of the Female Reproductive Axis
average 50% in women with postmenopausal osteoporosis.412,413 
Alendronate, risedronate, and zoledronic acid reduce the relative 
risk of new nonvertebral and hip fractures.411 Clinical trial exten­
sions of up to 10 years with alendronate and 7 years with rise­
dronate have shown that efficacy is maintained during long-term 
treatment.411 Moreover, discontinuation of long-term (≥5 years) 
alendronate therapy results in minimal bone loss over the ensuing 
3 to 5

ults in minimal bone loss over the ensuing 
3 to 5 years.414 As in the case of SERMs, definitive data are lacking 
on optimal doses, duration, timing, long-term effects, and effects 
in nonwhite women. Once-yearly zoledronic acid infusions appear 
to be an attractive choice because of better compliance and lack of 
esophagitis, a side effect associated with oral bisphosphonates.404
Osteonecrosis of the jaw is a rare but serious side effect of 
bisphosphonate therapies. Bisphosphonate users who p

sphosphonate therapies. Bisphosphonate users who plan to 
undergo tooth extraction should discuss this side effect with their 
dental surgeons, because tooth extraction may predispose them to 
osteonecrosis of the jaw.415 In premenopausal patients with estro­
gen-responsive early breast cancer, the addition of zoledronic acid 
to adjuvant endocrine therapy reduced disease recurrence in all 
body sites and improved disease-free survival.416 Further studies 
are required to explore the potential o

r studies 
are required to explore the potential of bisphosphonates in pre­
venting breast cancer in postmenopausal women with no previous 
history of breast cancer.
A number of case reports and a recent retrospective study 
suggested that the risk of esophageal cancer increased with oral 
bisphosphonate use over about a 5-year period. In Western coun­
tries, the incidence of esophageal cancer in patients over the age of 
60 is estimated to increase from 1 to 2 per 1000 population with 
5 years’

ase from 1 to 2 per 1000 population with 
5 years’ use of an oral bisphosphonate.417,418 Another retrospective 
study, however, did not find such a link.419 Until more accurate 
data are available, health care providers should avoid prescribing 
oral bisphosphonates to patients with Barrett esophagus, a known 
risk factor for esophageal cancer.417
Tibolone for Osteoporosis Prevention
Tibolone, a synthetic steroid with estrogenic, androgenic, and pro­
gestagenic properties, is approved in many co

pro­
gestagenic properties, is approved in many countries for the treat­
ment of menopausal symptoms and prevention of osteoporosis. 
Tibolone preserves BMD, reduces hot flashes, and may increase 
libido and vaginal lubrication in postmenopausal women.420 A 
randomized study showed that tibolone reduced the risks of frac­
ture and breast cancer and possibly colon cancer but increased 
the risk of stroke in older women with osteoporosis.420 Tibolone 
should not be used in breast cancer survivors,

ne 
should not be used in breast cancer survivors, as it increases breast 
cancer recurrence.404,421 Tibolone is not available in the United 
States.
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women. Am J Epidemiol. 2008;167:1207–1216.
	382.	 Vandenbroucke JP. The HRT controversy: observational studies 
and RCTs fall in line. Lancet. 2009;373:1233–1235.
	383.	 Shumaker SA, Legault C, Kuller L, et  al. Conjugated equine 
estrogens and incidence of probable dementia and mild cognitive 
impairment in postmenopausal women: Women’s Health Initiative 
Memory Study. JAMA. 2004;291:2947–2958.
	38

stin and lung cancer in postmenopausal women (Women’s 
Health Initiative trial): a post-hoc analysis of a randomised con­
trolled trial. Lancet. 2009;374:1243–1251.
	387.	 Martin KA, Manson JE. Approach to the patient with menopausal 
symptoms. J Clin Endocrinol Metab. 2008;93:4567–4575.
	388.	 Riis BJ, Hansen MA, Jensen AM, Overgaard K, Christiansen C. 
Low bone mass and fast rate of bone loss at menopause: equal 
risk factors for future fracture: a 15-year follow-up study. Bone. 
1996;19:9–1

teril. 1993;60:664–667.
	391.	 Woodruff JD, Pickar JH. Incidence of endometrial hyperplasia 
in postmenopausal women taking conjugated estrogens (Prema­
rin) with medroxyprogesterone acetate or conjugated estrogens 
alone. The Menopause Study Group. Am J Obstet Gynecol. 
1994;170:1213–1223.
	392.	 Archer DF, Pickar JH, Bottiglioni F. Bleeding patterns in post­
menopausal women taking continuous combined or sequential reg­
imens of conjugated estrogens with medroxyprogesterone acetate. 
Menopau

trogens with medroxyprogesterone acetate. 
Menopause Study Group. Obstet Gynecol. 1994;83:686–692.
	393.	 Effects of estrogen or estrogen/progestin regimens on heart disease 
risk factors in postmenopausal women. The Postmenopausal Estro­
gen/Progestin Interventions (PEPI) Trial. The Writing Group for 
the PEPI Trial. JAMA. 1995;273:199–208.
	394.	 Reed SD, Newton KM, Lacroix AZ. Indications for hormone ther­
apy: the post-Women’s Health Initiative era. Endocrinol Metab Clin 
North Am. 2004;33

tudy. JAMA. 2004;291:2959–
2968.
	397.	 Nand SL, Webster MA, Baber R, O’Connor V. Bleeding pattern 
and endometrial changes during continuous combined hormone 
replacement therapy. The Ogen/Provera Study Group. Obstet Gyne­
col. 1998;91:678–684.
	398.	 Korhonen MO, Symons JP, Hyde BM, Rowan JP, Wilborn WH. 
Histologic classification and pathologic findings for endometrial 
biopsy specimens obtained from 2964 perimenopausal and post­
menopausal women undergoing screening for continuous hor­
mon

women undergoing screening for continuous hor­
mones as replacement therapy (CHART 2 Study). Am J Obstet 
Gynecol. 1997;176:377–380.
	399.	 McGonigle KF, Karlan BY, Barbuto DA, Leuchter RS, Lagasse 
LD, Judd HL. Development of endometrial cancer in women on 
estrogen and progestin hormone replacement therapy. Gynecol 
Oncol. 1994;55:126–132.
	400.	 Karlsson B, Granberg S, Wikland M, et al. Transvaginal ultraso­
nography of the endometrium in women with postmenopausal 
bleeding––a Nordic multic

d risk of 
recurrence after hormone replacement therapy in breast cancer sur­
vivors. J Natl Cancer Inst. 2008;100:475–482.
	406.	 Brunner RL, Gass M, Aragaki A, et  al. Effects of conjugated 
equine estrogen on health–related quality of life in postmenopausal 
women with hysterectomy: results from the Women’s Health Initia­
tive Randomized Clinical Trial. Arch Intern Med. 2005;165:1976–
1986.
	407.	 Jordan VC, O’Malley BW. Selective estrogen-receptor modula­
tors and antihormonal resistance i

maker SA, et al. Effect of estrogen plus 
progestin on global cognitive function in postmenopausal women: 
the Women’s Health Initiative Memory Study: a randomized con­
trolled trial. JAMA. 2003;289:2663–2672.
	411.	 Bilezikian JP. Efficacy of bisphosphonates in reducing fracture risk 
in postmenopausal osteoporosis. Am J Med. 2009;122:S14–21.
	412.	 Jansen JP, Bergman GJ, Huels J, Olson M. Prevention of vertebral 
fractures in osteoporosis: mixed treatment comparison of bisphos­
phonate thera

0.	 Cummings SR, Ettinger B, Delmas PD, et al. The effects of tibolone 
in older postmenopausal women. N Engl J Med. 2008;359:697–708.
	421.	 Bundred NJ, Kenemans P, Yip CH, et al. Tibolone increases bone 
mineral density but also relapse in breast cancer survivors: LIBER­
ATE trial bone substudy. Breast Cancer Res. 2012;14:R13.
642
18
Hormonal Contraception
REBECCA H. ALLEN, ANDREW M. KAUNITZ, MARTHA HICKEY, AND 
ANNABELLE BRENNAN
P
revention of unplanned pregnancy continues to challenge 
wom

of unplanned pregnancy continues to challenge 
women, couples, and clinicians in developed and developing 
countries. For all couples, access to contraception is critical 
to be able to plan the timing and spacing of their childbearing for 
social, economic, and health reasons.1 Most women in the United 
States and other developed countries desire to have two children.2 
Therefore, on average, sexually active women devote more than three 
decades of their lives to avoiding unintended pregnancy. 

 of their lives to avoiding unintended pregnancy. However, 
nearly half (45%) of all pregnancies in the United States are unin­
tended.3 Women who use contraception consistently and correctly 
account for only 5% of unintended pregnancies, but women who 
use contraception inconsistently or incorrectly account for 41%, 
and women who do not use any method of contraception account 
for 54% of unintended pregnancies. Pregnancies that are unin­
tended or too closely spaced can lead to adverse matern

d or too closely spaced can lead to adverse maternal and child 
health outcomes.4 In developing countries, childbirth and unsafe 
abortion are important causes of maternal death and morbidity, and 
the unintended pregnancy rate is estimated at 65 per 1000 women.5
Contraceptive advice and provision should start before the com­
mencement of sexual activity and continue through the reproduc­
tive years. Although the incidence is declining, the United States 
still has one of the highest teenage pre

d States 
still has one of the highest teenage pregnancy rates in the devel­
oped world.6 Teenage parenting can have adverse consequences 
for the adolescent, as well as the adolescent’s children and family.7 
Programs to prevent unplanned pregnancy by promoting absti­
nence have largely been ineffective. Nonhormonal contraceptives, 
such as male condoms, may offer benefits in terms of protection 
against sexually transmitted infection (STI) but are highly user 
dependent and do not reliably pre

are highly user 
dependent and do not reliably prevent unplanned pregnancy in 
many populations. Female hormonal contraception, along with 
the copper intrauterine device (IUD), represents the most effective 
and acceptable reversible contraceptive options; male hormonal 
contraception has not achieved this goal. The ideal contraceptive 
would be safe, highly effective, discreet, inexpensive, long acting, 
easily reversible, and under the woman’s control. It would not 
need to be activated at or

 control. It would not 
need to be activated at or around the time of intercourse. The 
Choosing a Contraceptive Method, 643
Combined Estrogen and Progestin Contraceptives, 643
Progestin-Only Contraceptive Methods, 651
Emergency Contraception, 659
Clinical Challenges in Contraceptive Care, 661
KEY POINTS
 • 
Reducing health care system barriers to contraceptive initiation 
and continuation is important to reduce unintended pregnancy 
rates. Such barriers include requiring unnece

ncy 
rates. Such barriers include requiring unnecessary health screen­
ings, waiting until menstruation to begin methods, inappropri­
ate contraindications, and failure to provide adequate refills for 
prescription-based methods. The US Selected Practice Recom­
mendations for Contraceptive Use from the Centers for Disease 
Control and Prevention offers evidence-based guidelines to 
assist providers in caring for women who need contraception.
 • 
Long-acting reversible contraceptive methods, whi

Long-acting reversible contraceptive methods, which include 
the copper intrauterine device (IUD), the levonorgestrel IUD, 
and the etonogestrel subdermal implant, provide superior con­
traceptive effectiveness, equivalent to sterilization, and higher 
continuation and satisfaction rates compared with shorter-
acting methods. These methods can be offered to all women, 
including adolescents seeking contraception.
 • 
Emergency contraception should ideally be available to 
women at risk for unp

uld ideally be available to 
women at risk for unplanned pregnancy. The copper IUD is 
the most effective emergency contraceptive, followed by oral 
ulipristal acetate and oral levonorgestrel.
 • 
Besides offering highly effective contraception, the higher-dose 
levonorgestrel IUD represents a first-line treatment for heavy 
menstrual bleeding and an effective alternative to endometrial 
ablation and hysterectomy.
 • 
Pregnancy in women with underlying medical conditions is 
associated with hi

derlying medical conditions is 
associated with higher risks of maternal and perinatal morbid­
ity and mortality; therefore, achieving effective preconception 
and family planning care is particularly important in this setting. 
Health care providers should consult the US Medical Eligibil­
ity Criteria for Contraceptive Use from the Centers for Disease 
Control and Prevention when caring for these women.
643
CHAPTER 18  Hormonal Contraception
ideal contraceptive would also protect against STIs. 

al contraceptive would also protect against STIs. Because an 
ideal contraceptive does not exist, the challenge for clinicians is to 
tailor available methods to the medical, personal, and social needs 
of the woman and her partner and changing approaches when 
necessary as these needs evolve throughout her reproductive life 
span. The clinician must also learn to recognize and address barri­
ers to safe and effective implementation of the selected methods. 
Hormonal contraceptives are most effe

ed methods. 
Hormonal contraceptives are most effectively used by women who 
are well informed about the advantages and common side effects 
of the method, and who have actively participated in selecting 
the method. Guidance on medical eligibility for contraceptives 
can be found in two companion documents from the Centers for 
Disease Control and Prevention (CDC), entitled “U.S. Selected 
Practice Recommendations for Contraceptive Use, 2016”8 and 
“U.S. Medical Eligibility Criteria for Contrac

nd 
“U.S. Medical Eligibility Criteria for Contraceptive Use, 2016.”9
In addition to enhancing quality of life by allowing couples 
to choose whether and when they wish to bear children, effective 
contraception lowers health care costs.10 Male and female steriliza­
tion and long-acting reversible methods (e.g., IUDs, subdermal 
implants) constitute the most cost-effective contraceptive options, 
followed by other hormonal methods (e.g., oral contraceptives). 
Depot medroxyprogesterone acetate (

traceptives). 
Depot medroxyprogesterone acetate (DMPA) injection is more cost 
effective than oral contraception. Barrier and behavioral methods 
(i.e., male condom and withdrawal, respectively) are the least cost 
effective compared with other contraceptive options. Nevertheless, 
when compared with no method, they still prevent a large number 
of unintended pregnancies, leading to important cost savings.
Choosing a Contraceptive Method
Given the number of contraceptive options available to wo

he number of contraceptive options available to women, it 
is important providers concentrate their efforts on helping women 
choose the best contraceptive method for them and focus on coun­
seling that helps improve continuation. The best birth control 
method is one that provides the safest and most effective contra­
ceptive for a woman and is the method she chooses to use and has 
access to. This approach not only places a strong value on medical 
considerations but also includes consideratio

cal 
considerations but also includes consideration of a woman’s prefer­
ences, values, and level of prevention desired, as well as the afford­
ability of the method given her access to health insurance or ability 
to pay. When helping women make sound contraceptive decisions, 
clinicians should consider the patient’s age, lifestyle, and other rel­
evant circumstances, including the recognition that contraceptive 
needs are likely to change during different phases of reproductive 
life, and that

g different phases of reproductive 
life, and that the risks and benefits may alter according to age and 
background health factors. For those considering pregnancy in the 
future, contraceptive reversibility and time to return of fertility 
should be discussed. Affordability must be considered, because it 
may affect continuation rates and therefore affect efficacy.
Long-acting reversible contraceptives (i.e., IUDs and the 
implant) offer women the advantages of high contraceptive effi­
cacy an

the advantages of high contraceptive effi­
cacy and high rates of continuation. When discussing contracep­
tion, clinicians should present all suitable options to their patients 
but outline the options in terms of tiers of effectiveness (Fig. 18.1). 
Tier 1 includes long-acting reversible contraception and steriliza­
tion. Tier 2 includes combined methods (pill, patch, and ring), 
DMPA, and the progestin-only pill (POP). Tier 3 includes barrier 
methods, and tier 4 includes withdrawal and use o

methods, and tier 4 includes withdrawal and use of spermicides.
As with all medications, contraceptives have potential side effects. 
Candid discussion of these effects and other areas of anticipatory 
guidance may increase acceptability. The clinician should offer infor­
mation about adverse events that are individualized and provided in 
the context of how they compare with the effects of an unplanned 
pregnancy. Information about contraceptive failure and access to 
emergency contraception s

e failure and access to 
emergency contraception should be given. For those at risk for STIs, 
clinicians should encourage consistent condom use and minimizing 
the number of partners, regardless of contraceptive choice. No method 
of contraception is perfect. Each woman must consider the advantages 
and disadvantages of each method in making her decision. The most 
effective method is likely to be the one that she can use successfully. 
Combined Estrogen and Progestin 
Contraceptives
Methods co

 Estrogen and Progestin 
Methods combining estrogen and progestin offer the advantage of 
relatively regular bleeding patterns and high efficacy if used con­
sistently. These combined methods are available in oral, transder­
mal, and transvaginal preparations providing increased flexibility 
in choice of delivery system.
The Combined Oral Contraceptive Pill
Combined oral contraceptives (COCs) offer safe, reversible, and 
convenient birth control that is highly effective for those 

 birth control that is highly effective for those who take 
pills correctly and consistently. In many settings, oral contraception 
provides important noncontraceptive benefits (mentioned later) 
that should be discussed during counseling.11 By individualizing 
counseling and follow-up strategies based on relevant behavioral 
and medical considerations, clinicians can help women maximize 
efficacy of COCs. COCs have been available for more than 50 
years and have gradually been refined to improv

0 
years and have gradually been refined to improve safety, efficacy, 
and acceptability. COCs are used by almost 10 million women in 
the United States, making them the most widely used hormonal 
contraceptive method.12 Currently used methods contain estrogen, 
usually as ethinyl estradiol, at doses of 10 to 35 μg/day and pro­
gestins in different forms and doses. This section describes COCs 
available in the United States, focusing on education, counseling, 
and management measures to maximize

, counseling, 
and management measures to maximize contraceptive efficacy.
Composition and Formulations
Over time, the dose of estrogen and progestin in COCs has gradu­
ally decreased, and the types of progestins have changed. Currently, 
the highest-dose formulations marketed in the United States con­
tain 50 μg of estrogen, but the great majority of COCs prescribed 
contain 35 μg or less. Most modern COCs formulated with 35 μg 
of estrogen or less use ethinyl estradiol, which is a potent synth

ess use ethinyl estradiol, which is a potent synthetic 
estrogen with long half-life and slow metabolism and similar meta­
bolic effects (e.g., liver protein production) regardless of the route 
of administration (oral, transdermal, or vaginal ring).13 Estradiol 
valerate has been marketed as a component of one newer COC, 
Natazia (Bayer Healthcare Pharmaceuticals, Wayne, NJ). Estradiol 
valerate is a synthetic hormone extensively metabolized to estradiol 
and valeric acid before reaching the sy

estradiol 
and valeric acid before reaching the systemic circulation. A dose of 
estradiol valerate (2 mg daily) has biologic effects on the uterus, 
ovary, and hypothalamic-pituitary-ovarian axis similar to those of a 
20-μg dose of ethinyl estradiol.14 A COC formulated with micron­
ized estradiol (1.5 mg) and the progestin nomegestrol acetate has 
been approved for use in some European countries but is not cur­
rently marketed in the United States.15 Older COC preparations 
marketed in the Uni

tes.15 Older COC preparations 
marketed in the United States contain one of five progestins: nor­
ethindrone, norethindrone acetate, ethynodiol diacetate, norg­
estrel, or levonorgestrel. Newer formulations contain more potent 
progestins: norgestimate, desogestrel, drospirenone, and dienogest.
644
Many COC formulations are marketed in the United States 
(Table 18.1). These products do not appear to be different in 
terms of safety or efficacy if used

 different in 
terms of safety or efficacy if used correctly.16 COCs were originally 
formulated to mimic the normal menstrual cycle by providing 21 
days of combined estrogen and progestin, followed by a 7-day 
hormone-free interval (HFI) when withdrawal bleeding occurred. 
Although there is no evidence that an HFI provides any health 
benefits, most modern COCs continue to follow this approach 
and contain active pills for 21 to 24 days in each 28-day pack; in 
most formulations, pills contain

 28-day pack; in 
most formulations, pills containing no active medication are taken 
during the HFI.17 Less pituitary-ovarian activity during the HFI is 
noted in the 24/4 regimen compared with the 21/7 regimen.18,19 
COC formulations with a shorter HFI allow less opportunity for 
follicle development and possible ovulation; accordingly, such for­
mulations may be more effective.20 Some women may ovulate if 
the HFI is extended beyond 7 days.20,21
The HFI can be modified (shortened to 4 days) o

21
The HFI can be modified (shortened to 4 days) or eliminated to 
reduce hormone-withdrawal symptoms including pelvic pain, breast 
tenderness, and mood symptoms, and to improve treatment of prob­
lems such as dysmenorrhea, pelvic pain, and anemia. This approach 
may enhance convenience for some women.22 A recent meta-analysis 
showed lower rates of menstrual-related symptoms, including head­
ache, bloating, and pelvic pain with extended-dosing regimens.17 
Extended oral contraceptive formulati

regimens.17 
Extended oral contraceptive formulations are available as packs of 84 
pills, followed by 7 days of inert tablets or tablets containing ethinyl 
estradiol 10 μg in place of the conventional HFI. These formula­
tions offer women only four scheduled withdrawal bleeds per year. In 
addition, a continuous COC with no HFI is available.
COC regimens also differ with respect to the dose of sex ste­
roid and whether these remains constant throughout the hormon­
ally active pills. Monophasic

roughout the hormon­
ally active pills. Monophasic preparations have a constant dose of 
estrogen and progestin in each of the 21 or 24 active hormone 
tablets in each cycle pack. Phasic preparations alter the dose of 
the progestin and, in some formulations, the estrogen component 
within the active tablets. There is no evidence phasic preparations 
are superior to monophasic formulations in terms of efficacy or 
bleeding patterns.23-25 
Mechanism of Action, Efficacy, Administration, 
and Effec

sm of Action, Efficacy, Administration, 
and Effect on Pregnancy
Estrogen and progestin components of COCs contribute to 
inhibition of ovulation through suppression of the hypotha­
lamic-pituitary-gonadal axis. Suppression of pituitary release of 
follicle-stimulating hormone and luteinizing hormone (LH) dis­
courages folliculogenesis and prevents ovulation. Additionally, the 
exogenous progestin increases cervical mucous viscosity, causes 
endometrial atrophy, and impairs function of the fallo

metrial atrophy, and impairs function of the fallopian tube 
cilia to prevent fertilization.16
Sun Mon Tue Wed Thu
Fri
Sat
Female sterilization
IUD
Implants
Pills
LAM
Injectables
Fertility awareness
methods
Diaphragm
Male or female condoms
Spermicide
Withdrawal
More effective
Less than 1 pregnancy
per 100 women
in 1 year
Less effective
About 30 pregnancies
in 1 year
Vasectomy
1
2
MARCH
3
4
5
6
7
8
9 10 11 12
13 14 15 16 17 18 19
20 21 22 23 24 25 26
27 28 29 30 31
• Fig. 18.1  Comp

21 22 23 24 25 26
• Fig. 18.1  Comparing effectiveness of family planning methods. IUD, intrauterine device; LAM, lactational 
amenorrhea method. (Redrawn from World Health Organization Department of Reproductive Health and 
Research (WHO/RHR) and Johns Hopkins Bloomberg School of Public Health/Center for Communication 
Programs (CCP), Knowledge for Health Project. Family Planning: A Global Handbook for Providers (2011 
update). Baltimore and Geneva: CCP and WHO; 2011.)
  Oral Co

ltimore and Geneva: CCP and WHO; 2011.)
  Oral Contraceptive Formulations Available in the United States
Description
Name
Estrogen
Progestin
Progestin Dose (mg)
EE, 50 μg monophasic
Ovcon 50
EE
Norethindrone
1
Ogestrel 0.5/50
EE
Norgestrel
0.5
Zovia 1/50
EE
Ethynodiol diacetate
1
Norinyl 1/50a
Mestranol
1
EE, 35 μg monophasic
Femcon Fe chewable
EE
0.4
Modicona
EE
0.5
Brevicona
EE
0.5
Ovcon 35a
EE
0.4
Ortho-Cyclena
EE
Norgesti

35a
EE
0.4
EE
Norgestimate
0.25
Zovia 1/35a
EE
1
Ortho-Novum 1/35aa
EE
1
Norinyl 1 + 35a
EE
1
EE, 35 μg biphasic
Ortho-Novum 10/11a
EE
0.5/1
EE, 35 μg triphasic
Ortho-Novum 7/7/7a*
EE
0.5/0.75/1
Ortho Tri-Cyclena,b
EE
0.18/0.215/0.25
Tri-Norinyla
EE
0.5/1.0/0.5
Estrostepa,b
EE (20/30/35)
Norethindrone acetate
1
EE, 30 μg monophasic
Loestrin 1.5/30a
EE
Norethindrone 

0 μg monophasic
EE
1.5
Ortho-Cepta
EE
Desogestrel
0.15
Desogena
EE
0.15
Lo-Ovrala
EE
0.3
Nordettea
EE
Levonorgestrel
0.15
Levoraa
EE
0.15
Yasmina
EE
Drosperinone
3.0
EE, 30 μg triphasic
Triphasila
EE (30/40/30)
0.05/0.075/0.125
Trivoraa
EE, 30 μg extended cycle (84 estrogen/progestin 
tablets)
Seasonalea
EE
0.15
EE, 30 μg extended cycle (84 estr

onorgestrel
0.15
tablets, 7 tablets 10 μg EE)
Seasoniquea
EE
0.15
EE, 30 μg extended cycle triphasic (84 
estrogen/progestin tablets, 7 tablets 
10 μg EE)
Quartette
EE (20/25/30)
0.15
EE, 25 μg monophasic (24/4)
Generess Fe chewable
EE
0.8
EE, 25 μg triphasic
Cyclessaa
EE
0.10/0.125/0.15
Ortho Tri-Cyclen Loa
EE
EE, 20 μg monophasic
Loestrin 1/20a
EE

 20 μg monophasic
EE
1
Luteraa
EE
0.1
EE, 20 μg biphasic
Mircettea
EE (20/10)
0.15
EE, 20 μg 24/4 (24 estrogen/progestin tablets)
Yaza,b,c
EE
Drospirenone
3.0
Minastrin 24 Fe 
chewable
EE
1
EE, 20 μg extended cycle (84 estrogen/progestin 
tablets, 7 tablets 10 μg EE)
LoSeasoniquea
EE
0.1
EE, 20 μg extended cycle (all estrogen/progestin 
tablets)
Lybrela
EE
0.09
EE, 10 μg 24/4 (24 e

ybrela
EE
0.09
EE, 10 μg 24/4 (24 estrogen/progestin tablets, 
2 tablets 10 μg EE)
Lo Loestrin Fea
EE
1
Estradiol valerate, quadriphasic
Nataziad
EV (mg) (3/2/2/1)
Dienogest
2/3
Progestin only
Micronora
N/A
0.35
aGeneric versions available.
bIndicated for the treatment of acne in women desiring to use oral contraception.
cIndicated for the treatment of premenstrual dysphoric disorder in women desiring to use oral contraception.
dIndicated for the treatm

 use oral contraception.
dIndicated for the treatment of heavy menstrual bleeding in women desiring to use oral contraception.
EE, Ethinyl estradiol; EV, estradiol valerate.
   
TABLE 18.1
646
Daily use is critical to contraceptive efficacy of COC, so failure 
rates are largely attributable to poor adherence but may also be 
influenced by age and frequency of sexual intercourse. The Pearl 
index is a method of measuring contraceptive effectiveness in 

ethod of measuring contraceptive effectiveness in clin­
ical trials, with a lower Pearl index representing a lower chance of 
unintended pregnancy. Failure rates range from less than 1 per 
100 woman-years (Pearl index) with excellent adherence to more 
than 15 pregnancies per 100 woman-years with low adherence. 
Typical first-year combination oral contraception failure rates are 
estimated at 7 per 100 women.26 There does not appear to be any 
significant difference in contraceptive efficacy of

ignificant difference in contraceptive efficacy of COCs formu­
lated with different progestins.27
Traditionally, COCs have been started on the first day of men­
ses, but the pills can be safely started at any time if pregnancy has 
been excluded (the quick-start method).8 It is unclear whether this 
approach reduces unplanned pregnancies occurring while women 
are waiting until menses to start the pills.28 If the COCs are inad­
vertently taken during pregnancy, they do not appear to increase 
th

ring pregnancy, they do not appear to increase 
the rate of miscarriage or adversely affect the developing fetus.29 Of 
course, use of COCs may delay the diagnosis of an early pregnancy.
The importance of daily administration for contraceptive effi­
cacy must be communicated to patients. Some women link pill 
taking with a daily ritual (e.g., tooth brushing) or use daily remind­
ers through email or text messaging. Clear instructions for manag­
ing missed pills are an essential component of COC 

ng missed pills are an essential component of COC counseling. 
If a woman misses one tablet, she should take the missed pill as 
soon as possible even if it means taking two pills on the same day. 
She should then continue to take one tablet daily; no additional 
contraceptive protection is needed.8 If she has missed two or more 
consecutive tablets, she should take the most recent pill and con­
tinue taking the remaining pills at the usual time, even if it means 
taking two pills on the same da

 even if it means 
taking two pills on the same day. In this instance, she should 
also use an additional form of contraception (e.g., condoms) for 
7 days. If two or more pills are missed in the third week of the 
28-day pack, she can omit the HFI in the current pack and start 
a new pack. Emergency contraception can also be considered in 
these cases. These instructions also apply to women with vomiting 
and severe diarrhea while taking COCs. COCs are not suitable for 
women who consistently m

OCs are not suitable for 
women who consistently miss pills, because this undermines con­
traceptive efficacy. In these cases, a method that does not require 
daily adherence (e.g., contraceptive rings; patches; an injectable, 
intrauterine, or implantable method) should be considered.
After discontinuation of COCs, most women rapidly resume 
ovulation. In some women, ovulation may be temporarily delayed 
for several months after discontinuing oral contraception; how­
ever, 12-month conception r

al contraception; how­
ever, 12-month conception rates are no different in former pill 
users compared with women who discontinue other contraceptive 
methods.30 
Noncontraceptive Health Benefits
Educating women regarding potential noncontraceptive benefits 
of COC can assist patients in making a prudent contraceptive 
choice and can increase oral contraception adherence and con­
tinuation. For many users, COCs offer substantial noncontra­
ceptive health benefits, including enhanced menstrual re

e health benefits, including enhanced menstrual regularity; 
improved pelvic pain; reduced premenstrual symptoms; reduction 
in benign breast diseases, including fibroadenoma and fibrocystic 
changes; management of acne; and preservation of bone mineral 
density (BMD).11,31 The benefits to bone health are an impor­
tant consideration in perimenopause for women who have been 
hypoestrogenic and those with hypothalamic amenorrhea, where 
accelerated bone loss occurs in addition to the need for con

d bone loss occurs in addition to the need for contracep­
tion.32 As with most contraceptives, COC use reduces the inci­
dence of ectopic pregnancy, which is a common and potentially 
life-threatening condition.33
Reducing the risk of epithelial ovarian and endometrial cancer 
is an important noncontraceptive benefit of COC use. Although 
the incidence is low, ovarian cancer is the most common cause of 
death from gynecologic malignancy and the fifth most common 
cause of overall cancer death in

ifth most common 
cause of overall cancer death in women in the United States.34 
Meta-analysis data of cohort and case-control studies demon­
strated a reduction in ovarian cancer risk in women who have ever 
used COC, although 185 women needed to use COC to prevent 
one ovarian cancer. COC use of more than 10 years appeared to 
reduce the incidence of ovarian cancer by more than 50%. These 
results do not appear to be dependent on the dose of sex steroids 
administered.35 COCs appear to reduce

ex steroids 
administered.35 COCs appear to reduce risk of ovarian cancer 
in women at increased inherited risk of ovarian cancer through 
BRCA1 and BRCA2 genes.36-40 However, this benefit must be 
carefully weighed against the possible increase in breast cancer risk 
associated with COC use in BRCA1 and BRCA2 carriers, which is 
discussed further in the following.
Endometrial adenocarcinoma is the most common gynecologic 
cancer in US women, and rising obesity rates put more women at 
risk. The

 rising obesity rates put more women at 
risk. The risk of endometrial adenocarcinoma is reduced with COC 
use, with several meta-analyses demonstrating reduced risk of more 
than 50%.41-43 This protective effect appears to be time dependent, 
with risk of endometrial cancer reduced by almost 70% with 8 years 
of COC use and with protection persisting for at least 20 years after 
oral contraception discontinuation.43 It is unclear whether the level 
of protection varies among distinct COC formul

el 
of protection varies among distinct COC formulations.
Around 6% of reproductive-age women meet diagnostic crite­
ria for polycystic ovary syndrome (PCOS), and prevalence is likely 
to be higher in women considering COC use due to clinical fea­
tures such as irregular menstruation and hyperandrogenism.44 Use 
of COC leads to more regular vaginal bleeding patterns, prevents 
endometrial hyperplasia, and may reduce clinical and biochemical 
hyperandrogenism in PCOS women.45 The impact of COC on

randrogenism in PCOS women.45 The impact of COC on 
carbohydrate metabolism and cardiovascular risk in this popula­
tion is unclear. 
Contrary to popular perception, randomized, placebo-controlled 
trials have not shown COCs to cause weight gain, nausea, breast 
tenderness, or mood changes.46,47 Consequently, COCs are well 
tolerated by most users. Nevertheless, some individuals report side 
effects they attribute to COCs that may affect quality of life, con­
traceptive continuation

fect quality of life, con­
traceptive continuation, and patient satisfaction. Therefore, coun­
seling about side effects or lack thereof is an important aspect of 
contraceptive care and may improve patient tolerance and adher­
ence to COCs when they occur.48
Unscheduled vaginal bleeding is a common side effect attribut­
able to COC. Unscheduled bleeding affects 30% to 50% of COC 
users during the initial 3 months of use, but the incidence declines 
with ongoing use. Unscheduled bleeding is more

es 
with ongoing use. Unscheduled bleeding is more common with 
lower-dose (20 μg) than standard-dose (30 or 35 μg) ethinyl estra­
diol preparations, which increases discontinuation rates.49 The 
influence of progestin type is unclear. In addition, unscheduled 
bleeding is more common in women using extended COC for­
mulations in early cycles, but this bleeding will lessen over time. If 
unscheduled bleeding during extended COC use is bothersome, 
women may opt to stop active tablets for 3 days,

women may opt to stop active tablets for 3 days, thereby inducing 
647
withdrawal bleeding, and then continue with active pills. This 
strategy has been shown to reduce subsequent unscheduled bleed­
ing in this setting.50
Irregular bleeding may be a manifestation of pregnancy or 
disease such as cervical or endometrial infection, polyps, or neo­
plasia. Persistent or new-onset bleeding should therefore be inves­
tigated. Amenorrhea (absence of withdrawal blee

s­
tigated. Amenorrhea (absence of withdrawal bleeding) may occur 
with long-term COC use, and it may be more acceptable with 
counseling that provides appropriate reassurance.48 Women who 
experience absence of withdrawal bleeding or who for other rea­
sons suspect that they could be pregnant should use a urine preg­
nancy test.
Headaches are common, but there is a lack of high-quality 
evidence that COC use contributes to headaches.51 Some users 
may experience headaches in early cycles, which

s 
may experience headaches in early cycles, which tend to improve 
with ongoing use. Furthermore, estrogen withdrawal headaches 
that occur during the HFI may respond to a shorter duration 
HFI of 3 days.52,53 Any new-onset or worsening headache with 
COC use must be evaluated. A history of migraine with aura is 
a contraindication for use of COC because of increased risk of 
stroke (described in the following).54 COC use in women with 
a history of migraine without aura should be avoided in wo

y of migraine without aura should be avoided in women 
with other risk factors for stroke.52 Similarly, any COC users 
who experience increased frequency or intensity of any type 
of migraine headache should discontinue estrogen-containing 
contraceptives. 
Health Risks
Extensive studies in large populations have established the risks 
and benefits of COCs for most women.55 For the majority of 
women, use of COCs represents a safe contraceptive choice. 
However, clinicians should be aware of cir

hoice. 
However, clinicians should be aware of circumstances in which 
COC may pose health risks. The US Medical Eligibility Cri­
teria for Contraceptive Use (USMEC), most recently updated 
in 2016,9 were adapted from the World Health Organization 
(WHO) document Medical Eligibility Criteria for Contraceptive 
Use, 5th edition (Tables 18.2 and 18.3).55a The guidelines provide 
evidence-based advice on eligibility for use of hormonal contra­
ceptives. Although the WHO document covered many condi­

ves. Although the WHO document covered many condi­
tions, such as postpartum status, breastfeeding, smoking, obesity, 
cardiovascular disease, diabetes, and cancer, the USMEC added 
new categories: endometrial hyperplasia, rheumatoid arthritis, 
solid organ transplantation, inflammatory bowel disease, bar­
iatric surgery, and peripartum cardiomyopathy. The USMEC 
assists providers in deciding what contraceptive methods are 
appropriate for their patients to improve access to contracep­
tion, esp

patients to improve access to contracep­
tion, especially among women with medical problems for which 
providers may have been hesitant to prescribe contraception in 
the past.9
The safety of contraception in the USMEC is considered in 
four categories:
	
1.	 Conditions with no restriction on the use of the contracep­
tive method.
	
2.	 Conditions in which the advantages of the method usu­
ally outweigh the theoretical or proven risks. The method 
can generally be used, but careful follow-up m

od 
can generally be used, but careful follow-up may be 
required.
	
3.	 Conditions in which the theoretical or proven risks 
usually outweigh the advantages; examples of estro­
gen-containing methods include current gallbladder 
disease, diabetes with end-organ damage, controlled 
hypertension, and taking medications that may interfere 
with COC efficacy. The method is generally not recom­
mended unless other more appropriate methods are not 
available or acceptable.
	
4.	 Conditions that pre

vailable or acceptable.
	
4.	 Conditions that present an unacceptable health risk if the 
contraceptive method is used; examples of estrogen-con­
taining methods include delivery during the past 21 days, a 
personal history of deep venous thrombosis or pulmonary 
embolism, ischemic heart disease, stroke, known thrombo­
genic mutations, and migraine headaches with aura or other 
neurologic signs.
Thromboembolic Disease
The established increased risk of venous thromboembolism (VTE) 
is related to

isk of venous thromboembolism (VTE) 
is related to the estrogenic component, and although modern low-
dose preparations (≤35 μg) carry a lower risk than the original oral 
contraceptives, the incidence of VTE is still increased.56 COC use 
increases risk of VTE by more than twofold, with an incidence of 
approximately 9 to 10 per 10,000 women per treatment year57 
  US Medical Eligibility Criteria (USMEC) for 
Contraceptive Use Categories for the Use 
of Estrogen-Containing Contraception 
Accor

e Use 
According to Medical Condition
Condition
USMEC
Smoking age ≥35 years
  <15 cigarettes/day
Risks outweigh benefits
  ≥15 cigarettes/day
Unacceptable risk
Obesity (body mass index ≥30)
Benefits outweigh risks
  Controlled hypertension
  Elevated blood pressure
  Systolic 140–159 mm Hg or 
diastolic 90–99 mm Hg
  Systolic ≥160 mm Hg or 
diastolic ≥100 mm Hg
  Vascular disease

c ≥100 mm Hg
Diabetes
  No vascular disease
  Vascular disease or >20 
years’ duration
Either risks outweigh benefits or 
unacceptable risk (based on 
severity of condition)
Stroke
Current or history of ischemic 
heart disease
Multiple risk factors for cardio­
vascular disease (older age, 
smoking, obesity, diabetes, 
hypertension)
unacceptable ris

ither risks outweigh benefits or 
  Current
  Past and no evidence of 
disease for 5 years
Migraines
  Without aura
  With aura
TABLE 18.2
648
(Fig. 18.2). Comparatively, in otherwise healthy reproductive-age 
women, the risk of VTE ranges from 1 to 5 per 10,000 women 
per year, whereas VTE risk in pregn

 10,000 women 
per year, whereas VTE risk in pregnancy is almost 29 per 10,000 
women per year and even higher in the postpartum period.58 Other 
risk factors for VTE include, but are not limited to, age, obesity, 
smoking, and thrombogenic mutations.59-61 The best-quality pro­
spective epidemiologic studies have not shown any increased risk 
of VTE with newer progestins (desogestrel and drospirenone) and 
the vaginal ring (etonogestrel) compared with older progestins 
(levonorgestrel and noreth

 with older progestins 
(levonorgestrel and norethindrone).62-65 These prospective cohort 
studies addressed important baseline confounders, including age, 
family history, and body mass index (BMI); categorized con­
traceptives users by duration of use; maintained regular contact 
with users; and individually validated each diagnosis of VTE.66-68 
Because COCs containing estradiol valerate are relatively new, 
there is limited data on their effects on risk of VTE.
National guidelines contraindi

cts on risk of VTE.
National guidelines contraindicate COC use in those at 
increased risk for VTE, including those with a personal history 
of VTE, women immediately after delivery, women undergoing 
surgery with prolonged immobilization, and those with known 
inherited thrombophilic conditions.9 In the context of inherited 
thrombophilias, clinicians need to consider VTE risk based on the 
type of thrombophilia present and its association with other risk 
factors, such as coexistence of multip

other risk 
factors, such as coexistence of multiple thrombophilias, obesity, 
age, and any VTE events during prior periods of hormonal expo­
sure, such as pregnancy and estrogen-containing contraceptives.69 
Routine screening of the general population for familial thrombo­
philic markers is not recommended before initiating COC use.9 
  US Medical Eligibility Criteria for Contraceptive Use Categories for the Use of Progestin-Only 
Methods of Contraception According to Medical Condition
Conditi

ntraception According to Medical Condition
Condition
POP
DMPA
Implant
Levonorgestrel IUD
No restriction
Obesity ≥30 BMI
Menarche to age 18 years and 
≥30 BMI
Elevated BP
  Systolic 140–159 mm Hg o

restriction
  Systolic 140–159 mm Hg or 
diastolic 90–99 mm Hg
  Systolic ≥160 mm Hg or diastolic 
≥100 mm Hg
Vascular disease
Diabetes
Benefits outweigh ris

isks
  Vascular disease or >20 years’ 
duration
Stroke
I: Benefits outweigh risks
C: Risks outweigh benefits
Current or history of ischemic heart 
disease
Risks outweigh benefi

s
  Current
Contraindicated
  Past and no evidence of disease 
for 5 years
R

d
for 5 years
Migraines
No restrictions
  With aura
BMI, Body mass index; BP, blood pressure; C, continuation; DMPA, depot medroxyprogesterone acetate; I, initiation; IUD, intrauterine device; POP, progestin-only pill.
   
TABLE 18.3
649
CHAP

 POP, progestin-only pill.
   
TABLE 18.3
649
Myocardial Infarction and Thrombotic Stroke
Although arterial events are substantially less common than VTE 
in reproductive-age women, the sequelae of strokes and myocar­
dial infarctions are more devastating than those of VTE. The risk 
of arterial thrombosis including myocardial infarction and stroke is 
increased with COC use. The risk is estrogen dose dependent, with a 
1.6-fold increase in thromboses with estr

 with a 
1.6-fold increase in thromboses with estrogen doses of 20 μg and up 
to a 2.4-fold increase in risk with estrogen doses greater than 50 μg; 
risk appears to be unrelated to progestin type.70,71 This effect is esti­
mated to account for 19 thrombotic strokes and seven myocardial 
infarctions per 100,000 women per treatment year.71 Risk of arterial 
thrombosis does not appear to be increased with past COC use.72
The absolute risk of myocardial infarction and stroke increases 
with age. Th

dial infarction and stroke increases 
with age. Therefore, COC should be avoided in women older 
than 35 years with risk factors for myocardial infarction or stroke, 
including smoking, hypertension, long-standing diabetes, and 
migraine headache with aura.9 
Findings that exogenous estrogen-progestin combination hor­
mone therapy increases breast cancer risk in postmenopausal 
women raises concerns that COCs may increase breast cancer risk 
in premenopausal women. Recent findings 

cer risk 
in premenopausal women. Recent findings from a large national 
registry in Denmark including approximately 1.8 million women 
15 to 49 years of age over a period of almost 11 years reported a 
modest increased relative risk of invasive breast cancer in current 
hormonal contraception users (most of which were COC users) 
of 1.20 (95% confidence interval [CI], 1.14–1.26) compared 
with never users.73 The absolute increased risk of breast cancer 
was small, translating into approximately

 cancer 
was small, translating into approximately one extra breast cancer 
for every 7690 women using hormonal contraception for 1 year. 
The study adjusted for duration of hormonal contraceptive use, 
age, calendar year, education level, parity, PCOS, endometriosis, 
and family history of breast or ovarian cancer. However, meth­
odologic limitations should be considered when interpreting 
these findings. The Danish researchers utilized a database that 
did not include information addressing se

se that 
did not include information addressing several potential con­
founders, including age at menarche, history of breastfeeding, 
and alcohol consumption. In addition, investigators did not 
address possible differences between users and nonusers in breast 
cancer surveillance, such as screening mammography. Although 
the database had information on women 15 to 79 years of age, 
the analysis was limited to women younger than 50 years. This 
is unfortunate given that more than three quarters

is unfortunate given that more than three quarters of inva­
sive breast cancers are diagnosed in women age 50 years and 
older; not including this population eliminated the opportunity 
to assess the potential association of hormonal contraceptive 
use and risk of breast cancer during women’s postmenopausal 
years.74 A large prospective cohort study in the United Kingdom 
including more than 46,000 women followed for more than 40 
years also reported an increased risk of breast cancer but also a

rted an increased risk of breast cancer but also a 
reduced overall cancer risk (lower rates of colorectal, endome­
trial, ovarian, hematopoietic cancers) in ever users of COC com­
pared with nonusers.75 The authors concluded that most women 
who choose to use oral contraceptives do not expose themselves 
to long-term cancer harms; instead, with some cancers, many 
women benefit from important reductions of risk that persist for 
many years after stopping. These findings are consistent with a 
s

 stopping. These findings are consistent with a 
systematic review of oral contraceptive use and cancer risk that 
reported a reduction in cancer risk.41
In women with a family history of breast cancer, a systematic 
review of retrospective cohort studies and case-control studies 
show that COC use does not confer an increased risk of breast 
cancer.76 However, recent data suggest that COC use may increase 
risk of early-onset breast cancer in women carrying BRCA gene 
mutations,77 although prev

men carrying BRCA gene 
mutations,77 although previous reports have not found this asso­
ciation.37,78 According to the current USMEC guideline, there are 
no restrictions on use of hormonal contraception in women with 
a family history of breast cancer or women with breast cancer sus­
ceptibility genes (e.g., BRCA1 and BRCA2).9 
Cervical Cancer
For current COC users, the risk of invasive cervical cancer 
increases with increasing duration of use (relative risk for ≥5 years’ 
use vs. never use i

 (relative risk for ≥5 years’ 
use vs. never use is 1.90; 95% CI, 1.69–2.13), as demonstrated 
in a meta-analysis of pooled data from 24 studies including more 
than 16,000 women with cervical cancer and 35,000 without cer­
vical cancer. The risk declined after use ceased; however, after 10 
or more years, it returned to the risk level of never users. A similar 
pattern of risk was seen for invasive and in situ cancer, as well as 
for women who tested positive for high-risk human papillomavi­
ru

ested positive for high-risk human papillomavi­
rus strains.79
A more recent meta-analysis of case-control and cohort 
studies demonstrated an increased risk of cervical cancer 
beyond 5 years of treatment, but the effect was not statisti­
cally significant and the evidence was of insufficient strength.41 
Regular cervical cancer screening according to national pro­
tocols is advised for all sexually active women, regardless of 
contraceptive use. A history of cervical intraepithelial neoplasia 

. A history of cervical intraepithelial neoplasia 
or genital human papillomavirus infection is not a contraindi­
cation for COC use.9 
Use of Concomitant Medications With Combined Oral 
Contraceptives Pills
Certain anticonvulsants (phenytoin, carbamazepine, barbiturates, 
primidone, topiramate, and oxcarbazepine) that induce hepatic 
Postpartum
(12 weeks
only)
Pregnancy*
COC user
Nonpregnant
Non-COC
user
70
60
50
40
30
20
Ranges from
40 to 65
5 to 20
3 to 9
1

nges from
5 to 20
3 to 9
1 to 5
10
0
Number of women with a blood clot
out of 10,000 women years (WY)
• Fig. 18.2  Venous thromboembolism risk among women of reproductive 
age by various conditions. Pregnancy data based on actual duration of 
pregnancy in the reference studies. Based on a model assumption that 
pregnancy duration is 9 months, the rate is 7 to 27 per 10,000 women-
years. COC, combined oral contraceptives. (From U.S. Food and Drug 
Administration. FDA Drug 

From U.S. Food and Drug 
Administration. FDA Drug Safety Communication: Updated information 
about the risk of blood clots in women taking birth control pills containing 
drospirenone. Available at https://www.fda.gov/drugs/drug-safety-and-
availability/fda-drug-safety-communication-updated-information-about-
risk-blood-clots-women-taking-birth-control.)
650
enzymes may reduce contraceptive efficacy of COCs.9 Rifamycin 
antibiotics, including rifampic

 COCs.9 Rifamycin 
antibiotics, including rifampicin and rifabutin, can also interact 
with contraceptive hormones and reduce the efficacy of the con­
traceptive during and up to 4 weeks after use.80 With the excep­
tion of those in the rifamycin class, concomitant use of antibiotics 
has not been found to reduce efficacy of any hormonal contra­
ceptives.81 In addition, antiretrovirals may reduce efficacy of hor­
monal contraception, including some non-nucleoside reverse 
transcriptase inhibitor

me non-nucleoside reverse 
transcriptase inhibitors (efavirenz) and some ritonavir-boosted 
(/r) protease inhibitors (darunavir/r, fosamprenavir/r, lopinavir/r, 
saquinavir/r, and tipranavir/r).9 If oral contraceptives are used with 
these medications, a formulation containing at least 30 μg of ethi­
nyl estradiol and backup contraception in the form of condoms 
should be used.9 Alternatively, high-dose progestin-only methods 
such as DMPA or intrauterine contraceptives may be preferable 
becaus

rauterine contraceptives may be preferable 
because their efficacy is not reduced by medications that induce 
liver enzymes. 
Contraceptive Vaginal Ring and Transdermal 
Patch
The transdermal and transvaginal routes are safe and acceptable 
alternatives to COCs to deliver combined estrogen-progestin 
contraception. The contraceptive patch (Xulane, Mylan Pharma­
ceuticals, Morgantown, WV) and the contraceptive vaginal ring 
(NuvaRing, Merck & Co. Inc., Rockville, MD) offer women 
combination estr

Inc., Rockville, MD) offer women 
combination estrogen-progestin contraception without the need 
to take pills daily. In consistent users, contraceptive failure rates 
with the patch and ring are similar to those for oral contracep­
tives.82 There is currently insufficient evidence to determine how 
body weight affects the contraceptive efficacy of the vaginal ring 
and contraceptive patch.83
It is important to note that all ethinyl estradiol contraceptives 
elevate VTE risk, regardless of wheth

traceptives 
elevate VTE risk, regardless of whether the route of administra­
tion is oral, vaginal, or transdermal.84 The contraceptive patch 
generates higher circulating levels of ethinyl estradiol than either 
COC or the vaginal ring.85 It is uncertain whether use of the patch 
is associated with a higher risk of VTE than use of COC.67,86 
Contraindications to the patch and the vaginal ring are the same 
as those for COCs. In the absence of adequate evidence to deter­
mine how risks and nonc

dequate evidence to deter­
mine how risks and noncontraceptive benefits of the patch and 
the ring compare with those of COC, they are usually considered 
to be equivalent.
As with COC pills, the contraceptive patch and vaginal ring 
act primarily by suppression of ovulation. The patch and ring 
are immediately effective if commenced within the first 5 days of 
the onset of menstruation, but because therapeutic steroid levels 
are achieved over the course of several days, starting at any other 

he course of several days, starting at any other 
time requires 7 days of backup contraception, such as condoms or 
abstinence.8
Transdermal Contraceptive Patch
Xulane, the only contraceptive patch available in the United 
States, is a 4.5-cm tan square that releases 20 μg of ethinyl estra­
diol daily along with norelgestromin, the biologically active 
metabolite of the progestin norgestimate87 (Fig. 18.3). A new 
patch is applied each week on the same day for 3 weeks, followed 
by a patch-free 

e same day for 3 weeks, followed 
by a patch-free week, during which withdrawal bleeding is antici­
pated. Sweating associated with vigorous exercise, swimming, and 
use of a hot tub or sauna should not result in patch detachment. 
Unscheduled bleeding in patch users is similar to that seen with 
COCs.82 Although patch users are more compliant than pill users, 
they are more likely to report breast discomfort, dysmenorrhea, 
nausea, and vomiting.82,88 Mild local skin reactions are com­
mon. An i

82,88 Mild local skin reactions are com­
mon. An investigational contraceptive patch is being developed 
that releases less ethinyl estradiol than the Xulane transdermal 
contraceptive.89 
Contraceptive Vaginal Ring
The vaginal mucosa offers excellent absorption of sex steroids. 
The only contraceptive vaginal ring available in the United States 
is the NuvaRing, a flexible plastic ring that is 4 mm thick and 
has an outside diameter of 54 mm.90 The ring releases 15 μg of 
ethinyl estradiol dail

The ring releases 15 μg of 
ethinyl estradiol daily along with etonogestrel, the biologically 
active metabolite of the progestin desogestrel (Fig. 18.4). The con­
traceptive vaginal ring is inserted in the vagina for 3 weeks and 
then withdrawn for 1 week, during which withdrawal bleeding is 
anticipated. A new ring is required every 4 weeks. The ring does 
not require individual fitting; as long as it remains in the vagina, 
appropriate absorption of steroids occurs. The ring is well toler­
at

ion of steroids occurs. The ring is well toler­
ated, with increased physiologic vaginal discharge being the main 
bothersome side effect.88 Rates of unscheduled bleeding and spot­
ting are lower with the ring than with oral COC.82 Spontaneous 
expulsion is uncommon. The ring can also be used in extended-
cycle regimens.91
• Fig. 18.3  The contraceptive patch.
• Fig. 18.4  The contraceptive vaginal ring.
651
Women’s interest in using a contraceptive vaginal rin

en’s interest in using a contraceptive vaginal ring var­
ies; some women are highly motivated and comfortable with this 
method. Some users keep the ring in place during sexual relations; 
in this setting, male discomfort is uncommon. Other users prefer 
to remove the ring before intercourse, and removal for less than 
48 hours does not appear to impair efficacy.8 However, frequent 
removal for long periods is not recommended, and the label states 
a maximum of 3 hours is allowed.92 Backup contr

s 
a maximum of 3 hours is allowed.92 Backup contraception is 
required for 7 days if the ring is removed for 48 hours or longer. 
Progestin-Only Contraceptive Methods
Progestin-only contraceptives offer many advantages over estro­
gen-containing methods. There are fewer contraindications to 
progestin-only contraception (see Table 18.3). Progestin-only 
contraceptives may be appropriate for many women with con­
traindications to contraceptive doses of estrogen, including those 
at elevated base

nogestrel implant (Nexplanon [Merck & Co., White­
house Station, NJ]), and
 • levonorgestrel IUDs (LNG IUDs; the larger-frame devices 
Mirena [Bayer Healthcare Pharmaceuticals] and Liletta [Medi­
cines360, San Francisco, CA] and the smaller-frame devices 
Kyleena and Skyla [Bayer Healthcare Pharmaceuticals]).
These methods comprise various types of progestins, doses, and 
routes of administration. The dose of progestin in the “minipill” 
available in the United States (0.35-mg norethindrone) is

le in the United States (0.35-mg norethindrone) is one 
third that typically found in combined hormonal contraceptives 
formulated with this progestin. A stable serum concentration of 
150 to 200 pg/mL of levonorgestrel occurs after insertion of the 
52-mg LNG IUD,93 a concentration substantially lower than that 
seen in women using a 150-μg levonorgestrel-containing oral con­
traceptive pill (6.2 ng/mL).94
Although POPs, implants, and the LNG IUD do not appear 
to increase risk of VTE, data for

D do not appear 
to increase risk of VTE, data for DMPA are less clear.95-99 A 
twofold increased risk of VTE with progestin-only injectables 
was found in meta-analysis96 based on two studies; however, 
residual confounding weakens the result.100-102 If DMPA does 
increase VTE risk, the magnitude of this increase is likely 
less than that seen with COCs. The USMEC allows the use 
of DMPA in women with VTE and those at elevated risk for 
thromboembolism.
Progestin-Only Oral Contraceptive Pill
On

mbolism.
Only one POP formulation is marketed in the United States: 
norethindrone in 0.35-mg tablets (e.g., Micronor, Nor-QD, and 
generics). The progestin dose is lower than the dose in any com­
bination oral contraceptive. Due to the short half-life of POPs, 
serum steroid levels decline to near baseline as quickly as 24 hours 
after administration, and a dose is considered missed if it has been 
more than 3 hours beyond an expected dose.8 Norethindrone 

n 3 hours beyond an expected dose.8 Norethindrone 
POPs are dispensed in packs of 28 active pills, which are taken 
continuously (i.e., no HFI) (Table 18.4). Contraindications to 
POPs are few (see Table 18.3).
The mechanism of action of progestin-only oral contracep­
tives may include suppression of ovulation, thickening of cervi­
cal mucus, and induction of endometrial atrophy. Unlike with 
COCs, ovulation is not consistently suppressed with the 0.35-mg 
norethindrone POP, 

y suppressed with the 0.35-mg 
norethindrone POP, and the progestin effects on cervical mucus 
and the endometrium are critical factors in preventing concep­
tion.103 Within hours of administration, progestin-only oral con­
traceptives thicken cervical mucus, which acts as a barrier to stop 
sperm at the cervical canal; 48 hours of norethindrone POP use is 
necessary to achieve full contraceptive effects on cervical mucus.8 
In European countries, a POP containing 75 μg of desogestrel is 
availa

, a POP containing 75 μg of desogestrel is 
available. In contrast with the norethindrone POP available in the 
United States, the desogestrel POP inhibits ovulation, and contra­
ceptive efficacy is as high as with COCs.104 
Efficacy
The efficacy of progestin-only oral contraceptives is not well estab­
lished.105 National survey data used to estimate contraceptive fail­
ure rates with typical use have failed to distinguish between COCs 
(the larger group) and POPs. A potential confounding factor

er group) and POPs. A potential confounding factor is 
that women using progestin-only oral contraceptives may have 
reduced fertility as a result of breastfeeding or older reproductive 
age. It is possible that the typical first-year failure rates (7%) with 
progestin-only oral contraceptives are higher than those seen with 
COCs.26 In general, norethindrone 0.35-mg POPs should not be 
considered a first-line contraceptive except in women who should 
not or do not wish to use estrogen. Given co

ould 
not or do not wish to use estrogen. Given concerns regarding the 
efficacy of norethindrone 0.35-mg pills in women with normal 
fertility, some experts recommend that women with normal fer­
tility who choose to use POPs (e.g., a 35-year-old hypertensive 
woman) take two daily tablets (two tablets as a single dose, off-
label). A tablet combining 1 mg of norethindrone acetate and 5 
μg of ethinyl estradiol is marketed for treatment of menopausal 
symptoms.106 This dose of norethindrone acet

usal 
symptoms.106 This dose of norethindrone acetate appears adequate 
to consistently suppress ovulation; in addition, the dose of estro­
gen is small. Accordingly, some clinicians prescribe this combina­
tion medication off-label to women requiring contraception who 
medically are not candidates for conventional combination estro­
gen-progestin contraceptives. Such patients should be advised that 
the 1-mg norethindrone acetate/5-μg ethinyl estradiol combina­
tion is not approved for contrace

mize contraceptive efficacy.
	5.	 Menstrual irregularities are common in users of progestin-only oral 
contraception and represent the most frequent cause for contracep­
tive discontinuation.
TABLE 18.4
652
recommended. Of note is that the progestational impact of 1-mg 
norethindrone acetate is similar to that of 1-mg norethindrone. 
Starting the Progestin-Only Pill
Progestin-only oral contraceptives can be initiated within the first 
5 days of mense

can be initiated within the first 
5 days of menses, without need for backup contraception. Some 
clinicians initiate progestin-only oral contraceptives at any time 
in the cycle, as long as the clinician is reasonably certain that the 
patient is not pregnant. With this strategy, backup contraception 
should be used for the first 2 days of use.8 Because of the short 
duration of action and the short half-life of POPs, the pill must 
be taken at the same time each day to maximize contraceptive e

the same time each day to maximize contraceptive effi­
cacy. A backup contraceptive (e.g., condoms) should be used for at 
least 2 days if the progestin-only oral contraceptive is taken more 
than 3 hours late or forgotten on any day. The patient should 
also resume taking daily progestin-only oral contraceptives as soon 
as possible. Progestin-only oral contraceptives may be initiated 
immediately or within 7 days after induced or spontaneous abor­
tion. If not administered immediately after ab

or­
tion. If not administered immediately after abortion, the woman 
must use alternative contraception or abstinence for 2 days. After 
delivery, POPs can be started at any time, including immediately 
postpartum in breastfeeding and nonbreastfeeding women.107 If 
the woman is more than 21 days postpartum and is not using 
POPs to augment lactational amenorrhea, then backup contracep­
tion should be used for 2 days.8 
Side Effects of Progestin-Only Oral Contraceptives
Sonographic studies have s

nly Oral Contraceptives
Sonographic studies have shown that prominent ovarian follicular 
activity is more common among progestin-only oral contracep­
tive users than among other women, but this may wax and wane 
over time.108 Transvaginal ultrasound follow-up of women dur­
ing a 3-year clinical trial of the small-frame LNG IUD (Skyla) 
found that ovarian cysts (defined as >3 cm) were present in 1.6% 
of participants at screening and 1.1% to 2.4% of participants at 
each subsequent visit. Eighty

 of participants at 
each subsequent visit. Eighty-eight percent of these cysts were 5 
cm or less; none were greater than 8 cm. No ovarian cysts per­
sisted beyond 9 months, which is consistent with a functional eti­
ology.109 Other than reassurance and sonographic follow-up, no 
intervention is required in asymptomatic women found to have 
asymptomatic incidental ovarian cysts during use of progestin-
only contraceptives. If a follow-up sonogram in 6 to 8 weeks dem­
onstrates resolution or dec

m in 6 to 8 weeks dem­
onstrates resolution or decrease in size of ovarian cysts, no further 
evaluation is required.
Side effects with POPs other than changes in bleeding patterns 
are relatively uncommon.103 Weight gain has not been objectively 
documented, and headaches are uncommon. Unscheduled bleed­
ing, spotting, and amenorrhea are common menstrual patterns 
during progestin-only oral contraceptive use, and users should 
be counseled accordingly. Interpreting signs and symptoms of 
pregna

dingly. Interpreting signs and symptoms of 
pregnancy, whether intrauterine or extrauterine, can be challeng­
ing. Pregnancy testing is appropriate for POP users experiencing 
nausea, breast tenderness, a change in menstrual pattern, or lower 
abdominal pain (see Table 18.2). 
Other Effects
Most studies have reported that progestin-only oral contraceptives 
have little impact on carbohydrate metabolism.110 However, one 
study conducted in Latina women observed that lactating women 
with a histor

women observed that lactating women 
with a history of gestational diabetes who used progestin-only oral 
contraceptives after delivery had an almost tripled risk of diabetes 
compared with those who used low-dose COCs.111 All women 
with gestational diabetes, regardless of contraceptive used, should 
be screened for diabetes postpartum with a 75-g, 2-hour oral glu­
cose tolerance test.112
As with all contraceptives, POPs lower overall risk for ectopic 
pregnancy. A history of ectopic pregnancy 

ctopic 
pregnancy. A history of ectopic pregnancy does not contraindi­
cate progestin-only oral contraceptive use. However, if pregnancy 
occurs, the likelihood that the pregnancy is ectopic is higher in 
POP users than in nonusers of contraceptives.103 Bone density 
does not appear to be adversely affected by the POP. The only 
study assessing skeletal health in progestin-only oral contraceptive 
users was conducted in breastfeeding women; progestin-only oral 
contraceptives, in fact, protected

stin-only oral 
contraceptives, in fact, protected against small, reversible losses in 
BMD occurring during lactation.113 Finally, progestins usually 
suppress endometrial growth.108 However, few epidemiologic data 
address the effect of progestin-only oral contraceptives on endo­
metrial cancer risk or on any cancer risk. 
Progestin-Only Oral Contraceptives During Lactation
Progestin-only oral contraceptives do not interfere with the quality 
or quantity of breast milk.107 Very little progesti

r quantity of breast milk.107 Very little progestin is passed from 
nursing mothers into their breast milk, and no adverse impact on 
infant growth has been observed. Package labeling advises delay­
ing administration in lactating women until 6 weeks after delivery. 
A trial conducted in the United States randomized breastfeeding 
women to initiate progestin-only compared with combination 
estrogen-progestin oral contraceptives 2 weeks postpartum. At 8 
weeks postpartum, approximately two-thirds

. At 8 
weeks postpartum, approximately two-thirds of women contin­
ued to breastfeed in both groups.114 Based on the absence of data 
suggesting harm to the infant or mother and the contraceptive 
benefits of early initiation, some experts recommend initiation of 
POPs before hospital discharge and no later than the third post­
partum week, regardless of lactation status. 
Depot Medroxyprogesterone Acetate for 
Contraception
DMPA is an injectable, progestin-only contraceptive that provides 
eff

e, progestin-only contraceptive that provides 
effective, private, and reversible contraception. It is discrete, avoids 
the need for user action daily or near the time of sexual intercourse, 
and avoids the need for partner cooperation (Table 18.5).
Formulations and Pharmacology
DMPA is available in two formulations: 150 mg/1 mL for intra­
muscular injection and 104 mg/0.65 mL for subcutaneous injec­
tion. The injections can be given every 3 months because low 
solubility of the microcrystals a

ths because low 
solubility of the microcrystals at the injection site allows pharma­
cologically active drug levels to persist for several months. DMPA 
primarily acts by inhibiting follicular maturation and ovulation 
through inhibition of gonadotropin secretion. Unlike other pro­
gestin-only contraceptives, mean estradiol levels may be lower 
than normal for cycling premenopausal women.115
DMPA is an effective contraceptive. Following a 150-mg 
intramuscular injection, failure rates in clinic

intramuscular injection, failure rates in clinical trials have ranged 
from 0.0 to 0.7 per 100 woman-years. In clinical practice, typical-
user failure rates are 4 failures per 100 woman-years, reflecting the 
fact that some users do not return for their injections as sched­
uled.26 Because progestin levels are high, efficacy is not reduced 
by obesity or use of concurrent medications, such as anticonvul­
sants. For the 104-mg subcutaneous injection, no contraceptive 
failures were reported in

ction, no contraceptive 
failures were reported in phase III clinical trials.116 This formula­
tion is relatively new, so typical-user failure rates are not available, 
but they are expected to be similar to those for the intramuscular 
preparation.
653
Although users may be hypoestrogenic, vasomotor symptoms 
and vaginal atrophy are uncommon. Because of its progestin effect, 
DMPA also causes changes in cervical mucus that are hostile to 
sperm migration and e

l mucus that are hostile to 
sperm migration and endometrial atrophy. The newer subcutane­
ous injection is less painful and is available in a prefilled syringe, 
offering the potential for self-administration.117 Intramuscular 
DMPA is available as a generic formulation that is less costly than 
subcutaneous DMPA. Otherwise, benefits and risks are similar for 
intramuscular and subcutaneous administration.118 
Administration of DMPA
Starting Injections
The ideal time to initiate DMPA is within 

ections
The ideal time to initiate DMPA is within 7 days of the onset 
of menses.8 This approach ensures that the patient is not preg­
nant at the time of injection and prevents ovulation during the 
first month of use, so backup contraception is unnecessary. Most 
women have pharmacologically active drug levels and a poor cer­
vical mucus score within 24 hours after injection.119 The “same-
day,” “quick-start,” or “Depo-now” approach when a pregnancy 
test result is negative facilitates DMPA in

nancy 
test result is negative facilitates DMPA initiation for many users, 
and it may prevent some pregnancies.120 However, there is a 
small possibility of undiagnosed pregnancy despite a negative 
pregnancy test result. When the quick-start approach is used for 
initiating DMPA, backup contraception or abstinence should be 
used for 7 days and a repeat pregnancy test should be performed 
in 2 to 4 weeks. Nevertheless, DMPA given inadvertently during 
pregnancy does not appear to be teratogeni

during 
pregnancy does not appear to be teratogenic.121 DMPA may be 
initiated immediately after spontaneous or induced abortion or 
within the first 7 days.8 If given in the first 7 days after the event, 
then backup contraception should be used. After delivery, DMPA 
can be started at any time, including immediately postpartum in 
breastfeeding and nonbreastfeeding women.107,122 If the woman is 
more than 21 days postpartum and not breastfeeding exclusively, 
backup contraception should be use

g exclusively, 
backup contraception should be used for 7 days after the first 
DMPA injection.8 
Repeat Injections
Repeat injections of DMPA should be scheduled every 3 months 
(13 weeks). After a 150-mg injection, ovulation does not occur for 
at least 14 weeks. A 2-week grace period (repeat injection without 
pregnancy testing, up to 15 weeks following the prior injection) 
is appropriate for women receiving injections every 3 months.8 In 
women more than 2 weeks late for an injection, a urin

en more than 2 weeks late for an injection, a urine pregnancy 
test should be performed before administering further DMPA, 
and backup contraception for 7 days is advised. 
Side Effects of DMPA
Candid counseling regarding the side effects of DMPA and the 
need for timed injections must be provided. Women who are well 
informed when they choose this method of contraception are more 
likely to become satisfied users with high continuation rates.123 
Menstrual changes occur in almost all women usin

Menstrual changes occur in almost all women using DMPA and 
are the most frequent cause for discontinuation of injectable and 
all other progestin-only contraceptives.124 Proactive patient educa­
tion before the initiation of DMPA and supportive follow-up can 
improve tolerance of menstrual changes. During the first months 
of use, episodes of unpredictable bleeding and spotting lasting 7 
days or longer are common. Bleeding decreases with longer dura­
tion use, and at 1 year, 50% of women exp

er dura­
tion use, and at 1 year, 50% of women experience amenorrhea; 
this rate increases to 75% with long-term use.115 Similar bleed­
ing patterns are reported with the subcutaneous preparation.125 
Many women view amenorrhea (along with a reduction or elimi­
nation of menstrual cramps) as one of the advantages of using this 
method.
There are no established methods for predicting, preventing, or 
treating unscheduled bleeding in DMPA users. Small studies have 
shown that estrogen supplements 

all studies have 
shown that estrogen supplements (e.g., 1.25 mg of conjugated 
estrogen orally, 1–2 mg of micronized estradiol orally, or 0.1-mg 
estradiol patches for 10–20 days) may terminate a bleeding epi­
sode.126 Another treatment option endorsed by the US Selected 
Practice Recommendations for Contraceptive Use is nonsteroidal 
anti-inflammatory drugs for 5 to 7 days. However, a systematic 
review concluded that there is a lack of high-quality data to sup­
port routine clinical use of an

ality data to sup­
port routine clinical use of any interventions to treat persistent 
unscheduled bleeding with progestin-only contraceptives.125
Observational studies have not reported any consistent effects 
of DMPA on mood.127 Progestins may cause or exacerbate depres­
sive symptoms in certain subpopulations of women, including 
those with a history of premenstrual syndrome or mood disor­
ders. However, depression is not a contraindication to DMPA 
use.128 The impact of DMPA on weight has be

 DMPA 
use.128 The impact of DMPA on weight has been controversial.129 
Randomized controlled trials, although limited, fail to show that 
DMPA causes weight gain.115,130 Observational studies are dif­
ficult to interpret given that individuals tend to gain weight over 
time irrespective of contraceptive use. On average, the weight gain 
with DMPA use is small (2 kg); however, there is marked indi­
vidual variation. Weight gain with the use of DMPA may be asso­
ciated with user subgroups at part

gynecologic and nongynecologic disorders, such as heavy 
menstrual bleeding, dysmenorrhea, and iron deficiency anemia.
	9.	 There is no high-quality evidence that use of DMPA increases 
the risk of developing cancer, cardiovascular disease, or sexually 
transmitted infection. DMPA use significantly reduces the risk of 
developing endometrial cancer.
	10.	There is an association between current DMPA use and decreased 
bone mineral density; losses in bone mineral density are temporary, 
reverse 

s in bone mineral density are temporary, 
reverse after discontinuation of DMPA, and have not been linked to 
postmenopausal osteoporosis or fractures.
DMPA, Depot medroxyprogesterone acetate; IM, intramuscular; SC, subcutaneous.
   
TABLE 18.5
654
than those who chose other contraceptives. Weight changes varied 
among women choosing different progestin-only methods; how­
ever, being of the black race predicted weight gain regardless of 
the contracep

predicted weight gain regardless of 
the contraceptive chosen.132,133 It is reasonable to monitor weight 
over time in women using DMPA.8 
Risks and Benefits of DMPA
DMPA has been used to manage a variety of gynecologic and non­
gynecologic disorders. The tendency of DMPA to cause amenor­
rhea makes it a particularly appropriate contraceptive choice for 
women with heavy menstrual bleeding, dysmenorrhea, or iron-
deficiency anemia. DMPA is a useful means of suppressing men­
strual bleeding and m

ul means of suppressing men­
strual bleeding and managing menstrual hygiene in individuals 
with special needs (e.g., cognitive impairment, military personnel, 
wheelchair-bound individuals).115,134
Progestins inhibit endometriotic tissue growth by directly caus­
ing initial decidualization and eventual atrophy, and by inhibiting 
pituitary gonadotropin secretion and ovarian estrogen produc­
tion. Randomized trials show that DMPA is more effective than 
oral contraceptives and danazol and as eff

e than 
oral contraceptives and danazol and as effective as leuprolide injec­
tions for treatment of pain associated with endometriosis.130 In 
the United States, subcutaneous DMPA is approved for the treat­
ment of pain associated with endometriosis.
Effect on Cancer Risk
Large case-control studies conducted by the WHO have shown 
that use of DMPA is associated with an 80% reduced risk of 
endometrial cancer and does not affect the incidence of cervical 
cancer.135-137 A more recent Thai case-c

cervical 
cancer.135-137 A more recent Thai case-control study found that 
use of DMPA is associated with protection against epithelial ovar­
ian cancer, similar to that seen with use of COCs.138 Multicoun­
try data from the WHO and data from the United States, South 
Africa, and New Zealand provide reassurance that use of DMPA is 
not associated with an increased risk of breast cancer.139 
Effect on Cardiovascular Risk
DMPA has an adverse effect on circulating lipids but does not 
increase prod

 on circulating lipids but does not 
increase production of coagulation factors and has no adverse 
effect on blood pressure. No adverse clinical effects on cardiovas­
cular disease have been observed.102 Based on these findings, the 
USMEC allows DMPA and other progestin-only contraceptives 
for use in women with a history of VTE and those in whom use 
of combination estrogen-progestin contraceptive is contraindi­
cated.9 This recommendation is different from package labeling 
for DMPA (written

different from package labeling 
for DMPA (written in the 1960s), which indicates that a prior 
history of VTE is a contraindication to DMPA use. In women 
with multiple risk factors for cardiovascular disease (e.g., smoking, 
older age, hypertension, diabetes), the USMEC classifies DMPA 
as category 3 (see Table 18.3), indicating that the risks of use may 
exceed the benefits. The basis for this caution, which the authors 
do not support, seems to be the hypoestrogenic effects of DMPA 
and redu

to be the hypoestrogenic effects of DMPA 
and reduced high-density lipoprotein (HDL) levels. In addition, 
the effects of DMPA might persist for some time after discontinu­
ation, so it would not be immediately reversible if there were an 
adverse event. 
Effect on Skeletal Health
DMPA’s impact on BMD has generated much controversy. Bone 
mineral density is defined as the amount of mineral matter per vol­
ume of bone and directly correlates with bone strength.140 DMPA 
injections suppress the se

bone strength.140 DMPA 
injections suppress the secretion of gonadotropins, decreasing 
ovarian production of estrogen, which results in a decline in 
BMD. In 2004, the US Food and Drug Administration (FDA) 
added a “black box” warning to DMPA labeling about declines in 
BMD, which might discourage health care providers from initiat­
ing DMPA or limit long-term use.141 Pregnancy, breastfeeding, 
menopause, and use of hormonal contraceptives can all impact 
BMD by affecting sex hormones.
Both cro

ll impact 
Both cross-sectional and longitudinal studies using dual-
energy x-ray absorptiometry technology to evaluate current users 
of DMPA have observed lower BMD in current DMPA users 
compared with nonusers.142-149 Longitudinal studies report BMD 
losses of –3.1% to –5.8% and –4.1% to –5.7% at the hip and 
spine, respectively, after 2 years of use, and losses of –4.5% to 
–7.7% and –4.9% and –6.6% after 4 years of use.147,148 Most 
of the decline in BMD is ob

s of use.147,148 Most 
of the decline in BMD is observed within the first 2 years of use, 
suggesting that longer use may not progressively increase risk of 
osteoporosis.147 Decline in BMD appeared to be substantially or 
completely reversible, as seen in studies including both adults and 
adolescents, with a duration of DMPA use of 2 to 5 years and 
follow-up to 5 years after discontinuation.145,147,150-152
DMPA use and breastfeeding are both accompanied by 
hypoestrogenemia. The reversible de

ccompanied by 
hypoestrogenemia. The reversible declines in BMD associated 
with DMPA use, followed by recovery when the DMPA is discon­
tinued, parallels the BMD trends seen with breastfeeding.153,154 
Cross-sectional studies demonstrate that BMD in adult women 
who previously used DMPA is similar to that of women who 
never used injectable contraception, an observation that provides 
further reassurance that loss of BMD associated with DMPA use 
is reversible.155-157
Changes in BMD resulting f

Changes in BMD resulting from use of DMPA are clinically 
important if they increase risk for fracture. No published data 
address whether premenopausal DMPA impacts subsequent frac­
ture risk in postmenopausal women. Although the association 
between decline in BMD and risk of fracture is well established 
in postmenopausal women,158 the association is less robust in 
healthy premenopausal women.134 Observational studies have 
mixed findings on the association of DMPA us

have 
mixed findings on the association of DMPA use and fracture risk 
in reproductive-age women. Three observational studies used large 
national databases to examine the association between DMPA or 
LNG IUD use and fracture.159-161 Two of these were based on the 
same large United Kingdom database.159,160 Using case-control 
methods, ever users of DMPA had a higher risk of fracture com­
pared with never users (odds ratio [OR], 1.44).159 Using the same 
British database but a retrospective coho

he same 
British database but a retrospective cohort analysis, a second group 
also observed that DMPA users had an increased risk for fracture 
(OR, 1.41). However, this second report noted that the elevated 
risk was present at baseline, prior to DMPA use; accordingly, the 
elevated fracture risk could not have been caused by DMPA.160 A 
Danish case-control study also noted that ever use of DMPA was 
associated with increased risk for fracture (OR, 1.44). However, 
the authors of this Danish r

(OR, 1.44). However, 
the authors of this Danish report suggested that the subgroup of 
women choosing DMPA, 0.1% of the study sample, were not rep­
resentative of the larger Danish population, limiting attribution of 
their findings.161 These British and Danish studies raise the issue 
that women who choose DMPA are different from women who 
choose other methods of contraception (as noted also in the US 
CHOICE study132) and hypothesize that fracture risk associated 
with DMPA exposure may be d

cture risk associated 
with DMPA exposure may be due to unmeasured confounders 
in women who choose injectable contraception, including behav­
ioral differences. For example, authors of the Danish study point 
out the prevalence of alcoholism (a condition known to be associ­
ated with fractures from motor vehicle and other accidents) in 
women using DMPA was 14%, sevenfold higher than in women 
655
not using DMPA; furthermore, the Danish investigators note that

A; furthermore, the Danish investigators note that 
cases with fractures were some threefold more likely to be classi­
fied as alcoholics as control women.
When counseling women considering initiation or continu­
ation of injectable contraception, clinicians should discuss the 
benefits and the risks of DMPA, including the FDA black box 
warning, and use clinical judgment and shared decision making 
to assess appropriateness of use. The effect of DMPA on BMD 
on skeletal health should not preven

 DMPA on BMD 
on skeletal health should not prevent clinicians from recommend­
ing DMPA initiation or continuing use beyond 2 years (USMEC 
category 2 for adolescents <18 years of age and women >45 years 
of age; category 1 for women age 18–45 years).9 Routine assess­
ment of BMD is not recommended in adolescents and young 
women using DMPA. Although low-dose estrogen supplementa­
tion has been observed to limit BMD loss in adolescent DMPA 
users,162,163 estrogen supplementation during DMPA use 

,162,163 estrogen supplementation during DMPA use is not 
recommended due to potential adverse effects and a paucity of 
data from clinical trials assessing skeletal health outcomes.
For patients with comorbidities and conditions that may impact 
skeletal health, including an elevated risk of falls, wheelchair use, 
chronic corticosteroid use, renal disease, or malabsorption, indi­
vidualized counseling and clinical management along with shared 
decision making is appropriate. Age-appropriate ca

decision making is appropriate. Age-appropriate calcium and vita­
min D dietary intake, regular weight-bearing exercise, and smok­
ing cessation should be encouraged for all women. Although these 
recommendations may benefit the health of patients in general, 
studies demonstrating that these measures will improve skeletal 
health outcomes in women using injectable contraception are 
lacking. 
Effect on Sexually Transmitted Infections
The association between use of DMPA and risk of STIs is uncer

tion between use of DMPA and risk of STIs is uncer­
tain. Some studies show an increased risk of STI in DMPA users 
compared with other contraceptives, but it is unclear whether this 
reflects differences in sexual practices, such as lower condom use, 
in DMPA users.164,165 The relationship between DMPA use and 
human immunodeficiency virus (HIV) acquisition and transmis­
sion is also unclear, but several clinical trials show an increased 
risk.166 There are several plausible biologic mechanisms

66 There are several plausible biologic mechanisms through 
which DMPA may increase HIV transmission risk.167,168 DMPA 
is widely used in the areas of high HIV prevalence where alter­
native contraceptive options are limited. A systematic review 
of progestin injectables found a possible increased risk for HIV 
acquisition (adjusted hazard ratio, 1.4; 95% CI 1.2–1.6) among 
women at increased risk.166 It is unknown whether this associa­
tion is due to a real biologic effect, because there were m

ue to a real biologic effect, because there were methodo­
logic issues causing the evidence to be inconclusive. The current 
USMEC recommendation states that the advantages of DMPA 
continue to outweigh the theoretical or proven risks (USMEC 
category 2) among women at high risk for HIV infection but 
that women should be counseled regarding these issues.165 The 
USMEC emphasizes that women should not be denied DMPA 
because of these concerns, as there is a very real risk that unin­
tended pregn

 there is a very real risk that unin­
tended pregnancies and maternal morbidity and mortality would 
result. Women with HIV or those at risk for HIV should be 
counseled to always use condoms to prevent HIV and other STI 
transmission.165 
Effect on Return of Fertility
Although DMPA does not permanently impact endocrine func­
tion, return of fertility may be delayed after stopping DMPA 
use. Within 10 months of the last injection, 50% of women 
who discontinue DMPA to become pregnant will concei

ho discontinue DMPA to become pregnant will conceive. 
However, in some women, fertility is not reestablished until up 
to 18 months after the last injection.115 The persistence of ovu­
lation suppression after DMPA discontinuation is not related 
to the duration of use, but it is related to weight, as clearance 
is slower in heavier women.169 Before initiating DMPA con­
traception, clinicians should counsel candidates about the pos­
sible prolonged duration of action. Women who may want to 
bec

ged duration of action. Women who may want to 
become pregnant within the next year should choose an alterna­
tive contraceptive. 
Progestin-Releasing Intrauterine Devices
LNG IUDs provide highly effective, safe, convenient, and revers­
ible contraception.170 In the 1980s, intrauterine contraception use 
in the United States fell dramatically after earlier flawed studies 
reported an association between intrauterine contraception use 
and later tubal infertility. It is now acknowledged that mode

ubal infertility. It is now acknowledged that modern 
intrauterine contraception not only is highly effective but also is 
safe for most women to use.171 Use of IUDs has increased in the 
United States, with the prevalence of use rising from 2% to more 
than 10% from 2002 to 2012 among women using contracep­
tion. Most IUDs used by US women are progestin-releasing 
devices.172 The addition of progestin to the IUD increases con­
traceptive efficacy, and the 52-mg LNG IUD is approved in the 
Unite

y, and the 52-mg LNG IUD is approved in the 
United States not only for contraception but also for treatment 
of heavy menstrual bleeding. The LNG IUD provides other off-
label therapeutic benefits, including treatment of pain associated 
with endometriosis, treatment of symptoms associated with uter­
ine adenomyosis, treatment of endometrial hyperplasia or carci­
noma, and protection of the endometrium in women during use 
of menopausal estrogen therapy.
Contraceptive Uses
Mirena, Liletta, Kyle

 therapy.
Mirena, Liletta, Kyleena, and Skyla are the four LNG IUDs 
marketed in the United States. Mirena and Liletta are larger 
higher-dose T-shaped devices with a reservoir containing 52 
mg of levonorgestrel. These devices deliver 20 μg of levonorg­
estrel per day. Although Mirena is approved for up to 5 years 
of use, it provides contraceptive efficacy for a longer duration. 
In a US study of 496 women who had used the LNG IUD 
for 5 years, overall, two pregnancies occur

G IUD 
for 5 years, overall, two pregnancies occurred in the LNG IUD 
group during the sixth and seventh years of use. The failure rate 
in the sixth year of use was 0.25 per 100 women-years (95% 
CI, 0.04–1.42); in the seventh year of use, the failure rate was 
0.43 per 100 women-years (95% CI, 0.08–2.39).173 Accord­
ingly, women, particularly women older than 35 years, can be 
offered the opportunity to retain their 52-mg LNG IUDs for up 
to 6 or 7 years of use, recognizing that such use is of

or 7 years of use, recognizing that such use is off-label.174 
Although Liletta is approved for up to 4 years of use (as of early 
2018) but given that it has the same quantity of levonorgestrel 
as Mirena, it is expected to eventually be approved for 5 or more 
years of use.
Kyleena and Skyla are smaller and lower-dose LNG IUDs, 
containing 19.5 mg and 13.5 mg and releasing 17.5 μg/day and 
14 μg/day of levonorgestrel, respectively. Kyleena and Skyla are 
approved for up to 5 and 3 years of use

Skyla are 
approved for up to 5 and 3 years of use, respectively. These small-
frame devices are distinguished from Mirena and Liletta by their 
smaller size (30 mm × 28 mm compared with 32 mm × 32 mm) 
and a silver ring at the top of the vertical stem (visible with ultra­
sound). The inserter diameter is also narrower than Mirena (3.8 
mm vs. 4.5 mm). The smaller dimensions of the Kyleena and 
656
Skyla devices make them particularly suitable for wom

la devices make them particularly suitable for women with 
smaller uterine cavities or tight cervices.175 Data from large inter­
national studies confirm extremely low pregnancy rates with the 
various LNG IUD devices, ranging between 0.1 and 0.3 per 100 
woman-years.26,176,177
Despite endometrial suppression, fertility returns rapidly 
after contraceptive removal.178 The high contraceptive efficacy of 
LNG IUDs reflects the thickening cervical mucus and profound 
endometrial suppression caused 

ucus and profound 
endometrial suppression caused by locally high concentrations of 
progestin. Most LNG IUD users continue to ovulate, even when 
amenorrhea is present.176 There are few contraindications to IUDs, 
and most women are appropriate candidates—including adoles­
cents and nulliparous women.170 Contraindications to IUD use 
are pregnancy, active cervicitis or uterine infection, malignancy in 
the uterus or cervix, a distorted uterine cavity, unexplained abnor­
mal bleeding, and advers

avity, unexplained abnor­
mal bleeding, and adverse reaction to product ingredients.179 
Although systemic concentrations of levonorgestrel are very low 
in LNG IUD users, use of hormonal contraceptives is generally 
avoided after breast cancer. 
Expanding the Use of IUDs
There is growing consensus that women in the United States will 
benefit from broader use of IUDs, including the LNG IUD.171 
The Contraceptive CHOICE Project in St. Louis, Missouri, pro­
vided women (n = 5000) who did not desi

ouri, pro­
vided women (n = 5000) who did not desire pregnancy for at 
least 1 year with free reversible contraceptives for up to 3 years. 
This project has shown that with standardized counseling and 
no financial barriers to contraceptive acquisition, the majority of 
women (68%) will choose long-acting reversible contraceptive 
methods (45% LNG IUD, 10% copper IUD, and 13% etono­
gestrel implant).180 Continuation rates at 12 and 24 months 
were 88% and 79% for the LNG IUD, 84% and 77% for the

e 88% and 79% for the LNG IUD, 84% and 77% for the 
copper IUD, and 83% and 69% for the etonogestrel implant, 
respectively.181 Satisfaction rates were also higher for long-acting 
reversible methods compared with other methods of contra­
ception, such as oral contraceptives and the injectable DMPA. 
In addition, among women participating in the Contraceptive 
CHOICE Project, rates of unintended pregnancy, birth, and 
abortion were substantially lower than for other women in St. 
Louis and compa

lower than for other women in St. 
Louis and compared with women in the United States overall.182 
Adolescents in this study also frequently chose either IUDs or 
implants.183 With appropriate counseling and management 
regarding STI prevention and anticipated side effects, it is now 
accepted that IUDs can be used safely and effectively in adoles­
cents.184,185 Adolescents and young women have higher continu­
ation rates and lower repeat unintended pregnancy rates when 
using a method that does

ded pregnancy rates when 
using a method that does not require ongoing adherence.132 
Additionally, the use of long-acting reversible contraception 
immediately after abortion has been shown to decrease repeat 
abortion rates.186,187 
Abnormal Bleeding, Expulsion, and Uterine Perforation
As with DMPA and other progestin-only contraceptives, unpre­
dictable uterine bleeding is the most common reason for discontin­
uation of LNG IUDs.188 Unscheduled bleeding is most common 
during the early months

d bleeding is most common 
during the early months of LNG IUD use and tends to resolve 
with time. By 12 months, up to 50% of women have amenorrhea 
or infrequent bleeding. With the Kyleena and Skyla LNG IUD, by 
12 months, approximately 38% and 26% of women have amenor­
rhea or infrequent bleeding, respectively.189,190 Addressing patient 
preferences and assessing acceptance of menstrual disturbances are 
integral to efforts aimed at reducing early discontinuation rates. 
Adequate and specific 

rly discontinuation rates. 
Adequate and specific counseling about likely changes in bleed­
ing patterns before placement is essential for increasing patient 
acceptability.
Expulsion is the most common cause of IUD failure. The 
cumulative expulsion rate was 10.2 per 100 IUD users over 36 
months and did not vary among LNG and copper IUDs in a US 
study.191 Increased risk for expulsion occurs in nulliparous women, 
women with severe dysmenorrhea or uterine adenomyosis, those 
with uterine cavit

 or uterine adenomyosis, those 
with uterine cavity abnormalities, and women with insertions 
immediately after delivery. The authors’ anecdotal impression is 
that appropriate high fundal IUD insertion minimizes the likeli­
hood of expulsion.
The USMEC allows immediate insertion after first-trimester 
(category 1) and second-trimester (category 2) abortions.9 Post­
partum insertion is recommended after a 4-week interval (cat­
egory 1). Immediate postpartum insertion (following vaginal or 
cesar

 postpartum insertion (following vaginal or 
cesarean birth) is also an option (category 2) within 10 minutes of 
placental delivery as long as there is no evidence of chorioamnio­
nitis or puerperal sepsis.192 Expulsion rates with immediate post­
partum insertion range from 10% to 20%, with lower rates seen 
after cesarean delivery. Because expulsion usually occurs within 
the first few months, women are encouraged to follow up with 
their care provider within 12 weeks of insertion.
Uterine per

provider within 12 weeks of insertion.
Uterine perforation is an uncommon but potentially serious 
complication of LNG IUD placement. A large European prospec­
tive cohort study found a perforation incidence of 0.6 for nonlac­
tating women and 4.5 for lactating women per 1000 insertions.193 
No perforations caused injury of intra-abdominal or pelvic struc­
tures or serious morbidity.194 
Upper Genital Tract Infection and Infertility
The use of intrauterine contraception does not increase the ris

trauterine contraception does not increase the risk 
of pelvic infection.195 Similarly, there is no evidence intrauter­
ine contraception increases the risk of subsequent infertility.196 
Intrauterine methods are suitable for nulliparous and adoles­
cent women.184 Prophylactic antibiotics are not recommended 
for IUD insertion. A clinical history (including sexual history) 
should be taken as part of the routine assessment for intra­
uterine contraception to identify women at high risk for STIs;

raception to identify women at high risk for STIs; 
testing before insertion should be performed selectively, not 
routinely. Even among high-risk women, testing on the same 
day of insertion was found to be as safe as testing prior to inser­
tion in terms of subsequent pelvic inflammatory disease (PID) 
risk.197 An IUD should not be inserted in women with current 
PID, purulent cervicitis, or current chlamydial or gonorrheal 
infection.9 However, cervical or vaginal infection is not an 
indicat

, cervical or vaginal infection is not an 
indication to remove an IUD.8 If a woman contracts gonor­
rhea, chlamydia, or PID with an LNG IUD in place, treatment 
follows the CDC Sexually Transmitted Diseases Treatment 
guidelines, and if the patient responds to therapy, the IUD can 
remain in place.198 
Metabolic and Systemic Effects
Low but detectable circulating levels of progestin in LNG IUD 
users have raised concerns that glucose control, the lipid profile, 
and blood pressure may be negati

e lipid profile, 
and blood pressure may be negatively affected. However, these 
concerns have not been substantiated in high-quality clinical 
trials.176 The data support that the LNG IUD is safe to use in 
patients with diabetes, hyperlipidemia, or hypertension. Proges­
tins are not thought to increase the risk of thromboembolic dis­
ease, and the LNG IUD has not been associated with venous or 
arterial events.102 
657
Noncontraceptive Uses of the Levonorgest

raception
Noncontraceptive Uses of the Levonorgestrel-Releasing 
Intrauterine System
Heavy Menstrual Bleeding
The high-dose LNG IUD is a well-established and highly effective 
treatment intervention for heavy menstrual bleeding. In compara­
tive studies, it offers efficacy equal to or greater than other surgical 
uterine-conserving treatments (e.g., global endometrial ablation, 
transcervical endometrial resection), is an acceptable alternative to 
hysterectomy for many women, and offers compara

o 
hysterectomy for many women, and offers comparable improve­
ment in health-related quality of life for menstrual disorders.199-
202Although up to 43% of women using the LNG IUD to treat 
abnormal uterine bleeding will eventually undergo hysterectomy, 
the system still offers lower direct and indirect health care costs at 
5 years compared with immediate hysterectomy.201,203 
Symptomatic Fibroids and Uterine Adenomyosis
Data support that the LNG IUD is also an effective treatment for 
heavy me

G IUD is also an effective treatment for 
heavy menstrual bleeding associated with uterine fibroids.204,205 
However, submucous fibroids may be more likely to cause heavy 
bleeding and are more likely to lead to LNG IUD expulsion 
because they distort the uterine cavity.206 Therefore, effectiveness 
of the LNG IUD likely depends on fibroid number, size, and loca­
tion in the uterus.
Uterine adenomyosis also is a common, benign condition that 
may be associated with heavy menstrual bleeding and p

 be associated with heavy menstrual bleeding and pelvic pain. 
Limited data suggest that the LNG IUD reduces bleeding and 
pain in women with this condition.207-209 As with uterine fibroids, 
expulsion rates appear elevated when IUDs are placed in women 
with uterine adenomyosis.200 
A Cochrane review concluded that the 52-mg LNG IUD reduces 
pelvic pain and dysmenorrhea associated with endometriosis.210 
Advantages of the LNG IUD in this setting include the absence 
of hypoestroge

n this setting include the absence 
of hypoestrogenic effects associated with the use of gonadotropin-
releasing hormone agonists, as well as lower medication costs. Dis­
advantages of the LNG IUD include irregular bleeding associated 
with this approach. 
Endometrial Protection With Estrogen Replacement Therapy
Good-quality evidence supports long-term use of the LNG IUD 
for endometrial suppression during estrogen replacement therapy, 
and it is approved for this indication in other countries.2

 approved for this indication in other countries.211,212 
However, the 52-mg LNG IUD is relatively large and may be 
more difficult to insert into a postmenopausal woman’s uterus. 
The two smaller LNG IUDs may be better choices in menopausal 
women. Acceptability studies are needed, and more studies are 
necessary to determine the lowest effective dose of levonorgestrel 
required to achieve effective endometrial suppression.213 
Endometrial Protection With Tamoxifen Use
Tamoxifen is commonly use

ction With Tamoxifen Use
Tamoxifen is commonly used as adjuvant endocrine therapy in 
the treatment of estrogen receptor–positive breast cancer in pre­
menopausal women and in selected postmenopausal women. In 
postmenopausal women, tamoxifen increases the risk of endo­
metrial polyps, hyperplasia, and cancer. A systematic review of 
the 52-mg LNG IUD for prevention of endometrial disease in 
tamoxifen users concluded that the device reduced the risk of 
endometrial polyps, but it is not known w

risk of 
endometrial polyps, but it is not known whether LNG IUD 
reduces the risk of endometrial hyperplasia or cancer.214 LNG 
IUD users had a higher incidence of unscheduled bleeding, 
which may increase the need for diagnostic intervention in this 
high-risk group. The FDA lists a personal history of breast can­
cer as a contraindication to use of all progestational medications, 
including the LNG IUD. Reflecting the uncertain safety of LNG 
IUD use in women with breast cancer, guidance rele

IUD use in women with breast cancer, guidance released in 2014 
from the American College of Obstetricians and Gynecologists 
does not recommend that this intrauterine system be used in this 
patient population.215 
Treatment for Endometrial Hyperplasia or Carcinoma
Although hysterectomy represents the best treatment for early 
endometrial cancer, progestins are sometimes used to treat endo­
metrial hyperplasia with atypia or early endometrial cancer in 
women who wish to preserve future fertili

ncer in 
women who wish to preserve future fertility or who represent 
poor surgical candidates due to comorbidities. The 52-mg LNG 
IUD reverses endometrial neoplastic changes in most women 
with complex atypical hyperplasia and early-grade endometrial 
cancer.216 
Contraceptive Implants
Contraceptive implants provide long-acting, highly effective, con­
venient, and reversible contraception. All subdermal contracep­
tive implants for clinical use in humans employ progestins. These 
methods offe

e in humans employ progestins. These 
methods offer an excellent contraceptive option for women who 
have contraindications to combined hormonal methods and an 
option for any woman who desires long-term protection against 
pregnancy that is rapidly reversible. The only subdermal implant 
currently available to women in the United States is Nexplanon 
(Fig. 18.5), released in 2006 as Implanon. In 2011, Nexplanon (a 
radiopaque implant) replaced Implanon.
Nexplanon contains 68 mg of etonogestrel 

mplanon.
Nexplanon contains 68 mg of etonogestrel as a single-rod sub­
dermal implant.217 The device is approved for 3 years’ duration, 
provides excellent efficacy throughout its use, and is easy to insert 
and remove by trained practitioners. Nexplanon can be used dur­
ing lactation; may improve dysmenorrhea; does not significantly 
affect weight, acne, lipids, or liver enzymes; and has only modest 
effects on BMD.218 Like other progestin-only contraceptives, the 
etonogestrel implant commonly

contraceptives, the 
etonogestrel implant commonly causes irregular vaginal bleeding.
• Fig. 18.5  The subdermal etonogestrel implant.
658
Description and Pharmacology
The Nexplanon rod releases the gonane progestin etonogestrel, 
formerly known as 3-ketodesogestrel, the biologically active 
metabolite of desogestrel.218 Desogestrel is used in some oral 
contraceptives, and etonogestrel is the same progestin used in 
the contraceptive vaginal ring. Th

gestin used in 
the contraceptive vaginal ring. The implant is 4 cm long and 2 
mm in diameter, is radiopaque, and has a core made from a non­
biodegradable solid composed of ethylene vinyl acetate impreg­
nated with 68 mg of etonogestrel (see Fig. 18.5). The ethylene 
vinyl acetate copolymer of Nexplanon allows controlled hormone 
release over at least 3 years of use. Each implant is provided in a 
disposable sterile inserter for subdermal application. Maximum 
serum concentrations of etonogest

cation. Maximum 
serum concentrations of etonogestrel are usually seen by day 4 
after implant insertion. Etonogestrel levels decrease slightly by 1 
year and further by 3 years but remain above the threshold needed 
to suppress ovulation.217 After removal, serum levels are unde­
tectable by 1 week in most users, who resume ovulation within 6 
weeks of implant removal. Despite effective suppression of ovula­
tion, estradiol levels only fall into the early follicular level range, 
and the implant

the early follicular level range, 
and the implant does not cause hypoestrogenism. Consistent with 
that observation, based on limited data from clinical trials, there 
does not appear to be a clinically significant adverse effect of the 
etonogestrel implant on BMD.218 
Mechanism of Action and Efficacy
Nexplanon prevents fertilization primarily by inhibiting ovu­
lation, although some thickening of cervical mucus may also 
occur.219 The etonogestrel implant provides highly effective con­
tracep

trel implant provides highly effective con­
traception. In an integrated analysis of 11 international clinical 
trials that included more than 900 healthy women between 18 
and 40 years old, no pregnancies were reported while the etono­
gestrel implants were in place. Six pregnancies occurred during 
the first 14 days after etonogestrel implant removal. Including 
these six pregnancies, the cumulative Pearl index (number of 
pregnancies per 100 woman-years) was 0.38 (Pearl indices were 
0.27 and

oman-years) was 0.38 (Pearl indices were 
0.27 and 0.30 at 1 and 2 years, respectively).219 After the implant 
has been removed, normal ovulation and fertility rapidly return. 
Reported pregnancies among implant users have primarily been 
due to unrecognized pregnancies at the time of insertion and fail­
ure to insert the device. Postmarketing data regarding the etono­
gestrel implant from Australia found a real-world failure rate of 
1.07 per 1000 insertions.220 Of the 218 pregnancies identifie

00 insertions.220 Of the 218 pregnancies identified in 
the Australian study, data were insufficient to assess the reason for 
contraceptive failure in 45 women, and 46 women were deter­
mined to have been already pregnant prior to implant insertion. 
Of the remaining 127 cases, failure to insert the implant resulted 
in pregnancies in 84 women. Other reasons for contraceptive fail­
ure included incorrect timing of insertion (19 cases), expulsion of 
the implant out of the insertion site (3 case

 of 
the implant out of the insertion site (3 cases), and interaction with 
hepatic enzyme–inducing medications (8 cases). The remaining 
13 cases were classified as product failures. However, even when 
these method failures are accounted for, Nexplanon continues to 
have one of the highest efficacies of any available reversible or per­
manent contraceptive.
There are reports of implant failure in women using anticonvul­
sants, particularly carbamazepine. Accordingly, contraceptive implants 
ar

amazepine. Accordingly, contraceptive implants 
are not recommended in women taking anticonvulsants, certain anti­
retrovirals, or other medications that induce hepatic enzymes.220-222 
Overweight and obese women were not included in the trials for 
approval of the etonogestrel implant. However, a prospective cohort 
study that examined 1168 women (28% overweight and 35% obese) 
found that failure rates were not impacted by BMI.223
Package labeling indicates that Nexplanon can be used for up 
to

ng indicates that Nexplanon can be used for up 
to 3 years. However, the implant appears to retain its efficacy for 
up to 5 years.224 In a US study among 291 women who had used 
the implant for 3 years, no pregnancies occurred during a fourth 
or fifth year of use.173 Accordingly, and recognizing that such use 
is off-label, individuals can be offered the opportunity to extend 
use to as long as 5 years. 
Safety and Side Effect Profile
Nexplanon users commonly experience irregular and unpredict

 users commonly experience irregular and unpredict­
able bleeding patterns, similar to the experience of users of other 
continuous progestin-only contraceptives. Combined data from 
11 clinical trials show that the most common bleeding patterns 
with the etonogestrel implant are amenorrhea (22%), infrequent 
bleeding (34%), frequent bleeding (7%), and frequent or pro­
longed bleeding or both (18%).225 The number of bleeding days 
are usually not increased, but the pattern is unpredictable. Of 

 increased, but the pattern is unpredictable. Of 
clinical relevance is that bleeding patterns experienced during the 
initial 3 months predict future patterns for most women. Women 
with favorable bleeding patterns during the first 3 months tend to 
continue this pattern throughout the first 2 years of use. Women 
with unfavorable initial patterns have at least a 50% chance that 
the pattern will improve. Only 11.3% discontinue use because 
of bleeding irregularities, mainly because of prolonge

leeding irregularities, mainly because of prolonged flow and 
frequent irregular bleeding.225 Most women (77%) with baseline 
dysmenorrhea experience complete resolution of symptoms due 
to ovulation suppression. Effective preinsertion counseling on 
the possible changes in bleeding patterns may improve continua­
tion rates.126,226 A small, short-term clinical trial found that add­
ing COCs reduces bothersome bleeding in implant users.227 If a 
patient using a contraceptive implant desires treat

atient using a contraceptive implant desires treatment to reduce 
bothersome irregular bleeding, the US Selected Practice Rec­
ommendations for Contraceptive Use present two options: (1) 
nonsteroidal anti-inflammatory drugs for 5 to 7 days, or (2) low-
dose COCs or estrogen for 10 to 20 days.8 These treatments may 
shorten bleeding episodes and offer temporary relief but do not 
change long-term bleeding patterns. In selected patients experi­
encing bothersome bleeding due to the contraceptive 

cing bothersome bleeding due to the contraceptive implant and 
without contraindications to COC use, the authors recommend 
long-term use of COCs to reduce such bleeding if the patient 
desires this therapy.
Several studies observe a small (<1 kg) weight increase in 
etonogestrel implant users.219 However, only 3% to 7% of women 
choose to remove the implant because of weight changes. Similar 
to the effect seen in women using POPs, ovarian cysts occur in up 
to 15% of users. Most cysts regress 

 occur in up 
to 15% of users. Most cysts regress spontaneously and do not need 
additional treatment.228
Nexplanon appears safe for use during lactation.108 A 2015 
Cochrane review did not identify concerns regarding milk volume 
or composition of infant growth in implant users.229 Likewise, 
a Brazilian randomized trial of breastfeeding women found that 
infant growth was similar with early (prior to hospital discharge) 
implant placement compared with delayed placement, and that 
Nexplanon do

red with delayed placement, and that 
Nexplanon does not appear to change the content of breast milk 
and does not influence infant growth up to 3 years.230
Lipid measurements demonstrate an overall decrease in serum 
total cholesterol, HDL cholesterol, and low-density lipoprotein 
cholesterol. Some studies also find a decrease in triglyceride lev­
els. Minor reductions in the HDL/low-density lipoprotein ratio 
have been observed, although not into ranges thought to be clini­
cally significant. 

to ranges thought to be clini­
cally significant. Etonogestrel implant use does not significantly 
increase the risk of cardiovascular disease.231
659
Migration of contraceptive implants to the systemic vasculature, 
particularly the pulmonary artery, represents a rare complication 
of implant placement. Proper superficial placement of implants 
represents the key to avoiding this serious complication.232,233 
Patient Selection
Before recommending the contracep

atient Selection
Before recommending the contraceptive implant, providers 
should review the indications and contraindications for its use. 
Contraindications to etonogestrel implant use are few and include 
breast cancer, use of hepatic enzyme–inducing drugs, unexplained 
and unevaluated abnormal vaginal bleeding, severe cirrhosis, 
systemic lupus erythematosus (SLE) with positive or unknown 
antiphospholipid antibodies, and liver tumors.9 There are also 
reports describing etonogestrel implant

 are also 
reports describing etonogestrel implant failure in a woman tak­
ing the antiretroviral drug efavirenz.234 When counseling women 
regarding the etonogestrel implant, the clinician should address 
concerns and fears a woman may have about this method of con­
traception. Women may have particular concerns about implant 
removal, although removal problems with single-rod devices such 
as Nexplanon are uncommon. Side effects (particularly irregular 
bleeding) should be discussed, as an une

rregular 
bleeding) should be discussed, as an unexpected side effect may 
cause women to request early removal of the implant. The implant 
does not provide protection against STIs. Accordingly, and as is 
appropriate for all sexually active women, implant users should 
be reminded about safe sexual practices. Appropriate candidates 
for implantable contraception are women who desire long-term 
reversible birth control, have no contraindications to etonogestrel 
implant use, accept implant inse

 to etonogestrel 
implant use, accept implant insertion and removal, and are ready 
to accept a change in menstrual bleeding patterns. 
Insertion and Removal
Proper insertion and removal techniques are essential for clinical 
efficacy and for prevention of complications. Timing of insertion 
depends on the patient’s prior use of contraception and the clini­
cian’s evaluation of the appropriateness for the individual.8 In the 
United States, Nexplanon has been made available only to clini­
cians 

anon has been made available only to clini­
cians who have completed insertion and removal training pro­
vided by the manufacturer. Proper training of clinicians reduces 
the incidence of complications at insertion and removal. Com­
plications of Nexplanon insertion are rare (<2%) but may include 
local pain, infection, and bleeding. Migration of the implant into 
the systemic vasculature is described earlier.232,233 A follow-up 
visit following implant placement is not routinely indicated.
Befo

implant placement is not routinely indicated.
Before removal, the clinician needs to palpate the implant. 
Under sterile conditions and after injection of a local anesthetic, 
a 2- to 3-mm incision is made vertically over the implant. The 
rod is then removed using the pop-out technique described by 
Pymar and associates235 for Norplant System removal. If inserted 
correctly, removal is usually simple and should take less than 5 
minutes. The most common reason for difficulty in removal is 
plac

t common reason for difficulty in removal is 
placement of the implant too deeply. If the clinician cannot pal­
pate the implant, imaging techniques may be necessary before 
proceeding. 
Emergency Contraception
Emergency contraception (EC) is defined as a drug or device used 
to prevent pregnancy after unprotected sexual intercourse (includ­
ing sexual assault) or after a recognized contraceptive failure.236 
Although EC is sometimes called the morning-after pill, this label 
is confusing becaus

orning-after pill, this label 
is confusing because it implies that EC can be taken only the 
morning after unprotected sexual intercourse. EC, or postcoital 
contraception, can actually reduce the risk of unintended preg­
nancy up to 5 days after unprotected sex.
Oral EC is intended as a backup method for occasional use 
rather than a regular method of contraception. Discussions with 
patients regarding EC offer an opportunity to discuss longer-term 
contraception, sexual health and safety, and

term 
contraception, sexual health and safety, and other preventative 
women’s health measures such as Papanicolaou smears. EC should 
be readily available to all women at risk for unplanned pregnancy, 
including adolescents and victims of sexual assault.
Research shows that increased access to EC improves use, but 
it is not yet clear whether this access translates into a reduction 
in unintended pregnancy or abortion.237 Although access is an 
important factor, how women use EC may be a strong

important factor, how women use EC may be a stronger determi­
nant of its ultimate effect. Even when women have EC at home, 
they often fail to use it after unprotected sexual intercourse. The 
most common reason for this is a lack of recognition of the risk of 
pregnancy or neglect of the perceived risk.238 Although increasing 
knowledge of and access to EC is a public health priority, use of 
regular, ongoing contraception is a more effective way to reduce 
unplanned pregnancy and abortion.
Em

ay to reduce 
Emergency Contraception Regimens
The EC currently available to women in the United States 
includes 1.5-mg levonorgestrel (Plan B One-Step [Duramed Phar­
maceuticals, Pomona, NY], Next Choice [Watson Laboratories, 
Inc., Corona, CA]), 30-mg ulipristal acetate (UPA, Ella, Afaxys 
Pharma, LLC., Charleston, SC), and the copper T 380A IUD 
(Paragard [CooperSurgical, Trumbull, CT]).
In 2009, a one-dose 1.5-mg levonorgestrel tablet was intro­
duced (pref

.5-mg levonorgestrel tablet was intro­
duced (preferred over the previous 0.75-mg two-dose regimen), 
which should be taken as soon as possible after unprotected 
intercourse and is labeled for use up to 72 hours following 
intercourse.236 The medication is available without a prescrip­
tion. A prescription was previously required for those younger 
than 17 years, but there are now no age restrictions for access 
to levonorgestrel EC without a prescription following FDA 
approval in 2014. The pr

escription following FDA 
approval in 2014. The progestin-only oral EC regimen is better 
tolerated than the older Yuzpe method of EC that used high 
doses of COCs.
UPA was approved in 2010 for EC in the United States and is 
marketed as Ella.239 UPA is a progesterone receptor modulator in 
the same class as mifepristone and is more effective than levonorg­
estrel EC.236 UPA requires a prescription, and although it should 
be taken as soon as possible after unprotected intercourse, it has 
been 

sible after unprotected intercourse, it has 
been shown to be effective for up to 5 days.240
The copper T 380A IUD may be inserted for EC up to 5 days 
after unprotected intercourse, although some evidence suggests 
that it may still be effective when inserted up to 10 days post inter­
course.241-243 Although the copper T is a highly effective form of 
EC, with the advantage of providing ongoing longer-term contra­
ception for women,244 the requirement for specialist insertion and 
the cost of t

rement for specialist insertion and 
the cost of the device may act as a barrier to access. 
The mode of action of hormonal EC is multifactorial and not 
completely understood. Because sperm are viable in the female 
reproductive tract for up to 5 days but eggs can be fertilized only 
within 1 day of ovulation, the mechanism of action depends on 
when the oral EC regimen is given in relation to the time of inter­
course and time of ovulation.
660
SECTION V    Sexual Developme

me of ovulation.
660
Levonorgestrel EC prevents or delays ovulation if administered 
before the commencement of the LH surge and is ineffective if 
taken afterward.236,245 UPA is a selective progesterone receptor 
modulator with different proposed mechanisms when taken at dif­
ferent times of the menstrual cycle. UPA administered before the 
commencement of the LH surge inhibits follicular development, 
and when taken during the LH increase but prior 

and when taken during the LH increase but prior to the peak, it 
delays follicular rupture.246 Neither oral agent has been shown to 
interfere with implantation directly. Therefore, they are not consid­
ered abortifacients or teratogenic to a developing pregnancy.247,248
The mechanism of action of the copper IUD is thought to be 
the release of copper ions toxic to sperm function while associated 
inflammation potentially impairs transport of ovum or implanta­
tion of the fertilized egg.249 
E

vum or implanta­
Efficacy
The effectiveness of EC depends on the mechanism of action and 
when the EC regimen is given in relation to the time of unpro­
tected intercourse. The probability of pregnancy after a single act 
of unprotected intercourse varies (3–8%) according to the day of 
the menstrual cycle and the couple’s fertility status.250 Calculating 
the efficacy of EC is complex because it is impossible to know 
how many pregnancies would otherwise have occ

now 
how many pregnancies would otherwise have occurred. Therefore, 
an estimation of efficacy is the failure rate of the contraceptive 
method—that is, the rate of pregnancy following administration.
The most effective EC is the copper IUD, with a failure rate of 
less than 0.1%.241 With regard to oral EC, UPA has greater effi­
cacy over levonorgestrel because it more effectively delays follicular 
rupture and has a longer action well into the LH surge.251 A recent 
systematic review directly c

H surge.251 A recent 
systematic review directly comparing UPA and levonorgestrel dem­
onstrated that UPA almost halved the risk of pregnancy compared 
with levonorgestrel when given within 120 hours of unprotected 
sexual intercourse; the failure rate of UPA when administered within 
120 hours is 1.3%.252 Moreover, when given within 24 hours of 
intercourse, UPA reduces the risk of pregnancy by almost two thirds 
compared with levonorgestrel, with a failure rate of 0.9%.253
After a single episo

th a failure rate of 0.9%.253
After a single episode of unprotected intercourse, the use of 
levonorgestrel-only EC is more effective than no treatment at 
all.254 Taking a total of 1.5 mg of levonorgestrel in a single dose 
is as effective as the 0.75-mg split dose, and single dosing is more 
user-friendly.252 The timing of levonorgestrel EC influences its 
effectiveness. Waiting 12 hours to initiate treatment after unpro­
tected intercourse increases the odds of pregnancy by almost 50%, 
and i

reases the odds of pregnancy by almost 50%, 
and its efficacy decreases linearly with time.255
The efficacy of oral EC may be reduced with increased BMI. 
One study used a subanalysis to demonstrate the effect of BMI on 
contraceptive efficacy; levonorgestrel was shown to be ineffective 
in overweight women (with a BMI >26 kg/m2) compared with 
UPA, which was ineffective in obese women (with a BMI >35 kg/
m2).256 Women should not be denied access to EC based on BMI, 
but the potential impact of 

 to EC based on BMI, 
but the potential impact of weight on EC efficacy should be dis­
cussed and considered in agent selection. Some European authori­
ties have recommended increasing the dose of levonorgestrel EC 
to 3 mg in the setting of obesity and also when a woman is taking 
concomitant liver enzyme–inducing drugs.257 The efficacy of the 
copper IUD is not affected by BMI. 
EC is indicated for any woman at risk for unintended preg­
nancy from an episode of contraceptive failur

reg­
nancy from an episode of contraceptive failure or unprotected 
intercourse. Experts recommend the advance provision of EC to 
women.258 However, surveys have found that the level of knowl­
edge and use of EC in the United States by women and their 
health care providers is low.259
No clinical examination or pregnancy testing is required before 
using oral hormonal EC.9 Given the infrequent use and short 
duration of action of EC, the medical contraindications to regular 
oral contraceptives

 contraindications to regular 
oral contraceptives are not applicable. USMEC recommendations 
state that women with previous ectopic pregnancy, cardiovascular 
disease, migraines, or liver disease can use EC.9 Current guidelines 
advise that breastfeeding should be avoided for 1 week after using 
UPA; breastfeeding women should continue to express and dis­
card milk during this time.260
The only contraindication to the oral EC regimens is allergy 
to the active substance. Pregnancy is a relative

to the active substance. Pregnancy is a relative contraindication 
because EC is ineffective if a pregnancy is already established; as 
discussed earlier, there is no evidence demonstrating teratogenicity 
to a developing pregnancy. There may be reduced efficacy of oral 
EC in women with severe malabsorption syndromes and those 
taking hepatic enzyme–inducing drugs and certain antiretroviral 
drugs. Contraindications for the copper IUD for EC are the same 
as those for regular contraceptive us

re the same 
as those for regular contraceptive use.9 
The FDA has determined that levonorgestrel EC is safe enough to 
be available without a prescription in states with direct pharmacy 
access. UPA has an excellent safety profile and is available over 
the counter throughout Europe but requires a prescription in the 
United States. The side effects of the copper IUD for EC are the 
same as those for its use for long-term contraception.
No deaths or serious complications have been 

ion.
No deaths or serious complications have been directly linked to 
EC. The most common short-term side effects of oral EC include 
nausea, vomiting, and irregular bleeding, which affect up to 20% 
of users. Irregular bleeding caused by hormonal EC typically 
resolves by the next menstrual cycle. Other minor adverse effects 
reported by women in clinical trials include dizziness, fatigue, 
breast tenderness, headache, and abdominal pain.252 If vomiting 
occurs within 3 hours of taking oral EC,

vomiting 
occurs within 3 hours of taking oral EC, most experts recommend 
that the dose be repeated.236
After hormonal EC is taken, menses usually occurs within 1 week 
before or after the expected time. Women using levonorgestrel are 
more likely to experience early menstruation, whereas women using 
UPA are more likely to experience delayed menstruation.252 If the 
delay in the onset of menses is greater than 1 week or if the expected 
menses is lighter than usual, a pregnancy test should be 

is lighter than usual, a pregnancy test should be performed. 
A woman should also be advised to seek medical attention for con­
tinued irregular bleeding or abdominal pain, as these symptoms may 
be a sign of a spontaneous abortion or ectopic pregnancy.
Ongoing Contraception
As outlined previously, hormonal EC can postpone ovulation, 
making a woman vulnerable to pregnancy later in the cycle. 
Women should therefore be counseled to begin a regular method 
of contraception after using EC. However

r method 
of contraception after using EC. However, there is preliminary 
data demonstrating that hormonal contraception, particularly oral 
progestins, could reduce the efficacy of UPA.261 Further research is 
needed into the mechanism and risk profiles of various therapies. 
It has been suggested that commencement of regular hormonal 
contraception should be delayed for 5 days after the administra­
tion of UPA, and that women should either abstain from sexual 
intercourse or use barrier protec

ain from sexual 
intercourse or use barrier protection for that duration.8,261,262 
661
Clinical Challenges in Contraceptive Care
Hormonal Contraception for Adolescents
Although on the decline, rates of teenage pregnancy are higher 
in the United States than in other Western industrialized coun­
tries.6,263 Approximately 75% of these pregnancies are unplanned, 
of which about 38% end in induced abortion.3 Teenage preg­
nancy and parenting are risk factors for a

e preg­
nancy and parenting are risk factors for adverse medical, edu­
cational, and psychosocial outcomes for mother and child.264 
Children of teenage mothers are at greater risk for preterm birth, 
low birth weight, neonatal death, and later behavioral problems 
and poor academic performance.265 Teenage parenting contrib­
utes to the intergenerational transmission of poverty.266 Sexually 
active adolescents are more motivated to use contraception if they 
are academically successful, believe 

ion if they 
are academically successful, believe that pregnancy would be an 
impediment to their goals, and are involved in a stable relation­
ship with a sexual partner.267
Considerations for clinicians advising adolescents about con­
traception should include the potentially high fertility rates in 
these young women, their high rates of unprotected intercourse, 
elevated risk of sexual assault, and increased risk of STIs.268 Con­
sistent and correct use of contraceptive methods can be challe

correct use of contraceptive methods can be challeng­
ing for adolescents, and long-acting methods increase efficacy.132 
These methods, including the DMPA injection, contraceptive 
implants, and IUDs, should be made available to teens.184,269 
With proper patient selection and counseling, IUDs and implants 
can be successful contraceptive options for adolescents.184,270 Cli­
nicians should also be aware of the legal conditions surrounding 
prescribing contraception to minors in the state in whi

ribing contraception to minors in the state in which they are 
practicing.
Sexually active adolescents benefit from health guidance annu­
ally regarding responsible health behaviors, including abstinence, 
latex condoms to prevent STIs, and appropriate methods of birth 
control, along with instructions on ways to use them effectively.268 
Adolescents should also be counseled in a nonjudgmental manner 
about the need to use condoms for all types of intercourse, includ­
ing anal and oral intercour

f intercourse, includ­
ing anal and oral intercourse. Barriers to effective contraception 
use by adolescents include lack of forward planning, noncon­
sensual intercourse, lack of confidential care, fear of disapproval 
by parents and doctors, absence of adolescent-friendly services, 
language and cultural barriers, fear of pelvic examination, and 
cost.271 Misconceptions about contraception, including effects on 
weight gain, future fertility, acne, and risk of cancer, may also pre­
vent adole

acne, and risk of cancer, may also pre­
vent adolescents from using effective contraception and should be 
addressed in counseling. The National Campaign to Prevent Teen 
and Unplanned Pregnancy, now called Power to Decide, operates 
a website that provides excellent information, decision tools, and 
reminders specifically designed for young women at https://www.
bedsider.org.
Combined Hormonal Contraceptives in Adolescents
Although COCs, the patch, and the ring represent safe and effec­
tive me

ch, and the ring represent safe and effec­
tive methods for adolescents, many young women are not effec­
tively educated about correct use and anticipated side effects. They 
may use these short-acting hormonal contraceptives inconsistently, 
which will lead to very high pregnancy rates.271 During counsel­
ing, it is important to provide clear oral and written instructions 
about initiating these methods, ways to ensure correct use, and 
what to do if one or more pills are missed or if the patch

do if one or more pills are missed or if the patch or ring 
is not used correctly. By the same token, POPs are not favored 
for adolescents because they require stricter adherence for efficacy.
Initiating or continuing hormonal contraception does not 
require pelvic examination, cervical cancer screening, or STI 
screening in adolescents or older reproductive-age women.8 A his­
tory excluding contraindications, blood pressure measurement, 
and being reasonably sure the patient is not pregnant is

ing reasonably sure the patient is not pregnant is sufficient 
before initiating hormonal contraception.272 Age-appropriate 
screening for STIs or cervical cancer should be considered regard­
less of contraceptive use. Screening for chlamydial infection and 
gonorrhea, as recommended by the CDC for women under the 
age of 26 years, can be accomplished through urine tests or a vagi­
nal swab without a speculum.198 The use of condoms should be 
encouraged in conjunction with the use of hormonal co

ouraged in conjunction with the use of hormonal contracep­
tives to prevent STIs.
Initial follow-up should be 8 to 12 weeks after commencing 
the COCs to monitor correct use and adverse events and should 
be done thereafter at 6- to 12-month intervals. Education and 
ongoing counseling are essential to ensure correct usage. Many 
adolescents are concerned that the COCs will cause weight gain or 
acne, and these issues should be directly addressed.271 Chewable 
pills are available and may be more

.271 Chewable 
pills are available and may be more acceptable to young women 
who find it difficult to swallow pills. Unscheduled bleeding may 
be less acceptable to adolescents than adult women. Clinicians 
must consider the possibility that unscheduled bleeding represents 
cervicovaginal infection in this high-risk population and investi­
gate and treat it accordingly. 
Injectable Contraceptives in Adolescents
Discontinuation rates for DMPA in adolescents are high, with 
half of adolescents di

adolescents are high, with 
half of adolescents discontinuing the method by 12 months.273 
However, because DMPA suppresses ovulation for an extended 
period of time, prior use of DMPA protects many adolescents 
from unintended pregnancy despite inconsistent use. Weight gain 
is the most commonly cited reason for adolescents to discontinue 
DMPA, and it may be more common in obese adolescents.115 
Although loss of bone density has been a concern in adolescents 
using DMPA, the position statement

in adolescents 
using DMPA, the position statement of the Society for Adolescent 
Medicine is that DMPA represents an extremely effective contra­
ceptive, and that clinical concerns about loss of BMD must be 
placed within the context of likely bone recovery on discontinua­
tion, low risk of fractures, and benefits of preventing unintended 
pregnancy among adolescents.274 
Hormonal Contraception in Postpartum and 
Lactating Women
The postpartum period is a critical time period for initiating con

eriod is a critical time period for initiating con­
traception to help women achieve optimal interpregnancy inter­
vals.275 The immediate postpartum period offers an ideal time for 
women to initiate contraception because of patient access and 
convenience. Delaying initiation of contraception until the stan­
dard 6-week postpartum visit places many women at risk for unin­
tended pregnancy. By 6 weeks postpartum, up to 40% of women 
will have had unprotected intercourse, and nearly 50% will have

 unprotected intercourse, and nearly 50% will have 
ovulated if they are not breastfeeding.276,277 Furthermore, not all 
women seek postpartum medical care, resulting in a large propor­
tion of women with unmet contraceptive needs.278 Return of fer­
tility is, of course, postponed in breastfeeding women who meet 
criteria for the lactational amenorrhea method of contraception.
Postpartum women remain in a hypercoagulable state for weeks 
after childbirth.279,280 This elevated risk does not resol

ildbirth.279,280 This elevated risk does not resolve until 12 
weeks postpartum, although after 6 weeks the risk is low (22 cases 
per 100,000 deliveries within 6 weeks and falling to three cases 
662
per 100,000 deliveries in weeks 7–12).281 Risk factors for VTE 
in the postpartum period include age 35 years or older, previous 
VTE, thrombophilia, immobility, transfusion at delivery, peri­
partum cardiomyopathy, BMI 30 kg/m2 or higher, postpartum 
he

iomyopathy, BMI 30 kg/m2 or higher, postpartum 
hemorrhage, cesarean delivery, preeclampsia, and smoking.282 
The USMEC recommends deferring use of estrogen-containing 
methods (category 4) until 3 weeks after delivery in all women 
and until 4 weeks in breastfeeding women.9 The guidelines further 
state that the risks of using estrogen-containing methods outweigh 
the benefits in those with risk factors for VTE starting 6 weeks 
postpartum.
Traditionally, combined hormonal contraception has not

itionally, combined hormonal contraception has not been 
recommended as the first choice for lactating women because of 
concerns that the estrogenic component can reduce the volume 
of milk production and the caloric and mineral content of breast 
milk.229 However, use of COCs by well-nourished breastfeeding 
women does not appear to result in problems with infant devel­
opment. Data regarding this concern is limited and inconsistent, 
with two recent systematic reviews unable to draw firm conc

recent systematic reviews unable to draw firm conclu­
sions regarding the effect of COCs on breastfeeding duration and 
infant outcomes.229,283 On an individual level, it is important to 
note that some women might be at risk for breastfeeding difficul­
ties, and if full breastfeeding is desired, counseling regarding these 
concerns is warranted. USMEC guidelines allow use of estrogen-
containing methods after 30 days postpartum in breastfeeding 
women without risk factors for VTE.9
Evidence fro

Evidence from limited studies suggests that progestin-only 
contraception does not interfere with lactation or infant develop­
ment and does not increase risk of thromboembolic disease.102,107 
It appears reasonable to initiate progestin-only contraception, 
including DMPA, POPs, and implants, immediately after deliv­
ery, regardless of whether mothers are nursing their infants. There 
is a theoretical concern, nevertheless, that initiation of progestin 
met

n, nevertheless, that initiation of progestin 
methods immediately postpartum could preempt lactogenesis, 
given that progesterone withdrawal after delivery of the placenta is 
thought to trigger prolactin secretion.229 Although long-term data 
are limited, observational studies of progestin-only contraceptives 
suggest that they have no effect on successful initiation and con­
tinuation of breastfeeding or on infant growth and development 
even when started immediately postpartum.107 IUDs are a

when started immediately postpartum.107 IUDs are also an 
option either immediately after delivery of the placenta or as an 
interval insertion (4–6 weeks postpartum) provided that there is 
no evidence of chorioamnionitis or postpartum endometritis or 
sepsis. Higher risk of expulsion with postplacental IUD insertion 
compared with interval insertion should be balanced against the 
benefit of immediate access to a highly effective contraceptive 
method, especially in women at risk for not retur

method, especially in women at risk for not returning for postpar­
tum follow-up.284 
Hormonal Contraception in Women Older Than 
35 Years
There are no contraindications to use of hormonal contraceptives 
on the basis of age alone.9 However, risks associated with the use 
of some methods may increase with age and additional comor­
bid conditions. For example, although the incidence of VTE, 
myocardial infarction, and stroke is low, these risks increase with 
age, obesity, smoking, diabetes, mig

ncrease with 
age, obesity, smoking, diabetes, migraine headaches with aura, 
and hypertension.72,285 However, because lean, healthy, non­
smoking women are at low risk for these rare events, they can 
use any method, including combined (estrogen-progestin pills, 
patch, and vaginal ring) methods, until menopause.286 Large US 
population-based case-control studies have found no increased 
risk of myocardial infarction or stroke among healthy, nonsmok­
ing women older than 35 years who use COCs f

smok­
ing women older than 35 years who use COCs formulated with 
less than 50 μg of estrogen.287,288 Perimenopausal women may 
benefit from a positive effect on BMD,289 abnormal uterine 
bleeding,290 and a reduction in vasomotor symptoms291 offered 
by COCs. In addition, COC use is associated with a reduced risk 
of endometrial cancer and ovarian cancer, which is of particular 
importance to older reproductive-age women. Progestin-only hor­
monal contraceptives and IUDs are preferred for older 

l contraceptives and IUDs are preferred for older women at 
elevated risk for VTE, myocardial infarction, and stroke.292-294 It 
is unknown if long-term older DMPA users can regain BMD lev­
els to baseline before entering menopause. For this reason, DMPA 
in women older than 45 years is USMEC category 2, and the 
other progestin-only methods are category 1. The LNG IUD is 
particularly effective for perimenopausal bleeding.295,296 
Discontinuation of Hormonal Contraception at 
Menopause
When dec

n of Hormonal Contraception at 
Menopause
When deciding to stop use of a contraceptive method, the benefits 
and risks of the method must be evaluated in the context of the 
diminishing risk of pregnancy as the woman ages. The median 
age of menopause in North America is 51 years, and about 90% 
of women will have reached menopause by age 55.297 Although 
pregnancy is uncommon after age 44, spontaneous conception 
can occur. Assessment of follicle-stimulating hormone levels 
to determine when ho

e-stimulating hormone levels 
to determine when hormonal contraceptive users have become 
menopausal and thus no longer need contraception may be mis­
leading and should not be routinely performed.8 Until a well-vali­
dated tool to confirm menopause is available, it is appropriate for 
healthy, nonsmoking women using a hormonal contraceptive to 
continue until age 50 to 55 years, after weighing the risks and ben­
efits.9 Thus, most women will be able to use contraception safely 
until they are a

able to use contraception safely 
until they are assured of menopause. At the time they discontinue 
hormonal contraception, some midlife women will choose to ini­
tiate menopausal hormonal therapy. 
Contraception in Women With Underlying 
Medical Conditions
Because pregnancy in women with underlying medical condi­
tions is associated with higher risks of maternal and perinatal 
morbidity and mortality, achieving effective contraception is 
particularly important in this setting298 (Table 18.6).

cularly important in this setting298 (Table 18.6). Providers 
should remember that the risk of using a contraceptive must be 
balanced with the risk of pregnancy. Although numerous studies 
have addressed safety and effectiveness of hormonal contraceptive 
use in healthy women, data unfortunately are far less complete 
for women with underlying medical problems or other special 
circumstances. The US Medical Eligibility Criteria for Contra­
ceptive Use from the CDC provides an important evidence

ve Use from the CDC provides an important evidence-based 
resource for clinicians.9
Decisions regarding contraception for women with coexisting 
medical problems may be complicated. In some cases, medica­
tions taken for certain chronic conditions may alter the effective­
ness of hormonal contraception, and pregnancy in these cases may 
pose substantial risks to the mother and her fetus. Differences in 
content and delivery methods of hormonal contraceptives may 
affect patients with certain con

ntraceptives may 
affect patients with certain conditions differently. Because trans­
dermal and vaginal ring contraception are relatively new, less data 
address their use in women with medical concerns. In the absence 
663
of more extensive data, contraindications to the use of estrogen-
progestin COCs should also be considered as contraindications to 
the use of transdermal and vaginal ring contraception. Practitio­
ners should recognize that use of nonhormo

ctitio­
ners should recognize that use of nonhormonal forms of contra­
ception, such as the copper IUD, remain a safe, effective choice 
for many women with medical conditions. 
Hormonal Contraception in Obese Women
The proportion of American adult women who are obese (BMI of 
30 kg/m2 or higher) has increased to 38%.299 Studies show that 
obese women have similar levels of sexual activity and unintended 
pregnancy compared with normal weight women.300 No methods 
of contraception are contraindi

en.300 No methods 
of contraception are contraindicated in obese women.9,301 As noted 
earlier, oral EC may be less effective in obese women. Although the 
efficacy of oral contraceptives and the transdermal patch in obese 
women has been questioned, a 2016 Cochrane review found that 
there was generally no association between BMI and the effective­
ness of hormonal contraceptives.302 Similarly, the overall risk for 
unintended pregnancy in women using the combined hormonal 
pill, patch, or ring

 using the combined hormonal 
pill, patch, or ring was not significantly different across BMI catego­
ries, with an overall range of 8.4% to 11.0% at 3 years of use in a 
prospective cohort study of 1500 women.303 This study was not able 
to evaluate efficacy separately among pill, patch, and ring users.304 A 
recent large, prospective cohort study of more than 52,000 women 
reported a slight increase in failure rates of COCs as BMI increased 
(hazard ratio of 1.5 for BMI higher than 35; 95% CI,

azard ratio of 1.5 for BMI higher than 35; 95% CI, 1.3–1.8), 
adjusting for age, parity, and education.20 However, this difference 
disappeared when evaluating pills dosed in a 24/4 fashion compared 
with 21/7 regimens. The ability to detect small differences in con­
traceptive efficacy between normal weight and obese women may be 
obscured by adherence issues with short-term methods.305
Efficacy of COCs, contraceptive rings, and transdermal patches 
primarily reflects suppression of ovulation r

hes 
primarily reflects suppression of ovulation resulting from the dose 
of progestin. Pharmacokinetic studies show that, compared with 
normal weight women, it takes twice as long for obese women 
to reach steady-state therapeutic levels of contraceptive steroids 
when starting the pill or after the HFI due to changes in clear­
ance.306,307 Therefore, these episodes might represent a vulnerable 
time period for this population. It is possible that continuous oral 
contraceptive use or oral con

hat continuous oral 
contraceptive use or oral contraceptives with a shorter HFI may 
be more effective in obese women. Evidence evaluating the effec­
tiveness of the patch and vaginal ring among obese women is lim­
ited, but both provide more effective contraception than barrier 
methods alone.89,301,308,309 Among overweight and obese women, 
higher pregnancy rates have not been observed with use of the 
150-mg intramuscular DMPA or 104-mg subcutaneous formula­
tions116,310 or the etonogestrel 

taneous formula­
tions116,310 or the etonogestrel implant.311 Considering that IUDs 
work locally on the uterus and do not rely on systemic drug levels, 
their efficacy is not affected by BMI.305
Combined hormonal contraceptives are rated USMEC cate­
gory 2 for obese women due to theoretical concerns about arterial 
and VTE. Although obesity and use of combined hormonal con­
traceptives represent independent risk factors for VTE,285,312,313 
the absolute risk of VTE with combined contraceptives 

absolute risk of VTE with combined contraceptives in obese 
women is still less than the risk of VTE during pregnancy and the 
puerperium in obese women.314,315 The evidence of an increased 
risk of acute myocardial infarction or stroke in obese users of COC 
is conflicting,316 but the overall absolute risk of these events in 
reproductive-age women is low.72 That being said, obese women 
are more likely to have other comorbidities, such as hypertension, 
hypercholesterolemia, and diabetes. No p

tension, 
hypercholesterolemia, and diabetes. No published studies address 
the safety of combined hormonal contraceptives in obese women 
with these comorbidities. Accordingly, consideration should be 
given to progestin-only and intrauterine methods when counsel­
ing obese individuals regarding contraceptive choices, particularly 
when such patients are older than 35 years. Because obese women 
experience an elevated risk for abnormal uterine bleeding and 
endometrial neoplasia, use of the LNG

leeding and 
endometrial neoplasia, use of the LNG IUD represents a particu­
larly sound choice in this patient population.317,318
Individuals who undergo bariatric surgery that may compro­
mise the absorption of oral medications (Roux-en-Y gastric bypass 
or biliopancreatic diversion) should use oral contraceptives with 
caution (USMEC category 3).319 There are no similar concerns for 
women who have had restrictive types of bariatric surgery (verti­
cal banded gastroplasty, laparoscopic adjust

erti­
cal banded gastroplasty, laparoscopic adjustable gastric band, or 
laparoscopic sleeve gastrectomy). 
Hormonal Contraception in Women Taking 
Antiepileptic Drugs
Effective contraception is a critical component in management 
of the female patient with epilepsy because of increased risk of 
seizures in pregnancy, teratogenic effects of some antiepileptic 
drugs (AEDs), and the multitude of interactions between AEDs 
and hormonal contraception.320 AEDs that induce hepatic 
enzymes can decrea

n.320 AEDs that induce hepatic 
enzymes can decrease serum concentrations of one or both of the 
estrogen or progestin component of oral contraceptives.321 Risk 
of COC failure may be increased in the presence of CYP3A4 
enzyme inducing, such as carbamazepine, felbamate, lamotrig­
ine, phenobarbital, phenytoin, oxcarbazepine, primidone, topi­
ramate, and rufinamide. Concomitant use of AEDs that induce 
hepatic enzymes with combined hormonal contraceptives is rated 
USMEC category 3, given that t

en-containing contraceptives increase metabolism of 
lamotrigine, reducing drug levels.322,323 Therefore, dose adjust­
ments of lamotrigine will be needed if the two medications are 
used together. In addition, during the HFI, lamotrigine levels may 
increase.
As a high-dose contraceptive, injectable contraception (DMPA) 
appears effective with concomitant AED use, and IUDs are highly 
effective in women taking anticonvulsants. The USMEC recom­
mends that when women taking liver enzyme–inducing 

ends that when women taking liver enzyme–inducing medica­
tions use COC, a formulation containing a minimum of 30 μg 
of ethinyl estradiol should be used. In addition, expert opinion 
recommends selecting an oral contraceptive with a progestin that 
has a longer half-life (drospirenone, desogestrel, levonorgestrel) 
or using formulations with HFIs shorter than 7 days, or active 
pills in a continuous fashion, to potentially improve efficacy.20,324 
Given that the norethindrone POP available in t

4 
Given that the norethindrone POP available in the United States 
is dosed too low to consistently suppress ovulation, this contracep­
tion does not represent a sound choice for women taking enzyme-
inducing AEDs. Data are limited for the contraceptive patch and 
vaginal ring. Contraceptive failures have also been reported in 
women taking enzyme-inducing AEDs and using the etonogestrel 
implant system (Nexplanon).220-222 
Antibiotics
Although there have

n in Women Taking 
Although there have been many retrospective case series and 
anecdotal reports of COC failure in women taking concomitant 
antibiotics, pharmacokinetic evidence of lower serum steroid lev­
els exists only for rifampin.80,81 Women taking rifampin should 
not rely on oral, transdermal, vaginal ring, or implantable con­
traception alone for protection. In contrast to rifampin, use of 
ampicillin, doxycycline, fluconazole, metronidazole, miconazole, 
quinolones, and te

le, metronidazole, miconazole, 
quinolones, and tetracycline have not lowered steroid levels in 
women using COCs. The use of combined hormonal methods is 
not restricted among women taking broad-spectrum antibiotics, 
antifungals, or antiparasitics.9 
Hormonal Contraception in HIV-Positive Women
More than 17 million women, many of reproductive age, are 
infected with HIV.325 HIV transmission increasingly is linked 
to heterosexual intercourse. The role of hormonal contraception 
in HIV-positive

he role of hormonal contraception 
in HIV-positive women has been controversial. There may be 
an increased risk for acquiring or transmitting HIV in DMPA 
users, but DMPA is a needed contraceptive in areas of high HIV 
prevalence (sub-Saharan Africa) where there are limited alterna­
tive contraceptive options.166 The USMEC recently revised its rec­
ommendation for the use of DMPA among women at increased 
risk for HIV acquisition based on a systematic review of progestin 
injectables that found

ematic review of progestin 
injectables that found a possible increased risk for HIV acquisi­
tion (adjusted hazard ratio 1.4; 95% CI, 1.2–1.6) in this popu­
lation.166 The current recommendation is that the advantages of 
DMPA still outweigh the theoretical or proven risks (category 2) 
among women at high risk for HIV infection, but women should 
be counseled regarding these issues.165 POPs, combined hormonal 
methods, and implants are safe (category 1) for women at high 
risk for HIV infectio

tegory 1) for women at high 
risk for HIV infection or infected with HIV. Hormonal contra­
ceptives, including DMPA, do not appear to have an impact on 
HIV disease progression.326 For women on antiretroviral therapy, 
it is important to note there are several drug-drug interactions 
with hormonal contraceptives (particularly some non-nucleoside 
reverse transcriptase inhibitors and ritonavir-boosted protease 
inhibitors). These interactions might alter the safety and efficacy 
of both the hormo

t alter the safety and efficacy 
of both the hormonal contraceptive and the antiretroviral drug. 
The website https://www.hiv-druginteractions.org is kept up to 
date as new evidence emerges and new drugs are released. Case 
reports suggest that the antiretroviral drug efavirenz reduces con­
traceptive implant protection; IUDs and DMPA represent prefer­
able contraceptive choices for women taking this medication or 
others that induce hepatic enzymes.234
IUDs are allowed by the USMEC for women w

ymes.234
IUDs are allowed by the USMEC for women with HIV (cate­
gory 1), as there is no evidence that they are at higher risk for infec­
tious complications than HIV-uninfected women.166 The IUD 
has not been shown to affect disease progression or increase risk of 
transmission to sex partners. For women with acquired immuno­
deficiency syndrome, IUDs can be used if they are clinically well 
on antiretroviral therapy (category 1) and also considered if they 
are not (category 2). All IUD users 

ered if they 
are not (category 2). All IUD users with acquired immunodefi­
ciency syndrome should be closely monitored for pelvic infection. 
Hormonal Contraception and Chronic 
Blood pressure should be measured prior to initiating combined 
hormonal contraceptives.8,327 Although few women develop overt 
hypertension after starting combined hormonal contraceptives, 
blood pressure should be checked at follow-up visits.328 Modern 
low-dose COCs may increase systolic and diastolic bl

w-dose COCs may increase systolic and diastolic blood pres­
sure, on average, by 8 and 6 mm Hg, respectively.329 This increase 
is thought to be secondary to increased activation of the renin-
angiotensin system.330 Nevertheless, a systematic review of the 
literature found that only a small percentage of women developed 
incident hypertension in up to 2 years of follow-up after starting 
a COC.331
For women with preexisting hypertension, the concern with 
combined hormonal methods is the increa

cern with 
combined hormonal methods is the increased risk of arterial throm­
bosis leading rarely to myocardial infarction and stroke. In women 
with stage 1 hypertension (systolic 140–159 mm Hg or diastolic 
90–99 mm Hg), combined hormonal contraceptives should not 
be used unless no other method is appropriate or acceptable to 
the patient (USMEC category 3).9 Women with stage 2 hyper­
tension (systolic ≥160 mm Hg or diastolic ≥100 mg Hg) or vas­
cular disease should not use combined hormonal

as­
cular disease should not use combined hormonal contraceptives 
(USMEC category 4). Women on antihypertensive medication 
represent a separate category (see Table 18.2). Although the risk 
of cardiovascular disease in women with hypertension adequately 
controlled with medications should be reduced, there are no data 
on the use of combined hormonal contraceptives in this popula­
tion. Therefore, use of combined hormonal contraceptives in these 
women requires clinical judgment (USMEC categor

e 
women requires clinical judgment (USMEC category 3). Women 
with well-controlled and monitored hypertension who are age 35 
years or younger may be appropriate candidates for a trial of com­
bined contraceptives, provided that they are otherwise healthy, 
show no evidence of end-organ vascular disease, and do not smoke 
cigarettes. If blood pressure remains well controlled with careful 
monitoring several months after contraceptive initiation, use can 
be continued.
Progestin-only contracepti

 use can 
Progestin-only contraceptives, such as DMPA, POPs, 
the etonogestrel implant, or the LNG IUD, are appropriate 
options in women with hypertension. Unlike other progestins, 
the use of DMPA in women with stage 2 hypertension (sys­
tolic ≥160 mm Hg or diastolic ≥100 mm Hg) is generally not 
665
advised because of the theoretical risk of unfavorable lipopro­
tein changes (USMEC category 3).9 However, risks and ben­
efits of the method need 

 However, risks and ben­
efits of the method need to be weighed against risks of adverse 
pregnancy outcomes in women with hypertension. Blood pres­
sure measurement prior to or during the use of progestin-only 
methods is not necessary.8 
Hormonal Contraception in Women With 
Diabetes
A systematic review concluded that hormonal contraceptives 
have limited effect on carbohydrate metabolism in women 
without diabetes.332 Likewise, COCs do not appear to impair 
carbohydrate metabolism or affect v

ear to impair 
carbohydrate metabolism or affect vascular disease in diabetic 
women. A history of gestational diabetes is not a contraindica­
tion to hormonal contraceptives. However, studies are limited 
and do not inform the management of diabetic women who are 
overweight.333 Additionally, clinicians should consider the pres­
ence of other cardiovascular risk factors in a diabetic patient, 
such as obesity, smoking, hypertension, and age, when evaluat­
ing for hormonal contraception.
For wom

n evaluat­
For women with diabetes of more than 20 years’ duration 
or evidence of microvascular disease (retinopathy, nephropathy, 
or neuropathy), combined hormonal contraceptives are gener­
ally contraindicated (USMEC category 3 or 4 depending on the 
severity of the condition).9 Due to the concern about possible 
hypoestrogenism during use of injectable contraception and effect 
on HDL levels,334 DMPA is given a category 3 rating for women 
with diabetes of mo

a category 3 rating for women 
with diabetes of more than 20 years or evidence of microvascular 
disease, indicating that the risks usually outweigh the benefits.
The POP, IUDs, and implant are suitable for women with 
diabetes. A clinical trial found that metabolic control was similar 
in women with uncomplicated diabetes randomized to a copper 
IUD or the LNG IUD.335 
Hormonal Contraception in Women Awaiting 
Surgery
VTE with pulmonary embolism remains a major cause of 
fatalities associated w

 remains a major cause of 
fatalities associated with surgical (including gynecologic) pro­
cedures.336 There is concern that combined hormonal contra­
ception use around the time of surgery may increase this risk. 
The procoagulant changes of COCs take 6 weeks or longer to 
resolve after discontinuation.337 The benefits associated with 
stopping combined hormonal contraceptives 1 month or more 
before major surgery should be balanced against the risks of an 
unintended pregnancy. The USMEC reco

 risks of an 
unintended pregnancy. The USMEC recommends that COCs 
be discontinued before major surgery with an anticipated pro­
longed immobilization postoperatively (category 4).9 In addi­
tion, patients with risk factors for VTE, such as prior VTE or 
undergoing high-risk procedures (e.g., major abdominal-pelvic 
surgeries, major orthopedic surgeries, or cancer surgery), may 
consider discontinuing COCs. Otherwise, for any major surgery 
for which the patient is expected to be ambulatory imm

which the patient is expected to be ambulatory immediately 
after surgery, it is not necessary to discontinue combined hor­
monal contraceptives. Because of the low perioperative risk of 
VTE, it is not considered necessary to discontinue combined 
hormonal contraceptives before laparoscopic tubal sterilization 
or other minor surgical procedures not known to be associated 
with an elevated VTE risk. Progestin-only and intrauterine con­
traceptives are not expected to be associated with an incre

es are not expected to be associated with an increased 
risk of perioperative VTE. 
Hormonal Contraception in Women With a 
History of Thromboembolism
Women with an acute deep vein thrombosis or pulmonary embo­
lism, documented history of unexplained VTE, recurrent VTE or 
VTE associated with pregnancy, exogenous estrogen use, known 
thrombophilia or antiphospholipid antibody syndrome, or 
active cancer should not use combined hormonal contraceptives 
(USMEC category 4).9,338 With respect to wom

ives 
(USMEC category 4).9,338 With respect to women not using anti­
coagulants, the risk of recurrent VTE depends on whether the ini­
tial deep vein thrombosis was associated with a permanent (e.g., 
factor V Leiden) or reversible (e.g., surgery) risk factor.339 There­
fore, a COC candidate who had experienced a single episode of 
VTE years earlier associated with a nonrecurring risk factor (e.g., 
VTE occurring after immobilization following a motor vehicle 
accident) may not currently be at i

motor vehicle 
accident) may not currently be at increased risk for VTE. Accord­
ingly, the decision to initiate estrogen-containing contraceptives 
in such patients can therefore be individualized. Most studies have 
not found an increased VTE risk with use of progestin-only con­
traception; therefore, these methods and IUDs are not contraindi­
cated in women at increased risk of thromboembolism.102 
Anticoagulation Therapy
Long-term risks of warfarin for

oagulation Therapy
Long-term risks of warfarin for reproductive-age women include 
heavy or prolonged menstrual bleeding and rarely include hemo­
peritoneum after rupture of ovarian cysts. Warfarin is also a terato­
gen. Because the risk of recurrent thrombosis is reduced in women 
on therapeutic anticoagulation, some authorities recommend that 
use of estrogen-containing contraceptives can be considered for 
anticoagulated women with a prior history of venous thrombosis 
on a case-by-case basis

tory of venous thrombosis 
on a case-by-case basis, especially if alternative methods are not 
acceptable to the woman or contraindicated and they have a low 
risk of recurrence (USMEC category 3).340 Among women with 
a history of thromboembolic disease participating in clinical trials 
of anticoagulants, use of hormonal contraceptives, including com­
bination methods, does not elevate the risk of recurrent throm­
bosis among women using anticoagulants.340 Accordingly, among 
chronically antico

gulants.340 Accordingly, among 
chronically anticoagulated women, use of combination hormonal 
contraceptives may be appropriate. Because of its high contracep­
tive efficacy and ability to suppress uterine bleeding, use of LNG 
IUDs also represents a useful option for some anticoagulated 
women and is preferred over the copper IUD in this setting.341-345 
Intramuscular injection of DMPA consistently suppresses ovula­
tion, and limited evidence has not revealed injection site prob­
lems, such as

as not revealed injection site prob­
lems, such as hematoma, in anticoagulated women.346 Similar 
injections (e.g., influenza vaccine) in anticoagulated women have 
not shown risk of hematoma.347 Accordingly, DMPA represents a 
useful contraceptive for anticoagulated women. There are no data 
on the risk of hematoma formation at the insertion site of contra­
ceptive implants; however, it is unlikely to be a greater risk than 
that associated with intramuscular DMPA injections. 
Hormonal Contrace

 intramuscular DMPA injections. 
Hormonal Contraception for Women With 
Migraine Headaches
Headaches are common in women of reproductive age. Most of 
these headaches are tension headaches, not migraines.348 Some 
women with migraines experience improvement in their symp­
toms with use of hormonal contraceptives, and some wom­
en’s symptoms worsen. Because the presence of true migraine 
666
headaches with aura affects the decision to use estrogen-cont

ith aura affects the decision to use estrogen-containing 
contraception, careful consideration of the diagnosis is important. 
Most migraines occur without aura. Nausea, vomiting, photopho­
bia, phonophobia, or visual blurring occurring before or during a 
migraine headache do not constitute aura. According to the Inter­
national Headache Society, an aura is the complex of neurologic 
symptoms that occurs usually before the headache, but it may 
begin after the headache has begun.349 A typical a

begin after the headache has begun.349 A typical aura is revers­
ible, lasts less than 60 minutes, and can consist of visual symp­
toms such as a zigzag figure (fortification spectrum, so named due 
to its resemblance to the walls of a medieval fortress) spreading 
across the visual field, sensory symptoms such as pins and needles, 
speech disturbances, or motor weakness.
Although the absolute risk of ischemic stroke is low in women 
of reproductive age, migraine with aura is an independent risk

ive age, migraine with aura is an independent risk fac­
tor for stroke. Most studies note a higher risk of stroke in women 
who have migraine with aura than in those who have migraine 
without aura.350,351 The assumption is that aura is associated with 
ischemic changes.352 Use of combined hormonal contraception 
by itself also increases the risk of stroke in women, although the 
absolute risk is still low.353 The elevated risk of ischemic stroke is 
likely due to the hypercoagulable state cause

 is 
likely due to the hypercoagulable state caused by estrogen. The 
risk of stroke among women using combined hormonal contra­
ception rises with increasing age from 3.4 per 100,000 women-
years in adolescents to 64.4 events per 100,000 women-years 
among women age 45 to 49 years.72 In a recent case control study, 
investigators found combined hormonal contraceptive use by 
women with migraines with aura synergistically increased the risk 
of stroke.54 In contrast, use of combined hormonal con

troke.54 In contrast, use of combined hormonal contraception 
by women with migraines without aura did not increase the risk 
over baseline. Although cerebrovascular events occur rarely among 
women with migraine with aura who use COCs, the impact of a 
stroke can be so devastating that clinicians should consider the 
use of progestin-only or intrauterine contraceptives in this set­
ting. According to the USMEC, estrogen-containing contracep­
tives are contraindicated (category 4) in women migra

es are contraindicated (category 4) in women migraine sufferers 
with aura and should be discontinued in patients suffering from 
migraine without aura if aura symptoms appear. Of note, some 
headache specialists are suggesting that today’s ultralow-dose 
COC formulations (e.g., the COC formulated with ethinyl estra­
diol dosed at 10 μg) may be safe and also useful in preventing 
menstrual-related migraines in migraineurs who are otherwise 
healthy, normotensive nonsmokers.354 There are no contr

hy, normotensive nonsmokers.354 There are no contraindica­
tions to the use of any progestin-only method or IUD in women 
with migraines with or without aura (USMEC category 1).9 
Systemic Lupus Erythematosus
Although effective contraception is important for women with 
lupus, concerns about increasing disease activity and thrombosis 
have resulted in clinicians rarely prescribing combination estrogen-
progestin oral contraceptives to women with this disease

tin oral contraceptives to women with this disease. Indeed, 
women with SLE are at higher risk for ischemic heart disease, stroke, 
and VTE, especially in the presence of antiphospholipid antibod­
ies.355-357 Prior to initiating contraceptives in women with SLE, the 
level of disease activity and presence of antiphospholipid antibod­
ies and thrombocytopenia should be established. Combined hor­
monal contraception is contraindicated in women with SLE and 
positive antiphospholipid antibodies due

 SLE and 
positive antiphospholipid antibodies due to the increased risk of 
venous and arterial thromboembolism (USMEC category 4). In 
other SLE patients, decisions regarding use of combined hormonal 
contraceptives should consider the presence of other cardiovascular 
disease risk factors (e.g., older age, smoking, hypertension, diabetes, 
and hypercholesterolemia) and is rated USMEC category 2. Use 
of COC does not appear to worsen disease activity in women with 
inactive or stable active di

tivity in women with 
inactive or stable active disease.358 Although progestin-only meth­
ods are not considered to increase the risk of VTE, they are all rated 
category 3 (risks outweigh benefits) for SLE patients with antiphos­
pholipid antibodies. This is because the propensity for VTE is 
quite high in these patients, and one randomized controlled trial 
reported the development of thrombosis in two such women taking 
POPs.359,360 Although the category 3 classification also applies to 
the 

he category 3 classification also applies to 
the LNG IUD, systemic drug levels with this device would be mini­
mal. In other SLE patients without antiphospholipid antibodies, 
progestin-only methods are rated USMEC category 2. There are no 
restrictions on the use of the copper IUD in women with SLE and 
positive antiphospholipid antibodies. For women with thrombo­
cytopenia, the LNG IUD is preferred over the copper IUD. Many 
women with SLE take immunosuppressant medications, and, in 
the past

e immunosuppressant medications, and, in 
the past, the use of IUDs among immunocompromised women 
has been controversial. However, immunosuppression is no longer 
considered a contraindication to IUD use.361,362 
Hormonal Contraception in Women With Sickle 
Cell Disease
Similar to pregnancy in SLE patients, pregnancy in women with 
sickle cell disease increases maternal and fetal morbidity and mor­
tality rates. The safety of hormonal contraceptives in women with 
homozygous (SS) sickle cell di

ives in women with 
homozygous (SS) sickle cell disease has been controversial.363 Only 
one small randomized controlled trial has addressed this issue. 
Twenty-five patients were given DMPA or intramuscular saline 
placebo injections once every 3 months in a crossover design. 
DMPA users were less likely to experience painful sickle episodes 
(OR, 0.23; 95% CI, 0.05–1.02).364 No randomized studies have 
addressed estrogen-containing products.365 These limited data 
suggest that DMPA and other p

 These limited data 
suggest that DMPA and other progestin-only methods are a safe 
contraceptive option for women with sickle cell disease.363
No well-controlled study has assessed whether VTE risk in 
oral contraception users with sickle cell disease is higher than in 
other combination oral contraception users. However, a small 
case-control study conducted in the United States found that 
COC use was associated with a nonsignificantly elevated risk of 
VTE.366 Cross-sectional studies of wome

d risk of 
VTE.366 Cross-sectional studies of women with sickle cell disease 
have observed no differences in markers for platelet activation, 
thrombin generation, fibrinolysis, or red blood cell deformability 
between users of combination oral contraception methods, users 
of progestin-only methods, and nonusers of hormonal contracep­
tion.367 On the basis of these observations, studies of pregnant 
women with sickle cell disease, and small observational studies of 
women with sickle cell dise

ervational studies of 
women with sickle cell disease who use COCs, and on theoreti­
cal considerations, the USMEC concludes that pregnancy carries 
a greater risk than estrogen-containing contraceptive use.9 This 
recommendation would change, however, if the woman had con­
comitant cardiovascular risk factors or pulmonary hypertension—
a complication of sickle cell disease.
Although the lack of evidence on IUD use among women with 
sickle cell disease represents a major gap in the literature, t

isease represents a major gap in the literature, theoreti­
cal concerns about IUD use in this population are few. There is no 
current evidence to support limiting IUD use among women with 
sickle cell disease.363 The USMEC rates the LNG IUD category 1, 
indicating that there are no restrictions for this method in women 
with sickle cell disease. The copper IUD is considered category 2, 
667
in which benefits outweigh risks, because of the theoretical con­
cern

tweigh risks, because of the theoretical con­
cern about increased blood loss with menstruation. 
Hormonal Contraception in Depression
Depression and mood disorders are extremely common in women 
of reproductive age.128 The issues to consider are the impact of 
hormonal contraceptives on mood and the potential impact of 
treatments for depression on contraceptive efficacy. The use of any 
method of contraception is acceptable for women who suffer from 
depression, and there are no contraindicati

 from 
depression, and there are no contraindications (USMEC category 
1).9 There is no evidence that hormonal contraception including 
DMPA worsens a woman’s mood, regardless of whether or not 
she has baseline depression.127,368 Selective serotonin reuptake 
inhibitors and serotonin norepinephrine reuptake inhibitors, the 
most commonly used antidepressants, do not appear to interact 
with hormonal contraceptives.369,370 In contrast, a clinical trial 
observed that use of the herbal remedy St.

 trial 
observed that use of the herbal remedy St. John’s wort, a hepatic 
enzyme inducer, increased progestin and estrogen metabolism, as 
well as breakthrough bleeding and the likelihood of ovulation, in 
women using COCs.371,372 For this reason, concomitant use of St. 
John’s wort is rated USMEC category 2 for combined hormonal 
contraception, POPs, and the etonogestrel implant.
667.e1
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668
19
Testicular Disorders
ALVIN M. MATSUMOTO AND BRADLEY D. ANAWALT
T
he test

LVIN M. MATSUMOTO AND BRADLEY D. ANAWALT
T
he testes have critical physiologic roles during various 
stages of development. During early fetal life, production 
of testosterone and antimüllerian hormone (AMH) by the 
fetal testes is required for the differentiation and development of 
normal male internal and external genitalia. During puberty, acti­
vation of the hypothalamic-pituitary-testicular axis and testoster­
one production by the testes are necessary for the induction of 
secondary (adu

are necessary for the induction of 
secondary (adult) male sexual characteristics, stimulation of sexual 
function, and initiation of sperm production. In adults, testis pro­
duction of testosterone is required for the maintenance of adult 
male characteristics (virilization), sexual function, spermatogen­
esis, and fertility potential. Therefore, disorders of the testis may 
result in abnormalities in sexual development and function, body 
habitus and function, and fertility that have profound 

us and function, and fertility that have profound effects on 
health and well-being.
Disorders of the testis are common. Klinefelter syndrome is the 
most common human sex chromosome abnormality and the most 
common cause of primary hypogonadism, occurring in 1 in 500 
to 600 male births.1,2 Isolated disorders of sperm production are 
the main causes of male infertility, affecting approximately 5% to 
Functional Anatomy and Histology, 669
Testis Development, 673
Adult Physiology,

ogy, 669
Adult Physiology, 675
Male Hypogonadism, 690
KEY POINTS
 • 
The testes are critical for normal development of internal and 
external genitalia in the fetus; for secondary sexual characteris­
tics, sexual function, and initiation of spermatogenesis during 
puberty; and for development and maintenance of adult male 
phenotype, sexual function, and fertility.
 • 
Knowledge of the anatomy, physiology, and regulation of 
the hypothalamic-pituitary-testicular axis fo

 of 
the hypothalamic-pituitary-testicular axis forms the basis 
for understanding the clinical manifestations, diagnosis, and 
management of the main disorders of testis function, androgen 
deficiency and infertility.
 • 
Male hypogonadism is a clinical syndrome that results from the 
failure of the testes to produce adequate amounts of testos­
terone (androgen deficiency) and sperm, or from an isolated 
impairment of spermatogenesis.
 • 
The consequences of androgen deficiency vary depending

consequences of androgen deficiency vary depending 
on the stage of sexual development. In the fetus, androgen 
deficiency causes ambiguous genital development; in the 
child, it causes delayed puberty or eunuchoidism; and in adults, 
it causes sexual dysfunction, gynecomastia, infertility, and 
changes in body composition, muscle mass and strength, and 
bone density and strength that adversely affect health.
 • 
Male hypogonadism is common, but the diagnosis should only 
be made in men with sy

 the diagnosis should only 
be made in men with symptoms and signs of androgen deficien­
cy and consistently low serum testosterone concentrations, in 
the absence of conditions that transiently suppress testosterone.
 • 
In men with low serum total testosterone concentrations and 
conditions that alter sex hormone–binding globulin, an accu­
rate assessment of serum free testosterone is useful.
 • 
In men with hypogonadism, gonadotropin concentrations 
should be measured to distinguish between

­spermatogenesis and fertility in men with secondary 
­hypogonadism.
 • 
Prior to initiating testosterone replacement in men with obesity 
or metabolic syndrome, low serum testosterone concentrations, 
and symptoms of possible androgen deficiency, it is important 
to initiate lifestyle changes for weight loss. Drugs that might 
suppress the hypothalamic-pituitary-testicular axis should be 
stopped, if possible.
 • 
Efficacy and safety monitoring should be performed during 
testosterone treatm

ng should be performed during 
testosterone treatment.
669
Chapter 19  Testicular Disorders
6% of otherwise healthy men in the reproductive age group.3 Tes­
ticular disorders resulting in testosterone deficiency may contrib­
ute to complaints of reduced libido (sexual interest and desire), 
erectile dysfunction, gynecomastia (benign breast enlargement), 
and reduced bone mass (osteoporosis), all of which are common 
in men, particularly as they age. Finally, disordered hypothalamic-
pituitary-te

ge. Finally, disordered hypothalamic-
pituitary-testicular function is commonly associated with chronic 
systemic illnesses, wasting syndromes, morbid obesity, chronic 
use of certain medications (e.g., opioids and glucocorticoids), 
and aging. These conditions often result in testosterone deficiency 
that, if severe and prolonged, may contribute to clinical manifesta­
tions and morbidity.4,5
The treatment of testicular disorders usually results in sig­
nificant clinical improvements in function

in sig­
nificant clinical improvements in function and quality of life. In 
prepubertal boys and adults with severe testosterone deficiency, 
testosterone therapy results in dramatic transformations in body 
composition and function.6 In men with low serum testosterone, 
infertility, and impaired spermatogenesis due to gonadotropin 
deficiency, gonadotropin replacement therapy generally increases 
sperm and testosterone production and restores fertility. Finally, 
advances in assisted reproducti

rtility. Finally, 
advances in assisted reproductive technology (ART) have per­
mitted previously infertile men with testicular disorders to have 
children. For example, although men with Klinefelter syndrome 
and azoospermia (no spermatozoa in their ejaculate) were once 
thought to have untreatable infertility, testicular sperm extraction 
(TESE) using microsurgical techniques combined with intracyto­
plasmic sperm injection (ICSI) may permit some of these men to 
father children.7
Functional A

me of these men to 
Functional Anatomy and Histology
The Testis
Adult testes are paired, ovoid organs that hang from the inguinal 
canal by the spermatic cord (which is composed of a neurovascular 
pedicle, vas deferens, and cremasteric muscle); they are located 
outside the abdominal cavity within the scrotum. The left testis 
hangs lower in the scrotum than the right in about 60% of men, 
and the right testis hangs lower in approximately 30% of men. 
Each testis has a volume 

proximately 30% of men. 
Each testis has a volume of 15 to 30 mL and measures 3.5 to 5.5 
cm in length by 2.0 to 3.0 cm in width.8,9
The testis comprises two structurally and functionally dis­
tinct compartments: the seminiferous tubule compartment, which 
is composed of Sertoli cells and developing germ cells at various 
stages of spermatogenesis and accounts for 80% to 90% of the 
volume of the testis, and the interstitial compartment, which is 
composed of Leydig cells that secrete testostero

composed of Leydig cells that secrete testosterone, the main male 
sex steroid hormone, as well as peritubular myoid cells, fibroblasts, 
neurovascular cells, and macrophages10 (Fig. 19.1). Because germ 
cells constitute most of the testis volume, small testes are usually 
an indication of significantly impaired spermatogenesis.
The testis is surrounded by a fibrous capsule, the tunica albu­
ginea. Fibrous septa that emanate from the tunica albuginea sepa­
rate the parenchyma of the testis int

uginea sepa­
rate the parenchyma of the testis into lobules. The arterial blood 
supply of the testes is derived primarily from the testicular (inter­
nal spermatic) arteries that arise from the abdominal aorta and 
descend through the inguinal canal in the spermatic cord. Col­
lateral blood supply is provided by the cremasteric and deferential 
arteries. This collateral supply permits survival of the testis after a 
testicular artery ligation associated with surgical fixation of a high 
undesce

sociated with surgical fixation of a high 
undescended testis into the scrotum (orchiopexy). However, twist­
ing of the spermatic cord, known as testicular torsion, results in 
strangulation of the blood supply to the testis and causes testicular 
necrosis and infarction after 6 to 8 hours, making this condition a 
surgical emergency.11 A testis that has a bell-clapper deformity (i.e., 
is not attached to the scrotal wall) is more susceptible to testicular 
torsion. Lymphatic drainage from the t

testicular 
torsion. Lymphatic drainage from the testes follows the testicular 
arteries to periaortic lymph nodes; this is a common route for 
metastasis of testicular cancer.
A network of veins that compose the pampiniform plexus pro­
vides venous drainage from the testes. The pampiniform plexus 
coalesces into the testicular (internal spermatic) vein. The right 
testicular vein drains into the inferior vena cava, and the left 
testicular vein empties at a right angle into the left renal vein.

empties at a right angle into the left renal vein. 
One-way valves in testicular veins prevent backflow of blood into 
the scrotum. Abnormal enlargement of the venous plexus drain­
ing a testicle, known as a varicocele, occurs if valves are defective 
or absent or if there is extrinsic venous compression impeding 
normal venous drainage.12 Increased pressures associated with 
the backflow of blood and altered temperature regulation may 
contribute to testicular dysfunction associated with a vari

e to testicular dysfunction associated with a varicocele. 
Ninety-eight percent of varicoceles occur in the left scrotum, pos­
sibly because of absent or defective valves in the left testicular vein. 
The presence of a prominent unilateral right-sided varicocele or 
new-onset varicocele on either side should prompt evaluation for 
venous obstruction by an abdominal or pelvic malignancy (e.g., 
renal cell carcinoma) or lymphadenopathy; a chronic right-sided 
varicocele may also indicate situs inv

ight-sided 
varicocele may also indicate situs inversus. Rarely, an anatomic 
anomaly of the superior mesenteric artery that compresses the left 
renal vein causes a left-sided varicocele; this is known as the nut­
cracker syndrome.
ST
L
ST
L
• Fig. 19.1  Light photomicrograph of the seminiferous tubule and inter­
stitial compartments of the human testes. The seminiferous tubule (ST) 
compartment makes up the majority of the testis and is composed of 
developing germ cells enveloped by Sertoli c

d of 
developing germ cells enveloped by Sertoli cells. Spermatogonia line the 
basal lamina of the seminiferous tubules, spermatocytes at various stages 
of development are present in the middle layers of the tubules, and sper­
matids at various steps of maturation are present in the luminal aspect 
of the seminiferous tubules. Within each tubule, there are germ cells at 
different stages of spermatogenesis. In the interstitial compartment, there 
are prominent clusters of Leydig cells (L) nest

e 
are prominent clusters of Leydig cells (L) nestled between seminiferous 
tubules, peritubular myoid cells within the basal lamina of the tubules, 
and scattered blood vessels and macrophages. (From Matsumoto AM. 
Spermatogenesis. In: Adashi EY, Rock JA, Rosenwaks Z, eds. Reproduc­
tive Endocrinology, Surgery, and Technology. Philadelphia, PA: Lippincott-
Raven; 1996:359–384.)
670
Because the testes are located outside the abdominal cavity, 
they ar

are located outside the abdominal cavity, 
they are exposed to temperatures approximately 2°C lower than 
core body temperature. The position of the testes within the scro­
tum and the testicular temperature are regulated by the cremas­
teric muscle. The cremasteric muscle contracts when warming is 
needed, resulting in shortening of the spermatic cord and drawing 
of the testis toward the abdomen; when cooling is needed, the 
muscle relaxes, resulting in lowering of the testis into the scrotum.

ulting in lowering of the testis into the scrotum. 
In addition, the pampiniform venous plexus provides a counter-
current heat exchange mechanism to cool the testis by surrounding 
the testicular artery with cooler venous blood. A testis temperature 
slightly lower than core body temperature is important for normal 
spermatogenesis. Exposure of the testes to higher temperatures, 
such as with failure of the testes to descend normally into the 
scrotum (cryptorchidism) or excessive external heat

crotum (cryptorchidism) or excessive external heat exposure due 
to frequent, prolonged hot tub use may impair spermatogenesis. 
Seminiferous Tubule
Seminiferous tubules contain epithelium consisting of Sertoli cells 
that envelop and support germ cells undergoing progressive dif­
ferentiation and development into mature spermatozoa. Once 
released into the lumen, mature spermatozoa are transported 
within seminiferous tubules, which measure up to 70 cm in length 
and are tightly coiled within l

o 70 cm in length 
and are tightly coiled within lobules of the testis, to the rete testis, 
the efferent ducts, the epididymis, and, finally, the vas deferens for 
ejaculation. The seminiferous tubules are surrounded by a basal 
lamina composed of an extracellular matrix that serves to separate 
them from the interstitial compartment, provides structural integ­
rity to the tubules, and regulates the function of cells in contact 
with it. Histologic examination of a testis biopsy specimen in cro

gic examination of a testis biopsy specimen in cross 
section reveals many different seminiferous tubules surrounded by 
basal lamina and clusters of Leydig cells in the interstitial com­
partment between each tubule10 (see Fig. 19.1).
Sertoli cells extend from the basal lamina to the lumen of 
tubules, and adjacent Sertoli cells envelop and provide a struc­
tural scaffold for germ cells as they differentiate within the tubule9 
(Fig. 19.2). Undifferentiated spermatogenic stem cells, called sper

fferentiated spermatogenic stem cells, called sper­
matogonia, lie along the basal lamina at the periphery of tubules, 
interspersed between Sertoli cells. Adjacent Sertoli cells surround 
spermatogonia and form specialized junctional complexes or tight 
junctions that divide the seminiferous tubule into the basal com­
partment, in which spermatogonia reside, and the adluminal com­
partment, which is occupied by differentiating germ cells. Sertoli 
cell tight junctions impede the passage of larg

i 
cell tight junctions impede the passage of large molecules, steroids, 
and ions into the seminiferous tubule and constitute the cytologic 
basis of the blood-testis barrier, analogous to the blood-brain bar­
rier. In the adluminal compartment, spermatocytes derived from 
spermatogonia in the basal compartment undergo meiosis to 
form spermatids that progressively mature (spermiogenesis), with 
Cross section of
seminiferous tubule
Spermatozoa
Lumen
Tight junction
Early spermatid
Spermatocyte
A

umen
Adluminal
(inner)
compartment
Basil
(outer)
Spermatogonia
Myoid cell
Sertoli cell nucleus
• Fig. 19.2  Schematic diagram of the cells in the seminiferous tubule (top). The seminiferous tubule con­
sists of Sertoli cells that surround developing germ cells (middle). Sertoli cells extend from the basal lam­
ina to the lumen. Tight junctions between adjoining Sertoli cells separate the seminiferous tubule into 
basal and adlu

arate the seminiferous tubule into 
basal and adluminal compartments and are the anatomic basis for the blood-testis barrier (bottom). The 
basal compartment, which contains spermatogonia lining the basal lamina and peritubular myoid cells, is 
exposed to the interstitial compartment, which contains Leydig cells and blood vessels that deliver endo­
crine regulators of testis function (e.g., gonadotropins). The adluminal compartment contains developing 
spermatocytes, spermatids, and mature sperm

oping 
spermatocytes, spermatids, and mature spermatozoa that are released into the lumen of the seminiferous 
tubule. (From Matsumoto AM. The testis. In: Felig P, Frohman LA, eds. Endocrinology and Metabolism. 4th 
ed. New York, NY: McGraw-Hill; 2001:635–705.)
671
the more mature germ cells occupying positions closer to the 
lumen, until mature spermatozoa are released into the lumen of 
the tubule (spermiation).
Because of the blood-testis barrier, only Sertoli

Because of the blood-testis barrier, only Sertoli cells and sper­
matogonia are directly accessible to endocrine and paracrine 
regulation from the circulation and cells of the interstitial com­
partment, respectively. Sertoli cells need to synthesize and secrete 
many products, some of which are present in the circulation but 
not accessible to developing germ cells in the adluminal com­
partment, to nurture and regulate spermatogenesis. Sertoli cells 
contain receptors for follicle-stimulatin

i cells 
contain receptors for follicle-stimulating hormone (FSH) and 
androgens, and they mediate the regulation of spermatogenesis by 
circulating FSH and testosterone produced locally by Leydig cells 
in response to stimulation from circulating luteinizing hormone 
(LH). Sertoli cells produce extracellular matrix components and 
AMH that causes regression of müllerian ducts and prevents the 
development of female accessory sex organs in the male fetus and 
inhibin B, which is the most importa

le fetus and 
inhibin B, which is the most important testicular contributor to 
negative feedback suppression of FSH. 
In male humans, the process of spermatogenesis supports a pro­
duction rate of approximately 120 million mature spermatozoa 
per day by the testis (approximately 1000 per heartbeat!).13 Sper­
matogenesis, the process by which stem cells (spermatogonia) dif­
ferentiate into mature spermatozoa, proceeds in three functionally 
distinct phases: (1) the mitotic or pro

ctionally 
distinct phases: (1) the mitotic or proliferative phase, during which 
the majority of spermatogonia undergo mitosis to renew the stem 
cell pool and a minority become committed to further differen­
tiation to produce spermatocytes; (2) the meiotic phase, during 
which spermatocytes undergo successive meiotic divisions to pro­
duce haploid germ cells (spermatids); and (3) spermiogenesis, dur­
ing which immature, round spermatids differentiate into mature 
spermatozoa10,14 (Fig. 19.3).

entiate into mature 
Proliferative Phase
Based on chromatin staining and pattern, spermatogonia may 
be classified as A dark (Ad), A pale (Ap), or B spermatogonia. 
Because of their relatively low mitotic rate, Ad spermatogonia are 
thought to be the spermatogonial stem cells. Ad spermatogonia 
are relatively resistant to external insults (e.g., ionizing radiation), 
and in response to such insults, they undergo mitotic prolifera­
tion. However, severe or complete d

tic prolifera­
tion. However, severe or complete depletion of Ad spermatogo­
nia, such as that which occurs with high-dose x-irradiation or 
vascular compromise, results in irreversible impairment or loss 
of sperm production.
A small number of Ad spermatogonia undergo mitotic divi­
sions to form Ap and then B spermatogonia. In humans, the rate 
of formation of B spermatogonia is low, so only a small number of 
B spermatogonia are available to enter meiosis and undergo fur­
ther differentiation.

ter meiosis and undergo fur­
ther differentiation.15 This rate limits the efficiency of spermato­
genesis in humans. B spermatogonia are the most sensitive germ 
cells to the effects of ionizing radiation, and their numbers are 
reduced after irradiation of the testes.16
B spermatogonia that become committed to further differen­
tiation undergo mitotic division to form preleptotene or resting 
spermatocytes, which enter a prolonged meiotic phase of 24 days. 
Spermatogonia do not completely separ

of 24 days. 
Spermatogonia do not completely separate after mitosis (incom­
plete cytokinesis). Groups of spermatogonia remain connected via 
cytoplasmic bridges, forming a syncytium, and undergo meiosis 
and spermiogenesis in synchrony. 
Meiotic Phase
Preleptotene primary spermatocytes contain a diploid comple­
ment of chromosomes (46 chromosomes or 2N, where N is the 
number of haploid chromosomes), and they are the last germ cells 
to undergo DNA synthesis. Preleptotene spermatocytes undergo 

DNA synthesis. Preleptotene spermatocytes undergo 
an initial round of meiotic division (meiosis I), lasting longer than 
2 weeks, to form secondary spermatocytes that contain a haploid 
complement of chromosomes (1N). Secondary spermatocytes, 
which are present for only about 8 hours, undergo a second mei­
otic division (meiosis II) to form haploid spermatids.
Improper segregation of chromosomes (meiotic nondisjunc­
tion) resulting in an abnormal number of chromosomes (aneu­
ploidy) occurs in 0

l number of chromosomes (aneu­
ploidy) occurs in 0.7% of live births and 50% of first-trimester 
abortuses.17,18 Klinefelter syndrome is classically associated with a 
47,XXY karyotype caused by paternal meiotic nondisjunction in 
50% of cases.19,20 
Spermiogenesis
The final phase of spermatogenesis is the maturation of sper­
matids from round to elongated spermatids and then to mature 
spermatozoa; this process, known as spermiogenesis, is fol­
lowed by release of spermatozoa into the lumen of 

lowed by release of spermatozoa into the lumen of seminiferous 
tubules (spermiation). The major changes that occur during 
spermiogenesis include formation of the sperm head with con­
densation of chromosomes (DNA and nucleoproteins) and 
formation of the acrosomal cap, which contains proteolytic 
enzymes needed for sperm penetration of the ovum; formation 
of the sperm tail or flagellum (pointing into the lumen) that 
permits motility; phagocytic removal of excess spermatid cyto­
plasm (known 

ic removal of excess spermatid cyto­
plasm (known as the residual body) by Sertoli cells; and release 
of mature spermatozoa into the lumen. Progressive matura­
tion of spermatids is accompanied by progressive movement of 
more mature spermatids toward the lumen of the seminiferous 
tubule. Under the regulation of FSH and intratesticular tes­
tosterone, Sertoli cells nurture spermiogenesis by supporting 
spermatid maturation. 
Germ Cell Loss
Compared with most other species, the efficiency of sp

ared with most other species, the efficiency of spermatogen­
esis in humans is relatively poor, and the germ cell degeneration 
and loss that occur predominantly during mitosis and meiosis are 
major contributors to the low efficiency of sperm production.21 
Significant degrees of germ cell degeneration occurring during 
meiosis results in a loss of about 60% during formation of sperma­
tids from preleptotene spermatocytes. As men age, increased germ 
cell degeneration may result in lower daily 

germ 
cell degeneration may result in lower daily sperm production. It is 
hypothesized that germ cell degeneration prevents abnormal germ 
cells from further development, thereby serving an important 
quality control function. 
Organization of Spermatogenesis
Histologic examination of a human testis in cross section reveals 
that germ cells at particular phases of development cluster in six 
cellular associations, referred to as stages, which together constitute 
a complete cycle of spermatogen

gether constitute 
a complete cycle of spermatogenesis. In most mammals, stages are 
organized sequentially along the longitudinal axis of the tubules so 
that all of the germ cells present in a cross section of a tubule are in 
the same stage of spermatogenesis.22,23 In contrast, three or more 
different stages of a cycle may variably be present in a cross section 
of human testis. Although some have proposed a helical pattern of 
stages along the tubule to explain this seemingly chaotic arrang

he tubule to explain this seemingly chaotic arrange­
ment, this has not been confirmed by others.
672
In humans, the time for completion of spermatogenesis (from 
Ap spermatogonia to release of mature spermatozoa) is 74 ± 4 
days.24 The epididymal transit time of spermatozoa is approxi­
mately 12 to 21 days.25 Therefore, external insults to the testis (e.g., 
ionizing radiation) or induction of gonadotropin deficiency (e.g., 
by male hormonal contrace

ropin deficiency (e.g., 
by male hormonal contraceptive regimens) that inhibits early germ 
cell development and reduces spermatogenesis are not reflected in 
reduced sperm counts in the ejaculate for 3 months or more. 
Sperm Transport and Fertilization
Mature spermatozoa released into the lumen of the seminiferous 
tubule are transported to the rete testis, to the efferent ducts of the 
testis, and then to the caput epididymis primarily by peristaltic 
contractions and intratubular fluid flow. 

taltic 
contractions and intratubular fluid flow. In the epididymis, sperm 
undergo biochemical and functional modifications that result 
in the capacity for sustained forward motility. After ejaculation 
from the vas deferens and penis into the female reproductive 
tract, human sperm interact with uterine secretory products and 
undergo capacitation in the uterus; the resulting biochemical alter­
ations in the acrosomal cap increase the sperm membrane fluidity, 
calcium influx, and sperm motili

mbrane fluidity, 
calcium influx, and sperm motility (“hyperactivation”) so that the 
spermatozoa acquire the capacity to fertilize an ovum.26,27 After 
capacitation, as the spermatozoon meets an ovum in the ampulla 
of the fallopian tube, the sperm binds to the egg and releases 
hyaluronidase to penetrate the zona pellucida that surrounds the 
ovum, a process known as the acrosome reaction. Fertilization then 
occurs as the plasma membranes of sperm and ovum fuse. 
Phase of
spermatogenesis
Germ

perm and ovum fuse. 
Phase of
Germ cell
type
Ploidy
(N = haploid)
Type of
cell division
Stem cell
renewal
A dark
2N
2N
Mitotic
divisions
1st meiotic
division
2nd meiotic
division
2N
2N
2N
1N
1N
1N
A dark
A pale
Primary
spermatocyte
Secondary
Spermatid
Round
Spermatozoan
B
Elongated
Mitotic or
proliferative
phase
Meiotic phase
Preleptotene
Leptotene
Zygotene
Pachytene
Diplotene
• Fig. 19.3  Schematic diagram of human spermatogenesis. Sperm

 Schematic diagram of human spermatogenesis. Spermatogonial stem cells undergo self-
renewal by mitotic division. At the initiation of spermatogenesis, some spermatogonia undergo differentia­
tion into primary spermatocytes, which contain a diploid number of chromosomes (2N = 46 chromosomes). 
The primary spermatocytes then undergo two successive meiotic divisions to form spermatids, which 
contain a haploid number of chromosomes (1N = 23 chromosomes). Spermatids undergo spermiogenesis 
from rou

omes). Spermatids undergo spermiogenesis 
from round to elongated spermatids to form mature spermatozoa, which also contain a haploid number 
of chromosomes. (Modified from Matsumoto AM. Spermatogenesis. In: Adashi EY, Rock JA, Rosen­
waks Z, eds. Reproductive Endocrinology, Surgery, and Technology. Philadelphia, PA: Lippincott-Raven; 
1996:359–384.)
673
Morphologically, most ejaculated human spermatozoa are com­
posed of an oval-shaped head that cont

oa are com­
posed of an oval-shaped head that contains condensed chromatin 
and nucleoproteins, an acrosomal cap that covers approximately 
the anterior two thirds of the head, a short neck that contains 
centrioles important for attachment of the tail and cleavage of the 
zygote after fertilization, a middle piece that consists of axial fila­
ments surrounded by a spiral of mitochondria containing oxida­
tive enzymes that provide energy for motility, and a long tail or 
flagellum that permits n

lity, and a long tail or 
flagellum that permits normal progressive forward motility of the 
spermatozoa10 (Fig. 19.4). The flagellum consists of a microtu­
bule-based cytoskeleton, the axoneme, which has a characteristic 
structure composed of two central microtubules surrounded by 
nine microtubule doublets (9 × 2 + 2 pattern) that serves as a scaf­
fold for motor protein complexes (i.e., dynein arms).28
In humans, the normal sperm concentration in the ejaculate 
is greater than 15 million/mL,

n in the ejaculate 
is greater than 15 million/mL, with 4% or more having normal 
morphologic appearance by strict criteria (≥30% by previous cri­
teria) and 40% or more having total motility, according to the 
most recent World Health Organization (WHO) criteria.29,30 
Men with infertility may exhibit morphologic abnormalities of the 
sperm head (tapered, amorphous, or double-headed forms) or of 
the tail (coiled forms) and reduced or absent motility. Alterations 
in the structural or functiona

ility. Alterations 
in the structural or functional components of the axoneme (e.g., 
absence of dynein arms) result in altered motility, and deficiency 
of dynein adenosine triphosphatase results in primary ciliary dys­
kinesia (also known as immotile cilia syndrome), a syndrome asso­
ciated with lung disease, recurrent sinusitis, and otitis media, as 
well as male infertility.31 
Interstitium
The interstitial compartment of the testis contains clusters of 
Leydig cells, the primary sex steroid

clusters of 
Leydig cells, the primary sex steroid–producing cells of the tes­
tis, which make up only about 5% of testis volume23,32 (see Fig. 
19.1). Leydig cells produce testosterone, which acts as a paracrine 
regulator within the seminiferous tubules of the testis on Sertoli 
cells in close proximity to stimulate spermatogenesis. Testosterone 
is secreted into adjacent testicular capillaries and then into the 
general circulation to act as an endocrine signal on androgen tar­
get organs thr

n endocrine signal on androgen tar­
get organs throughout the body. Leydig cells also produce insulin-
like factor 3 (INSL3), a peptide hormone in the relaxin-insulin 
family, which plays an important role in the first phase of testicu­
lar descent from the abdomen into the scrotum during develop­
ment.33 INSL3 also may be an important autocrine regulator of 
Leydig cells and a paracrine regulator of germ cells directly.34
Peritubular myoid cells that surround the seminiferous tubules 
are contr

 that surround the seminiferous tubules 
are contractile, smooth muscle-like cells that serve to facilitate 
forward transport of spermatozoa and testicular fluid within the 
tubular lumen, provide structural integrity to the tubule, secrete 
extracellular matrix components and putative regulatory fac­
tors such as growth factors, and are involved in retinol metabo­
lism.23,35 These cells contain androgen receptors (ARs) and are 
thought to mediate some of the paracrine effects of testosterone 

te some of the paracrine effects of testosterone 
on Sertoli cells within the seminiferous tubules, but their precise 
role in human testicular physiology remains unclear.
The interstitial compartment also contains macrophages that 
may regulate Leydig cell steroidogenesis by secretion of cytokines 
and may play a role in phagocytosis of degenerating cells and 
necrotic debris. The interstitium contains arterioles and a rich net­
work of capillaries that permit secretion of testosterone and othe

ies that permit secretion of testosterone and other 
products into the circulation and delivery of circulating gonado­
tropins (e.g., LH and FSH). 
Testis Development
Fetal Development
During embryogenesis, the Y chromosome directs the develop­
ment of the testis from an undifferentiated anlage that has the 
potential to develop into either a testis or an ovary.36 The SRY 
(sex-determining region of the Y chromosome) gene, located on the 
pseudoautosomal region of the Y chromosome, encodes a tra

utosomal region of the Y chromosome, encodes a tran­
scription factor that increases the expression of SRY-box 9 (SOX9), 
which in turn drives the formation of Sertoli cells and testis dif­
ferentiation. SRY gene expression is activated by several factors, 
including steroidogenic factor 1 and the DNA-binding protein 
GATA4.37 SRY-independent SOX9 expression may also be driven 
by steroidogenic factor 1. SOX9 directs the expression of other 
genes that are essential in testis differentiation, su

s that are essential in testis differentiation, such as fibroblast 
growth factor 9 (FGF9) and AMH, and in repression of ovarian 
differentiation, such as WNT4 and NR0B1 (formerly known as 
DAX1). In the absence of SRY or SRY action, SOX9 is repressed 
by many factors, including β-catenin, and development of the fol­
licular cells and ovaries follows.
Primordial germ cells originate in the yolk sac and migrate to 
the genital ridges. Together with coelomic epithelial and mesen­
chymal cells that

h coelomic epithelial and mesen­
chymal cells that eventually differentiate, respectively, into Sertoli 
cells and interstitial cells (Leydig and peritubular myoid cells), 
they form the genital blastema by 6 weeks of gestation. Primor­
dial germ cells that fail to migrate normally explain the location 
of extragonadal germ cell cancers in men. Under the influence 
Acrosomal cap
Head
Neck
Mid-piece
Principal piece
End piece
Tail
• Fig. 19.4  Light photomicrograph of an ejaculated mature human sp

t photomicrograph of an ejaculated mature human sper­
matozoon, composed of a head, neck, mid-piece, and tail (principal piece 
and end piece). (From Matsumoto AM. Spermatogenesis. In: Adashi EY, 
Rock JA, Rosenwaks Z, eds. Reproductive Endocrinology, Surgery, and 
Technology. Philadelphia, PA: Lippincott-Raven; 1996:359–384.)
674
of gene products activated by SRY, primordial germ cells become 
surrounded by primitive Sertoli cells to form seminiferou

ded by primitive Sertoli cells to form seminiferous or sex 
cords that eventually develop into seminiferous tubules.
Under the influence of maternal human chorionic gonadotro­
pin (hCG) initially and later LH and FSH from the fetal pituitary 
gland, immature Leydig cells, Sertoli cells, and germ cells undergo 
differentiation, proliferation, and organization. Testosterone pro­
duction from fetal Leydig cells increases progressively and induces 
development of the epididymis, vas deferens, and se

evelopment of the epididymis, vas deferens, and seminal vesicles 
from wolffian or mesonephric ducts. Conversion of testosterone 
to 5α-dihydrotestosterone (DHT) in the urogenital tract leads to 
the formation of the prostate from the urogenital sinus, the penis 
from the genital tubercles and folds, and the scrotum from the 
urogenital swelling.38 In the absence of testosterone production or 
action, female internal and external genitalia develop. AMH secre­
tion from the fetal Sertoli cells ca

p. AMH secre­
tion from the fetal Sertoli cells causes regression of the müllerian 
or paramesonephric ducts and prevents the formation of a uterus 
and fallopian tubes.
Male phenotypic development is complete by about 15 weeks 
of gestation, after which the proliferation of Sertoli and germ cells 
arrests. The Leydig cells produce testosterone in the third trimester 
when fetal gonadotropin secretion rises, and this phase of andro­
gen action is required for normal penis size at birth. 
Testis 

 required for normal penis size at birth. 
Testis Descent
The developing testis is attached to the diaphragm by the cranio­
suspensory ligament and anchored to the inguinal region by a 
caudal ligament known as the gubernaculum. Descent of the testis 
occurs in two phases.33 During the initial transabdominal phase, 
the testis descends within the abdomen to the inguinal region; this 
occurs between 10 and 23 weeks of gestation. Studies in animals 
suggest that testis descent during this phase de

suggest that testis descent during this phase depends on two pro­
cesses: (1) regression of the craniosuspensory ligament, induced 
by testosterone, which frees the testes to descend, and (2) thick­
ening of the gubernaculum, which is controlled by INSL3 and 
its cognate receptor, relaxin-family peptide receptor 2 (RXFP2, 
also known as leucine-rich repeat–containing G protein–coupled 
receptor 8 or G protein–coupled receptor affecting testis descent). 
During the inguinoscrotal phase, which b

escent). 
During the inguinoscrotal phase, which begins at 26 to 28 weeks 
of gestation, the testis descends into the scrotum; this process is 
largely controlled by the effects of testosterone on gubernacular 
shortening and contractions. The effects of testosterone may be 
mediated in part by the neurotransmitter, calcitonin gene–related 
peptide, which is released by the genitofemoral nerve. The impor­
tance of testosterone, gonadotropins, and INSL3 in testis descent 
in humans is suggested b

 INSL3 in testis descent 
in humans is suggested by the occurrence of undescended tes­
tes (cryptorchidism) associated with fetal androgen deficiency 
or resistance, gonadotropin deficiency, and INSL3 or RXFP2 
mutations.34
Testis descent is usually complete, with the testes entirely 
within the scrotum, by 7 months of gestation to birth. During 
testis descent, a herniation of the abdominal cavity, the processus 
vaginalis, develops along the course of the gubernaculum, forming 
the inguinal ri

urse of the gubernaculum, forming 
the inguinal ring and canal and descending with the testis into the 
scrotum. As the abdominal wall and muscles develop, the ingui­
nal rings close, and the processus vaginalis obliterates to form the 
tunica vaginalis, which covers the anterior and lateral portion of 
the testes. Incomplete closure of the inguinal ring predisposes an 
individual to inguinal hernia, and incomplete obliteration of the 
processus vaginalis with accumulation of serous fluid result

vaginalis with accumulation of serous fluid results in a 
hydrocele; either of these conditions can manifest as a scrotal mass. 
Postnatal Development
During late gestational life, the male fetus is exposed to high con­
centrations of maternal estrogens from the placenta. With the 
decline in estrogen concentrations after birth, the hypothalamic-
pituitary-testicular axis is released from negative feedback suppres­
sion, resulting in a postnatal surge of gonadotropin that stimulates 
the testes 

surge of gonadotropin that stimulates 
the testes to produce testosterone and inhibin B.
LH and testosterone concentrations begin to increase at about 
1 week of life and peak 1 to 2 months later, reaching the equiva­
lent of adolescent concentrations in association with an increase 
in the number of Leydig cells; LH and testosterone then decline 
to prepubertal concentrations by about 6 months of age. Infants 
with complete androgen insensitivity who lack androgen action 
due to an AR mutation 

y who lack androgen action 
due to an AR mutation (see later discussion) do not demonstrate 
a postnatal gonadotropin surge and have low or undetectable LH 
and testosterone concentrations postnatally, suggesting that AR 
expression is required for the surge.39 In humans, there is no evi­
dence that the postnatal surge of LH and testosterone has an effect 
on adult Leydig cell function. However, in infants with hypogo­
nadotropic hypogonadism, the lack of third trimester fetal gonad­
otropins an

e lack of third trimester fetal gonad­
otropins and the neonatal surge in testosterone play an important 
role in the development of micropenis and cryptorchidism.40,41 In 
infants with micropenis, postnatal hormone testing may permit 
early identification and possible treatment of isolated hypogonad­
otropic hypogonadism or hypopituitarism.
FSH and inhibin B concentrations also begin to rise at 1 week 
of life, in association with an increase in proliferation of Sertoli 
cells, and peak at 3 mo

 proliferation of Sertoli 
cells, and peak at 3 months. FSH declines to prepubertal con­
centrations by 9 months of age. Because Sertoli cells continue to 
proliferate for some time after FSH declines, the inhibin B con­
centration declines more slowly and plateaus at approximately 
15 months of age. Sertoli cell number determines spermatogenic 
potential, and the postnatal gonadotropin surge is likely impor­
tant for quantitatively normal sperm production in adults. The 
postnatal testosterone 

production in adults. The 
postnatal testosterone surge also increases the formation of Ad 
spermatogonia (spermatogonial stem cells) from gonocytes during 
the first 3 months and increases testis size and seminiferous tubule 
length during the first year of life, providing further evidence for 
the importance of the gonadotropin surge on normal spermato­
genesis and fertility as an adult. In male infants with congenital 
gonadotropin deficiency, the lack of a postnatal surge in serum 
gonadotro

 the lack of a postnatal surge in serum 
gonadotropin concentrations results in lower numbers of Sertoli 
cells and spermatogonia. The lack of a postnatal gonadal surge 
explains why gonadotropin replacement therapy fails to produce 
normal sperm counts in the majority of men with congenital forms 
of hypogonadotropic hypogonadism who are treated as adults. 
Pubertal Development
At the onset of puberty, reactivation of hypothalamic GnRH 
secretion stimulates pituitary LH and FSH, initially only 

n stimulates pituitary LH and FSH, initially only dur­
ing nocturnal sleep (Fig. 19.5) and subsequently throughout the 
day.42,43 Increased circulating LH increases Leydig cell secretion 
of testosterone that induces male secondary sexual characteris­
tics. Increased serum LH and intratesticular testosterone concen­
trations together with increased FSH concentrations stimulate 
Sertoli cells to initiate spermatogenesis. With increasing germ 
cell numbers and expansion of seminiferous tubules, te

 numbers and expansion of seminiferous tubules, testis size 
increases progressively. Increase in testis size is the first clinical sign 
of puberty. In addition, with the release of mature spermatozoa 
into the lumen of seminiferous tubules and transport of sperm to 
675
the genitourinary tract, sperm begin to appear in the urine (sper­
marche) during early puberty (usually at 12–15 years of age).44 
Adult Physiology
Hypothalamic-Pituitary-Testicular Axis
The te

logy
The testis is controlled by classic positive feed-forward and nega­
tive feedback mechanisms (Fig. 19.6). The major positive regula­
tors of testis function are LH and FSH, which are synthesized 
and secreted from the anterior pituitary gland. Secretion of LH 
and, to a lesser extent, FSH, is pulsatile and driven primarily by 
episodic release of GnRH from neurons in the hypothalamus.45 
GnRH stimulates gonadotropin-producing cells of the anterior 
pit

 gonadotropin-producing cells of the anterior 
pituitary (gonadotrophs) to secrete LH and FSH every 90 to 120 
minutes in men.
LH acts on Leydig cells of the testes to stimulate production 
of testosterone, the main sex steroid hormone in males. In concert 
with FSH, testosterone acts locally on Sertoli cells within the sem­
iniferous tubules of the testes to initiate and maintain spermato­
genesis. Testosterone secreted into the circulation acts to mediate 
and promote androgen action on almost

 to mediate 
and promote androgen action on almost every tissue in the body, 
including negative feedback inhibition of pituitary LH and FSH 
secretion (primarily via conversion to estradiol) and suppression of 
GnRH production by the hypothalamus. FSH stimulates Sertoli 
cells to produce inhibin B, a peptide hormone that causes nega­
tive feedback inhibition of FSH secretion by the anterior pituitary.
Knowledge of the hypothalamic-pituitary-testicular axis is 
essential in understanding the cau

icular axis is 
essential in understanding the causes, classification, differen­
tial diagnosis, clinical consequences, and treatment of testicular 
disorders. 
Central Nervous System Regulation of 
Gonadotropin-Releasing Hormone Secretion
The brain plays a vital role in regulation of the testis and of 
reproductive function through production of the decapeptide, 
gonadotropin-releasing hormone (GnRH), by a relatively small 
number of neurons located primarily in the arcuate nucleus of 
the medi

ated primarily in the arcuate nucleus of 
the medial basal hypothalamus. GnRH is released episodically 
from axon terminals in the median eminence into capillaries of 
the hypothalamic hypophyseal portal system, through which it 
is carried to the anterior pituitary to stimulate synthesis and the 
release of LH and FSH. Like all hypothalamic trophic hormones, 
the concentration of GnRH in portal blood is low. As a result, 
GnRH concentration in peripheral blood is very low and can­
not be measur

eripheral blood is very low and can­
not be measured for clinical purposes (e.g., to diagnose GnRH 
300
200
100
0
(ng/100 mL)
LH (mIU/mL)
2200
0200
0600
1000
1400
1800
2200
Clock time
Start
Finish
Awake
REM
I
II
III
IV
16
14
12
10
8
6
4
Start
Finish
• Fig. 19.5  Sleep-associated secretion of luteinizing hormone (LH) (middle) and testosterone (bottom) 
related to sleep stage (top) in a boy entering puberty. REM, rapid eye movement sleep. (From Boyar RM, 
Rose

M, rapid eye movement sleep. (From Boyar RM, 
Rosenfeld RS, Kapen S, et al. Human puberty: simultaneous augmented secretion of luteinizing hormone 
and testosterone during sleep. J Clin Invest. 1974;54:609–618.)
676
The precise mechanism of synchronous episodic release of 
GnRH from many separate GnRH neurons into the hypophy­
seal portal system to provide pulsatile stimulation of pituitary 
gonadotrophs is not clear. There is evidence that pulse gene

hs is not clear. There is evidence that pulse generation 
is a direct consequence of intrinsic periodicity of GnRH or other 
neurons that synapse on GnRH neurons within the medial basal 
hypothalamus, such as neurons containing kisspeptin or excit­
atory or inhibitory neurotransmitters.
The frequency of pulsatile GnRH secretion is temporally cou­
pled with the episodic release of LH, free α-subunit (co-secreted 
with intact gonadotropins), and FSH.46 Because the half-lives of 
LH and free α-subu

H.46 Because the half-lives of 
LH and free α-subunit in the circulation are shorter than that 
of FSH, concentrations of LH and α-subunit demonstrate dis­
crete pulses, which are evident with frequent blood sampling (e.g., 
every 10 minutes for 12–24 hours), whereas FSH pulses are not 
as apparent. The frequency of LH or free α-subunit pulses reflects 
GnRH pulse frequency and serves as an indicator of synchronous 
GnRH neuronal activity (pulse generation) in the brain. The 
amplitude of LH or 

generation) in the brain. The 
amplitude of LH or free α-subunit pulses reflects both the ampli­
tude of GnRH pulses and the responsiveness of the gonadotrophs 
to GnRH stimulation. Normal men usually demonstrate 12 to 16 
LH pulses of varying amplitude over 24 hours (Fig. 19.7). In men 
with GnRH deficiency (congenital hypogonadotropic hypogo­
nadism [CHH], also referred to as idiopathic hypogonadotropic 
hypogonadism [IHH]), there is an absence of LH pulses (most 
commonly) or abnormalities in

 of LH pulses (most 
commonly) or abnormalities in LH pulsatility.47
Treatment of men who have GnRH deficiency with low-dose 
pulsatile GnRH normalizes LH and FSH secretion and testicular 
function. In contrast, continuous low-dose GnRH administration 
does not stimulate normal gonadotropin secretion in these men.48 
Administration of potent GnRH receptor agonists providing con­
tinuous, high-dose GnRH stimulation of the pituitary initially 
GnRH
GnRH
Inhibin B
LH
LH
T
T
T
E2
E2
DHT

GnRH
Inhibin B
LH
LH
T
T
T
E2
E2
DHT
AR
ER
Leydig cell
FSH
Interstitial
Sexual development
Skin and hair
Libido and erectile
function
Sperm production
Bone and muscle
Behavior and cognition
Gonadotropin secretion
Seminiferous
tubule
Sertoli cell
Germ
cells
Neuromodulators
Pulse generator
BRAIN
Androgen actions
Pituitary
Testis
• Fig. 19.6  Schematic diagram of the hypothalamic-pituitary-testicular axis. Hypothalamic gonadotro­
pin-releasing hormone (GnRH) st

halamic gonadotro­
pin-releasing hormone (GnRH) stimulates the pituitary to produce luteinizing hormone (LH) and follicle-­
stimulating hormone (FSH). LH stimulates (solid lines) Leydig cells of the testes to produce testosterone 
(T), which is actively metabolized to estradiol (E2) and dihydrotestosterone (DHT), which interact with 
androgen receptor (AR) and estrogen receptor (ER), respectively, resulting in several direct and indirect 
androgen actions. FSH stimulates Sertoli cells of the tes

n actions. FSH stimulates Sertoli cells of the testes, which, together with LH-stimulated testoster­
one, increases spermatogenesis. LH-stimulated testosterone and E2 exert negative feedback suppression 
(dashed lines) of GnRH production at the hypothalamus and LH and FSH production at the pituitary, and 
FSH-stimulated inhibin B exerts negative feedback suppression on FSH secretion by the pituitary. (Modi­
fied from Matsumoto AM. The testis. In: Felig P, Frohman LA, eds. Endocrinology and Metab

 Felig P, Frohman LA, eds. Endocrinology and Metabolism. 4th ed. 
New York, NY: McGraw-Hill; 2001:635–705.)
677
stimulates but then downregulates and profoundly suppresses 
gonadotropin secretion and testosterone production. This effect 
has been the basis for the use of potent GnRH agonists to produce 
medical castration (androgen deprivation therapy) in men with 
advanced prostate cancer. These findings underscore the critical 
importance of pulsatile GnRH cont

re the critical 
importance of pulsatile GnRH control of reproductive function 
in men.
GnRH neurons receive many excitatory and inhibitory inputs 
from other brain regions (e.g., from kisspeptin neurons), as well 
as feedback signals from the testes and other circulating endocrine 
signals. Therefore, the GnRH neuronal system serves an important 
integrative role in the regulation of reproductive and testis func­
tion. A large and complex ensemble of neuroregulators mediates 
GnRH secretion, ac

le of neuroregulators mediates 
GnRH secretion, acting directly on GnRH neurons themselves or 
indirectly on other neurons that in turn regulate GnRH neurons 
to stimulate or inhibit GnRH secretion. These central nervous 
system (CNS) neuromodulatory systems, together with periph­
eral endocrine regulators, provide the means by which GnRH 
secretion and testicular function may be altered by environmental 
factors such as stress (e.g., via corticotropin-releasing hormone, 
glucocorticoids), nutri

tropin-releasing hormone, 
glucocorticoids), nutritional compromise (e.g., via leptin), and 
medications (e.g., opioid drugs).
During embryogenesis, GnRH and olfactory neurons originate 
outside the CNS in the olfactory placode and migrate together 
along olfactory axons through the cribriform plate of the ethmoid 
bone to the olfactory bulb, where GnRH neurons diverge and 
continue to migrate to the medial basal hypothalamus.49 Abnor­
malities in the development of the olfactory bulbs and migra

n the development of the olfactory bulbs and migra­
tion of these neurons explain the association between CHH due 
to GnRH deficiency and the loss or impairment of the sense of 
smell (anosmia or hyposmia, respectively) that occurs in patients 
with Kallmann syndrome. Loss-of-function mutations occur in 
Normal male
A
5
4
3
2
1
0
LH (mIU/mL) LER 907
0
6
12
18
24
*
*
*
*
*
*
*
*
* **
*
*
*
*
*
*
* *
*
*
*
*
*
Hours
T = 1091 ng%
K.H.
36
Subset 1: Apulsatile pattern of LH release
B
5
4
3
2
1
0
LH (

pulsatile pattern of LH release
B
5
4
3
2
1
0
0
6
12
18
24
Hours
T = 72 ng%
E.B.
20
Subset 2: Sleep-entrained pattern
C
5
4
3
2
1
0
0
6
12
18
24
Hours
T = 119 ng%
Sleep
V.M.
19
Subset 3: Decreased amplitude pattern
D
5
4
3
2
1
0
0
6
12
18
24
Hours
T = 36 ng%
P.H.
27
• Fig. 19.7  Endogenous pulsatile secretion of luteinizing hormone (LH) in a normal man (A) and in men with 
idiopathic hypogonadotropic hypogonadism (B–D), assessed by bloo

ogonadotropic hypogonadism (B–D), assessed by blood sampling every 20 minutes for 24 
hours. In normal men (A), discrete pulses (*) of LH occur approximately every 2 hours, reflecting pulsatile 
release of gonadotropin-releasing hormone from the hypothalamus, and stimulate normal adult concentra­
tions of testosterone (T). Most men with idiopathic hypogonadotropic hypogonadism (B) demonstrate no 
detectable LH pulses and have prepubertal testosterone concentrations. Others exhibit primarily slee

rone concentrations. Others exhibit primarily sleep-
entrained LH pulses of reduced amplitude without significant LH pulsatility during the waking hours (C) or 
LH pulses of reduced amplitude throughout sleep and waking hours (D) with pubertal or prepubertal tes­
tosterone concentrations, respectively. (From Santoro N, Filicori M, Crowley WF Jr. Hypogonadotropic dis­
orders in men and women: diagnosis and therapy with pulsatile gonadotropin-releasing hormone. Endocr 
Rev. 1986;7:11–23.)
678
SECT

sing hormone. Endocr 
678
genes that play important roles in the migration and embryologic 
development of GnRH neurons, such as the genes for Kallmann 
syndrome 1 (ANOS1), KAL2 (now called fibroblast growth fac­
tor receptor 1 [FGFR1]), and prokineticin receptor 2 (PROKR2) 
and its ligand, prokineticin 2 (PROK2). Similarly, genes that are 
important in the regulation of GnRH neurons may sustain loss-
of-function mutations; example

s may sustain loss-
of-function mutations; examples include the genes for kisspeptin 
1 receptor (KISS1R [formerly GPR54]) and its ligand, kisspeptin 
1 (KISS1), also called metastin; the neurokinin B (tachykinin 3) 
receptor (TACR3) and its ligand (TAC3); and the GnRH recep­
tor (GNRHR) and its ligand (GNRH). All of these defects result 
in isolated GnRH deficiency associated with impaired pubertal 
development, often in combination with anosmia or hyposmia or 
other morphologic defects.50 
GnR

 or hyposmia or 
GnRH Regulation of Gonadotropin Secretion
GnRH released from the hypothalamus into the hypophyseal 
portal system binds to G protein–coupled GnRH receptors on 
anterior pituitary gonadotrophs.51 In humans, GnRH recep­
tors are coupled primarily to Gq/11 proteins, which activate 
phospholipase C-β to produce 1,2-diacylglycerol and inositol 
1,4,5-trisphosphate (IP3). Diacylglycerol activates protein kinase 
C (PKC), and IP3 mobilizes intracellular ca

inase 
C (PKC), and IP3 mobilizes intracellular calcium, which binds 
to the calcium-binding protein, calmodulin. Both PKC and 
calmodulin-dependent kinases phosphorylate and activate sev­
eral transcription factors, resulting in increased synthesis of the 
gonadotropin subunits LHβ, FSHβ, and the common α-subunit 
and release of intact LH and FSH and free α-subunits into the 
circulation. The GnRH receptor may also be coupled to Gs pro­
tein, which activates protein kinase A (PKA), resulting in

ich activates protein kinase A (PKA), resulting in the syn­
thesis and release of gonadotrophs.
LH and FSH, together with another anterior pituitary hor­
mone, thyroid-stimulating hormone (TSH), and a placental hor­
mone, hCG, are members of the glycoprotein hormone family. 
Glycoprotein hormones are heterodimers in which a common 
α-subunit is noncovalently linked to a unique β-subunit; this 
structure confers their ability to bind to their cognate receptors 
and their biologic specificity. In 

ate receptors 
and their biologic specificity. In the pituitary gonadotroph, the 
common α-subunit and the LHβ- and FSHβ-subunits are prod­
ucts of different genes that are synthesized and regulated differen­
tially.52 After the subunits are synthesized, an α-subunit combines 
noncovalently with either an LHβ- or FSHβ-subunit. After trans­
lation, the heterodimer undergoes variable glycosylation wherein 
oligosaccharide chains (glycans) are attached covalently to specific 
amino acids, resulting

hed covalently to specific 
amino acids, resulting in LH and FSH molecules with a high 
degree of microheterogeneity (i.e., many LH and FSH isoforms 
characterized by different glycosylation patterns). The gonado­
tropin α-subunit is produced in excess relative to the LHβ- and 
FSHβ-subunits; it too is glycosylated, and free α-subunit is co-
secreted into the circulation with LH and FSH. Many nonfunc­
tional and gonadotropin-secreting pituitary adenomas secrete 
excessive amounts of free α-subun

denomas secrete 
excessive amounts of free α-subunit into the circulation.53
The degree of glycosylation of gonadotropins and other gly­
coprotein hormones alters their clearance rate from the circula­
tion and their signal transduction after receptor binding, thereby 
affecting their biologic activity in vivo. The half-life in the circula­
tion of gonadotropins increases with greater degrees of glycosyl­
ation: hCG > FSH > LH > free α-subunit. In humans, the initial 
half-life of disappearance 

n humans, the initial 
half-life of disappearance for LH is about 40 minutes, and the sec­
ondary half-life of disappearance is about 120 minutes; for FSH, 
these periods are approximately 4 and 70 hours, respectively.54,55 
Variations in glycosylation of LH and FSH result in substantial 
microheterogeneity among circulating gonadotropin isoforms, 
which vary in half-life and biologic activity and may be altered 
by particular physiologic conditions, such as during puberty, with 
aging, and with

ons, such as during puberty, with 
aging, and with androgen deprivation.
Clinically, serum LH and FSH concentrations are measured 
with rapid, nonradioactive, highly sensitive immunoassays that 
use monoclonal antibodies recognizing two separate epitopes on 
the gonadotropin molecule. Gonadotropin measurements are 
essential in the evaluation of men with hypogonadism to dis­
tinguish those who have a primary testicular disorder (primary 
hypogonadism, in which gonadotropins are high) from those 

dism, in which gonadotropins are high) from those who 
have a secondary hypothalamic or a pituitary disorder (secondary 
hypogonadism, in which gonadotropins are low or normal). Spe­
cific immunoassays for free α-subunit are used to diagnose and 
monitor patients with nonfunctional and gonadotropin-secreting 
pituitary adenomas. 
Gonadotropin Control of Testicular Function
LH Regulation of Leydig Cells
Circulating LH binds to the G protein–coupled receptor for LH 
and hCG (termed LHCGR) on the s

d receptor for LH 
and hCG (termed LHCGR) on the surface of Leydig cells, result­
ing in receptor aggregation and a conformational change that acti­
vates Gs protein. Gs protein, in turn, results primarily in cyclic 
adenosine monophosphate (cAMP)-dependent activation of 
PKA.56 Activated PKA increases the production of proteins that 
regulate steroidogenesis and testosterone biosynthesis (Fig. 19.8). 
The main proteins regulated by LH-stimulated PKA are as follows:
	1.	 Steroidogenic acute reg

 PKA are as follows:
	1.	 Steroidogenic acute regulatory protein (StAR), a transport pro­
tein that regulates transfer of cholesterol from the outer to the 
inner mitochondrial membrane—the rate-limiting step for ste­
roid production
	2.	 Cytochrome P450 isoenzyme 11A1 (P450 11A1), also called cho­
lesterol side-chain cleavage enzyme, within the inner mitochon­
drial membrane, which catalyzes the conversion of cholesterol 
delivered by StAR protein to pregnenolone—the first and rate-
limiting 

tein to pregnenolone—the first and rate-
limiting enzymatic step in steroidogenesis
	3.	 P450 17A1, also called 17α-hydroxylase/17,20-lyase in the endo­
plasmic reticulum, which catalyzes the conversion of pregneno­
lone to 17α-hydroxypregnenolone—the second enzymatic step 
in testosterone biosynthesis.57
In humans, cholesterol is synthesized within Leydig cells from 
acetate by 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) 
reductase, or it is derived from circulating low-density lipoprotein

s derived from circulating low-density lipoprotein 
(LDL) cholesterol.
Clinically, rare inactivating mutations of the LHCGR cause 
Leydig cell hypoplasia, resulting in impaired male genital devel­
opment and 46,XY disorder of sex development (46,XY DSD, 
previously known as male pseudohermaphroditism; see Chap­
ter 23) resulting from insufficient testosterone production dur­
ing fetal development.58 Rare LHβ mutations cause failure of 
normal male pubertal development with micropenis but oth­
er

e pubertal development with micropenis but oth­
erwise normal genital development at birth, evidence that nor­
mal endogenous LH secretion is not required for male sexual 
differentiation during fetal development and that hCG stim­
ulation of testosterone production by fetal Leydig cells is the 
main driver.59 Activating mutations of the LHCGR have been 
found in boys with familial precocious puberty (testotoxicosis).60 
Inhibitors of HMG-CoA reductase (statins) used to treat hyper­
cholesterole

uctase (statins) used to treat hyper­
cholesterolemia do not significantly affect serum testosterone 
concentrations. 
679
Leydig Cell Production of Testosterone and INSL3
In the human testis, LH-stimulated transport of cholesterol into 
the inner mitochondrial membrane is followed by conversion of 
cholesterol to pregnenolone by CYP11A1 and conversion of preg­
nenolone to 17α-hydroxypregnenolone by CYP17A1. Testoster­
one biosynthesis then proceeds via a series 

ster­
one biosynthesis then proceeds via a series of further enzymatic 
steps in the endoplasmic reticulum, initially through the Δ5 ste­
roid biosynthesis pathway57,61 (see Fig. 19.8).
The 17,20-lyase (desmolase) activity of P450 17A1 catalyzes the 
further conversion of 17α-hydroxypregnenolone to DHEA. DHEA 
then is converted to Δ5-androstanediol by 17β-hydroxysteroid 
dehydrogenase 3 (17β-HSD3 or HSD17B3). DHEA and Δ5-
androstanediol are converted to the Δ4 steroids, Δ4-androstenedione 
and t

rted to the Δ4 steroids, Δ4-androstenedione 
and testosterone, respectively, by the enzyme 3β-hydroxysteroid 
dehydrogenase/Δ5-4 isomerase type 2 (3β-HSD2 or HSD3B2).
The early steroid precursors, pregnenolone and 17α-
hydroxypregnenolone, may also be converted to progesterone 
and 17α-progesterone, respectively, by HSD3B2 and then pro­
ceed down the Δ4 pathway to testosterone synthesis. However, 
in the human testis, the Δ5 pathway is the predominant steroid 
biosynthetic pathway for testostero

inant steroid 
biosynthetic pathway for testosterone production. Testosterone 
may be converted in the testes to the active metabolites, estra­
diol and DHT, by the enzymes aromatase (P450 19A1) and steroid 
5α-reductase type 1 (SRD5A1, the predominant isoform found in 
the testes), respectively.
Mutations in testosterone biosynthetic enzymes result in 
abnormalities of sexual differentiation and varying degrees of 
46,XY DSD, depending on the severity of androgen deficiency.62
Testosterone is t

verity of androgen deficiency.62
Testosterone is the major androgen produced by the testis. In 
humans, the average secretion rate of testosterone is approximately 
7000 μg/day. The testes also secrete significant but quantitatively 
smaller amounts of 17α-hydroxyprogesterone, pregnenolone, Δ4-
androstenedione, and progesterone. Very little estradiol (about 10 
μg/day) or DHT (about 69 μg/day) is secreted by the testes9,63 
(Fig. 19.9).
In response to pulsatile LH stimulation, testosterone is se

se to pulsatile LH stimulation, testosterone is secreted 
episodically into the spermatic vein and then into the general cir­
culation. However, testosterone pulses are less discrete, relatively 
low in amplitude, and concordant with LH pulses only after a 
lag time of 80 to 120 minutes, suggesting a relatively sluggish 
response of Leydig cells to LH stimulation.45 In addition to this 
ultradian variation, testosterone concentrations in young men 
exhibit a circadian variation characterized by 

n 
exhibit a circadian variation characterized by a maximum excur­
sion of 140 ng/dL, with peak testosterone concentrations occur­
ring at approximately 8 am and nadir concentrations at 8 pm.64 
The circadian variation in testosterone concentrations is blunted 
Endoplasmic
reticulum
∆4 Pathway
∆5 Pathway
Plasma cholesterol
StAR
CYP17
CYP17
CYP17
CYP17
17 OH-pregnenolone
DHEA
Androstenediol
First
side-
chain
cleavage
Second
side-
chain
cleavage
Reduction
of 17

eavage
Second
side-
chain
cleavage
Reduction
of 17-keto
group
A ring
oxidation
17 OH-progesterone
Cholesterol esters
Acetate
17
β
-HSD3
17
β
-HSD3
3
β
-HSDII
3
β
-HSDII
CYP11A1
Mitochondria
• Fig. 19.8  Testosterone biosynthetic pathways in the Leydig cell of the 
human testis. Cholesterol may be synthesized de novo from acetate within 
the Leydig cell or derived from hydrolysis of cholesterol esters or circu­
lating cholesterol. Cholesterol is transport

ircu­
lating cholesterol. Cholesterol is transported into the inner mitochondrial 
membrane by steroidogenic acute regulatory protein (StAR); there it is 
converted to pregnenolone by the cholesterol side-chain cleavage enzyme 
P450 11A1 (CYP11A1). Biosynthesis of testosterone in the human tes­
tis proceeds predominantly through the Δ5 pathway in the endoplasmic 
reticulum, where pregnenolone is converted to 17-hydroxypregnenolone 
and then to dehydroepiandrosterone (DHEA) by 17α-hydroxylase/17,

hydroepiandrosterone (DHEA) by 17α-hydroxylase/17,20 
lyase P450 17A1 (CYP17), which is converted to androstenediol by 
17β-hydroxysteroid dehydrogenase type 3 (17β-HSD3 or HSD17B3) 
and then to testosterone by 3β-hydroxysteroid dehydrogenase type II 
(3β-HSDII or HSD3B2). In the Δ4 pathway, pregnenolone is converted suc­
cessively to 17-hydroxyprogesterone, androstenedione, and testosterone. 
(Modified from Bhasin S. Testicular disorders. In: Kronenberg HM, Melmed 
S, Polonsky KS, et al, eds. W

onenberg HM, Melmed 
S, Polonsky KS, et al, eds. Williams Textbook of Endocrinology, 11th ed. 
Philadelphia, PA: Elsevier; 2008:645–698.)
Testicular steroid secretion rates
7000
5000
3000
1000
500
Micrograms/24 hours
T
17-OH
P
∆5
P
∆4
A
P
DHT
• Fig. 19.9  Relative steroid hormone secretion rates from the human testis. 
Secretion rates were calculated from arteriovenous (AV) differences across 
the testis of testosterone and other steroids, assuming a testosterone 
secretion rate of 7000 μg per 2

ng a testosterone 
secretion rate of 7000 μg per 24 hours and using the following formula: 
Secretion rate = Assumed testosterone secretion rate/Testosterone AV 
difference × AV difference of steroid in question. Testosterone (T) is the 
main steroid secreted by the testis; much lower amounts of 17-hydroxy­
progesterone (17-OHP), pregnenolone (Δ5P), androstenedione (Δ4A), pro­
gesterone (P), and dihydrotestosterone (DHT) are also secreted. (From 
Hammond GL, Ruokonen A, Kontturi M, et al. The si

Hammond GL, Ruokonen A, Kontturi M, et al. The simultaneous radioim­
munoassay of seven steroids in human spermatic and peripheral venous 
blood. J Clin Endocrinol Metab. 1977;45:16–24.)
680
but still present in older men, with a maximum excursion of 60 
ng/dL. The testosterone response to hCG (LH-like) stimulation 
is greater in the morning than in the evening, suggesting that 
diurnal variation in Leydig cell responsiveness may contribute to 
the c

eydig cell responsiveness may contribute to 
the circadian variation in testosterone concentrations.65 Both the 
ultradian and the circadian variation in testosterone concentra­
tions contribute to the variability in testosterone measurements 
within an individual; together with assay variability, this under­
scores the importance of repeating testosterone measurements 
during the clinical evaluation of patients with clinical manifesta­
tions of male hypogonadism.
INSL3 is a peptide hormone in t

male hypogonadism.
INSL3 is a peptide hormone in the relaxin-insulin family that 
is produced by Leydig cells and secreted into the circulation.66 
Serum concentrations of INSL3 reflect the number and differ­
entiation status of Leydig cells. During puberty, LH induces pro­
liferation and differentiation of Leydig cells and production of 
INSL3. Serum INSL3 concentrations increase progressively dur­
ing puberty, reaching adult concentrations at about 18 years of age 
and remaining stable until 3

bout 18 years of age 
and remaining stable until 35 to 40 years of age, after which they 
decline steadily with age.
Men with anorchia and bilateral orchidectomy, in whom Ley­
dig cells are absent, and men with chronic gonadotropin suppres­
sion induced by GnRH analogues or androgens have undetectable 
or very low concentrations of INSL3. In men with hypogonado­
tropic hypogonadism, INSL3 concentrations are undetectable; in 
these patients, hCG (LH-like) stimulation increases serum testos­
teron

LH-like) stimulation increases serum testos­
terone concentrations within 72 to 96 hours but has no stimu­
latory effect on INSL3 concentrations. However, chronic hCG 
treatment, presumably because of induction of Leydig cell differ­
entiation by longer-term LH-like stimulation, increases both tes­
tosterone and INSL3 concentrations in these men.67 In men with 
unilateral orchidectomy, INSL3 concentrations are intermediate 
between those in men with bilateral orchidectomy and those in 
normal me

ith bilateral orchidectomy and those in 
normal men, but testosterone concentrations are normal, support­
ing the importance of Leydig cell number for circulating INSL3 
FSH and Testosterone Regulation of Sertoli Cells
Circulating FSH binds to G protein–coupled FSH receptors on 
the surface of Sertoli cells, activating Gs protein that increases 
cAMP production.68 cAMP then activates PKA and other sig­
nal-transduction proteins (e.g., phosphatidylinositol 3-kinase, 
phospholipas

e.g., phosphatidylinositol 3-kinase, 
phospholipase A2, calcium channel proteins, mitogen-activated 
protein kinase [MAPK]). Activated PKA activates many proteins, 
including the transcription factor, cAMP response element–binding 
protein; these proteins, in turn, regulate gene expression and pro­
duction of Sertoli cell proteins that play important roles in sup­
porting and regulating spermatogenesis within the seminiferous 
tubules. In rodents, the expression of FSH receptors in Sertoli cells

, the expression of FSH receptors in Sertoli cells 
varies cyclically with the stage of spermatogenesis, being highest 
in stages XIII through I and lowest in stages VII and VIII.69 The 
precise roles of Sertoli cell products in human spermatogenesis 
are poorly understood and derive from studies using Sertoli cells 
obtained primarily from immature animals, mostly rodents.
Testosterone produced locally by Leydig cells binds to intracellu­
lar ARs in the cytoplasm of Sertoli cells; ligand-bound 

s in the cytoplasm of Sertoli cells; ligand-bound AR translo­
cates to the nucleus, where it binds to androgen response elements 
and interacts with coregulator proteins to regulate gene expression 
and production of Sertoli cell proteins that play vital roles in sup­
porting and regulating spermatogenesis. The expression of the AR 
also varies cyclically with the stage of spermatogenesis, being high­
est in stage VII, when FSH receptor expression is the lowest.70
The major functions of Sertoli 

n is the lowest.70
The major functions of Sertoli cells71,72 are (1) to maintain 
seminiferous tubule structure and compartmentalization; (2) to 
provide nutrients and growth factors to developing germ cells 
and spermatozoa; (3) to translocate, sculpt, and release develop­
ing germ cells; (4) to secrete seminiferous tubule fluid; and (5) to 
produce reproductive hormones.
Maintenance of Seminiferous Tubule Structure and 
Compartmentalization
The blood-testis barrier is formed by basal tight jun

 blood-testis barrier is formed by basal tight junctions between 
adjacent Sertoli cells; these serve to compartmentalize the seminif­
erous tubule into basal and adluminal compartments. Compart­
mentalization provides an environment in which developing germ 
cells are protected from external insults and the immune system.
Sertoli cells produce many junctional complex, structural, and 
extracellular matrix proteins, such as cell adhesion molecules (e.g., 
claudin 3, which is particularly importa

s (e.g., 
claudin 3, which is particularly important for the integrity of Ser­
toli cell tight junctions), cadherins, laminins, type I and type IV 
collagen, and proteoglycans including chondroitin and heparin. 
These proteins are important in maintaining the structural integ­
rity and support for developing germ cells, forming the blood-
testis barrier, mediating cell-to-cell interactions, and maintaining 
polarized secretion of products by Sertoli cells. 
Provision of Nutrients and Growth Fact

oli cells. 
Provision of Nutrients and Growth Factors to Developing 
Germ Cells and Spermatozoa
Although it is protective, the blood-testis barrier also isolates 
developing germ cells from nutrients, hormones, and growth fac­
tors that are present in the systemic circulation. The Sertoli cell 
has an essential role in producing vital nutrients, cofactors, and 
proteins that are needed for the normal progression of spermato­
genesis and support of spermatozoa being transported within the 
semini

f spermatozoa being transported within the 
seminiferous tubule lumen. Sertoli cells produce pyruvate and 
contain lactate dehydrogenase, which catalyzes the conversion of 
pyruvate to lactate, the preferred energy substrate of germ cells.
Most of the proteins produced by Sertoli cells are binding or 
transport proteins for substances (e.g., metals, vitamins, sphin­
golipids, androgens, hormones, growth factors) that serve as 
cofactors and regulators of germ cell development within the 
seminif

ators of germ cell development within the 
seminiferous tubule. Binding proteins produced by Sertoli cells 
include transferrin, an iron-binding protein; ceruloplasmin, a 
copper-binding protein; glycosphingolipid-binding protein; sul­
fated glycoprotein 2, also called clusterin, a lipid-binding protein 
with other biologic activities; androgen-binding protein (ABP); 
α2-macroglobulin, which serves as a binding protein for inhibin 
and activin; follistatin, a potent binding protein for activin; 

ollistatin, a potent binding protein for activin; and 
insulin-like growth factor–binding proteins (IGFBPs), which 
bind insulin-like growth factor type 1 (IGF1).
ABP is a testicular homologue of sex hormone–binding globu­
lin (SHBG), the major circulating ABP, which is synthesized by 
the liver and encoded by the same gene.73 Based on studies in 
rodents, ABP is thought to play a role in regulating local testos­
terone concentrations in the seminiferous tubule and epididymis. 
However, one stud

niferous tubule and epididymis. 
However, one study reported that human SHBG is expressed in 
germ cells but not in Sertoli cells and that a smaller isoform of 
SHBG is located between the outer acrosomal membrane and the 
sperm plasma membrane and is released during capacitation.74 
These findings suggest a potentially different role of SHBG/ABP 
in humans and rodents and underscore the hazard of extrapolating 
results from animal studies.
The Sertoli cell also produces several growth factors, 

ertoli cell also produces several growth factors, such as 
IGF1, basic fibroblast growth factor, activin A, transforming 
681
growth factor-α (TGFα) and TGFβ, interleukin 1α (IL1α) and 
IL6, stem cell factor (the c-KIT ligand), glial cell–derived neuro­
trophic factor, and polyamines (putrescine, spermine, and spermi­
dine), which act both as paracrine regulators of stem cell renewal, 
germ cell development, and Leydig cell and peritubular myoid cell 
function an

eydig cell and peritubular myoid cell 
function and as autocrine regulators. 
Translocation, Sculpting, and Release of Developing 
Germ Cells
Sertoli cells actively move developing germ cells from the basal 
compartment through the adluminal compartment and release 
spermatozoa from the seminiferous epithelium into the lumen 
(spermiation). During translocation, Sertoli cells remove degen­
erating germ cells, residual cytoplasm from late elongated sperma­
tids (residual body), and seminiferous t

d sperma­
tids (residual body), and seminiferous tubule fluid and contents 
by phagocytosis and pinocytosis. The Sertoli cell produces prote­
ases and protease inhibitors (e.g., testibumin or sulfated glycopro­
tein 1, tissue plasminogen activators, type IV collagenase, cystatin, 
α2-macroglobulin) that are involved in germ cell translocation, 
removal of degenerating germ cells, and spermiation. 
Secretion of Seminiferous Tubule Fluid
Seminiferous tubule fluid serves important roles in the deli

us tubule fluid serves important roles in the delivery 
of nutrients to developing germ cells within the seminiferous epi­
thelium, transportation of regulatory factors and nutrients within 
the seminiferous tubule lumen, and transportation of spermato­
zoa released into the seminiferous tubule lumen to the rete testis, 
efferent ducts, and epididymis. 
Production of Reproductive Hormones
Sertoli cells produce hormones that are important in male repro­
ductive differentiation and function. These

repro­
ductive differentiation and function. These include AMH, which 
causes regression of the müllerian ducts, preventing uterine and 
fallopian tube formation during embryogenesis; inhibin B and 
activin A, peptide hormones that, respectively, participate in nega­
tive feedback regulation of FSH secretion and potentially act as 
a paracrine regulator of spermatogenesis; and estradiol, a potent 
estrogen that is produced via aromatization of testosterone in 
immature Sertoli cells.
FSH acts di

stosterone in 
FSH acts directly and testosterone (secreted by Leydig cells in 
response to LH) acts directly and also indirectly (e.g., by stimulat­
ing peritubular myoid cells) to control Sertoli cell function and 
regulate spermatogenesis. Gene expression profiling studies using 
microarray analysis in rat Sertoli cells treated with FSH, in Sertoli 
cell–specific AR mutant mice, and in GnRH-mutant mice treated 
with testosterone have provided insight into specific Sert

tosterone have provided insight into specific Sertoli cell 
genes that are regulated directly by FSH and testosterone.75 How­
ever, similar studies in humans investigating the regulation of 
Sertoli cell gene expression, specifically by FSH and testosterone, 
have yet to be performed. 
Paracrine and Autocrine Regulation of Testis Function
As described earlier, with regard to testicular function, gonado­
tropins, LH, and FSH secreted by the pituitary are the major 
endocrine regulators, and testo

ary are the major 
endocrine regulators, and testosterone produced by Leydig cells 
in response to LH stimulation is the main paracrine regulator. 
However, there is evidence, mostly from studies in experimental 
animals and in vitro studies using isolated testis cell types from 
animals, that Leydig, Sertoli, and peritubular myoid cells and 
macrophages in the testis secrete other paracrine and autocrine 
factors that may be important modulators of testosterone and 
sperm production.
One of the

 of testosterone and 
One of the most important examples of paracrine regulation 
within the testis is the effect of testosterone, produced locally by 
Leydig cells, on Sertoli cell function and spermatogenesis. There 
is evidence that testosterone has both a direct effect on Sertoli cell 
function and spermatogenesis and an indirect effect on peritubu­
lar myoid cell ARs. In GnRH- and gonadotropin-deficient hpg 
mice in which the AR has been knocked out specifically in Sertoli

he AR has been knocked out specifically in Sertoli 
cells, DHT treatment is not able to stimulate spermatogenesis, 
suggesting that direct androgen action on Sertoli cells is needed 
to stimulate spermatogenesis.76 Animals with a peritubular myoid 
cell–specific AR knockout demonstrate impaired Sertoli function 
(i.e., reduced seminiferous tubule fluid and Sertoli cell androgen-
dependent gene expression), azoospermia, and infertility not 
explained by alterations in testosterone, LH, and FSH co

ned by alterations in testosterone, LH, and FSH concentra­
tions.77 These findings suggest that the paracrine effect of Leydig 
cell–produced testosterone on Sertoli cell function and regulation 
of spermatogenesis is mediated in part by androgen-driven inter­
actions between peritubular myoid cells and Sertoli cells. Whether 
these stromal-epithelial cell interactions occur in humans is not 
known. At present, the roles of paracrine and autocrine factors 
other than testosterone in the regulati

e factors 
other than testosterone in the regulation of human testis function 
are not clear. 
Hormonal Control of Spermatogenesis
LH and FSH are the main hormonal regulators of spermatogen­
esis in humans. The effect of FSH on spermatogenesis is mediated 
by a direct endocrine action on Sertoli cells, whereas that of LH 
is mediated by an action on Leydig cells to produce testosterone, 
which in turn acts locally within the testes in a paracrine man­
ner, with direct effects on Sertoli cells an

 man­
ner, with direct effects on Sertoli cells and possibly indirect effects 
through peritubular myoid cell regulation of Sertoli cells. The 
gonadotropin requirements necessary for the initiation of sper­
matogenesis at the time of puberty differ from those needed for the 
maintenance of sperm production, once initiated, in adults.10,78
Initiation of Spermatogenesis
Usually, both LH and FSH are required to initiate spermatogen­
esis at the time of puberty. In men with prepubertal gonadotro­
p

e of puberty. In men with prepubertal gonadotro­
pin deficiency (e.g., CHH), treatment with LH (hCG) is needed 
to stimulate sufficient intratesticular testosterone production to 
support spermatogenesis and seminal fluid production by acces­
sory sex glands (seminal vesicles and prostate gland). However, 
most such patients also require FSH treatment to initiate and 
complete the first wave of spermatogenesis and to produce sperm 
in the ejaculate.79 In some men with incomplete gonadotropin 
de

te.79 In some men with incomplete gonadotropin 
deficiency (usually with evidence of endogenous FSH secretion, 
e.g., larger testis volume), LH treatment alone is sufficient to initi­
ate and complete spermatogenesis. FSH treatment without LH 
(hCG) does not stimulate sperm production in men with prepu­
bertal complete gonadotropin deficiency—but might be sufficient 
in men with partial gonadotropin deficiency.
Natural inactivating mutations of gonadotropin β-subunits 
and receptors provide some

onadotropin β-subunits 
and receptors provide some insight into the roles of LH and FSH 
in initiating spermatogenesis. Men with inactivating mutations of 
LHβ usually have a lack of pubertal development and arrested 
spermatogenesis or azoospermia and infertility.59 Recently, how­
ever, a man with an LHβ mutation resulting in a partially active 
LH molecule (as evidenced by expression of steroidogenic enzymes 
in a few mature Leydig cells and low intratesticular testosterone 
concentrations) wa

w intratesticular testosterone 
concentrations) was reported to have complete and quantitatively 
normal spermatogenesis.80 This finding suggests that complete 
spermatogenesis may be initiated by low concentrations of LH 
682
and intratesticular testosterone in the presence of high serum FSH 
concentrations, as were present in this case.
Men with inactivating LH receptor mutations present with 
varying degrees of impaired sexual differentiation or 46

g degrees of impaired sexual differentiation or 46,XY DSD, 
ranging from ambiguous genitalia to perineoscrotal hypospa­
dias, and azoospermia, although defects in sperm production are 
confounded by the presence of cryptorchidism in many of these 
men.58 Recently, a man with a partially inactivating mutation of 
the LH receptor was reported to have micropenis, delayed puberty, 
low serum testosterone concentrations, and normal FSH concen­
trations but bilaterally normal-size, descended testes an

s but bilaterally normal-size, descended testes and normal 
sperm production, albeit with low sperm concentration (oligo­
zoospermia).81 This finding suggests that spermatogenesis may be 
initiated qualitatively by very low LH activity and intratesticular 
testosterone in the presence of normal FSH concentrations.
Men with inactivating mutations of FSHβ have been found 
generally to have azoospermia with low or low-normal testosterone 
and high LH concentrations.82-85 In contrast, men with inact

H concentrations.82-85 In contrast, men with inactivat­
ing FSH receptor mutations have been reported to have moderate 
to severely reduced sperm counts (but not azoospermia) with nor­
mal testosterone and normal to high LH concentrations.58,86 The 
reason for the apparent discrepancy in degree of spermatogenic 
impairment exhibited by men with FSHβ versus FSH receptor 
mutations is not clear. It is possible that residual receptor function 
in men with FSH receptor mutations results in persisten

n with FSH receptor mutations results in persistent small 
amounts of FSH signaling or that men with FSHβ mutations have 
greater Leydig cell dysfunction, as evidenced by lower serum con­
centrations of LH and intratesticular testosterone that result in 
greater impairment of spermatogenesis.
In summary, findings from the small number of reports of 
men with inactivating mutations of gonadotropin β-subunits and 
receptors suggest that initiation of the first wave of spermato­
genesis may require

of the first wave of spermato­
genesis may require only very low concentrations or activity of 
either LH (intratesticular testosterone) or FSH in the presence 
of adequate amounts of the other gonadotropin. Clinically, how­
ever, most men with prepubertal gonadotropin deficiency require 
treatment with both LH and FSH to initiate spermatogenesis dur­
ing puberty. Because FSH stimulates Sertoli cell proliferation and 
number during testis development, it plays a vital role in deter­
mining the c

ment, it plays a vital role in deter­
mining the capacity for quantitatively normal spermatogenesis. 
Maintenance of Spermatogenesis
In men with prepubertal gonadotropin deficiency (e.g., CHH), 
once spermatogenesis has been initiated with LH (hCG) and FSH 
treatment, sperm production may be maintained with LH treat­
ment alone without continued FSH administration.79 However, 
spermatogenesis is not stimulated by administration of FSH in 
combination with testosterone (that maintains normal conc

tion with testosterone (that maintains normal concen­
trations of serum testosterone but with continued low LH and 
intratesticular testosterone concentrations) in men with CHH. 
Spermatogenesis may be reinitiated with LH (hCG) alone in pre­
viously gonadotropin-treated men with CHH after a period of 
gonadotropin deficiency associated with exogenous testosterone 
replacement therapy. Furthermore, in men with gonadotropin 
deficiency and azoospermia acquired as an adult (e.g., secondary 
to a pi

mia acquired as an adult (e.g., secondary 
to a pituitary adenoma), spermatogenesis may be reinitiated and 
maintained with LH (hCG) treatment alone.79
In normal men with experimental gonadotropin deficiency 
induced by high-dose testosterone administration, spermato­
genesis may be reinitiated and maintained by either LH or hCG 
alone, despite markedly suppressed FSH concentrations, or by 
FSH alone, despite severely suppressed LH (and presumably low 
intratesticular testosterone) concentration

y low 
intratesticular testosterone) concentrations. However, sperm pro­
duction was not stimulated by either LH or FSH alone to the 
baseline concentrations that existed before experimental gonado­
tropin suppression.87 In this model of gonadotropin deficiency, 
treatment with both LH (hCG) and FSH restored sperm counts 
fully to baseline values. Finally, in support of the ability of FSH 
alone to stimulate sperm production, spermatogenesis was main­
tained despite undetectable serum gonadotrop

main­
tained despite undetectable serum gonadotropin concentrations 
in a hypophysectomized man who had an activating FSH receptor 
mutation.88
Together, these findings suggest that a normal concentration 
of either FSH or LH is sufficient for maintenance of qualitatively 
normal sperm production, but both gonadotropins are necessary 
for quantitatively normal spermatogenesis in male humans.
The effect of gonadotropins on specific stages of spermatogen­
esis has been studied in normal men with e

atogen­
esis has been studied in normal men with experimental gonado­
tropin suppression induced by the administration of high-dose 
progestin and testosterone. In these gonadotropin-deficient men, 
selective replacement of either FSH or LH (increasing intratestic­
ular testosterone) supported all stages of spermatogenesis, includ­
ing spermatogonial maturation, meiosis, spermiogenesis, and 
spermiation, but each agent had predominant actions on specific 
stages.89 FSH exerted a relatively great

specific 
stages.89 FSH exerted a relatively greater effect on maturation of 
spermatogonia (conversion of spermatogonia Ap to spermatogonia 
B), early meiosis, and maintenance of pachytene spermatocytes 
(conversion of spermatogonia to pachytene spermatocytes). LH 
(stimulating intratesticular testosterone) had predominant effects 
on the completion of meiosis (conversion of pachytene spermato­
cytes to round spermatids) and on spermiation (release of mature 
spermatozoa). LH and FSH (intratest

ase of mature 
spermatozoa). LH and FSH (intratesticular testosterone) exert 
similar effects on spermiogenesis (conversion of round to elon­
gated spermatids).
In normal men, LH stimulates intratesticular testosterone con­
centrations that are approximately 100- to 200-fold higher than 
serum testosterone concentrations and correlate with circulat­
ing LH concentrations. Administration of various combinations 
of exogenous testosterone, progestin, and GnRH antagonist to 
induce gonadotropin def

n, and GnRH antagonist to 
induce gonadotropin deficiency in male contraception trials sup­
pressed intratesticular testosterone by 98%, to concentrations 
comparable to those in serum, and reduced sperm production, 
producing severe oligozoospermia or azoospermia.90 Short-term 
administration of hCG (LH-like activity) in normal men with 
experimental gonadotropin deficiency resulted in a dose-depen­
dent increase in intratesticular testosterone.91
Testosterone replacement therapy in gonadotropi

91
Testosterone replacement therapy in gonadotropin-deficient 
men does not increase intratesticular testosterone sufficiently to 
support spermatogenesis. In fact, testosterone treatment sup­
presses endogenous gonadotropin concentrations and may sup­
press sperm production. However, this cannot be assumed to occur 
in all testosterone-treated gonadotropin-deficient men, especially 
if testosterone replacement is not adequate. In one study involving 
a small number of men with acquired hypogona

ving 
a small number of men with acquired hypogonadotropic hypogo­
nadism due to hypothalamic-pituitary disease, half had detectable 
sperm counts ranging from very low (1 million/mL) to normal 
(120 million/mL) on testosterone replacement therapy, mostly 
because of incomplete gonadotropin suppression associated with 
an subtherapeutic testosterone regimen (200–250 mg given by 
intramuscular [IM] injection every 3–4 weeks).92
Spermatogenesis is maintained with intratesticular testoster­
one con

maintained with intratesticular testoster­
one concentrations that are 10% of normal, but the minimum 
concentration needed to support sperm production is not known. 
Intratesticular testosterone is converted within the testes to its 
683
active metabolites, estradiol and DHT, by P450 19A1 (aroma­
tase) and 5α-reductase (SRD5A1), respectively. As with testoster­
one, intratesticular concentrations of estradiol are about 100-fold 
higher than serum estradiol conce

 about 100-fold 
higher than serum estradiol concentrations; however, intratesticu­
lar concentrations of DHT are only approximately 15-fold higher 
than concentrations in the circulation.93 The roles of these rela­
tively high concentrations of estradiol and DHT within the testes 
in the maintenance of spermatogenesis are not clear. 
Negative Feedback Regulation of 
Gonadotropin Secretion
As described earlier, the feed-forward regulation of testicular 
function involves hypothalamic GnRH stimul

icular 
function involves hypothalamic GnRH stimulation of pituitary 
gonadotropin secretion, which in turn stimulates the testes to 
secrete testosterone and increase sperm production (see Fig. 19.6). 
An important aspect of hypothalamic-pituitary-testicular axis 
regulation is the negative feedback suppression of hypothalamic 
GnRH and pituitary gonadotropin secretion by steroid and pep­
tide hormones produced by the testes. Testosterone, produced by 
Leydig cells of the testis, and estradiol,

ced by 
Leydig cells of the testis, and estradiol, its active metabolite, act at 
both the hypothalamus and the pituitary gland to inhibit GnRH 
and gonadotropin secretion. Inhibin B, produced by Sertoli cells 
within seminiferous tubules of the testis, acts primarily on the 
pituitary to suppress FSH secretion.
In a series of elegant prospective studies, normal men and 
GnRH-deficient men with CHH treated with physiologic doses 
of pulsatile GnRH (i.e., a GnRH clamp) underwent medical cas­
trat

H (i.e., a GnRH clamp) underwent medical cas­
tration induced by high-dose ketoconazole and treatment with 
physiologic doses of testosterone or estradiol. The effects of these 
interventions on production of LH and FSH were measured. 
These studies have helped to define the relative roles of testoster­
one and estradiol in regulating gonadotropin secretion and the 
sites of negative feedback by these steroids.94,95 It appears that 
both testosterone and estradiol (derived from aromatization of 

rone and estradiol (derived from aromatization of 
testosterone) exert negative feedback effects at the hypothalamus 
to suppress pulsatile GnRH secretion. These studies also demon­
strated that negative feedback inhibition of pituitary LH and FSH 
secretion by testosterone requires aromatization of testosterone to 
estradiol. The suppression of FSH by estradiol is modest when 
inhibin B concentrations are normal and testes are normal, sug­
gesting that inhibin B is the main physiologic negative

ng that inhibin B is the main physiologic negative feedback 
regulator of FSH secretion.96 When inhibin B concentrations are 
low, as in men with seminiferous tubule failure or anorchia, the 
negative feedback effect of estradiol derived from testosterone 
assumes a greater role in suppressing FSH.
Although active metabolism of testosterone to estradiol is 
important in the negative feedback actions of testosterone, con­
version of testosterone to DHT by 5α-reductase types 1 and 2 
does not play

o DHT by 5α-reductase types 1 and 2 
does not play a major role in steroid feedback. Men with muta­
tions in SRD5A2 exhibit only modest elevations in gonadotro­
pins and increases in LH pulse amplitude but not frequency.97 
Men with benign prostatic hyperplasia (BPH) and normal men 
treated with finasteride (an SRD5A2 inhibitor) or dutasteride (an 
SRD5A1 and SRD5A2 inhibitor) do not demonstrate increases 
in serum LH and FSH concentrations.98 These findings suggest 
a relatively minor role for 

ese findings suggest 
a relatively minor role for DHT in physiologic negative feedback 
regulation of gonadotropins. However, administration of supra­
physiologic amounts of DHT does suppress concentrations of LH 
(by 30–60%) and FSH (by 15–30%).99
The pituitary gonadotrophs contain estrogen receptor α 
(ERα), but GnRH neurons appear to lack both ERα and AR. The 
negative feedback actions of testosterone and estradiol are thought 
to be mediated indirectly by other neuronal systems that relay st

indirectly by other neuronal systems that relay ste­
roid feedback signals to GnRH neurons. Studies in animals sug­
gest that neurons that produce kisspeptin, a 54–amino acid peptide 
product of the KISS1 gene, may be candidate mediators of ste­
roid negative feedback.100,101 These neurons interact directly with 
GnRH neurons in the medial basal hypothalamus, the majority of 
which contains the kisspeptin receptor, KISS1R, and the release of 
kisspeptin thereby stimulates GnRH secretion. Kisspep

speptin thereby stimulates GnRH secretion. Kisspeptin neu­
rons may also interact with other neurons (e.g., γ-aminobutyric 
acid neurons) to indirectly regulate GnRH secretion. Kisspeptin 
neurons contain both AR and ERα. In experimental animals, 
castration increases kisspeptin expression, coinciding with an 
increase in GnRH and gonadotropin secretion; sex steroid treat­
ment with testosterone, estradiol, or DHT reverses these changes, 
and kisspeptin antagonists block the postcastration incre

speptin antagonists block the postcastration increase in 
LH secretion. In humans, mutations in KISS1 or KISS1R cause 
hypogonadotropic hypogonadism and impaired pubertal develop­
ment,102 and there is accumulating evidence in animals that kis­
speptin may have a key role in the initiation of puberty.
Inhibins are heterodimeric glycoproteins belonging to the 
TGFβ superfamily of proteins, which includes activins, inhibins, 
TGFβ, bone morphogenetic proteins, and growth and differentia­
tion fact

ic proteins, and growth and differentia­
tion factors such as AMH and myostatin.103 Inhibins are composed 
of an α-subunit connected by a disulfide bridge to either a βA or a 
βB subunit to form inhibin A or inhibin B, respectively. Inhibin B 
(α-βB heterodimer) is the physiologically relevant inhibin species 
in humans. Unlike most proteins in the TGFβ family, which act as 
local paracrine or autocrine regulators of diverse cellular functions, 
inhibin B acts as a circulating hormone. Inhibin B

inhibin B acts as a circulating hormone. Inhibin B is produced 
by the Sertoli cell in response to FSH stimulation. It binds to a 
coreceptor composed of the type III TGFβ receptor (TGFBR3 or 
betaglycan) and the type IIB activin receptor (ACVR2B) and is 
thought to be the main endocrine negative feedback suppressor of 
FSH secretion by pituitary gonadotrophs.
In humans, inhibin B concentrations rise progressively at the 
time of puberty, correlating with FSH concentrations and FSH-
stimulated S

ting with FSH concentrations and FSH-
stimulated Sertoli cell proliferation.103 Adult concentrations are 
reached by midpuberty. At that time, Sertoli cell function becomes 
intimately linked to the onset of spermatogenesis, and inhibin B 
concentrations assume an inverse relationship with FSH concen­
trations as the inhibin B–mediated negative feedback regulation 
becomes activated. For example, in boys with Sertoli cell–only 
syndrome, inhibin B concentrations are normal before puberty as 
a f

B concentrations are normal before puberty as 
a function of Sertoli cell proliferation but become undetectable at 
the time of puberty, reflecting the absence of germ cells and Sertoli 
cell dysfunction. In adults, inhibin B concentrations are inversely 
related to the degree of germ cell damage or loss and Sertoli cell 
dysfunction. This relationship suggests that germ cells regulate 
Sertoli cell function, although the precise cellular and molecular 
mechanisms underlying this regulation are 

ecular 
mechanisms underlying this regulation are not clear. Inhibin B 
concentrations have been used as biomarkers of spermatogenesis 
and Sertoli cell function in research and by some practitioners, but 
they have not yet been used in routine clinical practice.
Activins include homodimers consisting of two βA subunits 
(activin A) or two βB subunits (activin B) and a heterodimer of 
one βA and one βB subunit (activin AB).103,104 Activins are pro­
duced by gonadotrophs and bind to ACVR2B recept

o­
duced by gonadotrophs and bind to ACVR2B receptors. They 
act primarily as autocrine regulators to stimulate FSHβ synthe­
sis and sensitize gonadotrophs to GnRH stimulation, resulting in 
increased FSH secretion. Inhibin B acts as a selective antagonist 
of activins in gonadotrophs by binding to ACVR2B receptors. 
684
Follistatins are glycoproteins produced by gonadotrophs and by 
folliculostellate cells of the pituitary gland that bind and antago­

cells of the pituitary gland that bind and antago­
nize the actions of activin. They act as autocrine and paracrine 
regulators of FSH secretion. Activins and follistatins are also pro­
duced in Sertoli cells and germ cells and may act as autocrine and 
paracrine regulators of testis function.
Negative feedback suppression of gonadotropin production 
(by pharmacologic doses of androgens or combinations of andro­
gens and progestins or by GnRH antagonists) that is sufficient to 
result in suppres

agonists) that is sufficient to 
result in suppression of sperm production has been the basis for 
male hormonal contraceptive development strategies.105 
Testosterone Transport, Metabolism, 
and Actions
Circulating Testosterone
Like other steroid and thyroid hormones, testosterone secreted 
into the circulation by Leydig cells is mostly bound to plasma pro­
teins, primarily to SHBG and albumin. In the circulation, total 
testosterone is composed of 0.5% to 3.0% free testosterone unbound 
to pla

 of 0.5% to 3.0% free testosterone unbound 
to plasma proteins, 30% to 44% SHBG-bound testosterone, and 
54% to 68% albumin-bound testosterone106,107 (Fig. 19.10).
Clinically, biologic actions of testosterone, like those of other 
steroid hormones, are thought to conform to the free hormone 
hypothesis—that is, the biologic activity of testosterone is medi­
ated only by its free (unbound) concentration or the concen­
tration that is easily dissociable from plasma proteins in the 
circulation.107

iable from plasma proteins in the 
circulation.107,108 Testosterone is tightly bound to SHBG with 
such high affinity (1.6 × 10–9 mol/L) that it is not easily dissocia­
ble and available to target tissues for biologic action. In contrast, 
testosterone is loosely bound to albumin, with a binding affinity 
(1.0 × 10–4 mol/L) that is several orders of magnitude less than 
that of SHBG binding. Therefore, albumin-bound testosterone is 
dissociable and available to target tissues for action. Togethe

nd available to target tissues for action. Together, free 
testosterone and albumin-bound testosterone are referred to as bio­
available testosterone, because these fractions are available to diffuse 
into target tissues, bind AR, and affect gene transcription, result­
ing in androgen action in those tissues. One case was reported of 
a man who had undetectable SHBG and very low total testoster­
one concentrations due to a missense mutation of the SHBG gene 
that blocked its secretion.109 His fr

e SHBG gene 
that blocked its secretion.109 His free testosterone and gonadotro­
pin concentrations and semen analysis were normal, and he had 
normal objective evidence of sexual development. Although he 
did have subtle symptoms of male hypogonadism (sexual dysfunc­
tion, weakness, and fatigue), the overall findings in this man pro­
vide support for the free hormone hypothesis. In addition, data 
from the European Male Aging Study, a longitudinal cohort study, 
indicate that low serum free tes

al cohort study, 
indicate that low serum free testosterone concentrations are asso­
ciated with symptoms and objective findings of male hypogonad­
ism in men with normal total testosterone concentrations.110 In 
addition, incident symptoms of hypogonadism occur in men who 
have low serum free testosterone and total testosterone concentra­
tions but not in obese men with normal serum free testosterone 
and low serum total testosterone concentrations.111 Although the 
free testosterone hypothesis

ons.111 Although the 
free testosterone hypothesis remains controversial, the limited 
available data generally support it.112
SHBG, which is synthesized by hepatocytes of the liver, is a 
homodimeric β-globulin protein composed of a heavy and a light 
subunit that are identical in peptide sequence and encoded by a 
single gene but differ in their degree of glycosylation.73 The SHBG 
gene is also expressed in the testes and in most mammals, Sertoli 
cells produce a homologue of SHBG, known as AB

li 
cells produce a homologue of SHBG, known as ABP, under the 
control of FSH. In the human testis, however, SHBG appears to 
be expressed in germ cells rather than Sertoli cells and to pro­
duce a truncated form of SHBG (not ABP) that localizes to the 
acrosome of spermatozoa.74 SHBG has two binding sites per 
monomer; these sites bind DHT and testosterone with high affin­
ity and estradiol less avidly. Previous studies have assumed iden­
tical binding affinities of SHBG monomers for testoster

 binding affinities of SHBG monomers for testosterone and 
other ligands. Recent studies suggest that binding of testosterone 
to one monomer of the SHBG dimer affects the binding affin­
ity of testosterone to the second monomer and that binding can 
only be characterized by a complex dynamic, multistage, allosteric 
model.113 Glycosylation of SHBG is not involved in steroid bind­
ing but may prolong the plasma half-life of the protein in the 
circulation. SHBG production by the liver is increas

rculation. SHBG production by the liver is increased under the 
influence of estrogens and thyroxine and decreased by androgens 
and insulin.
Counter to the free hormone hypothesis, there is evidence that 
testosterone bound to SHBG may affect androgen action in some 
target tissues. In some tissues, such as the human prostate, SHBG 
may bind to a cell surface receptor; testosterone then binds to the 
SHBG-receptor complex, activating cAMP and affecting target 
organ function.114 Glycosylation o

fecting target 
organ function.114 Glycosylation of SHBG may be important for 
the interaction of steroid-SHBG complexes with plasma mem­
branes. Megalin is a member of the LDL receptor superfamily 
of proteins that serve as endocytic proteins facilitating the entry 
of steroids into cells (most notably 25-hydroxyvitamin D into 
proximal tubule cells of the kidney). Megalin is found in the kid­
ney and also in the epididymides, prostate, ovaries, and uterus. 
In vitro studies in cells that expre

 and uterus. 
In vitro studies in cells that expressed megalin but no other endo­
cytic receptors found that testosterone, DHT, and estradiol bound 
to SHBG were endocytosed into cells and activated AR-mediated 
transcription; these effects were blocked by a megalin antago­
nist.115 Furthermore, megalin-knockout mice demonstrated 
impaired testicular descent, an androgen-mediated process. These 
findings suggest that megalin may be important in mediating cel­
lular uptake of androgens into some 

ediating cel­
lular uptake of androgens into some tissues. However, the impor­
tance of testosterone bound to SHBG or endocytic proteins such 
as megalin in human physiology remains to be determined.
In normal men with an intact hypothalamic-pituitary-testic­
ular axis, alterations in SHBG concentrations and testosterone 
bound to SHBG do not influence the physiology and action of 
androgens at steady state. Any acute effects on free testosterone 
Albumin-
bound T
(weak)
54%–68%
Free T
0.5%–3.0%

Albumin-
bound T
(weak)
54%–68%
Free T
0.5%–3.0%
SHBG-bound
T (tight)
30%–44%
(free and weakly
bound T)
• Fig. 19.10  Fractions of circulating testosterone in blood. The majority of 
circulating testosterone (T) is bound to serum proteins: approximately 54% 
is weakly bound to albumin, and 44% is tightly bound to sex hormone–
binding protein (SHBG). Only about 2% of circulating T is free of protein 
binding. The combination of free and weakly bound (albumin-bound) T is 
referred

ee and weakly bound (albumin-bound) T is 
referred to as bioavailable testosterone.
685
concentration caused by changes in SHBG would alter the nega­
tive feedback regulation of gonadotropins, resulting in normal­
ization of free testosterone concentrations. For example, an acute 
increase in SHBG concentrations may transiently decrease free 
testosterone concentrations, but the consequent decrease in testos­
terone negative feedback increases pituitary LH secret

ne negative feedback increases pituitary LH secretion, which 
increases testosterone production by the testes to restore normal 
free testosterone concentrations. In contrast, alterations in SHBG 
concentrations may alter free testosterone concentrations in men 
with reproductive disorders who have impaired negative feedback 
regulation or are receiving testosterone replacement therapy.
SHBG concentrations may be decreased or increased in a vari­
ety of commonly encountered clinical conditions.1

ety of commonly encountered clinical conditions.116 Clinically, 
alterations in SBHG are extremely important to consider in the 
diagnosis of male hypogonadism (see later discussion). Because 
serum total testosterone measurements are affected by changes in 
SHBG concentrations, accurate measurements of free or bioavail­
able testosterone are needed to assess the adequacy of Leydig cell 
function, to determine whether a patient is hypogonadal, and to 
monitor testosterone replacement in patient

nd to 
monitor testosterone replacement in patients with alterations in 
circulating SHBG concentrations. 
Active Metabolism and Catabolism of Testosterone
An important aspect of the effects of testosterone action on target 
tissues is its active metabolism to 17β-estradiol (estradiol) by P450 
19A1 and to DHT by SRD5A1 and SRD5A2; these are the most 
potent of the endogenous estrogens and androgens9 (Fig. 19.11). 
Many of the biologic actions of testosterone are mediated by these 
active metabo

 testosterone are mediated by these 
active metabolites, acting through mechanisms that are depen­
dent on ERα and ERβ (estradiol) or on AR (DHT). These active 
metabolites are formed and act locally as paracrine or autocrine 
regulators, and they also are secreted and act as endocrine regula­
tors of target tissue function.
Aromatization of Testosterone to Estradiol
Aromatase (P450 19A1) catalyzes the conversion of testosterone 
to estradiol, as well as the conversion of the weaker androgen, 
Δ

 metabolism of testosterone. Testosterone may be converted to the potent estrogen 
17β-estradiol by the enzyme aromatase or P450 19A1 (CYP19) or to the more potent androgen dihy­
drotestosterone (DHT) by the enzyme 5α-reductase. The effects of testosterone on external genital dif­
ferentiation, prostatic growth, skin, and hair follicles in androgen-sensitive areas require 5α-reduction of 
testosterone to DHT. The effects of testosterone that require aromatization to estradiol are prevention of 

ire aromatization to estradiol are prevention of 
bone resorption, increase in bone mineral density, epiphyseal fusion, and gonadotropin secretion. Testos­
terone and DHT, but not estradiol, increase and maintain muscle mass and strength; testosterone, DHT, 
and estradiol play a role in normal male sexual function; and testosterone and DHT have opposing effects 
to estradiol on high-density lipoprotein cholesterol concentrations. (Modified from Bhasin S. Testicular dis­
orders. In: Kronenberg HM

hasin S. Testicular dis­
orders. In: Kronenberg HM, Melmed S, Polonsky KS, et al, eds. Williams Textbook of Endocrinology, 11th 
ed. Philadelphia, PA: Elsevier; 2008:645–698.)
686
these conversions occur predominantly in adipose tissue but also 
in other tissues, including brain, bone, breast, liver, blood ves­
sels, and testes (Sertoli cells and Leydig cells). Approximately 
40 to 50 μg of estradiol is produced daily, primarily by extra­
testicular a

s produced daily, primarily by extra­
testicular aromatization of testosterone to estradiol and of Δ4-
androstenedione to estrone (which is then converted to estradiol 
by various isoforms of the 17β-HSD enzymes). Approximately 
15% to 25% of circulating estradiol is produced by the testes, 
primarily by Leydig cells.117
Aromatization of testosterone to estradiol mediates the effects 
of testosterone on epiphyseal closure at the time of puberty, inhibi­
tion of bone resorption and maintenance of

inhibi­
tion of bone resorption and maintenance of bone mineral density 
(BMD), regulation of fat mass accumulation, negative feedback 
suppression of hypothalamic GnRH secretion and pituitary 
gonadotropin secretion, sexual function, and possibly regulation 
of high-density lipoprotein (HDL) cholesterol concentrations and 
some aspects of cognitive function and mood.118,119 Healthy young 
to middle-age men with experimental hypogonadism induced by 
a GnRH agonist were treated with placebo or va

by 
a GnRH agonist were treated with placebo or various dosages of 
testosterone with or without coadministration of an aromatase 
inhibitor to investigate the relative effects of experimental andro­
gen and estrogen deficiency, respectively.120,121 Decreases in lean 
body mass, muscle size, and strength were induced by androgen 
deficiency; fat mass accumulation and bone loss was induced by 
estrogen deficiency; and decline in sexual function was induced by 
both androgen and estrogen deficienc

s induced by 
both androgen and estrogen deficiency. In addition, estradiol defi­
ciency appears to be the primary cause of vasomotor symptoms 
(hot flashes) in men with acute severe hypogonadism.122
In men, circulating estradiol is regulated primarily by the amount 
of androgen substrates, testosterone and Δ4-androstenedione, and 
by aromatase activity in adipose tissue and other peripheral tis­
sues. Aromatase activity in Leydig cells is controlled primarily by 
LH. Stimulation of aromatase ac

lled primarily by 
LH. Stimulation of aromatase activity in gonadotropin-deficient 
men treated with hCG may result in relatively higher concen­
trations of serum estradiol compared with testosterone and may 
contribute to benign breast tenderness and enlargement (gyneco­
mastia) during treatment. As in women, men with ER-positive 
breast cancer may be treated with aromatase inhibitors to reduce 
estrogen synthesis.
The few men reported with inactivating mutations of the aro­
matase gene (CYP19A

tivating mutations of the aro­
matase gene (CYP19A1) have demonstrated tall stature, persis­
tent linear growth after puberty, eunuchoidal body proportion, 
delayed bone age, osteopenia or osteoporosis and progressive genu 
valgum, and variable impairments in glucose and lipid metabo­
lism, including insulin resistance, elevated triglyceride and low 
HDL cholesterol concentrations, abnormal liver enzymes and 
fatty liver, variably low sperm counts and infertility, and unde­
tectable estradiol co

s and infertility, and unde­
tectable estradiol concentrations with normal to elevated serum 
testosterone and gonadotropin concentrations.123 Furthermore, 
estradiol treatment results in closure of the epiphyses, increased 
BMD, and increased bone age. Men with a rare inactivating muta­
tion of ERα had a similar phenotype, but in contrast to men with 
aromatase deficiency, they had high serum estradiol, testosterone, 
and gonadotropin concentrations, consistent with estrogen resis­
tance.118 Th

ions, consistent with estrogen resis­
tance.118 These findings support potentially important roles of 
estradiol in bone, glucose, and lipid metabolism; liver function; 
and pituitary and testis function. 
5α-Reduction of Testosterone to DHT
Testosterone is converted to DHT, an androgen that is 2.5 to 3.0 
times more potent than testosterone, by SRD5A1 and SRD5A2. 
These two isoenzymes of 5α-reductase differ in the optimal pH 
for their activity and in their expression patterns.124 SRD5A2 is 
ex

and in their expression patterns.124 SRD5A2 is 
expressed most highly in prostate, epididymis, seminal vesicles, 
genital skin, and liver and at lower concentrations in other tissues, 
such as certain brain regions, nongenital skin, testis, and kidney. 
SRD5A1 is expressed most highly in nongenital skin (hair folli­
cles), liver, and certain brain regions and at lower concentrations in 
prostate, epididymis, seminal vesicles, genital skin, testis, adrenal, 
and kidney. Approximately 200 to 300 μ

, adrenal, 
and kidney. Approximately 200 to 300 μg of DHT is produced 
daily, mostly from 5α-reduction of testosterone in peripheral tis­
sues (predominantly skin and liver). The prostate and testis con­
tribute relatively little to concentrations of DHT in blood.
Men with inactivating mutations of SRD5A2 are born with 
severe 46,XY DSD with ambiguous genitalia (clitoris-like phal­
lus, bifid scrotum, pseudovaginal hypospadias, and rudimentary 
prostate gland) but normal wolffian duct different

prostate gland) but normal wolffian duct differentiation (normal 
seminal vesicles, epididymides, and vas deferens) and no müllerian 
duct structures, supporting the vital role of DHT in external geni­
tal differentiation and prostate development.125 Individuals with 
SRD5A2 deficiency are usually raised as girls. With the onset of 
puberty and increase in testosterone to adult male concentrations, 
the phallus grows, the scrotum develops, libido and erections are 
stimulated, and the gender rol

 and erections are 
stimulated, and the gender role may change from female to male. 
Cryptorchidism is common but not invariable and is associated 
with oligozoospermia or azoospermia. Testes may descend at 
the time of puberty. Normal sperm counts may occur in indi­
viduals with descended testes, and fertility has been reported in 
men with SRD5A2 deficiency. However, in adults, the prostate 
remains underdeveloped and is not palpable, facial and body 
hair are diminished, sebum is not produced

d body 
hair are diminished, sebum is not produced, and male-pattern 
baldness does not occur, supporting the importance of normal 
SRD5A2 activity and DHT for hair growth, sebaceous function, 
and prostate development. Serum DHT concentrations are low, 
testosterone concentrations are normal to slightly elevated, and 
gonadotropin concentrations are normal to modestly elevated.
Within the prostate gland, conversion of testosterone to DHT 
produces concentrations of DHT that are approximately 10

es concentrations of DHT that are approximately 10-fold 
higher than those in serum, serving to amplify androgen activ­
ity in the prostate. Intraprostatic androgen concentrations may 
contribute to prostate disease, such as BPH or prostate cancer.126
Inhibitors of SRD5A2 (finasteride) or of both SRD5A1 and 
SRD5A2 (dutasteride) are used to treat lower urinary tract symp­
toms (LUTS), improve urinary flow, and prevent complications 
related to BPH, as well as to treat male-pattern baldness and 

H, as well as to treat male-pattern baldness and 
androgenic alopecia.127 Treatment with finasteride or dutasteride 
reduces, respectively, the prevalence or the incidence of prostate 
cancer found on biopsy but is possibly associated with a greater 
number of cancers with high Gleason grade.128,129 An important 
aspect of the effect of androgens on the prostate is that intrapros­
tatic androgen concentrations are not reflected in serum con­
centrations, underscoring the importance of local para

rations, underscoring the importance of local paracrine and 
autocrine actions of androgens in the physiology and pathology of 
the prostate and probably other androgen target tissues. 
Catabolism of Testosterone
The primary site of catabolism of circulating testosterone and 5α-
DHT is the liver.130 Testosterone and 5α-DHT are taken up in the 
liver, and testosterone is converted to an inactive metabolite, 5β-
DHT, by the enzyme 5β-reductase. Both 5α- and 5β-DHT then 
undergo 3α-reduction by the

h 5α- and 5β-DHT then 
undergo 3α-reduction by the 3α-HSD enzymes to form 3α,5α-
androstanediol (also called 3α-diol) and 3α,5β-androstanediol, 
respectively; this is followed by 17β-oxidation by the enzyme 
17β-HSD type 2 to form androsterone and etiocholanolone as 
687
catabolic products. In peripheral tissues such as skin, 5α-DHT 
may also be converted to 3α-diol, which is further metabolized 
in the liver.
In the liver, testosterone, DHT, androsterone, etioch

the liver, testosterone, DHT, androsterone, etiocholano­
lone, and the 3α-androstanediols undergo glucuronidation and, 
to a lesser degree, sulfation, to form more hydrophilic conjugates 
that are released into the circulation and excreted in urine and 
bile. Metabolic inactivation of testosterone primarily involves its 
conversion to metabolites such as androstenedione (about 50%), 
androsterone (20%), and etiocholanolone (20%) glucuronides 
(as well as sulfates) and lesser conversion to 3α-dio

 well as sulfates) and lesser conversion to 3α-diol glucuronides 
(3α-diol Gs). Because 3α-diol comes mostly from skin, blood and 
urine measurements of 3α-diol G have been used as a marker of 
peripheral androgen action.130 In men with 5α-reductase defi­
ciency, 3α-diol G concentrations are reduced. The amount of 
body hair and acne correlates with 3α-diol G concentrations.
Epitestosterone (17α-hydroxy-4-androsten-3-one) is a biologi­
cally inactive 17α-hydroxy epimer of testosterone (17β-hydro

tive 17α-hydroxy epimer of testosterone (17β-hydroxy-
4-androsten-3-one) that is produced by the testes in response to 
LH.131 The production rate of epitestosterone is about 3% that 
of testosterone, but its clearance rate is 33% that of testosterone, 
and there is no interconversion of epitestosterone and testoster­
one. Like testosterone, epitestosterone is conjugated in the liver, 
primarily to glucuronides and sulfates, and excreted in the urine. 
Because epitestosterone conjugates are rapi

rine. 
Because epitestosterone conjugates are rapidly cleared in the urine, 
excretion rates of testosterone and epitestosterone are similar, and 
the ratio of urinary testosterone to epitestosterone (T/E ratio) is 
approximately 1:1.
Measurements of the T/E ratio and other metabolites in urine 
by sensitive gas chromatography/mass spectrometry methods are 
used to detect androgenic anabolic steroid doping, particularly 
testosterone, by competitive athletes.131 Administration of exog­
enous tes

ive athletes.131 Administration of exog­
enous testosterone suppresses LH and the production and clear­
ance of epitestosterone relative to the administered testosterone, 
resulting in an elevated T/E ratio in urine. The World and United 
States Anti-Doping Agencies have set a threshold T/E ratio of 
greater than 4:1 as suspicious for testosterone doping.
Testosterone is glucuronidated primarily by the enzyme uridine 
diphosphate glucuronyl transferase 2B17 (UGT2B17), whereas 
epitestosterone is

ferase 2B17 (UGT2B17), whereas 
epitestosterone is glucuronidated mostly by another UGT iso­
form, UGT2B7. Although testosterone may be glucuronidated by 
other UGT isoforms (e.g., UGT2B15), individuals with deletion 
of UGT2B17 (common in Asian populations) have reduced tes­
tosterone glucuronidation and clearance, resulting in lower T/E 
ratios with testosterone administration.130-133 Realization that 
there were populations with naturally low T/E led to a reduction 
in the T/E ratio cutoff fr

T/E led to a reduction 
in the T/E ratio cutoff from the previous threshold for suspicion 
of doping of (>6:1) to the present one (>4:1). In addition, there 
are individuals with naturally high T/E ratios, perhaps because of 
other genetic polymorphisms or environmental factors, such as 
excessive alcohol consumption, that may increase T/E ratio tran­
siently, particularly in women.134 In the absence of environmental 
perturbations, the T/E ratio in a single individual is remarkably 
stable over

 in a single individual is remarkably 
stable over time, and longitudinal measurements of urinary T/E 
ratio are used to detect illicit androgen use (known as the ath­
lete biologic passport). Coadministration of epitestosterone with 
testosterone (to maintain a normal T/E ratio) has been used by 
athletes to avoid detection.
If a urinary T/E ratio is suspicious for doping, exogenous 
androgen use is confirmed by gas chromatography combustion 
isotope ratio mass spectrometry, which can detect sm

otope ratio mass spectrometry, which can detect small differ­
ences in the ratio of carbon-13 to carbon-12 (13C/12C) isotopes of 
testosterone or its metabolites.131 Because synthetic androgens are 
synthesized from plant sources (yams or soy), their 13C/12C ratio is 
lower than that of endogenously produced testosterone and other 
steroids that reflect an animal source or dietary ingestion of both 
animal and plant products. However, 13C/12C isotope ratio mass 
spectrometry will not detect dopi

tope ratio mass 
spectrometry will not detect doping by administration of hCG or 
LH-like activity to stimulate endogenous testosterone production 
or administration of androgens derived from animal sources that 
have a 13C/12C ratio like that of endogenous testosterone. 
Mechanisms of Androgen Action
In androgen target tissues, testosterone and DHT in the circulation 
diffuse through the plasma membrane and bind to intracellular 
ARs.135 Binding of androgen to the AR induces a conformational 
c

 of androgen to the AR induces a conformational 
change in the AR that causes dissociation of heat shock proteins 
bound to AR, permitting translocation into the nucleus, induc­
tion of phosphorylation and homodimerization, and interaction 
with DNA, specifically on androgen response elements located 
in regulatory sites of target genes. The AR dimer actively recruits 
tissue-specific coregulators (coactivators and corepressors) to form 
the transcriptional apparatus necessary to control androge

scriptional apparatus necessary to control androgen-regu­
lated gene transcription and subsequent androgen-regulated pro­
tein synthesis.
The AR gene is located on the long arm of the X chromosome 
(Xq11-12). AR is organized into functional domains135 (Fig. 
19.12): (1) an N-terminal domain comprising two transactivation 
domains (AF1 and AF5) that mediate the majority of AR transcrip­
tional activity and coregulator interaction and two trinucleotide 
repeat segments (CAG and GGN, encoding polyg

tide 
repeat segments (CAG and GGN, encoding polyglutamine and 
polyglycine tracts, respectively) of varying number that modify AR 
transactivation; (2) a DNA-binding domain comprising two zinc 
finger motifs, the first mediating DNA recognition and binding 
and the second stabilizing DNA interaction and mediating dimer­
ization of AR; (3) a small hinge region (H); and (4) a ligand-binding 
domain that mediates high-affinity binding of androgen to the AR 
and also contains another transactivatio

o the AR 
and also contains another transactivation domain (AF2).
The AR is a member of the nuclear receptor superfamily that 
includes other steroid hormone receptors. It shares approximately 
80% sequence homology in the DNA-binding domain and 50% 
in the ligand-binding domain, with its most closely related ste­
roid receptors being the progesterone receptor, the glucocorticoid 
receptor, and the mineralocorticoid receptor.135 This similarity 
may explain why, for example, some progestins (e.g

may explain why, for example, some progestins (e.g., medroxypro­
gesterone acetate) have AR agonist activity, others (e.g., cyproter­
one acetate) have AR antagonist activity, and a mineralocorticoid 
antagonist, spironolactone, has AR antagonist activity. Compared 
with the binding of testosterone, DHT binds to AR with higher 
affinity, greater stability, and a slower rate of dissociation, confer­
ring greater androgen activity to DHT, the most potent endog­
enous androgen in humans.
Inactivati

potent endog­
Inactivating mutations of the AR may cause qualitative or 
quantitative abnormalities of receptor function, resulting in vari­
ably impaired androgen action.136 AR mutations manifest pheno­
typic variability, ranging from that of a 46,XY phenotypic female 
with normal female external genitalia and breast development 
(complete androgen insensitivity, formerly testicular feminization), 
which occurs in individuals with complete androgen insensitivity 
or re

iduals with complete androgen insensitivity 
or resistance, to that of an otherwise normal male with incomplete 
hypospadias, mild undervirilization, or infertility.
In a normal population, the number of trinucleotide CAG 
repeats in the first exon of the AR gene varies from 11 to 35. In 
688
general, the number of CAG repeats appears to correlate inversely 
with AR function and action, both in vitro and in vivo, in trans­
genic mice and humans. Kenne

nd in vivo, in trans­
genic mice and humans. Kennedy disease, or X-linked spinal and 
bulbar muscular atrophy, is a rare adult-onset neurodegenerative 
disease of motor neurons that results in progressive muscle weak­
ness; it is associated with a markedly expanded number of CAG 
repeats, varying from 40 to 62, which does not overlap with the 
normal population.137 Neurodegeneration in this disorder is 
thought to be caused by toxicity from intracellular aggregation 
of the AR and associated cof

cellular aggregation 
of the AR and associated cofactors that is worsened by androgen 
binding to the mutant AR and translocation into the nucleus.
Most men with Kennedy disease also manifest clinical findings 
of partial androgen resistance, including gynecomastia, reduced 
libido, erectile dysfunction, decreased facial hair, testicular atro­
phy, and oligozoospermia or azoospermia in association with high 
testosterone and high or normal gonadotropin concentrations.138 
The severity of the lat

tropin concentrations.138 
The severity of the latter biochemical indices of androgen insen­
sitivity is directly related to CAG repeat length. Although it is 
not found consistently, some studies have reported an associa­
tion between CAG repeat number and manifestations of andro­
gen action in normal men.139 In these studies, a low number of 
CAG repeats within the normal range was associated with higher 
androgenicity (e.g., earlier onset of prostate cancer, male-pattern 
baldness, lower HDL 

rostate cancer, male-pattern 
baldness, lower HDL cholesterol), and a high number of repeats 
within the normal range was associated with lower androgenicity 
(e.g., gynecomastia, impaired spermatogenesis, lower bone den­
sity, depressive symptoms). CAG repeat number also seems to be 
associated with the clinical manifestations of men with androgen 
deficiency due to Klinefelter syndrome and their response to tes­
tosterone treatment; men with Klinefelter syndrome with higher 
CAG repeat numbers

inefelter syndrome with higher 
CAG repeat numbers require relatively high doses of testosterone 
for full androgen effects.
Studies in vitro and in experimental animals suggest that some 
actions of androgens may occur within seconds to minutes, too 
rapidly to be caused by classic genomic effects of androgens act­
ing through the AR on gene transcription and subsequent protein 
synthesis, which usually take hours to produce effects.140 Rapid, 
nongenomic (also called nonclassical) effects of a

nongenomic (also called nonclassical) effects of androgens may be 
mediated by cell surface interactions and receptors and by the acti­
vation of conventional signal transduction mechanisms, including 
activation of PKA and PKC, which increase intracellular calcium 
and MAPK pathways. Binding of androgen to intracellular AR 
may also activate coregulators that do not require gene transcrip­
tion to signal, such as the tyrosine kinases (e.g., MAPK, ERK, 
and Akt) that are AR coactivators. Nongeno

, ERK, 
and Akt) that are AR coactivators. Nongenomic actions of andro­
gens have been described in testis (Sertoli cells), brain, muscle, 
cardiovascular tissue, prostate, and immune cells. In humans, the 
rapid vasodilatory effect of testosterone on myocardial ischemia 
in patients with coronary artery disease is attributed to a direct 
nongenomic effect of androgens (or estrogenic metabolites) on 
vascular cells. 
Androgen Effects at Various Stages of Sexual 
Development
Concentrations of tes

tages of Sexual 
Concentrations of testosterone and its actions differ at various 
stages of sexual development141 (Fig. 19.13). During fetal life, 
testosterone is secreted by the fetal testis beginning at 7 weeks 
of gestation.36 Testosterone secretion is primarily under the con­
trol of maternal hCG, initially, and then LH secreted by the fetal 
pituitary. During this time, testosterone concentrations increase 
to almost adult male concentrations, and testosterone and its con­
ver

 concentrations, and testosterone and its con­
version to DHT are critical for normal male internal and external 
genital differentiation (e.g., development of primary sexual char­
acteristics). Testosterone concentrations remain elevated through 
most of the second trimester and decline in the third trimester.
Shortly after birth, during neonatal life, LH secretion increases 
and stimulates a second rise in testosterone concentrations, almost 
to adolescent concentrations, between 3 and 6 month

o adolescent concentrations, between 3 and 6 months of age; this 
is followed by a fall to low prepubertal concentrations.142 The neo­
natal surge in testosterone concentrations may have a role in the 
development of normal phallus size and completion of testicular 
descent. The neonatal increase in testosterone and FSH concen­
trations at this time also stimulates Sertoli cell proliferation and 
NTD
DBD
LBD
100
100
100
AR
919
<15
80
53
PR
933
<15
77
50
GR
777
<15
77
52
MR
984
<15
51
20
ERα
395

5
77
50
GR
777
<15
77
52
MR
984
<15
51
20
ERα
395
<15
56
22
ERβ
530
• Fig. 19.12  Schematic diagram of the structure of the human androgen receptor (AR) gene and homology 
to other steroid hormone receptors: progesterone receptor (PR), glucocorticoid receptor (GR), mineralo­
corticoid receptor (MR), estrogen receptor α (ERα), and estrogen receptor β (ERβ). The AR is a 919–amino 
acid protein that is composed of three functional domains: a ligand-binding domain (LBD), a DNA-binding 
domain (DBD),

binding domain (LBD), a DNA-binding 
domain (DBD), and an N-terminal transactivation domain (NTD). The DBD shares the greatest degree of 
homology (>51% vs. AR) and the NTD the least degree of homology (<15% vs. AR) among steroid hormone 
receptors. (From Li J, Al-Azzawi F. Mechanism of androgen receptor action. Maturitas. 2009;63:142–148.)
689
spermatogonial development, which may play a role in determin­
ing spermatogenic capacity.
During puberty, testosterone 

rmatogenic capacity.
During puberty, testosterone concentrations increase to adult 
male concentrations (or higher) in response to activation of hypotha­
lamic GnRH secretion and its stimulation of pituitary gonadotropin 
secretion.43,143 The progressive increase in testosterone and its active 
metabolites, estradiol and DHT, is responsible for the development 
of secondary sexual characteristics (virilization or masculinization) 
and other changes. Pubertal changes induced by testosterone may 

bone marrow induction of 
erythroid differentiation and stimulation of erythropoietin 
secretion
 
• Reduction of HDL cholesterol.
Brain function changes mediated by testosterone and its active 
metabolites include stimulation of libido (sexual interest, desire, 
and motivation); increase in motivation, initiative, and social 
aggressiveness; and increase in aspects of cognitive function 
(e.g., visuospatial abilities). Sexual function changes mediated by 
testosterone and its active metabolite

ediated by 
testosterone and its active metabolites include initiation of sper­
matogenesis and acquisition of fertility potential and increase in 
spontaneous erections.
In adult life, normal adult male concentrations of testosterone 
serve to maintain many of the changes induced during puberty. 
This includes maintenance of body function changes such as 
normal amounts of androgen-dependent hair and male hair dis­
tribution, sebum production, BMD, muscle mass and strength, 
hematocrit in the m

MD, muscle mass and strength, 
hematocrit in the male range (higher than the female range), 
and HDL cholesterol in the male range (lower than the female 
range); brain function changes such as libido, motivation, initia­
tive, social aggressiveness, energy and vitality, mood, and possibly 
some aspects of cognitive function (e.g., visuospatial ability); and 
sexual function changes such as sperm production and fertility 
potential and spontaneous erections. Some of the masculinizing 
changes in

s erections. Some of the masculinizing 
changes induced by testosterone during puberty are permanent. 
Once some body changes are developed, testosterone is not nec­
essary for maintenance of penis size, scrotal development, linear 
growth, laryngeal size, vocal cord thickness, or voice pitch.116
With aging, there is a gradual and progressive decline in serum 
testosterone concentrations associated with reductions in mus­
cle mass and strength, BMD, libido, energy and vitality, mood, 
Fetal
Neon

MD, libido, energy and vitality, mood, 
Fetal
Neonatal
Puberty
Adult
Old
age
5.0
2.5
100
50
(ng/mL)
20
10
Plasma testosterone (nmol/L)
Sperm production (% of maximal)
17
10
1
40
60
1st
2nd
3rd
80
Year
Trimester
gestation
Birth
• Fig. 19.13  Schematic diagram of changes in serum testosterone concentration and sperm production 
during different phases of life. During fetal life, testosterone concentrations increase almost to adult male 
concentrations, peaking during the first trimester and remain

ons, peaking during the first trimester and remaining elevated throughout the second trimester, 
after which they decline. During neonatal life, testosterone increases almost to adolescent concentrations 
at 3 to 6 months of age, then declines to prepubertal concentrations. During puberty, testosterone con­
centrations and sperm production increase to adult male concentrations over several years. With aging, 
there is a variable, gradual, and progressive decline in serum testosterone concentrati

gressive decline in serum testosterone concentrations and sperm pro­
duction, beginning at age 40 years. (From Griffin JE, Wilson JD. The testis. In: Bondy PK, Rosenberg LE, 
eds. Metabolic Control and Disease, 8th ed. Philadelphia, PA: WB Saunders; 1980:1535–1578.)
690
aspects of cognitive function, sperm production and fertility, and 
erections.144 However, the contribution of the age-related decline 
in testosterone concentrations to these age-asso

in testosterone concentrations to these age-associated changes in 
function is not clear. 
Male Hypogonadism
The two major functions of the testis are to produce sufficient 
amounts of testosterone and of sperm to support the develop­
ment and maintenance of male sexual function, body function, 
and fertility. Male hypogonadism is a clinical syndrome that results 
from a failure of the testes to produce adequate amounts of testos­
terone; this syndrome is almost always associated with impaired

syndrome is almost always associated with impaired 
sperm production (androgen deficiency and impairment of sperm 
production), or an isolated impairment of sperm production or func­
tion with normal testosterone production. Hypogonadism is the 
most common disorder of testis function encountered in clinical 
practice.
Because testis function is controlled by the hypothalamus and 
the pituitary, male hypogonadism may be caused by a primary 
disorder of the testis (primary hypogonadism); it may b

der of the testis (primary hypogonadism); it may be secondary 
to a disorder of the pituitary or hypothalamus (secondary hypogo­
nadism); or in some instances, there may be defects at both levels 
(combined primary and secondary hypogonadism).
Identifying men with secondary hypogonadism has important 
clinical implications that may affect management.116 For example, 
secondary hypogonadism can be caused by a pituitary adenoma 
that may be associated with clinical manifestations related to 
tumor

ted with clinical manifestations related to 
tumor mass (e.g., headaches, visual field defects), to deficiency 
or excessive secretion of other anterior pituitary hormones, or to 
diabetes insipidus (polyuria) resulting from hypothalamic antidi­
uretic hormone deficiency. Such patients require management of 
the underlying hypothalamic or pituitary disorder in addition to 
testosterone replacement therapy. Secondary hypogonadism may 
be reversible with treatment of the underlying condition (e.g.

e with treatment of the underlying condition (e.g., 
nutritional deficiency) or discontinuation of an offending medica­
tion (e.g., glucocorticoids, opioids), or it may be associated with a 
chronic systemic illness that is not curable, such as chronic kidney 
disease (CKD). Impaired spermatogenesis and infertility caused 
by gonadotropin deficiency in men with secondary hypogonadism 
may be treated with gonadotropin or GnRH therapy, and sperm 
production and fertility may be restored. In contra

roduction and fertility may be restored. In contrast, infertility 
caused by primary testicular disease is usually not treatable with 
hormone therapy and requires other fertility options, such as the 
use of donor sperm, ART (e.g., ICSI), or adoption.
Because testosterone has different roles during fetal, prepubertal, 
and adult life, the manifestations of androgen deficiency differ 
depending on the stage of sexual development.9,116
Fetal Androgen Deficiency
During feta

opment.9,116
During fetal development, testosterone and its conversion to 
DHT have vital roles in directing male internal and external 
genital differentiation and development. Fetal androgen defi­
ciency (e.g., from congenital defects in testosterone biosynthe­
sis enzymes) or androgen resistance/insensitivity (e.g., from AR 
mutations or 5α-reductase deficiency) manifests at birth with 
varying degrees of ambiguous genitalia and 46,XY DSD32,62,145 
(Table 19.1). Depe

nitalia and 46,XY DSD32,62,145 
(Table 19.1). Depending on the severity of androgen deficiency 
or resistance/insensitivity, the phenotype of individuals with these 
disorders may range from that of a normal female to that of an 
otherwise normal male with microphallus, pseudovaginal peri­
neoscrotal hypospadias, bifid scrotum, and/or cryptorchidism of 
varying severity. These disorders are described in greater detail in 
Chapter 23. 
Prepubertal Onset of Androgen Deficiency
The increase in test

 Onset of Androgen Deficiency
The increase in testosterone concentrations that occurs at the 
time of puberty is responsible for development of secondary 
sexual characteristics; an increase in muscle mass and reduction 
and redistribution of body fat; long bone growth and eventually 
closure of epiphyses resulting in cessation of growth; stimula­
tion of sexual interest (libido), spontaneous erections, and sexual 
activity; and initiation of spermatogenesis and seminal fluid pro­
duction.146,14

rmatogenesis and seminal fluid pro­
duction.146,147 Prepubertal onset of androgen deficiency causes 
eunuchoidism146,147 (Fig. 19.14; see Table 19.1), which is charac­
terized most notably by infantile genitalia with a small penis and a 
poorly developed scrotum that lacks rugal folds and pigmentation. 
The testes are small, usually less than 2 cm in length and from 2 
mL to less than 4 mL in volume. Hair is thin and fine, and there is 
a lack of androgen-dependent hair growth (i.e., absence of 

 androgen-dependent hair growth (i.e., absence of a male 
hair pattern in all body areas) and no temporal hair recession. The 
pubic hair pattern is more typical of females, with the shape of an 
inverted triangle in the pubic area (female escutcheon) rather in 
a diamond shape with hair extending from the pubic area to the 
umbilicus (male escutcheon), and there is little hair extending to 
the thighs. Acne does not develop because sebum production is 
not stimulated due to androgen deficiency.

ion is 
Eunuchoidism is typified by a distinctive body habitus, charac­
terized by poor muscle mass development (especially in the shoul­
ders and chest), prepubertal fat distribution (predominantly in the 
face, chest, and hips), and excessively long arms and legs relative to 
height. Arm span exceeds height by greater than 5 cm, and the dis­
tance from the symphysis pubis to the floor exceeds the distance 
from the crown of the head to the symphysis p

nce 
from the crown of the head to the symphysis pubis by greater than 
5 cm. The voice is high pitched in the absence of androgen-depen­
dent laryngeal enlargement and vocal cord thickening. Relatively 
long arms and legs result from a failure of long bone epiphyses to 
close; epiphyseal closure is mediated normally by increased estra­
diol derived from aromatization of the increased testosterone pro­
duced at the time of puberty. In preliminary studies, aromatase 
inhibitors that inhibit the c

 studies, aromatase 
inhibitors that inhibit the conversion of testosterone to estradiol 
and delay epiphyseal closure have been used in combination with 
GH treatment in boys with idiopathic short stature; however, the 
long-term risks and benefits of this approach are not known.148
Androgen deficiency of prepubertal onset may not be recog­
nized or diagnosed until adulthood. Compromise in peak bone 
mass accrual due to androgen deficiency may manifest as low 
BMD for age, and prolonged severe 

anifest as low 
BMD for age, and prolonged severe androgen deficiency increases 
the risk of osteoporosis and fractures as these men become older. 
Despite the absence of pubertal development, these individuals 
may develop gynecomastia (benign breast enlargement) that is 
caused by androgen deficiency rather than by the relatively high 
production of estradiol concentrations associated with puber­
tal gynecomastia. (Both prepubertal and pubertal gynecomastia 
result from relatively high ratios 

 gynecomastia 
result from relatively high ratios of serum estradiol to testoster­
one concentrations but by different mechanisms.) Psychologi­
cal motivation and initiative may be reduced and, together with 
poor muscle mass and strength, may contribute to poor physical 
performance (e.g., in athletics or the military). These men have 
reduced sexual interest or desire (libido) and lack spontaneous 
erections at night or on awakening in the morning. Hematocrit 
691
Chapter 19  Testicular Disord

ing. Hematocrit 
691
  Clinical Manifestations of Androgen Deficiency
FETAL ANDROGEN DEFICIENCY
Symptoms
Signs
Ambiguous genitalia
Ambiguous genitalia (46,XY DSD)
Normal female genitalia
Microphallus (resembling clitoromegaly)
Pseudovaginal perineoscrotal hypospadias
Bifid scrotum
Cryptorchidism
ANDROGEN DEFICIENCY OF PREPUBERTAL ONSET
Symptoms
Signs
Eunuchoidism
Lack of sexual interest or desire (libido)
Infantile genitalia
Reduced nighttime or 

(libido)
Reduced nighttime or morning spontaneous erections
Small testes
Breast enlargement and tenderness
Lack of male hair pattern growth, no acne
Reduced motivation and initiative
Disproportionately long arms and legs relative to height
Diminished strength and physical performance
Prepubertal fat distribution
No ejaculate or ejaculation (spermarche)
Poorly developed muscle mass
Inability to father children (infertility)
Reduced peak bone mass, osteopenia

h-pitched voice
Reduced peak bone mass, osteopenia or osteoporosis
Gynecomastia
Small prostate gland
Aspermia, severe oligozoospermia or azoospermia
ADULT ANDROGEN DEFICIENCY
Symptoms
Signs
Incomplete sexual development
Small or shrinking testes
Loss of male hair (axillary and pubic hair)
Aspermia or a

ity to father children (infertility)
Aspermia or azoospermia or severe oligozoospermia
Height loss, history of minimal-trauma fracture
Low bone mineral density (osteopenia or osteoporosis)
Hot flushes, sweats
Height loss, minimal-trauma or vertebral compression fracture
Reduced shaving frequency
Unexplained reduction in prostate size or PSA
Less Specific Symptoms
Less Specific Signs
Decreased energy, vitality
Mild normocytic, normochromic anemia (normal female range)
Decreased motivation, self-c

(normal female range)
Decreased motivation, self-confidence
Depressed mood, mild depression or dysthymia
Feeling sad or blue, irritability
Reduced muscle bulk and strength
Weakness, decreased physical or work performance
Increased body fat or body mass index
Poor concentration and memory
Fine facial skin wrinkling (lateral to orbits and mouth)
DSD, Disorder of sex development; PSA, prostate-specific antigen.
   
TABLE 19.1
692
remains in the female ra

 Development and Function
remains in the female range due to inadequate androgen stimula­
tion of erythropoiesis. The prostate and seminal vesicles remain 
small without androgen stimulation, and seminal fluid produc­
tion is absent, resulting in aspermia (lack of ejaculate) and fail­
ure to undergo spermarche (first ejaculation). Seminal fluid may 
be present in men with mild or partial androgen deficiency of 
prepubertal onset or in those treated with androgens. However, 
these men usually hav

ed with androgens. However, 
these men usually have severe oligozoospermia or azoospermia, 
and most are infertile. 
Adult Androgen Deficiency
Some individuals with androgen deficiency of prepubertal onset 
who are not diagnosed or are inadequately treated as boys pres­
ent as adults with features of eunuchoidism and other manifesta­
tions of androgen deficiency of prepubertal onset (see Table 19.1). 
Their condition is usually clinically obvious because of inadequate 
sexual development for the

 because of inadequate 
sexual development for their chronologic age.
In adults, testosterone is needed to maintain sexual function, 
some secondary sexual characteristics, muscle and bone mass, and 
sperm production. Clinical manifestations of androgen deficiency 
are nonspecific and may be modified by the severity and dura­
tion of androgen deficiency, the presence of comorbid illnesses, 
previous testosterone treatment, or variations in target-organ sen­
sitivity to androgens. Therefore, the 

-organ sen­
sitivity to androgens. Therefore, the clinical diagnosis of androgen 
deficiency acquired as an adult can be challenging, particularly in 
older men.116
Some clinical symptoms and signs are suggestive of androgen 
deficiency. Adults most commonly present with sexual dysfunc­
tion (diminished libido as manifested by reduced sexual interest 
or desire, reduced spontaneous and sexually evoked erections, and 
erectile dysfunction), gynecomastia (benign breast enlargement 
that may be acc

mastia (benign breast enlargement 
that may be accompanied by tenderness), and infertility (inabil­
ity to father children despite unprotected intercourse) associated 
with oligozoospermia or azoospermia and small or shrinking tes­
tes with severe impairment in spermatogenesis. Secondary sexual 
characteristics do not regress to a prepubertal state; however, with 
long-standing, severe androgen deficiency, there may be loss of 
androgen-dependent hair, such as reduced facial hair associated 
wit

 hair, such as reduced facial hair associated 
with reduced shaving frequency and loss of axillary and pubic 
hair (Fig. 19.15). Men with rapid and profound decreases in tes­
tosterone concentrations (e.g., from GnRH agonist treatment of 
prostate cancer) may have hot flushes and sweats due to vasomo­
tor instability (like those experienced by menopausal women) 
due mostly to low estradiol production related to extremely low 
testosterone concentrations. Because testosterone and its active 
meta

rations. Because testosterone and its active 
metabolite, estradiol, have an important role in the maintenance 
of bone mass, men with chronic androgen deficiency may present 
with osteopenia or osteoporosis on BMD measurement (e.g., by 
dual-energy x-ray absorptiometry scanning) or with a minimal-
trauma bone or vertebral compression fracture that may be associ­
ated with height loss. An unexplained reduction in prostate size 
or in the level of prostate-specific antigen (PSA) is uncommon 
but 

 prostate-specific antigen (PSA) is uncommon 
but may occur as a result of long-term, severe acquired androgen 
deficiency.
Other symptoms and signs are much less specific for andro­
gen deficiency but may occur, commonly in conjunction with 
clinical manifestations described previously that are more sug­
gestive of androgen deficiency. Men with low testosterone 
C
D
A
B
E
F
• Fig. 19.14  A 19-year-old patient with androgen deficiency of prepubertal onset caused by congenital 
anorchia before (A

tal onset caused by congenital 
anorchia before (A, C, and E) and after (B, D, and F) 5 years of testosterone treatment. Before testosterone 
treatment, the patient had features of eunuchoidism, characterized by infantile genitalia (small penis and 
poorly developed scrotum); lack of chest, pubic, and facial hair; long arms and legs relative to height; and 
poorly developed muscle mass in the upper body with accumulation of fat in the face, chest, and hips. 
After testosterone treatment, there w

, and hips. 
After testosterone treatment, there was an increase in penis size; an increase in chest, pubic, and facial 
hair with scalp recession and development of acne; an increase in muscle mass, particularly in the upper 
body; and loss of fat in the face, chest, and hips. (From Matsumoto AM. The testis. In: Felig P, Frohman LA, 
eds. Endocrinology and Metabolism. 4th ed. New York, NY: McGraw-Hill; 2001:635–705.)
693
concentrations often complain of diminish

isorders
concentrations often complain of diminished energy and vital­
ity, poor motivation and social aggressiveness, depressed mood 
and irritability, or poor concentration and memory. Men with 
severe androgen deficiency may have a mild hypoproliferative 
normocytic, normochromic anemia within the female range in 
the absence of androgen stimulation of erythropoiesis. With 
long-standing deficiency, reduced muscle bulk and strength 
associated with weakness and reduced physical and work perfo

 with weakness and reduced physical and work perfor­
mance may occur. The latter symptoms may occur in conjunc­
tion with an increase in body fat, but androgen deficiency is not 
a cause of clinically obvious obesity per se. Skin changes and 
reduced sebum production with severe, long-standing androgen 
deficiency may be associated with fine facial wrinkling that is 
particularly noticeable on the lateral corners of the orbits (lat­
eral canthus) and mouth. Testis size may be small, especially 

 and mouth. Testis size may be small, especially 
with severe impairment of spermatogenesis, but in most men 
with acquired adult androgen deficiency, testis size is normal to 
slightly reduced.
Because clinical manifestations are nonspecific, older men 
may have several medical or comorbid conditions and medica­
tions that contribute to symptoms and signs that are consistent 
with androgen deficiency, presenting a particular diagnostic 
challenge (Fig. 19.16). Symptoms and signs of comorbid ill

e (Fig. 19.16). Symptoms and signs of comorbid ill­
nesses may mask, mimic, or contribute to clinical manifestations 
of androgen deficiency in older men. Elderly men may present 
with muscle loss and mobility impairment, fragility fracture 
or osteoporosis, and reduced vitality and depressed mood. On 
close examination, however, older men with severe, long-stand­
ing androgen deficiency usually manifest objective evidence of 
androgen deficiency. 
Isolated Impairment of Sperm Production or Func

. 
Isolated Impairment of Sperm Production or Function
Most men with male infertility have hypogonadism manifested by 
an isolated impairment of sperm production with normal androgen 
production. These men present as adults with infertility and demon­
strate oligozoospermia or azoospermia, sperm with abnormal mor­
phologic appearance (teratospermia) or reduced or absent motility 
(asthenospermia), or a combination of abnormalities on seminal fluid 
analysis. They do not have manifestations of an

d 
analysis. They do not have manifestations of androgen deficiency, 
and serum testosterone concentrations are normal. Testes may be 
small (if spermatogenesis is severely impaired) or normal size. Testes 
may not be palpable if cryptorchidism or anorchia is present. 
History and Physical Examination
Clinical evaluation of male hypogonadism involves a careful his­
tory and physical examination directed at determining whether 
there are symptoms and signs of androgen deficiency or isolated 
impa

and signs of androgen deficiency or isolated 
impairment of sperm production and at identifying potential 
common causes of hypogonadism.9 Because adults with androgen 
deficiency commonly present with sexual dysfunction, gyneco­
mastia, and infertility, the differential diagnosis of these condi­
tions and causes other than hypogonadism of these presenting 
complaints should be considered. Laboratory evaluation of serum 
testosterone, gonadotropins, and seminal fluid (in men who are 
concerned w

ns, and seminal fluid (in men who are 
concerned with infertility) are performed to confirm the diagno­
sis of hypogonadism and to determine whether there is predomi­
nantly primary or secondary hypogonadism.
The history should include inquiry regarding symptoms of 
androgen deficiency. Questions about the onset of these symp­
toms may be useful in determining whether the cause of androgen 
A
C
B
• Fig. 19.15  A 54-year-old man with adult androgen deficiency caused by hypopituitarism who present

n deficiency caused by hypopituitarism who presented 
with sexual dysfunction (reduced libido and erectile dysfunction); loss of chest, axillary, and pubic hair (A, 
B, and C); and gynecomastia (A). His penis and testes were normal in size (B). He had normal facial hair 
(C), but his shaving frequency was less.
694
deficiency occurred before birth, between birth and puberty, or 
after puberty. Note that symptoms or signs that suggest reduced 
androgen

t symptoms or signs that suggest reduced 
androgen effect compared with baseline generally indicate postpu­
bertal-onset of hypogonadism. These questions may be grouped in 
the following areas:
	
1.	 Development: Developmental markers of congenital andro­
gen deficiency include a history of genital abnormalities 
(hypospadias, microphallus, cryptorchidism); developmen­
tal markers of congenital or androgen deficiency of prepu­
bertal onset include a history of delayed sexual development 
or gro

de a history of delayed sexual development 
or growth and testosterone therapy to induce puberty; fam­
ily history of delayed puberty or reproductive disorders; 
psychological impact of delayed puberty or growth; diffi­
culty in school or learning disability (often associated with 
Klinefelter syndrome); inability or reduced ability to smell 
(suggesting CHH due to Kallmann syndrome)
	
2.	 Sexual function: Poor erections; reduced spontaneous, 
nighttime, or morning erections; inability to perfo

ighttime, or morning erections; inability to perform sexu­
ally; decreased sexual activity; inability to father children 
despite unprotected sexual intercourse (>1 year); small (sug­
gesting congenital or prepubertal onset of hypogonadism) 
or shrinking testes
	
3.	 Brain function: Poor general well-being; reduced sexual 
desire, interest, and motivation (libido); poor energy and 
vitality and excessive fatigue; poor motivation and initia­
tive, passivity, low self-confidence, and low self-est

, passivity, low self-confidence, and low self-esteem; 
depressed mood and irritability; difficulty sleeping; hot 
flashes and sweats; poor concentration and memory
	
4.	 Body function: Decreased muscle bulk and strength; reduced 
physical activity or performance; breast enlargement or ten­
derness, especially if recent in onset; height loss, history of 
low-trauma or vertebral compression fractures, osteopenia, 
or osteoporosis; body hair loss (chest, axillary, or pubic); 
reduced beard growth

(chest, axillary, or pubic); 
reduced beard growth and shaving frequency.
The initial history may also include inquiry concerning the 
potential cause of hypogonadism. With primary hypogonadism, 
there may be a history of mumps involving the testes; testicular 
trauma, irradiation, or surgery; complications of pelvic or scro­
tal surgery; medication use (spironolactone, ketoconazole, cyto­
toxic agents); or chronic liver or kidney failure. With secondary 
hypogonadism, headaches, visual complain

econdary 
hypogonadism, headaches, visual complaints or reduced peripheral 
vision, history of pituitary disease, chronic lung disease or conges­
tive heart failure (CHF), wasting conditions (e.g., acquired immu­
nodeficiency syndrome [AIDS], cancer), nutritional deficiencies, 
recent acute illness, morbid obesity, or use of certain medications 
(e.g., opioids, glucocorticoids, anabolic steroids, megestrol ace­
tate, medroxyprogesterone acetate, nutritional supplements) may 
be noted.
The patien

nutritional supplements) may 
be noted.
The patient should be questioned regarding conditions that 
are relative or absolute contraindications to testosterone treat­
ment, including a history of severe BPH and LUTS as assessed 
by the American Urological Association (AUA) symptom score 
or International Prostate Symptom Score (IPSS), history of pros­
tate or breast cancer, history or symptoms of untreated obstruc­
tive sleep apnea syndrome (daytime sleepiness, snoring with 
sleep disruption, wit

me sleepiness, snoring with 
sleep disruption, witnessed apnea episodes), history of CHF, and 
polycythemia.
In patients with suspected androgen deficiency of prepubertal 
onset, physical examination should include measurements of total 
arm span, height, and the distances from the crown of the head 
to the symphysis pubis and from the symphysis pubis to the floor 
to determine whether the patient has excessively long arms and 
legs (see Fig. 19.14). Eunuchoidal body proportions are character­
i

14). Eunuchoidal body proportions are character­
ized by an arm span that is at least 5 cm greater than height and 
a symphysis-to-floor distance that is at least 5 cm greater than 
crown-to-symphysis distance; such proportions are indicative of 
androgen deficiency of prepubertal onset. Men with Klinefelter 
syndrome may have disproportionately long legs relative to arms 
A
B
• Fig. 19.16  A 70-year-old man with severe androgen deficiency caused by Kallmann syndrome (hypogo­
nadotropic hypogona

 by Kallmann syndrome (hypogo­
nadotropic hypogonadism and anosmia) who presented to a geriatric evaluation and management unit 
with functional and mobility disability caused by upper (A) and lower extremity muscle wasting and severe 
back pain from multiple vertebral compression fractures (B) due to osteoporosis. He was noted to have 
gynecomastia and absence of chest, axillary, and pubic hair.
695
and a greater ratio of lower- to upper body segment measurement

 ratio of lower- to upper body segment measurements 
but a relatively normal ratio of arm span to height. Eunuchoid­
ism is also characterized by infantile genitalia (micropenis or 
small penis, unrugated and nonpigmented scrotum); small testes 
or, rarely, absence of the testes; cryptorchidism; sparse or absent 
facial, axillary, chest, extremity, and pubic hair; poorly developed 
upper body musculature; fat predominance in the face, chest, and 
hips; and gynecomastia. Patients with Kallmann sy

hips; and gynecomastia. Patients with Kallmann syndrome may 
have anosmia or hyposmia that may be tested with an odor iden­
tification and threshold test using readily identifiable, common 
household odorants (e.g., peppermint, cinnamon, cocoa, coffee, 
cigarette, orange, soap), or more formally, such as with the Uni­
versity of Pennsylvania scratch and sniff test.
The physical findings of androgen deficiency acquired in adult­
hood are usually subtler than those of androgen deficiency of 
prep

subtler than those of androgen deficiency of 
prepubertal onset (see Fig. 19.15). In patients with severe, long-
standing adult androgen deficiency, there may be loss of andro­
gen-dependent facial, axillary, chest, extremity, and pubic hair; 
however, there are ethnic variations in body hair in androgen-
dependent areas (e.g., less in Asians and Hispanics). The skin may 
be dry, and there may be fine wrinkling lateral to orbits or the 
mouth in patients with severe, long-standing androgen defic

patients with severe, long-standing androgen deficiency. 
Patients should be carefully examined for the presence of palpa­
ble breast tissue or gynecomastia; presence, size, and consistency 
of the testes; and palpable abnormalities in the scrotum, such as 
varicocele, epididymal enlargement, or tenderness or absence of 
the vas deferens. In older men, particularly those at high risk for 
prostate cancer (e.g., African-American ethnicity, family history of 
prostate cancer), a digital rectal exa

history of 
prostate cancer), a digital rectal examination (DRE) to screen for 
prostate cancer should be offered. If performed, the focus of the 
examination is to detect palpable abnormalities, such as a prostate 
nodule or induration and to assess the size of the prostate. Careful 
examination for kyphosis and measurement of height are useful for 
detecting significant height loss (>5 cm) associated with osteopo­
rotic vertebral compression fractures that may be asymptomatic.
Proper technique

actures that may be asymptomatic.
Proper technique is needed to examine the male breast. The 
thumb and index finger (or the fingers of both hands) are used to 
grasp and gently grasp the periareolar area of the breast and to pal­
pate glandular breast tissue, which is rubbery in consistency and 
firmer than the surrounding adipose tissue (Fig. 19.17). Breast 
tissue is generally discoid and has a firm edge that can be pal­
pated and “flipped up” to distinguish it from adipose tissue. With 
this

 to distinguish it from adipose tissue. With 
this technique, gynecomastia can usually be distinguished from 
excessive breast adipose tissue, called pseudogynecomastia, which 
is often associated with generalized obesity. Gynecomastia is usu­
ally bilateral and relatively symmetric, but occasionally it is asym­
metric and more prominent on one side. If present, asymmetric 
gynecomastia may suggest breast carcinoma, which is usually rock 
hard and irregular and may be associated with skin dimpli

d irregular and may be associated with skin dimpling (peau 
d’orange), nipple retraction or discharge, and axillary lymphade­
nopathy. The diameter of palpable breast tissue is used as an objec­
tive measure of gynecomastia. Gynecomastia of recent onset is 
usually tender on palpation, and men usually complain of nipple 
irritation associated with rubbing against clothing.
Examination of the testes and scrotum may be performed 
with the patient either lying on his back or standing, but the lat­

ither lying on his back or standing, but the lat­
ter position is preferred because it relaxes the scrotum, making 
some abnormalities (e.g., varicocele) easier to detect. In patients 
with retractile testes positioned high in the scrotum, it may be 
possible to palpate the testes only after placing the scrotum in 
warm water, after a warm bath, or by having the patient assume 
a squatting position. The testes may be very difficult to examine 
and palpate in morbidly obese men who have excessive

d palpate in morbidly obese men who have excessive folds of 
fat overlying the scrotum, in the presence of a large hydrocele, if 
the testis is tender (e.g., with epididymo-orchitis or testicular tor­
sion), or occasionally in some men who are sensitive to palpation 
for unclear reasons. In these instances, testicular ultrasound may 
be required to confirm the presence of the testis, estimate its size, 
and detect abnormalities.
Although ultrasonographic size estimates are more accurate, 
testis

ographic size estimates are more accurate, 
testis size can be estimated by measuring the length and width 
with a ruler or calipers or by comparing testis volume with that 
of ellipsoid models of known volume (Prader orchidometer) (Fig. 
19.18). Normal testis size varies with age and ethnicity. Normal 
prepubertal testis size is approximately 1.6 to 2.9 cm in length and 
1.0 to 1.8 cm in width, or 1 to 4 mL in volume. Testis size greater 
• Fig. 19.17  The proper method of examining the male br

 19.17  The proper method of examining the male breast is to use 
the thumb and index finger to grasp the periareolar area of the breast 
and to gently pinch the thumb and index finger together on either side of 
the breast toward the nipple. Glandular breast tissue feels like a rubbery 
disc of tissue that extends concentrically from under the nipple and sub­
areolar area and is firmer than the surrounding adipose tissue. The size of 
gynecomastia is estimated by measurement of the diameter of 

ia is estimated by measurement of the diameter of palpable 
breast tissue. (From Matsumoto AM. The testis. In: Felig P, Frohman LA, 
eds. Endocrinology and Metabolism. 4th ed. New York, NY: McGraw-Hill; 
2001:635–705.)
• Fig. 19.18  The assessment of testis size using a Prader orchidometer in 
a man with classic 47,XXY Klinefelter syndrome is characteristically very 
small (e.g., 2 mL). Care must be taken not to include the head of the epi­
didymis when estimating testis size.
696
SECTION V    S

is when estimating testis size.
696
than 3 mL (by orchidometer) suggests the onset of puberty.149 In 
adults, normal testes usually measure 3.5 to 5.5 cm in length and 
2.0 to 3.0 cm in width or 15 to 30 mL in volume.8,9 In addition to 
size, testes should be palpated for consistency or firmness and for 
presence of a mass representing a benign or malignant testicular 
tumor. The testicular examination in men with classical Klinefel­
ter syndrome is n

 in men with classical Klinefel­
ter syndrome is notable for very small (usually <3 mL), firm testes. 
Because sexual dysfunction, gynecomastia, and infertility are often 
presenting complaints in adults with androgen deficiency, it is 
important to consider the differential diagnosis of these conditions 
and to be familiar with other common causes of these manifesta­
tions when evaluating men who present with these complaints.
Sexual Dysfunction
Normal sexual function req

nts.
Normal sexual function requires successive, coordinated physi­
ologic events—libido, erection, ejaculation, orgasm, and detu­
mescence—that occur in a defined sequence and require normal 
psychological, CNS, peripheral nerve, vascular, and genital 
function.150
Sexual dysfunction may involve specific disorders of libido or 
sexual desire, erectile dysfunction, ejaculatory disorders, orgasmic 
dysfunction, or failure of detumescence. These components may 
occur in isolatio

umescence. These components may 
occur in isolation, but specific disorders of sexual function com­
monly occur together because these processes are interrelated and 
because a specific cause (e.g., testosterone and/or estrogen defi­
ciency due to hypogonadism) can affect more than one of the 
physiologic processes that mediate normal sexual function. Male 
sexual dysfunction is detailed in Chapter 20. Men with androgen 
deficiency often present with sexual dysfunction, and it is impor­
tant to 

with sexual dysfunction, and it is impor­
tant to consider the differential diagnosis of this complaint in the 
evaluation.
Androgen deficiency often results in reduced libido or sexual 
desire (hypoactive sexual desire disorder); loss or reduction of 
spontaneous evening and morning or sexually stimulated erec­
tions (erectile dysfunction); and, if severe, reduced or absent ejac­
ulation. In many men with androgen deficiency, erectile response 
to intense erotic stimuli (and, occasionally, spon

to intense erotic stimuli (and, occasionally, spontaneous erections) 
may be preserved, suggesting that the androgen requirement for 
sexual function is variable.151 However, persistent erectile dysfunc­
tion may cause performance anxiety, and, together with hypoac­
tive sexual desire and depressed mood associated with androgen 
deficiency, this may contribute to the eventual loss of erotically 
stimulated erections and, secondarily, to orgasmic dysfunction. 
Androgen deficiency may also affect 

dysfunction. 
Androgen deficiency may also affect nitric oxide (NO) produc­
tion and maximal smooth muscle relaxation and vasodilatation 
within the penis, reducing the ability to produce an erection that 
is sufficient to satisfactorily complete sexual intercourse and fur­
ther contributing to the severity of erectile dysfunction.152,153
Clinically, men with androgen deficiency most commonly 
present with hypoactive sexual desire disorder and erectile dys­
function. Men with severe androgen def

ectile dys­
function. Men with severe androgen deficiency may present with 
reduced ejaculation, but these individuals usually also complain of 
hypoactive sexual desire disorder and erectile dysfunction.
Hypoactive Sexual Desire Disorder and Erectile Dysfunction
Libido, the desire or drive for sexual activity, is generated by exter­
nal visual, auditory, and tactile stimuli, as well as internal psychic 
stimuli acting on cortical and subcortical brain regions such as 
the limbic system (amygdal

 brain regions such as 
the limbic system (amygdala, hippocampus, anterior thalamic 
nuclei, and prefrontal cortex) and the temporal lobe. Stimuli from 
these areas are relayed to the medial preoptic area, which serves to 
integrate central inputs and sends impulses to the paraventricular 
nuclei; these, in turn, send projections to the thoracolumbar and 
sacral spinal cord centers that regulate penile erection. This neu­
ral pathway explains why brain disorders that cause hypoactive 
sexual des

 brain disorders that cause hypoactive 
sexual desire disorder are usually accompanied by varying degrees 
of erectile dysfunction (see later discussion).150 In particular, there 
is a loss of the spontaneous evening and morning erections that are 
associated with brain activation of sexual neural pathways during 
rapid eye movement sleep and dreaming. Clinically, libido may 
be influenced by previous or recent sexual activity and by expe­
riences, psychosocial background, overall state of gener

s, psychosocial background, overall state of general health, 
androgen sufficiency, and brain function.
The neurotransmitter systems that regulate the physiology of 
normal libido are not precisely known. However, there is evidence 
that central dopamine neurotransmission may be important 
in mediating CNS regulation of sexual desire and erections. In 
humans, treatment with dopamine receptor agonists (e.g., bro­
mocriptine, pergolide) may stimulate spontaneous erections (and 
even hypersexual b

ate spontaneous erections (and 
even hypersexual behavior), and in 20% to 30% of men with Par­
kinson disease, levodopa therapy is associated with stimulation 
of libido and spontaneous erections. The use of pharmacologic 
agents with dopamine receptor antagonist activity is frequently 
associated with reduced libido and erectile dysfunction. However, 
these agents also affect many other neurotransmitter systems. 
Dopamine antagonism (e.g., by neuroleptic or antipsychotic 
agents) results in ele

uroleptic or antipsychotic 
agents) results in elevated prolactin concentrations that suppress 
endogenous gonadotropin and testosterone secretion and may 
contribute to reduced libido and erectile dysfunction.
Hypoactive sexual desire disorder is defined as persistent or recur­
rent deficiency or absence of desire for sexual activity resulting in 
marked personal distress, interpersonal difficulty, or both150,154,155 
and is estimated to affect more than 15% of men. The causes of 
hypoactive se

more than 15% of men. The causes of 
hypoactive sexual desire disorder are primarily disorders that affect 
normal brain function and are usually associated with erectile dys­
function, particularly with the loss of spontaneous evening or 
morning erections (Table 19.2). Erectile dysfunction is defined as 
the inability to achieve or maintain penile erection that is adequate 
for completion of satisfactory sexual intercourse or activity.156 
Erectile dysfunction is a common condition that increa

tile dysfunction is a common condition that increases with 
aging. It is estimated to affect fewer than 10% of men younger 
than 40 years of age but approximately 50% of men between 40 
and 70 years of age, with 35% of men in the latter age group hav­
ing moderate or complete erectile dysfunction.
Hypoactive Sexual Desire Disorder and Erectile Dysfunction 
Due to Brain Disorders. Psychogenic disorders commonly cause 
hypoactive sexual desire and erectile dysfunction. These disorders 
include str

erectile dysfunction. These disorders 
include stress or preoccupation associated with life circumstances 
or situations, illness, marital discord, or underlying maternal 
transference or gender identity issues; performance anxiety associ­
ated with fear of failure or preoccupation with the adequacy of 
erections during sexual intercourse; major depression or dysthy­
mia (moderate or complete erectile dysfunction occurs in 60% to 
90% of men with moderate to severe depression); and major psy­
ch

 moderate to severe depression); and major psy­
chiatric illness such as psychotic or personality disorders.150,154,155
Chronic systemic illness (chronic heart disease, respiratory ill­
ness, kidney or liver failure, or cancer) and poor general health 
are usually associated with reduced libido and spontaneous erec­
tions.150,154,155 Several CNS-active medications may cause hypo­
active sexual desire disorder and erectile dysfunction, including 
alcohol, centrally acting antihypertensive medicat

alcohol, centrally acting antihypertensive medications, narcotics, 
697
sedative-hypnotic drugs, anticonvulsants, antidepressants, and 
antipsychotic medications. In addition to their direct effects on 
brain neurotransmitter function, both chronic illness and CNS-
active medications may also be associated with androgen defi­
ciency. Structural brain disease, such as infiltrative or destructive 
lesions of the temporal lobe or limbic system, Parkinson or other 
n

poral lobe or limbic system, Parkinson or other 
neurodegenerative brain disease, or vascular brain disorders such as 
stroke or vasculitis, may reduce libido and spontaneous erections.
Androgen deficiency is commonly associated with reduction or 
loss of libido and spontaneous erections.157,158 Sexual dysfunction 
is usually a prominent presenting complaint in young men who 
are severely androgen deficient and in older men who are treated 
with medical therapies (e.g., GnRH agonist treatment) o

medical therapies (e.g., GnRH agonist treatment) or surgi­
cal castration for advanced prostate cancer. In contrast, older men 
with less severe androgen deficiency may have sexual dysfunction 
that is also related to underlying depression, chronic systemic 
illness, or use of certain medications.159 Comorbid conditions 
contribute to the nonspecificity of presenting complaints of 
androgen deficiency (e.g., sexual dysfunction) as men age. Tes­
tosterone treatment of severe androgen deficiency i

osterone treatment of severe androgen deficiency in young men 
usually improves sexual desire, interest, and thoughts; attentive­
ness to erotic stimuli; and the frequency, duration, and rigidity 
of spontaneous evening and morning erections.158,160,161 In older 
hypogonadal men with reduced libido, 1 year of a physiologic 
dose of transdermal testosterone improved sexual activity and 
desire compared with placebo.162
Other endocrine disorders can cause hypoactive sexual desire 
disorder and ere

n cause hypoactive sexual desire 
disorder and erectile dysfunction; examples include hyperprolac­
tinemia, Cushing syndrome (glucocorticoid excess), hyperthy­
roidism, and hypothyroidism. In addition to their direct effects 
on brain function, hyperprolactinemia and glucocorticoid excess 
also suppress GnRH and gonadotropin secretion and induce 
androgen deficiency that contributes to sexual dysfunction. Anec­
dotally, some men with androgen deficiency due to severe hyper­
prolactinemia who are

iciency due to severe hyper­
prolactinemia who are treated with testosterone alone do not fully 
recover sexual function and may require additional therapy with 
dopamine receptor agonists, but this has not been demonstrated 
conclusively. Dopamine receptor agonists lower elevated prolac­
tin concentrations and may also have direct affects in the brain 
to activate neuronal systems involved in stimulating libido and 
erections. 
Erectile Dysfunction Due to Spinal Cord or Peripheral Disor­
ders. 

ion Due to Spinal Cord or Peripheral Disor­
ders. External and internal erotic stimuli from the brain are relayed 
via descending neural pathways in the lateral spinal columns to 
stimulate the parasympathetic sacral (S2-S4) spinal erection cen­
ter, resulting in psychogenic erections. Efferent parasympathetic ner­
vous system stimulation from the sacral center travels via the nervi 
erigentes (pelvic splanchnic nerve) and the pelvic plexus and enters 
the penis via the cavernosal nerve. This st

nters 
the penis via the cavernosal nerve. This stimulation causes relax­
ation of the smooth muscles that form sponge-like interconnected 
trabecular spaces within the corpora cavernosa of the penis and 
vasodilation of the cavernosal arterioles and vascular sinusoids. As 
a result, blood flow and pressure into the trabecular spaces within 
the corpora increase several fold and cause engorgement of the 
penis (tumescence). Expansion of the trabecular spaces against 
the thick fibrous sheath (tu

cular spaces against 
the thick fibrous sheath (tunica albuginea) surrounding the cor­
pora compresses subtunical venules and impedes venous outflow, 
resulting in sustained penile tumescence (i.e., an erection).150,156
Afferent somatic (via the pudendal nerve) and parasympathetic 
impulses in response to sensory stimulation of the penis with sex­
ual intercourse or masturbation also act to stimulate erections via 
a reflex arc through the sacral spinal erection center, resulting in 
reflexogeni

 spinal erection center, resulting in 
reflexogenic erections. Pudendal nerve stimulation also triggers the 
reflex contraction of the ischiocavernosus and bulbocavernosus 
muscles, resulting in vascular compression at the base of the penis, 
further increasing cavernosal blood pressure and maximal penile 
rigidity, leading to the plateau phase of erection.
The primary neurotransmitter that mediates penile smooth 
muscle relaxation and erection is NO. In response to parasym­
pathetic cholinergic

s NO. In response to parasym­
pathetic cholinergic (acetylcholine-mediated) stimulation, NO is 
synthesized from its precursor, l-arginine, by the enzyme nitric 
oxide synthase and is released by corporal sinusoidal endothelial 
cells and postganglionic noncholinergic, nonadrenergic nerve 
terminals. NO then enters adjacent smooth muscle cells, where 
it activates guanylyl cyclase and increases intracellular cyclic 
guanosine monophosphate (cGMP). cGMP activates cGMP-
dependent protein kinase, w

. cGMP activates cGMP-
dependent protein kinase, which phosphorylates many proteins, 
including myosin light chains and ion channels that ultimately 
decrease intracellular calcium concentrations, causing smooth 
  Causes of Hypoactive Sexual Desire 
Disorder and Erectile Dysfunction
Cause
Examples
Brain Disorders
Psychogenic disorders
Stress or preoccupation, performance anxi­
ety, depression, major psychiatric illness
Chronic systemic illness
Heart, respiratory, kidney, or liver failure; 
can

cancer
CNS-active drugs
Alcohol; antihypertensive, narcotic, sedative-
hypnotic, anticonvulsant, antidepressant, 
antipsychotic medications
Structural brain disease
Temporal lobe or limbic system disorders, 
Parkinson or other neurodegenerative 
brain disease, vascular brain disorders
Androgen deficiency
Primary and secondary hypogonadism
Other endocrine 
disorders
Hyperprolactinemia, Cushing syndrome, 
hyperthyroidism, hypothyroidism
Spinal Cord an

e, 
Spinal Cord and Peripheral Disorders
Spinal cord disorders
Trauma, vascular compromise, spinal steno­
sis, epidural abscess, tumor, transverse 
myelitis, multiple sclerosis, other spinal 
cord lesions
Peripheral nerve 
disorders
Diabetes mellitus; pelvic, prostate, or 
retroperitoneal surgery or damage; other 
causes of peripheral neuropathy
PNS-active drugs
Anticholinergic, antihistamine, antidepressant, 
sympathomimetic, α-adrenergic agonist, 
β-adrenergic a

athomimetic, α-adrenergic agonist, 
β-adrenergic antagonist medications
Peripheral vascular 
disease
Aortoiliac atherosclerosis, diabetes mel­
litus, trauma, surgery, vasculitis, venous 
incompetence (venous leakage), smoking
Antihypertensive drugs
Diuretics, β-adrenergic antagonists, calcium 
channel antagonists
Penile abnormalities
Peyronie disease, chordee, trauma, pria­
pism, phimosis
CNS, Central nervous system; PNS, peripheral nervous system.
   
TABLE 19.2
698
SECTION V    Sexual Developm

m.
   
TABLE 19.2
698
muscle relaxation, increase in penile blood flow, and erection. 
cGMP is primarily hydrolyzed and inactivated by the enzyme 
phosphodiesterase type 5 (PDE5). In addition to cGMP, other 
neurotransmitters induce cavernosal smooth muscle relaxation, 
including prostaglandin E1 (PGE1) that activates adenylyl cyclase 
and increases cAMP and cAMP-dependent protein kinase.
Knowledge of the neurotransmitter systems that control erec­
ti

the neurotransmitter systems that control erec­
tions has been used to design pharmacologic treatments for erectile 
dysfunction156,163,164 (detailed in Chapter 20). The most com­
monly used treatments are oral PDE5 inhibitors, such as silde­
nafil, vardenafil, and tadalafil, which act to inhibit the breakdown 
of cGMP, resulting in more sustained smooth muscle relaxation 
and improved penile erection after erotic stimulation. Injection 
of intracavernosal alprostadil (PGE1) or insertion of intr

cavernosal alprostadil (PGE1) or insertion of intraurethral 
alprostadil pellets acts to increase cavernosal cAMP concentra­
tions and induce smooth muscle relaxation and penile erection 
even in the absence of sexual stimulation. Intracavernosal injec­
tions of papaverine, a nonspecific PDE inhibitor (which inhibits 
the breakdown of both cGMP and cAMP), combined with phen­
tolamine, an α1- and α2-adrenergic receptor antagonist vasodilator 
(bi-mix), or the two combined together with alprostadi

mix), or the two combined together with alprostadil (tri-mix), 
are also used to induce smooth muscle relaxation and erection.
Nonpharmacologic therapies include vacuum pumps that draw 
blood into the penis and induce tumescence. Vacuum pumps are 
often used with veno-occlusive elastic rings to maintain the erec­
tion. Some men are best treated with surgically implanted penile 
prostheses.
Studies in experimental animals and in vitro have found that 
androgen deficiency impairs penile nerve, tra

hat 
androgen deficiency impairs penile nerve, trabecular smooth 
muscle, vascular endothelial, and tunica albuginea structure and 
function; reduces both endothelial and neuronal NO synthase syn­
thesis and activity; and causes accumulation of adipocytes in the 
subtunical region of the corpora cavernosa.153 These changes are 
reversed with androgen administration, suggesting a direct penile 
effect of androgens in addition to their central role in maintaining 
penile erections. In humans, ARs 

 in maintaining 
penile erections. In humans, ARs are expressed in the corpora cav­
ernosa tissue. However, there is no conclusive evidence to support 
the notion that androgen treatment has a direct effect in the penis 
to enhance the response to PDE5 inhibitor therapy in androgen-
deficient men with erectile dysfunction. In practice, symptomatic 
men with androgen deficiency and sexual dysfunction are usually 
treated with testosterone replacement, which can variably improve 
erectile dysfunct

ent, which can variably improve 
erectile dysfunction, particularly in younger hypogonadal men 
with severe androgen deficiency and no comorbid illness. In the 
placebo-controlled Testosterone Trials, 1 year of physiologic trans­
dermal testosterone therapy improved erectile function in older 
men with hypogonadism, although the improvement was less 
than that observed with PDE5 inhibitors.162 If erectile dysfunc­
tion does not improve with testosterone treatment alone, patients 
are given addit

sterone treatment alone, patients 
are given additional therapy for erectile dysfunction (e.g., addition 
of a PDE5 inhibitor). In some hypogonadal men, PDE5 inhibitor 
treatment alone may be sufficient to improve erectile dysfunction 
but is not adequate to treat reduced libido or other symptoms of 
androgen deficiency.161
In addition to the brain disorders that cause hypoactive sexual 
desire disorder and erectile dysfunction, spinal cord and periph­
eral disorders (e.g., peripheral nervous sy

eriph­
eral disorders (e.g., peripheral nervous system disorders, periph­
eral vascular disease, medications that affect peripheral nerve and 
vascular function, penile abnormalities) may also cause erectile 
dysfunction that is usually not associated with hypoactive sexual 
desire (see Table 19.2). However, long-standing, severe erectile 
dysfunction may cause performance anxiety or depression, which 
may secondarily reduce libido. Furthermore, peripheral disorders 
that cause erectile dysfunct

peripheral disorders 
that cause erectile dysfunction may also affect brain function and 
alter sexual interest and drive, contributing to erectile dysfunc­
tion. Tricyclic antidepressants may affect both peripheral nervous 
system and CNS function.
Spinal cord disorders, such as spinal cord injury due to trauma, 
vascular compromise, spinal stenosis, epidural abscess, tumor, 
transverse myelitis, multiple sclerosis, or other spinal cord lesions, 
usually cause erectile dysfunction. In general, 

usually cause erectile dysfunction. In general, the severity of erec­
tile dysfunction associated with spinal cord injury and the response 
to treatment vary with the cord level involved, the severity of 
the lesion (i.e., complete vs. incomplete), and the time since the 
injury. Peripheral nerve disorders, particularly those that affect the 
autonomic nervous system, may disrupt the normal regulation of 
penile erectile tissue and cause erectile dysfunction. For example, 
erectile dysfunction

le dysfunction. For example, 
erectile dysfunction may be caused by diabetes mellitus or other 
diseases that cause peripheral neuropathy (e.g., amyloidosis, vas­
culitis, heavy metal toxicity, renal failure, multiple system atrophy, 
acute intermittent porphyria) or by pelvic, prostate, or retroperi­
toneal surgery or damage (e.g., abdominoperineal resection of the 
rectum, pelvic lymph node dissection, prostatectomy, aortoiliac 
bypass, lumbar sympathectomy). Peripheral nervous system medi­
ca

sympathectomy). Peripheral nervous system medi­
cations, including anticholinergic agents, antihistamines, antide­
pressants, sympathomimetic medications, α-adrenergic agonists, 
and β-adrenergic antagonists, often impair erectile function by 
affecting peripheral nervous system regulation of erectile tissue of 
the penis, and many also cause erectile dysfunction by altering 
neurotransmitter function in the nervous system and penis.
The blood supply of the penis is derived from the internal ili

pply of the penis is derived from the internal iliac 
(hypogastric) artery, a branch of the common iliac artery that 
bifurcates from the aorta.150 The internal iliac artery gives rise to 
the internal pudendal artery, which branches into the dorsal penile, 
bulbourethral, and cavernosal arteries. The cavernosal arteries run 
through the middle of the corpora cavernosa and give off cork­
screw-shape branches, the helicine arteries, that open directly into 
the lacunar spaces. Smooth muscle relax

ctly into 
the lacunar spaces. Smooth muscle relaxation of lacunar spaces 
increases blood flow into the corpora cavernosa, resulting in penile 
tumescence. Blood from the lacunar spaces or cavernosal sinusoids 
collects in the subtunical plexus and is delivered via emissary veins 
to the deep dorsal vein, which ultimately drains into the internal 
and common internal iliac veins and then into the inferior vena 
cava. With filling of the lunar spaces of the corpora cavernosa and 
penile tumescen

aces of the corpora cavernosa and 
penile tumescence against the fibrous tunica albuginea, venous 
outflow from the subtunical venous plexus is impeded, and sus­
tained tumescence or erection ensues. Disorders of arterial inflow 
or venous output may cause erectile dysfunction.
Peripheral vascular disease due to aortoiliac atherosclerosis is 
probably the most common cause of erectile dysfunction in aging 
men.150,156 These men usually have absent or severely diminished 
femoral artery pulses, a

t or severely diminished 
femoral artery pulses, and some present clinically with Leriche 
syndrome (absent femoral pulses, buttock or leg claudication, and 
erectile dysfunction). Other men with iliac atherosclerosis may be 
able to achieve an erection, but with penetration and use of the hip 
muscles for thrusting during sexual intercourse, blood is diverted 
from the penis to the hips, resulting in premature detumescence 
and loss of erection; this is known as the pelvic steal syndrome. 
Athe

 this is known as the pelvic steal syndrome. 
Atherosclerotic large- and small-vessel disease may contribute to 
erectile dysfunction in men with diabetes mellitus, hypertension, 
CKD, smoking, and other atherosclerotic risk factors. Erectile 
dysfunction occurs in about 50% of men with diabetes mellitus. 
Smoking, specifically nicotine, also causes direct vasoconstric­
tion of the corpora cavernosum and erectile dysfunction. Other 
699
conditions that compromise

9  Testicular Disorders
conditions that compromise aortoiliac circulation, such as pel­
vic trauma, irradiation, and vasculitis, are less common causes 
of erectile dysfunction. Chronic pressure on the pudendal artery 
from bicycle riding, especially with some bicycle seats, may cause 
penile ischemia and erectile dysfunction; in addition, pressure on 
the pudendal nerve may cause penile numbness and contribute 
to sexual dysfunction. Penile venous incompetence (venous leakage) 
may cause premat

us incompetence (venous leakage) 
may cause premature loss of erections and inability to maintain 
erections sufficient to complete intercourse.
Many antihypertensive medications, including diuretics, α- and 
β-adrenergic antagonists, angiotensin-converting enzyme inhibi­
tors, and calcium channel antagonists, have been implicated as 
causes of erectile dysfunction. Penile abnormalities, such as Peyro­
nie disease or chordee (fibrosis or scarring of the tunica albuginea 
resulting in bending of 

 of the tunica albuginea 
resulting in bending of the penis), micropenis or microphallus, 
penile trauma, phimosis (inability to retract the foreskin over the 
penis), and priapism (painful extended erections) may also cause 
erectile dysfunction. 
Evaluation of Erectile Dysfunction. The cause of erectile 
dysfunction is usually strongly suspected based on a careful 
medical, psychiatric, and medication history and physical exami­
nation.150,156 Erectile dysfunction of psychogenic origin usually

Erectile dysfunction of psychogenic origin usually 
occurs abruptly, is transient, is intermittent or associated with a 
stressful situation, occurs with only some partners but not with 
others, or does not occur with masturbation. Spontaneous eve­
ning and morning erections are usually maintained in psychogenic 
erectile dysfunction but lost with organic causes. Spontaneous 
erections may be detected by formal measurements of nocturnal 
penile tumescence in a sleep laboratory or by breakage of 

umescence in a sleep laboratory or by breakage of wires 
of different tensile strength in a snap gauge (RigiScan), but these 
assessments are not routinely performed in practice and usually 
are not necessary.
Patients with nonpsychogenic brain disorders, spinal cord or 
peripheral nervous system disorders, vascular disorders, or penile 
abnormalities that cause organic erectile dysfunction usually 
exhibit clinical manifestations of the underlying disorder, and 
offending drugs that impair erec

ng disorder, and 
offending drugs that impair erectile function are revealed with a 
careful review of medications. Androgen deficiency is a cause of 
reduced libido and erectile dysfunction and occurs in 15% to 20% 
of men who complain of sexual dysfunction in a general medical 
clinic.165 Therefore, evaluation of men who present with sexual 
dysfunction should include inquiry regarding other symptoms 
of androgen deficiency, examination for signs such as small testis 
size and gynecomastia, an

ns such as small testis 
size and gynecomastia, and confirmation of androgen deficiency 
by measurement of serum testosterone concentrations (see later 
discussion).
Peripheral pulses, particularly the presence of femoral pulses, 
should be tested to assess for peripheral vascular disease. Diagno­
sis of penile vascular insufficiency may be suspected by Doppler 
ultrasound measurement of the ratio of penile to brachial systolic 
blood pressure (penile/brachial index). A penile/brachial index 
gr

enile/brachial index). A penile/brachial index 
greater than 0.75 is normal, whereas an index of less than 0.60 is 
suggestive of vascular erectile dysfunction. If there is a clinical sus­
picion of spinal cord disease, perineal and penile sensation should 
be assessed. A cremasteric reflex (stroking of the inner thigh asso­
ciated with contraction of the ipsilateral cremasteric muscle and 
pulling up of the scrotum and testis) and a bulbocavernosus reflex 
(squeezing of the glans penis associat

sus reflex 
(squeezing of the glans penis associated with contraction of the 
anal sphincter) should be elicited to assess spinal cord levels L1-L2 
and S2-S4, respectively. Finally, the penis should be examined for 
abnormalities, such as penile plaques, angulation, or tight and 
unretractable foreskin. 
Ejaculatory Disorders and Orgasmic Dysfunction
After the plateau phase of erection is achieved, sympathetic ner­
vous system stimulation from the thoracolumbar (T10-L2) spinal 
erection center 

he thoracolumbar (T10-L2) spinal 
erection center travels via the hypogastric nerve and pelvic plexus, 
enters the penis via the cavernosal nerve, and causes α-adrenergic 
receptor–mediated contraction of the cauda epididymis, vas defer­
ens, accessory sex glands (the bulbourethral or Cowper glands and 
the urethral glands or glands of Littre), prostate, seminal vesicles, 
and ejaculatory ducts that moves sperm and semen into the pos­
terior urethra (emission). It also stimulates closure of the 

hra (emission). It also stimulates closure of the internal 
urethral sphincter to prevent retrograde ejaculation of sperm into 
the bladder.150 After emission, continued sensory stimulation of 
the penis with sexual intercourse or masturbation stimulates reflex 
rhythmic contractions of the ischiocavernosus and bulbocaver­
nosus muscles, resulting in expulsion of semen from the urethra 
(ejaculation).150
Like erection, ejaculation is under considerable control by 
higher brain centers, with both

erable control by 
higher brain centers, with both voluntary and involuntary regula­
tion.150 Premature ejaculation is ejaculation that occurs before or 
shortly after vaginal penetration during sexual intercourse and is 
followed by a rapid loss of erection.166 The cause of premature 
ejaculation is usually a psychological disturbance such as perfor­
mance anxiety; it is rarely the result of an organic cause. There 
is evidence that serotoninergic neurotransmission inhibits sexual 
function and

ic neurotransmission inhibits sexual 
function and ejaculation. Selective serotonin reuptake inhibitors 
retard ejaculation, an effect that is used therapeutically to treat pre­
mature ejaculation.166,167 Other men with psychological disorders 
such as excessive anxiety may have retarded ejaculation (inability 
to ejaculate), either in isolation or in combination with impaired 
libido and erections. The ejaculate is composed of spermatozoa 
(10%) and seminal fluid (90%), the latter derived mostl

 and seminal fluid (90%), the latter derived mostly from 
the seminal vesicles (65%) and the prostate gland (30%). Because 
secretions from these accessory sex glands are androgen depen­
dent, severe androgen deficiency may result in absent or reduced 
ejaculation. Absent or reduced ejaculation may also be caused 
by urethral abnormalities. Autonomic neuropathy, such as that 
caused by diabetes mellitus, sympatholytic drugs, thoracolumbar 
sympathectomy, extensive retroperitoneal or pelvic surge

hectomy, extensive retroperitoneal or pelvic surgery, or 
bladder neck surgery, may be associated with absent or reduced 
ejaculation by causing retrograde ejaculation into the bladder.
Orgasm, the pleasurable sensation associated with ejaculation, 
usually occurs simultaneously with ejaculation and is mediated by 
CNS activation via ascending pathways from the spinal cord erec­
tion centers to regions of the temporal lobe and limbic system.150 
Because of impaired libido and erectile dysfunctio

Because of impaired libido and erectile dysfunction, men with 
androgen deficiency may also fail to achieve an orgasm. Isolated 
absence of orgasm in the presence of normal libido, erections, and 
ejaculation is relatively rare and is almost always caused by a psy­
chological disorder.
Disorders of Detumescence. After ejaculation, the thoracolum­
bar sympathetic outflow acts via α-adrenergic receptor stimulation 
to cause contraction of trabecular smooth muscle, which results in 
collapse of lac

r smooth muscle, which results in 
collapse of lacunar spaces, vasoconstriction of arterioles of the cor­
pora cavernosa (reducing blood flow into the penis), and decompres­
sion of subtunical venules, leading to an increase in venous outflow 
and a flaccid penis (detumescence).150 Premature detumescence may 
contribute to erectile dysfunction, such as that caused by penile 
venous incompetence. Intracorporal injection of an α-adrenergic 
receptor antagonist, phentolamine, together with papaveri

r antagonist, phentolamine, together with papaverine and 
PGE1, causes sustained lacunar smooth muscle relaxation, arteriole 
vasodilatation, and penile tumescence and is used to treat erectile 
dysfunction caused by premature detumescence.
700
Priapism is failure of detumescence with persistence of erection 
lasting for longer than 4 hours that is unrelated to sexual stimu­
lation and is usually painful.150,168 An erection that persists for 
more tha

ul.150,168 An erection that persists for 
more than 4 hours is an emergency and may be complicated by 
ischemia, thrombosis, and vascular damage that contribute further 
to erectile dysfunction; if ischemia is severe, it can cause gangrene 
and eventual loss of the penis. Priapism may be idiopathic, or it 
may be caused by medications (e.g., intracavernosal injection ther­
apy for erectile dysfunction, phenothiazines, trazodone, cocaine), 
by hematologic disorders such as sickle cell disease or 

atologic disorders such as sickle cell disease or chronic 
myelogenous leukemia, by neurologic disorders such as spinal cord 
injury, or by infiltrative diseases such as amyloidosis. The initial 
treatment is administration of the α-adrenergic receptor agonist 
pseudoephedrine; if this is unsuccessful, aspiration of blood from 
the corpora cavernosa is performed with local anesthesia. 
Gynecomastia is benign enlargement of the male breast caused 
by proliferation of glandular breast

reast caused 
by proliferation of glandular breast tissue.169-171 On inspection, 
it is difficult to distinguish gynecomastia from increased adipose 
tissue deposition within the breast in the absence of glandular pro­
liferation (pseudogynecomastia), which is commonly present in 
obese men and boys. Detection of glandular breast tissue requires 
a careful and properly performed physical examination (see ear­
lier discussion), feeling for a firm, rubbery, finely lobular, freely 
mobile disc of t

 rubbery, finely lobular, freely 
mobile disc of tissue that extends concentrically from under the 
nipple and areola. Initially, gynecomastia of relatively recent and 
rapid onset may be painful and associated with tenderness. With 
time, glandular tissue is replaced by fibrous tissue and tenderness 
resolves, although palpable tissue remains. In contrast, pseudogy­
necomastia is soft, nondiscrete, and irregularly lobular, similar to 
subcutaneous (SC) fat in the abdomen.
Gynecomastia is usuall

us (SC) fat in the abdomen.
Gynecomastia is usually present bilaterally but may be asym­
metric in size and variably symptomatic. If palpable breast tissue 
is present unilaterally, the major concern is male breast cancer. 
Breast cancer is usually rock hard and indurated, eccentrically 
located from the nipple and areola, and fixed to underlying tissue; 
it may be associated with skin dimpling with retraction of hair 
follicles (peau d’orange), nipple retraction, nipple bleeding or dis­
charge,

nipple retraction, nipple bleeding or dis­
charge, or axillary lymphadenopathy.172 Other chest wall tumors 
may cause unilateral breast enlargement, including lipomas, seba­
ceous or dermoid cysts, hematomas, fat necrosis, lymphangiomas, 
neurofibromas, and lymphomas.
The primary hormones that regulate breast tissue development 
are estrogens, which stimulate the growth and differentiation of 
breast epithelium to form ducts (ductal hyperplasia), and pro­
gesterone, which controls acinar develop

 and pro­
gesterone, which controls acinar development and the formation 
of glandular buds (glandular formation).169,171 GH, IGF1, insu­
lin, thyroid hormone, and cortisol play permissive roles in breast 
development. Androgens inhibit the growth and differentiation of 
breast tissue. Prolactin stimulates differentiated breast acinar cells 
to produce milk, but high progesterone (and to a lesser extent high 
estradiol) concentrations inhibit lactogenesis. Therefore, milk pro­
duction requires a

togenesis. Therefore, milk pro­
duction requires a reduction in high progesterone (and to a less 
extent reduction in high estradiol) concentrations in the presence 
of high prolactin concentrations, as occurs in the first few days 
after parturition. Milk production (galactorrhea) is rarely seen in 
men with hyperprolactinemia and gynecomastia, because pro­
gesterone concentrations are not usually high enough for breast 
acinar development to occur, and progesterone and estradiol con­
centratio

cur, and progesterone and estradiol con­
centrations do not decline from baseline in men in the presence of 
high prolactin concentrations to stimulate lactogenesis.
Gynecomastia develops in clinical situations in which the con­
centrations or activity of estrogens is relatively high in comparison 
with androgens (i.e., high estrogen to androgen ratio). This hor­
monal milieu may result from high estrogen or low androgen con­
centrations or activity. Androgen deficiency, because it decreases 
th

ity. Androgen deficiency, because it decreases 
the inhibitory influence of androgens on breast development, is a 
major cause of gynecomastia. However, the differential diagnosis 
of other causes of gynecomastia should be considered in patients 
who present with breast enlargement with or without tenderness.
Causes of Gynecomastia
Physiologic gynecomastia occurs normally in neonatal and pubertal 
boys. Transient gynecomastia (neonatal gynecomastia) occurs in 
60% to 90% of neonatal boys as a re

ia) occurs in 
60% to 90% of neonatal boys as a result of exposure in utero to 
high concentrations of maternal estrogens; it resolves within sev­
eral weeks after delivery169-171 (Table 19.3). At the time of puberty, 
breast enlargement greater than 0.5 cm in diameter, which is often 
tender, initially occurs in 60% to 70% of boys by 14 years of age 
and then regresses within 1 to 2 years. This pubertal gynecomastia 
is thought to be caused by a transient rise in serum concentrations 
of estrog

 transient rise in serum concentrations 
of estrogen relative to testosterone during puberty.
Pathologic gynecomastia may result from excessive estrogen con­
centrations or action or from androgen deficiency or androgen 
resistance/insensitivity in isolation. In some conditions, both estro­
gen excess and androgen deficiency contribute to proliferation of 
glandular breast tissue.169-171 For example, in most conditions that 
cause gynecomastia as a result of excessive estrogen exposure, high 
ci

 a result of excessive estrogen exposure, high 
circulating estrogen concentrations inhibit endogenous gonadotro­
pin and testosterone secretion and cause secondary hypogonadism, 
which also contributes to the growth of breast tissue. In addition, 
some disorders of the testes that cause androgen deficiency (i.e., pri­
mary hypogonadism), such as Klinefelter syndrome, result in high 
circulating LH concentrations that stimulate aromatase activity in 
Leydig cells, leading to higher concentration

 in 
Leydig cells, leading to higher concentrations of estradiol relative to 
testosterone and contributing to the pathogenesis of gynecomastia.
Estrogen excess disorders that cause gynecomastia include expo­
sure to exogenous estrogens (e.g., diethylstilbestrol treatment of 
prostate cancer, contact with an estrogen-containing cream or cos­
metic, accidental occupational exposure to estrogens, ingestion of 
estrogen-containing nutritional supplements or excessive amounts 
of phytoestrogens) and

ments or excessive amounts 
of phytoestrogens) and exposure to ER agonists such as marijuana 
smoke (unidentified phenolic components but not active canna­
binoids173) or digitoxin. Ingestion of normal dietary amounts of 
phytoestrogens (e.g., soybean isoflavones) does not usually cause 
gynecomastia.174 Uncommonly, administration of testosterone or 
other aromatizable androgens, usually to prepubertal boys or men 
with long-standing, severe androgen deficiency, induces or wors­
ens gynecomastia

ogen deficiency, induces or wors­
ens gynecomastia by initially causing relatively higher estradiol 
than testosterone concentrations.
Increased peripheral aromatase activity with increased conversion 
of androgens to estrogens in excessive amounts of adipose tissue 
is thought to cause mild to moderate gynecomastia in men with 
obesity.169-171 In addition, increased aromatization of androgens 
to estrogens with increasing amounts of adipose tissue (including 
that within the breast) probably co

ue (including 
that within the breast) probably contributes substantially to the 
increased prevalence of gynecomastia with aging, which occurs 
in up to 65% of men from 50 to 80 years of age.169-171 Familial 
gynecomastia, an autosomal dominant or X-linked genetic disor­
der caused by constitutive activation of the CYP19A1 (aromatase) 
gene that results in increased peripheral conversion of androgen 
to estrogen, is a very rare cause of gynecomastia that manifests as 
prepubertal gynecomastia p

stia that manifests as 
prepubertal gynecomastia persisting into adulthood.
701
Estrogen-secreting tumors of the adrenal gland or testis are 
uncommon causes of gynecomastia. Feminizing adrenal tumors 
are usually malignant and large, manifesting with a palpable 
abdominal mass. In contrast, estrogen-secreting Leydig or Sertoli 
tumors are usually small and benign. Feminizing Sertoli tumors 
(particularly the large cell calcifying variety) may occur in isola­
tio

e cell calcifying variety) may occur in isola­
tion or in association with autosomal dominant disorders such as 
Peutz-Jeghers syndrome (multiple intestinal polyps and mucocu­
taneous pigmented macules) or the Carney complex (cardiac or 
cutaneous myxomas, pigmented skin lesions, and endocrinopathy, 
including functioning endocrine tumors of the adrenal and testis). 
hCG-secreting tumors (e.g., germ cell, lung, gastric, renal cell, or 
hepatic carcinomas in adults; hepatoblastomas in boys) or hC

rcinomas in adults; hepatoblastomas in boys) or hCG 
treatment of gonadotropin deficiency increases aromatase activity 
in Leydig cells and stimulates excessive secretion of estradiol rela­
tive to testosterone, causing relative rapid onset of symptomatic 
gynecomastia.
Disorders and drugs that cause androgen deficiency, such as 
conditions that cause either primary or secondary hypogonad­
ism (including medications such as cytotoxic agents) or andro­
gen resistance, are major causes of gynecoma

ndro­
gen resistance, are major causes of gynecomastia.169-171 Although 
prolactin acts on the breast to facilitate milk production in devel­
oped glandular tissue, the major mechanism by which hyperp­
rolactinemia causes gynecomastia is inhibition of endogenous 
gonadotropin and testosterone production (inducing androgen 
deficiency), which acts indirectly to stimulate breast develop­
ment by reducing the inhibitory influence of androgens on the 
breast. Hyperprolactinemia is the main reason th

breast. Hyperprolactinemia is the main reason that many CNS-
active medications, such as antipsychotics, antidepressants, and 
sedatives, are associated with gynecomastia. Drugs that interfere 
with androgen action, such as spironolactone (in contrast to 
eplerenone, a selective aldosterone receptor antagonist that does 
not cause gynecomastia), AR antagonists (e.g., flutamide, bicalu­
tamide, nilutamide and enzalutamide), marijuana, and histamine 
2 (H2) receptor antagonists, may cause gyneco

ine 
2 (H2) receptor antagonists, may cause gynecomastia.
Androgen deficiency contributes to the pathogenesis of gyne­
comastia in systemic disorders such as major organ failure—and 
particularly in hepatic cirrhosis and CKD, which are commonly 
associated with combined primary and secondary hypogonad­
ism—and in endocrine disorders such as acromegaly and Cushing 
syndrome, which may be associated with secondary hypogonad­
ism.169-171 In hepatic cirrhosis, there is reduced catabolism of Δ4-
andr

cirrhosis, there is reduced catabolism of Δ4-
androstenedione, resulting in increased peripheral conversion of 
Δ4-androstenedione to estrone and increased circulating estrogen 
concentrations. In addition, in both hepatic cirrhosis and hyper­
thyroidism, increased serum concentrations of SHBG, which 
binds testosterone with greater affinity than estradiol, result in rel­
atively higher free estradiol compared with free testosterone con­
centrations and thereby contribute to stimulation of breas

ons and thereby contribute to stimulation of breast tissue 
and development of gynecomastia. LH concentrations are often 
elevated in men with hyperthyroidism, which stimulates relatively 
more estradiol than testosterone secretion by Leydig cells of the 
testes. Excessive GH with acromegaly or GH treatment and exces­
sive cortisol with Cushing syndrome directly stimulate breast tis­
sue growth in addition to causing secondary hypogonadism, both 
of which contribute to the pathogenesis of gyneco

of which contribute to the pathogenesis of gynecomastia.
Gynecomastia often accompanies nutritional disorders, par­
ticularly during nutritional repletion after a period of starvation 
and weight loss (refeeding gynecomastia) and, analogously, dur­
ing recovery from chronic illness.169-171 In both starvation and 
severe chronic illness that is commonly associated with anorexia 
and weight loss, central GnRH production and concomitant 
  Causes of Gynecomastia
Cause
Examples

of Gynecomastia
Cause
Examples
Maternal estrogen exposure
Neonatal gynecomastia
Transient increase in 
estrogen to androgen 
concentrations
Pubertal gynecomastia
Estrogen Excess
Estrogens or estrogen 
­receptor agonists
Estrogens, digitoxin, testosterone or 
other aromatizable androgens
Increased peripheral 
­aromatase activity
Obesity, aging, familial
Estrogen-secreting tumors
Adrenal carcinoma, Leydig or Sertoli 
cell tumor
hCG-secreting tumors
Germ cell, lung, hepatic carci

CG-secreting tumors
Germ cell, lung, hepatic carcinoma
hCG treatment
Androgen Deficiency or Resistance
Androgen Deficiency
Primary or secondary hypogonadism
Hyperprolactinemia causing 
androgen deficiency
Androgen Resistance 
Disorders
Congenital and acquired androgen 
resistance
Drugs that interfere with 
androgen action
Spironolactone, androgen receptor 
antagonists, 5α-reductase inhibitors, 
histamine 2 receptor antagonists
Organ failure
Hepatic cirrhosis, chronic kidney 
d

disease
Endocrine disorders
Untreated hyperthyroidism, acromegaly, 
growth hormone treatment, Cushing 
syndrome
Nutritional disorders
Refeeding, recovery from chronic 
illness (hemodialysis, insulin, iso­
niazid, antituberculous medications, 
efavirenz)
Idiopathic Causes
Drugs
HAART, calcium channel antagonists, 
amiodarone, antidepressants (SSRIs, 
tricyclic antidepressants), alcohol, 
amphetamines, penicillamine, sulindac, 
phenytoin, omeprazole

s, penicillamine, sulindac, 
phenytoin, omeprazole, theophylline
Adult-onset idiopathic 
gynecomastia
Persistent prepubertal 
macromastia
HAART, Highly active antiretroviral therapy; hCG, human chorionic gonadotropin; SSRIs, 
selective serotonin reuptake inhibitors.
   
TABLE 19.3
702
gonadotropin and testosterone secretion are markedly suppressed. 
With refeeding or restitution of appetite and weight gain, there is 
activation of the hypothalamic-pit

gain, there is 
activation of the hypothalamic-pituitary-testicular axis and resto­
ration of gonadal function, similar to what occurs during puberty 
but occurring more rapidly (a “second puberty”), resulting in 
transiently higher concentrations of estrogen relative to andro­
gen concentrations and inducing gynecomastia. Refeeding gyne­
comastia was described initially in World War II prisoners who 
developed painful gynecomastia after liberation and nutritional 
repletion. Analogously, refeed

on and nutritional 
repletion. Analogously, refeeding-like gynecomastia may occur in 
stage 5 CKD with the initiation of hemodialysis, in type 1 diabe­
tes mellitus (T1DM) with insulin therapy, in tuberculosis with 
antituberculosis medications, and in human immunodeficiency 
virus (HIV) infection or acquired immunodeficiency syndrome 
with highly active antiretroviral treatment (HAART). As men­
tioned, these chronic systemic disorders also cause androgen defi­
ciency that may contribute to the 

 androgen defi­
ciency that may contribute to the pathogenesis of gynecomastia. 
HAART also may cause lipohypertrophy and fat accumulation in 
the breast (pseudogynecomastia); efavirenz, a component of some 
HAART regimens, has estrogenic activity that might cause true 
The mechanisms of gynecomastia’s causal relationship with 
many drugs are not entirely clear, and these cases are usually clas­
sified as idiopathic. Such drugs include calcium channel block­
ers (e.g., nifedipine, 

ude calcium channel block­
ers (e.g., nifedipine, verapamil), amiodarone, antidepressants 
(selective serotonin reuptake inhibitors, tricyclic antidepressants), 
alcohol, amphetamines, penicillamine, sulindac, phenytoin, 
omeprazole (much less commonly than H2-receptor antagonists), 
and theophylline.169-171,175,176
In many cases of adult-onset gynecomastia, the cause remains 
idiopathic. Most of these cases are probably caused by increased 
aromatization of androgens to estrogens associated wit

atization of androgens to estrogens associated with increased 
peripheral adiposity, enhanced breast production of estrogens, 
enhanced sensitivity to estrogens, or some combination of these 
factors. Rarely, boys may develop severe pubertal gynecomastia 
(female size breast development, Tanner stages III through V) 
that persists to adulthood (persistent pubertal macromastia). This 
disorder is not associated with specific hormonal or receptor 
abnormalities. 
Most gynecomastia is as

Most gynecomastia is asymptomatic and of mild degree but can be 
appreciated on a properly performed, careful physical examination 
(as described earlier). Asymptomatic gynecomastia (<5 cm) found 
incidentally on examination does not warrant evaluation. How­
ever, breast enlargement that is recent and rapid in onset, large (≥5 
cm), symptomatic (i.e., associated with breast pain, tenderness, or 
galactorrhea), asymmetric, or suspicious for malignancy (eccentri­
cally l

c, or suspicious for malignancy (eccentri­
cally located, rock hard, fixed to overlying or underlying tissues, 
or associated with bloody nipple discharge or lymphadenopathy) 
should evaluated.
A careful history, including medication history, and physical 
examination usually identify potential predisposing conditions 
or medications causing gynecomastia that in older men may be 
mulifactorial.169-171 Clinical evaluation should focus on evidence 
of androgen deficiency; assessment of prescriptio

of androgen deficiency; assessment of prescription and over-the-
counter medications, substance abuse, herbal or nutritional sup­
plement intake, cosmetic use, and usual dietary intake; symptoms 
and signs of systemic illness (e.g., hepatic or renal disease), malig­
nancy, or endocrine disorders (e.g., thyroid, GH, cortisol excess); 
and history of recent recovery from malnutrition, severe weight 
loss, or chronic illness. At a minimum, the initial laboratory 
evaluation comprises serum testost

ial laboratory 
evaluation comprises serum testosterone (calculated free testos­
terone if SHBG perturbation is suspected), LH, FSH, TSH, and 
renal and liver function tests. Evaluation also usually includes 
measurements of estradiol, prolactin, and βhCG, although clini­
cally important elevations of these hormones suppress serum 
gonadotropin concentrations. Breast enlargement suspicious for 
malignancy should be evaluated by mammography and biopsy. 
Treatment
Pubertal gynecomastia usually reg

opsy. 
Treatment
Pubertal gynecomastia usually regresses spontaneously without 
treatment in 1 to 2 years and by 18 years of age in about 90% 
of cases. In adults, spontaneous regression of symptoms (breast 
pain and tenderness, nipple sensitivity) associated with inflam­
matory glandular proliferation usually occurs within 6 months, 
after which progressive stromal fibrosis causes permanent palpable 
breast tissue and only partial regression of gynecomastia by 1 year.
Initial treatment of gynec

gynecomastia by 1 year.
Initial treatment of gynecomastia is directed at correction of 
the underlying cause of breast enlargement or discontinuation or 
replacement of a potentially offending medication.177 No treatment 
is effective for gynecomastia of 6 months or more; absence of breast 
tenderness generally indicates the chronic fibrotic stage of gyneco­
mastia that will not remit. Prophylactic low-dose breast irradiation 
(10–15 Gy over 1–3 days) may be used before androgen deprivation 
the

days) may be used before androgen deprivation 
therapy in men with prostate cancer to prevent the development 
of gynecomastia, which is more common in surgical orchidectomy 
and in AR antagonist monotherapy than in combined therapy with 
a GnRH agonist or antagonist. Testosterone replacement therapy 
in androgen-deficient men may result in partial regression of gyne­
comastia if breast enlargement is of recent onset. For gynecomastia 
of recent onset (<6 months), selective ER antagonists (tamox

onset (<6 months), selective ER antagonists (tamoxifen, 
10–20 mg daily, or raloxifene, 60 mg daily) are effective in treat­
ing pubertal and adult gynecomastia and preventing gynecomas­
tia induced by androgen deprivation therapy. For unclear reasons, 
aromatase inhibitors (e.g., anastrozole) generally are not effective. 
Although tamoxifen is not approved for treatment of gynecomas­
tia, it has been shown to be effective in the treatment of idiopathic 
gynecomastia, resulting in partial regres

opathic 
gynecomastia, resulting in partial regression in approximately 80% 
and complete regression in about 60% of cases. A gel formulation 
of DHT, a nonaromatizable androgen, is used to treat gynecomastia 
in some countries outside the United States.
Gynecomastia of recent onset, during the initial phase of duc­
tal proliferation, periductal inflammation and edema, and sub­
areolar fat accumulation, is usually responsive to medical therapy 
(e.g., androgen replacement in hypogonadal men, ER 

e.g., androgen replacement in hypogonadal men, ER antagonist 
therapy). With long-standing gynecomastia (>6 months), there 
is progressive stromal fibrosis of the breast that is not responsive 
to medical treatment. In these cases, surgical reduction mam­
moplasty (i.e., removal of breast tissue [SC mastectomy] with or 
without periareolar adipose tissue [liposuction]) is necessary, espe­
cially if breast enlargement is severe, painful, socially embarrass­
ing, or disfiguring. 
Infer

embarrass­
Infertility is defined as the inability of a sexually active couple to 
achieve conception despite 1 year of unprotected intercourse. 
The probability of conception in a sexually active young couple 
(both younger than 35 years) is approximately 85% by 1 year. 
Approximately 15% of couples in the reproductive age group are 
infertile, and a male factor contributes to the cause (either in isola­
tion or in combination with a female factor) in about hal

 in combination with a female factor) in about half of the 
cases. Therefore, male infertility is a common condition, affecting 
approximately 7% of men.178
703
Causes of Male Infertility
In about 80% to 90% of infertile men, infertility is caused by 
primary or secondary hypogonadism, manifested most commonly 
by an isolated impairment of sperm production or function; much 
less commonly by androgen deficiency and impaired spermato­
genesis; and rarely by androg

d impaired spermato­
genesis; and rarely by androgen resistance179,180 (Table 19.4). The 
evaluation and specific causes of hypogonadism are discussed in 
detail in subsequent sections. Most men with isolated impairment 
in sperm production have a primary disorder of the testes that 
is idiopathic in 60% to 70% of cases (if one includes idiopathic 
oligozoospermia or azoospermia and varicocele, given that rela­
tionship of varicocele to the pathogenesis of infertility is unclear). 
If isolated i

genesis of infertility is unclear). 
If isolated impairment of spermatogenesis is severe in men with 
primary hypogonadism, serum FSH concentrations may be selec­
tively elevated (FSH >8 IU/L) as a result of reduced negative feed­
back by inhibin B from Sertoli cells of the testis.
Disorders of spermatogenesis caused by primary hypogonadism 
may be associated with chromosomal or genetic disorders. There 
is an 8- to 10-fold increase in the prevalence of chromosomal 
abnormalities among infertile

ence of chromosomal 
abnormalities among infertile men with impaired spermatogen­
esis—specifically, sex chromosomal aneuploidy (e.g., Klinefelter 
syndrome) or Robertsonian translocations of two nonhomologous 
chromosomes, most commonly involving chromosomes 13 and 
14 or chromosomes 14 and 21.181 The long arm of the Y chro­
mosome (Yq), specifically the azoospermia factor (AZF) region 
(Yq11), contains many genes that encode for proteins that have 
important roles in spermatogenesis. This regi

ave 
important roles in spermatogenesis. This region contains highly 
homologous palindromic DNA repeat sequences that are sus­
ceptible to rearrangement and deletions. Small deletions in the 
AZF region (Y chromosome microdeletions) are the most common 
genetic cause of impaired sperm production and male infertility; 
they are found in 5% to 10% of men with azoospermia or severe 
oligozoospermia (sperm concentration <5 million/mL).181 Y chro­
mosome microdeletions have been identified in three 

some microdeletions have been identified in three regions: in 
the AZFa region, microdeletions are uncommon but are usually 
associated with azoospermia and Sertoli cell–only histology; in the 
AZFb region, they are usually associated with severe oligozoosper­
mia and germ cell arrest at the pachytene primary spermatocyte 
stage; and in the AZFc region, where the majority of Y chromo­
some microdeletions reside, they are usually associated with germ 
cell arrest at the spermatid stage or hypospe

erm 
cell arrest at the spermatid stage or hypospermatogenesis with 
some mature spermatids present. Occasionally, microdeletions in 
the AZFb and AZFc regions are associated with azoospermia and 
Sertoli cell–only histology. Genes encoding a number of candi­
date proteins for male infertility include DDX3Y (DEAD box Y, 
an ATP-dependent RNA helicase), RBMY (RNA-binding motif 
Y-linked, an RNA-binding protein), and DAZ (deleted in azo­
ospermia, another RNA-binding protein) in the AZFa, AZFb, an

another RNA-binding protein) in the AZFa, AZFb, and 
AZFc regions, respectively.181,182
Approximately 15% to 20% of male infertility is caused by 
disorders of sperm transport from the testes to the urethra, most 
commonly by genital tract obstruction. Congenital bilateral 
absence of the vas deferens (CBAVD) is present in 1% to 2% of 
men with infertility.181,183,184 Seventy-five percent of men with 
CBAVD are heterozygous for the cystic fibrosis transmembrane 
conductance regulator gene (CFTR)

s transmembrane 
conductance regulator gene (CFTR) that encodes for an epithelial 
chloride channel. They do not have obvious clinical manifesta­
tions of cystic fibrosis, although some manifest abnormalities on 
sweat chloride testing and sinopulmonary infections. Conversely, 
almost all men with clinical cystic fibrosis have CBAVD. CBAVD 
is also commonly associated with absence of the seminal vesicles, 
ejaculatory ducts, and epididymides due to fetal atrophy of these 
wolffian duct derivativ

to fetal atrophy of these 
wolffian duct derivatives; in 10% of cases, there is also renal agen­
esis or hypoplasia.
Other causes of genital tract obstruction include other congen­
ital defects of the epididymides and vas deferens (e.g., epididymal 
cysts associated with prenatal diethylstilbestrol exposure); vasec­
tomy (surgical ligation of the vas deferens); postinfectious fibro­
sis (e.g., associated with gonorrhea, Chlamydia infection, other 
sexually transmitted diseases; tuberculosis; lep

sexually transmitted diseases; tuberculosis; leprosy); and Young 
syndrome, a rare, congenital primary ciliary dyskinesia syndrome 
characterized by bronchiectasis, recurrent sinopulmonary infec­
tions, and obstructive azoospermia caused by thickened, inspis­
sated mucous secretions obstructing the epididymides.
Although a causal link to infertility has not been clearly 
established, other genital tract abnormalities may contribute to 
impaired sperm transport and the pathogenesis of infertili

 sperm transport and the pathogenesis of infertility in 
some men. Accessory gland dysfunction, such as reduced semi­
nal vesicle and prostate secretions associated with disorders that 
cause severe androgen deficiency or resistance, may contribute to 
reduced fertility, although the main effects of these disorders are 
  Causes of Male Infertility
Cause
Examples
Isolated impairment of 
sperm production or 
function
Androgen deficiency 
and impaired sperm 
Androgen resis

ncy 
Androgen resistance
Disorders of Sperm Transport
Genital tract obstruction
Congenital bilateral absence of the vas 
deferens, cystic fibrosis, other con­
genital defects, vasectomy, postinfec­
tious fibrosis, Young syndrome
Accessory gland 
­dysfunction
Androgen deficiency or resistance, 
infection or inflammation, antisperm 
antibodies (immunologic)
SNS dysfunction
Autonomic neuropathy, sympatholytic 
drugs, sympathectomy, retroperitoneal 
or abdominopelvic s

ympathectomy, retroperitoneal 
or abdominopelvic surgery, spinal cord 
injury or disease, vasovasostomy
Ejaculatory Dysfunction
Premature or retarded 
ejaculation
Retrograde ejaculation
Prostatectomy, bladder neck 
surgery, autonomic neuropathy, 
Reduced ejaculation
Androgen deficiency or resistance, SNS 
dysfunction, ureteral abnormalities
Coital Disorders
Erectile dysfunction
Defects in coital technique
Infrequent intercourse (< once weekly), 
poor timing in relation to ovulati

 once weekly), 
poor timing in relation to ovulation, 
premature withdrawal of penis
SNS, Sympathetic nervous system.
   
TABLE 19.4
704
to impair spermatogenesis and cause sexual dysfunction. Infection 
or inflammation of the epididymides, seminal vesicles, or prostate 
gland may affect fertility directly by impairing sperm maturation 
or function or secondarily by causing scarring of the genital tract 
or induction of antisperm antibodies in semen (

t 
or induction of antisperm antibodies in semen (resulting in sperm 
agglutination and impaired sperm function).185,186 Sympathetic 
nervous system dysfunction (e.g., associated with autonomic neu­
ropathy, sympatholytic drugs, sympathectomy, retroperitoneal 
or abdominopelvic surgery, spinal cord injury or disease, vasova­
sostomy) may contribute to impaired sperm transport and male 
infertility.
Ejaculatory dysfunction may cause or contribute to male infer­
tility by preventing normal or effi

to male infer­
tility by preventing normal or efficient deposition of sperm into 
the vagina and female genital tract. Premature or delayed ejacu­
lation may contribute to infertility if ejaculation occurs during 
arousal or foreplay before vaginal penetration or after withdrawal 
from the vagina. Retrograde ejaculation of semen into the bladder, 
rather than the urethra, occurs with neuromuscular failure of nor­
mal bladder sphincter contraction during ejaculation. Retrograde 
ejaculation may b

 during ejaculation. Retrograde 
ejaculation may be associated with prostatectomy or bladder neck 
surgery (e.g., transurethral resection of the prostate), autonomic 
neuropathy (e.g., complicating diabetes mellitus), or sympathetic 
nervous system dysfunction, and particularly with sympatholytic 
drugs (e.g., α-adrenergic receptor antagonists like prazosin or tera­
zosin), retroperitoneal or abdominopelvic surgery (e.g., retroperi­
toneal lymph node dissection), and spinal cord injury or diseas

node dissection), and spinal cord injury or disease. 
Reduced ejaculation caused sympathetic nervous system dysfunc­
tion or urethral abnormalities may contribute to reduced sperm 
delivery to the female genital tract.
Erectile dysfunction may contribute to male infertility by 
causing unsuccessful vaginal intercourse. Coital disorders are 
uncommon causes of male infertility, but they are potentially 
correctable with proper education. Infrequent vaginal inter­
course (less than weekly), interc

t vaginal inter­
course (less than weekly), intercourse during menses rather 
than just before or around the time of ovulation, and premature 
withdrawal of the penis during intercourse may contribute to 
reduced fertility. 
Because a coexisting female factor contributes to infertility in 30% 
of cases, it is important for the female partner to undergo evalu­
ation for ovulation (menstrual periods, androgenization) and for 
cervical disorders (postcoital testing) and uterine and tubal

sorders (postcoital testing) and uterine and tubal disor­
ders (hysterosalpingogram, pelvic ultrasound). In men, the history 
and physical examination are usually able to identify the potential 
cause of male infertility.179-181
In addition to an assessment of general health and medical 
comorbid conditions, the initial clinical evaluation should focus 
on the following:
 • Symptoms and signs of androgen deficiency or resistance (as 
detailed elsewhere in this chapter)
 • Scrotal examination f

o assess semen 
volume, sperm count and concentration, and sperm motility and 
morphologic appearance, with the aim of identifying men who 
have impaired sperm production or function—the major cause of 
male infertility. The presence of leukocytes in semen (>106/mL, 
termed leukospermia or pyospermia) may suggest genitourinary 
inflammation or infection, but routine cultures are not helpful in 
guiding treatment. Agglutination of spermatozoa in semen (i.e., 
sticking of motile sperm to each othe

emen (i.e., 
sticking of motile sperm to each other) suggests the presence of 
antisperm antibodies that can be measured in semen and might 
indicate an immunologic cause of male infertility, but it is unclear 
whether antisperm antibodies affect fertility.187
Seminal fluid fructose is derived mostly from the seminal ves­
icles (60%) and to a lesser extent from the prostate gland (30%). 
Absent or low seminal fluid fructose and low semen volume sug­
gest either congenital absence of the vas defe

ug­
gest either congenital absence of the vas deferens and seminal ves­
icles or obstruction of the ejaculatory ducts. Transrectal or scrotal 
ultrasonography to detect dilated seminal vesicles should be done 
to confirm ejaculatory duct obstruction. In men who have little or 
no ejaculate, a postejaculatory urine specimen should be collected 
and examined for the presence of sperm, indicating retrograde 
ejaculation.
Serum testosterone concentrations should be measured on at 
least two occasion

tions should be measured on at 
least two occasions to confirm androgen deficiency, and measure­
ments of LH and FSH should be performed to determine whether 
the patient has primary or secondary hypogonadism (see later 
discussion). Men with primary hypogonadism have low testos­
terone with elevated serum LH and FSH concentrations (FSH 
higher than LH concentrations). Identification of infertile men 
with secondary hypogonadism (low testosterone with low or inap­
propriately normal LH and FSH c

 with low or inap­
propriately normal LH and FSH concentrations) potentially has 
important therapeutic implications. In men with impaired sperm 
production due to gonadotropin deficiency, spermatogenesis may 
be stimulated and fertility restored with the use of gonadotropin 
or GnRH therapy. Secondary hypogonadism is one of few treat­
able causes of male infertility. Elevated concentrations of testos­
terone, LH, and FSH suggest androgen resistance: LH is usually 
higher than FSH.
Measurements 

nce: LH is usually 
Measurements of FSH concentrations specifically as a marker of 
Sertoli cell and seminiferous tubule function are useful in identify­
ing men with severe defects in spermatogenesis and impairment of 
seminiferous tubule and Sertoli cell function; such patients often 
demonstrate selective elevation in serum FSH concentrations with 
normal or high-normal LH concentrations due to loss of negative 
feedback inhibition of pituitary FSH secretion by inhibin B.188 

ition of pituitary FSH secretion by inhibin B.188 
In young men with proven fertility, the upper limit of normal 
for serum FSH is 8 IU/L.189 Many commercial laboratories have 
higher upper limits of normal because they have included men 
without proven fertility. However, men with less severe seminifer­
ous tubule dysfunction and impairment of spermatogenesis and 
those with azoospermia due to genital tract obstruction (obstruc­
tive azoospermia) have normal serum FSH concentrations.
Genetic di

) have normal serum FSH concentrations.
Genetic disorders make up a small but significant proportion of 
the causes of male infertility. Because ART, and specifically ICSI, 
which involves direct injection of spermatozoa into the cytoplasm 
of an ovum (discussed later), is commonly used to treat male 
705
infertility, the potential exists for transmission of genetic defects 
to offspring. Therefore, genetic testing and counseling should be 
offered to men who are

g and counseling should be 
offered to men who are considering ICSI, particularly for those 
with severe oligozoospermia or azoospermia.179-181
Men in whom bilateral congenital absence of the vas or geni­
tal tract obstruction is confirmed by transrectal or scrotal ultra­
sound and those who have unexplained obstructive azoospermia 
should undergo genetic testing for CFTR mutations and genetic 
counseling before ICSI. In men with severe oligozoospermia 
(sperm concentration <5 million/mL) or azo

permia 
(sperm concentration <5 million/mL) or azoospermia, testing for 
Y chromosome microdeletions in the AZF region should be per­
formed routinely. There is a high prevalence of sex chromosome 
and autosomal chromosome defects, often in the absence of other 
phenotypic abnormalities, in men with moderately impaired 
spermatogenesis and infertility. Therefore, karyotyping is recom­
mended before ICSI for infertile men with impaired sperm pro­
duction, particularly for those with a sperm conce

duction, particularly for those with a sperm concentration of less 
than 10 million/mL.
In azoospermic men with normal semen volume, a normal fruc­
tose level, and a normal FSH level in whom it is unclear whether 
azoospermia is caused by germ cell failure, genital tract obstruc­
tion, or both, surgical exploration of the scrotum and testis biopsy 
are needed. Microsurgical biopsy is also used to harvest sperm for 
ICSI, even in men with known severe impairment in spermato­
genesis, such as thos

vere impairment in spermato­
genesis, such as those with Klinefelter syndrome.179,180,190,191
Specialized tests of in vitro sperm function, such as detailed 
examination of sperm motility using computer-assisted semen 
analysis, cervical mucus penetration tests, acrosome reaction, and 
human zona pellucida binding tests, may be useful in some men 
who are considering intrauterine insemination (IUI) or in vitro 
fertilization (IVF). However, these tests should be performed only 
in highly special

 tests should be performed only 
in highly specialized laboratories that have demonstrated excellent 
quality control. Even in such laboratories, there is a high rate of 
clinical false-positive and false-negative results, limiting the clini­
cal utility of these tests. 
Treatment
In men with infertility caused by primary hypogonadism (whether 
due to androgen deficiency and impairment of sperm produc­
tion or to isolated impairment of sperm production or function), 
defects in sperm production 

uction or function), 
defects in sperm production are not effectively treated with drugs, 
although spontaneous recovery of spermatogenesis may occur at 
variable times after discontinuation of cytotoxic drugs or ionizing 
radiation. Because intratesticular testosterone concentrations are 
normally approximately 100-fold higher than serum concentra­
tions, exogenous testosterone treatment of men with androgen 
deficiency cannot deliver sufficient amounts of testosterone to 
support sperm product

 amounts of testosterone to 
support sperm production in the testis.
In men with secondary hypogonadism, however, sperm pro­
duction can be stimulated with gonadotropin or GnRH treatment, 
or spermatogenesis may recover sufficiently to restore fertility after 
discontinuation of drugs that suppress gonadotropins, such as 
androgenic anabolic steroids, progestins, glucocorticoids, opioids, 
and drugs causing hyperprolactinemia.
Most men with a varicocele and infertility have abnormal semi­
nal fl

icocele and infertility have abnormal semi­
nal fluid. However, varicocele repair has not been demonstrated to 
be effective in restoring fertility to these men. Therefore, unless a 
varicocele is very large or symptomatic, surgical repair is not recom­
mended.12,192 Although clinical trials have failed to demonstrate 
effectiveness,193 infertile men with leukospermia or sperm aggluti­
nation are sometimes treated empirically with a 14-day course of 
antibiotics, such as doxycycline, trimethopri

 of 
antibiotics, such as doxycycline, trimethoprim-sulfamethoxazole, 
or a fluoroquinolone. Although high-dose prednisone (40–60 mg 
for several months) has been recommended to treat infertility in 
men with sperm in the ejaculate and serum antisperm antibodies, 
a recent systematic review concluded that antisperm antibodies do 
not cause male infertility. It is likely that the “response” to cortico­
steroid therapy is due to the fact that some of these men eventually 
conceive without any ther

of these men eventually 
conceive without any therapy.187 Empiric high-dose corticosteroid 
therapy is not justified because of the high risk of toxicity.
Although ICSI is costly, it is used increasingly to treat male 
infertility, and it dramatically improves the prognosis for men with 
impaired sperm production regardless of the cause.190,191 Sperma­
tozoa that are ejaculated or obtained by testicular biopsy (TESE) 
or from the epididymis (microsurgical epididymal sperm aspiration 
[MESA]) are

osurgical epididymal sperm aspiration 
[MESA]) are used for ICSI and other ARTs. With ICSI, fertil­
ization rates of about 60% and pregnancy rates of approximately 
20% are achieved, irrespective of the cause of male infertility or 
source of spermatozoa. ICSI after TESE using microsurgical testis 
biopsy or fine-needle aspiration to retrieve sperm has been success­
ful in restoring fertility to men with primary hypogonadism who 
had severe impairments in spermatogenesis and azoospermia that 
we

rments in spermatogenesis and azoospermia that 
were previously thought to be untreatable (e.g., Klinefelter syn­
drome, prolonged azoospermia after chemotherapy).194 Because 
of the potential for chromosomal abnormalities and transmission 
of Y chromosome microdeletions and CFTR mutations that cause 
infertility in male offspring, genetic testing and counseling should 
be conducted if ICSI is being considered (see earlier discussion).181
Obstruction of the epididymides or the ejaculatory ducts 

tion of the epididymides or the ejaculatory ducts can 
be corrected surgically, such as with end-to-end anastomosis of the 
epididymides or transurethral resection of the ejaculatory ducts. 
More commonly, MESA is used to obtain spermatozoa that can 
be incubated with ova in vitro (IVF) or directly injected into an 
ovum (ICSI), and this method is more successful in restoring fer­
tility than surgical options. In contrast, microsurgical reanasto­
mosis of the vas (vasovasostomy) to reverse vasec

mosis of the vas (vasovasostomy) to reverse vasectomy is less costly 
and more successful in restoring fertility than MESA followed by 
IVF or ICSI. Return of sperm in the ejaculate occurs in approxi­
mately 90% of men who undergo vasectomy reversal, but restora­
tion of fertility occurs in only about 50%, probably because of 
stenosis or blockage of the previous vasovasostomy, epididymal 
blockage, or the development of antisperm antibodies in response 
to the vasectomy.195
In men with retrogr

response 
In men with retrograde ejaculation, collection of spermatozoa 
in alkalinized postejaculation urine, followed by extensive wash­
ing and IUI or ICSI, has been used successfully to treat infertility. 
With proper education, coital disorders that contribute to infertil­
ity may be corrected. In addition, the timing of intercourse may 
be optimized to occur a few days before and after ovulation (the 
period of highest probability for conception) based on basal body 
t

probability for conception) based on basal body 
temperature measurements or, more accurately, on commercially 
available rapid urinary LH kits to estimate the timing of ovulation 
in the female partner.
If the treatment options described previously are not available 
or affordable to infertile couples who desire children, artificial 
insemination with donor sperm or adoption may be considered. 
Diagnosis of Male Hypogonadism
Clinical Manifestations of Androgen Deficiency
The diagnosis of male h

ons of Androgen Deficiency
The diagnosis of male hypogonadism requires clinical manifes­
tations consistent with androgen deficiency and unequivocally 
low serum testosterone concentrations. In community-dwelling 
middle-age and older men, the crude prevalence of symptomatic 
706
androgen deficiency is 2% to 9%, depending on the constellation 
of symptoms and signs and the biochemical definition of andro­
gen deficiency used (i.e., the threshold used 

ro­
gen deficiency used (i.e., the threshold used for a single low tes­
tosterone concentration).196-198 This prevalence increases with age 
and is much higher in a primary care setting.199 In community 
populations, the prevalence of low testosterone concentrations 
alone, without consideration of symptoms and signs of androgen 
deficiency, is much higher than that of clinical androgen defi­
ciency. This underscores the importance of making a diagnosis of 
hypogonadism only in men who have clin

agnosis of 
hypogonadism only in men who have clinical manifestations and 
consistently low testosterone concentrations. Both the clinical and 
the biochemical diagnosis of androgen deficiency can be challeng­
ing, especially in older adults.
Although the manifestations of fetal androgen deficiency or 
resistance (ambiguous genitalia and 46,XY DSD) and those of 
androgen deficiency of prepubertal onset (eunuchoidism) are 
usually clinically obvious, the clinical diagnosis of androgen defi­
cienc

us, the clinical diagnosis of androgen defi­
ciency in adults is more difficult. As described previously, the 
symptoms and signs of androgen deficiency are nonspecific and 
have a broad differential diagnosis. Moreover, clinical manifesta­
tions may be modified by several factors, such as the severity and 
duration of androgen deficiency, age, comorbid illnesses, medica­
tions, previous testosterone treatment, and individual variations 
in androgen sensitivity, all of which contribute to variab

gen sensitivity, all of which contribute to variability in 
clinical presentation that may confound the diagnosis.116 Because 
the manifestations of androgen deficiency in adults are nonspe­
cific, other potential causes (e.g., depression, comorbid illness, or 
medications) should be considered in the differential diagnosis to 
explain clinical features in any individual patient.200
The degree and duration of androgen deficiency have impres­
sive effects on clinical manifestations. The severe an

 effects on clinical manifestations. The severe and relatively 
rapid suppression of testosterone concentrations in men with pros­
tate cancer treated with a GnRH agonist or orchidectomy results 
in prominent clinical manifestations with notable reductions in 
erectile function, libido, energy, and mood; hot flushes and sleep 
disturbances; infertility; decreases in muscle mass and strength, 
BMD (associated with an increase in fracture risk), and body hair; 
gynecomastia; increases in body fat;

d body hair; 
gynecomastia; increases in body fat; anemia; and possibly increases 
in the risks for diabetes mellitus and cardiovascular events.201,202 
In contrast, men with mild androgen deficiency may have few 
or no referable manifestations; such patients have “subclinical” 
androgen deficiency that may or may not be associated with clini­
cally significant outcomes. The latter situation is analogous to that 
observed in other endocrine disorders such as subclinical hypothy­
roidism or asymp

ders such as subclinical hypothy­
roidism or asymptomatic primary hyperparathyroidism.
Aging is associated with alterations in body functions, such as 
declines in sexual function, muscle mass and strength, and BMD, 
that result in clinical manifestations similar to those of androgen 
deficiency.144 These alterations associated with aging may also be 
caused in part by age-related androgen deficiency. To add to the 
clinical complexity in older men, age-associated comorbid ill­
nesses and medica

en, age-associated comorbid ill­
nesses and medications used to treat these illnesses may modify 
the symptoms and signs of androgen deficiency, and in many 
instances, they may also contribute to the cause of androgen defi­
ciency. Therefore, it is understandable why the diagnosis of clini­
cal androgen deficiency is challenging in older men, particularly 
in frail elderly men who have multiple comorbid illnesses and are 
taking numerous medications.
Previous testosterone treatment that has bee

ions.
Previous testosterone treatment that has been discontinued 
may affect the clinical manifestations of androgen deficiency, 
depending on the duration of therapy and the time since discon­
tinuation. It is also likely that clinical manifestations of androgen 
deficiency are affected by individual variations in androgen action 
on specific target organs. Alterations in androgen sensitivity may 
result from individual or tissue-specific differences in the activity 
of the AR or the ER and ass

ences in the activity 
of the AR or the ER and associated coregulators, or from differ­
ences in active metabolism of testosterone to estradiol or DHT or 
inactivation of testosterone.
In men with clinical manifestations suggestive of androgen 
deficiency, the diagnosis of hypogonadism is confirmed biochemi­
cally by measurement of consistently low serum testosterone con­
centrations116 (Fig. 19.19). 
Testosterone Measurements
As with the clinical manifestations, the biochemical confirmation 
of

l manifestations, the biochemical confirmation 
of androgen deficiency presents its own difficulties. Testosterone 
concentrations exhibit both biologic and assay variability. Total 
testosterone concentrations are affected by alterations in SHBG, 
and testosterone concentrations may be suppressed transiently 
with illness, certain medications, and some nutritional deficien­
cies.116 Therefore, the biochemical diagnosis of androgen defi­
ciency requires demonstration of consistently and unequivo

equires demonstration of consistently and unequivocally 
low serum testosterone concentrations in at least two blood sam­
ples obtained between 7 am and 10 am, preferably fasting. In men 
who have conditions that alter SHBG, accurate and reliable free or 
bioavailable testosterone measurements are needed to confirm the 
diagnosis of hypogonadism. Finally, the diagnosis of hypogonad­
ism should not be made during acute or subacute illness.
The threshold level of circulating total or free testoste

eshold level of circulating total or free testosterone 
below which symptoms and signs of androgen deficiency occur 
and for which testosterone treatment will improve clinical mani­
festations is not known. However, the concept of a single thresh­
old testosterone level is probably not valid, nor is it clinically 
useful, because thresholds vary with the specific symptom and the 
androgen target organ or tissue. In general, symptoms and signs 
of androgen deficiency are more likely to occur with

 androgen deficiency are more likely to occur with a total testos­
terone level that is below the lower limit of the normal range for 
young, nonobese, healthy men (approximately 265–280 ng/dL or 
2.65–2.8 ng/mL [9.2–9.7 nmol/L], when using an accurate and 
reliable assay).203 The likelihood and severity of clinical manifes­
tations of androgen deficiency increase with a greater decline in 
testosterone level below normal.
Variability in Testosterone Concentrations
Because serum testosterone con

rone Concentrations
Because serum testosterone concentrations exhibit both biologic 
and assay variability, a single measurement is not a reliable indica­
tor of an individual’s average concentration. Serum testosterone 
concentrations exhibit both ultradian and circadian variation, 
providing physiologic sources of biologic variability. Ultradian 
fluctuations in serum testosterone concentrations, characterized 
by peaks of incremental amplitude that average approximately 
240 ng/dL (40% fracti

 that average approximately 
240 ng/dL (40% fractional amplitude) with a 95-minute dura­
tion,45 have been reported in a small number of young men; more 
chaotic peaks with lower amplitude have been reported in older 
men.204 As described previously, the circadian variation in serum 
testosterone peaks at about 8 am and has a maximum excursion 
averaging 140 ng/dL.64 The circadian variation in testosterone is 
blunted but still present in older men, with a maximum excursion 
averaging 60 ng/dL. 

en, with a maximum excursion 
averaging 60 ng/dL. In young men, serum testosterone concentra­
tions are 20% to 25% lower at 4 pm than at 8 am (i.e., over the 
course of usual clinic hours).205 This difference decreases with age: 
in 70-year-old men, testosterone concentrations are 10% lower 
at 4 pm than at 8 am. Most importantly, many young and old 
men who have testosterone concentrations that are below normal 
Normal total T
or
Normal or low total T**
and normal free T
History and Physical (s

otal T**
History and Physical (symptoms and signs)
Exclude systemic illness, drugs, nutritional deficiency
Morning fasting total T* (and free T#, if altered SHBG or borderline total T)
Semen analysis (if fertility issue)
Low total T
or
and low free T
Confirm by repeating morning fasting total T
(and free T)
Low total T
or
Measure LH and FSH
Diagnosis of hypogonadism confirmed
Consider potentially reversible
function

confirmed
functional causes@
Prolactin, iron saturation
Evaluate other pituitary hormones (if
clinically indicated%)
Sella MRI (if indicated^)
Karyotype (if clinical Klinefelter
syndrome)
LH and FSH
low or inappropriately normal
(Secondary hypogonadism)
LH and FSH
high
(Primary hypogonadism)
Consider other causes of
symptoms and signs
• Fig. 19.19  Algorithm for the diagnosis and evaluation of suspected androgen deficiency. In men with clinical manifestations (sym

ficiency. In men with clinical manifestations (symptoms and signs) consis­
tent with androgen deficiency, a morning fasting total testosterone (T), and if a condition is present that causes an alteration in the level of sex hormone–
binding globulin (SHBG; see Table 19.5) or if total T is borderline (e.g., between 200 and 400 ng/dL), an accurate measurement of free T level should be 
obtained initially. Acute systemic illnesses, drugs, or nutritional deficiency that could lower T concentrations 

onal deficiency that could lower T concentrations should be excluded. If the initial total and/or free 
T level is low, they should be repeated to confirm a diagnosis of male hypogonadism. *The lower limit of the normal range for total T harmonized to Centers 
for Disease Control and Prevention (CDC) standards in healthy nonobese men age 19 to 39 years is 264 ng/dL (9.2 nmol/L); this limit could be used for 
total T assays that are CDC certified. Depending on the assay and reference population u

 Depending on the assay and reference population used, the lower limit of the normal range established in non–CDC-
certified laboratories may not accurately identify men with hypogonadism. #Free T should be measured by an accurate method, such as the equilibrium 
dialysis method or estimated from total T, SHBG with or without albumin concentrations, using a formula that accurately reflects free T by equilibrium 
dialysis. The normal range for free T in healthy young men is usually between 5 and 

e T in healthy young men is usually between 5 and 6 ng/dL (0.17–0.31 nmol/L) in most laboratories. **Total T may also be 
high in some conditions in which SHBG concentrations are very high, such as human immunodeficiency virus disease or with use of some anticonvulsants. 
Concentrations of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) should be obtained (in same sample as T measurements) to determine 
whether T deficiency is caused by secondary hypogonadism (low T with low or n

sed by secondary hypogonadism (low T with low or normal LH and FSH) or primary hypogonadism (low T with high LH and FSH). 
@In men with secondary hypogonadism, potentially reversible or treatable functional causes of primary and secondary hypogonadism that might be man­
aged without T treatment should be considered (denoted in Tables 19.7 and 19.8); serum prolactin and iron saturation should be assessed; and %if there 
is clinical indication of hypopituitarism or sella abnormality on imaging, ev

ypopituitarism or sella abnormality on imaging, evaluation of other pituitary hormones (e.g., free T4 and TSH) should be performed. 
^Pituitary imaging (sella magnetic resonance imaging [MRI]) to exclude pituitary and/or hypothalamic tumor or infiltrative disease is indicated if there is severe 
secondary hypogonadism (e.g., serum total T < 150 ng/dL [5.2 nmol/L]), panhypopituitarism, persistent hyperprolactinemia, or symptoms or signs of tumor 
mass effect (e.g., new-onset headache, visual impa

mass effect (e.g., new-onset headache, visual impairment, visual field defect, or cerebrospinal fluid rhinorrhea) are present. A sella computed tomography 
scan may be sufficient if macroadenoma is suspected or to assess parasellar bone involvement. (Redrawn from Bhasin S, Ponce JP, Cunningham GC, 
et al. Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2018;103:1715–1744.)
708
i

708
in the afternoon have consistently normal concentrations in the 
morning. Testosterone concentrations are also lowered by glucose 
infusion or food intake,206,207 and thus measurements should 
preferably be done in the fasting state. Given these findings and 
the fact that normal ranges of testosterone concentration are usu­
ally based on morning blood samples, testosterone measurements 
to confirm the diagnosis of hypogonadism should preferably b

 the diagnosis of hypogonadism should preferably be 
performed in the morning in the fasting state.
There is also substantial day-to-day variation in serum testoster­
one concentrations, underscoring the need to repeat the measure­
ment to confirm low concentrations, particularly if the first result 
was only moderately below normal. Among men with serum tes­
tosterone concentrations of less than 300 ng/dL on an initial test, 
30% to 35% were found to have a normal level on repeat test­
ing.208 

nd to have a normal level on repeat test­
ing.208 Among community-dwelling middle-age and older men 
who had an initial serum testosterone concentration of less than 
250 ng/dL, 20% were found to have an average testosterone level 
higher than 300 ng/dL (i.e., within the normal range) when six 
samples were drawn over the subsequent 6 months.209 However, 
none of the subjects who had an initial average serum testoster­
one concentration of less than 250 ng/dL on two separate samples 
obtained 1 

han 250 ng/dL on two separate samples 
obtained 1 to 3 days apart had an average level higher than 300 
ng/dL in six samples drawn over the next 6 months. These find­
ings support the need to measure testosterone on at least two occa­
sions to confirm the diagnosis of androgen deficiency. 
Total Testosterone Assays
Total testosterone assays are performed in most local laboratories 
and are readily available to clinicians. Therefore, total testoster­
one is recommended as the initial measurement 

er­
one is recommended as the initial measurement for the assessment 
of androgen deficiency. In local clinical laboratories, total testos­
terone is usually measured by automated platform-based direct 
immunoassays on unextracted serum or plasma. However, there 
is substantial variability in results from different assays, mostly 
because the accreditation of laboratories has been based on the 
reproducibility of results in comparison to other laboratories using 
the same method or kit, rather t

boratories using 
the same method or kit, rather than on the accuracy of results. For 
example, when identical quality control samples were assayed by 
different methods or kits, the reported measured values ranged 
from 160 to 508 ng/dL. Moreover, the lower limit of the normal 
range in some assays was as low as 132 to 210 ng/dL (clearly in 
the hypogonadal range for most conventional assays).210 In con­
trast, the lower limit of the normal range based on conventional 
radioimmunoassays after e

e based on conventional 
radioimmunoassays after extraction is approximately 280 to 300 
ng/dL. Most commercial reference laboratories now measure tes­
tosterone by liquid chromatography tandem mass spectrometry 
methods after solid phase extraction that are more accurate and 
precise than immunoassays.
To address the problems in the accuracy, quality control, and 
lack of standardization of total testosterone assays, the US Cen­
ters for Disease Control and Prevention (CDC) has developed a 
pro

 Control and Prevention (CDC) has developed a 
program to standardize and harmonize testosterone assays using 
accuracy-based quality control standards to which most commer­
cial reference laboratories participate.203,211 Recently, the College 
of American Pathologists also instituted an accuracy-based qual­
ity control program, but, unfortunately, neither of these programs 
is mandatory for certification. Recently, a harmonized reference 
range was established for total testosterone, cross-cali

was established for total testosterone, cross-calibrated to 
the CDC reference method and standards, in more than 9000 
community-dwelling men from four large cohorts in the United 
States and Europe.203 The harmonized reference range in healthy, 
nonobese men 19.39 years of age was 264 to 916 ng/mL. This 
range could be used for laboratories that perform a CDC-certified 
total testosterone assay but not necessarily for total testosterone 
assays that are not CDC certified for accuracy.
The prev

 that are not CDC certified for accuracy.
The prevalence of low testosterone concentrations is high in 
several clinical conditions, namely in the presence of a pituitary or 
sellar mass; irradiation; disease; use of certain medications, such 
as opiates or high-dose glucocorticoids; HIV disease with weight 
loss; late-stage CKD; withdrawal from long-term androgenic 
anabolic steroid use; infertility; BMD concentrations that reveal 
osteoporosis or low-trauma fracture; and low libido or erectile

or low-trauma fracture; and low libido or erectile 
dysfunction. If clinical manifestations consistent with androgen 
deficiency are present in men with these conditions, testosterone 
measurements should be performed.116 
Total Testosterone Affected by Alterations in SHBG
Because a substantial proportion (30–40%) of circulating tes­
tosterone is bound tightly to SHBG, alterations in SHBG con­
centration may affect total testosterone concentrations without 
altering free or bioavailable testoste

ns without 
altering free or bioavailable testosterone. Conditions that sup­
press SHBG concentrations (even within the broad normal range) 
lower total testosterone (sometimes to below the normal range) 
without affecting circulating free or bioavailable testosterone 
concentrations116 (Table 19.5). Common conditions that lower 
SHBG concentrations include moderate obesity, often associ­
ated with type 2 diabetes mellitus (T2DM); protein-losing states, 
such as nephrotic syndrome; administratio

states, 
such as nephrotic syndrome; administration of glucocorticoids, 
progestins, or androgens; untreated hypothyroidism; acromeg­
aly; familial SHBG deficiency; and SHBG gene polymorphisms. 
SHBG concentrations are increased with increasing age, hepatic 
cirrhosis and inflammation (hepatitis of any cause), estrogens, 
hyperthyroidism, anticonvulsants, HIV disease, and SHBG gene 
polymorphisms.
If conditions that affect SHBG concentration are present in 
a patient or if total testosterone con

present in 
a patient or if total testosterone concentrations are close to the 
lower limit of the normal range (e.g., total testosterone concentra­
tion 200–400 ng/dL), serum free or bioavailable testosterone mea­
surements should be obtained to confirm androgen deficiency. 
Unfortunately, accurate and reliable assays for free or bioavailable 
  Conditions Associated With Alterations 
in SHBG Concentrations
Decreased SHBG 
Increased SHBG Concentrations
Moderate obesity, type 2 

sed SHBG Concentrations
­diabetes mellitus
Aging
Hepatic cirrhosis and hepatitis
Glucocorticoids, progestins, 
and androgens
Estrogens
Hypothyroidism, untreated
Hyperthyroidism, untreated
Familial SHBG deficiency
HIV disease
Polymorphisms in SHBG gene
HIV, Human immunodeficiency virus; SHBG, sex hormone–binding globulin.
From Bhasin S, Brito JP, Cunningham GR, et al. Testosterone therapy in men wit

ningham GR, et al. Testosterone therapy in men with hypo­
gonadism; an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 
2018;103:1715-1744.
   
TABLE 19.5
709
testosterone are not performed routinely in most local clinical lab­
oratories. Although direct free testosterone assays using analogue 
immunoassay methods (that may be automated) are widely avail­
able and inexpensive, these assays are inaccurate and are affected 
by alterations in

are inaccurate and are affected 
by alterations in SHBG, so they are not recommended.212-214
The gold standard method for measurement of free testoster­
one concentrations is equilibrium dialysis or centrifugal ultra­
filtration. Free testosterone concentrations may be calculated 
accurately from measurements of total testosterone, SHBG, and 
albumin using affinity constants for binding of testosterone to its 
binding proteins and published formulas. Calculated free testos­
terone values are com

las. Calculated free testos­
terone values are comparable to those measured by equilibrium 
dialysis.213 However, calculated values depend on the specific 
testosterone and SHBG assays employed and the formula used 
to estimate free testosterone.215 There are several formulas used 
for calculation of free testosterone that all demonstrate some bias 
relative to the equilibrium dialysis method, largely as a result of 
assuming a single or two identical, noninteracting binding sites 
on SHBG.113 H

tical, noninteracting binding sites 
on SHBG.113 However, they all provide reasonable approxima­
tions of free testosterone in the normal to low range of values that 
are less prone to misinterpretation and overdiagnosis of hypogo­
nadism than total testosterone measurements when alterations in 
SHBG concentrations are present.
Bioavailable testosterone is measured by ammonium sulfate pre­
cipitation of SHBG-bound testosterone and measurement of free 
and albumin-bound testosterone in the supern

free 
and albumin-bound testosterone in the supernatant. Bioavailable 
testosterone concentrations may also be calculated from measure­
ments of total testosterone, SHBG, and albumin. In general, fewer 
studies used bioavailable versus free testosterone measurements, so 
many clinicians use free rather than bioavailable testosterone mea­
surements to diagnose hypogonadism. These accurate and reliable 
measurements of free and bioavailable testosterone are available in 
commercial laboratories an

erone are available in 
commercial laboratories and should be used to confirm androgen 
deficiency in men who have conditions that affect SHBG and in 
those with total testosterone concentrations near the lower limit 
of the normal range. In a recent report, 60% of more than 3600 
men in a single health care system who had low total testosterone 
concentrations were found to have normal calculated free testoster­
one concentrations using the laboratory reference range that was 
utilized by pract

ratory reference range that was 
utilized by practitioners to make clinical decisions. These findings 
highlight the potential clinical importance of using free testoster­
one to confirm a biochemical diagnosis of hypogonadism.216 
Transient Suppression of Testosterone
In evaluating men for a diagnosis of male hypogonadism, it is 
important to recognize that serum testosterone concentrations 
may be suppressed transiently during acute (particularly criti­
cal) and subacute illness and recovery; 

ly criti­
cal) and subacute illness and recovery; with the short-term use of 
certain medications, such as opioids, high-dose glucocorticoids, 
and CNS-active medications or recreational drugs that suppress 
gonadotropin and testosterone production; and during transient 
malnutrition, such as that associated with illness, eating disor­
ders, or excessive or prolonged strenuous exercise (resulting in low 
energy intake relative to energy expenditure). In such situations, 
measurement of serum tes

re). In such situations, 
measurement of serum testosterone should be delayed until the 
patient is completely recovered from the illness, the offending 
drugs are discontinued, the malnutrition is corrected, or the exces­
sive exercise is stopped.116 
Screening and Case Finding for Androgen Deficiency
In the absence of evidence for long-term clinically meaningful 
health benefits greater than risks for testosterone treatment of 
androgen deficiency, screening for androgen deficiency in the gen­

ncy, screening for androgen deficiency in the gen­
eral population or in all elderly men is not indicated. In addition, 
existing screening instruments lack sufficient specificity and sensi­
tivity to be clinically useful.116 In certain clinical conditions, case 
finding is warranted because hypogonadism is common. These 
conditions include hypothalamic-pituitary disease, mass, surgery, 
or radiation therapy; medications that suppress testosterone pro­
duction (e.g., opioids, glucocorticoids); w

e pro­
duction (e.g., opioids, glucocorticoids); withdrawal from long-
term androgenic anabolic steroid use; HIV-associated weight loss 
and other wasting syndromes; infertility; osteoporosis or minimal-
trauma fragility fracture, especially in young men; and low libido 
or erectile dysfunction.116 
Seminal Fluid Analysis
If infertility is a main complaint, whether or not androgen defi­
ciency is also present, seminal fluid analysis should be performed 
to determine the presence and degree of im

formed 
to determine the presence and degree of impairment of sperm pro­
duction. Seminal fluid analyses are performed on ejaculated semen 
samples obtained by masturbation after a standardized period (of 
at least 48 hours and not more than 7 days) of abstinence from 
ejaculation. Semen collection after withdrawal of the penis from 
the partner just before ejaculation during sexual intercourse (coitus 
interruptus) is usually incomplete and is not recommended, but it 
may be an option if mastur

ot recommended, but it 
may be an option if masturbation is not possible or is not permit­
ted for personal or religious reasons. Seminal fluid analyses should 
be performed in a specialized laboratory that employs standard­
ized procedures, such as those outlined by the WHO, and is certi­
fied and qualified to carry out these procedures.29
According to recently revised WHO criteria based on approxi­
mately 1800 to 1900 men from 14 countries whose partners 
became pregnant in 12 months or less (

e partners 
became pregnant in 12 months or less (Table 19.6), the lower 
limit of normal sperm concentration is 15 million/mL; semen vol­
ume is 1.5 mL; total sperm count is 39 million per ejaculate; total 
sperm motility (both progressive and nonprogressive) is 40% and 
progressive sperm motility is 32%; and the percentage of sperm 
with normal morphologic forms, using strict criteria to elimi­
nate spermatozoa with even mild abnormalities, is at least 4%.29 
Values below these lower limits ma

at least 4%.29 
Values below these lower limits may be classified as falling into 
the subfertile range, and such values are found in independent 
(unscreened) populations. In another study, the subfertile ranges 
were defined as follows: sperm concentration, less than 13.5 mil­
lion/mL; motility, less than 32%; and strict morphologic struc­
ture, less than 9%. The respective fertile ranges were greater than 
48 million/mL for sperm concentration, 63% for motility, and 
12% for strict morphologi

, 63% for motility, and 
12% for strict morphologic structure. Values between these ranges 
indicated indeterminate fertility.217
  Normal Seminal Fluid Analysis
Parameter
Normal Value
Sperm concentration
≥15 million/mL
Semen volume
≥1.5 mL
Sperm count
≥39 million per ejaculate
Sperm motility
≥40% (progressive sperm motility >32%)
Sperm morphology
≥4% normal forms (by strict criteria excluding 
sperm with mild abnormalities)
From WHO Laboratory Manual for the Examination and Processing of Human

Manual for the Examination and Processing of Human Semen. 5th ed. 
Geneva, Switzerland: World Health Organization; 2010.
   
TABLE 19.6
710
Sperm counts and concentrations exhibit extreme variability9 
(Fig. 19.20) for several reasons, including variations in sexual 
activity and abstinence, completeness of collection, recent illness 
(especially febrile illness) that may suppress spermatogenesis, and 
lifestyle factors such as frequent and prolonged 

lifestyle factors such as frequent and prolonged hot tub use. There­
fore, at least two or three seminal fluid analyses, separated by at 
least 1 to 2 weeks, should be performed to assess sperm produc­
tion adequately. In addition, to assess motility, freshly collected 
semen (within 1 hour of ejaculation) should be used, necessitating 
collection at or near the laboratory in which the analysis is to be 
performed. 
Androgen Deficiency and Impaired Sperm Production
The

rogen Deficiency and Impaired Sperm Production
The diagnosis of hypogonadism is confirmed in men with symp­
toms and signs consistent with androgen deficiency and in whom 
low testosterone concentrations are found on at least two occa­
sions. Testosterone concentrations should not be measured shortly 
after an acute illness, medication use, or nutritional deficiency 
that could transiently lower testosterone. Furthermore, an accu­
rate assay of free or bioavailable testosterone should be perform

ree or bioavailable testosterone should be performed 
for men who have conditions that alter SHBG or a total testoster­
one level near the lower limit of normal. In men with confirmed 
hypogonadism, measurements of serum gonadotropins, LH, and 
FSH should be measured to distinguish primary from secondary 
hypogonadism116 (see Fig. 19.19).
Men with primary hypogonadism caused by a disorder of the 
testis have low serum testosterone in association with elevated 
LH and FSH concentrations as a resu

with elevated 
LH and FSH concentrations as a result of reduced negative feed­
back suppression of gonadotropin secretion by testosterone and 
inhibin B (FSH concentrations are higher than LH concentra­
tions in men with primary hypogonadism) 9 (Fig. 19.21). In con­
trast, men with secondary hypogonadism caused by a disorder 
of the pituitary or the hypothalamus (or both) have low testos­
terone in association with low gonadotropin concentrations or 
inappropriately normal LH and FSH given the p

 or 
inappropriately normal LH and FSH given the presence of low 
testosterone concentrations9 (Fig. 19.22). In most local clinical 
laboratories, LH and FSH are measured by newer-generation 
nonradioactive immunoassays that have sufficient sensitivity to 
distinguish between normal and low concentrations. Although 
there are differences in reference ranges for LH and FSH among 
commercial gonadotropin assays, they are relatively minor, and 
a consensus reference range in healthy eugonadal young

nsensus reference range in healthy eugonadal young men for 
LH is 1.6 to 8.0 IU/L and FSH is 1.3 to 8.4 IU/L.189 Refer­
ence ranges in which the upper limit is higher than these have 
included older men or men with unrecognized impairment of 
spermatogenesis.
Aging, some systemic illnesses (e.g., iron overload syndromes), 
and certain medications (e.g., glucocorticoids) may cause defects 
in both the testes and the hypothalamus or pituitary, resulting 
in combined primary and secondary hypogonad

lting 
in combined primary and secondary hypogonadism. In most 
cases, a hormonal pattern consistent with either primary or sec­
ondary hypogonadism predominates. For example, men with 
hemochromatosis have defects in both the pituitary and the testes 
due to iron overload, but they usually have low testosterone and 
gonadotropin concentrations, a hormonal pattern consistent with 
secondary hypogonadism. Men with late-stage CKD have both 
testis and hypothalamic-pituitary dysfunction but usually

and hypothalamic-pituitary dysfunction but usually have 
low testosterone and elevated LH and FSH concentrations, the 
latter mostly due to reduced clearance of gonadotropins by the 
kidney. However, in the presence of comorbid illness, nutritional 
deficiency, or certain medications, men with CKD may have sup­
pression of gonadotropins and testosterone into the normal range 
and hormonal pattern consistent with secondary hypogonadism. 
Some men have more than one disorder influencing the gonada

have more than one disorder influencing the gonadal 
axis, one affecting the testis and another affecting the hypothala­
mus or pituitary. This may result in a hormonal pattern that is 
predominantly consistent with either primary or secondary hypo­
gonadism or in a combined pattern (e.g., very low testosterone 
concentrations with only slightly elevated or high-normal gonado­
tropin concentrations that are lower than expected given the pres­
ence of very low testosterone concentrations).
200
18

e of very low testosterone concentrations).
200
180
160
140
120
100
80
60
40
20
0
Sperm count (million/mL)
0
20
40
60
80
100
120
140
160
180
200
Weeks
• Fig. 19.20  Normal variation in sperm concentration (millions of spermatozoa per milliliter of ejaculate) in a 
healthy young man: results of frequent sampling over a period of 210 weeks. At several times during this 
period, sperm concentrations dropped below the normal range (15 million/mL). (From Matsumoto AM. The 
testis. In: Felig P, Frohma

rom Matsumoto AM. The 
testis. In: Felig P, Frohman LA, eds. Endocrinology and Metabolism, 4th ed. New York, NY: McGraw-Hill; 
711
Distinguishing primary from secondary hypogonadism helps 
define the specific cause of hypogonadism and has important 
clinical and therapeutic implications.9 Secondary hypogonad­
ism may be caused by a destructive process in the pituitary or 
hypothalamus, such as a pituitary adenoma (i.e., organic causes). 
A large pi

uitary adenoma (i.e., organic causes). 
A large pituitary adenoma (macroadenoma) may cause space-
occupying tumor mass effects such as headaches, visual field 
defects, hydrocephalus, or cerebrospinal fluid rhinorrhea, or it 
may result in impaired or excessive secretion of some anterior 
pituitary hormones, leading to clinical manifestations and thera­
peutic implications beyond the treatment of androgen deficiency 
alone. Secondary hypogonadism may be caused by disorders 
that are transient, s

 may be caused by disorders 
that are transient, such as an acute illness, certain medications 
(e.g., opioids, glucocorticoids), or malnutrition associated with 
illness (i.e., functional causes of gonadotropin suppression). In 
such cases, androgen deficiency may resolve with treatment of 
and recovery from the illness or malnutrition or discontinua­
tion of the offending medication. Finally, in men with secondary 
hypogonadism who have gonadotropin deficiency but otherwise 
normal testes, gon

ropin deficiency but otherwise 
normal testes, gonadotropin or GnRH treatment may be used 
to stimulate spermatogenesis and androgen production and to 
restore fertility in men who wish to father children. In contrast, 
infertility in men with primary hypogonadism is not treatable 
with hormone therapy. 
Most men with isolated impairment of sperm production have 
low sperm counts or abnormalities in sperm motility or mor­
phologic appearance (o

s in sperm motility or mor­
phologic appearance (or both) but no clinical manifestations of 
androgen deficiency and normal concentrations of testosterone 
and gonadotropins. Most men with isolated impairment of sperm 
production or function are classified as having primary hypogo­
nadism with an isolated defect in the seminiferous tubule com­
partment of the testes (Table 19.7); in such cases, there is no 
improvement in spermatogenesis with gonadotropin treatment 
like there is in secondary hy

adotropin treatment 
like there is in secondary hypogonadism. Men with severe semi­
niferous tubule failure and azoospermia or severe oligozoospermia 
may demonstrate a selective elevation in FSH concentrations as a 
result in reduced negative feedback from inhibin B with normal 
LH concentrations9 (Fig. 19.23). Occasionally, isolated impair­
ment in sperm production is caused by gonadotropin deficiency 
(i.e., secondary hypogonadism) (Table 19.8); this may occur in 
men who are taking high dose

); this may occur in 
men who are taking high doses of testosterone and in those who 
have androgen-secreting tumors; congenital adrenal hyperplasia; 
or, rarely, isolated FSH deficiency.
Men with nonfunctioning or gonadotropin-secreting pituitary 
macroadenomas often have secondary hypogonadism with clini­
cal manifestations of androgen deficiency and low testosterone 
concentrations.53 Many of these men secrete excessive amounts of 
GnRH
Inhibin B
LH
T
T
FSH
Interstitiu

n B
LH
T
T
FSH
PRIMARY HYPOGONADISM
Deficiency in androgen and sperm production
tubule
Germ
cells
BRAIN
Pituitary
Testis
• Fig. 19.21  Schematic diagram of alterations in the hypothalamic-pituitary-testicular axis with primary 
hypogonadism due to testicular disease, which results in both androgen deficiency and impairment of 
sperm production and elevated luteinizing hormone (LH) and follicle-stimulating hormone (FSH) c

mone (LH) and follicle-stimulating hormone (FSH) concentra­
tions (with FSH concentrations usually higher than LH concentrations) due to loss of negative feedback 
regulation of gonadotropins. GnRH, gonadotropin-releasing hormone; T, testosterone. (Modified from Mat­
sumoto AM. The testis. In: Felig P, Frohman LA, eds. Endocrinology and Metabolism. 4th ed. New York, 
NY: McGraw-Hill; 2001:635–705.)
712
biologically inactive but immunoassay-detectable 

biologically inactive but immunoassay-detectable intact FSH, free 
α-, FSHβ-, and LHβ-subunits but rarely immunoassay-detectable 
intact LH. Therefore, a gonadotropin-secreting pituitary tumor 
should be suspected in a man who has clinical manifestations of 
androgen deficiency, low testosterone, and elevated FSH but nor­
mal or low LH (or, rarely, elevated LH but normal or low FSH), 
which is an atypical gonadotropin pattern for men with androgen 
Rarely, disorders of androgen acti

en 
Rarely, disorders of androgen action or androgen resistance 
manifest in adults (Table 19.9). These men usually present with 
clinical manifestations similar to those of men with mild androgen 
deficiency, usually with an almost normal male phenotype and 
frequently with varying degrees of hypospadias, cryptorchidism, 
scrotal abnormalities, or impairment in sperm production. Usu­
ally, both serum testosterone and gonadotropin concentrations 
are elevated (with LH concentrations 

are elevated (with LH concentrations higher than FSH concentra­
tions) 9 (Fig. 19.24). 
Further Evaluation
Testosterone measurements and assessment of sperm production, 
combined with measurement of gonadotropin concentrations, 
allow classification of the causes of male hypogonadism into pri­
mary or secondary hypogonadism and subclassification of the 
latter into disorders causing both androgen deficiency and impair­
ment in sperm production and those causing isolated impairment 

production and those causing isolated impairment 
of sperm production or function (see Tables 19.7 and 19.8).
Once hypogonadism has been classified as primary or second­
ary hypogonadism, further evaluation includes a history (includ­
ing medication review), physical examination, and laboratory 
testing to identify a specific cause or causes of hypogonadism. In 
men with primary hypogonadism and suggestive clinical mani­
festations such as very small testes (e.g., <6 mL) and gynecomas­
tia, low 

mall testes (e.g., <6 mL) and gynecomas­
tia, low or low-normal testosterone, azoospermia, and markedly 
elevated gonadotropins, a karyotype may be obtained to confirm 
the diagnosis of Klinefelter syndrome.
In men with secondary hypogonadism, further evaluation may 
include measurements of serum prolactin (in almost all cases) to 
exclude hyperprolactinemia; iron saturation and ferritin to screen 
for hereditary hemochromatosis, especially in men with other 
manifestations of iron overload (e.g

n with other 
manifestations of iron overload (e.g., liver failure, diabetes, and 
CHF) and in young men with unexplained selective gonadotropin 
deficiency; further testing to exclude excessive secretion or defi­
ciency of anterior pituitary hormones; and magnetic resonance 
imaging (MRI) of the sella turcica to exclude a pituitary or hypo­
thalamic tumor or infiltrative disease. Computed tomography 
(CT) of the sella turcica usually detects a pituitary macroadenoma 
but is less sensitive than 

ituitary macroadenoma 
but is less sensitive than sella MRI in detecting smaller tumors 
and infiltrative disease; CT is more sensitive in detecting parasellar 
bone destruction.116
It is not cost effective to perform sella MRI in all men with 
secondary hypogonadism. This modality should be reserved for 
men with secondary hypogonadism and the following: serum 
GnRH
Inhibin B
LH
T
T
FSH
Normal-
Normal-
tubule
Germ
cells
BRAIN
Hypoth

ferous
tubule
Germ
cells
BRAIN
Pituitary
Testis
SECONDARY HYPOGONADISM
• Fig. 19.22  Schematic diagram of alterations in the hypothalamic-pituitary-testicular axis with secondary 
hypogonadism due to hypothalamic or pituitary disease, which results in both androgen deficiency and 
impairment of sperm production and inappropriately normal or low luteinizing hormone (LH) and follicle-
stimulating hormone (FSH) concentrations. Gn

icle-
stimulating hormone (FSH) concentrations. GnRH, gonadotropin-releasing hormone; T, testosterone. 
(Modified from Matsumoto AM. The testis. In: Felig P, Frohman LA, eds. Endocrinology and Metabolism, 
4th ed. New York, NY: McGraw-Hill; 2001:635–705.)
713
testosterone less than 150 ng/dL and low gonadotropin concen­
trations218-220; discordant LH and FSH concentrations (i.e., one 
gonadotropin markedly elevated but the other not); hyperprolac­
tinemia (even m

d but the other not); hyperprolac­
tinemia (even mild); clinical and biochemical evidence of excessive 
secretion of other pituitary hormones (e.g., free α-subunit secre­
tion, Cushing syndrome, acromegaly), panhypopituitarism, or 
diabetes insipidus; tumor mass effects (e.g., severe headache, new 
onset of blurry vision or visual impairment, visual field defects, or 
cerebrospinal fluid rhinorrhea); and no obvious functional cause 
of secondary hypogonadism (e.g., morbid obesity, long-acting 
o

hypogonadism (e.g., morbid obesity, long-acting 
opioid, or chronic high-dose glucocorticoid therapy).
In evaluating the etiology of secondary hypogonadism, it is 
important to consider potentially reversible/treatable causes of 
gonadotropin suppression that are more common than organic 
congenital or destructive causes of gonadotropin deficiency and 
might be managed by discontinuation of an offending medica­
tion (e.g., opioid pain medication), treatment of the underlying 
condition (e.g., ob

, treatment of the underlying 
condition (e.g., obesity), or organ transplantation (e.g., renal 
transplantation).
In men who have severe androgen deficiency caused by pri­
mary or secondary hypogonadism or who have sustained a low-
trauma or fragility fracture, dual-energy x-ray absorptiometry 
scanning to assess BMD should be performed to exclude osteope­
nia or osteoporosis.116 
Causes of Primary Hypogonadism
Androgen Deficiency and Impairment in Sperm 
Production
Congenital or Developmental 

 in Sperm 
Congenital or Developmental Disorders
Klinefelter Syndrome. Classically, Klinefelter syndrome is 
characterized by very small, firm testes; azoospermia and infer­
tility; varying degrees of androgen deficiency and eunuchoidism; 
and uniformly elevated gonadotropin concentrations.20,221,222 It 
is the most common sex chromosome abnormality and the most 
common cause of primary hypogonadism, resulting in androgen 
deficiency and impaired sperm production. It occurs prenatally

nd impaired sperm production. It occurs prenatally 
and neonatally in 1 of every 500 to 700 males, and the prevalence 
in adults is 1 in 2500.1,2 Because the syndrome does not cause 
premature death in boys, the low adult prevalence indicates that 
Klinefelter syndrome is often overlooked and underdiagnosed in 
men. Given the almost uniform finding of extremely small testes 
and other phenotypic abnormalities, it is surprising that about 
75% of men with Klinefelter syndrome are never diagnosed.

men with Klinefelter syndrome are never diagnosed. This 
might be due to failure to recognize mild phenotypes or failure to 
perform a testicular exam. The risk of having a child with Kline­
felter syndrome increases with both maternal and paternal age.
The chromosomal abnormality in Klinefelter syndrome is the 
presence of one or more extra X chromosomes due to maternal 
meiotic nondisjunction (mostly in meiosis I) in approximately 
50% of the cases or paternal meiotic nondisjunction in the 
re

ases or paternal meiotic nondisjunction in the 
remaining cases.20,221,222 The principal karyotype in 90% of men 
with Klinefelter syndrome is 47,XXY. Most of the remaining 10% 
have mosaic Klinefelter syndrome (47,XXY/46,XY), in which 
there is a 47,XXY karyotype in some tissues or cells and a nor­
mal 46,XY karyotype in other tissues or cells. Mosaicism occurs 
as a result of postfertilization mitotic nondisjunction. Men with 
mosaic Klinefelter syndrome usually demonstrate a variable and 
les

r syndrome usually demonstrate a variable and 
less severe phenotype that depends on the specific tissues in which 
an extra X chromosome is present. Some men with mosaicism 
have a normal karyotype in the testis with intact spermatogenesis 
and fertility. Rarely, men with Klinefelter syndrome have more 
  Causes of Primary Hypogonadism
Common Causes
Uncommon Causes
Androgen Deficiency and Impairment of Sperm Production
Klinefelter syndrome (XXY) and varian

al Disorders
Klinefelter syndrome (XXY) and variants
Myotonic dystrophy
Uncorrected cryptorchidism
Noonan syndrome
Bilateral congenital anorchia
Polyglandular autoimmune 
syndrome
Testosterone biosynthetic 
enzyme defects
CAH (TART)
Complex genetic syndromes
Down syndrome
LH receptor mutation
Bilateral surgical castration or trauma
Orchitis
Drugs (spironolactone, ketoconazole, 
abiraterone, enzalutamide, alcohol, 
chemotherapy agents)a
Ionizing radiation
Chr

agents)a
Chronic liver disease (hepatic 
cirrhosis)a,b
Malignancy (lymphoma, 
testicular cancer)
Chronic kidney diseasea,b
Sickle cell diseaseb
Agingb
Spinal cord injury
Vasculitis (polyarteritis)
Infiltrative disease (amyloi­
dosis, leukemia)
Varicocele
Sertoli cell–only syndrome
Y chromosome microdeletions
Primary ciliary dyskinesia
D

mosome microdeletions
FSH receptor mutation
Orchitis
Environmental toxins
Chemotherapy agents
Thermal trauma
Acute febrile illness
Malignancy (testicular cancer, Hodgkin 
disease)b
Idiopathic azoospermia or oligozoo­
spermia
aFunctional causes that are potentially reversible with discontinuation of offending medica­
tion, or with liver or renal transplantation.
bCombined primary a

iver or renal transplantation.
bCombined primary and secondary hypogonadism.
CAH, Congenital adrenal hyperplasia; FSH, follicle-stimulating hormone; LH, luteinizing hormone; 
TART, testicular adrenal rest tumor.
   
TABLE 19.7
714
than one extra X chromosome (e.g., 48,XXXY, 49,XXXXY). Men 
with these variants manifest a more severe phenotype than is seen 
in classic Klinefelter syndrome.
Infants with Klinefelter syndrome may manifest micropenis, 
hypo

linefelter syndrome may manifest micropenis, 
hypospadias, cryptorchidism, or developmental delay.222,223 Dur­
ing childhood, boys with the syndrome commonly have small 
testes and reduced penile length relative to age-matched normal 
individuals and may manifest relatively tall stature, clinodactyly, 
hypertelorism, gynecomastia, elbow dysplasia, high-arched pal­
ate, hypotonia, language delay or learning and reading disabilities 
requiring therapy, and behavioral problems.222,224 However, thes

apy, and behavioral problems.222,224 However, these 
manifestations may be mild and are often missed. Fewer than 
10% of boys with Klinefelter syndrome (usually those with the 
most severe phenotype) are diagnosed before puberty. At puberty, 
the testes fail to increase in size and become firm due to a progres­
sive loss of germ cells and seminiferous tubule hyalinization and 
fibrosis; Sertoli cell products, inhibin B, and AMH decline to very 
low or undetectable serum concentrations; circulati

ow or undetectable serum concentrations; circulating testoster­
one concentrations increase but to less than normal concentra­
tions in some boys, resulting in varying degrees of eunuchoidism 
and gynecomastia; and FSH concentrations are disproportion­
ately elevated relative to LH concentrations. Elevation of serum 
LH disproportionately to FSH is an indication of a disorder other 
than Klinefelter syndrome or any cause of primary hypogonad­
ism that affects both Sertoli cell and Leydig cell dy

 that affects both Sertoli cell and Leydig cell dysfunction—for 
example, rare diseases such as disorders of testosterone biosynthe­
sis or abnormalities of LH or its receptor should be considered.
In adults, the most prominent and consistent clinical feature of 
Klinefelter syndrome is very small testes, less than 4 mL in volume 
(<2.5 cm in length); this feature is easily detected on examina­
tion and should alert the clinician to the possibility of Klinefelter 
syndrome8,225 (see Fig. 19.18).

ty of Klinefelter 
syndrome8,225 (see Fig. 19.18). Men with this syndrome may pres­
ent with a complaint of infertility and subsequently be found to 
have azoospermia and very small testes.226 Other manifestations 
include varying degrees of androgen deficiency, eunuchoidism, 
and gynecomastia221 (Fig. 19.25). In contrast to the classic long 
arms and legs of eunuchoidism seen in patients with androgen 
deficiency of prepubertal onset, Klinefelter syndrome results in a 
disproportionate increase

r syndrome results in a 
disproportionate increase in lower- compared with upper extrem­
ity long bone growth. Gynecomastia occurs in 50% to 80% of 
men with the syndrome and may be quite prominent and embar­
rassing. Learning and developmental disabilities occur in about 
70% of men who are diagnosed with Klinefelter syndrome; the 
prevalence of cognitive impairment might be lower in men with 
undiagnosed Klinefelter syndrome. Character and personality dis­
orders and behavioral problems occur 

onality dis­
orders and behavioral problems occur commonly, possibly in part 
because of the psychosocial consequences of androgen deficiency 
and learning disabilities. Men with Klinefelter syndrome have 
intelligence quotient scores that are reduced by 10 to 15 points 
but not into the intellectual disability range. Taurodontism, 
GnRH
Inhibin B
T
Normal T
Normal 
or ↑ FSH
Normal LH
tubule
Germ
cells
BRAIN
Pituitary
Te

li cell
Germ
cells
BRAIN
Pituitary
Testis
ISOLATED DEFECT IN SPERM PRODUCTION
Normal or Isolated FSH elevation
• Fig. 19.23  Schematic diagram of alterations in the hypothalamic-pituitary-testicular axis with an isolated 
seminiferous tubule defect that results in isolated impairment of sperm production or function. Follicle-stim­
ulating hormone (FSH) concentrations may be normal or selectively elevated (if defect in spermatogenesis 
is severe) with normal testosterone and luteiniz

is severe) with normal testosterone and luteinizing hormone (LH) concentrations. GnRH, gonadotropin-
releasing hormone; T, testosterone. (Modified from Matsumoto AM. The testis. In: Felig P, Frohman LA, 
715
  Causes of Secondary Hypogonadism
Constitutiona

ongenital or Developmental Disorders
Constitutional delayed puberty
CHH due to genetic mutations
CHH, idiopathic (IHH)
Isolated LH deficiency, LHβ mutations
Hyperprolactinemiaa
Opioidsa
Androgenic anabolic steroids, progestins, estrogen excessa
GnRH agonist or antagonista
Pituitary or hypothalamic tumor
Surgical hypophysectomy, pituitary or cranial irradiation
Vascular com

omy, pituitary or cranial irradiation
Vascular compromise, traumatic brain injury
Granulomatous or infiltrative disease
Infection
Pituitary stalk disease
Lymphocytic or autoimmune hypophysitis
Acquired IHH
Glucocorticoid excess (Cushing syndrome)a,b
Chronic systemic illnessa,b
Chronic organ failurea,b
Chronic liver disease (hepatic cirrhosis), chronic kidney disease, chronic lung 
disease, chronic heart failurea
Transfusion-related iron overload (β-thalassem

rea
Transfusion-related iron overload (β-thalassemia)a
Type 2 diabetes mellitusa
Cystic fibrosis
Malignancya
Rheumatic disease (rheumatoid arthritis)a
HIV diseasea
Starvation,b malnutrition,b eating disorders, endurance exercisea
Morbid obesity, obstructive sleep apneaa
Acute and critical illnessa
Aging (comorbid illnesses associated with aging)a,b
Congenital 

Congenital adrenal hyperplasia (21-hydroxylase deficiency, 11β-hydroxylase 
deficiency)
Isolated FSH deficiency, FSHβ mutations
Testosterone, androgenic anabolic steroids
Androgen- or hCG-secreting tumors
Malignancy (Hodgkin disease, testicular cancer)b
aFunctional causes that are potentially reversible or treatable with discontinuation of offending medication, treatment of underlying cause of gonadotropin suppression o

 of underlying cause of gonadotropin suppression or organ transplantation.
CHH, Congenital hypogonadotropic hypogonadism; FSH, follicle-stimulating hormone; GnRH, gonadotropin-releasing hormone; hCG, human chorionic gonadotropin; HIV, human immunodeficiency virus; 
IHH, idiopathic hypogonadotropic hypogonadism; LH, luteinizing hormone.
   
TABLE 19.8
716
characterized by enlarged molar teeth resulting from

aracterized by enlarged molar teeth resulting from enlargement 
and extension of the pulp chamber, is present in 40% of men with 
Klinefelter syndrome.
As described previously, the AR gene is located on the X chro­
mosome, and the length of the highly polymorphic CAG repeat in 
exon 1 of the AR gene is inversely related to AR activity. In Kline­
felter syndrome, the X chromosome carrying the AR gene with a 
short CAG repeat length (i.e., greater AR activity) undergoes inac­
tivation preferential

AR activity) undergoes inac­
tivation preferentially.139 Klinefelter syndrome patients with short 
CAG repeat lengths were found to have more stable relationships, 
higher educational concentrations, and greater responses to testos­
terone treatment. In contrast, men with long CAG repeat length 
(i.e., reduced AR activity) had longer arms and legs, smaller tes­
tes, a greater degree of gynecomastia, and lower BMD. Therefore, 
skewed inactivation of the X chromosome resulting in preferential 
act

of the X chromosome resulting in preferential 
activity of the long CAG repeat may contribute to phenotypic 
severity and variability of Klinefelter syndrome.
Most men with mosaic Klinefelter syndrome exhibit less severe 
clinical manifestations than those with the classic syndrome. 
Men with more than two extra X chromosomes have more severe 
manifestations and a higher incidence of intellectual disability 
and somatic abnormalities such as hypospadias, cryptorchidism, 
and radioulnar synostosi

spadias, cryptorchidism, 
and radioulnar synostosis. Very rarely, some phenotypic males 
with a 46,XX karyotype exhibit typical clinical manifestations 
  Causes of Androgen Resistance
Kennedy disease (spinal and bulbar 
muscular atrophy)
Partial androgen insensitivity syndrome 
(PAIS; 40–60% with AR mutations)
5α-Reductase type 2 deficiency (not a 
syndrome of androgen insensitivity, 
but it presents similarly to PAIS)
Complet

ivity, 
Complete androgen insensitivity syn­
drome (female phenotype)
AR antagonists (bicalu­
tamide, nilutamide)
Celiac disease
Drugs (spironolactone, 
cyproterone acetate)
AR, Androgen receptor; PAIS, partial androgen insensitivity syndrome.
   
TABLE 19.9
GnRH
Inhibin B
T
T
FSH
LH
tubule
Germ
cells
BRAIN
Pituitary
Testis
ANDROGEN RESISTANCE
• Fig. 19.24  Schematic 

estis
• Fig. 19.24  Schematic diagram of alterations in the hypothalamic-pituitary-testicular axis with androgen 
resistance due to impaired androgen action (e.g., androgen receptor mutation), which results in high tes­
tosterone (T) concentrations but reduced androgen action. This leads to manifestations of androgen defi­
ciency, impaired sperm production, and elevated luteinizing hormone (LH) and follicle-stimulating hormone 
(FSH) concentrations (with LH concentrations tha

(FSH) concentrations (with LH concentrations that are higher than FSH concentrations) due to impaired 
androgen-mediated negative feedback regulation of gonadotropins. GnRH, gonadotropin-releasing hor­
mone. (Modified from Matsumoto AM. The testis. In: Felig P, Frohman LA, eds. Endocrinology and Metab­
olism. 4th ed. New York, NY: McGraw-Hill; 2001:635–705.)
717
of Klinefelter syndrome but have shorter stature and a greater 
incidence of cryptorchidism, gynecom

nd a greater 
incidence of cryptorchidism, gynecomastia, and androgen defi­
ciency.227 In most of these cases, there has been a translocation 
of an SRY-containing segment of the Y chromosome onto an X 
chromosome.
In addition to infertility, variable androgen deficiency, and 
gynecomastia, patients with Klinefelter syndrome have an approx­
imately 20- to 50-fold increased risk in breast cancer compared 
with normal men (although the absolute lifetime risk of <1% is 
low); such patients account 

etime risk of <1% is 
low); such patients account for approximately 4% of all cases of 
male breast cancer.228 Klinefelter syndrome is also associated with 
increased risk for mitral valve prolapse; lower extremity varicose 
veins, venous stasis ulcers, deep vein thrombosis, and pulmonary 
embolism; autoimmune diseases such as systemic lupus erythe­
matosus, rheumatoid arthritis, and Sjögren syndrome; other 
cancers such as extragonadal germ cell cancer and non-Hodgkin 
lymphoma; T2DM and the me

 cancer and non-Hodgkin 
lymphoma; T2DM and the metabolic syndrome; and psychiatric 
illnesses such as depression and schizophrenia. There is a small 
reduction in life expectancy of approximately 5 to 6 years in men 
with Klinefelter syndrome, associated with a higher rate of death 
due to a variety of common causes.222,229,230
Azoospermia is present in more than 95% of men with clas­
sic Klinefelter syndrome and infertility in more than 99%. 
Serum total testosterone concentrations are usually

erum total testosterone concentrations are usually low but may 
fall in the low-normal to mid-normal range in 40% to 50% of 
cases20,221,231 (Fig. 19.26). Relative to normal men, some men 
with Klinefelter syndrome have relatively high serum estradiol 
concentrations, which probably contributes to the development 
of gynecomastia, and increased SHBG concentrations. Increased 
SHBG concentrations provide a partial explanation for normal 
total testosterone concentrations in the presence of low co

tosterone concentrations in the presence of low concentra­
tions of free testosterone. Serum FSH concentrations are almost 
always elevated, and LH concentrations are usually elevated but 
may fall into the high-normal range in some men, consistent 
20
15
10
5
>55
40
35
30
25
0
IU/L
FSH
34
25
20
15
10
5
0
IU/L
LH
35
30
25
20
15
10
5
0
nmol/L
• Fig. 19.26  Serum concentrations of follicle-stimulating hormone (FSH, left), luteinizing hormone (LH, mid­
dle), and total testosteron

zing hormone (LH, mid­
dle), and total testosterone (T, right) in men with Klinefelter syndrome (dots) compared with normal men 
(normal range depicted by shaded boxes). A substantial proportion of men with Klinefelter syndrome have 
total T concentrations within the normal range, but almost all have elevated LH and FSH concentrations. 
(From Smyth CM, Bremner WJ. Klinefelter syndrome. Arch Intern Med. 1998;158:1309–1314.)
• Fig. 19.25  Variability in the degree of androgen deficiency manifested

ty in the degree of androgen deficiency manifested 
in men with Klinefelter syndrome. The man on the left, who has classic 
47,XXY Klinefelter syndrome, demonstrates androgen deficiency of prepu­
bertal onset with eunuchoidal body proportions, small penis, sparse chest 
and pubic hair, poor muscle development, prepubertal fat distribution, and 
very small testes (2 mL bilaterally). The man on the right, who has mosaic 
47,XXY/46,XY Klinefelter syndrome, demonstrates normal body propor­
tions, pe

ndrome, demonstrates normal body propor­
tions, penis size, and body hair but small testis size (8 mL bilaterally). 
(From Smyth CM, Bremner WJ. Klinefelter syndrome. Arch Intern Med. 
1998;158:1309–1314.)
718
with primary hypogonadism20,221,231 (see Fig. 19.26). Symptoms 
of androgen deficiency in the presence of normal total and free 
testosterone and elevated gonadotropin concentrations occur in 
some men with Klinefelter syndrome, suggesting relat

me men with Klinefelter syndrome, suggesting relative andro­
gen deficiency or resistance.
The diagnosis of Klinefelter syndrome is confirmed by karyo­
type analysis, which is usually performed on cultured peripheral 
blood lymphocytes. Occasionally, karyotype analysis is performed 
on cultured skin fibroblasts and testis tissue if mosaicism is sus­
pected. If a fetus is diagnosed on prenatal testing with 47,XXY 
Klinefelter syndrome, genetic counseling should be provided. 
Klinefelter syndrome 

nseling should be provided. 
Klinefelter syndrome has a generally good prognosis, but a minor­
ity of couples choose termination of a fetus with a prenatal diag­
nosis of Klinefelter syndrome.
Treatment of Klinefelter syndrome is directed at correction of 
androgen deficiency. Infants with micropenis may benefit from (sys­
temic or penile topical) testosterone treatment. For boys with Kline­
felter syndrome, early intervention with speech and reading therapy 
is important if speech delay and dys

ding therapy 
is important if speech delay and dyslexia are present. At puberty, 
testosterone treatment may be needed for adequate development of 
secondary sexual characteristics; peak bone mass and BMD; muscle 
mass and strength; and energy, motivation, mood, and behavior. 
Adults with Klinefelter syndrome who have clinical manifesta­
tions of androgen deficiency and consistently low concentrations of 
serum total or free testosterone (or both) should receive testosterone 
replacement therapy

) should receive testosterone 
replacement therapy. In addition, men who have symptoms and 
signs of androgen deficiency but normal total and free testosterone 
concentrations should be offered a trial of testosterone treatment.
Although azoospermia and infertility are not generally revers­
ible with medical therapy, TESE permits identification of the 
relatively few seminiferous tubules that contain active spermato­
genesis and harvesting of sperm from 50% to 70% of men with 
Klinefelter syndro

rm from 50% to 70% of men with 
Klinefelter syndrome for use in ICSI; this approach results in live 
birth rates of approximately 45% in specialized centers.232 Because 
there is an increased risk of sex chromosome and autosomal aneu­
ploidy, genetic counseling and prenatal or preimplantation testing 
should be provided to couples who undergo ICSI.
Chronic, nontender gynecomastia does not resolve with testos­
terone treatment and requires reduction mammoplasty. Psycho­
logical counseling for pat

on mammoplasty. Psycho­
logical counseling for patients and spouses and participation in 
support groups may be extremely helpful for men with Klinefelter 
syndrome. 
Myotonic Dystrophy. Myotonic dystrophy is an autosomal 
dominant, multisystem disorder that is characterized by progres­
sive muscle weakness and wasting (particularly in the lower legs, 
hands, neck, and face) and results in physical disability; myotonia 
(involuntary sustained contraction of muscles); cataracts; cardiac 
conducti

traction of muscles); cataracts; cardiac 
conduction defects; respiratory insufficiency; dysphagia; testicular 
atrophy, impaired spermatogenesis, infertility, and androgen defi­
ciency; premature frontal balding; and intellectual disability.233 
Features of the disease usually develop in young adults but can 
occur at any age, and their severity varies widely among affected 
individuals. Two types of myotonic dystrophies are caused by 
expansion of CTG trinucleotide repeats in two different gen

 of CTG trinucleotide repeats in two different genes. 
Myotonic dystrophy type 1 has more severe clinical features and 
is caused by CTG repeat expansion in the dystrophia myotonica 
protein kinase (DMPK) gene; it accounts for 98% of cases. Myo­
tonic dystrophy type 2 is less severe and is caused by CTG repeat 
expansion in the CCHC-type zinc finger, nucleic acid–binding 
protein (CNBP) gene.
Primary hypogonadism occurs in approximately 80% of young 
to middle-age men with myotonic dystrophy.234

ung 
to middle-age men with myotonic dystrophy.234,235 Most of these 
men have isolated impairment of sperm production or function 
with testicular atrophy, oligozoospermia or azoospermia, moder­
ate to severe testicular damage on testis biopsy, infertility, and dis­
proportionately elevated FSH compared with LH concentrations. 
Approximately 20% to 40% of men with myotonic dystrophy 
have variable degrees of androgen deficiency with low testosterone 
concentrations and elevated LH and FSH; test

rone 
concentrations and elevated LH and FSH; testosterone replace­
ment therapy is appropriate for these men. High-dose testoster­
one therapy has been demonstrated to improve muscle mass but 
has not been shown to affect muscle strength.236 Infertility due to 
myotonic dystrophy is not reversible with medical therapy, and 
patients who desire children need to pursue ART or other options; 
genetic counseling and preimplantation or prenatal testing should 
be provided, given the autosomal domina

ng should 
be provided, given the autosomal dominant nature of the disease. 
Cryptorchidism. Cryptorchidism is the failure of one or both 
testes to descend normally from within the abdomen through the 
inguinal canal into the scrotum. It is the most common congeni­
tal disorder of the male genital tract in children, affecting 2% to 
4% of full-term male infants.237-239 It is more common in pre­
mature, low-birth-weight, and small-for-gestational-age infants. 
Spontaneous descent of the testis o

-age infants. 
Spontaneous descent of the testis occurs during the first year in 
most infants (probably induced by the neonatal surge of gonado­
tropins and testosterone), so the prevalence of cryptorchidism in 
boys and adults is lower, approximately 0.3% to 1.0%. Because a 
patent processus vaginalis is usually also present, inguinal hernia 
is found in conjunction with cryptorchidism in 50% to 80% of 
cases. Both unilateral and bilateral cryptorchidism are associated 
with impaired sperm pro

torchidism are associated 
with impaired sperm production, infertility, and an increased risk 
of testicular cancer.
In contrast to cryptorchidism, ectopic testes are located outside 
the normal path of testicular descent.240 Ectopic testes may be 
located anywhere in the perineum or in the femoral or superfi­
cial inguinal regions. It is also important to distinguish cryptor­
chidism from retractile testes (pseudocryptorchidism). Retractile 
testes are located in the scrotum but withdraw into t

tes are located in the scrotum but withdraw into the inguinal 
canal or abdomen with minimal stimulation due to a hyperac­
tive cremasteric reflex; retractile testes are not usually associated 
with impaired sperm production, infertility, or increased risk of 
testicular cancer. However, impaired spermatogenesis and fertility 
have been reported in men with bilateral retractile testes that are 
located high in the inguinal canal or, sometimes, in the abdomen.
Bilateral cryptorchidism may be asso

 the abdomen.
Bilateral cryptorchidism may be associated with several disor­
ders causing primary hypogonadism (including Klinefelter syn­
drome variants and Noonan syndrome), secondary hypogonadism 
(including CHH, Kallmann syndrome, and complex genetic dis­
orders associated with multiple congenital anomalies or defects, 
e.g., Prader-Labhart-Willi syndrome or Laurence-Moon-Biedl 
syndrome), and androgen resistance syndromes (e.g., Reifenstein 
syndrome).241 Cryptorchidism may also be caused b

syndrome).241 Cryptorchidism may also be caused by mutations 
of the Leydig cell product, INSL3, which controls growth of the 
gubernaculum or of its receptor (RXFP2) in up to 5% of cases.
Unilateral or bilateral cryptorchidism that is not related to 
known causes of hypogonadism or androgen resistance usually 
results in isolated impairment of sperm production associated with 
low sperm counts; normal testosterone concentrations; a selective 
elevation in FSH concentrations; and, occasionally,

levation in FSH concentrations; and, occasionally, high LH con­
centrations.237,242 Rarely, cryptorchidism causes Leydig cell failure 
and androgen deficiency (e.g., in adults with uncorrected bilateral 
cryptorchidism), producing low serum testosterone with high LH 
and FSH concentrations.243,244 Azoospermia occurs in 50% to 
60% and oligozoospermia in 75% to 100% of men with bilateral 
cryptorchidism; among men with unilateral cryptorchidism, these 
719
figures

hese 
719
figures respectively are 15% to 20% and 20% to 40%. This sug­
gests that the function of both testes is compromised in unilateral 
cryptorchidism. An underlying developmental or environmental 
disorder affecting both testes (testicular dysgenesis) may contribute 
to impaired spermatogenesis in these individuals.237,242 The for­
mation of Ad spermatogonia from neonatal gonocytes is inhibited 
in undescended testes.245 Uncommonly, normal testicular descen

ed testes.245 Uncommonly, normal testicular descent 
is impeded by an anatomic abnormality, such as a large external 
inguinal hernia. In this instance, both testes function normally, 
and orchiopexy before puberty is generally recommended to pre­
serve spermatogenesis and fertility.
The risk of testicular cancer in an undescended testis is 2.5- to 
8-fold greater than in a scrotal testis, and the risk remains higher 
even after the testis is surgically relocated into the scrotum, support­
ing t

gically relocated into the scrotum, support­
ing the notion that cryptorchidism is a manifestation of a under­
lying testicular disorder (i.e., testicular dysgenesis).237,241,242,246 
Even though the incidence of testicular cancer is only 1 or 2 per 
100,000 males, the lifetime risk of malignancy in a cryptorchid 
testis is substantial. The prevalence of testicular carcinoma in situ, 
which is thought to precede testicular cancer, is about 3% in the 
cryptorchid testis. Testicular cancer usually

the 
cryptorchid testis. Testicular cancer usually occurs in men from 20 
to 40 years of age.
A careful physical examination should be performed to assess 
the location of testes within the scrotum or inguinal canal; the 
presence of an inguinal hernia, hydrocele, or other scrotal mass; 
the cremasteric reflex induced by stroking of the upper medial 
thigh; and penile size and the position of the urethral meatus. 
The examination should be performed with the patient stand­
ing, squatting, or sup

med with the patient stand­
ing, squatting, or supine with legs abducted. Scrotal examination 
may be difficult in a morbidly obese man with a large abdominal 
panniculus. Techniques that may be helpful in the detection of 
retractile testes include examination during a Valsalva maneuver, 
with pressure applied to the lower abdomen, or after scrotal warm­
ing with a warm towel to facilitate testicular descent. In addition, 
elicitation of the cremasteric reflex may cause a localized pucker­
ing 

masteric reflex may cause a localized pucker­
ing of the scrotal skin if a retractile testis is present in the scrotum. 
Often, a low undescended testis may be confirmed by pushing 
with one hand on the lower abdomen, with firm strokes from the 
anterior superior iliac spine through the groin and to the pubis, 
toward and into the scrotum, and then grasping the testis with the 
other hand. After the testis has been held in the scrotum until the 
cremasteric muscle fatigues, it is released. If th

cremasteric muscle fatigues, it is released. If the testis then retracts, 
it is considered a retractile testis; a low cryptorchid testis returns 
to its undescended position after release. Absence of a palpable 
testis in the scrotum after repeated examinations may be a result of 
cryptorchidism, an extremely atrophic testis, or anorchia (absent 
testis). Up to 50% of men with a unilateral, nonpalpable testis 
in the scrotum have a severely atrophic or absent testis instead of 
cryptorchidism; 

phic or absent testis instead of 
cryptorchidism; in these instances, the contralateral testis may be 
relatively large (by about 2 mL). High-resolution ultrasonography 
(or MRI) usually localizes testes that are not palpable.
Treatment for persistent cryptorchidism should be started 
before puberty, when greater germ cell degeneration occurs.237,242 
The exact timing of treatment is controversial, but recent recom­
mendations suggest that treatment should be instituted between 6 
and 12 months 

ent should be instituted between 6 
and 12 months or up to 24 months of age. Hormonal treatment 
with hCG in prepubertal boys is effective in stimulating descent of 
a cryptorchid testis in approximately 10% to 20% of cases. A trial 
of hCG therapy may be attempted in the hope of avoiding sur­
gery. If hormonal treatment is unsuccessful or not attempted, an 
orchiopexy (surgical relocation and fixation of the testis into the 
scrotum with ligation of the hernia sac at the external or internal 
i

n of the hernia sac at the external or internal 
inguinal ring) should be performed to allow examination of the 
testes (e.g., as in monitoring for malignancy) and preservation of 
remaining testis function. Despite orchiopexy, spermatogenesis 
remains impaired and fertility rates are reduced, particularly with 
bilateral cryptorchidism (65% paternity rate after orchiopexy). If 
orchiopexy is performed before puberty, the risk of testicular can­
cer is reduced but is still increased two- to thre

cer is reduced but is still increased two- to threefold. In patients 
with a history of unilateral cryptorchidism, the risk of malig­
nancy in the contralateral testis is also increased. Because the risk 
of malignancy is two to six times higher in men who underwent 
orchiopexy after puberty and the fertility potential is poor, some 
clinicians recommend orchidectomy for men with cryptorchidism 
discovered after puberty. The majority of testis cancers found in 
persistently cryptorchid testes ar

ncers found in 
persistently cryptorchid testes are seminomas, whereas those in 
cryptorchid testes after orchiopexy are mostly nonseminoma tes­
ticular cancers. 
Noonan Syndrome. Noonan syndrome is an autosomal 
dominant or occasional sporadic genetic disorder that is charac­
terized by short stature; unusual facial features (hypertelorism, 
downward-slanting eyes, ptosis, strabismus, low-set ears with 
thickened helices, high nasal bridge, micrognathia and trian­
gular-shaped face, high-arched

ognathia and trian­
gular-shaped face, high-arched palate, low hairline, dental 
malocclusion); short, webbed neck; shield-like chest, pectus 
excavatum or carinatum, scoliosis, cubitus valgus, and joint lax­
ity; intellectual disability; cardiac disease (pulmonary stenosis, 
hypertrophic cardiomyopathy); hepatosplenomegaly; lymph­
edema; and cryptorchidism.247,248
Because several these clinical features resemble those of females 
with Turner syndrome, Noonan syndrome was previously called 
male

drome, Noonan syndrome was previously called 
male Turner syndrome. However, the karyotype in these men is 
normal. Noonan syndrome affects approximately 1 in 1000 to 
2500 live births and is caused by mutations in genes in the Ras-
MAPK signaling pathway.247,248 Approximately 50% of men with 
Noonan syndrome have mutations in the protein tyrosine phos­
phatase nonreceptor type 11 (PTPN11) gene, and the remainder 
have mutations in the son of sevenless homolog 1 (SOS1), RAF1, 
or KRAS genes.
Men

enless homolog 1 (SOS1), RAF1, 
Men with Noonan syndrome may demonstrate primary hypo­
gonadism characterized by androgen deficiency and impaired 
sperm production with elevated gonadotropin concentrations; 
they usually present with delayed puberty.249 Cryptorchidism is 
present in more than 50% of men with the syndrome and contrib­
utes to the cause of hypogonadism. 
Bilateral Congenital Anorchia. Congenital anorchia (also 
known as functional prepubertal castrate or vanishing t

 as functional prepubertal castrate or vanishing testis syn­
drome) is a rare condition in which there is absence of one or both 
testes in a phenotypically and genotypically normal male.250,251 
Normal fetal testis function is needed for normal male internal 
and external genital differentiation and development during early 
gestation. The presence of otherwise normal male internal and 
external genitalia without müllerian duct–derived structures or 
descent of the spermatic cord structures (e.

s or 
descent of the spermatic cord structures (e.g., vas deferens, blood 
vessels) into the scrotum implies that normally functioning testes 
must have been present during the first 16 weeks of gestation and 
subsequently lost during fetal or neonatal life. The prevalence of 
bilateral congenital anorchia is 1 in 20,000, and that of unilateral 
congenital anorchia is 1 in 5000 males. The cause is not known 
but is probably heterogeneous. It is hypothesized that congenital 
anorchia may be cause

ypothesized that congenital 
anorchia may be caused by spermatic vascular compromise due to 
torsion or trauma during or after testicular descent.
Infants with bilateral anorchia present with micropenis 
in almost 50% of cases, supporting a prenatal origin of the 
720
disorder.250,251 Males with congenital anorchia usually present 
with prepubertal primary hypogonadism with delayed puberty 
and eunuchoidism (see Fig. 19.14), very low testosterone conc

idism (see Fig. 19.14), very low testosterone concen­
trations in the castrate range, and elevated gonadotropin concen­
trations. On examination, palpable testes are absent, but spermatic 
cords and epididymides are usually present. Normal testosterone 
and gonadotropin concentrations in pubertal or adult patients 
with absent testes exclude the diagnosis of congenital anorchia 
and should raise the possibility of bilateral cryptorchidism, which 
carries an increased risk for testicular malignan

carries an increased risk for testicular malignancy.
An hCG stimulation test may be performed to distinguish 
congenital anorchia from bilateral cryptorchidism. In patients 
with congenital anorchia, serum testosterone concentrations do 
not increase in response to prolonged hCG administration (e.g., 
1000–2000 IU three times weekly for 2 weeks), whereas most 
patients with bilateral cryptorchidism respond to hCG. How­
ever, lack of testosterone response to hCG administration for 6 
weeks has b

 response to hCG administration for 6 
weeks has been reported in men with bilateral cryptorchidism.252 
Serum AMH concentrations are usually undetectable in patients 
with congenital anorchia.253 Measurements of AMH concentra­
tions are more sensitive than those of testosterone concentrations 
but equally specific. If clinical examination and endocrine bio­
chemical tests do not distinguish bilateral anorchia from crypt­
orchidism, then imaging studies (e.g., MRI) and laparoscopy or 
surgical a

studies (e.g., MRI) and laparoscopy or 
surgical abdominal exploration may be necessary to confirm the 
Treatment of bilateral congenital anorchia consists of testos­
terone replacement to stimulate penile length in patients with 
micropenis and to induce and maintain sexual development in 
boys with delayed puberty and eunuchoidism. Implantation of 
testicular prostheses in the scrotum may be of psychological and 
cosmetic value. 
Autoimmune Polyglandular Syndrome. Autoimmune poly­
g

oimmune Polyglandular Syndrome. Autoimmune poly­
glandular syndromes are characterized by a clustering of organ-
specific autoimmune disorders that involve many endocrine and 
nonendocrine tissues and are associated with circulating autoanti­
bodies to components of these tissues. Autoimmune polyglandular 
syndrome type 1, also called autoimmune polyendocrinopathy-can­
didiasis-ectodermal dystrophy, is a rare autosomal recessive disorder 
that is caused by a mutation in the autoimmune regulator 

 caused by a mutation in the autoimmune regulator (AIRE) 
gene.254,255 Its main features are mucocutaneous candidiasis, 
hypoparathyroidism, primary adrenal insufficiency, and the pres­
ence of autoimmune disorders including primary hypogonadism. 
The prevalence of primary hypogonadism is much more common 
in females than in males. In females, hypogonadism is manifested 
as premature ovarian failure and is present in 35% to 70% of 
cases, whereas in males, it is present in 8% to 28% of cases and

 in males, it is present in 8% to 28% of cases and 
is manifested either by androgen deficiency and impairment in 
sperm production with elevated serum gonadotropin concentra­
tions or isolated impairment in sperm production (azoospermia) 
and isolated elevation of FSH concentrations.
Autoimmune polyglandular syndrome type 2 is a common 
polygenic disorder associated with the genes for human leukocyte 
antigens DR3 and DR4.254,255 It is characterized by autoimmune 
primary adrenal insufficiency,

ized by autoimmune 
primary adrenal insufficiency, thyroid disease (Hashimoto or 
Graves disease), and T1DM in addition to other autoimmune dis­
orders including primary hypogonadism (again, more common in 
females than in males). In autoimmune polyglandular syndrome 
type 2, primary hypogonadism is associated with circulating ste­
roid-producing cell autoantibodies and with specific autoantibod­
ies to P450 11A1 and P450 17A1 (cholesterol side-chain cleavage 
enzyme and 17α-hydroxylase/17,20-ly

hain cleavage 
enzyme and 17α-hydroxylase/17,20-lyase, respectively). 
Defects in Testosterone Biosynthetic Enzymes. Males with 
uncommon defects in 17,20-lyase/17α-hydroxylase, 17β-HSD/17-
ketoreductase type 3, or 3β-HSD type 2, resulting from mutations 
in the CYP17A1, HSD17B3, and HSD3B2 genes, respectively, 
usually present at birth as phenotypic females with partial viriliza­
tion or with ambiguous genitalia. However, patients with incom­
plete defects in these enzymes occasionally present 

ete defects in these enzymes occasionally present as phenotypic 
males with hypospadias, gynecomastia, and primary hypogonad­
ism with androgen deficiency manifested by delayed puberty.
Because 17,20-lyase and 17α-hydroxylase activities reside in the 
same enzyme, mutations in CYP17A1 usually cause deficiencies in 
both activities, leading to elevated concentrations of progesterone, 
corticosterone, and the aldosterone precursor 11-deoxycorticoste­
rone.256 Rarely, males exhibiting isolated 17,2

e­
rone.256 Rarely, males exhibiting isolated 17,20-lyase deficiency 
with elevated 17-hydroxyprogesterone concentrations have been 
reported.257 XY males with 17α-hydroxylase deficiency may have 
hypertension and hypokalemia due to excessive production of 
11-deoxycorticosterone that has potent mineralocorticoid activity, 
as well as variable degrees of sex steroid hormone deficiency; more 
severe 17α-hydroxylase deficiency is characterized by a female 
phenotype due to lack of testosterone eff

 female 
phenotype due to lack of testosterone effects on the wolffian ducts. 
Patients with 17α-hydroxylase deficiency usually do not mani­
fest adrenal insufficiency because of increased production of the 
cortisol precursor corticosterone that has glucocorticoid activity. 
Males with either partial, combined 17,20-lyase/17α-hydroxylase 
deficiency, or isolated 17,20-lyase deficiency have primary hypo­
gonadism with low testosterone and elevated LH and FSH con­
centrations (LH > FSH); they req

d LH and FSH con­
centrations (LH > FSH); they require testosterone treatment at 
the time of puberty.
Patients with 17β-HSD/17-ketoreductase type 3 deficiency 
may have ambiguous genitalia and be raised as females; however, 
at puberty, their testosterone concentrations increase sufficiently 
to induce virilization, resulting in gender reassignment (simi­
lar to individuals with low serum DHT concentrations due to 
5α-reductase deficiency).258 Serum testosterone concentrations 
are low to norma

erum testosterone concentrations 
are low to normal, but androstenedione and gonadotropin con­
centrations are increased.
Incomplete deficiency of 3β-HSD type 2 is a rare disorder 
that may manifest in adolescents with mild ambiguous genitalia, 
delayed virilization, gynecomastia, low testosterone concentra­
tions, and elevated LH and FSH concentrations.259 Concentra­
tions of pregnenolone, 17-hydroxypregnenolone, and DHEA 
are elevated. Spontaneous virilization and puberty due to direct 
effect

ous virilization and puberty due to direct 
effects of high concentrations of the weak androgen DHEA or 
conversion of DHEA to testosterone via 3β-HSD type 1 (or 
both) has been reported. A eugonadal male with partial 3β-
HSD deficiency who presented with gynecomastia has also been 
reported.260 
LH Receptor Mutations. Inactivating mutations of the LH 
receptor in XY males usually cause Leydig cell aplasia or hypo­
plasia. These patients usually present with DSD characterized by 
a female phenot

present with DSD characterized by 
a female phenotype without breast development at puberty (Ley­
dig cell aplasia) or genital ambiguity (hypoplasia) and cryptorchi­
dism.58 Rarely, partially inactivating LH receptor mutations result 
in a male phenotype with micropenis, hypospadias, delayed sexual 
development, undervirilization, low testosterone concentrations, 
impaired sperm production, and high serum LH concentrations. 
However, they usually have normal FSH concentrations. An indi­
vidual w

 have normal FSH concentrations. An indi­
vidual with an LH receptor mutation was reported to exhibit nor­
mal concentrations of testosterone and spermatogenesis after hCG 
stimulation, suggesting that hCG action via the LH receptor may 
be dissociated from that of LH.261 
721
Congenital Adrenal Hyperplasia. Adolescent and adult male 
patients with congenital adrenal hyperplasia due to 21-hydroxy­
lase deficiency may develop testicular adrenal rest tumors (TARTs)

may develop testicular adrenal rest tumors (TARTs) 
that resemble Leydig cell tumors but do not contain intracyto­
plasmic Reinke crystalloids that are commonly found (approxi­
mately 40–50%) on histologic analysis in the latter.262-264 TARTs 
may be large and easily palpable, or they may be detectable only 
on testicular ultrasonography. These tumors are thought to origi­
nate embryologically from aberrant adrenal tissue, and they are 
responsive to adrenocorticotropic hormone (ACTH). TARTs 
re

e to adrenocorticotropic hormone (ACTH). TARTs 
regress with supraphysiologic glucocorticoid therapy at a dosage 
that suppresses circulating ACTH concentrations, and they may 
grow with physiologic dosages of glucocorticoids.
21-Hydroxylase deficiency is an autosomal recessive condition 
caused by a mutation in the CYP21A2 gene. It is the most com­
mon enzymatic defect causing congenital adrenal hyperplasia. 
21-Hydroxylase deficiency produces an accumulation of steroid 
substrates (17-hydroxyp

n accumulation of steroid 
substrates (17-hydroxyprogesterone and progesterone) that results 
in excessive production of adrenal androgens (androstenedione, 
DHEA, and 11-hydroxyandrostenedione) that are subsequently 
converted to testosterone and 11-ketotestosterone. In addition, 
classic (severe) 21-hydroxylase deficiency causes reduced produc­
tion of cortisol and aldosterone, resulting in deficiency of glu­
cocorticoid and mineralocorticoid, respectively, and increased 
ACTH secretion (due t

respectively, and increased 
ACTH secretion (due to reduced cortisol negative feedback). 
Elevated ACTH secretion stimulates adrenal gland growth, 
resulting in adrenal hyperplasia. Glucocorticoid treatment lowers 
ACTH secretion, reduces excessive adrenal androgen production, 
treats clinical adrenal insufficiency, and prevents excessive adrenal 
hyperplasia.
Excessive adrenal androgen production due to untreated or 
inadequate glucocorticoid therapy for 21-hydroxylase deficiency 
suppresses go

erapy for 21-hydroxylase deficiency 
suppresses gonadotropin secretion by sex steroid negative feed­
back regulation and causes secondary hypogonadism.265 Because 
androgen concentrations are maintained by excessive adrenal 
production, men with 21-hydroxylase deficiency may not have 
clinical androgen deficiency. However, they usually have isolated 
impairment of sperm production due to suppression of circulating 
gonadotropin concentrations (due to the increased production of 
adrenal sex ster

e to the increased production of 
adrenal sex steroids and exogenous corticosteroid therapy). Glu­
cocorticoid therapy reduces excessive adrenal androgen produc­
tion and usually restores gonadotropin secretion and normalizes 
testis function, but supraphysiologic dosages of glucocorticoids 
may be required for these effects. Early glucocorticoid treatment 
may prevent or induce regression of adrenal rest tumors that may 
affect testicular function directly or cause mechanical obstruction 
of se

on directly or cause mechanical obstruction 
of seminiferous tubules.262-265
If TARTs are present, lack of suppression of circulating ACTH 
stimulates their growth, often resulting in very large tumors that 
can cause irreversible testicular damage.262-265 When testicular 
damage occurs, the patient will have irreversibly impaired sperm 
production due to loss of seminiferous tubules and may also have 
androgen deficiency due to loss of Leydig cells. With suppression 
of circulating ACTH with gl

lls. With suppression 
of circulating ACTH with glucocorticoid treatment, TARTs may 
regress completely, but the damaged testes remain small and serum 
testosterone concentrations and sperm counts low. Although 
excessive amounts of glucocorticoid therapy will suppress ACTH 
production and TART growth, they also suppress serum gonado­
tropins, serum testosterone, and sperm production. 
Down Syndrome. Down syndrome, or trisomy 21, is a chro­
mosomal disorder in which all or part of an extra chrom

al disorder in which all or part of an extra chromosome 
21 is present.266 It affects 1 of every 700 to 800 infants and is 
the most common cause of intellectual disability in children. 
Down syndrome is characterized by moderate to severe intel­
lectual disability; spontaneous, warm, and cheerful personality; 
short stature; characteristic facial features (most notably round 
facies with microgenia, upward-slanting and almond-shaped 
eyes resulting from bilateral epicanthal folds, macroglossia,

ing from bilateral epicanthal folds, macroglossia, and 
flat nasal bridge); congenital heart defects; primary hypothyroid­
ism; and defects affecting most other body systems. Males with 
Down syndrome usually manifest primary hypogonadism, most 
commonly characterized by isolated impairment in sperm produc­
tion with normal or selective elevation of FSH concentrations. 
Histologically, they may manifest hypospermatogenesis (moderate 
to severe reduction in all germ cell types), maturation arrest

duction in all germ cell types), maturation arrest, or 
Sertoli cell–only syndrome with loss of all germ cells. Less com­
monly, they demonstrate mild to moderate androgen deficiency 
with low or low-normal testosterone and elevated LH and FSH 
Complex Genetic Syndromes. Primary hypogonadism result­
ing in androgen deficiency and impaired sperm production or in 
isolated impairment in sperm production or function may occur 
as a manifestation of complex genetic syndromes, usuall

manifestation of complex genetic syndromes, usually in asso­
ciation with several congenital anomalies or defects and distinct 
morphologic developmental manifestations.267 Examples include 
the Alström, ataxia telangiectasia, Marinesco-Sjögren, Robinow, 
Rothmund-Thomson, Sohval-Soffer, Weinstein, Werner, and 
Wolfram syndromes.268-277 Prader-Labhart-Willi syndrome (asso­
ciated with cryptorchidism), Laurence-Moon-Bardet-Biedl syn­
drome, and Alström syndrome are disorders that are commonly 
as

ström syndrome are disorders that are commonly 
associated with secondary hypogonadism, but primary hypogo­
nadism may also occur in these disorders.278-281 
Bilateral Surgical Castration and Trauma. Bilateral surgical 
castration causes a rapid and profound decline in testosterone 
concentrations within hours, resulting in severe clinical mani­
festations of androgen deficiency, including hot flushes. Severe 
blunt trauma to the testes and associated vascular compromise 
may 

he testes and associated vascular compromise 
may result in testicular atrophy and loss of testis function, includ­
ing androgen deficiency and impaired spermatogenesis or isolated 
impairment in sperm production or function. 
Drugs and Ionizing Radiation. Certain drugs that affect andro­
gen production may cause androgen deficiency. Ketoconazole and 
abiraterone inhibit 17,20-lyase and 17α-hydroxylase activity at 
high doses (>400 mg daily and 1000 mg daily, respectively) and 
have been used, i

000 mg daily, respectively) and 
have been used, in conjunction with other agents, to lower both 
adrenal and testicular androgen production in the treatment of 
prostate cancer.282-284 Spironolactone, a nonspecific aldosterone 
antagonist, acts mainly as a competitive AR antagonist and inhib­
its androgen action.285 However, it also inhibits 17,20-lyase and 
17α-hydroxylase activity and testosterone biosynthesis at high 
doses.286,287 Enzalutamide is a competitive AR antagonist that 
also inhib

de is a competitive AR antagonist that 
also inhibits postreceptor AR action (translocation of AR into 
the nucleus, binding of AR cofactors, and DNA binding) that 
is used to treat castration-resistant prostate cancer.288 Chronic 
excessive intake of alcohol may directly inhibit testosterone pro­
duction, but it may also suppress gonadotropins; it may be asso­
ciated with nutritional deficiency or chronic liver disease that 
may contribute to androgen deficiency and impairment in sperm 
product

drogen deficiency and impairment in sperm 
production.289,290
In general, because spermatogenesis involves active cell rep­
lication, the germ cell compartment is much more sensitive to 
external or environmental influences (e.g., chemotherapy agents, 
722
ionizing radiation) than are Leydig cells. Exposure to such agents 
often results in primary hypogonadism characterized by an iso­
lated impairment in sperm production or function and elevated 
FSH 

in sperm production or function and elevated 
FSH concentrations. However, severe testis damage induced by 
these agents may cause Leydig cell dysfunction or damage, result­
ing in both androgen deficiency and impairment of sperm pro­
duction, and elevated LH and FSH (FSH > LH) concentrations.
Combination chemotherapy regimens that include alkylat­
ing agents (e.g., cyclophosphamide, ifosfamide, procarbazine, 
busulfan, chlorambucil), such as those used for the treatment of 
Hodgkin and non-Hodg

se used for the treatment of 
Hodgkin and non-Hodgkin lymphoma and leukemia, or use of 
alkylating agents, such as cyclophosphamide, in the treatment of 
systemic rheumatic disorders are particularly toxic to the testes and 
may result in androgen deficiency in up to 20% of patients.291,292 
High-dose chemotherapy and total-body irradiation before bone 
marrow transplantation may also cause androgen deficiency in a 
substantial proportion of men. In contrast, men with testicular 
cancer who unde

 In contrast, men with testicular 
cancer who undergo combination chemotherapy that includes 
platinum drugs (together with unilateral orchidectomy and, often, 
radiation therapy) have a relatively low prevalence of usually mild 
androgen deficiency with slightly low to low-normal testosterone 
concentrations and elevated LH concentrations.292
Exposure of testes to ionizing irradiation commonly suppresses 
spermatogenesis in a dose-dependent manner, and doses greater 
than 600 to 800 cGy may com

er, and doses greater 
than 600 to 800 cGy may compromise Leydig cell function and 
reduce testosterone production.16,293,294
Clinical practice guidelines for ongoing surveillance for testicu­
lar toxicity in childhood and young adult cancer survivors have 
been established by several groups.295,296 
Orchitis. Viral infection of the testes may result in testicular 
atrophy; impaired sperm production; and, in severe cases, andro­
gen deficiency. Mumps orchitis was common before the introduc­
tion

umps orchitis was common before the introduc­
tion of the measles/mumps/rubella vaccine in 1968, after which 
rates of mumps infection decreased profoundly.297,298 Because 
some people are fearful about potential side effects of vaccinations, 
there has been a resurgence of mumps infections and orchitis in 
adolescents and young adults. Prepubertal mumps orchitis is very 
uncommon and is not associated with subsequent testicular dys­
function. Orchitis is the most common complication of mumps 
i

chitis is the most common complication of mumps 
infection in pubertal boys and adults. It usually causes permanent 
seminiferous tubule damage; impaired spermatogenesis; and, in 
severe cases, Leydig cell failure and androgen deficiency.
Mumps infection usually manifests with headache, fever, and 
malaise followed by unilateral or bilateral parotid swelling due to 
parotiditis.297,298 Testis pain and swelling due to orchitis occurs in 
about 10 days or up to 6 weeks after the onset of parotidit

days or up to 6 weeks after the onset of parotiditis and 
may be subclinical in up to 50% of cases. Epididymitis accompa­
nies orchitis in 85% of cases. Mumps orchitis is usually unilateral 
but may be bilateral in 15% of cases in adolescents and in 30% 
of cases in young adults. Even if orchitis is clinically unilateral, 
degenerative changes may occur in the apparently unaffected 
testis. Germ cell sloughing occurs as a result of acute infection, 
inflammation, and ischemia resulting from pres

n, 
inflammation, and ischemia resulting from pressure induced 
by swelling of the testes within the tunica albuginea. The acute 
phase is followed by seminiferous tubule fibrosis and then testicu­
lar atrophy (30–50% of cases) during the next several months, 
resulting in impaired spermatogenesis in 25% to 40% of cases. In 
severe cases, prednisone may be used to reduce inflammation and 
swelling associated with mumps orchitis, but it does not prevent 
testicular damage.
Untreated or undertreat

revent 
Untreated or undertreated HIV infection may cause gonadotro­
pin suppression and secondary hypogonadism, particularly when 
associated with wasting and systemic illness. However, in 20% 
to 30% of cases, HIV infection causes primary hypogonadism 
characterized by low testosterone and elevated gonadotropin con­
centrations. The cause of primary hypogonadism is orchitis that 
is caused by HIV infection of the testis or occurs secondary to 
opportunistic infection (e.g., 

curs secondary to 
opportunistic infection (e.g., by cytomegalovirus [CMV], Myco­
bacterium avium intracellulare, Toxoplasma gondii) associated with 
the patient’s immunocompromised state.299,300
Other causes of infectious orchitis usually associated with epi­
didymitis include echovirus, arbovirus, or lymphocytic chorio­
meningitis infection; gonorrhea or Chlamydia infection in young 
adults and urinary pathogens such as Escherichia coli in older men; 
leprosy and tuberculosis; brucellosis, gla

r men; 
leprosy and tuberculosis; brucellosis, glanders, and syphilis; and 
parasitic infections such as filariasis and bilharziasis.299,301 
Low serum testosterone concentrations associated with symptoms 
and signs consistent with androgen deficiency are commonly asso­
ciated with chronic diseases affecting the liver, kidney, heart, and 
lungs.4,5 The underlying cause of both clinical and biochemical 
hypogonadism in these conditions is complex and multifactorial. 
The illnes

ditions is complex and multifactorial. 
The illness itself, associated complications, nutritional compro­
mise, and medications used to treat the illness may contribute to 
or confound the clinical manifestations of androgen deficiency 
and also may suppress gonadotropin and testosterone production, 
thereby playing an etiologic role. Chronic systemic diseases usu­
ally affect both testicular and hypothalamic-pituitary function and 
cause combined primary and secondary hypogonadism. Clini­
cally

d primary and secondary hypogonadism. Clini­
cally, however, measurements of gonadotropin concentrations 
usually suggest predominantly primary hypogonadism (i.e., ele­
vated gonadotropins) or secondary hypogonadism (i.e., normal or 
low gonadotropins). The benefits and risks of testosterone therapy 
in patients with these systemic disorders have not been evaluated 
in long-term randomized, controlled outcome studies.
Chronic Liver Disease. In men with chronic liver disease of 
any cause (and pa

n with chronic liver disease of 
any cause (and particularly in those with hepatic cirrhosis or liver 
failure), sexual dysfunction, gynecomastia, and testicular atrophy 
resulting in impaired androgen and sperm production occur com­
monly, affecting 50% to 75% of these patients.4,5,302-304 Total tes­
tosterone concentrations may be low but are often normal or high 
normal, because SHBG concentrations are increased substantially 
with cirrhosis and chronic active hepatitis. Therefore, measure­
m

nd chronic active hepatitis. Therefore, measure­
ments of free or bioavailable testosterone using accurate assay 
methods should be used to assess androgen deficiency. Free and 
bioavailable testosterone concentrations are usually low, and LH 
concentrations are usually elevated or in the high-normal range 
in patients with mild to moderate hepatic cirrhosis (Child-Pugh 
class A or B).
Estrogen (estrone and estradiol) concentrations are usually 
high due to increased production (e.g., induced by

high due to increased production (e.g., induced by alcohol excess) 
and reduced clearance of adrenal androgens (e.g., androstene­
dione), which provide increased substrates for aromatization of 
androgens to estrogens. High estrogen concentrations contribute 
to the development of gynecomastia (high estrogen-to-androgen 
ratio), palmar erythema and spider angiomas, and increased pro­
lactin concentrations. Treatment of ascites and edema with spi­
ronolactone may further lower testosterone concen

ronolactone may further lower testosterone concentrations and 
block androgen action, contributing to gynecomastia and other 
manifestations of androgen deficiency. Serum FSH and LH con­
centrations may be suppressed by high estrogen and prolactin 
concentrations and by the malnutrition that occurs commonly 
in men with hepatic cirrhosis and liver failure; these factors may 
723
contribute a hormonal pattern more consistent with secondary 
hypogonadism in some m

 consistent with secondary 
hypogonadism in some men. Oligozoospermia or azoospermia 
associated with abnormalities in sperm motility and morphologic 
appearance occurs in approximately 30% to 50% of men with 
chronic liver disease.
Testosterone treatment of androgen deficiency is usually well 
tolerated but occasionally may worsen gynecomastia and rarely 
may increase edema and ascites by causing fluid retention. A 
preliminary 
double-blind, 
randomized, 
placebo-controlled 
12-month trial in 

andomized, 
12-month trial in men with hepatic cirrhosis found that testos­
terone improved lean body and bone mass, reduced fat mass, and 
improved hemoglobin and hemoglobin A1c concentrations.305 
Because immunosuppressive medications such as prednisone and 
cyclosporine are used to prevent rejection, liver transplantation 
only partially reverses hypogonadism associated with chronic liver 
disease.302,306 
Chronic Kidney Disease. Late-stage CKD commonly causes 
combined pr

sease. Late-stage CKD commonly causes 
combined primary and secondary hypogonadism, resulting in 
androgen deficiency and impairment in sperm production in 
50% to 60% of these patients.4,5,307 Serum testosterone concen­
trations are low, and LH and FSH concentrations are high, in 
large part because of markedly reduced renal clearance of gonado­
tropins and increased secretion resulting from reduced negative 
feedback. SHBG concentrations are usually not affected by CKD 
unless nephrotic syndro

ually not affected by CKD 
unless nephrotic syndrome is present, in which case concentra­
tions of SHBG and, consequently, total testosterone may be low. 
In the latter situation, free or bioavailable testosterone measure­
ments should be performed to assess androgen deficiency. Sperm 
production is impaired, and sperm motility and the percentage 
of sperm with normal morphologic appearance are reduced. The 
Leydig cell response to hCG administration is reduced, consistent 
with primary testicul

tion is reduced, consistent 
with primary testicular dysfunction. The frequency and amplitude 
of pulsatile LH secretion are altered, suggesting an alteration in 
hypothalamic-pituitary function as well. Hyperprolactinemia, 
relative nutritional deficiency, uremia, the proinflammatory state, 
comorbid conditions that secondarily suppress gonadotropins, 
and zinc deficiency that affects testicular function may contribute 
to testicular dysfunction in men with CKD.
Randomized controlled trials tha

 in men with CKD.
Randomized controlled trials that assess the long-term clinical 
benefits and risks of testosterone treatment have not been con­
ducted.308 Hemodialysis and peritoneal dialysis do not improve 
testosterone or sperm production.307,309 Successful renal trans­
plantation usually brings testosterone and sperm production to 
almost normal concentrations, although immunosuppression by 
rapamycin inhibitors (e.g., sirolimus) may impair testis function 
slightly.307,310,311 
Aging. Aft

 testis function 
Aging. After age 40 years, there is a gradual and progressive 
decline in total testosterone concentrations (by approximately 
1% per year) such that an increasing proportion of older men 
have low serum testosterone concentrations in the hypogonadal 
range.312-314 Because SHBG concentrations increase with age, free 
and bioavailable testosterone concentrations decline even more 
rapidly (2–3% per year). Daily sperm production, sperm motility, 
percentage 

ily sperm production, sperm motility, 
percentage of sperm with normal morphologic forms, Sertoli cell 
number, and inhibin B concentrations also decline with aging.144 
Leydig cell number and testosterone production in response to 
stimulation by hCG are reduced, consistent with primary testicular 
dysfunction.144 In older men, circadian variation in testosterone 
concentration is present but blunted.64 Furthermore, pulsatile LH 
secretion is more irregular and disorderly and LH pulse amplitude

re irregular and disorderly and LH pulse amplitude 
is reduced in older men compared with those who are younger. 
Pulsatile GnRH administration normalizes pulsatile LH but not 
testosterone secretion, consistent with an impairment of hypotha­
lamic GnRH secretion occurring in combination with a primary 
defect in testosterone production by the testis.315,316
Serum LH and FSH concentrations increase (by about 1–2% 
per year) with aging but do not usually rise above the normal 
range until very ol

usually rise above the normal 
range until very old age (>70 years).312-314,317 Therefore, the most 
common hormonal profile associated with aging observed in mid­
dle-age to older men is low testosterone with normal LH and FSH 
concentrations, consistent with secondary hypogonadism; gonad­
otropin suppression is thought to be largely related to age-related 
comorbidities (e.g., obesity, illness, and medications). As men get 
older, gonadotropins continue to rise. A hormonal pattern of low 
test

 continue to rise. A hormonal pattern of low 
testosterone and elevated LH and FSH concentrations, consistent 
with primary hypogonadism, is more prevalent in very old men, 
particularly after 70 years of age.
As men age, they may develop chronic organ failure or systemic 
illnesses, take an increasing number of medications, and develop 
nutritional deficiency or wasting syndromes that are associated 
with low testosterone concentrations.318 It is likely that these 
comorbid conditions contribut

s likely that these 
comorbid conditions contribute to low testosterone and clinical 
hypogonadism associated with aging in men. Conversely, the age-
related decline in testosterone concentrations may contribute to 
the susceptibility to or severity of clinical hypogonadism observed 
in these conditions.
In community-dwelling middle-age to older men, the preva­
lence of low testosterone increased from 12% among men in their 
50s to 48% among men older than 80 years.313 However, the 
prevalence o

older than 80 years.313 However, the 
prevalence of clinical androgen deficiency (i.e., symptoms and signs 
consistent with androgen deficiency and low testosterone concen­
trations) was 6% to 9% and increased with age, reaching 18% to 
23% among men in their 70s.196,197 When more stringent crite­
ria for the diagnosis of androgen deficiency associated with aging 
were used (i.e., three sexual symptoms and a low testosterone level 
defined as late-onset hypogonadism), the prevalence was 2% and 

e-onset hypogonadism), the prevalence was 2% and 
also increased with age, reaching 5% among men in their 70s.198
The decline in testosterone concentrations with aging is asso­
ciated with alterations in body function that could be attribut­
able to androgen deficiency.144 These include a decline in muscle 
mass and strength associated with reduced physical function and 
performance; decreased BMD and increased risk of osteoporosis 
and fractures; increased body fat; reduction in sexual function

; increased body fat; reduction in sexual function and 
activity, including reduced libido and erectile dysfunction; decline 
in vitality, energy, mood, and cognitive function; and alterations 
in sleep quality. Similar changes occur in younger hypogonadal 
men and improve with testosterone treatment, raising the pos­
sibility that the decline in testosterone concentrations that occurs 
with aging may contribute to these age-associated changes in body 
function.
Previous relatively small, short-

 body 
function.
Previous relatively small, short-term (up to 3 years) studies of 
testosterone treatment in heterogeneous groups of older men with 
low or low-normal testosterone concentrations without regard to 
the presence of symptoms or signs of androgen deficiency have 
produced conflicting results. Most demonstrated beneficial effects 
of testosterone treatment on body composition, with increasing 
lean or muscle mass and decreasing fat mass, but less consistent 
effects on muscle strengt

ss, but less consistent 
effects on muscle strength and performance, BMD, sexual func­
tion, vitality, and cognitive function. The only adverse effect found 
in these studies was excessive erythrocytosis in some men.
The Testosterone Trials were seven coordinated, double-blind, 
placebo-controlled studies with allocation by minimization that 
evaluated the short-term effects of testosterone treatment for 1 
year on sexual function, physical function, vitality, cognitive 
724
SECTION V    Sexual 

ion, vitality, cognitive 
724
function, bone density, anemia, and coronary artery plaque vol­
ume in a large number (788) of older men (average age of 72 
years) with unequivocal hypogonadism (symptoms and signs of 
low libido, difficulty walking and slow walking speed, and/or low 
energy, as well as two morning testosterone concentrations of 
<275 ng/dL) for no apparent reason other than age. Compared 
with placebo, testosterone treatment for 1 year 

with placebo, testosterone treatment for 1 year that raised testos­
terone concentrations to the mid-normal range for young men 
improved sexual function (sexual activity, libido, and erectile dys­
function), anemia and hemoglobin, volumetric BMD, and esti­
mated bone strength; slightly improved walking distance, mood, 
and depressive symptoms; and did not improve multiple domains 
of cognitive function and vitality. In addition, testosterone treat­
ment caused erythrocytosis (hemoglobin conce

reat­
ment caused erythrocytosis (hemoglobin concentration >17.5 g/
dL) and an increase in PSA concentration of more than 1.0 ng/
mL in a small number of men and increased noncalcified coronary 
artery plaque volume on CT angiography. There was no difference 
in LUTS or cardiovascular or prostate adverse events in testoster­
one- and placebo-treated men (although the Testosterone Trials 
were underpowered for the latter outcomes).162,319-323
Previous studies of testosterone treatment in frail ol

ious studies of testosterone treatment in frail older men 
with low testosterone concentrations found beneficial effects on 
muscle strength and physical performance; there was an increase 
in self-reported cardiovascular adverse events in one small study 
but not in another similar study.324,325 Meta-analyses of testos­
terone treatment trials326-328 and pharmacoepidemiologic studies 
evaluating cardiovascular events associated with testosterone treat­
ment329-337 have also been conflicting. Me

 treat­
ment329-337 have also been conflicting. Meta-analyses of testoster­
one treatment trials, although underpowered, found no increased 
risk of prostate cancer,338 and pharmacoepidemiology studies have 
been inconsistent, showing no or possibly increased risk of low-
grade and reduced risk of high-grade, aggressive prostate cancer 
with testosterone treatment.337,339-342
Larger, long-term, randomized trials are needed to determine 
the balance of clinical benefits (e.g., related to frailty 

ce of clinical benefits (e.g., related to frailty and fractures) 
and risks (particularly as related to prostate cancer and cardiovas­
cular disease [CVD]) of testosterone treatment in elderly men. 
Until results from these outcome studies are available, testoster­
one treatment should be considered only for older men who have 
clinically significant manifestations of androgen deficiency (e.g., 
low libido and anemia) and unequivocally low serum testosterone 
concentrations, and only after a car

testosterone 
concentrations, and only after a careful discussion of the uncer­
tainty concerning the long-term benefits and risks of treatment.116 
Other Systemic Disorders. Primary hypogonadism with 
low testosterone concentrations or elevated LH concentrations 
(or both) occurs in up to 20% of men with malignancy such as 
advanced Hodgkin disease or testicular cancer before gonadotoxic 
chemotherapy and radiation therapy.343-345 Impairment of sperm 
production is found more frequently in abou

sperm 
production is found more frequently in about 30% to 50% of 
men with Hodgkin disease or testicular cancer before therapy. The 
mechanism of gonadal dysfunction before treatment is not clear.
Sickle cell disease is an autosomal recessive disorder caused by a 
point mutation in the β-globulin chain. It results in an abnormal 
hemoglobin (hemoglobin S) that polymerizes, leading to sickle-
shape, rigid, and fragile red blood cells. The disease is character­
ized by recurrent episodes of painf

 is character­
ized by recurrent episodes of painful, vaso-occlusive events in 
a variety of organs due to thrombosis, ischemia and infarction, 
and hemolysis. Sickle cell disease is a common disorder, affect­
ing approximately 1 in 700 African-American infants. Sickle cell 
disease may cause primary hypogonadism characterized by low 
to low-normal testosterone concentrations, clinical manifesta­
tions consistent with androgen deficiency, testicular atrophy and 
impaired spermatogenesis, and ele

lar atrophy and 
impaired spermatogenesis, and elevated gonadotropin concentra­
tions, possibly due to repeated testicular vaso-occlusive events and 
infarction.346-348 Hydroxyurea therapy and possibly zinc deficiency 
may contribute to impaired spermatogenesis. Men with sickle cell 
disease may experience priapism due to penile vaso-occlusion, and 
this may be precipitated by restoration of libido with testosterone 
treatment of hypogonadism.
Within the first few months to 1 year after a spinal

thin the first few months to 1 year after a spinal cord injury, 
testosterone concentrations and sperm production are suppressed, 
and gonadotropins are usually normal. However, in some men, 
LH or FSH concentrations or both may be elevated in the pres­
ence of low to low-normal testosterone, consistent with primary 
hypogonadism.349 Some men demonstrate low testosterone and 
elevated gonadotropin concentrations chronically after spinal cord 
injury.350-352 The latter condition may be caused by 

ury.350-352 The latter condition may be caused by hyperprolac­
tinemia associated with medications, as well as nutritional defi­
ciency, obstructive sleep apnea, or comorbidities associated with 
chronic spinal cord injury. Testis biopsy has revealed impaired 
spermatogenesis in approximately 40% of men with spinal cord 
injury, but mature sperm that could be used for TESE and ICSI 
were present in almost 90%.353
Vasculitis (e.g., polyarteritis nodosa, granulomatosis with 
polyangiitis, Henoch-S

odosa, granulomatosis with 
polyangiitis, Henoch-Schönlein purpura, Behçet disease) or 
infiltrative disease (e.g., systemic amyloidosis) involving both 
testes may cause testicular damage and may necessitate orchidec­
tomy, resulting in both androgen deficiency and impaired sperm 
production.354-356 
As described previously, cryptorchidism, myotonic dystrophy, 
and Down syndrome most commonly manifest with

hy, 
and Down syndrome most commonly manifest with primary 
hypogonadism characterized by isolated impairment in sperm 
production without androgen deficiency and selectively elevated 
FSH concentrations. In men with less severely impaired sper­
matogenesis, serum gonadotropin concentrations are normal, 
but it is most appropriate to classify these men as having primary 
hypogonadism with isolated impairment in sperm production, 
because gonadotropin treatment has not been demonstrated to 
impro

opin treatment has not been demonstrated to 
improve fertility. 
Varicocele is a dilatation of the pampiniform venous plexus sur­
rounding the spermatic cord in the scrotum. It is caused by retro­
grade blood flow into the internal spermatic vein, which is usually 
caused by defective or absent valves in spermatic veins or, rarely, 
by obstruction of normal venous drainage by extrinsic or intrinsic 
venous compression (e.g., from a tumor). It usually occurs on the 
left side, and most

or). It usually occurs on the 
left side, and most cases are asymptomatic. A varicocele is present 
in 10% to 15% of men in the general population and more fre­
quently in infertile men (up to 30–40%).192,357
The relationship of varicocele to impaired sperm production 
and infertility is unclear.192,357 Approximately 50% of men who 
have a varicocele demonstrate normal semen analyses, and most 
with varicoceles are fertile. Men with a large varicocele and infer­
tility usually exhibit low sperm 

ocele and infer­
tility usually exhibit low sperm counts with reduced motility and 
increased numbers of sperm with abnormal morphologic appear­
ance (e.g., tapered or amorphous sperm heads), but these abnor­
malities are not specific for varicocele. Varicocele is painful in 2% 
to 10% of men with infertility. Testis size and serum concentra­
tions of testosterone, LH, and FSH are usually normal, although 
725
there may be an isolated elevation of serum FSH in me

re may be an isolated elevation of serum FSH in men with 
decreased spermatogenesis. Testis biopsy in men with a varicocele 
and abnormal semen parameters reveals a spectrum of histopatho­
logic findings, including hypospermatogenesis, maturation arrest, 
and Sertoli cell–only histology.
It is unclear whether varicocele ligation improves fertility in 
men who present with infertility. Controlled trials to investigate 
the efficacy of varicocele ligation have not demonstrated improved 
fertility.

igation have not demonstrated improved 
fertility. However, these trials were generally small, heterogeneous, 
and of poor quality. A small number of controlled trials of infertile 
men with palpable varicocele and at least one abnormal semen 
parameter suggested improvement in the spontaneous pregnancy 
rate with varicocele ligation. Some organizations have recom­
mended surgical ligation for infertile men who have a large, pal­
pable varicocele with an abnormal seminal fluid analysis.192,357
Y

 with an abnormal seminal fluid analysis.192,357
Y Chromosome Microdeletion. Yq chromosome (long arm of 
the Y chromosome) microdeletions are the most common genetic 
cause of impaired sperm production and male infertility. They are 
found in 5% to 10% of men with severe oligozoospermia and in 
10% to 15% of men with azoospermia.181 As mentioned earlier, 
microdeletions have been identified in three regions of the long 
arm of the Y chromosome181,182,358 (Fig. 19.27).
Microdeletions in the AZFa 

,182,358 (Fig. 19.27).
Microdeletions in the AZFa region, which contains the DDX3Y 
and USP9Y (ubiquitin specific peptidase 9, Y-linked) genes, are 
usually associated with azoospermia and Sertoli cell–only histol­
ogy. Microdeletions in the AZFb region, which contains multiple 
copies of the RBMY and PRY (PTPM13-like, Y-linked) genes, 
are usually associated with severe oligozoospermia and germ cell 
arrest at the pachytene primary spermatocyte stage and occa­
sionally with hypospermatogenesis.

stage and occa­
sionally with hypospermatogenesis. Microdeletions in the AZFc 
region, which contains the DAZ (deleted in azoospermia) gene 
and is where the majority of Y chromosome microdeletions reside, 
are usually associated with germ cell arrest at the spermatid stage 
or with hypospermatogenesis with some mature spermatids pres­
ent. AZFc microdeletions are found in approximately 12% of men 
with nonobstructive azoospermia and in 6% of men with severe 
oligozoospermia. Microdeletions in b

 with severe 
oligozoospermia. Microdeletions in both the AZFb and the AZFc 
region are usually associated with azoospermia and Sertoli cell–
only histology.
Yq microdeletion analysis should be considered for couples 
who are contemplating ICSI, because these microdeletions have 
been shown to be transmitted to male offspring with the ICSI 
procedure.181 If ICSI is performed in men with Y chromosome 
microdeletions, genetic counseling and preimplantation or prena­
tal testing should be considere

antation or prena­
tal testing should be considered. 
Sertoli Cell–Only Syndrome (Germ Cell Aplasia). Sertoli cell–
only syndrome, or germ cell aplasia, is an uncommon histologic 
diagnosis in which the seminiferous tubules are completely devoid 
of germ cells and are lined only with Sertoli cells with little to 
no fibrosis or hyalinization.359 Men with this disorder present 
with infertility, normal androgenization, moderately small testes 
(10 to 20 mL in volume), azoospermia, normal testoste

 to 20 mL in volume), azoospermia, normal testosterone and 
LH concentrations, and selectively elevated FSH concentrations 
(indicating severe seminiferous tubule dysfunction).360 Occasion­
ally, LH concentrations are slightly high, and the testosterone 
response to hCG stimulation is reduced, suggesting mild Leydig 
cell dysfunction.
The cause of Sertoli cell–only syndrome is not known, but it 
is thought to result from congenital absence of germ cells due to 
a failure of gonocyte migration. I

m cells due to 
a failure of gonocyte migration. In some families, however, germ 
cells were present before puberty but were subsequently lost dur­
ing or after puberty. As described previously, a Sertoli cell–only 
histology may be associated with microdeletions in the long arm 
of the Y chromosome in the AZF regions.361 Severe germ cell 
Yq
AZFa
AZFa
AZFb
AZFc
P5/proximal-P1 (AZFb)
P5/distal-P1
P4/distal-P1
AZFc (b2/b4)
gr/gr
terminology
Semen profile
Azoospermia
Azoo-oligozoosperm

ology
Azoo-oligozoospermia
Histology
Hypospermatogenesis
Virtually nil
-50–70%
only
Germ cell
arrest
Testicular sperm
recovery
Yp
• Fig. 19.27  Schematic diagram of the short (Yp) and long (Yq) arms of the human Y chromosome. Dele­
tions in regions of Yq, specifically in azoospermia factor (AZF) regions, are associated with severe defects 
in sperm production. Microdeletions in the AZFa region are usually associated with azoospermia and 
Sertoli cell–only h

sociated with azoospermia and 
Sertoli cell–only histology. Microdeletions most commonly affect the AZFc region and may be associated 
with severe oligozoospermia or azoospermia. However, even if oligozoospermia or azoospermia is pres­
ent, sperm for use in intracytoplasmic sperm injection are recoverable from half of the patients by testicular 
sperm extraction at biopsy. (From McLachlan RI, O’Bryan MK. Clinical review: state of the art for genetic 
testing of infertile men. J Clin Endocrinol M

tic 
testing of infertile men. J Clin Endocrinol Metab. 2010;95:1013–1024.)
726
damage and loss occurring with Klinefelter syndrome, mumps 
orchitis, cryptorchidism, ionizing radiation, or alkylating agents 
may result in seminiferous tubules lined only with Sertoli cells. 
However, in these cases of acquired Sertoli cell–only syndrome, 
there is usually extensive seminiferous tubule sclerosis or hyalin­
ization, and the testes are usually smaller. In

n­
ization, and the testes are usually smaller. Infertility is irreversible 
in congenital Sertoli cell–only syndrome, but it may be reversible 
with time in some cases of acquired Sertoli cell–only syndrome. 
Primary Ciliary Dyskinesia (Immotile Cilia Syndrome). Pri­
mary ciliary dyskinesia, or immotile cilia syndrome, is a rare, 
heterogeneous, autosomal recessive genetic disorder of cilia. It is 
characterized primarily by recurrent respiratory infections (sinus­
itis and bronchitis) that lea

y infections (sinus­
itis and bronchitis) that lead to the development of bronchiecta­
sis, caused by impaired mucociliary clearance due to dyskinesia of 
respiratory tract cilia, and to infertility caused by asthenozoosper­
mia (nonmotile or poorly motile sperm) due to impaired sperm 
tail movement.28,362 In half of the cases, primary ciliary dyskinesia 
is associated with situs inversus and is known as Kartagener syn­
drome. Some men exhibit abnormalities of sperm motility in the 
absence of r

normalities of sperm motility in the 
absence of respiratory tract involvement.
Patients with primary ciliary dyskinesia and impaired sperm 
motility demonstrate ultrastructural abnormalities of the axo­
neme, the microtubule cytoskeleton of the sperm flagellum, espe­
cially in the dynein arms (motor protein complexes). Almost all 
men with primary ciliary dyskinesia have mutations of the genes 
encoding the dynein axonemal heavy chain 5 (DNAH5), dynein 
intermediate chain 1 (DNAI1), or dynein a

 dynein 
intermediate chain 1 (DNAI1), or dynein axonemal heavy chain 
11 (DNAH11). These men present with infertility and an isolated 
impairment in sperm motility with normal sperm counts and 
morphologic structure and normal testosterone and gonadotropin 
FSH Receptor Mutations. Rare inactivating FSH receptor 
mutations have been described in males.58,86 In contrast to females 
with FSH receptor mutations, who have primary amenorrhea and 
infertility, men have a more variable

norrhea and 
infertility, men have a more variable presentation. Some demon­
strate severe oligozoospermia, whereas others have moderate oligo­
zoospermia or normal sperm concentrations with abnormal sperm 
morphologic appearance, and some have normal fertility. Serum 
testosterone concentrations are normal, FSH concentrations are 
elevated, and LH concentrations are slightly high to normal. It 
is possible that normal testosterone production within the testes 
contributes to the persistence of 

hin the testes 
contributes to the persistence of spermatogenesis and fertility in 
the absence of FSH action. 
Because seminiferous tubule function is more susceptible to dam­
age than Leydig cell function, most men with primary hypo­
gonadism due to chemotherapy agents, ionizing radiation, or 
orchitis caused by mumps or other infections manifest isolated 
impairment of sperm production without androgen deficiency.
Alkylating agents (e.g., cyclophosphamide, ifosfamide, proca

 agents (e.g., cyclophosphamide, ifosfamide, procar­
bazine, busulfan, chlorambucil) used in combination chemother­
apy regimens to treat lymphoma and leukemia almost uniformly 
cause azoospermia that recovers after 5 years in up to two thirds of 
men.291,363-365 The effect of cyclophosphamide is dose dependent, 
and cumulative doses of cyclophosphamide of 10 g/m2 for malig­
nancy are likely to result in severe oligozoospermia or azoospermia 
that may be irreversible.365 High-dose chemotherapy a

t may be irreversible.365 High-dose chemotherapy and total-
body irradiation before bone marrow transplantation commonly 
cause irreversible germ cell damage associated with azoospermia or 
severe oligozoospermia and elevated FSH concentrations. Sperm 
production is suppressed initially in men with testicular cancer 
who undergo combination chemotherapy that includes platinum 
drugs, unilateral orchidectomy, and often radiation therapy.292 
However, sperm production recovers in 80% of men by 5 y

er, sperm production recovers in 80% of men by 5 years. 
Before chemotherapy begins, cryopreservation of sperm for sub­
sequent use in IUI, IVF, or ICSI should be offered to men who 
desire future fertility. Approaches aimed at suppressing gonado­
tropins with GnRH agonists or exogenous testosterone adminis­
tration have not been uniformly effective in preventing germ cell 
damage by chemotherapy.
Methotrexate and sulfasalazine may cause oligozoospermia 
and low sperm motility and contribute to 

spermia 
and low sperm motility and contribute to infertility.366,367 The 
processes of human spermatogenesis, and particularly the sper­
matogonia, are very sensitive to the effects of exposure to ionizing 
radiation.16,293,294 X-radiation doses as low as 15 cGy may sup­
press sperm production temporarily. The time required to recover 
spermatogenesis after x-irradiation is dose dependent. Recovery of 
sperm counts to baseline takes 9 to 18 months after exposure to 
100 cGy or less and up to 5 

hs after exposure to 
100 cGy or less and up to 5 years after 400 to 600 cGy. Although 
Leydig cell function is more resistant to ionizing radiation, x-irra­
diation of greater than 800 cGy may cause Leydig cell damage 
and androgen deficiency. As with chemotherapy, sperm banking 
before radiation therapy for later use in ART offers hope for sub­
sequent fertility.
Prolonged and repeated thermal trauma (e.g., with excessive 
hot tub use) to the testes may transiently suppress sperm produc­
tion.

estes may transiently suppress sperm produc­
tion.368,369 In addition, many chemical agents used in industry 
and laboratories have been implicated as direct toxins to the testes. 
Examples of environmental toxins include carbon disulfide, a solvent 
used in rayon production; dibromochloropropane, an insecticide; 
lead; deuterium oxide; ethyl glycol; cadmium; fluoroacetamide; 
nitrofurans; dinitropyroles; diamines; and α-chlorohydrin.370 
Furthermore, it is postulated that environmental or xenob

more, it is postulated that environmental or xenobiotic 
agents such as phthalates act as antiandrogens or estrogens to alter 
reproductive function; these agents have been termed endocrine 
disruptors.371 Such environmental agents have been implicated in 
causing the increased incidence of testicular dysgenesis syndrome 
(i.e., hypospadias, cryptorchidism, declining sperm counts, and 
testicular cancer).372 
An acute febrile illness may cause a temporary suppression of sper­

llness may cause a temporary suppression of sper­
matogenesis.373 In men with spinal cord injury resulting in tetra­
plegia or paraplegia, impaired sperm production may be caused in 
part by increased scrotal temperature due to loss of lumbar sympa­
thetic innervations.349-351,353 Men with malignancy, particularly 
Hodgkin lymphoma or testicular cancer, may have impaired sper­
matogenesis with azoospermia or oligozoospermia in 30% to 80% 
of cases before treatment (the former possibly related to

s before treatment (the former possibly related to B symp­
toms of fever, night sweats, and weight loss).331,343,345,364
In most men who present with infertility and an isolated 
impairment of sperm production, a cause cannot be identified. 
Idiopathic oligozoospermia or azoospermia occurs in 60% to 80% 
of cases (including men with varicocele).178-180 If isolated impair­
ment of spermatogenesis is severe in men with primary hypogo­
nadism, serum FSH concentrations may be selectively elevated. 

 FSH concentrations may be selectively elevated. 
As mentioned earlier, men with less severely impaired spermato­
genesis and normal serum gonadotropin concentrations are still 
classified as having a disorder of primary hypogonadism, because 
gonadotropin treatment has not been demonstrated to improve 
fertility in such cases. 
727
Causes of Secondary Hypogonadism
Const

uction
Constitutional Delay of Growth and Puberty. It is important 
to consider constitutional delay of growth and puberty (CDGP) in 
the differential diagnosis of secondary hypogonadism because it 
is a transient cause of secondary hypogonadism and is the most 
common cause of pubertal delay, usually associated with growth 
delay and short stature in boys.146,147,374,375
The initial endocrine event that precedes the phenotypic 
changes of puberty is activat

edes the phenotypic 
changes of puberty is activation of the CNS mechanisms that 
regulate GnRH production, which results in pulsatile LH fol­
lowed by testosterone secretion, initially at night (see Fig. 19.5) 
and then throughout the day.42 Under the influence of increas­
ing testosterone concentrations, secondary sexual characteristics 
appear between 9 and 13 years of age.
The first physical sign of pubertal development is an increase of 
testis size to greater than 3 mL in volume and thinni

tis size to greater than 3 mL in volume and thinning of the scrotal 
skin that is followed by development of rugal folds and increased 
pigmentation.146,147,149,374,375 Subsequently, penile length increases, 
and pubic hair develops over the next 1 to 2 years, followed by long 
bone growth (with peak height velocity occurring approximately 3 
years later) and development of other secondary sexual characteris­
tics, such as laryngeal enlargement and deepening of the voice. In 
boys, peak bone mas

nd deepening of the voice. In 
boys, peak bone mass usually is not reached until the third decade. 
There is considerable variability in the onset and progression of 
puberty and the degree of virilization, and these variations are attrib­
utable in large part to an individual’s genetic and ethnic background.
The absence of sexual maturation and testis size less than 3 mL 
at 14 years of age is diagnostic of delayed puberty.146,147,149,374,375 
Boys with delayed puberty often experience consider

oys with delayed puberty often experience considerable psycho­
social distress due to the lack of sexual and physical development 
that results in their being considered younger than their peer 
group and having difficulty competing in athletics. In addition 
to the lack of sexual development, both boys and their parents are 
usually also concerned about the boy’s failure to undergo a growth 
spurt and his short stature.
CDGP, or transient secondary hypogonadism, is the cause of 
delayed puberty

ary hypogonadism, is the cause of 
delayed puberty in approximately 65% of cases.146,147,149,374,375 
CDGP is usually clinically indistinguishable from permanent 
organic causes of secondary hypogonadism (e.g., due either to 
known genetic mutations or IHH) or acquired hypogonado­
tropic hypogonadism due to an identifiable cause (e.g., pituitary 
or hypothalamic disease). However, permanent causes of delay 
puberty occur in less than 10% of cases. Other causes of delayed 
sexual development and 

. Other causes of delayed 
sexual development and growth account for about 25% of cases, 
and they are usually clinically apparent on presentation. These 
causes include functional secondary hypogonadism (e.g., due to 
chronic systemic illness, hypothyroidism, medications) and pri­
mary hypogonadism (e.g., due to Klinefelter syndrome, mumps, 
chemotherapy, radiation therapy) or androgen resistance.
CDGP is a physiologic variant of normal puberty that is char­
acterized by a slowing of the growth

that is char­
acterized by a slowing of the growth rate and of the timing and 
tempo of pubertal development.146,147,149,374,375 Because there is 
an increased prevalence of CDGP in families with GnRH defi­
ciency (CHH), CDGP could represent a mild variant of CHH 
in some cases.376 However, a recent study found distinct genetic 
profiles in boys with CDGP and CHH.377 No genes or genetic 
polymorphisms have been identified that determine the variations 
in the timing of normal puberty; however, a

tions 
in the timing of normal puberty; however, a family history of 
delayed puberty or being a “late bloomer” is found in 80% of 
boys with CDGP.
Clinically, a boy with CGDP has congruent delays in growth, 
sexual development, and bone development. The height age (i.e., 
the age that corresponds to the child’s height when plotted at the 
50th percentile on a growth chart), stage of sexual development, 
and bone age are concordant and delayed compared with chrono­
logic age.146,147,149,374,375 

mpared with chrono­
logic age.146,147,149,374,375 Height velocity in boys with CGDP is 
slower than peers of the same chronologic age because puberty 
accelerates growth velocity.
In the absence of anosmia, hyposmia, or other morphologic 
abnormalities, CDGP cannot be distinguished clinically or 
biochemically from permanent CHH (see later discussion). If 
spontaneous puberty does not develop by 18 years of age in a nor­
mosmic patient, the diagnosis is usually CHH. It should be noted 
that reve

osis is usually CHH. It should be noted 
that reversal and relapse of hypogonadism after age 18 has been 
reported in a few older men with anosmic (Kallmann syndrome) 
or normosmic CHH.378-380
Eventually, boys with CDGP undergo normal growth and 
sexual development, although they do so several years after their 
peers.146,147,149,374,375 Normal height is usually attained, but target 
mid-parental height may not be achieved. In addition, acquisition 
of peak bone mass may be compromised in some m

on 
of peak bone mass may be compromised in some men. Boys with 
CDGP may experience severe emotional distress and social exclu­
sion or isolation resulting from immature sexual development and 
short stature. Therefore, after exclusion of organic causes of delayed 
puberty, low-dose testosterone treatment is usually initiated in boys 
with CDGP at age 14 years, or sometimes sooner, to induce sexual 
maturation and growth that is more consistent with that of their 
peers. Testosterone is usually

with that of their 
peers. Testosterone is usually started at a low dose and gradually 
increased over several years; it is stopped intermittently to assess 
whether spontaneous puberty occurs (see later discussion). 
Hereditary Hemochromatosis. Hereditary hemochromatosis is 
a common autosomal recessive disorder that is characterized by 
inappropriately high gastrointestinal iron absorption resulting in 
excessive iron storage in a number of tissues, most prominently 
those of the liver, pancre

sues, most prominently 
those of the liver, pancreas, heart, joints, skin, testes, and pitu­
itary gland.381,382 In most cases, hereditary hemochromatosis is 
caused by mutations in the hemochromatosis gene (HFE), most 
commonly homozygous C282Y/C282Y mutations (70–85%) or 
compound heterozygote C282Y/H63D mutations (5–10%), or 
rarely, by other mutations in iron-regulating genes such as transfer­
rin receptor 2 (TFR2), hepcidin antimicrobial peptide (HAMP), 
hemojuvelin (HJV), and solute carrie

tide (HAMP), 
hemojuvelin (HJV), and solute carrier family 40A1 (SLC40A1). 
Homozygous C282Y mutations occur in approximately 1 of every 
200 to 400 Caucasians of Northern European descent.
Regardless of the specific mutation causing hereditary hemo­
chromatosis, iron overload results from insufficient hepatic 
production of hepcidin, a peptide hormone that degrades the 
iron-exporter protein, ferroportin; this causes unregulated iron 
absorption in the duodenum and iron overload in tissues.381,

 in the duodenum and iron overload in tissues.381,382 
Initially, iron overload causes an increase in transferrin iron satu­
ration, followed by an increase in ferritin concentrations in most 
men with hemochromatosis. Therefore, the biochemical pene­
trance is high. Evaluation for hemochromatosis should be consid­
ered if iron saturation is greater than 45% in men. In the absence 
of inflammation or cancer, a serum ferritin level higher than 1000 
μg/L is associated with greater risk of hepatic

0 
μg/L is associated with greater risk of hepatic cirrhosis in patients 
with hemochromatosis, and a liver biopsy or MRI for hepatic iron 
content should be considered.
728
In contrast to biochemical abnormalities consistent with iron 
overload, the clinical penetrance of hereditary hemochromato­
sis is quite low (0.5–2.0%), and manifestations of iron overload 
(hepatic cirrhosis or carcinoma, diabetes mellitus, heart failure 
or arrhythmias, arthral

s mellitus, heart failure 
or arrhythmias, arthralgias or polychondritis, bronzing of skin, 
hypogonadism) are far less common.381,382 This may be related 
to the importance of secondary insults to end organs (e.g., alco­
holic liver damage) that contribute to clinical manifestations or to 
earlier diagnosis with increased awareness and screening. Clinical 
manifestations, when present, usually appear at between 40 and 
60 years of age. Hypogonadism, changes in skin color, and arthral­
gias of t

ism, changes in skin color, and arthral­
gias of the hands are usually the earliest clinical manifestations of 
iron overload.
Men with end-organ effects from hereditary hemochromatosis 
almost always have secondary hypogonadism resulting in andro­
gen deficiency and impairment in sperm production due to iron 
overload in the pituitary gland that causes selective gonadotro­
pin deficiency.383 Serum testosterone concentrations and sperm 
counts are low, LH and FSH concentrations are usually low, 

e low, LH and FSH concentrations are usually low, and 
gonadotropin response to GnRH stimulation is absent or mark­
edly attenuated. In the presence of cirrhosis caused by hemochro­
matosis, SHBG concentrations may be elevated; as a result, serum 
free testosterone concentrations may be low in the presence of 
normal total testosterone concentrations. Therefore, accurate and 
reliable assessment of free testosterone are needed to confirm bio­
chemical androgen deficiency in men with hepatic cirr

mical androgen deficiency in men with hepatic cirrhosis due 
to hemochromatosis. Iron deposition in the pituitary may be 
detected by MRI. Iron overload also occurs in the testes and may 
occasionally cause a modest reduction in testosterone response to 
gonadotropin stimulation, resulting in combined primary and 
secondary hypogonadism. However, in most cases, gonadotro­
pin treatment can stimulate normal testicular function, including 
spermatogenesis and fertility.
The prevalence of hypogonad

genesis and fertility.
The prevalence of hypogonadism in iron overload has declined 
from between 10% and 100% in older reports to approximately 
6% more recently due to earlier diagnosis and less severe iron 
overload. Hypogonadism is preventable and may be reversed with 
therapeutic phlebotomy early in the course of iron overload. 
Congenital Hypogonadotropic Hypogonadism. CHH, either 
due to known mutations (see the following discussion) or idio­
pathic (referred to as IHH, previously known a

io­
pathic (referred to as IHH, previously known as hypogonadotropic 
eunuchoidism), is an uncommon, clinically heterogeneous group 
of disorders characterized by isolated gonadotropin deficiency of 
varying degree with otherwise normal pituitary function.384,385 
Most cases of CHH are likely due to genetic mutations, many of 
which remain to be identified. Males with CHH fail to undergo 
normal puberty, resulting in incomplete sexual maturation or 
eunuchoidism, androgen deficiency with very lo

or 
eunuchoidism, androgen deficiency with very low testosterone 
(usually in the prepubertal range), low to low-normal LH and 
FSH concentrations, and impaired sperm production. Gonado­
tropin deficiency is caused by a defect in normal GnRH produc­
tion or action, as evidenced by absent or abnormal patterns of 
pulsatile LH secretion47 (see Fig. 19.7) and the ability of exog­
enous pulsatile GnRH treatment to restore normal gonadotropin 
secretion and testis function. Infants with CHH may have 

on and testis function. Infants with CHH may have micro­
penis or cryptorchidism. Rarely, a man with normal virilization 
may develop CHH as an adult.386
In approximately 60% of cases, CHH is associated with 
anosmia or hyposmia and is known as Kallmann syndrome.384 
Developmental failure of olfactory bulbs (detectable on brain 
MRI) is responsible for the anosmia or hyposmia. The remain­
ing 40% of men have normosmic CHH. In addition, some men 
with CHH exhibit other developmental defects, incl

with CHH exhibit other developmental defects, including syn­
kinesia (mirror movements), unilateral renal agenesis, cleft lip/
palate or high-arched palate, sensorineural hearing loss, digital 
skeletal abnormalities such as syndactyly or brachydactyly (fourth 
metacarpal), dental agenesis, eye movement abnormalities or color 
blindness (deuteranopia), and agenesis of the corpus callosum.
The prevalence of Kallmann syndrome is approximately 1 in 
8000 to 10,000 men, and there is a marked male pr

8000 to 10,000 men, and there is a marked male predominance 
(male-to-female ratio, 4:1–5:1).387 X-linked recessive, autoso­
mal dominant, and autosomal recessive modes of inheritance are 
observed, but many cases are sporadic. Family members of a man 
with Kallmann syndrome may have variable clinical expressions, 
such as normosmic CHH, isolated anosmia, or CDGP.50,385,388 
In approximately 30% to 40% of cases, Kallmann syndrome is 
caused by a known mutation of genes that play important roles

 known mutation of genes that play important roles in 
the migration of GnRH neurons from the olfactory placode to the 
hypothalamus and in normal development of the olfactory bulbs 
during fetal development. These include mutations of ANOS1 
(10–20%), FGFR1/KAL2 (10%), PROK2 (5%), and PROKR2 
(5%).50,385,388 (ANOS1 and ANOS2 were formerly known as 
KAL1 and KAL2). CHH without known gene mutation is some­
times called idiopathic hypogonadotropic hypogonadism (IHH).
The ANOS1 gene is located on t

hypogonadism (IHH).
The ANOS1 gene is located on the X chromosome and 
encodes an extracellular adhesion glycoprotein known as anosmin 
1.50,385,388 Mutation or deletion of ANOS1 results in failure of the 
normal migration of GnRH neurons from the olfactory placode 
to the hypothalamus, resulting in severe GnRH deficiency; it is 
the main cause of X-linked recessive Kallmann syndrome (type 1). 
The phenotype of Kallmann syndrome caused by ANOS1 muta­
tions is more severe and less variable than t

uta­
tions is more severe and less variable than that of other known 
genetic defects. Synkinesia or dysdiadokinesia is present in 80% 
and unilateral renal agenesis in 30% of cases. The FGFR1 gene 
encodes for a fibroblast growth factor receptor that also plays an 
important role in the migration of GnRH neurons during devel­
opment.50,385,388 Mutations in this gene cause a spectrum of phe­
notypes, from severe autosomal dominant Kallmann syndrome 
(type 2) and normosmic CHH to CDGP associated 

me 
(type 2) and normosmic CHH to CDGP associated with cleft lip/
palate (in 30% of cases), dental agenesis, and skeletal abnormali­
ties such as brachydactyly and syndactyly. PROK2 and PROKR2 
encode for a peptide and its G protein–coupled receptor that play 
important roles in normal GnRH neuronal migration and olfac­
tory bulb development.50,385,388 The clinical phenotypes in men 
with PROK2 or PROKR2 mutations are quite variable, ranging 
from severe Kallmann syndrome (type 4 or type 3, resp

m severe Kallmann syndrome (type 4 or type 3, respectively) 
to normosmic CHH.
In approximately 30% of cases, normosmic CHH is caused 
by mutations in genes involved in hypothalamic-pituitary func­
tion (particularly during puberty). These include mutations in 
GNRHR, the GnRH receptor (10–20%); KISS1R, which encodes 
the receptor for kisspeptin 1/metastin, an important GnRH stim­
ulatory neuropeptide particularly at the time of puberty (2–5%); 
TAC3, which encodes neurokinin B, another importan

TAC3, which encodes neurokinin B, another important GnRH 
stimulatory neuropeptide, and the gene for its receptor, TAC3R; 
FGFR1/KAL2 (2–5%); PROK2; the genes for leptin (LEP) and its 
receptor (LEPR), which are associated with massive obesity; and, 
rarely, the GnRH gene, GNRH1.50,385,388
Hypogonadotropic hypogonadism is a component of complex 
genetic syndromes associated with specific dysmorphic features or 
combined hormonal defects. For example, the CHARGE syndrome, 
characterized by Colobo

ple, the CHARGE syndrome, 
characterized by Coloboma of the eye or CNS anomalies, Heart 
anomalies, nasal choanal Atresia, growth Retardation, Genital 
defect (hypogonadism), and Ear anomalies (deafness, dysmorphic 
729
ears, and hypoplasia of the semicircular canals), may be associ­
ated with Kallmann syndrome or normosmic CHH.385,388-390 In 
approximately 60% of cases, CHARGE syndrome is caused by a 
mutation in the chromodomain helicase DNA-binding protein 7 

 the chromodomain helicase DNA-binding protein 7 
gene (CHD7), which encodes for a chromatin remodeling pro­
tein. CHD7 mutations have been found in approximately 3% to 
4% of men with Kallmann syndrome or normosmic CHH, and it 
is hypothesized that CHH may be a mild variant of the CHARGE 
syndrome.
X-linked congenital adrenal hypoplasia, characterized by adrenal 
insufficiency due to adrenal hypoplasia and normosmic CHH, 
is caused by a mutation in the dose-sensitive sex reversal–adre­
nal hypo

 in the dose-sensitive sex reversal–adre­
nal hypoplasia critical region on the X chromosome protein 1 
gene (DAX1), now known as NR0B1 (nuclear receptor subfam­
ily 0, group B, member 1), which encodes for an orphan nuclear 
receptor. Mutations of the SF-1 gene (NR5A1), which encodes 
another orphan nuclear receptor, the prohormone convertase 1 
gene (PC1), which encodes an enzyme involved in post-transla­
tional processing of pituitary prohormones and neuropeptides, 
and genes for many pituita

nes and neuropeptides, 
and genes for many pituitary transcription factors such as HESX1 
(septo-optic dysplasia or de Morsier syndrome), LHX3, LHX4, 
POUF1, and PROP1 cause deficiencies in multiple anterior pitu­
itary and other hormones in addition to gonadotropin deficiency. 
Mutations in the latter transcription factors may also be associated 
with specific dysmorphic features. Unlike men with CHH, those 
with multiple hormone defects usually fail to normalize gonad­
otropin and testicular f

 fail to normalize gonad­
otropin and testicular function in response to chronic pulsatile 
GnRH administration.50,385,388,391
In the absence of anosmia, hyposmia, or features of Kallmann 
syndrome (e.g., synkinesia), it is not possible to confidently dis­
tinguish an individual who has normosmic CHH not associ­
ated with a complex genetic syndrome from someone who has 
CDGP.146,147,374,375 In both conditions, there may be a family 
history of delayed puberty, CHH, or Kallmann syndrome; a his­
t

layed puberty, CHH, or Kallmann syndrome; a his­
tory of cryptorchidism; clinical manifestations of delayed sexual 
maturation or eunuchoidism; and low serum testosterone and 
low to low-normal gonadotropin concentrations. In contrast to 
boys with CHH, who have normal height for their chronologic 
age, those with CDGP usually manifest some growth delay and 
short stature. Boys with CHH may have a history of micropenis 
and usually demonstrate pubertal delay beyond 19 years of age, 
although som

bertal delay beyond 19 years of age, 
although some individuals undergo spontaneous puberty after 20 
years of age or reversal of CHH in adulthood; a subset of the latter 
may also subsequently relapse with CHH.378-380 Currently, there 
is no diagnostic test that completely and reliably distinguishes iso­
lated normosmic CHH from CDGP.
As discussed previously, after organic causes of delayed puberty 
(e.g., craniopharyngioma) have been excluded, low-dose testos­
terone therapy is usually initiat

low-dose testos­
terone therapy is usually initiated in boys with delayed puberty 
at about 14 years of age to induce sexual maturation and 
growth.146,147,374,375 Testosterone is usually discontinued inter­
mittently to assess whether spontaneous puberty occurs, as deter­
mined by an increase in testis size. Boys with CHH usually require 
continued testosterone treatment to achieve and maintain sexual 
maturation, whereas those with CDGP do not require further 
treatment after spontaneous secre

require further 
treatment after spontaneous secretion of gonadotropin and tes­
tosterone commences. Sustained reversal occurs after discontinu­
ation of therapy in up to 10% of men with Kallmann syndrome 
or normosmic CHH who present initially with absent or partial 
sexual maturation.378-380 Therefore, it is reasonable to discontinue 
treatment briefly to assess the reversibility of hypogonadotropic 
hypogonadism in all patients.
If fertility is desired, testosterone therapy is stopped and 
go

s desired, testosterone therapy is stopped and 
gonadotropin replacement therapy (or in some specialized cen­
ters, pulsatile GnRH) is initiated to stimulate sperm production. 
Previous treatment with testosterone might be expected to sup­
press endogenous gonadotropin production further; however, 
previous androgen therapy does not alter the subsequent overall 
response of spermatogenesis to gonadotropin therapy, although 
the response to gonadotropins might be slower.392-394 Gonado­
tropin the

tropins might be slower.392-394 Gonado­
tropin therapy is much more likely to stimulate spermatogenesis 
if there is some evidence of sexual maturation and larger testes 
at baseline and no history of cryptorchidism or another primary 
testicular disorder.386,395 Even in the absence of clinical evidence 
of hypothalamic-pituitary-testicular disorder, 10% to 20% of men 
with CHH do not have an adequate gonadotropin or testicular 
response to chronic pulsatile GnRH therapy, suggesting underly­
ing

ic pulsatile GnRH therapy, suggesting underly­
ing pituitary or testicular defects.396
Men with CHH have varying degrees of gonadotropin defi­
ciency, as evidenced by a persistent but abnormal pattern of pul­
satile LH secretion47 (see Fig. 19.7). In some men with CHH, 
FSH secretion predominates relative to LH secretion, resulting in 
some germ cell maturation and spermatogenesis and a rare vari­
ant form of CHH known as isolated LH deficiency, or the “fer­
tile eunuch syndrome.”397-399 This sy

or the “fer­
tile eunuch syndrome.”397-399 This syndrome is characterized 
by androgen deficiency of prepubertal onset or eunuchoidism 
caused by LH deficiency but pubertal or almost adult-size testes 
in which advanced-stage spermatogenesis is present because of 
relatively preserved FSH secretion. However, spermatogenesis is 
usually not completely normal in these men, and they are not 
fertile, as the name of the syndrome might imply. Because there 
is only relative gonadotropin deficiency an

there 
is only relative gonadotropin deficiency and some spermatogen­
esis is present, treatment with LH-like activity (hCG) stimulates 
Leydig cell testosterone production and ameliorates androgen 
deficiency, stimulating spermatogenesis sufficient for induction 
of fertility. Men with isolated FSH deficiency in the absence of an 
FSHβ mutation have been reported, but the degree and nature of 
the defect in pulsatile gonadotropin secretion have not been well 
documented.400-402
Men with inactiv

not been well 
Men with inactivating LHβ mutations usually demonstrate a 
lack of pubertal development, impaired spermatogenesis or azo­
ospermia, and infertility.59,403,404 Recently, however, a man with 
an LHβ mutation resulting in a partially active LH molecule (as 
indicated by expression of steroidogenic enzymes in a few mature 
Leydig cells and low intratesticular testosterone concentrations) 
was reported to have complete and quantitatively normal sper­
matogenesis.80 C

e and quantitatively normal sper­
matogenesis.80 Complete spermatogenesis was achieved in the 
presence of very low LH and intratesticular testosterone concen­
trations and high serum FSH concentrations in this man. Men 
with inactivating FSHβ mutations generally have been found to 
have azoospermia with undetectable FSH, low or low-normal tes­
tosterone, and high LH concentrations.80,82-85,405,406
Secondary hypogonadism may be present in several complex 
genetic syndromes, such as Prader-Labhar

 complex 
genetic syndromes, such as Prader-Labhart-Willi, Laurence-Moon-
Bardet-Biedl, Alström, Bjornstad, Börjeson-Forssman-Lehmann, 
Bosma, Chudley, Costello, Gordon-Holmes, Johnson-McMillin, 
Juberg-Marsidi, LEOPARD (multiple Lentigines, Electrocardio­
graphic conduction abnormalities, Ocular hypertelorism, Pul­
monary stenosis, Abnormal genitalia, growth Retardation, and 
sensorineural Deafness), Martsof, Moebius-Poland, Roifman, 
Rud, and Woodhouse-Sakati syndromes.267,280,407-422 Most of 

oodhouse-Sakati syndromes.267,280,407-422 Most of 
these syndromes are diagnosed by pediatricians and pediatric 
endocrinologists based on the clustering of specific dysmorphic 
features and congenital anomalies that are characteristic of the 
730
syndrome.267 Secondary hypogonadism in these disorders usu­
ally causes androgen deficiency of prepubertal onset. Many but 
not all of these syndromes are associated with CNS abnormalities 
or intellectual d

sociated with CNS abnormalities 
or intellectual disability. Obesity may contribute to the cause of 
hypogonadism and may alert clinicians to the potential presence 
of a complex genetic syndrome. Some, such as the Prader-Labhart-
Willi, Laurence-Moon-Bardet-Biedl, and Alström syndromes, 
have been reported to be associated with both primary and sec­
ondary hypogonadism.270,278,279,281 
Hyperprolactinemia. Hyperprolactinemia is a common cause 
of secondary hypogonadism. These 

 a common cause 
of secondary hypogonadism. These patients have low testosterone 
concentrations and low to low-normal gonadotropin concentra­
tions and present with sexual dysfunction (reduced libido and 
erectile dysfunction), infertility, and gynecomastia.423,424 Because 
the male breast usually is not exposed to a relatively high estro­
gen and progestin milieu needed to induce ductal hyperplasia and 
glandular formation in women, high prolactin concentrations 
in men rarely result in galact

tin concentrations 
in men rarely result in galactorrhea. Hyperprolactinemia causes 
gonadotropin deficiency primarily by suppressing pulsatile hypo­
thalamic GnRH secretion, as evidenced by reduced spontaneous 
LH pulse frequency and amplitude and by restoration of normal 
LH pulsatility and testosterone concentrations with dopamine 
agonist treatment or pulsatile GnRH administration.425
Common causes of hyperprolactinemia resulting in clini­
cal secondary hypogonadism are a prolactin-secreting

l secondary hypogonadism are a prolactin-secreting adenoma, 
pituitary stalk disease (e.g., stalk compression from a non–pro­
lactin-secreting adenoma, traumatic stalk section); hypothalamic 
disease (e.g., hypothalamic tumors, granulomatous disease); and 
medications.423,424
In contrast to women who usually present with microadeno­
mas, men with prolactin-secreting adenomas usually present with 
large macroadenomas because of a lack of overt symptoms or pos­
sibly a gender-specific difference i

ptoms or pos­
sibly a gender-specific difference in the biologic behavior of the 
tumor.426,427 In approximately 10% of cases, there is excessive 
co-secretion of prolactin and GH. In men with prolactin-secret­
ing macroadenomas, serum prolactin concentrations are usually 
higher than 250 ng/mL, and they can be higher than 1000 ng/
mL with tumors larger than 2 cm in diameter. Some patients with 
very large prolactin-secreting macroadenomas have only modestly 
elevated serum prolactin concentrati

nly modestly 
elevated serum prolactin concentrations, which is caused by an 
assay artifact from saturation of both the capture and detection 
antibodies used in two-site sandwich immunoassays.428 This phe­
nomenon is known as the pro-zone or “hook” effect and neces­
sitates dilution of serum samples prior to assay. Measurement of 
prolactin after serial dilution should be considered for all patients 
with large pituitary tumors and modest hyperprolactinemia.
Diseases affecting the pituitary st

prolactinemia.
Diseases affecting the pituitary stalk and hypothalamic dis­
eases may cause hyperprolactinemia because of disruption of the 
hypothalamic hypophyseal portal tract and transport of dopamine 
from the hypothalamus to the pituitary in the former condition 
or loss of hypothalamic dopamine-containing neurons in the lat­
ter. Suprasellar extension of a large, non–prolactin-secreting pitu­
itary macroadenoma that compresses the pituitary stalk usually 
causes hyperprolactinemia with se

y stalk usually 
causes hyperprolactinemia with serum prolactin concentrations in 
the 20- to 250-ng/mL range, although higher concentrations are 
occasionally seen.423,424
Medications that cause hyperprolactinemia (prolactin concen­
tration usually <100 ng/mL) interfere with hypothalamic dopamine 
production or action or affect the regulation of dopamine secretion 
by CNS neurotransmitters (e.g., serotonin).429,430 The medications 
that most commonly cause hyperprolactinemia are dopamine D2 
re

monly cause hyperprolactinemia are dopamine D2 
receptor antagonists, such as typical antipsychotic drugs (phenothi­
azines, thioxanthenes, and butyrophenones), some atypical antipsy­
chotic agents (e.g., risperidone, molindone), and gastrointestinal 
promotility agents (e.g., metoclopramide, domperidone). In con­
trast, newer atypical antipsychotic medications such as clozapine, 
olanzapine, quetiapine, ziprasidone, and aripiprazole much less 
commonly increase prolactin. Other medications that

ommonly increase prolactin. Other medications that cause hyper­
prolactinemia less commonly include some tricyclic antidepressants 
(e.g., clomipramine), monoamine oxidase inhibitors (e.g., pargyline; 
clorgiline, which is rarely used), and antihypertensive agents (vera­
pamil; α-methyldopa and reserpine, which are rarely used). Selective 
serotonin and serotonin/norepinephrine reuptake inhibitors in gen­
eral have minimal to no effect on prolactin concentrations.
Serum prolactin concentrations 

in concentrations.
Serum prolactin concentrations may be elevated in patients 
with CKD, in proportion to the degree of renal impairment, 
because of both increased secretion and decreased clearance, and 
this may contribute to the hypogonadism associated with chronic 
renal failure. The mild hyperprolactinemia associated with pri­
mary hypothyroidism does not usually suppress gonadotropin 
secretion significantly or cause clinical secondary hypogonadism 
in men. However, if primary hypothyroidi

gonadism 
in men. However, if primary hypothyroidism is severe and long 
standing, it may cause slight enlargement of the pituitary gland, 
which may be confused with a pituitary adenoma.
Treatment is aimed initially at the underlying cause of hyper­
prolactinemia. In men with prolactin-secreting macroadenomas, 
treatment is initiated with a dopamine agonist medication such 
as bromocriptine or carbergoline.423,424,426,431 Dopamine agonist 
therapy usually results in a reduction of serum prolact

py usually results in a reduction of serum prolactin, decreased 
tumor size, and improvement in visual field defects. Treatment 
with these agents may also improve sexual dysfunction, normalize 
testosterone concentrations, and improve semen quality. In men 
who remain persistently hypogonadal despite adequate dopamine 
agonist treatment, testosterone therapy may be initiated to treat 
manifestations of androgen deficiency. Testosterone is aromatized 
to estradiol, and this estrogen theoreticall

ized 
to estradiol, and this estrogen theoretically might increase prolac­
tin concentrations and tumor growth by direct action on pituitary 
lactotrophs; this effect may also cause resistance to dopamine ago­
nist therapy.432 Therefore, monitoring of tumor size is required 
during testosterone replacement therapy.
In some men who do not respond to dopamine agonist treat­
ment alone, gonadotropin or pulsatile GnRH therapy may be nec­
essary to stimulate spermatogenesis sufficient to induce ferti

imulate spermatogenesis sufficient to induce fertility. 
Surgery or radiation therapy may be needed for tumors that are 
resistant to dopamine agonists, and urgent surgery may be needed 
for pituitary apoplexy or rapidly progressive tumor mass effects 
such as visual loss. Medications that cause hyperprolactinemia 
may be stopped or switched to ones that do not elevate prolactin. 
For antipsychotic and antidepressant medications, such changes 
should be made in consultation with the patient’s ps

ould be made in consultation with the patient’s psychiatrist.429 
If discontinuing or switching drugs is not an option, testosterone 
treatment may be needed to treat androgen deficiency. Addition 
of a dopamine agonist while administration of an offending anti­
psychotic medication is continued should be done with extreme 
caution, because there is a risk of exacerbating psychosis. 
Opioids. Use of opiates or opioid medications, particularly 
potent, long-acting narcotic analgesics such as meth

tent, long-acting narcotic analgesics such as methadone (>30 
mg daily), controlled-release or intrathecal morphine sulfate or 
the transdermal fentanyl patch, and drugs of abuse such as heroin 
(diacetylmorphine) or time-released oxycodone (OxyContin), 
profoundly suppress gonadotropin secretion, resulting in severe 
androgen deficiency.433-437
731
Prolonged use of opioids causes symptomatic androgen defi­
ciency, resulting in sexual dysfunction and long-term co

, resulting in sexual dysfunction and long-term conse­
quences such as loss of BMD and increased risk of osteoporosis; 
this is a common cause of secondary hypogonadism associated 
with androgen deficiency and impairment in sperm production. 
Although the benefits and risks of testosterone treatment have not 
been evaluated in randomized controlled studies,438 testosterone 
treatment should be considered in cases of secondary hypogonad­
ism due to chronic use of opioids. In contrast, short-term 

o chronic use of opioids. In contrast, short-term use of 
opioids or use of short-acting opioids (e.g., postoperatively) may 
cause only a transient suppression of gonadotropins and testoster­
one that does not require treatment.
Administration of opioid antagonists such as naloxone or nal­
trexone results in an increase in LH pulse frequency in normal 
men, suggesting that endogenous opioid neuronal systems within 
the hypothalamus exert a negative regulation on pulsatile GnRH 
secretion.439 Th

ive regulation on pulsatile GnRH 
secretion.439 Therefore, exogenous administration of opioids most 
likely causes reduced gonadotropin secretion by suppressing hypo­
thalamic GnRH secretion. The action of exogenous opioids on 
GnRH secretion is probably mediated by μ-opioid receptors. In 
this regard, the pure μ-opioid receptor agonist methadone more 
commonly and severely suppresses gonadotropins and testos­
terone than does buprenorphine, which is a partial μ-receptor, 
δ-receptor, and opioid

h is a partial μ-receptor, 
δ-receptor, and opioid receptor–like 1/nociceptin receptor ago­
nist and κ-opioid receptor antagonist.434,440 Both methadone and 
buprenorphine are used clinically for detoxification and mainte­
nance treatment of opioid addiction.
In men taking chronic high-dose, long-acting opioids, serum 
testosterone, LH, and FSH are usually severely suppressed, and 
sperm production is impaired.436,441 The most prominent abnor­
mality demonstrated on seminal fluid analysis in men

lity demonstrated on seminal fluid analysis in men taking 
methadone is reduced sperm motility (asthenospermia), but 
abnormalities in sperm morphologic structure (teratospermia) 
and oligozoospermia are also seen.367 Functional δ-, κ-, and 
μ-opioid receptors have been reported to be present on human 
spermatozoa.442 Therefore, exogenous opioids may have a direct 
effect by slowing sperm motility, independent of their effects on 
the hypothalamic-pituitary-testicular axis. 
Sex Steroids. Admini

c-pituitary-testicular axis. 
Sex Steroids. Administration of sex steroids—androgens, pro­
gestins, or estrogens—suppresses gonadotropin secretion by nega­
tive feedback mechanisms at the hypothalamus or the pituitary 
gland or both; chronic administration may cause secondary hypo­
gonadism, resulting in androgen deficiency and impaired sperm 
production.
Synthetic androgens (androgenic anabolic steroids) and tes­
tosterone are used by a significant number of teenage boys and 
men to increase mu

ant number of teenage boys and 
men to increase muscle bulk and strength and enhance athletic 
performance or physical appearance. For these purposes, these 
androgens are used in extremely high doses in a variety of com­
binations and patterns for prolonged periods. The prevalence of 
anabolic steroid abuse ranges from approximately 1% to 6% in 
various populations, including high school and college students, 
weightlifters, body builders (who often have muscle dysphoria, an 
obsessive preoccup

ften have muscle dysphoria, an 
obsessive preoccupation with a muscular appearance), young rec­
reational athletes, and competitive athletes.131,443,444
During chronic administration of high-dose androgenic ana­
bolic steroids, serum concentrations of testosterone, LH, and FSH 
are very low, and sperm counts are usually suppressed to severe 
oligozoospermia or azoospermia.131,443-445 Unless testosterone is 
being administered, serum testosterone concentrations are low 
because synthetic androgen

concentrations are low 
because synthetic androgenic anabolic steroids do not cross-react 
in testosterone assays. Because of the androgenic effects of these 
anabolic steroids, individuals taking these agents may not com­
plain of androgen deficiency symptoms.
After discontinuation of even prolonged anabolic steroid use, 
recovery of the hypothalamic-pituitary-testicular axis usually 
occurs within weeks to months. However, for unclear reasons, 
some men experience a protracted period of sympto

some men experience a protracted period of symptomatic hypo­
gonadism that may last for several months to several years, par­
ticularly in older men.131,443-446 It usually is not possible to know 
whether these men had underlying hypogonadism before taking 
anabolic steroids; therefore, if secondary hypogonadism is severe, 
an appropriate workup, which might include sellar imaging (if 
hypogonadism is severe), is usually needed. Prolonged secondary 
hypogonadism after androgenic anabolic steroi

ary 
hypogonadism after androgenic anabolic steroid use often causes 
sexual dysfunction and depressed mood. Severe symptoms may 
lead to continued use of these agents and anabolic steroid depen­
dence. Treatment with testosterone to relieve symptoms of andro­
gen deficiency or with gonadotropins (hCG) to stimulate sperm 
production and induce fertility may be needed. Although poorly 
studied, off-label treatment with clomiphene citrate and aroma­
tase inhibitors have also been reported to stimu

­
tase inhibitors have also been reported to stimulate gonadotropin 
and testosterone secretion in these men.447
Chronic administration of high doses of progestins, such as 
megestrol acetate or depo-medroxyprogesterone acetate, or estro­
gens, such as diethylstilbestrol, also suppresses gonadotropins and 
testicular function, resulting in secondary hypogonadism. Meges­
trol acetate is used to stimulate appetite in wasting conditions 
such as cancer and HIV disease. At the doses used for this pu

cer and HIV disease. At the doses used for this purpose, 
it causes severe symptomatic androgen deficiency and suppression 
of sperm production.448 Weight gain induced by megestrol acetate 
is mostly fat rather than lean mass, in part because of the androgen 
deficiency that it causes.449 Most importantly, megestrol acetate 
may cause symptomatic and potentially life-threatening secondary 
adrenal insufficiency from simultaneous suppression of cortisol 
production.448 Both megestrol and depo-med

rtisol 
production.448 Both megestrol and depo-medroxyprogesterone 
acetate have been used to induce medical castration in patients 
with prostate cancer.450 Medroxyprogesterone acetate has also 
been used to reduce libido in psychiatric conditions manifested by 
deviant sexual behavior (paraphilia), and it is used in combination 
with testosterone (to prevent androgen deficiency) for suppression 
of spermatogenesis in male contraceptive development trials.105,451 
Administration of estrogens (e

ent trials.105,451 
Administration of estrogens (e.g., low-dose diethylstilbestrol for 
prostate cancer), exposure to estrogen-containing substances, or 
excessive estradiol production by estrogen-secreting tumors (e.g., 
Sertoli or Leydig cell tumors) suppresses gonadotropin and testos­
terone production and causes secondary hypogonadism, usually 
with prominent gynecomastia.452-455 
GnRH Analogues. GnRH analogues, both agonists and 
antagonists, severely suppress endogenous gonadotropin and 
t

, severely suppress endogenous gonadotropin and 
testosterone production (i.e., medical castration); they are used 
to treat androgen-dependent pathologic states such as locally 
advanced or metastatic prostate cancer and central precocious 
puberty.202,456,457 Administration of GnRH agonists (e.g., leup­
rolide, goserelin) produces an initial stimulation of gonadotropin 
and testosterone secretion (known as a flare), which is followed 
in 1 to 2 weeks by GnRH receptor downregulation and marked 

 weeks by GnRH receptor downregulation and marked 
suppression of gonadotropins and testosterone to castration con­
centrations.202 The initial surge in testosterone concentrations has 
been associated with clinical flares in metastatic prostate cancer, 
and there have been reports of increased bladder outlet obstruc­
tion, bone pain, pathologic fracture, spinal cord compression, and 
death. However, these complications are very uncommon, and it 
is not clear that they are directly related to th

is not clear that they are directly related to the initial increase in 
732
testosterone concentrations. To prevent the potential complica­
tions associated with the testosterone flare, AR antagonists (e.g., 
bicalutamide) are usually coadministered with a GnRH agonist 
for men with metastatic prostate cancer.458 In contrast to agonists, 
GnRH antagonists (e.g., degarelix) cause an immediate suppres­
sion of gonadotropin and testosterone secretion wi

sion of gonadotropin and testosterone secretion without a flare; 
whether clinically relevant outcomes differ between GnRH ago­
nists and antagonists is not clear.456,457
Continuous administration of GnRH agonists in men with 
locally advanced or metastatic prostate cancer induces castration 
or near-castration testosterone concentrations and causes symp­
toms of severe androgen deficiency, including sexual dysfunction 
with reduced libido and reduced spontaneous erections, dimin­
ished energy a

duced spontaneous erections, dimin­
ished energy and motivation, depressed mood and irritability, hot 
flushes and sleep disturbance, decreased memory and concentra­
tion, reduced in muscle mass and strength, increased fat mass 
and insulin resistance, decreased BMD resulting in osteopenia 
or osteoporosis, gynecomastia and loss of male hair pattern, and 
decreased hemoglobin and hematocrit, resulting in significant 
decline in quality of life.201,202 As a result, increasingly, GnRH 
agonist the

1,202 As a result, increasingly, GnRH 
agonist therapy is administered intermittently in the treatment of 
advanced prostate cancer. However, in a substantial number of 
men who stop GnRH agonist therapy, testicular function remains 
suppressed, and testosterone concentrations persist within the 
castrate or hypogonadal range for prolonged periods (up to 1–3 
years).459-461 Risk factors for prolonged testicular suppression are 
longer duration of GnRH agonist therapy, older age (>70 years), 
and

 GnRH agonist therapy, older age (>70 years), 
and possibly low testosterone concentrations and hypogonadism 
that was present before treatment.
Some, but not all, large observational population-based studies 
have found that prolonged GnRH agonist therapy increases the 
risk of diabetes mellitus, coronary heart disease, myocardial infarc­
tion, sudden cardiac death, stroke, and fractures. Consequently, 
the US Food and Drug Administration (FDA) has recommended 
that the risk factors for these d

has recommended 
that the risk factors for these diseases should be assessed and the 
benefits and risks of GnRH agonist therapy weighed before it is 
used and that monitoring for these conditions should be contin­
ued during treatment.202,462-465 A 2011 meta-analysis of random­
ized trials reported that androgen deprivation therapy of men with 
unfavorable-prognosis prostate cancer was not associated with an 
increased risk of cardiovascular mortality but was associated with 
an improved prosta

tality but was associated with 
an improved prostate cancer–specific mortality rate.466 However, 
another meta-analysis of cohort and randomized trials found that 
androgen deprivation therapy was associated with an increase in 
nonfatal cardiovascular events and acute myocardial infarction.467 
Hypopituitarism. A destructive or infiltrative lesion of the 
pituitary gland or hypothalamus commonly causes impaired pitu­
itary hormone production (hypopituitarism) and gonadotropin 
deficiency, resul

popituitarism) and gonadotropin 
deficiency, resulting in androgen deficiency and impairment in 
sperm production. The prevalence of hypopituitarism has been 
estimated to be approximately 1 in 2200.468,469
Hypopituitarism is most commonly caused by pituitary adeno­
mas and their treatment (hypophysectomy or radiation therapy) 
or by hypothalamic or parasellar tumors such as craniopharyngi­
oma, meningioma, optic glioma or astrocytoma, metastatic car­
cinoma (from the breast, lung, colon, or pro

 car­
cinoma (from the breast, lung, colon, or prostate), pinealoma, 
germinoma, chordoma, and ependymoma; together, these tumors 
account for approximately 90% of cases.468,469
Other conditions of the pituitary or hypothalamus (or both) 
that cause hypopituitarism include cranial radiation therapy 
(intracranial tumors, acute lymphoblastic leukemia prophylaxis, 
nasopharyngeal carcinoma, total-body irradiation), vascular 
compromise (traumatic brain injury,470-472 infarction or pituitary 
apopl

rain injury,470-472 infarction or pituitary 
apoplexy, subarachnoid hemorrhage, ischemic stroke, vascular 
malformation), granulomatous or infiltrative disease (sarcoidosis, 
histiocytosis X, granulomatosis with polyangiitis, hemochroma­
tosis, transfusion-induced iron overload), infection (tuberculosis, 
fungal infections like aspergillosis or coccidioidomycosis, basi­
lar meningitis, encephalitis, syphilis, Whipple disease), pituitary 
stalk disease (traumatic injury, e.g., basilar skull fract

sease (traumatic injury, e.g., basilar skull fracture or sur­
gical pituitary stalk section, granulomatous disease, lymphocytic 
infundibuloneurohypophysitis, infection, tumor), lymphocytic 
hypophysitis (particularly lymphocytic infundibuloneurohypoph­
ysitis, which is more common in men, rather than lymphocytic 
adenohypophysitis, which is more common in women), and auto­
immune hypophysitis (e.g., with cytotoxic T-lymphocyte–associ­
ated protein 4 inhibitors, e.g., ipilimumab therapy).470-475

in 4 inhibitors, e.g., ipilimumab therapy).470-475 These 
conditions are discussed in Chapter 9.
Destructive or infiltrative lesions of the pituitary gland (e.g., 
nonfunctioning pituitary adenoma) usually result in a gradual, 
progressive loss of anterior pituitary function. In these instances, 
GH and gonadotropin (LH and FSH) deficiency (i.e., second­
ary hypogonadism) usually occur initially, followed by deficien­
cies of TSH (secondary hypothyroidism) and, eventually, ACTH 
(secondary adren

hyroidism) and, eventually, ACTH 
(secondary adrenal insufficiency), resulting in panhypopituita­
rism.468,469 However, there are many exceptions to this order of 
loss, depending on the specific location of the pituitary lesion and 
the nature of the underlying disease process. For example, lym­
phocytic hypophysitis usually causes ACTH and TSH deficiency 
without impairment of gonadotropin production, and ACTH defi­
ciency is more common than TSH deficiency after radiation ther­
apy involving 

SH deficiency after radiation ther­
apy involving the hypothalamic-pituitary axis. Anterior pituitary 
hormone loss is even less predictable in disease processes involving 
the hypothalamus, in part because of the more disperse anatomic 
arrangement in the hypothalamus of nuclei that produce releasing 
factors for pituitary hormones. Acute destructive processes such as 
pituitary apoplexy usually cause panhypopituitarism.
Diseases of the hypothalamus or high in the pituitary stalk may 
be associ

amus or high in the pituitary stalk may 
be associated with diabetes insipidus, which is caused by destruc­
tion or retrograde degeneration of neurons producing arginine 
vasopressin in the supraoptic or the paraventricular nuclei, respec­
tively.476-479 Processes involving only the pituitary gland do not 
cause diabetes insipidus.
Hypothalamic and pituitary stalk diseases may cause hyper­
prolactinemia due to loss of dopamine-containing neurons or 
interruption of the hypothalamic hypophyseal p

or 
interruption of the hypothalamic hypophyseal portal tract and 
transport of dopamine from the hypothalamus to the pituitary. 
Pituitary microadenomas or macroadenomas may produce prolac­
tin, and suprasellar extension of nonsecretory pituitary macroad­
enomas or those secreting other hormones (e.g., GH) may cause 
hyperprolactinemia by interrupting the hypothalamic-hypophy­
seal portal system.
Prepubertal boys who have hypopituitarism resulting in gonad­
otropin deficiency present with delay

ng in gonad­
otropin deficiency present with delayed puberty and eunuch­
oidism, and men present with adult androgen deficiency and 
complaints of reduced libido and erectile dysfunction. However, 
in patients with secondary hypogonadism, clinicians must be alert 
to the possibility and clinical manifestations of deficiencies of other 
pituitary hormones (ACTH, TSH, GH, and arginine vasopressin); 
excessive pituitary hormone production by pituitary adenomas and 
resulting clinical syndromes, suc

ry adenomas and 
resulting clinical syndromes, such as excessive prolactin produc­
tion resulting in hyperprolactinemia, ACTH resulting in Cushing 
syndrome, GH resulting in acromegaly, gonadotropin and free α- 
and β-subunits (which usually do not result in a hormone excess 
733
syndrome but rarely cause precocious puberty), or, rarely, TSH 
resulting in hyperthyroidism; and tumor mass effects such as head­
ache, visual disturbance, and visual field defects (typ

 visual disturbance, and visual field defects (typically bilateral 
superior quadrantanopia or bitemporal hemianopia, but a unilat­
eral effect and a variety of visual field defects may be present) and, 
uncommonly, cerebrospinal fluid rhinorrhea, cranial nerve palsies, 
temporal lobe epilepsy, and personality changes.468,469,480,481 It is 
important to have a high index of suspicion for the presence of sec­
ondary adrenal insufficiency in patients with hypothalamic or pitu­
itary disease, becau

ts with hypothalamic or pitu­
itary disease, because it is a life-threatening and treatable condition 
that manifests with nonspecific symptoms and signs. In boys with 
hypopituitarism who present with a clinical picture of CDGP, GH 
deficiency may occur in conjunction with gonadotropin deficiency 
and may contribute to short stature and growth delay.
Usually in men with secondary hypogonadism due to hypopi­
tuitarism, the serum testosterone level and sperm count are very 
low, and LH and FSH co

l and sperm count are very 
low, and LH and FSH concentrations are distinctly low or, less 
commonly, in the low-normal to slightly low range. Men with 
panhypopituitarism may have more severe testosterone deficiency 
than those with CHH.482 The gonadotropin response to acute or 
chronic GnRH stimulation is not a clinically useful differential 
diagnostic test, because it does not reliably distinguish between 
pituitary and hypothalamic disease that cause gonadotropin defi­
ciency in hypopituita

hat cause gonadotropin defi­
ciency in hypopituitarism. If hypopituitarism is suspected on the 
basis of the initial clinical and laboratory evaluation, further evalu­
ation should include hypothalamic-pituitary imaging, preferably 
an MRI with gadolinium contrast enhancement, which can bet­
ter define the presence and extent of hypothalamic and pituitary 
disease compared with a CT scan (although CT is less expensive 
and equally sensitive for identifying a pituitary macroadenoma or 
microcalci

dentifying a pituitary macroadenoma or 
microcalcifications found frequently in craniopharyngioma, and it 
more accurately assesses parasellar bone destruction); formal visual 
field examination; and investigation of anterior pituitary hormone 
deficiency or excess.468,469
Treatment is aimed at the underlying cause of the hypopitu­
itarism and treatment of pituitary hormone deficiency, includ­
ing treatment of androgen deficiency secondary to gonadotropin 
deficiency with testosterone replacemen

adotropin 
deficiency with testosterone replacement therapy.468,469 With 
transsphenoidal surgical treatment of pituitary adenomas, pitu­
itary function is improved in approximately 50% of cases. Dopa­
mine agonist treatment of prolactin-secreting pituitary adenomas 
improves pituitary function in 60% to 75% of cases. If fertility is 
desired, testosterone treatment is stopped and gonadotropin ther­
apy is initiated, initially with hCG alone. In men with acquired 
gonadotropin deficiency, normal

men with acquired 
gonadotropin deficiency, normal testicular volumes and no coex­
isting testicular disease, hCG treatment alone (without addition 
of FSH therapy) often stimulates spermatogenesis to concentra­
tions sufficient to restore fertility.79 
Glucocorticoid Excess (Cushing Syndrome). Excessive concen­
trations of either exogenous or endogenous glucocorticoids (the 
latter due to pituitary Cushing disease, ectopic ACTH syndrome, 
or adrenal adenoma) is a common acqui

H syndrome, 
or adrenal adenoma) is a common acquired cause of secondary 
hypogonadism, resulting in symptomatic androgen deficiency and 
impaired sperm production.483-486 In contrast to those patients 
with adrenal adenoma, some men with glucocorticoid excess due 
to adrenal carcinoma secrete excessive amounts of androgens (and 
mineralocorticoids) and do not demonstrate manifestations of 
androgen deficiency.
Glucocorticoids act primarily to suppress gonadotropins via 
inhibition of hypothalam

ppress gonadotropins via 
inhibition of hypothalamic GnRH secretion, but they may also 
have direct suppressive effects on testis function and therefore pro­
duce combined primary and secondary hypogonadism. However, 
high-dose immunosuppressive glucocorticoid therapy is most 
commonly associated with a hormone pattern characterized by 
low testosterone and low-normal gonadotropin concentrations, 
consistent with secondary hypogonadism. Occasionally, in men 
receiving glucocorticoid treatment, g

lly, in men 
receiving glucocorticoid treatment, gonadotropins are high nor­
mal or slightly elevated, suggesting primary hypogonadism.
Although most commonly observed with high-dose glucocor­
ticoid treatment, daily doses as low as 5.0 to 7.5 mg of prednisone 
may cause hypogonadism, particularly in older men. Because high 
doses of glucocorticoids may suppress SHBG concentrations, it is 
important to confirm the biochemical diagnosis of hypogonadism 
using an accurate measurement of free testo

adism 
using an accurate measurement of free testosterone (i.e., calcu­
lated free testosterone or free testosterone by equilibrium dialysis). 
In preliminary studies of men receiving chronic glucocorticoid 
therapy, testosterone treatment was found to improve muscle 
mass, BMD, and quality of life.487 
Chronic Organ Failure. Chronic organ failure, such as in 
hepatic cirrhosis, CKD, chronic lung disease, or CHF, is a com­
mon cause of symptomatic secondary hypogonadism.4,5 As dis­
cussed previo

c secondary hypogonadism.4,5 As dis­
cussed previously, chronic systemic illness commonly affects the 
hypothalamic-pituitary-testicular axis at multiple sites and usu­
ally causes combined primary and secondary hypogonadism, but 
many disorders are associated with a hormone pattern character­
ized by low serum testosterone and low to low-normal gonadotro­
pin concentrations, indicative of secondary hypogonadism.
The cause of clinical and biochemical hypogonadism in these 
cases is multifactoria

ical hypogonadism in these 
cases is multifactorial and encompasses both the chronic disease 
itself and its associated conditions of malnutrition, wasting, a pro­
inflammatory state (with elevated proinflammatory cytokines like 
IL2, IL6, and tumor necrosis factor α [TNFα]),488 medication use 
(e.g., alcohol, opiates, glucocorticoids), chronic stress, and other 
comorbid illnesses. These associated factors play a large role in 
suppressing gonadotropin concentrations and contribute to the 
horm

otropin concentrations and contribute to the 
hormonal pattern of secondary hypogonadism associated with 
chronic organ failure. The degree to which these factors contribute 
to the clinical and biochemical manifestations of hypogonadism 
varies considerably among individuals. Furthermore, biochemical 
confirmation of low testosterone in patients with chronic organ 
failure or systemic illness may be confounded by alterations in 
SHBG. Therefore, accurate and reliable measurements of free tes­
t

 accurate and reliable measurements of free tes­
tosterone are needed to establish biochemical androgen deficiency 
in the presence of chronic systemic illness.
Hepatic cirrhosis from any cause (e.g., alcoholic or nonalcoholic 
liver disease) is commonly associated with a hormone pattern that 
is consistent with primary hypogonadism (i.e., low free testoster­
one, high LH, and normal to high-normal FSH concentrations) 
in mild to moderate disease (Child-Pugh class A or B) and with 
secondary hyp

 (Child-Pugh class A or B) and with 
secondary hypogonadism (i.e., low free testosterone and low-nor­
mal LH and FSH concentrations) in severe to end-stage liver dis­
ease (Child-Pugh class C).303,304 SHBG concentrations increase 
progressively with the severity of cirrhosis, resulting in normal or 
high serum total testosterone concentrations despite low free tes­
tosterone concentrations and clinical manifestations of androgen 
deficiency. In men with cirrhosis, sperm production is commonly 
i

en with cirrhosis, sperm production is commonly 
impaired, and sperm motility is reduced.
In alcoholic cirrhosis, serum estrone and estradiol concentra­
tions are relatively high as a result of increased production of adre­
nal androgens (e.g., androstenedione) induced by alcohol and its 
metabolite, acetaldehyde; reduced clearance of these androgens 
by the liver; and subsequent aromatization of androstenedione to 
734
estrone and its conversion to e

pment and Function
estrone and its conversion to estradiol.4,5 Relative hyperestrogen­
ism is responsible for several of the clinical manifestations com­
monly observed in men with alcoholic cirrhosis compared with 
nonalcoholic cirrhosis, including gynecomastia, palmar erythema, 
plethora, spider angiomas, and loss of male body hair (reduced 
axillary and pubic hair and a female escutcheon). Men with severe 
alcoholic cirrhosis usually have atrophic testes (usually soft in con­
sistency) due to

phic testes (usually soft in con­
sistency) due to direct toxic effects of alcohol.
In men with severe hepatic cirrhosis, pulsatile GnRH secretion 
and the pituitary response to GnRH are diminished, contribut­
ing to secondary gonadal failure.489 Spironolactone, which is used 
to treat edema and ascites associated with portal hypertension, 
is an AR antagonist and an androgen biosynthesis inhibitor. Its 
use may contribute to symptoms of androgen deficiency, gyne­
comastia, and hypogonadism. Pro

 deficiency, gyne­
comastia, and hypogonadism. Protein-calorie malnutrition, com­
plications of cirrhosis such as infection, and continued alcohol 
abuse contribute to the clinical manifestations and cause of low 
testosterone in these chronically ill men. A preliminary 12-month 
double-blind, placebo-controlled study in men with cirrhosis and 
low testosterone concentrations found that testosterone treatment 
was safe and increased lean body and bone mass and hemoglobin 
concentrations and redu

 bone mass and hemoglobin 
concentrations and reduced fat mass.305 Successful liver trans­
plantation improves but does not normalize gonadal function, 
probably because of chronic immunosuppressive treatment with 
glucocorticoids and other agents.490
As described earlier, CKD is commonly associated with a hor­
mone pattern of low serum testosterone and elevated gonadotropin 
concentrations resulting from reduced renal clearance, consistent 
with primary hypogonadism.4,5,307,309,311 With increas

 primary hypogonadism.4,5,307,309,311 With increasing stage 
of CKD, there is progressive reduction in sperm concentration 
and motility and alterations in sperm morphology.491 However, 
the amplitude of pulsatile LH secretion is reduced, suggesting 
impaired hypothalamic-pituitary function in men with CKD.492 
Gonadotropin secretion may also be suppressed by coexisting ure­
mia, hyperprolactinemia, malnutrition, a proinflammatory state, 
comorbid conditions (e.g., diabetes), and obesity, and so

d conditions (e.g., diabetes), and obesity, and some men 
demonstrate a hormone pattern that is more consistent with sec­
ondary hypogonadism (i.e., with low testosterone and normal to 
high-normal gonadotropin concentrations). Randomized con­
trolled trials to assess the long-term clinical benefits and risks of 
testosterone treatment in CKD patients who are high risk of car­
diovascular events and mortality have not been performed.308 Suc­
cessful renal transplantation usually normalizes conce

ful renal transplantation usually normalizes concentrations of 
testosterone and gonadotropins and sperm production, although 
spermatogenesis and Sertoli cell function recover more slowly and 
to a lesser extent.307,309,493
Men with chronic lung disease, especially chronic obstructive 
pulmonary disease (COPD), commonly have low serum testoster­
one concentrations.494-496 The prevalence of biochemical hypogo­
nadism depends on the population studied and varies from 12% 
in a community-based pop

died and varies from 12% 
in a community-based population to 38% in male veterans, the 
latter being a population with numerous comorbid conditions 
that lower testosterone concentrations. In the population of veter­
ans, approximately 75% of men with COPD with low serum tes­
tosterone have low or low-normal gonadotropin concentrations, 
consistent with secondary hypogonadism, and the remainder 
have elevated gonadotropins, indicative of primary hypogonad­
ism.495 Coexisting factors that contrib

hypogonad­
ism.495 Coexisting factors that contribute to the clinical symptoms 
and biochemical diagnosis of hypogonadism in men with severe 
COPD include muscle wasting, inactivity, and deconditioning; 
malnutrition and cachexia; chronic stress and inflammation; med­
ications (e.g., glucocorticoids); and hypoxia. Hypoxia suppresses 
gonadotropin and testosterone secretion independent of gluco­
corticoid therapy in men with COPD or idiopathic pulmonary 
fibrosis.494-497 Preliminary studies have 

monary 
fibrosis.494-497 Preliminary studies have demonstrated an increase 
in lean mass with testosterone treatment in men with COPD and 
low testosterone concentrations but inconsistent improvements in 
muscle strength, including respiratory muscle function, and no 
effects on endurance or quality of life.
CHF is associated with biochemical androgen deficiency in 
approximately 25% to 30% of cases.498-500 Men with CHF who 
have low serum testosterone usually have normal to low-normal 
gonadotr

terone usually have normal to low-normal 
gonadotropin concentrations, suggesting secondary hypogonad­
ism. However, it is unclear whether men with CHF and low tes­
tosterone differ from those with normal testosterone in regard to 
symptoms and signs or response to testosterone therapy.499 In lim­
ited initial clinical trials in men with CHF, testosterone treatment 
improved exercise tolerance, muscle strength, and oxygen capac­
ity in men with either low or normal testosterone concentrations, 

ither low or normal testosterone concentrations, 
suggesting a pharmacologic effect of testosterone independent of 
the presence of androgen deficiency. In a preliminary randomized 
controlled pilot study in men with CHF with reduced ejection 
fraction and low testosterone concentrations, testosterone treat­
ment was safe but did not improve functional capacity, walking 
distance, left ventricular ejection fraction, brain natriuretic pep­
tide concentrations, or quality of life.501 
Chronic Syst

ncentrations, or quality of life.501 
Chronic Systemic Illness. Several chronic systemic illnesses, 
such as T2DM, malignancy, rheumatic disease, and HIV disease, 
may also cause secondary hypogonadism characterized by low 
serum testosterone concentrations and low or low-normal gonad­
otropin concentrations.4,5 As in men with chronic organ failure, 
the cause of clinical and biochemical hypogonadism is multifacto­
rial due to the chronic illness itself and associated obesity (e.g., 
with diabet

 itself and associated obesity (e.g., 
with diabetes mellitus) or malnutrition (e.g., with malignancy), 
wasting, proinflammatory state, medication use (e.g., opiates, glu­
cocorticoids), chronic stress, or other comorbid illnesses. These 
factors suppress gonadotropin concentrations and contribute vari­
ably to the clinical and biochemical androgen deficiency seen in 
individuals with systemic illness. Because systemic illnesses and 
associated comorbid conditions and medications may alter SHBG

comorbid conditions and medications may alter SHBG 
concentrations, accurate measurements of free testosterone (i.e., 
calculated free testosterone or free testosterone by equilibrium 
dialysis) are needed to confirm biochemical androgen deficiency.
Low serum free testosterone and low or low-normal gonado­
tropin concentrations, consistent with secondary hypogonadism, 
occur in 30% to 50% of men with T2DM.502,503 Low testosterone 
concentrations are associated with nonspecific clinical manifesta

are associated with nonspecific clinical manifesta­
tions that may be caused by androgen deficiency, such as sexual 
dysfunction and reduced vitality. Insulin resistance and moderate 
obesity are commonly associated with T2DM. In patients with 
T2DM and moderate obesity, low total testosterone concentra­
tions commonly result from reduced SHBG concentrations 
caused by insulin resistance and action on the liver. Therefore, it 
is important to confirm biochemical androgen deficiency in these 
men

firm biochemical androgen deficiency in these 
men by using calculated free testosterone values or measurements 
of free testosterone by equilibrium dialysis.
Secondary hypogonadism and clinical manifestations of andro­
gen deficiency may also be caused by comorbid conditions and 
complications associated with diabetes, such as obesity, athero­
sclerotic vascular disease, the proinflammatory state of diabetes, 
diabetic neuropathy, and CKD. A meta-analysis of seven placebo-
controlled randomized

a-analysis of seven placebo-
controlled randomized trials of testosterone treatment in men with 
T2DM failed to improve glycemic control as assessed by hemoglo­
bin A1c and insulin resistance by homeostatic model assessment of 
735
insulin resistance 2.504 Therefore, testosterone treatment should 
only be considered only in those diabetic men with symptomatic 
androgen deficiency that has been confirmed by accurate free tes­
tosterone measurements. If symptoms do

e free tes­
tosterone measurements. If symptoms do not improve with an 
adequate trial of testosterone therapy (e.g., 6 months), discontinu­
ation of treatment should be considered, particularly in men who 
had borderline or slightly low testosterone concentrations before 
therapy.
Men with poorly controlled T1DM may have reduced serum 
testosterone and gonadotropin concentrations and reduced LH 
pulse amplitude and frequency, which are not present in those 
with well-controlled disease and in t

nt in those 
with well-controlled disease and in the absence of obesity.505,506
Men with malignancy commonly have secondary hypogonadism 
characterized by low serum testosterone concentrations and low or 
low-normal gonadotropin concentrations.364,507,508 As mentioned 
previously, some men with malignancy present with low testos­
terone and elevated gonadotropin concentrations, consistent with 
primary hypogonadism. Malnutrition, wasting (cancer cachexia), 
systemic inflammation, medication use 

cachexia), 
systemic inflammation, medication use (e.g., opioid pain medica­
tions, glucocorticoids), chronic stress, and concomitant comorbid 
illnesses contribute to clinical and biochemical hypogonadism in 
men with cancer. Because SHBG may be reduced as a result of 
these associated conditions, accurate free testosterone measure­
ments are needed to confirm androgen deficiency.
Primary or secondary hypogonadism may be present before 
systemic chemotherapy or radiation therapy, as well as aft

 chemotherapy or radiation therapy, as well as after treat­
ment. A low free testosterone concentration with normal or ele­
vated gonadotropin concentrations was found in 40% to 60% of 
men with advanced malignancy (i.e., metastatic cancer) present­
ing with malnutrition and men with various stages of Hodgkin 
disease before chemotherapy.344,345,364 Low concentrations of free 
testosterone and bioavailable testosterone were present, respec­
tively, in approximately 78% and 66% of men with a vari

y, in approximately 78% and 66% of men with a variety of 
cancers, excluding those with androgen-dependent cancer (pros­
tate, breast) or testicular cancer, most of whom received chemo­
therapy or radiation therapy or both.364,507 Low testosterone was 
associated with reduced quality of life and sexual function.508
Men with rheumatic diseases, particularly the systemic autoim­
mune disorder rheumatoid arthritis, may manifest symptoms of 
sexual dysfunction (reduced libido and erectile dysfunctio

ysfunction (reduced libido and erectile dysfunction) and 
low serum free or bioavailable testosterone and gonadotropin con­
centrations in approximately 30% of cases.509,510 Secondary hypo­
gonadism in rheumatoid arthritis may be due in part to systemic 
inflammation (with elevated proinflammatory cytokines like IL2, 
IL6, and TNFα), complications such as rheumatoid lung, and 
treatment with glucocorticoids, but it may also occur early in the 
course of rheumatoid arthritis in the absence of com

urse of rheumatoid arthritis in the absence of complications and 
before glucocorticoid therapy.509,510 In men with long-standing 
rheumatoid arthritis, low free testosterone concentrations do not 
normalize after marked suppression of inflammation induced 
by anti-TNF therapy.511 Testosterone treatment may improve 
symptoms of androgen deficiency, but it does not reduce disease 
activity.512
Men with systemic lupus erythematosus may also demonstrate 
low free testosterone concentrations in conj

rate 
low free testosterone concentrations in conjunction with nor­
mal or low-normal gonadotropin concentrations, indicative of 
secondary hypogonadism or elevated gonadotropins, consistent 
with primary hypogonadism.513-516 Factors that contribute to 
gonadotropin suppression and secondary hypogonadism include 
chronic systemic illness and inflammation, major organ involve­
ment or organ failure (heart, lung, brain, kidney), and glucocor­
ticoid therapy.516 Factors that contribute to primary t

d therapy.516 Factors that contribute to primary testicular 
dysfunction include systemic and local inflammation or vascu­
litis, organ failure, and treatment with cytotoxic agents such as 
cyclophosphamide.
In observational cohort studies, low testosterone concentra­
tions are associated with rheumatic autoimmune disease, including 
rheumatoid arthritis and systemic lupus erythematosus.517,518 It is 
possible that the hypogonadism contributes to the immunologic 
pathophysiology of rheumatic dis

 the immunologic 
pathophysiology of rheumatic disorders.519,520 Autoimmune dis­
eases, particularly autoimmune rheumatic diseases (e.g., Sjögren 
syndrome, systemic lupus erythematosus, rheumatoid arthritis), 
thyroid disease (Hashimoto disease, Graves disease), and autoim­
mune neurologic diseases (e.g., myasthenia gravis, multiple scle­
rosis), occur more commonly in women than in men. Sex steroid 
hormones, primarily estrogens and androgens, modulate immune 
function by direct actions on imm

modulate immune 
function by direct actions on immune cell function and may play 
a role in sex differences in autoimmunity and in the pathophysiol­
ogy of autoimmune disorders.
Men with HIV disease commonly have hypogonadism charac­
terized by symptomatic androgen deficiency and impaired sperm 
production with low free testosterone concentrations due to com­
bined primary and secondary hypogonadism.300,521 Hypogonad­
ism occurred in up to 50% of men with HIV wasting before the 
advent of HAART.

 men with HIV wasting before the 
advent of HAART. Although androgen deficiency is less common 
since the advent of HAART, it occurs in approximately 20% of 
HIV-infected men. In 75% to 90% of cases, low free testoster­
one is associated with low or low-normal gonadotropin concen­
trations, consistent with secondary hypogonadism.522,523 In the 
remaining 10% to 25% of cases, gonadotropin concentrations are 
elevated, indicating primary hypogonadism.
As in other chronic systemic illnesses, the ca

sm.
As in other chronic systemic illnesses, the cause of hypogo­
nadism in men with HIV disease is multifactorial.4,5 In addition 
to HIV infection itself, gonadotropin suppression and second­
ary hypogonadism may be caused by malnutrition, wasting, and 
cachexia; opportunistic infections affecting hypothalamic-pitu­
itary function (e.g., CMV, T. gondii); systemic inflammation (with 
elevated concentrations of cytokines like IL2, IL6, and TNFα); 
medications (e.g., opiates, glucocorticoids, mege

medications (e.g., opiates, glucocorticoids, megestrol acetate); 
ongoing substance abuse (e.g., alcohol); and acute and chronic ill­
nesses. Conditions that may play a role in causing primary hypo­
gonadism include opportunistic infections affecting the testes 
(CMV, M. avium intracellulare, T. gondii), malignancies involv­
ing the testes (Kaposi sarcoma, lymphoma), systemic inflamma­
tion, and medications (chemotherapy for secondary neoplasms, 
ketoconazole).300,521,522,524
Protein-calorie ma

Protein-calorie malnutrition may suppress SHBG concentra­
tions, and advancing HIV infection is associated with elevated 
SHBG concentrations. Therefore, accurate measurements of free 
testosterone concentrations should be used to evaluate men with 
HIV disease for androgen deficiency.523,525 Sperm production 
structure and testicular atrophy may be present. It is important 
to recognize that HIV may

resent. It is important 
to recognize that HIV may be present in semen even when it is 
undetectable in plasma.300
In small clinical trials, treatment with testosterone or an andro­
genic anabolic steroid has been demonstrated to improve libido 
and sexual function, increase muscle mass and strength and BMD, 
decrease fat mass, improve mood, well-being, and quality of life, 
and increase hematocrit in HIV-infected men with low serum tes­
tosterone concentrations.116
Men with chronic spinal cord 

e concentrations.116
Men with chronic spinal cord injury at any level resulting in 
tetraplegia or paraplegia may have secondary hypogonadism 
with low serum testosterone and low or normal gonadotropin 
736
concentrations.350,351 Less commonly, some men with chronic 
spinal cord injury demonstrate low testosterone and elevated 
gonadotropins concentrations (consistent with primary hypogo­
nadism) or combined primary and secondary hypogonadism. In 
men

mbined primary and secondary hypogonadism. In 
men with lower spinal cord injury causing paraplegia, there may 
be a transient suppression of testosterone concentrations within 4 
months after the injury that resolves in most cases.352,526 Gonado­
tropin suppression is caused in part by several conditions associ­
ated with spinal cord injury, such as acute and chronic trauma 
and stress associated with the injury and attendant complications; 
obstructive sleep apnea; obesity or nutritional compr

tructive sleep apnea; obesity or nutritional compromise; hyper­
prolactinemia (usually associated with medications); and medica­
tion use (e.g., glucocorticoids, opioids, CNS-active drugs). The 
benefits and risks of testosterone treatment in patients with spinal 
cord injury are not clear.
Thalassemia major, or β-thalassemia, is an autosomal recessive 
disorder characterized by absent or severely deficient synthesis of 
β-globulin chains of hemoglobin resulting in severe anemia that 
requires l

globin resulting in severe anemia that 
requires lifelong blood transfusions. It is common in the Medi­
terranean region, in India, and in Southeast Asia. Chronic blood 
transfusions in patients with β-thalassemia cause transfusion-related 
iron overload in tissues and produce clinical manifestations similar 
to those that occur in patients with hereditary hemochromatosis. 
Transfusion-related iron overload may also occur in patients with 
sickle cell anemia, refractory aplastic anemia, or myelo

 cell anemia, refractory aplastic anemia, or myelodysplastic 
syndrome.
Iron deposition in the testes and pituitary gland usually causes 
combined primary and secondary hypogonadism.527-529 How­
ever, males with transfusion-related iron overload usually exhibit 
a hormonal pattern indicative of secondary hypogonadism, with 
low serum free testosterone and low to low-normal gonadotropin 
concentrations in most cases. Hypogonadism due to transfusion-
related iron overload may manifest with androge

n-
related iron overload may manifest with androgen deficiency and 
impaired sperm production; affected boys usually have short stat­
ure and pubertal and growth delay.530 Early iron chelation ther­
apy with agents such as deferoxamine, deferasirox, or deferiprone 
may reverse hypogonadism and improve survival.531 In men with 
long-standing β-thalassemia, chelation therapy does not reverse 
hypogonadism.532
Sickle cell disease may be associated with low serum testosterone 
and low to low-normal 

ith low serum testosterone 
and low to low-normal gonadotropin concentrations, indicative of 
secondary hypogonadism.533-535 Gonadotropin suppression may 
be caused by transfusion-induced iron overload (although much 
less commonly than in men with β-thalassemia348,536), hypotha­
lamic-pituitary microinfarctions, medications (e.g., opioids for 
chronic pain), systemic inflammation, nutritional deficiencies, 
and chronic systemic illness and stress secondary to repeated pain­
ful vaso-occlusive e

s secondary to repeated pain­
ful vaso-occlusive events. As discussed earlier, men with sickle cell 
disease may present with primary hypogonadism due to testicular 
microinfarctions caused by vaso-occlusive events or iron overload 
affecting the testes. Priapism, which may occur with vaso-occlu­
sive events, has been reported in patients receiving testosterone 
therapy.531,537 As in β-thalassemia, early iron chelation therapy 
may reverse hypogonadism.
Uncommonly, boys or men with cystic fibros

nadism.
Uncommonly, boys or men with cystic fibrosis have low serum 
testosterone and low to low-normal gonadotropin concentrations, 
a hormonal pattern consistent with secondary hypogonadism.538 
Chronic systemic illness and inflammation, malnutrition, and 
glucocorticoid use may contribute to gonadotropin suppression. 
Nutritional Disorders or Endurance Exercise. Starvation, mal­
nutrition, and eating disorders (anorexia nervosa) suppress gonad­
otropin and testosterone secretion, resulting in

­
otropin and testosterone secretion, resulting in symptomatic 
secondary hypogonadism with androgen deficiency (usually man­
ifested by reduced libido, sexual activity, and performance) and 
impaired sperm production; these effects are reversed with restora­
tion of food/calorie intake and weight gain. Fasting for periods of 
3 to 5 days suppresses gonadotropin and testosterone secretion and 
decreases LH pulse amplitude and frequency.539-541 These changes 
are reversed completely by pulsatile 

ese changes 
are reversed completely by pulsatile GnRH administration or low-
dose recombinant human methionyl leptin replacement, suggest­
ing that short-term starvation suppresses leptin production, which 
in turn suppresses the hypothalamic GnRH pulse generator.542-544 
Severe protein-calorie malnutrition, often associated with other 
nutritional deficiencies, may cause severe suppression of testoster­
one and elevation of gonadotropin concentrations, indicative of 
primary hypogonadism.540,5

rations, indicative of 
primary hypogonadism.540,545
Chronic endurance exercise results in low serum testosterone, 
low to low-normal gonadotropin concentrations, and impaired 
sperm production and motility, consistent with secondary hypo­
gonadism.546-548 High-intensity endurance training and exercise, 
such as occurs in the military and with overtraining in athletes, 
is associated with relative calorie deprivation (resulting in energy 
expenditure that is greater than energy intake, i.e., a r

ture that is greater than energy intake, i.e., a relative energy 
deficit) and intense stress that causes more severe suppression of 
gonadotropin and testosterone concentrations and symptoms of 
androgen deficiency than is seen with chronic lower-intensity 
endurance exercise. This condition has been referred to as relative 
energy deficit in sport by the International Olympic Commit­
tee.539,546-549 Suppression of the hypothalamic-pituitary-testicu­
lar axis resolves with cessation of training

ticu­
lar axis resolves with cessation of training and increased calorie 
intake.546,547 Other than reduced spermatogenesis, the clinical 
consequences of androgen deficiency induced by endurance exer­
cise are not clear.550 In contrast to chronic endurance exercise, 
short-term endurance or resistance exercise in some men results in 
an acute and transient increase in testosterone concentrations that 
is modified by the intensity of exercise and prior training and is 
possibly related to hemoco

 prior training and is 
possibly related to hemoconcentration, reduced metabolic clear­
ance, or transient increases in serum LH concentrations.551
Mild to moderate obesity results in reduced SHBG and total 
testosterone concentrations. Free testosterone is usually nor­
mal but may be reduced in association with low or low-normal 
gonadotropin concentrations in some men, particularly in those 
with comorbid conditions such as T2DM or obstructive sleep 
apnea.552-555 In men with morbid obesity (b

sleep 
apnea.552-555 In men with morbid obesity (body mass index [BMI] 
>40 kg/m2) or massive obesity (BMI >45 kg/m2), serum free 
testosterone is low, gonadotropin concentrations are low or low 
normal, and LH pulse amplitude (but not frequency) is reduced, 
indicative of secondary hypogonadism.556 Morbidly and massively 
obese men often complain of reduced libido and sexual dysfunc­
tion, but these symptoms are confounded by obesity and comor­
bid conditions associated with morbid obesity, suc

bid conditions associated with morbid obesity, such as depression, 
diabetes, and obstructive sleep apnea. Gonadotropin suppression 
and androgen deficiency is likely due to proinflammatory cyto­
kines and dysregulated leptin signaling, as well as obesity-related 
comorbidities.555,557 Bariatric surgery and substantial weight loss 
increase serum gonadotropin and testosterone concentrations, but 
modest weight loss achieved by dieting is less effective in reversing 
obesity-related hypogonadism.

ective in reversing 
obesity-related hypogonadism.555,558-561 In a small, 56-week, ran­
domized, double-blind, placebo-controlled trial in men with mild 
to moderate symptoms of androgen deficiency and low testoster­
one concentrations, testosterone treatment plus a very low energy 
diet improved symptoms and reduced fat mass while maintaining 
lean mass versus diet alone that resulted in reductions in both fat 
and lean body mass.562,563
737
Morbid obesity may b

pter 19  Testicular Disorders
Morbid obesity may be complicated by obstructive sleep apnea 
syndrome. Men with untreated or inadequately treated obstruc­
tive sleep apnea have low gonadotropin and testosterone concen­
trations in part related to but also independent of obesity and 
age.553,564,565 Adequate treatment with continuous positive airway 
pressure (CPAP) may improve symptoms (e.g., erectile function) 
and reverse biochemical secondary hypogonadism in some but not 
all men. Treatment of

ypogonadism in some but not 
all men. Treatment of hypogonadism with relatively high doses of 
testosterone (e.g., doses associated with the use of parenteral tes­
tosterone esters) may induce or worsen obstructive sleep apnea in 
men with predisposing conditions such as obesity and in older 
men.553,566-570 Testosterone replacement therapy of obese men 
with obstructive sleep apnea that produce more physiologic testos­
terone concentrations has been demonstrated to improve sexual 
desire withou

been demonstrated to improve sexual 
desire without inducing or worsening obstructive sleep apnea.571 
Acute and Critical Illness. Acute and critical illnesses, includ­
ing medical and surgical illnesses requiring hospital or intensive 
care unit admission (e.g., myocardial infarction, respiratory ill­
ness, sepsis, burns, surgery, polytrauma, stroke, traumatic brain 
injury, liver disease, fractures), suppress gonadotropin and testos­
terone secretion as a result of combined primary and seconda

retion as a result of combined primary and secondary 
testicular dysfunction.572-575 However, the predominant hormone 
pattern during acute or critical illness is low serum testosterone 
with low or low-normal gonadotropin concentrations, suggesting 
secondary hypogonadism. Spontaneous LH pulse amplitude is 
reduced, but pulse frequency is maintained, and pulsatile GnRH 
administration only partially corrects secondary hypogonadism, 
underscoring the presence of concomitant pituitary and testicu

 the presence of concomitant pituitary and testicular 
defects.576 For unclear reasons, aromatization of testosterone to 
estradiol and serum concentrations of estradiol may be increased, 
sometimes markedly, in patients with acute or critical illness 
despite low testosterone concentrations.577 Elevated estradiol con­
centrations are associated with fatality in critically ill and injured 
patients.578-581 A caveat regarding these studies is that estradiol 
was measured by immunoassay, not by st

 estradiol 
was measured by immunoassay, not by state-of-the-art mass spec­
trometry-based assays that are more accurate, sensitive, and spe­
cific than immunoassays for estradiol measurements in men.582
The severity and duration of testosterone suppression are 
related to the severity of the acute or critical illness, the presence 
of underlying chronic systemic illnesses, age, and the medications 
used (e.g., glucocorticoids, opioids).572,573 Recovery of testos­
terone and gonadotropin concent

ecovery of testos­
terone and gonadotropin concentrations may take several weeks 
to months, depending on the severity and duration of the acute 
illness, duration of subacute recovery and rehabilitation, com­
plications including malnutrition, medications, and underlying 
chronic systemic illnesses or organ failure. In the presence of 
underlying chronic disease or organ failure, hypogonadism may 
persist long after recovery from the acute illness. For these reasons, 
evaluation for underlying 

ss. For these reasons, 
evaluation for underlying hypogonadism should not be performed 
during acute or subacute illness and recovery. Assessment should 
be delayed for several months until recovery to the individual’s 
baseline or near-baseline clinical condition has occurred. 
Aging. As discussed earlier, aging is associated with a gradual 
and progressive decline in total and free testosterone concentra­
tions; as a result, an increasing proportion of older men have low 
serum testosterone co

rtion of older men have low 
serum testosterone concentrations in the hypogonadal range.312-
314 The prevalence of clinical androgen deficiency is 6% to 9% 
and increases with age, reaching 18% to 23% among men in 
their 70s.196,197 Serum gonadotropin concentrations increase with 
aging but do not usually rise above the normal range until very 
old age, usually beyond 70 years of age.314 Therefore, the most 
common hormonal profile observed in clinically hypogonadal 
middle-aged to older men is 

inically hypogonadal 
middle-aged to older men is low testosterone with normal LH and 
FSH concentrations, indicative of secondary hypogonadism. Pul­
satile LH secretion is abnormal and is characterized by disorderly 
LH pulses of reduced amplitude; it is normalized by exogenous 
pulsatile GnRH administration, suggesting a defect in hypotha­
lamic GnRH secretion.316
The presence of obesity, chronic systemic illness or organ fail­
ure, medications, or malnutrition or wasting syndromes that occur 

, or malnutrition or wasting syndromes that occur 
more frequently with increasing age may contribute to suppres­
sion of gonadotropin and testosterone production in middle-aged 
to older men. Conversely, the age-related decline in testosterone 
may also contribute to the susceptibility and severity of clinical 
hypogonadism that occur in these conditions.200,318 Many of the 
comorbidities that contribute to low testosterone concentrations in 
older men are potentially reversible or treatable (f

der men are potentially reversible or treatable (functional hypo­
gonadism, see later discussion). Therefore, management of under­
lying causes of functional causes of hypogonadism and symptoms 
androgen deficiency in older men should be utilized as an initial or 
adjunctive approach to testosterone replacement therapy.583
Previous relatively small, short-term studies of testosterone 
treatment in heterogeneous populations of older men have pro­
duced conflicting results, with most finding benef

duced conflicting results, with most finding beneficial effects of 
testosterone treatment on body composition (increasing lean 
mass and decreasing fat mass) but less consistent effects on muscle 
strength and performance, BMD, sexual function, vitality, and 
cognitive function.
Recently, results of the Testosterone Trials, a coordinated set 
of seven double-blind, placebo-controlled studies that evaluated 
the short-term effects of testosterone treatment in 788 older men 
(average age of 72 ye

 treatment in 788 older men 
(average age of 72 years old) with unequivocal hypogonadism 
(symptoms and signs of low libido; difficulty walking and slow 
walking speed; and/or low energy; and average of two morning 
serum total testosterone concentrations <275 ng/dL) with no 
apparent cause of hypothalamic-pituitary-gonadal disease. Com­
pared to placebo, testosterone treatment for one year that increased 
and maintained testosterone concentrations to the mid-normal 
range for young men signific

ns to the mid-normal 
range for young men significantly improved sexual function (sex­
ual activity, libido and erectile dysfunction), anemia and hemo­
globin, volumetric BMD and estimated bone strength; slightly 
improved walking distance, mood and depressive symptoms; and 
did not improve multiple domains of cognitive function and vital­
ity. There has been controversy about the clinical importance of 
these findings. In addition, testosterone treatment caused eryth­
rocytosis (hemoglobin conc

treatment caused eryth­
rocytosis (hemoglobin concentration >17.5 g/dL) and an increase 
in PSA concentration >1.0 ng/mL in small number of men; and 
increased noncalcified coronary artery plaque volume on CT angi­
ography. There was no difference in LUTS or cardiovascular or 
prostate adverse events in testosterone- and placebo-treated men 
(although the Testosterone Trials were underpowered for the latter 
outcomes).162,319-323
the 

m, randomized trials are needed to determine 
the balance of clinical benefits and risks (particularly prostate 
cancer and cardiovascular risks) associated with testosterone treat­
ment in older men. For now, testosterone treatment should be 
considered on an individual basis only for older men who have 
clinically significant symptoms and signs of androgen deficiency 
and unequivocally low serum testosterone concentrations, and 
only after a careful discussion of the uncertainty regarding the 

reful discussion of the uncertainty regarding the 
benefits and risks of treatment.116 In addition, in older men, 
there should be consideration of treatment of underlying comor­
bidities and discontinuing medications that may cause functional 
hypogonadism. 
738
Congenital Adrenal Hyperplasia. If untreated or inadequately 
treated with glucocorticoids, congenita

dequately 
treated with glucocorticoids, congenital adrenal hyperplasia caused 
by deficiency of 21-hydroxlase or 11β-hydroxylase results in exces­
sive secretion of adrenal androgen precursors (including andro­
stenedione, DHEA, and 11-hydroxyandrostenedione) that are 
converted to testosterone and other androgens (e.g., 11-ketotes­
tosterone). Elevated circulating androgen concentrations suppress 
gonadotropin secretion by negative feedback regulation, which in 
turn decreases endogenous testo

ulation, which in 
turn decreases endogenous testosterone secretion and sperm pro­
duction, resulting in secondary hypogonadism. Excessive adrenal 
androgen production may offset testicular androgen deficiency, 
so secondary hypogonadism may be manifested by an isolated 
impairment of sperm production and function.584
Glucocorticoid therapy at dosages that suppresses serum 
ACTH concentrations to normal or low levels reduces exces­
sive adrenal androgen production and may restore gonadotropin 
s

ndrogen production and may restore gonadotropin 
secretion and may normalize testis function, including spermato­
genesis. As discussed previously, supraphysiologic glucocorticoid 
treatment is usually required to reduce adrenal androgen concen­
trations to normal, and the high dosages of glucocorticoids sup­
press the hypothalamic-pituitary-testicular axis and result in low 
gonadotropin, testosterone, and sperm concentrations in many 
men with congenital adrenal hyperplasia. In addition, some 

congenital adrenal hyperplasia. In addition, some men 
with congenital adrenal hyperplasia continue to manifest impaired 
sperm production and function due to irreversible testicular dam­
age caused by large TARTs. 
Isolated FSH Deficiency and FSHβ Mutations. Rare cases of men 
with isolated FSH deficiency in the absence of FSHβ gene mutations 
have been reported; these patients had isolated impairment in sperm 
production characterized by azoospermia or severe oligozoosper­
mia, and hypospermat

ermia or severe oligozoosper­
mia, and hypospermatogenesis or maturation arrest was found in 
the few who underwent testis biopsy.400-402 These men had normal 
virilization, normal concentrations of testosterone and LH, low to 
undetectable serum FSH concentrations with poor or no response 
to GnRH administration, normal LH, and normal inhibin B and 
activin A concentrations when tested. In one man, administration 
of recombinant human FSH (rhFSH) alone resulted in a robust 
increase in sperm co

) alone resulted in a robust 
increase in sperm counts and induced fertility on two occasions.
generally to have azoospermia with undetectable FSH, low or low-
normal testosterone, and high LH concentrations.82-85,405,406 In 
one man, rhFSH administration was demonstrated to increase tes­
tosterone concentrations, suggesting that FSH-stimulated Sertoli 
cells may enhance LH-induced Leydig cell production of testos­
terone via a paracrine m

ell production of testos­
terone via a paracrine mechanism.585 
Androgen Administration or Excess. Exogenous testosterone 
administration (in normal men or in men with partial hypogo­
nadism)586 or stimulation of endogenous testosterone production 
by hCG administration587 or ectopic hCG-secreting tumors (e.g., 
testis cancer, lung cancer)588 suppresses pituitary gonadotropin 
secretion by negative feedback regulation; this in turn suppresses 
spermatogenesis by the testes in 

turn suppresses 
spermatogenesis by the testes in the presence of normal or high 
serum testosterone concentrations (i.e., secondary hypogonadism 
with isolated impairment of sperm production).589,590 Use of some 
androgenic anabolic steroids (e.g., nandrolone) may also produce 
low gonadotropin concentrations and isolated reduction in sper­
matogenesis while providing sufficient androgen activity to avoid 
clinical androgen deficiency; however, endogenous testosterone 
production is usually als

endogenous testosterone 
production is usually also suppressed by androgenic anabolic ste­
roid use, resulting in low serum testosterone concentrations.
Discontinuation of androgen or hCG administration results in 
restoration of normal gonadotropin secretion and recovery of nor­
mal sperm production and testosterone production by the testis. 
Long-term anabolic steroid abuse in athletes has been reported to 
be associated with testicular atrophy and severe oligozoospermia or 
azoospermia that p

 and severe oligozoospermia or 
azoospermia that persists for several months to years after discontin­
uation of the performance-enhancing agents.445 Suppressed sperm 
production induced by anabolic steroids may respond to treatment 
with hCG or with clomiphene citrate (off-label use).447,591 Admin­
istration of testosterone in combination with progestins in normal 
men has been the main strategy used to suppress sperm production 
in recent hormonal male contraceptive development trials.105 
Mal

nal male contraceptive development trials.105 
Malignancy. Malignancies that occur commonly in men of 
reproductive age (e.g., testicular cancer, Hodgkin disease) mani­
fest with impaired sperm production and function before chemo­
therapy or radiation therapy in 30% to 80% of cases.343-345,364 
Among men with cancer who provided semen samples for cryo­
preservation before treatment, approximately 64% had abnormal 
semen parameters, and 12% had no viable sperm.592
In population studies, testicul

o viable sperm.592
In population studies, testicular cancer is associated with infer­
tility. This association may reflect abnormal testicular develop­
ment, termed testicular dysgenesis syndrome, caused by exposure to 
environmental gonadotoxins or endocrine disruptors (e.g., estro­
gens) or by an underlying genetic predisposition.343,364,593 Tes­
ticular dysgenesis syndrome is also associated with cryptorchidism 
and hypospadias, and the former is associated with an increased 
risk of testicul

 is associated with an increased 
risk of testicular cancer and abnormal spermatogenesis. Ectopic 
hCG secretion and possibly increased scrotal temperature associ­
ated with cancer within the testis may also contribute to impaired 
spermatogenesis in men with testicular cancer.593 Hodgkin disease 
and other lymphomas and leukemias may be associated with fever, 
weight loss, and systemic inflammation.593 These cancers may also 
involve the testis. All of these factors may play a role in impair­
i

 All of these factors may play a role in impair­
ing sperm production. Men with these cancers who have systemic 
disease, symptoms, or inflammation may present with low-nor­
mal testosterone, suppressed gonadotropin concentrations, and 
abnormal semen analysis, consistent with secondary hypogonad­
ism causing isolated impairment of sperm production or function. 
Hyperprolactinemia. Men with severe hyperprolactinemia 
(e.g., prolactin concentrations >200 ng/mL) develop secondary 
hypogonadism cau

ns >200 ng/mL) develop secondary 
hypogonadism causing androgen deficiency and impaired sperm 
production. Mild hyperprolactinemia may be associated with iso­
lated impairment of sperm production.594 In most of these cases, 
gonadotropin and testosterone concentrations are normal, and 
spermatogenesis is not improved with dopamine agonist treat­
ment.595 Therefore, hyperprolactinemia does not contribute to 
impairment in sperm production and probably is not clinically 
significant in most cases.

ably is not clinically 
significant in most cases. In most instances, abnormal sperm pro­
duction and function are caused by a primary testicular disorder, 
such as idiopathic oligozoospermia or azoospermia. Rarely, some 
men with moderate hyperprolactinemia (e.g., prolactin concen­
trations of 100–200 ng/mL) have low-normal testosterone and 
gonadotropin concentrations and isolated impairment of sperm 
production that respond to dopamine agonist treatment.594 
Androgen Resistance Syndromes
Cong

 treatment.594 
Congenital Disorders
Congenital androgen resistance and insensitivity syndromes are 
usually caused by defects in androgen action due to mutations 
739
in the AR gene or in the steroid 5α-reductase type 2 gene, 
SRD5A2.125,136,596,597 Males with severe defects in androgen 
action present at birth either as phenotypic females, as occurs with 
complete androgen insensitivity syndrome (CAIS, previously known 
as testicul

vity syndrome (CAIS, previously known 
as testicular feminization syndrome), or as males with ambiguous 
genitalia and 46,XY DSD (previously termed male pseudohermaph­
roditism).597 Individuals with partial androgen insensitivity syn­
drome (PAIS) present with varying degrees of impaired androgen 
action and mildly to severely disordered male sexual development.
CAIS usually presents as a 46,XY phenotypic female with nor­
mal breast development, primary amenorrhea, and absence of 
body hair.597 

primary amenorrhea, and absence of 
body hair.597 Severe androgen insensitivity results in absence of 
facial, axillary, and pubic hair; normal-appearing female external 
genitalia and distal two thirds of the vagina; and poorly devel­
oped or absent male internal genitalia (prostate, epididymides, 
seminal vesicles, and vasa deferentia) as a result of fetal andro­
gen resistance. In adults with CAIS, female breast development 
is present due to conversion of normal to high concentrations of 
te

conversion of normal to high concentrations of 
testosterone (which are secreted by the testes at puberty) to estra­
diol, which stimulates breast development. Because testes that are 
intra-abdominal or inguinal in location at birth secrete AMH nor­
mally during fetal development, female internal genitalia (proxi­
mal vagina, uterus, and fallopian tubes) are absent.
There is considerable variation in the presentation of individu­
als with PAIS.597 Some of these males present at birth with ambig

97 Some of these males present at birth with ambig­
uous genitalia, whereas others present at puberty or in adulthood 
with mild genital abnormalities or relatively normal genital 
development. Clinical manifestations of androgen deficiency and 
disordered male sexual development range from severe undervir­
ilization to near-normal virilization with infertility. In men with 
PAIS, manifestations that are indicative of disordered sexual devel­
opment, such as microphallus, hypospadias, scrotal ab

ent, such as microphallus, hypospadias, scrotal abnormalities 
(e.g., bifid scrotum), cryptorchidism, and gynecomastia, are com­
mon. Gynecomastia is present in almost all of these individuals.
As a result of the variability in clinical manifestations, PAIS 
encompasses many disorders, previously referred to as Reifenstein, 
Lubs, Rosewater, and Gilbert-Dreyfus syndromes. For example, 
Reifenstein syndrome is characterized by hypospadias, gyneco­
mastia, undervirilization, a small prostate gland

mastia, undervirilization, a small prostate gland, cryptorchidism, 
and impaired spermatogenesis. However, even in the same fam­
ily, different members may have different clinical manifestations. 
Some family members may not have hypospadias, and others may 
be normally virilized. Given this degree of variability in manifesta­
tions, the older eponyms are not clinically useful.597
Some men with PAIS have no evidence of disordered male sex­
ual development and present with isolated impairment of

evelopment and present with isolated impairment of sperm 
(idiopathic oligozoospermia or azoospermia), occasionally in 
association with gynecomastia, high to high-normal testosterone 
concentrations, and elevated LH concentrations. This disorder is 
referred to as minimal AIS.136,596
In CAIS and PAIS, serum testosterone concentrations are 
high or high normal with high serum LH concentrations, but 
FSH concentrations are usually normal. Most 46,XY individuals 
with CAIS have autosomal recessive

XY individuals 
with CAIS have autosomal recessive mutations of the AR gene 
on the X chromosome that alter its primary sequence and struc­
ture (almost always resulting in CAIS) or its function, resulting 
in impaired androgen binding to AR, AR binding to DNA, or 
AR transactivation.136,596 In men with PAIS, the correlation of 
AR genotype and clinical phenotype is relatively poor. In many 
men with PAIS or minimal AIS, no mutations in AR are identi­
fiable. These men may have high CAG repeat l

enti­
fiable. These men may have high CAG repeat length in the AR 
gene, misdiagnosis with unrecognized mutations of other genes 
such as SRD5A2, or mutations of coactivators or corepressors 
that regulate AR function.
An increase in the number of trinucleotide CAG repeats in 
the first exon of the AR gene results in expansion of the polyglu­
tamine tract in the N-terminal domain of the AR.139 The CAG 
repeat length is inversely correlated with AR function and action. 
Pathologic increase to mor

R function and action. 
Pathologic increase to more than 40 to 62 CAG repeats (normal 
range, 11–35) causes Kennedy disease (spinal and bulbar mus­
cular atrophy), a rare neurodegenerative disease that is thought 
to be caused by neurotoxicity from intracellular aggregation of 
the abnormal AR and associated coregulator proteins (see earlier 
discussion).137,138 Men with Kennedy disease have clinical mani­
festations of partial androgen resistance, including gynecomastia, 
sexual dysfunction, ol

e, including gynecomastia, 
sexual dysfunction, oligozoospermia or azoospermia, and infertil­
ity associated with high testosterone and high or normal gonado­
tropin concentrations. Higher CAG repeat numbers within the 
normal range have been reported to be associated with decreased 
virilization, impaired spermatogenesis and infertility, and gyneco­
mastia in some studies but not others.139
5α-Reductase deficiency, caused by an autosomal recessive 
mutation in SRD5A2, is a rare condition simila

ve 
mutation in SRD5A2, is a rare condition similar to PAIS.598 
Affected individuals typically present with markedly ambiguous 
genitalia, usually characterized by a clitoris-like phallus, a severely 
bifid scrotum, an apparent vaginal opening with perineal and 
scrotal hypospadias (termed pseudovaginal perineoscrotal hypospa­
dias), an atrophic prostate, and testes located in the inguinal canal 
or scrotum or sometimes intra-abdominally (cryptorchidism). In 
contrast to men with AISs, wolffian

rchidism). In 
contrast to men with AISs, wolffian duct differentiation is unaf­
fected, and patients with 5α-reductase deficiency have normal 
male internal genitalia (epididymides, seminal vesicles, ejacula­
tory ducts, and vasa deferentia). AMH secretion by the fetal testes 
causes regression of müllerian duct structures, so no female inter­
nal genitalia develop.
Because ambiguous genitalia resemble female more than male 
external genitalia, individuals with 5α-reductase deficiency are 
usua

individuals with 5α-reductase deficiency are 
usually raised as females.125 However, with the marked increase 
in testosterone production by the testes at puberty, partial viril­
ization occurs (i.e., penile growth, rugation and pigmentation of 
the scrotum, increased muscle mass and height, deepening of the 
voice, increase in libido, and spontaneous erections), and some of 
these individuals take on a male gender role at puberty, depending 
on complex psychosocial factors and cultural backgrou

complex psychosocial factors and cultural background. Sebum 
production is normal in these men. Because normal androgen 
action in the skin and prostate requires conversion of testoster­
one to DHT by 5α-reductase, men with 5α-reductase deficiency 
do not develop a male body hair pattern, and the prostate gland 
remains nonpalpable. Prostate cancer and BPH have not been 
reported in these men.
Men with 5α-reductase deficiency have high-normal to high 
serum testosterone and normal to slightly el

high 
serum testosterone and normal to slightly elevated LH and FSH 
concentrations (LH > FSH). Serum DHT concentrations are low, 
and the serum testosterone/DHT ratio is variably elevated. Sper­
matogenesis is impaired (oligozoospermia or azoospermia) as a 
result of cryptorchidism, but normal sperm production has been 
reported in some men with descended testes.599
Individuals with CAIS are usually raised as females and 
undergo orchidectomy (particularly if the testes are intra-abdom­
inal) a

articularly if the testes are intra-abdom­
inal) and treatment with estrogen replacement; more recently, 
testosterone therapy for reduced sexual functioning has been 
shown to be more effective than estrogen.597,600 In men with 
PAIS or 5α-reductase deficiency, virilization has been induced 
with high-dose testosterone treatment, which increases serum 
740
testosterone concentrations to above the normal range and nor­
malizes DHT concentrations.597 

al range and nor­
AR antagonists (flutamide, bicalutamide, and nilutamide) induce 
androgen resistance and are used to treat androgen-dependent 
prostate cancer.601 Drugs such as spironolactone, cyproterone ace­
tate, marijuana, and H2 receptor antagonists (specifically cimeti­
dine) have AR antagonist activity.602-606
Men with celiac disease or gluten-sensitive enteropathy may 
experience manifestations of androgen deficiency, including 
reduce

stations of androgen deficiency, including 
reduced virilization, sexual dysfunction, impaired sperm produc­
tion and function, and infertility. They may also demonstrate 
high to high-normal concentrations of serum total and free tes­
tosterone and high LH concentrations, indicative of androgen 
resistance.607-610 Manifestations of androgen deficiency and bio­
chemical androgen resistance may improve with dietary gluten 
restriction and improvement in small bowel atrophy in some 
men.609 Malnut

ent in small bowel atrophy in some 
men.609 Malnutrition, nutritional deficiencies, chronic systemic 
illness, and hyperprolactinemia may occur in men with celiac dis­
ease and may contribute to their clinical manifestations. Serum 
DHT concentrations may be low despite high testosterone con­
centrations in men with celiac disease, suggesting that partial 
acquired 5α-reductase deficiency may also be present and may 
play a role in androgen resistance. The main source of circulating 
DHT is from

tance. The main source of circulating 
DHT is from conversion of testosterone to DHT by 5α-reductase 
type 1 in the skin and liver.608 However, 5α-reductase is also pres­
ent in the gastrointestinal tract, so it is possible that loss of enzyme 
activity in the small bowel with active celiac sprue contributes to 
low DHT concentrations. 
Treatment of Androgen Deficiency
Functional Versus Organic Causes of Hypogonadism
Prior to initiating testosterone replacement therapy, it is important 
to consi

one replacement therapy, it is important 
to consider whether the cause of hypogonadism is functional or 
organic.200 Organic hypogonadism is caused by congenital/develop­
mental, destructive, or infiltrative disorders of the hypothalamus, 
pituitary gland, or testes that result in permanent hypogonad­
ism. Generally, organic hypogonadism presents with clinically 
unequivocal severe androgen deficiency (also referred to as clas­
sical hypogonadism). Most causes of primary hypogonadism and 
some 

m). Most causes of primary hypogonadism and 
some causes of secondary hypogonadism are organic (see Tables 
19.7 and 19.8). Functional hypogonadism is caused by nondestruc­
tive suppression of hypothalamic, pituitary, or, uncommonly, 
testis function that is potentially reversible without medical or 
surgical therapy. Drug-induced primary hypogonadism is a func­
tional cause. Many causes of secondary and combined primary 
and secondary hypogonadism are due to functional gonadotropin 

sm are due to functional gonadotropin 
suppression.
Management or treatment of functional causes of hypogonad­
ism might improve or resolve clinical and biochemical androgen 
deficiency and should be considered before initiating testoster­
one replacement therapy. For example, functional hypogonadism 
caused by hyperprolactinemia may be treated by discontinuation 
of medications that cause hyperprolactinemia or dopamine ago­
nist therapy; opioids, glucocorticoids, CNS-active medications, or 
pro

, glucocorticoids, CNS-active medications, or 
progestins may be reversed by discontinuation of the offending 
drug; nutritional deficiency may be corrected by nutritional sup­
plementation and weight gain; morbid obesity may be improved 
with diet-induced weight reduction or bariatric surgery; obstruc­
tive sleep apnea may be improved by CPAP therapy; T2DM may 
be improved by weight loss and reduction in insulin resistance; 
and alcohol abuse may be improved with treatment of alcohol 
dependenc

y be improved with treatment of alcohol 
dependence and abstinence. In many instances, however, func­
tional causes of hypogonadism cannot be treated or managed in a 
reasonable time frame (e.g., opioid or glucocorticoid therapy for 
chronic comorbid illnesses), so testosterone treatment should be 
considered. 
Testosterone Replacement Therapy
Therapeutic Goals and Management
The overall goal of testosterone replacement therapy is to correct 
or improve the clinical manifestations of androgen de

improve the clinical manifestations of androgen deficiency in 
men with primary or secondary hypogonadism. Because specific 
manifestations vary with the stage of sexual development, the spe­
cific goals of testosterone treatment vary depending on whether 
the patient is an adolescent or an adult.6,116
In boys with androgen deficiency of prepubertal onset and 
delayed puberty, the goals of testosterone treatment are the 
following146,147,374,375:
 
• To induce and maintain secondary sexual char

eal enlargement and deepening of the voice
 
• To increase red blood cell production into the normal adult 
male range.
Testosterone treatment also stimulates the growth of accessory 
sex glands (seminal vesicles and prostate), resulting in seminal 
fluid production and an increase in ejaculate volume, but it does 
not stimulate sperm production to a degree sufficient for induc­
tion of fertility. The most common cause of delayed puberty is not 
a pathologic condition but rather CDGP. Testoster

a pathologic condition but rather CDGP. Testosterone therapy in 
boys who present with delayed puberty utilizes low-dose testos­
terone to avoid premature epiphyseal closure and compromise of 
adult height, and treatment is given intermittently until spontane­
ous puberty occurs (see later discussion). If spontaneous puberty 
does not occur, the testosterone dose is increased gradually to 
adult concentrations.146,147,374,375
The goals of testosterone therapy in adult hypogonadism are the 
foll

restore male hair growth.
Recent-onset gynecomastia that is usually symptomatic may 
respond to testosterone treatment, but severe or long-standing 
gynecomastia requires surgical excision. Spermatogenesis requires 
relatively high intratesticular concentrations of testosterone that 
cannot be achieved by exogenous androgen administration. There­
fore, testosterone replacement therapy does not stimulate sperm 
production or increase testis size, nor does it restore fertility. Treat­
ment of infe

nor does it restore fertility. Treat­
ment of infertility in hypogonadal men is usually possible only 
in those men with secondary hypogonadism and gonadotropin 
741
deficiency; gonadotropin or GnRH therapy is used to induce sper­
matogenesis and fertility.611
The normal adult range of serum testosterone concentrations is 
broad and is usually based on results in healthy young men using 
blood samples drawn in the morning. In young men with androgen 
deficiency, 

 morning. In young men with androgen 
deficiency, testosterone replacement therapy produces beneficial 
clinical effects as serum testosterone concentrations are increased 
into this normal range. Serum testosterone concentrations decline 
gradually and progressively with age, but the physiologic signifi­
cance of this age-related decline is unclear. Initial studies in older 
men with low serum testosterone concentrations demonstrated 
some clinical beneficial effects with testosterone treatment

cal beneficial effects with testosterone treatment that 
increased testosterone concentrations into the normal young adult 
range.322,612 Therefore, the goal of testosterone treatment of hypo­
gonadism is to restore serum testosterone concentrations to within 
the normal adult range, irrespective of age.116
The dose-response effects of testosterone vary in different tar­
get organs and for different clinical outcomes.613 For example, 
the action of testosterone on muscle mass demonstrates a con­

of testosterone on muscle mass demonstrates a con­
tinuous dose-response relationship. With testosterone administra­
tion, muscle mass increases when testosterone concentrations are 
increased from below normal to within the normal range, and it 
continues to increase as concentrations are raised from within to 
above the normal range. In contrast, the actions of testosterone on 
libido exhibit threshold dose-response characteristics: testosterone 
administration increases libido when serum test

e 
administration increases libido when serum testosterone concen­
trations are increased from low to low-normal concentrations but 
does not continue to stimulate libido further as serum testosterone 
is increased to normal or supraphysiologic concentrations.
In men with severe, long-standing androgen deficiency, testos­
terone replacement therapy induces profound alterations in sexu­
ality, behavior, and physical appearance that may be upsetting to 
patients and their partners and may result i

g to 
patients and their partners and may result in serious adjustment 
problems. To reduce the likelihood of problems, it is important to 
inform and counsel hypogonadal men and their partners regard­
ing changes in body characteristics and behavior that are expected 
during testosterone replacement therapy. In some men with 
severe, long-standing hypogonadism, initiation of testosterone 
replacement with a low-dose regimen (e.g., testosterone enanthate 
or cypionate 100 mg every 2 weeks, testo

nanthate 
or cypionate 100 mg every 2 weeks, testosterone patch 2 mg daily, 
or 1.62% testosterone gel 20.25 mg daily) for several months, fol­
lowed by an increase to full testosterone replacement, may pro­
duce a more gradual symptomatic transition from hypogonadism 
to eugonadism and may result in fewer adjustment difficulties.614
Because the metabolic clearance rate of testosterone is reduced 
in older men with hypogonadism, therapeutic testosterone con­
centrations may be achieved with lowe

sterone con­
centrations may be achieved with lower dosages of testosterone.615 
In some clinical situations, such as severe symptomatic BPH or 
the presence of numerous comorbid illnesses, full testosterone 
replacement may be ill advised. In these instances, low-dose tes­
tosterone supplementation may also be more prudent than full 
testosterone replacement therapy. Low dosages of testosterone 
may be sufficient to induce some beneficial effects while minimiz­
ing the potential worsening of LU

s while minimiz­
ing the potential worsening of LUTS (although this has not been 
demonstrated as an adverse effect of testosterone therapy).
The potential effectiveness of low-dose testosterone supple­
mentation is suggested by studies of short-acting testosterone 
formulations (e.g., oral testosterone undecanoate and sublingual 
testosterone cyclodextrin) that produced anabolic effects despite 
serum testosterone concentrations that were not sustained within 
the normal range.616,617 An analog

stained within 
the normal range.616,617 An analogy may be made to the use of 
hydrocortisone for glucocorticoid replacement therapy, in which 
the duration of biologic action of hydrocortisone on tissues is not 
reflected by its serum concentrations. Short-term administration 
of low-dose testosterone enanthate (50 mg/week IM) was found 
to increase muscle strength and power in some young men in 
whom hypogonadism was induced by concomitant GnRH ago­
nist treatment.618
Hypogonadism due to gonad

 ago­
Hypogonadism due to gonadotropin deficiency may be caused 
by hypothalamic-pituitary disease that requires specific manage­
ment in addition to testosterone replacement. Therefore, careful 
evaluation to determine the cause of secondary hypogonadism 
should be performed before testosterone treatment is started. For 
example, pituitary or hypothalamic tumors may cause mass effects 
such as visual field defects, or they may be associated with defi­
ciency or excessive secr

 be associated with defi­
ciency or excessive secretion of other pituitary hormones. These 
tumors may require surgery or radiation therapy, additional hor­
monal replacement, medical therapy, or some combination of 
these treatments to reduce excessive pituitary hormone secretion. 
In some cases, treatment of the underlying cause of secondary 
hypogonadism corrects the androgen deficiency (e.g., stopping 
a medication that causes hyperprolactinemia or gonadotropin 
deficiency). In men with gona

mia or gonadotropin 
deficiency). In men with gonadotropin deficiency and otherwise 
normal testes who are interested in fathering children, gonadotro­
pin therapy may be used instead of testosterone replacement to 
stimulate sperm production, restore fertility, and correct andro­
gen deficiency. Similarly, men with secondary hypogonadism due 
to hypothalamic disease may be treated with pulsatile GnRH to 
stimulate testosterone and sperm production and restore fertility.
A comprehensive clinical

on and restore fertility.
A comprehensive clinical approach is important for optimal 
management of hypogonadism. It is important to consider causes 
other than androgen deficiency that might contribute to symptoms 
and signs, and to manage them appropriately. In hypogonadal 
men who complain primarily of sexual dysfunction, an underly­
ing neurovascular disease or use of certain medications is usually 
the major cause of erectile dysfunction. In these men, testoster­
one treatment alone is insu

 these men, testoster­
one treatment alone is insufficient to completely restore erections 
and permit satisfactory sexual intercourse. Additional treatment 
with a PDE5 inhibitor (sildenafil, vardenafil, or tadalafil),161,619 
intracavernosal or intraurethral alprostadil (MUSE), intracaverno­
sal alprostadil/papaverine/phentolamine (tri-mix), penile vacuum 
device, or penile prosthesis might be needed for a satisfactory 
clinical outcome. In hypogonadal men who present with osteopo­
rosis, it i

ogonadal men who present with osteopo­
rosis, it is critical to perform a thorough evaluation for other com­
mon causes of bone loss (e.g., vitamin D deficiency, alcohol abuse, 
smoking, medications, inactivity, primary hyperparathyroidism) 
and to treat them as well. It is also important to institute measures 
to prevent falls to reduce the risk of fractures. 
Testosterone Formulations
Testosterone formulations that are used to treat male hypogonad­
ism are summarized in Table 19.10.6,116 In th

nad­
ism are summarized in Table 19.10.6,116 In the United States, 
approved formulations include parenteral testosterone esters that 
are administered by long- and short-acting IM injection, transder­
mal testosterone patch and testosterone gels or solutions, a trans­
buccal testosterone tablet, and an intranasal testosterone gel.
Oral 17α-alkylated testosterone derivatives, such as methyltes­
tosterone and fluoxymesterone, should not be used for testoster­
one replacement therapy.6 It is diffi

r testoster­
one replacement therapy.6 It is difficult to achieve full androgen 
replacement with these oral formulations because they are weak 
androgens that have low bioavailability. They also have the poten­
tial for serious hepatotoxicity.620 17α-Alkylated androgens most 
commonly cause cholestasis that is reversible with discontinua­
tion. More concerning is the potential for these agents to cause 
742
  Treatment of Adult Male Hypogonadism
For

nction
Formulation
Dosage
Formulations Available in the United States
Parenteral Testosterone Esters
Testosterone enanthate 
or cypionate, IM 
injections
Adults: 150–200 mg IM every 2 wk or 
75–100 mg IM every wk
50 mg, 75 mg, or 100 mg SC by autoinjector
Prepubertal boys: 50–100 mg monthly or 
25–50 mg every 2 wk, increasing to 
50–100 mg every 2 wk and then to adult 
replacement dosage over 2–4 yr

 and then to adult 
replacement dosage over 2–4 yr or until 
spontaneous pubertal development occurs
Extensive clinical use
Inexpensive with self-injection
Some dose flexibility
IM injections, discomfort
Symptomatic fluctuation of T concentrations 
(supraphysiologic after injection to low 
normal or low before next injection)
Frequent IM injections to reduce fluctuations 
of T concentrations
More erythrocytosis than with transdermal T
Testosterone 
undecanoate, IM 
750 mg at 0 and 4 w

e 
750 mg at 0 and 4 wk, then every 10 wk
Less frequent IM injections
Maintenance of normal T concentra­
tions for a longer duration
No apparent fluctuations in symptoms
REMS: Slow, deep IM injection in clinic (no 
self-injection); 30-min observation for 
potential POME and anaphylaxis
Large-volume injection (3 mL)
Self-injection not possible
Rarely, cough immediately after injection
Prolonged maintenance of T level after dis­
continuation if

 maintenance of T level after dis­
continuation if adverse effects develop
Transdermal Testosterone
Testosterone patch 
(nonscrotal)
2 or 4 mg (one patch) or 6 mg (one 2-mg 
plus one 4-mg patch) applied daily over 
nonpressure areas
Low- to mid-normal physiologic T 
Mimics normal circadian variation 
when applied nightly
No injections
Less erythrocytosis than with paren­
teral T
Rapid withdrawal of T replacement if 
adverse effects occur
Frequent skin irritation
Low-normal T conce

 occur
Low-normal T concentrations: two patches 
may be needed
Skin adhesion poor with excessive sweating
Daily application
More expensive than parenteral T
Testosterone gels and 
solution
1% T gel: 5–10 g of gel (containing 
50–100 mg of T) applied daily over 
shoulders or upper arms; available in foil 
sachets of 2.5 or 5.0 g (containing 25 or 
50 mg of T, respectively), or a tube of 5 g 
(containing 50 mg of T)
1.62% T gel: 20.25–81.00 mg (containing 
20.25–81.00 mg o

 gel: 20.25–81.00 mg (containing 
20.25–81.00 mg of T) applied daily to 
shoulders or upper arms; available in 
20.25- and 40.50-mg packets (contain­
ing 20.25 and 40.50 mg of T, respec­
tively) or metered-dose pump delivering 
12.5 mg per pump depression
2% gel: 40–70 mg (containing 40–70 mg 
of T) applied daily to inner thighs; avail­
able in a metered-dose pump delivering 
10 mg per pump depression
2% solution: 30–120 mg applied to under­
arms; available in a metered-dose pump 
applicator del

; available in a metered-dose pump 
applicator delivering 30 mg per pump 
depression
Low- to high-normal steady-state 
physiologic T concentrations
Little skin irritation
Dose flexibility
For 1.62% or 2% gel: less gel amount 
in more concentrated formulations
Less gel amount in more concen­
trated formulations
For 2 % solution: absorption of solu­
tion not affected by deodorant or 
antiperspirant
Absorption of solution not

rant or 
Absorption of solution not affected by 
deodorant or antiperspirant
Potential for contact transfer of T to women 
or children
More expensive than parenteral T, especially 
with higher doses
Moderately high DHT concentrations
One formulation has a musk odor and 
another is associated with stickiness or 
skin dryness
Slight skin irritation in some men
Solution may drip under arms
Transbuccal testos­
terone
30-mg tablet applied between cheek and 
gum two ti

gum two times daily
Mid-normal steady-state physiologic 
T concentrations
No injections, patch or gel application, 
or their associated disadvantages
Twice-daily application
Gum irritation or inflammation
Altered or bitter taste
High learning curve for proper applica­
tion; requires careful instruction or poor 
acceptability occurs
Tablets may be difficult to remove or may fall 
off prematurely
No

ifficult to remove or may fall 
No dose flexibility
TABLE 19.10
Dosage
Testosterone nasal gel
11 mg (delivering 1.1 mg of T) three times 
daily (every 6–8 hours) for a total daily 
dose of 33 mg (delivering 3.3 mg of T) 
daily
Available in a metered-dose pump that 
delivers 5.5 mg (delivering 0.55 mg of T) 
per pump depression
No 

55 mg of T) 
No interaction with sympathomimetic 
nasal decongestants
Thrice-daily administration
Learning curve for proper administration
Fluctuation in T concentrations from lower- to 
upper-normal range after administration
No nose blowing or sniffing for 1 hour after 
administration
Discontinue with severe rhinitis
Nasal irritation
Not recommended with other intranasal 
drugs or chronic nasal conditions
Testosterone pellets
2–6 pellets (each 3.2 mm diameter 

sterone pellets
2–6 pellets (each 3.2 mm diameter × 9 mm 
in length pellet containing 75 mg of T, 
for a total of 150–450 mg of T delivered) 
implanted SC every 3–6 mo (usually 
3–4 mo)
Requires surgical incision
Extrusion, bleeding, and infection can occur 
uncommonly
Large number of pellets
Not easily removed; fibrosis may occur
Lack of ability for rapid withdrawal of T 
replacement if adverse effects occur
Infrequent use
Testoster

 if adverse effects occur
Testosterone Formulations Available Outside the United States
Oral testosterone 
undecanoate
40–80 mg PO with meals twice a day to 
three times daily
Oral administration is convenient 
for many
Twice- or thrice-daily administration
Variable T concentrations and clinical responses
Requires administration with meal
High DHT concentrations
Testosterone-in-adhe­
sive matrix patch
Two patches (delivering 4.8 mg of T per 
day) applied every 2 days
Low- to mid-n

 of T per 
Duration 2 days
Some skin irritation
Two patches needed
Treatment to Initiate and Maintain Sperm Production in Men With Hypogonadotropic Hypogonadism
Initially to Stimulate Testosterone and Potentially Sperm Production
hCG
500–2000 IU given SC two to three times 
weekly to maintain serum T concen­
trations within the normal range for 
6–12 mo
Effective in stimulating endogenous T 
In me

tive in stimulating endogenous T 
In men with acquired and some men 
with partial congenital hypogo­
nadotropic hypogonadism, sperm 
production may be stimulated with 
hCG treatment alone
SC injections easier than IM injections 
(smaller needle, injection not as deep)
Less fluctuation in T concentrations 
compared with IM T ester injections
No injection, patch, or buccal tablet
Injections two to three times weekly
Expensive
Higher doses needed in men with con­
comitant primary testicu

s needed in men with con­
comitant primary testicular disease (e.g., 
cryptorchidism)
Breast tenderness or gynecomastia second­
ary to high estradiol production by testes
May require dilution
Occasional burning sensation with injection
Ineffective in primary hypogonadism
Added to hCG to Stimulate Sperm Production
FSH
Human menopausal 
gonadotropin, 
human FSH, or 
recombinant human 
FSH
After 6–12 mo of hCG treatment alone 
resulting in normal T concentrations, 
add FSH 75–300 IU given SC three 

concentrations, 
add FSH 75–300 IU given SC three times 
weekly for an additional 6–12 mo or 
longer
Effective in stimulating sperm produc­
tion in men with hypogonadotropic 
hypogonadism
Injections three times weekly
Extremely expensive, prohibitive cost for most
In men with concomitant primary testicular 
disease (e.g., cryptorchidism), stimulati

ticular 
disease (e.g., cryptorchidism), stimulation 
of spermatogenesis is not likely
To Stimulate Testosterone and Sperm Production
GnRH
5–25 ng/kg SC every 2 hr by programmable 
infusion pump for 6–12 mo
Effective in stimulating both endog­
enous T and sperm production
GnRH not readily available
Requires pump use and management, usu­
ally in a specialized center
Expensive
Infrequently used except at certain sites
Rarely, local irritation, infection
DHT, Dihydrotestosterone; FSH, follicle-stim

ction
DHT, Dihydrotestosterone; FSH, follicle-stimulating hormone; GnRH, gonadotropin-releasing hormone; hCG, human chorionic gonadotropin; IM, intramuscular; PO, orally; POME, potential pulmonary oil 
microembolism; REMS, Risk Evaluation and Mitigation Strategy; SC, subcutaneous; T, testosterone.
   
  Treatment of Adult Male Hypogonadism—cont’d
744
peliosis hepatis (blood-filled cysts in the liver) or benign or malig­
nant hepatic tumor

 the liver) or benign or malig­
nant hepatic tumors. 17α-Alkylated androgens also lower HDL 
cholesterol and raise LDL cholesterol, causing a proatherogenic 
lipid profile, and they are relatively expensive. Therefore, these 
oral androgens carry greater potential risks with few therapeutic 
benefits compared with other testosterone formulations, and they 
should not be used to treat male hypogonadism.
Parenteral Testosterone Esters. Relatively long-acting paren­
teral 17β-hydroxyl esters of tes

ong-acting paren­
teral 17β-hydroxyl esters of testosterone, testosterone enanthate and 
testosterone cypionate, are administered by IM injection. These are 
effective, safe, and relatively practical and inexpensive prepara­
tions that have been used for testosterone replacement in hypogo­
nadal men for decades. Transdermal testosterone gel formulations 
provide more physiologic testosterone concentrations and are now 
used more commonly than testosterone ester injections. However, 

tosterone ester injections. However, 
testosterone esters are preferred over transdermal formulations by 
some hypogonadal men because they are the least expensive formu­
lation available, require less frequent administration, and usually 
produce more consistent but higher average serum testosterone 
concentrations. Given proper instruction, most hypogonadal men 
(or a family member) can self-administer IM testosterone ester 
injections. Otherwise, testosterone injections need to be adminis­
te

se, testosterone injections need to be adminis­
tered in a clinic setting.
Esterification of testosterone at the 17β-hydroxyl group 
increases its hydrophobicity and solubility within an oil vehicle 
(sesame oil for testosterone enanthate, cottonseed oil for testoster­
one cypionate). After IM injection, testosterone esters are released 
slowly from the oil vehicle within muscle and hydrolyzed rapidly 
to testosterone, which is released into the circulation, resulting 
in relatively high peak se

circulation, resulting 
in relatively high peak serum testosterone concentrations but an 
extended duration of release. Testosterone enanthate and testoster­
one cypionate have similar pharmacokinetic profiles, duration of 
action, and therapeutic efficacy, so they are considered therapeuti­
cally equivalent.621,622
In adults with hypogonadism, the usual starting dose of tes­
tosterone enanthate or cypionate is 150 mg to 200 mg via IM 
injection every 2 weeks. After IM administration of 200 mg o

every 2 weeks. After IM administration of 200 mg of 
testosterone enanthate, serum testosterone concentrations usu­
ally rise above the normal range for 1 to 3 days and then decline 
gradually over 2 weeks to the lower end of the normal range, or 
sometimes to below-normal concentrations, before the next injec­
tion623 (Fig. 19.28). The extreme rise and fall of serum testoster­
one concentrations may cause fluctuations in energy, mood, and 
libido that are disturbing to some men. Shortening the 

o that are disturbing to some men. Shortening the dosing 
interval to every 10 days and reducing the dose to 150 mg (i.e., 
150-mg IM injection every 10 days) may alleviate symptoms asso­
ciated with nadir testosterone concentrations occurring before the 
next injection. Alternatively, some patients prefer changing the 
dose of testosterone enanthate or cypionate to 75 to 100 mg IM 
every week to reduce swings in testosterone concentrations and 
associated symptoms (see Fig. 19.28). Administrati

associated symptoms (see Fig. 19.28). Administration of testos­
terone enanthate at doses of 300 mg IM every 3 weeks or 400 mg 
IM every 4 weeks produces extremely wide fluctuations in serum 
testosterone concentrations with markedly supraphysiologic con­
centrations for several days after an injection and concentrations 
below normal 3 weeks after an injection); these regimens are not 
recommended623.
Because CDGP in which puberty eventually occurs spon­
taneously is clinically indistinguishabl

urs spon­
taneously is clinically indistinguishable from delayed puberty 
caused by permanent hypogonadotropic hypogonadism (e.g., 
CHH),146,147,374,375 testosterone treatment usually is not initiated 
in boys with androgen deficiency due to a cause of prepubertal 
onset until they are about 14 years of age (with a bone age of at 
least 10.5 years). Testosterone therapy is administered intermit­
tently to allow determination of spontaneous puberty, if it occurs. 
Occasionally, testosterone thera

y, if it occurs. 
Occasionally, testosterone therapy is started at a younger age if 
delayed genital development and growth are causing severe psy­
chological distress in affected boys and their families.
In boys with androgen deficiency due to a cause of prepubertal 
onset, treatment is initiated with a very low dose of testosterone 
enanthate or cypionate (e.g., monthly IM injection of 50–100 mg 
or 25–50 mg every 2 weeks) to prevent premature closure of long 
bone epiphyses that would comprom

closure of long 
bone epiphyses that would compromise adult height.146,147,374,375 
These low dosages of testosterone are sufficient to induce some 
0
7
14
21
28
Days
1100
900
600
T (ng/dL)
300
35
42
49
56
63
70
• Fig. 19.28  Schematic diagram of serum testosterone (T) concentrations after chronic administration of 
T enanthate 200 mg intramuscularly every 2 weeks (red) or 100 mg every week (blue), T undecanoate 
750 mg intramuscularly every 10 weeks (black), and T gel 1.62% transdermally daily 

eeks (black), and T gel 1.62% transdermally daily (green). Dashed line 
denotes the reference range of serum T concentrations in adult men (300–1000 ng/dL).
745
virilization and long bone growth without interfering with the 
spontaneous puberty that occurs eventually in boys with CDGP. 
Testosterone treatment is continued for 3 to 6 months and then 
stopped for 3 to 6 months to assess whether spontaneous pubertal 
onset occurs. If there is indication that spontan

onset occurs. If there is indication that spontaneous puberty is 
occurring (e.g., testis size >8 mL), testosterone therapy is discon­
tinued. If there is no evidence of spontaneous puberty, intermit­
tent testosterone treatment is continued. The dose of testosterone 
enanthate or cypionate is increased gradually to 50 to 100 mg IM 
every 2 weeks and then to full adult replacement doses over the 
next several years to mimic the gradual increase in testosterone 
concentrations that occurs during

in testosterone 
concentrations that occurs during spontaneous puberty.
At present, transdermal testosterone formulations are not 
approved for use in boys with delayed puberty. However, because 
they circumvent the need for IM injections, low-dose transdermal 
testosterone patches and gels would provide very useful alterna­
tives for the treatment of androgen deficiency due to a cause of 
prepubertal onset in boys, but they are currently not approved for 
this indication.
A formulation of testo

roved for 
A formulation of testosterone undecanoate in castor oil (Aveed, 
Endo Pharmaceuticals, Malvern, PA) was approved in 2014 for 
use in the United States for treatment of male hypogonadism. 
This formulation is administered by slow IM injection into the 
gluteus muscle at a dose of 750 mg in 3 mL of castor oil initially, 
followed by another injection of the same dose 4 weeks later and 
then every 10 weeks to produce and maintain serum testoster­
one concentrations withi

maintain serum testoster­
one concentrations within the normal range in most hypogonadal 
men.624 Steady state is achieved after the third injection, and 
mean testosterone concentrations peak in the high-normal range 
at 7 days after injection and gradually decline over the next 10 
weeks to mean nadir concentrations just above the lower limit of 
the normal range (see Fig. 19.28). Despite this decline in testos­
terone concentrations, fluctuations in or recurrence of symptoms 
of androgen defi

ons in or recurrence of symptoms 
of androgen deficiency have not been reported. Although some 
men experience discomfort with large-volume injections, they are 
generally tolerated well and have the advantage of fewer injections 
than shorter-acting testosterone ester formulations.
A different formulation of testosterone undecanoate in a castor 
oil vehicle (Nebido, Bayer Schering Pharma AG, Berlin, Germany) 
has been approved and is used in Europe and other countries 
for testosterone replacem

ope and other countries 
for testosterone replacement therapy in hypogonadal men.625 It 
is administered at a dose of 1000 mg in 4 mL IM, followed by 
another injection of the same dose 6 weeks later and then every 
10 to 14 weeks.
Because of the large volume of drug administered and the need 
for proper injection technique, self-administration of IM tes­
tosterone undecanoate is not possible. Coughing may occur in 
a small number of men immediately after injection of testoster­
one undecanoate 

ely after injection of testoster­
one undecanoate (this also occurs with shorter-acting testoster­
one ester injections). Although there is no direct evidence for the 
cause of coughing, it is conjectured to be related to pulmonary oil 
microembolism (POME) emanating from the large volume of cas­
tor oil vehicle that is injected into the muscle with this formula­
tion. For these reasons, the FDA has required a risk evaluation and 
mitigation strategy for use of testosterone undecanoate (Aveed) i

tegy for use of testosterone undecanoate (Aveed) in 
the United States that requires training of personnel and certifica­
tion of the health care facility to ensure proper injection technique 
(slow IM injection) and adequate monitoring (for 30 minutes) 
and treatment capability for potential POME or anaphylaxis fol­
lowing injection.
In 2018, a testosterone enanthate formulation of 50 mg, 
75 mg, or 100 mg (0.5-mL solution) in a prefilled, single-use, 
small-needle autoinjector for weekly SC in

e-use, 
small-needle autoinjector for weekly SC injection was approved 
for use in testosterone replacement therapy of hypogonadal men 
(Xyosted, Antares Pharma, Ewing, NJ).626,627 
Transdermal Testosterone. Transdermal testosterone formu­
lations available for testosterone replacement therapy for male 
hypogonadism include an adhesive testosterone patch, two 1% 
testosterone gels, a 1.62% testosterone gel, a 2% testosterone gel, 
and a 2% testosterone solution (see Table 19.10). Transdermal 
de

terone solution (see Table 19.10). Transdermal 
delivery of testosterone is used in hypogonadal men who prefer 
this method or are unable to tolerate or self-administer IM injec­
tions of testosterone ester. Currently, testosterone gel is the most 
frequently used formulation for treatment of male hypogonadism 
in the United States.
In contrast to testosterone ester injections, which produce 
transient supraphysiologic testosterone concentrations, patch, gel, 
and solution formulations produce a

, patch, gel, 
and solution formulations produce a more physiologic range of 
testosterone concentrations; use of the patch results in a circadian 
variation in testosterone concentrations, and the gel formulations 
usually produce relatively constant steady-state serum testosterone 
Testosterone stimulates red blood cell production, and testos­
terone replacement therapy may result in excessive erythrocytosis. 
In men with hypogonadism, excessive erythrocytosis occurs less 
comm

nadism, excessive erythrocytosis occurs less 
commonly with testosterone patch therapy than with testoster­
one enanthate injections, suggesting that physiologic testosterone 
concentrations produced by transdermal testosterone therapy may 
be associated with fewer androgenic adverse effects.628 Compared 
with testosterone ester injections, transdermal formulations have 
a short half-life in SC tissue and in the circulation; consequently, 
their discontinuation results in a rapid fall in serum t

discontinuation results in a rapid fall in serum testosterone 
concentrations and a shorter duration of action. Therefore, an 
advantage of transdermal testosterone is the ability to withdraw 
androgen replacement relatively rapidly if excessive erythrocytosis 
develops or if prostate cancer is detected.
Disadvantages of transdermal formulations include the require­
ment for daily application, greater expense compared with tes­
tosterone ester injections, skin irritation or rash with testosteron

jections, skin irritation or rash with testosterone 
patches629 (less common with testosterone gels and solution), and 
the potential with gel and solution formulations for transfer of 
testosterone to others through skin contact at the application site.
The first transdermal testosterone delivery system for treatment 
of male hypogonadism was a scrotal testosterone patch.630 Treat­
ment required daily application of a relatively large, nonadhesive 
patch to clean, dry, and preferably shaven scr

ve 
patch to clean, dry, and preferably shaven scrotal skin and the 
use of brief-style underwear to hold them in place. These require­
ments were not acceptable to some hypogonadal men. In addition, 
in some men with congenital androgen deficiency, the scrotum 
was too small to accommodate even the smaller-size testosterone 
patch. Because of poor adherence to scrotal skin, thin adhesive 
strips were added as an option to this patch. Some men using 
this testosterone patch experienced skin irri

ing 
this testosterone patch experienced skin irritation and itching. The 
scrotal testosterone patch produced serum DHT concentrations 
in the upper-normal range or above the normal range as a result of 
high 5α-reductase activity within scrotal skin. The nonscrotal tes­
tosterone patch and testosterone gels have supplanted the scrotal 
testosterone patch for testosterone replacement therapy, and the 
latter is no longer available in the United States.
A nonscrotal testosterone patch, Androderm

States.
A nonscrotal testosterone patch, Androderm (Allergan USA, 
Madison, NJ), is available for testosterone replacement therapy in 
male patients with hypogonadism.631 This patch is composed of a 
central reservoir containing testosterone and permeation enhanc­
ers in an alcohol-based gel surrounded by an adhesive patch that is 
746
applied to the skin of the back, abdomen, upper arms, or thighs, 
avoiding areas over a bony prominence. When applied

voiding areas over a bony prominence. When applied at night, 
the testosterone patch produces serum testosterone concentrations 
that peak in the morning, mimicking the circadian variation of 
endogenous testosterone concentrations in normal men. Andro­
derm patches are available in two sizes delivering 2 mg (32 cm2) or 
4 mg (39 cm2) of testosterone daily. Long-term use of the testos­
terone patch usually maintains serum testosterone concentrations 
within the mid- to low-normal range and impro

ons 
within the mid- to low-normal range and improves the clinical 
manifestations of androgen deficiency. Usually, to achieve consis­
tent mid- to high-normal testosterone concentrations, application 
of two patches may necessary—one 2-mg patch plus one 4-mg 
patch or two 4-mg patches.632
The major limitation of Androderm is skin irritation or rash of 
varying severity; this side effect occurs in at least 30% to 60% of 
patients.629 Mild to moderate erythema and irritation are almost 
always pr

rate erythema and irritation are almost 
always present, probably because of a skin reaction to the perme­
ation-enhancing agent or adhesive. Uncommonly, severe contact 
dermatitis or burn-like skin reactions occur. Pretreatment of the 
skin under the reservoir of the patch with a topical corticosteroid 
such as triamcinolone acetonide 0.1% cream reduce the incidence 
and severity of skin irritation produced by testosterone patches.633
There are several testosterone gels on the market, and they 

several testosterone gels on the market, and they are 
the most frequently used testosterone formulations for the treat­
ment of male hypogonadism in the United States. The gels are 
dispensed into the palm of the hand and applied daily in the 
morning to clean, dry skin. The specified site of application var­
ies by gel product, but none of them are applied on the face or 
scrotum. The alcohol-based gel dries rapidly after application, and 
testosterone is absorbed into the SC space, where it i

osterone is absorbed into the SC space, where it is released 
steadily over the remainder of the day, producing relatively steady-
state testosterone concentrations. Residual testosterone remains 
on the surface of the skin of the hands and at the sites of applica­
tion. Therefore, the hands should be washed with soap and water 
after application, the sites of gel application should be covered 
with clothing, and skin contact with the application sites by oth­
ers (especially women and children)

 sites by oth­
ers (especially women and children) should be avoided for several 
hours or the site has been washed to prevent transfer of testoster­
one.634 Because of reports of contact transfer of testosterone to 
children, these instructions and caution to avoid contact transfer 
are now in an FDA black box warning in the prescribing informa­
tion for all transdermal testosterone formulations. Residual testos­
terone on the skin at application sites may be washed off (e.g., by 
showering or 

n sites may be washed off (e.g., by 
showering or bathing), but washing the sites should be avoided 
for a minimum of 1 hour and ideally for 6 hours or more after 
application (to maximize testosterone absorption).
In contrast to testosterone patches, local skin irritation with 
testosterone gel formulations is relatively uncommon, occurring 
in fewer than 5% of men, and is probably related mostly to dry­
ing of the skin by the alcohol. Some men complain of stickiness 
of the skin after the alco

complain of stickiness 
of the skin after the alcohol-based gel dries. Testosterone gels 
produce serum DHT concentrations at the upper end or above 
the normal range as a consequence of 5α-reductase activity in the 
relatively large surface area of skin over which the gel is applied. 
A major limitation of the use of testosterone gel for testoster­
one replacement therapy is the high cost. Generic testosterone 
gel products have been approved and are less costly than brand 
products, but their 

d are less costly than brand 
products, but their availability has been variable, and they are 
not inexpensive.
The first gel developed, AndroGel 1% (Abbott, Abbott Park, 
IL) was available initially in foil sachets at two different doses: 
2.5 g of gel (containing 25 mg of testosterone) or 5 g of gel 
(containing 50 mg of testosterone). (Because only about 10% of 
the medication is absorbed, these sachets deliver 2.5 or 5.0 mg of 
testosterone, respectively.) The starting dose of AndroGel is 5

 respectively.) The starting dose of AndroGel is 5 g 
daily. Based on testosterone concentrations or clinical response, 
approximately 2 weeks after initiation of therapy, the dose may 
be increased to 7.5 g (i.e., one 2.5-g packet plus one 5.0-g packet) 
or to 10.0 g (two 5.0-g packets) daily or decreased to 2.5 g daily. 
AndroGel is now available in a more concentrated reformulation 
of 1.62% that is delivered by a metered-dose pump that delivers 
12.5 mg of testosterone per actuation, or foil

rs 
12.5 mg of testosterone per actuation, or foil packets of 20.25 or 
40.5 mg of testosterone. With titration, the dosage of AndroGel 
1.62% ranges from 20.25 to 81 mg of testosterone.
Testim (Auxilium Pharmaceuticals, Chesterbrook, PA) is the 
other 1% hydroalcoholic testosterone gel that is available for treat­
ment of male hypogonadism.635 Like AndroGel, Testim is applied 
daily in the morning to intact, clean, dry skin over the shoulders 
and arms. In short-term, placebo-controlled trials,

nd arms. In short-term, placebo-controlled trials, Testim main­
tained steady-state physiologic serum testosterone concentrations 
in hypogonadal men and improved the clinical manifestations 
of androgen deficiency.636 After the initial application of Testim, 
serum testosterone concentrations are approximately 30% higher 
than those achieved after application of AndroGel. However, no 
direct comparison of steady-state testosterone concentrations with 
long-term use of the two testosterone gels 

 with 
long-term use of the two testosterone gels is available.
Testim is packaged in a 5-g tube containing 50 mg of testoster­
one and delivering approximately 5 mg of testosterone (i.e., 10% 
absorption). The starting dose of Testim is 5 g daily. Based on 
testosterone concentrations or clinical response approximately 2 
weeks after initiation of therapy, the dose may be increased to 10 g 
(two tubes) daily. In contrast to AndroGel, Testim is not available 
in a dose of 2.5 g or in a metered-d

ot available 
in a dose of 2.5 g or in a metered-dose dispenser, limiting dose 
adjustments with this formulation. Whereas AndroGel is odor­
less, Testim has a musk-like scent. Depending on the individual 
patient and his partners, this aroma might be pleasant or objec­
tionable. Testim contains a skin emollient and is less drying to the 
skin than AndroGel.
Two additional transdermal formulations for the treatment 
of male hypogonadism have been approved in the United States. 
Fortesta (Endo Ph

 approved in the United States. 
Fortesta (Endo Pharmaceuticals, Malvern, PA) is a 2% testoster­
one gel that is applied to the inner thighs at a daily dosage of 40 to 
70 mg (delivering 4–7 mg of testosterone) daily by a metered-dose 
pump that delivers 10 mg of testosterone per pump depression 
(delivering 1 mg of testosterone).637 Axiron (Eli Lilly, Indianapo­
lis, IN) is a 2% testosterone solution that is applied to axillary 
skin at a dosage of 30 to 120 mg (delivering 3-12 mg of testos­
te

 of 30 to 120 mg (delivering 3-12 mg of testos­
terone) daily using a metered-dose pump applicator that delivers 
30 mg (delivering 3 mg of testosterone) per pump depression.638 
Both of these transdermal testosterone formulations are able to 
achieve and maintain relatively steady-state serum testosterone 
concentrations within the normal range in hypogonadal men. The 
advantages and disadvantages of these formulations are similar 
to those of AndroGel and Testim gels. There might be a higher 

ndroGel and Testim gels. There might be a higher 
risk of clinically important secondary transfer of testosterone to 
sexual partners with the use of Fortesta, and some men complain 
of dripping of the 2% Axiron testosterone solution from the axilla. 
In addition, some men might need to shave their axilla to apply 
Axiron; secondary transfer might be less likely to occur with axil­
lary application, however.
Long-term use of testosterone gel in hypogonadal men usually 
maintains steady-state phy

ypogonadal men usually 
maintains steady-state physiologic serum testosterone concentra­
tions and improves the clinical manifestations of androgen defi­
ciency208,639,640 (see Fig. 19.28). With all transdermal testosterone 
747
formulations, serum testosterone concentrations may vary consid­
erably both within and between individuals and from day to day, 
and therefore it is difficult to judge the adequacy of a particular 
dose in maintaining testosterone concen

articular 
dose in maintaining testosterone concentrations using single mea­
surements of testosterone641 (Fig. 19.29).
In contrast to FDA-approved transdermal testosterone formu­
lations, testosterone concentrations contained in compounded 
testosterone gels and creams vary considerably and are inaccu­
rate. In one study, AndroGel contained testosterone amounts that 
were consistent and accurate; there was little variation within and 
between batches of AndroGel. However, in this same study, on

tches of AndroGel. However, in this same study, only 
30% to 50% of batches of compounded testosterone formulations 
from 10 different pharmacies contained testosterone amounts 
within 20% of the stated dose; one compounded formulation had 
virtually no testosterone.642 
Transbuccal Testosterone. A transbuccal testosterone tablet, 
Striant (Columbia Laboratories, Livingston, NJ), is available for 
testosterone replacement therapy in hypogonadal men643,644 (see 
Table 19.10). This formulation is 

en643,644 (see 
Table 19.10). This formulation is a small mucoadhesive tablet that 
contains 30 mg of testosterone in an oil-water emulsion carrier 
vehicle. The tablet contains polycarbophil, which, after applica­
tion, remains attached to buccal mucosa until epithelial cells turn 
over (approximately every 12–15 hours). The tablet is placed in the 
mouth between the inner cheek and gum, above the incisors, with 
the monoconvex side toward the gum and the flat side toward the 
cheek. After plac

um and the flat side toward the 
cheek. After placement, the tablet softens and swells with hydra­
tion and becomes gelatinous and sticky, causing it to adhere to 
the gum. Testosterone is released at a controlled and sustained 
constant rate from the tablet through the buccal mucosa into the 
systemic circulation, circumventing first-pass hepatic metabolism.
Striant tablets are placed on the buccal mucosa twice daily, with 
one tablet applied in the morning and removed after 12 hours and 
anoth

 the morning and removed after 12 hours and 
another applied in the evening on the opposite side.643,644 Use of 
Striant requires careful instruction to orient the tablet with the 
rounded side against the gum and to hold it firmly in place with a 
finger over the lip for approximately 30 seconds. If the tablet falls 
off or is dislodged, a new tablet should be applied and left in place 
until the next regularly scheduled dose. Swallowing of the tablet 
is not harmful. The buccal tablet is remov

tablet 
is not harmful. The buccal tablet is removed by gently sliding it 
downward toward the incisor to avoid scratching the gum.
400
300
200
100
0
P1
P2
P3
P4
P5
P6
P7
P8
P9
P10
P11
P12
P13
P14
P15
P16
P17
P18
P19
P20
Serum (ng/dL)
Placebo
1400
1200
800
200
0
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
T11
T12
T13
T14
T15
T16
T17
T18
T19
T20
T21
T22
T23
T24
T25
T26
T27
400
600
1000
Serum T (ng/dL)
T Gel
A
B
C
A
B
C
• Fig. 19.29  Large variability of serum testosterone (T) concentrations 2 hours after 16 ±

testosterone (T) concentrations 2 hours after 16 ± 2 weeks of trans­
dermal application of daily gel application at ambulatory clinic visits (A, open circles; B, closed squares) 
and an inpatient visit (C, shaded triangles) in participants in the Testosterone Trials who received T gel 1% 
(upper, participants T1–T27) and placebo gel (bottom panel, participants P1–P20). Within-subject varia­
tion is shown by a shaded vertical line between the highest and lowest T concentration for each man. 
The 

est and lowest T concentration for each man. 
The dashed line denotes the reference range of serum T concentrations in adult men (300–1000 ng/dL).
748
Application of a Striant tablet containing 30 mg of tes­
tosterone every 12 hours produces, on average, steady-state 
testosterone concentrations in the mid-normal range through­
out the day.643,644 Although a formal study has not been con­
ducted, unrestricted intake of food and beverage (including 
al

tricted intake of food and beverage (including 
alcohol), tooth brushing, mouth washing, and gum chewing 
did not appear to affect the absorption of testosterone in phar­
macokinetic studies. Contact transfer of testosterone in saliva 
to others has not been reported to occur. Like the transdermal 
testosterone gels, Striant produces high-normal to high serum 
DHT concentrations, probably because of 5α-reductase activ­
ity in the buccal mucosa. In general, transbuccal testosterone 
tablets are t

n general, transbuccal testosterone 
tablets are tolerated well. In one study, approximately 10% to 
15% of men developed gum or mouth irritation or inflam­
mation, and 5% experienced an altered or bitter taste in the 
mouth.643
Like the transdermal testosterone formulations, Striant is 
relatively expensive compared with testosterone ester injections. 
Initially, patients are aware and bothered by the tablet between 
their cheek and gum, resulting in premature discontinuation of 
the formulatio

ng in premature discontinuation of 
the formulation. However, with continued use, the unusual sen­
sation and awareness of the buccal tablet diminish and become 
less bothersome. Twice-daily application of Striant is required to 
sustain physiologic testosterone concentrations, and this makes 
compliance challenging. Informing patients that awareness of 
the buccal tablet diminishes over time and linking application 
of the transbuccal tablet to a routine daily activity such as morn­
ing and eve

a routine daily activity such as morn­
ing and evening tooth brushing may help improve and maintain 
compliance. 
Testosterone Nasal Gel. A 1% testosterone nasal gel formu­
lation, Natesto (Aytu Bioscience, Englewood, CO), has been 
approved by the FDA to treat men with hypogonadism.645 This 
formulation is administered intranasally at a dosage of 11 mg 
testosterone in 1.1 g gel (delivering 1.1 mg of testosterone in 
each nostril) three times daily (every 6–8 hours) by a metered-
dose pump that

ily (every 6–8 hours) by a metered-
dose pump that delivers 5.5 mg (delivering 0.55 mg of testoster­
one) per pump depression; the total daily dose is therefore 33 mg 
(delivering 3.3 mg of testosterone). It is advised that there should 
be no nose blowing or sniffing for 1 hour after administration, 
and treatment should be discontinued temporarily during epi­
sodes of severe rhinitis. Although there is no interaction with 
symphathomimetic nasal decongestants, potential interaction 
with other

l decongestants, potential interaction 
with other intranasally administered medications is not known, 
so use with other nasal drugs is not recommended. It is also not 
recommended for patients with chronic nasal conditions. When 
administered to hypogonadal men, mean serum testosterone 
concentrations peak in the high-normal range approximately 1 
hour after administration and decline over 6 to 8 hours to con­
centrations at or slightly below normal. 
Testosterone Pellets. SC testosterone pell

ormal. 
Testosterone Pellets. SC testosterone pellets are used infre­
quently in the United States but more commonly in Australia and 
some European countries for testosterone replacement therapy in 
men with hypogonadism.646,647 In the United States, Testopel Pel­
lets (Auxilium Pharmaceuticals, Chesterbrook, PA) are available 
for testosterone replacement (see Table 19.10). These are cylin­
drical pellets, 3.2 mm in diameter by 8 to 9 mm in length, that 
contain 75 mg of testosterone. Testopel

gth, that 
contain 75 mg of testosterone. Testopel Pellets are recommended 
at doses that range from 150 to 450 mg testosterone (i.e., two 
to six 75-mg pellets) and are implanted every 3 to 4 months.646 
However, a recent report of implantation of 6 to 12 pellets (450–
900 mg of testosterone) found that decline in serum testoster­
one concentrations to below normal occurred by 3 to 4 months 
after implantation regardless of BMI or the number of pellets 
implanted (from 6–9 or 10–12).648
Pharmac

pellets 
Pharmacokinetic profiles for testosterone pellets depend on the 
specific pellet formulation.647,649,650 In European studies using a 
different formulation, SC implantation of three to six 200-mg pel­
lets (for a total of 600–1200 mg of testosterone) produced the 
almost zero-order, sustained release of testosterone and main­
for 4 to 6 months in hypogonadal men.649 Testosterone pellets 

onths in hypogonadal men.649 Testosterone pellets are 
implanted subcutaneously with the use of a trocar that is intro­
duced through a small skin incision. This minor surgical proce­
dure is repeated three to four times yearly to maintain normal 
serum testosterone concentrations.
Although spontaneous extrusion of pellets and local bleeding 
or infection may occur occasionally, these problems are uncom­
mon in experienced hands. If adverse effects develop after implan­
tation, a major concern i

ts develop after implan­
tation, a major concern is that removal of the testosterone pellets 
will be difficult, if not impossible. Therefore, the use of testoster­
one pellets is inappropriate for testosterone replacement in older 
patients who are predisposed to erythrocytosis and prostate disease 
during treatment. 
Testosterone Formulations Available Outside the 
Oral Testosterone Undecanoate. In many countries outside the 
United States, an oral 17β-hydroxyl ester of testoster

ed States, an oral 17β-hydroxyl ester of testosterone, testoster­
one undecanoate (Andriol Testocaps, Organon, Oss, Netherlands), 
is available for testosterone replacement therapy in hypogonadal 
men.651,652 Testosterone undecanoate, which is formulated in a 
castor oil vehicle, is absorbed directly from the gastrointestinal 
tract into the lymphatic system and then into the systemic circula­
tion, thereby avoiding first-pass hepatic accumulation and inac­
tivation. Serum testosterone concentra

n and inac­
tivation. Serum testosterone concentrations peak approximately 
5 hours after administration of testosterone undecanoate and fall 
to pretreatment concentrations within 8 to 12 hours. For testos­
terone replacement therapy, it is administered at relatively high 
doses, 40 to 80 mg two to three times daily (total dose, 80–240 
mg daily). The frequency of administration makes compliance 
difficult for many men. Absorption of testosterone undecano­
ate requires concomitant food ingestio

e undecano­
ate requires concomitant food ingestion, and serum testosterone 
concentrations and clinical responses are highly variable. Because 
of 5α-reductase activity in the gastrointestinal tract, serum DHT 
concentrations are often very high.
The use of castor oil and propylene glycol laurate instead 
of oleic acid, the vehicle used in the original formulation, per­
mits storage at room temperature and extends the shelf life of 
Andriol Testocaps for up to 3 years while maintaining phar­
ma

ocaps for up to 3 years while maintaining phar­
macokinetic and pharmacodynamic characteristics similar to 
those of the original formulation. Testosterone concentrations 
fall quickly after discontinuation of testosterone undecanoate. 
Therefore, it may be particularly useful for testosterone replace­
ment therapy in older men with clinically significant prostate 
disease and comorbid conditions, in whom rapid withdrawal 
of androgen action is desirable if adverse effects develop, and 
in those

esirable if adverse effects develop, and 
in those for whom only low-dose testosterone supplementation 
is needed.
Transdermal Testosterone Formulations. A testosterone-in-
adhesive matrix patch (Testopatch, Pierre Fabre, Castres, France) is 
available in many countries in Europe for testosterone replacement 
therapy in patients with male hypogonadism.654,655 This patch is 
composed of an adhesive matrix that contains testosterone (0.5 
mg/cm2) and excipients that comes in three sizes—30, 45, an

nd excipients that comes in three sizes—30, 45, and 
749
60 cm2. Two 60-cm2 patches (delivering approximately 4.8 mg of 
testosterone daily) are applied to the skin of the arms, trunk, or 
thighs every 2 days to maintain serum testosterone concentrations 
in the normal range in hypogonadal men. Skin irritation has been 
reported to occur in about 20% of patients using this patch. 
Nontestosterone Therapies for Male Hypogonadism
Clomiphene and Aromatase Inhibitors

e Hypogonadism
Clomiphene and Aromatase Inhibitors. As noted previously, 
clomiphene and aromatase inhibitors (e.g., anastrozole) have 
been used off label to increase serum testosterone concentrations 
to enhance athletic performance or develop a muscular appear­
ance. These drugs are not approved for use in male hypogonadism. 
Because their mechanism of action is to increase pituitary secre­
tion of gonadotropins (thereby stimulating testicular production 
of testosterone), they will not be ef

 production 
of testosterone), they will not be effective in men with primary 
hypogonadism (who already have high gonadotropin concentra­
tions) and men with severe secondary hypogonadism who can­
not produce more gonadotropins. In addition, safety and efficacy 
of these drugs have not established for the treatment of male 
hypogonadism.116 Aromatase inhibitor therapy is associated with 
increased fat mass and decreased sexual function and is likely to 
decrease bone mass.120-122 
Selective AR 

kely to 
Selective AR Modulators. There is considerable interest in the 
development of selective AR modulators (SARMs), which are 
nonsteroidal molecules that interact with AR and have differen­
tial effects on various androgen target organs.656,657 The goal is to 
develop an orally active, nonsteroidal SARM that will maintain 
the beneficial anabolic and androgenic actions of testosterone on 
muscle, bone, sexual function, and mood but have reduced poten­
tial for a

ction, and mood but have reduced poten­
tial for adverse effects (e.g., on the prostate gland). These novel 
drugs are being developed primarily for use in muscle-wasting 
conditions such as age-related sarcopenia and cancer cachexia but 
not at present for treatment of male hypogonadism. The mecha­
nisms by which SARMs act in a tissue-specific manner are unclear. 
It is possible that SARMs have less stimulatory effect on the pros­
tate because they are not actively metabolized by 5α-reductase, 

hey are not actively metabolized by 5α-reductase, 
or they may act by unique interactions with tissue-specific AR 
coactivator and corepressor molecules.
Nonsteroidal SARMs have been developed that have anabolic 
actions in muscle and bone but reduced stimulatory effects on the 
prostate gland in animals.656,657 SARMs do not have intrinsic ER 
activity, and if they suppress endogenous gonadotropin, testoster­
one, and estradiol secretion, they will produce a state of relative 
estrogen deficienc

ll produce a state of relative 
estrogen deficiency. Therefore, studies evaluating the clinical ben­
efits and risks of an SARM must consider potential adverse effects 
on target actions that are regulated by estradiol in men, such as 
effects on BMD, fat mass, sexual function, and possibly lipids 
(HDL cholesterol), cardiovascular function, and brain function. 
The importance of estrogens on bone, for example, was under­
scored by a recent long-term study of the potent, nonaromatizable 
androge

term study of the potent, nonaromatizable 
androgen—DHT gel—administered to older men. Compared 
with placebo, DHT gel stimulated lean body mass and reduced 
fat mass but reduced BMD.99 
Monitoring Clinical Response and Testosterone 
The clinical responses to testosterone replacement and serum 
testosterone concentrations are used to monitor the adequacy of 
testosterone therapy in androgen-deficient men116 (Table 19.11). 
Symptoms and signs of androgen deficiency should be assess

 and signs of androgen deficiency should be assessed 
before the initiation of testosterone treatment, 3 to 12 months 
after starting testosterone, and then yearly. By 3 to 6 months, 
most hypogonadal men experience improvements in libido, sexual 
function and activity, energy, vitality, motivation, and mood.658 
Increases in body hair growth, muscle mass and strength, and 
BMD occur over the subsequent months to years of testosterone 
therapy.
Serum testosterone concentrations are monitored to 

erum testosterone concentrations are monitored to determine 
the adequacy of therapy and to avoid overreplacement or underre­
placement. This is particularly important in men treated with the 
transdermal testosterone patch, gels, and solution, as the bioavail­
ability of these formulations is highly variable among individu­
als; because of this variability, decisions regarding the adequacy 
of a given dosage of transdermal testosterone should not be made 
on a single testosterone measurement.64

t be made 
on a single testosterone measurement.641 The goal of testosterone 
replacement therapy is to achieve average serum testosterone con­
centrations within the normal range.116
For testosterone ester injections, testosterone concentrations 
should be measured at 3 to 6 months of treatment, midway 
between two injections (e.g., 1 week after an injection if the injec­
tions are given every 2 weeks). Serum testosterone concentrations 
measured at the nadir of the injection interval (i.e., ju

d at the nadir of the injection interval (i.e., just before the 
next injection) may help to document an inadequate dosing inter­
val. For the testosterone patch, testosterone concentrations should 
be measured after approximately 3 to 4 weeks of daily use, 8 to 
10 hours after application of a patch on the previous evening. For 
testosterone gels and solution, testosterone concentrations should 
be measured after about 2 weeks of daily use, at any time after 
application of the gel. For buccal 

ny time after 
application of the gel. For buccal testosterone, serum testosterone 
should be measured 4 to 6 weeks after initiation of therapy, at 
any time after application of the buccal tablet, preferably in the 
morning. 
Risks and Adverse Effects
Contraindications and Precautions. Testosterone treatment is 
contraindicated in men with metastatic prostate cancer or breast 
cancer.116 The primary concern is that testosterone administra­
tion could stimulate the growth of these androgen- or e

could stimulate the growth of these androgen- or estrogen-
dependent malignancies. Testosterone therapy is particularly risky 
in men with metastatic prostate cancer, in whom rapid growth of 
metastatic tumors may worsen bone pain or cause spinal cord com­
pression. In fact, the mainstay of treatment for metastatic prostate 
cancer is androgen-deprivation therapy to reduce endogenous tes­
tosterone production and action; this is achieved by GnRH ago­
nist and AR antagonist treatment or by surgic

ago­
nist and AR antagonist treatment or by surgical orchidectomy.659 
The effect of testosterone replacement in hypogonadal men with 
localized prostate cancer is not known. However, in the absence of 
evidence, testosterone treatment in men with clinical evidence of 
active prostate cancer should be avoided.
The safety of testosterone therapy in hypogonadal men who 
have been surgically cured of organ-confined, low-grade prostate 
cancer (Gleason score <7 or 3+4, PSA <10 or 20 ng/mL, and 
clin

on score <7 or 3+4, PSA <10 or 20 ng/mL, and 
clinical stage 1 or 2a) and have had clinically undetectable disease 
and an undetectable PSA level for several years also is not clear. 
Because these men would not have been treated with androgen-
deprivation therapy, testosterone replacement to restore eugonadal 
concentrations of testosterone seems reasonable. In these patients, 
a careful discussion of the potential benefits and risks of testos­
terone replacement should be undertaken, and thera

terone replacement should be undertaken, and therapy should be 
initiated only after informed consent and with careful monitoring 
by DRE and measurement of PSA concentrations. It is prudent to 
avoid testosterone treatment in men with organ-confined, high-
risk prostate cancer (e.g., Gleason score 8–10, PSA >20 ng/mL, 
and clinical stage ≥T3) despite undetectable PSA concentrations 
750
  Monitoring During Testosterone Treatment
Parameter
Timing
Fur

During Testosterone Treatment
Parameter
Timing
Further Management
Measures of Efficacy
Symptoms and signs of androgen 
At baseline, after 3–12 mo, and then yearly
Continue testosterone treatment in men with clinical 
improvement and no adverse effects.
Consider discontinuing testosterone treatment in 
men if no clinical improvement.
BMD
For men at high risk for fracture, BMD before treatment; for men 
with osteoporosis or minimal-trauma fracture, BMD after 1–2 yr
Institute appropriate

a fracture, BMD after 1–2 yr
Institute appropriate treatment for men with osteo­
porosis, including calcium and vitamin D.
Testosterone ester injection: After 3–6 mo, measured midway 
between injections or at end of dosing interval (if androgen 
deficiency symptoms are present at that time)
Testosterone patch: After 3–4 wk, at 8–10 hr after application
Testosterone gel: After 2 wk, at any time after application
Buccal testosterone: After 4–6 wk, at any time after application 

one: After 4–6 wk, at any time after application 
(preferably in the morning)
Testosterone pellets: At end of dosing interval
Oral testosterone undecanoate: After 1 wk, at 3–5 hr after oral dose
Testosterone undecanoate injection: At end of dosing interval
Adjust dose or dosing interval to achieve serum 
testosterone concentrations in the mid-normal 
range.
Adverse Effects
Hematocrit
At baseline, after 3–6 mo, and then yearly
If hematocrit is >54%, stop or reduce dosage of 
testosterone until he

%, stop or reduce dosage of 
testosterone until hematocrit declines to normal 
and reinitiate testosterone at a lower dosage.
Investigate for a hypoxic condition such as obstruc­
tive sleep apnea or chronic lung disease.
PSA level, with or without DRE 
(using shared decision mak­
ing, i.e., if a patient desires 
prostate cancer screening 
after discussion of risks and 
benefits of PSA screening 
and monitoring), in men 
>50 yr (>40 if risk factors for 
prostate cancer)
At baseline, after 3–6 mo,

 apnea, daytime 
somnolence, unexplained 
erythrocytosis, worsening 
hypertension or edema)
Evaluate for obstructive sleep apnea or adjustment 
of CPAP settings.
Evaluate for other causes of hypoxia.
Formulation-specific adverse 
effects
Discontinue and switch to another formulation.
Testosterone ester injections
Discomfort, bleeding, or hematoma with IM injections
Fluctuations in energy, mood, libido
Allergy 

ions
Allergy to oil vehicle (rare)
Reinstruct on the self-injection site technique.
Consider shortening the injection interval if the nadir 
testosterone level is low.
Testosterone patch
Skin irritation
Adhesion to skin
Coadministration of corticosteroid cream may 
reduce skin irritation.
Testosterone gel
Contact transfer to others
Skin dryness at site of application
Reinstruct on washing hands and covering applica­
tion area after gel dries or showering 4–6 

plica­
tion area after gel dries or showering 4–6 hr after 
application, avoiding prolonged skin-to-skin con­
tact of application site with women and children.
Buccal testosterone tablets
Poor adhesion to gums
Reinstruct on proper application and reassure to 
complete an adequate trial with the correct 
technique.
SC testosterone pellets
Pellet extrusion
Implantation-site infection, bleeding, fibrosis
Reimplant pellets.
Treat infection with 

 fibrosis
Treat infection with appropriate drainage and 
antibiotics.
BMD, Bone mineral density; CPAP, continuous positive airway pressure; DRE, digital rectal examination; IM, intramuscular; IPSS, International Prostate Symptom Score; PSA, prostate-specific antigen.
   
TABLE 19.11
751
after surgical or brachytherapy, as these individuals have high like­
lihood of recurrence and poor outcomes independent of testoster­
one therapy.
Before testo

ndependent of testoster­
Before testosterone replacement therapy is started in men older 
than 50 years (>40 years if the man has risk factors for prostate 
cancer), prostate cancer screening should be offered after a shared 
decision-making discussion of the potential risks and benefits of 
prostate cancer screening (including detection and treatment 
of low-grade prostate cancer that might not benefit from treat­
ment).116 Minimal prostate cancer screening consists of measure­
men

prostate cancer screening consists of measure­
ment of serum PSA; there is controversy about the added value of a 
DRE as an initial screening test. Hypogonadal men with an abnor­
mal DRE or a consistently elevated PSA value (e.g., >4 ng/mL in 
Caucasian men, >3 ng/mL in African-American men and men at 
high risk for prostate cancer) should have a urologic evaluation 
that may include a transrectal ultrasound study and biopsy of the 
prostate before starting testosterone. Older men, African-Amer

ore starting testosterone. Older men, African-American 
men, men with an abnormal DRE, and men with a history of a 
first-degree relative with prostate cancer have an increased risk of 
prostate cancer; men with a previous negative prostate biopsy and 
men who are taking a 5α-reductase inhibitor have a reduced risk. A 
prostate cancer risk calculator, based on data from placebo-treated 
men in the Prostate Cancer Prevention Trial, is available to assess 
the risk of clinical (biopsy-detectable) 

 assess 
the risk of clinical (biopsy-detectable) and high-grade (Gleason 
score ≥7) prostate cancer according to ethnicity, age, PSA level, 
family history of prostate cancer, normal or abnormal DRE, results 
of prior prostate biopsy, and use of a 5α-reductase inhibitor. This 
calculator is available at http://myprostatecancerrisk.com/.660,661 
Although this tool has not been validated for men with untreated 
androgen deficiency (who usually have lower PSA concentrations 
than eugonadal men), t

e lower PSA concentrations 
than eugonadal men), the prostate cancer risk calculator may be 
helpful in assessing the risk of clinical or high-grade prostate can­
cer that may be present in a hypogonadal man before testosterone 
replacement is instituted.
Although breast cancer in men is rare, some disorders that 
cause androgen deficiency, such as Klinefelter syndrome, are asso­
ciated with an increased risk of breast cancer.662 Therefore, a care­
ful breast examination for suspicious masses sh

e­
ful breast examination for suspicious masses should be performed 
in hypogonadal men before starting testosterone treatment. The 
conversion of testosterone to estradiol may stimulate growth of 
ER-positive breast cancer.
Relative contraindications to testosterone replacement therapy 
include the following116:
 
• Presence of a PSA level greater than 4 ng/mL (or >3 ng/mL 
in individuals at high risk for prostate cancer, e.g., African-
American men or those with a family history of prostate 

n men or those with a family history of prostate 
cancer), or prostate nodule or induration that has not been 
evaluated
 
• Baseline hematocrit in the high-normal range (e.g., hema­
tocrit >48 at or near sea level or >50% at high altitudes), 
because further stimulation of erythropoiesis induced 
by testosterone therapy may result in erythrocytosis and, 
potentially, hyperviscosity and vascular complications, par­
ticularly in elderly men with underlying vascular disease
 
• Untreated obstruc

ility in the near future
 
• Myocardial infarction or stroke within the past 6 months, 
because of an increased risk of a cardiovascular event inde­
pendent of testosterone treatment, or thrombophilia or 
hypercoagulability, although rare, that predisposes men to 
venous thromboembolism (VTE) events. 
Potential Adverse Effects and Monitoring. Monitoring for 
potential adverse effects of testosterone therapy is summarized in 
Table 19.11.116 
Hematocrit. Testosterone replacement stimulates eryth

atocrit. Testosterone replacement stimulates erythropoi­
esis in hypogonadal men, increasing the hemoglobin concentra­
tion and hematocrit from the female range into the normal adult 
male range.663 Occasionally, testosterone treatment causes exces­
sive erythrocytosis (e.g., hematocrit >54%) that may require tem­
porary discontinuation of therapy or lowering of the testosterone 
dose until the hematocrit declines to normal; reinitiation of tes­
tosterone at a lower dose; or, uncommonly, therape

tosterone at a lower dose; or, uncommonly, therapeutic phlebot­
omy.326 Excessive erythrocytosis occurs more commonly in older 
men612 and in those treated with parenteral testosterone esters 
rather than transdermal testosterone patches, probably related to 
the supraphysiologic testosterone concentrations that are present 
for a few days after administration and the higher average testos­
terone concentrations produced by IM testosterone.628 A greater 
erythropoietic response to testosterone i

greater 
erythropoietic response to testosterone is associated with stimula­
tion of erythropoietin and suppression of ferritin and hepcidin (a 
liver-derived peptide that is the main negative regulator of iron 
bioavailability), possibly resetting the relationship between eryth­
ropoietin and hemoglobin and increasing the availability and use 
of iron for red blood cell production.664,665
Adverse effects of erythrocytosis induced by testosterone are 
poorly documented. However, there is concern

 are 
poorly documented. However, there is concern that an excessive 
increase in red blood cell volume and blood viscosity could cause 
vascular complications such as thrombosis, especially in older 
men with underlying atherosclerotic CVD, resulting in stroke 
or myocardial infarction. Therefore hematocrit should be mea­
sured before testosterone therapy is initiated, 3 to 6 months after 
starting treatment, and then yearly.116 If erythrocytosis develops 
during testosterone replacement therap

s develops 
during testosterone replacement therapy, testosterone should be 
stopped, and an evaluation should be performed to determine 
whether an underlying predisposing condition, such as hypoxia 
due to obstructive sleep apnea syndrome or chronic lung disease, 
is stimulating or contributing to erythrocytosis. Subsequently, tes­
tosterone may be restarted at a lower dosage. 
CVD and VTE. The effects of testosterone replacement therapy 
in men with androgen deficiency on the risk of major ca

n with androgen deficiency on the risk of major cardiovas­
cular events (MACE: myocardial infarction, stroke, and cardiovas­
cular mortality) is not known. There are no adequately powered, 
large, long-term randomized controlled trials that have assessed 
the risk of MACE in hypogonadal men treated with testosterone. 
Pharmacoepidemiologic studies evaluating cardiovascular events 
associated with testosterone treatment329-337 have been conflicting, 
with studies showing increased, no, or decreas

g, 
with studies showing increased, no, or decreased risk of cardiovas­
cular events associated with testosterone treatment. Meta-analyses 
of testosterone treatment trials326-328,666-670 have largely (except for 
one study327) shown no increase in cardiovascular events associated 
with testosterone treatment; however, meta-analyses have been 
limited by heterogeneity of the study populations and testosterone 
treatment regimens; variability of study durations and reported 
outcomes including ad

tudy durations and reported 
outcomes including adverse events; and lack of sufficient sample 
size or prespecified, independent adjudication of MACE. Despite 
the lack of evidence, the FDA (but not the European Medicines 
Agency) required the addition of a warning regarding the potential 
752
increased risk of cardiovascular events to the prescribing informa­
tion for all testosterone formulations. A large, long-term, multi­
center, randomized, place

large, long-term, multi­
center, randomized, placebo-controlled trial is currently being 
initiated to assess the risk of MACE in hypogonadal men with 
preexisting CVD or risk factors for CVD treated with testosterone.
A pharmacoepidemiologic and case-control study and a 
recent meta-analysis of testosterone treatment trials did not 
find an increased risk of VTE associated with testosterone treat­
ment.671-673 However, there were too few VTE events in ran­
domized controlled trials of testoster

nts in ran­
domized controlled trials of testosterone treatment to adequately 
assess risk. Case series have suggested that there is an increased 
risk of VTE events in men with a history of unprovoked VTE 
and thrombophilia who are treated with testosterone, even in the 
absence of erythrocytosis.674 The FDA also required the addition 
of a warning regarding the possible risk of VTE to prescribing 
information for all testosterone formulations. 
Prostate. Prostate size is reduced in men with an

Prostate. Prostate size is reduced in men with androgen defi­
ciency, and testosterone replacement therapy increases prostate 
volume to that of age-matched eugonadal men.675 Testosterone 
therapy does not cause excessive prostate enlargement, and there 
is no evidence that it worsens LUTS, reduces urinary flow rate, 
causes urinary retention, or increases the need for invasive inter­
vention for BPH (e.g., transurethral resection of the prostate). In 
the Testosterone Trials, testosterone vers

e). In 
the Testosterone Trials, testosterone versus placebo treatment for 1 
year was not associated with an increase in IPSS score greater than 
19 (severe LUTS).162 However, long-term, controlled studies have 
not been performed to evaluate these outcomes in middle-age to 
older hypogonadal men, who are most at risk for development 
of clinically significant, symptomatic BPH. In hypogonadal men 
older than 40 years, LUTS may be monitored with the use of the 
IPSS or the AUA symptom score duri

the use of the 
IPSS or the AUA symptom score during testosterone treatment 
in accordance with accepted guidelines.116 Concomitant therapy 
for LUTS (e.g., α-adrenergic receptor antagonists, 5α-reductase 
inhibitors, surgical bladder outlet procedures) should be consid­
ered in hypogonadal men with bothersome severe LUTS that 
affects quality of life.
There is no evidence that testosterone treatment causes pros­
tate cancer. In middle-age to older men with androgen deficiency, 
there is concern

er men with androgen deficiency, 
there is concern that long-term testosterone therapy might stimu­
late previously unrecognized or clinically undetectable localized 
or metastatic prostate cancer or accelerate the growth of preex­
isting subclinical disease into clinically apparent prostate cancer. 
There is evidence that testosterone treatment initially stimulates 
the growth of metastatic prostate cancer, but its effect on progres­
sion of subclinical prostate cancer is not known. Meta-analys

clinical prostate cancer is not known. Meta-analyses 
of small, short-term, controlled studies (up to 3 years in dura­
tion) have not found an increased incidence of prostate cancer 
in older men treated with testosterone; however, these trials were 
underpowered to assess the risk of prostate cancer, particularly 
high-grade prostate cancer.326,338 Pharmacoepidemiology studies 
have been inconsistent, showing no or possibly increased risk of 
low-grade prostate cancer (although this effect was 

w-grade prostate cancer (although this effect was confounded 
by ascertainment bias and greater prostate cancer screening in 
testosterone-treated men) and reduced risk of high-grade, aggres­
sive prostate cancer with testosterone treatment.337,339-342 Larger, 
longer-term, prospective randomized controlled trials are needed 
to determine whether testosterone therapy stimulates the growth 
and progression of subclinical prostate cancer into clinically evi­
dent and significant high-grade, aggres

cally evi­
dent and significant high-grade, aggressive disease.
Along with reduced prostate size, serum PSA concentrations in 
androgen-deficient men are decreased, and testosterone replace­
ment increases PSA to concentrations observed in age-matched 
eugonadal men.675 Because testosterone replacement is a poten­
tially disease-modifying intervention that may alter the natural 
history of prostate cancer, prostate cancer screening and monitor­
ing of PSA concentrations should be offered to pati

ng of PSA concentrations should be offered to patients older than 
50 years (>40 years if the man has risk factors for prostate cancer) 
initiating testosterone treatment, after a discussion of risks and 
benefits of prostate cancer screening (i.e., shared decision mak­
ing),116 as PSA screening for prostate cancer has no effect or only 
modest effect on the mortality rate in the general population of 
men.676-678
In men older than 40 years who desire prostate cancer screen­
ing, PSA measurement

esire prostate cancer screen­
ing, PSA measurement (and DRE, depending on the skill of the 
practitioner and patient preference) should be performed before 
testosterone therapy is initiated, 3 to 12 months after starting 
treatment, and then in accordance with accepted guidelines. More 
intensive PSA monitoring should not be performed, because 
abnormal concentrations that would trigger a prostate biopsy are 
more likely to occur in older hypogonadal men on testosterone 
therapy, resulting in a

nadal men on testosterone 
therapy, resulting in an increased likelihood of detecting localized, 
low-grade prostate cancer, for which the management is unclear.679 
The diagnosis of subclinical or localized low-grade prostate cancer 
does not affect overall mortality; however, the potential medical 
and surgical treatment, psychological, and socioeconomic conse­
quences and morbidity of this diagnosis may be considerable.
Hypogonadal men receiving testosterone therapy who demon­
strate a verifi

ng testosterone therapy who demon­
strate a verified PSA level greater than 4 ng/mL or increase in PSA 
greater than 1.4 ng/mL within 12 months after starting therapy, 
an abnormal finding on DRE (nodule or induration), or an AUA 
symptom score or IPSS greater than 19 should undergo urologic 
evaluation.116 Elevated PSA concentrations should be confirmed 
with a repeat measurement. If prostatitis or urinary tract infec­
tion that can markedly elevate PSA concentrations is suspected, 
PSA measure

vate PSA concentrations is suspected, 
PSA measurements should be repeated after appropriate antibiotic 
Sleep Apnea. Testosterone treatment has been reported to 
induce or worsen obstructive sleep apnea, but the prevalence 
of clinically significant obstructive sleep apnea during testoster­
one replacement therapy is probably very low and may be dose 
related.553,567,568 Short-term, high-dose testosterone treatment in 
older hypogonadal men significantly worsened obstructive sleep 

dal men significantly worsened obstructive sleep 
apnea, increased the duration of hypoxemia, and shortened total 
sleep time as assessed by polysomnography. In contrast, older men 
treated with a scrotal testosterone patch for 3 years did not dem­
onstrate a significant worsening of sleep apnea as assessed by a por­
table device.680 As discussed earlier, sleep apnea may conversely 
cause gonadotropin and testosterone suppression and secondary 
hypogonadism, probably because of the stress of oxy

ypogonadism, probably because of the stress of oxygen desatura­
tion and sleep disturbance.
Sleep apnea is associated with significant morbidity and mor­
tality risks. Therefore, hypogonadal men, especially those at 
increased risk (e.g., obese men), should be monitored for symp­
toms of obstructive sleep apnea syndrome (e.g., loud snoring, 
apnea witnessed by a bed partner, daytime somnolence, unex­
plained erythrocytosis, worsening or recent onset of hyperten­
sion or edema) before starting te

set of hyperten­
sion or edema) before starting testosterone therapy, after 3 to 12 
months, and then yearly.116 If symptoms suggest sleep apnea, a 
formal sleep study (polysomnography) should be performed. If 
obstructive sleep apnea is confirmed, appropriate treatment (e.g., 
CPAP) should be instituted before testosterone treatment is 
started or continued. 
Reduced Sperm Production and Fertility. In men who have 
androgen deficiency and persistence of some sperm production 
753
Chapter 19  Te

tence of some sperm production 
753
with either partial primary or secondary hypogonadism, testos­
terone treatment suppresses gonadotropin production by negative 
feedback regulation, which in turn suppresses spermatogenesis 
and may further impair fertility.587 Suppression of sperm produc­
tion is most important in men with secondary hypogonadism and 
normal testes who wish to father children. In these men, testos­
terone therapy should be discontinued, and gon

os­
terone therapy should be discontinued, and gonadotropin therapy 
should be started, initially with hCG alone and then, if neces­
sary, with combined hCG and FSH treatment to stimulate sper­
matogenesis.79 In the absence of coexisting testicular disease (e.g., 
cryptorchidism), prior testosterone therapy does not impair—but 
might delay—the subsequent induction of sperm production with 
gonadotropins.392-394 
Acne and Oily Skin. Boys with pubertal androgen deficiency 
who are receiving testos

rtal androgen deficiency 
who are receiving testosterone therapy to induce puberty and men 
with severe hypogonadism who are treated with full replacement 
doses of testosterone may develop acne and increased oiliness of 
the skin due to increased sebum production.146,147,374,375 These 
conditions usually respond to local skin measures (e.g., benzoyl 
peroxide, retinoids) and antibiotics, reduction in testosterone 
dose, or both. 
Gynecomastia. Occasionally, breast tenderness or gynecomas­
tia d

ccasionally, breast tenderness or gynecomas­
tia develops or worsens during testosterone replacement therapy, 
particularly in boys who are receiving testosterone for induction 
of puberty, men with severe androgen deficiency who are treated 
with full replacement or high-dose testosterone, and hypogonadal 
men with predisposing conditions such as hepatic cirrhosis. Gyne­
comastia is commonly found in boys and men with androgen 
deficiency before the initiation of testosterone therapy. A careful

 the initiation of testosterone therapy. A careful 
breast examination should be performed before and again during 
testosterone replacement therapy to detect the presence or wors­
ening of gynecomastia (breast tenderness or enlargement), or the 
rare occurrence of breast cancer. 
Lipids. In hypogonadal men, testosterone replacement results 
in no change or only a slight decrease in HDL cholesterol and 
no change in total cholesterol or LDL cholesterol concentra­
tions.326,681 The reduction in H

sterol concentra­
tions.326,681 The reduction in HDL cholesterol is greater in men 
with more severe androgen deficiency and in those treated with 
supraphysiologic testosterone doses or with nonaromatizable, oral 
17α-alkylated androgens.620
The clinical significance of HDL cholesterol reduction induced 
by testosterone in terms of cardiovascular risk is not known. 
Understanding of the effects of testosterone therapy on major 
cardiovascular outcomes (including myocardial infarction, stroke, 

tcomes (including myocardial infarction, stroke, 
cardiovascular mortality risk) will require larger, longer-term ran­
domized controlled studies. Cardiovascular risk and lipid measure­
ments should be evaluated as recommended by available practice 
guidelines; at present, more intensive monitoring in hypogonadal 
men receiving therapy is not justified. 
Other Potential Adverse Effects. Frontal balding or androgenic 
alopecia may develop or worsen in genetically predisposed hypo­
gonadal men dur

n in genetically predisposed hypo­
gonadal men during testosterone replacement therapy. Mild to 
moderate weight gain usually occurs during testosterone treatment 
because of the anabolic actions of testosterone on muscle mass and 
mild fluid retention. Testosterone therapy usually does not cause 
clinically significant edema, except occasionally in hypogonadal 
men with underlying edematous states such as CHF or hepatic 
cirrhosis.
Stimulation of excessive libido and erections by testosterone i

f excessive libido and erections by testosterone is 
rare and usually occurs in boys or young men with severe, long-
standing androgen deficiency who are treated with full replace­
ment or high-dose testosterone therapy. These symptoms usually 
resolve spontaneously or with a reduction in testosterone dose. 
Contrary to popular opinion, testosterone treatment does not 
cause pathologic aggressiveness, anger, or rage.682,683 Instead, 
testosterone replacement therapy increases social aggressivene

 replacement therapy increases social aggressiveness, 
motivation, and initiative and reduces irritability and anger.
Profound behavioral and physical changes induced by testos­
terone treatment in men with severe, long-standing androgen 
deficiency may be upsetting to both patients and their partners. 
Therefore, changes that are expected to occur with testosterone 
replacement should be discussed with patients and their partners 
before and during treatment. Oral 17α-alkylated androgens may 
c

ing treatment. Oral 17α-alkylated androgens may 
cause cholestasis or potentially serious hepatotoxicity.620 However, 
liver toxicity does not occur with testosterone replacement ther­
apy, and routine monitoring of liver enzymes is not necessary in 
hypogonadal men. IM injection of testosterone esters, particularly 
with large volume injections of testosterone undecanoate or rarely 
with testosterone enanthate and cypionate, has been reported to 
cause coughing that has been hypothesized to be 

cause coughing that has been hypothesized to be due to POME, 
the significance of which is not known (see earlier discussion). 
Formulation-Specific Adverse Effects. Testosterone ester injec­
tions may cause local discomfort, bleeding, or hematoma at the 
site of IM injections.116 Instruction on proper injection tech­
nique minimizes the likelihood of these adverse effects. Fluctua­
tions in energy, mood, and libido associated with the peak and 
nadir swings of testosterone concentrations afte

nadir swings of testosterone concentrations after testosterone ester 
injections may be disturbing to some hypogonadal men and may 
require reduction of the dose injected and shortening of the injec­
tion interval or switching to transdermal testosterone. Rarely, an 
allergy may occur to the sesame oil (enanthate) or cottonseed oil 
(cypionate) vehicle used.
Testosterone patches frequently cause local skin erythema, irrita­
tion, itching, and contact dermatitis and occasionally lead to more 
s

ontact dermatitis and occasionally lead to more 
severe reactions.116 Use of a topical corticosteroid cream under the 
reservoir of the patch may reduce skin irritation and reactions, but 
often men prefer to switch to another testosterone formulation. 
Testosterone patches may adhere poorly to skin, particularly with 
excessive perspiration.
In contrast to testosterone patches, testosterone gels and solution 
cause little to no skin irritation. However, residual testosterone 
remains on the ski

However, residual testosterone 
remains on the skin surface at the application sites, and there is a 
potential for transfer of testosterone to women and children who 
have prolonged intimate contact with these sites.116 Precautions to 
avoid contact transfer of testosterone include washing the hands 
immediately after application of testosterone gel, covering the 
application site with clothing, washing off residual testosterone 
on skin by showering or bathing (4–6 hours after application), 
a

ering or bathing (4–6 hours after application), 
and avoiding prolonged skin-to-skin contact of the application 
site with women or children.
Buccal testosterone tablets may cause gum irritation, inflamma­
tion or recession, or an altered or bitter taste sensation, and they 
may adhere poorly to gums if not they are not properly applied.116
Nasal testosterone gel may cause nasal irritation, nasopharyngi­
tis, rhinorrhea, epistaxis, and nasal scabbing.
SC testosterone pellets may uncommonly extru

bing.
SC testosterone pellets may uncommonly extrude spontane­
ously; rarely, there may be bleeding or infection at the site of 
implantation.116 
Gonadotropin Therapy
Secondary hypogonadism manifests as prepubertal or adult 
androgen deficiency and impairment of sperm production due 
to gonadotropin deficiency. The primary goal of gonadotropin 
therapy in men with secondary hypogonadism is to initiate and 
754
maintain spermatogenesis to establish an

 Function
maintain spermatogenesis to establish and restore fertility.614 
Because gonadotropin therapy is more complex (requiring mul­
tiple injections weekly) and more expensive than testosterone 
replacement therapy, symptomatic androgen deficiency is usually 
treated with the latter. In patients with partial gonadotropin defi­
ciency, testosterone treatment may suppress remaining gonado­
tropin secretion by negative feedback regulation. When fertility is 
desired and stimulation of sperm pro

fertility is 
desired and stimulation of sperm production is needed in a man 
with secondary hypogonadism, testosterone is discontinued and 
gonadotropin therapy is initiated. Previous testosterone treat­
ment does not compromise subsequent stimulation of spermato­
genesis by gonadotropins, although sperm production may be 
slower.392-394
The gonadotropin preparations most commonly used to treat 
secondary hypogonadism are purified urinary gonadotropins. 
Human recombinant gonadotropin preparati

tropins. 
Human recombinant gonadotropin preparations are now avail­
able and have much higher purity than urinary preparations, 
but they are more expensive. Because urinary gonadotropins are 
highly effective in treating gonadotropin deficiency, they remain 
the most commonly used preparations for treatment of infertility 
from secondary hypogonadism.
hCG is used to provide LH-like activity because its half-life is 
longer than that of LH. In contrast to LH, which would require 
pulsatile admi

ontrast to LH, which would require 
pulsatile administration approximately every 2 hours, hCG is 
administered two to three times per week. Purified urinary prep­
arations of hCG (derived from urine of pregnant women) that 
contain LH-like activity are used almost exclusively in gonado­
tropin therapy. FSH activity is usually provided by purified uri­
nary human menopausal gonadotropin (hMG), which is derived 
from urine of menopausal women and contains both LH and 
FSH; by highly purified urina

ontains both LH and 
FSH; by highly purified urinary human FSH; or by rhFSH. FSH 
preparations are administered three times weekly. hCG and FSH 
preparations may be administered either intramuscularly or sub­
cutaneously.684,685 Both are equally effective, but the latter is bet­
ter tolerated and more easily self-administered.
In patients with prepubertal gonadotropin deficiency, ini­
tiation of sperm production requires treatment with both hCG 
and FSH79 (Fig. 19.30). Gonadotropin therapy is in

and FSH79 (Fig. 19.30). Gonadotropin therapy is initiated with 
administration of hCG alone at a dose of 500 to 2000 IU SC two 
to three times weekly to stimulate sufficient endogenous testoster­
one production to increase and maintain circulating testosterone 
concentrations in the normal range and to correct manifesta­
tions of androgen deficiency.6 The dose of hCG is increased until 
serum testosterone concentrations are within the eugonadal range. 
Despite the more frequent injections, some 

ange. 
Despite the more frequent injections, some men with secondary 
hypogonadism prefer hCG over testosterone treatment because 
costs are comparable (purified urinary hCG preparations com­
pared with testosterone gels), SC injections are better tolerated 
than IM injections, testis size is generally larger, and spermatogen­
esis is increased. In addition, there is no risk of secondary transfer 
to intimate contacts, and some men prefer twice weekly SC injec­
tions to daily gel application.
Me

eekly SC injec­
Men with gonadotropin deficiency who also have primary tes­
ticular disease (e.g., cryptorchidism) or CHH may require larger 
doses of hCG (e.g., up to 3000–5000 IU two to three times weekly 
or occasionally much more). Men with secondary hypogonadism 
who have coexisting severe testicular damage may be completely 
unresponsive to hCG treatment (see Fig. 19.30). Because hCG 
stimulates Leydig cell aromatase activity within the testes, serum 
estrad

romatase activity within the testes, serum 
estradiol concentrations may increase disproportionately relative 
to testosterone concentrations, resulting in breast tenderness or 
gynecomastia more frequently than with testosterone replacement 
120
80
40
(170)
2
6
14
3
5
8
10
14
11
7
12
9
13
0
Total sperm count (× 106/ejaculate)
Pre-
treatment
hCG
Postpubertal
onset
Pre-
treatment
hCG
hCG and
hMG
Prepubertal
onset
19.21
17.18
20
16
15
Pre
treatment
hCG
hCG and
hMG
Prepubertal onset
and cryptorchid

hCG
hCG and
hMG
and cryptorchidism
• Fig. 19.30  Total sperm count response to gonadotropin therapy with human chorionic gonadotropin 
(hCG) alone or in combination with human menopausal gonadotropin (hMG) in males with hypogonado­
tropic hypogonadism of postpubertal onset (left), prepubertal onset without cryptorchidism (middle), or 
prepubertal onset with cryptorchidism (right). Sperm production was induced by hCG alone in all men 
with postpubertal hypogonadotropic hypogona

l men 
with postpubertal hypogonadotropic hypogonadism and in some with prepubertal onset who did not 
have cryptorchidism. Both hCG and hMG treatment was required to increase sperm production above 
the lower limit of normal (dashed line) in patients with prepubertal hypogonadotropic hypogonadism who 
did not have cryptorchidism. With one exception, men with prepubertal hypogonadotropic hypogonad­
ism who also had cryptorchidism failed to respond to hCG alone or to combined hCG and hMG therapy.

d to hCG alone or to combined hCG and hMG therapy. 
(From Finkel DM, Phillips JL, Snyder PJ. Stimulation of spermatogenesis by gonadotropins in men with 
hypogonadotropic hypogonadism. N Engl J Med. 1985;313:651–655.)
755
therapy. Some men may complain of burning at the site of SC 
injections of hCG.
hCG stimulates Leydig cell testosterone production, resulting 
in relatively high intratesticular testosterone concentrations that 
cause Sertoli cells to mature and

entrations that 
cause Sertoli cells to mature and sperm production to be initiated 
to varying degrees. In a small proportion of patients with partial 
gonadotropin deficiency of prepubertal onset and in almost all 
men with acquired postpubertal gonadotropin deficiency, treat­
ment with hCG alone may stimulate sperm production79 (see 
Fig. 19.30). Evidence for partial gonadotropin deficiency includes 
bilateral testicular volume larger than 8 mL, physical examination 
evidence of partial andro

L, physical examination 
evidence of partial androgenization, low-normal serum gonado­
tropin concentrations, and low-normal serum inhibin B con­
centrations.686-690 Most patients with severe CHH require FSH 
treatment in addition to hCG to stimulate complete spermato­
genesis and induce fertility.691-693
If no sperm is present in the ejaculate after 6 to 12 months 
of treatment with hCG and serum testosterone concentrations in 
the eugonadal range, hMG or FSH at a dose of 75 IU three times 
wee

ge, hMG or FSH at a dose of 75 IU three times 
weekly (increasing up to as much as 300 IU given subcutaneously 
three times weekly) is added, in combination with the same dose 
of hCG, for an additional 6 to 12 months or longer, until sper­
matogenesis is induced.
The induction of sperm production by gonadotropin ther­
apy may take 12 to 24 months. Factors that limit the duration 
of gonadotropin treatment are impatience by patients and their 
partners and the expense of gonadotropins, particula

rtners and the expense of gonadotropins, particularly hMG 
or FSH preparations, which are very costly. Some couples may 
opt for alternative means to have children (e.g., adoption). In 
patients with CHH, sperm output is often low, possibly because 
of inadequate gonadotropin stimulation of Sertoli cell number 
and maturation during development.694 Despite quantitatively 
low sperm production during gonadotropin therapy, fertility 
may be possible, sometimes at very low sperm counts (e.g., <1 
m

e, sometimes at very low sperm counts (e.g., <1 
million/mL).686 Some patients who have very low sperm counts 
and remain infertile on gonadotropin treatment may elect to use 
ejaculated sperm for ICSI.
Once spermatogenesis has been initiated with combined hCG 
and FSH treatment in patients with prepubertal gonadotropin 
deficiency, sperm production may be maintained with hCG treat­
ment alone.611 In men with acquired postpubertal gonadotropin 
deficiency, reinitiation of spermatogenesis can usu

eficiency, reinitiation of spermatogenesis can usually be accom­
plished with hCG therapy alone.79
In men with secondary hypogonadism due to hypothalamic 
GnRH deficiency (e.g., CHH, Kallmann syndrome), pulsatile 
GnRH therapy may be used to stimulate production of endog­
enous gonadotropins (both LH and FSH) and testosterone and 
to initiate and maintain sperm production sufficient for fertility.47 
GnRH is administered subcutaneously with the use of a portable 
infusion pump that delivers smal

se of a portable 
infusion pump that delivers small doses (pulses) of GnRH (e.g., 
GnRH 5–25 ng/kg SC every 1.5–2.0 hours, increasing to higher 
doses if needed); this treatment mimics a near-normal physiologic 
stimulus to the pituitary gland. In men with CHH, pulsatile 
GnRH treatment is successful in stimulating gonadotropin, tes­
tosterone, and sperm production in approximately 75% of cases; 
in the other 25% of cases, men fail to respond due to concomi­
tant pituitary or testicular defects.

 to concomi­
tant pituitary or testicular defects.396 The effectiveness of pulsatile 
GnRH replacement in stimulating sperm production is compa­
rable to that of gonadotropin therapy. Practically, however, the 
use of pulsatile GnRH therapy is limited to specialized centers 
because GnRH is not readily available, the infusion pump requires 
additional expertise and management, and therapy is expensive.
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20
Sexual Dysfunction in Men and Women
SHALENDER BHASIN AND R

ysfunction in Men and Women
SHALENDER BHASIN AND ROSEMARY BASSON
M
en and women rate sexual health as highly important 
to their quality of life.1 Recognizing the importance of 
sexual function as a determinant of quality of life, the 
World Health Organization declared sexual health a fundamen­
tal right of men and women. Yet disorders of sexual function are 
highly prevalent in men and women.2 Epidemiologic surveys, such 
as the National Survey of Sexual Attitudes and Lifestyle (Natsal-3), 
in

 of Sexual Attitudes and Lifestyle (Natsal-3), 
indicate that 42% of men and 51% of women experienced some 
problem with sexual response in the previous year, causing distress 
in the lives of 10% to 11%.2 The 2015 systematic review by the 
International Consensus Committee on Sexual Medicine found 
that low sexual interest was the most prevalent sexual concern for 
KEY POINTS
 • 
In contrast to the earlier sexual response model depicting a 
linear progression of discrete phases, current resear

ear progression of discrete phases, current research concep­
tualizes sexual response as a motivation/incentive-based cycle 
comprising phases of physiologic response. These phases of the 
cycle overlap, and their order is variable.
 • 
In middle-age and older men, sexual dysfunction is often re­
lated to comorbid conditions, such as diabetes, coronary artery 
disease, or a hormonal problem.
 • 
Penile erection results from biochemical and hemodynamic 
events that are associated with activatio

dynamic 
events that are associated with activation of central nervous 
system sites, cavernosal smooth muscle relaxation, increased 
blood flow into cavernosal sinuses, and venous occlusion.
 • 
Corporal smooth muscle tone is regulated by transmembrane 
and intracellular calcium flux, which in turn is regulated by 
potassium channels; connexin43-derived gap junctions; and 
cholinergic, adrenergic, and nonadrenergic, noncholinergic 
mediators, including nitric oxide.
 • 
Testosterone regulates

ncluding nitric oxide.
 • 
Testosterone regulates sexual thoughts and desire, sexual arous­
al, attentiveness to erotic stimuli, and sleep-entrained erections. 
Testosterone deficiency is a treatable cause of hypoactive sexual 
desire in men. In randomized trials, testosterone replacement 
therapy has been shown to improve erectile function and satis­
faction with intercourse.
 • 
Selective phosphodiesterase 5 inhibitors are safe and effective 
and have emerged as first-line therapy for men wi

and have emerged as first-line therapy for men with erectile 
dysfunction.
 • 
Sexual response is understood to be incentive based; multiple 
reasons for sex motivate receptivity to sexual stimuli that can be 
appraised as sexually arousing.
 • 
Physical and subjective arousal may diverge. Women complain­
ing of low sexual arousal usually physically respond to sexual 
stimuli in a laboratory setting. In contrast, men with erectile 
dysfunction from endothelial or neurologic deficit—the most 

estibulodynia—a chronic pain disorder associated with central 
sensitization of the nervous system and occasionally precipi­
tated by low-dose combined contraceptives.
Human Sexual Response Cycle, 757
Physiologic Mechanisms of Human Sexual Response, 758
Role of Testosterone in Regulating Sexual Function in Men, 765
Physiology of Physical Sexual Arousal in Women: Genital 
Congestion, 766
Physiology of Orgasm, 767
Revised Definitions of Sexual Dysfunction in Men, 767
Current Defini

s of Sexual Dysfunction in Men, 767
Current Definitions of Sexual Disorders in Women, 770
Sexual Dysfunction in the Context of Endocrine Disease, 771
Assessment of Sexual Dysfunction, 775
Management of Sexual Dysfunction in Men, 779
Management of Sexual Dysfunction in Women, 790
757
Chapter 20  Sexual Dysfunction in Men and Women
women and erectile dysfunction (ED) the most common form 
of sexual dysfunction in men.3 Among women, the loss of plea­
sure and the loss of interest were the most comm

­
sure and the loss of interest were the most commonly reported 
problems during the previous year, and their incidence was similar 
among younger and older women.3 In contrast, middle-age and 
older men reported higher prevalence of sexual dysfunction than 
younger men; the prevalence of ED ranged from less than 20% 
among men younger than 50 years of age to 25% to 76% among 
men older than 70 years.3
Sexual response exemplifies the intricate blending of mind 
and body so that dysfunction is ne

ending of mind 
and body so that dysfunction is neither purely psychological nor 
entirely biologic, but psychological and biologic factors both con­
tribute to sexual dysfunction to varying degrees. Recent advances 
in our understanding of the physiologic and biochemical mecha­
nisms of penile erection and the development of mechanism-
specific therapies for ED have greatly clarified the importance of 
vascular health in maintaining erectile function. The 1980s and 
1990s witnessed remarkable p

ction. The 1980s and 
1990s witnessed remarkable progress in our understanding of the 
physicochemical mechanisms that lead to penile tumescence and 
rigidity. It was recognized that penile erections are the result of 
cavernosal smooth muscle relaxation and increased penile blood 
flow.4–6 The appreciation of nitric oxide as a key vasodilator in 
the vascular smooth muscle was a pivotal discovery, recognized 
later by awarding of the Nobel Prize in Physiology or Medicine 
to Robert F. Furchgott

 in Physiology or Medicine 
to Robert F. Furchgott, Louis J. Ignarro, and Ferid Murad. The 
recognition that nitric oxide caused cavernosal smooth muscle 
relaxation by simulating guanylyl cyclase provided the foundation 
for the discovery of highly effective oral therapies for the treat­
ment of ED.
The growing recognition that ED is commonly a manifestation 
of systemic disease4,7–12 and the availability of easy-to-use thera­
peutic options, including oral and intraurethral drugs, have duly 
p

cluding oral and intraurethral drugs, have duly 
placed sexual disorders in men within the purview of the endocri­
nologist and the primary care provider. In middle-age and older 
men, but less so for women, sexual dysfunction is often related to 
comorbid medical conditions.4,7–12 ED may signal asymptomatic 
coronary artery disease,13,14 diabetes, or depression.2 In women, 
sexual dysfunction is very strongly linked to mental health.2,14–17 
A similarly strong association was found between sexu

imilarly strong association was found between sexual dysfunc­
tion and relationship unhappiness in both men and women. The 
American Psychiatric Association’s clinical definitions of sexual 
disorders published in the fifth edition of Diagnostic and Statisti­
cal Manual of Mental Disorders (DSM-5) in 201318 attempted to 
quantify symptom duration and severity more precisely, address 
the overlap between sexual desire and arousal in women, and 
include the important entity of sexual motivation/in

clude the important entity of sexual motivation/interest. Also 
included in DSM-5 are previously ignored aspects of women’s sex­
ual response, such as mental sexual arousal/excitement, pleasure, 
ability to trigger arousal and desire from sexual stimuli, and physi­
cal genital and nongenital sexual sensations (Table 20.1).
This chapter describes the current conceptualization of sexual 
response in men and women, the underlying pathophysiologic 
mechanisms of sexual dysfunction, the sexual sequel

echanisms of sexual dysfunction, the sexual sequelae of various 
endocrine disorders, and clinical assessment and diagnosis of sex­
ual dysfunction and its management. Management strategies for 
sexual dysfunction stemming from hormonal and nonhormonal 
factors are also outlined.
Human Sexual Response Cycle
The sexual response can be conceptualized as a motivation/
incentive-based cycle comprising phases of physiologic response 
and subjective experience.19–22 The phases of the cycle overlap, 
a

perience.19–22 The phases of the cycle overlap, 
and their order is variable (Fig. 20.1). The motivations and 
incentives for sex are multiple and varied. A wish to both dem­
onstrate and enhance emotional intimacy between partners is 
important for both men and women.23 Depression is a major 
cause of reduced sexual motivation in otherwise healthy persons 
and in those with endocrine disease: comorbid depression has 
been identified as a major factor underlying sexual dysfunction 
in women with

actor underlying sexual dysfunction 
in women with diabetes.24–26 Endocrine disorders can mark­
edly lessen sexual self-image, especially when associated with 
altered appearances, infertility, or the ability to be gainfully 
employed.24,25,27,28
Sexual desire—as in lust or drive triggered deliberately, acci­
dently, even subliminally,29 by sexual stimuli—is only one of 
many reasons people engage in sex and may or may not be sensed 
initially: desire can be triggered by the sexual excitement (i

esire can be triggered by the sexual excitement (i.e., the 
subjective sexual arousal) in response to sexual stimuli.19,24,30,31 In 
both men and women, the relationship between desire and arousal 
is variable, complex, and often inseparable.32–34 This overlap of 
phases is consistent with the neuroimaging data of sexual arousal, 
which have led to the concept that motivation is one facet of sex­
ual arousal and desire is one component of motivation.35 Many 
psychological and biologic factors in

external sexual/erotic cues (e.g., written, verbal, visual)
	6.	 Absent/reduced frequency or intensity of genital or nongenital 
sensations during sexual activity on all or almost all (approximately 
75%) sexual encounters
Female Orgasmic Disorder
At least one of the two following symptoms, which must have been 
present for a minimum duration of approximately 6 months and be 
experienced on all or almost all (approximately 75%) occasions of 
sexual activity:
	1.	 Marked delay in, marked infreq

	3.	 Marked fear or anxiety either about vulvovaginal or pelvic pain on 
vaginal penetration
	4.	 Marked tensing or tightening of the pelvic floor muscles during 
attempted vaginal penetration
TABLE 20.1
758
arousal.16,23,24,26,30-32,34–42 The sexual and nonsexual outcomes 
influence future sexual motivation. The circle depicted in Fig. 
20.1 may be partially or completely repeated several times during 
any given sexual encounter.19 The sexual resp

ing 
any given sexual encounter.19 The sexual response cycle varies sub­
stantially among individuals and even within a person’s own sexual 
life, and is influenced by multiple factors, including the stage of 
life cycle, age, relationship duration and happiness, and mental 
health34 (see Fig. 20.1).
Even with sufficient sexual motivation and the presence of 
adequate stimuli, the arousal and pleasure may not occur if 
attention is not focused.38 The attentional processes play a cen­
tral role i

 The attentional processes play a cen­
tral role in facilitating the subjective and also the physiologic 
components of sexual arousal.38 Sexual information is pro­
cessed in the mind both automatically and consciously.39 The 
sexual nature of the stimuli is processed by the limbic system, 
allowing genital congestion (observed to be rapid and involun­
tary in women and slower but still involuntary in men).39 Con­
scious appraisal of sexual stimuli and contextual cues can lead 
to subjective aro

li and contextual cues can lead 
to subjective arousal.39-42 The latter may be further increased 
by awareness of the genital congestion of arousal, which is 
typically accurately registered and very relevant to men’s expe­
rience.19 For women, an important component of arousal is 
the change in perception of genital sensitivity such that touch 
is no longer neutral or even unpleasant but rather sexual and 
pleasurable. Subjective arousal is also cognitively appraised to 
evaluate whether sex is

 cognitively appraised to 
evaluate whether sex is pleasurable and safe or whether it is 
shameful or likely to have a negative consequence.39-42 These 
cognitive appraisals continually modify both the physiologic 
and subjective responses.39,40
Undue focus on nonerotic thoughts during sexual stimulation, 
generated possibly by anxiety, can be associated with sexual prob­
lems.40 A study of men and women in long-term relationships 
found that women tended to report nonerotic thoughts about 
thei

en tended to report nonerotic thoughts about 
their body image and the consequences of sexual activity, whereas 
men were more likely to report nonerotic thoughts about prob­
lematic sexual performance.41 A high frequency of nonerotic 
thoughts can be associated with increased likelihood of sexual dys­
function. Importantly, the more difficult it was to refocus back 
on an erotic thought, the more this predicted sexual difficulty 
occurred. This research is clearly relevant to patients with endo

research is clearly relevant to patients with endo­
crine disease, which frequently has negative impact on sexual self-
image and sexual function.24,25
The dual control theory for sexual appraisal by Bancroft and 
associates43 envisions a balance between sexual activation and 
sexual inhibition in an individual’s brain that determines whether 
sexual stimulation leads to arousal. In men, the inhibitions include 
the threat of performance failure and the threat of performance 
consequences; in wo

and the threat of performance 
consequences; in women, relationship importance (reflecting the 
need for sex to be within a specific type of relationship), con­
cerns about sexual function (worries and distractions about sexual 
function), and arousal contingency (the potential for arousal to 
be inhibited by contextual/situational factors) were inhibiting 
factors.43
Thus, the current conceptualization of men’s and wom­
en’s sexual responses is in marked contrast to an earlier model 
depicting 

in marked contrast to an earlier model 
depicting a linear invariable progression of discrete phases—
from desire to arousal to a plateau of high arousal, followed by 
orgasm/ejaculation and finally a phase of resolution.44 Women’s 
sexual dysfunction typically involves lessened arousal and desire 
and lessened frequency of orgasm, which is now embodied in 
DSM-5 as sexual interest/arousal disorder (SIAD)45,46 (see Table 
20.1). Although the focus in men has typically been on ED or 
premature ej

s in men has typically been on ED or 
premature ejaculation, they too may experience a more general­
ized sexual distress disorder affecting desire, erectile function, 
and ease of orgasm.10,42 
Physiologic Mechanisms of Human 
Sexual Response
Physiology of Desire and Arousal
Functional Brain Imaging of Sexual Arousal in 
Men and Women
Functional neuroimaging studies have clarified some of the neural 
correlates of sexual response.47–53 These brain imaging studies dur­
ing sexual stimulation—mos

in imaging studies dur­
ing sexual stimulation—most of which were performed in healthy 
male heterosexual volunteers—has revealed engagement of a com­
plex brain circuitry that involves activation of brain regions related 
to the different aspects of arousal and deactivation of other brain 
regions related to the inhibition of sexual arousal.47–53
In keeping with the current circular model of sexual response 
(depicting sexual incentives/motivations, information processing, 
overlap of arousal a

ons, information processing, 
overlap of arousal and desire, emphasis on both subjective and 
physiologic arousal, plus importance of reward), the model of 
sexual arousal emanating from the neuroimaging data comprises 
cognitive, motivational, emotional, and autonomic components35 
(Fig. 20.2). The cognitive component includes appraisal of poten­
tially sexual stimuli, focused attention on those stimuli appraised 
as erotic, and imagery of actual sexual activity. The activations of 
the right l

l sexual activity. The activations of 
the right lateral orbitofrontal cortex (OFC), of the right and the 
left inferior temporal cortices, of the superior parietal lobules, and 
of areas belonging to the neural network mediating motor imag­
ery (inferior parietal lobules, left ventral premotor area, right and 
Multiple incentives for sex
include desire/arousal
from sexual cues
Attention to
stimuli & cues
Appraisal
of stimuli
Biologic
factors
factors
Arousal:
Subjective
+ ANS respo

chological
factors
Arousal:
+ ANS response
Arousal
triggers/merges
with sense of
desire
Sexual
satisfaction
& absence
of pain
Other rewards—
emotional intimacy,
increased well-being
• Fig. 20.1  Circular human response cycle of vverlapping phases. Human 
sexual response is depicted as a motivation/incentive-based cycle of over­
lapping phases of variable order. A sense of desire may or may not be 
present initially: it can be triggered alongside the sexual arousal resulting 
from atte

 alongside the sexual arousal resulting 
from attending to sexual stimuli. Psychological and biologic factors influ­
ence the brain’s appraisal of the sexual stimuli. Sexual arousal comprises 
subjective (pleasure/excitement/wanting more of the same) and physical 
(genital and nongenital) responses. The merged desire and arousal influ­
ence the ongoing attention to and appraisal of further sexual stimulation. 
The sexual and nonsexual outcomes influence present and future sexual 
motivation. ANS

fluence present and future sexual 
motivation. ANS, autonomic nervous system. (Modified from Basson R. 
Human sexual response. Handb Clin Neurol. 2015;130:11–18.)
759
left supplementary motor areas, cerebellum) are considered to be 
the neural correlates of this cognitive component. The motiva­
tional component comprises the processes that direct behavior to 
a sexual goal, including the perceived urge to express overt sexual 
behavior. Thus, the m

rge to express overt sexual 
behavior. Thus, the motivational component is conceptualized 
as including the experience of sexual desire. Neural correlates are 
thought to involve the anterior cingulate cortex (ACC), claus­
trum, posterior parietal cortex, hypothalamus, substantia nigra, 
and ventral striatum. The emotional component is the brain activ­
ity underlying the pleasure from the mental excitement and the 
perception of genital and other physical responses. This pleasure 
comprises liki

 physical responses. This pleasure 
comprises liking and wanting.54 The left primary and secondary 
somatosensory cortices, the amygdalae, and the right posterior 
insula are conceived as neural correlates of this emotional com­
ponent. The autonomic and neuroendocrine component includes 
the various responses (e.g., genital, cardiovascular, respiratory, 
changes in hormonal plasma levels) that allow preparedness for 
sexual activity: activations of the ACC, anterior insulae, puta­
mens, and hyp

 of the ACC, anterior insulae, puta­
mens, and hypothalamus may contribute to this component.
The brain imaging studies during sexual arousal have high­
lighted three inhibitory components35:
	
1.	 Inhibition mediated by regions in the temporal lobes and 
the gyrus rectus of the OFC in the resting state. Patients 
with lesions in the gyrus rectus are noted to have excessive 
appetite for sexual and other pleasurable activities.55 The 
Klüver-Bucy syndrome, a rare neurologic disorder associ­
ate

y syndrome, a rare neurologic disorder associ­
ated with damage to both temporal lobes, is characterized 
by hypersexuality.56 The inhibitory regions of the temporal 
lobes are distinct from those activated in response to visual 
sexual stimuli.
	
2.	 Inhibition of arousal once it has begun, to limit its expres­
sion because the circumstances are inappropriate, is medi­
ated in the caudate nucleus and putamen. This is consistent 
with the case reports of hypersexuality associated with 
lesions 

eports of hypersexuality associated with 
lesions in the head of the caudate nuclei.57
	
3.	 Activation of the left OFC is thought to block the ability 
of sexual stimuli to arouse. The brain regions that mediate 
inhibition of sexual arousal have been found to be activated 
during tasks that involve moral judgment, guilt, and shame.
In general, men typically show greater responsiveness to visual 
sexually arousing stimuli than do women.58 Brain imaging in hypo­
gonadal men before and after tes

 imaging in hypo­
gonadal men before and after testosterone replacement therapy 
suggests that the left OFC might exert a testosterone-dependent 
inhibitory tonic control on sexual arousal and that this inhibitory 
control decreases on visual sexual stimulation.53 In addition, the 
response of the right anterior insula to visual sexual stimulation 
appears to be related to circulating testosterone levels.53
Functional imaging during penile or clitoral stimulation to 
orgasm indicates that women 

toral stimulation to 
orgasm indicates that women show more activation in left fron­
toparietal regions, notably in the posterior parietal cortex and the 
supplementary motor area—regions associated with making a 
mental representation of another person’s actions.49 These find­
ings may reflect gender differences in perspective and empathy. 
The brain responses during the orgasm(s) are similar in men and 
women, but men and women use different cerebral strategies to 
reach orgasm.49
The complexi

rebral strategies to 
The complexity and variability of these systems was reflected in 
a study of surgically menopausal women who were sexually active 
but were not receiving hormonal therapy.48 When these surgically 
menopausal women viewed erotica during functional magnetic 
resonance imaging (fMRI), they failed to display the brain activa­
tion observed in premenopausal women or when they were treated 
with testosterone and estrogen. However, these women reported 
sexual arou

trogen. However, these women reported 
sexual arousal from the erotic videos regardless of the hormonal 
supplementation.48
Functional imaging studies in women with DSM-IV hypoac­
tive sexual desire disorder (HSDD) show, unsurprisingly, reduced 
activity in sexual desire networks and increased activity in self-
referential brain networks, areas involved in moral judgments, 
shyness, and visual analyses.59 It is encouraging to note that even 
Emotional
Pleasure from subjective
arousal and from
se

ional
sensations including
genital congestion
Motivational
Expectation of
sexual and nonsexual
rewards
Desire triggered
Cognitive
Appraisal of
sexual stimuli
Focused attention
Imagery of
sexual acts
Censoring arousal
to prevent sexual
behavior
Devaluing
stimuli
Autonomic
Unconscious reflex
becomes a conscious
reinforcing stimulus
especially in men
• Fig. 20.2  Brain areas activated during sexual arousal. Brain areas activated during sexual arousal allow 

Brain areas activated during sexual arousal allow 
(1) continued focus on sexual stimuli, imaging of sexual behavior, evaluation/censorship and limitation or 
prevention of actual behavior despite arousal (all constituting a cognitive component of arousal); (2) sexual 
feelings (an emotional component); (3) anticipation of reward (a motivational component); and (4) an auto­
nomic/neuroendocrine response of physical sexual arousal. (Modified from Basson R, Weijmar Schultz W. 
Sexual sequelae of g

Basson R, Weijmar Schultz W. 
Sexual sequelae of general medical disorders. Lancet. 2007;369[9559]:409–424.)
760
structural changes associated with chronic pain can reverse with 
treatment, as these data suggest that both anatomic and functional 
changes associated with HSDD may be reversible with appropri­
ate treatment.52
Neurotransmitters and Hormones Involved in Sexual Desire 
and Subjective Arousal
A variety of hormones and peptides are involved 

l
A variety of hormones and peptides are involved in the sexual 
response. The interplay among androgens and neurotransmitters 
is complex60–70: androgens influence neurotransmitter release, 
and neurotransmitters may modulate androgen receptor signal­
ing.58,62,64 The role of testosterone in desire and arousal is well 
documented in men but far less so in women.63,64 Circulating lev­
els of testosterone or androgen metabolites do not correlate with 
women’s sexual function in large epidemiologi

ith 
women’s sexual function in large epidemiologic studies.65–67,71 The 
radioimmunoassays used to measure testosterone concentrations in 
many epidemiologic studies were designed to measure the substan­
tially higher levels of testosterone in men and lacked the sensitivity, 
precision, and accuracy in the low range prevalent in women. In 
studies that have used sensitive mass spectrometry–based assays to 
measure testosterone levels, no significant differences have been 
found in serum testost

cant differences have been 
found in serum testosterone levels between women with low desire 
and low subjective arousal and healthy controls.67,71,72 Addition­
ally, Labrie and associates68 proposed circulating levels of androgen 
metabolites—most notably androsterone glucuronide (ADT-G)—
as markers of ovarian plus intracrine androgen activity. Circulat­
ing ADT-G levels decrease with age in women.68 However, serum 
ADT-G levels have not been found to differ significantly between 
women with lo

und to differ significantly between 
women with low desire and women without low desire67,71 or to 
be associated with sexual dysfunction.72
ADT-G is viewed by many as a good marker of total androgenic 
activity. The C19 steroids 11β-hydroxyandrostenedione and tes­
tosterone can be converted to the androgenic steroid 11-ketotes­
tosterone (11-KT) that can induce androgen receptor–mediated 
transactivation. 11-KT can be converted to 11-ketodihydrotestos­
terone, which can also activate androgen r

testos­
terone, which can also activate androgen receptor signaling. The 
enzymes required for the production of 11-KT—P450 11B1 and 
HSD11B2—are expressed in the adrenal, peripheral tissues, and 
weakly in testes and the ovaries.73 The circulating concentrations 
of 11-KT are similar to those of testosterone73 in healthy women 
and have been reported to be increased in women with polycystic 
ovary syndrome and 21-hydroxylase deficiency.74 Although 11-KT 
is known to serve as a major androgen in

gh 11-KT 
is known to serve as a major androgen in teleosts,75 its precise role 
in androgenic disorders in women is incompletely understood. 
Animal Models
In animal models, sex steroid hormones modulate sexual arousal by 
directing the synthesis of the enzymes and the receptors for neu­
rotransmitters, such as dopamine, noradrenaline, melanocortins, 
and oxytocin.60,61,69,70,76,77 Systems that act within the hypothala­
mus and limbic regions of the brain are involved in arousal, atten­
tion, a

 the brain are involved in arousal, atten­
tion, and sexual behavior. It is thought that dopamine transmission 
in the medial preoptic area (MPOA) and the nucleus accumbens 
focuses the person’s attention on sexual stimuli (the incentives or 
motivations for sexual activity). It is postulated that the behavioral 
pattern stimulated by these systems and the subjective feelings that 
accompany them constitute the phenomenon commonly referred 
to as sexual desire or arousal. The main part of this n

 sexual desire or arousal. The main part of this neural pathway 
includes the MPOA and its outputs to the ventral tegmental area. 
The latter contains dopaminergic cell bodies that project to vari­
ous limbic and cortical regions, including the prefrontal cortex, the 
nucleus accumbens, the ACC, and the amygdala.
Brain pathways for sexual inhibition include opioid, endocan­
nabinoid, and serotonin neural transmissions feeding back to 
various levels of the excitatory pathways.61,62 It is thought

els of the excitatory pathways.61,62 It is thought that the 
behavioral pattern stimulated by the inhibitory pathways includes 
both refractoriness to sexual reward and satiety.
Exogenous opiates are sexually inhibiting independent of their 
inhibitory effect on luteinizing hormone (LH), gonadotropin-
releasing hormone (GnRH), and testosterone.76 Endogenous 
opioids modulate the feedback effects of sex steroids on the hypo­
thalamus and pituitary.76 β-Endorphin is synthesized in the ante­
rior p

.76 β-Endorphin is synthesized in the ante­
rior pituitary, the hypothalamus, and the nucleus of the tractus 
solitarius in the brainstem. The sexual inhibiting effects of opioids 
occur mainly through their action in the MPOA and the amyg­
dala.76 Low doses of opiates can have facilitating effects, possibly 
through actions in the ventral tegmental area to activate the meso­
limbic dopamine system. Exogenous opiates can induce an intense 
feeling of pleasure, which has been likened to orgasm, f

g of pleasure, which has been likened to orgasm, followed by 
a state of relaxation and calm.77
Melanocortins, derived from pro-opiomelanocortin, modulate 
sexual response through a specific receptor subtype, the melano­
cortin-4 receptor. Administration of melanocortin receptor ago­
nists has been associated with an increase in spontaneous erections 
in healthy men and in men with ED, and with increased desire, 
but not genital responses, in women.78,79
Oxytocin levels increase close to orgasm.

en.78,79
Oxytocin levels increase close to orgasm. This hormone is 
known to be involved in pair bonding in some animal species, but 
its relevance in humans is less clear. The physiologic role of prolac­
tin in the human sexual response remains uncertain.80–82 Because 
a generalized reduction of dopamine activity in the hypothalamus 
results in increased prolactin secretion, it has been difficult to dis­
tinguish between the effects of raised prolactin itself and the pos­
sible effects of the r

olactin itself and the pos­
sible effects of the reduced dopamine transmission. High levels of 
prolactin are associated with impaired sexual function in men and 
less clearly in women.76,82
The effects of the biologic factors are intertwined with those 
of the environmental and social factors. For instance, dopamine 
and progesterone, acting on their cognate receptors in the hypo­
thalamus, can increase sexual behavior in oophorectomized, estro­
genized female rats, and the presence of a male a

genized female rats, and the presence of a male animal alongside 
the cage can cause an identical stimulation of the sexual behavior 
without the administration of either progesterone or dopamine.83 
In rodents, birds, and fish, complex neural networks enable the 
animal to assess the context of potential sexual activity and relate 
it to past experience and to the expectation of reward.84 
Genital Sexual Congestion and Arousal
Men and women differ substantially with respect to the corre­
latio

fer substantially with respect to the corre­
lation between genital congestion and subjective sexual arousal 
(excitement). Whereas subjective arousal is typically concordant 
with genital congestion in men, there is highly variable but often 
poor correlation between subjective arousal and measures of geni­
tal congestion in women.85,86 There are some exceptions in men: 
sleep-related erections are mostly dissociated from erotic dreams 
or from subjective sexual arousal.87 In addition, psychoph

subjective sexual arousal.87 In addition, psychophysiologic 
studies have found that men can get erections in response to films 
of assault or rape while experiencing no subjective arousal.88 In 
contrast, a psychophysiologic study identified some 25% of men 
in a community sample with minimal penile response to an erotic 
video while their subjective arousal was similar to the remaining 
75% of men with recorded penile congestion. There is preliminary 
evidence that mindfulness practice may inc

minary 
evidence that mindfulness practice may increase concordance in 
association with improved sexual function.89
761
In contrast to men’s typically accurate assessment of their erec­
tions, women’s assessment of their degree of genital congestion 
is less accurate. It is thought that genital congestion in women 
is a prompt, automatic reflex that occurs within seconds of an 
erotic stimulus; it may not be deemed at all sexually arousing by 
the

may not be deemed at all sexually arousing by 
the woman, or it may even be deemed emotionally negative.90 
Viewing primates engaging in sexual activity subjectively arouses 
neither young men nor young women.85 However, the young 
women viewing primate sex display marked genital congestion, as 
measured by vaginal photoplethysmography, whereas no genital 
response occurs in the men. Similarly, heterosexual women view­
ing lesbian women engaged in sexual activity report mostly low 
subjective ar

n sexual activity report mostly low 
subjective arousal but show a prompt vasocongestive response; in 
contrast, heterosexual men viewing male same-sex activity show 
minimal genital or subjective response.85 
Physiologic Mechanisms of Penile Erection
Penile Anatomy and Blood Flow
The erectile tissue of the penis consists of two dorsally positioned 
corpora cavernosa and a ventrally placed corpus spongiosum.5,6,91,92 
The erectile tissue of both the corpora cavernosa and corpus spon­
giosum is c

the corpora cavernosa and corpus spon­
giosum is composed of numerous cavernous spaces separated 
by trabeculae.5,6,91,92 These trabeculae are composed mainly of 
smooth muscle cells that are arranged in a syncytium. Endothelial 
cells cover the surfaces of the trabeculae.
The penile arterial blood supply is derived from pudendal arter­
ies, which are branches of the internal iliac arteries (Fig. 20.3). The 
pudendal artery divides into cavernosal, dorsal penile, and bulbo­
urethral arteries. Th

l, dorsal penile, and bulbo­
urethral arteries. The cavernosal arteries and their branches, the 
helicine arteries, provide blood flow to the corpora cavernosa.5,6 
Dilatation of the helicine arteries increases blood flow and pres­
sure in the cavernosal sinuses.5,6,91,92 
Penile Innveration
The neural input to the penis consists of sympathetic (T11-L2), 
parasympathetic (S2-S4), and somatic nerves92 (Table 20.2). 
Sympathetic and parasympathetic fibers converge in the infe­
rior hypogastric ple

 fibers converge in the infe­
rior hypogastric plexus, where the autonomic input to the penis 
is integrated and communicated to the penis through caverno­
sal nerves. In men the inferior hypogastric ganglionic plexus is 
located retroperitoneally near the rectum.6,92
Prostate
Deep dorsal vein
Deep
dorsal vein
Dorsal nerve
(somatic)
Cavernous nerve
(autonomic)
Circumflex
artery
vein
vein
Flaccid
Tunica
albuginea
Sinusoidal
spaces
spaces
Corpora
cavernosa
Cavernou

paces
spaces
Corpora
cavernosa
Cavernous
artery
Helicine
arteries
Helicine
arteries
Trabecular
smooth muscle
Subtunical
venular plexus
Erect
Dorsal
artery
Dorsal artery
(somatic)
• Fig. 20.3  Anatomy and mechanism of penile erection. Corpora cavernosa are made up of trabecular 
spaces that are surrounded by cavernosal smooth muscle. Helicine arteries provide the arterial supply to 
the cavernosal spaces. The dorsal nerve provides the sensory innervation to the penis. Duri

rovides the sensory innervation to the penis. During erection, 
the relaxation of the trabecular smooth muscle and increased blood flow result in engorgement of the sinu­
soidal spaces in the corpora cavernosa. The expansion of the sinusoids compresses the venous return 
against the tunica albuginea resulting in entrapment of blood. This imparts rigidity to the tumescent penis. 
(Adapted from Lue TF. Erectile dysfunction. N Engl J Med. 2000;342[24]:1802–1813.)
762
SECTION V    Sexual Development

4]:1802–1813.)
762
Several brain regions, including the amygdala, MPOA, para­
ventricular nucleus of the hypothalamus, and periaqueductal gray 
matter, act coordinately to affect penile erections.92 The MPOA 
of the hypothalamus serves as the integration site for the central 
nervous system control of erections; it receives sensory input from 
the amygdala and sends impulses to the paraventricular nuclei of 
the hypothalamus and the periaqueductal gra

ei of 
the hypothalamus and the periaqueductal gray matter. Neurons in 
paraventricular nuclei project onto the thoracolumbar and sacral 
nuclei associated with erections.
The parasympathetic input to the penis is proerectile, and 
sympathetic input is mainly inhibitory.92 The stimuli from the 
perineum and lower urinary tract are carried to the penis through 
the sacral reflex arc.92 
Hemodynamic Changes During Penile Erection
Penile erection results from a series of biochemical and hemody­
nam

sults from a series of biochemical and hemody­
namic events that are associated with activation of central nervous 
system sites involved in regulation of erections, relaxation of cav­
ernosal smooth muscle, increased blood flow into the cavernosal 
sinuses, and venous occlusion resulting in penile engorgement and 
rigidity.5,91 Normal penile erection requires coordinated involve­
ment of intact central and peripheral nervous systems, corpora 
cavernosa and spongiosa, and normal arterial blood s

vernosa and spongiosa, and normal arterial blood supply and 
venous drainage.5,91
As cavernosal smooth muscle relaxes and the blood flow 
to the penis increases, the increased pooling of blood in the 
cavernosal spaces results in penile engorgement5,91 (see Fig. 
20.3). The expanding corpora cavernosa compress the venules 
against the rigid tunica albuginea, restricting the venous out­
flow from the cavernosal spaces.5,91 This facilitates entrapment 
of blood in the cavernosal sinuses, imparting

ent 
of blood in the cavernosal sinuses, imparting rigidity to the 
erect penis. 
Biochemical Regulation of Cavernosal Smooth 
Muscle Tone
The tone of the corporal smooth muscle cells determines the erec­
tile state of the penis.5,6,91 When the cavernosal smooth muscle 
cells are relaxed, the penis is engorged with blood and erect. 
When the cavernosal smooth muscle cells are contracted, there 
is predominance of sympathetic neural activity, and the penis is 
flaccid.92
The smooth muscle tone in

the penis is 
The smooth muscle tone in the corpora cavernosa is main­
tained by the release of stored intracellular calcium into the cyto­
plasm and influx of calcium through membrane channels.93–96 
The transmembrane influx of calcium in the cavernosal smooth 
muscle cells is mediated mostly by L-type voltage-dependent 
calcium channels, although T-type calcium channels are also 
expressed in cavernosal smooth muscle cells.93–96 An increase in 
intracellular calcium activates myosin

ncrease in 
intracellular calcium activates myosin light chain kinase, resulting 
in phosphorylation of myosin light chain, actin-myosin interac­
tions, and smooth muscle contraction.96
The transmembrane and intracellular calcium flux in the caver­
nosal smooth muscle cells is regulated by several cellular processes 
that involve K+ flux through potassium channels; connexin43-
derived gap junctions; and several cholinergic, adrenergic, and 
nonadrenergic, noncholinergic mediators93–101 (Figs. 20

renergic, noncholinergic mediators93–101 (Figs. 20.4–20.6). 
The nonadrenergic, noncholinergic mediators include vasoactive 
intestinal peptide (VIP), calcitonin gene–related peptide (CGRP), 
and nitric oxide.101
Prostaglandin E1 (PGE1) binding to its cognate recep­
tor results in generation of cyclic adenosine monophosphate 
  Innervation of the Penis
Types of Fibers
Location of Neurons 
in the Spinal Cord
Nerves Carrying the Fibers
General Function
Sympathetic
T10-L2
Prevertebral outflow thro

ction
T10-L2
Prevertebral outflow through the hypogastric and cavernous nerves; 
additionally, paravertebral outflow through the parasympathetic 
ganglia, and pudendal or pelvic and cavernous nerves
Generally antierectile; sympathetic 
innervation plays an important role 
in regulating seminal emission
Parasympathetic
S2-S4
Cavernosal and pelvic nerves
Proerectile
Somatic
S2-S4
Pudendal nerve
Penile sensation, contraction of the 
striated muscles during ejaculation
TABLE 20.2
PGE1
G

ated muscles during ejaculation
TABLE 20.2
PGE1
G
–
cAMP
PKA
+
+
K+
K+
[Ca2+]i
Ca2+
Ca2+
• Fig. 20.4  Regulation of cavernosal smooth muscle contractility by PGE1. 
The relaxation of the cavernosal smooth muscle is regulated by intracellu­
lar cAMP and cGMP. These intracellular second messengers by activation 
of specific protein kinases cause sequestration of intracellular calcium and 
closure of calcium channels, and opening of K+ channels. This results in 
a net decrease in intracellular calc

s results in 
a net decrease in intracellular calcium causing smooth muscle relaxation. 
PGE1 by binding to PGE1 receptor increases the intracellular concentra­
tions of cAMP, which activates PKA. PKA promotes the sequestration of 
intracellular calcium and inhibits calcium influx and stimulates K+ chan­
nels. The nest result is a decrease in intracytoplasmic calcium and smooth 
muscle relaxation. Prostaglandin E1 stimulates cAMP generation. cAMP, 
3′,5′-cyclic adenosine monophosphate; cGMP, cyc

, 
3′,5′-cyclic adenosine monophosphate; cGMP, cyclic guanosine mono­
phosphate; PGE1, prostaglandin E1; PKA, protein kinase A. (Adapted from 
Bhasin S, Benson GS. Male sexual function. In: De Kretser D, ed. Knobil 
and Neill’s Physiology of Reproduction, 3rd ed. Boston, MA: Academic 
Press; 2006:1173–1194 and Lue TF. Erectile dysfunction. N Engl J Med. 
2000;342[24]:1802–1813.)
763
(cAMP), which activates protein kinase A. Activated protein 
kinas

tivates protein kinase A. Activated protein 
kinase A gates K+ channels, resulting in K+ efflux from the cell 
(see Fig. 20.4). The protein kinase A–mediated processes also 
result in a net decrease in intracellular calcium, favoring smooth 
muscle cell relaxation.
Adrenergic pathways, acting through norepinephrine and α1-
adrenergic receptors, activate phospholipase C, which generates 
diacyl glycerol and inositol trisphosphate (IP3).96 Diacyl glycerol 
activates protein kinase C, which inhibit

lycerol 
activates protein kinase C, which inhibits K+ channels and activates 
transmembrane calcium influx by activating L-type calcium chan­
nels97,98 (see Fig. 20.5). IP3 increases intracellular calcium by promot­
ing the release of calcium from intracellular calcium stores.97,98 The 
net increase in intracellular calcium promotes actin-myosin interac­
tions, resulting in smooth muscle contraction and a flaccid penis.
Potassium Channels
At least three types of potassium channels—adenosine tri

st three types of potassium channels—adenosine triphos­
phate (ATP)-sensitive (KATP), voltage-gated (Kv), and calcium-­
sensitive K+ channels (referred to as BKCa or maxi-K channels)—are 
expressed in the cavernosal smooth muscle cells.99,100 Of these, the 
BKCa channels are the most important, as they account for 90% 
of K+ efflux from the cavernosal smooth muscle cells. Agents that 
open BKCa channels have been shown to relax cavernosal smooth 
muscle cells in  vitro.100 Thus, the strategies t

muscle cells in  vitro.100 Thus, the strategies that increase BKCa 
channel expression in vivo improve erectile capacity in diabetic 
and older rodents100–102 and are being explored as therapy for ED. 
A phase I human gene therapy trial using this approach reported 
the feasibility of this approach, but there has not been a follow-up 
trial since then.102 A phase 2a trial using a single intracavernosal 
injection of hMaxi K channel was initiated in 2016, but no data 
have been reported (NCT0271

 in 2016, but no data 
have been reported (NCT02713789). 
Connexin43 Gap Junctions
The smooth muscle cells in the corpora cavernosa are connected 
by connexin43 gap junctions that allow the ions and some signal­
ing molecules such as IP3 to diffuse freely across smooth muscle 
cells103 (Fig. 20.7). The ionic changes induced by a stimulus in 
one smooth muscle cell are communicated rapidly across other 
smooth muscle cells, resulting in coordinate regulation of the 
entire corpora cavernosa.103 T

 regulation of the 
entire corpora cavernosa.103 Thus, the corpora cavernosa can be 
viewed functionally as a syncytium of interconnected smooth 
muscle cells103 (see Fig. 20.7). 
Nitric Oxide
Nitric oxide, derived from the nerve terminals innervating the cor­
pora cavernosa, the endothelial lining of penile arteries, and the 
cavernosal sinuses, is an important biochemical regulator of caver­
nosal smooth muscle relaxation. Nitric oxide also induces arterial 
dilatation.104 The actions of nitri

uces arterial 
dilatation.104 The actions of nitric oxide on the cavernosal smooth 
muscle and the arterial blood flow are mediated through the acti­
vation of guanylyl cyclase, the production of cyclic guanosine 
monophosphate (cGMP), and the activation of cGMP-dependent 
protein kinase (also called protein kinase G [PKG]) (see Fig. 20.6). 
cGMP causes smooth muscle relaxation by lowering intracellular 
calcium. There is some evidence that nitric oxide inhibits Rho 
kinase–induced cavernosal sm

c oxide inhibits Rho 
kinase–induced cavernosal smooth muscle sensitivity to calcium.105 
NE
DAG
IP3
PKC
–
+
+
+
+
+
K+
K+
[Ca2+]i
Ca2+
Ca2+
• Fig. 20.5  Regulation of cavernosal smooth muscle contractility by 
norepinephrine. Norepinephrine (NE) that mediates adrenergic signals 
binds to adrenergic receptors and stimulates diacyl glycerol (DAG) and 
inositol-1,4,5-trisphosphate (IP3). DAG stimulates protein kinase C (PKC), 
which along with IP3 causes an increase in intracytoplasmic calcium and

causes an increase in intracytoplasmic calcium and 
inhibition of K+ channels. Increased intracellular calcium causes caverno­
sal smooth muscle contraction and loss of penile erection. (Adapted from 
NO
GTP
cGMP
PDE5
–
5'-GMP
PDE5
Proteins
Phosphopr

P
PDE5
–
5'-GMP
PDE5
Proteins
Phosphoproteins
Smooth muscle
relaxation
Sequestration of
intracellular calcium
Avanafil
Sildenafil
Tadalafil
Vardenafil
cGMP-dependent
kinases
Guanylyl
cyclase
• Fig. 20.6  Regulation of cavernosal smooth muscle relaxation by nitric 
oxide (NO). cGMP regulates cavernosal smooth muscle relaxation by pro­
moting sequestration of cytoplasmic calcium. NO is released from nonad­
renergic, noncholinergic nerve endings and possibly from the endothelium. 
NO act

endings and possibly from the endothelium. 
NO activates guanylyl cyclase that generates cGMP, which in turn acti­
vates cGMP-dependent kinases resulting in sequestration of intracellular 
calcium and smooth muscle relaxation. cGMP is degraded by cyclic nucle­
otide phosphodiesterases. Sildenafil, vardenafil, and tadalafil are selective 
inhibitors of PDE isoform 5 that is present in cavernosal smooth muscles. 
cGMP, cyclic guanosine monophosphate; GTP, guanosine triphosphate; 
PDE5, phosphodies

e; GTP, guanosine triphosphate; 
PDE5, phosphodiesterase 5. (Adapted from Bhasin S, Benson GS. Male 
sexual function. In: De Kretser D, ed. Knobil and Neill’s Physiology of 
Reproduction. 3rd ed. Boston, MA: Academic Press; 2006:1173–1194 
and Lue TF. Erectile dysfunction. N Engl J Med. 2000;342[24]:1802–1813.)
764
Cyclic Nucleotide Phosphodiesterases
Cyclic nucleotide phosphodiesterases hydrolyze cAMP and cGMP, 
thus reducing their concentrations wit

 and cGMP, 
thus reducing their concentrations within the cavernosal smooth 
muscle. Of the 13 or more isoforms of cyclic nucleotide phospho­
diesterases that have been identified, isoforms 2, 3, 4, and 5 are 
expressed in the penis.106–113 Only phosphodiesterase 5 (PDE5) is 
specific to the nitric oxide/cGMP pathway in the corpora caver­
nosa.106–113 Hydrolysis of cGMP by this enzyme results in reversal 
of the smooth muscle relaxation and reversal of penile erection (see 
Fig. 20.6). Sildenafi

sal of penile erection (see 
Fig. 20.6). Sildenafil, vardenafil, and tadalafil are potent and selec­
tive inhibitors of the activity of PDE5 that prevent breakdown of 
cGMP and thereby enhance penile erection4,5 (see Fig. 20.6). 
Regulation of Sensitivity to Intracellular Calcium 
by Rho A/Rho Kinase Signaling
Recently, considerable attention has focused on the role of Rho 
kinase in modulating the sensitivity of cavernosal smooth muscle 
to intracellular calcium.114 A growing body of evidence s

acellular calcium.114 A growing body of evidence suggests 
that sensitization to intracellular calcium is regulated by the bal­
ance between phosphorylation of the regulatory light chain of 
myosin II by a myosin light chain kinase and its dephosphoryla­
tion by a myosin light chain phosphatase114–119 (Fig. 20.8). Phos­
phorylation of regulatory light chain of myosin II is necessary for 
activation of myosin II adenosine triphosphatases by actin, and its 
dephosphorylation prevents activation of

 and its 
dephosphorylation prevents activation of myosin II adenosine tri­
phosphatases.114–119 The ratio of the activities of the kinase and the 
phosphatase is an important determinant of the contractile sensitiv­
ity of the cavernosal smooth muscle cell to intracellular calcium.116
Rho A is a guanosine triphosphatase (GTPase) of approximately 
20 kDa that modulates Rho kinase activity, myosin light chain phos­
phorylation, and calcium sensitivity in smooth muscle cells.114 The 
Rho A–GDP (gu

vity in smooth muscle cells.114 The 
Rho A–GDP (guanosine diphosphate) complex is associated with a 
GDP dissociation inhibitor (RhoGDI) in its inactive state. Several 
intracellular signals can promote an exchange of GDP for GTP 
on Rho A through the mediation of guanine nucleotide exchange 
factors.114–119 Rho A–GTP interacts with its downstream effector 
Rho kinase,114–119 increasing the sensitivity of vascular smooth 
muscle to intracellular calcium by inhibiting the myosin light 
chain phos

calcium by inhibiting the myosin light 
chain phosphatases. Although the Rho A/Rho kinase expression is 
not significantly different between young and older rats, the activ­
ity of Rho kinase is higher in older rats than in young rats118; the 
age-related increase in Rho kinase activity has been proposed as one 
possible mechanism to explain the age-related decrease in erectile 
capacity.118 Inhibition of Rho kinase activity in experimental ani­
mals increases cavernosal smooth muscle relaxation

mals increases cavernosal smooth muscle relaxation and improves 
intracavernosal pressures and penile erections. Therefore, inhibitors 
of Rho A/Rho kinase signaling promise to provide attractive targets 
for the development of therapies for ED.118,119 
Mechanisms of Ejaculation
The ejaculatory mechanisms consist of three processes: emission, 
ejection, and orgasm.120–123 Although orgasm and seminal fluid 
ejection often occur contemporaneously, these two processes 
are regulated by separate mec

these two processes 
are regulated by separate mechanisms. Emission, the deposition 
of seminal fluid into the posterior urethra, is dependent on the 
integrity of the vasa deferentia, seminal vesicles, prostate gland, 
and the bladder neck.122,123 Ejaculation refers to the ejection of 
seminal fluid containing sperm, and the secretions of seminal vesi­
cles, prostate, and bulbourethral glands from the posterior urethra 
out through the urethral meatus, and is regulated primarily by 
the central

meatus, and is regulated primarily by 
the central nervous system activation of the sympathetic nervous 
system.122 This emission is ejaculated out of the urethra by the 
contractions of the bulbospongiosus and levator ani muscles, the 
closure of the bladder sphincter due to sympathetic activation, 
and synchronized opening of the external urinary sphincter.118 
The sensation associated with the rhythmic contractions of these 
pelvic floor muscles is referred to as the orgasm.
The stria termina

es is referred to as the orgasm.
The stria terminalis, the posteromedial amygdala, the sub­
parafascicular thalamus, the MPOA of the hypothalamus, the 
periaqueductal gray matter in the midbrain region, and the 
K+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
• Fig. 20.7  Interconnection of cavernosal smooth muscle cells in the penis. 
Connexin43-derived gap junctions connect adjacent corporal smooth 
muscle cells and allow flow of ions among interconnected smooth muscle 
cells. Therefore, alterations in actio

oth muscle 
cells. Therefore, alterations in action potential and K-channel activity in any 
myocyte affect the adjacent myocytes. (Modified from Melman A, Christ 
GJ. Integrative erectile biology: the effects of age and disease on gap junc­
tions and ion channels and their potential value to the treatment of erectile 
dysfunction. Urol Clin North Am. 2001;28[2]:217–231, vii.)
Cavernosal smooth muscle
contraction and sensitivity
to intracellular Ca2+
MLCK
Ca.CaM
MLC
phosphatase
Inactive MLC
phos

Ca2+
MLCK
Ca.CaM
MLC
Myosin II RLC20
Rho kinase
Myosin II RLC20-P
• Fig. 20.8  The role of Rho A–Rho kinase in regulation of cavernosal smooth 
muscle sensitivity to intracellular Ca2+ is depicted. Sensitivity to calcium and 
smooth muscle contractility is regulated by the Rho A–Rho kinase sys­
tem. The balance between phosphorylation of myosin regulatory light chain 
(Myosin II RLC20) kinase and its dephosphorylation by a myosin light chain 
phosphatase is a

rylation by a myosin light chain 
phosphatase is a major determinant of the smooth muscle sensitization to 
Ca2+. By inhibiting the activity of myosin light chain (MLC) phosphatase, 
Rho kinase, the downstream effector of Rho A can regulate smooth muscle 
responsiveness to calcium. Ca.CaM, calcium-calmodulin; MLCK, myosin 
light-chain kinase. (Modified from Somlyo AP, Somlyo AV. Ca2+ sensitiv­
ity of smooth muscle and nonmuscle myosin II: modulated by G proteins, 
kinases, and myosin phosphatase

ed by G proteins, 
kinases, and myosin phosphatase. Physiol Rev. 2003;83[4]:1325–1358.)
765
paragigantocellular nucleus in the pons integrate seminal fluid 
emission and ejection during copulatory behavior in animals124,125 
(see Table 20.2). The paragigantocellular nucleus in the pons 
through serotonergic pathways inhibits the lumbosacral motor 
nuclei that are involved in ejaculation125,126; the input from the 
MPOA to the paragigantocellular nu

input from the 
MPOA to the paragigantocellular nucleus causes loss of this inhi­
bition, resulting in ejaculation.125,126 An ejaculation generator in 
the spinal cord integrates the central and peripheral sympathetic 
and parasympathetic signals to control ejaculation.120,125 The 
parasympathetic fibers from the spinal ejaculation generator feed 
into the sacral parasympathetic nucleus and are carried from there 
through the pelvic nerve and the major parasympathetic ganglion 
into the seminal 

 major parasympathetic ganglion 
into the seminal tract.125 Sympathetic fibers are carried from 
the spinal ejaculation generator into the dorsal gray commissure 
and intermediolateral cell column and then through the lumbar 
sympathetic chain, pelvic nerve, superior hypogastric plexus, and 
major pelvic ganglion onto the seminal tract.125
Neural pathways that utilize serotonin (5-hydroxytryptamine 
[5HT]) and dopamine as neurotransmitters play an important 
role in regulating ejaculation.122 Th

n important 
role in regulating ejaculation.122 Thus, administration of selec­
tive serotonin reuptake inhibitors (SSRIs) is being explored for 
the treatment of premature ejaculation.124–127 At least 14 different 
serotonin receptor subtypes have been identified in different brain 
regions; 5HT1A somatodendritic receptors in the mesencephalic 
and medullary raphe nuclei reduce ejaculatory latency.120 An 
improved understanding of the neurochemical mechanisms that 
regulate ejaculation may provi

al mechanisms that 
regulate ejaculation may provide mechanism-specific targets for 
treatment of ejaculatory disorders. 
Role of Testosterone in Regulating 
Sexual Function in Men
Testosterone regulates many domains of sexual function in men 
and women.27 Although androgen-deficient men can achieve 
penile erections in response to visual erotic stimuli, their overall 
sexual activity is decreased.128 Spontaneous but not stimulus-
bound erections are testosterone responsive128 (Table 20.3). 
Tes

 are testosterone responsive128 (Table 20.3). 
Testosterone promotes sexual thoughts and desire53,128–134 
and increases sexual arousal and attentiveness to erotic audi­
tory and other stimuli.129,130 Nocturnal erections, temporally 
related to peaks of nighttime testosterone secretion, are of lower 
amplitude and duration in androgen-deficient men, and tes­
tosterone therapy increases the frequency, fullness, and dura­
tion of nocturnal penile tumescence.134,135 Maximum rigidity 
may require a 

umescence.134,135 Maximum rigidity 
may require a threshold level of androgen activity.136–140 Tes­
tosterone regulates nitric oxide synthase (NOS) in the caver­
nosal smooth muscle,137,138 exerts trophic effects on cavernosal 
smooth muscle139 and ischiocavernosus and bulbospongiosus 
muscles, and is necessary for the veno-occlusive response.136–139 
Androgen-deficient men show delayed orgasm and low ejacula­
tory volume.27
Testosterone therapy in androgen-deficient men improves 
overall sexual

in androgen-deficient men improves 
overall sexual activity, sexual desire, erectile function,141,142 spon­
taneous sexual thoughts, and attentiveness to erotic auditory 
stimuli; frequency of nighttime and daytime erections; duration, 
magnitude, and frequency of nocturnal penile erections; and the 
volume of ejaculate.27,53,128–131,134-136,140,143–145 However, testos­
terone does not improve erectile response to visual erotic stim­
ulus128 or erectile function in men with ED who have normal 
t

rectile function in men with ED who have normal 
testosterone levels.140,143–147 Testosterone does not improve ejacu­
latory function in men with ejaculatory dysfunction and low tes­
tosterone levels.148 In male rats, in whom endogenous testosterone 
production has been suppressed by administration of a GnRH 
antagonist, testosterone restores all measures of mating behavior 
at doses that are insufficient to restore prostate and seminal ves­
icle weights.149,150 Similarly, in dose-response studi

 weights.149,150 Similarly, in dose-response studies in humans 
rendered testosterone deficient by the administration of a GnRH 
agonist, sexual function is restored by testosterone doses that raise 
serum testosterone levels just above the lower limit of the normal 
male range151; further increase in serum testosterone levels does 
not further improve measures of sexual function.
Brain imaging studies suggest that processing of sexual stimuli 
may be altered in androgen-deficient men with decre

ay be altered in androgen-deficient men with decreased activa­
tion in those brain areas that are typically activated in eugonadal 
men and in androgen-deficient men after testosterone replace­
ment in response to erotic stimuli.53
Acting on dopaminergic receptors in the MPOA of the hypo­
thalamus, testosterone elicits reward-seeking behavior in male 
mammals.61 This may be the basis for testosterone’s motivational 
effects on mammalian sexual behavior.61 The roles of cytochrome 
P450 19A1 (arom

havior.61 The roles of cytochrome 
P450 19A1 (aromatase) and steroid 5α-reductase systems in 
mediating androgen effects on sexual function remain incom­
pletely understood.63 Studies suggest that 5α-reduction of testos­
terone to 5α-dihydrotestosterone is not essential for mediating 
testosterone’s effects on desire or erectile function in eugonadal 
men.152 Additional investigations, including those in men with 
mutations of the CYP19A1 gene, suggest that aromatization to 
estradiol is importa

uggest that aromatization to 
estradiol is important in mediating testosterone’s effects on sexual 
desire.153–155 Both testosterone and estradiol are required to main­
tain sexual function.
In surveys of community-dwelling middle-age and older men, 
low libido, poor morning erections, and ED are associated with 
low total and free testosterone levels.156–158 However, androgen 
deficiency and ED are two independently distributed disorders 
that may coexist in middle-age and older men.159–161 Sel

phosphodiesterase inhibitors
	4.	 Ejaculatory dysfunction
	5.	 Orgasms
aTestosterone administration in androgen-deficient men improves overall sexual activity 
scores through its effects on specific domains of sexuality.
Modified from Bhasin S, Enzlin P, Coviello A, Basson R. Sexual dysfunction in men and women 
with endocrine disorders. Lancet. 2007;369(9561):597–611.
   
TABLE 20.3
766
PDE5 inhibitors are highly effective first-line therapies for 

ors are highly effective first-line therapies for ED. 
However, one third of men with ED do not respond to PDE5 
inhibitors; some of the men with ED who are PDE5 inhibitor 
nonresponders have low testosterone levels. Observations that tes­
tosterone stimulates penile NOS, increases penile blood flow, and 
has trophic effects on cavernosal smooth muscle and bulbospon­
giosus and ischiocavernosus muscles have led to speculation that 
testosterone therapy might improve erectile response to PDE5 
in

herapy might improve erectile response to PDE5 
inhibitors.162–165 Spitzer and associates166 evaluated whether the 
addition of testosterone to sildenafil improves erectile response in 
men with ED and low testosterone. In this randomized controlled 
trial (RCT), after optimization of sildenafil dose during an initial 
run-in period, subjects were assigned randomly to 14 weeks of 
daily testosterone or placebo gel. Sildenafil plus testosterone was 
not superior to sildenafil plus placebo in impr

s 
not superior to sildenafil plus placebo in improving erectile func­
tion in men with ED and low testosterone.166 In another placebo-
controlled trial (TADTEST) of men with ED who were deemed 
tadalafil (Cialis) failures,167 the primary analysis of all randomized 
subjects also did not show a significantly greater improvement 
in erectile function in the testosterone arm than in the placebo 
arm. However, in post hoc analysis, erectile function improved 
with the addition of testosterone in a 

 improved 
with the addition of testosterone in a subset of men with baseline 
testosterone of 10 nmol/L or less (300 ng/dL).167 Thus, random­
ized trials have failed to support the hypothesis that addition of 
testosterone to PDE5 inhibitor improves erectile function in men 
with ED. Sildenafil alone raises testosterone levels, presumably 
because of its direct effects on Leydig cell steroidogenesis.168 It is 
possible that testosterone may improve penile rigidity and overall 
satisfaction with

ove penile rigidity and overall 
satisfaction with sexual experience in men who are unequivocally 
hypogonadal. 
Physiology of Physical Sexual Arousal in 
Women: Genital Congestion
Several physical changes occur in response to sexual stimuli with 
varying degrees of women’s sexual excitement (i.e., their subjec­
tive sexual arousal), including genital swelling, increased vaginal 
lubrication, breast engorgement, and nipple erection; increased 
skin sensitivity to sexual stimulation; changes in h

in sensitivity to sexual stimulation; changes in heart rate, blood 
pressure, muscle tone, breathing, and temperature; and mottling 
of the skin with a sexual flush of vasodilatation over the chest and 
face.169 These changes are reflexive, mediated by the autonomic 
nervous system. Within seconds, there is increased blood flow 
to the vagina; vasodilatation of the arterioles in the submucosal 
plexus increases transudation of interstitial fluid from the capil­
laries across the epithelium and i

from the capil­
laries across the epithelium and into the vaginal lumen.169 Simul­
taneously, there is relaxation of smooth muscle cells around the 
clitoral sinusoids, which promotes congestion.
The clitoris comprises the head, the shaft, the rami that extend 
along the pubic arch, and the periurethral tissue in front of the 
anterior vaginal wall, as well as the bulbar tissue that surrounds 
the anterior distal vagina and is contiguous with the periurethral 
tissue.170 MRI studies have confirm

 periurethral 
tissue.170 MRI studies have confirmed the presence of extensive 
clitoral tissue far beyond the visible portion of the clitoris.170 As 
the clitoris becomes more swollen, it elevates to lie nearer the 
symphysis pubis. The vagina lengthens and dilates during arousal, 
elevating the uterus. The labia become swollen and darker red, and 
the lower third of the vagina swells.170
The autonomic nerves subserving the widespread genital con­
gestion are at risk from gynecologic surgeries 

n­
gestion are at risk from gynecologic surgeries that sever the cardi­
nal ligaments and the uterosacral ligaments to potentially injure 
Frankenhauser’s nerve plexus and the uterovaginal nerve plexus.171,172 
Surgical procedures for hysterectomy that do not involve cutting 
or clamping the cardinal ligament, uterosacral ligament, or fascia 
and spare the pelvic nerve plexuses are less likely to impair sexual 
function.171 Transvaginal tapes for urinary incontinence, especially 
transobturator 

 urinary incontinence, especially 
transobturator tapes, may also compromise the autonomic nerves 
between the anterior vaginal wall and bladder.172,173
The correlation between genital congestion and subjective 
arousal in response to visual erotic stimuli is highly variable49,82,83; 
this is true in sexually healthy women and in women reporting a 
lack of desire or arousal or sexual pain. Women reporting chronic 
lack of arousal show prompt increases in vaginal congestion that 
are comparable t

eases in vaginal congestion that 
are comparable to those in control women, but they report no 
subjective sexual excitement in response to the erotic stimulation. 
fMRI studies show that, unlike in men, activation of the areas 
organizing genital vasocongestion in women does not correlate 
with subjective excitement.49,174
The neurobiology of the genital vasocongestive response in 
women is complex and incompletely understood. Genital vaso­
congestion involves the release of nitric oxide and VI

estion involves the release of nitric oxide and VIP from the 
sacral autonomic nerves.175 Acetylcholine, which blocks norad­
renergic vasoconstricting mechanisms and promotes nitric oxide 
release from the endothelium, is also released. There is commu­
nication between the nitric oxide–containing cavernosal nerve to 
the clitoris and the distal portion of the somatic dorsal nerve of the 
clitoris from the pudendal nerve. Pelvic sympathetic nerves release 
primarily vasoconstrictive norepinephrin

 release 
primarily vasoconstrictive norepinephrine and epinephrine, but 
some release acetylcholine, nitric oxide, and VIP. The provoked 
anxiety in the laboratory situation can increase the vasocongestive 
response of the genitalia to erotic stimulation in sexually healthy 
women.87 The localization of NOS, cAMP, and cGMP-degrading 
PDE isoenzymes in human vaginal tissue is established, along 
with identification of cAMP and cGMP-binding proteins. The 
latter are co-localized with endothelial 

ns. The 
latter are co-localized with endothelial NOS (eNOS). Close prox­
imity to VIP-positive nerves suggests that cAMP and cGMP work 
synergistically to control vaginal blood flow.175 Neuropeptide Y 
(a vasoconstrictor), CGRP (possibly influencing capillary perme­
ability and sensation), and substance P (a sensory transmitter) 
also innervate the vaginal microcirculation. The melanocortin-4 
receptors and oxytocin also may be involved in clitoral and vaginal 
efferent pathways.175
Intermitten

ral and vaginal 
Intermittency of the vaginal microcirculation due to con­
traction and relaxation of precapillary sphincters in response to 
hypoxia and the release of metabolites (CO2, lactic acid, ATP) has 
been termed vasomotion. Vasomotion is present in the nonaroused 
state but decreases within seconds of a sexual stimulus, which 
increases arterial supply to recruit more capillaries and diminish 
vasomotion: vaginal vasocongestion follows. Slow oscillations in 
vagin

asocongestion follows. Slow oscillations in 
vaginal blood flow, independent of vaginal vasocongestion, have 
been shown to correlate with subjective arousal in healthy women 
and to be less marked in women with arousal disorder.174
Increased blood flow to the submucosal vaginal capillaries 
results in increased interstitial fluid production, which diffuses 
more quickly across the vaginal epithelial cells and onto the 
lumen: the lubrication fluid in the aroused state thereby contains 
less pot

id in the aroused state thereby contains 
less potassium and more sodium than in the nonaroused state. 
How important the contribution of permeability of the epithelial 
cells is to the process of lubrication is currently unclear. Relaxation 
of the vaginal wall smooth muscle that enables the vagina to move 
up into the pelvis is likely mediated by VIP.174
The clitoris is the most sexually sensitive area of the body. 
Immunohistologic studies have identified neurotransmitters 
767
Chapter 20  Se

e identified neurotransmitters 
767
thought to be associated with sensation (substance P and CGRP) 
that are concentrated immediately under the epithelium of the 
glans clitoris. Nerve terminals in the glans clitoris known as cor­
puscular receptors are thought to be involved. They are mechano­
receptors, and their density is variable but can be up to 14 times 
greater than the density of similar receptors on the glans penis.176 
The physiology of 

ceptors on the glans penis.176 
The physiology of nongenital physical changes and their correla­
tion with subjective excitement remain poorly understood. 
Physiology of Orgasm
Orgasm is largely a brain event typically triggered by genital stim­
ulation; however, it can also be induced by stimulation of other 
parts of the body, including the breast and nipple; fantasy; sleep; 
certain medications; and, in women with spinal cord injury, by 
vibratory stimulation of the cervix. Vaginal stimulatio

tory stimulation of the cervix. Vaginal stimulation involves 
all of the clitoral-urethral complex, including the clitoral rami, 
whereas direct stimulation of the clitoral shaft and glans does not 
involve the clitoral rami.169 During vaginal intercourse, the penis 
distends the vagina and stretches the root of the clitoris. During 
penile thrusting, the anterior wall of the vagina is apposed against 
the root of the clitoris, perhaps explaining why some women can 
achieve orgasm by vaginal int

 why some women can 
achieve orgasm by vaginal intercourse without simultaneous non­
vaginal stimulation.177 The root of the clitoris and the anterior 
wall of the vagina thus appear to be anatomically and functionally 
related.178
Orgasm is a subjective experience that is associated with 
ejaculation in healthy men and, in both men and women, with 
involuntary (reflexive) muscular contractions of the striated peri­
neal muscles.179 The characteristic fluctuations in the rectal pres­
sure that o

istic fluctuations in the rectal pres­
sure that occur during orgasm due to involuntary contractions 
of perineal muscles have been proposed as an objective measure 
that has a strong correspondence with the subjective experience 
of orgasm.179
Positron emission tomography (PET) studies during orgasm 
have shown largely similar brain activations and deactivations in 
men and women: activations mainly in the anterior lobe of the 
cerebellar vermis and deep cerebellar nuclei and deactivations in 

 and deep cerebellar nuclei and deactivations in 
the left ventromedial cortex and OFC. The only gender differ­
ence in the PET findings during orgasm was the activation in the 
periaqueductal gray matter in men but not in women.180,181 The 
lateral OFC is involved in urge suppression and behavioral release, 
whereas the medial parts encode hedonic experiences and become 
activated with increasing pleasure and deactivated with feelings 
of satiety. The medial OFC is part of the neuronal network 

y. The medial OFC is part of the neuronal network that 
includes the amygdala, whose deactivation during orgasm is associ­
ated with a more carefree state of mind.180 The variations in rectal 
pressure indicative of orgasm recorded using a rectal probe cor­
relate with widespread blood flow changes in the prefrontal cortex 
measured using PET scanning.181,182 The specific orgasm-related 
changes in the mid/anterior OFC during PET scanning suggest a 
role of the mid/anterior OFC in the experience

 a 
role of the mid/anterior OFC in the experience of pleasure. Failed 
orgasm due to excessive behavioral suppression is associated with 
enhanced left lateral OFC activity. Researchers have speculated 
that the orgasm-related OFC dynamics may be related to the sense 
of loss of control during orgasm. Prefrontal, but not temporal, 
perfusion is negatively coupled to rectal pressure fluctuations asso­
ciated with orgasm. Overall, the findings of reduced prefrontal 
metabolism during orgasm are i

reduced prefrontal 
metabolism during orgasm are in keeping with the critical role 
of the prefrontal cortex in behavioral and emotional control and 
with the experimental data relating exaggerated prefrontal activity 
with sexual dysfunction.
The role of oxytocin and prolactin in orgasm is unclear. Both 
hormone levels increase at the time of orgasm. PET scanning has 
confirmed increased pituitary blood flow in women, but not in 
men, at the moment of orgasm.183 Both hormones can cause uter­
in

ent of orgasm.183 Both hormones can cause uter­
ine and vaginal smooth muscle contraction, which may contribute 
to the sensations of orgasm. 
Revised Definitions of Sexual Dysfunction 
in Men
In May 2013, DSM-5 provided an updated classification and defi­
nitions of male sexual disorders.45 The salient differences of the 
new DSM-5 classification and definitions from DSM-IV are the 
following46:
	1.	 DSM-5 includes only four male sexual disorders, as opposed to 
six in DSM-IV.46 The four sexua

sion criterion: the disorder should 
not be better explained by a “nonsexual mental disorder or a 
consequence of severe relationship distress or other significant 
stressors.”45,46
Male Hypoactive Sexual Desire Disorder
HSDD is the persistent or recurrent deficiency (or absence) of 
sexual fantasies and desire for sexual activity that causes marked 
distress and that is not better explained by another disorder, direct 
physiologic effects of a substance (medication), or general medi­
cal condit

ubstance (medication), or general medi­
cal condition.184–187 A diagnosis of HSDD is appropriate only if 
the person reports distress due to low sexual desire.184-187 Low 
sexual desire is not necessarily pathologic, as low sexual desire may 
be an appropriate adaptation to relationship and health-related 
issues.184–187
Although DSM-5 limits a diagnosis of HSDD to causes other 
than medication, illness including androgen deficiency and depres­
sion, and relationship and addiction problems, in c

ion, and relationship and addiction problems, in clinical practice 
the term hypoactive sexual desire disorder is often used for situa­
tions involving multiple factors, including androgen deficiency, 
prolactin excess, the use of medications (SSRIs, antiandrogens, 
GnRH analogues, antihypertensives, cancer chemotherapeutic 
agents, anticonvulsants), systemic illness, depression and other 
psychological problems such as low self-esteem, fear of emotional 
intimacy, fear of loss, long-term habit 

motional 
intimacy, fear of loss, long-term habit of sex alone with or without 
pornography, other causes of sexual dysfunction, or relationship 
problems. Thus, in practice, androgen deficiency is an important, 
768
treatable cause of low desire in men and should be excluded by 
measuring serum total and free testosterone levels.184–187
The incidence and prevalence of HSDD in the general popula­
tion are unknown. In studies of referred patient popula

are unknown. In studies of referred patient populations, the 
prevalence has been estimated to be 5% in men.7,186–189 Preva­
lence increases with age.186,187,189 HSDD often coexists with other 
sexual disorders, such as ED, and may develop as a consequence of 
other sexual disorders.186,187,189
Appropriate evaluation and treatment of HSDD are impor­
tant because evaluation may lead to detection of treatable disease, 
including androgen deficiency, prolactin excess, or depression. 
Furthermore, l

, prolactin excess, or depression. 
Furthermore, low sexual desire may impede or reduce the effec­
tiveness of treatments for other sexual dysfunctions. 
Erectile Dysfunction
ED, previously referred to as impotence or male ED, is the inabil­
ity to attain or maintain an erection or to achieve penile rigid­
ity sufficient for satisfactory sexual intercourse.5,6,45,46 DSM-5 
requires that the inability to attain or maintain an erection should 
occur in 75% to 100% of encounters over a period of at

r in 75% to 100% of encounters over a period of at least 6 
months.45,46 Sexual dysfunction is a more general term that also 
includes libidinal, orgasmic, and ejaculatory dysfunction, in addi­
tion to the inability to attain or maintain penile erection. Epide­
miologic surveys,188–198 including the Massachusetts Male Aging 
Study (MMAS)190 and the National Health and Social Life Survey 
(NHSLS),188 revealed a surprisingly high prevalence of ED. ED 
significantly affects quality of life of both 

ED 
significantly affects quality of life of both the affected individual 
and his partner. In one study, ED had a negative impact on the 
sexual life of female partners, specifically on their sexual satisfac­
tion and sexual drive.191
Prevalence and Incidence
The best data on the prevalence of ED in men have emerged 
from two cross-sectional studies that have used population-based 
sampling techniques, namely the MMAS190,192,195,196 and the 
NHSLS.7,188 The MMAS was a cross-sectional and longit

LS.7,188 The MMAS was a cross-sectional and longitudinal, 
community-based epidemiologic survey in which 1709 men, 40 
to 70 years of age, residing in the greater Boston area, were sur­
veyed between 1987 and 1989.190 This survey revealed that 52% 
of men between the ages of 40 and 70 years were affected by ED 
of some degree; 17.2% of surveyed men reported minimal ED, 
25.2% moderate ED, and 9.6% complete ED.190,195,196 The 
NHSLS was a national probability survey of English-speaking 
Americans

 probability survey of English-speaking 
Americans, 18 to 59 years of age, living in the United States.7,188 
This survey also revealed a high prevalence of ED in men; the 
prevalence of ED increased with increasing age.7,188 These two 
studies and several other studies are in agreement that ED is a 
common problem worldwide.7,188–197 In the U.S. civilian popula­
tion, the prevalence of ED in men 20 to 39 years of age has been 
estimated to be 5.1%, and almost three times as high (14.8%) in 
men

1%, and almost three times as high (14.8%) in 
men age 40 to 59 years of age. Men suffering from other medical 
problems, such as hypertension, diabetes, cardiovascular disease 
(CVD), and end-stage renal disease, have a significantly higher 
prevalence of ED than healthy men.190
There is a paucity of longitudinal data on the incidence of ED in 
men. In the MMAS, the crude incidence of ED in white men in the 
Boston area was found to be 25.9 cases per 1000 person-years.192 
The incidence rates i

s per 1000 person-years.192 
The incidence rates increased from 12.4 cases per 1000 person-years 
for men 40 to 49 years of age to 29.8 cases per 1000 person-years 
for men 50 to 59 years of age and 46.4 per 1000 person-years for 
men 60 to 69 years of age.192 In another study, incidence rates were 
derived from a survey of men seen at a preventive medicine clinic.7 
This study found the incidence rates of ED to be less than 3 cases 
per 1000 person-years among men less than 45 years of age and 

son-years among men less than 45 years of age and 52 
cases per 1000 person-years among men 65 years of age or older. 
These studies suggest that 600,000 to 700,000 men in the United 
States develop ED each year.195,196 
Risk Factors for Erectile Dysfunction
The risk factors for ED include age, diabetes mellitus, hyperten­
sion, smoking, medication use, depression, dyslipidemia, and 
CVD.4-6,190,198–208 Advancing age is an important risk factor for 
ED in men4–6,188,190: less than 10% of men you

or 
ED in men4–6,188,190: less than 10% of men younger than 40 years 
and more than 50% of men older than 70 years have ED. In both 
the MMAS and the NHSLS, the prevalence of ED increased with 
each decade of life.188,190
Among the chronic diseases associated with ED, diabetes mel­
litus is the most important risk factor. In the MMAS, the age-
adjusted risk of complete ED was three times higher in men with 
history of treated diabetes mellitus than in those without history 
of diabetes mellitus.

an in those without history 
of diabetes mellitus.190,198 Fifty percent of men with diabetes mel­
litus will experience ED sometime during the course of their ill­
ness. In the MMAS, treated heart disease, treated hypertension, 
and hyperlipidemia were associated with a significantly increased 
risk of ED. Among men with treated heart disease and hyperten­
sion, the probability of ED was more than two times greater for 
smokers than for nonsmokers.4–6,188,190 Smoking also increases 
the risk of 

s.4–6,188,190 Smoking also increases 
the risk of ED in men taking medications for CVD. Cardiovas­
cular disorders, including hypertension, stroke, coronary artery 
disease, and peripheral vascular disease, are all associated with 
increased risk of ED. Physical activity is associated with reduced 
risk of ED.207
Several reviews have emphasized the relationship of prescrip­
tion medications and the occurrence of ED. In the MMAS, the 
use of antihypertensives, cardiac medications, and oral hypogl

ypertensives, cardiac medications, and oral hypoglyce­
mic drugs was associated with an increased risk of ED.190 Thiazide 
diuretics and psychotropic drugs used in the treatment of depres­
sion may be the most common drugs associated with ED, simply 
because of the high prevalence of their use. However, a variety 
of drugs, including almost all antihypertensives, digoxin, hista­
mine-2 receptor antagonists, anticholinergics, cytotoxic agents, 
and androgen antagonists, have been implicated in th

d androgen antagonists, have been implicated in the patho­
physiology of ED.190 
Erectile Dysfunction as a Marker of Cardiovascular 
Disease 
CVD and ED share common risk factors, such as diabetes mel­
litus, obesity, hypertension, smoking, and dyslipidemia.199–207 ED 
precedes the symptoms of coronary artery disease by 2 to 3 years 
and cardiovascular events such as myocardial infarction or stroke 
by 3 to 5 years.199–207 ED in men is associated with increased 
risk of death, particularly fatal

risk of death, particularly fatality due to CVD.201 The presence 
of ED is a good predictor of subsequent coronary artery disease, 
especially in younger men, independent of traditional coronary 
risk factors, although it does not enhance the predictive ability 
of models that include traditional risk factors, likely reflecting 
the common pathophysiologic mechanisms of ED and coronary 
artery disease.202 Men reporting ED are 1.3 to 1.6 times more 
likely to experience a cardiov

 to 1.6 times more 
likely to experience a cardiovascular event within 10 years than 
men without ED.199–207 
Lower Urinary Tract Symptoms and ED
Epidemiologic surveys have reported a strong association of lower 
urinary tract symptoms (LUTS) with ED208–214 even after adjust­
ing for age and other risk factors. The Cologne Male Survey and 
769
the Multinational Survey of the Aging Male revealed that the 
presence and severity of LUTS is an independ

 the 
presence and severity of LUTS is an independent predictor of 
ED independent of age.209 LUTS and age are stronger predic­
tors of ED than all other risk factors, including diabetes, dyslip­
idemia, and hypertension. Considering that LUTS and ED are 
two common conditions in middle-age and older men, it is pos­
sible that this association reflects the coexistence of two highly 
prevalent conditions. However, there is growing evidence that the 
two conditions may be mechanistically linked, a

e 
two conditions may be mechanistically linked, as the biochemical 
mechanisms that regulate bladder detrusor and cavernosal smooth 
muscle function share many similarities.214,215 K+ channels, espe­
cially calcium-sensitive K+ channels (BKCa channels), Rho A/Rho 
kinase signaling, L-type calcium channels, and gap junctions are 
important mediators of both detrusor and cavernosal smooth 
muscle contractility and relaxation.214,215 Increased myocyte con­
tractility that characterizes both bladde

yte con­
tractility that characterizes both bladder detrusor dysfunction and 
ED may be mechanistically related to increased Rho kinase activ­
ity, impairments of K+ channel function,215 α-adrenergic recep­
tor imbalance, and endothelial dysfunction. Additional proposed 
hypotheses include increased sympathetic activity and autonomic 
dysfunction, and alterations in nitric oxide generation or PKG 
activity in the detrusor and cavernosal smooth muscles.214–216 
Some therapies for LUTS, such as so

scles.214–216 
Some therapies for LUTS, such as some types of surgical proce­
dures and steroid 5α-reductase inhibitors, may worsen sexual dys­
function. α-Adrenergic blockers used in the treatment of LUTS 
can cause ejaculatory problems. Some PDE5 inhibitors have been 
approved for the treatment of LUTS.215–218 
Ejaculatory Disorders
Ejaculatory disorders include premature ejaculation, delayed ejac­
ulation, retrograde ejaculation, anejaculation, and painful ejacula­
tion.120–123 Recent surveys

, and painful ejacula­
tion.120–123 Recent surveys have highlighted the high prevalence 
and clinical importance of ejaculatory disorders.10,120–123,219,220 
Although the availability of oral PDE5 inhibitors has increased 
awareness of ED, ejaculatory disorders are at least as prevalent and 
may be even more prevalent than ED.10,220
Premature ejaculation, a common form of ejaculatory dysfunc­
tion in men, can be lifelong or acquired later in life, and it negatively 
affects interpersonal relatio

, and it negatively 
affects interpersonal relationships, sexual satisfaction, and quality 
of life.221,222 An ad hoc committee of the International Society for 
Sexual Medicine defined premature ejaculation as follows222:
a male sexual dysfunction characterized by (i) ejaculation that 
always or nearly always occurs prior to or within about 1 min­
ute of vaginal penetration from the first sexual experience (life­
long PE [premature ejaculation]) or a clinically significant and 
bothersome reduc

 or a clinically significant and 
bothersome reduction in latency time, often to about 3 minutes 
or less (acquired PE); (ii) the inability to delay ejaculation on 
all or nearly all vaginal penetrations; and (iii) negative per­
sonal consequences, such as distress, bother, frustration, and/or 
the avoidance of sexual intimacy.
This definition of premature ejaculation is based on a multina­
tional survey of men, 18 years or older, in stable heterosexual rela­
tionships, which found the median in

osexual rela­
tionships, which found the median intravaginal ejaculation latency 
time (IELT) to be 5.6 minutes (range, 0.55–44.1 minutes)223; in 
this survey, 2.5% of men had an IELT of less than 1 minute and 
6% of less than 2 minutes. Most men seeking treatment for life­
long premature ejaculation ejaculate within 1 minute after penetra­
tion, and nearly all ejaculate within 2 minutes.224 Compared with 
patients with lifelong premature ejaculation, the men with acquired 
premature ejaculation

tion, the men with acquired 
premature ejaculation are older and have a longer IELT.222 The 
acquired premature ejaculation is often also associated with ED 
and other comorbid conditions, such as diabetes and CVD.
Some men who seek treatment for premature ejaculation do 
not meet the preceding definition established by the International 
Society for Sexual Medicine but express distress due to their per­
ception of lack of control over the timing of ejaculation and the 
IELT to be shorter than t

 of ejaculation and the 
IELT to be shorter than they wish. Waldinger225 proposed the 
terms subjective premature ejaculation and variable premature ejacu­
lation to describe these men whose IELT is either consistently or 
inconsistently greater than 5 minutes but who are preoccupied 
with a perceived short ejaculatory latency or the lack of control 
over the timing of ejaculation. Specific duration criteria for defin­
ing premature ejaculation among men engaged in nonvaginal 
sexual activities 

mong men engaged in nonvaginal 
sexual activities have not been established.
The prevalence of premature ejaculation is not known; previous 
estimates of prevalence of 20% to 25% based on older definitions 
are likely inaccurate. Although there are no epidemiologic surveys 
based on the International Society for Sexual Medicine criteria or 
the DSM-5 definition of premature ejaculation, the experts have 
inferred from the data on the distribution of IELT that the preva­
lence is unlikely to be g

 of IELT that the preva­
lence is unlikely to be greater than 4%.221,226,227
The neurobiologic mechanisms of premature ejaculation are 
not well understood. 5HT (serotonin), oxytocin, and dopamine 
are involved as neurotransmitters in the central nervous system 
pathways that control ejaculation.228,229 In preclinical models, 
serotonin delays ejaculation, and dopamine and oxytocin appear 
to stimulate ejaculation; these data have led to speculation that 
men with lower ejaculatory thresholds ma

ion that 
men with lower ejaculatory thresholds may either have deficient 
serotonin neurotransmission or 5HT 2C receptor hyposensitivity. 
The serotonin hypothesis is supported by data showing that SSRIs 
and the tricyclic antidepressant clomipramine increase ejaculatory 
control and delay ejaculation in men with premature ejaculation. 
The earlier theories that premature ejaculation is largely due to 
psychological factors such as anxiety or sequelae of early life sexual 
experiences have not 

quelae of early life sexual 
experiences have not been supported by recent data from clinical 
trials of pharmacologic agents. Genetic studies of the polymor­
phisms in the serotonin transporter protein (5HTTLPR) gene, 
oxytocin, serotonin, and vasopressin genes, and tandem repeats of 
the dopamine transporter gene (DAT1) have not yielded consis­
tent association with premature ejaculation.230–234 
Delayed Ejaculation
DSM-5 defines delayed ejaculation as a marked involuntary delay 
in ejaculatio

ation as a marked involuntary delay 
in ejaculation or absence of ejaculation on almost all or all occa­
sions of partnered sexual activity, persisting for at least 6 months 
and causing distress to the individual.235 A timing-based defini­
tion of delayed ejaculation has not been established. Because the 
median intravaginal ejaculatory latency in a multinational popu­
lation-based survey was 5.4 minutes223 and the 97.5th percentile 
value was 23 minutes, some experts have suggested that intrav

3 minutes, some experts have suggested that intravagi­
nal ejaculatory latency greater than 25 or 30 minutes should be 
considered delayed ejaculation.236
Mild to moderate delay in ejaculation occurs with advancing 
age; older men with comorbid conditions are at a higher risk of 
delayed ejaculation.237,238 Epidemiologic surveys have consistently 
found LUTS to be strongly associated with delayed ejaculation, 
even after adjusting for age.208,238–240 Ejaculatory disorders are 
common after surgi

–240 Ejaculatory disorders are 
common after surgical treatment of benign prostatic hyperpla­
sia including transurethral resection of the prostate.241 Several 
drugs—SSRIs, highly selective α1-adrenergic blockers, steroid 
5α-reductase inhibitors—can cause delayed ejaculation. The use 
of highly selective α1-adrenergic blockers such as tamsulosin and 
770
silodosin can lead to marked delay or inability to ejaculate; ejacu­
latory dysfunction is less 

y to ejaculate; ejacu­
latory dysfunction is less common with nonselective α-adrenergic 
blockers such as alfuzosin, doxazosin, and terazosin. The use of 
steroid 5α-reductase inhibitors such as finasteride and dutasteride 
is associated with increased risk of ejaculatory disorder. SSRIs can 
prolong intravaginal ejaculatory latency and cause delayed ejacula­
tion in some middle-age and older men.242 Excessive exposure to 
pornography and sexual abuse may reduce sexual desire, impair 
arousal, a

abuse may reduce sexual desire, impair 
arousal, and lead to ejaculatory dysfunction.243 Serum testos­
terone levels have not been consistently associated with delayed 
ejaculation.244 
Retrograde Ejaculation
Retrograde ejaculation is the failure of the semen to be ejected out 
through the urethral meatus; instead, the semen is propelled back­
ward into the urinary bladder.120–123 Retrograde ejaculation can be 
the result of autonomic neuropathy associated with diabetes mel­
litus; sympathectomy

associated with diabetes mel­
litus; sympathectomy; therapy with adrenergic antagonists, some 
types of antihypertensives, antipsychotics, or antidepressants; 
bladder neck incompetence; or urethral obstruction. Retrograde 
ejaculation due to diabetes-associated autonomic neuropathy is 
the second most prevalent ejaculatory disorder.120–123 Following 
transurethral resection of the prostate, the bladder neck closure 
mechanism may be damaged. Patients remain continent because 
of a second, more 

ients remain continent because 
of a second, more distal, continence mechanism that is present in 
the region of the membranous urethra; however, many patients 
who have undergone transurethral resection of the prostate expe­
rience retrograde ejaculation. Ejaculatory disorders can lead to 
infertility among men.120–123 
Current Definitions of Sexual Disorders in 
Women
The current definitions for female sexual disorders in DSM-5 are 
shown in Table 20.1.45,46 A disorder is diagnosed only if the

ble 20.1.45,46 A disorder is diagnosed only if there is 
clinically significant distress or impairment; additionally, the sexual 
dysfunction should not be more attributable to a nonsexual psy­
chiatric disorder, to the effects of a substance (e.g., a drug of abuse, 
a medication), to a medical condition, or to relationship distress, 
partner violence, or other significant stressors.45,46 Disorders are 
identified as early onset (lifelong) versus late onset (acquired).
Sexual Interest/Arousal Di

 late onset (acquired).
Sexual Interest/Arousal Disorder
The DSM-5 definition addressing problematic desire merges sex­
ual interest (motivation) with arousal and deemphasizes the need 
to have initial anticipatory desire.245,246 Increasing evidence sug­
gests that desire ahead of and at the outset of sexual engagement, 
although welcomed probably by both partners, is not mandatory 
for a woman’s sexual enjoyment and satisfaction.21,247–249 It is the 
inability to trigger desire and arousal duri

 the 
inability to trigger desire and arousal during sexual engagement (as 
well as an absence of desire initially) that constitutes the disorder. 
Empirical support for the concept that arousal may precede desire 
and the two then coexist is now strong and includes data from 
older and younger women.21,24,248 Therefore, merging of sexual 
arousal and desire difficulties into one disorder appears logical. 
However, validated questionnaires used to assess sexual function 
are based on models of s

o assess sexual function 
are based on models of sexual response in which desire is assumed 
necessary at the outset of engagement. This is now acknowledged 
as a serious limitation to research,250 and the prevalence of what is 
currently understood to be disorder is not known.37,249–257 Studies 
that simply report “low desire and distress” indicate prevalence of 
about 10%, not increasing with age.37,255 Early research suggests 
that some two thirds of women diagnosed with HSDD would 
merit a d

irds of women diagnosed with HSDD would 
merit a diagnosis of SIAD as well. This research also indicated that 
women with both diagnoses have more severe and more numerous 
symptoms.258
Although, according to DSM-5, the SIAD diagnosis excludes 
situations where relationship issues, mood, medications, or illness 
are strong contributors to sexual dysfunction, in clinical practice 
most patients seeking help have risk factors including negative 
feelings for the partner and mood disorders.252 Depr

elings for the partner and mood disorders.252 Depression, either 
currently or in the past and even in the absence of a clinical diag­
nosis of depression, depressive and anxious thoughts and low self-
esteem are found significantly more commonly in women with 
SIAD than in women without SIAD.17,37,255 
The prevalence of women’s orgasmic disorder is not known, 
because many studies include women with low arousal who 
rarely reach orgasm.14,37,255 Risk factors include anx

ly reach orgasm.14,37,255 Risk factors include anxiety about 
the partner’s presence, fear of being vulnerable, fear of not being 
in control, and fear of intimacy.14 These factors often stem from 
childhood (nonsexual) experiences. 
Genitopelvic Pain/Penetration Disorder
The merging of former terms vaginismus and dyspareunia in 
DSM-5 has some merit. According to DSM-IV, the term vaginis­
mus described (phobic) avoidance, involuntary pelvic muscle con­
traction, and anticipation of, fear of, or

le con­
traction, and anticipation of, fear of, or the experience of pain in 
the absence of structural or other abnormalities on examination. 
Some women who reported typical phobic avoidance of penetra­
tion received an initial diagnosis of vaginismus; however, in many 
such women, a physical examination had to be deferred until 
therapy enabled a thorough pelvic examination to be performed. 
However, in some of these women, a subsequent careful introital 
examination enabled by successful the

l introital 
examination enabled by successful therapy could reveal allodynia 
of the vestibule. Thus, the initial diagnosis of vaginismus based 
on history would need to be changed to provoked vestibulodynia 
(PVD) after introital examination.
Reported prevalence of sexual pain varies between 20% and 
35%.259–263 PVD, which is the most common form of pain with 
penetration, affects some 16% of mostly premenopausal women, 
many of whom have had pain consistently from first attempts at 
penetrati

ain consistently from first attempts at 
penetration.253 Risk factors for PVD include some personality 
traits—perfectionism, reward dependency, fear of negative evalu­
ation—as well as harm avoidance, hypervigilance for pain, higher 
levels of trait anxiety, and shyness.253 In a small subset, vaginal 
infections, such as vaginal candidiasis, appear to precipitate and 
maintain the condition.
In DSM-5, vaginismus is now included within genitopelvic 
pain/penetration disorder (GPPPD).45 Risk fact

ic 
pain/penetration disorder (GPPPD).45 Risk factors for GPPPD 
include depression, anxiety, social phobia, somatization, and hos­
tility. Some studies identify increased catastrophic thinking in 
women with vaginismus compared with women without pain or 
women with other forms of pain: moreover, women with vagi­
nismus show greater disgust propensity.253 There is no evidence 
that vaginismus is related to religious orthodoxy, negative sexual 
upbringing, or concerns about sexual orientation. T

pbringing, or concerns about sexual orientation. Typically, there 
is extreme fear of vaginal entry, fear that harm will come from 
771
something the size of a penis entering the vagina, and fear of dam­
age by vaginal delivery. 
Persistent Genital Arousal Disorder
Not included in DSM-5 but clinically an increasingly common 
and poorly understood entity is persistent genital arousal disor­
der, which involves spontaneous, intrusive, and unwanted ge

h involves spontaneous, intrusive, and unwanted geni­
tal arousal (e.g., tingling, throbbing, pulsating) in the absence of 
sexual interest and desire. The awareness of subjective arousal is 
typically, but not invariably, unpleasant. The arousal is unrelieved 
by one or more orgasms, and the feelings of arousal may persist for 
hours or days.254 Persistent genital arousal disorder can be associ­
ated with a broad range of symptoms, of which the severity can 
vary from mild (and perhaps pleasant

severity can 
vary from mild (and perhaps pleasant) to intrusive and distressing, 
and which can interfere with life. Prevalence is unknown. 
Sexual Dysfunction in the Context 
of Endocrine Disease
We will focus on sexual sequelae of endocrine disease and its treat­
ment, but in any given person, nonendocrine factors may be more 
important. These factors include psychological, relational, contex­
tual, cultural, and nonendocrine medical influences—especially 
depression, hypertension, neurologic

s—especially 
depression, hypertension, neurologic disease, and LUTS.14 For 
patients with chronic disease, the disease itself, its treatment, and 
its psychological effects, plus interpersonal, personal, and contex­
tual issues, affect sexual response.14
In healthy women, factors such as attitudes toward sex, feel­
ings for the partner, past sexual experiences, duration of relation­
ship, and mental and emotional health have been shown to more 
strongly modulate desire and arousability than do 

strongly modulate desire and arousability than do biologic fac­
tors.14 Contrary to gender stereotypes, recent analysis of the 1035 
sexually active adults who participated in the NHSLS in 1992 
showed that men’s physical sexual pleasure was more closely linked 
to relational factors than was the case for women.37,188,191 Simi­
larly, in a recent international study of midlife and older couples, 
men rated the importance of sex for closeness and intimacy to their 
partner more highly than did th

intimacy to their 
partner more highly than did their female partners.37 Qualitative 
research also suggests that both men and women note that positive 
self-esteem and feeling attractive enhance desire and arousal.37,191 
Sexual context is also important for both men and women.191
Endocrine Disorders and Sexual 
Dysfunction in Men
Androgen Deficiency Syndromes
Androgen deficiency in men is a syndrome characterized by a 
constellation of signs and symptoms associated with consistently 
and unequ

d symptoms associated with consistently 
and unequivocally low testosterone levels due to disorders of the 
testes, pituitary, or hypothalamus.27,63,264 Androgen deficiency 
can occur either because of primary testicular dysfunction or as 
a result of disorders affecting the hypothalamus or the pituitary.63 
Common causes of primary testicular dysfunction include Kline­
felter syndrome, uncorrected cryptorchidism, radiation and can­
cer chemotherapy, Yq microdeletions, human immunodeficiency 
vi

apy, Yq microdeletions, human immunodeficiency 
virus (HIV) infection, orchitis, trauma, and torsion.63 Secondary 
testicular dysfunction can result from systemic illness; excessive 
exercise; recreational drugs, especially opiates, anabolic steroids, 
marijuana, cocaine, and alcohol; pituitary and suprasellar tumors; 
hemochromatosis; hyperprolactinemia; and infiltrative disorders. 
Exclusion of these secondary causes of hypogonadism may then 
justify a diagnosis of idiopathic hypogonadotropic 

ustify a diagnosis of idiopathic hypogonadotropic hypogonad­
ism, which is a heterogeneous group of disorders characterized 
by disordered GnRH secretion.63 Opioid use has emerged as an 
important cause of testosterone deficiency and sexual dysfunction. 
Similarly, in many men’s health clinics, a substantial proportion of 
men receiving testosterone replacement therapy are former ana­
bolic steroid users. The testosterone levels required to maintain 
sexual function in men are close to the lower

ain 
sexual function in men are close to the lower limit of the normal 
male range.65,144,155,158,265,266 Therefore, some men with pituitary 
tumors may remain asymptomatic until their tumor has grown 
substantially and testosterone levels have declined to a level below 
this threshold.
Androgen deficiency is an important treatable cause of HSDD 
in men. Therefore, men diagnosed with HSDD should be evalu­
ated for androgen deficiency by measurement of testosterone lev­
els using a reliable assay

nt of testosterone lev­
els using a reliable assay, preferably in an early morning fasting 
blood sample.63 Although ED and androgen deficiency in men are 
separate disorders with distinct pathophysiologic mechanisms, the 
two can coexist in the same patient. Testosterone levels should be 
measured in men presenting with any form of sexual dysfunction 
because androgen deficiency is treatable; furthermore, androgen 
deficiency may be a manifestation of another underlying disease, 
such as a pitu

ion of another underlying disease, 
such as a pituitary tumor, which may require additional evaluation 
and disease-specific intervention. 
Diabetes and Sexual Dysfunction in Men
Men with diabetes mellitus are at increased risk of ED, retro­
grade ejaculation, and low testosterone levels. Peyronie disease 
is an important comorbid condition in older diabetic men with 
ED.267–274 Men with diabetes have significantly lower scores for 
sexual desire, activity, arousal, and satisfaction,268–274 in p

, activity, arousal, and satisfaction,268–274 in part due 
to the medical and psychological factors associated with diabetes, 
such as the variations in glycemic control, reduced energy, and 
altered self-image. Diabetes also is associated with increased risk 
of low testosterone levels.63,275–280 In population studies, sex hor­
mone–binding globulin (SHBG) and total testosterone are more 
strongly associated with diabetes risk than free testosterone levels; 
these data suggest that the observed

rone levels; 
these data suggest that the observed association of testosterone 
with diabetes risk may be related to factors, such as obesity and 
inflammation, that alter SHBG.279,280
The prevalence of ED in men with diabetes increases with age 
and has been reported to be as high as 75% in some studies. ED 
in men with type 2 diabetes, even without other risk factors for 
coronary artery disease, may signal silent cardiac ischemia.280–284 
Among men with diabetes, those with ED are more likely

g men with diabetes, those with ED are more likely to 
be older and smokers, and to have a longer duration of diabe­
tes, poor metabolic control, untreated hypertension, neuropathy, 
microalbuminuria and macroalbuminuria, retinopathy, CVD, 
diuretic treatment, low testosterone levels, and psychological vul­
nerability.270–273 The risk of ED generally increases with chronic 
elevation of hemoglobin A1c.270 Increased physical activity and 
consumption of small amounts of alcohol have been found to

ion of small amounts of alcohol have been found to be 
associated with lower risk of ED.
Endothelial and smooth muscle dysfunction, autonomic neu­
ropathy, and psychological and interpersonal issues contribute to 
sexual dysfunction in men with diabetes.283,284 Endothelial dys­
function is evident in both penile blood vessels and nongenital 
vascular beds.212 eNOS is reduced, possibly due to overexpression 
of arginase or lack of nicotinamide adenine dinucleotide phos­
phate (NADPH), which is an

nine dinucleotide phos­
phate (NADPH), which is an essential cofactor for NOS.284–289 
Additionally, accumulation of oxygen free radicals, including 
772
those from advanced glycosylation end products, quench nitric 
oxide and attenuate the action of K+ channels.288,289 Men with 
long duration of diabetes may not be able to generate sufficient 
nitric oxide and cGMP within the cavernosal smooth muscle; 
thus, they may become unresponsive to PDE5 inhib

thus, they may become unresponsive to PDE5 inhibitors. The 
reduction in NADPH is also associated with increased diacylglyc­
erol and protein kinase C, and, consequently, increased smooth 
muscle contractility.286 An increased activation of the Rho A/Rho 
kinase pathway may increase the sensitivity of cavernosal smooth 
muscle to calcium.286 Autonomic neuropathy affecting the pelvic 
nerves may lead to both ED and ejaculatory dysfunction.287
Retrograde ejaculation and partial ejaculatory incom

trograde ejaculation and partial ejaculatory incompetence 
affect up to one third of men with diabetes.290 Autonomic neu­
ropathy in diabetes may be associated with dysfunction of the 
internal sphincter so that all or a part of the seminal fluid is 
propelled into the bladder.287 Partial ejaculatory incompetence 
refers to the condition in which ejaculatory emission remains 
intact but the expulsion phase is inhibited; consequently, the 
semen trickles out of the penis and the experience of org

rickles out of the penis and the experience of orgasm is 
altered in quality. The ejaculatory problems may be a cause of 
infertility. 
Sexual Dysfunction Associated With Therapies 
for Benign Prostatic Hypertrophy
Benign prostatic hyperplasia is frequently associated with LUTS 
and sexual dysfunction.208–214 Some α1-adrenergic receptor block­
ers, such as tamsulosin, are associated with ejaculatory dysfunc­
tion.291,292 Treatment of men with LUTS with 5α-reductase 
inhibitors has been associate

S with 5α-reductase 
inhibitors has been associated with increased risk of ejaculatory 
disorder, ED, and decreased libido.293,294
Several surveys have reported the development of sexual symp­
toms, including loss of libido, ED, depressive symptoms and even 
suicidality, and cognitive symptoms in a subset of young men 
who have taken finasteride for alopecia.295–298 These symptoms 
have been reported to persist even after discontinuation of fin­
asteride, and this condition has been referred to 

asteride, and this condition has been referred to as postfinaste­
ride syndrome. Our recent investigations of men who developed 
sexual symptoms after using finasteride and whose symptoms per­
sisted even after discontinuation of finasteride have revealed that 
these men do not have evidence of androgen deficiency, decreased 
peripheral androgen action, or persistent peripheral inhibition of 
SRD5A.299
Symptomatic finasteride users revealed depressed mood and 
fMRI findings consistent with those

ssed mood and 
fMRI findings consistent with those observed in men with depres­
sion.299 The causal role of finasteride and the pathophysiology of 
these symptoms remain to be established.299,300 
Hyperprolactinemia and Sexual Dysfunction
Hyperprolactinemic men often present with decreased libido or 
ED; 75% of men with macroprolactinomas and 50% of men with 
microprolactinomas report reduced desire or ED, and almost all 
have subnormal nocturnal penile erections.301–305 Hyperprolac­
tinemia aff

 penile erections.301–305 Hyperprolac­
tinemia affects 1% to 5% of men presenting with ED302; a frac­
tion of these men have prolactin-secreting pituitary adenomas. 
Prolactin lowers testosterone levels through its inhibitory effects 
on GnRH secretion and on the pituitary gonadotrope response 
to GnRH. Most, but not all, men with sexual dysfunction and 
hyperprolactinemia have low testosterone levels.301,302 Whether 
and how hyperprolactinemia directly affects erectile function 
through target 

irectly affects erectile function 
through target organ effects is not well understood. Erectile func­
tion generally improves in hyperprolactinemic men following 
treatment with dopamine agonists.304,305 
Sexual Dysfunction in Patients With Thyroid Disease
Hypothyroidism has been associated with increased risk of hypo­
active sexual desire and ED.306–310 The exact prevalence of sexual 
dysfunction in men with hypothyroidism is unknown. Free tes­
tosterone levels are lower in hypothyroid men tha

tosterone levels are lower in hypothyroid men than in control 
subjects and become normal after thyroxine replacement.306–310 
Serum LH and follicle-stimulating hormone levels are typically 
not elevated in men with primary hypothyroidism.309 Hyperpro­
lactinemia is noted in a small proportion of hypothyroid men.309
Free testosterone levels are typically normal in men with hyper­
thyroidism, but SHBG and estradiol levels are elevated, resulting 
in a high estradiol-to-testosterone ratio and gyn

in a high estradiol-to-testosterone ratio and gynecomastia in some 
hyperthyroid men.308 Hyperthyroidism has been observed in a 
small fraction of men with ED.310 
Sexual Dysfunction in Men With Metabolic Syndrome
Men with metabolic syndrome have a higher prevalence of ED 
than men without metabolic syndrome.311–314 The risk of ED is 
correlated with the number of identified components of metabolic 
syndrome.310–314 
Endocrine Disorders and Sexual Dysfunction 
in Women
Thyroid Disease in Women

al Dysfunction 
in Women
Both hyper- and hypothyroid states have been found to be risk 
factors for sexual dysfunction, which mostly remits with return to 
a euthyroid state.315–317 Studies are few and small, and the sexual 
dysfunction was assessed in these studies using neither modern 
psychometrically robust instruments nor the DSM-5 classification 
of sexual disorders. Comorbid depression is associated with sexual 
dysfunction in the context of thyroid disease.27,315

sfunction in the context of thyroid disease.27,315 There is some 
evidence that thyroid autoimmunity lessens sexual desire indepen­
dent of altered thyroid status: euthyroid women with Hashimoto 
thyroiditis may report persistent loss of desire.315–317 One research 
group found that women with nodular goiter have significantly 
more sexual dysfunction than control subjects. This group also 
had the highest body mass index.317 It is not clear whether the 
sexual dysfunction in patients with thyro

her the 
sexual dysfunction in patients with thyroid disease is related to 
the underlying thyroid disease or to comorbid conditions or body 
image problems. 
Hyperprolactinemia in Women
Hyperprolactinemia can be associated with increased risk of men­
strual irregularities, amenorrhea, hypoestrogenemia, and galactor­
rhea.79,318 Some studies find that women with hyperprolactinemia 
report greater overall dissatisfaction with sexual function and 
lower scores for sexual desire, arousal, lubricati

lower scores for sexual desire, arousal, lubrication, and orgasm 
than women with normal prolactin levels. The lower scores for 
sexual function and desire have also been found in women with 
hyperprolactinemia who have regular menses.318 However, normal 
menstruation, younger age, and smaller size of prolactinoma are 
more likely to be associated with normal sexual function.14 Pro­
lactin inhibits GnRH pulses, attenuates gonadotropin response 
to GnRH, and is associated with reduced ovarian sec

o GnRH, and is associated with reduced ovarian secretion of 
estradiol and androgens. Sexual outcomes in hyperprolactinemic 
women after treatment with dopamine agonists have not been 
well studied. 
Diabetes in Women
A recent meta-analysis319 including 3168 women with diabetes 
and 2823 control subjects from 26 studies confirmed that sexual 
773
dysfunction is more frequent in women with diabetes than in 
age-matched controls. Compared with women 

han in 
age-matched controls. Compared with women without diabetes, 
the risk of sexual dysfunction was 2.27 and 2.49 times higher in 
women with type 1 and type 2 diabetes, respectively.319 However, 
postmenopausal women with any form of diabetes did not dem­
onstrate an increased risk of sexual dysfunction.
Sexual dysfunction in women with diabetes is complex, and 
the interrelationships between body mass index, fat distribution, 
diabetic complications, insulin resistance, inflammation, CVD, 

lications, insulin resistance, inflammation, CVD, 
relationship satisfaction, and depression remain incompletely 
understood.14,320,321 The increased prevalence of sexual dysfunc­
tion and lower Female Sexual Function Index (FSFI) score in 
women with diabetes may be related to body weight. This associa­
tion would be in keeping with other studies showing an increased 
prevalence of sexual dysfunction in obese women320–322 and in 
women with metabolic syndrome.323,324 Unlike the situation in 
me

bolic syndrome.323,324 Unlike the situation in 
men with diabetes, sexual dysfunction has not been associated 
with the duration of diabetes or the presence of diabetic complica­
tions in the majority of studies. Most studies in women with type 
2 diabetes are small, but one larger study of 600 women with type 
2 diabetes confirmed depression and marital status to be indepen­
dent risk factors for sexual dysfunction.325
Women’s sexual response and satisfaction may be compromised 
by diabetes-ass

d satisfaction may be compromised 
by diabetes-associated changes in their well-being, mood, self-
image, weight gain, recurrent vaginitis from candidiasis, or infer­
tility.319,320,322,325–335 In addition, genital sexual response may be 
compromised by autonomic neuropathy, endothelial dysfunc­
tion, and microvascular disease. Some, but not all, studies show 
increased prevalence of dyspareunia, orgasmic difficulties, and 
sexual dissatisfaction.318 In women with type 1 diabetes, sexual 
dysfun

.318 In women with type 1 diabetes, sexual 
dysfunction is mostly correlated to psychological factors includ­
ing depression, anxiety, and marital status.322,326,327,329 A large 
prospective study of 625 women with type 1 diabetes confirmed 
depression as a major predictor of sexual dysfunction.333 Type 1 
diabetes also may be associated with loss of genital sexual sensitiv­
ity and subsequent loss of orgasm.336 Young women with diabetes 
on multiple insulin dose regimens had impaired arousal an

iple insulin dose regimens had impaired arousal and lubri­
cation, whereas those using an insulin pump had sexual function 
comparable to healthy age-matched women.337
Autonomic and somatic neuropathy may contribute to loss of 
genital sexual sensitivity. When there is less engorgement of the 
vascular sinusoidal tissue comprising the shaft, head, rami, and 
bulbs of the clitoris, massaging the structures during sexual stim­
ulation may fail to elicit typical sexual sensations, compromise 
arous

licit typical sexual sensations, compromise 
arousal, and limit the experience of orgasm.338,339
In animal studies, diabetes has been shown to impair vaginal 
smooth muscle relaxation responses to the neurotransmitters, 
particularly VIP and nitric oxide.338 These studies also report 
decreased clitoral and vaginal blood flow to nerve stimulation, 
diffuse fibrosis of the clitoral and vaginal tissues, and reduced 
muscular layer and epithelial thickness in vaginal tissue. Endothe­
lial dysfuncti

ickness in vaginal tissue. Endothe­
lial dysfunction and reduced clitoral blood flow have been docu­
mented in women with diabetes.339
Most studies have not found increased prevalence of dyspareu­
nia in women with diabetes. However, they are at higher risk of 
recurrent candidiasis, which may contribute to dyspareunia. 
Metabolic Syndrome in Women
Metabolic syndrome has been shown to have a deleterious effect on 
women’s sexuality, independent of diabetes and obesity.14,27,340,341 
This negativ

f diabetes and obesity.14,27,340,341 
This negative effect seems to be more prevalent in premenopausal 
women than in postmenopausal women.14,27,340,341 
Some studies have suggested that women with polycystic ovary 
syndrome may be less sexually satisfied and may regard themselves 
as less attractive than control subjects.342–345 The presumption 
is that obesity and androgen-related symptoms may contribute 
to poor body image, which may increase the risk of sexual dys­

image, which may increase the risk of sexual dys­
function.27,320 However, it is not clear whether polycystic ovary 
syndrome per se, as opposed to obesity, is a risk factor for sex­
ual dysfunction.345,346 There is no evidence of heightened sexual 
desire from the higher androgen levels. 
Whereas the classic form almost always presents at birth with 
adrenal insufficiency and virilization in girls, nonclassic forms 
of congenital adrenal hyperplasia (NC-CAH) may p

of congenital adrenal hyperplasia (NC-CAH) may present with 
signs of hyperandrogenism in childhood or adulthood, depend­
ing on the severity of the 21-hydroxylase enzyme deficiency.347 
The presenting features of 21-hydroxylase enzyme deficiency may 
include amenorrhea, anovulation, hirsutism, or oligomenorrhea 
with infertility. A recent study found women with NC-CAH had 
a lower total FSFI score, with lower sexual arousal, lubrication, 
sexual satisfaction, and more dyspareunia, plus higher

al satisfaction, and more dyspareunia, plus higher depres­
sive symptom scores compared with well-matched controls.347 
Women with classic and NC-CAH may show gender-atypical 
behavior348; in one study, male-typical role-playing in childhood 
correlated with reduced satisfaction with the female gender role 
and reduced heterosexual interest in adulthood.349 Disturbed 
body image, repeated genital examinations, and genital surgery 
may also affect sexual function in women with classic congenital 

 sexual function in women with classic congenital 
adrenal hyperplasia.348 Decreased sexual frequency, sexual satisfac­
tion, and sensuality, high rates of sexual avoidance, and difficulty 
with vaginal penetration and orgasm, regardless of whether they 
have undergone prior surgery, have been reported in women with 
classic and NC-CAH. Caring for these women requires careful 
individualized treatment of both androgen excess and psychosex­
ual counseling.350 
Pituitary Disease in Women
There is 

unseling.350 
There is limited research on sexual function in women who have 
deficiencies of various pituitary hormones. Women with pituitary 
disease and gonadotropin deficiency may suffer from amenor­
rhea or menstrual irregularity or problems with sexual function, 
including decreased sexual desire and problems with lubrica­
tion or orgasm.351 Although women with hypopituitarism have 
lower estradiol and testosterone levels than healthy menstruating 
women, the sho

e levels than healthy menstruating 
women, the short- and long-term effects of replacing testoster­
one in women with hypopituitarism who are receiving estrogen 
replacement have not been well studied.351,352 In one random­
ized trial of 51 women, testosterone therapy in women who were 
receiving estrogen therapy was associated with some benefit in sex­
ual function and mood, compared with placebo, but with a higher 
frequency of androgenic side effects than placebo.353 The effects 
of replacing

effects than placebo.353 The effects 
of replacing dehydroepiandrosterone (DHEA) on sexual func­
tion and mood in women with hypopituitarism are also poorly 
understood.351 
Adrenal Insufficiency in Women
In addition to deficiency of cortisol and aldosterone, women 
with adrenal insufficiency also have low levels of testosterone and 
DHEA.27,354–363 Adrenal insufficiency in women has been associ­
ated with low health-related quality of life.359 However, a 2010 
larger study comparing 174 women w

owever, a 2010 
larger study comparing 174 women with Addison disease with 
774
740 age-matched healthy control subjects and with 234 women 
who had received a risk-reducing bilateral salpingo-oophorectomy 
(BSO) demonstrated that despite subnormal levels of androgens 
and androgen metabolites, the women with Addison disease 
reported higher sexual pleasure and less discomfort with inter­
course than the normative control women.360 Clinical trials of 

he normative control women.360 Clinical trials of 
DHEA replacement in women with adrenal insufficiency have 
been small and mostly negative.27,354–358,361,363 An earlier small 
trial in women with primary or secondary adrenal insufficiency 
reported greater improvements in sexual interest and satisfaction 
and in mood for women receiving DHEA compared with pla­
cebo354; however, four subsequent studies did not find significant 
improvements in sexual function.355–358 In 2009, a meta-analysis 
o

exual function.355–358 In 2009, a meta-analysis 
of the 10 notably small studies concluded that DHEA therapy 
in adrenal insufficiency may result in small improvements in 
health-related quality of life and depression, but it had no effects 
on anxiety or sexual well-being.363 Thus, there are insufficient 
data to support the routine use of DHEA in women with adre­
nal insufficiency. Unfortunately, given that the typical multiple 
factors underlying women’s sexual dysfunctions and small studies 

ing women’s sexual dysfunctions and small studies 
with limited assessments of sexual function may not be helpful in 
evaluating benefit from absent hormones. In addition to being a 
prohormone for the intracellular production of testosterone and 
estrogen, DHEA is now known to have multiple direct actions, 
including the modulation of various receptors and synaptic trans­
mission in the brain, where its concentration can be six times 
higher than in serum.364,365 
Natural Menopause
A majority o

n in serum.364,365 
A majority of women who discontinue previously needed post­
menopausal estrogen supplementation develop signs of vulvo­
vaginal atrophy, now termed genitourinary syndrome of menopause 
(GSM), which is a risk factor for sexual dysfunction.259–263,366,367 
However, symptoms from GSM may remit spontaneously within 
1 year; risk factors for more severe symptoms are diabetes, younger 
age, and low body mass index.263,366,367 The traditional notion 
that maintaini

263,366,367 The traditional notion 
that maintaining sexual activity will prevent symptomatic GSM 
has been refuted.367 Subjective symptoms and objective signs of 
GSM correlate poorly.368 Epidemiologic studies have not shown 
an increase in the prevalence of dyspareunia with age.259–261 
Clearly, not all postmenopausal women develop sexual symptoms 
of estrogen deficiency: of 1525 women followed from age 47 to 
54 years, most did not experience sexual dysfunction.262 It is likely 
that multiple

sexual dysfunction.262 It is likely 
that multiple factors contribute to sexual symptoms, including 
variations in the intracrine production of estrogen from adrenal 
precursors, the number and sensitivity of estrogen receptors, and 
the degree of sexual arousal or excitement at the time of vulval 
stimulation and vaginal entry.369–371 Psychological factors rather 
than estrogen levels were shown to moderate symptoms when 
GSM is present.371
Most studies report a decrease in sexual desire with a

 studies report a decrease in sexual desire with advancing 
age372 that is not easily explained by hormonal deficiency alone. 
Adaptive changes occur in the brain in response to the reduc­
tions in circulating levels of sex hormones associated with age and 
menopause.373,374 Sex hormones are produced locally within the 
brain: in women, steroidogenic enzymes and sex-steroid receptors 
in the brain are upregulated in response to decreased circulating 
levels of sex hormones.373,374 We do not know

ing 
levels of sex hormones.373,374 We do not know whether there is 
biologic adaptation to reduced amounts of circulating sex hor­
mones. In studies of age, menopausal status, and sexual function, 
the postmenopausal state has generally been negatively associated 
with desire, mainly among women who experienced low emo­
tional intimacy with their partners. Similarly, the negative associa­
tion between age and sexual desire was particularly pronounced in 
women experiencing little intimacy.375 

unced in 
Surgical Menopause
Surgical menopause is a state of both androgen and estrogen 
depletion of sudden onset and has often been viewed as a risk 
factor for sexual dysfunction. However, most women undergoing 
bilateral BSO for benign clinical indications do not develop sexual 
dysfunction. Three prospective studies found that women choos­
ing BSO plus hysterectomy for benign indications did not develop 
sexual dysfunction over the next 1 to 3 years.

op 
sexual dysfunction over the next 1 to 3 years.376–378
A national survey of 2207 American women confirmed an 
increased prevalence of distress about low sexual desire in women 
with a recent BSO.372 Thus, in women undergoing nonelective 
surgery, the thematic context of bilateral oophorectomy may 
impair sexual desire and function. For example, women who are 
treated for malignant disease or those who desire to preserve their 
fertility may experience greater distress about low sexual desire 

perience greater distress about low sexual desire 
after BSO than those who undergo BSO for benign conditions. 
In the same survey, both older and younger women with a rela­
tively recent BSO reported low sexual desire, per se, as often as 
age-matched subjects with intact ovaries.372 Despite their contin­
ued hormonal deficit, women older than 45 years who underwent 
oophorectomy before menopause had fewer complaints of low 
desire than women of similar age with intact ovaries.372
A recent stud

 similar age with intact ovaries.372
A recent study of 1352 women showed no difference in the 
report of sexual ideation, sexual function, or sexual problems 
between women who have had and women who have not had 
bilateral oophorectomy.379 Having thoughts about sex is less likely 
to be affected by contextual details including the sexual relation­
ship than is sexual function or motivation for partnered sex.
The women who carry a BRCA1 or BRCA2 mutation and 
undergo BSO to lessen the risk of br

mutation and 
undergo BSO to lessen the risk of breast, ovarian, or fallopian 
tube cancer mostly remain satisfied with their decision for sur­
gery.380 However, after risk-reducing surgery, women reported sig­
nificantly less sexual pleasure, more discomfort, and less frequent 
sex compared with controls.381 
Aging-Associated Decline in Sex Hormone 
Precursors in Women
From the middle 30s to the early 60s, a woman’s adrenal produc­
tion of precursor hormones—DHEA, androstenedione, and DHEA 
sul

rsor hormones—DHEA, androstenedione, and DHEA 
sulfate—declines by 70%.66,68 However, the trajectories of decline in 
these precursor steroids vary among women.66–68 The relationship of 
the age-related decline in these circulating precursors to sexual func­
tion remains poorly understood. On the population level, variation in 
circulating levels of sex steroids and their precursors is related to vari­
ation in steroidogenic enzyme activities, such as 3β-hydroxysteroid 
dehydrogenase, 17β-hydrox

ch as 3β-hydroxysteroid 
dehydrogenase, 17β-hydroxysteroid dehydrogenase, 17,20-lyase, 
and aromatase, and to the variation in the plasma clearance of these 
hormones and precursors. Labrie and associates68,369,382 have pro­
posed that the androgen metabolites, most notably ADT-G, may 
serve as useful markers of ovarian and tissue production and activity 
of androgens in women. How the decline in total androgen activity 
with age may relate to sexual function has yet to be studied. 
Selective Es

xual function has yet to be studied. 
Selective Estrogen Receptor Modulators
Selective estrogen receptor modulators are a class of ligands that 
bind estrogen receptor subtypes and induce a unique profile of 
775
tissue-specific gene expression. Accordingly, each selective estro­
gen receptor modulator may also be associated with a unique set 
of clinical responses. Ospemifene has estrogen antagonist action 
on breast and endometrium. Ospemifene, b

t action 
on breast and endometrium. Ospemifene, but not raloxifene or 
tamoxifen, can ameliorate the genital sexual symptoms of lack of 
estrogen.383 Limited research suggests raloxifene and tamoxifen 
are not associated with sexual adverse effects. 
Hormonal Contraceptives
The estrogen in combined systemic contraceptives increases 
SHBG and thus decreases available free testosterone. The 
decrease in sexual desire and subjective arousability in some 
women receiving oral contraceptives has bee

 some 
women receiving oral contraceptives has been attributed to 
the decrease in free testosterone levels. However, to date, low 
desire has not been associated with testosterone levels, even 
when mass spectrometry methods are used.66,67 Dyspareunia is 
an occasional consequence of very low estrogen formulations. 
Hormonal contraceptives exert multiple psychological and bio­
logic actions, some of which may positively affect sexuality, for 
example, by reducing anxiety about unwanted pregnanc

ample, by reducing anxiety about unwanted pregnancy and 
diminishing dysmenorrhea.384 
Androgen Insensitivity Syndrome
Women with complete androgen insensitivity syndrome have 
a female phenotype with full breast development but variable 
shallow vagina, which may require surgical intervention or pro­
gressive dilatation. Small cross-sectional studies indicate that 
women with complete androgen insensitivity are capable of a 
healthy sexual response with orgasms of self-stimulation and of 
inter

nse with orgasms of self-stimulation and of 
intercourse to be possible despite complete androgen insensi­
tivity.385–389 However, these women are often confronted with 
complex psychosocial issues related to the mismatch between 
their genetic sex and their gender role, the timing of diagnosis 
and timing of disclosure to the woman, and infertility. Reduced 
sexual confidence, poor self-esteem, and depression are noted in 
these studies. 
Assessment of Sexual Dysfunction
Assessment of sexual fu

ment of Sexual Dysfunction
Assessment of sexual function is an important part of the evalu­
ation of patients with endocrine diseases. Open-ended, non­
judgmental questions such as “Many men with diabetes notice 
changes in their erections or ejaculation—are you having any 
difficulties?” can facilitate further discussion of sexual problems. 
When sexual problems are identified, sensitive and respectful 
inquiry into their nature and the sexual context is necessary. 
Evaluating both partners tog

ontext is necessary. 
Evaluating both partners together, as well as individually, can 
often uncover problems that may not be apparent in individual 
interviews.
Evaluation of Men With Sexual Dysfunction
There are four important considerations in the evaluation of men 
with HSDD. First, an important initial step in the evaluation 
is an interview of the couple to determine whether the patient 
primarily has ED or a sexual desire problem. Second, ascertain 
whether the couple has a relationship p

ascertain 
whether the couple has a relationship problem. Establish whether 
self-stimulation continues despite lack of desire for partnered sex. 
With the easy accessibility of Internet sites, sex alone, possibly on 
a frequent basis, may allow sexual expression despite relationship 
difficulties. Third, general health evaluation is necessary to exclude 
systemic illness, depression, and medication use. Serum total and 
free testosterone levels should be measured to exclude androgen 

should be measured to exclude androgen 
deficiency because androgen deficiency is an important treatable 
cause of HSDD.
The diagnostic workup of men with ED should start with an 
evaluation of general health5,6,390–396 (Tables 20.4 and 20.5). The 
presence of diabetes mellitus, coronary artery disease, peripheral 
vascular disease, hypertension, chronic kidney disease, stroke, 
spinal cord or back injury, a neurologic disorder, depression, 
and dementia should be verified. Information about use

dementia should be verified. Information about use of rec­
reational drugs such as alcohol, opioids, marijuana, cocaine, and 
tobacco, prescription medications, particularly antihypertensives, 
antiandrogens, antidepressants, antipsychotic drugs, and opioid 
analgesics, and nonprescription over-the-counter supplements is 
important because almost one fourth of all cases of impotence 
can be attributed to medications. A detailed sexual history should 
elicit the nature of relationships, partner e

uld 
elicit the nature of relationships, partner expectations, situational 
erectile failure, performance anxiety, and marital discord. It is 
important to distinguish between inability to achieve erection, 
changes in sexual desire, failure to achieve orgasm and ejaculation, 
and dissatisfaction with the sexual relationship, as the etiologic 
factors vary with the type of sexual disorder.
A directed physical examination should focus on secondary sex 
characteristics, the presence or absence of 

 sex 
characteristics, the presence or absence of breast enlargement and 
testicular volume; evaluation of femoral and pedal pulses; neu­
rologic examination to determine the presence of motor weak­
ness, perineal sensation, anal sphincter tone, and bulbocavernosus 
reflex; and examination of the penis to evaluate any unusual cur­
vature, palpable plaques, or superficial lesions.390–395
The initial diagnostic workup in most men presenting with ED 
usually includes the measurement of hemoglobin, 

usually includes the measurement of hemoglobin, blood glucose 
and hemoglobin A1C, blood urea nitrogen and creatinine, plasma 
lipids, and serum total and free testosterone levels. Further evalua­
tion using more invasive diagnostic testing is limited to those men 
who do not respond to an empiric trial of oral PDE5 inhibitors; 
these patients should be referred to a specialist for detailed uro­
logical evaluation.
Self-reporting questionnaires are useful because many men 
with ED do not volun

 are useful because many men 
with ED do not voluntarily disclose their sexual complaints to 
their health care provider.390–393 The International Index of Erec­
tile Function, for example, is a multidimensional scale consist­
ing of 15 questions that address relevant domains of male sexual 
function, including sexual desire, intercourse satisfaction, orgas­
mic function, and overall satisfaction390; a short form is also 
available.393
The diagnosis of androgen deficiency should be made only 
in

sis of androgen deficiency should be made only 
in men with consistent symptoms and signs and unequivocally 
low early morning serum testosterone levels that are consis­
tently below the lower limit of the normal range for healthy 
young men.6,27,65,264 Because of the diurnal, pulsatile, and 
interday variations in circulating testosterone levels, and the 
known suppressive effects of food and glucose intake, total tes­
tosterone concentrations should be measured in a fasting state 
on two separ

hould be measured in a fasting state 
on two separate mornings. Serum testosterone levels should be 
measured using a reliable assay, such as liquid chromatography 
tandem mass spectrometry (LC-MS/MS) assays, which have 
become widely available from many commercial and research 
laboratories, and which offer a higher level of accuracy and pre­
cision than immunoassays, especially at the low levels present 
in hypogonadal men. Ideally, testosterone levels should be mea­
sured using an assay that 

e levels should be mea­
sured using an assay that has been certified by an accuracy-based 
776
  Assessment of a Patient With Sexual Dysfunction
Assessment Questions
Comments
Questions Asked of One or Both Partners
	1.	 Sexual problems and reason for presenting at this time
Ask patients to describe sexual problems in their own words; clarify further with direct ques­
tions, giving options rather than leading questions, support and encouragement, ack

 leading questions, support and encouragement, acknowl­
edgement of embarrassment, and reassurance that sexual problems are common.
	2.	 Duration, consistency, and priority if more than one 
problem is present
Are problems present in all situations? Which problem is most troubling?
Context of sexual problems
Emotional intimacy between partners, activity or behavior just before sexual activity, 
privacy, sexual communication, time of day and fatigue level, birth control (adequacy, 
type), risk o

n Seen Alonea
	1.	 Partner’s own assessment of the situation
Sometimes it is easier to disclose symptom severity (e.g., total lack of desire) in the 
partner’s absence.
	2.	 Sexual response with self-stimulation
Also inquire about sexual thoughts and fantasies.
	3.	 Past sexual experiences
Positive and negative aspects.
	4.	 Developmental history
Relationships to others in the home while growing up; losses, traumas, to whom (if any­
one) was the patient close; was he or she shown physical affec

 patient close; was he or she shown physical affection, love, respect?
	5.	 Past or current sexual, emotional, and physical abuse
Explain that abuse questions are routine and do not necessarily imply causation of the 
problems; it is helpful to ask whether the patient ever felt hurt or threatened in the 
relationship and, if so, whether he or she wishes to give more information.
	6.	 Physical health, especially conditions leading to debility and 
fatigue, difficulty with mobility (e.g., in car

d 
fatigue, difficulty with mobility (e.g., in caressing a partner, 
performing self-stimulation), and difficulties with self-image 
(e.g., from obesity, Cushing syndrome, hypogonadism)
Specifically, ask about medications with known sexual side effects, including SSRIs, 
SNRIs, β-blockers, narcotics, antiandrogens, and GnRH agonists.
	7.	 Evaluation of mood
A significant correlation of sexual function and mood (including anxiety and depression) 
warrants routine screening for mood disorder using

warrants routine screening for mood disorder using either a questionnaire (e.g., Beck 
Inventory) or semistructured series of questions.
aItems 3 through 5 of the single-patient interview may sometimes be omitted (e.g., for a recent problem after decades of healthy sexual function).
STIs, Sexually transmitted infections; SSRIs, selective serotonin reuptake inhibitors; SNRIs, selective serotonin norepinephrine reuptake inhibitors; GnRH, gonadotropin-releasing hormone
Adapted from Basson R. Clinic

in-releasing hormone
Adapted from Basson R. Clinical Practice. Sexual desire and arousal disorders in women. N Engl J Med. 2006;354(14):1497–1506.
   
TABLE 20.4
standardization or quality control program (e.g., the Hormone 
Standardization Program for Testosterone from the Centers 
for Disease Control and Prevention (CDC).264 The advent of 
LC-MS/MS, the availability of a testosterone calibrator from 
the National Institute of Standards and Technologies, and the 
institution of the Hormone Stan

nologies, and the 
institution of the Hormone Standardization Program for Testos­
terone have greatly improved the accuracy of testosterone assays 
and reduced interlaboratory variability among CDC-certified 
laboratories.264,397–400
The reference ranges for testosterone in men reported by vari­
ous laboratories vary substantially because of assay differences, 
calibrator differences, and differences in the reference popula­
tions used to generate ranges. Recently, under the auspices of the 
End

e ranges. Recently, under the auspices of the 
Endocrine Society and the Partnership for the Accurate Testing of 
Hormones, we have published harmonized reference range based 
on data from community-dwelling men from four large cohorts in 
the United States and Europe.401 The harmonized reference range 
for total testosterone in healthy, nonobese young men (age 19–39 
years) was 264 to 916 ng/dL (9.2–31.8 nmol/L).401 This reference 
range can be applied to all CDC-certified total testosterone as

applied to all CDC-certified total testosterone assays.
Circulating testosterone is bound with high affinity to SHBG 
and with lower affinity to albumin, orosomucoid, and cortisol-
binding globulin; only 2 to 4% of circulating testosterone is 
unbound or free.402 The alterations in SHBG concentrations can 
alter the total testosterone concentrations; therefore, in conditions 
that alter SHBG concentrations, such as obesity, diabetes, aging, 
liver disease, hypo- and hyperthyroidism, and intake o

r disease, hypo- and hyperthyroidism, and intake of many 
medications, determination of free testosterone levels is necessary 
to assess androgen status.402 Free testosterone concentrations can 
be measured using the equilibrium dialysis assays or calculated 
from total testosterone, SHBG, and albumin concentrations.402 
The direct tracer analogue free testosterone assays are used in 
many hospital laboratories, but they are inaccurate and should 
not be used.
Zakharov and colleagues403 have sho

Zakharov and colleagues403 have shown that testosterone’s bind­
ing to SHBG is a dynamic multistep process that includes het­
erogeneity in circulating isoforms of SHBG dimer, an allosteric 
interaction between the two binding sites on SHBG such that the 
binding affinities of the two binding sites on SHBG are not equiva­
lent, and convergence to an energetically favored bound state in 
777
which both sites are occupied.403 The free 

 Women
which both sites are occupied.403 The free testosterone levels com­
puted using this dynamic multistep binding with allostery match 
closely the values measured directly by equilibrium dialysis.403 
These studies also showed that the published law of mass action 
equations based on a linear model of testosterone’s binding to 
SHBG, in which one molecule of SHBG binds one molecule of tes­
tosterone with a single binding affinity constant, are erroneous.403
In men found to be androgen defic

re erroneous.403
In men found to be androgen deficient, measurement of LH 
levels helps distinguish between testicular (LH elevated) and hypo­
thalamic-pituitary (LH low or inappropriately normal) defects.65 
Men with hypogonadotropic hypogonadism may require mea­
surement of serum prolactin, serum iron, and total iron-binding 
capacity; evaluation of other pituitary hormones; and a pituitary 
MRI.
There is considerable debate about the usefulness and cost-
effectiveness of hormonal evaluation a

s and cost-
effectiveness of hormonal evaluation and the extent to which 
androgen deficiency should be investigated in middle-age and 
older men presenting with ED. In cross-sectional surveys,65 only 
a small fraction of men with ED and low testosterone levels have 
been found to have space-occupying lesions of the hypothalamic-
pituitary region.301,302 The diagnostic yield of pituitary imaging to 
exclude pituitary tumor can be improved by limiting these proce­
dures to men whose total testost

ting these proce­
dures to men whose total testosterone level is less than 150 ng/dL 
or who have panhypopituitarism, persistent hyperprolactinemia, 
or symptoms of tumor mass.301,302
Between 8% and 10% of men with ED have low testoster­
one levels; the prevalence of androgen deficiency increases with 
advancing age.158,404–406 The prevalence of low testosterone levels 
is not significantly different among men who present with ED and 
in an age-matched population.159 These data are consistent wi

atched population.159 These data are consistent with 
the proposal that ED and androgen deficiency are two common 
but independently distributed disorders.159 However, it is impor­
tant to exclude androgen deficiency in this patient population. 
Androgen deficiency is a correctable cause of sexual dysfunction, 
and some men with ED and low testosterone levels will respond 
to testosterone replacement. Androgen deficiency may be a mani­
festation of serious systemic disease and may have additiona

of serious systemic disease and may have additional del­
eterious effects on the individual’s health; for instance, androgen 
deficiency might contribute to osteoporosis and loss of muscle 
mass and function.
If the history, physical examination, and laboratory tests do not 
identify medical problems needing further workup, then a cost-
effective approach is to prescribe a trial of oral PDE5 inhibitor, 
provided there are no contraindications (e.g., nitrate use).
Tests that evaluate the integrit

g., nitrate use).
Tests that evaluate the integrity of penile vasculature and blood 
flow407,408 are not needed in most patients with ED, are reserved 
for patients in whom the results of these tests would alter the man­
agement or prognosis, and should be performed only by those 
with considerable experience with their use. The penile-brachial 
blood pressure index is a simple and specific, but not a sensitive, 
index of vascular insufficiency. It is rarely used today in the evalu­
ation of ED.

It is rarely used today in the evalu­
Intracavernosal injection of a vasoactive agent such as PGE1 
can be useful as a diagnostic and a potential therapeutic modality. 
This procedure can reveal whether the patient will respond to this 
therapeutic modality and can facilitate patient education about the 
procedure and its potential side effects. Failure to respond to intra­
cavernosal injection can raise the suspicion of vascular insufficiency 
or a venous leak that might need furth

ufficiency 
or a venous leak that might need further evaluation and treatment.
Most men with ED do not need duplex color sonography, 
cavernosography, or pelvic angiography.5,6,394,395,407 For instance, 
angiography could be useful in a young man with arterial insuf­
ficiency associated with pelvic trauma. Similarly, suspicion of con­
genital or traumatic venous leak in a young man presenting with 
ED would justify a cavernosography. In each instance, confirma­
tion of the vascular lesion might 

ance, confirma­
tion of the vascular lesion might lead to consideration of surgery. 
Duplex ultrasonography can provide a noninvasive evaluation of 
vascular function.407
  Directed Diagnostic Evaluation of ED
History
Ascertain Psychosexual History
Nature of sexual dysfunction: whether the primary problem is decreased 
desire, erectile dysfunction, premature or delayed ejaculation, or 
difficulty in achieving orgasms
Strength of marital relationship and marital discord
Stress
Sexual 

ship and marital discord
Stress
Sexual performance anxiety
Knowledge and beliefs about sexuality
Ascertain Risk Factors
Presence of diabetes mellitus, hypertension, hyperlipidemia, coronary 
artery disease, end-stage renal disease, and peripheral vascular 
disease
History of spinal cord injury, stroke, Alzheimer disease, or other neuro­
logic disorder
Prostate or pelvic surgery
Pelvic injury
Medications such as antihypertensives, antidepressants, antipsychotics, 
opioid analgesics, an

epressants, antipsychotics, 
opioid analgesics, and inhibitors of androgen production or action
Use of recreational drugs such as alcohol, cocaine, opiates, and tobacco
Ascertain Factors That Might Affect Choice of Therapy 
and the Patient’s Response to It
Coexisting coronary artery disease and its symptoms and severity
Exercise tolerance
Use of nitrates or nitrate donors
Use of α-adrenergic blockers
Use of vasodilators for hypertension or congestive heart failure
Use of foods (e.g., cranberry j

tive heart failure
Use of foods (e.g., cranberry juice) or drugs (e.g., erythromycin, protease 
inhibitors, ketoconazole, and itraconazole) that might affect metabo­
lism of PDE5 inhibitors
Ascertain signs of androgen deficiency, such as loss of body hair, 
eunuchoidal proportions, small testicular volume, or breast enlarge­
ment
Genital and perineal sensation to evaluate neurologic deficit from 
spinal cord lesion, previous stroke, peripheral neuropathy, or other 
neurologi

stroke, peripheral neuropathy, or other 
neurologic disorder
Blood pressure and postural change in blood pressure
Evaluate femoral and pedal pulses and evidence of lower extremity 
ischemia
Penile examination to exclude Peyronie disease or other penile 
­deformities
Basic Laboratory Evaluation That Should Be Performed in 
All Men With ED
Fasting blood glucose
Plasma lipids
Serum total and free testosterone level using reliable assays
ED, Erectile dysfunction; PDE5, phosphodiesterase 5.
   
TABLE

 dysfunction; PDE5, phosphodiesterase 5.
   
TABLE 20.5
778
Nocturnal penile tumescence testing is rarely needed for 
patients being evaluated for ED and is recommended only for 
a limited number of patients with a high clinical suspicion of 
psychogenic ED or situational problems, to document preopera­
tively poor penile rigidity, or for medical-legal reasons. Although 
recording of nocturnal penile tumescence in a sleep laboratory 
for successive ni

umescence in a sleep laboratory 
for successive nights can help to differentiate organic from psy­
chogenic impotence, this test is expensive and labor intensive. 
The introduction of portable RigiScan (GOTOP Medical, St. 
Paul, MN) devices in 1985 has provided clinicians with a reliable 
means of continuously monitoring penile tumescence and rigidity 
at home.408 It is a multicomponent device that the patient wears 
at bedtime for two to three nights. It has two wire gauge loops 
that are place

ights. It has two wire gauge loops 
that are placed around the base and tip of the penis that record 
changes in penile circumference and rigidity. Data are stored and 
downloaded via a software program that allows for interpretation. 
For most cases, a careful history of nighttime or early morning 
erections provides a reasonable correlation with nocturnal penile 
tumescence and RigiScan studies.408 
Evaluation of Women With Sexual Dysfunction
Sexual dysfunction is diagnosed by clinical intervi

exual dysfunction is diagnosed by clinical interview and, when 
necessary, physical examination. After obtaining details of general 
and mental health, the sexual complaints are clarified (Table 20.6 
and Fig. 20.9).
Sexual function questionnaires can be used to monitor treat­
ment response but should not be used for establishing a diagnosis 
of sexual dysfunction in women.409–411 One such instrument is the 
FSFI,409,410 which was designed to monitor treatment response 
and not to diagnose sexua

itor treatment response 
and not to diagnose sexual dysfunction in women who have not 
been clinically assessed. Moreover, the FSFI is based on the DSM-
IV criteria of sexual disorders, which are grounded in the con­
ceptualization of female sexual response beginning with conscious 
desire leading to a phase of arousal, then orgasm, and then resolu­
tion—these phases being in set order, discrete, and necessary for 
normal function. The limitations of the FSFI include the lack of 
recognition of 

s of the FSFI include the lack of 
recognition of triggered desire and the normality of beginning an 
experience that is sexually neutral; the possibility of sexual satis­
faction without orgasm; a focus on partnered sex, which limits its 
use in women, who currently do not have a sexual partner;410 and 
the limited period of assessment (the last 4 weeks), which may 
increase the risk of erroneous results during transient periods of 
stress or the partner’s absence.412 Although the FSFI has been

e partner’s absence.412 Although the FSFI has been used 
in many studies of women’s sexual function, including those of 
women with endocrine disease, more contemporary instruments, 
such as the 17-question Natsal-SF, developed for the Natsal-3,411 
better reflect the importance of sexual satisfaction in contrast to 
many previous instruments. This attribute of the Natsal-SF instru­
ment is particularly important, because patients may report satis­
faction despite dysfunction, and dissatisfactio

s­
faction despite dysfunction, and dissatisfaction can occur in the 
context of a functional response413: women’s satisfaction may or 
may not include orgasms.246 A recent study confirms strong links 
between sexual satisfaction and sexual motivation.414
Physical examination, including pelvic and genital examination, is 
part of routine care and can be reassuring to the patient by con­
firming normal anatomy and overall health. Unless dyspareunia 
is involved, it is not oft

th. Unless dyspareunia 
is involved, it is not often that physical examination identifies 
the cause of sexual dysfunction. For some women with a history 
of coercive or abusive sexual experience, or previous traumatic 
medical experience, such examination may cause extreme anxiety 
and, unless sex is painful, may not be necessary. The reason for an 
examination—usually the presence of dyspareunia or loss of geni­
tal sexual sensitivity—and an explanation of what will and will 
not be done shoul

planation of what will and will 
not be done should be provided before the examination begins 
(Table 20.7 and Fig. 20.10, Parts 1 & 2 Assessment of GPPPD 
algorithm). If the woman prefers to invite her partner to be pres­
ent, then a careful examination can be highly educational for both 
partners. In women with GPPPD with a marked component of 
vaginismus, the vaginal examination should be delayed until psy­
chological therapy renders the examination possible and informa­
tive for both the pat

nation possible and informa­
tive for both the patient (and partner if present) and the clinician. 
  General Evaluation of Women Presenting 
With Sexual Dysfunction
Item
Comments
General health and past medical 
historya
  Medicationsa
Current and past medications
  Aspects of chronic disease 
relevant to sexual life
Pain, fatigue, continence, self-
image, mobility
  Mooda
Depression, depressive symptoms, 
current and past
  Relationshipa
Type(s), satisfaction
  Past sexual experiences
Positiv

s), satisfaction
Positive, negative, coercive, abusive
Partner’s sexual functiona
Previous therapy
Details and compliance
Motivation to address problems
Willingness to prioritize addressing 
sexual life now
aAmenable to questionnaire format.
   
TABLE 20.6
Why might she sometimes/always agree/instigate;
why sometimes/ always decline to do either?
Mood; fatigue; interpersonal issues; fear of negative outcome
Feelings for partner at that moment; energy; time of day; priva

partner at that moment; energy; time of day; privacy
Nonphysical; physical nongenital;
genital nonpenetrative
Distracting thoughts; mood; medications; chronic pain
Pleasure, subjective arousal/excitement;
genital sexual sensations; lubrication; orgasm
Pain; partner sexual dysfunction; minimal
arousal/pleasure; no orgasm
Motivations & disincentives
Willingness for partnered sex
Sexual context
Sexual stimuli
Attention/focus on stimuli
Outcome
Create patient’s sex response cycle sho

se
Outcome
Create patient’s sex response cycle showing problem areas
Ask for her sexual complaint(s)
• Fig. 20.9  Focused evaluation of women presenting with sexual com­
plaints.
779
Laboratory Testing
The laboratory testing plays a small role in women’s sexual evalu­
ation. Estrogen activity is best evaluated by history and exami­
nation. The commercially available estradiol radioimmunoassays 
lack the sensitivity and precision required to measure

 the sensitivity and precision required to measure the low con­
centrations present in the older woman; in addition, these assays 
do not measure estrone, the major estrogen after menopause. As 
discussed earlier, serum testosterone levels do not correlate with 
sexual function, even when LC-MS/MS assays are used.66,67 The 
circulating testosterone levels may not reflect intracrine produc­
tion, metabolism, or activity of androgens. Measurement of 
testosterone metabolites has been proposed as a

f 
testosterone metabolites has been proposed as a marker of intra­
crine plus gonadal production of testosterone,68 but the circulat­
ing levels of these metabolites have been shown to be similar in 
women with and without sexual dysfunction.67,71,72 The clinical 
usefulness of androgen metabolites remains to be demonstrated. 
Currently available evidence does not support the usefulness of 
measuring serum testosterone concentrations in women with low 
sexual desire.415 Fasting glucose, hemoglo

th low 
sexual desire.415 Fasting glucose, hemoglobin A1c, prolactin, and 
thyrotropin should be measured if clinically indicated by the gen­
eral health evaluation. 
Management of Sexual Dysfunction in Men
Treatment of Hypoactive Sexual Desire in Men
There is a need to focus on the couple when the patient has a 
sexual partner. Treating the sexual dysfunction in the male part­
ner improves the female partner’s sexual function and satisfac­
tion. Comorbid depression should be treated and relatio

 Comorbid depression should be treated and relationship 
difficulties addressed. The efficacy of cognitive and behavioral 
therapies has not been evaluated systematically in men with 
HSDD. Testosterone therapy should be considered in men with 
HSDD who have androgen deficiency, even though there are no 
randomized trials of testosterone in men with HSDD.
Much of the information about the effects of testosterone on 
sexual desire has emerged from open-label trials of testosterone in 
hypogonadal

 open-label trials of testosterone in 
hypogonadal men and a small number of randomized placebo-con­
trolled trials of testosterone in hypogonadal men.65,131,141,142,416–
420 Most testosterone trials recruited men based on the presence 
of low testosterone levels alone, whereas a small number of recent 
trials required men to have low testosterone levels and one or more 
symptoms of testosterone deficiency.131,141,142,146,416–420 Testos­
terone therapy in these trials has been associated with si

herapy in these trials has been associated with significant 
improvements in overall sexual activity, sexual desire, attention to 
erotic cues, and the duration and frequency of nocturnal penile 
erections.65,131,416–419
The Testosterone Trials were a set of several coordinated pla­
cebo-controlled trials, funded primarily by the National Institute 
on Aging, to determine the efficacy of testosterone in improving 
sexual function, physical function, and vitality in community-
dwelling older men,

on, and vitality in community-
dwelling older men, 65 years or older, with low libido, mobility 
limitation, and/or fatigue, and an unequivocally low fasting early 
morning total testosterone level.141 The eligible participants were 
required to have an average of two morning total testosterone lev­
els less than 275 ng/dL measured using a CDC-certified LC-MS/
MS assay.421 Low libido was defined as the Derogatis Interview 
for Sexual Functioning for Men sexual desire domain score of less 
than 2

for Men sexual desire domain score of less 
than 20. The eligible participants were assigned using the proce­
dure of minimization to receive either 1% transdermal testoster­
one gel or placebo gel daily for 1 year. The testosterone dose was 
adjusted to achieve and maintain on-treatment testosterone levels 
between 400 and 800 ng/dL. The primary outcome was overall 
sexual activity. Assignment to the testosterone arm was associated 
with greater improvements in overall sexual activity, sexual d

 improvements in overall sexual activity, sexual desire, 
and erectile function than placebo.141,142 Testosterone treatment 
improved most categories of sexual activity. The overall improve­
ments in sexual desire and erectile function were modest but were 
  Evaluation of Genitopelvic Penetration 
Disorder—Part (2): the Detailed Genital 
Examination
Diagnoses (May Be Multiple)
Findings on Examination
Anatomic variations
Congenital (e.g., hypoplasia), past 
surgery or genital mutilation
Provoke

asia), past 
Provoked vestibulodynia
No visual abnormalities on exami­
nation; Q-tip test to confirm 
allodynia of introital edge
Reflexive and chronic hypertonic 
pelvic muscles “vaginismus”
Palpation of pelvic muscles and 
their insertion into ischial spines 
confirms high tone, inability to do 
reverse Kegel maneuver
Genitourinary syndrome of 
menopause
Pallor, dryness, thinning, loss of 
turgor and elasticity and rugae, 
possible stenosis
Chronic vaginitis
Eryth

 rugae, 
Erythema, discharge, confirm by 
culture
Recurrent tearing of posterior 
fourchette
Taut fourchette, white line scars of 
past tears
Vulvar dystrophy (e.g., lichen 
sclerosus)
Abnormal mucosa typical of 
dystrophy
Deep pelvic pathology (e.g., 
endometriosis), bladder 
pathology
Tenderness, nodules in cul-de-sac, 
tender adnexae, pain with cervi­
cal movement or with bladder or 
uterine compression
TABLE 20.7
Consider differential diagnosis:
1. Provoke

E 20.7
1. Provoked vestibulodynia
2. Genitourinary syndrome of
menopause
3. Chronic vaginitis
4. Recurrent tearing of posterior
5. Vulvar dystrophy
Mostly meets this
description
1. Reflexive & chronic pelvic muscle
hypertonicity “vaginismus”
2. Deeper pelvic pathology (e.g.,
endometriosis)
3. Comorbid vestibulodynia 
4. Comorbid genitourinary 
syndrome of menopause
Obtain Details
Introital 

y 
Introital +/- deeper
Body tenses with attempt at entry
“Pressure” “hits a wall” “too tight”
Absent if no movement
May lessen as movement continues
Not ↑ with partner ejaculation
No postcoital burning
No postcoital dysuria
Introital
Burning/stinging/tearing
↑ with movement, but rarely
absent if no movement
↑ with partner ejaculation
Postcoital burning
Postcoital dysuria
• Fig. 20.10  Evaluation of genitopelvic penetration disorder—part (1): the 
detailed hi

ic penetration disorder—part (1): the 
detailed history.
780
deemed clinically meaningful.422 A carefully performed meta-
analysis of randomized testosterone trials in hypogonadal men, 
who met the Endocrine Society’s definition of testosterone defi­
ciency, confirmed that compared with placebo administration, 
testosterone treatment is associated with greater improvements 
in sexual activity, sexual desire, erectile function, and satisfaction 
with s

esire, erectile function, and satisfaction 
with sexual experience.423
Several points should be considered when initiating testoster­
one replacement therapy. First, testosterone treatment has been 
shown to improve some but not all domains of sexual function; 
testosterone improves sexual desire and erections but has not been 
shown to improve ejaculatory disorder. Second, testosterone treat­
ment has been shown to be efficacious in improving sexual func­
tion only in men with unequivocally low

xual func­
tion only in men with unequivocally low testosterone levels and 
low sexual desire; it has not been shown to improve sexual func­
tion in men who have normal or low-normal testosterone levels. 
Third, there is enormous variability in on-treatment testosterone 
levels, especially with transdermal testosterone gels; a substantial 
fraction of hypogonadal men started on the initial dose of a trans­
dermal testosterone gel will not raise their on-treatment levels into 
the target therapeu

heir on-treatment levels into 
the target therapeutic range, and 5% to 10% may have supraphys­
iologic levels due to substantial variations in transdermal absorp­
tion and plasma clearance of testosterone. Unsurprisingly, nearly 
half of testosterone-treated men discontinue testosterone treat­
ment after 3 months, and only about 25% are still on testosterone 
treatment after 12 months. Fourth, some men with HSDD may 
have relationship problems, which are not likely to be corrected by 
testostero

hich are not likely to be corrected by 
testosterone therapy. Finally, the long-term safety and efficacy of 
testosterone therapy has not been established. 
Treatment of Erectile Dysfunction
The current practice employs a stepwise approach that first uti­
lizes minimally invasive therapies that are easy to use and have 
fewer adverse effects and progresses to more invasive therapies that 
may require injections or surgical intervention after the first-line 
choices have been exhausted (Fig. 20.1

first-line 
choices have been exhausted (Fig. 20.11). The physician should 
discuss the risks, benefits, and alternatives of all therapies with 
the couple. The selection of the therapeutic modality should be 
based on the underlying cause, patient preference, the nature 
and strength of the relationship with his sexual partner, and the 
absence or presence of underlying CVD and other comorbid con­
ditions.5,6,394,395 All patients with ED can benefit from psycho­
sexual counseling.5,6,394,395,42

efit from psycho­
sexual counseling.5,6,394,395,424–428
In the execution of good medical practice, treatment of 
all associated medical disorders should be optimized. In men 
with diabetes mellitus, glycemic control should be optimized, 
although improving glycemic control may not improve sexual 
function. In men with hypertension, control of blood pressure 
should be optimized and, if possible, the therapeutic regimen 
may be modified to remove antihypertensive drugs that impair 
sexual functio

antihypertensive drugs that impair 
sexual function. This strategy is not always feasible because 
almost all antihypertensive agents have been associated with sex­
ual dysfunction; the frequency of this adverse event is less with 
converting-enzyme inhibitors and angiotensin-receptor blockers 
than with other agents.
First-Line Therapies
Psychosexual Counseling
The goals of psychosexual therapy are to reduce performance 
anxiety, develop the patient’s sexual skills and knowledge, modify 
negati

ient’s sexual skills and knowledge, modify 
negative sexual attitudes, and improve communication between 
partners.424 Counseling can be beneficial in both psychogenic and 
organic causes of sexual dysfunction424–430 (Table 20.8).
An individual’s focus on sexual performance rather than erotic 
stimulation is a major factor in the pathophysiology of psycho­
genic ED424,425; this behavior is referred to as spectatoring. Many 
experts recommend a sensate focus treatment approach, in which 
the coup

nsate focus treatment approach, in which 
the couple avoids intercourse and engages in nongenital, non­
demanding, pleasure-seeking exercises to reduce performance 
anxiety.424
Involving the partner in the counseling process helps to dispel 
misperceptions about the problem, decreases stress, enhances inti­
macy and the ability to talk about sex, and increases the chances 
of successful outcome.424 Counseling sessions are also helpful in 
uncovering conflicts in relationships, psychiatric proble

ing conflicts in relationships, psychiatric problems, alco­
hol and drug abuse, and sometimes significant misperceptions 
about sex. As many men and women may harbor misinforma­
tion and unrealistic expectations about sexual performance and 
age-related changes in sexual function, cognitive restructuring 
techniques are helpful in correcting sexual myths and beliefs.424 
There is a paucity of outcome data on the effectiveness of this 
psychobehavioral therapy, but meta-analyses have reported ben

ioral therapy, but meta-analyses have reported ben­
efit from group psychotherapy administered in conjunction with 
PDE5 inhibitors.427 
Selective Phosphodiesterase 5 Inhibitors
Selective PDE5 inhibitors are safe and effective and have 
become widely accepted as first-line therapy for patients with 
ED, except in men for whom these drugs are contraindi­
cated4-6,106,108,394,395,430–432 (Tables 20.9 and 20.10). Selective 
PDE5 inhibitors are contraindicated in men using nitrates on a 
regular bas

aindicated in men using nitrates on a 
regular basis, in those with severe heart disease in whom sexual 
activity may not be safe, and in those with nonarteritic anterior 
ischemic optic neuropathy.5,6,106,108,394,395,431,432
Mechanisms of Action. Four classes of enzymes—NOS, ade­
nylyl cyclase, guanylyl cyclase, and PDEs—play important roles 
in regulating the intracavernosal concentrations of cAMP and 
cGMP. Three isoforms of NOS catalyze the formation of nitric 
oxide from arginine using NADP

ormation of nitric 
oxide from arginine using NADPH and oxygen as substrates: 
neuronal NOS (nNOS or NOS1), inducible NOS (iNOS or 
NOS2), and endothelial NOS (eNOS or NOS3). Nitric oxide 
activates soluble guanylyl cyclase to stimulate the production of 
cGMP from GTP, which then activates cGMP-dependent pro­
tein kinases to induce myosin phosphatase, inositol 1,4,5-trispho­
sphate receptor 1–associated protein, and some cation channels 
resulting in smooth muscle relaxation. PDEs hydrolyze cGM

ng in smooth muscle relaxation. PDEs hydrolyze cGMP, 
thus reducing its concentrations within the cavernosal smooth 
muscle.107,108,111–113,433–436 Although PDE isoforms 2, 3, 4, and 
5 are expressed in the penis, only PDE5 is specific to the nitric 
oxide–cGMP pathway in the corpora cavernosa.436 PDE inhibi­
tors sildenafil, vardenafil, tadalafil, avanafil, and udenafil are 
relatively selective inhibitors of PDE5.107,108,111–113,433–459 These 
drugs block the hydrolysis of cGMP, thus promoting

drugs block the hydrolysis of cGMP, thus promoting cavernosal 
smooth muscle relaxation. The action of these drugs requires an 
intact nitric oxide response, as well as constitutive synthesis of 
cGMP by the smooth muscle cells of the corpora cavernosa. By 
selectively inhibiting cGMP catabolism in the cavernosal smooth 
muscle cells, PDE5 inhibitors restore the natural erectile response 
to sexual stimulation but do not produce an erection in the 
absence of sexual stimulation. 
Clinical Pharma

e 
Clinical Pharmacology. Although the four currently avail­
able PDE inhibitors have some structural similarities, they dif­
fer in their selectivity and pharmacokinetics (see Table 20.9). The 
781
• General health evaluation with a focus on
  cardiometabolic health
• Psychosexual counseling of the
  couple
• Lifestyle changes to improve
• Optimize treatment of comorbid
  conditions such as 

timize treatment of comorbid
  conditions such as diabetes,
  hypertension, and cardiovascular
  disease
• Evaluate cardiovascular risk, contraindications to PDE5I
  use, and potential for drug-drug interactions with PDE5Is
• Recommend Second-line therapies:
  vacuum constriction devices,
  intraurethral alprostadil, intra-
  cavernosal alprostadil
No contraindications to PDE5I use
PDE5I use contraindicated
Initiate PDE5I therapy; optimize
efficacy by dose titration
Satisfactory clinical
respons

cy by dose titration
response ro PDE5I
Unsatisfactory clinical
response
Unsatisfactory clinical response
to PDE5I
• Evaluate: appropriate use,
  adherence, couple relationship,
  AEs, or fear of AEs
Satisfactory
clinical
response
Unsatisfactory
clinical
response
Penile implants
• Escalate dose to maximum
  tolerated dose
Periodic Follow-Up
• Fig. 20.11  An algorithmic approach to the treatment of men with erectile dysfunction. AE, adverse event; 
PDE5I, phosphodiesterase 5 

n. AE, adverse event; 
PDE5I, phosphodiesterase 5 inhibitor.
782
common adverse effects of the available PDE5 inhibitors—head­
ache, visual problems, flush, and myalgias—are related to non­
selective inhibition of PDE isoforms 6 and 11 in other organ 
systems108 (see Table 20.9). The selectivity of a PDE5 inhibitor 
is the ratio of its inhibitory potency for PDE isoforms other than 
type 5 relative to its inhibitory potency for PDE isoform 5.108 For 

its inhibitory potency for PDE isoform 5.108 For 
PDE6, tadalafil is the most selective and sildenafil is the least selec­
tive; for PDE11, vardenafil is the most selective and tadalafil is the 
least selective.108 The retinal side effects of sildenafil are related 
to inhibition of PDE6 in the retina, whereas muscle aches expe­
rienced by a small fraction of men using tadalafil are related to 
inhibition of PDE11 in the skeletal muscle.108 
Pharmacokinetics. After oral administration of sildena

macokinetics. After oral administration of sildenafil, peak 
plasma concentrations are achieved within 30 to 120 minutes, 
after which plasma concentrations decline, with a half-life of 4 
hours107,433–442 (see Table 20.9). Vardenafil achieves peak con­
centrations within 0.7 to 0.9 hour and has a half-life of 4 to 5 
hours. The peak concentrations of tadalafil are achieved at 2 
hours, and its half-life of 16.9 hours in young men is significantly 
longer than the half-lives of sildenafil and va

y 
longer than the half-lives of sildenafil and vardenafil. The half-
life of tadalafil is even longer in older men (21.6 hours) than 
in young men (16.9 hours).107,433–436 Because of the relatively 
short half-lives of vardenafil and sildenafil, these drugs should be 
taken 1 to 4 hours before the planned intercourse; in contrast, 
tadalafil, because of its longer half-life, does not have to be taken 
on demand, although it can be.107,433–436 The second-generation 
PDE5 inhibitors avanafil and 

e second-generation 
PDE5 inhibitors avanafil and udenafil have a more rapid onset of 
action than the first-generation PDE5 inhibitors sildenafil, varde­
nafil, and tadalafil.437–440
Food, particularly a high-fat meal and large amounts of alcohol, 
can delay and decrease the absorption of sildenafil.441,442 How­
ever, early pharmacokinetic studies have not reported changes in 
maximum serum concentrations or absorption rates of vardenafil 
or tadalafil due to food or moderate alcohol ingestion.

dalafil due to food or moderate alcohol ingestion.436 
Efficacy. Orally active, selective PDE5 inhibitors—sildenafil, 
vardenafil, avanafil, udenafil, and tadalafil—have been shown 
to be effective and safe in randomized clinical trials of men with 
ED.4–6,439–465 In men treated with oral, selective PDE5 inhibitors, 
the rates of successful intercourse vary from 50% to 65%, and 
rates of improved erections vary from 70% to 75%.439–455 Selec­
tive PDE5 inhibitors are effective in men of all ethni

 PDE5 inhibitors are effective in men of all ethnic groups and 
ages439–465 who have ED due to a multitude of causes, although 
response rates vary in different patient subgroups.439–465
Introduced to the U.S. market in March 1998, sildenafil 
citrate (Viagra; Pfizer, New York, NY) was the first effective oral 
agent for the treatment of ED.466 The efficacy of sildenafil has 
been demonstrated in men with organic, psychogenic, or mixed 
ED in multiple RCTs443–450,466 and confirmed by meta-analys

tiple RCTs443–450,466 and confirmed by meta-analyses 
of randomized trials.446–448 In these trials, patients receiving 
sildenafil experienced greater increments in the number of suc­
cessful attempts per month, penile rigidity, frequency of vaginal 
penetration, and maintenance of erection than those receiving 
placebo.446–448 Increasing doses of sildenafil were associated with 
higher mean scores for the frequency of penetration and mainte­
nance of erections after sexual penetration. The mean

ce of erections after sexual penetration. The mean scores for 
orgasms, intercourse satisfaction, and overall satisfaction were also 
significantly higher in the sildenafil group than in the placebo 
group.446–448 Sildenafil also is an effective treatment for ED in 
men with diabetes mellitus.449,450 A meta-analysis of randomized 
clinical trials of sildenafil confirmed its efficacy in improving erec­
tile function in men with diabetes mellitus.450
In the vardenafil efficacy trials, 5-, 10-, and

50
In the vardenafil efficacy trials, 5-, 10-, and 20-mg doses of 
vardenafil were all superior to placebo in improving erectile func­
tion domain scores; the improvements in erectile function scores 
were dose related.451–457 Vardenafil improved rates of vaginal pen­
etration, penile rigidity, intercourse success, and satisfaction with 
sexual experience in men with ED from diverse causes.451–457
Similarly, in randomized clinical trials, 2.5-, 5-, 10-, and 
20-mg doses of tadalafil were each su

-, 10-, and 
20-mg doses of tadalafil were each superior to placebo in improv­
ing erectile function scores.458–462 The beneficial effects of tadalafil 
were dose related.458–462
Two new PDE5 inhibitors have been introduced recently in 
clinical practice. Avanafil has a very rapid onset of action because 
of its rapid absorption, which allows it to reach maximum circu­
lating concentration in about 30 to 45 minutes.437–439 Therefore, 
a majority of patients taking avanafil are able to engage in 

of patients taking avanafil are able to engage in sexual 
activity within 15 minutes.437–439 Udenafil also has a relatively 
rapid onset of action, with a time to maximum serum concentra­
tions of 1.0 to 1.5 hours. It has been approved in Korea, Russia, 
and the Philippines, but not in the United States.440
PDE5 inhibitors are effective in men with ED due to a vari­
ety of causes, including spinal cord injury and radical prosta­
tectomy.449,450 In general, baseline sexual function correlates 
po

n general, baseline sexual function correlates 
positively with response to PDE5 inhibitors, and patients with 
diabetes mellitus or previous prostate surgery respond less well 
than patients with psychogenic or vasculogenic ED.449,450 Because 
there is no baseline characteristic that predicts the likelihood of 
failure to respond to sildenafil therapy, a therapeutic trial of PDE5 
inhibitors should be tried in all patients, except in those in whom 
it is contraindicated.432 
Adverse Effects. In

om 
Adverse Effects. In clinical trials, the adverse effects that have 
been reported with greater frequency in men treated with PDE5 
inhibitors than in those treated with placebo include headaches, 
flushing, rhinitis, dyspepsia, muscle aches, and visual distur­
bances108,432,463–465 (Table 20.11). The occurrence of headache, 
flushing, and rhinitis, a direct consequence of nonselective PDE5 
inhibition in other organ systems, is related to the administered 
dose. Th

 systems, is related to the administered 
dose. These drugs do not affect semen characteristics.467,468 No 
cases of priapism were noted in the pivotal clinical trials.
Several cases of nonarteritic anterior ischemic optic neuropathy 
have been reported after oral PDE5 inhibitor use.469,470 This con­
dition is characterized by the sudden onset of monocular visual 
loss due to acute ischemia of the anterior portion of the optic 
nerve in the absence of demonstrable arteritis. This may progress 
t

ce of demonstrable arteritis. This may progress 
to partial or complete infarction of the optic nerve head, result­
ing in permanent visual loss or visual field cuts.469,470 Although 
a cause-and-effect relationship with PDE5 inhibitor use has not 
been established, patients with history of sudden visual loss should 
not be treated with PDE5 inhibitors without ophthalmologic 
  Goals of Psychosexual Therapy in Men 
 • Reduce performance anxiety; train the co

n and Women
  Clinical Pharmacology of Selective PDE5 Inhibitorsa
Feature
Tadalafil
Avanafil
Commercial name
Viagra
Levitra
Cialis
Stendra, Spedra
Tmax
0.5–2.0 h
0.7–0.9 h
2 h
30–45 min
T1⁄2
3–4 h
4–5 h
16.9 h (young)
21.6 h (old)
5 h
Onset of erection (min)
30–60
15–45
20–30 min
15 min
Muscle selectivity (ratio of PDE6 IC50/PDE5 IC50)
11 (most ­selective)
25
187 (least selective)
>100-fold
Retinal selectivity (ratio of PDE11/PDE5 IC50), higher 
number indicates greater PD

11/PDE5 IC50), higher 
number indicates greater PDE5 selectivity
780
1160 (most selective)
5 (least selective)
>10,000-fold
Effect of food and alcohol
Cmax decreased
Minimal change
No change
Absorption delayed
Protein binding
96%
94%
94%
98–99%
Bioavailability
41%
15%
Tadalafil
Avanafil
(R0) q
*
N
N
R3 R2
CH3
CH3
CH2CH2CH3
CH3CH2O
O3S
R1
X
Z
Y
*
*
N
N
HN
N
N
H
H
HOOC
OH
CO2H
CO2H
N
H
N
O
O
O
Cl
N
H
N
N
OH
O
N
N
N
N
H
O
O
O
N
N
N
S
aComparative ph

H
N
N
OH
O
N
N
N
N
H
O
O
O
N
N
N
S
aComparative pharmacokinetic data are provided on the four oral selective PDE5 inhibitors that have been approved by the U.S. Food and Drug Administration. Additional PDE5 inhibitors such as udenafil 
and mirodenafil have been approved for use in some other countries. Selectivity refers to the ratio of the IC50 for a PDE isoform other than PDE5 to the IC50 for PDE5. A higher number implies greater 
selectivity. Sildenafil is more selective than tadalafil for PD

Sildenafil is more selective than tadalafil for PDE5 relative to PDE11, but it is less selective than tadalafil for PDE6 relative to PDE5.
Cmax, Maximum plasma concentration; IC50, 50% inhibitory concentration; PDE, phosphodiesterase; T ½, half-life; Tmax, time to peak concentration.
Data from Haning H, Niewohner U, Bischoff E. Phosphodiesterase type 5 (PDE5) inhibitors. Prog Med Chem. 2003; 41:249–306; Bischoff E. Potency, selectivity, and consequences of nonselectivity of PDE 
inhibition. Int 

quences of nonselectivity of PDE 
inhibition. Int J Impot Res. 2004;16(Suppl 1):S11–S14; and Saenz de Tejada I, Angulo J, Cuevas P, et al. The phosphodiesterase inhibitory selectivity and the in vitro and in vivo potency of the new PDE5 
inhibitor vardenafil. Int J Impot Res. 2001;13:282–290.
   
TABLE 20.9
The U.S. Food and Drug Administration (FDA) has noted sev­
eral reports of sudden hearing loss with and without vestibular 
symptoms, such as tinnitus, vertigo, or dizziness, in temporal rela

 tinnitus, vertigo, or dizziness, in temporal rela­
tionship to administration of sildenafil, vardenafil, and tadalafil 
in postmarketing surveillance. Hearing loss was also reported in a 
few patients in clinical trials of these drugs.471,472 Hearing loss has 
been noted in patients using sildenafil for the treatment of pul­
monary arterial hypertension. Although a causal relationship has 
not been established, the temporal relationship between the use of 
PDE5 inhibitors and the onset of sudde

the use of 
PDE5 inhibitors and the onset of sudden hearing loss prompted 
the FDA to recommend changes in the product labeling for the 
drug class. A few observational studies have reported an associa­
tion of PDE5 inhibitor use with an increased risk of melanoma 
and basal cell carcinoma; however, due to the lack of association 
between melanoma risk and the number of prescriptions filled, a 
causal relation has not been established.473–475 
Cardiovascular and Hemodynamic Effects. In postmarke

rdiovascular and Hemodynamic Effects. In postmarket­
ing surveillance of adverse events associated with sildenafil use, 
several instances of myocardial infarction and sudden death were 
reported in men using sildenafil in temporal relation to the inges­
tion of the drug476; many of these deaths occurred in individuals 
who also were taking nitrates. Because most men presenting with 
ED also have high prevalence of cardiovascular risk factors, it is 
unclear whether these events were causally re

 is 
unclear whether these events were causally related to the inges­
tion of sildenafil, underlying heart disease, or both.476 In con­
trolled studies,477–480 oral administration of 100 mg of sildenafil 
to men with severe coronary artery disease produced only a small 
decrease in systemic blood pressure and no significant changes 
in cardiac output, heart rate, coronary blood flow, and coronary 
artery diameter. In a separate pooled analysis of five randomized, 
placebo-controlled trials of va

 five randomized, 
placebo-controlled trials of vardenafil,479 the overall frequency of 
cardiovascular events was similar in both vardenafil- and placebo-
treated men. However, vardenafil treatment was associated with a 
mild reduction in blood pressure (4.6 mm Hg decrease in systolic 
blood pressure) and a small increase in heart rate (two beats per 
minute).480 This led the American Heart Association and other 
experts to conclude that preexistent coronary artery disease by 
itself does not c

tent coronary artery disease by 
itself does not constitute a contraindication for the use of PDE5 
inhibitors459,480–485 (Table 20.12). 
Drug-Drug Interactions. Sildenafil is metabolized mostly 
by the CYP2C9 and the CYP3A4 pathways. Cimetidine and 
erythromycin, inhibitors of CYP3A4, increase the plasma con­
centrations of sildenafil. HIV protease inhibitors may also alter 
the activity of the CYP3A4 pathway and affect the clearance of 
sildenafil.486 Conversely, sildenafil is an inhibitor of 

fil.486 Conversely, sildenafil is an inhibitor of the CYP2C9 
metabolic pathway, and its administration could potentially affect 
784
the metabolism of drugs metabolized by this system, such as war­
farin and tolbutamide. Combined administration of sildenafil and 
ritonavir results in significantly higher plasma levels of sildenafil 
than sildenafil given alone.486 There are similar interactions with 
other drugs, including saquinavir and itraconazole

other drugs, including saquinavir and itraconazole. Therefore, the 
doses of PDE5 inhibitors should be reduced appropriately in men 
taking protease inhibitors or erythromycin.
Grapefruit juice can alter oral drug pharmacokinetics by differ­
ent mechanisms. Grapefruit juice given in normal amounts (e.g., 
200–300 mL) or as whole fresh fruit segments can irreversibly inac­
tivate intestinal CYP3A4, thus reducing presystemic metabolism 
and increasing oral bioavailability of PDE5 inhibitors.487 Al

ing oral bioavailability of PDE5 inhibitors.487 Although 
the magnitude of this problem in clinical practice is unknown, it 
seems prudent to warn men who are contemplating the use of PDE5 
inhibitors not to ingest more than a small amount of grapefruit juice.
The most serious interactions of PDE5 inhibitors are with 
the nitrates. The vasodilator effects of nitrates are augmented by 
PDE5 inhibitors; this also applies to inhaled forms of nitrates such 
as amyl nitrate or nitrites that are sold 

s such 
as amyl nitrate or nitrites that are sold under the street name “pop­
pers.” Concomitant administration of the two vasodilator drugs 
can cause a potentially fatal decrease in blood pressure.216–218
PDE inhibitors should be used carefully in men taking 
α-adrenergic blockers. In men with congestive heart failure or 
those receiving vasodilator drugs, or those who are using com­
plex regimens of antihypertensive drugs, blood pressure should be 
monitored after initial administration of PD

d be 
monitored after initial administration of PDE5 inhibitors.216–218 
Several trials have demonstrated the safety of administering PDE5 
inhibitors in combination with α-adrenergic blockers in men with 
ED and LUTS.216–218 
Therapeutic Regimens.488 Men with ED are at increased risk 
for cardiometabolic disorders (see Table 20.10). Baseline evalua­
tion should include general health evaluation and screening for 
diabetes and cardiovascular risk. Patients should be counseled for 
lifestyle modi

. Patients should be counseled for 
lifestyle modification to improve overall health and reducing car­
diometabolic risk. All men with ED can benefit from psychosex­
ual counseling. PDE5 inhibitors are the first-line therapy for ED. 
The men who are prescribed PDE5 inhibitors should be counseled 
on their appropriate use guided by their clinical pharmacology. 
Due to the substantial variation in metabolism of PDE5 inhibi­
tors among men, the dose should be titrated to the maximum 
tolerated dose

 should be titrated to the maximum 
tolerated dose to maximize efficacy while avoiding dose-related 
adverse effects.
In most men with ED, sildenafil is started at an initial dose of 
50 mg. If this dose is not effective yet does not produce any adverse 
effects, the dose should be titrated up to 100 mg or the maximum 
tolerated dose.106,431,432,489,490 The dose adjustment should be 
guided by the therapeutic response to therapy and occurrence of 
adverse effects. Vardenafil should be started at

adverse effects. Vardenafil should be started at an initial dose of 10 
mg; the dose should either be increased to 20 mg or the maximal 
tolerated dose or decreased490 to 5 mg depending on the clinical 
response and the occurrence of adverse effects. Tadalafil is started 
at an initial dose of 10 mg, with further adjustment of dose based 
on effectiveness and side effects. Tadalafil need not be taken more 
frequently than once every 48 hours. Avanafil should be started 
at an initial dose of 5

no-responders: hemodynamic and mor­
phometric studies. Eur Urol. 2005;48:136–139; Brock GB, McMahon CG, Chen KK, et al. 
Efficacy and safety of tadalafil for the treatment of erectile dysfunction: results of integrated 
analyses. J Urol. 2002;168:1332–1336; Morales A, Gingell C, Collins M, et al. Clinical safety 
of oral sildenafil citrate (Viagra) in the treatment of erectile dysfunction. Int J Impot Res. 
1998;10:69–73; Katz EG, Tan RB, Rittenberg D, Hellstrom WJ. Avanafil for erectile dysfunc

erg D, Hellstrom WJ. Avanafil for erectile dysfunc­
tion in elderly and younger adults: differential pharmacology and clinical utility. Ther Clin Risk 
Manag. 2014;10:701–711.
TABLE 20.11
  Guidelines for Optimizing the Use 
of Selective PDE5 Inhibitors
	1.	 Men with ED are at increased risk for cardiometabolic disorders. 
Baseline evaluation should include general health evaluation, as well 
as screening for diabetes and cardiovascular risk. Patients should 
be counseled for lifestyle modific

 men taking daily 
doses of nitrates.
	7.	 PDE5 inhibitors may produce hypotension when taken together with 
other vasodilator drugs, such as α-adrenergic blockers or antihyper­
tensive medications. These interactions should be considered when 
prescribing PDE5 inhibitors in men who are using α-adrenergic 
blockers or other vasodilators.
	8.	 In human immunodeficiency virus–infected men, consider potential 
drug-drug interactions between selective PDE5 inhibitors and anti­
retroviral drugs and

ive PDE5 inhibitors and anti­
retroviral drugs and antimicrobial agents.
	9.	 Some men who do not respond to an on-demand PDE5 inhibitor may 
respond to the use of daily tadalafil, which is a U.S. Food and Drug 
Administration–approved regimen.
Data from Burnett AL, Nehra A, Breau RH, et  al. Erectile dysfunction: AUA guideline. 
J Urol. 2018;200(3):633–641.
TABLE 20.10
785
Sildenafil and varden

Dysfunction in Men and Women
Sildenafil and vardenafil are taken at least 1 to 2 hours before 
sexual intercourse and not more than once in any 24-hour period; 
because of its longer half-life, tadalafil need not be taken immedi­
ately before intercourse. Avanafil is taken 30 minutes before sexual 
intercourse.
Based on the results of the randomized clinical trials,468,491,492 
the FDA has approved 2.5 or 5 mg of tadalafil once daily for the 
treatment of ED. In the pivotal trials, men using 2.5

atment of ED. In the pivotal trials, men using 2.5 or 5 mg of 
tadalafil once daily experienced greater improvements in erectile 
function compared with those taking a placebo.468,491,492 Subse­
quent open-label extension studies indicated that once-daily 5-mg 
tadalafil for up to 2 years was effective in maintaining improve­
ments in erectile function.468,491,492 The adverse events associated 
with once-daily administration of tadalafil included headaches, 
indigestion, back pain, muscle aches,

 headaches, 
indigestion, back pain, muscle aches, nasal congestion, and flush­
ing and were similar to those observed with on-demand tadalafil 
ingestion.468,491,492 Once-daily regimen of tadalafil had no signifi­
cant effect on semen or reproductive hormone levels.468 
Use of PDE5 Inhibitors in Men With Coronary Artery Dis­
ease.485 Excellent therapeutic guidelines have been published on 
this topic by expert panels from several societies.106,431,432,481,490 
To minimize the risk of hypotensio

31,432,481,490 
To minimize the risk of hypotension and adverse cardiovascular 
events during the use of PDE5 inhibitors, the American Heart 
Association/American College of Cardiology has published a list 
of recommendations (see Table 20.12), which should be followed 
rigorously.481
Before prescribing PDE5 inhibitors, cardiovascular risk fac­
tors should be assessed. If the patient has hypertension or symp­
tomatic coronary artery disease, the treatment of those clinical 
disorders should be a

treatment of those clinical 
disorders should be addressed first.485 The use of nitrates must 
be ascertained, because PDE5 inhibitors are contraindicated in 
individuals taking any form of nitrates regularly. PDE5 inhibitors 
should not be used within 24 hours of the use of nitrates or nitrate 
donors.5,6,484,485
Sexual activity can induce coronary ischemia in men with pre­
existing coronary artery disease482; therefore, men contemplating 
use of ED therapies should undergo assessment of their 

f ED therapies should undergo assessment of their exercise 
tolerance. One practical way to assess exercise tolerance is to 
have the patient climb one or two flights of stairs. If the patient 
can safely climb one or two flights of stairs without angina or 
excessive shortness of breath, he can likely engage in sexual inter­
course with a stable partner without inducing similar symptoms. 
Exercise testing may be indicated in some men with significant 
heart disease to assess the risk of inducin

icant 
heart disease to assess the risk of inducing cardiac ischemia dur­
ing sexual activity.481–483 Selective PDE5 inhibitors have been 
shown not to impair the ability of patients with stable coronary 
artery disease to engage in exercise at levels equivalent to that 
attained during sexual intercourse.481–483 Similarly, each of the 
three PDE5 inhibitors has been shown not to have significant 
adverse effects on hemodynamics and cardiac events in carefully 
selected men with ED who did not h

s in carefully 
selected men with ED who did not have any contraindication for 
the use of PDE5 inhibitors.459,479–485 None of the PDE5 inhibi­
tors adversely affects total exercise time or time to ischemia during 
exercise testing in men with stable angina.459,479–485 In one recent 
analysis, treatment with PDE5 inhibitors for ED after a first 
myocardial infarction was associated with a reduced mortality 
and hospitalization for heart failure.493 Another cohort study also 
reported reduced rat

93 Another cohort study also 
reported reduced rates of myocardial infarction or other CVD 
events during the first 3 years after initiation of treatment with a 
PDE5 inhibitor for ED.494 
Treatment of Patients Who Do Not Respond to PDE5 Inhibi­
tors. Although oral PDE5 inhibitor therapy has revolutionized the 
management of ED, not all men respond to this treatment. The 
cumulative probability of intercourse success with sildenafil citrate 
increases with the number of attempts, reaching a maxi

eases with the number of attempts, reaching a maximum after 
eight attempts.495 Based largely on these data,495 the failure to 
respond to PDE inhibitor therapy has been defined as the failure 
to achieve satisfactory response even after eight attempts of either 
the highest approved dose (e.g., 100–mg sildenafil) or the highest 
tolerable dose of PDE5 inhibitor, whichever is lower. Many factors 
may contribute to apparent treatment failure, including failure to 
take the medication as recommend

uding failure to 
take the medication as recommended, suboptimal dose, dose-lim­
iting adverse effects, absence of sexual stimulation, psychological 
issues, partner and relationship issues, incorrect diagnosis, and 
patient-specific pathophysiologic factors.495–497 In clinical trials of 
PDE5 inhibitors, treatment failures were reported predominantly 
in men who had diabetes mellitus, non–nerve-sparing radical 
prostatectomy, cavernosal nerve damage, venous leak, and high 
disease severity.495–

mage, venous leak, and high 
disease severity.495–497 In an evaluation of cavernosal smooth mus­
cle biopsies in sildenafil nonresponders, Wespes and coworkers497 
found severe vascular lesions, as well as cavernosal smooth muscle 
atrophy and fibrosis, to be the underlying pathologic processes.
Patients may not take the medication appropriately because of 
inadequate instructions, failure to understand the instructions, 
adverse effects, or fear of adverse effects.495–497 Oral PDE5 inhibi­
tors

 of adverse effects.495–497 Oral PDE5 inhibi­
tors are taken optimally 1 to 2 hours before planned intercourse. 
The medication is unlikely to be effective if it is taken immedi­
ately before intercourse; a high-fat meal and large amounts of 
alcohol may further affect the maximal serum concentrations of 
sildenafil citrate. Similarly, patients may not take the appropriate 
dose because of side effects or fear of side effects. The men who 
have been misdiagnosed as having ED and whose primary se

n men with preexisting coronary artery disease, assess the risk of 
inducing cardiac ischemia during sexual activity before prescribing 
PDE5 inhibitors. This assessment may include a stress test.
	6.	 In men who are taking vasodilators and diuretics for the treatment of 
hypertension or congestive heart failure, consider the potential risk 
of inducing hypotension because of potential interaction between 
PDE5 inhibitors and vasodilators, especially in patients with low 
blood volume.
PDE5, Ph

ially in patients with low 
PDE5, Phosphodiesterase isoform 5.
Data from Cheitlin MD, Hutter AM Jr, Brindis RG, et al. Use of sildenafil (Viagra) in patients 
with cardiovascular disease: Technology and Practice Executive Committee [published erra­
tum appears in Circulation. 1999;100(23):2389]. Circulation. 1999;99(1):168-177; Lue TF, 
Giuliano F, Montorsi F, et al. Summary of recommendations on sexual dysfunctions in men. 
J Sex Med. 2004;1:6-23.
TABLE 20.12
786

ed. 2004;1:6-23.
786
disorder is unresponsive to PDE5 inhibitors may be incorrectly 
deemed treatment failures. For instance, men with HSDD, Pey­
ronie disease, or orgasmic or ejaculatory disorder would not be 
expected to respond to PDE5 inhibitors. The anxiety associated 
with resumption of sexual activity and unresolved relationship 
and partner issues can attenuate response to treatment. The sexual 
partner may not be willing or able t

. The sexual 
partner may not be willing or able to engage in sexual activity 
because of relationship issues, sexual disorder, or real or perceived 
health issues. Because erotic stimulation is necessary for local pro­
duction of nitric oxide, which then stimulates the production of 
cGMP, PDE5 inhibitors do not induce spontaneous erections and 
may not be effective in inducing erections in the absence of sexual 
stimulation.
Patients who report lack of satisfactory response to initial 
adminis

 lack of satisfactory response to initial 
administration of PDE5 inhibitors should be asked about the time 
of drug administration, the dose taken, and adverse effects experi­
enced. Psychological and partner issues should be evaluated. The 
dose of PDE5 inhibitor should be increased gradually as tolerated. 
Some men who do not optimally respond to on-demand PDE5 
inhibitors may respond to daily low-dose tadalafil (2.5 or 5 mg), 
which is an FDA-approved regimen.498–500 Should the patient not 

-approved regimen.498–500 Should the patient not 
respond to maximal tolerable doses of PDE5 inhibitors, the PDE5 
inhibitors can be combined with vacuum devices or intraurethral 
therapy. Second-line therapies, such as intracavernosal injections, 
should be pursued. The men who are unresponsive to oral PDE5 
inhibitors and second-line therapies may find penile implant an 
acceptable alternative.495 
Cost-Effectiveness of Phosphodiesterase 5 Inhibitor Use for 
Erectile Disorder. Several studies 

bitor Use for 
Erectile Disorder. Several studies have evaluated the economic cost 
of treating ED in men in managed care health plans.501–505 One 
simulation estimated sildenafil citrate cost to be approximately 
$11,000 per quality-adjusted life year.503 This amount is less than 
that for many other accepted treatments for medical disorders that 
cost $50,000 to $100,000 per quality-adjusted life year; thus, the 
cost-effectiveness of PDE5 inhibitor therapy compares favorably 
with other accep

bitor therapy compares favorably 
with other accepted medical therapies. Other analyses have con­
cluded that PDE5 inhibitors and vacuum constriction devices are 
the most cost effective of all available therapeutic options.501–505 
Several recent analyses have shown that the financial burden 
imposed by patients with ED on managed care plans is surprisingly 
small.501–505 In one such cost-utility analysis, the monthly cost 
of providing ED-related treatment services in a health plan with 
100,0

ed treatment services in a health plan with 
100,000 members amounted to less than $0.10 per member.503 
Thus, the failure of many insurance companies to cover the cost of 
PDE5 inhibitor therapy is not informed by cost-utility analyses. 
Second-Line Therapies
Vacuum Devices for Inducing Erection
The vacuum devices typically consist of an acrylic cylinder, a 
vacuum pump, and an elastic constriction band.506–508 The cyl­
inder fits over the penis and is connected to a vacuum pump. 
The negative 

 and is connected to a vacuum pump. 
The negative pressure created by the vacuum within the cylinder 
draws blood into the penis, producing an erection. An elastic band 
slipped around the base of the penis traps the blood in the penis, 
maintaining an erection as long as the rubber band is retained. 
The constriction band should not be left in place for more than 
30 minutes. In addition, only vacuum devices with a pressure-
limiting mechanism should be recommended to prevent injury 
due to hig

hould be recommended to prevent injury 
due to high vacuum.
Limited data on the efficacy of vacuum devices from open-label 
trials indicate that these devices are safe, relatively inexpensive, and 
moderately effective.506–508 They can impair ejaculation, resulting 
in entrapment of semen. Some couples dislike the lack of sponta­
neity engendered by the use of these devices. Partner cooperation 
is important for successful use of these devices.508 
Intraurethral Therapies
An intraurethral system

8 
An intraurethral system for delivery of alprostadil called MUSE 
(medicated urethral system for erection; VIVUS, Menlo Park, 
CA) was released in 1997. Alprostadil is a stable, synthetic form 
of PGE1, which stimulates generation of cAMP and activation 
of protein kinase A. Activated protein kinase A stimulates K+ 
channels, resulting in K+ efflux from the cell. In addition, protein 
kinase A–mediated processes result in a net decrease in intracel­
lular calcium, favor

n a net decrease in intracel­
lular calcium, favoring smooth muscle cell relaxation.
Alprostadil, when applied into the urethra, is absorbed through 
the urethral mucosa into the corpus cavernosum. In comparison 
to intracavernosal injection of PGE1, intraurethral alprostadil is 
easier to administer and has a lower frequency of adverse effects, 
particularly penile fibrosis.
Alprostadil is available in 125-, 250-, 500-, and 1000-μg 
strengths. Typically, the initial alprostadil dose of 250 or 5

ypically, the initial alprostadil dose of 250 or 500 μg is 
applied in the clinician’s office to observe changes in blood pres­
sure or urethral bleeding secondary to misapplication of the device 
into the urethra.
Initial randomized, placebo-controlled studies reported 40% 
to 60% success rates, defined as having at least one successful sex­
ual intercourse during a 3-month study period.509–511 In clinical 
practice, only about a third of men using intrauretheral alprostadil 
will respond.512
C

ing intrauretheral alprostadil 
Common side effects of intraurethral alprostadil are penile 
pain and urethral burning in up to 30% of patients509–512; its use 
also may cause dizziness, hypotension, and syncope in some users. 
Intraurethral alprostadil can cause mild burning or itching in the 
vagina of the sexual partner. Intraurethral alprostadil should not 
be used by men whose partners are pregnant or planning to get 
pregnant. 
Intracavernosal Injection of Vasoactive Agent

nt. 
Intracavernosal Injection of Vasoactive Agents
The use of intracavernosal injections of vasoactive agents has been a 
cornerstone of the medical management of ED since the early 1980s 
(Table 20.13). Patients can be taught to inject a vasoactive agent 
into their corpora cavernosa using a 27- or 30-gauge needle prior to 
the planned intercourse. Erections occur typically 15 minutes after 
intracorporal injection and last 45 to 90 minutes. Although intra­
cavernosal injection therapy is high

though intra­
cavernosal injection therapy is highly effective,513–520 it is associated 
with higher complication rates than oral therapy and should be used 
only by practitioners who are experienced in the use of this therapy 
and who can provide emergency medical support to their patients in 
the event of a serious adverse event, such as priapism.
Although several different agents—PGE1, papaverine, and 
phentolamine—have been used alone or in combination,513–521 
only intracavernosal PGE1 has 

ombination,513–521 
only intracavernosal PGE1 has been approved for clinical use. 
The long-term data on the efficacy and safety of intracavernosal 
therapy are sparse.
Several formulations of alprostadil (PGE1) are commercially 
available (Caverject, Pharmacia, Stockholm, Sweden; Prostin VR, 
Pharmacia; Edex, Schwarz Pharma, Milwaukee, WI). PGE1 binds 
to PGE1 receptors on the cavernosal smooth muscle cells, stimu­
lates adenylyl cyclase, increases the concentrations of cAMP, and is 
a powerful

ses the concentrations of cAMP, and is 
a powerful smooth muscle relaxant. The usual dose is 5 to 20 μg, 
and response to therapy is dose related and should be titrated.513–519
In one placebo-controlled efficacy trial, the intracavernosal 
alprostadil injection resulted in satisfactory sexual performance 
787
after more than 90% of administrations, and approximately 85% 
of men and their partners reported satisfactory sexual activity.513 
Intracave

ported satisfactory sexual activity.513 
Intracavernosal alprostadil is more effective than intraurethral 
alprostadil.517
The common adverse effects of intracavernosal therapy include 
penile pain, hematoma, formation of corporal nodules, penile 
fibrosis, and prolonged erections.513–520 Despite the effectiveness 
of this approach in producing rigid erections, many patients do 
not relish injecting a needle into their penis; therefore, it is not 
surprising that long-term dropout rates are high

surprising that long-term dropout rates are high. Intracavernosal 
injections should not be used in patients with sickle cell disease, 
multiple myeloma or leukemia, hypercoagulable disorder, ana­
tomic deformation of the penis, or penile implants.
Intracavernosal injections of papaverine, phentolamine, for­
skolin, and VIP have also been used, although these agents are 
not approved by the FDA.521 Papaverine, derived originally from 
the poppy seed, is a nonspecific PDE inhibitor, which incre

 seed, is a nonspecific PDE inhibitor, which increases 
both intracellular cAMP and cGMP. It has a greater propensity to 
induce priapism and fibrosis with long-term use, and efficacy and 
long-term safety data from randomized, placebo-controlled trials 
are lacking. Therefore, there is insufficient information to evaluate 
its efficacy and safety.
Phentolamine is a competitive α1- and α2-adrenergic antag­
onist that contributes to smooth muscle relaxation. As a single 
agent it is minimally eff

relaxation. As a single 
agent it is minimally efficacious, but it has been used to potentiate 
the effects of papaverine, VIP, and PGE1.521 Randomized clinical 
trial data on its efficacy and safety are lacking. Therefore, there is 
insufficient information to evaluate its efficacy and safety.
Various combination of these drugs are available as “bi-mix” 
or tri-mix” depending on the number of drugs included in the 
formulation. Tri-mix, a combination of papaverine, phentol­
amine, and PGE1, obt

ation of papaverine, phentol­
amine, and PGE1, obtained from compounding pharmacies, is 
often used in urologic practices even though it is not an FDA-
approved drug.
Priapism is a serious concern with the use of intracavernosal 
injection therapy. In patients who develop a prolonged or painful 
erection with PGE1, either 5 mg of brethane or 60 mg of pseu­
doephedrine, administered orally, may be of benefit. If priapism 
persists longer than 4 hours, patients should be instructed to seek 
medica

urs, patients should be instructed to seek 
medical care in which aspiration alone or with the injection of an 
α-adrenergic agent is used to induce detumescence. If this fails, 
surgical therapy may be needed to reverse a prolonged erection; 
otherwise, anoxic damage to the cavernosal smooth muscle cells 
and fibrosis can occur. 
Third-Line Therapies
Penile Prosthesis
The penile prostheses are invasive and costly, but they can be an 
effective method for restoring erectile function for patients

ethod for restoring erectile function for patients with 
advanced organic disease who are unresponsive to other medical 
therapies, have significant structural disorders of the penis (e.g., 
Peyronie disease), or have suffered corporal loss from cancer or 
traumatic injury.522–524
Penile implants are paired supports that are placed in each 
of the two erectile bodies. There are two basic types of penile 
implants: (1) hydraulic (fluid filled), referred to as inflatable pros­
theses, and (2) mall

ferred to as inflatable pros­
theses, and (2) malleable, semirigid rods, which are bendable but 
always remain firm in the penis.522–524 Penile prostheses come in 
a variety of lengths and girths. Implantation surgery usually takes 
less than an hour and in most cases can be done as an outpatient 
procedure under general or regional anesthesia.
Infection, perforation of the corpora during device placement, 
extrusion or migration of the device, and mechanical malfunc­
tion are the most common pr

nd mechanical malfunc­
tion are the most common problems with penile prostheses. 
With recent improvements in materials and design, the chance of 
mechanical malfunction has decreased to 5% to 10% in the first 
10 years.522–524 Infection occurs in 1% to 3% of cases, but infec­
tion rates can be higher in revision surgery, especially in men with 
diabetes mellitus.
The total cost of penile prosthesis implantation varies from 
$3,000 to $20,000, depending on the type of device used and the 
commun

pending on the type of device used and the 
community in which the procedure is performed. There are no 
randomized efficacy trials, but retrospective analyses have reported 
that greater than 80% of patients and 70% of partners are pleased 
with their prosthesis.522–525 
Testosterone Replacement in Androgen-Deficient Men 
Presenting With Erectile Dysfunction
Randomized trials have shown that testosterone replacement therapy 
improves erectile function, sexual desire, overall sexual activity, an

nction, sexual desire, overall sexual activity, and 
satisfaction with intercourse in hypogonadal men who have low sex­
ual desire; meta-analyses of randomized trials have confirmed these 
findings. Testosterone treatment does not improve sexual function 
in men with ED who have normal testosterone levels.27,65,143,144,160 
It is not known whether testosterone replacement improves sexual 
function in men with ED and borderline serum testosterone levels. 
Not all men with ED and low testosterone 

levels. 
Not all men with ED and low testosterone levels will experience 
improvements in their libido and overall sexual activity with testos­
terone replacement therapy;27,65,143,144 the response to testosterone 
therapy even in this group of men is variable because of the coex­
istence of other disorders, such as diabetes mellitus, hypertension, 
CVD, and psychogenic factors.27,143,144,160
ED in middle-age and older men is a multifactorial disorder, 
often associated with other comorbid condi

order, 
often associated with other comorbid conditions such as diabe­
tes mellitus, hypertension, CVD, medications, peripheral vas­
cular disease, psychogenic factors, and chronic kidney disease. 
  Guidelines for Intracavernosal Therapy
	1.	 Do not prescribe intracavernosal therapy to men who have psychi­
atric disorders, hypercoagulable states, or sickle cell disease; those 
who are receiving anticoagulant therapy; or those who are unable to 
comprehend the risks or take appropriate action 

e first injection in the office and observe the blood 
pressure and heart rate response. This provides an excellent 
opportunity for educating the patient, observing adverse effects, 
and determining whether the patient will respond to intracavernosal 
therapy.
	5.	 Start with a low dose of alprostadil, and titrate the dose based on 
the erectile response and the duration of erection. Adjust the dose 
of alprostadil to achieve an erection that is sufficient for sexual inter­
course but that doe

s sufficient for sexual inter­
course but that does not last more than 30 minutes.
	6.	 If the erection does not abate in 30 minutes, the patient should be 
instructed to take a tablet of pseudoephedrine or terbutaline, or an 
intracavernosal injection of phenylephrine. If this is not effective, the 
patient should call the designated physician or the urologist, and go 
to the emergency room.
TABLE 20.13
788
Therefore, it is not surprising that testo

unction
Therefore, it is not surprising that testosterone treatment alone 
may not improve sexual function in all men with androgen defi­
ciency. Testosterone induces NOS activity,137,138 has trophic 
effects on cavernosal smooth muscle and ischiocavernosus and 
bulbospongiosus muscles,139 increases penile blood flow,163 and 
is essential for achieving venous occlusion.136 These observa­
tions have led to speculation that testosterone might improve 
response to PDE5 inhibitors; however, as discu

ve 
response to PDE5 inhibitors; however, as discussed earlier, data 
from randomized trials have not shown the superiority of testos­
terone over placebo in improving erectile function in men with 
ED who have low testosterone levels and in whom the PDE5 
inhibitor dose has been optimized.166 However, it is possible that 
in men with ED who have testosterone deficiency, correcting 
testosterone deficiency first by testosterone replacement therapy 
may augment subsequent response to PDE5 inhibit

y 
may augment subsequent response to PDE5 inhibitors. In addi­
tion, testosterone replacement therapy in hypogonadal men 
with ED may improve libido and overall satisfaction with sexual 
experience, and may have other beneficial effects in correcting 
anemia, as well as improving bone mineral density and bone 
strength. 
Therapies With Either Unproven Efficacy or Limited 
Efficacy Data
There are insufficient efficacy data to support the use of trazo­
done526 or yohimbine527 in men with ED. The 

razo­
done526 or yohimbine527 in men with ED. The literature on the 
effectiveness of herbal therapies is difficult to interpret because of 
lack of consistency in product formulations and potencies, con­
tamination of herbal products with PDE5 inhibitors, poor trial 
design, and paucity of randomized clinical trial data.528–532 One 
randomized trial of Korean red ginseng reported this product to 
be effective in the treatment of ED529; these data need further 
confirmation. Icariin is a flavono

a need further 
confirmation. Icariin is a flavonoid, derived from several species 
of plants, whose extracts have been known in herbal medicine 
to produce aphrodisiac effects and enhance erectile function.532 
Dipyridamole also inhibits PDE5 and can augment the effects of 
nitric oxide. 4-Methylpiperazine and pyrazolopyrimidine, com­
ponents of the lichen Xanthoparmelia scabrosa, have also been 
claimed to inhibit PDE5.531 The use of these or other herbal ther­
apies is not recommended.531 Apo

ther herbal ther­
apies is not recommended.531 Apomorphine also functions as a 
dopamine agonist and acts centrally to initiate erection; its main 
adverse effects are nausea and dizziness. 
Gene Therapy and Erectile Dysfunction
The goal of gene therapy for ED is to introduce novel genetic 
material into the cavernosal smooth muscle cells to restore normal 
cellular function and produce a therapeutic effect.533–535 Gene 
therapy has been proposed as a treatment option for diseases that 
have a v

 as a treatment option for diseases that 
have a vascular origin, such as arteriosclerosis, congestive heart fail­
ure, and pulmonary hypertension.533–535 ED may be particularly 
amenable to gene therapy because of the easily accessible external 
location of the penis,533–535 which permits direct injection into 
the corpora cavernosa. A tourniquet placed around the base of the 
penis limits entry of the injected material into the systemic circu­
lation. This is a distinct advantage of the gene t

lation. This is a distinct advantage of the gene therapy of penile 
diseases over gene therapy for other systemic diseases, because 
introduction of the genetic material into the systemic circulation 
can potentially induce adverse systemic effects due to insertion of 
the material into an incorrect organ or vascular bed. Additionally, 
in the penis, only a small number of cells need to be transfected, 
because the interconnection of smooth muscle cells in the corpus 
cavernosum by gap junctions

e cells in the corpus 
cavernosum by gap junctions allows second messenger molecules 
and ions to be transferred to other interconnected smooth muscle 
cells.533–535 The low turnover rate of the vascular smooth muscle 
cells of the penis allows the desired gene to be expressed for long 
periods of time.
The strategies of gene therapy for ED treatment have focused 
on the molecules that regulate corporal smooth muscle relax­
ation or neovascularization533–535 (Table 20.14). Several candi­
date ge

ation533–535 (Table 20.14). Several candi­
date genes have been explored, including the maxi-K+ channel, 
penile-inducible NOS, eNOS, VIP, CGRP, vascular endothelial 
growth factor (VEGF), the brain neurotrophic factor angiopoi­
etin 1, neurturin (a member of the glial cell line–derived neu­
rotrophic factor family), superoxide dismutase, IGF1, PKG1α, 
and Rho A/Rho kinase533–543 (see Table 20.14). Several vectors 
have been used to transfer exogenous genes, including adeno­
viruses, adeno-assoc

enous genes, including adeno­
viruses, adeno-associated viruses, retroviruses, sindbis viruses, 
replication-deficient retroviruses, liposomes, naked DNA, and 
gold nanoparticles.533–549
Garban and associates536 first demonstrated that gene therapy 
can be performed in the penis by utilizing naked complementary 
DNA (cDNA) encoding the penile-inducible NOS gene leading 
to physiologic benefit in the aging rat. Christ and colleagues537 
injected hSlo cDNA (which encodes the human smooth muscle 
m

Slo cDNA (which encodes the human smooth muscle 
maxi-K+ channel) into the rat corpora cavernosa and demon­
strated increased gap junction formation and enhanced erectile 
responses to nerve stimulation in the aged rat. Adenoviral con­
structs encoding the eNOS and CGRP cDNAs were shown to 
reverse age-related ED in rats.538,539 In these studies, both eNOS 
and CGRP expression were sustained for at least 1 month in the 
corpora cavernosa of the rat penis. Five days after transfection 
with the A

at penis. Five days after transfection 
with the AdCMVeNOS or AdRSVeNOS viruses, aged rats had 
significant increases in erectile function as determined by response 
to cavernosal nerve stimulation and pharmacologic injection of 
endothelium-dependent vasodilator acetylcholine and PDE5 
inhibitors.538,539 In one study, intracavernosal injection of adeno-
associated virus construct carrying the brain-derived neurotrophic 
factor gene improved erectile function after cavernosal nerve 
injury.543 T

tile function after cavernosal nerve 
injury.543 This neurotrophic factor purportedly restored neuronal 
NOS in the major pelvic ganglion, thus enhancing the recovery of 
erectile function after bilateral cavernous nerve injury.543 In other 
studies, intracavernosal VEGF injection and adeno-associated 
virus-mediated VEGF gene therapy were each shown to reverse 
venogenic ED in rats.540,541 These other preclinical studies using 
gene therapy targets such as CGRP, superoxide dismutase, and 
Rho A

ets such as CGRP, superoxide dismutase, and 
Rho A/Rho kinase provided proof of concept that in vivo gene 
transfer can be accomplished technically. The translation of these 
preclinical data into human trials has been slow and unsuccessful 
so far.
Ion Channel Innovations Inc. has completed a phase I trial 
of slow K+ channel gene therapy in men with ED.99,545,546 In 
this trial, hMaxi-K, a “naked” DNA plasmid carrying the human 
cDNA encoding for the gene for the α-subunit of the human 
smooth

or the gene for the α-subunit of the human 
smooth muscle maxi-K+ channel, was injected directly into the 
penises of 11 men with ED. Patients who received the highest dose 
of hMaxi-K experienced significant improvements in their erec­
tile function that was sustained for the 24-week duration of the 
trial. This trial demonstrated the feasibility and safety of injecting 
naked DNA into the human penis.545,546 A trial of hMaxi-K in 
patients with overactive bladders is ongoing. Phase I gene ther

 overactive bladders is ongoing. Phase I gene therapy 
trials using VEGF and hepatocyte growth factor have been con­
ducted in patients with peripheral vascular disease and chronic 
limb ischemia; these trials have reported low frequencies of seri­
ous adverse effects. However, phase II studies have not confirmed 
789
efficacy. Thus, the early therapeutic promise of gene therapy has 
yet to be realized. Successful gene therapy may require introduc­

zed. Successful gene therapy may require introduc­
tion of multiple gene products using vectors with higher efficiency 
of transfection of a larger number of target cells and more pro­
longed action than can be realized with the current generation 
of vectors. Additional concerns about long-term safety, sustained 
gene expression, and efficiency of gene delivery and expression 
remain to be fully addressed. 
Potential of Stem Cell Therapy for Erectile Dysfunction
The past decade has seen conside

ctile Dysfunction
The past decade has seen considerable interest in the transplanta­
tion of stem cells derived from bone marrow, adipose tissue, or 
skeletal muscle into the corpora cavernosa.550–558 However, even 
when injected within the corpora cavernosa, the stem cells escape 
rapidly from the penis and hone into the bone marrow.550–558 The 
mechanism of the reported improvements in intracavernosal pres­
sure after stem cell injection into the corpora cavernosa remains 
unclear.553 The safe

he corpora cavernosa remains 
unclear.553 The safety and efficacy of stem cell therapy in humans 
has yet to be demonstrated. It remains to be demonstrated whether 
transplanted human mesenchymal stem cells (hMSCs) can differ­
entiate into functional cavernosal smooth muscle cells and restore 
erectile capacity in men with ED. In addition, long-term out­
comes including the tumorigenic potential of these transplanted 
progenitor cells are unknown.
hMSCs may also be attractive gene delivery vehic

.
hMSCs may also be attractive gene delivery vehicles because 
these cells can replicate in vitro as well as in vivo, thus potentially 
providing a large pool of cells.554,555 Initial studies have demon­
strated that rat mesenchymal stem cells, expanded and transfected 
ex vivo and implanted into the corpora cavernosa, are capable of 
expressing the gene product of interest.550–555 Stem cell therapy 
using stem cells carrying angiogenic or neurotrophic genes or pro­
teins is also being explored.

rophic genes or pro­
teins is also being explored. Although several animal studies have 
reported improved erectile function with hMSC transplantation, 
few studies have shown evidence of long-term stem cell survival in 
the cavernosal smooth muscle or evidence of differentiation of the 
transplanted stem cells into endothelial cells or cavernosal smooth 
muscle cells.550–558 
Management of Retrograde Ejaculation
Case reports have claimed benefit from methoxamine, imipra­
mine, midodrine, and ep

 from methoxamine, imipra­
mine, midodrine, and ephedrine; however, randomized clinical 
trial data are lacking.559–561 Induction of fertility in men with 
retrograde ejaculation may require retrieval of sperm from the 
urinary bladder after sexual stimulation or electrostimulation of 
the prostatic nerve plexus per rectum plus assisted reproductive 
techniques, such as intrauterine insemination or in vitro fertiliza­
tion with or without intracytoplasmic sperm injection using the 
retrieved spe

toplasmic sperm injection using the 
retrieved sperm.562–565 
  Physiologic Targets for Gene Therapy
Gene Target
Vector and Mechanism
Reference
Nitric oxide isoforms
Increase eNOS, nNOS, and iNOS activity in the cavernosal smooth muscle
Champion 1999; Bivalacqua 2000, 2003, 
2005; Gonzalez-Cadavid 2004; Kendirci 
2005; Wessels 2006
PIN
Antisense and short hairpin RNA constructs targeting PIN
Magee et al., 2007
Maxi-K+ channel
Transfer of maxi-K+ channels using a plasmid vector that carries the 

 channels using a plasmid vector that carries the hSlo gene 
encoding the α-subunit of the maxi-K+ channel; first human trial demonstrated 
the safety and feasibility of gene therapy in humans
Christ et al., 2002, 2004, 2004; Melman 
2003, 2005, 2006, 2007, 2008; So et al., 
2007
PKG1α
Replication-deficient recombinant adenoviruses carrying PKG1α
Bivalacqua et al., 2007
VEGF
Transfer of VEGF cDNA into rat corpora cavernosa to promote neovascularization
Rogers et al., 2003; Buchardt et al., 2005;

zation
Rogers et al., 2003; Buchardt et al., 2005; 
Dall’Era et al., 2008
Angiopoietin 1
Adenovirus-mediated transfer of human angiopoietin 1
Ryu et al., 2006; Jin et al., 2010
BDNF
Transfer of BDNF using adeno-associated virus
Rogers et al., 2005; Gholani et al., 2003
Neurotrophin 3 gene
Transfer of neurotrophin 3 gene using HSV vector
Bennett et al., 2005
Neurturin
Neurturin, a member of glial cell line–derived neurotrophic factor family
Kato et al., 2009
VIP
Transfection of corpora cavernosa 

t al., 2009
VIP
Transfection of corpora cavernosa of streptozotocin-treated diabetic rats using 
pcDNA3 carrying VIP cDNA
Shen et al., 2005
CGRP
Adenoviral transfer of CGRP in aged rats
Bivalacqua et al., 2001; Deng 2004
Superoxide dismutase
Adenoviral-mediated gene transfer of extracellular superoxide dismutase injected 
into the corpora cavernosa
Bivalacqua 2003; Brown 2006; Lund 2007
IGF1
Adenoviral-mediated gene transfer of IGF1
Pu et al., 2007
BDNF, Brain-derived neurotrophic factor; cDNA, 

07
BDNF, Brain-derived neurotrophic factor; cDNA, complementary DNA; CGRP, calcitonin gene–related peptide; HSV, human syncytial virus; IGF1, insulin-like growth factor 1; NOS, nitric oxide synthase 
(epithelial [e], inducible [i], or neuronal [n] isoforms); PIN, protein inhibitor of NOS; PKG, protein kinase G; VEGF, vascular endothelial growth factor; VIP, vasoactive intestinal protein.
Data from Melman A, Davies K. Gene therapy for erectile dysfunction: what is the future? Curr Urol Rep. 2010;

function: what is the future? Curr Urol Rep. 2010;11(6):421–426; Harraz A, Shindel AW, Lue TF. Emerging gene and stem cell therapies for 
the treatment of erectile dysfunction. Nat Rev Urol. 2010;7(3):143–152; Strong TD, Gebska MA, Burnett AL, et al. Endothelium-specific gene and stem cell-based therapy for erectile dysfunction. Asian J 
Androl. 2008;10(1):14–22; Deng W, Bivalacqua TJ, Hellstrom WJ, Kadowitz PJ. Gene and stem cell therapy for erectile dysfunction. Int J Impot Res. 2005;17(Suppl 

ctile dysfunction. Int J Impot Res. 2005;17(Suppl 1):57–63.
   
TABLE 20.14
790
Management of Sexual Dysfunction in 
Women
Psychotherapeutic methods are the mainstay of management of 
female sexual dysfunction (Fig. 20.12). A systematic review of lit­
erature on the psychogenic aspects of sexual dysfunction by the 
2016 International Consultation on Sexual Medicine commit­
tees found strong literature support for the routine investigation 
of personal

support for the routine investigation 
of personal, interpersonal, and contextual factors in women.566 
This type of assessment facilitates treatments such as cognitive 
behavioral therapy (CBT), mindfulness-based cognitive therapy 
(MBCT), and sex therapy approaches.
Management of Low Desire and Arousal in 
Women
In the management of SIAD, the circular model of sexual response 
(see Fig. 20.1) can serve as a useful basis for discussing which areas 
are problematic with one, or preferably both, 

as 
are problematic with one, or preferably both, partners (Table 
20.15; see Fig. 20.1. When insufficient emotional intimacy is 
identified, the normality of low interest to be sexual can be clari­
fied, and referral for couple counseling may be indicated. When 
the lack of sexual context and stimuli are contributing factors, 
simply emphasizing the requirement of appropriate environment 
and sufficiently erotic stimuli is usually sufficient, but referral to 
a sex therapist may be appropriate.

t referral to 
a sex therapist may be appropriate. Nonsexual distractions, fears 
about sexual outcome, self-monitoring of sexual response, anxiety, 
low self-image, and depression can interrupt the mental appraisal 
of stimuli. These issues can be explained and addressed. The main 
modalities of treatment for SIAD are cognitive therapy, sex ther­
apy, and psychoeducation (see Table 20.15).
Psychoeducation
Psychoeducation includes providing information while simultane­
ously soliciting the woman

mation while simultane­
ously soliciting the woman’s thoughts and feelings, which are then 
addressed and processed. It also includes dispelling widespread 
myths about sexuality in women (e.g., the absence of desire pre­
ceding sexual activity denotes sexual dysfunction). Bibliotherapy 
with self-help reading material or videos can be helpful. For some 
women, information on anatomy and physiology is necessary. 
Cognitive Behavioral Therapy
CBT can assist the woman to recognize, challenge, and,

can assist the woman to recognize, challenge, and, in time, 
change her negative and often catastrophic self-view imposed by 
underlying illness, endocrine problems, or infertility. Some of 
the exaggerated or catastrophic thoughts amenable to cognitive 
therapy include “sex is only for healthy women,” “I am no lon­
ger fertile, so I am no longer sexually attractive,” and “if inter­
course is painful, then no one will want me.” Identifying these 
biased/maladaptive thoughts is the first step: CB

biased/maladaptive thoughts is the first step: CBT then attempts 
to practice changing such thoughts. A growing body of data 
has provided empiric support for the benefit of targeting cogni­
tions and emotions during sex to increase physical and subjective 
arousal.39,567–570 
Mindfulness-Based Cognitive Therapy
Mindfulness has been shown to be of benefit for women with 
sexual dysfunction,568,569,571 who are medically healthy, women 
with pelvic cancer,570 and in those with PVD.571 Mindfulness

c cancer,570 and in those with PVD.571 Mindfulness is 
an Eastern practice of meditation and relatively new to Western 
medicine: the learned skill is to be fully present and accepting of 
all that is sensed in that moment. Attention is enhanced by the 
gradual ability to identify thoughts arising in that moment, be 
they future- or past oriented, and not to engage in them but ulti­
mately view these negative or positive thoughts more distantly, as 
if they are just sensations not dissimilar to 

as 
if they are just sensations not dissimilar to physical sensations. The 
practice of mindfulness focuses the attention on and acceptance 
of sexual sensations rather than on self-monitoring. Functional 
brain imaging performed before and after mindfulness training 
supports the clinical finding that such training lessens self-ref­
erencing of sensations and emotions, including pain and anxi­
ety.573–575 Resources for mindfulness practice in the community 
and through the internet can be offer

e community 
and through the internet can be offered.
Recently, an adapted form of CBT blended with mindfulness 
was defined—the treatment is called mindfulness-based cognitive 
therapy, or MBCT. Regular mindfulness practice is an integral 
component of MBCT. As with CBT, the skill of detecting mal­
adaptive thoughts, including those that are critical or evaluative, 
is learned but with simple observation of their presence and an 
acknowledgment that they are just mental events, not necessarily 

that they are just mental events, not necessarily 
the truth; there is no focus here to change thoughts. MBCT has 
been used to treat anxiety disorders and depression and has been 
adapted to treat arousal and desire disorders and chronic pain of 
PVD.568,569,572,576
Detailed treatment manuals for CBT and MBCT for clini­
cians with separate versions for patients have been developed; 
these manuals provide sufficiently detailed information for non–
mental health professionals and non–sexual medic

–
mental health professionals and non–sexual medicine specialists to 
provide CBT and MBCT to small groups of women. This small 
group format is consistently rated as beneficial.577 
Sex Therapy
Sex therapy usually focuses on sensate focus exercises whereby 
each partner takes turns giving and receiving sensual and, later 
on, sexual touches, caresses, and kisses. Initially, genital areas 
and breasts are off-limits. The idea of any goal or expectation 
Explain sex response cycle:
Identify probl

tation 
Identify problematic area(s)
Treat/refer for current mood disorder &
defer therapy for sexual disorder
Refer for interpersonal problems & 
Psychological modalities are the mainstay of therapy, combinations are often needed
Consider medical adjuncts
Cognitive therapy
techniques: challenge
irrational thoughts/
myths re her sexuality
Mindfulness skills to lessen
low self-image,
self-criticism, and enhance
sexual physical sensatio

lf-criticism, and enhance
sexual physical sensations
Sex therapy approaches
to address problems with
context, stimuli,
communication, goal-
focused sex
vaginal estrogen, vaginal DHEA, optimal
antidepressants
• Fig. 20.12  Algorithmic approach to the management of female sexual 
dysfunction. Sexual dysfunction in women is strongly linked to mental 
health. Therefore, evaluation of mental health and relationship issues is 
an important component of the evaluation of women with sexual dysfunc­
tion

 the evaluation of women with sexual dysfunc­
tion. Psychological therapies are the mainstay of treatment of women’s 
sexual disorders with growing evidence of benefit from mindfulness-based 
cognitive therapy. Topical estrogens or vaginal dehydroepiandrosterone 
(DHEA) may be useful in a subset of women with menopausal urogenital 
symptoms.
791
is put aside. Together, the couple and the clinician decide as to 
when breasts and genital areas are to

ecide as to 
when breasts and genital areas are to be included. Ultimately, the 
act of intercourse (or vaginal penetration with a dildo) may be 
included—but not as the focus. 
Outcome of Psychological Treatments for 
Women’s Sexual Dysfunctions
Data are limited regarding the long-term effects of psychologi­
cal treatments for women’s sexual dysfunction. Systematic review 
and meta-analysis of controlled clinical trials for female and male 
sexual dysfunction published before 2009578 concluded 

al dysfunction published before 2009578 concluded that psy­
chosocial interventions for sexual dysfunction were efficacious. A 
more recent meta-analysis of the 11 reported trials of MBCT for 
women’s sexual dysfunction concluded that all aspects of sexual 
function and subjective sexual well-being exhibited significant 
improvement during mindfulness-based therapy, with moderate 
effect sizes.579
For women with sexual dysfunction associated with the hor­
monal changes arising from breast cancer

 the hor­
monal changes arising from breast cancer treatment, a systematic 
review suggests that the most effective interventions are couple-
based psychoeducational interventions that include an element 
of sex therapy.580 The American Society of Clinical Oncology 
2018 guideline recommends psychosexual counseling along with 
addressing estrogen deficiency–related dyspareunia.581 
Management of Women’s Orgasmic Disorder
Management is guided by the details of the difficulties reaching 
a high le

he details of the difficulties reaching 
a high level of arousal and being free to let the orgasm occur 
(Table 20.16).
Although CBT and mindfulness training have been used in 
clinical practice to treat women with orgasmic disorder, outcome 
research is sparse. A 2013 meta-analysis found evidence of benefit 
with CBT and mindfulness training in symptom severity and sex­
ual satisfaction for women with orgasmic dysfunction.578 To date, 
there is no pharmacologic treatment for orgasmic disorder. 

no pharmacologic treatment for orgasmic disorder. In one 
trial with highly selective inclusion criteria, sildenafil was reported 
to improve orgasm dysfunction associated with SSRI use.582 In a 
small trial, transdermal testosterone did not improve sexual func­
tion compared with placebo as measured by the Sabbatsberg Sex­
ual Self-Rating Scale as the primary outcome.583 The strategies to 
treat antidepressant-associated orgasmic disorder include the use 
of medications that are less likely to 

e the use 
of medications that are less likely to impair sexual function, such 
as vortioxetine, vilazodone, mirtazapine, and bupropion.584 
Management of Genitopelvic Pain/Penetration 
Disorder (Dyspareunia and Vaginismus)
The most common type of dyspareunia is PVD, which affects 
about 12% to 18% of women.585 PVD and the other common 
type of dyspareunia—that stemming from vulvovaginal atro­
phy—are typically associated with pelvic muscle hypertonicity 
and reflexive involuntary contractions w

tonicity 
and reflexive involuntary contractions when penetration is antici­
pated. DSM-5 therefore merges the previous terms of dyspareunia 
and vaginismus. The use of transperineal ultrasound for pelvic floor 
muscle morphometry in women with PVD has confirmed abnor­
malities both at rest (e.g., small levator hiatus area, small anorectal 
angle suggesting increased tone) and with maximal contraction. 
These findings are thought to be consistent with weakness and 
poor control of pelvic muscle.

 with weakness and 
poor control of pelvic muscle.586 Pelvic floor physiotherapy is fre­
quently a component of sexual pain management (see Table 20.7 
and Fig. 20.10)
Management of Provoked Vestibulodynia 
There is no definite association between PVD and endocrine sta­
tus, but recent investigation of the presence of polymorphism in 
the guanine triphosphate cyclohydrolase gene (GCH1) is of inter­
est. Specific single nucleotide polymorphisms in the GCH1 gene 
are associated with reduced pain s

 the GCH1 gene 
are associated with reduced pain sensitivity. Although no corre­
lation between PVD and the pain-protective GCH1 polymor­
phism was found, patients with PVD using oral contraceptives 
and carrying a specified GCH1 polymorphism had higher pain 
  Management of Sexual Interest and Arousal Disorder in Women
Problematic Phase of SRC
Details of Phase Problem
Management Options
Sexual context problematic
Interpersonal
Explain usual need for emotional closeness +/– counseling 
referral

d for emotional closeness +/– counseling 
referral
Explain usual need for privacy, personal preferences regard­
ing surroundings
Sexual stimuli insufficient
Nonphysical, physical nongenital, genital 
­nonpenetrative
Give information, ↑ sexual communication, consider sex 
therapy or referral
Deterrents (e.g., partner’s hygiene, alcohol use)
Information, consider couple counseling referral
Processing of stimuli problematic
Depression/dysthymia
Fatigue-chronic illness, poor sleep
Too 

dysthymia
Too late in day
Chronic pain
Medications: opioids, SSRIs, SNRIs
Refer to mental health
Non–goal-oriented sex, address sleep
Explanation
Time pain meds, +MBCT for pain Rx
Consider vortioxetine, bupropion, mirtazapine, vilazodone
↓ Genital sensations
Sensations muted or fleeting
Onset with menopause
Mindfulness (e.g., body scan)
Vaginal DHEA, vaginal estrogen
↓ Mental sexual arousal
Physical arousal too similar to feelings of anxiety
Distractions, “bus

 similar to feelings of anxiety
Distractions, “busy mind”
MBCT, mental health referral
Mindfulness practice
DHEA, Dehydroepiandrosterone; MBCT, mindfulness-based cognitive therapy; SNRIs, serotonin and norepinephrine reuptake inhibitors; SSRIs, selective serotonin reuptake inhibitors; SRC, sexual response 
cycle.
   
TABLE 20.15
792
sensitivity.587 This finding is in keeping with the clinical experi­
ence that some women with PVD benefit from disconti

nce that some women with PVD benefit from discontinuing oral 
contraceptives.588
Randomized trials of oral and topical medications for PVD 
including tricyclics, anticonvulsants, lidocaine, fluconazole, cro­
molyn, and nifedipine gave similar results in analgesic benefit 
compared with placebo.589 Investigational botulinum toxin was 
statistically inferior to placebo in reduction of sexual distress.589 
Because medical treatments for PVD have been unsatisfac­
tory, an interdisciplinary biopsycho

n unsatisfac­
tory, an interdisciplinary biopsychosexual approach is currently 
encouraged.590,591
Although rarely chosen by women, surgical vestibulectomy 
may be of benefit in a small subset of women; however, there 
are numerous exclusions for vestibulectomy,592 and benefit is 
obtained mostly in women with acquired as opposed to lifelong 
histories. Other common negative prognostic factors for success­
ful vestibulectomy include comorbid muscle tightening, wide­
spread allodynia of the intro

le tightening, wide­
spread allodynia of the introital margin, involvement of the Skene 
duct openings, unwillingness to have sex therapy if offered, and 
comorbid depression and anxiety. Negative prognostic factors for 
all treatment modalities include disgust and contamination sensi­
tivity, erotophobia (the tendency to respond with negative effects 
to sexual cues), and coexistence of depression and anxiety.593
Personality traits of women with PVD, including negative self-
evaluation and fear

 PVD, including negative self-
evaluation and fear of negative evaluation by others, predispose to 
self-labeling as sexually substandard or even sexually inadequate. 
The stress model of pain posits that sexual stress not only contrib­
utes to the chronic pelvic muscle hypertonicity but also maintains 
a heightened reactivity of the pain circuitry from top-down mod­
ulation afforded by neuroplasticity.589,593–595 The cause of the sen­
sitization within the nervous system has not been establishe

 within the nervous system has not been established with 
certainty, but internal stress appears to be a likely cause. Women 
with PVD report higher levels of premorbid depression and anxi­
ety, as well as perfectionism, reward dependency, fear of negative 
evaluation, increased prevalence of type D personality, self-dislike, 
harm avoidance, hypervigilance for pain, and shyness compared 
with women without PVD.596–600
Given that mood disorder is so commonly comorbid with PVD, 
management needs 

 so commonly comorbid with PVD, 
management needs to address both pain and depression/anxiety. 
There is evidence of sustained benefit from CBT.601 Catastrophic 
thinking, amenable to CBT approaches, is particularly common 
in women with PVD.602 The lack of self-acceptance apparent in 
women with PVD, considered to be a pain-maintaining stress, is 
potentially amenable to MBCT given that a key component of 
MBCT is acceptance. A brief mindfulness-based group interven­
tion was more efficacious i

-based group interven­
tion was more efficacious in reducing both cotton swab–induced 
vestibular pain and psychological measures of pain in comparison 
to a waitlist control.572,577 In a study of women with PVD, group 
MBCT was recently compared with group CBT, finding similar 
benefit in all secondary endpoints, but improvements in reported 
pain with vaginal penetration after MBCT were greater than those 
reported after CBT. 
Management of Phobic Reflex Pelvic Muscle Contractions 
Component o

bic Reflex Pelvic Muscle Contractions 
Component of Genitopelvic Pain/Penetration Disorder 
(Vaginismus)
Heightened pelvic muscle tone, often along with muscle tension 
elsewhere, may be the only physical findings in women reporting 
dyspareunia or painful but unsuccessful attempts at penetration. 
Often guided by pelvic floor physiotherapists, management involves 
progressive desensitization and progressive vaginal accommodation 
using a variety of relaxation techniques and vaginal inserts.603 

 of relaxation techniques and vaginal inserts.603 The 
term dilators is preferably avoided, because women fear that their 
therapy is going to (painfully) stretch the vagina. In-clinic physical 
therapy has been associated with better outcomes than traditional 
guidance for insert therapy at home.604 Psychotherapies, including 
mindfulness and CBT, are often used to reduce anxiety.605 
Testosterone Therapy for Women With 
As discussed earlier in this chapter, testosterone defi

scussed earlier in this chapter, testosterone deficiency has 
not been demonstrated in women diagnosed with sexual dysfunc­
tion, either by measurement of blood levels or by measurement 
of androgen metabolites. Several randomized trials of testosterone 
therapy have been conducted, mostly in postmenopausal women. 
These testosterone trials were conducted largely in women dis­
tressed by reduced sexual desire since their menopause. The eligi­
bility criteria for these trials did not meet the DSM

ity criteria for these trials did not meet the DSM-IV diagnostic 
criteria for HSDD, or for the newly coined SIAD of DSM-5.
The first series of randomized trials showed a statistically signif­
icant improvement in the numbers of satisfactory sexual encoun­
ters (SSEs) in women randomized to testosterone: on average, 
SSEs increased from two to three per month to five per month in 
women on active drug and to four per month in women receiving 
placebo. Testosterone was given transdermally in the 

cebo. Testosterone was given transdermally in the form of a 
patch with a nominal testosterone delivery of 300 μg/day. Doses 
of either 150 or 450 μg/day were not effective.606 Serum testoster­
one and dihydrotestosterone concentrations exceeded the upper 
limits of the target serum concentrations of these hormones in a 
significant number of women receiving testosterone.607 Women 
on an active drug reported further improvements in arousal, plea­
sure, orgasm, self-image, and responsiveness to a

sure, orgasm, self-image, and responsiveness to a statistically sig­
nificantly greater extent than did women receiving placebo.
These testosterone trials focused mostly on surgically meno­
pausal women, but one testosterone patch study included naturally 
menopausal women608 with comparable results. Two testosterone 
studies recruited naturally and surgically menopausal women who 
were not receiving estrogen therapy. One of these studies reported 
a significant increase in SSEs in the naturall

ed 
a significant increase in SSEs in the naturally menopausal women 
from the active drug but not in the smaller subgroup of surgi­
cally menopausal women.609 Only 464 of the 814 participants 
completed treatment, with similar distribution of high discon­
tinuation rates in all three arms. A second study of 272 naturally 
  Management of Orgasmic Disorder in 
Women
Etiology
Insufficient arousal
Manage as for sexual interest/arousal 
disorder
Antidepressant induced
Consider switch to

disorder
Consider switch to vortioxetine, bupropion, 
mirtazapine, vilazodone
Clitoris “too sensitive”
Information on deeper clitoral bulbs, non­
genital stimuli, G spot
Vibrator to vulva away from clitoris
General need to be in 
control
Mental health referral, use of vibrator may 
bypass this difficulty
Orgasmic alone but not 
with partners
Encourage couple sexual communication; 
consider psychology referral regarding 
fear of intimacy and fear of vulnerability
TABLE 20.1

r of intimacy and fear of vulnerability
TABLE 20.16
793
menopausal women, of whom a total of 73% of the participants 
were not receiving systemic estrogen therapy, showed a significant 
increase in SSEs.610
On the basis of these studies, all by the same sponsor, the 
transdermal testosterone patch was approved in Europe, but not 
in North America or elsewhere, for the treatment of surgically 
menopausal women with persistently distressing low sexua

usal women with persistently distressing low sexual desire 
despite adequate systemic estrogen therapy. Although approved, 
the patch is no longer available in Europe because of low sales.
In contrast to the previous studies, two large phase III RCTs 
by a different sponsor of 1172 postmenopausal women, approxi­
mately half of whom received systemic estrogen, showed no bene­
fit of transdermal testosterone in the form of a gel over placebo.611 
Full study details are not available because these 

ull study details are not available because these two studies have 
not been published. The entry criterion of distressingly reduced 
sexual desire after menopause was similar to the previous random­
ized trials; endpoints were the numbers of SSEs per month and 
the level of sexual desire as assessed using a daily diary.
A dose-response study of menopausal women with past hys­
terectomy with or without bilateral oophorectomy and with low 
testosterone levels (free testosterone <3.5 pg/mL) found 

erone levels (free testosterone <3.5 pg/mL) found that some 
aspects of sexual function improved compared with placebo, but 
only in those receiving the 25-mg weekly intramuscular testoster­
one dose and not the 12.5-mg or lower doses. All treatment doses 
increased circulating testosterone concentrations above those of 
regularly cycling women—the 25-mg dose leading to 10 and 20 
times the physiologic levels of total and free testosterone, respec­
tively.612 Even at these doses, sexual receptiv

c­
tively.612 Even at these doses, sexual receptivity, pleasure, orgasm, 
or problems with sexual function did not improve.
There is very little information available on the effects of tes­
tosterone in premenopausal women. One study of 261 women 
who experienced loss of their former sexual satisfaction reported 
minimal benefit from testosterone.613
A systematic review concluded that currently available evidence 
does not support the measurement of serum testosterone levels in 
the evaluation o

 of serum testosterone levels in 
the evaluation of women with low libido.415
Testosterone Plus a Phosphodiesterase Inhibitor
Studies have begun to evaluate the efficacy of a pharmacologic dose 
of testosterone (0.5 mg sublingually) to improve attentiveness for 
erotic cues in women with low desire.614 The testosterone was com­
bined with sildenafil, a PDE5 inhibitor, to facilitate genital conges­
tion. In those women who at baseline already showed high levels 
of subconscious attention bias for

ed high levels 
of subconscious attention bias for erotic cues (as measured by a 
masked version of the emotional Stroop task), this drug combina­
tion had no effect, and in fact, testosterone alone reduced attention 
to erotic cues. However, the women with lower arousability or sensi­
tivity to erotic cues at baseline showed increased physiologic genital 
congestion and increased awareness of the genital sensations and of 
sexual desire when they subsequently viewed an erotic video. Recent 
dos

y subsequently viewed an erotic video. Recent 
dose-finding studies found that 0.5 mg of sublingual testosterone, 
but not 0.25 mg, plus 50 mg of sildenafil increased SSEs compared 
with baseline and compared with either drug alone. For women 
with higher arousability but presumed to have more inhibiting 
thoughts, 0.5 mg of sublingual testosterone plus 10 mg of buspirone 
increased SSEs compared with baseline and compared with either 
drug alone.615 The safety of intermittent use of, albeit tem

.615 The safety of intermittent use of, albeit temporary, 
markedly supraphysiologic testosterone therapy is totally unknown. 
Limitations of Trials of Testosterone Therapy in Women
A major limitation of testosterone trials to date is the targeted pop­
ulation. Studies have recruited women with decreased desire since 
menopause, most of whom retained the ability to be aroused and 
sexually satisfied on at least some (on average 50%) occasions. Thus, 
an absence of sexual desire between sexual en

us, 
an absence of sexual desire between sexual encounters has been the 
focus. However, research confirms this to be well within the range of 
normal female sexual experience. The majority of 3250 multiethnic 
middle-age women in the SWAN cohort indicated that although 
moderately or extremely sexually satisfied, they never or very infre­
quently felt desire.616 In an online survey of 3687 younger women, 
1865 were assessed to be without evidence of sexual dysfunction, 
specifically confirming 

e of sexual dysfunction, 
specifically confirming their easy sexual arousal—close to one third 
of this group rarely or never began a sexual experience with a sense 
of sexual desire.617 As noted earlier in this chapter, an incentives/
motivations model of human sexual response is now considered to 
more accurately reflect sexual experience, desire for sex per se being 
just one of many reasons or incentives for sex. When absent at the 
beginning of a sexual encounter, desire can be triggered al

 of a sexual encounter, desire can be triggered along with 
arousal after effective stimulation.
Women able to have satisfactory sexual experiences 50% of the 
time are thought unlikely to have a biologic cause of dysfunc­
tion to merit any hormonal therapy.618–620 The trials did show 
improvements in desire and response domains using validated sex­
ual questionnaires; however, increasing the degree and frequency 
of pleasure and arousal currently experienced by study subjects 
does not necessar

y experienced by study subjects 
does not necessarily imply that improvements would be observed 
in women with a consistent absence of pleasure and arousal.618
There has been criticism of the use of statistical significance alone 
to evaluate the difference between the powerful placebo effects and 
active drug treatments in the area of women’s sexual dysfunction—
especially that of low desire.621,622 It is suggested that effects might 
be better reported in terms of the percentage of participant

reported in terms of the percentage of participants no lon­
ger meeting current criteria for sexual dysfunction.621 As noted, the 
women in the testosterone trials were not recruited on the basis of a 
clinical diagnosis of sexual dysfunction but based on confirmation 
of low desire after menopause that caused distress. 
Risks of Testosterone Therapy
Long-term safety data are lacking; published safety data from trials 
of up to 12 months’ duration are reassuring.622 There are theo­
retical reaso

n are reassuring.622 There are theo­
retical reasons to consider exogenous testosterone as either a risk 
factor or a protective factor for breast cancer; high endogenous 
testosterone may be associated with an increased risk.351,620 A high 
endogenous testosterone-to-estrogen ratio can increase the risk of 
metabolic syndrome and CVD.623 However, some data suggest 
that low SHBG may be related to the risk of diabetes, metabolic 
syndrome, and CVD.624 In the Melbourne Women’s Midlife 
Health Pro

D.624 In the Melbourne Women’s Midlife 
Health Project, weight gain and free androgen index, but not total 
testosterone, were strong predictors of CVD risk.625 Similar results 
were observed in a 9-year follow-up of the SWAN cohort.626 In 
this study, the free androgen index was positively and SHBG was 
negatively associated with the development of obesity. Weight 
gain preceded changes in the free androgen index and SHBG. The 
expert panel of the Endocrine Society noted that the association 
b

he Endocrine Society noted that the association 
between the free androgen index, CVD risk factors, and the meta­
bolic syndrome phenotype appears to be more driven by obesity 
and low SHBG rather than testosterone.351 In a very recent pro­
spective study of postmenopausal women, higher levels of testos­
terone were associated with an increased incidence of CVD and 
coronary artery disease, whereas higher estradiol levels were associ­
ated with a lower incidence of coronary artery disease.627
In

 lower incidence of coronary artery disease.627
In most randomized trials, testosterone therapy has been 
administered in the background of concurrent estrogen therapy.351 
However, the present advice is to only initiate estrogen therapy 
794
close to menopause, noting the increase in CVD when initiated 
after 10 years’ post menopause.628 This risk not only limits use 
of combination therapy but, theoretically, also testosterone alone 
given its aroma

etically, also testosterone alone 
given its aromatization to estrogen.
The Endocrine Society task force noted (1) the limited safety 
data (median follow-up, 4 months; range, 6 weeks to 2 years) and 
(2) the efficacy data focus on sexually responsive women with­
out the common comorbid conditions including depression or 
antidepressant treatment. The task force requested a meta-analysis 
of transdermal testosterone RCTs; the gel studies, however, were 
excluded because they are only published i

, were 
excluded because they are only published in abstract form. 
Needed Research in the Area of Testosterone 
Supplementation
Further research is needed in women with low sexual interest/incen­
tives and low arousal (and typically few orgasms) to reflect the preva­
lent clinical situation and to merit a diagnosis of SIAD. It is of note, 
however, that in the study of 125 women with and 125 women with­
out HSDD, when no group differences in androgen activity were 
found, 55% of the women with 

rogen activity were 
found, 55% of the women with HSDD also met criteria for SIAD.629
Women diagnosed with SIAD and in remission from depres­
sion but taking antidepressants and women who, despite treat­
ment, still score in the depressive range again reflect the clinical 
situation. Given that depression typically blunts sexual response, 
it has been an exclusion factor in clinical trials, as has the use of 
antidepressant therapy, but the reality is that mood disorder and 
its treatment common

ty is that mood disorder and 
its treatment commonly accompany complaints of low sexual 
desire.629–631 Not only is depression the factor most robustly 
linked to low desire in otherwise healthy women, but also depres­
sion frequently determines the presence of sexual dysfunction even 
when other medical conditions including diabetes are comorbid.14 
Oral Dehydroepiandrosterone for Sexual 
Dysfunction in Healthy Women
Small trials of DHEA have been conducted in older healthy 
women. A 2014 syste

en conducted in older healthy 
women. A 2014 systematic review and meta-analysis to evaluate 
the benefits and risks of systemic DHEA therapy for postmeno­
pausal women632 included 15 randomized trials that were gener­
ally considered at high risk of bias and were of short duration. 
Statistically, DHEA use was marginally significant for desire, and 
there were no other significant improvements to outcome. The 
quality of evidence was considered low to moderate for benefit 
and very low for long

low to moderate for benefit 
and very low for long-term harm. A more inclusive systematic 
review (38 studies but with 8 having fewer than 50 participants) 
found benefit in 11 studies, noting that the main benefit was for 
postmenopausal women.633 Importantly, recruitment has not 
focused on women clearly diagnosed with sexual disorder with 
or without relatively low serum DHEA.634,635 Recent research 
revealed benefits from 50 mg of DHEA daily to reduced genital 
sexual sensitivity attributed 

to reduced genital 
sexual sensitivity attributed to oral contraceptives.636 
Local Dehydroepiandrosterone  Therapy for 
Sexual Dysfunction in Healthy Women
Local vaginal DHEA therapy has recently been approved for post­
menopausal women with GSM. Treatment was found to improve 
vaginal symptoms of dryness and dyspareunia and all domains of 
sexual function. Moreover, all steroids, measured by mass spectrom­
etry methods, remained in the postmenopausal range. Specifically, 
ADT-G remained consta

pausal range. Specifically, 
ADT-G remained constant. This delivery of precursor hormones 
to the target tissue may allow strictly local estrogen and androgen 
actions and may be a preferable choice for women in whom any 
systemic estrogen therapy is undesirable, such as those receiving 
aromatase inhibitors for breast cancer, who can develop severe vul­
vovaginal atrophy. Rodent work suggests that local DHEA’s benefi­
cial effect on genital sexual sensitivity might stem from its potent 
stimula

al sensitivity might stem from its potent 
stimulatory effect on vaginal nerve fiber density.637 The increase 
in desire is interesting given lack of systemic increase in testoster­
one and minor increase in serum DHEA. Restored feedback from 
augmented genital response may be relevant. However, a caveat 
is that trial participants were recruited on the basis of symptoms 
from GSM, not on the diagnosis of SIAD. When part or all of the 
sexual dysfunction stems from loss of reward due to loss of 

function stems from loss of reward due to loss of genital 
+/– nongenital sexual sensations, vaginal DHEA can be tried after 
explaining that this use of an approved medication is off-label (see 
Table 20.15). 
Estrogen Therapy for Women With 
Local vaginal therapy is recommended for dyspareunia associated 
with GSM. Low doses of estrogen can be supplied by a Silastic 
vaginal ring, vaginal cream, or a mucoadhesive vaginal tablet with 
similar benefit and low systemic absorpti

et with 
similar benefit and low systemic absorption. Use of estradiol, 10 
μg twice weekly, and the Estring (Pfizer, New York, NY) (a Silastic 
ring containing estradiol, placed high in the vaginal vault) results 
in serum levels of 4.6 and 8.0 pg/mL, respectively. Progesterone 
is usually considered unnecessary for endometrial protection. 
Smaller doses of these formulations of estrogen are being inves­
tigated (e.g., 10 μg rather than 100 μg estradiol cream, 0.03 mg 
rather than 0.2 mg estrio

stradiol cream, 0.03 mg 
rather than 0.2 mg estriol vaginal pessaries) or have already been 
approved (e.g., 10-μg rather than 25-μg estradiol vaginal tablets). 
When local estrogen does not ameliorate GSM-associated dyspa­
reunia, comorbid PVD may be present.638
Of concern is that women using aromatase inhibitors and 
vaginal estrogen may show a small increase in serum estradiol lev­
els.639 Small increases in estradiol are reported from use of both 
the estradiol ring and the former 25-μg vagi

both 
the estradiol ring and the former 25-μg vaginal tablet, but not 
from a new 4-μg vaginal capsule and only on day 1 from a 10-μg 
capsule.640 A combination of 0.03 mg of estriol and lactobacilli 
was found to benefit GSM symptoms without systemic absorp­
tion.641 In another recent study of women receiving aromatase 
inhibitors, both 5 mg of vaginal testosterone three times a week 
or an estradiol-releasing ring showed benefit to signs of atrophy, 
sexual interest, and sexual dysfunction.642

ophy, 
sexual interest, and sexual dysfunction.642 A small number of 
patients in the testosterone arm sustained estradiol elevations and 
high treatment-induced testosterone levels. Systemic absorption 
of testosterone could increase serum estrogen through aromati­
zation. Previously, using a lower dose of testosterone (150–300 
μg daily), no significant estradiol elevation was reported at 4 
weeks.643 Vaginal DHEA is now being trialed in breast cancer 
patients, given the lack of systemic upta

 cancer 
patients, given the lack of systemic uptake of estrogen or testos­
terone. The American Society of Clinical Oncology recommen­
dations include offering vaginal DHEA to women on aromatase 
inhibitors who have current or past breast cancer and have not 
responded to previous treatment.581 A study of 300 μg of vaginal 
testosterone nightly showed benefit to dyspareunia without estro­
gen effect on maturation index or reduction of visible signs of 
GSM and without increase in serum testoste

gns of 
GSM and without increase in serum testosterone or estrogen. The 
benefit is presumed to be from vasodilatation alone.
Of particular relevance to women with past breast cancer is a 
2013 report of a hyaluronic acid vaginal gel improving dyspareunia 
in 85% of women, comparable to women receiving vaginal estriol.644
795
Vaginal Lubricants and Moisturizers
Nonhormonal preparations are often a first choice for women with 
and without histories 

irst choice for women with 
and without histories of estrogen-sensitive tumors. Other causes of 
reduced lubrication include hypothalamic hypogonadism, radia­
tion, non–nerve-sparing surgeries for pelvic cancer, and medica­
tions such as ultralow-dose combined contraceptives, GnRH 
agonists, antihistamines, and anticholinergics, as well as breastfeed­
ing or lack of any sexual arousal. Lubricants reduce friction and 
are applied just before intercourse. Moisturizers also reduce friction 
but are

course. Moisturizers also reduce friction 
but are used two or three times weekly to maintain moisture over 
the long term. Hyaluronic acid can be used alone or in combina­
tion with aloe vera and tea tree oil. There is some evidence that 
hyaluronic acid can benefit the urinary symptoms of GSM—such 
symptoms can lessen sexual motivation. Many preparations are 
marketed, but many clinicians recommend coconut oil, which is 
usually nonirritating, longer lasting, inexpensive, and thought to 
be ba

longer lasting, inexpensive, and thought to 
be bacteriostatic and fungistatic; however, it is not compatible with 
latex condoms. Water-based lubricants can dry out quickly. They 
may contain glycerin (to increase moisture retention), which may 
promote yeast infections or sexually transmitted infections such 
as herpes and HIV, and possibly promote chronic bacterial vagi­
nosis. Parabens may be added to deter microbial growth but may 
cause irritation and increase the risk of HIV or herpes tra

itation and increase the risk of HIV or herpes transmission, 
and these additives do have ultraweak estrogenic activity. Replens 
(Church and Dwight, Ewing, NJ), which contains a patented bio­
adhesive that attaches to vaginal cells and preserves moisture, has 
received more study than most products and was shown to increase 
elasticity, lower pH, and benefit dryness and dyspareunia. Silicone 
products are suitable for sensitive skin, do not alter vaginal pH, 
and are longer lasting, but they ar

r vaginal pH, 
and are longer lasting, but they are expensive and must be washed 
off with soap and water—and are also extremely flammable. Lubri­
cants that are iso-osmolar so as to preserve sperm motility are rec­
ommended when conception is wanted. 
Systemic Estrogen
When systemic estrogen is needed for other menopausal symp­
toms, it is sometimes necessary to give additional local estrogen 
for dyspareunia. In contrast, for some women, ultralow-dose 
(0.014 mg/day) systemic transdermal estra

ow-dose 
(0.014 mg/day) systemic transdermal estradiol may be sufficient 
for all menopausal symptoms, including dyspareunia.646 If sys­
temic supplementation improves insomnia and dyspareunia, then 
sexual motivation would logically be expected to increase, but this 
has not been vigorously studied. No significant differences were 
found between estrogen and placebo groups in reported sexual 
satisfaction in the Women’s Health Initiative trial.647 However, 
sexual dysfunction was not a primary 

47 However, 
sexual dysfunction was not a primary focus of that trial, women 
with marked menopausal symptoms were excluded, and the 
instruments used to assess sexual function were substandard. The 
Kronos Early Estrogen Prevention Study results suggest that the 
transdermal, rather than oral, route may be more likely to benefit 
sexual function, but again, the participants were not recruited on 
the basis of sexual dysfunction.648
Ospemifene—the only approved estrogen receptor modulator 
for G

e only approved estrogen receptor modulator 
for GSM—is another alternative to estrogen. Its estrogenic effects 
on vulvar and vaginal tissues restore the maturation index, vaginal 
pH, and dyspareunia.649 However, its theoretical safety on breast 
tissue is not established. 
Approved But Not Recommended Medication 
for the Former DSM-IV Hypoac­tive Sexual Desire 
Disorder 
Flibanserin—a 5HT1A agonist and a 5HT2A antagonist stud­
ied originally for potential benefit to depression—was trialed by 

or potential benefit to depression—was trialed by 
different pharmaceutical companies and received limited FDA 
approval for the treatment of HSDD. This approval is despite 
benefit being similar to placebo and serious risk of harm.650 
There are strict contraindications to alcohol and to medications 
that inhibit CYP3A4, including oral contraceptives and flucon­
azole. Safety concerns include hypotension, syncope, somnolence, 
fatigue, and potential carcinogenicity. Overall risks of sedation or

tial carcinogenicity. Overall risks of sedation or 
hypotension-related events were 28.6% with flibanserin versus 
9.4% with placebo. Two recent meta-analyses have been based on 
both published and unpublished RCTs.651 Meta-analysis showed 
that flibanserin led to a mean increase of 0.49 satisfying sexual 
events per month.652 There was an increase of 0.3 (range, 1.2–6.0) 
on the desire subscale of the validated questionnaire, which was 
considered not to differ from placebo. Recruited women rep

ed not to differ from placebo. Recruited women reported 
two to three rewarding sexual experiences each month at baseline 
(i.e., the participants did not have SIAD). Due both to safety con­
cerns and to the short duration of studies and questionable accu­
racy of 4-week recall along with marginal efficacy, approval had 
been denied and additional safety studies recommended. Surpris­
ingly, despite lack of additional efficacy data, but with data ruling 
out driving impairment and assessing alcoh

ruling 
out driving impairment and assessing alcohol’s enhancement of 
side effects in 23 men and 2 women, on June 4, 2015, an FDA 
advisory committee voted 18 to 6 to accept approval of fliban­
serin, although with reservations. Fifteen members endorsing its 
approval declared their reluctance to do so. A recent review of 
scientific knowledge on flibanserin to date seriously questioned its 
use, noting the tenuous risk-benefit profile along with prohibitive 
prescribing restrictions and doubtf

h prohibitive 
prescribing restrictions and doubtful presence of sexual disorder 
in trialed women.653
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. Curr Drug Metab. 2017;18(1):78–
85. https://doi.org/10.2174/1389200217666161026090333.
	653.	 Anderson R, Moffatt CE. Ignorance is not bliss: if we don’t under­
stand hypoactive sexual desire disorder, how can flibanserin treat 
it? Commentary. J Sex Med. 2018;15(3):273–283. https://doi.
org/10.1016/j.jsxm.2018.01.001.
796
21
Transgender Endocrinology
STEPHEN M. ROSENTHAL AND VIN TANGPRICHA
During the last decade there has been increasing public aware­
ness of individuals whose g

creasing public aware­
ness of individuals whose gender identity is not aligned with 
their physical sex characteristics. Concurrently there has been an 
increasing number of gender nonconforming/transgender indi­
viduals seeking medical services to enable development of physical 
sex characteristics consistent with their affirmed gender identity. 
This chapter will review relevant terminology/definitions, diag­
nosis and prevalence of gender dysphoria (GD), mental health 
comorbidities, studies

phoria (GD), mental health 
comorbidities, studies that shed light on the biological determi­
nants of gender identity, current treatment models, barriers to 
care, and priorities for research.
Definitions and Diagnosis of Gender Dysphoria
Terminology and definitions used in this chapter are summarized 
in Table 21.1. 
Prevalence of Gender Nonconforming Identity
Based on state-level population-based surveys, a 2017 report from 
the Williams Institute of the University of California–Los Angeles 

tute of the University of California–Los Angeles 
School of Law indicated that 0.6% of US adults and 0.7% of 
youth age 13 to 17 years identify as transgender.1 A recent inter­
national review reports transgender prevalence estimates ranging 
from 0.5% to 1.3% of birth-assigned males and from 0.4% to 
1.2% of birth-assigned females, representing an estimate of 25 
million transgender people worldwide.2 Of note, there has been a 
striking inversion in the sex ratio of adolescents with GD, with a 

n in the sex ratio of adolescents with GD, with a 
predominance of birth-assigned females in recent years.3 
Biological Determinants of Gender Identity
Compelling data have emerged supporting the concept that 
gender identity is not simply a psychosocial construct; rather, 
it reflects a complex interplay of biological, environmental, and 
Introduction, 796
Care of Transgender Youth, 798
Care of Transgender Adults, 801
Barriers to Care and Priorities for Research for Transgender 

asculine adults with confirmed gender dysphoria will 
start on testosterone therapy.
 • 
Only limited data currently exist regarding long-term outcomes 
of pubertal suppression and of gender-affirming hormone use 
in transgender youth.
 • 
The long-term risks of gender-affirming hormone therapy are 
relatively few when hormone levels remain in the physiologic 
range; however, monitoring of potential adverse events is still 
recommended until long-term risks can be better ascertained.
797
Chapt

ng-term risks can be better ascertained.
797
Chapter 21  Transgender Endocrinology
  Definitions From the Endocrine Society 2017 Guidelines
Term
Biological sex, biological 
male or female
These terms refer to physical aspects of maleness and femaleness. As these may not be in line with each other (e.g., a person 
with XY chromosomes may have female-appearing genitalia), the terms biological sex and biological male or female are impre­
cise and should be avoided.
Cisgender
This means 

Cisgender
This means “not transgender.” An alternative way to describe individuals who are not transgender is nontransgender people.
Gender-affirming 
(hormone) treatment
See “gender reassignment.”
Gender dysphoria
This is the distress and unease experienced if gender identity and designated gender are not completely congruent. In 2013, the 
American Psychiatric Association released the fifth edition of Diagnostic and Statistical Manual of Mental Disorders (DSM-V), 

 Statistical Manual of Mental Disorders (DSM-V), 
which replaced gender identity disorder with gender dysphoria and changed the criteria for diagnosis.
Gender expression
This refers to external manifestations of gender, expressed through one’s name, pronouns, clothing, haircut, behavior, voice, or 
body characteristics. Typically, transgender people seek to make their gender expression align with their gender identity, rather 
than their designated gender.
Gender identity/experi­
enced gender
Th

ed gender.
This refers to one’s internal, deeply held sense of gender. For transgender people, their gender identity does not match their sex 
designated at birth. Most people have a gender identity of man or woman (or boy or girl). For some people, their gender identity 
does not fit neatly into one of those two choices. Unlike gender expression (to come), gender identity is not visible to others.
Gender identity disorder
This is the term used for gender dys

tity disorder
This is the term used for gender dysphoria/gender incongruence in previous versions of DSM (see “gender dysphoria”). The 
International Statistical Classification of Diseases and Related Health Problems, Tenth Edition (ICD-10) still uses the term for 
diagnosing child diagnoses, but the upcoming ICD-11 has proposed using gender incongruence of childhood.
Gender incongruence
This is an umbrella term used when the gender identity and/or gender expression differs from what is typicall

or gender expression differs from what is typically associated with the 
designated gender. Gender incongruence is also the proposed name of the gender identity–related diagnoses in ICD-11. Not all 
individuals with gender incongruence have gender dysphoria or seek treatment.
Gender variance
See “gender incongruence.”
Gender reassignment
This refers to the treatment procedure for those who want to adapt their bodies to the experienced gender by means of hormones 
and/or surgery. This is also cal

eans of hormones 
and/or surgery. This is also called gender-confirming or gender-affirming treatment.
Gender-reassignment 
surgery (gender-
confirming/gender-
affirming surgery)
These terms refer only to the surgical part of gender-confirming/gender-affirming treatment.
Gender role
This refers to behaviors, attitudes, and personality traits that a society (in a given culture and historic period) designates as mas­
culine or feminine and/or that society associates with or considers typical of th

society associates with or considers typical of the social role of men or women.
Sex designated at birth
This refers to sex assigned at birth, usually based on genital anatomy.
Sex
This refers to attributes that characterize biological maleness or femaleness. The best-known attributes include the sex-determin­
ing genes, the sex chromosomes, the H-Y antigen, the gonads, sex hormones, internal and external genitalia, and secondary 
sex characteristics.
Sexual orientation
This term describes an in

tics.
This term describes an individual’s enduring physical and emotional attraction to another person. Gender identity and sexual 
orientation are not the same. Irrespective of their gender identity, transgender people may be attracted to women (gynephilic), 
attracted to men (androphilic), bisexual, asexual, or queer.
Transgender
This is an umbrella term for people whose gender identity and/or gender expression differs from what is typically associated with 
their sex design

hat is typically associated with 
their sex designated at birth. Not all transgender individuals seek treatment.
Transgender male (also: 
transman, female-
to-male, transgender 
male)
This refers to individuals assigned female at birth but who identify and live as men.
Transgender woman 
(also: transwoman, 
male-to-female, 
transgender female)
This refers to individuals assigned male at birth but who identify and live as women.
Transition
This refers to the process during which transgender perso

fers to the process during which transgender persons change their physical, social, and/or legal characteristics consistent 
with their affirmed gender identity. Prepubertal children may choose to transition socially.
Transsexual
This is an older term that originated in the medical and psychologic communities to refer to individuals who have permanently 
transitioned through medical interventions or desired to do so.
From Hembree WC, Cohen-Kettenis PT, Gooren L, et al. Endocrine treatment of gen

is PT, Gooren L, et al. Endocrine treatment of gender-dysphoric/gender-incongruent persons: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 
2017;102:3869–3903; by permission of the Endocrine Society.
   
TABLE 21.1
798
cultural factors. Studies that shed light on biological determinants 
of gender identity are derived principally from three biomedical 
disciplines: genetics, endocrinology, and neuroscience, highlights 
of w

 endocrinology, and neuroscience, highlights 
of which are summarized in this chapter. The intent in focusing on 
biological underpinnings of gender identity is not to pathologize 
or to identify a mechanism that can be “fixed.” Instead, the pur­
pose is to underscore biological variation in gender identity that 
may in turn destigmatize gender-variant individuals and decrease 
health disparities for gender minorities.
With respect to genetics, a recent study notes heritability esti­
mates for g

 recent study notes heritability esti­
mates for gender identity in the range of 30% to 60%.4 A study 
in twins supporting a role for genetic factors in gender identity 
outcome demonstrated a 39.1% concordance for what was previ­
ously called gender identity disorder (based on the Diagnostic and 
Statistical Manual of Mental Disorders [Fourth Edition] [DSM-
IV] criteria) in 23 monozygotic female and male twin pairs, but 
no concordance in 21 same-sex dizygotic female and male twin 
pairs or in 

e-sex dizygotic female and male twin 
pairs or in 7 opposite-sex twin pairs.5 A number of investigators 
have sought to identify polymorphisms in specific candidate genes 
(e.g., the androgen receptor, estrogen receptor, and aromatase) 
that may be more prevalent in transgender individuals in com­
parison to a control population of nontransgender individuals; 
however, such findings have been inconsistent and lacking strong 
statistical significance.6–9
With respect to endocrine considerations, 

nce.6–9
With respect to endocrine considerations, it should be noted 
that most transgender individuals do not have a disorder/differ­
ence of sex development (DSD). However, studies in individuals 
with DSDs have informed our understanding of the role that hor­
mones (prenatal/early neonatal androgens, in particular) may play 
in gender identity development. For example, in studies of 46,XX 
individuals raised female with virilizing congenital adrenal hyper­
plasia (CAH) caused by mutations in 

drenal hyper­
plasia (CAH) caused by mutations in the CYP21A2 gene, there 
was a greater degree of female-to-male transgenderism in compari­
son to what would be expected based on prevalence estimates in a 
control population.10,11 In a meta-analysis of 250 adults with this 
condition raised female, while nearly 95% accepted a female gen­
der identity, 5.2% reported either a male gender identity or gen­
der dysphoria.11 This rate is higher than what would be expected 
for a transgender prevalenc

hat would be expected 
for a transgender prevalence of 0.6% to 0.7% in recent population 
estimates.1 A relationship between severity of disease and gender 
identity outcome was demonstrated in a study of adult 46,XX 
individuals with classic 21-hydroxylase deficiency: Three of the 42 
patients (7.1%) with the salt-wasting form had gender dyspho­
ria; no gender dysphoria was reported in the less severely affected 
individuals.10 A study in 46,XX youth with the same condition 
found that of 43 in

youth with the same condition 
found that of 43 individuals with either salt-wasting or simple 
virilizing CAH, 12.8% demonstrated cross-gender identification 
that was considered to be independent of gender role behavior.12 
In contrast, no cross-gender identification was noted in a nearly 
equivalent number of 46,XY individuals with virilizing CAH 
due to mutations in the CYP21A2 gene or in a nearly equivalent 
number of controls (age-matched relatives).12 A role for prenatal/
early natal andr

elatives).12 A role for prenatal/
early natal androgens in gender identity development is also sup­
ported by studies in a variety of other hormonal and nonhormonal 
DSDs, as recently reviewed.13
With respect to brain and gender identity, some studies in 
transgender individuals carried out before treatment with gen­
der-affirming cross-sex hormones have demonstrated that some 
sexually dimorphic structures (both gray and white matter) are 
more closely aligned with a person’s gender identity th

closely aligned with a person’s gender identity than with 
their physical sex characteristics.14,15 In addition, some functional 
studies (e.g., analysis of hypothalamic blood flow in response to 
smelling odorous compounds or brain imaging studies carried out 
during mental rotation tasks) have demonstrated that patterns 
typically observed to be sexually dimorphic were more closely 
aligned with gender identity than with physical sex characteris­
tics, even before cross-sex hormone treatment i

s­
tics, even before cross-sex hormone treatment in both transgender 
adults and adolescents.16–18 
Care of Transgender Youth
Gender dysphoria that either worsens or emerges with onset of 
physical puberty implies a high likelihood of a transgender iden­
tity during adulthood.19 Given the complexity of gender dyspho­
ria—and of adolescence itself—it is essential that gender dysphoric 
youth undergo a thorough evaluation by a qualified mental health/
gender specialist. A psychodiagnostic evaluati

th/
gender specialist. A psychodiagnostic evaluation is important not 
only to determine the presence or absence of gender dysphoria 
but also to assess for the presence of other mental health concerns 
that may co-occur. While the majority of gender dysphoric chil­
dren and adolescents do not have an underlying severe psychiatric 
illness, it is important to recognize that there is an association of 
gender dysphoria and autism spectrum disorder.19–21
Based on pioneering work from the Netherlan

.19–21
Based on pioneering work from the Netherlands, care of 
transgender youth has been primarily informed by clinical prac­
tice guidelines (CPGs) from the Endocrine Society (ES) and 
cosponsoring organizations and by standards of care (SOCs) 
from the World Professional Association for Transgender Health 
(WPATH).22,23 These documents endorse the use of pubertal 
blockers in GD adolescents using gonadotropin-releasing hor­
mone (GnRH) agonists at Tanner stage 2 of pubertal development 
(test

s at Tanner stage 2 of pubertal development 
(testicular volume ≥4 mL for assigned males at birth; initial stage 
of breast budding for assigned females at birth). Considered fully 
reversible treatments, GnRH agonists, by pausing puberty in this 
clinical setting, provide additional time for gender-identity explo­
ration without the pressure of continued pubertal progression and 
prevent irreversible development of secondary sex characteristics 
associated with puberty that are not aligned with

associated with puberty that are not aligned with the person’s gen­
der identity. Such undesired physical changes include a promi­
nent Adam’s apple, lowered voice, male bone configuration, and 
potentially tall stature in assigned males at birth; conversely, breast 
development, female body habitus, and potentially short stature in 
assigned females at birth.22 A recommended baseline and follow-
up protocol for physical examination and laboratory monitor­
ing during pubertal suppression is out

ry monitor­
ing during pubertal suppression is outlined in Table 21.2. While 
GnRH agonists are the preferred option for pubertal suppression, 
this treatment is costly and often inaccessible. Other options for 
pubertal suppression include depot and oral progestins.22,24
Adolescents who continue to meet the criteria for gender 
incongruence and gender dysphoria may request phenotypic tran­
sition with sex steroids. While age-specific recommendations for 
initiation of sex steroids in gender dys

ions for 
initiation of sex steroids in gender dysphoric adolescents are not 
delineated in the WPATH SOCs, the most recent version of the 
ES CPGs recommends initiating treatment “using a gradually 
increasing dose schedule after a multidisciplinary team of medi­
cal and mental health professionals has confirmed the persistence 
of gender dysphoria/gender incongruence and sufficient mental 
health capacity to give informed consent, which most individuals 
have by age 16 years.”22 Table 21.3 pro

dividuals 
have by age 16 years.”22 Table 21.3 provides a protocol for induc­
tion of puberty in such adolescents; Table 21.4 provides recom­
mendations for baseline and follow-up physical examination and 
laboratory evaluation during induction of puberty.
An important change in the current ES guidelines is the 
acknowledgment that there may be compelling reasons to start 
799
sex hormone treatment before the age of 16 years in some adoles­
cents with gender

 age of 16 years in some adoles­
cents with gender incongruence and gender dysphoria.22 Delay­
ing sex hormone treatment until age 16 years in an adolescent 
whose puberty was blocked at Tanner stage 2 could not only be 
detrimental to bone health, but also keeping someone suspended 
in an early pubertal state until this age could isolate the individual 
further from age-matched peers, with potentially negative conse­
quences for emotional well-being.22
When puberty is initiated with gradually i

eing.22
When puberty is initiated with gradually increasing doses of 
sex steroids, the initial levels will not be sufficient to suppress 
endogenous sex steroids. In transgender females, it is therefore 
recommended that GnRH analog treatment (or an alternative 
antiandrogen) be continued until gonadectomy.22 Since some 
transgender young adults may opt not to have gonadectomy, 
long-term studies are necessary to examine the potential risks of 
prolonged GnRH analog treatment. In transgender ma

prolonged GnRH analog treatment. In transgender male adoles­
cents, GnRH analog treatment can typically be discontinued once 
adult levels of testosterone have been reached.22 If uterine bleed­
ing occurs, a progestin can be added.22,25
Some transgender adolescents first come to medical attention 
when they are already in the later stages of puberty or are postpu­
bertal. Transgender males can be treated with testosterone alone; 
transgender females are treated with a combination of estrogen 
an

les are treated with a combination of estrogen 
and an agent that blocks testosterone secretion and/or action 
using protocols similar to those used in transgender adults (see 
later discussion on hormone therapy in transgender adults).
With respect to estrogen treatment, 17β-estradiol (transdermal, 
oral, or parenteral) is preferred to conjugated (e.g., Premarin) or syn­
thetic estrogens (e.g., ethinyl estradiol) since conjugated and syn­
thetic estrogen levels cannot be monitored in the serum,

 estrogen levels cannot be monitored in the serum, and ethinyl 
estradiol is associated with an increased risk for venous thromboem­
bolic disease and death from cardiovascular causes in adult studies 
(see later discussion on hormone therapy in transgender adults).
Some transgender adolescents seek surgical procedures to bring 
their bodies into closer alignment with their gender identity. The 
  Endocrine Society Guidelines for Baseline 
and Follow-Up Protocol for Physical 
Examination and La

ollow-Up Protocol for Physical 
Examination and Laboratory Monitoring 
in Transgender Children During Pubertal 
Suppression With a GnRH Agonist
Every 3–6 Months
Anthropometry: height, weight, sitting height, blood pressure, Tanner 
stages
Every 6–12 Months
Laboratory: LH, FSH, E2/T, 25(OH)D
Every 1–2 Years
Bone density using DXA
Bone age on X-ray of the left hand (if clinically indicated)
25(OH)D, 25-Hydroxyvitamin D; DXA, dual-photon absorptiometry; E2, estradiol; FSH, follicle-
stimulating hor

try; E2, estradiol; FSH, follicle-
stimulating hormone; LH, luteinizing hormone; T, testosterone.
From Hembree WC, Cohen-Kettenis PT, Gooren L, et al. Endocrine treatment of gender-dys­
phoric/gender-incongruent persons: an Endocrine Society clinical practice guideline. J Clin 
Endocrinol Metab. 2017;102:3869–3903.
   
TABLE 21.2
  Endocrine Society Guidelines for Induction 
of Puberty in the Affirmed Gender
Induction of female puberty with oral 17β-estradiol, increasing the dose 
every 6 month

 17β-estradiol, increasing the dose 
every 6 months:
5 μg/kg/day
10 μg/kg/day
15 μg/kg/day
20 μg/kg/day
Adult dose = 2–6 mg/day
In postpubertal transgender female adolescents, the dose of 
17β-estradiol can be increased more rapidly:
1 mg/day for 6 months
2 mg/day
Induction of female puberty with transdermal 17β-estradiol, increasing 
the dose every 6 months (new patch is placed every 3.5 days):
6.25–12.5 μg/24 hours (cut 25-μg patch into quarters, then halves)
25 μg/24 hours
37.5 μg/24 hours
Ad

s, then halves)
Adult dose = 50–200 μg/24 hours
For alternatives once at adult dose, see Table 21.5.
Adjust maintenance dose to mimic physiologic estradiol levels 
(see Table 21.6).
Induction of male puberty with testosterone esters increasing the dose 
every 6 months (intramuscularly or subcutaneously):
25 mg/m2/2 weeks (or alternatively, half this dose weekly, or double 
the dose every 4 weeks)
50 mg/m2/2 weeks
75 mg/m2/2 weeks
100 mg/m2/2 weeks
Adult dose = 100

5 mg/m2/2 weeks
Adult dose = 100–200 mg every 2 weeks
In postpubertal transgender male adolescents, the dose of testosterone 
esters can be increased more rapidly:
	
75	 mg/2 weeks for 6 months
	
125 mg/2 weeks
Adjust maintenance dose to mimic physiologic testosterone levels 
(see Table 21.7).
From Hembree WC, Cohen-Kettenis PT, Gooren L, et  al. Endocrine treatment of gender-­
dysphoric/gender-incongruent persons: an Endoc

r-­
dysphoric/gender-incongruent persons: an Endocrine Society clinical practice guideline. J 
Clin Endocrinol Metab. 2017;102:3869–3903; adapted from Hembree WC, Cohen-Kettenis 
P, Delemarre-van de Waal HA, et al. Endocrine treatment of transsexual persons: an Endocrine 
Society clinical practice guideline. J Clin Endocrinol Metab. 2009;94:3132–3154.
   
TABLE 21.3
and Follow-Up Physical Examination and 
Laboratory Monitoring in Transgender 
Childre

and 
Children During Induction of Puberty
stages
In transgender males: hemoglobin/hematocrit, lipids, testosterone, 
25-hydroxyvitamin D
In transgender females: prolactin, estradiol, 25(OH)D
Bone mineral density (BMD) using dual-energy X-ray absorptiometry (DXA)
BMD should be mo

ft hand (if clinically indicated)
BMD should be monitored into adulthood (until the age of 25–30 years 
or until peak bone mass has been reached).
For recommendations on monitoring once pubertal induction has been 
completed, see Tables 21.5 and 21.6.
25(OH)D, 25-Hydroxyvitamin D.
Clin Endocrinol Metab. 2017;102:3869–3903

line. J 
   
TABLE 21.4
800
current ES guidelines recommend that gender-affirming genital 
surgery involving gonadectomy and/or hysterectomy not take 
place until the patient has reached at least 

ake 
place until the patient has reached at least 18 years of age or the 
age of legal majority in his or her country.22 With respect to breast 
surgery/mastectomy for transgender males, the current ES CPG 
notes that some adolescents may consider this procedure before 
age 18 years, though there is insufficient evidence to recommend 
a specific age requirement. Rather, the ES CPG suggests that cli­
nicians determine timing of this procedure based on the physical 
and mental health status of the

d on the physical 
and mental health status of the individual patient.22
Outcomes of Current Treatment Models for 
Transgender Youth and Potential Adverse Effects
Mental Health Outcomes
Only limited outcomes data are currently available to support the 
ES guidelines and WPATH SOCs for care of transgender youth. 
A study from the Netherlands evaluated the mental health in 55 
transgender adolescents/young adults at three time points: before 
the start of GnRH agonist treatment (average age 13.6 y

tart of GnRH agonist treatment (average age 13.6 years), at 
the start of cross-sex hormone treatment (average age 16.7 years), 
and 1 year after gender-reassignment surgery (average age 20.7 
years). Following the completion of this three-stage approach to 
care, gender dysphoria was resolved, general psychologic function­
ing steadily improved, and a sense of well-being based on stan­
dardized survey results was noted to be equal to or greater than 
that seen in age-matched controls from the g

than 
that seen in age-matched controls from the general population.26 
None of the study participants regretted treatment.26 
Potential Adverse Effects of Pubertal Blockers in Transgen­
der Youth
There are potential adverse effects of pubertal suppression with 
GnRH agonists in transgender youth. The primary risks include 
impaired bone mineral density (BMD), compromised fertil­
ity, and unclear effects on brain development, body mass index 
(BMI), and body composition, as recently reviewed.27

I), and body composition, as recently reviewed.27
Skeletal Health
With respect to skeletal health, a 6-year longitudinal study assessed 
BMD in 34 transgender adolescents (15 transgender females, 19 
transgender males) at the start of GnRH agonist treatment, at the 
start of cross-sex hormone treatment, and at age 22 years, follow­
ing gonadectomy at a minimum age of 18 years (at which time 
GnRH agonist was stopped and cross-sex hormone treatment 
was continued). During the 6-year study period,

t 
was continued). During the 6-year study period, there was a sig­
nificant decrease in lumbar spine areal BMD Z-scores (relative 
to natal sex) in the transgender females, with a similar trend in 
the transgender males, suggesting either a delay in attainment of 
peak bone mass or an attenuation of peak bone mass itself.28 The 
authors acknowledged potential study limitations, including a 
relatively small number of participants, use of relatively low doses 
of cross-sex hormones during the in

ely low doses 
of cross-sex hormones during the initial period of that phase of 
treatment, and a lack of information regarding vitamin D levels, 
dietary calcium intake, and weight-bearing exercise, all of which 
can influence BMD.28 Particularly during GnRH agonist treat­
ment, it would seem important to ensure adequate intake of cal­
cium and vitamin D to encourage weight-bearing exercise and to 
routinely monitor levels of 25(OH)D.24
A separate study investigated bone turnover markers as wel

te study investigated bone turnover markers as well as 
BMD in transgender adolescents who underwent GnRH agonist 
treatment followed by cross-sex hormone therapy (the latter at 
about the age of 16 years, similar to previous reports). At baseline, 
bone mineral apparent density (BMAD) in the lumbar spine was 
lower in young (bone age <15 years) transgender female adoles­
cents versus young (bone age <14 years) transgender male adoles­
cents.29 Bone turnover markers and BMAD Z-scores decreased 

one turnover markers and BMAD Z-scores decreased 
following GnRH agonist treatment in the younger transgender 
adolescents, while an increase in BMAD was seen after 24 months 
of cross-sex hormone treatment in the younger transgender ado­
lescents (bone age as previously noted) and older transgender ado­
lescents (bone age ≥15 years in transgender females and ≥14 years 
in transgender males).29
A 22-year follow-up study of the first described gender dys­
phoric adolescent, treated initially with

der dys­
phoric adolescent, treated initially with GnRH agonist and sub­
sequently with cross-sex hormones, found that BMD was in the 
normal range for both sexes when evaluated at 35 years of age.30 
Fertility
It is essential that an informed consent process and a discussion 
about implications for fertility precede any treatment of trans­
gender youth with pubertal blockers and/or gender-affirming sex 
hormones. Transgender adolescents may wish to preserve fertility, 
which may be otherwise co

 to preserve fertility, 
which may be otherwise compromised if puberty is suppressed at 
an early stage and the patient subsequently pursues physical tran­
sition with gender-affirming sex hormones. In vitro maturation 
of human germ cells has not yet been achieved, though promis­
ing studies have been carried out in mice.31 Late pubertal and 
fully pubertal adolescents have the option of cryopreservation of 
mature sperm or eggs. 
Brain
Few studies have thus far investigated potential adverse e

ies have thus far investigated potential adverse effects 
of GnRH agonists on the brain in transgender adolescents. A 
cross-sectional study in gender dysphoric adolescents treated with 
GnRH agonist, in comparison to untreated gender dysphoric 
adolescents, showed no significant compromise of executive func­
tioning, a developmental milestone thought to reflect prefrontal 
brain activation and to be typically achieved during puberty.32 
A 28-month longitudinal study in a single pubertal transge

th longitudinal study in a single pubertal transgender 
adolescent undergoing GnRH agonist treatment reported lack of 
significant variation in brain white matter fractional anisotropy, 
thought to be a measure of brain maturation, and a nine-point 
drop in operational memory testing after 22 months of pubertal 
suppression.33 Further longitudinal studies are needed to deter­
mine the impact of GnRH agonist treatment on brain develop­
ment and function in transgender adolescents. 
Body Mass Inde

nction in transgender adolescents. 
Body Mass Index and Body Composition
The impact of GnRH agonist treatment of transgender adoles­
cents on BMI and body composition has also been studied. While 
variable results have been reported with respect to BMI,28,34 
a decrease in lean body mass percentage and an increase in fat 
percentage were reported after 1 year of treatment with GnRH 
agonist in both transgender male and female adolescents.35 In 
an earlier study, 1 of 27 adolescents treated with 

n earlier study, 1 of 27 adolescents treated with GnRH agonist 
developed significant weight gain, though BMI was above the 
85th percentile before treatment.35 
Potential Adverse Effects of Gender-Affirming Sex Hor­
mones in Transgender Adolescents
Few studies have thus far evaluated potential adverse effects of 
gender-affirming sex hormones in transgender adolescents. A 
study from the Netherlands in 28 transgender female adolescents 
treated primarily with gradually increasing doses of 17β-e

primarily with gradually increasing doses of 17β-estradiol 
801
for 1 to 3 years described no change in blood pressure, BMI stan­
dard deviation score, lean body mass percentage, or fat percentage 
(despite the presence of a desired female pattern of fat distribu­
tion). No abnormalities were seen with creatinine or with liver 
enzymes, and there was no change in hematocrit or hemoglobin 
A1c. One individual developed hyperprolactinemia, though this 
was see

developed hyperprolactinemia, though this 
was seen in the context of high-dose ethinyl estradiol treatment to 
limit statural growth.36
A study from the United States assessed potential adverse effects 
of gender-affirming sex hormones in 116 transgender adolescents 
and young adults (44 transgender females and 72 transgender 
males treated with 17β-estradiol or testosterone, respectively). Fol­
lowing 17β-estradiol treatment, no abnormalities were reported 
in blood pressure, BMI, hemoglobin/h

ere reported 
in blood pressure, BMI, hemoglobin/hematocrit, lipids, renal and 
liver function studies, and prolactin. Following testosterone treat­
ment, there was an increase in BMI and hemoglobin/hematocrit 
(3 of 44 individuals developed supraphysiologic hematocrit lev­
els) and a decrease in high-density lipoprotein (HDL) cholesterol. 
No abnormalities were reported in blood pressure, renal and liver 
function studies, and hemoglobin A1c.37 
Care of Transgender Adults
Because adults are abl

Because adults are able to more clearly articulate their gender 
identity and most adults have a gender identity that is fixed, 
any physician with the knowledge and expertise to diagnose 
gender dysphoria can manage and prescribe gender-affirming 
hormone therapy in adults with gender incongruence/gender 
dysphoria (GI/GD). Endocrinologists and other physicians 
with expertise should evaluate individuals with GI/GD for eli­
gibility and readiness for hormone therapy 

r eli­
gibility and readiness for hormone therapy and should serve 
as a liaison between mental health providers and surgeons. 
Endocrinologists should follow updated national/international 
guidelines on how to initiate and monitor hormone therapy in 
GI/GD individuals and serve as a resource for medical profes­
sionals in the community. In some instances, co-management 
with an endocrinologist along with another hormone pre­
scriber may be necessary if there are other coexisting mental 
health

ssary if there are other coexisting mental 
health or medical comorbidities.
The initial encounter with a transgender adult should include 
a history of the duration and severity of the gender dysphoria and 
a careful review of chronic medical issues that can be exacerbated 
by hormone therapy. An assessment of the individual’s family and 
social support structure is important to determine if the timing 
of hormone therapy is appropriate (especially if not currently liv­
ing in the affirmed gend

lly if not currently liv­
ing in the affirmed gender role) or if additional social support is 
needed prior to starting hormone therapy. It is important to dis­
cuss any history of depression and to assess suicide since transgen­
der individuals are at increased risk.38,39 The prescription of an 
antidepressant medication or urgent referral to a mental health 
provider may be necessary. Finally, the endocrinologist should 
carefully review the letter provided from the mental health pro­
vider if

tter provided from the mental health pro­
vider if the individual has already been receiving counseling. It 
is not a requirement to have a mental health referral in adults as 
stated in the ES guidelines22; however, in cases where the severity 
and duration of gender dysphoria are not quite clear and where 
there may be other mental health concerns that impact the gender 
transition, a mental health referral may be helpful.
Follow-up visits with the endocrinologist should focus on the 
response

 the endocrinologist should focus on the 
response of the gender-affirming hormone therapy on the indi­
vidual’s gender dysphoria and mood. The endocrinologist should 
continue to assess the individual’s social support structure and 
environment. The endocrinologist should also assist in any legal 
matters that arise, such as gender or name change, and referral to 
surgical specialists for gender-affirming surgery when the timing 
is appropriate.
Hormone therapy protocols have been developed to 

Hormone therapy protocols have been developed to mimic 
the sex steroid (testosterone and estradiol) concentrations in the 
expected reference range for males and females. Many centers 
around the world have utilized different preparations and delivery 
methods of sex steroid hormones along with adjunctive treatments 
to achieve this goal. Over time, these protocols have been refined 
and adopted by other clinics around the world. These protocols 
involve careful titration of hormone doses acco

s 
involve careful titration of hormone doses according to the blood 
concentrations of sex steroid hormones until a steady-state con­
centration is achieved. The ES guidelines recommend a number 
of different hormone regimens that can be used for transgender 
  Endocrine Society Recommendations for 
Hormone Therapy in Transgender Adults
TRANSGENDER FEMALESa
Estrogen
Oral
Estradiol
2.0–6.0 mg/day
Transdermal
Estradiol transdermal patch (new 
patch placed every 3–5 days)
0.025–0.2 mg/day
Parente

ch placed every 3–5 days)
Parenteral
Estradiol valerate or cypionate
5–30 mg IM every 2 weeks
2–10 mg IM every week
Antiandrogens
100–300 mg/day
Cyproterone acetateb
25–50 mg/day
GnRH Agonist
3.75 mg SQ (SC) monthly
11.25 mg SQ (SC) 3-monthly
TRANSGENDER MALES
Parenteral testosterone
Testosterone enanthate or 
­cypionate 
100–200 mg SQ (intramuscu­
larly) every 2 weeks or SQ 
(subcutaneously) 50% per 
week
Testosterone undecanoatec 
1000 mg every 12 w

week
1000 mg every 12 weeks
Transdermal testosterone
Testosterone gel 1.6%d
50–100 mg/day
Testosterone transdermal patch
2.5–7.5 mg/day
aEstrogens used with or without antiandrogens or GnRH agonist.
bNot available in the United States.
c1000 mg initially followed by an injection at 6 weeks then at 12-week intervals.
dAvoid cutaneous transfer to other individuals.
dysphoric/gender-inco

ocrine treatment of gender-­
Clin Endocrinol Metab. 2017;102:3869–3903.
   
TABLE 21.5
802
adults (Table 21.5) and a protocol for monitoring for potential 
adverse events (Table 21.6). In general, when hormone therapy 
is taken under medical supervision, the risks of adverse events are 
low likely due to the careful attention not to exceed supraphysi­
ologic con

ful attention not to exceed supraphysi­
ologic concentrations.40 However, it is important to note that 
self-treatment with hormone therapy still remains common in 
countries where hormones can be obtained without prescription, 
particularly in the developing world.
Transfeminine Hormone Therapy
Estrogen
Several formations of estrogen have been used in the past, includ­
ing synthetic estrogens (e.g., diethylstilbestrol, ethinyl estradiol), 
conjugated estrogens (Premarin), or estradiol. Estrogen

gated estrogens (Premarin), or estradiol. Estrogens can be 
given by several routes of administration, including orally, trans­
dermally, or intramuscularly/subcutaneously (see Table 21.5). 
Studies have demonstrated that conjugated estrogens and syn­
thetic estrogens are associated with increased risks of thrombo­
embolism and thus are no longer recommended for use by the 
ES guidelines.22 Another issue with conjugated estrogens and syn­
thetic estrogens is their inability for physicians to mea

estrogens is their inability for physicians to measure these 
drugs in blood and/or estimate equivalent estradiol levels, which 
impairs efforts to avoid supraphysiologic exposure to estrogen.
The most commonly prescribed and common route of 
administration of estrogen is oral estradiol due to its conve­
nience, cost, and relatively good safety profile. The dose of oral 
estradiol is titrated to a serum estradiol range of 200 to 300 
pg/mL.41 Liang et al42 reported that oral estradiol doses in t

ng et al42 reported that oral estradiol doses in the 
range of 2 to 7.5 mg daily in combination with spironolactone 
will result in serum estradiol concentrations between 50 and 
200 pg/mL in transwomen followed in the United States for 
over 3 years. In transwomen followed in the European Net­
work for the Investigation of Gender Incongruence (ENIGI), 
oral estrogen is prescribed to those under age 40 at 4 mg daily 
in combination with cyproterone acetate; estradiol levels reach 
100 to 200 pg/

ne acetate; estradiol levels reach 
100 to 200 pg/mL.43
Transdermal estrogen, presumed to be less stimulatory of pro­
thrombotic proteins, is preferred in transgender women over the 
age of 40 due to the increased risk of thromboembolic disease.44 
In the Netherlands, all transgender women are switched to trans­
dermal estrogen after age 40.45 In an academic endocrinology 
practice in Austria, 162 transwomen were prescribed primarily 
transdermal 17β-estradiol. They found no cases of thromboem­

 17β-estradiol. They found no cases of thromboem­
bolism after a mean follow-up time of 64 months.46 However, 
in 676 transgender women prescribed oral estradiol followed in 
the United States, only one person developed a thromboembolism 
after a mean follow-up time of 1.9 years.47 Therefore it is unclear 
at this point if oral or transdermal formulations are equally as safe 
for use in transgender women. There have been no head-to-head 
studies that have studied these two methods of administrat

that have studied these two methods of administration. 
Testosterone-Lowering Agents
Most published transfeminine hormone regimens involve the use 
of a testosterone-lowering agent in combination with estrogen 
therapy (see Table 21.5). Testosterone-lowering agents allow for 
estrogen therapy to induce secondary sex characteristics unop­
posed from the actions of testosterone, and thus a lower dose of 
estrogen is required. There are three commonly prescribed testos­
terone-lowering agents, spir

ly prescribed testos­
terone-lowering agents, spironolactone, cyproterone acetate, and 
GnRH agonists.
Spironolactone is most commonly used in the United States 
for its widespread availability and ease of administration. It is 
taken orally once or twice a day in doses ranging from 100 to 
300 mg total daily (see Table 21.5). Spironolactone was devel­
oped for its antagonism of the mineralocorticoid receptor and 
as a potassium-sparing diuretic. Less is known about the precise 
mechanism by whi

 Less is known about the precise 
mechanism by which spironolactone lowers testosterone, but it is 
widely appreciated in cisgender men to cause gynecomastia likely 
due to antagonism of the androgen receptors, increased clearance 
of testosterone, increased conversion of testosterone to estradiol, 
and direct inhibition of testosterone production.48,49 Largely ret­
rospective studies based in the United States on the effectiveness of 
spironolactone to lower testosterone concentrations in trans

tone to lower testosterone concentrations in transgen­
der women report mixed results.42,50 Unfortunately, no prospec­
tive randomized controlled trial has been conducted to examine 
estrogen regimens with or without spironolactone.
In Europe and parts of Asia, cyproterone acetate has been 
commonly used as the testosterone-lowering agent. Cyproterone 
acetate is taken orally, between 25 and 50 mg daily. Cyproter­
one has progestin-like activity and lowers testosterone by lowering 

and lowers testosterone by lowering 
gonadotropin levels. A recent study in transgender girls found that 
cyproterone acetate monotherapy resulted in lower gonadotropins 
and lower total and free testosterone, which were associated with 
favorable physical changes, including less facial hair and increased 
breast growth.51 However, there have been some recent concerns 
associated with cyproterone acetate that are not seen with spirono­
lactone, including increased incidence of hyperprolactinemia

ncluding increased incidence of hyperprolactinemia and 
meningiomas.52–54
In the United Kingdom, GnRH agonists are primarily used to 
lower testosterone in transgender women.55 These medications are 
highly effective in lowering levels of gonadotropins and lowering 
testosterone to nearly undetectable levels. Some of the limitations 
of GnRH agonists are the high cost of therapy and the lack of 
long-term data with this approach. 
5α-Reductase Inhibitors
5α-Reductase inhibitors such as finasteri

nt).
	5.	 Consider bone mineral density testing at baseline. In individuals at 
low risk, screening for osteoporosis should be conducted at age 60 
years or in those who are not compliant with hormone therapy.
   
TABLE 21.6
803
since they do

hapter 21  Transgender Endocrinology
since they do not lower serum concentrations of testosterone and 
they have a higher cost. In addition, there have been concerns 
regarding an increased risk of sexual dysfunction and depression 
as reported in cisgender men.56–58 Since there is an increased rate 
of depression and suicide in the transgender population,38,39 these 
medications should not be used as antiandrogens in first-line 
therapies. However, 5α-reductase inhibitors may have a role in 
tr

er, 5α-reductase inhibitors may have a role in 
transgender women who have androgenetic alopecia and who do 
not achieve an adequate response to spironolactone.59 
Many transgender women request progesterone as part of their 
gender-affirming hormone regimen despite the lack of any high-
quality data to support its use. Several websites and forums have 
suggested that progesterone may enhance breast development 
and areolar darkening based on anecdotal reports. A population-
based s

 based on anecdotal reports. A population-
based survey of more than 100 transgender women living in New 
York City revealed that 17% of transgender women were taking 
progesterone.60 The Endocrine Society guidelines do not make a 
recommendation for the use of progesterone as part of a gender-
affirming hormone regimen. The Guidelines for the Primary and 
Gender-Affirming Care of Transgender and Gender Nonbinary 
People by the University of California–San Francisco (UCSF) 
Center of Excellence 

fornia–San Francisco (UCSF) 
Center of Excellence for Transgender Health mention the use of 
progesterone as a treatment option, with the caveat that there are 
limited high-quality data supporting its effectiveness.61 Concerns 
raised about the use of progesterone include data from cisgender 
postmenopausal women indicating increased risk of stroke when 
progesterone is taken in combination with conjugated estrogens.62 
There currently is insufficient evidence to support the use of pro­
gestero

icient evidence to support the use of pro­
gesterone for transgender women. Physicians and other clinicians 
should discuss the known risks of progesterone with transgender 
women who ask for progesterone therapy and make a shared deci­
sion of whether a trial of progesterone is indicated. 
Transmasculine Hormone Therapy
The main hormone used for transgender men for gender-affirm­
ing therapy is testosterone. Testosterone esters (cypionate or 
enanthate) can be given intramuscularly every 1 to 2

anthate) can be given intramuscularly every 1 to 2 weeks or 
subcutaneously weekly63 (see Table 21.5). In a small retrospective 
study of 22 transgender men who received both intramuscular 
and subcutaneous testosterone at some point during their gender 
transition, all preferred testosterone administration subcutane­
ously.63 Testosterone undecanoate is a longer-acting testosterone 
that can be given by intramuscular injection every 12 weeks and 
is also well tolerated among transgender men as 

is also well tolerated among transgender men as reported by a 
prospective study of 17 transgender men in Europe.64 Transder­
mal preparations of testosterone as gels or patches are also an 
option for treatment of transgender men. In Europe, but not 
in the United States, testosterone is available orally as testoster­
one undecanoate, which is becoming less commonly prescribed 
because of its short half-life, which requires dosing three to four 
times daily.
The goal of the testosterone thera

r 
The goal of the testosterone therapy is to reach serum tes­
tosterone concentrations in the range expected for cisgender 
men and to induce male secondary sex characteristics, including 
increased body and facial hair, deeper voice, increased muscle 
mass, and cessation of menses (see Table 21.6). Testosterone will 
also reduce (but not completely eliminate) the gender dysphoria, 
as the physical changes will better align with the patient’s gender 
identity. 
Potential Adverse Ef

 patient’s gender 
identity. 
Potential Adverse Effects of Gender-Affirming 
Sex Hormones in Transgender Adults
In general, gender-affirming hormone therapy is safe when pre­
scribed under medical supervision with monitoring of circulating 
hormone levels to ensure that these values remain in the physi­
ologic range (Table 21.7; see also Table 21.6).40 However, there 
have not been any head-to-head studies on which hormone prepa­
rations are the safest to use in this population, and increased ri

safest to use in this population, and increased risks 
of several conditions associated with gender-affirming hormone 
therapy have been identified and should be considered. 
Potential Risks Associated With Transfeminine 
Venous Thrombosis and Pulmonary Embolism
Thromboembolism is the most well-studied and most serious 
complication resulting from transfeminine hormone therapy, pri­
marily from estrogen. Earliest reports found a 45-fold increase in 
the expected incidence of veno

 serum testosterone every 3 months until levels are in the 
normal physiologic male rangea:
	
a.	 For testosterone enanthate/cypionate injections, the testosterone 
level should be measured midway between injections. The target 
level is 400–700 ng/dL to 400 ng/dL. Alternatively, measure 
peak and trough levels to ensure levels remain in the normal 
	
b.	 For parenteral testosterone undecanoate, testosterone should be 
measured just before the following injection. If the level is <

performed. If mastectomy is not performed, then 
consider mammograms as recommended by the American Cancer 
Society.
aAdapted from Lapauw B, Taes Y, Simoens S, et al. Body composition, volumetric and areal 
bone parameters in male-to-female transsexual persons. Bone. 2008;43:1016–1021; Ott J, 
Kaufmann U, Bentz EK, et al. Incidence of thrombophilia and venous thrombosis in transsexu­
als under cross-sex hormone therapy. Fertil Steril. 2010;93:1267–1272.
From Hembree WC, Cohen-Kettenis PT, Gooren

7–1272.
   
TABLE 21.7
804
estradiol and cyproterone acetate.44 After changing the route from 
oral to transdermal estrogen in transgender women above the age 
of 40, the risks of thromboembolism declined to 20-fold.45 A 
more recent re

oembolism declined to 20-fold.45 A 
more recent review of 10 cohort studies based in Europe and Can­
ada found a low incidence of thromboembolism in transgender 
women on oral estradiol in the range of 2 to 4 mg daily or trans­
dermal estradiol 0.1 to 0.2 mg/day.65 A cohort study from Aus­
tria found no thromboembolic events in 162 transgender women 
followed for a mean of 5 years exclusively treated with trans­
dermal estradiol, cyproterone acetate, and finasteride, suggest­
ing improved safety

ate, and finasteride, suggest­
ing improved safety with transdermal formulations of estrogen. 
In a systematic review of 23 studies, representing 3231 subjects, 
venous thromboembolism was found in 56 subjects and had a 
prevalence between 0% and 5% in the individual studies.66 The 
study subjects ranged between 19 and 44 years of age and were 
followed between 3 months and 41 years. In a European cohort 
of 214 transgender women, the cases of venous thromboembo­
lism before and after gender-aff

nous thromboembo­
lism before and after gender-affirming hormone therapy increased 
from 9.2 cases to 60.7 cases per 1000.66 In a large cohort based in 
the United States of more than 7000 transgender women followed 
in a managed health care system, the rates of venous thromboem­
bolism increased starting after 5 years following the initiation of 
estrogen therapy compared to a reference group of cisgender men 
and women.67
Estrogen is likely the cause for the increased incidence of 
thromboembo

 cause for the increased incidence of 
thromboembolism given its known mechanism for promoting 
prothrombotic factors and given evidence from clinical trials of 
cisgender women on estrogen therapy. However, there are other 
factors that increase the risk of thromboembolism in transgen­
der women, including the formulation of estrogen, the route of 
administration, postoperative state (especially following gender-
affirming surgery), and age.65 The ES guidelines recommend the 
use of estrogen co

he ES guidelines recommend the 
use of estrogen compounds that can be measured in blood and 
against synthetic or conjugated estrogens, which make monitor­
ing blood levels difficult.22 Transgender women should hold their 
estrogen therapy for a period of time prior to gender-affirming 
surgery, typically 2 to 3 weeks.65 
Myocardial Infarction and Cerebrovascular Accidents
Most cohort studies in transgender women show low rates of 
myocardial infarction (MI) or cerebrovascular accidents (CVA). I

arction (MI) or cerebrovascular accidents (CVA). In 
the Maraka et al systematic review,66 they found only 14 MIs and 
8 CVAs among 3231 subjects. However, in the European ENIGI 
cohort,67 the cases of MI and CVA found in transgender women 
initiating on gender-affirming hormone therapy increased from 
4.7 to 18.7 and 23.4 per 1000, respectively. In a cohort of over 
7000 transgender women followed in the United States, the risks 
of CVA appear to increase after 7 years following the initiation 

o increase after 7 years following the initiation 
of hormone therapy, while in contrast, there does not appear to 
be an increase in MI compared to cisgender women and men. 
The ES guidelines recommend that clinicians evaluate for risk of 
cardiovascular disease, but there are no current recommendations 
on prevention or treatment in transgender women who have risk 
factors.22 
In a systematic review of 29 studies, gender-affirming hormone 
therapy with estrogen in transgen

firming hormone 
therapy with estrogen in transgender women was associated with 
increased levels of triglycerides by 31.9 mg/dL (95% CI, 3.9–
59.9) at 24 months with no significant increase in low-density 
lipoprotein (LDL) or HDL cholesterol for up to 24 months. In 
transfeminine youth, no changes in triglycerides but an increase 
in HDL cholesterol were seen after 24 months.68 In another study 
of transfeminine youth, there were no changes seen in lipids after 
treatment for up to 35 months.3

n in lipids after 
treatment for up to 35 months.37 It is not known whether the 
changes in triglycerides seen in adult transgender women are clini­
cally meaningful and/or result in an increased risk of cardiovascu­
lar disease in the long term. 
Early reports of prolactinomas occurring in transgender women 
and animal studies suggesting that estrogen may induce the growth 
of lactotroph cells prompted the Endocrine Society to recommend 
screening for hyperprolactinemia in tr

 recommend 
screening for hyperprolactinemia in transgender women.69,70 
However, in the majority of these cases, the transgender women 
were also taking the antiandrogen cyproterone acetate. Defreyne 
et al demonstrated that prolactin levels declined significantly after 
discontinuation of cyproterone acetate therapy following orchiec­
tomy.53 Furthermore, in a study conducted in Canada, transgen­
der women who were prescribed cyproterone acetate had much 
higher increases in prolactin compared

e had much 
higher increases in prolactin compared to transgender women 
prescribed spironolactone as the antiandrogen drug.71 These 
studies suggest that cyproterone has a more stimulatory impact 
on prolactin secretion than estrogen. In fact, a recent study in 
the United States, where transgender women were treated with 
estrogen and spironolactone, demonstrated no significant rise in 
prolactin concentrations.54 
Adequate estrogen therapy is protective against bone loss in trans

n therapy is protective against bone loss in trans­
gender women. In an early study of 20 transgender women, lower 
luteinizing hormone (LH) and follicle-stimulating hormone 
(FSH) concentrations (markers of estrogen adequacy) were associ­
ated with a lower rate of decline in bone mineral density at the 
lumbar spine.72 In a cohort of 231 transgender women, 1 year 
of gender-affirming hormone therapy resulted in a significant 
increase in BMD at the lumbar spine and hip.73 A recent sys­
tematic 

the lumbar spine and hip.73 A recent sys­
tematic review of 13 studies representing 392 transgender women 
found that gender-affirming hormone therapy was associated 
with increased lumbar BMD at 12 and 24 months.74 However, 
despite the positive effects of gender-affirming hormone therapy 
on bone density, transgender women have been reported to have 
lower expected BMD prior to the initiation of hormone therapy.75 
Therefore BMD screening may be indicated in those transgender 
women who have r

e indicated in those transgender 
women who have risk factors for osteoporosis as recommended by 
the Endocrine Society.22 
Transgender women have been reported to develop breast can­
cer in a few case series, raising the concern that gender-affirming 
hormone therapy may be associated with increased risk of breast 
cancer.76 Data from the North American Association of Central 
Cancer Registries (NAACCR) report increased proportional inci­
dence ratios (PIR) of breast cancer compar

l inci­
dence ratios (PIR) of breast cancer compared to cisgender men 
(PIR 20.7: 95% CI, 15.0–27.9) but lower PIR of breast cancer 
compared to cisgender women (PIR 0.2: 95% CI, 0.1–0.2). Simi­
larly, in a large cohort of US transgender women followed at Kai­
ser Permanente, the risk of breast cancer was increased compared 
to cisgender men but the same as compared to cisgender women.77 
Liver Dysfunction
Early reports from Dutch transgender women found elevations 
in liver enzymes in 10% (88 o

en found elevations 
in liver enzymes in 10% (88 of 816) of the cohort initiated on 
gender-affirming hormone therapy. However, a third of these cases 
805
were related to hepatitis B infection and alcohol, and another 
quarter were only transient elevations lasting less than 6 months. 
The Endocrine Society initially recommended routine measure­
ment of liver enzymes in transgender women, but subsequent 
reports showed that most changes in liver function du

orts showed that most changes in liver function due to gender-
affirming hormone therapy were transient and did not result in 
any permanent liver abnormalities. Thus the recommendation 
for routine monitoring of liver enzymes was removed in the 2017 
guidelines update.22,37,78,79 
Potential Risks Associated With Transmasculine 
Erythrocytosis
Gender-affirming therapy with testosterone in transgender men 
is commonly associated with increases in red blood cell mass and 
hematocri

th increases in red blood cell mass and 
hematocrit due to stimulation of erythropoietin.80 Several cohort 
studies of transgender men report significant increases in hemoglo­
bin and hematocrit, but very few studies have demonstrated serious 
consequences from these increases.81–83 The ES guidelines recom­
mend maintaining hematocrit below 55% to avoid adverse events 
rising from increased red cell mass.22 Some clinics have advocated 
reduction of red cell mass with phlebotomy, but there are no

of red cell mass with phlebotomy, but there are no trials 
examining this approach in terms of improving safety.50 
Hyperlipidemia
A recent systematic review found that testosterone therapy in 
transgender men results in lowered HDL cholesterol, higher LDL 
cholesterol, and higher triglycerides.30 In transgender boys, testos­
terone therapy also results in lowered HDL cholesterol and higher 
triglycerides over a period of 2 years.34 However, there are insuf­
ficient data to determine if this adv

e are insuf­
ficient data to determine if this adverse pattern of lipids results 
in long-term cardiovascular events. Data from a cohort of trans­
masculine adults indicate that the risks of MI are not increased.67 
Routine pharmacologic therapy to improve lipid parameters is not 
indicated given the available evidence in low-risk individuals. 
Uterine and Cervical Cancer
The risk of cancers of the reproductive tract in transgender men 
appears to be comparable to the risk of cisgender women; th

o be comparable to the risk of cisgender women; therefore, 
the ES guidelines recommend screening based on remaining 
organs as recommended for the general population. Among trans­
masculine adults receiving care in the Kaiser Permanente system 
in the United States, the risk of cervical cancer was not increased 
compared to matched cisgender women.77 There were no cases of 
uterine cancer reported. 
Barriers to Care and Priorities for Research 
for Transgender Youth and Adults
While there have 

While there have been significant advances in our understanding 
and management of transgender youth and adults, many ques­
tions remain. There are limited efficacy and safety data regarding 
current treatment models, with minimal available data regard­
ing the use of GnRH agonists in gender-nonconforming youth 
younger than 12 years of age, of gender-affirming sex hormones 
in adolescents younger than 16 years of age, and long-term risks 
(>10 years) on cardiova

 age, and long-term risks 
(>10 years) on cardiovascular disease and cancer. Questions also 
arise regarding the dose and choice of hormone therapy in elderly 
transgender adults. Furthermore, GnRH agonists and gender-
affirming sex hormones are off-label for GI/GD youth and adults, 
can be expensive (GnRH agonists and transdermal preparations 
of hormones, in particular), and are often denied by insurance 
companies. In addition, while new clinical programs for the care 
of gender-incongruent y

cal programs for the care 
of gender-incongruent youth have emerged in many parts of the 
world in recent years, there are still relatively few such programs, 
and patients are often required to travel long distances to receive 
care. Gender-affirming surgical procedures are only performed at 
a few centers and are often not a covered benefit under insurance. 
The success and long-term complication rates of these procedures 
are not known. Finally, formal training for medical providers is 
not u

y, formal training for medical providers is 
not universally available, and there is often prejudice and mis­
understanding. Further training and further prospective studies 
focused on long-term safety and efficacy are needed to optimize 
health care for transgender youth and adults.
         
806.e1
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Heijboer AC. Effect of puber

ein MA, Rotteveel J, 
Heijboer AC. Effect of pubertal suppression and cross-sex hormone 
therapy on bone turnover markers and bone mineral apparent den­
sity (BMAD) in transgender adolescents. Bone. 2017;95:11–19.
	30.	 Cohen-Kettenis PT, Schagen SE, Steensma TD, de Vries AL, Delemarre-
van de Waal HA. Puberty suppression in a gender-dysphoric adolescent: 
A 22-year follow-up. Arch Sex Behav. 2011;40(4):843–847.
	31.	 Yokonishi T, Sato T, Komeya M, et al. Offspring production with 
sperm grown

a M, et al. Offspring production with 
sperm grown in vitro from cryopreserved testis tissues. Nat Com­
mun. 2014;5:4320.
	32.	 Staphorsius AS, Kreukels BP, Cohen-Kettenis PT, et al. Puberty sup­
pression and executive functioning: an fMRI-study in adolescents 
with gender dysphoria. Psychoneuroendocrinology. 2015;56:190–199.
	33.	 Schneider MA, Spritzer PM, Soll BMB, et al. Brain maturation, cog­
nition and voice pattern in a gender dysphoria case under pubertal 
suppression. Front Hum Neuros

der nonconforming youth compared with their peers. 
Pediatrics. 2018;141(5).
	39.	 Blosnich JR, Brown GR, Shipherd Phd JC, Kauth M, Piegari RI, 
Bossarte RM. Prevalence of gender identity disorder and suicide risk 
among transgender veterans utilizing veterans health administration 
care. Am J Public Health. 2013;103(10):e27–e32.
	40.	 Weinand JD, Safer JD. Hormone therapy in transgender adults is safe 
with provider supervision; A review of hormone therapy sequelae for 
transgender individual

rmone therapy sequelae for 
transgender individuals. J Clin Transl Endocrinol. 2015;2(2):55–60.
	41.	 Tangpricha V, Hannema SE, Irwig MS, Meyer 3rd WJ, Safer JD, 
Hembree WC. 2017 American Association of Clinical Endocrinolo­
gists/Endocrine Society update on transgender medicine: case dis­
cussions. Endocr Pract. 2017;23(12):1430–1436.
806.e2
	42.	 Liang JJ, Jolly D, Chan KJ, Safer JD. Testosterone levels achieved by 
medically treated transgender women in a United States endocrino

C, Feustel PJ, Joseph J. Hormonal treatment of transgen­
der women with oral estradiol. Transgend Health. 2018;3(1):74–81.
	51.	 Tack LJW, Heyse R, Craen M, et al. Consecutive cyproterone ace­
tate and estradiol treatment in late-pubertal transgender female ado­
lescents. J Sex Med. 2017;14(5):747–757.
	52.	 Mancini I, Rotilio A, Coati I, Seracchioli R, Martelli V, Meriggiola 
MC. Presentation of a meningioma in a transwoman after nine years 
of cyproterone acetate and estradiol intake: case r

 among male-to-female transgender 
persons in New York City. Am J Public Health. 2009;99(4):713–719.
	61.	 Deutsch MB, ed. Guidelines for the Primary and Gender-Affirming 
Care of Transgender and Gender Nonbinary People. 2nd ed. Center of 
Excellence for Transgender Health, Department of Family and Commu­
nity Medicine, University of California San Francisco; 2016. Avail­
able at. www.transhealth.ucsf.edu/guidelines.
	62.	 Wassertheil-Smoller S, Hendrix SL, Limacher M, et  al. Effect 
of estro

, Hendrix SL, Limacher M, et  al. Effect 
of estrogen plus progestin on stroke in postmenopausal women: 
the Women’s Health Initiative: a randomized trial. JAMA. 
2003;289(20):2673–2684.
	63.	 Spratt DI, Stewart II , Savage C, et al. Subcutaneous injection of 
testosterone is an effective and preferred alternative to intramuscular 
injection: demonstration in female-to-male transgender patients. J 
Clin Endocrinol Metab. 2017;102(7):2349–2355.
	64.	 Jacobeit JW, Gooren LJ, Schulte HM. Safety a

 M. Physiologic 
response to gender-affirming hormones among transgender youth. 
J Adolesc Health. 2018;62(4):397–401.
	69.	 Gooren LJ, Harmsen-Louman W, van Kessel H. Follow-up of pro­
lactin levels in long-term oestrogen-treated male-to-female transsex­
uals with regard to prolactinoma induction. Clin Endocrinol (Oxf). 
1985;22(2):201–207.
	70.	 Mueller A, Gooren L. Hormone-related tumors in transsexuals 
receiving treatment with cross-sex hormones. Eur J Endocrinol. 
2008;159(3):197–202.
	7

rmones. Eur J Endocrinol. 
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Differential effects of cyproterone acetate vs spironolactone on 
serum high-density lipoprotein and prolactin concentrations 
in the hormonal treatment of transgender women. J Sex Med. 
2016;13(11):1765–1772.
	72.	 van Kesteren P, Lips P, Gooren LJ, Asscheman H, Megens J. Long-
term follow-up of bone mineral density and bone metabolism in 
transsexuals treated with cross-sex ho

abolism in 
transsexuals treated with cross-sex hormones. Clin Endocrinol (Oxf). 
1998;48(3):347–354.
	73.	 Wiepjes CM, Vlot MC, Klaver M, et  al. Bone mineral density 
increases in trans persons after 1 year of hormonal treatment: a 
multicenter prospective observational study. J Bone Miner Res. 
2017;32(6):1252–1260.
	74.	 Singh-Ospina N, Maraka S, Rodriguez-Gutierrez R, et  al. Effect 
of sex steroids on the bone health of transgender individuals: a 
systematic review and meta-analysis. J C

ui TA, et  al. Cohort study 
of cancer risk among insured transgender people. Ann Epidemiol. 
2017;27(8):499–501.
	78.	 Roberts TK, Kraft CS, French D, et  al. Interpreting laboratory 
results in transgender patients on hormone therapy. Am J Med. 
2014;127(2):159–162.
	79.	 Wierckx K, Van Caenegem E, Schreiner T, et al. Cross-sex hormone 
therapy in trans persons is safe and effective at short-time follow-up: 
results from the European network for the investigation of gender 
incongruence. J S

for the investigation of gender 
incongruence. J Sex Med. 2014;11(8):1999–2011.
	80.	 Bachman E, Travison TG, Basaria S, et  al. Testosterone induces 
erythrocytosis via increased erythropoietin and suppressed hepcidin: 
evidence for a new erythropoietin/hemoglobin set point. J Gerontol 
A Biol Sci Med Sci. 2014;69(6):725–735.
	81.	 Ederveen EGT, van Hunsel FPAM, Wondergem MJ, van Puijen­
broek EP. Severe secondary polycythemia in a female-to-male trans­
gender patient while using lifelong hor

ale trans­
gender patient while using lifelong hormonal therapy: a patient’s 
perspective. Drug Saf Case Rep. 2018;5(1):6.
	82.	 Defreyne J, Vantomme B, Van Caenegem E, et al. Prospective evalu­
ation of hematocrit in gender-affirming hormone treatment: results 
from European network for the investigation of gender incongru­
ence. Andrology. 2018;6(3):446–454.
	83.	 Vita R, Settineri S, Liotta M, Benvenga S, Trimarchi F. Changes in 
hormonal and metabolic parameters in transgender subjects on 

d metabolic parameters in transgender subjects on cross-
sex hormone therapy: a cohort study. Maturitas. 2018;107:92–96.
808
Placental Development
Normal placentation requires a coordinated series of events begin­
ning with fertilization. The fertilization rate following unpro­
tected regular intercourse during a single menstrual cycle is 25% 
to 30%. However, in approximately one-third of conceptions 
there is either failure of implantation or clinical or subclinical 
spontaneous abortion.1
For

linical or subclinical 
For the first 5 days, preimplantation development takes place 
within the fallopian tube. During this period, the zygote under­
goes cleavage division and, at least through the eight-cell stage, the 
blastomeres remain totipotential. In the 16-cell stage, differentia­
tion of the innermost cells into the inner cell mass and the sur­
rounding cells into the trophectoderm occurs. The inner cell mass 
develops into the fetus, and the trophectoderm give

evelops into the fetus, and the trophectoderm gives rise to the 
placenta and membranes. On approximately day 5 or 6 after fer­
tilization, the blastocyst enters the uterus, but implantation does 
not occur for another 1 to 2 days. Implantation occurs after the 
zona pellucida disappears from around the embryo.2
Recent developments in embryo culture during in vitro fer­
tilization (IVF) have allowed embryos to mature to the blasto­
cyst stage, which in turn allows biopsy of the trophectoderm. Th

ich in turn allows biopsy of the trophectoderm. The 
trophectoderm cells are then tested for euploidy or aneuploidy 
and, if a suitable probe is available, single gene disorders.3
Implantation is a complex process that involves apposition of 
the microvilli present on the trophectoderm cells with pinocytes 
(fused microvilli) on the endometrial cells, followed by removal 
of fluid between the cells through pinocytosis by the endometrial 
cells, a process stimulated by progesterone.4 Progesterone

 process stimulated by progesterone.4 Progesterone synthesis 
by the corpus luteum is stimulated and sustained during this time 
and for the first 6 to 7 weeks of pregnancy by secretion of human 
chorionic gonadotropin (hCG) by the trophoblast cells. The hCG 
is first detected in the maternal serum 6 to 9 days after concep­
tion.5 Embryo attachment is enhanced through the expression of 
a variety of adhesion molecules, including mucins, integrins, and 
trophinin, a trophoblast-specific cell memb

, and 
trophinin, a trophoblast-specific cell membrane adhesion protein, 
as well as cytokines, growth factors, and a variety of transcription 
factors encoded by homeobox genes.4,6
Placental Development, 808
Maternal Adaptations to Pregnancy, 810
Maternal Endocrine Alterations, 812
Placental Hormone Production, 815
Endocrinology of Pregnancy and Parturition, 821
KEY POINTS
 • 
The impact of pregnancy on the endocrine system is profound 
and begins with very early production of 

ersal transplacental bidirectional trafficking of maternal, 
placental, and fetal cells and deoxyribonucleic acid (DNA) 
occurs. Detecting cell-free fetal DNA in maternal circulation 
enables noninvasive screening for fetal aneuploidy and other 
conditions. Persistent postpartum microchimerism has been 
linked to autoimmune endocrinopathies in later life in both 
mother and offspring.
22
Endocrine Changes in Pregnancy
JOSHUA F. NITSCHE, GLENN D. BRAUNSTEIN, AND SARAH L. BERGA
SECTION VI A    Mat

BRAUNSTEIN, AND SARAH L. BERGA
SECTION VI A    Maternal-Fetal
809
Chapter 22  Endocrine Changes in Pregnancy
After the trophoblast attaches to the endometrium during 
the “window of implantation” 6 to 10 days postovulation, the 
embryo invades the endometrium through a complex process 
involving matrix metalloproteinases and differentiation of the 
trophectoderm into cytotrophoblasts or syncytiotrophoblasts. The 
syncytiotrophoblasts are multinucleated cells formed by the 
fusion of cytotrophobl

leated cells formed by the 
fusion of cytotrophoblasts. The cytotrophoblasts form a column 
of cells that invade the endometrium, form anchoring villi, and 
enter the maternal vasculature, eventually replacing the endothe­
lial layer of the endometrial and myometrial spiral arterioles with 
a layer of cytotrophoblasts (vascular trophoblasts).7 This process 
converts the high-resistance, low-capacity uterine vessels into low-
resistance, high-volume vessels, essential for growth of the pla­
centa

me vessels, essential for growth of the pla­
centa and fetus.8 At the site of implantation, the endometrial cells 
undergo decidualization, enlarging and increasing their metabolic 
activity with enhanced production of tissue inhibitors or metal­
loproteinase, extracellular matrix proteins, cytokines, and growth 
factors that modulate the extent of trophoblast invasion and influ­
ence trophoblast function.4,5,9
The trophoblast cells secrete several angiogenic proteins, 
including vascular endoth

al angiogenic proteins, 
including vascular endothelial growth factor (VEGF), platelet-
derived growth factor (PDGF), and basic fibroblast growth fac­
tor (bFGF), which stimulate blood vessel development within 
the villi.10 The syncytiotrophoblasts form an outer layer of cells 
in the chorionic villi, between the cytotrophoblast cells and the 
maternal blood space on the exterior surface. Only three tissues 
separate fetal blood from maternal blood: (1) the endothelium 
of the fetal vessels in 

ood: (1) the endothelium 
of the fetal vessels in the villi, (2) connective tissue, and (3) the 
trophoblasts; this form of placentation is referred to as hemocho­
rial. Thus in addition to hCG secretion, which is responsible for 
maintaining early pregnancy, progesterone secretion, required for 
continuation of pregnancy after the luteal-placental shift, and the 
synthesis and secretion of other hormones and growth factors 
(Table 22.1), the syncytiotrophoblasts provide the major site for 
tran

cytiotrophoblasts provide the major site for 
transportation of oxygen and nutrients to and removal of waste 
from the fetus.
Substances are transferred across the placenta through tran­
scellular pathways that include carrier-mediated transport (e.g., 
immunoglobulin G through the Fcγ receptor) and simple extra­
cellular diffusion. The degree of transplacental passage of a hor­
mone from the mother to the fetus through diffusion depends on 
(1) the rate of placental blood flow; (2) the maternal

the rate of placental blood flow; (2) the maternal concentration 
of the free or readily dissociable hormone; and (3) the molecular 
mass, lipid solubility, charge, and degree of placental metabolic 
degradation of the hormone. Maternal-to-fetal transfer occurs for 
hormones smaller than 700 Da, but the placenta is not permeable 
to hormones larger than 1200 Da.11
The trophoblast also anchors the placenta and fetus to the 
uterus and helps to protect the fetus, which contains paternal anti­
gens

tect the fetus, which contains paternal anti­
gens, from rejection by the maternal immune system. This immu­
nologic protection may be mediated by high concentrations of 
progesterone at the trophoblast-maternal interface and the expres­
sion by the trophoblasts of a histocompatibility complex antigen, 
human leukocyte antigen G (HLA-G), which exhibits reduced 
polymorphism in comparison with other major HLA antigens.12 
The mass of the trophoblast increases logarithmically during the 
first tri

st increases logarithmically during the 
first trimester, followed by a more gradual increase throughout 
the remainder of pregnancy. Trophoblastic mass closely correlates 
with maternal serum concentrations of human placental lactogen 
(hPL) and pregnancy-specific β1-glycoprotein throughout preg­
nancy and with hCG during the first trimester but not during the 
subsequent trimesters.13
Sex Differences in the Placenta
There is now a substantial body of literature describing physi­
ologic differe

ody of literature describing physi­
ologic differences between men and women and how these dif­
ferences impact numerous acute and chronic conditions and 
diseases. Some of these sex differences, such as differences in life 
  Placental Products
Hypothalamic Analogs
Gonadotropin-releasing hormones
Urocortin
Growth hormone–releasing hormone
Ghrelin
Dopamine
Neuropeptide Y
Enkephalin
Pituitary Analogs
Chorionic gonadotropi

Chorionic gonadotropin
Placental lactogen
Chorionic corticotropin
α-Melanocyte-stimulating hormone
Placental variant growth hormone
Oxytocin
Estrogens (estriol, estradiol, estetrol)
Other
Activins
Inhibins
Relaxin
Leptin
Parathyroid hormone–related protein
Renin
Nitric oxide
Transforming growth factor β
Tumor necrosis factor α
Epidermal growth factor
Insulin-like

osis factor α
Insulin-like growth factor type 1
Insulin-like growth factor type 2
Insulin-like growth factor binding protein 1
Colony-stimulating factor 1
Basic fibroblast growth factor
Corticotropin-releasing hormone-binding protein
Platelet-derived growth factor
Vascular endothelial growth factor
Endothelin 1
Anandamide (endocannabinoid)
Hepatocyte growth factor
Oncomodulin
TABLE 22.1
810
SECTION VI    Endocrinology and the Life Span
span and differential risk for and o

the Life Span
span and differential risk for and outcomes related to chronic 
conditions such as cardiovascular and cerebrovascular disease, 
have been recognized for decades. Ongoing investigations have 
revealed a burgeoning list, suggesting that sex differences are the 
norm rather than the exception. There are sex differences in fre­
quency and manner of presentation as well as in pathophysiol­
ogy and pharmacology. Examples of diseases and conditions that 
demonstrate sex differences includ

onditions that 
demonstrate sex differences include systemic lupus erythematosus, 
thyroid disease, osteoporosis, diabetes mellitus, Alzheimer disease, 
and schizophrenia. Differences may be strictly biologic (sex) or 
due to sociocultural disparities (gender) interacting with biologic 
predilections.14 For example, women are at an increased risk of 
systemic lupus erythematosus, thyroid disease, and osteoporosis; 
men are at increased risk of cardiovascular disease and schizophre­
nia. There ar

rdiovascular disease and schizophre­
nia. There are also sex differences in responses to acute insults such 
as trauma, sepsis, and surgery, with women often responding more 
favorably than men.14–19
Two important determinants of pathophysiology of the con­
ditions and diseases are sex and age. We now recognize that sex 
differences begin during fetal and early neonatal life.20–25 Thus sex 
differences interact with aging from conception. The epidemiology 
surrounding the impact of these very ea

demiology 
surrounding the impact of these very early events is often referred 
to as the developmental origins of health and disease (DOHAD) or 
fetal origins of adult disease (FOAD). Sex differences in the funda­
mental processes mediating embryogenesis and intrauterine fetal 
development are just beginning to be understood. Insults during 
intrauterine life such as natural disasters, death of a family mem­
ber, and severe maternal anxiety or depression are associated with 
an increased risk o

epression are associated with 
an increased risk of neurodevelopmental disorders (depression, 
anxiety, schizophrenia, and autism) in the offspring.26 A classic 
epidemiologic example is the famine produced by the invasion of 
the Netherlands in 1940.27 Women in their second trimester dur­
ing this time had male but not female offspring with an increased 
risk of schizophrenia as adults. In most cases female offspring are 
more resilient and male offspring more susceptible to the negative 
effec

 offspring more susceptible to the negative 
effects of gestational insults. Recent data suggest that sex differ­
ences in developmental risk are mediated by sex differences in the 
placental response to common exposures and stressors. It is often 
not appreciated that because the placenta derives from the same 
cell mass as the embryo that the placenta has a sex. However, the 
placenta does not have a gender because gender refers to sociocul­
tural constructs and behaviors related to sex-relate

ral constructs and behaviors related to sex-related expectations 
and stereotypes.
Sex chromosomes and gonadal steroids are thought to play a large 
role in mediating sex differences in placental and fetal responses to 
exposures.28 Total and free testosterone levels are higher in male 
fetuses as expected; however, so are levels of the female sex hor­
mone estradiol.29 However, testosterone is a more potent inhibitor 
of cytokine production in explants from female compared to male 
placentas, s

xplants from female compared to male 
placentas, suggesting female fetuses may be more sensitive to the 
effects of testosterone than male fetuses.28 Indeed, sex-specific hor­
mone action may be the norm and not just confined to sex steroids.
In addition to global sex differences described earlier, evidence 
is accumulating that epigenetic modifications also play a role in 
gating differential sensitivity between the sexes to common expo­
sures. At baseline, female placentas demonstrate increase

At baseline, female placentas demonstrate increased expres­
sion of immune-regulating genes (JAK1, IL2RB, Clusterin, LTBP, 
CXCL1, and ILR1L1), while in males, placental genes reported 
to mediate graft versus host disease and inflammation (HLA-
DQB1, HLA-DOA1, HCP5, NOS1, FSTL3, among others) are 
more highly expressed.30 Overall the sex differences in physiology 
appear to produce a less reactive female and more reactive male 
placental phenotype. This would better equip female placentas to 
b

pe. This would better equip female placentas to 
buffer the embryo from insults during critical periods of develop­
ment. The protection provided by a female placenta is in keeping 
with the long-known improved mortality and morbidity due to 
prematurity in female compared to male neonates.31–33
Although sex differences in gene expression profiles that drive 
placental structure and function are now well described, epigen­
etic changes also play a role. Studies using the established mouse 
model

a role. Studies using the established mouse 
model of early prenatal stress have identified DNA methyltrans­
ferases (DNMT)34 and O-linked N-acetylglucosamine transferase 
(OGT)35,36 as top candidates for primary roles in the epigenetic 
changes. A major function of OGT is to stabilize enhancer of zeste 
homolog 2 (EZH2), a histone H3K27 methyltransferase that 
leads to global repression of transcription. As OGT is X-linked 
and escapes X inactivation in the placenta, levels are twice as 
high i

ation in the placenta, levels are twice as 
high in the female placenta.35,36 The lower levels of OGT in 
males would lead to a globally reactive phenotype in the placenta. 
More targeted regulation of expression may be mediated through 
DNMT1, which, in conjunction with the methyl-binding protein 
MeCP2, results in DNA methylation and repression of specific 
genes. DNMT1 is more highly expressed at baseline in the female 
placenta and is further increased with maternal stress in female but 
not

 increased with maternal stress in female but 
not in male placentas.34 Again this would further contribute to a 
reactive male placental phenotype that could leave male fetuses at 
greater risk of complications from maternal stress. 
Maternal Adaptations to Pregnancy
To some extent, every maternal organ system is altered in preg­
nancy. The impact of pregnancy on the endocrine system is pro­
found and begins very early with the production of hCG from 
the trophoblast at implantation. The effect

 from 
the trophoblast at implantation. The effects on many other organ 
systems are more gradual and may not appear until later in preg­
nancy. While the vast majority of these changes are hormonally 
mediated, some organ systems are also affected by the anatomic 
alterations caused by the enlarging uterus or the physiologic 
increase in maternal blood volume. It is critically important for 
the clinician to understand these changes when caring for preg­
nant women, as many physical exam findin

for preg­
nant women, as many physical exam findings, laboratory values, 
and imaging findings that would be viewed as abnormal and of 
great concern outside of pregnancy can be normal and reassuring 
in pregnant women.
Physiologic Adaptations
During pregnancy some amount of weight gain is expected, with 
overweight and obese women expected to gain less than normal or 
underweight women. Excessive gestational weight gain has been 
associated with a number of adverse neonatal37,38 and maternal 
o

 a number of adverse neonatal37,38 and maternal 
outcomes.39–43 Given this association and the increasing preva­
lence of obesity among women of childbearing age, the Institute 
of Medicine (IOM) recommended an updated set of guidelines 
regarding weight gain in pregnancy based on the woman’s pre-
pregnancy body mass index44 (Table 22.2).
In recent years many experts and medical professional organi­
zations with a stake in the obesity epidemic have proposed fur­
ther subdividing obesity (body ma

ve proposed fur­
ther subdividing obesity (body mass index [BMI] >30) into class 
I (BMI 30–34), class II (BMI 35–39), and class III (BMI ≥40).45 
The IOM guidelines did not further refine their recommenda­
tions for weight gain based on level of obesity, citing a lack of 
evidence regarding proper weight gain in women with very high 
BMI. While this more specific classification system is helpful for 
811
consistency in categorizing patients for epidemi

y
consistency in categorizing patients for epidemiologic or clinical 
studies, it is not clinically meaningful as the impact of obesity 
on maternal health has a much more linear relationship than the 
abrupt classification system would suggest. As with the BMI clas­
sification schema proposed by the IOM, the subclassification of 
obesity also groups large numbers of patients into the high end 
of the scale (i.e., class III). Such patients are often encountered in 
current clinical practice as t

ten encountered in 
current clinical practice as the prevalence of women of childbear­
ing age with BMI over 40 is approximately 7.5% overall and as 
high as 15% among certain ethnic groups.46
It is important to remember that only a small portion of the 
recommended weight gain is due to an increase in maternal adi­
pose stores. For example, in a normal weight woman with a weight 
gain of 12.5 kg, 9.25 kg are due to factors other than maternal adi­
pose stores (fetus accounts for about 3.4 kg, p

i­
pose stores (fetus accounts for about 3.4 kg, placenta for 0.65 kg, 
amniotic fluid for 0.8 kg, uterus for 1 kg, breasts for 0.4 kg, blood 
for 1.5 kg, extravascular fluid for 1.5 kg).14 Thus only 3.25 kg is 
due to the increase in maternal adipose stores. If one considers the 
weight gain goal for overweight and obese women (5–9 kg) and 
assumes that most will be expected to gain 9.25 kg of nonadipose 
weight during pregnancy, there should be a negligible increase and 
potentially a decrease

 a negligible increase and 
potentially a decrease in adipose stores in these women if IOM 
guidelines are followed.
The volume of the uterine cavity increases from 10 mL in the 
nonpregnant state to an average of 5 L at term, and blood flow 
through the uteroplacental circulation reaches 450 to 650 mL/
minute, approximately a 10-fold increase.47 To maintain appro­
priate perfusion of the mother and the fetal-placental unit, sys­
temic blood volume increases throughout pregnancy and is 40% 
to 4

me increases throughout pregnancy and is 40% 
to 45% higher at term than in the nonpregnant state. The plasma 
volume increases by about 45% to 50% as a result of aldosterone-
stimulated sodium and water retention. The red cell mass increases 
approximately 20% because of increased production, resulting 
from a twofold to threefold increase in erythropoietin secretion. 
The net effect is a physiologic decrease in the hematocrit by about 
15% at term.47
The increase in uterine blood flow, while e

erm.47
The increase in uterine blood flow, while essential to maintain 
the fetus and placenta during pregnancy, carries a risk of severe 
hemorrhage surrounding delivery. Laceration of a vessel during 
a vaginal delivery can lead to heavy bleeding due to the high rate 
of blood flow to the uterus, cervix, and vagina. Bleeding can be 
life threatening if it tracks into the retroperitoneum where a large 
amount of blood can accumulate before symptoms prompt an 
evaluation and confirmation of the 

oms prompt an 
evaluation and confirmation of the diagnosis. Damage to a branch 
of the uterine artery during a cesarean section can lead to bleed­
ing rapid enough to require intraoperative blood transfusion to 
replace ongoing blood loss. For these reasons, elective operations 
are typically avoided in the third trimester. Fortunately, there are 
also maternal adaptations that lessen the potential impact of these 
and more typical amounts of blood loss. The physiologic drop in 
hematocrit duri

ood loss. The physiologic drop in 
hematocrit during pregnancy leads to less red cell mass lost per a 
given volume of blood. This allows women to maintain oxygen-
carrying capacity after volumes of blood loss that would lead to 
advanced stages of shock in nonpregnant individuals.48
The renal blood flow and glomerular filtration rate (GFR) 
increase rapidly and peak during the second trimester, and a 50% 
increase in creatinine clearance results in a reduction in the serum 
creatinine level. At

 in a reduction in the serum 
creatinine level. Atrial natriuretic peptide (ANP) levels increase 
during pregnancy and may be in part responsible for increased 
renal blood flow, GFR, 24-hour urine volume, and natriuresis.49 
An alteration in the osmotic thresholds for the release of vaso­
pressin and activation of the hypothalamic thirst centers, possibly 
caused by an extragonadal effect of hCG, lead to an approximately 
4% reduction in serum osmolality (∼10 mOsm/kg).50
It is important to cons

smolality (∼10 mOsm/kg).50
It is important to consider the increase in GFR and blood vol­
ume when treating women with medications that are cleared in the 
kidney. Many medications, both prescription and over the coun­
ter, are cleared in this way, including aminoglycosides, β-lactams, 
antiviral agents, antifungal agents, histamine (H1 and H2) block­
ers, among others. Some of these medications require dose adjust­
ments due to increased GFR of pregnancy. Adjustments in dose 
are particularly r

pregnancy. Adjustments in dose 
are particularly relevant for antiepileptic drugs, the plasma levels 
of which can become subtherapeutic if not monitored and steadily 
increased over the course of pregnancy. Although all the reasons 
for the increased drug requirement in pregnancy are unclear, it is 
likely that the increase in the volume of distribution is a contrib­
uting factor.51 This is also true of low-molecular-weight heparins, 
a preferred anticoagulant in pregnancy. Although only a smal

d anticoagulant in pregnancy. Although only a small 
portion of the drug is cleared in the kidney, the increased GFR 
of pregnancy is thought to contribute to the much larger doses 
required in pregnant women to maintain effectiveness.52
Several hemodynamic changes are induced by the low-resis­
tance, high-capacity uteroplacental vasculature, which, in many 
respects, acts like an arteriovenous shunt. The large quantities of 
estrogens, progesterone, prostaglandins, and angiotensin present 
duri

one, prostaglandins, and angiotensin present 
during pregnancy are thought to mediate these changes. Other 
changes include an increase in the heart rate by 10 to 15 beats/
minute, a 30% to 50% increase in cardiac output, resulting from 
increased stroke volume in early pregnancy and heart rate during 
the third trimester, a reduction in diastolic blood pressure of 10 
to 15 mm Hg with little or no change in systolic pressure, and an 
approximately 20% reduction in peripheral vascular resistance

ly 20% reduction in peripheral vascular resistance.47
While these increases in cardiac demand are easily met by most 
pregnant women, for some, particularly those with preexisting 
  Weight Gain During Pregnancy
TOTAL WEIGHT GAIN
RATES OF WEIGHT GAIN
SECOND AND THIRD TRIMESTERS
Pre-pregnancy BMI
Range in kg
Range in lb
Mean (range) in kg/week
Mean (range) in lb/week
Underweight (<18.5 kg/m2)
12.5–18
28–40
0.51 (0.44–0.58)
1 (1–1.3)
Normal weight (18.5–24.9 kg/m2)
11.5–16
25–35
0.42 (0.35–0.50)

 (18.5–24.9 kg/m2)
11.5–16
25–35
1 (0.8–1)
Overweight (25–29.9 kg/m2)
7–11.5
15–25
0.28 (0.23–0.33)
0.6 (0.5–0.7)
Obese (≥30 kg/m2)
5–9
11–20
0.22 (0.17–0.27)
0.5 (0.4–0.6)
TABLE 22.2
812
coronary artery or structural heart disease, the added strain can 
threaten the health of both mother and baby. Although women with 
a history of myocardial infarction (MI) with normal left ventricular 
function tolerate pregnancy well, women with p

ar 
function tolerate pregnancy well, women with poor hemodynamics 
with significant left ventricular injury are discouraged from attempt­
ing pregnancy as maternal morbidity and mortality are markedly 
increased.53 In women with structural heart disease, particularly 
Eisenmenger syndrome, the increase in cardiac output and blood 
volume can overwhelm an already impaired heart leading to arrhyth­
mia, congestive heart failure, pregnancy loss, and maternal death.54
In pregnancy the pulmonary vas

d maternal death.54
In pregnancy the pulmonary vascular resistance is reduced by 
about one-third, and pulmonary tidal volume increases by about 
30%. The latter, which results in a respiratory alkalosis that is 
compensated by increased bicarbonate excretion by the kidneys, 
increases in minute ventilatory volume by 30% to 40%. There are 
no changes in respiratory rate, maximum breathing capacity, or 
forced or timed vital capacity. However, there is an approximately 
40% reduction in the expir

re is an approximately 
40% reduction in the expiratory reserve because of the elevation 
of the diaphragm by the enlarged uterus.47
While these changes do not increase the risk of pregnant 
women contracting respiratory infections such as influenza and 
bacterial pneumonia, their morbidity and mortality are notably 
increased during pregnancy. This was most recently noted in the 
2009 H1N1 pandemic, in which pregnant women accounted for 
5% of the deaths but made up only 1% of the population.55

he deaths but made up only 1% of the population.55 Preg­
nant women were also more likely to be hospitalized with pneu­
monia,56 but they did not appear to be at an increased risk of 
infection compared to the general population.57
Gastrointestinal tract function is altered during pregnancy. 
Gastric emptying time is unchanged until labor, when it becomes 
prolonged. Reduced lower esophageal sphincter tone and displace­
ment of the abdominal contents by the pregnant uterus result in a 
marked in

ents by the pregnant uterus result in a 
marked increase in gastroesophageal reflux. Motility of the intes­
tine is also reduced, contributing to nausea and vomiting early in 
pregnancy and constipation that is common later in pregnancy. 
Decreased motility of the gallbladder leads to an increased gall­
bladder volume and reduced emptying of bile after meals, produc­
ing a more lithogenic bile and increasing cholelithiasis risk during 
pregnancy.47 
Metabolic Adaptations
Numerous changes in mate

47 
Numerous changes in maternal metabolism occur in pregnancy 
to ensure that the fetus has a consistent supply of metabolic fuel 
during in utero development. These changes include hyperin­
sulinemia, insulin resistance, increased plasma lipids, and more 
efficient plasma amino acid transport.58 As a result of these altera­
tions, maternal energy requirements are met predominately by 
lipolysis, allowing glucose and other carbohydrates to supply the 
energy needs of the f

carbohydrates to supply the 
energy needs of the fetus. This shift in maternal fuel utilization 
can be conceptualized as an accelerated starvation. Although car­
bohydrates are readily available in the maternal circulation, insu­
lin resistance reduces entry of glucose into maternal cells, thereby 
limiting her use of carbohydrates for energy. To compensate, 
maternal cells turn toward lipid metabolism for energy-producing 
levels of ketones similar to that seen after prolonged fasting.59 The 

ilar to that seen after prolonged fasting.59 The 
increased maternal low-density lipoprotein (LDL) and very low-
density lipoprotein (VLDL) that occur in pregnancy60,61 are also 
thought to be the main cholesterol supply for steroid hormone 
production in the placenta, as the placenta has very low hydroxy­
methylglutaryl–coenzyme A (HMG-CoA) reductase activity and 
thus limited ability to produce cholesterol from acetate in situ.62
The underlying causes of the shift in metabolic fuels during pre

 causes of the shift in metabolic fuels during preg­
nancy are largely understood, but significant gaps in knowledge per­
sist. It is clear that the increasing levels of hPL and human placental 
growth hormone (hPGH) reduce insulin receptors and glucose 
transport.63 However, to what extent glucagon and cortisol, which 
demonstrate strong diabetogenic effects outside of pregnancy, play a 
role in this process has not been thoroughly investigated. 
Maternal Endocrine Alterations
T

The anterior pituitary gland enlarges by an average of 36% dur­
ing pregnancy, primarily because of a 10-fold increase in lacto­
troph size and number. This enlargement increases the height and 
convexity of the pituitary that is observed on magnetic resonance 
imaging (MRI). Somatotroph and gonadotroph numbers are 
reduced, and there are no changes in corticotroph or thyrotroph 
numbers.64 The size of the posterior pituitary gland diminishes 
dur

e of the posterior pituitary gland diminishes 
during pregnancy.65
The marked increase in estrogen levels during pregnancy 
enhances prolactin synthesis and secretion, and maternal serum 
prolactin levels increase in parallel with the enlargement of the 
lactotrophs. At term, the mean serum prolactin concentration is 
207 ng/mL (range, 35–600 ng/mL), in contrast to a mean of 10 
ng/mL in nonpregnant, premenopausal women.66 Prolactin also 
is present in the amniotic fluid and appears to be primar

ent in the amniotic fluid and appears to be primarily of 
decidual origin because the decidua actively synthesizes prolactin. 
Amniotic fluid prolactin levels are 10 to 100 times higher than in 
the maternal circulation in early pregnancy. Serum prolactin lev­
els return to the baseline of nonpregnancy approximately 7 days 
after delivery in the absence of breastfeeding. With breastfeeding, 
the basal prolactin levels remain elevated for several months but 
gradually decrease; however, with suck

months but 
gradually decrease; however, with suckling, there is a brisk rise in 
prolactin levels within 30 minutes.64
Although rare, pituitary tumors do occur in women of child­
bearing age. While most are diagnosed prior to pregnancy, some 
are diagnosed for the first time in pregnancy. The most common 
type of tumor is a prolactinoma, which (as the name implies) is 
due to an overgrowth of lactotrophs. Prolactinomas, and other 
pituitary tumors, are typically classified based on size, with t

rs, are typically classified based on size, with those 
less than 10 mm in diameter designated microadenomas and those 
greater than 10 mm in diameter designated macroadenomas. The 
increase in pituitary size during pregnancy can lead to complica­
tions in women with prolactinomas. In a study of 352 women 
with untreated microadenomas, 2.3% experienced visual dis­
turbances, 4.8% experienced headache, and 0.6% experienced 
diabetes insipidus. The rates of complications were much higher 
in 144 w

 rates of complications were much higher 
in 144 women with macroadenomas, with 15.3% experiencing 
visual disturbances, 15.3% experiencing headache, and 2.14% 
experiencing diabetes insipidus.67 In cases of symptomatic tumor 
expansion in pregnancy, the dopamine agonist bromocriptine 
can be safely used and is successful in reducing tumor size and 
symptoms in the majority of cases.68 Cabergoline, a more selective 
dopamine 2 receptor agonist and the preferred treatment outside 
of pregnancy, a

d the preferred treatment outside 
of pregnancy, also appears safe, but there is less experience with 
this drug in pregnancy. Transsphenoidal resection of the pituitary 
is occasionally needed if rapid tumor growth occurs or symptoms 
persist despite bromocriptine treatment.
Growth hormone (GH) levels in maternal serum remain 
unchanged throughout pregnancy, although the source of 
immunoreactive GH during gestation changes due to placental 
813
produc

production of the hormone. Relaxin, secreted by the corpus luteum 
of pregnancy, and estrogens stimulate GH secretion during early 
pregnancy.69
Pituitary GH is known as GH1 or hGH-N; its messenger ribo­
nucleic acid (mRNA) and GH1 secretion decrease after week 25 
of pregnancy, and beginning in the fourth month of gestation, 
the placental syncytiotrophoblasts secrete a variant of GH (called 
GH2 or hGH-V) in a nonpulsatile pattern. In concert with th

H-V) in a nonpulsatile pattern. In concert with the 
different sources of GH during the first and second halves of preg­
nancy, the GH response to provocative stimuli differs in each half. 
Insulin hypoglycemia or arginine infusion results in an enhanced 
GH response during the first half of gestation, and during the 
second half the response is decreased compared with the response 
in nonpregnant women.64
Maternal serum concentrations of insulin-like growth factor 
type 1 (IGF1) are elevated du

-like growth factor 
type 1 (IGF1) are elevated during the second half of pregnancy, 
probably through the combined effect of the placental GH vari­
ant and hPL, which is evolutionarily related to GH and prolactin. 
hPL has somatotropic biologic activity, and its serum concentra­
tion increases throughout pregnancy, paralleling that of IGF1.70 
It is likely that the suppression of GH1 synthesis and secretion is 
caused by the high IGF1 concentrations, which in late pregnancy 
are five times high

ions, which in late pregnancy 
are five times higher than those in nonpregnant women.71
Although the placenta synthesizes and secretes biologically 
active gonadotropin-releasing hormone (GnRH), pituitary 
gonadotropin production decreases throughout pregnancy, as 
indicated by a marked reduction in gonadotropin immunoreac­
tivity in the gonadotrophs beginning at 10 weeks of gestation and 
by a reduction in serum levels of luteinizing hormone (LH) and 
follicle-stimulating hormone (FSH).64 Suppr

) and 
follicle-stimulating hormone (FSH).64 Suppression is probably 
mediated through the elevated blood levels of ovarian and placen­
tal sex steroid hormones and by placental production of inhibin. 
Suppression is incomplete because administration of exogenous 
GnRH leads to release of gonadotropins, although the response is 
blunted compared with that of nonpregnant women and does not 
return to normal until a month after birth.71
The mean concentrations of human thyroid-stimulating hor­
mon

ncentrations of human thyroid-stimulating hor­
mone (thyrotropin or hTSH) during the first trimester are sig­
nificantly lower than in the second and third trimesters or in the 
nonpregnant state.72 Much of this early decrease may be accounted 
for by the intrinsic thyrotropic activity of hCG. The nadir in TSH 
in maternal serum corresponds to the peak concentration of hCG 
at 10 to 12 weeks after the last menstrual period, demonstrat­
ing a reciprocal relationship between the rising hCG levels 

procal relationship between the rising hCG levels and 
falling hTSH concentrations72,73 (Fig. 22.1A). Higher hCG levels 
are associated with higher T4 levels (see Fig. 22.1B). Despite the 
lower mean hTSH during early pregnancy, the hTSH response 
to exogenous thyrotropin-releasing hormone (TRH) is normal.64
Maternal adrenocorticotropic hormone (ACTH, or corticotro­
pin) levels rise during pregnancy, increasing fourfold over con­
centrations in the nonpregnant state between 7 and 10 weeks of 
ge

he nonpregnant state between 7 and 10 weeks of 
gestation. There is a further gradual rise to weeks 33 to 37, when 
a mean 5-fold increase over prepregnancy values is found, fol­
lowed by a 50% drop just before parturition and a marked 15-fold 
increase during the stress of delivery.74 The ACTH concentra­
tion returns to the prepregnancy level within 24 hours of deliv­
ery. The pituitary gland and placenta are sources of circulating 
ACTH during pregnancy, and exogenous corticotropin-releasing 

pregnancy, and exogenous corticotropin-releasing 
hormone (CRH) stimulates the release of ACTH from both tis­
sues in a dose-dependent manner.47 Biologically active CRH is 
synthesized and secreted by the placenta and, to a lesser extent, 
by the decidual and fetal membranes, but unlike the inhibitory 
effect on pituitary CRH, glucocorticoids stimulate the expression 
of placental CRH.75
The relationship between CRH and ACTH is altered during 
pregnancy. Although biologically active CRH would be

egnancy. Although biologically active CRH would be expected 
to stimulate ACTH production, the qualitative patterns of CRH 
production, which show an exponential rise during the sixth 
month of gestation, and ACTH secretion, which demonstrates 
a more gradual rise during pregnancy, are quite different. The 
lack of a significant correlation between maternal plasma CRH 
and ACTH during pregnancy suggests that factors such as the 
elevated levels of free cortisol in the maternal serum may modu­
la

f free cortisol in the maternal serum may modu­
late the response to CRH. The circadian rhythm and the ability 
to respond to stress are maintained throughout pregnancy; how­
ever, the ACTH response to exogenous CRH during the third 
trimester is blunted, whereas the responsiveness to vasopressin is 
maintained, suggesting that the elevation of CRH in the maternal 
serum downregulates the responsiveness to CRH.74
Arginine vasopressin (AVP) concentrations in the maternal 
serum are similar to tho

trations in the maternal 
serum are similar to those in nonpregnant women. During preg­
nancy, however, there is increased synthesis of AVP, which is off­
set by the increased metabolic clearance of the hormone through 
destruction by a trophoblast-derived cysteine aminopeptidase 
(i.e., vasopressinase), which rises throughout pregnancy in parallel 
hTSH (mU/L) (       )
hCG (IU/L×1000) (       )
hCG (IU/L×1000)
Weeks of gestation
0.5
0
0
A
B
Free T4 (ng/dL)
Free T4 (pmol/L)
10
20
30
40
50
40
30

e T4 (ng/dL)
10
20
30
40
50
40
30
20
10
0
20
40
60
80
100
1.0
1.5
1.0
1.5
2.0
15
20
25
• Fig. 22.1  (A) Concentrations of serum thyrotropin (hTSH, filled circles) 
and human chorionic gonadotropin (hCG, open circles) throughout preg­
nancy. Between 8 and 14 weeks of gestation, there is a significant nega­
tive correlation between the individual hTSH and hCG levels (p <0.001). 
Each point represents the mean (± standard error). (B) Linear regression 
of maternal serum free thyrox

) Linear regression 
of maternal serum free thyroxine (T4) and hCG concentrations during the 
first half of gestation (p <0.001). (From Glinoer D, de Nayer P, Bourdoux P, 
et al. Regulation of maternal thyroid during pregnancy. J Clin Endocrinol 
Metab. 1990;71:276–287.)
814
with the increase in trophoblastic mass.76,77 The osmolar set-
point for thirst is reduced, and the release of AVP is related to the 
10-mOsm/kg average decrease in plasma osmola

 the 
10-mOsm/kg average decrease in plasma osmolality during preg­
nancy, possibly reflecting an extragonadotropic effect of hCG.48 
Taking into account the reduced set-point, the AVP response to 
dehydration and water loading is normal.
Oxytocin levels progressively increase in the maternal blood and 
parallel the increase in maternal serum levels of estradiol and proges­
terone. The levels increase further with cervical dilation and vaginal 
distention during labor and delivery, stimulating c

istention during labor and delivery, stimulating contraction of the 
uterine smooth muscles and enhancing fetal expulsion.78 Uterine 
oxytocin receptors also increase throughout pregnancy, resulting in a 
100-fold increase in oxytocin binding at term in the myometrium.79 
Thyroid Gland
The thyroid gland enlarges by an average of 18% during preg­
nancy.72 Enlargement is associated with an increase in the size of 
the follicles with increased amounts of colloid and enhanced blood 
volume. This enl

ts of colloid and enhanced blood 
volume. This enlargement may be a response to the thyrotropic 
effect of hCG and asialo hCG, which may account for some of 
the increase in serum thyroglobulin concentrations observed dur­
ing pregnancy. Although clinically contraindicated, experimental 
evidence suggests that enhanced iodine (131I) uptake by the mater­
nal thyroid gland reflects the combined effects of hCG stimula­
tion and reduction of the blood levels of iodide by increased renal 
iodide clea

d levels of iodide by increased renal 
iodide clearance.72
Rising estrogen concentrations during pregnancy induce 
increased hepatic synthesis of thyroxine-binding globulin (TBG) 
and enhanced sialylation of TBG, which decreases its metabolic 
clearance rate.73 The results are a twofold increase in TBG and 
increased total thyroxine (T4) and triiodothyronine (T3) levels in 
maternal serum throughout pregnancy,73 whereas for most of the 
gestation, the free T4 and free T3 concentrations remain no

, the free T4 and free T3 concentrations remain normal. 
There are no significant changes in the levels of thyroxine-bind­
ing prealbumin, but albumin levels are decreased because of the 
increase in vascular volume.
While hyperthyroidism is rather rare, hypothyroidism is rela­
tively common in women of childbearing age. When managing a 
pregnancy complicated by hypothyroidism, it is important to note 
that the pregnant woman is the source of T4 and T3 for the fetus 
throughout the first trimest

and T3 for the fetus 
throughout the first trimester. As a result, the maternal thyroid 
hormone replacement requirements increase as early as the fifth 
week of gestation. Although requirements often plateau in the 
second trimester, they continue to increase well into the third tri­
mester in many patients. Increased doses of thyroid hormone are 
needed in 50% to 85% of women, and the total dose may increase 
as much as 50%.80,81 Thus in women with preexisting hypothy­
roidism, it is necessary

with preexisting hypothy­
roidism, it is necessary to check thyroid function at the first prena­
tal visit to determine if additional thyroid hormone replacement 
is necessary. Alternatively, the dose can be empirically increased 
when pregnancy is confirmed prior to any laboratory assessment 
of thyroid function.82 Intermittent monitoring throughout preg­
nancy is warranted every 4 to 8 weeks to make certain replace­
ment remains adequate as gestation progresses. Routine screening 
for undiagno

tation progresses. Routine screening 
for undiagnosed hypothyroidism in pregnancy is controversial 
and is not currently endorsed by the American Thyroid Associa­
tion (ATA), American College of Obstetricians and Gynecolo­
gists (ACOG), or the Endocrine Society.83–85 Instead, a targeted 
screening approach has been adopted based on risk factors such as 
family or personal history of thyroid disease, prior head or neck 
radiation, morbid obesity, or age greater than 30 years.
Parathyroid Glands
D

or age greater than 30 years.
During pregnancy, approximately 30 g of calcium is transferred 
from the maternal compartment to the fetus, with most of the 
transfer occurring during the last trimester. Maternal total serum 
calcium levels decrease during pregnancy, with a nadir at 28 to 32 
weeks related to the decrease in albumin levels that accompanies 
the increase in vascular volume. However, the albumin-adjusted 
total calcium and the ionized calcium concentrations actual

cium and the ionized calcium concentrations actually rise 
slightly above this level in the nonpregnant state.86 The urinary 
calcium excretion rate increases in parallel with the increased GFR, 
and intestinal calcium absorption undergoes a twofold increase.86
Although some studies have suggested that parathyroid hor­
mone (PTH) levels increase during pregnancy, measurements of 
intact PTH levels by two-site immunometric assays indicate that 
they are within the normal, nonpregnancy range throu

ey are within the normal, nonpregnancy range throughout preg­
nancy. In contrast, the circulating concentrations of PTH-related 
protein (PTHrp) increase throughout pregnancy.87 Many normal 
tissues produce this protein, and the source of the elevated levels 
during pregnancy is unclear, although the two most likely sites are 
the mammary tissue and the placenta.87 This protein is probably 
involved in placental and mammary gland calcium transport.
The serum levels of 25-hydroxyvitamin D are unc

t.
The serum levels of 25-hydroxyvitamin D are unchanged dur­
ing pregnancy, but the estrogen-induced rise in vitamin D–binding 
globulin results in a twofold increase in 1,25-dihydroxyvitamin D 
(1,25[OH]2D) concentrations in maternal serum.86 There is also a 
rise in the biologically active free fraction of 1,25(OH)2D, which 
may reflect both increased maternal renal 1α-hydroxylase activity 
and the synthesis and secretion of 1,25(OH)2D by the placenta.86 
This increase in the active metabolit

placenta.86 
This increase in the active metabolite of vitamin D may be respon­
sible in part for the enhanced intestinal calcium absorption.
Although hyperparathyroidism and hypoparathyroidism are 
uncommon, they both can lead to serious complications in preg­
nancy. Pregnancy is somewhat protective in women with hyper­
parathyroidism because calcium uptake by the fetus helps lower 
maternal calcium levels. Although women with hypercalcemia 
in pregnancy often have mild to moderate symptoms of 

pregnancy often have mild to moderate symptoms of nausea, 
vomiting, pain, and renal colic,88 some experience more serious 
complications such as nephrolithiasis, pancreatitis, hypertension, 
bone disease, and hypercalcemic crisis.89 A crisis can be difficult to 
treat as many of the agents used in nonpregnant patients, such as 
bisphosphonates and plicamycin, are avoided because of concern 
for the fetus. Calcitonin can potentially be used as it does not cross 
the placenta; however, its benefi

 does not cross 
the placenta; however, its benefits and safety in pregnancy have 
not been sufficiently investigated.90
Hypoparathyroidism typically poses fewer serious risks, but it 
can lead to both maternal and fetal fractures if sufficiently severe.91,92 
These complications can generally be prevented with sufficient oral 
calcium and vitamin D supplementation. As mentioned earlier, a 
significant amount of maternal calcium is consumed by the fetus 
during gestation. This often requires a s

e fetus 
during gestation. This often requires a steady increase in calcium 
(1–1.5 mg of elemental calcium per day) and vitamin D (up to 
50,000–100,000 units/day or more) throughout pregnancy to 
maintain normal maternal serum calcium homeostasis.93 
Pancreas
Hyperplasia and hypertrophy of the beta cells in the islets of Lang­
erhans are probably the result of stimulation by estrogen and pro­
gesterone.94 During early pregnancy, the glucose requirements of 
the fetus lead to enhanced transport

uirements of 
the fetus lead to enhanced transport of glucose across the placenta 
by facilitated diffusion, and maternal fasting hypoglycemia may 
be present. Although basal insulin levels may be normal, there is 
815
hypersecretion of insulin in response to a meal. Because the half-life 
of insulin is not altered during pregnancy,95 this increase represents 
an increase in synthesis and secretion. The results are enhanced 
glycogen storage and decreas

results are enhanced 
glycogen storage and decreased hepatic glucose production.
As pregnancy progresses, the levels of hPL rise, as do the levels 
of glucocorticoids, leading to the insulin resistance found during 
the last half of pregnancy.96 Thus, in late pregnancy, glucose inges­
tion results in higher and more sustained levels of glucose and 
insulin and a greater degree of glucagon suppression than in the 
nonpregnant state. 
Adrenal Glands
As a result of the hyperestrogenemia of pregnanc

s
As a result of the hyperestrogenemia of pregnancy, hepatic pro­
duction of cortisol-binding globulin is increased. The increased 
production results in a doubling of the maternal serum levels 
of cortisol-binding globulin, which in turn results in decreased 
metabolic clearance of cortisol and a threefold rise in total plasma 
cortisol by week 26, when the levels reach a plateau until they 
rise at the onset of labor.74,97 The rate of cortisol production and 
the plasma free cortisol concentra

production and 
the plasma free cortisol concentrations are also increased.92 The 
enhanced cortisol production is due to an increase in the mater­
nal plasma ACTH concentrations and hyperresponsiveness of the 
adrenal cortex to ACTH stimulation during pregnancy.74 Cortisol 
secretion follows that of ACTH, and the diurnal rhythm is main­
tained during pregnancy.97 Despite the elevated free cortisol lev­
els, pregnant women do not develop the stigma of glucocorticoid 
excess, possibly because of 

ma of glucocorticoid 
excess, possibly because of the antiglucocorticoid activities of the 
elevated concentrations of progesterone.
Levels of androstenedione and testosterone, whether they are 
of adrenal or ovarian origin, are elevated because of the estrogen-
induced increase in hepatic synthesis of sex hormone–binding 
globulin. However, the free androgen levels remain normal or low. 
The adrenal production rates of dehydroepiandrosterone (DHEA) 
and dehydroepiandrosterone sulfate (DHEAS) ar

EA) 
and dehydroepiandrosterone sulfate (DHEAS) are increased 
twofold, but the maternal serum concentration of DHEAS is 
reduced to one-third to one-half the nonpregnancy levels because 
of the enhanced 16-hydroxylation and placental utilization of 
16-hydroxydehydroepiandrosterone sulfate in estrogen forma­
tion. Adrenal medullary function remains normal throughout 
pregnancy. Thus 24-hour urine catecholamine and plasma epi­
nephrine and norepinephrine levels are similar to concentrations 
in 

nephrine levels are similar to concentrations 
in the nonpregnant state.98
Fortunately, hypercortisolism is rare in pregnancy as the dis­
order often leads to menstrual disturbance and infertility. While 
outside of pregnancy hypercortisolism is most commonly caused 
by an ACTH-producing pituitary99 adenoma (so-called Cushing 
disease), when encountered in pregnancy it is more likely due to 
primary adrenal hyperplasia (so-called Cushing syndrome). This 
discrepancy may be due to the lower degre

). This 
discrepancy may be due to the lower degree of menstrual distur­
bance in patients with primary adrenal hyperplasia.100 The high 
levels of cortisol in both conditions can lead to hypertension, dia­
betes, preeclampsia, and even maternal death. Cushing syndrome 
in pregnancy it also associated with a 43% risk of preterm delivery 
and a 6% risk of stillbirth.74 
Renin-Angiotensin System
Plasma renin substrate levels are increased as a consequence of the 
effects of estrogen on the liver. 

equence of the 
effects of estrogen on the liver. Renin levels are also increased, and 
increased renin activity results in increased levels of angiotensin 
II, which leads to an 8-fold to 10-fold increase in aldosterone 
production and serum aldosterone levels.74 The aldosterone levels 
peak in midpregnancy and are maintained until delivery.
Despite their baseline elevations, the various components of 
the renin-angiotensin-aldosterone system demonstrate normal 
responses to positional changes,

monstrate normal 
responses to positional changes, sodium restriction, and sodium 
loading. The elevated aldosterone levels do not lead to an increase 
in serum sodium, a decrease in serum potassium, or an increase 
in blood pressure, which again may reflect the high progesterone 
concentrations, which are capable of displacing aldosterone from 
its renal receptors. Another mineralocorticoid, 11-deoxycorti­
costerone, shows a 6-fold to 10-fold increase in concentration 
at term.45 Elevated level

crease in concentration 
at term.45 Elevated levels of this hormone are due to estrogen-
induced extraglandular 21-hydroxylation of progesterone pro­
duced by the placenta.101
The prevalence of hypertension in women of childbearing 
age has steadily increased over time and is now a common prob­
lem encountered by obstetric providers. Outside of pregnancy, 
angiotensin-converting enzyme (ACE) inhibitors and angioten­
sin receptor blockers (ARBs) are widely used in the treatment of 
hypertension, 

re widely used in the treatment of 
hypertension, particularly in people with diabetes, as they have 
been shown to slow the progression of diabetic nephropathy.102 
ACE inhibitors have been linked to oligohydramnios, fetal renal 
dysplasia, and fetal calvarial hypoplasia.103 As a result, this group 
of medications is avoided in pregnancy. Although ARBs have not 
been shown to have the same adverse fetal effects, they are also 
typically avoided in pregnancy.104 Patients on these medications 
sh

in pregnancy.104 Patients on these medications 
should be switched to alternative agents prior to or in the first 
trimester of pregnancy. Methyldopa is the best studied antihyper­
tensive medication in pregnancy and is safe for use at any point 
in gestation.105 Although less well studied, the calcium channel 
blocker nifedipine appears to be safe for the fetus and is often used 
in pregnancy.106 Beta blockers are often used, but these medica­
tions have been associated with an increased risk o

ions have been associated with an increased risk of fetal growth 
restriction107 
Placental Hormone Production
Sex Steroid Production From the Maternal-Fetal-
Placental Unit
Sex steroid production in the adult ovary has been classically 
described as a “two-cell process,” wherein the theca cells convert 
cholesterol to progesterone, testosterone, and other androgens but 
are then unable to produce estrogen as the theca cells lack the 
crucial enzyme aromatase. This enzymatic obstacle is overcome

yme aromatase. This enzymatic obstacle is overcome by 
the nearby granulosa cells, which have an abundance of aromatase 
and take up androgens and rapidly convert them to estrogens.108 
A similar strategy using multiple locations and cell types is also 
utilized in the massively increased sex steroid machinery of preg­
nancy. In this system, both the maternal and fetal adrenal glands 
interact with the placenta to produce very large amounts of pro­
gesterone and estrogens. This dependence has le

o­
gesterone and estrogens. This dependence has led to the concept 
of the maternal-fetal-placental unit.109 These interactions are out­
lined in Fig. 22.2.110,111
The placenta, like the theca cell of the ovary, has the enzymes 
necessary to produce progesterone. However, the placenta has very 
low levels of HMG-CoA activity, so it utilizes cholesterol from 
the maternal circulation as a substrate for progesterone produc­
tion.112 Cholesterol is delivered to the placenta by VLDL, LDL, 
and HDL, 

delivered to the placenta by VLDL, LDL, 
and HDL, as receptors for all these lipoproteins are present in 
the syncytiotrophoblast. Cholesterol is converted to pregneno­
lone by the enzyme CYP11A1,109,113 which is in turn converted 
816
to progesterone by 3β-hydroxysteroid dehydrogenase (3βHSD). 
The placenta also lacks the enzyme 17α-hydroxylase and cannot 
convert progesterone to androgens even though it has sufficient 
aromatase activity to convert

h it has sufficient 
aromatase activity to convert androgens into estrogens.114 Thus 
progesterone is not further modified in the placenta, and approxi­
mately 90% is released into the maternal circulation. However, 
placental progesterone is also an important substrate for produc­
tion of glucocorticoids and mineralocorticoids in the fetal adrenal 
glands.109,113
The trophoblast lacks 17α-hydroxylase and 17,20-lyase 
(CYP17) activities and therefore cannot directly convert pro­
gesterone to est

fore cannot directly convert pro­
gesterone to estrogen. Pregnenolone produced in the placenta 
enters the fetal compartment, where it is taken up by the fetal 
zone of the adrenal cortex, which also synthesizes pregnenolone 
from fetal LDL cholesterol. Pregnenolone is conjugated with sul­
fate by fetal steroid sulfotransferase in the fetal liver and adrenals 
to form pregnenolone sulfate and is converted in the fetal adre­
nals to 17α-hydroxypregnenolone sulfate and then DHEAS by 
17α-hydroxyla

gnenolone sulfate and then DHEAS by 
17α-hydroxylase and 17,20-lyase (CYP17) activities.115
The DHEAS enters the fetal circulation and undergoes hydrox­
ylation in the fetal liver to form 16α-hydroxy-DHEAS, which 
is converted to 16α-DHEA in the placenta through the action 
of placental sulfatase. Further metabolism in the trophoblast by 
3βHSD1, 17βHSD, and aromatase (CYP19) leads to the genera­
tion of estriol, which is quantitatively the major estrogen in the 
maternal circulation during preg

 estrogen in the 
maternal circulation during pregnancy. The maternal liver actively 
conjugates estriol with glucosiduronate and sulfate, which are 
excreted into the urine. Approximately 90% of the estriol pres­
ent in the maternal serum and urine is derived from fetal precur­
sors, and therefore measurement of estriol levels in serum or urine 
serves as an index of fetal well-being.115
DHEAS from both the fetus and mother is also taken up by 
the placenta and converted to estradiol by the act

the placenta and converted to estradiol by the actions of sulfa­
tase, 3βHSD1, 17βHSD, and aromatase or to estrone by sulfatase, 
3βHSD1, and aromatase. An estrogen unique to pregnancy, estet­
rol, is generated by 15α-hydroxylation of 16α-DHEAS in the fetal 
adrenal followed by enzymatic conversion by placental sulfatase, 
3βHSD1, 17βHSD, and aromatase.115 
Protein Hormones
Chemistry
Human chorionic gonadotropin is a glycoprotein composed of 
two dissimilar subunits,

glycoprotein composed of 
two dissimilar subunits, α and β, which are noncovalently linked 
through hydrophobic bonding. This molecule shares structural 
homology with the other glycoprotein hormones, human lutein­
izing hormone (hLH), hFSH, and hTSH. These hormones have 
α-subunits that contain the same sequence of 92 amino acids and 
differ only in their carbohydrate composition; the β-subunits 
differ in both amino acid and carbohydrate structure and are 
responsible for the biologic and immu

ure and are 
responsible for the biologic and immunologic specificity of the 
heterodimeric (intact) hormones. The 22,200-Da β-subunit of 
hCG is composed of 145 amino acids. Approximately 80% of the 
first 115 amino acids are homologous to those in the β-subunit 
of hLH. hCG has an additional 24 amino acids on its carboxyl-
terminal end that enhance its biologic activity.
Both subunits of hCG contain two oligosaccharide chains 
attached to asparagine residues through N-glycosidic linkages, and 

agine residues through N-glycosidic linkages, and 
the β-subunit contains in addition four O-serine–linked oligosac­
charide units in the carboxyl-terminal peptide. The carbohydrate 
composition of hCG contains microheterogeneity and affects 
hormone clearance and biologic activity. The tertiary structure of 
hCG is determined by the carbohydrate composition and mul­
tiple disulfide bonds within each subunit. The β-subunit contains 
five disulfide bonds; the β-subunit has six. In each of the sub

e bonds; the β-subunit has six. In each of the subunits, 
three of the disulfide bonds form a cysteine knot, similar to that 
found in PDGFβ and transforming growth factor β (TGFβ).116 
Maternal compartment
sulfate
Sulfatase
Sulfatase
DHEA sulfate
DHEA
Arom
Arom
Arom
Liver
Liver
E2
E3
E3 sulfate
E3 glucosiduronate
E3 sulfaglucosiduronate
T,A
E1
Adrenal
Adrenal
scc
16α-hydroxy DHEA
16α-hydroxy
DHEA sulfat

renal
scc
Acetate
Placenta
Fetal compartment
3  HSD
β
3  HSD
β
3  HSD
β
17  HSD
β
17  HSD
β
• Fig. 22.2  Steroidogenesis in the maternal-fetal-placental unit. AROM, aromatase-enzyme complex; 
DHEA, dehydroepiandrosterone; HSD, hydroxysteroid dehydrogenase; SCC, cholesterol side-chain cleav­
age enzyme.
817
Biosynthesis
The single α-subunit gene, located on chromosome 6, is actively 
expressed in both the cytotro

some 6, is actively 
expressed in both the cytotrophoblast and syncytiotrophoblast. In 
contrast, the β-subunit is encoded by a cluster of six genes located 
on chromosome 19 in proximity to the hLHβ gene. Three of the 
hCGβ genes are actively transcribed during pregnancy, primar­
ily in the syncytiotrophoblast, which synthesizes and secretes free 
subunits and intact hCG. After synthesis of the protein core, each 
subunit is glycosylated, undergoes further post-translational mod­
ification thro

oes further post-translational mod­
ification through trimming of the carbohydrate, then combines to 
form intact hCG.116
Secretion of hCG differs from that of many of the other pla­
cental proteins, whose secretory pattern parallels that of the tro­
phoblastic mass. hCG is first detected in maternal serum 6 to 9 
days after conception.5 The levels rise in a logarithmic fashion, 
peaking 8 to 10 weeks after the last menstrual period, followed by 
a decline to a nadir at 18 weeks, with subsequent

a decline to a nadir at 18 weeks, with subsequent levels remaining 
constant until delivery117 (Fig. 22.3). The placenta also secretes 
free subunits. During the first 13 weeks of pregnancy, relatively 
more β-subunit is synthesized than α-subunit, and throughout 
the remainder of pregnancy the opposite occurs.118 In addition, 
a hyperglycosylated form of α-subunit (big α) that is unable to 
combine with free α-subunit is secreted into the maternal serum.
The physiologic factors that regulate h

nal serum.
The physiologic factors that regulate hCG secretion in  vivo 
are unknown. Much of the data concerning factors that stimulate 
or inhibit hCG synthesis and secretion have been derived from 
in vitro studies and are difficult to extrapolate to the in vivo situa­
tion. There is strong circumstantial evidence that GnRH, synthe­
sized in both the cytotrophoblast and syncytiotrophoblast, may be 
an important factor in hCG secretion. This peptide is identical to 
hypothalamic GnRH and stimu

ptide is identical to 
hypothalamic GnRH and stimulates placental hCG production 
both in vitro and in vivo, whereas GnRH antagonists decrease 
basal hCG secretion.116,119,120
Immunohistochemical staining for GnRH in placental tissue 
is highest at 8 weeks of gestation and lower afterward,121 roughly 
paralleling the pattern of hCG production, as do the circulating 
levels of GnRH measured in maternal serum.120 In addition, the 
placenta contains GnRH receptors.122 Placental GnRH release is 
sti

 GnRH receptors.122 Placental GnRH release is 
stimulated by cyclic adenosine monophosphate (cAMP), prosta­
glandin E2 (PGE2), prostaglandin F2 (PGF2), epinephrine, epider­
mal growth factor, insulin, and vasoactive intestinal peptide (VIP), 
factors also noted to increase hCG secretion in vitro.116,119,123
Two other peptides synthesized by the cytotrophoblast, activin 
and inhibin, also modulate GnRH and hCG secretion; activin 
increases both, and inhibin inhibits the action of GnRH on the 
syn

nd inhibin inhibits the action of GnRH on the 
syncytiotrophoblast.119 Increases in hCG production have also 
been found after trophoblast exposure to FGF, calcium, gluco­
corticoids, and phorbol esters.119 Decreased production occurs 
with TGFβ, follistatin, and progesterone.119 The decidua may 
also influence hCG production through paracrine mechanisms.9 
Decidual interleukin-1 stimulates hCG secretion in cultured tro­
phoblasts,124 while decidual prolactin and an 8-kDa to 10-kDa 
decidual pro

ual prolactin and an 8-kDa to 10-kDa 
decidual protein inhibit hCG production.125
Finally, hCG may autoregulate its own production to some 
extent. hCG receptors are present on the surface of trophoblastic 
cells, and the addition of hCG to placental cells in culture stimu­
lates cAMP production as well as proliferation and differentiation 
of the cytotrophoblasts into syncytiotrophoblasts.116 Both hCG 
mRNA and hCG production are stimulated by analogues of cAMP 
or agents that activate adenylat

nalogues of cAMP 
or agents that activate adenylate cyclase, probably through a protein 
kinase.116,119 Thus the net effect of an increase in syncytiotropho­
blast mass and cAMP would be an enhancement of hCG secretion.
The placenta is not the only site of hCG synthesis. Immunore­
active hCG has been found by immunocytochemistry or by immu­
noassay of extracts of a wide variety of normal tissues, including 
the spermatozoa, testes, endometrium, kidney, liver, colon, gastric 
tissue, lung, spleen

idney, liver, colon, gastric 
tissue, lung, spleen, heart, fibroblast, brain, and pituitary gland,126 
and the hormone has been shown to be synthesized in some fetal 
tissues.116 The pituitary gland appears to be the major source of 
hCG or an hCG-like material present in nonpregnant individu­
als. Immunoactive and bioactive hCG have been partially purified 
from pituitary glands; the material is secreted in vitro by fetal pitu­
itary cells and is shown by immunocytochemistry to be present in 
g

s shown by immunocytochemistry to be present in 
gonadotroph-type cells that do not contain hLH or hFSH.126,127
Immunoreactive hCG has been measured in sera from normal, 
nonpregnant individuals, with the highest concentrations found 
in postmenopausal women.127,128 In postmenopausal women, 
this material is secreted in a pulsatile fashion in parallel with hLH 
pulses, and during the normal menstrual cycle, the immunore­
active hCG shows a midcycle peak concomitant with the hLH 
peak.129 In both

le peak concomitant with the hLH 
peak.129 In both men and postmenopausal women, GnRH stimu­
lates secretion of the hormone, whereas its secretion is inhibited 
by oral contraceptives in women and by a GnRH agonist in ago­
nadal men.128,130
Both gestational and nongestational trophoblastic tumors 
secrete hCG and its free subunits. The sources of hCG secretion 
in nongestational trophoblastic neoplasms are the syncytiotropho­
blastic cells and in seminomas are the trophoblastic giant cells.130 

 seminomas are the trophoblastic giant cells.130 
In many instances, the tumors produce incomplete forms of hCG 
or its subunits, and differences in carbohydrate content from the 
hCG in pregnancy have been especially apparent. A wide variety 
of nontrophoblastic tumors also secrete hCG, although the pre­
dominant moiety appears to be the free β-subunit of hCG.130,131 
After it is secreted, hCG exhibits a biexponential clearance from 
the circulation with a fast half-time (t1/2) of 6 

the circulation with a fast half-time (t1/2) of 6 hours and a slow t1/2 
of close to 36 hours. In contrast, the free β-subunit has a 41-min­
ute fast t1/2 and a slow t1/2 of 4 hours, and the free α-subunit has 
a 13-minute fast t1/2 and a 76-minute slow t1/2.132 Approximately 
22% of the intact hormone appears in the urine unchanged; the 
rest undergoes metabolic degradation (Fig. 22.4). One of the 
early steps is proteolytic cleavage (“nicking”) of the β-subunit at 
0
4
8
12
16
20
24
28
32
36
4

 of the β-subunit at 
0
4
8
12
16
20
24
28
32
36
40
10
5
0
15
20
25
30
35
40
45
50
55
hCG (IU/mL)
Weeks after last menstrual period
• Fig. 22.3  Mean (± standard error) levels of maternal serum human chori­
onic gonadotropin (hCG) throughout normal pregnancy. (From Braunstein 
GD, Rasor J, Danzer H, et al. Serum human chorionic gonadotropin levels 
throughout normal pregnancy. Am J Obstet Gynecol. 1976;126:678–681.)
818
Val44-Leu45 and Gly47-Val48. H

y and the Life Span
Val44-Leu45 and Gly47-Val48. Human leukocyte elastase, pres­
ent in macrophages and leukocytes, appears to be responsible for 
some of the nicking of the α-subunit.132
Nicked hCG is unstable and dissociates into free α-subunit 
and nicked free β-subunit. The latter is further metabolized, pri­
marily in the kidney, to produce the β-core fragment, which is 
composed of the β-subunit amino acids 6 to 40 disulfide bridged 
to amino acids 55 to 92, trimmed of a portion of carbohy

no acids 55 to 92, trimmed of a portion of carbohydrate, 
and has a molecular mass of 10,479 Da.133 This fragment is the 
major form of immunoreactive hCG present in the urine in 
pregnancy. In normal pregnancy, the urine also contains vari­
able quantities of the hyperglycosylated form of α-subunit, free 
α-subunit, free β-subunit, nicked hCG, nicked free β-subunit, 
carboxyl-terminal fragments of the β-subunit, and fragments of 
the α-subunit.132 
Physiologic Functions
Most, if not all, of the

32 
Most, if not all, of the physiologic functions of hCG occur after 
interaction of the hormone with the hLH-hCG receptor. The 
receptor gene is located on chromosome 2 and encodes for a G 
protein–coupled receptor with seven hydrophobic transmem­
brane domains and a large extracellular amino terminus that 
binds to hCG (and hLH). The receptor is part of a superfamily 
of receptors, including those for hFSH, hTSH, AVP, VIP, PTH, 
and receptors for a variety of biogenic am

, PTH, 
and receptors for a variety of biogenic amines and neurotransmit­
ters.116 The hCG-receptor interaction results in increased cAMP 
production and, in some tissues, increased phosphoinositide 
turnover.132
Because of the close structural homology of the hLH-hCG 
receptor with the other glycoprotein hormone receptors, hCG 
may interact with the hTSH and hFSH receptors and thus has 
weak intrinsic hTSH and hFSH biologic activity. As previously 
noted, the hTSH-like activity of hCG is clinic

ly 
noted, the hTSH-like activity of hCG is clinically manifested 
during normal pregnancy by the reciprocal decrease in maternal 
hTSH at the time of the hCG peak between 8 and 12 weeks after 
the last menstrual period. It is especially important in patients 
with hydatidiform moles and other forms of trophoblastic disease, 
in which hCG levels may exceed 100,000 IU/L and result in clini­
cal thyrotoxicosis72,73 (see Fig. 22.1).
One of the major functions of hCG during pregnancy is the 
“rescue

 functions of hCG during pregnancy is the 
“rescue” of the corpus luteum during the conception cycle.134 Dur­
ing a menstrual cycle without conception, progesterone concen­
trations in the serum increase for the first 6 to 7 days of the luteal 
phase, followed by a plateau of 3 to 4 days and then a decrease, 
resulting in the shedding of the endometrial lining. After concep­
tion and implantation, the corpus luteum continues to secrete pro­
gesterone and 17-hydroxyprogesterone for another 4 to 6

rone and 17-hydroxyprogesterone for another 4 to 6 weeks. 
The maternal serum progesterone and 17-hydroxyprogesterone 
concentrations then decrease, indicating a marked diminution 
in corpus luteum function.135 The fall in 17-hydroxyprogester­
one concentrations continues, but the drop in progesterone levels 
is only transient. This marks the transition from dependence on 
ovarian progesterone production to placental progesterone secre­
tion (the luteal-placental shift). As previously noted, lut

 luteal-placental shift). As previously noted, luteectomy 
during the first 50 days after the last menstrual period is associated 
with a decline in progesterone levels and expulsion of the products 
of conception. After a therapeutic abortion, progesterone levels 
also drop rapidly.
Thus the fetal-placental unit is responsible for the signal to 
maintain the corpus luteum. The data supporting the notion that 
hCG is that physiologic signal include the following:
 • The presence of hLH-hCG rece

 hCG 
during their luteal phase
The inability of hCG to prolong the life of the corpus luteum 
of pregnancy beyond the sixth to eighth week of pregnancy 
appears to be due to homologous desensitization of the adenylate 
cyclase system and the inhibitory effects of high estrogen levels on 
progesterone synthesis through inhibition of 3βHSD1 and Δ5-4-
isomerase in the corpus luteum.
Another physiologic role for hCG is in the differentiation of 
fetal male genitalia through stimulation of the hLH-h

al male genitalia through stimulation of the hLH-hCG recep­
tors on the fetal testicular Leydig cells during the period when 
differentiation of wolffian duct structures and development of the 
external genitalia occur. The maximum testosterone production 
per unit weight of the testes coincides with the maximum binding 
of 125I-labeled hCG to the fetal testicular receptors at 10 to 12 
weeks of development, and fetal Leydig cells produce cAMP and 
testosterone in vitro after exposure to hCG. Th

d 
testosterone in vitro after exposure to hCG. The hCG concentra­
tions in fetal serum parallel the fetal testicular testosterone levels 
at a time when the amount of fetal pituitary hLH is not sufficient 
to stimulate testosterone production.136
There are several other possible actions of hCG during normal 
pregnancy. In vitro, hCG stimulates the differentiation of cytotro­
phoblast to syncytiotrophoblast and hence may play an important 
Trophoblast
Blood
Kidney
Urine
Also contains α:Nβ, Nβ, C

blast
Blood
Kidney
Urine
Also contains α:Nβ, Nβ, CTP fragment, α fragments 
α mRNA
β-core
β-core
α
β
α:β
Big α 
β
α:β
α:Nβ
α
α
Nβ
Nβ
Big α 
β
α:β
Big α 
β-core
α:β
Big α 
β mRNA
β
Free
α 
Free
β 
HLE+?
• Fig. 22.4  Proposed pathways for metabolism of human chorionic 
gonadotropin (hCG). α:β, intact hCG; α:Nβ, hCG with nicked β-subunit; 
Big α, hyperglycosylated form of the α-subunit; CTP fragment, carboxy-
terminal fragment; HLE, human leukocyte elastase; mRNA, messenger 
RNA; Nβ, free nicked β-

elastase; mRNA, messenger 
RNA; Nβ, free nicked β-subunit. (From Braunstein GD. Physiologic func­
tions of human chorionic gonadotropin during pregnancy. In: Mochizuki M, 
Hussa R, eds. Placental Protein Hormones. Amsterdam: Elsevier Science; 
1988:33.)
819
paracrine role in regulating syncytiotrophoblast mass and produc­
tion of trophoblast hormones.125,137 Additional data supporting 
this autoregulatory effect of hCG include the in vitro stimula­
tion

y effect of hCG include the in vitro stimula­
tion of placental synthesis of cAMP, activation of glycogen phos­
phorylase, and incorporation of radiolabeled galactose and leucine 
into placental proteins upon exposure to hCG.136 hCG stimulates 
the secretion of VEGF from the cytotrophoblast, which may be 
important for placental angiogenesis.116 Vasodilation of myome­
trial blood vessels mediated by hCG binding to vascular hCG 
receptors may enhance uterine blood flow in early pregnancy.116 
The

nce uterine blood flow in early pregnancy.116 
The fetal zone of the adrenal releases DHEAS in response to hCG 
exposure in vitro, and therefore hCG may have adrenocortico­
tropic activities in concert with fetal pituitary ACTH and placen­
tal ACTH.136
It has also been suggested that hCG plays a role in the immu­
nosuppression that occurs during pregnancy. Many early studies 
on this topic were hampered by the use of impure preparations 
of hCG or the presence of preservatives such as phenol tha

r the presence of preservatives such as phenol that may 
alter the end-points of the test systems used to define immunosup­
pression. In addition, the immunosuppressive effects may be due 
to gonadal steroid secretion in response to the hCG in the in vivo 
models used in some of the studies.138 Relaxin secretion from the 
corpus luteum is stimulated by hCG both in vivo and in vitro.116
Finally, the decrease in osmotic threshold for thirst and AVP 
release during pregnancy is clearly related to h

release during pregnancy is clearly related to hCG.77 Whether 
this decrease is due to a direct effect of hCG or an indirect effect 
through stimulation of gonadal steroids or interaction with hLH-
hCG receptors present in vascular smooth muscle is unclear. 
Gestational Trophoblastic Disease
Gestational trophoblastic disease (GTD) includes complete and 
partial hydatidiform moles, choriocarcinoma, and placental-site 
trophoblastic tumor.139 Complete molar pregnancy is the most 
common variety,

plete molar pregnancy is the most 
common variety, occurring in 1 to 2 in 1000 pregnancies. Patients 
usually present with vaginal bleeding, a uterus that is larger than 
expected for the duration of pregnancy, anemia, and excessive 
vomiting. Pathologically, trophoblast hyperplasia, marked edema 
of the chorionic villi, and absence of fetal tissues are observed. In 
contrast, partial moles demonstrate focal trophoblast hyperplasia 
and villous swelling and often have fetal tissues with congenit

welling and often have fetal tissues with congenital 
malformations. Approximately 20% of patients with complete 
moles develop persistent trophoblastic disease, whereas only 2% 
to 4% of patients develop persistent disease after partial molar 
pregnancy. Persistent trophoblastic disease also can occur after a 
normal term pregnancy and in pregnancies that end in spontane­
ous or induced abortion.
Choriocarcinoma is the most aggressive malignant form of per­
sistent trophoblastic disease and may

form of per­
sistent trophoblastic disease and may involve complications from 
local uterine disease, such as bleeding and rupture of the uterus, or 
from the effects of metastases, especially those involving the liver, 
lungs, and brain. The least common form of GTD is placental-
site trophoblastic tumor, which is derived from the intermedi­
ate trophoblast and is often associated with vaginal bleeding and 
amenorrhea.139
All of these neoplasms secrete hCG, free β-subunit, and 
often additional

secrete hCG, free β-subunit, and 
often additional forms of these molecules. With the exception of 
placental-site trophoblastic tumor, which secretes relatively low 
amounts of hCG, the serum and urine concentrations of hCG 
roughly parallel the tumor burden and also provide prognostic 
information. Thus hCG measurements in concert with clini­
cal and radiologic findings, especially vaginal ultrasonography 
findings, are useful for making the diagnosis of GTD. On rare 
occasions, false-positive

agnosis of GTD. On rare 
occasions, false-positive, low level hCG results may be found in 
some women who have heterophilic antibodies and other inter­
fering substances in their sera. This may lead to a misdiagnosis 
of GTD. Since these substances are not excreted in the urine, 
a urine pregnancy test will be negative in the presence of such 
“phantom hCG.”139
Hydatidiform moles are initially treated with uterine dilata­
tion and evacuation with or without adjunctive single-agent che­
motherapy

 or without adjunctive single-agent che­
motherapy with methotrexate or actinomycin D. Approximately 
90% of patients with low-risk, persistent trophoblastic disease are 
cured by single-agent chemotherapy; 75% of patients with high-
risk, metastatic disease are cured by multiagent chemotherapy, 
including etoposide, methotrexate, actinomycin D, cyclophos­
phamide, and vincristine. Serial hCG measurements are invalu­
able for monitoring as they accurately reflect the effect of therapy 
on the tu

ccurately reflect the effect of therapy 
on the tumor.139 
Human Placental Lactogen
Also called chorionic somatomammotropin, hPL is a single-chain, 
nonglycosylated polypeptide composed of 191 amino acid resi­
dues and two disulfide bridges, with a molecular mass of 21,600 
Da.70 It is closely related chemically and biologically to both GH 
(85% amino acid homology) and prolactin (13% amino acid 
homology). The hGH-hPL gene cluster is located on the long 
arm of chromosome 17 and consists of fiv

the long 
arm of chromosome 17 and consists of five genes—one coding for 
pituitary hGH (hGH-N), one for placental hGH (hGH-V), and 
three for placental hPL (hPL-L, hPL-A, and hPL-B, of which only 
hPL-A and hPL-B are transcribed).140
hPL is synthesized and secreted by the syncytiotrophoblast and 
is detected in maternal serum between 20 and 40 days of gesta­
tion. The maternal serum levels rise rapidly and peak at 34 weeks, 
followed by a plateau70 (Fig. 22.5). Both the serum concentra­
tions a

u70 (Fig. 22.5). Both the serum concentra­
tions and placental hPL mRNA concentrations are closely cor­
related with placental weight and syncytiotrophoblastic mass.141 
The maternal serum concentrations at term average between 6 and 
7 μg/mL; at that time, on the basis of the 9-minute to 15-minute 
t1/2 of disappearance from the circulation, the placental produc­
tion rate of hPL is in excess of 1 g/day. The fetal serum levels are 
1/50 to 1/100 of the maternal levels.140
The physiologic in viv

 of the maternal levels.140
The physiologic in vivo regulation of hPL synthesis and secre­
tion, other than the constitutive production related to placental 
mass, is unknown. Several studies have examined the possible 
role of nutrients in hPL secretion in pregnant women. Neither 
acute hyperglycemia nor hypoglycemia appeared to alter the hPL 
concentrations, although prolonged glucose infusions decreased 
and prolonged fasting increased the concentrations.70,118 Arginine 
infusions, dexamethas

centrations.70,118 Arginine 
infusions, dexamethasone administration, and changes in plasma 
free fatty acid levels did not affect the maternal hPL concentra­
tions.142,143 Glucose, estrogens, glucocorticoids, prostaglandins, 
epinephrine, oxytocin, TRH, GnRH, and l-dopa have been 
examined in in vitro systems and found to be without consistent 
effects.144–147
Angiotensin II, IGF1, phospholipase A2, arachidonic acid, and 
epidermal growth factor stimulated hPL release in  vitro.119,148 
Epiderm

 stimulated hPL release in  vitro.119,148 
Epidermal growth factor probably enhances production through 
promotion of cytotrophoblast-to-syncytiotrophoblast differen­
tiation.148 Apolipoprotein AI also stimulated hPL synthesis and 
release through cAMP-dependent and arachidonic acid–depen­
dent pathways.70,149,150 Because changes in the maternal plasma 
apolipoprotein AI concentrations parallel those of hPL during 
pregnancy, it is likely that this apoprotein, alone and as part of 
circulating H

is apoprotein, alone and as part of 
circulating HDL, is important in the secretion of hPL.150
820
hPL has a number of biologic activities that are qualitatively 
similar to those of hGH and prolactin and can bind to both the 
hGH and prolactin receptors.151 In various bioassay systems, hPL 
had weak somatotropic and lactogenic effects.151,152 It appears to 
be a major regulator of IGF1 production, and during pregnancy 
hPL concentrations are correla

d during pregnancy 
hPL concentrations are correlated with those of IGF1.70,151 HPL 
also affects the metabolism of maternal nutrients. It stimulates 
pancreatic islet insulin secretion, both directly and after carbohy­
drate administration,151 and is a diabetogenic factor during preg­
nancy through its promotion of insulin resistance. It enhances 
lipolysis, leading to a rise in free fatty acids, which may in part be 
responsible for the insulin resistance.151
The various biologic activities of

 resistance.151
The various biologic activities of hPL have led to the hypothesis 
that the role of hPL during pregnancy is to provide the fetus with a 
constant supply of glucose and amino acids.151 The hPL-stimulated 
lipolysis allows the mother to utilize free fatty acids for energy during 
fasting, allowing glucose, amino acids, and ketone bodies to cross 
the placenta for use by the fetus. In addition, hPL has actions in 
the fetus, promoting amino acid uptake by muscle and stimulating 
pro

g amino acid uptake by muscle and stimulating 
protein production, IGF1 production, and glycogen synthesis.152
Despite the proposed importance of hPL in maternal and fetal 
metabolic homeostasis during pregnancy, its absence does not 
appear to impair pregnancy. Deficient or absent hPL production 
related to gene defects has been described in several women who 
experienced normal pregnancies and delivered normal infants.153 
Placental Growth Hormone
Placental GH, hGH-V, is synthesized and secret

one
Placental GH, hGH-V, is synthesized and secreted by the syncy­
tiotrophoblast.70 Alternate splicing of the hGH-V gene results in 
two nonglycosylated isoforms with molecular masses of 22 and 26 
kDa.70,140 The 22-kDa variant may also be glycosylated and circu­
late as a 26-kDa protein.140 HGH-V is detected in the maternal 
plasma from 10 weeks of gestation and peaks during the third 
trimester70,133,154 (Fig. 22.6).
HGH-V has somatotropic activity and stimulates IGF1 pro­
duction, and the in

ivity and stimulates IGF1 pro­
duction, and the increase in IGF1 concentrations may in turn 
be responsible for the suppression of maternal pituitary hGH 
secretion152 (see Fig. 22.6). Unlike pituitary hGH, hGH-V is not 
secreted in a pulsatile fashion, nor is it released from the tropho­
blast by growth hormone–releasing hormone (GHRH), but it 
is inhibited by glucose. It has been estimated that at term, 85% 
of the GH biologic activity in maternal serum is due to hGH-V, 
12% to hPL, and only 3

nal serum is due to hGH-V, 
12% to hPL, and only 3% to pituitary hGH.155 Within 48 hours 
of delivery, pituitary hGH secretion returns to normal. 
Human Chorionic Corticotropin
The syncytiotrophoblast synthesizes an ACTH-like peptide, 
human chorionic corticotropin (hCC), as well as several pro-
opiomelanocortin–derived peptides, including β-lipotropin, 
β-endorphin, and α-melanocyte–stimulating hormone.74 The 
maternal serum concentrations of ACTH increase as pregnancy 
progresses, and the elev

TH increase as pregnancy 
progresses, and the elevation of free cortisol levels during preg­
nancy may be related in part to both placental hCC and pituitary 
ACTH production.74
hCC secretion is stimulated by CRH, which is probably the 
most important factor regulating the local production of the 
peptide through paracrine or autocrine mechanisms, or both, 
because it is also produced by both the cytotrophoblast and the 
syncytiotrophoblast. Unlike the situation with the pituitary, glu­
cocortic

ke the situation with the pituitary, glu­
cocorticoids and oxytocin also stimulate hCC release from placen­
tal cultures.75 Indeed, the resistance of maternal plasma ACTH 
concentrations to suppression after glucocorticoid administration 
may reflect the placental hCC contribution to the total pool of 
circulating immunoreactive ACTH.74 
Hypothalamic Peptides
Both the cytotrophoblast and the syncytiotrophoblast synthesize 
and secrete GnRH, which has the same chemi

hesize 
and secrete GnRH, which has the same chemical structure and 
biologic activity as hypothalamic GnRH.119,121 Although the 
GnRH mRNA levels in the placenta are similar throughout gesta­
tion, the highest concentrations of the peptide in the placenta and 
serum are found during the first trimester and correlate with the 
mass of the cytotrophoblast and peak hCG concentrations.121,156
In vitro, GnRH production by placental explants or purified 
trophoblasts is stimulated by prostaglandins, 

ed 
trophoblasts is stimulated by prostaglandins, epinephrine, activin, 
insulin, epidermal growth factor, VIP, estradiol, and estriol, and 
secretion is reduced by inhibin, progesterone, and κ-opiate and 
μ-opiate agonists.155,157 The syncytiotrophoblast contains low-
affinity GnRH receptors, whose concentrations parallel the hCG 
secretory pattern.122
Placental weight (g)
100
0
200
300
400
500
4
8
12
0.02
0.04
0.06
16
20
24
28
1
2
3
4
5
6
7
32
36
40
Serum hPL (g/mL)
hPL
Pl. 

8
1
2
3
4
5
6
7
32
36
40
hPL
Pl. wt.
Placental
weight
hPL
hPL
Pl. wt.
• Fig. 22.5  Placental weight (Pl. wt.), maternal serum concentrations of human placental lactogen (hPL), 
and the ratio of hPL to Pl. wt. during pregnancy. (From Selenkow HA, Saxena BN, Dana CL. Measure­
ment and pathophysiologic significance of human placental lactogen. In: Pecile A, Finzi C, eds. The Feto-
Placental Unit. Amsterdam: Excerpta Medica; 1969:340.)
821

.)
821
Because GnRH stimulates hCG secretion by placental explants 
and purified trophoblast cells in vitro, with the response of early 
to midtrimester placentas being greater than that of term tropho­
blast, it is reasonable to conclude that GnRH is an important 
autocrine or paracrine regulator of hCG secretion.155 The hCG-
stimulatory effect of GnRH can be blocked by administration of a 
GnRH antagonist.158 Because GnRH stimulates metalloprotein­
as

ist.158 Because GnRH stimulates metalloprotein­
ases in cytotrophoblasts, the peptide may be important during 
implantation.159 
and secrete a 41–amino acid peptide that is identical to hypotha­
lamic CRH.119,160 CRH mRNA is first detected in trophoblast at 7 
weeks of gestation. The levels remain low during the first 30 weeks 
of pregnancy but rise 20-fold during the final 5 weeks, a pattern that 
i

0-fold during the final 5 weeks, a pattern that 
is parallel to the rise of CRH content in the placenta and concentra­
tions in maternal plasma.102 In maternal plasma, CRH circulates 
bound to a 37-kDa protein that is synthesized by the placenta, liver, 
and brain and that reduces the biologic activity of the CRH.75,160
In  vitro, placental CRF production is stimulated by prosta­
glandins (E2 and F2α), norepinephrine, acetylcholine, oxytocin, 
neuropeptide Y, AVP, angiotensin II, and interleukin

uropeptide Y, AVP, angiotensin II, and interleukin-1. Gluco­
corticoids have been shown to increase both CRH mRNA and 
peptide, whereas in the hypothalamus suppression is found. CRH 
secretion is reduced by progesterone and nitric oxide donors. The 
placenta contains CRH binding sites, and the addition of CRH 
to cultured placental cells results in a dose-dependent increase 
in hCC, β-endorphin, and α-melanocyte–stimulating hormone 
secretion.74,119,160 Thus it is likely that CRH has an autocrin

119,160 Thus it is likely that CRH has an autocrine or 
paracrine effect in the placenta.
Whether CRH has a physiologic effect on the maternal pitu­
itary secretion of ACTH is unclear; the circulating CRH may be 
biologically inactive because of the binding protein. However, just 
before parturition, the binding protein concentration decreases by 
approximately 50% and the CRH levels rise.74,160 At this time, 
CRH stimulates the synthesis and release of prostaglandins from 
the decidua, amnion, 

ease of prostaglandins from 
the decidua, amnion, and chorion, which enhances cervical ripen­
ing.161 The myometrium contains CRH receptors, and CRH may 
increase myometrial contractility.102 Thus CRH may have a role 
in initiating and promoting parturition. CRH may also stimu­
late the fetal pituitary production of ACTH, which in turn may 
lead to increased fetal adrenal DHEA production and ultimately 
estriol synthesis by the fetoplacental unit.160 In addition to CRH, 
the syncytiotrophoblast 

.160 In addition to CRH, 
the syncytiotrophoblast and fetal membranes secrete urocortin 
1, which in vitro stimulates placental ACTH, PGE2, and activin 
secretion through the CRH receptor.162 
Endocrinology of Pregnancy and 
Roles of Estrogens and Progesterone
Although it is an oversimplification, viewing the maintenance 
of pregnancy versus the initiation of labor as a balance between 
the effects of progesterone and estrogen, it is helpful to concep­
tualize this very complicated p

helpful to concep­
tualize this very complicated process. Progesterone has long been 
known to be essential for pregnancy; in fact its name is a contrac­
tion of the term progestational steroid hormone. It is essential from 
the very beginning of pregnancy as interruption of its production 
prior to 7 weeks of pregnancy leads to pregnancy loss.163 Later in 
pregnancy is it thought to maintain uterine quiescence by limit­
ing the production of prostaglandins and the expression of genes 
such as i

staglandins and the expression of genes 
such as ion channels, oxytocin receptors, prostaglandin receptors, 
and ion channels involved in the contraction machinery of the 
uterus.164,165 Progesterone’s role in maintaining uterine quies­
cence is also illustrated in the benefits of progesterone supplemen­
tation to prevent preterm birth and is now the standard of care in 
women at high risk for preterm birth.166
Estrogen clearly has a role in the endocrinology of pregnancy, 
but it appears less i

endocrinology of pregnancy, 
but it appears less important than progesterone both during preg­
nancy and labor. During pregnancy estrogen enhances the uptake 
of LDL in the syncytiotrophoblast to aid in steroid production, 
increases uterine blood flow to allow for adequate gas exchange 
and nutrient transport across the placenta, and causes hypertro­
phy of mammary tissue to prepare the breasts for lactation.109
In many animal models it has been clearly demonstrated that 
a decline in progester

 clearly demonstrated that 
a decline in progesterone concentration, either spontaneous or 
induced, is sufficient to initiate labor.167 However, in women 
there is no clear spontaneous drop in progesterone levels in the 
weeks leading up to labor. While this may seem contradictory 
at first, there is mounting evidence that functional progesterone 
withdrawal does occur in humans despite a constant level of the 
hormone throughout late gestation. The progesterone receptor 
(PR) has two distinct 

 The progesterone receptor 
(PR) has two distinct isoforms, PR-A and PR-B. PR-B is thought 
GH (mU/L)
Gestation (weeks)
GH 5 B4
GH K24
5
5
5
5
8–10
11–13
14–16
17–19
20, 21
22, 23
24, 25
26, 27
28, 29
30, 31
32, 33
34, 35
36, 37
38, 39
A
B
IGF1 (µg/L)
17
17
16
21
20
13
18
19
11
14
10
20
30
40
50
0
0
100
150
200
250
300
350
• Fig. 22.6  Mean (± standard error) of plasma human growth hormone 
(GH) (A) and insulin-like growth factor type 1 (IGF1) (B) levels through­
out pregnancy. The number of ind

) levels through­
out pregnancy. The number of individual assays of GH and IGF1 at each 
gestational stage is indicated in (A) at the top of the vertical bars. GH 5 
B4, placental GH (GH2); GH K24, pituitary GH (GH1). (From Mirlesse V, 
Frankenne F, Alsat E, et  al. Placental growth hormone levels in normal 
pregnancy and in pregnancies with intrauterine growth retardation. Pediatr 
Res. 1993;34:439–442.)
822
to mediate most of progesterone’s effects

ife Span
to mediate most of progesterone’s effects of uterine quiescence by 
activating progesterone responsive genes and suppressing estrogen 
receptor (ER) production. PR-A, on the other hand, predomi­
nantly acts as a repressor or PR-B. The onset of labor is associated 
with an increase in the relative levels of PR-A compared to PR-B 
in the myometrium, suggesting a functional decrease in progester­
one activity occurs in humans as it does in other mammals.168–172
This functional decrease occ

other mammals.168–172
This functional decrease occurs in concert with an increase 
in ER expression, which in turn produces a contractile uterine 
phenotype in preparation for labor, including an increase in myo­
metrial gap junctions and prostaglandin production. The nones­
sential role of estrogen in pregnancy and labor is demonstrated in 
pregnancies complicated by placental sulfatase deficiency. These 
pregnancies generally continue to term, despite the very low 
estrogen production througho

despite the very low 
estrogen production throughout pregnancy. Although there is a 
delay in the onset of labor and the uterus is relatively refractory to 
prostaglandins and oxytocin, labor does ultimately occur sponta­
neously and inductions of labor, although difficult, can be success­
fully carried out.173 
Role of Prostaglandins
There is overwhelming evidence for the role of prostaglandins as 
mediators of labor.164,165 Their production is carefully compart­
mentalized within the uterus, w

carefully compart­
mentalized within the uterus, with PGE2 confined to the fetal 
membranes, PGF2α in the decidua, and PGI2 in the myometrium. 
Although structurally very similar, the different prostaglandin spe­
cies can have opposing effects, adding to the complexity of how 
prostaglandins regulate uterine activity. While the PGE2 produced 
by the fetal membranes and PGI2 produced by the myometrium 
inhibit uterine activity, the PGF2α produced by the decidua is a 
potent uterotonic,174 an acti

by the decidua is a 
potent uterotonic,174 an action that is exploited in the treatment 
of postpartum hemorrhage. PGF2α production is suppressed in 
pregnancy with prostaglandin levels in the gravid uterus being 
lower than those found in the nongravid uterus at any time dur­
ing the menstrual cycle.175,176 Later in pregnancy, and particularly 
at term with the onset of labor, PGF2α levels of prostaglandins 
increase in maternal serum and amniotic fluid.174
The physiology of prostaglandins in t

ic fluid.174
The physiology of prostaglandins in the labor process is exten­
sively exploited in modern obstetrics. Procontractile prostaglan­
dins, such as PGE1, are commonly used for cervical ripening 
and induction of labor.177 Postpartum hemorrhage due to uter­
ine atony is very effectively treated by administration of PGE1 
or PGF2α.178 Furthermore, inhibitors of prostaglandin synthesis, 
such as indomethacin, are among the most potent tocolytic agents 
used in the treatment of preterm labo

ytic agents 
used in the treatment of preterm labor.179 
Role of Oxytocin
Oxytocin is a polypeptide hormone produced in the hypothala­
mus and stored in the posterior pituitary gland. It is one of the 
first endogenous compounds discovered to play a role in human 
labor, and it is one of the few medications to receive US Food 
and Drug Administration (FDA) approval for specific use in preg­
nancy. It has long been known to produce uterine contractions, 
and it is frequently used for induction of

tions, 
and it is frequently used for induction of labor and to treat post­
partum hemorrhage due to uterine atony.178 Oxytocin mediates 
its effects by binding to G protein–coupled receptors that are pres­
ent throughout the uterus. The receptors are preferentially distrib­
uted in the uterine fundus, with lower levels found in the lower 
uterine segment and cervix, corresponding to the increased con­
tractility of the fundus compared to the lower uterine segment.180 
Although the levels of oxy

er uterine segment.180 
Although the levels of oxytocin do not detectably increase as preg­
nancy progresses until late in the second stage of labor,180 receptor 
concentrations increase up to 100-fold in the first trimester and 
up to 300-fold in the third trimester of pregnancy.180 The increase 
in receptors, mediated predominantly by estrogen, leads to the 
increased sensitivity of the myometrium to oxytocin in the second 
trimester.180 
Use of Placental Hormones in Genetic Screening 
and Pre

f Placental Hormones in Genetic Screening 
and Pregnancy Outcomes
Modern biochemical screening for Down syndrome, other aneu­
ploidies, and less common genetic disorders rely heavily on assess­
ing hormones and other proteins produced by the placenta. For 
example, maternal levels of hCG and pregnancy-associated plasma 
protein-A (PAPP-A) are combined with a measurement of fetal 
nuchal translucency in a first-trimester screening. In general, hCG 
levels are higher in pregnancies affected by Dow

levels are higher in pregnancies affected by Down syndrome and 
lower in pregnancies affected by trisomy 18 than euploid preg­
nancies. PAPP-A levels are lower in pregnancies affected by the 
common aneuploidies (i.e., Down, trisomy 13, trisomy 18) than 
in euploid pregnancies.181,182 For screening in the second trimes­
ter, hCG is combined with estriol and inhibin-A—all of which 
are produced by the placenta—and alpha-fetoprotein (AFP)—
which is predominantly produced by the fetus. Estriol is

is predominantly produced by the fetus. Estriol is lower 
in pregnancies affected by Down syndrome and other trisomies, 
whereas levels of inhibin-A are higher in pregnancies affected by 
Down syndrome compared to euploid pregnancies.183 Although 
not designed to do so, the hormone levels assayed in these genetic 
screening tests can provide some clues to other genetic condi­
tions and even help predict future pregnancy complications. For 
example, very low levels of estriol are seen in pregnanc

e, very low levels of estriol are seen in pregnancies compli­
cated by placental sulfatase184 deficiency and Smith-Lemli-Opitz 
syndrome,185 a disorder in which an enzymatic defect in the cho­
lesterol synthesis pathway leads to a disturbance of all steroid hor­
mones, including estriol. These analyte patterns are summarized 
in Table 22.3.
It is now well established that certain abnormal maternal 
serum analytes used in genetic screening are associated with preg­
nancy complications later in pr

ociated with preg­
nancy complications later in pregnancy in euploid nonanomalous 
fetuses. The association is strongest for elevated maternal serum 
AFP (MSAFP) and inhibin-A, which are both associated with an 
increased risk of fetal growth disturbance and intrauterine fetal 
death. Elevated inhibin-A is also associated with an increased risk 
of preterm birth.184,186,187 Once discovered, these pregnancies are 
often monitored by serial fetal ultrasounds to assess for growth 
disturbance and o

ltrasounds to assess for growth 
disturbance and ongoing documentation of fetal well-being if 
abnormal growth is found. Although the association is less well 
established, an abnormally low PAPP-A in a euploid pregnancy 
is associated with growth restriction and stillbirth, preterm 
birth, and preeclampsia.188 Both low first trimester PAPP-A and 
increased second trimester inhibin-A have been associated with an 
increased risk of preeclampsia. The correlations between abnormal 
serum analytes a

he correlations between abnormal 
serum analytes and pregnancy complications are outlined in Table 
22.4. 
Noninvasive Prenatal Testing and 
Microchimerism
Transplacental bidirectional trafficking of maternal, placental, and 
fetal cells occurs universally in pregnancy, beginning around 6 to 
7 weeks of gestation and peaking at term. Both fetal and maternal 
823
cells persist indefinitely in the host.189 In addition to fetal cells, 
the maternal circula

 In addition to fetal cells, 
the maternal circulation also contains fetal cell free DNA. The cell 
free DNA from maternal circulation can be sequenced and used 
to screen for fetal aneuploidy. This approach is termed noninva­
sive prenatal testing (NIPT). The technique is generally thought 
to be highly sensitive and specific for fetal aneuploidy, but it 
also suffers from the same pitfalls as trophectoderm biopsy in 
that mosaicism may be difficult to detect and, when detected, to 
interpret. 

cult to detect and, when detected, to 
interpret. Nonetheless, the availability of NIPT has resulted in a 
marked increase in its use in high-risk patient populations defined 
as pregnant women older than 35 years and those with a prior 
abnormal NIPT screen, abnormal ultrasound, and family history 
of aneuploidy. The superior accuracy of NIPT compared with 
conventional screening methods has led to significant decreases 
in the number of invasive diagnostic procedures, in addition to 
a concomi

e diagnostic procedures, in addition to 
a concomitant decrease in the number of procedure-related fetal 
losses.190 The introduction of NIPT significantly decreased the 
use of screening using placental hormone patterns in high-risk 
but not in low-risk populations.191 While NIPT is more accurate, 
it is also more expensive. Furthermore, invasive fetal karyotyp­
ing is still recommended to confirm abnormal screens regardless 
of screening method.192 Improving screening technologies and 
decreas

.192 Improving screening technologies and 
decreasing costs are likely to propel the use of NIPT, but wise 
application will also require improved patient and provider appre­
ciation of its limitations.193
More intriguing than the utilization of fetal cell free DNA 
in maternal circulation for prenatal screening, however, is the 
putative contribution of persistent maternal and fetal microchi­
merism to the fetal origins of adult disease and long-term post­
partum maternal health. Indeed, microc

-term post­
partum maternal health. Indeed, microchimerism apparently 
persists indefinitely, as fetal cells have been found in maternal 
circulation decades after birth and can be transmitted from 
grandmother to grandchild.194 While both maternal and fetal 
microchimerism have been implicated as causes of autoimmune 
diseases, including endocrine disorders such as autoimmune 
thyroid disorders and type 1 diabetes,195 microchimerism may 
also provide benefits, including enhanced tissue repair o

ovide benefits, including enhanced tissue repair or rejuve­
nation and slowed aging via transmission of stem cells. Micro­
chimerism has been hypothesized to protect the host from the 
development of certain types of cancer.195 Thus acute and persis­
tent maternal and fetal microchimerism can be adverse, neutral, 
or beneficial, depending on factors such as HLA match or mis­
match, persistence, microchimeric cell mass, and other factors as 
yet to be determined. The “discovery” of persistent mic

o be determined. The “discovery” of persistent microchime­
rism years after parturition adds a new dimension to our under­
standing of the endocrine changes and consequences associated 
with pregnancy. 
The “Fourth Trimester” and the Parental Brain
The physiologic and homeostatic endocrine and metabolic 
adaptations of pregnancy profoundly impact all maternal and 
fetal tissues, including the maternal brain. While delivery is a 
relatively abrupt transition and represents the greatest risk for 

 transition and represents the greatest risk for 
  Maternal Serum Analyte Patterns for Genetic Disorders
Genetic Disorder
PAPP-A
hCG
AFP
uE3
Inh A
↓
↑
↓
↓
↑
Trisomy 18
↓
↓
↓
↓
—
Trisomy 13
↓
↓
—
—
—
Smith-Lemli-Opitz syndrome
—
—
↓
↓
—
Turner syndrome (45,XO) without hydrops
↓
↓
↓
↓
—
Turner syndrome (45,XO) with hydrops
↓
↑
↓
↓
↑
Placental sulfatase deficiency
—
—
—
↓
—
AFP, Alpha-fetoprotein; hCG, human chorionic gonadotropin; InhA, inhibin-A; PAPP-A, placenta-associated plasma

nhA, inhibin-A; PAPP-A, placenta-associated plasma protein-A; uE3, unconjugated estriol; ↓ = decreased; ↑ = increased.
   
TABLE 22.3
  Maternal Serum Analyte Levels and Pregnancy Outcomes
Growth Restriction
Fetal Demise
Preterm Birth
PAPP-A (<0.42 MoM)
↑
↑
↑
↑
Free hCG (<0.021 MoM)
—
↑
↑
—
AFP (>2 MoM)
—
↑
↑
↑
hCG (>2 MoM)
↑
—
↑
—
uE3 (<0.5 MoM)
—
↑
↑
Inh A (>2 MoM)
↑
↑
↑
↑
AFP, Alpha-fetoprotein; hCG, human chorionic gonadotropin; InhA, inhibin-A; MoM, multiples of the median; PA

 InhA, inhibin-A; MoM, multiples of the median; PAPP-A, placenta-associated plasma protein-A; uE3, unconjugated estriol; ↓ = decreased; 
↑ = increased.
   
TABLE 22.4
824
maternal and neonatal mortality and morbidity, a new set of 
endocrine, metabolic, psychologic, and sociocultural challenges 
follows delivery. Indeed, pregnancy permanently imprints both 
mother and child and determines the future health of both, both 
acutely and chronically. Acut

ealth of both, both 
acutely and chronically. Acutely, following delivery, lactation 
and infant sleeping and feeding patterns present an immediate 
and enormous set of challenges that elicits maternal endocrine, 
metabolic, and psychologic adaptations that differ remarkably 
from those of pregnancy. Placental hormone levels fall acutely 
and the neuroendocrine axis responds. The catch-phrase for the 
adaptations related to immediate adjustment following delivery 
is the “fourth trimester.” Shor

ollowing delivery 
is the “fourth trimester.” Short shrift has been given to the term 
and the many endocrine and other adaptations that follow. What 
we do know is that adverse pregnancy outcomes such as gesta­
tional diabetes and preeclampsia not only heighten the risk for 
maternal cardiovascular health195–200 but also pose sociocultural 
adjustments that are only just beginning to be identified. Not 
all women are equally able to meet the endocrine and metabolic 
challenges of normal, much l

ocrine and metabolic 
challenges of normal, much less complicated, pregnancy and 
delivery. While the hormones of pregnancy prepare the mater­
nal brain for “nesting” and facilitate lactation and care of her 
infant, we know far less about the endocrine and metabolic 
impact of pregnancy and parenthood on the longer term health 
of women and men. Common knowledge suggests that preg­
nancy poses differential challenges to each sex or, in the case of 
same-sex liaisons, to the gestational and nong

of 
same-sex liaisons, to the gestational and nongestational parent. 
The psychologic challenges of parenting are often termed “paren­
tal brain.”201 Postpartum maladaptation may result in syndromal 
postpartum psychiatric disorders, and this burden seems to fall 
exclusively to the gestational parent.202 Our current view is that 
the abrupt withdrawal from the hormonal milieu of pregnancy 
that is associated with parturition creates an endocrine challenge 
that overwhelms the endocrine, metabol

 challenge 
that overwhelms the endocrine, metabolic, and sociocultural 
adaptation of vulnerable individuals. Further, we are beginning 
to recognize that pregnancies complicated by gestational diabe­
tes, hypertension, and other maternal and fetal medical condi­
tions heighten the risk for future maternal disease. Particularly, 
we now know that gestational diabetes predisposes to maternal 
diabetes after gestation and that gestational hypertension is a 
harbinger of later maternal cardiovascu

sion is a 
harbinger of later maternal cardiovascular disease. Psychologic 
sequelae of pregnancy such as postpartum depression also appear 
to heighten the risk not only of later syndromal psychiatric con­
ditions but also endocrine and cardiovascular conditions. A team 
approach is needed at both scientific and clinical levels if we are 
to ensure both individual and population health. In particular, 
women should be queried by health care providers about medi­
cal and psychiatric complication

iders about medi­
cal and psychiatric complications during and after pregnancy so 
that risk assessment, screening, surveillance, and interventions 
are adjusted appropriately.203 
Summary
The endocrine, metabolic, psychologic, and sociocultural changes 
associated with pregnancy and parturition represent a coordinated 
and cooperative set of homeostatic responses that facilitate mater­
nal and fetal health. Normal pregnancy represents an enormous 
challenge, and complications of pregnancy such 

s 
challenge, and complications of pregnancy such as preeclampsia 
and gestational diabetes predispose the mother and her offspring 
to endocrine, metabolic, cardiovascular, immune, and other con­
ditions after pregnancy. A better understanding of the physiology 
and pathophysiology of pregnancy and its aftermath is required 
for appropriate medical and societal care.
824.e1
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olism, and clinical utility. In: Lustbader JW, Puett D, Ruddon RW, 
eds. Glycoprotein Hormones. New York: Springer-Verlag; 1994:293.
	133.	 Lonberg U, Damm P, Andersson AM, et al. Increase in maternal 
placental growth hormone during pregnancy and disappearance 
during parturition in normal and growth hormone-deficient preg­
nancies. Am J Obstet Gyneco

in alpha and placental lactogen mRNAs during 
development of the human placenta. J Cell Biol. 1982;93:190–198.
	142.	 Morris HH, Vinik AI, Mulvihal M. Effects of acute alterations in 
maternal free fatty acid concentration on human chorionic somato­
mammotropin secretion. Am J Obstet Gynecol. 1974;119:224–229.
	143.	 Ylikorkala O, Kauppila A. Effect of dexamethasone on serum levels 
of human placental lactogen during the last trimester of pregnancy. 
J Obstet Gynaecol Br Commonw. 1974;81:368–3

s of placental growth hormone 
and placental lactogen in the regulation of human fetal growth and 
development. J Pediatr Endocrinol Metab. 2000;13:343–356.
	153.	 Sideri M, De Virgiliis G, Guidobono F, et  al. Immunologically 
undetectable human placental lactogen in a normal pregnancy. 
Case report. Br J Obstet Gynaecol. 1983;90:771–773.
	154.	 Mirlesse V, Frankenne F, Alsat E, et al. Placental growth hormone 
levels in normal pregnancy and in pregnancies with intrauterine 
growth retardatio

n pregnancies with intrauterine 
growth retardation. Pediatr Res. 1993;34:439–442.
	155.	 Petraglia F, Lim AT, Vale W. Adenosine 3’,5’-monophosphate, 
prostaglandins, and epinephrine stimulate the secretion of immu­
noreactive gonadotropin-releasing hormone from cultured human 
placental cells. J Clin Endocrinol Metab. 1987;65:1020–1025.
	156.	 Kelly AC, Rodgers A, Dong KW, et al. Gonadotropin-releasing 
hormone and chorionic gonadotropin gene expression in human 
placental development. DNA Ce

T, et al. Progesterone withdrawal and 
estrogen activation in human parturition are coordinated by pro­
gesterone receptor A expression in the myometrium. J clin Endocri­
nol Metab. 2002;87:2924–2930.
	172.	 Ni X, Hou Y, Yang R, et al. Progesterone receptors A and B dif­
ferentially modulate corticotropin-releasing hormone gene expres­
sion through a cAMP regulatory element. Cell Mol Life Sci. 
2004;61:1114–1122.
	173.	 Shozu M, Akasofu K, Harada T, Kubota Y. A new cause of female 
pseudoherma

da T, Kubota Y. A new cause of female 
pseudohermaphroditism: placental aromatase deficiency. J Clin 
Endocrinol Metab. 1991;72:560–566.
	174.	 Fuchs AR. Plasma, membrane receptors regulating myometrial 
contractility and their hormonal modulation. Semin Perinatol. 
1995;19:15–30.
	175.	 Phaneuf S, Europe-Finner GN, Varney M, et al. Oxytocin-stim­
ulated phosphoinositide hydrolysis in human myometrial cells: 
involvement of pertussis toxin-sensitive and -insensitive G-pro­
teins. J Endocrinol.

imester maternal 
serum PAPP-A and free-beta subunit human chorionic gonado­
tropin concentrations and nuchal translucency are associated with 
obstetric complications: a population-based screening study (the 
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825
23
Endocrinology of Fetal Development
HARSHINI KATUGAMPOLA, EVELIEN F. GEVERS, AND MEHUL T. DATTANI
Placental Transfer of Hormones, 826
Ectopic Fetal Hormone Production, 827
Fetal Endocrine Systems, 828
Fetal Growth, 857
Neutralization of Hormone Actions in the Fetus, 860
Programming of Fetal Endocrine Systems, 861
T

l gland 
and liver to provide substrates for placental estrone and estra­
diol production.
 • 
The anterior and posterior pituitary lobes develop from oral ec­
toderm and ventral diencephalon. Pituitary hormone secretion 
starts at 8 to 10 weeks’ gestation.
 • 
Thyroid hormone synthesis starts at 11 weeks’ gestation. 
­Circulating T4 increases to maximal levels after 35 weeks’ 
gestation, but maturation of hypothalamic-pituitary control 
and response of thyroid gland to thyroid-stimulating hor

sponse of thyroid gland to thyroid-stimulating hormone 
develops in the third trimester.
 • 
In the presence of SRY, the sex-determining region of the Y 
chromosome, male gonadal differentiation starts at 7 weeks 
of gestation. Development of Leydig cells leads to an increase 
in fetal testosterone production from week 10 and stimulates 
differentiation of the primitive mesonephric ducts into bilateral 
ductus deferens, epididymides, seminal vesicles, and ejaculatory 
ducts. Dihydrotestosterone

Growth of the fetus involves complex interactions between 
environmental, epigenetic, and genetic factors. Insulin-like 
growth factors are important for fetal growth and are regu­
lated mainly by transplacentally derived nutritional substrate. 
Hormones most important for postnatal growth, including T4, 
growth hormone, and gonadal steroids, have a limited role in 
fetal growth.
 • 
Fetal hormonal systems are programmed to maintain anabo­
lism with minimal perturbation. Production of catabolic

with minimal perturbation. Production of catabolic and 
thermogenic hormones is limited, and effects of hormones 
altering metabolic substrate supply and distribution pathways 
are muted.
 • 
Transition to extrauterine life involves the adrenal cortex and 
autonomic nervous system in the immediate postnatal period. 
Longer term transition to an intermittent nutrient supply and 
transient substrate deficiency requires maturation of secretory 
control mechanisms for the parathyroid hormone–calcit

trol mechanisms for the parathyroid hormone–calcitonin 
system and the endocrine pancreas.
826
T
he unfolding of our understanding of mammalian preg­
nancy and fetal development represents one of the dra­
matic chapters of scientific progress during the past 
half-century. Human pregnancy necessitates an ensemble of auto­
crine, paracrine, and endocrine networks that coordinate mater­
nal-placental-fetal communication. The evolution of a pregnancy 
i

tal communication. The evolution of a pregnancy 
is accompanied by dramatic changes in the hormonal milieu. This 
is a time of potential vulnerability but tremendous plasticity and 
involves complex genetic, cellular, and hormonal interactions 
that serve to create a unique, protective intrauterine environment 
for the developing fetus (Table 23.1).1 An orchestra of signaling 
molecules, transcription factors, and epigenetic events regulates 
embryo implantation, placentation, and ultimately the

ryo implantation, placentation, and ultimately the mainte­
nance of pregnancy until the onset of parturition with transition 
of the fetus to extrauterine life. It is becoming increasingly clear 
that the intrauterine environment not only enables fetal develop­
ment; it also has a significant impact on the lifelong health of 
an individual, as well as exerting multigenerational effects. This 
chapter reviews our current understanding of the transplacen­
tal passage of hormones, fetal endocrine d

placen­
tal passage of hormones, fetal endocrine development and hor­
mone production, adaptations of the fetal endocrine systems to 
extrauterine life, and programming of developing fetal endocrine 
systems.
Placental Transfer of Hormones
The placenta forms the materno-fetal interface, delivering nutri­
ents and oxygen to the fetus and acting as a selective barrier. The 
placenta demonstrates remarkable capacity to adapt to adverse 
environments and lessen their impact on the fetus. Placental 

 and lessen their impact on the fetus. Placental 
transfer of hormones decreases with increasing molecular weight, 
and those larger than 0.7 to 1.2 kDa have little or no access to 
the fetal compartment.2 Maternal hormones therefore play a very 
limited role in the fetal endocrine milieu. Hormones that do cross 
the placenta may be metabolized en route (Fig. 23.1), including 
steroid hormones (cortisol), thyroid hormones (T3, T4), estradiol, 
and catecholamines.3–6
The concentration of maternal

d catecholamines.3–6
The concentration of maternal cortisol is almost 10-fold 
higher than that in the fetus. Placental cells contain an active 
11β-hydroxysteroid dehydrogenase type 2 (11βHSD2) that cata­
lyzes conversion of maternal cortisol to inactive cortisone.7 Syn­
thetic glucocorticoids, such as dexamethasone or betamethasone, 
can bypass this protective mechanism resulting in exposure of the 
fetus to steroid hormones. While this is used acutely for its ben­
eficial effects on fetal lun

 acutely for its ben­
eficial effects on fetal lung maturation in cases of threatened pre­
term delivery, chronic use may produce adverse effects on blood 
pressure, blood glucose, and memory, as demonstrated in rodent 
models,8–10 as well as impacting negatively on placental and fetal 
growth.11 Clinical use of single or multiple courses of glucocorti­
coid treatment in the obstetric management of threatened preterm 
delivery continues, but treatment for other proposed indications 
lacking robu

tment for other proposed indications 
lacking robust evidence (e.g., use in pregnancy to reduce viriliza­
tion of a fetus with congenital adrenal hyperplasia [CAH]) should 
take place only in a research setting with careful auditing.12,13
Estrogens modulate many intrauterine processes throughout 
gestation, and the balance between estrogens and progesterone in 
  Features of the Fetal Endocrine 
Environment
Estrogens
Growth factors
Neutral

Neutralization of Hormone Actions
Cortisol
Thyroxine
Unique Fetal Endocrine Systems
Fetal adrenal cortex
Para-aortic chromaffin system
Intermediate lobe of the pituitary
Prominent Fetal Hormones or Metabolites
Vasotocin
Cortisone
Reverse triiodothyronine (rT3)
Sulfated iodothyronines
Ectopic neuropeptides
Fetal Endocrine System Adaptations
Adrenal-placental interactions
Testicular control of male phenotypic diffe

ctions
Testicular control of male phenotypic differentiation
Developmentally regulated growth factor control of fetal growth
Neuropeptides and fetal water metabolism
Parathyroid glands and placental calcium transport
Catecholamine and vasopressin responses to hypoxia
Cortisol programming for extrauterine exposure
Catecholamine and cortisol control of extrauterine adaptation
Perinatal hormonal programming
TABLE 23.1
CAT
Estradiol
Estrone
Cortisone
MET
MET
rT3
rT3
T2
T2
Estrone
Cortisone
T4
T4
T3

MET
MET
rT3
rT3
T2
T2
Estrone
Cortisone
T4
T4
T3
MDI3
MDI3
MAO/COMT
17βHSD
11βHSD
Cortisol
Maternal
Fetal
Placenta
• Fig. 23.1  Placental neutralization of biologic activity of hormones dur­
ing maternal-fetal transfer. The neutralizing enzymes, 17β-hydroxysteroid 
dehydrogenase (17βHSD) and 11βHSD, are shown (see text for details). 
CAT, catecholamines; COMT, catechol O-methyltransferase; MAO, 
monoamine oxidase; MDI3, type 3 iodothyronine monodeiodinase; MET, 
metamachines; rT3, 3,3′5′ (rever

deiodinase; MET, 
metamachines; rT3, 3,3′5′ (reverse) triiodothyronine; T2, diiodothyronine; 
T3, 3,5,3′-triiodothyronine; T4, thyroxine.
827
CHAPTER 23  Endocrinology of Fetal Development 
utero is thought to be critical to the maintenance of pregnancy, fetal 
maturation, and the onset of parturition. The placenta produces 
vast amounts of estrogens in the form of estradiol, estrone, estriol, 
and estetrol. The estrogen products released from the placenta 
depend on the nature of the substrate 

e placenta 
depend on the nature of the substrate available. Estradiol is the 
primary estrogen circulating at term. In addition, significant levels 
of estriol and estetrol are also found in the maternal circulation, 
and they increase particularly late in gestation. These hydroxylated 
forms of estrogen are produced in the placenta, using substrates 
from the combined efforts of the fetal adrenal gland and liver. 
Though the primary site of estrogen biosynthesis is the placenta, the 
placenta 

rogen biosynthesis is the placenta, the 
placenta lacks the cytochrome P450 enzyme CYP17 and accord­
ingly is unable to synthesize estrogens de novo.14 Placental estrogen 
biosynthesis relies on a supply of C19 androgens, mainly dehydro­
epiandrosterone (DHEA) and its sulfoconjugate, DHEA sulfate 
(DHEAS), derived principally from the fetal and maternal adrenal 
cortex.14–16 By term, estradiol and estrone concentrations are 100-
fold higher than those of nonpregnant women, and estriol concen­
tr

those of nonpregnant women, and estriol concen­
trations are 1000-fold higher.16 The developing fetus is protected 
from excessive estrogen exposure by conversion of active estradiol to 
inactive estrone by placental 17β hydroxysteroid dehydrogenase.5
During the first trimester, transplacental passage of thyroid 
hormones from the maternal circulation to the fetus is important 
for neurodevelopment. Placental tissue contains an iodothyro­
nine inner ring monodeiodinase, which deiodinates most of

ner ring monodeiodinase, which deiodinates most of the 
thyroxine (T4) to inactive reverse triiodothyronine (rT3) and 
converts active 3,5,3′-triiodothyronine (T3) to inactive diiodothy­
ronine.6,17 Inappropriately high concentrations of iodothyronines 
are associated with fetal loss,18 and the quantity and complement 
of thyroid hormones passing across the placenta are therefore care­
fully regulated. There is, however, some transplacental passage of 
T4 to the fetus in early pregnancy,19–21 an

ge of 
T4 to the fetus in early pregnancy,19–21 and epidemiologic data 
report impairment to intellectual function and behavior in infants 
born to mothers with mild untreated or subclinical hypothyroid­
ism,22 although this may also be due to placental passage of thy­
roid peroxidase (TPO) antibodies.22
Catecholamine-degrading enzymes in placental tissue include 
monoamine oxidase and catechol O-methyltransferase, and both 
metanephrine and dihydroxymandelic acid metabolites of cate­
cholamines

droxymandelic acid metabolites of cate­
cholamines are present in placental homogenates (see Fig. 23.1).23 
Ectopic Fetal Hormone Production
A number of hormones appear to have a much broader tissue dis­
tribution in the fetus, although their functional roles in utero are 
uncertain, and indeed ectopic hormone production is not always 
associated with endocrine manifestations.
Human chorionic gonadotropin (hCG), a heterodimeric gly­
coprotein hormone produced in abundance by the placenta, is 
pr

mone produced in abundance by the placenta, is 
preferentially secreted into maternal circulation. hCG and lutein­
izing hormone (LH) (a structural and functional homolog of 
hCG, produced by the anterior pituitary gland) bind to the same 
transmembrane glycoprotein hCG/LH receptor; however, hCG 
binds with higher affinity. Maternal serum hCG concentrations 
exponentially increase and reach a peak by about the end of the 
first trimester, followed by a rapid decline.24 The fetal circulation 
als

d by a rapid decline.24 The fetal circulation 
also contains low concentrations of hCG; it has been shown that 
fetal kidney, liver, and testes from human fetuses 16 to 20 weeks 
old produce immunoreactive and bioactive hCG in  vitro.25,26 
hCG is also found in fetal ovary, kidney, lung, adrenal, thymus, 
spleen, and muscle27; hCG/LH receptors have been demonstrated 
in fetal kidney, liver, pancreas, lung, small and large intestines, 
and adrenals.28 Although the role of hCG in the fetus is larg

s.28 Although the role of hCG in the fetus is largely 
unknown, it is able to stimulate production of DHEA29 and addi­
tionally stimulates testosterone production by Leydig cells, which 
is essential for masculinization of fetal tissues.30 hCG signaling in 
other tissues may be important for their growth and differentia­
tion. The presence of fetal nongonadal hCG/LH receptors sug­
gests mediation of the pleiotropic actions of hCG in the growing 
human fetus.31
Ectopic production of adrenocortico

Ectopic production of adrenocorticotropic hormone (ACTH), 
derived presumably from a pituitary-independent pro-opiomela­
nocortin (POMC) parent molecule, has been shown with ACTH-
like immunoreactivity present in relatively high concentrations 
in neonatal rat pancreas and kidney.25 POMC is also secreted by 
the neuroendocrine cells of fetal lung. POMC is known to alter 
adrenal sensitivity to ACTH; this interplay between fetal pituitary, 
lung, and adrenal may regulate adrenal re

uitary, 
lung, and adrenal may regulate adrenal response to stress or the 
onset of parturition.32 The fetal lung neuroendocrine cells synthe­
size other hormones in addition to POMC, including vasoactive 
intestinal peptide (VIP) and serotonin, although the function of 
these is unknown.32
Hypothalamic neuropeptides (thyrotropin-releasing hormone 
[TRH], corticotropin-releasing hormone [CRH], growth hor­
mone–releasing hormone [GHRH]) are present in a variety of 
adult tissues, particularly pan

t in a variety of 
adult tissues, particularly pancreas and gut, as well as in fetal gut 
and tissues derived from it.33–35 In neonatal rat pancreatic and 
gastrointestinal tract tissues, high concentrations of TRH and 
somatostatin immunoreactivity have been reported, in contrast to 
low hypothalamic concentrations.36,37 Furthermore, pancreatec­
tomy, rather than encephalectomy, results in a significant reduc­
tion of circulating TRH concentrations in the neonatal rat. In 
the sheep fetus, thyr

ons in the neonatal rat. In 
the sheep fetus, thyroid hormones modulate pancreatic and gut 
TRH concentrations.38 TRH and somatostatin are also present 
in the human neonatal pancreas and detectable in the circula­
tion, with both hormones derived mostly from extrahypothalamic 
sources.39–42 These data suggest a role for extrahypothalamic TRH 
in the control of fetal pituitary thyroid hormone secretion before 
the near-term maturation of hypothalamic TRH. The role of 
extraneural somatostatin in

amic TRH. The role of 
extraneural somatostatin in the fetus remains uncertain.
Ghrelin is a hormone with potent GH-releasing activity, pro­
duced by rat and human gastric endocrine cells and by the pitu­
itary, hypothalamus, and placenta. Ghrelin protein has also been 
identified in the endocrine cells of the fetal lung in decreasing 
amounts from embryonic to late fetal periods, and its expression 
was maintained in neonates.43 Fetal lung is therefore an additional 
source of circulating ghrel

erefore an additional 
source of circulating ghrelin, whose functions at the respiratory 
tract level remains to be clarified.43,44
CRH is a peptide hormone released by the hypothalamus; 
however, human placenta, fetal membranes, and decidua also 
express CRH that is identical to hypothalamic CRH, and from the 
second trimester the placenta is the major source of CRH secre­
tion. The circulating concentrations of CRH increase 1000-fold 
as pregnancy progresses,45 reaching values of 0.5 to 1 nmol

ncy progresses,45 reaching values of 0.5 to 1 nmol/L at 
term; normal values in nonpregnant women are lower than 0.01 
nmol/L. In the last 12 weeks of gestation, CRH plasma concen­
trations rise considerably, peaking during labor and then falling 
precipitously after delivery.46,47 Umbilical cord blood levels and 
amniotic fluid levels of CRH are similarly increased in late gesta­
tion.48 Fetal CRH concentrations are lower than those in mater­
nal circulation (50 vs. 1000 pmol) but are still qui

l circulation (50 vs. 1000 pmol) but are still quite substantial 
compared with those in men and nonpregnant women. Unlike 
828
hypothalamic CRH, placental CRH gene expression and produc­
tion can be stimulated by glucocorticoids.49 This positive feed-
forward system is a unique feature of placental CRH and indicates 
a distinct role in pregnancy. The fetal adrenal and pituitary express 
CRH receptors.50,51 In vitro experiments using cultured primary

.50,51 In vitro experiments using cultured primary 
adrenal cells suggest that in vivo placental CRH stimulates pitu­
itary ACTH secretion, driving fetal HPA activation and directly 
stimulating adrenal steroid production, as well as affecting fetal 
adrenal responsiveness to ACTH, thereby indirectly promoting 
steroidogenesis.51–53 It appears that during most of pregnancy 
CRH-binding protein (CRH-BP) binds most of the circulating 
CRH in the fetal and maternal compartment, which likely serves 

tal and maternal compartment, which likely serves 
to tightly control the activity of placental CRH.54 At the end of 
pregnancy, there is increased bioavailability of CRH due to a fall 
in levels of its binding protein, which results in an exponential 
increase in maternal CRH concentrations from 35 weeks of gesta­
tion to term.45 The steep rise and peak in CRH has been proposed 
as an initiator of parturition by forming a feed-forward loop that 
leads to increased production of adrenal androgen

leads to increased production of adrenal androgens and hence pla­
cental estrogen.45 
Fetal Endocrine Systems
The fetus has a unique hormonal network that combines not only 
its own maturing endocrine systems but also that of the corpus 
luteum, placenta, and maternal hormones. Fetal endocrine organs 
develop from early in the first trimester in a carefully coordinated 
manner, and their abnormal development can impact on a wide 
range of other systems in the body.
Pituitary
Human Hypothalamic-

systems in the body.
Pituitary
Human Hypothalamic-Pituitary Development
The fetal forebrain is identifiable by 3 weeks’ gestation and the 
diencephalon and telencephalon by 5 weeks; Rathke pouch, the 
buccal precursor of the anterior pituitary gland, separates from the 
primitive pharyngeal stomodeum by 5 weeks’ gestation.55–57 The 
neural components (hypothalamus, pituitary stalk, and posterior 
pituitary) are largely developed by 7 weeks of gestation, and the 
bony floor of the sella turcica i

tation, and the 
bony floor of the sella turcica is also present by that time, separat­
ing the adenohypophysis from the primitive gut.
Hypothalamic neurons containing the neuropeptides soma­
tostatin (SS), growth hormone–releasing hormone, thyrotro­
pin-releasing hormone, and gonadotropin-releasing hormone 
(GnRH) are present in hypothalamic tissue by 10 to 14 weeks 
of gestation. Interconnecting fiber tracts are demonstrable by 15 
to 18 weeks. Maturation of the pituitary portal vascular syste

 Maturation of the pituitary portal vascular system 
continues through 30 to 35 weeks when the system becomes func­
tional with portal vascular extension into the hypothalamus.
The definitive Rathke pouch comprises proliferative progeni­
tors that gradually relocate ventrally. A proliferative zone contain­
ing SRY (sex-determining region Y-box 2 [SOX2]–expressing) 
progenitors is maintained in the mouse embryo in a periluminal 
area and persists in the adult.58–60 This can give rise to all cell 

in the adult.58–60 This can give rise to all cell types 
within the anterior pituitary. Specialized anterior pituitary cell 
types (lactotropes, somatotropes, corticotropes, thyrotropes, and 
gonadotropes) are present between 7 and 16 weeks.57 The cellular 
differentiation of the five distinct hormone-producing cell types in 
the anterior pituitary is a highly regulated and coordinated process; 
however, the final differentiation is still unclear. Secretory granules 
are present within anterior 

. Secretory granules 
are present within anterior pituitary cells by 10 to 12 weeks and 
all pituitary hormones can be identified by immunoassay between 
10 and 17 weeks.57,61 Thus the anatomy and biosynthetic mecha­
nisms that make up the hypothalamic-pituitary neuroendocrine 
transducer appear functional by 12 to 17 weeks of gestation. 
Anterior Pituitary and Target Organs
Two distinct components of the pituitary become evident during 
embryogenesis: the adenohypophysis (anterior and intermedi

nesis: the adenohypophysis (anterior and intermediate 
lobes) and neurohypophysis (posterior lobe).62–65 The three lobes of 
the mature pituitary gland have a dual embryonic origin: The ante­
rior and intermediate lobes are derived from oral ectoderm; the pos­
terior pituitary originates from the infundibulum, a specific region of 
the developing central nervous system (CNS) that forms in the mid­
line of the ventral diencephalon. Much information has been derived 
from the mouse as a model orga

n has been derived 
from the mouse as a model organism for pituitary development in 
mammals, but fate map studies have shown that these processes are 
similar in all vertebrate species studied, including zebrafish, amphibi­
ans, chicks, and rodents.66–69 In the mouse, the first signs of pituitary 
development occur at 7.5 days postconception (dpc) with the devel­
opment of the hypophyseal placode, a thickening of the ectoderm in 
the midline of the anterior neural ridge (Fig. 23.2). At approxim

the anterior neural ridge (Fig. 23.2). At approximately 
9 dpc the placode invaginates dorsally to form a rudimentary Rathke 
pouch, the primordium of the anterior and intermediate lobes. By 
10.5 dpc, the overlying neuroectoderm evaginates to form the infun­
dibulum from which the posterior pituitary and pituitary stalk will 
derive, which then comes into direct contact with Rathke pouch. 
The juxtaposition of Rathke pouch and the diencephalon is main­
tained throughout the early stages of pitu

s main­
tained throughout the early stages of pituitary organogenesis. This 
close relationship is required for tissue interactions between neural 
and oral ectoderm, which are critical for the initial stages of pitu­
itary specification. The iterative nature of the inductive interactions 
required for pituitary morphogenesis makes it very sensitive to both 
loss-of-function and gain-of-function mutations.70 
Intermediate Pituitary Lobe
The intermediate lobe of the pituitary gland is prominent i

mediate lobe of the pituitary gland is prominent in both the 
human and the sheep fetus. Intermediate lobe cells begin to disap­
pear near term and are virtually absent in the adult human pituitary, 
although the intermediate lobe in the adult of some lower species 
is anatomically and functionally distinct.71 The major secretory 
products of the intermediate lobe are α-melanocytic-stimulating 
hormone (α-MSH) and β-endorphin, both derived from cleavage 
of the POMC molecule.72 Cleavage of POMC 

leavage 
of the POMC molecule.72 Cleavage of POMC in the anterior lobe 
results predominantly in corticotropin and β-lipotropin formation. 
In rhesus monkeys and humans, the fetal pituitary contains high 
concentrations of compounds resembling α-MSH and corticotro­
pin-like intermediate lobe peptide (CLIP).73 In the human fetus, 
α-MSH levels decrease with increasing fetal age.74 The circulating 
concentrations of β-endorphin and β-lipotropin are high in the 
fetal lamb, and the ratio of β-endor

 high in the 
fetal lamb, and the ratio of β-endorphin to β-lipotropin increases 
during hypoxic stimulation of the anterior pituitary.75 Because 
hypoxia provokes corticotropin release and β-lipotropin produc­
tion from the anterior pituitary, these data have been interpreted 
to suggest that basal β-endorphin levels in the fetus originate in 
the intermediate lobe. α-MSH and CLIP may play a role in fetal 
adrenal activation, and α-MSH may play a role in fetal growth.76,77 
The processing of pi

a role in fetal growth.76,77 
The processing of pituitary POMC in the human fetus by the end 
of the second trimester is similar to that in the adult, but the role 
of these intermediate lobe peptides in the fetus remains obscure.78 
The posterior lobe comprises axonal projections of neurons, which 
traverse the pituitary stalk and median eminence at the base of the 
829
hypothalamus. The neurons originate from hypothalamic magn

amus. The neurons originate from hypothalamic magno­
cellular bodies termed the supraoptic, suprachiasmatic, and para­
ventricular nuclei. The former two release arginine vasopressin 
and the latter releases oxytocin.79
The fetal neurohypophysis is well developed by 10 to 12 weeks of 
gestation and contains both arginine vasopressin (AVP) (also called 
antidiuretic hormone, ADH) and oxytocin (OT).80,81 In addition, 
arginine vasotocin (AVT), the parent neurohypophyseal hormone 
in submammalian v

parent neurohypophyseal hormone 
in submammalian vertebrates, is present in the fetal pituitary and 
pineal glands and in adult pineal glands from several mammalian 
species, including humans.82 AVT is present in the pituitary dur­
ing fetal life from 11 to 19 weeks, is secreted by cultured human 
fetal pineal cells during the second trimester, and disappears in 
the neonatal period.81,82 In adult mammals, instillation of AVT 
into cerebrospinal fluid inhibits gonadotropin and corticotropin 
rel

fluid inhibits gonadotropin and corticotropin 
release, stimulates prolactin (PRL) release by the anterior pituitary, 
and induces sleep; however, the physiologic importance of these 
effects remains unclear. The role of AVT in the fetal pineal gland is 
unknown. Recent work shows a role for oxytocin in the develop­
ment of the neurovascular interface in the posterior pituitary.83 By 
40 weeks, the concentrations of AVP and OT approximate 20% of 
those in adults. Fetal pituitary oxytocin concent

those in adults. Fetal pituitary oxytocin concentration, detectable 
by 11 to 15 weeks, exceeds AVP concentration by 19 weeks. The 
AVP-oxytocin ratio falls progressively thereafter.
In the fetal sheep, the baseline fetal plasma AVP concentra­
tions are similar to maternal levels after midgestation. During 
the last trimester of gestation, fetal hypothalamic and pituitary 
responsiveness to both volume and osmolar stimuli for AVP secre­
tion are well developed, and AVP exerts antidiuretic effec

 well developed, and AVP exerts antidiuretic effects on 
the fetal kidney.80,81 Baseline plasma levels of AVT in fetal sheep 
during the last trimester approximate values for AVP and OT.82 
Presumably this AVT is derived from the posterior pituitary, but 
the stimuli for AVT secretion in the fetus are not defined. The 
neurohypophyseal peptides are synthesized as large precursor 
molecules (neurophysins) and processed to bioactive amidated 
peptides.84 Enzymatic processing involves progressive c

des.84 Enzymatic processing involves progressive cleavage 
of carboxyl terminal–extended peptides, sequentially producing 
(for OT) OT-glycine-lysine-arginine (OTGKR), OTGK, OTG, 
and OT. Similar progressive processing yields AVP-glycine and 
AVP from the AVP neurophysin. Enzymatic processing of neu­
rophysins matures progressively in the fetus so that early in gesta­
tion fetal plasma contains relatively large concentrations of the 
extended peptides.84 For OT, the ratio of OT-extended peptides

tides.84 For OT, the ratio of OT-extended peptides to 
OT in fetal sheep serum is approximately 35:1 early in gestation 
and 3:1 late in gestation.84
In the fetus, AVP appears to function as a stress-responsive hor­
mone. Perhaps the major potential stress for the fetus is hypoxia, 
and the response of AVP to hypoxia is increased compared with 
the maternal response and with fetal AVP responses to osmolar 
stimuli.81,85–87 Plasma AVP concentrations in human cord blood 
are elevated in associatio

ns in human cord blood 
are elevated in association with intrauterine bradycardia and meco­
nium passage.86 The vasopressor action of AVP may be important 
in the maintenance of fetal circulatory homeostasis during hemor­
rhage and hypoxia; AVP has a limited effect on fetoplacental blood 
flow.81,88 Fetal hypoxia is also a major stimulus for catecholamine 
release. There is little information on interactions between AVP 
and catecholamines during fetal hypoxia, but both fetal hypoxia 
and AVP st

 fetal hypoxia, but both fetal hypoxia 
and AVP stimulate anterior pituitary function.88 A role for AVP 
as a corticotropin-releasing hormone is established in the adult, 
and the ovine fetal pituitary responds separately and synergisti­
cally to AVP and CRH early in the third trimester.89 The role of 
AVP in controlling fetal corticotropin release seems to decrease 
PL
I
PL
O
N
IL
AL
SC
OC
P
DI
PO
Oral ectoderm
Rat
Mouse
8.5
8.0–8.5
11
9.5
14.5
12
19.5
17
Embryonic
days
Rudimentary pouch
Defini

12
19.5
17
Embryonic
days
Definitive pouch
NP
N
F
AN
I
MB
HB
RP
O
H
N
H
OM
I
PP
A
B
C
D
• Fig. 23.2  Schematic representation of the developmental processes implicated in pituitary development. 
Mouse pituitary development in sagittal section. Stages of development are indicated in dpc. AL, anterior 
lobe; AN, anterior neural pore; DI, diencephalon; F, forebrain; H, heart; HB, hindbrain; I, infundibulum; IL, 
intermediate lobe; MB, midbrain; N, notochord; NP, ne

ermediate lobe; MB, midbrain; N, notochord; NP, neural plate; O, oral cavity; OC, optic chiasma; OM, oral 
membrane; P, pontine flexure; PL, posterior lobe; PO, pons; PP, prechordal plate; RP, Rathke pouch; SC, 
sphenoid cartilage. (Based on Sheng HZ, Westphal H. Early steps in pituitary organogenesis. Trends Genet. 
1999;15:236–240. As adapted in Gevers EF, Fisher DA, Dattani MT. Fetal and neonatal endocrinology. In 
Jameson JL, De Groot LJ, eds. Endocrinology: Adult and Pediatric. 7th ed. Phil

. Endocrinology: Adult and Pediatric. 7th ed. Philadelphia: Elsevier; 2016.)
830
with gestational age. It is not known whether AVT functions as a 
corticotropin-releasing hormone in the fetus.
OT receptors have been demonstrated in human fetal mem­
branes at term, and AVP receptors have been found in renal med­
ullary membranes of newborn sheep.82,90,91 Both AVP and AVT 
evoke antidiuretic actions in the sheep fetus during the last third 
of gestatio

the sheep fetus during the last third 
of gestation, and both hormones act to conserve water for the 
fetus by inhibiting fluid loss into amniotic fluid through the lungs 
and kidneys.81,82 Water channel receptors (aquaporins 1, 2, and 3) 
are present in the human fetal and newborn kidney, and the ability 
of the newborn infant to regulate free water clearance in response 
to volume and osmolar stimuli has been demonstrated.92 Whether 
AVT exerts its effects through AVP receptors or separate fet

 its effects through AVP receptors or separate fetal AVT 
receptors is not clear. Maximal concentrating capacity by the fetal 
kidney is limited to about 600 mmol/L. This limitation is not 
related to inadequate AVP stimulation but to inherent immaturity 
of the renal tubules.
Lack of AVP is associated with diabetes insipidus (DI) and 
with a failure to retain water in the body, leading to polyuria and 
polydipsia. Although most cases of DI are from acquired causes 
such as pituitary germinoma, 

rom acquired causes 
such as pituitary germinoma, craniopharyngioma, or Langerhans 
cell histiocytosis, the condition may rarely be due to mutations in 
the AVP-neurophysin gene or to other congenital causes such as 
septo-optic dysplasia or holoprosencephaly. Mutations in WFS1 
are associated with autosomal recessive Wolfram syndrome that 
includes diabetes insipidus, diabetes mellitus, optic atrophy, and 
sensorineural hearing loss.93 
Hypothalamus and Pituitary Stalk
The anatomy of the develo

amus and Pituitary Stalk
The anatomy of the developed hypothalamus is well understood. 
It extends from the anteriorly located optic chiasm to the pos­
teriorly located mammillary body and is organized into distinct 
rostral-to-caudal regions: preoptic, anterior, tuberal, and mam­
millary. The organ is subdivided into three medial-to-lateral 
regions: periventricular, medial, and lateral.94 Contained within 
the medial region are the preoptic nucleus, the anterior hypothala­
mus, the dorsomedial

leus, the anterior hypothala­
mus, the dorsomedial nucleus, the ventromedial nucleus, and the 
mammillary nuclei. The lateral zone comprises the preoptic and 
hypothalamic areas.94
As Rathke pouch invaginates, part of the ventral diencepha­
lon evaginates ventrally to form the infundibulum and, later, the 
posterior pituitary lobe and pituitary stalk. The pituitary stalk 
acts as a physical connection between the pituitary gland and 
brain and contains the hypophyseal (hypothalamic-pituitary) 
p

ntains the hypophyseal (hypothalamic-pituitary) 
portal system as well as the neuronal connections traversing 
across the hypothalamic median eminence. These neurons origi­
nate from the supraoptic, suprachiasmatic, and paraventricu­
lar nuclei, which are large hypothalamic magnocellular bodies 
located within the periventricular region of the hypothalamus.94 
Within the median eminence itself at the base of the hypo­
thalamus is the capillary bed into which the widely dispersed 
hypothalamic pa

d into which the widely dispersed 
hypothalamic parvocellular neurons secrete hypophysiotrophic 
hormones. These stimulate the release of the seven anterior/
intermediate pituitary lobe hormones via the hypophyseal por­
tal system. The parvocellular neurons also secrete oxytocin and 
arginine vasopressin, although at much lower concentrations 
than the magnocellular neurons, with the parvocellular-derived 
arginine vasopressin acting synergistically with corticotropin-
releasing hormone in regul

lly with corticotropin-
releasing hormone in regulating ACTH release. It is therefore 
evident that it is the hypothalamus acting through the pituitary 
gland that is the central mediator of growth, reproduction, and 
homeostasis.55
Deciphering hypothalamic development during embryogenesis 
has proved difficult, perhaps due to its anatomic complexity and 
diverse collection of cell groups and neuronal subtypes. There is a 
dearth of data defining the genetics and the signaling and marker 
molecu

 the genetics and the signaling and marker 
molecules involved in their delineation and identification.95,96 A 
dual novel role for fibroblast growth factor (FGF)3/FGF10 was 
identified in innervation and vascularization of the prospective 
vertebrate hypothalamic region, acting as chemoattractive cues 
to the neurohypothalamic neurons.97 Little is known about the 
molecular mechanisms governing the formation of the hypothala­
mus, and genetic expression studies within the hypothalamus have 
kno

pression studies within the hypothalamus have 
knock-on effects on multiple neuronal subtypes and downstream 
physiologic processes. Studies are slowly elucidating hypothalamic 
development, and this will in time elaborate the processes involved. 
Genes Involved in Pituitary Disease
Complex genetic interactions dictate normal pituitary develop­
ment.55 A cascade of signaling molecules and transcription factors 
plays a crucial role in organ commitment, cell proliferation, cell 
patterning, and t

tment, cell proliferation, cell 
patterning, and terminal differentiation. The final product is a cul­
mination of this coordinated process (Fig. 23.3).
Initially, cells within the primordium of the pituitary gland 
are competent to differentiate into all cell types. After expression 
of the earliest markers of pituitary gland development, such as 
homeobox gene expressed in embryonic stem (ES) cells (Hesx1), 
further signaling pathways are established from within the gland 
and ventral dienceph

lished from within the gland 
and ventral diencephalon that direct these cells toward terminal 
differentiation into mature hormone-secreting cell types. Sig­
naling molecules and transcription factors are expressed sequen­
tially at critical periods of pituitary development, and expression 
of many of these factors is subsequently attenuated (Fig. 23.4). 
Genes expressed early are implicated in organ commitment but 
are also implicated in repression and activation of downstream 
target genes th

sion and activation of downstream 
target genes that have specific roles in directing the cells toward 
a particular fate.
Spontaneous or artificially induced mutations in the mouse 
have led to significant insights into human pituitary disease, and 
identification of mutations associated with human pituitary dis­
ease have in turn been invaluable in defining the genetic cascade 
responsible for the development of this embryologic tissue. Muta­
tions involved specifically in human hypothalamic-p

ions involved specifically in human hypothalamic-pituitary dis­
ease are listed in Table 23.2 and briefly discussed in this section.
Extrinsic molecules within the ventral diencephalon and sur­
rounding structures, such as bone morphogenetic proteins 2 
and 4 (BMP2, BMP4), FGF8, sonic hedgehog (SHH), wingless 
(Wnt4), thyroid transcription factor 1 (TTF1; also called Nkx2-
1), and molecules involved in Notch signaling, play critical roles 
in early organogenesis.55,56,97 Recent studies in the mo

ly organogenesis.55,56,97 Recent studies in the mouse have 
shown that a close interaction between oral ectoderm and neural 
ectoderm is critical for initial development of the pituitary gland. 
Rathke pouch develops in a two-step process that requires at least 
two sequential inductive signals from the diencephalon. First, 
induction and formation of the rudimentary pouch are depen­
dent upon BMP4; second, FGF8 activates two key regulatory 
genes, LIM homeobox 3 (LHX3) and LIM homeobox 4 (LHX4)

s, LIM homeobox 3 (LHX3) and LIM homeobox 4 (LHX4), 
that are essential for subsequent development of the rudimentary 
pouch into a definitive pouch. BMP4 and FGF8 are present only 
in the diencephalon, not in Rathke pouch. Murine mutations 
within the gene encoding TTF1/Nkx2-1 (also called thyroid-
specific enhancer binding protein), which is expressed only in 
the presumptive ventral diencephalon, can cause severe defects in 
the development of the diencephalon and the anterior pituitary 
831

 the diencephalon and the anterior pituitary 
831
gland. Conditional deletion of RBPJ, which encodes the major 
mediator of the Notch pathway, leads to conversion of the late 
(pituitary-specific transcription factor 1 [Pit1]) lineage into the 
early (corticotroph) lineage. Notch signaling is required for main­
taining expression of prophet of Pit1 (PROP1), which is required 
for generation of the Pit1 lineage. Attenuation of Notch signaling 
is ne

it1 lineage. Attenuation of Notch signaling 
is necessary for terminal differentiation in Pit1 cells and for matu­
ration and proliferation of the GH-producing somatotrophs.98
Mutations of the sonic hedgehog (SHH) signaling pathway 
(SHH, TGIF, ZIC2, PTCH1, GLI2) and the transcription fac­
tors sine oculis–related homeobox 3 (SIX3), teratocarcinoma-
derived growth factor 1 (TDGF1), and FOXH1/FAST1 have been 
identified in patients with holoprosencephaly, with and without 
hypothalamic-pituitary 

cephaly, with and without 
hypothalamic-pituitary defects.99–103 Mammalian SIX6 and SIX3 
are vertebrate homologues of Drosophila optix, and expression of 
these are restricted to the developing eye and brain. SIX6 knock­
out mice have hypoplastic pituitaries, may be blind, and fail to 
progress through the estrous cycle with resultant infertility. SIX3 
and SIX6 are transcription factors of importance for pituitary lin­
eage specifications, in particular gonadotropic cell lines.104
Mutations in

articular gonadotropic cell lines.104
Mutations in FGF8 have also been identified in association 
with both holoprosencephaly associated with diabetes insipidus 
and Kallmann syndrome (KS).62,105 Mutations in HESX1 have 
been identified in patients with septo-optic dysplasia (SOD; a 
VENTRAL DIENCEPHALON
Bmp4
Sox3
Wnt4
Shh
Sox2
Hesx1
Pitx1
Pitx1
Prop1
Pit1
Sf1,
LH
FSH
TSH
T4
GH
GSα
 PRL
TGF
ORAL ECTODERM
CAMP
CREB
ESTROGEN
Egfr
DRD2
Ghrhr
Gata2?
Tbx19/Tpit
ACTH
POMC
Lhx3
Lhx4
Nkx2.1
Otx2 Fgf8 Fg

ACTH
POMC
Lhx3
Lhx4
Nkx2.1
Otx2 Fgf8 Fgf10
Wnt5a
Sf1
Lhx4
Foxb1
Gata 4
TRH
DOPAMINE
Gnrh
Ghrh
Isl2, Gata2,
Foxl2, Egr1
Six3
Six6
Isl1
Isl2
Lhx2
Pax6
Dlx2
Pitx1
Pitx2
Lhx3
Lhx4
• Fig. 23.3  Schematic representation of the developmental cascade of genes implicated in human pituitary 
development with particular reference to pituitary cell differentiation.
Pituitary placode
E8–8.5
Extrinsic
Intrinsic
Rudimentary
Rathke pouch
E9.5
Definitive
E11
Adult
E15–17
B

ve
E11
Adult
E15–17
Bmp4
Ttf1
Bmp4
Fgf8
Wnt5a
+
+
–
Fgf8
Wnt5a
Hesx1
Six3
Pax6
Ptx1
Is11
Hesx1
Six3
Is11
Lhx3
Lhx4
Ptx1
Wnt4
Nkx3.1
Hesx1
Six3
Is11
Lhx4
Ptx1
Wnt4
Nkx3.1
Ptx2
αGSU
Prop1
Bmp4
Bmp2
Bmp7
Msx1
Brn4
P-Frk
Lhx3
Lhx4
Sf1
GATA2
Pit1
αGSU
• Fig. 23.4  Transcription factors and signaling molecules involved in anterior pituitary development.
  Comparison of Murine and Human Hypothalamic-Pituitary Development Phenotypes
Gene
Protein
Murine 
Loss-of-Function Phe

enotypes
Gene
Protein
Murine 
Loss-of-Function Phenotype
Human Phenotype
Inheritance (Murine and 
Human)
HESX1
HESX1
Anophthalmia or microphthalmia, agenesis 
of corpus callosum, absence of septum 
pellucidum, pituitary dysgenesis, or 
aplasia
Variable: SOD, CPHD, IGHD with EPP
Anterior pituitary hypoplastic or absent 
Posterior pituitary ectopic or eutopic
Frequency of mutations: <1%
Dominant or recessive in 
humans, recessive in 
mouse
OTX2
OTX2
Lack of forebrain and midbrain, olfactory 
placo

2
placode, optic placodes
Anophthalmia, APH, ectopic posterior pituitary, 
absent infundibulum
Frequency of mutations: 2%–3% of anophthal­
mia/microphthalmia cases
Heterozygous: haploinsuffi­
ciency/dominant negative
SOX2
SOX2
Homozygous null mutants: embryonic lethal
Heterozygous mice and further dose reduc­
tion: poor growth, reduced fertility, CNS 
abnormalities, anophthalmia; pituitary 
hypoplasia with reduction in all cell types
Hypogonadotropic hy

th reduction in all cell types
Hypogonadotropic hypogonadism; APH, abnormal 
hippocampi, bilateral anophthalmia/microph­
thalmia, abnormal corpus callosum, learning 
difficulties, esophageal atresia, sensorineural 
hearing loss, hypothalamic hamartoma
Frequency of mutations: 3%
De novo haploinsufficiency 
in humans, heterozygous 
mutation associated with 
haploinsufficiency in 
mouse
SOX3
SOX3
Poor growth, weakness, craniofacial 
abnormalities, ACC, hypothalamic and 
infundibular abnormalities
I

CC, hypothalamic and 
IGHD and mental retardation, hypopituitarism; 
APH, infundibular hypoplasia, EPP, midline 
Frequency of mutations: 6% (duplications), 1.5% 
(mutations)
X-linked recessive in both 
mice and humans
GLI2
GLI2
N/A
Holoprosencephaly, hypopituitarism, craniofacial 
abnormalities, polydactyly, single nares, single 
central incisor, partial ACC
Frequency of mutations: 1.5%
Haploinsufficiency in humans
FGF8
FGF8
Holoprosencephaly, reduction o

in humans
FGF8
FGF8
Holoprosencephaly, reduction of vasopressin 
and oxytocin, reduced GnRH neurons
Holoprosencephaly, hypogonadotrophic hypogo­
nadism, ACTH and TSH deficiencies, diabetes 
insipidus
AR in both human and 
mouse; AD in some cases
LHX3
LHX3
Hypoplasia of Rathke pouch
GH, TSH, gonadotropin deficiency with pituitary 
hypoplasia
ACTH insufficiency variable
Short, rigid cervical spine
Variable sensorineural hearing loss
Frequency of mutations: 1.3%
Recessive in both
LHX4
LHX4
Mild hyp

tations: 1.3%
LHX4
LHX4
Mild hypoplasia of anterior pituitary
GH, TSH, cortisol, and gonadotropin deficiency, 
persistent craniopharyngeal canal and abnor­
mal cerebellar tonsils; APH, ectopic/eutopic 
posterior pituitary, absent infundibulum
Frequency of mutations: 1.2%
Recessive in mouse, reces­
sive/dominant or reces­
sive in humans
PROP1
PROP1
Hypoplasia of anterior pituitary with reduced 
somatotropes, lactotropes, thyrotropes, 
corticotropes, and gonadotropes
GH, TSH, PRL

pes, 
GH, TSH, PRL, and gonadotropin deficiency
Evolving ACTH deficiency
Enlarged pituitary with later involution
Frequency of mutations: 1.1% sporadic cases, 
29.5% familial cases
POU1F1
POU1F1 
(PIT1)
Anterior pituitary hypoplasia with reduced 
somatotropes, lactotropes, and thyro­
tropes
Variable anterior pituitary hypoplasia with GH, 
TSH, and PRL deficiencies
Frequency of mutations: 3.8% sporadic cases, 
18% familial cases
Recessive in mouse

adic cases, 
Recessive in mouse, domi­
nant/recessive in humans
ARNT2
ARNT2
Anterior pituitary hypoplasia, TRH, soma­
tostatin, oxytocin and CRH deficiencies, 
reduced vasopressin neurons
TSH, GH, ACTH deficiencies, DI, small anterior 
pituitary, vesicoureteric reflux, renal impair­
ment, visual impairment, neonatal seizures 
with progressive microcephaly
Autosomal recessive
PNPLA6
PNPLA6
N/A
Oliver-McFarlane syndrome; trichomegaly, 
congenital hypopituitarism and retinal dege

galy, 
congenital hypopituitarism and retinal degen­
eration with choroidal atrophy; hypogonado­
trophic hypogonadism
TCF7L1
TCFL1
Forebrain, eye and pituitary deficits
Septo-optic dysplasia with variable hypopituita­
rism
Autosomal dominant
ACC, Agenesis of corpus callosum; ACTH, adrenocorticotropic hormone; APH, anterior pituitary hypoplasia; AR, androgen receptor; CNS, central nervous system; CPHD, combined pituitary hormone defi­
ciencies; EPP, ectopic posterior pituitary

e defi­
ciencies; EPP, ectopic posterior pituitary; GH, growth hormone; IGHD, isolated growth hormone deficiency; N/A, not applicable; PRL, prolactin; SOD, septo-optic dysplasia; TSH, thyroid-stimulating hormone.
   
TABLE 23.2
833
combination of pituitary, eye, and midline forebrain defects), 
combined pituitary hormone deficiencies (CPHDs), and isolated 
growth hormone deficiency (IGHD).55,106 Mutations in SOX2 
and OTX2 have been described in as

ations in SOX2 
and OTX2 have been described in association with severe eye 
defects, hypogonadotropic hypogonadism, and variable hypopitu­
itarism.107–111 Mutations and genomic duplications in SOX3 have 
been identified in patients with hypopituitarism with and without 
learning defects.112
Mutations in the gene encoding the LIM homeodomain 
transcription factor LHX3 have been identified in patients with 
hypopituitarism, neck abnormalities, and sensorineural deafness, 
whereas mutations in LHX

 sensorineural deafness, 
whereas mutations in LHX4 have been identified in patients with 
hypopituitarism, which can be lethal, and cerebellar abnormali­
ties.113–115 Mutations in genes expressed later in pituitary develop­
ment, such as PROP1 and POU1F1 (previously known as PIT1), 
are associated with more specific pituitary phenotypes (variable 
GH, thyroid-stimulating hormone [TSH], ACTH, PRL, and 
gonadotropin deficiencies and often a large anterior pituitary that 
later involutes with PROP

anterior pituitary that 
later involutes with PROP1 mutations; GH, TSH, and PRL defi­
ciencies with POU1F1 mutations), in keeping with a role for these 
genes in cellular proliferation and differentiation and hormone 
secretion.55,116–118 Mutations in the T-box transcription factor 
TBX19/TPIT have been described in patients with early-onset 
isolated ACTH deficiency.119 Mutations in TSHβ have been asso­
ciated with central hypothyroidism.120 More recently, mutations 
and deletions of IGSF1 have

e recently, mutations 
and deletions of IGSF1 have been associated with TSH and vari­
able GH and PRL deficiencies associated with macro-orchidism. 
Immunoglobulin superfamily, member 1 (IGSF1) is a glycopro­
tein that is expressed in the pituitary and testis. The function of 
the gene remains unclear but it may be involved in TRH signal­
ing.121,122 Central hypothyroidism may be more prevalent than 
previously thought, affecting up to 1 in 16,000 neonates in the 
Netherlands where screening is 

0 neonates in the 
Netherlands where screening is via a combination of T4 and TSH.
Rare recessive biallelic inactivating mutations in TRHR, 
namely p.S115-T117del and p.A118T, have been reported in 
three affected individuals from two unrelated pedigrees with cen­
tral congenital hypothyroidism (CCH), with absent TSH and 
prolactin responses to exogenous TRH.123,124 More recently, the 
p.P81R missense mutation described in isolated CCH highlights 
the importance of the second transmembrane helix

the importance of the second transmembrane helix in mediating 
TRH receptor activation via hormone binding, making it the 
first deleterious missense TRHR defect that gives rise to CCH.125 
In addition, a recently identified novel homozygous mutation, 
p.I131T, that decreases TRH affinity was identified in an over­
weight patient with CCH and normal stature.126
The X-linked transducin β-like protein 1 (TBL1X) gene is a 
component of the thyroid hormone receptor–corepressor com­
plex, mutations

 hormone receptor–corepressor com­
plex, mutations that have been previously associated with sen­
sorineural hearing loss.127,128 In a recent study, six mutations in 
unrelated pedigrees with isolated CCH have been identified.129 
Like IGSF1, TBL1X is associated with an X-linked form of TSH 
PCSK11 mutations result in ACTH deficiency, hypoglyce­
mia, hypogonadotrophic hypogonadism, obesity, GH deficiency, 
and DI. In a study of 13 children with proprotein convertase 1/3 
(PC1/3) defi

ldren with proprotein convertase 1/3 
(PC1/3) deficiency, an autosomal recessive disorder caused by 
rare mutations in the proprotein convertase subtilisin/kexin type 
1 (PCSK1) gene, obesity and severe malabsorptive diarrhea were 
associated with a number of endocrine abnormalities. PC1/3 is an 
endoprotease that processes many prohormones expressed in endo­
crine and neuronal cells. Failure of enteroendocrine cells to pro­
duce functional hormones resulted in generalized malabsorption. 
Additi

nes resulted in generalized malabsorption. 
Additional endocrine abnormalities that developed with disease 
progression included central DI, GH deficiency, primary male 
hypogonadism, adrenal insufficiency secondary to ACTH defi­
ciency, and hypothyroidism.130
The hypothalamic-pituitary-gonadal axis is the key regulator 
of sex development and reproduction, processes that are initiated 
through the decapeptide GnRH. GnRH is produced and released 
after the successful migration during embryogenes

after the successful migration during embryogenesis of GnRH 
neurons from the olfactory placode, across the cribriform plate, 
to the hypothalamic arcuate nucleus, where they are detectable 
from about 9 weeks of gestation. These neurons are then pro­
jected into the hypothalamic median eminence. It has recently 
been reported that the number of GnRH neurons in the brain 
at the end of the first trimester is fivefold higher (∼10,000) than 
previously described in the hypothalamus.131 New insigh

ously described in the hypothalamus.131 New insights from 
three-dimensional (3D) imaging include the presence of GnRH 
neurons in nonhypothalamic areas, raising the possibility of non­
reproductive roles for GnRH.131
A functional connection with respect to GnRH is detected 
by 16 to 20 weeks’ gestation. This hormone is released in a pul­
satile fashion and binds to its receptors on pituitary gonado­
trophs, which in turn respond by synthesizing and releasing 
the gonadotropins LH and FSH. These

and releasing 
the gonadotropins LH and FSH. These bind to their cognate 
receptors in the gonads, where they stimulate the production 
of sex steroids, such as androgens or estrogens, and stimulate 
gametogenesis. The sex steroids then regulate gonadotropin 
secretion via negative feedback at the level of the hypothalamus 
or pituitary. Congenital disorders of gonadotropin secretion 
include normosmic isolated GnRH deficiency causing hypogo­
nadotropic hypogonadism and KS, a disorder characteri

otropic hypogonadism and KS, a disorder characterized by 
hypogonadotropic hypogonadism, anosmia/hyposmia, cleft lip/
plate, sensorineural hearing loss, dental anomalies, synkinesia, 
and renal abnormalities.
Mutations in a number of genes have been reported to date 
in Kallmann syndrome, including KAL1, CCDC141, FEZF1, 
IL17RD, SEMA3A, SEMA3E, and SOX10.105,132–134 Pathogenic 
variants in CHD7, FGF8, FGF17, FGFR1, DUSP6, SPRY4, 
and FLRT3, HS6ST1, NSMF (NELF), PROK2, PROKR2, and 
WDR11 cause bo

1, NSMF (NELF), PROK2, PROKR2, and 
WDR11 cause both KS and normosmic hypogonadotropic hypo­
gonadism.134,135 Variants in encoding prokineticin receptor 2 and 
prokineticin 2 (PROKR2 and PROK2, respectively) have been 
identified in approximately 9% of patients with Kallmann syn­
drome.136 Prokineticins are secreted cysteine-rich proteins that 
possess diverse biologic activities, including effects on neuronal 
survival, gastrointestinal smooth muscle contraction, circadian 
locomotor rhythm, an

uscle contraction, circadian 
locomotor rhythm, and appetite regulation.136 Prokineticins 
PROK1 and PROK2 act through their G protein–coupled recep­
tors PROKR1 and PROKR2, which are expressed in the olfactory 
bulbs. PROK2 functions as a chemoattractant for neuronal pro­
genitors, which follow a rostral migratory stream. Prokr2−/− mice 
have reduced LH in the pituitary, small gonads, and abnormal 
olfactory bulb formation, but Prokr2 seems to be dispensable for 
normal pituitary formation.84,8

e dispensable for 
normal pituitary formation.84,85 Variations in PROKR2 have also 
been associated with hypopituitarism, pituitary stalk interrup­
tion syndrome, and septo-optic dysplasia but are unlikely to be 
causative in isolation and likely only contribute to the phenotype, 
including KS, in combination with other genetic mutations or 
environmental factors.
Heterozygous pathogenic variants or microdeletions in CHD7 
can also cause CHARGE syndrome, characterized by coloboma, 
heart abnorma

yndrome, characterized by coloboma, 
heart abnormalities, choanal atresia, retardation of growth and 
development, genital hypoplasia, and ear abnormalities.137 The 
834
genital abnormalities in CHARGE syndrome are caused by hypo­
gonadotropic hypogonadism and are frequently accompanied by 
olfactory defects and cleft lip/palate.138
Mutations in a number of genes have also been identified in 
association with normosmic hypogonadotropic hypogonadism, 

ion with normosmic hypogonadotropic hypogonadism, 
and these mutations include genes encoding GnRH1 and its 
receptor GnRHR, kisspeptin (KISS1) and its receptor KISS1R, 
and neurokinin B (encoded by tachykinin 3 [TAC3] ) and its 
receptor TACR3.134,139–142
Recently, mutations in genes such as RNF216, OTUD4, STUB1, 
and PNPLA6 have been found in syndromic forms of GnRH defi­
ciency that have associated features such as ataxia and dementia, as 
part of Gordon Holmes syndrome (RNF216, OTUD4, STUB1)

t of Gordon Holmes syndrome (RNF216, OTUD4, STUB1) 
and Boucher-Neuhäuser syndrome (PNPLA6).143–145 RNF216, 
OTUD4, and STUB1 are involved in protein ubiquitination; 
PNPLA6 encodes an enzyme involved in the production of the 
neurotransmitter acetylcholine.
Other genes implicated in syndromic forms of hypotha­
lamic-pituitary disease include GLI3, a component of the SHH 
signaling pathway, haploinsufficiency of which results in Pallis­
ter-Hall syndrome associated with polydactyly, hypothalamic

syndrome associated with polydactyly, hypothalamic 
disorganization, hypothalamic hamartoma, and hypopituita­
rism.146,147 PITX2 mutation has been identified as one cause of 
Axenfeld-Rieger syndrome, including ocular, dental, and hypo­
thalamic abnormalities.148 Pitx2-null mutant mice manifest 
pituitary hypoplasia and decreased GHRH receptor (GHRH-R), 
GH, FSH, LH, and TSH gene expression.148
Mutations in ARNT2 are associated with severe pituitary insuf­
ficiency, including GH, TSH, and ACTH d

ary insuf­
ficiency, including GH, TSH, and ACTH deficiencies, and diabe­
tes insipidus with progressive microcephaly, seizures, severe visual 
impairment, severe learning difficulties, and abnormalities of the 
renal and urinary tracts.149 Aryl hydrocarbon receptor nucleus 
translocator 2 (ARNT2) is a basic helix-loop-helix transcription 
factor critical for normal development of the paraventricular and 
supraoptic nuclei.
Deficient anterior pituitary with variable immune deficiency 
(DAVID) sy

uitary with variable immune deficiency 
(DAVID) syndrome is a rare disorder comprising central ACTH 
deficiency and common variable immune deficiency (which can 
be mild, absent, or develop at a later stage), and a small anterior 
pituitary has been described in this condition. The syndrome is 
caused by mutations in NFKB2.150
Rare isolated pituitary hormone deficiencies have been asso­
ciated with mutations in the respective hypothalamic-releasing 
hormones or the releasing hormone receptors (e

ng 
hormones or the releasing hormone receptors (e.g., familial GH 
deficiency due to GHRH-R mutations), TSH deficiency due to 
TRHR mutations, and gonadotrophin deficiency due to GNRHR 
mutations.123,140,151
Far fewer mutations have been identified to account for abnor­
malities of the posterior pituitary and stalk. Pituitary stalk inter­
ruption syndrome comprises a thin or absent pituitary stalk, an 
ectopic posterior pituitary, and small or absent anterior pitu­
itary, usually accompanied by

bsent anterior pitu­
itary, usually accompanied by pituitary hormone deficiencies. 
Roundabout (robo) genes, first identified in Drosophila, encode 
Robo proteins that belong to the immunoglobulin superfamily 
of protein secreted by midline glial cells and play a role in axon 
guidance. Whole exome sequencing recently identified ROBO1 
variants in 25 patients with pituitary stalk interruption syndrome 
(PSIS); however, functional studies were not performed to sup­
port causality.152
However, no 

 performed to sup­
However, no genetic etiology has been identified in most cases 
of congenital hypopituitarism, which suggests a role for other 
unidentified genes or environmental or epigenetic factors. 
Growth Hormone and Prolactin
The fetal pituitary gland can synthesize and secrete GH by 8 to 
10 weeks’ gestation. Pituitary GH content increases from about 
1 nmol (20 ng) at 10 weeks to 45 nmol (1000 ng) at 16 weeks. 
Fetal plasma GH concentrations in cord blood samples a

l plasma GH concentrations in cord blood samples are in 
the range of 1 to 4 nmol/L during the first trimester and increase 
to a mean peak of approximately 6 nmol/L in midgestation. 
Plasma GH concentrations fall progressively during the second 
half of gestation, to a mean value of 1.5 nmol/L at term.57 The 
responses of plasma GH to somatostatin and GHRH and to 
insulin and arginine are mature at term in human infants.57,153 
The high plasma GH concentrations after development of the 
pituita

H concentrations after development of the 
pituitary portal vascular system at midgestation may reflect 
unrestrained secretion.57 Whatever the mechanisms, control 
of GH secretion matures progressively during the last half of 
gestation and the early weeks of postnatal life, so that mature 
responses to sleep, glucose, and l-dopa are present by 3 months 
of age. GH secretion is already pulsatile soon after birth in 
humans,154 but trough concentrations are still higher than in 
later life so th

rations are still higher than in 
later life so that random GH sampling can be used to detect 
GH deficiency in the neonatal period, which is not possible at 
a later age.155
The ontogenesis of fetal plasma PRL differs significantly from 
that of GH; concentrations are low until 25 to 30 weeks’ gestation 
and increase to a mean peak value of approximately 11 nmol/L 
at term (Fig. 23.5).57 Pituitary PRL content increases progres­
sively from 12 to 15 weeks. PRL release increases in response to 
T

 15 weeks. PRL release increases in response to 
TRH and decreases in response to dopamine. Brain and hypo­
thalamic control of PRL matures late in gestation and during 
the first months of extrauterine life.57,153 The marked increase in 
fetal plasma PRL concentration in the last trimester parallels the 
increase in fetal plasma estrogen concentrations, although it lags 
by several weeks.57,153
Postnatally, GH acts through receptors in liver and other tis­
sues to stimulate production of IGF1 a

 other tis­
sues to stimulate production of IGF1 and, to a lesser degree, 
IGF2. Prenatally, GH receptor mRNA levels and receptor bind­
ing are low in fetal liver, although receptor messenger ribonucleic 
acid (mRNA) is present in other fetal tissues.57 The growth of 
anencephalic fetuses is almost normal, suggesting that factors 
other than GH stimulate fetal IGF production. Nutrition plays 
an important role.156,157 PRL receptors are present in most fetal 
tissues during the first trimester of

 most fetal 
tissues during the first trimester of gestation, and it is likely that 
lactogenic hormones have a significant role in organ and tissue 
development early in gestation.61,157
Human chromosome 17q22-24 contains a growth hormone/
placental lactogen (GH/PL) gene cluster containing five related 
genes: GH-N encodes pituitary growth hormone; GH-V encodes 
placental growth hormone; and hPL-A, hPL-B, and hPL-L encode 
placental lactogens (also called chorionic somatomammotropins 
[CS]). Th

lso called chorionic somatomammotropins 
[CS]). The major circulating placental lactogens derive from the 
hPL-A and hPL-B genes. GH-V differs in 13 of the 191 amino acids 
of GH-N and is produced in the syncytiotrophoblast.158 It rises 
sharply after midgestation to a peak at 34 to 37 weeks, and within 
1 hour after delivery of the placenta it disappears from the circula­
tion.159 Placental GH-V is secreted into the maternal circulation 
and reduces insulin sensitivity in the mother and so spar

uces insulin sensitivity in the mother and so spares glucose 
and other nutrients for transplacental delivery and fetal growth.
Human placental lactogen (hPL) is structurally homologous 
to GH but functionally closer to prolactin and is secreted directly 
into both fetal and maternal circulations. hPL is first detected in 
the mother at 6 weeks of gestation to reach a peak of 5000 to 7000 
ng/mL at 32 to 35 weeks; fetal hPL concentrations, however, 
835
CHAPTER 23  Endocrinology of Fetal Develop

r, 
835
approximate to 20 to 50 ng/mL at term. hPL concentrations 
increase with placental mass and are higher in twin than singleton 
pregnancies. Lactogens affect insulin production, hypothalamic 
gene expression, and leptin action in the mother and so maintain 
metabolic homeostasis while providing the substrates for nutri­
tion for the fetus and newborn infant. Studies in rodents suggest 
that maternal hyperphagia is mediated by progesterone an

aternal hyperphagia is mediated by progesterone and pro­
lactin.158 At least in rodents, evidence suggests that prolactin and 
placental lactogen signaling through the PRL receptor is essential 
for the increase in β-cell mass.160 
Adrenal
An appreciation of human adrenal development is essential to 
appropriately understand adrenal physiology as well as the patho­
logic basis of many congenital adrenal disorders. Furthermore, 
understanding the regulation of adrenal development sheds light 
on 

regulation of adrenal development sheds light 
on the emerging critical role the fetal adrenal plays as a compo­
nent of the fetoplacental unit during pregnancy. The uniqueness 
of the human fetal adrenal was largely overlooked until 1911 
when Starkel and Węgrzynowski, two Polish medical students, 
described the fetal zone (FZ) for the first time as an immature cor­
tex, which undergoes involution in the first years of life.161 At the 
same time, Elliott and Armour published the article, “The D

, Elliott and Armour published the article, “The Devel­
opment of the Cortex in the Human Suprarenal Gland and Its 
Condition in Hemicephaly,” which described the unique FZ.162 
The transient FZ is not present in most mammals and appears to 
be unique to humans and a few higher primates.
Embryology
The adrenal appears as the bipotential adrenogonadal primordium 
(AGP) at 28 to 30 dpc in humans163 due to expression of the tran­
scription factor, steroidogenic factor 1 (SF1, NR5A1), a nuclear 
rec

teroidogenic factor 1 (SF1, NR5A1), a nuclear 
receptor essential for adrenal development and steroidogenesis 
(Fig. 23.6). The adrenal cortex is derived from a thickening of the 
intermediate mesoderm, known as the gonadal ridge, in contrast 
to the adrenal medulla, which is derived from the ectoderm.
The AGP cells give rise to the steroidogenic cells of the adrenal 
gland and the gonad. The gonadal cells migrate caudally. Those 
cells that are more medial, expressing the highest levels of SF1,

more medial, expressing the highest levels of SF1, 
migrate retroperitoneally to the upper pole of the mesonephros to 
form the adrenal primordium at 33 dpc. At about 48 dpc, sym­
pathetic neural crest cells begin migrating to the area where the 
adrenal primordium (AP) is developing. These cells coalesce and 
differentiate into the catecholamine-producing chromaffin cells 
of the adrenal medulla after birth. Following neural crest inva­
sion, the AP becomes encapsulated, resulting in the format

e AP becomes encapsulated, resulting in the formation 
of a distinct organ just above the developing kidney. By 50 to 
52 dpc, there are two distinct zones within the cortex: The larger 
inner FZ contains eosinophilic cells with high levels of expres­
sion of steroidogenic enzymes; the smaller outer definitive zone 
(DZ) is composed of tightly packed cells with much lower lev­
els of steroidogenic enzyme expression. A third cortical zone, the 
transitional zone (TZ), becomes identifiable from ab

ansitional zone (TZ), becomes identifiable from about 14 weeks 
Plasma concentration (nmol/L)
Gestational age (wk)
IGF2
IGF1
hPL
GH
PRL
10
0
5
15
20
35
10
20
30
40
2
4
6
Postnatal age (wk)
• Fig. 23.5  Patterns of change of fetal plasma human placental lactogen 
(hPL), growth hormone (GH), prolactin (PRL), insulin-like growth factor 1 
(IGF1), and insulin-like growth factor 2 (IGF2) during gestation and in the 
neonatal period. The shaded area indicates the range of fetal plasma hPL 
concentrati

dicates the range of fetal plasma hPL 
concentrations. (Data from Bennett A, Wilson DM, Liu R, et al. Levels of 
insulin-like growth factors I and II in human cord blood. J Clin Endocri­
nol Metab. 1983;57:609–612; Kaplan SL, Grumback MM, Aubert ML. The 
ontogenesis of pituitary hormones and hypothalamic factors in the human 
fetus: maturation of central nervous system regulation of anterior pituitary 
function. Recent Prog Horm Res. 1976;32:161–243; Bala RM, Lopatka J, 
Leung A, et al. Serum im

243; Bala RM, Lopatka J, 
Leung A, et al. Serum immunoreactive somatomedin levels in normal adults, 
pregnant women at term, children at various ages and children with con­
stitutionally delayed growth. J Clin Endocrinol Metab. 1981;52:508–512.)
StAR
SF1
DAX1
SRY
Aorta
Omentum
Gonadal ridge
Hindgut
• Fig. 23.6  Hemi cross section of a 5-week human embryo showing the 
locations of the adrenal primordia (suprarenal cortices) and gonadal ridges. 
Steroidogenic factor 1 (SF1) is in

onadal ridges. 
Steroidogenic factor 1 (SF1) is involved in testicular and ovarian develop­
ment. SRY is the single critical regulator of testicular embryogenesis. Inac­
tivation of the DAX1 gene leads to adrenal hypoplasia. The steroidogenic 
acute regulatory protein (StAR) is the rate-limiting factor for adrenal ste­
roidogenesis (see text for details).
836
postconception (wpc). The TZ lies between the DZ and FZ, con­
taining cells with histologic 

the DZ and FZ, con­
taining cells with histologic appearances of both.14
The fetal adrenal undergoes tremendous growth as pregnancy 
progresses largely due to an increase in size of the FZ, which 
accounts for 80% to 90% of the gland’s mass by midgesta­
tion.164,165 The fetal adrenal itself forms 0.4% of body weight at 
term, weighing 3 to 5 g with a relative size 10-fold to 20-fold that 
of the adult adrenal.14 Growth is paralleled by functional devel­
opment, specifically steroidogenesis for p

 devel­
opment, specifically steroidogenesis for production of androgens 
from the FZ, with brief periods of increased cortisol secretion. 
These steroids maintain intrauterine homeostasis and prepare 
the fetus for life ex-utero. FZ cells robustly express cytochrome 
P450 17 alpha (P450c17α), a bifunctional enzyme with both 17 
hydroxylase and 17,20 lyase activities that converts pregnenolone 
to DHEA. DHEA is sulfated to DHEAS, which is subsequently 
aromatized by the placenta to estrogens.14

ently 
Immediately after birth the FZ rapidly involutes and remodels 
by a process involving apoptosis of cells in its inner region with a 
concomitant decrease in adrenal androgen secretion.163 The weight 
of the adrenal gland drops by 50% within the first 2 weeks after 
birth,166 and the FZ is absent by 6 months of age in most cases. 
Whether the timing of fetal adrenal involution is determined by 
gestation or by birth is controversial. FZ androgen p

tation or by birth is controversial. FZ androgen production has 
been reported to persist in infants born prematurely167; however, 
a more recent sonographic study demonstrated a similar pattern 
of adrenal involution within the first 2 weeks after birth in all 
neonates examined, regardless of their gestational age at birth.14
Cells with zona reticularis morphology are detectable in human 
adrenal cortex from around 3 years of age until a continuous ZR 
forms at around 6 years of age and adrena

uous ZR 
forms at around 6 years of age and adrenal androgen synthesis 
recommences, a stage referred to as adrenarche.168 The DZ and 
TZ give rise, respectively, to the zona glomerulosa and zona fas­
ciculata of the developing adult adrenal cortex. 
Transcriptional Regulation of Adrenal Development
Insight into the genetic regulation of adrenal development has 
been gleaned from human and animal models of disordered devel­
opment. The spatiotemporal expression of key genes is critical. 
Genes i

oral expression of key genes is critical. 
Genes involved in the formation of the intermediate mesoderm 
and urogenital ridge affect kidney, adrenal, and gonadal develop­
ment; those affecting AGP development affect both adrenal and 
gonadal development, and there are also those that specifically 
affect AP development.
The earliest stages of adrenal development appear to be regulated 
by a number of transcription factors (e.g., ODD1, SALL1, FOXD1/
FOXD2, WT1, SF1, DAX1), coregulators (e.g., CIT

1/
FOXD2, WT1, SF1, DAX1), coregulators (e.g., CITED2), signal­
ing molecules (e.g., SHH/GLI3, WNT3/WNT4/WNT11), matrix 
proteins (e.g., SPARC), and regulators of telomerase activity (e.g., 
ACD). The transcription factors most significant for early develop­
ment of the AGP have emerged as SF1 and DAX1 (dosage-sensitive 
sex reversal, adrenal hypoplasia congenita [AHC], X-chromosome 
factor, NR0B1).169–171 As well as expression in the adrenal cortex, 
these genes are also expressed in the gonads

tex, 
these genes are also expressed in the gonads, hypothalamus, and 
pituitary tissues. In the absence of SF1 expression, the adrenal gland 
does not form.163 Sf1-knockout mutant mice manifest adrenal 
and gonadal agenesis, gonadotropin deficiency, and absence of the 
hypothalamic ventromedial nucleus.170 Severe disruption of SF1 
in humans can cause adrenal dysfunction, although most patho­
genic variants in SF1 in humans cause impaired testicular develop­
ment and Leydig cell dysfunction rat

ular develop­
ment and Leydig cell dysfunction rather than adrenal insufficiency. 
Heterozygous loss-of-function mutations in SF1 in humans result 
in 46,XY and 46,XX DSD, complete gonadal dysgenesis, and 
primary adrenal failure.170 No cases of homozygous SF1 muta­
tions have been reported to date. Current data indicate that, while 
Wilms tumor 1 (WT1) regulates Sf1 expression in the AGP, Cbp/
p300-interacting transactivator with Glu/Asp-rich carboxy-termi­
nal domain 2 (CITED2) expression in t

rboxy-termi­
nal domain 2 (CITED2) expression in the AGP is necessary for 
proper differentiation of the AP.163
SF1 upregulates DAX1, which is itself a negative regulator 
of SF1 transcriptional activity and hence steroidogenesis. It is 
unclear how disruption of SF1 and its negative regulator can cause 
similar defects; however, DAX1 can function as a coactivator for 
SF1 transcriptional activity in steroidogenic cells when expressed 
at high levels. Inactivating Dax1/DAX1 mutations in mice and

vels. Inactivating Dax1/DAX1 mutations in mice and 
humans are associated with gonadotropin deficiency and adrenal 
hypoplasia.170 Knockdown of Dax1 results in premature differ­
entiation of mouse adrenocortical progenitor cells, and adrenal 
failure occurs as the reservoir of stem/progenitor cell pluripotency 
is depleted. Pathogenic variants or deletions of DAX1 in humans 
are well established as the cause of X-linked adrenal hypoplasia 
congenita (AHC).172
Transcription factors involved in th

ita (AHC).172
Transcription factors involved in the early formation of the 
rodent urogenital ridge include Odd1, Wt1, Sall1, Pbx4, Wnt4, 
FOxD1, and FoxD2. Wt1 knockout mice have renal and gonadal 
anomalies and lack adrenal glands, and in humans, germline 
WT1 mutations cause defects in gonad and kidney formation.171 
Pbx1 knockout mice die in utero with multiple organ defects that 
include adrenal agenesis and impaired testis development. Wnt4-
null mutant mice, in contrast, have impaired kid

-
null mutant mice, in contrast, have impaired kidney develop­
ment from 15 dpc, masculinization of XX females, müllerian duct 
agenesis, and abnormal adrenal glands with reduced steroido­
genic enzyme expression. A homozygous missense mutation in 
WNT4 in humans was shown to be associated with renal agenesis, 
gonadal defects, and adrenal hypoplasia from 19 weeks’ gestation. 
Sall1-null mutants have severe kidney dysgenesis or agenesis and 
hypoplastic adrenal glands at birth. Heterozygous muta

plastic adrenal glands at birth. Heterozygous mutations in 
SALL1 result in Townes-Brocks syndrome, which comprises renal 
and genital abnormalities. The murine model that phenocopies 
the deletion seen in Townes-Brocks syndrome shows absent adre­
nal glands and kidneys and hypoplastic gonads at 16 dpc. 
Signaling Pathways in Adrenal Development
Sonic hedgehog, a member of the vertebrate hedgehog (Hh) family 
of secreted ligands, performs numerous crucial roles during embry­
onic development and

s crucial roles during embry­
onic development and is required in the adult for tissue mainte­
nance, differentiation, and the regulation of stem cell populations. 
SHH signaling is required for normal adrenal development, at a later 
stage than SF1 and DAX1, and is expressed in the AP in cells just 
beneath the capsule.173–175 It has long been thought that undifferen­
tiated, pluripotent stem cells exist in the adrenal cortex to maintain 
homeostasis in the adult. The precise origin of adrenoco

tasis in the adult. The precise origin of adrenocortical cells is 
debatable, and their origin remains ambiguous. Studies focused on 
a downstream activator of the hedgehog pathway, glioma-associated 
oncogene homolog 1 (zinc finger protein) (Gli1), provide evidence 
that the adrenal capsular cells also give rise to the DZ. Gli1-express­
ing cells are specifically located in the adrenal capsule and do not 
express SF1. In vitro this subpopulation of cells is capable of giv­
ing rise to SF1-expre

 of cells is capable of giv­
ing rise to SF1-expressing differentiated adrenocortical cells during 
embryonic development.174 Shh expression marks cortical progeni­
tors,173 and Shh-expressing cells give rise to all steroidogenic cells in 
cortical zones. Homozygous deletion of Shh in murine embryos is 
lethal; however, analysis of their adrenal gland at 14.5 and 16.5 dpc 
indicates that the adrenal primordium forms but is much smaller 
than in the wild type.174,175
837
CHAPTER 23  Endocrinology

he wild type.174,175
837
Several lines of evidence suggest that β-catenin plays an impor­
tant role in adrenal zonation and maintenance. Activation of the 
β-catenin pathway is restricted to the ZG,176 and ectopic expres­
sion leads to the activation of ZG markers in ZF cells.87 Members 
of the R-spondin (Rspo) gene family have recently been identified 
to be expressed from E12.5 onward within mesenchymal cells sur­
rounding the forming adrenal.177

chymal cells sur­
rounding the forming adrenal.177 R-spondins are signaling mole­
cules that positively regulate the β-catenin signaling pathway. This 
study showed capsular RSPO3 signals to the underlying steroido­
genic compartment to induce β-catenin signaling and imprint 
glomerulosa cell fate. SHH in turn signals back to recruit capsular 
cells to form the adrenal cortex, at least during development. This 
study therefore identifies the adrenal capsule as a crucial signaling 
center that is

nal capsule as a crucial signaling 
center that is continuously required for proper zonation.
FGF signaling controls early developmental processes such 
as anterior/posterior patterning and organogenesis.178 FGFs are 
a large family of secreted glycoproteins that bind to four signal­
ing FGF receptors, FGFR1 to FGFR4. FGF signaling interacts 
with SF1 and SHH signaling.179 Fgfr2 and Fgfr4 are expressed in 
the developing adrenal cortex. Embryos with a global Fgfr2IIIb 
deletion have hypoplastic 

ith a global Fgfr2IIIb 
deletion have hypoplastic adrenal glands,179 and deletion of both 
isoforms of Fgfr2 from steroidogenic tissue recapitulates this 
phenotype and causes male-to-female sex reversal, implying that 
Fgfr2 is not necessary for AGP formation but is required for the 
subsequent growth and development of the adrenal gland.180 Epi­
dermal growth factor (EGF) stimulates proliferation of both the 
fetal and definitive zones. The fetal adrenal expresses high levels 
of IGF2 mRNA and

al adrenal expresses high levels 
of IGF2 mRNA and protein, which are responsive to ACTH.15 
IGF2 augments ACTH-stimulated expression of steroidogenic 
enzymes and stimulates steroid hormone production in fetal adre­
nal cortical cells.
Cyclin-dependent kinase inhibitor 1 C (CDKN1C, P57KIP2) 
is a paternally imprinted gene located on chromosome 11p.15, 
which encodes the CDKN1C protein, an inhibitor of cell cycle 
progression. Variations in CDKN1C or its genomic imprinting 
can lead to adrenal p

C or its genomic imprinting 
can lead to adrenal pathology.171 Loss of function of CDKN1C 
results in Beckwith-Wiedemann syndrome (BWS), an overgrowth 
syndrome with increased susceptibility to adrenal carcinoma. 
IMAGe (intrauterine growth restriction, metaphyseal dysplasia, 
AHC, and genital anomalies) syndrome is a rare multisystem dis­
order181 that mirrors the features of BWS and is caused by gain-
of-function mutations in CDKN1C. Most affected individuals are 
born small and develop skelet

ted individuals are 
born small and develop skeletal abnormalities. Males present with 
genital malformations, including micropenis and undescended 
testes and urethral anomalies, but the most clinically important 
component of the syndrome is adrenal insufficiency that causes 
salt wasting, hypoglycemia, and shock due to loss of both min­
eralocorticoid and glucocorticoid synthesis and that can be life 
threatening shortly after birth.181 
Fetal Adrenal Steroidogenesis
The fetal adrenal express

 Adrenal Steroidogenesis
The fetal adrenal expresses five steroidogenic apoenzymes as in the 
adult gland: CYP17A1 (P450c17, 17-hydroxylase/17-20-lyase), 
CYP21A2 (P450c21, 21-hydroxylase), CYP11A1 (P450scc, side-
chain cleavage), CYP11B1/CYP11B2 (P450c11/aldosterone syn­
thase). The fifth enzyme, expressed by the smooth endoplasmic 
reticulum, exhibits both 3βHSD and Δ4, Δ5-isomerase activi­
ties.14 The spatiotemporal expression of these results in differences 
in zonal activity of these enzyme

 in differences 
in zonal activity of these enzymes, and transcription of steroido­
genic genes is tightly regulated.14 Steroid production from the 
fetal adrenal has important roles in the maintenance of intrauter­
ine homeostasis and in the maturation of the fetus in preparation 
for postnatal adaptation to extrauterine life. Vast quantities of 
cholesterol are required as a precursor for steroid production; they 
are obtained through low-density lipoprotein receptors on the 
cell surface as w

ty lipoprotein receptors on the 
cell surface as well as intracellular synthesis of cholesterol from 
acetate. As in the adult adrenal, cholesterol use for steroids is 
tightly controlled by the expression of steroidogenic acute regula­
tory (StAR) protein, which regulates transport of cholesterol to 
the inner mitochondrial membrane, where CYP11A converts it 
to pregnenolone. Sphingosine-1-phosphate lyase (SGPL1) is an 
endoplasmic reticulum enzyme, ubiquitously expressed in tissues. 
This enzy

yme, ubiquitously expressed in tissues. 
This enzyme mediates irreversible cleavage of the lipid signaling 
molecule sphingosine-1-phosphate. Several recent studies report 
mutations in SGPL1 resulting in primary adrenal insufficiency 
and steroid-resistant nephrotic syndrome.182,183
During most of gestation, the fetal adrenal lacks 3βHSD2, 
preventing cortisol and aldosterone synthesis and directing ste­
roid production toward DHEA production. High expression of 
SULT2A1 in the FZ accounts for 

igh expression of 
SULT2A1 in the FZ accounts for sulfation of most of the Δ5 ste­
roids produced, including DHEA and pregnenolone. DHEA is 
sulfated in the FZ to DHEAS, which is then converted by placen­
tal aromatase to estrogens.14
The TZ produces cortisol, with an early peak from 8 to 9 wpc 
coinciding with transient expression of 3βHSD2.184 The hypotha­
lamic-pituitary-adrenal (HPA) axis is sensitive to glucocorticoid-
mediated feedback at this time; 46,XX fetuses with steroidogenic 
defect

his time; 46,XX fetuses with steroidogenic 
defects (e.g., in CYP21 or CYP11) lack cortisol and have an ele­
vated ACTH drive that results in excess production of fetal andro­
gens at a time when the genital and scrotal folds are sensitive to 
androgen exposure, resulting in virilization of female genitalia.184 
Cortisol can retard fetal and placental growth and so is converted 
to biologically inactive cortisone by the enzyme 11βHSD2 in pla­
cental and fetal tissues. In this way the fetus is pr

tal and fetal tissues. In this way the fetus is protected from 
high concentrations of cortisol in utero. Concentrations of circu­
lating cortisone in the fetus at midgestation are fourfold to fivefold 
higher than cortisol concentrations (Fig. 23.7).
The pattern of enzyme maturation in the fetal adrenal suggests 
that cortisol production does not occur de novo from cholesterol 
until 30 weeks’ gestation, but some production using progester­
one as precursor probably occurs earlier.15 During the

as precursor probably occurs earlier.15 During the third tri­
mester, expression/activity of 11βHSD2 drops and selected fetal 
tissues, including liver and lung, express 11βHSD1 so that gluco­
corticoids are available to the fetus to promote the maturation of 
fetal organs, including the lung and brain. This increase in fetal 
cortisol production has an important role for extrauterine survival 
(see “Transition to Extrauterine Life”). The number of glucocor­
ticoid receptors in the sheep fetal h

of glucocor­
ticoid receptors in the sheep fetal hypothalamus increases at term, 
at the time of increasing glucocorticoid concentrations, suggesting 
that some process in the fetus allows the normal autoregulation of 
glucocorticoid receptors to be overridden at term.185 Mice lacking 
GR function manifest enlarged and disorganized adrenal cortices, 
adrenal medullary atrophy, lung hypoplasia, and defective gluco­
neogenesis. They appear normal at birth but are not viable.
Postnatally, biologica

t birth but are not viable.
Postnatally, biologically active cortisol predominates over cor­
tisone in the circulation. Healthy term infants have initial pul­
satility of cortisol secretion and can develop a circadian rhythm 
from as early as 1 month after birth, although there is considerable 
individual variability. There is, however, no consensus about the 
definition of a “normal” cortisol concentration in preterm infants.
Antenatal steroids for a threatened preterm delivery have clear 
bene

for a threatened preterm delivery have clear 
beneficial effects, with a reduction in mortality and morbidity, but 
there are consequences of such early high steroid exposure, and 
long-term effects are unknown. The HPA axis is highly susceptible 
838
to programming during development. Unlike the endogenous glu­
cocorticoids that are inactivated by 11βHSD2 in the placenta, syn­
thetic glucocorticoids readily cross the placenta. In the fetal sheep, 
h

readily cross the placenta. In the fetal sheep, 
hypothalamic and pituitary glucocorticoid receptors are present 
at midgestation, and corticotropin suppressibility can be demon­
strated by the midpoint of the third trimester of gestation.185
Longitudinal follow-up of the offspring of women treated with 
betamethasone for threatened preterm birth has shown that indi­
viduals exhibit insulin resistance 30 years after treatment, particu­
larly women. Previous antenatal treatment of the fetus of a 

n. Previous antenatal treatment of the fetus of a mother 
whose previous child had CAH by administering dexamethasone 
orally to the mother at 8 to 12 wpc was found to reduce fetal 
androgens and hence the virilization of female fetuses, but due to 
paucity of high-quality evidence in terms of efficacy and safety this 
should be regarded as a highly controversial experimental treat­
ment.13 The constant dexamethasone dose used can result in cor­
tisol concentrations that exceed physiologic fetal

tisol concentrations that exceed physiologic fetal concentrations 
by around 60-fold. To date, a number of side effects and adverse 
associations have been reported, including neurologic abnormali­
ties in the offspring.13
Use of steroids postnatally to prevent or treat chronic lung dis­
ease in preterm infants facilitates earlier extubation and reduces 
the risk of chronic lung disease, but it is associated with hyper­
glycemia, hypertension, gastrointestinal bleeding and perforation, 
hypertro

strointestinal bleeding and perforation, 
hypertrophic cardiomyopathy, growth failure, and cerebral palsy 
and thus cannot be recommended.186 Trials involving more physi­
ologic doses of hydrocortisone are currently underway, but all sys­
temic and inhaled steroids are associated with adrenal suppression 
in neonates.
Early low-dose hydrocortisone treatment has been reported 
to improve survival in very preterm infants without broncho­
pulmonary dysplasia. In an analysis of secondary outcomes of

dysplasia. In an analysis of secondary outcomes of 
the PREMILOC (Early Low-Dose Hydrocortisone to Improve 
Survival Without Bronchopulmonary Dysplasia in Extremely 
Preterm Infants) randomized clinical trial of extremely preterm 
infants, early low-dose hydrocortisone was not associated with a 
statistically significant difference in neurodevelopment at 2 years 
of age.187 Further randomized studies are needed to provide defin­
itive assessment of the neurodevelopmental safety of hydrocorti­
so

of the neurodevelopmental safety of hydrocorti­
sone in extremely preterm infants.
The human fetal adrenal gland is capable of aldosterone 
secretion near term with the development of the zona glomer­
ulosa, and fetal plasma aldosterone concentrations in infants 
who are born by caesarean section are threefold or fourfold 
higher than maternal concentrations.188 Vaginal delivery 
and maternal salt restriction increase concentrations in both 
mother and infant. The increased aldosterone concentra

er and infant. The increased aldosterone concentrations 
in the fetus are a result of increased fetal adrenal secretion and 
persist during the first year of extrauterine life. There is poor 
correlation between plasma renin activity (PRA) and aldoste­
rone concentrations in cord blood.189 Mineralocorticoid recep­
tors (MRs) are present in fetal tissues from 12 to 16 weeks’ 
gestation.190 MR immunoreactivity is detectable in fetal kid­
ney, skin, hair follicles, trachea and bronchioles, esophagu

 hair follicles, trachea and bronchioles, esophagus, 
stomach, small intestine, colon, and pancreatic exocrine ducts. 
The role of MRs in these fetal tissues remains unclear. MR 
knockout mice appear normal at birth but demonstrate defects 
in mineralocorticoid and renin-angiotensin system functions 
in the postnatal period.191
Angiotensin II concentrations in the sheep fetus are similar to 
maternal values, and blockade of fetal production with angioten­
sin-converting enzyme (ACE) inhibitors d

angioten­
sin-converting enzyme (ACE) inhibitors decreases the fetal glo­
merular filtration rate.192 Both subtypes of angiotensin receptors, 
AT1 and AT2, are detectable in various tissues, including kidney, 
early in fetal development.191 Hormonal factors modulate fetal 
renal AT gene expression in sheep: Angiotensin II suppresses both 
AT1 and AT2, and cortisol increases AT1 gene expression in kid­
ney and lungs.193 The role of the fetal renin-angiotensin system is 
not clear; rather than mod

-angiotensin system is 
not clear; rather than modulating renal sodium excretion through 
aldosterone, it may maintain renal excretion of salt and water into 
amniotic fluid to prevent oligohydramnios.192 The mechanism for 
the high aldosterone concentrations in the fetal and neonatal peri­
ods remains unclear.
Aldosterone affects renal sodium excretion in the fetal sheep 
and in premature infants.189 Manifestations of mineralocorticoid 
deficiency in the newborn term infant can occur because of

cy in the newborn term infant can occur because of aldo­
sterone deficiency or competition for binding to renal MRs by 
other steroids such as 17-hydroxyprogesterone. Relatively reduced 
glomerular filtration in the newborn limits sodium loss initially, 
but by 1 week of age, aldosterone deficiency produces the charac­
teristic manifestations of hyponatremia, hyperkalemia, and vol­
ume depletion. 
ACTH (pmol/L) and steroids (nmol/L)
DHEAS
Cortisone
Cortisol
ACTH
50
0
100
150
200
2000
4000
6000
F

ne
Cortisol
ACTH
50
0
100
150
200
2000
4000
6000
Fetal age (wk)
10
20
30
40
2
4
6
• Fig. 23.7  Patterns of change of fetal plasma adrenocorticotropic hor­
mone (ACTH), cortisol, cortisone, and dehydroepiandrosterone sulfate 
(DHEAS) during gestation and in the neonatal period. The trend of average 
values is shown for each hormone in nanomoles per liter. Notice the bro­
ken scale for DHEAS. (Data from Geller DH, Miller WL. Molecular develop­
ment of the adrenal gland. In: Pesc

cular develop­
ment of the adrenal gland. In: Pescovitz OH, Eugster EA, eds. Pediatric 
Endocrinology. Philadelphia: Lippincott Williams & Wilkins; 2004:548–567; 
Winters AJ, Oliver C, Colston C, et al. Plasma ACTH levels in the human 
fetus and neonate as related to age and parturition. J Clin Endocrinol 
Metab. 1974;39:269–273; Murphy BEP. Human fetal serum cortisol levels 
related to gestational age: evidence of a midgestational fall and a steep 
late gestational rise, independent of sex or m

ep 
late gestational rise, independent of sex or mode of delivery. Am J Obstet 
Gynecol. 1982;144:276–282; Beitins IZ, Bayard F, Ances FIG, et al. The 
metabolic clearance rate, blood production, interconversion and transpla­
cental passage of cortisol and cortisone in pregnancy near term. Pediatr 
Res. 1973;7:509–513.)
839
Hormonal Regulation of Adrenal Development
The hormonal regulation of adrenal development is not under­
stood. A major stimulu

l development is not under­
stood. A major stimulus to fetal adrenal function is fetal pituitary 
ACTH, a 39–amino acid peptide secreted from the anterior pitu­
itary gland under control of CRH. ACTH binds to the transmem­
brane receptor melanocortin 2 receptor 2 (MC2R), specifically 
in adrenocortical cells, and exerts its effect through downstream 
cyclic adenosine monophosphate (cAMP) and Ras/MEK/ERK 
signaling pathways. The growth-promoting effects of ACTH are 
mediated in part through the s

ffects of ACTH are 
mediated in part through the stimulation of locally produced 
growth factors such as insulin-like growth factor 2 (IGF2) and 
fibroblast growth factor β (FGFβ).163 During development, as the 
outer DZ emerges, ACTH participates in the regulation of ste­
roidogenesis, cell differentiation, and cell growth.163 While adre­
nocortical growth and differentiation are independent of ACTH 
during the first trimester of human pregnancy, ACTH begins to 
play an essential role in the mo

, ACTH begins to 
play an essential role in the morphologic and functional develop­
ment of the adrenal gland after 15 weeks’ gestation.194
In vivo ACTH stimulates steroid production by activating StAR 
and increasing delivery of substrate cholesterol to P450scc.188 
In vitro ACTH directly stimulates DHEAS and cortisol produc­
tion, and in vivo excess ACTH is clearly involved in driving the 
high androgen concentrations seen in fetuses affected with CAH. 
The HPA axis clearly functions early in 

ith CAH. 
The HPA axis clearly functions early in fetal life, but anencephalic 
fetuses, which lack pituitary ACTH, have adrenals that contain 
a fairly normal complement of steroidogenic enzymes and retain 
their capacity for steroidogenesis.171 Furthermore, growth and ris­
ing steroid production by the fetal adrenal during pregnancy are 
not paralleled by increasing concentrations of fetal plasma ACTH. 
A possible explanation for this paradox is that there may be ges­
tational age–dependent al

s that there may be ges­
tational age–dependent alterations in responsiveness of the fetal 
adrenal tissue to ACTH. Thus it appears that both ACTH-depen­
dent and ACTH-independent mechanisms regulate fetal adrenal 
steroidogenesis.
The presence of all CRH peptides and their receptors in the 
adrenal suggests that the CRH system can function locally within 
human and rodent adrenals. An important source of CRH is the 
human placenta, which releases CRH into the fetal circulation. 
As gestation ad

s CRH into the fetal circulation. 
As gestation advances, CRH concentration increases. The remark­
able increase in placental CRH production at the end of gesta­
tion has been suggested to contribute to the parturition process 
by forming a feed-forward loop that leads to increased production 
of cortisol and DHEA/DHEAS in human fetal adrenals. Studies 
have shown that CRH stimulates cortisol production in primary 
cultures of fetal adrenocortical cells by elevating the mRNA lev­
els of steroido

l cells by elevating the mRNA lev­
els of steroidogenic acute regulatory protein and other steroido­
genic enzymes, including 3βHSD2, CYP21A2, and CYP11B1. In 
addition, CRH enhances the adrenal response to ACTH, further 
driving the production of cortisol and DHEA/DHEAS.51 CRH 
gene knockout in mice leads to neonatal death due to pulmonary 
hypoplasia, suggesting that CRH-stimulated glucocorticoid pro­
duction is essential for adrenergic chromaffin and normal lung 
development. Circulating CRH 

fin and normal lung 
development. Circulating CRH concentrations are elevated in 
the fetus, largely from extrahypothalamic and placental sources. 
The circulating concentrations of CRH increase 1000-fold as 
pregnancy progresses45 and reach values of 0.5 to 1 nmol/L at 
nmol/L. At the end of pregnancy, there is increased bioavailability 
of CRH due to a fall in levels of its binding protein. This results 
in an exponential increase i

rotein. This results 
in an exponential increase in maternal CRH concentrations from 
35 weeks’ gestation to term.45,195 Postpartum, CRH normalizes 
to nonpregnant levels within 24 hours of delivery in keeping with 
the fact that the placenta is the primary source.45,195 This placen­
tal CRH is bioactive, and concentrations correlate with maternal 
cortisol concentrations, suggesting that this circulating placental 
CRH plays a role in stimulating maternal corticotropin release. 
Midgestation fe

g maternal corticotropin release. 
Midgestation fetal plasma corticotropin concentrations average 
about 55 pmol/L (250 pg/mL), levels that maximally stimu­
late fetal adrenal steroidogenesis, and concentrations are higher 
throughout gestation than in postnatal life, although they fall near 
term (see Fig. 23.7).25,188 
The Fetal-Placental Unit
The paradox of human fetal adrenal function is that steroido­
genesis is programmed by the steroidogenic enzyme expression 
pattern (e.g., relative 3βHS

ic enzyme expression 
pattern (e.g., relative 3βHSD deficiency) to produce inactive 
products (including DHEA and pregnenolone and their sulfate 
conjugates).188 Much of the DHEA is converted to 16-hydroxy-
DHEAS by the fetal adrenal and fetal liver. As previously dis­
cussed, this is designed to provide a substrate for placental estrone 
and estradiol production: 16-hydroxy-DHEA undergoes metab­
olism to estriol in the placenta. Fetal DHEAS production and 
maternal estriol concentrations increa

uction and 
maternal estriol concentrations increase progressively to term; 
DHEAS production approximates to 200 mg/day near term.25 In 
pregnant baboons with placental estrogen production suppressed 
by administration of an aromatase inhibitor, the volume of the FZ 
of the fetal adrenal increased markedly.196 This effect was reversed 
by administration of inhibitor plus estrogen, suggesting that 
estrogen selectively suppresses FZ growth and development dur­
ing the second half of primate preg

velopment dur­
ing the second half of primate pregnancy. It is proposed that this 
represents a feedback system to regulate secretion of fetal adrenal 
DHEA, thereby maintaining normal fetal-placental function and 
development.196 
Adrenal Insufficiency
Adrenal insufficiency is a rare condition that may occur second­
ary to ACTH deficiency or is primary resulting from adrenal 
failure. The mature ACTH peptide is cleaved from the larger 
precursor molecule, POMC, together with other small peptide

 molecule, POMC, together with other small peptides 
such as β-endorphin and α-MSH and β-MSH. Defects in ACTH 
synthesis, processing, or release can lead to secondary adrenal 
hypoplasia resulting in neonatal hypoglycemia, prolonged jaun­
dice, or collapse. Given that mineralocorticoid secretion is largely 
independent of ACTH secretion, it is preserved, and salt loss is 
therefore unusual. Nevertheless, mineralocorticoid deficiency can 
be an issue in some patients with ACTH insensitivity. Low 

sue in some patients with ACTH insensitivity. Low serum 
concentrations of ACTH, absence of hyperpigmentation, and 
presence of associated features such as pale skin, red hair, diar­
rhea, and obesity (POMC/PC1 mutations) are important diag­
nostic clues.
The presence of multiple pituitary hormone deficiencies (e.g., 
GH, ACTH, TSH, gonadotropins, vasopressin, and PRL) may 
point to a diagnosis of multiple pituitary hormone deficiency, 
often in association with structural abnormalities of the p

association with structural abnormalities of the pituitary 
gland, eye abnormalities, and forebrain abnormalities (septo-
optic dysplasia). Signs such as those of congenital hypothyroid­
ism, hypoglycemia, congenital hypogonadotropic hypogonadism 
(micropenis, undescended testes), and severe postnatal growth 
failure may be suggestive of the diagnosis. A number of single-gene 
defects have now been associated with congenital hypopituitarism 
(e.g., mutations in HESX1, SOX3, OTX2, GLI2, ARNT2, LH

., mutations in HESX1, SOX3, OTX2, GLI2, ARNT2, LHX3, 
LHX4, and PROP1).55,149,197 Occasionally ACTH insufficiency 
may not be present at the time of diagnosis but may develop pro­
gressively with time.
840
Recessive mutations in the T-box factor TPIT (TBX19) have 
been identified in patients with severe, early-onset isolated ACTH 
deficiency with profound hypoglycemia, prolonged jaundice, and 
sudden neonatal death.118 TPIT is required for the speci

 neonatal death.118 TPIT is required for the specification, 
maturation, and maintenance of both precorticotroph and pre­
melanotroph populations and for the suppression of gonadotroph 
fate. It is also required to activate the expression of POMC in 
conjunction with the transcription factor PTX1. Murine trans­
genesis resulted in ACTH and glucocorticoid deficiencies, adrenal 
hypoplasia, and pigmentation defects in mice deleted for TPit.118 
Sixty-five percent of patients with severe congenital

ty-five percent of patients with severe congenital isolated 
ACTH deficiency are found to have TBX19 mutations, but muta­
tions are not found in partial or later-onset ACTH deficiency.198
ACTH resistance can occur in a number of well-defined enti­
ties, such as defects in the ACTH receptor (MC2R), familial glu­
cocorticoid deficiency (FGD) type 1 (FGD1), in MC2R accessory 
protein (MRAP, FGD2), or as part of the triple-A syndrome (alac­
rimia, achalasia, addisonism; also known as Allgrove syndro

halasia, addisonism; also known as Allgrove syndrome, 
caused by defects in ALADIN/AAAS). These disorders are charac­
terized by isolated glucocorticoid deficiency, hyperpigmentation, 
and markedly elevated concentrations of ACTH.199,200 Neverthe­
less, approximately 15% of individuals with triple-A syndrome 
have evidence of mineralocorticoid insufficiency, those with the 
most severe loss of function manifesting hyponatremia on pre­
sentation. Recently, mutations in NNT (nicotinamide nucleotid

Recently, mutations in NNT (nicotinamide nucleotide 
transhydrogenase), GPX1 (glutathione peroxidase 1), and MCM4 
(mini-chromosome maintenance-deficient-4 homologue) have 
been found as a cause of FGD.201–203 Mutations in MCM4 were 
found in an Irish Traveller community and result in late-onset, less 
severe glucocorticoid deficiency, short stature, and natural killer 
cell deficiency due to increased chromosomal breakage. Mutations 
in GPX1 and NNT, involved in detoxification of reactive oxyge

 NNT, involved in detoxification of reactive oxygen 
species, can also result in FGD.204
Primary adrenal failure may be caused by congenital adrenal 
hypoplasia (or AHC).205 This results in severe salt-losing primary 
adrenal failure in early infancy or childhood, although milder, 
delayed-onset forms of the condition exist. The most common 
form of the condition is X-linked. Patients have mutations in the 
nuclear receptor DAX1 (NROB1); in addition to adrenal failure, 
the males suffer from hyp

ion to adrenal failure, 
the males suffer from hypogonadotropic hypogonadism. Rarely, 
patients present with isolated mineralocorticoid deficiency with 
normal cortisol concentrations; however, glucocorticoid deficiency 
usually develops later. DAX1 is expressed in ES cells, steroidogenic 
tissues (gonads and adrenals), the ventromedial hypothalamus 
(VMH), and pituitary gonadotrophs. It acts as a transcriptional 
repressor of other nuclear receptor pathways but is also involved 
in maintenance 

tor pathways but is also involved 
in maintenance of pluripotency of stem cells.206
Heterozygous and homozygous mutations in SF1 have been 
associated with adrenal failure in 46,XY phenotypic females, as 
well as in at least one 46,XX girl, although the latter phenotype is 
rare.205 Mutations in SF1 have also been associated with gonadal 
dysgenesis in 46,XY individuals in the absence of adrenal insuf­
ficiency.207 Additionally, SF1 mutations have been associated with 
primary ovarian failure, b

e been associated with 
primary ovarian failure, but this is rare.208,209
Various forms of congenital adrenal hyperplasia may be associ­
ated with variable adrenal failure (e.g., mutations in CYP11A1, 
StAR, HSD3B2, CYP17, CYP21A2, CYP11B1) with varying 
degrees of genital ambiguity. The enzyme P450c11 (aldoste­
rone synthase), which is found in the ZG, has 11β-hydroxylase, 
18-hydroxylase, and 18-methyloxidase activities and catalyzes all 
the reactions needed to convert 11-deoxycorticosterone 

eactions needed to convert 11-deoxycorticosterone (DOC) 
to aldosterone. Mutations in the gene encoding the enzyme are 
associated with isolated mineralocorticoid deficiency. Functional 
mineralocorticoid deficiency with severe salt loss resulting in 
hyponatremia and hyperkalemia may also arise as a result of muta­
tions in the MR or in the gene encoding the epithelial sodium 
channel (ENaC).
Primary adrenal insufficiency due to adrenal hypoplasia can be 
classified into nonsyndromic and syndro

ia can be 
classified into nonsyndromic and syndromic (triple-A syndrome, 
IMAGe syndrome, Irish Travellers syndrome) types, and up to 
30% of patients have no cause identified. Whole exome sequenc­
ing of patients with syndromic primary adrenal insufficiency and 
a striking phenotype of a multisystem disorder, including myelo­
dysplasia, infection, growth restriction, adrenal hypoplasia, genital 
anomalies, and enteropathy, revealed mutations in the SAMD9 
gene (MIRAGE syndrome).210,211 A diagn

 the SAMD9 
gene (MIRAGE syndrome).210,211 A diagnosis of adrenal insuf­
ficiency must be prompt in view of the potential life-threatening 
consequences of glucocorticoid and mineralocorticoid deficiencies 
to the infant. 
Thyroid Development
The thyroid gland, one of the first endocrine organs to develop, 
is derived from contributions of two anlagen: By 22 dpc a mid­
line thickening of the pharyngeal floor (median anlage) forms as 
the precursor of the T4-producing follicular cells.

he precursor of the T4-producing follicular cells. Paired ultimo­
branchial bodies originating from the fourth pharyngobranchial 
pouches (lateral anlagen) give rise to the parafollicular calcitonin-
secreting (C) cells.21
The thyroid precursor cells migrate caudally between 28 and 
48 dpc, proliferate, and expand laterally, a process known as 
lobulation. The lateral and median anlage fuse at around 44 dpc, 
and during descent the developing thyroid retains an attachment 
to the pharynx by an e

roid retains an attachment 
to the pharynx by an epithelial stalk known as the thyroglossal 
duct.212 By 37 dpc this structure that connects the median anlage 
with the floor of the buccal cavity (a point later known as the fora­
men cecum on the developing tongue) has usually disappeared 
and the only remnant of the duct is the foramen cecum itself.212 
An ectopic thyroid and a persistent thyroglossal duct or cyst may 
occur because of abnormal thyroid descent.
By 51 dpc, the gland comprises tw

thyroid descent.
By 51 dpc, the gland comprises two lateral lobes with a con­
necting isthmus, and it reaches its final position below the thyroid 
cartilage by 9 wpc. At 10 wpc, the fetal thyroid gland weighs about 
80 mg and at term 1 to 1.5 g. Terminal differentiation begins at 
around 60 dpc, only after migration is complete. Differentiation 
is evidenced by expression of the genes encoding the TSH recep­
tor (TSHR), the sodium-iodide symporter (NIS), thyroglobulin 
(Tg), and thyroperoxidase

er (NIS), thyroglobulin 
(Tg), and thyroperoxidase (TPO), and results in formation of fol­
licles and functional capacity.213 Production of thyroid hormones 
is stimulated by TSH and requires an adequate supply of iodine. 
By 70 dpc colloid is visible histologically and the thyroid gland 
expresses Slc5a5, encoding the 13-transmembrane domain glyco­
protein NIS, enabling uptake of iodine and synthesis of thyroid 
hormones. Thyroid hormones can be detected in the thyroid from 
9 wpc and in fetal 

 detected in the thyroid from 
9 wpc and in fetal blood from 10 wpc.214–217 
Thyroid Hormone Biosynthesis
Pituitary and plasma thyrotropin (TSH) concentrations begin 
to increase during the second trimester in the human fetus, at 
about the time that pituitary portal vascular continuity develops 
(Fig. 23.8).
In the rat, expression of the Tshr gene is significantly upreg­
ulated on fetal day 17, and this is accompanied by significant 
841
growth an

 23  Endocrinology of Fetal Development 
growth and rapid development in terms of structure and func­
tion. The expression of Tg and TPO is increased, and thyroid 
follicles are seen with thyroid hormonogenesis, suggesting 
that the TSHR has an important role to play in these events. 
Murine mutation of the Tshr gene is associated with the hyt/
hyt phenotype, which exhibits severe hypothyroidism and a 
hypoplastic but normally located thyroid gland with a poorly 
developed follicular structure. 

nd with a poorly 
developed follicular structure. In humans, a similar phenotype 
is observed in babies of mothers with potent TSHR-blocking 
antibodies and in babies with severe loss-of-function muta­
tions in TSHR.
Although onset of fetal thyroid hormone synthesis starts at 
the end of the first trimester, levels are low and necessitate a high 
dependency on maternal thyroid hormones during pregnancy. 
Overt maternal hypothyroidism has been shown to be associated 
adversely with pregnancy outc

wn to be associated 
adversely with pregnancy outcomes, including miscarriage and 
premature delivery, as well as adverse perinatal and neurocognitive 
outcomes in offspring. The effects of subclinical maternal hypo­
thyroidism or hypothyroxinemia are less clear. Using a zebrafish 
model, the ontogeny of the hypothalamic-pituitary-thyroid axis 
and its negative-feedback regulation by thyroid hormones has 
been characterized. Embryonic exposure to excess thyroid hor­
mone during a developmental t

 excess thyroid hor­
mone during a developmental time window resulted in long-term 
dysregulation of pituitary function by apoptosis of developing 
thyrotrope cells.218 These findings are relevant in the context of 
maternal hyperthyroidism due to maternal Graves or overreplace­
ment of pregnant hypothyroid women in the first trimester with 
levothyroxine.
Plasma thyrotropin concentrations increase progressively dur­
ing the last half of gestation. Plasma concentrations of T4-binding 
globulin a

n. Plasma concentrations of T4-binding 
globulin and total T4 increase progressively, from low levels at 14 to 
16 wpc to maximal levels at term. Free T4 concentrations increase 
with the increase in T4 production. The increases in plasma TSH 
and T4 concentrations during the third trimester reflect progressive 
maturation of hypothalamic pituitary control and of thyroid gland 
responsiveness to TSH. Pituitary TSH secretion is responsive to 
hypothyroxinemia and to TRH early in the third trimest

thyroxinemia and to TRH early in the third trimester.215 The 
period of parallel increases in fetal TSH and free T4 concentra­
tions during the latter half of gestation is followed by sequential 
surges of TSH and free T4 in the early neonatal period and a final 
slow equilibration of the TSH/free T4 ratio to adult values during 
infancy and childhood.219–222 This sequence includes coordinated 
maturation of hypothalamic TRH secretion, pituitary TRH sensi­
tivity, thyrotropin negative-feedback c

TRH sensi­
tivity, thyrotropin negative-feedback control, and thyroid follicu­
lar cell responsiveness to TSH. Fetal serum TRH concentrations 
are higher than in maternal blood, which is the consequence of 
extrahypothalamic (placenta and pancreas) TRH production and 
decreased TRH degradation in fetal serum. Functionally, the fetus 
progresses from a state of both primary (thyroidal) and tertiary 
(hypothalamic) hypothyroidism at midgestation, through a state 
of mild tertiary hypothyroidism du

hrough a state 
of mild tertiary hypothyroidism during the final weeks in utero, 
to a fully mature hypothalamic-pituitary-thyroid axis by 2 months 
after birth.
The adult thyroid follicular cell can modify iodine transport or 
uptake with changes in dietary iodine intake, independent of vari­
ations in serum TSH concentrations.223,224 Before 36 to 40 weeks’ 
gestation, the thyroid gland lacks this autoregulatory mecha­
nism and is susceptible to iodine-induced inhibition of thyroid 
hormone syn

 iodine-induced inhibition of thyroid 
hormone synthesis.224,225 The fetal thyroid follicular cell, when 
exposed to high circulating levels of iodide, is unable to reduce 
iodide trapping and prevent the high intracellular iodide concen­
trations that produce the blockade of hormone synthesis referred 
to as the Wolff-Chaikoff effect. Failure of the immature thyroid to 
exhibit autoregulation is probably due to failure of downregula­
tion of thyroid cell membrane NIS units, which may be related

roid cell membrane NIS units, which may be related to 
the absence or reduced iodination of an 8-kDa to 10-kDa protein 
in the thyroid follicular cell.223,224 In addition to maturation of 
autoregulation, thyroidal responsiveness to TSH increases during 
the last trimester.21
The metabolism of thyroid hormones occurs through a pro­
gressive series of monodeiodinations.219,226 Three deiodinase 
enzymes act to remove an iodine atom from the outer (pheno­
lic) ring or the inner (tyrosyl) ring of th

pheno­
lic) ring or the inner (tyrosyl) ring of the tetraiodothyronine (T4) 
molecule thus, respectively, activating or inactivating the hor­
mone. The deiodinases are encoded by separate genes and share 
sequence homology. Most of the circulating biologically active T3 
in adults is derived by outer-ring monodeiodination of T4 in liver 
and other nonthyroidal tissues; biologically inactive rT3 derives 
from inner-ring deiodination of T4 in peripheral tissues. The type 
I enzyme (D1), an outer-r

heral tissues. The type 
I enzyme (D1), an outer-ring monodeiodinase, is a high–Michae­
lis constant (Km) enzyme inhibited by propylthiouracil (PTU) 
and stimulated by thyroid hormone. It deiodinates T4 to T3 and 
rT3 to T2. D1 also has inner-ring deiodinase activity, converting 
T3 to T2. Activity of D1 is low throughout gestation. The type 
2 outer-ring monodeiodinase (D2) is a low-Km enzyme that is 
insensitive to PTU and inhibited by thyroid hormone. It deiodin­
ates T4 to T3 and rT3 to T2, 

hormone. It deiodin­
ates T4 to T3 and rT3 to T2, and it is highly expressed in brain and 
pituitary. Type 3 monodeiodinase (D3) inactivates T4 and T3 via 
inner-ring deiodination of T4 to rT3 and of T3 to T2; it is highly 
expressed in fetal tissues and placenta. D1 is largely responsible 
1
2
3
4
50
0
100
150
200
2
0
4
6
8
10
rT3
TSH
T4S
T3S
T3
T4
rT3S
TSH (mU/L)
T4 (nmol/L)
T3, T3S, rT3, rT3S, T4S (nmol/L)
Fetal age
(wk)
0
20
30
40
2
4
6
Postnatal age
(wk)
• Fig. 23.8  Patterns of change of f

atal age
(wk)
• Fig. 23.8  Patterns of change of fetal plasma thyroid-stimulating hormone 
(TSH), thyroxine (T4), triiodothyronine (T3), reverse T3 (rT3), and iodothy­
ronine sulfates (T4S, rT3S, and T3S) during gestation and in the neona­
tal period. The patterns for T4S and rT3S are based on limited 30-week 
data. (Data from Roti E. Regulation of thyroid stimulating hormone [TSH] 
secretion in the fetus and neonate. J Endocrinol Invest. 1988;11:145–158; 
Fisher DA, Klein AH. Thyroid developmen

:145–158; 
Fisher DA, Klein AH. Thyroid development and disorders of thyroid func­
tion in the newborn. N Engl J Med. 1981;304:702–712; Santini F, Chiovato 
L, Ghirri P, et al. Serum iodothyronines in the human fetus and the new­
born: evidence for an important role of placenta in fetal thyroid hormone 
homeostasis. J Clin Endocrinol Metab. 1999;84:493–498.)
842
for the production of T3 that escapes from the cells, especially 
liver and kidney, into 

rom the cells, especially 
liver and kidney, into the circulation; D2 is responsible for the 
production of local tissue T3. Inactive rT3 also diffuses out of 
most tissues to appear in plasma.
Iodothyronine deiodinases belong to a family of selenoproteins. 
Selenium is an essential trace element required for the biosynthe­
sis of selenoproteins. Selenocysteine insertion sequence (SECIS) 
binding protein 2 (SBP2) represents a key factor for the insertion 
of selenocysteine into selenoproteins. R

nsertion 
of selenocysteine into selenoproteins. Recently, mutations in the 
SBP2 gene have been described that lead to a multisystem disorder 
with deficiencies of multiple selenoproteins, resulting in growth 
retardation, myopathy and skin photosensitivity, nonketotic 
hypoglycemia, colitis, and infertility.227,228 Due to abnormal D2 
activity, T3 is low, T4 and rT3 are increased, and TSH is slightly 
elevated. As selenoproteins also serve as antioxidants, tissue dam­
age may be related to inc

ntioxidants, tissue dam­
age may be related to increased concentrations of reactive oxygen 
species (ROS).228,229
The distribution of the deiodinases has been characterized in 
rodent and human tissues (Table 23.3).230 D2 is detectable by 
midgestation and plays an important role in supplying T3 to 
developing brain tissue, regulating thermogenesis in brown adi­
pose tissue in the neonatal period, and regulating pituitary TSH 
secretion. D3 activity is present in placenta, liver, and perhaps 
fe

ity is present in placenta, liver, and perhaps 
fetal skin, accounting for the higher concentrations of rT3 in the 
fetus and limiting the metabolic effects of thyroid hormones dur­
ing much of fetal life. There is little conversion of T4 to circulating 
T3 via D1 deiodination until midgestation in the human fetus; 
plasma T3 concentrations are low (<0.2 nmol/L or <15 ng/dL) 
until 30 weeks’ gestation, after which the mean value increases to 
0.7 nmol/L (50 ng/dL) at term (see Fig. 23.8).231 On 

 nmol/L (50 ng/dL) at term (see Fig. 23.8).231 On the other 
hand, fetal brain T3 concentrations are 60% to 80% of those in 
the adult by fetal age 20 to 26 weeks owing to D2 activity. In the 
presence of fetal hypothyroidism, D2 increases while D3 decreases 
in an effort to maintain near-normal brain T3 concentrations.
Sulfation is active in fetal tissues, and the predominant thy­
roid hormone metabolites in the fetus are iodothyronine sul­
fates.219,232,233 High levels of phenolsulfotransferas

s.219,232,233 High levels of phenolsulfotransferases (SULT) have 
been characterized in fetal liver, lung, and brain by midgestation. 
SULT activities decrease rapidly in the neonatal period.233 In the 
last third of gestation in fetal sheep, the mean plasma production 
rates for T4 and metabolites (in micrograms per kilogram of body 
weight per day) are as follows: T4, 40; T4 sulfate (T4S), 10; rT3, 
5; rT3S, 12; T3, 2; and T3S, 2. All metabolites are biologically 
inactive except T3 and perhap

es are biologically 
inactive except T3 and perhaps T3S, so 90% of the T4 metabolites 
in the fetus are biologically inactive.232 The sulfated metabolites 
accumulate in fetal serum as a result of the low D1 activity in fetal 
tissues and because the sulfated iodothyronines are not substrates 
for D3.223,234 The production rate of T3 increases progressively 
between 30 weeks’ gestation and term because of maturation of 
D1 activity in the liver and other tissues and because of decreasing 
D3 act

nd other tissues and because of decreasing 
D3 activity in placenta.21,233 In fetal sheep, hepatic D1 activity 
increases progressively during the last trimester.235
It has long been assumed that thyroid hormones passively dif­
fuse into cells. However, several classes of cell membrane iodo­
thyronine transporters have been described, questioning this 
hypothesis.236–238 These transporters belong to different families 
of organic anion, amino acid, and monocarboxylate solute carri­
ers, includin

d, and monocarboxylate solute carri­
ers, including the organic anion transporting polypeptide (OATP) 
family and the solute carrier family 21 (SLC21).236–238 The sig­
nificance of these transporters is not yet clear, but mutation of the 
human monocarboxylate transporter 8 (MCT8), a member of the 
SLC21 family shown to be a specific thyroid hormone transporter 
present in developing brain, leads to a syndrome of combined thy­
roid dysfunction and psychomotor retardation (X-linked Allan-
Herndon

d psychomotor retardation (X-linked Allan-
Herndon-Dudley syndrome).239,240 Mct8 expression in neonatal 
mice has been localized to neurons in the olfactory bulb, cerebral 
cortex, hippocampus, and amygdala. Presumably, all thyroid hor­
mone–sensitive cell populations express iodothyronine membrane 
transporters. A cell surface T4 receptor has been characterized as an 
αVβ3 integrin that serves as the initiation site for T4-induced acti­
vation of the mitogen-activated protein kinase (MAPK) path

f the mitogen-activated protein kinase (MAPK) pathway 
for angiogenesis and perhaps actin polymerization and neuronal 
migration.241 The ontogenesis and significance of these cell surface 
receptors and membrane transporters in fetal development remain 
to be further defined. Recent studies have suggested that MCT8 
is also required for normal thyroid hormone secretion, in addition 
to its role as a thyroid hormone transporter.242 Brain damage in 
human fetuses is already induced in utero.276 Tr

human fetuses is already induced in utero.276 Treatment options for 
patients with MCT8 deficiency are limited. PTU blocks thyroid 
hormone production and inhibits conversion of T4 to T3. PTU, 
in combination with levothyroxine, has shown limited metabolic 
benefits but no effect on neurologic features. To date, two thyromi­
metics, diiodothyropropionic acid (DITPA) and triiodothyroacetic 
acid (TRIAC), are under investigation.277,278 DITPA has been used 
in a small number of patients from age

 been used 
in a small number of patients from age 9 months; it has been shown 
that DITPA crosses the placenta and reaches neuronal target cells 
in mice, inducing a T3-like effect.277,278 DITPA may therefore be a 
future candidate for in utero treatment of MCT8 deficiency. 
Thyroid Hormone Action
Classic thyroid hormone actions are mediated via functional thy­
roid hormone nuclear receptors (TRs), members of the steroid/
retinoid/vitamin D family of nuclear transcription factors. Two 
genes co

ly of nuclear transcription factors. Two 
genes code for the receptors: THRA on chromosome 17 encodes 
  Deiodinase Expression in Human and 
Rodent Tissues
Tissue
D1
D2
D3
Brain
X
X
X
Pituitary
X
X
Thyroid
X
Xa
Liver
X
Xb
Kidney
X
Ovary
X
X
Ear
Xa
Heart
Xa
Muscle
Xb
Skin
X
X
Testes
X
X
Uterus
X
X
Brown fat
X
aExpressed in human only.
bExpressed only in fetus.
Modified from St. Germain DL, Hernandez A, Schneider MJ, et al. Insights into the role of 
deiodinases from studies of genetically modifi

of 
deiodinases from studies of genetically modified animals. Thyroid. 2005;15:905–916.
   
TABLE 23.3
843
TRα, and THRB on chromosome 3 encodes TRβ.243 These genes 
code for four classic receptor isoforms (TRα1, TRα2, TRβ1, and 
TRβ2), three of which bind thyroid hormones (T3/T4 affinity 
10:1) and bind to DNA to effect gene transcription. The TRα2 
isoform does not bind thyroid hormone but binds to DNA and 
can inhibit binding of other TRs. The T

o DNA and 
can inhibit binding of other TRs. The TRs exist as monomers, 
homodimers, and heterodimers with other nuclear receptor fam­
ily members such as retinoid X (RXR). Other TR transcripts, 
including TRΔα1 and TRΔα2, have been characterized; they do 
not bind DNA or T3 but can inhibit TR and retinoid receptor 
activities.244
The TRs are expressed developmentally and differentially in 
various fetal and adult tissues. TRα proteins are present in most 
tissues. TRβ1 is expressed in liver, ki

t in most 
tissues. TRβ1 is expressed in liver, kidney, and lung and in devel­
oping brain, cochlea, and pituitary. TRβ2 expression is restricted 
largely to the pituitary gland, retina, and cochlea.21,243 The recep­
tors function redundantly, as indicated by knockout studies in 
mice, but predominant effects of one or another TR have been 
characterized (Table 23.4). Knockout of both the TRα and TRβ 
genes in mice is not lethal but results in elevated TSH concen­
trations, deafness, bradycardia

evated TSH concen­
trations, deafness, bradycardia, and decreased postnatal growth 
with delayed bone maturation.21,243 Lethality occurs because of 
improper intestinal development associated with persistent TRΔα 
isoforms in TRα or combined TRα and TRβ knockout mice.244 
In the fetal rat brain, TRα1 mRNA and receptor binding are 
detectable by 12 to 14 days of gestation (term is 21 days), increas­
ing to maximal levels at birth. The TRβ1 isoform is detected at 
birth and increases approximately

 is detected at 
birth and increases approximately 40-fold in the early postnatal 
period.20,21 In human fetal brain, TRα1 and TRβ1 isoforms and 
receptor binding are present by 8 to 10 weeks’ gestation; TRα1 
transcripts and receptor occupancy increase 8-fold to 10-fold by 
16 to 18 weeks.21,245,246 Liver, heart, and lung receptor binding 
can be identified by 13 to 18 weeks.21,246,247
Heterozygous missense mutations in THRA, coding for the 
TRα1, have recently been described in association wit

1, have recently been described in association with short stat­
ure, developmental delay, and chronic constipation. The muta­
tions act in a dominant negative fashion. The clinical phenotype 
is in line with the TRα1 being the predominant subtype receptor 
in bone, the gastrointestinal tract, cardiac and skeletal muscle, and 
the CNS.248,249 
Ontogeny of Thyroid Hormone Secretion
The role of maternal thyroid hormone during fetal develop­
ment remains controversial. The high placental concentrati

ains controversial. The high placental concentration of 
D3 inactivates most of the thyroid hormone presented from the 
maternal circulation. The iodide released in this manner is used 
for fetal thyroid hormone synthesis. Despite the limited maternal-
fetal placental transfer of T4 and the predominant production of 
inactive thyroid hormone metabolites in the human fetus, sig­
nificant levels of free T4 are present in fetal fluids, from placen­
tal transfer early in gestation and from fetal thy

tal transfer early in gestation and from fetal thyroid production 
during later gestation.20,21 Early in gestation, placental transfer is 
the only source of T4 in fetal fluids and is essential for normal 
fetal neurodevelopment. T4 is detectable in human coelomic fluid 
at levels of 0.5 to 2 nmol/L between 6 and 11 weeks’ gestation, 
before the onset of fetal thyroid function.19 Low concentrations of 
T4 are detectable in the fetal brain at about 10 weeks of gestation. 
Significant placental tr

t 10 weeks of gestation. 
Significant placental transfer continues to term, when serum T4 
levels in the athyroid fetus range from 30 to 70 nmol/L (2.3–5.4 
μg/dL).250 Isotopic equilibrium studies with pregnant rats at term 
suggest that 15% to 20% of the T4 in fetal tissues is of maternal 
origin.251 As indicated, thyroid hormones cross the placenta early 
in gestation, supplying the low levels of free T4 that are essential 
for brain development between 12 and 20 weeks, before the onset 
of fe

t between 12 and 20 weeks, before the onset 
of fetal thyroid hormone production.20 Most thyroid hormone 
in the fetal compartment is inactivated to sulfated and deiodin­
ated analogues until the perinatal period.219,232 This neutraliza­
tion of active circulating thyroid hormone maintains the low T3 
metabolic state, facilitating fetal growth and programmed tissue 
maturation.
Thyroid hormone–programmed development of selective fetal 
tissues requires the interaction of local tissue D1, D2, thy

quires the interaction of local tissue D1, D2, thyroid 
receptors, receptor coactivators, and thyroid-responsive genes. In 
most responsive tissues, the timing of maturation events is con­
trolled by the state of the thyroid receptors acting as a molecu­
lar switch.244,252 In the absence of T3, the unliganded receptor 
(aporeceptor) recruits corepressors, repressing gene transcription. 
Non–T3-binding receptors also can repress transcription by inhib­
iting receptor DNA binding. Local tissue mat

nhib­
iting receptor DNA binding. Local tissue maturation events are 
initiated by the coincident availability of T3, liganded T3 receptor, 
T3-mediated receptor exchange of corepressor with coactivators 
for creation of an active holoreceptor, and activation of responsive 
gene transcription.
These programming events have been investigated in studies of 
transgenic mice, including brain, liver, heart, intestine, and bone 
tissues; thermogenesis; and spleen erythropoiesis.239,240,252–256 The 
ti

 and spleen erythropoiesis.239,240,252–256 The 
timing of these events in the mouse ranges from early midbrain 
neuronal development at gestational day 15; through perinatal 
activation of hepatic enzymes, cardiac ion channels, and spleen 
erythropoiesis; to postnatal brain, intestinal, and bone maturation 
and thermogenesis. Parturition occurs at a gestational age equiva­
lent to human midgestation. In hypothyroid mice, repressive 
effects of aporeceptors have been shown to delay tissue maturat

oreceptors have been shown to delay tissue maturation 
in brain, bone, intestine, spleen, and heart.240 The increase in cir­
culating T3 levels associated with parturition in mice and humans 
  Predominant Thyroid Hormone Receptor 
Subtype Functions in Developing Mice
Brain
TRα1, TRβ1
TRα1, TRβ1, TRβ2
Pituitary TSH secretion
Inner ear
TRα1, TRβ2
TRβ2, TRα1
Pituitary GH secretion
Retina
TRβ2
TRα1
Bone maturation
Intestine
TRα1
TRα1
Liver
Heart
TRβ1
TRα1
GH, Growth hormone; TR, thyr

Liver
Heart
TRβ1
TRα1
GH, Growth hormone; TR, thyroid hormone receptor; TSH, thyroid-stimulating hormone.
Modified from Yen P. Genomic and nongenomic actions of thyroid hormones. In: Braver­
man LE, Utiger RD, eds. The Thyroid. 9th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 
2005:135–150; Flamant F, Samarut J. Thyroid hormone receptors: lessons from knockout 
and knockin mutant mice. Trends Endocrinol Metab. 2005;14:85–90; Ortiga-Carvalho TM, 
Sidhaye AR, Wondisford FE. Thyroid hormone

ho TM, 
Sidhaye AR, Wondisford FE. Thyroid hormone receptors and resistance to thyroid hormone 
disorders. Nat Rev Endocrinol. 2014;10:582–591.
   
TABLE 23.4
844
normally triggers development of tissue functions essential to 
postnatal metabolism and homeostasis (e.g., hepatic, intestinal, 
and cardiac functions and brown fat thermogenesis). Thyroid hor­
mone–stimulated maturation of vision and hearing appear to be 
triggered by the local expression

ng appear to be 
triggered by the local expression of D2, mediating local T3 pro­
duction, postnatally in the mouse and probably toward the end of 
the second midtrimester in the human fetus.
In humans, T3-mediated maturation of fetal tissues, includ­
ing liver, heart, brown adipose tissue, and bone, allows them to 
become thyroid hormone responsive during late gestation and in 
the perinatal period. Paracrine actions of thyroid hormone are 
critical for normal fetal development (e.g., in the co

ical for normal fetal development (e.g., in the cochlea, where 
D2 is expressed in connective tissue immediately adjacent to the 
sensory epithelium, and in spiral ganglion, where thyroid hor­
mone receptors are located). This implies that D2-containing 
cells in the connective tissue take up T4 from the circulation, 
convert it to T3, and then release D3 to adjacent responsive cells. 
Similarly, in the brain, D2 is expressed predominantly in glial 
cells, whereas TRs are expressed in adjacent n

al 
cells, whereas TRs are expressed in adjacent neurons and oligo­
dendrocytes. In other areas of the brain, such as the pituitary 
gland, hippocampus, and caudate nucleus, there is coexpression 
of D2 and TRs. On the other hand, D3 is coexpressed with TRs 
in neurons, thereby protecting sensitive tissues from the effects of 
excess thyroid hormone.
The actions of thyroid hormones and their developmental reg­
ulation in the brain are complex. Functionally, thyroid hormones 
are critical for the

nctionally, thyroid hormones 
are critical for the establishment of neural circuits during a critical 
window of brain development. They provide inductive cues for 
the differentiation and maturation of a number of processes such 
as neurogenesis and neural cell migration (occurring between 5 
and 24 weeks of gestation), neuronal differentiation, dendritic and 
axonal growth, synaptogenesis, gliogenesis (late fetal period to 6 
months postpartum), myelination (second trimester to 24 months 
post

, myelination (second trimester to 24 months 
postpartum), and neurotransmitter enzyme synthesis. TRs are 
found in highest concentration in developing neurons and in 
multiple areas of the fetal brain, including the cerebrum, the cer­
ebellum, and the auditory and visual cortices. The hormones bind 
to receptors and stimulate a number of genes such as myelin, neu­
rotropins and their receptors, cytoskeletal components, transcrip­
tion factors, extracellular matrix proteins and adhesion molecule

xtracellular matrix proteins and adhesion molecules, 
intracellular signaling molecules, and mitochondrial and cerebel­
lar genes.
Thyroid hormone is also important for normal bone growth. 
T3 regulates endochondral ossification and controls chondrocyte 
differentiation in the growth plate both in vivo and in vitro. TRs 
are expressed on osteoblasts and growth plate chondrocytes, and 
T3 target genes have been identified in bone. T3 stimulates skull 
suture closure in vivo.257
In the perinatal p

ull 
In the perinatal period, thyroid hormone stimulates the tran­
scription of thermogenin (also known as uncoupling protein 1 
[UCP1]), a protein that uncouples nucleotide phosphorylation 
and the storage of energy as adenosine triphosphate (ATP), actions 
that are important for nonshivering thermogenesis by brown adi­
pose tissue. 
Genetic Regulation of Thyroid Development
At least five developmental genes are involved in thyroid and 
parathyroid gland embryogenesis

ed in thyroid and 
parathyroid gland embryogenesis. These include the genes for 
thyroid transcription factors Pax8 (paired box 8), Nkx2-1 (NK2 
homeobox 1, previously known as TTF1), Hhex (hemopoetically 
expressed homeobox), Foxe1 (forkhead box E1, previously known 
as TTF2), and Nkx2-5 (NK2 homeobox 5) (Fig. 23.9).
As well as mediating the formation of the thyroid bud, these 
transcription factors drive functional differentiation, regulate the 
expression of genes involved in thyroid hormone 

expression of genes involved in thyroid hormone biosynthesis, and 
play a role in the maintenance of the mature thyroid gland (see 
Chapter 11). Mutations in NKX2-1, PAX8, and FOXE1 are all 
associated with thyroid dysgenesis in humans.215,217,258–260 
Putative Contributing Mechanisms to Thyroid 
Morphogenesis
Animal studies in mouse and zebrafish suggest that extrinsic fac­
tors such as permissive signals from blood vessels provide guidance 
cues in thyroid morphogenesis.261 Genes known to co

ues in thyroid morphogenesis.261 Genes known to contribute 
to thyroid morphogenesis in animal models include the Notch 
ligand JAG1, the thyroid and cardiac transcription factor Nkx2-
5, and NTN1, which is implicated in zebrafish aortic arch artery 
formation and thyroid morphogenesis.262–265 Other autonomous 
factors that may play a role include Tbx,266 a member of the T-box 
family of transcription factors and the prime candidate gene miss­
ing in DiGeorge syndrome. In mice, Tbx1 is expressed

g in DiGeorge syndrome. In mice, Tbx1 is expressed in both 
the pharyngeal endoderm and subpharyngeal mesoderm but only 
its mesodermal activity promotes the generation of Nkx2-1+ 
progenitors in the thyroid placode.267 The effect of Tbx1 on the 
embryonic thyroid is mediated by Fgf8 that is also produced in the 
mesoderm.267 It is known that signaling molecules such as those 
in the Shh pathway are also implicated in murine thyroid develop­
ment.261 In late organogenesis, Shh appears to have an

.261 In late organogenesis, Shh appears to have an important 
role in the symmetric bilocation of the thyroid; it also suppresses 
the ectopic expression of thyroid follicular cells.268 
Thyroid Function in Preterm Infants
Compared to infants born at term, neonatal free T4 increments 
are reduced in premature infants born at 31 to 34 weeks of gesta­
tion, attenuated at 28 to 30 weeks, and absent in infants 23 to 
27 weeks.269 This reflects hypothalamic-pituitary-thyroid system 
Pharynx
Pouches
P

alamic-pituitary-thyroid system 
Pharynx
Pouches
Parathyroid
glands
Thyroid
gland
Foramen
caecum
HOXB3
HOXA3
TTF1
TTF2
PAX8
TSHR
TPO
Tg
2
3
4
HOX15:
PAX1, PAX3,
PAX9, EYA1,
HOXA3, SIX1,
SOX3, GCM2,
CRK1, and TBX1
and TBX2
• Fig. 23.9  Homeobox genes that program development of the thyroid 
and parathyroid glands. HEX is involved early in the integrated cascade 
that programs thyroid gland embryogenesis. HOXB3 and HOXA3 may be 
responsible for activation of thyroid transcription factors TTF1 and 

ivation of thyroid transcription factors TTF1 and TTF2, 
respectively, during early embryogenesis. PAX8 is essential in the cascade. 
These factors are also involved in thyroid follicular cell function, promoting 
thyroglobulin (Tg), thyroid peroxidase (TPO), and thyroid-stimulating hor­
mone receptor (TSHR) gene transcription. HOX15 gene knockout in mice 
causes parathyroid gland aplasia. See text for details.
845
immaturity inversely related to g

tal Development 
immaturity inversely related to gestational age.220 Age-specific 
changes in thyroid function in children with small for gestational 
age (SGA) and normal birth weight from birth to adulthood are 
available.270 Premature birth interrupts the changes in thyroid 
hormone metabolism that occur over the third trimester resulting 
in low T4 and T3 concentrations that increase in proportion to 
increasing gestational age at delivery in preterm infants.269 The 
thyroid hormone concentr

 preterm infants.269 The 
thyroid hormone concentrations appear to be even lower than 
those expected in utero271; the reasons for this are multifactorial, 
including loss of maternal fT4 in association with an immature 
TSH-T4 axis, low iodide stores, and reduced deiodinase 1 (DIO1) 
activity.
A natural postnatal nadir in thyroxine concentration occurs 
at 7 days of life with a subsequent rise thereafter.220,269,272 Sev­
eral patterns of thyroid dysfunction can occur in preterm 
neonates226,269

 dysfunction can occur in preterm 
neonates226,269,273:
	1.	 Transient hypothyroxinemia of prematurity—identified by a 
low free T4 and normal TSH and seen in 50% of preterms 
born less than 28 weeks—does not require treatment. Matura­
tion of the hypothalamic pituitary axis is likely to be delayed 
in prematurity and, although the postnatal TSH surge is seen 
at all gestational ages, it appears to be attenuated in the most 
premature infants.
	2.	 Primary hypothyroidism—very low birth weight 

2.	 Primary hypothyroidism—very low birth weight (VLBW) 
infants have a higher risk of primary hypothyroidism and can 
exhibit a pattern of delayed TSH rise, although in a consider­
able number of these infants the hypothyroidism is transient. 
Thyroid hormone treatment should be initiated and continue 
at least until thyroid hormone–dependent brain maturation 
has completed at approximately 3 years of age. At this stage 
thyroid function can be reassessed.
	3.	 Hypothyroidism due to iodine ex

 T3, and TSH. Treatment is not indicated, but repeat 
testing every 1 to 2 weeks is recommended. 
Congenital Hypothyroidism
Classic signs of congenital hypothyroidism (jaundice, lethargy, 
feeding difficulties, macroglossia, myxedema, hypothermia, 
growth retardation and progressive developmental delay, and 
intelligence quotient [IQ] deterioration) appear during the ini­
tial critical weeks and months of extrauterine life as maternal T4 
becomes unavailable and the non-CNS tissues become respon

 unavailable and the non-CNS tissues become respon­
sive to thyroid hormone.21,219 Rarely, hypothyroidism is associ­
ated with respiratory distress in the neonatal period. The period 
of brain dependency for thyroid hormone extends up to 3 years 
of age, and most countries have a rigorous screening program to 
ensure early diagnosis and treatment.280 Most cases of perma­
nent primary congenital hypothyroidism are due to thyroid dys­
genesis. Mutations in known genes account for less than 5% of 

tions in known genes account for less than 5% of 
patients with congenital hypothyroidism and thyroid dysgenesis. 
Carré and colleagues recently described the impact of BOREA­
LIN mutations as a genetic cause of thyroid dysgenesis in humans. 
BOREALIN expression in human embryonic thyrocytes was 
shown before and after the onset of thyroid hormone synthesis, 
and BOREALIN is involved in the adhesion and migration of 
the thyrocytes, in keeping with a phenotype of thyroid ectopy.281 
Lower neonat

th a phenotype of thyroid ectopy.281 
Lower neonatal screening thresholds have increased the incidence 
of congenital hypothyroidism with a normally located gland in 
situ, and recent studies reveal that a significant proportion of 
patients have multiple gene mutations in more than one thyroid-
specific gene, supporting the hypothesis of an oligogenic etiol­
ogy of congenital hypothyroidism.282 Known causative genes in 
congenital hypothyroidism with gland in situ include TG, TPO, 
DUOX2, DUOXA

 with gland in situ include TG, TPO, 
DUOX2, DUOXA2, SLC5A5, SLC26A4, IYD, and TSHR, and 
the molecular spectrum is increasing.
In congenital hypothyroidism there is an increased net flux of 
maternal thyroid hormone to the fetus, resulting in cord T4 con­
centrations that are 25% to 50% of normal. There is increasing 
evidence of maternal-fetal T4 transfer in the first half of pregnancy, 
when fetal thyroid hormone levels are low.283 The transplacen­
tal passage of thyroid hormone, in conjuncti

acen­
tal passage of thyroid hormone, in conjunction with adjustments 
in brain deiodinase activity, has a critical role in minimizing the 
adverse effects of fetal hypothyroidism and helps explain the nor­
mal or near-normal outcome of hypothyroid fetuses (provided 
that prompt and adequate treatment of hypothyroidism ensues 
postnatally) as well as the relatively normal clinical appearance of 
the majority of babies with congenital hypothyroidism at birth. 
On the other hand, in the presence o

sm at birth. 
On the other hand, in the presence of both maternal and fetal 
hypothyroidism—such as that observed in the presence of potent 
TSHR-blocking antibodies, maternal and fetal POU1F1 defi­
ciency, and severe iodine deficiency—there is severe neurocogni­
tive impairment despite early and adequate commencement of 
thyroid replacement. Importantly, the presence of maternal hypo­
thyroxinemia or inadequately controlled hypothyroidism is also 
associated with significant neurocognitive defi

o 
associated with significant neurocognitive deficit in the offspring 
that is not reversible by early postnatal therapy.284 Table 23.5 
shows mechanisms resulting in congenital hypothyroidism due 
to abnormal thyroid development, dyshormonogenesis, abnormal 
thyroid hormone transport or action, and genes involved in these 
mechanisms.285 International guidelines for treatment of congeni­
tal hypothyroidism are available.286 
Gonadal Development
Sex determination is the result of a series of mo

Sex determination is the result of a series of molecular events that 
direct the undifferentiated bipotential gonad to become a testis or 
ovary. Sex differentiation takes place once sex determination has 
occurred via factors produced by the gonads that determine the 
development of the phenotypic sex. The work of Alfred Jost, elu­
cidating the pathophysiology of an experiment of nature, the free­
martin calf, was seminal in the field of fetal sex development.287
The freemartin phenotype appea

sex development.287
The freemartin phenotype appears in a dizygotic twin preg­
nancy, where a genetically female fetus is masculinized in the 
presence of a male twin. The phenomenon of freemartinism is a 
disorder of sex development, explained by the endocrine effects 
of testosterone and antimüllerian factor from the male calf on the 
genital development of the female, made possible by the placental 
vascular anastomoses in cattle resulting in conjoined circulations 
of the twin calves. Subseq

conjoined circulations 
of the twin calves. Subsequent advances in uncovering the genetic 
control and the pathophysiology of sex development have revealed 
that there is potential for disruption of the sex differentiation pro­
cess at different stages, and clinical phenotype will depend on the 
nature of the disruption
The mammalian gonad originates from the intermediate meso­
derm, from which the bipotential urogenital ridge differentiates. 
Several genes (including those encoding S

tiates. 
Several genes (including those encoding SF1, WT1, EMX2, 
846
CBX2, and PBX1) are required for formation of the bipotential 
gonadal ridge.288 The gonad is derived from two tissue anlagen, 
the primordial germ cells of the yolk sac wall and the somatic, 
stromal cells that migrate from the primitive mesonephros.289,290
By 2 to 3 wpc, the germ cells have begun their migration from 
the yolk sac, and the gonadal ridge has appeared as a derivati

, and the gonadal ridge has appeared as a derivative of 
the mesonephros. The germ cells are incorporated into the devel­
oping urogonadal ridge during the fourth week postconception 
in the human embryo as a thickening of the mesothermic meso­
nephros covered by coelomic epithelium. The primitive gonad is 
composed of a surface epithelium, primitive gonadal cords contin­
uous with the epithelium, and a dense cellular mass referred to as 
the adrenogonadal primordium, which includes the steroido

enogonadal primordium, which includes the steroidogenic 
cell precursors.290 As this primordium grows, the cells delaminate 
from the coelomic epithelium and invade the underlying mesen­
chyme, with the cells adjacent to the mesonephros migrating dor­
solaterally to form the gonadal primordium.
Until the appearance of testicular cords made up of pre-Sertoli 
cells at 6 wpc, the fetal testis and ovary are indistinguishable. The 
development of a specific sex phenotype requires the action of 
netw

pecific sex phenotype requires the action of 
networks of transcription factors and a complex signaling cascade 
that regulates differentiation of the bipotential gonad into a testis 
or ovary from 6 wpc.
Embryogenesis of the gonads is programmed by genes encod­
ing the male sexual determinant SRY, as well as SF1, SOX9, and 
DAX1.291,292 SRY, thought to have evolved from SOX3, is a criti­
cal regulator of male gonadal differentiation. The X chromosomal 
SOX3 gene is not normally expressed in the

osomal 
SOX3 gene is not normally expressed in the developing gonad. 
Therefore, when ectopically expressed, it might substitute for SRY 
in driving testicular development. Loss-of-function mutations of 
SOX3 do not affect sex determination, but SOX3 overexpression 
in murine XX gonads leads to testis differentiation; in humans, 
SOX3 duplication or rearrangements of the SOX3 regulatory 
region have been found in 46,XX individuals with dysgenetic 
testes.293
SF1 is required for testicular and ov

SF1 is required for testicular and ovarian development and 
mediates müllerian-inhibiting hormone (i.e., antimüllerian hor­
mone [AMH]) gene expression and gonadotropin production. 
Both SF1 and DAX1 are required for normal gonadal development.
By 7 wpc, in the XY gonad, SRY is expressed in pre-Sertoli cells 
resulting in the upregulation of SOX9 expression that is further 
augmented by the synergistic action of SRY and SF1, leading to 
the initiation of definitive Sertoli cell diff

to 
the initiation of definitive Sertoli cell differentiation. The tran­
scriptional activation by SRY and SF1 is mediated by the bind­
ing of these two proteins to the testis-specific enhancer of SOX9 
core (TESCO) region, which lies approximately 13 kb upstream 
of SOX9. Genomic rearrangements (such as duplication and trip­
lication) affecting SOX9 regulatory elements, located significantly 
further upstream (500 kb) than the TESCO enhancer region, 
have been found in patients with 46,XX sex r

ion, 
have been found in patients with 46,XX sex reversal.288
Once SOX9 levels reach a critical threshold, several positive 
regulatory loops are initiated, including autoregulation of SOX9 
expression and formation of feed-forward loops via FGF9 or 
PGD2 signaling. During testicular development, SOX9 functions 
by regulating the production of AMH from Sertoli cells and pos­
sibly by repressing genes involved in ovarian development such as 
WNT4 and FOXL2. The DMRT1 transcription factor may also

and FOXL2. The DMRT1 transcription factor may also be 
involved in this process.288
WT1, the Wilms tumor suppressor gene, is also expressed in 
the bipotential gonadal ridge during its differentiation from the 
intermediate mesoderm and is critical for normal male sexual dif­
ferentiation. Located at 11p13, it is expressed in both the primitive 
kidney and the genital ridge. WT1 isoforms associate and syn­
ergize with SF1 to promote expression of AMH. WT1 missense 
mutations associated with 46XY

 AMH. WT1 missense 
mutations associated with 46XY disorders of sexual development 
(DSD) in Denys-Drash syndrome fail to synergize with SF1.294 In 
addition, WT1 binds to and activates the SRY promoter.
The expression of SRY is transcriptionally regulated by WT1, 
SF1, GATA-4 and its cofactor zinc finger protein FOG2 (also 
called ZFPM2), and chromobox protein homologue 2 (CBX2).295 
In the presence of SRY, male gonadal differentiation begins with 
organization of the gonadal blastema into inte

th 
organization of the gonadal blastema into interstitium and germ 
cell–containing testicular cords. The primitive cords lose their 
connections with the epithelium, primitive Sertoli cells and sper­
matogonia become visible within the cords, and the epithelium 
differentiates to form the tunica albuginea.296 Sertoli cells induce 
the development of fetal Leydig cells via a hedgehog-signaling 
pathway, which produces androgens and INSL3, a member of 
the insulin-like family, at 8 to 9 wpc.297 

er of 
the insulin-like family, at 8 to 9 wpc.297 INSL3 is required for 
  Mechanisms Involved in Congenital 
Thyroid Dysgenesis (1 in 4500)
Isolated thyroid aplasia, hemiagenesis, hypoplasia, or ectopy
Transcription factor defect (PAX8)
Unknowna
Associated with other developmental abnormalities
Transcription factor defect (TTF1, FOXE1 [TTF2], NKX2-5, SHH, Tbx1)
Inborn Errors of Thyroid Hormonogenesis (1 in 35,000)
Abnormal iodide uptake via Na-I transporter (NIS, SLC5A5)
Abnorma

 uptake via Na-I transporter (NIS, SLC5A5)
Abnormal concentration of iodine
Abnormal organification of iodine
Abnormal iodination of thyroglobulin catalyzed by thyroid peroxidase 
(TPO)
Abnormal H2O2 generation (THOX, DUOX2, DUOXA2)
Efflux of iodide into colloid via apical anion channel (Pendred syndrome, 
SLC26A4)
Defective thyroglobulin synthesis or transport
Abnormal iodotyrosine deiodinase (DEHAL1)
Abnormal thyroid hormone transport in the brain (MCT8)
Abnormal selenium incorporation (SECISB

ain (MCT8)
Abnormal selenium incorporation (SECISBP2)
Secondary and Tertiary Hypothyroidism (1 in 50,000–100,000)
Hypothalamic abnormality
Isolated TRH deficiency
Multiple hypothalamic hormone deficiencies
Isolated hypothalamic defect
Associated with other midline facial/brain dysmorphic features (e.g., 
SOD, cleft lip/palate)
Pituitary abnormality
Isolated TSH deficiency (IGSF1)
TRH resistance
Abnormal TSHβ molecule
Posterior pituitary eutopic (transcript

ficiencies
Posterior pituitary eutopic (transcription factor defect, e.g., POU1F1, 
PROP1, LHX3)
Posterior pituitary ectopic (idiopathic, transcription factor defect, 
e.g., HESX1, SOX3, LHX4, OTX2)
TSH resistance
TSH receptor gene mutation (TSHR)
Postreceptor defect?
Thyroid hormone resistance (1 in 100,000)
Gαs gene mutation (GNAS)
aMost common.
   
TABLE 23.5
847
testicular descent.288 Rare mutations have been described in crypt­
orchidism.298,2

ions have been described in crypt­
orchidism.298,299 Testosterone and AMH cause regression of mül­
lerian structures and differentiation of the wolffian duct into the 
epididymis, vas deferens, and seminal vesicles. In 46,XY males, 
testosterone is converted to 5-dihydrotestosterone (DHT) by the 
enzyme 5α reductase, resulting in the development of male exter­
nal genitalia. The activity of DHT is mediated by the nuclear tran­
scription factor, androgen receptor (AR), which has high affinity 
fo

ndrogen receptor (AR), which has high affinity 
for DHT.
The fetal testes grow from approximately 20 mg at 12 wpc to 
800 mg at birth; at 5 to 6 months they descend into the inguinal 
canal in association with the epididymis and the vas deferens.296 
The gonad, adrenal, and kidney all initially develop in close prox­
imity, and as the testes descend they may carry rests of adrenocor­
tical cells with them. These adrenal rests may become hyperplastic 
with resulting testicular enlargement if subj

tic 
with resulting testicular enlargement if subjected to prolonged 
ACTH stimulation (e.g., in patients with poorly controlled con­
genital adrenal hyperplasia).
SF1 mutations result in a number of phenotypes related to 
gonadal development and function. A recurrent specific heterozy­
gous mutation of SF1 accessory DNA-binding region was recently 
reported in XX individuals with varying degrees of testis develop­
ment.300 One individual, raised as female, was found to have a 
46,XY karyotype a

 as female, was found to have a 
46,XY karyotype and partial testicular dysgenesis. These findings 
highlight how a specific variant in a developmental transcription 
factor can switch organ fate from ovary to testis in mammals, 
which represents the first missense mutation causing isolated non­
syndromic 46,XX ovotesticular DSD in humans.
In human females, differentiation of ovaries begins during 
week 5 of conception, in the absence of SRY. In the XX gonad, 
the absence of SRY results in the i

the XX gonad, 
the absence of SRY results in the inability of SOX9 expression 
to reach a critical threshold, and together with the expression of 
factors such as RSPO1/WNT4 signaling, FST and FOXL2 lead 
to formation of the ovary, at least in part through suppression of 
the activity of “testis” genes. In the 46,XX female, the absence of 
androgens leads to the development of female genitalia; the wolff­
ian duct regresses and the müllerian duct is maintained and forms 
the oviduct, uterus, cer

 is maintained and forms 
the oviduct, uterus, cervix, and upper part of the vagina.288 The 
gonadal blastema differentiates into interstitium and medullary 
cords containing the primitive germ cells, referred to as oogonia. 
The cords degenerate and cortical layers of surface epithelium, 
containing individual small oogonia, appear. By 9 to 10 wpc, clus­
ters of dividing oogonia are surrounded by cord cells within the 
cortex; the medulla at this time consists largely of connective tis­
sue.301

s time consists largely of connective tis­
sue.301 At 10 wpc, primitive granulosa cells begin to replicate, and 
many of the large oogonia in the deepest layers of the cortex enter 
their first meiotic division.
Primordial follicles are first observed at about 16 wpc, and the 
number increases rapidly thereafter.302 However, the number of 
oocytes progressively declines from a peak of 3 to 6 million at 
5 months of gestation to approximately 2 million at term.25,302 
Germ cell proliferation and 

llion at term.25,302 
Germ cell proliferation and apoptosis occur simultaneously. Pro­
liferating oocytes cluster, but the clusters break down with the 
development of follicles because only those oocytes enfolded by 
developing granulosa cells as primordial follicles survive.25,302 By 
5 months of gestation and during the seventh month, stroma-
derived thecal cells develop around the primordial follicles as they 
mature to primary follicles. This process continues after birth pro­
gressing towa

s process continues after birth pro­
gressing toward the superficial layers.
Each fetal ovary weighs about 15 mg at 12 wpc and 300 to 350 
mg at term.301 The number of surviving primary follicles at birth 
correlates with the duration of subsequent postpubertal ovulation. 
Interstitial cells with characteristics of steroid-producing cells are 
present after 12 weeks and, during the third trimester, theca cells 
with steroidogenic capacity surround the developing follicles.25 
Significant aromata

d the developing follicles.25 
Significant aromatase activity also is present but few if any steroids 
are produced by the ovary during development.25,301
The specific genetic mechanisms dictating ovarian develop­
ment are being unraveled; some of the most powerful regulators 
include the WNT/FZD/β-catenin, FOXO/FOXL2, and TGFβ/
SMAD pathways.295,303 FOXL2, encoding a forkhead transcrip­
tion factor, is required for ovarian development.304 DMRT1, 
expressed in testis, prevents expression of FOXL

expressed in testis, prevents expression of FOXL2 and hence 
female programming in the postnatal testis.305 In the XX gonad, 
RSPO1, WNT4, CTNNB1, FOXL2, and FST are also expressed 
in a female-specific manner to promote ovarian development and 
repress testicular development. In humans and mice, R-spon­
din-1 (encoded by RSPO1) augments β-catenin signaling, pos­
sibly via WNT4.306 Homozygous mutations in RSPO1 result 
in a syndrome with palmoplantar hyperkeratosis and 46,XX 
DSD with sex reve

lantar hyperkeratosis and 46,XX 
DSD with sex reversal and dysgenetic testes or ovotestes.307 
Aberrations of WNT4 cause Mayer-Rokitansky syndrome or 
SERKAL syndrome (sex reversal, dysgenetic kidneys, adrenals, 
and lung).302,308 
Fetal Sex Steroid Production
In the male fetus, the development of Leydig cells leads to an 
increase in fetal testosterone production between 8 and 18 wpc 
(Fig. 23.10).296 Androgen receptors appear in the epithelium dur­
ing development at 7 to 10 wpc308 with no dif

dur­
ing development at 7 to 10 wpc308 with no difference in expres­
sion between sexes.
In vitro studies in rat tissue have shown that hCG binding to 
fetal testis cells does not downregulate LH receptors. Fetal LH 
may contribute to fetal Leydig cell function, but quantitatively 
hCG is the predominant gonadotropin. Testosterone itself, acting 
through the androgen receptor, stimulates differentiation of the 
primitive mesonephric ducts into bilateral vas deferens, epididy­
mides, seminal vesi

lateral vas deferens, epididy­
mides, seminal vesicles, and ejaculatory ducts. DHT stimulates 
male differentiation of the urogenital sinus and external genita­
lia, including differentiation of the prostate, growth of the genital 
tubercle to form a phallus, and fusion of the urogenital folds to 
form the penile urethra. DHT mediates the action of testosterone 
in the wolffian ducts.
The fetal testis also produces AMH, which causes dedifferentia­
tion of the müllerian duct system in the male fe

­
tion of the müllerian duct system in the male fetus.309,310 AMH is a 
glycoprotein with a monomer that has a molecular size of approxi­
mately 72 kDa and multimer sizes ranging from 145 to 235 kDa. 
It is a member of the transforming growth factor β (TGFβ) fam­
ily. It is produced by testicular Sertoli cells and reaches the mül­
lerian ducts largely by diffusion; duct regression in vitro requires 
an exposure to AMH of 24 to 36 hours. AMH is synthesized early 
in gestation, with production pea

nthesized early 
in gestation, with production peaking at the time of müllerian 
duct regression; biosynthesis continues throughout gestation and 
decreases after birth. AMH gene expression is activated by the SRY 
and SF1 genes.309 AMH also has autocrine and paracrine effects 
on testicular steroidogenic function during fetal life.310 Male phe­
notypic differentiation is mediated by testicular testosterone and 
AMH and occurs between 6 and 12 wpc.
In the female fetus, the müllerian duct system 

c.
In the female fetus, the müllerian duct system differentiates in 
the absence of AMH, the mesonephric ducts fail to develop in the 
absence of testosterone, and the undifferentiated urogenital sinus 
and external genitalia mature into female structures. Mutation of 
the AMH gene results in a persistent müllerian duct syndrome in 
the XY fetus.309
848
Estrogen effects are mediated by cognate receptors, members of 
the large family of steroid and th

rs, members of 
the large family of steroid and thyroid hormone, vitamin D, and 
retinoid receptors.311,312 Two receptors, ERα (encoded by ESR1 
on chromosome 6) and ERβ (encoded by ESR2 on chromosome 
14), have been identified, with 96% and 58% homology in the 
DNA-binding and ligand-binding domains, respectively. Expres­
sion profiles of mRNAs of both receptors have been character­
ized in the human fetus at 16 to 23 weeks. One or both receptor 
mRNAs are present in most tissues. The ERβ messa

mRNAs are present in most tissues. The ERβ message is predomi­
nant, particularly in testis, ovary, spleen, thymus, adrenal, brain, 
kidney, and skin. The ERα message is prominent in the uterus, 
with relatively low levels in most other tissues.311,312
The significance of ERs in fetal development is unclear. Knock­
out of the ERα gene in mice does not impair fetal development 
of any tissue, but adult females are infertile with hypoplastic 
uteri and polycystic ovaries while adult males manife

ri and polycystic ovaries while adult males manifest decreased 
fertility.312 ERβ knockout mice develop normally, and female 
adults are fertile with normal sexual behavior; adult males repro­
duce normally but have prostate and bladder hyperplasia.311 It is 
known that estrogens regulate DHEA production in the baboon 
and human fetal adrenal.311
Knockout of both ERα (ESR1) and ERβ (ESR2) genes has 
little impact on fetal development, but after birth, the uterus, 
fallopian tubes, vagina, and ce

irth, the uterus, 
fallopian tubes, vagina, and cervix in females are hypoplastic 
and unresponsive to estrogen.312 In humans, ESR1 mutations 
in males are associated with tall stature, osteoporosis, and insu­
lin insensitivity.313
Androgens and estrogens are both involved in the structural 
development of the rat brain.314 Gonadal hormones also control 
gonadotropin production in the brain that results in cyclic ovar­
ian function and normal function of the testes.315,316 Testosterone 
administ

ction of the testes.315,316 Testosterone 
administration to neonatal female rats produces permanent inhi­
bition of cyclic hypothalamic control through local aromatization 
to estradiol and ER binding. In primates and humans, estrogens 
seem to be more effective in this regard, but there is no evidence 
for permanent programming in the primate nor does there appear 
to be major tissue biochemical differences between the sexes in 
utero to account for sexual dimorphic behavioral or gonadotropic 

 for sexual dimorphic behavioral or gonadotropic 
programming.315 The mechanisms for these effects are not yet 
clear in the primate and human fetus.
A current view of the pathways for genes programming 
gonadal differentiation is shown in Fig. 23.11. The full menu of 
downstream gene targets remains to be defined, but the net result 
is the highly organized pattern of gonadal development and 
phenotypic sexual differentiation. Fetal pituitary gonadotropins 
0
15
10
5
Birth
Fetal age
(wk)
10
20

nadotropins 
0
15
10
5
Birth
Fetal age
(wk)
10
20
30
40
2
4
6
(wk)
Plasma LH, hCG (U/L)
Plasma E2, T (nmol/L)
T
hCG
LH
E2
1000
500
200
100
0
1500
2000
• Fig. 23.10  Patterns of change of plasma concentrations of human cho­
rionic gonadotropin (hCG), luteinizing hormone (LH), testosterone (T), and 
estradiol (E2) in a male fetus during gestation and in the neonatal period. 
(Data from Mann DR, Gould KG, Collins DC, et al. Blockade of neona­
tal activation of the pituitary-testicular

 neona­
tal activation of the pituitary-testicular axis: effect on peripubertal lutein­
izing hormone and testosterone secretion and on testicular development 
in male monkeys. J Clin Endocrinol Metab. 1989;68;600–607; Reyes FI, 
Boroditsky RS, Winter JS, et al. Studies on human sexual development: 
II. Fetal and maternal serum gonadotropin and sex steroid concentrations. 
J Clin Endocrinol and Metab. 1974;38:612–617; Kaplan SL, Grumbach 
MM, Aubert ML. The ontogenesis of pituitary hormones and 

ert ML. The ontogenesis of pituitary hormones and hypothalamic 
factors in the human fetus: maturation of central nervous system regula­
tion of anterior pituitary function. Recent Prog Horm Res. 1976;32:161–
243; Winter JS, Faiman C, Hobson WC, et al. Pituitary-gonadal relations 
in infancy: I. Patterns of serum gonadotropin concentrations from birth 
to four years of age in man and chimpanzee. J Clin Endocrinol Metab. 
1975;40:545–551; Forest MG, Cathiard AM. Pattern of plasma testoster­
one a

G, Cathiard AM. Pattern of plasma testoster­
one and Delta[4]-androstenedione in normal newborns; evidence for tes­
ticular activity at birth. J Clin Endocrinol Metab. 1975;41:977–980.)
AMH
InhB
Estrogens
Sertoli
cells
Theca
cells
SF1
SF1
CBX2
SOX8
SOX9
WT1
SOX9
DHH
FOG2
DAX1
GATA-4
DAX1
FOXL2
RSPO1
SRY
LHX9
LIM1
GATA4
EMX2
WT1
WNT1 CTNNB1
WNT4
DMRT1
germ cells
Gonadal
ridge
Bipotential
gonad
Steroidogenic
cells
Leydig
cells
Testis
Ovary
• Fig. 23.11

cular
cells
Leydig
cells
Testis
Ovary
• Fig. 23.11  Summary of molecular and cellular events of gonadal differ­
entiation. AMH, antimüllerian hormone or müllerian inhibiting substance; 
DHH, desert hedgehog; InhB, inhibin B. See text for details. (Molecular 
cascades developed from Harley VR, Clarkson MJ, Argentaro A. The 
molecular action and regulation of the testis-determining factors, SRY 
[sex-determining region of the Y chromosome] and SOX9 [SRY-related 
high-mobility group (HMG) box 9]). 

 [SRY-related 
high-mobility group (HMG) box 9]). Endocr Rev. 2003;24,466–487; Park 
SY, Jameson JL. Minireview: transcriptional regulation of gonadal develop­
ment and differentiation. Endocrinology. 2005;146:1035–1042.)
849
are not required for gonadal development or sexual differentia­
tion; LH or FSH receptor knockout mice are born phenotypically 
normal.317 
Disorders of Sex Development
The increasing knowledge about sex determination and diff

reasing knowledge about sex determination and differen­
tiation and society’s increasing fluidity about the nature of sexual 
identity and gender roles have led to a reappraisal of former man­
agement practice in DSD. Optimal care for infants with DSD 
requires an experienced multidisciplinary team that transitions 
into adulthood, and there is a need for greater data sharing and 
formal research and clinical expertise networks at several levels, as 
highlighted by the Chicago Consensus in 2005.

 as 
highlighted by the Chicago Consensus in 2005. Human muta­
tions in several genes programming gonadal differentiation have 
been described, and it is likely that future innovation in “omic” 
technologies (e.g., transcriptomics) will lead to improved under­
standing of the underlying abnormality as well as explain the vari­
ability in the phenotype. 
Fetal Autonomic Nervous System
The activity of the autonomic nervous system may be regarded 
as a landmark brain function reflecting overall abi

s a landmark brain function reflecting overall ability to regulate 
the fetal central nervous system. The interplay between the sym­
pathetic and parasympathetic nervous systems is effected via the 
cerebral cortex, the medulla oblongata, the sympathetic ganglia, 
and the vagus nerve. There is an increasing realization that neural 
function in the fetus serves specialized needs particular to devel­
opment and that the onset of maturity represents the loss of these 
unique patterns of neural acti

 the loss of these 
unique patterns of neural activity. Nowhere is this more evident 
than in the sympathetic nervous system and its endocrine coun­
terpart, the adrenal medulla.
As early as 4 to 5 wpc the primordia of the sympathetic trunk gan­
glia are visible in the human fetus. The sympathoadrenal (SA) cell 
lineage is a derivative of the neuroectodermal crest (NC), which 
gives rise to sympathetic neurons and neuroendocrine chromaffin 
cells of the adrenal medulla and extra-adren

ffin 
cells of the adrenal medulla and extra-adrenal chromaffin cells. 
Signals that are important for specification of these two types of 
cells are largely unknown. The SA progenitor cells that migrate 
in a dorsolateral direction form numerous extramedullary sympa­
thetic paraganglia,318 which are scattered throughout the abdomi­
nal and pelvic sympathetic plexuses.319 Each of the paraganglia 
may reach a maximal diameter of 2 to 3 mm by 28 wpc; the largest 
of these, the organs of Zuckerkand

c; the largest 
of these, the organs of Zuckerkandl near the origin of the inferior 
mesenteric arteries, enlarge to 10 to 15 mm in length at term. 
After birth, the paraganglia gradually atrophy and disappear by 3 
years of age.
The SA cells that migrate ventrally through the anterior sclero­
tome to reach the dorsal aorta320 give rise to tyrosine hydroxy­
lase-expressing, catecholaminergic neuronal progenitor cells in 
response to bone morphogenetic protein cues from cells of the 
wall of the 

netic protein cues from cells of the 
wall of the dorsal aorta and the surrounding mesenchyme.321 
Some cells then migrate from the dorsal aorta along nerves and 
blood vessels to enter the adrenal primordium at the cranial end 
from 6 wpc322,323 and acquire the phenotype of chromaffin cells. 
It was thought that these were SA cells that lost expression of neu­
ronal genes, but more recently it has been shown that at least some 
of the presumptive chromaffin cell population enter the adrenal 
pr

e chromaffin cell population enter the adrenal 
primordium still expressing neural crest markers and then gain 
tyrosine hydroxylase and dopamine β-hydroxylase expression but 
not neuronal markers.324 A variety of transcription factors have 
been identified in the development of the SA cell lineage, includ­
ing PHOX2B (paired-like homeobox 2b), MASH1 (mammalian 
achaete-scute homolog 1), PHOX2A (paired-like homeobox 2a), 
and HAND2 (heart and neural crest derivatives-expressed pro­
tein 2).318 T

ral crest derivatives-expressed pro­
tein 2).318 The PHOX2B gene is pivotal in the development of the 
autonomic nervous system. Phox2b-/- mice die in utero since auto­
nomic nervous system neurons either fail to develop or degen­
erate. Mutations in human PHOX2B cause a rare syndrome of 
dysautonomia (congenital central hypoventilation syndrome) and 
a predisposition to neuroblastoma.325
Nerve growth factor (NGF) is required for the survival of 
developing sympathetic neurons. In the rodent, NG

developing sympathetic neurons. In the rodent, NGF anti­
serum injected into neonatal rats resulted in degeneration of 
immature chromaffin cells, sympathetic cells, and pheochro­
moblasts.326 It is unclear whether NGF and other growth fac­
tors are involved in maturation of the human fetal autonomic 
nervous system. 
Functional Development of the Sympathoadrenal System
Chromaffin cells are initially scattered within the adrenal cortex 
and are mostly noradrenergic. At around 10 wpc, small isle

mostly noradrenergic. At around 10 wpc, small islets of 
chromaffin cells express phenylethanolamine N-methyltransferase 
(PNMT), the enzyme required to convert noradrenaline into 
adrenaline, to become adrenergic.327 With increasing gestational 
age, there is progressive growth of the adrenal medullae, increasing 
catecholamine content and progressive maturation of medullary 
functional capacity. Histologically the adrenal medullae are some­
what immature at birth, but by 1 year they resemble t

t immature at birth, but by 1 year they resemble the adult 
glands.
Catecholamines are present in the para-aortic chromaffin tis­
sue by 8 to 13 wpc, and concentrations increase until term. The 
predominant catecholamine is norepinephrine (NE), presumably 
because of low activity of PNMT in para-aortic chromaffin tissue. 
In contrast, the adrenal medulla has high levels of PNMT activ­
ity. This enzyme catalyzes the methylation of NE to epinephrine 
and is activated by the high levels of cortisol

e 
and is activated by the high levels of cortisol that diffuse into the 
adrenal medulla from the adrenal cortex.319,328
How the fetus withstands an in utero environment of relative 
hypoxia has been a vibrant area of research for over a century. 
The ovine model provides a comparative system for physiologic 
and developmental research in the fetus that is applicable to 
human medicine. Basal plasma epinephrine, NE, and dopamine 
concentrations decrease during the last trimester in sheep329,330

decrease during the last trimester in sheep329,330 
due to maturation of clearance mechanisms.330 This model has 
been used to study fetal adaptive responses to chronic oxygen 
and nutrient deprivation during placental insufficiency. The fetal 
sheep responds to hypoxia with increased catecholamine concen­
trations.331 This response is also present in human fetuses during 
the third trimester of gestation.332 Central and adrenal enkepha­
lins are also involved in fetal autonomic nervous system f

 also involved in fetal autonomic nervous system function; 
pretreatment with naloxone potentiates and methadone inhibits 
the catecholamine response to hypoxia.319,333
Catecholamines are critical for fetal cardiovascular function and 
fetal survival. Gene knockout studies in mice, targeting either tyro­
sine hydroxylase or dopamine β-hydroxylase, produced fetal catechol­
amine deficiency and midgestation fetal death in 90% of the mutant 
embryos.334,335 In addition, fetal catecholamines are the

.334,335 In addition, fetal catecholamines are the major stress 
hormones in the fetus.328,332 The fetal adrenal and the para-aortic 
chromaffin masses discharge large amounts of catecholamines directly 
into the circulation in response to fetal hypoxia.328 Moreover, the 
defense against fetal hypoxia involves catecholamine actions mediated 
850
through cardiac α-receptors that are unique to immature animals. 
α-Adrenergic receptors predominate in im

animals. 
α-Adrenergic receptors predominate in immature cardiac tissue and 
gradually decline in number as β-adrenergic receptors increase with 
maturation. Chromaffin tissue in the fetus is also innervated by opiate 
receptors and contains relatively large amounts of opiate peptides that 
appear to be co-secreted with the catecholamines.328 The extent to 
which these peptides or pituitary endorphins are involved in modu­
lating fetal catecholamine secretion remains unclear.
The human neonate r

ine secretion remains unclear.
The human neonate responds to parturition with an increase in 
plasma epinephrine and NE concentrations, and these responses 
are augmented by hypoxia and acidosis.331,334,335 In the newborn, 
catecholamine secretion also increases after cold exposure and 
hypoglycemia.328,332 Labor-induced catecholamine release may 
provide an important mechanism underlying the neonate’s process 
of adapting to extrauterine conditions and the recruitment of vital 
postnatal adapta

ons and the recruitment of vital 
postnatal adaptations. 
Endocrine Pancreas: Insulin and Glucagon
Pancreatic development represents a fascinating process in which 
two morphologically distinct tissue types must derive from one 
simple epithelium. These two tissue types, exocrine (including aci­
nar cells, centroacinar cells, and ducts) and endocrine (alpha cells 
secrete glucagon, beta cells secrete insulin, delta cells secrete soma­
tostatin, and PP or gamma cells secrete pancreatic

tostatin, and PP or gamma cells secrete pancreatic polypeptide), 
serve disparate functions. Pancreatic development is a multistep 
process defined by three major periods (primary and secondary 
transition and the postnatal period).
Embryogenesis of the pancreas is mediated by a series of 
homeobox genes and transcription factors that program pancre­
atic budding from the gut tube, development of branching ducts 
and undifferentiated epithelium, differentiation of exocrine and 
endocrine cell li

differentiation of exocrine and 
endocrine cell lineages that originate from endodermal tissue, and 
organization of the endocrine cells into islets of Langerhans.336 
Members of the EGF family of growth factors, laminin, and per­
haps other growth factors, including the IGFs, also contribute to 
pancreatic growth and differentiation.337,338
Cell lineage determination during pancreatic development 
involves regulation by many transcription factors. The process has 
been well studied in the mouse

s. The process has 
been well studied in the mouse and begins at day 8 of the 21-day 
gestation, extending to 2 to 3 weeks after birth (Fig. 23.12). An out­
growth of dorsal and ventral pancreatic buds from the dorsal fore­
gut endoderm occurs at around embryonic day 8.5 (e8.5).339 These 
cells express pancreatic and duodenal homeobox 1 (Pdx1) without 
which the pancreas is unable to form.339 This phenotype in mice is 
mimicked in humans with homozygous loss of PDX1 that is respon­
sible for pan

ozygous loss of PDX1 that is respon­
sible for pancreatic agenesis. Pdx1 expression is quickly followed 
by expression of the basic helix-loop-helix protein pancreas-specific 
transcription factor 1a (Ptf1a), necessary for further development of 
both the endocrine and exocrine pancreas in mice as well as humans, 
forkhead box O1 (FOXO1), NK2 homeobox 2 (NKX2.2), and 
NK6 homeobox 1 and 2 (NKX6.1 and NKX6.2, respectively).339,340 
While regulatory factor X 6 (Rfx6) is also expressed during these

y factor X 6 (Rfx6) is also expressed during these 
early time points in mice, it appears only to be required for endo­
crine pancreatic development, a phenotype shared in humans with 
mutations in RFX6.339 Motor neuron and pancreas homeobox 1 
(Mnx1), formerly known as Hlbx9 or Hb9, is expressed early but, 
unlike the previously mentioned transcription factors, Mnx1-null 
mice fail to develop a dorsal pancreas, and the resulting pancreas has 
fewer and dysfunctional islets.336,339,341 Similar p

wer and dysfunctional islets.336,339,341 Similar phenotypic results 
were observed in humans with mutations in MNX1.
Hes family BHLH transcription factor 1 (Hes1) is a Notch 
signaling pathway component, and its loss in mice disrupts nor­
mal development of the pancreas but still allows formation of 
exocrine and endocrine cells.340 During this period, the only 
hormone-positive endocrine cells produced are a small number of 
glucagon-positive cells. Whether these cells play a role in develop­
m

lls. Whether these cells play a role in develop­
ment and postnatal function and what that role might be is not 
understood.336
Starting at around e13.5 in the mouse, expression of the basic 
helix-loop-helix transcription factor neurogenin 3 (Ngn3) marks 
endocrine progenitor cells that will differentiate into the mature, 
hormone-producing cells of the islets of Langerhans.336,339,340 
Ngn3 is only highly expressed before hormone expression, and 
loss of Ngn3 prevents the development of endocr

d 
loss of Ngn3 prevents the development of endocrine progenitor 
cells and consequently pancreatic islets. Ngn3 knockout in the 
mouse leads to marked β-cell dysplasia and hypoplasia.341 Ngn3 
initiates a new set of developmental programs for endocrine cell 
lineage differentiation and maintenance. These include the LIM 
homeobox protein islet 1 (Isl1), paired box 4 and 6 (Pax4 and 
Pax6), aristaless related homeobox (Arx), and neuronal differentia­
tion 1 (Neurod1), which are all lost in Ngn3-

tia­
tion 1 (Neurod1), which are all lost in Ngn3-deficient mice. In 
mice, Ngn3 knockout leads to complete absence of endocrine cells, 
whereas knockout of the lower pathway genes shown in Fig. 23.12 
impairs specific islet cell differentiation.341 Nkx2.2, Nkx6.1, Pax4, 
or Pax6 loss of function results in endocrine cell agenesis or hypo­
genesis.342–344 The competency of progenitors changes with time 
such that Ngn3+ cells that arise early in development preferentially 
Common
bile duct
Gut
PA

evelopment preferentially 
Common
bile duct
Gut
PAX4
PDX1
MAF1
BRN4
MAF-B
PAX6
NKX2.2
NKX6.1
ISL1
NGN3
PDX1
HLXB9
HNF6
HES1
HES1
PTF1a
MIST1
Beta2
β cells
α cells
D cells
Endocrine
cells
Dorsal
pancreas
Ventral
pancreas
Pancreatic
islet
• Fig. 23.12  Expression of main transcription factors during pancreatic 
embryogenesis. Knockout of PDX1, MNX1 (previously called HLXB9), 
or ISL1 is associated with early arrest of pancreatic development. MNX1 
knockout leads to failure of the pancreatic dorsal

knockout leads to failure of the pancreatic dorsal bud to develop with 
decreased β-cell number in the remnant pancreas. HES1, PTF1A, or neu­
rogenin 3 (NGN3) disruption leads to aplasia or hypoplasia of the islets of 
Langerhans. Disruption of the downstream transcription factors impairs 
formation of the β cells or α cells. SOX9 and HNF3B (not shown) are 
required for early foregut formation and pancreas specification. (Data from 
Habener JF, Kemp DM, Thomas MK. Minireview: transcriptional reg

emp DM, Thomas MK. Minireview: transcriptional regulation 
in pancreatic development. Endocrinology. 2005;146:1025–1034.)
851
differentiate into glucagon-positive cells while later Ngn3+ cells 
give rise to insulin-positive or somatostatin-positive cells.339
During the later stages of β-cell differentiation, the MafB and 
MafA (V-maf musculoaponeurotic fibrosarcoma oncogene homo­
logs B and A) transcription factors play a prominent role. Upon 
β-ce

cription factors play a prominent role. Upon 
β-cell specification, the transcription of insulin is initiated and 
maintained by MafB, Pdx1, Neurod1, Pax6, and MafA. Addition­
ally, Pax4, NKX6.1, and Pdx1 inhibit glucagon expression thereby 
preventing the expression of glucagon by β cells.339
Arx is one of the first known transcription factors to specifically 
mark the glucagon-producing α-cell lineage, and its loss prevents 
development of α cells with a concomitant increase in β and δ cells.3

lls with a concomitant increase in β and δ cells.339 
Indeed, a competitive reciprocal interaction between Arx and Pax4 is 
involved in the decision of cell fate between α and β cells. While 
Arx is necessary for the early specification of the α cell and directly 
maintains α-cell mass, it is not directly involved in glucagon expres­
sion. The Pax4-related factor Pax6 is expressed by both α and β cells. 
Pax6-deficient mice lack α cells. In addition to its role in insulin tran­
scription in the 

ion to its role in insulin tran­
scription in the β cell, Pax6 also coordinates glucagon transcription in 
the α cell directly by binding to the glucagon promoter and indirectly 
by inducing expression of other transcription factors such as c-Maf, 
MafB, and Neurod1, which also activate glucagon expression. A third 
factor that is crucial for later α-cell development is forkhead box A2 
(Foxa2), which plays a role downstream of Arx and Pax6.339
A de novo heterozygous mutation in FOXA2 has been d

 de novo heterozygous mutation in FOXA2 has been described 
in a child with congenital hyperinsulinism and congenital hypo­
pituitarism with craniofacial dysmorphic features, choroidal colo­
boma, and endoderm-derived organ malformations in liver, lung, 
and gastrointestinal tract.345,346 Expression profiling in human 
embryos by immunohistochemistry showed strong expression of 
hFOXA2 in endoderm-derived organs, including the pancreas, 
and transfection studies and Western blot assays demonstra

sfection studies and Western blot assays demonstrated the 
causative role of FOXA2 in this syndrome. Upon successful speci­
fication of the α-cell lineage, pre-proglucagon expression is further 
promoted by forkhead box A1 (Foxa1), brain 4 (Brn4), and Isl1. 
Of all the transcription factors that can promote glucagon expres­
sion, expression of only two, Brn4 and MafB, is enriched in adult 
mouse α cells compared to β cells.339
More recent studies have compiled gene regulatory networks 
for pancr

 have compiled gene regulatory networks 
for pancreas development integrating data from multiple studies, 
including mutation analysis, gene expression, cell lineage tracing, 
and biochemical gene regulation. These networks include all the 
previously mentioned genes but also others such as Tle2, Dll, One­
cut1, BMP7, and SOX9.340 The transcription factor Glis-similar 3 
(Glis3) has been implicated in the development of neonatal type 
1 and type 2 diabetes.347 Loss of functional Glis3 leads to a

2 diabetes.347 Loss of functional Glis3 leads to a severe 
reduction in both β-cell and pancreatic polypeptide cell numbers 
postnatally, as well as formation of cystic pancreatic ducts. Ngn3, 
glis-, and Glis3 represent the relatively “late actors” as their expres­
sion is restricted to the establishment of defined endocrine pro­
genitor cells (or endocrine and ductal cells in the case of Glis3).
In humans, mutations in GLIS3, NeuroD1, PDX1, PTF1A, 
and GATA6 are associated with neonatal diabet

1A, 
and GATA6 are associated with neonatal diabetes mellitus due to 
pancreatic agenesis; mutations in EIF2AK3 (encoding translation 
initiation factor 2-alpha kinase 3), HNF1B (hepatocyte nuclear 
factor-1β), MNX1, NKX2.2, and RFX6 lead to neonatal diabetes 
due to pancreatic hypoplasia.348–351 Recessive mutations in a dis­
tal enhancer of PTF1A also cause pancreatic agenesis.352 
Functional Development of the Endocrine Pancreas
The human fetal pancreas is identifiable by 4 weeks of gestation 

 pancreas is identifiable by 4 weeks of gestation 
and α and β cells can be recognized by 8 to 9 weeks. Insulin, 
glucagon, somatostatin, and pancreatic polypeptide are measur­
able by 8 to 10 weeks of gestation.353 α Cells are more numer­
ous than β cells in the early fetal pancreas and reach a relative 
peak at midgestation; β cells continue to increase throughout the 
second half of gestation so that, by term, the ratio of α to β cells 
is approximately 1:1. Both α and β cells have opposing r

roximately 1:1. Both α and β cells have opposing roles in 
glucose homeostasis, although they derive from a common pro­
genitor and share many proteins important for glucose sensing 
and hormone secretion. Mouse models have underlined the simi­
larities between the two cell types by showing that β-to-α as well 
as α-to-β transdifferentiation can take place under certain experi­
mental circumstances, revealing a remarkable degree of plasticity 
among the different pancreatic endocrine cells.339
T

ong the different pancreatic endocrine cells.339
The β cell is functional by 14 weeks of human gestation, and 
fetal pancreatic insulin content exceeds that of the adult through­
out most of pregnancy. The insulin content of the pancreas 
increases from less than 3.6 pmol/g (0.5 U/g) at 7 to 10 weeks to 
30 pmol/g (4 U/g) at 16 to 25 weeks of gestation and 93 pmol/g 
(13 U/g) near term; the concentration in the adult pancreas is 
approximately 14 pmol/g (2 U/g).354 Endocrine cells are dis­
perse

 pmol/g (2 U/g).354 Endocrine cells are dis­
persed throughout the exocrine tissues by 20 weeks and the islets 
of Langerhans are clearly differentiated by 31 weeks.
Fetal glucose metabolism is largely independent of insulin and 
glucagon.355–357 In physiologic doses, glucagon does not increase 
hepatic glucose production, probably because of a paucity of fetal 
hepatic glucagon receptors.357 Insulin receptors are present in a 
variety of fetal tissues at levels generally exceeding those of the 

issues at levels generally exceeding those of the 
adult, but downregulation of insulin receptor binding does not 
occur during fetal hyperinsulinemia, in contrast with observations 
in adult animals. Acute hypoglycemia or hyperglycemia is not 
associated with significant alterations in either insulin or glucagon 
Although the fetal β cell is functional by 14 to 24 weeks of 
gestation, secretion of insulin into the bloodstream by the fetal 
pancreas is low. Insulin release from t

the fetal 
pancreas is low. Insulin release from the fetal rat pancreas in vitro 
in response to glucose or pyruvate is minimal but can be stimu­
lated by leucine, arginine, tolbutamide, or potassium chloride, 
indicating that parts of the secretory mechanism are functional 
in the fetus.354,355,358 Insulin secretion in adult islets is mediated 
by two or more mechanisms, including stimulation of the adenyl­
ate cyclase system with production of cAMP and inhibition of 
potassium efflux, which le

cAMP and inhibition of 
potassium efflux, which leads to depolarization of the cell mem­
brane and opening of voltage-dependent calcium channels. Cal­
cium channel activation does not occur in fetal islets in response 
to initiators of insulin release that cause depolarization of adult 
islet cells.358 Infusion of glucose or arginine to pregnant women 
fails to provoke fetal insulin secretion at midgestation or near 
term, and plasma insulin concentrations in the late human fetus 
are relatively

entrations in the late human fetus 
are relatively unresponsive to high glucose concentrations before 
the onset of labor.354 Similar observations have been made in the 
monkey: Neither glucose nor arginine stimulated fetal insulin 
release near term, but glucagon evoked prompt insulin produc­
tion.354 Late in gestation in the ovine fetus, epinephrine inhibits 
insulin release through a receptor pathway.354
In the anencephalic human fetus, the endocrine pancreas 
develops normally if maternal ca

docrine pancreas 
develops normally if maternal carbohydrate metabolism is not 
impaired; β-cell hypertrophy and hyperplasia do not occur in 
the anencephalic fetus or in decapitated fetal rabbits exposed to 
chronic hyperglycemia. This lack of β-cell response to hypergly­
cemia may be the result of deficiency of GH or IGF1, or both, 
because GH stimulates insulin gene expression and may play a 
permissive role in β-cell hyperplasia and hypertrophy.338 Chronic 
852
SECTION VI    Endocrinology an

hy.338 Chronic 
852
fetal hyperglycemia does evoke hyperinsulinemia and glucagon 
suppression, and chronic hypoglycemia may inhibit fetal insulin 
and promote fetal glucagon release.343
Pancreatic glucagon concentrations are relatively high in fetal 
plasma and increase progressively with fetal age.354,355 The fetal 
pancreatic glucagon content at midgestation is approximately 
6 μg/g, compared with an adult level of 2 μg/g. As is true for 
insulin, 

n adult level of 2 μg/g. As is true for 
insulin, the capacity for glucagon secretion is blunted in the fetus. 
Hyperglycemia does not suppress fetal plasma glucagon concen­
trations in rats, monkeys, or sheep, and acute hypoglycemia does 
not evoke glucagon secretion in the rat fetus. Amino acids, which 
are important secretagogues for insulin and glucagon in the adult, 
probably have little role in modulating insulin and glucagon secre­
tion in the preterm fetus, but infusion of alanine into w

 the preterm fetus, but infusion of alanine into women at 
term increases both maternal and cord blood glucagon concen­
trations, indicating a fetal glucagon response to amino acids in 
the term fetus. Catecholamines also evoke glucagon release in the 
near-term ovine fetus.354
Thus fetal pancreatic islet cells, although histologically mature 
and capable of hormone synthesis and hyperplasia, are relatively 
immature functionally at birth with regard to their capacity to 
secrete insulin and glu

gard to their capacity to 
secrete insulin and glucagon. The rapid maturation of respon­
siveness to glucose in the neonatal period in both premature and 
mature infants suggests that this blunted state may be a secondary 
result of the relatively stable fetal serum glucose concentrations 
maintained by placental transfer of maternal glucose rather than 
a primary, temporally fixed, maturation process. Alternatively, the 
lack of any enteric signal to the pancreas from feeding via release 
of in

al to the pancreas from feeding via release 
of incretins may also account for this stability. The blunted capac­
ity for insulin and glucagon secretion has been related to a defi­
cient capacity of the fetal pancreatic islet cells to generate cAMP 
or to rapid destruction of cAMP by phosphodiesterase, or both.354
In rodents, a period of a rapid increase in β-cell mass at the 
time of birth and shortly thereafter has been observed; this rapid 
change is attenuated by a period of apoptosis before

ange is attenuated by a period of apoptosis before adult mass 
is achieved. β-cell mass is more difficult to determine from a 
developmental standpoint in humans. At birth in the human, 
there are some 200 to 300 × 106 β cells, which is approximately 
one-third of the population present in adulthood. However, 
most of the actual mass change takes place in the newborn 
period and is associated with changes in β-cell size rather than 
number.359 Thereafter, there is rapid further expansion in term

ereafter, there is rapid further expansion in terms 
of cell numbers, but the waxing and waning of the β-cell mass, 
particularly with pregnancy, is poorly understood. How much 
β-cell mass is a determinant of progression to type 2 diabetes 
mellitus is unclear.
Insulin and glucagon are normally not necessary for substrate 
metabolism in the fetus.355 Fetal energy needs are met by a con­
tinuous intravenous supply of glucose across the placenta, fetal 
glucose uptake being directly related to bo

fetal 
glucose uptake being directly related to both the maternal blood 
glucose concentration and the transplacental gradient. There is lit­
tle endogenous glucose production. The fetal respiratory quotient 
is approximately 1, which suggests that glucose is the primary 
energy substrate for the fetus. Other substrates, such as amino 
acids and lactate, may also be used in the human as in the sheep 
fetus and these, together with glucose, are stored as fat and glyco­
gen in preparation for birt

ored as fat and glyco­
gen in preparation for birth. Early in gestation, hepatic metabo­
lism and substrate utilization appear to be independent of insulin 
and to be modulated in an autoregulatory fashion by glucose.354 
In addition, the constant supply of glucose normally precludes 
the necessity for endogenous gluconeogenesis, and gluconeogenic 
enzyme activities are low in the fetal liver.
Glycogen storage in the fetus is modulated by fetal glucocorti­
coids and probably by placental lactoge

lucocorti­
coids and probably by placental lactogen (hPL). Fetal insulin plays 
a role near term, when insulin also has the capacity to increase 
fetal glucose uptake and lipogenesis.354,355 Insulin receptors are 
present on most fetal cells in higher numbers than on adult cells; 
moreover, hyperinsulinemia fails to downregulate fetal insulin 
receptors.354 Fetal hepatic glucagon receptors, in contrast, are 
reduced in number, and fetal liver is relatively resistant to the gly­
cemic effect of g

relatively resistant to the gly­
cemic effect of glucagon. These conditions tend to potentiate the 
fetal anabolic milieu during the period of rapid growth in the last 
trimester of gestation. 
Pancreatic Regulation of Blood Glucose
During the neonatal period, the placental source of glucose is lost. 
A normal term infant has an immediate postnatal fall in blood 
glucose concentration during the first 2 to 4 hours from values 
close to maternal concentrations to around 2.5 mmol/L (45 g/
dL) post

oncentrations to around 2.5 mmol/L (45 g/
dL) postpartum.360,361 Low concentrations are usually transient, 
asymptomatic, and part of the normal adaptation to extrauter­
ine life. Counterregulatory hormones rapidly become active, 
and these changes result in stabilization of blood glucose con­
centrations, although adult concentrations are not reached until 
approximately 72 hours of life.360 Plasma glucagon concentrations 
rise in association with a rapid increase in functionally coupled 
gluca

th a rapid increase in functionally coupled 
glucagon receptors. The increase in plasma catecholamines coin­
cident with parturition may be responsible for some of these 
changes; catecholamines stimulate glucagon and inhibit insulin 
release.344,357,362 Phosphoenolpyruvate carboxykinase (PEPCK) 
activity also increases during this period. Thus gluconeogenesis is 
readily demonstrable in the newborn, where nearly 10% of glu­
cose utilization is accounted for by gluconeogenesis from alanine.
Plas

ccounted for by gluconeogenesis from alanine.
Plasma free fatty acid concentrations increase postnatally 
because of the effects of catecholamines and chemical thermogen­
esis. Oxidation of fatty acids probably provides cofactors (acetyl 
coenzyme A and the reduced form of nicotinamide adenine dinu­
cleotide) required for gluconeogenesis, as well as sparing glucose 
for utilization by crucially dependent tissues such as the brain.344 
Failure of this sequence of physiologic changes can lead to h

this sequence of physiologic changes can lead to hypo­
glycemia, which is most common in the first few hours after birth. 
It has been reported that lactate may be neuroprotective in at-risk 
babies with hypoglycemia within the first 48 hours after birth, a 
time period when ketone bodies are unlikely to provide protec­
tion.363 Neonatal hypoglycemia is common and is treated with 
supplemental feeds or intravenous glucose. Early feeding for pre­
vention of hypoglycemia in at-risk babies (includi

vention of hypoglycemia in at-risk babies (including infants of 
mothers with diabetes, small and large for gestational age, and 
preterm) is the mainstay of management. Dextrose gel is effective 
in treating neonatal hypoglycemia without affecting breastfeed­
ing. The Pre-hPOD Study (hypoglycemia prevention with oral 
dextrose) reports the first randomized controlled trial (RCT) of 
prophylactic oral dextrose in at-risk babies, showing reduced inci­
dence of neonatal hypoglycemia.364 
Neonatal 

nci­
Neonatal Diabetes
The neonatal diabetes phenotype encapsulates numerous sub­
types, which involve severe disruption in β-cell function. Neonatal 
diabetes can be permanent and require lifelong treatment or may 
be transient, in which case the diabetes may spontaneously remit 
before 18 months of age (60% of cases) (or be so mild as not to 
require treatment) but will often relapse, usually during adoles­
cence. Both have an incidence of approximately 1 in

ence. Both have an incidence of approximately 1 in 200,000 to 
250,000.365
853
Transient neonatal diabetes mellitus (TNDM) may be due to 
overexpression of an imprinted gene on chromosome 6q24 in the 
majority of cases, whereas activating mutations of KCNJ11 and 
ABCC8, encoding for the two subunits (Kir6.2 and sulfonylurea 
receptor 1 [SUR1]) of the pancreatic β-cell ATP-sensitive inward 
rectifier potassium (K-ATP) channel account for 25% of case

 potassium (K-ATP) channel account for 25% of cases. 
Mutations in the INS gene, encoding pre-proinsulin, are rarer, 
as are mutations in ZPF57, which regulates gene methylation. A 
genetic cause can be identified in approximately half of patients 
with NDM. Intrauterine growth retardation (IUGR) is another 
feature of TNDM, although not in TNDM due to K-ATP chan­
nelopathies. After remission, diabetes mellitus recurs in approxi­
mately 50% of cases.
Permanent neonatal diabetes mellitus (PNDM) i

ses.
Permanent neonatal diabetes mellitus (PNDM) is most often 
due to activating mutations in KCNJ11 or ABCC8, resulting in 
overactivation of potassium channels and thus impaired insulin 
secretion. These patients are responsive to sulfonylurea therapy 
and do not require insulin treatment. Other causes of PNDM 
include INS and GCK mutations. Mutations in EIF2AK3 (eukary­
otic translation initiation factor 2-alpha kinase 3) lead to Wol­
cott-Rallison syndrome of which other features include sk

llison syndrome of which other features include skeletal 
abnormalities, liver dysfunction, cardiac and renal abnormalities, 
developmental delay, epilepsy, and neutropenia. FOXP3 muta­
tions result in IPEX (immune dysregulation, polyendocrinopathy, 
on X chromosome), which can lead to PNDM. Other monogenic 
causes of PNDM include mutations in genes encoding for zinc 
finger protein GLIS3, PAX6, the thiamine transporter SLC19A2, 
glucose transporter GLUT2 (encoded by SLC2A2),366 NeuroG3, 
and Ne

ter GLUT2 (encoded by SLC2A2),366 NeuroG3, 
and NeuroD1, and genes important for pancreas development: 
PTF1A (hypoplasia of pancreas and cerebellum), PDX1 (pancreas 
agenesis, endocrine and exocrine hormone deficiency),367 RFX6, 
acting downstream of NeuroG3 (hypoplasia of pancreas, gall­
bladder, intestinal atresia, intractable diarrhea),368,369 GATA4,370 
and GATA6.367 Recently, mutations in MNX1 (previously called 
HLXB9) and NKX2-2, coding for key transcription factors for 
pancreas develop

or key transcription factors for 
pancreas development, were identified as monogenic causes for 
PNDM in humans.350 Neonatal diabetes that does not respond 
to sulfonylurea therapy is best treated with continuous subcutane­
ous insulin infusion via a pump using rapid-acting insulin ana­
logues. Dilution of insulin to fill the pump is often required for 
neonates and infants. Demand of insulin is so small, and feed­
ing so frequent, that treatment with multiple daily injections is 
difficult.371,

 with multiple daily injections is 
difficult.371,372
Although most cases of neonatal diabetes involve isolated dia­
betes, many of the known monogenic causes are characterized by a 
variety of syndromic features. Increased attention to the primarily 
genetic nature of early-onset diabetes has resulted in an expanding 
list of causal genes and an expansion of phenotypic characteristics 
in syndromic forms. 
Hyperinsulinemic Hypoglycemia of Infancy
Hyperinsulinemic hypoglycemia (HH) is the most f

y
Hyperinsulinemic hypoglycemia (HH) is the most frequent cause 
of persistent and recurrent hypoglycemia in neonates and infants. 
It is due to unregulated secretion of insulin and can be transient 
or permanent.373,374 Transient forms lasting for days are associated 
with maternal diabetes mellitus, maternal sulfonylurea treatment, 
and glucose infusions during labor. Poorly controlled maternal 
diabetes mellitus is associated with fetal macrosomia, an increased 
risk of spontaneous abortion, 

omia, an increased 
risk of spontaneous abortion, and fetal malformation. Maternal 
hyperglycemia also leads to hyperinsulinism and β-cell hyperpla­
sia in the infant. Infants of diabetic mothers are prone to polycy­
themia, renal vein thrombosis, hypocalcemia, respiratory distress 
syndrome, jaundice, persistent fetal circulation, cardiomyopathy, 
congenital heart disease, and malformations of other organs.
Transient hyperinsulinism due to intrauterine growth retarda­
tion, perinatal asphyxia, 

uterine growth retarda­
tion, perinatal asphyxia, and BWS can last for days to months 
and may require treatment.375 Permanent hyperinsulinism can 
histologically be divided into focal and diffuse forms, which are 
inherited in, respectively, a sporadic and autosomal manner. To 
date, causative mutations in 12 different genes involved in β-cell 
insulin release have been described in congenital hyperinsulinism 
(CHI), for example, ABCC8, KCNJ11, GLUD1, GCK, HADH, 
SLC16A1, HNF4A, and UCP2.374–37

GLUD1, GCK, HADH, 
SLC16A1, HNF4A, and UCP2.374–376 ABCC8 and KCNJ11 
encode for the subunits (Kir6.2 and SUR1) of the pancreatic 
β-cell ATP-sensitive K+ channel.377 However, the molecular basis 
of HH is unclear in up to 60% of patients.375,376,378 Owing to 
their sensitivity to ATP, K+ channels convert intracellular meta­
bolic signals into membrane excitability in the following cascade. 
Glucose is transported into the cell by glucose transporters; intra­
cellular glucokinase catalyzes an es

rters; intra­
cellular glucokinase catalyzes an essential irreversible step for gly­
colysis. Intracellular glucose metabolism raises the cytosol ATP/
adenosine diphosphate (ADP) ratio, which inhibits the SUR1 
component of the K+ channel, resulting in K+ channel closure, 
which subsequently results in membrane depolarization, opening 
of voltage-gated calcium channels, and calcium influx, which then 
triggers the release of insulin from stored granules. Mutations in 
ABCC8 and KCNJ11 can thus r

ranules. Mutations in 
ABCC8 and KCNJ11 can thus result in oversecretion of insulin. 
The hyperinsulinism/hyperammonemia (HI/HA) syndrome is 
due to gain-of-function mutations in glutamate dehydrogenase 
(GDH) and is characterized by both fasting hypoglycemia and 
hyperinsulinemic hypoglycemia due to protein ingestion.379 
Certain syndromes have been associated with hyperinsulinism: 
overgrowth syndromes (BWS, Sotos, Simpson-Golabi-Behmel), 
chromosomal abnormalities (trisomy 13, mosaic Turner s

mosomal abnormalities (trisomy 13, mosaic Turner syn­
drome), growth failure syndromes (Kabuki syndrome, Costello 
syndrome), syndromes with abnormal calcium homeostasis (Tim­
othy syndrome), congenital disorders of glycosylation, congeni­
tal hypoventilation syndrome, and a contiguous gene deletion, 
including ABCC8 (Usher syndrome).375
The rarest forms of CHI have recently been associated with 
inactivating mutations in the uncoupling protein 2 gene (UCP2), 
a mitochondrial inner membrane prot

 gene (UCP2), 
a mitochondrial inner membrane protein,380 and recently the 
spectrum of genetic causes for HH has been extended with the 
reported co-occurrence of congenital hyperinsulinism and con­
genital polycystic kidney disease, caused by a promoter mutation 
in the phosphomannomutase 2 gene (PMM2).381
At the time of hypoglycemia, a hypoglycemia screen, including 
glucose, insulin, C-peptide, acetoacetate, and β-hydroxybutyrate, 
will easily identify congenital hyperinsulinism.
Diagnostic 

y identify congenital hyperinsulinism.
Diagnostic criteria are a glucose infusion rate more than 8 mg/
kg/minute, a laboratory blood glucose less than 3 mmol/L with 
detectable serum insulin or C-peptide, low serum ketone bodies, 
and low serum fatty acids.
First-line treatment is diazoxide and chlorothiazide, and 
second-line treatment is octreotide and glucagon. Patients who 
are diazoxide insensitive need further investigation with a com­
bined computed tomography (CT)/18F-DOPA positron emiss

d computed tomography (CT)/18F-DOPA positron emission 
tomography (PET) scan to assess for focal or diffuse lesions. Focal 
lesions can be cured with surgery, whereas diffuse lesions require 
near-total pancreatectomy, which usually does not result in a cure 
and may lead to diabetes mellitus.377
Tremendous advances are being made in the field of hyperinsu­
linemic hypoglycemia, for example, use of rapid molecular genetic 
testing, application of novel imaging techniques (6-[fluoride-18]
854
SEC

 novel imaging techniques (6-[fluoride-18]
854
fluoro-levodopa [18F-DOPA] PET-CT, glucagon-like peptide 
1 [GLP1] receptor imaging), and development of new medical 
therapies (e.g., long-acting octreotide formulations, mammalian 
target of rapamycin [mTOR] inhibitors, GLP1 receptor antago­
nists) and laparoscopic surgical techniques.373,374 
Parathyroid/Calcitonin System
During the fifth to sixth week of intrauterine development, the 
embr

 sixth week of intrauterine development, the 
embryonic pharynx is marked externally by four branchial clefts 
of ectoderm origin and internally by five branchial pouches of 
endoderm origin. The branchial arches of mesoderm origin are in 
between them. Together these structures comprise the branchial 
apparatus, which normally undergoes involution leaving behind 
derivatives, including the thyroid gland, parathyroid glands, thy­
mus, ultimobranchial body, eustachian tube, middle ear, and 
exter

hial body, eustachian tube, middle ear, and 
external auditory canal.382
The parathyroid glands develop as epithelial thickenings of 
the dorsal endoderm of the third and fourth branchial pouches. 
The superior parathyroid glands are derived from the fourth bran­
chial pouch, which also gives rise to the thyroid gland. The third 
branchial pouch gives rise to the inferior parathyroid glands and 
the thymus. The parathyroid glands remain intimately connected 
with their respective branchial pouch

y connected 
with their respective branchial pouch derivatives, and parathyroid 
gland development from the third and fourth pharyngeal pouches 
proceeds in synchrony with thyroid embryogenesis.21,383
The third pouches encounter the migrating thyroid anlage, 
and the parathyroid anlagen are carried caudally with the thy­
roid gland, finally coming to rest at the lower poles of the thyroid 
lobes as the inferior parathyroid glands.21,383 The fourth pouches 
encounter the thyroid anlage later and 

h pouches 
encounter the thyroid anlage later and come to rest at the upper 
poles of the thyroid lobes as the superior parathyroid glands.21,383 
The individual parathyroid glands increase in diameter from less 
than 0.1 mm at 14 weeks of gestation to 1 to 2 mm at birth. 
Most of the parathyroid gland parenchyma is composed of chief 
cells (also known as the parathyroid gland principal cells). These 
play a critical role in calcium homeostasis by sensing changes in 
extracellular calcium concen

y sensing changes in 
extracellular calcium concentration and releasing the appropriate 
amount of PTH to correct or maintain normal blood calcium 
concentrations. The chief cells spend most of their time inactive 
due to normal calcium homeostasis. Near term, fetal parathyroid 
cells are largely composed of inactive chief cells, with only a few 
intermediate chief cells containing occasional secretory granules. 
The fifth pouches contribute paired ultimobranchial bodies that 
are incorporated i

ed ultimobranchial bodies that 
are incorporated into the developing thyroid gland as the parafol­
licular or C cells that secrete calcitonin. The calcitonin content 
of C cells in the neonatal thyroid gland is as high as 540 to 2100 
mU/g of tissue, values as much as 10 times those observed in the 
normal adult gland.384 Both endocrine systems are functional dur­
ing the second and third trimesters. 
Transcription Factors Involved in Development of 
­Parathyroid Glands
The molecular signaling p

 of 
The molecular signaling pathways that are involved in determining 
the differentiation of the pharyngeal pouch endoderm into para­
thyroid cells are being elucidated by studies of patients with hypo­
parathyroidism and appropriate mouse models. These studies have 
revealed important roles for several transcription factors, which 
include members of the homeobox (Hox) and paired box (Pax) 
families. Disruption of the Hox15 gene in mice results in para­
thyroid gland aplas

 gene in mice results in para­
thyroid gland aplasia, indicating that this gene functions as part 
of the genetic cascade programming normal thyroid-parathyroid 
gland development. Deletions of genes encoding several transcrip­
tion factors in mice result in developmental anomalies that include 
parathyroid defects, although such abnormalities have not yet been 
reported in man. The transcription factors include members of the 
Hox (Hoxa3, which results in parathyroid agenesis, and Pbx1, a 
prot

results in parathyroid agenesis, and Pbx1, a 
protein that acts as a cofactor of HOX transcription factors) and 
Pax (Pax 1,3,9) families, the eyes absent homologue (Eya1) and 
sine oculis homeobox homologue (Six1).385
Human disorders with congenital hypoparathyroidism have 
been found in association with mutations and deletions in GATA3 
(hypoparathyroidism, deafness, renal dysplasia [HDR] syndrome), 
AIRE1 (autoimmune polyendocrinopathy-candidiasis-ectodermal 
dystrophy [APECED] syndrome or au

asis-ectodermal 
dystrophy [APECED] syndrome or autoimmune polyglandular 
syndrome type 1 [APS1]), and tubulin folding cofactor E (TBCE) 
gene (Kenny-Caffey syndrome). TBX1 (a DNA-binding tran­
scriptional factor of the T-box family that is known to have an 
important role in vertebrate and invertebrate organogenesis and 
pattern formation) and CRK1 mutations have been associated 
with 22q11 deletion syndrome.386
Isolated hypoparathyroidism can be found in association with 
mutations in PTH, GCM

e found in association with 
mutations in PTH, GCMB (the human homologue of the mouse 
glial cells missing 2 [Gcm2] gene, which is expressed exclusively 
in the parathyroid glands) and the SRY-related HMG-box gene 
3 (SOX3).385 X-linked recessive hypoparathyroidism has been 
reported in two multigenerational, related kindreds with a dele­
tion-insertion downstream of SOX3 likely to exert a position effect 
on SOX3 expression. These findings point to a potential role for 
the SOX3 gene in the emb

 to a potential role for 
the SOX3 gene in the embryologic development of the parathy­
roid glands from the pharyngeal pouches. 
Fetal and Neonatal Calcium Metabolism
Parathyroid hormone produced by the parathyroid glands is an 84–
amino acid sequence that stimulates bone resorption, renal tubu­
lar absorption, and synthesis of calcitriol in the kidneys. There is a 
negative feedback loop with ionized calcium and calcitriol. PTH-
related protein (PTHrp) binds to and activates PTH receptors. 
Thi

(PTHrp) binds to and activates PTH receptors. 
This is expressed in the uterus, placenta, amnion, umbilic cord, 
lactating breast, and fetal parathyroid glands. In animal models, 
despite maternal hypocalcemia induced by a calcium-restricted 
diet, vitamin D deficiency, parathyroidectomy, or a vitamin D 
receptor deletion model, fetuses were reported to be normocalce­
mic. The calcium concentration in the fetus is therefore indepen­
dent of the maternal calcium concentration (Fig. 23.13).
Studie

aternal calcium concentration (Fig. 23.13).
Studies in fetal sheep and monkeys and measurements in 
human preterm and term infants indicate that the majority of 
calcium is transported in the third trimester. Concentrations of 
total calcium (averaging 2.75–3 mmol/L in the third trimester) 
and ionized calcium, as well as phosphate and magnesium, are 
higher in the fetal circulation compared to maternal, demonstrat­
ing active transport of calcium across the placenta against a con­
centration gr

m across the placenta against a con­
centration gradient.387,388 Nearly all the calcium transferred to the 
fetus is involved in mineralization of the fetal skeleton.389
Calcium enters the placental trophoblast at the brush-border 
membrane primarily via the TRPV6 channel. Transport is facili­
tated by calbindin D9K, and calcium is released to the fetal circu­
lation through the calcium pump PMCA3. Inorganic phosphate 
enters the trophoblast probably via the sodium-phosphate cotrans­
porter NaPi

ably via the sodium-phosphate cotrans­
porter NaPiIIb.387 A three-step model of transcellular placental 
calcium transport has been proposed387,390:
	1.	 Influx of calcium ions from the maternal circulation involv­
ing voltage-dependent calcium channels (primarily transient 
855
receptor potential cation channel, subfamily V, member 6 
[TRPV6], which is a calcium channel that opens in the mater­
nal-facing basement membrane of the syncytiotrophobl

l-facing basement membrane of the syncytiotrophoblast 
to enable calcium entry into cells) at the apical brush-border 
membrane of the trophoblast cells at the maternal-placental 
interface.
	2.	 Movement of calcium through trophoblastic cytosol, involv­
ing intracellular calcium-binding proteins (mainly calbindin 
D9K), which transport calcium to the basal membrane.
	3.	 Efflux of calcium ions from cytosol across the basolateral pla­
cental membrane involving a membrane ATP-dependent cal­
ciu

mbrane involving a membrane ATP-dependent cal­
cium pump (PMCA3, at the placental-fetal interface), which 
transports calcium into the fetal circulation.
Calcium and phosphate are maintained at a higher set-point in 
the fetus than in the pregnant mother, especially in the third tri­
mester, to meet the demands of fetal bone mineralization. In late 
gestation, fetal serum concentrations of calcium and phosphate 
are maintained above the maternal concentrations by approxi­
mately 1.2 to 2 mg/dL a

concentrations by approxi­
mately 1.2 to 2 mg/dL and 1.5 mg/dL, respectively.388 Although 
the fetal concentration of Mg is also increased compared with 
the maternal value, the gradient is less (∼0.12 mg/dL)388 than 
those of calcium and phosphate. Although the molecular mecha­
nism of transplacental calcium transport has been well studied, 
little is known about the transport mechanism of phosphate and 
magnesium.
Fetal serum phosphate is also higher than maternal serum con­
centrations. The t

higher than maternal serum con­
centrations. The type IIb sodium-dependent inorganic phosphate 
transporter (NaPiIIb) is expressed in embryonic visceral and pari­
etal endoderm as well as in labyrinthine cells of the placenta and 
likely has a role in fetal phosphate homeostasis.387 Little is known 
of the mechanism of placental magnesium transport.
The placenta is impermeable to PTH, PTHrp, and calcitonin, 
but 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D (cal­
citriol) are transported acros

roxyvitamin D (cal­
citriol) are transported across the placenta, and free vitamin D 
concentrations in fetal blood are similar to or higher than mater­
nal values.391,392
The fetal sheep has low circulating concentrations of PTH 
but can increase serum PTH concentrations in response to a 
fall in serum calcium concentration induced by ethylenediami­
netetraacetic acid (EDTA) and can respond promptly to infused 
calcium with increased serum calcitonin concentrations. In this 
model, fetal parath

tonin concentrations. In this 
model, fetal parathyroidectomy decreases placental calcium trans­
port and lowers fetal serum calcium.
Although PTH has no effect on placental calcium transport, 
PTHrp is present in fetal tissues and placenta and stimulates cal­
cium transport. The principal PTH receptor is PTHR1, which has 
equal affinity for PTH and PTHrp. A second receptor, PTHR2, is 
present in CNS and binds PTH but not PTHrp, whereas PTHrp 
action in the placenta occurs likely through a disti

tion in the placenta occurs likely through a distinct receptor that 
binds PTHrp but not PTH.387,393
The PTH concentration in the fetus near the end of gestation 
is lower (<4.72 pg/mL) than maternal or nonpregnant adult con­
centrations. In cord blood, the concentration of PTHrp is 15-fold 
higher than that of PTH at term and plays a critical role in peri­
natal calcium homeostasis and maintenance of the higher fetal 
calcium set-point.390,394 In the fetus, PTHrp is produced in many 
tissues, i

n the fetus, PTHrp is produced in many 
tissues, including the placenta. PTH plays an essential role in reg­
ulating calcium and bone homeostasis in the adult, but whether 
PTH is required at all for regulating fetal-placental mineral 
homeostasis and skeletal development is uncertain. It is unknown 
whether the human placenta produces PTH. Since maternal PTH 
concentrations are suppressed during pregnancy compared with 
those in the nonpregnant adult, fetal PTH concentrations appear 
to be stro

adult, fetal PTH concentrations appear 
to be strongly suppressed. Little PTH is detectable in fetal plasma 
using immunoassays, whereas bioassays showed significant bioac­
tivity.395 Parathyroid glands are active in the human fetus from 
about 12 weeks of gestation, and thyroparathyroidectomy in the 
fetal sheep causes a rapid decrease in fetal plasma calcium concen­
tration and a loss of the placental calcium gradient.391 Although 
the fetal PTH concentrations are very low, fetal PTH is though

H concentrations are very low, fetal PTH is thought to 
be important because fetal mice lacking parathyroid glands, PTH, 
or the PTH/PTHrp receptor are hypocalcemic and have under­
mineralized skeletons.389,396,397 The principal factor responsible 
for this bioactivity would therefore appear to be PTHrp rather 
than PTH itself.
Other factors are involved in maintenance of fetal serum cal­
cium concentrations because knockout of the mouse gene for 
PTH-PTHrp also results in hypocalcemia in the pr

PTH-PTHrp also results in hypocalcemia in the presence of 
normal or increased placental calcium transport.390,398 PTH and 
PTHrp through the PTH/PTHrp receptor presumably modulate 
fetal skeletal calcium flux, calcium excretion through the fetal 
kidney, and perhaps reabsorption of calcium from amniotic fluid. 
PTHrp has a major role in fetal bone development and metabo­
lism as well as fetal calcium homeostasis. PTHrp knockout mice 
display increased ossification of the basal portion of the 

ncreased ossification of the basal portion of the skull, long 
bones, vertebral bodies and pelvic bones, and mineralization of 
the normally cartilaginous portions of the ribs and sternum; as 
PTH
Fetal parathyroids
PTHrp
PTHrp
CT
1,25(OH)2D
PO4
Ca
• Fig. 23.13  Proposed actions of parathyroid hormone (PTH), PTH-related 
protein (PTHrp), and calcitonin (CT) in the fetus. PTHrp and perhaps PTH 
from the parathyroid glands and PTHrp from the placenta act on the 
placenta to promote calcium (Ca) an

ta act on the 
placenta to promote calcium (Ca) and phosphate (PO4) transport from 
the maternal to the fetal circulation to maintain the relative fetal hyper­
calcemia and the high rate of fetal bone formation during the last half of 
gestation. PTHrp also acts on the kidney to promote 1-hydroxylation of 
25-hydroxycholecalciferol to 1,25(OH)2D (1,25-dihydroxyvitamin D, cal­
citriol), which augments placental calcium transport and promotes fetal 
bone growth. High fetal CT levels tend to promot

bone growth. High fetal CT levels tend to promote bone accretion. See 
text for details. (From Kovacs CS. Bone metabolism in the fetus and 
neonate. Pediatr Nephrol. 2014;29:793–803.)
856
a result of the cartilaginous mineralization, the animals die of 
asphyxiation in the early neonatal period.390,394,399,400
The skeletal chondrodysplasia is more severe in combined PTH 
plus PTHrp or PTH/PTHrp receptor gene–ablated animals.397,401 
In the mouse, k

ptor gene–ablated animals.397,401 
In the mouse, knockout of the calcitonin gene or the vitamin D 
receptor gene leads to postnatal osteosclerosis or osteomalacia, 
respectively; the pups appear normal at birth.402 Studies have 
demonstrated that Pthrp-null fetuses had hypocalcemia (equal to 
maternal blood concentration), hypomagnesemia, hyperphospha­
temia, reduced fetal-placental calcium transfer, normal amniotic 
fluid calcium content, and normal skeletal mineral content,389 but 
the absence

rmal skeletal mineral content,389 but 
the absence of parathyroids caused a more substantial reduction 
in serum calcium and skeletal mineral content than absence of 
PTHrp despite the fact that PTH normally circulates at low con­
centrations in the fetus.
PTH-null mice have enlarged parathyroids that are incapable 
of making PTH, whereas Gcm2-null mice lack parathyroids but 
have PTH that arises from the thymus. PTH-nulls served as a 
model of complete absence of PTH, whereas Gcm2-nulls are a 

omplete absence of PTH, whereas Gcm2-nulls are a 
model of severe hypoparathyroidism. Simmonds and colleagues 
demonstrated that PTH contributes to fetal mineral homeosta­
sis because a fetal hypoparathyroid phenotype results with hypo­
calcemia, hypomagnesemia, hyperphosphatemia, low amniotic 
fluid mineral content, and reduced skeletal mineral content in 
its absence.389 PTH regulates the placental expression of genes 
involved in calcium and other solute transfer and may contribute 
to the re

ther solute transfer and may contribute 
to the regulation of placental calcium transfer. PTH may con­
tribute to placental gene expression and function through both 
endocrine/systemic (parathyroid-derived) and paracrine (placen­
tal-derived) pathways. Although parathyroid hormone–related 
protein (PTHrP) acts in concert with PTH to regulate fetal min­
eral homeostasis and placental calcium transfer, unlike PTH, it 
does not upregulate in response to fetal hypocalcemia.
The fetal parathyroid-pl

se to fetal hypocalcemia.
The fetal parathyroid-placental axis promotes maternal-fetal 
transfer of bone mineral and accretion of fetal bone mineral. It 
seems likely that fetal PTH and presumably PTHrP act on the 
fetal kidney to stimulate 1α-hydroxylation of 25-hydroxyvitamin 
D and that 1,25-dihydroxyvitamin D participates in modulating 
placental calcium transport. 1,25(OH)2D or 24,25(OH)2D also 
play a role in fetal cartilage growth and bone mineral accretion.403 
Thus PTHrP and to a lesser

 mineral accretion.403 
Thus PTHrP and to a lesser extent PTH in the ovine fetus appear 
to augment maternal-to-fetal calcium transport across the pla­
centa and thus provide for the high rate of bone mineral accretion 
in the latter half of pregnancy.
During pregnancy, maternal 25(OH)D crosses the placenta, 
so the fetal concentrations of 25(OH)D reach 75% to 100% 
of the maternal concentration at term.404 Fetal 1,25(OH)2D 
concentrations, however, are lower than that in the mother 
(<50%).405 

er, are lower than that in the mother 
(<50%).405 The maternal concentration of 1,25(OH)2D during 
pregnancy is twofold to threefold higher than that in the non­
pregnant adult. It is suggested that the maternal kidneys contrib­
ute to the abundance of 1,25(OH)2D in the pregnant mother.406 
Nephrectomy in the fetal sheep reduces fetal serum calcium con­
centrations, and this can be prevented by prior administration 
of 1,25-dihydroxyvitamin D. Moreover, infusion into the sheep 
fetus of antibody

reover, infusion into the sheep 
fetus of antibody to 1,25(OH)2D reduced the placental calcium 
gradient.391 The fetal kidney can produce 1,25(OH)2D and the 
placenta contains 1,25(OH)2D receptors, as well as a vitamin 
D–dependent calcium-binding protein.400 It is thought that the 
synthesis of 1,25(OH)2D in the fetus is suppressed by the high 
concentrations of calcium and phosphate and low concentra­
tions of PTH.
The high blood concentrations of calcitonin in the fetus, 
probably resulting f

 of calcitonin in the fetus, 
probably resulting from the chronic stimulation by fetal hyper­
calcemia, are thought to contribute to the fetal bone mineral 
accretion.391,397 The predominant effect of calcitonin is to inhibit 
bone resorption, and the high fetal serum calcium concentrations 
coupled with high circulating calcitonin promote bone mineral 
anabolism.397 Placental calcitonin production may contribute to 
the calcitonin in fetal plasma, but the persistence of high plasma 

a, but the persistence of high plasma 
concentrations in neonatal plasma argues for predominant fetal 
production. Thyroid C cells begin to differentiate at around week 
12 of gestation,407 and calcitonin is detectable at around week 
15 of gestation. Fetal circulating calcitonin concentrations are 
approximately twofold higher than those in the mother.388,408,409 
The trophoblasts of the placenta also produce calcitonin and sup­
ply it to the fetus.410 Since calcitonin does not cross the placen

tus.410 Since calcitonin does not cross the placenta 
in the mouse or rat,411 fetal circulating calcitonin is derived from 
fetal sources. The major source of fetal circulating calcitonin has 
not been determined. 
Calcium-Sensing Receptor (CaSR) and FGF23
The calcium-sensing receptor plays a pivotal role in systemic cal­
cium metabolism by modulating production and secretion of 
calcium-regulating hormones such as PTH, calcitonin, FGF23 
and vitamin D, and urinary calcium excretion.412,413 The 

amin D, and urinary calcium excretion.412,413 The CaSR 
is present in parathyroid glands, renal tubules, bone and cartilage, 
and many other tissues. Magnesium binds to the CaSR and influ­
ences PTH secretion. Mutations within the CaSR result in either 
inactivation or overactivation of the receptor resulting in hyper­
calcemia or hypocalcemia, respectively.414 Inactivating mutations 
lead to switching off of PTH secretion at a lower calcium con­
centration than usual and hypercalcemia ensues. R

centration than usual and hypercalcemia ensues. Renal calcium 
secretion is reduced.
Hypercalcemia disorders related to inactivating mutations 
of the CASR gene are either heterozygous (autosomal dominant 
familial benign hypercalcemia [FBH], still named hypocalciuric 
hypercalcemia syndrome type 1) or homozygous (severe neo­
natal hyperparathyroidism). The differential diagnosis consists 
of the hypocalciuric hypercalcemia syndrome, types 2 (involv­
ing GNA11) and 3 (involving AP2S1), hyperpar

volv­
ing GNA11) and 3 (involving AP2S1), hyperparathyroidism, 
abnormalities of vitamin D metabolism involving CYP24A1 and 
SLC34A1 genes, and reduced GFR.
Hypocalcemia disorders, which are rarer, are related to hetero­
zygous activating mutations of the CASR gene (type 1), consisting 
of autosomal dominant hypocalcemia (ADH), sometimes with a 
presentation of pseudo-Bartter syndrome. The differential diagno­
sis consists of the hypercalciuric hypocalcemia syndrome type 2, 
involving GNA11 and 

ypocalcemia syndrome type 2, 
involving GNA11 and other hypoparathyroidism etiologies. An 
online database to keep track of described mutations is available at 
http://www.casrdb.mcgill.ca.
Some conditions are particularly important in the neonatal 
period. When a neonate presents with abnormal calcium concen­
trations, assessment of calcium, albumin, phosphate, creatinine, 
alkaline phosphatase, vitamin D and urine Ca-creatinine ratio, and 
phosphate reabsorption in both the child and parents w

phate reabsorption in both the child and parents will help for­
mulate the underlying diagnosis. Hypocalcemia resulting from fetal 
and maternal vitamin D deficiency is relatively common. Occasion­
ally, severe neonatal vitamin D deficiency can be associated with a 
dilated cardiomyopathy, which is reversible.415,416 Asphyxia can result 
in subcutaneous fat necrosis, which is due to release of 1,25(OH)
D from macrophages and leads to hypercalcemia. Hyperhydration, 
steroid therapy, low calcium m

a. Hyperhydration, 
steroid therapy, low calcium milk, and occasionally bisphosphonate 
treatment may be needed to reduce calcium concentrations.
857
FBH and familial hypocalciuric hypercalcemia (FHH) can 
present in neonates with mild to moderately elevated calcium 
concentrations, high normal (nonsuppressed) PTH, and a low 
urine Ca-creatinine ratio.417 FHH may also present as neonatal 
hyperparathyroidism leading to bone disease and variable sym

hyroidism leading to bone disease and variable symp­
toms of hypercalcemia. This occurs especially if an affected infant 
has inherited the inactivating CaSR mutation from the father 
while the mother is normal. The fetus will consider the mother as 
hypocalcemic and therefore overproduce PTH. Cinacalcet ther­
apy may help bind calcium and settle the hyperparathyroidism 
but parathyroidectomy is occasionally necessary. Hypocalcemic 
and hypercalcemic disorders may also be due to mutations in G 

emic disorders may also be due to mutations in G 
protein subunit alpha 11 (GNA11), AP2S1 (involved in G protein 
internalization), and calcium channels TRPV5 and TRPV6.418,419
FGF23 is the principal hormone that regulates phosphate 
transport. It is primarily produced and secreted into the circula­
tion by osteocytes and, after its activation through glycosylation 
by GALNT3, acts through FGFR1c and FGFR4 on renal tubules 
to increase phosphate excretion through the sodium-phosphate 
exchanger 

excretion through the sodium-phosphate 
exchanger (NaPiIIC, SLC34A3).420 α-Klotho acts as a cofactor 
for FGF23 to bind to FGFR1c, increasing specificity of FGFR1c 
for FGF23. FGF23 is inactivated by cleavage by subtilisin/furin-
like enzyme. PHEX (phosphate regulating gene with homolo­
gies to endopeptidases on the X chromosome) regulates cleavage 
of FGF23 and PHEX mutations, and therefore renders FGF23 
constitutively active. Besides increasing renal tubular phosphate 
excretion, FGF23 inhibi

g renal tubular phosphate 
excretion, FGF23 inhibits 1α hydroxylase, thus reducing 1,25 
vitamin D activity. Loss of FGF23 or components of the FGF23 
network (GALNT3, FGF23, α-Klotho) cause hyperphosphatemia, 
extraskeletal calcifications, and early mortality; excess FGF23 
or pathway components (FGF23, PHEX, SLC34A3, subtilisins 
[PCSKs], phosphate transporters NPT2a and NPT2c) cause hypo­
phosphatemia with rickets or osteomalacia.421 However, FGF23 
may not be important during fetal developme

FGF23 
may not be important during fetal development. In a study by 
Ma and colleagues, FGF23 deficiency (Fgf23-null) and FGF23 
excess (Phex-null) did not alter fetal phosphorous or skeletal 
parameters.422 Although FGF23 is present in the fetal circulation 
at concentrations that may equal adult values, and there is robust 
expression of FGF23 target genes in placenta and fetal kidneys, 
FGF23 itself may not be an important regulator of fetal phospho­
rous metabolism. 
Fetal Growth
The hormone

hospho­
The hormones most important for postnatal growth, T4, GH, and 
gonadal steroids have a limited role in fetal growth, even though 
TR and GHR are expressed in many embryonic tissues, including 
the growth plates.1 Placental hormones, including the human GH 
variant and placental lactogens (hPL), play a limited role; hPL 
may promote early embryonic growth and may stimulate IGF 
and insulin production.1 IGF1 and IGF2 are produced by the 
placenta and may exer

nd IGF2 are produced by the 
placenta and may exert autocrine-paracrine actions on placental 
growth. IGF1, IGF2, and the IGF1 and IGF2 receptors are widely 
expressed in fetal tissues of mesenchymal, ectodermal, and endo­
dermal origin and play critical roles in modulating normal fetal 
growth, including the nervous system.423,424
Insulin-Like Growth Factors
IGFs are involved in regulation of uterine and placental growth 
during pregnancy. In early embryonic and fetal development, 
IGF1, EGF, a

rly embryonic and fetal development, 
IGF1, EGF, and estrogens are mitogens for endometrial stromal 
cells, and the endometrial contents of IGF1 and IGF1 mRNA 
are high at implantation and during early embryogenesis in the 
sow.425 Uterine IGF1 and IGF1 mRNA levels decrease progres­
sively with advancing gestation.425 Placental tissue also contains 
IGF1 and IGF2 mRNAs, significant concentrations of the respec­
tive proteins, and IGF1 receptors.25 Autocrine and paracrine roles 
for the IGFs in u

5 Autocrine and paracrine roles 
for the IGFs in uterine and placental tissues are postulated. IGF1 
and insulin are produced by embryonic tissues during the pre-
pancreatic stage of mouse development; both factors stimulate 
growth of embryonic mouse cells.426
IGF2 is genomically imprinted and paternally expressed in the 
fetus and placenta. The mature IGF2 protein is generated from the 
biologically inactive pro-IGF2 peptide by the action of proprotein 
convertase 4. Previous studies have show

oprotein 
convertase 4. Previous studies have shown a role of IGF2 in deter­
mining placental nutrient supply and hence fetal growth.427 In 
mutant mice lacking the imprinted placental-specific IGF2 tran­
script, growth of the placenta is altered from early gestation, but 
fetal growth is normal until late gestation, suggesting functional 
adaptation of the placenta to meet the fetal demands. It is believed 
that this adaptation may be mediated by the altered expression of 
placental transporter

y the altered expression of 
placental transporters GLUT3 and Slc38a4.428
Studies of transgenic mice with null mutations of the genes 
encoding IGF1, IGF2, or the IGF1 receptor have defined the role 
of these somatomedins; the birth weight of embryos lacking IGF1 
or IGF2 is only 60% that of control mice. When both genes 
are inactive, birth weight is reduced by another 30%, and mice 
lacking the IGF1 receptor have birth weights averaging 45% of 
control values.157 IGF2-deficient mice also manif

control values.157 IGF2-deficient mice also manifest intrauterine 
growth retardation in association with a small placenta. They have 
near-normal postnatal growth but delayed bone development.157 
IGF2-receptor knockout fetal mice are 30% overweight, suggest­
ing a negative growth-modulating effect of this receptor.
The normal growth in fetuses with both IGF1 receptor and 
IGF2 receptor knockout is caused by IGF1 signaling via the insulin 
receptor; combined IGF1, IGF2, and insulin receptor kn

ptor; combined IGF1, IGF2, and insulin receptor knockout 
results in severe intrauterine growth retardation and fetal death. 
Knockout of individual IGF-binding proteins (IGFBP) has little 
effect on fetal or placental growth.157 In humans, mutations in 
IGF1 or IGF1R are associated with intrauterine growth retar­
dation,429,430 suggesting that IGF1 signaling contributes sig­
nificantly to fetal growth. Apart from growth retardation, these 
children also suffer from developmental delay, microcep

ren also suffer from developmental delay, microcephaly, 
hypoglycemia, and sensorineural hearing loss.428,429 Indeed, 
there is a relation between umbilic IGF1 concentration and birth 
weight in humans; maternal smoking reduces both umbilic IGF1 
and birth weight.431,432
Recent work in mice suggests that IGF1 and IGF1R are required 
for late gestational lung maturation.433 Hypomethylation of the 
11p15 imprinted region has been associated with the phenotype 
of Silver-Russell syndrome (SRS).434 

e phenotype 
of Silver-Russell syndrome (SRS).434 This results in the relaxation 
of imprinting and biallelic expression of H19 and downregulation 
of IGF2. Additionally, abnormal processing of IGF2 by proprotein 
convertase 4 in the placenta has been implicated in the etiology of 
fetal growth restriction.435 Pregnant women carrying fetuses with 
intrauterine growth retardation (IUGR, low birth weight for gesta­
tional age) had higher concentrations of pro-IGF2 compared with 
controls. Recently

ions of pro-IGF2 compared with 
controls. Recently, a genomic mutation of the human IGF2 gene 
was described in four individuals from one family who exhibited 
severe intrauterine and postnatal growth retardation and signs of 
SRS.436 The defect was a nonsense mutation with a predicted severe 
truncation of the prepro-IGF2; the disorder was paternally inherited 
and affected children had diminished IGF2 serum concentrations.
858
Murphy and colleagues

ocrinology and the Life Span
Murphy and colleagues reported severe intrauterine growth retar­
dation and atypical diabetes mellitus secondary to insulin resistance 
in association with disruption of regulation of the IGF2 gene.437 On 
the other hand, overexpression of IGF2 as a result of loss of imprint­
ing associated with uniparental disomy, CDKN1C gene loss of func­
tion, alteration in the KvLQT1 differentially methylated region 
(DMR), or microdeletions in the human H19 DMR are associ­
ated 

rodeletions in the human H19 DMR are associ­
ated with overgrowth in the form of BWS.438 As highlighted previ­
ously, activating mutations in the proliferating cell nuclear antigen 
(PCNA)–binding domain of CDKN1C have been shown to cause 
IMAGe syndrome.181 Arboleda and colleagues demonstrated that 
missense mutations in the PCNA-binding domain have an inhibi­
tory effect on growth and differentiation in vivo, via loss of binding 
of PCNA to CDKN1C.181 The contrast between BWS and IMAGe 
mutati

N1C.181 The contrast between BWS and IMAGe 
mutations in CDKN1C highlights the dual and opposing effects of 
specific CDKN1C mutations.
IGF-binding proteins (BPs) are present as early as 5 weeks of 
gestation; prenatally as postnatally, the IGFs circulate in associa­
tion with binding proteins.157 High concentrations of circulating 
IGFBP1 are associated with fetal growth restriction in the mouse, 
as is overexpression of fetal IGFBP1 in the human.439,440 IGFBP4, 
expressed in the maternal decid

n.439,440 IGFBP4, 
expressed in the maternal decidua and cleaved by its protease PAPP-
A (pregnancy-associated plasma protein-A), inhibits IGF action. 
High maternal concentrations of IGFBP4 were recently shown to be 
related to fetal growth restriction.435 During fetal and postnatal life, 
plasma concentrations of IGFs are relatively high compared with tis­
sue concentrations.
In the fetus, IGF2 concentrations are five to six times higher 
than those of IGF1 in contrast to these concentrations 

those of IGF1 in contrast to these concentrations in children 
and adults; concentrations of both increase progressively through­
out gestation,441 and fetal concentrations at term are 30% to 50% 
of the adult concentrations. The metalloproteinase pregnancy-
associated plasma protein-A2 (PAPP-A2) regulates IGFBPs and is 
hypothesized to increase IGF1 bioactivity by specific proteolytic 
cleavage of IGFBP3 and 5. Mutations in PAPPA-A2 have recently 
been shown to cause short stature due to low IG

y 
been shown to cause short stature due to low IGF1 availability as 
part of a novel syndrome of growth failure.442 The two mutations 
described by Dauber and colleagues were shown to cause a com­
plete absence of PAPP-A2 proteolytic activity, a significant increase 
in bound IGF1 and decreased IGF1 concentrations, highlighting 
the critical role of PAPP-A2 in releasing IGF1 from its BPs.442
In most studies, cord blood IGF1 concentrations correlate with 
birth size. Despite the fetal growth–enh

ate with 
birth size. Despite the fetal growth–enhancing effects of IGF2, 
blood concentrations are only weakly related to size at birth, 
largely because of the inhibiting effect of soluble IGF2 receptor 
(IGF2R)424 but also because IGF2 appears to exert most of its 
growth effects in the earlier part of gestation. Soluble IGF2R is 
derived through proteolytic cleavage of the transmembrane region 
of the receptor in many tissues. IGF receptors have been identi­
fied as early as 5 weeks of gesta

ave been identi­
fied as early as 5 weeks of gestation and are widespread in fetal 
tissues.443 IGF1 stimulates glycogenesis in cultured fetal rat hepa­
tocytes and induces formation of myotubes in cultured myoblasts. 
IGF2 is active in cultured muscle and neonatal rat astroglial cells. 
Insulin receptors are increased in fetal cells and are resistant to 
downregulation.
The control of IGF production differs in fetal and postnatal 
life. GH receptors are present, but receptors for hPL predomi­
n

tors are present, but receptors for hPL predomi­
nate in fetal tissue61; GH, which stimulates IGF1 production 
after birth, has a limited role in fetal IGF production.423 GH 
does play a minor role in fetal growth, as reflected in the low IGF 
concentrations and slight reduction in birth weight and length 
in infants with GH resistance (Laron dwarfism) and growth 
hormone deficiency.444 hPL stimulates IGF1 production and aug­
ments amino acid transport and DNA synthesis in human fetal 
fibroblas

nsport and DNA synthesis in human fetal 
fibroblasts and muscle cells.61 IGF1 and IGF2 concentrations are 
reduced in fetuses of protein-starved pregnant rats, and the low 
IGF2 concentrations are reversed by hPL.445
Thyroidectomy of the third-trimester sheep fetus impairs 
skeletal muscle growth in association with a decrease in muscle 
GH receptor mRNA and IGF1 mRNA without an effect on IGF2 
concentrations.446 Glucocorticoids can inhibit fetal growth, pre­
sumably by inhibiting IGF gene trans

 growth, pre­
sumably by inhibiting IGF gene transcription, but may also affect 
growth plate chondrocytes directly.446 Indeed, nutrition is the 
major factor modulating IGF production in the fetus. IGF concen­
trations fall in suckling rats deprived of milk, and IGF1 and IGF2 
concentrations are reduced in fetuses of protein-starved pregnant 
rats and placentally restricted sheep.156,157 Recent work suggests 
that light stimuli alter circulating and brain IGF1 concentrations 
and control neuron

 and brain IGF1 concentrations 
and control neuronal migration through increased IGF1 signal­
ing.447 Weekly intra-amniotic injections increase fetal growth of 
intrauterine growth–restricted sheep.448 These data support the 
view that IGFs are important in embryonic and fetal growth and 
that in the fetus they are regulated, at least in part, by hPL and by 
nutritional substrate derived transplacentally. The high concentra­
tions of IGF2 in fetal rat serum, the high concentrations of IGF2 
mRNA

l rat serum, the high concentrations of IGF2 
mRNA in fetal tissues, and the presence of a truncated form of 
IGF1 in human fetal brain tissue suggest unique developmental 
actions of these peptides. 
Insulin
Insulin has been proposed to act as a fetal growth factor. Infants 
born to women with diabetes mellitus may have hyperinsu­
linemia associated with increased birth weight.449 Most of this 
increased weight is accounted for by body fat; there is little 
increase in body length, but some org

e is little 
increase in body length, but some organomegaly may occur. 
Infants with hyperinsulinemia caused by congenital hyperin­
sulinism or the BWS may also have increased somatic growth 
in utero; the human neonate is born large for gestational age, 
primarily due to increased lipogenesis mediated by insulin or 
IGF1 receptors. Conversely, the human fetus with pancreatic 
agenesis is small and has decreased muscle bulk and little or no 
adipose tissue.449
Mice with insulin or insulin recept

ose tissue.449
Mice with insulin or insulin receptor gene mutations have a 
10% decrease in birth weight and early neonatal death with hyper­
glycemia and ketonemia.157 Insulin receptor mutations in humans 
lead to severe intrauterine growth retardation and limited postna­
tal weight gain.157 In contrast to mice, the human fetus during the 
latter half of gestation has a significant increase in adipose mass, 
and adipose tissue is highly sensitive to insulin. Treatment with 
IGF1 improves the cl

e to insulin. Treatment with 
IGF1 improves the clinical condition to some extent.450 
Epidermal Growth Factor/Transforming 
Growth Factor
The EGF/TGFα system has been characterized in considerable 
detail.451,452 EGF is a 6-kDa peptide product of a large, 1207–
amino acid precursor molecule and acts through a 170-kDa mem­
brane receptor glycoprotein. This has intrinsic tyrosine kinase 
activity like the IGF receptor, and tyrosine kinase–mediated auto­
phosphorylation is a critical event in EGF 

 auto­
phosphorylation is a critical event in EGF signal transduction. 
TGFα, which has 35% amino acid homology with murine EGF 
and 44% homology with human EGF, also acts through the EGF 
receptor system.453,454 Several additional family members have 
859
been characterized, including amphiregulin, heparin-binding 
EGF, betacellulin, and neuregulins.452 Three additional receptors 
are referred to as ErbB2, ErbB3, and ErbB4 in animals; the human 
r

s ErbB2, ErbB3, and ErbB4 in animals; the human 
receptors are called human EGF receptor 2 (HER2), HER3, and 
HER4.452
EGF, pre-pro-EGF mRNA, and EGF receptors are present in 
most tissues in the postnatal rodent, but mRNA levels are high­
est in salivary glands and kidneys. EGF and pre-pro-EGF mRNA 
levels are absent or low in the fetal mouse and remain low in 
mouse tissues during the early neonatal period.453 Nonetheless, 
the EGF receptor knockout mouse exhibits epithelial immaturity 
and mu

ckout mouse exhibits epithelial immaturity 
and multiorgan failure with early death.453 Tissue concentrations 
of EGF mRNA increase in the mouse during the first 2 months 
of postnatal life; indeed, concentrations of EGF in the salivary 
glands increase 1000s-fold between 3 weeks and 3 months of age. 
Mouse urinary levels increase 200-fold and kidney concentra­
tions increase 10-fold between 1 week and 2 months of age. EGF 
concentrations in mouse ocular tissues increase 100-fold during 
the fir

e ocular tissues increase 100-fold during 
the first week of life.452 Liver EGF concentrations increase more 
slowly, as do serum concentrations, and there is a high degree of 
correlation between serum and liver EGF concentrations in the 
developing mouse.452 Therefore the production of EGF in the 
rodent is accelerated during the early neonatal period, and it is 
during this time that most hormone-stimulated growth and devel­
opment occur.
Fetal mouse and human tissues have high concentrations

l mouse and human tissues have high concentrations of 
TGFα.451,454,455 Immunoreactive TGFα concentrations in mice 
are measurable at relatively high levels in lung, brain, liver, and 
kidney tissues in the fetal/neonatal rat, and the ontogenic pattern 
of TGFα is tissue specific; most late fetal tissues studied contained 
TGFα, and levels persisted or increased in most tissues through 
the period of growth and development.454
In rodents and sheep, EGF provokes precocious eyelid opening 
and too

p, EGF provokes precocious eyelid opening 
and tooth eruption in neonatal animals, stimulates lung matura­
tion, promotes palatal development in organ culture, stimulates 
gastrointestinal maturation, evokes secretion of pituitary hor­
mones (including GH, PRL, and corticotropin), and stimulates 
secretion of chorionic gonadotropin and placental lactogen by the 
placenta.452,453
Both EGF and TGFα compete for binding to the EGF recep­
tor, and both factors accelerate eye opening and tooth eruptio

h factors accelerate eye opening and tooth eruption 
in the neonatal rodent, presumably through interaction with the 
same EGF receptor.452 Considerable evidence suggests a role for 
the EGF family of growth factors in mammalian CNS develop­
ment.456 EGF, TGFα, neuregulins, and the EGF receptors are 
widely distributed in the nervous system.452,457–460 EGF promotes 
proliferation of astroglial cells, acts as an astroglial differentiation 
factor, and enhances survival and outgrowth of selected n

 and enhances survival and outgrowth of selected neuro­
nal cells.457,458 Transgenic mice with a deficiency of neuregulin, 
ErbB2, ErbB3, or ErbB4 die in utero with cardiac anomalies and 
developmental anomalies of the hindbrain, midbrain, and ventral 
forebrain.459,460
EGF also plays an important role in rodent pregnancy. Mater­
nal salivary gland and plasma EGF concentrations in the mouse 
increase fourfold to fivefold during pregnancy.461 Removal of the 
salivary glands prevents the increase 

val of the 
salivary glands prevents the increase in plasma EGF and reduces 
the number of mice completing term pregnancy by 50%, decreases 
the percentage of live pups, and decreases the crown-rump length 
of fetuses delivered.461 Administration of EGF antiserum to preg­
nant mice without salivary glands further increases the abortion 
rate, whereas administration of EGF improves pregnancy out­
come.461 Because maternal EGF is too large a molecule to traverse 
the placental barrier, an effect o

le to traverse 
the placental barrier, an effect on maternal metabolism or on the 
placenta is likely.461 The placenta is richly endowed with EGF 
receptors, and placental tissue binds and degrades EGF to con­
stituent amino acids.452 TGFα is also produced by the maternal 
decidua in rodents and stimulates proliferation of decidual tissue 
and decidual PRL production. 
Other Factors
Additional growth factors involved in fetal growth and devel­
opment include hematopoietic growth factors, platele

ment include hematopoietic growth factors, platelet-derived 
growth factors (PDGFs), fibroblast growth factors, vascular endo­
thelial growth factor (VEGF), and members of the TGFβ fam­
ily.157,462 The TGFβ superfamily of extracellular growth factors 
comprises more than 35 members, including TGFβ, the bone 
morphogenetic proteins (BMPs), growth and differentiation fac­
tors, activins, inhibins, müllerian-inhibiting substance, Nodal, 
and Lefty.463 These ligands activate 12 transmembrane serine/

63 These ligands activate 12 transmembrane serine/thre­
onine kinase receptors expressed in a variety of tissues. The family 
is critical for early embryonic development, left-right asymmetry, 
heart and vascular system development, craniofacial development, 
nervous system development, and skeletal morphogenesis and 
plays an important role in body composition and growth.
Hematopoietic growth factors are also active in the fetus dur­
ing development; erythropoietin in fetal sheep is produced by

ment; erythropoietin in fetal sheep is produced by the 
liver rather than the kidney, and erythropoietin gene expression 
in fetal sheep is regulated by glucocorticoids.464 A switch to kid­
ney production occurs after parturition.465 Postnatally, thyroid 
hormones, testosterone, and hypoxia modulate erythropoietin 
production. PDGF represents a family of homodimers and het­
erodimers of PDGFA and PDGFB chains derived from two gene 
loci.466 Two PDGF receptors have been characterized, PDGFα 
and 

DGF receptors have been characterized, PDGFα 
and PDGFβ. The genes for PDGF and its receptors are expressed 
in many tissues. PDGFA gene inactivation in mice leads to defects 
in lung, skin, intestine, testes, and brain resulting in early postna­
tal death.466 PDGFB gene inactivation leads to microvessel disrup­
tion and leakage with hemorrhage, edema, and intrauterine death.
The FGF family of heparin-binding growth factors now 
includes 17 members with diverse effects on development, angio­
gen

rs with diverse effects on development, angio­
genesis, wound healing, and other biologic systems.467,468 These 
effects are mediated by ligand-activated tyrosine protein kinase 
receptors (FGFRs) transcribed from four related genes. Several 
receptor isoforms are products of alternative RNA splicing.157,451 
Targeted disruptions of FGF and FGFR genes in mice have defined 
critical roles in development.157,467 FGF3-deficient mice show tail 
and inner ear defects. Knockout of the FGF4 gene is let

nner ear defects. Knockout of the FGF4 gene is lethal, lead­
ing to early death. Knockout of the FGFR1 gene also leads to 
early fetal death. FGF10 knockout mice die at birth because of 
pulmonary agenesis. Deficiency of FGF4, FGF8, FGF9, FGF10, 
or FGF17 is associated with limb deformities. FGF8 deficiency 
leads to abnormal left-right axis patterning. In mice, FGFR3 
knockout results in chondrocyte hypertrophy and increased bone 
length.451 In humans, a variety of gain-of-function FGFR muta­
t

humans, a variety of gain-of-function FGFR muta­
tions are associated with chondrodysplasias and craniosynostosis 
syndromes.451 On the other hand, loss-of-function mutations in 
both FGFR1 and FGF8 are associated with Kallmann syndrome 
or hypogonadotrophic hypogonadism; FGF8 mutations are also 
associated with holoprosencephaly.62,469,470 FGF, like EGF, stimu­
lates the production of hCG from a choriocarcinoma cell line.465 
FGF21 is mainly produced by the liver but is also expressed in 
adipo

duced by the liver but is also expressed in 
adipocytes and pancreas, and regulates glucose and lipid metab­
olism through pleotropic actions in these tissues and the brain. 
860
During calorie restriction in mice, increased FGF21 causes growth 
attenuation and GH insensitivity. Additionally high concentra­
tions of FGF21 may directly suppress growth plate chondrocyte 
proliferation and differentiation. A negative correlation between 
FGF21 concentra

n. A negative correlation between 
FGF21 concentration and linear growth has been reported, and 
high levels of FGF21 were associated with growth failure in pre­
term infants.471 The mechanisms might involve GH resistance by 
GH-induced SOCS2 expression (which negatively regulates GH 
signaling) and inhibiting GH-induced signal STAT5 phosphory­
lation. These observations and the fact that the placenta contains 
FGF, NGF, TGFα, TGFβ, IGF1, and IGF2 suggest that the pla­
centa plays an important r

2 suggest that the pla­
centa plays an important role in modulating fetal growth.
Lastly, Wnt signaling, Notch signaling, BMP signaling, and 
hedgehog signaling play major roles in embryogenesis and fetal 
organ growth and development. These signaling pathways are also 
involved in bone development and growth and thus have a major 
effect on fetal size.472 
Neutralization of Hormone Actions in the 
Fetus
After the period of embryogenesis, the fetal milieu is programmed 
to optimize body growth a

al milieu is programmed 
to optimize body growth and organ development through an array 
of generalized and specialized growth factors with substrate supply 
maintained by the placenta. The endocrine and metabolic systems 
characterizing the extrauterine environment are programmed to 
maintain metabolic stability in a changing external environment. 
Hormonal systems in the fetus are programmed to maintain anab­
olism with minimal hormonal perturbation. Therefore production 
of catabolic and ther

ation. Therefore production 
of catabolic and thermogenic hormones is limited, and the effects 
of the hormones altering metabolic substrate supply and distribu­
tion pathways are muted (Table 23.6).
Limitation of Hormone Secretion
The human fetal pancreas is functional during the second trimes­
ter, but secretion of insulin in response to glucose or pyruvate is 
minimal until the neonatal period.354,355 Glucagon secretion is 
also blunted, although fetal blood glucagon concentrations are rel­
a

ugh fetal blood glucagon concentrations are rel­
atively high. Fetal islet hyperplasia and increased insulin secretion 
occur in response to chronic hyperglycemia (e.g., in the infant 
of a diabetic mother), and insulin release can be stimulated by 
acute fetal infusions of leucine, arginine, or tolbutamide.355,358 
Moreover, responsiveness of both insulin and glucagon secretion 
to glucose develops rapidly in the neonatal period. It is not clear 
whether the limited fetal islet cell responsiven

whether the limited fetal islet cell responsiveness results from the 
relatively stable fetal serum glucose concentrations or from a tem­
porally fixed maturation process. 
Production of Inactive Hormone Metabolites
Throughout the second half of gestation, cortisol is metabolized in 
fetal tissues to inactive cortisone through the activity of 11βHSD2. 
The placenta is permeable to steroid hormones, including cortisol. 
During midgestation, placental 11βHSD2 activity is low, and 
some cortisol 

ental 11βHSD2 activity is low, and 
some cortisol is transferred to the fetus. Placental 11βHSD2 activ­
ity increases during the second half of pregnancy under the control 
of placental estrogens, and enzyme activity near term is high.25,194 
Maternal-fetal cortisol transfer decreases progressively. In addi­
tion, although many adult tissues can convert cortisone to corti­
sol, conversion is limited during most of fetal life. Consequently, 
most of the cortisol that crosses the placenta or is pr

of the cortisol that crosses the placenta or is produced by the 
fetus is inactivated to cortisone by the placenta or by fetal tissues.
Concentrations of cortisone in fetal plasma exceed those of 
cortisol by threefold to fourfold until after 30 weeks’ gestation (see 
Fig. 23.5). Teleologically, this would help preserve the anabolic 
and growth-promoting milieu of the fetus and minimize prema­
ture maturational and parturitional effects of cortisol. After 30 
weeks, the ratio of cortisol to cort

ol. After 30 
weeks, the ratio of cortisol to cortisone in fetal tissues and plasma 
increases as a result of increased fetal secretion and decreased con­
version of cortisol to cortisone within the placenta and fetal tis­
sues.194 Cortisol has an important maturational action on several 
fetal tissues near term.
Fetal thyroid hormone metabolism is characterized by conver­
sion of active thyroid hormones to inactive rT3 and inactive sulfated 
iodothyronines and by limited receptor and postrecept

othyronines and by limited receptor and postreceptor respon­
siveness to thyroid hormone in selected tissues.219,232 The placenta 
contains an iodothyronine inner-ring monodeiodinase that catalyzes 
conversion of maternal T4 to rT3. The fetal sheep liver and kidney, 
in contrast to the adult liver and kidney, manifest low concentra­
tions of D1 outer-ring monodeiodinase activity, so conversion of T4 
to active T3 is limited, and large amounts of inactive iodothyronine 
sulfoconjugates accumulate

inactive iodothyronine 
sulfoconjugates accumulate.219,233 Consequently plasma T3 concen­
trations in the fetus remain low until the last few weeks of gestation 
(see Fig. 23.8). Selected fetal tissues (brain, brown adipose tissue) 
have active D2 outer-ring monodeiodinase activities that contribute 
to local tissue T3 concentrations; local T3 is important in develop­
ment, particularly in the hypothyroid fetus.219,473 Near term and 
in the neonatal period in the human fetus, the dramatic increa

tal period in the human fetus, the dramatic increase 
in plasma T3 concentrations, and presumably in T3 production, 
  Neutralization of Hormone Actions in the 
Fetus
PRODUCTION OF INACTIVE METABOLITES
Active Hormone
Inactive Metabolites
Cortisol
Cortisone
Thyroxine (T4)
rT3, T4S, rT3S
Triiodothyronine (T3)
T3S, T2
DELAYED EXPRESSION OR NEUTRALIZATION OF 
RECEPTORS
Receptor
GHR
TRα, TRβ
βAR
Estrogens
ERα, ERβ
Glucagon
GR
LIMITED 

amines
βAR
Estrogens
ERα, ERβ
Glucagon
GR
LIMITED HORMONE SECRETION
Secretory Cell
Insulin
Islet cell β
Glucagon
Islet cell α
AR, Adrenergic receptor; T2, diiodothyronine; rT3, reverse T3; T4S, T4 sulfate.
   
TABLE 23.6
861
heralds the onset of thyroid hormone actions on growth and devel­
opment and on metabolism (see Fig. 23.8). 
Neutralization of Receptor Response
Selected ovine fetal tissues seem relatively unresponsive to thy­
r

tal tissues seem relatively unresponsive to thy­
roid hormones. Fetal ovine liver and kidney thermogenesis as 
evidenced by oxygen consumption, sodium-potassium pump 
(Na+, K+-ATPase) activity, and mitochondrial α-glycerophosphate 
activity is unresponsive to exogenous T3 during the third trimes­
ter, and thyroid hormone responsiveness in a number of tissues 
(cardiac, hepatic, renal, and skin) develops only during the peri­
natal period.474 β-Adrenergic receptor binding in heart and lung 
of th

renergic receptor binding in heart and lung 
of the ovine fetus is unresponsive to T3 late in the third trimester 
but increases in response to T3 in the neonatal period.21,474 In 
rodent species, in which development at birth is comparable to 
human fetal development at midgestation, pituitary GH concen­
trations become responsive to thyroid hormone only during the 
first weeks of extrauterine life.475 Mouse submandibular gland 
EGF and NGF concentrations become responsive to thyroid 
hormone d

entrations become responsive to thyroid 
hormone during the second week of life, as do urine and kid­
ney EGF concentrations and hepatic EGF receptor levels.476,477 
Mouse skin EGF levels and EGF receptors are responsive during 
the first neonatal week.478,479 Therefore, despite the presence of 
nuclear T3 receptors in significant concentrations in developing 
rat and sheep fetuses, many thyroid hormone actions in these spe­
cies are delayed.480 The mechanism of this delayed thyroid hor­
mone re

The mechanism of this delayed thyroid hor­
mone responsiveness is not clear; developmental programming of 
iodothyronine monodeiodinase expression and gene expression 
programming via unliganded TRs or TR-interacting corepressors 
probably all play a role.
The effect of high circulating concentrations of GH in the 
fetus is also limited. Fetal somatic growth is only partially GH 
dependent; indeed, the GH-deficient fetus has little or no growth 
retardation.61,444 The paucity of fetal GH effects

retardation.61,444 The paucity of fetal GH effects is due to delayed 
maturation of GH receptors or postreceptor mechanisms. In ani­
mals such as sheep, hepatic GH receptor binding appears only 
during the neonatal period.61 Receptor deficiency may also be a 
factor in the limited PRL bioactivity in the fetus near term.61
There is less information on fetal hormone responsiveness in 
other systems. β-Adrenergic receptor binding in heart and lung 
of the sheep fetus is relatively low near term and

of the sheep fetus is relatively low near term and increases in the 
neonatal period in response to thyroid hormones.474 Premature 
lambs have an augmented plasma catecholamine surge at birth 
but have a relatively mild increase in plasma free fatty acid concen­
trations, which suggests reduced catecholamine responsiveness.481 
The high concentrations of progesterone and estrogens in fetal 
blood also seem to have limited effects in the fetus. Progesterone 
receptors are present in low concentra

ogesterone 
receptors are present in low concentration in fetal guinea pig kid­
ney, lung, and uterus at midgestation and increase progressively 
until term.482 ERs appear in neonatal rat uterus, oviduct, cervix, 
and vagina during the first 10 days of extrauterine life, and both 
ERα and ERβ mRNAs are present in human fetal tissues during 
the second trimester.312,483 The human neonate often manifests 
mild breast enlargement at birth, and vaginal estrogenization may 
be evident in female infan

al estrogenization may 
be evident in female infants at birth. Estrogen effects otherwise 
appear to be limited (see Table 23.6). 
Programming of Fetal Endocrine Systems
It is becoming increasingly evident that the early environment can 
have a significant impact on the health of an individual throughout 
a lifetime. During the past several decades, the concept of the plas­
ticity of fetal endocrine systems has evolved from experiments in 
several mammalian species indicating that hormonal progr

l mammalian species indicating that hormonal program­
ming occurs during a critical fetal or perinatal period of develop­
ment. Our understanding of the mechanisms of programming is 
expanding with epigenetic processes clearly being implicated.
There is a growing list of examples. In the female rodent, 
transient neonatal androgen administration masculinizes the pat­
tern of hypothalamic control of GnRH secretion and pituitary 
gonadotropin secretion. Furthermore, masculinization of adult 
behav

tion. Furthermore, masculinization of adult 
behavior and sexual activity is seen, and the pattern of GH secre­
tion is permanently altered with an increase in longitudinal bone 
growth and body weight.484,485 Prenatal androgens program the 
timing of neuroendocrine puberty in sheep: The higher the dose 
of prenatal testosterone, the earlier the initiation of the pubertal 
LH rise.486 Estrogen administration to pregnant rats during the 
last third of gestation produces cryptorchid male offspring

d of gestation produces cryptorchid male offspring and 
may permanently suppress spermatogenesis in adult males.487 
Transient levothyroxine administration to neonatal rodents leads 
to growth retardation, delayed puberty, decreased adult pituitary 
weight, decreased pituitary TRH concentrations, low serum 
thyrotropin levels, and decreased thyrotropin responsiveness to 
propylthiouracil challenge.488,489 Administration of insulin or 
alloxan to neonatal rats produces permanent alteration of glu

neonatal rats produces permanent alteration of glu­
cose tolerance.490 A single dose of vasopressin to the neonatal rat 
permanently enhances the adult response to vasopressin.5,32 Fetal 
exposure to high maternal glucocorticoid concentrations in the 
rat inhibits fetal growth and leads to subsequent hypertension in 
the offspring.9 Moreover, it has been observed that the permanent 
programming can be transmitted to later generations, leading to 
the concept of epigenetic effects.490,491
The con

The concept of fetal programming was extended with the 
observation of ecologic associations between fetal and early-life 
health indicators (e.g., birth size, infant mortality) and adult 
diseases. The concept advanced in the 1980s that adult diseases 
have fetal and perinatal genesis has been referred to as the Barker 
hypothesis.492 There is now extensive documentation of the asso­
ciation of IUGR with an increased risk of later hypertension, 
insuli

h an increased risk of later hypertension, 
insulin resistance, diabetes, and cardiovascular and coronary heart 
disease.493–500 The programming involves epigenetic, neuroendo­
crine, hormonal receptor, and metabolic alterations involving the 
placenta and fetus.
epigenetic and genetic factors. Furthermore the impact of the 
environment is being increasingly recognized. It has been shown 
that airborne pollutants such as cadmium and ciga

that airborne pollutants such as cadmium and cigarette smoke 
may alter thyroid hormone function, which is crucial for growth. 
A recent study examining the role of fine particle air pollution 
reported that fine particles with a diameter under 2.5 μm (PM2.5) 
during the third trimester impacted on fetal thyroid function.501 
In cord blood, TSH and FT4:T3 ratio decreased with increasing 
exposure to PM2.5 possibly via modulation of the hypothalamic-
pituitary-thyroid axis indirectly via glucoco

mic-
pituitary-thyroid axis indirectly via glucocorticoid activity as pre­
viously shown.502
An increasing range of epigenetic effects and corresponding 
clinical syndromes have been identified over recent years. Epigen­
etic effects include genetic imprinting. Imprinted genes are a class 
of genes in placental mammals and marsupials whose expression 
depends on the parental origin; they are expressed only from the 
paternal or the maternal gene copy but not biparentally. To date, 
more than 150

copy but not biparentally. To date, 
more than 150 human genes have been shown to be imprinted,503 
but there are likely to be more.
862
Four classes of molecular changes are described in most 
imprinted disorders: uniparental disomy, chromosomal imbal­
ances, aberrant methylation (epigenetic mutation), and genomic 
mutations in imprinted genes. These all alter expression of 
imprinted genes, but it is the parental allele affected by the muta­
tion t

s the parental allele affected by the muta­
tion that determines phenotype. So far, genomic point mutations 
in imprinted genes have been reported only for Beckwith-Wie­
demann, Silver-Russell, and Angelman syndromes, precocious 
puberty, and pseudohypoparathyroidism (PHP). Imprinting is 
controlled epigenetically (by factors such as nutrition) via DNA 
methylation, post-translational histone modification, chromatin 
structure, and noncoding RNAs. Imprinted loci often comprise 
several genes und

. Imprinted loci often comprise 
several genes under epigenetic regulation leading to stage-specific 
and tissue-specific transcriptional activity in cells with identical 
DNA sequences. In the majority of imprinted disorders only the 
disease-specific loci are affected, but an increasing number have 
been reported to exhibit multilocus methylation imprinting 
disturbance (MLID),504 the mechanism of which is at present 
unknown.
Many of the imprinted genes are involved in the control of fetal 
g

nted genes are involved in the control of fetal 
growth.505 Paternally expressed imprinted genes tend to enhance 
fetal growth, and maternally expressed genes tend to suppress it. 
Knockout of paternally expressed genes for IGF2, PEG1, PEG2, 
and insulin results in IUGR, whereas knockout of the maternal 
genes H19, IGF2R, or overexpression of IGF2 results in fetal over­
growth.505 More recently, the identification of an IGF2 mutation 
in patients with growth restriction indicates that IGF2 not o

 with growth restriction indicates that IGF2 not only 
mediates prenatal growth but also contributes to postnatal growth 
and has pleiotropic effects.436 A role of IGF2 mutations in both 
overgrowth and growth restriction phenotypes is conceivable, as 
has been shown for functionally opposing mutations in CDKN1C 
in 11p15.5.506 A loss of DNA methylation (LOM) at the H19/
IGF2 domain has been identified in over 50% of patients with 
SRS.507 In contrast, gain of methylation at this domain is found

trast, gain of methylation at this domain is found 
in 10% of patients with BWS. Other genetic alterations, includ­
ing modification of tandem repeats in the insulin gene, have been 
described.508
Hormones in the fetus are derived from the placenta, from the 
mother, from fetal endocrine glands, and from circulating precur­
sors in fetal or placental tissues. These extensive networks linking 
maternal-placental-fetal endocrine interactions and the apparent 
plasticity of developing endocrine and

e apparent 
plasticity of developing endocrine and metabolic systems facilitate 
endocrine system programming. As discussed earlier, the program­
ming may be relatively system limited. Other examples include the 
observation many years ago that diethylstilbestrol administration 
to pregnant women increased the prevalence of vaginal adenocar­
cinoma in female offspring during the second and third decades 
of life.498 More recently, it was shown that prenatal or neonatal 
diethylstilbestrol exposu

at prenatal or neonatal 
diethylstilbestrol exposure in hamsters and mice perturbs normal 
uterine development by affecting the genetic pathways program­
ming uterine differentiation and results in hyperplastic and neo­
plastic uterine lesions with increased levels of cJun, cFos, Myc, 
Bax, and Bclx.509,510
Excessive androgen exposure during fetal life has been asso­
ciated with later polycystic ovary syndrome.498 Hormonal pro­
gramming is also demonstrable in cell lines and in unicellular 
orga

emonstrable in cell lines and in unicellular 
organisms, in which a single exposure to a hormone can produce 
persistent alteration of the hormonal response characteristics or of 
prohormone processing.511,512 Undernutrition during pregnancy 
in the rat results in the development of obesity, hyperinsulinemia, 
and hyperleptinemia during adult life; this phenotype is potenti­
ated when the offspring are fed a high-fat diet. Neonatal leptin 
treatment normalized the programmed phenotype, indicatin

ent normalized the programmed phenotype, indicating 
that metabolic programming may be reversible during the period 
of developmental plasticity.513
The effects of maternal undernutrition and fetal IUGR extend 
to several systems, and it is hypothesized that excessive mater­
nal-fetal glucocorticoids play a significant programming role. 
Glucocorticoids have wide-ranging effects in the fetus, altering 
receptors, enzymes, ion channels, and transporters in a variety of 
cells and tissues in the l

orters in a variety of 
cells and tissues in the late-gestation fetus, and they can induce 
programming of other endocrine systems. Throughout gestation, 
they modify GLUT gene expression in placenta and fetus, influ­
ence IGF and glucocorticoid receptor gene expression in vari­
ous tissues, affect expression of several transcription factors, and 
affect a wide variety of enzymes in placenta, liver, kidney, intes­
tine, and lung.514 Intrauterine exposure to stress or high levels 
of endogenous o

exposure to stress or high levels 
of endogenous or exogenous glucocorticoids has been shown to 
have molecular and structural impact on brain development, par­
ticularly limbic regions, impairing cognition and increasing anxi­
ety and reactivity to stress.515 Furthermore, recent studies have 
begun to show transgenerational effects in the rat model where in 
utero exposure to glucocorticoids resulted in two generations of 
offspring exhibiting decreased weight at birth compared to con­
trols as

ecreased weight at birth compared to con­
trols as well as abnormal glucose tolerance and behavior modi­
fications.491,516 These programming effects were transmitted by 
either maternal or paternal lines implying an epigenetic mecha­
nism. The HPA axis seems to play an important role in the fetal 
programming of adult disease, but the pathologic mechanisms are 
largely unknown. In very preterm babies less than 32 weeks of 
gestation, a relative adrenal insufficiency is observed after birth, 
whi

drenal insufficiency is observed after birth, 
while adverse effects of chronic glucocorticoid excess are observed 
in adult life.517,518 Maternal undernutrition, stress, and placen­
tal dysfunction are associated with increased maternal and fetal 
glucocorticoid concentrations, which contribute importantly to 
IUGR and programmed alterations in adult endocrine systems 
and metabolism.496,514,519 Preweaning growth hormone treatment 
during a critical developmental window has been shown to preven

ical developmental window has been shown to prevent 
maternal undernutrition changes in postnatal growth pattern and 
related adiposity, normalizing body growth trajectory and revers­
ing metabolic dysregulation in adult offspring.520 These findings 
suggest that early intervention on the GH-IGF1 axis may attenu­
ate or prevent long-term consequences of fetal programming. 
Transition to Extrauterine Life
The transition to extrauterine life involves abrupt delivery from 
the protected intrauterin

es abrupt delivery from 
the protected intrauterine environment and succor by the placenta 
into the relatively hostile extrauterine environment. The neonate 
must initiate air breathing and defend against hypothermia, hypo­
glycemia, and hypocalcemia as the placental supply of energy and 
nutritional substrate is removed. Both the adrenal cortex and the 
autonomic nervous system, including the para-aortic chromaf­
fin system, are essential for extrauterine adaptation. Longer term 
transition re

xtrauterine adaptation. Longer term 
transition requires adaptation to an environment of intermittent 
nutrient supply and transient substrate deficiency and requires 
maturation of the secretory control mechanisms for the PTH-
calcitonin system and the endocrine pancreas.
Cortisol Surge
In most mammals, a cortisol surge occurs near term and is medi­
ated by increased cortisol production by the fetal adrenal and a 
decreased rate of conversion of cortisol to cortisone. Pepe and 
863
CHAPTER 23  

 cortisol to cortisone. Pepe and 
863
Albrecht have proposed that the preterm fetal cortisol surge is due 
to the progressive stimulation by estrogens of placental 11βHSD2 
activity and the subsequent increase in placental conversion of 
cortisol to cortisone.194 The resulting decrease in maternal-to-fetal 
cortisol transfer results in stimulation of fetal CRH and cortico­
tropin secretion through the negative-feedback control loop. The 
concomitan

he negative-feedback control loop. The 
concomitant estrogen-stimulated increase in 11βHSD2 activity in 
fetal tissues potentiates the relative fetal cortisol deficiency and the 
CRH-corticotropin response.194 Placental CRH may also poten­
tiate fetal adrenal activation. Recent data suggest an increase in 
11βHSD1 expression and activity in placenta and intrauterine 
fetal membranes during late gestation, with a consequent increase 
in local cortisol production in preparation for parturition.521

e ductus arteriosus to prostaglandins, 
facilitating ductus closure
 • Induces maturation of several enzymes and transport processes 
of the small intestine
 • Stimulates maturation of hepatic enzymes
In some cases, these events involve increased synthesis of spe­
cific proteins or enzymes. In other instances, such as the action on 
the ductus arteriosus, the mechanism remains obscure.
Secondary effects of cortisol also promote extrauterine adap­
tations. The increased T3 concentrations stimul

p­
tations. The increased T3 concentrations stimulate β-adrenergic 
receptor binding, potentiate surfactant synthesis in lung tissue, 
and increase the sensitivity of brown adipose tissue to NE. The 
significance of prenatal cortisol is demonstrated by the effects of 
gene-targeted CRH or glucocorticoid receptor deficiency in mice; 
the progeny of homozygous CRH-deficient or glucocorticoid 
receptor–deficient animals die in the first 12 hours with lung dys­
plasia and surfactant deficiency.525,5

h lung dys­
plasia and surfactant deficiency.525,526
The adaptational effects of the prenatal cortisol surge have led 
to the current recommendation for prenatal corticosteroid therapy 
in pregnancies threatened by the risk of preterm delivery. Gener­
ally, preterm infants prenatally exposed to augmented glucocor­
ticoid concentrations have lower overall morbidity and mortality 
than untreated infants. 
Catecholamine Surge
Parturition also evokes a dramatic catecholamine surge in the 
newborn, r

 a dramatic catecholamine surge in the 
newborn, resulting in extraordinarily high concentrations of NE, 
epinephrine, and dopamine in cord blood.319 As discussed ear­
lier, plasma NE concentrations exceed epinephrine concentrations 
because of peripheral and adrenomedullary and para-aortic cat­
echolamine release. Cord blood NE concentrations of 15 nmol/L 
(2500 pg/mL) and epinephrine concentrations of 2 nmol/L (370 
pg/mL) are common after spontaneous delivery of term infants.319 
Concentratio

taneous delivery of term infants.319 
Concentrations of 25 nmol/L (4200 pg/mL) of NE and 35 
nmol/L (640 pg/mL) of epinephrine are common in cord blood 
of premature infants. These changes evoke critical cardiovascular 
adaptations, including increased blood pressure and cardiac ino­
tropic effects and glucagon secretion, decreased insulin secretion, 
increased thermogenesis in brown adipose tissue, plasma free fatty 
acid concentrations, and pulmonary adaptation, including mobi­
lization of pul

monary adaptation, including mobi­
lization of pulmonary fluid and increased surfactant release.319,521 
Thermogenesis in Neonatal Brown 
Adipose Tissue
Brown adipose tissue is the major site of thermogenesis in the new­
born and is especially prominent in the mammalian fetus. The larg­
est accumulations of brown adipose tissue envelop the kidneys and 
adrenal glands, and smaller amounts surround the blood vessels of 
the mediastinum and neck.234 The mass of brown adipose tissue 
peaks at the ti

 The mass of brown adipose tissue 
peaks at the time of birth and gradually decreases during the early 
weeks of life. Surgical removal of this tissue leads to neonatal hypo­
thermia. NE, through β-adrenergic receptors, stimulates thermo­
genesis by brown adipose tissue, and optimal responsiveness of this 
tissue to NE is dependent on thyroid hormone.527 Brown adipose 
tissue is rich in mitochondria containing a unique 32-kDa protein 
(thermogenin) that uncouples oxidation and phosphorylation of

n) that uncouples oxidation and phosphorylation of 
adenosine diphosphate, reduces ATP production, and consequently 
enhances thermogenesis.234 Thermogenin is T3 dependent, and 
brown adipose tissue contains a 5′-monoiodothyronine deiodinase 
that deiodinates T4 locally to T3.234 Full maturation of catechol­
amine-stimulated cellular respiration in brown adipose tissue occurs 
before delivery in the ovine fetus and requires thyroid hormone.234 
Fetal thyroidectomy in this species leads to marked

etal thyroidectomy in this species leads to marked hypothermia, 
with low plasma free fatty acid concentrations and increased plasma 
epinephrine concentrations.528 Basal brown adipose tissue thermo­
genesis and NE-stimulated and dibutyryl cAMP-stimulated ther­
mogenesis are decreased by fetal thyroidectomy.
The rapid onset of thermogenesis in brown adipose tissue is 
essential for survival in newborn infants. Catecholamine release is 
the stimulus for brown adipose tissue thermogenesis in the e

us for brown adipose tissue thermogenesis in the early 
neonatal period, and responsiveness to catecholamines is markedly 
increased by cutting of the umbilical cord.527 Fetal hypoxia and 
placental inhibitors, including prostaglandin E2 and adenosine, 
appear to inhibit brown adipose tissue thermogenesis in utero.527 
Cord cutting, neonatal cooling, catecholamine stimulation, and 
augmented conversion of T4 to T3 in brown adipose tissue in the 
neonatal period are the essential features that me

neonatal period are the essential features that mediate and con­
dition newborn thermogenesis. It was previously thought that 
brown adipose tissue involuted soon after birth, but combined 
18F-fluorodeoxyglucose positron emission tomography (18F-FDG-
PET) and CT scans have identified active brown adipose tissue 
in adults and shown a strong positive correlation between brown 
adipose tissue activity and the basal metabolic rate.529–531 
Calcium Homeostasis
In utero, active calcium transport fro

Homeostasis
In utero, active calcium transport from the mother to the fetus 
takes place through the placenta, responsible for approximately 
20% to 30% of calcium stores in the fetus. The neonate must 
adjust rapidly from a high-calcium environment regulated by 
PTHrp and calcitonin to a low-calcium environment that requires 
regulation by PTH and vitamin D. With removal of the placenta 
in term infants, plasma total calcium concentration falls, reach­
ing a nadir of approximately 2.3 mmol/L (9

 reach­
ing a nadir of approximately 2.3 mmol/L (9 mg/dL),390 and the 
ionized calcium concentration reaches a low level of about 1.2 
mmol/L (4.8 mg/dL) by 24 hours of life.532 Plasma PTH concen­
trations are relatively low in the neonatal period and are minimally 
responsive to hypocalcemia during the first 2 to 3 days of life. 
Calcitonin concentrations are high in cord blood (∼2000 ng/L), 
increase further during the early neonatal period, and remain 
high for several days after birth.390,53

d remain 
high for several days after birth.390,533 The relatively obtunded 
PTH response and the high calcitonin concentrations lead to a 
period of 2 to 3 days of transient neonatal hypocalcemia.533,534 
864
Inhibition of calcitonin secretion and stimulation of PTH secre­
tion gradually result in increased serum calcium concentrations in 
the neonate. The disappearance of PTHrp in the neonatal lamb is 
approximately coincident with the time of rest

is 
approximately coincident with the time of restoration of calcium 
concentrations to the adult range.390 The mechanism of transition 
from PTHrp to PTH secretion by the neonatal parathyroid glands 
is not clear.
Calcium homeostasis is also affected in the human newborn 
by the low level of glomerular filtration that persists for several 
days.533,534 In addition, renal responsiveness to PTH is reduced in 
the first few days of life. These factors limit phosphate excretion 
and predispose the 

ors limit phosphate excretion 
and predispose the neonate to hyperphosphatemia, particularly if 
the diet includes high-phosphate milk such as unmodified cow’s 
milk. Premature infants compared with term infants tend to have 
lower PTH and higher calcitonin concentrations and more imma­
ture kidney function; in these infants, neonatal hypocalcemia may 
be more marked and prolonged, and the incidence of symptom­
atic hypocalcemia is higher. Birth asphyxia also predisposes the 
neonate to hypocalc

asphyxia also predisposes the 
neonate to hypocalcemia.534 Infants born to mothers with hyper­
calcemia related to hyperparathyroidism have a high incidence of 
symptomatic hypocalcemia. These infants have a more marked 
suppression of parathyroid function and a longer period of tran­
sient hypoparathyroidism in the neonatal period. PTH secretion 
and calcium homeostasis usually return to normal in 1 to 2 weeks 
in full-term infants and within 2 to 3 weeks in the small prema­
ture infant. 
Gluco

to 3 weeks in the small prema­
Glucose Homeostasis
Fetal energy needs are met by a continuous intravenous supply 
of glucose across the placenta, fetal glucose uptake being directly 
related to both the maternal blood glucose concentration and the 
transplacental gradient. There is little endogenous glucose produc­
tion. Glucose and other substrates are stored as fat and glycogen 
in preparation for birth. During labor and delivery, the secretion 
of stress hormones such as glucoco

 the secretion 
of stress hormones such as glucocorticoids and catecholamines 
Cortisol
Promote islet
maturation
NE to E
BAT
Promote
ductus closure
Glycogen
T4 to T3
Beta-adrenergic receptors
Surfactant synthesis
Surfactant release
Lung water
   resistance
Glucagon
Insulin
• Fig. 23.14  Actions of cortisol and catecholamines during fetal adaptation to the extrauterine environ­
ment. The prenatal cortisol surge p

erine environ­
ment. The prenatal cortisol surge promotes functional maturation of several organ systems. The neonatal 
catecholamine surge triggers or potentiates many of the extrauterine cardiopulmonary and metabolic func­
tional adaptations that are critical to extrauterine survival. BAT, brown adipose tissue; E, epinephrine; NE, 
norepinephrine; T3, triiodothyronine; T4, thyroxine.
865
causes a rise in fetal blood glucose concentrations, so tha

rise in fetal blood glucose concentrations, so that cord 
blood glucose concentrations are often high.
At delivery following clamping of the cord, the maternal supply 
of glucose ceases. A normal term infant has an immediate postna­
tal fall in blood glucose concentration during the first 2 to 4 hours 
from values close to maternal levels to around 2.5 mmol/L (45g/
dL) in the initial hours postpartum.354,355 Low concentrations are 
usually transient, asymptomatic, and part of the normal adapta­

ent, asymptomatic, and part of the normal adapta­
tion to extrauterine life. In term infants metabolic adaptation is 
compensated for by producing alternative fuels, and the transition 
to intermittent feeding and fasting with the introduction of milk 
feeds into the gut is accomplished with little external evidence of 
the magnitude of changes taking place. Counterregulatory hor­
mones rapidly become active with high catecholamine, glucagon, 
GH, and glucocorticoid concentrations and a fall in 

, and glucocorticoid concentrations and a fall in insulin.354,355 
Insulin concentrations are low at birth and tend to fall further 
with hypoglycemia. The early glucagon response is short lived, 
but concentrations remain at about 100 ng/L for the first 12 to 
24 hours; the glucagon/insulin ratio is high enough to stabilize 
glucose concentrations in the range of 2.8 to 4 mmol/L (50–70 
mg/dL) during this period. The early glucagon and catecholamine 
surges deplete hepatic glycogen stores, so t

mine 
surges deplete hepatic glycogen stores, so the return of plasma glu­
cose concentrations to normal after 12 to 18 hours requires matu­
ration of hepatic gluconeogenesis under the stimulus of a high 
plasma glucagon to insulin ratio.355 Glucagon secretion gradually 
increases during the early hours after birth, especially with pro­
tein feeding, which stimulates gut glucagon release and pancreatic 
glucagon secretion.354,355 Together these changes result in stabili­
zation of blood glucose 

hanges result in stabili­
zation of blood glucose concentrations, although adult concentra­
tions are not reached until approximately 72 hours of life.
Failure of this sequence of physiologic changes can lead to 
hypoglycemia, which is most common in the first few hours after 
birth. Severe hypoglycemia occurs if the normal relationship of 
these hormones is disturbed but the net effect of the changes is to 
stabilize the blood glucose concentration at a lower level during 
the first few hours w

ion at a lower level during 
the first few hours while milk feeding is initiated. The availability 
of ketone bodies allows a sparing effect of glucose for brain utili­
zation. The process of adaptation is, however, incomplete and is 
compromised when an infant is born prematurely or following 
intrauterine growth retardation.
Premature infants have more severe and more prolonged hypo­
glycemia because of reduced glycogen stores and impaired hepatic 
gluconeogenesis. Infants born to diabetic mot

tic 
gluconeogenesis. Infants born to diabetic mothers have more 
severe neonatal hypoglycemia because of relative hyperinsulinism. 
In the healthy term infant, glucose homeostasis is achieved within 
5 to 7 days of life; in premature infants, 1 to 2 weeks may be 
required. 
Other Hormonal Adaptations
Delivery of the placenta results in decreases in fetal blood con­
centrations of estrogens, progesterone, hCG, and hPL. The fall in 
estrogen concentrations presumably removes the major stimulus 
t

ntrations presumably removes the major stimulus 
to fetal pituitary PRL release, and PRL concentrations decrease 
within several weeks. The relatively delayed fall may be due to lac­
totrope hyperplasia in the fetal pituitary or to delayed maturation 
of hypothalamic dopamine secretion. The gradual fall of GH con­
centrations during the early weeks of life is due to delayed matura­
tion of hypothalamic-pituitary feedback control of GH release.57 
In the neonatal primate, there are concomitant de

In the neonatal primate, there are concomitant decreases in 
plasma GH concentrations and GH responsiveness to exogenous 
GHRH.535 The mechanisms remain unclear; changes in secretion 
or in pituitary sensitivity to GHRH or somatostatin, or both, may 
be involved. IGF1 and IGF2 concentrations fall to infantile values 
within a few days, presumably because of the removal of placental 
hPL and placental IGF production (see Fig. 23.5).
In male infants (see Fig. 23.10), after a transient fall in tes

ts (see Fig. 23.10), after a transient fall in tes­
tosterone concentrations as the hCG stimulus abates, pituitary 
LH secretion rebounds modestly, and there is a secondary surge 
of plasma testosterone that persists at significant levels for sev­
eral weeks.57,536 This surge is mediated by hypothalamic GnRH; 
blockade of neonatal activation of the pituitary-testicular axis with 
a GnRH agonist in neonatal monkeys ablates the neonatal incre­
ments in LH and testosterone.537 Such a blockade also 

s in LH and testosterone.537 Such a blockade also results in 
subnormal increments in plasma LH and testosterone concentra­
tions and subnormal testicular enlargement at puberty in these 
animals, suggesting that neonatal GnRH release with pituitary-
testicular activation may be critical for normal sexual maturation 
of male primates.537 In females, a transient, secondary surge in 
FSH may transiently elevate estrogen concentrations.
Delivery results in a reversal of the high fetal cortisone/cor

ults in a reversal of the high fetal cortisone/cortisol 
ratio, and plasma cortisol concentrations are higher in the neonate 
despite relatively lower plasma corticotropin concentrations (see 
Fig. 23.7). Presumably this increase is due to decreased inhibi­
tion of adrenal 3βHSD by estrogen and perhaps to removal of 
a placental CRH action on fetal pituitary corticotropin release. 
Plasma DHEAS and DHEA concentrations fall as the fetal adre­
nal atrophies.
The increase in serum thyrotropin conce

atrophies.
The increase in serum thyrotropin concentrations during the 
early minutes after birth is due to cooling of the neonate in the 
extrauterine environment.21,219 In term infants, the thyrotropin 
surge peaks at 30 minutes at a concentration of about 70 mU/L 
(see Fig. 23.7). This peak evokes increased secretion of T4 and T3 
by the thyroid gland. In addition, increased conversion of T4 to 
T3 by liver and other tissues maintains the T3 concentration in 
the extrauterine range of 1.6 to 

oncentration in 
the extrauterine range of 1.6 to 3.4 nmol/L (105–220 ng/dL). 
The re-equilibration of thyrotropin concentrations to the normal 
extrauterine range is probably a result of the readjustment of pre­
vailing serum T3 concentrations and maturation of feedback con­
trol of thyrotropin by thyroid hormones during the early weeks 
of life.21,538 Production of rT3 by fetal and neonatal tissues abates 
by 3 to 4 weeks of age, at which time serum rT3 reaches adult 
Maternal

serum rT3 reaches adult 
Maternal and Fetal Medicine
Advances in fetal imaging, genomics, and minimally invasive 
techniques, as well as a better understanding of the intrauter­
ine endocrine milieu and natural history of many endocrine dis­
eases, have revolutionized the management of fetal and neonatal 
endocrine conditions. This progress has set the stage for disease 
diagnosis, therapy for fetal endocrine and metabolic disorders, 
management of disorders of fetal growth, and

ers, 
management of disorders of fetal growth, and diagnosis and 
management of perinatal or neonatal endocrine dysfunction. 
In addition, understanding of developmental endocrinology is 
increasingly relevant to management strategies for premature 
infants, and infants and children with fetal growth retardation, 
and for understanding of the pathogenesis of adult endocrine and 
metabolic diseases. Up to 18% of babies are born preterm, and 
over the last decades survival has increased. Understan

the last decades survival has increased. Understanding the 
endocrine physiology in the extreme preterm and establishment 
of normal reference ranges has significant clinical value. To that 
effect, a biobank for blood samples of infants 24 to 32 weeks has 
recently been established.539
866
We are now entering an era of direct access to and manage­
ment of the intrauterine environment with provision of medical 
and surgical fetal therapy, entailing b

f medical 
and surgical fetal therapy, entailing both potential advantages and 
adverse effects.540 With expansion of the application and scope of 
amniotic fluid fetal cell sampling, maternal plasma DNA analysis, 
and the advent of fetal visualization and intrauterine fetal blood 
sampling, direct access for fetal diagnosis is now possible.541 Non­
invasive technologies for fetal evaluation are realizing the promise 
of lower risk yet robust diagnostics; examples include sampling 
and analysis 

gnostics; examples include sampling 
and analysis of free fetal DNA from maternal blood and analysis 
of fetal products accessible at maternal sites. After birth, definitive 
assessment of prenatal environmental and/or drug exposures to 
the fetus can be retrospectively assessed by analysis of meconium, 
hair, and other alternative matrices.542 Intrauterine diagnosis 
of fetal adrenal and thyroid disorders has become the standard 
of care,543,544 followed by the prospect of in utero treatment, 

 followed by the prospect of in utero treatment, 
although this is often controversial. Management of hypothyroid 
fetal goiter, for example, poses significant risks (including miscar­
riage) associated with both diagnostic amniocentesis/cordocen­
tesis and intra-amniotic thyroid hormone therapy.545 These risks 
often preclude large-scale studies of optimal antenatal treatment.
Intravenous nutritional supplementation of fetal sheep can 
prevent some forms of growth retardation, and chronic fetal

ome forms of growth retardation, and chronic fetal 
therapy through indwelling pumps is feasible in animal fetuses.546 
These approaches, coupled with increasing availability of synthetic 
hormones and growth factor agonists and antagonists, facilitate 
direct fetal endocrine therapy. In addition, intrauterine stem cell 
transplantation has been successful in the correction of congenital 
hematologic diseases. The fetus in early gestation is a favorable 
recipient of cellular therapy, and fetal 

vorable 
recipient of cellular therapy, and fetal cell transplantation may be 
applicable to therapy for selected endocrine and metabolic dis­
ease.547 Studies involving the endocrine pancreas, for example, 
have demonstrated long-term circulating human insulin follow­
ing intrauterine stem cell transplantation in sheep, with potential 
translation as diabetic cellular therapy.548
Finally, there is a growing experience with fetal and neonatal 
gene therapy in animals,549 although this field rema

ne therapy in animals,549 although this field remains a challeng­
ing frontier of medicine. Intraplacental gene therapy, for example, 
is a novel strategy that takes advantage of an organ that will be 
discarded at birth. The results of preliminary studies in the rabbit 
recently have demonstrated that placental gene therapy may be an 
effective therapy for intrauterine growth restriction, for which no 
treatment is currently available.550
The rapid evolution of maternal-fetal medicine raises co

pid evolution of maternal-fetal medicine raises complex 
ethical and medicolegal challenges related to what constitutes 
innovative treatment versus human experimentation, with or 
without the umbrella of “medical research.” There inevitably exists 
a grey zone between these black and white classifications, high­
lighting the need for clear, responsible guidelines and thoughtful, 
safe practice delivering excellence in care.551
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opportunities for new insights into bone biology. Cell Biochem 
Funct. 2013;31:91–11

o bone biology. Cell Biochem 
Funct. 2013;31:91–114.
	473.	 Ruiz de Oña C, Obregón M, Escobar del Rey F, Morreale de 
Escobar G. Developmental changes in rat brain 5′-deiodinase 
and thyroid hormones during the fetal period: the effects of fetal 
hypothyroidism and maternal thyroid hormones. Pediatr Res. 
1988;24:588–594.
	474.	 Polk D, Cheromcha D, Reviczky A, Fisher DA. Nuclear thyroid 
hormone receptors: ontogeny and thyroid hormone effects in 
sheep. Am J Physiol. 1989;256:E543–E549.
	475.

ni J, Sumida C, Gelly C. Progesterone receptors in the fetal 
uterus and ovary of the guinea pig: evolution during fetal develop­
ment and induction and stimulation in estradiol-primed animals. J 
Steroid Biochem. 1976;7:1031–1038.
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	483.	 Yamashita S, Newbold RR, McLachlan JA, Korach KS. Develop­
mental pattern of estrogen receptor expression in female mouse 
genital tracts. Endocrinology. 1989;125:2888–2896.
	484.	 Dohler K. The special case of hormonal imprinting: the neon

 The special case of hormonal imprinting: the neonatal 
influence on sex. Experientia. 1986;42:759–769.
	485.	 Resko J, Roselli C. Prenatal hormones organize sex differences in the 
neuroendocrine reproductive system: observations on guinea pigs 
and nonhuman primates. Cell Mol Neurobiol. 1997;17:627–648.
	486.	 Kosut SS, Wood RI, Herbosa-Encaración C, Foster DL. Prenatal 
androgens time neuroendocrine puberty in the sheep: effect of tes­
tosterone dose. Endocrinology. 1997;138:1072–1077.
	487

oidism results 
in hypothyroidism in the adult rat. Endocrinology. 1985;116:
2246–2250.
	490.	 Csaba G, Inczefi-Gonda A, Dobozy O. Hereditary transmission 
in the F1 generation of hormonal imprinting (receptor memory) 
induced in rats by neonatal exposure to insulin. Acta Physiol Hung. 
1984;63(93-99).
	491.	 Drake AJ, Walker BR, Seckl JR, et al. Intergenerational conse­
quences of fetal programming by in utero exposure to glucocorti­
coids in rats. Am J Physiol Regul Integr Com Physiol. 2005;

f familial Russell 
Silver syndrome. J Med Genet. 2013;50:823–830.
	507.	 Azzi S, Abi Habib W, Netchine I. Beckwith-Wiedemann and Rus­
sell-Silver syndromes: from new molecular insights to the compre­
hension of imprinting regulation. Curr Opin Endocrinol Diabetes 
Obes. 2014;21(1):30–38.
	508.	 Ibáñez L, Ong K, Potau N, et al. Insulin gene variable number 
of tandem repeat genotype and the low birth weight, precocious 
pubarche, and hyperinsulinism sequence. J Clin Endocrinol Metab. 
2001;86:

inism sequence. J Clin Endocrinol Metab. 
2001;86:5788–5793.
	509.	 Huang WW, Yin Y, Bi Q, et al. Developmental diethylstilbestrol 
exposure alters genetic pathways of uterine cytodifferentiation. Mol 
Endocrinol. 2005;19:669–682.
	510.	 Zheng X, Hendry 3rd WJ. Neonatal diethylstilbestrol treatment 
alters the estrogen-related expression of both cell proliferation and 
apoptosis-related proto-oncogene (c-jun, c-fos, c-myc, bax, bcl-2 and 
bcl-x) in the hamster uterus. Cell Growth Diff. 1997;8:

ic-pituitary-thyroid function of adult off­
spring. Endocrinology. 2004;145:4068–4072.
	520.	 Li M, Reynolds CM, Gray C, Vickers M. Preweaning GH treat­
ment normalizes body growth trajectory and reverses metabolic 
dysregulation in adult offspring after maternal undernutrition. 
Endocrinology. 2015;156(9):3228–3238.
	521.	 Alfaidy N, Li W, Macintosh T, et al. Late gestation increase in 
11beta-hydroxysteroid dehydrogenase 1 expression in human fetal 
membranes: a novel intrauterine source of 

len K. Blockade of neo­
natal activation of the pituitary-testicular axis: effect on peripu­
bertal luteinizing hormone and testosterone secretion and on 
testicular development in male monkeys. J Clin Endocrinol Metab. 
1989;68:600–607.
	538.	 Fisher DA, Nelson JC, Carlton EI, Wilcox RB. Maturation of 
human hypothalamic-pituitary-thyroid function and control. Thy­
roid. 2000;10:229–234.
	539.	 Greaves RF, Pitkin J, Ho CS, et al. Hormone modeling in preterm 
neonates: establishment of pituita

Powell C, Nicholas AK, et al. Intrauterine death fol­
lowing intraamniotic triiodothyronine and thyroxine therapy for 
fetal goitrous hypothyroidism associated with polyhydramnios and 
caused by a thyroglobulin mutation. Endocrinol Diabetes Metab 
Case Rep. 2017;2017.
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mentation ameliorates the development of embolization-induced 
growth retardation in sheep. Pediatr Res. 1987;22:55–61.
	547.	 Lin RY, Kubo A, Keller GM, Davies

rapy: benefits and pitfalls. Gene Ther. 2004;11:592–597.
	550.	 Keswani S, Balaji S, Katz A, et al. Intraplacental gene therapy with 
Ad-IGF-1 corrects naturally occurring rabbit model of intrauterine 
growth. Hum Gene Ther. 2015;26(3):172–182.
	551.	 Moon-Grady AJ, Baschat A, Cass D, et al. Fetal treatment 2017: 
the evolution of fetal therapy centers - a joint opinion from the 
International Fetal Medicine and Surgical Society (IFMSS) and the 
North American Fetal Therapy Network (NAFTNet). 

North American Fetal Therapy Network (NAFTNet). Fetal Diagn 
Ther. 2017;42(4):241–248.
867
24
YEE-MING CHAN, SABINE E. HANNEMA, JOHN C. ACHERMANN, AND IEUAN A. HUGHES
D
efined as “conditions in which chromosomal, gonadal, or 
anatomical sex is atypical,”1 disorders of sex development 
(or differences in sex development [DSDs]) represent 
a broad range of conditions that can present to many different 
health professionals at different stages of life.
In the newborn 

onals at different stages of life.
In the newborn period, approximately 1 in every 4500 babies 
has atypical (ambiguous) genitalia and cannot be immediately 
designated as male or female without further expert evaluation. 
However, DSDs can present in many other ways, such as discor­
dance between prenatal karyotype and appearance of the external 
genitalia at birth; bilateral inguinal hernias; during evaluation for 
associated syndromic features (e.g., renal) in childhood; viriliza­
tion, absen

 (e.g., renal) in childhood; viriliza­
tion, absence of pubertal development, or primary amenorrhea in 
teenage years; or even later in life with infertility. Therefore many 
different health professionals may be involved in DSDs, and all 
should be aware of the range of conditions, how they might pres­
ent, and the principles of their management.1,2
The investigation and management of DSDs require a mul­
tidisciplinary team with experience in these conditions.2,3 The 
pediatric endocrinologist 

ese conditions.2,3 The 
pediatric endocrinologist plays a key role within this team in 
the childhood and teenage years, whereas adult endocrinolo­
gists need to be involved in transitioning care and in the man­
agement of long-term issues such as hormone replacement 
and bone health. In addition to input from urologists, gyne­
cologists, biochemists, clinical pathologists, radiologists, and 
geneticists, it is becoming increasingly clear that experienced 
psychological support is essential for 

perienced 
psychological support is essential for individuals with DSDs 
and their families at key points in their lives. The past decade 
has seen changing terminology and attitudes (Table 24.1), but 
DSD still has significant stigma, and engagement with sup­
port groups and the DSD community is increasingly impor­
tant to define the best pathways of care at local and national 
levels.
In keeping with previous editions of this textbook, this chapter 
will first describe the development of the r

pter 
will first describe the development of the reproductive systems, 
then present an overview of the range of conditions that can be 
classified as DSDs, and finally consider approaches to investiga­
tion and management of DSDs at different ages.4
Development of the Reproductive Systems, 868
Disorders (Differences) of Sex Development, 884
Investigation and Management of Disorders of Sex 
Development, 924
KEY POINTS
 • 
Disorders of sex development (DSDs) can present to differ

rs of sex development (DSDs) can present to different 
health professionals at different ages, but pediatric and adult 
endocrinologists play a central role in diagnosis, support, and 
management.
 • 
DSDs represent a broad range of conditions with many underly­
ing causes; understanding the basic biology of sex development 
and steroidogenesis can help in elucidating the causes of these 
conditions.
 • 
Achieving a diagnosis is important for predicting the natural 
history of specific conditi

redicting the natural 
history of specific conditions, identifying associated features, 
monitoring endocrine function and tumor risk, advising about 
fertility potential, and counseling families about inheritance 
patterns. In some situations, a diagnosis might influence sex 
designation.
 • 
A range of special biochemical tests and genetic analyses can 
help reach a specific diagnosis in most steroidogenesis disor­
ders. Currently, a genetic diagnosis is reached in less than half of 
children

iagnosis is reached in less than half of 
children with gonadal dysgenesis.
 • 
A multidisciplinary team approach is key in providing coordinated 
management from diagnosis through later life. An experienced 
psychologist or allied professional can help support families and 
young people in the early years, as well as following transition to 
adult services. Support groups also play an important role.
 • 
Some DSDs present in teenage years or even in adulthood. 
Adult endocrinologists have a c

ven in adulthood. 
Adult endocrinologists have a crucial role to play in managing 
young people who present in adulthood, as well as for long-
term follow-up of individuals with DSDs diagnosed in child­
hood. A supportive and sensitive approach is essential.
SECTION VI B  Childhood
868
Development of the Reproductive Systems
Reproductive system development begins at 4 to 5 weeks after 
conception in humans and is complete with the achievement of 
sec

umans and is complete with the achievement of 
secondary sex characteristics and fertility (i.e., production of via­
ble gametes) after puberty. Sex development is a dynamic process 
that requires the interaction of many genes, proteins, signaling 
molecules, paracrine factors, and endocrine stimuli.5–9 Marked 
differences in the basic mechanisms of sex determination, differen­
tiation, and reproductive strategy have evolved in different species, 
with variability in sex chromosome complement, g

with variability in sex chromosome complement, gonad develop­
ment, and gametogenesis throughout the animal kingdom.10,11 In 
this chapter, we focus on the basic mechanisms of reproductive 
development in humans. We also include some important insights 
obtained from studies of normal and transgenic mice, although 
these are reviewed extensively elsewhere. More detailed explana­
tion of pituitary gonadotrope development is provided in Chapter 
23 and of normal and disordered puberty in Chapter

23 and of normal and disordered puberty in Chapter 26.
Sex Determination and Sex Differentiation
Sex determination is the process whereby the bipotential gonad 
develops into a testis or an ovary. Sex differentiation refers to the 
development of the internal and external genitalia, as directed by 
the production of peptide hormones and sex steroids by the devel­
oping gonad.
In the typical male, the process of sex differentiation involves 
androgenization of the external genitalia (leading to f

genization of the external genitalia (leading to formation of 
the penis and scrotum), regression of müllerian structures (leading 
to absence of the uterus, fallopian tubes, and the upper two-thirds 
of the vagina), stabilization of wolffian structures (leading to 
development of seminal vesicles, vasa deferentia, and epididymi­
des), and descent of the testes from their origin in the urogenital 
ridge to their final position in the scrotum (Fig. 24.1).
Ovarian hormonal secretion in fetal life 

. 24.1).
Ovarian hormonal secretion in fetal life has little if any effect 
on sex differentiation. At the time of puberty, estrogen synthe­
sis stimulates breast and uterine development, and eventually 
coordinated activity of the reproductive endocrine axis results in 
regular menstrual cycles. Defects in ovarian development there­
fore usually manifest in adolescence with absent puberty. Ovar­
ian development and differentiation have been viewed in the past 
as a “default” or passive process,

ed in the past 
as a “default” or passive process, but ovarian development in fact 
involves many active processes. Studies of gene expression show 
that a specific complement of genes is implicated in ovarian devel­
opment and integrity, some of which (e.g., RSPO1, NR2F2) may 
actively antagonize testis differentiation.6,12–14 Even the concepts 
of a fixed and quiescent population of ovarian germ cells at birth 
and of absent ovarian steroidogenesis have been challenged.15,16
In typical sex dev

esis have been challenged.15,16
In typical sex development, sex determination and sex dif­
ferentiation can be divided into three major components: chro­
mosomal sex (i.e., the complement of X and Y chromosomes), 
gonadal sex (i.e., presence of testes, ovaries, or both), and pheno­
typic or anatomic sex (i.e., presence of male and/or female external 
  Proposed Revised Nomenclature
Previous Terms
Proposed Terms
Intersex
Disorders of sex development 
(DSDs)
Male pseudohermaphrodite
Underviriliza

ent 
(DSDs)
Undervirilization of an XY male
Undermasculinization of an XY male
46,XY DSD
Female pseudohermaphrodite
Overvirilization of an XX female
Masculinization of an XX female
46,XX DSD
True hermaphrodite
Ovotesticular DSD
XX male or XX sex reversal
46,XX testicular DSD
XY sex reversal
46,XY complete gonadal 
dysgenesis
Reproduced with permission from Hughes IA, Houk C, Ahmed SF, et al. Consensus statement 
on management of intersex disorders. Arch Dis Child. 2006;9

ment of intersex disorders. Arch Dis Child. 2006;91:554–562.
   
TABLE 24.1
Mesoderm
Sertoli cells
Germ cell
migration
Müllerian duct
regression
Wolffian duct stabilization
Leydig cells
Testis descent
Male external
genital differentiation
External
genital growth
10
nmol/L
Weeks postconception
40
15
14
13
12
11
10
9
8
7
6
5
4
• Fig. 24.1  Events temporally related to sex differentiation in the male fetus. Mesoderm refers to the tissue 
source for Sertoli and Leydig cell formation. Th

source for Sertoli and Leydig cell formation. The continuous line depicts the rise in fetal serum testoster­
one, with a peak concentration of about 10 nmol/L (300 ng/dL).
869
Chapter 24  Disorders of Sex Development
and internal genitalia) (Fig. 24.2). Though consideration of sex 
development in terms of chromosomal sex, gonadal sex, and phe­
notypic (anatomic) sex can be a useful way of understanding the 
processes involved in reproductive development, none of these 
processes absolutely def

velopment, none of these 
processes absolutely defines a person’s sex, and gender and psycho­
sexual development are influenced by several biologic factors as 
well as by environmental and social influences.
Chromosomal Sex
Chromosomal sex describes the complement of sex chromosomes 
present in an individual (e.g., 46,XY or 46,XX). In humans, the 
usual complement of 46 chromosomes consists of 22 pairs of auto­
somes (identified numerically from 1 to 22 based on decreasing size) 
and a pair of s

1 to 22 based on decreasing size) 
and a pair of sex chromosomes (XX or XY) (Fig. 24.3). Other species 
have different numbers of chromosomes, and they may have differ­
ent types of sex chromosomes or sexually dimorphic autosomes.10,11
In humans, chromosomal sex is usually determined at the time 
of fertilization, when two haploid gametes (an ovum and a sperm, 
XY
XX
Chromosomal sex
Gonadal sex
Phenotypic sex
• Fig. 24.2  Division of typical sex development into three major compo­
nents pro­vide

development into three major compo­
nents pro­vides a useful framework for diagnosis and classification. Chro­
mosomal sex refers to the sex-chromosome complement. Gonadal sex 
refers to the presence of a testis or ovary after the process of sex determi­
nation. Phenotypic (anatomic) sex refers to the appearance of the external 
genitalia and internal structures after the process of sex differentiation.
12
11
10
9
15
8
1
7
1
4
1
16
13
8
7
6
Y
X
22
21
19
A
20
1
2
3
4
5
B
D
12
11
10
9
15
8
1
7
1
4

2
21
19
A
20
1
2
3
4
5
B
D
12
11
10
9
15
8
1
7
1
4
1
16
13
8
7
6
Y
X
22
21
1
2
3
4
5
19
20
C
• Fig. 24.3  Cytogenetic and fluorescence in situ hybridization (FISH) studies. (A) Typical male (46,XY) 
G-banded karyotype. (B) FISH analysis of a male (46,XY) using fluorescent probes directed against SRY 
(sex-determining region on the Y chromosome, spectrum red) and against the X centromere (spectrum 
green). (C) Typical female (46,XX) G-banded karyotype. (D) Photomicrograph shows the X chromatin bo

type. (D) Photomicrograph shows the X chromatin body 
(Barr body, arrow) in the nucleus of buccal mucosa cells from a 46,XX female (thionine stain; original mag­
nification, ×2000). (A–C, courtesy Lee Grimsley and Jonathan Waters, MD, North East London Regional 
Cytogenetics Laboratory, Great Ormond Street Hospital NHS Trust, London, UK.)
870
with 23 chromosomes each) fuse to generate a diploid zygote 
(with 46 chromosomes). Gametes are derived from 

(with 46 chromosomes). Gametes are derived from germ cells, 
which initially replicate their chromosome complement and then 
undergo a series of two meiotic divisions (meiosis I and meiosis 
II) to produce haploid ova or sperm. Normal ova have a single X 
chromosome. Normal sperm contain a single Y chromosome or 
a single X chromosome, resulting in a 46,XY or 46,XX zygote, 
respectively, after fertilization.
Nondisjunction is the failure of a pair of sister chromatids to 
separate during anaph

air of sister chromatids to 
separate during anaphase.17,18 Meiotic nondisjunction during 
gametogenesis can result in ova or sperm with gain or loss of sex 
chromosomal material. Fertilization by such gametes can give rise 
to a zygote with an imbalance in sex chromosome number, called 
sex chromosome aneuploidy. For example, a zygote with a single X 
chromosome (i.e., 45,X) has Turner syndrome, and the presence 
of an extra X causes Klinefelter syndrome (47,XXY) or triple X 
syndrome (47,XXX, 

 syndrome (47,XXY) or triple X 
syndrome (47,XXX, trisomy X).18 Zygotes with no X chromo­
somal material (e.g., 45,Y) are nonviable.
Mitotic nondisjunction can occur in the zygote (i.e., after fer­
tilization), resulting in an imbalance in sex chromosome number 
in a subset of somatic cells, and this is called sex chromosome mosa­
icism (e.g., 45,X/46,XY). In such cases, the two (or more) cell lines 
originate from a single zygote. This situation differs from chime­
rism, which is the existence 

 differs from chime­
rism, which is the existence of two or more cell lines with different 
genetic origins in one individual. Chimerism can occur through 
several mechanisms, including double fertilization (dispermy) of 
a binucleate ovum, fusion of two complete zygotes or morulae 
before implantation, or fertilization by separate sperm of an ovum 
and its polar body. If the different cell lines have different sex chro­
mosomes, a 46,XX/46,XY karyotype occurs. This form of true sex 
chromosome 

ryotype occurs. This form of true sex 
chromosome chimerism is very rare in humans. The consequences 
of some of these events in humans are discussed later (see “Sex 
Chromosome Disorders of Sex Development”).
The Y Chromosome
Although the Y chromosome was initially thought to be inert, 
detection of a 46,XY karyotype in males and a 47,XXY karyotype 
in men with Klinefelter syndrome provided evidence that the Y 
chromosome is responsible for male sex determination.
The human Y chromosome is appr

 sex determination.
The human Y chromosome is approximately 60 megabases 
(Mb) long and represents only 2% of the human genome (Fig. 
24.4).19,20 The Y chromosome consists of a heterochromatic 
region that is highly variable and largely genetically inactive, a 
conserved male-specific region, and autosomally derived regions 
that are estimated to have been added approximately 80 to 130 
million years ago. The male-specific regions have undergone rapid 
evolution, with marked differences even bet

rapid 
evolution, with marked differences even between humans and 
chimpanzees.21 It is thought that Y-chromosome genes encode 
around 57 proteins. Although some of these genes have putative 
roles in growth, cognition, and tooth development, several genes 
in the male-specific region are involved in reproductive develop­
ment, function, and pathology. For example, a cluster of genes at 
Yq11.22 (e.g., the AZFc region) is essential for spermatogenesis, 
and genes within the gonadoblastoma locus 

nesis, 
and genes within the gonadoblastoma locus (e.g., TSPY) increase 
the risk of malignancy when present in dysgenetic gonads (see 
Fig. 24.4).22,23
The euchromatic portion of the Y chromosome consists of 
a Y-specific segment and regions at the distal ends of the short 
and long arms, called the pseudoautosomal regions (PARs) (see 
Fig. 24.4).19,24 These PARs are homologous to the distal ends of 
the short and long arms of the X chromosome and are the only 
regions involved in pairing and r

nd are the only 
regions involved in pairing and recombination during meiosis. 
This process is essential for proper distribution of recombined sex 
chromosomal material to daughter cells as well as for maintaining 
dosage sensitivity of X-Y pairs, as these regions are not subject 
to dosage compensation (i.e., gene inactivation). PAR1 (distal 
short arm, Yp and Xp) contains at least 15 genes, including the 
homeobox gene SHOX. SHOX haploinsufficiency contributes to 
the short stature associated

iency contributes to 
the short stature associated with Turner syndrome, Xp– or Yp– 
deletions, and Léri-Weill syndrome (i.e., dyschondrosteosis).
The search for a testis-determining factor on the Y chromo­
some began more than 50 years ago. In 1987, Mardon and Page 
proposed that the sex-determining function of the Y chromosome 
is located within a 140-kb segment of the short arm, within the 
Y-specific euchromatic portion.25 The ZFY gene was the initial 
candidate in this region. However, in 1

e initial 
candidate in this region. However, in 1989, Palmer and colleagues 
described several 46,XX males who had Y-to-X translocations of 
Y-chromosomal material that was distal (telomeric) to the ZFY 
locus; this focused attention on a 35-kb region of the Y chro­
mosome close to the pseudoautosomal boundary.26 This region 
contained a gene encoding a putative transcription factor subse­
quently called sex-determining region Y (SRY) (see Fig. 24.4).
A series of elegant studies in mice and hum

24.4).
A series of elegant studies in mice and humans established 
SRY as the primary Y-chromosomal testis-determining gene.27–29 
The first definitive proof came with the generation of transgenic 
XX mice specifically expressing the SRY locus (14 kb); some 
of these mice had a male phenotype, developed testes (with­
out spermatogenesis), and showed male sexual mating behavior 
(Fig. 24.5).30 This work was supported by reports of deletions 
22
3
2
1
3
4
2
1
3
2
1
21
11
11
12
13
11.3
11.2
11.1
11

2
1
3
4
2
1
3
2
1
21
11
11
12
13
11.3
11.2
11.1
11.21
11.22
11.23
12
21
22
23
24
25
26
27
28
PAR1
PAR2
ANOS1/KAL1 
NR0B1/DAX1  
AR 
FMR 
DIAPH2 
ATRX 
POF1B 
BMP15 
ARX 
SOX3 
MAMLD1
SRY 
TSPY 
DAZ
AZF 
PAR1
PAR2
Heterochromatic 
region
p 
q
p
X
Y
q
• Fig. 24.4  Schematic diagrams of the X chromosome (left) and Y chro­
mosome (right) show key regions and genes involved in sex develop­
ment and reproduction. ANOS1/KAL1, Kallmann syndrome type 1; AR, 
Androgen receptor; ARX, aristaless-related hom

R, 
Androgen receptor; ARX, aristaless-related homeobox, X-linked; ATRX, 
α-thalassemia, X-linked mental retardation; AZF, azoospermia factor; 
BMP15, bone morphogenetic protein 15; DAZ, deleted in azoospermia; 
DIAPH2, human homolog of the Drosophila diaphanous gene; FMR, frag­
ile X, mental retardation; MAMLD1, mastermind-like domain containing 1 
(CXorf6); p, short arm; NR0B1/DAX1, dosage-sensitive sex reversal con­
genital adrenal hypoplasia critical region on the X chromosome type 1; 
PAR, 

critical region on the X chromosome type 1; 
PAR, pseudoautosomal region; POF1B, actin-binding protein, 34 kDa; q, 
long arm; SOX3, SRY-related HMG box 3; SRY, sex-determining region Y; 
TSPY, testis-specific protein Y.
871
and loss-of-function mutations in SRY in humans with 46,XY 
complete gonadal dysgenesis (Swyer syndrome) (see later discus­
sion).28,31,32 The structure and function of the SRY gene and SRY 
gene product are discussed later. 
The X Chr

 SRY 
The X Chromosome
The X chromosome is a relatively large and gene-rich chromo­
some compared with the Y chromosome, and it consists of about 
160 Mb of genomic deoxyribonucleic acid (DNA) (see Fig. 
24.4).19,33,34 This DNA contains 5% of the haploid genome and 
approximately 850 protein-encoding genes. Several genes on the 
X chromosome play an important role in sex development in 
males and females, gametogenesis, and hypothalamic-pituitary 
(gonadotrope)

genesis, and hypothalamic-pituitary 
(gonadotrope) function (e.g., androgen receptor [AR], ANOS1 
[also called KAL1], DAX1 [NR0B1], MAMLD1, SOX3). How­
ever, most X-linked genes are unrelated to reproductive function 
and have a diverse range of cellular functions.
The X chromosome contains PARs at the distal end of each 
arm, similar to the Y chromosome (see Fig. 24.4).19 These regions 
and several genes in their boundaries function with their homo­
logs on the Y-chromosome PARs in an autosomal

omo­
logs on the Y-chromosome PARs in an autosomal fashion. How­
ever, as large numbers of genes on the X chromosome are located 
outside the PARs and do not have homologs on the Y chromo­
some, a process must exist to maintain the balance in copy num­
ber (i.e., gene dosage) of these genes between males with a single X 
chromosome and females with two X chromosomes. This process 
is called X inactivation.
The first insight into X inactivation came after the identifica­
tion in 1949 of the X chr

me after the identifica­
tion in 1949 of the X chromatin body (i.e., Barr body) in a pro­
portion of cells in females (see Fig. 24.3). This X chromatin is 
derived from one of the two X chromosomes in interphase nuclei 
of these somatic cells. Grumbach and colleagues showed that the 
X chromosome giving rise to X chromatin completes DNA syn­
thesis later than any other chromosome.35 These findings led to 
the concept that only one X chromosome is genetically active dur­
ing interphase, whereas t

 genetically active dur­
ing interphase, whereas the other X chromosome is heterochro­
matinized and relatively inactive. This change in activation state 
occurs in early gestation in humans (12–18 days, late blastocyte 
stage) and is a multistep process, regulated by the gene XIST and 
the TSIX antisense transcript, leading to stable and epigenetic 
silencing of genes on all but one X chromosome (Lyon hypothe­
sis).36 However, female germ cells beyond the stage of oogonia are 
exempt from X ina

beyond the stage of oogonia are 
exempt from X inactivation, demonstrating a need for a second X 
chromosome for oocyte development.
X inactivation occurs randomly in different cells.37 After inac­
tivation has occurred, the inactive state of that particular X chro­
mosome is transmitted to all descendants of that cell so that XX 
individuals effectively function as genetic mosaics for X-linked 
traits. If the initial population of cells is small, skewed X inacti­
vation can occur as a chance ev

, skewed X inacti­
vation can occur as a chance event despite random inactivation. 
N
C
3
1
2
B
A
• Fig. 24.5  (A) The XXSRY+ mouse (right) has testis development and a male phenotype, providing con­
vincing evidence that SRY (sex-determining region on the Y chromosome) is a testis-determining gene. 
A normal XY male littermate is shown for comparison (left). (B) Model of the structure of the SRY high-
mobility group (HMG) box bound to DNA. The HMG domain contains three α-helices (red), which ad

MG domain contains three α-helices (red), which adopt 
an L-shaped conformation. Binding of this region of SRY to the minor groove of DNA (green) causes it 
to bend and unwind. (A, courtesy Professor Robin Lovell-Badge, National Institute of Medical Research, 
London, UK; B, from Harley VR, Clarkson MJ, Argentaro A. The molecular action and regulation of the 
testis-determining factors, SRY [sex-determining region on the Y chromosome] and SOX9 [SRY-related 
high-mobility group (HMG) box 9]. Endo

RY-related 
high-mobility group (HMG) box 9]. Endocr Rev. 2003;24:466–487, used with permission of The Endocrine 
Society, Copyright 2003.)
872
In these situations, heterozygous female carriers of an X-linked dis­
order may manifest symptoms of the condition. A subset of genes 
on the X chromosome may also be imprinted and only expressed 
from one allele. Furthermore, recent data suggest that several other 
X-chromosome genes (especially on the short

other 
X-chromosome genes (especially on the short arm) may escape 
X inactivation, potentially in a tissue-specific manner, and that 
sex chromosome gene dosage may regulate autosomal gene net­
works.38,39 All these phenomena might influence phenotype vari­
ability of X-linked conditions or sex chromosome aneuploidy. 
Gonadal Sex
Gonadal sex refers to whether the gonadal tissue developed as a 
testis or an ovary. The principal embryologic and morphologic 
changes involved in gonadal development

rphologic 
changes involved in gonadal development are shown in Fig. 24.6 
and have been described in detail elsewhere.6–8,40
The Bipotential Gonad
The primitive gonad arises together with the adrenal gland from 
a condensation of the medioventral region of the urogenital ridge 
at approximately 4 to 5 weeks after conception in humans (see 
Fig. 24.6). The primitive gonad separates from the adrenal pri­
mordium at about 5 weeks but remains bipotential (indifferent) 
until about 42 days after con

ntial (indifferent) 
until about 42 days after conception.
Several important genes are expressed in the developing urogenital 
ridge in mice that facilitate formation of the bipotential gonad; they 
include Emx2, Lim1, Lhx9, M33/Cbx2, Pod1, Six1/4, Map3k4, Wt1, 
and Nr5a1/Sf1.41–47 Deletion of these genes causes gonadal dysgenesis 
in mice and can be associated with abnormalities in other organs (e.g., 
kidney, brain). Only some of them have been associated with human 
DSDs to date (e.g., WT1, C

 associated with human 
DSDs to date (e.g., WT1, CBX2, NR5A1/SF1) (see “46,XY Disorders 
of Sex Development”) (Fig. 24.7).48 These factors may affect gonad/
testis development at multiple stages (Fig. 24.8).
WT1 (11p13) is a gene that encodes a four–zinc finger tran­
scription factor expressed in the developing genital ridge, kid­
ney, gonads, and mesothelium.49 Homozygous deletion of Wt1 
in mice prevents gonad and kidney development.50 It is thought 
that at least 24 WT1 isoforms exist due to 

ought 
that at least 24 WT1 isoforms exist due to mRNA splicing vari­
ants and complex post-translational modifications.51 The two 
most common variants are an isoform with alternative splicing 
of exon 5, resulting in insertion of an additional 17 amino acids 
in the middle of the protein, and an isoform that uses an alterna­
tive splice donor site for exon 9, resulting in the addition of three 
amino acids (lysine, threonine, and serine; called +KTS) between 
zinc fingers 3 and 4. It is though

 +KTS) between 
zinc fingers 3 and 4. It is thought that +KTS and –KTS isoforms 
have different cellular functions and differential effects on gonad 
and renal development.52 The ratio of +KTS to –KTS isoforms 
may be important in testis development, with the +KTS isoform 
having a cell-autonomous role in regulating SRY expression and 
influencing cellular proliferation and Sertoli cell differentiation.53 
Wt1 also regulates expression of Sf1 and Sox9 in mice and may 
oppose β-catenin (Ctnnb1) p

 Sox9 in mice and may 
oppose β-catenin (Ctnnb1) pathways.
In humans, WT1 transcripts can be detected in the indifferent 
gonadal ridge when it first forms at 32 days after ovulation.54 Dele­
tions or mutations of WT1 cause well-defined syndromes. Hap­
loinsufficiency of WT1 due to deletion of the chromosomal locus 
containing WT1 and PAX6 (11p13) causes WAGR syndrome 
(Wilms tumor, aniridia, genitourinary abnormalities, and mental 
retardation).55 Dominant-negative point mutations in WT1 cause 

55 Dominant-negative point mutations in WT1 cause 
Denys-Drash syndrome (gonadal dysgenesis, early-onset nephrop­
athy, and predisposition to Wilms tumor),56 whereas mutations in 
the exon 9 splice site of WT1, causing an altered ratio of +KTS 
to –KTS isoforms of WT1, result in Frasier syndrome (gonadal 
dysgenesis, late-onset nephropathy, and predisposition to gonado­
blastoma) (see “46,XY Disorders of Sex Development” and Fig. 
7 weeks
8-9 weeks
4 weeks
4-5 weeks
Wolffian
duct
Müllerian
duct

ks
4 weeks
4-5 weeks
Wolffian
duct
Müllerian
duct
Müllerian
duct
Testis
(Sertoli cells)
Coelomic
vessel
MIS/AMH
Testis
(Leydig cells)
Primitive
gonad 
Metanephros
(kidney)
Pronephros
Mesonephros
germ cell
migration
stabilizes
wolffian
structures
DHT androgenizes
external genitalia
• Fig. 24.6  Schematic representation of the principal morphologic and 
functional events during early gonad or testis development in humans. 
DHT, dihydrotestosterone; MI

velopment in humans. 
DHT, dihydrotestosterone; MIS/AMH, müllerian-inhibiting substance/anti­
müllerian hormone. (Modified from Achermann JC, Jameson JL. Testis 
determination. Top Endocrinol. 2003;22:10–14. Used with permission of 
Chapterhouse Codex.)
RSPO1
WNT4
DAX1/NR0B1
SF1/NR5A1
(R92W)
COUP-TFII/
NR2F2
ridge
gonad 
Ovary 
Testis
Male sex
differentiation
Sertoli cells 
Testicular
descent
46,XX
Müllerian
46,XY
INSL3
GREAT
+
hCG (in utero)
LH (pe

gression
46,XY
INSL3
GREAT
+
LH (perinatal) 
SF1/NR5A1
WT1
GATA4
FOG2/ZFPM2
CBX2
SRY
SOX9
SOX8
MAP3K1
ESR2/NR3A2
DMRT1
TSPYL1
DHH
SAMD9
ARX
MAMLD1
DHT
AMH
• Fig. 24.7  The flow chart provides an overview of the major events involved 
in sex determination and sex differentiation. Mutations or deletions in sev­
eral genes reported to cause disorders of sex development in humans 
are shown. hCG, human chorionic gonadotropin; LH, luteinizing hormone.
873
Chapter 24  Disor

in; LH, luteinizing hormone.
873
24.21 later).57,58 Some phenotypic overlap in the latter two condi­
tions exists.
Another key transcription factor expressed in the urogeni­
tal ridge is steroidogenic factor 1 (SF1, encoded by NR5A1).59 
SF1 is a member of the nuclear receptor superfamily and regu­
lates the transcription of at least 30 genes known to be involved 
in gonadal development, adrenal development, steroidogenesis, 
and reproduction. In mice, co

t, steroidogenesis, 
and reproduction. In mice, complete deletion of the gene encod­
ing Sf1 results in apoptosis of the developing gonad and adrenal 
gland during early embryonic development, resulting in impaired 
androgenization and persistence of müllerian structures in XY 
animals.60 Other features of these homozygous-deleted animals 
include hypogonadotropic hypogonadism, abnormalities of the 
ventromedial hypothalamus, and late-onset obesity in adult ani­
mals rescued by adrenal transplan

ty in adult ani­
mals rescued by adrenal transplantation.61 Heterozygous animals 
have reduced gonadal size and impaired adrenal stress responses.62 
Data for mice suggest that Sf1 plays a critical role in the genera­
tion of a population of Sf1+ progenitor cells in the gonad before 
sex determination occurs, as well as in promoting testis develop­
ment by facilitating SRY regulation of SOX9 expression (discus­
sion to come).63,64
SF1 is expressed during the early stages of urogenital ridge 
for

d during the early stages of urogenital ridge 
formation in humans (32 days after ovulation).54 Consistent 
with the mouse phenotype, heterozygous and homozygous loss-
of-function mutations affecting key DNA-binding motives have 
rarely been described in patients with primary adrenal failure and 
severe 46,XY gonadal dysgenesis (see later discussion).65,66 In con­
trast, haploinsufficiency of NR5A1 due to heterozygous disruptive 
variants is now established as a relatively frequent cause of 46,X

established as a relatively frequent cause of 46,XY 
DSD with normal adrenal function.67
Although SF1 was originally thought to play a less significant 
role in the ovary compared with the testis, studies in mice show 
that SF1 is also an important regulator of ovarian integrity and 
function as well as in the development of early progenitor cell 
populations in the ovary.68–70 Loss of function or haploinsuffi­
ciency of NR5A1 is associated with primary ovarian insufficiency 
(POI), whereas recu

primary ovarian insufficiency 
(POI), whereas recurrent heterozygous variants affecting the 
same codon in SF1 (p.Arg92) are associated with 46,XX testicu­
lar and ovotesticular DSD,71–76 suggesting that SF1 can also act 
as a switch between testis-development and ovary-development 
pathways. 
Primordial Germ Cell Migration
Primordial germ cells (PGCs) are the embryonic precursors of 
gametes (spermatocytes or ova). Surprisingly, in all species, PGCs 
arise some distance from the developing gona

PGCs 
arise some distance from the developing gonad and undergo a pro­
cess of migration during the early stages of embryogenesis.77,78 
In humans, PGCs arise from pluripotent epiblast cells and are 
initially located in the 24-day embryo in a region of the dorsal 
endoderm of the yolk sac, close to the allantoic evagination (see 
Fig. 24.6). After mitotic division, PGCs migrate into the primi­
tive gonad (between 4 and 5 weeks postconception [wpc]) under 
the influence of signaling molecules, r

c]) under 
the influence of signaling molecules, receptors, and extracellular 
matrix proteins such as KIT, the KIT ligand KITLG (formerly 
called Steel), β1-integrin, E-cadherin, WNT5A/ROR2, KIF13B, 
interferon-induced transmembrane protein 1 (IFITM1), and 
IFITM3.79,80 Hindgut expansion may also regulate or facilitate 
this process. Gonadal colonization is mediated by CXCL12 (pre­
viously called SDF1) and its receptor CXCR4 and influenced by 
CXCR7.
gonad
Fgf9
Fgfr2c
Gata4/Zfpm2
Wt

XCR7.
gonad
Fgf9
Fgfr2c
Wt1
Nr5a1
testis
Rspo1
Nr0b1
Rspo1
Ctnnb1
Ctnnb1
Foxl2
Sox9
Wt1–kts
Nr5a1
Lhx9
Cbx2
Emx2
Dmrt1
Wnt4
Sox8
Gata4
Sox8
Dmrt1
Foxl2
Fst
Sry
Nr5a1
Wt1+kts
Gadd45g
Map3k4
Six1, Six4
Igfr, Insr
Cbx2
Sox9
Ptgds
Fgf9
Fgfr2
Nr5a1
Dhh
Amh
Steroidogenesis
Cyp26b1
Retinoic acid
ERα
ERβ
ovary 
Nr5a1
Foxl2
• Fig. 24.8  Overview of some of the molecular events thought to be involved in the development of the 
bipotential gonad, as

d in the development of the 
bipotential gonad, as well as in testis determination and ovary development. These data are based mostly 
on studies in mice. Sry is the principal testis-determining factor, but many other factors interact down­
stream to support testis development and repress ovary development, or vice versa.
874
In the first few months of gestation, PGCs undergo multiple 
cycles of mitotic division. In the testis, a self-renewing popula

ic division. In the testis, a self-renewing population 
of germ cells exists. These undifferentiated PGCs are maintained 
by factors such as POU5F1 (also called OCT4), but they commit 
to differentiation in response to the expression of specific signaling 
molecules and transcription factors. After several cycles of mitotic 
division, these cells enter mitotic arrest.81 Subsequent testicular 
development can occur in the absence of this germ cell popula­
tion.82 Meiosis does not occur until the 

 popula­
tion.82 Meiosis does not occur until the onset of spermatogenesis 
in puberty (see Chapter 19).
In the developing ovary, primordial ova (oogonia) undergo 
mitotic expansion in the first few months of gestation (5–24 
weeks), followed by meiotic division (8–36 weeks) and a process 
of meiotic arrest (oocytes). Although it was originally thought 
that entry into meiosis occurred autonomously, data suggest that 
retinoic acid signaling from the mesonephros stimulates this pro­
cess.83,84 M

 the mesonephros stimulates this pro­
cess.83,84 Male germ cells may be protected from this signal by 
their location within the testis cord and by Sertoli cell expression 
of the cytochrome P450 isoenzyme 26B1 (CYP26B1), which 
breaks down retinoic acid. Meiotic arrest occurs in the first pro­
phase, when the chromatids of homologous pairs have begun to 
separate but are fixed by chiasmata (diplotene stage). The pres­
ence of these PGCs and subsequent meiotic oocytes is critical for 
the differ

equent meiotic oocytes is critical for 
the differentiation of prefollicular cells into follicular cells and for 
the maintenance of ovarian development (see Chapter 17). More 
detailed insight into the stages of human PGC development is 
emerging from single-cell transcriptomic studies.85
More than 6 million oogonia and prophase oocytes exist in 
the developing ovary at about 16 weeks of gestation, and forma­
tion of oogonia from PGCs ceases by the seventh month. At that 
stage, some oocytes re

the seventh month. At that 
stage, some oocytes remain in undifferentiated nests, whereas oth­
ers associate with somatic pregranulosa cells to form primitive or 
primordial follicles. However, approximately 80% of oogonia fail 
to form follicles and undergo apoptosis, so that only 1 million 
germ cells are present in the ovary at the time of birth. The rest­
ing primordial follicles can remain in that stage of development 
throughout the woman’s reproductive life, and meiosis progresses 
only i

 reproductive life, and meiosis progresses 
only in response to ovulation of the graafian follicle, which occurs 
approximately 400 times in a woman’s reproductive lifetime. 
Testis Determination
Testis determination begins about 6 wpc in humans and consists 
of several distinct genetic and morphologic events.5–7 One of the 
first and most significant events in testis determination is a tran­
sient wave of SRY expression through the hitherto undifferenti­
ated gonad at approximately 42 days post

fferenti­
ated gonad at approximately 42 days postconception (dpc) (Fig. 
24.9; also see Fig. 24.8).12,86 Initially, this occurs in the center of 
the gonad, followed by expression in cells located at the cranial 
and caudal poles. SRY expression must reach a certain threshold 
within a definite time window for testis development to occur.87,88 
XY
XX
Migration
Organization
Vascularization
gonad
G
MT
SOX9
BrdU
Brd

fferentiation
gonad
G
MT
SOX9
BrdU
BrdU
Scc
Scc
Sry
• Fig. 24.9  Key morphologic changes in the developing testis in mice. No morphologic differences between 
the XY and XX gonad are seen during the bipotential gonad stage at 10.5 to 11.5 days postconception 
(dpc) (far left). In XY gonads, SRY expression is followed by expression and nuclear localization of SOX9 
(blue) in pre-Sertoli cells (middle), resulting in Sertoli cell differentiation by 11.5 dpc (vasculature and germ 
cells 

ntiation by 11.5 dpc (vasculature and germ 
cells are labeled with platelet endothelial cell adhesion molecule [PECAM] and appear green). Between 
11.5 and 12.5 dpc, distinct changes occur in the XY gonad (near right column), which are not seen in the 
XX gonad (far right). These changes include proliferation of coelomic epithelial cells (measured by BrdU 
incorporation; red, arrow); migration of cells from the mesonephros (shown by recombinant culture of 
a wild-type gonad and a mesonephros in 

ulture of 
a wild-type gonad and a mesonephros in which the cells express green fluorescent protein); structural 
organization of testis cords (detected by laminin deposition, green); male-specific vascularization (by light 
microscopy with blood cells indicated by an arrow); and Leydig cell differentiation (detected by mRNA in 
situ hybridization for the steroidogenic enzyme P450scc). BrdU, bromodeoxyuridine; MT, basal lamina 
of mesonephric tubules; G, gonad. (From Brennan J, Capel B. One tiss

ules; G, gonad. (From Brennan J, Capel B. One tissue, two fates: molecular genetic 
events that underlie testis versus ovary development. Nat Rev Genet. 2004;5:509–521. Used with permis­
sion of Macmillan Publishers, Ltd.)
875
Expression levels peak from approximately day 44, when testicu­
lar cords are first visible. Thereafter, low-level SRY expression in 
humans is confined to Sertoli cells (day 52), where it persists into 
adulthood.
In humans, SRY is

ere it persists into 
In humans, SRY is a single-exon gene (Yp11.3) that encodes a 
204–amino acid, high-mobility group (HMG)–box transcription 
factor.87 Mutations in SRY have been reported in approximately 
10% to 15% of patients with sporadic or familial 46,XY gonadal 
dysgenesis (see “46,XY Disorders of Sex Development” and Fig. 
24.22 in later discussion). As described earlier, the presence of SRY 
(due to translocation of SRY to the X chromosome or an auto­
some) or transgenic e

the X chromosome or an auto­
some) or transgenic expression of SRY is sufficient to induce testis 
development in XX humans and mice (see “The Y Chromosome” 
and Fig. 24.5).
Mutations and deletions in SRY tend to cluster within the 
region encoding the HMG box. The HMG box is a 79–amino 
acid structure that has moderate homology with SRY in other spe­
cies (∼70%) and with the HMG box of related SOX (SRY-like 
HMG box) proteins (60%) (see “46,XY Disorders of Sex Devel­
opment” and Fig. 24.22 late

isorders of Sex Devel­
opment” and Fig. 24.22 later).87 The HMG box consists of three 
α-helices, which are able to adopt an L-shaped or boomerang-
shaped configuration (see Fig. 24.5). The HMG box binds to spe­
cific response elements (AACAAT/A and variants) in the minor 
groove of DNA and induces a 40-degree to 85-degree structural 
bend in its target, depending on the sequence. The precise func­
tion of protein-directed DNA bending is not known, although 
this interaction results in minor gro

n, although 
this interaction results in minor groove expansion, DNA unwind­
ing, and altered base stacking. These effects likely alter DNA 
architecture in chromatin and may permit the interaction of other 
protein complexes with the DNA, resulting in activation or repres­
sion. Other important domains in SRY are two nuclear localiza­
tion signals that can interact with calmodulin and importin-β to 
regulate cellular localization; several serine residues in the amino 
(N)-terminal of SRY that c

 residues in the amino 
(N)-terminal of SRY that can undergo phosphorylation and influ­
ence DNA binding; and a carboxy (C)-terminal, 7–amino acid 
motif that interacts with PDZ domains of SRY-interacting protein 
1 (SIP1).89–93
Although SRY was convincingly shown to be the primary testis-
determining gene more than 20 years ago, relatively little is known 
about the regulation of SRY expression. Some studies have shown 
that SF1, WT1, GATA4, and ZFPM2 (FOG2) can all regulate 
SRY promoter activ

 ZFPM2 (FOG2) can all regulate 
SRY promoter activity in vitro, and MAP3K4 and insulin-related 
signaling pathways may be important, but the exact mechanisms 
that activate SRY in vivo remain relatively poorly understood (see 
Fig. 24.8).47,94–98
The downstream targets of SRY are also not entirely eluci­
dated, but it seems likely that the most significant actions of SRY 
occur through upregulation of SOX9.12 SOX9 is an SRY-related 
HMG box factor comprising three exons (509 amino acids).99–102 

r comprising three exons (509 amino acids).99–102 
In humans, SOX9 becomes strongly localized to the develop­
ing sex cords at 44 to 52 days after ovulation and is expressed 
in Sertoli cells thereafter (Fig. 24.10; also see Fig. 24.9).86 SOX9 
is also expressed in developing cartilage under the regulation of 
parathyroid hormone–related protein (PTHrP)/Indian hedgehog 
signaling pathways. Heterozygous mutations or deletions in SOX9 
cause campomelic dysplasia, a severe skeletal dysplasia that i

elic dysplasia, a severe skeletal dysplasia that is 
A1
B
Pre-Sertoli
cells
Nr5a1+
progenitors
Fetal
Leydig
cells
Sertoli
cells
A
SOX9
AMH/MIS
NR5A1
• Fig. 24.10  (A) Expression of SOX9 (left panel) and AMH/MIS (right panel) in primitive seminiferous tubules 
of human fetal testis at 9 weeks postconception. NR5A1 (SF1) is shown in green. (B) Model for fetal testis 
lineage development based on single cell transcriptomic studies in the mouse. (A, modified from Del Valle 

tudies in the mouse. (A, modified from Del Valle 
I, Buonocore F, Duncan AJ, et al. A genomic atlas of human adrenal and gonad development. Wellcome 
Open Res. 2017;2:25; B, modified from Stévant I, Neirijnck Y, Borel C, et al. Deciphering cell lineage speci­
fication during male sex determination with single-cell RNA sequencing. Cell Rep. 2018;22:1589–1599.)
876
associated with variable gonadal dysgenesis in approximately 75% 
of patients.99,100 Mut

nesis in approximately 75% 
of patients.99,100 Mutations have been found in the HMG box of 
SOX9, in the C-terminal transactivation domain, and in a region 
that interacts with heat shock proteins (e.g., HSP70) (see “46,XY 
Disorders of Sex Development” and Fig. 24.22 later).101,102
The regulatory region of the SOX9 promoter is very large. 
Breakpoints have been reported up to 350 kb from the start of the 
SOX9 gene in patients with campomelic dysplasia and gonadal 
dysgenesis. Studies have show

splasia and gonadal 
dysgenesis. Studies have shown that Sry and Sf1 can synergistically 
regulate Sox9 expression in mice and in vitro through a testis-spe­
cific enhancer region (TESCO).64,103 Recently, several additional 
SOX9 enhancer regions have been characterized that are regulated 
by SRY/SF1 or through SOX9 autoregulation (e.g., Enh13)104 
and which may be involved in maintaining SOX9 expression at 
critical stages over time.
SOX9 can direct testicular development independently of 
SRY 

rect testicular development independently of 
SRY and function as a testis determination gene in its own right. 
Transgenic expression of Sox9 in mice results in testis develop­
ment in XX animals, and the XX odsex (Ods) mouse develops as a 
male owing to disruption of a regulatory element 1 Mb upstream 
of Sox9 that causes testis-specific overexpression of Sox9 during 
development.105,106 Similar disruption of an upstream cis-regula­
tory region resulting in SOX9 overexpression causes 46,XX tes

 resulting in SOX9 overexpression causes 46,XX tes­
ticular DSD,107 and overexpression of SOX9 due to duplication of 
17q24.3-q25.1 or duplications in the SOX9 promoter region have 
also been reported in 46,XX individuals with testicular or ovotes­
ticular DSD.108–110 In mice, Sox9 may mediate its effects through 
direct interactions with other target genes (e.g., Fgf9, Ptgds, Amh) 
to promote testis development pathways and through the degrada­
tion of β-catenin to suppress inhibitors of testis

tion of β-catenin to suppress inhibitors of testis development (see 
Fig. 24.8). Autoregulatory loops with the abovementioned target 
genes may also be important in maintaining SOX9 expression.
Around the time of SRY and SOX9 expression, the develop­
ing testis undergoes a series of distinct cellular and morphologic 
changes (see Fig. 24.9). Understanding of these processes has 
resulted largely from studies in mice, although data are also emerg­
ing from humans at a global level as well as from

ing from humans at a global level as well as from single-cell tran­
scriptomic studies.5,12,85,111 As outlined earlier, the first stage of 
testis development in mice involves a proliferation of Nr5a1/Sf1-
positive somatic cells that results in an increase in pre-Sertoli cell 
precursors and, ultimately, Sertoli cell differentiation. This process 
is influenced by growth factors such as fibroblast growth factor 
9 (Fgf9) and the receptor Fgfr2, as well as DMRT1 (double sex, 
Mab3–related transc

as well as DMRT1 (double sex, 
Mab3–related transcription factor 1; 9p24.3), a 373–amino acid 
protein that is homologous to the sex development doublesex gene 
of Drosophila and the Mab3 gene of Caenorhabditis elegans, and 
SOX8.112 These primitive Sertoli cells coalesce with peritubular 
myoid cells (i.e., flat, smooth muscle–like cells) to form primary 
sex cords, which then condense to form primitive seminiferous 
cords at about 7 wpc in humans.
Sex cord development is supported by a strikin

ns.
Sex cord development is supported by a striking reorganiza­
tion of the gonadal vasculature that occurs in the developing testis 
but not in the ovary (see Fig. 24.9).5,113 These changes include 
the development of a discrete coelomic vessel, restriction of endo­
thelial cells to the interstitial space between the sex cords, and 
increased branching of blood vessels. The development of these 
vascular systems is influenced by growth factor signaling systems, 
such as platelet-derived growth 

gnaling systems, 
such as platelet-derived growth factor receptor-α (PDGFRα)/
prostaglandin D2 synthase (PTGDS), and can be repressed by 
the Wnt4/β-catenin/follistatin (Fst) systems in mice.113 These 
changes in vascular architecture play an important role in deter­
mining cellular patterning and organization in the developing 
testis, in supporting paracrine interactions, and in the export of 
androgens from the developing fetal Leydig cells to the perineal 
and systemic circulation.
A fetal L

 the perineal 
A fetal Leydig cell population also differentiates around this 
time from in the developing testis, supported by Nr5a1/Sf1 and 
following the loss of Tcf21 and Nr2f2/Coup-TFII from intersti­
tial progenitors. Other key factors involved in this process that 
have been associated with human phenotypes include DHH (des­
ert hedgehog), ARX (aristaless-related homeobox, X-linked gene), 
and MAMLD1 (mastermind-like domain–containing 1, previ­
ously called CXORF

ike domain–containing 1, previ­
ously called CXORF6).114–118
Our understanding of the genetic events underlying cell fate 
is being advanced by newer techniques such as single-cell tran­
scriptomics. These events have been reviewed recently and new 
data are emerging rapidly in this field.40,63,111 These approaches 
are showing that the early Nr5a1+ common progenitor lineage 
first gives rise to a Sertoli cell lineage, followed by fetal Leydig cell 
lines, with a progenitor cell lineage maintain

ll 
lines, with a progenitor cell lineage maintained in the interstitium 
that has capacity for steroidogenic fate. A summary of these events 
is shown in Fig. 24.10. 
Ovary Development
For many years, ovary development was thought to be a constitu­
tive (default) process because the gonad differentiates as an ovary 
in the absence of a Y chromosome (and specifically lack of SRY). 
However, several studies in mice and humans have now shown 
that a similar number of genes are differentially expre

a similar number of genes are differentially expressed in the 
developing ovary at critical early stages of development compared 
to the testis.12,13 Therefore ovarian development can be viewed as 
an active process that requires expression of a set of specific genes 
and factors to promote this process as well as to prevent testicu­
lar development. Considerable mutual antagonism between these 
pathways exists (see Fig. 24.8).
The initial stages of ovary development require establishment 
of th

 of ovary development require establishment 
of the bipotential gonad. As in the testis, several key genes are 
required for supporting the early granulosa-cell lineage (Gata4/
Zfpm2, Wt1, Nr5a1) (see Fig. 24.8). Subsequently, WNT signaling 
pathways play a key role in promoting ovarian development as well 
as in repressing testis development, mostly through antagonism of 
SOX9. RSPO1/R-spondin 1 and WNT4 stabilize β-catenin. Loss 
of either RSPO1 or WNT4 has been reported in association with 
X

1 or WNT4 has been reported in association with 
XX ovotesticular DSD or hyperandrogenism in humans.119,120
Other key factors involved in ovary development in the mouse 
are Foxl2 and follistatin. Foxl2 is involved in maintaining or 
enforcing the ovary program and granulosa-cell fate, and loss of 
Foxl2 in mice is associated with postnatal transdifferentiation of 
ovary into testis.121 In humans, loss of FOXL2 is associated with 
blepharophimosis-ptosis-epicanthus inversus syndrome (BPES). 
Wom

s-ptosis-epicanthus inversus syndrome (BPES). 
Women with BPES can have variable POI, but ovotesticular DSD 
has not been reported. The role of follistatin in human ovary 
development is not well established.
Recent reports of ovotesticular DSD associated with specific 
variants in NR5A1/SF1 and NR2F2 suggest that both these tran­
scription factors play a role in maintaining the balance between 
ovarian and testicular differentiation, but more data regarding 
the specific mechanisms are needed,7

ta regarding 
the specific mechanisms are needed,71,122 as well as the role of 
related estrogen receptors (see Fig. 24.8). It has been proposed 
that NR2F2 and NR5A1 may be involved in supporting theca 
cell fate in the ovary by maintaining a stromal progenitor lineage 
(similar to that in the testis that gives rise to Leydig cells), but data 
from patients suggest that the ovotestis has more profound devel­
opment of testicular structures beyond just Leydig cells.
877
Chapter 24  Disorders of 

d just Leydig cells.
877
As with testis development, single-cell transcriptomics data and 
lineage-tracing studies are starting to map out the genetic events 
underlying development of germ cells and pregranulosa cells, as 
well as theca cell origins in the mouse and human ovary. Current 
knowledge has been reviewed extensively recently, but it is likely 
major new insights will emerge in the future in this area.40 
Phenotypic or Anatomic Sex
The developi

s area.40 
The developing gonad produces several steroid and peptide hor­
mones that mediate sexual differentiation and result in the phe­
notypic sex seen at birth. Alfred Jost first showed the importance 
of fetal testicular androgens in this process in 1947.123 In his clas­
sic experiments, Jost demonstrated that surgical removal of the 
gonads during embryonic development of the rabbit resulted in 
development of female reproductive characteristics, regardless of 

male reproductive characteristics, regardless of 
the chromosomal sex of the embryo.
Male Sex Differentiation
Sertoli Cells and Müllerian Regression. Sertoli cells play a key 
role in supporting germ cell survival, and they produce two impor­
tant peptide hormones: antimüllerian hormone (AMH also known 
as müllerian-inhibiting substance [MIS]) and inhibin B. AMH, a 
glycoprotein homodimer, is a member of the transforming growth 
factor-β (TGFβ) superfamily and is first secreted in humans from 
a

uperfamily and is first secreted in humans from 
about 7 to 8 wpc under the regulation of key transcription factors 
such as SOX9, SF1, WT1, and GATA4 (see Figs. 24.1, 24.6, and 
24.10).124,125 AMH causes regression of müllerian structures (i.e., 
fallopian tubes, uterus, upper two-thirds of the vagina) by its para­
crine action on the AMH type 2 receptor (AMHR2).
Müllerian structures appear to be maximally sensitive to AMH 
between 9 and 12 weeks after conception, a time when the devel­
oping t

s after conception, a time when the devel­
oping testis is producing peak concentrations of AMH but before 
the onset of significant AMH production by the developing ovary. 
Boys with mutations in the AMH or AMHR2 genes can present 
with persistent müllerian duct syndrome (PMDS), which can 
present with undescended testes but otherwise typical male exter­
nal genitalia. Severe forms of 46,XY gonadal dysgenesis can also 
result in persistent müllerian structures due to impaired Sertoli 
cell deve

rian structures due to impaired Sertoli 
cell development and AMH release. In some cases, a hemiuterus 
is present if AMH release is affected on only one side (the same 
side as the hemiuterus); but in these cases, androgenization of the 
external genitalia is usually also impaired so that these children 
present with atypical genitalia. In contrast, defects confined to 
Leydig cell steroidogenesis in 46,XY DSD are not associated with 
persistent müllerian structures because Sertoli cell product

 müllerian structures because Sertoli cell production of 
AMH is unaffected. In boys, a small müllerian remnant some­
times persists as a testicular appendage or as a prostatic utriculus 
or utricular remnant.
Inhibin B is also a member of the TGFβ superfamily and 
exerts negative feedback on pituitary follicle-stimulating hormone 
(FSH) secretion (but not secretion of luteinizing hormone [LH]), 
but it is unclear whether inhibin B has any local role during testis 
development. 
Fetal Leydig Cel

role during testis 
Fetal Leydig Cells and Steroidogenesis. Fetal Leydig cells 
develop within the interstitium of the developing testis from 
common precursor cells. There is a marked upregulation of ste­
roidogenic genes at around 53 to 57 dpc, followed by androgen 
synthesis and secretion (see Fig. 24.1).12,126 An expansion in fetal 
Leydig cells occurs between 14 and 18 weeks of gestation, result­
ing in increased testosterone secretion at about 16 weeks.126,127 
Fetal Leydig c

ecretion at about 16 weeks.126,127 
Fetal Leydig cell steroidogenesis is stimulated by placental human 
chorionic gonadotropin (hCG) during the first trimester of 
pregnancy, but the developing hypothalamic-gonadotrope system 
produces significant amounts of LH from about 16 to 20 weeks 
of gestation.128,129
The pathways of testicular steroidogenesis are shown in Fig. 
24.11 and overlap extensively with adrenal steroidogenesis path­
ways (see Chapter 15). The role of individual enzymes is discus

pter 15). The role of individual enzymes is discussed 
in relation to individual steroidogenic defects later in this chapter 
and in several excellent reviews.130 In brief, cholesterol is taken up 
into Leydig cells by low-density lipoprotein or high-density lipo­
protein receptors or is generated de novo by cholesterol synthesis 
pathways or from cholesterol ester. Stimulation of the LH/hCG 
receptor increases the ability of steroidogenic acute regulatory pro­
tein (StAR) to facilitate movement

regulatory pro­
tein (StAR) to facilitate movement of cholesterol from the outer to 
the inner mitochondrial membrane.131 The first and rate-limiting 
step in steroid hormone synthesis involves three distinct reactions: 
20α-hydroxylation, 22-hydroxylation, and cleavage of the cho­
lesterol side chain to generate pregnenolone and isocaproic acid. 
These steps are catalyzed by a single enzyme, P450scc (CYP11A1).
Pregnenolone is converted to progesterone by the microsomal 
enzyme 3β-hydroxysteroid

terone by the microsomal 
enzyme 3β-hydroxysteroid dehydrogenase type 2 (3βHSD2), or 
it can undergo 17α-hydroxylation by P450c17 (CYP17) to yield 
17-hydroxypregnenolone. CYP17 also has 17,20-lyase activity, 
which can cleave the C17,20 carbon bond of 17-hydroxypreg­
nenolone to generate dehydroepiandrosterone (DHEA). This 
17,20-lyase activity is favored by the presence of Δ5-substrates 
(e.g., 17-hydroxypregnenolone), redox partners such as P450 
oxidoreductase (POR) and cytochrome b5, and se

50 
oxidoreductase (POR) and cytochrome b5, and serine phos­
phorylation of CYP17. These reactions are facilitated by the rela­
tive abundance of these factors in Leydig cells in humans, and 
the main pathway to androgen production is through conversion 
of 17-hydroxypregnenolone to DHEA rather than through con­
version of 17-hydroxyprogesterone (17-OHP) to androstenedi­
one.132 Subsequent testosterone production can occur through 
conversion of DHEA to androstenedione by 3βHSD2, followed 
by th

DHEA to androstenedione by 3βHSD2, followed 
by the 17β-hydroxysteroid dehydrogenase action of 17βHSD3 
to generate testosterone, or through the intermediate metabolite 
androstenediol (see Fig. 24.11).
Testosterone undergoes conversion to dihydrotestosterone 
(DHT) by 5α-reductase type 2 in peripheral tissues. DHT’s high-
affinity action on the androgen receptor results in androgeniza­
tion of the external genitalia. Studies based on the phenotype of 
patients with POR deficiency and the fetal 

pe of 
patients with POR deficiency and the fetal tammar wallaby have 
proposed that an alternative pathway to DHT production may 
exist in the human fetal testis (see “P450 Oxidoreductase Defi­
ciency”): the so-called backdoor pathway (Fig. 24.12).133,134
Local production of testosterone stabilizes wolffian structures 
such as the epididymis, vas deferens, and seminal vesicle, whereas 
the potent metabolite DHT induces androgenization of the exter­
nal genitalia and urogenital sinus (Fig. 24.13

er­
nal genitalia and urogenital sinus (Fig. 24.13). In the male, the 
urogenital sinus gives rise to the prostate and prostatic urethra, the 
genital tubercle develops into the glans penis, the urogenital (ure­
thral) folds fuse to form the shaft of the penis, and the urogenital 
(labioscrotal) swellings fuse to form the scrotum (Figs. 24.14 and 
24.15).135
Testosterone and DHT mediate their effects through the andro­
gen receptor (AR). The AR is a member of the superfamily of 
ligand-dependent

s a member of the superfamily of 
ligand-dependent nuclear receptor transcription factors encoded 
by a gene (AR, also known as NR3C4) on the X chromosome 
(Xq11-q12).136 Like other nuclear receptors, it contains a highly 
conserved central DNA-binding domain (DBD) and a C-terminal 
ligand-binding domain (LBD) (Fig. 24.16). In addition, the AR 
uniquely contains an extended N-terminal domain (NTD) that 
878
includes polyglutamine (CAG) and polyglycin

e Span
includes polyglutamine (CAG) and polyglycine (GGN) repeats 
that can vary in number; the length of these repeats can modu­
late AR activity, with a longer stretch of repeats associated with 
decreased AR activity.
The DBD contains cysteine residues that coordinate zinc atoms 
to form two zinc fingers, the first of which contains a P-box that 
enters the major groove of DNA to form specific base-pair con­
tacts that are identical for all classic steroid receptors. The second 
zinc finger c

assic steroid receptors. The second 
zinc finger containing the D-box is involved in protein-protein 
interactions and stabilizes the unit for receptor dimerization. This 
binding requires preferential recognition of androgen response ele­
ments (AREs) composed of an inverted repeat of DNA sequences 
related to 5′-AGAACA-3′.137
The AR in the unliganded state is located in the cytoplasm 
and is complexed to heat shock proteins such as HSP70 and 
HSP90 and to co-chaperone proteins such as FKBP4 (a

SP90 and to co-chaperone proteins such as FKBP4 (also called 
FKBP52). Binding of ligand to its receptor results in dissociation 
from these complexes to allow translocation of the AR into the 
nucleus, where it binds to DNA target sequences as a homodimer.
The crystal structure of the AR LBD bound to natural and 
synthetic androgens is shown in Fig. 24.16C. Helix 12, in the 
presence of ligand, undergoes a conformational change to fold 
back on top of the ligand hydrophobic pocket to capture th

top of the ligand hydrophobic pocket to capture the 
ligand, thus slowing the rate of ligand-receptor dissociation. This 
trapping effect by helix 12 enables recruitment of transcriptional 
coactivators.
Activation of transcription involves motif activation func­
tion 1 (AF1) in the NTD and motif activation function 2 (AF2) 
in the LBD. AF1 is ligand independent, whereas AF2 is ligand 
dependent and interacts with p160 steroid receptor coactiva­
tors such as NCOA1, NCOA2, and NCOA3.138 A relativ

tors such as NCOA1, NCOA2, and NCOA3.138 A relatively 
unique feature of the AR is interaction between the N-terminal 
and C-terminal domains (N-C interaction). In the presence of 
its primary physiologic ligands, testosterone and DHT, the AR 
recruits diverse coregulators that cooperate in the activation 
of transcription of androgen-regulated genes (see Fig. 24.16). 
N-terminal sequences comprising amino acid residues 23 to 27 
(FQNLF) and residues 435 to 439 (WHTLF) participate in 
the AR N-C

dues 435 to 439 (WHTLF) participate in 
the AR N-C interaction to stabilize the AR and slow dissocia­
tion of the ligand. Further modulation of AR function occurs 
post-translationally by processes such as phosphorylation and 
sumoylation.
(outer surface mitochondria)
(inner surface mitochondria)
StAR
CYP11A1
CYP21A2
CYP11B1
(18-OH) CYP11B2
(18-Oxidase) CYP11B2
Deoxycorticosterone (DOC)
18-OH Corticosterone
∆5-Pre

orticoids
∆5-Pregnenolone
CYP21A2
CYP11B1
Cortisol
17-OH Progesterone
∆5-17-OH-Pregnenolone
Estrone
Gonadal steroids
∆4-Androstenedione
CYP19A1
CYP19A1
Estradiol
∆5-Androstenediol
CYP17A1
Lyase
CYP17A1
CYP17A1
Lyase
CYP17A1
HSD3B2
HSD17B3
HSD17B3
HSD17B3
HSD3B2
HSD3B2
HSD3B2
• Fig. 24.11  Schematic diagram shows the steroid biosynthetic pathways leading to androgen production 
in the testis. In

 leading to androgen production 
in the testis. In humans, the main pathway to androgen production is through conversion of 17-hydroxy­
pregnenolone to dehydroepiandrosterone (DHEA) rather than through conversion of 17-hydroxyprogester­
one to androstenedione. Subsequent testosterone biosynthesis can occur through conversion of DHEA 
to androstenedione (by 3β-hydroxysteroid dehydrogenase type 2 [3βHSD2]), followed by the actions of 
17α-hydroxysteroid dehydrogenase type 3 (17βHSD3) to generate t

eroid dehydrogenase type 3 (17βHSD3) to generate testosterone, or through the intermedi­
ate metabolite androstenediol. During male sex development, testosterone undergoes local conversion to 
dihydrotestosterone by 5α-reductase type 2 (not shown). The high-affinity action of dihydrotestosterone 
on the androgen receptor results in androgenization of the external genitalia. The pathways responsible for 
mineralocorticoid and glucocorticoid synthesis are present in the adrenal gland. Additional o

sis are present in the adrenal gland. Additional or alternative 
pathways to dihydrotestosterone production may exist in the fetal testis.
879
The action of the AR is further modulated by interaction with 
coregulatory proteins that function as coactivators or corepres­
sors.138 Agonist-induced changes in LBD organization enable 
coactivator recruitment through their LXXLL motifs.139 The 
AR exhibits a selective preference for the ARA70, ARA55, and 
ARA54

ective preference for the ARA70, ARA55, and 
ARA54 coregulators, which contain FXXLF motifs related to the 
aforementioned amino acid residues in the N-terminus.
Relatively little is known about AR targets in the developing 
wolffian structures (which are testosterone responsive) and in the 
key target tissues such as the developing external genitalia (which 
are DHT responsive). Studies in mice have revealed a number of 
factors that are necessary for development of the wolffian ducts 
(e.g., G

ry for development of the wolffian ducts 
(e.g., Gdf7, Bmps4, Bmps7, Bmps8a, Bmps8b, Hoxa10, Hoxa11) 
and for growth of the genital tubercle (e.g., Fgfs, Shh, Wnts, 
Hoxa13, Hoxd13, Bmp/noggin, ephrin signaling). Impaired 
androgen action occurs in several syndromic conditions and may 
reflect defects in genes that mediate target-tissue responsiveness 
and genital tubercle growth (e.g., HOXA10, HOXA13). 
Testicular Descent. Testicular descent is a two-stage process 
that starts at about 8 weeks 

a two-stage process 
that starts at about 8 weeks of gestation and is usually complete 
by the middle of the third trimester.140 The initial transabdomi­
nal stage of testicular descent (8–15 weeks) involves contraction 
and thickening of the gubernacular ligament and degeneration of 
the craniosuspensory ligament. This stage is mediated by the testis 
itself after secretion of factors such as insulin-like 3 (INSL3, a 
relaxin-like factor) and its G protein–coupled receptor, GREAT 
(also called 

s G protein–coupled receptor, GREAT 
(also called LGR8 or RXFP2).141 The subsequent transinguinal 
(or inguinoscrotal) phase of testicular descent (25–35 weeks) is 
primarily driven by androgens. The genitofemoral nerve and its 
neurotransmitter, calcitonin gene–related peptide (CGRP), have 
also been implicated in this process. 
Subsequent Testicular Development. During the second and 
third trimesters, the testes show several distinct morphologic 
changes, including a reduction in fetal Leydig

ic 
changes, including a reduction in fetal Leydig cell mass and 
elongation and coiling of seminiferous cords. There is no further 
significant development of germ cells during this time, and semi­
niferous cords do not canalize until later in childhood. Neverthe­
less, certain developmental insults can affect the testis at this stage. 
For example, testicular regression syndrome (also called vanishing 
or absent testis syndrome) is most likely a late fetal event, because 
boys with this condit

a late fetal event, because 
boys with this condition have adequate androgenization and no 
müllerian structures, indicating normal testicular function in early 
gestation (see “Anorchia and Cryptorchidism’’). 
Female Sex Differentiation
The processes of female sex differentiation do not involve sig­
nificant morphologic changes in the external genitalia. Müllerian 
structures persist to form the fallopian tubes, uterus, and upper 
portion of the vagina (see Fig. 24.13). Normal uterine develop­

 vagina (see Fig. 24.13). Normal uterine develop­
ment in mice occurs even in the absence of the ovary and requires 
a host of factors for uterine development (e.g., Pax2, Lim1, Emx2, 
Wnt4/Lp, Hoxa13) and differentiation (e.g., Wnt7a, Hoxa10, 
Hoxa11, Hoxa13, progesterone and estrogen receptors).142 The 
lack of local testosterone production leads to degeneration of wolff­
ian structures. The urogenital sinus develops into the urethra and 
lower portion of the vagina, which in the absence of te

gnenolone
StAR
CYP11A1
CYP17A1
POR
CYP17A1
POR
• Fig. 24.12  The classic and alternative (backdoor) pathways of dihydrotestosterone (DHT) synthesis. The 
classic pathway leading to synthesis of DHT is shown on the left. The alternative pathway potentially 
involved in DHT synthesis is shown on the right. The alternative pathway involves the actions of additional 
enzymes: 5α-reductase, type 1 (5α-reductase 1, encoded by SRD5A1), AKR1C2 (3α-re

(5α-reductase 1, encoded by SRD5A1), AKR1C2 (3α-reductase, type 3) 
and possibly AKR1C4 (3α-reductase, type 1), and RoDH (3-hydroxyepimerase, encoded by HSD17B6). 
DHEA, dehydroepiandrosterone; DHP, dihydroprogesterone; HSD, hydroxysteroid dehydrogenase; POR, 
P450 oxidoreductase; StAR, steroidogenic acute regulatory protein. (From Flück CE, Meyer-Böni M, Pan­
dey AV, et al. Why boys will be boys: two pathways of fetal testicular androgen biosynthesis are needed for 
male sexual differentiation.

hesis are needed for 
male sexual differentiation. Am J Hum Genet. 2011;89:201–218.)
880
become separated by the vaginal plate, resulting in two separate 
orifices for these structures. The genital tubercle develops into the 
clitoris, the urogenital (urethral) folds form the labia minora, and 
the urogenital (labioscrotal) swellings form the labia majora (see 
Figs. 24.13 and 24.14).
In contrast to the testis, the developing ovary does not express 

he testis, the developing ovary does not express 
FSH or LH/hCG receptors until after 16 weeks of gestation. At 
about 20 weeks of gestation, plasma concentrations of FSH reach 
a peak and the first primary follicles are formed.129 By 25 weeks 
of gestation, the ovary has developed definitive morphologic 
characteristics. Folliculogenesis can proceed, and a few graafian 
follicles have developed by the third trimester.8 Although some 
studies have suggested that the early fetal ovary can produce

e suggested that the early fetal ovary can produce ste­
roids and express aromatase, the amount of estrogen secreted 
by the developing ovary is likely to be insignificant compared 
with placental estrogen synthesis, and ovarian estrogen produc­
tion does not seem to have significant effects on sex development 
until the time of puberty.16
Several conditions can affect 46,XX sex development in utero. 
Exposure of the fetus to androgens results in androgenization of 
the external genitalia. The l

 androgenization of 
the external genitalia. The local testosterone concentration typi­
cally is not sufficient to stabilize wolffian structures because the 
androgens are usually adrenal in origin. Most frequently, andro­
genization of the 46,XX fetus results from disorders of adrenal 
steroidogenesis (e.g., deficiency of 21-hydroxylase [CYP21], 
11β-hydroxylase, POR, or 3β-hydroxysteroid dehydrogenase). 
Rare causes of androgenization include (placental) aromatase 
deficiency, glucocorticoid r

placental) aromatase 
deficiency, glucocorticoid resistance, testicular and ovotesticu­
lar DSD, and maternal virilizing tumors (e.g., luteoma of preg­
nancy). With the exception of testicular/ovotesticular DSD, these 
conditions result in exposure to androgens but not AMH, so a 
uterus is usually present. Exposure to certain chemical agents in 
pregnancy has been proposed as a cause of fetal androgenization, 
but data are limited. For other developmental abnormalities of 
the female genital tra

lopmental abnormalities of 
the female genital tract (e.g., Mayer-Rokitansky-Küster-Hauser 
[MRKH] syndrome) see “46,XX Disorders of Sex Development.” 
Psychosexual Development
Psychosexual development is traditionally viewed as having sev­
eral distinct components (Table 24.2). Gender identity refers to a 
person’s self-identification as male, female, or a nonbinary gender 
(e.g., intermediate to male and female, combined male and female, 
agender). For some individuals, gender identity may be 

er). For some individuals, gender identity may be fluid. Gen­
der role (sex-typical behaviors) describes the expression or portrayal 
of psychological characteristics and behaviors that are sexually 
dimorphic within the general population, such as toy preferences, 
physical aggression, and choices of activities and occupation. Gen­
der expression refers to whether outward appearance conforms 
more closely to male or female societal norms. Sexual orientation 
refers to romantic and erotic intere

 orientation 
refers to romantic and erotic interests (e.g., heterosexual, bisexual, 
homosexual) and includes behavior, fantasies, and attractions and 
may or may not correspond to actual sexual activity.
The past 50 years have seen a number of opposing theories 
about the origins of psychosexual development and debate about 
the relative contributions of chromosomes, hormones, brain struc­
ture, and societal and family influences on the various compo­
nents. Much of this work has focused on th

 compo­
nents. Much of this work has focused on the study of rodents and 
nonhuman primate species. For example, Young and colleagues 
first showed in 1959 that exposure of guinea pigs to testosterone 
during pregnancy resulted in altered mating behavior of female 
offspring.143 These effects may be most pronounced during a criti­
cal window of exposure and, in rodents and some primates, may 
depend in part on aromatization of the androgens to estrogens, 
receptor availability, and social enviro

trogens, 
receptor availability, and social environment.144 More recently, 
interest has focused on the role of genes and chromosomes in sex 
behavior. For example, studies of differential gene expression pat­
terns in the developing mouse brain have shown upregulation of 
different X and Y chromosomal genes in early embryonic life, even 
before the onset of significant androgen secretion by the develop­
ing testis in males.145 Studies of mice in which Sry was deleted 
(XYSRY–) or transgenically

 which Sry was deleted 
(XYSRY–) or transgenically expressed (XXSRY) showed certain 
neuroanatomic differences between XY and XX mice independent 
of gonad development and endocrine status.146 These findings 
suggest that factors related to chromosome complement have at 
least the potential to affect psychosexual development indepen­
dent of sex hormone action, and a model integrating both factors 
may be more appropriate.147
Understanding the complex issues related to psychosexual 
development 

mplex issues related to psychosexual 
development in humans is even more challenging, especially 
because gender identity cannot readily be assessed in non­
human species. For many years, it was thought that gender 
identity would be concordant with the sex designated at birth, 
provided that the child was raised unambiguously, and appro­
priate surgical procedures and hormone therapies were insti­
tuted concordant with the gender chosen. This theory assumed 
psychosexual neutrality at birth, bu

eory assumed 
psychosexual neutrality at birth, but it has been challenged 
by a refocusing on the potential importance of prenatal (e.g., 
endocrine) and innate (e.g., chromosomal) influences on psy­
chosexual development.148,149 Direct data to assess such effects 
Gonad
Wolffian duct
Epididymis
Testis
Prostate
Vas deferens
Seminal
vesicle
Ovary
Fallopian
tube
Vagina
Uterus
Indifferent stage
Female
Male
• Fig. 24.13  Embryonic differentiation of female and male genita

mbryonic differentiation of female and male genital ducts 
from wolffian and müllerian primordial tissue before descent of the testes 
into the scrotum. In females, müllerian structures persist to form the fal­
lopian tubes, uterus, and upper portion of the vagina. The lower portion 
of the vagina and urethra are derived from the urogenital sinus. In males, 
wolffian structures develop into the epididymides, vasa deferentia, and 
seminal vesicles, whereas the prostate and prostatic urethra are d

, whereas the prostate and prostatic urethra are derived 
from the urogenital sinus. In some cases, a small müllerian remnant can 
persist in males as a testicular appendage.
881
Genital
tubercle
Urethral
folds
slit
Labioscrotal
swelling
Genital tubercle
Urethral folds
Urogenital slit
Labioscrotal swelling
Anal pit
16.8 mm
Glans
Clitoris
Urethral meatus
Labia
minora
Vaginal
orifice
Labia
majora
Anus
Glans penis
Scrotum
Raphe
Anus
49.0 mm
45.0 m

Anus
Scrotum
Raphe
Anus
49.0 mm
45.0 mm
• Fig. 24.14  Differentiation of male and female external genitalia. (Modified from Spaulding MH. The devel­
opment of the external genitalia in the human embryo. Contrib Embryol Carnegie Inst. 1921;13:69–88.)
male
B
A
indifferent
gt
gs
gt
uf
sf
10 wpc
8 wpc
sf
*
*
• Fig. 24.15  Differentiation of male external genitalia in humans between 8 and 10 wpc. (A) Undifferenti­
ated human external genitalia at 8 wpc. (B) Differentiation of scrotal fold

alia at 8 wpc. (B) Differentiation of scrotal folds and fusion of the urethral folds 
(asterisks indicate patent regions on either side) at 10 wpc. gs, genital swelling; gt, genital tubercle; sf, 
scrotal folds; uf, urethral folds. Scale bars: 500 μm. (From Goto M, Piper Hanley K, Marcos J, et al. In 
humans, early cortisol biosynthesis provides a mechanism to safeguard female sexual development. J 
Clin Invest. 2006;116:872–874, used with permission of the American Society for Clinical Investig

sion of the American Society for Clinical Investigation, 
Copyright 2006.)
882
in humans are limited, but studies in women with complete 
androgen insensitivity syndrome (CAIS), in whom the karyo­
type is 46,XY but psychosexual development is almost always 
female, argue for a major role for androgens and a lesser role 
for Y chromosome genes alone in human psychosexual devel­
opment.150 However, the gender characteristics of individuals 
with a Y ch

gender characteristics of individuals 
with a Y chromosome and a partial degree of androgen expo­
sure or responsiveness (e.g., partial androgen insensitivity syn­
drome [PAIS], 17β-hydroxysteroid dehydrogenase deficiency, 
5α-reductase deficiency) can vary, and long-term gender iden­
tity can be difficult to predict.151,152 It has been suggested 
that the degree of androgenization of the external genitalia 
may serve as a marker of in utero testosterone exposure for 
not only the genitals but a

osterone exposure for 
not only the genitals but also the brain. Indeed, in a study 
of 46,XY individuals with PAIS, a greater degree of genital 
O
H
H
H
OH
CH3
CH3
O
H
H
H
OH
CH3
CH3
H
0
100
200
300
400
500
600
700
800
900
Phenotype
AR amino acid position
PAIS
CAIS
3L3X.pdb
NCoA3/
SRC3
H12
DHT
H4
N
001
AF1
FxxLF
5--reductase
[Gln]n
[Gly]n
hinge
AF2
Ligand Binding Domain
(LBD)
DNA Binding
Domain (DBD)
C
920
A
D
B
C
• Fig. 24.16  (A, B) Structure of the androgen 

C
• Fig. 24.16  (A, B) Structure of the androgen receptor (AR) in keeping with nuclear receptors in general. 
The AR uniquely has an extended amino-terminal domain, which contains the FxxLF, polyglutamine, and 
polyglycine motifs, which influence transactivation in response to testosterone and its 5α-reduced metab­
olite, dihydrotestosterone. (C) The crystal structure of the LBD (ligand-binding domain) is shown bound 
to the ligand, DHT, which initiates a conformational change involving helix 1

nitiates a conformational change involving helix 12 and enables recruitment of 
coactivators such as NCoA3/SRC3. (D) The gene distribution of mutations associated with CAIS and PAIS 
are shown (see also Fig. 24.31). CAIS, complete androgen insensitivity syndrome; PAIS, partial androgen 
insensitivity syndrome. (From Mongan NP, Tadokoro-Cuccaro R, Bunch T, et al. Androgen insensitivity 
syndrome. Best Pract Res Clin Endocrinol Metab. 2015;29:569–580.)
  Components of Psychosexual 
Te

80.)
Term
Gender identity
Self-identification of gender as male, 
female, or other
Gender expression/role
Expression of gender-stereotypical behavior
Aggression
Parenting rehearsal
Peer and group interactions
Labeling (e.g., “tomboy”)
Grooming behavior
Pattern of sexual and romantic attraction
TABLE 24.2
883
androgenization generally correlated with increased scores on a 
composite 

 correlated with increased scores on a 
composite gender index that incorporated assessments of both 
gender identity and gender-stereotypical behaviors, but there 
was wide variability for the most ambiguous phenotypes.153
Prenatal exposure to androgens can also influence psycho­
sexual development in 46,XX individuals.149,154 Girls with 
congenital adrenal hyperplasia (CAH) who have more marked 
genital androgenization are more likely to play more with boys’ 
toys, with long-term effects evide

ore with boys’ 
toys, with long-term effects evident into adulthood.155 Prenatal 
androgen exposure can also be associated with other psycho­
logical characteristics, such as sexual orientation.156 However, 
the association between high prenatal androgen exposure and 
gender identity is generally less marked, except perhaps in some 
cases of (ovo)testicular DSD and potentially severe deficiencies 
of aromatase or 11β-hydroxylase.157 Although gender dissatis­
faction (i.e., unhappiness with sex d

er dissatis­
faction (i.e., unhappiness with sex designated at birth) is more 
common in individuals with DSD, more than 90% of 46,XX 
individuals with CAH who are raised as girls later identify as 
female.149,158 The small number of 46,XX men with CAH were 
usually raised male and diagnosed late. However, more 46,XX 
individuals with CAH and raised as girls may later identify as 
male than originally thought.159,160
Social environment and gender of rearing may also play a role in 
psychosexual 

 of rearing may also play a role in 
psychosexual development. Evidence for this comes from case series 
of 46,XY individuals with genital anatomy that was disrupted either 
developmentally (e.g., due to pelvic wall anomalies such as cloacal 
exstrophy) or iatrogenically (e.g., penile ablation) but who other­
wise had normal testosterone production and responsiveness.161,162 
Historically, many of these individuals were raised as girls. Long-
term follow-up of these patients has shown that many 

m follow-up of these patients has shown that many continued 
to identify as female, though a significant proportion identified as 
male gender or were uncomfortable with a female gender identity. 
Of note, of those raised as boys, all continued to identify with the 
male gender in late adolescence and adulthood, and current practice 
favors raising 46,XY patients as boys.
Collectively, these studies show that karyotype, prenatal 
androgen exposure, and designated gender may all influence 
gender

e, and designated gender may all influence 
gender identity. However, gender identity cannot be reliably pre­
dicted from any of these factors. Furthermore, because gender 
identity, sex-typical behavior, and sexual orientation are sepa­
rate components of psychosexual development, it is important 
to appreciate that an interest in same-sex relationships (relative 
to sex of rearing) or strong cross-sex interest in an individual 
with DSD is not necessarily an indication of incorrect gender 
des

necessarily an indication of incorrect gender 
designation.3
Challenges in assessing gender identity in young children 
make it difficult to know when this is established, although 
it is thought to begin between 18 and 36 months and possi­
bly younger.163 Many of the sex differences in brain structures 
reported at late childhood, puberty, or adulthood are not seen 
in early childhood and are therefore not useful for guiding sex 
designation.164,165 There may be some plasticity in psychosex­
ua

,165 There may be some plasticity in psychosex­
ual development, as is evident from studies of some patients with 
conditions such as 5α-reductase deficiency and 17β-hydroxysteroid 
dehydrogenase deficiency who may change their gender role in 
adolescence. Furthermore, some individuals with DSD do not 
fit into a binary model of gender. In several countries, important 
legal changes have taken place to acknowledge this, such as the 
option not to define gender as male or female on passports. Nev

 define gender as male or female on passports. Nev­
ertheless, many societies and individuals still have a very binary 
view of gender, and freedom of expression remains difficult for 
many people who do not feel they fit a binary model. 
Development of the Hypothalamic-Pituitary-
Gonadal Axis in the Fetus
Development of the hypothalamic-pituitary-gonadal (HPG) axis 
is described in detail in Chapter 23; points that are particularly 
relevant to DSD are reviewed here.
Fetal hypothalamic-gonadotr

DSD are reviewed here.
Fetal hypothalamic-gonadotrope development occurs from 6 
wpc in humans and occurs in parallel with the processes of sex 
determination and differentiation. This involves migration of 
gonadotropin-releasing hormone (GnRH)–producing neurons 
from the olfactory placode through the cribriform plate to the 
fetal hypothalamus, development of the hypothalamic nuclei, for­
mation of the anterior pituitary from Rathke pouch, and specifi­
cation and maturation of pituitary gonado

specifi­
cation and maturation of pituitary gonadotropes that are capable 
of releasing LH and FSH as part of a functional HPG axis. Defects 
in these systems can result in a range of clinical conditions, such 
as Kallmann syndrome, congenital isolated hypogonadotropic 
hypogonadism, and gonadotropin insufficiency as part of a mul­
tiple pituitary hormone deficiency, which are reviewed elsewhere 
(Chapters 8, 23, and 26).
It is generally thought that pulsatile release of LH and FSH 
does not occ

that pulsatile release of LH and FSH 
does not occur and therefore does not influence the gonad until 
about 16 to 20 wpc, at which stage fusion of the penis is com­
plete.129 In male fetuses, the main effects of gonadotropins are to 
support the latter stages of testicular steroidogenesis involved in 
elongation of the penis and to support the transinguinal aspects 
of testicular descent. Consequently, boys with congenital gonado­
tropin insufficiency may have a small penis (micropenis) and/or 

ciency may have a small penis (micropenis) and/or 
bilaterally undescended testes. These children might need moni­
toring for important associated features such as renal agenesis in 
Kallmann syndrome or growth hormone or adrenocorticotropic 
hormone (ACTH) insufficiency (causing hypoglycemia) in pan­
hypopituitarism. Classically, boys with hypothalamic or pituitary 
gonadotropin insufficiency do not have hypospadias, as fusion of 
urethral folds has occurred before 20 weeks. Rarely, hypospadias

 has occurred before 20 weeks. Rarely, hypospadias 
occurs with HPG disorders, potentially reflecting loss of factors 
that influence the HPG axis at multiple levels.166,167 
The Hypothalamic-Pituitary-Gonadal Axis in 
Infancy and Childhood
At birth, the infant is removed from the influence of maternal and 
placental hormones and undergoes a series of distinct endocrine 
changes.
Postnatal Endocrine Changes in Boys
Testosterone can be detected at birth, with levels falling during 
the first few 

 birth, with levels falling during 
the first few days of life. Thereafter, a reactivation of the HPG 
axis occurs and results in serum testosterone that is typically mea­
surable by 1 to 2 weeks of life and peaks at midpubertal levels 
between 2 and 3 months after birth (Fig. 24.17).168,169 This peak 
of testosterone is associated with an acceleration in penile growth 
and may be linked in part to early postnatal maturation of gono­
cytes in the testis.169 The HPG axis then becomes relatively q

 testis.169 The HPG axis then becomes relatively qui­
escent from around 6 months of age until the onset of puberty, 
and serum concentrations of LH, FSH, and testosterone are low 
during this time.
Inhibin B concentrations are similarly high at birth and in 
infancy, fall during the first 2 years of life to low but still detect­
able levels, then rise again with the onset of puberty (see Fig. 
24.17).170,171 In contrast, AMH concentrations remain high 
from birth through childhood and only decl

n high 
from birth through childhood and only decline with the onset 
884
of puberty (see Fig. 24.17).124,125 Inhibin B and especially AMH 
can therefore be useful markers of active testicular tissue (specifi­
cally, Sertoli cells) in boys with cryptorchidism, anorchia, and 
46,XY DSD even when assessed between the periods of robust 
HPG activity in infancy and at puberty. INSL3 assays may pro­
vide a useful additional marker of Leydig cell function 

 useful additional marker of Leydig cell function in the 
future.172,173 
Postnatal Endocrine Changes in Girls
The early postnatal endocrine events in girls are less well charac­
terized. Placental estrogen exposure can result in breast develop­
ment before birth, and a small episode of menstrual bleeding 
can occur several days after birth from withdrawal of estrogen 
and progesterone. Girls also have a discrete activation of the 
HPG axis in infancy that declines by 1 to 3 years of age. Detect

fancy that declines by 1 to 3 years of age. Detect­
able concentrations of estradiol (5–20 pg/mL [20–80 pmol/L]) 
and inhibin B (50–200 pg/mL) can be measured in the first few 
months of life, and surprisingly high concentrations of FSH 
with marked interindividual variability can be found during 
infancy and early childhood (median, 3.8 IU/L; range, 1.2–18.8 
IU/L [2.5–97.5%] at 3 months of age in healthy term girls).174 
Throughout childhood, FSH remains detectable, but LH is low. 
AMH is very

SH remains detectable, but LH is low. 
AMH is very low until puberty. Inhibin A, which is made exclu­
sively by the ovary, has been proposed as a test of ovarian tissue 
in the newborn period in children with possible ovotesticular 
DSD, but this hormone is below the limits of detection in many 
normal term newborn girls, and FSH stimulation may be needed 
to detect it.175 
Disorders (Differences) of Sex Development
DSDs can have a wide range of presenting phenotypes, depend­
ing on the underlyi

presenting phenotypes, depend­
ing on the underlying condition and its severity. Individuals with 
these conditions can present to many different health care pro­
fessions, including neonatologists, geneticists, urologists, gyne­
cologists, and internists. The classic presentation is an infant 
born with ambiguous genitalia, but not all conditions are appar­
ent at birth. For example, 46,XY individuals with complete 
17α-hydroxylase/17,20-lyase deficiency may first present in early 
adolescence 

eficiency may first present in early 
adolescence with hypertension and delayed puberty, or a young 
woman with CAIS (46,XY) may first present to a gynecologist 
with amenorrhea.
Significant progress in understanding the molecular basis 
of gonadal development has occurred during the past 30 years. 
Several single-gene disorders causing gonadal dysgenesis in 
humans have been described, and many more candidate genes 
are emerging from studies in mice. The percentage of patients 
with disorders o

mice. The percentage of patients 
with disorders of gonadal development who can be diagnosed at 
the molecular level is increasing with the use of next-generation 
sequencing (NGS) of gene panels or whole exomes (currently 
around 25–40%),166,176–179 and molecular diagnosis of classic 
disorders of steroidogenesis can be reached in most situations. 
Defining the exact basis of a patient’s DSD can have important 
implications for sex designation, predicting response to treat­
ment (e.g., androgen

predicting response to treat­
ment (e.g., androgen supplementation), screening for associ­
ated features (e.g., adrenal dysfunction), assessing the risk of 
tumorigenesis, determining likely fertility options, and provid­
ing long-term counseling for the individual and family. How­
ever, long-term outcome studies are often inadequate, and an 
evidence-based approach to management is not possible in many 
cases.
Nomenclature and Classification of Disorders of 
Sex Development
DSD has been defined

Disorders of 
DSD has been defined as “congenital conditions in which devel­
opment of chromosomal, gonadal, or anatomic sex is atypical.”1 
This definition is wide ranging enough to cover nonendocrine 
conditions such as cloacal exstrophy but sufficiently specific not 
to embrace, for example, disorders of puberty. The consensus 
statement that introduced the term DSD further suggested clas­
sification of DSD by karyotype (Table 24.3): sex chromosome 
variations (sex chromosome 

24.3): sex chromosome 
variations (sex chromosome DSD), disorders of testis develop­
ment and of androgen synthesis and action (46,XY DSD), and 
androgen excess and disorders of ovary development (46,XX 
DSD). Although karyotype does not define a person’s gender, 
rapid assessment of karyotype can be a very useful stepping stone 
for focusing investigations and to start to counsel the family about 
likely causes and outcomes. 
Sex Chromosome Disorders of Sex Development
Differences in the number

rders of Sex Development
Differences in the number of sex chromosomes (i.e., sex chro­
mosome aneuploidy) can be considered sex chromosome DSDs. 
Testosterone (ng/dL)
600
400
200
0
Inhibin B (pg/mL)
400
300
200
100
0
AMH (ng/mL)
100
75
50
25
0
Ad
IV
III
II
I
1 2 3 4 5 6
7
8 9 10
Birth
6 m
• Fig. 24.17  Typical postnatal changes are shown for testosterone, anti­
müllerian hormone (AMH), and inhibin B in normal males from birth to 
adulthood. Pubertal stages (I–IV) are indicated. Conversions: test

tal stages (I–IV) are indicated. Conversions: testosterone, 
ng/dL × 0.0347 for nmol/L; AMH, ng/mL × 7.14 for pmol/L; ranges may 
vary with the assay, so only indicative values are provided to show trends 
over time.
885
These conditions include Klinefelter syndrome (47,XXY and 
variants), Turner syndrome (45,X and variants), trisomy X 
(47,XXX and variants), XYY syndrome (47,XYY and variants), 
45,X/46,XY mosaicism and its variants, and true sex chromo­

 mosaicism and its variants, and true sex chromo­
some chimerism (46,XY/46,XX) (Table 24.4). A 45,Y cell line 
is nonviable.
There can be ambiguous genitalia at birth if a Y chromosome 
or Y chromosome fragment is present in some but not all cells 
due to sex chromosome chimerism or mosaicism, but Klinefel­
ter syndrome (unless four or more X chromosomes are present) 
and classic Turner syndrome in which there is no Y chromo­
somal material do not typically cause ambiguous genitalia. In 
many ca

t typically cause ambiguous genitalia. In 
many cases of classic sex chromosome aneuploidy, the diagnosis 
is made in adolescence or adult life during evaluation of charac­
teristic features, impaired pubertal development, or infertility. 
More detailed descriptions of the features and long-term man­
agement of some of these conditions are provided in the relevant 
chapters (e.g., Turner syndrome in Chapter 26, Klinefelter syn­
drome in Chapters 19 and 26).
Klinefelter Syndrome and Its Variants

19 and 26).
Klinefelter syndrome and its variants are the most common 
forms of sex chromosome aneuploidy, with a reported inci­
dence of 1 in 500 to 1 in 1000 live male births.180 This inci­
dence may be increasing.181 The classic form of Klinefelter 
syndrome is associated with a 47,XXY karyotype and is caused 
by meiotic nondisjunction of the sex chromosomes during 
gametogenesis (Fig. 24.18).18 This abnormality occurs during 
spermatogenesis in approxima

mality occurs during 
spermatogenesis in approximately 50% of patients and dur­
ing oogenesis or postzygotic division in approximately 50%.180 
Mosaic forms of Klinefelter syndrome (46,XY/47,XXY) likely 
represent mitotic nondisjunction within the developing zygote 
and are thought to occur in approximately 10% of individu­
als with Klinefelter syndrome. Other chromosomal variants of 
Klinefelter syndrome (e.g., 48,XXXY) can be seen.
The clinical features of Klinefelter syndrome and its variants

 features of Klinefelter syndrome and its variants 
are summarized in Table 24.4.180 In the most obvious situations, 
a young man may be diagnosed because of small testes, gyneco­
mastia, poor androgenization at puberty, eunuchoid proportions, 
  Example of a DSD Classification System
Sex Chromosome DSD
46,XY DSD
46,XX DSD
A: 47,XXY (Klinefelter syn­
drome) and variants
B: 45,X (Turner syndrome) 
and variants
C: 45,X/46,XY (mosaicism) 
D: 46,XX/46,XY (chimerism)
A: Disorders of gon

nts
A: Disorders of gonadal (testis) development
A: Disorders of gonadal (ovary) development
Complete or partial gonadal dysgenesis (e.g., SF1/NR5A1, 
WT1, GATA4, FOG2/ZFPM2, CBX2, SRY, SOX9, SOX8, 
MAP3K1, ESR2/NR3A2, DMRT1, TSPYL1, DHH, SAMD9, 
ARX, MAMLD1/CXorf6)
Gonadal dysgenesis
Ovotesticular DSD (e.g. NR5A1, NR2F2, RSPO1)
Testicular DSD (e.g., SRY+, dup SOX9, dup SOX3, NR5A1, 
NR2F2, RSPO1, WNT4)
Testis regression
B: Disorders in androgen synth

D
B: Disorders in androgen synthesis or action
B: Androgen excess
Disorders of androgen synthesis
Luteinizing hormone (LH) receptor mutations
StAR protein mutations
Cholesterol side-chain cleavage (CYP11A1)
3β-hydroxysteroid dehydrogenase 2 (HSD3B2)
17α-hydroxylase/17,20-lyase (CYP17)
P450 oxidoreductase (POR)
Cytochrome b5 (CYB5A)
Aldo-keto reductase 1C2 (AKR1C2)
17β-hydroxysteroid dehydrogenase (HSD17B3)
5α-reductase 2 (SRD5A2)
Fetal
3β-hydroxystero

7B3)
Fetal
21-hydroxylase (CYP21A2)
11β-hydroxylase (CYP11B1)
Glucocorticoid receptor mutations
Fetoplacental
Aromatase (CYP19) deficiency
Oxidoreductase (POR) deficiency
Maternal
Maternal virilizing tumors (e.g., luteomas)
Androgenic drugs
C: Other
Syndromic associations (e.g., cloacal anomalies)
Müllerian agenesis/hypoplasia (e.g., MKRH)
Uterine abnormalities (e.g., MODY5)
Vaginal atresias (e.g., McKus

lities (e.g., MODY5)
Vaginal atresias (e.g., McKusick-Kaufman)
Labial adhesions
Disorders of androgen action
Androgen insensitivity syndrome
Drugs and environmental modulators
C: Other
Syndromic associations of male genital development (e.g., 
cloacal anomalies, Robinow, Aarskog, hand-foot-genital, 
popliteal pterygium)
Persistent müllerian duct syndrome
Vanishing testis syndrome
Isolated hypospadias
Cryptorchidism (INSL3, GREAT)
Environmental influences
CYP, Cytochrome P450 isoenzyme; DSD, diso

fluences
CYP, Cytochrome P450 isoenzyme; DSD, disorders of sex development; MODY5, maturity-onset diabetes of the young type 5; MRKH, Mayer-Rokitansky-Küster-Hauser; StAR, steroidogenic acute regula­
tory (protein).
   
TABLE 24.3
886
  Clinical Features of Sex Chromosome DSDs
Condition
Karyotype
Gonad
Internal Genitalia
Features
Klinefelter syndrome
47,XXY and variants
Hyalinized testes
No uterus
Small testis, azoospermia, hypoandrogenemia; tall st

all testis, azoospermia, hypoandrogenemia; tall stature 
and increased leg length; increased incidence of learn­
ing difficulties, language delay, obesity, breast tumors, 
varicose veins, impaired glucose tolerance
Turner syndrome
45,X and variants
Streak gonad or imma­
ture ovary
Uterus
Childhood: Lymphedema, shield chest, web neck, low 
hairline; cardiac defects and coarctation of the aorta; 
renal and urinary abnormalities; short stature, cubitus 
valgus, hypoplastic nails, scoliosis; otitis 

tus 
valgus, hypoplastic nails, scoliosis; otitis media and 
hearing loss; ptosis and amblyopia; nevi; autoimmune 
thyroid disease; visuospatial learning difficulties
Adulthood: Pubertal failure, primary amenorrhea; 
hypertension; aortic root dilatation and dissection; 
sensorineural hearing loss; increased risk of CVD, IBD, 
colon cancer, thyroid disease, glucose intolerance and 
diabetes mellitus, osteoporosis (note that some of 
these may be related to estrogen deficiency)
Mixed gonadal dys­

elated to estrogen deficiency)
genesis
45,X/46,XY and 
variants
Testis or dysgenetic 
gonad
Variable
Increased risk of gonadal tumors; short stature; features 
of Turner syndrome may be present
46,XX/46,XY chime­
rism
Testis, ovary, or 
ovotestis
Variable
Possible increased risk of gonadal tumors
CVD, Cardiovascular disease; DSDs, disorders of sex development; IBD, inflammatory bowel disease.
   
TABLE 24.4
12
11
10
9
15
8
1
7
1
4
1
16
13
8
7
6
Y
X
22
21
19
B

11
10
9
15
8
1
7
1
4
1
16
13
8
7
6
Y
X
22
21
19
B
A
20
1
2
3
4
5
1
2
3
4
5
12
11
10
9
15
8
1
7
1
4
1
16
13
8
7
6
Y
X
22
21
19
20
C
• Fig. 24.18  G-banded karyotypes of Klinefelter syndrome (47,XXY) (A) and Turner syndrome (45,X) (B). (C) 
Structural changes of the X chromosome seen in variants of Turner syndrome (from left to right): normal 
X; ring chromosome (r[X][p22.3q22]); short-arm deletion (del[X][p21]); long-arm deletion (del[X][q21.31]); 
isochromosome (I[X][q10]). (Images courtesy Lee

; 
isochromosome (I[X][q10]). (Images courtesy Lee Grimsley and Jonathan Waters, MD, North East London 
Regional Cytogenetics Laboratory, Great Ormond Street Hospital NHS Trust, London, UK.)
887
or infertility. Other features, such as learning difficulties, speech 
and language delay, behavioral issues, and altered motor devel­
opment, may occur, and early educational support focusing on 
any specific areas of difficulty is important.182,183 It is likely 

s of difficulty is important.182,183 It is likely that 
the clinical detection of Klinefelter syndrome based on postnatal 
karyotyping is biased toward detection of those individuals with 
a more severe phenotype, and it is estimated that as few as 25% 
of men with Klinefelter syndrome may be diagnosed throughout 
their life span. However, this pattern may change as diagnosis of 
Klinefelter syndrome on prenatal genetic testing becomes increas­
ingly common.
The development of testes and a male 

ngly common.
The development of testes and a male phenotype in individu­
als with Klinefelter syndrome provides important evidence for 
the key role of the presence of the Y chromosome (rather than 
X chromosome number) in testis determination and subsequent 
prenatal androgen production. However, testicular function 
may not be completely intact; micropenis and hypospadias may 
be presenting features in some cases, and some studies report 
mildly elevated FSH concentrations during the postnatal

y elevated FSH concentrations during the postnatal “mini­
puberty” together with testosterone levels in the lower half of the 
normal range.184
A more predictable elevation in gonadotropin concentrations 
(e.g., FSH, LH) occurs in the periadolescent period in patients 
with Klinefelter syndrome, after activation of the HPG axis.185,186 
By midadolescence, plasma concentrations of FSH are increased 
in 90% of patients with Klinefelter syndrome, and plasma con­
centrations of LH are increased in 8

d plasma con­
centrations of LH are increased in 80%. Other serum markers 
of testicular function (e.g., peripubertal inhibin B, midpuber­
tal INSL3) are often below the normal ranges.173,186 Although 
puberty usually starts at a normal age with an appropriate rise 
in testosterone in classic Klinefelter syndrome, serum testoster­
one levels off around midpuberty with an increase of gonadotro­
pins. In the majority of young adults, serum testosterone is in the 
lower half of the reference range.

rone is in the 
lower half of the reference range.185,187 Testes usually remain small 
and firm; the median length and volume are 2.5 cm and 3 mL, 
but most are smaller than 3.5 cm (range 1–7 mL).185 Testes typi­
cally appear inappropriately small for the degree of androgeniza­
tion. The serum estradiol concentration is often elevated, which 
contributes to the gynecomastia observed during the adolescent 
period.
Testicular biopsy is not warranted clinically because the diag­
nosis can usually b

d clinically because the diag­
nosis can usually be made on karyotyping from peripheral blood 
cells. However, studies in which testicular histologic examination 
has been obtained report germ cell depletion, progressive hya­
linization of seminiferous tubules, and Leydig cell hyperplasia 
after chronic LH stimulation.185 Testosterone levels need careful 
surveillance during puberty.186 A significant proportion of indi­
viduals with Klinefelter syndrome receive testosterone supplemen­
tation to 

yndrome receive testosterone supplemen­
tation to fully induce puberty and to support sex characteristics, 
libido, and bone mineralization into adult life. Psychological sup­
port and educational support may be needed, as well as attention 
to potential long-term issues such as diabetes. Management of 
Klinefelter syndrome in adolescence and adulthood is discussed 
in Chapter 19.
Although some cases of spontaneous fertility have 
been reported for mosaic forms of Klinefelter syndrome 
(46,XY/47

or mosaic forms of Klinefelter syndrome 
(46,XY/47,XXY), the prospects for fertility in classic Klinefel­
ter syndrome have historically been poor. Occasionally, sperm 
can be obtained from ejaculate, and this should always be 
assessed, but most men have azoospermia. However, in the past 
decade, testicular sperm extraction (TESE, or micro-TESE) 
has allowed successful sperm retrieval for approximately 40% 
to 50% of men with classic Klinefelter syndrome (47,XXY) 
in specialist centers, indepen

 syndrome (47,XXY) 
in specialist centers, independent of age, testicular volume, 
or hormonal profile.188 Sperm retrieval rates also seem to be 
similar in those with and without previous testosterone treat­
ment.189 Subsequent intracytoplasmic sperm injection (ICSI) 
has led to successful pregnancy and live birth in approximately 
40% to 50%.185,188 The risk of transmission of the sex chro­
mosome aneuploidy seems to be low, although an increase in 
some other chromosomal changes is seen, and 

e in 
some other chromosomal changes is seen, and preimplantation 
genetic diagnosis is often offered. 
Turner Syndrome and Its Variants
Turner syndrome is the second most frequent form of sex chromo­
some aneuploidy, with an incidence of approximately 1 in 2500 
live female births.190 The classic form of Turner syndrome is asso­
ciated with a 45,X karyotype, which occurs in approximately one-
half of individuals with this condition (see Fig. 24.18). Mosaic 
forms of Turner syndrome (45,X/46,XX)

18). Mosaic 
forms of Turner syndrome (45,X/46,XX) account for approxi­
mately one-fourth of the patients, and the remainder have struc­
tural abnormalities of the X chromosome such as long-arm or 
short-arm deletions, isochromosomes, or ring chromosomes (see 
Fig. 24.18).190
A 45,X chromosomal constitution may be the consequence 
of nondisjunction or chromosome loss during gametogenesis 
in either parent that results in a sperm or ovum that lacks a 
sex chromosome. Although errors in mitosis in

s a 
sex chromosome. Although errors in mitosis in normal zygotes 
often lead to mosaicism, a purely 45,X constitution may arise at 
the first cleavage division from anaphase lag with loss of a sex 
chromosome or, less often, from mitotic nondisjunction with 
failure of the complementary 47,XXX or 47,XYY cell line to 
survive.17 An estimated 2% of all zygotes have a 45,X karyo­
type, and approximately 7% of spontaneous abortuses have a 
45,X karyotype, making this the most frequent chromosomal 

otype, making this the most frequent chromosomal 
anomaly in humans.
The clinical features of Turner syndrome are highly variable, 
and the age at diagnosis can be similarly variable. For example, 
a prenatal diagnosis of Turner syndrome may be made inci­
dentally on prenatal genetic testing or after the detection of 
increased nuchal translucency on fetal ultrasound scanning.190 
In early infancy, the diagnosis should be considered in females 
with lymphedema, nuchal folds, low hair line, and/o

ith lymphedema, nuchal folds, low hair line, and/or left-sided 
cardiac defects. Unexplained growth failure or characteristic 
somatic features (e.g., abnormal nails, shield chest, abnormal 
carrying angle, recurrent ear infections) may point to the diag­
nosis during childhood, and Turner syndrome should be consid­
ered in all girls with pubertal delay or pubertal failure associated 
with hypergonadotropic hypogonadism. The clinical features of 
Turner syndrome are summarized in Table 24.4, and

Turner syndrome are summarized in Table 24.4, and diagnosis 
and management are discussed in Chapters 25 and 26. Growth 
hormone treatment can reduce the severity of short stature. 
Timely and appropriate introduction of estrogens is necessary 
in adolescence to ensure optimal growth, bone health, and psy­
chosocial development as well as adequate breast and uterine 
development, thereby optimizing the opportunity to potentially 
carry a pregnancy by ovum donation in the future.190–192 How­
eve

cy by ovum donation in the future.190–192 How­
ever, careful assessment of cardiovascular risk before and during 
pregnancy is essential.193 Women with Turner syndrome benefit 
from dedicated transition in adolescence and long-term follow-
up with a focus on issues such as cardiovascular, bone, and repro­
ductive health and hearing.190,194–198
Girls with classic Turner syndrome show ovarian dysgenesis. 
Studies of Turner syndrome embryos have shown normal germ 
888

wn normal germ 
888
cell migration and normal ovarian development until the third 
month of gestation.199 Then, accelerated germ cell apoptosis and 
subsequent oocyte atresia occur, resulting in progressive degenera­
tion of the ovary in the prenatal or postnatal period. With these 
gonadal changes, LH and FSH tend to rise in late childhood, after 
activation of the hypothalamic pulse generator, and sometimes 
even earlier. Nevertheless, sufficient e

ometimes 
even earlier. Nevertheless, sufficient estrogen synthesis for puberty 
to commence in adolescence occurs in approximately 25% of girls 
with Turner syndrome (10% of those with 45,X; 30–40% of those 
with 45,X/46,XX mosaicism), and menstruation occurs in about 
2% of cases.194,200 AMH may be a useful marker of ovarian reserve 
and a predictor of ovarian insufficiency.201 Gonadectomy is not 
usually required except when a Y fragment containing the TSPY 
locus is present; in such patients

ining the TSPY 
locus is present; in such patients, the risk of gonadoblastoma is 
increased.202
Oocyte cryopreservation has been used successfully in sev­
eral women with Turner mosaicism and is a fertility preservation 
option for young women with persistent ovarian function.190,203 
Ovarian cortical tissue preservation is feasible but requires surgery 
and is currently considered experimental. Routine fertility preser­
vation in girls under 12 years of age is not recommended.190
Risk from pre

 years of age is not recommended.190
Risk from pregnancy is high in women with Turner syndrome 
due to cardiovascular defects and other comorbidities, and detailed 
assessment and counseling prior to and during any potential preg­
nancy are mandatory.190,203 
45,X/46,XY Mosaicism and Variants
A mosaic 45,X/46,XY karyotype probably arises through ana­
phase lag during mitosis in the zygote, although Y chromosomal 
abnormalities are sometimes seen, and interchromosomal rear­
rangements with loss o

 and interchromosomal rear­
rangements with loss of structural abnormal Y material may be 
a common mechanism for variants of this condition. Although 
the classic form of this condition is associated with 45,X/46,XY 
mosaicism, 45,X/47,XYY, 45,X/46,XY/47,XYY, and other mosaic 
karyotypes can occur.
The true prevalence of this condition is unknown (see Table 
24.4). Because individuals with 45,X/46,XY mosaicism are more 
likely to be referred for further assessment if they have atypical 
genital

 further assessment if they have atypical 
genitalia, most series of patients reported in the literature have 
probably reflected this bias; studies based on nonselected prena­
tal karyotyping have shown that most children with 45,X/46,XY 
mosaicism appear male.204–206
The phenotypes associated with 45,X/46,XY mosaicism are 
highly variable. The gonads can develop as dysgenetic ovaries 
(ovarian-like stroma with sparse primordial follicles) or streak 
gonads (similar to those seen in Turner synd

reak 
gonads (similar to those seen in Turner syndrome), normal or 
dysgenetic testes, or very rarely as ovotestes (with testicular and 
dysgenetic ovarian components within a single gonad).
Depending on the degree of Leydig cell activity (if any), the 
gonads may be positioned anywhere along the pathway of tes­
ticular descent, with streak gonads more likely to be intraabdomi­
nal and well-formed testes more likely to be in the inguinoscrotal 
region. Marked differences in gonadal development a

egion. Marked differences in gonadal development and histologic 
appearance can sometimes be seen between the right and the left 
sides (referred to as mixed gonadal dysgenesis) or even within a sin­
gle gonad.207
All possible genital phenotypes have been reported, rang­
ing from typical female external genitalia or mild clitoromegaly 
through all stages of ambiguous genitalia to hypospadias or typi­
cal male external genitalia.204,207–210 Asymmetry of the external 
genitalia strongly suggests a

try of the external 
genitalia strongly suggests asymmetric gonadal development (i.e., 
mixed gonadal dysgenesis).
Müllerian structures may be present if there is absent or 
impaired AMH production by Sertoli cells, and these structures 
are also often asymmetric. The presence of a hemiuterus and fal­
lopian tube on the side of the more dysgenetic gonad in some 
cases of mixed gonadal dysgenesis provides important evidence for 
the paracrine actions of AMH on developing müllerian structures.
Som

ons of AMH on developing müllerian structures.
Somatic features associated with a 45,X/46,XY karyotype are 
also highly variable and do not always correlate well with the 
gonadal phenotype.204,207,209,210 Approximately 40% of children 
have clinical features reminiscent of Turner syndrome, such as 
short stature, nuchal folds, low-set hairline, and cardiac and renal 
abnormalities.207 In other cases, short stature may be the only 
somatic manifestation. Ongoing monitoring for features associ­
a

station. Ongoing monitoring for features associ­
ated with Turner syndrome (e.g., thyroid function, hearing, and 
cardiac anomalies; see Chapter 26) is therefore applicable for this 
group of individuals.190 Furthermore, many families benefit from 
the psychological support and education that can be linked to spe­
cialist services.
Sex designation can be difficult in individuals with 45,X/46,XY, 
and several factors should be considered, including probable gen­
der identity, genital appearance a

g probable gen­
der identity, genital appearance and urogenital anatomy, fertility 
and reproductive options, risk of gonadal malignancy, and poten­
tial need for hormone replacement. Recommendation for sex 
designation should be handled like any other intervention, with 
a careful discussion of risks, benefits, and potential outcomes. 
Most infants with female or minimally androgenized genitalia are 
raised as female, and the presence of a uterus or hemiuterus may 
allow the potential for pregn

s or hemiuterus may 
allow the potential for pregnancy by ovum donation in the future, 
though predicting future function can be difficult. Furthermore, 
as with girls with classic Turner syndrome, there may be risks asso­
ciated with pregnancy (by ovum donation) due to cardiovascular 
anomalies. Intraabdominal streak-like (i.e., flat, scar-like gonads) 
and dysgenetic gonads are thought to pose a significant risk of 
germ cell malignancy and should be removed.208,210,211 The risk 
of germ cell 

uld be removed.208,210,211 The risk 
of germ cell tumors in individuals with 45,X/46,XY DSD is esti­
mated to be 15% to 40%.212 Estrogen replacement is required to 
induce breast and uterine development in adolescence, and the 
addition of progestins allows menstruation if a uterus is present. 
Growth hormone treatment has been used when short stature 
is present, but no large trials have been performed to assess this 
group of patients. Usually there is a significantly reduced growth 
spurt dur

there is a significantly reduced growth 
spurt during puberty, and some studies have suggested use of GH 
from earlier in childhood may optimize growth.209,213 Similarly, 
limited long-term outcome data on gender identity or psycho­
sexual functioning are available.
Infants with hypospadias are usually raised as male. Testoster­
one can sometimes be given to promote phallic growth in infancy, 
and hypospadias repair is usually offered as a one-stage or two-
stage procedure, depending on the seve

age or two-
stage procedure, depending on the severity of the hypospadias. 
Attempts should be made to perform orchidopexy as a one-stage 
or two-stage procedure because there may be a significant risk of 
malignancy in these gonads. Gonadectomy should be considered 
for gonads that cannot be placed in a palpable position; some have 
advocated for close monitoring, but imaging is not able to detect 
early neoplastic changes in abdominal gonads.208,210,214 Gonads 
that can be secured within the s

8,210,214 Gonads 
that can be secured within the scrotum need careful monitoring 
by palpation/self-examination and imaging to detect changes such 
as microlithiasis.214 At the time of orchidopexy, and in adoles­
cence, biopsy can be used to assess for premalignant changes and 
estimate the risk of germ cell cancer. Puberty should be carefully 
monitored to ensure adequate endogenous testosterone produc­
tion. Most boys enter puberty spontaneously, but some then 
889
Chapter 24  Disorders of Sex

y, but some then 
889
develop androgen insufficiency requiring testosterone supplemen­
tation.210 Reduced final height is invariable, but there is a benefi­
cial response to growth hormone in some boys.215
Designation of sex and management of a 45,X/46,XY child 
with highly ambiguous genitalia is a difficult situation for fami­
lies and physicians, and long-term outcome data for this group 
are scant. Individuals raised female are infertile, usually have 

viduals raised female are infertile, usually have 
no uterus, require gonadectomy because of tumor risk, and are 
likely to undergo urogenital surgery. Those raised as male often 
undergo multiple hypospadias operations, may have poor corpo­
ral tissue, are infertile if dysgenetic gonads are present that need 
to be removed and often infertile even if the gonads are intact, 
and may have a significantly reduced height potential. There is 
limited data on gender identity outcomes; in one series o

 data on gender identity outcomes; in one series of 11 chil­
dren with mixed gonadal dysgenesis raised as girls, 6 later declared 
a male gender identity.216 Detailed assessment of each child is 
important, and an individualized approach by an experienced 
multidisciplinary team is important for long-term monitoring, 
management, and support. Long-term outcome data from larger 
studies may provide better guidance on the management of this 
group of individuals in the future.
A 45,X/46,XY mosaic 

of individuals in the future.
A 45,X/46,XY mosaic karyotype can also be associated with a 
male phenotype and apparently normal testis development. Ini­
tial cases of typical-appearing males with a 45,X/46,XY karyotype 
were described after screening of family members as potential 
bone marrow transplant donors, but later studies of amniocentesis 
showed that 90% of infants with a 45,X/46,XY karyotype have 
normal male genitalia and apparently normal testes, and postna­
tally these individuals m

rmal testes, and postna­
tally these individuals may be diagnosed because of short stature 
or infertility.205,206,217,218 There seems to be variable correlation 
between the degree of mosaicism on peripheral blood sampling 
and gonadal or somatic phenotype. Monitoring of gonadal func­
tion and growth, assessment for premalignant changes of the 
gonads, and cardiac and renal screening are advised in individuals 
with a typical male phenotype or minor genital anomalies because 
they are at risk f

inor genital anomalies because 
they are at risk for abnormal growth, Sertoli cell dysfunction, 
gonadal germ cell cancer, and cardiac and renal malformations.217 
Ovotesticular Disorders of Sex Development: 46,XX/46,XY 
Chimerism and Variants
The diagnosis of ovotesticular DSD (previously referred to as true 
hermaphroditism) requires the presence of ovarian tissue (con­
taining follicles) and testicular tissue in the same or the opposite 
gonad (Fig. 24.19; see Table 24.4). Gonadal stroma arra

 (Fig. 24.19; see Table 24.4). Gonadal stroma arranged in 
whorls, similar to what is found in a typical ovary but lacking 
oocytes, is a common finding in a dysgenetic or streak gonad and 
is not considered sufficient evidence to designate the rudimentary 
gonad as an ovotestis.
Ovotesticular DSD is an uncommon condition in the United 
States and Europe, occurring in less than 5% of babies referred 
because of  atypical genitalia, but is a common cause of atypical 
genitalia in South Africa.218

n cause of atypical 
genitalia in South Africa.218a Ovotesticular DSD can occur in sev­
eral different scenarios. The classic diagnosis is 46,XX/46,XY chi­
merism (sometimes caused by double fertilization or ovum fusion; 
see “Chromosomal Sex”), which is found in approximately 20% to 
30% of children with ovotestes in our center and occurs in all ances­
tral backgrounds (Table 24.5).219,220 Somewhat more prevalent is 
ovotesticular DSD associated with a 46,XX karyotype. In a propor­
tion of case

 with a 46,XX karyotype. In a propor­
tion of cases this can occur due to p.Arg92 variants in NR5A1/SF1, 
loss of function of RSPO1 or NR2F2/COUP-TFII, or upregula­
tion of expression of SRY, SOX9, or SOX3.71,72,122,221,222 Of note, 
46,XX ovotesticular DSD is much more prevalent in children from 
sub-Saharan Africa; the molecular etiology for this is not currently 
known (see Table 24.5). The molecular basis of these conditions will 
be discussed later in the relevant section (see “46,XX Ovotes

d later in the relevant section (see “46,XX Ovotesticu­
lar and 46,XX Testicular Disorders of Sex Development”). Finally, 
ovotestes have rarely been reported in several conditions associated 
with 46,XY gonadal dysgenesis (e.g., MAP3K1, SOX9), and fertile 
46,XY women have been described.223–225
Here we will discuss basic principles of management of ovotes­
ticular DSD based on 46,XX/46,XY chimerism, which are appli­
cable to the other forms of the condition. However, it is important 
to rememb

the condition. However, it is important 
to remember that a Y chromosome (containing the spermatogenic 
locus) is necessary for potential sperm production, so 46,XX chil­
dren with ovotestes will not be fertile males.
Patients with ovotesticular DSD often have asymmetric 
gonadal development and sometimes may be subclassified accord­
ing to the type and location of the gonads.4 Three forms are pos­
sible: lateral (20%) with a testis on one side and an ovary on the 
other, bilateral (30%) with te

nd an ovary on the 
other, bilateral (30%) with testicular and ovarian tissue present 
bilaterally, and unilateral cases (50%) with an ovotestis present on 
one side and an ovary or testis on the other. It has been suggested 
that the left gonad is more likely to develop as an ovary, whereas 
the right gonad is more likely to develop as a testis. An ovary is 
primordial
follicles
seminiferous
tubules
follicles
tubules
• Fig. 24.19  Ovotestis, showing immature seminiferous

g. 24.19  Ovotestis, showing immature seminiferous tubules lined with 
Sertoli cells and germ cells (upper left) and ovarian tissue with primordial 
follicles (lower right) (hematoxylin-eosin stain; original magnification, ×400). 
(Courtesy Neil Sebire, MD, Great Ormond Street Hospital NHS Trust, Lon­
don, UK.)
  Relative Frequency (%) of Different 
Karyotypes in Ovotesticular DSD (True 
Hermaphroditism)
Location
46,XX/46,XY
46,XX
46,XY
North America
21
72
7
Europe
41
52
7
Africa
—
97
—
DSD, Di

erica
21
72
7
Europe
41
52
7
Africa
—
97
—
DSD, Disorders of sex development.
Adapted from Krob G, Braun A, Kuhnle U. True hermaphroditism: geographical distribution, 
clinical findings, chromosomes and gonadal histology. Eur J Pediatr. 1994;153:2–10.
   
TABLE 24.5
890
likely to be in its normal anatomic position, whereas a testis or 
ovotestis can be anywhere along the pathway of testicular descent 
and is often found in the inguinal region.
Differ

Differentiation of the genital tract and development of sec­
ondary sex characteristics vary in ovotesticular DSD.226,227 Most 
patients who present early have ambiguous genitalia or signifi­
cant hypospadias. Cryptorchidism is common, but often at least 
one gonad is palpable, usually in the labioscrotal fold or inguinal 
region, and is often associated with an inguinal hernia. The dif­
ferentiation of the genital ducts usually follows that of the gon

 the genital ducts usually follows that of the gonad, 
and a hemiuterus or rudimentary uterus is often present on the 
side of the ovary or ovotestis.
Breast development at the time of puberty is common in ovo­
testicular DSD.227 Menses occur in a significant proportion of cases 
when a uterus is present, and ovulation and pregnancy have been 
reported in a number of patients with a 46,XX karyotype when an 
ovary is present. However, progressive androgenization can occur 
in girls with testicula

androgenization can occur 
in girls with testicular tissue, which can result in voice changes and 
clitoral enlargement during adolescence if left untreated. Individu­
als reared as male often present with hypospadias and undescended 
testes, although bilateral scrotal ovotestes have been reported. These 
individuals can experience significant estrogenization at the time of 
puberty and may have cyclic hematuria if a uterus is present. Sper­
matogenesis is rare, and interstitial fibrosis of the 

genesis is rare, and interstitial fibrosis of the testis is common. 
As noted, fertility requires Y chromosomal genes other than SRY, so 
boys with 46,XX ovotesticular DSD will be infertile.
Although ovotesticular DSD is rare, the diagnosis should 
be considered in all patients with ambiguous genitalia. A 
46,XX/46,XY karyotype strongly suggests the diagnosis, but the 
detection of a 46,XX or 46,XY karyotype does not exclude the 
diagnosis, especially if a 46,XX baby has genital asymmetry. Pelvi

ially if a 46,XX baby has genital asymmetry. Pelvic 
imaging with ultrasound or magnetic resonance imaging (MRI) 
is useful for visualizing internal genitalia. The presence of testicu­
lar tissue may be detected by measurement of basal testosterone, 
AMH, and inhibin B in the first months of life and by measuring 
basal AMH thereafter. Ovarian tissue is more difficult to detect 
in early childhood, although estradiol, inhibin A, and follicular 
response to repeated injection of recombinant human

esponse to repeated injection of recombinant human FSH can 
provide useful information.175 Examination under anesthesia and 
laparoscopy may provide the most detailed information about 
internal structures and allow a biopsy to confirm the diagno­
sis of ovotesticular DSD when other forms of DSD have been 
excluded.228,229 However, the biopsy sometimes does not sample 
all the tissues present in a gonad.229
The management of ovotesticular DSD depends on the age at 
diagnosis, genital development

ends on the age at 
diagnosis, genital development, internal structures, and reproduc­
tive capacity. Either male or female designation may be appropri­
ate for the young infant. Individuals with a 46,XX karyotype, little 
virilization, and a uterus are likely to have functional ovarian tis­
sue, and female designation is likely to be appropriate. If andro­
genization is not desired by the individual, functional testicular 
tissue could poten­tially be removed before puberty and monitored 
posto

lly be removed before puberty and monitored 
postoperatively by measuring serum AMH and testosterone levels. 
Although still experimental, options for cryopreserving gonadal tis­
sue are being explored. Potentially, GnRH agonists could be used 
to temporarily delay puberty if the young person needs more time 
to understand the condition and be involved in decisions about 
gonadectomy (whether complete or partial). The risk of malignant 
transformation in ovarian tissue of 46,XX patients is not k

ation in ovarian tissue of 46,XX patients is not known but 
is likely to be low given the absence of Y chromosomal material.
A male sex designation may be more appropriate if there is 
significant clitorophallic development and testicular tissue, and 
müllerian structures are absent or very poorly formed. Appropriate 
counseling should be provided and evidence of a likely male gen­
der seen before considering prepubertal removal of ovarian tissue 
within an ovotestis. Again, cryopreservation of 

within an ovotestis. Again, cryopreservation of gonadal tissue may 
emerge as an option but is currently experimental. GnRH agonists 
or aromatase inhibitors might be considered on a trial basis if more 
time is needed to make decisions. Remnant müllerian structures can 
be removed by an experienced surgeon if there are problems with 
hematuria or urinary retention, although extreme care is needed not 
to disrupt the blood supply to any testis/testes being preserved, and 
often there is no har

testes being preserved, and 
often there is no harm in leaving müllerian structures intact. The 
testicular tissue is usually well differentiated in 46,XX ovotesticular 
DSD so that the risk of germ cell tumors is estimated to be low.22
Gender identity is also an important consideration in patients 
with ovotesticular DSD who first present in late childhood or ado­
lescence due to androgenization in girls or estrogenization in boys. 
In most cases, gender identity is consistent with gender of re

s, gender identity is consistent with gender of rearing. 
After appropriate counseling, the discordant gonad and dysgenetic 
tissue can be removed to prevent unwanted androgenization in girls 
and estrogenization in boys. Sex hormone supplementation may be 
required for complete pubertal development and in adult life. 
46,XY Disorders of Sex Development
The 46,XY DSDs can be categorized as disorders of testis devel­
opment, disorders of androgen synthesis, disorders of androgen 
action, and othe

synthesis, disorders of androgen 
action, and other conditions affecting sex development (Table 
24.6; see also Fig. 24.7).
Disorders of Testis Development
Disorders of testis development can have a spectrum of pheno­
types and presentations. Complete testicular dysgenesis, a condition 
sometimes called Swyer syndrome, is associated with a complete 
lack of androgenization of the external genitalia and persistent 
müllerian structures due to insufficient AMH production. In con­
trast, partial go

fficient AMH production. In con­
trast, partial gonadal dysgenesis may be associated with a range of 
phenotypes, ranging from clitoromegaly to ambiguous or atypical 
genitalia to isolated hypospadias. A uterus or uterine remnant and 
vagina may be present. Subtle forms of testicular dysgenesis can 
present with testicular regression, a small penis, or male infertility 
in the absence of an external genital phenotype.
Several single-gene disorders have been described in patients 
with various de

s have been described in patients 
with various degrees of testicular dysgenesis. Table 24.6 summa­
rizes these factors, and the role of many of these factors in devel­
opment has already been discussed (see “Testis Determination”). 
Although associated features can sometimes help to direct genetic 
analysis, often there are no other findings, and a genetic diagnosis 
is currently reached in only about 20% to 40% of individuals with 
46,XY testicular dysgenesis.
Single-Gene Disorders
Steroidogen

ular dysgenesis.
Steroidogenic Factor 1: NR5A1. SF1 (encoded by NR5A1) is 
a member of the nuclear receptor superfamily that regulates the 
transcription of at least 30 genes involved in gonadal development, 
adrenal development, spleen development, steroidogenesis, and 
reproduction.59,230 NR5A1 mutations were first reported in two 
46,XY individuals with female external genitalia, persistent mül­
lerian structures, and primary adrenal failure.65,66 The first vari­
ant was

mary adrenal failure.65,66 The first vari­
ant was a de novo heterozygous p.Gly35Glu change in the P-box 
primary DNA-binding region of SF1; the second was a recessively 
inherited homozygous p.Arg92Gln mutation in the A-box sec­
ondary DNA-binding region (Fig. 24.20). These changes resulted 
891
  Overview of Important Genes Involved in DSDs
Gene
Protein
OMIM
Locus
Gonad
Müllerian 
Structures
External Genitalia
Associated Features/Variants
A

A.  Causes of 46,XY DSD
Disorders of Gonadal (Testicular) Development: Single-Gene Disorders
WT1
TF
607102
11p13
AD
Dysgenetic testis
±
Female, ambiguous 
or hypospadias
Wilms tumor, renal abnormalities, gonadal tumors 
(WAGR, Denys-Drash, and Frasier syndromes)
NR5A1 (SF1)
Nuclear 
receptor TF
184757
9q33.3
AD/AR (SLD)
Dysgenetic testis 
(variable)
±
Female, ambiguous, 
More severe phenotypes include primary adrenal fail­
ure; mild

phenotypes include primary adrenal fail­
ure; milder phenotypes have isolated partial gonadal 
dysgenesis or impaired androgenization or both
GATA4
TF
600576
8p23.1
AD (SLD)
−
or hypospadias/
micropenis
Cardiac defects (e.g., septal defects, tetralogy of Fallot)
ZFPM2 
(FOG2)
Coregulator
603693
8q23.1
AD
±
Female or ambigu­
ous
Congenital heart defects. Learning and language dif­
ficulties, autism spectrum disorder i

anguage dif­
ficulties, autism spectrum disorder in one individual.
CBX2
Polycomb 
protein
602770
17q25.3
AR
Ovary (one case)
+
Female
SRY
TF
480000
Yp11.3
Y
Dysgenetic testis or 
ovotestis
±
ous
SOX9
TF
608160
17q24-q25
AD
ovotestis
±
ous
Campomelic dysplasia (17q24 rearrangements have a 
milder phenotype than point mutations)
SOX8
TF
605923
16p13.3
AD, de novo
±
ous
16p deletion can have a

e)
±
ous
16p deletion can have additional features
MAP3K1
Signaling 
molecule
600982
5q11.2
AD
±
NR3A2 
(ESR2)
Nuclear 
601663
14q23.2
AD/AR
±
ous
Variable (anal atresia, blepharophimosis, dysmorphic 
features)
DMRT1
TF
602424
9p24.3
AD
+
Female
TSPYL1
? Chromatin 
remodeling
604714
6q22.1
AR
−
ous
Sud

1
AR
−
ous
Sudden infant death
DHH
Signaling 
molecule
605423
12q13.1
AR
Dysgenetic testis, 
testis (Leydig)
−
ous
Minifascicular neuropathy in several patients
SAMD9
Growth 
repressor
617053
7q21.2
AD, de novo
-
MIRAGE syndrome (myelodysplasia, infections, 
restriction of growth, adrenal hypoplasia, genital 
phenotypes, enteropathy)
ARX
TF
300382
Xp22.13
X
Dysgenetic testi

eropathy)
ARX
TF
300382
Xp22.13
X
(Leydig)
−
Ambiguous
X-linked lissencephaly, epilepsy, temperature instability
MAMLD1 
(CXORF6)
Unknown
300120
Xq28
X
Normal (Leydig cell 
dysfunction)
−
Hypospadias
TABLE 24.6
Continued
892
Disorders of Gonadal (Testicular) Development: Chromosomal Changes Involving Key Candidate Genesa
DMRT1
TF
602424
9p24.3
Monosomic 
deletion
±
ous
Mental retardation
ATRX
Hel

ous
ATRX
Helicase (? 
chromatin 
remodel­
ing)
300032
Xq13.3
X
−
or male
α-thalassemia, mental retardation
NR0B1 
(DAX1)
Nuclear 
300018
Xp21.3
dupXp21
ovary
±
ous
WNT4
Signaling 
molecule
603490
1p36.12
dup1p35
+
Ambiguous
Disorders in Hormone Synthesis or Action
DHCR7
Enzyme
602858
11q13.4
AR
Testis
−
Variable
Smith-Lemli-Opitz syn

11q13.4
AR
Testis
−
Variable
Smith-Lemli-Opitz syndrome: coarse facies, second-
third toe syndactyly, failure to thrive, developmental 
delay, cardiac and visceral abnormalities
LHCGR
G protein 
receptor
152790
2p16.3
AR
Testis
−
or micropenis
Leydig cell hypoplasia
STAR
Mitochondrial 
protein
600617
8p11.2
AR
Testis
−
Lipoid CAH (primary adrenal failure), pubertal failure
CYP11A1
Enzyme
118485
15q24.1
AR
Testis
−
Female or ambigu

Enzyme
118485
15q24.1
AR
Testis
−
ous
CAH (primary adrenal failure), pubertal failure
HSD3B2
Enzyme
201810
1p13.1
AR
Testis
−
Ambiguous
CAH, primary adrenal failure, ↑ Δ5 : Δ4 ratio
CYP17A1
Enzyme
202110
10q24.3
AR
Testis
−
CAH, hypertension due to DOC (except in isolated 
17,20-lyase deficiency)
POR (P450 
oxidore­
ductase)
CYP enzyme 
electron 
donor
124015
7q11.2
AR
Testis
−
Male or ambiguous
Mixed features of 21-hydroxylase deficiency, 
17α

17α-hydroxylase/17,20-lyase deficiency, and 
aromatase deficiency; sometimes associated with 
Antley-Bixler craniosynostosis
CYB5A
Cofactor
613218
18q22.3
AR
Testis
−
Ambiguous or hypo­
spadiac
Methemoglobinemia
AKR1C2 
(AKR1C4)
Enzyme
600450
10p15.1
AR (? 
digenic)
Testis
−
Variable
HSD17B3
Enzyme
605573
9q22.23
AR
Testis
−
ous
Partial androgenization at puberty, ↑ ratio of andro­
stenedione to testosterone
SRD5A2
Enzyme
607306
2p

SRD5A2
Enzyme
607306
2p23.1
AR
Testis
−
Ambiguous or 
Partial androgenization at puberty, ↑ ratio of testoster­
one to DHT
  Overview of Important Genes Involved in DSDs—cont’d
TABLE 24.6
Gene
Protein
OMIM
Locus
Gonad
Müllerian 
893
Androgen 
receptor 
(NR3C4)
Nuclear 
313700
Xq12
X
Testis
−
micropenis, or 
normal m

tis
−
normal male
Phenotypic spectrum from complete AIS (female exter­
nal genitalia) to partial AIS (ambiguous) to normal 
male genitalia/infertility
AMH
Signaling 
molecule
600957
19p13.3
AR
Testis
+
Persistent müllerian duct syndrome (PMDS)
AMH recep­
tor
Serine/threo­
nine kinase 
trans­
membrane 
receptor
600956
12q13.13
AR
Testis
+
Male external genitalia, bilateral cryptorchidism
B.  Causes of 46,XX DSD
Disorders of Gonadal (Ovar

Disorders of Gonadal (Ovarian) Development
SRY
TF
480000
Yp11.3
Testis or ovotestis
−
SOX9
TF
608160
17q24
dup17q24 or 
deletion of 
regulatory 
region
Not determined
−
SOX3
TF
313430
Xq27.1
dup Xq27 
or del of 
region
Testis (variable)
−
Male
Additional features if large duplication
Nuclear 
184757
9q33.3
AD or de 
novo 
(affecting 
codon 92)
-
NR2F

n 92)
-
NR2F2 
(COUP-
TFII)
Nuclear 
107773
15q26.2
AD or de 
novo
-
Cardiac defects, diaphragmatic hernia, BPES
RSPO1
Thrombospon­
din (Wnt 
signaling)
609595
1p34.3
AR
−
Male
Palmar-plantar hyperkeratosis, squamous cell carci­
noma
WNT4
Wnt signaling
611812
1p36.12
AR
−
SERKAL syndrome
HSD3B2
Enzyme
201810
1p13.1
AR
Ovary
+
Clitoromeg

HSD3B2
Enzyme
201810
1p13.1
AR
Ovary
+
Clitoromegaly (mild)
CAH, primary adrenal failure, partial androgenization 
due to ↑ conversion of DHEA
Continued
894
CYP21A2
Enzyme
201910
6p21.33
AR
Ovary
+
Ambiguous; rarely 
Prader V
CAH, phenotypic spectrum from severe salt-losing 
forms associated with adrenal failure to simple 
virilizing forms with compensated adrenal function, 
↑ 17-OHP
CYP11B1
Enzyme
202010
8q24.3
AR
Ovary
+
Prader 

02010
8q24.3
AR
Ovary
+
Prader V
CAH, hypertension due to ↑ 11-deoxycorticosterone
POR (P450 
oxidore­
ductase)
CYP enzyme 
electron 
donor
124015
7q11.23
AR
Ovary
+
Normal or ambigu­
ous
17α-hydroxylase/17,20-lyase deficiency and 
aromatase deficiency; associated with Antley-Bixler 
craniosynostosis
CYP19A1
Enzyme
107910
15q21.2
AR
Ovary
+
Ambiguous
Maternal androgenization during pregnancy, absent 
breast development at puberty e

pregnancy, absent 
breast development at puberty except in partial 
cases
Glucocor­
ticoid 
receptorb 
(NR3C1)
Nuclear 
138040
5q31.3
AR
Ovary
+
ous
↑ ACTH, 17-OHP, cortisol, mineralocorticoids, and 
androgens; failure of dexamethasone suppression
aChromosomal rearrangements likely to include key genes are included.
bNote: Patient heterozygous for a mutation in CYP21.
–, Absent; +, present; ACTH, adrenocorticotropic hormone; AD, autosomal dominant (often de novo mut

hormone; AD, autosomal dominant (often de novo mutation); AIS, androgen insensitivity syndrome; AR, autosomal recessive; BPES, blepharophimosis-ptosis-epicanthus inversus syndrome; CAH, congenital adrenal 
hyperplasia; CYP, cytochrome P450 enzyme; DHEA, dehydroepiandrosterone; DHT, dihydrotestosterone; DOC, 11-deoxycorticosterone; DSDs, disorders of sex development; 17-OHP, 17-hydroxyprogesterone; LHCGR, luteinizing hormone (LH) or human chorionic gonad­
otropin (hCG) receptor; OMIM, Online Mend

c gonad­
otropin (hCG) receptor; OMIM, Online Mendelian Inheritance in Man; SERKAL, sex-reversal with kidney, adrenal, and lung dysgenesis; SLD, sex-limited dominant; TF, transcription factor; WAGR, Wilms tumor, aniridia, genitourinary anomalies, and mental retardation.
Adapted from Achermann JC, Ozisik G, Meeks JJ, et al. Genetic causes of human reproductive disease. J Clin Endocrinol Metab. 2002;87:2447–2454, used with permission. © 2002 The Endocrine Society.
   
  Overview of Important Gene

docrine Society.
   
Gene
Protein
OMIM
Locus
Gonad
Müllerian 
TABLE 24.6
895
in impaired DNA binding. The combination of gonadal and adre­
nal phenotype is uncommon, and it initially seemed that disrup­
tion of SF1 would be a rare cause of DSD in humans.
However, the past decade has seen a different picture emerging 
as an ever-inc

 seen a different picture emerging 
as an ever-increasing number of monoallelic (heterozygous) non­
sense, frameshift, and missense mutations in NR5A1 have been 
associated with a spectrum of 46,XY DSD conditions in individu­
als with normal adrenal function (see Fig. 24.20).59,67,231,232 These 
changes usually result in haploinsufficiency of SF1. A range of 
phenotypes is seen, most commonly mild gonadal dysgenesis and 
significantly impaired androgenization, where alterations in SF1 
are found

drogenization, where alterations in SF1 
are found in approximately 15% of cases.59 These variations usu­
ally arise de novo but may be inherited from the mother in a sex-
limited dominant fashion (i.e., the mother carries the mutation 
but is unaffected) or even from the father and occasionally may be 
autosomal recessive.66 In some situations, there is a family history 
of ovarian insufficiency, and females in the family may be at risk of 
developing ovarian insufficiency in the future.68 Loss

loping ovarian insufficiency in the future.68 Loss of SF1 func­
tion is also reported in some boys with severe hypospadias and 
undescended testes (∼5%), bilateral anorchia, and a small penis 
and in men with infertility (∼2%).233–235 Oligogenic effects of 
other genes have also been proposed to influence phenotype.236,237 
Therefore variable loss of SF1 activity is associated primarily with 
different degrees of testicular or ovarian dysfunction in humans. 
Adrenal function may need to be monit

 in humans. 
Adrenal function may need to be monitored over time, but at 
present, adrenal insufficiency does not seem prevalent. In addition, 
splenic abnormalities have been described, ranging from asplenia 
to polysplenia.230,238 Although their prevalence among patients 
with NR5A1 mutations is currently unclear, assessment of spleen 
anatomy and function by ultrasound and search for Howell-Jolly 
bodies in blood has been recommended given the serious con­
sequences that (functional) asplenia

 serious con­
sequences that (functional) asplenia may have.238 Defining the 
molecular basis in different families can be important for coun­
seling, especially about the potential risk of ovarian insufficiency, 
Frameshift mutation
Nonsense mutation
46,XY DSD
46,XX Adrenal
insufficiency
46,XY Hypospadias
46,XX Primary ovarian
46,XY Adrenal
Male factor infertility
46,XX
Zn
I
G35E
C33S
C30W
Y25C
H24Y
R103Q
P97T
P131L
R191C
G212S
E237K
D238N
L245W
A154T
D257N
W279R
E30

191C
G212S
E237K
D238N
L245W
A154T
D257N
W279R
E304K
R313H
R313C
V355M
V424del
K372del
L437Q
R313H
D293N
R255C
E237K
P235L
P198L
A192F, A192S, A192V
P136L
S54R
R92W/Q
ovotestis
M1I
Y5D
G123AP129L
L231-L233del
R255L
G328V
I323T
R281P
G165R
D293N
R313C
C370W
K372del
L376F
N380Y
L231-L233dup
V20L
V15M
C13R
M1V
C65Y
C65Y
K63Q
R62S
R62C
G91S
G90R
R84H
R84C
M78I
V41G
R39P
R92Q
Zn
II
FtzF1
Box
Hinge region
Ligand binding domain
46,XY
1
461
AF2
• Fig. 24.20  Schematic diagram of steroidogen

AF2
• Fig. 24.20  Schematic diagram of steroidogenic factor 1 (SF1) shows key domains and mutations associ­
ated with a gonadal phenotype. The Gly35Glu (heterozygous) and Arg92Gln (homozygous) changes affect 
DNA-binding regions of the protein and are associated with marked underandrogenization, dysgenetic 
gonads, müllerian structures, and primary adrenal failure. Heterozygous frameshift (triangles) and non­
sense mutations (stars), as well as missense mutations, have been described in 46,XY in

issense mutations, have been described in 46,XY individuals with a 
spectrum of disorders of sex development (DSD) phenotypes (below the structure drawing) and in women 
with primary ovarian insufficiency (above the structure drawing). (From Suntharalingham JP, Buonocore F, 
Duncan A, et al. DAX-1 (NR0B1) and steroidogenic factor-1 (SF-1, NR5A1) in human disease. Best Pract 
Res Clin Endocrinol Metab. 2015;29:607–619. Based on Lin L, Philibert P, Ferraz-de-Souza B, et al. Het­
erozygous missense

 Ferraz-de-Souza B, et al. Het­
erozygous missense mutations in steroidogenic factor-1 [SF1/Ad4BP, NR5A1] are associated with 46,XY 
disorders of sex development with normal adrenal function. J Clin Endocrinol Metab. 2007;92:991–999, 
used with permission of The Endocrine Society, Copyright 2007.)
896
and for identifying those males who might need surveillance for 
potential androgen insufficiency or a decline in fertility with age. 
NR5A1 mutations 

a decline in fertility with age. 
NR5A1 mutations can also result in ovotesticular DSD (see “Ovo­
testicular Disorders of Sex Development”). 
Wilms Tumor 1 Gene: WT1. WT1 (11p13) is a four–zinc fin­
ger transcription factor expressed in the developing genital ridge, 
kidney, gonads, and mesothelium.49 The WT1 protein has several 
different isoforms that have complex roles in sex development, 
as outlined previously and shown in Fig. 24.21. The important 
role of WT1 in human testis development h

portant 
role of WT1 in human testis development has been confirmed 
through the description of various WT1 mutations in patients 
with WAGR syndrome, Denys-Drash syndrome, and Frasier 
syndrome.
WAGR syndrome is caused by deletion of a region of chro­
mosome 11p13.239 The resultant phenotype is likely to be the 
consequence of haploinsufficiency of WT1 together with loss 
of developmental genes such as PAX6, which is involved in eye 
development.55 Renal abnormalities include childhood Wilms 
t

.55 Renal abnormalities include childhood Wilms 
tumor in about half of children; renal agenesis has been reported, 
and renal dysfunction often occurs after adolescence. Genitouri­
nary abnormalities are usually relatively mild and include bilat­
eral cryptorchidism, micropenis, and occasionally hypospadias. 
Careful ophthalmic support is needed for the aniridia or iridic 
hypoplasia, and cataracts or corneal clouding can occur. Develop­
mental delay can occur with larger chromosomal deletions,

delay can occur with larger chromosomal deletions, and 
obesity may be a feature if the deletion is large and includes the 
BDNF gene (WAGRO syndrome).
Denys-Drash syndrome is characterized by gonadal dysgen­
esis, severe congenital or early-onset nephropathy (i.e., diffuse 
mesangial sclerosis), and predisposition to Wilms tumor.56 Most 
46,XY patients with Denys-Drash syndrome present with geni­
tal ambiguity or severe penoscrotal hypospadias in the newborn 
period, although typical male or fe

n the newborn 
period, although typical male or female genital appearance has 
been described. The presence or absence of müllerian structures 
depends on the degree of Sertoli cell dysfunction. Denys-Drash 
syndrome usually results from heterozygous de novo point muta­
tions in WT1 that have a dominant negative effect on the function 
of the wild-type protein. These point mutations usually affect the 
DNA-binding region (zinc fingers) of WT1. The risk of early-
onset renal failure is high, and 

e risk of early-
onset renal failure is high, and Wilms tumor usually develops in 
the first few years of life. There is an increased risk of germ cell 
tumors.212 The prevalence of WT1 mutations in boys with peno­
scrotal hypospadias and undescended testes has been reported to 
be as high as 7%.240
Frasier syndrome usually results from heterozygous muta­
tions in the donor splice site of exon 9 of WT1.57,58 These 
changes are predicted to result in an imbalance in the ratio of 
+KTS to –KTS iso

 in an imbalance in the ratio of 
+KTS to –KTS isoforms of WT1. Frasier syndrome is typi­
cally characterized by streak gonads, a 46,XY female phenotype 
with müllerian structures, and later-onset nephropathy (i.e., 
focal segmental glomerulosclerosis) that usually progresses to 
renal failure in the second decade of life. There is a high risk 
of gonadal tumors (gonadoblastoma and dysgerminoma) in 
patients with Frasier syndrome.241 In practice, Denys-Drash 
syndrome and Frasier syndrome may re

 Denys-Drash 
syndrome and Frasier syndrome may represent a continuum 
of phenotypes rather than distinct conditions.242 Severe forms 
of these conditions can present with congenital nephrotic syn­
drome, whereas milder variants have been reported, for exam­
ple, in a man with hypospadias and late-onset nephropathy.243 
Taken together, these cases highlight the importance of con­
sidering this diagnosis in 46,XY DSD and of performing uri­
nalysis for proteinuria in children with 46,XY DSD. Meach

 for proteinuria in children with 46,XY DSD. Meacham 
syndrome (i.e., DSD, cardiac defects, and diaphragmatic her­
nia) may also result from changes in WT1.244
Management of patients with WT1 mutations includes moni­
toring and treatment of renal function and assessment for Wilms 
tumor, as well as gonadectomy in individuals with Frasier syn­
drome and in patients with Denys-Drash syndrome who have a Y 
chromosome if the gonads cannot be monitored.22 
Gata-Binding Protein 4: GATA4. GATA4 is a fo

d.22 
Gata-Binding Protein 4: GATA4. GATA4 is a four–zinc fin­
ger transcription factor involved in early testis development and 
heart development. Haploinsufficiency of GATA4 or heterozygous 
loss-of-function changes are well established in a range of cardiac 
defects such as atrial and ventricular septal defects. More recently, 
heterozygous GATA4 mutations have been shown to cause tes­
ticular dysgenesis with or without cardiac anomalies.245 These 
mutations can be transmitted by a mother in

These 
mutations can be transmitted by a mother in a sex-limited domi­
nant manner. Reported changes may disrupt interactions between 
GATA4 and its cofactor, friend of GATA 2 (FOG2, also known 
as ZFPM2). 
Friend of GATA2 (FOG2): ZFPM2. FOG2 is a key coregulator 
of GATA4, and missense mutations in the gene encoding FOG2 
(ZFPM2) have been described in individuals with 46,XY gonadal 
dysgenesis.166,246,247 The point mutations affect interactions with 
GATA4, and some have been described previou

 with 
GATA4, and some have been described previously in patients with 
cardiac defects, highlighting the variable penetrance of gonadal 
and cardiac phenotypes following disruption of the GATA4/
FOG2 complex. 
Proline-glutamine–rich
trans-regulatory region
Alternative splice 1
(± 17 amino acids)
Alternative splice 2
(± KTS)
Zinc finger DNA binding domain
Zn-1
Zn
C
C
H
H
KTS
7
Zn-2
Zn
C
C
H
H
8
Zn-3
Zn
C
C
H
H
9
Zn-4
Zn
C
C
H
H
10
6
5
4
3
2
1
Exons
• Fig. 24.21  Schematic diagram shows the struc

ns
• Fig. 24.21  Schematic diagram shows the structure of WT1 and the changes associated with exon 5 and 
exon 9 (addition of lysine, threonine, and serine) in +KTS isoforms. Many point mutations associated with 
Denys-Drash syndrome are located within zinc fingers 2 and 3 (especially Arg394). Mutations affecting the 
exon 9 splice site are associated with Frasier syndrome. (Modified from Koziell A, Grundy R. Frasier and 
Denys-Drash syndromes: different disorders or part of a spectrum? Arch Dis

ifferent disorders or part of a spectrum? Arch Dis Child. 1999;81:365–369, 
used with permission of the BMJ Publishing Group, Copyright 1999.)
897
Chromobox Homolog 2: CBX2. Chromobox homolog 2 
(CBX2) is a human homolog of the polycomb protein M33. Dele­
tion of its gene causes XY sex reversal in mice. Loss-of-function 
mutations in CBX2 were described in a 46,XY girl with a uterus 
and ovaries who was diagnosed by prenatal karyotyping, but at 
present, 

agnosed by prenatal karyotyping, but at 
present, CBX2 variants in humans are rare.48 
Sex-Determining Region of the Y Chromosome: SRY. The 
sequence of events leading to the identification of SRY as 
the primary testis-determining gene and the actions of SRY 
in testis development were described earlier. SRY is a 204–
amino acid HMG box transcription factor that is encoded 
by a single exon on the Y chromosome (Yp11.3) (see Fig. 
24.5).87 The discovery of inactivating mutations in SRY in 
patie

scovery of inactivating mutations in SRY in 
patients with 46,XY gonadal dysgenesis confirmed the key 
role played by this factor in testis determination in humans.31
Approximately 10% of individuals with the complete form of 
46,XY gonadal dysgenesis have inactivating mutations in SRY.87 
Most of the mutations occur in the HMG box DNA-binding 
domain of the SRY protein (Fig. 24.22), a region that is involved 
in binding and bending of DNA.87 Rare mutations in the 5′ and 
3′ flanking regions occ

e mutations in the 5′ and 
3′ flanking regions occur.248 The HMG box contains at least two 
nuclear localization signals that bind calmodulin/exportin 4 and 
importin B. Mutations in these nuclear localization signal domains 
in the HMG box of SRY result in failure to transport the SRY 
protein into the nucleus, and subtle alterations in nuclear import 
and export can lead to XY gonadal dysgenesis.90 Most individuals 
with SRY mutations are girls who have no pubertal development 
(Swyer syndrome

 who have no pubertal development 
(Swyer syndrome). Testicular tumor risk in this group is high. 
SRY Box 9: SOX9. Heterozygous mutations of the autosomal 
SOX9 gene (17q24-q25) cause campomelic dysplasia.99,100 Fea­
tures of this condition include bowed long bones, hypoplastic 
scapulae, a deformed pelvis, 11 pairs of ribs, a small thoracic cage, 
cleft palate, macrocephaly, micrognathia, hypertelorism, and a 
variety of cardiac and renal defects. Death from respiratory dis­
tress often occurs

ts. Death from respiratory dis­
tress often occurs in the neonatal period, but long-term survival 
has been reported.
NLS
CaM
DNA-binding
HMG box
Section 58–137
1
A
B
58
137
204
COOH
NH2
58
137
SiP1
binding
NLS
imp-β
SRY
NLS
CaM
HMG box
1
103
144
182
229
103
182
Intron 1
Intron 2
402
509
COOH
NH2
HSP70 binding
domain
NLS
impβ
SOX9
• Fig. 24.22  Structure of human SRY and SOX9 proteins and a selection of reported mutations. (A) Dia­
gram of SRY. The high-mobility group

ons. (A) Dia­
gram of SRY. The high-mobility group (HMG) box is an 80–amino acid, DNA-binding domain with nuclear 
localization signals (NLS) at either end, one of which binds calmodulin (CaM) or exportin-4 and the other 
importin-β (imp-β). The last seven amino acids of SRY can bind to either of the PDZ domains found in 
SRY-interacting protein 1 (SiP1). The solid circles indicate missense mutations reported in the SRY protein 
that affect testicular development and cluster within the HMG box. 

cular development and cluster within the HMG box. Nonsense and frameshift mutations in 
SRY are indicated by solid triangles. (B) Diagram of SOX9. SOX9 has an HMG box with two NLSs, similar 
to SRY. However, SOX9 is encoded by three exons, binds to heat shock protein 70 (HSP70), and has a 
transactivation domain at the carboxy-terminal end, unlike SRY. Selected mutations causing 46,XY DSD 
and campomelic dysplasia are indicated by the solid circles (missense) and solid triangles (nonsense 
and f

es (missense) and solid triangles (nonsense 
and frameshift). Mutations that cause only campomelia or a bony phenotype in 46,XY males or affect 
46,XX females are indicated by open triangles.
898
SOX9 is an important testis-determining gene, and it is the key 
regulator of testis determination downstream of SRY (see “Testis 
Determination”). Consistent with this hypothesis, three-fourths 
of 46,XY patients with heterozygous SOX9 mutations have dysge­

ients with heterozygous SOX9 mutations have dysge­
netic gonads, but a wide range of genital phenotypes can be seen, 
from typically male to typically female appearance.87 Histologic 
examination of the gonads from 46,XY patients with ambigu­
ous or female external genitalia shows a similarly wide range of 
gonadal phenotypes, including streak gonads, dysgenetic testes, 
ovotestes, or even ovaries.223 Müllerian structures may or may not 
be present, depending on the degree of gonadal dysgenesis.

nt, depending on the degree of gonadal dysgenesis. 46,XX 
females with SOX9 mutations have normal external genitalia and 
apparently normal ovaries.
The locus for campomelic dysplasia with 46,XY DSD was 
mapped to 17q24.3-q25.1 after studies of three patients with bal­
anced de novo reciprocal translocations, and SOX9 was proposed 
as a candidate gene based on expression studies in the mouse. 
Subsequently, missense, nonsense, frameshift, and splice junction 
mutations have been detected in SOX9

ice junction 
mutations have been detected in SOX9 in patients with campo­
melic dysplasia with or without gonadal dysgenesis.87 These muta­
tions are usually heterozygous de novo changes. In one kindred, 
multiple siblings were affected due to germline mosaicism for a 
SOX9 mutation in a parent.223 The gonadal phenotype in this kin­
dred varied in the two affected 46,XY siblings: One of them had 
dysgenetic gonads; the other was reported to have normal ovaries.
The SOX9 gene has three exons and

 normal ovaries.
The SOX9 gene has three exons and two introns; it encodes a 
509-residue protein that contains an HMG box with 71% homol­
ogy to that of the SRY protein and a C-terminal, trans-activation 
domain (see Fig. 24.22). Unlike SRY, in which most mutations 
are located within the HMG box, SOX9 mutations are located 
throughout the protein, with little relation between functional 
domains and phenotype. Chromosomal translocations that dis­
rupt regulatory elements upstream of the SOX9 p

s­
rupt regulatory elements upstream of the SOX9 promoter can be 
associated with a less severe phenotype or campomelic dysplasia 
without gonadal abnormalities, or 46,XY DSD without a skeletal 
phenotype. Heterozygous changes that allow residual DNA bind­
ing and transactivation may also be associated with the acampo­
melic variant and variable or absent DSD. 
SRY Box 8: SOX8. SOX8 (16p13.3) is closely related to SRY 
and SOX9 and is coexpressed with NR5A1 and SOX9 in Sertoli 
and Leydig cells 

 with NR5A1 and SOX9 in Sertoli 
and Leydig cells during the early stages of human testis determi­
nation.249 Deletions and point mutations in SOX8 have recently 
been described in individuals with testicular dysgenesis, sometimes 
as part of a chromosome 16p-deletion syndrome. An enrichment 
of rare variants in SOX8 has also been found in individuals with a 
spectrum of reproductive phenotypes, including oligozoospermia 
and primary ovarian insufficiency.249 
Mitogen-Activated Protein Kinase Ki

fficiency.249 
Mitogen-Activated Protein Kinase Kinase Kinase-1: MAP3K1. 
MAP3K1 is one of several kinase signaling pathways involved in 
organogenesis, and heterozygous splice site or point mutations in 
MAP3K1 have been described in several families and individu­
als with impaired testis development.97 MAP3K1 is expressed 
in early testis cords and Sertoli cells in mice, and disruption of 
MAP3K1 has been described in diverse phenotypes ranging from 
46,XY complete testicular dysgenesis to hyp

 from 
46,XY complete testicular dysgenesis to hypospadias and micro­
penis with cryptorchidism. Familial cases have a clear autosomal 
dominant inheritance. MAP3K1 mutations may be one of the 
more common causes of 46,XY DSD, and routine screening has 
been recommended in all patients with 46,XY complete or partial 
gonadal dysgenesis.225 
Estrogen Receptor β: ESR2/NR3A2. A homozygous ESR2 
mutation has been reported in an individual with 46,XY DSD 
characterized by absence of gonads, uterus, a

DSD 
characterized by absence of gonads, uterus, and vagina, as well as 
dysmorphic features, eye abnormalities, and anal atresia; hetero­
zygous mutations were identified in two individuals with 46,XY 
complete and partial gonadal dysgenesis without syndromic fea­
tures.250 It was hypothesized that the ESR2 variants may lead to 
increased activity of the MAPK pathway through nonclassic sig­
naling, thereby altering gonadal fate. 
Doublesex-Related and Mab3-Related Transcription Factor 
1: DMRT1

ed and Mab3-Related Transcription Factor 
1: DMRT1. DMRT1, a gene with sex-specific homologs in Dro­
sophila (doublesex gene) and Caenorhabditis elegans (Mab3) that is 
expressed in early gonad development, plays an important role in 
supporting testis development.112,251 Deletions of the region of 
chromosome 9p24-pter containing DMRT1 are associated with 
46,XY gonadal dysgenesis.252 More recently, a partial deletion of 
DMRT1 and a dominant-negative point mutation in DMRT1 
have been seen in 

gative point mutation in DMRT1 
have been seen in patients with 46,XY DSD.253,254 
Testis-Specific Protein, Y-Linked–Like 1 Gene: TSPYL1. An asso­
ciation between 46,XY gonadal dysgenesis and sudden infant death 
syndrome was characterized in a large Amish kindred and named sud­
den infant death, dysgenetic testes (SIDDT).255 The gene responsible 
for this autosomal recessive condition, TSPYL1, encodes a protein of 
unknown function that may be involved in chromatin remodeling. 
Additional varia

nvolved in chromatin remodeling. 
Additional variants of TSPYL1 have been described in patients with 
46,XY DSD.256 
Desert Hedgehog: DHH. The hedgehog signaling pathways 
play an important role in many aspects of neuronal, skeletal, and 
endocrine development. A homozygous mutation in the desert 
hedgehog gene (DHH) was originally reported in a patient with 
partial gonadal dysgenesis and minifascicular neuropathy.116 Sub­
sequently, a number of DHH changes have been found in patients 
with com

 DHH changes have been found in patients 
with complete 46,XY gonadal dysgenesis or more often Leydig cell 
defects, with or without apparent neurologic features.117,166 
Sterile α-Motif Domain-Containing Protein 9: SAMD9. 
SAMD9 is an endosomal protein that negatively regulates growth 
and development. Gain-of-function variants in SAMD9 have 
been reported to cause impaired testicular function and 46,XY 
DSD as part of a complex multisystem growth restriction disor­
der, MIRAGE syndrome (myelod

th restriction disor­
der, MIRAGE syndrome (myelodysplasia, infections, restriction 
of growth, adrenal hypoplasia, genital phenotypes, enteropa­
thy).257,258 Children are often born preterm and have high mor­
tality. Somatic rescue due to monosomy 7 or secondary somatic 
loss-of-function changes in SAMD9 itself can modify the pheno­
type, but monosomy can be associated with the development of 
myelodysplastic syndrome. The testicular phenotype most likely 
represents a Leydig cell defect.258 
A

ost likely 
Aristaless-Related Homeobox, X-Linked Gene: ARX. ARX is 
a transcription factor that plays a central role in neuronal migra­
tion, and the Arx knockout mouse has a profound myelination 
defect. Mutations in ARX have been found in X-linked lissen­
cephaly ambiguous genitalia (XLAG) syndrome.114 This unusual 
form of lissencephaly is associated with severe epilepsy and ther­
mal instability. The genital abnormality most likely represents a 
defect i

tal abnormality most likely represents a 
defect in Leydig cell function. Additional ARX mutations have 
been described in patients with neurologic defects (e.g., infantile 
spasms) without significant DSDs. 
Mastermind-Like 
Domain-Containing 
1: 
MAMLD1. 
MAMLD1 (formerly called CXORF6) is a gene on the X chromo­
some that encodes a protein expressed in the developing testes. 
Hemizygous mutations in MAMLD1 were originally described 
in boys with isolated severe hypospadias, but a range of phe

th isolated severe hypospadias, but a range of pheno­
types has since been described, although it is unclear whether the 
reported MAMLD1 mutations suffice to explain the phenotype in 
899
all individuals.118,259,260 MAMLD1 disruption results in a defect 
in fetal Leydig cell development and function. 
Chromosomal Rearrangements Associated With Gonadal 
Dysgenesis
Abnormalities of genital development are associated with a num­
ber of chromosomal deletions

ssociated with a num­
ber of chromosomal deletions, duplications, and rearrangements. 
The most frequently seen changes are deletions of 9p24-pter 
(affecting DMRT1, described earlier), 10q25-qter, and Xq13 and 
duplications of Xp21.
Terminal deletions of chromosome 10 (10q25-qter) are fre­
quently associated with urogenital abnormalities and sometimes 
with complete gonadal dysgenesis.261 The gene in this locus has 
not been identified; FGFR2 is a candidate. Deletions of Xq13.3 
and of the tip 

 a candidate. Deletions of Xq13.3 
and of the tip of chromosome 16p cause α-thalassemia mental 
retardation (ATR) syndromes that may have gonadal dysgen­
esis as part of the phenotype.262 The Xq13.3 locus contains the 
transcription factor gene ATRX (also known as XH2 or XNP). 
The SOX8 gene is located on 16p, and rearrangements of this 
locus have been implicated in a variety of conditions with 
impaired gonadal development and/or function (see previous 
discussion).249,263
Duplications of the 

previous 
Duplications of the Xp21.3 region that contains the DAX1 
(NR0B1) gene can cause 46,XY partial or complete gonadal dys­
genesis.264 The role of DAX1 and the WNT4 pathway (duplication 
1p35) in opposing testis development was discussed previously 
(see “Development of Reproductive Systems”).264,265
Several other studies using array comparative genomic hybrid­
ization or single-nucleotide polymorphism (SNP) analysis have 
been reported and have proposed other candidat

ve 
been reported and have proposed other candidate regions for tes­
ticular dysgenesis.266,267 
Syndromic Causes of 46,XY Disorders of Sex Development
In addition to the specific syndromes outlined earlier, various 
degrees of testicular dysgenesis and impaired genital develop­
ment (e.g., hypospadias, cryptorchidism, scrotal transposition) 
are seen in many discrete syndromes.268,269 In some situations, 
a genetic basis has been identified, but in many cases the cause is 
unknown. Syndromic as

 in many cases the cause is 
unknown. Syndromic associations of 46,XY DSD may be more 
prevalent than originally thought and have been reviewed in detail 
elsewhere.268,269
46,XY DSD is often associated with intrauterine growth 
restriction (IUGR).270 Monozygotic twins can show disparate 
genital development, with the growth-restricted twin having 
ambiguous genitalia and the larger twin appearing as a normal 
male. The mechanism of this association is unclear and may rep­
resent a shared geneti

ion is unclear and may rep­
resent a shared genetic cause or a common epigenetic or somatic 
developmental event affecting fetal growth, placental function, 
and reproductive development. More common genetic causes of 
46,XY DSD (e.g., mutations of SRY, SF1, androgen receptor, and 
steroidogenic enzymes) are rarely found in this group of IUGR 
patients (personal observation). 
Genes Involved in Central Hypogonadism
Central causes of hypogonadism resulting in LH and FSH insuf­
ficiency do not typ

resulting in LH and FSH insuf­
ficiency do not typically cause hypospadias, as the early stages of 
fetal testis steroidogenesis are autonomous or driven by hCG. 
However, boys can have combined central defects together with 
hypospadias or DSD, and potential disease-causing variants in 
several genes have been reported, which may affect the HPG axis 
at multiple levels (e.g., CHD7, ANOS1/KAL1, WDR11, PROK2, 
PROKR2, FGF8, and FGFR1).166,167 
Potential Novel Genes and Oligogenic Effects
Whole-ex

ential Novel Genes and Oligogenic Effects
Whole-exome array and next-generation sequencing approaches 
have identified potential pathogenic variants in genes related to 
testis development (e.g., WWOX), but the significance of some of 
these findings is not yet clear. Furthermore, data emerging from 
these approaches suggest that oligogenic effects involving changes 
in more than one gene may also be associated with gonadal dys­
genesis or may influence the severity of the phenotype in some 
sit

fluence the severity of the phenotype in some 
situations.166,236,237 
Disorders of Androgen Synthesis
Defects anywhere along the pathway of androgen synthesis and 
target organ action can result in impaired androgenization and 
46,XY DSD (formerly referred to as male pseudohermaphrodit­
ism) (see Table 24.6 and Fig. 24.11). Because many of the steps of 
androgen synthesis overlap with those of glucocorticoid synthesis, 
some of the enzymatic deficiencies that cause androgen deficiency 
also cau

iciencies that cause androgen deficiency 
also cause congenital adrenal hyperplasia. In all of these condi­
tions, because Sertoli cell secretion of AMH is intact, müllerian 
structures are absent.
Cholesterol Synthesis Defects: Smith-Lemli-Opitz Syndrome
Smith-Lemli-Opitz syndrome is caused by a deficiency of 
7-dehydrocholesterol reductase (3β-hydroxysterol Δ7-reductase 
[DHCR7]), the phylogenetically conserved sterol-sensing 
domain–containing enzyme required for the last step in the 
biosynt

 enzyme required for the last step in the 
biosynthetic pathway from acetate to cholesterol. Cholesterol is 
necessary as a substrate for steroid-hormone synthesis, and inter­
mediates of cholesterol synthesis may also have important interac­
tions with hedgehog signaling pathways.
Smith-Lemli-Opitz syndrome has a broad phenotypic spec­
trum that typically includes microcephaly, developmental delay, 
cardiac defects, ptosis, upturned nose, micrognathia, cleft palate, 
polydactyly, syndactyly of 

nathia, cleft palate, 
polydactyly, syndactyly of toes (especially the second and third 
toes), severe hypospadias, micropenis, and growth failure.271 
Abnormalities of the external genitalia occur in approximately 
65% of 46,XY patients and vary from micropenis and hypospa­
dias to complete absence of androgenization, resulting in a female 
phenotype.
The syndrome is diagnosed by finding elevated plasma levels of 
7-dehydrocholesterol (7-DHC) and low levels of cholesterol. The 
DHCR7 gene maps 

d low levels of cholesterol. The 
DHCR7 gene maps to 11q12-q13, and more than 150 mutations 
have been described.272 Measurement of serum 7-DHC should 
be considered in all underandrogenized males with relevant phe­
notypic features, however mild. Testis development is apparently 
normal, and normal, elevated, or low concentrations of plasma 
testosterone have been described in affected male infants with 
intact HPG function. Compromised adrenal function can some­
times occur. 
Luteinizing Hormo

function can some­
Luteinizing Hormone Receptor Mutations
Mutations in the LH/hCG receptor cause impaired responsiveness 
to hCG and LH, resulting in Leydig cell agenesis or hypoplasia.273 
Phenotypically, the external genitalia vary from a female appear­
ance to a male with micropenis (Table 24.7). Rudimentary wolff­
ian derivatives may be present, even in some patients with severely 
underandrogenized external genitalia. This finding may reflect 
some early hCG-independent mechan

ing may reflect 
some early hCG-independent mechanisms of testosterone syn­
thesis between 8 and 10 weeks of gestation. Small, undescended 
testes are usually found in the inguinal region in the most severe 
forms of this Leydig cell hypoplasia. Patients with milder pheno­
types may have appropriately descended testes of relatively normal 
900
size because the Leydig cell population contributes to only about 
10% of testicular volume. On histologic e

y about 
10% of testicular volume. On histologic examination, the testes 
lack distinct Leydig cells in prepubertal patients, though Leydig 
cells may be difficult to visualize even in normal prepubertal testes. 
Postpubertal patients have absent or decreased numbers of Leydig 
cells without Reinke crystalloids, normal-appearing Sertoli cells, 
and discrete seminiferous tubules with spermatogenic arrest. This 
observation highlights the important role of intratesticular testos­
terone in the fin

 role of intratesticular testos­
terone in the final stages of sperm maturation.
The typical biochemical profile of patients with Leydig cell 
hypoplasia includes elevated basal and GnRH-stimulated LH 
(and FSH) levels in early infancy or at puberty due to loss of sex 
steroid negative feedback. In childhood, when the GnRH pulse 
generator is relatively quiescent, basal LH levels may still some­
times be detected above the normal range. Plasma levels of andro­
stenedione and testosterone are low

vels of andro­
stenedione and testosterone are low, with little or no response to 
prolonged hCG stimulation. Plasma LH falls after testosterone 
administration. Less marked biochemical changes can occur with 
milder forms of this condition.
More than 30 different homozygous or compound hetero­
zygous mutations have been reported in the LH/hCG receptor 
gene (LHCGR) in individuals with various forms of this condi­
tion (Fig. 24.23).274,275 The original reports of Kremer and col­
leagues274 and L

iginal reports of Kremer and col­
leagues274 and Latronico and associates275 described homozygous 
  Clinical Features of Leydig Cell 
Hypoplasia in 46,XY Individuals
Karyotype
46,XY
Autosomal recessive; mutations in LHCGR gene
Genitalia
Female, hypospadias, or micropenis
Wolffian duct deriva­
tives
Müllerian duct 
derivatives
Absent
Gonads
Testes
Biochemical and 
physiologic features
Underandrogenization with variable insuffi­
ciency of sex hormone production at puberty

suffi­
Hormone profile
Low T and DHT; elevated LH (and FSH); exag­
gerated LH response to LHRH stimulation; 
poor T and DHT response to hCG stimulation
DHT, Dihydrotestosterone; FSH, follicle-stimulating hormone; hCG, human chorionic gonado­
tropin; LH, luteinizing hormone; LHCGR, LH or hCG receptor; LHRH, luteinizing hormone–
releasing hormone; T, testosterone.
   
TABLE 24.7
Insert LLKLLLLLQ
Cys131Arg
Phe194Val
Glu354Lys
Cys343Ser
Trp491X
Ala593Pro
∆

Phe194Val
Glu354Lys
Cys343Ser
Trp491X
Ala593Pro
∆Leu608/Val609
Cys545X
Arg554X
Cys543Arg
COOH
Val144Phe
Ile152Thr
Ile374Thr
Thr392Ile
Leu502Pro
Ile625Lys
Ser616Tyr
Deletion of:
Exon 8
Exon 10
NH2
• Fig. 24.23  Diagram of the luteinizing hormone/human chorionic gonadotropin (LH/hCG) receptor with 
selected inactivating missense mutations shown by squares. Most of these changes are associated with 
marked underandrogenization. However, the changes at residues 616 and 625 in the seventh transmem­

 at residues 616 and 625 in the seventh transmem­
brane domain are associated with a milder phenotype of micropenis.
901
Ala593Pro and Arg554Ter mutations, respectively, in 46,XY phe­
notypic females with Leydig cell hypoplasia, hypergonadotropic 
hypogonadism, and no testosterone response to hCG stimulation. 
An affected 46,XX sister showed normal sexual maturation at 
puberty but an elevated LH level and amenorrhea, demonstrating 
that the LH receptor i

 amenorrhea, demonstrating 
that the LH receptor is not necessary for estrogen synthesis but is 
necessary for normal ovulation in females. These mutations impair 
hCG stimulation of intracellular cyclic adenosine monophosphate 
(cAMP) in vitro through disturbances in hCG binding, intracellu­
lar signaling, and/or receptor stability and trafficking, depending 
on the nature of the change. Defects in a cryptic exon of the LH 
receptor (exon 6A) have also been described as a cause of 46,XY 
DSD.27

ve also been described as a cause of 46,XY 
DSD.276 Partial loss-of-function mutations in the LH/hCG recep­
tor causing milder phenotypes such as micropenis have tended to 
localize within the seventh transmembrane domain (Ser616Tyr, 
Ile625Lys) (see Fig. 24.23).275,277 Individuals with complete Ley­
dig cell hypoplasia are usually reared as females and require estro­
gens at puberty. Gonadectomy is usually performed, though the 
risk of gonadal malignancy is unknown. If a male sex designation 

malignancy is unknown. If a male sex designation 
is chosen, testosterone supplements may be given in early infancy 
and to support puberty. 
Steroidogenic Acute Regulatory Protein Defects
Steroidogenic acute regulatory protein is a 30-kDa mitochondrial 
protein that is present in the adrenal gland and gonads. It plays 
a key role in facilitating the rapid movement of cholesterol from 
the outer to the inner mitochondrial membrane.131,278 This pro­
cess is necessary to allow de novo steroid bios

o­
cess is necessary to allow de novo steroid biosynthesis in response 
to ACTH or angiotensin II in the adrenal gland or an LH pulse 
in the gonad. Although a limited amount of cholesterol transfer 
is StAR independent (14%), this protein plays a central role in 
the acute regulation of adrenal and gonadal steroidogenesis. StAR 
is not necessary for placental progesterone production, unlike 
P450scc (CYP11A1, discussed next).
Consistent with these actions, patients with recessively inher­
ited 

se actions, patients with recessively inher­
ited defects in StAR develop a severe form of primary adrenal fail­
ure called lipoid congenital adrenal hyperplasia (lipoid CAH).279,280 
Patients with this condition tend to present with severe glucocor­
ticoid deficiency (e.g., hypoglycemia, hyperpigmentation) early 
in life and progressive mineralocorticoid insufficiency resulting 
in hyponatremia, hyperkalemia, dehydration, acidosis, and col­
lapse (Table 24.8). Presentation is often between 3 we

e (Table 24.8). Presentation is often between 3 weeks and 3 
months of age. Little or no C18, C19, and C21 steroids (which 
include estrogens, androgens, and progesterone/glucocorticoids/
mineralocorticoids, respectively) are detectable in plasma or urine, 
even after corticotropin or hCG stimulation. Females (46,XX) 
with lipoid CAH have typical female genitalia. In 46,XY indi­
viduals, mutations in STAR typically cause a marked deficiency in 
testosterone synthesis by fetal Leydig cells so tha

estosterone synthesis by fetal Leydig cells so that typical female 
genitalia are seen. Testes may be abdominal, inguinal, or labial. 
Müllerian structures have regressed, and there is a blind vaginal 
pouch. A karyotype should be performed in all apparent girls pre­
senting with early-onset adrenal failure.
The typical histologic finding in lipoid CAH is lipid accumula­
tion within steroidogenic cells. In the steroid-deficient state, the 
tropic drive by ACTH, angiotensin II, and LH causes incr

 drive by ACTH, angiotensin II, and LH causes increased 
cholesterol uptake and synthesis by steroidogenic cells. Coupled 
with the inability of StAR to facilitate cholesterol movement into 
mitochondria, this leads to marked accumulation of cholesterol in 
cells and results in the appearance of enlarged, lipid-laden adrenal 
glands sometimes seen on MRI or computed tomography (CT). 
Eventually, cholesterol accumulation causes engorgement and 
results in structural disruption superimposed on the

sults in structural disruption superimposed on the functional 
disruption—the “two-hit” hypothesis (Fig. 24.24).280
The two-hit hypothesis explains why 46,XX girls with lipoid 
CAH show evidence of estrogenization and breast development 
at puberty but have progressive hypergonadotropic hypogonad­
ism.280,281 Follicular cells are relatively quiescent in utero and 
before puberty and are therefore undamaged. They are recruited 
at the beginning of each cycle, and a small amount of estradiol 
can 

 each cycle, and a small amount of estradiol 
can be produced as a result of StAR-independent mechanisms. 
This can occur until the follicular cells are engorged and rendered 
nonfunctional. Puberty can occur, but any cycles are anovulatory. 
Without treatment, polycystic ovaries and progressive ovarian 
insufficiency failure usually ensue.
Although more than 40 different StAR mutations have been 
described in patients from around the world, lipoid CAH is espe­
cially prevalent in Japan and Kore

id CAH is espe­
cially prevalent in Japan and Korea, where it is the second-most 
common steroidogenic disorder after CYP21 deficiency (Fig. 
24.25). Most Japanese patients and virtually all Korean patients 
harbor the Gln258Stop mutation, which is estimated to be car­
ried by 1 in 300 Japanese people.282 Other geographic clusters 
include the Leu260Pro mutation in patients of Swiss ancestry, 
Arg182His in Eastern Saudi Arabia, and Arg182Leu in Palestin­
ians. Most of these mutations result in c

alestin­
ians. Most of these mutations result in complete loss of function. 
However, a nonclassic form of lipoid CAH has been described 
that is caused by point mutations in StAR; in this form, StAR 
retains approximately 20% function.283 These patients present 
with progressive glucocorticoid deficiency between 2 and 4 years 
of age; affected males have normal androgenization of the exter­
nal genitalia but may be at risk of hypoandrogenism or reduced 
fertility later in life. StAR mutations w

reduced 
fertility later in life. StAR mutations with intermediate function 
may also be associated with hypospadias and milder adrenal fail­
ure.284 These partial loss-of-function changes tend to affect amino 
acids (codons 187, 188, 192, 221) that lie around the cholesterol-
binding pocket.285
  Clinical Features of Lipoid CAH in 46,XY 
Individuals
Karyotype
46,XY
Autosomal recessive; mutations in STAR 
gene
Genitalia
Female; sometimes ambiguous, hypospa­
dias, or male
Wolffian du

imes ambiguous, hypospa­
Wolffian duct derivatives
Hypoplastic or normal
Müllerian duct derivatives
Absent
Gonads
Testes
Biochemical and physi­
ologic features
Severe adrenal insufficiency in infancy 
with salt loss; lack of pubertal develop­
ment; rare nonclassic cases associated 
with isolated glucocorticoid deficiency
Usually deficiency of glucocorticoids, 
mineralocorticoids, and sex steroids 
except in nonclassic cases in which a 
predominant defect in glucocor

c cases in which a 
predominant defect in glucocorticoid 
production is seen (similar to familial 
glucocorticoid deficiency)
CAH, Congenital adrenal hyperplasia; StAR, steroidogenic acute regulatory (protein).
   
TABLE 24.8
902
Management of classic lipoid CAH includes glucocorticoid 
and mineralocorticoid replacement and salt supplementation in 
early life. Gonadectomy is usually performed in individuals with 
a 46,XY karyotype; although limited d

iduals with 
a 46,XY karyotype; although limited data are available, neoplastic 
germ cell changes have been reported.286,287 Males with nonclassic 
lipoid CAH need careful follow-up to ensure adequate testoster­
one production in puberty and adulthood. Estrogen treatment is 
given to induce puberty and is administered to 46,XX females 
when gonadal insufficiency occurs. 
P450 Side-Chain Cleavage Enzyme Deficiency
P450scc (CYP11A1) is the mitochondrial enzyme that con­
verts cholesterol to pregn

ndrial enzyme that con­
verts cholesterol to pregnenolone by three distinct enzymatic 
reactions: 20α-hydroxylation, 22-hydroxylation, and cleavage 
of the cholesterol side chain. P450scc is therefore responsible 
for the first and rate-limiting step in steroid synthesis, which is 
necessary for pregnenolone and progesterone production by the 
placenta, for mineralocorticoid, glucocorticoid, and androgen 
production by the adrenal glands, and for sex steroid production 
by the gonads.
Although a

 sex steroid production 
Although a natural model of lipoid CAH due to P450scc 
deficiency exists in the rabbit, it was thought that severe loss of 
P450scc activity in humans would be incompatible with survival. 
Placental progesterone production is necessary to support preg­
nancy after the second trimester in higher primates (due to the so-
called luteoplacental shift) but not in rodents. However, complete 
disruption of P450 activity due to a homozygous frameshift muta­
tion i

tivity due to a homozygous frameshift muta­
tion in CYP11A1 has been reported in a 46,XY infant with female 
external genitalia and severe early-onset, salt-losing adrenal fail­
ure.288 An increasing number of other mutations in P450scc have 
also been described, initially in 46,XY phenotypic females with 
severe salt-losing adrenal failure (Table 24.9).289,290 More recently, 
A
Corticotropin
Low-density
lipoprotein
cAMP

w-density
cAMP
cAMP
cAMP
ATP
ATP
ATP
Steroid
Steroid
Lipid
droplet
Lipid
droplet
Lipid
droplet
Lysosome
Lysosome
StAR
StAR
StAR
StAR-independent
flow
flow
reticulum
reticulum
?
?
Nucleus
Nucleus
Nucleus
Normal
B
Early congenital lipoid
adrenal hyperplasia
C
Late congenital lipoid
• Fig. 24.24  Model of the steroid-synthesizing cell (adrenal/gonadal) showing conve

-synthesizing cell (adrenal/gonadal) showing conversion of cholesterol 
to steroids. (A) Cholesterol from low-density lipoprotein, from cholesterol esters stored in lipid droplets, 
and from endogenous synthesis in the endoplasmic reticulum is transported from the outer mitochondrial 
membrane to the inner membrane. This transport is facilitated by the steroidogenic acute regulatory pro­
tein (StAR) and by other StAR-independent mechanisms. In the mitochondria, steroid synthesis begins 
with the

e mitochondria, steroid synthesis begins 
with the conversion of cholesterol to Δ5 pregnenolone by the enzyme CYP11A1 (P450scc). (B) In patients 
with lipoid congenital adrenal hyperplasia, a mutation in the gene encoding StAR results in little or no 
activity of the mutant StAR, greatly reducing cholesterol transport into the mitochondria. Low levels of 
steroidogenesis by mechanisms independent of StAR can occur; however, increased secretion of cortico­
tropin (or luteinizing hormone or follic

 cortico­
tropin (or luteinizing hormone or follicle-stimulating hormone) results in cholesterol accumulation in the cells 
as lipid droplets. (C) Continued stimulation and resultant accumulation of cholesterol cause engorgement 
of these cells, with mechanical and chemical perturbation of cell function. This results in primary adrenal 
insufficiency and impaired androgen biosynthesis by fetal Leydig cells. Females with lipoid congenital 
adrenal hyperplasia feminize at puberty and menstruate bu

 hyperplasia feminize at puberty and menstruate but have progressive hypergonadotropic hypo­
gonadism. This may occur because the follicular cells are relatively quiescent in utero and before puberty 
and therefore are undamaged. At the beginning of each cycle, follicles are recruited, and a small amount of 
estradiol can be produced as a result of StAR-independent mechanisms. This can occur until the follicular 
cells are engorged and rendered nonfunctional. ATP, Adenosine triphosphate; cAMP, c

onfunctional. ATP, Adenosine triphosphate; cAMP, cyclic adenosine 
monophosphate. (From Bose HS, Sujiwara T, Strauss JF III, et al. The pathophysiology and genetics of 
congenital lipoid adrenal hyperplasia. N Engl J Med. 1996;335:1870–1878, used with permission of the 
Massachusetts Medical Society, Copyright 1996.)
903
partial loss-of-function defects in P450scc have been described in 
boys with hypospadias who developed adrenal failure in late child­
h

ias who developed adrenal failure in late child­
hood, and milder changes in CYP11A1 have been reported in chil­
dren with primary adrenal insufficiency without DSD.291–293 This 
condition is now well established as a cause of combined adrenal 
and gonadal failure as well as isolated adrenal insufficiency. Long-
term follow-up of patients with hypospadias or isolated adrenal 
insufficiency may be needed to monitor testicular function and 
fertility. Molecular analysis is needed to differentiate 

ty. Molecular analysis is needed to differentiate this P450scc 
deficiency from StAR deficiency, though it seems that patients 
with P450scc deficiency rarely have significant lipoid enlargement 
of the adrenal glands.290 
3β-Hydroxysteroid Dehydrogenase/Δ4,5-Isomerase Type 2 
3βHSD type 2 deficiency is a rare, autosomal recessive cause of 
CAH that affects both adrenal and gonadal steroid production. 
It is one of the few conditions that can cause atypical genitalia 
in both 46,XX an

hat can cause atypical genitalia 
in both 46,XX and 46,XY individuals. This autosomal reces­
sive disorder is a consequence of mutations in HSD3B2, the 
gene encoding the 3βHSD/Δ4,5-isomerase type 2 isozyme, 
which is expressed mainly in the adrenals and gonads. This 
enzyme catalyzes a crucial step in the biosynthesis of all ste­
roid hormones, the conversion of Δ5 steroids to Δ4 steroids 
(see Fig. 24.11).294 The other 3βHSD isoenzyme in humans, 
3βHSD1, is expressed in the placenta and in per

, 
3βHSD1, is expressed in the placenta and in peripheral tissues 
such as the skin (mainly sebaceous glands), breast, and pros­
tate; it is not associated with CAH.
Classic 3βHSD2 deficiency is subdivided into salt-losing and 
non–salt-losing forms (Table 24.10). Severely disruptive changes 
in 3βHSD2 cause salt-losing adrenal insufficiency soon after 
birth. In 46,XY patients, the external genitalia are usually atypical, 
with small penis, severe hypospadias, partial labioscrotal fusion, 
and 

re hypospadias, partial labioscrotal fusion, 
and urogenital sinus. Mutations that severely disrupt enzymatic 
activity usually present with salt loss (Fig. 24.26).295–297 In con­
trast, mutations such as Ala245Pro retain considerable enzyme 
activity (2–10%) and have been found in males with penoscro­
tal hypospadias and no salt loss. A late-onset form of 3βHSD2 
deficiency usually manifests as premature pubarche and idiopathic 
hirsutism in females. The biochemical profile of classic 3βHSD2 
d

ales. The biochemical profile of classic 3βHSD2 
deficiency is an elevated 17-hydroxypregnenolone, which is pres­
ent at levels greater than 100 nmol/L at baseline or after ACTH 
stimulation.298 An elevated ratio of Δ5 to Δ4 steroids is sometimes 
but not always seen; adrenal production of Δ4 steroids such as 
Gln212X
5′
3′
1
2
3
4
5
6
7
Gln128Arg
Trp147Arg
Asn148Lys
Glu169Lys
Glu169Gly
Arg182Leu
Arg182His
*Val187Met
*Arg188Cys
*Arg192Cys
Arg193X
Try250X
Gln258X
Leu260Pro
Phe267Ser
∆Arg272
Leu27

Try250X
Gln258X
Leu260Pro
Phe267Ser
∆Arg272
Leu275Pro
Met225Thr
*Gly221Ser
Gly221Asp
Ala218Val
• Fig. 24.25  Diagram of selected mutations identified in the STAR gene associated with lipoid congenital 
adrenal hyperplasia. Numbered solid boxes depict the exons. The three-letter abbreviations for amino 
acids are used to indicate the position of missense mutations; X indicates a nonsense (stop) mutation; 
insertions and deletions resulting in frameshift mutations (solid triangles) and splice sit

meshift mutations (solid triangles) and splice site mutations (open 
triangles) are shown. Although STAR mutations are common in Japan, Korea, and regions of the Middle 
East, an increasing number of sporadic changes in the steroidogenic acute regulatory (StAR) protein are 
being detected in other countries. Missense mutations at residues 187, 188, 192, and 221 (asterisks) have 
been associated with a nonclassic late-onset phenotype of glucocorticoid insufficiency.
  Clinical Features of CYP11A

coid insufficiency.
  Clinical Features of CYP11A1 Deficiency 
in 46,XY Individuals
Karyotype
46,XY
Autosomal recessive; mutations in 
CYP11A1 gene
Genitalia
Female; rarely ambiguous or hypospa­
dias
Absent
Gonads
Testes (or absent)
Biochemical and physiologic 
features
with salt loss ranging to milder 
adrenal insufficiency with onset in 
childhood; prematurity associ

iency with onset in 
childhood; prematurity associated in 
one case
mineralocorticoids, and sex steroids
CYP11A1, Cytochrome P450 side-chain cleavage enzyme.
   
TABLE 24.9
904
17-OHP and androstenedione is decreased, but serum concentra­
tions may be elevated in 3βHSD2 deficiency. This apparent para­
dox arises because of peripheral conversion of Δ5 to Δ4 steroids by 
3βHSD1 activity. In a neon

f Δ5 to Δ4 steroids by 
3βHSD1 activity. In a neonatal screening program, an elevated 
17-OHP level in a salt-losing person with 3βHSD2 deficiency 
may be potentially misdiagnosed as 21-hydroxylase CYP21A2 
deficiency.299,300 As the ratio of Δ5 to Δ4 steroids is not always 
elevated in 3βHSD2 deficiency, the ratio of 17-hydroxypregneno­
lone to cortisol has been proposed as a more sensitive screen for 
this condition.301 However, a 17-hydroxypregnenolone assay is 
not always readily available, a

nenolone assay is 
not always readily available, and early genetic testing may be an 
alternative.
Children with 3βHSD deficiency need glucocorticoid and 
sometimes mineralocorticoid replacement. A recent study of the 
Amish population, in which the c.35G>A founder effect is seen, 
has suggested that some children need only physiologic doses of 
glucocorticoids to suppress ACTH and, in turn, adrenal andro­
gen production, whereas others need higher, supraphysiologic 
doses.300 Undertreatment can

er, supraphysiologic 
doses.300 Undertreatment can be associated with advanced skel­
etal maturation and early puberty due to sex steroid excess in 
childhood, whereas overtreatment can cause iatrogenic compli­
cations.300 Gynecomastia can occur at puberty in affected males. 
This is presumably the result of 3βHSD1-mediated peripheral 
conversion of Δ5C19 steroids to Δ4C19 steroids and aromatiza­
tion to estrogens.302 Normal puberty has been reported in males 
with a null mutation in the HSD3B2 

rted in males 
with a null mutation in the HSD3B2 gene, but puberty needs 
close monitoring to ensure adequate androgens are produced, and 
data on fertility are still limited.302 
17α-Hydroxylase/17,20-Lyase Deficiency
P450c17 (CYP17) is a microsomal enzyme with 17α-hydroxylase 
and 17,20-lyase activity that is expressed in the adrenals and 
gonads but not in the placenta or in ovarian granulosa cells.303,304 
The 17α-hydroxylase action of P450c17 catalyzes the conversion 
of pregnenolone (Δ5) 

17 catalyzes the conversion 
of pregnenolone (Δ5) to 17-hydroxypregnenolone and the conver­
sion of progesterone (Δ4) to 17-OHP (see Fig. 24.11). The 17,20-
lyase action of P450c17 can convert 17-hydroxypregnenolone 
  Clinical Features of HSD3B2 Deficiency 
Karyotype
46,XY
HSD3B2 gene
Genitalia
Ambiguous; hypospadias
Normal
Absent
Gonads
Testes
ologic 

bsent
Gonads
Testes
Severe adrenal insufficiency in infancy; 
poor virilization at puberty with gyne­
comastia
Mild form: no mineralocorticoid defi­
ciency, premature adrenarche → mild 
virilization
Increased concentrations of Δ5C21 and 
C19 steroids (e.g., 17-hydroxypreg­
nenolone/cortisol ratio response to 
corticotropin, 17-hydroxypregnenolone, 
DHEA suppressible by dexamethasone)
DHEA, Dehydroepiandrosterone; HSD3B2, 3β-hydroxysteroid de

hydroepiandrosterone; HSD3B2, 3β-hydroxysteroid dehydrogenase type 2.
   
TABLE 24.10
Non salt losers
Salt losers
Ser123Gly
Gly129Arg
Pro155Lys
Ala167Val
Leu173Arg
Ser213Gly
Lys216Glu
Pro222His
Leu236Ser
Ala245Pro
Gly250Val
Tyr254Asp
Gly294Val
Leu6Phe
Ala10Glu
Gly15Asp
Asn100Ser
Ala82Thr
Ala82Pro
Leu108Trp
Glu135X
Glu142Lys
Trp171X
Pro186Leu
Leu205Pro
Pro222Gln
Pro222Thr
Trp355X
Pro341L
Arg335X
Gln334X
Tyr308X
Thr259Arg
Thr259Met
Tyr253Asn
Arg249X
Val248Asn
5′
3′
1
2
3
4
• Fig. 24.26  Diagram of

X
Val248Asn
5′
3′
1
2
3
4
• Fig. 24.26  Diagram of the 3β-hydroxysteroid dehydrogenase type 2 gene (HSD3B2) shows the muta­
tions that result in deficiency of the enzyme. Changes associated with salt loss are shown below and 
non–salt loss above. The numbered solid boxes depict the exons. Mutations are subdivided according to 
their association with salt-losing and non–salt-losing states. The three-letter abbreviations for amino acids 
are used to indicate the position of missense mutations; X i

o indicate the position of missense mutations; X indicates a nonsense (stop) mutation; insertions 
and deletions resulting in frameshift mutations (solid triangles) and splice site mutations (open triangles) 
are shown.
905
(Δ5) to DHEA and 17-OHP (Δ4) to androstenedione (see Fig. 
24.11). P450c17 is bound to the smooth endoplasmic reticulum, 
where it accepts electrons from a specific flavoprotein, reduced 
nicotinamide adenine dinucleotide phosphate (NA

d 
nicotinamide adenine dinucleotide phosphate (NADPH)–P450 
oxidoreductase (POR). The 17,20-lyase activity of P450c17 is 
favored by the presence of Δ5 substrates, redox partners such as 
POR and cytochrome b5, and serine phosphorylation. Unlike in 
rodents, the 17,20-lyase activity of human P450c17 on Δ5 sub­
strates is 50 times more efficient than its activity on Δ4 substrates; 
therefore very little androstenedione is formed directly from 
17-OHP, and the principal pathway to androgen produc

-OHP, and the principal pathway to androgen production is 
through DHEA.132 P450c17 also has 16α-hydroxylase activity.
Defects in P450c17 action can result in two different forms of 
CAH. Most often, combined 17α-hydroxylase/17,20-lyase defi­
ciency is seen, but rare cases of isolated 17,20-lyase deficiency have 
been reported (Tables 24.11 and 24.12).305
Combined 17α-hydroxylase/17,20-lyase deficiency is a 
rare, autosomal recessive form of CAH, although an increas­
ing number of cases are bein

 although an increas­
ing number of cases are being reported from many different 
countries.306 A prevalence of approximately 1 case per 50,000 
individuals is reported in some areas, but in general, the prev­
alence is lower than this. The classic phenotype of complete 
combined 17α-hydroxylase/17,20-lyase deficiency is that of a 
phenotypic female (46,XX, or underandrogenized 46,XY) who 
presents with an absence of secondary sex characteristics and 
hypergonadotropic hypogonadism at puberty an

 and 
hypergonadotropic hypogonadism at puberty and is found to 
have low-renin hypertension and hypokalemic alkalosis (see 
Table 24.11).
The classic phenotype and underlying biochemistry can be 
explained by the enzyme deficiency (see Fig. 24.11).305 A defect 
in 17α-hydroxylation in the adrenal cortex and gonads results 
in impaired synthesis of 17-OHP and 17-hydroxypregnenolone 
and therefore of cortisol, androgens, and estrogens. Decreased 
cortisol synthesis causes increased ACTH secretion

cortisol synthesis causes increased ACTH secretion, which 
results in excessive secretion of 17-deoxysteroids by the adrenal 
cortex, including the mineralocorticoids 11-deoxycorticosterone 
(DOC), corticosterone, and 18-hydroxycorticosterone. Excess 
DOC secretion eventually leads to hypertension, hypokale­
mic alkalosis, and suppression of the renin-angiotensin system. 
Diminished aldosterone synthesis and secretion are sometimes 
reported. Corticosterone is a weak glucocorticoid; the high 
pl

ticosterone is a weak glucocorticoid; the high 
plasma concentrations in this disorder prevent the signs and 
symptoms of cortisol deficiency (e.g., hypoglycemia) and mod­
erate the secretion of ACTH.
Affected 46,XX females have normal typical female internal 
and external genital tracts, but the ovaries cannot secrete estro­
gens at puberty, resulting in absent breast development and hypo­
gonadism with elevated plasma FSH and LH levels. The lack of 
adrenal and ovarian androgens can result in 

k of 
adrenal and ovarian androgens can result in little or no growth of 
pubic and axillary hair. In affected 46,XX individuals, the ovaries 
have a high proportion of atretic follicles, and some ovaries con­
tain enlarged follicular cysts.
Affected 46,XY individuals with complete combined 
17α-hydroxylase/17,20-lyase deficiency who are diagnosed in 
adolescence usually have female external genitalia and a blind 
vaginal pouch (see Table 24.11). Testes may be intraabdominal, 
in the inguinal ca

 Testes may be intraabdominal, 
in the inguinal canal, or in the labioscrotal folds. Inguinal hernias 
are common, müllerian structures are absent, and wolffian deriva­
tives are hypoplastic. Bone age is frequently delayed because of 
decreased sex steroid production, and prolonged linear growth can 
lead to tall stature. Pubic and axillary hair is absent or sparse, and 
hypergonadotropic hypogonadism results in a failure to develop 
secondary sex characteristics at puberty. Excessive secretion 

x characteristics at puberty. Excessive secretion of 
DOC and corticosterone usually leads to low-renin hypertension 
and hypokalemic alkalosis, as in 46,XX girls with this condition.
Complete 17α-hydroxylase/17,20-lyase deficiency is asso­
ciated with a variety of pathogenic variants in the CYP17 gene 
that cause complete loss of function in assays of enzyme activity. 
These changes include a range of missense, frameshift, and non­
sense mutations (Fig. 24.27). A common mutation is the 4-bp 
  
ns (Fig. 24.27). A common mutation is the 4-bp 
  Clinical Features of Combined CYP17 
Deficiency in 46,XY Individuals
Karyotype
46,XY
Autosomal recessive; mutations in CYP17 
gene
Genitalia
Female, ambiguous, or hypospadias
Absent or hypoplastic
Absent
Gonads
Testes
Physiologic features
Absent or poor virilization at puberty, 
gynecomastia, hypertension
Decreased T; increased LH and FSH; 
increased plasma deoxycor

; increased LH and FSH; 
increased plasma deoxycorticosterone, 
corticosterone, and progesterone; 
decreased plasma renin activity
Low renin hypertension with hypokalemic 
alkalosis
CYP17, 17α-Hydroxylase/17,20-lyase; FSH, follicle-stimulating hormone; LH, luteinizing 
hormone; T, testosterone.
   
TABLE 24.11
  Clinical Features of Isolated 17,20-Lyase 
Karyotype
46,XY
gene, usually affecting key redox 
domain

n CYP17 
domains
Genitalia
Absent
Gonads
Testes
Absent or poor virilization at puberty; 
Decreased plasma T, DHEA, androstenedi­
one, and estradiol; abnormal increase 
in plasma 17-hydroxyprogesterone and 
17-hydroxypregnenolone; increased 
LH and FSH; increased ratio of C21 
deoxysteroids to C19 steroids

reased ratio of C21 
deoxysteroids to C19 steroids (DHEA, 
androstenedione) after hCG stimulation
CYP17, 17α-Hydroxylase/17,20-lyase; DHEA, dehydroepiandrosterone; FSH, follicle-stimulat­
ing hormone; hCG, human chorionic gonadotropin; LH, luteinizing hormone; T, testosterone.
   
TABLE 24.12
906
duplication in exon 8, which is shared by Mennonites and indi­
viduals in the Friesland region of the Netherlands and is attrib­
uted to a founder effect or

erlands and is attrib­
uted to a founder effect originating in Friesland. Other geographic 
clusters include an in-frame deletion of residues 487 to 489 in 
Southeast Asia and the Arg362Cys and Trp406Arg missense muta­
tions found among Brazilians of Portuguese and Spanish ancestry, 
respectively.306 However, many different changes occur in other 
populations and are located throughout the enzyme.307
Partial forms of combined 17α-hydroxylase/17,20-lyase defi­
ciency have been described. This con

0-lyase defi­
ciency have been described. This condition most frequently 
manifests in a 46,XY infant with ambiguous genitalia or severe 
hypospadias for whom the steroid profile is consistent with the 
diagnosis of P450c17 deficiency. Hypertension may or may not 
be present in partial forms of combined 17α-hydroxylase/17,20-
lyase deficiency, and aldosterone secretion may be normal or even 
elevated. Corticosterone levels, which are usually 50-fold to 100-
fold higher than normal, provide adequ

old to 100-
fold higher than normal, provide adequate glucocorticoid effects 
and prevent symptoms of cortisol deficiency. The development of 
male secondary sex characteristics at puberty may be incomplete, 
and gynecomastia is often seen. This rare condition has been asso­
ciated with a phenylalanine deletion at codon 53 or 54 and several 
missense changes.305,308
Isolated 17,20-lyase deficiency has been reported in a small 
number of patients.305,309,310 These 46,XY individuals usually 
have 

305,309,310 These 46,XY individuals usually 
have genital ambiguity, normal secretion of glucocorticoids and 
mineralocorticoids, and marked reduction in sex steroid syn­
thesis (see Table 24.12). The first two patients shown to have a 
molecular defect in P450c17 harbored homozygous point muta­
tions (Arg347His, Arg358Gln) that specifically interfered with 
17,20-lyase activity by changing the distribution of surface 
charges in the redox-partner binding site.311 Other patients have 
been repor

r binding site.311 Other patients have 
been reported with similar mutations or with a point mutation 
(Glu305Gly) that alters the conformation of the substrate-bind­
ing site.312
The diagnosis of 17α-hydroxylase deficiency is strongly sup­
ported by the discovery of low-renin hypertension and hypoka­
lemic alkalosis and by a lack of secondary sex characteristics at 
puberty. Plasma concentrations of corticotropin, DOC, corti­
costerone, and progesterone are high, and those of 17α-OHP, 
cortisol

esterone are high, and those of 17α-OHP, 
cortisol, and gonadal steroids are low. Replacement therapy with 
physiologic doses of glucocorticoids suppresses DOC and corti­
costerone secretion and normalizes serum potassium levels, blood 
pressure, and plasma renin and aldosterone levels. The hypokale­
mia can be associated with life-threatening cardiac arrhythmia, so 
careful monitoring and treatment in the acute phase are needed. 
Gonadectomy is usually performed in 46,XY patients who have 
a fe

usually performed in 46,XY patients who have 
a female gender identity and can consent. A gonadoblastoma 
and an invasive mixed germ cell tumor have been reported in the 
gonads of two 46,XX females with this condition.306,313 Appropri­
ate gonadal steroid replacement therapy is indicated at puberty. 
Cytochrome b5 Deficiency
A splice site mutation in the 17,20-lyase redox partner cytochrome 
b5 was first reported in a 46,XY child with ambiguous genitalia 
and methemoglobinemia, although extensi

genitalia 
and methemoglobinemia, although extensive hormone data were 
not reported.314 Homozygous nonsense and missense mutations 
in this gene (CYB5A) have now been described in 46,XY chil­
dren with severe hypospadias and a biochemical profile consistent 
ATG   ACG
Trp17X
Arg21Leu
Ala82Asp
Gly90Asp
Phe93Cys
Arg96Trp
Arg96Gln
Tyr64Ser
∆Phe53 or 54
Arg347Cys
Ser106Pro
Lys110X
Arg239Gln
Arg239X
Ala174Glu
Tyr201Asn
Glu194X
Val178Asp
Asn177Asp
Glu305Gly
Arg362His
Arg362Cys
Arg358X
Ala355Thr
Asp48

305Gly
Arg362His
Arg362Cys
Arg358X
Ala355Thr
Asp487
Ser488
Phe489
Arg440His
Arg440Cys
Leu465Pro
Pro409Arg
Trp406Arg
Trp406Leu
Ala398Val
Arg416Cys
Phe417Cys
Pro428Leu
His373Leu
His373Asn
His373Asp
Ile332Thr
Tyr329Asp
Tyr329X
Arg496His
Arg496Cys
Gln461X
Arg347His
Arg358Gln
Pro342Thr
Asp116Val
Phe114Val
+Ile112
Gly111Ser
5′
3′
1
2
3
4
5
6
7
8
∆
Ile299Asp
• Fig. 24.27  Diagram of selected mutations in the CYP17 gene causing 17α-hydroxylase/17,20-lyase defi­
ciency. The numbered solid boxes depict th

e defi­
ciency. The numbered solid boxes depict the exons. The three-letter abbreviations for amino acids are 
used to indicate the position of missense mutations; X indicates a nonsense (stop) mutation; insertions and 
deletions resulting in frameshift and splice site mutations are shown by solid triangles and open triangles, 
respectively. All of these mutations cause 17α-hydroxylase deficiency. Several missense mutations, such 
as those at codons 305, 347, and 358 (boxes), have been associate

ons 305, 347, and 358 (boxes), have been associated with isolated 17,20-lyase deficiency.
907
with isolated 17,20-lyase deficiency and in a family with multiple 
affected members and a range of genital phenotypes.315,316 Met­
hemoglobin was elevated above the normal range in all these cases, 
but it did not result in clinical signs. 
P450 Oxidoreductase Deficiency
POR is a membrane-bound flavoprotein that plays a central 
role in electron transfer from NA

plays a central 
role in electron transfer from NADPH to P450 enzymes (Fig. 
24.28).317 POR is crucial in the 17,20-lyase reaction of P450c17, 
and it interacts with all 57 microsomal P450 enzymes, including 
P450c21 (21-hydroxylase) and P450c19 (aromatase), and with 
many others involved in hepatic drug metabolism.
A potential role for POR in human steroidogenesis emerged 
after the description of several patients with apparent combined 
deficiencies of CYP17 and CYP21,318 who presented not onl

ncies of CYP17 and CYP21,318 who presented not only 
with ambiguous genitalia and unusual patterns of combined ste­
roidogenic defects but also with Antley-Bixler syndrome, a form 
of skeletal dysplasia that is characterized by craniosynostosis, 
brachycephaly, midface hypoplasia, proptosis, choanal steno­
sis, radioulnar or radiohumeral synostosis, femoral bowing, and 
arachnodactyly.
The first recessively inherited human mutations in POR were 
described in 2004, and a wide range of phenotypes 

described in 2004, and a wide range of phenotypes has been 
described in patients (Table 24.13).133,319,320 At the most severe 
end of the spectrum is ambiguous genitalia due to apparent com­
bined CYP17 and CYP21 deficiency, with or without Antley-
Bixler syndrome. Milder defects in POR have also been seen in 
women with a phenotype resembling polycystic ovary syndrome 
and in men with mild gonadal insufficiency, sometimes with 
subtle skeletal features. Two common mutations are emerging: 
Arg2

features. Two common mutations are emerging: 
Arg287Pro is the most prevalent mutation in patients of Euro­
pean ancestry, whereas the Arg457His mutation is common in 
Japan.320,321 Activating mutations in fibroblast growth factor 
receptor 2 (FGFR2) have also been reported in association with 
Antley-Bixler syndrome; these patients do not have ambiguous 
genitalia or steroidogenic defects.
Most patients with POR deficiency have normal electrolytes 
and mineralocorticoid function (see Table 24.1

es 
and mineralocorticoid function (see Table 24.13). Cortisol insuf­
ficiency may be present, or if basal levels are adequate, the response 
to ACTH stimulation is reduced.321 The serum 17-OHP concen­
tration is usually elevated, with a variable response to ACTH stim­
ulation, and levels of sex steroids tend to be low. POR deficiency 
can be associated with ambiguous genitalia in both 46,XY and 
46,XX individuals. Underandrogenization of 46,XY males can 
occur due to disturbed 17,20-lyase activ

ales can 
occur due to disturbed 17,20-lyase activity during fetal Leydig cell 
steroidogenesis. The partial androgenization of 46,XX infants is 
more prevalent and may be the result of a disturbance in aroma­
tase activity, because POR is an electron donor for this enzyme 
and aromatase deficiency causes prenatal androgenization of the 
developing 46,XX fetus (see “Aromatase Deficiency”). Alterna­
tively or additionally, a “backdoor” pathway of androgen biosyn­
thesis has been described in cert

androgen biosyn­
thesis has been described in certain species, such as the tammar 
wallaby.133,134 In this model system, 17-OHP can be converted 
to DHT without the use of androstenedione or testosterone as an 
intermediate. Emerging data indicate that this pathway may also 
be functional during human development. Puberty can be vari­
able in children with POR deficiency, with delayed or disordered 
puberty that is especially common in girls. This diagnosis should 
be considered in any child wit

s diagnosis should 
be considered in any child with skeletal features and hypospa­
dias (46,XY) or clitoromegaly (46,XX) or who does not progress 
through puberty.322 
3α-Reductase Type 3 and 3α-Reductase Type 1: AKR1C2 and 
AKR1C4
The “backdoor” pathway from 17-OHP to DHT involves the 
activity of AKR1C2 (aldo-keto reductase family 1 member C2) 
(see Fig. 24.12). Variants in AKR1C2, sometimes in combination 
NADP+
NADPH
FAD
POR
FMN
P450
Fe
+
+
++
+
–
–
–
–
b5
– PO4
• Fig. 24.28  Role of P450 ox

+
+
–
–
–
–
b5
– PO4
• Fig. 24.28  Role of P450 oxidoreductase (POR) in electron transfer to 
microsomal (type II) P450 enzymes. Reduced nicotinamide adenine dinu­
cleotide phosphate (NADPH) interacts with POR, which is bound to the 
endoplasmic reticulum, and transfers a pair of electrons to the FAD moiety. 
This change in charge results in altered conformation, which allows the 
electrons to pass from the FAD to the FMN moiety. After further realign­
ment, the FMN domain can interact with the 

align­
ment, the FMN domain can interact with the redox partner binding site of 
the P450 enzyme (e.g., P450c17, P450c21, P450c19), permitting electron 
transfer to the active heme group of the enzyme, which results in substrate 
catalysis. The interaction of POR and the P450 enzymes is coordinated by 
negatively charged acidic residues on the surface of the FMN domain of 
POR and positively charged basic residues in the redox partner binding 
site of the P450 enzyme. In the case of human P450c1

te of the P450 enzyme. In the case of human P450c17, this interac­
tion is facilitated by the allosteric action of cytochrome b5 and by serine 
phosphorylation of P450c17. FAD, flavin adenine dinucleotide; FMN, flavin 
mononucleotide. (From Miller WL. Minireview: regulation of steroidogen­
esis by electron transfer. Endocrinology. 2005;146:2544–2550, used with 
permission of The Endocrine Society, Copyright 2005.)
  Clinical Features of P450 Oxidoreductase 
Karyo

doreductase 
Karyotype
46,XY
Autosomal recessive; mutations in POR 
gene
Genitalia
Ambiguous; hypospadias or male
Absent
Gonads
Testes
Variable androgenization at birth; variable 
virilization at puberty, especially in 
girls; glucocorticoid deficiency; no 
severe mineralocorticoid deficiency; 
features of Antley-Bixler syndrome 
(craniosyn

y; 
(craniosynostosis, skeletal dysplasia) 
in some cases
Evidence of combined CYP17 and CYP21 
insufficiency; normal or low cortisol 
with poor response to ACTH stimula­
tion; elevated 17-OHP; T low
ACTH, Adrenocorticotropic hormone; CYP, cytochrome P450 enzyme; 17-OHP, 17-hydroxy­
progesterone; POR, P450 oxidoreductase; T, testosterone.
   
TABLE 24.13
908
with changes in AKR1C4, have been describ

ife Span
with changes in AKR1C4, have been described in several members 
of a family with various degrees of 46,XY underandrogenization 
who were originally thought to have 17,20-lyase deficiency.323 
Compound heterozygous variants or rearrangements in AKR1C2 
were found in another unrelated child with 46,XY DSD. These 
variants were inherited in a sex-limited recessive manner.134,323 
These findings were taken as evidence that the “backdoor” path­
way plays a role in fetal androgen synthesis in

h­
way plays a role in fetal androgen synthesis in humans as well as 
in wallabies. These studies also highlight that biochemical 17,20-
lyase deficiency can have several molecular causes: CYP17A1, 
CYB5A, POR, and possibly AKR1C2/4.310 
17β-Hydroxysteroid Dehydrogenase Type 3 Deficiency
The 17β-hydroxysteroid dehydrogenase reaction (also called the 
17-ketosteroid reductase reaction) is mediated by isozymes that 
catalyze the reduction of androstenedione, DHEA, and estrone to 
testosterone, Δ5 

tenedione, DHEA, and estrone to 
testosterone, Δ5 androstenediol, and estradiol, respectively, as well 
as the reverse reaction (see Fig. 24.11). The family of 17βHSD 
comprises at least 14 isoenzymes that have physiologic relevance 
in a range of human tissues and disorders such as breast cancer, 
prostate cancer, and endometriosis. The HSD17B3 gene on chro­
mosome 9q22 contains 11 exons and encodes the type 3 isoen­
zyme, which is expressed primarily in the testes, where it favors 
the convers

marily in the testes, where it favors 
the conversion of the weak androgen androstenedione to the more 
biologically active testosterone (see Fig. 24.11). 17βHSD isoen­
zymes other than 17βHSD3 play a role in estrogen synthesis in 
the ovary.
Deficiency of 17βHSD3, which was originally called 17-keto­
steroid reductase, was first reported as an autosomal recessive cause 
of 46,XY DSD by Saez and colleagues (Fig. 24.29).324 Many cases 
have been reported, and the phenotype is well characterized (

eported, and the phenotype is well characterized (Table 
24.14).325,326 Most affected 46,XY children have female-typical 
external genitalia at birth, although a few infants present with cli­
toromegaly or ambiguous genitalia. The testes are usually located 
in the inguinal canal. Müllerian structures are absent, and there is 
a blind vaginal pouch. The wolffian ducts are often stabilized to 
form epididymides, vasa deferentia, seminal vesicles, and ejacula­
tory ducts, likely due to paracrine a

and ejacula­
tory ducts, likely due to paracrine androgenic effects of high con­
centrations of androstenedione.327,328 Affected infants are usually 
designated female and may mistakenly be assumed to have CAIS. 
The phenotype of 17βHSD3 deficiency is limited to 46,XY indi­
viduals; 46,XX individuals with biallelic HSD17B3 mutations are 
asymptomatic and fertile. The chances of having an affected child 
are twice as high if the mother has biallelic changes, which is why 
affected 46,XY siblings 

ic changes, which is why 
affected 46,XY siblings with 17βHSD3 deficiency are more com­
mon than might be expected.
Significant virilization usually occurs at puberty in 46,XY 
17βHSD3 deficiency in the form of clitoral enlargement, hir­
sutism, deepening of the voice, and muscle development. These 
symptoms and signs may be the presenting features of this condi­
tion if it was not diagnosed earlier in life.329 These changes are 
largely due to a pubertal increase in androstenedione produc­
tion

 pubertal increase in androstenedione produc­
tion by the gonad and subsequent extraglandular conversion of 
androstenedione to testosterone. It has been speculated that this 
is mediated by genetic or environmental induction of enzyme 
activities of other 17βHSD isoenzymes, such as the type 5 iso­
enzyme (also known as 17βHSD5 and AKR1C3).330 The testes 
may also have partial 17βHSD3 activity in some cases. In a large 
cohort of patients from a consanguineous population in the Gaza 
Ser65Leu
As

consanguineous population in the Gaza 
Ser65Leu
Asn74Thr
Ala56Thr
Trp50X
Val31Ile
Arg80Gln
Arg80Trp
Asn130Ser
Met235Val
Cys268Tyr
Gly289Ser
Pro282Leu
Tyr287X
Ser232Leu
Glu215Asp
Ala203Val
5′
3′
1
2
3
4
5
6
7
8
9
10
11
Leu212Glu
Phe208Ile
Cys206X
Val205Glu
Ala188Val
Trp192X
Met197Lys
Gln176Pro
His155Pro
• Fig. 24.29  Diagram of the 17β-hydroxysteroid dehydrogenase type 3 gene (HSD17B3) shows the muta­
tions that result in deficiency of the enzyme. The numbered solid boxes depict the exons. The th

 The numbered solid boxes depict the exons. The three-letter 
abbreviations for amino acids are used to indicate the position of missense mutations; X indicates a non­
sense (stop) mutation; insertions and deletions resulting in frameshift mutations (solid triangles) and splice 
site mutations (open triangles) are shown.
  Clinical Features of HSD17B3 Deficiency 
Karyotype
46,XY
HSD17B3 gene
Genitalia
Female → ambiguous; blind 

Genitalia
Female → ambiguous; blind vaginal pouch
Present
Absent
Gonads
Testes (usually undescended)
Virilization at puberty (phallus enlarge­
ment, deepening of voice, and 
development of facial and body hair); 
gynecomastia variable
Increased plasma estrone and andro­
stenedione; decreased ratio of plasma 
testosterone/androstenedione after hCG 
stimulation test; increased plasma FSH 
and LH

CG 
and LH levels
FSH, Follicle-stimulating hormone; hCG, human chorionic gonadotropin; HSD17B3, 
17β-hydroxysteroid dehydrogenase 3; LH, luteinizing hormone.
   
TABLE 24.14
909
Strip, the phallic lengths were described as reaching 4 to 8 cm.331 
The 17βHSD3 mutation reported in this population (Arg80Gln) 
is associated with 15% to 20% retention of normal 17βHSD3 
activity (see Fig. 24.29). Gynecomastia occurs at p

ctivity (see Fig. 24.29). Gynecomastia occurs at puberty because 
of estrogens derived from the conversion of androstenedione to 
estrone by aromatase in extraglandular tissue and the subsequent 
conversion of estrone to estradiol by the action of the 17βHSD1 
or 17βHSD2 isoenzymes.
The typical biochemical profile in 17βHSD3 deficiency is an 
elevated androstenedione level relative to testosterone (see Table 
24.14). Expressed as a ratio of testosterone to androstenedione 
(after an hCG stimulat

sterone to androstenedione 
(after an hCG stimulation test before puberty), values less than 
0.8 are typically found in these patients; however, this is not a 
universal finding presumably because of the extratesticular action 
of other 17βHSD isoenzymes, and diagnosis based on hormone 
measurements alone can be challenging.326,332,333 Testicular vein 
sampling at the time of gonadectomy shows a markedly increased 
androstenedione gradient relative to testosterone, though this test 
is not rout

ive to testosterone, though this test 
is not routinely performed. Early genetic analysis can be useful and 
can avoid the need for hCG stimulation in some children.334,335
The range of mutations reported in patients with 17βHSD3 
deficiency is shown in Fig. 24.29. Most are missense mutations. 
Expression studies of the mutant enzymes in heterologous cells usu­
ally show complete absence of activity in the conversion of andro­
stenedione to testosterone compared with the normal enzyme. One 
rece

sterone compared with the normal enzyme. One 
recent study from a single center specializing in adult DSD found 
that approximately one-fourth of a cohort of women with partially 
virilized 46,XY DSD had mutations in HSD17B3.336 The degree 
of androgenization was less than that typically seen in 5α-reductase 
deficiency, and many had been labeled as having PAIS. The diagno­
sis could only be made by genetic analysis.
Sex designation can be particularly difficult in children with 
17βHSD3 deficie

ularly difficult in children with 
17βHSD3 deficiency because the external genital appearance cor­
relates even less well with future gender identity than in other 
DSDs. Some babies have had early gonadectomy and been brought 
up as girls. In other settings (e.g., in Gaza), where affected patients 
typically adopted a male identity at puberty, some patients were 
designated male at diagnosis and were treated early with testos­
terone.337 An alternative approach would be to raise the child as 
a

rnative approach would be to raise the child as 
a girl initially and delay any interventions until late childhood 
or early adolescence so that the family and child have more time 
to consider the options and the child’s gender is clearer. Careful 
monitoring is needed for the onset of puberty and androgen pro­
duction if the child identifies as female, and gonadectomy may be 
considered to prevent undesired androgenization.
Those children who first present with virilization at puberty may 
dec

irst present with virilization at puberty may 
declare a male gender identity in some cases. Gender changes at 
puberty have been reported in 39% to 64% of patients reared as 
girls in one study, but in our experience the rates are lower.151,152,336 
Other young people may identify as having a nonbinary gender. If 
androgenization is undesired, gonadectomy offers a definitive treat­
ment; another potential option is short-term, reversible blockade 
of androgenization with an antiandrogen or pube

e 
of androgenization with an antiandrogen or puberty suppression 
with a GnRH agonist to provide time for appropriate counseling 
and to ensure the young person is involved in the decision making. 
Careful assessment and support from a multidisciplinary team and 
experienced psychologist are important, and long-term follow-up is 
important for hormone replacement and support. 
Steroid 5α-Reductase Type 2 Deficiency
Steroid 5α-reductase type 2 deficiency in 46,XY individuals is also 
characteriz

ficiency in 46,XY individuals is also 
characterized by normally differentiated testes and male internal 
ducts, but external genitalia may be more ambiguous at birth than 
in 17βHSD deficiency. Classic features of this enzyme deficiency 
are summarized in Table 24.15.
The description of a genetic variant in SRD5A2, the gene that 
encodes steroid 5α-reductase type 2, in the Dominican Republic 
and Mexico and analysis of the biochemical and molecular fea­
tures underline the importance of DHT in 

lar fea­
tures underline the importance of DHT in the development of 
the male phenotype.338–340 At birth, there is typically a bifid scro­
tum, a urogenital sinus, a blind vaginal pouch, and a clitoris-like 
hypospadiac phallus. Testes differentiate normally and are located 
in the inguinal canal or in the labioscrotal folds. No müllerian 
structures are present. The wolffian ducts are stabilized so that the 
epididymides, vasa deferentia, and seminal vesicles are well dif­
ferentiated; the eja

eminal vesicles are well dif­
ferentiated; the ejaculatory ducts usually terminate in the blind 
vaginal pouch. The prostate is hypoplastic. Up to a third of cases 
may present with isolated hypospadias.341
At puberty, there is a striking degree of virilization: The voice 
deepens, muscle mass increases, the phallus lengthens to 4 to 8 
cm, the bifid scrotum becomes rugated and pigmented, and the 
testes enlarge and descend into the labioscrotal folds. Postpubertal 
affected individuals do not h

folds. Postpubertal 
affected individuals do not have acne, temporal hair recession, or 
enlargement of the prostate. They rarely develop gynecomastia, in 
contrast to individuals with 17βHSD3 deficiency or AIS. There is 
normal libido with penile erections. Histologic examination of the 
testes shows Leydig cell hyperplasia and decreased spermatogene­
sis. Infertility is caused by a composite of failure to transform sper­
matogonia into spermatocytes, the adverse effect of a cryptorchid 
testis

cytes, the adverse effect of a cryptorchid 
testis, and the specific role of DHT in regulating semen volume 
and viscosity.342 Nevertheless, some individuals with 5α-reductase 
deficiency have a normal sperm count. One man in the Domini­
can cohort fathered a child after intrauterine insemination, and 
two affected brothers in a Swedish family were spontaneously 
fertile after hypospadias repair performed in childhood.343,344 
Gender changes occur frequently in 5α-reductase deficiency, par­
ticu

r frequently in 5α-reductase deficiency, par­
ticularly in individuals who do not undergo gonadectomy. Rates 
of social gender change in three cohorts ranged from 12% to 50% 
and seemed related to the age at diagnosis, with social gender 
  Clinical Features of 5α-Reductase Type 2 
Karyotype
46,XY
SRD5A2 gene
Genitalia
Usually ambiguous with small, hypospa­
diac phallus; blind vaginal pouch

us; blind vaginal pouch
Normal
Absent
Gonads
Normal testes
Decreased facial and body hair, no tempo­
ral hair recession, prostate not palpable
Decreased ratio of 5α/5β C21 and C19 
steroids in urine; increased T/DHT ratio 
before and after hCG stimulation; mod­
est increase in plasma LH; decreased 
conversion of T to DHT in vitro
DHT, Dihydrotestosterone; hCG, human chorionic gonadotropin; LH, luteinizing h

G, human chorionic gonadotropin; LH, luteinizing hormone; 
SRD5A2, steroid-5α-reductase type 2; T, testosterone.
   
TABLE 24.15
910
change being less common in the cohort where individuals were 
diagnosed at a younger (prepubertal) age.345 Male sex designa­
tion is now increasingly chosen when the diagnosis is made in 
infancy.345,346 An alternative approach similar to that sometimes 
used for 17β-hydroxysteroid dehydrogenase deficiency would be 
to

droxysteroid dehydrogenase deficiency would be 
to rear the child as female with the gonads kept intact pending 
further informed discussions before puberty. Females (46,XX) 
homozygous for 5α-reductase deficiency undergo normal puberty 
and have normal fertility.347
The biochemical profile in 5α-reductase deficiency typically 
shows an elevated testosterone-to-DHT ratio basally or after hCG 
stimulation (see Table 24.15). Various ratios have been suggested as 
diagnostic, but none is unequivoca

n suggested as 
diagnostic, but none is unequivocal. A ratio exceeding 10:1 (when 
testosterone and DHT are expressed in the same units) had a sen­
sitivity and specificity of 78% and 72%, respectively, in a cohort 
of 90 patients investigated for this enzyme deficiency.348 Serum 
LH and FSH levels may be normal or elevated after puberty. The 
most reliable biochemical test (undertaken after 3 months of age) 
is analysis of a urinary steroid profile by gas chromatography and 
mass spectrometry t

ile by gas chromatography and 
mass spectrometry to demonstrate a diminished ratio of urinary 
5α/5β-reduced C19 and C21 steroids.349,350 The diagnosis can 
still be confirmed biochemically after gonadectomy because of 
persistent effects on 5α/5β-reduced C21 steroids.351 Early genetic 
testing is also useful to make the diagnosis or to confirm it when 
suspected from the biochemical profile.
Early diagnosis of 5α-reductase type 2 deficiency is important 
because of its bearing on sex designatio

mportant 
because of its bearing on sex designation. The natural history of 
this condition, with a tendency for change to a male gender role 
with virilization at puberty, indicates that male sex designation is 
now a frequent option even when the external genitalia are rela­
tively severely underandrogenized at birth.352 The enzyme defi­
ciency can masquerade as PAIS in newborns.353 DHT, which may 
be applied topically increases penile length and facilitates repair of 
the hypospadias, but com

nd facilitates repair of 
the hypospadias, but commercial availability may be limited.354
Two microsomal 5α-reductase enzymes catalyze the NADPH-
dependent conversion of testosterone to DHT. 5α-reductase type 
2 is a 254–amino acid protein encoded by the SRD5A2 gene on 
chromosome 2p23. The type 2 isozyme is expressed predomi­
nantly in the primordia of the prostate and external genitalia but 
not in the wolffian ducts until after their differentiation into the 
male internal genital ducts.355 T

iation into the 
male internal genital ducts.355 The type 1 isoenzyme is expressed 
in skin, including human genital skin fibroblasts. The action of 
this isoenzyme may contribute to the virilization that occurs in 
5α-reductase–deficient patients at puberty.356
5α-reductase type 2 deficiency is transmitted as an autoso­
mal recessive trait. 5α-reductase type 2 deficiency is genetically 
heterogeneous, and the more than 60 mutations detected in the 
SRD5A2 gene are distributed among all five exo

he 
SRD5A2 gene are distributed among all five exons (Fig. 24.30). 
Most are missense mutations; a complete gene deletion is found 
in the New Guinea population. There is a predominance of muta­
tions in exon 4, mostly localized between codons 197 and 230, 
where the effect is complete inactivity of the mutant enzyme. 
Consanguinity is common, but a significant number of cases are 
compound heterozygotes. Male heterozygotes are not affected. 
Disorders of Androgen Action
Androgen insensitivity s

orders of Androgen Action
Androgen insensitivity syndrome (AIS) is the paradigm of a clinical 
disorder resulting from hormone resistance.136 A detailed description 
Arg168Cys
Asp164Val
Asn160Asp
Gly158Arg
Glu200Lys
Gly203Ser
Ala207Asp
Pro212X
Pro212Arg
Leu221Pro
His231Arg
His230Pro
Ala228Thr
Arg227Gln
Arg227X
Leu226Pro
Leu224Pro
Leu224His
Gly123Arg
Gln126Arg
Cys133Gly
Arg145Trp
Gly149Ser
Val3Leu
Cys5Arg
Gln6X
Val10Gly
Ser14Arg
Leu20Pro
Tyr26X
Ala62Glu
Val63Met
Gly85Asp
Tyr91Asp
Pro59Arg
Glu57Gl

a62Glu
Val63Met
Gly85Asp
Tyr91Asp
Pro59Arg
Glu57Gln
Gln56Arg
Leu55Gln
Trp53X
Ala49Thr
Pro47Arg
Gln34Arg
Thr120Pro
Phe118Leu
Gly115Asp
Leu113Val
Ile112Asn
Arg103X
Gly183Ser
Pro181Leu
Arg171Ser
Phe186Leu
Val189Ala
Gly191Glu
Ala192Thr
Asn193Ser
Gly196Ser
Glu197Asp
Tyr235X
Tyr235Phe
X255Ser
X255Gln
Ser245Tyr
Arg246Trp
Arg246Gln
Ala248Val
U.A.E. Deletion
New Guinea Deletion
1
2
3
4
5
∆Met157
• Fig. 24.30  Diagram of the 5α-reductase type 2 gene (SRD5A2) shows the mutations that result in 
5α-reductas

2) shows the mutations that result in 
5α-reductase deficiency. The numbered solid boxes depict the exons. The three-letter abbreviations for 
amino acids are used to indicate the position of missense mutations; X indicates a nonsense (stop) muta­
tion; insertions and deletions resulting in frameshift and splice site mutations are shown by solid triangles 
and open triangles, respectively. Large deletions found in affected United Arab Emirates (U.A.E.) and New 
Guinea populations are shown. (Ada

.A.E.) and New 
Guinea populations are shown. (Adapted from Grumbach MM, Hughes IA, Conte FA. Disorders of sex 
differentiation. In: Larsen PR, Kronenberg HM, Melmed S, et al, eds. Williams Textbook of Endocrinol­
ogy, 10th ed. Philadelphia, PA: Saunders; 2003, with additional data provided courtesy of Dr. Julianne 
Imperato-McGinley, Department of Medicine, Weill Medical College of Cornell University, Ithaca, NY.)
911
of the role of androgens in sex deve

x Development
of the role of androgens in sex development and the molecular actions 
of the androgen receptor are provided earlier in this chapter.
Complete Androgen Insensitivity Syndrome
CAIS is an X-linked condition that typically presents in an ado­
lescent female who has breast development and a pubertal growth 
spurt but who has not had menarche (Table 24.16). Pubic and 
axillary hair is absent or scant, and the uterus is absent as a result of 
normal AMH action, although there may be müll

 of 
normal AMH action, although there may be müllerian remnants. 
The wolffian ducts are stabilized in many patients, with a well-
developed vas deferens and epididymis observed when gonadec­
tomy is performed.328 Estimates of the prevalence of CAIS range 
from 1 case in 20,400 to 1 in 99,000 individuals with a 46,XY 
karyotype.357 The main differential diagnosis in an adolescent 
with a 46,XY karyotype is complete gonadal dysgenesis (Swyer 
syndrome), which is distinguished from CAIS by (1) ab

drome), which is distinguished from CAIS by (1) absence of 
gonadal sex steroid production, resulting in poor breast develop­
ment, and (2) absence of AMH production, resulting in retention 
of müllerian structures in Swyer syndrome.358 CAIS may present 
in early infancy with bilateral inguinal or labial swellings that on 
evaluation are found to be partially descended testes. 17βHSD3 
deficiency (or rarely 5α-reductase deficiency or SF1 deficiency) 
may also present this way. Bilateral inguinal

cy) 
may also present this way. Bilateral inguinal hernias are rare in 
girls, of whom 1% to 2% are estimated to have CAIS.359 Girls 
with this type of hernia should undergo evaluation for the pres­
ence of a Y chromosome by fluorescence in situ hybridization 
(FISH) or a full karyotype. If the hernial sac contains gonads, 
a biopsy can be performed together with cytogenetic studies.360 
Presently, with fetal chromosomal analysis being commonplace, 
a mismatch between chromosomal sex and the ext

e, 
a mismatch between chromosomal sex and the external genital 
phenotype at birth can be the presenting feature of CAIS. 
Partial Androgen Insensitivity Syndrome
PAIS is characterized by incomplete androgenization resulting 
from a partial biologic response to androgens (Table 24.17). The 
phenotype for PAIS is highly variable and may include penoscro­
tal hypospadias, micropenis, and bifid scrotum. The testes may be 
undescended. The most severe form of PAIS manifests as isolated 
clitoromega

re form of PAIS manifests as isolated 
clitoromegaly. The milder end of the spectrum includes isolated 
hypospadias; PAIS does not present as isolated micropenis. As with 
other forms of AIS, müllerian structures are absent. Gynecomastia 
is common at puberty. The differential diagnosis for a PAIS-like 
phenotype is lengthy and includes partial gonadal dysgenesis (e.g., 
due to an SF1/NR5A1 mutation or 45,X/46,XY mosaicism) and 
partial defects in androgen biosynthesis (e.g., due to LH receptor,

n androgen biosynthesis (e.g., due to LH receptor, 
17βHSD, or 5α-reductase deficiencies). 
Minimal or Mild Androgen Insensitivity Syndrome
This category of AIS is generally associated with typical male geni­
talia but can present with gynecomastia at puberty and infertility 
in adulthood.361 Cancer of the breast is rare in males, but the risk 
is increased in MAIS as well as in those with PAIS raised male. 
In contrast, breast cancer has not been reported in women with 
CAIS. A mild form of and

n reported in women with 
CAIS. A mild form of androgen insensitivity is also found in spi­
nal and bulbar muscular atrophy (Kennedy syndrome), a condi­
tion caused by hyperexpansion of the N-terminal polyglutamine 
repeat region of the androgen receptor, which can be associated 
with gynecomastia and reduced fertility.362 
Hormone Profiles in Androgen Insensitivity Syndromes
In pubertal and adult individuals with CAIS, serum testoster­
one is within or above the adult male range with a moderate

thin or above the adult male range with a moderately 
but inappropriately increased concentration of LH due to loss 
of negative feedback from androgens (though negative feed­
back from estrogens remains intact).363 Serum estradiol is usu­
ally within and sometimes above the adult male reference range 
due to aromatization of androgens.363 The concentration of sex 
hormone–binding globulin (SHBG), which is suppressed by 
  Clinical Features of Complete Androgen 
Insensitivity Syndrome
Karyotype

omplete Androgen 
Karyotype
46,XY
X-linked recessive; mutations in AR gene
Genitalia
Female with blind vaginal pouch
Often present, depending on mutation 
type
Absent or vestigial
Gonads
Testes
Physiologic findings
Scant or absent pubic and axillary hair; 
breast development at puberty; 
primary amenorrhea
Hormone and metabolic 
profile
Increased LH and testosterone levels; 
normal or increased estradiol (for

terone levels; 
normal or increased estradiol (for 
male reference range); FSH levels 
often normal or slightly increased; 
resistance to androgenic and meta­
bolic effects of testosterone
AR, Androgen receptor; FSH, follicle-stimulating hormone; LH, luteinizing hormone.
   
TABLE 24.16
  Clinical Features of Partial Androgen 
Karyotype
46,XY
X-linked recessive; mutations in AR 
gene
External genitalia
Ambiguous with blind vaginal pouch → 
isolated hypospadias

s with blind vaginal pouch → 
isolated hypospadias → male with 
infertility (mild AIS)
Often normal
Absent
Gonads
Decreased to normal axillary and pubic 
hair, beard growth, and body hair; 
gynecomastia common at puberty
profile
Increased LH and testosterone con­
centrations; normal or increased 
estradiol (for men); FSH levels may 
be normal or slightly increased
Partial

evels may 
Partial resistance to androgenic and 
metabolic effects of testosterone
AIS, Androgen insensitivity syndrome; AR, androgen receptor; FSH, follicle-stimulating hor­
mone; LH, luteinizing hormone.
   
TABLE 24.17
912
androgens and enhanced by estrogens, is similar to that found 
in 46,XX females. Serum AMH concentration is typically in 
or above the normal male range in CAIS, a finding that distin­
guishes CAI

 range in CAIS, a finding that distin­
guishes CAIS from gonadal dysgenesis, in which AMH levels 
are low.364
During the first few months of life, male infants have an active 
reproductive endocrine system with robust production of LH and 
testosterone; this period is known as minipuberty. For reasons that 
are unclear, this early reproductive endocrine activity is absent in 
most infants with CAIS (but not in PAIS),365 and this may be a 
distinguishing feature of CAIS. 
Molecular Pathogenesis o

uishing feature of CAIS. 
Molecular Pathogenesis of Androgen Insensitivity Syndromes
Information about mutations that affect the AR is recorded on 
an International Mutation Database at McGill University366 (Fig. 
24.31). More than 400 germline mutations are recorded for AIS; 
the database also records somatic mutations associated with pros­
tate cancer. There is no specific hot spot for mutations, but cer­
tain locations, such as exons 5 and 7 within the LBD, are affected 
more frequently. Appr

ithin the LBD, are affected 
more frequently. Approximately 20% of mutations are located in 
the DBD. The most common functional AR defect results from 
disruption of the hydrophobic ligand-binding pocket, which is 
necessary for repositioning of helix 12 to form the AF2 coregula­
tor interaction surface.
The phenotype of individuals with AIS can vary according to 
different substitutions at the same codon. For example, phenyl­
alanine at codon 754 in helix 5 has a side chain that points away 
f

54 in helix 5 has a side chain that points away 
from the ligand-binding pocket. When substituted for a valine, 
the mutant AR is transcriptionally inactive and causes CAIS.367 
In contrast, serine and leucine substitutions result in a PAIS 
phenotype, explained by some residual transcriptional activity 
as measured in vitro. Linking the DBD to the LBD is a flexible 
hinge region defined by residues 628 to 669. This region stabilizes 
receptor interaction with selective androgen response element

teraction with selective androgen response elements 
and signals nuclear localization. Deletion of the hinge region by 
site-directed mutagenesis results in enhanced gene transcription, 
suggesting that it has a role in repression.368
There can be considerable heterogeneity in the phenotypic 
expression of a particular mutation, sometimes even within 
families. A mutation at codon 703 in exon 4 of the LBD, which 
changes a serine to a glycine, is reported in four individuals listed 
in the McGil

 reported in four individuals listed 
in the McGill database. One patient had typical female genitalia 
consistent with CAIS. The other three all had ambiguous geni­
talia consistent with PAIS, but the degree of androgenization of 
Androgen receptor gene mutations, 30-7-03
∆P125P:172X
ζR127R:172X
E153X
∆208K:232X
∆G272G:301X
L172X
L172X
∆P102P:172X
Q80X
58
E2K
L54S
Q112H
(W798X)
(F891L)
+L57Q
Q64R
(L830P)
Aa substitution mutations
Location of splicing and untranslated region mutations
c+14a
Intr

icing and untranslated region mutations
c+14a
Intron
1
2
3
4
5
6
7
Location of intron mutations
g>a
∆6kb-18
a+5t
g+6c
1
2
3
4
5
6
7
8
AR-
TIS II
g+2t
∆Exon 2
I-11a
g+1a
g+1a
-2820
g+1t
a+3t
∆Exon 3
Legend
CaP, Somatic
PAIS, Constitutional
CAIS, Constitutional
MAIS, Constitutional
When more than
one mutation present
in patient, other
mutations in brackets
K180R
Q194R
S178S
E198G
E202E
R210R
E211E
P269S
S298R
M266T
(L574P)
L255P
D233K
N222D
(A877T)
G214R
E211E
E211E
P533S
L547F
(L741

N222D
(A877T)
G214R
E211E
E211E
P533S
L547F
(L741C)
L547F
P548S
D528G
G524S
G524D
P514S
C559Y
G568W
G563V
Y571C
Y571H
A573D
T575A
C576R
C576F
G577R
S578T
C579Y
C579F
K580R
V581F
R629Q
K630T
(K717Q)
A645D
S647N
(G724N)
(L880Q)
(A898T)
T649T
E653K
L677P
E681L
P682T
G683A
V684I
C686R
A687V
G688Q
H689P
D695N
R774H
R774H
R774C
R779W
M780I
S782K
C784Y
M787V
L790F
E793D
D879T
D879G
L881V
M886V
S888L
S888S
V889M
V889M
P892L
M895T
I898T
Q902R
V903M
P904S
P904H
L907F
G909R
G909E
K910R
V911L
P913S
F916L
Q

P904H
L907F
G909R
G909E
K910R
V911L
P913S
F916L
Q919R
F794S
Q798E
Q798E
Q798E
C806Y
M807V
M807R
S814N
G820A
L821V
R831Q
R831L
Y834C
R840C
R840H
I841S
I842T
I842T
R854K
R855C
R855H
L863R
R788S
Q802R
G803L
F804L
L812P
N819Q
Q824L
F827V
L830V
L838L
R846G
F856L
D864N
D864G
V866L
V866M
V866M
V866M
V866E
I869M
A870G
A870V
R871G
H874Y
H874R T877S
T877A
F725L
R726L
N727K
L728S
V730M
D732N
D732Y
Q733H
I737T
Y739D
W741R
M742V
M742I
G743V
L744F
L744F
M745T
V746M
A748D
A748V
M749V
M749I
G750S
G750D
W751R
R

V746M
A748D
A748V
M749V
M749I
G750S
G750D
W751R
R752Q
F754V
F754L
W741R
G743R
A748T F754L
T755A
N756S
V757A
N758T
S759F
S759P
L762F
Y763H
Y763C
Y763C
F764L
A765T
N756D
A765V
P766A
P766S
D767E
L768P
L768M
N771H
E772G
E772A
D695N
D695V
D695H
L700M
L701P
L701I
L701H
S702A
S703G
S703G
N705S
N705S
N705Y
L707R
G708A
G708V
E709E
V715M
K720E
A721T
L722F
P723S
G724S
G724D
R710T
Q711E
L712F
I664N
Q870R
(S791P)
P671H
I672T
F582Y
F582S
R585K
A588V
A587S
A596T
S597G
(R617P)
C601F
D604Y
R607Q
R607Q
*
*
*
R60

6T
S597G
(R617P)
C601F
D604Y
R607Q
R607Q
*
*
*
R608K
R608K
D610T
C611Y
R615H
R615P
R615G
L616R
L616P
L616P
R617P
C619Y
P390L
(N758D)
G491S
V511V
P390S
P390R
(Q443R)
(E211E)
P340L
S334P
1
2
3
4
5
6
7
8
78
538
586
625
724
772
816
868
919
Q113X
∆E473G:499X
Q403X
G371X
E353X
∆G461G
∆P766 0 807X
W751X
∆G743G
W741X
R752X
T800T:807X
R786X
E772X
W796X
S853X
R831X
Y85 7X K883X
Y480X
Y534X
W502X
∆G4880
A7210:766X
∆N692 0
∆D690 0
W718X
∆F5820
∆P554P
∆C5790:619X
∆R6150
K590X
R607X
Q657X
Q640X
Premature term

619X
∆R6150
K590X
R607X
Q657X
Q640X
Premature termination mutations or 1-6 bp ∆ or ∆
M868
∆
N 8480:879X
∆
E2020:232X
∆
Q80Q:80X
∆
A215G:232X
∆
G510:180X
∆
t+3
∆
Q85Q:91X
∆
5′
3′
• Fig. 24.31  Overview of androgen receptor (AR) mutations that cause different forms of androgen insen­
sitivity syndrome (AIS). CAIS, complete androgen insensitivity syndrome; MAIS, minimal or mild androgen 
insensitivity syndrome; PAIS, partial androgen insensitivity syndrome. (From the McGill Androgen Receptor 
Gene 

yndrome. (From the McGill Androgen Receptor 
Gene Mutation Database. Available at http://www.androgendb.mcgill.ca.)
913
the external genitalia was sufficiently variable that two were raised 
male and the third was raised female.369 About 30% of mutations 
in AIS are de novo. Such mutations arise from a single mutational 
event in a parental germ cell (the mother in the case of AIS) or as 
germ cell mosaicism in the maternal gonad. When the mutation 
arise

sm in the maternal gonad. When the mutation 
arises at the postzygotic stage, the index case is a somatic mosaic. 
Perhaps one-third of de novo mutations arise at the postzygotic 
stage, giving rise to expression of mutant and wild-type AR in 
different target tissues. This may explain some of the phenotypic 
variability in PAIS.370
Implicating a variant in AR as the cause of AIS generally 
requires proof of pathogenicity when it is novel and particularly 
with a PAIS phenotype. A commonly used 

ticularly 
with a PAIS phenotype. A commonly used assay assesses transcrip­
tional activity using a promoter-luciferase system after transient 
transfection of the variant AR in an androgen receptor–negative 
cell line (such as Cos-1, Hela, CV1, or the PC3 prostate cancer 
cell line). Fig. 24.32 illustrates the application of such an assay 
for analysis of three different amino acid substitutions at codon 
582 in the DBD.136 A range of phenotypes from CAIS to severe 
and mild PAIS is found, depe

 from CAIS to severe 
and mild PAIS is found, depending on the nature of the substi­
tution. Furthermore, the data demonstrate that high doses of 
androgens can overcome the resistance of the mutant AR. This has 
particular relevance for patients with PAIS who are raised male 
and may require large doses of androgens to induce puberty or 
to stimulate spermatogenesis. In addition to functional studies 
in vitro, structure-guided modeling of the androgen receptor can 
be used to predict the likel

ndrogen receptor can 
be used to predict the likely consequences of an AR mutation (see 
Fig. 24.16C). However, the N-terminal domain has a disordered 
flexible structure that has yet to be analyzed by crystallography. 
Nonsense mutations have long been identified in the NTD, but 
improved sequencing techniques have now identified a number 
of missense mutations generally associated with milder forms of 
AIS. It is possible that the structural flexibility of the NTD enables 
the AR to tolerate a

exibility of the NTD enables 
the AR to tolerate amino acid substitutions with little functional 
consequence. Analysis of such mutations provides a means to bet­
ter define the structure-function relationship of the N-terminal 
domain.371 The discovery of novel mutations is more common 
now that full sequencing of the AR gene is more straightforward.372 
Androgen Insensitivity Syndromes Without an Androgen 
Receptor Mutation
An AR gene mutation is identified in about 95% of individuals 
when th

is identified in about 95% of individuals 
when the clinical and biochemical phenotypes are consistent with 
CAIS.373 There may also be mutations outside the AR coding 
region, such as the mutation in the 5ʹ untranslated region that was 
recently reported in two unrelated individuals with CAIS, which 
generates an upstream open reading frame and results in reduced 
AR protein levels.374 In addition, in some individuals with a phe­
notype consistent with AIS, no AR mutation is identified, but 
as

nt with AIS, no AR mutation is identified, but 
assessment of DHT-dependent transcriptional induction of an 
No hormone
0.01 nmol/L Mib
0.1 nmol/L Mib
1.0 nmol/L Mib
10 nmol/L Mib
100 nmol/L Mib
Wild type
V582F
(CAIS)
V582G
(PAIS)
V582L
(PAIS)
Wild type (%)
0
20
40
60
80
100
120
140
G
D
N
R
S A
C
T
Proximal box
Distal box
Zn
Zn
First zinc
finger
Second
zinc finger
C-terminal
extension
V582
L
A
G
Y
H
C
C
I
L
C
T
K
Q
P
P
G
D
E
A
S
G
F F K R A A E G K Q K C
L R K C Y E A G M T L
C
C
T
I
D
K F R
R
K

 K Q K C
C
C
T
I
D
K F R
R
K
N
C
P
S
C
R
L
S
C
K
V
• Fig. 24.32  Phenotypes differing according to mutation at codon 582 in first zinc finger of the DNA-binding 
domain (DBD). The effects of substitution of a valine (V) with phenylalanine (F), glycine (G), and leucine (L) 
are shown in a transactivation assay (upper panel) with the synthetic androgen, mibolerone. The linear 
stretch of amino acids composing the DBD is shown in the lower panel. CAIS, complete androgen insen­

in the lower panel. CAIS, complete androgen insen­
sitivity syndrome; PAIS, partial androgen insensitivity syndrome. (From Hughes IA, Davies JD, Bunch TI, 
et al. Androgen insensitivity syndrome. Lancet. 2012;380:1419–1428.)
914
AR target gene (i.e., apolipoprotein D) in cultured genital skin 
fibroblasts shows a pattern consistent with functional androgen 
insensitivity.375
In contrast, the likelihood of finding a pathogenic AR variant in 
those wit

d of finding a pathogenic AR variant in 
those with a PAIS phenotype is much lower, even when other causes 
of a similar phenotype have been excluded.369 The genital pheno­
type in patients without an AR mutation, as based on a validated 
external masculinization score (EMS), is no different than that in 
infants with PAIS with a proven AR mutation, but gynecomastia 
is much less common.376–378 Boys with an AR mutation are more 
likely to have poorer outcome than those without a mutation.378 
Bi

orer outcome than those without a mutation.378 
Birth weight for gestational age is significantly lower in mutation-
negative patients compared with infants with PAIS who have an AR 
gene mutation.270,379 It has been suggested that this form of 46,XY 
DSD with fetal growth restriction is a discrete category that as yet 
remains unexplained. The term PAIS is best reserved for individu­
als with AR mutations. Individuals with 17βHSD3, 5α-reductase 
deficiency, and especially SF1 mutations might al

deficiency, and especially SF1 mutations might also have been mis­
labeled as PAIS in the past. Although there is evidence that the AR 
can interact with more than 200 unique coregulator proteins, no 
mutations have been identified when screening these proteins in 
AIS, in contrast to findings in prostate cancer.380 
Management of Androgen Insensitivity Syndromes
Sex designation and gender of rearing are uniformly female in 
CAIS. Inguinal hernias need repair when presenting in infancy. 
There 

as need repair when presenting in infancy. 
There is an option then to perform gonadectomy at this early 
stage, but most patients, families, and providers now favor late 
gonadectomy, as this approach enables spontaneous puberty to 
occur, allows the patient to be involved in the decision-making 
process, and does not substantially increase the risk for malig­
nancy (discussed later in chapter). If gonadectomy is performed 
before puberty, hormone induction of puberty is similar to 
that used f

ne induction of puberty is similar to 
that used for girls with other causes of hypogonadism, typically 
starting around 10 to 11 years of age; the precise timing should 
consider the patient’s wishes, family history of pubertal timing, 
growth and the degree of skeletal maturation as assessed by bone 
age X-ray, and whether gonadotropins are starting to rise. In the 
A
B
Arg172X
Arg342Trp Arg471His
Arg59Cys
Arg54Cys
Gly40X
Arg80X
Arg97X
Gly265Arg
Gly142Val
Arg303Trp
His282Gln
Asp426Gly
Arg407X

g
Gly142Val
Arg303Trp
His282Gln
Asp426Gly
Arg407X
Arg406Gln
Cys500Tyr
Asp491His
Arg504Cys
(∆)27nt
Val458Ala
Val12Gly
Leu70Pro
Gly101Val
Gly101Arg
Gln128X
Arg194Cys
Arg206Asp
Thr193Ile
Cys488Tyr
Val477Ala
Glu382X
Phe148Leu
Pro151Ser
Tyr167Cys
1
5′
3′
5′
3′
2
3
4
5
1
2
3
4
5
6
7
8
9
10
11
ATG    ATT
AMH
AMHR2
Arg302Gln
Ala405Pro
gly326Val
Gln496His
His506Gln
Val508Ala
Cys525Tyr
Arg191X
Gln325Arg
Arg95X
Leu118Pro
Arg123Trp
• Fig. 24.33  (A) Diagram of selected mutations in the antimüllerian hormone

of selected mutations in the antimüllerian hormone (AMH) gene that cause per­
sistent müllerian duct syndrome. The numbered solid boxes depict the exons. The three-letter abbrevia­
tions for amino acids are used to indicate the positions of missense mutations; X indicates a nonsense 
(stop) mutation; insertions and deletions resulting in frameshift and splice site mutations are shown by 
solid triangles and open triangles, respectively. (B) Diagram of selected mutations in the gene for the AMH 

am of selected mutations in the gene for the AMH 
receptor type 2 (AMHR2). The numbered solid boxes depict the exons. Exons 1 to 3 encode the extracel­
lular domain of the receptor. Exon 4 (diagonal lines) encodes the transmembrane domain, and exons 5 
through 11 encode the intracytoplasmic domain. Different forms of mutation are depicted, as in (A). The 
Δ27nt (open box) is a 27-nucleotide deletion, the most common AMHR2 mutation causing the persistent 
müllerian duct syndrome. (A, redrawn from

sistent 
müllerian duct syndrome. (A, redrawn from Imbeaud S, Carré Eusebe D, Rey R, et al. Molecular genetics 
of the persistent müllerian duct syndrome: a study of 19 families. Hum Mol Genet. 1994;3:125–131, used 
with permission of Oxford University Press; B, redrawn from Imbeaud S, Belville C, Messike-Zeitoun L, 
et al. A 27 base-pair deletion of the antimüllerian type II receptor gene is the most common cause of the 
persistent müllerian duct syndrome. Hum Mol Genet. 1996;5:1269–1277, used 

t syndrome. Hum Mol Genet. 1996;5:1269–1277, used with permission of Oxford 
University Press.)
915
absence of a uterus, hormone replacement can be continuous, 
unopposed estrogen, with no need for a progestin. Some adults 
with CAIS report improved well-being when androgen replace­
ment is also given; the mechanism of this androgenic effect is not 
understood. One double-blind crossover trial found increased 
sexual desire with testosterone compared with

sed 
sexual desire with testosterone compared with estradiol treatment 
but otherwise found no difference in mental-health outcomes.381 
Bone mineral density of the lumbar spine in women with CAIS 
with intact gonads is lower than in typical 46,XX women, indica­
tive of the direct effect of androgens on bone mineralization,382 
and women who have undergone gonadectomy have lower bone 
mineral density than those with intact gonads. Optimal estrogen 
therapy is essential as well as adequate calciu

n 
therapy is essential as well as adequate calcium, vitamin D, and 
physical exercise. A small increase in fractures has been reported in 
AIS, although this study did not show any difference in underly­
ing osteoporosis.383
There is an increased risk of gonadal germ cell tumors in 
46,XY DSD, including AIS. Germ cell maturation is delayed in 
the abnormal microenvironment. Some of the immature germ 
cells may subsequently mature, and while many may undergo 
apoptosis, some may retain fetal cha

 may undergo 
apoptosis, some may retain fetal characteristics and become pre­
malignant/malignant. Within the testicular environment, the 
premalignant lesion is termed germ-cell neoplasia in situ (GCNIS) 
(previously called carcinoma in situ [CIS] or intratubular germ cell 
neoplasia unclassified [ITGNU]). GCNIS may progress to inva­
sive germ cell tumors, most commonly a seminoma. Recently, the 
stage between maturational delay and GCNIS has been termed 
pre-GCNIS.384 GCNIS cells express immu

een termed 
pre-GCNIS.384 GCNIS cells express immunohistochemical mark­
ers such as placenta-like alkaline phosphatase (PLAP), c-KIT (the 
receptor for the stem cell factor SCF), POU5F1 (a transcription 
factor also called OCT3/4), and NANOG. However, in individu­
als with DSD, germ cells frequently show delayed maturation 
and prolonged expression of these markers. Therefore additional 
histologic criteria have been developed to distinguish matura­
tional delay from GCNIS (see “Tumor Risk and D

ra­
tional delay from GCNIS (see “Tumor Risk and DSDs” later 
in the chapter). In addition, SCF (or KIT ligand [KITLG]) has 
been proposed as a marker specific for germ cell neoplasia that 
is not expressed in germ cells with maturational delay.384 Nearly 
all GCNIS eventually progress to invasive germ cell cancer in 
individuals without DSD, but it is hypothesized that this pro­
gression requires androgenic activity and may therefore be much 
more uncommon in AIS.384,385 In CAIS the risk of dev

re uncommon in AIS.384,385 In CAIS the risk of developing 
GCNIS or a germ cell tumor before puberty is low (in the order 
of 0.8–2%).386,387 Hence there is a general preference to delay 
gonadectomy until young adulthood to enable spontaneous 
puberty to occur. The risk of pre-GCNIS, GNIS, or malignancy 
is estimated to be 10% to 14% in adolescent and adult individu­
als with CAIS.385,387 Some adults with CAIS elect to retain the 
gonads, though monitoring can be difficult, especially in case o

 monitoring can be difficult, especially in case of 
abdominal gonads (see “Tumor Risk and DSDs”).
Surgery to bring abdominal gonads to a more superficial loca­
tion is a potential option; a biopsy can be taken at the time of 
surgery to assess the risk of germ cell cancer.214 GCNIS and 
seminoma usually do not secrete tumor markers such as βhCG 
and α-fetoprotein (AFP), and GCNIS cannot be detected using 
ultrasound or MRI.388 Noninvasive monitoring may potentially 
become available with the de

ring may potentially 
become available with the development of techniques to measure 
embryonic microRNAs in serum, but this method is also currently 
unable to detect GCNIS.389 Common findings in CAIS gonads 
on MRI include Sertoli cell adenomas and epididymal cysts.388
Surgery in CAIS is generally confined to gonadectomy. For 
vaginal hypoplasia, self-dilatation is the first-line approach.390,391 
This technique is effective in lengthening the vagina for most 
CAIS women but requires time; thu

vagina for most 
CAIS women but requires time; thus it should be started when a 
girl is thinking about sexual relations and when there is clear focus 
and motivation. A specialist nurse or gynecologist with experience 
of vaginal dilatation is important to provide support and guid­
ance, and several useful written materials are becoming available 
(e.g., www.dsdteens.org). In situations in which self-dilatation is 
not appropriate or possible, alternative forms of vaginoplasty, such 
as the lap

lternative forms of vaginoplasty, such 
as the laparoscopic Vecchietti procedure, may be offered.390
The early management of PAIS centers on sex designation and the 
subsequent plan and timing for surgery. There has been an increas­
ing trend in recent decades to choose a male gender of rearing.392 
A recent prospective study of boys with PAIS showed the value of 
the EMS at birth as a predictor of pubertal development.393 Those 
with an EMS of 5 or more started puberty spontaneously. Further 
s

 or more started puberty spontaneously. Further 
studies on this cohort of 27 boys with PAIS involved functional 
analysis of the mutant ARs using reporter gene assays. In contrast to 
the EMS as a predictor, the results were not a reliable indicator of 
whether androgens would be required to induce puberty. However, 
such assays may have a place in guiding the magnitude of high-dose 
androgens sometimes required in adulthood in men with PAIS. 
Several surgical procedures may be required to corr

everal surgical procedures may be required to correct hypospadias 
and relocate the testes in a scrotal sac. Gynecomastia is an almost 
universal finding as boys with PAIS reach puberty. This raises the 
possibility of prevention by the use of antiestrogens and aromatase 
inhibitors and thereby avoids the need for surgical reduction mam­
moplasty, but long-term safety has not been established. The risk 
of germ cell tumor of the testis had previously been reported to be 
high in PAIS, but more r

usly been reported to be 
high in PAIS, but more recent reports suggest a lower risk, compa­
rable to that in CAIS.378,394 When scrotal in position, testes should 
be monitored by regular self-examination and periodic imaging by 
ultrasound. It has been suggested that bilateral testicular biopsies 
should be undertaken at the time of orchidopexy and after puberty 
to determine the presence of GCNIS.395 In view of the risk of malig­
nant progression, gonadectomy is advised if GCNIS is discovered,

on, gonadectomy is advised if GCNIS is discovered, 
although an alternative may be irradiation of bilateral in situ lesions.
When an infant with PAIS is designated a female sex, gonad­
ectomy will be required before puberty if the adolescent wishes 
to prevent virilization. If gonadectomy is performed, estrogen 
replacement is needed to induce puberty and for hormone replace­
ment thereafter. Whether and when to perform genitoplasty is a 
subject of controversy and is discussed in “Surgery and D

 of controversy and is discussed in “Surgery and DSDs.” In 
MAIS, clinical presentation occurs either in adolescence because 
of gynecomastia or in adulthood because of male factor infertility. 
Reduction mammoplasty is required for the gynecomastia. There 
is one report of successful fertility after testicular sperm extraction 
in a man with MAIS.396,538 
Other Conditions Affecting 46,XY Sex Development
Persistent Müllerian Duct Syndrome
AMH is a glycoprotein homodimer that is secreted by the S

a glycoprotein homodimer that is secreted by the Sertoli 
cells of the developing testes from approximately 7 wpc. It acts 
through the AMH type 2 receptor between 8 and 12 wpc to cause 
regression of the müllerian duct (see “Development of Reproduc­
tive Systems”).124
AMH is encoded by a 2.75-kb gene containing 5 exons in the 
region of chromosome 19p13.3. The AMHR2 is a serine/threo­
nine kinase with a single transmembrane domain that is encoded 
by an 11-exon gene on 12q13. Exons 1 to 3 code 

d 
by an 11-exon gene on 12q13. Exons 1 to 3 code for the signal 
sequence and the extracellular domain of the AMHR2, exon 4 
codes for the transmembrane domain, and exons 5 through 11 
code for the intracellular serine/threonine domain.
916
Persistent müllerian duct syndrome is a condition in which 
46,XY males have well-developed testes and normal male external 
genitalia but also have müllerian duct derivatives. The diagnosis is 
often not made un

t derivatives. The diagnosis is 
often not made until a fallopian tube and uterus are incidentally 
encountered in patients undergoing inguinal hernia repair, orchi­
opexy, or abdominal surgery. In the more prevalent form, a boy 
presents with bilaterally undescended testes, and the uterus, tubes, 
and testes are in the pelvis. In other situations, a hernia contains 
a partially descended or scrotal testis, and the ipsilateral fallopian 
tube and uterus are in the hernia, which is known as herni

 uterus are in the hernia, which is known as hernia uteri 
inguinale. In some instances, the contralateral testis and tube also 
are present in the hernial sac, known as transverse testicular ectopia.
Approximately one-half of all genetically proven cases of PMDS 
result from defects in AMH, and half are caused by mutations in 
its receptor (Fig. 24.33).397–399 AMH gene mutations are most 
commonly found in Mediterranean and North African coun­
tries,400 and most familial mutations are homozygou

ries,400 and most familial mutations are homozygous. In contrast, 
mutations in the gene encoding AMHR2 are more commonly 
found in Northern Europe and the Middle East. Europeans are 
often compound heterozygous for mutations involving a 27-bp 
deletion in exon 10.401,402 Measurement of serum AMH levels can 
provide a useful means for guiding genetic analysis; patients with 
PMDS caused by mutations of the AMH gene typically have low 
or undetectable levels of serum AMH (except for the p.Gln496H

able levels of serum AMH (except for the p.Gln496His 
mutation thought to affect receptor binding), whereas AMH con­
centrations are normal for age or elevated in patients with muta­
tions of AMHR2. Approximately 12% of patients with PMDS 
have an idiopathic form, with no mutation in AMH or AMHR2; 
they often have associated malformations.399
The main aim of management of PMDS is to prevent tes­
ticular cancer and preserve fertility in males, which can be dif­
ficult because of the anatomic find

ch can be dif­
ficult because of the anatomic findings. An increased prevalence 
of testicular degeneration has been described, which probably 
results from torsion of the testes and/or their anatomic location. 
Anatomic abnormalities of the epididymis and the vas deferens 
are common. Infertility may result from late orchiopexy or from 
mechanical problems associated with entrapment of the vas defer­
ens in the müllerian derivatives. Treatment consists of early orchi­
dopexy, which often requir

onsists of early orchi­
dopexy, which often requires removal of mullerian structures, 
although it should not be attempted to remove these in toto as 
this will likely damage the genital ducts or blood supply. Men 
with high pelvic testes rarely have successful orchidopexy, and 
many of these individuals are androgen deficient. Increasing rates 
of testicular malignancy have been reported in adult men with this 
condition, which further complicates long-term management of 
gonads that cannot be 

es long-term management of 
gonads that cannot be orchidopexed.399 Fertility is rare, but suc­
cessful TESE-ICSI has been reported in isolated cases.399 
Hypospadias is incomplete fusion of the penile urethra due to 
an arrest in development of the urethral spongiosum and ventral 
prepuce.403 The normal embryologic resolution of penile curva­
ture may also be interrupted, resulting in chordee. It is a common 
congenital anomaly with an estimated birth prevalence of 3 to 4 
cases per 

n estimated birth prevalence of 3 to 4 
cases per 1000 live births. Although it has been suggested that 
the rate of hypospadias has been increasing, this is not borne out 
in contemporaneous studies.404 The cause is unknown in most 
cases, but it is assumed that there is an interplay of genetic and 
environmental factors.405–407 Mutations in candidate genes such 
as MAMLD1, NR5A1, and the AR are found in a minority of 
cases.408 There is a familial clustering of cases in hypospadias, 
with a 7%

ial clustering of cases in hypospadias, 
with a 7% incidence of one or more additional affected family 
members.409 Four chromosomal regions have been identified in 
familial hypospadias by fine-mapping analysis of genes involved in 
steroid metabolism.410 A genome-wide association study involv­
ing a total of 2978 cases and 7298 controls of European ancestry 
identified 18 genetic loci associated with hypospadias at genome-
wide significance.407 None of these loci lay near known 46,XY 
DSD gene

 None of these loci lay near known 46,XY 
DSD genes, so distinct pathways may underlie the susceptibility 
to hypospadias.
Hypospadias is associated with increased maternal age, mul­
tiple births, maternal exposure to diethylstilbestrol in utero, 
paternal subfertility, maternal vegetarian diet, maternal smoking, 
assisted reproductive techniques, paternal exposure to pesticides, 
and fetal growth restriction.411 The association with low birth 
weight is remarkably consistent across all studies 

eight is remarkably consistent across all studies of idiopathic 
hypospadias.412
Hypospadias may be the only phenotype in mild forms of 
46,XY DSD, but it is not entirely clear when an evaluation for 
DSD is needed in a child with isolated hypospadias. Recom­
mendations have been made to conduct such an evaluation if the 
hypospadias is severe (midshaft or more proximal) or if there are 
additional genital findings (e.g., unilateral or bilateral cryptorchi­
dism, micropenis, anomalies of the scr

cryptorchi­
dism, micropenis, anomalies of the scrotum), but the prevalence 
of DSD conditions with each of these presentations has yet to be 
reported.
Management is generally surgical. The urethral meatus is relo­
cated to the glans, and the penis is straightened by correcting any 
chordee to give a forward-directed urinary stream and the ability 
to have sexual intercourse. Numerous techniques are described; 
repairs may require more than one stage, usually starting at 6 to 
18 months of age 

stage, usually starting at 6 to 
18 months of age or deferred until 4 years or later. Delaying sur­
gery allows the child to be involved in decisions regarding sur­
gery, but older age at surgery has been reported to be a risk factor 
for complications.413 A course of testosterone is often given to 
induce penile growth prior to surgery, particularly if micropenis 
is present. The severity of the hypospadias may not be apparent 
until release of the chordee. Complications include fistula forma­

the chordee. Complications include fistula forma­
tion, meatal stenosis, urethral stricture, and residual chordee. Fol­
low-up studies in adulthood indicate a less satisfactory cosmetic 
appearance compared to control subjects, shorter penile length, 
more dysfunction with voiding, and a lower urinary flow rate.414 
These problems were more evident in severe forms of hypospadias. 
Anorchia and Cryptorchidism
The term testicular regression syndrome (also called vanishing tes­
tis syndrome) is use

e (also called vanishing tes­
tis syndrome) is used to describe the phenotype of bilateral anor­
chia in a male infant with otherwise typical-appearing genitalia. 
The phenotype suggests the presence of testes in early gestation 
functioning appropriately to differentiate male external genita­
lia, induce müllerian duct regression, and stabilize wolffian duct 
development. Bilateral anorchia with a normally differentiated but 
small phallus (micropenis) is a variant of the syndrome and may 
repr

openis) is a variant of the syndrome and may 
represent a form of testicular dysgenesis. The cause of testicular 
regression syndrome is unknown, but an interruption of the testis 
blood supply by a torsion or vascular occlusion event in utero has 
been proposed. Familial cases suggest that there may be a genetic 
cause in some cases. Surgical exploration and histologic findings 
typically show nubbins of fibrous tissue devoid of any testicular 
tissue attached to a blind-ending vas deferens.
Di

Diagnosis is based on a combination of biochemical tests, 
imaging studies, and/or surgical exploration. An undetectable 
serum AMH level coupled with elevated serum levels of gonado­
tropins (particularly FSH) is highly predictive of the absence of 
917
testes.415 If such findings are present, there may be no need for 
surgical exploration. Androgen replacement is required to induce 
puberty at around 11 to

ent is required to induce 
puberty at around 11 to 12 years of age. Testicular prostheses can 
be inserted if and when the young person desires. It is unclear 
whether the testicular nubbins pose a risk for gonadal tumors, but 
most studies suggest that the risk is low; whether nubbins should 
be removed surgically remains a subject of debate.
Testes that have not descended at birth (i.e., cryptorchidism) 
present the most common congenital abnormality in boys, affect­
ing 2% to 9% of term infan

mality in boys, affect­
ing 2% to 9% of term infants.140,416 Because testicular descent 
occurs late in gestation, undescended testes are a common finding 
in premature baby boys. There is epidemiologic evidence of an 
increasing prevalence to 6% in populations studied in the United 
Kingdom.417 Longitudinal studies demonstrate the expected spon­
taneous descent of the testis by early infancy but an unexpected 
reascent of the testis to a cryptorchid position in later childhood. 
It is now recog

chid position in later childhood. 
It is now recognized that there are just as many boys with acquired 
cryptorchidism as there are with congenital cryptorchidism.418 
There is a strong association with low birth weight, disorders of 
the pituitary-gonadal axis, and a number of syndromes, but in the 
majority of situations the cause of maldescent is unknown. Unilat­
eral cryptorchidism does not require an evaluation to identify an 
underlying cause, but bilateral cryptorchidism could be a sign o

se, but bilateral cryptorchidism could be a sign of 
hypogonadotropic hypogonadotropism, which could in turn be a 
sign of combined pituitary hormone deficiency that would require 
prompt diagnosis and treatment to avoid neurocognitive deficits 
due to untreated central hypothyroidism and hypoglycemia due 
to ACTH and growth hormone deficiency. In addition, bilateral 
nonpalpable gonads can rarely be the presenting feature of 46,XX 
congenital adrenal hyperplasia. Unilateral and bilateral crypto

renal hyperplasia. Unilateral and bilateral cryptor­
chidism in the context of other signs of underandrogenization, 
such as hypospadias, also warrants a DSD evaluation. Mutations 
in INSL3 and the gene for its receptor, RXFP2—factors involved 
in the transabdominal phase of testis descent—were reported in a 
few boys with cryptorchidism.419 Cord blood levels of INSL3 but 
not testosterone are decreased in idiopathic cryptorchidism, par­
ticularly in infants whose testes descended by 3 months of

y in infants whose testes descended by 3 months of age.420
Congenital and acquired forms of cryptorchidism differ in 
their management. Orchidopexy is the recommended treatment 
in the former after 6 months but preferably no later than 12 to 
18 months of age according to recent guidelines.421,422 There is 
no consensus on the role of hormonal treatment with GnRH or 
hCG due to poor-quality evidence and lack of long-term data.422 
It is not routinely recommended because complete testicular 
desc

nely recommended because complete testicular 
descent is often not achieved. Both positive and negative poten­
tial effects of hormonal treatment on germ cells and testicular 
volume have been reported. The European Urology Association/
European Society for Paediatric Urology guidelines recommend 
offering hormonal treatment to boys with bilateral undescended 
testes to possibly improve further fertility potential, whereas the 
Canadian Urological Association–Pediatric Urologist of Canada 
guide

l Association–Pediatric Urologist of Canada 
guideline considers such treatment experimental.422,423 hCG has 
been shown to induce descent to a lower level in the majority of 
testes, so as neoadjuvant therapy it may improve testis position 
before surgery to increase the chances of successful orchidopexy.424 
Some also recommend orchidopexy for acquired cryptorchidism, 
but an alternative approach is to wait until puberty because in 
more than 50% of cases the testes descend spontaneously at th

0% of cases the testes descend spontaneously at this 
time.425 There is substantial evidence to indicate that the relative 
risk of germ cell tumor development in adulthood associated with 
cryptorchidism is more than halved if orchidopexy is undertaken 
before puberty.426 Early orchidopexy is also beneficial for testis 
development and spermatogenesis, although the evidence that this 
is translated into improved fertility is lacking.427 
Anatomic Defects of the Pelvis and Penis
Cloacal exstroph

c Defects of the Pelvis and Penis
Cloacal exstrophy is an important developmental condition that 
can affect genital development as part of a complex anatomic 
anomaly involving the bladder, usually with associated gastroin­
testinal, anorectal, and skeletal features. Penile agenesis is a very 
rare isolated defect in penile development. In both these condi­
tions, there is usually typical androgen synthesis by the testes, and 
androgen responsiveness in the brain, so a male gender identity is 

eness in the brain, so a male gender identity is 
most likely. 
Endocrine Disruptors
The proposed increase in male reproductive tract disorders (testis 
cancer, abnormal spermatogenesis, cryptorchidism, and hypospa­
dias) led to the concept that testicular dysgenesis syndrome (TDS) 
of fetal origin might be triggered by lifestyle factors and exposure 
to endocrine-disrupting chemicals (EDCs) operating during the 
fetal and perinatal periods.428,429 The US Environmental Protec­
tion Agency has de

29 The US Environmental Protec­
tion Agency has defined an EDC as an exogenous agent that inter­
feres with synthesis, secretion, transport, metabolism, binding 
action, or elimination of natural blood-borne hormones that are 
present in the body and are responsible for homeostasis, reproduc­
tion, and developmental processes.430 The types of compounds 
generically include detergents, pesticides, and cosmetics with pos­
tulated modes of action comprising estrogenic and antiandrogenic 
effects. S

mprising estrogenic and antiandrogenic 
effects. Such a broad definition and the ubiquity of thousands of 
compounds in the environment, which may have endocrine-dis­
rupting effects, pose an enormous challenge for scientific studies 
to prove that adverse health effects occur in humans, and, if so, 
what measures can be taken for prevention. Analgesics commonly 
used during pregnancy, such as paracetamol/acetaminophen and 
ibuprofen, have also been suggested as potential endocrine disrup­
tors 

een suggested as potential endocrine disrup­
tors interfering with testicular and ovarian development.431–434
Classic toxicological methods used in animals are not appo­
site for human studies, but application of a phenotypic marker 
long used by toxicologists testing EDCs in animals has been vali­
dated in humans for epidemiologic studies.435 Anogenital distance 
(AGD) is significantly reduced in isolated hypospadias and crypt­
orchidism in comparison with normative data (Fig. 24.34).436 As 
an

arison with normative data (Fig. 24.34).436 As 
an exemplar of the application of this marker in epidemiologic 
studies in reproductive development, prenatal exposure to phthal­
ates widely used as plasticizers in many products was associated 
with reduced AGD in males at birth.437,438 Xenografts of human 
fetal testis tissue in animals is a recent developed methodology 
which usefully informs on chemical effects relevant to human 
reproductive development.439 Another approach is the study of 
o

evelopment.439 Another approach is the study of 
occupational exposure where there is evidence of increased risk of 
hypospadias and cryptorchidism in offspring of parents working 
with pesticides, industrial chemicals, cosmetics, and several other 
potentially toxic substances.440,441
The relevance of these epidemiologic findings to the assessment 
of an individual child with DSD remains uncertain. There is no 
consensus yet about low-dose effects on health from the combina­
tion of a myriad of

ts on health from the combina­
tion of a myriad of chemicals to which humans are exposed over 
the lifespan or the optimal toxicological methodology to measure 
exposure.442,443 An expert panel has identified compelling evi­
dence for male infertility attributable to phthalate exposure and 
estimated the financial burden that accrues to the European Union 
from the consequences of needing additional assisted reproductive 
technology procedures.444 A similar analysis can be undertaken to 
918
SEC

4 A similar analysis can be undertaken to 
918
estimate the additional cases of cryptorchidism requiring orchi­
dopexy. Adopting the precautionary principle to reduce exposure 
to chemicals to as low as is reasonably practicable is a sensible 
approach in the face of so much uncertainty about the adverse 
effects of EDCs on human health. This applies particularly to 
pregnant women who may need to take an analgesic periodically. 
46,XX Disorders of S

e an analgesic periodically. 
46,XX Disorders of Sex Development
46,XX DSDs can be divided into disorders of ovarian development, 
disorders of steroid hormone synthesis leading to androgen excess, 
and other conditions affecting sex development (see Table 24.3).
Disorders of Ovarian Development
Disorders of ovarian development (i.e., ovarian dysgenesis or 
resistance) usually manifest as absent puberty due to the absence 
of estrogen production. Other disorders of gonadal development 
can resul

 Other disorders of gonadal development 
can result in a 46,XX individual with an ovary containing tes­
ticular tissue (i.e., 46,XX ovotesticular DSD) or a testis capable 
of producing enough testosterone for a male phenotype and/or 
sufficient AMH to regress the uterus (i.e., 46,XX testicular DSD).
Ovarian Dysgenesis
Ovarian dysgenesis most often occurs as a result of sex chromo­
some aneuploidy (e.g., Turner syndrome; 45,X and variants) 
with progressive follicular apoptosis. Mutations in many

rogressive follicular apoptosis. Mutations in many genes 
can result in POI, but each accounts for only a small proportion, 
and often no cause of POI is identified. Two recent reviews pro­
vide an excellent overview of the various groups of genes that are 
involved.445,446 Some of these genes are important for formation 
of the indifferent gonad; these include NR5A1 and WT1, which 
have been discussed earlier. FOXL2 plays a role in early ovarian 
AGD  
p  0.0001 
Penile length p  0.0001
Healt

 cephalad insertion of the penis (AP) and from the junction 
of the scrotal and perineal (AS) skin to the midpoint of the anus in males are commonly used methods 
to determine AGD in epidemiologic studies. (B) Measurements of penile length and AGD in boys with 
cryptorchidism and hypospadias compared with normal male infants. (Redrawn from Thankamony A, Lek 
N, Carroll D, et al. Anogenital distance and penile length in infants with hypospadias or cryptorchidism; 
comparison with normative data. 

 cryptorchidism; 
comparison with normative data. Environ Health Perspect. 2014;122:207–211, used with permission of 
Environmental Health Perspectives.)
919
determination, and mutations are associated with POI, usually 
but not always, as part of the blepharophimosis, ptosis, epicanthus 
inversus syndrome. Other genes are involved in meiosis and DNA 
repair, such as cohesins (e.g., STAG3), and various helicases (e.g., 
MCM8 and HFM1). Mutations in these 

icases (e.g., 
MCM8 and HFM1). Mutations in these genes disrupt meiotic pro­
gression of oocytes and result in oocyte degeneration. They form 
an important group of genes to consider in women with POI, as 
mutations in these genes are associated with an increased risk of 
various cancers. A group of genes involved in mitochondrial func­
tions cause sensorineural deafness in combination with POI, also 
known as Perrault syndrome and other neurosensory syndromes 
(e.g., POLG). Other syndromic caus

sory syndromes 
(e.g., POLG). Other syndromic causes include premature aging 
syndromes (e.g., LMNA) and skeletal syndromes (e.g., BMPR1B). 
Mutations in genes involved in the formation and maturation of 
the primordial follicles, such as SOHLH1, SOHLH2, NUP107, 
FIGLA, and NOBOX, as well as those involved in the next hor­
mone-dependent step, maturation and growth from primary to 
ovulatory follicles, such as ESR1, FSHR, LHCGR, and BMP15, 
cause nonsyndromic POI. One of the most common monogeni

 nonsyndromic POI. One of the most common monogenic 
causes is a premutation of FMR1, found in approximately 2% to 
6% of women with sporadic POI. SOX8 mutations have recently 
been identified in 5% of a cohort of females with POI.249 
46,XX Ovotesticular and 46,XX Testicular Disorders of Sex 
In rare situations, the developing ovary may contain testicular 
tissue (i.e., 46,XX ovotesticular DSD) or may even develop as a 
functioning testis (i.e., 46,XX testicular DSD). In 46,XX ovote

estis (i.e., 46,XX testicular DSD). In 46,XX ovotes­
ticular DSD, the patient usually presents with ambiguous genita­
lia at birth and progressive virilization at puberty if the testicular 
component is not removed. In contrast, 46,XX testicular DSD 
is usually associated with a male phenotype at birth and absence 
of müllerian structures. However, infertility occurs in both situa­
tions as crucial spermatogenesis genes on the Y chromosome are 
absent, and testicular androgen production may not 

bsent, and testicular androgen production may not be sufficient 
to support full pubertal development or may decrease with age, so 
careful monitoring is needed. In some circumstances ovotesticular 
DSD and testicular DSD may represent a spectrum of the same 
molecular condition.
The presentation, endocrinology, and management of 46,XX 
ovotesticular DSD is similar to those of ovotesticular DSD due 
to 46,XX/46,XY chimerism, as described earlier (see “Sex Chro­
mosome Disorders of Sex Developmen

(see “Sex Chro­
mosome Disorders of Sex Development”). However, 46,XX males 
cannot achieve male fertility. As discussed, a 46,XX karyotype 
is the most frequent finding in ovotesticular DSD, especially in 
patients from sub-Saharan Africa.219,220 Familial cases have been 
described, and it is likely that a genetic basis exists in some situa­
tions. Indeed, mutations affecting a specific residue (p.Arg92Trp 
or p.Arg92Gln) of NR5A1 (SF1) have recently been identified 
in a number of individuals 

ently been identified 
in a number of individuals with ovotesticular/testicular DSD 
from diverse ancestral backgrounds (see “Gonadal Sex”) (see Fig. 
24.20).71,72 These specific changes initiate a testis development 
pathway, possibly through loss of repressor activity, and account 
for up to 10% of ovotesticular DSD. Disruption of another 
nuclear receptor, NR2F2 (COUP-TFII), has also been reported in 
ovotesticular/testicular DSD and cardiac defects, sometimes with 
additional features (e.g.,

efects, sometimes with 
additional features (e.g., diaphragmatic hernia, BPES).122 Homo­
zygous mutations of the ovarian-testis repressor gene RSPO1 
(encoding R-spondin 1) can cause ovotesticular/testicular DSD 
with eye and skin abnormalities and hearing impairment.222,447
In contrast, patients with 46,XX testicular DSD often present 
first with male factor infertility, although sometimes hypospadias 
is present. In some cases, a family history is found. Because dif­
ferent family members can 

 is found. Because dif­
ferent family members can have different phenotypes, it is pos­
sible that 46,XX testicular DSD represents the most severe end 
of the spectrum of ovarian transdifferentiation phenotypes. Up 
to 80% of 46,XX males harbor translocations of Y-chromosomal 
material containing SRY, the testis-determining factor. This find­
ing helped considerably in mapping the SRY gene in the first 
instance (see “The Y Chromosome”), and in some situations, 
residual ovarian tissue (ovotesti

ome situations, 
residual ovarian tissue (ovotestis) develops. Individuals with SRY 
translocations can be diagnosed by FISH analysis using a probe 
directed to that gene (see Fig. 24.3). A number of SRY-negative 
cases have been reported. Sometimes, these arise from the same 
mutations in NR5A1 or NR2F2 that can cause 46,XX ovotes­
ticular DSD. Duplication of the genetic locus containing SOX9 
or deletions or rearrangements in distant regulatory regions that 
result in SOX9 overexpression,107–1

 regions that 
result in SOX9 overexpression,107–109 and similar alterations in 
the X chromosome resulting in overexpression of SOX3, have also 
been described.448 In addition, it has been hypothesized that over­
expression of SOX10 may be the cause of 46,XX ovotesticular/
testicular DSD in two individuals with an increased copy number 
of the 22q13 area.449 Loss-of-function mutations in RSPO1 have 
been reported in individuals with 46,XX testicular DSD as well 
as in ovotesticular DSD. Severe 

ular DSD as well 
as in ovotesticular DSD. Severe disruption of WNT4 can result 
in 46,XX ovotesticular/testicular DSD as part of the SERKAL 
syndrome, which also encompasses renal, adrenal, and lung 
dysgenesis.450 
Disorders of Androgen Excess
The steroid biosynthetic disorders causing androgen excess and 
46,XX DSD are summarized in Table 24.6. Although CYP21 
deficiency is by far the most common cause of this condition, 
alternative diagnoses should be considered because approaches to 
couns

 should be considered because approaches to 
counseling and management may vary.
3β-Hydroxysteroid Dehydrogenase Type 2 Deficiency
3βHSD/Δ4,5-isomerase catalyzes the conversion of Δ5 steroids to 
Δ4 steroids and is required for the generation of mineralocorti­
coids, glucocorticoids, and more potent androgens (e.g., testoster­
one, DHT) by the adrenal glands and gonads (see Fig. 24.11). The 
actions of 3βHSD and the consequences of deficiency of the type 
2 enzyme (3βHSD2) were described earlier

the type 
2 enzyme (3βHSD2) were described earlier because, in 46,XY 
individuals, this deficiency results in adrenal insufficiency with 
variable defects in androgenization.
Defects in 3βHSD2 can also cause a genital phenotype in 
46,XX females. Severe, recessively inherited defects in enzyme 
function can cause mild clitoral enlargement at birth in girls 
with glucocorticoid deficiency with or without salt loss. This 
mild androgenization occurs not as a direct androgenic effect 
of the excess

s not as a direct androgenic effect 
of the excess DHEA but as a result of conversion of DHEA and 
other Δ5-3β-hydroxy-C19-steroids to testosterone by 3βHSD1 
in the placenta and in the peripheral tissues of the fetus. This 
conversion, coupled with the limited capacity of the placenta to 
aromatize androgens to estrogens early in gestation, can increase 
circulating levels of androgens in the female fetus and cause 
modest clitoromegaly in a minority of patients. Other girls have 
normal genita

ority of patients. Other girls have 
normal genitalia but show adrenal features. Milder, nonclassic 
forms of 3βHSD2 deficiency can cause premature pubarche 
in girls. Breast development can occur at puberty in affected 
females, presumably by peripheral conversion of Δ5C19 steroids 
to Δ4C19 steroids by 3βHSD1 expressed principally in the liver 
and peripheral tissues and by the subsequent aromatization of 
androgens to estrogens.
920
The diagnosis 

    Endocrinology and the Life Span
The diagnosis of 3βHSD2 deficiency can be challenging in non­
classic forms of this condition.299 The levels of Δ5 steroids (e.g., 
17-hydroxypregnenolone, DHEA, and its sulfate, DHEAS) usually 
are increased, and the ratio of Δ5 steroids to Δ4 steroids (e.g., the 
ratio of 17-hydroxypregnenolone to cortisol) is markedly increased, 
especially after stimulation with intravenous ACTH. The urinary 
steroid profile can also be informative because 17-ketosteroids 

e can also be informative because 17-ketosteroids and 
especially DHEAS and 16-hydroxy-DHEAS are elevated. Although 
adrenal production of 17-OHP is reduced due to the enzymatic 
block, plasma concentrations of 17-OHP can actually be increased 
as a result of peripheral conversion of Δ517-hydroxypregnenolone 
to 17-OHP by the 3βHSD type 1 enzyme. This finding may lead to 
confusion with other forms of CAH such as CYP21A2 deficiency or 
POR deficiency. Mild forms of 3βHSD2 deficiency may manifest

ency. Mild forms of 3βHSD2 deficiency may manifest 
in a fashion similar to that of virilizing adrenal tumors. Suppres­
sion of the increased plasma and urinary levels of C19 and C21 
3β-hydroxysteroids by glucocorticoids distinguishes 3βHSD defi­
ciency in such cases. Treatment for 3βHSD deficiency is glucocor­
ticoid, mineralocorticoid, and salt supplementation, as appropriate, 
and estrogen replacement to induce puberty, if needed.300 
CYP21A2 deficiency (see Fig. 24

oxylase Deficiency
CYP21A2 deficiency (see Fig. 24.11) is one of the most common 
causes of ambiguous genitalia of the newborn. It is a disorder of 
adrenal steroidogenesis that affects around 1 in 15,000 children, 
with most 46,XX patients presenting at birth due to virilization of 
the genitalia (see Chapter 15).451,452
A 46,XX fetus with CYP21A2 deficiency can become andro­
genized to various degrees, as illustrated by the Prader clas­
sification (Fig. 24.35). High fetal concentrations of 17-

ion (Fig. 24.35). High fetal concentrations of 17-OHP, 
androstenedione, and testosterone are the hallmark of CAH due 
to CYP21A2 deficiency. More than 75% of patients with clas­
sic CAH, characterized by cortisol deficiency, are also salt losers 
because of their inability to synthesize sufficient mineralocorti­
coids. This is sometimes referred to as salt-wasting 21-hydroxylase 
deficiency. Other children do not develop such significant salt 
loss and are termed simple virilizing. A third form

oss and are termed simple virilizing. A third form is nonclassic 
21-hydroxylase deficiency, which is characterized by mild subclin­
ical mineralocorticoid and glucocortiocoid deficiency but varying 
degrees of excess androgen production, and usually presents with 
premature pubarche, irregular menses, or hirsutism but may also 
be asymptomatic. Salt loss is usually caused by mutations/dele­
tions of the CYP21A2 gene on chromosome 6p21.3 that result in 
a completely inactive enzyme. The concorda

ult in 
a completely inactive enzyme. The concordance between genotype 
and phenotype for the single-gene disorder is reasonably strong 
but not invariable (Fig. 24.36). In patients who are compound 
heterozygotes, the phenotype is usually determined by the milder 
allele. More than 90% of cases result from CYP21A2 deletion or 
one of nine mutations derived from recombination with the non­
functional pseudogene CYP21P (see Fig. 24.36). About 5% of 
CYP21A2 mutations arise spontaneously.453 World

f 
CYP21A2 mutations arise spontaneously.453 Worldwide, around 
100 non–pseudogene-derived mutations are reported for the 
CYP21A2 allele.
The diagnosis of CYP21A2 deficiency in a newborn with 
ambiguous genitalia is readily confirmed by markedly elevated 
serum 17-OHP concentrations (>300 nmol/L) after the first 48 
hours. Sick and/or preterm infants can have moderately increased 
steroid levels, so ACTH stimulation may be needed to resolve any 
Scoring external genitalia
Prader stage
Normal
I

Normal
I
II
III
IV
V
Normal
• Fig. 24.35  Prader classification of the degree of androgenization in a 
46,XX individual with congenital adrenal hyperplasia.
Phenotype
1
0%
30%
60%
A
Pro30Leu
Ala339His
% 21-Hydroxylase activity
expression in transfected cells
Pro453Ser
(20–50%)
Ile172Asn
(3–7%)
Val281Leu
(18±9%)
Arg356Trp
(2%)
Ile236Asn
Val237Glu
Met239Lys
G
306+t
Gln318x
Arg484Profs
∆8nt
5'
3'
2
3
4
5
6
7
8
9
10
NC/SV
SV/SW
SV/SW
SW
SW
SW
SW
NC
SW
NC
NC
S

8
9
10
NC/SV
SV/SW
SV/SW
SW
SW
SW
SW
NC
SW
NC
NC
SV
• Fig. 24.36  Diagram of the CYP21 gene and locations of the mutations that cause more than 90% of 
cases of 21-hydroxylase deficiency. The numbered boxes depict the exons. The three-letter abbreviations 
for amino acids are used; X indicates a nonsense (stop) mutation. An adenine (A) to guanine (G) transition 
in intron 2 causes a common splice site mutation. Other mutations include an 8-nucleotide deletion (Δ8nt) 
in exon 3, a thymidine inser

tide deletion (Δ8nt) 
in exon 3, a thymidine insertion at codon 306 (306+t), and a guanine (G) to cytosine (C) transition at codon 
484. The activities of the mutant enzymes, expressed as a percentage of the wild type, are indicated on 
the vertical axis and in parentheses for some missense mutations. Historic codon numbering is shown. 
Current classification of codon numbering is usually n + 1 (e.g., Pro30Leu is Pro31Leu, Ile 172Asn is 
Ile173Asn). NC, nonclassic form; SV, simple virilizing; SW

n). NC, nonclassic form; SV, simple virilizing; SW, salt wasting.
921
diagnostic confusion. The affected male usually has no alerting 
clinical signs at birth and, if a salt loser, does not decompensate 
with hyponatremia, hyperkalemia, and weight loss until 1 to 2 
weeks after birth.452 Newborn screening programs to measure 
blood spot 17-OHP concentrations soon after birth have been 
introduced in some countries.454 Elevated 17-OHP can also be 
seen in 

ountries.454 Elevated 17-OHP can also be 
seen in conditions such as 3βHSD deficiency, 11β-hydroxylase 
deficiency, and P450 oxidoreductase deficiency, although the lev­
els are usually not as high as in 21-hydroxylase deficiency. 
Treatment of 21-Hydroxylase Deficiency
Prompt diagnosis and treatment of 21-hydroxylase deficiency are 
required to replace adrenal steroids and to suppress androgen pro­
duction. Details of adrenal treatment are discussed in Chapter 15, 
and the approach to diagnosis

ssed in Chapter 15, 
and the approach to diagnosis and initial management are pro­
vided later (see “Investigation and Management of Disorders of 
Sex Development”). 21-hydroxylase deficiency is a potentially life-
threatening condition, and the atypical genitalia give an impor­
tant sign that this diagnosis should be considered.
Of relevance to DSD is the potential to prevent ambiguous 
genitalia of the newborn through prenatal treatment with dexa­
methasone, although this treatment is consider

xa­
methasone, although this treatment is considered experimen­
tal.455,456 Androgenization can be prevented if dexamethasone is 
administered to the mother early enough in gestation, indicating 
an intact fetal pituitary-adrenal axis that is responsive to negative-
feedback regulation. Treatment is effective if dexamethasone is 
started by 6 to 7 weeks of gestation but has side effects in treated 
mothers and fetuses. Traditionally, karyotype analysis and CYP21 
genotype analysis have not been 

alysis and CYP21 
genotype analysis have not been available until weeks 9 to 11, 
when chorionic villus biopsy can be performed. This means that 
seven unaffected or male fetuses would need to be treated with 
dexamethasone during this initial period for every one affected 
female fetus who might benefit. However, recent advances in the 
detection of circulating free fetal DNA in the maternal serum 
have increased the sensitivity and specificity of determining fetal 
chromosomal sex much earlier

of determining fetal 
chromosomal sex much earlier in pregnancy.457 Furthermore, 
using parental haplotype analysis by targeted massively parallel 
sequencing of the CYP21 locus, it has been possible to determine 
the genotype of the fetus earlier in pregnancy so that fewer unaf­
fected females would be treated.458 With this approach CAH can 
be diagnosed using a maternal blood sample drawn at 6 weeks 
of gestation, although this awaits validation in larger studies.459 
These are all potentially

n in larger studies.459 
These are all potentially promising advances to prevent expo­
sure of unaffected babies to dexamethasone, if parents do decide 
to have dexamethasone treatment in pregnancy in a center that 
offers it.
Several concerns have been raised about the potential effects of 
dexamethasone on the developing fetus. Although cognitive and 
motor development based on questionnaires appeared not to be 
adversely affected by exposure to dexamethasone, direct examina­
tion of children 

o dexamethasone, direct examina­
tion of children who had been exposed to dexamethasone from 
gestational age 6 to 7 weeks did show adverse effects on verbal 
working memory compared with control subjects.460–463 All of 
these studies were observational, and a meta-analysis empha­
sized the paucity of good-quality, long-term follow-up data to 
inform a risk-benefit analysis.464 Cognitive function may be more 
affected in girls than in boys.465 Limited long-term data in older 
children, adolescen

mited long-term data in older 
children, adolescents, and adults have suggested that any changes 
may not persist.466,467 However, this aspect should be considered 
when counseling families about prenatal treatment. Clinical prac­
tice guidelines have suggested that prenatal therapy should only 
be undertaken using protocols approved by institutional review 
boards and at centers capable of collecting long-term outcome 
data as part of multicenter studies.468,469 

r studies.468,469 
role in electron transfer from NADPH to all microsomal P450 
enzymes, including CYP17, CYP21, and CYP19 (aromatase) (see 
Fig. 24.28). Defects in POR can cause apparent combined CYP17 
and CYP21 deficiency, with or without Antley-Bixler syndrome, a 
form of craniosynostosis. These conditions were described earlier 
(see “Disorders of Androgen Synthesis”).320,322 POR deficiency 
can be ass

gen Synthesis”).320,322 POR deficiency 
can be associated with ambiguous genitalia in both 46,XX and 
46,XY infants. The androgenization of 46,XX fetuses may result 
from a defect in aromatase activity or occur through a proposed 
backdoor pathway of DHT production that does not involve 
androstenedione or testosterone as intermediates.134 Children 
with this condition usually have cortisol deficiency, but mineralo­
corticoid function is relatively preserved. Careful monitoring of 
puberty is ne

ly preserved. Careful monitoring of 
puberty is needed as this can be delayed or disrupted.320 
11β-hydroxylase (CYP11B1) deficiency can cause profound 
androgenization in an affected 46,XX fetus and accounts for 
approximately 5% of patients with CAH.470,471 11β-hydroxylase 
deficiency typically presents with ambiguous genitalia in the 
46,XX newborn, although in some situations the androgenization 
may be so marked that the baby is thought to be a boy with a 
small p

at the baby is thought to be a boy with a 
small penis and undescended testes.472 Subsequently in childhood 
there can be hyperandrogenism and precocious puberty, with 
accelerated growth and skeletal maturation. Hypertension occurs 
in two-thirds of cases as prolonged accumulation of 11-deoxycor­
ticosterone, which has some mineralocorticoid activity, leads to 
salt and fluid retention, and plasma renin is suppressed. Milder 
changes in 11β-hydroxylase cause a nonclassic form of the con­
dition

oxylase cause a nonclassic form of the con­
dition, similar to nonclassic 21-hydroxylase deficiency, which 
presents mostly with hyperandrogenism.473 The biochemi­
cal diagnosis can be made by urine steroid profile and through 
a pattern of reduced cortisol and elevated intermediates such as 
11-deoxycorticosterone.
11β-hydroxylase is encoded by the CYP11B1 gene, which is 
located on chromosome 8q21-22 in tandem with CYP11B2, which 
encodes aldosterone synthase, the enzyme catalyzing the conver­

terone synthase, the enzyme catalyzing the conver­
sion of deoxycorticosterone to corticosterone and then to aldo­
sterone. Some CYP11B1 mutations that cause 11β-hydroxylase 
deficiency are shown in Fig. 24.37. Most changes are missense 
mutations. Arg448 appears to be a relative hot spot for muta­
tions, with Arg448His possibly a founder mutation in the Moroc­
can Jewish population. Treatment of 11β-hydroxylase deficiency 
includes glucocorticoid replacement to reduce ACTH drive and 
in some si

d replacement to reduce ACTH drive and 
in some situations antihypertensive treatment. 
Familial Glucocorticoid Resistance
Glucocorticoid resistance is a rare disorder that is usually caused 
by sporadic heterozygous mutations in the glucocorticoid receptor 
α-isoform (GRα), encoded by the NR3C1 gene.474 Partial end-
organ insensitivity to glucocorticoid action coupled with impaired 
feedback mechanisms results in excess ACTH secretion and ele­
vated circulating cortisol levels without the clini

ated circulating cortisol levels without the clinical features of 
Cushing syndrome. In many cases, elevated levels of mineralocor­
ticoids cause hypertension and hypokalemia, and elevated levels 
of adrenal androgens cause hirsutism and acne. Although classic 
glucocorticoid resistance is not associated with atypical genitalia, 
922
one homozygous NR3C1 mutation (Val571Ala) was reported in a 
Brazilian girl who had a large clitoris, posterior labios

an girl who had a large clitoris, posterior labioscrotal fusion, 
and a urogenital sinus at birth.475 This mutation caused marked 
reduction in GRα function without complete loss of receptor 
activity. She also harbored a heterozygous mutation in CYP21, 
which may have influenced the phenotype. 
Aromatase Deficiency
Aromatase (CYP19A1), a cytochrome P450 enzyme, is the only 
enzyme known to catalyze the conversion of androgens (C19 
steroids) to estrogens (C18 steroids) in vertebrate species.476

estrogens (C18 steroids) in vertebrate species.476 
Aromatase is expressed in many tissues, including the placenta, 
ovary, brain, bone, vascular endothelium, breast, and adipose tis­
sue, where it is regulated by a number of tissue-specific promoters 
to convert testosterone to estradiol and androstenedione to estrone 
(Fig. 24.38). Aromatase plays a crucial role in the local production 
of estrogens and in the synthesis of circulating estrogens from the 
ovary at the time of puberty.
Aromatase

 from the 
Aromatase deficiency due to recessively inherited mutations in 
CYP19 has been described in approximately 40 girls with 46,XX 
DSD (see Fig. 24.38). The clinical and biochemical features of 
this condition underscore the key role aromatase plays in the 
Val129Met
His125Arg
Trp116X
Trp116Cys
Gln19X
Pro42Ser
Arg43Gln
Leu83Ser
Pro94Leu
Asn133His
Pro135Ser
Phe139Leu
Arg141X
Leu158Pro
Lys174X
Leu186Val
Thr196Ala
Trp247X
Gln265X
Arg384Gly
Arg384Gln
Arg386Val
Le

a
Trp247X
Gln265X
Arg384Gly
Arg384Gln
Arg386Val
Leu461Pro
Arg448His
Arg448Cys
Gly444Asp
Val441Gly
Arg427His
Tyr423X
Leu489Ser
Thr318Pro
Thr318Met
Thr318Arg
Gly314Arg
Ala306Val
Leu299Pro
Gly267Asp
Gly267Arg
Thr319Met
Phe321Val
Ala331Val
Gln338X
Arg341Ser
Gln356X
Ala368Asp
Glu371Gly
Arg374Gln
1
2
3
4
5
6
7
8
9
5′
3′
• Fig. 24.37  Diagram of the CYP11B1 gene and locations of selected mutations causing 11β-hydroxylase 
deficiency. The numbered solid boxes depict the exons. The three-letter abbreviat

boxes depict the exons. The three-letter abbreviations for amino acids are 
used; X indicates a nonsense (stop) mutation. A deletion of cytosine (C) at codon 32 and the addition of 
two nucleotides at codon 394 cause frameshift mutations; insertions and deletions resulting in a frameshift 
and splice site mutations are shown by solid triangles and open triangles, respectively.
Promoter
I.1
Promoter
I.4
Phe234
Tyr81Cys
Ala306_Ser314dup
Met127Arg
Promoter II
ATG
Asp411Ser
Arg375His
Arg375Cys
Val37

romoter II
ATG
Asp411Ser
Arg375His
Arg375Cys
Val370Met
Arg365Gln
Arg192His
Arg192Cys
Arg435Cys
Cys437Tyr
Leu451Pro
Arg457X
AATAAA
ATTAAA
I.1
2a
I.4
I.2
I.3 II
III
IV
V VI
VII VIII IX
X
5′
3′
• Fig. 24.38  Diagram of the CYP19 gene and selected mutations causing aromatase deficiency. The num­
bered solid boxes represent translated exons. The septum in the open box in exon II represents the 3′ 
acceptor splice junction for the untranslated exons. Multiple alternative promoters and the untranslated

ultiple alternative promoters and the untranslated 
exons (open boxes) are indicated. The three-letter abbreviations for amino acids are used to indicate the 
positions of missense mutations; X indicates a nonsense (stop) mutation; insertions and deletions resulting 
in frameshift and splice site mutations are shown by solid triangles and open triangles, respectively. In addi­
tion to the mutations causing classic aromatase deficiency, a homozygous Arg435Cys mutation and dele­
tion of a phenylal

us Arg435Cys mutation and dele­
tion of a phenylalanine residue at position 234 are both associated with a partial aromatase insufficiency 
phenotype. (Modified from Morishima A, Grumbach MM, Simpson ER, et al. Aromatase deficiency in male 
and female siblings caused by a novel mutation and the physiological role of estrogens. J Clin Endocrinol 
Metab. 1995;80:3689–3698, used with permission of The Endocrine Society, Copyright 1995.)
923
fetoplacental uni

24  Disorders of Sex Development
fetoplacental unit, and this condition is sometimes referred to as 
placental aromatase deficiency. Aromatase plays a critical role in 
protecting the fetus and mother from excessive androgen exposure 
during gestation. In the absence of aromatase, estrogen cannot 
be synthesized by the placenta, and large quantities of placental 
testosterone and androstenedione are transferred to the fetal and 
maternal circulation, resulting in androgenization of the female 
f

ion, resulting in androgenization of the female 
fetus and virilization of the mother during pregnancy.477
Females (46,XX) with aromatase insufficiency are born with 
clitoromegaly, various degrees of posterior fusion, scrotalization of 
the labioscrotal folds, and in some infants a urogenital sinus with 
a single perineal orifice.478–480 There is often a striking history of 
maternal virilization after the second trimester of pregnancy (e.g., 
acne, hair growth, voice changes) coupled with elev

cne, hair growth, voice changes) coupled with elevated maternal 
androgen levels, which usually resolve after the infant is born, but 
maternal virilization does not always occur.481,482 As expected for 
a steroidogenic defect, affected girls (46,XX) have normal mül­
lerian structures because AMH is not produced. The histologic 
picture of the ovaries in infancy is normal, but under increased 
FSH stimulation in the absence of ovarian aromatase, multiple 
enlarged follicular cysts develop. At pu

multiple 
enlarged follicular cysts develop. At puberty, affected females have 
hypergonadotropic hypogonadism, typically fail to develop female 
secondary sex characteristics, and exhibit progressive virilization. 
Plasma androstenedione and testosterone levels are elevated, and 
estrone and estradiol levels are low or not measurable. The hyper­
gonadotropism and the multiple ovarian cysts respond to estrogen 
replacement therapy, but in some cases, temporary treatment with 
an antiandrogen is 

ses, temporary treatment with 
an antiandrogen is necessary.
In addition to a role in the reproductive axis, aromatase defi­
ciency has implications for bone development, metabolism, and 
immune function, as determined by the long-term follow-up of 
the small numbers of women (46,XX) and men (46,XY) with 
aromatase deficiency and by studies of aromatase-knockout mice. 
Males present only after puberty and have tall stature, delayed 
bone maturation and delayed epiphyseal fusion, and osteopenia, 

on and delayed epiphyseal fusion, and osteopenia, 
demonstrating that estrogens are essential for normal skeletal mat­
uration and proportions and for the prevention of osteoporosis 
in males and females.483 Hyperinsulinemia and abnormal plasma 
lipids have also been reported in aromatase deficiency, which may 
in part reflect estrogen insufficiency and/or testosterone overpro­
duction but may also reflect specific actions of aromatase itself. 
The finding of apparently normal psychosexual devel

he finding of apparently normal psychosexual development in 
the three aromatase-deficient adolescent or adult patients and in a 
man with an estrogen receptor defect suggests that estrogen does 
not play a critical role in sex differentiation of the human brain, as 
has been reported in nonprimate mammals.
CYP19A1 mutations cause aromatase deficiency in males and 
females and are inherited in a recessive fashion (see Fig. 24.38). 
Functional assays of aromatase activity have shown severe loss o

ays of aromatase activity have shown severe loss of 
enzyme function (∼0.3%) in nearly all cases associated with clas­
sic aromatase deficiency; in one case, approximately 1% activity 
was found for an Arg435Cys change in a compound heterozygous 
state together with a null Cys437Tyr mutation.480 Partial aroma­
tase deficiency has also been described; a homozygous Arg435Cys 
change has been reported in a girl who presented with andro­
genized genitalia at birth and showed some limited breast deve

talia at birth and showed some limited breast devel­
opment during puberty, and deletion of a single phenylalanine 
residue (Phe234del) causing partial loss of aromatase activity was 
described in an androgenized 46,XX individual who achieved sig­
nificant breast development (to Tanner stage 4) during puberty.484
Thus a spectrum of phenotypes may be seen with aromatase 
insufficiency in humans. This important diagnosis should be 
considered in all androgenized 46,XX infants when more com­
mon fo

l androgenized 46,XX infants when more com­
mon forms of CAH (e.g., CYP21 deficiency) and ovotesticular 
DSD have been excluded. A history of maternal virilization in 
pregnancy should always be sought but may not always be pres­
ent, and increased levels of Δ4 androstenedione, testosterone, 
and DHT and low levels of plasma estriol, urinary estriol, and 
amniotic fluid estrone, estradiol, and estriol may be detected. The 
diagnosis can be difficult to make during childhood years, when 
the HPG 

ult to make during childhood years, when 
the HPG axis is relatively quiescent, and genetic analysis may be 
needed. 
Maternal Androgen Excess
Maternal sources of androgens that may virilize a female fetus may 
be endogenous from adrenal and ovarian tumors or exogenous 
from maternal exposure to androgenic compounds. Danazol, a 
synthetic derivative of ethisterone with androgenic, antiestrogenic, 
and antiprogestogenic activities, is used in diverse conditions such 
as endometriosis, benign fibr

rse conditions such 
as endometriosis, benign fibrocystic breast disease, and heredi­
tary angioedema and in women with unexplained subfertility. It 
crosses the placenta and is contraindicated in pregnancy in view 
of reports that a female fetus may become androgenized.485 Ovar­
ian causes of virilization include primary malignancy and benign 
lesions such as luteoma and hyperreactio luteinalis. Recurrence in 
subsequent pregnancies and maternal virilization can occur with 
luteomas.486 A case 

 virilization can occur with 
luteomas.486 A case of a similar pattern of recurrent hyperandro­
genization without evidence of placental aromatase deficiency has 
been reported.487 
Other Conditions Affecting 46,XX Sex Development
Several syndromic associations can cause developmental genital 
abnormalities in 46,XX females. Complex pelvic anomalies such 
as cloacal exstrophy can affect both sexes and require major recon­
structive surgery for bladder and bowel function and for the lower 
genita

adder and bowel function and for the lower 
genital system.
Abnormalities in uterine development can result in bicornate 
uterus (i.e., Fryns syndrome), uterine hemiagenesis or hypopla­
sia, or uterine agenesis. These conditions can be part of Mayer-
Rokitansky-Küster-Hauser syndrome, which may be associated 
with vaginal and/or ovarian agenesis as well as with renal, cardiac, 
and cervical spinal abnormalities, as in MURCS (müllerian, renal, 
and cervical spine syndrome).488,489 The cause of th

d cervical spine syndrome).488,489 The cause of these condi­
tions is unknown for most women, although some familial cases 
have been described, and a mutation in WNT4 was reported in a 
patient with absent müllerian structures (uterus and upper vagina), 
unilateral renal agenesis, and mild hyperandrogenemia.120 Uter­
ine abnormalities have also been associated with maturity-onset 
diabetes of the young type 5 (MODY5, HNF1B) and with vagi­
nal abnormalities in patients with hand-foot-genital syn

normalities in patients with hand-foot-genital syndrome 
(HOXA13) and McKusick-Kaufman syndrome (MKKS, formerly 
called BBS6). Because a prominent clitoris can be associated with 
conditions such as Fraser syndrome (not to be confused with Fra­
sier syndrome) or neurofibromatosis, careful evaluation is neces­
sary before a hyperandrogenic cause is diagnosed.
Other common conditions that may be mistaken for a more 
serious underlying disorder include apparent clitoromegaly seen 
in premature or f

ude apparent clitoromegaly seen 
in premature or formerly premature babies or when little labial 
adipose tissue is present. Assessment by a surgeon or physician 
with experience in recognizing normal variability in clitoral size 
is important. Sometimes this form of isolated clitoromegaly can 
persist into childhood.490 Labial adhesions are a common find­
ing in female infants. They often resolve spontaneously, although 
estrogen cream can be used to accelerate the process. Transient 
924
SECTI

ed to accelerate the process. Transient 
924
menstrual bleeding during the first week of life can be seen fre­
quently in female infants with the withdrawal of large amounts of 
estrogens and progesterones after birth. This can be alarming for 
parents but rapidly resolves, and no treatment is needed. 
Investigation and Management of Disorders 
of Sex Development
DSDs can present at many ages and to a wide range of different 
health professionals. So

 wide range of different 
health professionals. Some of the most common presentations 
are shown in Table 24.18, and the range of conditions discussed 
in our center over a 2-year period is shown in Table 24.19. The 
potential diagnosis of DSD has major implications for the child 
and the family. Most people will never have heard of DSD or 
have thought that they may be faced with, for example, a situ­
ation in which they cannot be told immediately whether their 
newborn is a boy or a girl. Talk

ly whether their 
newborn is a boy or a girl. Talking about such issues can be dif­
ficult, and sensitive and positive support and education from all 
health professionals is essential to provide parents with knowl­
edge and confidence.491
Specialist multidisciplinary (or interdisciplinary) team (MDT) 
involvement with families, young people, and adults with DSD 
is important.1–3,348,492 The pediatric (or adult) endocrinologist 
plays a central role in the team with support from psychologists, 

ole in the team with support from psychologists, 
urologists, and other health professionals (Fig. 24.39).3,492,493 The 
structure and available expertise vary depending on the center. 
Specialist MDTs are usually based only at large centers, so effective 
communication between the team and those health professionals 
managing the initial presentation is crucial. Sometimes regional or 
national networks (e.g., the Scottish Genital Anomalies Network) 
are needed to provide support where the popul

rk) 
are needed to provide support where the population is scattered 
geographically or to link experts in different centers together, with 
meetings held by teleconference rather than in person.494 It is 
important to keep the primary care physician informed. It is also 
useful for families to have a clearly defined point of contact if they 
need to ask further questions.
Although there is often uncertainty at the start, a general diag­
nosis can usually be reached following a series of careful

n usually be reached following a series of careful assess­
ments and basic investigations. Reaching a more specific diagnosis 
can be important in some situations and may take longer and 
involve more detailed investigations or genetic tests (discussed 
later). Every person is different, and many DSD-related condi­
tions can have a spectrum of presentations, so it is also important 
to take an individualized approach to the child and the family.
The following is a discussion of the investigation

The following is a discussion of the investigation and manage­
ment of DSDs at different ages. Several overarching themes are 
subsequently highlighted. More extensive details for specific con­
ditions are discussed earlier in the chapter.
Prenatal Diagnosis
Prenatal diagnosis of DSD is increasingly common as fetal ultra­
sonography improves and as more people have karyotype anal­
ysis or cell free fetal DNA for various reasons in pregnancy.495 
Although some prospective parents choose not to kn

Although some prospective parents choose not to know the sex of 
their baby, many couples ask whether they are having a boy or a 
girl, and issues can arise when the genital appearance is unclear on 
the scan or when the appearance of the genitals is discordant with 
the known karyotype. These situations can generate great anxiety, 
and early contact with the DSD MDT is helpful to support the 
parent(s). Sometimes prenatal genetic testing such as a microarray 
is considered, especially when mult

 a microarray 
is considered, especially when multiple congenital anomalies are 
suspected on ultrasound. However, prenatal genetic testing can 
occasionally produce inaccurate results that are not confirmed on 
karyotyping of the baby (e.g., in cases of twin fetal demise). Also, 
genital appearance at birth can differ from that seen on prena­
tal scans. Support to deal with the uncertainty and explanations 
of what will happen if their baby does have atypical genitalia are 
important, and discu

 have atypical genitalia are 
important, and discussion of specific diagnoses is best deferred 
until after birth. Nonetheless, prenatal suspicion of DSD can have 
benefits as it gives parents time to learn about possible conditions 
and to think how they will deal with the situation as long as sup­
port is provided in a sensitive manner. 
The Newborn With Atypical Genitalia
The most established presentation of DSD occurs when a new­
born baby has atypical (or ambiguous) genitalia, and it is not

s atypical (or ambiguous) genitalia, and it is not 
possible to say immediately whether the baby is a boy or a girl. 
Sometimes atypical genital features are first discovered on the first 
full newborn examination rather than immediately at birth. Fur­
ther investigations such as a karyotype are usually needed, and 
specialist review may be required. It is estimated that around 1 in 
4500 babies cannot have a sex designated immediately at birth, 
although the exact incidence is not known. Some p

although the exact incidence is not known. Some presentations 
  Common Presentations of DSDs at 
Different Ages
Feature
Examples
Prenatal
Karyotype-
phenotype 
discordance
Complete androgen insensitiv­
ity, 46,XY complete gonadal 
dysgenesis, 46,XX testicular 
DSD with SRY translocation
Neonatal
Atypical genitalia
21-hydroxylase deficiency 
(46,XX), 45,X/46,XY 
mosaicism, partial AIS, 
5α-reductase deficiency, 
SF1/NR5A1 (46,XY)
Salt-losing crisis
21-hydroxylase deficiency, 
3βHS

alt-losing crisis
3βHSD deficiency (46,XX)
StAR, CYP11A1, 3βHSD defi­
ciency, SF1/NR5A1 (46,XY)
Childhood
Inguinal hernia
Complete AIS
Androgenization
11β-hydroxylase deficiency
features
Wilms tumor
Puberty
17βHSD deficiency, 
5α-reductase deficiency 
(rarely SF1/NR5A1, partial 
AIS, ovotestes)
Absent puberty
Complete gonadal dysgen­
esis (Swyer syndrome), 
17α-hydroxylase deficiency
Postpuberty
Comple

droxylase deficiency
Adult
Tumors
Minimal AIS, SF1/NR5A1, etc.
AIS, Androgen insensitivity syndrome; CYP, cytochrome P450 enzyme; DSDs, disorders/dif­
ferences of sex development; HSD, hydroxysteroid dehydrogenase; SF1, steroidogenic factor 
1; StAR, steroidogenic acute regulatory (protein).
   
TABLE 24.18
925
  The Range of Diagnoses of DSD Seen in a Single Center Over 2 Years
Sex Chromosome DSD (n = 13)
46,XY DSD (n

r 2 Years
46,XY DSD (n = 65)
46,XX DSD (n = 23)
A: 47,XXY (Klinefelter syndrome and 
variants) (2)
B: 45,X (Turner syndrome and vari­
ants) (2) (both Y fragment)
C: 45,X/46,XY mosaicism (mixed 
gonadal dysgenesis and variants) 
(8)
D: 46,XX/46,XY (chimerism/mosa­
icism) (1)
Complete gonadal dysgenesis (7)
Partial gonadal dysgenesis (8)
Steroidogenic factor 1 (2)
Ovotesticular DSD 

 of gonadal (ovary) development
Ovotesticular DSD (1)
Testicular DSD (1)
Disorders of androgen biosynthesis
StAR (1)
17βHSD3 (2)
5α-reductase 2 (6)
CAIS (6)
PAIS (5)
Fetal
21-hydroxylase (10)
11β-hydroxylase (2)
Maternal
C: Other
Syndromic associations of male genital development 
(11) (e.g., chromosomal variants, skeletal, lung, 
skin, gastrointestinal, Cornelia de Lange, CHARGE)
Cloacal a

rointestinal, Cornelia de Lange, CHARGE)
Cloacal anomalies (1)
IUGR/preterm/hypospadias (4)
Persistent müllerian duct syndrome (3)
Vanishing testis syndrome (2)
Isolated severe hypospadias (4)
Micropenis (1)
Bilateral undescended testes (2)
C: Other
Syndromic associations (e.g., cloacal anomalies) (1)
Müllerian agenesis (1)
Clitoromegaly, possible clitoromegaly or clitoral vari­
ants (6)
Ovarian cysts (1)
CAIS, Complete androgen insensitivity syndrome; CHARGE, coloboma, heart defect, atresia cho

drome; CHARGE, coloboma, heart defect, atresia choanae, retarded growth and development, genital hypoplasia, ear anomalies (deafness); DSDs, disorders of 
sex development; IUGR, intrauterine growth restriction; PAIS, partial androgen insensitivity syndrome; StAR, steroidogenic acute regulatory (protein).
From Brain CE, Creighton SM, Mushtaq I, et al. Holistic management of DSD. Best Pract Res Clin Endocrinol Metab. 2010;24:335–354.
   
TABLE 24.19
Biochemist
Nurse specialist
Endocrinologist
Urol

eighton SM, Mushtaq I, et al. 
Holistic management of DSD. Best Pract Res Clin Endocrinol Metab. 2010;24:335–354.)
926
may be mistaken for a DSD condition, such as labial swelling after 
a breech delivery, undescended testes, or mild hypospadias, which 
can affect up to 1 in 300 male infants. Examples of genital features 
that should trigger consideration of a DSD condition are outlined 
in Table 24.20. In particular, any child with nonpalpable teste

0. In particular, any child with nonpalpable testes 
needs careful review because this child could—rarely, but impor­
tantly—be a 46,XX baby with severe, life-threatening CAH.
History and Examination
A careful history is important but must be done sensitively and 
rarely points to an immediate diagnosis. Key points to consider 
include aspects of the pregnancy such as growth restriction and 
preeclampsia, maternal acne or hair growth in pregnancy (which 
would suggest aromatase or POR deficiency

 (which 
would suggest aromatase or POR deficiency), and any prenatal 
tests or scans performed. Theoretically, medications and drugs 
may have an effect on the fetus (e.g., 5α-reductase inhibitors), 
and assisted reproduction or the need for fertility treatment might 
indicate an underlying and variably penetrant genetic cause. A 
careful family history should include any history of adrenal condi­
tions, known CAH or infant death, other known DSD, hypospa­
dias, or infertility (with attention t

D, hypospa­
dias, or infertility (with attention to X-linked patterns that could 
suggest androgen insensitivity syndrome), but such issues may 
not be known or widely discussed. Consanguinity is associated 
with an increased risk of autosomal recessive conditions such as 
many of the steroidogenic enzyme deficiencies. It is important to 
understand the parents’ thoughts and beliefs and not to generate 
additional anxiety by insensitive questions.
A careful physical examination is needed but sho

.
A careful physical examination is needed but should be done 
sensitively and only after the parents have had some time to be 
with their baby. Key features to note are the length of the phallus, 
consistency of the corpora and glans, extent of chordee; scrotal 
transposition (extension of the scrotal folds above the phallus), 
features of the foreskin (hooded, excess skin), position of the ure­
thral opening; presence or absence of a separate vaginal opening, 
rugosity (wrinkling of the scrota

aginal opening, 
rugosity (wrinkling of the scrotal skin, also called scrotalization) 
and degree of midline fusion of the labioscrotal folds, and size 
and position of any palpable gonads (likely to be testes). Geni­
tal appearances can sometimes be misleading because of chordee 
(bending and tethering of the phallus) and/or a large suprapubic 
fat pad, so palpation as well as inspection is important. Various 
scales such as the external masculinization score, the Prader scale, 
and the Quigley

nization score, the Prader scale, 
and the Quigley scale can be used to summarize findings and 
are particularly useful for research purposes, but it is also impor­
tant to note and document individual features (Fig. 24.40).376 
Asymmetry of the genitalia can suggest mixed gonadal dysgen­
esis associated with 45,X/46,XY mosaicism and can also be seen 
in partial gonadal dysgenesis or ovotesticular DSD. Other find­
ings such as low birthweight; position of the anus; skeletal, car­
diac, or other 

sition of the anus; skeletal, car­
diac, or other congenital anomalies; and dysmorphic features are 
important to note. Hyperpigmentation is a rare but important 
sign of primary adrenal insufficiency due to elevated ACTH, but 
increased labioscrotal pigmentation can often be seen in babies of 
Asian or African ancestry. 
The Initial Approach to the Baby With Atypical Genitalia
A useful mnemonic for the initial approach is BASIC: bonding, 
adrenal insufficiency, sex designation, imaging, and cyt

l insufficiency, sex designation, imaging, and cytogenetics. 
Each of these is introduced briefly here and discussed more exten­
sively elsewhere.
Bonding between the parents and baby can be disrupted by 
the shock of the situation or if the newborn is taken away to an 
intensive care nursery without good reason. Allowing parents 
time to focus on their new baby is important, and putting the 
genital issue aside at times is key. At the same time, the impact of 
the situation should not be trivia

 the impact of 
the situation should not be trivialized; striking the right balance 
requires an individualized approach involving frequent feedback 
from parents. Nursing staff can play a crucial role in helping fami­
lies initially by engaging with them and acknowledging the stress 
that uncertainty can bring while also addressing routine newborn 
issues such as establishing feeding, changing diapers, and sleeping. 
  Problems in Newborns That Merit 
Investigation for DSDs
Ambiguous (atypical

 Merit 
Ambiguous (atypical) genitalia
Apparent female genitalia
Enlarged clitoris
Posterior labial fusion
Inguinal/labial mass
Apparent male genitalia
Nonpalpable testes
Isolated penoscrotal hypospadias
Severe hypospadias, undescended testes, micropenis
Family history of DSD, such as complete AIS
Discordance between genital appearance and prenatal karyotype
AIS, Androgen insensitivity syndrome; DSDs, disorders of sex development.
   
TABLE 24.20
A
B
External masculinizati

opment.
   
A
B
External masculinization score
EMS
3
2
1
0
0
0.5
1
1.5
Left
gonad
Scrotal
fusion
Micro-
penis
Urethral
meatus
Norm
N
Y
Glan
Pen
Per
Abs
Abs
Abd
Abd
Ing
Ing
Scro
Scro
Y
N
Right
gonad
Normal
I
II
III
IV
V
Normal
• Fig. 24.40  (A) Prader staging for scoring the degree of androgenization of 
the external genitalia in a female infant with congenital adrenal hyperplasia. 
(B) The external masculinization score (EMS) is used to assess 

nal masculinization score (EMS) is used to assess the degree 
of underandrogenization in an individual with 46,XY DSD. The score is 
based on the presence or absence of a micropenis and bifid scrotum, 
the location of the urethral meatus (normal, glandular, penile, perineal), 
and the position of the testes (scrotal, inguinal, abdominal/absent). (B, 
from Ahmed SF, Khwaja O, Hughes JA. The role of a clinical score in the 
assessment of ambiguous genitalia. BJU Int. 2000;85:120–124.)
927
Chapter 

genitalia. BJU Int. 2000;85:120–124.)
927
Staff members themselves may feel uncomfortable and may also 
need support and guidance.
Adrenal insufficiency is associated with some causes of DSD 
and is potentially life threatening if not promptly recognized and 
treated. The common scenario is a 46,XX baby with 21-hydroxy­
lase deficiency, which occurs in about 15% to 20% of babies with 
atypical genitalia, more in some studies. Adrenal insufficiency 
can al

ore in some studies. Adrenal insufficiency 
can also occur in 46,XY babies (e.g., StAR, CYP11A, HSD3B2, 
CYP17A1, NR5A1) but is rare. Monitoring for hypoglycemia, 
hyponatremia, and hyperkalemia is necessary, but the electrolyte 
changes usually take several days to manifest. During this time the 
infant can be kept with its mother if otherwise well. Micropenis 
and/or undescended testes may be caused by gonadotropin defi­
ciency, which can be associated with deficiency of other pituitary 
hormo

sociated with deficiency of other pituitary 
hormones, including ACTH.
Sex designation should be undertaken as soon as possible but 
should not be rushed. Parents sometimes have strong preconcep­
tions about their baby, especially if they have been told prenatally 
to expect a boy or a girl, and/or may want a child of a specific sex 
to fit their desired family structure. If wrong decisions are made, 
it can be very traumatic to reverse them, so keeping an open mind 
from the start and avoiding 

keeping an open mind 
from the start and avoiding premature recommendations is criti­
cal.496 In the interim, it is advisable to use neutral terms such as 
“phallus” rather than “penis” or “clitoris,” and to refer to the child 
as “your baby,” “your child,” or “little one” rather than “he” or 
“she.”
Imaging is useful but can be operator dependent, and results 
can sometimes be deceptive. Pelvic ultrasound or MRI can show 
internal structures, but structures are not always clearly visualized, 
a

t structures are not always clearly visualized, 
a utricular remnant (commonly found in boys with hypospadias) 
can be confused with a uterus, and lymph nodes can be confused 
with testes.228,497 Parents and providers need to know the limita­
tions of these tests.
Cytogenetics, or a rapid assessment of sex chromosome com­
plement, is essential. This testing has been done traditionally by 
fluorescence in situ hybridization for X-chromosome and Y-chro­
mosome markers (see Fig. 24.3), but in some 

chro­
mosome markers (see Fig. 24.3), but in some centers is now being 
performed by quantitative fluorescent polymerase chain reaction 
(qfPCR) on DNA. The relevant laboratories should be informed of 
the urgency of the test, and samples should be transported quickly. 
An attempt to look for mosaicism should be made by determin­
ing the genotype in a sufficient number of cells, but FISH on a 
second tissue (such as a buccal smear) may be necessary to detect 
mosaicism. Rapid karyotyping can hel

ry to detect 
mosaicism. Rapid karyotyping can help with the next stages of 
management and investigation, and a result is usually available 
within 2 working days. 
Support for the Parents
Providing parents with adequate support in the first days while 
awaiting results is essential.491 It can be useful to stress that many 
other parents are in a similar position each year, that specialists 
who have knowledge and experience are available, and that the 
situation will become clearer in the next

hat the 
situation will become clearer in the next days. Expressions of 
anxiety by the staff may exacerbate anxiety in the parents. On 
the other hand, excessive nonchalance may be viewed as trivial­
izing a serious situation. Parents need to be provided with infor­
mation about the tests performed and a realistic expectation of 
when results will be available. They should be encouraged to ask 
questions and to rediscuss information that is not clear or dif­
ficult to understand. Sometimes reco

clear or dif­
ficult to understand. Sometimes recording conversations can be 
helpful so that families have an opportunity to listen to informa­
tion again.
Being repeatedly asked by family and friends if they have a 
boy or a girl can be highly stressful for parents. There is no single 
strategy that suits all parents, but many seek the safe haven of the 
hospital and will try to delay announcing the birth until the situa­
tion is clearer. It is reasonable to ask friends for some private time 

 reasonable to ask friends for some private time 
and to say that they will be in touch soon with more information. 
It often helps parents to confide in one or two close friends or fam­
ily members; sometimes parents can be under pressure with work, 
and siblings may have questions or need care. Several resources 
have been developed to help support parents during this time.498 
It is important to emphasize to parents that they need to look after 
themselves, too. 
Sex Designation
Most people w

r 
Most people wish to raise their child as either a boy or a girl. 
Sufficient information should be gathered so that parents can 
be informed and educated to make the best decisions for their 
child. This should be done as quickly as practical but without 
rushing decisions. The karyotype per se does not determine 
gender identity, but it is a useful starting point for focusing 
investigations and reaching a diagnosis (see Table 24.3 for clas­
sification of DS

agnosis (see Table 24.3 for clas­
sification of DSDs). A broad diagnosis is often sufficient for 
designating sex, and a more specific diagnosis can be reached 
later. Within the context of the karyotype and diagnosis, use­
ful points to consider are the child’s future gender identity, 
functional anatomic aspects such as likely urologic and sexual 
function, potential endocrine function (whether consistent 
or inconsistent with designated sex), fertility options, and 
the risk of gonadal tumors

fertility options, and 
the risk of gonadal tumors. Many of these questions are dif­
ficult to answer, and in some situations more investigations 
may be undertaken to inform the parents, but often the choice 
is relatively straightforward based on findings from physical 
examination, imaging studies, and karyotype. Current recom­
mendations are for 46,XX individuals with congenital adrenal 
hyperplasia to be raised as girls, though some have suggested 
that those with highly androgenized genita

gested 
that those with highly androgenized genitalia may be raised 
as boys.499 A female sex designation is appropriate for 46,XY 
individuals with complete androgen insensitivity syndrome 
or complete gonadal dysgenesis (Swyer syndrome), who have 
female-appearing external genitalia and are highly likely to 
identify as women in adulthood; a female sex designation is 
probably also appropriate for those with minimal androgen­
ization (e.g., isolated clitoromegaly). Particularly challenging 
si

lated clitoromegaly). Particularly challenging 
situations arise in babies with limited phallic development and 
a 46,XY karyotype who have intact testosterone responsive­
ness (such as 17β-hydroxysteroid dehydrogenase deficiency, 
5α-reductase deficiency, and partial testicular dysgenesis) or 
with mixed internal and external phenotypes (e.g., 45,X/46,XY 
or various forms of ovotesticular DSD). In the past there was a 
greater focus on the genitalia and a tendency to raise children 
with a very

alia and a tendency to raise children 
with a very small phallus as female. More recently, there has 
been a trend to raise more 46,XY children as male and to look 
at other aspects of the child such as likely gender identity, pos­
sible fertility, and endocrine function.392 Sometimes, cultural 
influences can be important for the parents.500 In most coun­
tries there is a time limit to register the birth of a baby (e.g., 42 
days in England and Wales), but this can usually be deferred or 
done 

Wales), but this can usually be deferred or 
done without defining sex (e.g., in Germany). 
928
Investigations for DSDs
Once the initial analysis of sex chromosomes is available, sev­
eral investigations can be undertaken to reach a more specific 
diagnosis or to obtain a more detailed assessment of urogenital 
anatomy and future function.2,492 It is difficult to develop rigid 
guidelines for investigations because local availability and afford­
abil

ations because local availability and afford­
ability of tests around the world vary greatly.501 Details of some 
of the commonly used tests for DSD are shown in Table 24.21, 
and potential investigations based on karyotype are shown in Fig. 
24.41. It is important to consider what the likelihood of a spe­
cific diagnosis might be. An overview of diagnoses discussed in 
our setting is shown in Table 24.19, but in areas of the world 
with greater consanguinity, the relative prevalence of recessiv

consanguinity, the relative prevalence of recessive 
conditions such as CAH is greater. There may also be hotspots 
of specific diagnoses due to founder effects (e.g., 5α-reductase in 
the Dominican Republic), so knowledge of the local prevalence 
of certain conditions is important. Genetic testing is more widely 
available and may sometimes avoid the need for invasive tests or 
prolonged endocrine stimulation, but it has certain drawbacks, 
which are discussed later.335 
Chromosomal DSD
In our 

h are discussed later.335 
In our experience, approximately 10% to 15% of newborn refer­
rals have a chromosomal DSD (usually 45,X/46,XY), and an 
individualized approach is needed.3,207 This includes assessment 
of anatomy and gonadal function and, if indicated, screening for 
Turner syndrome–associated features (e.g., renal and cardiac con­
ditions) (see Fig. 24.41). Sometimes an early examination under 
  Potential Investigations for DSDs
Approach
Test
Uses
Genetics
FISHa (X-

ons for DSDs
Approach
Test
Uses
Genetics
FISHa (X-specific and 
Y-specific probes)
Rapid analysis of sex chromosome complement on cells
qfPCRa
Rapid analysis of sex chromosome signal in DNA
Karyotypea
Analysis of sex chromosomes and autosomes in cells with ability to look for mosaicism by screening 
multiple cells, as well as detection of major deletions, duplications, and balanced translocations
Array CGH or SNP microar­
raya
Analysis of chromosome signal across the genome, with ability to dete

ome signal across the genome, with ability to detect smaller copy number variants but 
not balanced translocations, using DNA
Multiple ligation probe-
dependent amplification
Analysis of the loss or gain of specific exons or whole genes on a predefined panel of probes, such as for 
DSD genes, using DNA
Single-gene analysis
Sanger sequencing and analysis of individual genes that are highly likely to be the cause of DSD based on 
incidence and clinical and biochemical features (e.g., CYP21A2)
Targ

ical and biochemical features (e.g., CYP21A2)
Targeted panel sequencing
Analysis of large numbers of known DSD-causing genes using high throughput sequencing of DNA
Whole-exome sequencing
Analysis of all the coding exons in the DNA, which may show changes in known, putative, or novel DSD-
associated genes, using high-throughput sequencing
Endocrine
Routine serum biochemis­
trya; urinalysisa
May reveal a salt-losing crisis or associated renal disorder (e.g., WT1)
17-hydroxyprogesterone,a 
11-deox

rder (e.g., WT1)
11-deoxycortisol, 
17-hydroxypregnenolone
May help to diagnose CAH or reveal a specific block in an adrenal pathway relevant to DSD
Renin, ACTH
May show a salt-losing state or primary adrenal insufficiency
Testosterone,a androstene­
dione, DHT; in the basal 
state or following hCG 
stimulation
Indicates the degree of androgen production and ratios of androgens and may help to diagnose a block in 
androgen production consistent with a specific diagnosis 

n production consistent with a specific diagnosis (e.g., 17βHSD or 5α-reductase deficiencies); 
can also reveal androgen production in ovotesticular DSD
May indicate an underlying block in steroidogenesis or androgen insensitivity (LH), or impaired Sertoli cell 
function (FSH), or gonadal dysgenesis (FSH and LH)
AMH, inhibin B
Can be useful markers of testicular integrity: AMH is detectable throughout childhood and is reduced 
in testicular dysgenesis or absent in case of streak go

sticular dysgenesis or absent in case of streak gonads or anorchia; AMH may be high in AIS or 
reduced androgen production due to steroidogenic defects; AMH may help to reveal the presence of 
testicular tissue in 46,XX (ovo)testicular DSD
Urinary steroids by GC/MS
Can be used to diagnose specific steroidogenic defects in the newborn period (e.g., 21-hydroxylase defi­
ciency, 11β-hydroxylase deficiency, 3β-hydroxysteroid dehydrogenase deficiency, P450 oxidoreductase 
deficiency, 17α-hydroxylase 

 P450 oxidoreductase 
deficiency, 17α-hydroxylase deficiency); can reveal 5α-reductase deficiency only after 3–6 months of 
life
Dynamic tests: ACTH 
Used to assess the adrenal gland stress response (quantitative) and can be coupled with measurement of 
steroid metabolites or poststimulation urine steroid analysis to study ratios of metabolites (diagnostic)
hCG stimulation
Used in short (e.g., 3 day) or prolonged (e.g., 3 week) formats to assess androgen production (quantitative) 
an

s to assess androgen production (quantitative) 
and androgen biosynthesis pathways (diagnostic); can also be used to assess the presence of testicular 
tissue (e.g., anorchia, ovotestis), although AMH is now more often used initially
FSH stimulation test
Rarely used to investigate the presence of ovarian tissue by measuring inhibin A and estradiol response
TABLE 24.21
929
anesthesia and laparoscopy is performed to assess anatomy and to 
determine gonadal 

ormed to assess anatomy and to 
determine gonadal characteristics and histologic features. 
46,XX DSD
The majority of  babies with a 46,XX DSD have 21-hydroxy­
lase deficiency, although other rarer forms of CAH, ovotesticular 
DSD, and clitoromegaly associated with prematurity are seen. 
Children have variable genital appearances; sometimes the baby 
may have a clitoris that is naturally large (Table 24.22) or appears 
slightly more prominent or with excess clitoral skin. However, any 
46,XX bab

with excess clitoral skin. However, any 
46,XX baby with atypical genitalia must be assumed to be at risk 
of adrenal insufficiency until proved otherwise. Serial electrolytes 
should be monitored and tests to confirm or refute the diagnosis 
Approach
Test
Uses
Imaging
Abdominopelvic and renal 
ultrasounda
Can reveal the size, position, and structure of gonads (especially testes), the presence of müllerian struc­
tures, and associated changes (such as renal size or anomalies)
MRI
Sometimes used 

ch as renal size or anomalies)
MRI
Sometimes used to assess internal structures, especially in adolescence
Cystourethroscopy, sino­
gram
Can reveal the structure of the bladder, vagina, and common channel
Surgical
Laparoscopy
Can reveal internal structures by direct visualization, such as gonads and müllerian structures
Gonadal biopsies
Can be used to determine the nature of gonads, especially if dysgenetic testes or ovotestes are suspected
aIndicates first-line investigations for which results 

cates first-line investigations for which results are available within days. For images of G-banded karyotypes and FISH analysis, see Fig. 23.3.
ACTH, Adrenocorticotropic hormone; AIS, androgen insensitivity syndrome; AMH, antimüllerian hormone; CAH, congenital adrenal hyperplasia; CGH, comparative genomic hybridization; DHT, dihy­
drotestosterone; DSDs, disorders of sex development; FISH, fluorescent in situ hybridization; FSH, follicle-stimulating hormone; GC-MS, gas chromatography–mass spectr

ing hormone; GC-MS, gas chromatography–mass spectrometry; hCG, human chorionic 
gonadotropin; LH, luteinizing hormone; MRI, magnetic resonance imaging; qfPCR, quantitative fluorescent polymerase chain reaction; SNP, single-nucleotide polymorphism.
   
  Potential Investigations for DSDs—cont’d
46,XX
If CAH suspected (common):
PRIOR TO TREATMENT: glucose,
electrolytes, cortisol, PRA, aldosterone,
17OHP, A4, DHEAS, USP; consider ACTH
stimulation test (after d3)
Aromatase deficiency (v

timulation test (after d3)
Aromatase deficiency (very rare),
also clitoromegaly of prematurity and
structural variants
Ovotesticular DSD (rare):
AMH, inhibin B, testosterone, consider early 
EUA and laparoscopy
45,X/46,XY
AMH, inhibin B, testosterone, LH/FSH;
consider hCG stimulation test, or early
EUA/laparoscopy if uncertainty about
assignment; ECHO, TFTs, renal USS,
Turner syndrome screening, audiology, etc.
46,XY
Ensure adrenal function adequate
(rarely insufficient): glucose,
electrolytes, 

ate
electrolytes, ACTH, cortisol; consider
ACTH stimulation test
Is it testicular dysgenesis,
steroidogenic defect or androgen
resistance?
AMH, inhibin B, testosterone, LH/FSH,
USP; consider 3-day/3-week hCG
stimulation test
46,XX/46,XY (very rare)
Individualized approach; consider early
assignment
Initial karyotype
(FISH, qfPCR)
• Fig. 24.41  Overview of potential investigations for a newborn with atypical genitalia or disorde

s for a newborn with atypical genitalia or disorder of sex 
development (DSD) once the initial karyotype or assessment of sex chromosome complement has been 
made. A4, androstenedione; ACTH, adrenocorticotropic hormone; AMH, antimüllerian hormone; CAH, 
congenital adrenal hyperplasia; DHEAS, dehydroepiandrosterone sulfate; ECHO, echocardiogram; EUA, 
examination under anesthetic; FISH, fluorescent in situ hybridization; FSH, follicle-stimulating hormone; 
hCG, human chorionic gonadotropin; LH, l

hormone; 
hCG, human chorionic gonadotropin; LH, luteinizing hormone; 17-OHP, 17-hydroxyprogesterone; PRA, 
plasma renin activity; qfPCR, quantitative fluorescent polymerase chain reaction; TFT, thyroid function test; 
USP, urine steroid profiling; USS, ultrasound screening.
930
should be undertaken according to local protocol and assay avail­
ability (such as 17-OHP, basal and stimulated cortisol, ACTH, 
plasma renin activity, and urine steroid prof

TH, 
plasma renin activity, and urine steroid profiling). Results can 
be inaccurate before 3 days of age using conventional assays due 
to interfering substances, although liquid chromatography/tan­
dem mass spectrometry (LC/MS-MS) assays may be more useful 
if available.502,503 Steroid replacement treatment can be started 
once the diagnosis is clear and any additional samples obtained. 
Of note, 17-OHP can be mildly elevated in other forms of CAH 
(e.g., 11β-hydroxylase deficiency, 3βHSD, POR

CAH 
(e.g., 11β-hydroxylase deficiency, 3βHSD, POR), and investiga­
tion of steroid intermediates (e.g., 11-deoxycortisol, 11-deoxy­
corticosterone, 17-hydroxypregnenolone) and urine steroid 
profiling can be useful in these situations.350 If all adrenal inves­
tigations are negative, rare diagnoses such as 46,XX ovotesticular 
DSD and aromatase deficiency should be considered and AMH 
measured to investigate for testicular tissue.492 The presence of 
ovarian tissue can be difficult to assess in

e of 
ovarian tissue can be difficult to assess in early infancy without 
laparoscopy or even biopsy. Occasionally, follicles can be seen 
on imaging, or there can be a rise in estradiol and inhibin A fol­
lowing recombinant FSH stimulation, but such tests are rarely 
performed.175 
46,XY DSD
The most common newborn karyotype in our experience 
is 46,XY, which occurs in approximately 60% of babies with 
atypical genitalia. These children can have a range of conditions, 
including variable forms 

 a range of conditions, 
including variable forms of partial testicular dysgenesis, disor­
ders of androgen synthesis and action, or other diagnoses such 
as proximal hypospadias of unknown cause or a DSD-associated 
syndrome. In some situations, the diagnosis might influence 
decision making. For example, some clinicians would advocate 
raising a baby with 5α-reductase deficiency as a boy and giv­
ing dihydrotestosterone cream, whereas a child with androgen 
resistance causing a more severe for

ith androgen 
resistance causing a more severe form of PAIS might be raised 
as a girl. Basic investigations in the 46,XY baby can include 
karyotype or array analysis looking for established chromo­
somal alterations (e.g., 9q deletion, 10p deletion, Xp21 duplica­
tion) and basal assessment of testis integrity (e.g., AMH/MIS, 
androgens).2,125 Additional diagnostic tests to assess androgen 
synthesis include basal androgens (DHEA, androstenedione, 
testosterone, DHT) and short-term and long-ter

e, 
testosterone, DHT) and short-term and long-term hCG-stim­
ulated androgens.504 Basal androgens may be low initially and 
can be reassessed during minipuberty, around age 6 to 8 weeks. 
The ratios of intermediates after hCG stimulation can be use­
ful in diagnosing specific blocks, such as 17β-hydroxysteroid 
dehydrogenase deficiency (testosterone to androstenedione ratio 
often <0.8) and 5α-reductase deficiency (testosterone to dihy­
drotestosterone ratio often >10–20).349 However, these ass

sterone ratio often >10–20).349 However, these assays 
may not be accurate in the first few days of life unless performed 
by LC/MS-MS, and diagnosing these conditions by androgen 
ratios or urine steroid profiling can be difficult in the first few 
  Anthropometric Measurements of the External Genitalia (Mean ± SD)
Population
Age
Stretched Penile Length (cm) or 
Clitoral Length (mm)
Penile Width (cm) or 
Clitoral Width (mm)
Testicular Volume (mL) or 
Perineum Length (mm)
Male
30 

L) or 
Male
30 weeks GA
2.5 ± 0.4 cm
Full term
3.5 ± 0.4 cm
1.1 ± 0.1 cm
0.52 mL (median)
Japan
Term
14 years
2.9 ± 0.4 cm
8.3 ± 0.8 cm
Australia
24–36 weeks GA
2.27 + (0.16 × GA) cm
China
Term
3.1 ± 0.3 cm
1.07 ± 0.09 cm
India
Term
3.6 ± 0.4 cm
1.14 ± 0.07 cm
Term
3.4 ± 0.3 cm
1.13 ± 0.08 cm
Europe
Adult
13.3 ± 1.6 cm
16.5 to 18.2 mL
Female
Full term
4.0 ± 1.24 mm
3.32 ± 0.78 mm
Adult, nulliparous
15.4 ± 

± 0.78 mm
15.4 ± 4.3 mm
Adult
19.1 ± 8.7 mm
5.5 ± 1.7 mm
31.3 ± 8.5 mm
GA, Gestational age; SD, standard deviation.
Data from Cheng PK, Chanoine JP. Should the definition of micropenis vary according to ethnicity? Horm Res. 2001;55:278–281; Feldman KW, Smith DW. Fetal phallic growth and penile standards 
for newborn male infants. J Pediatr. 1975;86:395–398; Fujieda K, Matsuura N. Growth and maturation in the male genitalia from birth to adolescence.

n in the male genitalia from birth to adolescence. II. Change of penile length. Acta Paediatr Jpn. 
1987;29:220–223; Lloyd J, Crouch NS, Minto CL, et al. Female genital appearance: “normality” unfolds. BJOG. 2005;112:643–646; Oberfield SE, Mondok A, Shahrivar F, et al. Clitoral size in full-term 
infants. Am J Perinatol. 1989;64:53–54; Schonfield WA, Beebe GW. Normal growth and variation in the male genitalia from birth to maturity. J Urol. 1942;48:759–777; Tuladhar R, Davis PG, Batch J, 
et al.

48:759–777; Tuladhar R, Davis PG, Batch J, 
et al. Establishment of a normal range of penile length in preterm infants. J Paediatr Child Health. 1998;34:471–473; Verkauf BS, Von Thron J, O’Brien WF. Clitoral size in normal women. Obstet Gynecol. 
1992;80:41–44; Zachmann M, Prader A, Kind HP, et al. Testicular volume during adolescence: cross-sectional and longitudinal studies. Helv Paediatr Acta. 1974;29:61–72; from Hughes IA, Houk C, 
Ahmed SF, et al. Consensus statement on management of inters

et al. Consensus statement on management of intersex disorders. Arch Dis Child. 2006;91:554–562, used with permission.
   
TABLE 24.22
931
months of life, possibly because of other pathways or isoenzymes 
influencing the results.350 Adrenal disorders do occur but are 
rare. Most children with complete blocks in steroid production 
(e.g., StAR, CYP11A1) do not present with atypical genitalia 
but rather present as a phenotypic girl with a salt-losing cri­

sent as a phenotypic girl with a salt-losing cri­
sis, and the 46,XY karyotype is only found afterward. Atypical 
genitalia at birth can be a sign of partial defects in high ste­
roidogenic enzymes (e.g., partial StAR, CYP11A1) or partial 
17α-hydroxylase insufficiency (CYP17A1), as well as classic 
presentations of 3β-hydroxysteroid dehydrogenase deficiency 
(3βHSD2) or P450 oxidoreductase deficiency (POR). It is cur­
rently unclear whether detailed investigations of adrenal func­
tion are just

iled investigations of adrenal func­
tion are justified in all boys with 46,XY DSD, but an adrenal 
diagnosis should at least be considered. 
Genetic Testing and DSDs
Genetic testing plays an increasingly important role in the diag­
nostic process.505 It is important for reaching a specific diagnosis 
(for both the individual and other affected family members), as 
well as counseling about the risk of recurrence in future children, 
defining the need to look for associated features, and in some 

need to look for associated features, and in some situ­
ations providing guidance on issues such as potential gender iden­
tity, endocrine function and fertility, and tumor risk.334 Reaching 
a specific genetic diagnosis can provide a sense of resolution and 
reduce uncertainty but may generate anxiety if no cause is found 
or if incidental genetic information is uncovered. Parents may 
experience guilt or blame for transmitting a mutation to their 
child. Individuals and families should have a 

ir 
child. Individuals and families should have a clear understanding 
about what genetic tests are being performed and what the poten­
tial benefits and disadvantages might be. Genetic testing can be 
expensive and may not be available or affordable locally, especially 
as a clinical service.334
Approaches to genetic testing are changing rapidly with new 
technologies for the analysis of chromosomes as well as individual 
genes. In some centers, sex chromosome FISH is being replaced 
by qfPCR, 

 sex chromosome FISH is being replaced 
by qfPCR, and microarray or array-based comparative genomic 
hybridization (CGH) approaches are being used to replace or 
supplement traditional karyotype analysis. These technologies 
can detect much smaller copy number variations than traditional 
G-banded karyotypes but do not detect balanced translocations 
and, as with traditional karyotyping, may miss low levels of mosa­
icism. Multiplex ligation-dependent probe amplification (MLPA) 
is a method of l

ndent probe amplification (MLPA) 
is a method of looking for specific copy number changes (dele­
tions or duplications) in known DSD genes (e.g., duplication of 
NR0B1, or loss of exons of other genes) that are too small to be 
detected on arrays.
Analysis of single genes is also changing with the development 
of next-generation sequencing approaches. Traditionally, candi­
date genes for a condition were amplified one by one by PCR and 
analyzed by Sanger sequencing. This direct sequencing appro

by Sanger sequencing. This direct sequencing approach 
is still the method of choice when there is a clear candidate gene 
from biochemical analysis (e.g., HSD17B3, SRD5A2), especially 
when the presence of a pseudogene can make sequencing diffi­
cult (e.g., CYP21A2).334 However, this approach is relatively time 
consuming and expensive, particularly if there are several candi­
date genes. Newer approaches involve using NGS to sequence 
panels of genes relevant to DSD or sequencing all of the ex

 genes relevant to DSD or sequencing all of the exome 
or even whole genomes.176,506 These methods can provide high 
throughput analysis but are still expensive, generate large amounts 
of data (including so-called bystander or incidental data for other 
genes, which may or may not be desired), and are still mostly only 
available as research tools, although increasingly these approaches 
are used in clinical testing. Positive findings can be validated by 
focused clinic testing, which can then 

idated by 
focused clinic testing, which can then provide useful information 
for the family and clinician. Interpretation of identified variants 
in DSD genes can be challenging and may require detailed infor­
mation on anatomy and endocrine function in the affected indi­
vidual, genetic analysis in family members (to look at segregation 
of the variant with the phenotype), or (in vitro) functional studies 
of the variant.
Given the advances in genetics and increasing recognition that 
DSD can 

genetics and increasing recognition that 
DSD can be a feature of many well-defined syndromes, a clinical 
geneticist can be a very valuable member of the MDT, especially 
when a child needs to be examined for additional syndromic fea­
tures or when the family needs to be counseled about genetic test­
ing and the significance of any results. 
Presentation During Childhood
DSD can occasionally present during the childhood years. 
Sometimes a parent may notice genital differences for the first 
ti

t may notice genital differences for the first 
time in childhood or feel inguinal masses that turn out to be 
testes. A small proportion of inguinal hernias in girls are found 
to contain testes, most often in patients with CAIS. Sometimes 
associated features present for the first time in childhood; for 
example, a small proportion of boys with penoscrotal hypo­
spadias develop an abdominal mass due to a Wilms tumor and 
are found to have Denys-Drash syndrome (due to mutations 
in WT1). 
Prese

-Drash syndrome (due to mutations 
in WT1). 
Presentation During Adolescence
Another common time for DSD to present is during adolescence 
in one of three well-recognized ways: (1) a girl who experiences 
spontaneous virilization at puberty, (2) a girl with absence of 
pubertal development, and (3) a girl who has primary amenorrhea 
after having normal breast development.2 Although the situation 
is not as urgent as in the newborn who may be at risk of salt-
wasting CAH, it is equally if not mor

isk of salt-
wasting CAH, it is equally if not more important to handle the 
situation with sensitivity, as the young person is aware of the issues 
as well as the parents and must be involved in decision making 
and consent.
The conditions that are typically associated with virilization at 
puberty are 5α-reductase deficiency type 2 and 17β-hydroxysteroid 
dehydroxygenase deficiency type 3. Alternative diagnoses include 
cases in which previous clitoromegaly may have been overlooked 
in partial

clitoromegaly may have been overlooked 
in partial testicular dysgenesis (e.g., mutations in SF1/NR5A1),507 
partial androgen insensitivity syndrome, or testicular/ovotesticu­
lar DSD. Usually the diagnosis is relatively straightforward based 
on serum concentrations and ratios of androgens, urine steroid 
profiling, and genetic testing.2 It is best to avoid further stimula­
tion with hCG tests if virilization is undesired, and some have 
proposed that puberty can be blocked for a period with a 

d that puberty can be blocked for a period with a GnRH 
analog, or if not available, antiandrogens to provide time to 
obtain a genetic diagnosis, educate and counsel the young person 
and the family, and give them time make a decision. An MDT 
approach is invaluable, and the input of an experienced psycholo­
gist or expert in gender medicine is key. A proportion of young 
people with 5α-reductase deficiency will choose to transition 
from female to male, and this has also been reported for some

 to male, and this has also been reported for some 
individuals with 17-HSD deficiency.151,152 Many girls with these 
conditions continue to identify as females and often want to 
have their gonads removed; appropriate counseling about risks, 
932
benefits, and fertility options is essential. If gonads are dysfunc­
tional or removed, puberty will need to be induced with estrogens. 
Gathering and sharing the information and providing psychologi­
cal s

ng the information and providing psychologi­
cal support over a period of time can help ensure that the young 
person makes the best choices for the future.
Another presentation in adolescence is a girl who does not have 
any pubertal development. This situation may represent many 
diagnoses in a 46,XX girl, including constitutional pubertal delay, 
hypogonadotropic hypogonadism, and congenital or acquired (e.g., 
autoimmune) primary ovarian insufficiency. Absence of puber­
tal development may a

fficiency. Absence of puber­
tal development may also be the presenting feature of sex chro­
mosomal DSD (Turner syndrome) or a 46,XY DSD (complete 
gonadal dysgenesis or 17α-hydroxylase/17,20-lyase deficiency). In 
complete testicular dysgenesis (Swyer syndrome), both Leydig cell 
production of testosterone and Sertoli cell production of AMH are 
impaired. As a result, müllerian structures are typically present. The 
immature uterus may be difficult to see, even on MRI scan, but 
may become mor

cult to see, even on MRI scan, but 
may become more easily visualized following estrogen treatment 
(the “clandestine” uterus).508 Dysgenetic gonads, which may appear 
streak-like (i.e., flat and scar-like), have a high tumor risk even in 
prepubertal children and should therefore be removed. True streak 
gonads (i.e., gonads that consist of fibrous tissue and lack germ cells 
and support cells) are not at risk of germ cell cancers (GCC) by 
definition, but this can only be confirmed at histolog

nition, but this can only be confirmed at histologic examina­
tion. The underlying cause of complete gonadal dysgenesis may be a 
range of genetic conditions (see Table 24.6), but a cause is often not 
found. Puberty can be induced with estrogens. In contrast, 46,XY 
girls with 17α-hydroxylase/17,20-lyase deficiency have intact Ser­
toli cell function and AMH production, and as a result they do not 
have a uterus. The block in adrenal steroidogenesis can be associ­
ated with hypertension and hyp

esis can be associ­
ated with hypertension and hypokalemia, which can cause arrhyth­
mias, so this is a rare but important diagnosis to make. Estrogen 
treatment is needed. The gonads are typically undescended but 
not dysgenetic, and the risk for GCC in pubertal children is likely 
to be low, but there may be an increased risk of germ cell tumors 
in late adolescence and adulthood. Two large studies of gonadal 
tumors in XY DSD individuals, which included 42 females with 
17α-hydroxylase/17,20-

h included 42 females with 
17α-hydroxylase/17,20-lyase deficiency, reported one dysgermi­
noma, two Sertoli cell tumors, and one Leydig cell tumor.509,510 A 
decision about gonadectomy should be made after discussing these 
risks.
The third common presentation is with primary amenorrhea 
in a girl who has experienced breast development. Acquired hypo­
gonadotropic hypogonadism should be considered, and primary 
ovarian insufficiency may also be associated with some breast 
development but absen

associated with some breast 
development but absent menarche. Hormone measurements, a 
karyotype, and pelvic ultrasound are often useful initial investiga­
tions.2 If a uterus is visualized, obstruction of the outflow tract is 
likely, such as imperforate hymen; this is usually accompanied by 
cyclic abdominal pain. In the absence of a uterus, the diagnosis is 
most likely a form of uterine agenesis, such as MRKH syndrome if 
the karyotype is 46,XX and complete androgen insensitivity syn­
drome 

XX and complete androgen insensitivity syn­
drome (CAIS) if the karyotype is 46,XY. Girls with CAIS often 
have reduced pubic and axillary hair. Detailed discussion about 
the management of these conditions is provided earlier in this 
chapter (see “Complete Androgen Insensitivity Syndrome”). 
Presentation During Adulthood
Occasionally, DSD may first present in adulthood. Examples 
include mild variants in DSD genes associated with male factor 
infertility or ovarian insufficiency (e.g., NR5A1),

nfertility or ovarian insufficiency (e.g., NR5A1), conditions such 
as persistent müllerian duct syndrome when a uterine-like struc­
ture is found incidentally in a man, or when a person with a more 
typical form of DSD presents to health professionals later in life, 
sometimes after migration from a medically underserved area or 
from one country to another. 
Information Sharing, Transitioning, and Adult 
Services
Information sharing (disclosure) is an important part of educat­
ing people about

) is an important part of educat­
ing people about their condition and giving them insight into 
the future. In the past, there was more secrecy around DSD, but 
most research supports sharing information in an age-appropriate 
manner over time.511 Sometimes parents need considerable help 
in how to deal with questions and concerns, and psychological 
support is important to allow them to feel confident in discuss­
ing these issues with their child.491,512,513 Education is an ongoing 
process an

ld.491,512,513 Education is an ongoing 
process and must account for ongoing changes with the child’s 
cognitive development.511,514 Different families may prefer dif­
ferent strategies, but being prepared for an open dialogue when 
opportunities arise usually works well. Sometimes a psychologist 
or pediatric endocrinologist can provide information at key stages 
and support the family in reinforcing information or answering 
questions as they come up. It is important to discuss what termi­
nol

ome up. It is important to discuss what termi­
nology the child and parents prefer; most people prefer the use of 
the specific diagnosis rather than the general term DSD.515
During childhood, growth and development are monitored. 
In adolescence, hormone replacement therapy may be indi­
cated in cases of gonadal dysfunction, or as discussed, in some 
situations puberty may be blocked to delay the progression of 
virilization.
Transitioning the care of young people from pediatric to adult 
servi

are of young people from pediatric to adult 
services is another important aspect of the care of DSD man­
agement.516,517 Often the pediatric endocrinologist will induce 
puberty before transfer to an adult endocrinologist, but it may be 
useful to involve the adult provider at an earlier stage so that the 
young person can build trust with the doctor. In childhood and 
later, any genital examinations should be performed only when 
necessary and ideally by someone with experience who will be 
in

ideally by someone with experience who will be 
involved in long-term care. Photography should be avoided unless 
absolutely necessary and then only with consent.
As more young people transition into adult services, the need 
for specialist adult centers for DSDs is becoming apparent. Stan­
dardized follow-up has been proposed to detect and treat health 
problems associated with DSD in adulthood.514 The team still 
largely includes an endocrinologist, psychologist, urologist, and 
gynecologist, 

gist, psychologist, urologist, and 
gynecologist, and radiologists can be important team members, 
especially when discussing young people with retained gonads 
or complex anatomy. Multidisciplinary clinics in which all team 
members are available are useful for coordinating care and reduc­
ing the number of hospital visits and time away from work. The 
key issues in long-term management of DSDs include hormone 
replacement, bone health, sexual activity, relationship issues, 
genital/urinary iss

ctivity, relationship issues, 
genital/urinary issues, psychological well-being, fertility/family 
building, and other factors that contribute to overall quality of 
life.518–520 Some data are now emerging on outcomes in adult­
hood for general medical issues and quality of life (see later). 
Support Groups and Information
Support groups can also have a very important role in providing 
additional information for children, adolescents, and adults with 
933
Chapter 24  Disorders of Sex Developmen

 with 
933
DSD and for bringing members of the community together.521 
There are well-established support groups in several countries for 
general aspects of DSD as well as for specific conditions such as 
CAH and AIS (for a recent overview, see Lee and colleagues522). 
Increasingly accessible information about DSD is becoming avail­
able through the internet, such as the biology of DSDs,523 sup­
port for families (e.g., dsdfamilies.org),498,524 and infor

families (e.g., dsdfamilies.org),498,524 and information for 
young people about DSDs (e.g., dsdteens.org).525 It is useful to 
guide young people and families in how to search for information, 
as general search terms such as sex can result in many inappropri­
ate hits. Patients and families should also be counseled that there 
is great variability within and between DSD conditions, and thus 
some information may not be applicable. 
Tumor Risk and DSDs
The gonads of individuals with DSD with Y 

nd DSDs
The gonads of individuals with DSD with Y chromosomal mate­
rial containing the GBY region are at risk of malignant change, but 
the incidence varies widely, depending on the underlying diagnosis 
(Table 24.23). Precise risk assessment per diagnosis is hampered by 
the lack of a molecularly confirmed diagnosis in various studies, 
report bias, inconsistent criteria to define precursor lesions, and the 
practice of early gonadectomy that has limited information on the 
natural history. In

as limited information on the 
natural history. In general, forms of DSD with impaired or arrested 
gonadal development have a high risk of germ cell cancers, whereas 
forms with normal testis development but reduced androgen syn­
thesis or action have a lower risk, especially in childhood.384 Scrotal 
testes are thought to be at lower risk of GCC development than 
undescended testes.384 True streak gonads (i.e., gonads that consist 
of fibrous tissue and lack germ cells) are by definition not a

issue and lack germ cells) are by definition not at risk of 
GCC, but this can only be concluded at histologic examination. 
Streak-like gonads (i.e., gonads that appear scar-like and lack endo­
crine function) may in fact contain germ cells, and these dysgenetic 
gonads are at risk of GCC.
Various subtypes of GCC exist: seminomatous cancers, on 
the one hand, classified as seminoma in the testis, dysgermi­
noma in the ovary and dysgenetic gonad, and germinoma when 
located extragonadally; nonse

 and germinoma when 
located extragonadally; nonseminomatous cancers, on the other 
hand, including teratoma, yolk sac tumor, choriocarcinoma, and 
embryonal carcinoma.384 The development of GCC is preceded 
by abnormal germ cell development.
In poorly differentiated gonads, germ cells may be present 
without the formation of follicles or seminiferous tubules; this 
is known as undifferentiated gonadal tissue. These germ cells may 
develop into a gonadoblastoma, which is the precursor lesion of 

 gonadoblastoma, which is the precursor lesion of 
a dysgerminoma.384 If the gonad is more testicularized, the germ 
cells are present in seminiferous tubules, although these may 
remain immature due to reduced synthesis or action of andro­
gens. Delayed germ cell maturation is often seen in DSD and is 
characterized by luminal gonocytes that show prolonged expres­
sion of markers such as POU5F1 (also known as OCT3/4) that 
are normally only expressed during early life. Once these germ 
cells ha

essed during early life. Once these germ 
cells have migrated to the basal membrane but continue to express 
POU5F1, coexpress TSPY in a heterogeneous pattern, and show 
focal KITLG expression, they are classified as pre-GCNIS.384 
If germ cells show cytonuclear atypia with homogeneous TSPY 
expression and diffuse KITLG expression, then GCNIS is diag­
nosed.384 GCNIS is the precursor lesion of GCC in an underviril­
ized testis. GCNIS was previously known as carcinoma in situ, 
testicular intraep

ly known as carcinoma in situ, 
testicular intraepithelial neoplasia, or intratubular germ cell neopla­
sia, unclassified.
In the general population, GCNIS is thought to progress 
to invasive GCC in the majority of cases, but this progression 
seems rare in CAIS, suggesting that androgens influence this 
progression.384,385
The risk of GCC can be estimated using information from 
clinical assessment, endocrine and genetic evaluation, and imag­
ing. Gonadal biopsy is the gold standard, although i

g. Gonadal biopsy is the gold standard, although it is important 
  Risk of Germ Cell Malignancy According to Diagnosis
Risk Group
Disorder
Malignancy Risk (%)
Recommended Action
No. Studies
No. Patients
High
GDa (+Y)b intraabdominal
15–35
Gonadectomyd
12
350
Partial AISc nonscrotal
50
2
24
Frasier syndrome
60
1
15
Denys-Drash syndrome 
(+Y)
40
1
5
Turner syndrome (+Y)
12
11
43
17βHSD
28
Watchful waiting
2
7
GD (+Y)b scrotal
Unkno

HSD
28
2
7
Unknown
Biopsye and irradiation?
0
0
Partial AIS scrotal gonad
Unknown
0
0
Low
Complete AISf
2
Biopsye and ?
2
55
3
Testis tissue removal?
3
426
Turner syndrome (–Y)
1
None
11
557
No (?)
5α-reductase
0
Unresolved
1
3
0
1
2
aGD including disorders not further specified, 46XY, 45X/46XY, mixed, partial, and complete.
bGBY region positive, including the TSPY gene.
cMore recent re

positive, including the TSPY gene.
cMore recent reports suggest a lower risk, comparable to that in CAIS.378,394
dAt time of diagnosis.
eAt puberty, allowing investigation of at least 30 seminiferous tubules, preferentially diagnosis based on OCT3/4 immunohistochemistry.
fThis risk estimate concerns postpubertal individuals; the risk is lower before puberty.387
AIS, Androgen insensitivity syndrome; DSDs, disorders of sex development; GD, gonadal dysgenesis; 17βHSD, 17β-hydroxysteroid dehydrogena

dysgenesis; 17βHSD, 17β-hydroxysteroid dehydrogenase.
Reproduced with permission from Hughes IA, Houk C, Ahmed SF, et al. Consensus statement on management of intersex disorders. Arch Dis Child. 2006;91:554–562.
   
TABLE 24.23
934
that appropriate immunohistochemical markers are used and that 
the pathologist has a high level of expertise, and one should be 
aware that a biopsy may not be representative of the entire gonad. 
Genetic predisposition t

ive of the entire gonad. 
Genetic predisposition to GCC development is thought to play a 
role; SNPs in candidate genes for disease development have been 
investigated as predictors of GCC risk but were found to have 
insufficient discriminatory value to use in clinical practice.394
The highest risk for early gonadoblastoma occurs with Frasier 
syndrome (caused by mutations in WT1), for which the risk may 
be as high as 60%.212
In individuals with gonadal dysgenesis due to a 45,X/46,XY 
karyotyp

h gonadal dysgenesis due to a 45,X/46,XY 
karyotype, the prevalence of gonadoblastoma has been estimated 
to be 15% to 40%.212
The risk of GCC is lower in AIS385 (see “Disorders of Andro­
gen Action”). Fewer data are available on the risk in conditions 
affecting steroidogenesis, but the risk seems relatively low but not 
negligible.212 Other tumors such as Sertoli cell adenomas are com­
mon in CAIS.
When counseling patients and their family about surveillance 
and/or treatment, the MDT should t

t surveillance 
and/or treatment, the MDT should take into account the risk of 
gonadal GCC based on diagnosis, age of the individual, location 
of the gonad and results of gonadal biopsy if available, options 
for relocating the gonad, options for reliable surveillance, endo­
crine potential of the gonad and whether this endocrine function 
is desired by the individual, fertility potential of the gonad, risk of 
surgery, and side effects of hormone replacement therapy.214,384,526 
Because of th

one replacement therapy.214,384,526 
Because of the high risk for early tumor development, pro­
phylactic gonadectomy in infancy or early childhood is typically 
recommended for dysgenetic gonads and streak-like gonads that 
are undescended and cannot be placed in a lower position where 
they can be monitored. Serial monitoring with physical exami­
nation, and for older individuals self-examination, and imaging 
(e.g., ultrasound) may be more appropriate for gonads that are 
descended or can be 

ropriate for gonads that are 
descended or can be placed in an extraabdominal, preferably scro­
tal position, and that exhibit more robust endocrine function. In 
conditions such as CAIS with a lower risk of germ cell tumors and 
where GCC generally do not occur before puberty, gonadectomy 
is deferred until after puberty; after appropriate counseling, some 
individuals may prefer to leave the gonads in situ in adulthood, 
although monitoring of abdominal gonads is difficult. GCNIS 
cannot be de

abdominal gonads is difficult. GCNIS 
cannot be detected directly on ultrasound, although microlithia­
sis, especially when associated with inhomogeneous testis paren­
chyma, may be suggestive.385 Abdominal gonads may not be 
visualized on ultrasound, and while MRI has been suggested as an 
alternative, this cannot reliably detect GCNIS either.385 An alter­
native may be to move abdominal gonads to a more superficial 
location214 (see “Complete Androgen Insensitivity Syndrome”). 
Standard circul

ndrogen Insensitivity Syndrome”). 
Standard circulating tumor markers (e.g., α-fetoprotein, βhCG, 
lactate dehydrogenase) are of limited use in detecting GCNIS or 
seminomas. Novel serum biomarkers such as microRNAs (e.g., 
miR371-3 and miR302/367) may be useful to detect malignant 
lesions but currently cannot detect premalignant lesions.385 
Surgery and DSDs
Surgical approaches to DSD were once regarded as standard man­
agement, but there is much greater consideration now about the 
need for s

ch greater consideration now about the 
need for surgery and its timing.527,528
Surgery can potentially be performed on the gonads, the uro­
genital tract, and the genitals. The indication, timing, and choice 
of any potential procedure should be made on an individual basis 
and can be influenced by the underlying diagnosis. The MDT 
should inform the patient and family of all treatment options and 
their potential benefits and risks and should take into account 
medical, psychological, social, 

ake into account 
medical, psychological, social, and cultural aspects so that a well-
informed shared decision can be made.529
Some procedures are functionally important (e.g., to prevent 
hematocolpos or to mitigate high tumor risk), whereas others such 
as clitoral reduction surgery might be viewed as cosmetic. Some 
people believe that early surgery can be beneficial because the tis­
sues are easier to operate on and heal better and that parents can 
make decisions for their child, acting in

nts can 
make decisions for their child, acting in the child’s best interest, 
whereas others feel that surgery should be deferred until a time 
when a young person can be part of the decision-making and con­
sent process.528–531 The psychological implications of these deci­
sions can be great. Some parents experience great distress because 
of the appearance of their child’s genitals.532 Performing early sur­
gery to make the appearance of the genitalia more typical may 
make some parents feel 

enitalia more typical may 
make some parents feel more comfortable, but the child may later 
regret the parents’ decisions, especially if there is impaired sexual 
function or sensation or if further surgeries are required. On the 
other hand, bringing up a child with very atypical genitalia pro­
tects the child’s rights to bodily integrity but needs strong positive 
parenting and support, and it is not well understood how well 
young people cope in these situations as they mature.
For a child b

 in these situations as they mature.
For a child born with atypical genitalia and raised female, the 
parents should be guided through a balanced discussion about 
all options. The most common scenario where clitoral reduc­
tion surgery is considered is in 46,XX girls with CAH. In a baby 
with CAH there is often considerable reduction in the size of the 
enlarged clitoris once androgen exposure is reduced by adrenal 
suppression, and the clitoris may also become less prominent as 
the child grow

 may also become less prominent as 
the child grows, so a period of observation is appropriate before 
decisions about surgery are made.530 Psychological support in 
dealing with worries such as diaper changing should be provided, 
and in many situations surgery may not be considered. Outcome 
studies involving assessment of genital sensitivity and sexual func­
tion in women with CAH show impairment related to previous 
feminizing genital surgery.531,533 Although surgical procedures 
have change

.531,533 Although surgical procedures 
have changed, with greater emphasis on nerve sparing, these data 
have had a bearing on decisions taken during infancy and child­
hood for the surgical management of conditions such as CAH, 
and long-term outcome data are awaited for newer approaches. 
In centers that offer surgical options, it is generally accepted that 
clitoroplasty should be reserved only for the most severe degree of 
clitoromegaly, and in many situations vaginal surgery is deferred 
u

 in many situations vaginal surgery is deferred 
until after puberty. In adulthood, vaginal lengthening in con­
ditions such as CAIS and the MRKH syndrome can often be 
achieved by dilator treatment alone.390 If a surgical vaginoplasty is 
performed, the Vecchietti procedure is often preferred.
Subjects with DSD raised male usually undergo surgery to correct 
hypospadias and orchidopexy for testis maldescent. Hypospadias 
repair can be scheduled between 6 and 18 months of age or deferred 
until 

between 6 and 18 months of age or deferred 
until approximately 4 years of age or later. More than one procedure 
is often required, and complications such as stenosis, fistula, or need 
for redo surgery are relatively frequent. The surgeon should provide 
complete information on these procedures to the parents and ensure 
that their expectations are realistic.534 Many boys with severe hypo­
spadias undergo surgery in early life, as performing these approaches 
in adolescence may be more difficu

ese approaches 
in adolescence may be more difficult and associated with a higher 
rate of complications, but there is a lack of data for optimal timing 
and outcomes.535 Open discussions with the parents and later with 
the child are needed, and boys may need more support in childhood 
and adolescence than is usually currently offered.
935
The topic of gonadectomy before puberty in 46,XY individuals 
raised female to prevent virilization and in others wi

ed female to prevent virilization and in others with DSD to 
avoid gonadal tumors has previously been mentioned. The irre­
versible nature of such a procedure has created uncertainty among 
professionals, particularly when the procedure is performed before 
the affected child can be engaged in discussions and in situa­
tions where there could be a potential for future fertility or endo­
crine function. Temporary suppression of puberty using GnRH 
analogues may be used to delay a decision on gona

analogues may be used to delay a decision on gonadectomy.528 
Gonadectomy is another issue that requires the collective discus­
sion of the multidisciplinary team and may include input from 
an ethicist. The practice of cryopreserving excised gonads with 
unrealistic expectations for preservation of reproductive potential 
should be viewed cautiously, especially because current knowledge 
is based primarily on the gonadal effects of cancer therapies.536 If 
tissue is preserved, there needs to b

pies.536 If 
tissue is preserved, there needs to be a clear dialogue about likely 
outcomes and options. However, germ cell number and viabil­
ity may decline with time, so there could be a limited window 
of opportunity.536 Furthermore, reproductive technologies are 
changing rapidly, there are highly publicized stem cell approaches 
such as germ cell reprogramming reported in animals, and success­
ful birth following cryopreservation of (normal) prepubertal testis 
has been achieved in monkeys

) prepubertal testis 
has been achieved in monkeys.537 Whether these technologies will 
eventually provide useful clinical approaches remains to be seen. 
Fertility/Family Building
Many forms of DSD are associated with reduced fertility or infer­
tility. Successful use of assisted reproductive techniques has been 
reported in some cases, such as TESE and/or ICSI in men with PAIS 
or 5α-reductase deficiency,396,538,539 and a few pregnancies using 
donor oocytes have been reported in women with 46

donor oocytes have been reported in women with 46,XY DSD who 
have a uterus.540,541 Uterus transplantation has recently been shown 
to make pregnancy possible for women who do not have a uterus, 
resulting in live birth in a woman with MRKH syndrome.542 Alter­
native options to build families include the use of donor sperm, 
adoption, or foster care. It is important to provide individuals with 
information on the possibilities in their specific situation; a recent 
study found that provision of

situation; a recent 
study found that provision of such information is suboptimal.543 
DSD in Resource-Limited Countries
Many of our approaches to DSD are being developed within 
the context of large medical centers and experienced MDTs 
with ready access to biochemical assays such as AMH, imaging, 
and state-of-the-art genetic tests. In reality, many children and 
adults with DSD live in countries with less access to resources 
or with different social and cultural pressures.501 Each clinician 

 social and cultural pressures.501 Each clinician 
needs to work with the tests and treatments that are available 
and affordable to optimize their ability to assess, diagnose, and 
treat individuals with DSD. Knowledge of the local prevalence 
of certain forms of DSD and genetic hotspots can be very impor­
tant, as well as relevant social, cultural, and religious factors 
that might influence the way a child and the family view DSD. 
Limited education about DSD is available to clinicians in som

cation about DSD is available to clinicians in some 
countries, but tools such as the European Society for Paediatric 
Endocrinology (ESPE) e-learning may be valuable.544 The Pae­
diatric Training Centres in Africa (PETCA) established in 2008 
to provide specialist training in endocrinology and diabetes for 
qualified pediatricians is beginning to pay dividends with more 
than 100 additional qualified specialists distributed among 13 
sub-Saharan African countries.545 
Outcome Studies
There is s

 African countries.545 
There is such a wide range of conditions included under the 
umbrella of DSD that it is difficult to obtain an overarching 
assessment of the health and quality of life in adulthood for indi­
viduals who have lived with DSD from childhood. Standardizing 
longitudinal assessment in individuals with DSD across centers 
should facilitate future outcome studies.514
Most data regarding sexual function and qual­ity of life come 
from women with CAH who had vario

ty of life come 
from women with CAH who had various genitoplasty procedures 
during their childhood.546,547 The debate continues regarding the 
timing of any surgery for girls with CAH and whether one-stage or 
two-stage surgery is the preferred option.548 Adult endocrinologists 
are now undertaking detailed studies of the comorbid conditions 
associated with CAH and its medical treatment (see also Chapter 
15). The UK Congenital Adrenal Hyperplasia Adult Study Execu­
tive (CaHASE) studies on c

asia Adult Study Execu­
tive (CaHASE) studies on cohorts of more than 200 adults show 
poorer health status relating to increased obesity, hypertension, 
insulin resistance, osteoporosis, impaired fertility, and reduced 
quality of life.549–551 Despite improvements in growth achieve­
ment in recent years, shorter adult height was one of the adverse 
outcomes, particularly in relation to hypertension. Similar find­
ings were found in outcome studies of a Swedish population of 
adults with CAH.552

es of a Swedish population of 
adults with CAH.552 The Swedish CAH population also had lower 
educational achievement and reduced income, had increased dis­
ability allowance and sick leave, were less likely to be married, and 
had fewer children compared to control subjects.553 The medical 
comorbid conditions may result from long-standing inadequate 
control of CAH by replacement glucocorticoids, estimated to be 
the case in more than a third of adults. Studies of modified-release 
hydrocortis

f adults. Studies of modified-release 
hydrocortisone preparations that mimic more closely the cortisol 
circadian rhythm are showing promising results, with a reduction 
in the total hydrocortisone daily dose required for adequate adre­
nal androgen suppression.554
There are fewer outcome studies available for adults with con­
ditions within the category of 46,XY DSD, but data are beginning 
to emerge. A nationwide study of females with 46,XY DSD in 
Denmark showed that morbidity is not increas

D in 
Denmark showed that morbidity is not increased when diagno­
ses directly related to the DSD itself were excluded.383 A Swedish 
study reported a higher rate of psychiatric morbidity in women 
with CAIS and CGD compared to controls but similar to that in 
women with primary ovarian insufficiency.555
Quality-of-life studies have recently been reported in popula­
tions of German, Italian, and Brazilian individuals with 46,XX 
DSD and 46,XY DSD and of either male or female social gender, 
so s

 and of either male or female social gender, 
so study cohorts are quite heterogeneous.556–558 In general, quality 
of life was satisfactory, there was no evidence of gender dysphoria, 
and psychosocial adjustment was better in younger people when 
compared with an older generation of DSD individuals. Among 
the 46,XY DSD group in the Brazilian population, those with 
a male social gender had a better quality of life than those with 
a female social gender (e.g., the group with CAIS). There were

ial gender (e.g., the group with CAIS). There were 
more difficulties overall in partnerships and sexual relationships. 
Another study from Germany, which included individuals with 
both 46,XX and 46,XY DSD, showed evidence of dissatisfaction 
with health care services available, especially in the group with 
46,XY DSD.559 In a study that focused on comparing women 
with either CAIS or MRKH syndrome, there was greater lack of 
sexual confidence and sexual satisfaction in women with CAIS, 
wherea

nd sexual satisfaction in women with CAIS, 
whereas those with MRKH syndrome, while apprehensive in 
sexual situations, reported being satisfied with their sex life.560 
Danish women with 46,XY DSD had lower rates of cohabitation 
936
and motherhood compared to female controls, but education and 
income were similar to or higher than in controls.383 In contrast, 
Danish men with 46,XX DSD had poorer education and reduced 
income in some age groups.56

ducation and reduced 
income in some age groups.561
As discussed, a recent study of more than 400 46,XY subjects 
subdivided according to clinical diagnoses of PAIS, disorders of 
gonadal development (partial gonadal dysgenesis), and disorders 
of androgen synthesis (such as 5α-reductase deficiency) has shown 
an increasing trend for babies with atypical genitalia to be raised as 
boys.392 Therefore outcome studies are needed for these groups of 
individuals. A recent review of 46,XY DSD provide

individuals. A recent review of 46,XY DSD provided mainly anec­
dotal data on issues such as gender identity and sex redesignation, 
whether males were satisfied with the appearance of their genitalia, 
and the prevalence of sexual dysfunction.562 Overall, males were 
satisfied with their gender, and rarely did male-to-female gender 
redesignation occur in adulthood. Most males had some concern 
about the appearance of the genitalia, and many were dissatisfied 
with sexual function. Another sma

re dissatisfied 
with sexual function. Another small Dutch study reported poor 
outcome in terms of penile size and sexual function, although 
their overall body image and psychosexual functioning was no dif­
ferent from control subjects.563
There is a paucity of information about outcome in specific 
causes of 46,XY DSD, such as PAIS.565,566 Often this label is 
applied without confirmation that the phenotype is due to a 
mutation in the AR gene; as a result, other diagnoses may have 
been incl

; as a result, other diagnoses may have 
been included inadvertently. A French multicenter study reported 
on 15 adults with PAIS who had been monitored since birth.567 
The EMS (see Fig. 24.40) was very low and penile length was 
markedly reduced in adulthood irrespective of additional andro­
gen treatment. All boys developed gynecomastia in adolescence, 
and sexual function was severely impaired. An international study 
of 29 males with PAIS who were at least 16 years old and a more 
recent st

o were at least 16 years old and a more 
recent study of 27 males with PAIS studied prospectively also 
confirmed universal development of gynecomastia.378,393 Micro­
penis was common at initial presentation but less so when assessed 
after puberty. Onset of puberty was often spontaneous, although 
androgen treatment may be required subsequently.
There has been considerable progress in many aspects of DSD 
since the previous edition of this chapter. What is clearly needed 
now are outcome studie

er. What is clearly needed 
now are outcome studies in larger cohorts of individuals with 
DSD, particularly within the 46,XY DSD category. A major step 
toward that goal is the establishment of international registries 
for DSD studies (I-DSD) and allied activities such as evaluation 
of long-term care and outcomes in DSD (DSD-Life), interactive 
networks for activities in DSD (DSDnet), European Reference 
Networks for rare conditions such as DSD, and a DSD Trans­
lational Research Network (DSD

D, and a DSD Trans­
lational Research Network (DSD-TRN).568 Such networking 
should provide the means to gather more data on the individual 
rare causes of DSDs across the lifespan that can then be translated 
into improved management and outcome.
We recognize the major contribution made to this chapter by the 
late Melvin Grumbach MD and his close colleague, Felix Conte 
MD. John C. Achermann holds a Wellcome Trust Senior Research 
Fellowship in Clinical Science (209328/Z/17/Z).

h 
Fellowship in Clinical Science (209328/Z/17/Z). Ieuan A. Hughes 
is supported by the NIHR Cambridge Biomedical Research Cen­
tre for his work on disorders of sex development. Yee-Ming Chan 
is supported by the US National Institutes of Health and the 
Eunice K. Shriver National Institute of Child Health and Human 
Development (R01 HD089521).
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937
25
Normal and Aberrant Growth 
in Chil

87–393.
937
25
in Children
DAVID W. COOKE, SARA A. DIVALL, AND SALLY RADOVICK
Normal Growth
Overview
Growth is a fundamental, intrinsic aspect of childhood health. It 
is also a complex yet tightly regulated process. An individual’s final 
height and the path taken to reach that end point are significantly 
determined by that person’s genetic composition. But growth and 
final height can also be affected by external factors, including the 
quality and quantity of nutr

ctors, including the 
quality and quantity of nutrition, and by psychosocial factors. 
This process is regulated by multiple hormones and growth fac­
tors interacting with an array of membrane receptors that activate 
seemingly redundant intracellular signaling cascades. Yet, as com­
plex as this process is, 1 standard deviation (SD) of adult height 
represents about only 4% of the mean adult height.
Whether linear growth occurs as a continuous process or with 
periodic bursts of growth and arre

rocess or with 
periodic bursts of growth and arrest1–4 has been hard to charac­
terize definitively. There do appear to be seasonal variations of 
growth, with slower growth in autumn and winter and greater 
growth in spring and early summer.5,6 Some normal children have 
a broad growth channel, with many showing diverse but charac­
teristic growth tracks.7 Nonetheless, even though the process of 
growth is multifactorial and complex, children usually grow in 
a remarkably predictable manner. D

sually grow in 
a remarkably predictable manner. Deviation from such a normal 
pattern of growth can be the first manifestation of a wide vari­
ety of disease processes, including endocrine and nonendocrine 
disorders and involving virtually any organ system of the body. 
Therefore frequent and accurate assessment of growth is of pri­
mary importance in the care of children. 
Assessment of growth requires accurate and reproducible deter­
minations of height. Supine length is routinel

er­
minations of height. Supine length is routinely measured in chil­
dren younger than 2 years of age, and erect height is assessed in 
older children. It can be useful to measure both length and height 
in children between 2 and 3 years of age to allow comparisons 
with prior length measurements and to begin to record height 
measurement for ongoing comparisons. The inherent inaccura­
cies involved in measuring length in infants are often obscured 
by the rapid skeletal growth during this peri

red 
by the rapid skeletal growth during this period. For measurement 
of supine length (Fig. 25.1), it is best to use a firm box with an 
inflexible board, against which the head lies, and a movable foot­
board, on which the feet are placed perpendicular to the plane of 
the supine length of the infant. Optimally, the child should be 
Normal Growth, 937
Endocrine Regulation of Growth, 944
Pathologic Basis of Growth Retardation, 961
Pathologic Basis of Excess Growth, 985
Evaluati

61
Evaluation and Treatment of Growth Abnormalities, 987
KEY POINTS
 • 
Height is an important vital sign to obtain during childhood 
because deviations from the normal linear growth pattern may 
indicate an underlying disorder.
 • 
An intact hypothalamic (growth hormone–releasing hormone 
[GHRH])/pituitary (growth hormone [GH])/insulin-like growth 
factor 1 (IGF1) axis, adequate nutrition, and absence of sig­
nificant systemic disease are requirements fo

sig­
nificant systemic disease are requirements for normal linear 
growth.
 • 
A comprehensive past medical, family, and social history with 
assessment of an accurate growth velocity is required for the 
­initial investigation of abnormal growth. Laboratory and radio­
logic investigations include an evaluation for occult systemic 
disease and exclusion of hormonal abnormalities.
 • 
Successful treatment of the underlying disorder or correction of 
hormone deficiency(ies) improves linear growt

 of 
hormone deficiency(ies) improves linear growth.
 • 
Treatment of short stature with growth-promoting agents may 
improve linear growth in select patients with intact GHRH/GH/
IGF1 axis function.
938
relaxed, the legs should be fully extended, and the head should 
be positioned in the Frankfurt plane, with the line connecting 
the outer canthus of the eyes and the external auditory meatus 
perpendicular to the long axis of the trunk.
When childr

dicular to the long axis of the trunk.
When children are old enough (and physically capable) to 
stand erect, it is best to use a wall-mounted Harpenden sta­
diometer similar to that designed by Tanner and Whitehouse 
for the British Harpenden growth study. The traditional mea­
suring device of a flexible arm mounted to a weight balance 
is notoriously unreliable and does not provide accurate serial 
measurements.
As with length measurements in infants, positioning of the 
child in the stadiomet

nfants, positioning of the 
child in the stadiometer is critical (Fig. 25.2). The child should be 
fully erect, with the head in the Frankfurt plane; the back of the 
head, thoracic spine, buttocks, and heels should touch the vertical 
axis of the stadiometer, and the heels should be together. Every 
effort should be made to correct discrepancies related to lordosis 
or scoliosis. Ideally, serial measurements should be made at the 
same time of day, because standing height may undergo diurnal 
v

ay, because standing height may undergo diurnal 
variation.
Height determinations should be performed by a trained 
individual rather than an inexperienced member of the staff. We 
recommend that lengths and heights be measured in triplicate, 
that variation should be no more than 0.3 cm, and that the mean 
height should be recorded. For determination of height velocity 
when several measurements are being made within a short period, 
the same individual should perform the determinations to elim

dividual should perform the determinations to elimi­
nate interobserver variability. Even when every effort is made to 
obtain accurate height measurements, a minimum interval of 6 
months is necessary for meaningful height velocity computation. 
Nine to 12 months of data are preferable so that errors of measure­
ment are minimized and the seasonal variation in height velocity is 
assimilated into the data. 
Growth Charts
Evaluation of a child’s height must be done in the context of nor­
mal sta

height must be done in the context of nor­
mal standards. Most American pediatric endocrine clinics use the 
cross-sectional data provided by the National Center for Health 
Statistics (NCHS), which were originally introduced in 1977. 
Revised and updated growth charts have been available on the 
Centers for Disease Control and Prevention (CDC) website (www.
cdc.gov/growthcharts) since 2000 (Figs. 25.3 through 25.8).8 
The data for these charts encompass measurements obtained in 
the United Stat

ncompass measurements obtained in 
the United States between 1963 and 1995, and they include a 
broader representation of the US population for all measures than 
was available in earlier charts. Growth charts based on the World 
Health Organization (WHO) data gathered from 1997 through­
out 2003 should be used to monitor the growth of children under 
2 years of age. While the data on length are very similar between 
the CDC and WHO curves, the CDC curves describe higher 
weight gain that, among

DC curves describe higher 
weight gain that, among other things, reflects the lower prevalence 
of breastfed infants in the CDC data and is not felt to represent 
optimal growth.9
• Fig. 25.1  Technique for measuring recumbent length. (A device suitable for measurement of length of 
infants can be purchased from Raven Equipment Limited, Essex, UK.) (Photograph courtesy Noel Cam­
eron.)
• Fig. 25.2  Technique for measuring erect height using the Harpenden sta­
diometer with direct digital display

arpenden sta­
diometer with direct digital display of height. (Devices of this type are avail­
able from Holtain Ltd., Wales, UK, and Seritex Inc., Carlstadt, NJ.)
939
Chapter 25  Normal and Aberrant Growth in Children
Birth
AGE (MONTHS)
L
E
N
G
T
H
15
40
7
6
5
4
3
2
kg
45
50
55
60
65
70
75
80
85
90
95
100
cm
90
95
95
90
75
50
25
10
5
95
90
75
50
25
10
5
100
cm
16
14
12
10
8
6
lb
8
9
10
11
12
13
14
15
16
17
kg
16
18
20
22
24
26
28
30
32
34
36
38
lb
16
17
18
19
20
21
22
23
24
25
26
2

32
34
36
38
lb
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
in
35
36
37
38
39
40
41
in
Birth
3
6
9
12
Mother’s Stature
Father’s Stature
Date
CDC
Revised April 20, 2001.
SOURCE:  Developed by the National Center for Health Statistics in collaboration with
                   the National Center for Chronic Disease Prevention and Health Promotion (2000).
                   http://www.cdc.gov/growthcharts
Age
Birth
Weight
Length
Head Circ.
Gestational
Age
Weeks
Comm

eight
Length
Head Circ.
Age
Weeks
Comment
15
18
21
24
27
30
33
36
W
E
I
G
H
T
L
E
N
G
T
H
W
E
I
G
H
T
3
6
9
12
15
18
21
24
27
30
33
36
Birth to 36 months: Boys
Length-for-age and Weight-for-age percentiles
Name
Record #
• Fig. 25.3  Length-for-age and weight-for-age percentiles for boys (birth 
to 36 months). (Developed by the National Center for Health Statistics in 
collaboration with the National Center for Chronic Disease Prevention and 
Health Promotion, 2000. http://www.cdc.gov

on and 
Health Promotion, 2000. http://www.cdc.gov/growthcharts.)
CDC
Birth
LENGTH
H
E
A
D
C
I
R
C
U
M
F
E
R
E
N
C
E
1
2
3
4
cm
kg
in
30
32
34
36
38
40
42
44
46
48
50
52
42
44
46
48
50
52
cm
cm
2
4
6
8
10
12
14
16
18
20
22
24
lb
22
21
20
19
18
17
16
15
14
13
12
12
14
16
18
20
22
24
26
28
30
32
34
36
38
40
42
44
46
48
50
11
10
9
8
7
6
5
11
10
9
8
7
6
5
kg
cm
in
100
98
96
94
92
90
88
86
84
82
80
78
76
74
72
70
68
66
64
lb
12
13
14
15
16
17
18
19
20
17
18
19
20
in
in
Date
Age
Weight
Le

6
17
18
19
20
17
18
19
20
in
in
Date
Age
Weight
Length
Head Circ.
Comment
W
E
I
G
H
T
W
E
I
G
H
T
3
6
9
12
15
18
21
24
27
30
33
36
Name
Record #
95
90
75
50
25
10
5
95
90
75
50
25
10
5
                  the National Center for Chronic Disease Prevention and Health Promotion (2000).
                  http://www.cdc.gov/growthcharts
Head circumference-for-age and
Weight-for-length percent

ircumference-for-age and
Weight-for-length percentiles
26
18 19 20 21 22 23 24
62
60
58
56
54
52
50
48
46
27 28 29 30 31 32 33 34 35 36 37 38 39 40 41
H
E
A
D
C
I
R
C
U
M
F
E
R
E
N
C
E
• Fig. 25.4  Head circumference-for-age and weight-for-length percentiles 
for boys (birth to 36 months). (Developed by the National Center for Health 
Statistics in collaboration with the National Center for Chronic Disease Pre­
vention and Health Promotion, 2000. http://www.cdc.gov/growthcharts.)
These charts al

 http://www.cdc.gov/growthcharts.)
These charts allow comparison of individual children with the 
3rd, 10th, 25th, 50th, 75th, 90th, and 97th percentiles of normal 
American children. There are, however, two limitations of these 
charts when applied to the individual child. First, they do not sat­
isfactorily define children below the 3rd or above the 97th per­
centiles—the very children in whom it is most critical to define 
the degree to which they deviate from the normal growth centiles. 
How

they deviate from the normal growth centiles. 
However, the NCHS data tables (also available on the CDC web­
site) can be used to compute standard deviation scores (SDS). For 
example, a short child below the 3rd percentile can be described 
more precisely as being, for example, approximately 4.2 SDS below 
the mean for age. A height SDS for age is calculated as follows: The 
SDS equals the child’s height, minus the mean height for normal 
children of the child’s age and gender, divided by the S

en of the child’s age and gender, divided by the SD of the 
height for normal children of this age and gender. Second, cross-
sectional data are of greater value during infancy and childhood 
than in adolescence, because differences in the timing of pubertal 
onset can considerably influence normal growth rates. To address 
this issue, Tanner and Davies10 developed longitudinal growth 
charts in an effort to construct the curve shapes with centile widths 
obtained from a large cross-sectional su

e widths 
obtained from a large cross-sectional survey, thus accounting for 
variability in the timing of puberty. Such charts are of particular 
value in assessing growth during adolescence and puberty and for 
plotting sequential growth data for any individual child.
The data from cross-sectional and longitudinal growth studies 
have been used to develop height velocity standards (Figs. 25.9 and 
25.10). It is important to emphasize that carefully documented 
height velocity data are invaluabl

lly documented 
height velocity data are invaluable in assessing a child with abnor­
malities of growth. There is considerable variability in normal 
height velocity of children at different ages; however, between the 
age of 2 years and the onset of puberty, children grow with remark­
able fidelity relative to the normal growth curves. Any crossing of 
percentile curves on the height chart during this age period should 
be considered abnormal and warrants further evaluation.
Syndrome-specific g

d warrants further evaluation.
Syndrome-specific growth curves have been developed for 
a number of clinical conditions associated with growth failure, 
such as Turner syndrome (TS),11 achondroplasia,12 and Down 
­syndrome.13,14 Such growth profiles are invaluable for tracking 
the growth of children with these clinical conditions. Deviation 
of growth from the appropriate disease-related growth curve sug­
gests the possibility of a second underlying cause, such as acquired 
autoimmune hypothyro

ying cause, such as acquired 
autoimmune hypothyroidism in children with Down syndrome 
or TS. 
Body Proportions
Many abnormal growth states, including both short stature and 
excessive stature, are characterized by disproportionate growth. The 
940
following determinations should be made as part of the evaluation 
of short stature:
	1.	 Occipitofrontal head circumference
	2.	 Lower body segment: distance from top of pubic symphysis to 
the floor
	
stance from top of pubic symphysis to 
the floor
	3.	 Upper body segment: the difference between total height and 
lower body segment (it can also be measured as the sitting 
height, subtracting the height of the chair or stool)
	4.	 Arm span
Published standards exist for these body proportion measure­
ments, which must be evaluated relative to the patient’s age.15 The 
ratio of upper segment to lower segment ranges from 1.7 in the 
neonate to slightly less than 1.0 in the adult (Fig. 25.11). 

slightly less than 1.0 in the adult (Fig. 25.11). 
Parental Target Height
Genetic factors are important determinants of growth and height 
potential. Therefore it is useful to assess a patient’s stature relative 
to that of siblings and parents. Tanner and associates developed a 
growth chart that factored parents’ heights into the evaluation of 
the heights of children ages 2 to 9 years.16 One can also calculate 
a child’s expected final height based on the parents’ heights by 
calculating the 

based on the parents’ heights by 
calculating the midparental height. This is the average of the par­
ents’ heights, after accounting for the average difference in height 
between adult men and women (13 cm). In other words, a boy’s 
midparental height equals the average of his parents’ heights plus 
6.5 cm, and a girl’s midparental height equals the average of her 
parents’ heights minus 6.5 cm.
Because of regression to the mean,17,18 children of short par­
ents are likely to be less short than

f short par­
ents are likely to be less short than their parents, and children of 
tall parents are likely to be less tall than their parents. Therefore a 
child’s genetic target height range centers on the point that repre­
sents 80% of the difference between the child’s midparental height 
and the mean adult height for the child’s gender.17 For example, if 
a boy’s father is 168 cm tall and his mother is 153 cm tall, his mid­
parental height is 167 cm, which is 10 cm below the mean height 
of 

 167 cm, which is 10 cm below the mean height 
of adult men (177 cm). Therefore the boy’s target height range 
centers on 169 cm, which is 8 cm below the mean adult height for 
men. In more than 95% of children, the adult height falls within 
10 cm of the point thus calculated.16,17 In children with extremely 
short stature (≥3 SD), the father’s height may more strongly cor­
relate with the patient’s height, and the mother’s height may more 
greatly influence birth length.19 
Skeletal Maturation

tly influence birth length.19 
The growth potential in the tubular bones can be assessed by eval­
uation of the progression of ossification within the epiphyses. The 
Birth
15
40
7
6
5
4
3
2
kg
45
50
55
60
65
70
75
80
85
90
95
100
cm
90
95
100
cm
16
14
12
10
8
6
lb
8
9
10
11
12
13
14
15
16
17
kg
16
18
20
22
24
26
28
30
32
34
36
38
lb
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
in
35
36
37
38
39
40
41
in
12
Mother’s S

39
40
41
in
35
36
37
38
39
40
41
in
12
Date
Age
Birth
Weight
Length
Head Circ.
Age
Weeks
Comment
15
18
21
24
27
30
33
36
3
6
9
12
15
18
21
24
27
30
33
36
95
90
75
50
25
10
5
95
90
75
50
25
10
5
L
E
N
G
T
H
Birth
3
6
9
CDC
                   htt

nd Health Promotion (2000).
W
E
I
G
H
T
L
E
N
G
T
H
W
E
I
G
H
T
Birth to 36 months: Girls
Name
Record #
• Fig. 25.5  Length-for-age and weight-for-age percentiles for girls (birth 
CDC
Birth
AGE 

. http://www.cdc.gov/growthcharts.)
CDC
Birth
LENGTH
H
E
A
D
C
I
R
C
U
M
F
E
R
E
N
C
E
H
E
A
D
C
I
R
C
U
M
F
E
R
E
N
C
E
1
2
3
4
cm
kg
in
30
32
34
36
38
40
42
44
46
48
50
52
42
44
46
48
50
52
cm
cm
2
4
6
8
10
12
14
16
18
20
22
24
lb
22
21
20
19
18
17
16
15
14
13
12
12
14
16
18
20
22
24
26
28
30
32
34
36
38
40
42
44
46
48
50
11
10
9
8
7
6
5
11
10
9
8
7
6
5
kg
cm
in
100
98
96
94
92
90
88
86
84
82
80
78
76
74
72
70
68
66
64
lb
12
13
14
15
16
17
18
19
20
17
18
19
20
in
in
Date
Age
Weigh

15
16
17
18
19
20
17
18
19
20
in
in
Date
Age
Weight
Length
Head Circ.
Comment
W
E
I
G
H
T
W
E
I
G
H
T
3
6
9
12
15
18
21
24
27
30
33
36
Name
Record #
26
62
60

-length percentiles
26
62
60
58
56
54
52
50
48
46
95
90
75
50
25
10
5
95
90
75
50
25
10
5
• Fig. 25.6  Head circumference-for-age and weight-for-length percentiles 
for girls (birth to 36 months). (Developed by the National Center for Health 
941
Chapter 25  Normal and Aberrant Growth 

arts.)
941
ossification centers of the skeleton appear and progress in a pre­
dictable sequence in normal children, and this skeletal maturation 
can be compared with normal age-related standards. This forms 
the basis of the bone age or skeletal age, a quantitative determina­
tion of net somatic maturation that serves as a mirror of the tempo 
of growth and maturation. The bone age also reflects the degree 
of growth plate senescence and is the

 the degree 
of growth plate senescence and is therefore a useful adjunct in 
estimating growth opportunity (i.e., the ultimate adult height), as 
discussed later in this chapter.
Not all of the factors that determine the normal pattern of 
skeletal maturation have been identified, but genetic factors and 
multiple hormones, including thyroxine, growth hormone (GH), 
and gonadal steroids, are involved.20 Ultimately, growth cessa­
tion occurs after exhaustion of the proliferative capacity of the 

r exhaustion of the proliferative capacity of the 
growth plate chondrocyte.21 Estrogen plays an important role in 
this process: Animal studies have indicated that estrogen acceler­
ates growth plate senescence,22 and studies in patients with muta­
tions of the gene for the estrogen receptor23 or for the aromatase 
enzyme24,25 demonstrated that estrogen is primarily responsible 
for epiphyseal fusion.26 
Phases of Normal Growth
Growth occurs at differing rates during intrauterine life, early an

differing rates during intrauterine life, early and 
middle childhood, and adolescence and then ceases after fusion of 
long bone and vertebral epiphyseal growth plates. Karlberg and asso­
ciates resolved the normal linear growth curve into three additive, 
partially superimposable phases27,28: an “infancy” phase, starting in 
midgestation and then rapidly decelerating until about 3 to 4 years 
of age; a “childhood” phase, of slowly decelerating growth through 
early adolescence; and a sigmoid-s

growth through 
early adolescence; and a sigmoid-shaped “puberty” phase that com­
prises the adolescent growth spurt. Prenatal growth averages 1.2 to 
1.5 cm per week but varies dramatically (Fig. 25.12); the midgesta­
tional length growth velocity of 2.5 cm per week falls to almost 0.5 
cm per week immediately before birth. Growth velocity (see Figs. 
25.9 and 25.10) averages about 15 cm per year during the first 2 
years of life; it then slows to approximately 6 cm per year during 
middle chil

to approximately 6 cm per year during 
middle childhood. During this time, a normal child’s height, plotted 
on a growth curve, typically remains within a given growth channel; 
that is, it does not cross percentile lines on the growth curve.
Prepubertal growth is similar between boys and girls. The 
height difference between men and women, an average of 13 cm, 
is accounted for by two factors. First, boys grow for an average 
of 2 years longer than girls, because girls have an earlier onset of 

han girls, because girls have an earlier onset of 
puberty and, consequently, earlier cessation of growth. Therefore 
prepubertal growth is greater for boys; they are 8 to 10 cm taller 
when their puberty starts, compared with girls’ heights when their 
puberty starts.17 Second, boys achieve a greater maximal pubertal 
growth velocity than girls, giving them 3 to 5 cm greater pubertal 
growth. The time of onset of puberty varies in normal children, 
CDC
AGE (YEARS)
S
T
A
T
U
R
E
kg
1

n, 
CDC
S
T
A
T
U
R
E
kg
10
15
20
25
30
35
80
85
90
95
100
105
110
115
120
125
130
135
140
145
150
155
160
cm
190
185
180
175
170
165
160
155
150
105
100
95
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
cm
30
40
50
60
70
80
lb
kg
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
210
220
230
lb
in
60
62
64
66
68
70
72
74
76
in
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Revised and corrected November 28, 2000.
SOURCE:  Developed by the National Cen

r 28, 2000.
W
E
I
G
H
T
S
T
A
T
U
R
E
W
E
I
G
H
T
20
19
18
17
16
15
14
13
12
2 to 20 years: Boys
Stature-for-age and Weight-for-age percentiles
Name
Record #
Date
Age
Weight
Stature
BMI*
30
32
34
36
38
40
42
44
46
48
50
52
54
56
5

e
BMI*
30
32
34
36
38
40
42
44
46
48
50
52
54
56
58
60
62
3
4
5
6
7
8
9
10
11
95
90
75
50
25
10
5
95
90
75
50
25
10
5
• Fig. 25.7  Stature-for-age and weight-for-age percentiles for boys (2 to 20 
years). (Developed by the National Center for Health Statistics in collabora­
tion with the National Center for Chronic Disease Prevention and Health 
Promotion, 2000. http://www.cdc.gov/growthcharts.)
CDC
S
T
A
T
U
R
E
kg
10
15
20
25
30
35
80
85
90
95
100
105
110
115
120
125
13

20
25
30
35
80
85
90
95
100
105
110
115
120
125
130
135
140
145
150
155
160
cm
190
185
180
175
170
165
160
155
150
105
100
95
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
cm
30
40
50
60
70
80
lb
kg
30
40
50
60
70
80
90
100
110
120
130
140
150
160
170
180
190
200
210
220
230
lb
in
60
62
64
66
68
70
72
74
76
in
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
       
or Health Statistics in collaboration with
W
E
I
G
H
T
S
T
A
T
U
R
E
W
E
I
G
H
T
20
19
18
17
16
15
14
13
12
2 to 20 years: Girls
Name
Record #
Date
Age
Weight
Stature
BMI*
30
32
34
36
38
40
42
44
46
48
50
52
54
56
58
60
62
3
4
5
6
7
8
9
10
11
95
90
75
50
25
10
5
95
90

62
3
4
5
6
7
8
9
10
11
95
90
75
50
25
10
5
95
90
75
50
25
10
5
• Fig. 25.8  Stature-for-age and weight-for-age percentiles for girls (2 to 20 
942
Boys
cm/yr
1
2
3
3
4
5
6 7
8
9 10 11 12 13 14 15 16 17
97
50
18 19
23
22
21
20
19
18
17
16
1

 14 15 16 17
97
50
18 19
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
Height velocity
• Fig. 25.9  Height velocity chart for boys constructed from longitudinal 
observations of British children. The 97th, 50th, and 3rd percentile curves 
define the general pattern of growth during puberty. Shaded areas indicate 
velocities in those children with peak velocities occurring up to 2 stan­
dard deviations before or after the average age depicted by the percen­
tile lines. (Up arrows, d

 depicted by the percen­
tile lines. (Up arrows, diamonds, and down arrows mark, respectively, the 
97th, 50th, and 3rd percentiles of peak velocity when the peak occurs 
at these early or late limits.) (Modified from charts prepared by J.M. Tan­
ner and R.H. Whitehouse from data published in references 10 and 30. 
Reproduced with permission of J.M. Tanner and Castlemead Publications, 
Ward’s Publishing Services, Herts, UK.)
Girls 
Age (years) 
cm/yr 
1 2 3 
3 
4 5 6 7 8 9 10 11 12 13 14 15 16 1

m/yr 
1 2 3 
3 
4 5 6 7 8 9 10 11 12 13 14 15 16 17 
97 
50 
18 19
23 
22 
21 
20 
19 
18 
17 
16 
15 
14 
13 
12 
11 
10 
9 
8 
7 
6 
5 
4 
3 
2 
1 
Height velocity 
• Fig. 25.10  Height velocity chart for girls constructed from longitudinal 
velocities in those children with peak velocities occurring up to 2 standard 
deviations before or afte

urring up to 2 standard 
deviations before or after the average age depicted by the percentile lines. 
(Up arrows, diamonds, and down arrows mark, respectively, the 97th, 
50th, and 3rd percentiles of peak velocity when the peak occurs at these 
early or late limits.) (Modified and reproduced with permission of J.M. Tan­
ner and Castlemead Publications, Ward’s Publishing Services, Herts, UK.)
Female
Male
Ratio U/L segment
1.7
1.6
1.5
1.3
1.2
1.4
1.1
1.0
0
18
16
12
14
10
8
6
4
2
0
• F

1.3
1.2
1.4
1.1
1.0
0
18
16
12
14
10
8
6
4
2
0
• Fig. 25.11  Upper/lower segment ratio from birth to 18 years of age. (Data 
from Wilkins L. The Diagnosis and Treatment of Endocrine Disorders in 
Childhood and Adolescence. Springfield, IL: Charles C. Thomas; 1957.)
Crown-heel length velocity
Weight velocity (kg/yr)
Age (weeks)
Birth
2
4
6
8
10
20
30
40
10
20
30
40
10
12
14
2
4
6
8
10
(cm/4 wk)
• Fig. 25.12  Rate of linear growth and weight gain in utero and during first 
40 weeks after birth. Le

n utero and during first 
40 weeks after birth. Length velocity is expressed in centimeters per week. 
The solid line depicts the actual linear growth rate; the dashed line con­
necting the prenatal and postnatal length velocity lines depicts the theo­
retic curve for no uterine restriction late in gestation. The lighter dashed line 
depicts weight velocity. (From data in Tanner JM. Fetus Into Man. Cam­
bridge, MA: Harvard University Press; 1978.)
943
Chapter 25  Normal and Aberrant Growth in Ch

)
943
resulting in a normal variation in the timing of the pubertal 
growth spurt. However, in most normal children, the final height 
is not influenced by the chronologic age at onset of the pubertal 
growth spurt because the additional time for prepubertal growth 
that occurs when puberty is late is balanced by the fact that puber­
tal growth is smaller the later it occurs (see Figs. 25.9 and 25.10). 
After puberty, chondrocyte proliferation i

5.10). 
After puberty, chondrocyte proliferation in the growth plate slows 
and senescence occurs due to depletion of stem-like cells in the 
resting zone of the growth plate.29,30
There are two variants of normal growth whose characteristic 
patterns are such that children exhibiting these growth variants 
are often evaluated for a growth disorder. These two variants are 
crossing linear percentiles of infancy and constitutional delay of 
growth and development (CDGD). In many cases, it is diff

 and development (CDGD). In many cases, it is difficult 
to differentiate children with these normal growth variants from 
children with a growth disorder.
Crossing Linear Percentiles of Infancy
As in postnatal growth, genetic and environmental factors are 
important determinants of fetal growth and of the size of an infant 
(weight and length) at birth. However, there are significant differ­
ences between the factors affecting birth size and those affecting 
childhood growth and adult stature. 

se affecting 
childhood growth and adult stature. There is much less correlation 
between length at birth and ultimate adult height than between 
length at birth and height in the later years of childhood.31
Parental stature affects birth length, as it does adult stature; this 
is an indicator of the genetic influence on growth. Although mater­
nal and paternal stature contribute equally to childhood growth and 
adult stature, the effect of maternal stature may predominate over that 
of paternal

nal stature may predominate over that 
of paternal stature on length at birth.32,33 However, some studies have 
found equal effects of maternal and paternal stature on birth size.34 
Maternal nutrition and health have significant effects on fetal growth. 
Maternal weight has a positive effect, and smoking during pregnancy 
has a negative effect. Maternal diabetes has a strong positive effect on 
fetal growth. It is possible for the prenatal determinants of growth 
to result, for example, in a ch

minants of growth 
to result, for example, in a child who will ultimately grow to below-
average stature but is above average size at birth. For this reason, it is 
common for infants’ lengths to cross percentiles on the growth curve. 
Indeed, it is more common for the growth of an infant to cross per­
centiles than to follow a single percentile from birth into childhood: 
Approximately one-third of infants have lengths that cross percentiles 
moving upward on the growth curves, and approximatel

ving upward on the growth curves, and approximately one-third 
have lengths that cross percentiles moving downward.33,35 Most nor­
mal infants crossing percentiles do so in the first 6 to 12 months, 
although some normal infants cross percentiles after 1 year of age.
Infants who are small at birth can be divided into (1) those who 
are small only because of prematurity and are therefore of a size 
appropriate for gestational age (AGA) and (2) those who are small 
for gestational age (SGA). SGA i

se who are small 
for gestational age (SGA). SGA is usually defined as a birth weight 
or length (or both) below the 3rd percentile (or, sometimes, below 
the 10th percentile) for gestational age.36 Some infants born SGA 
represent the small percentage of individuals whose genetic poten­
tial leads to small size at birth, and they can be expected to remain 
small throughout childhood and adulthood. However, many 
infants are SGA due to intrauterine growth retardation (IUGR) 
and have a genetic p

ne growth retardation (IUGR) 
and have a genetic potential that would not be expected to result 
in small adult stature. Most infants who are born small, either 
AGA or SGA, have catch-up growth and achieve lengths greater 
than the 3rd percentile within the first 2 years of life. However, up 
to 10% of SGA infants do not show such catch-up growth.37,38 
The pathologic aspects of IUGR and of SGA in infants who do 
not have catch-up growth are discussed later. 
Constitutional Delay of Growth and 

cussed later. 
Constitutional Delay of Growth and Development
CDGD is a normal variant of growth.39 It describes the growth 
pattern of children who will experience a later than average tim­
ing of puberty. Their birth size is normal, and their final height 
is within their genetic potential. However, during most of their 
childhood, they grow at a height percentile below that expected 
based on their genetic potential. Typically, these children have a 
low growth velocity during the first years

have a 
low growth velocity during the first years of life, crossing down­
ward on the length percentile growth curves, so that by 2 years of 
age their heights are at or slightly below the 5th percentile. After 
age 3 years, their growth rate is typically normal, so that their 
height growth usually remains parallel to the 5th percentile until 
adolescence, although the height SDS may gradually drift slightly 
lower during the middle childhood years in some cases.39,40 Their 
height diverges ev

ears in some cases.39,40 Their 
height diverges even further from that of average children during 
the early teen years due to the declining prepubertal growth veloc­
ity of these children compared with the accelerating growth rate of 
children with an average timing of the onset of puberty.
Ultimately, children with CDGD have a late growth spurt, 
consistent with their late puberty, and this brings their height into 
the normal adult range. Final height is often in the lower part 
of the parent

l height is often in the lower part 
of the parental target height range, and few patients exceed the 
parental target height,41–43 although this finding is probably at 
least in part the result of a selection bias of the children exam­
ined for such studies. However, there is evidence that a delayed 
growth spurt may adversely affect growth of the spine, resulting 
in a decrease in the final ratio of the upper to the lower segment 
and perhaps contributing to a limited final height.44 Studies h

ontributing to a limited final height.44 Studies have 
also reported that prepubertal boys with CDGD have decreased 
bone mineral density (BMD),45 although by young adulthood the 
majority of the BMD deficit is lost.46,47 
Secular Changes in Height
There are surprisingly few data concerning the stature of modern 
humans before the measurement of military recruits became cus­
tomary in the 18th century. Skeletal remains from the last ice age 
appear to indicate that adult stature 10,000 to 20,000

ar to indicate that adult stature 10,000 to 20,000 years ago 
was not substantially different from that of contemporary adults, 
although this record is obviously fragmentary.48 It has been sug­
gested that a reduction in stature was observed with the intro­
duction of agriculture approximately 5000 years ago, with growth 
attenuation resulting from the combined effects of nutrient defi­
ciency, population growth, and spread of infectious diseases.
Military recruits in the 18th and 19th centurie

s.
Military recruits in the 18th and 19th centuries were clearly 
shorter than those of today, although it must be recognized that 
soldiers were commonly recruited from the lower socioeconomic 
classes, and poor health and nutrition would have contributed 
to both poor growth and late maturity.49 Whereas men in the 
20th century averaged 5 to 10 cm greater in height than those in 
the 18th century for whom we have records, much of this height 
gain has occurred over the past 100 years and proba

ain has occurred over the past 100 years and probably reflects 
the dramatic improvement in overall nutrition and health seen 
in the Western world. This upward trend in height appears to 
have stopped in many developed countries in the early 21st 
century.50
Therefore secular changes in height appear to reflect fundamen­
tal alterations in the standard of living rather than major genomic 
differences among populations; future economic advances in 
developing countries can be predicted to lead t

n 
developing countries can be predicted to lead to improvement 
in adult stature and a reduction in international differences in 
growth. 
944
Endocrine Regulation of Growth
The Hypothalamic-Pituitary Axis: Embryogenesis 
and Anatomy
The pituitary gland is central to the regulation of mammalian 
growth. The pituitary gland develops from oral ectoderm in 
response to inductive signals from the neuroepithelium of the 
ventral diencephalon and intrinsi

ithelium of the 
ventral diencephalon and intrinsic signaling gradients determin­
ing expression patterns of pituitary-specific transcription factors 
in the developing anterior pituitary gland.51 The primordium of 
the anterior pituitary, Rathke pouch, forms as an upward invagi­
nation of a single-cell-thick layer of ectoderm that contacts the 
neuroectoderm of the primordium of the ventral hypothalamus 
at embryonic day 8.5 (E8.5) in the mouse embryo52 and can be 
identified by the third week 

embryo52 and can be 
identified by the third week of pregnancy in humans. The neu­
rohypophysis (posterior pituitary) originates in the neural ecto­
derm of the floor of the forebrain, which also develops into the 
third ventricle. During anterior pituitary development, overlap­
ping but regionally specific and temporally distinct patterns of 
homeobox transcription factor expression lead to the sequential 
appearance of the terminally differentiated cell types from E12.5 
to birth.52
The initia

ated cell types from E12.5 
The initiation of anterior pituitary gland development 
depends on the competency of the oral ectoderm to respond to 
inducing factors from the neural epithelium of the ventral dien­
cephalon.53 The bone morphogenetic protein 4 (BMP4) signal 
from the ventral diencephalon is the critical dorsal neuroepithe­
lial signal required for organ commitment of the anterior pitu­
itary gland. Wnt5a and fibroblast growth factor 8 (FGF8) are 
also expressed in the die

wth factor 8 (FGF8) are 
also expressed in the diencephalon in distinct overlapping pat­
terns with BMP4. Subsequently, a BMP2 signal arises from the 
boundary of a region of oral ectoderm in which Sonic hedgehog 
(SHH) expression, initially expressed uniformly in the oral ecto­
derm, is selectively excluded from the developing Rathke pouch. 
Also expressed are Gli1 and 2, Lhx3, and Pitx1 and 2, which 
are important in progenitor pituitary cell type development. The 
ventral-dorsal BMP2 signal a

ype development. The 
ventral-dorsal BMP2 signal and the dorsal-ventral FGF8 signal 
appear to create opposing activity gradients that are suggested 
to dictate overlapping patterns of specific transcription factors 
underlying cell lineage specification. The various extensions of 
these transcription factors in their fields are theorized to combi­
natorially determine the specific cell types. The FGF8 gradient 
determines the dorsal cell phenotypes,53,54 and dorsally expressed 
transcription fa

pes,53,54 and dorsally expressed 
transcription factors include Hesx1, Nkx-3.1, Six3, Pax6,55 and 
prophet of Pit1 (PROP1).56 The attenuation of Hesx1 is required 
for the expression of PROP1. Temporally specific attenuation 
of the BMP2 signal is required for terminal differentiation of 
the ventral cell types, and ventrally expressed transcription fac­
tors include islet-1 (Isl1), Brn4, P-Frk, and GATA2.53,56,57 Pit1 
(encoded by the gene POU1F1) is expressed upon the attenuation 
of PROP1 exp

1) is expressed upon the attenuation 
of PROP1 expression and required for somatotroph, lactotroph, 
and thyrotroph development,58,59 whereas the orphan nuclear 
receptor steroidogenic factor 1 (SF1) is selectively expressed in 
the gonadotrophs.60,61
The ventral-dorsal gradient induces GATA2 in a correspond­
ing gradient in presumptive gonadotrophs and thyrotrophs, and 
high levels of GATA2 in the most ventral aspect of the develop­
ing anterior pituitary directly or indirectly restrict express

 pituitary directly or indirectly restrict expression of 
POU1F1 out of the presumptive gonadotrophs. In the absence 
of Pit1, GATA2 expression appears sufficient to induce the entire 
set of transcription factors that are typical of the gonadotroph cell 
type, including SF1, P-Frk, and Isl1. Conversely, the absence of 
GATA2 dorsally is critical for differentiation of Pit1-positive cells 
to somatotroph/lactotroph fates. It is hypothesized that the level 
of GATA2 expression in the thyrotrophs 

the level 
of GATA2 expression in the thyrotrophs is below the threshold 
required to inhibit activation of the POU1F1 gene early enhancer, 
permitting the emergence of a Pit1+, GATA2+ cell that results 
in the thyrotroph fate.57 Pax6 has a role in the sharp boundary 
of attenuation of the ventral signals that dictate thyrotroph and 
gonadotroph cell lineages. In the absence of Pax6, the ventral lin­
eages, particularly thyrotrophs, become dorsally extended at the 
expense of somatotroph and lac

ly extended at the 
expense of somatotroph and lactotroph cell types,55 and Pax6 
mutant mice are GH and PRL deficient.62
The earliest marker of the anterior pituitary is expression of the 
glycoprotein α-subunit (αGSU), which appears at E11.5 in the 
mouse. These αGSU-positive cells also express the transcription 
factor Isl1 and mark a population of differentiating thyrotrophs 
that disappears after birth.54,58,63,64 αGSU is expressed in mature 
thyrotrophs and gonadotrophs. At E12.5, corticot

thyrotrophs and gonadotrophs. At E12.5, corticotrophs start to 
differentiate and produce pro-opiomelanocortin (POMC).54,64 
Intensified cell proliferation within Rathke pouch results in for­
mation of a visible nascent anterior pituitary lobe on E12.5.52 
Definitive thyrotrophs are observed at 14.5 days postconception 
(dpc), characterized by the expression of Tshb at E14.5 and fol­
lowed by the expression of GH and prolactin (PRL) in somato­
trophs and lactotrophs, respectively, at E15.5. Th

trophs and lactotrophs, respectively, at E15.5. The gonadotrophs 
are the last cell type to develop, at E16.5, marked by expression 
of luteinizing hormone (LH) and later by follicle-stimulating 
hormone (FSH). Eventually the mature gland is populated by at 
least five highly differentiated cell types: Ventrally to dorsally, they 
are the gonadotrophs, thyrotrophs, somatotrophs, lactotrophs, 
and corticotrophs.53 Ultimately, some of these same transcrip­
tion factors are also involved in the cel

anscrip­
tion factors are also involved in the cell-specific expression and 
regulation of the gene products of these pituitary cell types, with 
corticotrophs producing adrenocorticotropin hormone (ACTH), 
thyrotrophs producing thyrotropin (thyroid-stimulating hormone 
[TSH]), gonadotrophs producing gonadotropins (LH and FSH), 
somatotrophs producing GH, and lactotrophs producing PRL. 
The developmental factors that play an in vivo role in pituitary 
gland development and differentiation are sh

tary 
gland development and differentiation are shown in Fig. 25.13.
In humans, GH-producing cells can be found in the anterior 
pituitary gland by 9 weeks of gestation,65 and vascular connections 
between the anterior lobe of the pituitary and the hypothalamus 
develop at about the same time,66 although hormone production 
can occur in the pituitary in the absence of connections with the 
hypothalamus. Somatotrophs can be demonstrated in the pitu­
itary in anencephalic newborns.67
In the newbor

u­
In the newborn, the pituitary weighs about 100 mg. In the 
adult, the mean weight is about 600 mg, with a range of 400 to 
900 mg; the pituitary is slightly heavier in women than in men 
and increases during pregnancy.68 The mean adult pituitary size is 
13 × 9 × 6 mm.69 The anterior pituitary normally constitutes 80% 
of the weight of the pituitary. The pituitary resides in the sella tur­
cica, immediately above and partially surrounded by the sphenoid 
bone

ove and partially surrounded by the sphenoid 
bone. The volume of the sella turcica is a good index of pituitary 
size and may be reduced in the child with pituitary hypoplasia. The 
optic chiasm is located superior to the pituitary gland, so suprasel­
lar growth of a pituitary tumor may initially manifest with visual 
complaints or evidence of decreases in peripheral vision. Further­
more, development of the neurohypophysis and the pituitary are 
intimately related, leading to potential anatomi

intimately related, leading to potential anatomic associations of 
central nervous system (CNS) abnormalities with pituitary hypo­
plasia. For example, septo-optic dysplasia is associated with several 
945
CNS anatomic abnormalities and pituitary hormone deficiencies. 
For this reason, children with congenital blindness or nystagmus 
should be monitored for hypopituitarism.
The anterior pituitary receives controlling signals from 
the hypothal

ry receives controlling signals from 
the hypothalamus through the portal circulatory system (Fig. 
25.14).66 The hypothalamus integrates signals from other brain 
regions and the environment, resulting in the release of factors that 
control pituitary hormone synthesis and secretion. Hypothalamic 
neurons that synthesize peptides terminate in the infundibulum, 
enter the primary plexus of the hypophyseal portal circulation, 
and are transported via the hypophyseal portal veins to the capil­
lar

via the hypophyseal portal veins to the capil­
laries of the anterior pituitary. This portal system in the pituitary 
stalk provides a means of communication between the neurons of 
the hypothalamus and the anterior pituitary.
A
Nkx2.1
Wmt5a
BMP4
Tebp
FGF8
TRH
Gsh1
GHRH
SF1
GnRH
Pitx1
Lhx4
NeuroD1
Tbx19
POMC
Gli1,2
Lhx3,4 Otx2
Pitx1/2
Shh
Pitx1/2
Lhx3/4
POU1F1
GATA2
Pitx1/2
Lhx3/4
Pit1
Pitx1/2
Lhx3/4
SF1
B
Pitx1/Pitx2
Rpx/Hesx1
Lhx3/Lhx4
Prop1
SF1
Pit1
Krox24
e7
Stomodeum
Anterior p

1
Pit1
Krox24
e7
Stomodeum
e9
e11
e13
e15
e17
e19
Pitx1/2
Lhx3/4
SF1
GATA2
LH,FSH
Pitx1/2
Lhx3/4
POU1F1
Zn15
GH
TSH
Pitx1/2
Lhx3/4
POU1F1
ER/Ets
PRL
Isl1 Pax6
Pitx1/2
Rpx Six3,6
Lhx3
Lhx4
Pitx1/2
Pitx1/2
Lhx3/4
Prop1
αGSU
Dopamine
• Fig. 25.13  Development of pituitary cell lineages. (A) Schematic representation of pituitary cell precursors 
shows the expression of prevalent transcription factors at each stage of development. Terminally differenti­
at

ch stage of development. Terminally differenti­
ated cells are shown as larger and shaded circles together with the hormones produced (lineage-specific 
transcription factors are highlighted in bold in these cells). The interaction with transcription factors and 
signaling molecules in the hypothalamus is also depicted. Transcription factors are represented in lower­
case (except for SF1 and GATA2), whereas signaling molecules appear in uppercase. (B) The timing of 
appearance and disappearance 

. (B) The timing of 
appearance and disappearance of pituitary transcription factors during mouse embryogenesis. αGSU, 
glycoprotein α-subunit; BMP4, bone morphogenic protein 4; e, embryonic day; ER, estrogen recep­
tor; FGF8, fibroblast growth factor 8; FSH, follicle-stimulating hormone; GATA2, GATA binding factor 2; 
GHRH, growth hormone–releasing hormone; GnRH, gonadotropin-releasing hormone; Hesx1, homeobox 
expressed in ES1 cells (Rathke pouch homeobox, Rpx); LH, luteinizing hormone; Nkx2.1

ch homeobox, Rpx); LH, luteinizing hormone; Nkx2.1, NK2 homeobox 
1; POMC, pro-opiomelanocortin; PRL, prolactin; SF1, steroidogenic factor 1; Tebp, telomere binding pro­
tein; TRH, thyrotropin-releasing hormone; TSH, thyroid-stimulating hormone (thyrotropin); Wmt5a, wing­
less type MMTV integration site family, member 5A. (Reprinted with permission from Lopez-Bermejo A, 
Buckway CK, Rosenfeld RG. Genetic defects of the growth hormone-insulin-like growth factor axis. Trends 
Endocrinol. 2000;11:4

 growth factor axis. Trends 
Endocrinol. 2000;11:43.)
946
Growth hormone–releasing hormone (GHRH) is a 44–amino 
acid peptide hormone70,71 secreted by neurons located in the arcu­
ate nucleus (ARC) of the hypothalamus.72 GHRH is the main 
hypophysiotropic neuropeptide responsible for the generation and 
maintenance of pulsatile GH secretion.73,74 Regulation of GH 
production by GHRH is mediated largely at the level of

uction by GHRH is mediated largely at the level of transcrip­
tion and is enhanced by increases in intracellular cyclic adenosine 
monophosphate (cAMP) levels. Evidence includes administration 
of GHRH antagonists resulting in an impairment of GH pulsa­
tility, ablation of the ARC nucleus resulting in loss of GH secre­
tion,74,75 and administration of synthetic GHRH increasing GH 
secretion.75 GHRH binds to GHRH receptors (GHRH-R) in 
somatotropes to activate multiple intracellular signaling pat

s to activate multiple intracellular signaling pathways 
to stimulate somatotroph cell proliferation, differentiation, and 
growth76,77 as well as to stimulate the secretion and synthesis of 
GH (Fig. 25.15).78,79 The GHRH receptor is a member of the 
G protein–coupled receptor (GPCR) family BIII.80 GH signal­
ing pathways engaged by GHRH-R activation include adenylate 
cyclase (AC), increasing cAMP production, which in turn leads 
to an increase in protein kinase A (PKA) activity,81–83 intracel

 in protein kinase A (PKA) activity,81–83 intracellu­
lar and extracellular Ca2+, NOS/NO/GC/cGMP, and PKC/PLC 
pathways.81,84 In a dwarf transgenic mouse model with dimin­
ished GHRH production, pituitary somatotroph proliferation is 
markedly decreased.85,86 Transgenic mice that overexpress GHRH 
grow at a faster rate than control mice.87 
SST or somatotropin-release inhibitory factor (SRIF) is derived 
from a 116 precursor that produces two different cyclic forms, 
somatosta

at produces two different cyclic forms, 
somatostatin-14 and somatostatin-28, by alternative post-trans­
lational processing. Somatostatin binds to at least five receptor 
subtypes (SST1–5 receptors), which are GPCRs with seven trans­
membrane domains. Subtypes 2 and 5 are the predominant types 
found in the pituitary.88–90 SST binding results in the recruitment 
of several downstream signaling pathways, including AC, protein 
phosphatases, cyclic guanosine monophosphate (cGMP)–depen­
dent prote

c guanosine monophosphate (cGMP)–depen­
dent protein kinases, and calcium in addition to other ion chan­
nels.91–93 SST inhibits GH release and antagonizes the stimulatory 
effects from GHRH or ghrelin.73 
Human GH is produced as a single-chain, 191–amino acid, 
22-kDa protein containing two intramolecular disulfide bonds 
(Fig. 25.16). GH shares sequence homology with PRL, chorionic 
somatomammotropin (CS, placental lactogen), and a 22-kDa 
GH variant (GH-V) that is secreted only

 a 22-kDa 
GH variant (GH-V) that is secreted only by the placenta and 
Mamillary
body
Median eminence
Posterior
Optic
chiasm
Artery
Primary capillary
plexus
portal vessels
Sinuses
Anterior
Vein
• Fig. 25.14  The main components of the hypothalamic-pituitary portal 
system. (From Guyton AC, Hall JC. Human Physiology and Mechanisms 
of Disease, 6th ed. Philadelphia: WB Saunders; 1997:600, used with per­
mission.)
Adenyl cyclas

; 1997:600, used with per­
mission.)
GHRH
GS
Gi
ATP
cAMP
PKA
CREB
P
CREB
GH2
GH1
GH
GH promoter
CBP
P
POU1F1
POU1F1
• Fig. 25.15  Schematic diagram summarizing the main signaling pathways in the somatotroph involved in 
the response to GHRH and somatostatin. GHRH binding to its receptor increases intracellular cAMP. Acti­
vation of the PKA pathway results in binding of POU1F1 to specific promoter sequences on the GH gene 
to increase transcription. Somatostatin bindin

ne 
to increase transcription. Somatostatin binding to its receptor decreases intracellular cAMP.
947
differs from pituitary GH by 13 amino acids. The genes encoding 
these proteins most likely evolved from a common ancestral gene, 
despite being located on different chromosomes (chromosome 
6 for PRL and chromosome 17 for GH).94 The genes for GH, 
PRL, and placental lactogen share a common structural organiza­
tion, with four introns separating

tural organiza­
tion, with four introns separating five exons. The GH subfam­
ily contains five members, with genes located on a 78-kilobase 
(kb) section of chromosome 17; the 5′ to 3′ order of the genes is 
GH, a CS pseudogene, CS-A, GH-V, and CS-B. Normally, about 
75% of GH produced by the pituitary is of the mature, 22-kDa 
form. Alternative splicing of the second codon results in deletion 
of amino acids 32 through 46, yielding a 20-kDa form that nor­
mally accounts for 5% to 10% of pituit

m that nor­
mally accounts for 5% to 10% of pituitary GH.94 The remainder 
of pituitary GH includes desamidated and N-acetylated forms and 
various GH oligomers.
Growth Hormone Pulsatility
The pulsatile pattern characteristic of GH secretion reflects the 
interplay of the two hypothalamic regulatory peptides, GHRH 
and somatostatin, with modulation by other putative GH-releas­
ing factors.95,96 Somatostatin appears to have its major effect on 
the timing and amplitude of pulsatile GH secretion a

e timing and amplitude of pulsatile GH secretion and lesser 
effects on the regulation of GH synthesis. The pulsatile secretion 
of GH in vivo is believed to result from a simultaneous reduc­
tion in hypothalamic somatostatin release and increased GHRH 
release.97 Conversely, a trough of GH secretion occurs when 
somatostatin is released in the face of diminished GHRH activity. 
The coordinated secretion of GH pulses in response to GH secre­
tagogues results from a continuum of GH cells geograph

gues results from a continuum of GH cells geographically 
connected by adherens junctions as revealed by three-dimensional 
reconstructive microscopy.98
Regulation of the reciprocal secretion of GHRH and soma­
tostatin is imperfectly understood. The hypothalamus integrates 
signals for stress, sleep, hemorrhage, fasting, hypoglycemia, and 
exercise through the secretion of multiple neurotransmitters and 
neuropeptides to regulate the release of these hypothalamic factors 
and ultimately to influ

hese hypothalamic factors 
and ultimately to influence GH secretion. This physiologic phe­
nomenon forms the basis for a number of GH-stimulatory tests 
used in the evaluation of GH secretory capacity or reserve. GH 
secretion is also influenced by a variety of nonpeptide hormones, 
including androgens,99 estrogens,100 thyroxine,101 and glucocor­
ticoids.102,103 The mechanisms by which these hormones regu­
late GH secretion may involve actions at the hypothalamus and 
the pituitary. For example,

 the hypothalamus and 
the pituitary. For example, hypothyroidism and glucocorticoid 
excess may each blunt spontaneous and provocative GH secre­
tion. Gonadal steroids appear to be responsible for the rise in GH 
secretion that characterizes puberty. 
Ghrelin
Synthetic hexapeptides capable of stimulating GH secretion are 
termed GH secretagogues.80 These peptides stimulate GH release 
and enhance the GH response to GHRH, although they work 
at receptors distinct from those for GHRH, at hypothal

eceptors distinct from those for GHRH, at hypothalamic and 
pituitary sites. Kojima and colleagues104 identified a natural ligand 
called ghrelin, a 28–amino acid protein with the serine 3 residue 
n-octanoylated. It is produced mainly by the oxyntic cells of the 
stomach (and throughout the gastrointestinal tract105) and in the 
hypothalamus, heart, lung, and adipose tissue.106,107 Ghrelin has 
a potent, dose-related, GH-releasing effect108 and potentiates 
the GHRH-dependent secretion of GH. A

potentiates 
the GHRH-dependent secretion of GH. At the hypothalamic 
level, it increases GHRH secretion and inhibits somatostatin 
neurons.81,109–111 GH release results from the binding of ghre­
lin to the growth hormone secretagogue 1a receptor (GHSR1a) 
on somatotrophs in the pituitary108 and on GHRH-containing 
neurons in the hypothalamus.112 The coupling of ghrelin to the 
GHSR1a receptor results in activation of multiple signaling cas­
cades, including phospholipase C (PLC), protein kinase

s, including phospholipase C (PLC), protein kinase C (PKC), 
PKA,113 intracellular and extracellular Ca2+, or mitogen-activated 
H2N
COOH
Phe
1
5
10
15
20
25
Pro Thr
Ile
Pro Leu Ser Arg Leu Phe Asp Asn Ala Met Leu Arg
Ala His
Arg
Leu
His
Gln
Leu
Ala
Phe
30
Asp
Thr
Tyr
Glu
35
Phe
Glu
Glu
Ala
Tyr
40
Ile
Pro
Lys
Glu
Gln
Gln
45
Lys
Tyr
Ser
Phe
Leu
50
Gln
Asn
Pro
Gln
Thr
55
Ser
Leu
CyS
Phe
Ser
60
Glu
Ser
Ile
Pro
Thr
65
Pro
Ser
Asn
Arg
Glu
70
Glu
Thr
Gln
Gln
Lys
75
Ser
Asn
Leu
Gln
Leu
80
Leu
Arg
Ile
S

ln
Gln
Lys
75
Ser
Asn
Leu
Gln
Leu
80
Leu
Arg
Ile
Ser
Leu
85
Leu
Leu
Ile
Gln
Ser
90
Trp Leu Glu Pro Val
95
Gln Phe Leu Arg Ser Val
100
105
110
115
120
Phe Ala Asn Ser Leu Val Tyr Gly Ala
Ser
Asn
Ser
Asp
Val
Tyr
Asp
Leu
Leu
Lys
Asp
Leu
Glu
Glu
Glu
Gly
125
Ile
Gln
Thr
Leu
Met
130
Gly
Arg
Leu
Glu
Asp
135
Gly
Ser
Pro
Arg
Thr
140
Gly
Gln
Ile
Phe
Lys
145
Gln
Thr
Tyr
Ser
Lys
150
Phe
Asp
Thr
Asn
Ser
155
His
Asn
Asp
Asp
Ala
160
Leu
Leu Lys Asn Tyr
165
Gly Leu Leu Tyr CyS
170
Phe Arg Lys
Asp
Met
175
Asp
Ly

Leu Leu Tyr CyS
170
Phe Arg Lys
Asp
Met
175
Asp
Lys
Val
Glu
Thr
180
Phe
Leu
Arg
Ile
Val
185
Gln
CyS
Arg
Ser
Val
190
Glu
Gly
Ser
CyS
Gly
Phe
• Fig. 25.16  Covalent structure of human growth hormone. (From Chawla RK, Parks JS, Rudman D. 
Structural variants of human growth hormone: biochemical, genetic and clinical aspects. Annu Rev Med. 
1983;34:519–547.)
948
protein kinases.110,114,115 Many studies have demonstrated that 
ghrelin has a wide range of 

ve demonstrated that 
ghrelin has a wide range of effects, including on immune function, 
cognition, other anterior pituitary hormones (including gonadal 
axis regulation), bone metabolism, gastrointestinal motility, cell 
proliferation, and the cardiovascular system.111,116–123
However, it is difficult to separate the direct effects of ghrelin 
from those related to GH secretion. Although ghrelin has docu­
mented physiologic effects in vivo, GHSR1a knockout mice have 
a phenotype similar to tha

R1a knockout mice have 
a phenotype similar to that of wild-type animals, suggesting that 
ghrelin does not have a role in growth. However, compensatory 
mechanisms may provide an explanation for these findings.124,125
More recently, studies have documented a positive correla­
tion between ghrelin and anthropometric parameters in the first 
months of life, a finding that strengthens the hypothesis that ghre­
lin exerts an influence on growth.126 Two reports of mutations in 
GHSR1a in familial sh

Two reports of mutations in 
GHSR1a in familial short stature provide evidence to the con­
trary.127,128 Further studies indicate that the aging process may be 
associated with decreased expression of GHSRs in the hypothala­
mus129 and with systemic concentration of ghrelin.130 In addi­
tion to direct effects on linear growth, ghrelin has been shown 
to increase energy stores by stimulating appetite and affecting 
peripheral glucose and lipid metabolism.131–133 These data sug­
gest that ghrelin 

abolism.131–133 These data sug­
gest that ghrelin is an important stimulus for nutrient allocation 
for growth and metabolism and a central component of the GH 
regulatory system. Orally active ghrelin analogues have been con­
sidered as therapeutic agents in the treatment of GH deficiency 
(GHD), because they may provide a more physiologic approach 
to increasing the pulsatile release of endogenous GH compared 
with a single daily dose of recombinant human GH. However, 
there has been no defini

inant human GH. However, 
there has been no definite evidence of the therapeutic efficacy of 
ghrelin analogues in the treatment of GHD states. 
Pituitary Adenylate Cyclase–Activating Polypeptide
Pituitary adenylate cyclase–activating polypeptide (PACAP) is 
a hypothalamic peptide that has been shown to be effective in 
releasing GH from cultured pituitary cells. It belongs to a super­
family of hormones that includes glucagon, secretin, glucagon-
like peptide 1 (GLP1), GLP2, GHRH, vasoactive in

-
like peptide 1 (GLP1), GLP2, GHRH, vasoactive intestinal 
polypeptide (VIP), peptide histidine methionine (PHM), and 
glucose-dependent insulinotropic polypeptide (GIP). Studies on 
the role of PACAP on GH release have been contradictory, some 
reporting a stimulatory action while others showing no effect 
on GH release.73 PACAP increased both cAMP production and 
GH release in human somatotrope tumor cell lines134 involving 
activation of voltage-operated/gated Ca2+ channels and signaling 
th

age-operated/gated Ca2+ channels and signaling 
through the AC/PKA pathway.73,135 However, studies in humans 
did not reveal an induction of GH release after intravenous 
PACAP administration.136
PACAP binds to several G protein–coupled receptors with 
differential affinity for PACAP or VIP. PACAP type 1 receptors 
(PAC1R) are more specific for PACAP, while VPAC1 and VPAC2 
receptors bind VIP and PACAP isoforms with similar affinity. Fur­
ther complexity is added by at least five alternative spl

mplexity is added by at least five alternative splice products 
of PAC1R alternative splicing, which have differential affinity for 
PACAP isoforms and induce different signaling pathways.137,138 
PACAP receptors (PAC1R) are found throughout the brain and 
in peripheral tissues.137,138 Gene knockout of the specific PACAP 
receptor (PAC1R) resulted in a 60% mortality rate in the PAC1R 
null mice in the first 4 weeks after birth, providing insight into the 
importance of PACAP, even though other s

into the 
importance of PACAP, even though other superfamily members 
may compensate some of its functions.139 The surviving knock­
out mice showed reduced glucose-stimulated insulin release and 
glucose intolerance. This observation suggests that PACAP is 
important in carbohydrate metabolism, potentially through GH.
The synthesis and secretion of GH are also regulated by the 
insulin-like growth factor (IGF) peptides. Receptors specific for 
IGF1 and IGF2 have been identified in the hypothalam

F1 and IGF2 have been identified in the hypothalamus and 
pituitary.140 Inhibition of GH secretion by IGF1 or IGF2, or 
both, has been demonstrated,141,142 and spontaneous GH secre­
tion is diminished in humans treated with synthetic IGF1.143 
Growth Hormone Secretion in Humans
The episodic release of GH from the pituitary somatotrophs 
results in intermittent increases in serum levels of GH separated 
by periods of low or undetectable levels, during which time GH 
secretion is minimal.97,144 Th

ring which time GH 
secretion is minimal.97,144 The pulsatile nature of GH secretion 
has been demonstrated by frequent serum sampling coupled with 
the use of sensitive immunofluorometric or chemiluminescent 
assays of GH.144 GHRH is primarily responsible for the gen­
eration and maintenance of pulsatile/episodic GH secretion.73,74 
Under normal circumstances, serum GH levels are less than 0.04 
mg/L between secretory bursts. It is impractical to assess GH 
secretion by random serum sampling. E

o assess GH 
secretion by random serum sampling. Extensive sampling stud­
ies at different ages in normal persons and in many abnormal 
conditions have defined GH pulses, basal secretion, and diurnal 
variability. Computer programs have been developed to indicate 
whether changes in GH levels in various life periods and under 
diverse clinical circumstances occur because of a change of secre­
tory mass or pulse frequency, altered clearance, or a combination 
of these processes.144,145 Deconvolut

combination 
of these processes.144,145 Deconvolution techniques allow accurate 
estimates of the quantity of GH secreted per burst, GH clearance 
kinetics, and pulse amplitudes and frequencies, as well as an overall 
calculation of endogenous GH production. Approximate entropy, 
a model-free measure, is applied to quantify the degree of orderli­
ness of GH release patterns.146 The impact of the specific nature of 
pulsatile GH secretion on its biologic actions is under study.144,145 
For exampl

iologic actions is under study.144,145 
For example, it appears that better statural growth is associated 
with large swings of GH output of relatively uniform magnitude 
in an irregular sequence (high approximate entropy).147,148
GH-secreting cells have been identified by 9 to 12 weeks of 
gestation, and immunoreactive pituitary GH is present by 7 to 
9 weeks of gestation.149 Fetal pituicytes secrete GH in vitro by 
5 weeks, before the hypothalamic-portal vascular system is dif­
ferentiated. Pi

mic-portal vascular system is dif­
ferentiated. PitT1 mRNA and Pit1 protein are expressed by at 
least 6 weeks of gestation; their abundant presence early in ges­
tation suggests an important role in cytodifferentiation and cell 
proliferation.150 GH can be identified in fetal serum by the end 
of the first trimester, with peak levels of approximately 150 mg/L 
in midgestation.149 Serum levels fall throughout the latter part of 
pregnancy and are lower in full-term than in premature infants, 
pe

 lower in full-term than in premature infants, 
perhaps reflecting feedback by the higher serum levels of IGF pep­
tides that characterize the later stages of gestation.151
Mean levels of GH decrease from values of 25 to 35 mg/L in 
the neonatal period to approximately 5 to 7 mg/L through child­
hood and early puberty.152 Twenty-four-hour GH secretion peaks 
during adolescence, undoubtedly contributing to the high serum 
levels of IGF1 that are characteristic of puberty. The increase 
in GH prod

haracteristic of puberty. The increase 
in GH production during middle to late puberty is caused by 
enhanced pulse amplitude and increased mass of GH per secre­
tory burst, rather than by a change in pulse frequency (Figs. 25.17 
and 25.18).145,146,152 Greater irregularity in GH secretion corre­
sponds to greater linear growth.153 In the face of stable levels of 
the growth hormone–binding protein (GHBP),154 the enhanced 
pubertal GH production appears to be associated with higher 
949
Chapter 

appears to be associated with higher 
949
levels of “free” GH (Fig. 25.19), potentially facilitating the deliv­
ery of IGF1 to target tissues. This enhanced activity of the GH-
IGF axis contributes to the insulin resistance that occurs during 
puberty.155 Production of GH and IGF begins to decline by late 
adolescence and continues to fall throughout adult life. Normal 
young adult men experience 6 to 10 GH secretory bursts per 24 
hours, a valu

 6 to 10 GH secretory bursts per 24 
hours, a value similar to that observed in younger children and 
adolescents.144,152 On the other hand, 24-hour GH production 
rates for normal men range from 0.25 to 0.52 mg/m2 surface 
area,103 about 20% to 30% of pubertal levels; this is largely due to 
decreased GH pulse amplitude with age.152 Indeed, puberty may 
be considered, with some justification, a period of “physiologic 
acromegaly,” whereas aging, with its decrease in GH secretion, has 
been term

 with its decrease in GH secretion, has 
been termed the somatopause.100,156
Physiologic states that affect GH secretion, in addition to 
maturation and aging, include sleep,157 nutritional status,95,158 
fasting, exercise,159 stress,159 and gonadal steroids. Maximal GH 
secretion occurs during the night, especially at the onset of the 
first slow-wave sleep period (stages III and IV). Rapid eye move­
ment (REM) sleep, on the other hand, is associated with low GH 
secretion.157 A circadian rhyth

iated with low GH 
secretion.157 A circadian rhythm of somatostatin secretion, upon 
which is superimposed episodic bursts of GHRH release, may 
help explain the nocturnal augmentation of GH production.160
When testosterone was administered to boys with delayed 
puberty, spontaneous GH release was enhanced, but such a change 
was not duplicated by administration of nonaromatizable andro­
gens, emphasizing the possible unique importance of estrogen in 
GH secretion.161 The effects of testosterone

n in 
GH secretion.161 The effects of testosterone on serum IGF1 levels 
may in part be independent of GH because individuals with muta­
tions of the GH receptor (GHR) still experience a modest rise in 
serum IGF1 during puberty.162 With a combination of deconvo­
lution analysis, approximate entropy, and cosine regression analy­
sis, Veldhuis and associates146,161 carefully evaluated intensive GH 
sampling data derived from measurements in sensitive GH assays 
20
15
10
5
0
24-h mean GH
(µg/L)
Pu

ive GH assays 
20
15
10
5
0
(µg/L)
Pulse area
(min*µg/L × 10–2)
C
GH pulses
(per 24-h)
a
b
b
b
a,b
Pre
Early Late
Post
Adult
20
15
10
5
0
A
a
a
a
a
b
18
12
6
0
B
a
a
a
a
b
• Fig. 25.18  (A) The 24-hour and standard error (+SE) levels of growth hor­
mone (GH) for groups of normal boys at varied stages of pubertal matura­
tion. (B) The mean (+SE) area under the GH concentration-vs.-time curve 
for individual GH pulses, as identified by the cluster pulse detection algo­
rithm. (C) The 

 the cluster pulse detection algo­
rithm. (C) The number of GH pulses (+SE), as detected by the cluster 
algorithm, in the 24-hour GH concentration profiles for boys in each of the 
pubertal study groups. Notice that the mean 24-hour GH concentration 
changes are largely mediated by changes in the amount of GH secreted 
per pulse, rather than the frequency of pulses. In each panel, bars bear­
ing the same letter are statistically indistinguishable. (From Martha PM Jr, 
Rogol AD, Veldhuis JD, et 

e. (From Martha PM Jr, 
Rogol AD, Veldhuis JD, et al. Alterations in the pulsatile properties of cir­
culating growth hormone concentrations during puberty in boys. J Clin 
Endocrinol Metab. 1989;69:563–570.)
24-h mean GH (µg/L)
20
20
15
15
10
10
5
5
0
0
8
9
10
11 12
13
14
15
16 18 Adult
Growth velocity (cm/yr)
Chronologic age (years)
• Fig. 25.17  Relation between 24-hour mean growth hormone (GH) levels 
and age in boys and men. The bars represent the 24-hour mean and 
standard error (+SE) valu

nt the 24-hour mean and 
standard error (+SE) values of GH (left axis) obtained from 60 24-hour GH 
profiles of healthy boys and men subdivided according to chronologic age. 
An idealized growth velocity curve reproduced from the 50th percentile 
values for whole-year height velocity of North American boys16 is superim­
posed. (From Martha PM Jr, Rogol AD, Veldhuis JD, et al. Alterations in the 
pulsatile properties of circulating growth hormone concentrations during 
puberty in boys. J Clin End

concentrations during 
puberty in boys. J Clin Endocrinol Metab. 1989;69:563–570.)
350
250
150
50
GHBP or GH conc (pmol/L)
<11
Bone age (years)
GHBP
GH
11–
11.9
12–
12.9
13–
13.9
14–
14.9
15–
15.9
>16
• Fig. 25.19  Levels of growth hormone (GH) and growth hormone–bind­
ing protein (GHBP) measured in normal pubertal boys throughout ado­
lescence. The GHBP levels do not significantly change during puberty, 
but there is a significant increment of GH production and therefore of 
GH levels during th

H production and therefore of 
GH levels during this same time. These data suggest that there may be 
greater amounts of “free GH” during this period, leading to greater pro­
duction of insulin-like growth factor 1. (Based on data from Martha PM Jr, 
Rogol AD, Blizzard RM, et al. Growth hormone-binding protein activity is 
inversely related to 24-hour growth hormone release in normal boys. J 
Clin Endocrinol Metab. 1991;73:175–181; Martha PM Jr, Rogol AD, Veld­
huis JD, et al. Alterations in the

Rogol AD, Veld­
huis JD, et al. Alterations in the pulsatile properties of circulating growth 
hormone concentrations during puberty in boys. J Clin Endocrinol Metab. 
1989;69:563–570.)
950
in prepubertal and pubertal children of both genders. In addition 
to the amplified secretory burst mass caused by jointly increased 
GH pulse amplitude and duration, they found that sex steroids 
selectively affect facets of GH neurosecretory control: Estrogen 
i

t facets of GH neurosecretory control: Estrogen 
increases the basal GH secretion rate and the irregularity of GH 
release patterns, whereas testosterone stimulates greater GH secre­
tory burst mass and greater IGF1 concentrations.
Obesity is characterized by lowered GH production, reflected 
by a diminished number of GH secretory bursts and shorter 
half-life duration.163,164 Obesity in childhood and adolescence is 
characterized by decreased GH production with normal IGF and 
increased GHBP le

 production with normal IGF and 
increased GHBP levels and often by increased linear growth.164 
The hyperinsulinism associated with obesity causes lowered levels 
of IGF-binding protein 1 (IGFBP1) and, perhaps, higher levels 
of “free” IGF1.165 Endogenous GH secretion and levels achieved 
during provocative tests in obese subjects166 approximated the 
diagnostic range of GHD. Fasting increased both the number 
and the amplitude of GH secretory bursts, presumably reflecting 
decreased somatostat

ursts, presumably reflecting 
decreased somatostatin secretion and enhanced GHRH release, 
while lowering GHBP concentrations. Rapid changes in levels of 
IGFBPs in response to altered nutrition and changes in insulin 
levels may modify the effect of IGF1 on its negative feedback and 
effector sites.158,164 Body mass also influences GH production in 
normal prepubertal and pubertal children and adults.167–169 
Peripheral Modulators of Growth Hormone
Conflicting studies have shown

one
Conflicting studies have shown that glucocorticoids (GCs) are 
able to either stimulate or inhibit GH secretion, depending on 
specific conditions.170–178 During a short-term (1-hour) incuba­
tion, GCs inhibited GHRH-stimulated GH secretions due to an 
increase in SST secretion.172,179–181 After a 3-hour treatment with 
dexamethasone in normal human subjects, GH release was stimu­
lated, followed by an inhibition after 12 hours.173,174 After 4 days 
of treatment with predniso

s.173,174 After 4 days 
of treatment with prednisone, GHRH- stimulated GH secretion 
was blunted.175 Signaling is thought to involve the activation of 
cAMP/PKA or PKC signaling pathways and intracellular free cal­
cium mobilization.182 
Several studies have shown that an increase in thyroid hormone 
(TH) levels in humans inhibits GH release due to either an increase 
in SST tone or a direct effect to decrease GHRH release.183,184 
THs have also been shown to inhibit human growt

4 
THs have also been shown to inhibit human growth hormone 
(hGH) gene expression and secretion of GH in somatotroph cell 
cultures.185 T3 treatment also decreased hGH RNA levels without 
an effect on GH protein in transgenic (171hGH/CS-TG) mice 
overexpressing the human GH gene.186 
Insulin and IGF1
IGF1 and IGFBP3 have been shown to positively affect sponta­
neous 24-hour GH secretion in human volunteers.187 IGF1 has 
a negative feedback role to the hypothalamus and pituitary to 
regulate GH 

to the hypothalamus and pituitary to 
regulate GH secretion. Low doses of recombinant IGF1 infusion 
resulted in a blunted response to the fasting-stimulated GH secre­
tion,188 and its administration at physiologic doses decreased the 
GH response to GHRH without altering spontaneous GH lev­
els.189 Further, free and not total IGF1 levels may be the primary 
mediator of GH secretion190; however, a single dose of recombi­
nant IGF1 does not affect basal or pulsatile GH release.191
Similarly, insu

 basal or pulsatile GH release.191
Similarly, insulin reduces GH response to GHRH,192 and the 
elevated insulin associated with obesity is thought to decrease GH 
levels.193 IGF1 and insulin have a direct effect on the somatotropes 
to decrease GH mRNA levels and GH secretion.194 Somatotrophs 
are suppressed by IGF1-mediated signalings through the PI3K, 
mTORC1, and MEK pathways and insulin by the PI3K signaling 
pathway.176,195,196 Deletion of the IGF1 receptor from somato­
trophs in a genetica

he IGF1 receptor from somato­
trophs in a genetically engineered mouse line resulted in mice of 
normal weight and length with only slightly elevated GH messen­
ger RNA (mRNA) and serum GH levels.197 
Free Fatty Acids
Free fatty acids (FFAs) inhibit GHRH-stimulated GH secretion 
by suppressing GHRH and/or stimulating SST secretion, or a 
direct effect on somatotropes.198–205 
Adipokines
The adipokines (leptin, adiponectin, and resistin) are members of 
the cytokine family of proteins, which are 

rs of 
the cytokine family of proteins, which are released from adipose 
tissues. Although studies have reported an effect of leptin or adi­
ponectin on somatotrophs, the precise relevance is unknown. Adi­
ponectin has been associated with GH pulse secretion; however, 
whether this is a direct or indirect effect is unknown.206 Adiponec­
tin has also been shown to decrease GHRH-stimulated, but not 
ghrelin-stimulated, GH release. Leptin and resistin both increase 
GH secretions. The adipokines al

in both increase 
GH secretions. The adipokines all utilize the AC/PKA signaling 
pathway; leptin also signals through intracellular/extracellular cal­
cium and PLC/PKC, adiponectin through intracellular/extracel­
lular calcium, and resistin through the mTOR pathway.207 
Estrogens
Estrogen receptors are expressed in somatotropes, and treatment 
with estradiol decreased IGF1 and elevated basal GH concentra­
tions in men.208 In postmenopausal women, estrogen treatment 
increased GH secretions and 

, estrogen treatment 
increased GH secretions and decreased IGF1 levels. These effects 
may be mediated directly or indirectly by negative feedback effects of 
IGF1.209 In rodents, deletion of the estrogen receptor alpha resulted 
in a decrease in GH mRNA expression and secretion.210 Both estro­
gen receptor α and β stimulate pituitary GH gene expression. 
The Growth Hormone Receptor and Growth Hormone–
Binding Protein
The gene for the human GHR is located on chromosome 5p13.1-
p12, where it spa

is located on chromosome 5p13.1-
p12, where it spans more than 87 kb.211 The GHR gene (GHR) 
contains 10 exons: Exon 1 contains the 5′ untranslated region; 
exons 2 through 10 encode the three domains of the GHR—the 
extracellular ligand-binding domain, a single transmembrane 
domain, and a cytoplasmic domain for signal transduction.212 The 
highest level of GHR expression is in the liver, followed by muscle, 
fat, kidney, and heart. The receptor is 638 amino acids long, with 
a predicted molecu

 is 638 amino acids long, with 
a predicted molecular weight of 70 kDa before glycosylation.
Two isoforms of the GHR have been found in humans—a full-
length form and a form that has a deletion of exon 3 (GHRd3) 
with a loss of a 22–amino acid segment of the extracellular domain 
of the receptor.213 The GHRd3 allele is present in approximately 
33% of the general population.213 Several studies have investi­
gated the clinical significance of the GHRd3 allele on growth and 
GH responsiveness in s

GHRd3 allele on growth and 
GH responsiveness in specific disease states.
GH must be bound to a homodimer complex of GHR to acti­
vate its intracellular signaling pathways (Fig. 25.20). Whether 
dimerization of the GHR subunits occurs before or after GH bind­
ing is a matter of debate. It was initially thought that dimerization 
would occur only after GH binding; GH would bind to the first 
subunit, after which the GH-GHR complex would diffuse within 
951
Chapter 25  Normal and Aberrant Growth i

thin 
951
P
GH
P
P
P
P
P
P
P
Jak
2
Jak
2
S
T
A
T
P
P
S
T
A
T
P
P
S
T
A
T
P
GH
P
P
P
Jak
2
Jak
2
Jak
2
S
T
A
T
• Fig. 25.20  A model depicting intracellular signaling intermediates 
induced by binding of growth hormone (GH) with the GH receptor (GHR). 
Jak, Janus kinase; P, phosphorylation; STAT, signal transducer and acti­
vator of transcription. (From Le Roith DC, Bondy S, Yakar J-L, et al. The 
somatomedin hypothesis. Endocrine Rev. 2001;22:53

somatomedin hypothesis. Endocrine Rev. 2001;22:53–74.)
the membrane until contacted by a second subunit, leading to 
receptor activation.214 However, it was shown in live cells that the 
subunits of the GHR are constitutively dimerized in an inactive 
(i.e., unbound) state.215 The GH binding sites on the extracellular 
domains of the two subunits are placed asymmetrically. Therefore 
once GH binds to both subunits, it induces rotation of the two 
subunits of the dimer that is transmitted via th

subunits of the dimer that is transmitted via the transmembrane 
domain to the intracellular domain, allowing downstream kinase 
activation by transphosphorylation.
After binding to its receptor, GH stimulates phosphorylation of 
Janus kinase 2 (JAK2), a tyrosine kinase associated with the GHR 
(see Fig. 25.20). On recruitment or activation, the JAK2 molecule 
causes phosphorylation of critical tyrosines on the intracellular 
portion of the GHR, a sort of transphosphorylation. The phos­
phoryl

, a sort of transphosphorylation. The phos­
phorylated tyrosines on the GHR provide docking sites for critical 
intermediary STAT (signal transducers and activators of transcrip­
tion) proteins.214 STAT proteins dock, via their Src homology 2 
(SH2) domain to phosphotyrosines on ligand-activated receptors, 
such as the GHR. After docking, phosphorylation occurs on sin­
gle tyrosines at the carboxy-terminus (C-terminal) of the protein. 
Then STATs dissociate from the GHR, dimerize, translocate to

 dissociate from the GHR, dimerize, translocate to the 
nucleus, and bind to DNA through their DNA-binding domain 
to regulate gene transcription. There are seven known mammalian 
STATs; of these, STAT5B appears to be most critically involved in 
mediating the growth-promoting actions of the GHR.216
GHR signaling also leads to activation of extracellular-reg­
ulated kinase 1 (ERK1) and ERK2 to increase transcription.217 
How GHR activation promotes ERK1/2 activation is a matter of 
debate. A JAK

es ERK1/2 activation is a matter of 
debate. A JAK2-independent but Src-dependent phosphorylation 
has been modeled,167 as has a JAK2-dependent but Src-indepen­
dent mechanism.218 Experiments were conducted in different cell 
lines, so it is possible that the mechanisms of ERK1/2 activation 
by GHR are different in different cells. It is unclear what role these 
pathways play in the GH stimulation of growth.
GHBP prolongs the half-life of GH, presumably by impairing 
glomerular filtration, and m

sumably by impairing 
glomerular filtration, and modulates its binding to the GHR. It 
binds GH with high specificity and affinity but with low capacity; 
only about 45% of circulating GH is bound.154 GHBP is derived 
from proteolytic cleavage of the extracellular domain of the recep­
tor.219 GHBP levels reflect GHR levels and activity; that is, low 
levels are associated with states of GH insensitivity.154 Levels of 
GHBP are low early in life, rise through childhood, and plateau 
during the pu

rise through childhood, and plateau 
during the pubertal years and adulthood.220 Impaired nutri­
tion, diabetes mellitus, hypothyroidism, chronic liver disease, 
and inherited abnormalities of the GHR are associated with low 
levels of GHBP, whereas obesity, refeeding, early pregnancy, and 
estrogen treatment are associated with elevated levels of GHBP.154 
In general, GHBP levels reflect GHR levels and activity. Patients 
with GH insensitivity due to defects of the extracellular domain of 
the 

ue to defects of the extracellular domain of 
the GHR have low GHBP levels, and GHBP levels can therefore 
be useful in identifying these individuals. Patients with GH insen­
sitivity due to nonreceptor abnormalities, defects of the intracel­
lular domain of the GHR, or inability of the receptor to dimerize 
may have normal levels of GHBP.162
GHR signaling affects the transcription of many genes imme­
diately (<3 hours after stimulation) and other genes over a longer 
period of stimulation. Afte

r genes over a longer 
period of stimulation. After acute GH stimulation in GH-deficient 
rats, hepatic genes immediately induced by GH included signal 
transducers (STAT3, gp130, p38), DNA repair proteins, recep­
tor proteases, and metabolic regulators such as Igf1, Igfbp3, and 
Mct1.221 GHR signaling is involved in regulation of expression of 
genes involved in carbohydrate, fat, and steroid metabolism.222
The study of mice that lack Ghr or downstream components of 
GHR signaling has shed ligh

nstream components of 
GHR signaling has shed light on the role of GH in normal physi­
ology. Ghr−/− mice exhibit normal size at birth but have attenu­
ated postnatal growth, with body weight about half of normal and 
length about two-thirds of normal.223 Ghr−/− mice also exhibit 
delayed pubertal maturation, longer life span, and increased 
insulin sensitivity compared with a control group. Mice with 
Ghr deleted only in the liver have low IGF1 levels and high GH 
levels. These mice exhibit nor

levels and high GH 
levels. These mice exhibit normal growth but have significantly 
lower bone density than controls. They also exhibit liver steatosis 
with insulin resistance and elevated serum free fatty acid levels,224 
showing the metabolic function of excessive GHR signaling inde­
pendent of serum IGF1 signaling.
Deletion of Jak2 in mice is embryonically lethal,225 as is dele­
tion of Stat3. Deletion of Stat1 or Stat5a does not affect body size. 
However, deletion of Stat5b leads to decre

 size. 
However, deletion of Stat5b leads to decreased size in male but not 
female mice. The Stat5a/b−/− mouse is smaller than the Stat5b−/− 
mouse.226 The combined Igf1 and Ghr−/− mouse had a more severe 
attenuation of postnatal growth than mice with knockout of 
either gene alone, indicating that GH and IGF1 promote growth 
by both common and independent functions.227 The liver-specific 
IGF1 knockout mouse with extremely low serum IGF1 levels has 
normal linear growth, suggesting IGF-indepe

s has 
normal linear growth, suggesting IGF-independent actions of GH 
or paracrine production and effects of IGF1, or both.228
Historically, the anabolic actions of GH were thought to be 
mediated entirely by the IGF peptides (the so-called somatomedin 
hypothesis).164 Although a majority of GH actions are mediated 
by IGFs, the opposite effects of GH and IGFs on metabolism and 
in knockout mouse models suggest that there are IGF-indepen­
dent actions of GH. Indeed, the “diabetogenic” actions o

ctions of GH. Indeed, the “diabetogenic” actions of GH 
are contradictory to the glucose-lowering effects of IGFs. In vitro 
studies suggest potential IGF-independent action of GH in the 
following tissues:
	1.	 Epiphysis—stimulation of epiphyseal growth
	2.	 Bone—stimulation of osteoclast differentiation and activity, 
stimulation of osteoblast activity, and increase in bone mass 
through endochondral bone formation
952
Promoter1 Promoter2
5′
Gene

logy and the Life Span
5′
Gene
mRNAs
Exon
IGF-IA
IGF-IB
1
2
3
1 kb
4
5
6
3′
• Fig. 25.21  Structure and expression of the human insulin-like growth factor 1 (IGF1) gene. The structure 
of the different human IGF1 messenger RNAs (mRNAs) is displayed below the map of the gene. Sites of 
pre-mRNA processing are indicated by the thin lines. Sites of differential polyadenylation are marked at 
the 3′ end of the gene by vertical arrows and in the mRNAs by horizontal boxes of varyin

ows and in the mRNAs by horizontal boxes of varying length. (From 
Rotwein P. Structure, evolution, expression and regulation of insulin-like growth factors I and II. Growth 
Factors. 1991;5:3–18.)
	3.	 Adipose tissue—acute insulin-like effects, followed by 
increased lipolysis, inhibition of lipoprotein lipase, stimulation 
of hormone-sensitive lipase, decreased glucose transport, and 
decreased lipogenesis
	4.	 Muscle—increased amino acid transport, increased nitrogen 
retention, increased l

nsport, increased nitrogen 
retention, increased lean tissue, and increased energy expenditure 
Historic Background. The IGFs (somatomedins) are a fam­
ily of peptides that are, in part, GH dependent and mediate 
many of the anabolic and mitogenic actions of GH. They were 
originally identified in 1957 by their ability to stimulate [35S]-
sulfate incorporation into rat cartilage and were called sulfation 
factors.164 In 1972, that term was replaced with somatomedin,16

n 1972, that term was replaced with somatomedin,169 
and purification of somatomedin from human serum yielded a 
basic peptide (somatomedin C) and a neutral peptide (somatome­
din A).229 In 1978, Rinderknecht and Humbel230,231 isolated two 
active somatomedins from human plasma and, after demonstrat­
ing a striking structural resemblance to proinsulin, renamed them 
insulin-like growth factors (IGFs). 
IGF Genes and Protein Structure. There are two IGFs circulat­
ing in humans, IGF1 and IGF2. IG

wo IGFs circulat­
ing in humans, IGF1 and IGF2. IGF1 is a basic peptide of 70 
amino acids, and IGF2 is a peptide of 67 amino acids. The two 
peptides share 45 of 73 possible amino acid positions and have 
50% amino acid homology to insulin.164,230,231 Like insulin, both 
IGFs have A and B chains connected by disulfide bonds. The con­
necting C-peptide region is 12 amino acids for IGF1 and 8 amino 
acids for IGF2, bearing no homology for the C-peptide region of 
proinsulin. The structural simila

ptide region of 
proinsulin. The structural similarity to insulin explains the ability 
of both IGFs to bind to the insulin receptor and the ability of 
insulin to bind to the type I IGF receptor (encoded by IGF1R). 
On the other hand, structural differences probably explain the 
failure of insulin to bind with high affinity to the IGFBPs. 
Insulin-Like Growth Factor 1
Gene Regulation. The human IGF1 gene (IGF1) is located on 
the long arm of chromosome 12 and contains at least six exons 
(Fig. 

omosome 12 and contains at least six exons 
(Fig. 25.21). Exons 1 and 2 encode alternative signal peptides, 
each containing several transcription start sites; that is, the mul­
tiple existing IGF1 transcripts consist of either exon 1 or exon 2. 
Two different promoters regulated in a tissue-specific manner232 
control the use of exon 1 or 2. Exons 3 and 4 encode the remain­
ing signal peptide, the remainder of the mature IGF1 molecule, 
and part of the trailer peptide (E peptide). Exons 5 and 6

 of the trailer peptide (E peptide). Exons 5 and 6 encode 
alternatively used segments of the trailer peptide and 3′ untrans­
lated sequences with multiple different polyadenylation sites. As a 
result, multiple mRNA species exist, allowing for tissue-specific, 
developmental, and hormonal regulation of IGF1 gene expression.
GH is the primary regulator of Igf1 transcription, resulting in a 
20-fold rise in Igf1 mRNA. There may be tissue-to-tissue variabil­
ity in the extent of GH-induced express

 variabil­
ity in the extent of GH-induced expression of Igf1 mRNA.233,234 
Estrogen also regulates IGF1 transcription, as its administration to 
a Ghr−/− mouse can stimulate hepatic Igf1 synthesis and growth.223 
The sex steroid effects on IGF1 transcription play a role in the 
pubertal rise of IGF1 levels in humans235 (see later discussion).
The mechanisms involved in regulation of IGF gene expres­
sion include the existence of multiple promoters, heterogeneous 
transcription initiation within

rs, heterogeneous 
transcription initiation within each of the promoters, alternative 
splicing of various exons, differential RNA polyadenylation, and 
variable mRNA stability. The transcription factor STAT5B is the 
most critical mediator of GH-induced activation of IGF1 tran­
scription, as detailed earlier. Two adjacent STAT5B binding sites 
are present in the second intron of the rat Igf1 gene, within a 
region that undergoes acute changes in chromatin structure upon 
GH treatment.216
Once t

 chromatin structure upon 
Once translated, IGF1 pre-propeptides require processing 
to form the mature IGF1 peptide (Fig. 25.22). Proteases from 
the subtilisin-related proprotein convertase family (SPC) cleave 
pro-IGF1.236 
Serum Levels. In human fetal serum, IGF1 levels are relatively 
low and positively correlated with gestational age.237 There is a 
reported correlation between fetal cord serum IGF1 levels and birth 
weight,238 but this relationship is controversial.239 IG

,238 but this relationship is controversial.239 IGF1 levels in 
newborn serum are typically 30% to 50% of adult levels and rise 
953
during childhood. During puberty, IGF1 levels rise to two to three 
times the adult range. Levels during adolescence correlate better 
with Tanner stage or bone age than with chronologic age.240 The 
pubertal rise in gonadal steroids may stimulate IGF1 production 
indirectly by contributing to a rise in GH secretio

ndirectly by contributing to a rise in GH secretion and directly 
by augmenting liver synthesis and secretion of IGF1. Girls with 
gonadal dysgenesis show no adolescent increase in serum IGF1, 
providing evidence of the association of the pubertal rise in IGF1 
with the production of gonadal steroids.241 As further evidence, 
patients with GH insensitivity due to GHR mutations exhibit a 
modest rise in IGF1 during puberty despite a decline in GH levels. 
After 20 to 30 years of age, serum IGF1 l

levels. 
After 20 to 30 years of age, serum IGF1 levels gradually and pro­
gressively fall, and this decline is implicated in the negative nitro­
gen balance, decrease in muscle mass, and osteoporosis of aging.242 
Insulin-Like Growth Factor 2
Gene Regulation. The gene for IGF2 (IGF2) is located on 
the short arm of chromosome 11, adjacent to the insulin gene, 
and contains 9 exons (Fig. 25.23). Exons 1 through 6 encode 5′ 
untranslated RNA, exon 7 encodes the signal peptide and most of 
the mat

 7 encodes the signal peptide and most of 
the mature protein, and exon 8 encodes the C-terminal portion of 
the protein. As with IGF1, multiple mRNA species exist to allow 
for developmental and hormonal regulation of expression. IGF2 
mRNA expression is high in fetal life and has been detected as 
early as the blastocyst stage in mice.243 Fetal tissues generally have 
high IGF2 mRNA levels that decline postnatally.
IGF2 is imprinted—that is, only one allele is active, depend­
ing on parental o

ly one allele is active, depend­
ing on parental origin. In the case of IGF2, only the paternally 
expressed allele is active. Most imprinted genes occur in clusters 
with reciprocally imprinted genes, and IGF2 is no exception. The 
noncoding gene H19 is downstream from IGF2 and is oppositely 
imprinted, meaning that only the maternal allele is expressed and 
the paternal allele is inactive. The promoters of IGF2 and H19 
share a set of enhancers that act on either gene. On the paternal 
allele,

 that act on either gene. On the paternal 
allele, the H19 promoter region is methylated and thus inactivated 
(so-called epigenetic regulation of expression).244 The IGF2 pro­
moter does not contain regions that can be methylated. Instead, 
upstream from the H19 and IGF2 promoter region is a so-called 
differentially methylated region (DMR). When it is methyl­
ated, binding of CCCTC binding factor (CTCF) is prevented, 
allowing enhancers to act on the IGF2 promoter to activate tran­
scription.2

on the IGF2 promoter to activate tran­
scription.245 On the maternal chromosome, the DMR is not 
methylated, allowing CTCF to bind and preventing transcription 
(Fig. 25.24).246
The fact that IGF2 is monoallelically expressed emphasizes the 
importance of gene dosage to normal physiology and development. 
Loss of imprinting of IGF2 can lead to constitutively expressed 
IGF2 mRNA and excessive IGF2. IGF2 mRNA is expressed con­
stitutively in a number of mesenchymal and embryonic tumors, 
includin

ber of mesenchymal and embryonic tumors, 
including Wilms tumor,247 rhabdomyosarcoma, neuroblastoma, 
pheochromocytoma, hepatoblastoma, leiomyoma, leiomyosar­
coma, liposarcoma, and colon carcinoma.248 Loss of imprinting 
of IGF2 resulting in biallelic gene expression occurs in Beckwith-
Wiedemann syndrome (BWS), which is characterized by fetal 
and neonatal overgrowth and an increased risk of childhood 
B-Domain
A-Domain
C-Domain
S
S
S
S
S-S
COOH
NH2-
D-Domain
• Fig. 25.22  Structure of the ins

H
NH2-
D-Domain
• Fig. 25.22  Structure of the insulin-like growth factor 1 (IGF1) peptide. See 
discussion in text. (From Yakar S, Wu Y, Setser J, et al. The role of circulat­
ing IGF-1. Endocrine. 2002;19:239–248.)
5′
Gene
mRNAs
Exon
4
1
5
2
6
3
1 kb
7
8
9
3′
• Fig. 25.23  Structure and expression of the human insulin-like growth factor 2 (IGF2) gene. The structure 
of different human IGF2 messenger RNAs (mRNAs) is displayed below the map of the gene. The patterns 
of mRNA processing are indic

e gene. The patterns 
of mRNA processing are indicated by the thin lines. Sites of differential polyadenylation are marked at 
the 3′ end of the gene by vertical arrows and in the mRNAs by horizontal boxes of varying length. (From 
Rotwein P. Structure, evolution, expression and regulation of insulin-like growth factors 1 and 2. Growth 
954
IGF1
receptor
Cysteine-
rich
domain
Tyrosine
kinase
domain
SS
SS
SS
SS
SS
SS
SS
SS
SS
SS

rosine
kinase
domain
SS
SS
SS
SS
SS
SS
SS
SS
SS
SS
SS
SS
SS
SS
SS
SS
domain
Plasma
membrane
Cytoplasm
Insulin
receptor
Insulin receptor
Related receptor
Insulin-IGF1
hybrid receptor
IGF2-M6P
receptor
• Fig. 25.25  Structure of the insulin-like growth factor (IGF) receptors. The insulin receptor and the IGF 
type 2 receptor are both heterotetrameric complexes composed of extracellular α-subunits that bind the 
ligands and β-subunits that anchor the receptor in the membrane and conta

that anchor the receptor in the membrane and contain tyrosine kinase activity 
in their cytoplasmic domains. The tyrosine kinase domain of the insulin receptor–related receptor (IRR) 
is homologous to the tyrosine kinase domains of the insulin and IGF1 receptors. The carboxyl-terminal 
domain is deleted in the IRR. Hybrids consist of a hemireceptor from both insulin and IGF1 receptors. The 
IGF type 2/mannose-6-phosphate (M6P) receptor is not structurally related to the IGF1 and insulin recep­
t

ucturally related to the IGF1 and insulin recep­
tors or to the IRR, having a short cytoplasmic tail and no tyrosine kinase activity. (From LeRoith D, Werner 
H, Geitner-Johnson D, et al. Molecular and cellular aspects of the insulin-like growth factor I receptor. 
Endocr Rev. 1995;16:143–163.)
cancers. Loss of imprinting in BWS results from mutations that 
affect imprinting in the region of chromosome 11 that contains 
IGF2,249 methylation defects that cause hypermethylation of the 
region or d

ts that cause hypermethylation of the 
region or duplication of the expressed paternal allele, resulting in 
increased IGF2 expression, or paternal uniparental disomy (i.e., 
inheritance of only the expressing paternal allele). 
Serum Levels. Human newborn levels of IGF2 are typically 
50% of adult levels. By 1 year of age, adult levels are attained, and 
there is little if any subsequent decline, even up to the seventh or 
eighth decade.250 
Insulin-Like Growth Factor Receptors
There are two ty

ulin-Like Growth Factor Receptors
There are two types of IGF receptors, type I and type II (Fig. 
25.25). Structural characterization of these receptors has provided 
documentation of the differences between these two forms.251 
The type I IGF receptor is closely related to the insulin receptor; 
both are heterotetramers comprising two membrane-spanning 
α-subunits and two intracellular β-subunits. The α-subunits con­
tain the binding sites for IGF1 and are linked by disulfide bonds. 
The β-subu

GF1 and are linked by disulfide bonds. 
The β-subunits contain a transmembrane domain, an adenosine 
triphosphate (ATP)–binding site, and a tyrosine kinase domain, 
which constitute the presumed signal transduction mechanism for 
the receptor.
Although the type I IGF receptor has been commonly termed 
the IGF1 receptor, this receptor binds both IGF1 and IGF2 with 
high affinity, and both IGF peptides can activate tyrosine kinase 
by binding to the type I receptor. The affinity of the type I IGF 

e type I receptor. The affinity of the type I IGF 
receptor for insulin is usually 100-fold less, explaining the rela­
tively weak mitogenic effect of insulin.
The mature IGF receptor peptide has 1337 amino acids with a 
predicted molecular mass of 151,869 kDa (Fig. 25.26). The trans­
lated αβ heterodimer is cleaved at an Arg-Lys-Arg-Arg sequence at 
positions 707 through 710. The released α-subunit and β-subunit, 
linked by disulfide bonds, then form the mature (αβ)2 receptor, in 
which two α c

 form the mature (αβ)2 receptor, in 
which two α chains are joined by secondary disulfide bonds. The 
α-subunits are extracellular and contain a cysteine-rich domain 
that is critical for IGF binding. The β-subunit has a short extra­
cellular domain, a hydrophobic transmembrane domain, and the 
intracellular tyrosine kinase domain with the ATP-binding site.
The gene for the type I IGF receptor (IGF1R) spans more than 
100 kb of genomic DNA, with 21 exons; the genomic organi­
zation resembles tha

21 exons; the genomic organi­
zation resembles that of the insulin receptor gene.252 Exons 1 
through 3 encode for the 5′ untranslated region and the cysteine-
rich domain of the α-subunit that is involved in ligand binding. 
The remainder of the α-subunit is encoded by exons 4 through 10. 
The peptide cleavage site involved in generation of the α-subunit 
and β-subunit is encoded by exon 11, and the tyrosine kinase 
domain of the β-subunit is encoded by exons 16 through 20. It is 
in the latter

coded by exons 16 through 20. It is 
in the latter region that IGF1R and the insulin receptor gene share 
the greatest sequence homology, ranging from 80% to 95%.
Paternal
Maternal
Enhancers
IGF2
DMR
H19
Enhancers
IGF2
DMR
H19
CTCF
CH3
• Fig. 25.24  Schematic of the imprinted region of the insulin-like growth 
factor 2 (IGF2)-H19 locus. CTCF, CCCTC binding factor; CH3, meth­
ylation; DMR, differentially methylated region. (Adapted from Chao W, 
D’Amore P. IGF2: epigenetic regulation and role in 

’Amore P. IGF2: epigenetic regulation and role in development and dis­
ease. Cytokine Growth Factor Rev. 2008;19:111–120.)
955
IGF1R mRNA has been identified in virtually every tissue 
except liver.253 The mRNA is most abundant in embryonic tis­
sues and appears to decrease with age. IGF1R becomes widely 
expressed after implantation, consistent with the observation that 
this receptor is essential for normal fetal growth.
As with other growth f

al for normal fetal growth.
As with other growth factor receptor tyrosine kinases, binding 
of ligand (IGF1 or IGF2) induces receptor autophosphorylation 
of critical tyrosine residues in the type I IGF receptor.221 Specifi­
cally, ligand binding to the α-subunits leads to activation of the 
tyrosine kinase domain of the β-subunits. Autophosphorylation 
appears to occur by transphosphorylation of sites on the opposite 
β-subunit.254 A tyrosine proximal to the tyrosine kinase domain, 
Tyr 950, is

oximal to the tyrosine kinase domain, 
Tyr 950, is part of a motif that, when deleted, reduces receptor 
autophosphorylation, affects receptor internalization, and inhibits 
postreceptor signaling; the adapter proteins Shc and insulin recep­
tor substrate 1 (IRS1) bind to this domain.
Autophosphorylation and activation of the cytoplasmic region 
of the IGF type I receptor promotes recruitment or activation of 
several docking proteins, each of which activate distinct signal­
ing pathways, with s

ich activate distinct signal­
ing pathways, with some overlap (Fig. 25.27). Proteins that dock 
onto the activated IGF type I receptor include members of the 
IRS family, Shc, p85 subunit of PI3 kinase, tyrosine phosphatase 
PTP1D, and mGRB10. Of these docking proteins, the pathways 
involving IRS and Shc are best characterized.
IRS1 is a 185-kDa protein that, when phosphorylated, con­
tains specific phosphotyrosine motifs that can associate with pro­
teins containing Src homology 2 domains such

 pro­
teins containing Src homology 2 domains such as the p85 subunit 
of PI3 kinase, growth factor receptor–bound protein 2 (Grb2), 
Syp (a phosphotyrosine phosphatase), and Nck (an oncogenic 
protein). Activation of Shc and Grb2 results in activation of the 
Ras, Raf, MAP kinase kinase, and S6 kinase pathways.255 Binding 
of the p85 subunit of PI3 kinase leads to activation of the p110 
subunit of PI3 kinase. This process then activates downstream 
phospholipid signal transduction pathways tha

eam 
phospholipid signal transduction pathways that include Akt. 
Activation of Akt leads to regulation of diverse cellular processes, 
including apoptosis, glucose transport and metabolism, protein 
synthesis, mitosis, and differentiation.255 Phosphorylated Shc is a 
docking site for the SH2 domain of Grb2. Grb2 then binds SOS, a 
guanine nucleotide exchange factor that converts inactive Ras gua­
nosine diphosphate (GDP) into guanosine triphosphate (GTP). 
GTP-bound Ras then recruits Raf, and R

ate (GTP). 
GTP-bound Ras then recruits Raf, and Raf subsequently activates 
MAP kinase and mitogen and extracellular signal-related kinases 
(MEK) 1 and 2. Activation of these proteins ultimately regulates 
gene transcription. Given that insulin and IGF peptides activate 
similar signaling pathways through their own specific receptors, 
it is unclear how the cell distinguishes among these overlapping 
ligands. Whether the consequences merely reflect the relative levels 
of receptors and whether

lect the relative levels 
of receptors and whether divergent downstream pathways exist for 
insulin and IGF action remain questions for future investigation.
The α-subunit and β-subunit of the IGF receptor can form het­
erodimer with the α-subunit and β-subunit of the insulin receptor, 
forming hybrid receptors (see Fig. 25.25). Hybrid receptors have 
been found in significant amounts in most tissues. The hybrid 
receptor can bind either insulin of IGF1, but IGF1 may bind with 
higher affinity. 

of IGF1, but IGF1 may bind with 
higher affinity. No matter the ligand bound, autophosphorylation 
of the β-subunit of the insulin or IGF1 receptor occurs.256 The 
physiologic significance of these hybrid receptors is unknown as 
studies thus far have been performed in vitro.
The type II IGF receptor bears no structural homology with 
either the insulin receptor or type I IGF receptors (see Fig. 25.25). 
The receptor does not contain an intrinsic tyrosine kinase domain 
or any other recognized s

 tyrosine kinase domain 
or any other recognized signal transduction mechanism. The type 
II IGF receptor is identical to the cation-independent mannose-
6-phosphate (M6P) receptor, a protein involved in the intracellu­
lar lysosomal targeting of acid hydrolases and other mannosylated 
proteins.257 This common receptor is often referred to as the IGF2/
M6P receptor. The gene for the IGF2/M6P receptor (IGF2R) is 
located on the long arm of chromosome 6. Exons 1 through 46 
encode the extracellula

ome 6. Exons 1 through 46 
encode the extracellular region of the receptor, which contains 15 
repeat sequences of 147 residues each. Exons 47 and 48 encode 
the 23-residue transmembrane domain and a small cytoplasmic 
domain consisting of only 164 residues.258 Unlike IGF2, gene 
expression is biallelic in humans. Like IGF2, IGF2R expression 
is highest early in fetal development and declines postnatally.258
The IGF2/M6P receptor has an apparent molecular weight of 
220,000 under nonreducing con

molecular weight of 
220,000 under nonreducing conditions and 250,000 after reduc­
tion, indicating that it is a monomeric protein.259 The 15 repeat 
sequences contain cysteines to form intramolecular disulfide 
bonds necessary for receptor folding.258 Repeat 11 binds IGF2, 
whereas repeats 3, 5, and 9 bind M6P. Because of receptor folding, 
it appears that the IGF2 binding site is on the opposite face to the 
M6P binding site.260
The IGF2/M6P receptor binds a variety of M6P-containing 
proteins

ceptor binds a variety of M6P-containing 
proteins, including lysosomal enzymes, transforming growth 
factor-β (TGFβ),261 and leukemia inhibitory factor (LIF).262 The 
IGF2/M6P receptor binds only IGF2 with high affinity, IGF1 
binds to this receptor with lower affinity, and insulin does not 
bind at all.263 The receptor participates in lysosomal enzyme traf­
ficking between the trans-Golgi network and the extracellular 
space, regulates extracellular IGF2 and LIF levels, and plays a role 
in TG

lular IGF2 and LIF levels, and plays a role 
in TGFβ activation (reviewed by El-Shewy and colleagues264). 
Mice null for the type II IGF receptor have macrosomia and fetal 
death, implicating a role in IGF2 degradation.265
The mitogenic and metabolic actions of IGF2 is mediated 
through the type I IGF receptor, because monoclonal antibodies 
directed against the IGF1 binding site on the type I IGF receptor 
inhibit the ability of both IGF1 and IGF2 to stimulate thymi­
dine incorporation and cell

F2 to stimulate thymi­
dine incorporation and cell replication.266 Polyclonal antibodies 
that block IGF2 binding to the IGF2/M6P receptor do not block 
IGF2 actions.267,268 
NH2
Signal
peptide
(-30–1)
Cysteine
rich
(148–302)
Proteolytic
cleavage
(707–710)
(906–929)
Tyrosine
kinase
(973–1229)
COOH
α Subunit (1–706)
β Subunit (711–1337)
• Fig. 25.26  Structure of the human insulin-like growth factor type I recep­
tor precursor. Molecular cloning of human IGF1 receptor complementary 

ular cloning of human IGF1 receptor complementary 
DNAs isolated from a placental library revealed the presence of an open 
reading frame of 4101 nucleotides. The 1367–amino acid polypeptide 
contains, at its NH2-terminus, a 30–amino acid hydrophobic signal pep­
tide that is responsible for the transfer of the nascent protein chain into the 
endoplasmic reticulum. After digestion by endopeptidases at a proteo­
lytic cleavage site (Arg-Lys-Arg-Arg) located at residues 707 through 710, 
α-subunit 

) located at residues 707 through 710, 
α-subunit and β-subunit are released and linked by disulfide bonds to yield 
the configuration of the mature heterotetrameric receptor. Also shown are 
the cysteine-rich domain of the α-subunit and the transmembrane and 
tyrosine kinase domains of the β-subunit. (From LeRoith D, Werner H, 
Beitner-Johnson D, et al. Molecular and cellular aspects of the insulin-like 
growth factor I receptor. Endocr Rev. 1995;16:143–163.)
956
SECTION VI    Endocrinology and

5;16:143–163.)
956
Function-Targeted Disruption of IGF and IGF 
Receptor Genes
The in vivo role of the IGF axis in prenatal and postnatal growth 
was firmly established by a series of studies involving IGF and IGF 
receptor gene null mutations.269 Unlike Gh or Ghr null mice that 
are near-normal size at birth, mice with deletions of Igf1 or Igf2 
have birth weights approximately 60% of normal.269 Fetal size is 
proportionately reduced, but mice with 

l size is 
proportionately reduced, but mice with deletion of Igf1 have a 
higher neonatal death rate than mice with Igf2 deletion. Postna­
tally, mice with deletion of Igf1 that survive the neonatal period 
continue to have growth failure, with weights 30% of normal by 
2 months of age. When both Igf1 and Igf2 were disrupted, weight 
at birth was only 30% of normal and all animals died shortly after 
birth, apparently from respiratory insufficiency secondary to mus­
cular hypoplasia (Fig. 25.28

ncy secondary to mus­
cular hypoplasia (Fig. 25.28).
Mice lacking Igf1 and the Ghr are only 17% of normal size.227 
Therefore both IGF1 and IGF2 are important in fetal growth, but 
GHR signaling may have IGF-independent actions on growth as 
well. The postnatal growth of Igf1−/− mice was poorer than that 
observed in mice with Ghr or Ghrh receptor mutations, indicating 
that both GH-dependent and GH-independent effects of IGF1 
are necessary for normal growth.
Specific ablation of hepatic IGF1 p

normal growth.
Specific ablation of hepatic IGF1 production through the Cre/
LoxP recombination system confirmed that the liver is the principal 
source of circulating IGF1, but the resulting 80% reduction in serum 
IGF1 levels had no effect on postnatal growth,228,270 suggesting that 
postnatal growth is relatively independent of hepatic IGF1 pro­
duction. Presumably, production of IGF1 from local chondrocytes 
or other tissues maintains adequate endocrine sources of IGF1 to 
account for growth

e endocrine sources of IGF1 to 
account for growth preservation. Only modest decrement of postna­
tal growth is seen in mice null for Igfals (the gene encoding ALS).271
IGF1R
PI3-kinase
pathway
Raf
MEK
Erk1
Ras
Ras
Erk2
GTP
Sos
P
P
P
P
P
P
P
P
P
P
P
P
Shc
IRS
1-4
Fyn Syp
Nck
p85 p110
SEK1
???
???
???
???
p38
(MAPK)
JNK
PH
PH
PH
KD
KD
KD
Phospholipid
5'-
RD
RD
RD
Akt
PP2A
Inactive
Active
Protein
synthesis
Apoptosis
14-3-3
BAD
BAD
BCL2
Different

optosis
14-3-3
BAD
BAD
BCL2
GSK3
GLUT4
(glucose
transport)
PDK1
PDK2
PI -4,5-P2
PI -3,4-P2
PI -3,4,5-P3
GDP
• Fig. 25.27  Schematic representation of intracellular signaling pathways of the insulin-like growth factor 
type 1 (IGF1) receptor. On binding of IGF1, the IGF receptor undergoes autophosphorylation at multiple 
tyrosine residues. The intrinsic kinase activity of the receptor also phosphorylates insulin receptor substrate 
1 (IRS1) at multiple tyrosine residu

or substrate 
1 (IRS1) at multiple tyrosine residues. Various SH domain–containing proteins, including phosphatidylino­
sitol 3 (PI3) kinase, Syp, Fyn, and Nck associate with specific phosphotyrosine-containing motifs within 
IRS1. These docking proteins recruit diverse other intracellular substrates, which then activate a cascade 
of protein kinases, including Raf1, and one or more related kinases, including mitogen-activated protein 
kinase (MAPK), mitogen and extracellular signal-related kina

PK), mitogen and extracellular signal-related kinase (MEK), and others. These protein kinases, 
in turn, activate various other elements, including nuclear transcription factors. Alterations in expression of 
various IGF1-responsive genes result in longer-term effects of IGF1, including growth and differentiation. 
This model of signal transduction cascades also shows a potential mechanism for inhibition of apoptosis. 
BAD, BCL2-associated agonist of cell death; Erk, extracellular signal-regulat

t of cell death; Erk, extracellular signal-regulated kinase; GDP, guanosine 
diphosphate; GLUT4, glucose transporter 4; GTP, guanosine triphosphate; JNK, c-Jun N-terminal kinase; 
KD, catalytic kinase domain of Akt; MEK, mitogen-activated protein kinase; P, phosphorylation; PH, pleck­
strin homology domain of Akt; PP2A, protein phosphatase 2A; RD, regulatory C-terminal tail of Akt; SEK1, 
serum-inducible and glucocorticoid-inducible protein kinase 1. (From Le Roith D, Bondy C, Yakar S, et al. 
T

e 1. (From Le Roith D, Bondy C, Yakar S, et al. 
The somatomedin hypothesis. Endocr Rev. 2001;22:53–74.)
957
By crossing the liver-derived IGF1 gene–deleted mice with 
ALS gene–deleted mice, 85% to 90% reduction in serum IGF1 
occurred, with early postnatal growth retardation.272 These find­
ings suggest that postnatal growth is dependent on both hepatic 
and tissue IGF1, although definite conclusions are problematic 
in the face of elevated GH 

sions are problematic 
in the face of elevated GH production and perturbations of the 
IGFBP system observed in these studies.
Deletion of Igf1r resulted in birth weights 45% of normal 
with 100% neonatal lethality.273 Concurrent deletion of Igf1 
and Igf1r resulted in no further reduction in birth size com­
pared with deletion of Igf1r alone; this is consistent with the 
hypothesis that all IGF1 actions are mediated via the IGF type 
I receptor.
Deletion of Igf2r in mice causes an increase in b

Deletion of Igf2r in mice causes an increase in birth weight but 
death in late gestation or at birth.265 Because this receptor nor­
mally degrades IGF2, increased growth reflects excess IGF2 act­
ing through the IGF type I receptor, with variable accumulation 
of IGF2 in tissues. Deletion of Igf2r plus Igf2 results in a birth 
weight 60% of normal, similar to the size of mice with knockout 
of Igf2 alone, with no effect on neonatal survival.274 Simultaneous 
knockout of the genes for Igf2 and 

 Simultaneous 
knockout of the genes for Igf2 and Igf1r causes further reduction 
of birth size to 30% of normal, suggesting that some of the fetal 
anabolic actions of IGF2 are mediated by another mechanism 
such as via placental growth or interactions with the insulin recep­
tor. Indeed, specific deletion of Igf2 in the placenta causes small 
placenta and growth retardation.275
These studies allow the following conclusions: (1) IGF1 is 
important for both fetal and postnatal growth; (2) IGF2 i

nt for both fetal and postnatal growth; (2) IGF2 is a 
major fetal growth factor but has little role in postnatal growth; 
(3) the IGF type I receptor mediates anabolic actions of both 
IGF1 and IGF2; (4) the IGF type II receptor is bifunctional, 
serving to target lysosomal enzymes and to enhance IGF2 turn­
over; (5) IGF2 deletion results in impaired placental growth; 
and (6) IGF1 is the major mediator of GH effects on postnatal 
growth, although GH and GHR may have small IGF1-indepen­
dent ef

gh GH and GHR may have small IGF1-indepen­
dent effects. 
Insulin-Like Growth Factor–Binding Proteins
In contrast to insulin, the IGFs circulate in plasma complexed to 
a family of binding proteins that extend the serum half-life of the 
IGF peptides, transport the IGFs to target cells, and modulate the 
interactions of the IGFs with surface membrane receptors.276 In 
addition, the IGFBPs have effects on cells independent of IGFs. 
The presence and various actions of the IGFBPs provide layers 
o

nd various actions of the IGFBPs provide layers 
of regulation to the GH-IGF axis, greatly increasing its complex­
ity (Fig. 25.29). In the following paragraphs, the structure of the 
IGFBP family is described first, followed by the role of IGFBPs 
in IGF physiology and the characteristics of individual IGFBPs.
Structure of IGFBPs
The six IGFBPs share structural homology in the N-terminal and 
C-terminal domains but have highly variable midsections, which 
probably accounts for the more speciali

ns, which 
probably accounts for the more specialized properties of the indi­
vidual IGFBPs, such as cell dissociation, IGF-binding enhance­
ment, and IGF-independent actions. In each of the conserved 
N-terminal and C-terminal domains, there are a high number of 
cysteine-rich residues with a conserved spatial order, implying that 
the disulfide bond–dependent secondary structure of the IGFBPs 
is also conserved.277 Reduction of the disulfide proteins results in 
loss of IGF binding, further de

oteins results in 
loss of IGF binding, further demonstrating the importance of the 
cysteine-rich region.
The N-terminal domain contains residues that have been 
shown to be essential for IGF binding to IGFBPs 1, 3, and 4.278 
The exact sequences in the C-terminal domain that are responsible 
for binding may differ among the IGFBPs. The highly variable 
midregion segment of the IGFBPs is the site of post-translational 
modifications such as glycosylation and phosphorylation and 
of proteolysis.

osylation and phosphorylation and 
of proteolysis. Both the primary structure of IGFBPs and their 
post-translational modifications are responsible for the differen­
tial targeting to tissues: Glycosylation can affect cell interactions, 
phosphorylation can affect IGF1 binding affinity and suscepti­
bility to proteases, and proteolysis can affect IGF/IGF receptor–
dependent and IGF/IGF receptor–independent actions.279
The three-dimensional structure of IGFBPs 1, 2, 4, and 5 
details IGF-binding 

ure of IGFBPs 1, 2, 4, and 5 
details IGF-binding sites in the N-terminal and C-terminal 
domains. Crystallography indicates that the structure of the 
N-terminal domain is critical for IGF binding, whereas the C-ter­
minal domain is important for inhibiting interactions between 
IGF1 and its receptor.280,281
Analysis of IGFBPs is complicated by the presence of IGFBP 
proteases that degrade IGFBP (Fig. 25.30).282 Proteolysis of 
IGFBPs complicates their assay and must be taken into consid­
erati

s their assay and must be taken into consid­
eration when measuring the various IGFBPs in biologic fluids. 
Multiple IGFBP proteases exist, including calcium-dependent 
serine proteases, kallikreins, cathepsin, and matrix metalloprote­
ases.283,284 Proteolysis of IGFBPs releases IGFs to interact with 
cell surface receptors, thereby enhancing the mitogenic and ana­
bolic effects of IGF peptides. 
Role of IGFBPs in IGF Physiology
IGFBPs as Carrier Proteins
The IGFBPs complex almost all of the cir

 Proteins
The IGFBPs complex almost all of the circulating IGF1 and 
IGF2 secondary to their high affinity for the IGFs (10−10 to 10−11 
mol/L).285 In adults, 75% to 80% of IGFs are carried in a ter­
nary complex consisting of one molecule of IGF plus one mol­
ecule of IGFBP3 plus one molecule of the protein ALS.286 This 
ternary complex is too large to leave the vascular compartment 
and thus extends the half-life of IGF peptides from approximately 
lgf1–/–
30%
30%
45%
45%
75%
130%
100%
lgf2p–

ely 
lgf1–/–
30%
30%
45%
45%
75%
130%
100%
lgf2p–
lgf1r–/–
lgf2rm–
60%
60%
• Fig. 25.28  Effects of the disruption of one or more genes of the insu­
lin-like growth factor (Igf) system on fetal growth in mice, expressed as 
percentage of normal body weight. Igf1−/−, Igf1 gene null mice; Igf2p−, 
Igf2 paternal allele null mice; Igf1r−/−, Igf1r gene null mice; Igf2rm−, Igf2r 
maternal allele null mice. Mice with two source arrows are the combined 
genotype of the source arrow mice. (From Gicquel C

genotype of the source arrow mice. (From Gicquel C, LeBouc Y. Hormonal 
regulation of fetal growth. Horm Res. 2006;65:28–33.)
958
10 minutes for IGF alone to 12 to 15 hours for IGF in the ter­
nary complex.287 Binding of IGF to IGFBP3 in a binary complex 
extends the half-life of IGF to 1 to 2 hours as diffusion out of the 
vascular compartment may occur.288
Both IGFBP3 and ALS are GH dependent, providing an 
additional mechanism for GH regulation of

ding an 
additional mechanism for GH regulation of the IGF axis. GH 
administration to GH-deficient patients shifts IGF to the ternary 
complex.289 However, after IGF1 treatment alone, there is no 
increase in IGFBP3 levels, and ALS levels may decrease; thus IGF 
does not shift to the ternary complex.290 In serum of patients with 
GHD or GH insensitivity, little IGF is present in the 150-kDa 
ternary complex; the IGF present is in the IGF-IGFBP3 binary 
complex or is bound by other IGFBPs such a

binary 
complex or is bound by other IGFBPs such as IGFBPs 1, 2, 4, 
or 5.291 
IGFBPs as Modulators of IGF Action
IGFBPs also regulate biologic actions by modulating the availabil­
ity of IGFs (Fig. 25.31). The binding affinity of IGFs for IGFBPs 
is higher than for IGF receptors, and dissociation of IGFs from 
IGFBPs is required for IGFs to interact with IGF receptors.285 
Additionally, the concentration of IGFBPs is in excess, com­
pared with IGFs, in many bodily fluids.292 Dissociation of IGF

GFs, in many bodily fluids.292 Dissociation of IGFs 
from IGFBPs is achieved by mass action, proteolysis, or other 
unknown mechanisms. Inhibition of IGF1 action by the presence 
of IGFBP4 occurs in vitro and in vivo.293 IGFBPs 1, 3, and 5 
have potentiating effects on IGF action, perhaps by facilitating the 
delivery of IGF to target receptors, as is for IGFBP3.294
The enhancing effects of IGFBP5 involve binding of IGFBP5 
to extracellular matrix proteins, which reduces the affinity of 
IGFBP5 

x proteins, which reduces the affinity of 
IGFBP5 for IGF. Possibly, IGFBP5 binding to the extracellu­
lar matrix brings IGF close to the cell surface in a low-affinity 
complex, from which it can be released slowly to bind neigh­
boring IGF receptors.295 To complicate matters, it appears that 
the same IGFBP can potentiate or inhibit IGF action in vitro, 
depending on cell culture conditions, cell type, IGFBP dose, 
and post-translational modifications such as phosphorylation 
state.296
IGFBP p

cations such as phosphorylation 
state.296
IGFBP protease
Type 1 IGF
receptor
IGFBP
IGFBP
fragment
IGF
• Fig. 25.30  Schematic representation of the effect of insulin-like growth 
factor (IGF)–binding protein (IGFBP) proteases on IGF action. In this model, 
proteolysis of IGFBPs results in a reduction in their affinity for IGF ligands, 
leading to enhanced binding of IGF peptides by IGF receptors. (From 
Cohen P, Rosenfeld RG. The IGF axis. In Rosenbloom AL, ed. Human 
Growth Hormone: Basic and 

senbloom AL, ed. Human 
Growth Hormone: Basic and Scientific Aspects. Boca Raton, FL: CRC 
Press; 1995:43–58.)
High affinity IGF binders
Low  affinity IGF binders
IGFBP1 IGFBP2
IGFBP3
IGFBP4
IGFBP5
IGFBP6
150 kDa complex
130 kDa complex
IGFBP-rPs
IGFBP proteolyzed
fragments
IGFBP
protease
Type 1 IGF
receptor
IGFBP-rP(s)
receptor(s)
Type 2 IGF
receptor
IGFBP(s)
IGF1/2
M6P
?
?
α α
β
β
• Fig. 25.29  Schematic representation of the insulin-like growth factor (IGF) system, including IGF l

n-like growth factor (IGF) system, including IGF ligands 
(IGF receptors 1 and 2), binding proteins (both high-affinity and low-affinity binders), IGF-binding pro­
tein (IGFBP) proteases, type 1 and type 2 IGF receptors, and potential receptors for other IGFBPs and 
IGFBP-related proteins (IGFBP-rPs). M6P, mannose-6-phosphate. (From Hwa V, Oh Y, Rosenfeld RG. The 
insulin-like growth factor–binding protein [IGFBP] superfamily. Endocr Rev. 1999;20:761–787, copyright 
© Endocrine Society.)
959
Cha

:761–787, copyright 
959
It has also been proposed that IGFBP-IGF complexes can be 
stored in tissues to act in a paracrine manner. This has been dem­
onstrated for IGFBP5 and bone matrix. IGFBP5 is produced by 
osteoblasts, and the IGFBP5-IGF complex can bind hydroxyapa­
tite, possibly participating in bone turnover.295
IGFBP proteases, which degrade IGFBPs, are postulated to 
play a role in altering IGF availability by lo

to 
play a role in altering IGF availability by lowering affinities of 
IGFBPs for their ligand, thereby increasing the availability of 
IGFs to cell membrane receptors. The best characterized protease 
is pregnancy-associated plasma protein-A2 (PAPP-A2). Papp-a2 
knockout mice of postnatal growth restriction297 and mutation 
of PAPP-A2 causes growth retardation with high IGF1 levels in 
humans,298 thus indicating the importance of these proteases for 
tissue IGF1 availability. 
IGF-Independent 

s for 
IGF-Independent Actions of IGFBPs
IGFBPs also have IGF-independent actions, such as growth inhi­
bition of certain cell types, growth stimulation of tissues, direct 
induction of apoptosis, and modulation of the effects of other 
non-IGF growth factors. The mechanisms of IGF-independent 
actions are slowly being unraveled and include binding to IGFBP-
specific cell surface receptors, binding to other cell surface recep­
tors, and interaction with nuclear recepto

 recep­
tors, and interaction with nuclear receptors (Fig. 25.32).
IGF-independent actions have been characterized for IGFBPs 
1, 2, 3, and 5. IGFBP1 is implicated in cell motility and adhesion; 
IGFBP2 has mitogenic actions independent of IGF1 binding. 
In vitro, IGFBP3 administration or overexpression inhibits DNA 
synthesis and cell proliferation in conditions in which IGF1 or the 
IGF type I receptor is neutralized, suggesting IGF-independent 
inhibition by IGFBP3.285 IGFBP5 has IGF-independ

inhibition by IGFBP3.285 IGFBP5 has IGF-independent effects 
on osteoblasts.279
The IGFBPs also have IGF-independent effects via binding 
to IGFBP-specific membrane receptors for IGFBP3, IGFBP5, 
or other receptors such as the TGFβ receptor type V.279 The 
downstream signaling pathways activated by the IGFBP-specific 
receptors are unknown, as are the possible interactions with IGF 
receptor signaling. 
Characteristics of IGFBPs 1 Through 6
IGFBP1 was the first of the IGFBPs to be purified and

BP1 was the first of the IGFBPs to be purified and to have 
its cDNA cloned. The gene is located on the short arm of chro­
mosome 7, comprises four exons,299 and is strongly expressed in 
decidua, liver, and kidney. It is the major IGFBP in fetal serum in 
early gestation, reaching levels as high as 3000 μg/L by the second 
trimester. Levels of IGFBP1 in newborn serum are inversely cor­
related with birth weight, suggesting an inhibitory role on fetal 
IGF action.
IGFBP1 may be involved in repro

fetal 
IGFBP1 may be involved in reproductive function, including 
endometrial cycling, oocyte maturation, and fetal growth.300 It 
also appears to have an important metabolic role in that its gene 
expression is enhanced in catabolic states250 while insulin sup­
presses IGFBP1 expression. Serum IGFBP1 levels correlate with 
insulin sensitivity. Low levels are associated with lower insulin 
sensitivity. Fasting levels predict risk of diabetes development in 
longitudinal cohorts.301 

diabetes development in 
longitudinal cohorts.301 IGFBP1 null mice have normal glucose 
homeostasis, but IGFBP1 transgenic mice are protected against 
insulin resistance when challenged with a high-fat diet. The cellu­
lar target(s) of IGFBP1 that mediate these effects are not known, 
as IGFBP1 can have IGF1-independent effects.
The IGFBP2 gene is located on the long arm of chromo­
some 2 and is highly expressed in fetal tissues, particularly in the 
CNS.302 It is the second most abundant IGFBP 

the 
CNS.302 It is the second most abundant IGFBP in the serum, 
behind IGFBP3. Unlike IGFBP1, its expression does not change 
with feeding. IGFBP2 transgenic mice have diminished postnatal 
weight gain and slightly reduced fasting insulin levels with protec­
tion against age-related insulin resistance.301
The IGFBP3 gene is located on chromosome 7. IGFBP3 is 
composed of the conserved N-terminal and C-terminal domains 
and the variable midsection. The midsection is the site of N-linked 
glycosy

n. The midsection is the site of N-linked 
glycosylation, which is not present in IGFBP1 or IGFBP2 and is 
the site of phosphorylation. This midsection is the site responsible 
for interaction with cell surfaces.303
IGFBP3 is the predominant IGFBP in adult serum, where it 
carries approximately 75% of the total IGF, primarily as part of the 
150-kDa ternary complex consisting of IGF1, IGFBP3, and ALS. 
IGFBP3 and IGFBP5 are the only IGFBPs to form this complex. 
It is believed that formation of 

m this complex. 
It is believed that formation of this ternary complex limits IGF 
access to target cells, while at the same time prolonging serum 
half-lives of both the IGF peptide and its binding protein.304 
Serum levels of IGFBP3 and ALS are reduced in patients with 
GHD or GH insensitivity, conditions in which assays for serum 
IGFBP3 have important diagnostic value. IGFBP3 is increased in 
states of GH excess and acromegaly.
IGFBP3 action is GH dependent, either directly or through 
regul

is GH dependent, either directly or through 
regulation by IGF. IGF1 treatment of patients with GH insen­
sitivity does not greatly alter serum IGFBP3 levels,143 and GH 
treatment of GH-deficient patients does not increase serum levels. 
Whether these observations mean that GH has a direct effect on 
IGFBP
IGF1/2
Type 1 IGF
Receptor
IGF
inhibitory
IGF
enhancing
?
IGF
independent
• Fig. 25.31  Theoretic mechanisms of cellular actions of insulin-like growth 
factor (IGF)–binding proteins (IGFBPs).

ke growth 
factor (IGF)–binding proteins (IGFBPs). See text for details.
IGF
IGFBP3
Type 1 IGF
receptor 
IGFBP3
receptor
Cell growth
• Fig. 25.32  Schematic diagram of insulin-like growth factor (IGF)–depen­
dent and IGF-independent actions of IGF-binding protein 3 (IGFBP3), the 
latter being mediated through a putative membrane-associated IGFBP3 
receptor.
960
IGFBP3 or reflects GH regulation of ALS and ternary complex 
format

s GH regulation of ALS and ternary complex 
formation is unclear, although it appears likely that both factors 
are contributory.
IGFBP3 associates with cell membrane proteins in a specific, 
cation-dependent manner and with high affinity.279 Whether the 
cell membrane proteins constitute genuine IGFBP3 receptors 
remains to be determined, although they may mediate IGF-inde­
pendent actions of IGFBP3.
IGFBP3 expression can be induced by cell cycle regulators and 
growth-inhibitory factors such a

e regulators and 
growth-inhibitory factors such as TNFα, TNFβ, retinoic acid, 
vitamin D, antiestrogens, and antiandrogens.305 Like many genes, 
IGFBP3 expression is affected by methylation and histone modi­
fication. Abnormal methylation or histone modification of the 
IGFBP3 gene is present in many different types of human cancers 
(reviewed by Jogie-Brahim and colleagues305).
The IGFBP4 gene is located on chromosome 17 and contains 
four exons. It is widely expressed in embryonic tissues, fi

s. It is widely expressed in embryonic tissues, fibroblasts, 
osteoblastic cells, prostatic cells, ovarian cells, and liver. The cir­
culating form is derived mostly from the liver. IGFBP4 inhibits 
IGF action in most tissues.306 The inhibitory effect of IGFBP4 is 
reduced upon proteolysis by PAPP-A, as proteolysis of IGFBP4 
results in an increase of free IGF1. Circulating IGF1 or IGF2 acti­
vates IGFBP4 proteolysis by inducing a conformational change 
allowing PAPP-A to access the cleavage sit

change 
allowing PAPP-A to access the cleavage site.306
The IGFBP5 gene is located on chromosome 5 and contains 
four exons. IGFBP5 binds extracellular matrix proteins such as 
types III and IV collagen, laminin, and fibronectin, and it does 
so in response to binding of IGF1.307 The affinity of IGFBP5 is 
reduced about sevenfold when the binding protein is associated 
with extracellular matrix, providing a potential mechanism for 
release of IGFs to cell surface receptors. Association of IGFBP5

s to cell surface receptors. Association of IGFBP5 
with extracellular matrix protects it from proteolysis.308 Unlike 
proteolysis of IGFBP4, which is enhanced by the addition of 
IGFs, degradation of IGFBP5 is inhibited by the binding of IGF 
peptides.309
The IGFBP6 gene is located on chromosome 12 and contains 
four exons. Although IGFBP6 binds both IGF1 and IGF2, it has a 
significantly greater affinity for IGF2.310 IGFBP6 is found in high 
levels in cerebrospinal fluid, as is IGFBP2, which a

vels in cerebrospinal fluid, as is IGFBP2, which also binds IGF2 
with high affinity. IGFBP6 may have a role in regulating ovarian 
activity, perhaps by functioning as an antigonadotropin.311
Mice null for the individual IGFBPs exhibit modest, if any, 
alterations in growth. Modest decreases in organ size have been 
noted in IGFBP2 null mice,312 whereas growth in IGFBP4 null 
mice is 85% to 90% of normal. Increases in other IGFBP levels 
were noted in these mice. Triple knockout mice null for IG

ed in these mice. Triple knockout mice null for IGFBPs 
3, 4, and 5 exhibit lengths 80% of normal with IGF1 levels 45% 
of wild type.313 Transgenic mice overexpressing the IGFBPs 1, 2, 
3, and 4314 exhibit growth retardation to varying degrees, demon­
strating the role of the IGFBPs in sequestering IGF1 or inhibit­
ing its actions. Mice overexpressing IGFBP1 and IGFBP3 exhibit 
impaired glucose tolerance and decreased fertility, further impli­
cating a role for IGF1 or a separate role for these 

ting a role for IGF1 or a separate role for these IGFBPs in glu­
cose metabolism and reproduction. 
Androgens and estrogens affect growth predominantly through 
two mechanisms: regulation of the GH-IGF1 axis and matura­
tion of the epiphyseal growth plates. The adolescent rise in serum 
gonadal steroid levels is an important part of the pubertal growth 
spurt. In addition, it is the stimulation of epiphyseal fusion by 
the pubertal rise of gonadal steroid production that results

al rise of gonadal steroid production that results in the 
ultimate cessation of linear growth. The details of the roles of 
androgens and estrogens in enhancing GH secretion and directly 
stimulating IGF1 production were discussed earlier.
Both androgens and estrogens increase skeletal maturation and 
accelerate growth plate senescence. However, most of these effects 
are due to the action of estrogen, with androgens acting indirectly 
after their conversion to estrogens by aromatase in extragl

ir conversion to estrogens by aromatase in extraglandular 
tissues, including locally within the growth plate. The primacy of 
the role of estrogen comes from animal studies22 as well as reports 
of human subjects with mutations. A mutation of the estrogen 
receptor in a man was associated with tall stature and open epiph­
yses,23 and similar findings were reported in patients with muta­
tions of the gene encoding the aromatase enzyme.315 In addition, 
variants of the estrogen receptor have been

tion, 
variants of the estrogen receptor have been associated with height 
in males316 and in females,317 and increased aromatase expression 
results in short adult stature in males.318,319
Whereas most of the effects of androgen on growth are medi­
ated through actions that occur after their aromatization to 
estrogen, there is evidence of androgen-specific effects. Notably, 
dihydrotestosterone, a nonaromatizable androgen, can accelerate 
linear growth in boys. This effect of androgen does not

r growth in boys. This effect of androgen does not appear 
to be mediated by either GH or circulating IGF1, but it may be 
mediated by an increase in local IGF1 production.320
Gonadal steroids, along with GH and IGF1, contribute to the 
attainment of peak bone mass in adults. Again, this sex hormone 
effect is largely mediated by estrogen action.321–323
Thyroid hormone is of relatively little importance in the growth 
of the fetus, but it has significant effects on postnatal grow

, but it has significant effects on postnatal growth and 
bone maturation. Patients with hypothyroidism have decreased 
spontaneous GH secretion and blunted responses to GH pro­
vocative tests. Thyroid hormone also has a direct effect on chon­
drocytes and osteoblasts, which both express thyroid hormone 
receptors. Thyroid hormone regulates chondrocyte prolifera­
tion and stimulates terminal differentiation, mineralization, and 
angiogenesis.324,325 In particular, thyroid hormone is essential fo

325 In particular, thyroid hormone is essential for 
hypertrophic chondrocyte differentiation.326 Postnatally, hypo­
thyroidism can cause growth failure and delayed skeletal matura­
tion, whereas hyperthyroidism can accelerate linear growth and 
skeletal maturation. 
Glucocorticoids have both stimulatory and inhibitory effects on 
GH secretion, with the absolute effect depending on the timing and 
the glucocorticoid concentration. Glucocorticoid deficiency, as in 

 Glucocorticoid deficiency, as in 
Addison disease, leads to a decrease in GH secretion due to decreased 
expression of GHRH and GH secretagogue receptors.170 Acute 
exposure to supraphysiologic levels of glucocorticoids decreases GH 
secretion within 1 hour followed by a subsequent transient increase 
in GH secretion.170,327 Ongoing glucocorticoid excess then causes 
ongoing suppression of GH secretion. This decrease in GH secre­
tion is due to an increase in somatostatin tone.170 Glucocorticoi

an increase in somatostatin tone.170 Glucocorticoids 
can also impair growth through direct actions at the growth plate 
by inhibiting local IGF1 production through suppression of chon­
drocyte GHR expression, impairment of chondrocyte IGF1 recep­
tor expression,170,328 alterations in IGFBP levels, and impairment 
of intracellular signaling.329 Finally, glucocorticoids may stimulate 
apoptosis of growth plate chondrocytes.320
Additional indirect effects of excess glucocorticoids on growth 
can r

effects of excess glucocorticoids on growth 
can result from glucocorticoid inhibition of calcium absorption 
961
and reabsorption, with the development of secondary hyper­
parathyroidism.325 In pubertal children, glucocorticoid excess 
can induce sex hormone deficiency, causing a loss of the normal 
growth stimulatory effect of these hormones.325 
Pathologic Basis of Growth Retardation
A classification of growth retardation is presented in Tabl

ication of growth retardation is presented in Table 25.1. 
Growth disorders can also be subdivided into (1) disorders of the 
hypothalamic-pituitary axis resulting in deficiency of GH, (2) dis­
orders resulting in deficiency or resistance to the action of IGF1, 
(3) growth disorders that primarily affect the growth plate or are 
caused by chronic disease, and (4) idiopathic short stature (ISS), 
which is considered separately but may have a pathogenic basis in 
the GH-IGF1 axis or at the growth 

genic basis in 
the GH-IGF1 axis or at the growth plate. A schematic diagram of 
known defects in the GH-IGF1 axis is shown in Fig. 25.33, and 
the known involved genes are listed in Table 25.2.
Disorders of the GH-IGF1 Axis
Although it is not always possible to establish definitively whether 
hypothalamic or pituitary dysfunction is responsible for the hor­
mone deficiency, these compartments are differentiated to facili­
tate discussion of the pathology. Table 25.3 in

i­
tate discussion of the pathology. Table 25.3 indicates possible 
causes for deficiency in the GH-IGF axis. The term idiopathic is 
often used to designate lack of understanding of the basis for the 
GHD. Developmental or functional abnormalities of the hypo­
thalamus account for most idiopathic cases of hypopituitarism, 
and recent molecular studies have begun to elucidate the molecu­
lar bases of these disorders. It is anticipated that most cases of 
idiopathic hypopituitarism will be define

ases of 
idiopathic hypopituitarism will be defined at the genetic level in 
the future. 
The Hypothalamus
The primary hypothalamic neuropeptide responsible for synthesis 
and release of GH is GHRH. Somatostatin plays an antagonis­
tic role in GH release. Synthesis of these two hypothalamic pro­
teins is controlled by a series of neurochemicals, and the balance 
between them is responsible for the tight neuroendocrine control 
of GH biosynthesis. Mutations of some of the gen

of GH biosynthesis. Mutations of some of the genes encoding 
hypothalamic peptides explain some cases of GHD due to hypo­
thalamic dysfunction.
As noted earlier, patients with early-diagnosed congenital 
GHD frequently have an abnormal pituitary stalk, ectopia of the 
posterior pituitary, and hypoplasia of the anterior pituitary (Fig. 
25.34). Anencephaly results in a pituitary gland that is small or 
abnormally formed and is frequently ectopic. Despite the loss of 
hypothalamic regulation, so

. Despite the loss of 
hypothalamic regulation, somatotrophs differentiate and prolifer­
ate, although in diminished overall mass.67 During intrauterine 
life, serum GH and IGF1 levels are 30% to 50% of the normal 
range,151 and pituitary GH content at birth is about 15% to 20% 
of normal,149,151 with similarly low neonatal plasma GH levels.330
In most patients, so-called idiopathic hypopituitarism or GHD 
is presumed to be due to abnormalities of synthesis or secretion 
of hypothalamic hypophys

f synthesis or secretion 
of hypothalamic hypophysiotropic factors.331,332 In a number of 
reports, idiopathic GHD is associated with magnetic resonance 
imaging (MRI) findings of an ectopic neurohypophysis, pituitary 
stalk dysgenesis, and hypoplasia or aplasia of the anterior pituitary. 
Overall, those patients with the most striking abnormalities of the 
hypothalamic-pituitary region, largely those with combined pitu­
itary hormone deficiency (CPHD), had the smallest anterior pitu­
itary glan

ituitary-IGF axis: sites of established 
mutations. ALS, acid-labile subunit; CPHD, combined pituitary hormone 
deficiency; GH, growth hormone; GHRH, GH-releasing hormone; GHRHR, 
GHRH receptor; IGF1, insulin-like growth factor 1; IGF1R, IGF1 recep­
tor; JAK, Janus kinase; NPR2, natriuretic peptide receptor 2; SHOX, 
short homeobox; STAT, signal transducer and activator of transcription; v, 
ventral. (Reprinted with permission from Lopez-Bermejo A, Buckway CK, 
Rosenfeld RG. Genetic defects of t

 an etiologic role for these 
occurrences,335 the syndrome of pituitary stalk dysgenesis with 
congenital hypopituitarism is probably the result of abnormal 
development, and the perinatal difficulties are likely the conse­
quence rather than the cause of the abnormalities. Findings of 
a similar MRI appearance in patients with septo-optic dyspla­
sia,149,336 in association with type I Arnold-Chiari syndrome 
and syringomyelia,149,333 and possibly in holoprosencephaly,149 
and the occurrence of 

y in holoprosencephaly,149 
and the occurrence of micropenis with congenital hypopituita­
rism149,337,338 all support the concept that congenital hypopitu­
itarism is a genetic or developmental malformation, not a birth 
injury. Further indirect evidence in studies339 of isolated, com­
plete anterior pituitary aplasia indicates that hypothalamic hypo­
pituitarism and breech delivery are consequences of congenital 
midline brain defects, although perinatal residua of breech deliv­
ery may exacerb

perinatal residua of breech deliv­
ery may exacerbate ischemic damage to the hypothalamic-pitu­
itary unit.
The MRI findings described for early-diagnosed patients with 
hypopituitarism are also found in children diagnosed at a later 
age. Most of these children have hypothalamic dysfunction as the 
cause of diminished pituitary hormone secretion. In the older 
group, as in the infants, structural, acquired hypothalamic, stalk, 
or pituitary abnormalities must be considered.
Holoprosencephaly. H

rmalities must be considered.
Holoprosencephaly. Holoprosencephaly, which is caused by 
abnormal midline development of the embryonic forebrain, usu­
ally results in hypothalamic insufficiency and has been associated 
with mutations in developmental proteins.340–344 These mutations 
are associated with diminished signaling by SHH, a critical factor 
in forebrain development.343 Hedgehog ligands bind to and acti­
vate the transmembrane receptor Patched (PTCH), which results 
in release of the cor

tched (PTCH), which results 
in release of the coreceptor Smoothened (SMO), activating GLI 
transcription factors. SHH and FGF8 play a role in the induction 
of BMP2 and LHX, which are important for proliferation in the 
developing pituitary gland and are influenced by loss-of-function 
mutations of the GL12 gene.342,343
Facial dysmorphism of holoprosencephaly ranges from cyclo­
pia to hypertelorism and is accompanied by absence of the nasal 
septum, midline clefts of the palate or lip, and some

tum, midline clefts of the palate or lip, and sometimes a single 
central incisor. GHD may be accompanied by other pituitary hor­
mone insufficiencies.345 The incidence of GHD is increased in 
cases of simple clefts of the lip or palate (or both),346 and children 
with cleft palates who grow abnormally require further evaluation. 
Factor
Gene Function
Affected Cell Types
Clinical Phenotype
Mode of 
SOX2
 • Member of SOXB1 subfamily as SOX1 
and SOX3 expressed early in develop­
ment

 IGF, insulin-like growth factor; LH, luteinizing hormone; MRI, magnetic resonance imaging; PRL, prolactin; SHH, Sonic hedgehog; TSH, thyroid-stimulating 
hormone (thyrotropin).
   
  Genetic Defects of the GH-IGF Axis Resulting in Somatotroph Dysfunction and GH Deficiency—cont’d
TABLE 25.2
964
Septo-Optic Dysplasia. In its complete form, the rare syndrome 
of septo-optic dysplasia (SOD) combines hypoplasia or absence of 
the optic chiasm or optic n

oplasia or absence of 
the optic chiasm or optic nerves (or both), agenesis or hypopla­
sia of the septum pellucidum or corpus callosum (or both), and 
hypothalamic insufficiency.336,347,348 The extent of the anatomic 
and functional abnormalities can vary but usually are in parallel 
to each other.347,348 GHD can occur by itself or in combination 
with deficiencies of TSH, ACTH, or gonadotropins. About 50% 
to 70% of children with severe anatomic defects have hypopi­
tuitarism or, at least, ide

c defects have hypopi­
tuitarism or, at least, identifiable abnormalities of the GH-IGF 
axis,349 and the diagnosis should be considered in any child who 
has growth failure associated with pendular or rotatory nystagmus 
or impaired vision and a small optic nerve disc. In some patients, 
a hypoplastic or interrupted pituitary stalk and ectopic poste­
rior pituitary placement have been identified by MRI.149,336,347 
An increasing number of mutations in developmental transcrip­
tion factors, incl

ons in developmental transcrip­
tion factors, including HESX1, SOX2, SOX3, and OTX2, are 
implicated in the pathogenesis of septo-optic dysplasia. Recently, 
mutations in genes known to be associated with hypogonado­
tropic hypogonadism, prokineticin receptor 2 (PROKR2), FGF8, 
and FGF receptor 1 (FGFR1) have been found to be associated 
with other pituitary hormone deficiencies along with septo-optic 
dysplasia.350
The varying conditions of environment and genetics likely 
contribute to the var

ronment and genetics likely 
contribute to the variable phenotype.351,352 There is an increased 
incidence in offspring of young mothers, in first-born children, in 
areas of high unemployment, and in babies exposed to intrauter­
ine medications, smoking, alcohol, or diabetes.353 
HESX1. The first homozygous missense mutation (N53C) has 
been detected within the homeobox domain of HESX1 in two 
siblings born to consanguineous parents with SOD.354–361 Muta­
tions of HESX1, a paired-like homeodoma

–361 Muta­
tions of HESX1, a paired-like homeodomain gene that is expressed 
early in pituitary and forebrain development, are associated with 
familial forms of septo-optic dysplasia.351,354,362,363 HESX1, also 
referred to as RPX (Rathke pouch homeobox), is a member of 
the paired-like class of homeobox genes and is essential for nor­
mal forebrain and pituitary formation.364 It is one of the earli­
est known specific markers for the pituitary primordium and 
encodes for a developmental repres

primordium and 
encodes for a developmental repressor with localization to the 
Rathke pouch.357 TLE1 (the mammalian ortholog of the Dro­
sophila protein Groucho) and the nuclear corepressor NCOR1 
bind to HESX1 to exert repression.365,366 HESX1 downregulation 
is required for cell determination mediated by PROP1 to occur.367 
The LIM homeodomain proteins LHX1 and LHX3 are required 
for activation of the HESX1 promoter.368
Hesx1 null mutant mice display abnormalities in the forebrain, 
eyes, and

display abnormalities in the forebrain, 
eyes, and other anterior structures such as the pituitary.354 Mouse 
embryos heterozygous for both Hesx1 and Six3 null mutations 
have a mild phenotype, suggesting that these developmental fac­
tors control a switch in progenitor proliferation between repres­
sion by HESX1 and activation of PROP1.369 These defects have 
similarity with human phenotypes such as septo-optic dysplasia 
and CPHD. Patients with septo-optic dysplasia may present with 
a wide sp

 septo-optic dysplasia may present with 
a wide spectrum of phenotypes associated with congenital hypopi­
tuitarism. Several homozygous and heterozygous mutations have 
been identified in HESX1 (Online Mendelian Inheritance in Man 
[OMIM] no. 601802) in patients with hypopituitarism with vari­
able phenotypes.354,356,357,359,363,370–373
Two siblings (born to consanguineous parents) with a severe 
septo-optic dysplasia phenotype, including anterior pituitary 
hypoplasia, an ectopic posterior lobe

r pituitary 
hypoplasia, an ectopic posterior lobe, agenesis of the corpus cal­
losum, absent septum pellucidum, and optic nerve hypoplasia, 
were found to have a homozygous mutation at a highly conserved 
arginine residue of the homeodomain (p.R160C) that resulted in 
loss of DNA binding of the mutant protein. Another homozy­
gous mutation, a threonine/isoleucine substitution at residue 26 
(p.I26T), was identified in a child with GH and gonadotropin 
deficiency that later evolved to deficienci

ropin 
deficiency that later evolved to deficiencies of ACTH and TSH. 
She had no forebrain abnormalities and normal optic nerves, but 
on MRI had hypoplasia of the anterior pituitary and an unde­
scended posterior pituitary. This mutation lies in a highly con­
served engrailed homology domain (eh-1) that is required for 
transcriptional repression. Loss of repressor function was found 
to be caused by impaired interaction with the TLE1 corepressor. 
To determine the mechanisms by which these mu

or. 
To determine the mechanisms by which these mutations cause the 
disorder, mice homozygous for these mutations were generated.374 
Mice homozygous for the p.R160C mutation displayed pituitary 
and forebrain defects similar to those in the Hesx1 null embryos, 
indicating the critical role of HESX1 interactions with DNA in 
transcriptional repression during development. Mice homozygous 
for the p.I26T allele displayed pituitary defects and ocular abnor­
malities suggestive of a hypomorph allel

ar abnor­
malities suggestive of a hypomorph allele, indicating the impor­
tant role of TLE interaction in pituitary and ocular development.
Heterozygous mutations of HESX1 have been identified in 
patients with hypopituitarism and septo-optic dysplasia and are 
usually associated with less affected phenotypes. Approximately 
850 patients were studied for mutations in HESX1, including 
more than 300 patients with septo-optic dysplasia; 410 with iso­
lated pituitary dysfunction, optic nerve hypop

so­
lated pituitary dysfunction, optic nerve hypoplasia, or midline 
neurologic abnormalities; and 126 with familial inheritance. The 
incidence of coding region mutations in HESX1 within this popu­
lation was about 1%, suggesting that mutations in HESX1 are a 
rare cause of hypopituitarism and septo-optic dysplasia.370
Most of the HESX1 screenings performed in large CPHD 
cohorts failed to detect mutations. The only mutations 
  GH-IGF1 Deficiency Syndromes
Congenital Growth Hormone Deficiency

ncy Syndromes
GH deficiency resulting from hypothalamic dysfunction
Holoprosencephaly and septo-optic dysplasia
GH deficiency resulting from pituitary dysfunction
GH deficiency resulting from mutations of the GHRH receptor and GH
GH deficiency resulting from CPHD
Acquired Growth Hormone Deficiency
GH deficiency resulting from CNS tumors, trauma, or inflammation
Circulating antibodies to GH that inhibit GH action
Congenital IGF1 Deficiency
Abnor

inhibit GH action
Abnormalities of the GHR
STAT5 mutations
ALS mutations
IGF1 mutations
IGF1 receptor mutations
Acquired IGF1 Deficiency
Circulating antibodies to the GHR
Chronic disease states
ALS, Acid-labile subunit; CHPD, combined pituitary hormone deficiencies; CNS, central ner­
vous system; GH, growth hormone; GHR, growth hormone receptor; GHRH, growth hormone–
releasing hormone; IGF1, insulin-like growth factor 1.
   
TABLE 25.3
965
Chapter 25  Normal and Aberra

.
   
TABLE 25.3
965
identified were detected in sporadic subjects with an estimated 
global ­frequency of 0.45%.375 
OTX2. Mutations in other genes have been associated with 
CNS anatomic abnormalities and hypopituitarism. OTX2 is a 
homeobox gene that is expressed earliest in the neuroectoderm376 
cells of the forebrain and midbrain and encodes a transcription 
factor belonging to the orthodenticle family. This factor also plays 
a role in ocu

icle family. This factor also plays 
a role in ocular development. Murine models harboring mutant 
Otx2 genes demonstrate abnormal primitive streak organization 
and a headless phenotype.376 OTX2 has a role in regulating early 
expression of HESX1 and is expressed in the pituitary to regulate 
POU1F1 (Pit1). Dateki and colleagues377 identified a frameshift 
OTX2 mutation in a patient with bilateral anophthalmia and par­
tial isolated GHD (IGHD) with minimal transactivation activity. 
A heterozyg

ith minimal transactivation activity. 
A heterozygous OTX2 mutation was described in two unrelated 
patients with hypopituitarism; although initial studies dem­
onstrated normal binding to HESX1 binding sites, the mutant 
OTX2 gene was demonstrated to have decreased activation of the 
HESX1 promoter, suggesting a dominant negative effect leading 
to CPHD.378 This relationship between the transcription fac­
tors OTX2 and HESX1 emphasizes the complexities of pituitary 
development and suggests tha

exities of pituitary 
development and suggests that genetic causes may be multifac­
torial. Further studies have revealed two frameshift mutations, a 
nonsense mutation in two unrelated patients and a heterozygous 
microdeletion in a fifth patient.377–385 
SOX3. A syndrome of X-linked hypopituitarism and mental 
retardation involving duplications of Xq26-27 encompassing 
SOX3 (OMIM 313430) has been described in several pedigrees. 
SOX3 is a member of the SOX (SRY-related high mobility group 
box

r of the SOX (SRY-related high mobility group 
box) family of transcription factors expressed in neuroepithe­
lial progenitor and stem cells beginning in the earliest stages of 
embryogenesis involved in different developmental processes, such 
as gastrulation, neural induction, specification, and the differentia­
tion of many cell types. Both the overexpression and underexpres­
sion of SOX3 can cause different phenotypes, ranging from IGHD 
to CPHD with ectopic neurohypophysis, anomalies of the

PHD with ectopic neurohypophysis, anomalies of the corpus 
callosum with or without intellectual disability.386–388 Microdupli­
cations and deletions of genomic regions containing SOX3 as well 
as expansion of a polyadenine tract (in one case, a deletion) have 
been reported.389,393,394 The affected males have anterior pitu­
itary and infundibular hypoplasia with an undescended posterior 
pituitary and abnormalities of the corpus callosum. Other abnor­
malities may include anophthalmia or microo

abnor­
malities may include anophthalmia or microopthalmia, esoplageal 
atresia, and sensorineural hearing loss.395,398,399 The GHD may 
also be associated with deficiencies of ACTH, TSH, or gonadotro­
pins. Because Sox3 is not expressed in Rathke pouch in the mouse, 
the anterior pituitary developmental defects are probably second­
ary to disruption of infundibular development.351,386–389 Female 
carriers appear to be clinically unaffected, and no mutations have 
been found in patients with sex

no mutations have 
been found in patients with sex reversal, gonadal dysgenesis, or 
A
B
• Fig. 25.34  Magnetic resonance imaging of infundibular dysgenesis. (A) T1-weighted sagittal and coro­
nal images of the hypothalamic-pituitary area in a normal 8-year-old girl. The anterior (AP) and posterior 
pituitary (PP) lobes and the pituitary stalk (PS) are marked. (B) T1-weighted sagittal and coronal images of 
the hypothalamic-pituitary area of a 17-year-old boy with isolated growth hormone deficie

-year-old boy with isolated growth hormone deficiency. The anterior 
pituitary lobe (AP) is hypoplastic, the posterior pituitary (PP) is ectopic, and the pituitary stalk is absent. 
(From Root AW, Martinez CR. Magnetic resonance imaging in patients with hypopituitarism. Trends Endo­
crinol Metab. 1992;3:283–287.)
966
infertility.388–394 Synonymous changes have been found in chil­
dren who inherited the variant from a clinically unaffected par­
ent.39

e variant from a clinically unaffected par­
ent.395,396 A patient was reported with isolated hypogonadotropic 
hypogonadism and a normal anterior pituitary suggesting that 
SOX2 may be involved in development and control of the hypo­
thalamic neuronal reproductive axis. Indeed, SOX2 expression has 
been colocalized with LHX3 and HESX1.374,395 
SOX2. Heterozygous mutations within SOX2 in males have 
been associated with anophthalmia or microphthalmia and ante­
rior pituitary hypoplasia. The resul

mia and ante­
rior pituitary hypoplasia. The resulting hormonal abnormalities 
include GH and gonadotropin deficiency. Some patients also 
present with genital abnormalities.395 A wide variety of additional 
abnormalities may be present, including hypoplasia of the corpus 
callosum, hypothalamic hamartoma, hippocampal malformation, 
esophageal atresia, sensorineural hearing loss, and learning dif­
ficulties.395–399 In addition to de novo heterozygous mutations, 
nonsynonymous changes have been i

gous mutations, 
nonsynonymous changes have been identified in individuals who 
inherited the variant from a clinically unaffected parent.395,396 A 
perplexing case of a patient with isolated hypogonadotropic hypo­
gonadism but without anterior pituitary hypoplasia suggests that 
SOX2 may be independently involved in hypothalamic neuronal 
development. The expression of SOX2 has been found to overlap 
with that of LHX3 and HESX1 within Rathke pouch.374,395,400 
GLI2. Heterozygous frameshift or n

ch.374,395,400 
GLI2. Heterozygous frameshift or nonsense GLI2 mutations 
have been reported in patients with holoprosencephaly. In addition, 
nonsynonymous GLI2 variants in patients with congenital hypopi­
tuitarism with CPHD and an ectopic posterior pituitary lobe with­
out holoprosencephaly were found. Several patients with pituitary 
abnormalities, polydactyly, and abnormal facial features, also with­
out holoprosencephaly, were reported to have truncating mutations. 
The pattern of inherita

ave truncating mutations. 
The pattern of inheritance of these mutations has been dominant 
with incomplete penetrance and variable expression.401–403 
PROKR2. Patients with septo-optic dysplasia with CPHD 
have been found to have mutations in PROKR2, a known cause 
of isolated idiopathic hypogonadotropic hypogonadism and anos­
mia. Recently, a novel heterozygous substitution (c.742C>T;p.
R248W) was identified in PROKR2 in a patient with panhypopi­
tuitarism and pituitary dysplasia.350,404–406 

i­
Inflammation of the Brain or Hypothalamus. Bacterial, viral, 
or fungal infections may result in hypothalamic/pituitary insuf­
ficiency, and the hypothalamus or pituitary (or both) may also be 
involved in sarcoidosis.407 
Tumors of the Brain or Hypothalamus. Brain tumors are a 
major cause of hypothalamic insufficiency,408 especially midline 
brain tumors such as germinomas, meningiomas, gliomas, epen­
dymomas, and gliomas of 

ningiomas, gliomas, epen­
dymomas, and gliomas of the optic nerve.409 Although short stat­
ure and GHD are most often associated with suprasellar lesions 
in neurofibromatosis, they may also exist without such lesions. 
Whether growth impairment antedates the pathologic findings is 
not yet clear.408 Metastases from extracranial carcinomas are rare 
in children, but hypothalamic insufficiency can result from local 
extension of craniopharyngeal carcinoma or Hodgkin disease of 
the nasopharynx. T

arcinoma or Hodgkin disease of 
the nasopharynx. The laboratory diagnosis of GHD in children 
with brain tumors can be difficult because levels of IGF1 type I 
and IGFBP3 are poor predictors, especially in pubertal patients.410 
Craniopharyngiomas and histiocytosis can cause hypothalamic 
dysfunction (see later discussion). 
Trauma of the Brain or Hypothalamus. Head trauma, result­
ing from boxing and various injuries, can cause IGHD or mul­
tiple anterior pituitary deficiencies. Some series of 

e anterior pituitary deficiencies. Some series of patients with 
GHD have indicated an increased incidence of birth trauma, 
such as breech deliveries, extensive use of forceps, prolonged 
labor, or abrupt delivery. Although GHD can be a consequence 
of a difficult delivery or hypoxemic perinatal period, it is more 
commonly associated with developmental deficiencies (discussed 
previously) or head trauma later in life. In a series of 22 head-
injured adolescents and adults, almost 40% had some 

jured adolescents and adults, almost 40% had some degree of 
hypopituitarism.411–413 
Psychosocial Dwarfism. An extreme form of failure to thrive 
is termed psychosocial dwarfism or emotional deprivation dwarf­
ism.414–417 Most cases of failure to thrive can be traced back to a 
poor home environment and inadequate parenting, with improved 
weight gain and growth upon removal of the infant from the dys­
functional home. Some children have been reported, however, 
to show dramatic behavioral mani

ported, however, 
to show dramatic behavioral manifestations beyond those in the 
typical failure-to-thrive infant, namely, bizarre eating and drink­
ing habits such as drinking from toilets, social withdrawal, and 
primitive speech.414 Hyperphagia and abnormalities of GH pro­
duction may be associated.415 GH secretion is low in response to 
pharmacologic stimuli but returns to normal upon removal from 
the home. Concomitantly, eating and behavioral habits returned 
to normal, and a period of ca

ral habits returned 
to normal, and a period of catch-up growth ensued. Careful 
assessment of endogenous GH secretion showed reversal of the 
GH insufficiency within 3 weeks, including enhancement of GH 
pulse amplitude and a variable increase of pulse frequency.415 The 
reversibility of GH secretory defects and the later growth incre­
ment in the context of the clinical findings described previously 
confirm the diagnosis of psychosocial dwarfism.415
The neuroendocrinologic mechanisms involved

sm.415
The neuroendocrinologic mechanisms involved in psychoso­
cial dwarfism remain to be elucidated. GH secretion is abnor­
mal, and ACTH and TSH levels may also be low, although some 
patients have high plasma cortisol levels. Even when GH secre­
tion is reduced, treatment with GH is not usually of benefit until 
the psychosocial situation is improved. Management of the envi­
ronmental causes of the growth failure is imperative and often 
associated with substantial growth. In our experience,

ciated with substantial growth. In our experience, although 
psychosocial dysfunction is a common cause of failure to thrive in 
infancy, the constellation of bizarre behaviors described in psycho­
social dwarfism is rare.
The fact that GH production is impaired in adults with var­
ied psychiatric disorders and the growth aberrations of functional 
GHD with psychosocial dwarfism together suggest that children 
with emotional problems may have impaired GH secretion and 
growth.416 Indeed, depress

aired GH secretion and 
growth.416 Indeed, depression in children, as adults, can lower 
GH production,417 and in girls, anxiety disorders predict a mod­
est height loss in adults.416 
The Anterior Pituitary
As discussed earlier, many of the disease processes that impair 
hypothalamic regulation of GH secretion also impair pituitary 
function. Another group of abnormalities specifically affect pitu­
itary somatotroph development and function.
Congenital Disorders. As many as 3% to 30% of patient

genital Disorders. As many as 3% to 30% of patients with 
GHD have an affected parent, sibling, or child.418 Inborn errors of 
the genes for nuclear transcription factors affecting hypothalamic-
pituitary development, the GHRH receptor, or the GH gene can 
cause GHD and IGF deficiency. 
Combined Pituitary Hormone Deficiency. During pituitary 
development, a series of transcription factors are expressed in a 
specific timeframe and in a spatial context. The result of cell dif­
ferentiation and pr

ntext. The result of cell dif­
ferentiation and proliferation is a mature anterior pituitary gland 
with five distinct cell types.352 
967
PITX2. PITX2 (also known as RIEG) is a member of the bicoid-
like homeobox transcription factor family that is closely related to 
the mammalian OTX genes expressed in the rostral brain during 
development; it is required at many stages of pituitary develop­
ment.352 Studies have shown that activation of the 

ent.352 Studies have shown that activation of the WNT signal­
ing pathway or constitutive activation of β-catenin can induce 
PITX2 expression. Further, protein inhibitors of activated STAT 
(PIAS) modulate PITX2 expression.419 PITX2 is expressed in thy­
rotrophs, gonadotrophs, somatotrophs, and lactotrophs but not 
in corticotrophs.420
PITX2 acts to activate the promoters of pituitary hormone tar­
get genes.421,422 Homozygous loss of PITX2 results in early embry­
onic lethality with pituitary d

ts in early embry­
onic lethality with pituitary development severely affected.423–426 
This is thought to be related to the control of cell cycle regula­
tory genes by PITX2.427,428 Furthermore, the lack of PITX2 also 
results in excessive cell death during early pituitary development, 
suggesting a role in cell survival.429 A mouse line expressing a 
hypomorphic allele of Pitx2 provided evidence that the extent of 
pituitary hypoplasia and cellular differentiation is proportional to 
the reduc

ular differentiation is proportional to 
the reduced dosage of Pitx.426,430 In this model, the gonadotroph 
lineage was primarily affected and the numbers of differentiated 
somatotrophs and thyrotrophs were reduced, but the corticotroph 
population was unaffected.430
In a Pitx2 overexpression model, the gonadotroph population 
was expanded, probably because of the role of Pitx2 in the expres­
sion of gonadotroph-specific transcription factors GATA2, EGR1, 
and NR5A1 (SF1).429 Mutations of PITX2

TA2, EGR1, 
and NR5A1 (SF1).429 Mutations of PITX2 have been described in 
patients diagnosed with Rieger syndrome, an autosomal dominant 
condition with variable manifestations, including anomalies of 
the anterior chamber of the eye, dental hypoplasia, a protuberant 
umbilicus, mental retardation, and pituitary abnormalities. PHX2 
has been further associated with a role in left-right signaling, 
odontogenesis, cardiac development, and atrial fibrillation.431–433 
Mutations of PITX2, located w

ibrillation.431–433 
Mutations of PITX2, located within the homeodomain respon­
sible for DNA binding, have been described, and several of these 
mutations show loss of DNA binding capacity.434 A heterozygous 
mutation that changes the lysine at position 50 to glutamic acid in 
the homeodomain has been found to impart a dominant negative 
effect leading to a pronounced phenotype.435 
SOX2. Heterozygous mutations in SOX2 (sex-determining 
region Y box 2) have been associated with eye abnormalitie

 box 2) have been associated with eye abnormalities (i.e., 
anophthalmia, microphthalmia, and coloboma) and hypopituita­
rism characterized by anterior pituitary hypoplasia associated with 
GH and gonadotropin deficiency. Numerous nonsense, frame­
shift, and missense mutations, as well as chromosome deletions 
of SOX2, resulting in expression of SOX2 proteins with impaired 
function, have been identified.352 
LHX3. LHX3 is a member of the LIM-type homeodomain pro­
tein family of transcription fa

e homeodomain pro­
tein family of transcription factors that feature two LIM domains 
in their amino terminus (N-terminal) and a centrally located 
homeodomain required for interaction with specific DNA ele­
ments on target genes. During development, expression of LHX3, 
which persists in the adult pituitary, is seen in the anterior and 
intermediate lobes of the pituitary, spinal cord, and medulla.436 
Murine models with targeted disruption of Lhx3, reporter trans­
genic mice, and mutants of LH

hx3, reporter trans­
genic mice, and mutants of LHX3437 show depletion of thyro­
trophs, gonadotrophs, and somatotrophs, suggesting that Lhx3 is 
important for cell specification and proliferation.438 Three LHX3 
isoforms have been identified in humans: LHX3a, LHX3b, and 
M2LHX3.439 Of these, LHX3a displays the greatest ability to 
activate the promoters of pituitary genes. LHX3 interacts with 
components of the inhibitor of histone acetyltransferase (INHAT) 
to modulate chromatin structure.440 

rase (INHAT) 
to modulate chromatin structure.440 Patients with reported muta­
tions in LHX3 have deficits of GH, PRL, TSH, and gonadotro­
pins, as well as abnormal pituitary morphology along with a rigid 
cervical spine that limits head rotation.441–443 Other associated 
symptoms can include hearing loss and ACTH deficiency.
LHX3 mutations are a rare cause of reported hypopituita­
rism, and one study reported that the incidence of a homozygous 
LHX3 mutation in patients studied with CPHD was 2.

LHX3 mutation in patients studied with CPHD was 2.2%.444 
LHX3 mutations showed very low mutation frequency in genetic 
screening (0.3% in sporadic and 11.1% in familial cases). The 
two most northern counties of Sweden have an unexpectedly 
high prevalence of mutations, with carrier frequency estimated 
at 1 in 50, in apparently unrelated individuals, likely explained 
by a common ancestor dating back to the 17th century. Several 
other novel LHX3 mutations in patients with CPHD demon­
strating

X3 mutations in patients with CPHD demon­
strating autosomal recessive inheritance have been reported and 
characterized.441,442,444–448 
LHX4. LHX4 is another LIM homeodomain protein with 
homology to LHX3, and it is also expressed in the developing 
brain, including the cortex, pituitary, and spinal cord.449 It con­
tains two LIM domains in its N-terminus and a DNA-binding 
homeodomain.
Despite similarities in protein structure, the role of LHX4 in 
development is distinct from that of LHX3, a

4 in 
development is distinct from that of LHX3, as demonstrated by 
single and combined gene-deletion targeting in mice. Murine 
models with targeted deletion of Lhx4 form a definitive Rathke 
pouch that arrests and results in a hypoplastic pituitary. In con­
trast to Lhx3 gene knockout mice, Lhx4−/− mice contain all five 
differentiated cell types.450,451 Lhx3 expression is impaired in the 
Lhx4 mutants, suggesting that Lhx4 is required for cell survival, 
expansion of the pouch, and different

l survival, 
expansion of the pouch, and differentiation of pituitary-specific 
cell lineages. Since proper expression of Lhx4 is also crucial for the 
normal development of other organs such as lungs, Lhx4−/− mice 
die shortly after birth due to pulmonary failure, whereas heterozy­
gous appear to be normal.
Several reports have described CPHD patients with evidence of 
a hypoplastic pituitary who harbored LHX4 mutations.436,452,453 
These heterozygous mutations have been shown to result in pro­

zygous mutations have been shown to result in pro­
teins that are unable to bind DNA and activate pituitary gene 
expression.454 Further studies have demonstrated a functional 
relationship between POU1F1 and LHX4 in the regulation of 
POU1F1 expression in specific pituitary cell types.455 Also, sev­
eral studies have suggested that LHX4 and PROP1 have overlap­
ping functions in pituitary development.450 Finally, in addition to 
pituitary hormone deficiencies, LHX4 mutations have been impli­
cat

 deficiencies, LHX4 mutations have been impli­
cated in structural abnormalities; patients with an LHX4 muta­
tion have been reported with abnormal MRI findings, including a 
hypoplastic anterior pituitary, ectopic posterior lobe, poorly devel­
oped sella turcica, and Chiari malformation.456 In the nine cases 
described in the literature to date, large interfamilial and intrafa­
milial variabilities are present.454,457–459 Recently a novel recessive 
mutation has been reported (pT126M), which is

ive 
mutation has been reported (pT126M), which is associated with 
a lethal form of congenital hypopituitarism. However, functional 
assay failed to show any difference between the mutant and the 
wild-type LHX4, making it difficult to implicate the mutation 
with pathogenicity.460 
SIX6. SIX6 is a member of the SIX/sine oculis family of 
homeobox genes that is expressed in retina, optic nerve, hypo­
thalamus, and pituitary.461 Murine expression studies of the TCF/
LEF family of transcription f

 studies of the TCF/
LEF family of transcription factors during pituitary development 
demonstrated that Six6 plays a role in proliferation of cells during 
early formation of Rathke pouch.462 SIX6 has also been shown 
968
to interact with the Groucho family of transcriptional repres­
sors.463,464 Mice lacking SIX6 demonstrate infertility.465 SIX6 
has been mapped to chromosome 14q22-23, and patients with 
deletions of this chromosomal region display

with 
deletions of this chromosomal region display bilateral anophthal­
mia and pituitary anomalies.466,467 Patients with anophthalmia/
microphthalmia were shown to have several frequent polymor­
phisms of Six6 and one potential causative missense mutation.461 
One case report implicated SIX6 haploinsufficiency as being 
responsible for ocular and pituitary maldevelopment. Despite its 
importance in early development, further studies are required to 
determine whether SIX6 mutations are present 

 to 
determine whether SIX6 mutations are present in patients with 
pituitary hormone deficiency. 
ISL1. ISL1 is a member of the LIM homeodomain family of 
transcription factors, which are characterized by two tandemly 
repeated cysteine/histidine-rich LIM domains that are involved 
in protein-protein interactions. ISL1 has been shown to be a 
transcriptional regulator of LHX3.468 Its expression is restricted 
to cells that will express the pituitary of glycoprotein hormone 
α-subunit (αGSU) gen

tary of glycoprotein hormone 
α-subunit (αGSU) gene.469,470 Homozygous loss of Isl1 in mice 
results in developmental arrest with no pouch formation.471 To 
date, no human mutations in ISL1 have been identified. 
PROP1. Mutations in PROP1, a paired-like homeodomain 
transcription factor with expression restricted to the anterior 
pituitary during development, have also been found to result in 
CPHD.472 Mutation of this gene is responsible for a form of murine 
pituitary-dependent dwarfism known 

orm of murine 
pituitary-dependent dwarfism known as the Ames mouse.473 The 
pituitary gland appears to be enlarged in mice bearing mutations 
in Prop1, although the mechanism is unclear.473–476 In the end, a 
decrease in proliferation and apoptosis results in pituitary hypo­
plasia, similar to that seen in humans.402,450,474–478 The switch 
from repression of target gene expression by HESX1 to activation 
by PROP1 is important for development of the POU1F1 (GH, 
PRL, and TSH) and gonadotroph li

 the POU1F1 (GH, 
PRL, and TSH) and gonadotroph lineages.366,476,479 PROP1 
and β-catenin have been shown to form a complex that represses 
Hesx1 while activating Pou1f1 expression.480 PROP1 bends to 
a palindrome TAAT sequence as a dimer to actuate target gene 
expression.481,482 The gonadotropin deficiency in the Ames dwarf 
remains unexplained, although treatment with thyroxine (T4) or 
GH (or both) restored fertility in some male mice and restored 
sexual maturation, but not fertility, in fe

tored 
sexual maturation, but not fertility, in female mice.483,484
Mutations of PROP1 in humans result in GH, PRL, and 
TSH deficiencies, although failure in all cell lineages, including 
gonadotrophs and corticotrophs, has been reported.485–487 The 
characterization of PROP1 mutations is complex, because the 
phenotypes are variable and dynamic and hormone deficiencies 
may develop over time even in patients with similar genetic back­
grounds.485,488,489 Gonadotropin abnormalities are particul

85,488,489 Gonadotropin abnormalities are particularly 
diverse in that approximately 30% of patients have spontaneous 
pubertal development, including menarche, before ultimately 
developing hypogonadotropic hypogonadism.485,490 Apparently 
normal growth without GH has also been found in a child with 
PROP1 deficiency.491,492 The ACTH deficiency may develop in 
the fourth or fifth decade of life.493 Striking variability has been 
described in pituitary size, with very large glands, possibly ari

tuitary size, with very large glands, possibly aris­
ing from the intermediate lobe,494 producing a hyperintense 
T1-weighted signal occasionally demonstrated by MRI.486,495,496 
These glands may undergo involution, leaving a large empty sella 
in patients with complete anterior hypopituitarism, including 
ACTH deficiency.497–499
Many PROP1 (chromosome 5q35, OMIM 601538) abnormal­
ities have been identified, including missense, frameshift, and splic­
ing mutations. A GA repeat in exon 2 (295-CGA

lic­
ing mutations. A GA repeat in exon 2 (295-CGA-GAG-AGT-303) 
has been reported to be a “hot spot” in PROP1; any combination 
of a GA or AG deletion in this repeat region results in a frame­
shift in the coding sequence and premature termination at codon 
109.490,500 Similar abnormalities result from homozygous lesions 
at other sites on exon 2 and affect codons 73, 88, and 149.490,500 
Compound heterozygosity for two mutations was detected in 
36% of children from four families; two differen

36% of children from four families; two different common dele­
tions both led to a stop codon at position 109.501 These mutations 
are predicted to result in loss of the DNA-binding and C-terminal 
transactivating domains of PROP1. Some missense mutations 
have been shown to retain partial activity.486,487,502 Two mutations 
in the transactivating domain, not the DNA binding domain 
(W194 X Prop1, S156 insTProp1), were shown to differentially 
affect DNA binding and transactivation.503
To date

To date, PROP1 mutations represent the most commonly 
known genetic cause of CPHD both in sporadic (6.7%) and 
familial cases (48.5%) with a global mutation frequency of 11% 
considering all the patients. A screen of 73 subjects (36 families) 
diagnosed with CPHD by Deladoey and associates identified 35 
patients with PROP1 gene defects, including three different mis­
sense mutations, two frameshift mutations, and one splice site 
mutation. In 12 of 36 

tions, and one splice site 
mutation. In 12 of 36 unrelated families, defects were located in 
the region nt296 through nt302, suggesting a possible hot spot 
for PROP1 mutations in CPHD.490 Although PROP1 muta­
tions appear to be rare in sporadic cases, their prevalence is 
29.5% in familial cases of CHPD, as reported by Turton and col­
leagues.391,504 The mutation rate varies considerably among the 
different geographic areas: While Western European, US, Austra­
lian, and Japanese cohorts pres

opean, US, Austra­
lian, and Japanese cohorts presented a mutation prevalence lower 
than 1%, the Eastern European and Russian cohorts showed a 
much higher frequency, reaching 64.8% in the Lithuanian popu­
lation. Two variants, namely c.301_302delAG and c.150delA, 
are the most common mutations found in PROP1 and represent 
more than 90% of the mutated alleles in the Eastern European 
cohorts. Recently Dusatkova et al.505 genotyped 21 SNPs flanking 
a 9.6-Mb region around PROP1 and demonstrated

ing 
a 9.6-Mb region around PROP1 and demonstrated an ancestral 
origin for both variants that are carried on haplotypes spanning 
0.2e0.3 Mb, showing that the most frequent PROP1 variants 
are not mutation hot spots as previously assumed but founder 
variants.375,488,501,506–511
There does not appear to be a strong correlation between phe­
notype and genotype.490 Nyström and associates reported com­
pound heterozygous mutations in the PROP1 gene in twins with 
hypopituitarism and late ACTH defi

 in twins with 
hypopituitarism and late ACTH deficiency.512 
POU1F1. The POU1F1 gene (chromosome 3p11, OMIM 
173110) encodes Pit1, a member of a large family of transcrip­
tion factors referred to as POU-domain proteins that is respon­
sible for pituitary-specific transcription of genes for GH, PRL, 
TSH, and the GHRH receptor.59,513–515 Pit1, a 290–amino acid 
protein, contains two domains, the POU-specific domain and 
POU-homeo domain; both are necessary for DNA binding and 
activation of GH 

e necessary for DNA binding and 
activation of GH and PRL genes and for regulation of the PRL, 
TSHβ, and POU1F1 genes.516 Its expression is restricted to the 
anterior pituitary to control differentiation, proliferation, and 
survival of somatotrophs, lactotrophs, and thyrotrophs.418,515–517 
Pit1 regulates target genes by binding to response elements and 
recruiting coactivator proteins, such as cAMP response ele­
ment–binding protein (CREB)–binding protein (CBP).518 Gene 
expression microarra

nding protein (CBP).518 Gene 
expression microarray assays combined with chromatin immuno­
precipitation (CHIP) were used to detect targets of POUIFI.519
Two mouse models were first reported to have GH, PRL, and 
TSH deficiencies associated with mutations or rearrangements of 
969
the Pit1 gene; these were the Snell (dw/dwS) and the Jackson (dw/
dwJ) dwarf mice.520,521 Many different mutations of the POU1F1 
gene have been found internationally 

 the POU1F1 
gene have been found internationally in families with GHD and 
PRL deficiency and variable defects in TSH expression.522–525 
These mutations are transmitted as autosomal recessive or domi­
nant traits and cause variable peptide hormone deficiencies with 
or without anterior pituitary hypoplasia.522–528
The most common mutation, present in about the 30% of the 
patients that carry a POU1F1 mutation, is an R271W substitu­
tion that affects the POU homeodomain, encoding a mutant pro­

fects the POU homeodomain, encoding a mutant pro­
tein that binds normally to DNA and acts as a dominant inhibitor 
of transcription.528–531 Vertical transmission of the R271W muta­
tion was shown, emphasizing the importance of diagnostic and 
therapeutic management during pregnancy.532 Evidence from a 
patient with the R271W mutation suggests that Pit1 may have 
a role in cell survival.531 Indeed, the mutation was used to tar­
get cell proliferation tumoral model systems. A patient diag­
nosed 

tion tumoral model systems. A patient diag­
nosed with GHD, along with dysregulation of PRL and TSH, 
was reported to have a lysine-to–glutamic acid mutation at codon 
216 (K216E).516 This mutant Pit1 binds to DNA and appears not 
to inhibit basal activation of GH and PRL genes; however, the 
mutant is unable to support retinoic acid induction of POU1F1 
gene expression. Another report suggested that CBP (p300) 
recruitment and Pit1 dimerization are necessary for target gene 
activation and that

are necessary for target gene 
activation and that disruption of this process may account for the 
pathogenesis of CPHD.533 All of the reported mutations involve 
sites affecting POU1F1 DNA-binding, dimerization, or target 
gene transactivation.524,525,528,534–542
Phenotypic variability occurs among patients with apparently 
similar genotypes. It does not appear that ACTH or gonadotropin 
deficiencies occur, as is frequently the case with PROP1 defects,504 
but adrenarche has been reported to be

efects,504 
but adrenarche has been reported to be absent or delayed in patients 
with a POU1F1 mutation.543 Circulating antibodies against Pit1 
have been identified to be responsible for hypopituitarism similar 
to that caused by mutations.544 Of the sporadic cases of CPHD, 
1.6% are found to have POU1F1 mutations with no significant 
differences among populations. A higher frequency of POU1F1 
mutations (21.6%) has been detected in familial cases.375 
ARNT2
Six members of a consanguineous fam

ses.375 
ARNT2
Six members of a consanguineous family with CPHD and micro­
cephalia were found to have a homozygous frameshift mutation 
(c1373_1374dupTC) in aryl-hydrocarbon receptor nuclear trans­
locator 2 (ARNT2).545 
GRP161
Recently a homozygous mutation in GRP161, encoding the 
orphan G protein–coupled receptor 161, was identified in a fam­
ily with CPHD and pituitary stalk interruption.546
Isolated Growth Hormone Deficiency. The incidence of IGHD 
is estimated to be 1 in every 3480 to 10,

of IGHD 
is estimated to be 1 in every 3480 to 10,000 live births.86,547–549 
In most children with IGHD, no cause can be identified, and this 
group is often referred to as having idiopathic GHD. However, 
there is increasing recognition that genetic defects underlie some 
cases of GHD. It has been estimated that mutations in the GH 
and growth hormone receptor genes can be detected in up to 34% 
of familial cases of IGHD. Patients with GHD may develop defi­
ciencies of additional anterior pitu

develop defi­
ciencies of additional anterior pituitary hormones.550 Four forms 
of IGHD have been reported (see Table 25.2).551 The classification 
system is based on clinical characteristics, inheritance patterns, 
and GH secretion but not necessarily on disease causation. IGHD 
was most recently reviewed by Alatzoglou and associates.552
The gene encoding GH (GH1) is located on chromosome 
17q23 in a cluster of five highly homologous genes that includes 
two chorionic somatomammotropin genes C

 includes 
two chorionic somatomammotropin genes CHS1 and CHS2, the 
placentally expressed growth hormone gene GH2, and a pseudo­
gene, CSHP1. GH1 and GH2 differ in mRNA splicing pattern: 
GH1 generates 20-kDa and 22-kDa proteins (of approximately 
equal bioactivity), whereas GH2 yields a protein that differs from 
GH1 by 13 amino acid residues.
The 22-kDa isoform includes the five exons with the complete 
biologic activity of GH. A cryptic in-frame splice site within exon 
3 gives rise to alter

ame splice site within exon 
3 gives rise to alternative spliced transcripts lacking the first 45 
bp of exon 3 and produces a 20-kDa peptide missing aa 32-46.94 
The 17.5-kDa form results from the complete skipping of exon 3 
lacking aa 32-71 representing 1% to 5% of the total GH1 tran­
scripts. Two isoforms lacking exons 3 and 4 encoding a 11.3-kDa 
and exons 2 through 4 encoding a 7.4-kDa peptide have been 
detected.553–559 
IGHD Type I. IGHD type IA results primarily from large dele­
tions, 

type IA results primarily from large dele­
tions, with rare frameshift and nonsense mutations of the GH1 
gene that prevents synthesis or secretion of the hormone.552,560 
IGHD IA is inherited as an autosomal recessive trait, and affected 
individuals have profound congenital GHD, including hypogly­
cemia in infancy and severe dwarfism by 6 months of age.561 It has 
been reported that about 10% to 15% of subjects with a height 
below 4.5 SDS and severe IGHD carry GH1 gene deletions; how­
ever, g

severe IGHD carry GH1 gene deletions; how­
ever, geographic and patient selection criteria are present between 
studies. Frameshift and nonsense GH1 mutations have also been 
found in subjects with the IGHD IA phenotype.561
Because GH is not produced even in fetal life, patients are 
immunologically intolerant of GH and typically develop anti-GH 
antibodies when treated with either pituitary-derived or recom­
binant DNA–derived GH. When antibodies prevent patients 
from responding to GH, IGHD IA

s prevent patients 
from responding to GH, IGHD IA can be viewed as a form 
of GH insensitivity, and such patients are candidates for IGF1 
therapy.554,561–563
The less severe form of autosomal recessive GHD, termed 
IGHD type IB, may also result from mutations or rearrange­
ments of the GH1 gene that causes production of an aberrant GH 
molecule that retains some function or at least generates immune 
tolerance. The phenotypic variability is greater than in IGHD 
IA.551,552 These patients usual

ater than in IGHD 
IA.551,552 These patients usually respond to exogenous GH ther­
apy without antibody production. The very low frequency (1.7%) 
of GH1 gene mutations in familial type IB IGHD suggests the 
importance of studying the GH1 gene promoter region in patients 
with unexplained GHD.564
Mutations in GHRHR are also classified as IGHD type IB. 
Mutation of the gene for Ghrhr in its ligand-binding domain has 
been identified in the little mouse (lit/lit)565 and results in dwarf­
ism and d

mouse (lit/lit)565 and results in dwarf­
ism and decreased numbers of somatotrophs.515,556,566 In this 
model, the fetal somatotroph mass is normal, and hypoplasia of 
the somatotrophs is evident only postnatally.515,517,556,566 Such 
data suggest that GHRH is not an essential factor for fetal dif­
ferentiation of the somatotrophs and that GHRH-independent 
cells persist or that mutation does not cause total loss of GHRH 
function.567–569
Wajnrajch and colleagues reported the first human cases o

ch and colleagues reported the first human cases of a 
mutation in the GHRHR gene in two cousins in a consanguineous 
Indian Muslim family with IGF deficiency and profound growth 
failure.570 The gene defect, a nonsense mutation that introduced 
a stop codon at position 72 (E72X), resulted in a markedly trun­
cated GHRHR protein that lacked the membrane-spanning 
regions and the G protein–binding site. The affected children had 
970
undetectable GH r

 Endocrinology and the Life Span
undetectable GH release during standard provocative tests and 
after exogenous GHRH administration but responded to GH 
treatment. The same mutation was also identified in a reportedly 
unrelated Tamoulean kindred in Sri Lanka,571 in a consanguine­
ous kindred in Pakistan (“dwarfism of Sindh”),572,573 and in 17 
patients from one Muslim and four Hindu families in Western 
India.574 The largest kindred in which a mutation of GHRHR was 
identified was a Brazilian f

utation of GHRHR was 
identified was a Brazilian family with a homozygous donor splice 
mutation (G to A at position +1) of exon 1.86 This mutation dis­
rupts the highly conserved consensus GT of the 50-donor splice 
site, generating a truncated GHRHR.575,576
A GHRHR missense mutation in exon 11 (R357C) was 
described in two consanguineous Israeli Arab families.577 Patients 
in all of the groups had early growth failure with short stature 
(–4.5 to –8.6 SD), a high-pitched voice, and increased a

to –8.6 SD), a high-pitched voice, and increased abdominal 
fat accumulation.572,576 As expected, all of the patients demon­
strated severely reduced or undetectable serum concentrations of 
GH in response to provocative GH stimulation, as well as very 
low serum concentrations of IGF1, IGFBP3, and ALS.576 The 
adults manifest an unfavorable cardiovascular risk profile, which 
includes increased levels of low-density lipoprotein cholesterol and 
total cholesterol, elevated C-reactive protein, el

total cholesterol, elevated C-reactive protein, elevated blood pres­
sure, and abdominal obesity. However, a perplexing study found 
no evidence of premature atherosclerosis or premature myocar­
dial ischemia in these patients with GHRHR mutations.578 The 
patients respond well to exogenous GH without antibody forma­
tion. Heterozygotes may have minimal height deficits and may 
show moderate biochemical deficiencies of the GH-IGF axis.572 
Despite extensive study, the geographic separation and e

e extensive study, the geographic separation and ethnic dif­
ferences among these patient groups do not suggest recent (>200 
years) contact among the families from the Indian subcontinent. 
At present, the likely explanation for all four families is that of 
a “founder effect” or one-time mutation in each group followed 
by propagation within a geographically isolated gene pool.86 In 
an analysis of 30 families with IGHD type IB, Salvatori and col­
leagues579 found new missense mutations in tra

ol­
leagues579 found new missense mutations in transmembrane and 
intracellular domains of the GHRHR in three families (10%), with 
two affected members in each. Transfection experiments indicated 
normal cellular expression of these mutant receptors. Mutations 
in GHRH were recently reviewed by Corazzini and Salvatori.580
A recent dominant mutation with incomplete penetrance was 
reported due to a missense substitution (Val10Gly) in the signal 
peptide in three unrelated patients, resulting in 

peptide in three unrelated patients, resulting in a defect in recep­
tor processing and translocation to the cell surface.
Most of the reported GHRHR mutations occur within con­
sanguineous families, and of those a minority of the affected are 
compound heterozygotes. In a group of 65 children with IGHD 
IB, the GHRH receptor gene (GHRHR) was normal in domains 
coding for the extracellular region,581 but mutations in transmem­
brane and intracellular gene domains were found in 10% of fami­
lies 

ular gene domains were found in 10% of fami­
lies with IGHD IB.86,556,577,579,582–587 
IGHD Type II. IGHD type II is inherited as an autosomal 
dominant trait. The most common cause appears to be mutations 
that inactivate the 5′ splice donor site of intron 3 of the GH1 
gene, which contains multiple cis-acting splicing enhancers in the 
exon and intron required to activate canonic splice sites for intron 
2 and 3 and silencing of the cryptic sites. The skipping of exon 3 
results in a 17.5-kDa 

es. The skipping of exon 3 
results in a 17.5-kDa GH isoform mutant that has been shown 
to function in a dominant-negative manner to suppress intracel­
lular accumulation and secretion of wild-type GH.565,588,589 In 
patients with missense mutations in exon 4 or 5, clinical presenta­
tion is quite variable, with some evidence for reversibility of the 
impairment of intracellular GH storage and secretion by GH 
treatment.590 Mullis and coworkers591 studied 57 subjects from 19 
families and found

91 studied 57 subjects from 19 
families and found that patients with IGHD type II not only have 
a variable phenotype in terms of onset, severity, and progression 
of GHD but also may demonstrate later onset of ACTH or TSH 
deficiencies and pituitary hypoplasia. An extensive assessment of 
GH1 gene mutations in short children with and without GHD 
revealed a substantial number of heterozygous mutations.591 
Recent studies characterizing the mechanism of the GH secretory 
defects and the increas

anism of the GH secretory 
defects and the increased expression of the 17.5-kDa isoform may 
lead to novel molecular therapies.592 
IGHD Type III. IGHD type III, transmitted as an X-linked 
trait with associated hypogammaglobulinemia (XLA),593 has not 
yet been related to a mutation of the GH1 gene. A large Australian 
kindred demonstrated GHD with a variable spectrum of pituitary 
hormonal deficiencies that may have been caused by duplication 
of the Xq25.Xq28 region.594
A splicing mutation wit

f the Xq25.Xq28 region.594
A splicing mutation within the Bruton tyrosine kinase (BTK) 
gene in patients with XLA and IGHD has been reported. Muta­
tions in BTK, a cytoplasmic tyrosine kinase expressed in B lym­
phocyte and myeloid cells, have also been identified in several 
patients with XLA but without IGHD. 
SOX3. Mutations in SOX3, a member of the SOX (SRY-
related high mobility group box) family of transcription factors 
on Xq27.1, have been found in families with IGHD with ectopic 
neuroh

n found in families with IGHD with ectopic 
neurohypophysis, anomalies of the corpus callosum, and intellec­
tual disability.388 Several patients with GH deficiency and complex 
phenotypes with duplications in the proximal Xq chromosome 
have been reported.595,596 A complete description of SOX3 can be 
found in the section on septo-optic dysplasia earlier in the chapter. 
Bioinactive GH. Serum GH exists in multiple molecular 
forms, reflecting the consequences of alternative post-transcrip­
tion

e consequences of alternative post-transcrip­
tional or post-translational processing of the mRNA or protein, 
respectively. Some of these forms are presumed to have defects in 
the amino acid sequences required for the binding of GH to its 
receptor, and different molecular forms of GH may have varying 
potencies for stimulating skeletal growth, although this remains to 
be rigorously proved. Short stature with normal GH immunore­
activity but reduced biopotency has been suggested,597,598 but t

educed biopotency has been suggested,597,598 but the 
molecular abnormalities have been characterized in relatively few 
patients, and many cases of suspected bioinactive GH have not 
been rigorously proved.599,600
In one child with extreme short stature (–6.1 SDS), a mutant 
GH caused by a single missense heterozygous mutation (cysteine 
to arginine, codon 77 of GH1) bound with greater affinity than 
normal to GHBP and the GHR and inhibited the action of nor­
mal GH. The child grew more (6 vs. 

action of nor­
mal GH. The child grew more (6 vs. 3.9 cm/year) during a period 
of exogenous GH in moderate dosage. The father was found to 
have the same genetic abnormality but did not express the mutant 
hormone. In a second patient600 with marked short stature (–3.6 
SDS), a heterozygous alanine-to-glycine substitution in exon 4 of 
GH led to a substitution of glycine for arginine. This mutation 
was located in site 2 of GH molecular binding with its receptor 
and apparently led to failure o

with its receptor 
and apparently led to failure of appropriate molecular rotation of 
the dimerized receptor and subsequent diminished tyrosine phos­
phorylation and the GH-mediated intracellular cascade of events. 
Bioactivity determined in a mouse B-cell lymphoma line was 
about 33% of immunoreactivity.601 Exogenous GH substantially 
increased growth velocity (from 4.5 to 11 cm/year).
An Ile179Met substitution found in a short child was charac­
terized by normal STAT5 activation but a 50% dec

­
terized by normal STAT5 activation but a 50% decrement in ERK 
activation.217 This novel finding demonstrated the complexity of 
971
the functional interaction of GH with its receptor, but because 
STAT5B is clearly the major (if not the sole) GH-dependent 
mediator of IGF1 gene transcription, the role of this mutation is 
not clear. Six GH heterozygous variants with evidence of impair­
ment of JAK/STAT activation were found by screening short

 JAK/STAT activation were found by screening short 
children, suggesting that further studies are needed to determine 
the mechanism of interaction of GH with its receptor.602 Because 
these variants occur in heterozygotes, the genotype-phenotype 
correlations are unclear. In one of the more convincing cases of 
bioinactive GH reported to date, Besson and associates603 found 
a homozygous missense mutation (G705C) in a short child (–3.6 
SDS) that resulted in the absence of two disulfide bridges

t resulted in the absence of two disulfide bridges. Both 
GHR binding and JAK2/STAT5 signaling activity were markedly 
reduced.
Some patients demonstrate a decrease in bioactivity (when 
measured by sensitive in vitro assays) but not in immunoreactiv­
ity. The absence of mutations suggests that abnormal post-transla­
tional modifications of GH or other peripheral mechanisms may 
be responsible.604,605 
Craniopharyngiomas and Other Tumors. Many tumors that 
impair hypothalamic 

her Tumors. Many tumors that 
impair hypothalamic function also affect pituitary secretion of 
GH. In addition, craniopharyngiomas are a major cause of pitu­
itary insufficiency. These tumors arise from remnants of Rathke 
pouch, the diverticulum of the roof of the embryonic oral cavity 
that normally gives rise to the anterior pituitary. The diagnosis 
and treatment of craniopharyngioma have been recently reviewed. 
This tumor is a congenital malformation present at birth and 
gradually grows o

lformation present at birth and 
gradually grows over the ensuing years. The tumor arises from 
rests of squamous cells at the junction of the adenohypophysis and 
neurohypophysis and forms a cyst as it enlarges; the cyst contains 
degenerated cells and may calcify but does not undergo malig­
nant degeneration. The cyst fluid ranges from the consistency of 
machine oil to a shimmering, cholesterol-laden liquid, and the 
calcifications may be microscopic or gross. About 75% of cranio­

oscopic or gross. About 75% of cranio­
pharyngiomas arise in the suprasellar region; the remainder resem­
ble pituitary adenomas. Mutations in β-catenin have been found 
in patients with adamantinomatous craniopharyngiomas.606
Craniopharyngiomas can cause manifestations at any age from 
infancy to adulthood but usually manifest in middle childhood. 
The most common presentation results from increased intracra­
nial pressure and includes headaches, vomiting, and oculomotor 
abnormalities. Visual 

, vomiting, and oculomotor 
abnormalities. Visual field defects result from compression of the 
optic chiasm, and papilledema or optic atrophy may be present. 
Visual and olfactory hallucinations have been reported, as have 
seizures and dementia. Most children with craniopharyngiomas 
have evidence of growth failure at the time of presentation, and 
they are often found retrospectively to have had reduced growth 
since infancy.607 GH and the gonadotropins are the most com­
monly affected pituit

adotropins are the most com­
monly affected pituitary hormones in children and adults, but 
deficiency of TSH and ACTH may also occur, and diabetes insipi­
dus is present in 25% to 50% of patients.607 Between 50% and 
80% of patients have abnormalities of at least one anterior pitu­
itary hormone at diagnosis.
Cystic and solid components can be identified by MRI, and 
anatomic relationships can be delineated to help plan a rational 
operative approach. Operative intervention via craniotomy or 
t

roach. Operative intervention via craniotomy or 
transsphenoidal resection may result in partial or almost complete 
removal of the lesion. Postoperative irradiation is commonly used, 
especially if tumor resection was incomplete. In some patients, 
particularly those who become obese, a syndrome of normal linear 
growth without GH may occur. The metabolic syndrome, with 
evidence of insulin insensitivity and increased body mass index 
(BMI), is common and is a predictor of potential major long-

common and is a predictor of potential major long-term 
morbidity.607–609 The long-term childhood and adolescent conse­
quences of craniopharyngioma are substantial, with many qual­
ity-of-life issues exacerbating the hypopituitarism. Patients with 
a history of hypothalamic obesity associated with craniopharyn­
giomas undergoing brain surgery were favored to have sustained 
weight loss.
Pituitary adenomas (see Chapter 9) are infrequent during child­
hood and adolescence, accounting for fewer th

ild­
hood and adolescence, accounting for fewer than 5% of patients 
undergoing surgery at large centers. Almost two-thirds of tumors 
immunochemically stain for PRL, and a small number stain for 
GH. GH-secreting pituitary adenomas are exceedingly unusual in 
youth. There is a variable experience as to the invasive nature of 
pituitary adenomas, but the prevailing opinion is that they are less 
aggressive in children than in adults. In 56 patients at the Mayo 
Clinic with non–ACTH-secreting ade

ts at the Mayo 
Clinic with non–ACTH-secreting adenomas removed transsphe­
noidally, macroadenomas were about one-third more frequent 
than microadenomas, with cases in girls outnumbering those in 
boys 3.3 to 1. The patients with macroadenoma had an approxi­
mately 50% incidence of hypopituitarism, compared with zero 
incidence in those patients with microadenomas; long-term cure 
rates were between 55% and 65% for both tumor sizes. Familial 
isolated pituitary adenoma (FIPA) accounts for about

olated pituitary adenoma (FIPA) accounts for about 2% of pitu­
itary adenomas and is associated with aryl hydrocarbon receptor 
interacting protein (AIP) gene mutations. Gigantism is a feature 
of some patients with somatotropinomas.610 
Histiocytosis X. The localized or generalized proliferation of 
mononuclear macrophages (histiocytes) characterizes Langerhans 
cell histiocytosis, a diverse disorder that occurs in patients of all 
ages, with a peak incidence at ages 1 to 4 years. Endocrinologi

peak incidence at ages 1 to 4 years. Endocrinologists 
are more familiar with the term histiocytosis X, which includes 
three related disorders: solitary bony disease (eosinophilic granu­
loma), Hand-Schüller-Christian disease (chronic disease with dia­
betes insipidus, exophthalmos, and multiple calvarial lesions), and 
disseminated histiocytosis X (Letterer-Siwe disease, with wide­
spread visceral involvement). These syndromes are characterized 
by an infiltration and accumulation of Langerhan

by an infiltration and accumulation of Langerhans cells in the 
involved areas, such as skull, hypothalamic-pituitary stalk, CNS, 
and viscera. Although these disorders, especially Hand-Schüller-
Christian disease, are classically associated with diabetes insipidus, 
approximately 50% to 75% of patients in selected series have 
growth failure and GHD at the time of presentation. The degree 
of pituitary stalk thickness has been shown to correlate with long-
term risk outcomes.611 In contrast, 

te with long-
term risk outcomes.611 In contrast, a French national registry (n = 
589) found GHD in 61 subjects, with overall endocrine dysfunc­
tion in 148. In the latter group, an evolving neurodegenerative 
syndrome (identified in 10% of patients with 15-year follow-up) 
seemed to be associated with pituitary involvement.612 Only 1% 
of unselected children with Langerhans cell histiocytosis living in 
Canada during a 15-year period had GHD.613 
Growth Hormone Insensitivity
Mutations in GHR S

3 
Mutations in GHR Signaling Proteins and ALS. GH insensitiv­
ity, also known as primary IGF1 deficiency, encompasses a variety 
of genetic conditions characterized by growth failure, high serum 
GH levels, and very low serum IGF1 levels.614 These findings, 
first described in siblings in 1966 by Laron and colleagues,615 are 
also known as Laron syndrome. Most individuals with a diagnosis 
of classic GH insensitivity are from pedigrees of Mediterranean 
or Middle Ea

 are from pedigrees of Mediterranean 
or Middle Eastern descent. Members of a well-described pedigree 
972
from Ecuador were from a consanguineous population with Med­
iterranean origins.616
The phenotypic characteristics of classic GH insensitivity 
include growth failure evident at birth615 with postnatal subnor­
mal growth velocities and stature –4 to –10 SD below the mean 
(Fig. 25.35).614 Patients also have subnormal head circumference, 
protrud

s also have subnormal head circumference, 
protruding forehead, abnormal upper-body to lower-body ratio, 
short extremities, and sparse hair (Table 25.4). There is delayed 
motor development, indicating the importance of IGF1 in cerebral 
development. The genitalia are small and puberty is delayed, but 
fertility is normal. Metabolically, the most striking feature of IGF1 
deficiency is hypoglycemia with later development of obesity, rela­
tive hyperinsulinemia, and insulin resistance.617 Indivi

yperinsulinemia, and insulin resistance.617 Individuals do not 
respond to exogenous GH, as determined by growth velocity, hypo­
glycemia incidence, or serum IGF1 or IGFBP1 levels.618 GHBP 
activity is usually undetectable in sera of these patients,619,620 but 
measurable levels correspond to higher final heights.
Liver biopsy performed on two affected patients demonstrated 
that microsomal cells did not bind recombinant GH, suggesting 
a defect in the GHR.621 Both deletions and homozygous point

in the GHR.621 Both deletions and homozygous point 
mutations (missense, nonsense, and abnormal splicing) in the 
GHR have been described in persons with GH insensitivity.622–624 
Described gene deletions involve exons 3, 5, and 6.625 Deletion of 
exons 5 and 6 results in a frameshift and a premature translational 
stop signal with consequent encoding of a receptor lacking most 
of the extracellular GH-binding domain.
More than 70 mutations of the GHR gene resulting in GH 
insensitivity have bee

e GHR gene resulting in GH 
insensitivity have been reported to date.626 Most of the mutations 
are in the extracellular (GH-binding) domain of the receptor and 
result in impaired ability of GH to bind to the receptor; this also 
leads to a deficiency of circulating GHBP, which is derived from 
the extracellular domain of the receptor. One reported mutation 
of the extracellular domain does not affect GH binding to the 
receptor but prevents dimerization of the receptor.627 Homozy­
gous mutatio

erization of the receptor.627 Homozy­
gous mutations have also been reported in the transmembrane 
domain.628,629 These mutations result in GH insensitivity with 
normal GH binding but lack of receptor transduction. Because 
the extracellular domain is intact and the mutant receptor protein 
Girls
Height in cm
Age in months
50 percentile
5 percentile
95
90
85
80
75
70
65
60
55
50
45
40
0
3
6
9
12
15
18
21
24
27
30
33
36
Girls
Age in years
50
2
3
4
5
6
7
8

years
50
2
3
4
5
6
7
8
9
10
12
14
15
16
170
160
150
140
130
120
110
100
90
80
70
60
Boys
3 percentile
50
0 1 2 3 4 5 6 7 8 9 10 11 1213 1415 16 17 1819
170
180
160
150
140
130
120
110
100
90
80
70
60
• Fig. 25.35  Height measurements for Ecuadorian children with insulin-like growth factor (IGF) deficiency 
resulting from growth hormone (GH) insensitivity. (From Rosenfeld RG, Rosenbloom AL, Guevara-Aguirre J. 
Growth hormone [GH] 

bloom AL, Guevara-Aguirre J. 
Growth hormone [GH] resistance due to primary GH receptor deficiency. Endocr Rev. 1994;15:369–390.)
973
apparently becomes detached from the cell receptor surface, 
GHBP levels are normal to elevated.
Mutations affecting the intracellular domain of the GHR also 
occur; these directly involve the intracellular domain leading to 
dominantly inherited GH insensitivity.630–636 In the heterozygous 
mutations reported, tr

30–636 In the heterozygous 
mutations reported, truncation of the GHR results in absence of 
the intracellular domain. In vitro, this truncated GHR molecule 
behaves in a dominant negative manner, presumably by retaining 
an ability to dimerize with the normal GHR and thereby inhibit­
ing GH-induced tyrosine phosphorylation of STAT5. Mutations 
resulting in C-terminal deletions of the intracellular domain of the 
GHR exhibit normal GH binding and JAK2 phosphorylation but 
impaired phosphorylatio

 JAK2 phosphorylation but 
impaired phosphorylation of STAT5B.637,638
That a dominant negative effect has been described for some 
mutations raises the question of whether heterozygosity for defects 
of the extracellular domain can also result in short stature. Het­
erozygosity for defects of GHR has been reported to cause some 
degree of relative GH insensitivity, with modest growth improve­
ment occurring only with high doses of GH.639–642 In addition, 
a truncated GHR splice variant has been 

ddition, 
a truncated GHR splice variant has been described that functions 
as a dominant negative inhibitor of the full-length receptor and 
results in large amounts of GHBP, further downregulating GHR 
function.643
The most extensively studied polymorphism in the GHR gene 
is the deletion of exon 3 (GHRd3), which is present in up to 50% 
of Caucasians. It has been proposed that GHRs without exon 3 
bind GH with comparable affinity644,645 but may transduce the 
signal with a different intensity

y transduce the 
signal with a different intensity in vitro.644,645 Upon GH treat­
ment, children with GHD, TS, or SGA with one or two copies 
of the GHRd3 variant grow faster than those without the vari­
ant after correction for GH dosing. However, different studies 
reported different findings on whether GH-deficient children 
with the GHRd3 variant experience faster growth.646–648 Different 
GH dosing among studies and differences among the populations 
studied could account for the varying r

pulations 
studied could account for the varying results.
Some patients with the phenotype of GH insensitivity but 
without mutations of the GHR gene may have mutations in 
downstream GHR signaling molecules. Homozygous mutations 
in the Stat5B gene cause GH insensitivity either by decreasing 
the phosphorylation of tyrosine649 with resultant inability to dock 
with phosphotyrosines on GH-activated receptors, or to stably 
bind DNA,650 or by causing an insertion in exon 10 leading to 
early prot

ing an insertion in exon 10 leading to 
early protein termination.651 A mutation in the SH2 domain of 
Stat5b has also been described, leading to an inability to induce 
gene transcription.652 Patients with mutations in Stat5b have 
immune dysfunction and recurrent pulmonary infections, as 
Stat5B is involved in downstream signaling for multiple cytokines. 
Heterozygous STAT5b variants in the DNA-binding domain or 
the coiled-coil domain can lead to disruption of GHR signal­
ing.653 The mutated 

 to disruption of GHR signal­
ing.653 The mutated protein acts in a dominant negative manner 
because it can dimerize with wild-type STAT5b but cannot bind 
DNA to induce transcription. Affected patients had short stature, 
but immune dysfunction is not as pronounced as in persons with 
homozygous mutations in STAT5b.
Markedly reduced serum concentrations of IGF1 and IGFBP3 
are present in persons with mutation in the ALS gene.654 IGF1 
and IGFBP3 are low as in patients with classic GH insensiti

3 are low as in patients with classic GH insensitivity, 
but the height deficit is less severe, with reported patients with 
heights between –3.6 and –0.3 SDS.655 Affected male patients 
have delayed puberty and varying degrees of bone mineral density. 
Insulin insensitivity is present. Found mutations in the IGFALS 
gene are homozygous or compound heterozygous, all in exon 2. 
Since ALS levels are undetectable in affected patients, it is believed 
that the mutation prevents the protein from bei

d 
that the mutation prevents the protein from being expressed, 
secreted, or stable in serum.655 Whether the relatively normal 
growth of these patients reflects the greater importance of locally 
produced IGF1 or altered kinetics of serum IGF1 in the face of 
reduced concentrations of binding proteins remains uncertain. 
The mechanism underlying the observed insulin resistance is also 
unknown but is postulated to be due to GH excess.655 
Abnormalities of IGF1 and IGF1 Receptor Signaling. Wood

malities of IGF1 and IGF1 Receptor Signaling. Woods 
and colleagues656 described a 15-year-old boy with deletion 
of exons 4 and 5 of the IGF1 gene that resulted in a truncated 
IGF1 molecule. The boy exhibited severe prenatal and postna­
tal (approximately –7 SDS) growth retardation unresponsive to 
GH in addition to sensorineural deafness, mental retardation, 
and microcephaly. He had normal IGFBP3 and GHBP levels, 
  Clinical Features of Growth Hormone 
Insensitivity
Parameter
Clinical Findi

th Hormone 
Parameter
Clinical Finding
Birth weight
Near-normal
Birth length
May be slightly decreased
Postnatal growth
Severe growth failure
Bone age
Delayed, but may be advanced relative to 
height age
Genitalia
Micropenis in childhood; normal for body 
size in adults
Puberty
Delayed 3 to 7 years
Sexual function and fertility
Normal
Craniofacies
Hair
Sparse before age 7 years
Forehead
Prominent; frontal bossing
Skull
Normal head circumference; craniofacial 

ing
Skull
disproportion due to small facies
Facies
Small
Nasal bridge
Orbits
Shallow
Dentition
Delayed eruption
Sclerae
Blue
Voice
High pitched
Musculoskeletal/Metabolic/Miscellaneous
Blood glucose
Hypoglycemia in infants and children; 
fasting symptoms in some adults
Walking and motor mile­
stones
Delayed
Hips
Dysplasia; avascular necrosis of femoral 
head
Elbow
Limited extensibility
Skin
Thin, prematurely aged
Bone mineral density
Osteopenia

, prematurely aged
TABLE 25.4
974
undetectable IGF1 levels, and hyperinsulinism. On treatment 
with IGF1, the child experienced growth and improved metabolic 
parameters.657 In a kindred of members with height SDS –4.0 
and low but detectable IGF1, heterozygous mutation in the IGF1 
gene caused splicing of exon 4 and resultant protein truncation.658 
A child with height of –2.7 SDS and an IGF1 level in the low 
normal 

 of –2.7 SDS and an IGF1 level in the low 
normal range was found to have a 260-kb heterozygous deletion 
of chromosome 12, which includes the IGF1 gene.659 
Inactivating Mutation of the IGF1 Gene. An adult with the 
same phenotype as the boy with the IGF1 deletion but with mark­
edly elevated serum IGF1 levels660 had a homozygous point muta­
tion in the IGF1 gene. This mutation resulted in an IGF1 molecule 
with markedly reduced affinity for the IGF1 receptor that poorly 
stimulated autophospho

 IGF1 receptor that poorly 
stimulated autophosphorylation of the IGF1 receptor and activa­
tion of AKT or ERK.661 Family members heterozygous for this 
mutation have significantly lower birth weight, final height, and 
head circumference, suggesting an effect of heterozygosity for this 
mutation on IGF1 function. 
Primary Defects of IGF Transport and Clearance. Siblings 
from two families with high IGF1 and IGF2 levels, normal to high 
IGFBP3 levels, normal ALS levels, and height SDS scores bet

vels, normal ALS levels, and height SDS scores between 
–2.2 and 1.06 were found to have mutation in the pregnancy-
associated plasma protein A2 (PPAP-A2) gene.298 The described 
mutations generate truncated proteins that cannot cleave IGFBP3 
or IGFBP5. Because PAPP-A2 cleavage of IGFBP3 releases IGF1 
from the ternary complex, free IGF1 levels in the affected persons 
were low. Serum from affected patients could not stimulate IGF1 
receptor in vitro, further providing evidence of the loss of f

vitro, further providing evidence of the loss of function 
induced by the mutation. That the IGF1 levels were elevated sug­
gests that GH signaling was amplified. This amplification of GH 
signaling may account for the mild degree of height deficit seen in 
these patients compared to those with GH receptor defects. Treat­
ment of the children with this mutation with rIGF1 resulted in 
increased growth rate after 1 year.662 rIGF1 treatment’s effect on 
adult height is unknown. 
Primary Defects of

t on 
Primary Defects of IGF1 Receptor Production or Responsiveness. 
Patients with IUGR and postnatal growth failure, microcephaly, 
and mental retardation with normal to elevated serum IGF1 lev­
els have been reported to have mutations in the IGF1 receptor 
gene leading to reduced binding of IGF1 to its receptor.663,664 
Fang and associates published a report of a patient with a mis­
sense mutation in IGF1R and height –5.9 SDS, elevated IGF1 
levels, and resultant dec

–5.9 SDS, elevated IGF1 
levels, and resultant decreased activation of downstream signaling 
pathways.665
In leprechaunism, a syndrome of growth failure and insulin 
receptor dysfunction, there is variable IGF1 insensitivity.666,667 The 
profound abnormality of the insulin receptor suggests that het­
erodimeric insulin and IGF1 receptor combinations might lead to 
failed activation of the IGF1 signaling cascade. The IGF1 receptor 
gene resides on 15q, so persons with deletions of the distal long

 15q, so persons with deletions of the distal long arm 
of chromosome 15 or ring chromosome 15 have heterozygosity of 
the IGF1 receptor.667,668 These patients may have IUGR and post­
natal growth failure, but lack of a biologic response to IGF1 has not 
been conclusively demonstrated.668 Therefore whether the growth 
failure is caused by altered levels of IGF1 receptor or is a result of 
the loss of other genes located on 15q remains to be determined. 
Disorders Outside the Growth Hormone–IGF 

mined. 
Axis
Many systemic disorders, if severe enough, can cause growth fail­
ure in children. Those that primarily alter hormones that directly 
regulate growth (e.g., thyroid hormone, glucocorticoids) can be 
understood based on the known actions of those hormones. Even 
in those disorders in which the pathology is not primarily within 
the endocrine system, there is often an underlying hormonal 
abnormality contributing to the growth failure. In some

mality contributing to the growth failure. In some cases, the 
underlying disorder produces a secondary hormone deficiency. 
Those disorders in which a hormone deficiency cannot be iden­
tified may be thought of as demonstrating hormone resistance, 
because these children have growth failure in the presence of nor­
mal GH production.
Malnutrition
Given the worldwide presence of undernutrition, inadequate 
intake of energy (calories), protein, or both is the most common 
cause of growth failure. 

both is the most common 
cause of growth failure. Marasmus refers to an overall deficiency 
of calories, including protein malnutrition. Subcutaneous fat is 
minimal, and protein wasting is marked. Kwashiorkor refers spe­
cifically to inadequate protein intake, although it may also be 
characterized by some caloric undernutrition. Frequently, the two 
conditions overlap. Decreased weight growth usually precedes 
the failure of linear growth by a very short time in the neonatal 
period and by sev

very short time in the neonatal 
period and by several years at older ages. Stunting of growth due 
to caloric or protein malnutrition in early life often has lifelong 
consequences, including diminished skeletal growth.669
Both acute and chronic malnutrition affect the GH-IGF1 sys­
tem. The impaired growth seen in malnutrition is usually associated 
with elevated basal or stimulated serum GH levels,670,671 although 
in some cases of generalized malnutrition (marasmus), the GH lev­
els are norma

malnutrition (marasmus), the GH lev­
els are normal or low.672 In both conditions, serum IGF1 levels are 
reduced.671,673 The increase in GH levels is caused by a decrease in 
negative feedback by IGF1 and a decrease in somatostatin tone.674 
Malnutrition also results in increased ghrelin levels,675,676 which 
could also contribute to an increase in GH secretion, although the 
role of ghrelin in regulating GH secretion remains unclear.677 With 
serum IGF1 levels reduced despite normal or elevate

erum IGF1 levels reduced despite normal or elevated GH levels, 
malnutrition is a form of GH insensitivity.671 One cause of this 
insensitivity is a decrease in GHR expression, which is reflected in 
decreased serum GHBP levels.670,678 In addition, fasting increases 
expression of FGF21, which may cause GH insensitivity by 
inhibiting STAT5.679 This GH insensitivity may be an adaptive 
response, diverting scarce energy resources from growth toward 
use for acute metabolic needs. The low IGF1 min

d 
use for acute metabolic needs. The low IGF1 minimizes stimula­
tion of anabolism, whereas the direct actions of the elevated GH 
levels (e.g., lipolysis, insulin antagonism) may increase the avail­
ability of energy substrates.671,680,681 These adaptive mechanisms 
are accompanied by changes in serum IGFBPs that further limit 
IGF action during periods of malnutrition.670,682
Inadequate calorie or protein intake complicates many chronic 
diseases that are characterized by growth failure. Anor

ses that are characterized by growth failure. Anorexia is a 
common feature of renal failure and inflammatory bowel disease, 
and it also occurs with cyanotic heart disease, congestive heart 
failure, CNS disease, and other illnesses. Some of these conditions 
may be further characterized by deficiencies of specific dietary 
components, such as zinc, iron, and vitamins necessary for normal 
growth and development.
Undernutrition may also be voluntary, as occurs with diet­
ing and food fads (Fig.

tary, as occurs with diet­
ing and food fads (Fig. 25.36).682 Caloric restriction is especially 
common in girls during adolescence, in whom it may be associ­
ated with anxiety concerning obesity, and in gymnasts and ballet 
dancers. Anorexia nervosa and bulimia are extremes of “voluntary” 
caloric deprivation that are commonly associated with impaired 
growth before epiphyseal fusion, which may result in diminished 
975
Stature (cm)
Weight (kg)

errant Growth in Children
180
170
160
150
140
130
120
110
100
90
80
70
2
4
6
8
10
12
14
16
18
90
80
70
60
50
40
30
20
10
• Fig. 25.36  Curves of weight and height for a child who had growth failure 
resulting from prolonged self-imposed caloric restriction due to a fear of 
becoming obese. Notice that crossing of percentiles on the weight curve 
preceded that on the height curve, and that, after caloric intake was nor­
malized (arrow), gain in weight occurred

 was nor­
malized (arrow), gain in weight occurred before improvement in linear 
growth. At the end of the prolonged period of caloric restriction, weight 
age (10.2 years) was less than height age (12 years). (From Pugliese MT, 
Lifshitz F, Grad G, et  al. Fear of obesity: a cause of short stature and 
delayed puberty. N Engl J Med. 1983;309:513–518. Reprinted by permis­
sion of the New England Journal of Medicine.)
final adult height.683,684 Adolescent bone mineral accretion is 
impaired, and 

olescent bone mineral accretion is 
impaired, and significant osteopenia may persist into adult­
hood.685 Later in adolescence, malnutrition may cause delayed 
puberty or menarche (or both), as well as a variety of metabolic 
alterations. In anorexia nervosa, hormonal profiles are similar to 
those in protein-energy malnutrition,683,684,686 with high basal 
levels of GH. However, in contrast to chronic critical illness, in 
which there is an increase in nonpulsatile secretion but a decrease 
in 

ease in nonpulsatile secretion but a decrease 
in pulsatile GH secretion, in anorexia both nonpulsatile and pul­
satile secretion of GH are increased.674 The GH secretion stimu­
lated by insulin-induced hypoglycemia, dopaminergic agents, or 
acute administration of dexamethasone is impaired in patients 
with anorexia nervosa, whereas clonidine and arginine elicit nor­
mal GH responses. Finally, a paradoxical increase in GH release 
to intravenous glucose infusion has been described.674 As in mal

 glucose infusion has been described.674 As in mal­
nutrition in general, low levels of IGF1 and IGFBP3 are found 
in anorexia nervosa, indicating GH resistance. Similarly, GHBP 
levels are decreased,674 indicating a decrease in GHR expression as 
a contributing factor to the GH resistance. The hormones of the 
GH-IGF1 axis return to normal levels with refeeding.670,683,687
A rare cause of failure to thrive in infants and young children 
is the diencephalic syndrome.688 This syndrome is characte

iencephalic syndrome.688 This syndrome is characterized 
by a marked impairment of weight gain, or even weight loss, 
but with normal linear growth (at least initially). It is caused by 
hypothalamic tumors. Similar to the findings in other causes of 
low weight for length or height, increased GH levels have been 
found in patients with diencephalic syndrome. As in patients with 
anorexia nervosa, GH levels paradoxically increase in response to 
a glucose load. However, in contrast to the increa

a glucose load. However, in contrast to the increased GH secretion 
seen in malnutrition or anorexia nervosa, IGF1 levels are normal 
rather than decreased.688 Therefore diencephalic syndrome does 
not demonstrate GH resistance in the same way as these other 
Chronic or Systemic Diseases
Malabsorption and Gastrointestinal Diseases
Intestinal disorders that impair absorption of calories or protein 
cause growth failure, for many of the same reasons as malnu­
trition per se.680,689,690

 same reasons as malnu­
trition per se.680,689,690 Growth retardation may predate other 
manifestations of malabsorption or chronic inflammatory bowel 
disease. Celiac disease (gluten-induced enteropathy) and regional 
enteritis (Crohn disease) should be considered in the differential 
diagnosis of unexplained growth failure. Serum levels of IGF1 
may be reduced,680,691 reflecting malnutrition, and it is critical 
to discriminate between these conditions and disorders related to 
GHD. Documentat

nditions and disorders related to 
GHD. Documentation of malabsorption requires demonstration 
of fecal wasting of calories, especially fecal fat, along with other 
measures of gut dysfunction such as the d-xylose or breath hydro­
gen studies.
In celiac disease, an immune-mediated disorder in which the 
intestinal mucosa is damaged by dietary gluten (Fig. 25.37), 
impaired linear growth may be the first manifestation of disease.680 
The degree of growth impairment may be similar in patients with

 growth impairment may be similar in patients with 
or without gastrointestinal symptoms.680 The incidence of child­
hood celiac disease in the United States is about 0.9%,692 but its 
Height (cm)
190
180
170
160
150
140
130
120
110
100
90
80
70
60
50
0
19
18
17
97
50
3
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
Gluten-
free diet
• Fig. 25.37  Catch-up growth in a girl with gluten-induced enteropathy 
(celiac disease). After 8 years of growth impairment, the patient was pl

r 8 years of growth impairment, the patient was placed 
on a gluten-free diet and demonstrated substantial catch-up growth, 
returning to the previous growth percentiles. (Courtesy J.M. Tanner.)
976
prevalence is approximately 5% in children being evaluated for 
short stature.693 Celiac disease also has an increased incidence in 
individuals with TS, insulin-dependent (type 1) diabetes mellitus 
(IDDM), Down syndrome, or Williams syndrome.693 The ons

), Down syndrome, or Williams syndrome.693 The onset 
and progression of puberty may be delayed, and menarche may be 
late.694 Measurement of an immunoglobulin A (IgA) tissue trans­
glutaminase antibody (tTG-IgA)695 is the currently recommended 
screening test. This test has a sensitivity of up to 99%, with 
much higher specificity than IgA gliadin antibodies (AGA-IgA). 
However, tTG-IgA has significantly lower sensitivity in children 
younger than 18 months of age,696 in whom it may be preferab

an 18 months of age,696 in whom it may be preferable 
to also measure AGA-IgA. When measuring IgA antibodies, it is 
necessary to measure total IgA levels to exclude IgA deficiency, 
particularly because IgA deficiency has an increased incidence in 
patients with celiac disease.697 (In subjects with IgA deficiency, 
tTG-IgG may be used.697) Nonetheless, the diagnosis of celiac 
disease ultimately requires demonstration of the characteristic 
mucosal flattening in small bowel biopsy.692 Gluten wi

sal flattening in small bowel biopsy.692 Gluten withdrawal is 
a highly effective treatment for celiac disease and results in rapid 
catch-up growth and decreased clinical symptoms during the first 
6 to 12 months of treatment.680,694 Low IGF1 and IGFBP3 lev­
els return to normal during this period.691,698 Most children who 
receive appropriate dietary management ultimately achieve a nor­
mal final height.699,700
Growth failure in Crohn disease, which correlates with dis­
ease severity,701 is pr

which correlates with dis­
ease severity,701 is probably due to a combination of malnutrition 
from malabsorption, anorexia, nutrient loss, chronic inflamma­
tion,690,701 inadequacy of trace minerals in the diet, and use of 
glucocorticoids. IGF1 levels are low, especially with impaired 
growth.680,690 In an animal model, approximately one-half of the 
decrease in IGF1 levels was accounted for by undernutrition, with 
the other half attributable to the effects of inflammation.702 One-
third to t

to the effects of inflammation.702 One-
third to two-thirds or more of children with Crohn disease have 
impaired growth at diagnosis. In some patients, the growth fail­
ure precedes clinical symptoms of bowel disease by a few years, 
with a significant number showing linear growth failure before 
any weight loss.680,690,701,703,704 Adequate nutritional supplemen­
tation and surgical resection of the diseased intestine can lead 
to improved growth, but surgery is not always an option.705–707 
Os

h, but surgery is not always an option.705–707 
Osteopenia is common.708,709 An elevated erythrocyte sedimenta­
tion rate, anemia, and low serum albumin are useful clues, but 
diagnosis of Crohn disease ultimately requires colonoscopy and 
biopsy, along with gastrointestinal imaging studies. Permanent 
impairment of linear growth and deficits of final height may occur 
in 30% of patients.710 Approximately 20% of patients at adult 
height are more than 8 cm below the midparental target height.711

 than 8 cm below the midparental target height.711 
Small, mostly uncontrolled trials of GH treatment in children 
with Crohn disease have had conflicting results: Some have shown 
improved growth velocity,712,713 but others have not.714 Some 
have shown improved body composition and BMD.713 None of 
the reports, however, has extended beyond 2 years of treatment to 
determine whether there is a long-term benefit. 
Chronic Liver Disease
Chronic liver disease in childhood can cause impaired linear

ver disease in childhood can cause impaired linear 
growth. Decreased food intake, fat and fat-soluble vitamin mal­
absorption, and trace element deficiencies contribute to growth 
failure.715–718 In addition, these children show evidence of GH 
resistance, having decreased levels of IGF1 and IGFBP3 and 
increased GH secretion.719–722 IGF2 levels are also decreased, and 
IGFBP1 levels are increased.719–721 Although the low IGF1 levels 
might be due to the impaired synthetic capacity of the liver

ue to the impaired synthetic capacity of the liver, there 
is decreased expression of the GHR in cirrhotic liver, just as in 
malnutrition.723 However, despite provision of adequate calories, 
insensitivity to the action of GH persists,718,720 suggesting that 
the GH resistance of liver failure is not due solely to malnutri­
tion. Liver transplantation prolongs life expectancy, and linear 
growth is variably improved in the early post-transplantation 
years.717,721,724,725 Exogenous glucocortico

tion 
years.717,721,724,725 Exogenous glucocorticoid administration 
presumably plays a major role in the continued growth retarda­
tion717,725; GH and IGF1 production are normal, but the amount 
of “free IGF” may be decreased because IGFBP3 levels are rela­
tively high.721,724 Post-transplantation growth is inversely corre­
lated with age and directly correlated with the degree of growth 
impairment at transplantation.716,717,724 Exogenous GH treat­
ment enhances growth rates and increases medi

eat­
ment enhances growth rates and increases median height SDS by 
0.3 to 0.6 unit after 1 year of treatment,472,726,727 with continued 
increases in height SDS for up to 5 years during treatment.472 
Cardiovascular Disease
Congenital heart disease with cyanosis or chronic congestive heart 
failure can cause growth failure.728–730 Although some children 
with congenital heart disease will have growth failure related to 
an underlying genetic disorder, the disease itself can cause growth 
failur

order, the disease itself can cause growth 
failure. The growth failure in children with CHD is often due 
to malnutrition, which can have a number of causes in children 
with CHD. Frequently this is due to inadequate calorie intake due 
to feeding difficulties associated with the heart disease.728,729,731 
In addition, chronic congestive heart failure is associated with 
malabsorption that includes protein-losing enteropathy, intes­
tinal lymphangiectasia, and steatorrhea. Greater cardiac and 

ngiectasia, and steatorrhea. Greater cardiac and 
respiratory work requirement and the relatively higher ratio of 
metabolically active, energy-utilizing brain and heart tissue to the 
growth-retarded body mass (cardiac cachexia) cause an increased 
basal metabolic rate in these children.732,733 Thus food intake that 
appears adequate for the child’s weight is often inadequate for nor­
mal growth. Finally, in children with cyanotic CHD, the hypox­
emia impairs cellular metabolism and growth. Dec

­
emia impairs cellular metabolism and growth. Decreased levels of 
IGF1 and IGFBP3733–735 and normal levels of GH and hepatic 
GHRs in chronically hypoxemic newborn sheep735 suggest GH 
insensitivity distal to the GHR.
In the past, up to 30% of children with congenital heart dis­
ease had heights and weights that fell below the 3rd percentile for 
age.730 In the developing world, up to 90% of children with CHD 
continue to suffer growth failure.729 In contrast, in the developed 
world the impac

729 In contrast, in the developed 
world the impact of CHD on the growth of children has been 
nearly eliminated owing to the ability to make an early diagno­
sis, as well as on improvements in supportive care and the early 
surgical correction of these lesions. The nutritional management 
of these infants before surgical correction includes the use of cal­
orie-dense feedings because of the need to restrict fluids, calcium 
supplementation because of the use of diuretics that can cause cal­
ciu

se of the use of diuretics that can cause cal­
cium loss in the urine, and iron to maintain an enhanced rate of 
erythropoiesis. Early surgical correction restores normal growth, 
frequently after a phase of catch-up growth with normalization of 
energy expenditure.728,730,732,736 
Renal Disease
All conditions that impair renal function can impair growth.737–740 
Uremia and renal tubular acidosis can cause growth failure before 
other clinical manifestations become evident. The growth impair­
me

nifestations become evident. The growth impair­
ment results from multiple mechanisms, including inadequate 
formation of 1,25-dihydroxycholecalciferol (1,25-dihydroxyvi­
tamin D3, calcitriol) with resultant osteopenia, decreased caloric 
977
intake, loss of electrolytes necessary for normal growth, meta­
bolic acidosis, protein wasting, insulin resistance, chronic ane­
mia, compromised cardiac function, and impairment of GH and 
IGF production 

unction, and impairment of GH and 
IGF production and action. In nephropathic cystinosis, acquired 
hypothyroidism contributes to the inadequate growth.741 Between 
60% and 75% of patients with chronic renal failure treated before 
the GH therapeutic era had final adult heights more than 2 SD 
below the mean.742
The effects of renal failure on the GH-IGF1 axis are complex, 
and there is evidence for both GH and IGF1 resistance. Chil­
dren and adolescents have normal or elevated circulating level

lescents have normal or elevated circulating levels of 
GH, depending on the degree of renal failure.737,738,743–745 The 
increased GH levels result from both an increase in GH secre­
tory bursts and a decrease in renal GH clearance.746 Serum IGF1 
and IGF2 levels are usually normal in patients with renal fail­
ure.737,745,747,748 Early reports of decreased serum IGF levels in 
uremia were an artifact caused by inadequate separation of IGF 
from IGFBPs before assay.749 However, the normal IGF1 l

IGFBPs before assay.749 However, the normal IGF1 levels in 
the face of elevated GH levels denotes GH resistance, which is also 
indicated by the finding of decreased hepatic IGF1 production.750 
The mechanism for GH resistance includes decreased GHR gene 
expression in the liver and in the growth plate.747,751 There is also 
evidence that the uremic state causes a postreceptor defect in GH 
signal transduction by diminishing phosphorylation and nuclear 
translocation of GH-activated STAT protei

nuclear 
translocation of GH-activated STAT proteins.746,752 Although a 
defect in IGF1 receptor signaling has been demonstrated in renal 
failure,751 the more important mechanism for decreased IGF1 
action in renal failure is alterations in the serum level of IGFBPs 
that decrease the bioavailability of IGF1. IGFBPs 1, 2, 4, and 6 
are increased.737,738,751,753–756 In addition, low-molecular-weight 
IGFBP3 fragments, which have decreased affinity for IGF1, accu­
mulate as a result of reduced re

y for IGF1, accu­
mulate as a result of reduced renal clearance.751 In nephrotic syn­
drome, an additional contribution to growth failure may come 
from reduced serum levels of IGF1 and IGFBP3 resulting from 
urinary loss of IGF-IGFBP complexes.739 Finally, glucocorticoid 
therapy that may be used for treatment of the renal disease can 
exacerbate growth retardation by diminishing GH release and 
blunting IGF1 action at growth plates.755,757,758
After successful renal transplantation, growth may

After successful renal transplantation, growth may completely 
return to normal.759,760 Based on data from the large cohort 
of patients in the North American Pediatric Renal Transplant 
Cooperative Study (NAPRTCS), children who underwent trans­
plantation before 6 years of age showed catch-up growth for the 
subsequent 1 to 2 years, followed by a plateauing of their growth 
rate.759 In contrast, children older than 6 at the time of transplant 
have not shown catch-up growth; their final height 

ave not shown catch-up growth; their final height is determined 
by their height at the time of transplantation.759 In spite of this, 
final height in children requiring renal transplant has improved 
dramatically in the past 25 years so that the final heights of chil­
dren in the NAPRTCS registry that were transplanted in the years 
2002 to 2010 had a final height of –0.94 SDS, compared with 
final heights averaging –1.93 SDS in those transplanted between 
1987 and 1991. This improvement appear

ed between 
1987 and 1991. This improvement appears to be driven almost 
completely by the improved height at the time of transplantation: 
–1.5 SDS in 2009 versus –2.5 SDS in 1987.759 The importance 
of height at the time of transplantation in determining final adult 
height, despite the complex post-transplantation health issues, 
confirms the value of improving growth velocity and absolute 
height before transplantation. Additional factors that impact 
growth after transplantation include the

t impact 
growth after transplantation include the function of the allograft 
and the use of glucocorticoids in the immunosuppressive regimen. 
Immunosuppressive regimens with alternate-day glucocorticoid 
treatment, rather than daily treatment,760,759 utilizing a glucocor­
ticoid withdrawal approach759,761 and regimens that avoid gluco­
corticoids have all been associated with improved post-transplant 
growth rates.759 There are conflicting data on whether sirolimus 
impairs growth in compariso

 on whether sirolimus 
impairs growth in comparison to tacrolimus.759 Growth-retarded 
post-transplantation children receiving daily or alternate-day glu­
cocorticoid treatment have decreased GH secretion, normal levels 
of IGF1 and IGFBP1, and elevated levels of IGFBP3. They differ 
from patients with end-stage renal disease in that IGFBP1 levels are 
not strikingly elevated, perhaps because of altered glucose tolerance 
and hyperinsulinism due to chronic glucocorticoid therapy.755 
Hematologic

to chronic glucocorticoid therapy.755 
Hematologic Disorders
Chronic anemias, such as sickle cell disease, are characterized by 
growth failure.762 In general, the decrease in height and weight is 
greater in adolescent years than earlier because the onset of the 
adolescent growth spurt is delayed and menarche is late.762–764 
However, the adolescent growth and final adult height in patients 
with sickle cell disease may be normal.764 The causes of growth 
retardation probably include suboptima

 of growth 
retardation probably include suboptimal nutrition and hypogo­
nadism.765 Impaired oxygen delivery to tissues, increased work of 
the cardiovascular system, and energy demands of increased hema­
topoiesis are likely contributors to impaired nutrition. Long-term 
chronic transfusion therapy as part of stroke prevention treatment 
is associated with enhanced growth.766 The GH-IGF1 system 
probably does not have a primary role in the growth impairment 
of sickle cell anemia, although abn

th impairment 
of sickle cell anemia, although abnormalities in the GH-IGF1 sys­
tem have been described.763
In thalassemia, in addition to the consequences of chronic ane­
mia, endocrine deficiencies can result from chronic transfusions 
and accompanying hemosiderosis.767 Despite vigorous efforts to 
maintain hemoglobin levels near normal and to avoid iron over­
load, growth failure is still a common feature of thalassemia, 
especially in male adolescents.768 The patients tend to show body 
dis

dolescents.768 The patients tend to show body 
disproportion, with truncal shortening but normal leg length. It 
is likely that anemia, impaired IGF1 synthesis, hypothyroidism, 
gonadal failure, and hypogonadotropic hypogonadism all contrib­
ute to growth failure in this disorder. GH insensitivity in some 
cases is suggested by generally adequate GH production with low 
IGF1 levels.769,770 Several groups have reported data on treatment 
of thalassemia patients in whom GH production seemed dimin­

semia patients in whom GH production seemed dimin­
ished. In most patients, GH treatment increased growth, at least 
initially.769,771 In a longer term study (average duration, 59 months) 
starting with young patients (7.2 years of age), an increased growth 
velocity was maintained throughout the treatment period772; when 
treatment was initiated at an older age (13.6 years), final height was 
not improved.773 A small number of adults with thalassemia are 
found to have continued GHD, so GH trea

semia are 
found to have continued GHD, so GH treatment for cardiac and 
bone health reasons may be important in this disease.774
About half of the patients in the International Fanconi Anemia 
Registry have short stature. GHD was demonstrated by provoca­
tive testing (22 of 48 patients) or with assessment of endogenous 
secretion (13 of 13) in a group with a mean height of –2.23 SDS.775 
In pure red blood cell aplasia,776 approximately 30% of patients 
demonstrated growth retardation. The frequ

tients 
demonstrated growth retardation. The frequency increased with 
age (42% in individuals >16 years) and with treatment programs 
such as chronic transfusions or glucocorticoids. 
Inborn Errors of Metabolism
Inborn errors of metabolism are often accompanied by growth 
failure that may be pronounced. Glycogen storage disease, the 
mucopolysaccharidoses, glycoproteinoses, and mucolipidoses 
978
are characterized by poor growth. Many inborn metabol

 characterized by poor growth. Many inborn metabolic dis­
orders are also associated with significant skeletal dysplasia. In a 
small number of patients with organic acidoses, such as meth­
ylmalonic and propionic acidurias, IGF1 levels are low and GH 
levels are normal, suggesting a possible state of GH insensitivity 
related to nutritional status.777 Preliminary data suggest that exog­
enous GH treatment may improve the metabolic status of such 
children.777,778 
Pulmonary Disease
Growth can b

Growth can be retarded in children with asthma, including 
those who have not received glucocorticoid therapy. The lowered 
growth in asthmatic children does not appear to be associated with 
abnormalities of the GH-IGF1 axis.779 Impaired nutrition and 
increased energy requirements, along with chronic stress, espe­
cially with nocturnal asthma and enhanced endogenous gluco­
corticoid production, cause poor linear growth. However, despite 
this growth impair

inear growth. However, despite 
this growth impairment, there is at most only a small difference 
in final height achieved by those with asthma, with the majority 
of the growth failure due to a delay in puberty.780 Systemic glu­
cocorticoid therapy, which is rarely required, can be expected to 
impair growth. Intermittent glucocorticoid therapy is usually not 
associated with impaired final height. Alternate-day or aerosolized 
glucocorticoid therapy often ameliorates growth retardation and 
ca

erapy often ameliorates growth retardation and 
can be associated with an accelerated catch-up phase.781,782 The 
large Childhood Asthma Management Program investigated the 
effect of inhaled corticosteroids (ICS) on the growth of children 
with asthma. Although growth rate declined with the initiation of 
ICS, it subsequently normalized, and the final height in children 
treated with ICS for 4 to 6 years was only 1.2 cm below that 
in control subjects.783 ICS can, however, suppress the hypotha­

bjects.783 ICS can, however, suppress the hypotha­
lamic-pituitary axis, and investigation for this is appropriate in 
children who have a more marked suppression of growth with ICS 
treatment.780
Bronchopulmonary dysplasia (BPD), a sequela of neonatal 
respiratory distress syndrome and prematurity, has an incidence as 
high as 35% in infants with very low birth weight (<1500 g).784 The 
use of dexamethasone in the treatment of BPD in neonates causes 
a transient cessation of growth785 and has e

uses 
a transient cessation of growth785 and has engendered long-term 
concern for neurodevelopment and somatic growth.786 Growth 
in surviving infants is poor through early childhood,787–789 but 
the defect usually disappears by 8 years of age.790–792 Long-term 
hypoxemia, poor nutrition, chronic pulmonary infections, and 
reactive airway disease are responsible for the poor early growth.
In patients with cystic fibrosis (CF), chronic pulmonary infec­
tion with bronchiectasis, pancreatic insuff

infec­
tion with bronchiectasis, pancreatic insufficiency with exocrine 
and endocrine inadequacy, malabsorption, and malnutrition all 
contribute to decreased growth and late sexual maturation. Over 
20% of individuals with CF under 25 years of age will have 
heights or weights below the 10th percentile.793 With newborn 
screening for CF, the failure to thrive that was a common present­
ing feature is now less likely to be seen, although these infants still 
demonstrate some growth failure.794 

nfants still 
demonstrate some growth failure.794 Infants who are not iden­
tified by newborn screening often show marked growth failure 
before diagnosis followed by catch-up growth after diagnosis. The 
linear growth rate during childhood is generally normal before 
puberty,795 followed by delayed puberty, including a delayed and 
attenuated pubertal growth spurt. Adult height in individuals 
with CF is slightly decreased compared with population norms 
by approximately –0.2 to –0.7 SD.795–797

on norms 
by approximately –0.2 to –0.7 SD.795–797 The GH-IGF1 axis 
in CF patients shows evidence for some degree of acquired GH 
insensitivity with lowered mean IGF1 and elevated GH levels.798 
The degree of growth retardation is related most closely to the 
severity and variability of the pulmonary disease rather than to 
pancreatic dysfunction.799,800 While the degree of steatorrhea does 
not correlate well with growth impairment, improved nutrition 
programs do enhance the overall clinical 

trition 
programs do enhance the overall clinical picture.801,802 Endocrine 
abnormalities, such as failure of both alpha and beta islet cells 
with decreased glucagon and insulin production, do not seem to 
influence prepubertal growth patterns in children with CF. Altera­
tions of vitamin D metabolism, while potentially affecting skeletal 
mineralization, do not diminish growth.803
There is the potential for the anabolic effects of GH treatment 
to improve the health in patients with CF. A num

to improve the health in patients with CF. A number of short-
term trials of up to 12 months with GH treatment have been 
reported.793,804,805 These studies have demonstrated that GH 
treatment of CF patients increases height, weight, and lean body 
mass. In addition, some measures of lung function are slightly 
improved, notably forced vital capacity, although this improve­
ment did not always exceed that expected based on the improved 
growth.805 Although some studies have demonstrated a dec

.805 Although some studies have demonstrated a decreased 
rate of hospitalization in GH-treated patients, there has not been 
clear evidence of a decrease in pulmonary exacerbations.805,804 
These studies, as well as reports of uncontrolled treatments of up 
to 4 years, have not raised serious safety concerns of GH treat­
ment in CF patients. Specifically, there has not been an increase in 
glucose intolerance or diabetes mellitus, although in some studies 
fasting glucose levels were increased.

me studies 
fasting glucose levels were increased.804–806 Although the current 
data demonstrate the possibility that GH treatment will benefit 
patients with CF, there are not yet sufficient data to indicate that 
GH treatment improves long-term health outcomes in these 
patients. 
Chronic Inflammation and Infection
Poor growth is a characteristic feature of chronic inflammatory 
disease and recurrent serious infection. The impaired growth 
associated with such disorders as Crohn disease, CF, a

ciated with such disorders as Crohn disease, CF, and asthma, 
in which inflammatory processes may be significant, has already 
been discussed. Inflammatory states are associated with increased 
levels of numerous cytokines. Interleukin 6 (IL6), specifically, has 
been implicated in this growth impairment. De Benedetti and col­
leagues,807 studying juvenile rheumatoid arthritis in humans and 
in a transgenic murine model expressing excessive IL6, demon­
strated an IL6-mediated decrease in IGF1 pr

demon­
strated an IL6-mediated decrease in IGF1 production. IL6 has 
been demonstrated to activate SOCS3; this provides a pathway 
for inflammation to inhibit IGF1 production because SOCS3 is 
a negative regulator of the JAK2/STAT5 GH signal transduction 
pathway.702,808,809 IL6 may also cause a decrease in serum IGF1 
levels by increasing its clearance through a decrease in IGFBP3 
levels.702 In addition, cytokines can affect the endocrine system at 
many other levels,810,811 impairing mineral 

m at 
many other levels,810,811 impairing mineral and nutrient metabo­
lism and the growth and remodeling of bone.812
Exposure to human immunodeficiency virus (HIV) in chil­
dren and adolescents occurs through perinatal transmission, blood 
transfusions, drug usage, and sexual contact. Growth failure is a 
cardinal feature of childhood acquired immunodeficiency syn­
drome (AIDS).813–817 However, HIV-infected infants and children 
show growth failure even before demonstrating severe immune 
dysfu

ure even before demonstrating severe immune 
dysfunction.818,819 Weight, length, and head circumference are 
all affected, although weight-for-height may be normal.815,817,820 
Before the era of highly active antiretroviral therapy (HAART), 
height growth velocity was associated with survival, indepen­
dent of either CD4+ T-cell lymphocyte count or viral load, with 
HAART therapy normalizing growth in most studies.820 Studies of 
979
the GH-IGF1

Normal and Aberrant Growth in Children
the GH-IGF1 axis in HIV-infected children have shown evidence 
of decreased GH secretion, GH resistance, and IGF1 resistance; 
both normal and low levels of GH and IGF1 are seen.818 Lipo­
dystrophy associated with HAART therapy occurs in children, 
although less commonly than in adults, and decreased GH secre­
tion has been demonstrated in HIV-associated lipodystrophy,818 
potentially contributing to impaired growth. HIV-infected chil­
dren frequently have 

d growth. HIV-infected chil­
dren frequently have delayed puberty, which could contribute to 
their linear growth failure. In a short-term treatment trial with 
standard doses of GH, height and weight growth increased and 
protein catabolism diminished, without any adverse effect on viral 
burden.821 
Endocrine Disorders
Untreated severe congenital hypothyroidism results in profound 
growth failure. With proper treatment, however, children with 
congenital hypothyroidism reach a h

children with 
congenital hypothyroidism reach a height appropriate for their 
genetic potential.822
Acquired hypothyroidism during childhood may also result in 
growth failure that can range from subtle to profound, depending 
on the severity and duration of the hypothyroidism. Growth fail­
ure may be the most prominent manifestation of hypothyroidism 
in children.823 The poor growth is more apparent in height than 
in weight gain, so these children tend to be overweight for height. 
Rivkees an

dren tend to be overweight for height. 
Rivkees and coworkers823 reported a mean 4.2-year delay between 
the slowing of growth and the diagnosis of hypothyroidism; at 
diagnosis, girls were 4.04 SD below and boys 3.15 SD below 
the mean height for their age. Skeletal maturation is delayed in 
those children in whom the hypothyroidism was sufficient to 
retard growth, with the bone age at diagnosis corresponding to 
the age at onset of the hypothyroidism.823 Body proportion is 
immature, with an 

roidism.823 Body proportion is 
immature, with an increased upper-body to lower-body segment 
ratio. Although chronic hypothyroidism is usually associated with 
delayed puberty, precocious puberty and premature menarche can 
occur in hypothyroid children (see Chapter 26).
In those children with severe growth failure, treatment with 
thyroid hormone results in rapid catch-up growth. This growth 
is typically accompanied by marked skeletal maturation. In cases 
of prolonged severe hypothyroidism, 

on. In cases 
of prolonged severe hypothyroidism, the advancement of skeletal 
maturation with treatment can exceed the growth acceleration, 
resulting in a compromised adult height.823 The deficit in adult 
stature correlates with the duration of hypothyroidism before ini­
tiation of treatment. Catch-up growth may be particularly com­
promised if therapy is initiated near puberty.824
As expected, hyperthyroidism has effects on growth opposite 
to those of hypothyroidism: It results in accelerat

o those of hypothyroidism: It results in accelerated growth and 
epiphyseal maturation. Children with hyperthyroidism present 
with an increased height and advanced bone age. In neonates, 
hyperthyroidism can result in craniosynostosis. However, despite 
the advanced bone age at diagnosis, the final height of children 
treated for hyperthyroidism remains normal in relation to genetic 
potential.825 
Diabetes Mellitus
Although weight loss may occur immediately before the onset of 
clinically appa

r immediately before the onset of 
clinically apparent IDDM, children with new-onset diabetes are 
frequently taller than their peer group, possibly because GH and 
insulin levels are increased during the preclinical evolution of the 
disease.826–828 Most children with IDDM, even those with mar­
ginal control,829 grow quite normally, especially in prepubertal 
years, although growth velocity may decrease during puberty.830 
However, growth failure can occur in diabetic children with long-
standi

e can occur in diabetic children with long-
standing poor glycemic control.831,832 The Mauriac syndrome833 
describes children with poorly regulated IDDM, severe growth 
failure, and hepatosplenomegaly due to excess hepatic glycogen 
deposition. This type of growth retardation has become increas­
ingly rare with modern diabetes care.
Many pathophysiologic processes, including malnutrition, 
chronic intermittent acidosis, increased glucocorticoid produc­
tion, hypothyroidism, impaired calcium bal

produc­
tion, hypothyroidism, impaired calcium balance, and end-organ 
unresponsiveness to either GH or IGF, may contribute to growth 
failure in IDDM.826,834–836 IGF1 and IGFBP3 levels are dimin­
ished in the face of enhanced GH production,837–841 reflecting 
acquired GH insensitivity. GHBP levels are decreased,834,840 
supporting the concept of impaired GHR number or function. 
Furthermore, IGFBP1 is normally suppressed by insulin, and 
hypoinsulinemia results in elevated serum IGFBP1 levels t

ulinemia results in elevated serum IGFBP1 levels that 
may inhibit IGF action.612,840,842–844 In contrast to the situa­
tion in adolescents and adults, IGFBP1 levels are not elevated in 
well-growing prepubertal children.844 On the contrary, increased 
IGFBP3 proteolysis may enhance the bioactivity of the available 
IGF1.613,841 Most children with IDDM attain normal cellular 
nutrition and growth factor action despite intermittent hypoinsu­
linemia and derangements of peripheral indices of the G

ia and derangements of peripheral indices of the GH-IGF1 
system.
Even though glycemic control is inversely correlated with 
IGF1 level,835,837,840,845 the correlation between glycemic control 
and growth is weak. After conflicting reports as to the influence 
of ­glycemic regulation on growth,826,827,846,847 a longitudinal 
study847 of 46 children whose diabetes began before age 10 indi­
cated that initial heights at diagnosis were normal and that the 
final height SDS was minimally reduced fro

at the 
final height SDS was minimally reduced from that at onset. In 
boys, despite a delay of about 2.5 years in onset of puberty, total 
pubertal height gain was normal. In girls with diabetes, however, 
total pubertal height gain was diminished and the age at men­
arche was delayed; the effects of altered insulin and IGF1 levels on 
ovarian function have not been assessed in such patients. Chronic 
metabolic control did not correlate with the pubertal height gain 
or with the normal final he

 pubertal height gain 
or with the normal final height. Nevertheless, good glycemic con­
trol may improve growth at certain maturational periods such as 
puberty.826,839,848 
Cushing Syndrome: Glucocorticoid Excess
Glucocorticoid excess impairs skeletal growth, interferes with 
normal bone metabolism by inhibiting osteoblastic activity, and 
enhances bone resorption.849–851 These effects are related to the 
duration of steroid excess,852 regardless of whether the Cushing 
syndrome is due to ACTH

ss of whether the Cushing 
syndrome is due to ACTH hypersecretion, adrenal tumor, or glu­
cocorticoid administration.
Even modest doses of oral glucocorticoids can inhibit growth; 
these doses may be as low as 3 to 5 mg/m2 per day of prednisone 
or 12 to 15 mg/m2 per day of hydrocortisone (i.e., only slightly 
above what is considered physiologic replacement).853 The “toxic” 
effects of glucocorticoids on the epiphysis may persist to some 
degree after correction of chronic glucocorticoid excess

 after correction of chronic glucocorticoid excess, and 
patients frequently do not attain their target height.854,855 The 
longer the duration and the greater the intensity of glucocorticoid 
excess, the less likely it is that catch up growth will be completed. 
Alternate-day glucocorticoid treatment decreases but does not 
eliminate the risk of growth suppression.782,856,857 Inhaled gluco­
corticoids given for the treatment of asthma have an even lower 
risk of growth suppression, but even at 

en lower 
risk of growth suppression, but even at modest doses (e.g., 400 μg/
day of beclomethasone,858 200 μg/day of fluticasone,858 or 400 
980
μg/day of budesonide783), they can cause at least temporary slow­
ing of growth. However, inhaled corticosteroids do not appear to 
significantly impair final height.783
GH therapy can overcome some of the growth-inhibiting 
effects of excess glucocorticoids. The GH-induced increase in 
growth rate is inver

. The GH-induced increase in 
growth rate is inversely related to the glucocorticoid dose,859 with 
one small study finding no benefit of GH treatment for pred­
nisone doses greater than 0.35 mg/kg per day.860 GH or IGF1 
administration can diminish many of the catabolic effects of 
excess glucocorticoids.850,861,862
Adrenal tumors secreting large amounts of glucocorticoids can 
produce excess androgens, which may mask the growth-inhibi­
tory effects of glucocorticoids. In addition, Cushing synd

ects of glucocorticoids. In addition, Cushing syndrome in 
­children may not cause all the clinical signs and symptoms associ­
ated with the disorder in adults and may manifest with growth 
arrest. However, Cushing syndrome is an unlikely diagnosis in 
children with obesity because exogenous obesity is associated 
with normal or even accelerated skeletal growth, and growth 
deceleration is usually evident by the time other signs of Cushing 
syndrome appear (Fig. 25.38). In a series of 10 childre

ome appear (Fig. 25.38). In a series of 10 children and ado­
lescents treated for Cushing disease with surgery and cranial irra­
diation, post-therapy GHD was common and mean final height 
was –1.36 SDS.863 
Pseudohypoparathyroidism: Albright Hereditary 
Osteodystrophy
Albright hereditary osteodystrophy (AHO) is caused by mutations 
in the stimulatory GTP-binding protein, Gαs. It exists both with 
and without multihormone resistance—termed pseudohypopara­
thyroidism type 1A (PHP1A) and pseudo-ps

hypopara­
thyroidism type 1A (PHP1A) and pseudo-pseudohypoparathyroidism 
(pseudo-PHP). This condition is discussed in detail in Chapter 29 
but is included here because short adult height is a common fea­
ture.864 AHO is characterized by obesity (more marked in PHP1A 
than in pseudo-PHP865), short metacarpals, subcutaneous ossifi­
cations, round facies, and cognitive impairments. Patients with 
AHO, whether they have PHP1A or pseudo-PHP, typically have 
short adult stature. Many patients with P

ly have 
short adult stature. Many patients with PHP1A demonstrate 
GHD due to resistance to GHRH.866,867 Interestingly, patients 
with PHP1B, which is caused by a methylation defect in the 
GNAS gene that encodes Gαs, have PTH and TSH resistance 
like PHP1A patients, but have neither the AHO phenotype nor 
GHRH resistance.868 Although AHO patients have evidence of 
GHD, their growth pattern suggests that there is another con­
tributor to growth failure as they have modest growth failure in 
ear

failure as they have modest growth failure in 
early and middle childhood, with early epiphyseal fusion contrib­
uting to their short final height.866,869 Consistent with this likely 
multifactorial cause for short stature, preliminary data in patients 
with PHP1A found an increase in growth rate with GH treat­
ment, but without an increase in final height, as the few children 
reaching final height appeared to have an absence of a pubertal 
growth spurt.870 
Rickets
In the past, hypovitaminosis

th spurt.870 
Rickets
In the past, hypovitaminosis D was a major cause of short stature 
and was often associated with other causes of growth failure, such 
as malnutrition, prematurity, malabsorption, hepatic disease, or 
chronic renal failure (see Chapter 29). In isolated nutritional vita­
min D deficiency, breastfed infants typically have poor exposure 
to sunlight and are not nutritionally supplemented with vitamin 
D. Characteristic skeletal manifestations of rickets include frontal 
bossin

 manifestations of rickets include frontal 
bossing, craniotabes, rachitic rosary, and bowing of the legs. With 
vitamin D supplementation, whether due to prescribed supple­
mentation or a broadened diet, amelioration of the early decrease 
of linear growth velocity typically occurs. The association of vita­
min D receptor gene polymorphism with birth length, growth 
rate, adult stature, and BMD871–874 emphasizes the importance of 
vitamin D in normal growth. 
Hypophosphatemic Rickets
Hypophosph

ormal growth. 
Hypophosphatemic rickets is typically an X-linked dominant 
disorder caused by decreased renal tubular reabsorption of phos­
phate related to mutations in the phosphate-regulating endo­
peptidase gene, PHEX, located on chromosome Xp22.1. Other 
hypophosphatemic syndromes include autosomal-dominant 
hypophosphatemic rickets, hereditary hypophosphatemic rickets 
with hypercalciuria, and tumor-induced osteomalacia (see Chap­
ter 29).875 In all of these condit

acia (see Chap­
ter 29).875 In all of these conditions, there are increased levels of 
FGF23, a major phosphaturic agent.876 The features are usually 
more severe in boys and include short stature,776,877 prominent 
bowing of the legs, and sometimes rachitic signs.878
The metabolic and skeletal abnormalities cannot be overcome 
by vitamin D therapy alone. Treatment requires oral phosphate 
replacement, but such therapy may result in poor calcium absorp­
tion from the intestine. The addition of c

absorp­
tion from the intestine. The addition of calcitriol to oral phosphate 
increases intestinal phosphate absorption and prevents hypocalce­
mia and secondary hyperparathyroidism. Such combined therapy 
190
180
170
160
150
140
130
120
110
100
90
80
70
60
50
0
19
18
17
97
50
3
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
• Fig. 25.38  Growth curves of two boys with obesity. The boy depicted by 
the circles had cortisol excess related to Cushing disease. He had onset 
of rapi

 related to Cushing disease. He had onset 
of rapid weight gain associated with a decrease in linear growth velocity 
at age 7. The diagnosis was made, and an adrenalectomy (arrow) was 
performed at age 9.5 years, with an almost immediate increase in growth 
rate and striking catch-up growth. The boy whose growth is depicted by 
triangles had exogenous obesity. At age 9.5 years, his weight was approxi­
mately the same as that of the patient with Cushing disease, but his height 
was at the 97th p

Cushing disease, but his height 
was at the 97th percentile, reflecting the enhancement of linear growth in 
individuals with exogenous obesity.
981
improves the rickets but may not correct growth.875,879,880 Early 
initiation of this therapy in early infancy may lead to achievement 
of greater childhood and adult height.877 There is no clear associa­
tion between endogenous GH secretion, IGF1, or phosphate lev­
els and height in this disorder.8

r phosphate lev­
els and height in this disorder.881–883 Nevertheless, GH therapy in 
eight trials, including 83 patients, resulted in an enhancement of 
skeletal growth and improvement in BMD.884–886 In 14 of those 
patients, treatment for 4 to 5 years resulted in a height gain of 
up to 1.2 SDS. However, a Cochrane review did not find con­
clusive evidence that GH treatment improved growth in patients 
with hypophosphatemic rickets.887 One report cautioned that 
GH treatment appeared to exagge

rt cautioned that 
GH treatment appeared to exaggerate disproportionate truncal 
growth.888
Treatment with the FGF23 monoclonal antibody burosumab 
increased serum phosphate and improved linear growth rates after 
64 weeks of treatment.889 This therapy may supplant current ther­
apies with phosphate supplementation and calcitriol. 
Osteochondrodysplasias
The osteochondrodysplasias encompass a heterogeneous group of 
disorders characterized by intrinsic abnormalities of cartilage and 
bone.890 Th

rinsic abnormalities of cartilage and 
bone.890 These disorders include abnormalities in the size or shape 
of bones in the limbs, spine, or skull, often with abnormalities 
seen on radiographic evaluation. More than 100 osteochondro­
dysplasias have been identified based on physical characteristics 
and radiographic characteristics (Table 25.5).
Diagnosis of osteochondrodysplasias can be difficult, with clin­
ical and radiologic evaluation central to the diagnosis. The fam­
ily history is criti

al to the diagnosis. The fam­
ily history is critical, although many cases are caused by de novo 
mutations, and this is generally the case in autosomal-dominant 
achondrodysplasia and hypochondrodysplasia. Measurement of 
body proportions should include arm span, sitting height, upper 
and lower body segments, and head circumference. Radiologic 
evaluation should be used to determine whether involvement is of 
the long bones, skull, or vertebrae and whether abnormalities are 
primarily at the e

 and whether abnormalities are 
primarily at the epiphyses, metaphyses, or diaphyses. The osteo­
chondrodysplasias most commonly encountered in endocrine 
practice are discussed in the following paragraphs.
Achondrodysplasia is the most common of the osteochondro­
dysplasias, with a frequency of 1 in 26,000 individuals. Charac­
teristic abnormalities of the skeleton include megalocephaly, low 
nasal bridge, lumbar lordosis, short trident hand, and rhizomelia 
(shortness of the proximal legs and 

d rhizomelia 
(shortness of the proximal legs and arms) with skin redundancy. 
Radiologic findings include small, cuboid-shaped vertebral bod­
ies with short pedicles and progressive narrowing of the lumbar 
interpedicular distance. The small foramen magnum may lead to 
hydrocephalus, and spinal cord and root compression may result 
from kyphosis, stenosis of the spinal canal, or disk lesions.891 
Diminished growth velocity is present from infancy, although 
short stature may not be evident unti

y, although 
short stature may not be evident until after 2 years of age. Mean 
adult heights in males and females are 130 and 120 cm, respec­
tively. GH secretion is comparable to that in normal subjects.
Achondrodysplasia is caused by mutations in the transmem­
brane domain of the FGF receptor 3 gene (FGFR3).892 It is trans­
mitted in an autosomal dominant manner, but 80% to 90% of 
cases are caused by de novo mutations. Most of the cases are the 
result of activating mutations at nucleotide 1

he 
result of activating mutations at nucleotide 1138 of the FGFR3 
gene, which creates new recognition sites for restriction enzymes, 
thus easing the molecular diagnosis. The mutation rate reported at 
this site is very high, and FGFR3 has been labeled the most muta­
ble gene in the genome. As a result of the upregulation of receptor 
activity, there is abnormal chondrogenesis and osteogenesis during 
endochondral ossification, leading to the typical phenotypic find­
ings. The homogeneity of t

ypical phenotypic find­
ings. The homogeneity of the mutation in achondroplasia prob­
ably explains the minimal heterogeneity in the phenotype. Infants 
homozygous for the mutation have severe disease, typically dying 
in infancy from respiratory insufficiency due to a small thorax.
Hypochondroplasia is also autosomal dominant, and 70% of 
affected individuals are heterozygous for mutations in the FGFR3 
gene, frequently at amino acid position 1620.893 The facial fea­
tures of achondroplasia are

20.893 The facial fea­
tures of achondroplasia are absent, and both short stature and 
rhizomelia are less pronounced. Adult heights are typically in the 
range of 120 to 150 cm. Poor growth may not be evident until 
after 2 years of age, but stature then deviates progressively from 
normal. Occasionally, the disproportionate short stature is not 
apparent until adulthood. Outward bowing of the legs may be 
accompanied by genu varum. On radiologic evaluation, lumbar 
interpedicular distances are

c evaluation, lumbar 
interpedicular distances are diminished between L1 and L5, and 
there may be flaring of the pelvis and narrow sciatic notches. The 
diagnosis can be difficult to make, with mild variants of the syn­
drome difficult to distinguish from normal.
The short stature homeobox-containing (SHOX) gene is located 
in the pseudoautosomal region of distal Xp and Yp. Mutations or 
deletion of SHOX is associated with syndromes of poor growth and 
skeletal dysplasia, including Léri-Weill d

th and 
skeletal dysplasia, including Léri-Weill dyschondrosteosis (LWD), 
TS, and Langer mesomelic dysplasia (LMD).894 Findings include 
short stature, Madelung deformity, increased carrying angle, tibial 
bowing, scoliosis, and high arched palate. The auxologic finding 
of relatively short limbs suggests a defect in SHOX, because the 
SHOX protein may affect cellular proliferation and apoptosis of 
chondrocytes at the growth plate.895 Indeed, the skeletal manifes­
tations have been associated 

fibrous components 
of the skeleton
	III.	 Idiopathic osteolysis
TABLE 25.5
982
defects, whereas TS and LMD are caused by haploinsufficiency. 
The profound findings in LWD, compared with TS, may reflect 
the impact of pubertal estrogen exposure in LWD.894
Endochondral growth is regulated by multiple endocrine, para­
crine, and autocrine factors, and many inborn errors have been 
identified. In the syndrome of acromesomelic dysplasia, growth 
is rema

ndrome of acromesomelic dysplasia, growth 
is remarkably impaired, leading to adult heights that may be 
more than 5 SD below the mean. A homozygous mutation in 
the homodimeric transmembrane natriuretic peptide receptor B 
(NPR-B) that impairs binding of the ligand C-type natriuretic 
peptide has been found in some affected individuals.896 Obligatory 
heterozygotes are significantly shorter than normal but have normal 
skeletal anatomy.897 Therefore heterozygous mutations of NPR-B 
demonstrate 

fore heterozygous mutations of NPR-B 
demonstrate that a subset of children with unexplained short stat­
ure may in fact have mild forms of osteochondrodysplasias. 
Chromosomal Abnormalities
Abnormalities of autosomes or sex chromosomes can cause growth 
retardation without evidence of skeletal dysplasia, frequently with 
somatic abnormalities and developmental delay. In many cases, 
the precise cause of growth failure is not clear because the genetic 
defects do not affect known components of t

netic 
defects do not affect known components of the GH-IGF1 system. 
Chromosomal lesions may directly influence normal tissue growth 
and development or indirectly modulate local responsiveness to 
IGF or other growth factors at the growth plate.
Down Syndrome
Trisomy 21, or Down syndrome, is probably the most common 
chromosomal disorder associated with growth retardation, affect­
ing approximately 1 in 600 neonates. On average, newborns with 
Down syndrome have birth weights 500 g below norma

Down syndrome have birth weights 500 g below normal and are 2 
to 3 cm shorter than normal. Growth failure continues postnatally 
and is typically associated with delayed skeletal maturation and a 
delayed and incomplete pubertal growth spurt. Adult heights range 
from 135 to 170 cm in men and 127 to 158 cm in women.14 
The cause of growth failure in Down syndrome is unknown, and 
attempts to find underlying hormonal explanations for growth 
retardation have been unsuccessful. Growth retardation

rdation have been unsuccessful. Growth retardation may be 
due to GH deficiency associated with hypothalamic dysfunction. 
A morphometric study of patients with Down syndrome revealed 
fewer neurons in the arcuate and ventromedial nuclei, which are 
areas responsible for GHRH neurosecretory function.898 Fourteen 
children with Down syndrome (aged 10 months to 5 years, height 
SD scores [SDS] varying between −1.3 and −4.9) were studied 
and found to have a lack of response to levodopa and clonidi

to have a lack of response to levodopa and clonidine 
provocative testing, while responding to GH-releasing hormone 
(GHRH), indicating a likely hypothalamic deficiency.899 Marginal 
levels of GH secretion and low-normal serum levels of IGF1 have 
been reported in patients with Down syndrome. Exogenous GH 
has been tried in some patients with Down syndrome and has pro­
duced increases in growth velocity, but the long-term effects on final 
adult height have not been studied. In addition, no impr

height have not been studied. In addition, no improvements 
in gross motor or mental development were noted.900,901 Hashi­
moto thyroiditis is common in individuals with Down syndrome 
and should be sought and treated promptly. Because of the concern 
for development of leukemia, which is more common in individu­
als with Down syndrome, GH use is generally not recommended. 
Turner Syndrome
In girls with TS, short stature is the single most common feature, 
occurring more frequently than delayed 

 feature, 
occurring more frequently than delayed puberty, cubitus valgus, or 
webbing of the neck.902–904 Short stature occurs in 95% to 100% of 
girls with a 45,X karyotype.905–908 Mean adult heights in the United 
States and Europe range from 142 to 146.8 cm, with important 
genetic and ethnic influences on growth of girls in different regions. 
Parental height correlates well with final adult height,908,909 and a 
cross-cultural study in 15 countries demonstrated a very strong cor­
relation 

 of growth27,28,913 and 
progressive decline in height velocity from age 3 years until 
approximately 14 years, resulting in further deviation from 
normal height percentiles
	4.	 A prolonged adolescent growth phase, characterized by a par­
tial return toward normal height, followed by delayed epiphy­
seal fusion
These girls have many features of skeletal dysplasia, such as 
Madelung deformity and are haploinsufficient for the SHOX 
gene, which is located in the pseudoautosomal region of the sh

is located in the pseudoautosomal region of the short 
arm of the X chromosome.914 When heights of girls with TS are 
compared with those of girls with LWD, which involves a SHOX 
deletion, it appears that the SHOX defect may account for about 
two-thirds of the height deficit in TS.915 Girls with TS have nor­
mal GH and IGF levels during childhood; reports of low levels of 
GH or IGF, or both, in adolescents are likely due to low serum 
levels of gonadal steroids.916 Multiple studies have shown

f gonadal steroids.916 Multiple studies have shown that GH 
therapy is capable of accelerating short-term growth and increas­
ing final adult height.907,917,918 GH treatment in TS is discussed 
in detail later in the chapter. 
Noonan Syndrome
Individuals with Noonan syndrome have postnatal growth fail­
ure, right-sided cardiac abnormalities (most often pulmonary 
valve abnormalities), webbing of the neck, low posterior hair­
line, ptosis, cubitus valgus, and malformed ears. Microphallus 
and cry

 valgus, and malformed ears. Microphallus 
and cryptorchidism are common, and puberty may be delayed or 
incomplete. Cognitive delay of variable degrees is present in about 
25% to 50% of patients. Although this disorder shares phenotypic 
features with TS, the two are clearly distinct.919,920 In Noonan 
syndrome, the sex chromosomes are normal, and transmission is 
autosomal dominant, although about 50% of cases are sporadic. 
Noonan syndrome is caused by heterozygous activating mutations 
in t

 caused by heterozygous activating mutations 
in the genes of proteins in the RAS-MAPK pathway, including 
PTPN11, SOS1, RAF1, KRAS, and NRAS, with approximately 
half of patients having mutations in PTPN11.921,922 Through 
much of childhood, mean growth in length and weight is below 
the third percentile.923–925 GH secretory abnormalities do not 
account for the short stature, although endogenous GH pro­
duction may be slightly reduced.925,926 The protein product of 
the PTPN11 gene is the nonr

he protein product of 
the PTPN11 gene is the nonreceptor type 2 tyrosine phospha­
tase (SHP2). SHP2 dephosphorylates JAK2, suggesting that the 
growth failure of Noonan syndrome might be due to growth hor­
mone resistance from the enhanced action of SHP2.927–929 How­
ever, SHP2 is a positive regulator of the RAS-MAPK pathway, and 
the development of Noonan syndrome from activating mutations 
in other proteins in this pathway suggests that alterations in RAS-
MAPK signaling are responsible for t

tions in RAS-
MAPK signaling are responsible for the phenotype. GH therapy 
has been used in the treatment of patients with Noonan syndrome 
who have short stature, as discussed in detail later. 
983
Prader-Willi Syndrome
Prader-Willi syndrome (PWS), a neurogenetic developmental 
disorder, was initially described by Prader, Willi, and Labhart 
in 1956. This syndrome is characterized by obesity, hypotonia, 
hyperphagia, delayed motor skill develo

ypotonia, 
hyperphagia, delayed motor skill development, short stature, men­
tal retardation, hypothalamic dysfunction, and hypogonadism.930
PWS has an incidence of 1 in every 12,000 births, making 
it the most common syndrome causing obesity. PWS is caused 
by the absence of expression of genes on the paternally inherited 
chromosome 15 (q11–13). In about 70% to 75% of cases, PWS is 
due to deletion of the paternal allele; in 25% of cases, it is due to 
the presence of maternal uniparental diso

 due to 
the presence of maternal uniparental disomy (UPD). Other rarer 
causes of PWS are due to translocations, molecular defects, or 
imprinting errors.931,932 Although several genes and gene products 
of the PWS locus have been identified, the specific gene(s) respon­
sible for its pathogenesis are not completely understood.933,934
In PWS, growth failure may be evident at birth but is more 
pronounced postnatally.935 The hypothalamic dysfunction in 
PWS is associated with endocrinopathies th

ion in 
PWS is associated with endocrinopathies that include short stat­
ure, hyperphagia and obesity with abnormal body composition, 
deficient growth hormone (GH) secretion, central hypothyroid­
ism, and hypogonadism.936–938
Interestingly, a recent study showed that GH secretion in the 
patients with UPD was significantly lower than those with dele­
tions.939 Although infants with PWS typically have hypotonic and 
poor weight gain, the percentage of body fat is increased before the 
onset of h

ge of body fat is increased before the 
onset of hyperphagia and obesity.940 This syndrome is discussed at 
length in a later section of this chapter. 
Other Syndromes
Other syndromes associated with moderate to profound growth 
failure include Bloom syndrome, de Lange syndrome, lepre­
chaunism, Ellis–van Creveld syndrome, Aarskog syndrome, 
Rubinstein-Taybi syndrome, mulibrey nanism, Dubowitz syn­
drome, progeria, Cockayne syndrome, and Johanson-Blizzard 
syndrome.941 
Small for Gestational Age

-Blizzard 
Historically, infants born SGA have composed a heterogeneous 
group with birth weight or length below the 3rd, 5th, or 10th 
percentile for gestational age, depending on the study.36 As the 
growth and metabolic consequences of being born SGA have 
been observed and characterized, studies have more consistently 
used the definition of SGA as birth weight or length (or both) at 
least 2 SD below the mean for gestational age (usually at or below 

he mean for gestational age (usually at or below 
the 2.3 percentile for a population). The term IUGR has been 
used interchangeably with SGA to describe these infants. How­
ever, it has been proposed that, because IUGR implies a known 
underlying pathologic process, that term should be reserved for 
infants whose abnormal prenatal growth has been confirmed by 
intrauterine growth assessments and whose growth restriction can 
be attributed to a specific cause.942 The reason for abnormal fetal 
g

pecific cause.942 The reason for abnormal fetal 
growth is unclear in up to 40% of cases943; known underlying 
reasons are listed in Table 25.6. Accurate assessment of an infant 
as SGA depends on accurate gestational dating and weight and 
length measurements, which can be difficult in both developed 
and developing countries.944 However, recent technology has 
allowed ultrasound measurements of fetal size in utero to become 
increasingly more precise.945
Most SGA infants exhibit catch-up growt

recise.945
Most SGA infants exhibit catch-up growth (as defined by a 
growth velocity greater than the median for chronologic age and 
gender) by 2 years of age. Catch-up growth occurs during the first 
6 months of life in approximately 80% of infants born SGA.37 
Approximately 10% to 15% of infants born SGA exhibit slow, 
attenuated growth with persistent height deficits in childhood and 
adolescence. The remaining 5% to 10% exhibit a slower catch-up 
growth pattern, reaching heights 2 SD below

ch-up 
growth pattern, reaching heights 2 SD below the mean between 3 
and 5 years of age. These estimations vary by study; in a popula­
tion of severely affected infants with SGA who required care in 
a neonatal intensive care unit, 27% had not achieved catch-up 
growth by 6 years of age.946
Low-birth-weight premature babies who are appropriate for 
gestational age invariably experience catch-up growth in the first 2 
years of life. Final adult height of all children born SGA is −0.8 to 
−0.9 S

height of all children born SGA is −0.8 to 
−0.9 SDS, a mean deficit of 3.6 to 4 cm when adjusted for fam­
ily stature.947 It has been estimated that the 10% to 15% of SGA 
children with short stature account for as much as 20% of all short 
children. In the United States, 2.3% of the population fitting the 
definition of SGA represents roughly an incidence of 1 in 43 neo­
nates. Therefore SGA children have a fivefold to sevenfold greater 
possibility of short stature than AGA children.36
Normal

ility of short stature than AGA children.36
Normal fetal growth depends on a complex interplay of mater­
nal and fetal genetic and external environmental influences. 
Abnormal intrauterine growth can result from pathologic pro­
cesses in the fetus, the placenta, or the mother. Growth in length 
occurs early in fetal life, whereas weight gain occurs later in fetal 
life948; first-trimester growth failure has been closely associated 
with low birth weight and low-birth-weight percentile.949 Becaus

l infections, or toxic effects; asymmetric IUGR results from 
an insult occurring late in gestation, often due to fetoplacental 
insufficiency. Historically, it was thought that infants with sym­
metric IUGR do not experience catch-up growth, whereas those 
with asymmetric IUGR who have normal head circumference and 
length yet low birth weight usually experience catch-up growth 
postnatally.948 However, studies have suggested that infants with 
asymmetric IUGR have worse perinatal outcomes than

asymmetric IUGR have worse perinatal outcomes than those with 
symmetric IUGR950,951 and that both types of growth restrictions 
can arise in the second trimester of pregnancy.952,953 Therefore the 
subclassification of IUGR, like the term IUGR itself, is controver­
sial. A recent study found that 16% of dysmorphic children born 
SGA with postnatal growth retardation had pathologic genomic 
copy number variants.954 In a recent study, 14% of children born 
SGA with advanced skeletal maturation ha

ren born 
SGA with advanced skeletal maturation had an ACAN gene 
mutation.955
Endocrine-related causes account for a small fraction of the 
many contributors to fetal growth abnormalities, but hormonal 
disorders associated with fetal and neonatal growth restriction 
shed light on the endocrine mechanisms of growth in the fetus. 
Although GH plays a major role in postnatal growth, it has less 
of a role in fetal growth; infants with neonatal GHD are typi­
cally −0.5 to −1.5 SD below the mean in

 are typi­
cally −0.5 to −1.5 SD below the mean in length and are heavy 
for this length.338,956 An adequate nutrient supply is the main 
determinant of fetal growth, but growth factors such as insulin, 
IGF1, and IGF2 also play a role. IGF1 and IGF2 are the major 
hormonal regulators of fetal growth and can compensate for each 
other.265,273,957–960 The growth-promoting effects of the IGFs are 
mediated by the type I or IGF1 receptor.273 During fetal life, the 
IGF system operates largely indep

fetal life, the 
IGF system operates largely independently of GH. Genetically 
modified mouse models have shown that approximately 70% 
of body size is due to IGF, half of which are mediated by GH-
induced changes in IGF concentration and half due to direct IGF 
effects.227 The direct effect of GH on body size is relatively minor. 
Neonates who are SGA exhibit hormonal patterns consistent with 
insensitivity to GH and IGF1 and insulin action. In neonates with 
IUGR, GH levels are elevated961 and

neonates with 
IUGR, GH levels are elevated961 and IGF1 and IGFBP3 levels are 
low.151,961–964 IGFBP1 and IGFBP2 levels have been reported to 
be higher than in AGA infants,291 a pattern seen in individuals 
with insulin resistance. Similar patterns are found in the first week 
of life after severe fetal malnutrition.965 Exogenous GH treatment 
has little or no effect on growth, body composition, or energy 
expenditure966,967 in the neonatal period. In most infants with 
SGA, normalization of GH

od. In most infants with 
SGA, normalization of GH, IGF1, and IGFBP3 levels occurs by 3 
months of age, with normal response to GH stimulation testing in 
childhood.968 In measurement of spontaneous GH secretion, high 
pulse frequency with attenuated pulse amplitude and elevated 
trough values of GH have been noted in SGA children.969–971 
Serum IGF1 levels in children with short stature born SGA are 
slightly but significantly lower than in children with catch-up 
growth.968,969,971
Defects of 

dren with catch-up 
Defects of insulin secretion or action are associated with 
impaired fetal growth.948 Congenital defects of insulin secretion 
such as glucokinase deficiency972 or pancreatic agenesis973 are 
associated with severe IUGR. Leprechaunism is caused by defects 
in the insulin receptor and is associated with severe insulin resis­
tance and IUGR.27 The initial case of a deletion of the IGF1 gene 
had profound intrauterine growth failure.974,656 Polymorphisms 
of t

uterine growth failure.974,656 Polymorphisms 
of the IGF1 gene have also been reported to be associated with 
IUGR.974–976
SGA children who have striking weight gain during the first 
several years of life can develop endocrine disorders later in 
childhood, including premature adrenarche, insulin resistance, 
polycystic ovary syndrome, and an attenuated growth spurt.977 
This subset of SGA children have increased risks of hyperten­
sion, maturity-onset diabetes, and cardiovascular disease later

y-onset diabetes, and cardiovascular disease later in 
life.978–980 This is consistent with the Barker hypothesis, which 
states that fetal metabolic responses to a nutritionally hostile intra­
uterine environment may lead to inappropriate extrauterine con­
sequences.979,981 These problems do not appear to occur in SGA 
babies without catch-up growth, although insulin resistance has 
been described.982 Whether being SGA is causally related to these 
disorders or is a symptom of an underlying inb

se 
disorders or is a symptom of an underlying inborn metabolic dis­
order is not yet known.
Russell-Silver syndrome (RSS) was independently described by 
Russell983 and by Silver.984 Findings include IUGR with postna­
tal growth failure, congenital hemihypertrophy, and small, trian­
gular facies.985 Other nonspecific findings include clinodactyly, 
delayed closure of the fontanel, delayed bone age, and precocious 
puberty.986–988 Adults are short, with final heights approximately 
–4 SD below t

t, with final heights approximately 
–4 SD below the mean.985 The incidence is between 1 in 50,000 
and 1 in 100,000 live births. Endogenous GH secretion in prepu­
bertal RSS children is similar to that in other short IUGR children 
and less than in AGA short children.969,989 Maternal uniparental 
disomy of chromosome 7 is present in 7% to 10% of cases.985,990 
Although there are a number of imprinted genes or factors involved 
in growth and development on chromosome 7, numerous stud­
ies have n

lopment on chromosome 7, numerous stud­
ies have not detected any pathologic mutations in the candidate 
genes. A gene on chromosome 7p, GRB10, is involved in regula­
tion of insulin and IGF1 receptor signaling and is mainly expressed 
from the maternal allele; loss of the maternal allele results in fetal 
and placental overgrowth.990,991 Mutations that could cause over­
expression of GRB10 have not been found in patients with RSS.
A small number of patients with RSS have duplication of the 
mat

 of patients with RSS have duplication of the 
maternal allele of the 11p15 region992; duplication of the paternal 
allele in this region is associated with BWS and overexpression of 
IGF2. The 11p15 region contains two imprinting control regions 
(ICR), ICR1 and ICR2 (Fig. 25.39). ICR1 comprises the IGF2 
and H19 genes. The noncoding gene H19 is downstream from 
IGF2 and is oppositely imprinted, meaning that only the mater­
nal allele is expressed and that the paternal allele is inactive. The 

ed and that the paternal allele is inactive. The 
promoters of the IGF2 and H19 genes share a set of enhancers 
that act on either gene. On the paternal allele, the H19 promoter 
region is methylated and therefore inactivated.244 Upstream from 
the H19 and IGF2 promoter region is a paternally methylated 
region that prevents binding of CTCF, allowing enhancers to act 
on the IGF2 promoter to activate transcription.245 On the mater­
nal chromosome, this region is not methylated, allowing CTCF to 

, this region is not methylated, allowing CTCF to 
bind and preventing transcription.246 In this ICR1 region, muta­
tions causing hypomethylation of the H19 promoter region have 
been described in approximately 40% of patients with RSS.993 
Oppositely, hypermethylation of the ICR1 region has been associ­
ated with BWS. Disruption of ICR2 has been described in BWS 
(described later) but not in RSS. Reduced IGF2 expression has 
been demonstrated in fibroblasts of patients with RSS in vitro,993 
bu

 fibroblasts of patients with RSS in vitro,993 
but serum levels of IGF2 in patients with RSS are normal.994 Mice 
with a null mutation of IGF2 have prenatal growth retardation 
but normal postnatal growth; how reduced expression of IGF2 
contributes to postnatal growth failure in RSS has yet to be elu­
cidated. A number of other rare genetic defects have also been 
associated with RSS, such that a known molecular abnormality 
may be identified in 60% of these individuals.995
985
Chapter 25  Nor

n 60% of these individuals.995
985
Maternal and Placental Factors
Maternal factors and placental insufficiency can impair fetal 
growth and likely account for most cases of asymmetric IUGR. 
Maternal nutrition is an important contributor to fetal growth 
and to growth during the first year of life.996 Fetal growth retarda­
tion may result from use of alcohol,997 cocaine,998 marijuana,998 or 
tobacco999 during pregnancy. The mechanisms for drug-i

cco999 during pregnancy. The mechanisms for drug-induced 
fetal growth retardation are unclear but may include uterine vaso­
constriction and vascular insufficiency, placental abruption, or 
premature rupture of membranes. The maternal hormonal milieu 
is affected by placental steroids and peptides. Maternal IGF1 
affects placental function and may facilitate transport of nutrients 
to the fetus. Maternal IGF1 levels have been found to correlate 
with fetal growth.1000,1001 Increased levels of f

with fetal growth.1000,1001 Increased levels of free IGF1 are found 
during normal human pregnancy.1002
The placenta has multiple functions, including the transport 
of nutrients, oxygen, and waste and the production of hormones. 
It consumes oxygen and glucose brought by the uterine circula­
tion. Placental GH affects maternal IGF production, which in 
turn affects placental function. A woman with GHD due to PIT1 
mutation exhibited normal levels of placental GH and IGF1, 
demonstrating the in

ls of placental GH and IGF1, 
demonstrating the independent production of GH and IGF1 by 
the placenta.1003 Human placental lactogen (hPL) is a major regu­
lator of glucose, amino acid, and lipid metabolism in the mother, 
aiding in the mobilization of nutrients for transport to the fetus. 
Damage to the placenta resulting from vascular disease, infection, 
or intrinsic abnormalities of the syncytiotrophoblasts can impair 
these important functions. At times, examination of the placenta 
may yie

ns. At times, examination of the placenta 
may yield causal information about fetal growth retardation.
An X-linked homeobox gene, ESX1, detected only in extraem­
bryonic tissues and human testes, is a chromosomally imprinted 
regulator of placental morphogenesis.1004–1006 Heterozygous and 
homozygous mutant mice are born 20% smaller than normal and 
have large edematous placentas.1004 Vasculature is abnormal at the 
maternal-fetal interface, presumably causing the growth retardation. 
Pathologi

sumably causing the growth retardation. 
Pathologic Basis of Excess Growth
Although by definition there are as many children with heights 
greater than 2 SD above the mean as those with heights less than 2 
SD below the mean, tall stature as a chief complaint is encountered 
much less often in endocrine practice. Nevertheless, it is critical 
to identify those situations in which tall stature or an accelerated 
growth rate provides clues of an underlying disorder (Table 25.7).
Statural Overgrowt

derlying disorder (Table 25.7).
Statural Overgrowth in the Fetus
Maternal diabetes mellitus is the most common cause of large-for-
gestational-age (LGA) infants. LGA is defined as length or weight 
greater than the 90th percentile for gestational age. Even in the 
absence of clinical symptoms or family history, the birth of an 
excessively large infant should lead to evaluation for maternal or 
gestational diabetes. Two syndromes, Sotos syndrome and BWS, 
can also cause LGA infants.
Paternal
Mat

and BWS, 
Paternal
Maternal
Normal
ICR1 epimutation
RSS
H19
IGF2
CDKN1C
KCNQ1
CH3
H19
IGF2
CDKN1C
KCNQ1
CH3
H19
IGF2
CDKN1
KCNQ1
H19
IGF2
CDKN1
KCNQ1
BWS
H19
IGF2
CDKN1
KCNQ1
H19
IGF2
CDKN1
KCNQ1
ICR2 epimutation
H19
IGF2
CDKN1
KCNQ1
H19
IGF2
CDKN1
KCNQ1
Uniparental disomy
H19
IGF2
CDKN1
KCNQ1
H19
IGF2
CDKN1
KCNQ1
H19
IGF2
CDKN1
KCNQ1
H19
IGF2
CDKN1
KCNQ1
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
• Fig. 25.39  Schematic representation of the 11p15 region and the epigenetic 

esentation of the 11p15 region and the epigenetic mutations associated with 
Russell-Silver syndrome and Beckwith-Wiedemann syndrome. Red and pink boxes represent genes of 
imprinting control region 1 (1CR1), and green boxes represent genes of 1CR2. The arrow represents the 
presence of gene transcription. Dark triangles represent antisense transcripts that can repress transcrip­
tion of ICR2 genes. When methylated (represented by CH3), the transcripts cannot be formed, allowing 
transcription o

cripts cannot be formed, allowing 
transcription of ICR2 genes and reciprocal suppression of transcription of downstream genes (i.e., IGF2). 
(Adapted from Eggerman T. Silver-Russell and Beckwith-Wiedemann syndromes: opposite [epi]mutations 
in 11p15 result in opposite clinical pictures. Horm Res. 2009;71:S30–S35.)
986
Sotos Syndrome
Children with cerebral gigantism (Sotos syndrome) are typically 
above the 90th percentile for length and weight at bi

ve the 90th percentile for length and weight at birth.1007–1009 
Clinical features also include a prominent forehead; dolichoceph­
aly; macrocephaly; high arched palate; hypertelorism with unusu­
ally slanting eyes; prominent ears, jaw, and chin; large hands and 
feet with thickened subcutaneous tissue; cognitive delay; and 
motor incoordination. Children continue to grow rapidly dur­
ing early childhood, but puberty is usually early, with premature 
epiphyseal fusion. Therefore most children wi

ure 
epiphyseal fusion. Therefore most children with Sotos syndrome 
have a final height within the normal range.1009 GH secretion 
and serum IGF levels are normal, and no specific cause of the 
overgrowth has been identified. About 80% of patients have a 
loss-of-function mutation in the NSD1 gene whose product is a 
nucleus-localized basic transcription factor.1010 
Beckwith-Wiedemann Syndrome
BWS is the most common (1 of every 13,700 live births) of the 
overgrowth disorders, the group of dis

hs) of the 
overgrowth disorders, the group of disorders associated with exces­
sive somatic and specific organ growth. It is characterized by fetal 
macrosomia with omphalocele1011 and other clinical features sec­
ondary to organomegaly, such as macroglossia, renal medullary 
hyperplasia, and neonatal hypoglycemia due to islet cell hyper­
plasia.1012 Excessive childhood growth ultimately leads to earlier 
puberty and early epiphyseal fusion with resultant normal adult 
height.1013
Various lines

 resultant normal adult 
Various lines of evidence have shown that BWS is associated 
with loss of imprinting of the genes on chromosome 11p15.5, 
home of the IGF2 gene (see Fig. 25.39). Under normal condi­
tions, the paternally derived IGF2 gene is expressed and the 
maternally transmitted gene is not active, as described in detail 
earlier in this chapter. BWS has been associated with uniparental 
disomy or duplication of the paternal 11p15 region and resultant 
overexpression of t

al 11p15 region and resultant 
overexpression of the IGF2 gene.1014 The ICR1 region of 11p15 
contains the IGF2 and H19 genes; the noncoding gene H19 is 
located downstream from IGF2 and is oppositely imprinted. The 
region that prevents bindin

paternally methylated 
nal chromosome, this region is not methylated, allowing CTCF 
to bind and preventing transcription.246 Hypermethylation of the 
H19 promoter region with resultant loss of imprinting and bial­
lelic expression of IGF2 has been associated with fewer than 10% 
of the cases of BWS.1015,1016 Point mutations in the ICR1 region 
have been identified in

ations in the ICR1 region 
have been identified in patients with BWS. These mutations alter 
binding of OCT (octamer) transcription factors to the region 
leading to hypermethylation of the promoter.1017 Hypomethyl­
ation of this region is associated with RSS, a syndrome associated 
with prenatal and postnatal growth failure (see earlier discussion).
ICR2, located 5′ of ICR1, contains the genes for cyclin-depen­
dent kinase inhibitor 1C (CDKN1C) and potassium channel 
KQT family member 1 (KCNQ1)

and potassium channel 
KQT family member 1 (KCNQ1), among others that are meth­
ylated on the maternal allele. Associated with these two genes is 
an antisense transcript with paternal expression that may suppress 
transcription; it has been postulated that this cluster of genes is 
in “expression competition” with the IGF2/H19 cluster.1018 Up to 
25% of familial cases of BWS are associated with mutations in the 
CDKN1C or KCNQ1 gene,1019 but there is debate about whether 
loss of imprinting of 

re is debate about whether 
loss of imprinting of the IGF2 gene convincingly occurs with most 
of the mutations. Four children with somatic overgrowth but not 
the diagnostic features of BWS had IGF2 gene overexpression.1020 
Additionally, mutations in GPC3, a glypican gene that codes for an 
IGF2 neutralizing membrane receptor, cause the related Simpson-
Golabi-Behmel overgrowth syndrome.1021,1022 
Postnatal Statural Overgrowth
As in the case of the child with growth attenuation, crossing of he

 the child with growth attenuation, crossing of height 
percentiles between infancy and the onset of puberty is an indica­
tion for further evaluation because it can indicate serious underlying 
pathology. As with short stature, children with tall stature must be 
evaluated in the context of familial growth and pubertal patterns.
Tall Stature
GH secretion and levels of IFG1 and IGFBP3 in familial tall 
stature are often in the upper-normal range.1023 Tauber and col­
leagues1024 divided 65 childr

1023 Tauber and col­
leagues1024 divided 65 children with familial tall stature into a sub­
set with high GH secretion rates and frequent secretory bursts and 
another subset with lower GH secretion and fewer episodic spikes. 
IGF1 levels were higher among those producing more GH and 
normal in the low-GH group. The authors postulated that both 
enhanced secretion of GH and greater efficiency of GH-mediated 
IGF1 production might be potential causes of familial tall stature.
Tall stature is also

ses of familial tall stature.
Tall stature is also a characteristic of certain syndromes. Mar­
fan syndrome, an autosomal dominant disorder of collagen 
  Differential Diagnosis of Statural 
Overgrowth
Fetal Overgrowth
Maternal diabetes mellitus
Cerebral gigantism (Sotos syndrome)
Weaver syndrome
Beckwith-Wiedemann syndrome
Other insulin-like growth factor 2 (IGF2) excess syndromes
Postnatal Overgrowth Leading to Childhood Tall Stature
Familial (constitutional) tall stature
B

(constitutional) tall stature
Exogenous obesity
Excess growth hormone (GH) secretion (pituitary adenoma with gigan­
tism)
McCune-Albright syndrome or multiple endocrine neoplasia (MEN) as­
sociated with excess GH secretion
Marfan syndrome
Klinefelter syndrome (XXY karyotype)
Fragile X syndrome
Homocystinuria
XYY karyotype
Postnatal Overgrowth Leading to Adult Tall Stature
Familial (constitutional) t

 to Adult Tall Stature
Androgen or estrogen deficiency/estrogen resistance (in males)
Testicular feminization
Excess GH secretion
TABLE 25.7
987
metabolism, is characterized by hyperextensible joints, dislocation 
of the lens, kyphoscoliosis, dissecting aortic aneurysm, and long, 
thin bones that result in arachnodactyly and moderately tall 

that result in arachnodactyly and moderately tall stat­
ure. It is caused by mutations in the fibrillin-1 gene (FBN1). The 
abnormal FBN1 monomers from the mutated gene disrupt the 
normal aggregation of FBN1, impairing microfibril formation. 
Homocystinuria is an autosomal recessive disorder that pheno­
typically resembles Marfan syndrome, but patients also have cog­
nitive disabilities. Tall stature has also been found in patients with 
familial ACTH resistance due to a defective ACTH receptor

l ACTH resistance due to a defective ACTH receptor.1025
Dosage effects of the SHOX gene may result in tall stature.1026 
In females with three copies of the SHOX gene and gonadal dys­
genesis, adult stature was +2 to +2.9 SDS.1027 In women with 
the 47,XXX karyotype, mean final heights are 5 to 10 cm taller 
than population means, and men with the 47,XXY karyotype 
(Klinefelter syndrome) are about 3.5 cm taller than population 
means.31,1028 Males with an XYY karyotype may also have moder­
ately

s with an XYY karyotype may also have moder­
ately tall stature. In addition to SHOX effects, the variable degree 
of estrogen production in these syndromes may also influence 
skeletal maturation and final height.26
Failure to enter puberty and complete sexual maturation may 
result in sustained growth during adult life with ultimate tall stat­
ure and a characteristic eunuchoid habitus. The description of 
tall stature with open epiphyses resulting from mutation of the 
estrogen receptor or fr

ting from mutation of the 
estrogen receptor or from aromatase deficiency23,24 underscores 
the fundamental role of estrogen in promoting epiphyseal fusion 
and termination of normal skeletal growth. 
Obesity
Obesity is frequently associated with rapid skeletal growth and 
early onset of puberty.1029 Rapid early postnatal weight gain has 
been associated with taller stature at 8 years of age than that pre­
dicted from the midparental target height.1030 Others have shown 
that early postnatal gro

t.1030 Others have shown 
that early postnatal growth is associated with altered tempo of 
development, but sudden weight gain in middle childhood has 
little effect on height trajectory.1031 Patients with obesity tend 
to have diminished overall GH production but normal to high 
GHBP, and IGF1 levels appear to be capable of maintaining 
adequate or enhanced linear growth velocity. Bone age is usually 
modestly accelerated, so that both puberty and epiphyseal fusion 
occur early and adult height

nd epiphyseal fusion 
occur early and adult height is normal. This association between 
obesity and rapid growth is so characteristic that a child with obe­
sity and short stature should always be evaluated for underlying 
pathology, such as hypothyroidism, GHD, Cushing syndrome, 
or PWS. 
Tumors
Pituitary gigantism is a rare condition, analogous to acromegaly 
in the adult (see Chapter 9).1032–1034 GH-secreting tumors of 
the pituitary are typically eosinophilic or chromophobic adeno­
mas. Thei

ally eosinophilic or chromophobic adeno­
mas. Their cause is uncertain, although many result from somatic 
mutations that generate constitutively activated G proteins with 
reduced GTPase activity (see Chapter 9).1035 The resulting increase 
in intracellular cAMP in the pituitary leads to increased GH secre­
tion. Somatotropic tumors with excess GH secretion may occur in 
McCune-Albright syndrome, which is caused by mutations result­
ing in constitutive activation of G proteins.1036,1037 GH-secr

itutive activation of G proteins.1036,1037 GH-secreting 
tumors have also been reported in multiple endocrine neoplasia 
(MEN) and in association with neurofibromatosis and tuberous 
sclerosis.1038
GH excess that occurs before epiphyseal fusion results in 
rapid growth and attainment of adult heights above expected 
adult potential. When GH hypersecretion is accompanied by 
gonadotropin deficiency, accelerated linear growth may persist for 
decades, as was the case for the Alton giant, who reach

es, as was the case for the Alton giant, who reached a height 
of 280 cm by the time of his death in his 20s.1039 Manifestations 
typical of acromegaly may also appear, such as soft tissue swelling; 
enlargement of the nose, ears, and jaw with coarsening of facial 
features; pronounced increases in hand and foot size; diaphoresis; 
galactorrhea; and menstrual irregularity. 
Evaluation and Treatment of Growth 
Abnormalities
Clinical Evaluation of Growth Retardation
The most important parameter in

Growth Retardation
The most important parameter in assessing children with growth 
failure is careful clinical evaluation, including accurate serial assess­
ment of height and height velocity. Consideration of a growth 
disorder is raised when a child’s length or height falls below the 
normal range (<3rd percentile), the growth velocity is subnormal 
(indicated by length or height measurements that cross percentiles 
on the growth curve or by an annual growth velocity less than the 
3rd percent

 annual growth velocity less than the 
3rd percentile for age), or the child’s height is below the range 
expected based on the parents’ heights. To grow along the 3rd 
percentile for height, a child must maintain a height velocity at 
the 25th percentile for age.1040 Therefore a height velocity consis­
tently below the 25th percentile in a short child suggests abnormal 
growth. However, because of the greater error intrinsic to assess­
ment of growth velocity compared with height velocity,1041 

rowth velocity compared with height velocity,1041 a sin­
gle height velocity measurement above the 25th percentile for age, 
even based on annual height data, cannot fully exclude a growth 
abnormality in a short child.1041
GH is relatively less important as a growth factor prenatally 
compared with its role postnatally. However, GH is not without 
impact on prenatal growth, so that although many of these chil­
dren have lengths and weights within the normal range, the aver­
age size of infants 

n the normal range, the aver­
age size of infants with congenital GH deficiency is decreased, 
with a mean birth length of –1.3 SDS and a mean birth weight of 
–1.0 SDS.1042 Growth velocity over the first year averages near –2 
SD, so that over half of these infants have a length more than 2 SD 
below the mean at 1 year of age.1042
Table 25.8 provides an algorithm for evaluation of the child 
with growth failure. Although one-third of healthy infants will 
cross downward on the length percentile

ants will 
cross downward on the length percentile growth curve (discussed 
previously), this is normal only in relatively large infants born 
to small parents or in those infants who are following a growth 
pattern of CDGD (see later discussion). In other situations, the 
infant who is crossing downward on the length percentile curve 
should be investigated in the same way as other children with sub­
normal growth velocities.
The many illness-related causes of d

l Examination
The many illness-related causes of diminished growth were dis­
cussed in earlier sections of this chapter. A growth pattern in 
which weight gain is impaired before linear growth or in which 
there is greater impairment of weight gain than of length/height 
gain suggests an impairment of nutrition, such as inadequate 
intake, malabsorption, or increased energy requirements. Non­
hormonal causes of growth failure should be investigated based on 
data obtained from a careful history 

ed based on 
data obtained from a careful history and physical examination. In 
addition, careful evaluation of the growth curve in the context of 
the family history may suggest a normal variant growth pattern, 
988
  Clinical and Biochemical Evaluation of 
Growth Failure: Evaluating the GH-IGF1 
Axis
Step 1: Defining the Risk of Disorders of the GH-IGF1 Axis
Auxologic abnormalities
Severe short stature (height SDS <–3 SD)
Severe growth deceleratio

ture (height SDS <–3 SD)
Severe growth deceleration
Height <–2 SD and height velocity <–1.0 SD over 2 years
Height <–1.5 SD and height velocity <–1.5 SD over 2 years
Risk factors
History of a brain tumor, cranial irradiation, or other documented 
organic or congenital hypothalamic-pituitary abnormality
Incidental finding of hypothalamic-pituitary abnormality on MRI
If any of the above exists, proceed with step 2; if not, follow clinically 
and return to step 1 in 6 months
Step 2: Screening for D

turn to step 1 in 6 months
Step 2: Screening for Disorders of the GH-IGF1 Axis and 
Other Diseases
 A. Order a laboratory panel, including a bone age, free T4, TSH, chromo­
somes (in females), and nonendocrine tests; if indicated, refer back to 
primary care physician or treat diagnosed conditions as appropriate.
 B. Order an IGF1 and IGFBP3 level.
If IGF1/IGFBP3 are both above the –1 SD, follow clinically and return 
to step 1 in 6 months.
If IGF1/IGFBP3 are both below –2 SD, proceed to step 

IGF1/IGFBP3 are both below –2 SD, proceed to step 4. If MRI is 
abnormal, GH provocative testing is optional.
Otherwise, proceed to step 3. If this is a patient with delayed adoles­
cence, consider sex steroid treatment prior to step 3.
Step 3: Testing GH Secretion
This step can be bypassed if a clear GHD risk factor and severe IGF 
deficiency are identified.
Perform two of the following GH stimulation tests (if appropriate, estro­
gen prime) (see Table 25.11):
Clonidine
Arginine
Insulin
Glucago

e Table 25.11):
Clonidine
Arginine
Insulin
Glucagon
l-Dopa
Propranolol
If all GH levels are <7 μg/L, go to step 4.
If peak GH >15 μg/L, obtain GHBP; if GHBP <–2 SD, consider an IGF-
generation test and, if abnormal, IGF treatment.
If peak GH >15 μg/L and GHBP is normal, follow clinically and return to 
step 1 in 6 months.
If peak GH is between 7 and 15 μg/L, return to step 1 in 6 months.
Step 4: Evaluating the Pituitary
Perform MRI, with particular emphasis on hypothalamic-pituitary anatomy.
Tes

ar emphasis on hypothalamic-pituitary anatomy.
Test HPA, if not already done (CRH stimulation or ITT), and teach cortisol 
supplementation as needed (must do this if MRI is abnormal).
Consider molecular evaluation of GH, GHR, or GHRHR and other potential 
genetic defects (see Fig. 25.2).
Step 5: Treating for Growth Promotion
Initiate GH treatment at appropriate dose levels.
If GHIS is suspected, consider IGF therapy, if available.
Regularly evaluate growth parameters, IGF1, and IGFBP3, as well a

ate growth parameters, IGF1, and IGFBP3, as well as 
compliance and safety (see Table 25.14).
GH secretion should be retested, according to adult GH assessment 
protocols, at the end of growth.
CRH, Corticotropin-releasing hormone; GH, growth hormone; GHD, growth hormone defi­
ciency; GHIS, growth hormone insensitivity; GHR, growth hormone receptor; GHRHR, growth 
hormone–releasing hormone receptor; HPA, hypothalamic-pituitary axis; IGF1, insulin-like 
growth factor 1; IGFBP3, IGF-binding protei

-like 
growth factor 1; IGFBP3, IGF-binding protein 3; ITT, insulin tolerance test; MRI, magnetic 
resonance imaging; SD, standard deviation; SDS, standard deviation score; T4, thyroxine; 
   
TABLE 25.8
  Key History and Physical Examination 
Findings That Suggest the Diagnosis of 
Growth Hormone Deficiencya
Findings That Suggest the Diagnosis of GHD
In the Neonate
Prolonged jaundice
Hepatitis
Microphallus
Traumatic delivery
In a Chi

Hepatitis
In a Child With Short Stature or Growth Failure
Head trauma or central nervous system infection
Consanguinity or an affected family member
Craniofacial midline abnormalities
Findings That Support Immediate Investigation for GHD
In a Child With Short Stature
Signs indicative of an intracranial lesion
Neonatal symptoms and signs of GHD
Auxologic findings
Severe short stature (<–3 SD)
Height <–2 SD and height velocity over 1 year of <–1 

ght <–2 SD and height velocity over 1 year of <–1 SD
A decrease in height SD of >0.5 over 1 year in children >2 years of 
age
A height velocity <–2 SD over 1 year
A height velocity >1.5 SD below the mean sustained velocity 
over 2 years
Signs of multiple pituitary hormone deficiency (MPHD)
aThe Growth Hormone Research Society 2000 Criteria.
GHD, Growth hormone deficiency; SD, standard deviation.
From Growth Hormone Research Society. Consensus guidelines for the diagnosis and treat­
ment of growt

delines for the diagnosis and treat­
ment of growth hormone (GH) deficiency in childhood and adolescence: summary statement 
of the GH research society. J Clin Endocrinol Metab. 2000;85:3990–3993.
   
TABLE 25.9
such as crossing linear percentiles of infancy, familial short stat­
ure, or CDGD. In such cases, careful observation may be all that 
is required. One-third of all infants have growth parameters that 
cross percentiles downward on the growth curve, and 3% of all 
children have a length 

owth curve, and 3% of all 
children have a length or height below the 3rd percentile. Most of 
these children have no disease or growth disorder and will dem­
onstrate this by having a normal growth velocity on continued 
observation.
The physical examination should look for evidence of an 
underlying organ-specific or systemic disease. It should also evalu­
ate for clues specific to growth disorders, such as findings sugges­
tive of genetic disorders (e.g., Noonan syndrome, RSS, or TS). In 
add

rders (e.g., Noonan syndrome, RSS, or TS). In 
addition, body proportions should be measured because skeletal 
disproportion suggests a skeletal dysplasia.
Findings on the history and examination may point to an 
increased likelihood of the presence of GHD (Table 25.9). Micro­
penis in a male newborn should always lead to an evaluation of the 
GH-IGF1 axis. Nystagmus, indicating neonatal blindness, sug­
gests hypopituitarism due to its association with optic nerve hypo­
plasia in the syndrome of

n with optic nerve hypo­
plasia in the syndrome of septo-optic dysplasia. A history of other 
midline defects, such as cleft lip and cleft palate,1043 or a single 
central incisor increases concern for hypopituitarism. Unexplained 
neonatal hypoglycemia, hepatitis, or prolonged jaundice should 
prompt an evaluation of pituitary function. Older children with 
GHD have less impaired weight gain than height gain, resulting in 
989
an increased weig

 and Aberrant Growth in Children
an increased weight for height; they are often described as having 
a “cherubic” appearance. Increased weight for height with growth 
failure is also characteristic of hypothyroidism. If the weight gain 
is dramatic, Cushing syndrome should be considered. (However, 
linear growth acceleration with excess weight gain is not consistent 
with Cushing syndrome.) Finally, GHD should be suspected in 
children with known or suspected CNS pathology (e.g., tumors, 
prior 

 or suspected CNS pathology (e.g., tumors, 
prior radiation therapy to the CNS, malformations, infection, 
trauma) or with documented deficiency of TSH, ACTH, antidi­
uretic hormone, or gonadotropin. 
If the history and physical examination do not suggest a specific 
disorder causing growth failure, and the growth pattern and fam­
ily history do not provide sufficient reassurance that the growth 
is following a normal variant growth pattern, it is necessary to 
perform laborat

rowth pattern, it is necessary to 
perform laboratory testing. In many cases, the testing does not 
identify an abnormality, and the child either is ultimately found to 
have a normal variant growth pattern or falls into the classification 
of ISS. The laboratory tests can be divided into those screening 
for disorders outside the GH-IGF1 axis and those evaluating that 
axis.
Screening Tests
Because a number of illnesses can cause growth failure either 
before or in the absence of other signs or

either 
before or in the absence of other signs or symptoms, it is nec­
essary to screen for these disorders in children with abnormal 
growth. A complete blood count looks for evidence of anemia, 
chronic infection, and inflammation. A complete blood chem­
istry panel provides evidence for silent renal disease (including 
renal tubular acidosis), liver disease, and disorders of calcium 
and phosphorus. The erythrocyte sedimentation rate is measured 
to look for evidence of disorders involving c

red 
to look for evidence of disorders involving chronic inflamma­
tion, such as presymptomatic juvenile idiopathic arthritis and 
inflammatory bowel disease. A urinalysis is obtained to look for 
renal disease and chronic urinary tract infection. Tissue trans­
glutaminase IgA (and total serum IgA) is measured to screen for 
celiac disease. In girls in whom no other explanation for short 
stature is found, a karyotype should be obtained to exclude TS. 
This is done even in the absence of other p

e TS. 
This is done even in the absence of other physical features of TS, 
because growth failure may be the only evident feature, particu­
larly in cases of significant mosaicism.
Hypothyroidism should be screened for in children with 
growth failure. Because of the importance of thyroid hormone 
on brain development in infants, this possibility should be 
considered early in the evaluation of an infant with growth 
failure to correct identified hypothyroidism quickly. In addi­
tion, hypothyroi

 hypothyroidism quickly. In addi­
tion, hypothyroidism results in lower serum IGF1 levels and 
decreases GH levels during provocative testing.1044–1046 There­
fore it is necessary to ensure that thyroid function is normal 
before evaluating for GHD. TSH is measured because it is the 
most sensitive indicator of primary hypothyroidism. However, 
because central hypothyroidism must also be considered as a 
cause for growth failure in children, the thyroxine level should 
also be measured.
Bone Age

thyroxine level should 
Bone Age
After the neonatal period, a bone age determination can be useful 
in the evaluation of children with growth disorders. A radiograph 
of the left hand and wrist is commonly used for comparison with 
the published standards of Greulich and Pyle.1047 An alternative 
method for assessing bone age from radiographs of the left hand 
involves a scoring system for developmentally identified stages 
of each of 20 individual bones,1048 a technique that h

ach of 20 individual bones,1048 a technique that has been 
adapted for computerized assessment.1049,1050 The left hand is 
used because radiographs of the entire skeleton would be tedious 
and expensive and would involve additional radiation exposure. 
However, the hand does not contribute to height, and accurate 
evaluation of growth potential may require radiographs of the 
legs and spine.
Although the bone age result does not identify a specific 
diagnosis in a child with a growth disorder, i

ic 
diagnosis in a child with a growth disorder, it can be used to 
classify the child’s growth in relation to groups of diagnoses. 
Growth disorders caused by an underlying illness or hormone 
disorder (e.g., renal disease, malnutrition, glucocorticoid 
excess) are associated with a delayed bone age—that is, a bone 
age that is younger than the patient’s chronologic age. Similarly, 
hypothyroidism and GHD are associated with a delayed bone 
age. If short stature is intrinsic to the condition, h

 If short stature is intrinsic to the condition, however, bone 
age is not delayed and is within the range of normal for the 
chronologic age. This is true for genetic disorders such as TS, 
Noonan syndrome, and RSS, and also for familial short stature. 
In CDGD, the bone age is delayed, consistent with the future 
delay in puberty and late epiphyseal fusion. Given the lack of 
precise diagnostic laboratory tests for GHD, a lack of bone age 
delay argues against this diagnosis. On the other hand

y argues against this diagnosis. On the other hand, the fact 
that patients with CDGD have a delayed bone age does not help 
in the sometimes difficult discrimination between this condi­
tion and GHD.
A number of important caveats concerning bone age must 
be considered. Experience in the determination of bone age is 
essential to minimize intraobserver variance, and clinical studies 
involving bone age benefit from having a single reader perform 
all interpretations. The normal rate of skeletal

all interpretations. The normal rate of skeletal maturation differs 
between boys and girls and among different ethnic groups. The 
standards of Greulich and Pyle are separable by sex but were devel­
oped in American white children between 1931 and 1942. Both 
those and the Tanner and Whitehouse standards are based on nor­
mal children1051 and may not be applicable to children with skel­
etal dysplasias, endocrine abnormalities, or other forms of growth 
retardation or acceleration. 
Predictio

of growth 
Prediction of Adult Height
The extent of skeletal maturation observed in an individual can 
be used to predict the ultimate height potential. Such predic­
tions are based on the observation that the more delayed the bone 
age (relative to the chronologic age), the longer the time before 
epiphyseal fusion prevents further growth. The most commonly 
used method for height prediction, based on Greulich and Pyle’s 
Radiographic Atlas of Skeletal Development,

yle’s 
Radiographic Atlas of Skeletal Development,1047 was developed 
by Bayley and Pinneau1052 and relies on bone age, height, and 
a semiquantitative allowance for chronologic age (Table 25.10). 
The system of Tanner and colleagues1048 uses measurements of 
height, bone age, chronologic age, and, during puberty, height 
and bone age increments during the previous year, as well as men­
archal status. Roche, Wainer, and Thissen1053 used the combina­
tion of height, bone age, chronologic age, mid

na­
tion of height, bone age, chronologic age, midparental height, 
and weight (RWT method). Attempts have been made to cal­
culate final height predictions without requiring the determina­
tion of skeletal age1054 by using multiple regression analyses with 
available data such as height, weight, birth measurements, and 
midparental stature. These systems are, by nature, empiric and 
are not absolute predictors. Indeed, the 90% confidence inter­
vals for the predictions are approximately ±6 cm a

vals for the predictions are approximately ±6 cm at younger ages. 
The more advanced the bone age, the greater the accuracy of the 
990
  Prediction of Adult Stature
Bone Age (yr-mo)
FRACTION OF ADULT HEIGHT ATTAINED AT EACH BONE AGEa
GIRLS
BOYS
Retarded
Average
Advanced
Retarded
Average
Advanced
6-0
0.733
0.720
0.680
6-3
0.742
0.729
0.690
6-6
0.751
0.738
0.700
6-9
0.763
0.751
0.709
7-0
0.770
0.757
0.712
0.718
0.695
0.670
7-3
0.779
0.765
0.722
0.728

0
0.853
0.828
12-9
0.957
0.950
0.935
0.869
0.863
0.839
13-0
0.964
0.958
0.945
0.880
0.876
0.850
13-3
0.971
0.967
0.955
0.890
0.863
13-6
0.977
0.974
0.963
0.902
0.875
13-9
0.981
0.978
0.968
0.914
0.890
14-0
0.983
0.980
0.972
0.927
0.905
14-3
0.986
0.983
0.977
0.938
0.918
14-6
0.989
0.986
0.980
0.948
0.930
14-9
0.992
0.988
0.983
0.958
0.943
15-0
0.994
0.990
0.986
0.968
0.958
15-3
0.995
0.991
0.988
0.973
0.967
15-6
0.996
0.993
0.990
0.976
0.971
TABLE 25.10
991
Chapter 25  Normal and Aberrant Growth

E 25.10
991
adult height prediction because a more advanced bone age places 
a patient closer to his or her final height.
All methods of predicting adult height are based on data from 
normal children, and none has been documented to be accurate 
in children with growth abnormalities. For this kind of precision, 
it would be necessary to develop disease-specific atlases of skeletal 
maturation (e.g., for achondroplasia or TS). In addition, heigh

.g., for achondroplasia or TS). In addition, height 
predictions will clearly be inaccurate in predicting the final height 
in the case of a growth-impairing process that is not adequately 
treated. In addition, height predictions must be used with care 
in assessing height outcomes during treatment. For example, 
in patients with precocious puberty treated with GnRH ago­
nists, height prediction is known to overpredict the actual final 
height.1055 Thus height predictions should only be conside

055 Thus height predictions should only be considered a 
reasonable estimate. 
Tests of the GH-IGF1 Axis
In children with growth retardation in whom other causes have 
been excluded, the possibility of GHD should be considered. 
However, these tests can have poor specificity, so clinical assess­
ment must always play an important part in the evaluation of 
abnormalities in the GH-IGF1 axis. For example, a child growing 
consistently just below the 3rd percentile for height, with a growth 
rate t

e 3rd percentile for height, with a growth 
rate that is accordingly above the 25th percentile for age, is very 
unlikely to be GH deficient. Abnormal test results in this situation 
would most likely represent false-positive results.
Testing of the GH-IGF1 axis begins with “static” testing of 
GH function, measuring IGF1. It may also be helpful to mea­
sure IGFBP3. In some cases—if the clinical presentation is highly 
suggestive of GHD (see Table 25.9)1056 and the IGF1 level (and 
the IGFBP3 le

e 25.9)1056 and the IGF1 level (and 
the IGFBP3 level, if obtained) also indicates a high likelihood of 
GHD—it is appropriate to proceed directly to the dynamic tests 
of GH secretion. In most cases, however, unless there is an abnor­
mality identified on the screening tests, it is appropriate to moni­
tor the child’s growth for a period of 6 to 12 months to accurately 
assess the child’s growth rate. Then, based on the complete clinical 
picture, including the growth rate and the IGF1 level, t

e, including the growth rate and the IGF1 level, the decision 
is made to proceed with dynamic testing of GH secretion or to 
continue monitoring the child’s growth.
There is no test that definitively diagnoses GHD. Because 
there is no gold standard, it is impossible to precisely define the 
sensitivity or specificity of any test for GHD. Some informa­
tion on specificity can be obtained by comparing the results with 
those obtained in normal children, although, for the more com­
plicated tests

ildren, although, for the more com­
plicated tests, these data can be difficult to obtain in children. 
Sensitivity relies on comparing positive results from one test with 
those of another—for example, comparing low IGF1 concentra­
tions to failed results on provocative GH tests. Poor sensitivity 
of IGF1 as an indicator of GHD has been based on results from 
children with normal IGF1 concentrations who have abnormal 
results on provocative GH tests.1057 However, because of the 
known limited s

ests.1057 However, because of the 
known limited specificity of these provocative tests (discussed 
later), one cannot determine whether the discrepant results are 
due to the poor sensitivity of the IGF1 test or the poor specific­
ity of the provocative GH test. Again, it is critical to interpret all 
results together and in the context of the clinical data.
GHD is associated with low serum concentrations of IGF1 (Fig. 
25.40). Unlike GH levels, which rise and fall 

g. 
25.40). Unlike GH levels, which rise and fall with its pulsatile 
secretion, the IGF1 level in blood has minimal diurnal variation. 
The GH dependency of the IGFs was established in the initial 
GIRLS
BOYS
Retarded
Average
Advanced
Retarded
Average
Advanced
15-9
0.997
0.994
0.992
0.980
0.976
16-0
0.998
0.996
0.993
0.982
0.980
16-3
0.999
0.996
0.994
0.985
0.983
16-6
0.999
0.997
0.995
0.987
0.985
16-9
0.9995
0.998
0.997
0.989

97
0.995
0.987
0.985
16-9
0.9995
0.998
0.997
0.989
0.988
17-0
1.00
0.999
0.998
0.991
0.990
17-3
0.993
17-6
0.9995
0.9995
0.994
17-9
0.995
18-0
1.00
0.996
18-3
0.998
18-6
1.00
aThe column headed “Retarded” is used when bone age is >1 year below chronologic age; the column headed “Advanced” is used when bone age is >1 year greater than chronologic age.
Table derived from Post EM, Richman RA. A condensed table for predicting adult stature. J Pediatr. 1981;989:440–442 based on the data of Bayley and

. 1981;989:440–442 based on the data of Bayley and Pinneau.1052 Predicted final height is calculated 
by dividing the current height by the fraction of adult height achieved determined from the table.
   
  Prediction of Adult Stature—cont’d
992
report from Salmon and Daughaday164 and was further clarified 
with the development of sensitive and specific immunoassays that 
distinguish between IGF1 and IGF2.1058 IGF1 levels are more 
GH de

een IGF1 and IGF2.1058 IGF1 levels are more 
GH dependent than IGF2 levels and are more likely to reflect 
subtle differences in GH secretory patterns. However, serum 
IGF1 levels are also influenced by age,240,1059,1060 degree of sexual 
maturation, and nutritional status. Therefore IGF1 levels must be 
compared with age-specific ranges (Fig. 25.41) and with ranges 
defined by stage of sexual maturation. Some clinicians evaluate 
IGF1 results against the reference range based on bone age (rathe

ainst the reference range based on bone age (rather 
than the chronologic age). This may improve the specificity of this 
test for GHD, although there are no data to address the validity 
of this approach.
IGF1 levels in normal children younger than 5 or 6 years of age 
are low. This leads to poor sensitivity of IGF1 levels for identifying 
GHD in young children. As few as 40% to 50% of young, short 
children with evidence of GHD on provocative tests have IGF1 
levels below the lower level of th

ests have IGF1 
levels below the lower level of the reference range.1061,1062 IGF1 
levels increase with age, resulting in better separation of IGF1 lev­
els in GH-deficient children from those in normal children and 
a higher sensitivity of IGF1 levels for GHD. However, although 
sensitivities of 85% to 100% have been reported in some stud­
ies,1062,1063 in others it has averaged approximately 70%.1044,1063–
1066 Again, however, the lack of a gold standard means that some 
of the false-negative

ld standard means that some 
of the false-negative findings obtained by measurement of IGF1 
levels might rather represent false positives on provocative GH 
testing.
IGF1 levels also suffer from a lack of specificity in diagnosing 
GHD. In general, IGF1 levels have higher specificity in younger 
children, with declining specificity in older children. Juul and 
Skakkebaek1066 found that IGF1 levels had a specificity for GHD 
of 98% in children younger than 10 years of age but a specificity 
600

unger than 10 years of age but a specificity 
600
500
400
300
200
100
0
Normal subjects
Normal short stature subjects
Growth hormone–deficient subjects
C
F
1200
0
4
8
12
16
20
0
4
8
12
16
20
600
500
400
300
200
100
0
B
E
600
900
300
0
A
D
• Fig. 25.40  Serum levels of insulin-like growth factor 1 (IGF1) in normal 
patients and in patients with growth disorders. Circles represent IGF1 lev­
els in normal subjects (A and D), normal short stature su

normal subjects (A and D), normal short stature subjects (B and E), 
and growth hormone–deficient subjects (C and F). The lines represent the 
5th, 50th, 95th percentiles for log-normalized IGF1 levels in normal sub­
jects. (From Rosenfeld RG, Wilson DM, Lee PD, et al. Insulin-like growth 
factors I and II in the evaluation of growth retardation. J Pediatr. 1986;109:
428–433.)
IGF1 mean and SD by age for males
IGF1 mean and SD by age for females
10
0
B
5
10

10
0
B
5
10
15
20
25
100
1000
10
0
A
5
10
15
20
25
100
1000
Mean + 3SD
Mean + 2SD
Mean + 1SD
Mean - 1SD
Mean - 2SD
Mean - 3SD
Mean
Mean + 3SD
Mean + 2SD
Mean + 1SD
Mean - 1SD
Mean - 2SD
Mean - 3SD
Mean
• Fig. 25.41  Normal serum levels of insulin-like growth factor 1 (IGF1) for 
males (A) and females (B). SD, standard deviation. (Data courtesy Diag­
nostic Systems Laboratories, Inc., Webster, TX.)
993
Chapter 25  Normal and Aberrant Growth in Chi

993
of only 67% in those older than 10 years. Similarly, Cianfarani 
and colleagues1061 found a specificity of 100% in children younger 
than 9 years of age that dropped to 76% in older children. Thus 
the overall specificity of a low IGF1 measurement may only be 
approximately 70%.1044,1063,1064
In addition to the specific challenges of accurately quantitating 
IGF1 levels in serum (see later discussion), the nutritional depen­
dence of IGF1 l

iscussion), the nutritional depen­
dence of IGF1 levels significantly affects the accuracy of this test in 
evaluating for GHD. Even a few days of decreased caloric intake 
can lower IGF1 levels.1044,1067 This probably contributes to the 
finding that IGF1 levels can vary by as much as 38% from day to 
day in a given patient.1067,1068
Early quantitation of IGFs used bioassays based on [35S]-sul­
fate incorporation (hence “sulfation factor” as an early synonym 
for IGF/somatomedin C); on stimulat

early synonym 
for IGF/somatomedin C); on stimulation of the synthesis of 
DNA, RNA, or protein; or on glucose uptake.1069,1070 Develop­
ment of specific antibodies permitted the development of accu­
rate and specific measurement of IGF1 and IGF2.1058,1064,1071,1072 
However, the presence of IGFBPs results in a significant technical 
challenge for the accurate quantitation of IGF1 (and IGF2) in 
serum.620,1044,1069 Interference is a particular problem in condi­
tions with a relatively high IGFBP

oblem in condi­
tions with a relatively high IGFBP/IGF peptide ratio and at the 
extremes of the assay (i.e., GHD, acromegaly). In uremia, IGFBPs 
artifactually lower IGF1 levels and increase IGF2 levels in assays 
that do not eliminate this interference.749
The most effective way to deal with IGFBPs is to separate 
them from IGF peptides by chromatography under acidic con­
ditions.1073 This labor-intensive procedure has occasionally been 
replaced by an acid ethanol extraction procedure.1074 Al

ed by an acid ethanol extraction procedure.1074 Although 
the latter method may be reasonably effective for most serum 
samples, it is problematic in conditions of high IGFBP/IGF pep­
tide ratios, such as conditioned media from cell lines and sera 
from newborns or from subjects with GHD or uremia. A num­
ber of alternative approaches to addressing the interference from 
the IGFBs have been developed, including the use of tracers that 
do not bind to IGFBPs1075 and “sandwich” assay methods.1076 

d to IGFBPs1075 and “sandwich” assay methods.1076 
Automated IGF1 immunometric (IRMA) or immunochemilu­
minometric (ICMA) assays typically add an excess of IGF2 to 
the assay to displace the IGFBPs and use highly specific IGF1 
antibodies.1044,1069,1077 Although current IGF1 assays signifi­
cantly minimize the interference from IGBPs, it is important 
that each assay develop reference ranges that match the clinical 
samples to be tested because ethnic variations and nutritional 
and environmenta

thnic variations and nutritional 
and environmental factors can affect “normal” serum IGF1 con­
centrations. However, even with assay-specific reference ranges, 
the variability in IGF1 assays can result in a lack of concordance 
across assays when levels are near the upper or lower edge of the 
reference range, potentially misclassifying the results as “normal” 
or “abormal.”1078
A number of assays have been developed that purport to 
measure “free” or “free dissociable” IGF1 as a means of asse

ree” or “free dissociable” IGF1 as a means of assess­
ing concentrations of IGF1 peptides that circulate unbound to 
IGFBPs.1079 Both the accuracy and the physiologic relevance of 
these determinations remain open to debate.240,1044,1079 
Insulin-Like Growth Factor–Binding Protein 3
Measurement of the serum level of IGBP3, the major serum car­
rier of IGF peptides, is a potential additional tool for evaluating 
GH function.754,1080,1081 The advantages of assaying IGFBP3 
concentration include th

approximates the sum of the 
molar concentration of IGF1 and IGF2.)
	5.	 Serum IGFBP3 levels vary with age to a lesser degree than is 
the case for IGF1 (Fig. 25.42). Even in infants, serum IGFBP3 
levels are sufficiently high to allow discrimination of low values 
from the normal range.
	6.	 Serum IGFBP3 levels are less dependent on nutrition than 
serum IGF1, reflecting the “stabilizing” effect of IGF2 levels.
Like IGF1 levels, determination of the sensitivity and specific­
ity of IGBP3 leve

of the sensitivity and specific­
ity of IGBP3 levels for identifying GHD suffers from the lack 
of a gold standard. With that limitation in mind, Blum and col­
leagues754 initially found that low IGFBP3 levels had both high 
sensitivity (97%) and high specificity (95%) for GHD. Most sub­
sequent studies confirmed the high specificity, with values gener­
ally greater than 80% to 90%.1044,1061,1063,1065,1066 However, most 
0
A
5
10
15
20
25
15,000
10,000
8000
6000
5000
4000
3000
2500
2

0
25
15,000
10,000
8000
6000
5000
4000
3000
2500
2000
1500
1000
800
600
500
IGFBP3 mean and SD by age for males
IGFBP3 mean and SD by age for females
IGFBP3 (ng/mL)
0
B
5
10
15
20
25
15,000
10,000
8000
6000
5000
4000
3000
2500
2000
1500
1000
800
600
500
Mean + 3SD
Mean + 2SD
Mean + 1SD
Mean - 1SD
Mean - 2SD
Mean - 3SD
Mean
Mean + 3SD
Mean + 2SD
Mean - 2SD
Mean + 1SD
Mean - 1SD
Mean - 3SD
Mean
• Fig. 25.42  Normal serum levels of insulin-like growth factor–binding pro­

levels of insulin-like growth factor–binding pro­
tein 3 (IGFBP3) for males (A) and females (B). (Data courtesy Diagnostic 
Systems Laboratories, Inc., Webster, TX.)
994
studies have found that many children diagnosed with GHD on 
the basis of provocative GH testing actually have normal IGFBP 
levels, and the sensitivity of this test is less than 60%. In a study by 
Cianfarani and colleagues,1061 only 7% (2/28) of children younger 
than 14 years of a

 7% (2/28) of children younger 
than 14 years of age with GHD had an IGFBP3 level more than 
1 SD below the mean. Low IGFBP3 levels may be an indicator of 
more severe GHD. For example, in one study the sensitivity of the 
IGFBP3 assay was 93% if the peak GH was less than 5 μg/L on 
provocative testing, but when the peak GH was 5 to 10 μg/L, the 
sensitivity was only 43%.1082 
IGF2 levels are higher than those of IGF1. Levels increase rapidly 
after birth, but therea

. Levels increase rapidly 
after birth, but thereafter IGF2 levels are less age dependent than 
those of IGF1. However, although GHD is associated with low 
IGF2 levels, IGF2 is much less GH dependent than IGF1 (Fig. 
25.43). In two studies, IGF2 levels were more than 2 SD below 
the mean in only 21% and 31% of children defined as GH defi­
cient based on provocative GH testing.1061,1083
Rosenfeld and colleagues1064 found low IGF2 levels in 52% 
of GH-deficient children and in 35% of normal short

f GH-deficient children and in 35% of normal short children. 
However, only 4% of GH-deficient children and 11% of normal 
short children had normal plasma levels of both IGF1 and IGF2, 
showing a similar sensitivity and specificity to combined IGF1/
IGFBP3 testing. 
Assessment of pituitary GH production is difficult because GH 
secretion is pulsatile, with the most consistent surges occurring at 
times of slow-wave electroencephalographic rhythms during stage 
3 and stage 4 of sl

ographic rhythms during stage 
3 and stage 4 of sleep. Spontaneous GH secretion varies with gen­
der, age, pubertal stage, and nutritional status, all of which must 
be factored into the evaluation of GH production.
Between normal pulses of GH secretion, serum GH levels are 
low (often <0.1 μg/L), below the limits of sensitivity of most con­
ventional assays (typically <0.2 μg/L). Accordingly, measurement 
of random serum GH concentrations is almost useless in diagnos­
ing GHD but may be useful 

ost useless in diagnos­
ing GHD but may be useful in the diagnosis of GH insensitivity 
and GH excess. Measurement of GH “secretory reserve” relies on 
the use of physiologic or pharmacologic stimuli, and such provoc­
ative tests have formed the basis for diagnosis of GHD for more 
than 40 years.1084,1085
Assay Limitations. One of the biggest confounders in the 
evaluation of GH secretion is the variability of measured GH 
levels across different assays. Many studies have demonstrated as 
much a

 assays. Many studies have demonstrated as 
much as threefold variability in the measurement of serum GH 
levels among established laboratories.1086,1087 A more recent study 
found an interassay coefficient of variation of 24.3% among nine 
assays for values within the range of 5 to 10 μg/L and a maximal 
difference for a given sample as high as 11.4 μg/L.1088 This vari­
ability is explained, in part, by the presence of several molecular 
forms of GH in serum. Circulating GH consists of monomers

f GH in serum. Circulating GH consists of monomers of 
the two secreted GH isoforms (with ∼43% existing as the mono­
meric 22-kDa isoform and 8% as the monomeric 20-kDa iso­
form), along with dimers and higher-order oligomers of the two 
isoforms, acetylated forms of GH, desamidated GH, and peptide 
fragments of GH.1044 Therefore much of the variability in results 
is due to the use of different monoclonal or polyclonal antibod­
ies, including variability in recognition of the different circulat

riability in recognition of the different circulat­
ing forms of GH by these antibodies. Additionally, variations in 
the choice of standards, labeling techniques, and assay buffers 
(matrix) are also contributory.1044 Consequently, a child who is 
determined to have GHD by one assay may be considered normal 
by another.1088 
Provocative Tests. Because random GH levels cannot be used 
to diagnose GHD, evaluation of GH secretion requires that 
samples be obtained after an expected stimulation of 

ples be obtained after an expected stimulation of GH secre­
tion. Physiologic stimuli include fasting, sleep, and exercise, and 
pharmacologic stimuli include levodopa, clonidine, glucagon, 
1500
1200
900
600
300
0
IGF2 (ng/mL)
0
4
8
12
16
20
1500
1200
900
600
300
0
1500
1200
900
600
300
0
• Fig. 25.43  Serum insulin-like growth factor 2 (IGF2) levels in normal 
patients and in p

actor 2 (IGF2) levels in normal 
patients and in patients with growth disorders. (From Rosenfeld RG, Wil­
son DM, Lee PD, et al. Insulin-like growth factors I and II in the evaluation 
of growth retardation. J Pediatr. 1986;109:428–433.)
995
propranolol, arginine, and insulin. Standard provocative GH 
tests are summarized in Table 25.11. GH levels are measured 
on multiple specimens obtained after the stimulus. Failure to 
increase serum GH abov

r the stimulus. Failure to 
increase serum GH above a defined cutoff level is believed to 
indicate GHD.
Although provocative GH testing has been the foundation 
for the diagnosis of GHD since GH assays first became avail­
able, they have a number of weaknesses when used to identify 
GHD.1084,1089,1090 Most important, there is no clear cutoff level 
that discriminates a normal response from a deficient response; 
second, these provocative tests have poor specificity. 
Determination of the “Subno

ave poor specificity. 
Determination of the “Subnormal” Response to Provocative 
Tests. GH secretion has a continuous distribution; there is not a 
bimodal distribution of either spontaneous GH secretion or peak 
GH levels after provocative tests that would clearly separate nor­
mal from deficient secretion. The initial levels of GH during pro­
vocative testing that were used to define GHD were based on the 
study of patients with profound classic findings or organic destruc­
tion of the adenohy

c findings or organic destruc­
tion of the adenohypophysis.1091 In addition, partly because of the 
limited supply of GH, one goal was to identify the most severely 
affected children.
Initially, a cutoff level of 2.5 μg/L was used; this was later 
increased to 5 μg/L and subsequently to 7 μg/L. After the devel­
opment of recombinant DNA-derived GH eliminated the lim­
its on GH supply, many pediatric endocrinologists considered a 
peak GH level lower than 10 μg/L to be indicative of GHD. The 
hi

ower than 10 μg/L to be indicative of GHD. The 
higher cutoff level can be partially justified by the desire to iden­
tify children with GHD that is less complete (i.e., partial GHD). 
Nonetheless, there have been few data validating the higher cutoff 
values.1090,1092
Further confounding the determination of an evidence-based 
cutoff level is the variability in GH levels measured across differ­
ent assays (discussed earlier). Many new GH assays give results 
that are 33% to 50% lower than earli

give results 
that are 33% to 50% lower than earlier assays, but there has not 
been a systematic reassessment of the “new normal” GH cutoff 
levels, nor has there been a critical evaluation by many endocri­
nologists of which assay their center might be using.1093,1094 The 
same cutoff level has been applied without regard to the stimulus 
used. While insulin and arginine result in similar GH peaks, 
clonidine1095 and GHRH1096–1098 stimulate significantly higher 
levels of GH, thereby indicatin

nificantly higher 
levels of GH, thereby indicating a need for a higher cutoff value 
for such tests. 
Specificity of Provocative Tests for Growth Hormone Defi­
ciency. The data that are available suggest a low specificity for 
the provocative tests, with a substantial number of normal chil­
dren having peak GH levels lower than 7 to 10 μg/L.1089 Ghigo 
and colleagues1097 studied 472 healthy, normal children, includ­
ing 296 with normal stature and 177 with normal short stature. 
Excluding tests

nd 177 with normal short stature. 
Excluding tests that used GHRH (which has generally not been 
used in the evaluation of GH function in children), they found 
that between 10% and 25% of their subjects had peak GH levels 
lower than 7 μg/L and 23% to 49% had peak levels lower than 
10 μg/L. Similar results were found in other studies,1099–1102 with 
the 5th percentile for peak GH response to most provovactive 
stimuli being less than 5 μg/L in normally growing children.1103 
Because of this po

normally growing children.1103 
Because of this poor specificity, failure on two provocative tests 
should be obtained before GHD is diagnosed on the basis of pro­
vocative testing. This approach significantly improves the specific­
ity of provocative testing, although it remains imperfect. In the 
study of Ghigo and colleagues,1097 two tests were performed on 
  Tests to Provoke Growth Hormone Secretion
Stimulus
Dosage
Times Samples Are Taken (min)
Comments
Sleep
Obtain sample from indwelling 

min)
Comments
Sleep
catheter
60–90 min after onset of sleep
Exercise
Step climbing; exercise cycle for 10 
minutes
0, 10, 20
Observe child closely when on the 
steps
Levodopa
<15 kg: 125 mg
10–30 kg: 250 mg
>30 kg: 500 mg
0, 60, 90
Nausea, rarely emesis
Clonidine
0.15 mg/m2
0, 30, 60, 90
Tiredness, postural hypotension
Arginine HCl (IV)
10% arginine HCl in 0.9% NaCl over 
30 min
0, 15, 30, 45, 60
Insulin (IV)a
0.05–0.1 unit/kg
0, 15, 30, 60, 75,

0, 15, 30, 60, 75, 90, 120
Hypoglycemia, requires close 
supervision
Glucagon (IM)
0.03 mg/kg (max 1 mg)
0, 30, 60, 90, 120, 150, 180
Nausea, occasional emesis
GHRH (IV)b
1 μg/kg
0, 15, 30, 45, 60, 90, 120
Flushing, metallic taste
Patients must be euthyroid at the time of testing. Tests should be performed after an overnight fast. For prepubertal children, pretreating with sex hormones increases the specificity of the tests (see text).
aInsulin-induced hypoglycemi

the tests (see text).
aInsulin-induced hypoglycemia is a potential risk of this procedure, which is designed to lower the blood glucose by at least 50%. Documentation of appropriate lowering of blood glucose is recommended. 
If GHD is suspected, the lower dosage of insulin is usually administered, especially in infants. A solution of 10% dextrose in water (D10W) and glucagon should be available.
bThe cutoff used for tests involving GHRH is higher than that for other tests. (Maghnie M, Cavigioli 

 than that for other tests. (Maghnie M, Cavigioli F, Tinelli C, et al. GHRH plus arginine in the diagnosis of acquired GH deficiency of childhood-onset. J Clin 
Endocrinol Metab. 2002;87:2740–2744; Pandian R, Nakamoto JM. Rational use of the laboratory for childhood and adult growth hormone deficiency. Clin Lab Med. 2004;24:141–174.)
   
TABLE 25.11
996
78 children, and 10% had peak GH levels lower than 10 μg/L on 
both tests (2.6% had peak GH levels

an 10 μg/L on 
both tests (2.6% had peak GH levels <7 μg/L on both tests).
The specificity of provocative GH tests can be increased by 
using a lower cutoff point to define a normal response. However, 
this is undesirable if it excludes individuals with less severe degrees 
of GHD. Because of the continuous nature of GH secretion across 
individuals and the lack of a gold standard test for GHD, the 
conflict between specificity and sensitivity cannot be completely 
resolved. However, two reports

nnot be completely 
resolved. However, two reports have examined multiple clinical 
and laboratory characteristics of short children who were divided 
into three groups based on their results on GH stimulation test­
ing: a group whose results were low (<5 or <7 μg/L), fulfilling 
criteria for GHD; a group whose results were greater than 10 
μg/L and therefore were believed not to have GHD; and a group 
whose results were intermediate between these high and low cutoff 
points. In both of these st

e high and low cutoff 
points. In both of these studies, the group with the lowest peak 
GH levels differed significantly from both the intermediate-level 
and the high-level GH groups on numerous measures.1104,1105 
However, the intermediate group was indistinguishable from the 
group without GHD, rather than having characteristics interme­
diate between the other two groups, as might be expected if they 
had a less severe degree of GHD. As noted earlier, 5 μg/L is the 5th 
percentile for GH le

d earlier, 5 μg/L is the 5th 
percentile for GH levels on provocative testing of normally grow­
ing children; this same level of 5 μg/L identifies those children 
who will have the highest first-year growth response to GH treat­
ment.1103 Finally, a recent study using clonidine as the stimulus 
identified 3 μg/L (on an immunochemiluminescent assay) as the 
optimal cutoff to identify GHD in children and adolescents,1106 
while another identified cutoffs of 5.1 to 6.8 μg/L depending on 
the stimul

utoffs of 5.1 to 6.8 μg/L depending on 
the stimulus, with sensitivities of 88% to 93% and specificities of 
92% to 97%.1107 
Sex Hormone Priming. Serum GH levels rise during puberty, 
with GH secretion stimulated by the rise in estrogen produced 
from the ovary or from aromatization of testosterone.152,1108 This 
same process results in higher peak GH levels during provoca­
tive testing in pubertal children (compared with prepubertal chil­
dren)1100,1109 and in children who have received treatm

1100,1109 and in children who have received treatment with 
estrogen or testosterone.1057,1100,1109–1111 In a study by Marin and 
colleagues,1100 61% of normal prepubertal children and 44% of 
normal children in early puberty (Tanner stage II) had GH levels 
lower than 7 μg/L on provocative testing; based on these results 
(but not on their heights), they would have met criteria for a diag­
nosis of GHD. However, after 2 days of treatment with estrogen, 
95% of these children had a peak GH level

trogen, 
95% of these children had a peak GH level higher than 7 μg/L. 
Therefore the specificity of GH provocative tests can be improved 
by pretreating (“priming”) the pediatric patients with exogenous 
gonadal steroids.1112,1113 In a placebo-controlled comparison, the 
specificity of GH provocative tests (using 9 μg/L as a cutoff point 
on a polyclonal GH assay) increased from 80% to 98% after estro­
gen priming.1057 In a study of 50 growth-retarded boys who had 
subnormal results on provocat

tarded boys who had 
subnormal results on provocative GH tests without testosterone 
priming but normal results after priming, final height (without 
intervention) was greater than the midparental height, consistent 
with normal GH function in these children.1114 The most recent 
guidelines from the Pediatric Endocrine Society recommends sex 
hormone priming in prepubertal boys older than 11 years and in 
prepubertal girls older than 10 years.1103
Another factor to consider when evaluating provo

3
Another factor to consider when evaluating provocative GH 
testing is the impact of body weight on GH secretion. In both 
adults144,1115–1117 and children,1118,1119 obese individuals have 
decreased spontaneous and stimulated GH levels, compared 
with nonobese individuals. Even within the normal range, the 
BMI SDS is inversely associated with peak GH level on pro­
vocative tests in children.1120 Therefore particular care must be 
taken when interpreting GH stimulation test results in obese 
i

terpreting GH stimulation test results in obese 
individuals.
Although there are limitations to the information gained from 
provocative GH tests, they continue to be helpful in evaluating 
a child for GHD. The tests should be performed after an over­
night fast, and the patient needs to be euthyroid at the time of 
testing. Testing should not be performed if the patient is taking 
supraphysiologic doses of glucocorticoid (i.e., >15 mg/m2 per day 
of hydrocortisone or the equivalent of a synthet

of hydrocortisone or the equivalent of a synthetic glucocorticoid) 
because these drugs can suppress the GH response. The tests are 
generally safe, although appropriate precautions must be taken. 
Specifically, tests involving insulin administration carry the risk 
of hypoglycemia and seizures and should be performed only by 
experienced medical personnel and under appropriate patient 
supervision. Deaths have been reported from insulin-induced 
hypoglycemia and from its overly vigorous corre

d 
hypoglycemia and from its overly vigorous correction with par­
enteral glucose.1121 The specificity of the tests can be increased by 
pretreating the child with estrogen or testosterone (e.g., 1–2 mg of 
micronized estradiol1057 or 50–100 μg/day of ethinyl estradiol for 
3 consecutive days before testing, or 100 mg of depot testosterone 
3–7 days before testing) and by carefully selecting the lower limit 
for a normal response. 
Tests of Spontaneous Growth Hormone Secretion. Another 
diagnost

aneous Growth Hormone Secretion. Another 
diagnostic approach to evaluate GH secretion involves measure­
ment of spontaneous GH secretion. This can be done either by 
multiple sampling (every 5–30 minutes) over a period of 12 
to 24 hours or by continuous blood withdrawal over 12 to 24 
hours.103,1122–1124 The former method allows one to evaluate 
and characterize GH pulsatility, whereas the latter only permits 
determination of the mean GH concentration. Both methods are 
subject to many of the

ntration. Both methods are 
subject to many of the same limitations as provocative GH test­
ing. The problems of expense and discomfort are obvious, and, 
although it was thought that this approach might be more repro­
ducible than provocative GH tests, variability remains a prob­
lem.1095,1125,1126 The ability of such tests to discriminate between 
children with GHD and those with normal short stature is very 
limited because of significant overlap of each of the parameters 
measured between no

lap of each of the parameters 
measured between normal children and children with GHD. Rose 
and coworkers1127 reported that measurement of spontaneous GH 
secretion identified only 57% of children with GHD as defined by 
provocative testing. Similarly, Lanes1128 reported that one-fourth 
of normally growing children have low overnight GH levels, and a 
longitudinal study of normal boys through puberty demonstrated 
a wide intersubject variance, including many “low” 24-hour GH 
production rates 

including many “low” 24-hour GH 
production rates in children with fully normal growth.152,1129 
Therefore measurement of spontaneous GH secretion does not 
appear to offer advantages over alternative means of evaluating 
GH function. 
Summary. Despite the many problems associated with GH 
measurement methods, there continues to be value in deter­
mining GH secretory capacity in the diagnostic evaluation of a 
child with growth failure. Documentation of GH levels as being 
decreased, normal, or 

tion of GH levels as being 
decreased, normal, or increased helps in discriminating among 
GHD, non–GH/IGF1-related growth failure (including ISS), 
and GH insensitivity. Results supporting the presence of GHD 
alert the clinician to the possibility of other pituitary deficien­
cies. The presence of pituitary dysfunction mandates clinical 
and radiologic evaluation for evidence of congenital or acquired 
structural defects of the hypothalamus or pituitary, including 
the possibility of intracran

pituitary, including 
the possibility of intracranial tumors. Finally, documentation 
997
of GHD, either alone or combined with other pituitary defi­
ciencies, may warrant evaluation for molecular defects of GH 
Growth Hormone–Binding Protein
Mutation of the GHR impairs GH signaling, resulting in GH 
insensitivity. The most severe mutations cause profound growth 
failure (Laron dwarfism). The extracellular portion of the GHR is 
cle

ism). The extracellular portion of the GHR is 
cleaved from the remaining portion and circulates in the blood as 
GHBP, which can be measured in serum. Low levels, particularly 
undetectable levels, may be diagnostic of GH insensitivity due to 
GHR mutations.
GHBP levels are not low in all forms of GH insensitivity. The 
levels may be normal, or even increased, with mutations in the 
GHR that do not alter the GHBP portion of the protein (i.e., 
mutations in the transmembrane or intracellular dom

utations in the transmembrane or intracellular domains) or with 
defects that are downstream of the GHR. 
IGF1 and IGFBP Generation Tests
IGF1 and IGFBP3 generation tests are designed to evaluate for the 
presence of GH insensitivity.1130 When patients with GH insen­
sitivity are treated with GH for several days, the levels of IGF1 
and IGFBP do not increase as they would in normal individu­
als.1131,1132 Criteria for a response indicating GH insensitivity have 
included a rise in IGF1 of less t

ensitivity have 
included a rise in IGF1 of less than two times the intraassay varia­
tion (∼10%)617 or a failure to increase IGF1 by at least 15 μg/L.750 
Given the different protocols and cutoffs used to adequate define 
IGF1 generation, the recent Pediatric Endocrine Society guidelines 
suggest caution when using this diagnostic test and to combine it 
with other diagnostic tests, including genetic testing.1103 
Interpretation of Tests
Neonate
GH levels in the neonate are much higher than lev

GH levels in the neonate are much higher than levels seen after 
this period. Levels are highest in cord blood and within the first 
days of life.1133 Cornblath and colleagues reported in 1965 an 
average GH concentration in cord blood of 66 μg/L; this fell over 
the first week to an average of 16 to 20 μg/L in infants 7 to 55 
days of age.1133 However, even under the assay conditions used, 
the range of levels reported included measurements as low as 1 
μg/L. Subsequent studies confirmed the h

low as 1 
μg/L. Subsequent studies confirmed the high GH levels in neo­
nates, with many documenting cord blood levels in the range of 
20 to 40 μg/L, although other studies using similar assays found 
levels of 13 to 18 μg/L.1134 Other studies also confirmed that GH 
levels fall during the first week of life so that low basal values are 
seen by 1 to 2 months of age.1134,1135 GH levels in preterm infants 
have generally been found to be even higher than those in full-
term infants.1133
The neon

her than those in full-
The neonatal period is one period during which random mea­
surement of GH levels may be useful. However, low values can be 
found in healthy neonates.1135 Therefore, although a high value 
can exclude GHD, a single low value is not diagnostic of GHD—
except, perhaps, for the first few days of life in infants with a high 
probability of GHD.1103 An IGFBP3 measurement is of value for 
the diagnosis of neonatal GHD and should be measured in infants 
with su

tal GHD and should be measured in infants 
with suspected GHD; IGF1 levels are rarely helpful.1136 
Both clinical and laboratory evaluation must be utilized when 
considering the diagnosis of GHD in a child being evaluated 
because of short stature or growth failure.1056 As discussed earlier, 
there is no definitive test for the diagnosis for GHD. In addition, 
the laboratory tests for GHD have poor specificity and should 
be performed only in children who have a clinic

d 
be performed only in children who have a clinical presentation 
consistent with GHD. Short children who have well-documented 
normal height velocities usually do not require evaluation of GH 
function. Therefore the evaluation starts with identifying those 
children who may have GHD based on risk factors or growth 
parameters (see Tables 25.8 and 25.9).
In a child with a history and growth pattern that indicate 
a risk for GHD and in whom other causes for growth failure 
(including hypothyroi

r causes for growth failure 
(including hypothyroidism) have been excluded, testing for 
GHD begins with measurement of the IGF1 level and proceeds 
to GH provocation tests in selected patients (see Table 25.8). 
In some cases, particularly in younger children, it may also be 
helpful to measure the IGFBP3 level. If IGHD is suspected, two 
GH provocation tests (given sequentially or on separate days) 
are required. A patient is diagnosed with classic GHD when the 
IGF1 (or IGFBP3) level is below

ssic GHD when the 
IGF1 (or IGFBP3) level is below the normal range (i.e., >2 SD 
below the mean for age and pubertal status) and peak GH levels 
on two provocative tests are below the cutoff value that supports 
a diagnosis of GHD. The diagnosis is more firmly established if 
the child is pretreated with estrogen or testosterone before the 
provocative GH tests. In some patients, provocative testing of 
GH levels is not required to make the diagnosis. This includes 
children with growth failure

nosis. This includes 
children with growth failure and a pituitary hormone deficiency 
other than growth hormone in the setting of a known risk of 
hypopituitarism, such as anatomic abnormalities of the pituitary 
(such as ectopic posterior pituitary and pituitary hypoplasia with 
an abnormal stalk), tumor, or irradiation.1103 Similarly, a diagno­
sis of GHD due to congenital hypopituitarism can be diagnosed 
if random GH levels are below 5 ng/dL in the first week of life 
in an infant with hypo

 in the first week of life 
in an infant with hypoglycemia and either one other pituitary 
hormone deficiency or classic imaging findings (ectopic poste­
rior pituitary and pituitary hypoplasia with abnormal stalk).1103 
In patients who have had cranial irradiation or malformations of 
the hypothalamic-pituitary unit, GHD may evolve over years, 
and its diagnosis may require serial testing.
Some patients with auxology suggestive of GHD may have 
IGF1 or IGFBP3 levels below the normal range on re

IGF1 or IGFBP3 levels below the normal range on repeated tests 
but GH responses in provocation tests above the cutoff level. Such 
children do not have classic GHD but may have an abnormality of 
the GH-IGF1 axis. The child with a history of cranial irradiation, 
decreased height velocity, and reduced serum levels of IGF1 and 
IGFBP3 may have GHD (or GH insensitivity) even in the face 
of normal provocative tests.1137 Other children may have GHD 
that is not supported by the results of nonphysi

that is not supported by the results of nonphysiologic provocative 
tests (perhaps a milder degree of GHD than in those who fail the 
provocative tests), or they may have GH insensitivity (discussed 
later). Systemic disorders affecting the synthesis or action of IGF1 
must again be considered and excluded.
A difficult clinical situation to resolve is that of the short child 
with a persistently subnormal growth velocity whose IGF1 and 
IGFBP3 levels are normal. One can reasonably exclude cons

levels are normal. One can reasonably exclude consider­
ation of GHD if the IGF1 level is higher than 1 SD below the nor­
mal mean. However, because up to 30% of children with GHD 
identified by GH stimulation testing have had IGF1 levels that 
were not low,1044,1063–1066 it is appropriate to consider provocative 
GH testing in those children with persistent growth failure who 
have an IGF1 level between –1 SD and –2 SD for age and puberty 
status, with abnormal results on two tests indicating a

s, with abnormal results on two tests indicating a diagnosis 
of GHD. In this situation in particular, it would be appropriate 
to pretreat the child with testosterone or estrogen to maximize the 
specificity of the provocative tests.
998
A cranial MRI scan with particular attention to the hypotha­
lamic-pituitary region should be carried out in any child who is 
diagnosed as having GHD. In addition, documentation of abnor­
mal pituitary GH secretion

documentation of abnor­
mal pituitary GH secretion should prompt evaluation for other 
pituitary hormone deficiencies. Based on the clinical scenario, one 
may also consider molecular evaluation of GH, GHR, GHRHR, 
and other potential genetic defects. 
GH insensitivity is characterized by low serum IGF1 concentra­
tions in the presence of normal (or increased) production of GH 
(Figs. 25.44 and 25.45). As discussed earlier, GH insensitivity can 
be caused by defects 

rlier, GH insensitivity can 
be caused by defects of the GHR (Laron syndrome), the GH signal­
ing cascade, IGFBPs, IGF1, or the IGF1 receptor. GH insensitiv­
ity should be considered in a patient with growth failure who has 
a low IGF1 level but evidence of increased GH secretion based on 
increased basal or stimulated GH levels (e.g., basal GH levels >5 μg/L 
or stimulated levels >15 μg/L). This pattern is also seen with malnu­
trition, a form of physiologic GH insensitivity. If GH insensitivit

f physiologic GH insensitivity. If GH insensitivity is 
suspected, GHBP and ALS can be measured: Low values support the 
diagnosis of GH insensitivity due to mutations in the genes for the 
GHR or ALS, respectively. Growth attenuation with elevated IGF1 
levels and IGFBP3 levels may indicate disruption in IGF1 bioavail­
ability, as is seen in persons with mutation in the PAPP-A2 gene.298
The Pediatric Endocrine Society Guidelines suggest basing the 
diagnosis of primary IGF1 deficiency/GH insens

he 
diagnosis of primary IGF1 deficiency/GH insensitivity syndrome 
on a combination of factors, including testing of serum IGF1 and 
GHBP, exclusion of secondary IGF1 deficiency due to malnutri­
tion, liver disease, of GH deficiency, and possibly IGF1 genera­
tion test.1103 The IGF1 generation test has limitations and must 
be interpreted cautiously as there is limited normative data or stan­
dardization of such tests. Given the growing cadre of genes known 
to be involved in the GH-IGF1 growth

 genes known 
to be involved in the GH-IGF1 growth cascade, genetic testing 
may play a vital role in diagnosis in the near future. However, there 
are currently few clinical laboratories offering mutation analysis of 
the genes involved in the GH-IGF1 growth cascade.
<2.5
0–16
16.1– 67
100
90
80
70
60
50
40
30
20
10
0–16
16.1– 67
<STD
340
320
280
240
200
160
120
80
40
Basal growth hormone µg/L serum
Growth hormone–binding protein pmol/L serum
• Fig. 25.44  GH assays were performed 

e in years
• Fig. 25.44  GH assays were performed by radioimmunoassay (squares) 
and by immunoradiometric assay (triangles). For GHBP levels, open cir­
cles represent females, and closed circles represent males. Standard error 
of the mean (±SD) for GHBP levels in adult Ecuadorian control subjects 
are shown. STD, Assay standard. (From Rosenfeld RG, Rosenbloom AL, 
Guevara-Acquirre J. Growth hormone [GH] resistance due to primary GH 
receptor deficiency. Endocr Rev. 1994;15:369–390.)
0–16
16.1– 

iciency. Endocr Rev. 1994;15:369–390.)
0–16
16.1– 67
250
225
200
175
150
125
100
75
50
25
0
0–16
16.1– 67
700
600
500
400
300
200
100
0
0–16
16.1– 67
2800
2600
2400
2200
2000
1800
1600
1400
1200
1000
800
600
400
200
0
0–16
16.1– 67
600
550
500
450
400
350
300
250
200
150
100
50
7
3
8
13
7
3
8
13
IGF1 µg/L serum
IGF2 µg/L serum
IGF-binding protein—3 µg/L serum
IGF-binding protein—2% of control serum value
• Fig. 25.45  Serum levels of insulin-like growth factors (IGF) and IGF-binding 

insulin-like growth factors (IGF) and IGF-binding proteins in patients from 
Ecuador with growth hormone receptor deficiency. Open circles represent females, closed circles repre­
sent males. Control values for Ecuadorian adults are shown as mean ±SD for males and females, except 
for IGFBP3, for which large circles represent pooled male (closed circle) and female (open circle) control 
sera. Numbers adjacent to circles represent normal Ecuadorian children 3, 7, 8, and 13 years of age. (From 
Ro

n children 3, 7, 8, and 13 years of age. (From 
Rosenfeld RG, Rosenbloom AL, Guevara-Acquirre J. Growth hormone [GH] resistance due to primary GH 
999
Additional considerations related to GH insensitivity include 
the following:
	1.	 The presence of IUGR (in addition to severe postnatal growth 
attenuation) and developmental delay suggests IGF1 gene dele­
tions, bioinactive IGF1, or IGF type I 

sed in patients 
with molecular defects of IGF1.
	5.	 STAT5B defects should be suspected in a child with evidence 
of GH insensitivity who has evidence of immune deficiency; 
an elevated PRL level has also been reported in the presence of 
STAT5B mutation.1138 
The term constitutional delay of growth and development describes 
children who have a normal variant of maturational tempo charac­
terized by short stature with relatively normal growth rat

by short stature with relatively normal growth rates during 
childhood, delayed puberty with a late and attenuated pubertal 
growth spurt, and attainment of a normal adult height that is also 
within the target height range based on parental heights (Table 
25.12).39,1139 In childhood, such patients have heights that are 
lower than expected based on parental heights. CDGD aggre­
gates in families.1140 The diagnosis of CDGD may be suspected 
in a child with short stature if one or both parents h

 child with short stature if one or both parents have a his­
tory of a late timing of puberty. Bone age is usually delayed, so 
the predicted final height is in the normal range and is within 
the child’s target height range, although the correlation between 
predicted and final height is imperfect and must be viewed with 
caution.44,1141,1142 The predicted final height, especially when 
the skeletal age is extremely delayed, is greater than that usually 
achieved but is difficult to reliably an

 usually 
achieved but is difficult to reliably anticipate.44,1142,1143 Although 
the findings described can lead to a presumptive diagnosis of 
CDGD, the diagnosis can only be made retrospectively, once the 
child demonstrates a late timing for puberty and completes his or 
her growth in the normal range and at a height consistent with the 
parents’ heights.
Because of the estrogen stimulation associated GH secre­
tion that occurs during puberty, children with CDGD can be 
expected to have decr

, children with CDGD can be 
expected to have decreased GH secretion and lower IGF1 lev­
els compared with their pubertal, age-matched peers. However, 
IGF1 levels should be normal for pubertal stage, and GH levels 
on provocative tests should be normal after pretreatment with 
gonadal steroids.1112,1143–1145 Overnight GH secretion is usu­
ally normal in these children when control groups are carefully 
matched.1146
It is often difficult to differentiate CDGD from GHD. Both 
groups of patients h

rentiate CDGD from GHD. Both 
groups of patients have height SDS below their target height SDS 
range, and both have delayed bone ages. As the child with CDGD 
enters the usual age for puberty, his or her growth rate can be too 
low to exclude GHD on that basis. (The growth rate normally 
declines throughout childhood until it increases at the time of the 
pubertal growth spurt; a child with CDGD and a delayed pubertal 
growth spurt may have an additional 2 or more years of declining 
growth vel

dditional 2 or more years of declining 
growth velocity, which can result in growth velocities <4 cm/year.) 
If there is a clear family history of CDGD, the likelihood is high 
that the child also has CDGD, and careful continued observation 
may be all that is needed. An increase in growth velocity after 
treatment with a short course of sex hormone can be taken as evi­
dence against the presence of GHD.1147 In some cases, however, 
a laboratory investigation to exclude GHD is necessary. In such

investigation to exclude GHD is necessary. In such 
children, levels of IGF1 and IGFBP3 that are low for pubertal 
stage or skeletal age and a poor GH response to provocative testing 
after priming with gonadal steroids would raise concern for GHD 
and would mandate the appropriate investigation for underlying 
disease (e.g., intracranial tumor). 
Genetic (Familial) Short Stature
The control of growth in childhood and the final height attained 
are polygenic in nature. Familial height affects th

re polygenic in nature. Familial height affects the growth of an 
individual, and evaluation of a specific growth pattern must be 
placed in the context of familial growth and stature. As discussed 
at the beginning of this chapter, calculations can be made to deter­
mine whether a child’s growth pattern is appropriate based on his 
or her parents’ heights. As a general rule, a child who is growing at 
a rate that is inconsistent with that of siblings or parents warrants 
further evaluation.
Gen

lings or parents warrants 
Genetic short stature (GSS), also called familial short stature, is 
a normal growth pattern that describes the growth of healthy indi­
viduals who fall at the lower extreme of the distribution of height 
(i.e., below the 3rd percentile). Their height is appropriate for 
their genetic potential based on the parents’ heights; that is, their 
height SDS is within the target height SDS range. Particularly 
when the midparental height is significantly a

ly 
when the midparental height is significantly above or below the 
mean, it is important to include adjustment for regression toward 
the mean when calculating this target height range.
Although the differentiation may not be complete, children 
with GSS are generally distinguished from children with CDGD: 
Those with GSS will have a final height that is below the 3rd per­
centile, whereas children with CDGD will achieve a final height 
in the normal range. The midparental height of children w

normal range. The midparental height of children with 
GSS is lower than that of children with CDGD; often, height 
in both parents is below the 10th percentile. Growth in children 
with GSS is at or below the 3rd percentile, but the velocity is usu­
ally normal. The onset and progression of puberty are normal, so 
the skeletal age is concordant with chronologic age. Final heights 
in these individuals are in the target height range for the family.43 
By definition, the GH-IGF1 system is normal 

females, >150 cm (59 inches)
TABLE 25.12
1000 SECTION VI    Endocrinology and the Life Span
of the GHR gene; GH gene deletions; mutations of the PROP1, 
POUF1, or SHOX genes; pseudohypoparathyroidism; and some 
forms of hypothyroidism. Inherited nonendocrine diseases char­
acterized by short stature include osteochondrodysplasias and 
dysmorphic syndromes associated with IUGR (both described 
earlier), inborn errors of metabolism, renal disease, and thalas­
semia. Identifying a patient’s short 

 and thalas­
semia. Identifying a patient’s short stature as inherited does not, 
by itself, relieve the clinician of responsibility for determining 
the underlying cause of growth failure. Moreover, a parent’s short 
stature may be the result of an uncharacterized genetic differ­
ence that has been transmitted to the child and is causing the 
child’s short stature. Because height is normally distributed in 
the population, it can be arbitrary whether one characterizes such 
genetic differences 

ether one characterizes such 
genetic differences as mutations or as allelic variants. However, 
the further the parents’ and the child’s heights are from the mean, 
the more reasonable it is to consider such genetic alterations as 
abnormal. 
Idiopathic Short Stature
ISS is defined as “a condition in which the height of the individual 
is more than 2 SD below the corresponding mean height for a 
given age, sex and population group, and in whom no identifi­
able disorder is present.”1138 This de

o identifi­
able disorder is present.”1138 This definition includes children with 
CDGD, those with GSS, and short children who will not have 
delayed puberty and whose height is not consistent with parental 
heights. Therefore this definition includes both normal, healthy 
children (those with GSS and CDGD) and children who are pre­
sumed to have an unidentified disorder impairing their growth. 
It is not always a simple matter to distinguish among these possi­
bilities: CDGD can be definitivel

ong these possi­
bilities: CDGD can be definitively diagnosed only at the comple­
tion of growth, and GSS does not exclude inherited disorders of 
growth.
Children with ISS may have undiagnosed disorders outside 
the GH-IGF1 axis (e.g., an uncharacterized chondrodystrophy) 
or they may have subtler disorders of the hypothalamic-pitu­
itary-IGF axis than those identified by the currently available 
diagnostics tests.640,1148 Because of the lack of a gold standard 
for the diagnosis of GHD, the di

 a gold standard 
for the diagnosis of GHD, the distinction between isolated 
partial GHD and ISS is somewhat arbitrary, relying heavily on 
the results of the nonphysiologic provocative stimulation tests. 
The activity of different GH promoter haplotypes can differ as 
much as sixfold.602,1149 Some children with ISS may have GH 
neurosecretory dysfunction that cannot be detected with cur­
rent diagnostic tests.1122,1123 Similarly, although the severe GH 
insensitivity of Laron syndrome can be i

evere GH 
insensitivity of Laron syndrome can be identified by laboratory 
testing, partial GH insensitivity may be an unrecognized cause 
of ISS.1150
Heterozygous mutations in the GHR have been found in sig­
nificant numbers of children with ISS.639,642,1140 In heterozygotes, 
protein from the mutant allele may disrupt the normal dimer­
ization and rotation that is needed for normal GHR activation, 
leading to diminished GH action and growth impairment.215 In 
addition, GHBP expression may be d

airment.215 In 
addition, GHBP expression may be decreased in patients with 
ISS, 20% of whom have serum levels of GHBP below the normal 
range.1150–1152 Other potential causes for partial GH insensitivity 
in ISS include heterozygous mutations in other components of 
the growth system, a relatively greater preponderance of blockers 
of the GH-signaling cascade (e.g., enhanced intracellular phos­
phatase activity, production of such signaling factors as SOC2 and 
CIS), gene-mediated alterations 

tors as SOC2 and 
CIS), gene-mediated alterations in patterns of GH or IGF pro­
duction, or other possibilities yet to be discovered.1153 
Treatment of Growth Failure
When growth failure is the result of a chronic underlying disease, 
such as renal failure, CF, or malabsorption, therapy must first 
be directed at treatment of the underlying condition. Although 
growth acceleration may occur in such children with GH or 
IGF1 therapy, complete catch-up requires correction of the pri­
mary medical 

h-up requires correction of the pri­
mary medical problem. If treatment of the underlying condi­
tion involves glucocorticoids, growth failure may be profound 
and is unlikely to be correctable until steroids are reduced or 
discontinued.
Correction of growth failure associated with chronic 
hypothyroi­dism requires appropriate thyroid replacement. As dis­
cussed earlier, thyroid therapy causes dramatic catch-up growth 
but also markedly accelerates skeletal maturation, potentially lim­
iting ad

tes skeletal maturation, potentially lim­
iting adult height.
Treatment of Constitutional Delay
CDGD is a normal growth variant with delayed pubertal matu­
ration and a normal adult height. Most subjects can be managed 
by careful evaluation to rule out other causes of abnormal growth 
or delayed puberty combined with appropriate explanation and 
counseling. A family history of CDGD is frequently a source of 
reassurance. The skeletal age and Bayley-Pinneau table are often 
helpful in explaining

ley-Pinneau table are often 
helpful in explaining the potential for normal growth to the 
patient and parents. The predicted final height is usually greater 
than that achieved, especially when the skeletal age is extremely 
delayed, but this is difficult to reliably anticipate.44,1141,1154
On occasion, the stigmata of short stature and delayed matura­
tion are psychologically disabling for a preadolescent or teenage 
patient. Some adolescents with delayed puberty have poor self-
image and limi

ith delayed puberty have poor self-
image and limited social involvement.1155 In such patients and 
in some in whom pubertal delay is predicted based on the overall 
clinical picture, there may be a role for judicious short-term treat­
ment with androgen.
Androgen (Oxandrolone and Testosterone)
Two aspects of CDGD in boys are addressed by androgen treat­
ment: short stature, especially in boys 10 to 14 years of age, and 
delayed puberty after age 14. In the younger group, in whom 
CDGD is the pr

 14. In the younger group, in whom 
CDGD is the presumed cause of short stature, the orally admin­
istered synthetic androgen oxandrolone has been used extensively 
to accelerate growth so that height increases into (or closer to) 
the normal range sooner than it would without treatment.1156 
In several controlled studies,1139,1157–1161 oxandrolone therapy 
for 3 months to 4 years increased linear growth velocity by 3 
to 5 cm/year without adverse effects and without decreasing 
either actual116

e effects and without decreasing 
either actual1161–1163 or predicted1158,1162,1164 final height. (This 
treatment does not increase the final height of these boys.) The 
growth-promoting effects of oxandrolone appear to be related to 
its androgenic and anabolic effects rather than to augmentation of 
the GH-IGF1 axis.1165,1166 The lack of a measurable effect on the 
GH-IGF1 axis probably reflects the fact that oxandrolone cannot 
be aromatized to estrogen. Currently recommended treatment is 
0

to estrogen. Currently recommended treatment is 
0.05 to 0.1 mg/kg orally per day.
Oxandrolone is a relatively weak androgen, and its use stim­
ulates only minimal pubertal masculinization. In older boys in 
whom the delayed pubertal maturation is highly stressful and 
anxiety provoking, testosterone enanthate has been adminis­
tered intramuscularly with success.1156,1157,1167 Criteria for treat­
ment in such adolescents should include (1) a minimum age of 
1001
Chapter 25  Normal and Aberrant G

mum age of 
1001
14 years, (2) height below the 3rd percentile, (3) prepubertal or 
early Tanner G2 stage with an early-morning serum testosterone 
level of less than 3.5 nmol/L (<1 ng/mL), and (4) a poor self-
image that does not respond to reassurance alone. Therapy con­
sists of intramuscular testosterone enanthate, 50 to 100 mg every 
3 to 4 weeks, for a total of four to six injections.1155,1165,1167,1168 
Patients typically show early secon

165,1167,1168 
Patients typically show early secondary sex characteristics by the 
fourth injection and grow an average of 10 cm in the ensuing 
year. Testosterone enhances growth velocity by direct actions, 
increases GH production, and may have a direct effect on IGF1 
secretion.970,1108,1165,1169–1171 Brief testosterone regimens do not 
cause overly rapid skeletal maturation, compromise adult height, 
or suppress pubertal maturation.1172 It is important to emphasize 
to the patient that he is

 important to emphasize 
to the patient that he is normal, that therapy is short term and is 
designed to provide some pubertal development earlier than he 
would on his own, and that treatment will not increase his final 
adult height. In such situations, the combination of short-term 
androgen therapy, reassurance, and counseling can help boys with 
CDGD to cope with a difficult adolescence.
The availability of several new forms of testosterone, which are 
approved for adults with hypogonadism

, which are 
approved for adults with hypogonadism, provides adolescents with 
an opportunity for a choice among different androgen replace­
ment therapies. Testosterone gel is painless and easy to apply and 
has proved popular since its release.1173 However, there are con­
cerns about environmental contamination, including reports of 
precocious puberty in children caused by topical testosterone use 
by an adult in the household.1174–1176 If topical testosterone is pre­
scribed, careful instruc

ical testosterone is pre­
scribed, careful instruction must be given to avoid such inadver­
tent exposure of others. Testosterone patches also avoid the need 
for injections, but they are often poorly tolerated because of local 
skin reactions. The most recent testosterone products include a 
solution applied in the axilla and a nasal gel. Another disadvan­
tage of these topical testosterone preparations and the transdermal 
patches is the need for daily application. The dosing of these alter­
n

or daily application. The dosing of these alter­
native forms of therapy in children and adolescents has not been 
established, and care must be taken to avoid treating with too high 
a dose, which risks compromising final height.
Patients must be reevaluated to ensure that they enter “true” 
puberty. One year after testosterone treatment, boys should have 
testicular enlargement and a serum testosterone level in the puber­
tal range. If this is not the case, a diagnosis of hypothalamic-pitu­
it

not the case, a diagnosis of hypothalamic-pitu­
itary insufficiency or hypogonadotropic hypogonadism should be 
considered. Although the diagnosis of constitutional growth delay 
remains most likely in such patients, some eventually prove to be 
gonadotropin deficient, especially if they are still prepubertal late 
in adolescence.
Referrals for CDGD are more common in boys than girls. 
When CDGD is a problem in girls, short-term estrogen ther­
apy may be used, but the acceleration of skeletal ma

y may be used, but the acceleration of skeletal maturation is a 
greater hazard at doses that enhance growth velocity and sexual 
maturation. 
The final height of a child with CDGD will be in the normal range 
and appropriate for the child’s genetic potential. No treatment is 
needed for these children to achieve a normal height. However, 
as discussed earlier, the diagnosis of CDGD cannot be confirmed 
until a late puberty and normal height are achieved. Therefore it 
can be diff

mal height are achieved. Therefore it 
can be difficult in some cases to distinguish these children from 
children with ISS or GHD. In such cases, there may be uncer­
tainty about whether the final height will be in the normal range, 
and treatment to try to increase final height may be considered. If 
laboratory studies support a diagnosis of GHD, then GH treat­
ment would be appropriate. If there is no evidence of GHD, the 
consideration regarding treatment with GH would be the same as 
that r

ing treatment with GH would be the same as 
that regarding the use of GH to treat ISS (discussed later).
The Food and Drug Administration (FDA) indication for GH 
treatment includes in its definition of ISS the criterion that the 
child has “growth rate[s] unlikely to permit attainment of normal 
adult height.” A child with CDGD, who is expected to achieve a 
normal adult height, would not fit within this definition. None­
theless, partly because of the uncertainty of the diagnosis and the 
inab

 of the uncertainty of the diagnosis and the 
inability to perfectly predict final height, the database on out­
comes of children treated with GH for ISS includes within it data 
on the treatment of children with CDGD. ISS GH treatment tri­
als may include children with significant bone age delay,1177,1178 
at least some of whom have CDGD. For this reason, the outcome 
data for GH treatment of ISS (see later) can be taken as an indica­
tion of the expected outcome of GH treatment of CDGD. More­

e expected outcome of GH treatment of CDGD. More­
over, because estimated final height gain with GH treatment of 
ISS is proportional to the bone age delay at the time of initiation 
of treatment,1179 height gain with GH treatment of CDGD may 
be greater than that for treatment of ISS. However, a retrospec­
tive study specifically reporting the outcome of GH treatment in 
CDGD found no difference in adult height between those treated 
with GH and those receiving either no treatment or testostero

 those receiving either no treatment or testosterone 
treatment.1180 
Aromatase Inhibitor
In view of the important role of estrogen in the process of skeletal 
maturation, aromatase inhibitors could be used in conjunction 
with androgen therapy to prevent an acceleration of bone age and 
further enhance final adult height.1181,1182 The one report of com­
bined use of an aromatase inhibitor (letrozole) and testosterone 
did not provide a clear answer as to whether the addition of an 
aromatase in

wer as to whether the addition of an 
aromatase inhibitor increased final height in boys with CDGD. 
Whereas the near-final height in the boys treated with letrozole 
plus testosterone was higher than that of boys treated with tes­
tosterone alone, the letrazole-treated boys were a year older at the 
time of these height measurements and had higher pretreatment 
and midparental heights.1182 In addition, the boys treated with 
testosterone alone achieved a near-final height within the normal 
ran

chieved a near-final height within the normal 
range (consistent with the diagnosis of CDGD). Given the lack of 
long-term safety data on the use of aromatase inhibitors in puber­
tal boys, there are insufficient data to suggest a role for aromatase 
inhibitors in CDGD. 
Treatment of Growth Hormone Deficiency
Nomenclature and Potency Estimation
The nomenclature for the various biosynthetic GH preparations 
reflects the source and the chemical composition of the product. 
Somatropin refers to GH 

position of the product. 
Somatropin refers to GH of the same amino acid sequence as that 
in naturally occurring human GH. Somatropin from human pitu­
itary glands is abbreviated GH or pit-GH; somatropin of recombi­
nant origin is termed recombinant GH or rGH. Somatrem refers to 
the methionine derivative of recombinant GH and is abbreviated 
met-rGH. Although the latter is a more antigenic preparation, 
that propensity is not clinically relevant; despite the presence of 
anti-GH antibodies, gr

t; despite the presence of 
anti-GH antibodies, growth responses to met-rGH were similar 
to those in patients treated with rGH.1183,1184 This derivative of 
GH is no longer available for use. In this discussion, we refer to 
the biosynthetic preparations as GH.
1002
The biopotency of commercially available biosynthetic GH 
preparations, expressed as international units per milligram of the 
second WHO recombinant GH reference reagent for somatropin 

O recombinant GH reference reagent for somatropin 
98/574, is 3 IU/mg.1185 It was necessary to standardize the early 
GH preparations by bioassay because of variable production tech­
niques (e.g., extraction, column purification). The most common 
bioassays have been the hypophysectomized rat weight-gain assay, 
the tibial width assay, and the more sensitive Nb2 rat lymphoma 
proliferation assay.1186–1188 With the availability of purified and 
essentially equivalent recombinant GH products, the 

sentially equivalent recombinant GH products, the require­
ment for bioassays has become an FDA requisite to substantiate 
biologic activity rather than to assess potential differences among 
preparations. The bioassays are likely to be replaced by in vitro 
binding assays using GHRs or GHBP derived from molecular 
techniques. 
Because untreated patients with GHD have profound short 
stature (averaging almost –5 SDS1189,1190), the clinical urgency 
to use GH therapy as soo

0), the clinical urgency 
to use GH therapy as soon as it became available is under­
standable.1092 The action of GH is highly species specific, and 
humans do not respond to animal-derived GH (except that 
from other primates).1191 Human cadaver pituitary glands 
were for many years the only practical source of primate GH 
for treatment of GHD, and more than 27,000 children with 
GHD worldwide were treated with pit-GH.1192 The limited 
supplies of pit-GH, the low doses used, and interrupted tre

of pit-GH, the low doses used, and interrupted treat­
ment regimens resulted in incomplete growth increments; usu­
ally, therapy was discontinued in boys who reached a height of 
5 feet 5 inches and in girls who reached 5 feet. Nonetheless, 
this treatment did increase linear growth and in many patients 
enhanced final adult height. The dose-response relationship 
and the relation of age to GH response were recognized during 
this period.1193
Distribution of pit-GH was halted in the United State

tribution of pit-GH was halted in the United States and 
most of Europe in 1985 because of concern about a causal rela­
tionship with Creutzfeldt-Jakob disease (CJD), a rare and fatal 
spongiform encephalopathy that had been previously reported 
to be capable of iatrogenic transmission through human tissue. 
In North America and Europe, this disorder has an incidence 
of approximately 1 case per 1 million in the general popula­
tion, and it is exceedingly rare before the age of 50 years. To 
dat

ceedingly rare before the age of 50 years. To 
date, more than 200 young adults who had received human 
cadaver pituitary products have been diagnosed with CJD, with 
the sad likelihood that all affected patients will die of the dis­
ease.1194–1196 The onset of CJD has occurred 5 to 42 years after 
treatment, with a mean incubation of 17 years.1196 No cases of 
CJD have been identified in Americans who began treatment 
after new methods of purifying the hormone came into use in 
the United State

ing the hormone came into use in 
the United States in 1977.
By the time the risks of pituitary-derived GH were discovered, 
biosynthetic GH was being tested for safety and efficacy.1183,1197,1198 
The original recombinant GH mimicked pit-GH in regard to both 
anabolic and metabolic actions and was scrupulously scrutinized 
for monoisomerism, antigenic bacterial products, and toxins of 
any sort. GH has universally replaced pit-GH as the accepted treat­
ment for children with GHD. 
Treatment Reg

 treat­
Treatment Regimens
The recommended therapeutic starting dose of GH in children 
with GHD is 0.16 to 0.24 mg/kg body weight per week, admin­
istered in seven daily doses.1103,1199 Alternative regimens include 
a 6-day/week and a 3-day/week schedule, with the same weekly 
dosage, but they are not as successful. In general, the growth 
response to GH is a function of the log-dose given, so increas­
ing dosages further enhance growth velocities,1192,1193,1200 but

rther enhance growth velocities,1192,1193,1200 but 
daily dosing may be the most important treatment parameter.1201 
Subcutaneous and intramuscular administration has equivalent 
growth-promoting activity1202; the former is now used exclusively. 
At this time, all of the commercially available GH preparations 
yield comparable growth outcomes.
Growth responses to exogenous GH vary, depending on the 
frequency of administration, dosage, age (greater absolute gain in 
a younger child, although not

er absolute gain in 
a younger child, although not necessarily greater growth veloc­
ity SDS), weight, GHR type and amount (as assessed by serum 
GHBP levels), and, perhaps, seasonality.646,1203–1206 Nevertheless, 
the general regimen of daily GH at the recommended doses typi­
cally accelerates growth in a GH-deficient child from a pretreat­
ment rate of 3 to 4 cm/year to 10 to 12 cm/year in year 1 of 
therapy to 7 to 9 cm/year in years 2 and 3. Progressive waning 
of GH efficacy occurs and is p

Progressive waning 
of GH efficacy occurs and is poorly understood. The importance 
of dosage frequency is illustrated (Figs. 25.46 and 25.47) by data 
from a carefully done assessment of growth responses in prepu­
bertal naïve GH-deficient children randomly assigned to receive 
thrice-weekly or daily GH at the same total weekly dose (0.30 mg/
kg per week).1201 The mean total height gain during this period 
was 9.7 cm greater in the patients treated daily (38.4 vs. 28.7 
cm, p < 0.0002), with si

ted daily (38.4 vs. 28.7 
cm, p < 0.0002), with similar increments in skeletal maturation 
and no acceleration of the onset of puberty. Mean height SDS 
at the end of 4 years was +0.2 or at the midpoint of normal for 
Growth rate (cm/yr)
QD
TIW
n = 27
23
18
12
10
n = 29
28
22
14
13
Pretx
Year 1
Year 2
Year 3
Year 4
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
• Fig. 25.46  Annual growth velocity (mean ± standard deviation) for pre­
pubertal patients with growth hormone (GH) deficiency before and dur­
i

h growth hormone (GH) deficiency before and dur­
ing 4 years of GH treatment, contrasting results from daily (QD) versus 
thrice-weekly (TIW) injections. The mean annual growth velocity in the QD 
group was significantly greater than that in the TIW group during each 
year, although significance diminished from year 1 to year 4. (From Mac­
Gillivray MH, Baptista J, Johanson A. Outcome of a four-year randomized 
study of daily versus three times weekly somatropin treatment in prepu­
bertal naive 

eekly somatropin treatment in prepu­
bertal naive growth hormone deficient children. J Clin Endocrinol Metab. 
1996;81:1806–1809. Reproduced by permission of M.H. MacGillivray.)
1003
age. Studies using varying dosing of GH based on gender, growth 
responsivity, and growth factor concentrations have suggested the 
need for greater sophistication and individualization of the cur­
rent treatment regimens.1207,1208
Sophisticated mathematical models1

imens.1207,1208
Sophisticated mathematical models1200,1209 have exam­
ined many laboratory and auxologic parameters that influence 
response to GH therapy. Because age at onset of treatment is 
inversely correlated with growth responses, and because the 
smaller, lighter child requires less GH (with marked economic 
benefit), it is important to assess growth data in children treated 
early. In short-term studies of 134 patients1210–1212 treated before 
age 3 years, marked early catch-up growth o

efore 
age 3 years, marked early catch-up growth occurred, with a mean 
height gain of about 3 SDS by 4 years of therapy, allowing most 
children to reach their normal height range by middle childhood. 
Mean height in one study1210 reached –0.4 SDS after 8 years of 
treatment. Near-adult height in 13 patients treated before 5 years 
of age1213 did not differ significantly from the midparental target 
height (–0.9 vs. –0.7 SDS). In a group of 25 children treated 
before 12 months of age,1214 adul

hildren treated 
before 12 months of age,1214 adult height also matched the target 
height despite low dosage and less frequent administration. In 
an analysis of postmarketing data for development of a growth 
prediction model, a greater height gain per GH amount occurred 
in the very young children, but a seemingly lowered sensitivity to 
endogenous GH in early infancy adds complexity to interpreta­
tion of these data.1215 
Adult Height Outcomes
Treatment of growth-deficient children with pit-

s
Treatment of growth-deficient children with pit-GH significantly 
increased their final heights, although the majority had final 
heights below –2 SD.1216 This was likely due to the lower doses 
used, as well to treatment interruptions and the shorter duration 
compared to current treatment.
Patients treated with biosynthetic GH1092,1213,1216–1225 have 
improved final adult height SDS, with average final height in 
more than 1400 patients approximating –1.3 SD. Data from the 
two largest datab

ximating –1.3 SD. Data from the 
two largest databases,1217,1219,1224–1226 representing the North 
American and European experiences as reported by pediatric 
endocrinologists, are shown in Table 25.13.
Despite the use of GH therapy, long-term studies still show 
that most patients fail to reach their genetic target heights. Evalua­
tion of adult heights in 121 patients with childhood GHD treated 
in the Genentech GH research trials indicated a mean adult 
height in both male and female patients

ean adult 
height in both male and female patients of –0.7 SDS, with 106 
patients being within 2 SDS for normal adult Americans.1222 Even 
in these closely monitored patients, however, a difference of –0.4 
to –0.6 SDS from midparental target height still occurred. The 
achievement of the genetic target is possible, however: A Swedish 
subgroup (in the Kabi International Growth Study [KIGS] data­
base) of consistently treated patients reached a median final height 
SDS of –0.32, which was equiv

median final height 
SDS of –0.32, which was equivalent to the midparental target 
height.1226 A more recent study of the Swedish subgroup actually 
found a final height SDS greater than midparental target height by 
+0.2 (see Table 25.13).1217
Factors that have been found to correlate with enhanced 
adult height in GH-treated, GH-deficient children include base­
line height, younger age at onset of treatment, longer treatment 
duration (especially during prepubertal years), and a greater 
growt

ly during prepubertal years), and a greater 
growth velocity during the first year of treatment (Figs. 25.48 
and 25.49).1225 Increased height velocity and subsequent superior 
adult height outcome, although with considerable overlap, were 
demonstrated in children with GHD who carried one or both 
GHRd3 alleles (i.e., with an exon 3 deletion),646,1227,1228 although 
no difference in outcome was seen with GH treatment for ISS 
based on the GHRd3 allele polymorphism.1229 Data from the 
National C

allele polymorphism.1229 Data from the 
National Cooperative Growth Study (NCGS) and KIGS showed 
that the height gained during puberty in patients with GHD was 
generally comparable to that in healthy children with delayed 
bone age.1224,1230 As might be expected based on that observation, 
final height correlates with height at the onset of puberty in GHD 
patients.1205,1221,1231–1233 Therefore every effort should be made to 
enhance growth before puberty, and delays in diagnosis and ini­
tiat

ore puberty, and delays in diagnosis and ini­
tiation of therapy probably contribute to the compromised adult 
heights still reported in many studies.
GH treatment of children who develop GHD as a result of 
cranial irradiation represents a special case when evaluating final 
height outcomes. In these children, who have received cranial 
irradiation for the treatment of malignancy, GH treatment is 
not initiated until there is no evidence of active tumor; this 
results in a delay between the dia

ve tumor; this 
results in a delay between the diagnosis of GHD and the ini­
tiation of treatment. Final height is negatively correlated with 
the length of that lag time.1234 In addition, those who have 
received spinal irradiation in addition to cranial irradiation have 
a lower final height due to impairment of spinal growth after 
irradiation.1234,1235
In an effort to increase final height in GHD patients, the use 
of high-dose GH during puberty has been studied, based on the 
rationale that

rty has been studied, based on the 
rationale that GH secretion normally rises twofold to fourfold 
during the pubertal growth spurt, with dramatic concomitant 
Height SDS
QD
TIW
n = 27
23
18
12
10
n = 29
28
22
14
13
Pretx
Year 1
Year 2
Year 3
Year 4
2.0
1.0
0.0
−1.0
−2.0
−3.0
−4.0
−5.0
• Fig. 25.47  Height standard deviation score (SDS) for prepubertal patients 
with growth hormone (GH) deficiency before and during 4 years of GH 
treatment, contrasting results (mean ± standard deviation) with d

rasting results (mean ± standard deviation) with daily (QD) 
versus thrice-weekly (TIW) injections. The mean SDS in the QD group was 
significantly greater throughout the treatment period. Younger patients 
had the greatest increase in height SDS, and the effect of age was more 
marked in the QD group. (From MacGillivray MH, Baptista J, Johanson 
A. Outcome of a four-year randomized study of daily versus three times 
weekly somatropin treatment in prepubertal naive growth hormone defi­
cient chi

n prepubertal naive growth hormone defi­
cient children. J Clin Endocrinol Metab. 1996;81:1806–1809. Reproduced 
by permission of M.H. MacGillivray.)
1004 SECTION VI    Endocrinology and the Life Span
increases in serum IGF1 levels, and the pubertal growth spurt 
normally accounts for approximately 17% of adult male height 
and 12% of adult female height. Earlier studies by Stanhope and 
associates1236,1237 indicated that little difference in height gain 
could be observed when adolescent patien

ght gain 
could be observed when adolescent patients were treated with 
30 versus 15 IU/m2 per week of GH (∼0.04 vs. 0.02 mg/kg per 
day). Mauras and colleagues1238 evaluated higher pubertal GH 
doses (0.1 vs. 0.043 mg/kg per day) and found that the higher 
dosage resulted in a 4.6-cm increase in near-final height. Mean 
height SDS achieved in the group receiving 0.043 mg/kg per 
day of GH (as in the earlier report1222) was –0.7 ± 0.9, but in the 
group receiving 0.1 mg/kg per day, it was 0.0 ± 

group receiving 0.1 mg/kg per day, it was 0.0 ± 1.2. The higher 
GH dosage did not result in more rapid acceleration of skeletal 
  Adult Height in Children With Growth Hormone Deficiency Treated With 
Biosynthetic Growth Hormone
Study
Gender
N
GH Dose (mg/kg/wk)
Duration (yr)
Age (yr)
Height SDS
Change in 
Height SDS
Height vs. 
MPH
KIGSa
M
351
0.22
7.5
18.2
–0.8
+1.6
–0.2
F
200
0.20
6.9
16.6
–1.0
+1.6
–0.5
KIGS (Sweden)b
M
294
0.23
8.4
18.5
–0.9
+1.8
+0.2
F
107
0.23
8.5
17.4
–0.

23
8.4
18.5
–0.9
+1.8
+0.2
F
107
0.23
8.5
17.4
–0.7
+2.1
+0.2
NCGSc
M
2095
0.28
5.2
18.2
–1.1
+1.4
–0.7
F
1116
0.29
5.0
16.7
–1.3
+1.6
–0.9
F, Female; GH, growth hormone; KIGS, Pharmacia Kabi International Growth Study; M, male; MPH, midparental target height; NCGS, Genentech National Cooperative Growth Study; SDS, standard devia­
tion score.
aData from Reiter EO, Price DA, Wilton P, et al. Effect of growth hormone (GH) treatment on the near-final height of 1258 patients with idiopathic GH defic

l height of 1258 patients with idiopathic GH deficiency: analysis of a large international database. 
J Clin Endocrinol Metab. 2006;91:2047–2054.
bData from Westphal O, Lindberg A, Swedish KIGS National Board. Final height in Swedish children with idiopathic growth hormone deficiency enrolled in KIGS treated optimally with growth hormone. Acta 
Paediatr. 2008;97:1698–1706.
cData from August GP, Julius JR, Blethen SL. Adult height in children with growth hormone deficiency who are treated with bi

 growth hormone deficiency who are treated with biosynthetic growth hormone: the National Cooperative Growth Study experience. 
Pediatrics. 1998;102(2 Pt 3):512–516; updated NCGS data from Dana K, Baptista J, Blethen SL (personal communication, 2001).
   
TABLE 25.13
First-year height gain (SDS)
Total height gain (SDS)
8
6
4
2
0
-2
-4
6
4
2
0
-2
n = 667
r = 0.53
p < 0.001
KIGS data
• Fig. 25.48  Relationship between first-year change in height standard 
deviation score (SDS) and total change in 

andard 
deviation score (SDS) and total change in height SDS between start of 
growth hormone (GH) treatment and near-final height in children with 
idiopathic isolated GH deficiency. KIGS, Kabi International Growth Study 
database. (Modified with permission from Reiter EO, Price DA, Wilton P, 
et al. Effect of growth hormone [GH] treatment on the final height of 1258 
patients with idiopathic GH deficiency: analysis of a large international 
database. J Clin Endocrinol Metab. 2006;91:2047–2054.

abase. J Clin Endocrinol Metab. 2006;91:2047–2054.)
Prepubertal height gain (SDS)
8
6
4
2
0
-2
-4
6
4
2
0
-2
n = 245
r = 0.57
p < 0.001
KIGS data
• Fig. 25.49  Relationship between prepubertal change in height standard 
database. (Modified with permission from 

h Study 
database. J Clin Endocrinol Metab. 2006;91:2047–2054.)
1005
Another approach that has been taken to attempt to increase 
final height in GH-treated patients has been to adjust the dose 
of GH to reach a target IGF1 l

to adjust the dose 
of GH to reach a target IGF1 level rather than treating with a 
fixed, weight-based dose.1239,1240 A study reported by Cohen 
and colleagues found that targeting a high IGF1 level (reach­
ing a mean IGF1 SDS of approximately +1.5) resulted in an 
increase in growth rate compared with the two comparator 
groups, which had mean IGF1 SDS of approximately +1.1239 It 
required a GH dose approximately three times higher to reach 
this IGF1 level (0.11 mg/kg per day vs. 0.041 and 0.

is IGF1 level (0.11 mg/kg per day vs. 0.041 and 0.033 mg/kg 
per day for the comparator groups).1239 In the study reported 
by Marchisotti and colleagues,1240 the difference in IGF1 levels 
between groups (+0.8 SDS vs. –0.3 SDS) was even greater than 
that in the report by Cohen and colleagues,1239 but the Marchi­
sotti study did not find a difference in growth rate between 
the two groups. The difference in GH dose was much smaller, 
however (0.038 vs. 0.30 mg/kg per day). Therefore, as would 

038 vs. 0.30 mg/kg per day). Therefore, as would 
be expected, a higher GH dose results in a higher growth rate, 
but it is not yet known whether targeting a specific IGF1 level 
results in an increased final height; nor is it known whether these 
higher doses have adverse effects not seen with the more usual 
doses currently used. An interesting finding was the wide range 
of doses required to reach a given IGF1 level: The high-IGF1 
targeted group required GH doses ranging from less than 0.025

oup required GH doses ranging from less than 0.025 
mg/kg per day to greater than 0.25 mg/kg per day, whereas the 
low-IGF1 targeted group required doses ranging from less than 
0.025 to 0.15 mg/kg per day.1239
The impact of treatments to alter sex hormone levels (e.g., 
GnRH agonists, aromatase inhibitors) with the aim of improv­
ing final height in GHD and other conditions is discussed 
later. 
Benefits of GH Treatment Other Than Improved Growth
GH has effects beyond its impact on growth in ch

h
GH has effects beyond its impact on growth in childhood. Many 
non–growth-related benefits are discussed in the section on GH 
treatment in adulthood. There have been some data to suggest 
non–growth-related health benefits of GH treatment in child­
hood. A small study found treatment of GHD in childhood 
improved impaired cardiac function and structure identified in 
these subjects and improved body composition as seen with treat­
ment of adult GHD.1241 
Combined Pituitary Hormone Deficiencie

t GHD.1241 
Combined Pituitary Hormone Deficiencies
If GHD is part of a combined pituitary deficiency syndrome, it 
is necessary to address each endocrine deficiency, both for gen­
eral medical reasons and to ensure maximal effect of GH therapy. 
TSH deficiency is often “unmasked” during the initial phase of 
therapy, and thyroid function should be assessed before the onset 
of therapy during the first 3 months of GH treatment1242 and at 
least on an annual basis thereafter. The pituitary-adrena

n an annual basis thereafter. The pituitary-adrenal axis can 
be evaluated during the insulin stimulation test in the workup for 
GHD or separately if GHD is identified with the use of a differ­
ent provocative test. If ACTH secretion is impaired, patients may 
be placed on the lowest safe maintenance dose of glucocorticoids, 
certainly no more than 10 mg/m2 per day of hydrocortisone and 
less if possible. Higher doses impair the growth response to GH 
therapy but should be given during times of

to GH 
therapy but should be given during times of stress. It is critical 
to monitor the long-term evolution of glucocorticoid deficiency, 
especially in those with PROP1 mutations (discussed earlier). 
However, Lange and colleagues studied 24 adults with a prior his­
tory of idiopathic isolated childhood GHD and identified adrenal 
insufficiency in 10 of them, half of whom did not have evidence of 
ongoing GHD1243; this suggests a need to consider monitoring for 
impaired ACTH secretion even i

der monitoring for 
impaired ACTH secretion even in those patients with presumed 
IGHD.
Gonadotropin deficiency may be evident in the infant with 
microphallus. This can be treated with three or four monthly 
injections of 25 mg of testosterone enanthate in the first months 
of life.1244 Management at puberty can be more complicated, in 
that the physical and psychologic benefits of promoting sexual 
maturation must be balanced against the effects of epiphyseal 
fusion. When GH therapy is initia

s of epiphyseal 
fusion. When GH therapy is initiated in childhood and growth is 
normal before adolescence, it is appropriate to promote pubertal 
development with gonadal steroid replacement. 
Monitoring Growth Hormone Therapy
Patients treated with GH should be seen every 3 to 6 months to 
monitor their response to treatment (Table 25.14). An increase in 
linear growth velocity should be seen within the first 6 months. 
Height SDS should increase at least 0.25 SDS in the first year.1199 
Treat

se at least 0.25 SDS in the first year.1199 
Treatment models have been developed that predict the expected 
growth rate in response to GH treatment.1200,1209,1245–1247 A 
model1200 explaining 61% of growth response variability in the 
first year of therapy includes inverse relationships with maximum 
GH response during provocative testing, age, and height SDS 
minus midparental height SDS and positive correlation with body 
weight SDS, GH dose, and birthweight SDS.1248,1249 The single 
most imp

and birthweight SDS.1248,1249 The single 
most important predictive factor for growth in years 2 through 4 
is the first-year height velocity. These models can be used to quan­
tify whether the individual patient is responding appropriately to 
GH treatment. If a patient’s initial growth response is lower than 
predicted, the clinician should consider whether the diagnosis 
of GHD was correct, whether additional growth disorders (e.g., 
hypothyroidism) are present, and whether there is lack of c

idism) are present, and whether there is lack of compli­
ance with the treatment.1246
It is appropriate to monitor the IGF1 levels after initiation 
of GH treatment and perhaps yearly thereafter.1199,1250 A failure 
of IGF1 levels to increase into the normal range with treatment, 
together with an inadequate growth response, suggests compli­
ance failure or the presence of GH insensitivity. Because of the 
association of elevated IGF1 levels with certain cancers (see later 
discussion), a reduct

 certain cancers (see later 
discussion), a reduction in GH dose should be considered for 
patients with serum IGF1 levels substantially above the normal 
range after the first 2 years of therapy.1199 Whether the IGF1 level 
should be used to guide GH dosing awaits additional studies (see 
earlier discussion).
CPHD may evolve over several years, so children who are ini­
tially diagnosed with IGHD may develop CPHD. Thyroxine lev­
els should be assessed after GH treatment has been initiated, and 

essed after GH treatment has been initiated, and 
  Monitoring Growth Hormone Therapy
Close follow-up with a pediatric endocrinologist every 3–6 months
Determination of growth response (change in height SDS)
Evaluation of compliance
Screening for potential adverse effects
Interval measurements of serum IGF1 and IGFBP3
Annual assessment of thyroid function
Consideration of dose adjustment based on IGF values, growth 
response, and comparison with growth prediction models
Periodic reevaluation of

 growth prediction models
Periodic reevaluation of adrenal and thyroid function
IGBP, Insulin-like growth factor–binding protein; IGF, insulin-like growth factor; SDS, standard 
deviation score.
   
TABLE 25.14
1006 SECTION VI    Endocrinology and the Life Span
annually thereafter, to identify the development of central hypo­
thyroidism during treatment. Periodic reevaluation for ACTH 
deficiency should be performed. It is not necessary to routinely 
monitor fasting insulin and glucose levels, b

ely 
monitor fasting insulin and glucose levels, but if impaired glucose 
control is suspected, a fasting glucose level and glycosylated hemo­
globin (HbA1c) should be measured.
The growth response to GH typically attenuates after several 
years but should continue to be equal to or greater than the nor­
mal height velocity for age throughout treatment. Use of statistical 
growth treatment models can prove valuable in judging therapeu­
tic efficacy.1251–1253 A suboptimal response to GH can resul

cy.1251–1253 A suboptimal response to GH can result from 
several causes: (1) poor compliance, (2) improper preparation of 
GH for administration or incorrect injection techniques, (3) sub­
clinical hypothyroidism, (4) coexisting systemic disease, (5) exces­
sive glucocorticoid therapy, (6) prior irradiation of the spine, (7) 
epiphyseal fusion, (8) anti-GH antibodies, or (9) incorrect diag­
nosis of GHD as the explanation for growth retardation (particu­
larly in a patient with idiopathic IGHD 

(particu­
larly in a patient with idiopathic IGHD and normal findings on 
MRI). Although 10% to 20% of recipients of recombinant GH 
develop anti-GH antibodies, growth failure is rarely caused by 
such antibodies, except in the case of individuals who have GHD 
as a result of GH gene deletion.1254–1256 
Treatment During the Transition to Adulthood and in 
Adulthood
A growing challenge in the management of patients with GHD 
has been the issues surrounding their care after the growth process 
has

rrounding their care after the growth process 
has ceased.1257 This period, from the middle to late teenage years 
until the mid-20s, is a normal physiologic phase during which 
peak bone and muscle mass is achieved and the independence and 
self-sufficiency characteristics of adulthood are attained. It is also 
a time during which care of pediatric patients is transferred to 
endocrinologists who treat adults.
Clinical consequences of GHD in adults and the potential 
benefits of GH therapy in s

lts and the potential 
benefits of GH therapy in such patients have already been 
described.1258,1259 Signs and symptoms of adult GHD include 
reduced lean body mass and musculature, increased body fat, 
reduced BMD, reduced exercise performance, and increased 
plasma cholesterol. Adults with GHD have a significantly 
increased risk of death from cardiovascular causes, a finding 
potentially linked to increased visceral adiposity and other car­
diovascular risk factors.1260 Adults with GHD have 

iovascular risk factors.1260 Adults with GHD have been found 
to have “impaired psychological well-being and quality of 
life.”1261 Several placebo-controlled studies have demonstrated 
that GH therapy for adult GHD results in marked alterations 
in body composition, fat distribution, bone density, and sense 
of well-being.1259
Given the metabolic derangements associated with untreated 
GHD, documentation of persistent GHD and continuation of 
GH treatment in late adolescence in the patient who 

 treatment in late adolescence in the patient who shows persis­
tent GHD is important. Among nearly 500 patients with IGHD, 
207 (44%) had normal GH levels on provocative retesting.1262,1263 
In contrast, approximately 96% of patients with CPHD, with or 
without structural abnormalities of the hypothalamic-pituitary 
area, had sustained GHD.1262,1263 The PES Guidelines on GH 
treatment recommend that persons with more than two pitu­
itary hormone deficiencies be diagnosed with persistent GH 
def

 deficiencies be diagnosed with persistent GH 
deficiency.1103 Persons with two or less pituitary hormone defi­
ciencies, idiopathic GHD, idiopathic GHD with a small pituitary 
or isolated ectopic posterior pituitary, or persons postradiation 
require diagnostic evaluation for persistent GHD.
Many patients do not wish to continue the daily GH regimen, 
but the data support consideration for sustained treatment. After 
1 to 2 years off GH therapy, IGF1 and IGFBP3 levels decrease 
substantially be

 IGF1 and IGFBP3 levels decrease 
substantially below baseline levels.1258,1264,1265 Resumption of 
GH normalizes these levels, although there is a strong suggestion 
of a gender-based difference in GH requirements, with females 
needing higher GH doses.1258,1264,1265 Loss of energy and strength 
may occur, and some quality-of-life data suggest age-specific psy­
chologic issues in patients with untreated severe GHD during 
the transition to adulthood.1266 Quality-of-life data with rigor­
ous stu

hood.1266 Quality-of-life data with rigor­
ous study designs are lacking in GH-treated childhood GHD.1267 
In untreated patients, total body fat and abdominal fat increase 
significantly and lean body mass is lost, compared with control 
subjects, comparable GH-treated patients, and patients who have 
reinstituted therapy.1159,1268,1269
Because bone mass accrual is not completed until the third 
decade, late adolescence is an important time for GH sufficiency, 
to prevent later osteopenia.708,12

H sufficiency, 
to prevent later osteopenia.708,1257 There have been numerous GH 
treatment studies carried out in the transition age group to assess 
the impact on bone mineralization.1265,1268,1270,1271 Differences in 
age at onset of retreatment, duration of therapy, GH dosage, and 
gender distribution have led to variations in results. In general, 
however, the data affirm the concept that reinstitution of treat­
ment for 2 years in the transition period prevents deterioration 
in bone miner

ition period prevents deterioration 
in bone mineral density as experienced by untreated young adults 
with persistent GHD.
These studies support the continuation of GH treatment in late 
adolescence, albeit at lower doses than in childhood, to prevent 
the development of adverse cardiovascular risk, diminished bone 
mineralization, and a deterioration in quality of life. Whether the 
diversity of treatment and response data relates to the efficacy of 
the childhood GH therapeutic process is not

cy of 
the childhood GH therapeutic process is not clear, but the period 
of time off GH and the degree of persistent GHD would seem 
likely predictors of clinical status at the time of reinitiation of GH 
After the adolescent with GHD has completed skeletal growth, 
the persistence of GHD is evaluated.1103 Completion of skeletal 
growth is defined as growth rate less than 2 to 2.5 cm per year and 
bone age of 14 in females or 17 in males. GH at childhood doses 
should be discontinued

les. GH at childhood doses 
should be discontinued, and if evaluation for persistent GHD is 
needed, GH therapy should be halted for 1 to 3 months, followed 
by a thorough reevaluation. A recommended algorithm for this 
evaluation is shown in Fig. 25.50.1272
As in childhood GHD, laboratory testing is imperfectly precise 
for diagnosis of ongoing GHD. Some patients have results that 
neither indicate nor eliminate ongoing GHD but are intermedi­
ate, suggesting either probable or partial GHD. If o

e, suggesting either probable or partial GHD. If ongoing GHD 
is considered to be present or probable, assessment of body com­
position, BMD, and fasting lipid levels should be determined. The 
decision to reinitiate GH treatment is then based on a discussion 
with the patient and family regarding the risks and benefits in 
light of the laboratory test results and the risk for metabolic conse­
quences (see Fig. 25.50).1272 In addition, caution should be exer­
cised in considering whether to cont

ould be exer­
cised in considering whether to continue GH therapy when there 
is a known risk of diabetes or malignancy. This is also an oppor­
tunity for a thorough clinical reassessment and a determination 
of the need for replacement of other hormones. The transition to 
adult GH replacement should be arranged as a close collaboration 
between endocrinologists who treat pediatric patients and those 
who treat adult patients and should include discussion with the 
patient and family. 
1007
Cha

discussion with the 
1007
High probability of
persistent GHD
Acquired/congenital CPHD or
congenital isolated GHD with
structural abnormalities or
confirmed mutations
Moderate probability of
Acquired GHD from tumor,
irradiation, surgery, idiopathic CPHD
Low probability of
Idiopathic, isolated GHD
Off GH 2-3 months*
Low IGF1
Normal IGF1
GHRH-Arginine or insulin provocative testi

ider treatment
with GH 12 g/kg/day
Monitor BMD, fat mass,
LBM, quality of life
Monitor clinically
or discharge
• Fig. 25.50  Algorithm for evaluating patients diagnosed with GHD during childhood at the completion 
of growth. BMD, bone mineral density; CPHD, combined pituitary hormone deficiencies; GH, growth 
hormone; GHD, growth hormone deficiency; LBM, lean body mass. (From Molitch ME, Clemmons DR, 
Malozowski S, et al. Evaluation and treatment of adult growth hormone deficiency: an Endocrine

t of adult growth hormone deficiency: an Endocrine Soci­
ety clinical practice guideline. J Clin Endocrinol Metab. 2006;91:1621–1634; Growth Hormone Research 
Society. Consensus guidelines for the diagnosis and treatment of growth hormone [GH] deficiency in 
childhood and adolescence: summary statement of the GH Research Society. J Clin Endocrinol Metab. 
2000;85:3990–3993. *, Based on consensus and clinical practice guidelines. #, Threshold values that 
achieve a sensitivity of 95% and specific

es that 
achieve a sensitivity of 95% and specificity 91% to 92% for diagnosing GHD in adults; from Biller BMK, 
Samuels MH, Zagar A, et al. Sensitivity and specificity of six tests for the diagnosis of adult GH deficiency. 
J Clin Endocrinol Metab. 2002;87:2067–2079. †, Based upon data indicating that patients with irradiation-
induced CPHD can have hypothalamic dysfunction causing GHD; from Darzy KH, Aimaretti G, Wieringa G, 
et al. The usefulness of the combined growth hormone [GH]-releasing 

ess of the combined growth hormone [GH]-releasing hormone and arginine stimulation 
test in the diagnosis of radiation-induced GH deficiency is dependent on the post-irradiation time interval. 
J Clin Endocrinol Metab. 2003;88:95–102. ‡, Those at high risk for metabolic complications include 
patients with abnormal BMD, high fat mass, low LBM. ††, Those at low risk for metabolic complications 
include patients with normal BMD, fat mass, and LBM.)
1008
SECTION VI    Endocrinology and the Life Spa

1008
Growth Hormone Treatment of Other Forms of Short 
Stature
Growth failure due to PWS is an approved indication for hGH 
therapy in the United States and disordered body composition 
is an additional indication in Europe. In addition to promoting 
growth, hGH therapy improves body composition and physical 
strength, and increases energy expenditure.1273–1276
Guidelines for the clinical management of PWS were pub­
lished afte

e clinical management of PWS were pub­
lished after an international multidisciplinary expert meeting in 
2013.1277 Short stature due to GH insufficiency is almost always 
present in children with PWS. In a large cohort of 1135 children 
with PWS starting GH treatment, median height SDS was –2.2 
(range, –4.1 to –0.3) at a median age of 6.4 years (range, 1.3 to 
12.9 years).1278 Growth standards for infants with PWS have been 
pubished.1279
Serum levels of IGF1 are reduced in most children with 

 levels of IGF1 are reduced in most children with 
PWS.938,1280–1282 Studies have shown that spontaneous GH 
secretion is reduced, and 70% of children with PWS have GH 
peaks of less than 10 μg/L during pharmacologic stimulation 
tests.1273 Most experts agree that prior GH testing is not required 
before GH treatment is initiated. Numerous clinical trials have 
documented the efficacy of GH therapy to increase growth and 
improve body composition, energy expenditure, and strength and 
agility,12

, energy expenditure, and strength and 
agility,1283–1287 as well as increase growth and enhance gain of 
motor skills in infants and toddlers with PWS.1288,1289
In randomized, controlled studies there were significant 
increases in height and growth velocity and a significant decrease 
in percent body fat, as well as increased percentage of fat-free 
mass, improved muscle strength and agility, and increased fat oxi­
dation during the first year of GH treatment.1281,1290 Stabilization 
of these 

of GH treatment.1281,1290 Stabilization 
of these parameters occurs after the second year of therapy. Lean 
body mass increased significantly during the first 2 years in chil­
dren with PWS who received GH treatment, compared with no 
treatment.1220,1284 GH therapy for 2 additional years resulted in 
continued beneficial effects on body composition. When the dose 
of GH was reduced to 0.3 mg/m2 per day, these improvements 
were not maintained.1284 Beneficial effects continued after 8 years 
of G

4 Beneficial effects continued after 8 years 
of GH treatment.1287
BMD improves in children with PWS treated with GH. In 
two German cohorts, the mean spontaneous adult heights were 
reported in one study as 162 cm in boys and 150 cm in girls1291 
and the other as 159 cm in boys and 149 cm in girls.1292 In the 
KIGS database, 33 patients (21 boys and 12 girls) reached adult 
height, and two-thirds of them were above –2 SDS; the median 
adult height was –1 SDS after a mean duration of 8.4 years.1

ht was –1 SDS after a mean duration of 8.4 years.1293 
Another study with 21 adults (13 males, 8 females) revealed a 
mean adult height of 0.3 SDS after a mean duration of 7.9 years 
of GH treatment.1293 In this cohort, the strength and agility that 
were evident during the initial 2 years continued into adulthood. 
The patients also reported a higher quality of life and reduced 
depression.1294 In 55 children monitored during 4 years of con­
tinuous GH treatment (1 mg/m2 per day), body composit

uous GH treatment (1 mg/m2 per day), body composition was 
significantly improved, mean height normalized, head circumfer­
ence increased, and BMI significantly decreased. GH treatment 
had no adverse effects on bone maturation, blood pressure, glu­
cose homeostasis, and serum lipids.1295 Improved body compo­
sition and metabolic status were demonstrated in adults treated 
with GH in childhood and adolescence.1296
Before GH therapy is begun, screening for hypothyroid­
ism is indicated because of

ening for hypothyroid­
ism is indicated because of the association with primary and 
central hypothyroidism.1297,1298 Continued screening is also 
recommended. Treatment with GH beginning as early as 2 
years of age is recommended, but there may be additional ben­
efits to starting therapy between 6 and 12 months of age. Sev­
eral studies have found improvements in motor development, 
muscle tone, head circumference, and, possibly, cognition and 
behavior.1288,1299–1301
A recent review documents

A recent review documents the benefits of GH therapy, includ­
ing an increase in female height, improved body composition, and 
increased exercise tolerance.1302 Cognitive function was improved 
in children with treated GH.1303
Since October 2002, several reports of unexpected deaths in 
infants and children with PWS have been published.1304–1306 
Between 1985 and 2006, the NCGS monitored the safety and 
efficacy of recombinant human GH in 54,996 children. Two 
deaths we

binant human GH in 54,996 children. Two 
deaths were reported in patients with PWS.1307 Although death 
from presumed obesity-induced hypoventilation or apneic events 
in PWS without GH treatment is well described, the occur­
rence of such deaths during GH administration raises the ques­
tion of whether GH exacerbates this condition.1308,1309 Tonsillar 
hypertrophy and fluid retention associated with GH therapy are 
potential risk factors. Most of the deaths, with or without GH 
treatment, were 

f the deaths, with or without GH 
treatment, were related to obesity or to a complicated course of a 
relatively mild respiratory tract infection, sleep apnea, adenoid or 
tonsillar hypertrophy (or both), hypoventilation, and aspiration. 
The obesity-hypoventilation syndrome is the more likely etiologic 
factor, suggesting that ventilatory and pulmonary function should 
be assessed with polysomnographic studies before and during GH 
treatment.1308,1310
In a review that included 64 children (42 b

8,1310
In a review that included 64 children (42 boys and 22 girls, 
28 receiving GH treatment), the highest death risk occurred 
during the first 9 months of GH treatment.1306 Therefore it is 
recommended that GH treatment be started at a low dose (e.g., 
0.25–0.30 mg/m2 per day or 0.009–0.012 mg/kg per day) and 
increased during the first weeks and months to reach the standard 
replacement dosage of approximately 1 mg/m2 per day or 0.24 
mg/kg per week. Patients should be monitored for sleep a

per week. Patients should be monitored for sleep apnea and 
IGF1 levels. The GH dose should be decreased if there is evidence 
of high IGF1 levels, especially if associated with edema, worsening 
or new onset of snoring, headache, or acromegalic clinical fea­
tures. A longitudinal observational study evaluated 75 children 
with PWS and showed an increase in the apnea-hypoxia index 
(AHI) after GH treatment.1311
Five prospective studies have evaluated the effects of GH treat­
ment on breathing di

ated the effects of GH treat­
ment on breathing disorders in PWS.1285,1312–1314 Carbon dioxide 
responsiveness, resting ventilation, and airway occlusion pressure 
improved during 6 to 9 months of GH treatment,1314 and the 
inspiratory and expiratory muscle strength improved during 12 
months of GH treatment, compared with control subjects.1312 
In a double-blind, placebo-controlled, crossover study, AHI was 
found to decrease in 12 children with PWS, compared with con­
trol subjects, after 6 mo

 PWS, compared with con­
trol subjects, after 6 months of GH therapy although the differ­
ence was not statistically significant.1285 Another study found a 
decrease in AHI in most of the adults and children studied after 
6 weeks of GH therapy.1315 A subset of patients had an increased 
AHI with more obstructive events, but most of these latter 
patients had upper respiratory tract infections and adenoid/tonsil 
hypertrophy, and two of them had high IGF1 levels. In another 
study in 35 prepuber

high IGF1 levels. In another 
study in 35 prepubertal children with PWS, the AHI did not sig­
nificantly change during 6 months of GH therapy.1316 However, 
four of these children had an increase in the number of episodes 
1009
of obstructive apnea during an upper respiratory illness; these epi­
sodes were not present after recovery. Therefore it is recommended 
that obesity-related sleep and breathing problems be evaluated 
before and vigilantl

athing problems be evaluated 
before and vigilantly monitored after GH treatment begins. Poly­
somnography and ear, nose, and throat evaluations should be per­
formed as necessary. A longitudinal review of polysomnography 
data suggests that children younger than 2 years of age are most 
vulnerable to sleep-related disordered breathing (SRDB) after ini­
tiation of GH therapy.1317
A recent study1318 showed that 60% of PWS patients had cen­
tral adrenal insufficiency. This may explain the high rat

renal insufficiency. This may explain the high rate of sud­
den death, especially during infection-related stress. The authors 
concluded that patients with PWS should be treated with hydro­
cortisone during stress until adrenal insufficiency can be ruled 
out.1318,1319
Between 30% and 70% of children with PWS have scolio­
sis.1320–1325 Weight control is a vital part of its prevention and 
management. Therefore, before GH treatment is initiated, spinal 
radiographs and, if appropriate, orthopedi

spinal 
radiographs and, if appropriate, orthopedic assessment are recom­
mended. Reports of scoliosis worsening during GH treatment 
reflect the natural history of this condition rather than a side 
effect of treatment in most cases, and discontinuation of GH is 
not indicated.
In view of the childhood findings of low lean body mass and 
high fat content, osteopenia, and some degree of glucose intoler­
ance ameliorated by GH, the issue of long-term therapy through 
adulthood must be considered 

erm therapy through 
adulthood must be considered and studied.1326–1328 After growth 
is complete, attainment of a normal peak bone mass, continued 
improvement of muscle mass and strength, reduction of body fat, 
prevention of cardiovascular morbidity, and improvement in well-
being and quality of life are potential benefits of continued GH 
therapy.1329 Adult GHD and low IGF1 levels have been reported 
in patients with PWS.1282,1330 Short-term GH treatment in GH-
naïve adults with PWS has been

GH treatment in GH-
naïve adults with PWS has been reported to modestly improve 
body composition, cognition, motor performance, and social sta­
tus.1330 Further long-term studies are needed on adolescents with 
PWS transitioning to adult therapies. 
Chronic Renal Disease
Chronic renal disease (CRD) is regarded as an indication for the 
commencement of GH administration. Short stature is more 
severe in children with congenital renal disorders than in chil­
dren with acquired renal diseases.738,

an in chil­
dren with acquired renal diseases.738,1331,1332 Even after renal 
transplantation, final height is below the lower limit of normal 
in about 50% of children.1333 GH treatment is able to increase 
height velocity and height SDS748,1334,1335 and significantly 
improves final height1336–1338 in patients with CRD (Figs. 25.51 
and 25.52). The therapy should be implemented if short stature 
persists for longer than 6 months and in subjects with marked 
deceleration of growth velocity, and

 with marked 
deceleration of growth velocity, and it should be continued until 
transplantation is performed.1339 Typical growth patterns for the 
year after GH initiation have been devised. A growth velocity less 
than –1 SDS for age (in a child without dialysis or transplant) 
is inadequate and indicates presence of confounding factors that 
require investigation.1340
GH accelerates growth in children with CRD, at least over 5 
years of therapy.1341–1344 Using a GH dosage of 0.05 mg/kg per 
d

y.1341–1344 Using a GH dosage of 0.05 mg/kg per 
day, Fine and colleagues1334 reported a mean first-year growth rate 
of 10.7 cm in GH recipients and 6.5 cm in the placebo group; 
in the second year, GH-treated patients had a mean growth rate 
of 7.8 cm/year compared with 5.5 cm/year in placebo recipients, 
resulting in an improvement of height SDS from –2.9 to –1.5. 
Twenty patients who were treated for 5 years reached a normal 
height SDS of –0.7, with a mean height increase of 40 cm.1344 
The

.7, with a mean height increase of 40 cm.1344 
The youngest patients (<2.5 years of age) had the most impressive 
growth response to GH therapy (14.1 cm/year). Doses recom­
mended in children with CRD are higher than in GHD patients 
(i.e., about 0.35 mg/kg per week) because growth patterns in these 
Growth rate (cm/year)
Treatment year
0
1
2
3
4
5
6
12.2
8.3
6.8
6.5
6.2
16
12
8
4
0
14
10
6
2
• Fig. 25.51  Annual growth velocity (mean ± standard deviation) in 20 
growth-retarded prepubertal pati

deviation) in 20 
growth-retarded prepubertal patients with chronic renal insufficiency who 
were treated with growth hormone. (From Fine RN, Kohaut E, Brown 
D, et  al. Long-term treatment of growth retarded children with chronic 
renal insufficiency with recombinant human growth hormone. Kidney Int. 
1996;49:781–785. Reproduced with permission of R.N. Fine.)
Standardized height (SDS)
72
3
2
1
0
−1
−2
−3
−4
−5
−2.6
−1.4*
−1.0*
−0.9*
−0.8*
−0.7*
−12
0
12
24
36
48
60
• Fig. 25.52  H

*
−0.8*
−0.7*
−12
0
12
24
36
48
60
• Fig. 25.52  Height standard deviation score (SDS) in 20 growth-retarded 
prepubertal patients with chronic renal insufficiency (mean ± standard 
deviation). Note that the basal height is outside the normal range (at –2.6 
SDS), enters the normal range within 1 year of treatment initiation, and is 
not different from the mean by the fifth year of growth hormone therapy. 
(From Fine RN, Kohaut E, Brown D, et al. Long-term treatment of growth 
retarded children 

 Long-term treatment of growth 
retarded children with chronic renal insufficiency with recombinant human 
growth hormone. Kidney Int. 1996;49:781–785. Reproduced with permis­
sion of R.N. Fine.)
1010
patients are dose dependent. Deleterious effects on renal function 
and progression of osteodystrophy were not observed.1345 This 
treatment regimen does not adversely affect renal graft function 
after transplantation, nor is there significant “catch-d

transplantation, nor is there significant “catch-down” growth 
after the transplantation.1346 The final height in 38 German chil­
dren treated with GH for an average of 5.3 years was –1.6 ± 1.2 
SDS, an increment of 1.4 SDS over the pretreatment baseline. 
The final height of an untreated control group was –2.1 ± 1.2, or 
0.6 SDS below baseline.1337
More recently, adult heights of 178 French patients treated 
with GH were reported. Mean adult height was 162.2 cm in 
men and 152.9 cm in women, an

ght was 162.2 cm in 
men and 152.9 cm in women, and 46% were more than –2 SDS 
for height. Adult height SDS was correlated with height SDS 
and spontaneous height velocity before treatment and with the 
effect of treatment. These adult heights were significantly better 
when compared with historic cohorts of patients not treated with 
GH.1347
In 2009, a review of longitudinal data from 7189 patients 
enrolled in the chronic renal insufficiency registry of the North 
American Pediatric Renal Tria

gistry of the North 
American Pediatric Renal Trials and Collaborative Studies revealed 
that 827 patients (11.5%) had received GH. A total of 787 chil­
dren with CRD who were previously naïve to GH treatment and 
received recombinant human GH for 1 to 4 years (median, 1.5 
years) were paired with 787 control patients and monitored for 
4 years. The GH-treated group had a significantly greater height 
velocity SDS than the control group at 2.5 years. Among 220 
pairs evaluated at 2 years, the he

ars. Among 220 
pairs evaluated at 2 years, the height SDS of the GH-treated group 
was 0.56 SDS higher than that of the control group (p <0.05). 
Treatment with GH had no significant impact on BMI or esti­
mated glomerular filtration rate (GFR).1348
The magnitude of response to GH treatment depends on the 
GH dosage. A dose of 0.35 mg/kg per week appears to be opti­
mal for short patients with CRD; half of this dose was less effec­
tive, and doubling did not significantly improve the response i

bling did not significantly improve the response in 
double-blind studies.1342 Long-term GH treatment has also been 
shown to be safe and effective for extremely short (–4.0 SDS) 
children with nephropathic cystinosis and should be considered 
if nutrition and cysteamine treatment do not prevent growth 
failure.1332 Because children are often short at the time of renal 
transplantation, have hormone findings of relative GH insensi­
tivity, and receive chronic prednisone therapy, GH is sometimes 

ceive chronic prednisone therapy, GH is sometimes 
administered in the post-transplantation period. Data after 1 and 
2 years of treatment of such children and adolescents1349 indicated 
a large increment of growth velocity at year 1 and a smaller benefit 
at year 2. As with GH treatment of CRD, the pharmacologic regi­
men overcomes the relative GH insensitivity. Individual patients 
show a wide variation in response, and predictors include age, 
GFR, need for dialysis treatment, target height, 

GFR, need for dialysis treatment, target height, and pretreatment 
growth rate.1350
A mathematic model for prediction of the individual response 
to GH in prepubertal children with CRD was developed from 
the KIGS.11 Thirty-seven percent of the variation in the first-year 
growth response was explained by this model, with the greatest 
first-year response in younger children who had no weight reduc­
tion, no hereditary renal disorder, and high residual renal func­
tion. There was a small GH dos

esidual renal func­
tion. There was a small GH dose effect during the first treatment 
year.11 Such models, using clinical variables, may allow individual­
ized GH treatment decisions in children with CRD.
Considerable assessment must yet be undertaken to demon­
strate whether there is increased growth over a longer term, that 
renal function does not deteriorate during therapy, and that the 
risk of rejection is not enhanced. GH treatment does not appear 
to cause an accelerated decline of allo

ot appear 
to cause an accelerated decline of allograft function1351–1353 or 
changes in histopathologic findings.1354 Use of nonsteroid-based 
immunosuppressive regimens may obviate the need for post-
transplantation GH treatment.
A short-term study in chronic, well-nourished dialysis patients 
showed that combined administration of moderate doses of GH 
with recombinant human IGF1 (rhIGF1) had a complementary 
effect on protein metabolism.1355 This approach is theoretically rea­
sonable; howev

This approach is theoretically rea­
sonable; however, the safety profile of this intervention is unknown. 
Juvenile Idiopathic Arthritis
Juvenile idiopathic arthritis (JIA) is often complicated by growth 
deficiency. The decrease in linear growth usually correlates with 
the severity of the disease, although catch-up growth may not 
occur during remissions. The final height is less than –2 SDS 
in 11% of patients with polyarticular JIA and in 40% of those 
with systemic JIA.1356 Serum GH levels 

 of those 
with systemic JIA.1356 Serum GH levels are normal or low, usu­
ally with low plasma IGF1 levels.1357 The pathogenesis of short 
stature is thought to be GH insensitivity associated with both 
the inflammatory process and glucocorticoid treatment.1358 
Several trials have been conducted to determine whether GH 
replacement therapy is effective in patients with JIA.1359–1361 In 
trials of GH replacement, using GH dosages ranging from 0.1 to 
0.46 mg/kg per week increased serum levels of

 to 
0.46 mg/kg per week increased serum levels of IGF1 and linear 
growth velocity. However, the trials all had significant interindi­
vidual variability.1362 GH therapy for up to 3 years was notable 
for a decrease in the loss of growth velocity associated with the 
active phase of JIA.1363
In a randomized, controlled trial of GH therapy (0.33 mg/kg 
per week for 4 years) in 31 children with JIA, a height increase 
of 1 SDS was found, compared with a decrease of 0.7 SDS in 
the untreated contr

with a decrease of 0.7 SDS in 
the untreated control group.1361 The GH-treated patients had a 
significantly greater final height compared with untreated control 
patients (–1.6 ± 0.25 SDS vs. –3.4 ± 2.0 SDS; p <0.001).1364 
As expected, the severity of inflammation negatively affected the 
efficacy of GH therapy.1361,1364 In a randomized trial of normal-
height children with JIA treated with high-dose GH therapy (0.46 
mg/kg per week) starting within 12 to 15 months after initia­
tion of glucoc

ithin 12 to 15 months after initia­
tion of glucocorticoid therapy, the 3-year follow-up showed that 
the heights of GH-treated children remained normal (–1.1 SDS 
at baseline and –0.3 SDS after 3 years) while linear growth of 
the control patients declined from –1.0 SDS to –2.1 SDS.1359 
In the randomized, controlled trials published to date, patients 
were followed for up to 7 years, and no substantial differences 
were found in disease activity variables, including worsening of 
preexisting b

y variables, including worsening of 
preexisting bone deformities, between GH-treated and control 
patients.1358,1360,1361,1363
In a longitudinal study of patients treated for a mean of greater 
than 5 years, GH increased and normalized final height and 
improved bone and muscle mass.1365 Because chronic inflamma­
tion, glucocorticoid treatment, and GH therapy are all associated 
with decreased sensitivity to insulin, children with JIA are at risk 
for impaired glucose homeostasis.328,1362,1366 

k 
for impaired glucose homeostasis.328,1362,1366 Therefore monitor­
ing of glucose tolerance using blood glucose, fasting insulin, and 
HbA1c assays at intervals of 6 to 12 months is recommended. 
More intensive monitoring with oral glucose tolerance tests at 
baseline and then once yearly during treatment may be indicated. 
Patients with TS have a final height averaging 143 cm in the United 
States,11,907 about 20 cm lower than the mean final height of normal 
women.905 The goa

the mean final height of normal 
women.905 The goals of growth-promoting therapies are to attain 
1011
a normal height for age as early as possible, to progress through 
puberty at a normal age, and to attain a normal adult height. A delay 
in diagnosis of TS is often the greatest obstacle for girls with TS to 
obtain a normal height. A 7-year delay in diagnosis was reported 
in one study in patients who averaged 2.9 SD below the mean in 
height

ents who averaged 2.9 SD below the mean in 
height and were below the 5th percentile for height for an average of 
5.3 years.1367 Growth should be assessed frequently after the diagno­
sis is made and plotted using TS-specific growth charts to monitor 
for associated growth issues and effectiveness of therapy.11,912,1368
Before the availability of recombinant GH, there were conflict­
ing data concerning its efficacy in this disorder, but the ability of 
GH to accelerate growth has now been demon

ity of 
GH to accelerate growth has now been demonstrated in multiple 
reports. Growth responses are not affected by the karyotype. In 
1983, a randomized, controlled North American study of treat­
ment with GH (at a dose of 0.375 mg/kg per week) with or with­
out added oxandrolone was initiated, with a mean age at onset 
of treatment of approximately 9 years.1369 Analysis of all 62 girls 
enrolled in the study at near-final height showed a mean stature of 
152.1 cm in the group treated with GH 

stature of 
152.1 cm in the group treated with GH plus oxandrolone (a gain 
of 10.3 cm compared with the height predictions derived from 
Lyon and colleagues),11 whereas girls receiving GH alone averaged 
150.4 cm (a gain of 8.4 cm) (Fig. 25.53).907 In another arm of this 
study, addition of estrogen to the GH regimen before age 15 years 
lowered the final height gain from 8.4 cm to 5.1 cm.1370 In a reas­
sessment of North American data in the NCGS, early initiation 
of GH treatment was shown to

GS, early initiation 
of GH treatment was shown to allow estrogen administration at a 
physiologic age without loss of adult height.1371,1372
Several other studies917,1373 using higher doses of GH showed 
even greater gains in adult height outcomes. In a multicenter trial, 
Sas and coworkers used a maximum GH dose of approximately 
0.63 mg/kg per week for 4.8 estrogen-free GH treatment years 
beginning at a mean age of 8.1 years, resulting in a gain of 16 cm 
over the modified Lyon and colleague

ain of 16 cm 
over the modified Lyon and colleagues’ projection.11,917 In the same 
study, the group receiving a GH dose similar to that in the Ameri­
can studies achieved a height gain of 12.5 cm by age 16 with 4.8 
estrogen-free GH treatment years starting at 7.9 years. In these girls, 
induction of puberty at a normal (not delayed) age was associated 
with these excellent height outcomes.1374 Carel and colleagues,1373 
using 0.7 mg/kg per week in a group that received 5.1 estrogen-
free GH tr

 in a group that received 5.1 estrogen-
free GH treatment years beginning at 10.2 years, gained 10.6 cm 
over the projections of Lyon and colleagues.11 The group receiving 
a conventional dose (0.3 mg/kg per week) gained only 5.2 cm with 
3.0 estrogen-free GH treatment years starting at 11 years. Because 
girls with TS usually have a normal GH secretory pattern, provoca­
tive GH testing should be performed only in those whose growth 
is clearly abnormal relative to that expected for patients wit

bnormal relative to that expected for patients with TS, as 
determined by plotting lengths and heights on TS-specific growth 
curves.11,910,914,1375
Although it was well established that GH therapy was effec­
tive in increasing final adult height, the magnitude of the gain in 
height varied in the earlier studies, depending on study design and 
treatment parameters. In 2005, in the first randomized, controlled 
trial to follow GH-treated subjects with TS to final height, the 
Canadian Growth Hor

 with TS to final height, the 
Canadian Growth Hormone Advisory Committee corroborated 
the increases in adult stature reported by studies with historic 
control subjects.907,1374,1376,1377 In the Canadian study, girls with 
TS (age 7–13 years) who were randomized to receive GH (0.3 
mg/kg per week; maximum weekly dose, 15 mg) achieved a final 
adult stature 7.2 cm taller than that seen in the control group after 
an average of 5.7 years. Factors predictive of taller adult stature 
include a rel

 predictive of taller adult stature 
include a relatively tall height at initiation of therapy, tall paren­
tal heights, young age at initiation of therapy, a long duration of 
therapy, and a high GH dose.917,1372,1378,1379
Although the optimal age for initiation of GH treatment has 
not been established, preliminary data from the Toddler Turner 
Study, in which 88 girls between the ages of 9 months and 4 years 
were randomized to receive GH or no GH therapy, showed that 
GH therapy is effective

o GH therapy, showed that 
GH therapy is effective beginning as early as 9 months, with a 
safety profile similar to that in older children with TS.1380 Treat­
ment with GH should be considered as soon as growth failure is 
evident. GH therapy in the United States is typically initiated at 
the FDA-approved dose of 0.375 mg/kg per week. The dose can 
be changed in response to the patient’s growth response and IGF1 
levels. Growth prediction models may be helpful in determining 
the potential eff

s may be helpful in determining 
the potential effects of changes in dosing.1378
Studies have shown that higher doses produce a relatively small 
gain in final height, although there is no apparent increase in 
short-term adverse events.1381 In a study by the Dutch Working 
Group, the mean gain in final height with 4 IU/m2 per day (0.045 
mg/kg per day), 6 IU/m2 per day, and 8 IU/m2 per day averaged 
11.9 ± 3.6, 15.7 ± 3.5, and 16.9 ± 5.2 cm, respectively.1374 How­
ever, when GH was given at the

pectively.1374 How­
ever, when GH was given at the higher doses, IGF1 levels were 
often above the normal range, which theoretically could lead to 
long-term adverse effects.1382
In girls naïve to GH who are older than 9 years of age and in 
those with extreme short stature, consideration can be given to 
using higher doses of GH and adding a nonaromatizable ana­
bolic steroid, such as oxandrolone.907 The maximum suggested 
dose of oxandrolone is 0.05 mg/kg per day, because higher doses 
18
16
1

 0.05 mg/kg per day, because higher doses 
18
16
14
12
10
8
6
4
2
0
−2
−4
−6
−8
Most recent minus projected height (cm)
0.0
Control
n = 25
Protropin
n = 17
Combination
n = 45
8.4
10.3
• Fig. 25.53  Adult heights of patients with Turner syndrome treated with 
growth hormone (GH) or with a combination of GH plus oxandrolone and 
of historic control subjects, relative to each subject’s projected adult height 
(indicated by the dotted zero line). The mean increments in relative adult 
height are ind

 mean increments in relative adult 
height are indicated. The diamonds in the combination group indicate two 
subjects with poor compliance who terminated treatment early. (From 
Rosenfeld RG, Attie KM, Frane J, et al. Growth hormone therapy of Turner 
syndrome: beneficial effect on adult height. J Pediatr. 1998;132:319–324.)
1012
have been associated with virilization and accelerated skeletal 
maturation. Liver enzymes should be monitored. Therapy m

tion. Liver enzymes should be monitored. Therapy may 
be continued until a satisfactory height has been attained (bone 
age >14 years) or until the yearly growth velocity falls to less than 
2 cm/year. The children should be monitored at intervals of 3 to 
6 months.1383
The substantial variations in GH-induced growth increments 
in these studies were presumably related to GH dose, duration 
of estrogen-free GH treatment years, and age at initiation of GH 
and estrogen administration, as well as 

on of GH 
and estrogen administration, as well as the population and paren­
tal adult heights. Additionally, the GHRd3 polymorphism has 
been reported to be associated with increased responsivity to GH 
in girls with TS.1384 Transdermal or depot estrogen administra­
tion, as opposed to oral estrogen, may also contribute positively 
to growth outcomes.1385,1386 In these higher-dose treatment stud­
ies, hyperinsulinism with presumed insulin resistance was evident 
but reversible.1373,1387 Other da

nce was evident 
but reversible.1373,1387 Other data suggest that impaired insu­
lin secretion may ultimately be the significant issue in patients 
with TS,1388,1389 although long-term GH therapy has been well 
tolerated.
None of these studies were placebo-controlled to adult height, 
and some studies have yielded much poorer height outcomes,918 
so some uncertainty existed until the randomized, controlled, 
multicentered Canadian study, in which the mean height gain 
ascribed to GH treatment wa

 the mean height gain 
ascribed to GH treatment was 7.3 cm at 1 year after cessation 
of the treatment protocol. Estrogen was initiated at 13 years of 
age.1376 Another temporally matched control study from Italy 
showed a gain of 8.1 cm.1377 These studies corroborated and sup­
ported the information that had accumulated from historic data 
on natural growth in TS juxtaposed to the available GH treatment 
results. Such data, in aggregate, provides convincing support that 
GH can both accelerate 

s convincing support that 
GH can both accelerate growth velocity and increase adult height.
Recommendations include seeking the diagnosis vigorously 
at any age in every girl with otherwise unexplained short stature 
and initiating therapy at that young age (i.e., at diagnosis or in 
early childhood). One must bear in mind that growth velocity 
in girls with untreated TS can be slowed as early as ages 1 to 3 
years old.911,913 Treatment with the FDA-approved GH dosage of 
0.375 mg/kg per week s

 FDA-approved GH dosage of 
0.375 mg/kg per week should be initiated, and compliance should 
be monitored with auxologic and IGF1 measurements; contin­
ued normal thyroid status should be assured. In overweight girls, 
the use of body weight for calculating dosage may result in the 
delivery of excess GH. Adult height was compared in two Dutch 
trials that used BSA-based dosing and five Swedish trials that used 
body weight (BW)–based dosing,1390 finding that adult height 
was greater in girls t

 finding that adult height 
was greater in girls treated with BSA-based dosage versus those 
using a BW-based dosing. A dose 0.375 mg GH/kg per week is 
approximately equivalent to 1.38 mg GH/m2 per day.
Oxandrolone may be added to the regimen in the late-diag­
nosed girl. Initiation of estrogen therapy at 12 years of age, rather 
than at age 14 years and above, has recently been shown to improve 
adult height.1374,1386
A 2011 randomized, placebo-controlled study (double placebo, 
estrogen alone

-controlled study (double placebo, 
estrogen alone, GH alone, or GH and estrogen) that followed girls 
ages 5 to 12.5 to adult height showed that very low-dose ethinyl 
estradiol given prior to age 12 years, along with growth hormone 
treatment, had the greatest impact on increasing adult height.
The diminished areal bone density in TS is enhanced by GH, 
but estrogen therapy is needed to normalize volumetric density 
(i.e., not simply size).1391 Absence of an adverse impact of GH 
on aortic dia

 Absence of an adverse impact of GH 
on aortic diameters is reassuring, given the predisposition of such 
patients for dissecting aneurysms.543 Use of statistical prediction 
models for long-term growth in TS may permit a more quantita­
tive assessment of individual therapeutic efficacy.1378
Despite evidence for aggressive GH treatment of girls with TS 
to achieve adequate adult heights and relatively high quality-of-
life assessments in adulthood,1392,1393 health-related issues con­
tinue to im

d,1392,1393 health-related issues con­
tinue to imperil outcomes. Carel and colleagues1394 reviewed 
health outcomes of 568 adult French women (in their mid-20s) 
with GH-treated TS who had a mean height of 150.9 cm (having 
gained about 9 cm over prediction) and found that neither height 
nor height gain was associated with quality-of-life scores. Rather, 
issues regarding cardiologic and otologic health concerns and delay 
of pubertal initiation beyond 15 years were of greater concern. In 
con

n beyond 15 years were of greater concern. In 
contrast, a long-term follow-up assessment of 49 women from the 
Netherlands who had reached a mean adult height of 160.7 cm (a 
gain of >15 cm), suggested that the high quality-of-life scores were 
related to height gain and adequate estrogenization.1395
The NCGS recently published data on the efficacy and safety 
of GH therapy in 5220 children with TS treated during the past 
20 years. A total of 442 adverse events were reported for these 
patient

42 adverse events were reported for these 
patients, including 117 that were considered serious. Seven deaths 
occurred, including five from aortic dissection or rupture. The 
incidences of intracranial hypertension, slipped capital femoral 
epiphysis, scoliosis, and pancreatitis were increased compared with 
other patients in the registry who did not have TS. Ten new-onset 
malignancies occurred, including six in patients without known 
risk factors. The number of patients who developed IDDM al

tors. The number of patients who developed IDDM also 
appeared to be increased. It is believed that these adverse events are 
likely to be unrelated to treatment with GH. The incidence of aor­
tic dissection or rupture reflects the higher baseline risk in TS.1396
Two echocardiography studies reported normal left ventricular 
morphology and function in girls with GH-treated TS,1397,1398 and 
two MRI studies found no deleterious effect of GH treatment on aortic 
diameter.532,1383,1399,1400 Because

ent on aortic 
diameter.532,1383,1399,1400 Because GH treatment can alter craniofacial 
proportions, all girls with TS treated with GH should receive periodic 
orthodontic follow-up.1401 These studies were not controlled, but their 
findings emphasize the broad range of health concerns in women with 
TS and affirm the need for multidisciplinary follow-up care.
Recommendations:
	1.	 Early diagnosis and initiation of treatment.
	2.	 GH therapy offered to all girls with TS with a subnormal pre­
d

GH therapy at an earlier age, once decreasing height 
percentiles on the normal growth curve are noted (the current 
average age is around 9 years), since a longer time on therapy 
will result in improved adult height.
	6.	 Therapy should be continued until an appropriate adult height 
is attained or the growth rate falls below 2 to 2.5 cm per year.
	7.	 Early treatment with ultralow-dose estrogen may improve 
growth. Pubertal estradiol should begin at approximately 12 
years of age. Oxandrolo

begin at approximately 12 
years of age. Oxandrolone at a dose of approximately 0.03 mg/
kg per week may be considered in girls older than 8 years. 
Studies using GH in short children born SGA have been ham­
pered by the heterogeneity of this group of patients, whose poor 
1013
growth may reflect maternal factors, chromosomal disorders, dys­
morphic syndromes (e.g., RSS, Dubowitz syndrome), toxins, and 
idiopathic facto

 Dubowitz syndrome), toxins, and 
idiopathic factors.1402 A 2001 consensus statement recommended 
that SGA be defined as a birth weight or length at least 2 SD below 
the mean.1403 Almost 90% of infants born SGA undergo catch-
up growth within the first 2 years of life, and those who do not 
would then be eligible for GH treatment.37,38,947,989,1402,1404,1405 
Based on these proportions, and because the prevalence of “short 
SGA” is 2 to 3 per 1000 children, between 1 in 300 and 1 in 500 
childr

00 children, between 1 in 300 and 1 in 500 
children are eligible for GH treatment, as approved in the United 
States in 2001 and in Europe in 2003. It appears that GH mediates 
the postnatal catch-up growth.1406 Children born SGA appear to 
grow at a low-normal growth velocity during childhood,1407 but 
puberty tends to occur at a somewhat earlier age and to progress 
rapidly, resulting in decreased pubertal height gain and an adult 
height about 1 SD (∼7 cm) below the mean38,987,989,1408–1411 

t 1 SD (∼7 cm) below the mean38,987,989,1408–1411 and 
about 4 cm below the midparental target height.947
The impaired postnatal growth observed in children born SGA is 
probably due to many factors, including generalized cellular hypo­
plasia,1412 altered diurnal GH secretion patterns,237,969,971,1407,1413 
and, potentially, abnormalities in the GH-IGF axis resulting from 
GH sensitivity as determined by the presence or absence of exon 
3 of the GHR.1414–1416 Deletions and mutations of the IGF1

 GHR.1414–1416 Deletions and mutations of the IGF1 gene 
have been reported in patients with profound IUGR, microceph­
aly, deafness, and postnatal growth failure.974,660,1417 SGA patients 
have also been reported to have IGF1 gene polymorphisms or 
missense mutations resulting in low serum IGF1 concentra­
tions.975,976,989,1416,1418,1419 Reduced levels of IGF2 expression, 
associated with hypomethylation of the telomeric domain of chro­
mosome 11p15, were found in a cohort of SGA patients who h

1p15, were found in a cohort of SGA patients who had 
RSS.993 Mutations in the type I IGF receptor gene (one compound 
heterozygous, one nonsense) were found in two patients with 
IUGR and poor postnatal growth (one with elevated IGF1).663
The low levels of IGF1 and IGFBP3 in many infants with 
IUGR, apparently related to fetal malnutrition, do not seem 
to predict the degree of subsequent growth impairment,968 
although continuing low levels are associated with poor catch-
up growth.1420 Short 

 associated with poor catch-
up growth.1420 Short children born SGA make up a substantial 
portion of growth-retarded patients seen in pediatric endocrine 
practices.947,989,1421 Because these children may have heights in 
the range seen in IGF deficiency syndrome, therapeutic attempts 
are appropriate, assuming that the insulin resistance noted in these 
thin, small children982 does not become a clinical issue. Encour­
aging growth responses have been obtained with GH treatment.
The FDA approva

e been obtained with GH treatment.
The FDA approval of GH for the long-term treatment of 
growth failure in children born SGA who fail to manifest catch-
up growth by age 2 was based on data obtained from four ran­
domized, controlled, open-label, clinical trials that enrolled 209 
patients between the ages of 2 and 8 years. Height velocity was 
determined after 1 year without treatment, and then patients were 
randomized to receive either GH treatment (34 or 69 mg/kg per 
day [0.24 or 0.48 mg/k

atment (34 or 69 mg/kg per 
day [0.24 or 0.48 mg/kg per week]) or no treatment for 2 years, 
after which a crossover design occurred. Children who received 
the higher GH dose had an increase of about 0.5 SDS in height 
after 2 years, compared with the children who received the lower 
dose, although both treated groups had significant increases in 
height velocity compared with untreated children (prescribing 
information for Genotropin somatropin of recombinant DNA 
origin for injection, Pfizer

n of recombinant DNA 
origin for injection, Pfizer, Inc., New York, 2006). In Europe, the 
criteria for treatment differ from those in the United States: height 
SDS below –2.5, height velocity SDS below 0 during the previous 
year, and age greater than 4 years.
The data documenting efficacy in regard to adult height contin­
ues to be limited despite European regulatory authorities’ requir­
ing this evidence before approval. A meta-analysis of final height 
data for 56 children born SGA was perf

nal height 
data for 56 children born SGA was performed at the request of 
European authorities and demonstrated a mean height increase 
of 1.9 SDS for subjects treated with GH at 34 mg/kg per day 
versus 2.2 SDS for 69 mg/kg per day. A meta-analysis of three 
randomized studies of 28 patients reported that GH treatment 
for 7 to 10 years, initiated at doses of 34 to 69 mg/kg per day, can 
be expected to increase adult height by about 1.0 to 1.4 SDS.1422 
A larger, randomized, controlled study o

SDS.1422 
A larger, randomized, controlled study of 91 GH-treated and 
33 untreated French adolescents (mean age, 12 years) reported a 
lower intergroup difference of 0.6 SDS in adult height after 2.7 
years of treatment.1423
The many studies documenting efficacy of GH therapy in thou­
sands of children with SGA are far too numerous to report in detail 
here and have been reviewed elsewhere.1422,1424,1425 Although 
results vary, it appears that GH treatment can be expected to result 
in a 1-cm i

H treatment can be expected to result 
in a 1-cm increment in height gain each year. The factors that can 
increase the gain in height include a greater height deficit relative 
to the midparental height, a higher GH dose, initiation of therapy 
at an earlier age, longer treatment duration, and the proposed pres­
ence of the GHRd3 allele.1384,1403,1424,1426 The addition of a GnRH 
analogue was shown to improve adult height in a select group of 
children born SGA.1427 In a prediction model derive

hildren born SGA.1427 In a prediction model derived from KIGS 
SGA data, Ranke and coworkers1426 found that age and GH dose 
were strong predictors of initial growth but that growth achieved 
during the first year was a powerful predictor of later growth. 
Although from these studies it appears that relatively high doses 
may be required for the growth response (the FDA-approved dose 
is 70 μg/kg per day), initial GH dosing at 35 to 50 μg/kg per day is 
reasonable since the response in individua

day is 
reasonable since the response in individual patients is highly vari­
able. The dose may be increased if the growth rate is insufficient.
Routine measurement of serum IGF1 has been recommended 
for patients receiving GH as a safety marker; the rationale being 
that elevated levels of IGF1 may result in an increased risk of side 
effects and potential for future malignancy.1199,1428,1429 However, 
there is no documented evidence of such a link, and some children 
with SGA may have evidence

ink, and some children 
with SGA may have evidence of IGF1 resistance, requiring higher 
serum IGF1 levels to optimize growth.1430 Further, using IGF1 
levels to inform GH dosing has been shown to be less efficacious 
than standard dosing.1431 This is likely due to the wide range of 
“normal” IGF1 levels in children and that serum IGF1 levels are 
relatively poor indicators of IGF1 sensitivity.
Children born SGA, especially those who have rapid postnatal 
weight gain, have reduced insulin-mediat

ostnatal 
weight gain, have reduced insulin-mediated glucose uptake and 
are more likely to have insulin resistance.982,1432–1437 However, 
there have been no solid data documenting an increased risk of 
non-IDDM.1436 There have been reports of several children in 
GH registration studies who had mild, transient hyperglycemia 
(prescribing information for Genotropin somatropin of recom­
binant DNA origin for injection, Pfizer, Inc., New York, 2006). 
Insulin resistance during GH treatment in chi

6). 
Insulin resistance during GH treatment in children born SGA 
has been reported1381,1438,1439 and typically resolves after discon­
tinuation of GH.1440 In a large database of children treated with 
GH, no differences in glucose regulation were found in 1900 chil­
dren born SGA compared with children with ISS.1441 With 6000 
patient-years of exposure, no cases of diabetes mellitus have been 
reported during GH treatment.1441–1444
The European product labeling for GH treatment of children 
bor

product labeling for GH treatment of children 
born SGA reads, “The management of these patients should follow 
accepted clinical practice and include safety monitoring of fasting 
1014
insulin and blood glucose before treatment and annually during 
treatment.”1425 Since GH treatment of children who are SGA 
reduces insulin sensitivity but only rarely causes glucose intoler­
ance, and insulin resistance returns to normal following cessation 
of GH th

ce returns to normal following cessation 
of GH therapy, there is currently no indication to routinely moni­
tor metabolic parameters.1445,1446 In addition to insulin resistance 
alone, children born SGA have an increased risk of metabolic syn­
drome.1443,1447–1449 The current literature provides no informa­
tion on whether GH treatment in childhood increases risks for 
adult metabolic disease. Long-term follow-up studies are needed 
to determine the metabolic risk associated with GH treatment. 

 the metabolic risk associated with GH treatment. 
GH therapy has been studied in several skeletal dysplasias. The 
largest published study in achondroplasia involved 40 children; 
during the first year of treatment, the height velocity increased 
from 3.8 to 6.6 cm per year, and in the second year it decreased 
to approximately 5 cm per year.1450 A modest improvement was 
seen in the ratio of lower limb length to height. Although GH was 
well tolerated, atlantoaxial dislo

lthough GH was 
well tolerated, atlantoaxial dislocation during GH therapy was 
reported in one patient. In another study, normal growth velocity 
was achieved for up to 6 years in 35 subjects, with a significant 
increment in height SDS for at least 4 years1450,1451; in that study, 
vertebral growth was disproportionately greater than limb growth.
Bridges and Brook1452 reported on the effects of GH therapy in 
27 patients with hypochondroplasia; response was maximal dur­
ing the first year of t

 response was maximal dur­
ing the first year of treatment, but substantial benefit was seen 
through 4 years of treatment in pubertal subjects.
Experience with GH treatment is limited in other skeletal dis­
orders, such as dyschondrosteosis, hereditary multiple exostoses, 
osteogenesis imperfecta, and Ellis–van Creveld syndrome.
SHOX Haploinsufficiency and Léri-Weill Syndrome. Patients 
with mutations or deletions of the SHOX gene have variable 
degrees of short stature with or without mesomeli

degrees of short stature with or without mesomelic skeletal dys­
plasia. If untreated, short patients with SHOX deficiency remain 
short in adulthood. The SHOX gene is located in the pseudoauto­
somal region 1 (PAR1) on the distal end of the X and Y chromo­
somes at Xp22.3 and Yp11.3.1453 Because genes in PAR1 do not 
undergo X inactivation, normal individuals express two copies of 
the SHOX gene. This gene encodes a homeodomain transcription 
factor that is expressed during early fetal life in

actor that is expressed during early fetal life in the growth plate 
and functions in the regulation of chondrocyte differentiation 
and proliferation.895,1454 SHOX haploinsufficiency (or deficiency) 
is the primary cause of short stature in patients with LWD (see 
earlier discussion).941,1455 SHOX mutations and deletions are also 
found in patients with short stature without clinical evidence of 
dyschondrosteosis.1456
Clinical manifestations of SHOX deficiency include bowing of 
the forearms a

 SHOX deficiency include bowing of 
the forearms and lower legs, cubitus valgus, Madelung deformity 
or partial dislocation of the ulna at the wrist and elbow, short 
fourth and fifth metacarpals, and a high-arched palate along with 
characteristic radiologic signs.894,941,1455 In a randomized 2-year 
study of 52 prepubertal subjects with a SHOX gene defect, the 
first-year height velocity in the GH-treated group was significantly 
greater than in the untreated control group (mean ± standard err

n the untreated control group (mean ± standard error 
of the mean [SE], 8.7 ± 0.3 vs. 5.2 ± 0.2 cm/year; p < 0.001) and 
was similar to the first-year height velocity to GH-treated subjects 
with TS (8.9 ± 0.4 cm/year; p < 0.592). GH-treated subjects with 
SHOX deficiency also had significantly greater second-year height 
velocity (7.3 ± 0.2 vs. 5.4 ± 0.2 cm/year; p < 0.001), second-year 
height SDS (–2.1 ± 0.2 vs. –3.0 ± 0.2; p < 0.001), and second-year 
height gain (16.4 ± 0.4 vs. 10.5 ± 0.4 c

ond-year 
height gain (16.4 ± 0.4 vs. 10.5 ± 0.4 cm; p < 0.001) compared 
with untreated subjects.1457 Data on 28 persons with SHOX defi­
ciency who received GH for a mean of 4.5 years through to adult 
height indicate that adjusted final height was –2.1 SDS, similar 
to girls with TS enrolled in the study. Calculated height SDS gain 
was 1.3, again similar to girls with TS.1458 
Turner Syndrome and Langer Mesomelic Dysplasia. Homozy­
gous mutation of the SHOX gene results in the Langer type of 

on of the SHOX gene results in the Langer type of 
mesomelic dwarfism (i.e., LMD). A child with LMD and TS was 
found to have a SHOX gene abnormality resulting from a down­
stream allele deletion in her normal X chromosome. Although 
there have been numerous studies documenting growth improve­
ment with GH therapy in patients with TS907,1373,1396 and in 
those with heterozygous SHOX gene deletions,1455 only one case 
of growth response to GH therapy in the rare condition of LMD 
with TS has been

apy in the rare condition of LMD 
with TS has been found. Growth rates of 3.46, 3.87, 2.3, and 0.7 
cm per year were observed in the first, second, third, and fourth 
years of GH therapy, respectively, and there was no clinical dete­
rioration of the skeletal deformities in this patient. Because there 
was a failure to achieve growth improvement with GH therapy, 
the authors concluded that GH therapy was not beneficial in 
patients with the severe short stature caused by combined TS and 
LMD res

e short stature caused by combined TS and 
LMD resulting from homozygous SHOX deficiency.1459 
Since 1994, when a gene for Noonan syndrome was mapped to 
chromosome 12 (12q24.1) and a mutation in the protein tyrosine 
phosphatase nonreceptor type 11 (PTPN11) was identified and 
characterized in familial cases, at least three other gene mutations 
have been identified.922,923,1460–1463 While contributing to the 
broad heterogeneity of NS, these mutations have not completely 
local

 of NS, these mutations have not completely 
localized the cause of the short stature. However, several geno­
type-phenotype correlations are relevant to patient management. 
Patients with PTPN11 mutations often have normal or elevated 
GH levels and low serum IGF1 concentrations.928 The PTPN11 
genetically modified mouse model had a GH-stimulated increase 
in ERK activation, and growth was stimulated by inhibition of 
ERK1/2.1464 Several studies have reported effects of GH on near-
adult height

 have reported effects of GH on near-
adult height in patients with Noonan syndrome,1465–1469 but all 
involved small numbers of patients with varied enrollment ages, 
treatment durations, GH doses, and responses. The largest study 
of the response to GH in children with Noonan syndrome came 
from the NCGS, a postmarketing observational study of recom­
binant human GH treatment in children with various disorders. 
A total of 370 patients with Noonan syndrome (mean enrollment 
age, 11.6 years) re

nan syndrome (mean enrollment 
age, 11.6 years) received GH (mean dosage, 0.33 mg/kg per week) 
for a mean of 5.6 years. In the 65 patients with data to derive 
near-adult height, the mean gain above the projected height was 
10.9 ± 4.9 cm in boys and 9.2 ± 4.0 cm in girls. Duration of 
prepubertal recombinant human GH therapy and height SDS 
at puberty were important contributors to the near-adult height. 
No new adverse events were observed. The authors suggested that 
greater growth optimizat

e authors suggested that 
greater growth optimization would be possible with earlier initia­
tion of therapy.1470 This height increase was similar to the change 
observed in patients with TS but significantly less in those with 
idiopathic GHD in the same study.
Most experience with GH treatment of short stature in Noonan 
syndrome has been limited to small, uncontrolled studies in which 
few patients reached final height.920,1471,1472 The clinical diagnosis 
of a dysmorphic syndrome potentially

al diagnosis 
of a dysmorphic syndrome potentially makes the treatment groups 
heterogeneous, although identification of a mutant PTNN11 
gene1473 helps with characterization. Overall, treatment results for 
3 to 4 years are similar to those attained in patients with TS; mean 
1015
growth velocities improve by 2 to 4 cm per year (over baseline rates 
of ∼4 cm/year) during the first 4 years of therapy, with patients 
gaining from about –3.5 to –1

rapy, with patients 
gaining from about –3.5 to –1.7 SDS of stature without inordinate 
advancement of bone age.919,920,929,1472 Those children with identi­
fiable PTPN11 mutations are reported to have a poorer response to 
GH treatment in terms of growth and IGF production, suggesting 
impaired efficiency of phosphorylation-dependent GH signaling 
pathways.927–929 Although the initial anecdotal experience sug­
gested progression of ventricular hypertrophic cardiomyopathy, this 
was not confirme

ypertrophic cardiomyopathy, this 
was not confirmed in larger, carefully monitored studies.920,1474,1475 
However, it has been suggested that children with Noonan syn­
drome receiving GH be monitored for hypertrophic cardiomy­
opathy and hematologic abnormalities.921 Although neoplasia is a 
theoretic concern, there are no studies associating GH therapy with 
malignancy. A recent report of brain tumor growth after several 
years of GH therapy1476 suggests caution when treating with GH. 
Idiopath

 suggests caution when treating with GH. 
Idiopathic Short Stature (Subtle Errors Throughout the 
Growth Axis)
Although children with ISS, by definition, are without an identi­
fiable cause of growth retardation, the term clearly encompasses 
a heterogeneous group that may include children with constitu­
tional growth delay, genetic short stature, or subtle defects of the 
GH-IGF axis. Even though they are often grouped in one cat­
egory, children with ISS have been shown to have a broad range 

n with ISS have been shown to have a broad range 
of provocative GH responses, ranging from normal to elevated, 
and a wide range of serum IGF concentrations, from normal to 
IGF deficient. This group may also include children with uniden­
tified syndromes or unidentified chronic illnesses or endocrine 
disorders. Children with ISS may experience stressful behavioral 
circumstances, but studies suggest that the relationship of psycho­
social problems to the short stature is variable.1477–1479 No

lems to the short stature is variable.1477–1479 Nonethe­
less, hormonal intervention to enhance growth, with the aim of 
diminishing such difficulties, has been used.
Although the specific etiologies are often unknown, GH treat­
ment has been used widely. Important questions have been raised 
about the financial, ethical, and psychosocial impact of GH ther­
apy in “normal” short children.1474,1480,1481 Given the cost of GH, 
the financial implications of treating such children are consider­
able

tions of treating such children are consider­
able. The point is well taken that 5% of the population will always 
be below the 5th percentile, whether we treat with GH or not, 
and that focusing on short stature could potentially handicap an 
otherwise normal child psychologically and socially. No convinc­
ing data have been presented to show that GH treatment of short 
children definitively improves psychologic, social, or educational 
function.1481–1483 A possible exception is the improved in

.1481–1483 A possible exception is the improved intellec­
tual function in SGA children treated with GH.1484 Furthermore, 
final adult height in the subset of children with CDGD (probably 
a common inclusion, although not ISS as defined in the FDA 
approval) may be adequate without any treatment.41–43
Finally, the known and unknown treatment risks of GH 
therapy in otherwise apparently normal children, even if exceed­
ingly small, are a legitimate concern.1485 The failure to report lev­
els of I

e concern.1485 The failure to report lev­
els of IGF1, IGFBP3, and GHBP in many studies and differing 
interpretations of endogenous GH secretion studies (e.g., assay 
variance, control group size), as well as the heterogeneity of the 
patient groups, confounds assessment of response data. Neverthe­
less, it is clear that many children who do not meet conventional 
criteria for the diagnosis of GHD and who fall under the heading 
of ISS have as great a degree of growth retardation as children wi

reat a degree of growth retardation as children with 
bona fide GHD and might be considered suitable candidates for 
growth-promoting therapy.
Members of the Growth Hormone Research Society, the 
Lawson Wilkins Pediatric Endocrine Society, and the European 
Society for Pediatric Endocrinology meeting in 2007 published 
a manuscript detailing their agreement on the evidence-based 
evaluation and management of ISS in children. They agreed that 
the primary goal of therapy should be the attainment 

 primary goal of therapy should be the attainment of normal 
adult height. They also agreed that the expectation from patients 
and their families that taller stature is associated with positive 
changes in quality of life should be discouraged.1480 Physicians 
should inform families about the available therapeutic options 
and provide a realistic expectation for height gain with therapy, 
including the fact that there may be variability in the outcome, 
and the psychologic counseling options sh

utcome, 
and the psychologic counseling options should be discussed. In 
addition, the patient and family should be aware that therapy may 
be discontinued if the growth response is poor or if the child no 
longer provides assent. The physician is responsible for continued 
monitoring of efficacy and safety and should provide flexibility in 
treatment options.
The studies that ultimately led to approval of GH treatment in 
the United States included the long-term, randomized, double-
blind, plac

ded the long-term, randomized, double-
blind, placebo-controlled trial at the National Institutes of Health 
by Leschek and associates1178 and the randomized dosing trial of 
the European Idiopathic Short Stature Study Group.1486 In the 
former trial, which used a less optimal regimen of 0.22 mg/kg per 
week given thrice weekly in subjects with a mean starting height of 
almost –3 SDS, the treated children had mean final height gain of 
3.7 cm over the control group.1178 In the latter randomized

er the control group.1178 In the latter randomized study, 
GH doses of 0.24 and 0.37 mg/kg per week, given as a regimen of 
six doses per week in children with a mean starting height of –3.0 
to –3.4 SDS, the improvement in the higher-dose group exceeded 
that in the lower-dose group by 3.6 cm, with subjects achieving a 
final height of –1.12 SDS.1486 When the data from the two trials 
were combined, there was a cumulative gain of 7.3 cm in the group 
treated with 0.37 mg/kg per week, compared w

roup 
treated with 0.37 mg/kg per week, compared with placebo.
In a meta-analysis looking at an aggregate group of 1089 chil­
dren, there were four controlled trials that presented adult height 
data showing treatment benefits ranging from 0.54 to 0.84 SDS, 
corresponding to a mean effect on growth of 5 to 6 cm.1487
Concerns had been raised that GH treatment might acceler­
ate pubertal onset and progression in children with ISS, resulting 
in failure to improve height SDS for bone age and offset

lure to improve height SDS for bone age and offsetting the 
positive responses observed during the early years of GH treat­
ment. In the European ISS study,1488 there was no evidence of 
accelerated pubertal or skeletal maturation in the group receiving 
the higher dose, in contrast to data reported by Kamp and col­
leagues1489 with a 30% greater GH dose. This hypothetical effect 
of advancing maturation has not been substantiated by additional 
studies.1490–1493 Taken together, these data show 

studies.1490–1493 Taken together, these data show that GH treat­
ment of prepubertal children with ISS increases growth velocity 
and final height.
The auxologic criteria for GH therapy in the United States are 
based on FDA approval of GH treatment for children shorter than 
–2.25 SDS (1.2 percentile). Some believe that children should be 
treated if their height is below –2.0 SDS and more than 2.0 SDS 
below their midparental target height or if their predicted height 
is below –2.0 SDS (or bo

f their predicted height 
is below –2.0 SDS (or both). Although there is no consensus on 
the age for initiating treatment, most studies of GH therapy in 
children with ISS have involved children from 5 years to early 
puberty. There are no accepted biochemical criteria for initiating 
GH treatment in ISS. The bone age is used to predict adult height, 
although in a longitudinal study of ISS subjects, bone age delay 
had an effect on the precision of the prediction. In children with 
1016
SECTIO

n of the prediction. In children with 
1016
a bone age delay of about 2 years, the average adult height was 
almost equal to the predicted height; in children with no bone 
age delay, the adult height was greater than the initial predicted 
height; and if the bone age was delayed by more than 2 years, the 
adult height was significantly below the predicted adult height.1494 
The psychologic benefits of GH therapy in children with ISS are 
unproved,14

f GH therapy in children with ISS are 
unproved,1495 but medical and psychologic interventions should 
be considered for the child who seems to suffer from his or her 
short stature.
The clinical trials of the treatment of ISS with GH in the lit­
erature often did not contain long-term control groups, hence the 
results show variable growth responses.1485,1496 Most short children 
treated with GH experience growth acceleration that is usually 
sustained over the first several years of therapy, w

stained over the first several years of therapy, with attenuation 
of the treatment response over time. In general, a slower pretreat­
ment growth velocity and a higher weight-to-height ratio—factors 
suggestive of GHD and, to a lesser degree, bone age retardation—
are associated with better early growth responses. Three thousand 
children were classified as having ISS in the KIGS database, with 
153 having reached final height.1491 GH treatment (0.2–0.25 mg/
kg per week) resulted in achievement

(0.2–0.25 mg/
kg per week) resulted in achievement of target height in patients 
with familial short stature, although they continued to have short 
stature as adults (–1.7 SDS in males and –2.2 SDS in females), 
with a mean gain during therapy of 0.6 to 0.9 SDS. In children 
who did not have familial short stature, the mean final height was 
greater for males (–1.4 SDS), although not for females (–2.3 SDS), 
with average gains of 1.3 and 0.9 SDS, respectively. These attained 
heights were quite

, respectively. These attained 
heights were quite different from the midparental target heights, 
which were near 0 SDS. Hintz and colleagues1177 assessed adult 
height in 80 North American children with ISS who were treated 
for up to 10 years at a GH dose of 0.3 mg/kg per week. At the 
conclusion of the study, the mean height SDS was –1.4 SDS with 
a gain of 1.3 SDS, results quite similar to those from the KIGS 
database. Although the Hintz study was not placebo controlled, 
the data were com

udy was not placebo controlled, 
the data were compared with predicted and actual final heights 
in two groups of untreated short children followed for similar 
periods. Compared to untreated children, treated boys achieved a 
mean of 9.2 cm and girls 5.7 cm stature above predicted heights.
Several additional studies have documented the increase in 
final height with GH therapy in children with ISS, although the 
responses have been highly variable and dose dependent. The 
mean increase in adult

le and dose dependent. The 
mean increase in adult height in children with ISS is 3.5 to 7.5 cm 
(with an average duration of therapy of 4–7 years) when compared 
with historic control subjects,1177,1497 with patients’ pretreatment 
predicted adult heights, or with nontreatment or placebo control 
groups.1178,1487 Multiple factors affect the growth response to 
GH, and the best response is seen in children who are younger 
or heavier, those receiving higher GH doses, and those who are 
shortest 

ving higher GH doses, and those who are 
shortest relative to their target height. Adult height outcome is 
influenced negatively by age at initiation of therapy and positively 
by midparental height, initial height, delayed bone age, and the 
magnitude of the first-year response to GH.1498,1494 A 2-year study 
suggested that an increase in IGF1 correlates with height gain.1178 
Recently, a review of the literature on ISS was published, conclud­
ing that GH therapy increases height in some child

ing that GH therapy increases height in some children; however, 
the cost-benefit ratio and long-term safety are concerning.1499
Studies using monotherapy with GnRH agonists have shown 
only a small and variable effect on final adult height, so GnRH ago­
nist therapy is not recommended. Furthermore, GnRH agonists 
have been shown to have short-term adverse effects on BMD.1500 
However, combination therapy with a GnRH agonist and GH has 
potential value in increasing final height, but only with t

 value in increasing final height, but only with treatment 
lasting at least 3 years.1501,1502 However, in a randomized study, 
children treated with a combination of GnRH agonist and GH 
and control subjects had no difference in adult height.1502 A recent 
meta-analysis of eight selected studies showed a significant differ­
ence in adult height and predicted adult height comparing GnRHa 
and GnRHa/GH groups, but there was no significant difference 
in final height SDS and initial height SDS bet

ce 
in final height SDS and initial height SDS between the groups. 
The results suggest adding GH to GnRHa therapy may increase 
adult height in girls with early puberty. The study was limited by 
the small number of studies, its focus on early puberty in girls, and 
was not stratified to assess associations between patient age and 
predicted adult height at initiation of therapy and adult height.1503
Monitoring of GH treatment should include height, weight, 
pubertal development, and adverse ef

ght, weight, 
pubertal development, and adverse effects (scoliosis, tonsillar 
hypertrophy, papilledema, and slipped capital femoral epiphysis) 
two to three times per year. After 1 year of therapy, the height 
velocity SDS and the change in height SDS should be calculated. 
The bone age may be obtained to reassess height prediction and 
if one is considering intervention to delay puberty. IGF1 levels 
may be helpful in guiding GH dose adjustment. Because elevated 
blood glucose levels in patien

. Because elevated 
blood glucose levels in patients with GH-treated ISS have not been 
reported, routine monitoring of glucose metabolism is not advised.
The FDA-approved dose for GH in ISS is 0.3 to 0.37 mg/kg 
per week.1504 The dose may be increased if the growth response 
is inadequate and compliance was assured. However, there are 
no data regarding the long-term safety of GH doses higher than 
0.5 mg/kg per week in children with ISS. IGF1 levels may be 
helpful in assessing compliance and 

evels may be 
helpful in assessing compliance and GH sensitivity. IGF1 levels 
that are elevated (>2.5 SDS) should prompt consideration of GH 
dose reduction. IGF-based dose adjustments in children with 
ISS increased short-term growth when higher IGF targets were 
selected, but long-term studies with respect to efficacy, safety, and 
cost effectiveness have not been done.1489 If the growth rate con­
tinues to be inadequate, GH treatment should be stopped and 
alternative therapies considered. T

e stopped and 
alternative therapies considered. The duration of treatment is con­
troversial, with consideration given for stopping treatment when 
near-adult height is achieved (height velocity <2 cm/year or bone 
age >16 years in boys or >14 years in girls) if height is in the nor­
mal adult range (above –2 SDS). Stopping therapy may be influ­
enced by satisfaction of the patient and family with the resulting 
height, an ongoing cost-benefit analysis, or desire of the child to 
stop for other

nalysis, or desire of the child to 
stop for other reasons (e.g., dissatisfaction with daily injections).
The possible side effects in GH-treated children with ISS are 
similar to those previously reported in children receiving GH 
therapy for other indications, with no long-term adverse effects 
documented.1505,1506 Posttreatment surveillance with a focus on 
cancer prevalence and metabolic side effects is advisable.
The average cost of GH treatment of children with ISS is 
$10,000 to $20,000 p

ment of children with ISS is 
$10,000 to $20,000 per centimeter of growth improvement.1497 
The benefits are less clear,1497 because it is unknown whether a 
gain in height improves quality of life. Therefore recommenda­
tions for treatment that increases adult height should be balanced 
against the high cost of these therapies. 
Miscellaneous Causes of Growth Failure
In addition to the clinical conditions described earlier, GH has 
been used in the treatment of short stature associated with a v

the treatment of short stature associated with a vari­
ety of other conditions involving postnatal growth failure. In gen­
eral, such trials have been uncontrolled and have not included 
sufficient numbers of subjects for efficacy to be evaluated. Exami­
nation of such treatments should be continued in the large inter­
national databases.
1017
Down Syndrome. The encouraging results of GH trials in TS 
led to studies of GH therapy in children wit

n TS 
led to studies of GH therapy in children with Down syndrome. 
In several preliminary studies, GH accelerated growth in such 
patients, although ethical issues were raised concerning the appro­
priateness of such therapy.901,1507–1509 In the uncontrolled NCGS 
experience, 23 children experienced a 1.3-SDS height gain over 
the first 4 years of GH therapy.1471 No convincing data exist that 
GH improves neurologic or intellectual function in such patients. 
The increased risks of diabetes mel

uch patients. 
The increased risks of diabetes mellitus and leukemia in children 
with Down syndrome could be augmented by GH therapy.1510 A 
review of GH therapy given to 15 children with Down syndrome 
reported an increase in mean height but no effect on mental devel­
opment. They do not recommend GH therapy and call for further 
controlled studies.900 
Normal Aging and Other Catabolic States. Detailed consider­
ation of the potential use of GH in normal aging is beyond the 
scope of this chap

 in normal aging is beyond the 
scope of this chapter. The rationale for such therapy is based on 
the concept of the somatopause, a term that highlights the progres­
sive decline in GH secretion after 30 years of age, as reflected in 
decreasing IGF1 levels. Aging can be viewed as a catabolic state, 
with the potential that GH therapy might reverse or retard the loss 
of muscle mass and strength and the decrease in bone density that 
occur with aging. There is no evidence that GH supplementatio

aging. There is no evidence that GH supplementation 
during aging improves muscle strength or quality of life.1511
Growth failure, often with impaired final adult height, is a 
characteristic clinical finding in endogenous or exogenous Cush­
ing syndrome. Excess glucocorticoids cause a catabolic state char­
acterized by increased proteolysis, decreased protein synthesis, 
lowered osteoblastic and increased osteolytic activity, and insu­
lin resistance.1254 GH treatment blunts some of these catab

tance.1254 GH treatment blunts some of these catabolic 
actions but increases the insulin resistance.1255 Mauras and Beau­
frere862 showed that IGF1 therapy similarly induces an anabolic 
response, despite excess glucocorticoids, but does not cause insu­
lin resistance. GH treatment after transplantation1351–1353 and in 
other glucocorticoid-treated children851 causes some height incre­
ments but does not produce as good a response as in individuals 
not taking glucocorticoids. GH does enhance b

als 
not taking glucocorticoids. GH does enhance bone formation and 
increases osteoblastic activity in such children.1512 The marked 
increase in IGF1 levels during GH treatment may be sufficient to 
overcome the local insensitivity to IGF action.300,756,757,1513
GH therapy is also being investigated in catabolic states such 
as burns, tumor cachexia, major abdominal surgery, AIDS, sepsis, 
metabolic acidosis, and situations requiring total parenteral ali­
mentation. GH should not be used in cr

nteral ali­
mentation. GH should not be used in critically ill patients, because 
a randomized, controlled trial demonstrated increased morbidity 
and mortality rates with GH treatment of such patients.1514 The 
FDA-approved indications for the use of GH for purposes other 
than stimulation of growth are (1) GHD in adults, (2) AIDS-
associated wasting or cachexia, and (3) short-bowel syndrome 
requiring total parenteral nutrition. 
Adverse Effects of Growth Hormone
Pituitary-derived human GH had

s of Growth Hormone
Pituitary-derived human GH had an enviable safety record for 
a quarter of a century but proved to be an agent for transmis­
sion of the fatal spongiform encephalopathy, CJD.1194–1196 
Although pit-GH was removed from use in the United States in 
1985, and later throughout the world, more than 200 patients 
with GH-derived CJD have been identified.1194–1196 Although 
recombinant DNA–derived GH does not carry this risk, the 
experience with pit-GH serves as a grim reminder of 

perience with pit-GH serves as a grim reminder of the poten­
tial toxicity that can reside in products used for physiologic 
replacement.
Extensive experience with recombinant GH over nearly 30 
years has been encouraging.1092,1307,1430,1515–1517 Concerns have 
been raised about a number of potential complications, which 
clearly require continued follow-up and assessment. This evalu­
ation has been facilitated by the extensive databases that have 
been established by GH manufacturers, for examp

e 
been established by GH manufacturers, for example, Genentech 
(NCGS)1307 and Pharmacia (KIGS).1442,1518 An important limi­
tation of these registries, however, is that although they have pro­
vided valuable information about the risks of disease during GH 
use, they were not designed to provide data on adverse effects after 
discontinuation of GH use.
Development of Leukemia and Other Malignancies
A report in 1998 of five cases of leukemia that developed in GH-
treated Japanese children raise

t developed in GH-
treated Japanese children raised concern that GH therapy could 
increase the risk of leukemia.1519 One difficulty in assessing the 
role of GH treatment in this disorder is that many GH-deficient 
children have conditions that predispose to the development of 
leukemia, such as prior malignancy, a history of irradiation, or 
concurrent syndromes that themselves are associated with the 
development of leukemia (e.g., Bloom syndrome, Down syn­
drome, Fanconi anemia). After the i

ome, Down syn­
drome, Fanconi anemia). After the initial concerns about the role 
of GH treatment in the development of leukemia, long-term data 
have shown that GH treatment does not increase the incidence 
of leukemia in children who are not already at an increased risk 
of leukemia,1307,1520–1522 does not increase the risk of leukemia 
as a subsequent neoplasm in cancer survivors,1523 and does not 
increase the rate of leukemia relapse.1442,1520,1524 Although it is 
not possible to determine 

20,1524 Although it is 
not possible to determine if GH treatment increases the incidence 
of leukemia in children at increased risk for reasons other than 
prior malignancy, the incidence of leukemia in these at-risk chil­
dren does not appear to be out of line with the expected rate.1521 
Nonetheless, care should be used in prescribing GH therapy for 
children with a past history of leukemia or lymphoma or other 
disorders conveying an increased risk of leukemia.
With respect to malignancies o

d risk of leukemia.
With respect to malignancies other than leukemia, data from 
more than 88,000 GH-treated patients with more than 275,000 
patient-years at risk did not reveal an increased risk for nonleu­
kemic extracranial neoplasms.1307,1452,1516 However, an increased 
risk of post-transplant lymphoproliferative disease (odds ratio 
1.88; 95% confidence interval [CI] 1.00–3.55, p = 0.05) was 
found in children treated with GH before transplant for growth 
failure related to their end-stage

ant for growth 
failure related to their end-stage renal disease.1525 And a statistically 
nonsignificant increase in malignancies was found in GH-treated 
girls with TS (standardized incidence ratio compared with the age-
matched general population of 2.1; 95% CI, 0.76–4.49).1396 
Recurrence of Central Nervous System Tumors
Because many recipients of GH have acquired GHD due to CNS 
tumors or their treatment, the possibility of tumor recurrence 
with therapy is of obvious importance. Estimates 

with therapy is of obvious importance. Estimates of CNS tumor 
recurrence rates in non–GH-treated children and adolescents are 
difficult to obtain, bearing in mind the vast array of treatment 
programs used in the past four decades. In a total of 1083 patients 
compiled from 11 reports who were not treated with GH, 209 
(19.3%) had recurrences.1251,1524,1526–1534 Such data in a het­
erogeneous group, including patients with craniopharyngiomas, 
gliomas, ependymomas, medulloblastomas, and germ 

gliomas, ependymomas, medulloblastomas, and germ cell tumors, 
provide a background for assessing recurrence rates in GH-treated 
youth. Reports from nine centers, encompassing 390 patients, 
indicated recurrence in 64 patients (16.4%) at the time of pub­
lication,1524,1526,1534–1539 which is not much different from the 
1018
recurrence rate observed in a much larger number of untreated 
patients. In a particularly well-done comparative study at thr

 a particularly well-done comparative study at three 
pediatric neuro-oncology centers comprising 1071 brain tumor 
patients, 180 patients were treated with GH for a mean of 6.4 
years, and 31 of them were monitored for more than 10 years; the 
relative risk of recurrence or death was similar with and without 
GH treatment.1538 In a study of 361 cancer survivors (includ­
ing 172 brain tumor patients), disease recurrence of all cancers 
in GH-treated children did not differ from recurrence in tho

ted children did not differ from recurrence in those 
children not treated with GH.1523 A single center study of 110 
previously irradiated brain tumor patients found no difference in 
recurrence in GH-treated patients compared to matched control 
subjects after approximately 15 years of follow-up.1540 Finally, 
extensive analysis of 4410 patients with a history of brain tumor or 
craniopharyngioma before GH therapy in the NCGS and KIGS 
databases1442,1516,1541 showed a similar lack of increased

s1442,1516,1541 showed a similar lack of increased tumor 
recurrence. In the NCGS series, recurrence rates of the most 
common CNS neoplasms, craniopharyngioma (6.4%), primary 
neuroectodermal tumors (medulloblastoma and ependymoma, 
7.2%), and low-grade glioma (18.1%) were lower than or similar 
to those reported in non–GH-treated children.1252,1524,1542 
Development of Subsequent Neoplasms
Children who have survived cancer are at an increased risk of 
developing a second neoplasm because of ei

isk of 
developing a second neoplasm because of either an underlying 
genetic predisposition or the consequences of the treatment for 
the primary malignancy, including radiation therapy and treat­
ment with alkylating agents. Because of the mitogenic and anti­
apoptotic effects of GH and IGF1, there is concern that GH 
treatment could increase the rate of subsequent neoplasms. This 
has been monitored through both the large NCGS and KIGS GH 
treatment databases, as well as through the Childhood

atment databases, as well as through the Childhood Cancer 
Survivor Study (CCSS) group. The CCSS studies followed 361 
GH-treated cancer survivors, and the initial report found an over­
all increased risk of 3.21 (95% CI, 1.88–5.46) for second neo­
plasms.1523 Although the majority of these second neoplasms were 
meningiomas, there were three children who developed osteo­
genic sarcoma out of 122 leukemia/lymphoma survivors who 
had been treated with GH. A similar increased risk for second 
neop

with GH. A similar increased risk for second 
neoplasms, also largely meningiomas, was found in a study of 60 
GH-treated adult patients.1543 In 2500 GH-treated cancer survi­
vors in the NCGS database, there was also an increased incidence 
in secondary malignances, predominantly CNS tumors (glio­
blastoma/glioma, astrocytoma, meningioma) and osteogenic sar­
coma.1307 However, the most recent follow-up of the CCSS found 
that the rate of meningioma in those children not treated with 
GH had rise

ma in those children not treated with 
GH had risen over time to equal that of survivors treated with 
GH. Therefore the CCSS data found that the rate of subsequent 
CNS neoplasms was not increased by GH treatment,1544 with an 
adjusted rate of 1.0 for any CNS secondary neoplasm, 0.8 for 
meningiomas, and 1.9 (95% CI, 0.7–4.8; p = 0.21) for gliomas. 
The equalization of the rate ratio for meningiomas with longer 
follow-up raised the possibility that GH treatment may accelerate 
the manifestatio

that GH treatment may accelerate 
the manifestation of these tumors, rather than increase the abso­
lute rate. Another possible explanation, however, is that the GH-
treated patients had more intense surveillance because of their GH 
treatment and were therefore identified at an earlier stage. 
Pseudotumor Cerebri
Pseudotumor cerebri (idiopathic intracranial hypertension) has 
been reported in GH-treated patients.1307,1442,1516,1545 The disor­
der may develop within months after treatment starts

r may develop within months after treatment starts or as long as 
5 years into the course; it appears to be more frequent in patients 
with renal failure than in those with GHD.1307,1516 The mech­
anism for the effect is unclear but may reflect changes in fluid 
dynamics within the CNS. Pseudotumor has also been described 
after thyroid hormone replacement in patients with hypothy­
roidism. In any case, clinicians should be alert to complaints of 
headache, nausea, dizziness, ataxia, or visual c

headache, nausea, dizziness, ataxia, or visual changes. Significant 
fluid retention with edema or hypertension is rare.1546 Because of 
the possible association of pseudopapilledema with GHD, per­
haps representing a variant of optic nerve hypoplasia,1547 careful 
ophthalmologic evaluation should be undertaken in patients with 
suspected GH therapy–associated pseudotumor cerebri to avoid 
overdiagnosis and invasive treatments. 
Slipped Capital Femoral Epiphysis
Slipped capital femoral epiphys

 Femoral Epiphysis
Slipped capital femoral epiphysis is associated with both hypo­
thyroidism and GHD. Whether GH therapy plays a role in this 
disorder has been difficult to determine, in part because the inci­
dence of slipped capital femoral epiphysis varies with age, gender, 
race, and geographic locale. The reported incidence is between 2 
and 142 cases per 100,000 population; the data in the KIGS and 
NCGS studies are in this range.1307,1442,1516 Accordingly, although 
slipped capital femo

2,1516 Accordingly, although 
slipped capital femoral epiphysis cannot be attributed to GH ther­
apy per se, complaints of hip and knee pain or limp should be 
evaluated carefully. The occurrence of such pain in a GH-treated 
child with an exceedingly rapid growth rate should lead to consid­
eration of this diagnosis. 
Scoliosis
Both progression of preexisting scoliosis and new-onset scoliosis 
have been described in children treated with GH.1307 The num­
bers are extremely small, with only 238 

 The num­
bers are extremely small, with only 238 reported cases among 
54,996 patients in the NCGS database. There is specific concern 
for GH effects on scoliosis in TS and PWS patients, in whom 
the underlying rate of scoliosis is increased. Although the rate of 
scoliosis in GH-treated TS patients was higher than in other GH-
treated patients, it remained rare, being reported in only 0.69% of 
patients.1396 PWS by itself has a rate of scoliosis of 30% to 80%, 
and GH treatment does not appea

is of 30% to 80%, 
and GH treatment does not appear to increase the rate or severity 
of scoliosis in PWS patients.1325,1548 
The association of GH treatment with insulin resistance is well 
documented.1549 In addition, candidates for GH therapy include 
some who are known to have increased risk of type 2 diabetes mel­
litus (T2DM), including patients with TS, PWS, or those born 
IUGR. Data from the NCGS database estimated the incidence 
of T2DM in GH-treated patients as approx

ncidence 
of T2DM in GH-treated patients as approximately 14 cases per 
100,000 patient-years,1307,1442 and data from KIGS and from 
GeNeSIS (Genetic and Neuroendocrinology of Short Stature 
International Study; another large pharmaceutical company–
sponsored postmarketing surveillance database) found the inci­
dence of T2DM in GH-treated children to be approximately 30 
cases per 100,000 patient-years.1550,1551 The incidence found in 
the KIGS and GeNeSIS studies was 6 to 6.5 times higher than 

nd GeNeSIS studies was 6 to 6.5 times higher than the 
expected incidence.1550,1551 Many of these children did have risk 
factors for T2DM, although not all of them were obese.44,45,1516 
In some, but not all, of the subjects who developed T2DM, there 
was resolution of the diabetes on discontinuation of GH treat­
ment,1520,1550,1551 likely highlighting the underlying risk of dia­
betes in these individuals and supporting a causative role of GH 
treatment in the development of diabetes. Therefor

treatment in the development of diabetes. Therefore the reduction 
1019
of insulin sensitivity induced by GH is a concern that demands 
close assessment, particularly in high-risk patients such as those 
with PWS or TS or a history of IUGR. Reassuringly, however, 
the prevalence of diabetes in young adults who had previously 
been treated with GH in childhood was not increased in a large 
population-based study.1552 
Miscellaneous Side Effects13

ion-based study.1552 
Miscellaneous Side Effects1307,1442,1516
Other potential side effects of GH therapy include prepubertal 
gynecomastia,1553 pancreatitis,1554 growth of nevi1555,1556 (although 
typically without evidence of malignant degeneration1520), behav­
ioral changes, worsening of neurofibromatosis, hypertrophy of 
tonsils and adenoids, and sleep apnea. GH treatment increases 
the conversion of cortisol to cortisone, which may trigger a need 
for glucocorticoid replacement therapy in t

 need 
for glucocorticoid replacement therapy in those with subclinical 
central adrenal insufficiency.1430 GH treatment also increases the 
conversion of thyroxine to triiodothyronine, which could either 
increase levothyroxine dose requirements in those treated for cen­
tral hypothyroidism or unmask preexisting but undiagnosed central 
hypothyroidism.1430 A report1557 of reduced testicular volume and 
elevated gonadotropin levels in four young adult men previously 
treated with GH for ISS was 

adult men previously 
treated with GH for ISS was not confirmed by a double-blind, pla­
cebo-controlled study,1558 nor by the international databases.1559 
This list of side effects is only partial. It is best for the clinician 
to remember that GH and the peptide growth factors it regulates 
are potent mitogens with diverse metabolic and anabolic actions. 
All patients receiving GH treatment, even as replacement therapy, 
must be carefully monitored for side effects.
For the most part, the side

ored for side effects.
For the most part, the side effects of GH are minimal and rare. 
When they do occur, a careful history and physical examination 
are adequate to identify their presence. Management of these side 
effects may include transient reduction of dosage or temporary 
discontinuation of GH.1056 In the absence of other risk factors, 
there is no evidence that the risk of leukemia, brain tumor recur­
rence, or slipped capital femoral epiphysis is increased in recipi­
ents of long-ter

epiphysis is increased in recipi­
ents of long-term GH treatment. Any patient receiving GH who 
has a second major medical condition (e.g., being a tumor survi­
vor) should be followed up in conjunction with an appropriate 
specialist such as an oncologist or neurosurgeon. Whereas GH has 
been shown to increase the mortality rate of critically ill patients 
in intensive care units,1514 there is no evidence that GH replace­
ment therapy needs to be discontinued during intercurrent illness 
in chi

e discontinued during intercurrent illness 
in children with GHD.
Recent in vitro studies have found that growth hormone could 
play a role in the progression of certain cancer types through the 
induction of an epithelial to mesenchymal transition.1560 This 
effect would potentially increase the risk of cancer metastasis; 
therefore it strongly argues against GH treatment in patients with 
active malignancies.
While the impact of GH treatment on long-term mortality is 
complex (see upcoming dis

 long-term mortality is 
complex (see upcoming discussion), there is no evidence that GH 
treatment in childhood increases the acute mortality risk.1561 
The Question of Long-Term Cancer Risk
Several epidemiologic studies have suggested an association 
between high serum IGF1 levels and an increased incidence of 
malignancies.1562–1564 Such studies have found an association of 
higher IGF1 levels with breast, prostate, and colon cancer.1564–1566 
In contrast, higher IGFBP3 levels have been assoc

In contrast, higher IGFBP3 levels have been associated with a 
decreased risk of prostate and breast cancer, although other stud­
ies found no association of IGFB3 levels and cancer1564 or found 
a positive association.1564,1566 Because GH increases production 
of both IGF1 and IGFBP3, the studies that found the highest 
association with cancer with high IGF1 and low IGFBP3 levels 
do not suggest a simple relationship between GH and cancer risk.
Epidemiologic studies assessing the risk of malig

Epidemiologic studies assessing the risk of malignancy in 
patients with acromegaly found differing results, with some,1566–
1569 but not others,1570,1571 identifying significant associations 
between acromegaly and colon cancer risk. The small size and 
uncontrolled retrospective nature of these studies and the mul­
tiple possible sources of bias make these reports difficult to inter­
pret. The largest study to date, reviewing more than 1000 patients, 
indicated no overall increased cancer inc

tients, 
indicated no overall increased cancer incidence in acromegaly.1572 
Although colon cancer risk was also not increased in that study, 
the mortality rate was higher, suggesting an effect of GH or IGF 
on established tumors.1573 A prospective analysis of colon can­
cer and colonic polyps in acromegalics did not observe an asso­
ciation between these two diseases when using either autopsy 
series or prospective colonoscopy screening series for the control 
population.1574 Acromegaly is ass

for the control 
population.1574 Acromegaly is associated with a marked increase 
in the incidence of benign hyperplasia in several organs, includ­
ing colonic polyps.1575 Such findings suggest that the GH-IGF1 
axis may lead to symptomatic benign proliferative disease, which 
could be associated with symptoms, such as rectal bleeding, that 
would lead to a potential detection (or ascertainment) bias.
Children receiving GH do not appear to have a greater risk 
of de novo or recurrent tumors.1092

 greater risk 
of de novo or recurrent tumors.1092,1307,1442,1516,1576 A cohort 
of 1848 patients treated with GH in the United Kingdom was 
assessed after as long as 40 years and found to have increased 
rates of colorectal cancer and Hodgkin disease, but the tumor-
associated deaths were so few that a single patient death would 
markedly alter the risk ratios.1577,1578 No increased incidence 
of cancer was found in GH recipients among adults who were 
treated for GHD.1579,1580 A large European

o were 
treated for GHD.1579,1580 A large European study found no over­
all increase in cancer risk in adults who were treated with growth 
hormone as children and who did not have an apparent underly­
ing cancer risk.1576 However, that study did find an increased 
incidence of bone and bladder cancer, along with possible evi­
dence of an increase in Hodgkin lymphoma risk in these adults. 
In addition, although not found in all studies,1581 there have 
been some data suggesting an increased risk

 have 
been some data suggesting an increased risk of cancer mortality 
after childhood GH treatment.1576,1581,1582 One of these studies 
found an increased mortality rate from bone tumors,1581 while 
a large European study found an increased cancer mortality risk 
associated with the daily GH dose in patients treated with GH 
after previous cancer.1576 These reports represent imperfect, 
uncontrolled studies. The overall data do not indicate a clear 
association of GH therapy with the future de

lear 
association of GH therapy with the future development of neo­
plasms in the absence of other risk factors, but there remains 
some uncertainty over the risk of specific cancers and perhaps 
over cancer mortality risk.
The use of IGF1 and IGFBP3 in the monitoring of GH 
recipients, both adult and pediatric, has been recommended and 
endorsed by international bodies such as the Growth Hormone 
Research Society,1055 the Drug and Therapeutics Committees of 
the Lawson Wilkins Pediatric Endocri

ommittees of 
the Lawson Wilkins Pediatric Endocrine Society, and the Euro­
pean Society of Pediatric Endocrinology.1577,1578 Until the issue of 
cancer risk in GH therapy is fully resolved, the prudent approach 
appears to be regular monitoring of both IGF1 and IGFBP3 and 
alteration of the GH dose to ensure that the theoretic risk profile 
induced by GH therapy is favorable. This can be done by avoiding 
the unlikely situation of a GH-treated patient with an IGF1 level 
at the upper end of the

atient with an IGF1 level 
at the upper end of the normal range and an IGFBP3 level at the 
lower end of the normal range. 
1020
Long-Term Mortality with GH Treatment
The effect of GH deficiency and GH treatment on overall mortal­
ity is complex. Subjects with untreated congenital GH deficiency 
from mutations in PROP1 appear to have increased lifespan.1564 
In contrast, individuals with GH deficiency from GH1 gene 
deletion have a decreased lifespan

 from GH1 gene 
deletion have a decreased lifespan, with increased cardiovascular 
and infectious disease mortality rates,1564 yet subjects with GH 
deficiency from GHRH mutations have no change in mortality 
rate compared with the local population.1564,1583 The background 
mortality rates of the gene deletion and GHRH mutation ethnic 
populations were quite different, which may account for some of 
the mortality discrepancy. Untreated Laron patients have been 
found to have a marked decrease in

ents have been 
found to have a marked decrease in cancer mortality rate.1564 
Mortality data on GH-treated Laron patients are not available. 
Mortality risk is increased in adults with untreated GH deficiency 
(due to increased mortality rate from cardiovascular disease) but 
also in adults with GH excess (due to an increase in cardiovascu­
lar, cerebrovascular, and respiratory deaths).1564 In 2012, investi­
gators reported on the all-cause mortality rate of French children 
treated with GH for

ality rate of French children 
treated with GH for isolated GH deficiency. GH treatment in this 
cohort was initiated between 1985 and 1996, with a mean age at 
follow-up of 28 years. They found an increase in all-cause mortal­
ity rate when compared to the French general population, with a 
standardized mortality ratio of 1.33 (95% CI 1.08–1.64).1581 The 
diseases contributing to this increase in mortality rate were dis­
eases of the circulatory system and subarachnoid or intracerebral 
hemorrh

 system and subarachnoid or intracerebral 
hemorrhage. In response to this report, investigators looked at all-
cause mortality rate in GH-treated children in Sweden, Belgium, 
and the Netherlands.1582 These patients started treatment between 
1985 and 1997 and had a mean age of 27 to 29 years at follow-up. 
In this cohort, no subjects died from cancer or cardiovascular dis­
ease. Another report also found no increase in mortality in 3847 
adults in Sweden treated with GH in childhood for IGHD, 

 in Sweden treated with GH in childhood for IGHD, ISS, 
or SGA.1584 Thus the data do not give a clear answer to the long-
term effects of GH treatment on longevity, including whether the 
impact would differ based on the indication for GH treatment. 
IGF1 Treatment
In 2005, the FDA approved the use of rhIGF1 for “long term 
treatment of growth failure in children with severe primary IGF1 
deficiency and in children with GH gene deletions who have 
developed neutralizing antibodies to GH. Severe 

developed neutralizing antibodies to GH. Severe primary IGF1 
deficiency is defined by height SD score less than –3.0 and a basal 
IGF1 SD score less than –3.0 and normal or elevated GH. Severe 
primary IGF1 deficiency includes patients with mutations in the 
GHR, post-GHR receptor signaling pathway, and IGF1 gene 
defects” (Increlex [rhIGF1] prescribing information, 2009, Ter­
cica, Inc.). Clinical trials to study the safety and efficacy of rhIGF1 
therapy have been conducted mostly in indivi

IGF1 
therapy have been conducted mostly in individuals with proven 
mutations in the GHR; fewer than 10% of enrollees have had GH 
insensitivity due to GH antibodies. Because of the rarity of GHR 
mutations, the clinical trials have been small.
A number of short-term growth-related studies with sub­
cutaneous IGF1 treatment at varied doses have been reported. 
Despite slight differences in IGF1 dosage and inclusion criteria, 
the short-term effects of IGF1 on growth in GH insensitivity were 
co

cts of IGF1 on growth in GH insensitivity were 
consistent: Growth increased from pretreatment rates of 3 to 5 
cm per year to 8 to 11 cm per year on average. In five children 
with GHR deficiency, single daily doses of 150 μg/kg for 3 to 10 
months resulted in growth acceleration to rates of 8.8 to 13.6 cm 
per year.1585 Wilton1253 reported collaborative data on the treat­
ment of 30 patients, ages 3 to 23 years, with GH insensitivity due 
to GHR deficiency or IGHD type IA with anti-GH antibodi

R deficiency or IGHD type IA with anti-GH antibodies; 
the dose of IGF1 varied from 40 to 120 μg/kg twice daily. With 
the exception of the two oldest individuals, growth rates increased 
in Wilton’s subjects by at least 2 cm per year. A mean increment 
of more than 4 cm per year in growth velocity was found in 11 
prepubertal children treated with 80 μg/kg twice daily.1585 This 
study also demonstrated a significant inverse relationship between 
growth response to exogenous IGF1 and severity of

growth response to exogenous IGF1 and severity of the GH insen­
sitivity phenotype.
Longer-term studies of IGF1 treatment of GH insensitiv­
ity demonstrated a progressive waning effect of IGF1 on growth 
velocity.1586 Data from 17 patients in a European collaborative 
trial who were treated for at least 4 years showed an increase in 
mean height SDS from –6.5 to –4.9, with two adolescents reach­
ing the 3rd percentile. Children who received rhIGF1 for 2 years 
or less had a higher growth veloci

F1 for 2 years 
or less had a higher growth velocity than those children who 
received rhIGF1 for longer periods. In the longest treatment study 
of 58 children with GH receptor mutations, GH antibodies, or 
biochemical evidence of GH insensitivity without documented 
mutation,1587 outcomes were similar to those in the European 
study, with an initial burst of growth (from 3.1 to 8.0 cm/year) 
followed by slowing to just above baseline (4.8 cm/year) by the 
fourth year of therapy. Height SDS imp

ar) by the 
fourth year of therapy. Height SDS improved from –6.5 to –5.2 
after 5 to 6 years of therapy. Data on 21 of these patients at or near 
adult height with average treatment duration of 10 years ranged 
from –8.0 to –6.2 SDS, lower for those persons with GH antibod­
ies as the underlying cause for their GH resistance.1588
These limited data indicate that rhIGF1 is effective in increas­
ing statural growth, but the growth response is neither as robust 
nor as sustained as the growth resp

her as robust 
nor as sustained as the growth response to GH among children 
with GHD. The suboptimal growth response to rhIGF1 compared 
with recombinant human GH has been attributed to (1) inability 
of rhIGF1 to increase IGFBP3 and ALS levels, leading to decreased 
delivery of IGF1 to target tissues; (2) lack of GH-induced pro­
liferation of prechondrocytes in the growth plate; (3) absence of 
GH-induced local IGF1 at the growth plate; and (4) difficulty in 
using higher doses of IGF1 because

 difficulty in 
using higher doses of IGF1 because of the risk of hypoglycemia.
IGF1 treatment administered to individuals with IGF1 gene 
deletion656 and post-GHR signaling defects with growth pat­
terns was similar to those of children with GH insensitivity due to 
GHR deficiency. First-year results from an open-label trial of 124 
children with low serum IGF1 and height SDS below –2 with­
out biochemical evidence of GH insensitivity indicate that growth 
velocity increased for the year of the

hat growth 
velocity increased for the year of therapy (from 5 to 7 cm/year).1256
The most common adverse effect of rhIGF1 is hypoglycemia. 
In some studies, it occurred in almost half of the patients with 
GH insensitivity treated with rhIGF1,1587 usually within the first 
month of therapy. Importantly, individuals with GHR defects, the 
highest proportion of enrollees in these treatment studies, have 
baseline hypoglycemia. In a 6-month, placebo-controlled trial, 
67% of children receiving pla

o-controlled trial, 
67% of children receiving placebo experienced hypoglycemia, 
compared with 85% of children receiving rhIGF1—a difference 
that was not statistically significant.1589 In a study of 23 patients 
with GH insensitivity, hypoglycemia was noted before the start of 
treatment; after 3 months, 2.6% of glucose values in those receiv­
ing placebo were less than 50 mg/dL, compared with 5.5% of glu­
cose values in those receiving rhIGF1.1587 The hypoglycemia was 
not to be avoidable wit

1587 The hypoglycemia was 
not to be avoidable with adequate food intake. In children with 
low IGF1 but without GH insensitivity, 20% of subjects who 
received the higher IGF1 dose experienced symptoms attributed 
to hypoglycemia; documentation of hypoglycemia was only noted 
in one subject.1256
1021
Patients with GH insensitivity treated with rhIGF1 have also 
experienced lymphoid tissue hypertrophy, encompassing tonsil­
lar/adenoidal growth a

rophy, encompassing tonsil­
lar/adenoidal growth and associated snoring and sleep apnea, and 
thymic and splenic enlargement.1587 The hypertrophy occurred in 
22% of patients, with 10% of patients requiring tonsillectomy/
adenoidectomy. Coarsening of the face was observed in patients 
of pubertal age.1587 Studies have reported an increase in BMI 
with treatment and a twofold to threefold increase in body fat 
as assessed by dual-energy x-ray absorptiometry (DEXA).1590 The 
study to adult height 

rptiometry (DEXA).1590 The 
study to adult height did not note this significant increase in body 
fat.1588 Intracranial hypertension with associated papilledema has 
also been observed in as many as 5% of treated patients. In the 
study by Midyett and coworkers, 2 of 100 patients receiving IGF1 
had documented intracranial hypertension.1256 Anti-IGF1 anti­
bodies develop in about half of treated patients during the first 
year, but they have no effect on the response.1587 As with recombi­
nant h

ffect on the response.1587 As with recombi­
nant human GH treatment, an increased cancer risk with rhIGF1 
treatment is an unknown but legitimate concern, considering the 
role of IGF1 in neoplasias and the increased cancer risk associated 
with hypersomatotropism.1591
Approval of rhIGF1 by the FDA and the European Agency 
for the Evaluation of Medical Products has ignited debate about 
expansion of the diagnosis of severe IGF1 deficiency and conse­
quent expanded use of rhIGF1 beyond those pati

se­
quent expanded use of rhIGF1 beyond those patients with neu­
tralizing GH antibodies or documented mutations in the GHR, 
GHR signaling pathways, or the IGF1 gene. Some advocate that 
individuals with ISS, low serum IGF1, or poor response to a GH 
trial may have unidentified subtle alterations in GHR signaling 
and could benefit from treatment with rhIGF1.1592 Others express 
reservations about expanding the diagnosis of IGF1 deficiency 
and rhIGF1 use, postulating that the response of child

rhIGF1 use, postulating that the response of children with 
ISS to rhIGF1 may be minimal considering the lower long-term 
growth response of children with GHR defects treated with IGF1 
compared with GH-deficient children treated with GH. Another 
theoretic concern regards negative feedback of rhIGF1 on GH 
secretion, which leads to diminished IGF1-independent effects of 
GH on growth.1593 
Other Treatments to Promote Growth
GnRH Agonists
Increasing Adult Height of Children with Idiopathic Short

ing Adult Height of Children with Idiopathic Short 
Stature. The effect of GnRH agonist therapy on adult height has 
been studied in girls with ISS and normal puberty (8–10 years 
of age); there was a mean gain of 0 to 4.2 cm, compared with 
predicted height.1485,1594–1605 In boys with rapidly progressing 
puberty, GnRH agonist therapy increased adult height compared 
with predicted height,1606,1607 with mean gains of 4.4 to 10 cm in 
those receiving the combination therapy, compared with 0.5 to

ving the combination therapy, compared with 0.5 to 
6.1 cm in untreated control subjects.1487,1608
In these studies, the effects of GH cannot be separated defini­
tively from those of the GnRH agonist. In two randomized studies 
of adopted girls with normal puberty, treatment with a GnRH 
agonist plus GH was compared with GnRH agonist alone, and 
a 3-cm height gain was demonstrated with the combination 
therapy.1609,1610
Disadvantages of the use of GnRH agonists in children with 
ISS include abs

of GnRH agonists in children with 
ISS include absence of pubertal growth acceleration, delayed 
puberty with potential psychosocial disadvantage, and decreased 
BMD. Long-term follow-up studies are lacking. A panel of experts 
concluded that GnRH agonist therapy alone in children with ISS 
and normally timed puberty is minimally effective in increasing 
adult height, may compromise BMD, and cannot be suggested for 
routine use.1611 Combined GnRH agonist and GH therapy leads 
to a significant he

 agonist and GH therapy leads 
to a significant height gain but may have adverse effects. Routine 
use of GnRH agonists in children with ISS being treated with GH 
cannot be suggested (level of evidence, C-III). 
Increasing Adult Height of Children Born SGA. Short children 
born SGA usually have a normal pubertal timing, although some 
have rapidly progressing puberty and may be treated with GH.1445 
Data on the additional effect of treatment with GnRH agonists 
are limited. Routine use of the c

h GnRH agonists 
are limited. Routine use of the combination of a GnRH agonist 
and GH in children born SGA cannot be suggested. 
Increasing Adult Height of Children with Growth Hormone 
Deficiency. Some children with GHD are short at the start of 
puberty and are at risk for short adult stature. Retrospective stud­
ies evaluating the addition of GnRH agonists to GH therapy 
involved a limited number of subjects and provided controversial 
results.1612,1613 Three prospective studies that reporte

s.1612,1613 Three prospective studies that reported near-adult 
or adult heights showed a height gain of –1 SDS.1614–1616 Rou­
tine use of combined therapy with a GnRH agonist and GH in 
GH-deficient children with low predicted adult height at onset of 
puberty cannot be suggested (level of evidence, C-III). 
Men with estrogen deficiency due to aromatase gene defects or 
estrogen resistance due to inactive estrogen receptors experience 
growth into the third decade, demonstr

experience 
growth into the third decade, demonstrating the role of estrogen 
in growth plate fusion.23,315 The aromatase enzyme catalyzes aro­
matization of testosterone and androstenedione to estradiol and 
estrone, respectively. Various studies have been conducted to 
explore the efficacy of the aromatase inhibitor class of drugs delay­
ing growth plate fusion and increasing height in disorders associ­
ated with short stature in boys. Studies to date have been small, 
and many have used adjuv

to date have been small, 
and many have used adjuvant treatments, including GH, GnRH 
agonist, or testosterone, so the singular effect of the aromatase 
inhibitor has been unclear. Additionally, most studies to date have 
been short term and have measured changes in predicted adult 
height, with only one study investigating the effect on final adult 
height.
Aromatase inhibitors were first tried in disorders of sex steroid 
excess and precocious puberty, with only modest, if any, effects 
on pre

puberty, with only modest, if any, effects 
on predicted adult height. Treatment of boys with familial male-
limited precocious puberty with testolactone, a first-generation 
aromatase inhibitor, resulted in an improvement in predicted 
adult height only after 5 to 6 years of treatment.1617 The improve­
ment in predicted adult height after 6 years was robust, however, 
at 12.9 cm compared with untreated control subjects. In contrast, 
first-generation and second-generation aromatase inhibitors h

ation and second-generation aromatase inhibitors have 
not significantly affected predicted adult height in patients with 
McCune-Albright syndrome.1618–1620 Final height data are not 
available.
Aromatase inhibitors have also been used in boys with CDGD. 
Available studies had a small number of subjects enrolled, and 
boys with CDGD who received the second-generation aroma­
tase inhibitor letrozole, in combination with testosterone, for 
12 months experienced an increase in predicted adult heig

hs experienced an increase in predicted adult height of 
5.1 cm compared with control subjects treated with testosterone 
alone.1181 When followed to adult height, those boys who received 
letrozole in addition to testosterone achieved a final adult height 
5.7 cm higher than that of boys receiving testosterone alone.1182 
In GH-deficient boys treated with GH, the addition of anastro­
zole for 3 years increased predicted adult height by 6.7 cm, com­
pared with an increase of 1 cm in boys treated

om­
pared with an increase of 1 cm in boys treated with GH alone.1498 
The results of these preliminary small trials indicate that aromatase 
1022
inhibitors may be able to increase adult height, but larger, longer 
trials are needed to determine the optimal conditions and dura­
tion of treatment to significantly increase adult height. In addi­
tion, longer follow-up will be needed to demonstrate the safety of 
such treatments in peripubertal and pub

safety of 
such treatments in peripubertal and pubertal boys.
Given the observation of decreased BMD in males with defects 
in estrogen synthesis or action, the effects of short-term aromatase 
inhibitor therapy on BMD were investigated in these treatment 
studies. None of the studies found a difference in BMD as assessed 
by DEXA scans in patients who received aromatase inhibitor or 
placebo for up to 3 years.1498,1621 However, mild vertebral abnor­
malities were noted in boys who received the 

nor­
malities were noted in boys who received the aromatase inhibitor 
compared with placebo1622; it is unknown whether the deformi­
ties are due to treatment effects and whether they resolve upon 
longer duration letrozole therapy. In light of these effects, aroma­
tase inhibitors must be used with caution. Clearly, longitudinal 
follow-up is needed to better characterize the safety and efficacy of 
aromatase inhibitors to promote growth. 
Oxandrolone
Oxandrolone, an anabolic steroid, has been 

rolone
Oxandrolone, an anabolic steroid, has been used to increase 
growth velocity in a number of disorders. Because it cannot be 
aromatized to estrogen, it should not accelerate skeletal matura­
tion. In general, studies evaluating the effect of oxandrolone on 
growth have found that it increases growth velocity but is not asso­
ciated with an increase in final height.
Clinical Trials of Efficacy. Numerous studies have investi­
gated oxandrolone therapy in boys with CDGD. The studies 
have fo

ne therapy in boys with CDGD. The studies 
have found that oxandrolone increases growth velocity in these 
boys.1139,1158 The typical response is an increase in growth velocity 
from approximately 4 to 4.5 cm per year up to 8 to 9 cm per year. 
Although treatment does not decrease final height, as might occur 
with accelerated skeletal maturation from excessive sex hormone 
exposure, neither does it increase final height.1158 Therefore oxan­
drolone can be used to accelerate the growth of boys w

one can be used to accelerate the growth of boys with CDGD 
to allow them to increase their height sooner than they would 
naturally, but it does not increase their final adult height. There 
have been no trials comparing efficacy of oxandrolone versus tes­
tosterone treatment in boys with CDGD.
Oxandrolone has been studied in girls with TS, both as a single 
agent and as combined therapy with GH. As in boys with CDGD, 
oxandrolone increases the growth rate in girls with TS.1623,1624 
Although s

growth rate in girls with TS.1623,1624 
Although some studies found no effect on final height with oxan­
drolone treatment alone,1625 others found a mean increase in final 
height of up to 5.2 cm with oxandrolone treatment.1624 Results of 
three controlled trials testing the effects of combination therapy 
with GH versus GH alone on adult height were reviewed.1626 The 
studies had different average age at treatment start and used dif­
ferent GH and oxandrolone doses.1627–1629 In general, the stu

nd oxandrolone doses.1627–1629 In general, the studies 
reported a positive effect of oxandrolone when added to standard 
GH therapy, with the effect on adult height varying between 2.3 
and 4.6 cm. 
Side Effects. No significant side effects have been reported in 
boys treated with oxandrolone for CDGD. Although oxandrolone 
has significantly fewer androgenic effects than testosterone, mild 
virilization has been reported in girls taking oxandrolone, includ­
ing clitoromegaly. This is less of a 

one, includ­
ing clitoromegaly. This is less of a concern at lower doses.1626 There 
are also reports of a delay in breast development that improves 
upon higher estrogen dosing. Hepatic dysfunction has been 
reported with oxandrolone treatment, manifested by alterations in 
HDL cholesterol, and thus monitoring of lipids is suggested.1626 
Diagnosis and Treatment of Excess Growth and 
Diagnosis
The normal distribution of height predicts that 2.5% of the 
population will be taller th

cts that 2.5% of the 
population will be taller than 2 SD above the mean. The most 
common cause of tall stature is familial, and the diagnostic eval­
uation centers on distinguishing tall or constitutional stature 
from rare pathologic causes of tall stature. As with short stature, 
children with tall stature must be evaluated relative to familial 
growth patterns and parental target heights. When a family his­
tory of tall stature is available and the growth rate and physical 
examination find

and the growth rate and physical 
examination findings are normal, support and reassurance are 
frequently all that is needed without further testing. A careful 
assessment of pubertal status and bone age facilitates prediction 
of adult height and discussions with the patient and family. If 
the history suggests an underlying disorder or growth rate is 
accelerated, additional testing should be done to investigate the 
GH-IGF1 axis, chromosomal disorders, puberty, or other rare 
causes.
In GH e

disorders, puberty, or other rare 
causes.
In GH excess, serum IGF1 levels are elevated, although high 
IGF1 levels may be a normal manifestation of puberty. Basal 
serum GH levels may be normal to increased, but serum GH 
is not suppressed by administration of glucose (1.75 g/kg body 
weight up to a maximum of 100 g). Although abnormalities of 
the sella turcica can be evident on lateral skull films, the demon­
stration of increased GH-IGF secretion should lead to radiologic 
evaluation of the 

tion should lead to radiologic 
evaluation of the hypothalamus and pituitary by MRI or com­
puted tomography. 
Treatment
Definitive treatment of GH-secreting tumors requires surgical 
ablation, either transsphenoidally or with a more aggressive sur­
gical approach if large. As described in Chapter 9, somatostatin 
analogues, dopamine agonists, and GH-receptor antagonists are 
important components of treatment programs for GH excess.
In the past, most patients treated for familial tall stature we

most patients treated for familial tall stature were 
females. The number of patients treated in the United States has 
fallen markedly over the past four decades as tall stature in girls 
has become increasingly acceptable socially and psychologically. 
Treatment regimens were generally with estrogen prior to puber­
tal onset to induce early epiphyseal maturation1630 and consid­
ered girls with predicted heights greater than 183 cm (6 feet 0 
inches). Treatment regimens varied considerably, and

ches). Treatment regimens varied considerably, and there are no 
randomized trials testing treatment effectiveness. Controversy sur­
rounds the treatment of girls with tall stature, especially in light 
of long-term studies that raised the possibility of effects on fer­
tility.1631 One-fifth of pediatric endocrinologists reported use of 
estrogens for treatment of tall stature in 1999,1632 a percentage 
that is likely diminishing. In males, androgens have been used to 
accelerate skeletal matura

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1023
26

esc 
1023
26
Physiology and Disorders of Puberty
DENNIS M. STYNE
P
uberty is not a de novo event but rather a phase in the 
continuum of development of gonadal function and the 
ontogeny of the hypothalamic-pituitary-gonadal system 
from the fetus to full sexual maturation and fertility. During 
puberty, secondary sexual characteristics appear, and the pubertal 
growth spurt occurs; this results in the striking sex dimorphism 
of mature individuals, and fertility is ac

rphism 
of mature individuals, and fertility is achieved.1 These changes 
result from stimulation of the gonads by pituitary gonadotro­
pins and a subsequent increase in gonadal steroid output. The 
term puberty is characteristically used to refer to the physical and 
endocrine changes of this period, while adolescent usually refers 
to the profound psychologic changes encountered during this 
time.
Puberty and Evolution
Human beings evolved into the most reproductively successful 
of mammals, a

 the most reproductively successful 
of mammals, and many anthropologists attributed this success to 
the prolonged pattern of human growth and development and to 
the delay in attaining full sexual maturity.2 The human scheme 
of growth involves a childhood stage and a pubertal stage that 
includes a pubertal growth spurt (Fig. 26.1). Not even our closest 
biologic relative, the chimpanzee, which matures twice as rapidly 
as the human, unequivocally exhibits these two stages, and chim­
panzees 

ally exhibits these two stages, and chim­
panzees lack the unique human pubertal growth spurt.3 Learning 
and practice of adult behaviors related to sex and childrearing, par­
ticularly provisioning children (not just infants) with food, which 
Puberty and Evolution, 1023
Fetal Origins of Adult Disease, 1025
Determinants of the Age of Puberty and Menarche, 1025
Secondary Sexual Characteristics and Physical Changes of 
Puberty, 1032
Central Nervous System Anatomy, Function, Psycho

2
Central Nervous System Anatomy, Function, Psychology, and 
Electroencephalographic Rhythm in Puberty, 1047
Hormonal and Metabolic Changes in Puberty, 1051
Central Nervous System and Puberty, 1059
Adrenal Androgens and Adrenarche, 1079
Disorders of Puberty, 1082
KEY POINTS
 • 
Puberty is not a de novo event but rather a phase in the con­
tinuum of development of gonadal function and the ontogeny 
of the hypothalamic-pituitary-gonadal system from the fetus to 
full sexual maturation and fertili

m the fetus to 
full sexual maturation and fertility.
 • 
During puberty, secondary sexual characteristics appear, and 
the pubertal growth spurt occurs, resulting in the striking sex 
dimorphism of mature individuals; fertility is achieved, and 
profound psychologic effects ensue.
 • 
Gonadarche, or the awakening of gonadal sex steroid secretion, 
and adrenarche, or the awakening of adrenal androgen secre­
tion, are separate processes that must be evaluated individually, 
as the customary chr

t be evaluated individually, 
as the customary chronologic relationship between the events 
may vary considerably among individuals.
 • 
The age of onset of thelarche, or breast development, has de­
creased over the past several decades according to international 
data; the obesity epidemic may be one of the factors respon­
sible for this secular trend toward earlier female development, 
although other factors are suspected.
 • 
The discovery of new genes involved in regulation of the hypo­
th

f new genes involved in regulation of the hypo­
thalamic control of pubertal development greatly expands our 
understanding of the cause of delayed and precocious puberty, 
but the complex mechanisms that initiate normal pubertal 
development at a given age remain largely unknown.
 • 
Evaluation of puberty in children and adolescents should include 
the use of highly specific and sensitive assays with pediatric stan­
dards; sex steroids should be measured with high-performance 
liquid chromatog

e measured with high-performance 
liquid chromatography tandem mass spectroscopy (HPLC-MS/MS) 
methods, unless painstaking extraction and manual immunoassays 
are invoked.
 • 
The remarkable success in treatment of childhood neoplasia 
has led to secondary effects on pubertal development, either 
advancing or delaying the age of onset of puberty and often 
impairing subsequent fertility.
1024
is unique to human beings, is considered a critical part 

ue to human beings, is considered a critical part of human 
success: “the building of a better, healthier body and the develop­
ing of greater biological, behavioral, and cultural resilience prior 
to sexual maturity that leads to greater adult health, fitness, and 
longevity.”2 Toolmaking preceded the evolutionary development 
of adolescence, suggesting that the evolution and value of human 
childhood and adolescence and this unique pattern of growth and 
development have had significant roles 

rowth and 
development have had significant roles in the reproductive success 
of humans.
In the developed world, reproductive maturity occurs years 
earlier than psychosocial maturation, causing a mismatch 
between biologic stages and psychosocial expectations and 
roles (Fig. 26.2).4 In past eras, such as the Neolithic, Greek, 
or Roman periods, there was not such a mismatch because 
menarche occurred at an age close to that of reproductive 
maturity.
With increased population, the advent of a

turity.
With increased population, the advent of agriculture, and 
the growth of cities and later urban centers, menarche occurred 
later, and the complexity of life led to a delay in the attain­
ment of an adult role in society. In Roman Britain, 2000 years 
ago, pubertal growth as an indication of puberty occurred at 
ages similar to those of today, but puberty progressed at a slower 
pace apparently leading to a later attainment of reproductive 
maturity.5
Infancy
Childhood
Juvenile
Adolescen

e 
Infancy
Childhood
Juvenile
Adolescent
Adult
H. erec. 2
H. erec. 1
H. habilis
A. africa*
Pan/A. afar.*
~400
0
*~3.0 >1.5 × 106 years ago
2
4
6
8
10
12
14
16
18
20
~600–800
up to 1100
~1400
Brain size
(mL):
H. sapiens
• Fig. 26.1  Evolution of the human pattern of postnatal growth and development during the first 20 years of 
life. Specimens include A. afar, Australopithecus afarensis, a “bipedal chimpanzee”; A. africa, Australopithe­
cus africanus; H. habilis, Homo habil

stralopithe­
cus africanus; H. habilis, Homo habilis, the toolmaker; H. erec. 1, early Homo erectus; H. erec. 2, late Homo 
erectus; H. sapiens, Homo sapiens. The early hominid australopithecine specimens from South Africa date 
to about 3 to 1.5 million years ago. H. afarensis, although a hominid (family of all human species), retained 
many anatomic features of nonhominid species, such as an adult brain size of about 400 mL compared 
with H. habilis (650–800 mL), early H. erectus (850–900 mL),

bilis (650–800 mL), early H. erectus (850–900 mL), late H. erectus (up to 1100 mL), and mod­
ern H. sapiens (about 1400 mL). Infancy is the period when the mother’s breast milk is the sole or most 
important source of nutrition, and in preindustrialized societies, it ends at about 36 months. Childhood is 
the period after weaning, when the child depends on others for food and protection; this period ends when 
the growth of the brain in weight is almost complete, at about age 7 years. The juveni

 almost complete, at about age 7 years. The juvenile stage is defined as 
prepubertal individuals who are no longer dependent on their parents for survival. The adolescent stage, 
which begins with the onset of puberty, ends when adult height is attained (Moggi-Cecchi; Conroy and 
Kuykendall). The pattern in A. afarensis is no different from that of the chimpanzee (Pan troglodytes). Notice 
the first appearance of the childhood stage in H. habilis (arising about 2 million years ago) and the firs

s (arising about 2 million years ago) and the first 
appearance of the adolescent stage in H. erectus 2 (about 500,000 years ago); H. sapiens arose about 
120,000 to 150,000 years ago. (Modified from Bogin B. Growth and development: recent evolutionary and 
biocultural research. In: Boaz NT, Wolfe LD, eds. Biological Anthropology: The State of the Science. Bend, 
OR: International Institute for Human Evolutionary Research; 1995:49–70. Additional data from Moggi-
Cecchi J. Questions of growth. Na

data from Moggi-
Cecchi J. Questions of growth. Nature. 2001;414:595–597; Conroy GC, Kuykendall K. Paleopediatrics: or 
when did human infants really become human? Am J Phys Anthropol. 1995;98:121–131.)
20
10
0
Present
Social
complexity
and nutritional
overload
Industrial
revolution
and social
Agriculture
settlement
Mismatch
Menarche
Psychosocial
2000
years
ago
200
years
ago
20,000
years
ago
• Fig. 26.2  The relationship between the likely range of ages of mena

ationship between the likely range of ages of menarche 
(purple) and achievement of psychosocial maturity (pink) from 20,000 years 
ago to the current day. The mismatch in timing between these two processes 
is a novel phenomenon. (From Gluckman PD, Hanson MA. Evolution, devel­
opment, and timing of puberty. Trends Endocrinol Metab. 2006;17:7–12.)
1025
Chapter 26  Physiology and Disorders of Puberty
In modern times, the age of menarche has decreased, but the 
age of social adulthood still occurs

sed, but the 
age of social adulthood still occurs later, causing a discrepancy that 
probably has not occurred previously in human history. The study 
of human evolution adds to the understanding of many modern 
medical conditions as well as puberty, and the discipline of evo­
lutionary biology is now recommended as a required premedical 
course by the American Association of Medical Colleges.6,7 
Fetal Origins of Adult Disease
There are long-lasting effects of abnormalities in fetal and neo­
n

sting effects of abnormalities in fetal and neo­
natal growth. As seen in longitudinal studies, low birth weight 
followed by rapid weight gain in infancy (i.e., catch-up growth) 
leads to tall childhood stature and early pubertal development. 
Poor prenatal nutrition and small-for-gestational-age (SGA) sta­
tus tend to advance the age of puberty in girls and the age of 
adrenarche8–10; a secondary effect of increased postnatal nutri­
tion, often leading to overweight or obesity, also lowers the

 leading to overweight or obesity, also lowers the age 
of puberty. Fig. 26.3 describes the relationship between exces­
sive adipose tissue and early puberty. A prospective study dem­
onstrated that a lower expected birth weight ratio (i.e., ratio of 
observed infant’s birth weight to median birth weight appropri­
ate for maternal age, weight, height, parity, infant sex, and gesta­
tional age) and a higher body mass index (BMI) at 8 years led to 
an earlier age of menarche.11 Girls who are longe

an earlier age of menarche.11 Girls who are longer and lighter at 
birth and subsequently have greater BMI values at 8 years tend 
to have earlier menarche.12 Rapid weight gain in the second to 
ninth months but not thereafter correlated with a greater BMI 
at 10 years and with earlier menarche in a longitudinal study.13 
On the other hand, higher birth weight is related to a later age 
of menarche, judging from data across several countries.14 Large-
for-gestational-age (LGA) infants tend to h

Large-
for-gestational-age (LGA) infants tend to have an earlier age of 
takeoff of the pubertal growth spurt.15 Many international studies 
find a relationship between low birth weight or catch-up growth 
and chronic diseases in adulthood; this phenomenon, the Barker 
hypothesis, is found in a wide range of populations.16
Birth weight and rate of postnatal growth—not prematurity 
alone—are inversely related to cardiovascular mortality risk and 
prevalence of insulin resistance syndrome (i.e., m

prevalence of insulin resistance syndrome (i.e., metabolic syn­
drome or syndrome X), which consists of hypertension, impaired 
glucose tolerance, and elevated triglyceride levels, among a grow­
ing list of other findings. This outcome is attributed to fetal and 
neonatal metabolic programming, in which early adjustments to 
enhance survival in difficult intrauterine circumstances set the 
stage for later disorders. Insulin resistance, which may be the 
basis for most of these complications or m

be the 
basis for most of these complications or may be just one feature 
of the syndrome, may spare nutrients from use in muscle, leav­
ing them available for the brain. This mechanism can minimize 
central nervous system (CNS) damage in the fetus during periods 
of malnutrition. 
Determinants of the Age of Puberty and 
Menarche
Although historic records show that puberty occurs at an earlier 
age today, most evidence derives from reports of the age of men­
arche (Table 26.1).17,18 Age of menar

 age of men­
arche (Table 26.1).17,18 Age of menarche is removed by several 
years from the first sign of secondary development in girls, and 
modern studies demonstrate correlation coefficients of only 0.37 
between age of menarche and age of onset of puberty, suggesting 
both unique and similar factors exerting effects on these ages.19 
Changes in health and socioeconomic status in regions where data 
were collected during different decades lead to complexity in the 
interpretation of modern n

d to complexity in the 
interpretation of modern national data.
Age of menarche is determined in various ways. Recalled age 
of menarche is considered to be accurate within 1 year (in 90% 
of cases) up to 30 years after the event.20 Contemporaneous 
recordings are performed with the probit method of asking for a 
response of “yes” or “no” to the question, “Are you menstruating?” 
However, the results are subject to social pressures of the culture 
and socioeconomic group considered.21
Physical e

Physical examination with palpation of gonads or breasts by 
trained observers is the most accurate method of assessment of 
pubertal development. Detection of the onset of stage 2 breast 
development in an overweight girl on physical examination may 
be difficult even for a trained observer (although stage 3 usually 
is obvious). Visual observation of the stage of development in 
person (not by photographs) is one step removed from physical 
examination a

) is one step removed from physical 
examination and palpation; errors in the evaluation of breast tis­
sue in obese girls or the stage of testicular enlargement in boys 
may occur. Visual observation by multiple observers was used in 
the Third National Health and Nutrition Examination Survey 
(NHANES III) (and in the National Health Examination Sur­
vey [NHES] that occurred more than 20 years earlier), and visual 
observation is subject to interobserver variation. In a study by 
Pediatric Rese

robserver variation. In a study by 
Pediatric Research in the Office Setting (PROS), a network fos­
tered by the American Academy of Pediatrics, specially trained 
pediatricians, nurse practitioners, or physician assistants in 225 
offices used palpation for 30% of the study population and visual 
inspection for all members of a convenience sample of 17,070 
girls across the United States.22
Prenatal growth restriction
Adrenals:
  Androgens
Liver:
  SHBG
Ovary:
  Androgens
Fat cells:
  Aromatase

:
  SHBG
Ovary:
  Androgens
Fat cells:
  Aromatase
Promotes earlier
adrenarche, pubarche,
and/or thelarche
Triggers earlier
central precocious
puberty
Obesity, visceral fat
Bioavailability of sex steroids
?
Insulin resistance and
compensatory hyperinsulinemia
• Fig. 26.3  Proposed endocrine pathways linking childhood obesity and 
insulin resistance to early pubertal onset and maturation. Childhood obe­
sity and the predisposition to visceral adiposity after intrauterine growth 
restraint lead to

osity after intrauterine growth 
restraint lead to insulin resistance and peripheral hyperinsulinemia. Insulin 
acts on various organs, including the adrenals, liver, ovary, and fat cells, to 
increase sex steroid bioavailability. Elevated circulating and tissue sex ste­
roid levels in obese prepubertal children can have only mild local effects or 
activate early hypothalamic-pituitary puberty and early reproductive matu­
ration. SHBG, sex hormone–binding globulin. (From Ahmed ML, Ong KK, 
Dunge

e–binding globulin. (From Ahmed ML, Ong KK, 
Dunger DB. Childhood obesity and the timing of puberty. Trends Endocri­
nol Metab. 2009;20:237–242.)
1026
  Comparison of Menarcheal Ages Reported by Various Studies
Study
Year
Plan
n
AGE AT MENARCHE (YR)
Comments
Overall
W
B
M
Britain
1969
Long
Probit
192 (W)
13.5
NHES III
1963-1970
Cross
Recalled
3272
Born 1940-1960: 
12.8
Born 1890-1910: 
13.5
12.52
NHANES III
1988-1994
Cross
Yes/No
Probit
3

5
12.52
NHANES III
1988-1994
Cross
Yes/No
Probit
330 (W)
419 (B)
419 (M)
12.7
12.3
12.5
Men. age black < 
white
NHANES III
1988-1994
Cross
Yes/No
2510
12.43
white
Probit
710 (W)
917 (B)
883 (M)
12.6
12.06
12.3
PROS
1992-1993
Cross
Status Quo
Probit
17077 (W)
1638 (B)
12.9
12.16
white
NHLB 
Growth
1987-1997
Long
Recalled
1092 (W)
1164 (B)
12.7
12.1
white
BMI inversely propor­
tional to men. age
Bogalusa
1973-1974
Cross
5552
12.7
12.9
1992-1994

Bogalusa
1973-1974
Cross
5552
12.7
12.9
1992-1994
Cross
12.5
12.1
1973-1994
Long
2508
12.6
12.3
white
NHES
1963-1970
Status Quo
Yes/No
Status Quo
3272
12.75
12.8
12.48
1988-1994
Status Quo
Median
Yes/N
1414
12.54
12.6
12.14
Men. age NHANES III 
< NHES
Growing difference in 
men. age white-
black
B, Black; BMI, body mass index; Cross, cross sectional; Long, longitudinal; M, Mexican American; men.,

al; Long, longitudinal; M, Mexican American; men., menarcheal; NHANES, National Health and Nutrition Examination Surveys; NHES, National Health 
Examination Survey; NHLB Growth, National Heart, Lung, and Blood Institute Growth and Health Study; PROS, Pediatric Research in Office Settings; W, white.
From Styne DM. Puberty, obesity and ethnicity. Trends Endocrinol Metab. 2004;15:472–478.
   
TABLE 26.1
The availability of only limited numbers of trained personnel 
for examination or subjects’ refu

ained personnel 
for examination or subjects’ refusal of embarrassing examinations 
sometimes leads to the use of proxy measures. Photographs or 
drawings of pubertal development allow self-reporting or parental 
reporting of pubertal progress, but correlations range widely from 
0.48 to 0.91 compared with physicians’ examinations or visual 
observation of subjects.23 The answers to self-assessment may be 
influenced by subjects’ wishes to conform with their understand­
ing of normal development

m with their understand­
ing of normal development and may be less accurate in some eth­
nic groups than others, similar to reported menarcheal age. Obese 
girls may overestimate breast developmental stage, and boys may 
overestimate pubic hair development. Individuals with learning 
disabilities, chronic diseases (e.g., cystic fibrosis, Crohn disease), 
or psychologic conditions (e.g., anorexia nervosa) may have less 
accurate pubertal staging.24 Self-report is related to testosterone 
values f

4 Self-report is related to testosterone 
values for boys and girls and is said to be accurate enough if preci­
sion is not needed.25 With all of its difficulties, physical exami­
nation remains the best method to accurately evaluate pubertal 
development, although self or parent report can likely determine 
prepuberty from puberty.26,27
The Secular Trend in Puberty and Menarche
Review of classic writings gives us insight into the changes in age 
of puberty and menarche through the epochs, altho

of puberty and menarche through the epochs, although much of 
the data are inferred.28,29
During Paleolithic and Neolithic eras, menarche was estimated 
to occur between 7 and 13 years of age, equal to or earlier than 
1027
the present average. It was postulated that early menarche allows 
reproduction in the face of an early death. Writings during Greek, 
Roman, and Indian history suggest an age of menarche of 12 to 
14 years and age of ejaculati

f menarche of 12 to 
14 years and age of ejaculation in boys to be similar. Records from 
medieval Europe suggest a menarche at age 12 to 15 years. Up to 
the 20th century menarche age appeared to increase to 14 to 16 
years.
The Developed World
The average age of menarche in industrialized European countries 
has decreased by 2 to 3 months per decade over the past 150 years; 
in the United States, the decrease has been approximately 2 to 3 
months per decade during the past century17,30 (Fig. 2

hs per decade during the past century17,30 (Fig. 26.4). How­
ever, this secular trend slowed in developed countries such as the 
United States, Australia, and Western Europe between approxi­
mately 1940 and 1970, presumably due to improved socioeco­
nomic status, better health, and the benefits of urbanization. There 
is a relatively small range of ages of menarche in the well-off devel­
oped world, where lower socioeconomic classes do not have an 
increased burden of disease or malnutrition. Ch

n 
increased burden of disease or malnutrition. Chronic diseases pre­
viously increased the age of menarche, and delay in menarche is still 
associated with serious conditions (e.g., celiac disease, asthma) that 
are not adequately treated. The standard deviation of the mean age 
of menarche also decreased, suggesting a diminished number of 
those maturing very late, as might be found among disadvantaged 
people.31 Teasing out the various factors involved in subtler secular 
trends will require 

 involved in subtler secular 
trends will require further long-term study and newer methodo­
logic approaches in areas where nutrition and health are optimal 
or close to it.32 Remarkably, a reverse secular trend is reported in 
certain areas of Europe, leading to a later age of menarche. This has 
been attributed to a resurgence of physical and psychologic stress, 
as was seen in previous eras (e.g., World War II).32
There are cross-sectional and longitudinal data from the late 
20th century de

d longitudinal data from the late 
20th century demonstrating a resurgence of a secular trend in the 
United States, including ethnic influences.33,34 The median age 
of menarche in the United States was 12.8 years according to the 
1973 US National Center for Health Statistics National Health 
Education Study, and data from NHANES III (1988–1994) indi­
cate the median age at that time was 12.43 years, 0.37 year earlier 
than in 1973.35 During a 20-year period in the Bogalusa Heart 
Study, the m

20-year period in the Bogalusa Heart 
Study, the median menarcheal age decreased by approximately 9.5 
months among African-American girls compared with 2 months 
among white girls,36 leading to a 4-month difference. Between the 
ages of 5 and 9 years, African-American girls in the Bogalusa study 
had taller stature and greater weight, factors that were predictive 
of menarche before age 11 years. African-American girls have 
advanced secondary sexual development compared with white 
American gi

exual development compared with white 
American girls of the same age during the first three stages of 
puberty, and they have an advanced bone age; this may be related 
to the higher prevalence of obesity among African-American girls 
and to an ethnic-specific genetic influence.37–39
Obesity is defined as a BMI (calculated as weight in kilograms 
divided by height in meters squared) equal to or above the 95th 
percentile for age, and overweight is defined by the Centers for Dis­
ease Control an

is defined by the Centers for Dis­
ease Control and Prevention (CDC) as a BMI equal to or greater 
than the 85th percentile for age. Many studies have reported on 
the effect of the epidemic of childhood obesity and overweight on 
age of menarche.40,41 Most cross-sectional and longitudinal stud­
ies found an inverse relationship between menarcheal age or other 
stages of puberty and BMI or other reflections of adiposity.42,43 
If the population in the 1970s had had the same range of BMI 
values 

n the 1970s had had the same range of BMI 
values as was found in the 1990s, the projected age of menarche 
would have been the same in the 1970s as it was in the 1990s.44 
Girls had a greater prevalence of overweight or obesity between 
the ages of 8 and 10 years if they were in puberty or had advanced 
breast development.45 A study of the age at onset of puberty in 
US boys of various ethnic groups indicated an earlier age of onset 
between 6 months and 2 years depending upon indicator and eth

onths and 2 years depending upon indicator and eth­
nic group.46 Except for a delay in pubertal development with BMI 
less than 15% there was no effect of overweight or obesity in these 
boys as was found in girls in other studies.
Longitudinal studies are limited in the United States but of 
great importance in the evaluation of a secular trend. The lon­
gitudinal National Heart, Lung, and Blood Institute (NHLBI) 
Growth and Health Study followed 1266 white and 1313 Afri­
can-American girls fro

d 1266 white and 1313 Afri­
can-American girls from 9 or 10 years of age for 10 or more 
years. A recent report of a sample of 1239 girls from the study 
noted that 10.4% of white, 23.4% of black non-Hispanic, and 
14.9% of Hispanic girls had attained breast stage 2 at 7 years; 
18.3%, 42.9%, and 30.9%, respectively, attained breast stage 2 at 
8 years, younger than data from 10 to 30 years ago. In a sample 
of 1155 girls, the mean age of menarche among whites was 12.7 
years, and among African 

e among whites was 12.7 
years, and among African Americans it was 12.1 years; a direct 
relationship was found between weight and BMI and the age of 
menarche.38 Of these girls, 51.6% started puberty with only one 
manifestation. Those who experienced breast development first 
(i.e., thelarche pattern), rather than the appearance of pubic hair 
(i.e., adrenarche pattern), had an earlier menarche (12.6 vs 13.1 
years); this was associated with a greater BMI and body weight, 
which was not true f

greater BMI and body weight, 
which was not true for those girls manifesting the adrenarche pat­
tern.47 These findings support the analysis of NHANES III, in 
which girls with earlier breast development had greater BMI val­
ues at the time of menarche than those with adrenarche occurring 
first.48
Sweden
Norway
Finland
Denmark
Holland
U.S.A.
Age at menarche (years)
Year of menarche
13
14
12
13
14
15
16
17
1840
1860
1880
1950
1960
1970
1900
1920
1940
1960
1980
• Fig. 26.4  The cha

1970
1900
1920
1940
1960
1980
• Fig. 26.4  The changes in age at menarche between 1840 and 1978 
illustrate the advance in the age at menarche in Western Europe and the 
United States since 1840 and slowing of this trend since about 1965. 
(Modified from Tanner M, Eveleth PB. Variability between populations in 
growth and development at puberty. In: Berenberg SR, ed. Puberty, Bio­
logic and Psychosocial Components. Leiden, The Netherlands: H.E. Sten­
fert Kroese; 1975:256–273.)
1028
SECTION VI  

ten­
1028
African-American girls in the National Longitudinal Study 
of Adolescent Health (Add Health) were 1.55 times more likely 
than white girls to have menarche before 11 years49 of age, and 
Mexican Americans were 1.76 times more likely to do so than 
whites. Asians were 1.65 more likely than whites to mature later 
than 14 years. Those undergoing early menarche were twice as 
likely to be overweight, and African-Ame

twice as 
likely to be overweight, and African-American girls had a 2.57-
fold greater risk for overweight if they had menarche before they 
were 11 years old. Among early menarcheal African Americans, 
57.5% had a BMI greater than the 85th percentile, and 32.5% 
had a BMI greater than the 95th percentile. A longitudinal study 
of 180 girls indicated earlier pubertal development in these cases: 
between the ages of 5 and 9 years with a higher percentage of 
body fat at 5 years; those with a high

centage of 
body fat at 5 years; those with a higher percentage of body fat, 
higher BMI percentile, or larger waist circumference at 7 years; 
those with larger increases in the percentage of body fat from 5 
to 9 years; and those with larger increases in waist circumference 
from 7 to 9 years.50
During the past 40 years, menarcheal age in the white sub­
jects in the Fels Longitudinal Study has remained stable even as 
BMI has increased, with no relationship shown between the two. 
Subjects wit

 relationship shown between the two. 
Subjects with early menarche had a tendency to increase BMI 
after menarche,51 suggesting that increased weight appears to be 
a consequence rather than determinant of the age at menarche 
and that secular changes in BMI and in mean age at menarche 
may be independent phenomena. There was a rise in BMI, waist 
circumference, hip circumference, and serum levels of luteinizing 
hormone (LH), androstenedione, testosterone, and dehydroepi­
androsterone sulfate (

stosterone, and dehydroepi­
androsterone sulfate (DHEAS) during the years immediately after 
menarche in a shorter longitudinal study.52
International data support the effects of BMI on puberty, 
but earlier onset of pubertal growth and a tall stature in child­
hood do not lead to a taller adult height.53 Children studied 
longitudinally in Denmark showed earlier breast development 
(estimated mean age of 9.86 years compared to 10.88 years) and 
menarche (13.42 years compared to 13.13 years) in 

menarche (13.42 years compared to 13.13 years) in 2006 com­
pared to 1991, which could not be explained by BMI or even by 
a difference in gonadotropin values suggesting other factors were 
operative.54 
The Developing World
The interaction of socioeconomic conditions, nutrition, energy 
expenditure, states of health, and puberty is of particular impor­
tance in areas of the world where nutrition is suboptimal; where 
the most rapid improvement is found, such as in Oaxaca, Green­
land, or South 

s found, such as in Oaxaca, Green­
land, or South Korea, the age of menarche decreases more rapidly. 
Where standards of living do not change, the age of menarche is 
more stable. In South America and Africa, some rural children 
fare better and have earlier puberty and taller stature than urban 
children, demonstrating a trend of adverse health and nutritional 
conditions in crowded urban centers. Malnourished individuals 
have later age of menarche across the world.55
As a whole, these reports

rche across the world.55
As a whole, these reports indicate that populations existing 
in the most difficult conditions who experience improvement in 
socioeconomic status demonstrate a greater decrease in the age of 
menarche. Once a minimum nutritional status or state of health 
is reached, the effects of socioeconomic status on the age of men­
arche are minimized or eliminated, but increased BMI can lower 
the age of menarche further. However, there are other factors, 
apparently environmenta

 there are other factors, 
apparently environmental factors, that are postulated to affect the 
age of thelarche, which may not have as much effect on the age 
of menarche. 
Factors Affecting the Age of Puberty and Menarche
The composition of diet and the number of calories in the diet 
may relate to menarche.56 The prospective, longitudinal Harvard 
Longitudinal Studies of Child Health and Development found 
that girls had earlier menarche if they were taller and consumed 
more animal protein a

ey were taller and consumed 
more animal protein and less vegetable protein as early as 3 to 5 
years old and that they had earlier peak growth velocity if they had 
higher dietary fat intake at 1 to 2 years old and higher animal pro­
tein intake at 6 to 8 years. Peak height velocity (PHV) increased, 
controlling for body size, if more calories and animal protein were 
consumed 2 years before peak growth. The correlation between 
higher animal protein intake and lower age of menarche is con­
fir

otein intake and lower age of menarche is con­
firmed in Germany and Britain.57,58 A positive relationship 
between high fiber intake and age of menarche held true in a com­
parison of 46 countries,59 and a longitudinal study demonstrated 
a later onset of puberty and PHV in girls with higher isoflavone 
intakes, suggesting a lower risk for breast cancer with a high fiber 
prepubertal diet.60 On the other hand, lifelong vegetarian dietary 
intake does not affect the age of menarche,61 and a low-

 does not affect the age of menarche,61 and a low-fat diet 
in otherwise healthy, prepubertal children 8 to 10 years old with 
elevated low-density lipoprotein (LDL) cholesterol produced no 
difference in age of menarche or pubertal progression.62 Phytoes­
trogen (flavonol but not lignan) intake delays breast development, 
particularly in girls with lower BMI values.63,64
Another influence of the age of menarche is the maternal envi­
ronment. Macrosomia due to maternal diabetes or obesity is ass

rosomia due to maternal diabetes or obesity is asso­
ciated with increased BMI in childhood, which itself is related to 
earlier puberty and menarche. Increased or decreased weight gain 
in pregnancy likewise decreases the age of menarche according to 
data from the Nurses’ Health Study II.65 Heavy maternal smoking 
of cigarettes but not of marijuana leads to earlier male puberty 
but not female puberty in offspring.66 Maternal tea drinking but 
not coffee drinking was associated with a later ag

not coffee drinking was associated with a later age of menarche 
and puberty in their daughters. Daughters of mothers who were 
more physically active during pregnancy had a modest delay in 
menarche.67
Stress and Puberty
Life history theories aiming to explain influences on the age of 
puberty address energetics, stress suppression, psychosocial accel­
eration, paternal investment, and child development, each of 
which may have various effects on the timing and progression of 
pubertal developm

n the timing and progression of 
pubertal development.68 This approach acknowledges the fact that 
evolution optimizes allocation of limited resources to maximize 
fitness and allow reproductive success.69
Stress is variably reported to increase or decrease the age of 
menarche. The absence of a father or lower parental education 
increases the likelihood of early menarche; however, there is evi­
dence that the exposure to paternal psychopathology that precedes 
the absence of the father might b

y that precedes 
the absence of the father might be causative in such situations.70 
War increases the age of voice breaking in boys (e.g., in Bach’s 
choir during the War of Austrian Succession in 1727–1749), and 
the age of menarche increased during World War II and during 
more recent hostilities in the former Yugoslavia.71 Sexual abuse 
is associated with earlier onset of puberty and earlier menarche 
compared with a control population in a large US national study, 
although it is difficult 

arge US national study, 
although it is difficult to eliminate the effect of family dysfunc­
tion in this situation as well.72 Child abuse of various types is 
associated with delayed puberty and menarche in a large British 
study, and sexual abuse in this cohort led to both early and late 
menarche.73 
1029
Genetic Effects on Puberty and Menarche
There remains a difference in age at attainment of stages of puberty 
in different countries even with

tages of puberty 
in different countries even with stability in socioeconomic fac­
tors; for example, Japanese boys undergo changes in testicular size 
about 1 year earlier than Swiss boys do.74 When socioeconomic 
and environmental factors lead to good nutrition, general health, 
and infant care, 60% to 80% of the determination of the age at 
onset of puberty in normal children appears to be due to genetic 
causes.31
The important role played by genetic factors in the onset of 
puberty is illus

 genetic factors in the onset of 
puberty is illustrated by the similar age of menarche in members 
of an ethnic population and in parent-child and sibling or twin 
pairs.75,76 The correlation between mother and daughter patterns 
should theoretically be equal to sister-sister age at menarche if 
only genetic factors are operative, but because sister-sister cor­
relations are higher than mother-daughter correlations, environ­
mental influences must provide an additional influence beyond 
genetic

st provide an additional influence beyond 
genetic factors; the secular trend and nutritional factors may be 
invoked.
Concordance of ages of pubertal developmental stages and 
menarche is closer between monozygotic than dizygotic twins, 
supporting the influence of genetic factors. Monozygotic twins 
reared together have more similar ages of menarche than those 
reared apart, and dizygotic twins reared together are less similar 
than either of the monozygotic groups, pointing to environmen­
tal

he monozygotic groups, pointing to environmen­
tal influences on genetic factors. Some twin research suggests that 
additive genetic factors account for 96% of the variance in the age 
of puberty in girls and 88% of the variance in boys (although other 
sources from the United States, Australia, Great Britain, Finland, 
and Norway have found genetic effects accounting for between 
50% and 80% of the variance), with the remainder resulting from 
shared and nonshared environmental influences.77,78

hared and nonshared environmental influences.77,78
A large study of 180 twins, 132 monozygotic and 48 dizygotic, 
showed a high level of hereditability of DHEAS sulfate levels and 
pubertal development with a higher estimate in monozygotic 
twins.79
Genome-wide association study (GWAS) techniques approach 
the greatly complex question of the genetic control of puberty 
and growth. Alleles near 6q21 (T) at rs7759938 (LIN28B) cor­
related with earlier puberty and shorter prepubertal childhood 
hei

ier puberty and shorter prepubertal childhood 
height, as well as age of menarche.80,81 A meta-analysis of GWAS 
data for 17,510 women demonstrated the strongest signal related 
to age of menarche × 10−9, where the nearest genes include 
TMEM38B, FKTN, FSD1L, TAL2, and ZNF462, and the next 
strongest signal near the LIN28B gene (rs7759938; p = 7.0 × 
10−9), which also influences adult height and cancer risk.82 Stud­
ies of thousands of European individuals linked locus upstream 
of myocardin-lik

ndividuals linked locus upstream 
of myocardin-like 2 (MKL2) (p = 8.9 × 10−9) with earlier puberty 
with reduced pubertal growth (p = 4.6 × 10−5) and short adult 
stature (p = 7.5 × 10−6) in both males and females.83 Variants 
near MAPK3, PXMP3, and VGLL3 linked taller prepubertal 
stature with earlier menarche, and a variant near ADCY3-POMC 
was associated with increased BMI, reduced pubertal growth, 
and earlier puberty.83
Gonadotropin-releasing hormone type I (GnRH-I) and its 
receptor (GnRHR

g hormone type I (GnRH-I) and its 
receptor (GnRHR) genes are only modestly related to the age of 
menarche.84 The LEP1875 and XbaI and PvuII polymorphisms 
of the estrogen receptor α (ERα) gene (ESR1) and maternal 
age at birth (i.e., >30 or <30 years) were associated with age of 
menarche.85,86 High-activity CYP17 alleles involved in estro­
gen formation and high-activity CYP1A2 and CYP1B1 alleles, 
whose gene products metabolize estradiol, are not associated with 
pubertal stage,87 whereas th

 not associated with 
pubertal stage,87 whereas the high-activity CYP3A4* 1B/1B girls 
had an earlier age of onset of normal puberty alleles. The vari­
ant rs10235235 of CYP3 is associated with a reduction in breast 
cancer risk for women who had their menarche age at 15 years or 
older but not for those with menarche younger than 11 years.88 
Girls with longer (>8) TAAAA repeats in their sex hormone–bind­
ing globulin gene (SHBG) have a later age of menarche than those 
with fewer repeats.89 A 

e of menarche than those 
with fewer repeats.89 A GWAS study showed the common genetic 
variants contribute to early puberty in girls more than late puberty, 
and they contribute in boys to delayed puberty more than early 
puberty. It suggested that this may be an explanation why preco­
cious puberty is more likely to be idiopathic than organic in girls 
and delayed puberty more likely to be constitutional in boys.90 
This study also found that early puberty regardless of BMI was 
associated wit

arly puberty regardless of BMI was 
associated with higher risks of breast, ovarian, and endometrial 
cancers in women. 
Cancer and Age of Puberty
There is divergent data on puberty and prostatic cancer. The pre­
ceding study showed increased risk of prostate cancer in men 
with early puberty, which was confirmed in a study of 2927 cases 
of prostatic cancer that found that older onset of puberty led to 
a lower risk of prostate cancer, especially aggressive types, and 
another study of 757 men.

y aggressive types, and 
another study of 757 men.91 Conflicting results occurred in a 
study of 1088 cases of prostate cancer, which demonstrated an 
increased risk of 6% for each year of recalled delayed puberty 
with an odds ratio (OR) of 1.35 if puberty commenced after 15 
years.92
There are genetic effects upon the age of onset of testicular 
growth in boys as well. Danish and Chilean boys had a later onset 
of puberty when they had genetic alteration in follicle-stimulating 
hormone (FSH) 

alteration in follicle-stimulating 
hormone (FSH) beta production (FSHβ c.-211G>T), which was 
further enhanced with a genetic variation in FSH receptor sensi­
tivity (FSHR c.-29G>A).93 There was a 1.5% to 1.7% variance 
explained by the genetic modifications in contrast to the effect of 
BMI, which explained 7.2% variance in Chilean boys and 17.2% 
variance in Denmark boys. 
Seasonality of menarche was observed in a US cohort of 3000 
college students; those born after 1970 had an

00 
college students; those born after 1970 had an earlier age of men­
arche and a more pronounced frequency peak in July. Factors 
hypothesized to contribute to seasonality of menarche include 
stress and the photoperiod.94 In Peru, puberty begins at a later 
age, and pubertal development lasts longer at high altitudes than 
at low altitudes even when nutritional status is similar. There is a 
north-to-south decrease in the age of menarche in Europe25 that 
may result from environmental factors

rope25 that 
may result from environmental factors or genetic influences.31 
The Comorbid Conditions of Early Puberty
Many international studies show that earlier age at menarche is 
associated with a greater risk of development of breast cancer by 
a factor of 1.050 (95% confidence interval [CI] 1.044-1.057; p 
< 0.0001) for every year younger at menarche.95 The risk of pre­
menopausal breast cancer decreases 9% per year of delay in men­
arche, and the risk for postmenopausal breast cancer decr

and the risk for postmenopausal breast cancer decreases 
4%.96 This trend in which a longer period of reproductive func­
tion increases the risk for breast cancer is more due to early 
menarche than to later menopause and has been found in other 
studies as well. In disease-discordant monozygotic twins, the one 
with cancer recalled puberty to be earlier; in disease-concordant 
1030
twins, the one with earlier menarche had the earlier diagnosis 
of b

h earlier menarche had the earlier diagnosis 
of breast cancer.97 Women with breast cancer were taller and 
leaner in childhood and had increased height velocity at age 4 to 
7 years and age 11 to 15 years; higher BMI increased this risk. 
These variables were particularly significant in women with early 
menarche (at age <12.5 years).98 Remarkably, another study of 
117,415 women found that increased birth weight, height at 8 
years of age, peak growth at an early age, taller stature at 14 year

 growth at an early age, taller stature at 14 years 
of age, and low BMI at 14 years of age were independent risk 
factors for breast cancer, but there was no effect on age of men­
arche.99 However, increased body weight at 8 years appeared to 
decrease the risk of breast cancer in another study of 90, in 50% 
of women.100
There is indirect evidence relating earlier menarche to increas­
ing likelihood of hepatocellular carcinoma. On the other hand, 
later age of menarche (>14 years) is associate

d, 
later age of menarche (>14 years) is associated with an increased 
risk of glioma or non-Hodgkin lymphoma.101,102
Evidence from the Fels Longitudinal Study of white females 
revealed that girls with self-reported menarcheal age of less than 
11.9 years (classified as early menarche; 23% of the sample) had 
adverse cardiovascular risk factors such as elevated blood pressure 
and glucose intolerance unrelated to body composition.103 There 
was increased carotid artery intima media thickness in

increased carotid artery intima media thickness in women 
with an age of menarche less than 11 years compared to those with 
the age of menarche over 11 years in 461 women in the Bogalusa 
Heart Study.104 
National Trends in Pubertal Development
Limits of Normal Pubertal Development
The NHES enrolled subjects who were 12 years old; although it 
is useful for defining the upper limits of normal pubertal devel­
opment, the survey is uninformative about the lower limits of 
the age at onset of pube

bout the lower limits of 
the age at onset of puberty.37,105 A longitudinal study (Tables 
26.2 and 26.3) that enrolled 9.5-year-old white boys and girls 
added much to the determination of the mean age at attainment 
of stages of puberty106; however, it started too late to include 
normal children entering puberty at an earlier age. Two stud­
ies used data from NHANES III. One found onset of stage 2 
breast development for African Americans, Mexican Americans, 
and whites to occur at 9.5, 9.8, 

ican Americans, 
and whites to occur at 9.5, 9.8, and 10.3 years, respectively, and 
pubic hair stage 2 to occur at 9.5, 10.3, and 10.5 years, respec­
tively, using a sample of 1623 girls.107 The other study reported 
that the ages for onset of stage 2 breast development were 9.5 
years for African Americans, 9.8 years for Mexican Americans, 
and 10.4 years for white Americans, whereas the onset of stage 2 
pubic hair occurred at 10.4 years for white Americans, 9.4 years 
for African Americans, 

hite Americans, 9.4 years 
for African Americans, and 10.6 years for Mexican Americans in 
a sample of 2145 girls.108
Because the PROS started at 3 years of age but ended at 12 
years of age, it excluded a proportion of normal girls who enter 
puberty at a later age, although the probit statistical method can 
estimate events even when only a portion of the population has 
achieved the event.22 This study was criticized because the subjects 
from the convenience sample were not matched for the m

 the convenience sample were not matched for the many fac­
tors considered in a national study such as NHANES. The stan­
dard deviation of the longitudinal study described earlier was low, 
1.0 year or less in most cases, whereas the cross-sectional study 
PROS had a larger standard deviation of approximately 2 years. 
Any study of puberty will demonstrate a limit in the spread of the 
upper end of the age at onset of puberty curve, because rarely do 
individuals with the most severe constitutio

y do 
individuals with the most severe constitutional delay of puberty 
(CDP) spontaneously enter puberty after 18 years of age. There 
may be a skewing of the normal age at onset of puberty to an 
earlier spread of ages.
A study of the age of pubertal stages in children in NHANES 
revealed that in those with normal BMI values, pubertal signs 
occurred before 8 years of age in fewer than 5% of the non-
Hispanic white female population, although thelarche occurred 
before age 8 in 12% to 19% of n

helarche occurred 
before age 8 in 12% to 19% of normal-BMI, non-Hispanic black 
and Mexican-American girls; the 5th percentile for menarche was 
0.8 year earlier for non-Hispanic black than non-Hispanic white 
subjects.43 Although the appearance of pubic hair occurred in up 
to 3% of 8-year-old girls with normal BMI values in all ethnici­
ties, it did occur significantly earlier in the minority groups. Girls 
with the higher BMI values had a significantly higher prevalence 
of thelarche between

nificantly higher prevalence 
of thelarche between the ages of 8 and 9.6 years and pubarche 
from the ages of 8 to 10.2 years, compared with girls with normal 
BMI values. Menarche was significantly more likely to occur in 
younger girls with elevated BMI values. In boys with normal BMI 
values, pubic hair appeared in fewer than 2% before 10 years. The 
study authors concluded that pubertal development in girls with 
normal BMI values before 8 years of age is premature. These data 
support other

ars of age is premature. These data 
support other analyses of NHANES III, in which the heaviest girls 
tended to have thelarche before pubarche and the heaviest boys 
had pubarche before gonadarche.48
The latest and largest multiracial and ethnic study of the age at 
onset of puberty in a convenience sample of 4131 American boys 
between the ages of 6 and 16 years found the mean ages for onset 
of Tanner 2 genital development for non-Hispanic white boys was 
10.14, African-American boys was 9.1

ite boys was 
10.14, African-American boys was 9.14, and Hispanic boys was 
10.04 years; and for stage 2 pubic hair, the ages were 11.47, 10.25, 
and 11.43 years, respectively. Mean ages for achieving testicular 
volumes of greater than 3 mL were 9.95 for white, 9.71 for Afri­
can-American, and 9.63 for Hispanic boys; and for greater than 4 
mL ages were 11.46, 11.75, and 11.29, respectively.46
Weight affects the onset of puberty in boys but not in a lin­
ear manner. Overweight boys have earlier

in a lin­
ear manner. Overweight boys have earlier puberty than normal 
weight boys, while obese boys start puberty later.109
Spanish investigators demonstrated that the earlier normal 
girls entered puberty, the longer the duration of puberty before 
menarche. In one of these studies, girls who started puberty at 9, 
10, 11, 12, and 13 years of age experienced menarche 2.77, 2.27, 
1.78, 1.44, and 0.65 years later, respectively, demonstrating a nor­
malizing trend that keeps the age of menarche

nor­
malizing trend that keeps the age of menarche relatively stable in 
the group as a whole.110 These data may suggest that earlier onset 
of the first stages of puberty may not exert major effects on the age 
of menarche. This contrasts with a suggestion of a decrease in the 
time required to transit puberty from start to end in Dutch and 
Swedish boys and girls.31
In sum, the data show that African-American girls develop 
before white girls, regardless of socioeconomic issues. There is 
evid

regardless of socioeconomic issues. There is 
evidence that rising BMI values in childhood are associated with 
earlier pubertal maturation, which may explain some of the 
increasing age differences between these ethnic groups. Environ­
mental disruptors may also play a role. Although there is evidence 
for earlier menarche, a secular trend toward earlier puberty in girls 
in the absence of increased BMI cannot be supported from these 
data because of the different ages studied and lack of compa

se of the different ages studied and lack of comparable 
studies over past decades.34,111
The United States is lacking a comprehensive, large, longitu­
dinal study that would start early enough to include the young­
est normal pubertal subjects and last long enough to include the 
oldest and that would be based on direct physical examination 
1031
  Descriptive Statistics for the Timing of Sexual Maturity Stages in Females
BREAST STAGES
ONSET OF S

aturity Stages in Females
ONSET OF STAGE
MEAN AGE FOR STAGE
Stage
Mean
SD
Mean
SD
Stage 2
Roche et al (Ohio)
11.2
0.7
11.3
1.1
Herman-Giddens et al (United States)
  African American
8.9
1.9
  White
10.0
1.8
Stage 3
12.0
1.0
12.5
1.5
10.2
1.4
  White
11.3
1.4
Stage 4
12.4
0.9
TANNER PUBIC HAIR
Tanner Stage 2
11.0
0.5
  African-America

an-Giddens et al (United States)
  African-American
8.8
2.0
  White
10.5
1.7
Tanner Stage 3
11.8
1.0
10.4
1.6
  White
11.5
1.2
Tanner Stage 4
12.4
0.8
MENARCHE
Herman-Giddes et al (United States)
12.2
1.2
  White
12.9
1.2
Percent Menstruating
At Age 11
AT AGE 12
African American
27.9%a
62.1%
White
13.4%a
35.2%
Onset of Axillary Hair (Stage 2)
10.1 ± 2.0
White
11.8 ± 1

tage 2)
10.1 ± 2.0
White
11.8 ± 1.9
aAfrican-American girls enter puberty approximately 1 to 1.5 years earlier than white girls and begin menses 8.5 months earlier.
Data from Roche AF, Weilens R, Attie KM, et al. The timing of sexual maturation in a group of US white youths. J Pediatr Endocrinol. 1995;8:11–18; Herman-Giddens ME, Slora EJ, Wasserman RC, et al. 
Secondary sexual characteristics and menses in young girls seen in office practice: a study from the Pediatric Research 

ice practice: a study from the Pediatric Research in Office Settings network. Pediatrics. 1997;99:505–512.
   
TABLE 26.2
1032
rather than observation. Such a study must be balanced in terms 
of ethnic groups, and the planners must use the predicted increase 
of certain ethnic populations in the United States to avoid the 
unfortunate position we now have as we look back and try to draw 
conclusions about secular trends without sufficient data from v

bout secular trends without sufficient data from vari­
ous ethnic groups.34
From all of the longitudinal and some of the cross-sectional 
data, we may consider the mean age at onset of puberty in boys to 
be 11 years, with the normal limits being 9 to 14 years.106 It is pos­
sible that some normal boys, especially African-American boys, 
will enter puberty or adrenarche between 8 and 9 years of age. The 
influence of BMI must be taken into account. Obese boys charac­
teristically enter puberty e

t. Obese boys charac­
teristically enter puberty early, especially when there is an MCR4 
mutation, but obesity is also associated with delayed puberty.
Guidelines for the normal variation in pubertal development 
for girls in the United States are controversial. In the cross-sec­
tional, convenience sample study, 3% of white girls had stage 2 
breast development in their sixth year and 5% by the seventh year, 
whereas 6.4% of African Americans had stage 2 breast develop­
ment by the sixth year 

ad stage 2 breast develop­
ment by the sixth year and 15.4% by the seventh year. African-
American girls have an earlier onset of pubertal development by 
about 1 year, even though their average age at menarche in the 
cross-sectional study was only 8.5 months different (12.2 years for 
African Americans and 12.9 for whites). We may combine these 
findings and set the normal range for age at puberty in white girls 
at 7 to 13 years and in African-American girls at 6 to 13 years. 
However, some g

-American girls at 6 to 13 years. 
However, some girls in this early range may have a loss-of-func­
tion MKRN3 mutation, possibly in a familial pattern; testing is 
available only in research laboratories at present but may be avail­
able for clinical diagnosis in the future.
These guidelines help the decision of which children with 
early onset of puberty are candidates for expensive diagnostic 
tests and for consideration of long-term therapy, because many 
of those children who appeared to ha

because many 
of those children who appeared to have mild sexual precocity in 
years past may now be considered to represent a normal variation. 
We emphasize that family history, the rapidity of development 
of secondary sex characteristics, the rate of growth, and the pres­
ence or absence of CNS or other types of diseases must enter into 
the decision to evaluate a child. We recommended these ideas in 
previous editions of this textbook, and the Drug and Therapeu­
tics and Executive Committee

he Drug and Therapeu­
tics and Executive Committees of the Lawson Wilkins Pediatric 
Endocrine Society supported such a revision of the lower limit 
of the normal age at onset of puberty to age 7 for white girls and 
age 6 for African-American girls, with no changes in the current 
guidelines for evaluating boys, which target those with signs of 
puberty developing before 9 years of age.112
Several studies indicate the likelihood of missing serious endo­
crine disorders if the new guidelines are

us endo­
crine disorders if the new guidelines are followed.113–115 With all 
of these studies, it may be inferred that if the examining physician 
looks for signs and symptoms of disease rather than just relying on 
the age criteria, less than 10% of true precocious puberty will be 
missed; of those cases, some (probably many) will be so mild as to 
not need intervention and may represent variations of normal. A 
multinational study from Europe suggested that valid indications 
for magnetic res

suggested that valid indications 
for magnetic resonance imaging (MRI) of the CNS in the diagno­
sis of precocious puberty are puberty onset in girls before 6 years 
of age, in agreement with our recommendations, and an estradiol 
value higher than the 45th percentile (in the laboratory perform­
ing the diagnostic tests) for girls with central precocious puberty 
(CPP), a new criterion.116
As noted, there are variations of pubertal development that do 
not demonstrate pubertal elevations of LH v

at do 
not demonstrate pubertal elevations of LH values. For instance, 
early breast development with bone age advancement and 
increased growth without elevation of LH may be described as 
“thelarche variant”117 or “early normal variant puberty.”118 These 
classifications are not clearly explained in the literature. Obese 
girls may have local production of estrogen leading to thelarche 
without elevation of LH, but some studies, particularly from Den­
mark, demonstrate thelarche without elevat

om Den­
mark, demonstrate thelarche without elevation of LH in girls who 
do not have elevated BMI values. 
Secondary Sexual Characteristics and 
Physical Changes of Puberty
Female Development
Two distinct phenomena occur in the female. The development of 
the breast and its modified apocrine glands is primarily under the 
control of estrogens secreted by the ovaries (Fig. 26.5); the growth 
of pubic and axillary hair (Fig. 26.6) is mainly under the influence 
of androgens secreted by the adrena

the influence 
of androgens secreted by the adrenal cortex and the ovary. Breast 
cancer develops in rodents exposed to environmental toxins (e.g., 
endocrine disruptors) that alter normal mammary development, 
and this same relationship is postulated to occur in girls exposed 
to endocrine-disrupting chemicals, especially if development 
occurs early.119 Aromatase is present in adipose tissue, and estro­
gen produced in excess adipose tissue may stimulate breast devel­
opment at an earlier age 

 stimulate breast devel­
opment at an earlier age in obese girls. Peripubertal obese girls also 
demonstrated elevated values of androgens, especially just before 
the onset of puberty and in the early stages of puberty, and there 
is also decreased LH secretion compared to nonobese girls.120,121
The five stages of breast development described by Tanner are 
the most widely used staging mechanism (see Fig. 26.5). Initial 
breast development may be unilateral for several months, causing 
unfounde

e unilateral for several months, causing 
unfounded concern by girls or parents. Needless surgical biopsies 
  Descriptive Statistics for the Timing of 
Sexual Maturity Stages in White Males 
(Ohio)
Stage
AGE AT ONSET OF 
STAGE (YR)
AGE FOR STAGE (YR)
Mean
SD
Mean
SD
Genital Stage
2
11.2
0.7
11.3
1.0
3
12.1
0.8
12.6
1.0
4
13.5
0.7
14.5
1.1
5
14.3
1.1
—
—
Pubic Hair Stages
2
11.2
0.8
11.3
0.9
3
12.1
1.0
12.4
1.0
4
13.4
0.9
13.7
0.9
5
14.3
0.8
14.8
1.0
6
15.3
0.8
—
—
From Roche AF, Wellens R, Att

8
1.0
6
15.3
0.8
—
—
From Roche AF, Wellens R, Attie KM, et al. The timing of sexual maturation in a group of US 
white youths. J Pediatr Endocrinol. 1995;8:11–18.
   
TABLE 26.3
1033
are carried out for this normal variation, and an ultrasound exami­
nation may preempt unfounded concerns about breast cancer. If 
concern arises about breast cancer during puberty (a rare event), 
ultrasound evaluation is suggested because of the dense nature of 
the

n is suggested because of the dense nature of 
the tissue at this stage. Inherited or sporadic agenesis of the breast 
allows no glandular or fat enlargement, regardless of the level of 
estrogen stimulation. Virginal breast hypertrophy, an extreme and 
rapid increase in breast size at the onset of puberty, is rare but is 
attributed in part to increased sensitivity to estrogen action or to 
increased local estrogen synthesis and growth factors.
Changes in the diameter of the papilla of the nipp

Changes in the diameter of the papilla of the nipple are sequen­
tial and are linked to stages of pubertal development.122 Nipple 
papilla diameter (3–4 mm) does not increase much during pubic 
hair stage 1 to 3 or breast stage 1 to 3 but does increase after breast 
stage 3, providing an objective method of differentiating stage 4 
from123 stage 5 (final diameter, ∼9 mm).
The stage of breast development usually progresses along 
with the stage of pubic hair development in normal girls, but 
beca

 pubic hair development in normal girls, but 
because different endocrine organs control these two processes, 
discordance can occur. Therefore breast and pubic hair develop­
mental stages should be classified separately for greatest accu­
racy (Fig. 26.7; also see Table 26.2).
A dulling and thickening of the vaginal mucosa from the 
prepubertal reddish, glistening appearance occurs as the lin­
ing cells cornify, changing the mucosa to a pink appearance, 
and the secretion of clear or whitish di

earance, 
and the secretion of clear or whitish discharge increases in the 
B-1
B-2
B-3
B-4
B-5
• Fig. 26.5  Stages of breast development according to Marshall and 
Tanner (1969). Stage 1: preadolescent; elevation of papilla only. Stage 
2: breast bud stage; elevation of breast and papilla as a small mound, 
with enlargement of the areolar diameter. Stage 3: further enlargement 
of the breast and areola, with no separation of their contours. Stage 4: 
projection of the areola and papilla to form

e 4: 
projection of the areola and papilla to form a secondary mound above 
the level of the breast. Stage 5: mature stage; projection of the papilla 
only, resulting from recession of the areola to the general contour of the 
breast. (Photographs from Van Wieringen JD, Wafelbakker F, Verbrugge 
HP, et al. Growth Diagrams 1965 Netherlands: Second National Survey 
on 0-24 Year Olds. Netherlands Institute for Preventative Medicine TNO. 
Groningen, The Netherlands: Wolters-Noordhoff; 1971. Addition

The Netherlands: Wolters-Noordhoff; 1971. Additional data 
from Marshall WA, Tanner JM. Variations in pattern of pubertal changes in 
girls. Arch Dis Child. 1969;44:291–303.)
P-1
P-2
P-3
P-4
P-5
P-6
• Fig. 26.6  Stages of female pubic hair development according to Marshall 
and Tanner (1969). Stage 1: preadolescent; the vellus over the pubes is 
not further developed than that over the anterior abdominal wall; there is 
no pubic hair. Stage 2: sparse growth of long, slightly pigmented, downy 
ha

arse growth of long, slightly pigmented, downy 
hair that is straight or only slightly curled, appearing chiefly along the labia. 
This stage is difficult to see on photographs. Stage 3: hair is considerably 
darker, coarser, and curlier. The hair spreads sparsely over the junction of 
the pubic region. Stage 4: hair is adult in type, but the area covered by it 
is still considerably smaller than in most adults. There is no spread to the 
medial surface of the thighs. Stage 5: hair is adult in q

surface of the thighs. Stage 5: hair is adult in quantity and type, dis­
tributed as an inverse triangle of the classic feminine pattern. The spread 
is to the medial surface of the thighs but not up the linea alba or elsewhere 
above the base of the inverse triangle. (Photographs from Van Wieringen 
JD, Wafelbakker F, Verbrugge HP, et al. Growth Diagrams 1965 Nether­
lands: Second National Survey on 0-24 Year Olds. Netherlands Institute 
for Preventative Medicine TNO. Groningen, The Netherlands

ventative Medicine TNO. Groningen, The Netherlands: Wolters-
Noordhoff; 1971. Additional data from Marshall WA, Tanner JM. Variations 
in pattern of pubertal changes in girls. Arch Dis Child. 1969;44:291–303.)
1034
months before menarche as a result of estrogen action. Girls 
may notice light-colored discharge on their underwear at this 
stage. The vaginal pH decreases as menarche approaches due to 
the increase in lactic acid produced by lactobacill

he increase in lactic acid produced by lactobacilli in the vaginal 
flora. The length of the vagina increases from about 8 cm at 
onset of puberty to 11 cm at menarche. Thickening, protrusion, 
and rugation of the labia majora and minora occur. Fat is depos­
ited in the area of the mons pubis, and the appearance of the 
labia majora becomes wrinkled. Occasionally, the labia minora 
may enlarge on one or both sides enough to suggest a tumor; 
childhood asymmetric labium majus enlargement is a dis

dhood asymmetric labium majus enlargement is a disorder of 
prepuberty or early puberty.124 The clitoris enlarges slightly, and 
the urethral opening becomes more prominent. Photographic 
atlases of normal female prepubertal genitalia are available and 
include standards for the variation in appearance of the hymenal 
opening; this information is invaluable in the evaluation of a 
victim of suspected child abuse.
Ovarian Development
The peak cohort of germ cells in the fetal ovary is attained at

rt of germ cells in the fetal ovary is attained at 
16 to 20 weeks of gestation. Primordial follicles start to appear 
at 20 weeks of fetal life, and primary follicles soon follow; they 
constitute the lifelong store of follicles for the individual, which 
decreases with development and aging.125 Follicle-stimulating 
hormone receptors have not been detected in midtrimester human 
fetal ovaries; fetal pituitary FSH is not required for proliferation 
of oogonia, oocyte differentiation, or formati

on 
of oogonia, oocyte differentiation, or formation of primordial fol­
licles.126 During fetal life and childhood, follicular growth to the 
large antral stage occurs, but before menarche all developing fol­
licles are destined to undergo atresia (Fig. 26.8). Large preovula­
tory follicles are rarely present before puberty.
The ultrasound appearance of the prepubertal ovary changes 
with pulsatile gonadotropin secretion, and a multicystic appear­
ance occurs with more than six follicles of at l

­
ance occurs with more than six follicles of at least 4 mm in diame­
ter; this appearance differs from that found in the polycystic ovary 
syndrome (PCOS). During prepuberty, the ovarian volume is 0.2 
to 1.6 mL on ultrasound scans, and after the onset of puberty the 
volume increases to 2.8 to 15 mL. Tall girls have greater ovarian 
volume than average-size girls.
The uterus grows until the age of 16 years under the influence 
of estradiol, progesterone, growth hormone (GH),127 and insulin-
li

gesterone, growth hormone (GH),127 and insulin-
like growth factor 1 (IGF1). Ultrasound studies (Fig. 26.9) show 
that the corpus of the uterus increases during pubertal progression, 
from an initial tubular shape to a bulbous structure; the length of 
the uterus increases from 2 to 3 cm to 5 to 8 cm, and the volume 
increases from 0.4 to 1.6 mL to 3 to 15 mL.128 Decreased uterine 
size is found in patients with Turner syndrome, childhood expo­
sure to radiotherapy, and abnormalities in HOX and 

ure to radiotherapy, and abnormalities in HOX and WNT gene 
expression; maternal cigarette smoking can decrease uterine size 
at adolescence. Smaller uterine size is associated with an increased 
risk of miscarriage and failed implantation.
Uterine ultrasonography measurements are proposed to aid 
the clinician in differentiating premature thelarche from preco­
cious puberty. The addition of color Doppler studies may improve 
accuracy of the diagnosis of precocious puberty and can differen­
tiat

nosis of precocious puberty and can differen­
tiate the condition from premature thelarche. A Doppler study 
showed the lowest impedance of the uterine artery to be in girls 
with established CPP.129
Endometriosis is considered to be an estrogen-dependent pro­
cess, although it has been reported in premenarcheal girls. It was 
suggested that this cause for chronic abdominal pain is more com­
mon than previously considered. One proposed explanation for 
early-onset endometriosis is that the condi

n for 
early-onset endometriosis is that the condition results from mül­
lerian rests.130 
Menarche and Teenage Pregnancy
Menarche usually occurs in the 6 months preceding or following 
the epiphyseal fusion of the second and first distal phalanges and 
the appearance of the sesamoid bone; this corresponds to Tanner 
stage 4 in most cases. The 95th percentile for menarche is 14.5 
years, although many textbooks define primary amenorrhea as 
absence of menses at 16 years. Reconsideration of the a

ce of menses at 16 years. Reconsideration of the age at onset 
of female puberty may lead to a reconsideration of the definition 
of primary amenorrhea. Anovulatory cycles are common in the 
first years after menarche. There is a reported prevalence of 55% 
anovulation in the first 2 years after menarche that decreases to 
20% anovulatory cycles by the fifth year; others have observed 
a lower number of ovulatory events shortly after menarche and 
5 years after the event. With the high prevalenc

5 years after the event. With the high prevalence of PCOS, it is 
unclear how often delayed regularity is an early sign of PCOS or 
normal variation. 
Male Development
The growth and maturation of the penis usually correlate closely 
with pubic hair development, because both features are under 
androgen control. However, the stages of pubic hair development 
and genital development should be determined independently, 
because discordant stages provide clues to potential disease states 
of the 

provide clues to potential disease states 
of the adrenal gland or testes (Figs. 26.10 and 26.11; also see Table 
26.3).
Growth of the testes is usually the first sign of puberty in 
the male, and it begins approximately 6 months after the aver­
age chronologic age of initiation of breast development in girls 
(see Fig. 26.4). Pubertal testicular enlargement is indicated when 
the longitudinal measurement of a testis is greater than 2.5 cm 
(excluding the epididymis) or the volume is greater tha

uding the epididymis) or the volume is greater than 3 mL. 
8
91/2–141/2
10 –161/2
8 –13
8 –14
Height spurt
Menarche
Breast
Pubic hair
9
10
11
2
3
4
5
2
Bud
3
4
5
12
13
14
15
16
8
9
10
11
12
13
14
15
16
• Fig. 26.7  The sequence of events at puberty in females. Diagram of the 
sequence of events at puberty in males. An average is represented in rela­
tion to the scale of ages; the range of ages within which some of the 
changes occur is indicated by the numbers below. The ages are fro

s indicated by the numbers below. The ages are from 
British girls 40 years in the past, so the sequence of changes, rather than 
the ages, is the important factor. (From Marshall WA, Tanner JM. Variations 
1035
The testicular volume index ([length × width of right testis] + 
[length × width of left testis]/2) and testicular volume, measured 
by comparing the testes with ell

volume, measured 
by comparing the testes with ellipsoids of known volume (orchi­
dometer), correlate with the stages of puberty.131,132 A longi­
tudinal study supported the utility of adding a stage 2a when 
testicular volume is 3 mL; further pubertal progression occurred 
within 6 months in 82% of boys who had reached this 3-mL 
phase133 (Table 26.4). The most significant changes in serum 
testosterone and calculated free testosterone occur at the transi­
tions of testicular volume between 1 a

the transi­
tions of testicular volume between 1 and 2 mL, 2 and 3 mL, 
6 and 8 mL, and 10 and 15 mL, suggesting the denotation of 
stages pre-1 (testis, 1 mL), pre-2 (testis, 2 mL), early (testis, 3–6 
mL), middle (testis, 8–12 mL), late-1 (testis, 15–25 mL, the 
boy has not reached final height), and late-2 (testis, 15–25 mL, 
the boy has reached final height).134 The right testis is normally 
larger than the left testis, and the left testis is located lower in the 
scrotum than the right test

 located lower in the 
scrotum than the right testis.
The phallus should be measured while stretched and in the flac­
cid state because there is much variation among individuals in the 
length of the unstretched penis. The length of the erectile tissue 
(excluding the foreskin) increases from an average of 6.2 cm in the 
prepubertal state to 12.4 ± 2.7 cm in the white adult. Ethnic dif­
ferences have been identified; the mean value in African-American 
men is 14.6 cm, and in Asians it is 10.6 cm

rican 
men is 14.6 cm, and in Asians it is 10.6 cm.135
As in girls, the areolar diameter increases in boys during 
puberty, with a distinct separation between the sexes occurring in 
stage 4, when female areolar diameter increases much more than 
in the male. In gynecomastia, the areolar diameter increases to 
above-normal values.
Male Testicular Development in Puberty
The testes are active during the prepubertal period albeit at a 
lower level than during pubertal development.136 During puber­

han during pubertal development.136 During puber­
tal development, the testes increase in size, principally because 
of the growth of the seminiferous tubules associated with the 
onset of spermatogenetic activity and mitosis of Sertoli cells, and 
testosterone production increases (Fig. 26.12 and Table 26.5). 
The Sertoli cells are the major cell type in the seminiferous cords 
in prepuberty and early puberty, but in the adult, germ cells 
predominate. During progression through puberty, the Se

minate. During progression through puberty, the Sertoli 
cells cease to undergo mitosis, differentiate into adult-type Ser­
toli cells, and form occlusive junctions with the development of 
the blood-testes barrier. Although Leydig cells are found in early 
gestation and during the neonatal period of testosterone secre­
tion, the interstitial tissue is composed principally of undifferen­
tiated mesenchyme-type cells during childhood. With pubertal 
development and rising serum LH levels, adult-t

l 
development and rising serum LH levels, adult-type Leydig cells 
appear (Table 26.6). It is suggested that three phases of Leydig 
Small
(non-growing)
follicles
Small
antral
follicles
Large
antral
follicles
25
<25
80
80
80
80
<80
50 µm
200 µm
500 µm
>6000 µm
Preantral
follicles
Type 1
Type 2
Type 3
≤25 cells
Type 4
26 –100 cells
Type 5
101– 300 cells
Type 6
301– 500
cells
Type 7
501–1000 cells
Type 8
>1000 cells
A t r
e s
i a
10
00
– 6
00
0 µ
m
• Fig. 26.8  Schematic representation of the gro

m
• Fig. 26.8  Schematic representation of the growth of ovarian follicles during infancy and childhood. Type 1 
(primordial follicle) and type 2 (primary follicle) are composed of a small oocyte and a few to a ring of flat granu­
losa cells. In the diplotene (nesting) stage of prophase, primary follicles are the predominant form of oocyte 
and constitute the reservoir of cells from which follicular growth occurs. Types 3 to 5 (preantral follicles) are fol­
licles that have entered the growth ph

s) are fol­
licles that have entered the growth phase; the oocyte is enlarging and is surrounded by a zona pellucida, and 
granulosa cells increase in number and differentiate. The growth of the oocyte is complete by the end of the 
preantral stage, and the increased follicular size is caused by follicular growth and fluid accumulation. Types 6 
to 8 represent antral follicles (graafian follicles) and contain a fully grown oocyte, a large number of granulosa 
cells, a fluid-filled cavity, and a 

of granulosa 
cells, a fluid-filled cavity, and a well-developed theca external to the basement membrane. Large preovula­
tory follicles are absent (10,000–15,000 μm). Follicular growth and atresia take place throughout childhood. 
All follicles that enter the growth phase become atretic, and this can occur at any stage in their development 
but mainly involves large antral follicles. (From Peters H, Byskov AG, Grinsted J. Follicular growth in fetal and 
prepubertal ovaries of humans and other p

tal and 
prepubertal ovaries of humans and other primates. Clin Endocrinol Metab. 1978;7:469–485.)
1036
cell maturation correspond with ages of increased testosterone 
production: 14 to 18 weeks of fetal life, 2 to 3 months after 
birth, and puberty through adulthood.137 The seminal vesicle 
enlarges through childhood to puberty to hold 3.4 to 4.5 mL, 
or 70% of the seminal fluid. The mean blood flow in the testes 
increases to adult values (measured

in the testes 
increases to adult values (measured by Doppler sonography) in 
boys, with a testicular volume greater than 4 mL.
Testicular size is generally measured at the longest diameter 
excluding the epididymis or volume as determined by comparing 
the testis to a Prader orchidometer. New smoothed curves of tes­
ticular volume versus age are available as based on physical exami­
nation and ultrasonography in 769 healthy Dutch boys.138 
The first histologic evidence of sperma

atogenesis
The first histologic evidence of spermatogenesis appears between 
ages 11 and 15 years (Fig. 26.13; also see Figs. 26.6 and 26.9). 
Spermaturia may be the first sign of pubertal development, but 
the presence of sperm in urine is intermittent and therefore not 
a reliable indicator in all boys. Spermaturia is more prevalent 
in early puberty than in late puberty, suggesting that there may 
be a continuous flow of sperm through the urethra in early 
puberty but that ejaculation is nece

hra in early 
puberty but that ejaculation is necessary for sperm to appear 
in the urine in late puberty. Spermaturia in the first-morning 
urine specimen occurs at a mean chronologic age of 13.3 years 
and at a mean pubic hair stage 2 to 3 in one study (or 16 years 
in another study), but may be found in normal boys with bilat­
eral testicular volumes of only 3 mL and no signs of puberty.139 
Normospermia (i.e., normal sperm concentration, morphologic 
appearance, and motility) is not present 

hologic 
appearance, and motility) is not present until a bone age of 17 
years. The first conscious ejaculation occurs at a mean chrono­
logic age of 13.5 years in normal boys and at a mean bone age 
of 13.5 years in boys with delayed puberty.140 The age of the 
first ejaculation, spermarche, decreased in China between 1995 
and 2010, and a higher BMI led to an earlier age of spermarche; 
this pattern mirrors to a degree the secular trend of menarche 
in girls.141,142
The potential for fertilit

arche 
The potential for fertility is reached before an adult phenotype 
is attained, before adult plasma testosterone concentrations are 
reached, and before PHV occurs. 
Other Physical and Biochemical Changes of 
Puberty
The gender difference in voice develops during puberty. In the 
peripubertal period, the length of the vocal cords in boys and girls 
is about 12 to 15 mm, of which the membranous portion is 7 to 
8 mm.143 In adult men, the vocal cords attain a length of 18 to

dult men, the vocal cords attain a length of 18 to 
23 mm (membranous portion, 12–16 mm), whereas in women, 
the cords enlarge only slightly (13–18 mm). During puberty, the 
male larynx, cricothyroid cartilage, and laryngeal muscles enlarge, 
• Fig. 26.9  High-resolution pelvic ultrasonography. Top left, Prepubertal uterus. Top right, Prepubertal 
ovary demonstrating four small follicular cysts (arrows). Bottom left, Pubertal postmenarchal uterus. Bot­
tom right, Ovarian cyst in a girl with true

. Bot­
tom right, Ovarian cyst in a girl with true precocious puberty.
1037
leading to the appearance of an Adam’s apple. The largest changes 
in singing and speaking frequencies occur between Tanner genital 
stages 3 and 4; breaking of the voice occurs at approximately 13 
years, and the adult voice is achieved by about 15 years. Higher 
BMI is associated with earlier voice breaking. Voice breaking is 
suggested to be a noninvasive method to monit

 is 
suggested to be a noninvasive method to monitor pubertal devel­
opment in boys.144
Facial hair in boys is first apparent on the corners of the upper 
lip and the upper cheeks; it then spreads to the midline of the 
lower lip and finally to the sides and the lower border of the 
chin. The first stage of facial hair development usually occurs 
during pubic hair stage 3 (average age of 14.9 years in the United 
States), and the last stage occurs after pubic hair stage 5 and 
genital stage 5.
A

s after pubic hair stage 5 and 
Axillary hair appears at approximately 14 years in boys. Ninety-
three percent of African-American girls have axillary hair by age 
12, in contrast to 68% of white girls.22 Axillary sweat glands begin 
to function as the hair appears. The appearance of circumanal hair 
slightly precedes that of axillary hair in boys.
Comedones, acne, and seborrhea of the scalp appear as a result 
of the increased secretion of gonadal and adrenal sex steroids.145 

ecretion of gonadal and adrenal sex steroids.145 
Early-onset acne correlates with the development of severe acne 
later in puberty. Acne vulgaris, the most prevalent skin disorder 
in adolescence, appears at a mean age of 12.2 ± a standard devia­
tion (SD) of 1.4 years (range, 9–15 years) in boys and progresses 
with advancement through puberty. However, acne vulgaris can 
be the first notable sign of puberty in a girl, preceding pubic hair 
and breast development.145 Analysis of the Nurses’ He

 breast development.145 Analysis of the Nurses’ Health Study 
II indicates that intake of milk and skim milk is related to the 
development of acne, an association suggested to reflect the hor­
mone content of milk.146 At late prepuberty, comedones are pres­
ent in many boys, and 100% of boys have comedones by genital 
stage 5.
Facial morphologic appearance changes with pubertal devel­
opment, leading to the mature appearance. The mandible and 
nose enlarge more in boys, but they and the maxilla

ose enlarge more in boys, but they and the maxilla, brow, fron­
tal sinuses, and middle and posterior fossae enlarge in both sexes, 
mainly during the pubertal growth spurt. Children with isosexual 
precocity (ISP) have the facial appearance of older children, and 
G-1
G-2
G-3
G-5
G-4
• Fig. 26.10  Stages of male genital development and pubic hair devel­
opment according to Marshall and Tanner (1969). Genital development: 
stage 1: preadolescent. Testes, scrotum, and penis are about the same 
si

 Testes, scrotum, and penis are about the same 
size and proportion as in early childhood. Stage 2: the scrotum and tes­
tes have enlarged; the scrotal skin shows a change in texture and some 
reddening. Stage 3: growth of the penis has occurred, at first mainly in 
length but with some increase in breadth; there is further growth of the 
testes and scrotum. Stage 4: the penis is further enlarged in length and 
breadth, along with development of the glans. The testes and scrotum are 
further enl

the glans. The testes and scrotum are 
further enlarged. The scrotal skin has further darkened. Stage 5: genitalia 
are adult in size and shape. No further enlargement takes place after stage 
5 is reached. Pubic hair development: stage 1: preadolescent; the vellus 
over the pubic region is not further developed than that over the abdomi­
nal wall; there is no pubic hair. Stage 2: sparse growth of long, slightly 
pigmented, downy hair that is straight or slightly curled, appearing chiefly 
at th

aight or slightly curled, appearing chiefly 
at the base of the penis. Stage 3: hair is considerably darker, coarser, and 
curlier and spreads sparsely over the junction of the pubes. Stage 4: hair 
is adult in type, but the area it covers is still considerably smaller than in 
most adults. There is no spread to the medial surface of the thighs. Stage 
5: hair is adult in quantity and type, distributed as an inverse triangle. The 
spread is to the medial surface of the thighs but not up the line

e medial surface of the thighs but not up the linea alba 
or elsewhere above the base of the inverse triangle. Most men will have 
further spread of the pubic hair. (Photographs from Van Wieringen JD, 
Wafelbakker F, Verbrugge HP, et al. Growth Diagrams 1965 Netherlands: 
Second National Survey on 0-24 Year Olds. Netherlands Institute for Pre­
ventative Medicine TNO. Groningen, The Netherlands: Wolters-Noordhoff; 
1971. Additional data from Marshall WA, Tanner JM. Variations in pattern 
of puber

all WA, Tanner JM. Variations in pattern 
of pubertal changes in girls. Arch Dis Child. 1969;44:291–303.)
10
101/2–16
13 –171/2
11–141/2
131/2 –17
10 –131/2
141/2 –18
10 –15
Height
spurt
Apex strength
spurt
Penis
Testis
Pubic
hair
14 –18
11
12
13
2
3
4
5
14
15
16
17
18
10
11
12
13
14
15
16
17
18
• Fig. 26.11  Diagram of the sequence of events at puberty in males. An 
average is represented in relation to the scale of ages, and the range of 
ages within which some of the changes occur

ge of 
ages within which some of the changes occur is indicated by the num­
bers below. The ages are from British boys 40 years in the past, so the 
sequence of changes, rather than the ages, is the important factor. (From 
Marshall WA, Tanner JM. Variations in the pattern of pubertal changes in 
boys. Arch Dis Child. 1970;45:13–23.)
1038
individuals with delayed puberty have faces of younger children. 
There is a greater change in various measuremen

. 
There is a greater change in various measurements of the face com­
pared with measurements of the skull, with the jaw showing the 
most increase.147
The size of the thyroid gland evaluated by ultrasonography 
normally increases roughly 40% to 50% with growth in height, 
weight, surface area, and fat-free mass during puberty but not with 
BMI.148 This may lead an examiner to wrongly conclude there is 
a thyroid disorder due to the physiologic change. Lymphoid tis­
sue growth reaches a maximum 

hange. Lymphoid tis­
sue growth reaches a maximum at about age 12 and thereafter 
decreases with pubertal progression.
A host of other physiologic and biochemical measurements 
change with the onset of puberty and must be interpreted in 
terms of the stage of pubertal development. Age-related standards 
should be used in all laboratories but often are not, and the inter­
preting clinician must turn to a textbook of pediatrics or The Har­
riet Lane Handbook. For example, hemoglobin levels increas

e Handbook. For example, hemoglobin levels increase at 
puberty in boys; the effect appears to be mediated by androgen, 
as treatment of boys with CDP with testosterone and letrozole (to 
block aromatization to estrogen) resulted in increased hemoglobin 
levels, even in the absence of a rise in IGF1.149 
Adolescent Growth
Pubertal Growth Spurt
The pubertal growth spurt may be divided for purposes of com­
parison into three stages: the time of prespurt minimal growth 
velocity in peripuberty just

spurt minimal growth 
velocity in peripuberty just before the spurt (takeoff velocity); the 
time of most rapid growth, or PHV; and the stage of decreased 
velocity and cessation of growth at epiphyseal fusion. The great­
est postnatal growth occurs in infancy; growth decreases to the 
nadir known as the minimal prespurt velocity, the slowest period 
of growth in childhood, immediately before the pubertal growth 
spurt.
During puberty, boys and girls experience a growth velocity 
greater than at

irls experience a growth velocity 
greater than at any postnatal age since infancy (but paling in com­
parison to fetal growth, when one fertilized cell grows to 7 pounds 
in 9 months). Boys reach PHV approximately 2 years later than 
girls and are taller at takeoff (Fig. 26.14); PHV occurs during stage 
3 to 4 of puberty in most boys (see Fig. 26.11) and is completed 
by stage 5 in more than 95% of boys. Boys achieve a PHV of 9.5 
cm per year at a mean of about 13.5 years, with a greater PHV in

 a mean of about 13.5 years, with a greater PHV in 
those who mature earlier than in those who mature later.150 The 
pubertal growth spurt in girls (PHV in girls is ∼8.3 cm/year at a 
mean chronologic age of 11.5 years) occurs between stages 2 and 3 
(see Fig. 26.7). Boys grew a mean of 28 cm, and girls grew 25 cm 
between takeoff and cessation of growth in a study in the United 
Kingdom.151 Although the 8 to 11 cm of increased height gained 
during the pubertal growth spurt in boys has been hel

ing the pubertal growth spurt in boys has been held mainly 
responsible for the difference in adult height between the sexes,150 
twin studies of bone morphologic appearance indicate that the 
difference in adult height results more from the later onset of 
pubertal growth in boys than a difference in growth rate between 
the genders. The difference in bone widths between boys and girls 
is in large part established before puberty.152
A mathematical model that attempts to define the various 
sta

cal model that attempts to define the various 
stages of the pubertal growth curve based on longitudinal data 
separates the infancy, childhood, and pubertal phases of growth 
and allows evaluation of growth despite variation in the age at 
onset of puberty. A slowly decelerating childhood component 
is the base, with a sigmoidal pubertal component added during 
secondary sexual development (see Fig. 26.14C). The infancy-
childhood-puberty (ICP) model detected the onset of the puber­
tal growth 

model detected the onset of the puber­
tal growth spurt, predicted the actual magnitude of the pubertal 
growth spurt, and predicted adult height using only the age at 
onset of puberty and a measurement of height.153 Tanner and 
Davies constructed growth curves for American children using 
data from the National Center for Health Statistics, and data cal­
culated from theoretical growth curves can be adjusted for time 
of PHV.154
Daily, meticulous observations of girls during puberty over 120 

us observations of girls during puberty over 120 
to 150 days show stasis periods in each girl (three to seven events 
lasting between 7 and 22 consecutive days); steep changes in each 
girl (one to four episodes, with the sum of these steep changes 
calculated as a percentage of total growth during the study period 
ranging from 15.3–42.9%); and continuous growth the remainder 
of time, with no rhythms or cycles found.155 Clinicians observe an 
integrated growth rate during puberty, rather than

integrated growth rate during puberty, rather than these various 
complex patterns occurring during shorter periods of observation.
In a large Swedish registry, faster linear growth during infancy 
and childhood was associated with earlier PHV during ado­
lescence but less height gain between 8 and 18 years, although 
greater height and BMI at birth were associated with later PHV 
in adolescence and more height gain between the ages of 8 and 
18 years.156
Because girls reach PHV about 1.3 years 

years.156
Because girls reach PHV about 1.3 years before menarche, 
there is limited growth potential after menarche; most girls grow 
only about 2.5 cm taller after menarche, although there is a varia­
tion from 1 to 7 cm. The ages at menarche, takeoff, and PHV 
are not good predictors of adult height because the duration of 
  Correlation of Testicular Volume (TV) With 
Stage of Pubertal Development
Parameter
PUBERTAL STAGE
1
2
3
4
5
TV Indexa
Burr et al and 
August 
et al
1.8
4.5
8.2
10.5
—

August 
et al
1.8
4.5
8.2
10.5
—
Volume (cm3)
Zachmann 
et alb
2.5
3.4
9.1
11.8
14
Waaler et alc
1.8
4.2
10.0
11.0
15
Waaler et ald
1.8
5.0
9.5
12.5
17
aTV index calculated as follows: [(length × width of right testis) + (length × width of left 
testis)] ÷ 2.
bVolume estimated by comparison with ellipsoid of known volume (orchidometer) that is equal 
to or smaller than the testes.
cVolume by comparison with orchidometer.
dMeasurement with calipers and average volume of both test

ment with calipers and average volume of both testes calculated as follows: 0.52 × 
longitudinal axis × transverse axis.
Data from August GP, Grumbach, MM, Kaplan SL. Hormonal changes in puberty: correlation 
of plasma testosterone, LH, FSH, testicular size, and bone age with male pubertal develop­
ment. J Clin Endocrinol. 1972;34:319–326. No abstract available; Burr IM, Sizonenko PC, 
Kaplan SL, et al. Hormonal changes in puberty: correlation of serum luteinizing hormone and 
follicle stimulati

 serum luteinizing hormone and 
follicle stimulating hormone with stages of puberty, testicular size, and bone age in normal 
boys. Pediatr Res. 1970;4:25–35. No abstract available; Waaler PE, Thorsen T, Stoa KF, et al. 
Studies in normal male puberty. Acta Paediatr Scand Suppl. 1974;249:1–36; Zachmann M, 
Prader A, Kind HP, et al. Testicular volume during adolescence. Cross-sectional and longitudi­
nal studies. Helv Paediatr Acta. 1974;29:61–72.
   
TABLE 26.4
1039
Chapter 26  Physiology and Di

TABLE 26.4
1039
Gonocyte
Spermatocytes
Prespermatogenesis
Prespermatogonia
Type A
spermatogonia
B
A
Prespermatogonium
Primitive
germ cell
Spermatogonium
(type A)
Indifferent gonad
• Fig. 26.12  (A) The diagram shows the developmental stages of testicular germ cells based on electron 
microscopic findings in the rabbit. Notice the differences between prespermatogonium and spermatogo­
nium. (B) The diag

espermatogonium and spermatogo­
nium. (B) The diagram shows maturation of testicular cell types in the rabbit from prepubertal appearance 
(left) to onset of spermatogenesis (right). Interstitial cells undergo changes in shape, size, and arrangement 
in the process of Leydig cell differentiation. (From Gondos B. Testicular development. In: Johnson AD, 
Gomes WR, eds. The Testis, vol 4. New York, NY: Academic Press; 1977:1–37.)
  Mean Values of Various Body Measurements and Serum Hormone Levels 

arious Body Measurements and Serum Hormone Levels by Pubertal Stage in Boys 
(Ohio)a
Variable
PS1
PS2a
PS2b
PS3
PS4
PS5
Age (yr)
11.44 (NS)
12.18
12.79
13.74
14.63
15.19
144.2
149.8
154.6
162.3
169.9
173.3
38.18
41.65
47.27
54.67
61.11
66.88
Body mass index (kg/m2)
18.1
18.4
19.5
20.6
21.0
22.2
Testosterone (nmol/L [ng/dL])
  Blacks
0.8 (23)
3.0 (86)
4.9 (141)
11.5 (331)
13.4 (338)
15.5 (449)
  Whites
0.6 (16)
2.9 (83)
4.6 (132)
9.7 (281)
13.3 (383)
14.6 (422)
Free testos

 (132)
9.7 (281)
13.3 (383)
14.6 (422)
Free testosterone (pmol/L [ng/dL])
11 (0.33)
60 (1.74)
114 (3.28)
294 (8.49)
413 (11.9)
504 (14.5)
DHEAS (μmol/L [μg/dL])
2.71 (99.7)
3.31 (121.8)
4.04 (148.7)
4.75 (175.0)
5.08 (187.0)
5.89 (217.0)
TeBG (nmol/L)
34.6 (NS)
33.3 (NS)
28.4
21.5
14.4
10.7
aSubjects were 515 boys from Ohio, including 237 blacks and 278 whites, aged 10 to 15 years at intake, who were monitored every 6 months for 3 years. All values were significant by Duncan post hoc 
analysis a

es were significant by Duncan post hoc 
analysis at p <0.01 except those marked NS. Pubertal stages were defined as follows: PS1, absence of pubic hair, testicular volume <3 mL; PS2a, absence of pubic hair, testicular volume ≥3 mL; PS2b, 
Tanner stage 2 pubic hair; PS3 to PS5, Tanner pubic hair stages 3 to 5.
DHEAS, Dehydroepiandrosterone sulfate; NS, not significant; TeBG, testosterone-binding globulin.
Modified from Biro FM, Lucky AW, Huster GA, et al. Pubertal staging in boys. J Pediatr. 1995

, et al. Pubertal staging in boys. J Pediatr. 1995;127:40–46.
   
TABLE 26.5
1040 SECTION VI    Endocrinology and the Life Span
  Cellular Activity in Human Testis at Various Stages of Development
Stage
Germ Cells
Sertoli Cells
Leydig Cells
Prespermatogenic cells are present
Predominant cells in seminiferous 
cords
Scattered, partially differentiated cells are 
present
Pubertal
Initiation of spermatogenesis
Increased complexity, formation of 
occlusive junctions
Fully differentiated

ation of 
Fully differentiated cells appear
Adult
Active spermatogenesis, predominant cells
Individual cells associated with 
groups of germ cells
Groups of fully differentiated cells are 
present
From Gondos B, Kogan S. Testicular development during puberty. In: Grumbach MM, Sizonenko PC, Aubert ML, eds. Control of the Onset of Puberty. Baltimore, MD: Williams & Wilkins; 1990:387–398. 
© 1990, the Williams & Wilkins Co., Baltimore.
   
TABLE 26.6
The 

imore.
   
TABLE 26.6
The yin and the yang
Prepubertal juvenile pause
(Intrinsic CNS inhibition)
neurotransmitters
GABA and MKRN3
neurons
Augmentation
GnRH
pulse generator
GnRH
LH/FSH
Early puberty
Pituitary gonadotropes
Neuroexcitatory
amino acids
amino acids (glutamate)
Noradrenergic pathways
Neurotrophic factors
Growth peptides
GnRH
neuron
+
–
GABA and M

pothalamus
GnRH
neuron
+
–
neurons
GnRH
GnRH
LH/FSH
+
–
+
–
• Fig. 26.13  The yin and yang of the neuroendocrinology of the prepubertal juvenile pause and its intrinsic central inhibition of the gonadotropin-releasing 
hormone (GnRH) pulse generator and the reversal of this inhibition and termination of the juvenile pause, which leads to the onset of puberty. The GABAer­
gic neuronal network a

set of puberty. The GABAer­
gic neuronal network and its neurotransmitter γ-aminobutyric acid (GABA) constitute the most ubiquitous inhibitory transmitter in the hypothalamus and 
the brain. During the prepubertal juvenile pause, this neurotransmitter system appears to play the major neural role in inhibiting the GnRH pulse generator. 
Suppression of GABA inhibition during this period promptly results in reactivation of the suppressed GnRH pulse generator in the rhesus monkey. With 
the approach

generator in the rhesus monkey. With 
the approach of puberty, GABA inhibition of the GnRH pulse generator wanes, and its reactivation gradually occurs. This reactivation likely is augmented 
by stimulatory neurotransmitters (e.g., kisspeptin, excitatory amino acids), some of which depend on increased gonadal steroids for their activation, and 
by neurotrophic factors and growth peptides. A critical component of the reawakening of the GnRH neuronal network is the increase, independent of sex 
st

al network is the increase, independent of sex 
steroids, in KISS1 messenger ribonucleic acid (mRNA) expression in kisspeptinergic neurons in the medial basal hypothalamus and the secretion of kis­
speptins, the cognate ligands for the kisspeptin receptor (KISS1R, formerly GPR54) on the surface of the GnRH neuron (Shahab et al). As a consequence, 
the amplitude and, to a lesser extent, the frequency of GnRH pulses increase, which leads to increased pulsatile secretion of follicle-stimulating hor

ed pulsatile secretion of follicle-stimulating hormone 
(FSH) and luteinizing hormone (LH) and the activation of the ovary and testis. As shown experimentally in monkeys, the GnRH pulse generator can func­
tion in the absence of hypothalamic stimulatory factors. The nature of and factor(s) responsible for this transition from central inhibition and the postulated 
dominance of GABA in the release of inhibition and reactivation of the GnRH pulse generator are unknown. (Additional data from Shahab

enerator are unknown. (Additional data from Shahab M, Mastronardi 
C, Seminara SB, et al. Increased hypothalamic GPR54 signaling: a potential mechanism for initiation of puberty in primates. Proc Natl Acad Sci U S A. 
2005;102:2129–2134.)
1041
2
A
B
C
D
4
6
8
10 12
14 16 18
Attained height (cm)
Height velocity (cm/year)
Combined
Infancy
Childhood
Puberty
180
160
140
120
100
80
60
40
20
2
4
6
8 10 12 14 16 18
Combined
Infancy

4
6
Combined
Infancy
Childhood
Puberty
50
40
30
20
10
180
170
160
150
140
130
120
110
100
90
80
70
60
50
40
30
0
Estradiol
(and thyroid hormone)
Fetal growth
−1
1
3
5
7
9
11
13
15
17
19
20
18
16
14
12
10
8
6
4
2
0
Estradiol
Estradiol
Infancy
Childhood
Juvenile
Adolescence
2
4
6
8
10 12 14 16 18 20 22
• Fig. 26.14  The infancy-childhood-puberty (ICP) model of Karlberg for mean attained heigh

ty (ICP) model of Karlberg for mean attained height (A) and height velocity (B) for boys. The mean value for 
each component (infancy, childhood, and puberty) and their sums (combined growth [A] and combined velocity [B]) are plotted. The growth curve for an 
individual represents the additive effect of the three biologic phases of the growth process (ICP). Karlberg has provided mathematical functions for each 
component of his model. Infancy: This component starts before birth and falls off by 

is component starts before birth and falls off by age 3 to 4 years. It can be described by the exponential function y 
= a + b[1 − exp(−ct)]. Average total gain in height for Swedish boys is 79 cm (44% of final height) and for girls is 76.8 cm (46.2%). Childhood: This phase 
begins at the end of the first year of life and continues to mature height. A second-degree polynomial function describes this component: y = a + bt + ct2. 
Average total gain in height for boys is 85.2 cm (47.4%) and for gi

n in height for boys is 85.2 cm (47.4%) and for girls is 78.4 cm (47.3%). Puberty: The model for the pubertal growth spurt is a logistic func­
tion: y = a/[1 + exp(−b(t − tv))]. Average total gain in height for boys is 15.4 cm (8.6%) and for girls is 10.9 cm (6.5%); y designates attained height at time t 
in years from birth; a, b, and c are constants; tv is the age at peak height velocity. (C) A schematic male growth chart shows the features of the ICP pattern 
overlaid and illustrates the pred

the ICP pattern 
overlaid and illustrates the predominant endocrine mechanisms controlling each phase of growth. The first shaded area emphasizes the decreasing veloc­
ity of infantile growth as the individual leaves the rapid growth phase of fetal life. The open area is the childhood phase, which continues and magnifies 
the decreased velocity of growth into a plateau of rather constant growth during childhood. These two phases depend largely on the effects of growth 
hormone (GH) and thyroid h

 the effects of growth 
hormone (GH) and thyroid hormone, with no or little effect derived from gonadal steroids. In the next period of the pubertal growth spurt, gonadal steroids 
exert their direct and indirect effects. Gonadal steroids exert direct effects on the bone by stimulating the generation of insulin-like growth factor 1 (IGF1) 
and other growth factors locally, and they exert indirect effects by stimulating increased GH secretion, which exerts its own effects on bone and stimulates 

ch exerts its own effects on bone and stimulates 
the production of IGF1. In the female, the major gonadal steroid involved in the pubertal growth spurt is estradiol, whereas in the male, testosterone and 
estradiol (arising mainly from the aromatization of testosterone) are the major gonadal steroids. (D) The adolescent growth spurt in girls and boys (growth 
velocity curves). Notice the later onset of the pubertal growth spurt in boys and the approximately 2-year difference in peak height velo

pproximately 2-year difference in peak height velocity and the greater 
magnitude of peak height velocity compared with girls. The timing of the effects of estradiol is indicated. Progressive epiphyseal fusion terminates the 
growth spurt and leads to final or adult height. (A and B, modified from Karlberg J. On the construction of the infancy-childhood-puberty growth standard. 
Acta Paediatr Scand Suppl. 1989;356:26–37; C and D, from Grumbach MM. Estrogen, bone, growth, and sex: a sea change in

. Estrogen, bone, growth, and sex: a sea change in conventional wisdom. J 
Pediatr Endocrinol Metab. 2000;13:S1439–S1455.)
1042
pubertal growth is the more important determinant. Later onset 
of puberty and consequent increase in height at takeoff of the 
pubertal growth spurt can be balanced by a decrease in actual 
height achieved during PHV and result in no net change in adult 
height. However, early onset of puberty can diminish ultimate 
adult s

ly onset of puberty can diminish ultimate 
adult stature, prolonged delay of puberty can increase stature, and 
an older age at menarche leads to taller adult height in women. 
The age at PHV and the age at initiation of puberty correlate well 
with the rate of passage through the stages of pubertal develop­
ment in normal children. Physical examination of a boy can reveal 
that he is likely to have significant growth left if he is in early 
puberty, whereas limited growth is likely in boys who 

rty, whereas limited growth is likely in boys who are in late 
puberty.
Stature and the upper-to-lower (U/L) segment ratio, defined 
as the length from the top of the pubic ramus to the top of the 
head divided by the distance from the top of the pubic ramus to 
the sole of the foot, change markedly during the peripubertal and 
early pubertal periods because of the elongation of the extremi­
ties.157 At birth, the U/L ratio is about 1.7; at 1 year, it is 1.4; at 
10 years, it is 1.0 in a normal 

r, it is 1.4; at 
10 years, it is 1.0 in a normal healthy individual. The legs begin 
to grow before the trunk, although late in puberty, during the 
growth spurt, growth of the legs is similar to growth of the upper 
torso.158
The mean U/L segment ratio of white adults is 0.92, and that of 
African-American adults is 0.85. There are no differences in U/L 
segment ratio between the sexes. In general, hypogonadal patients 
have delayed epiphyseal fusion and lack a pubertal growth spurt; 
therefor

fusion and lack a pubertal growth spurt; 
therefore their extremities grow for a prolonged period, leading to 
a decreased U/L segment ratio and an increased span for height, 
a condition known as eunuchoid proportions. Eunuchoid propor­
tions are found in subjects with defects in estrogen synthesis and 
estrogen receptor deficiency, but normal proportions occur in 
patients with complete androgen insensitivity syndrome, demon­
strating the primary role of estrogen in mitigating or establishing 

ry role of estrogen in mitigating or establishing 
these proportions.159–161 The distal parts of the extremities, the 
hands and feet, grow before the proximal parts; a rapid increase 
in shoe size is a harbinger of the pubertal growth spurt. Boys with 
Klinefelter syndrome have long legs but not long arms, a physical 
feature that can assist diagnosis before the onset of puberty. The 
shoulders become wider in boys, and the hips enlarge more in 
girls. The female pelvic inlet widens, mainly bec

girls. The female pelvic inlet widens, mainly because of growth of 
the os acetabuli. The size of the head approaches the adult size by 
age 10 years, and the brain reaches 95% of adult size by the onset 
of puberty. 
Bone Age
Skeletal maturation is assessed by comparing radiographs of the 
hand, the knee, or the elbow with standards of maturation in 
a normal population.162,163 Ossification centers appear in early 
life, the bones mature in shape and size and develop articulation 
of surfaces;

pe and size and develop articulation 
of surfaces; ultimately, the epiphyses or growth plates fuse with 
their shafts in the process of epiphyseal fusion. Bone age, an 
index of physiologic maturation, does not have a well-defined 
relationship in normal children to the onset of puberty because 
it appears to be more variable than chronologic age.164 In addi­
tion, the bone age evaluator must have experience with the 
method or an erroneous reading may occur; smaller hospitals/
radiology offices

ng may occur; smaller hospitals/
radiology offices will not likely have such experience. However, 
bone age is still used for predicting the age of menarche, and in 
boys, the onset of normal, premature, and delayed puberty in a 
general sense.
A difference between bone age and chronologic age must 
exceed 2 SDs to be of biologic significance. Standard deviations 
may range from a few months in infancy to 1 year in later ado­
lescence; a 2-year variation of bone age from chronologic age is 
with

ariation of bone age from chronologic age is 
within normal limits in teenage years. As commonly estimated, 
bone age is imprecise and is a qualitative rather than a quantitative 
measure.165 The development of techniques for scanning radio­
graphs coupled with computer analysis is now approved in Europe 
and may improve accuracy of assessment.166
African-American children have reportedly had slightly more 
advanced bone ages than white children of the same chronologic 
age,167 but recent eviden

of the same chronologic 
age,167 but recent evidence suggests that Asian and Hispanic chil­
dren have more advanced bone ages than African Americans or 
Caucasians at the same chronologic age.168
Bone age, height, and chronologic age can be used for pre­
diction of adult height from the Bayley-Pinneau tables169 or by 
the use of the Roche-Wainer-Thissen,170 Khamis-Roche171 (for 
healthy whites only), or Tanner-Whitehouse techniques TW2158 
and TW3.172 Skeletal maturation is more advanced in girl

3.172 Skeletal maturation is more advanced in girls than 
in boys of the same chronologic age because the early pubertal 
bone ages of 11 years in girls and 13 years in boys are equivalent 
stages of bone maturation by the hand-wrist method. There are 
reportedly considerable variations in height prediction between 
the methods, so the subjective manner of prediction must be 
borne in mind.173 
Skeletal Density
Prevailing views locate the determinants of adult bone density in 
large part in gene

nants of adult bone density in 
large part in genetic tendencies and the appropriate acquisition 
of bone mineral in childhood during growth. Osteoporosis and 
osteopenia are important conditions of the adult that are held to 
have antecedents in youth, and increasing interest focuses on bone 
health in children and adolescents, including the effects of age 
of menarche, nutrition, exercise, and genetics on normal skeletal 
development.174,175 A relationship of bone density exists between 
gener

relationship of bone density exists between 
generations if the effects of age and puberty are eliminated, as 
60% to 80% of variance in peak bone mass is attributed to genetic 
factors.176 Clinical studies and animal models lead to the conclu­
sion that distant-past patterns of bone growth are less important 
than recent conditions and that childhood bone growth does not 
exert a strong effect on adult bone density.177 Thus even with poor 
diet in childhood and adolescence, improvement in bone 

in childhood and adolescence, improvement in bone density 
may be possible later in life.
Areal bone mineral density (BMD) represents a two-dimen­
sional image and is a function of the size of bone; this is the 
measurement most often available clinically with commercial 
dual-energy x-ray absorptiometry (DXA) devices. Smaller bones 
attenuate the radiation beam less than larger bones, and this factor 
must be considered in the interpretation when measuring bone 
density using commercial DXA dev

n measuring bone 
density using commercial DXA devices. BMD of the total body, 
lumbar spine, and femoral neck measured by DXA increases at a 
mean annualized rate of 0.047 g/cm2 for boys and 0.039 g/cm2 for 
girls (Fig. 26.15A). Longitudinal studies of total-body DXA assess­
ments indicate that boys accumulate 407 g per year and girls 322 
g per year of mineral (i.e., 359 mg/day for boys and 284 mg/day 
for girls); 26% of adult calcium is laid down during the adolescent 
years of peak calcium a

own during the adolescent 
years of peak calcium accretion—14 years (mean) for boys and 
12.5 years for girls.178 BMD approaches a maximum accretion in 
girls by the age of 16 years, and in boys by about 17 years, with the 
difference in timing related to the disparity in PHV; the rate then 
decreases, reaching a plateau in the third decade of life.179
1043
Quantitative computed tomography (CT) is at present a 
research technique but provides impor

t present a 
research technique but provides important information about the 
material aspects of bone.180 Although quantitative CT demon­
strates an increase in the cortical bone density of the lumbar spine 
with age, less increase in cancellous bone density with age occurs 
until the later stages of puberty.
Increased BMD correlates well with height, weight (a main 
determinant of bone density in adolescent and postpubertal 
females), age, pubertal development, and BMI but has less rela­
tions

rtal development, and BMI but has less rela­
tionship with serum IGF1 levels. Increased bone density in a lon­
gitudinal study of 227 girls was associated with an earlier onset of 
puberty.181
Some consider that the concept of age at peak bone density 
attainment is too simplistic and prefer to consider the strength of 
the bone and its geometry.182
Volumetric bone density (bone mineral apparent density 
[BMAD]) represents the amount of bone within the perios­
teal envelope and is of more physio

in the perios­
teal envelope and is of more physiologic importance, because 
it does not rely on the size of the bone that is changing during 
growth, particularly pubertal growth (see Fig. 26.15A). Vol­
umetric bone density grows in a region-specific pattern, and 
conditions in childhood and adolescence that affect the accrual 
of bone mineral have different effects based on the length of 
time the affected bone has left to achieve its maximum bone 
mineral content (BMC); deficits may occur in 

one 
mineral content (BMC); deficits may occur in limb dimen­
sions (prepuberty), spine dimensions (early puberty), or volu­
metric bone density by interference with mineral accrual (late 
puberty).183
Calculations for BMAD are made as follows:
BMAD = BMC ÷ the area of the region under consideration
The patterns of areal or volumetric bone density differ during 
development.184
DXA device manufacturers do not always provide standards for 
children and adolescents. Children are often referred for

n and adolescents. Children are often referred for evalua­
tion of osteoporosis because their DXA results are compared with 
young adult values, although they have not come close to reach­
ing maximal bone density characteristic of young adults. Bone 
reaches its adult size and PHV occurs before maximal BMC is 
reached185; these factors may result in a period of increased fragil­
ity and susceptibility to trauma characteristic of adolescence. Lean 
body mass (LBM) is related to skeletal density 

n 
body mass (LBM) is related to skeletal density (stronger in boys 
than girls) and fat mass and skeletal density (stronger in girls than 
in boys).186 Standards are available to interpret BMC in terms of 
LBM, which appears to be better related to normal bone growth 
than to chronologic age; muscle stress is an important factor in 
the development of bone.187 Normative data for pediatric DXA 
studies are available from various centers.179,187,188
Quantitative ultrasound standards are available

88
Quantitative ultrasound standards are available for children 
and adolescents.189 Because they do not require radiation expo­
sure, they may achieve wider use in childhood.
Seeman presented what he considers to be two fallacies in the 
interpretation of densitometry.184 The first is the concept that 
volumetric BMD increases during growth. It does not. Growth 
builds a bigger, not denser, skeleton. Second is the idea that 
peak volumetric BMD is higher in men than in women. It is 
not. Bone s

is higher in men than in women. It is 
not. Bone size is greater and is underestimated in patients with 
larger bones than controls. The misconceptions occur because 
the result of areal bone density is the BMC per unit projected 
bone area of bone in the coronal plane, or an areal BMC (g/
cm2). Too often, the “areal” element is deleted, and “content” 
is replaced by density, so that BMC per unit projected area is 
A
B
Metacarpal length (mm)
10
8
6
4
2
0
25
20
15
10
5
0
Periosteal di

th (mm)
10
8
6
4
2
0
25
20
15
10
5
0
Periosteal diameter
Puberty
Boys
Boys
Girls
Girls
Medullary diameter
Boys
Girls
Puberty
Effect of delayed puberty
• Fig. 26.15  (A) Periosteal diameter of the metacarpal bones does not differ before puberty in boys and 
girls. During puberty, the periosteal diameter expands in boys and ceases to expand in girls, whereas 
medullary diameter remains fairly constant in boys throughout growth but contracts in girls. (B) In boys, 
delayed puberty may reduce perios

s. (B) In boys, 
delayed puberty may reduce periosteal apposition, leaving a smaller bone with a thinner cortex but normal 
medullary diameter (top). In girls, delayed puberty may result in reduced endocortical apposition, leaving a 
normal or larger bone (if periosteal apposition continues in the absence of the inhibitory effect of estrogen) 
with a thinner cortex and larger medullary diameter (bottom). (From Seeman E. Pathogenesis of bone 
fragility in women and men. Lancet. 2002;359:1841–1850

ility in women and men. Lancet. 2002;359:1841–1850.)
1044
called BMD, even though volumetric bone density is the desired 
measurement. Although BMC may normally be higher in boys 
than in girls and rises with development, volumetric bone den­
sity of the long bones is identical in boys and girls. In contrast to 
the long bones, volumetric bone density increases at the spine in 
both sexes.179,183,184
The increase in BMD during the prepubertal and pub

The increase in BMD during the prepubertal and pubertal years 
reflects the increase in the size of the long bones. Legs grow more 
rapidly than the trunk in prepubertal girls, but during puberty 
there is more truncal growth. Boys develop greater bone size due 
to increased periosteal apposition (increasing bone strength) and 
endosteal resorption compared with girls; girls add bone on the 
endocortical surface, which may serve as a reservoir for calcium 
for later lactation and pregnancy.190
B

r calcium 
Birth weight, weight gain during infancy, and weight gain 
during the years 9 through 12 influence bone mass achieved at 
21 years.191 The BMD at the beginning of puberty predicted 
the peak bone mass at sexual maturity and appeared to predict 
the likelihood of osteoporosis as an adult in longitudinal studies, 
suggesting a method of identification of those most in need of 
intervention192 (Fig. 26.16). The mechanostat concept posits that 
develo

6.16). The mechanostat concept posits that 
developmental changes in bone strength result from the increasing 
loads imposed by larger muscle forces, which stimulate bone min­
eral acquisition. In a longitudinal study, a rise in LBM occurred 
before peak BMC, and fat mass later exerted more influence.193 
Increased physical activity is generally beneficial for bone health, 
but excessive running, gymnastics, and cheerleading are progres­
sively more likely to lead to stress fractures,194 and exc

ore likely to lead to stress fractures,194 and excessive exer­
cise can lead to the female athletic triad (discussed later). Femoral 
head strength increases markedly during puberty, and the femoral 
neck increases in density more with impact load sports such as 
running (compared with active load sports such as swimming); 
only 3 to 12 minutes of daily exercise increases femoral bone den­
sity in early pubertal children,195 with greater increases occurring 
during puberty.
Prepubertal girls eng

s occurring 
Prepubertal girls engaged in gymnastics have increased bone 
density in the limbs that are more often used, and this occurs 
in a dose-response manner.196 A longitudinal study of gymnasts 
and their mothers found that these effects do not mainly result 
from genetic influences. Female adolescent athletes have increased 
bone density, although the effects last only as long as the activity 
continues.
Calcium intake during puberty has been documented to affect 
bone de

ing puberty has been documented to affect 
bone density later in life in most studies,176 but the effect of 
increased ingestion of calcium may last only as long as the calcium 
is administered. Pubertal girls are estimated to get well below the 
recommended intake levels, and even recommended levels may be 
too low for optimal mineralization. Two cups of milk or equivalent 
is shown to provide adequate calcium intake for children 8 to 16 
years of age.197 Children who avoid dairy products and a

of age.197 Children who avoid dairy products and are with­
out calcium supplementation have an increased prevalence of frac­
tures in the prepubertal period, even with minor trauma.198 Early 
pubertal girls cannot increase gastrointestinal calcium absorption 
enough to compensate for a poor diet, as older individuals may. 
African-American children retain more calcium than white chil­
dren do, and the bone structure is thicker in African-American 
children; the difference in vertebral bone densi

children; the difference in vertebral bone density between ethnic 
groups appears to develop by late puberty. Randomized, placebo-
controlled clinical trials between 1985 and 2005 that enrolled 
normal children for at least 3 months revealed a small effect of 
calcium supplementation in the upper limb, but the increase in 
BMD was not thought to influence the likelihood of a fracture 
later in life.199 However, the lack of effects applies only to normal 
children, and studies of subjects with 

to normal 
children, and studies of subjects with disorders of puberty that 
affect bone development may reveal other findings. Remarkably, 
calcium intake is directly related to the rate of bone age advance­
ment to a degree.200 Increased sodium intake at the expense of cal­
cium intake adversely affects bone accretion. Adequate zinc intake 
is another factor related to BMD in puberty.
Vitamin D status is a concern, because 32% of girls with low 
calcium intake were also vitamin D deficient and

calcium intake were also vitamin D deficient and had elevated 
serum concentrations of parathyroid hormone (PTH) and thy­
roid hormone receptor–associated protein 5b (TRAP5b), as well 
as significantly lower cortical volumetric BMD of the distal radius 
and tibia shaft, and another 46% had low-normal concentrations 
of vitamin D.201 Deficient calcium and vitamin D can lead to 
secondary hyperparathyroidism in adolescence.
Girls with heterozygote ERα genotype (Pp) and high levels 
of physical a

e ERα genotype (Pp) and high levels 
of physical activity had significantly higher bone mass, higher 
BMD, and thicker cortex at loaded bone sites (compared with 
the distal radius, which is not a weight-bearing bone) than their 
counterparts202 with low physical activity. These results suggest 
that high physical activity benefits those with heterozygous ER 
genotypes, and the less favorable Pp genotype may be compen­
sated by increasing the amount of leisure-time physical activity at 
early pu

unt of leisure-time physical activity at 
early puberty.
Studies of male athletes are less common than those of girls, 
but athletic boys age 16 to 19 years can still gain more bone mass 
in the spine and femora than nonathletic control subjects.203 
Abnormalities of puberty impair bone accretion in both sexes and 
are mainly consequences of estrogen deficiency due to decreased 
secretion or peripheral aromatization of androgens (see Fig. 
26.15B). A 1.9-year increase in mean age at menarche in 

). A 1.9-year increase in mean age at menarche in young 
women was associated with lower radial areal BMD T-scores; lower 
trabecular number, thickness, and spacing; and cortical thickness 
without a reduction in cross-sectional area, a finding compat­
ible with less endocortical accrual and a possible explanation of 
Pituitary
Bone
+
+
+
+
+
+
+
+
+
?
−
GnRH
LH, FSH
Estrogens
Growth plate
IGF1
GRF
SRIF
GH
IGF1
Endocrine
Autocrine
Paracrine
• Fig. 26.16  Interactions of 

Autocrine
Paracrine
• Fig. 26.16  Interactions of the major growth-promoting hormones during 
puberty. Plus (+) indicates stimulatory action; minus (−) indicates inhibitory 
action. Circulating insulin-like growth factor 1 (IGF1) arises mainly from liver, 
but other tissues also contribute (i.e., endocrine action). Growth hormone 
(GH) and gonadal steroids have a direct stimulatory effect on the generation 
of IGF1 (i.e., paracrine action) locally in bone and cartilage cells. For simpli­
ficatio

y in bone and cartilage cells. For simpli­
fication, the feedback loops for IGF1 and gonadal steroids on the hypotha­
lamic pituitary unit are omitted. FSH, follicle-stimulating hormone; GnRH, 
gonadotropin-releasing hormone; GRF, growth hormone–releasing factor; 
LH, luteinizing hormone; SRIF, somatotropin release–inhibiting factor.
1045
how late menarche is a risk factor for forearm osteoporosis.204 
Urinary adrenal hormone metabolites are relate

04 
Urinary adrenal hormone metabolites are related to the achieve­
ment of increased proximal radial diaphyseal bone strength; the 
level of urinary androstenediol at about 8 years of age is an early 
predictor of diaphyseal bone strength in late puberty (about 16 
years of age).205 Peripheral conversion of adrenarchal dehydro­
epiandrosterone (DHEA) by 17β-hydroxysteroid dehydrogenase 
(17βHSD) to androstenediol may be associated with radial bone 
accretion during growth.
Testosterone administ

ne 
Testosterone administration to normal prepubertal boys 
increases calcium retention and bone growth and also increases 
bone density in adolescents with CDP, testosterone-deficient 
Klinefelter syndrome, or male hypogonadotropic hypogonad­
ism. Bone density is increased in females with excess androgens, 
whereas girls with anorexia nervosa, hypothalamic amenorrhea, or 
ovarian failure have decreased bone density.
Serum levels of inorganic phosphate, alkaline phosphat

m levels of inorganic phosphate, alkaline phosphatase, 
serum osteocalcin (Gla-protein level), collagen type I cross-linked 
N-terminal telopeptides (NTX),206 and procollagen type I C-ter­
minal propeptide (PICP); the cross-linked C-terminal telopeptide 
of type I collagen (ICTP), procollagen type III N-terminal pro­
peptide (P3NP), and tartrate-resistant acid phosphatase isoform 
5b; and urinary pyridinoline, deoxypyridinoline, and galactosyl-
hydroxy-lysine excretion reflect the increased oste

ydroxy-lysine excretion reflect the increased osteoblastic activity 
and growth rate during childhood and pubertal growth in both 
sexes, with values reaching a peak at midpuberty and decreasing 
thereafter.207 
Just as endocrine changes bring about remarkable changes in sec­
ondary sexual development and growth, body composition is dra­
matically affected. GH and gonadal steroids play major roles in 
this process.208 LBM, skeletal mass, and body fat are equal in pre­
pubertal b

al mass, and body fat are equal in pre­
pubertal boys and girls, but by maturity, men have 1.5 times the 
LBM and almost 1.5 times the skeletal mass of women, whereas 
women have twice as much body fat (25%) as men (13%), pro­
ducing a gynecoid (woman-like) or android (man-like) appear­
ance.209 LBM increases by the age of 6 years in girls and 9.5 years 
in boys. Boys acquire fat-free mass more quickly and for a longer 
period than girls during puberty; stability is attained by 15 to 16 
years i

uberty; stability is attained by 15 to 16 
years in girls and 2 to 3 years later in boys.150 Fat mass increases 
in girls at an average rate of 1.14 kg per year, but the fat mass 
does not change in boys during the pubertal years, leading to the 
greater fat value in girls than boys with age.150
The generalized distribution of fat in males (central fat, apple 
shaped, android), which is different from that in females (lower 
body fat predominance, pear shaped, gynecoid), develops largely 
during

, pear shaped, gynecoid), develops largely 
during puberty as males become more android than they were in 
prepuberty, although girls start and remain gynecoid. There are 
ethnic differences in the pattern of change, and Asians have the 
most significant changes.210
A strength spurt occurs during puberty after the pubertal 
growth spurt. Muscle mass is 54% of body weight in adolescent 
boys and 42% of body weight in adolescent girls, with the differ­
ence partly due to the presence of more muscl

fer­
ence partly due to the presence of more muscle cells and larger 
muscle cells in men. There is little gender difference before 8 years 
of age, but by 14 years, boys usually have developed greater lean 
leg mass and greater power than girls.211 
Obesity, Puberty, and the Metabolic Syndrome
Between NHANES II and NHANES III, a period of roughly 20 
years, there was more than a tripling of the prevalence of children 
and adolescents equal to or above the 95th percentile in weight 
(denoted by 

 above the 95th percentile in weight 
(denoted by the CDC as overweight), to a prevalence of 17.1%, 
and a 50% increase in those equal to or above the 85th percentile 
(denoted as obese).212
Weight is not an adequate reflection of body fat. The BMI is 
now invoked in describing the shape of the body in age-adjusted 
terms and is used as a better, if imperfect, reflection of body fat 
in childhood and adolescence.40 BMI changes with age, and there 
is no specific number indicating normal or abnor

is no specific number indicating normal or abnormal BMI at all 
stages of development, as there is for adults. Reference charts of 
BMI compared with age and gender between the 3rd and 97th 
percentiles are available online for interpretation of BMI values.213 
There are racial differences in the correlation of BMI with the 
amount of body fat, which decreases its accuracy.
Increased visceral fat (i.e., intra-abdominal adipose tissue 
[IAAT]) in obese teenagers is associated with hypertriglyce

n obese teenagers is associated with hypertriglyceri­
demia, decreased high-density lipoprotein (HDL) cholesterol, 
and small, dense, cholesterol-laden very low-density lipoprotein 
(VLDL) particles. Subcutaneous fat is associated with large, lipid-
laden VLDL particles, which are removed directly from the cir­
culation and pose less risk. The subcutaneous adipose tissue that 
leads to visibly different body forms is only an imperfect reflection 
of this internal distribution of fat cells, becau

of this internal distribution of fat cells, because increased IAAT 
may cause metabolic derangements without increasing total-body 
fat. Studies support the role of increased intra-abdominal visceral 
fat in children as a cause of insulin resistance and dyslipidemia, 
with small adipocytes demonstrating limited storage ability, lead­
ing to increased ectopic fat deposition in myocytes and hepato­
cytes.214,215 Waist-to-hip ratios may not reflect IAAT in children 
and adolescents, because subcut

 IAAT in children 
and adolescents, because subcutaneous abdominal fat may be low 
despite increased visceral fat.214 Although there is interest in waist 
circumference in children as a reflection of IAAT and standards 
are available, there is question if the waist circumference adds to 
the determination of IAAT more than BMI alone.216
DXA is used to determine the percentage of body fat, water, 
and bone mineral with great accuracy but cannot differentiate vis­
ceral from subcutaneous adipose t

fferentiate vis­
ceral from subcutaneous adipose tissue. CT was used until the val­
idation of MRI to determine intra-abdominal and subcutaneous 
fat distribution without the use of radiation. Excessive body fat 
during childhood and adolescence has significant medical effects 
early and later in life. Obesity, glucose intolerance, and hyperten­
sion in childhood are strongly associated with increased rates of 
premature death among Native Americans, but childhood hyper­
cholesterolemia is not.2

ans, but childhood hyper­
cholesterolemia is not.217
Serum Lipids in Normal Puberty and in Obesity and the 
Metabolic Syndrome
In normal puberty, testosterone increases serum levels of LDL 
cholesterol and decreases HDL cholesterol concentrations, 
accounting for the adverse LDL to HDL ratio in men compared 
with women.218 Postheparin hepatic lipase activity is increased by 
exogenous androgens (and decreased by estrogens), accounting for 
the decrease in HDL after androgen treatment or after a 

crease in HDL after androgen treatment or after a rise in 
endogenous androgen secretion. Standards for lipid levels in child­
hood differ from adult values.219
The epidemic of obesity has led to the advent of the meta­
bolic syndrome in youth. Diagnosis of metabolic syndrome varies 
among studies, and a generally accepted definition is needed.220 
Elevated cholesterol levels in children and adolescents track to 
adult values in longitudinal studies. Although familial hypercho­
lesterolemia lead

ies. Although familial hypercho­
lesterolemia leads to carotid intimal plaques by puberty, random 
autopsies demonstrate macroscopic or microscopic evidence 
1046
of arteriosclerosis in normal youth without familial hypercho­
lesterolemia, and the tendency is increased by obesity. By 15 
to 19 years, 2% of autopsied males had advanced (American 
Heart Association grade 4 or 5) atherosclerotic coronary artery 
lesions associated with increased serum c

 artery 
lesions associated with increased serum cholesterol, obesity, and 
hypertension.221 
Insulin and Insulin Resistance
Insulin resistance is a hallmark of obesity and is thought to be the 
cause of or an associated factor in the metabolic syndrome asso­
ciated with cardiac disease.222 Euglycemic clamp techniques are 
the gold standard for measurement of insulin resistance. Fasting 
insulin concentrations offer little insight into insulin resistance 
in an individual, and equations such as 

sistance 
in an individual, and equations such as the homeostatic model 
assessment (HOMA) that are based on fasting insulin levels offer 
little more; however, fasting insulin values are used in epidemio­
logic studies and offer more useful information.223 The fasting 
insulin concentration increases twofold to threefold with PHV, 
insulin secretion after a glucose load increases over prepubertal 
levels, and insulin-mediated glucose disposal in peripheral tissues 
decreases in the hyperinsulin

 peripheral tissues 
decreases in the hyperinsulinemic euglycemic clamp or the mini­
mal model frequently sampled intravenous glucose tolerance test 
(IVGTT), showing increased insulin resistance during normal 
puberty. Insulin sensitivity is inversely related to pubertal stage 
and BMI.
The response of insulin to an oral glucose tolerance test is 
greater in African-American subjects than in white subjects at all 
stages of pubertal development; this ethnic difference in insulin 
resistance is 

 this ethnic difference in insulin 
resistance is suggested as a cause for the increased incidence of 
type 2 diabetes among African-American adults compared with 
white adults and appears to offer a similar explanation of the eth­
nic disparity in youth, with white teenagers having greater insu­
lin sensitivity than African-American or Hispanic youth.224,225 
Insulin resistance occurs early in the course of Turner syndrome 
and thalassemia major, but even in Turner syndrome, in which 
there is 

, but even in Turner syndrome, in which 
there is an underlying increase in insulin resistance, there seems 
to be low or no risk of these conditions developing with GH 
With the increased prevalence of type 2 diabetes (non–insulin-
dependent diabetes mellitus [NIDDM]), proposed screening cri­
teria are being evaluated. Currently, children with a BMI higher 
than the 85th percentile should be screened if they (1) have a fam­
ily history of type 2 diabetes, (2) have signs of insulin re

y of type 2 diabetes, (2) have signs of insulin resistance 
(e.g., acanthosis nigricans, functional ovarian hyperandrogenism, 
hypertension, dyslipidemia), or (3) belong to one of several spe­
cific ethnic groups (e.g., African American, Native American, His­
panic American, Asian American). If a fasting plasma glucose level 
is higher than 126 mg/dL or a 2-hour postprandial value is higher 
than 200 mg/dL, or if there are symptoms such as weight loss, 
polyuria, or polydipsia and a casual plasm

 loss, 
polyuria, or polydipsia and a casual plasma glucose level higher 
than 200 mg/dL, the diagnosis of diabetes is likely, and determi­
nation of the type of diabetes (type 1 or 2) is appropriate.
Patients with type 1 diabetes (insulin-dependent diabetes mel­
litus [IDDM]) usually require an increase in the dose of insulin 
for euglycemic control at puberty. The cause of insulin resistance 
has been attributed in part to increased fat oxidation at puberty, 
which correlates with rising serum

on at puberty, 
which correlates with rising serum IGF1 levels and may be linked 
to increased GH secretion. However, there is no evidence that GH 
treatment alone increases the likelihood of development of type 
2 diabetes or impaired glucose tolerance. Weight gain increases 
in children with type 1 diabetes during puberty, leading to a 
higher incidence of obesity in children with IDDM than would 
be expected from family patterns. Some adolescents with IDDM, 
predominantly girls, reduce their 

nts with IDDM, 
predominantly girls, reduce their insulin use to lose weight, with 
dire consequences. Retinopathy due to IDDM characteristi­
cally appears in the teenage years or later, but duration and con­
trol of diabetes in the prepubertal years are contributing factors. 
The American Diabetes Association recommends screening for 
microalbuminuria, an indicator of the development of diabetic 
nephropathy during puberty.
A normal individual adapts to the changes in the physiologic 
rise in p

dapts to the changes in the physiologic 
rise in pubertal insulin resistance, but an individual at genetic risk 
for type 2 diabetes, with the accompanying defect in pancreatic 
beta-cell function,226 may not adapt to the insulin resistance and, 
with the additional insulin resistance characteristic of obesity, may 
develop clinical type 2 diabetes during the pubertal years or ear­
lier. Type 2 diabetes in children or adolescents should not be con­
fused with the various forms of monogenetic dia

n­
fused with the various forms of monogenetic diabetes (previously 
called maturity-onset diabetes of the young [MODY]), which are 
inherited as autosomal dominant traits.227
Although PCOS is common, several rare syndromes of severe 
insulin resistance combine hyperglycemia and virilization.228 
The Kahn type A syndrome features include a lean, muscular 
adolescent female phenotype with acanthosis nigricans, hirsut­
ism, oligomenorrhea or amenorrhea, and ovarian hyperthecosis 
and stromal hyper

rhea, and ovarian hyperthecosis 
and stromal hyperplasia associated with abnormalities of the 
insulin receptor gene. Hyperandrogenism, insulin resistance, 
acanthosis nigricans (HAIR-AN) syndrome, and PCOS are 
less severe than Kahn type A and usually manifest in adolescent 
females. Persons with Rabson-Mendenhall syndrome have severe 
insulin resistance (possibly leading to diabetic ketoacidosis), 
dysmorphic facies, acanthosis nigricans, thickened nails, hirsut­
ism, dental dysplasia, abdomin

ened nails, hirsut­
ism, dental dysplasia, abdominal distention, and phallic or cli­
toral enlargement. The Rabson-Mendenhall syndrome, similar 
to the Donahue (leprechaunism) syndrome, which shares some 
of these features, is caused by homozygous or compound het­
erozygote defects in the insulin receptor gene. Kahn type B syn­
drome is caused by inhibitory or stimulatory antibodies to the 
insulin receptor and sometimes occurs with acanthosis nigricans 
and ovarian hyperandrogenism. This syndro

gricans 
and ovarian hyperandrogenism. This syndrome can occur with 
ataxia-telangiectasia syndrome or in otherwise normal adoles­
cents. Individuals with the Berardinelli-Seip syndrome com­
bine lipodystrophy and severe insulin resistance and complete 
or partial absence of subcutaneous fat with increased growth 
and skeletal maturation, muscle hypertrophy, acanthosis nigri­
cans, hypertrichosis, organomegaly, and mild hypertrophy of the 
external genitalia.
Most of these NIDDM syndromes can be

al genitalia.
Most of these NIDDM syndromes can be treated with oral 
hypoglycemic agents initially; progression of the disorder may 
require the use of insulin. Several girls with these syndromes of 
insulin resistance have been described to have low serum gonado­
tropin values during puberty without response to GnRH but with 
enlarged ovaries, suggesting a direct role for insulin in stimulat­
ing the growth of the ovary. The hypoleptinemic state found in 
various degrees of lipodystrophy does 

e found in 
various degrees of lipodystrophy does not appear to affect pubertal 
progression, but administration of leptin has led to resumption of 
menstrual periods in some females and adjustment of testosterone 
production toward normal levels in males.229 
Blood Pressure
Blood pressure is related to the age, gender, and height of the 
child using appropriate standards.230 Blood pressure increases 
with pubertal maturation, related to increased stature and syn­
chronized with the pubertal gro

d stature and syn­
chronized with the pubertal growth spurt, suggesting some 
1047
relationship in the control of the two processes.231 Hypertension 
is becoming common in puberty as a comorbid condition of obe­
sity. Increased blood pressure at puberty depends on BMI and 
height, factors that are interrelated. Blood pressure in childhood 
and adolescence is predictive of adult blood pressure (tracking). 
Blood pressure rises in African-American ch

ing). 
Blood pressure rises in African-American children at lower BMIs 
than in white children, making the problem worse in the African-
American population. In sexual precocity, blood pressure rises 
above prepubertal levels to values commensurate with body size 
and BMI. 
Central Nervous System Anatomy, Function, 
Psychology, and Electroencephalographic 
Rhythm in Puberty
Brain anatomy and function change substantially during late 
childhood and adolescence (Fig. 26.17). Puberty is the time of

d adolescence (Fig. 26.17). Puberty is the time of 
appearance of the ability to solve complex problems in a mature 
manner. An increase in cortical metabolic rate in infancy is fol­
lowed by a late childhood decline to adult levels; this decline 
ceases by the end of the second decade. The prefrontal association 
cortex, an area of the brain that is concerned with forward plan­
ning and regulatory control of emotional behavior, continues to 
develop until the age of 20 to 25 years.232 Stress at

elop until the age of 20 to 25 years.232 Stress at various stages 
of development, even in early childhood, may cause psychologic 
manifestations during puberty.
Behavior or psychopathology that becomes evident at this time 
has its basis in these changes and exposures dating from early life 
and the prenatal period, all interacting against a genetic back­
ground. The anatomic changes revealed by functional MRI stud­
ies of the prefrontal cortex, an area that is involved with emotional 
regulati

 an area that is involved with emotional 
regulation and planning, occur during a time of physical matura­
tion and are likely to relate to many of the characteristic behavior 
changes of puberty. The volume of white matter increases linearly 
between 4 and 22 years of age owing to an increase in myelination 
during development.233 A reduction in cortical synaptic density 
and neuronal density, analogous to programmed cell death, occurs 
between 2 and 16 years of age, and this pruning of synapse

2 and 16 years of age, and this pruning of synapses appears 
to be linked to improved memory. This change in gray matter 
follows an inverted U-shaped curve of increase from the age of 
6 years. Longitudinal studies using dynamic mapping of human 
cortical development demonstrate that higher order association 
cortices (e.g., those involved in executive function, attention, and 
motor coordination) mature after lower-order somatosensory, 
motor, and visual cortices mature, and those areas that a

and visual cortices mature, and those areas that are phy­
logenetically older mature before newer ones.234,235 Intrauterine 
excess of androgens leads to enlargement of the amygdala, and 
girls reach greater mass of gray matter 2 years before boys dur­
ing puberty, demonstrating aspects of the effects of sex steroids 
on brain growth and remodeling in human beings. Gray matter 
volume, at least in girls, appears to be related to a pubertal increase 
in estradiol236 levels. Increased LH levels ar

se 
in estradiol236 levels. Increased LH levels are related to areas of 
increased white matter density, including the cingulum, middle 
temporal gyrus, and splenium of the corpus callosum; there is a 
genetic overlay to this relationship. Brain plasticity decreases dur­
ing puberty. Examples include the inability to learn to speak a 
foreign language without an accent after puberty and recovery of 
a child from the effects of a CNS injury that in an older teenager 
or an adult might have led to

n an older teenager 
or an adult might have led to aphasia. Loss of plasticity may be 
maladaptive to our rapidly changing world and extended life span 
compared with prehistoric times.237 Plasticity allowed develop­
mental learning before puberty, but lack of plasticity and a stan­
dard response to conditions in the adult allowed success in that 
static environment of the past.
Mania, depression, obsessive-compulsive disorders, and schizo­
phrenia are more common after puberty. They are postula

ia are more common after puberty. They are postulated to be 
related to alterations of the normal changes in brain architecture 
and function that occur during puberty.
Sleep Patterns in Puberty
Increased sleep is characteristic of the period of growth and 
development across species. Because sleep is a time of vulner­
ability, threats leading to stress are antithetical to normal sleep, 
and a feeling of safety is thought to be necessary to allow sleep 
to proceed normally as the adolescent is p

 sleep 
to proceed normally as the adolescent is preparing for indepen­
dence and increased self-care in a possibly hostile world. Ris­
ing complexity of brain function during puberty is reflected in 
increases in the amplitude and frequency of delta waves (0–3-
Hz electroencephalographic waves) found during deep sleep, 
which appears more related to age than to pubertal develop­
ment or growth.238 The function of deep sleep (i.e., slow-wave 
or non–rapid eye movement sleep) is thought to be res

non–rapid eye movement sleep) is thought to be restorative 
to learning and other activities of the waking state, and the 
most restorative portion is high-amplitude delta-wave sleep. 
During adolescence, the time spent in deep (stage 4) sleep 
declines by 40% to 50%, and increased (19.7%) stage 2 sleep 
occurs with pubertal development. The decline of slow-wave 
sleep during adolescence may reflect developmental changes of 
the brain.
When an individual is allowed to “run free,” the period is 

dividual is allowed to “run free,” the period is 
entrained (synchronized) close to the earth’s 24-hour light-dark 
cycle. Because human beings had little to do after dark, evolution 
favored an early bedtime, but within this schedule, developmental 
changes occur. One-year-old infants sleep an average of 11 hours 
per day, and by age 18, if circumstances permit, the mean is 8 
hours. Older people have earlier waking times and rate themselves 
as more morning-like than adolescents or young adult

 more morning-like than adolescents or young adults; because 
children are also morning-like, there is an inverted U-shaped 
curve of preferred times of awakening across the span of develop­
ment.239 This change to evening from morning alertness during 
puberty appears to be related to biologic factors, in contrast to 
social factors; in the past, social factors were thought to be more 
important.
Without the pressure of work or school, adolescents would 
stay up longer and awaken hours later th

ts would 
stay up longer and awaken hours later than a normal weekday 
schedule would dictate, a schedule far different from the one 
they followed at a younger age. Adolescents with early school 
starts awaken earlier than those with later school starts, but they 
do not change the time they go to sleep, leading to great varia­
tion in the amount of sleep attained. Data from the Add Health 
study showed decreased self-reported sleep duration during 
self-reported pubertal development, with girl

ing 
self-reported pubertal development, with girls reporting more 
problems with sleep (e.g., insomnia, insufficient sleep, awaken­
ing tired) as puberty progressed but with no such relationship 
seen in boys.240 There is an increase in daytime sleepiness dur­
ing adolescence, particularly during midpuberty up to stage 3 
to 4, even if total sleep time is held constant during longitudi­
nal multiple-year studies.241 With voluntary sleep deprivation 
(e.g., with late-night homework habits), slee

ion 
(e.g., with late-night homework habits), sleepiness can reach 
levels seen in narcolepsy and sleep apnea. Adolescents adapt 
more poorly to changes in sleep patterns than other age groups; 
1048
Superior frontal sulcus
Superior temporal sulcus
p < 0.01 (inverted U curve)
Nonlinear
coefficient
p < 0.0000008 (U curve)
Gray matter density
0.70
0.55
0.40
0.25
0.10
0 10 20 30 40 50 60 70 80 90
0.70
0.55
0.40
0.25
0.

 50 60 70 80 90
0.70
0.55
0.40
0.25
0.10
0.00010
0.00008
0.00006
0.00004
0.00002
0.00000
-0.00002
-0.00004
-0.00006
-0.00008
-0.00010
A
H
B
F
E
C
D
L
O
P
Q
N
M
K
J
I
A
B
C
D
E
F
G
H
I
J
K
L
M
N
S
T
Y
R
W
O
P
U
X
V
Q
>0.5
0.4
0.3
0.2
0.1
0.0
5 years
Age
20 years
Gray matter
volume
A
B
C
D
• Fig. 26.17  Mapping brain change over time. Brain changes in development can be identified by fitting time-dependent statistical models to data coll

ing time-dependent statistical models to data collected 
from subjects cross sectionally (i.e., across a group of subjects at a particular time) or longitudinally (i.e., following individual subjects as they aged), or 
both. Measurements such as cortical thickness are then plotted onto the cortex using a color code. Trajectory of gray matter loss over the human life span 
is based on a cohort of 176 subjects between 7 and 78 years of age.13 (A) represents a region in which the gray matter densit

epresents a region in which the gray matter density decreases rapidly during 
adolescence (i.e., superior frontal sulcus in which the decrease in gray matter is described by a quadratic equation represented by an inverted U-shaped 
curve), or follows a more steadily declining time course during the life span (i.e., superior temporal sulcus in which the decrease in gray matter is described 
by a quadratic equation represented by a U-shaped curve). (B, C) plots superimposed on the brain show how g

 (B, C) plots superimposed on the brain show how gray matter density decreases for particular 
regions with age, with the regions denoted by different letters. Brain maturation and change in gray matter density is mapped by year of age in (D) with 
fractional change in gray matter shown by color coding (C, D). (From Sowell ER, Peterson BS, Thompson PM, et al. Mapping cortical change across the 
human life span. Nat Neurosci. 2003;6:309–315; Toga AW, Thompson PM, Sowell ER. Mapping brain maturati

AW, Thompson PM, Sowell ER. Mapping brain maturation. Trends Neurosci. 2006;29:146–159.)
1049
this is manifested in the difference in hours awake between the 
school week and the weekend. Adolescents are able to shift to 
a later schedule more easily than to an earlier schedule. When 
self-selected bedtimes are late during summer but have to be 
changed to allow for school attendance, the adjustment is par­
ticularly lengthy and difficult.
After st

 is par­
After study of 27,000 individuals, it was proposed that the 
point of inflection from evening alertness during adolescence 
(after a morning-like pattern in childhood) to morning alertness 
in adulthood might be used as a marker of the end of adolescence, 
a sign that developmental remodeling of brain pathways is com­
pleted.242 The age of this inflection point is about 20.9 years in 
males and 19.5 years in females, who have an earlier change in this 
a

 in females, who have an earlier change in this 
and other aspects of puberty than males (Fig. 26.18). 
Characteristics of Adolescence
Most of this chapter deals with the biochemical and physical 
changes of the period we denote as puberty, but there are also pro­
found psychosocial changes during this period, usually denoted as 
adolescence. Although the attainment of an adult role in society 
occurs within a few years after achievement of reproductive matu­
rity in non-Western societies, the m

ductive matu­
rity in non-Western societies, the more technologically advanced 
the society, the more protracted the time allowed for adolescent 
psychosocial development. The prolonged period of adolescence 
in current society, ranging from 11 to 20 years in the United 
States, arose recently in human history, beginning no more than 
100 years ago in Western society.
As expressed by Remschmidt, the most important psychologic 
and psychosocial changes in adolescence are the emergence of 
abstrac

anges in adolescence are the emergence of 
abstract thinking, the growing ability to absorb the perspectives or 
viewpoints of others, an increased capacity for introspection, the 
development of personal and sexual identity, the establishment of 
a system of values, increasing autonomy from family and personal 
independence, greater importance of peer relationships of some­
times subcultural quality, and the emergence of skills and coping 
strategies to overcome problems and crises.243
Adolesce

egies to overcome problems and crises.243
Adolescence may be divided into three periods (early, middle, 
and late) by chronologic age. However, these periods may be 
reached at different maturational ages, because rates of physiologic 
maturation differ among individuals in these age groups.244
Early adolescence (age 11–15 years) encompasses most of the 
physical changes of puberty and in American society includes a 
profound social change from the sheltered, single-classroom envi­
ronment of el

he sheltered, single-classroom envi­
ronment of elementary school to the multiple-classroom and mul­
tiple-teacher experience of middle school or junior high school. 
The individual develops a maturing, but not mature, capacity for 
abstract thought and decision-making processes in contrast to the 
concrete reasoning of childhood.
Middle adolescence (ages 15–17 years) is the period of the 
high school years, a calmer period than early adolescence. The 
school experience is not a striking change,

. The 
school experience is not a striking change, and many of the most 
A
90
80
70
60
50
40
30
20
10
0
N
1200
900
600
300
0
B
Chronotype (MSFsc)
Late
Early
12
0
1
2
3
4
5
6
7
8
9 10 11
N
4000
3000
2000
1000
0
C
90
*
*
*
*
*
*
*
80
70
60
50
40
30
20
10
0
7
6
5
4
3
2
1
Late
Early
D
60
50
40
30
20
10
0
Late
5
4
3
Early
Chronotype (MSF)
p < 0.001
• Fig. 26.18  Assessment of chronotype (time of awakening in the absence of external cues) using th

wakening in the absence of external cues) using the 
MCTQ database (N ≈ 25,000). Age distribution within the database (A). Distribution of chronotypes (B). 
Age-dependent changes (C) in average chronotype (± SD [standard deviation]) are highly systematic 
(except for the age groups of 19, 21, 22, and 23, all other age-dependent averages ± SD are significantly 
different from that of age group 20; t-test, p < 0.001). Age-dependent changes of the chronotype (D) are 
different for males and females

hronotype (D) are 
different for males and females (filled circles and black line: females; open circles and gray line: males). 
Gray areas indicate significant male-female differences (t-test, p < 0.001). (From Roenneberg T, Kuehnle T, 
Pramstaller PP, et al. A marker for the end of adolescence. Curr Biol. 2004;14:R1038–R1039.)
1050
prominent biologic and physical changes of puberty have been 
accomplished. There is acceptance of some increased auto

lished. There is acceptance of some increased autonomy 
(e.g., driver’s permit and license are allowed), but the individual 
still lives at home. The individual emotionally moves away from 
the family and is less influenced by his or her peer group than are 
early adolescents; friendships assume an increasingly important 
role.
Late adolescence starts at the senior year of high school and 
is the age of acceptance of adult roles in work, family, and com­
munity. If the individual attends college

and com­
munity. If the individual attends college, this stage is prolonged. 
Behavior and Normal Puberty
Almost 100 years ago, G. Stanley Hall, without using what would 
be considered contemporary research techniques, characterized 
the maturing child as experiencing Sturm und Drang (i.e., storm 
and stress), which is normally restrained by cultural influences.245 
Contrary to this view, most recent empiric studies describe ado­
lescent development as a continuous, adaptive phase of emotional 

ent as a continuous, adaptive phase of emotional 
growth characterized more by stability than disorder and more 
by harmonious relationships between generations than conflict. 
Although mood changes are normally more rapid (occurring over 
hours or days) and more clearly marked in teenagers than in adults, 
these shifts must be differentiated from long-standing mood and 
behavior changes associated with serious psychopathology.
Turmoil, or truly tumultuous behavior, in adolescence is not 
a norm

tumultuous behavior, in adolescence is not 
a normal phase but may reflect psychopathology that requires 
diagnosis and treatment. It is often misdiagnosed as a temporary 
problem of adjustment reactions of adolescence. When the con­
ditions extend from adolescence to the adult stage, they are more 
severe. A 4-year longitudinal study of normal first-year US high 
school students showed that 25% experienced continuous growth, 
characterized by smooth, well-adjusted functioning despite stress­
fu

ooth, well-adjusted functioning despite stress­
ful situations; 34% experienced surgent growth, demonstrating 
good adaptation in general and short periods of difficulty and 
distress after some stressful situations. Twenty-one percent were 
judged to be in turmoil, characterized by mood swings, anxiety, 
and depression; these teenagers mainly came from homes char­
acterized by conflict, familial mental illness, and socioeconomic 
distress.246 Many with adolescent turmoil had not “grown out of 

ny with adolescent turmoil had not “grown out of 
it” when studied 5 years later and eventually were diagnosed with 
unipolar and bipolar depressive disorders. It may be concluded 
that 80% to 90% of adolescents do well psychologically during 
puberty and are happy individuals, but 10% to 20% have signifi­
cant difficulties.
Mood and Self-Image in Puberty
Young girls at the beginning of puberty frequently exhibit a nega­
tive self-image but a positive body image; positive peer relation­
ships an

itive body image; positive peer relation­
ships and superior adjustment improvement are observed with 
continued breast development. Mood in adolescence is not closely 
related to stage of puberty, but a significant curvilinear trend is 
seen for depressive affect (i.e., an increase followed by a decrease), 
for impulse control (a decrease followed by an increase), according 
to the level of serum estradiol. These data suggest that hormonal 
changes may be more important than physical changes as

ges may be more important than physical changes as determi­
nants of certain mood and behavior patterns during adolescence. 
Behavior in Variations of the Normal Age at Onset of 
Puberty
Within the normal limits of pubertal development, early-matur­
ing girls and late-maturing boys have the greatest prevalence of 
adjustment reactions in puberty and thereafter. Boys and girls who 
mature earlier are at increased risk for abuse.247
Early-developing boys are perceived to be more mature, attrac­
ti

g boys are perceived to be more mature, attrac­
tive, and smart and are given more leadership roles; late-devel­
oping boys are more insecure, more susceptible to lower levels of 
self-esteem and body image, and more vulnerable to peer pressure, 
especially in working-class and minority groups. Most of these dif­
ficulties of late maturation focus on the decreased height of the 
individual rather than the lack of sexual development. Social mat­
uration lags even after androgen treatment in patie

ration lags even after androgen treatment in patients with severe 
CDP. Delayed social maturation may put boys at risk for missing 
educational opportunities.
In contrast, early-maturing girls tend to experience more dif­
ficulty, especially in the junior high school setting, where they 
may attract the attention of older, more mature boys and have a 
higher prevalence of internalizing symptoms and disorders. They 
are more often the target of bullying but also more often the per­
petrator of bu

llying but also more often the per­
petrator of bullying than average-maturing girls.248
Early puberty may lead to negative body image in girls, com­
pared with boys, in whom the effect is positive. Early maturation 
may increase a propensity to violent behavior, which is fostered by 
living in a disadvantaged neighborhood. Early pubertal matura­
tion in girls may be related to a small intelligence quotient (IQ) 
advantage over late-maturing girls. Late-maturing girls are often 
more comfortable

s. Late-maturing girls are often 
more comfortable, remaining with the support of their families 
longer, and they are less often brought to medical attention than 
late-maturing boys. Early-maturing and late-maturing girls have a 
tendency to engage in health-risking behaviors involving strategies 
to lose weight, strategies to increase muscle, disordered eating, use 
of food supplements and steroids, and exercise dependence—ten­
dencies not found in boys at the same developmental stages. 
Risk

nd in boys at the same developmental stages. 
Risk-Taking Behavior
Adolescents who function at lower levels of cognitive complexity 
or concrete thinking and have an early onset of puberty demon­
strate an increase in risk-taking behavior. The age at onset of ciga­
rette smoking and alcohol use is proportional to the age at onset 
of puberty in girls: early-maturing girls partake earlier, and boys 
may follow the same pattern. 
Sexuality During Puberty
The social pressures are more mixed in thei

uberty
The social pressures are more mixed in their messages to girls, 
both encouraging sexuality and restricting it in a way more 
disparate than that encountered by boys. The earlier onset of 
puberty today compared with previous centuries has had a pro­
found effect on societal norms of sexual behavior.4 For example, 
early onset of puberty is associated with earlier onset of sexual 
intercourse.249
The percentage of adolescents who have had sex rises from less 
than 2% by 12 years of age, 1

sex rises from less 
than 2% by 12 years of age, 16% by age 15, 33% by 16, 48% by 
17, 61% by 18, and 71% by 19-year-olds for either sex. Teenagers 
are waiting longer to have sex than they did in the recent past. Fac­
tors such as the onset of puberty, weak self-concept, having tried 
smoking or drinking, and not being overweight were significantly 
associated with early sexual activity in girls. For boys, older age, a 
poor relationship with parents, low household income, and hav­
ing tried sm

rents, low household income, and hav­
ing tried smoking were factors significantly associated with sexual 
activity. Add Health revealed that adolescents at the upper and 
lower ends of the intelligence distribution were less likely to have 
sex.
Fertility is reached well before adult phenotype is acquired in 
boys and girls. The number of pregnancies for US teenagers 15 
1051
to 19 years old decreased almost 50% since 1991 and 22% since 
2009; it 

sed almost 50% since 1991 and 22% since 
2009; it was 31.3 per 1000 women in 2011.250 However, there 
is an ethnic-specific difference as the rate for Hispanic was 49.6 
per 1000, non-Hispanic black was 47.3, American Indian/Alaska 
Native (AIAN) was 36.1, non-Hispanic white was 21.7, and 
American Pacific Islanders (API) was 10.2. The decline is attrib­
uted to pregnancy prevention messages, and there is evidence of 
increased use of contraception.
Sexuality appears to be correlated with testos

on.
Sexuality appears to be correlated with testosterone production 
in boys in some studies, but in others it appears to be modified 
by the social effects of pubertal maturation. Religious activity may 
decrease the likelihood of sexual activity.
A randomized, double-blind, placebo-controlled, crossover 
clinical trial of boys and girls with delayed puberty address­
ing the effects of administration of oral conjugated estrogen to 
girls and testosterone enanthate to boys at three dose levels t

tosterone enanthate to boys at three dose levels that 
were intended to simulate early, middle, and late pubertal lev­
els demonstrated modest or no effects on sexual behavior. Boys 
had increased nocturnal emission and touching behaviors at the 
middle and high doses but no other effects. Girls demonstrated 
a significant increase in necking related to the administration of 
estrogen only at the late pubertal dose and no other effects. 
Hormonal and Metabolic Changes in Puberty
Increased amplit

 and Metabolic Changes in Puberty
Increased amplitude and alterations of the patterns of GnRH 
secretion at puberty initiate and regulate the sequential increases 
in secretion of pituitary gonadotropins and gonadal steroids that 
culminate in fertility.
Because of the pulsatile secretion of GnRH, gonadotropin secre­
tion is also episodic. The newer immunometric supersensitive 
assays allow accurate measurement in small pediatric samples. The 
results are lower than previously repo

mples. The 
results are lower than previously reported by the older assays.
During the first 2 years after birth, plasma levels of LH and 
FSH rise intermittently to adult values (the “mini puberty of 
infancy”) and occasionally higher but then remain low until 
puberty. In healthy Danish girls below 3 years of age LH increases 
up to 9.2 IU/L after GnRH test with lower values after 3 years.251 
Ultrasensitive assays and third-generation assays for LH and FSH 
confirm earlier evidence of pulsati

or LH and FSH 
confirm earlier evidence of pulsatile secretion of the gonadotro­
pins in prepuberty and indicate that the basal immunoreactive 
levels of LH are much lower than previously reported.252,253 The 
serum FSH level is higher than the LH level in prepubertal boys 
and girls.254 There is a more striking rise in serum LH amplitude 
by at least 1 year before the onset of puberty (i.e., the peripu­
bertal period immediately preceding the appearance of signs of 
sexual maturation) that reac

pearance of signs of 
sexual maturation) that reaches an early plateau, whereas FSH 
rises more consistently through male puberty rather than before, 
with increased pulse amplitude. An increased amplitude of LH 
and FSH secretion occurs at night in prepubertal boys and girls by 
5 years of age; the amplitude and frequency of such peaks increase, 
and daytime secretion increases with the progression of pubertal 
development.
In girls, FSH levels rise during the early stages of puberty, 
and LH l

rise during the early stages of puberty, 
and LH levels tend to rise in the later stages; from beginning 
to late puberty, the LH concentration rises more than 100-fold 
(Figs. 26.19 and 26.20). Disorderly patterns of secretion of LH, 
but not FSH, were noted just before the onset of puberty; this 
was followed by, first, increased orderliness in early puberty and, 
FSH
LH
<7
17β Estradiol
pg/mL
20
40
60
3.0
2.0
1.0
0.5
1
Stage of puberty
2
3
4
5
Bone age
7.0
10.5
11.6
13.0
14.0
FSH (LER 869)
LH

Bone age
7.0
10.5
11.6
13.0
14.0
LH (LER 960)
ng/mL
• Fig. 26.19  Mean plasma estradiol, follicle-stimulating hormone (FSH), 
and luteinizing hormone (LH) concentrations in prepubertal and pubertal 
females by pubertal stage of maturation (1, prepubertal; 5, menstruating 
adolescents) and the mean bone age for each stage. Single daytime 
values of gonadotropins have limited usefulness because of pulsatil­
ity of gonadotropin release and the increased amplitude of LH pulses 
during

e and the increased amplitude of LH pulses 
during sleep throughout puberty. The gonadal steroid values, however, 
are useful in determining the stage of pubertal development. To convert 
FSH values (LER-869) to international units per liter, multiply by 8.4. To 
convert LH values (LER-960) to international units per liter, multiply by 
3.8. To convert estradiol values to picomoles per liter, multiply by 3.671. 
(From Grumbach MM. Onset of puberty. In: Berenberg SR, ed. Puberty, 
Biologic and So

y. In: Berenberg SR, ed. Puberty, 
Biologic and Social Components. Leiden, The Netherlands: H.E. Sten­
fert Kroese; 1975:1–21. Reprinted by permission of Kluwer Academic 
Publishers.)
FSH
LH
<20
200
400
600
3.0
2.0
1.0
0.5
1
2
3
4–5
Adults
Bone age
7.7
12
13.7
15.7
(ng/mL) 
Plasma testosterone
(ng/100 mL) 
• Fig. 26.20  Mean plasma testosterone (after solvent extraction and 
chromatography) and gonadotropin levels in normal boys by stage of 

opin levels in normal boys by stage of 
maturation (1, prepubertal) and mean bone age for each stage (see Fig. 
26.19). To convert testosterone values to nanomoles per liter, multiply 
by 0.03467. (From Grumbach MM. Onset of puberty. In: Berenberg SR, 
ed. Puberty, Biologic and Social Components. Leiden, The Netherlands: 
H.E. Stenfert Kroese; 1975:1–21. Reprinted by permission of Kluwer Aca­
demic Publishers.)
1052
then, increased disorderliness aga

d the Life Span
then, increased disorderliness again in later puberty. This suggests 
that a more integrated feedback system operates in early puberty 
and is then followed by less stability.255
Doses of exogenous GnRH that are relatively ineffective in 
stimulating gonadotropin or gonadal steroid secretion before 
puberty become effective with the onset of puberty; an amplifi­
cation occurs in the hypothalamic-pituitary-gonadal axis with 
progression of puberty.256 Although the GnRH or GnRH ago

ssion of puberty.256 Although the GnRH or GnRH agonist 
test usually requires multiple sampling after the administration of 
GnRH, a single determination at 60 or 24 minutes now suffices 
with the new, sensitive assays.257–260 The basal values of serum LH 
and FSH measured in modern supersensitive assays are reported 
to predict the onset of pubertal development as well as GnRH 
testing does; a value of serum LH greater than 4 mIU/mL mea­
sured by immunochemiluminometric assay (ICMA) is consiste

y immunochemiluminometric assay (ICMA) is consistent 
with the onset of puberty. Moreover, the use of these ultrasensi­
tive assays to determine concentrations of LH and FSH in urine 
reveals a pattern of a 5-fold rise in urinary FSH in boys and girls, 
with a 50-fold rise in urinary LH in boys and a 100-fold rise in 
girls during puberty.261
In addition to the well-defined quantitative changes that occur 
in the pattern of FSH and LH in the pituitary gland, serum, and 
urine during development,

itary gland, serum, and 
urine during development, qualitative changes occur. The pattern 
of glycosylation of the α-subunit and β-subunit of the gonadotro­
pins is influenced by maturation, GnRH secretion, and the effects 
of gonadal steroids on the pituitary gonadotrophs. Variation in 
glycosylation that affects the size and charge of the hormone is 
the principal cause of the heterogeneity of FSH and LH and the 
large number of isoforms that varies according to the more acidic 
or more basic 

aries according to the more acidic 
or more basic charge.262 This pleomorphism has an important 
effect on biologic half-life and biologic activity and provides an 
additional mechanism of regulating the biologic activity of the 
gonadotropins. Discrepancies between serum bioactivity and 
immunoactivity of LH during pubertal development have been 
reported by some but not all researchers. However, a change in 
the isoforms of FSH released during puberty may favor the secre­
tion of increased bio

puberty may favor the secre­
tion of increased bioactive FSH, which may favor reproductive 
development.262 
Only recently has it been appreciated that many actions on lin­
ear skeletal growth, skeletal maturation, and accretion of bone 
mass thought to be due to testosterone in the male are mainly 
attributable to its peripheral aromatization to estrogen (Table 
26.7).
The Leydig cells of the testes produce testosterone and, in 
lesser amounts, androstenedione, α5-

rone and, in 
lesser amounts, androstenedione, α5-androstenediol, dihy­
drotestosterone, and estradiol, although a small amount of 
testosterone is derived from extraglandular conversion of 
androstenedione secreted by the testes and the adrenal. In 
the female, extraglandular conversion of ovarian and adre­
nal androstenedione accounts for almost all of the circulating 
testosterone.
Previous methods of determination of low levels of sex ste­
roids have been demonstrated to be inaccurate.263,26

ids have been demonstrated to be inaccurate.263,264 This insen­
sitivity is mainly due to the presence of interfering substances 
and relative insensitivity of antibodies used in assays. Now, larger 
national laboratories are beginning to use HPLC-MS/MS, which 
allows the accurate measurement of extremely low values pres­
ent in pediatric samples.265 These newer techniques may lead to 
revision of some of the results (described here) obtained by older 
methods. Measurement of testosterone and es

older 
methods. Measurement of testosterone and estradiol by HPLC-
MS/MS in laboratories with pediatric standards is the preferred 
method to measure gonadal steroids to investigate disorders of 
puberty.
Prepubertal boys and girls have plasma testosterone concen­
trations of less than 0.3 nmol/L except during the first 3 to 5 
months of infancy in the male, when pubertal levels are found. 
Nighttime elevations of serum testosterone levels are detectable 
in the male by 5 years of age, before th

tectable 
in the male by 5 years of age, before the onset of physical signs 
of puberty, and increase during early puberty after the appear­
ance of sleep-entrained secretion of LH and increased pituitary 
sensitivity to GnRH. The 60-minute lag between the peak of 
LH and the increase in testosterone is presumably due to syn­
thesis and secretion of the steroid. In the daytime, increases in 
testosterone levels are detectable after the testis volume is greater 
than 4 mL, with a consistent incre

ume is greater 
than 4 mL, with a consistent increase throughout puberty. The 
steepest increment in testosterone concentration occurs between 
pubertal stages 2 and 3 in males (see Fig. 26.20 and Table 26.5). 
The ratio of testosterone to epitestosterone in the urine, which 
is used to evaluate doping of athletes, may be elevated normally 
  Early Clinical Clues to the Effect of 
Estrogen on Growth and Skeletal 
Maturation in the Male
Complete androgen insensitivity (resistance

e Male
Complete androgen insensitivity (resistance) syndrome (Zachmann M, 
Prader A, Sobel EH, et al. Pubertal growth in patients with androgen 
insensitivity: indirect evidence for the importance of estrogen in 
pubertal growth of girls. J Pediatr. 1986;108:694–697.)
Short-term estradiol administration increased rate of ulnar growth in 
prepubertal boys (Caruso-Nicoletti M, Cassorla FG, Skerda MC, et al. 
Short term, low dose estradiol accelerates ulnar growth in boys. J 
Clin Endocrinol Metab.

es ulnar growth in boys. J 
Clin Endocrinol Metab. 1985;61:896–898.)
Aromatase inhibitor decreased rapid growth and skeletal maturation 
in testotoxicosis, whereas antiandrogen had no effect on skeletal 
maturation (Laue L, Jones J, Barnes K, et al. Treatment of familial 
male precocious puberty with spironolactone and deslorelin. J Clin 
Endocrinol Metab. 1993;76:151–155.)
Aromatase excess syndrome in boys associated with increased rate of 
growth and skeletal maturation, elevated plasma estrog

th and skeletal maturation, elevated plasma estrogen concen­
trations, but prepubertal testosterone values (Stratakis CA, Vottero A, 
Brodie A, et al. The aromatase excess syndrome is associated with 
feminization of both sexes and autosomal dominant transmission 
of aberrant P450 aromatase gene transcription. J Clin Endocrinol 
Metab. 1998;83:1349–1357.)
Estrogen-secreting tumors: adrenal and testicular neoplasms (especially 
Peutz-Jeghers syndrome) (Bulun SE, Rosenthal IM, Brodie AM, et al. 
U

ome) (Bulun SE, Rosenthal IM, Brodie AM, et al. 
Use of tissue-specific promoters in the regulation of aromatase 
cytochrome P450 gene expression in human testicular and ovarian 
sex cord tumors, as well as in normal fetal and adult gonads. J Clin 
Endocrinol Metab. 1993;77:1616–1621.)
From Grumbach MM. Estrogen, bone, growth, and sex: a sea change in conventional wisdom. 
J Pediatr Endocrinol Metab. 2000;13:S1439–S1455.
   
TABLE 26.7
1053
Free te

er 26  Physiology and Disorders of Puberty
Free testosterone measurements may be determined by dialysis 
or by calculation using testosterone values and available protein 
binding sites; low accuracy of the testosterone assay can make 
free testosterone measurements inaccurate.265 Nonetheless, if the 
total testosterone concentration on which the free testosterone 
level is based is measured by a highly specific assay, free or bio­
available testosterone determinations are helpful in evaluation 

osterone determinations are helpful in evaluation of 
PCOS or nonclassic congenital adrenal hyperplasia (CAH) in 
girls.
A sensitive mammalian cell recombinant bioassay for 
androgen bioactivity strongly correlates with serum immu­
noreactive testosterone concentration and demonstrates a rise 
with pubertal development in concert with progression of 
pubic hair and penile development in patients with CDP.266 
In contrast to this specific bioassay, a novel, highly sensitive 
transcriptional andro

, a novel, highly sensitive 
transcriptional androgen receptor (AR)–mediated bioassay 
system demonstrated higher circulating values of bioactive 
androgen in menopausal women and may be directed toward 
children in the future.267
The values of sex steroids measured in saliva are much lower 
than in the serum, but trauma (even tooth brushing) that leads 
to blood in the specimen can influence the results, and the accu­
racy of the basic assay is critical (see earlier discussion).265 The 
steroid

critical (see earlier discussion).265 The 
steroid level in saliva is not a direct representation of free steroid 
in the serum, as is often claimed. Testosterone in saliva is said in 
some reports to correlate well with serum levels of testosterone 
in normal subjects and in patients with chronic disease (e.g., 
cystic fibrosis). Salivary progesterone is said to rise with the pro­
gression of puberty. Salivary DHEA is higher after the onset of 
puberty than before. Salivary steroid measurements

puberty than before. Salivary steroid measurements, if accurate, 
can increase the ability of investigators to address the relation­
ship between development and behavior in a noninvasive man­
ner, but it may take the use of LC/MS-MS in salivary assays to 
achieve such accuracy. 
Estrogens
In the female, estradiol is secreted principally (90%) by the ovary; 
a small fraction of circulating estradiol arises from the extrag­
landular conversion of testosterone and androstenedione. In the 
male, ap

testosterone and androstenedione. In the 
male, approximately 75% of estradiol is derived from extraglan­
dular aromatization of testosterone and (indirectly) androstenedi­
one, and 25% is from testicular secretion.
In the fetus and at term, estrogen is high due to conversion of 
fetal and maternal adrenal C19-steroids to estrogen by the pla­
centa, but they drop precipitously during the first few days of life. 
Plasma estradiol levels are so low in prepuberty that detection by 
standard immunoa

 in prepuberty that detection by 
standard immunoassays is difficult, but a rise through puberty 
and a diurnal rhythm is seen with a sensitive radioimmunoassay 
(Table 26.8; see Fig. 26.19). Estrone levels rise early and reach 
a plateau by midpuberty. A highly sensitive bioassay demon­
strated higher estradiol concentrations in girls than in boys before 
puberty, with a rise through puberty until the pubertal growth 
spurt and a decrease thereafter. Today, estrogen measurements 
by high-perfor

fter. Today, estrogen measurements 
by high-performance liquid chromatography (HPLC) tandem 
mass spectroscopy is preferred. There is a significant correlation 
between peak growth velocity and the rise in estradiol concentra­
tion; the rise is earlier in girls than in boys, but bioactive estradiol 
levels are equivalent at peak growth velocity.160,268 The higher 
estrogen levels in girls may be an important factor in the more 
advanced levels of skeletal maturation in girls compared with boys 

 skeletal maturation in girls compared with boys 
and may play a part in their earlier onset of sexual maturation. A 
human cell bioassay measuring total estrogenic bioactivity (rather 
than estradiol alone) in children has an extremely sensitive detec­
tion limit of less than 1 pg/mL.269
The daily peak of estradiol in early pubertal girls occurs about 6 
to 9 hours after the peak of serum LH detected during the night, 
apparently related to time required for synthesis. In all stages of 
puberty

 required for synthesis. In all stages of 
puberty, boys have higher concentrations of estrone than estradiol, 
and levels of both estrogens are lower than those measured in girls 
at comparable stages. 
Protein Products of the Gonads
Inhibin, Activin, and Follistatin
Inhibin, a heterodimeric glycoprotein product of the Sertoli cell 
of the testes and the ovarian granulosa cell (as well as the placenta 
and other tissues), exerts negative feedback action on the secretion 
of FSH from the pituita

k action on the secretion 
of FSH from the pituitary. Inhibin is composed of an α-subunit 
and one of two β-subunits, βA or βB, which, respectively, form 
inhibin A or inhibin B, dimers with apparently identical function. 
Inhibin is a member of the transforming growth factor-β (TGFβ) 
superfamily that includes antimüllerian hormone (AMH) and the 
dimers of two inhibin subunits, activin A and activin B, which 
stimulate the release of FSH from pituitary cells.270 Synthesis and 
secretion of gona

tuitary cells.270 Synthesis and 
secretion of gonadal inhibin are induced by FSH. Inhibin plays a 
role in the feedback regulation of FSH secretion during puberty 
in males and females.
Like inhibin, follistatin inhibits, whereas activin stimulates, 
FSHβ-subunit expression and therefore affects FSH biosynthesis 
and secretion. Inhibin may also be an inhibitor of LH release in 
the follicular phase.271 These hormones are synthesized in a vari­
ety of tissues in addition to the gonads and have di

y of tissues in addition to the gonads and have diverse activities 
apart from those on the reproductive apparatus.
Two distinct binding proteins for inhibin and activin are 
present in the circulation, the gonads, and other tissues: α2-
macroglobulin, a high-capacity, low-affinity binding protein; and 
follistatin, a glycosylated, single-peptide chain that functions as a 
high-affinity binding protein and as a regulator of activin bioactiv­
ity (e.g., in the pituitary gland, a site of synthesis

(e.g., in the pituitary gland, a site of synthesis of activin and 
follistatin).
During pregnancy, the placenta secretes inhibin A, and the 
fetal membranes secrete inhibin A and inhibin B, but for at 
least the first 20 weeks of gestation, only inhibin A is detected 
in maternal serum. In umbilical cord serum from term female 
newborn infants, no inhibin dimer was detected, whereas cord 
serum from male newborns contained inhibin B, the only inhibin 
detected in adult males.272 In the human fet

ibin 
detected in adult males.272 In the human fetal testis, α-subunit 
and βB-subunit (but not βA-subunit) were present in Sertoli and 
Leydig cells at 16 weeks of gestation; by 24 weeks of gestation, 
immunoexpression of both subunits was greater in the Sertoli 
cells. Postnatally the expression of both subunits was decreased 
by 4 months of age. Inhibin subunits were not detected in the 
fetal ovary, nor was immunoreactive follistatin present in fetal or 
neonatal gonads.273
In large, cross-s

in fetal or 
In large, cross-sectional studies using highly specific inhibin B 
and inhibin A immunoassays that correlate with the bioactivity 
of inhibin and distinguish inhibin B from inhibin A, the mean 
concentration of serum inhibin B in males increased between pre­
puberty (a stage when it is higher than the undetectable levels in 
castrated men)274 and the first stage of puberty; when the strong 
1054
  Plasma Gonadal Ster

crinology and the Life Span
  Plasma Gonadal Steroid Values in Children
Steroid and Assay
Age
NORMAL VALUES
Sample Volume 
(Pediatric Minimums)
Males
Females
Testosterone, by LC-MS/MS 
(ng/dL)
Term infant
75-400
20-64
0.18 mL serum (Quest)
0.5 mL serum (Esoterix)
0.15 mL serum (ARUP)
1-7 mo
Levels decrease rapidly in first week 
to 20-50, then increase to 60-400 
between 20 and 60 days; levels then 
decline to prepubertal range by 7 mo
Levels decrease during 
the first month to <10 
and remain 

crease during 
and remain at that 
level until puberty
7-12 mo
<16
<11
Tanner stage I
<16
<16
Tanner stage II
<167
<40
Tanner stage III
7-762a
<60
Tanner stage IV
25-912
<62
Tanner stage V
110-975
<68a
Androstenedione, by RIA after 
extraction (ng/dL)
20-290; levels decrease to 10-80 after 
1 week
20-290 ng/dL; levels 
decrease to 10-80 
after 1 week
0.25 mL serum (Esoterix)
0.5 mL serum (Quest)
1-11 mo
6-68
6-68
8-50
8-50
Tanner I
8-50
8-50
Tanner

68
8-50
8-50
Tanner I
8-50
8-50
Tanner II
31-65
42-100
Tanner III
50-100
80-190
Tanner IV
48-140
77-225
Tanner V
65-210
80-240
DHT, by extraction chromatog­
raphy, RIA (ng/dL)
1-6 mo
12-85
<5
0.5 mL serum (Esoterix)
1.1 mL serum (Quest)
<5
<5
Tanner II-III
3-33
5-19
Tanner IV-V
22-75
3-30
Estradiol, by LC-MS/MS (ng/dL)
Newborn
Levels are markedly elevated and fall 
during first week to <1.5
Levels are markedly 
elevated and fall dur­
ing first week to <1.5
1.2 mL serum (E

d fall dur­
1.2 mL serum (Esoterix)
1-11 mo
Levels increase to 1-3.2 between 1 
and 2 mo, then decrease to <1.5 
by 6 mo
Levels increase to 0.5-5 
between 1 and 2 mo, 
then decrease to <1.5 
during the first year
<1.5
<1.5
Tanner I
0.5-1.1
0.5-2
Tanner II
0.5-1.6
1-2.4
Tanner III
0.5-2.5
0.7-6
Tanner IV
1-3.6
2.1-8.5
Tanner V
1-3.6
3.4-17
Estradiol, by extraction chroma­
tography, RIA (ng/dL)
0.3-1.5
0.5-1
0.6 mL (Quest)
0.3-1
0.5

1.5
0.5-1
0.3-1
0.5-11.5
0.5-1.5
0.5-18
0.3-4
2.5-34.5
1.5-4.5
2.5-41
TABLE 26.8
1055
Age
Males
Females
Estradiol, chemiluminescent 
immunoassay (ng/dL)
0-8
0.7-0.8
0.7-1.4
0.2 mL serum (ARUP)
9-10
0.7-1.1
0.7-3.2
11-12
0.7-2.2
0.7-3.8
13-14
0.7-2.4
1-9.1
15-16
1.1-3.3
1.7-18.1
17-40
1.8-6.7
2.3-17.0
Ext

15-16
1.1-3.3
1.7-18.1
17-40
1.8-6.7
2.3-17.0
Extrone, by LC-MS/MS (ng/dL)
Levels are markedly elevated at birth, 
then decrease during first week to 
<1.5
elevated at birth, then 
decrease during first 
week to <1.5
<1.5
<1.5
Tanner I
0.5-1.7
0.4-2.9
Tanner II
1.0-2.5
1-3.3
Tanner III
1.5-2.5
1.5-4.3
Tanner IV
1.5-4.5
1.6-7.7
Tanner V
2-4.5
2.9-10.5
aBecause this chart combines values from different laboratories, the ranges are larger in

m different laboratories, the ranges are larger in the aggregate than would be found in the specific laboratories’ standards. Please consult the laboratory being 
used to interpret results for clinical decisions.
DHT, Dihydrotestosterone; LC-MS/MS, liquid chromatography tandem mass spectrometry; RIA, radioimmunoassay.
From Albrecht L, Styne D. Laboratory testing of gonadal steroids in children. Pediatr Endocrinol Rev. 2007;5:S599–S607.
   
  Plasma Gonadal Steroid Values in Children—cont’d
TABL

sma Gonadal Steroid Values in Children—cont’d
TABLE 26.8
correlation with chronologic age was taken into account, a correla­
tion with LH and testosterone values remained. From genital stage 
2 of puberty on, inhibin B levels were relatively constant, despite 
a rise in the mean concentration of serum FSH between stages 2 
and 3, after which the FSH value was relatively unchanged. The 
rise in inhibin B is mirrored by a drop in AMH in early puberty, 
apparently reflecting Sertoli cell maturation

ty, 
apparently reflecting Sertoli cell maturation.275 By genital stage 
3, a negative partial correlation between inhibin B and FSH was 
found that persisted as puberty advanced, and by genital stage 4, 
there was a clear negative correlation of inhibin with serum FSH. 
Dimeric inhibin B rises twice during development, reflecting the 
two periods of Sertoli cell proliferation in infancy and in early 
puberty, whereas an inverse relationship between inhibin and FSH 
is seen at midpuberty and the

een inhibin and FSH 
is seen at midpuberty and thereafter, indicating the development 
of negative feedback inhibition. In the early stages of puberty, 
inhibin B values are closely related to LH and testosterone levels, 
but in stage 3, when inhibin B values peak, this relationship is 
lost, and inhibin B values become more closely related to FSH 
levels.276
Serum levels of inhibin A and B increase early in puberty in 
girls, although there are individual increases in the prepubertal 
period di

individual increases in the prepubertal 
period directly related to FSH levels, demonstrating sporadic 
follicular development in the infant and child due to FSH 
stimulation. Inhibin B is predominant in the follicular phase, 
as is inhibin A during the luteal phase. Inhibin A and inhibin 
B peak in midpuberty, and inhibin B is thereafter decreased. 
During the early stages of puberty, inhibin B values are related 
to estradiol and FSH values, but these relationships dimin­
ish with the progress

t these relationships dimin­
ish with the progression of puberty.277 Although there is no 
significant change in activin during female puberty, follistatin 
decreases from a midpuberty peak to later values that fall below 
prepubertal values.
Serum values of FSH-regulating proteins follow circadian 
patterns, with higher values of LH and FSH overnight, just after 
a nadir of inhibin B. Follistatin concentrations were found to 
reach their greatest value in early morning, and activin A con­
centr

t value in early morning, and activin A con­
centrations declined coincident with the nighttime increase in 
FSH levels in pubertal girls.278 Diurnal variation of inhibin B in 
boys in the peripubertal or early pubertal period demonstrates a 
fall in inhibin during the night as LH and, subsequently, testos­
terone rise, showing the negative feedback effect of testosterone 
on inhibin B secretion.279 Recombinant FSH treatment, which 
raises testosterone secretion, suppresses inhibin B, demonstrat

terone secretion, suppresses inhibin B, demonstrating 
the ability of testosterone to negatively influence inhibin B secre­
tion.280 Administration of a GnRH agonist led to an increase in 
the FSH level by 30 minutes as well as an increase in the inhibin 
B level in girls older than 5 years of age by 8 hours and in boys by 
20 hours.281 The baseline inhibin B concentration was greater in 
boys than in girls, baseline activin A concentrations were greater 
in girls, and activin did not change wit

 greater 
in girls, and activin did not change with GnRH administration. 
Testosterone administration to boys in Tanner stage 2 led to 
decreased FSH and LH, increased activin, and decreased inhibin 
B levels but caused no change in follistatin. Estradiol admin­
istered to girls in Tanner stage 1 or 2 led to decreased LH and 
1056
FSH and increased activin A levels but no change in inhibin B 
or follistatin concentrations, whereas administration of e

statin concentrations, whereas administration of estradiol 
to girls with Tuner syndrome led to decreased serum levels of 
FSH, although the effectively nondetectable levels of activin and 
inhibin did not change.282
A low concentration of inhibin B in men and pubertal boys is 
an indicator of impaired seminiferous tubule function.283 Early 
pubertal boys with testicular defects have higher FSH concentra­
tions and low inhibin levels. Inhibin B is the form most closely 
related to testicular fun

s the form most closely 
related to testicular function, and it is absent in orchidectomized 
men. Inhibin B is related to Sertoli cell function in prepuberty, 
but a developmental change occurs during puberty so that later 
in life, inhibin B concentration is related to spermatogenesis. Pre­
pubertal boys with the Sertoli cell–only syndrome had normal 
inhibin B levels, whereas postpubertal affected boys and men with 
Sertoli cell–only syndrome and early-stage spermatogenic arrest 
had undetect

and early-stage spermatogenic arrest 
had undetectable or low levels of inhibin B, and those with late-
stage spermatogenic arrest or obstructive azoospermia had normal 
or near-normal levels of serum inhibin B.274 In prepuberty, the α 
and the βB inhibin subunits are expressed in Sertoli cells, but dur­
ing puberty and in men, fully differentiated Sertoli cells express 
only the α-subunit; the β-subunit is expressed in germ cells. 
Inhibin B in the adult appears to be a product of germ and Sert

the adult appears to be a product of germ and Sertoli 
cells. In prepubertal boys, basal plasma inhibin B concentrations 
have a high correlation with the incremental testosterone response 
to administration of human chorionic gonadotropin (hCG), and 
they provide a useful assessment of the presence of testes and their 
function.284 
Antimüllerian Hormone
AMH (also called müllerian inhibiting substance [MIS] or factor 
[MIF]), a 14-kDa homodimeric glycoprotein that is structurally 
related to th

c glycoprotein that is structurally 
related to the subunit of inhibin and TGFβ, is produced by the 
Sertoli cells of the fetal testis after 7 weeks and the prepubertal 
testis and causes the regression of the müllerian ducts in boys 
during early fetal development and later in gestation by granu­
losa cells of the fetal ovary. AMH production is first stimulated 
by SOX9, SF1, GATA4, and WT1 before FSH rise but later is 
stimulated by FSH.285 Immunoassayable concentrations of AMH 
values rise fr

unoassayable concentrations of AMH 
values rise from birth to relatively high levels during the first year 
of life in males, decrease by age 10 years, and decrease further 
during puberty.286 Newborn females have low or nondetectable 
serum levels of AMH, which rise only slightly thereafter; serum 
AMH concentrations are virtually nondetectable in most girls 
just before puberty. Serum levels of AMH and of estradiol after 
GnRH analogue stimulation are increased in low-birth-weight 
and high-bi

ion are increased in low-birth-weight 
and high-birth-weight female infants, suggesting altered follicu­
lar development.287 Increased poststimulation FSH levels and 
low adiponectin concentrations are observed only in high-birth-
weight infants, indicating that altered ovarian function occurs 
by a different mechanism than that found in low-birth-weight 
infants.
Serum levels of AMH and inhibin B are inversely related to 
androgen concentrations in pubertal boys,275 and values in boys 
with CPP

 in pubertal boys,275 and values in boys 
with CPP are appropriate for pubertal stage rather than chron­
ologic age.286 Elevated serum AMH concentrations occur in 
androgen resistance. Treatment with recombinant FSH and hCG 
in hypogonadotropic hypogonadism increases testosterone and 
decreases the elevated levels of serum AMH (due to immature 
Sertoli cells) and inhibin B, further demonstrating this relation­
ship.280 AMH is slightly higher in boys with delayed puberty than 
in pubertal age-mat

oys with delayed puberty than 
in pubertal age-matched control subjects and lower in those with 
testicular dysgenesis associated with impaired virilization than 
in normal boys. Boys with isolated cryptorchidism have normal 
values of AMH, and AMH and inhibin B are absent in anor­
chia, allowing a differential diagnosis during the first month after 
birth.288 Dysgenetic testes secrete only low serum AMH levels; the 
testosterone response to hCG indicates the presence of testicular 
tissue.289
A

indicates the presence of testicular 
AMH is elevated in girls with PCOS and in girls with oligo­
menorrhea without classic AMH. This finding suggests that oligo­
menorrheic adolescents may have increased antral follicle number, 
similar to that observed in girls with PCOS.290 AMH is a useful 
gonadal tumor marker because values are elevated in males with 
primitive Sertoli-like tumors and in girls and women with granu­
losa cell tumors. 
Adrenal Androgens
A progressive increase in pl

s. 
A progressive increase in plasma levels of Δ5-steroids, DHEA, and 
DHEAS in boys and girls begins before age 8 (skeletal age of 6–8 
years) and continues through early adulthood (Table 26.9). The 
increase in the secretion of adrenal androgen and its precursors 
is known as adrenarche, and the appearance of pubic hair caused 
by adrenarche is known as pubarche. Plasma DHEA levels have 
a diurnal rhythm similar to that of cortisol, but plasma levels of 
DHEAS show less varia

tisol, but plasma levels of 
DHEAS show less variation and are a useful biochemical marker 
of adrenarche.
  Mean Serum Concentrations (mmol/L [ng/mL]) of DHEAS During Childhood
6-8 yr
8-10 yr
10-12 yr
12-14 yr
14-16 yr
16-20 yr
By Chronologic Age
Boys
0.5 (188)
1.6 (586)
3.4 (1260)
3.6 (1330)
7.2 (2640)
7.2 (2640)
Girls
0.8 (306)
3.2 (1170)
3.1 (1130)
4.6 (1690)
6.9 (2540)
6.3 (2320)
By Bone Age
Boys
0.98 (360)
1.6 (574)
3.4 (1250)
5.8 (2150)
10.9 (4030)
Girls
0.73 (276)
3.1 (1130)
4.33 (1560)

0.9 (4030)
Girls
0.73 (276)
3.1 (1130)
7.1 (2610)
3.9 (1450)
DHEAS, Dehydroepiandrosterone sulfate.
Modified from Reiter EO, Fuldauer VG, Root AW. Secretion of the adrenal androgen, dehydroepiandrosterone sulfate, during normal infancy, childhood, and adolescence, in sick infants, and in children with 
endocrinologic abnormalities. J Pediatr. 1977;90:766–770.
   
TABLE 26.9
1057
DHEA is the predominant precursor to more potent andro­
ge

the predominant precursor to more potent andro­
gens in females, but DHEAS cannot be converted. DHEAS is pro­
duced from DHEA by the action of the sulfotransferase enzyme 
SULT2A1, mainly in the adrenal glands and the liver. The sulfate 
donor phosphoadenosine phosphosulfate (PAPS) is required by 
SULT2A1, and in human beings, PAPS is synthesized by the two 
isoforms of PAPS synthase, PAPSS1 and PAPSS2.291 
Testosterone-Binding Globulin
Between 97% and 99% of circulating testosterone and estradi

7% and 99% of circulating testosterone and estradiol is 
reversibly bound to testosterone-binding globulin (TeBG) (or sex 
steroid–binding globulin [SHBG]); prepubertal levels of TeBG 
are approximately equal in boys and girls, but a decrease in TeBG 
level occurs with advancing prepubertal age and the concomi­
tant increase in the plasma gonadal steroid levels.292 At puberty, 
there is a small decrease in TeBG levels in girls; as a consequence 
of testosterone, there is a greater decrease in bo

of testosterone, there is a greater decrease in boys, although the 
drop observed in normal boys is attenuated by treatment with 
tamoxifen, even with advancing pubertal development. The rise 
in adrenal androgen levels at adrenarche may explain the early 
drop in TeBG levels, which allows more circulating free hormone 
at a given concentration of testosterone. Although the plasma 
concentration of testosterone is 20 times greater in men than in 
women, the concentration of free testosterone is 

women, the concentration of free testosterone is 40 times greater. 
Boys with hypogonadotropic hypogonadism and patients with the 
androgen resistance syndrome show the same characteristic fall in 
TeBG levels at puberty, but values are intermediate between those 
of normal adult males and females. 
Prolactin
Prolactin levels rise in girls during puberty. Prepubertal mean (± 
standard error) plasma prolactin concentrations are less than 10 
μg/L in boys and 3 to 12 μg/L in girls. Late pubertal 

 in boys and 3 to 12 μg/L in girls. Late pubertal girls and 
adult women have higher concentrations of prolactin (3–20 μg/L 
and 3–20 μg/L, respectively), whereas the mean concentration in 
adult men is 2 to 18 μg/L.293 This sex difference is probably a 
consequence of the higher estradiol levels during puberty in girls 
and in women. 
Insulin-Like 3 Protein
During puberty, serum levels of insulin-like 3 (INSL3), a pro­
tein produced by the Leydig cells, rise in normal boys under LH 

g cells, rise in normal boys under LH 
stimulation and in those in whom increased secretion is induced 
by letrozole treatment.294,295 Values do not increase in Klinefelter 
syndrome, in which the initial rise levels off during midpuberty.295 
INSL3 is said to be as sensitive as testosterone as an indication of 
Leydig cell function.296 
Prostate-Specific Antigen
Prostate-specific antigen (PSA) is detectable in male and female 
cord blood and in the serum of infants, but PSA concentrations 
decr

the serum of infants, but PSA concentrations 
decrease to undetectable levels during childhood. PSA concentra­
tions rise to the measurable range with the onset of puberty in the 
male and correlate with the progression of pubertal stage, the size 
of the testes, serum LH and testosterone concentrations, and, pre­
sumably, the size of the prostate.297,298 PSA values are increased to 
the pubertal range in boys with idiopathic CPP, and they decrease 
with GnRH agonist treatment. 
Hormonal Control

se 
Hormonal Control of the Pubertal Growth Spurt
Postnatal growth follows a specific pattern: An extremely high 
growth rate just after birth is followed by a deceleration that con­
tinues until 3 years of age; next, there is a slower phase of decel­
eration until puberty. The subsequent pubertal growth spurt, the 
second greatest period of postnatal growth, is followed by matura­
tion of the spine and long bones until adult height is reached.151 
Many factors infl

til adult height is reached.151 
Many factors influence the growth plate.299 The adolescent growth 
spurt in normal girls and boys depends on estradiol and GH lev­
els, among other factors.
Hormonal control of the pubertal growth spurt is com­
plex (see Figs. 26.15 and 26.16). GH is involved in increasing 
growth at puberty through stimulation of IGF1 production. 
Gonadal steroids have two effects on pubertal growth: (1) 
induction of an increase in GH secretion, with a consequent 
increase in I

 in GH secretion, with a consequent 
increase in IGF1 production, thereby indirectly stimulat­
ing pubertal growth; (2) a direct effect on cartilage and bone 
through stimulation of local production of IGF1 and other 
local factors.160,300
Gonadal Steroids159,160
In the developing human skeleton, gonadal steroids have 
growth-promoting and maturational effects on chondrocytes, 
osteoblasts, and other bone constituents.159,160 This action, 
which eventually leads to epiphyseal fusion and the cess

eventually leads to epiphyseal fusion and the cessation 
of longitudinal growth in boys and girls, is mediated mainly 
by estrogen that is directly secreted (in girls) or arises from the 
conversion of testosterone and androstenedione to estrogen in 
peripheral tissues by aromatase (see Table 26.8). Detection of 
estrogen resistance resulting from a null mutation in the gene 
encoding the estrogen receptor and from derangements in the 
CYP19A1 gene, leading to severe cytochrome P450 aromatase 
d

ne, leading to severe cytochrome P450 aromatase 
deficiency, has highlighted the cardinal role of estradiol (but 
not testosterone) in both boys and girls in the pubertal growth 
spurt, completion of epiphyseal maturation, and normal skel­
etal proportions and mineralization. Individuals with a muta­
tion in the ERα gene (ESR1) or the CYP19A1 gene encoding 
aromatase continue to grow, lack a pubertal growth spurt, and 
have open epiphyses and osteopenia.301–303 Estrogen treatment 
of men with ar

eopenia.301–303 Estrogen treatment 
of men with aromatase deficiency leads to epiphyseal closure, 
cessation of growth, and a striking increase in bone mass.304–306 
Patients with aromatase excess, who produce excess estrogen, 
have advanced skeletal maturation and rapid growth and ulti­
mately reach short adult stature.307
Although estradiol secreted by the ovary has long been recog­
nized as the major sex steroid responsible for the pubertal growth 
spurt, skeletal maturation, and bone mineral

owth 
spurt, skeletal maturation, and bone mineral accrual in females, 
until the detection of the rare human genetic defects in estrogen 
synthesis or action, conventional wisdom dictated that testos­
terone mediated these maturational changes during puberty in 
males. Now it is known that estrogen (not androgen) is the critical 
sex hormone in males and females in the pubertal growth spurt, 
skeletal maturation, accrual of peak bone mass, and maintenance 
of bone mass in the adult. Estrogen st

aintenance 
of bone mass in the adult. Estrogen stimulates chondrogenesis in 
the epiphyseal growth plate, increasing pubertal linear growth.270 
At puberty, estrogen promotes skeletal maturation and the gradual 
progressive closure of the epiphyseal growth plate.159 The use of a 
supersensitive assay for plasma estradiol in prepubertal and puber­
tal boys revealed a high positive correlation between estradiol con­
centrations and peak growth velocity (but not serum GH level), 
which was greates

ocity (but not serum GH level), 
which was greatest about 3 years after the onset of puberty,268 
1058
further implicating estrogen in the pubertal growth spurt and 
skeletal maturation of boys and girls.
There are estrogen receptors, both ERα and ERβ, in the growth 
plate chondrocytes.299 Histologic studies of the bone and carti­
lage of rodents treated with corticosteroids or estrogen and clinical 
evaluations of children with precocious puberty su

evaluations of children with precocious puberty support the the­
ory that senescence of the growth plate occurs because of estrogen 
exposure in precocious puberty, causing decreased growth during 
treatment with GnRH agonists.308
The high rate of bone turnover in early puberty followed by a 
decrease in periosteal apposition and endosteal resorption within 
cortical bone and decreased bone remodeling within cortical and 
cancellous bone mediated by apoptosis of chondrocytes in the 
growth plate

 by apoptosis of chondrocytes in the 
growth plate and osteoclasts within cortical and cancellous bone 
is mediated in part by estrogen. This leads to a reduction in bone 
turnover markers at menarche, reflecting the closure of the epiph­
yseal growth plates.309
Girls with Turner syndrome without estrogen exposure retain 
elevated markers of bone turnover. Prepubertal girls with Turner 
syndrome tend to lose bone, but that ceases when estrogen ther­
apy begins.310 During puberty and into the thi

er­
apy begins.310 During puberty and into the third decade, estrogen 
has an anabolic effect on the osteoblast and an apoptotic effect on 
the osteoclast, increasing bone mineral acquisition in the axial and 
appendicular skeletons. Evolutionary theory suggests that positive 
effects of estrogen on bone density, added to mechanical loading, 
allow women to carry increased weight for pregnancy and lacta­
tion; this process is unnecessary after reproduction, and osteopo­
rosis becomes more common

production, and osteopo­
rosis becomes more common at menopause.311
Testosterone may also have a direct action on bone in the 
human male, because ARs are found in human tibial growth 
plates in osteoblasts and chondrocytes, osteocytes, mononuclear 
cells, and endothelial cells of blood vessels in the bone marrow.312 
Androgens that cannot be aromatized to estrogen still cause an 
increase in growth rate, presumably due to interaction with these 
receptors. The greater increase in periosteal bon

receptors. The greater increase in periosteal bone deposition, the 
resultant thickening of cortical bone and greater bone strength, 
and the greater bone dimensions in boys probably result from 
direct effects of testosterone. Androgens may protect men against 
osteoporosis by maintenance of cancellous bone mass and expan­
sion of cortical bone.
A pubertal growth spurt leading to adult height close to that 
of genotypic men occurs in individuals with the complete form 
of androgen resistance, 

s with the complete form 
of androgen resistance, demonstrating the critical role of estrogen 
rather than androgen in the adolescent growth spurt in boys. A 
modest decrease in BMD Z-scores occurs in the spine but not 
in the hip based on age-specific female standard values, but the 
reductions are greater when male standards are used. Affected 
women have an increased prevalence of fractures, even with estro­
gen replacement. This suggests that lack of a direct effect of tes­
tosterone on the 

 lack of a direct effect of tes­
tosterone on the skeleton, especially the spine, has a part in the 
defects in bone mineralization observed in women with complete 
androgen insensitivity313 (see Table 26.8). 
Growth Hormone and Growth Factors
GH secretion approximately doubles during puberty in boys 
and girls in the basal state or after stimulation but decreases after 
pubertal development. Remarkably, peak values after hexarelin, a 
6–amino acid GH-releasing peptide (or GH secretagogue) stimu

id GH-releasing peptide (or GH secretagogue) stimu­
lates as much GH secretion in prepuberty as in puberty. The greater 
elevation in girls starts at an earlier age and pubertal stage than in 
boys due to the earlier onset of puberty in girls. GH secretion 
increases coincident with the onset of breast development (Tanner 
stage 2) and is maximal at Tanner stage 3 to 4 breast development; 
in boys, GH rises later and peaks at stage 4 genital development. 
GH secretion and IGF1 levels decrease af

lopment. 
GH secretion and IGF1 levels decrease after late puberty in both 
sexes. Adolescents of normal height have an inverse relationship 
between weight and GH levels. Increased GH pulse amplitude 
and content of GH secreted per pulse (but not frequency, meta­
bolic clearance rate, or intersecretory burst interval and half-life of 
GH) in the basal state are mainly responsible for the augmented 
GH levels.314
The increase in estradiol at puberty, which in boys results from 
testicular secret

rty, which in boys results from 
testicular secretion and extraglandular synthesis from testosterone 
and androstenedione and in girls from secretion by the ovaries, 
is the principal mediator of the increase in pulse amplitude and 
amount of GH secreted per pulse. Administration of exogenous 
androgens in delayed puberty raises GH secretion. Transdermal 
application of testosterone increases spontaneous GH secre­
tion overnight independent of growth hormone–releasing hor­
mone (GHRH), because i

owth hormone–releasing hor­
mone (GHRH), because infusion of GnRH antagonist does not 
affect this phenomenon.315 The effect of testosterone is mediated 
mainly through its conversion to estradiol, because treatment of 
late pubertal boys with tamoxifen, an estrogen receptor blocker, 
causes smaller GH secretory peaks and fewer GH secretory epi­
sodes. Exogenous estrogen increases the peak GH reached after 
insulin-induced hypoglycemia, exercise, and arginine, a priming 
effect that is used in c

 and arginine, a priming 
effect that is used in clinical practice, because estrogen adminis­
tered before a provocative test in prepubertal subjects increases the 
GH response. Androgens that cannot be aromatized to estrogen 
(e.g., oxandrolone, dihydrotestosterone) have less effect on GH 
secretion; however, androgen blockade with flutamide increases 
GH secretion. Dihydrotestosterone, which is not aromatized to 
estrogen, does not increase GH secretion or the plasma concen­
tration of IGF1 an

secretion or the plasma concen­
tration of IGF1 and may decrease the integrated GH secretion, 
but it still stimulates increased growth rate, suggesting a possible 
direct effect of androgen on pubertal growth independent of GH 
or estradiol.300 Increased GH secretion also occurs in sexual pre­
cocity. GH secretion decreases with the fall in gonadal steroid lev­
els after treatment of CPP with potent GnRH agonists.316
GH deficiency or GH resistance causes an attenuated puber­
tal growth spurt, i

ce causes an attenuated puber­
tal growth spurt, indicating the importance of GH and IGF1 
in this process. Severe primary or secondary hypogonadism 
leads to a minimal or absent growth spurt, demonstrating the 
primary role of gonadal steroids in pubertal growth. Hypopitu­
itary patients deficient in GH and gonadotropins do not have 
an adolescent growth spurt when GH alone is replaced; gonadal 
steroids must also be given, substantiating the interaction of GH 
and gonadal steroids in the puber

teraction of GH 
and gonadal steroids in the pubertal growth spurt. In normal 
puberty, neither the magnitude of the increase in GH secretion 
nor the concentration of plasma IGF1 correlates with the PHV 
of the pubertal growth spurt. Although a threshold level of GH 
secretion is necessary, the extent of the growth spurt correlates 
with gonadal sex steroid secretion. Individuals with both CPP 
and GH deficiency (usually as a consequence of cranial irradia­
tion for a brain tumor) have a growth

ial irradia­
tion for a brain tumor) have a growth spurt clinically indistin­
guishable from that of CPP and normal GH secretion.300 After 
treatment with a GnRH agonist for sexual precocity, growth 
velocity in patients with GH deficiency and CPP is decreased 
and pubertal progression is suppressed, illustrating the direct 
effect of gonadal steroids, principally estradiol, on the pubertal 
growth spurt.
Urinary GH excretion reflects serum levels and changes occur­
ring with pubertal developmen

s and changes occur­
ring with pubertal development. A peak is reached at pubertal 
stage 3 to 4. The level is higher in boys than in girls. 
1059
Growth hormone–binding protein (GHBP) has the same amino 
acid sequence as the extracellular component of the GH recep­
tor (GHR), and serum concentrations are directly related to the 
number of GHRs. In normal children, the plasma GHBP level is 
inversely related to 24-hou

 plasma GHBP level is 
inversely related to 24-hour GH secretion. The serum GHBP level 
rises early in childhood and also through puberty in some cross-
sectional studies, but not in others. Because plasma GHBP does 
not change appreciably with the onset of puberty, at the time of 
the pubertal growth spurt there is a relative increase in unbound 
(free) GH in relation to GH bound to GHBP. GHBP is related to 
adiposity, and it may be this factor that accounts for the increased 
levels of GHBP in

that accounts for the increased 
levels of GHBP in girls compared with boys, the rise in GHBP in 
girls with precocious puberty, and the negative influence of testos­
terone on GHBP levels.317 
Insulin-Like Growth Factor Type 1
Concentrations of IGF1 rise during puberty to levels higher 
than those of prepuberty or adulthood; they remain elevated 
past the time of PHV, with a peak attained 1 or 2 years after the 
pubertal growth spurt (later in boys than in girls) and then fall 
to normal adult 

oys than in girls) and then fall 
to normal adult levels.316,318 The pattern of the GH-dependent 
serum levels of IGF-binding protein 3 (IGFBP3) in pubertal 
development is similar to that of serum319 IGF1. However, 
serum IGFBP3 concentrations correlate with BMI even though 
IGF1 does not. Measurement of free IGF1 shows the same pat­
tern of change with development as does that of total IGF1, 
a slow rise in serum free IGF1 in prepuberty followed by a 
steeper rise during puberty. A decrease of

d by a 
steeper rise during puberty. A decrease of free IGF1 is associ­
ated with age in the later stages of puberty.319,320 The increase in 
the serum ratio of IGF1 to IGFBP3 at the time of the pubertal 
growth spurt appears to result from production, because prote­
olysis of IGFBP3 does not change in puberty in normal chil­
dren. The testosterone level in boys and the estradiol level in 
girls correlate with the rise in IGF1 concentration, but gonadal 
steroids are not the direct cause of the 

gonadal 
steroids are not the direct cause of the increase in circulating 
IGF1 levels; rather, GH secretion approximately doubles dur­
ing puberty owing to the effect of estrogen causing augmented 
release of GH.
Plasma IGF1 concentrations are high for chronologic age 
in sexual precocity and low in delayed puberty. Estrogen medi­
ates the pubertal increase in IGF1 concentration through 
increased secretion of GH, with an additional effect through 
the gonadal steroid–induced local generation o

gh 
the gonadal steroid–induced local generation of IGF1 in carti­
lage and bone. Treatment with GnRH agonist in a 16-year-old 
boy with a homozygous mutation in the WSXWS-like motif 
of the human GHR causing Laron syndrome led to a further 
decrease in the already low serum levels of IGF1 and IGFBP3, 
which did not reverse with dihydrotestosterone treatment, sug­
gesting a direct effect of estradiol on IGF1 production.299,321 
Children with CPP treated with a GnRH agonist showed sup­
pression o

treated with a GnRH agonist showed sup­
pression of the untreated elevated serum GH concentrations 
and a decrease in plasma IGF1 concentrations, although not 
to prepubertal values, supporting the concept that GH is the 
major (but not the only) factor that raises circulating IGF1 
levels in puberty.316
A confounding factor is the relative roles of hepatic-generated 
circulating IGF1 (i.e., endocrine role) and of locally produced 
IGF1 (i.e., paracrine/autocrine role) in linear growth. For exam

racrine/autocrine role) in linear growth. For exam­
ple, mice with a totally deleted hepatic IGF1 gene have strikingly 
reduced circulating levels of IGF1 but normal postnatal body and 
bone growth.322
GH stimulates local IGF1 production in resting zone chon­
drocytes, located at the epiphyseal end of the growth plate in the 
area known as the reserve zone or stem cell zone, through GHRs 
in the chondrocytes. This IGF1 production stimulates, through 
autocrine and paracrine effects, the clonal e

ugh 
autocrine and paracrine effects, the clonal expansion of proliferat­
ing chondrocytes derived from the resting chondrocyte or germ 
cells. GH and IGF1 can reduce the stem cell cycle time, proliferat­
ing cell cycle time and duration of the hypertrophic phase, a phase 
that leads to apoptosis, leaving the cells serving as a scaffold for the 
mineralization and production of new bone. 
Other Hormones
There are glucocorticoid receptors in human growth plates, mostly 
in hypertrophic chondrocyt

 growth plates, mostly 
in hypertrophic chondrocytes. However, children with chronic 
adrenal insufficiency who receive appropriate replacement therapy 
have a normal pubertal growth spurt despite deficient adrenal 
androgen secretion, indicating a minimal impact of these adrenal 
androgens on normal growth at puberty.290
Hypothyroid subjects lack a pubertal growth spurt even when 
the disorder is accompanied by sexual precocity.323 Thyroid hor­
mone has a permissive role in the pubertal growth 

mone has a permissive role in the pubertal growth spurt but is a 
requisite for normal growth. Hypothyroidism decreases GH secre­
tion and affects growth indirectly. However, thyroid hormone also 
interacts with the thyroid hormone receptors α1 and β, whose pro­
teins are found in early proliferating chondrocytes of the human 
growth plate and in the messenger ribonucleic acid (mRNA) 
found in other developing stages of chondrocytes and osteoblasts. 
Thyroid hormones also interact with the local

ts. 
Thyroid hormones also interact with the local effects of IGF1 and 
GH at the growth plate.299 A longitudinal cohort study of 323 
children demonstrated that higher prepubertal free T4 (FT4) and 
total T4 (TT4) are associated with earlier pubarche.324 
Central Nervous System and Puberty
Two independent but associated processes (controlled by differ­
ent mechanisms but closely linked temporally) are involved in the 
increased secretion of sex steroids in the peripubertal and pubertal 
periods

steroids in the peripubertal and pubertal 
periods. In the first process, adrenarche, the increase in adrenal 
androgen secretion301,325 precedes by approximately 2 years the 
second process, gonadarche, which is a consequence of the puber­
tal reactivation of the hypothalamic-pituitary gonadotropin-
gonadal apparatus.290,325,326
The onset of puberty is a consequence of maturational changes, 
including the development of secondary sexual characteristics, the 
adolescent growth spurt, the attainm

ristics, the 
adolescent growth spurt, the attainment of fertility, and psychoso­
cial changes, all emanating from the disinhibition or reaugmenta­
tion of the hypothalamic GnRH pulse generator and gonadotropin 
secretion, causing an increase in gonadal steroid secretion256,327 
(Table 26.10). The events characterizing the development of 
gonadal function can be viewed as a continuum extending from 
sexual differentiation and the ontogenesis of the hypothalamic-
pituitary gonadotropin-gonadal sy

he hypothalamic-
pituitary gonadotropin-gonadal system during fetal life and early 
infancy,126,254,326,327 through a juvenile pause (in which the sys­
tem is suppressed to a low level of activity,326 discussed later), 
to the attainment of full sexual maturation and fertility during 
puberty, leading to the ability to procreate (Fig. 26.21). In this 
light, puberty does not represent the initiation or first occurrence 
of pulsatile secretion of GnRH or pituitary gonadotropins but the 
reactivat

GnRH or pituitary gonadotropins but the 
reactivation or disinhibition of GnRH neurosecretory neurons in 
the medial basal hypothalamus and the endogenous, apparently 
self-sustaining oscillatory secretion of GnRH after the period of 
quiescent activity during childhood. An increase in the pulsatile 
1060 SECTION VI    Endocrinology and the Life Span
release of GnRH heralds the onset of puberty in primates and 
other mammals.126,327,328 The CNS, and not the hypothalamic 
GnRH pulse generator, pi

and not the hypothalamic 
GnRH pulse generator, pituitary gland, gonads, or gonadal ste­
roid target tissues, restrains activation of the hypothalamic-pitu­
itary-gonadal system during the prepubertal years.
Certain CNS lesions involving the hypothalamus and nearby 
structures can advance or delay the onset of human puberty.256,326 
CPP, including cyclic ovulation in girls and spermatogenesis in 
boys, can result from a variety of CNS disorders. Several regula­
tory systems control puberty (Fig.

Several regula­
tory systems control puberty (Fig. 26.22):
	1.	 In primates, the neural component controlling gonadotropin 
secretion resides in the medial basal hypothalamus, including the 
arcuate region. There are about 1500 to 2000 transducer GnRH 
neurosecretory neurons, which are not segregated into a specific 
nucleus but are functionally interconnected. These GnRH neu­
rons comprise the GnRH pulse generator, which drives and con­
trols the pituitary gonadal components, stimulates the re

he pituitary gonadal components, stimulates the release of 
LH and FSH, and translates neural signals into a periodic, oscil­
latory chemical signal, GnRH, in a coordinated manner. These 
pulses appear to be generated by a propagated depolarization, 
the firing of action potentials in individual cells, and the result­
ing influx of calcium through L-type calcium channels.329
	2.	 In response to the GnRH rhythmic signal, the pituitary gonad­
otrophs, which contain the seven-transmembrane domain 

phs, which contain the seven-transmembrane domain Gs-
coupled LH/hCG receptors (LHCGRs),330 release LH and 
FSH in a pulsatile manner. Each LH and FSH pulse is induced 
by a pulse of GnRH.
  Hypothesis of the Control of the Onset of 
Human Puberty
	1.	 Central dogma: The CNS exercises the only major restraint on the 
onset of puberty. The neuroendocrine control of puberty is mediated 
by the hypothalamic GnRH-secreting neurosecretory neurons in 
the medial basal hypothalamus, which act as an e

the medial basal hypothalamus, which act as an endogenous pulse 
generator (oscillator).
	2.	 The development of reproductive function is a continuum extending 
from sexual differentiation and the ontogeny of the hypothalamic-
pituitary-gonadal system in the fetus to the attainment of full sexual 
maturation and fertility.
	3.	 In the prepubertal child the GnRH pulse generator, operative in the 
fetus and infant, functions at a low level of activity (the juvenile 
pause) because of steroid-i

ctivity (the juvenile 
pause) because of steroid-independent and steroid-dependent 
inhibitory mechanisms.
	4.	 Puberty represents the reactivation (disinhibition) of the CNS 
suppressed GnRH pulse generator characteristic of late infancy 
and childhood, leading to increased amplitude and frequency of 
GnRH pulsatile discharges, to increased stimulation of the pituitary 
gonadotropes, and finally to gonadal maturation. Hormonally, puberty 
is initiated by the recrudescence of augmented pulsatil

itiated by the recrudescence of augmented pulsatile GnRH and 
gonadotropin secretion, mainly at night.
CNS, Central nervous system; GnRH, gonadotropin-releasing hormone.
From Grumbach MM, Kaplan SL. The neuroendocrinology of human puberty: an ontoge­
netic perspective. In: Grumbach MM, Sizonenko PC, Aubert ML, eds. Control of the Onset 
of Puberty. Baltimore, MD: Williams & Wilkins, 1990:1–68. © 1990, the Williams & Wilkins 
Co., Baltimore.
   
TABLE 26.10
A
NK3R
MKRN3?

ABLE 26.10
A
NK3R
MKRN3?
KNDy
neu
neu
neu
eu
eu
euron
GnR
GnR
G
H
neu
neu
eu
neu
euron
ron
ron
ron
ro
ro
ron
r n
Hig
Hig
Hig
Higher
he
he -order
der
er
der
er
er
er
er
er
e
neu
neu
neu
neu
neu
eu
ne
n
ron
r
MKRN3?
MKRN3?
KISS1R
GnRH
Pituitary
Gon
Go
Go
Go
Go
Go
Go
Go
Go ad
adotroph
ph
p
Gonads
Luteinizing hormone
Follicle-stimulating hormone
Puberty begins
INF
ME
POA
• Fig. 26.21  Timing of puberty. A pivotal event in the onset of puberty in mammals is the re

event in the onset of puberty in mammals is the resumption of pulsatile release of gonadotropin-releasing hormone 
(GnRH) from neurons of the hypothalamus. Known influences on the timing of the onset of puberty in mammals include the photoperiod, leptin levels, and 
the increased expression of neurokinin. (A) Kisspeptin, and their receptors (NK3R and KISS1R, respectively). Abreu and associates427 implicate MKRN3, 
a protein that is believed to mediate ubiquitination, in puberty onset. In contras

diate ubiquitination, in puberty onset. In contrast with kisspeptin and neurokinin B, which stimulate the commencement 
of puberty, MKRN3 seems to inhibit puberty: Abreu and associates427 show that mutations in MKRN3 predicted to cause loss of function of the protein 
cause central precocious puberty. INF, infundibular nucleus; KNDy, kisspeptin–neurokinin B–dynorphin; MBH, medial basal hypothalamus; ME, median 
eminence; POA, preoptic area. 
1061
F

Fetus
20 weeks’ gestation
Intrinsic CNS inhibitor
and sex steroid feedback
GnRH pulses
Pituitary
B
Gonadotropes
FSH and LH pulses
MBH GnRH
Neurosecretory neurons
(pulse generator)
MBH GnRH
MBH GnRH
Prepubertal child
CNS inhibition and
estrogen or testosterone
Late pubertal
male and female
estr

e pubertal
90 min pulses
initially mainly
sleep associated
FSH
LH
0
Fetus and early infancy
Prepuberty
Puberty
4
8
0
4
8
Hours
C
0
4
8
• Fig. 26.21, cont’d  (B) Postulated ontogeny of the dual mechanism for the inhibition of puberty. Dashed arrows indicate inhibition. Notice the action of both 
components during the juvenile pause (i.e., prepuberty) (see Fig. 26.45 for the relative roles of the

rty) (see Fig. 26.45 for the relative roles of these two mechanisms during development). (C) Change 
in the pattern of pulsatile follicle-stimulating hormone (FSH) and luteinizing hormone (LH) secretion in the fetus and early infancy, prepuberty, and puberty 
(data from Waldhauser et al). Notice the pulsatile secretion in the fetus and infant and the striking difference in the amplitude of FSH and LH pulses between 
male and female infants. After infancy, the amplitude and frequency of gonadotro

 infancy, the amplitude and frequency of gonadotropin pulses decrease greatly for almost a decade (i.e., juvenile pause) until 
the onset of puberty. (A, from Hughes IA. Releasing the brake on puberty. N Engl J Med. 2013;368:2513–2515; B and C, modified from Grumbach MM, 
Kaplan SL. The neuroendocrinology of human puberty: an ontogenetic perspective. In: Grumbach MM, Sizonenko PC, Aubert ML, eds. Control of the 
Onset of Puberty. Baltimore, MD: Williams & Wilkins; 1990:1–68. Additional data from

illiams & Wilkins; 1990:1–68. Additional data from Waldhauser F, Weissenbacher G, Frisch H, et al. Pulsatile secretion 
of gonadotropins in early infancy. Eur J Pediatr. 1981;137:71–74.)
	3.	 The gonads, which are modulated primarily by the amplitude 
of the gonadotropin pulse, transmit the episodic gonadotropin 
signal into pulsatile secretion of gonadal steroids.331
This control mechanism is common to all mammalian spe­
cies. At the last two levels—the pituitary gland and the gonad—
the targe

evels—the pituitary gland and the gonad—
the target cells contain receptors for the peptide hormones 
that mediate the cellular response to the signal.330,331 Diverse 
adaptive mechanisms and strategies have evolved among species 
and between the sexes that influence the biology and timing of 
puberty. Photoperiodicity and seasonal breeding, biologic clocks, 
and pheromones are integral parts of the pubertal process in some 
species but not humans. The most enlightening studies on the 
neuroendo

s. The most enlightening studies on the 
neuroendocrinology of human puberty have emerged from stud­
ies of humans and nonhuman primates.328,332
1062
Pattern of Gonadotropin Secretion
Tonic Secretion
Tonic, or basal, secretion is regulated by a negative, or inhibitory, 
feedback mechanism in which changes in the concentration of cir­
culating gonadal steroids and inhibin result in reciprocal changes 
in the secretion of pituitary gonadotropins. This 

in the secretion of pituitary gonadotropins. This is the pattern of 
secretion in the male and one of the control mechanisms in the 
female. Clinical studies reveal that testosterone and estradiol in 
the male have independent effects on LH secretion. Inhibition of 
LH by testosterone requires aromatization for its pituitary but not 
its hypothalamic effects, and estradiol-induced negative feedback 
on LH occurs at the level of the hypothalamus.333
In the female, cyclic secretion involves a posi

33
In the female, cyclic secretion involves a positive, or stimu­
latory, feedback mechanism in which an increase in circulating 
estrogens, to a critical level and of sufficient duration, initiates 
the synchronous release of LH and FSH (i.e., preovulatory LH 
surge) that is characteristic of the normal adult woman before 
menopause. 
Pulsatile Secretion
The GnRH Pulse Generator
GnRH. Generation of the GnRH pulse is an intrinsic property 
of the GnRH neurosecretory neuronal network, and other f

 GnRH neurosecretory neuronal network, and other factors 
modulate the fundamental autorhythmicity of the GnRH neu­
ron, including the downstream effects of cyclic adenosine mono­
phosphate (cAMP)–gated cation channels on the regulation of 
pulsatility by cAMP.334 The immortalized GnRH neurosecretory 
neuronal cell line and cultured monkey GnRH-I neurons exhibit 
spontaneous pulsatile release of GnRH at a frequency similar to 
that observed in  vivo. Patch-clamped primary GnRH neurons 
show diso

ivo. Patch-clamped primary GnRH neurons 
show disordered patterns of release that are sensitive to increased 
extracellular potassium; firing activity can be stimulated by expo­
sure to estrogen in a manner that appears to function through the 
estrogen receptor.335
The secretion of FSH and LH is always pulsatile or episodic, 
regardless of developmental stage, due to the pulsatility of the 
GnRH pulse generator. The pulsatile secretion of immunoreac­
tive FSH in normal adults is less prominent 

reac­
tive FSH in normal adults is less prominent than that of LH; 
this is attributed in part to the longer half-life of FSH compared 
with LH, to differences in the factors that modulate the action of 
GnRH on FSH and LH release by the gonadotrophs (especially 
gonadal steroids, inhibin, and possibly activin and follistatin), 
and to intrinsic differences in the secretory pattern of the two 
gonadotropins. For example, a change in the frequency of GnRH 
pulses can modify the ratio of FSH to LH

 of GnRH 
pulses can modify the ratio of FSH to LH released; midfollicu­
lar-phase concentrations of estradiol and adult male concentra­
tions of plasma testosterone have a greater inhibitory effect on the 
response of FSH to pulsatile injections of GnRH, compared with 
that of LH.
Intermittent or pulsatile administration (e.g., GnRH, 1 μg/
minute for 6 minute/hour) induces pulsatile release of LH and 
FSH in adult monkeys in which hypothalamic lesions have 
obliterated the arcuate nucleus regio

esions have 
obliterated the arcuate nucleus region and eliminated endoge­
nous GnRH secretion.336 Continuous infusion of GnRH inhib­
its gonadotropin secretion because of desensitization of GnRH 
receptors on the gonadotroph. Pulsatile GnRH administra­
tion reestablished gonadotropin secretion in animals in which 
gonadotropin secretion was suppressed by the continuous infu­
sion of GnRH (Fig. 26.23). The GnRH signal to the pituitary 
gonadotrophs of the adult is frequency coded.
The GnRH neuro

hs of the adult is frequency coded.
The GnRH neurosecretory neurons of the hypothalamic 
GnRH pulse generator that arise in the olfactory placode exhibit 
spontaneous autorhythmicity and function intrinsically as a 
neuronal oscillator for entrainment of the repetitive release of 
GnRH. The autorhythmicity in the GnRH neurosecretory neu­
rons involves cAMP and cyclic nucleotide-gated cation channels 
associated with oscillatory increases in intracellular calcium ions 
(Ca2+), a hallmark of neuro

cellular calcium ions 
(Ca2+), a hallmark of neurosecretion and gap junctional commu­
nication.337 Moreover, the immortalized GnRH neuronal cell line 
contains neuronal nitric oxide (NO) synthase, and NO gener­
ated by GnRH neurons may act as an intercellular or intracellular 
MBH
GnRH
neurons
GnRH Oscillator
(Pulse generator):
   Frequency coded:
   largely synchronous
   intermittent discharge
Portal vessels
GnRH
Hormonal signal: pulsatile
Pituitary
Frequency and ampl

ulsatile
Pituitary
Frequency and amplitude
modulated
Gonads
Testis
Ovaries
Amplitude modulated
Estradiol
ACT via gonadal steroid
receptor
FSH LH
Signal
   Pulsatile secretion
Activation of gonadal
gonadotropin receptors
• Fig. 26.22  Organization and characteristics of the hypothalamic-pituitary 
gonadotroph-gonadal system. The medial basal hypothalamus (MBH) 
contains the transducer gonadotropin-releasing hormone (GnRH) neu­
rosecretory neu

opin-releasing hormone (GnRH) neu­
rosecretory neurons. These neurons translate neural signals into a peri­
odic, oscillatory chemical signal, GnRH. This MBH complex functions as a 
GnRH pulse generator (oscillator), which is frequency coded and releases 
GnRH from its axon terminals at the median eminence as a largely syn­
chronous, intermittent discharge into the primary capillary plexus of the 
hypothalamic-hypophyseal portal circulation. The GnRH pulse generator 
is influenced by biogenic am

GnRH pulse generator 
is influenced by biogenic amine neurotransmitters, peptidergic neuro­
modulators, neuroexcitatory amino acids, and neural pathways. During 
the follicular phase in women and men, a GnRH pulse (estimated indirectly 
by monitoring luteinizing hormone [LH] pulses in peripheral blood) occurs 
approximately every 90 to 120 minutes throughout the day. Changes in 
the frequency and probably in the amplitude of the GnRH secretory epi­
sodes modulate the pattern of LH and follicle-s

i­
sodes modulate the pattern of LH and follicle-stimulating hormone (FSH). 
The major site of action of testosterone and progesterone is on the GnRH 
pulse generator, because these two classes of steroids decrease LH pulse 
frequency, but a pituitary site of action has also been described. Estrogens 
have major direct inhibitory and stimulatory effects on the GnRH-primed 
pituitary gonadotroph; the inhibitory or negative feedback action is associ­
ated with a decrease in the frequency and the a

i­
ated with a decrease in the frequency and the amplitude of pituitary LH 
secretion. Evidence supports the negative and positive feedback action 
of estrogen on the GnRH pulse generator. Inhibin has a direct inhibitory 
effect on the pituitary gland and the secretion of FSH. The secretion of 
gonadal steroids by the gonads is controlled mainly by the amplitude of 
the gonadotropin signal. (Modified from Grumbach MM, Kaplan SL. The 
neuroendocrinology of human puberty: an ontogenetic perspectiv

nology of human puberty: an ontogenetic perspective. In: 
Grumbach MM, Sizonenko PC, Aubert ML, eds. Control of the Onset of 
Puberty. Baltimore, MD: Williams & Wilkins; 1990:1–68.)
1063
messenger.338 GnRH acting as an autocrine factor may play a 
role in the synchronization mechanism. GnRH is synthesized in 
these neurons and released episodically from axon terminals at the 
median eminence into the primary plexus of the hypothalamic-
hypophyseal 

e primary plexus of the hypothalamic-
hypophyseal portal circulation; it is then transported by the portal 
vessels to the anterior pituitary gland to produce the pulsatile LH 
and FSH secretion. Remarkably, GnRH neurons exhibit charac­
teristics of both axons and dendrites and are known as dendrons.339 
Ionotropic γ-aminobutyric acid (GABA) and glutamate receptors 
on the dendrons can be stimulated to depolarize the cells to pro­
duce action potentials. 
Gonadotropin-Inhibitory Hormone. Gonadot

entials. 
Gonadotropin-Inhibitory Hormone. Gonadotropin-inhibi­
tory hormone (GnIH), a peptide first described in quail but now 
homologues of GnIH known in most animals, is a member of the 
RFamide family that has an RFamide (Arg-Phe-NH2) motif at 
the C-terminus.340 The family is called RFamide–related peptides 
(RFRPs); kisspeptin is one of the neurotransmitters belonging 
to this family. The human homologue is known as RFRP3 and is 
coded for by the NPVF gene. The mammalian receptor for RFP3

 by the NPVF gene. The mammalian receptor for RFP3 
is GPR147, a seven-transmembrane domain G protein–coupled 
receptor.341 RFP3 and its receptor are found in the human dorso­
medial hypothalamus with axons terminating both on the GnRH 
neuron and the median eminence, suggesting a role on the hypo­
thalamus and the pituitary in human beings. Surprisingly, the first 
survey of isolated hypogonadotropic hypogonadism (IHH) and 
CPP in a well-characterized cohort of patients noted a 3-nucleotide 
in

erized cohort of patients noted a 3-nucleotide 
in-frame deletion in the NPVF gene (p.I71K72), associated with a 
reduced risk for the occurrence of CPP.342 Further study is needed 
to determine if RFP3 may play a role in the treatment of disorders 
Kisspeptins and KISS1R. Kisspeptins and their receptors 
(KISS1R or GRP54) in the CNS hypothalamic-pituitary-gonadal 
axis play a major role in reproduction (Figs. 26.24 and 26.25; see 
Fig. 26.21).343,344 KISS1 is a human metastasis sup

ig. 26.21).343,344 KISS1 is a human metastasis suppressor gene at 
gene map locus 19p13.3, and KISS1 mRNA is found in placenta, 
testes, pancreas, liver, small intestine, and the brain, mainly in the 
hypothalamus and basal ganglion.345,346 KISS1 mRNA is pres­
ent in the primate in the medial arcuate nucleus only, but in the 
mouse it occurs in the arcuate, periventricular, and anteroventral 
periventricular (AVPV) nuclei, regions important in reproductive 
function.347,348 While Kiss neurons do

productive 
function.347,348 While Kiss neurons do not affect the migration of 
GnRh neurons from the olfactory placode, under the influence of 
steroids they connect to the GnRH neurons once they reach the 
hypothalamus. The product of the KISS1 gene is a 145–amino 
acid peptide, but the cleaved and secreted product is a 54–amino 
acid protein known as metastin or kisspeptin, which binds to an 
endogenous receptor.
The KISS1 receptor, KISS1R (formerly called GPR54), is a G 
protein–coupled rece

rmerly called GPR54), is a G 
protein–coupled receptor found in the brain, mainly in the hypo­
thalamus and basal ganglia, and in the placenta, from which it was 
first isolated and sequenced. KISS1R is coexpressed within GnRH 
neurons in the rat, in the medial and lateral sections of the arcuate 
nucleus and the ventral aspect of the ventromedial hypothalamus 
in mice, and in primates.347,349 Although the expression of KISS1R 
mRNA does not increase with development in the mouse, kiss­
peptin m

ease with development in the mouse, kiss­
peptin mRNA increases dramatically in the AVPV nucleus, and the 
number of receptors responsive to kisspeptin increase with develop­
ment.350 Activation of GnRH neurons by kisspeptin at puberty in 
the mouse reflects a dual process involving an increase in kisspeptin 
input from the AVPV and a post-transcriptional change in KISS1R 
signaling within the GnRH neuron. Increases in kisspeptin lead to 
increased GnRH release by means of an interneuron, rather

ed GnRH release by means of an interneuron, rather than 
acting directly on the GnRH-secreting neurons.351 It is released in 
a pulsatile manner as is GNRH.
Mice transfected with mutant KISS1R genes exhibited hypo­
gonadotropic hypogonadism, although they had normal content 
of GnRH in their hypothalamus and were responsive to GnRH 
or gonadotropin administration, suggesting normal function 
of the gonadotroph GnRH receptors and the gonadal LH and 
FSH receptors despite the mutation.352 The inta

d 
FSH receptors despite the mutation.352 The intact mouse also 
releases significant LH boluses after kisspeptin administration, an 
effect that is abolished in the KISS1R−/− mouse, which lacks the 
receptor.349
Underfed prepubertal mice have decreased hypothalamic 
KISS1, but kisspeptin administration leads to increases in KISS1R 
mRNA, which in turn leads to increased in vivo LH secretion 
and in vitro GnRH secretion.353 Chronic kisspeptin administra­
tion to these underfed mice restores vagi

inistra­
tion to these underfed mice restores vaginal opening and enhances 
gonadotropin and estrogen responses.
Administration of kisspeptin into the rostral preoptic area 
(RPOA), medial preoptic area (MPOA), paraventricular nuclei 
(PVN), and arcuate nuclei of the hypothalamus of male adult 
rats increased plasma LH and testosterone substantially,354 and 
intracerebral kisspeptin administration stimulated the release of 
FSH, albeit at a far higher dose than needed to stimulate LH 
release.35

igher dose than needed to stimulate LH 
release.355,356 Because the release of FSH is abolished with block­
ade of GnRH, GnRH modulates the central actions of kisspeptin 
in the rodent. In the rat, the mRNA for kisspeptin and its receptor 
increases at puberty, and administration of intracerebral injection 
of kisspeptin in prepubertal female rats caused large peaks of LH 
and advanced vaginal opening as a sign of pubertal development 
and premature ovulation.357,358 The ovulation elicited by pe

ure ovulation.357,358 The ovulation elicited by periph­
eral kisspeptin in the prepubertal female rat is abolished by block­
ade of GnRH.359
10
5
0
5
10
15
20
25
30
35
20
200
15
150
10
100
5
50
0
0
999
Pulsatile
Pulsatile
LH (ng/mL)
Days
• Fig. 26.23  The Knobil paradigm. Effect of pulsatile administration of 
gonadotropin-releasing hormone (GnRH) in contrast to continuous infu­
sion of GnRH in adult oophorectomized rhesus monkeys in which gonado­
tropin secretion has been

monkeys in which gonado­
tropin secretion has been abolished by lesions that ablated the medial 
basal hypothalamic GnRH pulse generator. Notice the high concentrations 
of plasma luteinizing hormone (LH) and follicle-stimulating hormone (FSH) 
in monkeys given one GnRH pulse per hour, the suppression of gonado­
tropin secretion by continuous infusion of GnRH even though the total 
dose of GnRH was the same, and the restoration of FSH and LH secretion 
when the pulsatile mode of GnRH administrat

etion 
when the pulsatile mode of GnRH administration was reinitiated. (From 
Belchetz PE, Plant TM, Nakai Y, et al. Hypophysial responses to continu­
ous and intermittent delivery of hypothalamic gonadotropin releasing hor­
mone. Science. 1978;202:631–633.)
1064
KISS1 and KISS1R mRNA expression is found in the pos­
terior two-thirds of the arcuate nucleus of the monkey.360 Kis­
speptin-containing axons make only infrequent contacts with 
GnRH neuron

ns make only infrequent contacts with 
GnRH neurons in the medial basal hypothalamus, whereas in 
the median eminence, kisspeptin and GnRH axons were found 
extensively, and intimate association with GnRH contacts on 
kisspeptin perikarya and dendrites was observed. Nonsynaptic 
pathways of communication in the median eminence may offer 
a possible mechanism of kisspeptin regulation of GnRH release 
and provide an anatomic basis for reciprocal control of kisspeptin 
neuronal activity by GnRH.361

ntrol of kisspeptin 
neuronal activity by GnRH.361 Although KISS1 increases with 
puberty in intact male and female monkeys, KISS1R mRNA 
levels increase in intact females but not in agonadal male mon­
keys. Administration of KISS1 through intracerebral catheters 
to GnRH-primed juvenile female rhesus monkeys stimulates 
GnRH release, but this release is abolished by infusion of GnRH 
antagonist. These findings have led to the postulation that KISS1 
signaling through KISS1R of primate hypothala

SS1 
signaling through KISS1R of primate hypothalamus may be 
activated at the end of the juvenile pause and may contribute to 
the pubertal resurgence of pulsatile GnRH release at puberty.347 
Short-term administration of kisspeptin raises gonadotropin 
secretion in men and nonhuman primates (Fig. 26.26A). Just as 
continuous infusion of GnRH suppresses GnRH release, con­
tinuous infusion of kisspeptin decreased the response of gonado­
trophs in the agonadal male monkey to boluses of kisspeptin

 the agonadal male monkey to boluses of kisspeptin. 
However, the release of FSH and LH after a bolus of N-methyl-
d-aspartate (NMDA) or GnRH was maintained demonstrated 
that the desensitization of the KISS1Rs was selective for kiss­
peptin administration362 (see Fig. 26.26B). This is in contrast to 
the finding that continuous infusion of kisspeptin in men and 
a woman with TAC3 mutations and hypogonadotropic hypogo­
nadism experienced renewed pulsatile release of gonadotropins, 
and the men h

pulsatile release of gonadotropins, 
and the men had increased secretion of inhibin B.363 Treatment 
with kisspeptin for a variety of disorders of puberty and repro­
duction appears promising, but the pattern of optimal adminis­
tration is not yet clear.
Administration of testosterone to castrated young male mon­
keys led to decreased kisspeptin mRNA in the mediobasal hypo­
thalamus but not in the preoptic area. There was no change in 
KISS1R expression. This suggests that feedback inhibition of

ression. This suggests that feedback inhibition of 
Zn
1
95
C3H1
C3H1
C3H1
Zn
Zn
Zn
COOH
NH3
Zn
RING finger
C3C4
MKRN-specific
Cys-His domain
Gln226Thrfs*6
MKRN3 mutations:
Frameshift (6)
Missense (3)
Nonsense (1)
Arg213Glyfs*73
Tyr391*
B
A
Ala162Glyfs*14
Pro161Argfs*10
Pro161Argfs*16
Glu256Glyfs*36
Phe417Ile
Arg365Ser
Cys340Gly
Zn
122
238
265 266
293
311
365
394
423 507 aa
T32I
Q11K
N10K
L102P
Q106R
E90K
A129D
A171T
S217R
Y284C
L314X
S168R
C223R
C279Y
P320L
A139H
L266R
R331X
X399R
L148S
C200Y
R

C279Y
P320L
A139H
L266R
R331X
X399R
L148S
C200Y
R297L
• Fig. 26.24  (A) Inactivating mutations of the gonadotropin-releasing hormone receptor GnRHR (closed 
circles) and the kisspeptin receptor (KISS1R, formerly GPR54) (open circles) were identified in patients with 
isolated hypogonadotropic hypogonadism. The dashed line indicates the intracellular domain of KISS1R. 
The seven-transmembrane G protein–coupled receptor model is used for illustration of both receptors. (B) 
Schematic illustration

ion of both receptors. (B) 
Schematic illustration of mutations found in the MKRN3 gene causing precocious puberty. The amino acid 
positions in the protein are noted by the numbers and frameshift mutations are indicated by red arrows, a 
nonsense mutation is indicated by a black arrow, and the three missense mutations are indicated by green 
arrows. (A, from de Roux N. GnRH receptor and GPR54 inactivation in isolated gonadotropic deficiency. 
Best Pract Res Clin Endocrinol Metab. 2006;20:515–52

st Pract Res Clin Endocrinol Metab. 2006;20:515–528; B, from Macedo DB, Brito VN, Latronico AC. New 
causes of central precocious puberty: the role of the genetic factors. Neuroendocrinology. 2014;100:1–8.)
1065
gonadotropin secretion by testosterone is mediated by kisspeptin 
upstream of the GnRH network.364
KISS1R mRNA is expressed in the pituitary gland, and there 
is evidence that kisspeptin can act directly on the gonadotroph 
to prompt LH sec

 act directly on the gonadotroph 
to prompt LH secretion.355 In sheep, kisspeptin colocalizes to a 
high proportion of GnRH receptor cells in the preoptic area as 
well as various neuronal fibers within the external neurosecretory 
zone of the median eminence. This raises the possibility that both 
kisspeptin and GnRH are secreted into the pituitary portal sys­
tem to affect the pituitary gland.365 There are also single-labeling 
KISSP1R cells in the preoptic area, and the number rises with 
ova

 the preoptic area, and the number rises with 
ovariectomy.
In addition to neurons that specifically secrete kisspepetin, 
other infundibular nucleus neurons in the human being that 
coexpress kisspeptin and neurokinin B as well as B-dynorphin 
(an opioid inhibitor) are known as kisspeptin-neurokinin (KNDy) 
neurons.366 KNDy cells have receptors for neurokinin B and 
B-dynorphin axons and connect with their own KNDy cell and 
other KNDy cells as well as GnRH neurons. It appears that the 
stimula

well as GnRH neurons. It appears that the 
stimulatory role of neurokinin B and the inhibitory action of dyn­
orphin autosynaptically coordinate the pulsatile release of kiss­
peptin, which in turn stimulates the pulsatile release of GnRH.366 
There are no steroid receptors on GnRH neurons, but there are 
steroid receptors on kisspeptin and KNDy neurons, which appear 
to modulate both the feedback inhibition and positive feedback 
effects of estrogen on gonadotropin secretion.
Spontaneous mutati

ogen on gonadotropin secretion.
Spontaneous mutations in the KISS1/KISS1R axis in human 
beings are rare but instructive in elucidating the role of KISS1 
in pubertal development; hypogonadotropic hypogonadism 
and CPP occur with different mutations of KISS1R (see later). 
Plasma concentrations of kisspeptin are higher in children than 
adults, and preliminary studies indicate that a rise occurs at 
puberty in boys and girls.367 These cases, augmented with stud­
ies of various animal species, de

ented with stud­
ies of various animal species, demonstrate that kisspeptin act­
ing through the KISS1R is an afferent influence that stimulates 
GnRH secretion.
With the demonstrated importance of KISS/KISSR in human 
reproduction, it is interesting to note that in mice alterative path­
ways operate. Thus elimination of KISS signaling by knockout 
of KISS (5% remained) and of KISSR genes (90% remained) 
did not eliminate puberty or fertility in developing female mice, 
although acute ablation o

developing female mice, 
although acute ablation of the axis in adult mice did eliminate fer­
tility.368 Alternative studies demonstrated that retention of 5% of 
the KISS/KISSR axis in male mice allowed fertility but conversely 
demonstrated decreased fertility in female mice.368 The discrep­
ancy in the female mouse results may be due to variations in tech­
niques used in the studies but is not yet resolved. However, it does 
appear clear that there are far more KISS or KISSR neurons than 
are

there are far more KISS or KISSR neurons than 
are needed for fertility and that redundancy is invoked to ensure 
the vital role of reproduction can occur. 
Ontogeny
In all vertebrates examined, GnRH neurons arise in the embryo 
from the epithelium of the olfactory placode and migrate in a 
rostrocaudal direction by an ordered spatiotemporal course along 
the pathway of the nervus terminalis-vomeronasal complex to 
the forebrain.369 The terminalis-vomeronasal complex also origi­
nates in the olf

s-vomeronasal complex also origi­
nates in the olfactory placode and forms a connection between 
the nasal septum and the forebrain. This contrasts to the pattern 
of growth hormone–releasing factor (GRF), thyrotropin-releasing 
factor (TRF), or corticotropin-releasing factor (CRF) neurosecre­
tory neurons, which originate from ventricular zones within the 
embryonic forebrain.370
The GnRH green fluorescent protein model demonstrates an 
increase in dendritic and somal spines in adult mice compa

 in dendritic and somal spines in adult mice compared 
with juveniles, suggesting an increase in direct excitatory inputs 
to GnRH neurons and increased glutamatergic stimulation of 
GnRH neurons across the time of puberty.371 Embryonic GnRH 
neurons of the olfactory placode and the hypothalamus coexpress 
mRNAs for GnRH and the type 1 GnRH receptor. These neurons 
demonstrate spontaneous electrical pulsatile activity, which can be 
stimulated by GnRH agonist and abolished by GnRH antagonist 
in

 GnRH agonist and abolished by GnRH antagonist 
in the same pattern as GnRH pulses, all in a calcium-dependent 
manner (i.e., the intracellular calcium responses are stimulated by 
the agonist and inhibited by the antagonist).353
Human Fetus
GnRH immunoreactivity was observed in the epithelium of the 
medial aspect of the olfactory placode of the normal human fetus 
by 42 days of gestation but not at 28 to 32 days369 (Table 26.11). 
No GnRH neurosecretory neurons were found in the brain, includ­

secretory neurons were found in the brain, includ­
ing the hypothalamus, of a 19-week gestational male human fetus 
with Kallmann syndrome,372 and the olfactory bulbs were absent. 
However, dense clusters of GnRH cells and fibers were present in 
the nose, including the nasal septum and cribriform plate, and 
within the dural layers of the meninges under the forebrain. The 
GnRH neurosecretory neurons migrate from the olfactory plac­
ode, where they arise outside of the central nervous system fr

hey arise outside of the central nervous system from 
neural crest and ectodermal progenitors, to the hypothalamus in 
normal humans and other mammals.373 As they reach the hypo­
thalamus they detach from the olfactory axons that guided them 
there and some end at the median eminence where they can release 
their products into the hypothalamic pituitary portal system.
Numerous genes are involved in the migration of the gonado­
tropin-releasing hormone neurons. They include fibroblast growth 
fac

rmone neurons. They include fibroblast growth 
factor 8 (FGF8), chromodomain helicase DNA-binding protein 7 
(CHD7), and sex-determining region Y-box 10 (SOX10). Other 
1
NH2
NH2
NH2
NH2
Kp54
Kp14
Kp13
Kp10
GnRH
GAP
Pro-convertase
carboxypeptidase E
PAM
PHM
22 23 32
68
121
1 19 20
65
68
121 124
145
• Fig. 26.25  Post-translational maturation of gonadotropin-releasing hor­
mone (GnRH) (top) and kisspeptins (Kp). Doublets of basic residues and 
glycine are indicat

oublets of basic residues and 
glycine are indicated by shaded vertical bars. Enzymes involved in the nor­
mal maturation are indicated. GAP, GnRH-associated peptide; PAM, pep­
tidyl glycine α-amidating monooxygenase; PHM, peptidyl α-hydroxylating 
monooxygenase. (From de Roux N. GnRH receptor and GPR54 inactiva­
tion in isolated gonadotropic deficiency. Best Pract Res Clin Endocrinol 
Metab. 2006;20:515–528.)
1066
genes influencing the migration are

 the Life Span
genes influencing the migration are the Kallmann protein (now 
called anosmin-1 with gene ANOS1) as well as prokineticin-2 
and prokineticin receptor 2 (PROK2 and PROKR2), WD repeat 
domain 11 (WDR11), semaphorin-3A (SEMA3A), FEZ family 
zinc finger 1 (FEZF1), NELF,374 DAX1 (or NR0B1), LEP, LEPR, 
KISS1, KISS1R, TAC3, TACR3, and GNRH1.
The number of the GnRH neurons and the GnRH mRNA 
levels in nonhuman primates and mice do not appear to change 
during pubertal development. The ab

ear to change 
during pubertal development. The ability of the GnRH neu­
ron to respond to electrical or neurochemical (e.g., glutama­
tergic, kisspeptinergic) stimuli does not change with pubertal 
development.375
A
Time (min)
LH (U/L)
−60
480
420
360
300
240
180
120
60
0
1440
14
0
2
4
6
8
10
12
Saline
B
Time (min)
LH (ng/mL)
40
30
20
10
0
50
0
50
0
50
0
50
0
50
0
Continuous metastin 45–54 or vehicle
Met
Met
NMDA
GnRH
Met
*
*
*
*
Day 1
Day 4
Day 5
• Fig. 26.26  (A) Effects of kisspep

1
Day 4
Day 5
• Fig. 26.26  (A) Effects of kisspeptin-54 (4 pmol/kg/min) or saline infusion in male volunteers (n = 6) on 
mean plasma. Each volunteer received both kisspeptin-54 and saline infusions and acted as their own 
controls. (B) Effect of single sequential boluses of human metastin 45-54 (Met-Kisspeptin 10) (black arrow), 
N-methyl-d-aspartate (NMDA) (gray arrow), and gonadotropin-releasing hormone (GnRH) (white arrow) 
on plasma luteinizing hormone (LH) concentrations (mean ± SEM [stan

zing hormone (LH) concentrations (mean ± SEM [standard error of the mean]) during the 
last 3 hours of the 98-hour intravenous infusion (shaded box) of human metastin 45-54 at a dose of 100 
μg/hour (closed circles) or vehicle compared with the LH response to the same bolus of human metastin 
45-54 1 hour before (day 1) and 21 hours after (day 5) the termination of continuous human metastin 45-54 
or vehicle infusion. Infusion of human metastin 45-54 (asterisk) was significantly different (p < 0

5-54 (asterisk) was significantly different (p < 0.05) from 
the preinjection mean; n = 3. (A, from Dhillo WS, Chaudhri OB, Patterson M, et al. Kisspeptin-54 stimulates 
the hypothalamic-pituitary gonadal axis in human males. J Clin Endocrinol Metab. 2005;90:6609–6615; 
B, from Seminara SB, Dipietro MJ, Ramaswamy S, et al. Continuous human metastin 45-54 infusion 
desensitizes G protein–coupled receptor 54-induced gonadotropin-releasing hormone release monitored 
indirectly in the juvenile male 

elease monitored 
indirectly in the juvenile male rhesus monkey (Macaca mulatta): a finding with therapeutic implications. 
Endocrinology. 2006;147:2122–2126.)
1067
The pituitary gland develops from cells originating in Rathke 
pouch and ultimately contains five different cell types. Devel­
opment of the pituitary gland is influenced by multiple genes, 
including HESX1, LHX3, LHX4, POU1F1, PROP1, SIX6, OTX2, 
PITX1, PITX1, PTX2, GLI2, GLI1, and SOX

X6, OTX2, 
PITX1, PITX1, PTX2, GLI2, GLI1, and SOX3.376
Gonadotropin cells are affected by TBX19 (or TPIT), GATA2, 
and SF1 (or NR5A1). Luteinizing hormone can be detected in 
human fetal pituitary glands from gestational week 12; follicle-
stimulating hormone can be detected by week 14.254
GnRH has been detected in human embryonic brain extracts 
by 4.5 weeks and in the fetal hypothalamus by 6 weeks (see 
Table 26.11); the fetal pituitary gonadotrophs are responsive 
to GnRH. The hypothalamic-h

trophs are responsive 
to GnRH. The hypothalamic-hypophyseal portal system is 
functional by 11.5 weeks of gestation, and by 16 weeks, axon 
fibers that contain GnRH are present in the median emi­
nence and terminate in contact with capillaries of the portal 
system.126,254,326,327 The available data are consistent with the 
development of a human fetal hypothalamic GnRH pulse gen­
erator by at least the end of the first trimester.127,327
The adreno-gonadal primordium gives rise to the testes, w

eno-gonadal primordium gives rise to the testes, which 
can be detected by 7 weeks of gestation. Sertoli cells under the 
influence of FSH after the first few weeks of gestation proliferate 
and produce antimüllerian hormone as well as inhibin B, which 
rises by midgestation. Leydig cells differentiate 1 week later than 
Sertoli cells and initially secrete testosterone under the influence 
of chorionic gonadotropin during the first third of gestation. By 
the second third of gestation, LH is the

tion. By 
the second third of gestation, LH is the major driving force toward 
Leydig cells producing testosterone. Both human chorionic 
gonadotropin and LH utilize the LH-CG receptor, which when 
stimulated causes Leydig cell proliferation, testosterone secretion, 
and secretion of INSL3.
The human fetal gonad is affected by placental gonadotropins 
and by fetal pituitary FSH and LH.327 The placental gonadotro­
pin hCG may play an important role in the secretion of testoster­
one by the Leydig

e in the secretion of testoster­
one by the Leydig cells of the fetal testes during masculinization 
of the wolffian ducts and the external genitalia at 8 to 13 weeks of 
gestation. However, it is uncertain whether functional hCG/LH 
and FSH receptors are present in the fetal testis by 12 weeks of 
gestation and whether the early fetal testis responds to hCG. Fetal 
Leydig cells are a unique population of Leydig cells limited to the 
fetus and infant, which regress to be followed by the differen

fant, which regress to be followed by the differentia­
tion of adult-type Leydig cells in the peripubertal period.377 Fetal 
testosterone and hCG levels, but not LH levels, were decreased in 
fetuses studied after elective second-trimester abortions between 
11 and 19 weeks of gestation, with no significant change in tes­
ticular responsiveness. The proportion of nonfunctional LH-CGR 
transcripts in fetal testes was 2.3-fold lower than in adults, so 
available ligand may exert a greater effect. 

 so 
available ligand may exert a greater effect. Fetal hCG was reduced, 
and the ratio of inactive to active LH-CGR isoforms was low­
ered by maternal smoking. Second-trimester fetal testosterone 
levels appear to decrease due to decreasing maternal hCG because 
Leydig cell LH/hCG responsiveness remains constant. Even with 
the decrease of fetal hCG caused by maternal cigarette smok­
ing, because the ratio of inactive to active LHCGR isoforms is 
reduced, testosterone remains normal due to feta

reduced, testosterone remains normal due to fetal gonadotropin 
  Early Development of the Human Fetal Pituitary and Hypothalamus
Gestational 
Age (wk)
Pituitary
Portal Circulation
3
Forebrain appears
4
Rathke pouch in contact with stomodeum
5
Diencephalon differentiated
Rathke pouch separated from stomodeum and in 
contact with infundibulum; pituitary in culture can 
secrete corticotropin, prolactin, GH, FSH
6
Premammillary preoptic nucleus; GnRH detected
Intermedia

millary preoptic nucleus; GnRH detected
Intermediate-lobe primordia; cell cords penetrate 
mesenchyme around Rathke pouch
7
Arcuate, supraoptic nucleus
Sphenoidal plate forms
8
Median eminence differentiated: TRH detected
Basophils appear
Capillaries in mesenchyme
9
Paraventricular nucleus; dorsal medial nucleus
Pars tuberalis formed: β-endorphin detecteda
10
Serotonin and norepinephrine detecteda
Acidophils appear
11
Mammillary nucleus; primary (hypothalamic) 
portal plexus present; β-endorphin

(hypothalamic) 
portal plexus present; β-endorphin and 
opioidergic neurons detecteda
Secondary (pituitary) portal plexus present; catechol­
amines detected by IF
Functional hypothalamic 
hypophyseal portal 
system
12
Dopamine present
13
Corticotropin-releasing hormone detecteda
α-Melanocyte-stimulating hormone detected
14
Fully differentiated hypothalamus
Adult form of hypophysis developed
aHormone is detected at this gestational age but may be present earlier.
FSH, Follicle-stimulating hormone

present earlier.
FSH, Follicle-stimulating hormone; GH, growth hormone; GnRH, gonadotropin-releasing hormone; IF, immunofluorescence; TRH, thyrotropin-releasing hormone.
Modified from Gluckman P, Grumbach MM, Kaplan SL. The human fetal hypothalamus and pituitary gland. In: Tulchinsky D, Ryan KJ, eds. Maternal-Fetal Endocrinology. Philadelphia: WB Saunders, 
1980:196–232.
   
TABLE 26.11
1068
Compared with the adult type, fetal Leydig cells form tight

with the adult type, fetal Leydig cells form tightly 
opposed clusters joined by gap junctions and lack Reinke crystals; 
they are resistant to hCG/LH-induced desensitization (hCG/LH 
produce upregulation of LH-CGRs), and they contain little aro­
matase activity and few estradiol receptors.
In contrast to the fetal testis, FSH receptors in the fetal 
ovary appear only late in the second trimester, well after com­
pletion of male phenotypic differentiation, demonstrating a 
sex difference in the 

entiation, demonstrating a 
sex difference in the stage of gestation at which fetal pituitary 
gonadotropins have an important effect on the development of 
the fetal gonad. In the anencephalic fetus (which is deficient 
in hypothalamic GHRH, resulting in deficiency of pituitary 
gonadotropins), the testes appear hypoplastic by early in the 
third trimester; however, the ovaries in this disorder are normal 
until at least 32 weeks of gestation.126,327 Androgen receptor 
mRNA expression is lower,

6,327 Androgen receptor 
mRNA expression is lower, and AMH mRNA expression is 
higher in fetal testicular Sertoli cells than in adult testes. This 
may explain the failure of testicular testosterone to support 
spermatogenesis and to suppress AMH at that stage; patients 
with androgen insensitivity syndrome also have an increase in 
circulating testosterone and AMH and have combined gonado­
tropin stimulation that is consistent with a failure of testoster­
one to repress AMH in the absence of AR

testoster­
one to repress AMH in the absence of AR signaling.378 AR was 
expressed in peritubular and Leydig cells.
The human fetal pituitary gland contains FSH and LH by 
10 weeks, secretion begins by 11 to 12 weeks, and the gonado­
tropin content increases until approximately 25 to 29 weeks of 
gestation.126,254,327 Fetal serum LH and FSH concentrations rise 
to peak levels by midgestation and then decrease to low values 
in umbilical venous blood at term. The serum concentrations 
of FSH and 

ood at term. The serum concentrations 
of FSH and LH and of bioactive FSH at 17 to 24 weeks of 
gestation are strikingly higher in female than in male fetuses, 
and in both sexes, they decrease remarkably between 25 and 40 
weeks of gestation. Mean FSH and LH concentrations are ele­
vated at the beginning of the third trimester and decrease with 
advancing gestational age to undetectable values in term fetuses. 
Mean FSH values are higher in female fetuses between 26 and 
36 weeks, whereas the m

le fetuses between 26 and 
36 weeks, whereas the mean LH level is higher in males. In the 
ovine fetus, LH and FSH are secreted in a pulsatile manner in 
response to the episodic secretion of fetal hypothalamic GnRH; 
human fetal pituitary gonadotropins are probably released in the 
same mode. The mean FSH and LH content of fetal pituitary 
glands is greater in female than in male fetuses at midgestation. 
This difference has been ascribed to the higher concentration 
of plasma testosterone betw

 higher concentration 
of plasma testosterone between 11 and 24 weeks in the male 
fetus (the only major difference in gonadal steroids between the 
male and female fetus) and to fetal testicular inhibin.126,327 The 
decrease in serum FSH and LH concentrations during late ges­
tation and near term is attributed to maturation of the nega­
tive feedback mechanism, the development of gonadal steroid 
receptors in the hypothalamic-pituitary unit,254 and the effect 
of inhibin.126,327
In vitro studie

and the effect 
In vitro studies indicate that the human fetal pituitary gland 
is responsive to GnRH as early as 10 weeks of gestation; the 
GnRH-stimulated release of LH is greater in second-trimester 
fetal pituitary cells cultured from females than in those cultured 
from males and is augmented by estradiol in both sexes.354 In vivo 
studies during middle and late gestation demonstrate the stimu­
lating action of exogenous GnRH on fetal FSH and LH release 
by 16 weeks, wit

GnRH on fetal FSH and LH release 
by 16 weeks, with a striking sex difference in the FSH response 
and a fall in responsiveness to GnRH in late gestation. Anen­
cephalic infants and some infants with neonatal hypothalamic 
hypopituitarism have an absent or diminished gonadotropin 
response to GnRH,126,254,327 in contrast to the brisk increase dem­
onstrated in the normal infant.
A pattern of increasing synthesis and secretion of FSH and 
LH leading to peak serum concentrations at castrate levels

ng to peak serum concentrations at castrate levels—
probably the result of relatively autonomous, unrestrained 
activity of the fetal hypothalamic GnRH pulse generator and 
subsequent stimulation of the fetal gonadotrophs by GnRH—is 
followed by a decline after midgestation that persists to term, 
probably due to maturation of the negative feedback mecha­
nism and increasing sensitivity of the GnRH pulse generator to 
the inhibitory effects of the high concentration of sex steroids 
(estrogens a

e high concentration of sex steroids 
(estrogens and progesterone from the placenta and, in the male, 
testosterone from the fetal testes) in the fetal circulation; in the 
male fetus, there is also a contributory effect on the decrease in 
FSH by testicular inhibin in late gestation. The increasing CNS 
control of gonadotropin secretion seems to require the matu­
ration of gonadal steroid receptors (intracellular or on the cell 
surface, or both) in the fetal hypothalamus and in the pituitary 

) in the fetal hypothalamus and in the pituitary 
gonadotrophs.254 
Sheep Fetus
In fetal sheep, the hypothalamus secretes GnRH in a pulsatile 
manner.326 By 0.6 gestation, the secretion of fetal LH and FSH 
is pulsatile, mediated by the hypothalamic GnRH pulse gen­
erator.326 A sex difference in gonadotropin secretion occurs in 
ovine and human fetuses, and orchiectomy (but not oophorec­
tomy) in the ovine fetus leads to increased pulsatile secretion 
of LH and, to a lesser degree, of FSH.379 Op

tion 
of LH and, to a lesser degree, of FSH.379 Opioidergic neurons 
have a tonic suppressive effect on the pulsatile release of GnRH 
in the ovine fetus,380 and the excitatory amino acid analogue 
NMDA evokes an LH pulse mediated by GnRH.381 The excit­
atory amino acids, glutamate and aspartate, can stimulate the 
GnRH pulse generator directly or indirectly. Glutamate is pres­
ent in abundance in the hypothalamus and is released from 
glutamatergic neurons by exocytosis in an adenosine triphos­

gic neurons by exocytosis in an adenosine triphos­
phate–dependent and calcium-dependent process. FSH stimu­
lates inhibin synthesis by the ovine fetal testis and ovary, and 
administration of an inhibin-rich extract inhibits fetal FSH but 
not LH secretion, evidence of the functional capacity of the 
FSH-fetal gonadal inhibin feedback system.382 These observa­
tions provide support for the central role of this process in the 
regulation of the CNS in the hypothalamic GnRH-pituitary 
gonadotropi

NS in the hypothalamic GnRH-pituitary 
gonadotropin unit. 
Human Neonate and Infant291,326
In both sexes, the concentration of plasma FSH and LH is low in 
cord blood as a consequence of the inhibitory effect of the high 
levels of placental-derived estrogens, but within a few minutes after 
birth, the concentration of LH increases abruptly in peripheral 
blood (about 10-fold) in the male neonate, but not in the female; 
this is followed by an increase in serum testosterone concentration 
during

crease in serum testosterone concentration 
during the first 3 hours that persists for 12 hours or longer.326,383 
After the fall in circulating levels of steroids of placental origin 
(especially estrogens) during the first few days after birth, the 
serum concentrations of FSH and LH increase and exhibit a pul­
satile pattern with wide perturbations during the first few months 
of life. FSH pulse amplitude is much greater in the female infant 
and is associated with a larger FSH response to Gn

and is associated with a larger FSH response to GnRH throughout 
childhood; LH pulses are of greater magnitude in the male. This 
striking sex difference also is present in agonadal male and female 
infants and in infant rhesus monkeys.326,384,385 This sex difference 
1069
may in part be related to the effect of testosterone in the male fetus 
on the development and function of the hypothalamic-pituitary 
apparatus.126,327
The high gonadotropin con

itary 
The high gonadotropin concentrations are associated with a 
proliferation of Sertoli cells and gonocytes (and their transfor­
mation into spermatogonia) and with a transient second wave 
of differentiation of fetal-type Leydig cells and increased serum 
testosterone levels in male infants during the first few postnatal 
months and increased estradiol levels intermittently during the 
first year of life and part of the second year in females.377 The 
mean FSH concentratio

ond year in females.377 The 
mean FSH concentration is higher in females than in males dur­
ing the first few years of life. By approximately 6 months of age 
for boys and 2 to 3 years for girls, the concentration of plasma 
gonadotropins decreases to the low levels that are present until 
the onset of puberty (earlier in boys than girls) in the juvenile 
pause.
The neonatal-to-midinfancy surge in pulsatile gonadotropin 
secretion, sex hormones, and inhibin—the postnatal surge or 
minipuberty of

ation of circulating inhibin B (low in hypogo­
nadotropic hypogonadism)
	4.	 Sertoli cell number, including postnatal proliferation, as a 
determinant of spermatogenic function
The increase in circulating testosterone in the normal male 
infant may lead to facial comedones and even to acneiform 
lesions, and the increase in gonadotropins may lead to a 
transient increase in testicular size, but there may be subtler 
changes. A detailed study of 18,570 infants in Finland (51% 
were boys) demonst

18,570 infants in Finland (51% 
were boys) demonstrated more rapid growth in boys and girls 
during minipuberty with the increase in the rate of growth 
in boys compared to girls being similar to that noted during 
pubertal growth.386
The minipuberty of infancy offers an excellent time to evalu­
ate children who have features that have been described as red 
flags of congenital hypogonadotropic hypogonadism.387 Missing 
an opportunity to measure testosterone in a sensitive pediatric 
assay at th

testosterone in a sensitive pediatric 
assay at this age may delay diagnosis of hypogonadism until well 
past the pubertal period. An LH and testosterone surge is absent 
in this period in those with the complete androgen insensitivity 
syndrome.
The postnatal surge apparently is not essential for mascu­
line-typical psychosexual development. The brain in patients 
with congenital hypogonadotropic hypogonadism, including 
Kallmann syndrome, is masculinized by testosterone therapy at 
puberty des

sculinized by testosterone therapy at 
puberty despite the lack of an infantile surge in gonadotropins 
and testosterone.
The transient postnatal to midinfancy function of the GnRH 
pulse generator in the male infant may be related to future 
spermatogenic function and fertility.388,389 Sertoli cells and 
germ cells proliferate for about 100 days after birth (indicating 
mitotic activity and the transformation of gonocytes into adult 
dark [Ad] spermatogonia, the stem cells for spermatogenesis),

permatogonia, the stem cells for spermatogenesis), 
with a subsequent decrease, mainly by apoptosis, after about 6 
months of age, coincident with the waning of gonadotropins 
and testosterone. 
Maturation of the CNS is the outcome or consequence of the 
totality of environmental and genetic factors that retard or 
accelerate the onset of puberty. It is a provocative but unproven 
hypothesis that a metabolic signal related to body composition is 
an important factor in the maturat

composition is 
an important factor in the maturation or activation of the hypo­
thalamic GnRH pulse generator and not a result of the early hor­
monal and body composition changes in human puberty. In either 
event, clinical and experimental data support the contention that 
the factors influencing the timing of puberty are expressed finally 
through CNS regulation of the onset of puberty.256,326 In humans, 
the pineal gland and melatonin do not appear to have a major 
effect on this control sy

 appear to have a major 
effect on this control system.326,390 
Timing and Onset of Puberty
Genetic Neural Control
Because many levels control the onset of pubertal development, 
a systems biology approach holds promise in characterizing the 
complex components of this neural and neuroendocrine network. 
It appears that normal and some types of abnormal puberty are 
under polygenic control. Although the increased pulsatile release 
of GnRH is most frequently considered, this change is caused by 

t frequently considered, this change is caused by 
a balance in the inhibitory and excitatory factors through coor­
dinated changes in transsynaptic and glial-neuronal communica­
tion, increased stimulatory factors (most prominently glutamate 
and kisspeptin), and decreased inhibitory tone, mostly through 
GABAergic neurons (i.e., those secreting γ-aminobutyric acid), 
opioidergic neurons, and MKRN3 expression all controlled by 
gene expression. Glial cells affect GnRH secretion through growth 

Glial cells affect GnRH secretion through growth 
factor–dependent cell-cell signaling coordinated by numerous 
unrelated genes. A second level of genes is postulated to con­
trol cell-cell interaction. The third highest level of control occurs 
through transcriptional regulation of the subordinate genes by 
other higher level genes that maintain the function and integra­
tion of the network.
The activation of GnRH release at puberty involves many dif­
ferent cellular phenotypes and intracellula

y dif­
ferent cellular phenotypes and intracellular and cell-cell signaling 
molecules, which is controlled by a highly coordinated and inter­
active regulatory system involving hundreds of gene products; a 
systems biology approach is proposed to explain how these net­
works are organized in a hierarchical fashion.391 This theory states 
that there are transcriptional regulators that direct expression of 
downstream subordinate genes to allow stability and to coordi­
nate the cellular networks 

ability and to coordi­
nate the cellular networks involved in controlling the onset of 
puberty. Epigenetic mechanisms sensitive to external inputs such 
as nutrition or endocrine disruptors are posited to integrate the 
response of these gene networks.392 Endocrine disrupting chemi­
cals discussed elsewhere in this chapter appear to exert their effect 
through epigenetic mechanisms.
Although the action of multiple genes (i.e., quantitative or 
polygenic inheritance) on the time of onset of pube

olygenic inheritance) on the time of onset of puberty (e.g., on 
stature) has long been recognized, little is known about the gene 
loci involved in this complex quantitative trait or the effect of 
gene interactions on this paradigm of complex traits.393 Genetic 
factors are estimated to account for 50% to 80% of the varia­
tion in the onset of normal puberty. These complex traits have 
been analyzed by linkage analyses (in which quantitative trait loci 
have been shown to relate to the age of 

ait loci 
have been shown to relate to the age of menarche) and by large-
scale haplotype-based association studies. Pedigree analyses have 
revealed relative risks for delay of puberty in kindreds with his­
tories of CDP compared with those without such a history; for 
1070
example, for first-degree relatives, the risk of a 2-SD delay in the 
onset of puberty is 4.8. There are clear genetic influences on the 
time of onset of puberty in monogenetic 

s on the 
time of onset of puberty in monogenetic disorders, such as KAL1, 
KISS, KISS1R (GRP54), MKRN3, and mutations and other influ­
ences, that can prevent or advance pubertal development (see ear­
lier discussion of genomic effects on the onset of puberty and later 
discussion of specific genomic conditions). 
Nutrition and Metabolic Control
The genetic effects on the time of onset of puberty and its course 
are influenced by environmental factors (e.g., socioeconomic fac­
tors, nutrition, 

actors (e.g., socioeconomic fac­
tors, nutrition, general health, geography) operating through the 
CNS. It has long been postulated that some alteration of body 
metabolism linked to energy metabolism may affect the CNS 
restraints on pubertal onset and progression, because of the earlier 
age of menarche in moderately obese girls, delayed menarche in 
states of malnutrition and chronic disease and with early rigorous 
athletic or ballet training, and changes in gonadotropin secretion 
and amen

g, and changes in gonadotropin secretion 
and amenorrhea in girls with anorexia nervosa, voluntary weight 
loss, and strenuous physical conditioning.
An invariant mean weight (48 kg) for initiation of the pubertal 
spurt in weight, the maximal rate of weight gain, and menarche 
in healthy girls regardless of chronologic age was proposed in the 
1970s, but the concept generated controversy and criticism, in 
part because the empiric estimations and the equations used to 
determine fat mass were c

d the equations used to 
determine fat mass were challenged and because no direct mea­
surements supported the theory.394 A more recent, 5-year longitu­
dinal study of 469 girls revealed that there was a similar percentage 
of body fat associated with the onset of puberty, although there 
was a spread of chronologic ages at onset.395 Body composition 
in the 2 years before puberty had a modest impact on the age of 
pubertal growth, but higher fat mass led to more rapid progress 
through pubertal

 mass led to more rapid progress 
through pubertal stages.396
The relationships of adipose tissue mass, fat metabolism, and 
energy balance to reproduction were illuminated by the discovery 
of the genes encoding leptin, an adipocyte satiety factor,397,398 and 
its receptor.399 Considerable interest has focused on the potential 
role of leptin in the control of the onset of puberty—from a pro­
posal that it was an essential, if not a key, factor in triggering the 
onset of puberty to one in whic

in triggering the 
onset of puberty to one in which it had a more subsidiary role. 
Leptin is a well-established afferent satiety factor in humans; it 
acts on the hypothalamus, including nuclei controlling appetite, 
to suppress appetite. Leptin reflects body fat and therefore energy 
stores and has an important role in the control of body weight and 
the regulation of metabolism.
Ob/ob mice (which lack leptin) and db/db mice (which lack 
leptin receptors) are obese and exhibit hypogonadotropic

 receptors) are obese and exhibit hypogonadotropic hypo­
gonadism, providing evidence for an important role of leptin in 
reproduction. Administration of recombinant leptin to hypo­
gonadal ob/ob mice and to rats experiencing pubertal delay 
associated with food restriction in the rat partially reverses the 
hypogonadism. However, leptin administration to normal prepu­
bertal rats did not advance the time of onset of puberty. A critical 
threshold level of leptin was necessary for puberty to beg

d level of leptin was necessary for puberty to begin and 
advance, but leptin alone (as in administration to normal rodents) 
was insufficient to promote puberty; it was but one among several 
permissive factors.400
The site of action of leptin in its effect on the hypothalamic 
GnRH-pituitary gonadotropin apparatus was clarified by experi­
ments in the mouse.401 Even though leptin receptors (LepR) 
were reported to be expressed in immortalized mouse GnRH 
(GT1) cells, leptin receptors are not e

ouse GnRH 
(GT1) cells, leptin receptors are not expressed on GnRH neurons 
in vivo.402 Mice with selective deletion of the long form (signal­
ing) of the leptin receptor underwent normal puberty and fertility. 
Kiss1 neurons express the leptin receptor (Ob-Rb isoform), and 
the suggestion was advanced that the effect of leptin on puberty 
onset and fertility was mediated through the Kiss1/Kiss1n com­
plex. Mice with selective deletion of the leptin receptor from 
Kiss1 neurons underwent normal 

tin receptor from 
Kiss1 neurons underwent normal puberty and fertility, strongly 
suggesting that the action of leptin on the hypothalamic GnRH-
pituitary gonadotropin system is not mediated through Kiss1 
neurons.403,404 The leptin receptor is expressed at a high level in 
ventral premammillary neurons, which express the excitatory neu­
rotransmitter glutamate, which stimulates either GnRH neurons 
or GnRH terminals in the median eminence or both sites. These 
observations dissociate the actio

th sites. These 
observations dissociate the action of leptin on reproductive func­
tion and on nutrition (Fig. 26.27).
Leptin levels in the male rhesus monkey were similar during 
the advancement of prepuberty to puberty. In peripubertal rhesus 
monkeys age 3 to 5 years and fasted for 2 days, the administra­
tion of leptin prevented the decrease in plasma gonadotropins 
detected in the untreated animals. Continuous infusion of leptin 
into the lateral ventricle of agonadal male monkeys failed t

ateral ventricle of agonadal male monkeys failed to evoke 
an increase in GnRH on gonadotropin secretion.332
Is leptin the peripheral, somatic trigger for the onset of puberty 
to the CNS, or does it have a permissive role, signaling the hypo­
thalamus and the GnRH pulse generator that a critical energy 
store has been attained? A longitudinal study of serum leptin 
levels in prepubertal and pubertal boys and girls showed leptin 
Leptin
mRNA
Adipocytes
Leptin
Leptin
mRNA
Ventral pre

ocytes
Leptin
Leptin
mRNA
Ventral premammillary
nucleus neurons
GABA
GnRH
Pituitary
gonadotropes
Leptin
mRNA
LH, FSH
• Fig. 26.27  The leptin receptor is expressed in ventral premammillary neu­
rons (PMV). Leptin secreted by adipocytes evokes release of the excitatory 
neurotransmitter glutamate from the PMV leading to activation of gonad­
otropin-releasing hormone (GnRH) neurons. Leptin functions as a permis­
sive factor, not a trigger, in the onset o

permis­
sive factor, not a trigger, in the onset of human puberty. In rodents, leptin 
advances the onset of puberty and plays a key role in initiating puberty 
and infertility. The hypothalamic pathway that regulates energy expendi­
ture and food intake by leptin is independent of the effect of leptin on 
reproduction. FSH, follicle-stimulating hormone; LH, luteinizing hormone; 
mRNA, messenger ribonucleic acid.
1071
to increase gradually during t

isorders of Puberty
to increase gradually during the prepubertal years, with similar 
levels in the two sexes.405 During puberty, leptin continued to 
rise in girls, whereas in boys, the leptin mean levels peaked at 
Tanner stage 2 and decreased to prepubertal concentrations by 
genital stage 5.406 This decrease is attributed to the effect of tes­
tosterone on leptin secretion.407 Adipose tissue mass, percentage 
of body fat, and age correlated with leptin levels, among other 
variables,405,408 

ith leptin levels, among other 
variables,405,408 but there was no correlation between 24-hour 
serum estradiol and leptin concentrations in nonobese and obese 
prepubertal and early pubertal girls. Serum leptin levels rose after 
administration of pulsatile GnRH administration for 36 hours 
to children with delayed puberty but not after a single dose of 
buserelin, a GnRH agonist, suggesting that, rather than puberty 
being triggered by leptin, pubertal increase in GnRH pulsatility 
increases l

pubertal increase in GnRH pulsatility 
increases leptin.409
Leptin circulates in both a free form and a high-molecular-
weight, bound form. Leptin-binding activity in serum is highest 
in childhood and decreases to relatively low levels during puberty. 
Free leptin is postulated to have more relevance to reproductive 
development than total leptin measured in the circulation; the 
soluble leptin receptor appears to be higher in males than in 
females and is inversely related to leptin levels lat

ales and is inversely related to leptin levels later in develop­
ment in females.410 A study of a 132 monozygotic female twin 
pairs and 48 dizygotic female twin pairs demonstrated a rise in 
leptin throughout puberty and a decrease in the soluble leptin 
receptor between stages 1 and 2, leading to a rise in the free 
leptin index between stages 1 and 2; there was greater heredita­
bility for the soluble leptin receptor than leptin,411 and there was 
also high hereditability of free IGF1 values.

was 
also high hereditability of free IGF1 values. There is no known 
relationship between leptin and leptin receptor gene polymor­
phism and CDP; however, the presence of a short allele of leptin 
was associated with heavier weight, whereas those who were thin, 
had significant bone age delay, and had increased frequency of 
parental pubertal delay were less likely to have this leptin short 
allele.412
As in the ob/ob mouse, human beings with a homozygous 
mutation in the leptin gene413 or the 

homozygous 
mutation in the leptin gene413 or the leptin receptor414 have 
morbid obesity and a striking delay in puberty because of hypo­
gonadotropic hypogonadism. In a pedigree affected by a stop 
codon mutation in the gene encoding leptin, a 23-year-old man 
failed to attain puberty because of hypogonadotropic hypogo­
nadism, and two affected women were prepubertal and amenor­
rheic, one until 29 years of age, after which she began to have 
irregular, scanty periods, and the other until age 

rregular, scanty periods, and the other until age 36 years, at 
which time she began to menstruate monthly. A 9-year-old girl 
with a bone age of 13 years who was affected with congenital 
leptin deficiency lost weight and had an early pubertal pattern 
of LH release in response to the administration of GnRH after 
treatment with recombinant leptin.415 A 4-year-old affected 
relative of this 9-year-old girl benefited from the metabolic 
improvement that occurred with leptin administration but di

nt that occurred with leptin administration but did 
not undergo early pubertal development, indicating the permis­
sive nature of leptin on puberty.
Boys with CDP have lower mean levels of leptin than 
expected and can enter puberty without an increase in circulat­
ing leptin. Two women with congenital lipoatrophic diabetes 
(Berardinelli-Seip syndrome), which is associated with absence 
of subcutaneous and visceral adipose tissue, did not have a 
delay in menarche despite severe hypoleptinemia

a 
delay in menarche despite severe hypoleptinemia, and one of 
the women had three unaffected children.416 Severe leptin defi­
ciency causes hypogonadotropic hypogonadism, suggesting 
that a critical level of leptin and a leptin signal are required to 
achieve puberty, but a rise in leptin is not required to trigger 
puberty. In summary, leptin is a permissive factor (tonic media­
tor) and not a trigger (phasic mediator) in the onset of human 
puberty (Table 26.12).
In a longitudinal study of b

uberty (Table 26.12).
In a longitudinal study of boys leading up to an increase in 
morning salivary testosterone concentrations, they had a relatively 
constant ratio of basal metabolic rate (BMR) to LBM but an 
increase in the ratio of BMR to total daily energy expenditure. A 
subtle energy-dependent process is in play, possibly related to an 
increase in brain BMR as a secondary phenomenon at the initia­
tion of puberty or indicating that a central rise in BMR is a signal 
for the onset of pu

ntral rise in BMR is a signal 
for the onset of puberty.
Ghrelin is the natural ligand for the GH secretagogue recep­
tor but also serves as an orexigenic signal that is increased after 
food deprivation; ghrelin is usually negatively correlated with 
BMI. Ghrelin administration delays pubertal development in rats 
because of gonadal and central effects, suggesting a link between 
malnutrition and the decrease in reproductive development or 
function.417
Adiponectin is an adipocytokine produced 

tion.417
Adiponectin is an adipocytokine produced in fat cells that has 
antidiabetic, antiatherogenic, and anti-inflammatory effects. Adi­
ponectin decreases in the face of excess fat mass in obesity and 
is suppressed by rising testosterone and DHEAS levels. The con­
centration of adiponectin falls during pubertal development in 
males but remains rather stable in females with advancing Tanner 
stage.418
Resistin is an adipocytokine belonging to the resistin-like 
molecular family of cysteine-

o the resistin-like 
molecular family of cysteine-rich molecules (RELMs). Values 
increase with pubertal development in boys, and this appears to be 
true in girls as well, although the evidence is weaker. Because resis­
tin serum levels were elevated in mouse models of obesity, it was 
considered to be a potential link between insulin resistance and 
obesity, but serum resistin levels appear to relate more to pubertal 
development than to insulin resistance.419 
  Leptin and Puberty: A Permiss

n resistance.419 
  Leptin and Puberty: A Permissive Factor, 
Not a Trigger for the Onset of Puberty
Pro-Trigger Evidence
Congenital leptin deficiency or congenital leptin resistance related to 
mutations is associated with delayed puberty and gonadotropin 
deficiency, evidence that the virtual absence of leptin or the leptin 
signal leads to severe hypogonadotropic hypogonadism.
In congenital leptin deficiency, administration of leptin led to a reduc­
tion in weight and an early pubertal patte

 reduc­
tion in weight and an early pubertal pattern of luteinizing hormone 
release in an affected prepubertal girl.
Pro-Permissive Evidence
A sharp rise in circulating leptin does not occur at the onset of puberty.
In prepubertal and early pubertal girls, the rise in serum leptin did not 
correlate with the increase in serum estradiol.
In constitutional delay in growth and adolescence, an increase in prepu­
bertal leptin levels is not essential for the onset of puberty.
In congenital lipoatrop

 for the onset of puberty.
In congenital lipoatrophic diabetes, despite the absence of subcutane­
ous and visceral adipose tissue and, as a consequence, severe 
hypoleptinemia, puberty can occur at the usual age and fertility is 
reported.
Supportive experimental data exist in rodents, sheep, and nonhuman 
primates.
From Grumbach MM. The neuroendocrinology of human puberty revisited. Horm Res. 
2002;57:S2–S14.
   
TABLE 26.12
1072
Mechanisms of Contr

ndocrinology and the Life Span
Mechanisms of Control
Diverse strategies and adaptive mechanisms have evolved to con­
trol puberty in different species.332,385 In rodents, exteroceptive 
factors and cues, including light, olfaction, and pheromones, have 
an important influence by way of the CNS on gonadotropin secre­
tion. In seasonal-breeding species such as sheep, the length of the 
light-dark cycle is critical, and the pattern of gonadotropin secre­
tion is different. In contrast, male and fem

ecre­
tion is different. In contrast, male and female primates exhibit an 
estrogen-provoked LH surge.
In humans and nonhuman primates, after the initial 
development and function in the fetus, the infantile surge of 
increased LH and FSH secretion occurs; this is followed by a 
decade of suppression (but not absence) of activity of the hypo­
thalamic GnRH pulse generator and the resulting quiescence of 
the pituitary gonadotropin-gonadal axis, known as the prepu­
bertal period or juvenile pause

nown as the prepu­
bertal period or juvenile pause (Table 26.13).126,326,332,385 Then 
there is gradual disinhibition and reactivation, mainly at night 
during late childhood,326,332,385,420 and, finally, the increased 
amplitude of the GnRH pulses, reflected in the progressively 
increased and changing pattern of circulating LH pulses that 
occurs with the approach of and during puberty. Two inter­
acting mechanisms have been proposed to explain the juvenile 
pause326,420 (Fig. 26.28).
Gonadal 

 the juvenile 
Gonadal Steroid–Dependent Negative Feedback Mechanism
There are three lines of evidence for an operative negative feedback 
mechanism in prepubertal256,326 children258,327:
	1.	 The pituitary of the prepubertal child secretes small amounts 
of FSH and LH, showing a low level of activity of the hypotha­
lamic-pituitary-gonadal complex.
	2.	 In agonadal infants and prepubertal children (e.g., in Turner 
syndrome), secretion of FSH and, to a lesser degree

yndrome), secretion of FSH and, to a lesser degree, LH is 
increased, suggesting that even low levels of hormones secreted 
by the normal prepubertal gonad inhibit gonadotropin secre­
tion by a sensitive, functional, tonic, negative feedback mecha­
nism256,326,420 (Fig. 26.29).
	3.	 The low level of gonadotropin secretion in childhood is shut 
off by administration of small amounts of gonadal steroids, 
showing that the hypothalamic-pituitary gonadotropin unit is 
highly sensitive (approximatel

nadotropin unit is 
highly sensitive (approximately 6–15 times more sensitive than 
in the adult) to the feedback effect of gonadal steroids.258,327 
Gonadal Steroid–Independent (Intrinsic) Central Nervous 
System Inhibitory Mechanism
The diphasic pattern of basal and GnRH-induced FSH and LH 
secretion from infancy to adulthood is similar in normal indi­
viduals and in agonadal patients, but in the latter, gonadotropin 
concentrations are higher, except during the middle childhood 
nadir.392,393

 except during the middle childhood 
nadir.392,393,420 The high plasma concentrations of FSH and LH 
in agonadal children between infancy and about 4 years of age 
and the increased gonadotropin reserve reflect the absence of 
gonadal steroid inhibition (see Fig. 26.29) of the hypothalamic-
pituitary unit by the low plasma levels of gonadal steroids.326,327 
However, the striking fall in gonadotropin secretion between the 
ages of 4 and 11 years suggests the presence of a CNS inhibi­
tory mechan

suggests the presence of a CNS inhibi­
tory mechanism that restrains the hypothalamic GnRH pulse 
generator, independent of gonadal steroid secretion. The result­
ing fall in gonadotropin secretion in agonadal children does not 
result from gonadal steroid feedback (because functional gonads 
are lacking) nor from increased secretion of adrenal steroids 
(because concentrations are low, and glucocorticoid suppression 
of the adrenal does not augment the concentration of circulating 
gonadotropin

ent the concentration of circulating 
gonadotropins).326,329,327 A CNS steroid-independent inhibi­
tory mechanism for suppression of the hypothalamic GnRH 
pulse generator seems to be the dominant factor in restraint of 
puberty between326,421 the ages of 4 and 11 years,327,394 and a 
gradual loss of this intrinsic CNS inhibitory mechanism leads 
to disinhibition or reactivation of the GnRH pulse generator at 
puberty. 
  Potential Components of the Intrinsic 
Central Nervous System Inhibitory 

itude)
Pituitary
FSH and LH
• Fig. 26.28  Postulated dual mechanism of restraint of puberty involves 
gonadal steroid-dependent and gonadal steroid-independent processes 
(i.e., intrinsic central nervous system [CNS] inhibitory mechanism). FSH, 
follicle-stimulating hormone; GnRH, gonadotropin-releasing hormone; LH, 
luteinizing hormone; MBH, medial basal hypothalamus. (Modified from 
Grumbach MM, Kaplan SL. The neuroendocrinology of human puberty: an 
ontogenetic perspective. In: G

 human puberty: an 
ontogenetic perspective. In: Grumbach MM, Sizonenko PC, Aubert ML, 
eds. Control of the Onset of Puberty. Baltimore, MD: Williams & Wilkins; 
1990:1–68.)
1073
Plasma LH
ng/mL (LER 960)
Plasma FSH
ng/mL (LER 869)
B
2d
3d
Juvenile pause
Gonadal
steroid
negative
feedback
Intrinsic
CNS
3+
Dom
?
±     1+
3+
Dominant
±     ?
3+
Dominant
220
200
100
60
20
10
0
2
4
10
12
16
20
B
2d
3d
2
6
10
14
2
4
10
12
16
20
• F

B
2d
3d
2
6
10
14
2
4
10
12
16
20
• Fig. 26.29  Interaction of the negative feedback mechanism and the putative intrinsic central nervous 
system (CNS) inhibitory mechanism in restraining puberty as extrapolated from the pattern of change in 
the concentrations of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) in agonadal infants, 
children, and adolescents. Triangles designate patients with the 45,X karyotype. Circles indicate Turner 
syndrome patients with X chrom

es indicate Turner 
syndrome patients with X chromosome mosaicism or structural abnormalities of the X chromosome, or 
both. Notice the values for the 2-day-old and 3-day-old infants. The solid line represents a regression line 
of best fit. The hatched area indicates the mean plasma values for normal females. To convert FSH values 
to international units per liter, multiply by 8.4. For about the first 3 years of life, the sensitive gonadal steroid 
negative feedback mechanism has a dominant rol

id 
negative feedback mechanism has a dominant role in restraining gonadotropin secretion, as exemplified 
by the high gonadotropin concentrations in this age group in the absence of gonads (and gonadal steroid 
feedback). A major role of the intrinsic CNS inhibitory mechanism in this age group is unlikely in light of the 
rise in gonadotropins to castrate levels in the absence of functional gonads. From 4 to 6 years of age, the 
postulated intrinsic CNS inhibitory mechanism is dominant, as indi

nsic CNS inhibitory mechanism is dominant, as indicated by the fall in FSH and LH concen­
trations in the absence of gonads. Even in this age group, the augmented gonadotropin response evoked 
by gonadotropin-releasing hormone (GnRH) and the slightly higher mean basal gonadotropin concentra­
tions in agonadal individuals support a role, although a subsidiary one, for gonadal steroid negative feed­
back in the suppression of gonadotropin secretion during this period of the juvenile pause. The inv

 during this period of the juvenile pause. The investigators 
suggested that the intrinsic CNS inhibitory mechanism suppresses the functional GnRH pulse generator. 
After about 10 years of age, the CNS inhibition gradually wanes, resulting in disinhibition of the GnRH pulse 
generator. The gonadal steroid negative feedback mechanism with an adult-type set-point and inhibin plays 
a dominant role in regulating the GnRH pulse generator and pituitary gonadotropin system. For conversion 
to SI units

y gonadotropin system. For conversion 
to SI units, see Fig. 26.19. (Modified from Grumbach MM, Kaplan SL. The neuroendocrinology of human 
puberty: an ontogenetic perspective. In: Grumbach MM, Sizonenko PC, Aubert ML, eds. Control of the 
Onset of Puberty. Baltimore, MD: Williams & Wilkins; 1990:1–68; Conte FA, Grumbach MM, Kaplan SL. 
A diphasic pattern of gonadotropin secretion in patients with the syndrome of gonadal dysgenesis. J Clin 
Endocrinol Metab. 1975;40:670–674. Copyright by The End

rinol Metab. 1975;40:670–674. Copyright by The Endocrine Society.)
1074
Interaction of the Negative Feedback Mechanism and the 
Intrinsic Central Nervous System Inhibitory Mechanism
The negative feedback mechanism and the intrinsic CNS inhibi­
tory mechanism appear to interact in restraining puberty (see 
Fig. 26.29). During the first 2 to 3 years of life, the gonadal-
steroid negative feedback mechanism seems to be dominant, 
but beginning at about 

ism seems to be dominant, 
but beginning at about 3 years of age, the intrinsic CNS inhibi­
tory mechanism becomes dominant and remains so during the 
rest of the juvenile pause, as evidenced by the fall in FSH and 
LH levels between the ages of 3 and 10 years despite the lack 
of functional gonads. The negative feedback mechanism remains 
operative during the juvenile pause; agonadal patients in this age 
group have higher mean plasma FSH levels than normal prepu­
bertal children and a greater 

 than normal prepu­
bertal children and a greater FSH and LH response to the acute 
administration of GnRH.392,393,420 As puberty approaches, the 
CNS inhibitory mechanism gradually wanes, initially during 
nighttime sleep, and the hypothalamic GnRH pulse generator 
becomes less sensitive to gonadal steroid negative feedback (see 
Fig. 26.29).326,327 After the onset of puberty, gonadal-steroid 
negative feedback becomes the dominant mechanism in restrain­
ing gonadotropin secretion (along with i

restrain­
ing gonadotropin secretion (along with inhibin), as reflected in 
the increased gonadotropin concentrations that are characteristic 
of the adolescent with severe primary hypogonadism.332 The pos­
tulated ontogeny of this dual mechanism of restraint of puberty 
is illustrated in Fig. 26.29. 
Potential Components of the Intrinsic Central Nervous 
The intrinsic CNS inhibitory mechanism has long remained elu­
sive.385 In the rhesus monkey, despite the damping o

ve.385 In the rhesus monkey, despite the damping of the GnRH 
pulse generator during the juvenile pause, the content of hypotha­
lamic GnRH and GnRH mRNA during this phase is similar to 
that in the infant or the adult monkey. Low-amplitude LH and 
FSH pulses are detectable by sensitive and specific immunoradio­
metric assays in the juvenile pause, demonstrating a low level of 
activity of the GnRH pulse generator.329,385 The end of the juve­
nile pause is marked by an increase in LH pulse ampli

e pause is marked by an increase in LH pulse amplitude that is 
most evident during the early hours of sleep.
Children with CPP associated with posterior hypothalamic 
neoplasms (usually a pilocytic astrocytoma), irradiation of the 
CNS, midline CNS developmental abnormalities such as septo-
optic dysplasia with deficiency of one or more pituitary hormones, 
or other CNS lesions provide indirect evidence for an inhibitory 
neural component located in or projecting through the posterior 
hypothal

d in or projecting through the posterior 
hypothalamus. These lesions compromise the neural pathway, 
which inhibits the hypothalamic GnRH pulse generator and 
results in its disinhibition and activation leading to CPP.326 For 
example, a suprasellar arachnoid cyst can cause CPP by compress­
ing and distorting the hypothalamus,326 but the puberty is reversed 
with regression of the hormonal and physical features of puberty 
after decompression of the cyst due to reversal of the disinhibi­
tion o

 the cyst due to reversal of the disinhibi­
tion of the CNS inhibitory mechanism of the posterior pituitary 
(Fig. 26.30). Precocious sexual maturation can be induced in the 
juvenile female rhesus monkey by posterior hypothalamic lesions; 
such lesions advance the age at onset of a pubertal increase in 
LH secretion and the time of the first positive feedback effects of 
estrogen.385
The GnRH-secreting hypothalamic hamartoma, a hetero­
typic mass of nervous tissue that contains GnRH neurosecre­

s of nervous tissue that contains GnRH neurosecre­
tory neurons attached to the tuber cinereum or the floor of the 
third ventricle, can cause CPP.422,423 The GnRH neurons within 
the hamartoma with their axon fibers projecting to the median 
eminence secrete GnRH in pulsatile fashion. We consider the 
hypothalamic hamartoma to be an ectopic GnRH pulse genera­
tor that functions independently of the CNS inhibitory mecha­
nism, which normally restrains the hypothalamic GnRH pulse 
generator (Fig.

rains the hypothalamic GnRH pulse 
generator (Fig. 26.31).326,423 An analogy can be drawn between 
the GnRH-secreting hypothalamic hamartoma and the rescue of 
fertility in GnRH-deficient hypogonadal (hyp/hyg) mice by trans­
plantation of fetal or neonatal hypothalamic tissue into the third 
ventricle. Some rare, large hypothalamic hamartomas that cause 
CPP contain TGFα, an astroglia-derived growth factor, with few 
or no GnRH neurosecretory neurons, raising the possibility that 
the secretion 

rons, raising the possibility that 
the secretion of TGFα may interact directly or indirectly to stimu­
late GnRH release.
The recent discovery of families with precocious puberty asso­
ciated with loss of function mutations in the MKRN3 gene has 
led to important insights into the CNS “brake” that intrinsically 
restrains puberty.424,425 MKRN3 decreases in mice at the same 
time that KISS and neurokinin B rise to promote puberty.
MKRN3 is imprinted as the maternal allele is methylated and 
supp

ted as the maternal allele is methylated and 
suppressed in normal individuals and only the paternal gene func­
tions; with the mutation in the paternal MKRN3 there is no longer 
function. MKRN3 encodes makorin ring finger protein 3, which 
is involved with ubiquitination and cell signaling, and is expressed 
in the arcuate nucleus, a location where other genes involved with 
puberty are found. MKRN3 concentrations in blood decrease in 
both boys and girls prior to the onset of puberty.426,427,9

 and girls prior to the onset of puberty.426,427,984
At present, more than 10 different loss-of-function mutations 
of MKRN3 have been described in CPP, including eight frame­
shift defects, three missense mutations, one nonsense mutation, 
and a most recent heterozygous deletion in the MKRN3 promoter 
region428,429 (see Fig. 26.24B).
The following affect the hypothalamic GnRH pulse genera­
tor: noradrenergic, dopaminergic, serotoninergic, and opioidergic 
pathways; inhibitory neurotransmitters 

ioidergic 
pathways; inhibitory neurotransmitters (e.g., GABA); excitatory 
amino acids (e.g., glutamic acid, aspartic acid); nitrergic trans­
mitters; other brain peptides, including neurotrophic and growth 
factors; and corticotropin-releasing hormone (CRH) (see Table 
26.13). Melatonin is not a critical restraining factor in primates, 
nor are endogenous opioid peptides.326,330,384
The GnRH pulse generator is inhibited by GABA (the most 
important inhibitory neurotransmitter in the primate br

tant inhibitory neurotransmitter in the primate brain) and 
GABAergic neurons during prepuberty, but exogenous adminis­
tration of GABA in prepuberty is ineffective because of the high 
local endogenous GABA levels328,329 (Fig. 26.32). Both GAD65 
and GAD67 forms of glutamic acid decarboxylase (GAD), the 
enzyme that catalyzes the conversion of glutamate to GABA, are 
present in the mediobasal hypothalamus, the site of the GnRH 
pulse generator. Antisense oligodeoxynucleotides for GAD67 and 
GAD

Antisense oligodeoxynucleotides for GAD67 and 
GAD65 mRNAs infused into the stalk median eminence of pre­
pubertal monkeys induced a striking increase in GnRH release, 
whereas nonsense d-oligos did not. These studies provided addi­
tional support for GABA arising from interneurons as the intrinsic 
CNS inhibitor during the juvenile pause of prepuberty.
GABA acting through GABAA and GABAB receptors affects 
GnRH secretion in the perifused mouse GT1 GnRH-releasing 
neuronal cell line. Stimulation

T1 GnRH-releasing 
neuronal cell line. Stimulation of GABAB receptors also inhibits 
kisspeptin secretion in the adult mouse.430 Conversely, chronic, 
repetitive administration of bicuculline, a GABA inhibitor, into 
the base of the third ventricle of a prepubertal monkey caused pre­
mature menarche and the onset of the first ovulation.328 Although 
GABA is inhibitory in the juvenile and adult brain, it is excit­
atory early in brain development through the postnatal period.431 
1075
Chapter 26 

through the postnatal period.431 
1075
A
B
C
D
• Fig. 26.30  (A) True precocious puberty in a 2.75-year-old girl is caused by a large, bilateral, congenital suprasellar arachnoid cyst. Signs of sexual precoc­
ity were observed during the preceding year. The head circumference was +5 SD above the mean value for age, and frontal bossing was present. Breasts 
were Tanner stage 3. The serum estradiol level was 26 pg/mL, the estrone level was 38 pg/mL, 

vel was 26 pg/mL, the estrone level was 38 pg/mL, and the dehydroepiandrosterone sulfate (DHEAS) level 
was less than 3 μg/dL. The serum luteinizing hormone (LH) concentration rose from 1.4 to 8.7 ng/mL (LER-960) after intravenous administration of gonad­
otropin-releasing hormone (GnRH), which constitutes a pubertal response. Bone age was 3.5 years. Pelvic ultrasonography showed pubertal-size uterus 
and ovaries. To convert estrone values to picomoles per liter, multiply by 3.699. To convert DH

omoles per liter, multiply by 3.699. To convert DHEAS values to micromoles per liter, multiply by 0.02714. For 
other conversions, see Fig. 26.19. (B) Cranial computed tomography (CT) scans show a low-density fluid collection in the middle cranial fossa, thinning 
of the cortex, and striking compression of the lateral and third ventricles. (C) Cranial CT scans 8 months later, after decompression of the arachnoid cyst 
and creation of a communication between the cyst and the basal cerebrospinal f

ion between the cyst and the basal cerebrospinal fluid cisterns and a cystoperitoneal shunt. Notice the striking decrease in 
size of the fluid collections and expansion of the cerebral cortex. (D) Basal and peak LH and follicle-stimulating hormone (FSH) concentrations after GnRH 
administration in SM and serum estradiol values before surgical decompression and 2 weeks and 9 months after surgical decompression of the arachnoid 
cyst. Notice the prepubertal LH response to GnRH and fall in serum e

repubertal LH response to GnRH and fall in serum estradiol level by 9 months after surgery. The bone age had increased by 3 years over 
an 11-month period, but the velocity returned to normal. The patient remained prepubertal during follow-up. LRF, luteinizing hormone-releasing factor; 
SD, standard deviation. (From Grumbach MM, Kaplan SL. The neuroendocrinology of human puberty: an ontogenetic perspective. In: Grumbach MM, 
Sizonenko PC, Aubert ML, eds. Control of the Onset of Puberty. Baltimor

ML, eds. Control of the Onset of Puberty. Baltimore, MD: Williams & Wilkins; 1990:1–68.)
1076
Intrinsic CNS
inhibitor
(–)
GnRH pulse
generator
OR
Hamartoma
Hamartoma
(–)
GnRH pulse
generator
hamartoma
• Fig. 26.31  Hypothalamic hamartoma as an ectopic gonadotropin-releasing hormone (GnRH) pulse gen­
erator that escapes the intrinsic central nervous system (CNS) inhibitory mechanism and results in true 
precocious puberty. Two possible me

sults in true 
precocious puberty. Two possible mechanisms are proposed. Left, The GnRH neurosecretory neurons in 
the hamartoma are functioning as a GnRH pulse generator without activation of the suppressed, normally 
located GnRH pulse generator. Right, The hamartoma acts as an ectopic GnRH pulse generator but com­
municates with and activates (possibly through axonic connections or by GnRH itself) the normally located 
hypothalamic GnRH pulse generator, which then functions synchronously with

generator, which then functions synchronously with the hamartoma.
The switch from dominance of the gonadal steroid-dependent 
negative feedback mechanism in infancy and early childhood to 
dominance of the intrinsic CNS inhibitory mechanism may be 
associated with the developmental switch of GABAergic synaptic 
transmission from excitatory to inhibitory.
The onset of puberty in the rhesus monkey is characterized by 
a decrease in GABAergic (and possibly neuropeptide Y [NPY]) 
inhibition of the h

ossibly neuropeptide Y [NPY]) 
inhibition of the hypothalamic GnRH pulse generator and 
increased release of glutamate,329 the major excitatory amino acid 
neurotransmitter in the hypothalamus.328 The sensitivity to the 
stimulatory glutamatergic input into the GnRH pulse generator 
increases strikingly after the onset of puberty, but it is the reduc­
tion in GABAergic inhibition that is the critical factor in disinhi­
bition of the GnRH pulse generator.329
A persistent question has been how a s

nerator.329
A persistent question has been how a single central signal can 
activate GnRH neurons to cause LH release and bring about ovu­
lation by simultaneous suppression of GABA and stimulation of 
glutamate release, both of which converge in the AVPV nucleus. 
Most neurons in the AVPV of female rats express both vesicular 
glutamate transporter 2 (VGLUT2), a marker of hypothalamic 
glutamatergic neurons, and GAD and vesicular GABA transporter 
(VGAT), markers of GABAergic neurons. These dua

r 
(VGAT), markers of GABAergic neurons. These dual-phenotype 
neurons are twice as prevalent in females than in males and are 
the main targets of the E2 binding site in the region.432 More­
over, dual-phenotype synaptic terminals contact GnRH neurons, 
and at the time of the surge, VGAT-containing vesicles decrease 
and VGLUT2-containing vesicles increase in these terminals. 
Dual-phenotype GABA/glutamate neurons may act as central 
transducers of hormonal and neural signals to GnRH neurons to

 of hormonal and neural signals to GnRH neurons to 
simultaneously decrease GABA and increase glutamate release.
NELL2, a protein containing epidermal growth factor (EGF)–
like repeats, is selectively expressed in the glutamatergic neurons 
containing VGLUT1 and in those expressing VGLUT2 in the 
postnatal rodent brain. NELL2 mRNA abundance increases selec­
tively in the medial basal hypothalamus of the female rat, reaching 
a peak at the end of the juvenile period, and declines at the time 
of 

the juvenile period, and declines at the time 
of puberty with less change observed in the preoptic area. Intra­
ventricular administration of antisense oligodeoxynucleotides 
to NELL2 reduced GnRH release from the medial basal hypo­
thalamus and delayed the initiation of female puberty. Therefore 
NELL2 plays an important role in glutamate-dependent processes 
of neuroendocrine regulation in puberty.433
Excitatory NMDA stimulates LH release in neonatal and adult 
rats, fetal sheep, and prepuber

eonatal and adult 
rats, fetal sheep, and prepubertal and adult rhesus monkeys, and 
its receptors are widely distributed throughout the CNS, includ­
ing the hypothalamus. NMDA evoked GnRH secretion from rat 
hypothalamic explants from a GnRH neuronal cell line and acutely 
stimulated the GnRH pulse generator in the ovine fetus but did 
not have a direct effect on pituitary gonadotrophs.332,381,385,434
Immortalized GnRH neurons contain inotropic NMDA 
receptors that mediate the release of GnRH b

NMDA 
receptors that mediate the release of GnRH by NMDA.404 The 
prepubertal male rhesus monkey may be forced to enter puberty 
by repetitive intravenous administration of NMDA; in the prepu­
bertal and pubertal female rhesus monkey, NMDA administered 
centrally and peripherally induced the release of GnRH.435
NPY has been suggested as a component of the central 
restraint mechanism in the male rhesus monkey. A small study 
of girls demonstrated higher NPY concentrations in those with 
CDP than

 higher NPY concentrations in those with 
CDP than in those with normal onset of puberty, supporting this 
relationship.332,436
These observations provide additional evidence that the hypo­
thalamic GnRH neurosecretory neuron is not a limiting factor in 
puberty, as is the GnRH pulse generator; the anterior pituitary 
gland, gonads, and gonadal steroid end organs are functionally 
intact in the fetus and prepubertally and can be fully activated 
by the appropriate stimulus. CNS restraint of pube

by the appropriate stimulus. CNS restraint of puberty lies there­
fore above the level of the autorhythmic GnRH neurosecretory 
neurons in the hypothalamus. Fig. 26.13 contrasts the direct and 
indirect effects of the GABA inhibitory and excitatory amino acid 
stimulatory neurotransmitters (as represented by NMDA and 
1077
other glutamate receptors) on GnRH release. In the primate, the 
GABA hypothalamic neural network seems to be the major com­
po

lamic neural network seems to be the major com­
ponent of the intrinsic CNS inhibitory mechanism during the 
juvenile pause. 
Sleep-Associated Luteinizing Hormone Release and Onset 
of Puberty
In sensitive radioimmunoassays, a diurnal rhythm of serum LH, 
FSH, and testosterone is already demonstrable in short but oth­
erwise normal girls age 5 to 6 years, demonstrating that prepa­
ration for the changes of puberty starts long before the physical 
features and classic endocrine markers of puberty

features and classic endocrine markers of puberty appear.252,326 
Although adult men and women during most phases of the men­
strual cycle have little difference in the amplitude or frequency of 
LH pulses over a 24-hour period, sleep-associated pulsatile release 
of LH is prominent in early and midpuberty; only in late puberty 
are prominent LH-secretory episodes detected during the day, 
but they are still less than during sleep until the adult pattern is 
achieved. Augmented LH release durin

t pattern is 
achieved. Augmented LH release during sleep leads to a rise in 
the plasma concentration of testosterone at night in boys, in chil­
dren with CPP, in glucocorticoid-treated children with CAH who 
have an advanced bone age and early onset of true puberty, and 
in agonadal patients during the pubertal age period, suggesting 
that it does not depend on gonadal function. There is significantly 
increased excretion of urinary LH in prepubertal children at night 
compared with the day.
S

bertal children at night 
Sleep-enhanced LH secretion can be viewed as a maturational 
phenomenon related to changes in the CNS and in the hypotha­
lamic restraint of GnRH release. Episodic release of gonadotropins 
is suppressed by anti-GnRH antibodies and by the administra­
tion of gonadal steroids or certain catecholaminergic agonists and 
antagonists and is augmented by the opioid antagonist naloxone. 
Naloxone does not alter the testosterone-mediated suppression 
of L

 alter the testosterone-mediated suppression 
of LH, nor does it alter the testosterone effect on LH pulsatility 
in boys in early to middle puberty. We have suggested that an 
increase in endogenous GnRH secretion at puberty has a priming 
effect on the gonadotroph and leads to increased sensitivity of the 
pituitary to endogenous or exogenous GnRH.256 In monkeys, a 
striking increase in pulse amplitude and a lesser increase in pulse 
frequency occurs between prepuberty and puberty.328,332 Slee

occurs between prepuberty and puberty.328,332 Sleep-
associated LH release in the peripubertal period correlates with 
increased sensitivity of the pituitary gonadotrophs to administra­
tion of GnRH in the peripubertal period and in puberty and is an 
indication that the hypothalamic GnRH pulse generator initially 
is less inhibited during sleep, even in prepubertal children. 
Pituitary and Gonadal Sensitivity to Tropic Stimuli
Endogenous GnRH secretion is estimated indirectly and quali­
tativel

cretion is estimated indirectly and quali­
tatively by determining the pulsatile pattern of LH and by the 
gonadotropin response to exogenous GnRH at different stages425 
and in disorders of the hypothalamic-pituitary-gonadal system. 
The release of LH after administration of GnRH is minimal in 
prepubertal children beyond infancy, increases during the peripu­
bertal period and puberty (Fig. 26.33), and is still greater in adults 
(depending on the phase of the menstrual cycle in women).256 
The

he phase of the menstrual cycle in women).256 
These results support the concept that the prepubertal state is 
characterized by functional GnRH deficiency. FSH release after 
the administration of GnRH is comparable in prepubertal, puber­
tal, and adult males, indicating similar pituitary sensitivity to 
GnRH, but females release more FSH than males at all stages of 
sexual maturation. There is a striking reversal of the FSH/LH ratio 
after administration of GnRH to males or females between pre

nistration of GnRH to males or females between prepu­
berty and puberty (see Fig. 26.33).
These observations suggest a striking change in pituitary sen­
sitivity to GnRH in prepubertal and pubertal individuals and 
indicate a sex difference in the dynamic reserve of pituitary FSH 
because the pituitary gonadotrophs of prepubertal females are 
more sensitive to GnRH than those of prepubertal males, even 
though the concentration of circulating gonadal steroids is very 
low in both sexes at that s

adal steroids is very 
low in both sexes at that stage of maturation.256 Prepubertal girls 
have a larger readily releasable pool of pituitary FSH than pre­
pubertal or pubertal males, possibly related in part to the higher 
concentration of inhibin B in prepubertal boys (see Fig. 26.33). 
These may be factors in the higher frequency of idiopathic CPP in 
girls and in the occurrence of premature thelarche.437 The avail­
able data are consistent with the hypothesis that less GnRH is 
required for

ith the hypothesis that less GnRH is 
required for FSH than for LH release.
This change in responsiveness of the gonadotrophs is appar­
ently mediated by increased pulsatile secretion of GnRH256,326; 
the increased LH response to synthetic GnRH is one of the earli­
est hormonal markers of puberty onset. Studies of the effects of 
acute and chronic administration of synthetic GnRH in hyper­
gonadotropic hypogonadism, hypogonadotropic hypogonadism, 
constitutionally delayed growth and adolescence,

constitutionally delayed growth and adolescence, and idiopathic 
GABA
GABA (ng/mL)
Early
prepubertal
Midpubertal
**
**
2.0
1.0
0.0
Early
**
*
3.0
1.0
2.0
0.0
GnRH
• Fig. 26.32  The striking developmental changes in γ-aminobutyric acid 
(GABA) and gonadotropin-releasing hormone (GnRH) release between the 
prepubertal and the pubertal rhesus monkey as measured in 10-minute 
perfusate samples from the stalk of the median eminence. Multi

mples from the stalk of the median eminence. Multiple sam­
ples were obtained from each animal. Mean ± SEM (standard error of the 
mean); **p < 0.01; *p < 0.05 vs prepubertal monkeys. (From Mitsushima 
D, Hei DL, Terasawa E. γ-Aminobutyric acid is an inhibitory neurotransmit­
ter restricting the release of luteinizing hormone-releasing hormone before 
the onset of puberty. Proc Natl Acad Sci U S A. 1994;91:395–399.)
1078
precocious puberty indicate t

gy and the Life Span
precocious puberty indicate that the degree of previous exposure 
of gonadotrophs to endogenous GnRH appears to affect both the 
magnitude and the quality of LH responses—a self-priming phe­
nomenon.256 With the approach of puberty, the derepression of 
the hypothalamic GnRH pulse generator and the increased pul­
satile secretion of GnRH augment pituitary sensitivity to GnRH 
and enlarge the reserve of LH. Reduction in the frequency of 
exogenous GnRH pulses (from one per ho

equency of 
exogenous GnRH pulses (from one per hour to one every 3 hours) 
in adult rhesus monkeys with ablative hypothalamic lesions that 
eliminated endogenous GnRH secretion increased the FSH/LH 
ratio, suggesting that GnRH pulse frequency is one factor affect­
ing relative secretion of FSH and LH. Inhibin and endogenous 
gonadal steroids may also affect this ratio through action on the 
hypothalamus or the pituitary or both.
Pulsatile administration of GnRH to prepubertal monkeys 
promptly 

stration of GnRH to prepubertal monkeys 
promptly initiates puberty (and ovulatory menstrual cycles in 
females) and restores complete gonadal function in adult mon­
keys with hypothalamic lesions.332,438 Similar studies in humans 
yielded comparable results for prepubertal children, patients with 
anorexia nervosa, and adults with hypothalamic-hypogonado­
tropic hypogonadism.439 These results provide further support for 
reactivation of the hypothalamic GnRH pulse generator as the 
first hormon

thalamic GnRH pulse generator as the 
first hormonal change in the onset of puberty.
Responsiveness of the gonads to gonadotropins increases dur­
ing puberty. For example, the augmented testosterone secretion in 
response to administration of hCG at puberty in boys is probably 
a consequence of the priming effect of the increase in endogenous 
secretion of LH (in the presence of FSH) on the Leydig cells. 
Maturation of Positive Feedback Mechanism
Estrogen exerts suppressive effects from late fet

Estrogen exerts suppressive effects from late fetal life to peripu­
berty, when the positive action of endogenous (or exogenous) 
estradiol on gonadotropin release is not demonstrated.256,326 A 
positive feedback effect, which is required for ovulation, is a late 
maturational event in puberty and probably does not occur before 
midpuberty in normal girls. The positive feedback effect requires 
an increased concentration of plasma estradiol for a sufficient 
length of time during the latter par

a sufficient 
length of time during the latter part of the follicular phase in later 
pubertal and adult women.336
Among the requirements for the positive feedback action of 
estradiol on gonadotropin release at puberty are ovarian follicles 
that are primed by FSH to secrete sufficient estradiol to reach and 
maintain a critical level in the circulation, a pituitary gland that is 
sensitized to GnRH and contains a large enough pool of releasable 
LH to support an LH surge, and sufficient GnRH s

LH to support an LH surge, and sufficient GnRH stores for the 
GnRH neurosecretory neurons to respond with an acute increase 
in GnRH release in addition to the usual adult pattern of pulsatile 
GnRH secretion (this last requirement is controversial in humans 
but not in lower animals).256
Estrogen exerts effects at the anterior pituitary and the hypo­
thalamus.440 In the rhesus monkey, positive and negative feedback 
can occur in adult ovariectomized females in whom the medial 
basal hypothal

tomized females in whom the medial 
basal hypothalamus is surgically disconnected from the remainder 
of the CNS.332 In monkeys with hypothalamic lesions, unvary­
ing, intermittent GnRH administration leads to sufficient estra­
diol release from the ovary to induce an ovulatory LH surge in 
the absence of an increase in the dose of the GnRH pulses. 336,438 
Estradiol has a positive feedback effect directly on the pituitary 
gland in normal women, and prolonged administration of estra­
diol is ac

 and prolonged administration of estra­
diol is accompanied by an augmented LH response to GnRH 
administration. The fact that the major positive feedback action 
on the pituitary gland is demonstrable in the absence of an 
increase in pulsatile GnRH secretion suggests that the failure to 
elicit positive feedback action with administration of estradiol to 
prepubertal girls may be related to the inadequate GnRH pulses 
or insufficient LH reserve or both.
0
1
2
3
4
5
6
-15
Minutes
Hours
0 15 30

 or both.
0
1
2
3
4
5
6
-15
Minutes
Hours
0 15 30
60
2
3
4
LH
(ng/mL) 
(LER 960)
FSH
(ng/mL)
(LER 869)
100 µg GnRH
0
1
2
3
4
5
6
−15
Minutes
Hours
0 15 30
60
2
3
4
GnRH
Pubertal
Adult 
&
&
n = 39 (10 µg/M2, 100 µg GnRH)
n = 32 (10 µg/M2, 100 µg GnRH)
n = 14 (5 µg/M2, 100 µg GnRH)
n = 13
n = 5
Pubertal
Adult 
n = 15
n = 9
• Fig. 26.33  Changes in plasma luteinizing hormone (LH) (top) and follicle-
stimulating hormone (FSH) (bottom) levels in prepubertal, pubert

rmone (FSH) (bottom) levels in prepubertal, pubertal, and adult 
individuals. Notice the limited LH response in prepubertal children compared 
with that of pubertal and adult subjects. The FSH response to gonadotro­
pin-releasing hormone (GnRH) is similar in prepubertal, pubertal, and adult 
males. In females, the FSH response is significantly greater than that of pre­
pubertal, pubertal, or adult males. For conversion to SI units, see Fig. 26.19. 
(Modified from Grumbach MM, Roth JC, Kaplan SL,

. 
(Modified from Grumbach MM, Roth JC, Kaplan SL, et al. Hypothalamic 
pituitary regulation of puberty in man: evidence and concepts derived from 
clinical research. In: Grumbach MM, Grave GD, Mayer FE, eds. Control of 
the Onset of Puberty. New York, NY: John Wiley & Sons; 1974:115–166.)
1079
Gonadotropin cyclicity and estradiol-induced positive feed­
back can be demonstrated by midpuberty and before menarche 
but may be insufficient to induce an

ore menarche 
but may be insufficient to induce an ovulatory LH surge even 
when there is an adequate pituitary store of readily releasable 
LH and FSH.1,256 The ovary does not secrete estradiol at a 
high level or long enough to induce an ovulatory LH surge. 
We visualize the process leading to ovulation as a gradual one 
in which the ovary (i.e., the zeitgeber for ovulation336) and the 
hypothalamic-pituitary-gonadal complex become progressively 
more integrated and synchronous until an ovary 

y 
more integrated and synchronous until an ovary primed for 
ovulation secretes sufficient estradiol to induce an ovulatory 
LH surge.
As many as 55% to 90% of cycles are anovulatory during the 
first 2 years after menarche, but the proportion decreases to less 
than 20% of cycles by 5 years after menarche.441 The mechanism 
of ovulation seems unstable and immature, and it does not appear 
to have attained the fine tuning and synchronization that are req­
uisite for maintenance of regular ovula

t are req­
uisite for maintenance of regular ovulatory cycles. However, the 
prevalence of PCOS adds to the irregularity of menses and anovu­
lation in puberty. 
Overview of Current Concept
Puberty is not an immutable process; it can be arrested or reversed. 
Environmental factors and certain disorders that affect the onset 
or progression of puberty mediate their effects by direct or indirect 
suppression of the hypothalamic GnRH pulse generator and its 
periodic oscillatory signal, GnRH. Table

 and its 
periodic oscillatory signal, GnRH. Table 26.14 lists some of these 
factors. 
Adrenal Androgens and Adrenarche
Speculation has focused on the mechanism of adrenarche (the 
adrenal component of pubertal maturation), the fact that adre­
narche occurs earlier than gonadarche (the maturation of the 
hypothalamic-pituitary-gonadal system), and the interaction 
between adrenal and gonadal hormones at puberty.290,325,442
Nature and Regulation of Adrenal Androgens
The major adrenal androgen pr

of Adrenal Androgens
The major adrenal androgen precursors secreted by the adrenal 
cortex are DHEA, DHEAS, and androstenedione, which can 
undergo extraglandular metabolism to produce physiologically 
active testosterone and estradiol159; however, adrenal andro­
gens do not directly activate the AR. DHEA and especially 
DHEAS (which binds avidly to serum proteins, particularly 
albumin) are useful biochemical markers of adrenal androgen 
secretion and the onset of adrenarche. Androstenedione is

on and the onset of adrenarche. Androstenedione is the 
major androgen secreted by the ovary during and after puberty, 
and it is more readily converted to potent androgens than 
DHEA or DHEAS.
Cross-sectional and longitudinal studies have demonstrated a 
progressive increase in the plasma concentration of DHEA and 
DHEAS in boys and girls starting by 3 years of age and becomes 
more noticeable approximately 2 years before the increase in 
gonadotropin and gonadal steroid secretion that continue

tropin and gonadal steroid secretion that continues 
through puberty (13–15 years),290,325,443,444 reaches a peak at age 
20 to 30 years, and then gradually decreases (Fig. 26.34).442 The 
increase is not associated with increased sensitivity of the pitu­
itary gonadotrophs to GnRH430 or with sleep-associated LH 
secretion and occurs at an age when the hypothalamic-pituitary-
gonadal complex is functioning at a low level.290,437 The impor­
tance of adrenarche is a matter of long-term debate. DHE

of adrenarche is a matter of long-term debate. DHEA is a 
neurosteroid with parallel patterns of increase along with cortical 
maturation from approximately age 6 years to the middle 20s, 
suggesting that adrenarche affects brain development. DHEAS 
may increase activity of the amygdala and hippocampus and pro­
mote synaptogenesis within the cortex, with effects on fearfulness, 
  Postulated Ontogeny of the 
Hypothalamic-Pituitary-Gonadal Circuit
Fetus
Medial basal hypothalamic GnRH neurosecret

t
Fetus
Medial basal hypothalamic GnRH neurosecretory neurons (pulse genera­
tor) operative by 80 days of gestation
Pulsatile secretion of FSH and LH by 80 days of gestation
Initially unrestrained secretion of GnRH (100–150 days of gestation)
Maturation of negative gonadal steroid feedback mechanism by 150 
days of gestation—sex difference
Low level of GnRH secretion at term
Early Infancy
Hypothalamic GnRH pulse generator highly functional after 12 days of 
age
Prominent FSH and LH episodic disc

12 days of 
age
Prominent FSH and LH episodic discharges until approximately age 
6 mo in males and 18 mo in females, with transient increases in 
plasma levels of testosterone in males and estradiol in females
Late Infancy and Childhood
Intrinsic CNS inhibition of hypothalamic GnRH pulse generator operative; 
predominant mechanism in childhood; maximal sensitivity by ap­
proximately 4 years of age
Negative feedback control of FSH and LH secretion highly sensitive to 
gonadal steroids (low set-p

n highly sensitive to 
gonadal steroids (low set-point)
GnRH pulse generator inhibited; low amplitude and frequency of GnRH 
discharges
Low secretion of FSH, LH, and gonadal steroids
Late Prepubertal Period
Decreasing effectiveness of intrinsic CNS inhibitory influences and 
decreasing sensitivity of hypothalamic-pituitary unit to gonadal 
steroids (increased set-point)
Increased amplitude and frequency of GnRH pulses, initially most promi­
nent with sleep (nocturnal)
Increased sensitivity of go

with sleep (nocturnal)
Increased sensitivity of gonadotrophs to GnRH
Increased secretion of FSH and LH
Increased responsiveness of gonad to FSH and LH
Increased secretion of gonadal hormones
Puberty
Further decrease in CNS restraint of hypothalamic GnRH pulse generator 
and in the sensitivity of negative feedback mechanism to gonadal 
steroids
Prominent sleep-associated increase in episodic secretion of GnRH 
gradually changes to adult pattern of pulses about every 90 min
Pulsatile secretion of 

 pulses about every 90 min
Pulsatile secretion of LH follows pattern of GnRH pulses
Progressive development of secondary sexual characteristics
Spermatogenesis in males
Middle to late puberty—operative positive feedback mechanism and 
capacity to exhibit an estrogen-induced LH surge
Ovulation in females
CNS, Central nervous system; FSH, follicle-stimulating hormone; GnRH, gonadotropin-releas­
ing hormone; LH, luteinizing hormone.
Modified from Grumbach MM, Roth JC, Kaplan SL, et al. Hypothalamic

umbach MM, Roth JC, Kaplan SL, et al. Hypothalamic-pituitary regulation 
of puberty in man: evidence and concepts derived from clinical research. In: Grumbach MM, 
Grave GD, Mayer FE, eds. Control of the Onset of Puberty. New York: John Wiley & Sons; 
1974:115–166.
   
TABLE 26.14
1080
anxiety, and memory that increase social interaction with unfamil­
iar individuals and shape cognitive development.445
Associated with the increase in adrenal secretio

5
Associated with the increase in adrenal secretion of DHEA 
and DHEAS (and independent of a change in the secretion 
of cortisol or aldosterone) is the appearance and growth of 
the zona reticularis (i.e., the principal source of DHEA 
and DHEAS) that occurs coincidently with adrenarche 
(Fig. 26.35).
In contrast to the zona glomerulosa and fasciculata, four main 
features distinguish the zona reticularis:
	1.	 There is a low level of expression of 3βHSD/Δ4,5-isomerase 
type 2 and CYP21 mRNAs 

tigens, which are not expressed 
in the fetal zone of the fetal adrenal cortex.
In contrast to the zona fasciculata, the zona reticularis has an 
increased ratio of 17,20-lyase to 17α-hydroxylase. An Arg347Ala 
mutation in human CYP17 resulted in strikingly decreased 17,20-
lyase activity but retention of 17α-hydroxylase activity. Two XY 
phenotypic females with hypergonadotropic hypogonadism and 
normal mineralocorticoid and glucocorticoid function had iso­
lated 17,20-lyase deficiency due to h

ion had iso­
lated 17,20-lyase deficiency due to homozygous mutations at the 
Arg347 or the Arg358 residue in CYP17.
In contrast to these observations of loss of 17,20-lyase activ­
ity with retention of 17α-hydroxylase activity, the ratio of human 
17,20-lyase to 17α-hydroxylase activities was increased by 
increased phosphorylation of serine and threonine residues on the 
CYP17 enzyme and by the increased abundance of redox partners 
such as cytochrome P450 oxidoreductase and by cytochrome b5, 

ochrome P450 oxidoreductase and by cytochrome b5, 
which preferentially promotes 17,20-lyase activity by allosterically 
affecting the interaction between CYP17 and P450 oxidoreduc­
tase.446 These studies provided a provisional hypothesis of the 
mechanisms that appear to be involved in the relatively increased 
17,20-lyase activity of the zona reticularis, although not its regula­
tion (see Fig. 26.35).
Regulation of adrenal androgen secretion in the zona reticu­
laris is postulated to be based

n the zona reticu­
laris is postulated to be based on a dual-control mechanism. First, 
corticotropin (adrenocorticotropic hormone [ACTH]) is obliga­
tory, as evidenced by the findings in cases of ACTH deficiency 
or resistance. Second, the mechanism requires the action of an 
unidentified adrenal androgen-stimulating factor, possibly pitu­
itary in origin or from a nonadrenal source or an intra-adrenal 
event.301 This concept is illustrated in Fig. 26.36.443
CRH has been advanced as an adrenal 

ig. 26.36.443
CRH has been advanced as an adrenal androgen secreta­
gogue that has stimulatory action on the zona reticularis. The 
intravenous infusion of human CRH into dexamethasone-sup­
pressed young men increased DHEA, DHEAS, and androstene­
dione secretion within 3 hours. Similar results were obtained 
in adolescent girls with hyperandrogenism and a history of 
premature adrenarche. CRH directly stimulates DHEAS secre­
tion and the expression of CYP17 by the fetal adrenal cortical 
cells.4

on of CYP17 by the fetal adrenal cortical 
cells.447 Leptin in vitro vigorously stimulates 17,20-lyase activ­
ity and transiently stimulates 17α-hydroxylase activity of the 
microsomal enzyme CYP17, implying a role in adrenarche,448 
but no clinical evidence suggests a pivotal role of leptin in adre­
narche. Therefore a distinct hormone or factor that in addition 
to ACTH stimulates the zona reticularis and adrenal androgen 
secretion has not been isolated, and the mechanism regulating 
adrenarc

n isolated, and the mechanism regulating 
adrenarche remains unknown.325
Dhom, 1973
Reiter et al, 1977
DHAS (µg/dL)
% of subjects with
continuous zona reticularis
Change of serum DHAS with age related
to growth of zona reticularis
Development of the zona reticularis
Growth of the adrenal
Adult cortex
Medulla
Swinyard, 1943
Adrenal volume (mm3)
% of subjects
Years
No reticular zone
Focal reticular zone
Continuous reticular zone
Dhom, 1973
100
50
0
18
16
14
12
10
8
6
4
2
0
300
200
100
0
A
B
C
2
4

18
16
14
12
10
8
6
4
2
0
300
200
100
0
A
B
C
2
4
6
8
10
12
14
16
1000
2000
3000
4000
Years
Years
18
16 17
14
12
10
8
6
4
2
15
13
11
9
7
5
3
300
50
0
• Fig. 26.34  Relation of plasma dehydroepiandrosterone sulfate (DHEAS 
[DHAS] ) levels to growth of the zona reticularis and increase in adrenal 
volume with age. (A) The close correlation between the development of the 
zona reticularis and the increase in the plasma DHEAS level. (B) The age 
at which focal islands of reticular tissue or a contin

hich focal islands of reticular tissue or a continuous reticular zone was 
found in a series of patients with sudden death who had not had an ante­
cedent illness. (C) The increase in adrenal volume at the time of puberty. For 
conversion to SI units, see Fig. 26.19. (From Grumbach MM, Richards HE, 
Conte FA, et al. Clinical disorders of adrenal function and puberty: assess­
ment of the role of the adrenal cortex and abnormal puberty in man and 
evidence for an ACTH-like pituitary adrenal androg

evidence for an ACTH-like pituitary adrenal androgen stimulating hormone. 
In: James VHT, Serio M, Giusti G, et al, eds. The Endocrine Function of the 
Human Adrenal Cortex. New York, NY: Academic Press; 1978:583–612.)
1081
A distinct adrenal androgen-stimulating factor, whether of 
pituitary, intra-adrenal, or other origin, may explain the following 
observations301:
	1.	 The spurt in adrenal growth and the differentiation and growth 
of the zona

th and the differentiation and growth 
of the zona reticularis at adrenarche occur independently of an 
increase in ACTH or cortisol secretion but correlate with the 
increase in plasma levels of DHEAS (see Fig. 26.35).
	2.	 Cortisol and adrenal androgen secretions vary independently 
with age, during normal and premature adrenarche, and in 
Cushing disease, starvation, malnutrition, anorexia nervosa, 
and chronic disease.
	3.	 Unlike cortisol secretion, the secretion of DHEA and DHEAS 
in res

secretion, the secretion of DHEA and DHEAS 
in response to ACTH administration varies with age.
	4.	 Dissociation of adrenarche and gonadarche occurs in a vari­
ety of disorders of sexual maturation (see Fig. 26.36), includ­
ing premature adrenarche (i.e., onset of pubic or axillary hair 
before 8 years of age), chronic adrenal insufficiency, CPP 
(when the onset is before age 6 years), primary hypogonadism, 
isolated gonadotropin deficiency, and anorexia nervosa.442
A longitudinal study of 42 

d anorexia nervosa.442
A longitudinal study of 42 children demonstrated that an 
increase in BMI (but not the value itself at any age) was related 
to the rise in urinary excretion of DHEAS, suggesting that a 
change in nutritional status is one physiologic regulator of 
adrenarche.444,449
The increase in DHEA before and at adrenarche might suggest a 
similarity to the actions of the fetal adrenal zone; indeed the histo­
logic finding and biochemistry are similar in both the fetal adrenal 
zone 

istry are similar in both the fetal adrenal 
zone and the zona reticularis. However, the fetal adrenal zone under­
goes involution at birth; in addition biomarkers such as enzyme 
AKR1C3 (17β-hydroxysteroid dehydrogenase type 5 [17βHSD5]) 
are higher in the zona reticularis than the fetal adrenal zone.325 
Adrenal Androgens and Puberty
The earlier onset of adrenarche than gonadarche and the contri­
bution of adrenal androgens to the growth of pubic and axillary 
hair have led some to suggest tha

ic and axillary 
hair have led some to suggest that adrenal androgens in normal 
children are an important factor in the onset of puberty and the 
maturation of the hypothalamic-pituitary-gonadal complex.
Although CPP may occur when the prepubertal child has 
previously been exposed to excessive levels of androgens from 
an endogenous or exogenous source (e.g., after the initiation of 
glucocorticoid therapy in congenital virilizing adrenal hyperpla­
sia or after removal of a sex steroid–secreti

pla­
sia or after removal of a sex steroid–secreting adrenal or gonadal 
neoplasm290,450), there is little evidence that adrenal androgens 
play an important qualitative or rate-limiting role in the onset 
of puberty in normal children.290 Most patients with premature 
adrenarche, who secrete excessive amounts of adrenal androgens 
for their age, enter puberty and experience menarche within the 
normal age range.290 Moreover, prepubertal children who have 
congenital or acquired chronic adrenal 

 who have 
congenital or acquired chronic adrenal insufficiency (i.e., Addison 
disease) and consequently have deficient or absent adrenal andro­
gen secretion usually have a normal onset and normal progression 
through puberty when given appropriate glucocorticoid and min­
eralocorticoid replacement therapy.290 Studies of children with 
chronic adrenal insufficiency, isolated gonadotropin deficiency, 
hypergonadotropic hypogonadism, or androgen resistance sug­
gest that adrenal androgens in gir

resistance sug­
gest that adrenal androgens in girls and boys are not essential for 
the adolescent growth spurt, whereas gonadal steroids secreted by 
the testis and ovary are essential and act in concert with GH.290 
The transient increase in height velocity (about 1.5 cm/year in 
both sexes) that occurs in middle childhood (6–7 years) and lasts 
about 2 years terminates while the serum DHEAS level continues 
Zona Fasciculata Cell
Zona Reticularis Cell
17-OH-Pro

ris Cell
Cortisol
CYP17
DHEA
DHEAS
CYP17
CYP17
17-Hydroxylase
3β-Hydroxysteroid
dehydrogenase 2
11β-Hydroxylase
Steroid
sulfotransferase
17,20-Lyase
Augmented serine
phosphorylation
of P450c17
Increased
abundance of the
electron-donating
redox partner:
and of
Cytochrome b5
• Fig. 26.35  Adrenarche and the zona reticularis. The rise in circulating dehydroepiandroste

ularis. The rise in circulating dehydroepiandrosterone sulfate 
(DHEAS) levels is the biochemical hallmark of adrenarche. The diagram compares and contrasts the 
major steroidogenic pathway in the zona fasciculata with that in the zona reticularis. In contrast to the 
zona fasciculata, the expression of 3β-hydroxysteroid, Δ4,5 isomerase type 2 messenger ribonucleic 
acid (mRNA), and its activity (the enzyme that irreversibly traps Δ5 precursors into Δ4 steroids) is very 
low in the zona reticula

nto Δ4 steroids) is very 
low in the zona reticularis, whereas the expression of and activity of steroid sulfotransferase is high. A 
single gene, CYP17 (now designated CYP17A1), encodes a single enzyme that has 17α-hydroxylase 
and 17,20-lyase activity, but the ratio of 17,20-lyase to 17α-hydroxylase activity is relatively high in the 
zona reticularis compared with that in the zona fasciculata. Some of the factors that seem to amplify the 
increased 17,20-lyase activity of CYP17 are the augmen

eased 17,20-lyase activity of CYP17 are the augmented serine phosphorylation of the enzyme and 
the apparent increased abundance of the electron-donating redox partner, including P450 reductase and 
of cytochrome b5.
1082
to increase. This increase in height velocity is related to the cyclic 
pattern of prepubertal growth and to genetic regulation of growth 
rather than to an increase in adrenal androgen or GH secretion.451 
Disorders of Puberty
Dela

gen or GH secretion.451 
Delayed Puberty and Sexual Infantilism
The upper limits of the normal age of onset of puberty are 2 SD 
above the mean and are 14 years for boys (although some sources 
say 13.5 years) and 13 years for girls (Table 26.15). Functionally, 
delayed puberty can be divided into disorders that affect the oper­
ation of the GnRH pulse generator, the pituitary gland, or the 
gonad.
Idiopathic or Constitutional Delay in Growth and Puberty
Otherwise healthy gi

l Delay in Growth and Puberty
Otherwise healthy girls who spontaneously enter puberty after 
the age of 13 years and boys who begin after 14 years may have 
constitutional delay in growth and adolescence (CDP), the most 
common diagnosis for delayed puberty. Affected individuals usu­
ally are short (2 SD below the mean value of height for age) at 
evaluation and have been shorter than their classmates for years, 
although growth velocity and height are usually appropriate for 
bone age (Fig. 26.

ht are usually appropriate for 
bone age (Fig. 26.37 and Table 26.16). Family history in as many 
as 77% of cases reveals a mother who had delayed menarche or a 
father (or sibling) who entered puberty late (i.e., age 14–18 years), 
and the pattern in some cases suggested dominant inheritance 
with incomplete penetrance.452–454
Recent GWAS studies demonstrate a low prevalence of genes 
found in CHH in CDP455,456; remarkably, genes usually in 
CHH were found in 17% of controls. However, there was

 were found in 17% of controls. However, there was only 
2% oligogenicity in controls compared to 15% oligogenicity in 
CHH subjects, demonstrating the importance of this genetic 
phenomenon (oligogenicity is discussed later). Recently muta­
tions in immunoglobulin superfamily member 10 (IGSF10) 
were found in males with CDP.457 This gene is important in the 
earliest stage of GnRH neuron migration from the nasal mes­
enchyme to the border with the telencephalon in mice; muta­
tions in this gene

he telencephalon in mice; muta­
tions in this gene appear to inhibit the normal migration of the 
neurons. While the IGSF10 gene is not solely responsible for 
IHH, it appears to play a role in partial HH found in hypotha­
lamic amenorrhea. Finding mutations in CDP may demonstrate 
a relationship with IHH.
Constitutional delay in development is physiologic immatu­
rity with a slow tempo of maturation; full sexual maturity will 
be reached, but the process takes longer than usual but almost 
alwa

e process takes longer than usual but almost 
always occurs by 18 years. Because of delay in reactivation of the 
GnRH pulse generator, there is functional deficiency of GnRH 
for chronologic age but not for the stage of physiologic develop­
ment. Adrenarche and gonadarche occur later in individuals with 
constitutional (idiopathic) delay in growth and adolescence,452 
whereas adrenarche reportedly occurs at a normal age in patients 
with isolated gonadotropin deficiency.442 Bone age is delayed 

d gonadotropin deficiency.442 Bone age is delayed at 
presentation, but after a bone age of approximately 12 to 14 years 
for boys or 11 to 13 years for girls is achieved, sexual maturation 
begins (although bone age is variable and not a reliable indicator). 
Patients achieve sexual maturation by 18 years of age.
Most of these patients with CDP are thin, but 25% are above 
the 85th percentile in BMI for age, and the bone age in these 
heavier boys is less delayed than in classic thin patients w

ys is less delayed than in classic thin patients with con­
genital disorders of glycosylation (CDG) (i.e., they tend to achieve 
taller adult stature452). Prepubertal boys who meet the criteria 
for diagnosis with delayed bone age have greater increased total 
energy expenditure (TEE) per kilogram fat-free mass (FFM) and 
increased nutritional needs possibly due to an alteration in mito­
chondrial metabolism or increased nonexercise activity thermo­
genesis (NEAT); increased nutrition increased 

mo­
genesis (NEAT); increased nutrition increased energy intake but 
TEE rose as well, continuing the need for high energy intake.458,459 
There is no impairment of olfaction, as in Kallmann syndrome, 
and undescended testes occur in the rate found in the general 
population. Plasma gonadal steroid levels are low in sensitive pedi­
atric assays at the time of presentation, but as bone age advances, 
the serum gonadotropin concentration and the amplitude of LH 
Adrenarche
CRF
Pituita

plitude of LH 
CRF
Pituitary
?
–
+
–
+
–
+
+
+
–
+
–
+
–
–
Fasciculata Reticularis
Cortisol
DHEA
DHEAS
Premature
adrenarche
Primary adrenal
Idiopathic precocious
puberty (onset <6 yrs)
puberty (onset >6 yrs)
Syndrome of gonadal
Isolated gonadotropin
Constitutional delayed
adolescence
DHEA
DHEAS
ACTH
Adrenal
Adrenal
androgen-
stimulating
hormone
“AASH”
Gonadarche
1. Hypothalamus
GnRH
3. Gonad
E2
T
2. Pituitary
    g

GnRH
3. Gonad
E2
T
    gland
FSH
LH
• Fig. 26.36  Hypothesis of the control of pituitary adrenal androgen secre­
tion by a putative separate adrenal androgen-stimulating hormone acting 
on a corticotropin (ACTH)–primed adrenal cortex. Although this diagram 
suggests that adrenal androgen-stimulating hormone (AASH) arises from 
the pituitary gland, a distinct pituitary factor with AASH activity has not 
been isolated; an extrapituitary factor is not excluded. The lower p

extrapituitary factor is not excluded. The lower part of 
the diagram shows the relationship of adrenarche to gonadarche, includ­
ing dissociation in various clinical disorders of sexual development (+, 
present; −, absent). CRF, corticotropin-releasing factor; DHEA, dehydro­
epiandrosterone; DHEAS, DHEA sulfate; E2, estradiol; FSH, follicle-stimu­
lating hormone; GnRH, gonadotropin-releasing hormone; T, testosterone. 
(Modified from Sklar CA, Kaplan SL, Grumbach MM. Evidence for dis­
sociation 

plan SL, Grumbach MM. Evidence for dis­
sociation between adrenarche and gonadarche: studies in patients with 
idiopathic precocious puberty, gonadal dysgenesis, isolated gonadotropin 
deficiency, and constitutionally delayed puberty. J Clin Endocrinol Metab. 
1980;51:548–556. Copyright by The Endocrine Society.)
1083
pulses increase (initially at night); the basal serum gonadotropin 
concentrations measured by third-generation assays and the LH 
r

 measured by third-generation assays and the LH 
response to GnRH or GnRH agonists reflect maturation of the 
hypothalamic-pituitary system.
The first signs of secondary sexual development occur within 1 
year after LH rises to pubertal levels in response to administration 
of intravenous synthetic GnRH or subcutaneous GnRH agonist 
or within 1 year after gonadotropin and testosterone or estradiol 
concentrations begin to increase spontaneously above prepubertal 
values.256,326 An 8 am serum tes

bove prepubertal 
values.256,326 An 8 am serum testosterone value of 0.7 nmol/L (20 
ng/dL) heralds the development of phenotypic puberty in boys 
within 12 to 15 months.460
CDP is more common in boys and may be the opposite of idio­
pathic CPP, a condition that is many times more common in girls. 
Familial short stature is a physiologic variant of growth in which 
the velocity of development and bone age are normal but stature 
is decreased, in contrast to CDP, which is a disorder of tempo that

contrast to CDP, which is a disorder of tempo that 
secondarily impairs growth. The combination of CDP and familial 
short stature leads to conspicuous shortness during adolescence, 
especially when other children increase their growth velocity, 
IDIOPATHIC (CONSTITUTIONAL) DELAY IN GROWTH AND 
PUBERTY (DELAYED ACTIVATION OF HYPOTHALAMIC LRF 
PULSE GENERATOR)
HYPOGONADOTROPIC HYPOGONADISM: SEXUAL 
INFANTILISM RELATED TO GONADOTROPIN DEFICIENCY
CNS Disorders
Tumors
Germinomas
O

S Disorders
Tumors
Other germ cell tumors
Hypothalamic and optic gliomas
Astrocytomas
Pituitary tumors (including MEN1, prolactinoma)
Langerhans histiocytosis
Postinfectious lesions of the CNS
Vascular abnormalities of the CNS
Congenital malformations especially associated 
with craniofacial anomalies
Isolated Gonadotropin Deficiency
With hyposmia or anosmia
Without anosmia
LHRH re

e
LHRH receptor mutation
Congenital adrenal hypoplasia (DAX1 mutation)
Isolated LH deficiency
Isolated FSH deficiency
Prohormone convertase 1 deficiency (PCI)
Idiopathic and Genetic Forms of Multiple Pituitary Hormone 
Deficiencies Including PROP1 Mutation
Miscellaneous Disorders
Laurence-Moon and Bardet-Biedl syndromes
Functional gonadotropin deficiency
Chronic systemic disease and malnutrition
Ac

alnutrition
Acquired immunodeficiency syndrome (AIDS)
Chronic gastroenteric disease
Chronic renal disease
Bulimia
Psychogenic amenorrhea
Impaired puberty and delayed menarche in female athletes and ballet 
dancers (exercise amenorrhea)
Marijuana use
Gaucher disease
HYPERGONADOTROPIC HYPOGONADISM
Males
The syndrome of seminiferous tubular dysgenesis and its variant

of seminiferous tubular dysgenesis and its variants 
(Klinefelter syndrome)
Other forms of primary testicular failure
Testicular steroid biosynthetic defects
Sertoli-only syndrome
Anorchia and cryptorchidism
Trauma/surgery
Females
The syndrome of gonadal dysgenesis (Turner syndrome) and its variants
XX and XY gonadal dysgenesis
Familial and sporadic XX gonadal dysgenesis and its variants
Familial and sporadic XY gonadal dysgenesis and its varia

l and sporadic XY gonadal dysgenesis and its variants
Aromatase deficiency
Other forms of primary ovarian failure
Premature menopause
Autoimmune oophoritis
Galactosemia
Glycoprotein syndrome type 1
Resistant ovary
LH/hCG resistance
Polycystic ovarian disease
Noonan or pseudo-Turner syndrome
Ovarian steroid biosynthetic defects
   
  Classification of Delayed Puberty and Sexual Infantilism
TABLE 26.15
CNS, Central nervous system

nfantilism
CNS, Central nervous system; hCG, human chorionic gonadotropin; LHRH, luteinizing hormone–releasing hormone; LRF, luteinizing hormone–releasing factor; MEN, multiple endocrine neoplasia.
   
1084
and referrals occur more often with this combination than with 
either condition alone. Because no single test reliably distinguishes 
between CDP and IHH, watchful waiting is usually in order.
The growth rate before the actual onset o

n order.
The growth rate before the actual onset of puberty in constitu­
tional delay is often suboptimal for chronologic age, but growth 
velocity usually increases to normal levels after puberty begins.461 
Affected boys seem to be more distressed by short stature than by 
delay in sexual development.
GH release in the basal state and in response to GH secreta­
gogues, including the administration of GHRH, is low for age and 
may be decreased further in children with CDP, but the amplitude 
of

urther in children with CDP, but the amplitude 
of GH secretion and the GH response to GHRH is greater after 
administration of exogenous (aromatizable) androgens or estro­
gens. Therefore CDP may constitute a temporary state of func­
tional GH insufficiency for chronologic age but not for bone age. 
Patients with CDP are sometimes mistakenly diagnosed with 
growth hormone deficiency. IGF1 interacts with gonadotropins 
in the ovary and testis, and the relatively low secretion of GH (and 
presuma

d the relatively low secretion of GH (and 
presumably intragonadal IGF1) in CDP may impair the gonadal 
response to gonadotropins.
Patients with constitutional delay in adolescence and growth 
often do not reach their predicted height. When the genetic ten­
dency for growth is greater, subjects with CDP reach taller adult 
stature, but the patients most likely to be referred are those who 
combine genetic short stature with CDP. Girls with CDP have a 
mean deficit in adult height of 2.4 cm below

ve a 
mean deficit in adult height of 2.4 cm below the mean predicted 
height, although the range of adult height varies about 10 cm 
above or below predictions. The magnitude of the catch-up in lin­
ear growth during puberty in boys is a major determinant of adult 
height. Heavier individuals with CDP reach greater height than 
those who are thinner.452 Adult height is lower if affected indi­
viduals show decreased height standard deviation scores (SDS) in 
earlier life.462,463
Because 15% to 2

es (SDS) in 
Because 15% to 20% of adult height is gained during puberty, 
many approaches aim to increase stature in otherwise normal, 
short children. Delaying the onset or progression with puberty by 
the use of GnRH agonists was suggested by some, but a decrease 
in bone density 1 year after cessation led to warnings that routine 
administration of this treatment carries substantial risk.464 The 
additional psychologic risk of delaying puberty in otherwise nor­
mal child

sk of delaying puberty in otherwise nor­
mal children must also be considered, and this treatment is neither 
established nor recommended.465
The US Food and Drug Administration (FDA) approved GH 
treatment for children who are predicted to reach an adult height 
less than the 1st percentile (160 cm), which includes some chil­
dren with CDP (but it is not specifically approved for CDP itself), 
and some studies reported an increase in adult height with this 
treatment. Because endogenous GH leve

t with this 
treatment. Because endogenous GH level rises during pubertal 
development, the FDA approved GH therapy in larger doses for 
subjects with GH deficiency during puberty. However, there are 
only moderate effects of increased GH doses during puberty on 
adult height. Male gender has a positive effect and age at onset 
of puberty has a negative effect on this increased dose on adult 
height.466
The cost of GH for treatment of non–GH-deficient short stat­
ure is exceptionally high: $14,0

cient short stat­
ure is exceptionally high: $14,000 per cm or $35,000 per inch 
gained.467 Payers are reluctant in many cases to cover the cost 
of the GH therapy for those without confirmed GH deficiency. 
There are few controlled studies of adult height, but some avail­
able results demonstrate an increase in adult height of several 
centimeters; more studies are strongly recommended to better 
determine the efficacy of this treatment in short, normal children 
with CDP.468
The combination of

, normal children 
The combination of GnRH agonist therapy with GH treat­
ment in attempts to increase adult height in children who were 
  Constitutional Delay in Growth and 
A variation of normal
Males more often seek assistance
Family history of delayed menarche or delayed secondary sexual 
characteristics
Height is often below the 5th percentile, but growth rate is normal for 
skeletal age
Onset of adrenarche is delayed
The combination of genetic short stature and c

yed
The combination of genetic short stature and constitutional delay leads 
to more profound short stature
Final height is less than predicted
TABLE 26.16
• Fig. 26.37  A boy age 16 years and 2 months has constitutional delay 
in growth and puberty. His height is 149.5 cm (4 SD [standard deviation] 
below the mean value for age); upper-to-lower body ratio is 1.1 (retarded 
for age); phallus is 6.0 × 1.6 cm; testes are 2.5 × 1.4 cm; and the scrotum 
showed early thinning. At a chronologic age of

um 
showed early thinning. At a chronologic age of 15 years and 4 months, the 
bone age was 11 years and the sella turcica was normal. The plasma con­
centration of luteinizing hormone (LH) was 0.7 ng/mL (LER-960); the con­
centration of follicle-stimulating hormone (FSH) was 0.5 ng/mL (LER-869). 
On gonadotropin-releasing hormone (GnRH) testing, the plasma concen­
tration of LH increased to 2.2 ng/mL (an increment of 1.5 ng/mL), and 
the testosterone level rose from 52 to 77 ng/dL. The testes s

erone level rose from 52 to 77 ng/dL. The testes subsequently 
spontaneously enlarged, and the patient progressed through puberty. For 
conversion to SI units, see Figs. 26.19 and 26.20. (From Styne DM, Grum­
bach MM. Puberty in the male and female: its physiology and disorders. 
In: Yen SCC, Jaffe RB, eds. Reproductive Endocrinology. 2nd ed. Philadel­
phia, PA: WB Saunders; 1986:313–384.)
1085
normal except for genetic short stature and in SGA chi

al except for genetic short stature and in SGA children led 
to inconclusive results or to increased predicted or near-final 
height, which does not necessarily translate into increased adult 
height. This approach to treatment remains experimental. Review 
of a large database from a postmarketing survey did not support 
the efficacy of this approach,469 and there was no good follow-up 
evaluation of adult height. This combination therapy cannot be 
supported by substantial evidence.470
When the

be 
When the critical role of estradiol in skeletal maturation was 
appreciated, treatment with a potent aromatase inhibitor to 
increase adult height by inhibiting skeletal maturation159,160,303 
aroused interest. A double-blind, randomized, placebo-controlled 
study enrolled boys with CDP who were treated with 6 months 
of monthly testosterone or testosterone plus an added 12-month 
trial of daily oral letrozole (a potent fourth-generation aromatase 
inhib

ozole (a potent fourth-generation aromatase 
inhibitor). The results revealed a mean increase in predicted adult 
height of 5.1 cm in the letrozole plus testosterone group; a sub­
sequent study supported these promising effects on increasing 
the time of pubertal growth height without affecting the devel­
opment of male secondary sex characteristics, but there are few 
data on actual adult height.471 The boys in the group treated with 
testosterone and letrozole developed increased bioactive tes

ne and letrozole developed increased bioactive testoster­
one, analyzed by a cellular assay, compared with control boys.266 
Serum testosterone measured by HPLC-MS/MS can reach levels 
of more than 1000 ng/dL. However, markers of bone turnover 
decrease, and vertebral abnormalities may develop with the use 
of these agents in idiopathic short stature.472 A 1-year study of 
the aromatase inhibitor anastrozole plus GH, compared with 
GH treatment alone, in boys demonstrated no ill effects on body 

lone, in boys demonstrated no ill effects on body 
composition, plasma lipids, bone metabolism, or the tempo of 
puberty (although estrogen decreased, as expected, with the use 
of anastrozole), and the predicted height was increased.473 This 
treatment has not been supported by long-term studies observing 
patients up to adult height. Concerns about possible effects on 
bone density and morphology must be addressed, probably until 
the patients are at least 20 to 25 years old, as well as a decr

are at least 20 to 25 years old, as well as a decrease 
in serum HDL cholesterol levels and increase of erythrocytosis 
before this off-label therapy can routinely be recommended.474,475 
Hypogonadotropic Hypogonadism: Sexual Infantilism 
Related to Gonadotropin Deficiency
Insufficient pulsatile secretion of GnRH and the resulting FSH 
and LH deficiency lead to delayed sexual maturation, which may 
be permanent. The phenotype in hypogonadotropic hypogonad­
ism can vary from severe sexual infanti

ism can vary from severe sexual infantilism to apparent CDP. Both 
conditions may be found in the same family, or hypogonadotropic 
hypogonadism may first appear to be CDP (e.g., a homozygous 
R262Q mutation in GNRHR presented with constitutional delay 
but later was associated with oligospermia).476 There may be an 
absolute or relative quantitative deficiency of pulsatile GnRH, 
or the deficiency may be qualitative, especially in females; it may 
involve abnormalities in amplitude o

ales; it may 
involve abnormalities in amplitude or frequency of GnRH pulses 
or in both components (Fig. 26.38). The many forms of this con­
dition occur in about 1 in 4000 males and three to five times less 
in females.
Patients with IHH usually are of normal height in early or 
middle adolescent years, whereas patients with CDP usually have 
a normal growth rate for bone age but are short for chronologic 
age. In contrast to CDP patients, those with hypogonadotropic 
hypogonadism usually do n

e with hypogonadotropic 
hypogonadism usually do not respond to GnRH stimulation, nor 
do they have a pulsatile LH profile commensurate with bone age. 
Although serum concentrations of plasma FSH and LH and uri­
nary gonadotropins are low, the differences are relative rather than 
absolute and are not diagnostic for an individual.
Hypogonadotropic hypogonadism can involve puberty and 
reproduction alone or can be a manifestation of a life-threatening 
condition such as a brain tumor. Hypogonadot

ning 
condition such as a brain tumor. Hypogonadotropic hypogonad­
ism may result from a genetic or developmental defect present at 
birth but remain undetected until the age of expected puberty, or it 
may be caused by a tumor, inflammatory process, vascular lesion, 
irradiation, or trauma to the hypothalamus. Similarly, hypogo­
nadotropic hypogonadism may arise from lesions or defects that 
involve the pituitary gland directly. When GH is affected as well 
as gonadotropins, impaired growth is 

ted as well 
as gonadotropins, impaired growth is manifested by decreased 
growth velocity, especially during the expected pubertal growth 
spurt, and short stature.
Isolated Hypogonadal Hypogonadism
A defect involving the GnRH pulse generator or gonadotrophs 
without an anatomic lesion causes selective deficiency of gonado­
tropins, producing IHH (Tables 26.17 and 26.18).477 Puberty 
fails to begin by 14 years in boys and 13 years in girls, or pubertal 
maturation is incomplete or transient. In

ubertal 
maturation is incomplete or transient. In boys, micropenis (the 
penis is normally formed but stretched length is less than 2 cm in 
length, which is 2.5 SD below the mean length in average new­
born males) or undescended testes or both findings are evidence of 
a fetal testosterone deficiency caused by gonadotropin deficiency. 
Prepubertal concentration of gonadal sex steroid values (testoster­
one in boys; estradiol in girls) and low serum gonadotropin levels 
or values within the nor

erum gonadotropin levels 
or values within the normal range (i.e., normal in the basal state 
but not in the stimulated state) are characteristic. Concentrations 
of gonadal sex steroids and gonadotropins are low, pulsatile LH 
secretion is often virtually absent, and the LH response to GnRH 
or GnRH agonist administration is deficient in the severe form. 
The testes are small (1–2 mL), and serum inhibin B, an estimate 
of seminiferous tubule function, is lower than in CDP in initial 
studies.47

ction, is lower than in CDP in initial 
studies.478
IHH may occur in families (about 20–30% of patients), or it 
may occur sporadically. There are a growing list of gene defects 
that cause hypogonadotropic hypogonadism.479 Some patterns of 
inheritance are rare and follow the classic mendelian schema; oth­
ers are oligogenetic, whereby multiple defects of common genes 
synergize to bring about a condition no single one of them could 
cause alone.454,480
Oligogenicity is more common in IHH than 

.454,480
Oligogenicity is more common in IHH than CDP, and IHH 
genes are more common in IHH than CDP, suggesting a different 
genetic architecture in IHH compared to CDP.456
In contrast to CNS tumors (in which patients usually have 
GH deficiency and growth failure) and to CDP (in which 
patients are short for chronologic age), height is appropriate 
for age in patients with IHH (Fig. 26.39). Because the levels of 
estradiol are too low to cause epiphyseal fusion at the normal 
age, increased a

 epiphyseal fusion at the normal 
age, increased arm span for height and a decreased ratio of upper 
to lower body segments (i.e., eunuchoid body proportions) are 
present. If the condition is left untreated, growth continues, and 
adult height is tall.481–483
Remarkably, about 22% of affected patients with IHH and 
severe delay of puberty spontaneously increase their testicular size 
and enter full puberty.484 Five men who experienced reversal once 
again became hypogonadal. Thus long-term foll

nce 
again became hypogonadal. Thus long-term follow-up is essential.
Kallmann Syndrome. Anosmia or hyposmia resulting from 
agenesis or hypoplasia of the olfactory lobes or sulci is associated 
1086
with GnRH deficiency in Kallmann syndrome, the most com­
mon form of IHH485 (see Table 26.18). The condition was first 
observed in 1856 during the autopsy of a 40-year-old man with 
micropenis, small cryptorchid testes, and absence of the olfac­
tory bu

ptorchid testes, and absence of the olfac­
tory bulbs, but Kallmann described a familial pattern in 1944 
(Fig. 26.40). The prevalence is 1 of every 10,000 males and 1 of 
every 40,000 females. Although the loss of olfaction usually corre­
lates with the degree of GnRH deficiency, even in complete anos­
mia (found in the classic KAL1 form), the GnRH deficiency may 
be partial (see “Isolated Luteinizing Hormone Deficiency”).486 
Because affected individuals often do not notice impaired olfac­
tio

ndividuals often do not notice impaired olfac­
tion, testing with graded dilutions of pure scents is necessary to 
determine partial anosmia. Patients with Kallmann syndrome 
have diminished or absence of nocturnal pulses of gonadotropins 
found in normal prepubertal boys, although daytime values are 
equal. Undescended testes are common in this and all types of 
hypogonadotropic hypogonadism in boys.483 The magnitude of 
the GnRH deficiency correlates with the size of the testes. Micro­
penis o

elates with the size of the testes. Micro­
penis occurs in about one-half of males with Kallmann syndrome 
because of absence of the elevated pituitary gonadotropins charac­
teristic of the normal midgestation fetus (Fig. 26.41).
Associated defects that are inconsistently present include 
cleft lip, cleft palate, imperfect facial fusion, seizure disorders, 
short metacarpals, pes cavus, neurosensory hearing loss (rarely 
found in the X-linked form), cerebellar ataxia and nystag­
mus, ocular moto

m), cerebellar ataxia and nystag­
mus, ocular motor abnormalities, unilateral or rarely bilateral 
renal aplasia or dysplasia, and mirror movements of the upper 
extremities (i.e., bimanual synkinesia), limited to the X-linked 
form (see Table 26.18).
KAL1. In classic, X-linked KAL1, fetal GnRH neurosecre­
tory neurons do not migrate from the olfactory placode to the 
medial basal hypothalamus, where they normally constitute the 
K.H.
36
B
Hours
T = 1091 ng%
Subse

mIU/mL) LER 907
K.H.
36
B
Hours
T = 1091 ng%
Subset 2: Sleep entrained pattern
V.M.
19
Sleep
*
*
**
*
*
*
*
T = 119 ng%
E.B.
20
5
4
3
2
1
0
0
6
12
18
24
T = 72 ng%
D
Hours
5
4
3
2
1
0
0
6
12
18
24
P.H.
27
T = 36 ng%
A
Hours
*
*
*
* *
* *
*
* *
*
*
*
* *
*
5
4
3
2
1
0
0
6
12
18
24
C
Hours
5
4
3
2
1
0
0
6
12
18
24
• Fig. 26.38  Various patterns of pulsatile l

8
24
• Fig. 26.38  Various patterns of pulsatile luteinizing hormone (LH) secretion that can occur in isolated 
hypogonadotropic hypogonadism (B–D) are compared with LH secretion in a normal man (A). (A) Discrete 
LH pulses occur about every 2 hours in a normal 36-year-old man. (B) Typical apulsatile LH pattern is asso­
ciated with a low testosterone (T) concentration usually found in isolated hypogonadotropic hypogonad­
ism. (C) Pattern of developmental arrest with low-amplitude nocturnal LH pu

opmental arrest with low-amplitude nocturnal LH pulses is apparent only during 
sleep. (D) Low-amplitude LH pulse pattern occurs during sleep and wake periods. To convert LH values to 
international units per liter, multiply by 1.0. (From Spratt DI, Crowley WF. Hypogonadotropic hypogonad­
ism: GnRH therapy. In: Krieger DT, Bardin CW, eds. Current Therapy in Endocrinology and Metabolism, 
1985-1986. Toronto, Canada: BC Decker; 1985:155–159.)
  Isolated Gonadotropin Deficiency
Males more commonly

olated Gonadotropin Deficiency
Males more commonly affected
Familial (more common in females) or sporadic (more common in males)
Height normal for age; tall adult height if untreated
Eunuchoid skeletal proportions
Delayed bone age
Small, often cryptorchid testes: diameter <2.5 cm prepubertal size; 
phallus may be small
Normal adrenarche
Examine for anosmia or hyposmia (Kallmann syndrome)
Look for associated malformations (facial, central nervous system, 
skeletal, renal)
TABLE 26.17
1087
Chapter

system, 
1087
GnRH pulse generator, but instead end in a tangle around the 
cribriform plate and in the dural layers adjacent to the menin­
ges beneath the forebrain.372 Abnormal or absent olfactory 
bulbs or folds are seen on MRI scans with other changes in 
brain morphometry resulting.487 Coronal and axial cranial MRI 
scans of the olfactory bulbs and sulci, unilaterally or bilater­
ally, reflect this defect in about 

ly or bilater­
ally, reflect this defect in about 90% of cases and can point to 
the diagnosis, especially in affected infants and prepubertal-age 
children (Fig. 26.42).
An increasing number of genes are implicated in Kallmann 
syndrome, starting from the classic ANOSM1 (previously KAL1) 
gene in the X-linked form first described, but only 30% of patients 
can be shown to have an identifiable gene defect. Further digenetic 
and oligogenetic inheritance is now recognized and may explain 
the gen

ritance is now recognized and may explain 
the genetic heterogeneity of the syndrome and the variation in 
phenotype encountered within a family—that is, 10% to 20% 
of IHH are explained with oligogenetic inheritance. Recently, 
mutations in genes “synexpressed” with fibroblast growth factor 8 
were found in IHH and Kallmann syndrome (FGF17, IL17RD, 
DUSP6, SPRY4, and FLRT3), supporting this oligogenetic nature 
of the condition.488
A variety of deletions and mutations of the ANOSM1 gene 
have b

deletions and mutations of the ANOSM1 gene 
have been described, including large and small (exon) deletions, 
point mutations, and a variety of nonsense mutations leading to 
frameshift and premature stop codons. The ANOSM1 muta­
tions are more prevalent in Japanese than in white patients, and 
they can be associated with normal olfactory function.489 The 
defect in some rare patients with no ANOSM1 mutation but 
X-linked inheritance may be located in the promoter region of 
the ANOSM1 gene. Kal

ed in the promoter region of 
the ANOSM1 gene. Kallmann syndrome associated with X-linked 
ichthyosis caused by steroid sulfatase deficiency, mental retarda­
tion, and chondrodysplasia punctata occurs in a contiguous gene 
syndrome. Only 14% of familial cases and 11% of sporadic cases 
involve mutations in the ANOSM1 gene on the X chromosome, 
  Features of Kallmann Syndrome
Clinical
GnRH deficiency: absent or arrested puberty
Anosmia or hyposmia
In infancy: microphallus; cryptorchidism
Normal 

a
Normal stature and growth in childhood
Eunuchoid proportions
Associated midline defects (e.g., cleft lip, cleft palate, midline cranial 
anomalies)
MRI: aplasia or hypoplasia of olfactory bulbs and/or sulci
Approximately 1 in 7500 males, 1 in 50,000 females; 10% prevalence of 
Sporadic and familial cases; genetic heterogeneity
X linked
X-linked recessive (Kallmann et al)
X chromosome deletion

d recessive (Kallmann et al)
X chromosome deletion: Xp22.3 (Meitinger et al)
Autosomal
Dominant (sex limitation) (Santen and Paulsen; Merriam et al)
Recessive (White et al)
Anatomy
Developmental field defect
Aplasia or hypoplasia of olfactory bulb and sulcus
Arrested migration of GnRH neurosecretory neurons from olfactory 
placode to medial basal hypothalamus
GnRH, Gonadotropin-releasing hormone; MRI, magnetic resonance imaging.
Data from Kallmann F, Schonfeld WA, Barrera SW. Genetic aspects of 

n F, Schonfeld WA, Barrera SW. Genetic aspects of primary eunuchoidism. 
Am J Ment Defic. 1944;48:203–236; Meitinger T, Heye B, Petit C, et al. Definitive local­
ization of X-linked Kallmann’s syndrome (hypogonadotropic hypogonadism and anosmia) to 
Xp22.3: close linkage to the hypervariable repeat sequence CRI-S232. Am J Hum Genet. 
1990;47:664–669; Merriam GR, Beitins IZ, Bode HH. Father-to-son transmission of hypogo­
nadism with anosmia: Kallmann’s syndrome. Am J Dis Child. 1977;131:1216–1219

ann’s syndrome. Am J Dis Child. 1977;131:1216–1219; Santen 
RJ, Paulsen CA. Hypogonadotropic eunuchoidism. I. Clinical study of the mode of inheritance. 
J Clin Endocrinol Metab. 1973;36:47–54; White BJ, Rogal AD, Brown KS, et al. The syndrome 
of anosmia with hypogonadotropic hypogonadism: a genetic study of 18 new families and a 
review. Am J Med Genet. 1983;15:417–435.
   
TABLE 26.18
• Fig. 26.39  A girl aged 18 years and 8 months has isolated gonadotropin 
deficiency (i.e., sexual infantili

d gonadotropin 
deficiency (i.e., sexual infantilism and primary amenorrhea). Her height is 
173 cm (+1 SD [standard deviation]), weight was 66.5 kg (+1 SD), and 
the skeletal age was 13 years. Adrenarche with pubic hair development 
occurred at age 13.5 years. At the time of the photograph, pubic hair was 
in stage 3, and there was slight breast and nipple development resulting 
from a previous short course of estrogen therapy. Immature labia minora 
and majora were observed, and no estrogen ef

nora 
and majora were observed, and no estrogen effect was seen on the vaginal 
mucosa. Olfactory testing results were normal. The plasma luteinizing hor­
mone (LH) (LER-960) level after gonadotropin-releasing hormone (GnRH) 
administration rose from 0.5 to 1.8 ng/mL (a prepubertal response). Serum 
estradiol was undetectable. The dehydroepiandrosterone sulfate (DHEAS) 
level was 92 μg/dL (appropriate for pubic hair stage 2). Notice the discrep­
ancy between adrenarche and gonadarche. For conver

ancy between adrenarche and gonadarche. For conversion to SI units, 
see Figs. 26.19 and 26.30. (From Styne DM, Grumbach MM. Puberty in 
the male and female: its physiology and disorders. In: Yen SCC, Jaffe RB, 
eds. Reproductive Endocrinology. 2nd ed. Philadelphia, PA: WB Saun­
ders; 1986:313–384.)
1088
but these patients are more likely to have complete absence of 
gonadotropin secretory pulses and absence of migration of GnRH 
neurons to the hypot

absence of migration of GnRH 
neurons to the hypothalamus.490 Hypogonadotropic hypogo­
nadism is rarely caused by a mutation in the ANOSM1 gene in 
females. 
KAL2. The autosomal dominant form is known as Kallmann 
syndrome type 2 (KAL2), and the associated gene is fibroblast 
growth factor receptor 1 (FGFR1, previously KAL2) with a gene 
map locus of 8p11.2-p11.1 and its ligand FGF8. Mutations 
result in autosomal dominant Kallmann syndrome, autosomal 
dominant normosmic hypogonadotropic hypogon

somal 
dominant normosmic hypogonadotropic hypogonadism, or 
delayed puberty. Evaluation of FGF8 in patients with idiopathic 
hypogonadotropic hypogonadism revealed inactivating muta­
tions of FGF17, IL17RD, DUSP6, SPR-Y4, and FL-RT3.488 
A Kallmann syndrome patient had a premature termination of 
FGFR1 resulting in a soluble receptor with dominant negative 
effect, which produced a truncated FGFR1 without the trans­
membrane and intracellular domains.491 Intracerebral adminis­
tration of anti-F

mains.491 Intracerebral adminis­
tration of anti-FGFR1 antibodies causes weight loss in rodents; 
FGF21, which is secreted by the liver under the stimulatory 
effects of beta-klotho (KLB), an obligate coreceptor mediating 
FEF21 signaling, is also involved in nutritional metabolism. 
While there is to date no report of mutation FGF21 causing 
hypogonadism, mutations in KLB impair FGF21 signaling lead­
ing to GnRH deficiency of various degrees of severity.492 Since 
KLB and FGF21 genes are expres

severity.492 Since 
KLB and FGF21 genes are expressed on GnRH neurons, they 
may provide a means in which GnRH neurons can detect nutri­
tional status and determine whether it is appropriate to allow 
reproduction to progress or cease.493
KAL2 is associated with developmental delay, choanal atresia, 
short stature, congenital heart defects, and sensorineural hearing 
loss, and its presentations are more varied than those of KAL1.
• Fig. 26.40  A boy aged 15 years and 10 months had isolated gonad

boy aged 15 years and 10 months had isolated gonadotro­
pin deficiency and anosmia (Kallmann syndrome). He had undescended 
testes, but after administration of 10,000 U of human chorionic gonado­
tropin (hCG), the testes descended and were palpable in the scrotum. His 
height was 163.9 cm (+1.5 SD [standard deviation]); the upper-to-lower 
body ratio was 0.86, which is eunuchoid. The phallus measured 6.3 × 
1.8 cm, and the testes were 1.2 × 0.8 cm. The concentration of plasma 
luteinizing hormon

m. The concentration of plasma 
luteinizing hormone (LH) was less than 0.3 ng/mL; the follicle-stimulating 
hormone (FSH) level was 1.2 ng/mL; and the testosterone level was 16 ng/
dL. After 100 μg of gonadotropin-releasing hormone (GnRH), the plasma 
LH level (LER-960) was 0.7 ng/mL, and the FSH level (LER-869) was 2.4 
ng/mL. For conversion to SI units, see Figs. 26.19 and 26.20. (From Styne 
DM, Grumbach MM. Puberty in the male and female: its physiology and 
disorders. In: Yen SCC, Jaffe RB,

 physiology and 
disorders. In: Yen SCC, Jaffe RB, eds. Reproductive Endocrinology. 2nd 
ed. Philadelphia, PA: WB Saunders; 1986:313–384.)
FSH
Normal prepubertal males
n = 45
Isol. gonadotropin defic.
(prepubertal testes)
n = 9
(testes larger than prepub.)
n = 16
Normal pubertal males
n = 16
0
Basal
+sem
Peak
+sem
* = p < 0.05
1
2
3
4
5
6
LH
n = 45
n = 9
(testes

al testes)
n = 9
n = 16
n = 16
0
Basal
+sem
Peak
+sem
* = p < 0.05
1
2
3
*
*
*
*
*
4
5
6
LH (ng/mL)
• Fig. 26.41  Serum luteinizing hormone (LH) and follicle-stimulating hor­
mone (FSH) responses to the administration of gonadotropin-releasing 
hormone (GnRH) in 25 males with an isolated gonadotropin deficiency 
with or without anosmia were segregated according to whether the 
volume of the testes was prepubertal or gre

r the 
volume of the testes was prepubertal or greater than 2.5 cm3. Tes­
ticular volume in those with testes larger than 2.5 cm3 was as large 
as 4 cm3. Basal and GnRH-stimulated gonadotropin levels after the 
intravenous injection of 100 μg GnRH (peak value) are shown (p <0.05). 
For conversion to SI units, see Fig. 26.16. sem, standard error of the 
mean. (From Van Dop C, Burstein S, Conte FA, et al. Isolated gonado­
tropin deficiency in boys: clinical characteristics and growth. J Pediatr. 

 clinical characteristics and growth. J Pediatr. 
1987;111:684–692.)
1089
The loss-of-function mutation of the FGFR1 gene interferes 
with migration of the olfactory cells to the olfactory bulb. Anos­
min-1, a neuronal protein, may act through FGFR1 to bring 
about FGF signaling. ANOSM1 partially escapes inactivation 
in females, and it is postulated that enough ANOSM1 may be 
produced in affected females, despite FGFR1 haploinsufficiency, 
to main

emales, despite FGFR1 haploinsufficiency, 
to maintain adequate FGF signaling and allow olfactory func­
tion and GnRH neuron migration. One kindred of Kallmann 
syndrome contained four women with FGFR1 mutations that 
were transmitted to affected male offspring, although the mothers 
had normal reproduction and olfaction.494
Although gain-of-function mutations of the FGFR1 gene are 
associated with craniosynostosis, a loss-of-function mutation is 
not associated with lack of fusion of the crania

not associated with lack of fusion of the cranial sutures. One kin­
dred with an FGFR1 mutation (Arg622X) in the tyrosine kinase 
domain, in which some of the manifestations were temporary, is 
A
B
C
D
• Fig. 26.42  Comparison of the brain and nasal cavities of a normal 19-week-old male fetus (A) and those 
of a male fetus of similar age with Kallmann syndrome caused by an X chromosome deletion at Xp22.3 
(B). In the normal fetal brain, the gonadotropin-releasing hormone (GnRH) neurosecretory 

adotropin-releasing hormone (GnRH) neurosecretory neurons (black 
dots) are located in the hypothalamic area, including the medial basal hypothalamus, the anterior hypo­
thalamic area, and of interest regarding hypothalamic hamartoma as an ectopic GnRH pulse generator, 
the premammillary and retromammillary areas. A small cluster of GnRH neurons exists among the fibers of 
the terminalis nerve on the floor of the nasal septum. In the male fetus with Kallmann syndrome, no GnRH 
neurons were detec

ith Kallmann syndrome, no GnRH 
neurons were detected in the hypothalamic region, including the basal hypothalamus, median eminence, 
and preoptic area. The GnRH cells fail to migrate to and enter the brain from their origin in the nose; these 
cells end in a tangle beneath the forebrain on the dorsal surface of the cribriform plate and in the nasal 
cavity. AC, anterior commissure; CG, crista galli; IN, infundibular nucleus; NT, terminalis nerve; OC, optic 
chiasm; POA, preoptic area. (C and D)

; OC, optic 
chiasm; POA, preoptic area. (C and D) show magnetic resonance imaging scans of brain (coronal section, 
TI-weighted image). (C) Normal olfactory sulci (open arrows) and bulbs (solid arrows) in a 15-year-old boy. 
(D) Absent olfactory sulci (open arrows) and bulbs in a 17-year-old, anosmic, sexually infantile boy with 
Kallmann syndrome.
1090 SECTION VI    Endocrinology and the Life Span
reported; the mother of the proband had delayed puberty, and the 
maternal grandmother had anosmi

 puberty, and the 
maternal grandmother had anosmia, whereas the proband with 
KAL2 exhibited normal LH levels, testosterone production, and 
spermatogenesis after prior testosterone therapy.495
Studies of mice revealed that the early emergence of GnRH neu­
rons from the embryonic olfactory placode requires FGF8 signal­
ing as a ligand mediated through FGFR1.496 About 30% of FGF8/
FGFR1 loss-of-function mutations are associated with cleft palate, 
whereas FGFR1 mutations may rarely lead to carti

whereas FGFR1 mutations may rarely lead to cartilage abnormali­
ties in the ears, nose, or digits.496 An unusual kindred with a pro­
band demonstrating severe ear anomalies, mandibular hypoplasia, 
thoracic dystrophia, and other usual findings was associated with an 
Arg622 mutation in the FGFR1 gene, and investigation for hypogo­
nadism is indicated when such facial abnormalities occur.497 Preva­
lence of FGFR1 mutations in the Japanese population is equal to the 
prevalence among whites. Alt

tion is equal to the 
prevalence among whites. Although renal aplasia is characteristic of 
ANOSM1 mutations, and cleft palate and dental agenesis are char­
acteristic of FGFR1 mutations in KAL2, these findings can occur 
in Kallmann patients without ANOSM1 or FGFR1 mutations.489 
KAL3. Apparent autosomal recessive inheritance characterizes 
other kindreds with Kallmann syndrome type 3 (KAL3), for which 
the affected gene is PROKR2. Unilateral renal agenesis, hypo­
telorism, cleft lip and palate

nal agenesis, hypo­
telorism, cleft lip and palate, and a midline cranial fusion defect 
occur. Fibrous dysplasia, sleep disorder, severe obesity, synkinesia, 
and epilepsy are described in patients with PROK2 or PROKR2 
mutations, and 3% of IHH patients were affected in one study. 
Knockout mice lacking the PROK2 gene (formerly called KAL4), 
which encodes prokineticin 2, an 81–amino acid peptide that sig­
nals through the G protein–coupled product of the PROKR2 gene 
(formerly called KAL3), ha

uct of the PROKR2 gene 
(formerly called KAL3), had defective development of the olfac­
tory bulbs and failed migration of GnRH neurons.498 This model 
led to demonstration of loss-of-function mutations in PROKR2 
or PROK2 in 9% of patients with Kallmann syndrome,499 who 
were mostly heterozygous, although homozygous and compound 
heterozygous mutations were also described. Some families appear 
to have autosomal dominant inheritance. 
Other Forms of Kallmann Syndrome. The human equivalent of 
t

s of Kallmann Syndrome. The human equivalent of 
the mouse nasal embryonic GnRH factor gene (Nelf) is NELF; a 
mutation of this gene is found in patients with IHH and in Kall­
mann syndrome.500
A mutation in the binding domain of the membrane corecep­
tor neuropilin-1 (SEMA3A) gene is found in Kallmann syndrome 
in an autosomal dominant pattern.501 This mutation was found 
in some patients who already had mutations in other Kallmann 
syndrome genes, further suggesting the oligogenic nature of th

es, further suggesting the oligogenic nature of the 
disorder. Semaphorin 3E (SEMA3E) is a protein involved with 
axonal growth, and missense mutations were found in two broth­
ers with Kallmann syndrome.502
About 30% of Kallmann syndrome patients have hearing 
impairment, and of those about 38% have heterogeneous muta­
tions of the SOX10 gene.503 Deafness may also be found in the 
CHARGE syndrome (discussed later), but in the absence of other 
CHARGE findings a search for a mutation in SOX10 is

HARGE findings a search for a mutation in SOX10 is indicated.
Mutations in heparin sulfate 6-O-sulfotransferase 1 (HS6ST1) 
are found in patients and families with IHH and Kallmann syn­
drome.504,505 This abnormality is found in complex inheritance 
patterns as patients may also have mutations in FGF1 and NELF. 
Thus HS6ST1 mutations may not be sufficient to cause disease 
but may add to it.
Recently, Kallmann syndrome has been linked to mutations 
in HESX1 (found in septo-optic dysplasia, discu

s 
in HESX1 (found in septo-optic dysplasia, discussed later)506 and 
FEZF1.507
Congenital hypogonadotrophic hypogonadism can be associ­
ated with many other clinical features not involving the repro­
ductive system. The CHARGE syndrome is characterized by 
coloboma or microphthalmia, coloanal stenosis or atresia, middle 
and inner ear defects, various cranial nerve defects (including 
anosmia or hyposmia [including absent olfactory bulbs]), and 
IHH. In autosomal dominant familial cases, causat

IHH. In autosomal dominant familial cases, causative mutations 
were found in CHD7, which encodes a chromatin-remodeling 
factor, which in a small percentage of cases were found in patients 
with Kallmann syndrome and in some patients with IHH due to 
apparent loss-of-function mutations. Hypogonadotropic hypogo­
nadism may be more frequent if there is deafness and hypoplasia 
of the semicircular canals.508
Patients with demyelinating sensorimotor polyneuropathy, 
developmental delay, and severe

r polyneuropathy, 
developmental delay, and severe hypoglycemia initially, with later 
nonautoimmune insulin-dependent diabetes mellitus, had muta­
tions in DMXL2, which codes for rabconnectin-3α.509
Kallmann syndrome has been reported in patients with hetero­
zygous mutations in the WDR11 gene, which is involved in the 
development of olfactory neurons.502,510
The protein CCDC141 is expressed in GnRH neurons and in 
the back in activating mutations led to normal osmotic hypogo­
nadotropic hypog

ons led to normal osmotic hypogo­
nadotropic hypogonadism.511 Mutations in FEZF1 were found 
in two families with Kallmann syndrome.507 This gene promotes 
the expression of a protease, which allows the olfactory receptor 
neurons to enter the brain along with GnRH neurons.
Mutations in SMC-HD1 led to the absence of the nose in 
41 individuals, most of whom had hypogonadism along with 
anosmia.512
The Gordon Holmes syndrome presents with congenital hypo­
gonadotropic hypogonadism, normosmia, and

al hypo­
gonadotropic hypogonadism, normosmia, and cerebellar atrophy/
ataxia; this syndrome is associated with mutations in the OTUD4, 
RNF216, and PNPLA6 genes.513
The various forms of Kallmann syndrome result from het­
erogeneous mutations in which the phenotype can vary. For 
example, a 20-year-old man with the complete picture of 
Kallmann syndrome had an identical twin brother (proved 
by genetic fingerprinting) with anosmia but a normal adult 
phenotype and normal plasma testosterone and 

ult 
phenotype and normal plasma testosterone and gonadotropin 
Other postulated defects that may interfere with GnRH neu­
ron migration are mutations in the genes for neural cell adhe­
sion molecules (NCAM) and related proteins, such as tenascin, 
laminin, and phosphacan. Various glycoconjugates may also be 
involved. 
Other Forms of Isolated Hypogonadotropic Hypogonadism. 
Only about 15% of normosmic hypogonadotropic patients have 
a definable genetic defect. The combination of

ve 
a definable genetic defect. The combination of human genetic 
studies and mouse models has led to the discovery of many genes 
involved in gonadotropin regulation.514 Inheritance of hypogo­
nadotropic hypogonadism (Table 26.19) with none of the other 
features of Kallmann syndrome may be found in autosomal 
dominant (gene map locus 19p13.3, 9q34.3), autosomal reces­
sive (8p21-p11.2), or X-linked recessive (Xp21)515,516 disorders. 
Males with cerebellar ataxia and deficient gonadotropin prod

 cerebellar ataxia and deficient gonadotropin produc­
tion are reported in kindreds with X-linked inheritance (possibly 
a variant form of Kallmann syndrome), and hypogonadotropic 
hypogonadism may be associated with the multiple lentigines and 
basal cell nevus syndromes.
Gonadotropin-Releasing Hormone Gene Mutations. The GnRH 
gene (GNRH1) would seem a likely candidate for the cause of hypo­
gonadotropic hypogonadism, but although mutations of the GnRH 
1091
Chapter 26  Physiology and Disorder

the GnRH 
1091
receptor gene (GNRHR) were identified years ago, mutations in the 
GNRH1 gene were not demonstrated until 2009. An autosomal 
recessive form had been described in the mouse (hyg/hyg), in which 
there is a deletion of part of the GNRH gene. There appears to be 
normal migration of GnRH neurons to the medial basal hypothala­
mus based on information from the mouse model.517
GNRH1 mutation is a very rare condition with a prevalence of 

on is a very rare condition with a prevalence of 
under 1% in isolated populations and probably far lower in the 
general population. A homozygous GNRH1 frameshift mutation 
in human beings, characterized by the insertion of an adenine at 
nucleotide position 18 (c.18-19insA) in the sequence encoding 
the N-terminal region of the signal peptide–containing protein 
  Molecular Basis for Developmental Disorders Associated With Hypogonadotropic Hypogonadism
Gene
Phenotype
Complex Phenotype
ISOLATE

ogonadism
Gene
Phenotype
ISOLATED HYPOGONADOTROPIC HYPOGONADISM
Kallmann Syndrome or Normosmic IHH (With the Same Mutant Gene)
KAL1 (Xp22.3)
X-linked Kallmann syndrome
Anosmia/hyposmia, renal agenesis, dyskinesia
FGFR1 (KAL2) (8p11.2)
Autosomal dominant Kallmann syndrome 
(± recessive)
Anosmia/hyposmia, cleft lip/palate
FGF8 (ligand for FGFR1) (10q25)
NELF (9p34.3)
Autosomal dominant (?) Kallmann syndrome
PROK2 (3p21.1)
Autosomal recessive Kallmann syndrome
PROKR2 a (20p12.3)
C

l recessive Kallmann syndrome
CHD7 (8p12.1)
Autosomal dominant (some)
CHARGE syndrome includes hyposmia
Normosmic Isolated Hypogonadotropic Hypogonadism
GNRH1 (8p21-11.2)
GNRHR a (4q13.2-3)
Autosomal recessive (± dominant)
GPR54 a (19p13.3)
SNRPN
Lack of function of paternal 15q11-
q13 region or maternal uniparental 
disomy
Obesity
LEP (7q31.3)
Obesity
LEPR (1p31)
Obesity
NR0B

esity
Obesity
NR0B1 (DAX1) (X21.3-21.2)
TAC3 (12q13-12)
TACR3 (4q25)
Multiple Pituitary Hormone Deficiencies
PROP1 (POU1F1)
Autosomal recessive GH, PRL, TSH, and LH/FSH 
(less commonly, later-onset ACTH deficiency)
HESX1 (RPX)
Autosomal recessive; and heterozygous mutations
Multiple pituitary deficiencies, including diabetes 
insipidus, but LH/FSH uncommon
LHX3
Auto

diabetes 
LHX3
Autosomal recessive GH, PRL, TSH, FSH/LH
Rigid cervical spine
PHF6
X linked; GH, TSH, ACTH, LH/FSH
Börjeson-Forssman-Lehmann syndrome: mental 
retardation; facies
aA G protein–coupled receptor.
ACTH, Corticotropin; CHD7, chromatin-remodeling factor; DAX1, dosage-sensitive sex reversal-adrenal hyperplasia congenita critical region on the X chromosome, gene 1; FGF, fibroblast growth factor; 
FSH, follicle-stimulating hormone; GH, growth hormone; GNRH, 

le-stimulating hormone; GH, growth hormone; GNRH, gonadotropin-releasing hormone; GPR54, kisspeptin G protein–coupled receptor 54; HESX1, homeobox gene expressed in ES cells; IHH, 
idiopathic hypogonadotropic hypogonadism; LEP, leptin; LH, luteinizing hormone; LHX3, lim homeobox gene 3; NELF, nasal embryonic luteinizing hormone–releasing factor; NR0B1, nuclear receptor family 
0, group B, member 1; PHF6, plant homeodomain–like finger gene; PRL, prolactin; PROK2, prokineticin 2; PROP1, prophet of

rolactin; PROK2, prokineticin 2; PROP1, prophet of Pit-1; R, receptor; SNRPN, small nuclear ribonucleoprotein polypeptide SmN; 
TAC3, neurokinin 3; TSH, thyroid-stimulating hormone.
Modified from Semple RK, Topaloglu AK. The recent genetics of hypogonadotrophic hypogonadism—novel insights and new questions. Clin Endocrinol. 2010;72(4):427–435.
See text for details.
   
TABLE 26.19
1092
precursor of GnRH (prepro-GnRH), was found in a teenage 
brother 

RH (prepro-GnRH), was found in a teenage 
brother and sister who had normosmic IHH.518 When expressed 
in vitro, the mutant peptide did not demonstrate immunoreactive 
GnRH. Only 1 of 310 patients with severe, congenital, normos­
mic IHH (with micropenis, bilateral cryptorchidism, and absent 
puberty) had a homozygous frameshift mutation that is predicted 
to disrupt the three C-terminal amino acids of the GnRH decapep­
tide and to produce a premature stop codon.517 Of four patients 
with normos

ature stop codon.517 Of four patients 
with normosmic IHH, one had a nonsynonymous missense muta­
tion in the eighth amino acid of the GnRH decapeptide; one had 
a nonsense mutation that causes premature termination within 
the GnRH-associated peptide (GAP), which lies C-terminal to 
the GnRH decapeptide within the GnRH precursor; and two had 
sequence variants that cause nonsynonymous amino acid substitu­
tions in the signal peptide and GnRH-associated peptide. 
Gonadotropin-Releasing Hormone R

ociated peptide. 
Gonadotropin-Releasing Hormone Receptor Mutations. Muta­
tions of the gene encoding the type 1 GnRH receptor (GNRHR, 
gene map locus 4q21.2) that affect the G protein–coupled, seven-
transmembrane segments lead to various degrees of familial and 
sporadic hypogonadotropic hypogonadism with normosmia. 
A GNRHR mutation is found in about 40% to 50% of cases of 
familial, autosomal recessive, normosmic IHH and in about 17% 
of sporadic cases of normosmic IHH.
Mutations in hypergon

adic cases of normosmic IHH.
Mutations in hypergonadal hypogonadism with amino 
acid substitutions in the extracellular N-terminal domain 
(Thr32Ile), the second extracellular loop (Cys200Tyr), the 
third intracellular loop (Leu266Arg), and the sixth transmem­
brane helix (Cys279Tyr) were found to affect specific GnRH 
binding.519,520 Except for Thr32Ile, there was no significant 
inositol phosphate accumulation after GnRH stimulation, 
demonstrating loss of function even if binding was accom­
p

ting loss of function even if binding was accom­
plished. However, an increased dose of GnRH allowed stimula­
tion of the gonadotropin subunit and GnRHR promoters and 
the ability to partially activate extracellular signal-regulated 
kinase 1 and stimulate cAMP response element (CRE)–lucif­
erase activity. A higher dose of GnRH caused the Cys200Tyr 
mutant to stimulate gonadotropin subunit and GnRHR pro­
moter activity because this mutant reduces cell surface recep­
tor expression.
Another human

 cell surface recep­
Another human GnRH receptor (GNRHR) gene mutation of a 
highly conserved sequence located in the second-transmembrane 
helix impairs GNRHR effector coupling due to loss of surface 
expression of the receptor and leads to a severe manifestation of 
IHH.521
Certain GnRH receptor defects may be rescued by membrane-
permeant pharmacologic agents that can act as a folding template 
(chaperones) that rescue the structural defects caused by the muta­
tions and allow

ctural defects caused by the muta­
tions and allow function to occur (i.e., ligand binding and restora­
tion of receptor coupling to effector).522 This approach may allow 
a therapeutic approach to conditions caused by this and other 
mutations in receptors that result in protein misfolding.
The clinical presentation of patients with mutations in the 
GnRH receptor is heterogeneous, and impairment of signal trans­
mission is highly variable (e.g., severe features of IHH, sexual 
infantilism, lon

, severe features of IHH, sexual 
infantilism, long-delayed puberty with reversal in adulthood, rela­
tively mild hypogonadism and infertility), even within the same 
pedigree and especially in patients with compound heterozygous 
mutations. For example, two women with a normal pubertal his­
tory with hypothalamic amenorrhea had mutations in the GNRHR 
gene523 while the other had mutations in the FGFR1, PROKR2, 
and ANOSM1 genes. When the mutation is homozygous, the phe­
notype is related to the

n is homozygous, the phe­
notype is related to the number of mutations. However, when it 
is monoallelic, there is no relationship, which indicates that other 
genes interact with the single allelic mutation in GNRH1.524 Bial­
lelic mutations in GNRHR were found in two girls (compound 
heterozygous c.317A>G p.[Gln106Arg] and c.924_926delCTT 
p.[Phe309-Del] and homozygous c.785G>A p.[Arg262Gln]) with 
incomplete pubertal development or secondary amenorrhea, and 
twin boys with incomplete pubertal

menorrhea, and 
twin boys with incomplete pubertal development had compound 
heterozygous for GNRHR mutations c.317A>G p.(Gln106Arg) 
and c.785G>A p.(Arg262Gln).525 With the increasing number of 
mutations of the GnRH receptor found in partial and reversible 
normosmic IHH, this gene is suggested to be the first to evaluate 
in such cases.526 In all types of congenital gonadotropin deficien­
cies, male patients are likely to manifest micropenis (penile length 
<2 cm at birth and in infancy) due 

penile length 
<2 cm at birth and in infancy) due to lack of fetal gonadotropin 
stimulation of fetal testes during the last half of gestation. Boys 
with congenital GH deficiency have micropenis even if gonado­
tropin function is normal. Because testosterone therapy is effective 
in increasing penile size (see later discussion), sex reversal is not 
indicated in these cases of microphallus.527 
KISS1/KISS1R Axis Mutations. The KISS1/KISS1R axis plays a 
role in the increased amplitude of GnRH s

plays a 
role in the increased amplitude of GnRH signaling in puberty. KISS1/
KISS1R axis mutations are rare but instructive. Of 30 normosmic 
subjects with hypogonadotropic hypogonadism who were evaluated, 
one person had two missense mutations in KISS1R (Cys223Arg in 
the fifth transmembrane helix and Arg297Leu in the third extracel­
lular loop); the former had no activity, and the latter manifested as 
mildly decreased signaling ability.528 Homozygous deletions of 155 
nucleotides in the KISS

mozygous deletions of 155 
nucleotides in the KISS1R gene encompassing the splicing acceptor 
site of the intron 4–exon 5 junction and part of exon 5 were found 
in all affected family members with IHH,529 but unaffected family 
members had no deletion or only one mutant allele. Another kin­
dred had a Leu48Ser mutation in the second intracellular loop (IL2) 
of KISS1R, and another had two separate mutations in the gene,352 
Arg331Xaa and Xaa399Arg. The latter patient had decreased secre­
tion o

rg. The latter patient had decreased secre­
tion of GnRH and decreased response to GnRH administration. A 
line of mice transfected with the affected gene exhibited hypogo­
nadotropic hypogonadism with decreased GnRH in the hypothala­
mus but were responsive to GnRH or gonadotropin administration. 
Agonist stimulation may stabilize the switch II region of Gα to pro­
mote the opening of Gα switch II to facilitate exchange of guano­
sine diphosphate (GDP) and guanosine triphosphate (GTP).530 The 

e (GDP) and guanosine triphosphate (GTP).530 The 
Leu148Ser mutation does not affect the expression, ligand-binding 
properties, or protein interaction network of KISS1R, but diverse 
KISS1R functional responses are markedly inhibited. 
X-Linked Congenital Adrenal Hypoplasia and Hypogonado­
tropic Hypogonadism. A rare deletion or mutation in the dosage-
sensitive sex reversal-A (DSS) adrenal hypoplasia congenita gene on 
the X chromosome gene 1 (NR0B1, formerly called DAX1; gene 
map locus Xp21.

NR0B1, formerly called DAX1; gene 
map locus Xp21.3-p21.2) leads to an X-linked recessive disorder of 
adrenocortical organogenesis.531 The gene encodes an orphan recep­
tor, a member of the nuclear receptor superfamily, that is a puta­
tive transcriptional repressor mapping to the Xp21 locus. A double 
dose of NR0B1 is associated with a female phenotype or ambiguous 
genitalia in 46,XY males. The NR0B1 protein has a novel domain 
in the N-terminus that contains two putative unique zinc finger 

us that contains two putative unique zinc finger 
motifs, and the C-terminus contains a conserved ligand-binding 
domain that binds DNA, localizes in the nucleus, and contains 
a transcriptional silencing domain that antagonizes the steroido­
genic factor 1 (SF1, also called NR5A1) transactivation function. 
NR0B1 has an SF1 response element in the 5′-promoter region that 
is another orphan member of the nuclear hormone receptor super­
family. Both NR0B1 and SF1 are expressed in the adrenals, go

th NR0B1 and SF1 are expressed in the adrenals, gonads, 
1093
pituitary, and hypothalamus, raising the possibility of an important 
interaction between these two genes and their products.
Rare abnormalities of NR0B1 are characterized by severe glu­
cocorticoid, mineralocorticoid, and, at puberty, androgen defi­
ciency. The abnormal structure of the adrenal cortex resembles 
that of the fetal zone because it consists of disorganized, vacu­
olated, c

cause it consists of disorganized, vacu­
olated, cytomegalic cells with a normal mature cortex. The severe 
primary adrenal insufficiency with hyponatremia, hyperkalemia, 
acidosis, and hypoglycemia is characterized by failure to thrive, 
vomiting, poor feeding, dehydration, circulatory collapse, and 
increased pigmentation and is lethal if not treated early in life in 
affected boys.
Adrenoleukodystrophy may manifest with adrenal failure long 
before neurologic symptoms develop, and some cases 

efore neurologic symptoms develop, and some cases of X-linked 
Addison disease may represent this diagnosis. This condition is in 
the differential diagnosis of adrenal hypoplasia. Plasma renin activ­
ity is high; plasma cortisol and aldosterone levels are low. Symp­
tomatic adrenal insufficiency may first manifest in later childhood. 
In male infants, signs of salt wasting are usually the most promi­
nent feature, but cortisol deficiency is detectable, and adrenal 
insufficiency includes defici

ctable, and adrenal 
insufficiency includes deficient secretion of the zona reticularis 
steroids, DHEA and DHEAS. An early sign of elevated ACTH is 
increased skin pigmentation. The testes are undescended in fewer 
than one-half of patients; micropenis is rare, but urogenital abnor­
malities and hearing loss occasionally are present. Boys who do not 
present with clinical evidence of adrenal insufficiency in infancy 
often have a more insidious onset during childhood or adulthood.
In a pedigree

onset during childhood or adulthood.
In a pedigree in which two affected boys had a hemizygous 
NR0B1 nonsense mutation and neonatal onset of adrenal insuf­
ficiency, a maternal aunt who was homozygous for the mutation 
had sexual infantilism and primary amenorrhea, but even after 
decades of follow-up, she maintained normal adrenal function. A 
maternal grandfather who carried the same mutation was asymp­
tomatic.532 This pedigree highlights the limitations and complexi­
ties of genotype and ph

 limitations and complexi­
ties of genotype and phenotype correlations. Most commonly, 
due to hypogonadotropic hypogonadism, signs of sexual matura­
tion at the age of puberty (e.g., pubic and axillary hair, testicular 
enlargement) are lacking, and the concentrations of serum FSH, 
LH, and testosterone are low. Delayed puberty is a manifestation 
in some female carriers of a NR0B1 mutation.
Intragenic mutations in NR0B1 (i.e., frameshift mutations, 
nonsense mutations, and missense mutations) 

ons, 
nonsense mutations, and missense mutations) indicate that the 
hypogonadotropic hypogonadism is an intrinsic characteristic of 
the disorder, a manifestation of the single-gene mutation and not 
a result of involvement of a contiguous gene. The NR0B1 gene 
is expressed in the adrenal cortex, in testes (and weakly in the 
ovary), and in the hypothalamus and pituitary. There is evidence 
of GnRH deficiency and an abnormality in the gonadotrophs, 
yielding a mixed picture of hypothalamic and 

hs, 
yielding a mixed picture of hypothalamic and intrinsic gonado­
troph defects with absent or erratic pulsatile secretion of LH. Even 
if basal immunoreactive LH and FSH levels are normal, gonado­
tropins seem to lack bioactivity. In some affected boys, the GnRH 
pulse generator and pituitary gonadotropin apparatus is intact and 
functional in infancy and early childhood, and the GnRH-gonad­
otroph defects do not manifest until later in childhood or during 
the peripubertal period. Azoospermi

ood or during 
the peripubertal period. Azoospermia unresponsive to gonadotro­
pin treatment was detected in a few affected men.508,515,533,534
A deletion of the adrenal hypoplasia congenita locus (at Xp21) 
can include the glycerol kinase (GK) and Duchenne muscular 
dystrophy (DMD) genes if it extends centromerically or produce 
developmental delay if there is extension toward the telomere, 
leading to contiguous gene syndromes.
Other mutations of the X chromosome may be associated with 
IHH. T

of the X chromosome may be associated with 
IHH. Two brothers were reported who had hypogonadotropic 
hypogonadism, obesity, and short stature associated with a mater­
nally inherited pericentric inversion (X) (p11.4q11.2). Because 
the breakpoint is not related to other genes associated with puber­
tal disorders, it is not clear whether this is a functional relationship 
or a coincidence.535
The TAC3 gene codes for tachykinin-3, which is cleaved to 
neurokinin B, a member of the tachykinin supe

 to 
neurokinin B, a member of the tachykinin superfamily of 127 
neuropeptides that includes substance P and neurokinin A. Its 
cognate G protein–coupled receptor 126 is neuromedin-K recep­
tor 3 (NK3R), which is encoded by TACR3. There are neurons in 
the arcuate nucleus of the hypothalamus that coexpress kisspeptins 
and NKB, the kisspeptin/NKB/Dyn (KNDy) neurons that also 
express dynorphin A (endogenous opioid peptide) and NK3R.536 
Loss-of-function mutations occur in this system in familia

function mutations occur in this system in familial, con­
genital hypogonadotropic hypogonadism; administration of pul­
satile GnRH caused resumption of gonadotropin secretion537–539 
(Fig. 26.43). A survey of 345 patients with normosmic hypo­
gonadotropic hypogonadism found 13 rare, distinct nucleotide 
sequence coding variants (three nonsense, six nonsynonymous, 
and four synonymous mutations [one predicted to affect splicing]) 
in TACR3 and one homozygous single-base-pair deletion, result­
in

e homozygous single-base-pair deletion, result­
ing in complete loss of neurokinin B. Of the 16 males for whom 
phenotype information was available, 15 had microphallus, and 
none of the females had spontaneous thelarche. When subjects 
were assessed after discontinuation of therapy, six of seven males 
and four of five females demonstrated evidence for reversibility of 
their hypogonadotropism.540 Continuous infusion of kisspeptin 
to two patients with loss-of-function mutations in neurokinin B

ts with loss-of-function mutations in neurokinin B 
(TAC3) or its receptor (TAC3R) led to pulsatile gonadotropin 
secretion, indicating that pulsatile administration of kisspeptin is 
not necessary to institute pulsatile GnRH secretion.363
Mutations in the prohormone convertase 1 gene (PCSK1, also 
called PC1) lead to extreme childhood obesity, hypocortisolemia, 
defects in conversion of proinsulin to insulin leading to hypogly­
cemia, diabetes insipidus, and isolated partial hypogonadotropic 
h

nsipidus, and isolated partial hypogonadotropic 
hypogonadism, allowing spontaneous pubertal development but 
primary amenorrhea. The hypogonadal hypogonadism probably 
resulted from impaired processing of GnRH or neuropeptides 
involved in its secretion. Findings in another subject extended 
to gastrointestinal disturbance, small intestinal malabsorption 
related to monosaccharide and fats, and elevation in progastrin 
and proglucagon levels, showing that prohormone processing in 
enteroendocri

owing that prohormone processing in 
enteroendocrine cells was abnormal.541 
Isolated Luteinizing Hormone Deficiency. Isolated LH defi­
ciency (fertile eunuch syndrome) is associated with deficient 
testosterone production (which responds to hCG administra­
tion) and decreased virilization in the presence of mature tes­
ticular size and variable spermatogenesis; the disorder may be 
idiopathic or may result from a hypothalamic pituitary neo­
plasm. A homozygous Gln106Arg mutation in the first ex

m. A homozygous Gln106Arg mutation in the first extra­
cellular loop of the GnRH receptor at gene map locus 4q21.2 
was associated with normal testicular volume (17 mL) but with 
apulsatile, low gonadotropin values and low testosterone542 val­
ues in one subject. After hCG stimulation, he developed ade­
quate spermatogenesis to father a child, and after cessation of 
hCG treatment, he demonstrated adult testosterone values and 
pulsatile gonadotropin secretion, an example of reversibility of 
th

opin secretion, an example of reversibility of 
the syndrome. 
Isolated Follicle-Stimulating Hormone Deficiency. Homozy-
gous or compound heterozygous mutations in the FSH β-subunit 
1094
have been reported in three females and two males with delayed 
puberty or poorly developed secondary sex characteristics and with 
primary amenorrhea but normal adrenarche in the women.543 
The LH concentration was elevated, the serum level of estradiol 
was low, a

elevated, the serum level of estradiol 
was low, and immunoactive FSH was absent. Two of the three 
women had a homozygous nonsense mutation (Val61X) in the 
FSH β-subunit gene at gene map locus 11p13, and the other was 
a compound heterozygote (Cys51Gly/Val61X). The women had 
antral follicles but no progression, demonstrating that FSH action 
may not be necessary for development to the antral stage. The two 
men had azoospermia; small, soft testes; and absence of serum 
FSH. One had normal pub

tes; and absence of serum 
FSH. One had normal puberty and normal LH and testosterone 
values, with a missense mutation (Cys82Arg), and the other had 
slightly delayed puberty, low testosterone and inhibin B levels, 
high LH levels, and a nonsense mutation (Val61X). The low tes­
tosterone may indicate a necessity for FSH to promote testoster­
one production. 
Follicle-Stimulating Hormone Receptor Mutations. Hypergo­
nadotropic hypogonadism is noted with rare mutations in FSH 
receptors.544 Women

ed with rare mutations in FSH 
receptors.544 Women may have several mutations, but only five 
men are described as homozygous for the Finnish p.Ala189Val 
FSHR gene mutation. Although the affected women had amen­
orrhea, some of the men were fertile and all progressed through 
puberty with normal or slightly small testes. The defect in the 
A
B
G18D
L58L
V98V
W208X
W275X*
R295S
Y256H*
P353S*
Y315C
T246T*
1249V
S448S
S27X*
* Homozygous variants
M A T L F A A E T W L O L G L O V
A D A V N L
T A
L

A T L F A A E T W L O L G L O V
A D A V N L
T A
L
A
A
G
A
A
I
G
A
V
E
T
G
A
I
O
I
L
O
O
A
G
N
L
S
S
S
P
S
A
G
L
P
V
A
S
P
A
P
S
O
P
A
A
N
L
T
N
O
P V
OP
S
W
N
I A L A
S L A V
G V V V
M
A
A
I S N L
I V I A
A
I
M
I
P
N
A
V R T V
T
N
V
V
A
O
M
A
T L I
N
F
I
N
A
L
I S F A T F G
A
N
V
O
N
F
F
I
F
I
V
I A V
O
N
Y
V
A
I
I
D F L K P N
L S
A
T
A T
K
I V I
I
S
G
I
L
L
F
A
I A K
C
Q
P
L V S
K
T
K
V
M
P
G
R
I
T C F V Q A P E G
P
K
Q
H
V
Y
N
I V I
I L V
C F P I
L I M
G I T
Y T I
V G I
T
L
G
G
I
I
P G D T C D

 I
L I M
G I T
Y T I
V G I
T
L
G
G
I
I
P G D T C D K Y H E Q L
K
A
N
K
V
V
A
M M I
I
T
M
V
V
F A I C
P
L
W
H I Y
L
I
I
T A I
Y
Q
Q
I N
N W K
Y
I
Q
G
V
Y L A
W
F
S
L A M
I
S
S
V Y N
I
Y C C
L
N
Y
R
P
R
A
G
F
I
I
A
F
R
W
C
P
F
I
K
V
S
S
Y
D
E
L
E
L
K
T
T
R
F
H
P
N
R
Q
S
S
M
Y
T
V
S
R
V
T
S
M
I
V
I
F
D
P
N
D
A
D
T
T
R
S
S
R
K
K
H
A
T P A D P S F N G C S R R N S K S A
A T S S F I S S P Y I S V D E T S
I
I A
V S
A S
T A V
F P
O N
F N
A
A S
P S
N L
P L
25
20
15
10
5
0
Maximal IP stimulation
(fold incr

25
20
15
10
5
0
(fold increase)
EV
Y315C
R295S
Y256H
1249V
G18D
WT
a
b
b
b
a
c
a
NK3-R
• Fig. 26.43  (A) Schematic of mutations in the NK3-R, which is encoded by TAC3. (B) Effects of mutations in TACR3 on neurokinin B (NKB)–mediated 
activation of signal transduction. COS7 cells transfected with wild-type (WT) G18D, I249V, Y256H, R295S, and Y315C NK3R or empty vector (EV) 
were treated with NKB (10-7 M) for 1 hour. A significant increase in inositol phosphate (IP) accumul

ficant increase in inositol phosphate (IP) accumulation occurred in cells transfected with WT, G18D, 
or I249V NK3R. In contrast, there was a marked reduction in NKB-stimulated IP production in cells transfected with Y256H, R295S, or Y315C NK3-R, 
or with EV. a, b, and c denote significantly different fold increases in IP accumulation. (From Gianetti E, Tusset C, Noel SD, et al. TAC3/TACR3 muta­
tions reveal preferential activation of gonadotropin-releasing hormone release by neurokinin B in neo

n-releasing hormone release by neurokinin B in neonatal life followed by reversal in adulthood. J Clin 
Endocrinol Metab. 2010;95:2857–2867.)
1095
FSH receptor appears to be less severe than the absence of the 
ligand. Variations in FSHR are associated with age of thelarche in 
healthy Danish girls, while variance in LIN28B is related to age of 
menarche and thelarche; this demonstrates a differential impact of 
specific gene loci on age of thelarc

al impact of 
specific gene loci on age of thelarche and menarche.545 
Developmental Defect of the Midline
Septo-optic or optic dysplasia is caused by abnormal development 
of the prosencephalon, leading to small, dysplastic, pale optic discs 
with a double outline and pendular (evenly moving side to side) 
nystagmus; blindness may occur. The prevalence is 1.9 and 2.5 per 
100,000 births.546 A midline hypothalamic defect may cause GH 
deficiency, diabetes insipidus, and ACTH, TSH, and gonadotrop

 diabetes insipidus, and ACTH, TSH, and gonadotropin 
deficiency. Short stature and delayed puberty may result, although 
CPP is an alternative.546 The septum pellucidum is often absent 
in association with optic hypoplasia or dysplasia, which is readily 
demonstrable by imaging techniques.547 In the University of Cali­
fornia at San Francisco (UCSF) series, the syndrome was associated 
with decreased maternal age; recent series found maternal age of 20 
to 24 years to carry the highest risk.545

e of 20 
to 24 years to carry the highest risk.545 The pituitary may be hypo­
plastic because of the lack of hypothalamic stimulatory factors, and 
the neurohypophysis may have an ectopic location identified by 
the location of the posterior pituitary hot spot on MRI.
Abnormalities of the corpus callosum and cerebellum are com­
mon on MRI. Four groups are described: those with normal MRI 
results, those with abnormalities of the septum pellucidum and 
with a normal hypothalamic pituitary area, t

 and 
with a normal hypothalamic pituitary area, those with abnor­
malities of the hypothalamic pituitary area and a normal septum 
pellucidum, and those with abnormalities in both areas.548 No 
endocrine abnormalities were described in the first group, but the 
others had progressively more endocrine abnormalities, with pre­
cocious puberty most common in the second group. Early diag­
nosis is important because of the risk of sudden death associated 
with adrenal insufficiency.
The condition is

ated 
The condition is usually sporadic, but inherited cases involv­
ing transcription factors HESX1, SOX2, SOX3, and OTX2 are 
reported.549
The solitary median maxillary incisor syndrome is associated 
with the eponymous midline defect and with a prominent mid­
palatal ridge (torus palatinus) and hypopituitarism. The defect in 
this autosomal dominant condition is in the sonic hedgehog gene 
(SHH) at gene map locus 7q3.550
Other congenital midline defects ranging fro

3.550
Other congenital midline defects ranging from complete dys­
raphism and holoprosencephaly to cleft palate or lip are associ­
ated with hypothalamic-pituitary dysfunction. Delayed puberty is 
rarely described in duplication of the hypophysis. Myelomeningo­
cele (myelodysplasia) is associated with endocrine abnormalities, 
including hypothalamic hypothyroidism, hyperprolactinemia, 
and elevated gonadotropin concentrations, and with CPP.
Long-term follow-up of hypogonadotropic hypogonadism 
o

term follow-up of hypogonadotropic hypogonadism 
or partial pubertal development revealed that even with defined 
genetic defects, restoration of normal gonadal function, includ­
ing pulsatile LH secretion and spermatogenesis in adulthood, may 
occur in over 20% of cases.551,552 A recent series of 308 patients 
demonstrated that 22% underwent reversal.484 One had unde­
tectable olfactory bulbs demonstrating that this anatomic find­
ing does not necessarily lead to long-term infertility. There wa

ecessarily lead to long-term infertility. There was a 
higher prevalence of neurokinin B signaling (10%) of the subjects 
undergoing reversal, but none had mutations in KAL1. Five men 
undergoing reversal reverted back to hypogonadotropic hypogo­
nadism. Long-term follow-up is necessary, even for patients with 
established anatomic and genetic defects. 
Idiopathic Hypopituitary Dwarfism
In addition to HESX1 mutations, autosomal recessive mutations 
in homeobox genes encoding transcription factor

s 
in homeobox genes encoding transcription factors involved in the 
early aspects of pituitary development lead to hypogonadotropic 
hypogonadism and other pituitary hormone deficiencies.553,554 
PROP1 mutations at gene map locus 5q cause GH and TSH defi­
ciency and produce delayed puberty or late onset of secondary 
hypogonadism in adulthood and rarely cause ACTH deficiency 
(Fig. 26.44).555 In one study of 73 patients with idiopathic multi­
ple pituitary hormone deficiencies, 35 had a mutatio

e pituitary hormone deficiencies, 35 had a mutation in PROP1. 
Homozygous Arg73Cys mutation of PROP1 allowed spontaneous 
puberty in 2 of 10 affected family members. ACTH deficiency 
may develop later than other deficiencies and is more rarely a fea­
ture of PROP1 deficiency.
Homozygous mutations occur in the LHX3 gene at gene 
map locus 9q34.3. It encodes a member of the LIM class of 
• Fig. 26.44  A 20-year-old man with idiopathic hypopituitary dwarfism and 
deficiencies of gonadotropins, thyr

 dwarfism and 
deficiencies of gonadotropins, thyrotropin, corticotropin, and growth hor­
mone had a history of arrested hydrocephalus. His height was 129 cm (−8 
SD [standard deviation]), the phallus was 2 cm long, and the testes mea­
sured 1.5 × 1 cm. He had received thyroid hormone and glucocorticoid 
replacement. The basal luteinizing hormone (LH) level was less than 0.2 ng/
mL (LER-960), follicle-stimulating hormone (FSH) level was 0.5 ng/mL (LER-
869), and testosterone level was less than 

 (LER-
869), and testosterone level was less than 0.1 ng/mL. In response to 100 μg 
of gonadotropin-releasing hormone (GnRH), the plasma LH concentration 
increased slightly to 0.6 ng/mL, and there was no increase in the plasma 
testosterone level. The excretion of urinary 17-ketosteroids was 1.1 mg/24 
hours. The bone age was 10 years, and the volume of the sella turcica was 
small on skull radiographs. For conversion to SI units, see Figs. 26.19 and 
26.20. (From Styne DM, Grumbach MM. Puberty

9 and 
26.20. (From Styne DM, Grumbach MM. Puberty in the male and female: its 
physiology and disorders. In: Yen SCC, Jaffe RB, eds. Reproductive Endo­
crinology. 2nd ed. Philadelphia, PA: WB Saunders; 1986:313–384.)
1096
homeodomain proteins, which are associated with multiple pitu­
itary hormone deficiencies, including LH and FSH, and often 
with severe restriction of head rotation.556 LH4 and GLI2 muta­
tions may cause isolated GH deficiency or c

 muta­
tions may cause isolated GH deficiency or combined pituitary 
hormone deficiencies, including gonadotropin deficiency.557,558
The familial forms of multiple pituitary hormone deficiencies 
with autosomal recessive or X-linked inheritance are less common. 
The degree of hormone deficit and the age at onset of pituitary 
hormone deficiencies may vary within a single kindred having the 
same genetic defect.
The X-linked form of hypopituitarism can be associated with 
duplication of the SOX3 

m can be associated with 
duplication of the SOX3 gene.559 Deficiency of SOX2, a transcrip­
tion factor involved in early hypothalamic-pituitary embryonic 
development, leads to anterior pituitary hypoplasia. Patients with 
SOX2 mutations have major eye abnormalities, including anoph­
thalmia, microphthalmia, and coloboma. They also have hypogo­
nadotropic hypogonadism as the most common pituitary defect, 
in contrast to most other types of pituitary hypoplasias, demon­
strating GH deficiency mo

tary hypoplasias, demon­
strating GH deficiency most frequently.560
There is an association between breech delivery (especially 
for male infants), perinatal distress, and idiopathic hypopituita­
rism.126 Malformations of the pituitary stalk demonstrable by 
MRI are common in these patients. Other types of birth traumas 
or complications may lead to hypopituitarism as well.
Common to many patients with congenital hypopituitary 
dwarfism is early onset of growth failure; late onset of diminished 

onset of growth failure; late onset of diminished 
growth is an ominous finding, suggesting the presence of a CNS 
tumor.
Isolated GH deficiency allows spontaneous pubertal develop­
ment when the bone age reaches the pubertal stage of 11 to 13 
years, usually after the corresponding chronologic age is reached. 
Associated gonadotropin deficiency does not allow spontaneous 
puberty, even when the bone age advances to the pubertal stage 
during GH therapy. 
Miscellaneous Conditions
Prader-Willi Sy

therapy. 
Prader-Willi Syndrome. Prader-Willi syndrome is an autoso­
mal dominant disorder that combines a tendency for intrauterine 
growth retardation, delayed onset and poor fetal activity, infantile 
central hypotonia, and lethargy, followed by early-onset child­
hood hyperphagia, pathologic obesity, and carbohydrate intoler­
ance (leading to type 2 diabetes in 25% of patients at a mean age 
of 20 years). Features include short stature, small hands and feet, 
mild to

lude short stature, small hands and feet, 
mild to moderate mental retardation, and emotional instability, 
including perseveration, obsessions, and compulsions. Almond-
shaped eyes, a triangular mouth, and narrow bifrontal diameter 
combined with delayed puberty and hypogonadotropic hypo­
gonadism caused by combined hypothalamic and gonadal dys­
function are characteristic.561 Despite the late or absent puberty, 
there is a tendency to early adrenarche (14%) or even precocious 
puberty rarely (

enarche (14%) or even precocious 
puberty rarely (3.6%).
Affected boys usually have a micropenis and cryptorchidism 
(100% in a large series562), and an underdeveloped scrotum 
(69%) is common. In a study of 37 adults with Prader-Willi syn­
drome, none achieved full genital development, and primary tes­
ticular defects were suggested.563 Serum AMH levels were near the 
lower limits of normal, inhibin B levels were consistently low or 
undetectable, and in the adults, FSH levels were high, althou

e, and in the adults, FSH levels were high, although 
LH levels were normal. Two adults had undetectable levels of LH 
and FSH, but in contrast to the others, they had high AMH lev­
els. Female subjects exhibit underdevelopment of the labia majora, 
labia minora, or clitoris (76%). Amenorrhea occurs in about 
one-half of cases (53%), and irregular menses or spotting is com­
mon in others. Weight reduction may lead to menarche in some 
females, because severe obesity may play a role in the impair

cause severe obesity may play a role in the impaired 
puberty in some patients. Dietary therapy during years 2 through 
10 can provide effective treatment of obesity but may decrease 
growth, although contemporaneous GH therapy may overcome 
slow growth.564
The role of relative GH deficiency in this disorder is uncertain 
and controversial. The FDA approved Prader-Willi syndrome as 
an indication for recombinant human GH treatment without a 
requirement for assessing GH secretion. Genetic testin

irement for assessing GH secretion. Genetic testing is used 
to confirm the clinical diagnosis of the syndrome. GH treatment 
(in a dose of 0.24 mg/kg/week subcutaneously given six to seven 
times per week) was shown in long-term, randomized, con­
trolled trials to decrease body fat; increase fat utilization, LBM, 
linear growth, and energy expenditure; and possibly improve 
physical strength and motor development.565 Children with 
Prader-Willi syndrome are at risk for sudden death due to gas­

syndrome are at risk for sudden death due to gas­
trointestinal, respiratory, or cardiac complications.566 However, 
the report of sudden deaths due to respiratory complications 
during GH treatment led to a recommendation for evaluation 
for sleep apnea or respiratory difficulties before instituting GH 
therapy. More recent data cast doubt on the beneficial effects of 
GH on body composition and BMI, but higher parental edu­
cational status was correlated with better clinical outcome.986
This d

correlated with better clinical outcome.986
This distinct genetic disorder, with a frequency of about 1 
case in 15,000 to 30,000 individuals, is rarely familial (i.e., the 
recurrence risk depends on the type of the genetic defect). It is 
caused by abnormalities involving the long arm of chromosome 
15 in the q11-q13 region. Approximately 70% of Prader-Willi 
cases are caused by a paternal deletion of 15q11-q13 (commonly 
3–5 megabase pairs long); 20% to 25% of cases involve mater­
nal unipare

g); 20% to 25% of cases involve mater­
nal uniparental disomy (isodisomy or heterodisomy) in which 
both chromosomes 15 are derived from the mother, possibly by 
nondisjunction during maternal meiosis, representing a striking 
example of genomic imprinting.561 In 2% to 5% of cases, an 
imprinting center defect has been detected. Lack of a functional 
paternal 15q11-q13 region, caused by any of a variety of genetic 
mechanisms, can result in the syndrome. One imprinted gene, 
that for small nucle

yndrome. One imprinted gene, 
that for small nuclear ribonucleoprotein-associated polypeptide 
SmN (SNRPN), which is implicated in splicing pre-mRNA, is 
expressed in the brain, including the hypothalamus, and has 
been advanced as one explanation of some characteristics of the 
syndrome.
Elevated serum concentrations of the GH secretagogue and 
orexigenic gastrointestinal hormone ghrelin are found in the basal 
state in Prader-Willi syndrome.567 Increased levels are identified 
after meals, whe

 Increased levels are identified 
after meals, when values should be suppressed, and are a possible 
cause of the insatiable appetite. Administration of the somatosta­
tin analogue octreotide leads to a decrease in basal ghrelin values 
and some decrease in values after meals, but no change in appetite 
was demonstrated as yet. 
Laurence-Moon and Bardet-Biedl Syndromes. The Laurence-
Moon syndrome and the Bardet-Biedl syndrome were previously 
separated as rare autosomal recessive traits, with r

parated as rare autosomal recessive traits, with retinitis pigmen­
tosum and hypogonadism of various types. Currently the condi­
tions are considered as one, and the present term is Bardet-Biedl 
syndrome. The estimated incidence is 1 in 160,000 in northern 
European populations and 1 in 13,500 in some Arab popula­
tions.568 The findings are developmental delay, spastic paraplegia, 
postaxial polydactyly, onset of obesity (usually in early infancy), 
and renal dysplasia. Hypgonadism is character

y), 
and renal dysplasia. Hypgonadism is characteristic, and males are 
1097
infertile as are most females. The genetically and phenotypically 
heterogenous Bardet-Biedl syndrome is linked to 16 genes, which 
account for 80% of cases. The basic defect is a ciliopathy. The 
Biemond syndrome II has similar features, with iris coloboma, 
hypogenitalism, obesity, polydactyly, and developmental delay, 
but it is a distinct entity. 
Functional Gonadotrop

ut it is a distinct entity. 
Functional Gonadotropin Deficiencies and Other Chronic 
Conditions. The effects of malnutrition, which can lead to func­
tional hypogonadotropic hypogonadism, should be separated 
from the primary effects of chronic systemic disease, some of 
which have direct effects on the function of the hypothalamic-
pituitary unit or the gonads. Weight loss from any cause to less 
than 80% of ideal weight for height can lead to gonadotropin defi­
ciency and low serum leptin leve

onadotropin defi­
ciency and low serum leptin levels; weight regain usually restores 
hypothalamic-pituitary gonadal function over a variable period, 
although the weight needed to restart menstrual periods varies 
among individuals and is related to the weight at which menstrua­
tion first ceased.569
If adequate nutrition and body weight are maintained in 
patients with regional enteritis or chronic pulmonary disease, 
gonadotropin secretion is usually adequate. Cystic fibrosis is also 
associa

usually adequate. Cystic fibrosis is also 
associated with delay in puberty and the age of PHV, in large 
part through malnutrition; improved management and new­
born screening have increased PHV and advanced the age of 
PHV, but neither has reached normal values.454,570,571 The age 
of menarche in girls with cystic fibrosis is related to maternal 
age, as expected, but is delayed by approximately 1 year com­
pared with menarche in the mother, an effect that is mainly 
related to nutritional sta

 effect that is mainly 
related to nutritional status.570,987 However, even with normal 
pubertal progression, boys with cystic fibrosis almost universally 
have oligospermia caused by obstruction of the spermatic ducts, 
which is unrelated to nutritional status. The greater prevalence 
of reproductive difficulties in male patients with cystic fibrosis 
compared with female patients may reflect the greater prevalence 
of the cystic fibrosis transmembrane regulator (CFTR) in male 
reproductive ti

membrane regulator (CFTR) in male 
reproductive tissues (e.g., epididymis, vas deferens), and more 
viscid luminal contents, which ultimately damage the testes and 
can lead to absence of the epididymides and the vasa deferentia. 
Normal ovaries do not express CFTR, and endometrial tissue 
expresses it only after puberty, with variable levels found in cer­
vical epithelium and the fallopian tubes. Even though the CFTR 
gene and its protein are expressed in the human hypothalamus, 
mutations in t

pressed in the human hypothalamus, 
mutations in the corresponding gene did not appear to affect 
LH and FSH secretion in an immortalized mouse hypothalamic 
GnRH-secreting cell line.
Boys and girls with sickle cell disease have delayed pubertal 
development by about 2 years and delayed menarche of about 
6 months even with modern treatment techniques.572 This may 
be due to nutritional status. Boys with sickle cell anemia often 
exhibit impaired Leydig cell function caused by ischemia of the 
t

 Leydig cell function caused by ischemia of the 
testes or gonadotropin deficiency or both.
Thalassemia carries the risk of hemochromatosis due to trans­
fusional iron deposition in the pituitary and hypothalamus; as a 
consequence, 60% to 80% patients may have hypogonadotropic 
hypogonadism and impairment of growth.573 The gonads can be 
stimulated by exogenous gonadotropins, and satisfactory sexual 
development, including fertility, can be promoted by the use of 
hCG and human FSH in many pati

oted by the use of 
hCG and human FSH in many patients without gonadal damage, 
although pituitary and gonadal damage may be severe in children 
with poorly controlled disease. Primary hypothyroidism is preva­
lent in this condition, but it is only part of the problem of sexual 
maturation. Desferrioxamine therapy may cause skeletal dysplasia 
and compromise pubertal growth, and growth failure due to GH 
deficiency may also affect pubertal growth.574 Decreased BMD in 
thalassemia makes early rec

.574 Decreased BMD in 
thalassemia makes early recognition and treatment of the problem 
all the more important.
Cytotoxic effects of the alkylating agents used to prepare 
patients for bone marrow transplantation in this condition add 
to the problem. Treatment after the onset of puberty is safer for 
gonadal function in boys but not necessarily in girls. Girls with 
early bone marrow transplantation and apparently normal puber­
tal development have elevated serum FSH levels and menstrual 
abno

have elevated serum FSH levels and menstrual 
abnormalities ranging up to amenorrhea,575 suggesting that 
gonadal impairment is universal in girls with thalassemia major 
after bone marrow transplantation.
Puberty is significantly delayed in children with prenatally 
acquired human immunodeficiency virus (HIV) infection com­
pared to those exposed but not affected.988 Modern treatment 
regimens are predicted to decrease this delay.
Chronic gastrointestinal disease (e.g., Crohn disease) is often 

intestinal disease (e.g., Crohn disease) is often 
accompanied by delayed puberty, and therapy to restore nutri­
tion, if successful, enables puberty to progress. The pubertal 
growth spurt is compromised by active inflammatory bowel 
disease, especially if glucocorticoid therapy is necessary. Celiac 
disease decreases the growth rate in childhood and adolescence, 
but with appropriate dietary restrictions, adult height appears to 
be normal.
Chronic renal disease is associated with delayed pube

onic renal disease is associated with delayed pubertal devel­
opment and decreased pulsatile gonadotropin secretion due to a 
decrease in the mass of bioactive and immunoactive LH secreted 
rather than an alteration of the frequency. Successful renal trans­
plantation usually restores gonadotropin secretion and improves 
growth. Immunoreactive gonadotropin concentrations may be 
elevated, presumably because of impaired renal clearance, but the 
response to GnRH is blunted in severe renal impairm

esponse to GnRH is blunted in severe renal impairment. TeBG 
is elevated in chronic renal failure, and the level of free testoster­
one is low. Survivors of renal transplantation who are undergoing 
immune suppression and alternate-day steroid treatment often 
have delayed onset of puberty and decreased pulsatility of GH 
and gonadotropins at night.
Patients with nephrotic syndrome have poor pubertal growth, 
poor secondary sexual development, and deficient gonadotro­
pin secretion in a pattern 

d deficient gonadotro­
pin secretion in a pattern resembling CDP. Glomerulonephritis 
treated with alternate-day glucocorticoid therapy leads to a late, 
diminished, but prolonged pubertal growth spurt that can result 
in a normal final height.
Children with early onset of leukemia and early long-term 
remission experience puberty at an appropriate age or with only 
a slight delay, whereas patients with initial symptoms of leuke­
mia in late childhood may have considerable delay of puber­
tal de

dhood may have considerable delay of puber­
tal development. Radiation treatment to the CNS may cause 
hypogonadotropic hypogonadism or GH deficiency or both, 
and irradiation of the abdomen or pelvis and certain types 
of chemotherapies, especially if administered during puberty, 
may impair gonadal function and cause primary hypogonad­
ism, although ovarian function may return even in the face of 
elevated serum gonadotropin levels.568 Total-body irradiation 
for bone marrow transplantation ex

dy irradiation 
for bone marrow transplantation exerts the most significant 
effects, such as severe GH deficiency in 50%, hypothyroid­
ism in 56%, and hypogonadism in 83% of males, and 100% 
of women had ovarian failure; insulin resistance was found 
in 83% and dyslipidemia in 61%.576 Children with leukemia 
treated with CNS irradiation had a diminished pubertal growth 
spurt and diminished final height. Long-term follow-up studies 
1098
demonstrate

 VI    Endocrinology and the Life Span
demonstrate the rising incidence of the metabolic syndrome 
survivors and high lifetime risk for cardiovascular disease in 
childhood cancer.577 
Other Endocrine Conditions and Puberty. Hypothyroidism 
may delay the onset of puberty or menarche (except in extreme 
cases in which puberty starts early); treatment with levothyroxine 
reverses this pattern, but there is likely to be a permanent loss of 
height if the diagnosis is delayed.
Diabetes mellitus type

f the diagnosis is delayed.
Diabetes mellitus type 1 is associated with delayed men­
arche.578 Remarkably, this may occur no matter the degree of gly­
cemic control. Prepubertal children are most vulnerable to poor 
glycemic control, and pubertal subjects exhibit normal growth 
unless severe hyperglycemia occurs. The degree of control neces­
sary to avoid these complications cannot be exactly quantified, 
but adolescents with even moderately poor control frequently 
manifest some degree of growt

 control frequently 
manifest some degree of growth impairment and delayed puberty 
or irregular menses. Mauriac syndrome is characterized by poorly 
controlled diabetes, hepatomegaly with fatty infiltration of the 
liver, and delayed puberty that is likely related to poor nutritional 
status.579
Cushing disease can be associated with delayed onset or arrest 
of gonadarche, although excessive virilization is an alternative 
finding. 
Anorexia Nervosa and Variants.580,581,990
Anorexia Nervosa. An

vosa and Variants.580,581,990
Anorexia Nervosa. Anorexia nervosa,582 a common cause of 
gonadotropin deficiency in adolescence, is a functional disorder. 
Prevalence is increased among girls (it is the third most com­
mon chronic disease of adolescent girls), but it is rarer in boys. 
This condition has the highest mortality rate of all psychiatric 
disorders583; weighted mortality rates (i.e., deaths per 1000 per­
son-years) were 5.1 for anorexia nervosa, and 1.7 standardized 
mortality ratios 

a nervosa, and 1.7 standardized 
mortality ratios were 5.86 for anorexia nervosa. It is character­
ized by a distorted body image, obsessive fear of obesity, and 
food avoidance that can cause severe self-induced weight loss 
(to less than 85% of normal weight for age and height or a BMI 
<17.5 kg/m2 after cessation of growth), primary or secondary 
amenorrhea in affected females, widespread endocrine disor­
ders, and even death. Specific diagnostic details are provided 
in the Diagnostic and St

tic details are provided 
in the Diagnostic and Statistical Manual of Mental Disorders, 
Fourth Edition (DSM-IV) criteria of the American Psychiatric 
Association581:
	A.	 Restriction of energy intake relative to requirements, leading 
to a significantly low body weight in the context of age, sex, 
developmental trajectory, and physical health. Significantly 
low weight is defined as a weight that is less than minimally 
normal or, for children and adolescents, less than that mini­
mally expect

and adolescents, less than that mini­
mally expected.
	B.	 Intense fear of gaining weight or of becoming fat, or persistent 
behavior that interferes with weight gain, even though at a 
significantly low weight.
	C.	 Disturbance in the way in which one’s body weight or shape is 
experienced, undue influence of body weight or shape on self-
evaluation, or persistent lack of recognition of the seriousness 
of the current low body weight.
There are subtypes:
   
Restricting type and binge-eating/

e subtypes:
   
Restricting type and binge-eating/purging type
   
The onset of amenorrhea may precede the onset of severe weight 
loss. Other common features include onset in middle adolescence, 
hyperactivity, defective thermoregulation with hypothermia and 
sensitivity to cold, constipation, bradycardia and hypotension, 
decreased BMR, dry skin, fine or downy hypertrichosis, periph­
eral edema, and parotid enlargement. The pathogenesis is mul­
tifactorial and includes a genetic factor and a w

tifactorial and includes a genetic factor and a well-characterized 
psychologic component. Before the diagnosis of anorexia nervosa 
is made, organic disease must be excluded; for example, a girl 
with macroprolactinoma may present with signs consistent with 
anorexia nervosa. The prevalence of anorexia nervosa is increased 
among individuals with Turner syndrome.
Anorexia nervosa has considerable endocrine ramifications.584 
The concentrations of plasma FSH, LH, leptin, and estradiol 
and the 

of plasma FSH, LH, leptin, and estradiol 
and the excretion of urinary gonadotropins are characteristically 
low. There may be a reversion to a circadian rhythm of LH secre­
tion and to the sleep-associated increase in episodic LH secre­
tion or LH response to GnRH characteristic of early puberty, 
or the amplitude of the pulsatile episodes may be diminished, 
as in the pattern of prepubertal children, if onset occurs during 
puberty. Pulsatile administration of intravenous GnRH at inter­
vals o

dministration of intravenous GnRH at inter­
vals of 90 to 120 minutes can produce LH pulses that are indis­
tinguishable from the normal pubertal pattern, demonstrating 
functional GnRH deficiency. Serum leptin levels are low, con­
sistent with the strikingly decreased mass of adipose tissue, and 
increase with regain of weight. Other hormonal changes include 
increased mean concentrations of plasma GH and plasma cor­
tisol; low levels of plasma IGF1, DHEAS, and triiodothyronine 
(T3) with norma

IGF1, DHEAS, and triiodothyronine 
(T3) with normal levels of thyroxine (T4) (unless the low thy­
roxine syndrome is present) and TSH; a decreased rise in serum 
prolactin after administration of thyrotropin-releasing hormone 
(TRH) or insulin-induced hypoglycemia; and a diminished 
capacity to concentrate urine. This condition must be consid­
ered in the differential diagnosis of growth failure in younger 
subjects.
Lower heart rates are characteristic and a dangerous sign of 
severe disease an

eristic and a dangerous sign of 
severe disease and not to be confused with an athletic bradycardia 
in view of the excessive exercise the patient engages in. A lower 
systolic blood pressure, lower body temperature, anemia, and leu­
kopenia are found in persons with anorexia nervosa. The ratio of 
bone age to chronologic age is significantly lower in girls with 
anorexia nervosa and correlates positively with duration of illness 
and markers of nutritional status. All measures of BMD are lower,

nutritional status. All measures of BMD are lower, 
and the most significant predictors of bone density are LBM, 
BMI, and age at menarche. Treatment of decreased bone density 
in these individuals is accomplished with improved nutrition, and 
a degree of catch-up in bone density occurs, although reversion to 
normal may not occur.
Normal endocrine and metabolic function may follow weight 
gain, but amenorrhea may persist for months, suggesting persis­
tent hypothalamic dysfunction. In view of t

ersis­
tent hypothalamic dysfunction. In view of the associated mor­
tality rate, parenteral alimentation may be indicated in resistant 
patients with severe weight loss, especially in those with infection 
or an electrolyte imbalance. However, refeeding syndrome with 
attendant hypophosphatemia may result if not handled appro­
priately. Treatment of this disorder requires skillful management, 
understanding, patience, and psychiatric consultation in a team 
approach. Unfortunately, an evidence-

n in a team 
approach. Unfortunately, an evidence-based approach to optimal 
treatment is not plentiful, and the difficulty in obtaining therapy 
in uninsured individuals remains.585
Functional hypothalamic amenorrhea is defined as the absence 
of menses, low or normal gonadotropin levels, and normal 
gonadotropin response to GnRH stimulation but lack of or an 
inadequate midcycle LH surge and a decrease in normal pulsatile 
secretion (amplitude or frequency, or both) of gonadotropins and 
hypoe

or frequency, or both) of gonadotropins and 
hypoestrogenemia without organic abnormality.523 This condition 
1099
can occur in women with normal weight but decreased percentage 
of body fat. These patients have higher than average cortisol val­
ues; decreased levels of free T4, free T3, and total T4 with normal 
TSH levels; and decreased leptin concentrations that are probably 
caused by subtle dysfunction of eating patterns and altered energy 
ex

function of eating patterns and altered energy 
expenditure. The consequences range from severe estrogen defi­
ciency to anovulation to a short luteal phase. Reduced bone den­
sity is a concern. Genes involved in the cause of hypogonadotropic 
hypogonadism such as GNRHR, KAL1, and PROKR2 are found 
in heterozygous patterns in women with functional hypothalamic 
amenorrhea, suggesting a genetic susceptibility to the condition 
that is brought out by various types of stresses523; thus functional 

by various types of stresses523; thus functional 
amenorrhea may not be simply functional.586,587 
Bulimia Nervosa. Bulimia nervosa is now separated from the 
diagnosis of anorexia nervosa582; DSM-IV diagnostic criteria are 
	A.	 Recurrent episodes of binge eating. An episode of binge eating 
is characterized by both of the following:
	
1.	 Eating, in a discrete period of time (e.g., within any 2-hour 
period), an amount of food that is definitely larger than 
what most individuals

s are useful clinical markers. Abuse of 
laxatives, diet pills, and diuretics is frequent. Although weight 
loss is not frequent, amenorrhea is common. Bulimia is especially 
prevalent in female high school and college students. A history 
of childhood sexual abuse is more common than in unaffected 
adolescents. Cessation of growth can occur in infants and young 
children with psychosocial dwarfism. Stressful social situations 
can also inhibit growth and physical pubertal development at 
adoles

rowth and physical pubertal development at 
adolescence. 
Exercise, Hypo-ovarianism, and Amenorrhea: The Female Ath­
lete Triad. In 1992, the American College of Sports Medicine 
defined the female athletic triad as primary or secondary amen­
orrhea, disordered eating, and osteoporosis.582 Although there 
are substantial endocrine effects of excessive athletic training in 
girls, elite prepubertal and pubertal female athletes suffer relatively 
few physical injuries. Because there is no demonstr

ew physical injuries. Because there is no demonstrable effect on 
pubertal development from moderate exercise in subelite female 
runners, moderate exercise should not be discouraged during ado­
lescence. However, extensive training (10–12 hours/week) may be 
excessive for prepubertal girls.
Bulimia, anorexia nervosa, or anorexia athletica is most often 
found in girls engaged in sports that emphasize weight.587 Teenage 
ballet dancers are lighter, have less body fat, and have a high inci­
dence

r, have less body fat, and have a high inci­
dence of delayed puberty and of primary and secondary amenor­
rhea than less physically active girls. Factors other than decreased 
body weight can impair pubertal progression and delay menarche 
through inhibition of the hypothalamic GnRH pulse generator in 
healthy ballet dancers and female athletes such as swimmers who 
are not underweight.
Athletes who began strenuous training before menarche have 
a delay in menarcheal age.588 However, genetic in

a delay in menarcheal age.588 However, genetic influences overlie 
changes due to weight or activity because there is a positive cor­
relation between the delayed menarche found in athletic girls 
and the age of menarche of their mothers. Although some stud­
ies of artistic and rhythmic gymnasts found delayed menarche 
when compared with their mothers and sisters, with a more sig­
nificant delay in artistic gymnasts,589–591 a recent survey of the 
literature by a committee of the Scientific Com

e 
literature by a committee of the Scientific Commission of the 
International Gymnastics Federation found the following: “(1) 
Adult height or near adult height of female and male artistic 
gymnasts is not compromised by intensive gymnastics training. 
(2) Gymnastics training does not appear to attenuate growth 
of upper (sitting height) or lower (legs) body segment lengths. 
(3) Gymnastics training does not appear to attenuate pubertal 
growth and maturation, neither rate of growth nor the ti

 and maturation, neither rate of growth nor the timing 
and tempo of the growth spurt. (4) Available data are inade­
quate to address the issue of intensive gymnastics training and 
alterations within the endocrine system.”592 The implication is 
that these athletes may be shorter and delayed in puberty due to 
self-selection. There are studies with findings at odds with these 
conclusions.
Higher bone density is reported in the femurs of gymnasts 
compared with those of ballet dancers and contr

s 
compared with those of ballet dancers and control subjects, but 
lower radial bone density in the gymnasts and ballet dancers 
reflects the effect of the application of force on bone remodeling; a 
positive relationship between serum leptin and tibial bone density 
is found.593 The bone density of female long-distance runners is 
greater in those with regular menses than in those with menstrual 
irregularities. Osteopenia in later life may result from amenorrhea 
in ballet dancers, even with 

ult from amenorrhea 
in ballet dancers, even with estrogen replacement, and nutrition 
therapy is considered important to improve outcome.594
Thinness and strenuous physical activity appear to act syner­
gistically, but strenuous exercise training by itself may inhibit the 
GnRH pulse generator, mediated in part by endogenous opioider­
gic pathways involving β-endorphin. When the strenuous physi­
cal activity is interrupted (e.g., by injury), puberty advances, and 
menarche often occurs within a

erty advances, and 
menarche often occurs within a few months in those with amen­
orrhea, in some cases before a significant change in body composi­
tion or weight. Even though gonadarche is retarded, adrenarche 
is not delayed.
Female athletes of normal weight who have less fat and more 
muscle than nonathletic girls (e.g., ice skaters, swimmers) are also 
at risk for delayed puberty and for primary and secondary amen­
orrhea. However, the mechanism apparently is different from the 
hypothalami

nism apparently is different from the 
hypothalamic amenorrhea in runners and ballet dancers. In swim­
mers, menstrual cycles frequently are irregular and anovulatory 
rather than absent, and the plasma concentrations of DHEAS 
and LH were higher than normal, but plasma estrogen levels were 
normal.
Prospective study of gymnasts contrasted with swimmers 
demonstrated decreased growth velocity, stunting in leg length 
growth, and in some studies decreased height prediction in the 
gymnasts.595
Pr

ecreased height prediction in the 
Prolactin levels may be elevated in women athletes and may 
contribute to the delayed menarche found in this group. Osteo­
penia can result from the associated chronic hypoestrogenism.588
Scoliosis in girls usually develops during the pubertal growth 
spurt and more often occurs in girls with a more rapid pubertal 
growth spurt. Ballet dancers have a higher incidence of scoliosis 
1100
than the general 

than the general population and often have delayed puberty and 
delayed menarche. These girls have decreased leptin levels, but 
soluble leptin receptor and adiponectin levels were increased 
throughout puberty; these changes were related to changes in 
LBM rather than BMI. Idiopathic scoliosis in the general popula­
tion is associated with a statistically earlier age of menarche and 
an early adolescent growth spurt. The strongest association with 
scoliosis is 

urt. The strongest association with 
scoliosis is taller stature at the time of the pubertal growth spurt. 
Adult height in familial constellations of scoliosis does not vary 
from the family norm.
Although men are less affected than women, men may also be 
affected by rigorous physical training. They may have decreased 
LH response to GnRH and decreased spontaneous LH pulse fre­
quency and amplitude; the serum testosterone level is normal or 
low with extreme activity levels. 
Other Causes of D

w with extreme activity levels. 
Other Causes of Delayed Puberty
Marijuana use has been associated with gynecomastia and is a 
putative cause of pubertal delay. Untreated Gaucher disease causes 
delay in pubertal development, but early treatment with enzyme 
replacement allows puberty to begin on time.596 Girls with famil­
ial dysautonomia have delayed menarche and often have a severe 
premenstrual syndrome. The condition is ultimately compat­
ible with pregnancy. Chronic infections may delay th

le with pregnancy. Chronic infections may delay the onset of 
puberty. 
Central Nervous System Tumors
Extrasellar masses may interfere with GnRH synthesis, secre­
tion, or stimulation of pituitary gonadotrophs. Most patients 
with hypothalamic-pituitary tumors causing gonadotropin defi­
ciency have one or more additional pituitary hormone deficien­
cies (or an increased serum prolactin level with prolactinomas). 
Those with GH deficiency due to a neoplasm have late onset of 
growth failure compa

 neoplasm have late onset of 
growth failure compared with those who have idiopathic and 
familial hypopituitarism, in which growth failure starts early in 
life. The presence of anterior and posterior pituitary deficien­
cies in infancy suggests a midline developmental defect, but the 
development of this combination after infancy ominously sug­
gests an expanding CNS lesion.
Craniopharyngioma. Craniopharyngioma is a rare embryonic 
malformation of nonglial origin in childhood (0.5–2 new cases 

 of nonglial origin in childhood (0.5–2 new cases 
per 1 million population annually, or 1.2–4% of pediatric intra­
cranial tumors), a common CNS neoplasm.597 It is, however, the 
most common brain tumor associated with hypothalamic-pitu­
itary dysfunction and sexual infantilism and accounts for 80% 
to 90% of neoplasms found in the pituitary and up to 15% of 
all intracranial tumors in childhood.598 Symptoms usually arise 
before the age of 20 years with a peak incidence between the 
ages of 6 

ears with a peak incidence between the 
ages of 6 and 14 years with about 30% to 50% occurring in 
the pediatric age range. Harvey Cushing introduced the term 
craniopharyngioma and considered them “the most formidable 
of intracranial tumors.”599
Various theories of the embryologic origin of this nonglial 
intracranial tumor are current: One theory favors development 
from ectodermal remnants of Rathke pouch and another develop­
ment from residual embryonic epithelium of the anterior pituitary 

al embryonic epithelium of the anterior pituitary 
gland and of the anterior infundibulum. Craniopharyngiomas 
may reside within or above the sella turcica, or more rarely, they 
may be found in the nasopharynx or the third ventricle.
Craniopharyngioma appears to be a monoclonal tumor, and 
about 50% have cytogenetic abnormalities such as gains in 1q, 
12q, and 17q. About 70% of cases of craniopharyngioma in 
childhood are the adamantinomatous type with cyst formation. 
These types have dysregul

pe with cyst formation. 
These types have dysregulation of the Wnt signaling pathway and 
a mutation in the β-catenin gene (CTNNB1) in contrast to the 
papillary type of craniopharyngioma that has BRAF mutations 
and is more often found in adult patients.600,601
CNS signs of craniopharyngiomas develop as the tumor 
encroaches on surrounding structures. Symptoms of craniopha­
ryngioma include headache, visual disturbances, short stature, 
diabetes insipidus, vomiting, and weakness of one or more 

 insipidus, vomiting, and weakness of one or more limbs. 
Visual defects (including bilateral temporal field deficits), optic 
atrophy or papilledema, and signs of GH deficiency, delayed 
puberty, and hypothyroidism are features of craniopharyngio­
mas. Although most patients are below the mean in height and 
height velocity at diagnosis, a long, indolent course is possible. 
Deficiencies of gonadotropins, GH, thyrotropin (TSH, thyroid-
stimulating hormone), ACTH, and arginine vasopressin are 
c

ng hormone), ACTH, and arginine vasopressin are 
common. The serum concentration of prolactin is normal or 
increased. Delayed bone age is common and may point to the 
onset of tumor growth.
About 70% of patients with a craniopharyngioma have 
suprasellar or intrasellar calcification (found in fewer than 1% 
of normal individuals) and an abnormal sella turcica, which are 
sometimes found on radiographs taken for other indications, 
including orthodontia. CT (but not MRI) reveals fine calcifi­
ca

odontia. CT (but not MRI) reveals fine calcifi­
cations that are not apparent on lateral skull radiographs. MRI 
scans before and after gadolinium are the diagnostic procedures 
of choice for suspected craniopharyngioma and can determine 
whether the tumor is cystic or solid and indicate the presence 
of hydrocephalus; if necessary a CT scan can be used to search 
for calcifications, and CT or MRI with contrast can determine 
whether the tumor is cystic or solid and indicate the presence of 
hyd

 cystic or solid and indicate the presence of 
hydrocephalus (Fig. 26.45).
Smaller craniopharyngiomas, usually intrasellar, can be 
treated by transsphenoidal microsurgery, but larger or suprasel­
lar masses usually require craniotomy, and the approach must be 
individualized. The reported postsurgical 5-year overall survival 
rate is 88% to 94%, and the reported 10-year overall survival 
rate is 70% to 92% with a 20-year survival rate of 76%. The 
combination of limited tumor removal and radiat

e 
combination of limited tumor removal and radiation therapy 
leads to a satisfactory neurologic prognosis, better cognitive out­
come, and better endocrine outcome compared with attempts at 
complete surgical extirpation. Frequent and early tumor relapse 
after apparently complete resection and tumor progression after 
incomplete resection suggest the wisdom of radiation therapy 
after surgery. Alternative approaches include proton beam 
therapy; in mainly cystic craniopharyngioma cases, insti

y; in mainly cystic craniopharyngioma cases, instillation 
of radioisotopes or sclerosing substances such as bleomycin or 
interferon-α are being investigated.602 Nonetheless, the pre­
ferred manner of treatment to retain the best quality of life is 
not yet established, but longitudinal studies such as the random­
ized multinational trial KRANIOPHARYNGEOM 2007 may 
answer this question.602
Postoperative hyperphagia and obesity (BMI >5 SD above nor­
mal) can be striking and correlate with the ma

or­
mal) can be striking and correlate with the magnitude of hypo­
thalamic damage on cranial MRI. Injury to the hypothalamic 
ventromedial nuclei (associated with increased parasympathetic 
activity and hyperinsulinemia) or to the PVN may cause these 
findings, and insulin suppression may be helpful. Short-term 
follow-up studies demonstrate the efficacy of bariatric surgery in 
the management of obesity in affected patients.601 Hypothalamic 
1101

01
sparing surgery decreases the risk of postoperative hyperphagia 
and obesity.602 Aberrant sleep patterns and even narcolepsy and 
daytime somnolence may follow surgical treatment of craniopha­
ryngiomas, with melatonin improving sleep patterns in some.603 
Although the endocrine complications are more manageable, the 
combination of antidiuretic hormone insufficiency (i.e., diabetes 
insipidus) and impaired sense of thirst remains a complex man­

nd impaired sense of thirst remains a complex man­
agement problem.
A Rathke cleft cyst is often discovered as an incidental finding 
on MRI, but it can produce symptoms and signs indistinguishable 
from those of a craniopharyngioma, such as precocious or delayed 
puberty.604 Surgical drainage and excision of the cyst wall are cus­
tomary approaches. 
Other Extrasellar Tumors
Germinomas. Germinomas (i.e., pinealomas, ectopic pine­
alomas, atypical teratomas, or dysgerminomas) and other germ 
cel

l teratomas, or dysgerminomas) and other germ 
cell tumors of the CNS are the most common extrasellar tumors 
that arise in the suprasellar hypothalamic region and in the pineal 
region that commonly cause sexual infantilism. Germinomas 
constitute 66% of all intracranial germ cell tumors (GCTs), 
which make up 3% to 11% of pediatric brain tumors. About 
84% are found in the pineal and the neurohypophyseal regions. 
Peak incidences occur in the second decade and during infancy. 
They are found m

econd decade and during infancy. 
They are found more often in males.605 Polydipsia and polyuria 
are the most common symptoms, followed by visual difficulties 
and abnormalities of growth and puberty or movement disorders. 
Diagnosis may be delayed for months to years because the find­
ings are attributed to psychiatric disorders. Deficiencies of vaso­
pressin and GH are most common, but other anterior pituitary 
hormone deficiencies (including gonadotropin deficiency) and 
elevated serum prola

gonadotropin deficiency) and 
elevated serum prolactin levels are also frequent. Determination 
of the concentration of hCG in spinal fluid and in serum and 
assessment of α-fetoprotein levels provide useful tumor mark­
ers in children and adolescents with germ cell tumors. Germ 
cell tumors in boys cause isosexual GnRH-independent sexual 
precocity (GISP) by secretion of hCG (see “Sexual Precocity”). 
Tumors secreting hCG cause precocious puberty in boys, and 
there has been one case report of 

y in boys, and 
there has been one case report of an affected girl.
Subependymal spread of germ cell tumors along the lining of 
the third ventricle is common, and seeding may involve the lower 
spinal cord and corda equina. MRI with contrast enhancement 
is useful in the detection of isolated enlargement of the pituitary 
stalk, an early finding that requires periodic MRI monitoring, 
especially in patients with diabetes insipidus.481 The size of the 
pituitary gland increases by 100% between y

f the 
pituitary gland increases by 100% between year 1 and year 15, 
but the pineal gland does not normally change in size after the first 
year of life; any later enlargement indicates a mass lesion. Pineal 
cysts are a rare cause of CPP.
Irradiation is the preferred treatment for pure germ cell tumors 
such as germinomas; surgery is rarely indicated, except for biopsy 
to establish a tissue diagnosis.481 However, attempts to decrease 
the long-term morbidity of radiation therapy lead to consi

-term morbidity of radiation therapy lead to consider­
ation of chemotherapy. Chemotherapy alone is inadequate, but 
the combination of chemotherapy and radiation therapy can be 
successful,485,606 and both treatment methods are recommended 
for a mixed germ cell tumor. Because testicular germ cell tumors 
are occasionally found years after successful therapy for CNS germ 
cell tumors, long-term surveillance is indicated.557,607
Hypothalamic and optic gliomas or astrocytomas, occurring 
as part 

optic gliomas or astrocytomas, occurring 
as part of neurofibromatosis (von Recklinghausen disease) or aris­
ing independently, can also cause sexual infantilism. Gliomas and 
meningiomas are the most common CNS tumors to develop in 
childhood cancer survivors treated with CNS radiation, often the 
young adult or even late teenage years.608 
A
B
• Fig. 26.45  Craniopharyngioma in a short 5-year-old girl with a history of frontal headaches, impaired vision, and poor growth. (A) Midline, sagittal,

ed vision, and poor growth. (A) Midline, sagittal, 
T1-weighted image shows a hyperintense region superiorly and an inferior hypointense region. The combination of hyperintense and hypointense areas 
in a non–­contrast-enhanced examination is the most characteristic finding for craniopharyngioma. Notice the erosion of dorsum sellae (solid arrow) and 
posterior pituitary bright spot. (B) Coronal, T1-weighted image shows tumor extending upward to the inferior frontal horns, narrowing the foramen o

he inferior frontal horns, narrowing the foramen of 
Monro, and causing mild hydrocephalus. The open arrows indicate the upper border of the hyperintense area of the tumor.
1102
Pituitary Adenomas. Only 2% to 6% of all surgically treated 
pituitary tumors occur in childhood and adolescence, with about 
1 in 1 million children affected.598 Most functional pituitary ade­
nomas are ACTH secreting, with prolactinomas or GH secret­
ing or nonfunctioning a

rolactinomas or GH secret­
ing or nonfunctioning adenomas occurring less commonly. Most 
pituitary tumors are monoclonal lesions caused by mutations of 
GNAS. Adolescent onset of pituitary team tumors may be the first 
manifestation of multiple endocrine neoplasia type I or famil­
ial isolated pituitary adenoma (FIPA).609 With higher sensitive 
imaging techniques the presence of a pituitary incidentaloma, a 
previously unsuspected pituitary lesion that is discovered on an 
imaging study performe

n that is discovered on an 
imaging study performed for an unrelated reason, may be discov­
ered.610 Evaluation of secretory activity of such a lesion, consid­
eration of mass effects, and follow-up to monitor a change in size 
are important, but some lesions detected will not be related to the 
pubertal abnormality.
Incidence of prolactinoma is low in childhood but one in 
five present in the age group 15 to 24 years.611 A survey of 44 
cases reported that 61% of prolactinomas were macroadenoma

ported that 61% of prolactinomas were macroadenomas 
(more often in boys; hypopituitarism and growth failure were 
common) and 39% were microadenomas (more often in girls; 
delayed puberty was common).612 Only 2 of these 29 patients 
had delayed onset of puberty, although primary amenorrhea was 
the presenting symptom in 13 of 20 pubertal females. Present­
ing symptoms included oligomenorrhea and galactorrhea in the 
girls and headache in the boys. Galactorrhea may be demonstra­
ble only by manu

s. Galactorrhea may be demonstra­
ble only by manual manipulation of the nipples (blood samples 
for prolactin should be obtained before examination or many 
hours later, because manipulation of the nipples raises prolactin 
levels).
Dopaminergic therapy is often successful in decreasing pro­
lactin values. The dopamine agonist bromocriptine may decrease 
serum prolactin concentrations and decrease tumor size, which 
is a useful approach before surgery of large macroprolactinomas 
is undertaken 

urgery of large macroprolactinomas 
is undertaken and when resection of the adenoma is incomplete. 
Transsphenoidal resection of microprolactinomas in children 
and adolescents is an effective treatment. Pubertal progression 
and normal menstrual function in girls usually follow reduction 
of serum prolactin levels. Pituitary apoplexy followed cabergoline 
treatment of a macroprolactinoma in a 16-year-old girl613; this 
complication has been seen in adults treated with bromocrip­
tine, and tricu

 in adults treated with bromocrip­
tine, and tricuspid regurgitation may be a cumulative effect.614 
High serum levels of macroprolactin, a complex of immuno­
globulin G and monomeric prolactin with little biologic activ­
ity in vivo, cross-react in commercial prolactin assays, leading 
to a finding of pseudohyperprolactinemia; high prolactin values 
should be rechecked with subfractionation after polyethylene 
glycol precipitation.615 
Other Central Nervous System Disorders Leading to Delayed 

tral Nervous System Disorders Leading to Delayed 
Puberty
Langerhans Cell Histiocytosis. Langerhans cell histiocytosis 
(i.e., Hand-Schüller-Christian disease or histiocytosis X) is a 
clonal proliferative disorder of Langerhans histiocytes or their 
precursors. It is characterized by the infiltration of lipid-laden 
histiocytic cells or foam cells in skin, viscera, and bone.616 The 
cause is not clear because there are features of a neoplasm and 
features of a reactive immunologic disorder. Dia

features of a reactive immunologic disorder. Diabetes insipi­
dus, caused by infiltration of the hypothalamus or the pituitary 
stalk, is the most common endocrine manifestation, with GH 
deficiency and delayed puberty possible. The lung, liver, and 
spleen, cystlike areas in flat and long bones, and the dorsolum­
bar spine may be involved. “Floating teeth” within rarefied bone 
of the mandible, absent or loose teeth, and exophthalmos due 
to infiltration of the orbit are seen. Mastoid or temp

nfiltration of the orbit are seen. Mastoid or temporal bone 
involvement may lead to chronic otitis media. Treatment with 
glucocorticoids, antineoplastic agents, and radiation therapy is 
promising in terms of survival, but more than 50% of patients 
have late sequelae or disease progression. The natural waxing and 
waning course of this rare disease makes evaluation of therapy 
difficult and highlights the importance of national treatment 
protocols.616 
Postinfectious Inflammatory Lesions of 

ocols.616 
Postinfectious Inflammatory Lesions of the Central Nervous 
System, Vascular Abnormalities, and Head Trauma. Tuberculous 
or sarcoid granulomas of the CNS are associated with delayed 
puberty. Hydrocephalus may cause delayed puberty that can be 
reversed with decompression, as may pressure from a subarach­
noid cyst. 
Irradiation of the Central Nervous System. Irradiation of the 
CNS for treatment of tumors, leukemia, or neoplasms of the head 
and face may result in the gradual onset 

he head 
and face may result in the gradual onset of hypothalamic-pituitary 
failure.617 Although GH deficiency is the most common hormone 
disorder resulting from irradiation, gonadotropin deficiency, 
hypothyroidism, and decreased bone density also occur.618,619 
Exposure of the hypothalamic pituitary axis to 30 Gy carries a 
recurrence rate of about 6 for premature ovarian failure in women 
and requirement for testosterone treatment in men.620
Self-reported fertility was reported to be lower 

Self-reported fertility was reported to be lower in women who 
received CNS radiotherapy for acute lymphoblastic leukemia 
at about the time of menarche,621 although the average age of 
women in this long-term study was in the early 20s, and longer 
follow-up of fertility may change the results. Irradiation of the 
CNS in early life predisposes the patient to later onset of second­
ary CNS tumors sometimes in just a few years after treatment of 
the first tumor.608
Fröhlich Syndrome. Fröhlich s

Fröhlich Syndrome. Fröhlich syndrome or adiposodysgenesis is 
a constellation of endocrine abnormalities, combining findings of 
obesity and hypogonadism due to a hypothalamic-pituitary disor­
der. Remarkably, the original description involved a patient with 
tuberculosis involving the hypothalamic-pituitary axis. 
Hypergonadotropic Hypogonadism: Sexual Infantilism 
Caused by Primary Gonadal Disorders
The most common forms of primary gonadal failure are associ­
ated with sex

 primary gonadal failure are associ­
ated with sex chromosome abnormalities and characteristic physi­
cal findings.590,622 Testicular or ovarian dysfunction as an isolated 
finding is less commonly a cause of pubertal hypergonadotropic 
hypogonadism.
Boys
Klinefelter Syndrome and Its Variants (see Chapter 24). Kline­
felter syndrome (i.e., syndrome of seminiferous tubular dysgen­
esis) and its variants occur in approximately 1 in 1000 males, and 
they are the most common forms of male hypogonadi

they are the most common forms of male hypogonadism.622–624 
The invariable clinical features include small, firm testes as an adult 
(<3.5 cm long); impaired spermatogenesis; and a male phenotype, 
usually with gynecomastia and long legs but not long arms622 
(Fig. 26.46). Prepubertally, patients can be detected by the dis­
proportionate length of the extremities, decreased U/L body ratio 
without an increase in arm span rather than eunuchoid propor­
tions, in which arm span and leg length are

ropor­
tions, in which arm span and leg length are increased; however, 
less than 10% are diagnosed prepubertally. With increasing use 
1103
of karyotype determination in children with behavior problems 
and in pregnancy for various reasons, diagnosis earlier in life may 
increase. Tall stature for family size is common in this disorder 
due to the disproportionate growth of the legs and appears to be 
related to multiple copies of the SHOX gene624

be 
related to multiple copies of the SHOX gene624; however, normal 
or short stature does not eliminate the diagnosis.
Infants with Klinefelter syndrome have normal INSL3 levels 
transiently increased at 2 to 3 months of age during the mini­
puberty of infancy, testosterone levels were below the median 
of control subjects but within the normal range, and inhibin 
B and AMH levels were also within normal range.624 However, 
FSH levels were above normal in 25% of patients, indicating 
early evid

 normal in 25% of patients, indicating 
early evidence of testicular deficiency. Muscle tone is also lower 
in some.625
There is a normal increase in the concentrations of testosterone, 
INSL3, and inhibin B before puberty. By midpuberty testosterone 
and INSL3 concentrations remain in the low-normal range, AMH 
levels were undetectable in XXY adolescents but decreased at an 
age later than in normal boys, and inhibin B levels decreased from 
normal to the low levels characteristic of adult Klin

mal to the low levels characteristic of adult Klinefelter syn­
drome during late puberty after an unequivocal increase in serum 
testosterone (>2.5 nmol/L) levels and degeneration of Sertoli 
• Fig. 26.46  47,XXY Klinefelter syndrome in 17-year-old identical twins. At age 15, gynecomastia was 
observed. The twins had a eunuchoid habitus and poorly developed male secondary sexual charac­
teristics. Both were 187 cm tall; arm spans were 187 cm and 189.5 cm; the voices were high pitched; 
the teste

189.5 cm; the voices were high pitched; 
the testes measured 1.8 × 1.5 cm; and penis length was 7.5 cm. Gynecomastia and signs of andro­
gen deficiency were more evident in the twin on the left. Urinary gonadotropin levels were greater 
than 50 mU/24 hours. The testes exhibited extensive tubular fibrosis, small dysgenetic tubules, and 
clumping or pseudoadenomatous formation of Leydig cells; germ cells were rare. The microscopic 
appearance was typical of seminiferous tubule dysgenesis. (Patient

ypical of seminiferous tubule dysgenesis. (Patient data from Grumbach MM, Barr 
ML. Cytologic tests of chromosome sex in relation to sexual anomalies in man. Recent Prog Horm 
Res. 1958;14:255–324.)
1104
cells.626 Serum LH and FSH usually rise by midpuberty. Rarely, 
low gonadotropin concentrations occur when hypogonadotropic 
hypogonadism is associated with 47,XXY Klinefelter syndrome or 
coexisting constitutional delay.
Prepubertal testes show only

constitutional delay.
Prepubertal testes show only subtle histologic changes, although 
the testes are small, and the germ cell content is reduced, whereas 
Sertoli cells are normal in abundance and appearance before 2 
years of age. Older prepubertal subjects have normal seminifer­
ous tubules. Hyalinization and fibrosis of the seminiferous tubules 
and pseudoadenomatous changes of the Leydig cells develop after 
puberty; adult-type spermatogonia are found in peripubertal boys, 
but older boys 

a are found in peripubertal boys, 
but older boys had no germ cells; and the testes degenerate in an 
accelerated manner at the onset of puberty.627 However, there is a 
44% success rate for sperm retrieval from testicular tissues and a 
55% success rate for use of microdissection testicular sperm aspi­
ration. Intracytoplasmic sperm injection led to the birth of 101 
children, in whom there was no apparent increase in congenital or 
genetic defects.628 Cryopreservation of testicular tissue is r

cts.628 Cryopreservation of testicular tissue is recom­
mended before degeneration progresses,629 although coverage for 
such procedure may be difficult to obtain.
There is variation in Leydig cell function among childhood and 
pubertal subjects, but the plasma concentration of testosterone 
fails to rise to normal adult levels. The onset of puberty usually is 
not delayed, but impaired Leydig cell reserve and low testosterone 
levels may lead to slow progression or arrest of pubertal changes. 

 slow progression or arrest of pubertal changes. 
Testosterone replacement should be considered when the LH level 
rises above the normal range of values but is not necessary in all 
subjects in early puberty.630 Serum estradiol-to-testosterone ratios 
and TeBG levels are higher than those in normal males, which 
indicates an increased estrogen effect and decreased testosterone 
effect that may account in part for the gynecomastia characteris­
tic of Klinefelter syndrome. Testosterone administra

c of Klinefelter syndrome. Testosterone administration does not 
appear to reduce the gynecomastia, but dihydrotestosterone may 
help. Aromatase inhibitors or estrogen receptor antagonists do not 
seem to be effective treatment for the gynecomastia of Klinefelter 
syndrome. If the gynecomastia does not regress within 2 years, 
reduction mammoplasty is required. Monitoring for breast cancer 
is important in these susceptible individuals.
The AR gene is located on the X chromosome and encodes 
a l

ne is located on the X chromosome and encodes 
a ligand-dependent transcription factor with highly polymor­
phic CAGn trinucleotide repeats in the coding sequence of the 
first exon. The length of the translated polyglutamate tract in 
the N-terminal transactivation domain of the resulting protein 
and the length of this polyglutamate tract are inversely propor­
tional to receptor transactivation activity. The shorter the repeat 
sequence within the range of normal variation, the more active 
th

the range of normal variation, the more active 
the AR, and small changes in this activity may result in more 
significant effects in Klinefelter syndrome, in which testoster­
one secretion may already be impaired. A negative correlation 
between CAGn repeat length and penile length, but not tes­
ticular size, in children has been reported.581,631 Another study 
found a positive association between CAGn repeat length and 
body height, an inverse relationship to bone density and arm 
span to body

relationship to bone density and arm 
span to body height, and the presence of longer CAGn repeats 
for gynecomastia and smaller testes632 in adults. A paternal ori­
gin of the extra X chromosome is associated with later onset of 
puberty and longer CAG repeats.627 
Behavior and Development in Klinefelter Syndrome. Neurobe­
havioral abnormalities, primarily in language, speech, learning, 
and frontal executive functions, are common, even universal, in 
Klinefelter syndrome, but severe retardatio

l, in 
Klinefelter syndrome, but severe retardation is uncommon.244 
These problems may lead to evaluation in childhood and the 
prepubertal recognition of the syndrome. The prevalence of adjust­
ment problems in adolescence is increased. Adults with Klinefelter 
syndrome have shorter education, lower income, earlier retire­
ment, and increased unemployment, and more rarely marry than 
the national average. Fatality among Klinefelter syndrome men 
was significantly increased (hazard ratio, 1.9).

was significantly increased (hazard ratio, 1.9).633 The crime rates 
for sexual abuse and arson are significantly increased, whereas traf­
fic offenses and drug-related crime are significantly decreased.
The global IQ in unselected populations of Klinefelter syn­
drome subjects is normal or near normal, but verbal IQ, in con­
trast to that of patients with Turner syndrome, is usually lower 
(e.g., 10–20 points) than performance IQ.
Prepubertal Klinefelter syndrome patients have reduced left 
h

Klinefelter syndrome patients have reduced left 
hemisphere specialization for verbal tasks and enhanced right 
hemisphere specialization for nonverbal tasks. However, these 
abnormalities tended to normalize after puberty began, suggest­
ing hemispheric reorganization during puberty. Hypotheses are 
advanced supporting the effect of prenatal testosterone on cerebral 
dominance and on language and reading pathology.
There is controversy about the indication for testosterone 
treatment of infants

 indication for testosterone 
treatment of infants or adolescents with Klinefelter syndrome. 
Although there is a growing feeling among parents that testos­
terone treatment in the infancy or early pubertal period improves 
language, reading, behavior, and self-image in boys with Klinefel­
ter syndrome, no well-controlled studies supporting this conten­
tion are available, and long-term studies are needed.634,991 
Other Aspects of Klinefelter Syndrome. Conditions associated 
with Klinefelter syn

drome. Conditions associated 
with Klinefelter syndrome include aortic valvular disease and rup­
tured berry aneurysms (6 times the normal rate); breast carcinoma 
(20 times the rate in normal men and one-fifth that of women); 
other malignancies such as acute leukemia, lymphoma, and germ 
cell tumors at any midline site; systemic lupus erythematosus; and 
osteoporosis in about 25% of affected adults. There is an increased 
risk of diabetes mellitus, thyroid disease, fatigue, varicose veins, 
an

tus, thyroid disease, fatigue, varicose veins, 
and essential tremor.
About 20% of mediastinal germ cell tumors are associated 
with Klinefelter syndrome, and they occur at a younger age than 
the mediastinal germ cell tumors that are not associated with the 
syndrome. With rare exceptions, these germ cell tumors, which 
may be located in the midline anywhere from the CNS to the 
pelvis, secrete hCG and induce sexual precocity. Klinefelter syn­
drome needs to be considered in boys with hCG-secre

rome needs to be considered in boys with hCG-secreting germ 
cell tumors, especially if the tumor is located in the mediastinum 
or CNS. 
Other Forms of Primary Testicular Failure
Cancer Survivors.  Chemotherapy  Chemotherapy and direct 
radiotherapy affect testicular function, and as more children 
survive with effective therapy for cancer, delayed puberty and 
adult infertility will result. Cancer therapy, especially irradiation 
of the gonads or the use of alkylating chemotherapeutic agents, 

or the use of alkylating chemotherapeutic agents, 
affects testicular function and can lead to adult infertility.635 
Some courses of therapy can cause severe damage to germinal 
cells without apparent effect on Leydig cells. Chemotherapy for 
childhood Hodgkin disease, including chlorambucil, vinblastine, 
Mustargen (mechlorethamine), Oncovin (vincristine), procarba­
zine, and prednisone (COPP/MOPP), may allow spontaneous 
progression through puberty, but FSH and LH concentrations 
may be eleva

berty, but FSH and LH concentrations 
may be elevated, and the inhibin B concentrations decrease dur­
ing puberty. The basal serum FSH level and rise in LH and FSH 
levels after GnRH correlate with the dose of cyclophosphamide. 
COPP/MOPP chemotherapy for Hodgkin disease can cause se­
vere damage to Sertoli and germinal cells, but it has less effect 
1105
on Leydig cells, even if therapy occurred in the prepubertal pe­
riod. Lower dosing or limitin

 the prepubertal pe­
riod. Lower dosing or limiting therapy to less than three courses 
is suggested to decrease these complications.636 Normal basal 
LH values may raise hope that Leydig cell function is normal, 
but if there is an exaggerated rise of the LH level after GnRH, 
compensated Leydig cell damage is present. The combination of 
Adriamycin (doxorubicin), bleomycin, vinblastine, and dacarba­
zine (ABVD regimen) can cause germ cell depletion. Although 
initially it was thought that some

tion. Although 
initially it was thought that some degree of prepubertal gonadal 
maturation was necessary before these drugs could cause gonadal 
damage, gonadal damage can occur earlier as a result of therapy 
in the prepubertal period but may not be demonstrable until the 
age of puberty. Chemotherapeutic agents used in the treatment 
of nephrotic syndrome or leukemia, such as cyclophosphamide 
or chlorambucil, have led to Sertoli cell, Leydig cell, and germ 
cell damage in prepubertal patien

 cell, and germ 
cell damage in prepubertal patients; these effects are sometimes 
reversible, which is indicative of gonadal damage. 
Radiation Therapy. Radiation to the gonads can cause primary 
testicular failure, usually resulting in azoospermia, although nor­
mal testosterone secretion may be associated with elevated LH and 
FSH values (compensated Leydig cell failure); the gonads must be 
shielded from the treatment, if possible. Direct radiation doses of 
more than 2 Gy will likely impair

iation doses of 
more than 2 Gy will likely impair spermatogenesis. Exposure to 
more than 20 Gy carries a recurrence rate of almost 11 of requir­
ing testosterone as an adult.620
Preservation of sperm by freezing is being investigated for pedi­
atric cancer patients (in Klinefelter syndrome patients), but the 
ethical and logistics problems of collecting sperm from adoles­
cents remain substantial, all the more in prepubertal children. Not 
all sperm may be affected by treatment, and functional

sperm may be affected by treatment, and functional sperm may 
be recovered by microdissection of seminiferous tubules even in 
individuals who appear to have testicular failure. This may allow 
fertilization by successful intracytoplasmic sperm injection even if 
the quantity of sperm is low. The genes responsible for apoptosis 
are activated when DNA damage from radiation or chemotherapy 
occurs; manipulation of such genes may allow reestablishment of 
fertility in the future. Oncofertility is 

ent of 
fertility in the future. Oncofertility is a new field of investigation 
that aims to preserve fertility in children and teenagers subject to 
cancer therapy, and new methods will undoubtedly appear.
Survey of adults who were not offered fertility preservation or 
with whom such an issue was not even discussed at time of diag­
nosis shows that they feel considerable distress. However, sperm 
banking in puberty or prepuberty presents ethical concerns as to 
the procedure, the cost of banki

l concerns as to 
the procedure, the cost of banking, and the uncertainty of out­
come and is not presently standard therapy.637
Concern for decreased bone density and risk for fractures is 
also raised in cancer survivors who have undergone radiation 
treatment of the hypothalamic-pituitary axis. 
Testicular Biosynthetic Defects. The 46,XY disorder of sex 
development is caused by 17α-hydroxylase/17,20-lyase defi­
ciency resulting from mutations in CYP17A1 at gene map locus 
10q24.3; it is asso

 in CYP17A1 at gene map locus 
10q24.3; it is associated with sexual infantilism and a female 
phenotype.638 The testosterone biosynthetic defect blocks the 
synthesis of testosterone and adrenal androgens, impairing mas­
culinization at all stages of development. Associated cortisol 
deficiency and increased mineralocorticoid secretion in this con­
dition lead to hypertension, decreased serum potassium levels, 
and metabolic alkalosis. Elevated serum progesterone levels and 
decreased plasma re

serum progesterone levels and 
decreased plasma renin activity are helpful diagnostic features.639 
Glucocorticoid replacement suppresses ACTH and mineralo­
corticoid excess and corrects the electrolyte abnormalities, but 
no sexual development occurs unless exogenous gonadal steroids 
are administered. Less severe deficiencies are associated with 
ambiguous genitalia. CYP17A1 mutations leading to isolated 
17,20-lyase deficiency are rare.
A rare autosomal recessive condition is steroidogenic ac

 autosomal recessive condition is steroidogenic acute 
regulatory (StAR) protein deficiency, in which the ability to pro­
duce C21, C19, and C18 steroids is lost and a severe impairment 
of the conversion of cholesterol to pregnenolone results; severely 
affected patients have lipid-laden adrenal glands.640 The large 
adrenal glands may be visualized on ultrasound, CT, or MRI. 
Death often occurs in infancy if untreated because of unrecog­
nized glucocorticoid and mineralocorticoid deficiencies.

glucocorticoid and mineralocorticoid deficiencies. Affected 
individuals physically appear to be sexually infantile females, 
whether their karyotype is 46,XY or 46,XX; because of the 
absence of gonadal or adrenal androgen production, the affected 
XY phenotypic females do not develop secondary sexual char­
acteristics, including pubic hair.622 However, XX females with 
a null mutation develop female sex characteristics at puberty, 
including pubic hair and multicystic ovaries, but they also ha

bic hair and multicystic ovaries, but they also have 
primary or secondary amenorrhea. In contrast to the fetal testis, 
the fetal ovary, which is insensitive to FSH and steroidogenically 
inactive, is undamaged in fetal life and remains so until the onset 
of puberty. Under FSH stimulation during puberty and with 
recruitment of ovarian follicles, the ovaries undergo progressive 
damage and cyst formation. Ovarian damage and impairment of 
ovarian StAR-independent steroidogenesis appear to be r

an StAR-independent steroidogenesis appear to be related 
to lipid deposition in the ovary.641 
Luteinizing Hormone Resistance. Presumptive evidence of LH 
resistance caused by an LH receptor abnormality on the Leydig 
cell was reported in an 18-year-old boy with a male phenotype, 
no male secondary sexual development, gynecomastia, elevated 
plasma LH levels, and early pubertal plasma testosterone concen­
trations that did not increase after hCG administration; there was 
no elevation of testos

 administration; there was 
no elevation of testosterone precursor levels.642 The testes were 
prepubertal in size and had the microscopic appearance of nor­
mal prepubertal testes. Plasma membrane receptor preparations 
from the testes bound only one-half as much radiolabeled hCG 
as control testes.
In affected males, this autosomal recessive disorder is caused by 
a mutation in LHCGR, the gene encoding the G protein–coupled, 
seven-transmembrane LH/hCG cell receptor at gene map locus 
2p21. Mu

e LH/hCG cell receptor at gene map locus 
2p21. Mutations that cause a more severe compromise in LHCGR 
function are associated with XY disorders of sex development. 
Homozygous deletion of exon 10 or the homozygous missense 
mutations Ser616Tyr and Ile625Lys of the LH receptor are associ­
ated with micropenis (but not hypospadias) due to partial impair­
ment of LH receptor function, leading to a discordance with a 
poor response to LH but not to hCG.
Nephropathic cystinosis in boys leads to hyp

 hCG.
Nephropathic cystinosis in boys leads to hypergonadotropic 
Anorchia and Cryptorchidism. Cryptorchidism is the condi­
tion in which one or both testes have not reached the bottom of 
the scrotum before birth. Animal and human data demonstrate 
that testicular descent is influenced by testosterone and INSL3; 
for example, androgen resistance leads to lack of descent, as also 
occurs in transgenetic mice without INSL3. When testes are not 
descended, they may be located in hig

tes are not 
descended, they may be located in high scrotal, suprascrotal, or 
inguinal positions or can be nonpalpable, which includes ecto­
pic testes as well.643 Data collection is variable, but analysis of 
the literature shows the undescended testis rate in term appro­
priate weight for gestational age (AGA) babies is 1.0% to 4.6%, 
whereas in premature or SGA infants it is 1.1% to 4.3%.644 By 
1 year the undescended testis rate in term AGA boys was 1.0% 
1106

boys was 1.0% 
1106
to 1.5%, at 6 years 0.0% to 2.6, at 11 years 0.0% to 6.6%, and 
at 15 years 1.6% to 2.2% of boys. Testes may ascend after birth, 
ascensus testis, which leads to a higher prevalence of undescended 
testes in prepuberty than at birth in several international reports, 
and these patients are not always included in cryptorchidism 
surveys. Study of more than 1 million Danish boys showed the 
concordance rate of cryptorchidism was 3.4

ed the 
concordance rate of cryptorchidism was 3.4% for paternal half-
brothers, 6% for maternal half-brothers, 8.8% for full brothers, 
24.1% for dizygotic twin brothers, and 27.3% for monozygotic 
twin brothers demonstrating effects of genetics and intrauterine 
environment.645
A 46,XY male without palpable testes may have intra-abdom­
inal testes, which carry an increased risk of malignant degenera­
tion; anorchia (i.e., vanishing testes syndrome, caused by perinatal 
torsion), in which no te

ome, caused by perinatal 
torsion), in which no testes are found at laparotomy; or retractile 
testes, a variation of normal. About 50% of bilateral, nonpalpable 
testes are undescended, and the other 50% are testicular remnants 
from vanishing testes that usually do not contain germ cells, are 
found in the scrotum, are not at risk for carcinoma, and need not 
be removed if the history is certain.646 If there is a male pheno­
type and male internal ducts, functioning fetal testes capable of 
se

nal ducts, functioning fetal testes capable of 
secreting testosterone and AMH were present early during fetal 
life but degenerated thereafter. Serum gonadotropins follow the 
normal U-shaped curve of high values in infancy and puberty with 
lower values in midchildhood in anorchia, although the values are 
above normal.647 Serum AMH and inhibin B are nondetectable in 
anorchia.648 Ultrasound is often used to locate undescended tes­
tes, but the accuracy is poor: A meta-analysis demonstrated th

 accuracy is poor: A meta-analysis demonstrated that a 
positive ultrasound result increases and negative ultrasound result 
decreases the probability that a nonpalpable testis is located within 
the abdomen from 55% to 64% and 49%, respectively.649 Lapa­
roscopy, however, is recommended in diagnosis of nonpalpable 
testes.650
Discovery of unilateral cryptorchidism may represent the pres­
ence of a descended testis on one side and none on the other side, 
and this presents a diagnostic dilemma. 

er side, 
and this presents a diagnostic dilemma. Compensatory hyper­
trophy occurs if there is no contralateral testis, and this aids the 
diagnosis. A Japanese study demonstrated that the mean contra­
lateral testicular length and volume in the boys with an absent 
testis were 22.4 mm and 2.20 mL compared with 16.6 mm and 
1.10 mL in boys with a testis present and 16.6 mm and 1.18 mL 
in control subjects; the optimal cutoff value of 21 mm in length 
and 1.6 mL in volume led to a predictive acc

ngth 
and 1.6 mL in volume led to a predictive accuracy for an absent 
testis of 87.3%, sensitivity of 81.8%, and specificity of 95.5%, 
with 85.5%, 84.8%, and 86.4% for the volume, respectively.651 
Because the finding does not universally predict monorchia, lapa­
roscopy can be used for diagnosis of this condition if ultrasound 
is unsuccessful. If there is a unilateral undescended testis, the 
increased relative risk of carcinoma in the contralateral descended 
testis is 1.74 (95% CI, 1.01–2.

lateral descended 
testis is 1.74 (95% CI, 1.01–2.98), which is lower than in the 
unilateral undescended testes (6.33; 95% CI, 4.30–9.31).652
Hormonal treatment of undescended testes may be successful 
in causing descent, but evidence is scarce and meta-analysis is dif­
ficult owing to variation in dosage and length of treatment. There 
is no compelling evidence that hormonal therapy in the short 
term is harmful, but unsuccessful medical therapy should not be 
allowed to significantly delay su

y should not be 
allowed to significantly delay surgical therapy. In one schema, a 
single dose of 1500 IU hCG is administered intramuscularly, and 
serum levels of LH, FSH, testosterone, inhibin B, and AMH are 
measured at baseline and at 72 hours afterward. The test was posi­
tive in Prader-Willi syndrome when the maximum testosterone 
level after 72 hours was 2 to 20 times higher than baseline levels 
in infants aged 3 to 12 months and 5 to 10 times higher, between 
2.5 and 9 nmol/L, in infan

 times higher, between 
2.5 and 9 nmol/L, in infants aged 1 to 4 years.653 Administration 
of an hCG dose, 250 IU for infants aged 3 to 12 months and 500 
IU intramuscularly for infants aged 1 to 4 years, twice a week 
for 6 weeks led to a descent of testes in 62% who had a positive 
stimulation test, and 23% of the testes reached a stable scrotal 
position. There are studies of longer term administration of hCG 
or GnRH to either foster a rise in testosterone for diagnosis or 
for descent of te

n testosterone for diagnosis or 
for descent of testes for treatment of boys without Prader-Willi 
syndrome with variable results. The lack of a rise in testosterone 
concentration, in conjunction with an increased plasma concen­
tration of FSH and LH or an augmented gonadotropin response 
to GnRH, and decreased AMH and inhibin B are evidence for 
the diagnosis of bilateral anorchia. Because testicular descent 
normally occurs by 1 year of age, orchidopexy is recommended 
between 6 and 12 months

rchidopexy is recommended 
between 6 and 12 months in those who have testes that do not 
descend with medical treatment or upon discovering cryptor­
chidism after 12 months of age.654 One study linked testicular 
descent to an adequate neonatal surge of LH and testosterone by 
4 months in AGA infants and by 6 months in premature infants; 
these study authors recommended treatment be considered earlier 
than at 1 year.
Cryptorchidism is associated with an increased risk of infertil­
ity. Cryptorc

 with an increased risk of infertil­
ity. Cryptorchid testes may demonstrate congenital abnormalities 
and may not function normally even if brought into the scrotum 
early in life. Two critical steps in the maturation of germ cells 
are described in the normal prepubertal testis that do not occur 
in the unilaterally undescended testes. First, at 2 to 3 months of 
age, the gonocytes (primitive spermatogonia) in the fetal stem 
cell pool transform into the adult dark spermatogonia, which 
become

m into the adult dark spermatogonia, which 
become the adult stem pool (possibly related to the early infancy 
surge in LH, FSH, and testosterone). Second, at 4 to 5 years 
of age, meiosis begins, and primary spermatocytes appear. The 
contralateral descended testis is affected but less so than the unde­
scended testis. Identification of the gonocyte transformation has 
influenced recommendations on the timing of orchidopexy. Post­
pubertal orchidopexy is associated with a greater than 85% preva

dopexy is associated with a greater than 85% preva­
lence of azoospermia or oligospermia. It has been surmised that 
cryptorchid testes, even if replaced in the scrotum, may never 
have normal spermatogenic function as a consequence of an early 
abnormality in germ cell maturation, vascular damage to the tes­
ticular circulation during orchidopexy, or an intrinsic testicular 
defect. Fertility potential varies depending upon preoperative 
history and laboratory results. Testicular dysgenesis may

 and laboratory results. Testicular dysgenesis may be indi­
cated by increased gonadotropin levels, and early surgery may be 
indicated; normal gonadotropin levels and a decreased germ cell 
number may indicate transient hypothalamus-pituitary-gonadal 
hypofunction with a poor fertility prognosis; if there are normal 
gonadotropins, inhibin B, and germ cell number, there is a good 
fertility prognosis, too.655 A study of men with cryptorchidism 
reported a paternity rate of 65% for men who had h

reported a paternity rate of 65% for men who had had bilateral 
cryptorchidism, compared with 90% for the formerly unilateral 
cryptorchid men and 93% for control men; the reduction in 
fertility was supported by semen and hormone analyses.656 Suc­
cessful fertilization was reported by the use of intracytoplasmic 
injection of sperm extracted from the testes of cryptorchid men 
who had orchiopexy after puberty.657 Patients undergoing orchi­
opexy may sustain subtle damage to the vas deferens, le

 may sustain subtle damage to the vas deferens, leading to 
the later production of antibodies to sperm that may result in 
1107
Cryptorchidism is associated with an increased risk of cancer 
of the testes, which is rising. The incidence of testicular carci­
noma in England at all ages increased from 2 per 100,000 in 
1909 to 4.4 per 100,000 in 1999, with a rise noticed during 
puberty.658 Such data are nation specific, and there is an

658 Such data are nation specific, and there is an approx­
imately 10-fold range between countries.646 A recent Swedish 
study reported the overall relative risk of testicular carcinoma 
in cryptorchidism is 2.5 to 8, with the highest risk associated 
with intra-abdominal rather than inguinal testes and higher risks 
observed for those with abnormal chromosomes, syndromes, or 
late or no orchiopexy. Undescended testes remain at a higher 
temperature than descended testes, and undescended testes 

ure than descended testes, and undescended testes have 
a maturation arrest at the conversion of the gonocytes to sper­
matogonia, which appears to direct the testes toward malignant 
degeneration. There is a very small risk of carcinoma of the tes­
tes in prepuberty, but the absence of carcinoma in situ in pre­
puberty is not an assurance that carcinoma will not develop in 
adult life. Periodic sonography of the testis of affected patients is 
recommended after the onset of puberty.
Adverse env

ecommended after the onset of puberty.
Adverse environmental factors may be important in the appar­
ent increase in testis cancer, cryptorchidism, hypospadias, and low 
semen quality, which are all qualities described in the testicular 
dysgenesis syndrome (TDS).659 The complexities of clustering 
such findings without a pathophysiologic explanation has led to 
criticism of the concept.654 This remains an important area of 
research.
Retractile testes can descend into the scrotum but then reas­

estes can descend into the scrotum but then reas­
cend. They are considered a normal variation, but a requirement 
for orchiopexy was reported for 22.7% in a series of 150. The find­
ing of one case of testicular carcinoma in a boy with spontaneous 
descent in this series led to a suggestion of following such cases in 
the long term.660 The risk of breast cancer associated with gyne­
comastia is increased in men with a history of undescended testes, 
orchiopexy, orchitis, testicular injury, infe

es, 
orchiopexy, orchitis, testicular injury, infertility, or any cause of 
delayed puberty. 
Small for Gestational Age. SGA predisposes males to repro­
ductive problems and is also associated with the TDS. Males 
born SGA tend to have smaller testes and lower testosterone and 
higher LH661 levels, suggesting an impairment of fertility, as is 
found in SGA females. Adult males born SGA have increased 
aromatase and 5α-reductase activities, leading to elevated lev­
els of estradiol and dihydrotes

g to elevated lev­
els of estradiol and dihydrotestosterone.662 Of most concern is 
the fact that elevated estradiol levels in males increase the risk 
of testicular cancer and do not provide the protective effect on 
cardiovascular health as found in females. The level of inhibin 
B is elevated significantly in SGA boys (i.e., mean birth weights 
>2 SD below the mean for gestational age), although other stud­
ies with higher-birth-weight SGA boys demonstrated no such 
chages.647,663 
Girls
Synd

s demonstrated no such 
Girls
Syndrome of Gonadal Dysgenesis and Its Variants.622. The 
most common form of hypergonadotropic hypogonadism in the 
female is the syndrome of gonadal dysgenesis (Turner syndrome 
and its variants), a sporadic disorder with an incidence of 1 per 
2500 liveborn girls in which all (i.e., X chromosome monosomy 
with haploinsufficiency) or part of the second sex chromosome 
(i.e., partial sex chromosome monosomy) is absent.590,646 About 
99% of 45,X conc

onosomy) is absent.590,646 About 
99% of 45,X conceptuses abort spontaneously, and 1 in 15 spon­
taneous abortions has a 45,X karyotype. The 45,X karyotype is 
associated with female phenotype, short stature, sexual infantil­
ism, and various somatic abnormalities.
Sex chromosome mosaicism or structural abnormalities of 
an X or Y chromosome (affects about 40% of individuals with 
Turner syndrome) may modify the features of this syndrome. The 
syndrome of gonadal dysgenesis and its variants are 

ndrome of gonadal dysgenesis and its variants are found in a 
continuum ranging from the typical 45,X phenotype to a normal 
male or female phenotype.590,622 Comprehensive recommenda­
tions for the diagnosis and management of Turner syndrome were 
presented by an international committee.649 
45,X Turner Syndrome.993 Short stature and sexual infantil­
ism are typical features of sex chromatin–negative 45,X gonadal 
dysgenesis (Turner syndrome), which is the karyotype found in 
approximately 60% o

ich is the karyotype found in 
approximately 60% of cases622 (Fig. 26.47). The short stature 
is caused by loss of a homeobox-containing gene that is located 
on the pseudoautosomal region (PAR1) of the short arms of 
the X (p22) and Y (p11.3) chromosomes and encodes an osteo­
genic factor. This short stature homeobox-containing (SHOX) 
gene664 was previously called the pseudoautosomal homeobox 
osteogenic gene (PHOG). Because it is located on the PAR1 of 
the short arm of the X and Y chromosome

e PAR1 of 
the short arm of the X and Y chromosomes, it escapes X-chro­
mosome inactivation. SHOX haploinsufficiency is responsible 
for, in addition to abnormal growth, mesomelic growth retarda­
tion and Madelung deformity of the wrist (i.e., bilateral bow­
ing of the radius with a dorsal subluxation of the distal ulna as 
found in Leri-Weill dyschondrosteosis [i.e., SHOX haploinsuf­
ficiency]). Langer mesomelic dysplasia, which includes severe 
dwarfism with striking hypoplasia or aplasia of t

dwarfism with striking hypoplasia or aplasia of the ulnar and 
fibula, is caused by SHOX nullizygosity. SHOX haploinsuffi­
ciency appears to be responsible for 2 SD of the approximately 
3 SD deficit in stature and the skeletal abnormalities in Turner 
syndrome. On the other hand, patients with complete gonadal 
dysgenesis and tall stature had a 45,X/46,X der(X) and three 
doses of the SHOX gene due to the SHOX duplication on the 
der(X) chromosome.
A diagnosis of Turner syndrome may be straigh

ome.
A diagnosis of Turner syndrome may be straightforward in its 
classic sense, but often there is a delay in diagnosis. For instance, 
the Danish study found that about 15% are diagnosed below 1 
year of age, 33% during the adolescent years, and over 38% dur­
ing adulthood.665 The age of diagnosis may be coming down with 
increased education of providers and the general public, however. 
The standardized mortality rate for all causes of death for Turner 
syndrome was 2.86 with the highest pre

for Turner 
syndrome was 2.86 with the highest prevalence among females 
with 40 5X and 45 isoXq karyotypes.
Turner syndrome may be recognized in the newborn period 
or before. More than 90% of Turner fetuses spontaneously abort. 
The 45,X abortuses have edema and large hygromas of the neck 
that may be seen on prenatal ultrasound studies. This lymphatic 
defect is the basis for the loose skinfolds that ultimately form the 
webbed neck (i.e., pterygium colli). Affected newborn infants 
may also 

rygium colli). Affected newborn infants 
may also have lymphedema of the extremities; the term Bonnevie-
Ullrich syndrome has been applied to newborn infants with these 
features of Turner syndrome.
Frequent features are distinct facies with micrognathia, a fish-
mouth appearance, high-arched palate with dental abnormali­
ties, epicanthal folds, ptosis, low-set or deformed ears, short neck 
with low hairline, webbing (i.e., pterygium colli), and recurrent 
otitis media, often leading to impaired

recurrent 
otitis media, often leading to impaired hearing (25% of affected 
adults require hearing aids622). A broad, shieldlike chest leads 
to the appearance of widely spaced nipples, and the areolae are 
often hypoplastic. Skeletal defects include short fourth metacar­
pals and cubitus valgus (which may develop after birth), Mad­
elung deformity of the wrist (in about 7%), genu valgum, and 
1108
scoliosis. There are extensive pigmented nevi,622 a

coliosis. There are extensive pigmented nevi,622 a tendency to 
keloid formation, and hypoplastic nails. Lymphatic obstruction 
leads to the infantile puffiness of extremities and pterygium colli 
and to a distinctive shape of the ears. Cardiovascular anomalies 
affect the left side of the heart and include coarctation of the 
aorta in about 10% (40% of these have associated webbing of 
the neck), aortic stenosis, and bicuspid aortic valves; the latter 
individuals are at risk for a dissecting a

latter 
individuals are at risk for a dissecting aortic aneurysm. An echo­
cardiogram of the cardiovascular system must be performed, and 
prophylactic antibiotics are indicated if an anatomic abnormal­
ity is demonstrated. Elevated values on the ambulatory arterial 
stiffness index (AASI) may be another risk factor for later serious 
cardiac disease.666 Long-term regular follow-up of adults with 
Turner syndrome with echocardiography is recommended every 
5 years in adulthood.659 Abnormal pelvi

ded every 
5 years in adulthood.659 Abnormal pelvicalyceal collecting sys­
tems, abnormal position or alignment of the kidneys, and an 
abnormal vascular supply to the kidney are encountered in 30% 
to 60% of patients. Recurrent urinary tract infections are com­
mon. Defects of the gastrointestinal system include intestinal tel­
angiectasias and hemangiomatoses that rarely can lead to massive 
gastrointestinal bleeding. The prevalence of inflammatory bowel 
disease, chronic liver disease, and co

tory bowel 
disease, chronic liver disease, and colon cancer is increased. 
Autoimmune diseases, such as Hashimoto thyroiditis (16-fold 
relative risk) and Graves disease, are common, and an associa­
tion with juvenile rheumatoid arthritis and psoriatic arthritis has 
been described.
A
B
• Fig. 26.47  (A) A girl aged 14 years and 10 months with the typical form of the syndrome of gonadal dys­
genesis (Turner syndrome). The X chromatin pattern was negative, and the karyotype was 45,X. She was 
sh

 negative, and the karyotype was 45,X. She was 
short (height, 134.5 cm; height age, 9 years and 5 months) and sexually infantile except for the appear­
ance of sparse pubic hair. She exhibited characteristic stigmata of the syndrome: a short, webbed neck; 
shield-like chest with widely separated nipples; bilateral metacarpal signs; puffiness over the dorsum of the 
fingers; cubitus valgus; increased number of pigmented nevi; characteristic facies; and low-set ears. The 
bone age was 13.5 years;

s; and low-set ears. The 
bone age was 13.5 years; the urinary 17-ketosteroid level was 5.1 mg/day; and the urinary gonadotropin 
level was greater than 100 mU/day. Vaginal smears and the urocytogram showed an immature pattern in 
which cornified squamous cells were absent. Female secondary sexual characteristics were induced with 
estrogen therapy, and the cyclic administration resulted in periodic estrogen withdrawal bleeding. (B) A 45,X 
girl aged 9 years and 11 months with Turner syndrome. A

aged 9 years and 11 months with Turner syndrome. Apart from short stature (height, 118 cm; height 
age, 6 years and 11 months), increased pigmented nevi, and subtle changes in the fingers and toes, she 
had few somatic anomalies. In contrast to the patient on the left, the main clinical feature was short stature.
1109
The age of diagnosis of Turner syndrome continues to be 
delayed, with the exception of newborns with the striking phe­
notype of th

on of newborns with the striking phe­
notype of the Bonnevie-Ullrich syndrome or those diagnosed on 
amniocentesis. It is recommended that all prepubertal age girls 
below −2 SD who have at least two somatic stigmata of the syn­
drome have a karyotype analysis; early diagnosis is key for optimal 
management of the growth failure and the detection of occult fea­
tures of the syndrome.
Pelvic ultrasonography or MRI usually permits the detection of 
even a small, infantile uterus and reveals streak

even a small, infantile uterus and reveals streak gonads. Ultrasen­
sitive estrogen bioassays can confirm decreased ovarian function 
in girls with Turner syndrome because estradiol values are signifi­
cantly lower than those found in average girls in puberty. Long-
term follow-up of affected women previously treated with GH 
and estrogen demonstrated normal adult uterine length only in 
those with 45,X/46,XX karyotypes, whereas those with pure 45,X 
karyotypes had a smaller uterus length and v

45,X 
karyotypes had a smaller uterus length and volume.667 The streak 
gonads result in sexual infantilism, but in about 10% of cases, 
puberty, menarche, and (rarely) pregnancy may occur.667 Women 
with some of the variants have been able to achieve fertility and 
deliver normal infants, although cardiovascular risks increase the 
danger. Affected adults can undergo hormone replacement to pre­
pare the uterus to receive a donated embryo and proceed to deliv­
ery. Unfortunately, some patients r

ceed to deliv­
ery. Unfortunately, some patients receiving a donated ovum have 
died because of dissection or rupture of the aorta, and caution 
should be used in considering this technique.622
Intrauterine growth retardation with a mean deficit in birth 
length of 2.6 cm (−1.24 SD) and a slow childhood growth rate 
result in a loss of about 8 to 9 cm (−3 SD) by age 3 years in girls 
with Turner syndrome.660,668 A major portion of the height defi­
cit occurs during the first 3 years of life. Dec

­
cit occurs during the first 3 years of life. Decreased growth rate 
occurs at the time of expected puberty, and the pubertal growth 
spurt is absent in those without pubertal development. Untreated 
individuals with Turner syndrome in the United Kingdom and 
United States have a mean adult height of approximately 142 to 
143 cm, which is about 20 cm less than the average height of 
typical women; the adult stature of these patients correlates with 
midparental height and with the height of una

ith 
midparental height and with the height of unaffected women of 
the same ethnic group. Haploinsufficiency of the SHOX gene is 
estimated to contribute two-thirds of the height deficit. It is pos­
tulated that a second gene on the short arm of the X chromosome 
that does not undergo X-chromosome inactivation contributes 
the other one-third of the deficit. In girls with Turner syndrome 
with spontaneous puberty, pubertal height velocity was transiently 
higher than in girls with amenorrhea, b

ansiently 
higher than in girls with amenorrhea, but adult height was not 
different.669 Specific growth curves are available for plotting the 
growth of affected children.622
GH treatment is approved by the FDA for Turner syndrome 
to increase growth and adult height.670 The addition of estrogen 
therapy at low doses in addition to GH has a modest additive 
effect on adult height; untreated height was 144.6 ± 5.5 cm, 140.8 
± 5.0 cm for estrogen treatment alone, 147.9 ± 7.2 cm for the 
GH treat

 treatment alone, 147.9 ± 7.2 cm for the 
GH treatment alone, and 149.3 ± 6.6 cm for GH and estradiol 
combined over an average 7.2 ± 2.5-year treatment/observation 
period.992 The average height gain in various studies has varied 
from 4 to 16 cm, and a systematic review of the literature shows 
a 5-cm gain to be the most likely outcome.671 This variability in 
gain in height is incompletely understood, but many factors have 
been implicated, including the age of initiation of therapy, dose 
du

cluding the age of initiation of therapy, dose 
duration, age (especially number of years from beginning hGH 
treatment) at the beginning of estrogen replacement, number of 
injections per week, compliance, and whether the last measured 
height represented the adult height. Early initiation of hGH 
therapy (e.g., 2–8 years of age) leads to greatest effect. GH treat­
ment of Turner patients has been safe, and untoward events are no 
more common than in other conditions treated with GH. There is 

an in other conditions treated with GH. There is 
some degree of improvement of the abnormal body proportions of 
Turner syndrome with hGH treatment, but the disproportionate 
growth of the foot may dissuade some girls from continuing treat­
ment to maximal benefit on height. Five-year follow-up of young 
adults with Turner syndrome demonstrated continued beneficial 
effects of GH on blood pressure, lipid levels, and increased adult 
height.672
A nationwide survey of 632 Danish girls with Turner

 nationwide survey of 632 Danish girls with Turner syndrome 
demonstrated an increased prevalence of fractures, mainly in the 
forearm, compared with control subjects. The prevalence was 
higher still in the absence of ovarian function and in girls with 
family history of fractures and presumed familial disorders of bone 
density.673 It appears that estrogen therapy is critical for the pre­
vention and repair of osteoporosis, but for adolescents and adults, 
the optimal dose preparation and site

and adults, 
the optimal dose preparation and site of delivery for the preven­
tion of osteoporosis are not known. GH treatment of Turner 
syndrome for at least 1 year showed no difference in volumetric 
BMD, although LBM was higher and fat mass was lower than in 
the control subjects.674
About 50% of patients with Turner syndrome have a tendency 
toward impaired glucose tolerance without GH treatment; in 
some, this may be caused by associated obesity, and risk of type 
2 diabetes mellitus is i

besity, and risk of type 
2 diabetes mellitus is increased. Although glucose values do not 
change with GH therapy, insulin levels reversibly rise during treat­
ment, indicating an additional degree of insulin resistance caused 
by the GH.675 Turner syndrome patients as young as 11 years can 
already have elevated serum cholesterol concentrations before 
treatment with GH or estrogen.
The biphasic pattern of gonadotropin secretion in normal 
infancy and childhood is exaggerated in Turner syndrom

ncy and childhood is exaggerated in Turner syndrome420 (see 
Fig. 26.27).
The appearance of pubic hair (i.e., pubarche) is often delayed 
in the syndrome of gonadal dysgenesis, even though adrenarche, 
as assessed by the increase in concentration of plasma DHEAS, 
occurs at the normal age.442 Girls with ovarian failure demonstrate 
early adrenarche, and therefore higher serum values of DHEAS, 
but later pubarche, whereas those who demonstrate at least begin­
ning breast development follow a cour

least begin­
ning breast development follow a course of adrenarche similar to 
that of unaffected girls. This suggests that ovarian function is nec­
essary to convert DHEA to the active androgens responsible for 
the appearance of pubic hair in normal girls. The pubic hair of 
affected individuals is sparse, but estrogen therapy increases the 
growth of pubic hair despite a lack of increase in adrenal androgen 
secretion, and estrogen affects pubic hair appearance. 
Behavior and Development of T

ic hair appearance. 
Behavior and Development of Turner Syndrome. Counseling 
and a peer support group are exceedingly important components 
of long-term management. Girls with Turner syndrome younger 
than 6 years did not perceive that they had a problem with height, 
but by 7 to 12 years and especially by 13 to 15 years, affected 
Turner girls have a strong desire for GH therapy and even unre­
alistic expectations of what GH therapy can accomplish in terms 
of adult height. GH therapy improved

ish in terms 
of adult height. GH therapy improved self-esteem even if there 
remained a significant difference in height between Turner girls 
and the normal range. Height gained with GH therapy did not 
affect quality of life, although cardiac defects and otologic com­
plications did.676 Girls who have discontinued GH therapy after 
reaching adult height showed no evidence of depression but still 
had remaining problems with self-perception and bodily attitude 
1110
SECTION VI    Endocrinology

 bodily attitude 
1110
despite significant height gains. Psychologic problems in Turner 
syndrome are not necessarily diminished with GH treatment and 
an increase in adult height.
Turner syndrome girls resemble normal girls in verbal and 
language skills, and IQ is normal when verbal ability, includ­
ing comprehension and vocabulary, are considered, but visual-
constructional or visual-perceptual spatiotemporal processing, 
visuomotor coordination, 

tiotemporal processing, 
visuomotor coordination, and mathematical ability (particularly 
in geometry) may be impaired, leading to a decrease in the per­
formance of IQ tests due to mistakes on operation and alignment 
processes.677 Girls with 45,X mosaicism associated with a 46,XX 
cell line, 45,X/46,XX, scored closer to normal than those with 
other types of mosaicisms. Only 3.3% of girls with Turner syn­
drome have developmental delay in the absence of a variant of 
Turner syndrome caused by 

bsence of a variant of 
Turner syndrome caused by a ring X chromosome. It is useful to 
monitor the patient’s progress in high school mathematics. There 
are consistent MRI abnormalities in the right parietal lobe and 
the occipital lobes, and decreased volumes in these areas are impli­
cated in defects in visual-spatial processing. These anatomic data 
relate to the difficulties in visual-spatial skills found in most stud­
ies of girls with Turner syndrome, because these difficulties are 
most 

er syndrome, because these difficulties are 
most closely linked to the right parietal region.
There is an increased risk of impaired social adjustment in 
Turner syndrome. Although there was evidence that the origin of 
the remaining X chromosome in classic Turner syndrome affected 
behavior due to imprinting, recent evidence could not support the 
effect of imprinting on social behavior.678
Transition of girls with Turner syndrome to adult care is best 
carried out by an experienced team, whic

 is best 
carried out by an experienced team, which is ideally composed 
of an endocrinologist, cardiologist, nephrologist, reproduc­
tive endocrinologist, audiologic physician, ENT (ear, nose, and 
throat) surgeon, plastic surgeon, dentist, and psychologist because 
of the multiplicity of complications that affected individuals may 
encounter in the areas of growth failure, cardiovascular disease, 
gonadal failure, and learning disabilities.677 
Sex Chromatin–Positive Variants of the Syndrome o

Sex Chromatin–Positive Variants of the Syndrome of Gonadal 
Dysgenesis. Mosaicism of 45,X/46,XX; 45,X/47,XXX; or 
45,X/46,XX/47,XXX chromosomes is associated with a chroma­
tin-positive buccal smear and usually with fewer manifestations of 
the syndrome of gonadal dysgenesis. Likewise, structural abnor­
malities of the X chromosome can be associated with fewer phe­
notypic features of the syndrome. Lack of genetic material on the 
long or the short arm of the second X chromosome can cause 
decr

ort arm of the second X chromosome can cause 
decreased gonadal function; loss of all or part of the short arm 
of the X leads to the physical findings of Turner syndrome.622 
Depending on the location and extent of the deletion on the short 
arm of the X chromosome, these patients may be more likely to 
have modest pubertal growth and some spontaneous pubertal 
Sex Chromatin–Negative Variants of Gonadal Dysgenesis. 
These variants include 45,X/46,XY mosaicism and structural 
abnor

include 45,X/46,XY mosaicism and structural 
abnormalities of the Y chromosome. Affected individuals have 
phenotypes that vary from those of classic gonadal dysgenesis to 
those of ambiguous genitalia to phenotypic males.622 Patients may 
present with short stature, delayed puberty, and a history of hypo­
spadias repair. There is variable testicular differentiation, ranging 
from a streak gonad to functioning testes. Patients with mosa­
icism involving a Y cell line or abnormalities of the Y ch

volving a Y cell line or abnormalities of the Y chromosome 
are at risk for neoplastic transformation of the dysgenetic testes. 
Gonadoblastomas, which are benign, nonmetastasizing tumors, 
may arise within the gonad and produce testosterone or estrogens. 
The neoplasm may become calcified sufficiently to be detected 
on an abdominal radiograph. The appearance of feminization or 
virilization in a patient with dysgenetic gonads and a Y cell line 
may indicate gonadoblastoma formation. Of greater

may indicate gonadoblastoma formation. Of greater significance 
is the increased prevalence of malignant germ cell tumors, aris­
ing within the dysgenetic gonad or gonadoblastoma. Examples are 
dysgerminomas, mature teratomas, and testicular intraepithelial 
neoplasia.679 These tumors occur more often in postpubertal sub­
jects and rarely in children.621 
46,XX and 46,XY Gonadal Dysgenesis. The term pure gonadal 
dysgenesis refers to phenotypic females with sexual infantil­
ism and a 46,XX or 4

females with sexual infantil­
ism and a 46,XX or 46,XY karyotype without chromosomal 
abnormalities.621 
Familial and Sporadic 46,XX Gonadal Dysgenesis and Its Vari­
ants. The usual phenotype of 46,XX gonadal dysgenesis includes 
normal stature, sexual infantilism, bilateral streak gonad, normal 
female internal and external genitalia, and primary amenorrhea. 
The streak gonad occasionally produces estrogens or androgens, 
but malignant transformation is rare. Incomplete forms of this 
condition

ation is rare. Incomplete forms of this 
condition may result in hypoplastic ovaries that produce enough 
estrogen to cause some breast development and a few menstrual 
periods, followed by secondary amenorrhea. This heterogeneous 
syndrome occurs sporadically or with autosomal recessive inheri­
tance,621 and in some instances it is associated with other congeni­
tal malformations. Some familial cases have been associated with 
sensorineural deafness (i.e., Perrault syndrome). 
Familial and Spor

ness (i.e., Perrault syndrome). 
Familial and Sporadic 46,XY Gonadal Dysgenesis and Its Vari­
ants. A phenotype that includes female genitalia with or without 
clitoral enlargement, normal or tall stature, bilateral streak gonads, 
normal müllerian structures, sexual infantilism, and a eunuchoid 
habitus is typical of 46,XY gonadal dysgenesis. About 15% of the 
patients have a deletion or mutation in the sex-determining region 
Y (SRY) gene. If the dysgenetic testes produce significant amounts 

he dysgenetic testes produce significant amounts 
of testosterone, slight clitoral enlargement may occur at birth, and 
virilization may ensue at puberty. The incomplete form of 46,XY 
gonadal dysgenesis may involve any degree of ambiguity of the 
external genitalia and internal ducts. All patients with dysgenetic 
gonads and a Y chromosome or an SRY gene are susceptible to 
neoplasia or carcinoma formation in the gonad. The evidence on 
optimal therapy is not supported by high-quality evidence,

therapy is not supported by high-quality evidence, but 
gonadectomy is indicated at diagnosis, especially if the gonad can­
not be palpated.673
The disorder is usually transmitted as an X-linked or sex-lim­
ited autosomal dominant trait or less commonly as an autosomal 
recessive trait.621 
Other Causes of Primary Ovarian Failure. The prevalence of 
primary ovarian failure is increasing as a consequence of the long-
term effects of cytotoxic chemotherapy and radiation therapy as 
these agents pr

otherapy and radiation therapy as 
these agents prolong the lives of children and adolescents with 
cancer. The same pattern occurs for males with testes that have 
been treated with these modalities. 
Chemotherapy. Successful treatment of childhood acute lym­
phoblastic leukemia has become commonplace. Chemotherapy 
and radiation therapy to the CNS or ovaries exert damage in a 
dose-dependent manner.680 Chemotherapy known to adversely 
affect ovaries carries about a recurrence rate of 6 for pre

aries carries about a recurrence rate of 6 for premature 
ovarian failure.620
Attempts to protect the gonads by suppressing the pituitary-
gonadal axis with gonadal steroids or GnRH agonists are prob­
ably ineffective.681 Oncofertility is a new field of investigation 
that aims to preserve fertility in children and teenagers subject 
to cancer therapy and is leading to improved methods of pres­
ervation.682,683 A major problem is lack of discussion of possible 
1111

on of possible 
1111
method of preservation by oncologists with the families. Careful 
endocrine follow-up of children and adolescents treated with che­
motherapy or radiation therapy is essential. 
Radiation Therapy. Ovarian transposition, moving the ovaries 
out of the radiation field if they are not the target of therapy, before 
radiation therapy is compatible with normal menses, pubertal 
development, and pregnancy in the few cases reported.68

opment, and pregnancy in the few cases reported.684 The 
uterus may be affected by radiation and may not expand normally 
during pregnancy. Hypothalamic-pituitary exposure to 30 Gy car­
ries a recurrence rate of 6 for ovarian insufficiency. 
Autoimmune Oophoritis. Premature menopause may occur at 
any age before the normal climacteric and has been reported in 
adolescent girls. Cessation of ovarian function usually manifests 
as secondary amenorrhea.685 Autoimmune oophoritis can cause 
ovarian f

hea.685 Autoimmune oophoritis can cause 
ovarian failure leading to primary amenorrhea, oligomenorrhea, 
arrest of puberty, and occasionally cystic enlargement of the ova­
ries. Most often it is associated with other autoimmune endocri­
nopathies, especially autoimmune Addison disease, in which it 
may precede the onset of adrenal insufficiency, but it rarely occurs 
in isolated premature ovarian failure. Autoimmune oophoritis 
occurs in more than 20% of patients with autoimmune adrenal 
insuffi

n 20% of patients with autoimmune adrenal 
insufficiency. Various autoantibodies have been detected in auto­
immune oophoritis, including autoantibodies to cytochrome 
P450 steroidogenic enzymes; some are organ specific, whereas 
others react with antigens in more than one tissue and more than 
one cell type. Glucocorticoid therapy may improve, at least tem­
porarily, ovarian function.
Type I autoimmune polyglandular insufficiency, also known as 
autoimmune polyendocrinopathy-candidiasis-ectoder

autoimmune polyendocrinopathy-candidiasis-ectodermal dystro­
phy (APECED), is a rare systemic autoimmune disorder with an 
array of clinical features, including hypoparathyroidism, adrenal 
insufficiency, gonadal failure, diabetes mellitus, pernicious ane­
mia, hypothyroidism, chronic hepatitis, mucocutaneous candidi­
asis, dystrophic nail hypoplasia, vitiligo, alopecia, keratinopathy, 
and intestinal malabsorption. Thirty-six percent of women with 
APECED exhibited ovarian failure before age 2

ith 
APECED exhibited ovarian failure before age 20, whereas only 
4% of affected men had testicular failure by that age. This autoso­
mal recessive disorder is caused by more than 42 mutations in the 
CDK14 (AIRE1) gene at gene map locus 21q22.3. 
Homozygous Galactosemia. Homozygous galactosemia due to 
mutation in the galactose-1-phosphate uridylyltransferase (GALT) 
gene is associated with primary ovarian failure, from failure to 
develop puberty to primary or secondary amenorrhea and pre­
ma

rty to primary or secondary amenorrhea and pre­
mature menopause, but puberty is usually normal in males, and 
the risk of testicular dysfunction is low; compound heterozygotes 
have normal onset of puberty. Dietary restriction programs have 
not prevented the ovarian failure, nor are other means of avoiding 
ovarian failure effective.686 The pathogenesis of galactose-induced 
ovarian toxicity remains unclear but probably involves galactose 
itself and its metabolites, such as galactitol and uri

lf and its metabolites, such as galactitol and uridine diphos­
phate galactose. Most cases are detected by newborn screening 
programs. 
Haploinsufficiency of the FOXL2 Gene. A rare autosomal dom­
inant disorder involving eyelid dysplasia and premature ovarian 
failure is caused by haploinsufficiency of the FOXL2 gene, a mem­
ber of the winged helix/forkhead family of transcription factors.687 
The eyelid abnormalities include small palpebral fissures, ptosis, 
and a small skinfold extending inw

sures, ptosis, 
and a small skinfold extending inward and upward from the lower 
lid (i.e., epicanthus inversus). The gene is expressed in the fol­
licular cells, and the mutations that lead to haploinsufficiency are 
associated with an increased rate of follicular atresia. The degree 
of ovarian failure varies from primary amenorrhea to irregular 
menses and premature ovarian failure, with ultrasound findings 
ranging from normal-appearing ovaries to streak gonads with 
an inconsistent number o

es to streak gonads with 
an inconsistent number of primordial follicles found on ovarian 
biopsy. The infertility component of the syndrome is limited to 
females. Animal studies provide insights into other genetic muta­
tions that may cause premature ovarian failure in humans, includ­
ing mutations of BMP15, FMR1, POF1B, and FOXO3A.688 
Congenital Disorders of Glycosylation-1: Carbohydrate-Defi­
cient Glycoprotein Syndrome Type Ia. The congenital disorders 
of glycosylation-1 (i.e., carbohydra

al disorders 
of glycosylation-1 (i.e., carbohydrate-deficient glycoprotein syn­
drome type Ia) include an autosomal recessive disorder associated 
with circulating glycoproteins deficient in their terminal carbohy­
drate moieties, including a wide range of glycoproteins, enzymes, 
binding proteins, and coagulation factors.689 A typical isoform 
pattern of serum transferrin detected by isoelectric focusing is 
used as a diagnostic test. The dominant clinical feature is the neu­
rologic manifesta

ant clinical feature is the neu­
rologic manifestations of involvement of the central and periph­
eral nervous system. Among the other organ systems affected is the 
pituitary-gonadal system.
The hypergonadotropic-hypogonadism is more severe in 
females because males virilize at puberty. The ovary and the 
pituitary are affected. Affected girls have sexual infantilism; the 
ovaries are hypoplastic or atrophic. High serum FSH and LH lev­
els exhibited normal electrophoretic isoform patterns, but 

ited normal electrophoretic isoform patterns, but they 
appeared to have decreased but not absent FSH bioactivity in an 
FSH bioassay. 
Follicle-Stimulating Hormone Receptor Resistance: Gene 
Mutations and Hypergonadotropic Hypogonadism. The FSH 
receptor is a member of the G protein–linked receptor, seven-
transmembrane superfamily. It has a large, extended extracellu­
lar ligand-binding domain. An autosomal recessive disorder due 
to a mutation in the extracellular ligand-binding domain of the

 in the extracellular ligand-binding domain of the 
FSH receptor in affected females in six Finnish families mainly 
from the north central region resulted in delayed (40%) or normal 
puberty but primary amenorrhea, elevated gonadotropin levels, 
and hypergonadotropic ovarian dysgenesis with arrest of ovarian 
follicular development at the primary follicle stage and continued 
atresia.642 The clinical features are very similar to the findings in 
FSH-deficient mice generated by targeted disrupti

FSH-deficient mice generated by targeted disruption of the gene 
encoding the FSHβ-subunit. This disorder likely is responsible 
for most cases of the resistant ovary syndrome. The FSH receptor 
gene contains nine small exons (1–9) that encode the extracellu­
lar ligand-binding domain and one large exon 10 that designates 
the remainder of the receptor, including the seven-transmembrane 
and intracellular domains. The Finnish mutation, an Ala1989Val 
substitution, is in the extracellular domain

9Val 
substitution, is in the extracellular domain. Expression of the 
mutation in transfected cells indicated a small FSH effect on 
cAMP production, a striking reduction of FSH-binding capacity, 
but apparently normal binding affinity.
The FSH receptor mutation in the Finnish patients is not a null 
mutation. It remains to be determined whether the loss or com­
plete inactivation of the FSH receptor leads to failure of puberty 
and sexual infantilism or to estrogen synthesis by the immature 
o

tilism or to estrogen synthesis by the immature 
ovarian follicles described in the FSHβ-subunit knockout mouse. 
Affected males in these families are normally masculinized at 
puberty but tend to have small testes. They have a variable degree 
of spermatogenic insufficiency, but not azoospermia, increased 
plasma concentrations of FSH and LH, decreased inhibin B levels, 
and normal plasma testosterone values.690 
Luteinizing Hormone and Human Chorionic Gonadotropin 
Resistance. LH/hCG resistanc

orionic Gonadotropin 
Resistance. LH/hCG resistance due to mutations in the gene 
encoding the seven-transmembrane LH-CGR is discussed in 
1112
Chapter 24. In the affected XY individual, this autosomal recessive 
disorder leads to various degrees of male pseudohermaphroditism; 
the mildest form is represented by an isolated micropenis.691 Less 
severe mutations of the LHCGR may be associated with delayed 
puberty. In affected females, LH/hCG resistan

yed 
puberty. In affected females, LH/hCG resistance does not affect 
pubertal maturation but does lead to amenorrhea with high serum 
LH levels but normal FSH and estradiol concentrations. 
Polycystic Ovary Syndrome. PCOS, or functional ovarian 
hyperandrogenism, does not delay the onset of puberty but often 
delays menarche or causes menstrual abnormalities.692 It can have 
serious long-term metabolic consequences such as dyslipidemia 
and insulin resistance over and above androgen excess and 

lin resistance over and above androgen excess and repro­
ductive difficulties (see upcoming discussion). 
Noonan Syndrome. Individuals with Noonan syndrome (i.e., 
pseudo-Turner syndrome, Ullrich syndrome) have webbed neck, 
ptosis, down-slanting palpebral fissures, low-set ears, short stat­
ure, cubitus valgus, and lymphedema, which explains why this 
phenotype has been called pseudo-Turner syndrome.622 Features 
that differentiate these individuals from those with Turner syn­
drome include tri

uals from those with Turner syn­
drome include triangular facies, pectus excavatum, right-sided 
heart disease (e.g., pulmonary stenosis, often with valve dyspla­
sia; atrial septal defect) compared with the left-sided heart disease 
in Turner syndrome, hypertrophic cardiomyopathy, varied blood 
clotting defects, and an increased incidence of developmental 
delay. Females with Noonan syndrome have normal ovarian func­
tion. Males have normal differentiation of external genitalia but 
may have un

erentiation of external genitalia but 
may have undescended testes; germinal aplasia or hypoplasia and 
impaired Leydig cell function may be present. Puberty is often 
delayed as 35% of boys enter puberty after age 13.5 years and 44% 
of girls enter puberty after age 13 years.693 Stature is decreased 
after normal birth length and weight, with a mean adult height of 
162.5 cm (63.9 inches) for men and 152.7 cm (60.1 inches) for 
women, usually following the −2 SD curve. The pubertal growth 
spur

llowing the −2 SD curve. The pubertal growth 
spurt is often delayed or attenuated.
Noonan syndrome is inherited as an autosomal dominant 
trait.622 A gene implicated in Noonan syndrome has been local­
ized to the long arm of chromosome 12 (12q24.2-q24.31), and 
a mutation in PTPN11 was identified, but at least three other 
gene mutations have been identified. Noonan syndrome is linked 
to the chromosomal band 12q24.1, and eight genes in the RAS-
MAPK signaling pathway cause Noonan syndrome: PTP

MAPK signaling pathway cause Noonan syndrome: PTPN11, 
SOS1, KRAS, NRAS, RAF1, BRAF, SHOC2, and CBL.693 This 
disorder with a mutation in the (RAS) rat sarcoma-mitogen-acti­
vated protein kinase pathway is classified as one of the RASopa­
thies.694 The incidence is estimated at 1 case in 1000 to 1 in 2500 
individuals. One parent may have features of the syndrome in 
40% to 60% of cases. About 50% of patients are thought to have 
new mutations.
Administration of hGH is approved by the FDA for u

Administration of hGH is approved by the FDA for use in 
Noonan syndrome. Response to GH is decreased with PTN11 
mutations in some but not all695 reports, and there is an increase 
in risk for neoplasia in this genotype as well.696 GH therapy can 
allow affected children to reach the lower normal range of stature. 
Frasier Syndrome. Germline mutations in exon 8 or 9, cod­
ing for zinc fingers 2 or 3 of the Wilms tumor suppressor gene, 
WT1, leads to Denys-Drash syndrome, which includes Wilms 
t

s to Denys-Drash syndrome, which includes Wilms 
tumor, male pseudohermaphroditism, and early nephrotic syn­
drome with diffuse mesangial sclerosis. Frasier syndrome includes 
male pseudohermaphroditism, nephrotic syndrome with focal 
and segmental glomerular sclerosis, and the development of 
gonadoblastoma and is associated with mutations in the second 
splice donor site in intron 9 leading to a decrease in the KTS+ 
isoforms. Although most patients with Frasier syndrome present 
with ambiguou

ients with Frasier syndrome present 
with ambiguous genitalia, this diagnosis should be considered for 
any phenotypic female with end-stage renal disease (due to focal 
segmental glomerulosclerosis) and sexual infantilism. The karyo­
type may be 46,XY or 46,XX. 
Williams-Beuren Syndrome. Williams-Beuren syndrome is 
a microdeletion disorder, or contiguous-gene-deletion disorder 
caused by deletion of the Williams-Beuren syndrome chromo­
some region; it was previously known as Williams syndrome.

ion; it was previously known as Williams syndrome. 
The prevalence is about 1 in 10,000 persons. While it is generally 
known for its characteristic facial appearance and unusual person­
ality, many other features, including endocrine conditions, occur. 
While the guidelines of the Academy of Pediatrics state that preco­
cious puberty is rare in this condition,697 CPP is reported in about 
18% of affected girls in a recent series of 24 patients, and these 
cases responded to GnRH agonist therapy

and these 
cases responded to GnRH agonist therapy.698 
Diagnosis of Delayed Puberty and Sexual 
Infantilism
When girls remain prepubertal at 13 years or boys remain pre­
pubertal at 14 years (although some say 13.5 years), the physi­
cian must make a clinical judgment about who are variants of 
the norm and who require extensive evaluation and treatment 
(Figs. 26.48 and 26.49; Tables 26.20 and 26.21). A boy who has 
not completed secondary sexual maturation within 4.5 years after 
onset of pub

al maturation within 4.5 years after 
onset of puberty or a girl who does not menstruate within 5 years 
after onset may have a hypothalamic, pituitary, or gonadal disor­
der. The diagnosis of hypergonadotropic hypogonadism is readily 
established by elevation of random plasma LH and FSH concen­
trations. However, differentiating the diagnosis of hypogonado­
tropic hypogonadism from constitutional delay in growth and 
adolescence remains difficult in spite of decades of study owing 
to the overl

t in spite of decades of study owing 
to the overlap in physical and laboratory findings for the two 
conditions (see Table 26.21).699 Most boys with pubertal delay 
have a self-limited variant in the tempo of growth and pubertal 
onset, CDP.
Medical history must elicit all symptoms of chronic or inter­
mittent illnesses and all details pertaining to growth and develop­
ment. Questions about the patient’s sense of smell are essential. 
Has puberty failed to occur, or did it begin but failed to p

y failed to occur, or did it begin but failed to progress 
or even regress? Disorders of pregnancy, abnormalities of labor 
and delivery, and birth trauma, if part of the patient’s history, sug­
gest that a congenital or neonatal event may be related to the delay 
in puberty. Poor linear growth and poor nutritional status dur­
ing the neonatal period and childhood may reflect long-standing 
abnormalities of development. Family history may reveal disor­
ders of puberty or infertility, anosmia, or

disor­
ders of puberty or infertility, anosmia, or hyposmia in relatives 
and delay in the age at onset of puberty in parents or siblings. 
Recalled age of pubertal onset is relatively reliable in women but 
less often accurate in men. A history of consanguinity is important 
in the detection of autosomal recessive disorders.
The physical examination starts with accurate determination 
of height, weight, and BMI. A growth chart is plotted to rep­
resent graphically growth velocity from birth (se

resent graphically growth velocity from birth (see Chapter 25). 
Late-onset growth failure usually indicates a serious condition 
requiring immediate evaluation. Weight is plotted to determine 
states of malnutrition. BMI should be calculated and plot­
ted by age and gender in this era of epidemic obesity to fur­
ther determine nutritional status. The height velocity should be 
documented over a period of at least 6 months, preferably 12 
months. The U/L segment ratio and the arm span are measu

. The U/L segment ratio and the arm span are measured 
1113
and compared with the height. The signs of puberty are assessed, 
and the stage of secondary sexual development is determined by 
physical examination according to the standards presented earlier 
(see Figs. 26.5, 26.6, and 26.10). Questionnaires with pictures 
are used to allow a child to determine his or her own stage of 
puberty in some studies but do not replace the physical examina­
t

studies but do not replace the physical examina­
tion, as there is a tendency to overestimate development early 
in puberty and underestimate it late in puberty. The length and 
width of the testes are measured in boys, or the volume is assessed 
using an orchidometer. The length and diameter of the gently 
stretched penis are determined in boys, and the diameter of glan­
dular breast tissue and areolar size are determined in girls. The 
presence or absence of galactorrhea is documented. Obese b

 or absence of galactorrhea is documented. Obese boys 
often appear to have a small penis because of excessive adipose 
tissue surrounding the phallus; only when the fat is retracted can 
the full extent of phallic development be assessed. This is among 
the most common causes of inappropriate referral for hypogo­
nadism. The extent of pubic and axillary hair is assessed, as is the 
degree of acne or comedones. The possibility of cryptorchidism 
or retractile testes should be determined if no te

or retractile testes should be determined if no testes are palpated 
in the scrotum. Neurologic examination, including examina­
tion of the optic discs and visual fields by frontal confrontation 
perimetry, may reveal findings, suggesting the presence of a CNS 
neoplasm or a developmental defect, and may suggest that more 
accurate ophthalmologic evaluation is needed. Determination 
of olfaction is important because many patients with Kallmann 
syndrome wait years for the correct diagnosis to be

yndrome wait years for the correct diagnosis to be made even 
in the presence of classic findings; physicians must remain alert 
to the possibility of this diagnosis and to the fact that congenital 
anosmia may not be noted by the patient or family for years. 
The stigmata of gonadal dysgenesis (i.e., Turner syndrome) or the 
small testes and gynecomastia of Klinefelter syndrome may sug­
gest a karyotypic abnormality. Complete physical examination, 
including the lungs, heart, kidney, and gastro

n, 
including the lungs, heart, kidney, and gastrointestinal tract, is 
important in the search for a chronic disorder that may delay 
puberty.
Laboratory studies (Table 26.22) include determination of 
plasma LH and FSH concentrations by sensitive third-generation 
assays in pediatric endocrine laboratories, measurement of the rise 
in the LH level after GnRH or GnRH agonist administration, and 
determination of testosterone concentrations in boys and estradiol 
levels in girls in pediatric end

ys and estradiol 
levels in girls in pediatric endocrine laboratories using HPLC-MS/
MS, all with pediatric standards for that laboratory. One of the 
national endocrine laboratories should be used for determinations 
of the hormones of puberty, because most local laboratories are 
interested only in differentiating the normally higher adult values 
Normal
46,XY
Primary
testicular
failure
Bone age delayed
MRI
Klinefelter
syndrome
(onset of puberty
often normal but
poor progression
follows)
Const

y
follows)
Constitutional delay
in growth and
puberty
Consistently short
for chronologic
age but appropriate
for bone age
Normal without
growth spurt
Late-onset growth
failure especially
with concurrent
diabetes insipidus
Mass
CNS
tumor
Kallmann
syndrome
(cryptorchidism
and/or
microphallus
make this
more likely)
Absence of
olfactory bulb
and/or sulcus
Anosmia or
hyposmia
Sense of smell
Normal
Normal
Isolated
and/or
microph

dotropin
and/or
make this
Karyotype
47,XXY
Growth pattern
Delay in onset of secondary sexual
development after 13.5 years in boys
Elevated
Low
Serum gonadotropins
• Fig. 26.48  Flow chart for the evaluation of delayed puberty in boys. CNS, central nervous system; MRI, 
1114
from inappropriately low levels, and they cannot determine the 
gradations of the low levels found

 determine the 
gradations of the low levels found in puberty.265 Results of com­
mercial immunochemiluminometric assays for LH and FSH are 
reported to be more sensitive for use in pediatrics than immuno­
fluorometric assays (IFMA).700 Several national laboratories began 
using liquid chromatography with tandem mass spectrometry 
methods for improved sensitivity and specificity and determina­
tions in children (and women) for increased accuracy.265 Newer 
ultrasensitive bioassays are not yet av

265 Newer 
ultrasensitive bioassays are not yet available commercially for the 
determination of low values of testosterone and estradiol or for 
total androgen or estrogen. Measurements of T4 and TSH con­
centrations in boys and girls are usually necessary, and prolactin is 
often useful as well.
Genetic testing is available either by targeted evaluation or by 
panels developed by the commercial laboratories for hypogonado­
tropic hypogonadism. Targeted evaluation may be carried out by 
looking

Targeted evaluation may be carried out by 
looking for mutations of FGF8 in affected patients with cleft pal­
ate or lip or skeletal abnormalities, signs of CHARGE syndrome 
in evaluating for mutations in CHD7, bimanual synkinesia or 
renal agenesis in looking for mutations of KAL1, or in those with 
severe obesity looking for mutations in leptin and leptin receptors 
or PCSK1.374 Genetic counseling should be offered to the affected 
families. This is more easily carried out when there is an aut

is is more easily carried out when there is an autoso­
mal dominant, autosomal recessive, or X-linked condition than 
for oligogenetic conditions at present.
Radiographic examination may include bone age determi­
nation and, if the history or physical examination is consistent 
with a CNS lesion, an MRI of the brain, with specific atten­
tion to the pituitary and hypothalamic area using contrast. MRI 
(T2-weighted) coronal views of the olfactory tract will be useful in 
the diagnosis of Kallmann

tract will be useful in 
the diagnosis of Kallmann syndrome. CT can detect calcification, 
in contrast to standard MRI scans, although calcification can be 
found on plain radiographs, in some cases. Ultrasound evaluation 
of the uterus and ovaries provides useful information about the 
state of development of these structures but only if the ultrasonog­
rapher has experience with children and young adolescents. DXA 
evaluation of bone density should be considered in hypogonado­
tropic hypogonad

ould be considered in hypogonado­
tropic hypogonadism.
Assessment of karyotype should be considered for all undiag­
nosed short girls, even in the absence of somatic signs of Turner 
syndrome and especially if puberty is delayed or unexplained short 
stature is involved. Karyotype assessment should be performed for 
boys with suspected Klinefelter syndrome stigmata or behavior.
A presumptive diagnosis of constitutional delay in growth and 
adolescence is made if the history and growth chart reve

cence is made if the history and growth chart reveal a his­
tory of short stature but a consistent growth rate for skeletal age 
(and no signs or symptoms of hypothalamic lesions), if the fam­
ily history includes parents or siblings with delayed puberty, if 
the physical examination (including assessment of the olfactory 
threshold) is normal, if optic discs and visual fields are normal, 
Normal
46,XX
MRI
Constitutional
delay in growth
and puberty
for chronol

 growth
Mass
CNS
tumor
Kallmann
syndrome
Absence of
Anosmia or
hyposmia
Normal
Normal
Isolated
Karyotype
development after 13 years in girls
Elevated
Low
Karyotype
Absence or abnorma

w
Karyotype
Absence or abnormality
of one X chromosome or
mosaicism with 45,X
cell line
Primary ovarian
failure–autoimmune
or other etiology
Turner syndrome (may
or may not have somatic
stigmata of Turner
syndrome but short stature
is extremely common)
Signs of increased
intracranial
pressure or visual
46,XY
Syndrome
of
androgen
Normal secondary sexual
development with primary amenorrhea
Anatomic
defect or
in LH
pulsatility
Normal
• Fig. 2

t or
in LH
Normal
• Fig. 26.49  Flow chart for the evaluation of delayed puberty in girls. CNS, central nervous system; LH, 
luteinizing hormone; MRI, magnetic resonance imaging.
1115
  Differential Diagnostic Features of Delayed Puberty and Sexual Infantilism
Condition
Stature
Plasma 
GnRHa Test 
LH Response
Plasma 
Gonadal 
Steroids
Plasma DHEAS
Karyotype
Olfaction
Constitutional 
delay in 
growth and 
adole

action
delay in 
growth and 
Short for chron. 
age, usually 
appropriate 
Prepubertal, 
later pubertal
Prepuber­
tal, later 
pubertal
Low, later 
normal
Low for chron. age, 
appropriate for 
bone age
Normal
Normal
Hypogonadotropic Hypogonadism
Isolated 
Normal, absent 
pubertal 
Low
Prepubertal or 
no response
Low
Appropriate for 
chron. age
Normal
Normal
drome
Normal, absent
Low
no respo

drome
Low
Low
chron. age
Normal
Anosmia; pubertal 
growth spurt or 
hyposmia
Idiopathic mul­
tiple pituitary 
hormone 
deficiencies
Short stature and 
poor growth 
since early 
childhood
Low
Low
Normal
Normal
pituitary 
tumors
Late-onset 
decrease 
in growth 
velocity
Low
Low
Normal or low for 
chron. age
Normal
Normal
Primary Gonadal Failure
Syndrome of 

Normal
Normal
gonadal 
dysgen­
esis (Turner 
syndrome) and 
variants
Short stature 
since child­
hood
High
Hyperresponse 
for age
Low
Normal for chron. 
age
45,X or 
variant
Normal
Klinefelter 
syndrome and 
variants
Normal to tall
High
at puberty
Low or 
normal
age
47,XXY or 
variant
Normal
Familial XX or 
XY gonadal 
Normal for age
High
Hyperresponse
Low
age
46,XX or 
46,XY
Normal
chron., Chron

r chron. 
age
46,XX or 
46,XY
Normal
chron., Chronologic; DHEAS, dehydroepiandrosterone sulfate; GnRH, gonadotropin-releasing hormone; LH, luteinizing hormone.
   
TABLE 26.20
  Endocrine Diagnosis of Constitutional 
Delayed Adolescence and 
No single test reliably discriminates between the two diagnoses.
Onset of puberty in boys is indicated by 
Testes >2.5 cm in diameter
Serum testosterone concentration >50 ng/dL
Pubertal LH response to GnRH bolus
Pubertal patter

Pubertal pattern of LH pulsatility
GnRH, Gonadotropin-releasing hormone; LH, luteinizing hormone.
   
TABLE 26.21
  Endocrine and Imaging Studies in 
Delayed Adolescence
Initial assessment
Plasma testosterone or estradiol
Plasma FSH and LH
Plasma thyroxine (and prolactin)
Bone age and lateral skull roentgenograph
Test of olfaction
Follow-up studies
Karyotype (short, phenotypic females)
MRI with contrast enhancement
Pelvic ultrasonography (females)
GnRH test 
h

ment
GnRH test 
hCG test (males)
Pattern of pulsatile LH secretion
Visual acuity and visual fields
FSH, Follicle-stimulating hormone; GnRH, gonadotropin-releasing hormone; hCG, human 
chorionic gonadotropin; LH, luteinizing hormone; MRI, magnetic resonance imaging.
   
TABLE 26.22
1116
and if the bone age is significantly delayed. In classic cases, MRI 
of the hypothalamic-pituitary region may not be necessary. The 
r

amic-pituitary region may not be necessary. The 
rate of growth in these patients is usually appropriate for bone 
age; a decrease in growth velocity occurs in some normal chil­
dren just before the appearance of secondary sexual characteris­
tics and may awaken concerns if such a pattern occurs in these 
subjects. The onset at puberty correlates better with bone age 
than with chronologic age in delayed puberty, although bone 
age is not any better at estimating the onset of puberty in normal 

ter at estimating the onset of puberty in normal 
boys than is chronologic age and cannot be considered a highly 
accurate test.164 Elevated concentrations of gonadotropins and 
gonadal steroids to early pubertal levels precede secondary sexual 
development by several months; measurements of serum LH, 
FSH, estradiol, or testosterone levels in appropriate assays may 
help to predict future development. The GnRH stimulation test 
was the gold standard for the determination of whether a child 
is 

dard for the determination of whether a child 
is in puberty, but GnRH is not available in the United States. 
Under 3 years of age, children may have peaks of gonadotropins 
and sex steroids similar to pubertal subjects. In healthy Dan­
ish girls below 3 years of age LH increases up to 9.2 IU/L after 
GnRH test, and the stimulated LH/FSH ratio did not exceed 
0.43. The third-generation LH assays are sufficiently sensitive to 
allow in most boys the determination of the onset of endocrine 
puber

the determination of the onset of endocrine 
puberty with a single blood sample, but a dynamic GnRH test 
is still often performed using GnRH agonists. The measurement 
of gonadotropins 1 hour (or 4 hours in some reports) after a 
subcutaneous injection of GnRH agonist is the current method 
of testing dynamic secretion of gonadotropins in the absence of 
native GnRH supplies.
Measurement of an 8 am serum testosterone level provides an 
accurate indication of impending pubertal development; a va

indication of impending pubertal development; a value 
of greater than 0.7 nmol/L (20 ng/dL) predicts enlargement of 
testes to greater than 4 mL by 12 months in 77% of cases and in 
15 months in 100% of cases, whereas of those with a value less 
than 0.7 nmol/L, only 12% entered puberty in 12 months and 
only 25% entered puberty in 15 months. This technique may help 
predict spontaneous pubertal development, but it still requires 
considerable watching and waiting.460 A single inhibin B value 

atching and waiting.460 A single inhibin B value 
under 35 pg/mL holds promise in eliminating CDP in boys.260,488 
A method has been proposed to reliably separate constitutional 
delay in puberty from idiopathic hypogonadotropic hypogonad­
ism in girls; a basal inhibin B value of less than 20 pg/mL or a 
GnRH agonist stimulated follicle-stimulating hormone less than 
11 IU/L at 4 hours favors IHH.701 For boys a method to reliably 
separate CDP from IHH is proposed to be a basal LH of less 
than 

om IHH is proposed to be a basal LH of less 
than 0.3 IU/L or a stimulated LH at 4 hours after GnRH agonist 
of less than 5.3 IU/L or an inhibin B less than 111 pg/mL.702 A 
recent series of 174 boys with delayed puberty found that a his­
tory of cryptorchidism carried the highest risk of hypogonadism 
of all types and the combination of small testes (<1.1 mL) and low 
inhibin B (10–50 ng/L) carried the highest risk of CHH.703 Low 
AMH values are found in severe IHH.703 Functional hypogonad­
otr

 found in severe IHH.703 Functional hypogonad­
otropic hypogonadism or CDP were more likely if growth veloc­
ity was less than 3 cm per year. These methods require continued 
evaluation before they become standard clinically. Watchful wait­
ing may remain the procedure of choice when a patient does not 
fulfill the delineated criteria and does not fall into a diagnosable 
grouping.
The presence of red flags is suggested to be a strong indication 
of gonadotropin deficiency rather than CDP. Thus 

of gonadotropin deficiency rather than CDP. Thus lack of prior 
minipuberty of infancy (such as cryptorchidism or micropenis) or 
the presence of congenital defects, including anosmia, deafness, 
mirror movements, renal agenesis, dental/digital anomalies, cleft­
ing or coloboma, or findings of CHARGE syndrome, make the 
diagnosis of gonadotropin deficiency more likely.704 Genetic 
testing for many of the genes that cause Kallmann syndrome is 
available; it is suggested that priorities for such 

ailable; it is suggested that priorities for such testing may be 
best focused upon subjects with anosmia or hyposmia, synkinesia, 
dental agenesis, digital bony abnormalities, and hearing loss.705 
Genetic testing for PROP mutations is commercially available for 
combined hypopituitarism.
A typical patient with isolated gonadotropin deficiency is of 
average height for age and has eunuchoid proportions; has low 
plasma concentrations of gonadal steroids, LH, and FSH; and has 
no increase or a b

steroids, LH, and FSH; and has 
no increase or a blunted response of LH after GnRH or GnRH 
agonist administration. The amplitude and usually the frequency 
of LH pulses are decreased when serial blood samples are studied 
over a 24-hour period. In some forms of Kallmann syndrome, 
the sense of smell is absent or impaired. However, as stated, dif­
ferentiation of isolated gonadotropin deficiency in the absence of 
hyposmia or anosmia from CDP may be difficult at initial study. 
Gonadotropin-defi

 be difficult at initial study. 
Gonadotropin-deficient patients may be as short as those with 
constitutional delay in growth and adolescence, and concentra­
tions of LH and FSH in hypogonadotropic hypogonadism may 
be indistinguishable from those of normal prepubertal children or 
children with constitutional delay. Sometimes, years of observa­
tion are necessary to detect the appearance of spontaneous and 
progressive signs of secondary sexual development or to document 
rising concentrations

 development or to document 
rising concentrations of gonadotropins or gonadal steroids before 
the diagnosis is clear. There is a tendency for hypogonadotropic 
patients to undergo adrenarche at a normal age and to have higher 
DHEAS concentrations than those with constitutional delay in 
growth, and this pattern is helpful in making the differential diag­
nosis.442 In most cases, absence of the first signs of sexual matura­
tion or failure of a rise in gonadotropins or gonadal steroid levels 

 rise in gonadotropins or gonadal steroid levels 
by age 18 years in the presence of a normal concentration of serum 
DHEAS for chronologic age supports the diagnosis of isolated 
gonadotropin deficiency.
Patients with deficiency of gonadotropins combined with 
deficiency of other pituitary hormones require careful evalua­
tion for a CNS neoplasm, especially if the defects are acquired. 
Visual field or optic disc abnormalities support the diagnosis of 
CNS tumor; even if these tests are normal,

sis of 
CNS tumor; even if these tests are normal, cranial MRI should 
be done to evaluate the pituitary gland and stalk and the hypo­
thalamic region. MRI appears superior to CT for detecting mass 
lesions and developmental abnormalities of the hypothalamic-
pituitary region. 
Treatment of Delayed Puberty and Sexual 
Patients with constitutional delay in growth and adolescence ulti­
mately have spontaneous onset and progression through puberty. 
Often, reassurance and continued obse

gh puberty. 
Often, reassurance and continued observation to ensure that the 
expected sexual maturation occurs are sufficient. However, the 
stigma of appearing less mature than one’s peers can cause psy­
chologic stress. These individuals may be unable to participate in 
the dating activities their friends are starting; smaller size may lead 
them to avoid participation in athletics; immature appearance 
may lead to ridicule, especially in the locker room; and school­
work may suffer because o

locker room; and school­
work may suffer because of their poor self-image. Some children 
feel such intense peer pressure and low self-esteem that only the 
appearance of signs of puberty can reassure them and enable them 
to participate in sports and social activities with their peers. Poor 
1117
self-image in late-maturing boys may carry into adulthood, even 
after normal puberty ensues. Growth retardation appears more 
often responsible for most

tardation appears more 
often responsible for most of the stress rather than the delay in 
pubertal development itself.
For psychologic reasons, for boys 14 years old or older who 
show no signs of puberty, a 3-month to 6-month course of tes­
tosterone enanthate, cypionate, or cyclopropionate (50-mg dose 
given intramuscularly every 4 weeks) may be helpful. Because 
starting with the higher dose of 100 mg can lead to priapism in 
treatment-naïve boys, care, lower dosage, and short-acting prepa­

boys, care, lower dosage, and short-acting prepa­
rations are advisable. Decades of experience confirm no effect on 
adult height of low dosages in the short term.706 The low dose of 
testosterone enanthate is considered to be safe but can raise apo­
lipoprotein B (apoB) and decrease HDL cholesterol and apoAI 
levels (estradiol increases HDL cholesterol and decreases triglycer­
ides, LDL cholesterol, and apoB). Although the use of exogenous 
androgens may improve self-image and start the seconda

ogens may improve self-image and start the secondary sexual 
changes of puberty, low-dose androgen use does not increase final 
height. Low-dose estrogen may be invoked in the treatment of 
CDP in girls in a similar manner.
If during the 3 to 6 months after discontinuing gonadal steroid 
therapy spontaneous puberty does not ensue or the concentra­
tions of plasma gonadotropins and plasma testosterone in boys 
or plasma estradiol in girls do not increase, the treatment may be 
repeated. Only one 

ncrease, the treatment may be 
repeated. Only one or two courses of therapy usually are necessary. 
When treatment is discontinued after bone age has advanced, for 
example, to 12 to 13 years in girls or 13 or 14 years in boys, patients 
with constitutional delay usually continue pubertal development 
on their own, whereas those with gonadotropin deficiency do not 
progress and may regress. Oral testosterone undecanoate was suc­
cessfully used for less than 1 year in a large study of boys with 

r less than 1 year in a large study of boys with 
constitutional delayed puberty with apparently no ill effects.707
Functional hypogonadotropic hypogonadism associated with 
chronic disease is treated by alleviating the underlying problem. 
Delayed puberty in this situation is usually a result of inadequate 
nutrition and low weight or excessive energy expenditure. When 
weight returns to normal values, puberty usually occurs spontane­
ously, although there will likely be months of delay. Treatm

hough there will likely be months of delay. Treatment 
with T4 allows normal pubertal development in hypothyroid 
patients with delayed puberty.
The transition of care from the range of age normally consid­
ered to be adolescence, administered by a pediatric practitioner, 
to adult life, administered by a practitioner experienced in adult 
endocrinology, presents important implications for a change in 
the manner in which medical care is presented. Changes in the 
individual’s autonomy and impli

d. Changes in the 
individual’s autonomy and implications of legal distinctions arise, 
and some have suggested that this transition period should be con­
sidered another stage of the life span.708
Congenital or acquired gonadotropin deficiency as a 
result of a lesion or surgery requires replacement therapy at 
an age approximating the normal age of onset of puberty 
(Tables 26.23 and 26.24). Present practice in hypogonadotropic 
hypogonadism is replacement with gonadal steroid therapy, but 
th

 replacement with gonadal steroid therapy, but 
there is research into the use of gonadotropins or GnRH to stimu­
late the adolescent age gonad in a more physiologic manner; at 
present it is not clear which is the optimal approach in teenagers, 
but sex steroid replacement is most often used.
An exception may occur when GH deficiency coexists with 
gonadotropin deficiency; if bone age advancement and epiphy­
seal fusion are brought about by testosterone or estradiol replace­
ment before therapy

osterone or estradiol replace­
ment before therapy with GH causes adequate linear growth, 
adult height will be compromised. However, if puberty is not 
initiated early enough, the patient may suffer psychologic damage. 
It is advisable to initiate puberty in these patients with low-dose 
gonadal steroids by age 14 in boys and age 13 in girls, regard­
less of the definitive diagnosis of gonadotropin deficiency. Isolated 
GH-deficient patients may have a delayed onset of puberty; with 
GH adminis

 have a delayed onset of puberty; with 
GH administration, puberty usually occurs at an appropriate age 
but may progress faster than in normal individuals. Children with 
GH deficiency showed a correlation between the age of onset of 
induced puberty and adult height for those who were also gonad­
otropin deficient, whereas those who underwent spontaneous 
puberty, which occurred earlier than the age of hormone-induced 
puberty in the gonadotropin-deficient children, had a decreased 
adult heig

in-deficient children, had a decreased 
adult height. This supports the advisability of waiting to initiate 
puberty in GH-deficient and gonadotropin-deficient children. 
Height at the onset of puberty is also correlated with adult height 
in GH-deficient children. However, artificially delaying puberty 
with a GnRH analogue to attempt to achieve a greater final height 
has been attempted in isolated cases of GH deficiency or with nor­
mal-variant short stature, but concern over decreased bone d

t short stature, but concern over decreased bone density 
seen in subjects treated with GnRH analogues led to warnings 
about the use of this agent in GH-deficient patients. There is inad­
equate evidence of efficacy to recommend this therapy.470
Microphallus due to hypothalamic deficiencies may be treated 
with one or two 3-month courses of testosterone enanthate, with 
25 mg per month given intramuscularly to enlarge the size of the 
penis.327 Although concern was raised that early testosteron

Although concern was raised that early testosterone ther­
apy might not allow the attainment of a normal adult penile size, 
experience has shown otherwise. The concern that the penis might 
not respond to androgens later in life if exposed to testosterone in 
childhood, a pattern observed in the rat, proved incorrect. Positive 
psychologic outcomes and attainment of normal stretched penile 
  Management and Treatment of Delayed 
Puberty
OBJECTIVE
Determine site and cause of abnormality.
Induce

VE
Induce and maintain secondary sexual characteristics.
Induce pubertal growth spurt.
Prevent the potential short-term and long-term psychologic, personality, 
and social handicaps of delayed puberty.
Ensure normal libido and potency.
Attain fertility.
THERAPY
Concerned But Not Anxious or Socially Handicapped 
Reassurance and follow-up (tincture of time)
Repeat evaluation (including serum testosterone or estradiol) in 6 mo
Psychosocial Handica

sterone or estradiol) in 6 mo
Psychosocial Handicaps, Anxiety, Highly Concerned
Therapy for 4 mo
Boys: testosterone enanthate 100 mg IM q4wk at 14-14.5 yr of age, or 
overnight transdermal testosterone patch
Girls: ethinyl estradiol 5-10 mg/day PO or conjugated estrogens 0.3 mg/
day PO or overnight ethinyl estradiol patch at 13 yr of age
No therapy for 4-6 mo; reevaluate status including serum testosterone 
or estradiol; if indicated repeat treatment regimen
IM, Intramuscularly; PO, orally; q, e

ment regimen
IM, Intramuscularly; PO, orally; q, every.
   
TABLE 26.23
1118
length have been reported.527,709 It is appropriate to treat male 
infants and children with micropenis due to gonadotropin or GH 
deficiency with short courses of androgens to enlarge the penis 
into the normal childhood range. Patients with isolated congenital 
GH deficiency occasionally have micropenis that may be success­
fully treated with GH replacement alone. It is no

fully treated with GH replacement alone. It is not appropriate to 
sex reverse a male infant because of microphallus caused by fetal 
testosterone or GH deficiency.
Episodic administration of GnRH elicits pulsatile LH and FSH 
release and gonadal stimulation in prepubertal children or hypo­
gonadotropic patients. Portable pumps are used to administer 
episodic GnRH over prolonged periods. Pulsatile GnRH therapy 
can induce puberty and promote the development of secondary 
sexual characteristics

e development of secondary 
sexual characteristics and spermatogenesis in men and ovulation 
in women,710 but it is not practical for the routine induction of 
puberty in adolescent boys and girls with gonadotropin deficiency. 
Pregnancy can be achieved with this regimen in women and sper­
matogenesis in men with hypogonadotropic hypogonadism. A 
lower frequency of GnRH administration favors FSH secretion, 
whereas a faster frequency favors LH secretion and ultimately has 
been associated with a

cretion and ultimately has 
been associated with a PCOS-like picture.
Human menopausal gonadotropin and hCG can be used as 
effective substitutes for recombinant human pituitary LH and 
FSH to produce full gonadal maturation, but this regimen is 
cumbersome and expensive. At present, long-term gonadal ste­
roid replacement therapy is the treatment of choice for hypo­
thalamic or pituitary gonadotropin deficiency until fertility is 
desired.
Hypergonadotropic hypogonadism is treated by replacemen

gonadotropic hypogonadism is treated by replacement 
of testosterone in boys and estradiol in girls. For treatment of 
gonadal dysgenesis, estrogen therapy should be initiated when 
the patient is age 13 (bone age >11 years) to allow secondary 
sexual development at an appropriate chronologic age. Klinefel­
ter syndrome is compatible with various degrees of spontaneous 
masculinization at puberty; some patients require testosterone 
replacement. The concentration of plasma testosterone and LH 
s

The concentration of plasma testosterone and LH 
should be monitored every 6 months during puberty and yearly 
thereafter. If the LH level rises more than 2.5 SD above the mean 
value or the testosterone level decreases below the normal range 
for age, testosterone replacement therapy is indicated. FSH is 
increased due to lack of inhibin from affected testes and may not 
decrease with testosterone treatment.
Patients receiving gonadal steroid replacement follow the 
same treatment regimen wheth

placement follow the 
same treatment regimen whether the diagnosis is hypogonado­
tropic hypogonadism or hypergonadotropic hypogonadism (see 
Table 26.24). Various testosterone preparations are available with 
several routes of administration. Alkylated testosterone prepara­
tions should be avoided because of the risk of peliosis hepatis (i.e., 
hemorrhagic liver cysts), which is not related to dose or dura­
tion of treatment; although regression is possible with discon­
tinuation of testosteron

 is possible with discon­
tinuation of testosterone treatment, progression to liver failure 
can occur. Males may receive testosterone enanthate, propionate, 
or cypionate (50 mg every 4 weeks intramuscularly) at the start, 
although priapism has been reported with the higher starting dose 
(100 mg) in a testosterone-naïve boy; later, the dosage is gradu­
ally increased to 200 to 300 mg every 2 to 3 weeks. Low-dose 
replacement therapy is appropriate until well into the pubertal 
D

iate until well into the pubertal 
Dermal preparations of testosterone have been used in studies 
of delayed puberty. Testosterone may be administered by cutane­
ous patch on nonsexual skin to cause secondary sexual develop­
ment in hypogonadal adolescents; patches may be given at night 
to recreate the diurnal variation of testosterone seen in early 
puberty. Physiologic values of serum testosterone may be reached 
with these patches, along with secondary sexual development. A 
te

es, along with secondary sexual development. A 
teenage boy may be less likely to apply a patch daily, and biweekly 
or monthly injections may allow better compliance; nonetheless, 
2.5-mg and 5-mg dermal testosterone patches may be useful in 
motivated teenagers. Testosterone gel preparations, usually rubbed 
onto the forearms or shoulders, are approved for adults but not for 
adolescents. Contact with the skin, clothing, or towels used by a 
patient treated with androgen gel can cause viriliza

tient treated with androgen gel can cause virilization in young 
children or women. Testosterone ointment may be used as therapy 
for microphallus to enlarge the size of the phallus intentionally, 
but a normal infant or child contacting the skin of an individual 
treated with testosterone gel (before it is absorbed) runs the risk of 
unplanned testosterone effects.711 A dose of 10 mg of 2% trans­
dermal testosterone given for 6 months achieved similar effects 
to 50 mg of intramuscular testoste

imilar effects 
to 50 mg of intramuscular testosterone in terms of height veloc­
ity, although the control group in comparison achieved testicular 
enlargement while the treated groups did not.712 A 1% testoster­
one gel given daily to patients age 12 to 17 years with Klinefel­
ter syndrome or anorchia raised their serum testosterone levels to 
the pubertal range over a 6-month period of treatment: Cough 
was the major adverse reaction.713 Transdermal testosterone is not 
approved for use in tee

ermal testosterone is not 
approved for use in teenagers with delayed puberty.
  Hormonal Substitution Therapy in 
Boys
Goal: to approximate normal adolescent development when diagnosis is 
established
Initial therapy: at 13 yr of age, testosterone enanthate (or other long-
acting testosterone ester) 50 mg IM every month for about 9 mo 
(6-12 mo)
Over the next 3-4 yr: gradually increase dose to adult replacement dose 
of 200 mg q2-3wk
Testosterone gel is coming into widespread use 

wk
Testosterone gel is coming into widespread use as discussed in the text
Begin replacement therapy in boys with suspected hypogonadotropic 
hypogonadism by bone age ≤14 yr
To induce fertility at appropriate time in hypogonadotropic hypogonad­
ism: pulsatile GnRH or FSH and hCG therapy
Girls
With a firmly established diagnosis of hypogonadism (e.g., girls with 
45,X gonadal dysgenesis), begin hormonal substitution therapy at 
12-13 yr of age
Goal: to approximate normal adolescent development
In

l: to approximate normal adolescent development
Initial therapy: ethinyl estradiol 5 mg by mouth or conjugated estrogen 
0.3 mg (or less) by mouth daily for 4-6 mo or preferably estradiol 
transdermally
After 6 mo of therapy (or sooner if breakthrough bleeding occurs), begin 
cyclic therapy:
Estrogen: first 21 days of month
Progestogen (e.g., medroxyprogesterone acetate 5 mg PO) 12th to 
21st day of month
Gradually increase dose of estrogen over next 2-3 yr to conjugated 
estrogen 0.6-1.25 mg or

next 2-3 yr to conjugated 
estrogen 0.6-1.25 mg or ethinyl estradiol 10-20 mg daily for first 
21 days of month or estradiol patch
In hypogonadotropic hypogonadism, to induce ovulation at appropriate 
time: pulsatile GnRH or FSH and hCG therapy
chorionic gonadotropin; PO, orally; q, every.
   
TABLE 26.24
1119
The type and route of estrogen administration for girls

ype and route of estrogen administration for girls are 
being reconsidered. Many suggest the use of 17β-estradiol rather 
than ethinyl estradiol owing to more physiologic effects; there 
is evidence in the treatment of transgender individuals that the 
use of ethinyl estradiol or conjugated estrogen carries a higher 
risk for venous and arterial thrombosisis.704 Transdermal estro­
gen administration is presently recommended to avoid the first 
pass effect and hepatic clotting factor production a

ss effect and hepatic clotting factor production as injected 
estrogen transits the liver and stimulates the production of 
proteins such as C-reactive protein, angiotensin precursor, and 
activated protein C, which are involved in cardiac complica­
tions; adult women treated with injectable estrogen had expe­
rienced increased risk of deep vein thrombosis and pulmonary 
thromboembolism, breast cancer, decreased serum insulin-like 
growth factor-I concentrations, and reduced lean body mass but 

I concentrations, and reduced lean body mass but 
increased fat mass. This contrasts with dermal estrogen, which is 
administered in lower dosages and reaches its therapeutic targets 
relatively unchanged and in lower, more physiologic concentra­
tions; there is no change in these proteins with dermal estrogen 
administration.714,715 For girls 13 years of age or older, transder­
mal estrogen has been used in clinical trials for decades in the 
treatment of delayed puberty or hypogonadism with be

eatment of delayed puberty or hypogonadism with beneficial 
results on physical development and bone density and is enter­
ing widespread use.716
Initially, girls 12 to 13 years old are given estradiol alone in 
an attempt to mimic normal estrogen exposure characteristic of 
puberty. Estrogen patches, fractions of Evorel patches (0.05–0.07 
μg/kg in younger girls or 0.08–0.12 μg/kg in older girls every 
night) where it is desirable to have more rapid breast develop­
ment, demonstrate achievement

apid breast develop­
ment, demonstrate achievement of reasonable levels of serum 
estrogen.716 Evorel patch is available in Europe; in the United 
States, one-sixth to one-eighth of a Vivelle-Dot patch is adminis­
tered once or twice per week.677 There is individual variation up to 
10% in absorption, but in an individual similar plasma levels are 
achieved using 2 mg daily oral 17β-estradiol, 2 mg daily gel, and 
100-mg patch.704 However, these patches are not FDA approved 
for such use at this

patches are not FDA approved 
for such use at this time. From a clinical experience 50 girls were 
given dosage that increased every 6 months until an adult dose 
of 100 to 200 μg daily or one to two pumps of 0.06% estro­
gen gel daily demonstrated efficacy. Estradiol gel in an increasing 
dose from 0.1 to 1.5 mg given over a 5-year period is reported to 
be safe and effective as replacement therapy in girls with Turner 
syndrome.717 As with testosterone, children must not be in con­
tact with t

osterone, children must not be in con­
tact with the preparation because untoward estrogen effects may 
occur. However, it is important to note that to date dermal estro­
gen preparations are not approved for teenagers. If oral estradiol is 
used an initial dose might be 0.1 mg increasing toward adulthood 
to 1 to 2 mg daily.
Progestational agents are added after breakthrough bleeding 
occurs or when the girl reaches Tanner stage 4 after the start of 
cyclic estrogen therapy to avoid causing end

t of 
cyclic estrogen therapy to avoid causing endometrial hyperplasia 
and to enhance artificially induced menstrual periods along with 
estradiol. Progestational agents may be initiated as 5 to 10 mg of 
progesterone first given 5 days per month increasing to 14 days 
per month toward adulthood. Estradiol might be administered 
the first 21 days of the cycle with progesterone added the last 14 
days of the artificial cycle.
The maintenance dose should be the minimal amount to main­
tain second

 should be the minimal amount to main­
tain secondary sexual characteristics, sustain withdrawal bleeding, 
and prevent osteoporosis. Undesirable effects are uncommon but 
may include weight gain, headache, nausea, peripheral edema, 
and mild hypertension.
There is a concern about the increased risk of endometrial and 
breast carcinoma in patients receiving chronic estrogen replace­
ment therapy, including patients with Turner syndrome.718 The 
use of progestational agents to antagonize the effe

se of progestational agents to antagonize the effect of estrogens 
reduces the risk of endometrial cancer, but the best answer about 
the optimal dose of estrogen, route of administration, and dose 
of progesterone to enhance development without unduly increas­
ing the risk of cancer must come from future studies. Estrogen 
replacement is important for its antiosteoporosis action on bone. 
Bone density is decreased in Turner syndrome in part because of 
hypogonadism at puberty, and this tendency

use of 
hypogonadism at puberty, and this tendency becomes more severe 
with age in patients who discontinue or do not receive estrogen 
replacement therapy. Transdermal estrogen can increase bone den­
sity in subjects with Turner syndrome who have finished statural 
growth.719 We lack adequate controlled studies on optional sex 
steroid replacement regimens in young adolescent women.
Biosynthetic hGH therapy in Turner syndrome causes an 
increase in growth rate, with an increase in adult height

e in growth rate, with an increase in adult height approach­
ing or reaching the lower range of the normal growth curves pos­
sible, as stated earlier.
Patients with hypopituitarism may complain of sparse pubic 
hair growth or, in girls, total absence of pubic hair. Pubic hair 
thickens further in affected males with hCG treatment, which 
adds the testicular contribution of testosterone to the exogenous 
testosterone therapy. GH therapy in GH-deficient and gonado­
tropin-deficient males enhances

icient and gonado­
tropin-deficient males enhances the steroidogenic response of the 
testes to hCG administration. Adolescent or young adult women 
have been given a low dose (25 mg) of long-acting intramuscu­
lar testosterone every 4 weeks to stimulate the growth of pubic 
hair without virilization. Oral DHEAS treatment is suggested to 
improve pubic hair growth in hypopituitary girls.720 However, 
these androgens are not presently standard practice.
Aromatase inhibitors have been used to enha

ctice.
Aromatase inhibitors have been used to enhance growth in short 
children, including those with constitutional delay in puberty, by 
decreasing estrogen levels that would ordinarily advance bone 
age and limit future growth.471 Unfortunately there are limited 
data on actual adult height achieved as most studies predict adult 
height at a point before the child actually ceases growth.721
As in the male, psychologic counseling is beneficial for girls to 
deal with the impact of chronic medi

for girls to 
deal with the impact of chronic medical visits, infertility after an 
extensive medical treatment, and perhaps a less than ideal physical 
development compared to peers. Adults affected by hypogonado­
tropic hypogonadism comment that while their providers may 
attend to their medical treatment, they often do not address their 
emotional needs. Further problems occur when patients are lost to 
follow-up as they make the transition to adult care.722 
Sexual Precocity994
If iso-sexual

 adult care.722 
If iso-sexual precocity results from premature reactivation of the 
hypothalamic GnRH pulse generator/pituitary gonadotropin-
gonadal axis, the condition is GnRH dependent and is termed CPP 
(i.e., central [or previously, complete or true] precocious puberty). 
Pulsatile LH release has a pubertal pattern, and the rise in the con­
centration of LH after GnRH or GnRH agonist administration 
is indistinguishable from the normal pubertal pattern of serum 
LH. If 

from the normal pubertal pattern of serum 
LH. If extrapituitary secretion of gonadotropins or secretion of 
gonadal steroids independent of pulsatile GnRH stimulation 
leads to virilization in boys or feminization in girls, the condi­
tion is termed incomplete, pseudoprecocious, or peripheral preco­
cious puberty. The production of excessive estrogens in males leads 
to inappropriate feminization, and the production of increased 
1120
androgen level

androgen levels in females leads to inappropriate virilization; these 
conditions are termed contrasexual precocity or heterosexual precoc­
ity. Disorders causing sexual precocity are therefore separated into 
those in which the increased secretion of gonadal steroids depends 
on GnRH stimulation of pituitary gonadotropins and those in 
which it is unrelated to activation of the hypothalamic GnRH 
pulse generator.
In all forms of sexual precocity, increased go

or.
In all forms of sexual precocity, increased gonadal steroid 
secretion increases height velocity, somatic development, and 
the rate of skeletal maturation; because of premature epiph­
yseal fusion, sexual precocity can lead to the paradox of tall 
stature in childhood but short adult height (Table 26.25). 
Untreated females with idiopathic CPP demonstrated a mean 
adult height of 151 to 155 cm.723 In the few reports of adult 
height in boys with untreated precocious puberty, mean adult 
sta

with untreated precocious puberty, mean adult 
stature was 155.4 cm ± 8.3 SD, with all subjects well below 
midparental height and far below the father’s height from the 
data available.724,725
Serum alkaline phosphatase reflects growth, and IGF1 con­
centrations reflect the degree of sexual development rather than 
chronologic age, as do most chemistry and hematology values. 
The serum concentrations of the propeptide of type III procolla­
gen (P-III-NP) in normal puberty and in CPP parallel th

P-III-NP) in normal puberty and in CPP parallel the normal 
pubertal growth curve and also parallel the changes in growth rate 
in children treated with GnRH agonists. Blood pressure matches 
that of normal subjects of the same height and gender after cor­
recting for bone age rather than chronologic age according to the 
latest standards for blood pressure.726
In boys (Fig. 26.51), the testes usually enlarge under gonado­
tropin stimulation before any other signs of puberty are seen; in 
girls,

re any other signs of puberty are seen; in 
girls, an increase in the rate of growth, the appearance of breast 
development, enlargement of the labia minora, and maturational 
changes in the vaginal mucosa are the usual presenting signs, with 
variable manifestations of pubic hair depending on the age at 
onset. Progression of secondary sexual maturation may be more 
rapid than normal, but a waxing and waning course of devel­
opment may occur. Spermatogenesis in males and ovulation in 
females o

ermatogenesis in males and ovulation in 
females often occur, and fertility is possible. The rapid growth is 
associated with the increased GH secretion and elevation of serum 
IGF1 levels resulting from stimulation by estradiol. The ratio of 
bone age to chronologic age and the rise of IGF1 above normal 
values for chronologic age are predictive of outcome: More mildly 
affected children progress less rapidly and tend to maintain their 
target height, and this may represent a benign entity.
Cen

eight, and this may represent a benign entity.
Central Precocious Puberty: Complete Isosexual Precocity
As noted earlier, the classic lower age limit beneath which the diag­
nosis of precocious puberty is applied was 8 years in girls. However, 
this limit was reconsidered leading into the amended diagnosis of 
sexual precocity (Table 26.26) as the appearance of any sign of sec­
ondary sexual maturation before 7 years for white girls and 6 years 
for African-American girls. These newer lower cuto

for African-American girls. These newer lower cutoffs assume that 
there are no signs or symptoms of CNS disorders or other con­
current diseases that might cause sexual precocity, as evaluation is 
indicated in those cases regardless of age. Further, the newer lower 
limits usually applied to girls with elevated body mass index values 
and if the subject does not have an increased BMI evaluation may 
still be indicated. Thus careful evaluation is essential at these lower 
age ranges in girls wh

s essential at these lower 
age ranges in girls who should have only minimal slowly progres­
sive signs of sexual precocity. These newer limits remain controver­
sial, so the cautions described must be heeded.
In the UCSF series of more than 200 patients with CPP, girls 
had true precocious puberty (i.e., GnRH-dependent CPP) five 
times more commonly than boys, and idiopathic CPP was eight 
times more common in girls than in boys (Table 26.27). Others 
reported a 10-fold increased prevalence of 

thers 
reported a 10-fold increased prevalence of precocious puberty in 
girls compared with boys. CNS abnormalities occurred at least 
as often as idiopathic CPP in boys, whereas in girls neurologic 
lesions were one-fifth as common as idiopathic disorders. There­
fore, it is essential to search for a CNS cause for CPP, especially in 
boys, because sexual precocity may be the only manifestation of 
a CNS tumor (Tables 26.28 and 26.29). However, most children 
referred for evaluation have the be

most children 
referred for evaluation have the benign variants leading to prema­
ture thelarche or premature adrenarche.
  Historical Controls of Untreated Children With True Precocious Puberty
Study
No. Patients (Women/Men)
FINAL HEIGHT (CM)a
Women
Men
Thamdrup
Sigurjonsdottir and Hayles
26/8
151.3 ± 8.8
155.4 ± 8.3
40/11
152.7 ± 8.0
156.0 ± 7.3
Werder et al
4/0
150.9 ± 5.0
Lee
15/0
155.3 ± 9.6
8/4
153.8 ± 6.8
159.6 ± 8.7
Total
93/23
152.7 ± 8.6
155.6 ±

 ± 6.8
159.6 ± 8.7
Total
93/23
152.7 ± 8.6
155.6 ± 7.7
aMean ± 1 SD (standard deviation).
From Paul D, Conte FA, Grumbach MM, et al. Long-term effect of gonadotropin-releasing hormone agonist therapy on final and near-final height in 26 children with true precocious puberty treated at a 
median age of less than 5 years. J Clin Endocrinol Metab. 1995;80:546–551.
Data from Lee PA. Medroxyprogesterone therapy for sexual precocity in girls. Am J Dis Child. 1981;135:443–445; Sigurjonsdottir TJ, Hayle

Child. 1981;135:443–445; Sigurjonsdottir TJ, Hayles AB. Precocious puberty: a report of 96 cases. Am J Dis Child. 
1968;115:309–321; Thamdrup E. Precocious Sexual Development: A Clinical Study of 100 Patients. Springfield, IL: Charles C Thomas; 1961; Werder EA, Murset G, Zachmann M, et al. Treatment of 
precocious puberty with cyproterone acetate. Pediatr Res. 1974;8:248–256.
   
TABLE 26.25
1121
Idiopathic True or Central Precocious 
Puberty159,16

iopathic True or Central Precocious 
Puberty159,160,316,423,726,727
In otherwise healthy girls, those with onset of puberty at 6 to 8 
years of age often represent one end of the normal range of age 
at puberty onset; those with constitutional delay in growth and 
adolescence fall at the opposite end of the normal range of varia­
tion. These girls with low normal age of onset of puberty are often 
considered to have early puberty; a meta-analysis demonstrated 
that treatment with GnRH agonist is

 demonstrated 
that treatment with GnRH agonist is not needed to achieve mid­
parental height.728,729
Children may develop CPP with no familial tendency and no 
signs of organic disease; these children have idiopathic CPP. This 
condition, which may manifest in infancy (see Table 26.27), is 
commonly associated with electroencephalographic abnormali­
ties. The age at onset is 6 to 7 years (the new boundaries for age of 
onset of normal puberty suggest some of the 7-year-olds may be 
in the norma

ggest some of the 7-year-olds may be 
in the normal range) for about 50% of affected girls, 2 to 6 years 
for about 25%, and during infancy for 18% (Fig. 26.50). Patients 
with organic forms of CPP, especially if associated with hypotha­
lamic hamartoma, have an earlier mean age at onset than those 
with the idiopathic form.
Many girls in this subset have clinical and hormonal features 
that fall between those of premature thelarche and CPP (e.g., more 
rapid growth rate) and are typical of neit

, more 
rapid growth rate) and are typical of neither condition730; this is 
  Classification of Sexual Precocity
TABLE 26.26
TRUE PRECOCIOUS PUBERTY OR COMPLETE ISOSEXUAL 
PRECOCITY (GNRH-DEPENDENT SEXUAL PRECOCITY OR 
PREMATURE ACTIVATION OF THE HYPOTHALAMIC GNRH 
Idiopathic true precocious puberty
CNS tumors
Optic glioma associated with neurofibromatosis type 1
Hypothalamic astrocytoma
Other CNS disorders
Developmental abnormalities including hypothalamic hamartoma of 
the t

alities including hypothalamic hamartoma of 
the tuber cinereum
Encephalitis
Static encephalopathy
Brain abscess
Sarcoid or tubercular granuloma
Arachnoid cyst
Myelomeningocele
Vascular lesion
True precocious puberty after late treatment of congenital virilizing adre­
nal hyperplasia or other previous chronic exposure to sex steroids
True precocious puberty due to gain of function mutations: 
in KISS1R/GRP54 gene
in KISS1 gene
INCOMPLETE ISOSEXUAL PR

R/GRP54 gene
INCOMPLETE ISOSEXUAL PRECOCITY (HYPOTHALAMIC 
GNRH-INDEPENDENT)
Males
Gonadotropin-secreting tumors 
hCG-secreting CNS tumors (e.g., chorioepitheliomas, germinoma, 
teratoma)
hCG-secreting tumors located outside the CNS (hepatoma, teratoma, 
choriocarcinoma)
Increased androgen secretion by adrenal or testis
Congenital adrenal hyperplasia (CYP21 and CYP11B1 deficiencies)
Virilizing adrenal neoplasm
Leydig cell adenoma
Familial testotoxicosis (sex-limited autosomal domin

milial testotoxicosis (sex-limited autosomal dominant pituitary 
gonadotropin-independent precocious Leydig cell and germ cell 
maturation)
Cortisol resistance syndrome
Females
Ovarian cyst
Estrogen-secreting ovarian or adrenal neoplasm
Peutz-Jeghers syndrome
Both Sexes
Iatrogenic or exogenous sexual precocity (including inadvertent expo­
sure to estrogens in food, drugs, or cosmetics)
VARIATIONS OF PUBERTAL DEVELOPMENT
Premature thelarche
Premature isolat

L DEVELOPMENT
Premature isolated menarche
Premature adrenarche
Adolescent gynecomastia in boys
Macroorchidism
CONTRASEXUAL PRECOCITY
Feminization in Males
Adrenal neoplasm
Chorioepithelioma
CYP11B1 deficiency
Late-onset adrenal hyperplasia
Testicular neoplasm (Peutz-Jeghers syndrome)
Increased extraglandular conversion of circulating adrenal androgens to 
estrogen
Iatrogenic (exposure to estrogens)
Virilization in Females
CYP21 deficiency
CYP11B

enital adrenal hyperplasia
3βHSD deficiency
Virilizing adrenal neoplasm (Cushing syndrome)
Virilizing ovarian neoplasm (e.g., arrhenoblastoma)
Iatrogenic (exposure to androgens)
   
CNS, Central nervous system; CYP11B1, 11-hydroxylase; CYP21, 21-hydroxylase; GnRH, gonadotropin-releasing hormone; hCG, human chorionic gonadotropin; 3βHSD, 3β-hydroxysteroid dehydrogenase 
4,5-isomerase; KISS1R/GPR54, kisspeptin/G 

genase 
4,5-isomerase; KISS1R/GPR54, kisspeptin/G protein–coupled receptor 54.
Modified from Grumbach MM. True or central precocious puberty. In: Kreiger DT, Bardin CW, eds. Current Therapy in Endocrinology and Metabolism, 1985-1986. Toronto, Canada: BC Decker; 1985:4–8.
1122
denoted exaggerated thelarche. About 10% of girls with apparently 
classic premature thelarche progress to definite CPP, but there are 
no signs at the time of first presentatio

ere are 
no signs at the time of first presentation to differentiate them from 
girls who continue with the pattern of premature thelarche and 
even biochemical measurements may not define them.731 In most 
patients in this situation, the onset of breast development was first 
noticed after 2 years of age. Therapy may not be indicated if a 
pubertal pattern of pulsatile LH secretion during sleep is not pres­
ent or if the basal LH measured by ultrasensitive assay or the LH 
response to exogenous

rasensitive assay or the LH 
response to exogenous GnRH or GnRH agonist is prepubertal or 
the uterus on sensitive ultrasound is less than 34 mm. (The upper 
limit of uterine length in the prepubertal state is 35 mm, as stated 
later.) Regular follow-up is thus important.
The nature of the striking sex difference in the prevalence of 
idiopathic CPP (females > males), compared with that of consti­
tutional delay in growth and puberty (males > females) is poorly 
understood, although genomic data

ales) is poorly 
understood, although genomic data are accumulating. There may 
be a history of early maturation in families of affected subjects. 
CPP may be transmitted as an autosomal dominant trait with 
incomplete dominance in boys and girls.726
Before beginning treatment, it is essential to establish the pro­
gressive nature of the sexual precocity. In a subset of girls, the tempo 
is relatively slow and the sexual precocity may not be sustained. 
A small proportion of patients with CPP pr

ained. 
A small proportion of patients with CPP proven by a pubertal 
response of LH to GnRH and increased pulsatile LH secretion 
at night may revert spontaneously to a more immature pubertal 
state, persist without further progression, or fluctuate between 
progression and regression. There is a continuum in girls with 
CPP from premature thelarche, through unsustained or slowly 
progressive precocious puberty, to the relatively rapid progression 
of sexual maturation once it has begun.732 If 

on 
of sexual maturation once it has begun.732 If the growth rate slows 
to normal for age, skeletal maturation progresses in accordance 
with chronologic age, and there is little to no risk of impairment 
of adult height733; estrogen and IGF1 concentrations are normal 
or only slightly elevated. If height prediction is normal at the time 
of diagnosis rather than reduced, the patient does not require 
therapy; the efficacy of treatment with GnRH in those with onset 
after 6 years of age is inco

 in those with onset 
after 6 years of age is inconsistent and rarely leads to increased 
adult stature (some sources say the effects of GnRH treatment are 
decreased after initiation of treatment after 8 years).470 In some 
girls, we have observed within a period of 1 to 2 months the return 
of a prepubertal pattern of LH pulsatility during sleep, a prepuber­
tal LH response to GnRH, and a concentration of plasma estradiol 
equivalent to the prepubertal state. Unlike typical patients, such 
gir

pubertal state. Unlike typical patients, such 
girls do not exhibit the initial hyperresponse of plasma estradiol 
and LH to the GnRH agonist or the physical changes of the estro­
gen effect, and they tend to have lower serum IGF1.
The uterus and ovaries increase in size in CPP. The ovaries 
also may develop a multicystic appearance (but not a polycystic 
appearance) that may remain even after successful treatment with 
a GnRH agonist. The pituitary gland undergoes hypertrophy in 
early infancy,

ary gland undergoes hypertrophy in 
early infancy, puberty, and pregnancy and also increases in size on 
MRI imaging in patients with CPP (see later).
CPP in females does not lead to premature menopause. How­
ever, there is increased risk in girls for the development of carci­
noma of the breast in adulthood as noted earlier.734 Psychosexual 
development is advanced modestly in patients with sexual precoc­
ity (about 1.5 years in girls with idiopathic CPP).
The gonadotropin and gonadal steroid c

athic CPP).
The gonadotropin and gonadal steroid concentrations in plasma, 
the LH response to GnRH administration, and the amplitude and 
frequency of LH pulses are in the normal pubertal range in CPP 
(Figs. 26.52 and 26.53). The third-generation gonadotropin assays 
allow diagnosis of CPP by the determination in a single serum 
sample of LH in the basal state in most girls.260 Gonadotropin 
determination 60 or 240 minutes after a single subcutaneous dose 
of GnRH or GnRH agonist can diagnose 

aneous dose 
of GnRH or GnRH agonist can diagnose CPP with high specific­
ity and sensitivity. Adrenarche usually does not accompany gonad­
arche in girls with CPP who are younger than 5 or 6 years of age442: 
Pubic hair is sparse or absent initially in such girls. When the onset 
of CPP occurs after age 6 years, it is usually associated with adre­
narche that is early for chronologic age but not for bone age.
In the UCSF experience and that of others, normal pregnan­
cies have occurred in women

thers, normal pregnan­
cies have occurred in women with idiopathic CPP, CPP triggered 
by a CNS abnormality, or premature menarche.735 Pregnancy is 
reported in the lay press in a patient with CPP at 5 years of age, an 
unfortunate result of childhood sexual abuse and CPP.
Long-term follow-up of 142 women up to 50 years of age who 
had central precocious puberty demonstrated no significant health 
conditions compared to the control population.736 Treatment with 
gonadotropin-releasing hormone ag

 Treatment with 
gonadotropin-releasing hormone agonist or lack of treatment did 
not change this conclusion. In an upcoming discussion, however, 
a concern over the occurrence of PCOS is raised.
An increase in the prevalence of CPP girls in Korea between 
2014 and 2010 is reported.737 There was a lesser but measurable 
increase in prevalence in boys as well. 
Gain-of-Function Mutations as Cause of Central Precocious 
Puberty
KISS and KISSIR/GRP54 Mutations
KISS Mutations. Whereas inactivating m

4 Mutations
KISS Mutations. Whereas inactivating mutations in the KISS1R 
receptor cause hypogonadotropic hypogonadism, recently the first 
detected mutations in the gene encoding KISS and KISSR associ­
ated with precocious puberty were reported.727 Two novel KISS1 
missense mutations were discovered in three children with idio­
pathic CPP; these mutations were absent in 400 control alleles. 
The first was an adopted Brazilian girl who had slowly progressing 
thelarche since birth, but accelerat

 progressing 
thelarche since birth, but acceleration of growth, bone age matu­
ration, and secondary sexual development occurred at 7 years of 
age and was reversed with GnRH agonist treatment and had bor­
derline pubertal GnRH-stimulated LH peak levels.738 She had a 
Female
Male
Number of patients
30
20
10
0
<1
1–2
2–3
3–4
4–5
5–6
6–7
7–8 9–11
• Fig. 26.50  Age at onset of idiopathic true precocious puberty in 106 
children. At all ages, the frequency is greater in females than in 

ages, the frequency is greater in females than in males. 
The peak prevalence in girls is between ages 6 and 8 years. (From Kaplan 
SL, Grumbach MM. The neuroendocrinology of human puberty: an 
1123
  Distribution by Sex of Children With Idiopathic and Neurogenic Precocious Puberty
Series
IDIOPAT

c and Neurogenic Precocious Puberty
Series
IDIOPATHIC
NEUROGENIC
Male
Female
Male
Female
Thamdrup (1961)
4
34
7
11
Wilkins (1965)
13
67
10
5
Sigurjonsdottir and Hayles (1968)
8
54
16
16
University of California, San Francisco (1981)a
13
121
26
45
aUnpublished data.
Data from Sigurjonsdottir TJ, Hayles AB. Precocious puberty: a report of 96 cases. Am J Dis Child. 1968;115:309–321; Thamdrup E. Precocious Sexual Development: A Clinical Study of 100 Patients. 
Springfield, IL: Charles C Thomas; 1961

Patients. 
Springfield, IL: Charles C Thomas; 1961; Wilkins L. The Diagnosis and Treatment of Endocrine Disorders in Childhood and Adolescence. Springfield, IL: Charles C Thomas; 1965.
   
TABLE 26.27
A
B
• Fig. 26.51  (A) A boy aged 2 years and 5 months with idiopathic precocious puberty. He had pubic hair and 
phallic and testicular enlargement by 10 months of age. At 1 year, his height was 86 cm (+4 SD [standard 
deviation]); the phallus measured 10 × 3.5 cm, and the testes measured 2.5 × 1.5

red 10 × 3.5 cm, and the testes measured 2.5 × 1.5 cm. The plasma luteinizing 
hormone (LH) level was 1.9 ng/mL (LER-960); the follicle-stimulating hormone (FSH) level was 1.2 ng/mL (LER-
869); and the testosterone level was 416 ng/dL. After 100 μg of gonadotropin-releasing hormone (GnRH), 
the plasma LH level increased to 8.4 ng/mL, and the FSH level increased to 1.8 ng/mL, a pubertal response. 
When photographed, the patient had been treated with medroxyprogesterone acetate for 1.5 years. His 

th medroxyprogesterone acetate for 1.5 years. His height 
was 95.2 cm (+1 SD), the phallus was 6 × 3 cm, and the testes were 2.4 × 1.3 cm. Basal concentrations of 
LH (LER-960) were 0.9 ng/mL; the FSH (LER-869) level was 0.8 ng/mL; and the testosterone level was 7 ng/
dL. After 100 μg of GnRH, LH concentrations rose to 2.3 ng/mL, whereas FSH concentrations did not change 
when he was on treatment with medroxyprogesterone acetate. For conversion to SI units, see Figs. 26.19 
and 26.20. (B) A girl

o SI units, see Figs. 26.19 
and 26.20. (B) A girl aged 3 years and 3 months with idiopathic true precocious puberty had recurrent vaginal 
bleeding since she was 9 months old. Her height age was 4 years and 5 months; bone age was 8 years and 
10 months. (A, from Styne DM, Grumbach MM. Puberty in the male and female: its physiology and disorders. 
In: Yen SCC, Jaffe RB, eds. Reproductive Endocrinology. 2nd ed. Philadelphia: WB Saunders; 1986:313–384.)
1124
SECTION VI    Endocrinology and the Lif

384.)
1124
  Etiology of True Precocious Pubertya
Etiology
No. 
Females
No. Males
121
13
CNS: hypothalamic tumors, including 
hamartomas
11
15
2
1
6
1
Head trauma (child abuse)
1
Perinatal asphyxia, cerebral palsy
3
1
Encephalitis or meningitis
3
1
Sex chromosome abnormalities (47,XXY; 
48,XXXY)
2
Nonspecific seizure disorder or mental 
retardation
26
16
Degenerative CNS disease
3
Congenital virilizing adrenal

rative CNS disease
3
Congenital virilizing adrenal hyperplasia 
with secondary true precocious puberty
3
aData from University of California, San Francisco, Pediatric Endocrine Clinic.
CNS, Central nervous system.
From Kaplan SL, Grumbach MM. Pathogenesis of sexual precocity. In Grumbach MM, 
Sizonenko PC, et al, eds. Control of the Onset of Puberty. Baltimore, MD: Williams & Wilkins; 
1990:620–660. © 1990, The Williams & Wilkins Co., Baltimore.
   
TABLE 26.28
  Classification of CNS Tumors As

  Classification of CNS Tumors Associated 
With Isosexual Precocity at UCSF
10% of all true precocious puberty patients: CNS tumors, hypothalamic 
(n = 26)
Males—IPP/organic precocious puberty = 13/15 (0.9:1)
Females—IPP/organic precocious puberty = 121/11 (12:1)
GnRH-dependent true precocious puberty
Astrocytoma: 3 M, 5 F
Hamartomas: 3 M, 3 F
Neurofibromatosis: 5 M, 1 F
Craniopharyngioma: 2 F
GnRH-independent incomplete sexual precocity 
hCG-secreting tumor,a 4 M
aCNS and extra

recocity 
aCNS and extra-CNS neoplasms.
CNS, Central nervous system; F, female; GnRH, gonadotropin-releasing hormone; hCG, 
human chorionic gonadotropin; IPP, idiopathic true or central precocious puberty; M, male; 
UCSF, University of California, San Francisco.
   
TABLE 26.29
heterozygous activating mutation in the KISS1R (p.Arg386Pro), 
which led to prolonged activation of intracellular signaling path­
ways in response to kisspeptin.
A 1-year-old boy with CPP had a h

e to kisspeptin.
A 1-year-old boy with CPP had a heterozygous missense muta­
tion in KISS1 and a c.C369T transition in exon 3 of the KISS1 
gene, resulting in substitution of a proline by a serine at position 
74 of kisspeptin-1(1-145) (p.Pro74Ser). His KISS1 product was 
thought to be more resistant to degradation, leading to greater 
kisspeptin bioavailability and leading to a greater capacity to stim­
ulate signal transduction compared to the wild-type kisspeptin; 
there was no indication of 

wild-type kisspeptin; 
there was no indication of a change of affinity for the receptor. 
Because his mother and maternal grandmother, who had normal 
pubertal development, also carried the p.Pro74Ser mutation in the 
heterozygous state, the mutation was thought to have incomplete 
sex-dependent penetrance.727 Two unrelated girls with CPP had 
homozygous mutations in c.417 C3G in exon 3, leading to the 
substitution of a histidine by an aspartic acid at position 90 of kis­
speptin-1 (p.H90D). It

acid at position 90 of kis­
speptin-1 (p.H90D). It was not clear what biologic changes this 
mutation brought about to cause the precocious puberty, but het­
erozygous H90D mutations were previously found in rare patients 
with IHH. The prevalence of KISS mutations causing CPP in chil­
dren previously thought to have idiopathic CPP remains to be 
determined but appears to be low. No new mutations of KISS1 
causing CPP have been found at this time.428 
Loss-of-Function Mutations as Cause of Centr

.428 
Loss-of-Function Mutations as Cause of Central Precocious 
Puberty
MKRN3. Ten different loss-of-function mutations of MKRN3, 
an imprinted gene located on the long arm of chromosome 15q11.2 
in the Prader-Willi syndrome critical region, which encodes the 
makorin ring finger protein 3, was found in 22 patients, mostly 
girls, with CPP, all of whom inherited their mutation from their 
fathers as the maternal allele of this gene is silenced by methyla­
tion, a case of monoallelic expression.

y methyla­
tion, a case of monoallelic expression.428 No patient had findings 
of Prader-Willi syndrome. There was no family history reported in 
most cases in spite of the genetic inheritance of the trait; this is sug­
gested to be an example of the underreporting of the familial nature 
of CPP in many cases as mothers more often are the parent to bring 
the child to the doctor. This mutation represents the most common 
cause of familial CPP. The age range of the onset of secondary sex­
ual dev

e age range of the onset of secondary sex­
ual development in affected girls was 3 to 6.5 years with a mean of 
5.9 years and median 6 years and in boys was 5.9 to 9.7 years with 
a mean of 8.5 years and median 8.8 years; this genetic dimorphism 
is reminiscent of the earlier age of normal puberty in girls compared 
to boys and demonstrates that girls are more affected by the muta­
tion. A single girl diagnosed prepubertally had pubertal develop­
ment by about 6 years.739 These ages sometimes ov

ment by about 6 years.739 These ages sometimes overlap with the 
new guidelines for precocious puberty in girls, which start at 7 years 
in white girls and 6 years in African-American girls with high BMI 
values, indicating that some girls starting puberty at these lower 
limits may have familial CPP. As the age of onset of puberty in 
affected patients was years after birth it is speculated that MKRN3 
does not play a role in suppression of pubertal development after 
the minipuberty of infanc

ertal development after 
the minipuberty of infancy but does play a role in the reactivation 
of the GnRH pulse generator closer to the normal age of pubertal 
onset. Affected patients are often but not always successfully treated 
with GnRH agonists. A single girl is described with a genetic altera­
tion in the 4-nt proximal promoter region of MKRN3. Her mother 
had menarche at 10 years but did not have the same 4-nt muta­
tion.429 A boy and a girl with mutations in the MKRN3 gene were 
asympto

irl with mutations in the MKRN3 gene were 
asymptomatic carriers.740 A normal-functioning MKRN3 gene is 
considered to be a brake for the onset of puberty, and absence of 
this brake leads to precocious puberty.425 
DLK1. Delta-like 1 homolog (DLK1) is another paternally 
imprinted gene recently reported to be associated with precocious 
puberty in girls.741 
Central Nervous System Tumors Causing True Precocious 
Puberty
Sexual precocity may be the first manifestation of a hypothalamic 
tumor of

he first manifestation of a hypothalamic 
tumor of any cell type when it arises in or impinges on the poste­
rior hypothalamus. Neurologic symptoms such as headaches and 
1125
Plasma LH
(LER 960) ng/mL
Plasma FSH
(LER 1364) ng/mL
Basal
Peak
Delta
increment
True precocious puberty
n = 39
Normal pubertal females
n = 17
Normal prepubertal females
n = 42
12
9
6
3
0
Basal
Peak
Delta
increment
n = 39
n = 17

ious puberty
n = 39
n = 17
n = 42
12
9
6
3
0
A
B
• Fig. 26.52  (A) Mean basal plasma luteinizing hormone (LH) level (LER-960) and mean peak and incre­
ment after intravenous gonadotropin-releasing hormone (GnRH) (100 μg) in normal prepubertal and puber­
tal females and in females with idiopathic true precocious puberty. The mean peak and increments of 
plasma LH are higher in true precocious puberty than in normal puberty. (B) Basal follicle-sti

rty than in normal puberty. (B) Basal follicle-stimulating 
hormone (FSH) level (LER-1364) and mean peak and increment after intravenous GnRH (100 μg) in nor­
mal prepubertal and pubertal females with true precocious puberty. The concentration of FSH and the 
response to GnRH were greater in females with true precocious puberty and normal puberty than in 
prepubertal females. (From Kaplan SL, Grumbach MM. Pathogenesis of sexual precocity. In: Grumbach 
MM, Sizonenko PC, Aubert ML, eds. Control o

mbach 
MM, Sizonenko PC, Aubert ML, eds. Control of the Onset of Puberty. Baltimore, MD: Williams & Wilkins; 
1990:620–660.)
Plasma estradiol (pg/dL)
Plasma testosterone (ng/dL)
B.L.
Sleep
Time in hours
60
40
20
0
0800
1600
2400
0800
1000
2400
600
400
200
0
0400
2400
1400
1000
J.C.
400
200
0
1400 2400
1200
1000
0800
M.D.
400
200
0
0800
0200
0800
100
80
60
40
20
0
0800
1600
2400
0800
1000
2400
250
200
150
100
50
0
0800
0200
2000
1300
1200
A
B
• Fig. 26.53  (A) Serial determinations 

1200
A
B
• Fig. 26.53  (A) Serial determinations of plasma estradiol in three girls with idiopathic true precocious 
puberty. Notice the striking fluctuations in values. (B) Serial determinations of plasma testosterone in three 
boys with true precocious puberty (B.L. and J.C. have a hypothalamic hamartoma; M.D. has the idiopathic 
form). For conversion to SI units, see Figs. 26.19 and 26.20. (From Kaplan SL, Grumbach MM. Patho­
genesis of sexual precocity. In: Grumbach MM, Sizonenko PC, Aubert

l precocity. In: Grumbach MM, Sizonenko PC, Aubert ML, eds. Control of the Onset of 
Puberty. Baltimore, MD: Williams & Wilkins; 1990:620–660.)
1126
visual disturbances may develop, and children may have diabetes 
insipidus, hydrocephalus, or optic atrophy caused by an enlarg­
ing tumor in addition to precocious puberty. CPP resulting from 
CNS tumors (see Table 26.29) has about the same prevalence in 
boys as in girls in the population; however, boy

boys as in girls in the population; however, boys have a lower over­
all prevalence of precocious puberty, and neurologic abnormalities 
account for two-thirds of those with CPP. It is more likely to find 
a CNS tumor in a boy with CPP than a girl. In the UCSF experi­
ence, a CNS tumor was present in at least half of this group.
A CNS neoplasm must be considered in the differential diag­
nosis of any patient with CPP. The location of CNS tumors caus­
ing CPP makes surgical removal difficult. A

 caus­
ing CPP makes surgical removal difficult. A conservative approach 
calls for biopsy of the neoplasm and radiation or chemotherapy or 
both, depending on the pathologic findings. Optic and hypotha­
lamic glioma (often associated with neurofibromatosis), astrocy­
toma, ependymoma, and, rarely, craniopharyngioma may cause 
CPP by impinging on the neural pathways that inhibit the GnRH 
pulse generator in childhood or as a consequence of cranial irra­
diation for treatment of a brain tumor. Pi

l irra­
diation for treatment of a brain tumor. Pineal neoplasms may be 
associated with loss of upward gaze (Parinaud syndrome) due to 
brainstem compression.
The prevalence of CPP is increased after cranial irradiation 
for local tumors or leukemia even if radiotherapy targets the 
pituitary gland.300 There is a 29% prevalence of CPP following 
cranial irradiation of the hypothalamic pituitary area for CNS 
tumors; there is a risk of decreased adult height, often due to 
delayed diagnosis and 

adult height, often due to 
delayed diagnosis and treatment.742 Further, late onset gonado­
tropin deficiency is reported in about 33% of the irradiated sub­
jects. The combination of GH deficiency and CPP can occur in 
children previously subjected to therapeutic irradiation of the 
CNS in association with a CNS neoplasm and in those with 
a variety of other CNS abnormalities, including developmental 
malformations and head trauma.300 The lack of GH may not 
be apparent because of increased gro

f GH may not 
be apparent because of increased growth resulting from the ele­
vated gonadal steroid levels; GH-deficient children with CPP 
grow more slowly than GH-sufficient children with CPP but 
faster than GH-deficient children without sexual precocity. GH-
deficient children with CPP have IGF1 concentrations that are 
intermediate between the higher levels found in GH-sufficient 
children with sexual precocity and the lower levels found in pre­
pubertal GH-deficient children.
GH deficiency

pre­
GH deficiency and CPP can occur with CNS radiation doses 
of only 18 to 47 Gy, whereas gonadotropin deficiency, TSH 
deficiency, ACTH deficiency, and hypoprolactinemia usually 
occur with doses greater than 40 Gy.743 The advance in the age 
of onset of puberty is positively correlated with the age of diag­
nosis of the condition for which the radiation therapy was given 
(i.e., earlier age at onset of puberty with earlier age at diagnosis) 
and is positively 

with earlier age at diagnosis) 
and is positively correlated with BMI at diagnosis. Newer radia­
tion treatment regimens using lower doses of radiation for vari­
ous malignancies may have less influence on advancing the age of 
menarche and may lead to less long-term morbidity. Treatment 
with a combination of GH and GnRH agonist is indicated in 
these patients and results in better growth and improved height 
prognosis when compared with the use of GnRH agonist alone. 
Because GH secretion is r

 of GnRH agonist alone. 
Because GH secretion is related to BMI, it is important to rule 
out a decrease in GH secretion caused by increased BMI in CPP 
before interpreting the decrease as evidence of GH deficiency.
Hamartomas of the Tuber Cinereum. Hamartomas are con­
genital malformations composed of a heterotopic mass of ner­
vous tissue containing GnRH neurosecretory neurons, fiber 
bundles, and glial cells; they are frequently associated with CPP 
(Fig. 26.54), which usually manifests befor

h CPP 
(Fig. 26.54), which usually manifests before the patient is 3 years 
of age (Table 26.30).423,718,744 Hypothalamic hamartomas may 
be sessile or pedunculated and are usually attached to the poste­
rior hypothalamus between the tuber cinereum and the mammil­
lary bodies. These masses project into the suprasellar cistern, and 
the pedunculated hamartoma has a distinct stalk. They present a 
characteristic appearance that does not change with time. Ham­
artomas of the tuber cinereum are not 

 time. Ham­
artomas of the tuber cinereum are not true neoplasms,395,396,744 
as long-term follow-up demonstrated lack of growth on moni­
toring by periodic CT or MRI.396,745,746 Hamartomas appear 
on CT or MRI scans as an isodense, abnormal fullness of the 
interpeduncular, prepontine, and posterior suprasellar cisterns, 
occasionally with distortion of the anterior third ventricle. Their 
appearance and location relate to the clinical manifestation, with 
distortion of the third ventricle more

tion, with 
distortion of the third ventricle more closely associated with the 
occurrence of seizures. There is no enhancement with contrast 
material, and T2-weighted MRIs provide the best visualization 
of the lesion (Fig. 26.55).396 However, the solid component of 
hamartomas may be missed when associated with a subarach­
noid cyst if lower resolution MRI studies are invoked.747
The etiologic development of the hypothalamic hamartoma 
may be the converse of the lack of migration of GnRH neur

the converse of the lack of migration of GnRH neurons in 
Kallmann syndrome due to absent production of adhesion mole­
cules coded by the KAL1 gene. We may postulate that in hypotha­
lamic hamartoma, the KAL1 protein, among other axon-guiding 
factors, may cause most of the total complement of about 1500 
GnRH neurons to migrate to the hamartoma; alternatively, there 
may be a stimulus to progenitor cells that are capable of synthesiz­
ing GnRH to do so while located in the hamartoma.
Hamartomas

o do so while located in the hamartoma.
Hamartomas associated with CPP contain ectopic GnRH neu­
rosecretory cells that are similar to the GnRH-containing neurons 
in the medial basal hypothalamus. This developmental abnormal­
ity exerts its endocrine effects through elaboration and pulsatile 
release of GnRH. GnRH-containing fibers have been identified 
passing from the hamartoma toward the median eminence. We 
have suggested that the GnRH-containing neurosecretory neu­
rons in the tumor are un

ining neurosecretory neu­
rons in the tumor are unrestrained by the intrinsic CNS mecha­
nism that inhibits the normal GnRH pulse generator and act as 
an ectopic GnRH pulse generator,326 either independently or in 
synchrony with the GnRH neurosecretory neurons in the medial 
basal hypothalamus, to produce intermittent secretory bursts of 
GnRH326 (see Fig. 26.29). The GnRH is transported to the pitu­
itary by way of the portal circulation and elicits pulsatile release 
of LH. If the hamartoma 

licits pulsatile release 
of LH. If the hamartoma were to secrete GnRH in a continu­
ous fashion, CPP would not occur, because the GnRH receptors 
would be desensitized. About 10% of hypothalamic hamartomas 
are not associated with CPP. The hamartomas associated with 
CPP tend to be more likely to contact the infundibulum or tuber 
cinereum and less likely to distort the third ventricle and were 
larger than those that are not, but the size of the hamartoma is not 
related to the age of onset of

e hamartoma is not 
related to the age of onset of puberty.748
Hypothalamic hamartomas are postulated to elicit their effects 
via neurons that are able to produce GnRH within the tumor, 
through the ability to control neurons synaptically connected 
to GnRH neurons, or through neuronal networks that include 
GnRH neurons in the hypothalamic hamartoma itself or signal­
ing-competent astrocytic and ependymoglial cells.749 The preco­
cious sexual development can be controlled by treatment with 
Gn

evelopment can be controlled by treatment with 
GnRH agonist therapy.750
All hypothalamic hamartomas in a recent series contained 
GnRH, GnRHR, and TGFα, and some contained KISS1, 
GPR54, and GRM1A (which encodes the isoform of the 
1127
A
C
B
• Fig. 26.54  (A) A 17-month-old male infant with hamartoma of the tuber cinereum and true precocious 
puberty. When the patient was 8 months of age, secondary sexual development was observed, and the 
patien

y sexual development was observed, and the 
patient was misdiagnosed as having congenital virilizing adrenal hyperplasia. He was treated with gluco­
corticoids, which slowed his growth but did not affect his sexual development and bone age advance­
ment. When he was first seen at 17 months, his height was 84.2 cm, his weight was 14.8 kg, pubic hair 
was stage 2, penis was 10.4 × 2.2 cm, testes were 1.5 × 2.8 cm, and the scrotum was thinned and 
rugated. The bone age was 4.25 years. After gonadot

ugated. The bone age was 4.25 years. After gonadotropin-releasing hormone (GnRH) administration, the 
luteinizing hormone (LH) level rose from 0.5 to 3.1 ng/dL (LER-960), the follicle-stimulating hormone (FSH) 
level rose from 0.5 to 1.2 ng/mL (LER-869), and the testosterone level rose from 409 to 450 ng/dL. The 
dehydroepiandrosterone sulfate (DHEAS) was 17 μg/dL (preadrenarchal value). The patient was treated 
with a potent, long-acting LH-releasing hormone (LHRH) agonist, deslorelin (d-Trp6Pr

asing hormone (LHRH) agonist, deslorelin (d-Trp6Pro9NEt-GnRH), which 
resulted in arrest of his pubertal advancement and a striking decrease in the plasma concentration of tes­
tosterone, LH pulses, and the response to exogenous GnRH. (B) Computed tomography demonstrated a 
1.5-cm mass posterior and rostral to the dorsum sella, which depressed the flow of the third ventricle. For 
conversion to SI units, see Figs. 26.19, 26.20, and 26.30. (C) Sagittal, T1-weighted magnetic resonance 
image shows

ittal, T1-weighted magnetic resonance 
image shows a hypothalamic hamartoma (white arrow) in a 4-year-old boy with true precocious puberty. 
The posterior pituitary hot spot is designated by the black arrow. (B, from Styne DM, Grumbach MM. 
Puberty in the male and female: its physiology and disorders. In: Yen SCC, Jaffe RB, eds. Reproductive 
Endocrinology. 2nd ed. Philadelphia: WB Saunders; 1986:313–384.)
1128
metabotropic glutamate receptor 1A, whi

 Life Span
metabotropic glutamate receptor 1A, which rises in the female 
monkey with puberty) with no relationship as to whether CPP 
occurred or not.748 Earlier two young girls with rapidly progres­
sive CPP did not contain immunoreactive GnRH neurons, but 
the mass showed a network of astroglial cells containing TGFα.751 
This suggests that some hypothalamic hamartomas, by virtue of 
increased production of TGFα and of neuregulins synthesized by 
hypothalamic and astroglial cells through para

by 
hypothalamic and astroglial cells through paracrine mechanisms, 
effect the release of bioactive factors, including prostaglandin E2, 
that act on GnRH neurons to increase GnRH secretion. How­
ever, these hamartomas were much larger than the typical hypo­
thalamic hamartoma associated with CPP; the mass bulged into 
the third ventricle in one girl, and in the other girl, the pituitary 
gland was hypertrophic and bulged through the diaphragma sel­
lae. Activation of the GnRH pulse generator t

sel­
lae. Activation of the GnRH pulse generator through a mass effect 
and compromise of restraint mechanisms may be the mechanism 
of CPP rather than TGFα signaling in these cases.
Before 1980, only 37 patients were in the literature with ham­
artomas of the tuber cinereum, but many more have been reported 
since the advent of CT and MRI brain scans (see Fig. 26.54 and 
Table 26.30). Of girls with CPP studied at the National Institutes 
of Health, 16% had a hypothalamic hamartoma, 40% had othe

th, 16% had a hypothalamic hamartoma, 40% had other 
CNS abnormalities, and 60% had idiopathic CPP. Among boys 
with CPP, 10% had idiopathic CPP, 50% had a hypothalamic 
hamartoma, and the rest had other CNS abnormalities, including 
hypothalamic neoplasms.752
Hypothalamic hamartomas that cause CPP can be associated 
with laughing (gelastic), petit mal, or generalized tonic-clonic sei­
zures; developmental delay; behavioral disturbances; and dysmor­
phic syndromes beginning as early as the neona

or­
phic syndromes beginning as early as the neonatal period. Males 
are more likely to have seizures with these lesions, and seizures can 
be caused by a hamartoma in the absence of precocious puberty. 
The occurrence of seizure is uncommon when the mass diameter 
of the hamartoma is less than 10 mm, whereas a larger mass is 
associated with a higher risk.
Although some have advocated neurosurgical removal of 
these hamartomas because of the occurrence of precocious 
puberty alone, we do not re

urrence of precocious 
puberty alone, we do not recommend neurosurgical extirpation 
in the absence of strong evidence of growth of the mass or of 
an associated complication such as intractable seizures or hydro­
cephalus.423,753 Gelastic seizures are less amenable to antiepi­
leptic therapy than other hamartoma-related seizures and may 
require surgical treatment; endoscopic technique is increasingly 
used,754,755 and Gamma Knife procedures are used, especially 
for small lesions.756 Refractor

 used, especially 
for small lesions.756 Refractory seizures have replaced precocious 
puberty as the main reason to perform surgery in cases of ham­
artoma of the tuber cinereum. The endocrine result of surgery on 
these lesions is rarely reported, but in one series of 29 subjects, 
hypernatremia, low T4, low GH, and weight gain were encoun­
tered.757 Prior unsuccessful surgery may increase the risk for 
endocrinopathy in these patients. The postoperative endocrine 
disturbances appear to be tr

toperative endocrine 
disturbances appear to be transient and mild or asymptomatic, 
but increased appetite and weight gain appeared in 25% of the 
subjects and may present an enduring problem. Although there 
are cases in which removal of a hypothalamic hamartoma led 
to reversal of the pubertal process, deaths have been reported 
after attempted operative removal. Medical therapy with GnRH 
agonist in lieu of surgery is the optimal treatment of precocious 
puberty associated with these hamarto

 precocious 
puberty associated with these hamartomas if seizures are absent 
or under control.
The Pallister-Hall syndrome is associated with hypothalamic 
hamartomas, precocious puberty (but rarely) with or without sei­
zures, polydactyly, imperforate anus, bifid epiglottis, and hypopi­
tuitarism with seizures.758 
  Clinical and Laboratory Characteristics 
of Children With True Precocious 
Puberty Caused by Hypothalamic 
Hamartoma
UCSF 
(6 M, 6 F)
Hochman et  al.a 
(18 M, 9 F)

tic
UCSF 
(6 M, 6 F)
(18 M, 9 F)
Age at Onset of Pubertal Signs
Birth to 1 yr
4
6
1-2 yr
4
17
2-4 yr
3
6
7 yr
1
1
Neurologic Signs
Seizures, including gelastic type
3/12
11/24
Headache and visual symptoms
1/12
5/24
None
7/12
7/24
aData from Hochman HI, Judge DM, Reichlin S. Precocious puberty and hypothalamic hamar­
toma [literature review]. Pediatrics. 1981;67:236–244.
F, Female; M, male; UCSF, University of California, San Francisco.
   
TABLE 26.30
3
2
1
0
2400
0400
0200
240

ncisco.
   
3
2
1
0
2400
0400
0200
2400
0400
0200
Before
R.E.
J.H.
LH
FSH
LH
FSH
Time
LH     FSH
(ng/mL)
On
3
2
1
0
2400
0400
0200
2400
0400
0200
Before
LH
FSH
LH
FSH
Time
LH     FSH
(ng/mL)
On
A
B
• Fig. 26.55  Pulsatile luteinizing hormone (LH) secretion before and during gonadotropin-releasing hor­
mone (GnRH) agonist therapy in a boy (B) and a girl (A) with true precocious puberty resulting from a 
hypothalamic hamartoma. For conversion to SI units, see Fig. 26.19. FSH, follicle-

ersion to SI units, see Fig. 26.19. FSH, follicle-stimulating hormone.
1129
Neurofibromatosis Type 1. Neurofibromatosis type 1 (NF1 
or von Recklinghausen disease) is associated with a propensity 
to develop the optic chiasmal tumors that are the most common 
cause (but not the only cause) of the development of CPP in a 
child with neurofibromatosis.759 Most optic gliomas appear dur­
ing the first decade of life, but only 20% to 30% become symp­
to

ecade of life, but only 20% to 30% become symp­
tomatic; these tumors rarely progress in the years after diagnosis. 
The tumor suppressor NF1 gene, located on the long arm of chro­
mosome 17 (q11.2), which has a high mutation rate, encodes a 
327-kDa protein, neurofibromin, that is widely expressed even 
though NF1 involves mainly tissues derived from the neural 
crest. A wide variety of mutations of the NF1 gene are reported, 
especially deletions, nonsense mutations, and truncating muta­
tions

ns, nonsense mutations, and truncating muta­
tions distributed over the coding region. In sporadic cases, the 
new mutation originates in the paternally derived NF1 allele in 
most instances, suggesting a role for genomic imprinting. Con­
centrations of midkine (MK) and stem cell factor, but not epi­
dermal growth factor, are substantially increased in the serum of 
NF1 patients compared with healthy control subjects and serve 
as a diagnostic feature.760 Serum MK levels increase dramatically 
i

ature.760 Serum MK levels increase dramatically 
in patients older than 18 years of age, apparently as a feature of 
pubertal development. Because serum from patients with NF1 
enhances proliferation of human neurofibroma-derived primary 
Schwann cells and endothelial cells, enhanced levels of circulating 
growth factors contribute to diffuse tumorigenesis in NF1.
NF1 is characterized by multiple pigmented areas and over­
growth of nerve sheaths and fibrous tissue elements (Fig. 26.56). 
Multipl

and fibrous tissue elements (Fig. 26.56). 
Multiple café au lait spots are frequent and are smoother in out­
line (coast of California appearance) than those of the McCune-
Albright syndrome (coast of Maine appearance). The diagnosis is 
made if two or more of the following are observed761:
	1.	 Six or more café au lait macules, the greatest diameter being 
more than 5 mm in prepubertal subjects or more than 12.5 
mm in postpubertal subjects
	2.	 Two or more neurofibromas of any type or one pl

kin in neurofibromatosis may be sub­
cutaneous, sessile or deep, plexiform masses in children; pedun­
culated lesions develop in later childhood. Internal neurofibromas 
cause most of the complications. Bone abnormalities such as 
cysts and pseudoarthrosis, hemihypertrophy, bowing, scolio­
sis, and skull and facial defects are common (20% of patients); 
• Fig. 26.56  A boy aged 8 years and 8 months with neurofibromatosis and precocious puberty resulting 
from a hypothalamic glioma. He had tonic-

sulting 
from a hypothalamic glioma. He had tonic-clonic seizures at 2.5 years and rapid growth starting at 4 years; 
an enlarged penis and testes and the presence of pubic hair were first noticed at 7.5 years. At this time, 
his height was 139.9 cm (+1.4 SD [standard deviation]); the phallus was 9 × 3 cm, the right testis mea­
sured 5.5 × 3.2 cm, and the left measured 5.4 × 2.9 cm. He had stage 3 pubic hair and 24 large café au 
lait spots. Computed tomography and pneumoencephalography revealed

uted tomography and pneumoencephalography revealed a 1.5 × 2.5–cm, hypothalamic 
mass, which was treated with irradiation. The plasma concentration of luteinizing hormone (LH) was 0.5 
ng/mL (LER-960); the follicle-stimulating hormone (FSH) level was 0.4 ng/mL (LER-869); and the testos­
terone level was 221 ng/dL. After 100 μg of intravenous LH-releasing hormone (LHRH), the peak concen­
tration of LH was 4.9 ng/mL, and that of FSH was 1.4 ng/mL, a pubertal response. For conversion to SI 
units, 

a pubertal response. For conversion to SI 
units, see Figs. 26.19 and 26.20. (From Styne DM, Grumbach MM. Puberty in the male and female: its 
physiology and disorders. In: Yen SCC, Jaffe RB, eds. Reproductive Endocrinology. 2nd ed. Philadelphia: 
WB Saunders; 1986:313–384.)
1130
dumbbell-shaped tumors of spinal nerve roots may cause pain, 
sensory and motor dysfunction, and bone erosions; gliomas or 
neurofibromas of any part of the CNS, including t

neurofibromas of any part of the CNS, including the optic nerves 
and the hypothalamus, may calcify. Lisch nodules of the iris are 
frequent, particularly in adults. Sarcomatous degeneration occurs 
in 5% to 15% of patients. Other neoplasms include CNS astro­
cytomas that often involve the visual pathways, ependymomas, 
meningiomas, neurofibrosarcomas, rhabdomyosarcomas, and 
nonlymphocytic leukemias. Pheochromocytoma may develop in 
affected adults.
The clinical manifestations of neurofibromat

ults.
The clinical manifestations of neurofibromatosis include 
seizures, visual defects, and either delayed puberty or CPP, 
although many have growth hormone deficiency.759 Although 
some manifestations of NF1 are quite common (e.g., café au 
lait spots, which were found in 99% of a series of 297 subjects), 
precocious puberty is rarer in some reports (found in 3.2% of 
that same series)762 but found in about 33% in a report of 40 
patients.759 GH deficiency is possible at presentation, but af

 GH deficiency is possible at presentation, but after 
radiation therapy for associated optic glioma, GH, TSH, ACTH, 
and gonadotropin deficiency may develop. Developmental delay 
occurs more often in this population but usually is not severe; 
there is also an increased incidence of psychiatric disease. Most 
affected children have some manifestations of the disease by 1 
year of age. Screening MRI scans are recommended for early 
detection of CNS tumors. 
Other Central Nervous System Condition

NS tumors. 
Other Central Nervous System Conditions
CPP may occur secondary to encephalitis, static cerebral enceph­
alopathy, brain abscess, or sarcoid or tuberculous granulomas 
of the hypothalamus, with or without tuberculous meningitis. 
CPP can occur after severe head trauma (usually in girls), and 
it has been associated with cerebral atrophy or focal encephalo­
malacia occurring after cerebral edema (causing cerebral injury), 
complicating diabetic ketoacidosis. Children with nontumor 
hy

 diabetic ketoacidosis. Children with nontumor 
hydrocephalus, even if shunted, experience earlier pubertal 
development, and those who have not been adequately treated 
may develop CPP. Delayed puberty is an alternative outcome 
in a minority of affected children. The growth pattern of chil­
dren with severe hydrocephalus often includes poor prepubertal 
growth and an early pubertal growth spurt leading to decreased 
final height.
Arachnoid Cysts. Arachnoid cysts arising de novo, after infec­
t

s. Arachnoid cysts arising de novo, after infec­
tion or after surgery, can cause premature sexual development, 
possibly with associated GH deficiency.326,763 Head nodding, 
abnormal gait, and abnormalities of visual fields are reported in 
30% to 40% of cases. Erosion or enlargement of the sella tur­
cica into a J shape may occur. Decompression and extirpation of 
a suprasellar arachnoid cyst can reverse the sexual precocity (see 
Fig. 26.30). Late endocrine effects such as growth hormone defi

Late endocrine effects such as growth hormone defi­
ciency can be found after surgery to approach suprasellar arach­
noid cyst.764 
Other Central Nervous System Abnormalities. Other CNS 
abnormalities associated with CPP but without demonstrable 
lesions on imaging study include epilepsy, laughing seizures, devel­
opmental delay, cerebral palsy, and the posttraumatic state.765 
Septo-optic dysplasia (described earlier) may be associated not 
only with multiple pituitary hormone deficiencies and 

 with multiple pituitary hormone deficiencies and delayed 
puberty but also, rarely, with CPP.546 There may be coexisting 
deficiencies of some pituitary hormones and excessive secretion of 
others, including prolactin.
Patients with myelomeningocele (myelodysplasia) have an 
increased prevalence of endocrine abnormalities, including 
hypothalamic hypothyroidism, hyperprolactinemia, and elevated 
gonadotropin concentrations, which in some patients is associated 
with CPP. 
C

s is associated 
with CPP. 
Central Precocious Puberty in Children Adopted From Devel­
oping Countries. There was a 15-fold to 20-fold increased prev­
alence of CPP reported among children (with established birth 
dates) from developing countries who were adopted into Den­
mark, and other countries have reported similar tendencies. In 
Sweden, adopted Indian children had pubertal growth spurts sim­
ilar to those of Swedish children, but adult height was decreased, 
with the 

ildren, but adult height was decreased, 
with the loss of height in childhood and the early puberty appar­
ently being responsible. Environmental influences, many related 
to the present obesity epidemic, are posited to exert effects in the 
reported decrease in age at menarche among children from the 
developing world. These children, who suffer poor prenatal nutri­
tion and often are SGA, are adopted into families in developed 
countries, and in that environment of affluence they experience 
p

n that environment of affluence they experience 
precocious puberty.4,766
It is not clear why adopted children may develop precocious 
puberty, and some even question this phenomenon itself; under­
reported age of adopted children fits an explanation of the ten­
dency to ascribe precocious puberty to adoption and more critical 
analysis of the data is sought.767 Foreign children who immigrated 
to Belgium with their biologic families from developing countries 
had greatly elevated concentrations

ing countries 
had greatly elevated concentrations of p,p′-DDE, a derivative of 
the organochlorine pesticide dichlorodiphenyltrichloroethane 
(DDT), raising the possibility of a role for endocrine disrupters 
in their CPP,768 although this is contradicted in the experience of 
other counties. Older age at adoption and immigration are risk 
factors for precocious puberty. One study showed that children of 
immigrant groups who were born in their new country may have 
earlier puberty than childre

new country may have 
earlier puberty than children of the predominant ethnic group of 
that country; these influences might be genetic, or they might be 
related to cultural and dietary differences but could complicate 
the analysis.31 However, a detailed analysis in Spain showed that 
the relative risk of CPP in domestic and internationally adopted 
children compared with those born in Spain was 27.82 (19.99 
– 38.77), whereas the relative risk among immigrants was 1.55 
(0.97 – 2.38); the aut

 among immigrants was 1.55 
(0.97 – 2.38); the authors suggest that psychologic factors may be 
the cause of this difference rather than immigration, nutrition, or 
pesticide exposure.744 Remarkably, children adopted from South 
Korea do not appear to have the onset of puberty as early as those 
from other countries that were studied.744
The use of GnRH agonist in addition to GH treatment to 
increase adult height is reported in adopted children with CPP, 
but the combination is not supported by

with CPP, 
but the combination is not supported by adequate evidence to 
recommend this combination in adopted children or any children 
with CPP.470 The combination of adoption, living in a culture 
foreign to the background at birth, and precocious puberty makes 
these children vulnerable to psychic trauma, which must always 
be considered. 
True Precocious Puberty After Virilizing Disorders. Correction 
of long-standing virilization may be followed by development of 
CPP with activation of th

lowed by development of 
CPP with activation of the hypothalamic-pituitary gonadotropin-
gonadal system. This secondary CPP occurs in congenital virilizing 
adrenal hyperplasia with advanced bone age when glucocorticoid 
replacement therapy starts after 4 to 8 years.290,727 CPP has also 
been documented in children who received or were exposed to 
androgens or estrogens for long periods during early childhood for 
a variety of medical conditions. 
1131
Chapter 26  Physiology and Disorders of Pub

1131
Marfan Syndrome. Marfan syndrome may be associated with 
tall stature and early PHV and menarche compared with North 
American averages.745 
Management of Central Precocious Puberty
Table 26.31 addresses the major psychosocial and clinical goals of 
therapy for CPP. Three principal agents were used in the medical 
treatment of idiopathic or neurologic CPP: medroxyprogesterone 
acetate, cyproterone acetate, and superactive GnRH agonists.
Medr

erone acetate, and superactive GnRH agonists.
Medroxyprogesterone Acetate and Cyproterone Acetate. 
Medroxyprogesterone and cyproterone reversed or arrested the 
progression of secondary sexual characteristics but had no appar­
ent effect or only a small effect on adult height, especially in 
affected girls.622,746 Medroxyprogesterone acetate inhibits gonad­
otropin secretion by its action on the hypothalamic GnRH pulse 
generator/pituitary gonadotropin unit and has a direct suppressive 
effect 

otropin unit and has a direct suppressive 
effect on gonadal steroidogenesis through 3βHSD2. Medroxy­
progesterone acetate has glucocorticoid action and can suppress 
ACTH and cortisol secretion, increase appetite and lead to exces­
sive weight gain, and induce hypertension and a cushingoid facies 
and appearance.
Cyproterone acetate has been used outside the United States for 
the treatment of CPP with advantages and disadvantages similar 
to those of medroxyprogesterone acetate. Cyproterone ac

ose of medroxyprogesterone acetate. Cyproterone acetate has 
antiandrogenic, antigonadotropic, and progestational properties. 
Cyproterone acetate suppresses the secretion of ACTH and the 
plasma concentration of cortisol. Fatigue and weakness are com­
mon side effects, probably as a consequence of secondary adrenal 
insufficiency. This agent lacks gluconeogenic activity and does not 
appear to produce cushingoid features.
The long-term effects of these agents on fertility are not known. 
For th

f these agents on fertility are not known. 
For the treatment of CPP, medroxyprogesterone acetate and 
cyproterone acetate have now been replaced by the much more 
effective GnRH agonists; however, they may be used as backup 
agents for the occasional patients who develop untoward effects 
from GnRH agonist therapy. 
Superactive Gonadotropin-Releasing Hormone Agonists. 
The GnRH agonists, synthetic analogues of the amino acid 
sequence of the natural GnRH decapeptide, are the treatment 
of choic

ural GnRH decapeptide, are the treatment 
of choice for CPP of any cause (Tables 26.32 and 26.33). After 
initial stimulation, these pharmacologic agents suppress pulsatile 
LH and FSH release, gonadal steroid output, and gametogene­
sis, similar to the effects of continuous administration of natural 
GnRH, which suppresses gonadotropin secretion after an initial, 
brief stimulation of gonadotropin release.336,747 The agonist binds 
to the GnRH receptor on gonadotrophs, and this is followed by 

eceptor on gonadotrophs, and this is followed by 
desensitization of the gonadotroph to GnRH and downregula­
tion and loss of receptors. Desensitization persists after receptor 
levels return to normal as a result of uncoupling of the receptors 
from the intracellular signaling effector pathway. This regimen 
functions as a selective, highly specific pharmacologic clamp on 
the secretion of gonadotropins without interfering directly with 
release of the other pituitary hormones. In essence, the 

 of the other pituitary hormones. In essence, the regimen 
produces a reversible medical gonadectomy (see Table 26.32).
The superactive agonist analogues of GnRH have about 15 to 
200 times the potency of the natural GnRH decapeptide, pro­
longed action, and low toxicity (see Table 26.33). Replacing the 
glycine-amide terminus of GnRH with alkyl amines (i.e., eth­
ylamine [NEt]), as in (Pro9-NEt)GnRH; substituting certain 
d-amino acids at position 6, as in (d-Trp6)GnRH; or making bulky 
hydroph

on 6, as in (d-Trp6)GnRH; or making bulky 
hydrophobic alterations at position 6, as in (d-Nal[2]6)GnRH 
increases the potency and duration of action. These changes make 
the molecule more resistant to enzymatic degradation, increase the 
binding affinity of the analogue for the receptor on the pituitary 
gonadotroph, increase hydrophobicity, and, with some analogues, 
increase binding to plasma proteins.
The suppressive effects of the GnRH agonists on gonadotropin 
secretion make them useful in

sts on gonadotropin 
secretion make them useful in the treatment of CPP, and they are 
also used in endometriosis, prostatic carcinoma, and in delaying 
puberty during consideration of definitive treatment in transgen­
der use. Every-4-week and every-12-week formulations of leupro­
relin (leuprolide acetate) have been FDA approved for treatment 
of CPP.769 Long-term studies have established the efficacy and 
safety of the every-4-week injection, but only short-term studies 
are available for the

but only short-term studies 
are available for the every-12-week version. Triptorelin has been 
recently approved for the treatment of central precocious puberty 
in children; it can be given as an injection every 6 months.770 The 
bioavailability of agonists given intranasally is much reduced, as 
reflected in the need to use a high dose at more frequent intervals, 
and they are rarely used at present. A subcutaneous implant of 
histrelin was approved for 12-month treatment of CPP, and recent 

proved for 12-month treatment of CPP, and recent 
studies show efficacy in gonadotropin suppression for 2 years after 
implant.771 Increased predicted adult height is demonstrated 
with this treatment.772 The effectiveness of GnRH agonists in the 
  Objectives for the Management and 
Treatment of True Precocious Puberty
Detection and treatment of an expanding intracranial lesion
Arrest of premature sexual maturation until the normal age at onset of 
puberty
Regression of secondary sexual charac

 of 
puberty
Regression of secondary sexual characteristics already present
Attainment of normal mature height; suppression of the rapid rate of 
skeletal maturation
Prevention of emotional disorders and handicaps and alleviation of 
parental anxiety; promotion of understanding by counseling, early 
sex education, and acceleration of social age
Reduction of risk of sexual abuse and early sexual debut
Prevention of pregnancy in girls
Preservation of future fertility
Diminishment of the increased 

of future fertility
Diminishment of the increased risk of breast cancer associated with 
early menarche
From Grumbach MM. True or central precocious puberty. In: Krieger DT, Bardin CW, eds. 
Current Therapy in Endocrinology and Metabolism, 1985-1986. Toronto, Canada: BC Decker; 
1989:4–8.
   
TABLE 26.31
  Action of Gonadotropin-Releasing 
Hormone Agonists in True Precocious 
Puberty
A selective, highly specific pharmacologic clamp on the secretion of 
gonadotropin that produces a medical gonad

eleasing hormone; LH, luteinizing 
hormone.
   
TABLE 26.32
1132
treatment of CPP varies with the potency of the analogue, dose, 
route of administration, and compliance.
Treatment of CPP with a potent GnRH agonist results in 1 
to 3 days of increased FSH and LH release and a rise in circulat­
ing gonadal steroid levels, followed after 7 to 14 days of treat­
ment by suppression of pulsatile secretion of LH and FSH and of 
the pubertal LH response to 

of LH and FSH and of 
the pubertal LH response to the administration of native GnRH 
(Figs. 26.57 and 26.58). The isoforms of gonadotropins tend 
toward a more basic charge. A plasma estradiol concentration of 
less than 18 pmol/L (5 pg/mL) in girls or a plasma testosterone 
level of less than 0.7 nmol/L (20 ng/dL) in pediatric assays using 
HPLC-MS/MS in boys indicates adequate gonadal suppression; 
this occurs within about 2 to 4 weeks in girls and 6 weeks in boys. 
GnRH agonist therapy does n

 and 6 weeks in boys. 
GnRH agonist therapy does not affect the secretion of adrenal 
androgens or sexual hair growth.750,773
Changes in secondary sexual characteristics within the first 6 
months of therapy (Fig. 26.59) include reduction in breast size 
and decrease in pubic hair, cessation of menses if present before 
treatment, and decreased size of the uterus and ovaries as assessed 
by pelvic sonography in girls. Some girls have recurrent episodes 
of hot flushes and moodiness. In boys, pub

isodes 
of hot flushes and moodiness. In boys, pubic hair thins, the testes 
decrease in size, acne and seborrhea regress, penile erections and 
masturbation become much less frequent, the high energy level 
and aggressive behavior diminish, and self-esteem improves.
  Gonadotropin-Releasing Hormone Agonists: Pharmacologic Treatment of True Precocious Pubertya
Hormone
Potency
Formula
Dosage Form
Dose
Structure of Natural GnRH
Substitutions in GnRH Agonist Analogues
Deslorelin: d-Trp6

tions in GnRH Agonist Analogues
Deslorelin: d-Trp6, -Net
150
(d-Trp6Pro9NEt)GnRH
SQ
Depot-IM
4-8 mg/kg/day
Grumbach and Kaplan, Kaplan 
and Grumbach, Styne et al,750 
Pescovitz et al,752 Boepple 
et al,773 Comite et al (1986),421 
Comite et al (1981), Oerter
Nafarelin: d-Nal(2)6
150
[d-Nal(2)6Pro9NEt]GnRH
SQ
Intranasal
4 mg/kg/day
800-1600 mg/day
and Grumbach, Comite et al 
(1981)
Leuprolide: d-Leu6, -Net
20
(d-Leu6Pro9NEt)GnRH
SQ
Depot-IM
20-50 mg/kg/day
140-300 mg/

o9NEt)GnRH
SQ
Depot-IM
140-300 mg/kg/mo
Boepple and Crowley, Eshet et al, 
Kaplan and Grumbach (1991)b
Buserelin: d-Ser(tBu)6, -Net
20
[d-Ser(tBu)6Pro9NEt]GnRH
SQ
20-40 mg/kg/day
1200-1800 mg/day
Drop et al, Bourguignon et al, 
Holland et al, Rappaport et al, 
Suwa et al, Luder et al, Don­
aldson et al, Rime et al
Triptorelin: d-Trp6
35
(d-Trp6)GnRH
SQ
Depot-IM
60 mg/kg/mo
Kauli et al, Roger et al
Histrelin: d-His(Bzt)6, -Net
150
[d-His(Bzt)6NEt]GnRH
SQ

lin: d-His(Bzt)6, -Net
150
SQ implant
12-mo pellet
aSuperscripted numbers indicate substitution at that position of the specified amino acid; -NEt indicates replacement of the terminal glycine-amide with ethylamine.
bUnpublished data.
GnRH, Gonadotropin-releasing hormone; IM, intramuscular; SQ, subcutaneous.
Modified from Grumbach MM, Kaplan SL. Recent advances in the diagnosis and management of sexual precocity. Acta Paediatr Jpn. 1988;30:S155–S175.
Data from Boepple PA, Cr

r Jpn. 1988;30:S155–S175.
Data from Boepple PA, Crowley WFJ. Gonadotrophin-releasing hormone analogues as therapeutic probes in human growth and development: evidence from children with central precocious puberty. 
Acta Paediatr Scand Suppl. 1991;372–338; Bourguignon JP, Van Vliet G, Vandeweghe M, et al. Treatment of central precocious puberty with an intranasal analogue of GnRH (buserelin). Eur J Pediatr. 
1987;146:555–560; Comite F, Cutler GBJ, Rivier J, et al. Short-term treatment of idiopath

 Rivier J, et al. Short-term treatment of idiopathic precocious puberty with a long-acting analogue of luteinizing hormone-releasing hormone: a preliminary 
report. N Engl J Med. 1981;305:1546–1550; Donaldson MD, Stanhope R, Lee TJ, et al. Gonadotrophin responses to GnRH in precocious puberty treated with GnRH analogue. Clin Endocrinol (Oxf). 
1984;21:499–503; Drop SL, Odink RJ, Rouwe C, et al. The effect of treatment with an LH-RH agonist (buserelin) on gonadal activity growth and bone maturati

elin) on gonadal activity growth and bone maturation in children with central precocious 
puberty. Eur J Pediatr. 1987;146:272–278; Eshet R, Duz Z, Silbergeld A, et al. Erythrocytes from patients with low concentrations of IGF1 have an increase in receptor sites of IGF1. Acta Endocrinol. 
1991;125:354–358; Grumbach MM, Kaplan SL. Recent advances in the diagnosis and management of sexual precocity. Acta Paediatr Jpn. 1988;30:S155–S175; Holland FJ, Fishman L, Costigan DC, 
et al. Pharmacokinetic c

 Fishman L, Costigan DC, 
et al. Pharmacokinetic characteristics of the gonadotropin-releasing hormone analog D-Ser (tBU) 6Pro9NEt luteinizing hormone-releasing hormone (buserelin) after subcutaneous and intranasal adminis­
tration in children with central precocious puberty. J Clin Endocrinol Metab. 1986;63:1065–1070; Kaplan SL, Grumbach MM. True precocious puberty: treatment with GnRH-agonists. In: Delemarre-Van 
de Waal H, Plant TM, van Rees GP, et al, eds. Control of the Onset of Puberty. Am

P, et al, eds. Control of the Onset of Puberty. Amsterdam, The Netherlands: Elsevier; 1989:357–373; Kauli R, Pertzelan A, Ben-Zeev A, et al. Treatment of precocious 
puberty with LHRH analogue in combination with cyproterone acetate: further experience. Clin Edocrinol (Oxf). 1984;20:377–387; Luder AS, Holland FJ, Costigan DC, et al. Intranasal and subcutaneous 
treatment of central precocious puberty in both sexes with a long-acting analog of luteinizing hormone-releasing hormone. J Clin Endocri

einizing hormone-releasing hormone. J Clin Endocrinol Metab. 1984;58:966–972; Oerter KE, Manasco P, Barnes 
KM, et al. Adult height in precocious puberty after long-term treatment with deslorelin. J Clin Endocrinol Metab. 1991;73:11235–11240; Rappaport R, Fontoura M, Brauner R. Treatment of central pre­
cocious puberty with an LHRH agonist (buserelin): effect on growth and bone maturation after three years of treatment. Horm Res. 1987;28:149–154; Rime JL, Zumsteg U, Blumberg A, et al. Long-term 

 Rime JL, Zumsteg U, Blumberg A, et al. Long-term 
treatment of central precocious puberty with an intranasal HLRH analogue: control of pituitary function by urinary gonadotrophins. Eur J Pediatr. 1988;147:263–269; Roger M, Chaussain JL, Berlier P, 
et al. Long term treatment of male and female precocious puberty by periodic administration of a long-acting preparation of D-Trp6-luteinizing hormone-releasing hormone microcapsules. J Clin Endo­
crinol Metab. 1986;62:670–677; Suwa S, Hibi I, Kato K

nol Metab. 1986;62:670–677; Suwa S, Hibi I, Kato K, et al. LH-RH agonistic analog (buserelin) treatment of precocious puberty: collaborative study in Japan. Acta Paediatr Jpn. 1988;30:S176–S184.
   
GnRH (potency 1): <Glu-His-Pro-Ser-Trp-Gly-Leu-Arg-Pro-Gly-NH2>
 1 
2 
3 
4 
5 
6 
7 
8 
9 
10
TABLE 26.33
1133
Height velocity decreases by about 60% during the first year 
of therapy, with greater decreases found in those with the most 
advanced bone 

eases found in those with the most 
advanced bone age and taller relative height.774 Skeletal matu­
ration slows dramatically during the first 3 years, to a rate that 
often is less than the progression in chronologic age. From the 
second year on, height velocity for bone age is usually appropri­
ate (Fig. 26.60). Bone age is suggested to represent a surrogate 
for growth plate senescence due to prior exposure to estrogens. 
Those with the longest courses before treatment commences, the 
most a

st courses before treatment commences, the 
most advanced physical findings, and the most rapid bone age 
advancement before therapy have the lowest growth velocities on 
The rate of growth during treatment with GnRH agonists is 
inversely related to the bone age at the start of therapy308; best 
results occur when treatment is begun soon after the onset of pre­
cocity and when the bone age is advanced by only a few years.775 
There is a striking benefit for those children treated bef

 a striking benefit for those children treated before 5 years 
of age (girls’ height, 164.3 ± 7.7 cm) compared with those treated 
after age 5 years (157.6 ± 6.6 cm) or with untreated patients 
(152.7 ± 8.6 cm).722 Adult height in children treated with GnRH 
agonists is improved, especially when therapy starts before 6 years 
of age rather than after 8 years of age470,724,776 (Table 26.34). 
The literature on treatment in children with onset of puberty 
after 6 years of age varies but generally 

uberty 
after 6 years of age varies but generally shows no proven efficacy 
in increasing adult height.777 An adult height within the target 
height occurs with therapy in about 90% of girls and boys.778 
We recommend treatment of all affected children with onset of 
puberty before 6 years of age to ensure an optimal prognosis for 
adult height. The optimal age at which to discontinue therapy is 
undetermined because the posttreatment growth spurt is impor­
tant in determining adult height.470
T

t is impor­
The addition of hGH treatment to the GnRH regimen is a 
consideration when growth velocity is reduced sufficiently over a 
6-month period to compromise predicted final height but must 
be considered experimental.779 This is in contrast to the concern 
raised regarding therapy with GnRH agonist and GH in nor­
mal variant short stature, which is not proved by evidence-based 
study.470
In a preliminary study, the GnRH antagonist, cetrorelix, 
appeare

y study, the GnRH antagonist, cetrorelix, 
appeared to bring about more rapid suppression of gonadotropin 
secretion and to eliminate the flare-up of gonadotropin secretion 
after administration of GnRH agonist; it is used in assisted repro­
ductive treatment.777 There are no reports at present concerning 
the use of cetrorelix as a sole treatment of precocious puberty, but 
in one unusual case, a girl with purported gonadotropin-indepen­
dent precocious puberty responded to this treatment, brin

ecocious puberty responded to this treatment, bringing up 
a possible direct effect of the agent on ovarian function.780 This 
agent suppresses ovarian but not adrenal function and was pro­
posed to be useful as a test to differentiate the origin of severe 
hyperandrogenism in adolescence.
Girls with CPP have a tendency toward obesity that is unre­
lated to treatment with GnRH agonist.470 Serum leptin values in 
patients with precocious puberty remain in line with the average 
for children of si

emain in line with the average 
for children of similar BMI and pubertal development.781 There 
is conflicting data on weight gain while on treatment with GnRH 
agonists. A longitudinal study 101 girls, 23% obese at start of 
therapy, suggested that, with adequate gonadotropin suppression, 
BMI for age may improve after at least 2 years of therapy.782 A 
study of 117 girls showed the increased BMI during treatment 
often reverted to normal after cessation of treatment.783 However, 
a longitudina

cessation of treatment.783 However, 
a longitudinal study of 333 girls, some to adult height, demon­
strated increasing BMI SDs with treatment continuing to adult­
hood.784 These conflicting findings make it difficult to predict the 
effect of GnRH agonist on BMI in individuals.
The IGF1 concentration in CPP correlates best with the stage 
of puberty and the plasma concentration of testosterone or estra­
diol. Treatment with GnRH agonist reduces the level of IGF1 to 
the normal range for bone ag

the level of IGF1 to 
the normal range for bone age but not for chronologic age.316 
Gonadal steroids increase plasma IGF1 concentrations in CPP, as 
in normal puberty. Secretion of GH is increased in CPP to a level 
comparable to that observed in normal puberty. Treatment with 
GnRH agonist usually results in decreased secretion of GH, most 
Pulsatile LH secretion
Before therapy
6 wks of therapy
Plasma LH
(LER 960)
(ng/mL)
Plasma LH
(ng/mL)
Estradiol
Plasma
estradiol
(pg/mL)
LH response to LHRH

adiol
Plasma
estradiol
(pg/mL)
LHRH
LHRH
LHRH
Before
LHRH
agonist
1 wk of
therapy
Before
therapy
1 wk of
therapy
M. Mey.
C. Age: 5 5/12Y
B. Age: 13Y
0
A
B
C
8
4
0
200
100
1000 1400
2200 0200
1000 1400
2400 0400
Hour
10
30
20
5
0
0
4
2
1
0
4
2
1
0
4
2
1
• Fig. 26.57  Effect of administration of the gonadotropin-releasing hor­
mone (GnRH) agonist deslorelin (4 μg/kg/day subcutaneously) on pulsatile 
secretion of luteinizing hormone (LH) (A), LH

ile 
secretion of luteinizing hormone (LH) (A), LH response to GnRH (B), and 
plasma concentration of estradiol (C) in a 5-year, 1-month-old girl with 
idiopathic true precocious puberty. This patient, who had a bone age 
of 13 years when treatment was begun, has been administered deslo­
relin for 7 years. During this period, the estimated predicted final height 
increased by 15 cm. Surprisingly, the bone age advanced by only about 
6 months on serial examinations for several years. LHRH, LH-rel

erial examinations for several years. LHRH, LH-releasing 
hormone. (Modified from Grumbach MM, Kaplan SL. Recent advances 
in the diagnosis and management of sexual precocity. Acta Paediatr Jpn. 
1988;30:S155–S175.)
1134
strikingly during sleep, and in a decreased GH response to pro­
vocative stimuli.
The depot formulations of GnRH agonists provide continuous 
exposure to the drug with a single intramuscular injection every 
4 to 26 weeks (as describ

uscular injection every 
4 to 26 weeks (as described earlier) and minimize the problem 
of compliance. However, irregular or inadequate treatment or 
poor compliance results in persistent or intermittent increase in 
the concentration of plasma gonadal steroids, leading to decreased 
growth but advancing bone age.
Regular assessment is essential, initially at intervals of 3 to 6 
months with the use of the injectable formulations, and should 
include periodic determinations of plasma testosteron

lude periodic determinations of plasma testosterone levels 
in boys and estradiol levels in girls by HPLC-MS/MS; change 
in basal concentrations of LH and FSH as measured by third-
generation assays with pediatric standards or in the LH and FSH 
response to exogenous GnRH or GnRH agonists; growth, bone 
age, and secondary sexual characteristics; and, in girls, serial evalu­
ations of ovarian morphologic appearance and uterine size by pel­
vic sonography. A decrease in the size of the ovaries and

ography. A decrease in the size of the ovaries and uterus 
on pelvic sonography occurs with successful GnRH agonist treat­
ment.750 Because native GnRH is not available, the rise in serum 
LH and FSH is evaluated 60 or 240 minutes after administra­
tion of the GnRH agonist.257–259 The LH and FSH responses to 
GnRH agonist are suppressed with effective therapy, but because 
standards differ among laboratories, actual cutoff values may dif­
fer in clinical practice, depending on location. A 1-year

clinical practice, depending on location. A 1-year Silastic 
implant of the histrelin GnRH agonist, although it involves a sur­
gical procedure, eliminates the need to titrate the dose and repeat 
testing for maximal effect; a second treatment year is reported to 
be effective.771 There may be low pubertal values of LH even with 
successful suppression of advancement of puberty.785
Urinary excretion of LH correlates with the stage of puber­
tal development in normal subjects and is elevated in C

evelopment in normal subjects and is elevated in CPP; these 
methods are proposed as a noninvasive manner to monitor or 
diagnose CPP.786
When treatment is discontinued, even after 8 years, gonadal 
suppression is reversed within a few weeks to months, as mani­
fested by a rise in the concentration of plasma gonadal steroids, 
progression of sexual maturation, and return of menses. Menarche 
occurred at an average of 1.2 to 1.5 years after discontinuation of 
therapy (range, 0–60 months). Ovulat

tinuation of 
therapy (range, 0–60 months). Ovulation had occurred in 50% 
of girls by 1 year after menarche and in 90% of those studied 
2 or more years after menarche, and pregnancies are reported in 
GnRHA-treated girls.735,787 Mean ovarian volume was found to 
remain greater than in normal subjects, and the LH response to 
GnRH was less than the normal response.
0
10
LH
LH
Females (n = 11)
Males (n = 6)
LH or FSH
response to LHRH
Peak
Maximum
increment
ng/mL
ng/mL
Estradiol
0
10
8
8
6
6
4
4

increment
ng/mL
ng/mL
Estradiol
0
10
8
8
6
6
4
4
2
2
0
FSH
FSH
LH or FSH
Peak
Maximum
increment
ng/mL
ng/mL
0
6
6
4
4
2
2
Maximal
unstimulated
estradiol and
0
Weeks of GnRH-A treatment
1
2
6
12
0
1
2
6
12
0
pg/mL
ng/dL
0
400
75
50
200
25
• Fig. 26.58  Deslorelin treatment (4 μg/kg/day subcutaneously) of girls and boys with true precocious 
puberty. Effects were seen during the first 12 weeks of treatment on the luteinizing hormone (LH) and 
follicle-st

t on the luteinizing hormone (LH) and 
follicle-stimulating hormone (FSH) response to a challenge with gonadotropin-releasing hormone (GnRH) 
(mean peak response and maximum increment) and on the maximal unstimulated concentration of plasma 
estradiol in the girls and of plasma testosterone in the boys. Notice the relatively rapid change from puber­
tal values to prepubertal values. For conversion to SI units, see Figs. 26.19 and 26.20. LHRH, LH-releasing 
hormone. (From Styne DM, Harris DA, Egl

releasing 
hormone. (From Styne DM, Harris DA, Egli CA, et al. Treatment of true precocious puberty with a potent 
luteinizing hormone releasing factor agonist: effect on growth, sexual maturation, pelvic sonography, and 
the hypothalamic pituitary gonadal axis. J Clin Endocrinol Metab. 1985;61:142–181. Copyright by The 
1135
The reversible nature of the therapy has also been confirmed 
in boys with CPP, because gonadotropins in

firmed 
in boys with CPP, because gonadotropins in the basal or GnRH-
stimulated state return to normal pubertal values by 1 year after 
cessation of therapy.788 Testicular size may take longer to reach 
normal values.
A report of 46 women studied 12.5 years after cessation of 
treatment with GnRH agonist revealed an adult height 1.6 cm, 
or 0.3 SD, below target height, with no evidence of reproduc­
tive impairment or apparent PCOS or hirsutism.789 However, 
an increased prevalence of PCOS was r

89 However, 
an increased prevalence of PCOS was reported in young women 
(mean age, 18.1 years) with a history of CPP, with onset at a 
mean age of 7.65 years.790 Of the patients, 32% had PCOS 
according to the Rotterdam definition, and 30% had PCOS 
according to the Androgen Excess Society; the most common 
presentation was clinical or biochemical hyperandrogenism, 
or both, and polycystic ovarian morphologic appearance. The 
study authors did not find any other predictive factors for the 
dev

not find any other predictive factors for the 
development of PCOS when the diagnosis of CPP was made. 
These girls would not have qualified for the diagnosis of CPP 
based on the newest criteria, so it is of concern that these girls 
with early rather than precocious puberty had such a high preva­
lence of PCOS. GnRH agonist treatment may actually decrease 
the incidence of PCOS compared to the incidence in untreated 
patients.735 Thus the prevalence of PCOS after Lupron for CPP 
is unclear.777

lence of PCOS after Lupron for CPP 
An international consensus conference convened by the Law­
son Wilkins Pediatric Endocrine Society and the European Soci­
ety for Pediatric Endocrinology reviewed the world literature on 
the use of GnRH to determine, in an evidence-based approach, 
the appropriate use of the agent in precocious puberty. However, 
few controlled prospective studies have been performed with 
GnRH analogues in children, so many of the conclusions relied 
• Fig. 26

dren, so many of the conclusions relied 
• Fig. 26.59  A girl aged 2 years and 5 months with true precocious puberty 
after 6 weeks of deslorelin therapy (4 μg/day subcutaneously). Notice the 
regression in the size of the breasts; however, the rapid rate of growth had 
not decreased. At the end of 1 year of therapy, the growth rate was sup­
pressed to 4 cm/year, and bone age advanced only 1 year. BA, bone age; 
CA, chronologic age; HT, height; WT, weight. (From Styne DM, Grumbach 
MM. Puberty i

T, weight. (From Styne DM, Grumbach 
MM. Puberty in the male and female: its physiology and disorders. In: Yen 
SCC, Jaffe RB, eds. Reproductive Endocrinology. 2nd ed. Philadelphia: 
Height velocity (cm/y)
Height SD for bone age
Years on therapy
D-TRP6-PRO9-NEt-GnRH
Nafarelin acetate
D-TRP6-PRO9-NEt-LHRH
12
10
8
6
4
2
0
1
–3
–2
–1
0
0
0.5
1
2
3
4
5
6
1
0
2
3
4
5
6
7
8
A
B
• Fig. 26.60  Effect of gonadotropin-releasing hormone (GnRH) 

  Effect of gonadotropin-releasing hormone (GnRH) agonist therapy in true precocious puberty 
on growth. (A) Changes in mean height velocity (cm/year ± 1 SE [standard error]) after the initiation of 
GnRH agonist therapy with d-Trp6Pro9Net (deslorelin) (darker bars) or with nafarelin (lighter bars). A sharp 
decrease in height velocity occurred within 1 year. (B) Mean (±1 SE) height for bone age before and during 
GnRH agonist treatment. The discrepancy between height and the more advanced bone 

repancy between height and the more advanced bone age decreases 
(reverts to normal) with chronic GnRH agonist treatment. SD, standard deviation. (From Kaplan SL, Grum­
bach MM. True precocious puberty: treatment with GnRH agonists. In: Delemarre-Van de Waal H, Plant 
TM, van Rees GP, et al, eds. Control of the Onset of Puberty. 3rd ed. Amsterdam, The Netherlands: 
Elsevier; 1989:357–373.)
1136
in part on collective expert opinion. The major conclusi

The use of GnRH analogues for conditions other than CPP, such 
as to increase adult height in children with idiopathic short stature 
or SGA, or with GH treatment in children, is not recommended.
Affected girls do not appear to manifest increased stress with 
this condition, although mothers do in preliminary studies.791 
Although psychosocial factors and parental anxiety that adversely 
affect the well-being of the child need to be assessed in the deci­
sion to initiate GnRH agonist treatment, 

he deci­
sion to initiate GnRH agonist treatment, this therapy cannot be 
routinely recommended for such concerns (Table 26.35).
Adverse Effects. Rare reactions to GnRH agonists include local 
and systemic allergic reactions, including asthmatic episodes 
when the agent is given intranasally. Rare cases of anaphylaxis 
occur in response to gonadotropin-releasing hormone ago­
nist.792 The prevalence of a sterile abscess at the site of intramus­
cular injection of long-acting repository preparatio

lar injection of long-acting repository preparations, including 
  Effect of GnRH Agonist Treatment on Adult or Near-Adult Height and Height Gain
Chronologic Age at Start 
of Therapy
MEAN CURRENT HEIGHT (CM)
Mean Height Gain (cm)a
Female
Male
Untreatedb
Total
116
152.7 ± 8.6
155.6 ± 7.7
<5 yr
41
150.2 ± 7.6
153.3 ± 7.1
>5 yr
75
153.4 ± 8.4
161.3 ± 6.0
GnRH-Treatedc
UCSF
26
160.5 ± 6.6
166.3 ± 12.2
<5 yr
11
164.3 ± 7.7
172.1
10.0 F; 11.1 M
>5 yr
15
157.6 ± 6.6
163.3 ± 13.0
4.0 F; 6.

11.1 M
>5 yr
15
157.6 ± 6.6
4.0 F; 6.0 M
Reference Untreated Study
Oerter et al
40
157.8 ± 5.9
168.8 ± 8.3
5.2 F; 6.7 M
Kauli et al
8
151.2 ± 5.9
5.8 F
Boepple and Crowley
26
154.4
4.1 F
aFinal predicted height minus initial predicted height (Bayley-Pinneau method).
bFinal height.
cFinal or near-final height.
F, Female; GnRH, gonadotropin-releasing hormone; M, male; UCSF, University of California, San Francisco.
From Paul D, Conte FA, Grumbach MM, et al. Long-term effect of gonadotr

, Grumbach MM, et al. Long-term effect of gonadotropin-releasing hormone agonist therapy on final and near-final height in 26 children with true precocious puberty treated at a 
Data from Boepple PA, Crowley WFJ. Gonadotrophin-releasing hormone analogues as therapeutic probes in human growth and development: evidence from children with central precocious puberty. Acta 
Paediatr Scand Suppl. 1991;372–338; Kauli R, Pertzela

diatr Scand Suppl. 1991;372–338; Kauli R, Pertzelan A, Ben-Zeev A, et al. Treatment of precocious puberty with LHRH analogue in combination with cyproterone acetate: further experience. Clin Endocrinol 
(Oxf). 1984;20:377–387; Oerter KE, Manasco PK, Barnes KM, et al. Effects of luteinizing hormone-releasing hormone agonists on final height in luteinizing hormone-releasing hormone-dependent precocious 
puberty. Acta Paediatr Suppl. 1993;388:62–68; discussion 69.
   
TABLE 26.34
  Indications for

ld’s psychosocial well-being is adversely affected
In children with neurogenic or organic true precocious puberty, espe­
cially those with associated GH deficiency, the course is almost 
invariably progressive and LHRH treatment should not be delayed.
GH, Growth hormone; LHRH, luteinizing hormone–releasing hormone; SD, standard deviation.
   
TABLE 26.35
1137
leuprorelin and triptorelin, is clearly increased (5–10%); these 
reactions are unpredicta

increased (5–10%); these 
reactions are unpredictable and intermittent, and in most 
instances are related to the polylactic and polyglycolic polymer 
and not to the GnRH agonist itself. Switching to daily subcuta­
neous injections of nondepot preparations or to intranasal prep­
arations is rarely associated with a recurrence. A small increase 
in serum prolactin above normal limits was described in girls 
after treatment with GnRH agonist, but galactorrhea was not 
observed. Volumetric BMD and 

alactorrhea was not 
observed. Volumetric BMD and peak bone mass are normal dur­
ing and after discontinuation of GnRH therapy. Calcium and 
vitamin D intake must be ensured during treatment to achieve 
optimal skeletal health. However, high fruit and vegetable intake 
(defined as >3 servings per day, lower than the US government 
recommended intake of 3 vegetables and 2 fruit in younger chil­
dren) is desirable in all children and may serve in early puberty as 
a factor to increase bone density

arly puberty as 
a factor to increase bone density, possibly because of decreasing 
calcium excretion in the urine.793
Four patients were reported to have developed slipped capi­
tal femoral epiphyses during or just after treatment of CPP with 
GnRH agonist.794 Slipped capital epiphyses occur mostly dur­
ing the earliest phase of puberty, when growth is beginning to 
increase, and not after fusion of the triradiate cartilage, so these 
cases may have a different etiologic course than that found 

have a different etiologic course than that found in aver­
age pubertal children.
There is a concern over maintenance of bone density when sex 
steroids are reduced with the use of GnRH agonists. There is evi­
dence, however, that adolescents previously treated with gonad­
otropin-releasing hormone agonist for precocious puberty have 
normal bone density.795
Recently, reports of young women complaining of various 
maladies who were previously treated for precocious puberty 
with GNRH agonist app

ated for precocious puberty 
with GNRH agonist appeared. They speak of pain in the jaw, 
fibromyalgia, degenerative disc symptoms, thinning bones and 
depression, or other psychologic symptoms. These have generally 
appeared on lay websites but the FDA is gathering such reports 
for ongoing surveillance to determine whether these are long-term 
effects related to Lupron well after treatment.796 
Other Treatment for Precocious Puberty. The GnRH ago­
nists are useful in conjunction with GH in the 

o­
nists are useful in conjunction with GH in the management 
of organic or neurogenic CPP with associated GH deficiency 
(usually as a result of irradiation of the brain). However, the 
use of GnRh agonists has been advocated even in the absence 
of precocious puberty to allow a longer period of GH treatment 
before epiphyseal fusion. A few, usually short-term studies have 
evaluated the combination, with variable results. This regimen 
is experimental, and its cost effectiveness needs to be co

imental, and its cost effectiveness needs to be considered 
(see earlier discussion).470
Aromatase inhibitors (e.g., letrozole) decrease or eliminate 
the effect of estrogen on bone age advancement. They have been 
useful to increase height prognosis in familial sexual precocity in 
boys (see upcoming discussion).787 A study of Greek girls with 
decreased predicted adult height due to early puberty demon­
strated that the combination of an aromatase inhibitor with a 
gonadotropin-releasing hormo

ase inhibitor with a 
gonadotropin-releasing hormone agonist will increase predicted 
adult height.797 Long-term controlled studies are necessary to 
establish the safety and efficacy and, especially, the effects on adult 
bone density, which remains a worrisome subject in the use of 
aromatase inhibitors in children475 (Table 26.36). 
Psychosocial Aspects. Psychologic management is a critical 
aspect of the care of children with CPP. With the advanced physi­
cal maturation for chronologic age, 

vanced physi­
cal maturation for chronologic age, these children tend to seek 
friends closer to their size, strength, and physical development. 
Difficulties may arise because they lack the social skills of older 
children. Sex education of the child and the family is essential 
and must be given in a skillful, sensitive, and explicit manner; the 
risks of sexual abuse (in both sexes) and of pregnancy need to be 
discussed.798
The parents need to be informed about the management of 
menses. The

o be informed about the management of 
menses. The onset of sexual activity may be earlier than average 
but usually remains within the normal range. Menarche before 
or at 11 years increases the risk of higher delinquency rates and 
stronger negative peer influences.799
It is imperative to provide support in handling the increased 
height, the advanced sexual maturation, and the effects of gonadal 
steroids on behavior, activity, and emotional stability. The unre­
alistic demands and expectatio

tability. The unre­
alistic demands and expectations that arise from the discrepancy 
between the child’s physique and his or her chronologic, mental, 
and psychosexual age require wise counseling, as do the reactions 
to ridicule by peers and the concern about being different from 
age-mates. Some of these problems have been mitigated by school 
acceleration, in which the child is advanced by one or two grades, 
if this is consistent with the mental and emotional development. 
These comments ar

ntal and emotional development. 
These comments are applicable to children with all forms of sexual 
precocity; the effectiveness of GnRH agonists has reduced but not 
eliminated many of these issues in CPP.
The psychological evaluation of the effects of precocious 
puberty children must take into account their family situation and 
other social stressors lest these factors create effects on the child 
and are incorrectly attributed to precocious puberty.800 A series of 
10 girls suggested an in

 puberty.800 A series of 
10 girls suggested an increased level of stress before treatment for 
central precocious puberty with a diminution of the stress with 
appropriate treatment.801 However, one detailed study of 15 girls 
found remarkable similarity between girls with precocious puberty 
and controls, except that those with precocious puberty had some 
tendency to increased emotional lability.802 
  Potential Use of Aromatase Inhibitors 
or Estrogen Receptor Antagonists 
in Disorders of G

atr Endocrinol Metab. 2000;13:S1439–S1455.
   
TABLE 26.36
1138
Peripheral Precocious Puberty or Incomplete Isosexual 
Precocity: Gonadotropin-Releasing Hormone–Indepen­
dent Sexual Precocity995
In incomplete or peripheral forms of precocious puberty (IPP), 
such as GISP, the secretion of testosterone in boys and estrogen in 
girls is independent of the hypothalamic GnRH pulse generator 
(see Table 26.26). There is no pubertal LH response to GnRH or 

.26). There is no pubertal LH response to GnRH or 
GnRHA, and there is no pubertal pattern of pulsatile LH secre­
tion. Patients do not respond to chronic GnRH agonist therapy 
with suppression of gonadal steroid output. ISP is a consequence 
of gonadal or adrenal steroid secretion that is independent of 
GnRH, iatrogenic exposure to gonadal steroids, or, in boys, rare 
hCG-secreting or LH-secreting tumors. It can be considered a 
paraneoplastic syndrome caused by tumor production of hor­
mones 

syndrome caused by tumor production of hor­
mones or a pathologic alteration of gonadotropin secretion.803 
Measures of tumor markers such as hCG or AFP may be helpful 
in the evaluation of peripheral precocious puberty but not of CPP.
Boys
Chorionic Gonadotropin–Secreting Tumors. Several types of 
germ cell tumors secrete hCG, which may cross-react in some 
polyclonal LH assays (although they are rarely used currently) and 
will lead to a positive pregnancy screen. Boys with hCG-secreting 
neop

ve pregnancy screen. Boys with hCG-secreting 
neoplasms have slightly enlarged testes (although not to a size con­
sistent with the size of the phallus and other male secondary sex 
characteristics, because the seminiferous tubules are not affected), 
and it may be difficult to differentiate these patients from boys in 
the early stages of CPP on the basis of physical examination alone. 
However, plasma hCG levels are elevated without an increase in 
the concentration of FSH or LH measured in sp

 in 
the concentration of FSH or LH measured in specific assays and 
will lead to a positive pregnancy screen.
Boys with hepatoma or hepatoblastoma present with hepato­
megaly or firm, irregular liver nodules, anemia, and precocious 
puberty804; these are among the most serious of hCG-secreting 
tumors (Fig. 26.61). hCG is localized to multinucleated tumor 
giant cells; α-fetoprotein was found in the embryonal-type tumor 
cells of the hepatoblastoma in one case. The mean age at onset is 2 
years

oma in one case. The mean age at onset is 2 
years, 8 months, but average survival time was only 10.7 months 
after diagnosis in previous series, but newer chemotherapy can 
lead to longer survival.805
Infantile choriocarcinoma is also associated with elevated 
hCG and is thought to originate in the placenta; infants may 
be diagnosed at 1 month of age, with a survival time of only 3 
months.806 About 20% of mediastinal germ cell tumors occur in 
boys with 47,XXY or mosaic Klinefelter syndrome, 

boys with 47,XXY or mosaic Klinefelter syndrome, a prevalence 
30 to 50 times more common than in unaffected boys. Plasma 
α-fetoprotein is a useful additional marker for yolk sac (endoder­
mal sinus) or mixed germ cell tumors; the cells in the tumor that 
secrete α-fetoprotein appear to differ from those that secrete hCG. 
Rarely, the germ cells contain enough aromatase activity to convert 
A
B
• Fig. 26.61  A boy aged 1 year and 5 months with a human chorionic gonadotropin (hCG)–secreting hep

a human chorionic gonadotropin (hCG)–secreting hepa­
toblastoma. Notice the outline of the large liver (A) and the penile enlargement (B). The testes were 2 × 1 cm, 
and pubic hair was stage 2. The plasma hCG level was 50 mIU/mL; the plasma testosterone level was 168 
ng/dL; and the plasma α-fetoprotein level was 160,000 ng/mL. Metastatic lesions in both lungs were seen 
on the chest radiograph. To convert testosterone values to SI units, see Fig. 26.19. To convert hCG values 
to international u

. 26.19. To convert hCG values 
to international units per liter, multiply by 1.0. To convert α-fetoprotein values to micrograms per liter, multiply 
by 1.0. (From Kaplan SL, Grumbach MM. Pathogenesis of sexual precocity. In: Grumbach MM, Sizonenko 
PC, Aubert ML, eds. Control of the Onset of Puberty. Baltimore, MD: Williams & Wilkins; 1990:620–660.)
1139
circulating C19 precursors (of adrenal origin after adrenarche) to 
estradiol, which in some i

n after adrenarche) to 
estradiol, which in some instances is sufficient to induce breast 
development.807
Some teratomas, chorioepitheliomas, and mixed germ cell 
tumors in the hypothalamic region (or in the mediastinum, 
lungs, gonads, or retroperitoneum); certain pineal tumors (usu­
ally a germ cell tumor or mixed germ cell tumor); and, less com­
monly, a chorioepithelioma or its variants cause sexual precocity 
in boys by secreting hCG rather than by activating the pituitary 
gonadotropin-go

 than by activating the pituitary 
gonadotropin-gonadal axis via the hypothalamic GnRH pulse 
generator.
Intracranial germ cell tumors account for 3% to 11% of malig­
nant CNS tumors in children and adolescents, with a predomi­
nance in the Far East. Germ cell tumors of the hypothalamus or 
pineal region constitute fewer than 1% of primary CNS tumors in 
Western countries but account for 4.5% of such tumors in Japan. 
The prevalence of intracranial germ cell tumors is 2.6 times greater 
in males

al germ cell tumors is 2.6 times greater 
in males than in females, but germ cell tumors in the suprasellar-
hypothalamic region do not exhibit a sex predominance and are 
generally associated with pituitary hormone deficiencies, includ­
ing diabetes insipidus and delayed puberty.481 Germ cell tumors 
do not cause gonadotropin-induced ISP in females because of the 
paucity of effects of hCG in prepubertal females. However, CPP 
may occur through disinhibition of the hypothalamic GnRH 
pulse gene

disinhibition of the hypothalamic GnRH 
pulse generator by local mass effects.808 Calcification of the pineal 
is found in 8% to 11% of children age 8 to 11 years, and by itself 
is not indicative of a germ cell tumor.
Germ cell tumors that secrete hCG are rarely located in the 
thalamus and basal ganglia. In “true” pure CNS germ cell tumors 
(germinomas), hCG cannot be readily detected in the circulation 
but may be detected in the cerebrospinal fluid.481 In mixed germ 
cell tumors, hCG is comm

 fluid.481 In mixed germ 
cell tumors, hCG is commonly found in the blood as well as in 
cerebrospinal fluid. Extremely elevated levels of hCG in a CNS 
tumor suggest a primary intracranial choriocarcinoma or germ cell 
tumor with a high risk of tumor hemorrhage during biopsy, and 
surgical removal or debulking rather than diagnostic biopsy may 
be the initial operative approach.809
Mixed germ cell tumors and especially pure germinomas are 
radiosensitive, and if the bone age is less than 11 yea

sensitive, and if the bone age is less than 11 years, sexual 
precocity may regress, only to progress later into normal puberty. 
Long-term survival is reported in 88% of patients with CNS germ 
cell tumors after appropriate therapy. However, testicular germ 
cell tumors are occasionally found years after successful therapy 
for CNS germ cell tumors, so long-term surveillance is always 
indicated.606
Pineal cysts are a rare cause of CPP.810 All pituitary adenomas, 
including gonadotropin-secreti

ituitary adenomas, 
including gonadotropin-secreting pituitary adenomas, are exceed­
ingly rare in children. An LH-secreting pituitary adenoma (basal 
serum LH of 900 IU/L with no rise after GnRH) and a prolactin-
secreting pituitary adenoma (215 μg/L) caused sexual precocity 
in two boys with serum testosterone levels of 7 nmol/L (200 ng/
dL).811 Prepubertal values returned after removal of these chromo­
phobe adenomas with suprasellar extension.
Precocious Androgen Secretion Caused by the Adre

.
Precocious Androgen Secretion Caused by the Adrenal Gland
Virilizing Congenital Adrenal Hyperplasia. Virilizing CAH 
caused by a defect in 21-hydroxylation (CYP21 deficiency) leads 
to elevated androgen concentrations and masculinization and 
is a common cause of GISP in boys.812 Approximately 75% of 
patients with CYP21 deficiency have salt loss resulting from im­
paired aldosterone secretion as well as low serum sodium and 
high serum potassium concentrations. Increased plasma concen­
tratio

um concentrations. Increased plasma concen­
trations of 17-hydroxyprogesterone, increased levels of urinary 
17-ketosteroids and pregnanetriol, and advanced bone age and 
rapid growth are characteristic. Recent discovery of alternative 
steroidogenic pathways toward the production of virilizing an­
drogens in this condition, the backdoor pathway, holds prom­
ise for new methods of evaluation of optimal treatment regi­
mens.813,814
Treatment with glucocorticoids suppresses the abnormal 
androgen 

glucocorticoids suppresses the abnormal 
androgen secretion and arrests virilization; treatment with min­
eralocorticoids, when necessary, corrects the electrolyte imbal­
ance. Virilizing CAH accompanied by hypertension occurs in 
11β-hydroxylase deficiency (CYP11B1 deficiency); the progres­
sive virilization ceases and the blood pressure falls to normal with 
glucocorticoid therapy. All forms of CAH are inherited as autoso­
mal recessive traits. Untreated virilizing CAH causes anovulatory 
amen

 Untreated virilizing CAH causes anovulatory 
amenorrhea in females and oligospermia in males; these condi­
tions are reversible with treatment. Delayed treatment of virilizing 
CAH may reveal GnRH-dependent CPP (secondary CPP) as a 
consequence of the advanced somatic and hypothalamic matu­
ration resulting from long exposure to adrenal androgen. Fur­
ther difficulty is presented in the treatment of CAH during the 
pubertal years, when androgen secretion normally increases, and 
increased clear

secretion normally increases, and 
increased clearance of glucocorticoids at puberty in girls may alter 
dosing requirements.
Adrenal rests, or heterotopic adrenal tissue in the testes, 
are found in over 90% of boys with CAH. They may enlarge 
(sometimes to massive size) and may mimic bilateral or unilat­
eral interstitial cell tumors. The size of these testicular adrenal 
rest tissues (TART) was thought to be related to poor control 
of CAH during childhood and adolescence, but recent evidence

ing childhood and adolescence, but recent evidence 
shows no relationship between biochemical control or plasma 
ACTH concentration and growth patterns, raising the likeli­
hood that other control mechanisms perhaps originating in fetal 
life determine the size of these tumors.815 MRI sonography, 
including Doppler flow studies of the testes, is useful to define 
the extent and nature of the testicular masses. These tumors can 
significantly decrease fertility, but surgical management, includ­
i

ease fertility, but surgical management, includ­
ing enucleation of the tumor(s), has been useful to prevent fur­
ther damage to the testes and improve the potential for fertility 
in some but not all studies.
As with most chronic diseases originating in childhood, transi­
tion of care to adult providers during late teenage years is essential. 
Comprehensive care clinics are suggested to offer the best method 
to provide care to children and adults with CAH.816 
Virilizing Adrenal Tumor. Viriliz

ts with CAH.816 
Virilizing Adrenal Tumor. Virilizing adrenal carcinomas or ad­
enomas secrete large amounts of DHEA and DHEAS and, on 
occasion, testosterone. Glucocorticoids do not suppress the in­
creased secretion of adrenal androgens to the normal range for 
age in carcinoma, as they do in CAH. Cushing syndrome result­
ing from adrenal carcinoma may cause ISP and growth failure in 
boys. Rarely, an adrenal adenoma produces both testosterone and 
aldosterone, leading to sexual precocity and 

and 
aldosterone, leading to sexual precocity and hypertension with hy­
pokalemia. 
NR0B1 (DAX1) Gene Mutations. Two cases of NR0B1 frame­
shift mutations demonstrated adrenal failure and GISP that were 
suppressible by glucocorticoid therapy but not by GnRH agonist. 
The exceedingly high ACTH levels, possibly acting through the 
human melanocortin 1 receptor present in human Leydig cells, 
may have been the underlying cause of the increased steroidogen­
esis and testosterone secretion that were

oidogen­
esis and testosterone secretion that were reversed by glucocorti­
coid treatment. Because NR0B1 inhibits transactivation of SF1, a 
regulator of steroidogenic genes, loss of NR0B1 inhibition of SF1 
transcriptional activity also may have had a role.817,818 
1140
Leydig Cell Tumor. Testicular tumors are rare in childhood, 
representing 1% to 2% of all pediatric solid tumors, and Leydig 
cell tumors make up only 1.5% of those.819 Androgen-prod

umors make up only 1.5% of those.819 Androgen-producing 
Leydig cell tumors rarely are malignant and are slow growing but 
must be treated or epiphyseal fusion will limit height and preco­
cious puberty will affect social development. They derive from pri­
mordial mesenchyme, are classified as interstitial cell tumors, and 
occur most frequently around the age of 4 to 5 years. Unilateral 
enlargement (often nodular) of the testis usually occurs in boys 
with this neoplasm (although 5–10% of case

n boys 
with this neoplasm (although 5–10% of cases are bilateral); in 
contrast, both testes are usually of normal size (small) for chrono­
logic age in boys with CAH or a virilizing adrenal tumor.820 Al­
though LH receptor–activating mutations were detected in several 
boys with sporadic Leydig cell adenomas,779 no known mutations 
can be found in others. There is a trend toward testis sparing sur­
gery or radiographic observation rather than orchidectomy in be­
nign testicular lesions.821 
Pi

rchidectomy in be­
Pituitary Gonadotropin–Independent Familial Premature Leydig 
Cell and Germ Cell Maturation: Familial or Sporadic Testotoxico­
sis.331,779,822,823,997. Pituitary gonadotropin–independent familial 
premature Leydig cell and germ cell maturation, or testotoxico­
sis,822,824,825 causes boys to develop secondary sexual characteris­
tics with penile enlargement, which may be present at birth, and 
bilateral enlargement of testes to the early or midpuber

ral enlargement of testes to the early or midpubertal range, 
although the testes often are smaller than expected in relation to 
penile growth and pubertal maturation (Fig. 26.62). Premature 
maturation of Leydig and Sertoli cells and spermatogenesis occur. 
Leydig cell hyperplasia may occur; the Leydig cells in affected boys 
produce dimeric inhibin B as well as testosterone, and Leydig cells 
and spermatogonia stain positively for the α and βB segments of 
inhibin. The rate of linear growth i

 segments of 
inhibin. The rate of linear growth is rapid, skeletal maturation is 
advanced, and muscular development is prominent. The presence 
of prepubertal basal and GnRH-stimulated gonadotropin concen­
trations, lack of a pubertal pattern of LH pulsatility (as measured 
by immunologic or bioassay techniques), and normal pubertal or 
adult testosterone levels and clearance of testosterone are charac­
teristic (Table 26.37). The onset of adrenarche and its biochemi­
cal marker, serum DHEAS, 

narche and its biochemi­
cal marker, serum DHEAS, correlate with bone age rather than 
chronologic age.
Treatment with an GnRH agonist does not suppress testicular 
function or maturation in the initial phase.822 In late childhood 
or early adolescence, fertility is achieved and an adult pattern 
of LH secretion and response to GnRH is demonstrable825; 
secondary GnRH-dependent CPP may be superimposed on 
A
B
C
• Fig. 26.62  (A) A boy aged 5.5 years and his 28-year-old father with familial testo

ars and his 28-year-old father with familial testotoxicosis. The boy 
exhibited signs of sexual precocity by 3 years of age. His height was 130.6 cm (+4.8 SD [standard devia­
tion]), and his bone age was 12.5 years. The plasma testosterone level was 267 ng/dL; the dihydrotes­
tosterone level was 46 ng/dL; and the dehydroepiandrosterone sulfate (DHEAS) level was 23 μg/dL. The 
plasma luteinizing hormone (LH) and follicle-stimulating hormone (FSH) levels were low, and neither rose 
after treatment

levels were low, and neither rose 
after treatment. Pulsatile LH secretion was not demonstrable. Treatment with deslorelin, a gonadotropin-
releasing hormone (GnRH) agonist, had no effect. The father had begun sexual maturation by 3 years of 
age and had reached a final height of 162.6 cm in his early teens. The plasma testosterone level was 294 
ng/dL; the LH level was 0.5 ng/mL (LER-960); and the FSH level was 0.5 ng/mL (LER-869). The father 
had an adult-type LH and FSH response to GnRH; the 

ad an adult-type LH and FSH response to GnRH; the LH level increased to 7.5 ng/mL, and the FSH level 
increased to 2 ng/mL. At least 28 male family members over nine generations are affected. To convert 
dihydrotestosterone values to nanomoles per liter, multiply by 0.03467. For other conversions to SI units, 
see Figs. 26.19 and 26.20. (B) External genitalia of the 5.5-year-old boy. The penis measured 12 × 2.8 cm, 
the right testis was 4 × 2 cm, and the left testis was 3.5 × 2.5 cm. (C) Testis 

 and the left testis was 3.5 × 2.5 cm. (C) Testis of the boy showed Leydig cell 
maturation without Reinke crystalloids and spermatogenesis (Mallory trichrome stain).
1141
the substrate of testotoxicosis. In some adults, impaired sper­
matogenic function is associated with elevated concentrations 
of plasma FSH. Testotoxicosis may occur sporadically, probably 
as a consequence of a germline mutation or even a postzygotic 
one, but it is usually inh

 or even a postzygotic 
one, but it is usually inherited as a sex-limited autosomal domi­
nant trait. A kindred with nine generations of affected males has 
been reported; obligatory female carriers of the trait were unaf­
fected, because constitutional activation of the LH receptor on 
the ovary causes no ill effects.826
Heterozygous activating mutations of the heterotrimeric Gs 
protein–coupled LH-CGR that in concert transduce the LH/
hCG signal to the main effector, adenyl cyclase, are the ca

l to the main effector, adenyl cyclase, are the cause of 
testotoxicosis827 (Fig. 26.63). The LH receptor cloned from the 
human is a glycoprotein of 80 to 90 kDa that belongs to a sub­
family of the seven transmembrane–spanning, G protein–coupled 
receptors. The gene is localized to chromosome 2p21 (the same as 
for the FSH receptor); it spans at least 70 kilobases and contains 
11 exons separated by 10 introns. The large glycosylated N-ter­
minal extracellular hormone binding domain of the 701

al extracellular hormone binding domain of the 701–amino 
acid LH-CGR is encoded by exons 1 through 10. A single exon, 
the large exon 11, encodes the entire G protein–linked trans­
membrane domain with its seven α-helic segments connected by 
alternating extracellular and intracellular loops, the intracellular 
domain, and the three untranslated regions—almost two-thirds 
of the receptor.828
Fourteen constitutive activating heterozygous missense muta­
tions are reported in more than 60 patients

 muta­
tions are reported in more than 60 patients827 (see Fig. 26.63). 
Nine mutations were located between amino acid residues 542 
and 581, suggesting a mutation hot spot. There appears to be a 
limited repertoire of mutations in American boys, consistent with 
a founder effect. European pedigrees are more diverse. A model 
of the transmembrane domain of the receptor provides novel sug­
gestions for the structural and functional effects of these activat­
ing mutations.829 Transfected cultured

se activat­
ing mutations.829 Transfected cultured cells with these mutations 
exhibited increased basal cAMP production in the absence of ago­
nist, observations consistent with a constitutive activating muta­
tion.823 Various possibilities for the conformational changes in the 
LH receptor that lead to its constitutive activation have been con­
sidered. Inactivating mutations of the LH-CGR and their clinical 
consequences are discussed earlier in this chapter.
Although affected boys do not res

in this chapter.
Although affected boys do not respond to chronic admin­
istration of a GnRH agonist with suppression of testosterone 
  Testotoxicosis: Clinical and Laboratory 
Sex-limited autosomal dominant inheritance; activating mutation in the 
gene encoding the LH receptor
Early onset of sexual precocity in boys, with bilateral testicular enlarge­
ment
Prepubertal immunologic and biologic LH response to GnRH; prepubertal 
LH pulse secretory pattern
Concentration of plasma 

H pulse secretory pattern
Concentration of plasma testosterone in pubertal range
Premature Leydig cell and seminiferous tubule maturation
No CNS, adrenal, or testicular abnormalities demonstrable by radiologic 
or hormonal studies
Lack of suppression of plasma testosterone or physical signs of puberty 
by GnRH agonist
CNS, Central nervous system; GnRH, gonadotropin-releasing hormone; LH, luteinizing hor­
mone.
   
TABLE 26.37
A
B
NH3
I
II
III
IV
V
VII
VI
COOH
Insert L

r
I
II
III
IV
V
VII
VI
COOH
Insert LLKLLLLLQ->
Ala373Val
Met398Thr
Leu457Arg
Arg554Stp
Cys543Arg
Cys545Stp
Asp564Gly
- COOH
Cys343Ser
Cys131Arg
Phe194Val
Glu345Lys
Trp491Stp
Ala593Pro
Ile542Leu
Cys581Arg
Ser616Tyr
Asp578Tyr
/Glu/Gly/His
Ile 625Lys
Thr577Ile
Ile575Leu
Ala572Val
Met571Ile
Ala568Val
Exon 8 ->
Exon 10 ->
- NH2
• Fig. 26.63  (A) The serpentine, seven-transmembrane, Gs protein–cou­
pled human luteinizing hormone/human chorionic gonadotropin (h

uteinizing hormone/human chorionic gonadotropin (hLH/hCG) 
receptor with its large extracellular domain and the intracellular domain. 
The seven-helic transmembrane domains are indicated by roman numer­
als. (B) Mutations in the LH receptor protein are shown in the schematic 
structure of the LH receptor protein and localization of the inactivating 
(open squares) and activating (blue-filled circles) mutations in the human 
LH receptor. Notice the cluster of mutations in the VI transmembrane hel

e cluster of mutations in the VI transmembrane helix 
and third cytoplasmic loop. The Asx578Gly mutation is the most common 
activating mutation. The short lines across the amino acid chain separate 
the 11 exons. (A, redrawn from Yano K, Kohn LD, Saji M, et al. A case of 
male limited precocious puberty caused by a point mutation in the second 
transmembrane domain of the luteinizing hormone choriogonadotropin 
receptor gene. Biochem Biophys Res Commun. 1996;220:1036–1042; 
B, from Themmen APN,

 Commun. 1996;220:1036–1042; 
B, from Themmen APN, Huhtaniemi IT. Mutations of gonadotropins and 
gonadotropin receptors. Endocr Rev. 2000;21:551–583.)
1142
secretion, testosterone secretion, height velocity and rate of bone 
maturation, and aggressive and hyperactive behavior have report­
edly been decreased by treatment with oral medroxyprogesterone 
acetate822 (Table 26.38).
Ketoconazole, an orally active substituted imidazole deriva­
tive, suppre

 active substituted imidazole deriva­
tive, suppresses gonadal and adrenal biosynthesis by inhibiting 
the enzyme CYP17, which regulates both 17-hydroxylation and 
the scission (17,20-lyase) of 17α-hydroxypregnenolone (Δ5-17P) 
to DHEA (see “Nature and Regulation of Adrenal Androgens”). 
In the dosage used for treatment of testotoxicosis (median dose 
16.2 mg/kg/day to a maximal dose of 700 mg/day830), the agent 
produces a mild transient decrease in cortisol secretion and inter­
feres with bind

e in cortisol secretion and inter­
feres with binding of testosterone to TeBG. Secondary CPP often 
occurs when the bone age advances to or has already reached the 
pubertal range (usually >11.5 years), at which time the addition 
of a GnRH agonist is appropriate.827 Ketoconazole can cause 
hepatic injury, which is usually mild and reversible, but in rare 
cases hepatotoxicity is severe. Reversible renal injury, rash, and 
interstitial pneumonia were reported in a patient who tolerated 
lower do

were reported in a patient who tolerated 
lower doses, suggesting a dose-response effect. Nonetheless, five 
patients treated with ketoconazole experienced no side effects 
other than one mild and transient elevation of liver enzymes; they 
had appropriate age of onset of true puberty and reached an adult 
height almost identical to the target height (a mean increase of 8 
cm over the initially predicted height), suggesting great benefit of 
this therapy in this condition.830,831
The antiandroge

 therapy in this condition.830,831
The antiandrogen (and antimineralocorticoid) spironolac­
tone, in combination with testolactone, an inhibitor of aroma­
tase (CYP19), the key enzyme in the conversion of androgens to 
estrogens, is also used to treat testotoxicosis.832 The addition of a 
GnRH agonist is a useful step to suppress pituitary gonadotro­
pin secretion and secondary CPP that may later develop.829 More 
potent nonsteroidal antiandrogens (e.g., flutamide, nilutamide) 
and aromatase inh

s (e.g., flutamide, nilutamide) 
and aromatase inhibitors (e.g., letrozole) inhibit the rate of skeletal 
maturation and linear growth by suppressing estradiol synthesis, 
potentially with greater therapeutic efficacy.159,160 The combi­
nation of bicalutamide and anastrozole is proposed as another 
approach.833 Report of 20-year follow-up of these therapies dem­
onstrated efficacy with few adverse effects. Table 26.38 lists the 
various agents used in the treatment of testotoxicosis.
A study of 

ed in the treatment of testotoxicosis.
A study of an untreated boy with testotoxicosis (GISP) had the 
expected pattern of rapid growth and early cessation, and the adult 
height of 174 cm was within target height range (171.5–188.5 
cm), indicating the critical importance of individual approaches to 
affected boys when considering treatment to maximize height.832
Follow-up of boys with testotoxicosis indicates an increased 
risk of seminoma in adult life and Leydig cell adenoma in later 
childh

dult life and Leydig cell adenoma in later 
childhood.779,834 One boy with testotoxicosis developed nodular 
Leydig cell hyperplasia at 10 years of age.835 These cases support a 
relationship between an activating mutation of the gene encoding 
the LH receptor and of Leydig cell tumors.836 
Gonadotropin-Independent Sexual Precocity and Pseudohypoparathy­
roidism Type Ia. A mutation in Gsα can constitutively activate or in­
activate adenyl cyclase.837 Two boys who presented in infancy with 
class

.837 Two boys who presented in infancy with 
classic pseudohypoparathyroidism type Ia (PHPIa), a disorder 
characterized by resistance to hormones whose action is mediated 
by cAMP, developed signs of sexual precocity with the hormonal 
characteristics of testotoxicosis (i.e., gonadotropin-independent 
sexual precocity) at about 24 months of age. Whereas the alanine 
residue is usually absolutely conserved in all heterotrimeric G pro­
  Pharmacologic Therapy for Sexual Precocity
Disorder
Treatm

logic Therapy for Sexual Precocity
Disorder
Treatment
Action and Rationale
GnRH Dependent
True or central precocious puberty
GnRH agonists
Desensitization of gonadotrophs; blocks action of 
endogenous GnRH
GnRH Independent
Incomplete sexual precocity
Girls
Autonomous ovarian cysts
Medroxyprogesterone acetate
Inhibition of ovarian steroidogenesis; regression of cyst 
(inhibition of FSH release)
Medroxyprogesterone acetatea
Inhibition of ovarian steroidogenesis; regression

Third-generation aromatase inhibitor 
(e.g., letrozole)
Inhibition of P450 aromatase; blocks estrogen synthesis
Boys
Familial testotoxicosis
Ketoconazolea
Inhibition of CYP17 (mainly 17,20-lyase activity)
Flutamide or bicalutamide and letrozole 
or anastrozole
Antiandrogen
Inhibition of aromatase; blocks estrogen synthesis
Inhibition of testicular steroidogenesis
aIf true precocio

on of testicular steroidogenesis
aIf true precocious puberty develops, an LHRH agonist can be added.
CYP17, Cytochrome P450 17α-hydroxylase/17,20-lyase; FSH, follicle-stimulating hormone; GnRH, gonadotropin-releasing hormone; LHRH, luteinizing hormone–releasing hormone.
Modified from Grumbach MM, Kaplan SL. Recent advances in the diagnosis and management of sexual precocity. Acta Paediatr Jpn. 1988:30:S155–S175.
   
TABLE 26.38
1143
teins, both of 

hysiology and Disorders of Puberty
teins, both of these patients had a unique Ala366Ser mutation in 
one allele of the Gsα gene. PHPIa is caused by a wide variety of 
inactivating mutations in Gsα that lead to an approximately 50% 
reduction in Gsα activity in functional assays.
The paradox of a Gsα mutation causing both inactivation 
with PHP and constitutive activation with testotoxicosis was 
resolved by the in vitro demonstration that, unlike other activat­
ing mutations of Gsα, which involv

 other activat­
ing mutations of Gsα, which involve mutations inhibiting its 
intrinsic GTPase activity and decreasing the rate of hydrolysis 
of GTP to GDP, the mutation in these two boys caused acceler­
ated dissociation of GDP at 33°C in transfected Leydig cells but 
was rapidly degraded at 37°C in a lymphoma cell line838 and at 
33°C and 37°C in skin fibroblasts transfected with the mutation. 
These observations explain the clinical consequences of increased 
Gsα activity in the testis, whic

ces of increased 
Gsα activity in the testis, which are 3°C to 5°C cooler than the 
body, and the tissue specificity and temperature dependency 
of the mutation. The mother of one patient appeared to be a 
mosaic for the Gsα mutation; in the other boy, a germline muta­
tion was likely. 
Girls
GISP in girls (see Table 26.26) is caused by autonomous estro­
gen secretion by an ovarian cyst or tumor or an adrenal neoplasm 
or by inadvertent exposure to estrogen. A series of 129 patients 

to estrogen. A series of 129 patients 
demonstrated that the prevalence of non-gonadotropin–depen­
dent precocious puberty was 14 in 1000 children if congenital 
adrenal hyperplasia was excluded. Girls were affected four times 
more than boys due to the prevalence of ovarian cysts followed by 
McCune-Albright syndrome in girls.837 Girls harboring a teratoma 
or teratocarcinoma (or a CNS germ cell tumor) that secretes hCG 
have experienced sexual precocity caused by concurrent estrogen 
secretion

precocity caused by concurrent estrogen 
secretion by the tumor but no effect from the hCG alone; these 
girls also may have galactorrhea, especially if chorionic somato­
mammotropin (hCS or hPL) is also secreted.
Autonomous Ovarian Follicular Cysts. The most common 
childhood estrogen-secreting ovarian mass and ovarian cause 
of sexual precocity is the follicular cyst.838 Antral follicles up to 
about 8 mm in diameter are common in the ovaries of normal 
prepubertal girls and may be seen in thi

f normal 
prepubertal girls and may be seen in third trimester fetuses and 
newborn infants.839 They may appear and regress spontaneously. 
Larger follicular cysts may be discovered because of the presence 
of an abdominal mass or abdominal pain, especially after torsion 
or as an unexpected finding on pelvic sonography performed for 
other reasons. Occasionally, the antral follicles secrete estrogen 
and may form large masses, or the follicular cysts may recur and 
cause recurrent signs of sexu

cysts may recur and 
cause recurrent signs of sexual precocity and acyclic vaginal bleed­
ing. Enlarged antral follicles or cysts occur in premature thelar­
che, CPP, and transient or incomplete sexual precocity. In some 
patients with ovarian follicular cysts, the transient or recurrent 
sexual precocity is GnRH independent (Fig. 26.64). The concen­
tration of estradiol fluctuates, usually correlating with changes in 
the size of the follicular cyst(s) when monitored by pelvic sonog­
raphy, and

cyst(s) when monitored by pelvic sonog­
raphy, and may increase to levels found in granulosa cell tumors, 
although values may also be in the pubertal range.
These patients do not have increased plasma granulosa cell 
tumor markers such as AMH or inhibin.840 The concentration 
of LH is suppressed, a pubertal pattern of pulsatile LH secretion 
is absent, and the LH rise induced by GnRH is prepubertal. A 
constitutive activating mutation of the FSH receptor has not been 
described in a female, but

 receptor has not been 
described in a female, but a heterozygous mutation, Asp567Gly, 
was detected in the third intracellular loop of the FSH receptor 
in a hypophysectomized man who, despite the gonadotropin 
• Fig. 26.64  A girl aged 4 years and 10 months with recurrent, “autonomous” follicular cysts of the ovary. 
For conversion to SI units, see Fig. 26.19. BA, bone age; CA, chronologic age; E2, estradiol; FSH, follicle-
stimulating hormone; LH, luteinizing hormone; LRF, luteinizing hormone

 LH, luteinizing hormone; LRF, luteinizing hormone-releasing factor; MPA, medroxy­
progesterone acetate (oral); P.E., physical examination; Rx, treatment. (From Kaplan SL, Grumbach MM. 
Pathogenesis of sexual precocity. In: Grumbach MM, Sizonenko PC, Aubert ML, eds. Control of the Onset 
of Puberty. Baltimore, MD: Williams & Wilkins; 1990:620–660.)
1144
deficiency, was fertile and had normal-size testes.827 Accordingly, 
the possibility that some gir

es.827 Accordingly, 
the possibility that some girls with recurrent ovarian cysts harbor 
an activating mutation of the FSH receptor seems worthy of study. 
The McCune-Albright syndrome may lead to recurrent ovarian 
cysts even in the apparent initial absence of other features of this 
disorder due to somatic activating mutations in the gene encoding 
the α-subunit of the heterotrimeric Gs protein. The luteinization 
of follicular cysts may be related to subtle elevations and increased 
pulses o

lated to subtle elevations and increased 
pulses of plasma FSH. Ovarian cysts and sexual precocity have 
been associated with the fragile X syndrome in girls.841
Estradiol-secreting ovarian cysts occur in preterm infants born 
before 30 weeks of gestation; they are associated with edema of 
the labia majora and, in some instances, of the lower abdominal 
wall.842 The LH and FSH response to GnRH in these patients 
suggests GnRH dependence, and treatment with medroxyproges­
terone acetate is assoc

atment with medroxyproges­
terone acetate is associated with regression of the cysts. A case of 
massive ovarian edema associated with ovarian cysts found in a 
6-month-old with breast and pubic hair development has been 
reported.843
GnRH agonists are useful in the treatment of ovarian follicular 
cysts associated with CPP (GISP) but not of so-called autono­
mous cysts. However, autonomously functioning ovarian follicu­
lar cysts, whether recurrent or manifesting in an isolated episode, 
often 

ent or manifesting in an isolated episode, 
often respond to treatment with oral medroxyprogesterone acetate 
(or cyproterone acetate outside of the United States), which seems 
to prevent recurrence, to accelerate involution of the follicular 
cysts,843 and to reduce the risk of torsion. The use of a potent 
aromatase inhibitor such as letrozole to reduce estradiol secretion 
is another potential approach to treatment and has been success­
fully used in girls with no stigmata of McCune-Albright

 used in girls with no stigmata of McCune-Albright syndrome 
other than autonomous ovarian cysts.844,845 Surgical intervention 
is rarely indicated; a large or persistent cyst can be reduced by 
puncture at laparoscopy, and the size of the cyst can be monitored 
readily by pelvic sonography.
While it is a rare cause of ovarian tumors, an FSH-secreting and 
TSH-secreting adenoma of the pituitary gland led to extremely 
large cysts in a 9-year-old girl’s ovaries but was only discovered 
after meno

girl’s ovaries but was only discovered 
after menopausal FSH levels led to the diagnosis of the tumor at 
19 years of age.846 
Ovarian Tumors. Ovarian tumors are the most common 
genitourinary tumors of girls,847,848 accounting for about 1% 
of all tumors in girls younger than 17 years of age, but they 
are rare in the prepubertal period. Most are benign according 
to most reports,849 but a referral center might find a majority of 
tumors are malignant.850 Most ovarian tumors arise from germ 
ce

ignant.850 Most ovarian tumors arise from germ 
cells or sex cord–stromal cells in childhood with fewer than 20% 
being of epithelial origin, whereas in adults, most are of epi­
thelial origin.851 Early diagnosis of most childhood tumors of 
the ovary allows successful cure, unlike ovarian cancer in adult 
women. Most of these girls present with pain or an abdominal 
mass. Tumors smaller than 5 cm at diagnosis are more likely to 
be nonneoplastic, and those larger than 10 cm are more likely 
to 

, and those larger than 10 cm are more likely 
to be neoplastic.838,852 Malignant ovarian tumors evaluated by 
ultrasonography, computed tomography, or magnetic resonance 
imaging are mostly solid or heterogeneous and are larger than 
benign tumors.853 The successful use of tumor markers for diag­
nosis varies by cause. For example, in one series, cystic terato­
mas resulted in lactate dehydrogenase (LDH) elevation and an 
increased erythrocyte sedimentation rate; immature teratomas 
produced el

edimentation rate; immature teratomas 
produced elevated levels of LDH, α-fetoprotein, and cancer 
antigen 125 (CA125), and granulosa cell tumors had elevated 
sex steroids (estradiol or testosterone850 or both).854
Granulosa cell tumor of the ovary is rare in childhood, 
although theca cell tumors are even less common.855 Because most 
granulosa cell tumors produce estrogen, incomplete isosexual pre­
cocity occurs in the youngest girls affected. Characteristic histo­
logic features of juvenile 

 Characteristic histo­
logic features of juvenile granulosa cell tumors include nodular 
architecture, follicle formation, abundant interstitial and intrafol­
licular acid mucopolysaccharide-rich fluid, irregular microcysts, 
individual cell necrosis, and high mitotic activity (mean activity, 
11 mitotic figures per 10 high-power fields). Size can vary from 
2.5 to 25 cm, with a mean diameter of 12 cm. The interstitial 
mucinous fluid consists predominantly of hyaluronic acid. Prog­
nosis is goo

edominantly of hyaluronic acid. Prog­
nosis is good, with the mortality rate being about 3%, but delay 
in treatment leads to substantial complications. In one series, girls 
who presented with ISP and were correctly diagnosed had no 
intra-abdominal spread and had Federation of Gynecology and 
Obstetrics (FIGO) stage IA disease, whereas those who presented 
with acute abdominal symptoms had 50% prevalence of intra-
abdominal spread and two recurrences after surgery. When the 
diagnosis was made

rences after surgery. When the 
diagnosis was made after normal puberty had begun, some girls 
experienced virilization or abdominal symptoms; 80% had intra-
abdominal spread, and 30% had recurrence with FIGO stage IIC.
Approximately 80% of granulosa cell tumors can be palpated 
on bimanual examination, whereas fewer than 5% are bilateral 
or clinically malignant. The concentration of plasma estradiol 
may increase to high levels, and serum FSH and LH concen­
trations are usually suppressed. AMH

nd LH concen­
trations are usually suppressed. AMH and inhibin are sensitive 
tumor markers and are used to screen for metastases especially 
if elevated at diagnosis856; an elevated estradiol concentration in 
an affected patient younger than 9 years of age or an abnormal 
rise in concentration of plasma AMH or inhibin at any age sug­
gests recurrence or metastasis. Girls affected with stage I disease 
may be treated by surgery alone, but a more advanced grade will 
require chemotherapy.857
Occ

 advanced grade will 
Occasionally, gonadoblastomas in streak gonads, rare lipoid 
tumors, cystadenomas, and ovarian carcinomas secrete estrogens, 
androgens, or both. Even with successful resection of a gonadal sex 
steroid–secreting neoplasm, the child is at risk for secondary CPP 
in the future. Gonadal tumors composed of a mixture of germ 
cells and sex cord–stromal cells are distinct from gonadoblastoma. 
They may be benign or malignant, with the Sertoli-Leydig cell

 benign or malignant, with the Sertoli-Leydig cell 
tumors reported to be most aggressive.858 α-Fetoprotein and other 
tumor markers aid in diagnosis. However, a 558-g ovarian dysger­
minoma was found in a 7-year-old girl with precocious puberty 
and no elevation of serum tumor markers.859 
Peutz-Jeghers Syndrome. Peutz-Jeghers syndrome, an autoso­
mal dominant syndrome, is usually caused by mutations in the 
gene located on 9p13.3 that encodes a serine/threonine protein 
kinase, STK11, leading 

 serine/threonine protein 
kinase, STK11, leading to haploinsufficiency of this novel tumor-
suppressing gene.88,860 This condition is characterized by mucocu­
taneous pigmentation of the lips, buccal mucosa, fingers, and toes; 
gastrointestinal hamartomatous polyposis; and a predisposition to 
It is associated with a rare, distinctive sex cord tumor with 
annular tubules in both boys and girls. Estrogen secretion by the 
tumor may lead to feminization in girls and incomplete sexual 

ad to feminization in girls and incomplete sexual 
precocity in boys. Less frequently, a feminizing Sertoli-Leydig cell 
tumor has been found in patients with Peutz-Jeghers syndrome. 
The rare feminizing large-cell calcifying Sertoli cell tumors of the 
testes present with gynecomastia in boys and are found in Peutz-
Jeghers syndrome and Carney complex, the latter of which is most 
often caused by PRKAR1A mutations, the gene encoding regula­
tory subunit type 1 of protein kinase A.
1145
Chapter 

 subunit type 1 of protein kinase A.
1145
Sex cord–stromal cell tumors derive from the celomic epi­
thelium or mesenchymal cells of the embryonic gonads and are 
composed of granulosa, theca, Leydig, and Sertoli cells. Estrogen 
secretion from these tumors can cause ISP in girls, and androgen 
secretion can cause virilization. Inhibin A and B activin are pro­
duced, as is AMH; all serve as useful tumor markers. Sex cord–
stromal cell tumors not ass

mor markers. Sex cord–
stromal cell tumors not associated with Peutz-Jeghers syndrome 
are malignant in 25% of cases; these tumors can grow quite large, 
but those associated with Peutz-Jeghers syndrome are often small 
and multiple, and they contain calcifications.861 Estrogen excess 
in Peutz-Jeghers syndrome1008 has been treated with aromatase 
inhibitors.474
Girls with this disorder should be examined at regular intervals 
for the presence of gonadal tumors by pelvic sonography. Peutz-
Jeghe

 gonadal tumors by pelvic sonography. Peutz-
Jeghers syndrome should be considered in boys with unexplained 
gynecomastia, especially if in the prepubertal years. 
Adrenal Tumors. Adrenocortical tumors are rare in childhood 
(0.6% of all childhood tumors and 0.3% of all malignant child­
hood tumors), but most produce steroid hormones, whereas those 
in adults usually do not. The median age at diagnosis is 4 years, 
but 41% of these tumors manifest before 2 years and 71% before 
5 years of age. M

t before 2 years and 71% before 
5 years of age. Most cause virilization or Cushing syndrome, but 
adrenal tumors may produce estrogen as well as androgens and 
can cause sexual precocity in a girl or gynecomastia in a boy.862 
One adrenal adenoma found in a 7-year-old girl expressed the 
gene for aromatase, demonstrating that the tumor could directly 
produce estrogen863 to a level of 145 pg/mL, within the range 
found in adrenal carcinomas. 
Boys and Girls
McCune-Albright Syndrome. McCune-Albr

ys and Girls
McCune-Albright Syndrome. McCune-Albright syn­drome846,1003 
occurs about twice as often in girls than in boys; it is sporadic 
and is caused by somatic activating mutations (Cys or His to 
Arg201) in exon 8 of the gene (GNAS1) encoding the α-subunit 
of the trimeric GTP-binding protein (Gsα) that stimulates adenyl 
cyclase.864,865 This leads to a constitutive ligand-free activation of 
cellular function in a mosaic distribution, leading to a high vari­
ability of organ involvement 

ding to a high vari­
ability of organ involvement and degree of severity.474 It is charac­
terized by the triad of irregularly edged hyperpigmented macules 
(café au lait spots of the coast of Maine type); a slowly progres­
sive bone disorder, polyostotic fibrous dysplasia, that can involve 
any bone and is frequently associated with facial asymmetry and 
hyperostosis of the base of the skull; and, more commonly in girls, 
GISP (Fig. 26.65 and Table 26.39). At least two of the features 
must be 

ble 26.39). At least two of the features 
must be present to consider the diagnosis.
Autonomous hyperfunction most commonly involves the 
ovary, but other endocrine involvement includes the thyroid 
gland (nodular hyperplasia with thyrotoxicosis or euthyroid sta­
tus), adrenal gland (multiple hyperplastic nodules with Cushing 
syndrome that may occur in the neonatal period and may be fol­
lowed by adrenal insufficiency),866 pituitary gland (adenoma or 
mammosomatotroph hyperplasia with gigantism

ma or 
mammosomatotroph hyperplasia with gigantism, acromegaly, 
and hyperprolactinemia), and parathyroid glands (adenoma or 
hyperplasia with hyperparathyroidism).867,868 Hypophosphatemic 
vitamin D–resistant rickets or osteomalacia occurs either because 
of overproduction of a phosphaturic factor, phosphatonin, that is 
secreted by the bone lesions or because of an intrinsic renal abnor­
mality leading to the excess generation of nephrogenous cAMP 
in the proximal tubule and, as a result, decr

AMP 
in the proximal tubule and, as a result, decreased reabsorption of 
phosphate. Hepatocellular dysfunction may occur due to expres­
sion of the mutant activating gene in liver cells, which leads to 
jaundice associated with hepatobiliary disease, and pancreatitis. 
Another nonendocrine manifestation is cardiac disease, and 
patients carry the risk of cardiac arrhythmia and sudden death. 
This is a sporadic condition that can be concordant or discordant 
in monozygotic twins.
Considering chil

discordant 
Considering children with at least one of the signs of 
McCune-Albright syndrome, 24% had the classic triad, 33% 
• Fig. 26.65  A girl aged 7 years and 4 months with gonadotropin-releasing 
hormone (GnRH)–independent sexual precocity associated with McCune-
Albright syndrome. She had breast development since infancy, and it 
increased noticeably at about 3 years of age; 6 months later, episodes of 
recurrent vaginal bleeding began. Growth of pubic hair was notic

nal bleeding began. Growth of pubic hair was noticed at about 
4 to 5 years. At age 5.5 years, the bone age was 6 years and 11 months; 
height was +1 SD (standard deviation) above the mean value for age. By 
6.5 years, when she was seen at the University of California, San Francisco, 
the bone age had advanced to 9 years, and height was at +1 SD. Breasts 
were at Tanner stage 4, and pubic hair at stage 3. Extensive, irregular café 
au lait macules cover the right side of the face, left lower abd

s cover the right side of the face, left lower abdomen and 
thigh, and both buttocks. A bone survey showed widespread involvement 
of the long bones with typical polyostotic fibrous dysplasia, the floor of the 
anterior fossa of the skull was sclerotic, and the diploic space had wid­
ened. She has had two pathologic fractures through bone cysts in the right 
upper femur. Notice the osseous deformities. Plasma estradiol concentra­
tions were consistently in the pubertal range; the luteinizing hor

stently in the pubertal range; the luteinizing hormone (LH) 
response to GnRH was prepubertal. Results of thyroid function studies 
were normal, including the thyrotropin response to thyrotropin-releasing 
hormone administration, and antithyroid antibodies were not detected. 
Treatment with oral medroxyprogesterone acetate suppressed menses and 
arrested pubertal development but did not slow skeletal maturation. Her 
final height is 142 cm (−2.5 SD). Menstrual cycles are regular.
1146
SECTION VI

SD). Menstrual cycles are regular.
1146
  Clinical Manifestations of McCune-Albright Syndrome in 158 Reported Patientsa
Manifestation
% OF PATIENTS
Mean Age at 
Diagnosis (yr) and 
Range
Comments
Total ( N = 158)
Male (n = 53)
Female (n = 105)
97
51
103
7.7 (0-52)
Polyostotic more common than 
monostotic
Café au lait lesion
85
49
86
7.7 (0-52)
Variable size and number of 
lesions, irregular border 
(coast of Maine)
Sexual precocity

rregular border 
52
8
74
4.9 (0.3-9)
Common initial manifestation
Acromegaly/gigantism
27
20
22
14.8 (0.2-42)
17/26 of patients with 
adenoma on MRI/CT
15
9
14
16 (0.2-42)
23/42 of acromegalic patients 
with PRL
19
7
23
14.4 (0.5-37)
Euthyroid goiter is common
Hypercortisolism
5
4
5
4.4 (0.2-17)
All primary adrenal
Myxomas
5
3
5
34 (17-50)
Extremity myxomas
Osteosarcoma
2
1
2
36 (34-37)
At site of fibrous dysplasia, not 
relate

 (34-37)
related to prior radiation 
therapy
Rickets/osteomalacia
3
1
3
27.3 (8-52)
Responsive to phosphorus plus 
calcitriol
Cardiac abnormalities
11
8
9
(0.1-66)
Arrhythmias and CHF reported
Hepatic abnormalities
10
6
10
1.9 (0.3-4)
Neonatal icterus is most com­
mon
aEvaluations include clinical and biochemical data; other rarely described manifestations include metabolic acidosis, nephrocalcinosis, developmental delay, thymic and splenic hyperplasia, and col

tal delay, thymic and splenic hyperplasia, and colonic 
polyps.
CHF, Congestive heart failure; CT, computed tomography; MRI, magnetic resonance imaging; PRL, prolactin.
Modified from Ringel MD, Schwindinger WF, Levine MA. Clinical implication of genetic defects in G proteins: the molecular basis of McCune-Albright syndrome and Albright hereditary osteodystrophy. 
Medicine. 1996;75:171–184.
   
TABLE 26.39
had two signs, and 40% had only one classic sign. The mutation 
was detected in 46% of bloo

ic sign. The mutation 
was detected in 46% of blood samples from patients present­
ing the classic triad but in only 21% and 8% of samples from 
patients with two signs or one sign, respectively. If an affected 
tissue was available, the mutation was found in more than 90% 
of the patients no matter what the number of signs. The muta­
tion was found in 33% of the 39 cases of isolated peripheral 
precocious puberty. Patients with monostotic fibrous dysplasia, 
isolated peripheral precocious puber

s dysplasia, 
isolated peripheral precocious puberty, neonatal liver cholesta­
sis, or the classic McCune-Albright syndrome all had the same 
molecular defect.818 Whereas most endocrine organs involved 
in McCune-Albright syndrome were not associated with par­
ent specificity,869 pituitary adenomas secreting GH expressed 
NESP55 transcripts, which are monoallelically expressed from 
the maternal alleles rather than from the exon 1A paternal allele. 
Mutation of GNAS1 involving Arg201His replacem

e. 
Mutation of GNAS1 involving Arg201His replacement is associ­
ated with apparent premature or exaggerated thelarche and early 
menarche.870,871
Most patients have pigmented skin lesions in infancy that usu­
ally increase in size along with body growth.872 The irregularly 
bordered café au lait macules usually do not cross the midline, but 
they may; they often are located on the same side as the main bone 
lesions and have a segmented distribution.
The skeletal lesions in the cortex are dyspl

tion.
The skeletal lesions in the cortex are dysplastic and are filled 
with spindle cells with poorly organized collagen support; they 
take the form of scattered cystic areas of rarefaction on radi­
ography and often result in pathologic fractures and progres­
sive deformities (Fig. 26.66). Technetium bone scintigraphy has 
been the most sensitive approach to the detection of bone lesions 
before they are visible radiographically. Fractures are most com­
mon between the sixth and the tenth yea

 most com­
mon between the sixth and the tenth years but decline thereafter, 
and they are more frequent if phosphaturia is present.873 Patients 
referred for fibrous dysplasia of the bone in one or more locations 
are often found to have endocrine or dermatologic manifesta­
tions of McCune-Albright syndrome as well as GNAS1 muta­
tions, so suspicion should be kept high when evaluating fibrous 
lesions.874 If the skull is involved, there may be entrapment and 
compression of optic or auditory ne

ntrapment and 
compression of optic or auditory nerve foramina, which can lead 
to blindness, deafness, facial asymmetry, and ptosis. Asymmetry 
of the jaw is another manifestation of McCune-Albright syn­
drome. Fifty percent of affected children in one series manifested 
bone abnormalities by 8 years of age.872 Increased serum GH 
1147
A
B
C
• Fig. 26.66  Bone lesions in McCune-Albright syndrome. (A) The skull has severe thickening primarily at 
t

A) The skull has severe thickening primarily at 
the base due to fibrous dysplasia. The auditory and optic nerves can be caught in narrowed foramina, but 
that is not the case in these patients. (B, C) Distortions of the long bones can develop into a “shepherd’s 
crook” appearance. Notice the multiple bone cysts. (D) Bone scan shows the areas of remodeling that 
“light up,” depending on the area affected in individual patients. There are examples of patients primarily 
affected in the craniofaci

 of patients primarily 
affected in the craniofacial area or in the appendicular area, or both areas, and the axial skeleton. (Courtesy 
Michael T. Collins, M.D., National Institutes of Health, Bethesda, MD, and Sandra Gorges, M.D., University 
of California, Davis, CA.)
Continued
1148
D
• Fig. 26.66  cont’d See page 1147 for legend.
levels occur in up to 30% of cases and have an adverse effect 
on the skull deformities, depending on the age at onset

e skull deformities, depending on the age at onset; soma­
tostatin analogues are usually effective in suppressing GH.875,876 
Irradiation of the hypothalamic-pituitary area may be invoked, 
but it carries a risk of later occurrence of sarcoma. Rapid control 
of elevated GH can be achieved by the use of long-acting soma­
tostatin agonists such as pegvisomant, which is often but not 
always successful.877
The sexual precocity often begins during the first 2 years of 
life and is frequently heralde

e first 2 years of 
life and is frequently heralded by menstrual bleeding; the cause 
is autonomously functioning luteinized follicular cysts of the 
ovary in girls (Table 26.40).865 The ovaries contain no corpora 
lutea and commonly exhibit asymmetric enlargement as a result 
of a large solitary follicular cyst that characteristically enlarges 
and then spontaneously regresses, only to recur (Fig. 26.67).865 
Serum estradiol is elevated (at times to extraordinary levels); 
in contrast, the LH r

s to extraordinary levels); 
in contrast, the LH response to GnRH is prepubertal, and the 
pubertal pattern of nighttime LH pulses is absent at the onset 
and during the initial years. Later in the course of the sexual pre­
cocity, when the bone age approaches 12 years, the GnRH pulse 
generator becomes operative and ovulatory cycles ensue. How­
ever, adults carry the risk of acyclic hyperestrogenemia, infertil­
ity, and a potential risk of estrogen-dependent cancer, including 
breast cancer.878

gen-dependent cancer, including 
An affected girl may progress from GnRH-independent 
puberty to GnRH-dependent puberty (see Table 26.40). GnRH 
agonists are not effective for treatment in the GnRH-independent 
stage. Testolactone, fadrozole, and anastrozole have had equivocal 
or no success in controlling manifestations in initial studies.475 
After a single case report of treatment with tamoxifen, an anties­
trogen, showed decreases in bone age advancement, growth rate, 
mens

reases in bone age advancement, growth rate, 
menses, and pubertal development, a multicenter trial demon­
strated the utility of this agent in decreasing vaginal bleeding and 
decreasing the rate of bone age advancement and growth rate in 
1149
  A Patient With McCune-Albright Syndrome and Recurrent Ovarian Cysts
Chronologic 
Age (yr)
Bone Age (yr)
Physical Signs
Basal and Post-
LHRH (ng/mL)a,b
Plasma Estradiol 
(pmol/L [pg/mL])
Radio

ng/mL)a,b
Radiography 
(Long Bones)
14/12
13/12
81.1
Café au lait pigmentation, B2, 
PH1
LH 0.6-1.3
FSH 1.9-3.2
40 (11)
Normal
Vaginal bleeding (×2 mo)
DHEAS <50 ng/mL 
(<0.14 mmol/L)
18/12
B1, PH1
26/12
26/12
92.4
B2, PH2
LH 0.6-1.1
55-66 (15-18)
Normal
Vaginal bleeding
31/12
98.3
B1, PH1
310/12
310/12
B2, PH1
LH 1.1-2.0
51-95 (14-26)
Normal
FSH 1-1.7
43/12
B1, PH1
7.3-7.3 (20-20)
Polyostotic fibrous 
dysplasia of 
f

3-7.3 (20-20)
femurs
511/12
6
123.4
B3, PH2
LH 1.1-4.3
FSH 1.0-2.0
66/12
710/12
128.5
B3, PH2
Oral medroxyprogesterone 
acetate, 10 mg bid stated
<5
710/12
810/12
136.8
87/12
142.2
aMatched standard reagents were LER-960 for LH and LER-869 for FSH. To convert ng/mL to IU/L, multiply LH value by 3.8 and FSH value by 8.4.
bNote the prepubertal LH response to GnRH consistent with GnRH-independent sexual precocity until age 5 11/12 yr and t

endent sexual precocity until age 5 11/12 yr and the pubertal LH response at 5 11/12 yr, consistent with the development of 
secondary true precocious puberty (GnRH-dependent). Note discrepancy between gonadarche and adrenarche as evidenced by preadrenarchal concentration of DHEAS.
B, Breast stage; bid, twice a day; DHEAS, dehydroepiandrosterone sulfate; FSH, follicle-stimulating hormone; GnRH, gonadotropin-releasing hormone; LER, matched standard reagent; LH, luteinizing 
hormone; LHRH, luteini

d reagent; LH, luteinizing 
hormone; LHRH, luteinizing hormone–releasing hormone; PH, pubic hair stage.
   
TABLE 26.40
affected girls.879 However, ovarian and uterine volumes remained 
elevated. The longest term study demonstrated that letrozole can 
suppress bone age advancement and preserve adult height far over 
controls.880
Sexual precocity is rare in boys with McCune-Albright syn­
drome865; however, testicular disease occurs as frequently in 
affected boys as ovarian disease in girls.881 A

n 
affected boys as ovarian disease in girls.881 Affected boys may 
have asymmetric enlargement of the testes in addition to signs 
of sexual precocity. The histologic changes and hormonal find­
ings are reminiscent of those observed in testotoxicosis: The 
seminiferous tubules are enlarged and exhibit spermatogenesis, 
and Leydig cells may be hyperplastic, the most common histo­
logic finding.865,881 A 3.8-year-old boy with McCune-Albright 
syndrome (several café au lait lesions on the back and

rome (several café au lait lesions on the back and polyos­
totic fibrous dysplasia) had an Arg201His mutation detected in 
bone and testis tissue and the unusual feature of macroorchi­
dism (right testis, 9 mL; left testis, 7 mL), but sexual precocity 
was absent. Basal and GnRH-stimulated gonadotropins and 
sex steroid levels were prepubertal, but serum inhibin B and 
AMH concentrations were strikingly elevated. On histologic 
examination of the testes, most of the seminiferous tubules 
were sl

 testes, most of the seminiferous tubules 
were slightly increased in diameter and filled with Sertoli cells 
but lacked a lumen. The tubules stained intensively for inhibin 
βB-subunit; mature Leydig cells were absent.882 An increased 
incidence of the rare condition testicular microlithiasis was 
described in boys with McCune-Albright syndrome evaluated 
by ultrasonography.883,884
McCune-Albright syndrome may occur concordantly or dis­
cordantly in monozygotic twins; familial cases have not be

y in monozygotic twins; familial cases have not been con­
vincingly described. In 1986, Happle885 posited that the disorder 
1150
• Fig. 26.67  Serial pelvic ultrasonograms at 2-week intervals of a 6-year-old girl with McCune-Albright 
syndrome. Breast development and vaginal bleeding coincided with the enlargement of the ovarian cyst; 
white arrows denote the decreasing size of the cyst. With the spontaneous regression of the large ovar­
ian cyst, t

ntaneous regression of the large ovar­
ian cyst, the breasts regressed in size, and vaginal bleeding ceased. (From Kaplan SL, Grumbach MM. 
is caused by an autosomal dominant lethal gene that results in 
loss of the zygote in utero and that cells bearing this mutation 
survive only in embryos mosaic for the lethal gene. Early somatic

 embryos mosaic for the lethal gene. Early somatic 
mutation would lead to a mosaic cell pattern of the distribution of 
cells containing the mutation. The severity of the disorder would 
depend on the proportion of mutant cells in various embryonic 
tissues. The description of somatic mutations in human endocrine 
tumors that convert the peptide chain of the Gs protein into a 
putative oncogene (referred to as a gsp mutation) raised the possi­
bility of a similar defect in McCune-Albright syndr

ility of a similar defect in McCune-Albright syndrome that both 
affects a differentiated function such as a signaling pathway and 
mediates the regulation of proliferation. These hypotheses are now 
established, because mutations in the gene encoding the α-subunit 
of the stimulatory G protein for adenyl cyclase was identified in 
the tissues of children with the McCune-Albright syndrome.
The heterotrimeric guanine nucleotide–binding proteins (G 
proteins) are a subfamily within the large super

proteins) are a subfamily within the large superfamily of GTP-
binding proteins and serve to transduce signals from a large 
number of cell surface receptors with a common structural motif 
of seven-transmembrane–spanning domains to their intracellu­
lar effector molecules, including enzymes and ion channels; in 
essence, they couple serpentine cell surface receptors to effectors 
(Fig. 26.68). For Gs, the stimulatory G protein, the effector is 
adenyl cyclase, which is controlled by Gs and an

adenyl cyclase, which is controlled by Gs and an inhibitory G pro­
tein,836 Gi. The heterotrimer is composed of an α-subunit (39–45 
kDa) that binds GTP, has intrinsic GTPase activity, and converts 
GTP to GDP; a β-subunit (35–36 kDa); and a smaller β-subunit 
(7–8 kDa). The latter two subunits are tightly but noncovalently 
associated with each other. Each of the subunits is encoded by a 
distinct gene. The G proteins function as conformational switches. 
The GDP-liganded α-subunit is bound t

l switches. 
The GDP-liganded α-subunit is bound to the βγ-subunits and is 
in an inactivated state. When the cell surface receptor is activated 
by its ligand or agonists, the GDP is catalytically released from 
the α-subunit, enabling GTP to bind. This leads to dissociation 
of the GTP-activated α-subunit from the bound βγ-subunits and 
activation of the effector, adenyl cyclase. When GTP is hydrolyzed 
by the intrinsic GTPase activity of Gsα, the α-subunit and βγ-
subunit reassociate, and the

the α-subunit and βγ-
subunit reassociate, and the α-subunit is in the off or inactive con­
formation. The three-dimensional structure of the heterotrimeric 
G proteins has been determined.
Activating heterozygous mutations in the Gsα-subunit that 
occurred as an early postzygotic event are described in the 
McCune-Albright syndrome. The somatic constitutive activat­
ing mutation, which leads to excess cAMP production and, in 
some tissues, cAMP-induced hyperplasia, has a mosaic pattern; 
the pr

induced hyperplasia, has a mosaic pattern; 
the proportion of the hyperactive mutant compared with nor­
mal cells varies in different tissues, contributing, at least in part, 
to the varied clinical findings, the severity, the sporadic nature 
of the syndrome, and the discordant occurrence in monozy­
gotic twins. A germline mutation is presumed to be lethal to 
the embryo. Two gain-of-function somatic missense mutations 
have been described in this disorder, both of which involve 
the arginine 2

is disorder, both of which involve 
the arginine 201 residue of the α-subunit.836 This is the site of 
covalent modification by cholera toxin: Either a cysteine or a 
histidine is substituted for arginine 201 (see Fig. 26.68). The 
arginine 201 residue is critical for α-subunit GTPase activity, 
and each of these two mutations decreases the GTPase activ­
ity of the Gsα-subunit, leading to constitutive activation. These 
activating mutations have been found in all tissues affected by 
the syndrom

been found in all tissues affected by 
the syndrome, including bone lesions.
1151
Adults with this syndrome have continued hyperfunction of 
the endocrine organs, but other comorbidities are reported. For 
example, a GNAS-positive breast cancer is reported in a woman, 
two GNAS-positive muscle myomas, platelet dysfunction in four, 
and various types of respiratory and cardiovascular complications 
were found in a series of 16 affected adults.886 Th

ere found in a series of 16 affected adults.886 There are reports of 
fertility in adults with McCune-Albright syndrome. 
Juvenile Hypothyroidism. Long-standing untreated primary 
hypothyroidism, usually a consequence of Hashimoto thyroid­
itis, is an uncommon cause of incomplete ISP in both girls and 
boys and occurs in association with impaired growth and delayed 
skeletal maturation.323 If the concentration of plasma prolactin 
is elevated, galactorrhea may be demonstrable, more commonly 
in 

lactorrhea may be demonstrable, more commonly 
in affected girls than boys (Figs. 26.69 and 26.70). Girls have 
breast development, enlarged labia minora, and estrogenic 
changes in the vaginal smear, usually without the appearance of 
pubic hair; some girls have irregular vaginal bleeding that could 
proceed to metrorrhagia, and solitary or multiple ovarian cysts 
may be demonstrable by pelvic sonography or on physical exam­
ination.887 It is important to recognize the condition to avoid 
unnec

portant to recognize the condition to avoid 
unnecessary surgery for the ovarian cysts or the accompanying 
pituitary enlargement, which would be a tragic mistake in view 
of the success of medical management. In about 80% of boys 
with juvenile hypothyroidism, the testes are enlarged because of 
an increase in the size of the seminiferous tubules, but signs of 
virilization and Leydig cell maturation are absent, and the plasma 
concentration of testosterone is prepubertal. Enlargement of the 
s

testosterone is prepubertal. Enlargement of the 
sella turcica and the pituitary gland in the face of hypersecretion 
of TSH (see Fig. 26.70) as well as associated galactorrhea has led 
to the misdiagnosis of a pituitary neoplasm. The hypothyroid­
ism, incomplete sexual maturation, galactorrhea, and pituitary 
enlargement are reversed or corrected by levothyroxine therapy 
within a few months.
In 1960, Van Wyk and Grumbach323 suggested that the syn­
drome resulted from hormonal overlap in negati

yn­
drome resulted from hormonal overlap in negative feedback regu­
lation with increased secretion of gonadotropins, prolactin, and 
TSH as a consequence of the chronic hypothyroidism. With the 
advent of radioimmunoassays for pituitary hormones, increased 
prolactin secretion was documented in children and adults with 
primary hypothyroidism and in affected girls with the syndrome. 
GH release is usually decreased, as in uncomplicated primary 
However, the explanation for the s

However, the explanation for the sexual maturation remains 
uncertain. Pubertal development in primary hypothyroidism is 
usually delayed and is only rarely advanced for chronologic age. 
By the use of radioimmunoassays or other methods for FSH 
and LH in which the cross reaction with TSH is negligible, an 
increased (pubertal) concentration of plasma immunoreactive 
and bioactive FSH, but not LH, has been detected. Bioactive 
LH activity is also low. Increased FSH pulsatility, m

activity is also low. Increased FSH pulsatility, mainly at 
night, but not LH release was demonstrated in patients with the 
syndrome and in some children with primary hypothyroidism 
who did not exhibit premature sexual maturation. The increased 
FSH release888 and the high FSH/LH ratio (in contrast to that 
observed in normal puberty) seem to account for the increased 
ovarian estrogen secretion in girls and for the enlarged testes 
without signs of virilization in affected boys; the suggestio

ns of virilization in affected boys; the suggestion 
is that FSH-induced Sertoli cell proliferation is an important 
determinant of mature testis size. Prolactin is postulated to sup­
press LH in this condition.
Pulsatile TSH release is increased at night, and administra­
tion of TRH appears to increase FSH release in normal chil­
dren (but not in adults). Moreover, the FSH response to TRH, 
but not GnRH, is augmented in primary hypothyroidism, and 
this response can occur in gonadotropin-secret

nd 
this response can occur in gonadotropin-secreting pituitary 
adenomas. It is likely that the incomplete sexual precocity and 
the increased prolactin secretion and galactorrhea are a conse­
quence of the increased release of TRH, the increased sensitiv­
ity of the mammotrophs and gonadotroph to TRH, or both.323 
This mechanism, which has gained support, would explain 
the relatively rapid and complete reversal of the syndrome by 
levothyroxine treatment. Human recombinant TSH at a dose 
abou

e treatment. Human recombinant TSH at a dose 
about 1000-fold greater than that of human FSH evoked a dose-
dependent cAMP response in COS7 cells transfected with the 
1. Synthesis and targeting
2. Receptor
by agonist
Agonist
Receptor
Agonist
Receptor
GTP
GDP
3. Receptor activation
of Gs protein
4. Gs protein-effector
interaction
5. GTPase
Effector
cAMP
McCune-Albright
Arg 201 His
Arg 201 Cys
PHP+testotoxicosis
Ala 366 Ser
Membrane
Cytoplasm
β
γ
α
Receptor

r
Membrane
Cytoplasm
β
γ
α
Receptor
β
γ
α
GTP GDP
Effector
AC
Effector
AC
β
γ
α
• Fig. 26.68  The G protein guanosine triphosphatase (GTPase) cycle. The 
heterotrimeric guanine nucleotide-binding proteins (G proteins), which are 
composed of three subunits (α, β, γ), couple cell surface receptors con­
sisting of a single serpentine polypeptide having seven-helic membrane-
spanning domains with an effector. In this instance, adenylate cyclase (AC) 
catalyzes the transformation of ad

e cyclase (AC) 
catalyzes the transformation of adenosine triphosphate (ATP) to cyclic 
adenosine monophosphate (cAMP). The G protein stimulation subunit α 
(Gs) mediates the stimulation of cAMP generation. In the inactive, unstimu­
lated state, the G protein is a heterotrimer, and guanosine diphosphate 
(GDP) is tightly bound to the α-subunit. When the cell surface receptor 
is activated by its cognate agonist, the receptor catalyzes the release of 
the tightly bound GDP, which enables GTP to b

 of 
the tightly bound GDP, which enables GTP to bind to the α-subunit. The 
GTP-bound α-subunit (αGTP) dissociates from the tightly bound βγ dimer, 
and both play a role in the G protein activation of the effector, adenylate 
cyclase. The intrinsic GTPase activity of the α-subunit ends the stimula­
tion of the effector by converting the bound αGTP to αGDP; as a conse­
quence, the α-subunit again returns to its inactive state and reassociates 
with high affinity with the βγ subunit, yielding the

th high affinity with the βγ subunit, yielding the α, β, γ heterotrimer. Dis­
orders of signal transduction can arise from germ cell or somatic muta­
tions at any of the five stages of the cycle. The gain-of-function, activating 
somatic mutations in the GNAS1 gene that encodes the G Gsα-subunit 
and leads to McCune-Albright syndrome (shown in the bracket), involves 
the highly conserved arginine 201 residue. These mutations inhibit the 
intrinsic GTPase activity of the α-subunit and therefore t

c GTPase activity of the α-subunit and therefore the conversion of 
the bound GTP to GDP. The Ala366Ser mutation (shown in the bracket) 
was detected in two boys, both of whom had pseudohypothyroidism 1a 
(PHP1a) and testotoxicosis. The mutant protein was constitutively acti­
vated in the Leydig cells at the scrotal temperature (32–33°C), leading to 
testotoxicosis, but it was rapidly degraded at body temperature, 37°C, 
which led to PHP1a. (Modified from Spiegel AM. Mutations in G proteins 
and

fied from Spiegel AM. Mutations in G proteins 
and G protein-coupled receptors in endocrine disease. J Clin Endocrinol 
Metab. 1996;81:2434–2442.)
1152
A
B
C
• Fig. 26.69  (A, B) Severe, chronic hypothyroidism of Hashimoto thyroiditis in a girl aged 7 years and 1 
month with sexual precocity (without pubic or axillary hair), episodic vaginal bleeding, and galactorrhea. 
She had symptoms of hypothyroidism and a sharply decreased rate of growth over th

ism and a sharply decreased rate of growth over the previous 2 years 
(height, −1 SD [standard deviation]; bone age, 5 years and 3 months). Breast development was Tanner 
stage 3, the labia minora were enlarged, and the vaginal mucosa was dull pink, thickened, and rugated, 
with evidence of an estrogenic effect. No acne, seborrhea, or hirsutism was present. The uterus was of 
adolescent size, and the endometrial mucosa was in a proliferative phase. Urinary gonadotropins were 
barely detectable b

e. Urinary gonadotropins were 
barely detectable by bioassay. (C) Striking change in appearance after 8 months of thyroid hormone treat­
ment. She had grown 7 cm in height and lost 8.1 kg in weight. The breasts had decreased in size, galactor­
rhea was no longer demonstrable, the labia minora had regressed, and the vaginal mucosa was pink and 
glistening (no estrogen effect). Ten weeks after the initiation of thyroid hormone replacement therapy, she 
developed a right slipped capital femoral epi

she 
developed a right slipped capital femoral epiphysis that was repaired surgically; recovery was uneventful.
human FSH receptor, which suggests another possible but less 
likely mechanism for the FSH-dependent (or FSH-like–depen­
dent) but GnRH-independent sexual precocity.889 A direct effect 
of severe hypothyroidism on the prepubertal testis that leads to 
overproliferation of Sertoli cells also has been advanced as an 
explication of the macroorchidism.
The syndrome is rare but a few cases

croorchidism.
The syndrome is rare but a few cases combine the Van Wyk-
Grumbach syndrome with the Kocher-Debre-Semelaigne 
syndrome, which manifests as lower extremity muscle pseudo­
hypertrophy, elevation of serum creatinine and creatinine kinase 
concentrations, delayed contraction and relaxation of reflexes, and 
myxedema; both manifestations of hypothyroidism can be treated 
with appropriate doses of thyroxine.890,891 
Iatrogenic Sexual Precocity and Endocrine Disruptors. Pre­
pubertal chil

ocity and Endocrine Disruptors. Pre­
pubertal children are remarkably sensitive to exogenous gonadal 
steroids and may show signs of sexual maturation resulting from 
overlooked sources of androgens or estrogens, such as ingested 
or absorbed tonics, lotions, or hair creams or hair straighteners 
that contain or are inadvertently contaminated with an estro­
gen.892 Dermal exposure to estrogen may add up to more than 
300 μg, far in excess of a therapeutic dose and possibly greater 
in infants an

erapeutic dose and possibly greater 
in infants and children exposed to estrogen dermal gel. Com­
pounds containing tea tree and lavender oils caused gynecomas­
tia in three prepuberal boys and demonstrated estrogenic activity 
in vitro.893,894 A short course of application of estrogen cream is 
used to treat labial adhesions, but long courses may lead to breast 
development or even withdrawal bleeding. In addition to breast 
development, pigmentation of the areolae and the linea alba and 
the a

ation of the areolae and the linea alba and 
the appearance of pubic hair may be seen in children exposed to 
dermal estrogen. Children who touch the skin or the towels of 
men using androgen gel therapy may themselves develop viriliza­
tion.895 The administration of hCG to boys with undescended 
testes may induce secretion of testosterone sufficient to cause 
incomplete sexual precocity.
FDA guidelines define a limit of not more than 1% of nor­
mal daily estrogen production in prepubertal child

mal daily estrogen production in prepubertal children as a safe 
intake of estrogen835; this is equivalent to 0.43 ng/day for boys 
and 3.24 ng/day for girls, based on data from extremely sensitive 
estrogen assays, but food is still a suspected source of endocrine 
disruptors.835,892 Epidemics of gynecomastia in boys and thelar­
che in girls among schoolchildren in Italy were suspected to be 
caused by contaminated meat. During a 10-year period, more 
than 600 cases of gynecomastia in boys and 

 more 
than 600 cases of gynecomastia in boys and premature thelarche 
or incomplete sexual precocity in girls were discovered in Puerto 
Rico; this is the highest prevalence reported in the world, about 
10 to 15 times higher than that measured in a survey in Olmsted, 
Minnesota.896,897 Maternal ovarian cysts were demonstrated in 
two-thirds of affected Puerto Rican girls. The clandestine use of 
1153
S.D. 141-14-30
cm
90
80
70
60
50
40
30
20
10
1

rty
cm
90
80
70
60
50
40
30
20
10
1 2 3 4 5
7 8 9 10 11 12 13 14 15 16 17
6
190
180
170
160
150
140
130
120
110
100
90
80
70
60
50
40
100
50
0
kilos
Triiodothyronine µg/d
I  + 21/2 S.D.
II + 1 S.D.
III- 1 S.D.
IV- 21/2 S.D.
I
II
III
IV
I
II
III
IV
A
B
• Fig. 26.70  (A) Radiograph of the skull of a patient with hypothyroidism shows an enlarged pituitary fossa 
in the lateral view. The dorsum sellae was thin and demineralized, and the floor had a double contour line. 
T

lized, and the floor had a double contour line. 
The area of the sella turcica was 150 mm2. Pneumoencephalography showed a suprasellar mass imping­
ing on the cisterna chiasmatica. After thyroid hormone treatment for 8 months, the volume of the sella had 
decreased 30% to 100 mm2, the dorsum sellae had remineralized, and the double floor was no longer 
evident. (B) Growth curve illustrates the decrease in growth rate despite sexual precocity and the catch-up 
growth induced by thyroid hormone th

the catch-up 
growth induced by thyroid hormone therapy. (From Van Wyk JJ, Grumbach MM. Syndrome of precocious 
menstruation and galactorrhea in juvenile hypothyroidism: an example of hormonal overlap in pituitary 
feedback. J Pediatr. 1960;57:416–435.)
estrogen preparations in animals to stimulate weight gain, leading 
to ingestion of estrogen-contaminated meat from these animals, 
was raised as a possible cause, but this was neither confirmed nor 
excluded by selected analyses of meat, poultry

or 
excluded by selected analyses of meat, poultry, and milk in Puerto 
Rico by the US Department of Agriculture.
There are rising concerns that endocrine-disrupting chemicals 
(EDCs), defined as “an exogenous substance that causes adverse 
health effects in an intact organism, or its progeny, secondary to 
changes in endocrine function” exert many effects on growth and 
development of children, including pubertal development.852,898 
Far more conclusive evidence of an adverse effect of EDCs com

nclusive evidence of an adverse effect of EDCs comes 
from animal, rather than human, studies, and some of the data on 
human beings derive from industrial accidents and very high-level 
exposure rather than the lower level exposures most individuals 
experience, so EDCs must be studied in more detail in human 
beings. Method of ascertainment, laboratory methods, activity 
and half-life of various molecules of interest, and the frequent 
difficulties in drawing conclusions from associations rath

ties in drawing conclusions from associations rather than 
controlled studies present complexities in interpreting data.
Girls who were breastfed or exposed during intrauterine life 
to polybrominated biphenyls (PBBs) after an accidental exposure 
of their mothers in Michigan experienced early menarche (by 
about 1 year of age) and early appearance of pubic hair but not 
breast development, compared with girls who were not exposed 
or breastfed.
The NHANES 2003-2004 data demonstrate a slightly l

The NHANES 2003-2004 data demonstrate a slightly lower 
age at menarche with higher serum polybrominated diphenyl 
ether (PBDE, a flame retardant) concentrations.899,900 On the 
contrary, a recent longitudinal study in Hispanic American chil­
dren demonstrated a later age of menarche while boys had an 
earlier onset of pubic hair development with in utero exposure 
to PBDE.901
Exposure to polychlorinated biphenyls (PCBs) in inner-city 
girls with reduced BMI values reportedly delayed thelarche.6

 reduced BMI values reportedly delayed thelarche.63 
There was a 9.56 increase in relative risk for precocious puberty 
treated by GnRH agonist in a localized area in Italy, compared 
with surrounding areas, suggesting the presence of an endocrine 
disruptor in the area.902 Recent longitudinal studies demonstrate 
no effect on secondary sexual development in girls with a slight 
decrease in the age of puberty in boys by elevated levels of phthal­
ate excretion in urine.900 A Chinese study found 

ate excretion in urine.900 A Chinese study found elevated levels 
of LH which corresponded to decreased levels of testosterone in 
children with constitutional delay in puberty who had elevated 
phthalates.903
Widespread exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin 
(TCDD), an extremely potent antiestrogenic xenobiotic, in Italy 
revealed that girls younger than 8 years of age at the time of expo­
sure, who presumably had the highest dose per BMI compared 
with older girls, showed a tendency

 BMI compared 
with older girls, showed a tendency for a decrease in age at men­
arche, suggesting that age of exposure to environmental endocrine 
disruptors modulates their effects.904 Blood concentrations of 
the phenols, 2,5-dichlorophenol and 2,4-dichlorophenol, were 
inversely proportional to the age of menarche in 440 girls included 
in the NHANES for 2003 to 2008.904 Follow-up of adolescents 
1154
who were exposed to di-(2-ethylhexyl)-phthala

Span
who were exposed to di-(2-ethylhexyl)-phthalate (DEHP), a com­
ponent of polyvinyl chloride (PVC) that is used in plastic tub­
ing and medical devices, when they underwent extracorporeal 
membrane oxygenation (ECMO) as neonates, demonstrated no 
effects on pubertal development despite findings of disruption of 
development in animals exposed to this substance, demonstrating 
difficulties in translating animal data to human beings.905
Increased lead levels in Mohawk girls living near the bor

ed lead levels in Mohawk girls living near the border 
of New York and Canada delayed the age of menarche, whereas 
increased levels of PCB promoted it906; surprisingly, variations in 
BMI exerted no effect of these toxic substances, which are con­
centrated in adipose tissue. In the NHANES III survey, a serum 
lead level of 0.7 to 2.0 μg/dL delayed menarche and pubic hair 
development, and African Americans exposed to 3 μg/dL also 
experienced delay in breast development.908 A long-term study 

elay in breast development.908 A long-term study 
of puberty in Chapaevsk, Russia, an area highly contaminated 
with industrial waste, demonstrated 43% reduced odds of enter­
ing stage 2 genitalia among boys age 8 to 9 years with serum lead 
levels equal to or greater than 5 μg/dL.909
There is a high frequency of reproductive problems among 
adult Danish men, including impaired semen quality, testicu­
lar cancer, and increased rate of infantile testicular cancer; these 
disorders occur in a patt

esticular cancer; these 
disorders occur in a pattern that is attributed to environmental 
disruptors, described in the testicular dysgenesis syndrome.669 A 
higher rate of hypospadias in Danish newborns and smaller tes­
tes with lower serum inhibin B, compared with Finnish infants, 
suggested that environmental agents played a role. Phthalates 
were found in the breast milk of mothers from both countries, 
and although there was no relationship to the finding of hypo­
spadias, there was an indi

to the finding of hypo­
spadias, there was an indication of altered reproductive hor­
mones in the boys in a pattern suggesting effects on Leydig cells. 
There may be a genetic component to susceptibility to the TDS 
based upon GWAS analysis, indicating a gene X environment 
basis for the disorder.848
Insulin-like peptide 3 (INSL3) is a testicular product associ­
ated with testicular descent in a chronologic pattern; amnionic 
values of INSL3 during the development of normal genitalia are 
affec

ing the development of normal genitalia are 
affected by exposure to phthalates, which is postulated to offer a 
potential mechanism of the etiology of TDS, although this is not 
proven to date.910,911
Longitudinal observation of girls in Copenhagen, Denmark, 
using physical examination of breast tissue, showed a significant 
decline in the age of Tanner stage 2 breast development (estimated 
mean age, 9.86 years in 2006–2008 vs 10.88 years in 1991) and 
the age at menarche (13.13 vs 13.42 years

91) and 
the age at menarche (13.13 vs 13.42 years, respectively); there 
was no change in serum gonadotropins, but the serum estradiol 
concentration was decreased in 2006.54 Changes in BMI did not 
occur in this study between the cohorts, leaving open the possibil­
ity of endocrine-disrupting chemicals as an explanation.
Boys exposed to PCBs and polychlorinated dibenzofurans 
(PCDFs) in utero from contamination of rice ingested by their 
mothers had decreased testosterone and defects in postpu

s had decreased testosterone and defects in postpubertal 
sperm production in a preliminary study, as well as increased 
estrogen compared with control subjects, although there was 
no difference on physical examination or in age at onset of 
puberty.912 Boys exposed to DDT in utero did not demonstrate 
any abnormalities in puberty in a study from Philadelphia, 
Pennsylvania.854
Proximity to traffic in a longitudinal cohort study of 437 
girls found an earlier onset of signs of puberty of 2 to 9

und an earlier onset of signs of puberty of 2 to 9 months 
compared to unexposed girls. This suggests exposure to air pol­
lutants affects the onset of puberty, although no pollutants were 
measured in this study.913 The complexities of the study of endo­
crine disrupters on pubertal timing are complicated by type, 
amount, developmental age at exposure, and other factors that 
are difficult to tease apart.914 In the United States, chemicals can 
enter the environment before their safety is prov

enter the environment before their safety is proved, leading to 
an ever-expanding list of potential EDCs without data to prove 
their effects. Environmental disrupters have a wide range of del­
eterious effects upon neurodevelopment as well as the endocrine 
system.915
Well-designed longitudinal studies are needed in this area.910 
Diagnosis of Sexual Precocity
As with delayed puberty the majority of patients referred for pre­
cocious puberty will prove to have variations of normal develop­
me

ill prove to have variations of normal develop­
ment or to be inappropriately referred due to a misinterpretation 
of the normal ages of puberty by the referring physicians. How­
ever, precocious puberty can certainly be the outward sign of a 
serious condition. Thus the separation of patients with self-lim­
ited benign disorders, such as premature adrenarche, premature 
thelarche, or normal but early puberty, from those with serious 
or even potentially fatal disorders is the first step in eval

entially fatal disorders is the first step in evaluation 
(Figs. 26.71, 26.72, and 26.73; Table 26.41). The history may 
reveal symptoms suggesting perinatal CNS abnormalities or inju­
ries, previous infections, adventitious ingestion of or exposure to 
gonadal steroids, or the presence of similar conditions in family 
members. Previous measurements should be plotted on a growth 
chart to determine height velocity and the age at onset of any 
increase in the rate of growth.
Important aspects of 

rease in the rate of growth.
Important aspects of the physical examination include 
description of the secondary sexual development according to 
Tanner (sexual maturation) stages; measurement of the penis 
(length gently stretched and width) and the testes in boys (great­
est diameter without the epididymis or volume by comparison 
with an orchidometer) and of the breast tissue in girls (areo­
lar and glandular diameters); and examination for comedones 
and acne, oily skin, facial and body hair

medones 
and acne, oily skin, facial and body hair, pubic and axillary hair 
development, axillary apocrine gland odor, muscular develop­
ment, and galactorrhea. A careful examination of the external 
genitalia should be done with a nonrelated chaperone present. 
A thorough neurologic examination is indicated, with empha­
sis on assessment of the visual fields and optic discs in a search 
for signs of increased intracranial pressure; evaluation for skin 
lesions associated with the McCune-Albrig

or skin 
lesions associated with the McCune-Albright syndrome or neu­
rofibromatosis; and examination for abdominal, gonadal, or 
adnexal masses and for coexisting endocrine disease. Bone age 
is determined in all cases, although it is an imperfect measure 
and the reader must be experienced. Dental development can be 
determined as a rough index of pubertal development916 but is 
rarely invoked clinically except in forensic cases.917
Ultrasonography of the ovary and uterus is exceedingly useful

aphy of the ovary and uterus is exceedingly useful 
in the evaluation of affected girls, because standards are avail­
able for shape and volume of the uterus and the ovaries.918 The 
largest measurements of uterine size by sonography in infants 
and children are found at puberty and in the neonatal period. 
The upper limit of uterine length in the prepubertal state is 3.5 
cm. A uterine volume of greater than 1.8 mL is specific for the 
onset of normal puberty, but increased ovarian volume is le

normal puberty, but increased ovarian volume is less 
specific. Patients with premature thelarche were indistinguish­
able from age-matched control subjects when this sonographic 
standard was used.128 The presence of microcysts and macro­
cysts of the ovary also can be detected on ultrasound examina­
tion. Cysts may be found in the ovaries in patients with CPP or 
1155
Pubic hair
Yes
No
Yes
No
Pubertal
Possibility of
exogenous estrogen

rtal
No
Pubertal
exogenous estrogen?
MRI
Normal
Free T4
Skeletal scintigraphy
Basal LH or LH response to GnRH
Breast development before 7 yrs
Yes
Premature
thelarche
Basal and LHRH-
stimulated LH
(some girls with
CPP will not have
pubic hair)
True precocious
puberty
Exogenous
estrogen
exposure
CNS tumor
or other
lesion
Hamartoma
of the tuber
cinereum
Prepubertal or suppressed
Serum estradiol
Elevated
Ultrasound of ovary and uterus
Ovarian cyst, ovarian tumor
Abnormality
High TSH
N

High TSH
Normal
Polyostotic fibrous dysplasia
central
precocious
puberty
Primary hypo-
or tumor
Café au lait macules likely
• Fig. 26.71  Flow chart for diagnosing sexual precocity in girls. CNS, central nervous system; CPP, cen­
tral precocious puberty; FSH, follicle-stimulating hormone; GnRH, gonadotropin-releasing hormone; LH, 
luteinizing hormone; LHRH, LH-releasing hormone; MRI, magnetic reso

ne; LHRH, LH-releasing hormone; MRI, magnetic resonance imaging; T4, thyroxine; TSH, 
thyroid-stimulating hormone.
GnRH-independent ISP; they usually are smaller than 9 mm 
in the former and larger than 9 mm in the latter919 condition. 
Ovarian volume is reportedly the best indicator of precocious 
puberty, and uterine length was best for the differentiation of 
premature thelarche from premature puberty.920 However, in 
the earliest stages of pubertal development ultrasonography of 
the pelvis 

bertal development ultrasonography of 
the pelvis may not differentiate the patient from the prepubertal 
state, although gonadotropin values will do so.921 The presence 
of an endometrial stripe was indicative of precocious puberty.128 
Ultrasonography of the breast is reported as a method of deter­
mining rapidly progressive versus slowly progressive or transient 
precocious puberty; accuracy is increased when breast ultra­
sound findings are added to those of uterine ultrasound and the 
other

dded to those of uterine ultrasound and the 
other factors discussed previously.922
CPP in males usually begins with enlargement of the testes, 
followed by other signs of secondary sexual maturation. A Ley­
dig cell tumor usually causes asymmetric enlargement of the tes­
tes, whereas an extragonadal hCG-secreting tumor is associated 
with less marked testicular enlargement than occurs at the same 
stage of masculinization in CPP. Testicular adrenal rest tissues may 
enlarge in boys with CAH and

nal rest tissues may 
enlarge in boys with CAH and may be bilateral, although they are 
unlikely to closely mimic normal pubertal testicular development. 
An elevated hCG level with prepubertal gonadotropin values or 
GnRH test indicates an ectopic, autonomous, gonadotropin-
secreting tumor. If this tumor is in the CNS, abnormalities will 
likely be present on MRI or CT brain scans. Enlargement of the 
liver or a mediastinal, retroperitoneal mass in boys with sexual 
precocity suggests an hCG-pr

 in boys with sexual 
precocity suggests an hCG-producing hepatic or germ cell tumor; 
the possibility of Klinefelter syndrome needs to be considered in 
the latter case.
It is essential that laboratory tests for sex steroids and gonado­
tropin be carried out by appropriate methods in the experienced 
laboratories. Simply ordering LH or FSH levels in a standard 
laboratory will usually lead to a determination of whether there 
are high levels with gonadal failure such as menopause or normal 
lev

h gonadal failure such as menopause or normal 
levels for adults, but rarely can such an assay determine subtle 
changes that are characteristic of childhood and adolescence. A 
highly sensitive third-generation assay with pediatric standards 
1156
Yes
Clitoral enlargement
No
DHEAS
Early pubertal
Premature
Yes
DHEAS
Cortisol
High
Virilizing adrenal
tumor
Normal or
slightly elevated
Elevated
17-OH-P
High
Cortico

17-OH-P
High
Corticotropin stimulation
Elevated
No signs of
Normal rise
in 17-OH-P
Free
High
17-OH-P rises
abnormally high
Virilizing ovarian
tumor
Late-onset CYP21
Pubic hair before 7 years of age
• Fig. 26.72  Flow chart for the evaluation of pubic hair in normal phenotypic girls before 7 years. DHEAS, 
dehydroepiandrosterone sulfate; 17-OH-P, 17-hydroxyprogesterone.
must be ordered. Further, stand

droxyprogesterone.
must be ordered. Further, standard testosterone assays can differ­
entiate between a normal male and one with gonadal failure but 
cannot determine the low levels characteristic of the early stages of 
puberty. For children and adolescents (and indeed for women) an 
HPLC-MS/MS method with pediatric standards is necessary; the 
same type of assay must be used for the determination of estradiol 
levels in girls.266
Measurements of basal plasma gonadotropin concentrations 
and th

f basal plasma gonadotropin concentrations 
and the LH response to administration of GnRH (presently not 
available) or GnRH agonist or the amplitude and frequency of 
LH pulses, especially at night, using third-generation assays with 
pediatric standards, as well as measurements of the plasma con­
centration of testosterone in boys or of estradiol in girls using LC/
MS-MS assays, are of primary importance in diagnosis. Patients 
with a basal LH level 0.3 IU/L or higher had subsequent pubertal 

level 0.3 IU/L or higher had subsequent pubertal 
progression, whereas 39 of 41 patients with a basal LH 0.2 IU/L 
or lower did not progress, resulting in 100% specificity (95% CI 
92–100%) and 90.5% sensitivity (69.6–98.8%) in one report.261 
Girls early in the course of CPP may have elevation of estradiol 
(this is not always noted, however) associated with increasing LH 
levels but not necessarily an increase in the concentration of FSH. 
Boys will have rising values of testosterone as pubert

 will have rising values of testosterone as puberty commences. 
Pubertal concentrations of LH and FSH, a pubertal mode of 
pulsatile LH secretion (initially during sleep), or pubertal LH 
response to GnRH or GnRH agonist confirms the diagnosis of 
CPP (and in boys differentiates CPP from familial testotoxicosis). 
Determination of T4 concentration (usually free T4) is indicated 
when hypothyroidism as a cause of sexual precocity is suspected.
A CNS tumor must be considered as a potential cause o

NS tumor must be considered as a potential cause of pre­
mature activation of the hypothalamic GnRH pulse generator, 
especially in boys. The evaluation for a CNS tumor as a cause of 
CPP is similar to the investigation of an hCG-secreting tumor of 
the CNS.923 Although CT scanning is a well-established proce­
dure for determining the presence of a CNS abnormality, MRI 
with contrast is more sensitive for the detection of small tumors 
in the hypothalamus, such as a hamartoma of the tuber cinere

othalamus, such as a hamartoma of the tuber cinereum 
(see Fig. 26.55). The use of contrast adds to diagnostic certainty 
and is recommended for MRI of the CNS. All boys with CPP 
1157
Phallic enlargement before age 9 years
No
Pubic hair present
Serum DHEAS
Possibility of exogenous
androgens or hCG?
value
<2.5 cm longest
diameter or 2.5 mL
Exogenous androgen or hCG
Elevated
Normal
Pubertal
Positive

size
Elevated
Normal
Pubertal
Positive
Normal
Elevated
Basal 17-OH-P
CYP21
OH
Small increase over
2.5 cm or 3 mL
Elevated testosterone
17-OH-P
Adrenal etiology
Definite increase over
2.5 cm or 4 mL
enlargement
Elevated
17-OH-P
Elevated
DHEAS
Cortisol
elevated
elevated
Elevated
No signs of
such as weight gain
or growth failure
Serum
hCG
Basal LH or LH
Adrenal
tumor
hCG-
secret

H or LH
Adrenal
tumor
hCG-
secreting
tumor
True
puberty
Gluco-
corticoid
Elevated
17-OH-P
response
Testo-
toxicosis
tumor
Premature
CYP21-
deficient
heterozygote
or
late-onset
CAH
TART
with
CYP21-
deficient
CAH
Yes
No
Yes
• Fig. 26.73  Flow chart for diagnosing sexual precocity in a phenotypic male. CAH, congenital adrenal 
hyperplasia; DHEAS, dehydroepiandrosterone sulfate; hCG, human chorionic gonadotropin; LH, lutein­
izing hormone;

chorionic gonadotropin; LH, lutein­
izing hormone; LHRH, LH-releasing hormone; 17-OH-P, 17-hydroxyprogesterone; TART, testicular adrenal 
rest tissue.
should have CNS MRI evaluation, but girls do not always receive 
the same recommendation because a CNS tumor is less likely in 
girls than in boys to be the cause of CPP. However, studies using 
MRI or CT brain scans indicate that the hypothalamic hamar­
toma is more prevalent in both boys and girls with so-called idio­
pathic CPP than was previou

s with so-called idio­
pathic CPP than was previously suspected. An unselected group of 
girls with precocious puberty and no other symptoms underwent 
CNS MRI; 15% were found to have intracranial trauma, and the 
investigators found no clinical difference between those girls and 
the other 85% of girls studied, supporting the use of CNS MRI 
for girls with precocious puberty.924
The height of the pituitary gland on MRI correlates with 
advancing age and with pubertal development925; patients wi

 age and with pubertal development925; patients with 
CPP had pituitary heights exceeding 6 mm on average, whereas 
those with precocious thelarche had lower heights. The shape of 
the pituitary gland is also of importance: A convex appearance 
rather than a flat top is associated with CPP of any cause.749 Physi­
ologic enlargement of the pituitary gland characteristic of puberty 
is the source of many needless neurosurgery referrals.926 The size 
and shape of the pituitary gland do not decrease

and shape of the pituitary gland do not decrease with successful 
GnRH therapy.
T1-weighted images indicate a convex upper border of the 
pituitary gland in both normal patients and those with CPP, 
indicating the similarity in the physiologic changes in both puber­
tal conditions. Pituitary gland hyperplasia (height >1 cm) is a 
rare finding reported in CPP. The empty sella syndrome is less 
frequently observed in patients with CPP than in patients with 
pituitary hypofunction. Empty sella is

ients with 
pituitary hypofunction. Empty sella is found in 10% of children 
who are imaged for suspected hypothalamic-pituitary disorders, 
including hypogonadotropic hypogonadism.927
The premature appearance of pubic hair, phallic enlargement, 
and other signs of virilization in a male without enlargement of 
the testes or the liver suggests the diagnosis of virilizing CAH, 
virilizing adrenal tumor, or, more rarely, Cushing syndrome. 
Measurement of plasma 17-hydroxyprogesterone and DHEAS 
co

ent of plasma 17-hydroxyprogesterone and DHEAS 
concentrations and their suppressibility with glucocorticoids 
will distinguish 21OH-deficient CAH from a virilizing adrenal 
tumor. If growth rate is suppressed, the possibility of primary 
hypothyroidism or Cushing syndrome is raised; elevated plasma 
concentrations of cortisol, urinary free cortisol, 17-hydroxycor­
ticosteroid, or salivary cortisol after suppression with dexametha­
sone confirm the latter diagnosis. The appearance in a girl of p

he latter diagnosis. The appearance in a girl of pubic 
hair and other signs of virilization, such as clitoral enlargement, 
acne, deepening voice, muscular development, or growth spurt, 
is caused by virilizing CAH, virilizing adrenal tumor, or virilizing 
ovarian tumor. Cushing syndrome caused by an adrenocortical 
1158
  Differential Diagnosis of Sexual Precocity
Disorder
Plasma 
LH Response to 
GnRHa
Serum Sex Steroid 

GnRHa
Gonadal Size
Gonadotropin Dependent
True precocious 
puberty
Prominent LH 
pulses (prema­
ture reactivation 
of GnRH pulse 
generator)
Pubertal LH 
response initially 
during sleep
Pubertal values of tes­
tosterone or estradiol
Normal pubertal tes­
ticular enlargement 
or ovarian and uterine 
MRI of brain to rule out 
CNS tumor or other 
abnormality; skeletal 
survey for McCune-
Albright syndrome (by 
US)
Incomplete Se

r McCune-
US)
Incomplete Sexual Precocity (Pituitary Gonadotropin-Independent)
Males
Chorionic gonado­
tropin-secreting 
tumor in males
High hCG, low LH
Prepubertal LH 
response
Pubertal value of testos­
terone
Slight to moderate 
uniform enlargement 
of testes
Hepatomegaly suggests 
hepatoblastoma; 
CT scan of brain if 
chorionic gonadotropin-
secreting CNS tumor 
suspected
Leydig cell tumor in 
males
No LH response
Very high testosterone
Irregular, asymmetric 

onse
enlargement of testes
Familial testotoxi­
cosis
Pubertal values of 
Testes symmetric and 
>2.5 cm but smaller 
than expected for 
pubertal development; 
spermatogenesis 
occurs
Familial; probably sex-
limited, autosomal 
dominant trait
Virilizing congenital 
adrenal hyper­
plasia
response
Elevated 17-OHP in 
CYP21 deficiency or 
elevated 11-deoxy­
cortisol in CYP11B1 

levated 11-deoxy­
Testes prepubertal
Autosomal recessive, may 
be congenital or late-
onset form, may have 
salt loss in CYP21 defi­
ciency or hypertension 
in CYP11B1 deficiency
Virilizing adrenal 
tumor
response
High DHEAS and andro­
stenedione values
CT, MRI, or US of abdomen
Premature adre­
narche
response
Prepubertal testosterone, 
DHEAS, or urinary 
17-ketosteroid values 

S, or urinary 
appropriate for pubic 
hair stage 2
Onset usually after 6 yr of 
age; more frequent in 
CNS-injured children
Females
Granulosa cell 
tumor (follicular 
cysts may pres­
ent similarly)
response
Very high estradiol
Ovarian enlargement on 
physical examination, 
CT, or US
Tumor often palpable on 
physical examination
Follicular cyst
response
Prepubertal to very high 
estradiol
Ovarian enlar

estradiol
CT, or US
Single or recurrent epi­
sodes of menses and/
or breast development; 
exclude McCune-
Albright syndrome
Feminizing adrenal 
tumor
response
High estradiol and 
DHEAS values
Ovaries prepubertal
Unilateral adrenal mass
Prepubertal LH, 
pubertal
Prepubertal or early 
estradiol response
Onset usually before 3 yr 
of age
TABLE 26.4

ertal
of age
TABLE 26.41
1159
carcinoma can result in virilization associated with growth fail­
ure, and a virilizing adrenocortical carcinoma can manifest with so 
much androgen effect that the signs of Cushing syndrome are not 
apparent, as rapid growth and virilization are first noted; estradiol 
may be secreted by these tumors as well as androgens. Virilizing 
ovarian tumors may be detected by pelvic ultrasonography.

tumors may be detected by pelvic ultrasonography.
The appearance of pubic hair without other signs of puberty in 
boys or girls is usually a result of premature adrenarche but may 
alternatively be the first sign of sexual precocity or of adrenal viril­
ism from other causes.
In a girl, breast development associated with dulling and 
thickening of the vaginal mucosa leading to a more pink than red 
appearance and enlargement of the labia minora indicate signifi­
cant estrogen secretion or iatrog

ndicate signifi­
cant estrogen secretion or iatrogenic exposure to estrogen. The dif­
ferential diagnosis includes CPP, an estrogen-secreting neoplasm, 
aromatase excess, and a cyst of the ovary. If the plasma concentra­
tions of gonadotropins are in the pubertal range, if LH pulses of 
pubertal amplitude are detected, or if a pubertal LH response to 
GnRH or GnRH agonist is elicited, CPP is present. In one report, 
a child had pubertal-level serum LH due to heterophile antibodies 
that interfer

um LH due to heterophile antibodies 
that interfered with the LH assay and fallaciously elevated the val­
ues in the basal and stimulated state; after addition of antimouse 
antibody, LH values decreased.928 As always, clinical observation 
should be congruent with laboratory findings, and the assays 
should be highly sensitive and associated with valid pediatric stan­
dards. Estradiol concentrations in girls early in normal puberty or 
CPP are in the prepubertal range for much of the day, and a

n the prepubertal range for much of the day, and a single 
determination may be inadequate to reflect ovarian function.160
If the concentration of plasma estradiol is elevated but gonadotro­
pin levels are low, an estrogen-secreting cyst or neoplasm is present 
or exogenous estrogens are the cause. Ovarian tumors of moderate 
size can be palpated by bimanual examination. Advances in pelvic 
sonography allow the delineation of ovarian cysts or tumors and the 
determination of uterine size and the

ors and the 
determination of uterine size and the size of the endometrial stripe.128 
An estrogen-secreting neoplasm of the ovary is usually accompanied 
by high estradiol concentrations, but some ovarian cysts are associated 
with concentrations of estradiol as high as those in granulosa cell 
tumors; the differential diagnosis between these cysts and ovarian 
neoplasms rarely requires exploratory laparotomy or laparoscopy and 
usually can be resolved by pelvic sonography and the use of tumor 

esolved by pelvic sonography and the use of tumor 
markers. Breast development in the absence of other estrogen effects 
is almost always a result of premature thelarche. 
Contrasexual Precocity: Feminization in Boys and 
Virilization in Girls
Boys
Feminization in a boy before the age of puberty is rare. Rarely, 
an estrogen-secreting adrenal adenoma or a chorionepithelioma 
causes gynecomastia. Gynecomastia has been reported in a 1-year-
old boy with 11β-hydroxylase deficiency and in boys with 

 with 11β-hydroxylase deficiency and in boys with late-
onset CAH.929
Aromatase Excess Syndrome. Gynecomastia in prepuber­
tal boys can be caused by increased extraglandular aromatiza­
tion of C19 steroids of adrenal origin, such as androstenedione, 
and subsequent increased extraglandular estrogen production in 
sporadic or familial cases. The autosomal dominant form leads 
to excess estrogen synthesis from C19 precursors due to aroma­
tase overexpression, especially in fat and skin; it is a co

expression, especially in fat and skin; it is a consequence 
of gain-of-function mutations of CYP19, the gene that encodes 
aromatase, resulting from a chromosome arrangement that gives 
rise to a cryptic promoter.930 An autosomal dominant pattern of 
prepubertal gynecomastia and adult hypogonadism but not short 
stature in the presence of elevated serum estrone (with little or 
no elevation of estradiol) has been reported; the mutations in the 
CYP19 gene in these patients appeared to be differ

CYP19 gene in these patients appeared to be different from those 
in the families described earlier with gain-of-function mutations 
resulting from gene inversions.931 In a Turkish kindred, there was 
a potential rearrangement between CYP19 and TRPM7 genes on 
chromosome 15q21.2 as a cause of aromatase excess syndrome. 
Aromatase excess syndrome is caused by duplications involving 
CYP19A1 and simple and complex rearrangements generated by 
Disorder
Plasma 
GnRHa
Se

der
Plasma 
GnRHa
narche
response
Prepubertal estradiol; 
DHEAS or urinary 
brain-injured children
Late-onset virilizing 
congenital adre­
nal hyperplasia
response
basal or corticotropin-

sponse
basal or corticotropin- 
stimulated state
In Both Sexes
McCune-Albright 
syndrome
Sex steroid pubertal or 
higher
Ovarian (on US); slight 
testicular enlargement
Skeletal survey for polyos­
totic fibrous dysplasia 
and skin examination 
for café au lait spots
Primary hypothy­
roidism
LH prepubertal; FSH 
may be slightly 
elevated
Prepubertal FSH may 
be increased
Estradiol may be 
pubertal
Testicular enlarge

ased
pubertal
Testicular enlargement; 
ovaries cystic
TSH and prolactin elevated; 
T4 low
CNS, Central nervous system; CT, computed tomography; CYP, P450 cytochrome isoenzyme; DHEAS, dehydroepiandrosterone sulfate; GnRH, gonadotropin-releasing hormone; hCG, human chorionic 
gonadotropin; LH, luteinizing hormone; MRI, magnetic resonance imaging; 17-OHP, 17-hydroxyprogesterone; T4, thyroxine; TSH, thyrotropin; US, ultrasonography.
   
  Differential Diagnosis of Sexual Precocity—cont’d
1160
both recombination-mediated and replication-mediated mecha­
nisms, independent of the known rearrangement-inducing DNA 
features or late-replication timing.932 
Feminizing Testicular Tumors. Feminizing testicular tumors 
may cause gynecomastia in boys younger than 6 years of age who 
have the Peutz-Jeghers syndrome.860,933 Aromatase is absent or 
is present in barely detectabl

atase is absent or 
is present in barely detectable amounts in prepubertal testes, but 
maximal amounts appear in late puberty. In normal testes, aroma­
tase is predominantly present in the Leydig cells, but in testicular 
tumors of Sertoli cells or Leydig cells (e.g., associated with the 
Peutz-Jeghers syndrome), the Sertoli cells of the tumor express 
aromatase. Both testes may be enlarged, and the histologic exami­
nation indicates sex cord or Sertoli cell tumors that form annular 
tubules an

 Sertoli cell tumors that form annular 
tubules and often have areas of calcification; increased estradiol 
secretion is noted in the basal state, and a further rise occurs after 
hCG administration. Otherwise, feminizing Sertoli cell tumors 
are very rare in boys.934 Sonography or MRI of the testes may be 
useful in making the diagnosis.
In one series, 5% of 581 boys referred for evaluation of gyne­
comastia were prepubertal at diagnosis (mean age, 9 years), and in 
93% no underlying cause was 

ge, 9 years), and in 
93% no underlying cause was identified.935 Spontaneous resolu­
tion was recorded in 6 boys, no change was found in 15, and fur­
ther breast enlargement was found in 6. Prepubertal gynecomastia 
can also be caused by neurofibromatosis. 
Girls
Adrenal Causes of Virilization. CAH resulting from 21-hydrox­
ylase or 11β-hydroxylase deficiency or from androgen-producing 
tumors of the adrenal can cause virilization (see earlier discussion 
of their occurrence in males). Nonclassi

scussion 
of their occurrence in males). Nonclassic or late-onset forms of 
CAH do not demonstrate ambiguous genitalia, but there is evi­
dence of androgen effect in prepuberty or the teenage years. The 
etiology is usually 21OH deficiency. 3βHSD/Δ4,5-isomerase 
deficiency is a rare type of CAH characterized by elevated levels 
of Δ5-17P, DHEA, and DHEAS, as well as decreased secretion 
of aldosterone and cortisol in the severe form. Severely affected 
patients have mineralocorticoid and glucoco

ected 
patients have mineralocorticoid and glucocorticoid deficiencies 
and may die in infancy. Excess adrenal androgens lead to viril­
ization in utero and to ambiguous external genitalia, including 
clitoral enlargement in females with continued virilization after 
birth. Milder forms of this disorder can cause hirsutism in women. 
Women with a 46,XY phenotype and incomplete forms of andro­
gen resistance syndrome or 17βHSD3 deficiency may have vir­
ilization as well as breast development at t

 vir­
ilization as well as breast development at the time of expected 
puberty. Aromatase deficiency due to mutations in the CYP19 
gene, which encodes aromatase, is associated with intrauterine 
masculinization of the external genitalia in affected 46,XX indi­
viduals and with progressive virilization, lack of female secondary 
sex characteristics, multicystic ovaries at the age of puberty, tall 
stature, and osteopenia.272,274,301,303
Cushing syndrome resulting from adrenal carcinoma usually 

yndrome resulting from adrenal carcinoma usually 
manifests as growth failure with or without virilization, obesity, 
and moon facies; striae may not appear until months to years later. 
Syndrome of Glucocorticoid Resistance. The syndrome of glu­
cocorticoid resistance has variable manifestations. Some patients 
demonstrate hyperandrogenic signs such as acne, hirsutism, male-
type baldness, menstrual irregularities, and oligoanovulation 
and infertility.936 Dexamethasone decreases the excessive 

rtility.936 Dexamethasone decreases the excessive adrenal 
androgen secretion, virilization, and advancing bone age found in 
general glucocorticoid resistance. 
Virilizing Ovarian Tumors. Arrhenoblastoma, also called Sertoli 
tumor of the ovary, is the most common virilizing ovarian tumor, 
but it is rare in children.871 Recently somatic missense mutations 
affecting the RNase IIIb domain of DICER1 that alter DICER1 
function in a manner that perturbs microRNA processing to 
become oncogenic ar

rturbs microRNA processing to 
become oncogenic are common in these nonepithelial ovarian 
tumors.937 Lipoid-cell tumors of the ovary and gonadoblastomas of 
the ovary are even more unusual sources of androgens. 
Variations of Pubertal Development
Premature Thelarche
Unilateral or bilateral breast enlargement without other signs of 
sexual maturation (e.g., sexual hair, growth of the labia minora, 
growth of the uterus) is not uncommon in infancy and childhood 
and is termed premature thelarche.

 and childhood 
and is termed premature thelarche. The disorder usually occurs by 
age 2 (>80% of cases) and rarely after age 4.938 In a retrospec­
tive study in Minnesota, the incidence of premature thelarche was 
21.2 per 100,000 patient-years, 60% of cases were identified in 
patients between 6 months and 2 years of age, and most cases 
regressed within 6 months to 6 years after diagnosis, although a 
few persisted until puberty. The prevalence of premature thelarche 
appears to be increasing

e of premature thelarche 
appears to be increasing in Denmark.940 When 10-year to 35-year 
follow-up of premature thelarche was available, no untoward 
effects on later health, growth, or fertility were evident.897 Breast 
enlargement usually regresses after a few months but occasionally 
persists for years or until the onset of normal puberty; in about 
half of affected girls, the breast development, which is character­
istically cyclic, lasts 3 to 5 years, and this is found with age of 
onset 

 to 5 years, and this is found with age of 
onset over 2 years.939 Usually significant nipple and areola devel­
opment is absent, and estrogen-induced thickening and dulling 
of the vaginal mucosa is uncommon. Enlargement of the uterus 
on ultrasonography (volume >1.8 mL, length >36 mm) is rare. 
Measurement of the ellipsoid volume of the uterus (V = longitu­
dinal diameter × anteroposterior diameter × transverse diameter 
× 0.523) is the most sensitive and specific discriminator between 
premat

nsitive and specific discriminator between 
premature thelarche and early128 CPP and provides better early 
discrimination than the LH response to GnRH or GnRH agonist. 
Growth in stature is normal.
This is a benign, self-limited disorder that is compatible 
with normal pubertal development at an appropriate age; usu­
ally, only reassurance and follow-up are necessary. However, the 
appearance of premature thelarche can be the harbinger of fur­
ther sexual maturation in a minority of cases. Alth

her sexual maturation in a minority of cases. Although onset 
occurring soon after birth and before 2 years of age carries a 
higher prognosis for regression, 14% of two large series did 
progress no matter the age of onset of premature thelarche.940 
Because the breast development may be unilateral, it is impor­
tant to consider the condition in girls who have unilateral breast 
development so that needless worry about a breast neoplasm is 
not stimulated in the parents and no unnecessary surgi

stimulated in the parents and no unnecessary surgical pro­
cedure is carried out. Removal of tissue in premature thelarche 
may leave the child with no possibility of future breast develop­
ment. In selected instances, sonography of the breast is useful 
to distinguish unilateral premature thelarche from less benign 
conditions but is of limited value in differentiating premature 
thelarche from precocious puberty.941 The most common cause 
of a breast mass in a pubertal girl is fibroadenoma, an

breast mass in a pubertal girl is fibroadenoma, and although 
metastatic disease may locate in the pubertal breast, breast car­
cinoma is exceedingly rare in young patients. Plasma estradiol 
levels are prepubertal in most standard assays but were slightly 
higher for age in patients with premature thelarche determined 
by a highly sensitive estrogen bioassay.942,943 However, there is 
usually no significant increase in plasma levels of TeBG or in 
1161
Chapter 26  Physiology and Disorders of Pu

n 
1161
thyroxine-binding globulin, which are indicators of estrogen 
action on circulating plasma proteins, and TeBG is suggested as 
one method of differentiating premature thelarche from PP.940
The urocytogram often reveals an estrogen effect on squamous 
epithelial cells in the urine in premature thelarche.840,944
Some endocrine measurements are proposed to differenti­
ate PT from CPP. The concentration of serum FSH may be in 
the pubertal rang

ntration of serum FSH may be in 
the pubertal range, nocturnal FSH pulsatility has been detected, 
and the rise in FSH elicited by administration of GnRH may be 
augmented for chronologic age, with an FSH/LH ratio higher in 
precocious thelarche than in normal individuals or in girls with 
CPP. Girls under 48 months with mild premature thelarche had 
17βE2 levels below 32 pmol/L using extraction RIA, while those 
with CPP have values over 70.935 However, these methods are not 
infallible in diff

However, these methods are not 
infallible in differentiating the two conditions.
As postulated for some recurrent ovarian cysts, premature the­
larche appears to result from the ovarian response to transient 
increases in FSH levels and possibly from variations in ovarian 
sensitivity to FSH. The LH response to GnRH is prepubertal in 
all cases.437,945 Plasma inhibin B and FSH levels are higher in girls 
with precocious thelarche than in control subjects, in a range simi­
lar to that observed i

 subjects, in a range simi­
lar to that observed in patients with precocious puberty. Activin 
concentrations have not been reported. The possible role of a 
paracrine-acting pituitary factor in stimulating FSH independent 
of GnRH is not known.
Sonograms of the ovary may show one or several cysts larger 
than 0.5 cm that disappear and reappear, usually correlating with 
changes in the size of the breasts, but the volume of the ovary and 
uterus is prepubertal.128 In clinical practice, it is rar

is prepubertal.128 In clinical practice, it is rare to find a cyst 
at the time of presentation and on ultrasonic study.
Exaggerated thelarche is described as premature thelarche with 
the added findings of advanced bone age and increased growth 
rate, which are estrogen effects. The endocrine measurements 
in the basal state are in the normal prepubertal range, but after 
GnRH agonist stimulation, the level of FSH (but not of LH) rose 
higher than in control subjects or in patients with CPP. Mu

an in control subjects or in patients with CPP. Mutation 
of GNAS1 involving Arg201His is associated with apparent pre­
mature or exaggerated thelarche and early menarche.730
Unfortunately, there are no guidelines that can determine 
which girl will undergo progression from premature thelarche to 
precocious puberty. Clinical follow-up is essential to determine 
which course will occur. 
Premature Isolated Menarche
Rarely, girls begin periodic vaginal bleeding at between 1 and 9 
years of age wi

ginal bleeding at between 1 and 9 
years of age without any other signs of secondary sexual develop­
ment. The bleeding can recur for 1 to 6 years and then cease. At 
the normal age of puberty (3–11 years later), secondary sexual 
development and menses ensue and follow a normal pattern, as 
does stature.946
Fertility was later demonstrated after a normal onset of puberty 
in women with this variant of pubertal development. The cause 
is uncertain, but it may be a counterpart of premature thelar

n, but it may be a counterpart of premature thelarche. 
There is a predominance of FSH secretion, but the gonadotropin 
secretion pattern is not characteristic of CPP.947 Isolated men­
arche may appear before other manifestations of sexual precoc­
ity in patients with the McCune-Albright syndrome and in those 
with the premature sexual maturation that can occur in juvenile 
Before the diagnosis of premature menarche is accepted, all 
other causes of vaginal bleeding and precociou

ll 
other causes of vaginal bleeding and precocious estrogen secre­
tion and of exposure to exogenous estrogens should be excluded, 
including neoplasms, granulomas, infection of the vagina or cer­
vix, and presence of a foreign body. A careful examination for 
trauma, such as that caused by sexual abuse, is indicated. Urethral 
prolapse may be misdiagnosed as vaginal bleeding. 
Premature Adrenarche
Premature adrenarche causes the precocious appearance of pubic 
hair (i.e., pubarche), axillary h

arance of pubic 
hair (i.e., pubarche), axillary hair and apocrine axillary odor, com­
edones, and acne, without other signs of puberty or virilization. 
It is characterized by premature and mild adrenal hyperandrogen­
ism.444,948 The term premature adrenarche refers to the rise in serum 
concentrations of adrenal androgens that cause the appearance of 
the pubic hair, which is termed premature pubarche. In the past, 
this designation was assigned when these clinical features appeared 
before ag

d when these clinical features appeared 
before age 8 years in girls or 9 years in boys. Although in boys the 
9 years still seems appropriate as a cutoff point, the age of 8 years 
may no longer be appropriate for girls, according to the results 
of previously quoted studies showing earlier onset of puberty in 
recent years (mean ages are shown in Table 26.1). We recommend 
that the diagnosis of premature pubarche should be limited to 
African-American girls younger than 6 years of age and Cauc

merican girls younger than 6 years of age and Cauca­
sian American girls younger than 7 years, which should affect the 
age at which laboratory studies are initiated unless there are other 
signs of virilization, such as clitoromegaly or rapid growth.
Premature adrenarche is about 10 times more common in 
girls than in boys.289,949,950 The prevalence is increased in chil­
dren with CNS abnormalities without a clear sex difference; the 
electroencephalogram may be abnormal in the absence of other

ephalogram may be abnormal in the absence of other 
neurologic findings. Familial transmission is uncommon. Prema­
ture adrenarche is commonly slowly progressive and does not have 
an untoward effect on either the onset or the normal progression 
of gonadarche or final adult height.444 Nonetheless, there is a 
relationship between reduced fetal growth leading to intrauter­
ine growth retardation and subsequent SGA and the increased 
prevalence of premature adrenarche, and probably ovarian hyper­

 premature adrenarche, and probably ovarian hyper­
androgenism in life, although the relationship may vary among 
different ethnic groups.951
Plasma concentrations of DHEA, DHEAS, androstenedione, 
testosterone, 17-hydroxyprogesterone, and Δ5-17P are usually 
comparable to values normally found in pubic hair stage 2. How­
ever, some patients have low concentrations of adrenal andro­
gens, indicating that conversion of these molecule to more active 
androgens or a rise in androgen receptor activi

e 
androgens or a rise in androgen receptor activity is as important in 
the development of premature pubarche as the adrenal androgen 
secretion itself.444 An ACTH stimulation increases serum DHEA 
and DHEAS concentrations and the excretion of urinary 17-keto­
steroids, but the concentrations of plasma 17-hydroxyprogester­
one and Δ5-17P do not increase to the levels found in individuals 
with virilizing forms of CAH.884 Shorter AR gene CAG number, 
indicative of increased androgen sensitivity,

er, 
indicative of increased androgen sensitivity, is reported in some 
girls with precocious adrenarche, suggesting that increased sen­
sitivity to low androgen levels may be the basis rather than high 
androgen values per se. As in CAH, dexamethasone suppresses 
adrenal androgen and androgen precursor secretion.952
Serum gonadotropin levels in the basal state and after GnRH 
are in the prepubertal range in patients with premature adre­
narche.437 Premature adrenarche occurs independently of go

37 Premature adrenarche occurs independently of gonad­
arche and results from some unknown factor other than increased 
secretion of GnRH or ACTH. Bone age, height, and weight gain 
are slightly advanced for chronologic age, but normal adult height 
is commonly achieved, except, rarely, in some individuals with 
unusually high levels of adrenal androgens, hirsutism, acne, and a 
1162
bone age more than 2 years advanced or 2.5 SD above the mean 
value

n 2 years advanced or 2.5 SD above the mean 
value for chronologic age.953 In a follow-up study of 20 girls, the 
functional adrenal hyperandrogenism in premature adrenarche 
was limited to childhood.
Premature adrenarche may be considered to be a developmen­
tally regulated, normal variation in the differentiation, growth, 
and function of the zona reticularis of the adrenal cortex, marked 
biochemically by the precocious increase in the concentration of 
plasma DHEAS to more than 40 μg/dL.290 

ration of 
plasma DHEAS to more than 40 μg/dL.290 The latter is prob­
ably related to the independent increase of 17,20-lyase activity in 
the developing zona reticularis, which is mediated by increased 
phosphorylation of serine and threonine residues on the CYP17 
enzyme, and the increased abundance of cytochrome b5 and of 
electron-donating redox partners such as cytochrome P450 oxi­
doreductase and cytochrome b5, which are essential for the 17,20-
lyase activity of this functional microsomal

7,20-
lyase activity of this functional microsomal enzyme (see Fig. 
26.36).884 A rise in intra-adrenal cortisol may inhibit 3βHSD 
activity, thereby increasing DHEA according to in  vitro evi­
dence.948 Nonetheless, the factors stimulating development and 
function of the zona reticularis, independent of ACTH, remain 
elusive.
The appearance of premature pubarche can be a manifesta­
tion of nonclassic 21-hydroxylase deficiency CAH caused by 
homozygous or compound heterozygous missense mutation

ozygous or compound heterozygous missense mutations in 
the CYP21 gene.954 This condition can readily be detected by a 
plasma 17-hydroxyprogesterone above 200 ng/dL (6 nmol/L) in 
the basal state or above 2000 ng/dL or 72 nmol/L in response to 
ACTH, which is at least 10 SD above the mean value for age. 
The prevalence of nonclassic 21-hydroxylase deficiency in children 
apparently presenting with premature adrenarche is low except in 
some ethnic groups (e.g., Hispanics, Italians, Ashkenazi Je

ic groups (e.g., Hispanics, Italians, Ashkenazi Jews), in 
which the prevalence may be as high as 20% to 30%.
The cause of the observed mild deficiency in 3βHSD activ­
ity is unknown, but it may be multifactorial and may lead to a 
wide range in secretory capacity of the zona reticularis. A family 
constellation was described with a dominant pattern of inheri­
tance of elevated adrenal androgens and androgen precursors 
that manifested as premature pubarche949; later-affected indi­
viduals devel

re pubarche949; later-affected indi­
viduals developed hirsutism and anovulation. Several investi­
gators joined to propose hormonal standards for the diagnosis 
of 3βHSD deficiency in cases of apparent premature pubarche 
and stated that ACTH-stimulated Δ5-17P values must exceed 
294 nmol/L for Tanner stage 2 (17 ± 5 nmol/L) or the ratio 
of Δ5-17P to cortisol (F) must be at least 363. Studies relating 
genotype and hormonal analyses in basal and ACTH-stimulated 
conditions have confirmed that 

d ACTH-stimulated 
conditions have confirmed that significant elevations of the Δ5-
17P-to-F ratio are necessary to prove true 3βHSD deficiency 
in genetically proven disease and that this is a rare disorder in 
patients presenting with putative premature adrenarche.955 
However, patients with a constellation of findings indicating 
PCOS may have subtler elevations of these values and present a 
picture of adrenal impairment of 3βHSD activity in the absence 
of mutations in the gene coding for t

the absence 
of mutations in the gene coding for the enzyme; these children 
presenting with premature pubarche are postulated to develop 
clinical PCOS at a later age.
The phenotype of premature pubarche is also associated 
with the rarer nonclassic 11β-hydroxylase deficiency. Mutation 
of HSD3B2 or HSD3B1 is an uncommon cause of premature 
pubarche, exaggerated adrenarche, and hirsutism in adolescent 
girls and women.
DHEA stimulates sebaceous gland activity,956 and prepubertal 
acne or comedo

gland activity,956 and prepubertal 
acne or comedones may appear in association with elevated serum 
DHEAS concentrations in some children without the appearance 
of pubic hair, suggesting that a variant of premature adrenarche 
may manifest in this manner.145 More significant androgen effects 
(e.g., clitoral or penile enlargement, rapid growth, hirsutism, 
deepening of the voice) exclude premature adrenarche and indi­
cate a more severe form of hyperandrogenism.
Although premature adrenarche w

 hyperandrogenism.
Although premature adrenarche was usually considered to be a 
benign condition with no substantial long-term risk, accumulat­
ing observations indicate that girls with premature adrenarche are 
at increased risk of developing functional ovarian hyperandrogen­
ism and PCOS, hyperinsulinism, acanthosis nigricans, and dyslip­
idemia in adolescence and adult life, especially if fetal growth was 
reduced and the birth weight was low.950 Affected girls have BMI 
values similar to th

.950 Affected girls have BMI 
values similar to those of control subjects but differing distribu­
tion of fat; they are more likely to have increased waist circumfer­
ence along with measures of insulin resistance.957
The concept of exaggerated adrenarche was first advanced in 
relation to a postulated childhood antecedent of PCOS. It has 
been extended to include rare instances of premature adrenarche 
associated with excessive responses of Δ5-17P, DHEAS, and 
androstenedione to ACTH found in w

7P, DHEAS, and 
androstenedione to ACTH found in women with functional adre­
nal hyperandrogenism.
A report of a patient with premature adrenarche, advanced 
bone age, excessive acne, hyperandrogenic anovulation, very 
low DHEAS levels, and increased androgen levels demonstrated 
a mutation in PAPSS2, an enzyme that generates the sulfate 
donor 3′-phosphoadenosine-5′-phosphosulfate (PAPS), which 
is required for conversion of DHEA to DHEAS by the enzyme 
SULT2A1. Although the child was described

 enzyme 
SULT2A1. Although the child was described as having prema­
ture pubarche, the androgen effects were greater than those usu­
ally encountered in this condition. The DHEA level was not 
elevated for age, but androstenedione was high owing to inad­
equate formation of DHEAS that decreases the conversion of 
DHEA to androstenedione. This presentation would ordinarily 
suggest an ovarian cause of the virilization. This monogenetic 
defect must be added to the differential diagnosis of premat

t be added to the differential diagnosis of premature 
adrenarche.292 
Polycystic Ovary Disease958,959
PCOS is the most common endocrine disease of reproductive age 
women; it is estimated to affect 10% of women.960 The hallmarks 
of this condition are hyperandrogenism, hirsutism,961 anovu­
lation, amenorrhea or oligomenorrhea, and insulin resistance; 
there is compensatory hyperinsulinemia, with its attendant risk 
of major metabolic sequelae, including type 2 diabetes mellitus, 
dyslipidemia, 

ncluding type 2 diabetes mellitus, 
dyslipidemia, an increased propensity to coronary heart disease, 
and, in about 50% of affected women, obesity. PCOS is consid­
ered to be equivalent to the metabolic syndrome in men with 
its many manifestations in females. A 2013 review of diagnos­
tic criteria supports the use of the Rotterdam criteria for diagno­
sis, noting the weakness of the method as well. Thus diagnosis 
rests upon the presence of two of the following criteria: andro­
gen excess, ovul

of the following criteria: andro­
gen excess, ovulatory dysfunction, or polycystic ovaries.691 Some 
prefer hyperinsulinemic androgen excess as a better indicator of the 
basis of the disorder in view of the fact that polycystic ovaries are 
not required for diagnosis.950 Indeed criteria for the diagnosis of 
polycystic ovary morphologic appearance may require a greater 
number of follicles (>25 on ultrasound) than recommended by 
the Rotterdam criteria, and standards may yet change.933 The use 

riteria, and standards may yet change.933 The use 
of AMH in the diagnosis of PCOS is under consideration but 
remains controversial owing to differences in study criteria and 
in the assays used to report AMH values.962 At least it can be said 
1163
that AMH appears to correlate with polycystic appearance and 
androgen manifestations in many studies and in the future may 
become a diagnostic criterion.
Premature adrenarche in some populations is a

ion.
Premature adrenarche in some populations is a risk factor 
for the later development of the PCOS and functional ovarian 
hyperandrogenism in adolescent and adult women; the magni­
tude of this risk is unknown, but it appears to be rare, except in 
girls with a history of SGA.963 However, catch-up growth after 
SGA may be as important a factor in the development of PCOS, 
and even prematurity may be a risk. In an 880-member cohort 
of 8-year-olds, serum androstenedione and DHEAS levels were 

olds, serum androstenedione and DHEAS levels were 
directly related to weight gain between 1 and 3 years and cur­
rent weight and inversely related to birth weight.878 However, 
a Dutch study could not confirm such a relationship between 
increase in premature adrenarche and SGA birth weight in 
181 subjects born SGA compared with 170 subjects born with 
AGA.964
In certain ethnic groups, and especially in African-American 
and Hispanic girls, there is a greater association of premature 
adrenarc

re is a greater association of premature 
adrenarche with the metabolic syndrome (obesity, hyperinsulin­
ism, dyslipidemia, and other factors that increase the risk of later 
coronary heart disease) and the development of PCOS in late 
adolescence and early adulthood, especially if decreased insulin 
sensitivity and acanthosis nigricans accompany the premature 
adrenarche.965
As discussed earlier, hyperinsulinism is associated with many 
metabolic and endocrine conditions and functional ovarian 

c and endocrine conditions and functional ovarian 
hyperandrogenism that, in some cases, is heralded by premature 
adrenarche. Lifestyle modification is the primary approach to 
the treatment of PCOS and although success is achievable,966 
the failure rate is substantial. Oral contraceptives are often 
invoked but may be considered to treat the effects rather than 
the cause based on insulin resistance and excessive adiposity; 
nonetheless, the use of oral contraceptives with progestational 
and

se of oral contraceptives with progestational 
and antiandrogen effects can regulate menstrual periods,951 
although long-term studies of progestation-only contraceptives 
in teenagers with PCOS are lacking.967 Therapeutic approaches 
to reduce insulin resistance, especially the use of insulin sensitiz­
ers, have been introduced into the therapy for PCOS. The most 
widely used drug is metformin because of its low prevalence of 
adverse effects and therapeutic efficacy. Substantial data sup­
port

d therapeutic efficacy. Substantial data sup­
port the safety and efficacy of its use in adolescents with insulin 
resistance,951 although metformin is not FDA approved for such 
use. Although flutamide in higher doses has caused hepatotoxic­
ity, low-dose flutamide (1 mg/kg) is reportedly safe and effec­
tive in hirsute young women (but has not been proved to have 
such a safety profile in obesity or steatohepatitis)968; abstinence 
or contraception is essential when using this teratogenic agen

tion is essential when using this teratogenic agent. 
Thiazolidinediones are not recommended because of safety 
concerns.691
In a trial involving girls just past menarche who had a history 
of low birth weight and premature adrenarche and were there­
fore at risk for development of PCOS, metformin prevented this 
predicted course.963 Treatment of 8-year-old girls who had simi­
lar risk factors appeared to diminish the risk during short-term 
studies. The beneficial effects on body composition, d

ies. The beneficial effects on body composition, dyslipidemia, 
insulin resistance, and other parameters were present only during 
therapy; they reverted to increased risk factors after discontinua­
tion of metformin. Weight loss is documented with the combina­
tion of metformin and oral contraceptives.969
The Endocrine Society recommends the oral contraceptives pill 
(OCP) as the first-line treatment for PCOS. 
Adolescent Gynecomastia970
Normal boys, usually in the early stages of puberty, may 

boys, usually in the early stages of puberty, may have 
either unilateral breast enlargement (∼25% of boys)971 or bilateral 
breast enlargement (∼50–65% of boys to varying degrees); this 
commonly occurs between chronologic ages 14 and 14.5 years 
or with pubic hair stages 3 and 4.972 The physical exam aims to 
detect glandular tissue, but due to the epidemic of childhood and 
adolescence obesity differentiation of glandular tissue from adi­
pose tissue has become more difficult. “Pinching” the 

 tissue has become more difficult. “Pinching” the tissue under 
the areole between the thumb and forefinger and comparing this 
to other areas of the chest helps in the determination of glandular 
tissue.973 After such palpation, determination of serum prolactin 
might be misleading high, so blood samples must be taken before 
or well after.
In affected boys, the plasma concentrations of testosterone and 
estrogen are normal for their stage of puberty. Some have sug­
gested that the ratio of est

berty. Some have sug­
gested that the ratio of estrogen to androgen or an increase in 
the ratio of testosterone to dihydrotestosterone is a cause. In a 
prospective study, adolescent boys with gynecomastia had a lower 
mean free testosterone concentration, lower weight, higher plasma 
TeBG levels, and a tendency toward earlier onset of puberty and 
more rapid progression through puberty.971 In one study, a signifi­
cant decrease in the concentration ratio of plasma androstenedi­
one to estrone 

tion ratio of plasma androstenedi­
one to estrone and estradiol and a similarly low ratio of DHEAS to 
estrone and estradiol were described in boys with pubertal gyneco­
mastia who had normal ratios of plasma estrone and estradiol to 
testosterone. It was postulated that decreased adrenal production 
of androgens or (more likely) increased peripheral conversion of 
adrenal androgens to estrogens was a factor in the development of 
pubertal gynecomastia.974
Trials of estrogen receptor antagonists

mastia.974
Trials of estrogen receptor antagonists (e.g., tamoxifen, raloxi­
fene) show promise, but more study is required.975,976 Aromatase 
inhibitor administration had mixed results in gynecomastia. One 
study of boys (average age, 13 years) with gynecomastia involving 
a mean of 7 months of treatment with anastrozole demonstrated 
a substantial decrease in breast area (63%) and volume (53%), 
as measured manually and by ultrasonography, compared with 
watchful waiting977; however, larger st

ared with 
watchful waiting977; however, larger studies are recommended to 
prove the utility of this approach because another study found no 
such result.
Pubertal gynecomastia is due to ductular proliferation in 
fibrous connective tissue.978 The condition usually resolves spon­
taneously within 1 to 2 years after onset,972 and reassurance and 
continued observation are often adequate treatment. Nevertheless, 
some boys have conspicuous gynecomastia, and if it lasts longer 
than 2 years (5–20%

astia, and if it lasts longer 
than 2 years (5–20% in various studies), it is likely to become 
permanent. These children may have sufficient psychologic dis­
tress to warrant a reduction mammoplasty. Indeed the psychologic 
stress appears unrelated to the duration or severity, and counsel­
ing should be considered in appropriate boys.979 Liposuction is an 
alternative approach, but its efficacy in adolescent gynecomastia 
remains to be established. Untreated persistent gynecomastia per­
sists i

ed. Untreated persistent gynecomastia per­
sists into adulthood. The histologic examination of physiologic 
gynecomastia tissue rarely reveals carcinoma, so routine disease 
examination may be unnecessary.980
Gynecomastia is a component of Klinefelter syndrome, anor­
chia, primary and secondary hypogonadism, biosynthetic defects 
in testosterone synthesis, increased aromatase activity in adipose 
and other tissues (aromatase excess syndrome),1008 Sertoli cell 
tumors, adventitious exposure to es

 Sertoli cell 
tumors, adventitious exposure to estrogens in meat or cosmet­
ics, and variants of the androgen resistance syndromes, including 
Rosewater syndrome (familial hypogonadism and gynecomastia) 
1164
and Reifenstein syndrome (hypospadias, hypogonadism, and 
gynecomastia). These disorders usually have characteristic find­
ings or environmental circumstances that allow ready differentia­
tion from the normal gynecomastia of puberty.622 Gyneco

from the normal gynecomastia of puberty.622 Gynecomastia 
has been described in association with the administration of drugs 
such as cimetidine, spironolactone, digitalis, atypical antipsychot­
ics, and phenothiazines; with GH therapy; and with the use of 
marijuana. The aromatase excess syndrome is described earlier. 
Macroorchidism is defined as testes twice the normal size for 
age without androgenization. It is a rare manifestation of the 
McCune-Albright syndrome884 and an o

ation of the 
McCune-Albright syndrome884 and an occasional finding in pre­
pubertal boys with long-standing primary hypothyroidism (see 
Van Wyk-Grumbach syndrome). This form of testicular enlarge­
ment appears to result from increased FSH secretion, indepen­
dent of a pubertal increase in LH secretion or a pubertal LH 
response to GnRH. Testicular adrenal rests in CAH can cause 
bilateral macroorchidism, usually with nodules, as can lym­
phoma. In the McCune-Albright syndrome, an activating mu

 In the McCune-Albright syndrome, an activating muta­
tion in the Gsα gene primarily expressed in the Sertoli cells can 
cause macroorchidism due to Sertoli cell proliferation and hyper­
function with increased concentration of serum inhibin B and 
AMH but without increased testosterone levels due to Leydig 
cell hyperplasia, elevated gonadotropins, or signs of puberty.327 
Macroorchidism is a feature of severe aromatase deficiency in 
young male adults303 and in men with an FSH-secreting pitu­

 adults303 and in men with an FSH-secreting pitu­
itary macroadenoma. Bilateral megalotestis (testicular volume, 
26 mL) in adults can occur as a normal variant.981 One may 
speculate that some instances of bilateral macroorchidism are the 
result of a heterozygous constitutive activating mutation of the 
FSH receptor. As noted earlier, prepubertal enlargement of the 
testes was reported with a single-base-pair deletion at codon 434 
(1301delT) of the NR0B1/DAX1 gene and led to prepubertal 
test

f the NR0B1/DAX1 gene and led to prepubertal 
testosterone and gonadotropin values.818
Immunoglobulin superfamily 1 (IGSF1) gene stimulates 
transcription of the thyrotropin-releasing hormone receptor by 
decreasing activity of the TGFβ1-Smad signaling pathway while 
increasing synthesis and biopotency of TSH; it also downregu­
lates the activin-Smad pathway so that FSHB decreases. Thus with 
mutation in the IGSF1 gene, central hypothyroidism and mac­
roorchidism develop.982
Testes may enlarge d

 mac­
Testes may enlarge during treatment with aromatase inhibitors.
The fragile X syndrome is associated with developmental delay, 
a long face and large prominent ears, and macroorchidism in 
80% of affected pubertal boys. Macroorchidism may be evident 
only after careful measurements. The enlarged testes are caused 
by increased interstitial volume and excessive connective tissue, 
including increased peritubular collagen fibers, rather than by an 
increase in the semi

en fibers, rather than by an 
increase in the seminiferous tubules. Enlargement of the testes is 
demonstrable in the prepubertal period in most patients with frag­
ile X syndrome, but the onset of true macroorchidism (>4 cm) 
occurs only in the later prepubertal period.983
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1165
27
Hormones and Athletic Performance
FABIO LANFRANCO, EZIO GHIGO, AND CHRISTIAN J. STRASBURGER
Effect of Athletic Performance on 
Hormonal Systems
Norepinephrine and epinephrine are closely coupled in their 
actions and respond rapidly to exercise to redistribute blood flow 
to meet metabolic demands. Norepinephrine increases from a 
resting level of 1.2 to 3.0 nmol/L to levels as high as 12.0 nmol/L 
at maximal exer

 to levels as high as 12.0 nmol/L 
at maximal exercise. Resting concentrations of epinephrine are 
380 to 655 pmol/L. With maximal exercise, epinephrine concen­
trations can increase up to 3300 pmol/L. Both of these hormones 
progressively increase as workload increases.1 Following exercise, 
plasma levels of catecholamines return to resting levels in a matter 
of minutes.
Mild exercise produces little or no response in catecholamines, 
whereas at moderate exercise levels norepinephrine signific

t moderate exercise levels norepinephrine significantly 
increases with minimal change in circulating epinephrine. At 
intense or prolonged exercise levels, both hormones increase 
significantly. Acute, short-duration maximal exercise can signifi­
cantly increase norepinephrine and epinephrine levels. This rapid 
response suggests that the levels are primarily regulated via neural 
release mediated by activation of the sympathetic nervous system. 
Spillover from active muscle during exercise app

Spillover from active muscle during exercise appears to be the pri­
mary contributor, but the kidneys are a possible other source.1 
Moreover, alteration in the ratio of norepinephrine to epineph­
rine, with a greater increase in the release of epinephrine from the 
adrenal medulla during exercise, suggests a possible hypothalamic 
mediation in the response to exercise.
Graded exercise produces a lower catecholamine response than 
continuous prolonged exercise. The responses are directly relat

olonged exercise. The responses are directly related 
to workload and oxygen uptake, and they are greater with small 
muscle groups than with large muscle groups.
Many studies report a higher adrenaline response to exercise in 
endurance-trained subjects compared with untrained subjects in 
response to intense exercise at the same relative intensity as all-out 
exercise. This higher capacity to secrete adrenaline was observed 
both in response to physical exercise and to other stimuli such as 
h

 physical exercise and to other stimuli such as 
hypoglycemia and hypoxia.2 This phenomenon can partly explain 
the higher physical performance observed in trained subjects com­
pared with untrained subjects. More recently, these findings have 
also been reported in anaerobic-trained subjects in response to 
supramaximal exercise.
The combination of mental and physical challenges can elicit exac­
erbated catecholamine responses above that of a single challenge alone. 
Webb and colleagues3 demons

ngle challenge alone. 
Webb and colleagues3 demonstrated that acute mental challenges and 
Effect of Athletic Performance on Hormonal Systems, 1165
Performance-Enhancing (Ab)use of Hormones, 1171
KEY POINTS
 • 
Physical activity exerts an important influence on the endocrine 
system, modulating synthesis and secretion of several hor­
mones. Almost every organ and system in the body is affected 
by physical activity and exercise, mainly mediated through the 
endocrine and neuroen

among pro­
fessional and recreational athletes. Anabolic androgenic ste­
roids, growth hormone and growth hormone secretagogues, 
insulin-like growth factor 1, insulin, erythropoietin, and glu­
cocorticosteroid properties are used and misused in sports. 
Specific methods to detect hormone abuse are described.
 • 
Professionals involved in sports medicine and in endocrinology 
are presented with an elucidation of the complex interactions 
between physical activity and the endocrine system. Hormo

 physical activity and the endocrine system. Hormone 
abuse in competitive and recreational sports exerts negative 
consequences for long-term health.
SECTION VI C    Adult
1166
physical stress together elicit a greater catecholamine and cardiorespi­
ratory response than a single challenge, and this response is affected 
by fitness level (being higher in individuals with higher fitness levels). 
These findings suggest that fitness may be of benefit t

 findings suggest that fitness may be of benefit to those engaged 
in situations where dual-stressors are common (emergency respond­
ers, military personnel, etc.), as greater activation of the sympathoad­
renal axis may be essential to safety and survival.
Interestingly, studies in women remain scarce; the results are 
more conflicting than in men, and the physical training type (aer­
obic or anaerobic) effects on catecholamine response remain to be 
specified.
Epinephrine and norepinephrine ar

o be 
Epinephrine and norepinephrine are responsible for many 
adaptations both at rest and during exercise. These include cardio­
vascular and respiratory adjustments and substrate mobilization 
and utilization.2 Redistribution of circulation to working muscles 
and to the skin for heat loss and sweating is mediated through 
changes in catecholamines directly or indirectly via other interme­
diate hormones. Moreover, catecholamines may mediate mental 
performance improvement that occ

y mediate mental 
performance improvement that occurs through exercise.2 
Fluid Homeostasis–Vasopressin–Renin-
Angiotensin-Aldosterone System
During physical exercise, there is considerable loss of water and 
electrolytes in sweat, which is necessary to maintain body temper­
ature by dissipating heat generated from muscle use. The rate of 
fluid loss owing to sweating may be as high as 1500 mL/hour. The 
loss of fluids is replaced by the subsequent ingestion of liquids, 
which is modulated by th

nt ingestion of liquids, 
which is modulated by thirst. The replacement of electrolyte is the 
result of the normal intake of food. Renal function is the major 
mechanism by which electrolytes are conserved following exercise.
The maintenance of fluid and electrolyte homeostasis during 
physical exercise depends on the action of arginine vasopressin 
(AVP), natriuretic peptides, the renin-angiotensin-aldosterone 
(RAA) axis, and catecholamines. These hormonal systems are 
modified in response to

hese hormonal systems are 
modified in response to exercise, with different patterns depend­
ing on the amount of relative work performed, the duration of 
exercise, and the training status. Other factors influencing the 
response of hormones to exercise include the mode of exercise, 
environmental factors, age and gender of the subjects, and sev­
eral medical/physiologic conditions.1,4 Hormones involved in the 
regulation of fluid and electrolyte homeostasis show a relatively 
consistent respon

e homeostasis show a relatively 
consistent response among individuals.
AVP concentrations increase during exercise up to 24 pg/mL, 
and elevated levels persist for more than 60 minutes following 
maximal exercise. The stimulus for the increase in AVP during 
exercise is the increase in plasma osmolality and reduction in blood 
volume. Animal experiments have demonstrated an increase in 
activation of hypothalamic neurons indicating an increased vaso­
pressin content and performance above the an

vaso­
pressin content and performance above the anaerobic threshold.5 
Thus, the response of vasopressin appears to be associated with the 
onset of anaerobic metabolism, which is also related to increases in 
stress hormones such as cortisol and adrenocorticotropic hormone 
(ACTH).
A study in runners using pharmacologic manipulation with a 
receptor-specific agonist and antagonist to activate and inhibit the 
vasopressin type 2 receptor (V2R) has provided indirect evidence that 
AVP response du

s provided indirect evidence that 
AVP response during exercise regulates water losses from body fluids 
only in the kidney with no effects on sweat and salivary glands.6
Atrial natriuretic peptide (ANP) and brain natriuretic peptide 
(BNP), which may be altered by exercise,1 also elicit a natriuretic 
effect. ANP is increased with exercise in a linear response. In case 
of prolonged exercise, ANP undergoes an initial increase, a subse­
quent fall, and then a re-elevation of levels, persisting u

l, and then a re-elevation of levels, persisting until com­
pletion of the exercise.1 The increase in ANP with exercise appears 
to be related to stretch of the atrium due to volume changes, neu­
rologic inputs, and sodium intake.1 BNP response to exercise is 
modulated by sodium intake, hydration status, and hypoxia.1,7 
BNP is not consistently altered in normal subjects in response to 
acute exercise except under conditions of high altitude/hypoxia, 
whereas it increases with long-duration exe

oxia, 
whereas it increases with long-duration exercise such as a 100-km 
ultramarathon.7
The RAA system responds to acute exercise with a significant 
increase of activity. In fact, increased values of plasma renin activ­
ity (PRA) are reported following maximal exercise.1 The increase 
in PRA occurs at submaximal workloads of more than 60% to 
70%. With the increase in PRA during exercise, there is a concom­
itant increase in angiotensin II (A-II), which partially mediates the 
increase in cir

II), which partially mediates the 
increase in circulating aldosterone concentrations up to 250 to 
3300 pmol/L. Elevated levels of aldosterone may persist for days 
after the end of exercise, depending on water and sodium intake. 
The primary activator of the RAA system during exercise is the 
sympathetic nervous system. Stimulation of the release of renin is 
modulated by changes in renal sympathetic nerve activity resulting 
in an increase in local norepinephrine. The increase in aldosterone 

local norepinephrine. The increase in aldosterone 
with exercise is assumed to be mediated by the increase in A-II in 
response to activation of the RAA system. However, inhibition of 
angiotensin-converting enzyme does not attenuate the increase in 
aldosterone with maximal exercise in healthy subjects.8 Other fac­
tors involved in the activation of aldosterone production include 
sodium intake, potassium balance, and levels of ACTH. Elevated 
levels of aldosterone may persist for days after th

evels of aldosterone may persist for days after the end of exercise 
depending on water and sodium intake.1
The persistent increase of aldosterone long after the end of 
exercise may be associated with reductions in plasma osmolality 
and sodium concentrations due to ingestion of water to replace 
total body water losses.1 Thus, the interaction of several regulating 
factors is involved in mediating the response of aldosterone.
In contrast with acute exercise, PRA is reduced after exercise 
trai

cute exercise, PRA is reduced after exercise 
training, whereas no effect is observed on serum aldosterone or 
A-II. Significant reductions in systolic and diastolic blood pressure 
following exercise training have been observed with no relation 
between PRA changes and changes in blood pressure.4 
Hypothalamus-Pituitary-Adrenal Axis
Since the pioneering studies of Davies and Few,9 it has been 
known that exercise of an appropriate intensity is a potent stimu­
lus for cortisol se

e intensity is a potent stimu­
lus for cortisol secretion. Glucocorticoids exert many beneficial 
effects in exercising humans, increasing the availability of meta­
bolic substrates for the need of energy of muscles, maintaining 
normal vascular integrity and responsiveness, and protecting the 
organism from an overreaction of the immune system in the face 
of exercise-induced muscle damage.10
During acute exercise, the hypothalamus-pituitary-adrenal 
(HPA) axis responds to numerous stimuli demo

enal 
(HPA) axis responds to numerous stimuli demonstrating the regu­
latory and integrative functions of the HPA axis: neuronal homeo­
static signals (chemoreceptor, baroreceptor, and osmoreceptor 
stimulation), circulating homeostatic signals (glucose, leptin, and 
ANP), and inflammatory signals (interleukin 1, interleukin 6, and 
tumor necrosis factor α).11,12
In humans, repeated brachial blood samplings have shown 
that the dynamics of HPA axis activation during exercise associate 
1167
Chap

is activation during exercise associate 
1167
Chapter 27  Hormones and Athletic Performance
stimulation of hypothalamic corticotropin-releasing hormone and 
AVP secretion (with a prominent role of corticotropin-releasing 
hormone), synthesis, and release of ACTH from pituitary cortico­
troph cells preceding the increase of cortisol.13 
Response to Endurance Exercise
Two major factors modulate the HPA axis response to endurance 
exercise: the intensity and the duration of exercise.10,13 The mini­

nsity and the duration of exercise.10,13 The mini­
mum intensity of exercise necessary to produce a cortisol response 
from the HPA axis is 60% of maximal oxygen uptake ( ˙VO2max), 
with a linear increase between the intensity of exercise and the 
increase in plasma cortisol concentrations for exercises above 60% 
˙VO2max.13,14 Below this intensity threshold, such as during light 
and prolonged exercise (<60% ˙VO2max), ACTH and cortisol 
concentration may increase, with a duration threshold arou

ation may increase, with a duration threshold around 90 
minutes of exercise at 40% ˙VO2max.14 These thresholds are inde­
pendent of training.
Several other factors modify the response of the HPA axis to 
physical activity. The cortisol response to exercise is modulated by 
hypohydration, meals, and time of day. Independently of exter­
nal thermal stress, hypohydration (up to 4.8% body mass loss) 
greatly amplifies the exercise-induced cortisol response to exer­
cise. This enhancement probably r

esponse to exer­
cise. This enhancement probably results from an increased core 
temperature and cardiovascular demand consecutive to decreased 
plasma volume.15 Meals also stimulate cortisol release in humans. 
Exercise performed immediately after food ingestion results in 
a blunted cortisol response to the exercise stimulus. Finally, the 
cortisol response to exercise is significantly modulated by time of 
day, which influences the incremental response to exercise (greater 
increase in the ev

 response to exercise (greater 
increase in the evening than in the morning) but not the peak 
response to exercise.13
Endurance training has been compared with chronic stress in 
humans. When the HPA axis is repeatedly challenged by exercise, 
humans demonstrate modifications in the activity of the HPA 
axis suggesting an adapting process to endurance training. Several 
studies have shown that in endurance-trained subjects, the HPA 
axis activity in resting conditions is similar to that of heal

y in resting conditions is similar to that of healthy sed­
entary subjects.14,16 Accordingly, plasma cortisol at 0800 hours, 
nycthemeral cortisol rhythm, overnight and 24-hour urinary 
free cortisol, seasonal rhythmicity of cortisol excretion, and cor­
tisol response to the dexamethasone suppression test in resting 
endurance-trained subjects are similar to those of age-matched 
untrained subjects.13,14,16
However, when the HPA axis is challenged, endurance-trained 
subjects demonstrate a decre

d, endurance-trained 
subjects demonstrate a decreased pituitary sensitivity to the nega­
tive feedback of glucocorticoids, which explains their capacity to 
successfully achieve a second bout of exercise separated by a short 
rest period.14 Different mechanisms can be involved in this adap­
tation. At the central nervous system level, neuropeptides and cor­
ticosteroid receptors (glucocorticoid receptors, mineralocorticoid 
receptors) in the brain and anterior pituitary play a major role in 
th

in and anterior pituitary play a major role in 
the regulation of circulating cortisol levels. At the peripheral level, 
tissue sensitivity to glucocorticoids may also be different between 
endurance-trained and sedentary subjects.17 Altogether, these 
adaptation processes are finalized to protect the body from the 
severe metabolic and immune consequences of increased cortisol 
levels. 
Response to Resistance Exercise
Resistance (strength) exercise is also a potent stimulus to the HPA 
axis. In

ise is also a potent stimulus to the HPA 
axis. In humans, a bout of resistance exercise acutely increases cor­
tisol secretion and then plasma cortisol concentrations.18
Two major factors modulate HPA axis response to resistance 
exercise: the intensity and the volume (sets × repetitions × inten­
sity) of the exercise performed. Resistance protocols of moderate 
to high intensity, high in volume, stressing large muscle mass, and 
using short rest intervals produce the greatest acute hormonal 
e

t intervals produce the greatest acute hormonal 
elevations (ACTH and cortisol).18
Cortisol has multiple roles during resistance exercise: meeting 
the greater metabolic demand of resistance training; remodeling 
of muscle in response to a bout of exercise, as cortisol regulates 
muscle protein content by inhibiting muscle protein synthesis; 
and stimulating protein degradation likely via activation of the 
ubiquitin pathway.13 Moreover, cortisol can also influence neuro­
muscular function (i.e.

 can also influence neuro­
muscular function (i.e., neuronal activity, intracellular signaling, 
muscle force) through various rapid or short-term mechanisms. 
Cortisol may contribute to the muscular properties of the periph­
eral nervous system by rapidly modulating calcium influx in skel­
etal muscle cells.19 
The RAA system is closely coupled and responds to acute exer­
cise. PRA values increase following maximal exercise.1 Progres­
sively higher increases in aldosterone le

Progres­
sively higher increases in aldosterone levels have been observed 
with increasing degrees of exertion. Elevated levels of aldosterone 
may persist for days after the end of exercise depending on water 
and sodium intake.1 The interaction of several regulating factors is 
involved in mediating the response of aldosterone. These include 
the sympathetic nervous system, renin activity, A-II, sodium 
intake, potassium balance, blood volume reductions, and levels 
of ACTH.1 In contrast with 

eductions, and levels 
of ACTH.1 In contrast with acute exercise, PRA is reduced after 
endurance training, whereas no effect is observed on serum aldo­
sterone or A-II.4 
Endorphins
Exercise influences the release of β-endorphins depending on 
intensity and duration of the physical activity. If a threshold 
intensity is exceeded, endogenous opiate levels start to increase. 
Incremental graded exercise tests elevate β-endorphin levels 1.5- 
to 7-fold.20 Several studies suggest that circulating β

fold.20 Several studies suggest that circulating β-endorphins 
can increase with an appropriate minimal intensity of exercise 
(>60% ˙VO2max) but this is not always the case.20 Instead of a 
critical intensity related to aerobic capacity, other studies related 
the increase in β-endorphins to the lactate threshold.20 Moreover, 
other factors, such as diet and immune function, can influence the 
β-endorphin response. Very little is known about the influence of 
training status on the release of β

 influence of 
training status on the release of β-endorphins, and study results 
are often inconsistent.
Interestingly, the psychological and physiologic stress related 
to competitive practice has been proposed to stimulate the secre­
tion of endorphins to counter the negative effects of competitive 
stress.21 A physiologic purpose of endogenous opiate increase in 
athletes can be the modulation of pain and the improvement 
of mood.22 Altogether, the action of endogenous opiates can be 
descri

r, the action of endogenous opiates can be 
described as a rewarding system that makes the athlete continue 
physical activity. 
Hypothalamic-Pituitary-Gonadal Axis
Male Gonadal Axis
The effects of physical activity on the hypothalamic-pituitary-
gonadal (HPG) axis in males vary with the intensity and duration 
of the activity, the fitness level of the individual, and his nutri­
tional-metabolic status. Relatively short, intense exercise usually 
1168

1168
increases serum testosterone levels, whereas more prolonged exer­
cise usually decreases them.23-25 Increased serum testosterone levels 
have been reported during relatively strenuous free and treadmill 
running, weight training, rock climbing, and ergometer cycling.26
The testosterone response increases with increased exercise 
load.27 Similar workloads produce similar responses regardless of 
whether the load is aerobic or anaerobic.28 Increa

whether the load is aerobic or anaerobic.28 Increased and decreased 
ambient temperature, altitude, and dehydration have no effect on 
testosterone response to intense exercise.29 Acute exercise-induced 
testosterone increments are also seen in older men despite their 
different hormonal milieu.30
Exercise-associated increase in circulating testosterone is considered 
not to be mediated by luteinizing hormone (LH) due to the inconsis­
tent LH response and to the evidence that testosterone levels

ponse and to the evidence that testosterone levels increase 
more quickly than LH in response to exercise. Possible mechanisms 
such as hemoconcentration, reduced clearance, and/or increased tes­
tosterone synthesis may be involved.26 However, the timing of tes­
tosterone response differs from that of other circulating steroids (e.g., 
androstenedione and dehydroepiandrosterone increase simultaneously 
with cortisol), thus suggesting that specific testicular mechanisms are 
involved.31 These mec

c testicular mechanisms are 
involved.31 These mechanisms may include the activation of the sym­
pathetic system, which stimulates testicular testosterone production 
during exercise via a direct neural pathway in some species.
In contrast to the short-term testosterone increment, a suppres­
sion of serum testosterone levels occurs during and subsequent 
to more prolonged exercise, and to some extent in the hours fol­
lowing intense short-term exercise.32,33 A variety of systems could 
influence

ercise.32,33 A variety of systems could 
influence the decrease of testosterone synthesis during and subse­
quent to prolonged exercise. A lack of significant elevation in basal 
LH, coupled with a reduced testosterone concentration, has been 
reported, reflecting hypogonadotrophic-hypogonadism character­
istics.34,35 Moreover, altered basal prolactin has been described in 
response to prolonged exercise.34,36
Leptin and ghrelin levels, which may be influenced by physi­
cal exercise, have been s

 be influenced by physi­
cal exercise, have been suggested to be altered in exercise-hypogo­
nadal men. Leptin is an adipocyte-released hormone associated in 
part with communicating satiety and energy reserve status to the 
hypothalamus.37 It is also linked to reproductive function in both 
women and men. Acute and chronic exercise can reduce leptin 
concentrations independent of changes in body adiposity.38,39
Ghrelin is another hormone associated with appetite regula­
tion. Evidence from seve

ted with appetite regula­
tion. Evidence from several experimental studies in animals and in 
humans suggests that ghrelin may function as a metabolic modu­
lator of the HPG axis, with predominant inhibitory effects in line 
with its role as a signal of energy deficit.40 Acute and chronic exer­
cise has been shown to increase ghrelin concentration levels.38,41 
However, no research has yet examined whether ghrelin levels in 
exercise-hypogonadal men are normal.
Another potential disruptive hormo

men are normal.
Another potential disruptive hormone to the HPG axis is cor­
tisol. Cumming and associates42 have demonstrated that the direct 
infusion of cortisol in men results in concurrent declines in tes­
tosterone levels. However, in the hormonal profile studies report­
ing the existence of low testosterone in trained men, none have 
reported elevated resting cortisol levels.26 Thus, at this time, the 
role of cortisol in the changes found in the gonadal axis of trained 
men is in need of

 in the gonadal axis of trained 
men is in need of further study.
Reductions in testosterone levels, if extensive and prolonged, 
can compromise a person’s health status. Relative to athletes, these 
reductions can negatively impact on the adaptation process asso­
ciated with skeletal muscle (i.e., myoplasticity), which is funda­
mental to the exercise training progression and improvement in 
performance.43,44
It is important to note that hormonal changes involved in 
overtraining and/or decreas

l changes involved in 
overtraining and/or decreased energy availability can decrease the 
production of testosterone. Participation in sports where leanness 
is considered a competitive advantage, such as running, cycling, 
wrestling, lightweight rowing, and gymnastics, has been linked to 
lower body mass index, eating disorders, and low energy availabil­
ity.33,45 Low energy availability in the context of anorexia nervosa 
has been associated with low testosterone levels in males.46 Hag­
mar a

ith low testosterone levels in males.46 Hag­
mar and associates47 evaluated athletes from 26 different sports 
and divided them into those who participated in leanness sports 
and those who did not. The leanness sport athletes had lower body 
fat, higher spinal bone mineral density (BMD), lower serum free 
testosterone and leptin, and higher insulin-like growth factor 1 
(IGF1) binding protein.
Research in exercising men demonstrates the existence of a 
select group who, through chronic exposure

e of a 
select group who, through chronic exposure to endurance exer­
cise training, developed alterations in their reproductive hormonal 
profile (i.e., persistently low basal resting testosterone concentra­
tions).34 In particular, the majority of these men exhibit clinically 
“normal” testosterone concentrations, but these concentrations 
are at the low end of the normal range or even reach subclini­
cal status. The health consequences of such hormonal changes are 
increased risk of abnormal 

 hormonal changes are 
increased risk of abnormal spermatogenesis, male infertility prob­
lems, and compromised bone mineralization.34 The prevalence of 
such health problems seems low, but investigative studies exam­
ining this condition and its consequences are few in number.34 
The specific terminology used to refer to this condition has not 
been universally agreed on. In 2005, Hackney and associates48 
proposed use of the phrase “the exercise-hypogonadal male” as a 
label for this condition

e-hypogonadal male” as a 
label for this condition.
The effects of intense training on male fertility are controver­
sial. Some studies have found changes in semen from exercise, not­
ing decreases in sperm motility, quality, and number,49-51 whereas 
others have not found such alterations.52,53 Several studies, how­
ever, did not control for energy availability. Thus, like much of 
the work in female athletes, research in male athletes will be more 
easily interpretable in the future if energy 

ore 
easily interpretable in the future if energy availability is accurately 
quantified and factored into the analysis.33 
Female Gonadal Axis
The endocrine equilibrium that regulates the reproductive func­
tion in women can be affected by physical and psychological fac­
tors. In females, short-term consequences of physical exercise on 
the HPG axis have not been as widely studied as in males. Differ­
ences in hormonal responses between the sexes are believed to be 
dependent on basal hormone c

s are believed to be 
dependent on basal hormone concentrations (i.e., sex differences 
in basal estradiol and testosterone).33 A few studies have demon­
strated increases in total testosterone after acute bouts of resis­
tance and endurance training in women.54-57 Studies evaluating 
acute changes in estradiol are challenging given that estradiol lev­
els change widely during a typical menstrual cycle. Nevertheless, 
multiple investigations have found an acute increase in estradiol 
immediately

 found an acute increase in estradiol 
immediately after exercise.54,58 This increase in estradiol seems to 
be responsible for greater lipid and less carbohydrate oxidation 
in women compared with men during exercise and seems to be 
protective for muscle during the stress of exercise.33
The long-term impact of exercise and training on hormone lev­
els has been more widely studied in females, particularly in the 
setting of decreased energy availability. Women who engage in 
regular high-intens

lability. Women who engage in 
regular high-intensity exercise may be at risk of developing men­
strual disturbances such as delayed menarche, oligomenorrhoea, 
amenorrhea, and luteal phase defects.59
1169
Although factors such as physical and/or psychological stress 
of competition have been postulated to underlie exercise-induced 
reproductive disorders, evidence accumulated to date indicates 
that negative energy balance is the primary cause of th

negative energy balance is the primary cause of the impair­
ment of normal reproductive function commonly observed in 
female athletes.60,61 In 1939, Selye62 reported that “the ovaries 
undergo atrophy and more or less permanent anestrus ensues” 
when young female rats were forced to exercise for prolonged peri­
ods. Earlier, Selye63 observed a general adaptation syndrome also 
involving hypertrophy of the adrenal glands, cessation of growth 
and lactation, shrinkage of the liver, loss of muscul

 lactation, shrinkage of the liver, loss of muscular tone, a fall 
in body temperature, and the disappearance of adipose tissue. In 
1980, Warren60 was the first to suggest that menstrual disorders in 
dancers are disrupted by an “energy drain.” In 1984, Winterer and 
associates64 hypothesized that lack of sufficient metabolic fuels to 
meet the energy requirements of the brain causes an alteration in 
brain function that disrupts the gonadotropin-releasing hormone 
(GnRH) pulse generator, altho

n-releasing hormone 
(GnRH) pulse generator, although the mechanism of this altera­
tion was unknown. The energy availability hypothesis is supported 
by endocrine observations of athletes. Amenorrheic athletes dis­
play low 24-hour blood glucose, low 24-hour insulin, and high 
24-hour insulin-like growth factor binding protein 1 (IGFBP1),65 
loss of the leptin diurnal rhythm,66 and low T3 levels in the morn­
ing.67 Loucks and associates68 found that low energy availability 
reduced LH pulse fre

that low energy availability 
reduced LH pulse frequency and increased LH pulse amplitude, 
and that exercise stress had no suppressive effect on LH pulsatility 
beyond the impact of the energy cost of exercise on energy avail­
ability. LH pulsatility was disrupted regardless of whether energy 
availability was reduced by extreme energy restriction alone, by 
extreme exercise energy expenditure alone, or by a combination of 
moderate dietary energy restriction and moderate exercise energy 
expen

gy restriction and moderate exercise energy 
expenditure. Dietary supplementation prevented the suppression 
of LH pulsatility by exercise energy expenditure.
To investigate the dose-response relationship between energy 
availability and LH pulsatility in exercising women, Loucks and 
Thuma69 manipulated the energy availability of 29 regularly men­
struating, habitually sedentary, young women, administering a 
clinical dietary product to set energy availability at 45 and 10, 20, 
or 30 kcal/kg f

gy availability at 45 and 10, 20, 
or 30 kcal/kg fat-free mass (FFM) for 5 days. LH pulsatility was 
disrupted only below 30 kcal/kg FFM per day. This was consistent 
with many studies of amenorrheic runners, all of which indicated 
energy availabilities less than 30 kcal/kg FFM per day,70 and with 
the only prospective study of the refeeding of amenorrheic athletes, 
in which menstrual cycles were restored in runners by increasing 
energy availability from 25 to 31 kcal/kg FFM per day.71
In fem

bility from 25 to 31 kcal/kg FFM per day.71
In female athletes, low energy availability is a component of the 
female athlete triad, a term used to describe the interrelationship 
of decreased energy availability, subsequent HPG axis inhibition 
leading to menstrual irregularity, and decreased BMD.45 The triad 
was first described by the American College of Sports Medicine 
in the 1990s. In 2007, the American College of Sports Medicine 
published a revised position stand on the female athlete tr

 a revised position stand on the female athlete triad,72 
correcting the former misunderstanding of the triad as a narrow 
syndrome consisting of disordered eating, amenorrhea, and osteo­
porosis by describing the triad more broadly as the harmful effects 
of low energy availability on menstrual function and BMD.61 The 
International Olympic Committee (IOC) proposed an expansion 
of the concept of the female athlete triad to include males and 
coined the term relative energy deficiency in sport.

ined the term relative energy deficiency in sport.73 The develop­
ment of that term had three main purposes: (1) to draw awareness 
to the fact that energy restriction can have negative consequences 
in men in addition to women; (2) to highlight other potential 
negative health and performance consequences of low energy 
availability in athletes besides bone problems; and (3) to encourage 
expansive research into the potential myriad effects of low energy 
availability in various populations, in

ow energy 
availability in various populations, including Paralympic athletes.
The discovery of leptin in 1994 was fundamental in clarifying 
the relationship between negative energy balance and reproductive 
dysfunction. Various sets of data suggest that leptin may serve as 
a signal to the CNS with information on the critical amount of 
adipose tissue stores necessary for GnRH secretion and pubertal 
activation of the HPG axis. Several unfavorable metabolic situa­
tions are associated with low

ble metabolic situa­
tions are associated with low plasma leptin, increased secretion of 
hypothalamic neuropeptide Y, and hypogonadism, and a causal 
relationship has been evoked. Severe dietary restriction in juvenile 
female rats is associated with low plasma leptin and sexual imma­
turity. Cessation of food restriction leads to immediate increase in 
plasma leptin followed by sexual maturation.74 Leptin administra­
tion for the relative leptin deficiency in women with hypothalamic 
amenorrhe

n deficiency in women with hypothalamic 
amenorrhea improves reproductive, thyroid, and growth hormone 
(GH) axes and markers of bone formation, confirming that leptin 
tion75 and can restore ovulatory cycles.76
Impaired production of gonadotropins, which leads to luteal 
phase deficiency and anovulation, is a common hormonal finding 
with exercise-induced menstrual disturbances, but several other 
hormones may show significant alterat

veral other 
hormones may show significant alterations.
The HPA axis activation may be involved in the gonadal axis 
functional disruption during physical exercise. The so-called stress 
hypothesis holds that exercise activates the HPA axis, which dis­
rupts the GnRH pulse generator by another unknown mechanism. 
Amenorrheic athletes may display mildly elevated cortisol levels, 
and this observation is the basis for attributing their amenorrhea 
to stress. However, since cortisol is a glucoregul

to stress. However, since cortisol is a glucoregulatory hormone 
activated by low blood glucose levels, the mild hypercortisolism 
observed in amenorrheic athletes may reflect a chronic energy 
deficiency rather than exercise stress.61
The involvement of endogenous opioid peptides and catecho­
lestrogens in provoking menstrual irregularities in women athletes 
has been also suggested.77 In basal circumstances, β-endorphins 
may decrease LH levels by suppressing hypothalamic GnRH; 
some catechole

 by suppressing hypothalamic GnRH; 
some catecholestrogens may suppress LH levels, whereas others 
seem to potentiate and induce LH surge. The activities of both 
β-endorphins and catecholestrogens depend on the essential pres­
ence of a sufficiently estrogenic environment. In addition, both 
endogenous opioid peptides and some of the catecholestrogens 
appear to be able to suppress prolactin release, probably by inter­
fering with its inhibiting factor, dopamine. The increased plasma 
concentra

 factor, dopamine. The increased plasma 
concentrations of β-endorphin, which are found after physical 
exercise, give rise to speculations as to their involvement in the 
frequently appearing menstrual irregularities in female athletes.77
Hyperandrogenism has been suggested as a possible alterna­
tive mechanism underlying oligomenorrhea or amenorrhea in 
some female athletes with menstrual disturbances.78-80 Sports that 
emphasize strength over leanness, such as swimming and rowing, 
are not as

leanness, such as swimming and rowing, 
are not associated with low weight and restrictive eating patterns, 
yet athletes engaged in these sports are vulnerable to menstrual 
irregularities as well. The endocrine profile of athletes engaged in 
these sports is characterized by mildly elevated LH levels, elevated 
LH/follicle-stimulating hormone ratios, and mild hyperandrogen­
ism rather than hypoestrogenism. Interestingly, hyperandrogenic 
female athletes have a more anabolic body composition an

 athletes have a more anabolic body composition and higher 
˙VO2max and performance values than female athletes with men­
strual disturbances but normal androgen concentrations.78 
1170
Prolactin
Blood prolactin levels increase during exercise, and this response 
appears proportional to the exercise intensity.36 Provided that 
the intensity is adequate, the increase in prolactin is quite rapid. 
Nonetheless, short-term, graded exercise may result in 

heless, short-term, graded exercise may result in a peak hor­
monal response after the exercise ends. In a prolonged exercise 
session, the prolactin response is proportional to the intensity at 
which it is performed. However, extending the duration of exer­
cise can augment the magnitude of the prolactin response.36
The mechanisms by which prolactin increases with exercise are 
unclear. Prolactin levels may increase when the anaerobic thresh­
old is reached, perhaps concomitantly with a GH inc

ld is reached, perhaps concomitantly with a GH increase.81 In 
some situations, such as competitive sports, excessive emotional 
stress can cause an anticipatory increase in prolactin even before 
the initiation of exercise.82 Moreover, the prolactin increase may 
be related to changes in body temperature and dehydration, is 
reduced with habituation and hypoxia, and is unresponsive to 
metabolic events.83
Vigorous, high-intensity anaerobic exercise results in greater 
prolactin response than th

ise results in greater 
prolactin response than that typically seen in submaximal steady-
state aerobic exercise.28 The effect of resistance exercise, which 
is intermittent in nature, on prolactin has not been studied 
extensively.
The chronic effects of exercise training on basal, resting pro­
lactin levels are yet unclear and in need of further research. Some 
studies have found increases in resting levels, whereas others have 
found decreased levels.36 These contradictions seem to be related

 levels.36 These contradictions seem to be related 
to differences in training protocols (intensity, frequency, and dura­
tion of training sessions). 
GH/IGF1 Axis
In 1963, Roth and associates84 demonstrated that plasma levels of 
GH increase during exercise, and it was later shown that exercise is 
the most potent physiologic stimulus to GH release.85 Most of the 
current knowledge regarding the GH response to exercise is based 
on studies of the effect of aerobic-type exercise.
The GH response

e effect of aerobic-type exercise.
The GH response to exercise is dependent on the duration 
and intensity of the exercise bout, the fitness level of the exer­
cising subject, the refractoriness of pituitary somatotroph cells to 
the exercise stimuli, and other environmental factors.86 Lactate 
and nitric oxide are suggested to be afferent stimulation for the 
exercise-induced GH response.87 A linear dose-response relation­
ship between exercise intensity and the GH secretory response was 
demon

intensity and the GH secretory response was 
demonstrated, with escalating GH release across the range of exer­
cise intensities (25–175% of lactate threshold).
The exercise duration should be at least 10 minutes, because 
exercise of shorter duration both below and above the lactate 
threshold was not accompanied by increases in circulating GH 
levels.86 Exercise-induced GH peak occurs 25 to 30 minutes after 
the start of exercise, irrespective of the exercise duration.86 Thus, 
when the task i

of the exercise duration.86 Thus, 
when the task is brief, a peak may be reached after its cessation, 
but when the task is long (e.g., 45 minutes), the GH peak occurs 
while the individual is still exercising.
The nature of the exercise may also influence the GH response. 
Although the continuous exercise protocols may be comparable to 
competition events, the endurance-type training undertaken by 
many athletes involves intermittent or interval exertion. Compar­
ing exercise at equivalent tota

 exertion. Compar­
ing exercise at equivalent total workloads, GH levels are lower with 
continuous protocols (40–45% ˙VO2max) as opposed to interval 
protocols, with twice the work rate for half the time, reflecting 
the greater metabolic stress and lactate levels in the latter.88 With 
resistance exercise, incremental GH responses have also been 
described.89 The important determinants appear to be the relation­
ship between load and frequency of individual repetitions. Greater 
GH increments 

of individual repetitions. Greater 
GH increments have been reported following “hypertrophy” pro­
tocols (moderate loads, high number of repetitions) than “strength” 
protocols (heavy loads, low repetitions) in both men and women.90
There is conflicting evidence regarding the neuroendocrine 
pathways that regulate GH secretion during exercise. Mechanisms 
involving cholinergic, serotoninergic, α-adrenergic, dopaminer­
gic, and opioidergic pathways have been proposed.91 There may be 
interactions

s have been proposed.91 There may be 
interactions among the pathways, and they may operate at differ­
ent exercise intensities. In young males, regular but not acute exer­
cise is associated with higher GH production and also augments 
GH stimulation of GH release by GH-releasing hormone.92 It 
has been hypothesized that this is due to decreased hypothalamic 
somatostatinergic activity and higher GH pulsatility.
The GH response to exercise is greater in women than in 
men—as is the unstimulated

eater in women than in 
men—as is the unstimulated GH output over time—and declines 
with aging.91 In fact, even in early middle age (mean age, 42 
years), the GH response to exhaustive exercise is greatly attenuated 
compared with younger subjects (mean age, 21 years).93 However, 
it is difficult to separate the effects of aging from changes in body 
composition, because body fat increases with aging and GH secre­
tory rates are reduced in overweight subjects.94
Environmental and nutritional fa

eight subjects.94
Environmental and nutritional factors, as well as some patho­
logic states, may interfere with the GH response to exercise. Cap­
pon and associates95 showed that a high-fat meal could inhibit 
the magnitude of GH response to exercise, the inhibition of the 
exercise-induced GH response being correlated with circulating 
levels of somatostatin.
High ambient temperature may increase circulating GH lev­
els,96 whereas low temperature attenuates GH release.97 Obesity 
and/or polycy

re attenuates GH release.97 Obesity 
and/or polycystic ovarian syndrome are characterized by attenu­
ated GH response to exercise.98
Exercise exerts acute effects on other components of the GH/
IGF1 axis. The effect of exercise on circulating IGF1 has been 
examined by several investigators with differing results depend­
ing on the intensity, duration, and type of exercise performed. 
Schwarz and associates99 demonstrated IGF1 increases follow­
ing 10 minutes of exercise of both below and above 

ng 10 minutes of exercise of both below and above the lactate/
anaerobic threshold. This study suggests that the increase in IGF1 
accompanying exercise is not related to GH.
The transient nature of IGF increases suggests that hemodynamic 
or metabolic effects of exercise per se might play a role. Exercise in 
humans is accompanied by the rapid “hemotransfusion” of hemo­
concentrated blood from the spleen into the circulation by increased 
blood flow to the exercising muscle and by loss of plasm

flow to the exercising muscle and by loss of plasma water. 
These phenomena might explain, at least in part, an increased IGF 
concentration by changes in IGF flux and/or volume of distribution.
Longer periods of exercise training, however, stimulate IGF1 
gene expression both in the central neuroendocrine and local 
tissue components of the GH-IGF1 system. Eliakim and associ­
ates100 showed that muscle IGF1 protein concentrations in rats 
can increase with endurance training despite the lack of

crease with endurance training despite the lack of change in 
muscle IGF1 mRNA or serum IGF1.
Few studies have investigated the response of IGFBPs to exer­
cise. IGFBP1 levels have been shown not to change during 30 
minutes of moderate exercise90 but to increase transiently after 
acute exercise.101 The physiologic role of the postexercise increase 
in IGFBP1, given IGFBP1 inhibition of IGF1 metabolic actions, 
may be to prevent late hypoglycemia.
1171
Chapter 27  Hormones and Athletic Performa

a.
1171
Schwarz and associates99 have demonstrated that IGFBP3 lev­
els increased with both low- and high-intensity exercise and that 
high-intensity exercise increased IGFBP3 proteolysis. A transient 
increase in IGFBP3 levels in response to an acute exercise has also 
been confirmed by Wallace and associates,101 who described an 
acute increase of all components of the ternary complex, IGF1, 
IGFBP3, and the acid-labile subunit.
Eliakim and associa

, and the acid-labile subunit.
Eliakim and associates102 have described that functional and 
structural indices of fitness were correlated with mean overnight 
GH levels, growth hormone–binding protein (GHBP), and serum 
IGF1 levels in late pubertal adolescent girls. Moreover, thigh mus­
cle volume was inversely correlated with IGFBP2 and IGFBP4.
The acute increase in serum GHBP in response to acute exer­
cise has also been described by Wallace and associates101; con­
sidering that GHBP at rest 

nd associates101; con­
sidering that GHBP at rest acts as a damper on GH oscillation, 
these authors speculate that the postexercise GHBP increase may 
prolong the GH signal, increasing the GH-mediated signal for 
postexercise protein synthesis, tissue repair, and muscle glycogen 
replenishment. The increment in the serum GHBP concentration 
may represent either increased synthesis from the liver or reduced 
clearance.101 
Hypothalamus-Pituitary-Thyroid Axis
Exercise has effects on thyroid funct

Thyroid Axis
Exercise has effects on thyroid function, which can be viewed as an 
adaptive mechanism associated with enhanced performance pos­
sibly serving to provide a better balance between energy consump­
tion and expenditure.103 Short-term incremental exercise (≤20 
minutes) increases blood TSH (thyroid-stimulating hormone, or 
thyrotropin) levels, with a critical intensity threshold of approxi­
mately 50% or more of ˙VO2max necessary to induce significant 
changes.36 Even though TSH is ele

uce significant 
changes.36 Even though TSH is elevated, most research involv­
ing short-term exercise indicates that total and free thyroxine (T4) 
and T3 are not immediately affected.36 However, total T4 and T3 
levels can increase following such exercise, although these findings 
appear primarily due to exercise-induced hemoconcentration.104
The influence of more prolonged submaximal exercise 
(approaching 60 minutes) on thyroid hormones is controversial. 
Some investigations report no effect

ntroversial. 
Some investigations report no effect on blood TSH levels, whereas 
others have found TSH and/or free T3 to increase progressively 
with high-intensity steady-state workloads.36 These divergent 
findings are difficult to interpret due to the highly variable exercise 
sessions (i.e., different durations and intensities of exercise) and 
the varying blood sampling protocols employed.
Energy balance plays an important role in the body’s thyroid 
hormone response to exercise. Loucks and

 thyroid 
hormone response to exercise. Loucks and Heath105 found a 
decrease in T3 and free T3 along with an increase in reverse T3 in 
healthy women undergoing aerobic exercise testing with low caloric 
intake. This “low T3 syndrome” was not seen in individuals receiv­
ing a higher caloric diet. An increase in reverse T3 concentrations 
has been described as one of the more consistent findings, particu­
larly when a caloric energy deficiency is associated with exercise.103 
Insulin and Glucose

 associated with exercise.103 
Insulin and Glucose Metabolism
Physical activity affects the metabolism of glucose and other 
intermediate substrates in normal subjects and in subjects with 
diabetes. The effects of exercise on carbohydrate metabolism are 
complex and involve type, intensity, and duration of exercise; 
changes in body composition; alterations in other behaviors, such 
as food intake; degree of insulin deficiency; and a complex time 
course of the glucose-insulin response.106
In h

e 
In healthy individuals, no major alterations in blood glucose 
levels are usually seen during exercise, despite the increase in 
glucose utilization by skeletal muscle. With the onset of activ­
ity, activation of the α-adrenergic system results in inhibition of 
insulin release from the pancreas. This results in an increased rate 
of lipolysis in the periphery, as well as a stimulation of hepatic 
glucose output. As glucose levels begin to fall, gluc

cose output. As glucose levels begin to fall, glucagon levels rise, 
further stimulating hepatic glucose output. Finally, as plasma glu­
cose drops toward hypoglycemic levels, epinephrine is released, 
further stimulating hepatic glucose production and increasing 
lipolysis in the periphery. The increased availability of free fatty 
acid for muscle metabolism helps to restrain the rate of glucose 
utilization. It has been shown that when one of these mechanisms 
fail, the others can largely comp

hese mechanisms 
fail, the others can largely compensate, avoiding the development 
of hypoglycemia.106
Training induces a reduction in basal insulin levels and in exer­
cise-associated changes in glucagon and insulin, increases insulin 
sensitivity at rest and in response to a glucose load, and reduces 
insulin decline during acute exercise.107 Regular exercise has 
become an integral part of the treatment recommendations for 
patients with type 2 diabetes, because it improves insulin sensitiv­

 2 diabetes, because it improves insulin sensitiv­
ity and reduces average blood glucose concentrations.108,109 Phys­
ical training results in an increase in insulin-stimulated glucose 
disposal and improves glucose control in type 2 diabetes. How­
ever, the increase in insulin sensitivity is rapidly lost if exercise is 
not performed on a regular basis. Exercise may also be effective in 
delaying or preventing the development of type 2 diabetes.109,110 
Plasma erythropoietin (EPO

109,110 
Plasma erythropoietin (EPO) levels are not usually affected by sin­
gle bouts of strenuous exercise at sea level. The lack of major effects 
of normoxic exercise on EPO production is plausible, because 
the oxygen sensor in control of EPO synthesis is not located in 
the skeletal muscle or the heart but in the kidneys.111 Generally, 
there are no major differences in basal red blood cell (RBC) count, 
hematocrit, blood hemoglobin concentration, and mean corpus­
cular hemo

oglobin concentration, and mean corpus­
cular hemoglobin concentration values in athletes compared with 
nonathletes. Hemoglobin mass has been described to be increased 
in elite endurance athletes.112 However, sportsmen, particularly 
those in endurance sports, may present with relatively low hemo­
globin concentrations and hematocrit values. This “sports anemia” 
is usually a pseudoanemia due to an enlarged plasma volume. If 
there is an increased rate of intravascular hemolysis caused by 
com

sed rate of intravascular hemolysis caused by 
compression, the RBC lifespan will be reduced.111 
Performance-Enhancing (Ab)use of 
Hormones
Anabolic Androgenic Steroids
The use of ergogenic (performance-enhancing) substances by ath­
letes continues to be a growing problem, and androgens are among 
the most frequently abused drugs.113 The global lifetime preva­
lence rate of using anabolic-androgenic steroids (AASs) is 6.4% 
for males and 1.6% for females.114 Classical sport disciplines are 
not

r females.114 Classical sport disciplines are 
not the only ones involved; the phenomenon is similarly wide­
spread among bodybuilders, and thus the drugs used have been 
collectively classified as appearance- and performance-enhancing 
drugs (APEDs).
1172
Although doping has been practiced since antiquity, often with 
placebo or toxic effects, truly effective APEDs only became available 
with the rise of modern pharmacology, and particularly followi

e of modern pharmacology, and particularly following 
the isolation and synthesis of testosterone and AASs. Testosterone 
came into clinical use shortly after its synthesis in 1935,115 and its 
first documented use for doping was by German rowers in 1952 (to 
maintain their marital duties during exhausting training) and by 
Russian weight lifters in 1954 to enhance their power.116
The IOC introduced antidoping regulations for the first time 
in 1967 and performed the first antidoping testing in 

967 and performed the first antidoping testing in the 1972 
Munich Olympics. In 1976, androgens were placed on the IOC 
doping list.113
Interestingly, an early and comprehensive review of previous 
results concluded that there was little evidence for supraphysio­
logic doses of testosterone or synthetic AASs having any apprecia­
ble effect on muscle size or strength in healthy men.117 However, 
many of the studies reviewed had a lack of adequate control and 
standardization. Conclusions from mor

control and 
standardization. Conclusions from more recent reviews suggested 
that the administration of AASs could consistently result in signif­
icant increases in strength if male athletes satisfied certain criteria, 
including the timing of doses and dietary factors.118,119 In 1996, 
Bhasin and associates120 demonstrated that the administration of 
supraphysiologic doses of testosterone in combination with exer­
cise in male weight lifters induces a greater increase in muscle size 
and stren

duces a greater increase in muscle size 
and strength than exercise alone or testosterone treatment alone 
(i.e., the effects of combining supraphysiologic doses of testos­
terone with exercise are additive). Subsequent work showed that 
increases in FFM, muscle size, strength, and power are highly 
dose dependent and correlated with serum testosterone concen­
trations.121,122 In fact, significant increases in muscle size and 
strength occur only with doses of 300 mg per week and higher.121 
The

 with doses of 300 mg per week and higher.121 
The increase in muscle size is due to a hypertrophy that results 
from an increase in cross-sectional areas of both type I and type II 
muscle fibers and an increase in myonuclear number.123
The anabolic effects of androgens are primarily mediated through 
androgen receptor signaling. Androgen receptors are expressed in 
the satellite cells and other stem-like cells in the interstitium of the 
skeletal muscle fibers.124 A growing body of evidence su

al muscle fibers.124 A growing body of evidence supports the 
hypothesis that testosterone and dihydrotestosterone promote myo­
genic differentiation of mesenchymal, multipotent stem cells and 
inhibit their differentiation into the adipogenic lineage.125,126
The AASs used for nontherapeutic purposes are endogenous 
androgens (e.g., androstenedione, dehydroepiandrosterone [DHEA]), 
17β-esters of testosterone (e.g., cypionate, enanthate, heptylate, pro­
pionate, undecanoate, buciclate), 17α-alkyl

, pro­
pionate, undecanoate, buciclate), 17α-alkyl derivatives of testosterone 
(e.g., methyltestosterone, fluoxymesterone, oxandrolone, stanozolol), 
19-nortestosterone (nandrolone), 17β-esters of 19-nortestosterone 
(e.g., decanoate, phenpropionate), 19-norandrostenedione and 19-
norandrostenediol, and tetrahydrogestrinone. More than 100 differ­
ent AASs have been developed, with most of them being used ille­
gally, synthesized in clandestine laboratories, commercialized without 
medical presc

aboratories, commercialized without 
medical prescription or safety controls, and sometimes unknown to 
the scientific world127 (Table 27.1).
The updated list of AASs is available on the following website: 
https://www.wada-ama.org/en/prohibited-list.128
Intramuscular injections are used far more frequently than oral for­
mulations. The dose of AASs used by athletes varies considerably and is 
often thought to exceed 10 to 40 times the recommended therapeutic 
dose. In addition, combinations of 

d therapeutic 
dose. In addition, combinations of androgens are used more frequently 
than single agents. Multiple androgens may be combined in a prac­
tice known as stacking, in which two or more androgens are added in 
progressively increasing doses over a period of several weeks. Athletes 
also often use a practice called cycling, in which weeks of androgen 
use are followed by periods of drug holiday; this routine is based on 
the unproven premise that cycling prevents desensitization to mas

emise that cycling prevents desensitization to massive 
doses of androgen. The phrase “building a pyramid” refers to the pro­
gressive increase in the doses of androgens during a cycle. Toward the 
end of a cycle, athletes may reduce the doses of androgens or switch to 
other drugs (e.g., human chorionic gonadotropin or aromatase inhibi­
tors or estrogen antagonists) that they believe will reduce the likeli­
hood of testicular suppression. In most surveys, the duration of steroid 

t surveys, the duration of steroid 
administration or steroid cycle lasts between 4 and 12 weeks.129
The side effects associated with AAS use are numerous and involve 
multiple organ systems113,116,130 (Table 27.2). Systematic investi­
gations of the adverse effects of androgens in athletes and recre­
ational body builders have been difficult for many reasons. With 
the exception of the association between hepatic dysfunction and 
the use of some oral AASs, many of the reports of

the use of some oral AASs, many of the reports of serious side 
effects in otherwise healthy individuals have come from anecdotal 
case studies or small retrospective studies based largely on self-
reported data.130 Confounding factors, such as undiagnosed pre­
existing conditions, family history, and concurrent use of other 
drugs, further dampen the credibility of case reports. Moreover, 
because most anabolic steroids are obtained on the black market 
and are of dubious quality, there is pot

k market 
and are of dubious quality, there is potential for adverse medi­
cal events independent of steroid use. However, data from larger 
observational studies suggest that the majority (88–96%) of AAS 
users experience at least one minor subjective side effect, including 
acne (40–54%), testicular atrophy (40–51%), gynecomastia (10–
34%), cutaneous striae (34%), and injection site pain (36%).131
AAS-induced hypogonadism (ASIH) is common among former 
AAS abusers and usually presents as hypog

 former 
AAS abusers and usually presents as hypogonadotropic hypogonad­
ism due to abrupt decreases in plasma androgen levels following 
AAS cessation.132 Scientific data on ASIH are limited, but the con­
dition is characterized by symptoms and signs of hypogonadism 
(e.g., testicular atrophy, low plasma testosterone levels, impaired 
spermatogenesis, erectile dysfunction, fatigue, decreased libido, and 
depressive symptoms) and is considered to resolve spontaneously 
within 6 to 12 months.116,

 resolve spontaneously 
within 6 to 12 months.116,133 However, studies investigating the 
recovery phases of young men with ASIH are scanty. A growing 
number of studies reporting cases in which ASIH manifestations 
persisted years after AAS cessation suggest that ASIH is a more 
permanent condition in a substantial proportion of former AAS 
abusers.132 This emerging group of young men may become a con­
siderable public health concern in the coming years.
  Performance-Enhancing Hormones
Anabol

ng years.
17β-Esters of testosterone (cypionate, enanthate, heptylate, propionate, 
undecanoate, buciclate)
17α-Alkyl derivatives of testosterone (methyltestosterone, fluoxymester­
one, oxandrolone, stanozolol)
19-Nortestosterone (nandrolone)
17β-Esters of 19-nortestosterone (decanoate, phenpropionate)
19-Norandrostenedione
19-Norandrostenediol
Tetrahydrogestrinone
Peptide Hormones
Insulin-like growth factor 1
Insulin

owth hormone
Insulin
TABLE 27.1
1173
In a recent cross-sectional case-control study involving 37 cur­
rent AAS abusers, 33 former AAS abusers, and 30 healthy con­
trol participants, Rasmussen and associates132 demonstrated that 
the group of former AAS abusers exhibited significantly lower 
plasma total and free testosterone, had smaller testicular sizes, 
and featured a higher proportion of participants wi

nd featured a higher proportion of participants with depressive 
symptoms, fatigue, erectile dysfunction, and decreased libido 
than the control group more than 2 years after AAS cessation.
AASs may also induce dyslipidemia, cardiovascular disease, 
insulin resistance, glucose intolerance, diabetes mellitus, and liver 
disease.113,130 In general, the ingestion of oral C17α-alkylated 
anabolic steroids causes an average 30% decrease in high-density 
lipoprotein and an average 30% increase in low-

y 
lipoprotein and an average 30% increase in low-density lipopro­
tein.134 The mechanisms for this effect are unknown but appar­
ently include an increase in the activity of hepatic triglyceride 
lipase that catabolizes high-density lipoprotein particles. Most 
studies indicate that injectable non-C17α-alkylated anabolic ste­
roids, such as testosterone and nandrolone esters, exert minimal 
adverse effects on blood lipids.129,134 It is unclear if the adverse 
changes in blood lipids as a functi

f the adverse 
changes in blood lipids as a function of testosterone use actually 
lead to an increase in the incidence of coronary artery disease.
AASs may also influence platelet aggregation and the myocar­
dium, although the relationship between these effects and cardio­
vascular disease is unclear. Occasional reports of cardiomyopathies 
and arrhythmias associated with steroid use have been published, 
and several mechanisms have been proposed.130
Liver disease is a well-documented side effe

d.130
Liver disease is a well-documented side effect of most, but not 
all, C17α-alkylated AASs, the exception being oxandrolone. In 
contrast, most non-C17α-alkylated steroids exert minimal hepa­
toxicity. Liver pathologies associated with anabolic steroids include 
cholestasis, peliosis hepatis, hepatocellular adenoma and carci­
noma, and hepatic angiosarcoma and cholangiocarcinoma.113,130
Potential effects on the reproductive system include infertility 
and testicular atrophy in men, and mens

fertility 
and testicular atrophy in men, and menstrual and genital tract 
alterations and infertility in women.116 Although all AASs sup­
press the hypothalamic-pituitary axis to some extent, the result­
ing infertility in males is generally reversible. The effects of AAS 
use on fertility in women is unknown.116 Menstruation is either 
diminished or absent in steroid users, but ovulation may occur.135
Balding is common in those who use AASs that undergo 5α 
reduction to potent androgens. In so

t undergo 5α 
reduction to potent androgens. In some male steroid users, gyneco­
mastia may be caused by increased circulating estrogen associated 
with the use of aromatizable androgens and/or human chorionic 
gonadotropin, with decreased clearance of circulating estrogens as 
a result of impaired hepatic function, and/or with a temporary 
state of hypotestosteronemia following AAS withdrawal.130
Several studies have suggested that AAS use may lead to sig­
nificant psychological morbidity. Anxi

ead to sig­
nificant psychological morbidity. Anxiety, psychosis, irritability, 
increased aggression, and antisocial and violent behavior have 
been described to be associated with AASs. In addition, depen­
dence, withdrawal symptoms, and depression may accompany or 
follow the nonmedical use of AASs.130
Finally, there are concerns about potential effects of androgens 
on the risk of prostate disease, although the long-term effects of 
supraphysiologic doses of androgens on the risk of prostate

iologic doses of androgens on the risk of prostate cancer, 
benign prostatic hypertrophy, and lower urinary tract symptoms 
remain unknown.116 
Detection
All known AASs can be detected via urinalysis for a period of time 
following the last dose. Detection of the misuse of exogenous sub­
stances is preferably done using gas chromatography/mass spectrom­
etry (GC-MS) complemented by liquid chromatography/tandem 
mass spectrometry.136 To improve the selectivity and sensitivity, tra­
ditional GC-MS

e selectivity and sensitivity, tra­
ditional GC-MS methods are accompanied by high-resolution mass 
spectrometry and/or tandem mass spectrometry techniques.137
Endogenous androgens and their metabolites occur naturally 
in the human body, and thus specific indicators for the detection 
  Side Effects of Anabolic Androgenic Steroids
TABLE 27.2
Lipid disorders (decreased HDL, increased LDL)
Increased platelet aggregation
Increased hematocrit
Elevated blood pressure
C

ion
Cosmetic
Acne
Hair loss
Cutaneous striae
Reproductive-Endocrine
Libido changes
Subfertility
In Males
Testicular atrophy
Impaired spermatogenesis
Prostate diseases
In Females
Hirsutism
Breast atrophy
Voice deepening
Virilization (clitoromegaly)
Menstrual disturbances
Hepatic
Cholestasis
Steatosis
Tumors
Hepatocellular adenoma and carcinoma
Hepatic angiosarcoma and cholangiocarcinoma
Mood sw

holangiocarcinoma
Mood swings
Anxiety
Psychosis
Irritability
Dependence
Injection Related
Infection
Bruising
Fibrosis
Injection site pain
HDL, High-density lipoprotein; LDL, low-density lipoprotein.
1174
of the exogenous administration of these steroids are required.137 
For screening purposes, a set of urinary concentrations of several 
endogenous steroids or metabolites is determined by the GC-MS 
meth

ds or metabolites is determined by the GC-MS 
method used for the detection of steroid abuse. The method of ste­
roid profiling was first introduced into routine doping control by 
Donike and associates138 in 1983 (testosterone to epitestosterone 
ratio). The most important steroid profile parameters in doping 
control are the ratios of testosterone/epitestosterone, androster­
one/etiocholanolone, androstane/testosterone, and 5α/5β forms 
of androstane-3,17-diol. The administration of steroids l

ostane-3,17-diol. The administration of steroids like tes­
tosterone or its precursors (e.g., androstenediol, androstenedione, 
or DHEA) or metabolites (e.g., dihydrotestosterone or epitestos­
terone) has been proved to alter one or more of the parameters 
of the urinary steroid profile.138 Consequently, monitoring the 
steroid profile parameters allows to screen for potential misuse. 
GH has been used as a drug of abuse in sport since the early 
1980s, although the first scientif

ince the early 
1980s, although the first scientific studies demonstrating a clear-
cut physiologic role for GH in adults were only published in the 
peer-reviewed medical literature in 1989.139,140
Human GH (hGH) misuse in sports predates its use in adult 
endocrinology by 10 years, although cadaveric GH had been used 
in children since the 1950s. This practice was stopped by clini­
cians (but not athletes) when cases of Creutzfeldt-Jakob disease 
were discovered in patients who had previously 

e 
were discovered in patients who had previously received hGH. 
The clinical use of GH resumed following US Food and Drug 
Administration approval of the first synthetic recombinant hGH 
(rhGH) in 1985. Other formulations subsequently followed.141
GH has several properties that are attractive to athletes wishing 
to improve their performance, but it is mainly misused because 
of its anabolic and lipolytic effects. GH administration leads to 
increased muscle mass and a decrease and redistributi

reased muscle mass and a decrease and redistribution of body 
fat from central to peripheral depots, thereby potentially improv­
ing the power to weight ratio of an athlete.141
In 2008, Liu and associates142 completed a systematic review 
of 44 randomized controlled trials that compared GH treatment 
with no GH treatment in healthy adults. Their review suggests 
that although GH administration may increase FFM, strength 
and exercise performance did not improve with GH, and edema 
and fatigue we

did not improve with GH, and edema 
and fatigue were more common in the GH-treated subjects. The 
authors concluded that claims that GH improves athletic perfor­
mance are not supported by the available scientific literature.
Graham and associates143 performed a well controlled study 
that showed an effect on strength and peak power output in a 
group of well-defined abstinent steroid abusers using rhGH. A 
study in recreational athletes demonstrated that in the short term, 
GH significantly inc

ated that in the short term, 
GH significantly increased lean body mass, reduced fat mass, and 
improved sprint capacity but not strength, power, or endurance.144
Because GH is a banned substance, the doses that athletes use 
are difficult to evaluate. It has been suggested that athletes abusing 
GH take rhGH three to four times per week at a dose of 10 to 25 
IU per day to increase their lean body mass.145
GH abuse extends beyond professional sports and is also present 
among adolescents engage

orts and is also present 
among adolescents engaged in sports in schools. This widespread use 
presents a public health problem because GH use is accompanied by 
adverse effects, and long-term use can lead to serious morbidity.146
The side effects of GH treatment in GH-deficient adults are well 
documented and include edema, arthralgias, myalgias, sweating, 
fatigue, and dizziness.141,147 Athletes typically take pharmacologic 
doses that are likely 10 or more times greater than t

es that are likely 10 or more times greater than typical replace­
ment doses. Although the long-term adverse effects of these mega­
doses are not known at present, some insight may be obtained 
from patients with acromegaly. Acromegalic patients have an 
increased risk of insulin resistance and diabetes, hypertension, 
cardiomyopathy, and certain forms of cancer (colorectal, thyroid, 
breast, and prostate).148
Finally, the risk of infections such as human immunodeficiency 
virus/acquired immunod

 as human immunodeficiency 
virus/acquired immunodeficiency syndrome or hepatitis due to 
the use of nonsterile or contaminated syringes145 and the avail­
ability on the black market of GH that comes from extracts of 
human pituitary glands and represents a source for Creutzfeldt-
Jakob disease must be taken into account. 
Detection
Two different strategies have been proposed to detect GH dop­
ing in sports.149 For the marker method “pharmacodynamic end­
points of GH use,” the GH-2000 and GH-200

mic end­
points of GH use,” the GH-2000 and GH-2004 consortiums 
identified biochemical parameters of the IGF system, such as 
IGF1, IGFBP3, and the acid-labile subunit, as suitable markers of 
GH use in combination with procollagen cleavage products that 
also show a clear-cut increase following GH use. The combination 
of IGF1 and the procollagen III N-terminal extension peptide is 
proposed to provide a set of markers that allow the detection of 
GH abuse in athletes for up to 2 weeks after t

of 
GH abuse in athletes for up to 2 weeks after the last application.
The “GH-isoform method” exploits the difference in iso­
form composition between recombinant GH consisting mostly 
of monomeric 22-kilodalton (kDa) hGH while the pituitary 
secretes a variety of GH isoforms including a 20-kDa form lack­
ing 14 amino acids, as well as amidated and acylated isoforms, and 
dimers, oligomers, and fragments of GH. After peripheral injec­
tion of recombinant 22-kDa hGH, the pituitary’s production o

combinant 22-kDa hGH, the pituitary’s production of 
GH isoforms is reduced by negative feedback via IGF1. Subject­
ing serum samples to two immunoassay analyses—one of which 
is specific for 22-kDa monomeric hGH, whereas the other recog­
nizes the majority of isoforms released from the pituitary—allows 
the calculation of an isoform ratio (Fig. 27.1).
The isoform test is an excellent strategy to detect GH doping, 
provided that it is administered shortly after the last GH dose 
(within 24–36 ho

d shortly after the last GH dose 
(within 24–36 hours, depending on the dose),150 realistically 
probably within 12 to 24 hours.146 Due to this short window of 
opportunity, the test is not well suited for in-competition test­
ing. Its use in unannounced out-of-competition testing, however, 
should be more successful in catching GH abusers, and recently it 
has been used mostly in that setting.146 
GH Secretagogues
GH secretagogues are peptides or nonpeptidic agents that act to 
release GH from 

r nonpeptidic agents that act to 
release GH from the pituitary. There is evidence that they are being 
used by athletes as an indirect method for GH doping. Secreta­
gogues include GH-releasing hormone and its analogues, ghrelin 
analogues (known as GH-releasing peptides) or GHSs (GH secreta­
gogues in a narrower sense), and amino acids (e.g., arginine or orni­
thine). GHSs have short-lived effects and provide a relatively weak 
boost in GH exposure compared with what can be achieved by 
direct

sure compared with what can be achieved by 
direct GH administration.146 Nevertheless, GHSs may be attractive 
to athletes who want to avoid detection because the GH released is 
endogenous and therefore not detectable by the GH isoform test.146
Currently, there are no published studies of the effects of GHSs 
on performance, but investigations have shown that they increase 
GH secretion and potentially could improve athleticism through 
this mechanism.141 
1175
Chapter 27  Hormones and Athletic

hanism.141 
1175
Insulin-like Growth Factor 1
Recombinant human IGF1 (rhIGF1) is used in clinical practice, but 
a variety of IGF1 compounds and IGF1 analogues are also advertised 
on the internet, and many have been available on the black market for 
several years.151 The rationale for using rhIGF1 as an ergogenic aid dif­
fers little from that of rhGH. The potential benefits include increased 
muscle protein synthesis, increased glycogen synthesis,

e protein synthesis, increased glycogen synthesis, and free fatty 
acid availability,152 but in contrast to GH, IGF1 is not lipolytic.
Abuse of IGF1 is more recent than that of rhGH. In fact, until 
recently, IGF1 was only available in limited supply, partly because 
there is no natural source for this hormone and partly because 
there were no pharmaceutical preparations. This situation changed 
with the development of two compounds, mecasermin (Increlex) 
by Tercica (Brisbane, CA) and mecasermi

Increlex) 
by Tercica (Brisbane, CA) and mecasermin rinfabate (iPLEX) by 
Insmed (Richmond, VA). Both preparations received US Food and 
Drug Administration approval for clinical use in the treatment of 
growth failure in children with severe primary IGF1 deficiency or 
with GH gene deletion who have developed neutralizing GH anti­
bodies.153 Although there are no confirmed cases of IGF1 misuse 
by athletes, the drug seems to be already popular among amateur 
bodybuilders. IGF1 is extensively di

mong amateur 
bodybuilders. IGF1 is extensively discussed on internet body­
building forums. The list of purported benefits include increases in 
muscle size and strength, improvements in energy and endurance, 
benefits to the immune system, and increased bone density.151
Very few studies have been undertaken to examine the effects 
of IGF1 on performance. Guha and associates154 demonstrated a 
9% increase in ˙VO2max in recreational athletes following 28 days 
of administration of rhIGF1/rhIGFBP

owing 28 days 
of administration of rhIGF1/rhIGFBP3 despite no change in body 
composition. The mechanism underlying this change is unknown, 
but previous studies have shown that the intravenous administra­
tion of rhIGF1 caused an increase in cardiac output, heart rate, and 
stroke volume in healthy adults.155 IGF1 may have a role in the reg­
ulation of vascular tone through alterations in nitric oxide synthesis, 
allowing improved intramuscular blood flow. Serum IGF1 corre­
lates positively wi

 blood flow. Serum IGF1 corre­
lates positively with hemoglobin concentration, which may enhance 
oxygen delivery to exercising muscle. Finally, an anabolic effect on 
respiratory muscle strength may improve inspiratory effort.141
The adverse effects that have been reported with Increlex include 
hypoglycemia, jaw pain, headache, myalgias, and fluid reten­
tion.156 The most common adverse effects observed so far with the 
use of mecasermin rinfabate include local injection site e

ecasermin rinfabate include local injection site erythema 
and lipohypertrophy, although headaches, increased liver and kid­
ney size, and altered liver function tests have also been reported.153
It also seems reasonable to hypothesize that IGF1 could result 
in adverse effects similar to acromegaly.152 
Detection
At present, there is no specific test to detect IGF1 abuse, and detec­
tion of this form of doping poses many challenges. IGF1 is excreted 
in urine at low concentrations, and methods 

eted 
in urine at low concentrations, and methods for measuring urinary 
Pituitary hGH
Pref.
22 kD
Pref.
pituitary hGH
22 kD hGH and
various other
isoforms
22 kD hGH
exclusively
Recombinant hGH
Incubate sample with 22 kD specific
or permissive mAb
Add detection mAb to measure
amount of bound hGH isoforms
A
B
Pref.
22 kD
Pref.
Wash to remove unbound hGH
Pref.
22 kD
Pref.
C
D
• Fig. 27.1  (A) While the pituitary gland secretes a variety of molecular forms of growth horm

cretes a variety of molecular forms of growth hormone (GH), recombinant GH consists predominantly of mono­
meric 22 kD GH. (B) Serum samples are incubated with immobilized monoclonal antibodies (mAbs) against GH preferentially binding to either 22 kD GH 
(left) or the other pituitary-derived molecular forms (right). (C) Unbound GH forms and other sample constituents are washed away before (D) incubation 
with an independently labeled anti-GH antibody binding to all GH molecular forms. The result

body binding to all GH molecular forms. The result of the differential immunoassay is then expressed as the ratio 
between the two different isoform measurements. hGH, human growth hormone.
1176
IGF1 are complex and time consuming. Several factors contribute 
to renal clearance of IGF1: a significant increase in urinary IGF1 
concentration is observed as part of the proteinuria that occurs in 
response to exercise. Blood sample collection, rather tha

e to exercise. Blood sample collection, rather than urine, 
will thus be required for IGF1 doping tests.151
The GH-2004 research group has been investigating methods 
for detection of IGF1 misuse, and a test is being developed based 
on principles of the successful GH-2000 marker method. Com­
mercial immunoassays for these markers have been validated for 
antidoping purposes, but new methods, including IGF1 measure­
ment by use of mass spectrometry, are hoped to improve the per­
formance of the 

ry, are hoped to improve the per­
formance of the tests and to help in the detection of athletes who 
are doping with these peptide hormones.151,157 
Insulin
Insulin stimulates the uptake of glucose into muscle and fat by mak­
ing available an increased number of glucose transporters (Glut-4) 
at the cell membrane. However, its main effect is inhibitory to lipol­
ysis, glycolysis, gluconeogenesis, ketogenesis, and proteolysis.158
Insulin regulates hepatic glucose output by inhibiting gluconeo­
g

s hepatic glucose output by inhibiting gluconeo­
genesis and promoting glycogen storage. Similarly, in muscle cells, 
insulin-mediated glucose uptake enables glycogen to be synthesized 
and stored, and for carbohydrates, rather than fatty acids or amino 
acids, to be utilized as the immediately available energy source for 
muscle contraction. Although insulin stimulates the uptake of amino 
acid into cells and promotes protein synthesis in a range of tissues at 
high insulin concentrations, the 

e of tissues at 
high insulin concentrations, the major action of insulin is to inhibit 
proteolysis, which occurs at lower insulin concentrations.152
The theoretic performance benefits of insulin are mediated by an 
increase in muscle glycogen storage and the inhibition of proteoly­
sis but have not been demonstrated in clinical or scientific trials.152 
The first suggestions of insulin as an anabolic agent were published 
in two bodybuilding magazines in 1996, which were commented 
on in the B

gazines in 1996, which were commented 
on in the British Journal of Sports Medicine in 1997.159 At the Win­
ter Olympic Games in Nagano in 1998, a Russian medical officer 
inquired as to whether the use of insulin was restricted to type 1 
diabetes.160 This drew attention to its role as a potential perfor­
mance-enhancing drug and led to its ban in 1999 by the IOC.153
Various reports since then have described the misuse of insulin 
to improve performance and muscle strength, and synthetic ana­
l

formance and muscle strength, and synthetic ana­
logues were the subject of several studies describing the beneficial 
effects of biotechnologically modified insulins.161
The most common adverse effect of insulin use is hypoglycemia. 
Most athletes who abuse insulin usually balance the ingestion of car­
bohydrate when injecting rapidly acting insulin analogues. Another 
problem associated with insulin is weight gain, although most com­
petitive athletes are accustomed to diet and

 com­
petitive athletes are accustomed to diet and follow training regimens 
that allow them to have strict control over weight gain.152,153 
Detection
So far, no screening tool is available providing fast and reliable 
information on possible insulin misuse. Only sophisticated pro­
cedures including immunoaffinity purification followed by liquid 
chromatography and tandem mass spectrometry have enabled the 
detection of synthetic insulins in doping control blood or urine 
samples.162 Insulin an

ing control blood or urine 
samples.162 Insulin analogues have been designed to alter their 
pharmacokinetics by genetic substitutions of one or two amino 
acids from human insulin. These small differences can be utilized 
to differentiate between native and exogenous insulin.153 
Erythropoietin and the Erythropoietin System
EPO, a glycoprotein hormone naturally produced in the kidney 
and the liver, is an essential growth factor for the erythrocytic pro­
genitors in the bone marrow. Tissue hypo

ytic pro­
genitors in the bone marrow. Tissue hypoxia is the physiologic 
stimulus for EPO expression and erythropoiesis. Once released, 
it serves to stimulate an increase in hemoglobin. In this way, it 
increases the oxygen-carrying capacity of the blood.111
Successful cloning of the human EPO gene allowed for the 
production of recombinant human EPO (rhEPO) and later the 
approval to treat patients with anemia. More recently, several 
newer generations of EPO analogues have been produced.163

nerations of EPO analogues have been produced.163
Because hemoglobin mass correlates with the aerobic capac­
ity, some athletes seek to increase erythropoiesis by pharmacologic 
means. Treatment with rhEPO may enhance performance by rais­
ing hemoglobin concentration. Studies show that when rhEPO is 
applied to healthy volunteers in low dosages, ˙VO2max is increased 
by 6% to 12% when the hematocrit is raised to approximately 
0.50. In addition, the time to exhaustion (in the laboratory) is 
inc

the time to exhaustion (in the laboratory) is 
increased by up to 50% at a given level of ˙VO2max.164 When 
rhEPO administration is discontinued, ˙VO2max remains elevated 
for at least 3 weeks.111
Apart from rhEPO, several other erythropoiesis-stimulating 
agents have been developed. Darbepoetin alfa is an rhEPO gly­
cosylation analogue approved for use in 2002 in the European 
Union and United States. It has the same mechanism of action as 
rhEPO—binding and activation of the EPO receptor—but i

O—binding and activation of the EPO receptor—but it has 
a longer serum half-life and an increased in vivo potency. Another 
long-acting erythropoiesis-stimulating agent is methoxy polyeth­
ylene glycol-epoetin beta (methoxy PEG-epoetin beta). The half-
life of methoxy PEG-epoetin beta amounts to 130 to 140 hours 
on intravenous injection. The prolonged in vivo survival of dar­
bepoetin alfa and methoxy PEG-epoetin beta is in part due to a 
reduced EPO-R binding affinity.111
EPO mimetic peptides

ed EPO-R binding affinity.111
EPO mimetic peptides are synthetic cyclic peptides of about 
20 amino acids that signal through the EPO-R but show no 
sequence homology to EPO. The clinically most advanced product 
was peginesatide, a pegylated homodimer of two EPO mimetic 
peptides, which was initially approved in the United States for 
treatment of anemic patients with chronic renal failure but sub­
sequently recalled after lethal acute hypersensitivity reactions 
occurred in several patients.16

itivity reactions 
occurred in several patients.165
Sotatercept (ACE-011) is a novel recombinant erythropoietic 
protein. It is composed of the extracellular part of the activin type 
II receptor and the Fc domain of human immunoglobulin G1. In 
a clinical phase I study, single intravenous injections of sotatercept 
proved to increase blood hemoglobin levels, RBC numbers, and 
the hematocrit.166 However, the mechanisms underlying the effect 
of sotatercept or of analogue compounds on erythropoie

otatercept or of analogue compounds on erythropoiesis are still 
poorly understood.111
The expression of the EPO gene (chromosome 7q22) is under 
the control of several transcription factors, including GATA-2, 
which inhibits EPO expression. GATA antagonists are organic 
small molecule compounds that prevent GATA-2 from suppress­
ing the EPO promoter. GATA antagonists may be misused in 
sports, because they have been shown to increase EPO concen­
trations, hemoglobin levels, and endurance perfor

trations, hemoglobin levels, and endurance performance in mice, 
although human studies have not yet been reported.111,167
Hypoxia-inducible factor (HIF) stabilizers, such as α-ketoglutarate 
competitors, stimulate erythropoiesis by increasing EPO expres­
sion in the kidneys and at extrarenal sites. Advantages of HIF sta­
bilizers—compared with recombinant proteins—include the oral 
1177
route of administration. However, the HIFs can activate more 

inistration. However, the HIFs can activate more 
than 1000 genes apart from EPO, including genes for proteins 
that are involved in tumor growth, such as vascular endothelial 
growth factor. However, some of the HIF-activated genes encode 
proteins that may increase physical performance independent 
from the stimulation of erythropoiesis.111
EPO gene transfer is theoretically possible but medically little 
advanced. In  vivo allogeneic EPO transfer can cause immune 
reactions. Furthermore, anti

fer can cause immune 
reactions. Furthermore, antidoping research has provided tech­
niques to specifically amplify intronless DNA sequences allowing 
the detection of small amounts of transgenic DNA in blood.168 
Ex vivo EPO transfer is performed by use of transfected autolo­
gous or allogeneic cells that are transferred from in vitro cultures 
into the individual. The autologous ex vivo approach has been 
investigated in clinical trials.169 However, the method has made 
little progress and has

ever, the method has made 
little progress and has not stepped beyond the clinical trial phase, 
even after many years of study. Taken together, for technical dif­
ficulties, gene doping with EPO is probably not applied in sports, 
at least not yet.111
Artificially raising hemoglobin levels can have dangerous conse­
quences. In contrast to the effect of endurance training, which 
results in an increased plasma volume, the administration of 
rhEPO produces a selective increase in 

ration of 
rhEPO produces a selective increase in red cell mass. If the hema­
tocrit exceeds 0.50, blood viscosity and cardiac afterload increase 
significantly. The main risk of erythrocytosis with a hematocrit 
greater than 0.55 include heart failure, myocardial infarction, 
seizures, peripheral thromboembolic events, and pulmonary 
embolism.170 Moreover, EPO withdrawal may be implicated in 
neocytolysis—that is, the hemolysis of young red blood cells in 
the presence of an increased hematocri

d cells in 
the presence of an increased hematocrit.171 
Detection
The authorities of sports forbade the use of EPO in 1990, and 
now any analogue or mimetic is also included in the list of prohib­
ited substances of the World Anti-Doping Agency (WADA). The 
challenge of detecting the misuse of EPO has given strength to 
the proposal of several strategies.172 The marker methods rely on 
the measurement of some hematologic and serum parameters, and 
their comparison with populational or individua

d 
their comparison with populational or individual limit values.173 
An indirect test based on the measurement of five blood param­
eters, including reticulocyte hematocrit, serum EPO, hematocrit, 
soluble transferrin receptor, and the percentage macrocytes, was 
developed by Parisotto and associates.174 Two models were devel­
oped. The “on” model used all parameters to detect recent use of 
EPO. The “off” model used three parameters to detect EPO use 
with more retrospectivity.173 An advantage

PO use 
with more retrospectivity.173 An advantage of indirect methods to 
detect rhEPO is its universal coverage of different types of ana­
logues and mimetics, a field in clear expansion.
The most powerful way to directly discriminate between 
endogenous and rhEPO is probably based on the glycosylation 
differences existing between both types of molecules.175 In 1995, 
Wide and colleagues176 proposed, for the first time, a method able 
to separate both types of molecules in blood and urine. Th

ate both types of molecules in blood and urine. This tech­
nique was reliable, as it allowed to clearly identify the presence 
of rhEPO in urine and blood. However, although the proposed 
method was powerful as long as the biologic samples were col­
lected within 24 hours after the last rhEPO injection, it appeared 
to be far less sensitive on samples collected later after injection.
In June 2000—a few weeks before the Sydney Olympic Games—
Lasne and de Ceaurriz177 presented in Nature an innovat

 and de Ceaurriz177 presented in Nature an innovative test 
based on the isoelectric separation of urinary EPO isoforms on a 
polyacrylamide gel followed by a double blotting process.178 In 
the past 10 years, this test has been adapted to several recombinant 
EPO forms, such as darbepoetin-α, whose isoforms are located in 
the most acidic part of the gel, epoetin-δ, or more recently, generic 
(biosimilar) or “copy” EPOs. 
Glucocorticosteroids
Glucocorticosteroids (GCs) were first purified and m

ucocorticosteroids (GCs) were first purified and manufactured 
in the 1930s and 1940s following the discovery of their potent 
anti-inflammatory actions.179 GCs are widely used in medicine 
and have shown unchallenged therapeutic potential in several 
chronic inflammatory and other diseases.
GCs are also widely used in sports medicine for the treatment 
of conditions such as asthma and acute injuries. Nevertheless, their 
beneficial effect in certain conditions in sports, where inflamma­
tion is

tain conditions in sports, where inflamma­
tion is only a secondary reaction, remains to be validated.180
According to the World Anti-Doping Code, all orally, rectally, 
intravenously, and intramuscularly administered GCs are prohib­
ited, and their medical use requires a standard therapeutic use 
exemption. Administration of GCs by all other routes requires 
an abbreviated therapeutic use exemption, except dermatologic 
preparations, which are not prohibited.180 Because of the com­
plexity of G

ot prohibited.180 Because of the com­
plexity of GCs, determining the boundaries between their medical 
use and abuse is a constant challenge for antidoping organizations.
The first use of GCs as performance-enhancing drugs was 
described in the 1960s.179 With regard to athletes, the most inter­
esting systemic effect of GCs is energy production by stimula­
tion of gluconeogenesis and mobilization of amino acids and 
fatty acids. A possible increase in cardiovascular performance is 
a matter of 

ase in cardiovascular performance is 
a matter of debate, as there is no evidence for such effects. As 
a consequence, systemic GCs have been misused for decades to 
enhance performance, and they once belonged to the group of 
most commonly used doping substances in sports.180 Indeed, the 
expected effects of the use and abuse of GCs are numerous: neuro­
stimulatory effects at cerebral GC receptors could attenuate cen­
tral impressions of fatigue, and anti-inflammatory and analgesic 
effects cou

, and anti-inflammatory and analgesic 
effects could inhibit sensations of muscle pain on effort and raise 
the fatigue threshold.181
Few studies have examined the effects of GCs on exercise per­
formance. An extensive review of the scientific literature performed 
by Duclos182 showed two types of results: studies supporting the 
hypothesis that there is no relationship between performance and 
corticosteroid use in humans (negative studies) and studies sup­
porting the hypothesis that there are

studies sup­
porting the hypothesis that there are relationships between perfor­
mance and corticosteroid use in humans (positive studies).
Inconsistencies found regarding the ergogenic effect of GC 
administration in humans may be attributed to (1) the GC admin­
istration dosage, route, and mode (acute or short term); (2) the 
type, duration, and intensity (submaximal, maximal) of exercise 
tested; (3) the participants (highly trained or professional vs. recre­
ational trained); (4) the differe

ional vs. recre­
ational trained); (4) the differences in diet, such as whether or not 
experiments are food controlled and whether or not subjects fasted; 
and (5) GC intake coupled or not with intensive training.182
It is noteworthy that data reported by the “negative studies” 
mainly was derived from acute administration of GCs. Animal 
and human studies performed using higher doses of GCs and/
or longer periods of administration (positive studies) clarified 
the effects of GC based on scient

dies) clarified 
the effects of GC based on scientific evidence and clearly dem­
onstrated that GCs have ergogenic effects in both animals and 
humans.182
1178
GCs have pleiotropic effects, causing several adverse effects, espe­
cially at higher doses and for long periods, such as osteoporosis, 
insulin resistance, and cardiovascular conditions (e.g., hyperten­
sion and atherosclerosis).183 Moreover, a major (possibly life-
threatenin

).183 Moreover, a major (possibly life-
threatening) complication can arise on the withdrawal of GCs: 
acute adrenal insufficiency. 
Detection
The detection of the administration of GCs is complicated by the 
fact that the human body produces these steroids naturally. Sev­
eral groups have described protocols and screening methods for 
the analysis of endogenous GC urinary metabolites using liquid 
chromatography/mass spectrometry. Some methods allow for the 
analysis of a large number of analyt

llow for the 
analysis of a large number of analytes from most of the classes 
in the WADA Prohibited List, including anabolic agents, β2-
agonists, hormone antagonists and modulators, diuretics, stimu­
lants, narcotics, glucocorticoids, and β-blockers, and do so while 
meeting the WADA sensitivity requirements.184
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York: Springer; 2013:501–511.
	107.	 O’Rahilly SO, Hosker JP, Rudenski AS, et  al. The glucose 
stimulus-response curve of the beta-cell in physically trained 
humans, assessed by hyperglycemic clamps. Metabolism. 1988;37:
919–923.
	108.	 Colberg SR, et al. Exercise and type 2 diabetes: the American College of 
Sports Medicine and the American Diabetes Association: joint position 
statement executive

t al. Develop­
ment of models to predict anabolic response to testosterone admin­
istration in healthy young men. Am J Physiol Endocrinol Metab. 
2003;284:E1009–E1017.
	123.	 Sinha-Hikim I, Artaza J, Woodhouse L, et  al. Testosterone-
induced increase in muscle size in healthy young men is associ­
ated with muscle fiber hypertrophy. Am J Physiol Endocrinol Metab. 
2002;283:E154–E164.
	124.	 Sinha-Hikim I, Taylor WE, Gonzalez-Cadavid NF, et al. Andro­
gen receptor in human skeletal muscle and c

Andro­
gen receptor in human skeletal muscle and cultured muscle satel­
lite cells: up-regulation by androgen treatment. J Clin Endocrinol 
Metab. 2004;89:5245–5255.
	125.	 Singh R, Artaza JN, Taylor WE, et al. Androgens stimulate myo­
genic differentiation and inhibit adipogenesis in C3H 10T1/2 
pluripotent cells through an androgen receptor-mediated pathway. 
Endocrinology. 2003;144:5081–5088.
	126.	 Singh R, Bhasin S, Braga M, et al. Regulation of myogenic dif­
ferentiation by androgens: cr

ion of myogenic dif­
ferentiation by androgens: cross talk between androgen receptor/ 
beta-catenin and follistatin/transforming growth factor-beta signal­
ing pathways. Endocrinology. 2009;150:1259–1268.
	127.	 Di Luigi L, Romanelli F, Leni A. Androgenic-anabolic steroids 
abuse in males. J Endocrinol Invest. 2005;28:81–84.
	128.	 World Anti-Doping Agency (WADA). The World Anti-Doping 
Code International Standard: Prohibited List, January 2018. Avail­
able at: http:www.wada-ama.org. Accessed 

 J, Ernst A, et al. High-sensitivity chemilumi­
nescence immunoassays for detection of growth hormone doping 
in sports. Clin Chem. 2009;55:445–453.
	151.	 Guha N, Cowan DA, Sönksen PH, Holt RI. Insulin-like growth 
factor-I (IGF-I) misuse in athletes and potential methods for detec­
tion. Anal Bioanal Chem. 2013;405:9669–9683.
	152.	 Richmond EJ, Rogol AD. Hormones as performance enhancing 
agents. In: Constantini N, Hackney AC, eds. Contemporary Endo­
crinology: Endocrinology of Physical Act

ary Endo­
crinology: Endocrinology of Physical Activity and Sport. 2nd ed. New 
York: Springer; 2013:535–546.
	153.	 Erotokritou-Mulligan I, Holt RIG. Insulin-like growth factor I and 
insulin and their abuse in sport. Endocrinol Metab Clin North Am. 
2010;39:33–43.
	154.	 Guha N, Nevitt SP, Francis M, et  al. The effects of recombi­
nant human insulin-like growth factor-I/insulin-like growth fac­
tor binding protein-3 administration on body composition and 
physical fitness in recreational at

I, Bartlett C, et  al. Biochemi­
cal markers of insulin-like growth factor-I misuse in athletes: the 
response of serum IGF-I, procollagen type III amino-terminal pro­
peptide, and the GH-2000 score to the administration of rhIGF-
I/rhIGF binding protein-3 complex. J Clin Endocrinol Metab. 
2014;99:2259–2268.
	158.	 Ho RC, Alcazar O, Goodyer LJ. Exercise regulation of insulin 
action in skeletal muscle. In: Kraemer WJ, Rogol AD, eds. The 
Endocrine System in Sports and Exercise, in Collaboratio

as A, Schänzer W. Detecting peptidic drugs, drug 
candidates and analogs in sports doping: current status and future 
directions. Expert Rev Proteomics. 2014;11:663–673.
	163.	 Lamon S, Robinson N, Saugy M. Procedures for monitoring 
recombinant erythropoietin and analogs in doping. Endocrinol 
Metab Clin North Am. 2010;39:141–154.
	164.	 Thomsen JJ, Rentsch RL, Robach P, et al. Prolonged administra­
tion of recombinant human erythropoietin increases submaximal 
performance more than maximal a

control: past, present and 
future. Bioanalysis. 2012;4:1565–1575.
	173.	 Segura J, Zorzoli M. Distinction between endogenous and exog­
enous erythropoietin: marker methods. In: Ghigo E, Lanfranco F, 
Strasburger CJ, eds. Hormone Use and Abuse by Athletes. New York: 
Springer; 2011:151–161.
	174.	 Parisotto R, Gore CJ, Emslie KR, et  al. A novel method utilis­
ing markers of altered erythropoiesis for the detection of recom­
binant human erythropoietin abuse in athletes. Hematologica. 
2000;85

gh pressure liquid chro­
matography coupled to high resolution mass spectrometry for fast 
screening in high throughput doping control. J Chromatogr A. 
2013;1288:82–95.
1179
28
Endocrinology and Aging
ANNEWIEKE W. VAN DEN BELD AND STEVEN W.J. LAMBERTS
Aging and Physical Frailty, 1180
The Endocrinology of Aging, 1180
Menopause, 1182
Andropause Versus Late-Onset Hypogonadism, 1187
Adrenopause, 1191
Somatopause , 1192
The Concept of Successful Aging, 1194
KEY POINTS
 • 
Treatment 

 Successful Aging, 1194
KEY POINTS
 • 
Treatment of subclinical hypothyroidism in older individuals is 
not associated with clinical benefits. In younger individuals, it is 
associated with fewer ischemic heart disease events, but this is 
not evident in older people (>70 years).
 • 
Subclinical hyperthyroidism is associated with an increased risk 
of total and ischemic heart disease mortality, as well as incident 
atrial fibrillation. Currently, treating subclinical hyperthyroidism 
is recomm

y, treating subclinical hyperthyroidism 
is recommended when the thyroid-stimulating hormone level is 
<0.1 mU/L, and confirmed after a 3- to 6-month period, in all in­
dividuals 65 years of age or older; postmenopausal women not 
on bisphosphonates or estrogen; and patients with cardiac risk 
factors, heart disease, or osteoporosis. At a thyroid-stimulating 
hormone level between 0.1 and 0.5 mU/L, treatment should be 
considered only for individuals 65 years of age or older and in 
patients wit

uals 65 years of age or older and in 
patients with cardiac disease or hyperthyroid symptoms.
 • 
Aging is associated with late-day and evening increases in 
cortisol levels; earlier morning cortisol maximum (phase ad­
vance)—for example, 6:30 am (older) versus 9:00 am (younger); 
lower circadian amplitude; and more irregular cortisol secretion 
patterns.
 • 
Estrogen hormone therapy is currently only recommended 
in the perimenopausal period in those women suffering from 
menopausal symptoms.

n those women suffering from 
menopausal symptoms. Hormone therapy is not indicated for 
cardioprotection for women in their 70s.
 • 
The current recommendation is not to treat asymptomatic older 
men with age-related decline in testosterone levels. Clinicians 
can consider offering testosterone therapy on an individualized 
basis to older men with low testosterone levels and clini­
cally significant symptoms of androgen deficiency. Control of 
prostate size, prostate-specific antigen levels, a

prostate size, prostate-specific antigen levels, and hematocrit is 
mandatory.
 • 
During the aging process, growth hormone–insulin-like growth 
factor 1 axis activity declines. At present, there is no evidence 
to recommend medical intervention in the growth hormone–
insulin-like growth factor 1 axis as an antiaging effort to prolong 
life or rejuvenate healthy elderly people.
G
lobal life expectancy at birth is currently 69.1 years for 
men and 73.8 years for women.1 Twenty-nine countries 
ha

d 73.8 years for women.1 Twenty-nine countries 
have an average life expectancy of 80 years or higher. 
Life expectancy increases accelerated in most regions from 2000 
onward, and overall there was a global increase of 5 years in life 
expectancy between 2000 and 2015. Average global life expec­
tancy at birth is predicted to further increase by 4 years by 2030.1 
Between 1950 and 2050, the number of people in the world older 
than 80 years is expected to increase from 14.5 million to 394.7 
mi

xpected to increase from 14.5 million to 394.7 
million.2
It is not clear, however, whether these additional years will be 
satisfactory. Healthy life expectancy in high-income countries at 
birth in 2015 for males and females combined was 69.8 years, 8.9 
years lower than total life expectancy at birth. In general, healthy 
life expectancy is on average 11.7% shorter than life expectancy 
(ranging from 9.3–14.7% between countries).1 Most data indi­
cate a modest gain in the number of healthy ye

di­
cate a modest gain in the number of healthy years lived but a far 
greater increase in years of compromised physical, mental, and 
social function.3 The number of days with restricted activity and 
admissions to hospitals and nursing homes increases sharply after 
70 years of age.4 The compression-of-morbidity hypothesis5 sug­
gests that it may be possible to reduce cumulative lifetime mor­
bidity. Since chronic illness and disability usually occur late in 
life, cumulative lifetime disabili

 occur late in 
life, cumulative lifetime disability could be reduced if primary pre­
vention measures postpone the onset of chronic illness. Indeed, 
smoking, body mass index, and exercise patterns in midlife and 
late-adulthood are important predictors of subsequent disability.6 
Not only do persons with better health habits live longer, but in 
such persons, disability is postponed and compressed into fewer 
years at the end of life.
1180
Aging an

ION VI    Endocrinology and the Life Span
Aging and Physical Frailty
Throughout adult life, all physiologic functions start to decline 
gradually.7 There is a diminished capacity for cellular protein syn­
thesis, a decline in immune function, an increase in fat mass, a 
loss of muscle mass and strength, and a decrease in bone min­
eral density.7 Most older adults die of atherosclerosis, cancer, or 
dementia, but in an increasing number of the “healthy” oldest old, 
loss of muscle strength is the

althy” oldest old, 
loss of muscle strength is the limiting factor that determines their 
chances of an independent life until death.
Age-related disability is characterized by generalized weakness, 
impaired mobility and balance, and poor endurance. In the old­
est old, this state is termed physical frailty, defined as “a state of 
reduced physiological reserves associated with increased suscepti­
bility to disability.”8 Clinical correlates of physical frailty include 
falls, fractures, impairm

hysical frailty include 
falls, fractures, impairment in activities of daily living, and loss of 
independence. Falls contribute to 40% of admissions to nursing 
homes.9
Loss of muscle strength is an important factor in the develop­
ment of frailty. Muscle weakness can be caused by aging of muscle 
fibers and their innervation, osteoarthritis, and chronic debili­
tating diseases.10 A sedentary lifestyle, decreased physical activ­
ity, and disuse, however, are also important determinants of the 

 however, are also important determinants of the 
decline in muscle strength.
In a study of 100 frail nursing home residents (average age 
87 years), lower extremity muscle mass and strength were closely 
related.11 Supervised resistance exercise training (45 minutes 
three times a week for 10 weeks) doubled muscle strength and 
significantly increased gait velocity and stair-climbing power. 
This finding demonstrates that frailty in the elderly population 
is not an irreversible effect of aging

population 
is not an irreversible effect of aging and disease but can be influ­
enced and perhaps even prevented.11 Further, in nondisabled 
elderly persons living in the community, objective measures of 
lower extremity function are highly predictive of subsequent dis­
ability.12 Prevention of frailty may be achieved by caloric and 
protein support, vitamin D supplementation, reduction in poly­
pharmacy, and working (training).13 However, exercise is diffi­
cult to implement in the daily routi

ise is diffi­
cult to implement in the daily routine of the aging population, 
and the number of dropouts from exercise programs is very high.
Part of the aging process involving body composition (i.e., loss 
of muscle [strength] and bone, increase in fat mass) might also be 
related to changes in the endocrine system.14 Current knowledge 
has shed light on the effects of long-term hormone replacement 
therapy on body composition, as well as on atherosclerosis, cancer 
formation, and cognitive f

therosclerosis, cancer 
formation, and cognitive function. 
The Endocrinology of Aging
The two most important clinical changes in endocrine activ­
ity during aging involve the pancreas and the thyroid gland. 
Approximately 40% of individuals age 65 to 74 years and 50% 
of those older than 80 years have impaired glucose tolerance or 
diabetes mellitus, and in nearly 50% of elderly adults with dia­
betes, the disease is undiagnosed.15 These adults are at risk for 
development of secondary, mainly 

are at risk for 
development of secondary, mainly macrovascular, complications 
at an accelerated rate. Pancreatic, insulin receptor, and postre­
ceptor changes associated with aging are critical components 
of the endocrinology of aging. Apart from decreased (relative) 
insulin secretion by the beta cells, peripheral insulin resistance 
related to poor diet, physical inactivity, increased abdominal fat 
mass, and decreased lean body mass contributes to the deterio­
ration of glucose metabolism.

utes to the deterio­
ration of glucose metabolism.16 Dietary management, exercise, 
hypoglycemic agents, and insulin are the four components of 
treatment for these patients, whose medical care is costly and 
intensive (see Chapter 27).
Age-related thyroid dysfunction is also common.17 Lowered 
plasma thyroxine (T4) and increased thyrotropin concentrations 
occur in 5% to 10% of elderly women.17 These abnormalities are 
primarily caused by autoimmunity and are thus an expression of 
age-associat

munity and are thus an expression of 
age-associated disease rather than a consequence of the aging pro­
cess. Normal aging is accompanied by an increase in serum thy­
rotropin levels and seems to be dependent on the regional iodine 
intake.18-21 Serum free T4 levels remain largely unaffected dur­
ing aging, but decreased peripheral degradation of T4 results in 
a gradual age-dependent decline in serum triiodothyronine (T3) 
concentrations.17,22 The magnitude and pattern of changes in thy­
roid 

The magnitude and pattern of changes in thy­
roid function during aging, however, are highly variable among 
individuals.21,23 This slight decrease in plasma T3 concentrations 
occurs largely within the broad normal range of the healthy elderly 
population and has not been convincingly related to functional 
changes during the aging process. It remains to be clarified why 
serum thyroid-stimulating hormone (TSH) levels increase with 
advancing age. Several mechanisms, like a changed pituitary se

e. Several mechanisms, like a changed pituitary sen­
sitivity or affected TSH glycosylation and thus TSH bioactivity, 
have been proposed.19 The detrimental effects of overt thyroid 
dysfunction in elderly individuals are clearly recognized. However, 
the clinical relevance of mild forms of hypo- and hyperthyroidism 
are a matter of debate.
Subclinical hypothyroidism (SCH) is present in about 4% to 
8.5% of adults in the United States without known thyroid dis­
ease.24 Subtle thyroid dysfunction

wn thyroid dis­
ease.24 Subtle thyroid dysfunction in the oldest-old fraction of the 
elderly population (i.e., those >85 years) is often present. Although 
SCH in younger individuals is associated with an increased risk of 
atherosclerosis,25 such an association is not present in elderly sub­
jects older than 65 years.26,27 In addition, an observational study 
of real-life practice performed from data obtained from the United 
Kingdom General Practitioners Research Database showed that 
treatme

actitioners Research Database showed that 
treatment of SCH with levothyroxine was associated with fewer 
ischemic heart disease events in younger individuals, but this was 
not evident in older people (>70 years).26 Treatment with levothy­
roxine in persons with SCH older than 65 years for 1 year did not 
provide improvement of hypothyroid symptoms or tiredness.28 In 
fact, in 85-year-old “healthy” individuals, hypothyroidism was, in 
the subsequent 4 years, accompanied by lower all-cause and c

uent 4 years, accompanied by lower all-cause and cardio­
vascular mortality when compared with euthyroid individuals.29 
In a group of 400 males with a mean age of 78 years, low serum 
free T4 and T3 (with normal reverse T3 [rT3]) concentrations were 
associated with better physical performance and 4-year survival, 
whereas subjects with low serum T3 and high rT3 concentrations 
(i.e., fulfilling the criteria for “low T3 syndrome”) did not show a 
survival advantage and had lower physical activi

survival advantage and had lower physical activity23 (Fig. 28.1). 
These findings were confirmed in other studies that link lowered 
thyroid hormone levels to less frailty.30,31 These studies support 
the concept that some degree of physiologically decreased thyroid 
activity at the tissue level might even have favorable effects in the 
oldest-old subjects, but caution should be exercised when inter­
preting the predictive value of thyroid dysfunction in old subjects, 
which may give a double-

unction in old subjects, 
which may give a double-faced “Janus response” if not considered 
in the appropriate context.32,33
Subclinical hyperthyroidism seems to be associated with an 
increased risk of total and ischemic heart disease mortality, and 
incident atrial fibrillation, as was concluded by recently pooled 
individual data from 10 prospective cohort studies.34 In addi­
tion, subclinical hyperthyroidism is associated with an increased 
1181
Chapter 28  Endocrinology and Aging
fracture r

181
fracture risk.35 Controversy persists about whether treatment is 
warranted. The latest consensus guidelines recommend treating 
subclinical hyperthyroidism after diagnosing the underlying dis­
order, based on the level of TSH suppression. When the TSH is 
less than 0.1 mU/L, and confirmed after a 3- to 6-month period, 
treatment is advised in individuals with hyperthyroid symptoms; 
all individuals 65 years of age or older; postmenopausal women 
not on bi

s of age or older; postmenopausal women 
not on bisphosphonates or estrogen; and patients with cardiac risk 
factors, heart disease, or osteoporosis. At a TSH level between 0.1 
and 0.5 mU/L, treatment should be considered only for individu­
als 65 years of age or older and in patients with cardiac disease, 
osteoporosis, or hyperthyroid symptoms.36 Furthermore, a recent 
systematic review concluded that there is a substantial body of 
evidence to support the association between subclinical hype

o support the association between subclinical hyper­
thyroidism and cognitive impairment. There is, however, lack of 
evidence to suggest that antithyroid treatment might delay the 
development of dementia.37
In addition, cortisol homeostasis is influenced by age, mostly 
via unknown mechanisms. Most clinical studies show circadian 
cortisol changes with age (reviewed by Veldhuis and colleagues38). 
Aging is associated with late-day and evening increases in cortisol 
levels; earlier morning cort

ncreases in cortisol 
levels; earlier morning cortisol maximum (phase advance)—for 
example, 6:30 am (older) versus 9:00 am (younger); lower cir­
cadian amplitude (24-hour decrement for peak minus nadir or 
attenuated wake-evening slopes); and more irregular (less orderly) 
cortisol secretion patterns.
The question remains to which degree these alterations reflect 
or cause aging-associated changes in functional ability, cognition, 
and possibly depression. A few studies have investigated associ

depression. A few studies have investigated associa­
tions between parameters of the hypothalamic-pituitary-adrenal 
axis and functional parameters. More dynamic activity of the 
hypothalamic-pituitary-adrenal axis (i.e., a greater diurnal decline) 
seems to be associated with better physical performance in later 
life,39 as well as better cognitive function.40 In a longitudinal set­
ting, higher urinary cortisol levels are associated with an increased 
risk in Alzheimer disease pathology.41 Fur

reased 
risk in Alzheimer disease pathology.41 Further, data from a pro­
spective cohort study of more than 400 men and women, with a 
mean age of 61 years, demonstrated that a flatter slope in cortisol 
levels across the day was associated with increased risk of all-cause 
6-year mortality.42 Results from the Longitudinal Amsterdam 
Study of Aging showed that higher morning salivary cortisol levels 
were associated increased mortality in men, and that higher night-
time salivary cortisol levels

d that higher night-
time salivary cortisol levels were associated with increased mortal­
ity in women.43 Future clinical research is necessary to determine 
whether measuring diurnal cortisol measures is relevant in identi­
fying those who might benefit from intervention therapies.
Three other hormonal systems exhibit lowered circulating hor­
mone concentrations during normal aging, and these changes 
have thus far been considered mainly physiologic (Figs. 28.2 and 
28.3). Hormone replacement s

ogic (Figs. 28.2 and 
28.3). Hormone replacement strategies have been developed, but 
many aspects remain controversial, and replenishing hormone 
blood levels to those found in 30- to 50-year-old patients has not 
yet uniformly proved to be beneficial and safe.
The most dramatic and rapidly occurring change in women 
around age 50 years is menopause.44 Cycling estradiol production 
during the reproductive years is replaced by very low, constant 
Reverse T3 (nmol/L)
Upper limit of normal range
o

of rT3 (0.32 nmol/L)
Lower limit of normal
range of T3 (1.35 nmol/L)
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
T3 (nmol/L)
2.2
Age 77.3
[76.8; 77.8]
n  137,
2.05
[1.87; 2.22]
Disease
8.5
[8.1; 8.9]
PPS
2.30
[2.25; 2.36]
ADL
104.8 [101.4; 108.2]
LES
35.1
[34.0; 36.2]
IGS
0.89
[0.86; 0.91]
NeckBMD
51.9
[51.0; 52.7]
Lean mass
21.2
[20.2; 22.2]
Fat mass
Age 77.6
[76.7; 78.5]
n  66,
1.91
[1.66; 2.16]
Disease
9.0
[8.5; 9.6]
PPS
2.30
[2.23; 2.38]
ADL
105.7 [

e
9.0
[8.5; 9.6]
PPS
2.30
ADL
105.7 [100.9; 110.6]
LES
35.1
[33.5; 36.7]
IGS
0.90
[0.87; 0.94]
NeckBMD
53.5
[52.2; 54.7]
Lean mass
21.7
[20.3; 23.1]
Fat mass
Age 77.8
[77.2; 78.5]
n  123,
2.16
[1.98; 2.35]
Disease
8.4
[8.0; 8.8]
PPS
2.30
ADL
101.9
[98.4; 105.4]
LES
33.8
[32.7; 35.0]
IGS
0.88
[0.85; 0.90]
NeckBMD
50.6
[49.7; 51.5]
Lean mass
21.1
[20.1; 22.1]
Fat mass
Age 79.1
[78.0; 80.2]
n  66,
2.33
[2.07; 2.59]
Disease
7.9
[7.3; 8.5]
PPS
2.36
[2.28; 2.44]
ADL
100.2
[

0
0.8
0.6
0.4
• Fig. 28.1  Overview of the values of triiodothyronine (T3) and reverse T3 (rT3) within a population of 403 
elderly men. The spotted lines indicate the normal values of T3 and reverse T3. PPS, physical performance 
score; ADL, number of problems in activities of daily living; LES, maximum leg extensor strength; IGS, 
isometric grip strength; BMD, bone mineral density. (From van den Beld AW, Visser TJ, Feelders RA, et al. 
Thyroid hormone concentrations, disease, physical function

hormone concentrations, disease, physical function, and mortality in elderly men. J Clin Endocrinol 
Metab. 2005;90[12]:6403–6409.)
1182
estradiol levels. For many years, the prevailing view was that meno­
pause resulted from exhaustion of ovarian follicles. An alternative 
perspective is that age-related changes in the central nervous sys­
tem and the hypothalamic-pituitary unit initiate the menopausal 
transition. The evidence that both the ovary a

al 
transition. The evidence that both the ovary and the brain are key 
pacemakers in menopause is compelling.44
Changes in the activity of the hypothalamic-pituitary-gonadal 
axis in men are slower and more subtle. During aging, a grad­
ual decline in serum total and free testosterone levels occurs.45 
Andropause is characterized by a decrease in testicular Leydig cell 
numbers and their secretory capacity, as well as by an age-related 
decrease in episodic and stimulated gonadotropin secretion

 in episodic and stimulated gonadotropin secretion.46,47 
The primary site of the aging effect appears to be the Leydig cell’s 
ability to respond to luteinizing hormone (LH) with increased tes­
tosterone production.
The second hormonal system demonstrating age-related 
changes is adrenopause, a term that describes the gradual decline 
in circulating levels of dehydroepiandrosterone (DHEA) and 
its sulfate (DHEAS).48,49 Adrenal secretion of DHEA gradu­
ally decreases over time, whereas corticotr

gradu­
ally decreases over time, whereas corticotropin secretion, which 
is physiologically linked to plasma cortisol levels, remains largely 
unchanged. The decline in DHEA and DHEAS levels in both 
sexes therefore contrasts with the maintenance of plasma cortisol 
levels and seems to be caused by a selective decrease in the number 
of functional zona reticularis cells in the adrenal cortex instead of 
being regulated by a central (hypothalamic) pacemaker of aging.50
The third endocrine system 

 pacemaker of aging.50
The third endocrine system that gradually declines in activity 
during aging is the growth hormone (GH)–insulin-like growth 
factor 1 (IGF1) axis51 (see Fig. 28.3219). Mean pulse amplitude 
and duration and fraction of GH secreted, but not pulse fre­
quency, gradually decrease during aging. In parallel, a progressive 
drop in circulating IGF1 levels occurs in both sexes.51,52 There is 
no evidence for a peripheral factor in this process of somatopause, 
and its triggering 

 this process of somatopause, 
and its triggering pacemaker seems mainly localized in the hypo­
thalamus because pituitary somatotropes, even in the oldest old, 
can be restored to their youthful secretory capacity by treatment 
with GH-releasing peptides (see later discussion).
It is unclear whether changes in gonadal function (menopause, 
andropause) are interrelated with the processes of adrenopause and 
somatopause, which occur in both men and women. In addition, 
functional correlates (decr

d women. In addition, 
functional correlates (decrease in muscle size and function and 
in fat and bone mass, progression of atherosclerosis, and decline 
of cognitive function) have not been demonstrated to be directly 
causally related to these changes in endocrine activity. However, 
several effects of normal aging closely resemble features of (iso­
lated) hormonal deficiency (hypogonadism, GH deficiency), 
which in subjects in middle adulthood are successfully reversed 
by replacement of the

d are successfully reversed 
by replacement of the appropriate hormone.53,54 Although aging 
does not simply result from a variety of hormone deficiency states, 
medical intervention in the processes of menopause, andropause, 
adrenopause, or somatopause might prevent or delay some aspects 
of the aging process. 
Menopause
Menopause is the permanent cessation of menstruation result­
ing from the loss of ovarian follicular function and is diag­
nosed retrospectively after 12 months of amenorrhea.

sed retrospectively after 12 months of amenorrhea. In most 
women, vasomotor reactions, depressed mood, and urogenital 
complaints accompany this period of estrogen decline. In the 
subsequent years, the loss of estrogens is followed by a high inci­
dence of cardiovascular disease, loss of bone mass, and cogni­
tive impairment. The average age of menopause (51.4 years) has 
not changed over time and seems to be largely determined by 
genetic factors.
In the past decade, antimüllerian hormone (AM

ors.
In the past decade, antimüllerian hormone (AMH) has been 
proposed as a marker to predict age at natural menopause. In 
women, AMH is exclusively produced by granulosa cells of 
ovarian follicles during the early stages of follicle development. 
After an initial increase until early adulthood, AMH concen­
trations slowly decrease with increasing age until becoming 
12
6
9
3
2
4
5
7
8
10
11
1
Pacemaker
of aging?
GH
LH/FSH
PRL =
TSH
IGF-I
During aging:
ACTH =
DHEA
Cortisol =
Menopause:
Androp

ng aging:
ACTH =
DHEA
Cortisol =
Andropause:
Adrenopause:
Somatopause:
E2 / T
E2
T
DHEA 
GH/IGF-I
T4 =
• Fig. 28.2  During aging, declines in the activities of several hormonal sys­
tems occur. PRL, prolactin; T4, thyroxine; TSH, thyroid-stimulating hor­
mone. Left. A decrease in growth hormone (GH) release by the pituitary 
gland causes a decrease in the production of insulin-like growth factor 1 
(IGF1) by the liver and other organs (somatopause). Middle. A decrease in 
release of g

(somatopause). Middle. A decrease in 
release of gonadotropin luteinizing hormone (LH) and follicle-stimulating 
hormone (FSH) and decreased secretion at the gonadal level (from the 
ovaries, decreased estradiol [E2] from the testicle, decreased testosterone 
[T]) cause menopause and andropause, respectively. (Immediately after the 
initiation of menopause, serum LH and FSH levels increase sharply.) Right. 
The adrenocortical cells responsible for the production of dehydroepian­
drosterone (DHEA

r the production of dehydroepian­
drosterone (DHEA) decrease in activity (adrenopause) without clinically 
evident changes in corticotropin (adrenocorticotropic hormone [ACTH]) 
and cortisol secretion. A central pacemaker in the hypothalamus or higher 
brain areas (or both) is hypothesized, which together with changes in the 
peripheral organs (the ovaries, testicles, and adrenal cortex) regulates the 
aging process of these endocrine axes.
1183
A
B
C
D
E
F
Es

A
B
C
D
E
F
Estrogen excreted in urine
(µg/24 h)
400
300
200
100
0
4000
3000
2000
1000
0
DHEAS (ng/mL)
500
400
300
200
100
0
IGF-1 (ng/mL)
Puberty
Menopause
Females
20
40
60
80
100
6000
4000
2000
0
20
40
60
80
100
20
40
0
60
80
100
500
400
300
200
100
0
20
40
0
60
80
100
28
24
20
16
12
8
4
0
20
30
50
40
80
70
60
90
100
Free testosterone (ng/100 mL)
0
12
13
40
50
60
Males
• Fig. 28.3  Changes in the hormone le

50
60
Males
• Fig. 28.3  Changes in the hormone levels of normal women (left) and men (right) during the aging process. 
A and B, Estrogen secretion throughout an individual normal woman’s life (expressed as urinary estrogen 
excretion) (A) and mean free testosterone (T) index (the ratio of serum total testosterone to sex hormone–
binding globulin levels) during the life span of healthy men (B). C and D, Serum dehydroepiandrosterone 
sulfate (DHEAS) concentrations in 114 healthy women (C) and 16

AS) concentrations in 114 healthy women (C) and 163 healthy men (D). E and F, The course 
of serum insulin-like growth factor 1 (IGF1) concentrations in 131 healthy women (E) and 223 healthy 
men (F) during aging. Note the difference in the distribution of ages in the different panels. (A and B from 
Guyton A. Textbook of Medical Physiology, 8th ed. Philadelphia: Saunders; 1991:899. C and D modified 
from Ravaglia G, Forti P, Maioli F, et al. The relationship of dehydroepiandrosterone sulfate [D

 relationship of dehydroepiandrosterone sulfate [DHEAS] to 
endocrine-metabolic parameters and functional status in the oldest-old. Results from an Italian study on 
healthy free-living over-ninety-year-olds. J Clin Endocrinol Metab. 1996;81[3]:1173–1178. E and F modi­
fied from Corpas E, Harman SM, Blackman MR. Human growth hormone and human aging. Endocr Rev. 
1993;14[1]:20–39.)
1184
undetectable about 5 years before menopause when the stock 
of pr

out 5 years before menopause when the stock 
of primordial follicles is exhausted.55 In a long-term follow-up 
study, 257 women were followed for 11 years. It was demon­
strated that using age and AMH, the age range in which meno­
pause will subsequently occur can be individually calculated.56,57 
A study of 401 women participating in the Penn Ovarian Aging 
Study demonstrated that in women with a baseline AMH level 
below 0.20 ng/mL, the median time to menopause was 5.99 
years (95% confidence 

time to menopause was 5.99 
years (95% confidence interval [CI], 4.20–6.33) in the 45- to 
48-year age group and 9.94 years (95% CI, 3.31–12.73) in the 
35- to 39-year age group. With higher baseline AMH levels 
above 1.50 ng/mL, the median time to menopause was 6.23 
years in the oldest age group and more than 13.01 years in the 
youngest age group. Smoking significantly reduced the time to 
menopause.58
Perimenopausal Use of Hormone Therapy
Typical symptoms that result from the sudden decrease

ical symptoms that result from the sudden decrease in estro­
gen production around menopause are menstrual cycle disorders, 
vasomotor changes (hot flushes, night sweats), and urogenital 
complications (atrophic vaginal irritation and dryness, dyspareu­
nia, atrophic urethral epithelium leading to micturition disorders). 
Additional symptoms are irritability, mood swings, joint pain, 
and sleep disturbances. Frequency, severity, onset, and duration 
of symptoms vary widely between individuals an

on 
of symptoms vary widely between individuals and between ethnic 
groups. About 75% of women in Western societies experience so 
few troublesome symptoms during the menopausal transition that 
hormone therapy (HT) is not needed or requested.59 HT rapidly 
alleviates the symptoms of menopause. Hot flushes and vasomo­
tor instability, as well as symptoms of urogenital atrophy, rapidly 
disappear on the start of HT. 
Long-Term Hormone Replacement Therapy
Because life expectancy is increasing, the

Therapy
Because life expectancy is increasing, the time a woman spends 
after menopause constitutes more than one third of her life. A 
decade ago, long-term use of HT (5–10 years) was considered to 
offer advantages with regard to the prevention of the three chronic 
disorders most common in the elderly: cardiovascular diseases, 
osteoporosis, and dementia. In the early 1900s, several cross-sec­
tional and prospective studies demonstrated a statistically signifi­
cant reduction in coronary hear

istically signifi­
cant reduction in coronary heart disease in menopausal women 
taking HT. Grady and colleagues60 presented a meta-analysis of 
published observational studies and reported that HT was associ­
ated with a reduction in fatal coronary heart disease by one third. 
A meta-analysis of 25 observational studies conducted between 
1976 and 1996 showed that the relative risk for coronary heart 
disease in women who ever used HT compared with never users 
was 0.70.61
The Nurse’s Health St

ith never users 
The Nurse’s Health Study was a comprehensive investigation 
conducted in 121,700 female nurses age 30 to 55 years. In the 
latest report, compiled with data from 70,533 postmenopausal 
nurses followed for 20 years, the overall risk of coronary heart 
disease in current users of HT was reduced, with a relative risk of 
0.61.62
Over the past 15 years, however, findings from several pro­
spective randomized controlled trials (RCTs) have fully changed 
the attitudes conc

ials (RCTs) have fully changed 
the attitudes concerning the benefits and harms of HT. The 
Women’s Health Initiative (WHI) trial comprised two large, 
randomized, placebo-controlled clinical trials, including estro­
gen-only and combined estrogen-progestin studies in more 
than 161,000 “healthy” postmenopausal women, age 50 to 79 
years.63 It was expected that the WHI would definitively answer 
whether or not estrogen is cardioprotective. However, the estro­
gen-progestin versus placebo trial, 

r, the estro­
gen-progestin versus placebo trial, which involved more than 
16,000 women, was discontinued early because of an increase 
in cardiovascular complications (coronary heart disease, stroke, 
and venous thromboembolism) and increased incidence of breast 
cancer in the treatment group.63 Although important benefits 
were also seen (risk reduction for fractures and colon cancer), 
there was concern that the risks of combined estrogen-progestin 
outweighed the benefits. The estrogen-only

gestin 
outweighed the benefits. The estrogen-only versus placebo trial 
included nearly 11,000 women who had undergone hysterec­
tomy and therefore did not require a progestin. This trial was 
also stopped early because a small increase in breast cancer and 
coronary heart disease risk was seen, although hip fracture risk 
was reduced.64
Three other RCTs supported the absence of benefit of HT for 
prevention of coronary heart disease and ischemic stroke.65,66 
These studies were carried out in 

c stroke.65,66 
These studies were carried out in postmenopausal women with 
documented ischemic stroke or transient ischemic attack,67 in 
women after documented myocardial infarction,65 or in women 
with documented coronary heart disease.66
Taken together, the WHI, which studied women presumed 
healthy at recruitment, and the 3 other trials carried out in women 
with documented cardiovascular disorders argue strongly against 
the earlier assumptions made on the basis of observational studies 

tions made on the basis of observational studies 
that estrogen users had a 30% to 40% reduced risk of coronary 
heart disease mortality and morbidity relative to nonusers.
A series of commentaries has addressed the differences in 
outcome between the observational studies and the randomized 
trials.68,69 Healthy user bias, the age at which study participants 
started HT, the different estrogen and progestin preparations, and 
dose all have been mentioned as possible confounders.
Subsequently, r

mentioned as possible confounders.
Subsequently, reassessment of the data from the WHI and 
other studies has led to a different interpretation of the data, in 
that groups of peri- and early postmenopausal women may in fact 
derive cardiovascular benefits from hormone replacement ther­
apy. In this regard, much interest has been focused on the “tim­
ing hypothesis,” which states that estrogens are atheroprotective if 
used in an early phase of atherosclerosis development. In an arm 
of the WHI 

therosclerosis development. In an arm 
of the WHI in which 50- to 59-year-old women were treated with 
conjugated estrogens, coronary artery calcium scores were slightly 
but significantly lower.70 The timing hypothesis is also supported 
by a recent Danish study in which it was shown that participants 
who were assigned to HT and were younger than 50 years at 
baseline had a 65% lower risk of a combined outcome of all-cause 
mortality, heart failure, or myocardial infarction. Unfortunately, 
th

lure, or myocardial infarction. Unfortunately, 
there was no placebo, and the trial was not blinded.71 Further, an 
independent subanalysis from the WHI also supported the tim­
ing hypothesis. This study also demonstrated that the duration 
of treatment is of importance. Comparison of the rate ratios for 
years 1 through 6 versus years 7 through 8 showed a statistically 
significant (p = 0.003) reduction in cardiovascular disease risk 
after more than 6 years of use of conjugated equine estrogen

 than 6 years of use of conjugated equine estrogens 
(CEE) versus placebo.72 In summary, the majority of long-term 
large observational studies and several small RCTs strongly sug­
gest that menopausal HT should be protective against atheroscle­
rosis if initiated early but is potentially harmful if administered to 
women who already have mature at-risk plaque.73
Subsequently, several studies have confirmed the risk of breast 
cancer, which increases with longer duration of HT.74-76 In the 
Mill

ases with longer duration of HT.74-76 In the 
Million Women Study, current users of estrogen had an increased 
1185
risk of incident invasive breast cancer of 30%, whereas in women 
using estrogen plus progestin, this risk had doubled. Breast cancer 
risk was unchanged in both the older and younger women with 
a prior hysterectomy treated with estrogen only. Past users of HT 
also had no increased risk.77
In addition, the earlier expectations from observationa

dition, the earlier expectations from observational studies 
that estrogen use might prevent cognitive decline were not con­
firmed by randomized, placebo-controlled trials. Estrogen therapy 
alone did not reduce dementia or mild cognitive impairment in 
women 65 years of age or older, but the estrogen-progestin com­
bination resulted in slightly increased risks for both end points.78
The efficacy of HT in the prevention of osteoporotic fractures 
remains undisputed with regard to both hip and o

remains undisputed with regard to both hip and other fractures79 
(Table 28.1).
The findings of the WHI trial are so important and have been 
so broadly publicized that they have created the perception that 
HT, in general, always carries risks that exceed its benefits. Inter­
estingly, recently, data have been published presenting the analysis 
of two combined studies of the WHI with extended duration of 
follow-up of 18 years. Compared with placebo, no differences were 
found in all-cause mo

lacebo, no differences were 
found in all-cause mortality (cardiovascular and cancer related) in 
the CEE plus medroxyprogesterone acetate treated group (treated 
for a mean of 5.6 years) nor in the CEE trial only (treated for a 
mean of 7.2 years). Mortality risk is similar compared with the 
placebo group despite an increased risk of breast cancer in the 
CEE plus medroxyprogesterone acetate treated group.80 However, 
it remains unclear whether longer duration of treatment would be 
of benefit

 longer duration of treatment would be 
of benefit and outweigh the risks.
Given the noted uncertainties, HT can be considered in 
the perimenopausal period in those women suffering from 
menopausal symptoms.81 For menopausal women older than 
60 years or more than 10 years past menopause with bother­
some vasomotor symptoms who do not have contraindications 
or excess cardiovascular or breast cancer risk, it is suggested to 
initiate estrogen therapy for those without a uterus and estro­
gen pl

erapy for those without a uterus and estro­
gen plus progesterone for those with a uterus. An association 
between endometrial cancer and estrogen use was observed 
many years ago. Ten years of unopposed estrogen use increases 
the risk for endometrial cancer 10-fold.60 For this reason, the 
HT regimens were supplemented with progestogens, which 
almost completely prevented this excess risk for endometrial 
cancer.
Presently advised doses of estrogen were originally designed 
to prevent bone los

ogen were originally designed 
to prevent bone loss, and progestogen regimens were opposed 
to prevent endometrial cancer. Several estrogen and progestogen 
preparations are available for HT.59 Components of available 
preparations vary in their effects on different target tissues. Com­
mercial preparations differ in their clinical effect by design, and 
individual women differ in their responses. HT can be adminis­
tered orally, transdermally, topically, intranasally, or as subcutane­
ous impla

opically, intranasally, or as subcutane­
ous implants.
Although HT in the perimenopausal state can cause some 
symptoms (e.g., vaginal discharge, uterine bleeding, and breast 
tenderness), it relieves many other symptoms, including hot 
flushes and the severity of night sweats.82 Grady has estimated 
that about one serious adverse event will occur among every thou­
sand 50-year-old women using HT for 1 year. The HT regimen 
used in the WHI trial combined 0.625 mg/day of CEE and 2.5 
mg/day of me

combined 0.625 mg/day of CEE and 2.5 
mg/day of medroxyprogesterone acetate. The dose of estrogen 
necessary to diminish perimenopausal symptoms, however, can 
be lower in many women. A dose of 0.3 or 0.45 mg/day of CEE 
is effective at diminishing the number and the intensity of hot 
flushes as well.83 
In the search for optimal hormone replacement therapy dur­
ing menopause, it was observed that tamoxifen has variable 
antiestrogenic and estrogenic action

has variable 
antiestrogenic and estrogenic actions in different tissues.84,85 
Tamoxifen suppresses the growth of estrogen receptor–posi­
tive breast cancer cells. Long-term treatment of menopausal 
patients with breast cancer with tamoxifen also lowered the 
incidence of new (contralateral) breast cancer by 40%. In addi­
tion, the number of cardiovascular incidents decreased by 70%, 
and the age-related decrease in bone mineral density was par­
tially prevented.86
  Absolute Risks and Benefit

  Absolute Risks and Benefits of Clinical Events With Estrogen-Progestin and Estrogen-Only Therapy 
Compared With Placebo in the Women’s Health Initiative Trial.a
ESTROGEN-PROGESTIN THERAPY
ESTROGEN THERAPY
Absolute Risk (per 10,000 
women/yr)
Absolute Benefit (per 
10,000 women/yr)
Absolute Risk (per 
Health Event
8
—
—
3
Stroke
8
—
11
—
8
—
—
8
18
—
8
—
Col

cancer
8
—
—
8
18
—
8
—
—
7
1
—
Hip fracture
—
5
—
6
Any fracture
—
47
—
56
New-onset diabetes
—
15
—
14
aFor overall hazard ratio, 95% confidence interval, and adjustments, see the original article.
Modified from Hodis HN. Assessing benefits and risks of hormone therapy in 2008: new evidence, especially with regard to the heart. Cleve Clin J Med. 2008;75 Suppl 4:S3–S12.
   
TABLE 28.1
1186
These initially puz

These initially puzzling observations were explained by the 
fact that tamoxifen and other compounds such as raloxifene have 
selective estrogen receptor–modulating effects, exerting antiestro­
genic actions on normal and cancerous breast tissue but agonistic 
actions on bone, lipids, and blood vessel walls.87 These effects of 
tamoxifen and raloxifene may be explained by differential sta­
bilization of the conformation of the estrogen receptor, which 
facilitate i

tion of the estrogen receptor, which 
facilitate interactions with coactivator or corepressor proteins, 
and subsequently initiate or suppress transcription of target genes. 
These specific interactions in the target cell lead to tissue selec­
tive actions.88 Several other selective estrogen receptor modulators 
(SERMs) have been evaluated for the treatment of breast cancer, 
osteoporosis, and menopausal symptoms.89
The efficacy and safety of raloxifene for the prevention of osteo­
porosis in po

oxifene for the prevention of osteo­
porosis in postmenopausal women were demonstrated in a study 
that found a 2.5% increase in bone mineral density in the lum­
bar spine and hip in a group of postmenopausal nonosteoporotic 
women treated with raloxifene for 2 years.90 A significant reduc­
tion of vertebral fracture risk by raloxifene was subsequently dem­
onstrated.91 A meta-analysis confirmed the effects of raloxifene 
treatment on reduction of vertebral fracture risk in postmeno­
pausal wome

f vertebral fracture risk in postmeno­
pausal women without an effect on nonvertebral fracture risk.92
In a placebo-controlled clinical trial of 10,000 postmenopausal 
women with an increased risk of coronary heart disease followed 
for 5 or 6 years, raloxifene had an overall neutral effect on the inci­
dence of cardiovascular events.93 The incidence of all strokes did 
not differ between the raloxifene participants and placebo-treated 
participants. However, in the raloxifene-treated individual

nts. However, in the raloxifene-treated individuals, there 
was a higher incidence of fatal stroke, especially in smokers, as 
well as venous thromboembolic events (incidence rates per 100 
women-years 0.22 vs. 0.15 [p < 0.05] and 0.39 vs. 0.27 [p = 0.02], 
respectively).94 Raloxifene, in contrast to tamoxifen and estrogen, 
does not stimulate endometrial thickness or vaginal bleeding.90 
With regard to side effects, raloxifene causes an increased incidence 
of leg cramps and hot flushes.95
Endo

d incidence 
Endocrine approaches to breast cancer prevention have been 
increasingly successful in recent years. Four trials of tamoxifen 
administration for 5 years or longer in women at increased risk 
of breast cancer showed an about 50% reduction in breast cancer, 
but only for estrogen receptor–positive disease (Fig. 28.4220) (refer 
to Howell96 for further discussion). Follow-up indicates that there 
was a carryover effect of tamoxifen after the completion

carryover effect of tamoxifen after the completion of treat­
ment at 5 years, so the preventive effect at 10 years is significantly 
greater than at 5 years.
Raloxifene has been compared with placebo in three trials: one 
in women with osteoporosis,91,97 one in women with or at risk of 
cardiac disease,93 and one in which raloxifene was compared with 
tamoxifen in women at high risk of breast cancer98: a 66%, a 44%, 
and a 50% risk reduction of breast cancer was noted after 4 to 5 
years of ralo

reast cancer was noted after 4 to 5 
years of raloxifene therapy, respectively. The last trial showed that 
raloxifene was as effective as tamoxifen.98 As with the previous 
tamoxifen studies, raloxifene was only reducing the risk of estro­
gen receptor α–positive tumors.
In the United States, a 60-mg dose of raloxifene is now indi­
cated for the treatment and prevention of osteoporosis in post­
menopausal women, for reduction in risk of invasive breast 
cancer in postmenopausal women with osteo

 breast 
cancer in postmenopausal women with osteoporosis, and in post­
menopausal women at high risk for invasive breast cancer.99,100 
Although approved for use in the European Union for the pre­
vention of vertebral fractures, lasofoxifene was never marketed. In 
addition, bazedoxifene is approved for use in the European Union. 
Four RCTs show that bazedoxifene decreases the incidence of new 
vertebral fractures (relative risk, 0.69: 95% CI, 0.52–0.93). Baze­
dozifene treatment was not associ

.52–0.93). Baze­
dozifene treatment was not associated with an increased risk of 
breast cancer or with increases in endometrial thickness or carci­
noma.101-103 Tibolone is a compound that also regulates estrogen 
activity selectively at different tissues. It has a high efficacy in the 
treatment of climacteric symptoms and the prevention of bone 
loss. The compound is approved in many countries but not in the 
United States. In a 5-year trial, tibolone (2.5 mg/day) was associ­
ated with a 45% 

tibolone (2.5 mg/day) was associ­
ated with a 45% decrease in vertebral fractures, a 26% decrease in 
nonvertebral fractures, a 68% decrease in invasive breast cancer, 
and a 69% decreased risk of colon cancer.104 However, the tibo­
lone-treated group had an increased risk of stroke (relative hazard, 
2.19 [p = 0.02]).
Androgen Replacement
In premenopausal women, androgen production originates 
equally from the adrenal glands and the ovaries. Androgen pro­
duction in women declines with aging. A

rogen pro­
duction in women declines with aging. After menopause, circulat­
ing androgen levels decrease by more than 50%.
At present, there is increasing awareness of the impact of 
low androgen levels on emotional and sexual well-being in peri­
menopausal women. No single androgen level is predictive of 
low female sexual function, and the majority of 1423 women 
age 18 to 75 years with low DHEAS levels had normal sexual 
function.105
The efficacy and safety of testosterone treatment for hypoa

acy and safety of testosterone treatment for hypoactive 
sexual desire disorder in postmenopausal women was studied in 
a double-blind, placebo-controlled, 52-week trial in which more 
than 800 women participated.106 Treatment with a patch deliver­
ing 300 μg of testosterone per day resulted in a modest but mean­
ingful improvement in sexual function. Similar results were found 
in a smaller study among 272 women treated with a testoster­
one patch for 6 months.107 The long-term effects of testo

ch for 6 months.107 The long-term effects of testosterone, 
including effect on the breast, remain uncertain. 
HT, SERMs, or No Treatment?
The issue of hormonal therapy in postmenopausal women is 
controversial, and many aspects remain unresolved. The idea 
that HT is a global risk reduction strategy has been abandoned. 
1.5
1.0
0.5
0.0
0
1
2
3
4
Placebo (n = 2576)
Raloxifene (n = 5129)
Years since randomization
% of randomized patients
• Fig. 28.4  Effect of raloxifene administration (60–120 mg

.4  Effect of raloxifene administration (60–120 mg/day) on the 
cumulative incidence of breast cancer in 7705 postmenopausal women 
(mean age, 66.5 years) with osteoporosis. Statistical significance of the dif­
ference between the groups was p < 0.001. (Modified from Cummings SR, 
Eckert S, Krueger KA, et al. The effect of raloxifene on risk of breast cancer 
in postmenopausal women: results from the MORE randomized trial. Multi­
ple Outcomes of Raloxifene Evaluation. JAMA. 1999;281[23]:2189–219

Raloxifene Evaluation. JAMA. 1999;281[23]:2189–2197.)
1187
Although the general benefits of HT in the short term during and 
after the menopausal transition are evident in women suffering 
from estrogen withdrawal symptoms, the balance of the effects 
of long-term HT after menopause points in general to a nega­
tive outcome with more harm than benefits. The evolution of the 
association of HT and cardiovascular risk from protection to harm 
and now to possible

 risk from protection to harm 
and now to possible protection again has resulted in controversy 
and confusion.108,109
HT is usually prescribed for women 45 to 60 years of 
age who are experiencing vasomotor symptoms. HT is not 
indicated for cardioprotection for women in their 70s or 
for women who do not suffer from vasomotor symptoms or 
urogenital atrophy. Data clearly demonstrate, however, that 
clinicians can prescribe and women can use (low-dose) HT 
confidently during the time when thera

w-dose) HT 
confidently during the time when therapy is most needed. The 
debate regarding the potential benefits of HT on cardiovas­
cular risk and/or mortality for women who start therapy in 
proximity to the menopausal transition is not resolved. The 
benefit and risk of any therapy, including HT, should be reas­
sessed periodically in each individual based on future scientific 
evidence.109
Currently, a vast armamentarium of other pharmacologic 
treatments to reduce cardiovascular and bone r

ic 
treatments to reduce cardiovascular and bone risks is available; 
these include cholesterol-lowering statins, β-blockers, SERMs, 
and bisphosphonates. An optimal choice of these different life­
style drugs for menopausal women requires individualization of 
the treatment decision. Coronary artery disease, for example, is 
a complex disorder resulting from an interaction of genetic pre­
disposition and environmental factors. Risk factor modification 
(diet, smoking, physical activity) should 

cation 
(diet, smoking, physical activity) should be advised. Secondary 
prevention of coronary artery disease and atherosclerosis includes 
lipid-lowering drugs, aspirin, nitrates, and β-blockers.110 For 
women with existing osteoporosis, HT is effective. However, 
SERMs and especially bisphosphonates come close or are better in 
their fracture-reducing effects. Recognition of an increased risk for 
breast cancer in menopausal women is an important consideration 
in the choice for SERMs. Chemop

ant consideration 
in the choice for SERMs. Chemoprevention of breast cancer with 
raloxifene has become a major consideration in the pharmacologic 
choice for risk reduction in the long-term preventive treatment of 
postmenopausal women. 
Andropause Versus Late-Onset Hypogonadism
Role of Testosterone During Aging
Age-associated hypogonadism does not develop as clearly in men 
at andropause as in women at menopause. The key difference is 
the gradual, often subtle change in androgen levels in me

dual, often subtle change in androgen levels in men versus 
the precipitate fall of estrogen production in women. It is gen­
erally agreed that as men age, there is a decline in serum total 
testosterone concentration that begins after the age of 40 years. 
In cross-sectional studies, the annual decline in total and free tes­
tosterone is 1.0% and 1.2%, respectively. The higher decline in 
free testosterone levels is related to the increase in sex hormone–
binding globulin (SHBG) levels with age

x hormone–
binding globulin (SHBG) levels with age.45,111 Part of these 
changes attributed to aging may be confounded by health-related 
factors such as obesity. It was also reported that serum testoster­
one was stable across age strata among men self-reporting very 
good health.112 It remains unclear whether the well-known bio­
logic changes occurring during aging in men (e.g., reduced sexual 
activity, muscle mass and strength, and skeletal mineralization) are 
causally related to these chan

ineralization) are 
causally related to these changes in testosterone bioactivity. The 
gradual decline in testosterone levels is called late-onset hypogonad­
ism (LOH). In recent years, there has been major disagreement on 
how to define androgen deficiency in the elderly man. According 
to practice guidelines,113-115 the diagnosis of androgen deficiency 
in the elderly or LOH, for which testosterone treatment might be 
considered, should be based on the concurrent presence of con­
sistent symp

ed on the concurrent presence of con­
sistent symptoms of hypogonadism and unequivocally low serum 
(free) testosterone.
The biochemical part consists of measuring morning serum 
total testosterone levels (two times) or determining a “free” tes­
tosterone concentration by additionally measuring SHBG lev­
els. Most elderly men have testosterone levels within the normal 
range, with prevalence estimates of “low” (e.g., <10.4 nmol/L = 
<300 ng/dL) serum testosterone concentrations generally between

erum testosterone concentrations generally between 
10% and 25%.116,117 Most of these men with low testosterone lev­
els will not come to clinical attention because testosterone levels 
are not routinely measured in clinical practice.118 An important 
problem with a biochemical approach of the diagnosis of LOH 
in elderly men is that men with low testosterone may not exhibit 
clinically significant symptomatology, raising the possibility that 
large numbers of men, simply by virtue of falling be

rge numbers of men, simply by virtue of falling below an arbi­
trary threshold, are diagnosed as needing testosterone replacement 
therapy.
In addition, the clinical part to the diagnosis of androgen defi­
ciency of the aging male has important drawbacks.119 All symp­
toms and signs of androgen deficiency are nonspecific and readily 
accounted for or modified by comorbidities and medications. Leth­
argy, reduced concentration, sleep disturbance, irritability, and 
depressed mood may relate to ph

irritability, and 
depressed mood may relate to physical illness (and side effects of 
treatment), obesity, and/or lack of physical exercise and other life­
style issues (e.g., alcohol or drug use), relationship difficulties, and 
occupational or financial stresses. Indeed, existing screening tools 
for androgen deficiency lack adequate specificity and sensitivity to 
be reliably employed in directing clinical diagnosis and treatment.
Araujo and colleagues118 defined the prevalence of symp­
toma

colleagues118 defined the prevalence of symp­
tomatic androgen deficiency in men by studying the association 
between symptoms of androgen deficiency (low libido, erec­
tile dysfunction, osteoporosis or fracture, or two of the follow­
ing symptoms: sleep disturbance, depressed mood, lethargy, or 
diminished physical performance) and low serum total (<10.4 
nmol/L = <300 ng/dL) and free (<0.17 nmol/L = <5 ng/dL) tes­
tosterone concentrations. In nearly 1500 men (age 30–79 years), 
they found 24% 

early 1500 men (age 30–79 years), 
they found 24% with total testosterone less than 10.4 nmol/L 
and 11% with free testosterone levels less than 0.17 nnol/L. The 
prevalence of symptoms was as follows: low libido (12%), erec­
tile dysfunction (16%), osteoporosis/fracture (1%), and two or 
more of the nonspecific symptoms (20%). Although low testos­
terone levels were associated with symptoms, many men with low 
testosterone levels were asymptomatic (e.g., in men older than 
50 years: 47.6%). In 

ic (e.g., in men older than 
50 years: 47.6%). In Fig. 28.5, the interrelationships are shown: 
symptomatic androgen deficiency with low serum testosterone 
levels (<10.4 nmol/L) was observed in 4.2% of men younger than 
50 years and in 8.4% of men older than 50 years. This prevalence 
rapidly increases with age, amounting to 18.4% among 70-year-
old men.
In addition, Wu and colleagues120 tried to better identify 
elderly men with LOH. They surveyed a random population 
sample of 3369 men betwee

yed a random population 
sample of 3369 men between the ages of 40 and 79 years (the 
European Male Aging Study). They proposed minimal diagnostic 
criteria, consisting of the “syndromic” simultaneous presence of 
three sexual symptoms (i.e., poor morning erections, decreased 
sexual interest, and erectile dysfunction), together with a serum 
1188
total testosterone levels less than 11 nmol/L (320 ng/dL) and free 
testosterone levels less than 0.22 n

dL) and free 
testosterone levels less than 0.22 nmol/L (6.4 ng/dL); men fulfill­
ing these criteria with serum total testosterone levels less than 8 
nmol/L (230 ng/dL) are considered to have severe LOH. Preva­
lence of such defined LOH in the European Male Aging Study 
population increased with age, body mass index, and number of 
coexisting illnesses and was 5.1% in men 70 to 79 years of age.120 
Men with LOH had lower hemoglobin and muscle and bone mass 
and poorer physical performance and g

d bone mass 
and poorer physical performance and general health compared 
with their peers; men with low testosterone only, irrespective of 
sexual symptoms, showed lesser magnitude of associations with 
the same end points.121
In recent years, it has become clear that an association exists 
between LOH and adverse metabolic conditions, such as obe­
sity, metabolic syndrome, and type 2 diabetes mellitus.122-124 An 
obesity-induced estrogen increase seems to play a major role in 
determining the 

se seems to play a major role in 
determining the negative feedback at the pituitary level, therefore 
inducing hypogonadotropic hypogonadism with low, or inappro­
priately normal, gonadotropin levels. In addition, insulin resistance 
may contribute to the low testosterone levels seen in obese men.122 
Comorbidity was also associated with a lower total testosterone 
level. Three independent meta-analyses published in 2011, as well 
as a recent large observational study of nearly 2600 men partici

rge observational study of nearly 2600 men participat­
ing in the European Male Aging Study, demonstrated an association 
between hypogonadism and overall and cardiovascular mortality, 
but they failed to find any statistical association with incident car­
diovascular events.124-127 These findings support the concept that 
cardiovascular disease is associated with male hypogonadism. 
Many persuasive reports in the literature demonstrate that tes­
tosterone replac

 literature demonstrate that tes­
tosterone replacement in men of all ages (young, adult, and 
old) with clear clinical and severe biochemical hypogonad­
ism instantly reverses vasomotor activity (flushes and sweats); 
improves libido, sexual activity, and mood; increases muscle 
mass, strength, and bone mineralization; prevents fractures; 
decreases fat mass; and decreases fatigue and poor concentra­
tion.54,111,128 In addition, the treatment of normal adult men 
with supraphysiologic doses of 

 normal adult men 
with supraphysiologic doses of testosterone, especially when 
combined with resistance exercise training, increased fat-free 
mass and muscle size and strength.129
Most studies reporting the results of androgen therapy in 
older men were small, short term, noncontrolled, and without 
uniform end points. The results of one of the first large random­
ized studies in healthy elderly men seem representative for effects 
expected of androgen therapy.130,131 Ninety-six men (mean age

 androgen therapy.130,131 Ninety-six men (mean age 
73 years) wore a testosterone patch on their scrotum (6 mg of 
testosterone per 24 hours) or a placebo patch for 36 months. 
Mean serum testosterone concentrations in the men treated with 
testosterone increased from 12.7 ± 2.9 nmol/L (367 ± 7.9 ng/
dL) before treatment to 21.7 ± 8.6 nmol/L (625 ± 249 ng/dL; p 
< 0.001) at 6 months of treatment and remained at that level for 
the duration of the study. The decrease in fat mass (–3.0 ± 0.5 kg) 

 study. The decrease in fat mass (–3.0 ± 0.5 kg) 
in the testosterone-treated men during the 36 months of treat­
ment was significantly different from the decrease (–0.7 ± 0.5 kg) 
in the placebo-treated men (p < 0.001) (Fig. 28.6A). The increase 
in lean mass (1.9 ± 0.3 kg) in the testosterone-treated men was 
significantly different from that in the placebo-treated men (0.2 
± 0.2 kg; p < 0.001).
Changes in knee extension and flexion strength, hand grip, 
walking speed, and other parameters of

hand grip, 
walking speed, and other parameters of muscle strength and 
function were not significantly different in the two groups. 
Bone mineral density in the lumbar spine increased in both the 
testosterone-treated (4.2% ± 0.8%) and placebo-treated (2.5% 
± 0.6%) groups, but mean changes did not differ between 
groups (see Fig. 28.6C). However, the lower the pretreat­
ment serum testosterone concentration, the greater the effects 
of testosterone treatment on lumbar spine bone density after 

rone treatment on lumbar spine bone density after 
36 months (p = 0.02). A minimal effect (0.9 ± 1.0%) of tes­
tosterone treatment on bone mineral density was observed in 
men with a pretreatment serum testosterone concentration of 
13.9 nmol/L (400 ng/dL), but an increase of 5.9% ± 2.2% was 
found in men with a pretreatment testosterone concentration of 
6.9 nmol/L (200 ng/dL).
The subjective perception of physical functioning decreased 
significantly during the 36 months of treatment in the pl

cantly during the 36 months of treatment in the placebo-
treated group (p < 0.001) but not in the testosterone-treated 
group. Interestingly, the effect of testosterone treatment on the 
perception of physical functioning varied inversely with the pre­
treatment serum testosterone concentration (p < 0.01). There 
was no significant difference between the two treatment groups 
with regard to the subjective perception of energy or sexual 
functions.
To determine definitively whether testosterone t

.
To determine definitively whether testosterone treatment 
of elderly men with low testosterone is efficacious in improv­
ing symptoms and objective measures of age-associated condi­
tions, a coordinated set of seven clinical trials were designed. The 
seven trials included 790 men with measurements of physical 
function, sexual function, vitality, cognitive function, anemia, 
Symptoms
present
18.8
Age <50 y, n = 869
Low free T
5.4
4.2
0.1
2.1
10.6
1.5
1.9
8.4
6.6
6.1
7.0
Low total T
Low free T

0.6
1.5
1.9
8.4
6.6
6.1
7.0
Low total T
Low free T
Low total T
Age 50+ y, n = 606
Symptoms
present
30.8
58.8
37.7
• Fig. 28.5  Venn diagrams showing the interrelationships among symp­
toms, low total testosterone (<10.4 nmol/L [<300 ng/dL]), and low free 
testosterone (<0.17 nmol/L [<5 ng/dL]) among men younger than 50 years 
(left) and those 50 years of age or older (right). Numbers displayed are 
percentages within each area. Positive symptom reports and low total 
and free testosterone were m

eports and low total 
and free testosterone were more common in older men. The presence 
of symptoms was related more strongly to testosterone levels in older as 
compared with younger men as indicated by a greater degree of overlap 
between symptom presence and low total and free testosterone among 
older (52.4% of men with low total or free testosterone had symptoms) 
compared with younger (43.1% of men with low total or free testosterone 
had symptoms) men. The intersection of symptoms and lo

symptoms) men. The intersection of symptoms and low total and free 
testosterone levels was more common in older men (prevalence of symp­
tomatic androgen deficiency was 4.2% among men <50 years of age and 
8.4% among men 50 years of age or older). Note: Circles for the Venn 
diagrams are proportional within age strata. T, testosterone. (From Araujo 
AB, Esche GR, Kupelian V, et al. Prevalence of symptomatic androgen 
deficiency in men. J Clin Endocrinol Metab. 2007;92[11]:4241–4247.)
1189
Chapt

docrinol Metab. 2007;92[11]:4241–4247.)
1189
cardiovascular, and bone parameters.132 These trials show that in 
treatment of men younger than 65 years for 12 months with tes­
tosterone levels below 9.5 nmol/L (275 ng/dL) with a testosterone 
gel in an adjusted dose to maintain testosterone levels in the nor­
mal range of young men, self-reported sexual function increased 
more with testosterone treatment than with placebo. There was a 
small difference between

ith placebo. There was a 
small difference between the testosterone group and the placebo 
group in the percentage of men whose 6-minute walking distance 
increased by at least 50 m131 (Fig. 28.7). Further, testosterone 
treatment showed no significant benefit over placebo with respect 
to vitality, as determined by an increase of at least 4 points in the 
FACIT-Fatigue score131 (Fig. 28.8).
With regard to the potential adverse effects of testosterone 
treatment in healthy elderly men, the studi

erone 
treatment in healthy elderly men, the studies by Snyder and col­
leagues131,133 seem representative. The mean serum prostate-spe­
cific antigen concentration did not change during treatment in 
the placebo-treated group but increased by a small but statistically 
significant (p < 0.001) amount in the testosterone-treated group. 
Three cases of prostate cancer were observed in the testosterone-
treated group versus one in the placebo group. The urine flow rate 
and volume of urine in the b

 The urine flow rate 
and volume of urine in the bladder after voiding were similar in 
the two groups. Hemoglobin and hematocrit did not change in 
the placebo-treated group, but both increased significantly (p < 
0.001) in the testosterone-treated group. Seven men treated with 
testosterone developed persistent erythrocytosis (hemoglobin 
>17.5 g/dL; hematocrit >52%) during treatment.
Numerous studies of large populations of healthy men have 
shown a marked rise in the incidence of impotence t

hown a marked rise in the incidence of impotence to more than 50% 
in men 60 to 70 years old.134 Although this increased rate occurs in 
the same age group who show a clear decline in serum (free) testos­
terone levels, no causal relationships have been demonstrated. A sys­
tematic review and meta-analysis of randomized, placebo-controlled 
trials concluded that testosterone use in normal men is associated 
with a small improvement in satisfaction with erectile function and 
moderate improvement

n with erectile function and 
moderate improvements in libido.135 Other factors, such as athero­
sclerosis, alcohol consumption, smoking, and the quality of personal 
relationships, seem to be more important.136,137 Only in the case of 
clear hypogonadism is the decrease in libido and potency restored by 
testosterone replacement therapy.114,128,133
No RCTs have been large enough or long enough to determine 
the effects of testosterone replacement therapy on major adverse 
cardiovascular events.

t therapy on major adverse 
cardiovascular events. A meta-analysis suggested that testoster­
one therapy increases cardiovascular-related events among men. 
The risk of testosterone therapy was particularly marked in tri­
als not funded by the pharmaceutical industry.138 The Testoster­
one Trials,132 which were published after this meta-analysis, have 
investigated the effects of testosterone therapy in older men on 
cardiovascular-related parameters. Treatment with testosterone 
gel for 1 year 

ters. Treatment with testosterone 
gel for 1 year in 394 men with symptomatic hypogonadism and 
testosterone levels less than 275 ng/dL was associated with small 
reductions in cholesterol and insulin but not with glucose markers 
or markers of inflammation or fibrinolysis or with troponin com­
pared with placebo.139 However, testosterone treatment in a sub­
set of 73 men was associated with a significantly greater increase 
in coronary artery noncalcified plaque volume compared with pla­
cebo.1

ncalcified plaque volume compared with pla­
cebo.140 In addition, in the Testosterone Effects on Atherosclerosis 
in Aging Men Trial, a placebo-controlled, randomized, double-
blind trial among 308 community-dwelling men older than 60 
years, insulin sensitivity did not improve in a subset of 134 non­
diabetic men treated with testosterone for 36 months.141
The Endocrine Society Clinical Practice Guideline on testos­
terone therapy in adult men with androgen deficiency syndromes 
summarized the 

ith androgen deficiency syndromes 
summarized the observed effects and the adverse outcomes of ran­
domized, placebo-controlled trials of testosterone administration 
over 1 to 3 years in older men with low-normal to low testosterone 
concentrations.115
105
104
103
102
101
99
100
2.5
2.0
1.5
1.0
0.5
–0.5
0.0
1.0
0.0
–1.0
–2.0
–3.0
–4.0
Bone mineral density,
L2–L4 (% basal)
Change in lean mass (kg)
Change in fat mass (kg)
A
B
C
Placebo
0
Placebo
0
6
12
18
24

e
0
Placebo
0
6
12
18
24
30
36
0
12
24
36
0
12
24
36
Placebo
• Fig. 28.6  A to C, Mean (± standard error) change from baseline in fat 
mass, lean mass, and bone mineral density of the lumbar spine (L2–L4) as 
determined by dual-energy x-ray absorptiometry in 108 men older than 65 
years who were treated with either testosterone or placebo (54 men each). 
The decrease in fat mass (p < 0.005) and the increase in lean mass (p < 
0.

(p < 0.005) and the increase in lean mass (p < 
0.01) in the testosterone-treated subjects were significantly different from 
those in placebo-treated subjects at 36 months. Bone mineral density 
increased significantly in both groups. (A and B from Snyder PJ, Peachey 
H, Hannoush P, et al. Effect of testosterone treatment on body composi­
tion and muscle strength in men over 65 years of age. J Clin Endocrinol 
Metab. 1999;84[8]:2647-2653; C from Snyder PJ, Peachey H, Hannoush 
P, et al. Effect 

 Snyder PJ, Peachey H, Hannoush 
P, et al. Effect of testosterone treatment on bone mineral density in men 
over 65 years of age. J Clin Endocrinol Metab. 1999;84[6]:1966–1972.)
1190
Which Elderly Men Should Be Treated?
A key lesson from the Heart and Estrogen/Progestin Replace­
ment and WHI studies is that conventional medical practice 
should not precede substantiation with reliable clinical evidence 
of safety and efficacy.142 Androgen replacement

e 
of safety and efficacy.142 Androgen replacement in older men is 
the male counterpart of hormonal therapy in postmenopausal 
women but differs crucially in that a clear syndrome of andro­
gen deficiency is lacking. On the basis of several suggestive clini­
cal features collected from the history, symptoms, or signs of an 
elderly man, the biochemical confirmation of androgen deficiency 
30
Walking distance-
men enrolled in the physical function trial 
Walking distance- all men
p = 0.20
p = 0.

n trial 
p = 0.20
p = 0.003
% of men who walked ≥50 m
further than at baseline
20
10
0
Testosterone (n)
Placebo (n) 196
191
179
179
171
174
159
172
165
172
0
3
6
Month
9
12
30
20
10
0
0
3
6
Month
9
12
Placebo (n)
389
392
356
368
339
358
320
348
326
346
A
B
• Fig. 28.7  Effect of testosterone on walking distance. Graphs showing percentage of (A) men taking 
testosterone or placebo and enrolled in the Phy

g 
testosterone or placebo and enrolled in the Physical Function Trial and (B) all men enrolled in testosterone 
trials whose distance walked in 6 minutes increased by 50 or more meters greater than baseline. Data 
presented as means and 95% confidence intervals. (Redrawn from Snyder PJ, Bhasin S, Cunningham GR, 
et al. Lessons from the Testosterone Trials. Endocr Rev. 2018;39[3]:369–386.)
Vitality-fatigue
men enrolled in the vitality trial
p = 0.30
% of men who scored ≥4 higher than
at baseline

.30
at baseline on the FACIT-fatigue scale
Placebo (n) 238
236
207
219
196
217
188
206
191
203
Month
100
80
60
40
20
0
0
3
6
9
12
Vitality-fatigue all men
p = 0.22
Placebo (n) 394
394
337
351
329
350
317
337
316
333
Month
100
80
60
40
20
0
0
3
6
9
12
A
B
• Fig. 28.8  Effect of testosterone on vitality and fatigue. Graphs showing percentage of (A) men taki

fatigue. Graphs showing percentage of (A) men taking 
testosterone or placebo and enrolled in the Vitality Trial and (B) all men enrolled in testosterone trials whose 
score on the FACIT-Fatigue scale increased by 4 or more points greater than baseline. Data presented as 
means and 95% confidence intervals. (Redrawn from Snyder PJ, Bhasin S, Cunningham GR, et al. Les­
sons from the Testosterone Trials. Endocr Rev. 2018;39[3]:369–386.)
1191
is sought. In previo

r 28  Endocrinology and Aging
is sought. In previous discussions of testosterone replacement in 
older men,136,143 it was suggested that the biochemical diagnosis 
of “true” hypogonadism seems certain if the serum total testos­
terone concentration is less than 6.9 nmol/L (200 ng/dL). This 
cutoff remains arbitrary and does not answer the question whether 
healthy elderly men with testosterone levels between 6.9 and 10.4 
nmol/L are hypogonadal, or whether such men would benefit 
from replacemen

or whether such men would benefit 
from replacement therapy with testosterone. There are not enough 
data to determine whether low testosterone levels in the elderly 
are just a marker of poor health and should be treated with tes­
tosterone replacement. It has been demonstrated that intercurrent 
diseases frequently result in a transient, sharp drop in serum tes­
tosterone concentrations,144 whereas frail, elderly men generally 
tend to have testosterone levels 10% to 15% lower than those of 
h

tosterone levels 10% to 15% lower than those of 
healthy, age-matched control subjects.145 In addition, functional 
hypogonadism in the elderly is potentially reversible.146 Lifestyle 
modifications, particularly weight loss, as obesity is the strongest 
risk factor for low testosterone levels in older adults, can lead to 
higher testosterone levels.147
The current recommendation115 is not to treat asymptomatic 
older men with age-related decline in testosterone levels. The 
guideline suggests t

 in testosterone levels. The 
guideline suggests that clinicians consider offering testosterone 
therapy on an individualized basis to older men with low tes­
tosterone levels measured on more than one occasion and clini­
cally significant symptoms of androgen deficiency, after explicit 
discussion of uncertainty about the risks and benefits of testos­
terone therapy. Among experts, there is disagreement on serum 
levels below which testosterone therapy should be offered to older 
men with sympt

therapy should be offered to older 
men with symptoms. When a serum testosterone concentration 
is found to be low, an additional evaluation with measurements 
of serum gonadotropins and prolactin is mandatory to exclude 
pituitary pathology.
If one decides to start testosterone replacement, the guideline 
suggests that clinicians aim at achieving total testosterone levels 
in the lower part of the normal range of young men (400–500 
ng/dL (14.0–17.5 nmol/L). The dose should thus be titrated 
ac

17.5 nmol/L). The dose should thus be titrated 
according to serum levels. Considerations concerning the choice 
of testosterone preparation, as well as the route of administration 
(oral, injectable, implantable, or transdermal), are discussed in 
Chapter 8.
At present, the duration of testosterone administration is 
uncertain. Control of prostate size, prostate-specific antigen lev­
els, and hematocrit is mandatory. The identification of elderly 
men who might benefit most from testosterone tr

y 
men who might benefit most from testosterone treatment remains 
uncertain, and the risks to the prostate and increased blood viscos­
ity require further study. 
Adrenopause
Role of DHEA During Aging
Humans are unique among primates and rodents because the 
human adrenal cortex secretes large amounts of the steroid 
precursor DHEA and its sulfate derivative DHEAS.148 Serum 
DHEAS concentrations in adult men and women are 100 to 500 
times higher than those of testosterone and 1000 to 10,000 ti

r than those of testosterone and 1000 to 10,000 times 
higher than those of estradiol. In normal subjects, serum concen­
trations of DHEA and its sulfate are highest in the third decade 
of life, after which the concentrations of both gradually decrease; 
therefore, by the age of 70 to 80 years, the values are about 20% 
of peak values in men and 30% of peak values in women49 (see 
Fig. 28.3).
DHEA and DHEAS seem to be inactive precursors that are 
transformed within human tissues by a complicat

e 
transformed within human tissues by a complicated network of 
enzymes into androgens or estrogens, or both (Fig. 28.9). The 
key enzymes are aromatase, steroid sulfatase, 3β-hydroxysteroid 
dehydrogenases (3β-HSD-1 and 3β-HSD-2), and at least seven 
organ-specific 17β-hydroxysteroid dehydrogenases (17β-HSD-1 
to 3β-HSD-7). Labrie and colleagues148 introduced the term 
intracrinology to describe this synthesis of active steroids in periph­
eral target tissues in which the action is exerted in 

 target tissues in which the action is exerted in the same cells in 
which synthesis takes place, without release into the extracellular 
space and general circulation.
In postmenopausal women, nearly 100% of sex steroids are 
synthesized in peripheral tissues from precursors of adrenal origin, 
except for a small contribution from ovarian or adrenal testos­
terone and androstenedione. Thus, in postmenopausal women, 
virtually all active sex steroids are made in target tissues by an 
intracrine 

oids are made in target tissues by an 
intracrine mechanism. In elderly men, the intracrine production 
of androgens is also important; less than 50% of the androgen 
supply is derived from testicular production.
The high secretion rate of adrenal precursor sex steroids in 
men and women differs from that in laboratory animal models, 
in which the secretion of sex steroids occurs exclusively in the 
gonads. In rats and mice, long-term administration of DHEA 
prevented obesity, diabetes mellitus,

ion of DHEA 
prevented obesity, diabetes mellitus, cancer, and heart disease, and 
enhanced immune function as well.48,50,149
These experimental animal data have been used to argue 
that DHEA administration in adult or elderly individu­
als prolongs the life span and might be an “elixir of youth.” 
Supportive data in humans are few, however, and highly 
Steroid
sulfatase
Steroid
sulfatase
Aromatase
Aromatase
Steroid
sulfatase
DHEAS
DHEA
4-DIONE
5-DIOL
Testo
3β-HSD-1
3β-HSD-2
3β-HSD-1/3β-HSD-2
5α

-DIOL
Testo
3β-HSD-1
3β-HSD-2
5α-reductase-1
5α-reductase-2
DHT
E1
E1–S
E2
E2–S
17β-HSD-3
17β-HSD-5
17β-HSD-2
17β-HSD-7
17β-HSD-1
17β-HSD-2
17β-HSD-4
17β-HSD-1
17β-HSD-2
17β-HSD-4
• Fig. 28.9  Human steroidogenic enzymes in peripheral intracrine tis­
sues. DHEA, dehydroepiandrosterone; DHEAS, DHEA sulfate; DHT, 
dihydrotestosterone; 5-DIOL, androsterone-5-ene-3β,17β-diol; 4-Dione, 
androstenedione; E1, estrone; E2, estradiol; 3β-HSD, 3β-hydroxysteroid 
dehydrogenase; 17β-HSD, 1

-HSD, 3β-hydroxysteroid 
dehydrogenase; 17β-HSD, 17β-hydroxysteroid dehydrogenase; Testo, 
testosterone. (Modified from Labrie F, Luu-The V, Lin SX, et al. Intracri­
nology: role of the family of 17 beta-hydroxysteroid dehydrogenases in 
human physiology and disease. J Mol Endocrinol. 2000;25[1]:1–16.)
1192
controversial (see the following). Epidemiologic studies 
indeed point to a mild cardioprotective effect of higher 
DHEAS levels in both men and 

ve effect of higher 
DHEAS levels in both men and women.150 In the Rotter­
dam study, DHEA and DHEAS concentrations are inversely 
associated with the risk of diabetes.151 Functional parameters 
of activities of daily living in men older than 90 years were 
lowest in those with the lowest serum DHEAS concentra­
tions49; in healthy elderly individuals, there was an association 
between the ratio of cortisol to DHEAS levels and cognitive 
impairment.152
CYP3A7, expressed in the human fetal liver a

t.152
CYP3A7, expressed in the human fetal liver and normally 
silenced after birth, plays a major role in the 16α-hydroxylation of 
DHEA, DHEAS, and estrone. A common polymorphism in the 
CYP3A7 gene (CYP3A7*1C) causes persistence of the enzymatic 
activity of the gene during adult life. Between 6% and 8% of the 
population are heterozygous carriers of this polymorphism, result­
ing in almost 50% lower DHEAS levels compared with homozy­
gous carriers of the reference allele.153 Interestingly, n

riers of the reference allele.153 Interestingly, no evidence 
was found that such lowered levels are associated with an accelera­
tion of the aging process. 
DHEA Replacement Therapy
A physiologic functional role of DHEA in women has been ascer­
tained in a careful double-blind study. In women with adrenal 
insufficiency,154 DHEA administration (50 mg/day) normalized 
serum concentrations of DHEA, DHEAS, androstenedione, and 
testosterone. DHEA significantly improved overall well-being, as 
well

ignificantly improved overall well-being, as 
well as scores for depression and anxiety, the frequency of sexual 
thoughts, sexual interest, and satisfaction with both mental and 
physical aspects of sexuality.
Several short controlled trials with DHEA in small groups of 
elderly individuals provided ambiguous results.155,156 A 2-year 
placebo-controlled trial showed no effect of the oral administra­
tion of DHEA (at a dose of 75 mg/day in men and 50 mg/day in 
women) on body composition, muscle

d 50 mg/day in 
women) on body composition, muscle strength, or insulin sen­
sitivity, as compared with placebo.157 In this study involving 87 
elderly men with low levels of DHEAS and bioavailable testos­
terone and 57 elderly women with low levels of DHEAS, DHEA 
levels increased in both sexes during DHEA administration by 
about 9.5 μmol/L.
These results are in line with a previous study158 in 280 healthy 
subjects 60 to 79 years of age, in which 50 mg of DHEA daily 
for 1 year did not improv

ich 50 mg of DHEA daily 
for 1 year did not improve body composition or muscle strength, 
although in women an increase in libido was noted. In addition, 
a double-blind RCT with 50 mg of DHEA for 36 weeks in 50 
elderly men, 70 years of age or older, with low scores on muscle 
strength tests did not improve isometric grip strength, leg extensor 
power, and physical performance.159
With regard to one beneficial effect of DHEA administration, 
the results seem rather consistently positive: increa

 results seem rather consistently positive: increases in bone 
mineral density in women were repeatedly reported.157,158,160,161 
These positive effects, however, were very small and not more than 
approximately half those observed with current osteoporosis ther­
apies, such as estrogens and bisphosphonates, and therefore they 
are unlikely to have a significant effect on the risk of fracture.157 
Review of placebo-controlled RCTs does not show benefits of oral 
DHEA in postmenopausal women for 

enefits of oral 
DHEA in postmenopausal women for impaired sexual function, 
well-being, and cognitive performance, nor does it show favorable 
effects on lipids and carbohydrate metabolism.162,163 A study from 
Weiss and colleagues,164 however, of both elderly men (n = 92) 
and women (n = 51) showed that DHEA supplementation in a 
dose of 50 mg/day with a follow-up of 12 months reduces arterial 
stiffness.
Results from a meta-analysis showed that among the 644 
elderly men enrolled in 8 RCTs me

t among the 644 
elderly men enrolled in 8 RCTs measuring bone parameters, 
DHEA supplementation with a mean follow-up of 52 weeks did 
not improve lumbar or femoral bone mineral density nor forma­
tion or resorption of bone turnover markers.165 The same meta-
analysis including a total of 25 RCTs with 1353 elderly men with 
a mean follow-up of 36 weeks showed that DHEA supplementa­
tion of 50 to 100 mg/day was associated with a reduction of fat 
mass. No effect was observed for lipid and glycem

mass. No effect was observed for lipid and glycemic metabolism, 
sexual function, or quality of life.
Local delivery of DHEA, however, may be beneficial, because 
daily intravaginal administration of 0.50% (6.5 mg) DHEA has 
shown to improve moderate to severe vaginal dryness at 12 weeks 
and decrease pain at sexual activity.166 
DHEAS is a universal precursor for the peripheral local produc­
tion and action of estrogens and androgens in target tissues such as 
brain, bone, skin, an

s in target tissues such as 
brain, bone, skin, and adipose tissue. However, the importance of 
these pathways remains undefined, particularly in men, who have 
a relatively much higher production of testosterone from testicu­
lar origin. DHEA administration in the elderly, compared with 
placebo, increases serum DHEAS, testosterone, free testosterone, 
estrone, estradiol, and IGF1 concentrations, and lowers SHBG 
levels.157,159,160 It is not known whether this increase in sex steroid 
levels in

wn whether this increase in sex steroid 
levels induced by DHEA administration in the long term is safe 
with regard to the development or growth of ovarian, prostate, or 
other types of steroid-dependent cancers. The addition of DHEA 
(50 mg/day) to the existing large pool of DHEA and DHEAS in 
elderly individuals, even if they have been selected on the basis of 
low(ered) circulating levels of these steroids, has very limited clini­
cally meaningful effects, if any.
DHEA, which is currently av

ngful effects, if any.
DHEA, which is currently available as a dietary supplement, is 
widely used within the United States as an unapproved preventive 
treatment against aging. There are no convincing arguments at 
present to recommend the (routine) use of DHEA for delaying or 
preventing the physiologic consequences of aging, and its safety is 
unknown.167,168
Somatopause 
Role of GH and IGF1 During Aging
Elderly men and women secrete GH less frequently and at lower 
amplitude than do young pe

requently and at lower 
amplitude than do young people.20 In fact, GH secretion declines 
approximately 14% per decade in normal individuals.169,170 In 
parallel, serum levels of IGF1 (see Fig. 28.3) are 20% to 80% 
lower in healthy elderly individuals than in healthy young 
adults.171 However, whether this decline has adverse or beneficial 
effects is controversial. In invertebrates and rodents, an attenu­
ation of the insulin/IGF signaling pathway has led to an exten­
sion of the life span.172

way has led to an exten­
sion of the life span.172 In addition, in humans, genetic defects 
in the somatotropic axis resulting in lower or a decreased IGF1 
signaling may be associated with increased survival.173 Paradoxi­
cally, low IGF1 concentrations in humans have been associated 
with increased risk for cardiovascular disease, stroke, type 2 diabe­
tes mellitus, and osteoporosis. Many studies over the past decade 
have looked for associations between IGF1 levels and mortality 
risk. Some st

s between IGF1 levels and mortality 
risk. Some studies have found no association174-176; however, 
1193
other studies have found that lower levels of serum IGF1177-179 or 
serum IGF1 bioactivity180 were associated with greater all-cause 
mortality, whereas still others found that low IGF1 levels were 
predictive of extended survival.181 One study found that higher 
IGF1 levels were associated with a higher mortality risk,182 and 
two studies found a higher ri

tality risk,182 and 
two studies found a higher risk in subjects with both high and 
low IGF1 levels.183,184 A meta-analysis indicated that both high 
and low IGF1 concentrations are associated with increased cancer 
and cardiovascular-related mortality in the general population.185
IGF1 and IGF2 actions can be modulated by IGF binding pro­
teins (IGFBPs). IGFBP2 is the second most abundant IGFBP in 
the circulation. This protein has received a lot of attention over the 
past decade due to its p

ot of attention over the 
past decade due to its potential role in metabolic syndrome and 
several forms of cancer. IGFBP2 levels increase with increasing 
age and decrease with increasing body mass index (BMI).186,187 A 
strong association between low IGFBP2 levels and metabolic syn­
drome and low insulin sensitivity has been identified.187-190 Sur­
prisingly, however, although it was found that higher IGFBP2 was 
associated with favorable risk factors, including lower fasting glu­
cose and low

factors, including lower fasting glu­
cose and lower fasting insulin, two studies found a strong associa­
tion between higher IGFBP2 and mortality in older adults.187,189 
In both studies, the association appeared to be independent of 
acute-phase reactants (C-reactive protein and interleukin 6). Ear­
lier, it was published that high serum IGFBP2 concentrations 
were associated with worse physical function, low muscle mass, 
and low bone density in a population of elderly men.191 The asso­
ciati

in a population of elderly men.191 The asso­
ciation of high IGFBP2 concentrations and increased 8-year mor­
tality was independent of physical function. The clinical features 
of subjects with high IGFBP2 levels are similar to subjects with 
protein energy malnutrition. Interestingly, these associations were, 
however, present in a population with a normal BMI and in sub­
jects who lived independently.
IGFBP2 concentrations are overexpressed in several forms of 
malignancies.192 Thus far, it is

several forms of 
malignancies.192 Thus far, it is unclear what role IGFBP2 plays 
in the development and progression of human cancers.192 In 
the studies mentioned previously, the associations between high 
IGFBP2 levels and increased mortality were independent of the 
presence of malignancy. The role of IGFBP2 in the aging process 
remains to be elucidated. The concept that this decline in GH and 
IGF1 secretion contributes to the decline of functional capacity 
in elderly people (somatopause)

nctional capacity 
in elderly people (somatopause) is mainly derived from studies in 
which GH replacement therapy in GH-deficient adults was shown 
to increase muscle mass, muscle strength, bone mass, and quality 
of life. A beneficial effect on the lipid profile and an important 
decrease in fat mass were also observed in these patients.53,193,194 
As in hypogonadal individuals, adult GH deficiency can thus be 
considered a model of normal aging because several catabolic pro­
cesses that are c

g because several catabolic pro­
cesses that are central in the biology of aging can be reversed by 
GH replacement. 
GH Therapy
Rudman and colleagues,195 after a ground-breaking RCT of 
healthy men, 61 to 81 years old, with serum IGF1 concentra­
tions in the lower third for their age, reported in 1990 that GH 
treatment (30 mg/kg three times weekly for 6 months) restored 
the men’s IGF1 levels to “normal.” In the treatment group, lean 
body mass rose by 8.8% and lumbar vertebral density increas

 rose by 8.8% and lumbar vertebral density increased by 
1.6%. The magnitudes of these initial changes were equivalent to 
a reversal of the age-related changes by 10 to 20 years. However, 
during continuation of this study to 12 months, the significant 
positive effect on bone mineral density at any site was lost.196
In the subsequent years, it became clear that GH administra­
tion in healthy elderly individuals frequently caused acute adverse 
effects, such as carpal tunnel syndrome, gynecomas

effects, such as carpal tunnel syndrome, gynecomastia, fluid reten­
tion, and hyperglycemia, which were severe enough for an appre­
ciable number of individuals to drop out of these studies. The most 
disappointing aspect, however, was that no positive effects of GH 
administration were observed on muscle strength, maximal oxygen 
consumption, or functional capacity. In contrast, when GH was 
administered in combination with resistance exercise training, a 
significant positive effect on muscle 

raining, a 
significant positive effect on muscle mass and muscle strength was 
recorded that did not differ from that seen with placebo treatment, 
which suggests that GH does not add to the beneficial effects of exer­
cise.197,198 A representative example of a well-controlled study199 of 
GH administration in unselected elderly men is given in Table 28.2.
In a systematic study of 31 articles describing 18 unique well-
defined study populations, the safety and efficacy of GH in the 
healthy eld

 the safety and efficacy of GH in the 
healthy elderly were reviewed.200 A total of 220 participants who 
received GH for 107 person-years completed the study. Partici­
pants’ mean age was 69 years, and they were overweight (mean 
BMI, 28 kg/m2). Initial daily GH dose (mean, 14 μg/kg body 
weight) and treatment duration (mean, 27 weeks) varied. Overall 
fat mass decreased by 2.1 kg and lean body mass increased by 2.1 kg 
in those treated with GH, whereas total cholesterol levels decreased 
by 0.

 whereas total cholesterol levels decreased 
by 0.29 mmol/L. Disappointingly, no consistent changes in muscle 
strength, physical activity, or psychosocial outcomes were observed.
GH is associated with substantial adverse effects.200 To obtain 
insights into what can be expected, the details of one particularly 
well carried out placebo-controlled study in healthy women (n = 
  Effects of Growth Hormone 
Administration in Healthy Older Men.
MEAN CHANGE IN 
VARIABLE
Parameter
GH (n = 26)
Placebo

 CHANGE IN 
VARIABLE
Parameter
GH (n = 26)
Placebo 
(n = 26)
p Value
119.2
7.6
<0.0001
Body Weight and Composition
Weight, kg
0.5
1.0
>0.2
Lean mass, %
4.3
–0.1
<0.001
Fat mass, %
–13.1
–0.3
<0.001
Bone mineral content, %
0.9
–0.1
0.05
Skin thickness, %
13.4
1.1
0.09
Muscle Strength, %
Knee extension
3.8
1.3
>0.2
Knee flexion
10.0
8.2
>0.2
Hand grip
–1.5
3.8
0.11
Maximum Oxygen 
Consumption, %
2.5
–2.0
>0.2
GH, 30 μg/kg three times a week, was administered for 6 months to 52 healthy

 week, was administered for 6 months to 52 healthy 69-year-old 
men with well-preserved functional ability but low levels of IGF1.
From Papadakis MA, Grady D, Black D, et al. Growth hormone replacement in healthy older 
men improves body composition but not functional ability. Ann Intern Med. 1996;124:708–
716.
   
TABLE 28.2
1194
57) and men (n = 74) 65 to 88 years of age is presented201: GH 
a

= 74) 65 to 88 years of age is presented201: GH 
administered subcutaneously at an initial dose of 30 μg/kg, three 
times per week, then reduced to 20 μg/kg for 26 weeks, was asso­
ciated with carpal tunnel syndrome in 38% of women versus 7% 
of those taking placebo, and in 24% of men versus 0% taking 
placebo; edema in 39% of women (0% for placebo) and 30% of 
men (12% for placebo); and arthralgias in 46% of women (7% for 
placebo) and 41% of men (0% for placebo). Troublesome was the 
report th

n (0% for placebo). Troublesome was the 
report that 18 men treated with GH developed glucose intolerance 
or diabetes compared with 7 men in the nontreatment group.201
In an RCT, Blackman and colleagues201 studied the synergetic 
effects of a combination of GH and sex steroids in healthy elderly 
women and men. A slight increase in lean body mass and mus­
cle strength was observed in elderly men treated with GH and 
testosterone but not in women who received GH and estrogen. 
In a study of heal

 who received GH and estrogen. 
In a study of healthy elderly men, coadministration of low-dose 
GH with testosterone resulted also in slightly beneficial changes 
(muscle strength, quality of life) compared with GH or testoster­
one alone.202 In a partially placebo controlled trial of 112 older 
men, testosterone and recombinant human GH administration 
appeared not to worsen cardiometabolic risk in healthy older men 
after 4 months.203 Earlier studies demonstrated that pharmaco­
logic doses of

studies demonstrated that pharmaco­
logic doses of GH prevent the autocannibalistic effects of acute dis­
eases on muscle mass.204 Confirmation is needed, however, before 
GH can gain a place in the treatment of acute catabolic states in 
frail elderly people.
Other components in the regulation of the GH–IGF1 axis 
are effective in activating GH and IGF1 secretion. Long-acting 
derivatives of the hypothalamic peptide growth hormone–releas­
ing hormone, given twice daily subcutaneously for 14 day

rmone, given twice daily subcutaneously for 14 days to 
healthy 70-year-old men, increased GH and IGF1 levels to those 
encountered in 35 year olds.51 These studies suggest that somato­
pause is driven primarily by the hypothalamus and that pituitary 
somatotropes retain their capacity to synthesize and secrete high 
levels of GH.
Two ghrelin-mimetic GH secretagogues (GHSs) have also 
been demonstrated to be able to restore levels of GH and IGF1 
in elderly individuals to those of young adults.2

in elderly individuals to those of young adults.205 Ghrelin, an 
octanoylated 28–amino acid peptide, stimulates GH secretion 
via a distinct, endogenous GHS receptor but also has appetite-
stimulating activity.206 These GHSs slightly increased fat-free 
mass without changing fat mass, muscle strength, and func­
tion or quality of life,207 and changed body composition and 
increased appetite that was accompanied by small but significant 
improvements in some measures of physical function208 in o

ents in some measures of physical function208 in older 
adults.
The long-term safety of activating GH and IGF1 levels 
in older people has become a concern because of reports of 
an association between serum IGF1 concentrations and can­
cer risk. Individuals with high IGF1 levels (or low IGFBP3 
levels) within the broad normal range have an increased risk 
of prostate, colon, and breast cancer.209-211 These epidemio­
logic studies, together with experimental data, suggest that the 
IGF1 system i

experimental data, suggest that the 
IGF1 system is involved in tumor development and progres­
sion. However, no causal relationship between IGF1 levels 
and cancer risk has yet been established, and possible medical 
intervention directed at increasing IGF1 bioactivity in elderly 
people will in most instances be given toward the end of life, 
presumably not allowing enough time to affect tumor develop­
ment or progression. 
During the aging process, GH–IGF1 axis activity declines. 

he aging process, GH–IGF1 axis activity declines. It is 
unclear whether changes in body composition and functional 
capacity are directly related. GH administration in older adults 
causes an increase in lean body mass and an appreciable loss 
of fat mass. However, GH treatment does not improve mus­
cle strength and functional capacity in elderly people, despite 
restoration of circulating IGF1 concentrations to young adult 
levels. Furthermore, most dose regimens of GH cause appre­
ciable adve

 most dose regimens of GH cause appre­
ciable adverse effects, and long-term safety with regard to tumor 
development and progression remains uncertain. Oral ghrelin 
mimetics are also capable of restoring GH and IGF1 levels in the 
elderly population, together with an increase in appetite. Modest 
functional improvement was observed in one study after 2 years 
administration.
In the near future, clinical trials with such orally active mol­
ecules in frail elderly people or in elderly individual

s in frail elderly people or in elderly individuals with clearly 
lowered IGF1 levels, or both, should be able to delineate the pre­
cise role of the GH–IGF1 axis in the aging process. In such trials, 
much emphasis must be given to safety aspects. At present, there 
is no evidence to recommend medical or other intervention in 
the GH–IGF1 axis as an antiaging effort to prolong life or rejuve­
nate healthy elderly people.212,213 Only elderly patients with GH 
deficiency caused by organic disease

ents with GH 
deficiency caused by organic diseases, such as pituitary adenomas, 
clearly benefit from GH replacement therapy.214 
The Concept of Successful Aging
There is considerable variation in the effects of aging on healthy 
individuals, with some people exhibiting greater and others 
exhibiting few or no age-related alterations in physiologic func­
tions. It has been suggested that it might be useful to distinguish 
between usual and successful patterns of aging.215 Genetic fac­
tors, lif

ssful patterns of aging.215 Genetic fac­
tors, lifestyle, and societal investments in a safe and healthful 
environment are important aspects of successful aging.216 Tradi­
tionally, the aging process, including the development of physical 
frailty toward the end of life, has been considered physiologic and 
unavoidable.
It has recently become evident, however, that it might not be 
necessary to accept the grim stereotype of aging as an unalterable 
process of decline and loss.215 As life expect

le 
process of decline and loss.215 As life expectancy rises further in 
the coming decades, the overarching goal should be “an increase in 
years of healthy life with a full range of functional capacity at each 
stage of life.”217 Such a compression of morbidity can be achieved 
by adapting lifestyle measures, but several aspects of the aging pro­
cess of the endocrine system invite the development of routine 
medical intervention programs offering long-term replacement 
therapy with one or mor

ing long-term replacement 
therapy with one or more hormones to delay the aging process and 
to allow humans to live for a longer period in a relatively intact 
state.218 However, unfortunately, it should be concluded at pres­
ent that hormonal interventions with sex steroids, DHEA, GH, 
and/or oral ghrelin agonists have very limited effects on physical 
capacity in elderly individuals, whereas adverse effects are often 
considerable.
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 Effect 
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1196
29
Hormones and Disorders of Mineral 
F. RICHARD BRINGHURST, MARIE B. DEMAY, AND HENRY M. KRONENBERG
Basic Biolog

RIE B. DEMAY, AND HENRY M. KRONENBERG
Basic Biology of Mineral Metabolism: Roles 
of the Mineral Ions
Calcium (Ca) and phosphorus (P) are the principal constituents 
of bone, and together they compose 65% of its weight. Bone, in 
turn, contains nearly all of the calcium and phosphorus and over 
half of the magnesium in the human body. The quantitatively 
minor amounts of each of these ions in the extracellular fluid and 
within cells play crucial roles in normal physiology (Fig. 29.1).
Ninety-ni

 roles in normal physiology (Fig. 29.1).
Ninety-nine percent of total body calcium resides in bone, of 
which 99% is located within the crystal structure of the mineral 
phase. The remaining 1% of bone calcium is rapidly exchange­
able with extracellular calcium; this calcium is equally distributed 
between the intracellular and extracellular fluids. Extracellular cal­
cium is the principal substrate for the mineralization of cartilage 
and bone, but it also serves as a cofactor for many extrace

 but it also serves as a cofactor for many extracellular 
enzymes, most notably the enzymes of the coagulation cascade, 
and as a source of calcium ions that serve as signaling molecules 
for a great diversity of intracellular processes. These processes 
include automaticity of nerve and muscle; contraction of cardiac, 
skeletal, and smooth muscle; neurotransmitter release; and various 
forms of endocrine and exocrine secretion.
In blood, approximately 50% of total calcium is bound to 
proteins,

mately 50% of total calcium is bound to 
proteins, mainly albumin and globulins. The ionized calcium 
concentration in serum is approximately 1.2 mmol/L (5 mg/dL), 
and it is this ionized fraction that is biologically active and that is 
tightly controlled by hormonal mechanisms. Because intracellular 
cytosolic free calcium concentrations typically are in the range of 
only 100 nM, a very large chemical gradient (i.e., 10,000:1), aug­
mented by the large negative electrical potential, favors ca

the large negative electrical potential, favors calcium 
entry into cells through calcium channels. This gradient is main­
tained by the limited conductance of resting calcium channels and 
Basic Biology of Mineral Metabolism: Roles of the Mineral Ions, 
1196
Parathyroid Hormone, 1198
Calcitonin, 1210
Vitamin D, 1211
Fibroblast Growth Factor-23, 1216
Calcium and Phosphate Homeostasis, 1217
Laboratory Assessment of Mineral Metabolism, 1218
Hypercalcemic Disorders, 1220
Hypocalcemi

sm, 1218
Hypocalcemic Disorders, 1238
Disorders of Phosphate Metabolism, 1248
Disorders of Magnesium Metabolism, 1251
KEY POINTS
 • 
Parathyroid hormone (PTH), vitamin D, and fibroblast growth 
factor 23 (FGF23) together regulate the levels of calcium 
and phosphorus in the bloodstream to keep them relatively 
constant and slightly above the inherent solubility product of 
calcium and phosphate. These hormones act largely on cells in 
intestine, kidney, bone, and p

lood are usually caused by diseases of the kidney.
 • 
Magnesium disorders usually reflect abnormalities of influx of 
magnesium across the intestine, excretion of magnesium by 
the kidney, or shifts among the intracellular and extracellular 
compartments.
1197
Chapter 29  Hormones and Disorders of Mineral Metabolism
by the energy-dependent extrusion of calcium into the extracel­
lular fluid via high-affinity Ca2+-adenosine triphosphatases and 
H+-adenosine triphosphatases (ATPases) and low-aff

H+-adenosine triphosphatases (ATPases) and low-affinity sodium-
calcium (Na+-Ca2+) exchangers.
More than 99% of intracellular calcium exists in the form of 
complexes within the mitochondrial compartment, bound to the 
inner plasma membrane, or associated with the inner membranes 
of the endoplasmic reticulum and other compartments. Release 
of calcium from membrane-bound compartments transduces cel­
lular signals and is tightly regulated. The mechanisms responsible 
for translocations of intrac

chanisms responsible 
for translocations of intracellular calcium between the cytosol and 
these sequestered regions have become better understood with the 
identification of specific receptors for calciotropic signaling mol­
ecules such as the inositol triphosphate (IP3) receptor and ryano­
dine receptors.
Phosphate is more widely distributed to nonosseous tissues 
than is calcium. Eighty-five percent of body phosphate is in the 
mineral phase of bone, and the remainder is located in inorganic 

f bone, and the remainder is located in inorganic or 
organic form throughout the extracellular and intracellular com­
partments. In human serum, inorganic phosphate (Pi) is present 
at a concentration of approximately 1 mmol/L and exists almost 
entirely in ionized form as either H2PO4– or HPO42–. Only 12% 
of serum phosphate is protein bound, and an additional small 
fraction is loosely complexed with calcium, magnesium, and other 
cations. Intracellular free phosphate concentrations are gener

racellular free phosphate concentrations are generally 
comparable to those in the extracellular fluid (i.e., 1–2 mmol/L), 
although the inside-negative electrical potential of the cell creates 
a significant energy requirement for translocation of phosphate 
into cells. This process generally is accomplished through sodium-
phosphate cotransport driven by the transmembrane sodium gra­
dient. A number of sodium-phosphate cotransporters have been 
cloned; various cells and tissues use different s

cloned; various cells and tissues use different species of such trans­
porters with distinctive regulatory characteristics.
Organic phosphate is a key component of virtually all classes 
of structural, informational, and effector molecules that are essen­
tial for normal genetic, developmental, and physiologic processes. 
Phosphate is an integral constituent of nucleic acids; phospholip­
ids; complex carbohydrates; glycolytic intermediates; structural, 
signaling, and enzymatic phosphoproteins;

ctural, 
signaling, and enzymatic phosphoproteins; and nucleotide cofac­
tors for enzymes and G proteins. The need for the large amounts of 
phosphate incorporated into cellular constituents during cell pro­
liferation explains why phosphate levels in the blood are regulated 
by insulin-like growth factor 1 (IGF1), in addition to regulation 
by hormones of bone mineralization. Of particular importance are 
the high-energy phosphate ester bonds present in molecules such 
as adenosine triphosphate

esent in molecules such 
as adenosine triphosphate (ATP), diphosphoglycerate, and cre­
atine phosphate that store chemical energy. Phosphate plays a par­
ticularly prominent role as the key substrate or recognition site in 
numerous kinase and phosphatase regulatory cascades. Cytosolic 
phosphate per se also directly regulates a number of crucial intra­
cellular reactions, including those involved in glucose transport, 
lactate production, and synthesis of ATP. In light of these diverse 
roles, 

nthesis of ATP. In light of these diverse 
roles, it is not surprising that disorders of phosphate homeostasis 
associated with severe depletion of intracellular phosphate lead 
to profound and global impairment of organ function. Note that 
none of these roles for intracellular phosphate involves actions of 
intracellular calcium; the reason we are discussing these together 
results solely from their intimate relationship in regulating bone 
mineralization outside cells.
Magnesium is the fourth

eralization outside cells.
Magnesium is the fourth most abundant cation in the body. 
Roughly half is found in bone and half in muscle and other 
soft tissues. As much as half of the magnesium in bone is not 
sequestered in the mineral phase but is freely exchangeable 
with the extracellular fluid and therefore may serve as a buffer 
against changes in extracellular magnesium concentration. Less 
than 1% of all magnesium in the body is present in the extra­
cellular fluid, where the magnesium co

 the extra­
cellular fluid, where the magnesium concentration is approxi­
mately 0.5 mmol/L. The concentration of magnesium in serum 
normally is 0.7 to 1 mmol/L, of which roughly one-third is 
protein bound, 15% is loosely complexed with phosphate or 
other anions, and 55% is present as the free ion. Over 95% 
of intracellular magnesium is bound to other molecules, most 
notably ATP, the concentration of which is approximately 5 
mmol/L. The intracellular cytosolic free magnesium concentra­
tio

racellular cytosolic free magnesium concentra­
tion is approximately 0.5 mmol/L (i.e., 1000-fold higher than 
that of calcium) and is maintained by an active sodium-magne­
sium antiporter. The mechanism(s) whereby magnesium enters 
cells, presumably down a favorable electrochemical gradient, is 
unknown, although some evidence for regulated channels has 
been obtained.1
Intracellular magnesium, like phosphate, is necessary for a 
wide range of cellular functions. It is an essential cofactor in e

llular functions. It is an essential cofactor in enzy­
matic reactions, including most of the same glycolytic, kinase, and 
phosphatase pathways that also involve phosphate. Magnesium 
serves to directly stabilize the structures of a variety of macromol­
ecules and complexes, including deoxyribonucleic acid (DNA), 
ribonucleic acid (RNA), and ribosomes; is a key activator of the 
many ATPase-coupled ion transporters; and plays a direct role in 
mitochondrial oxidative metabolism. As a result, ma

itochondrial oxidative metabolism. As a result, magnesium is 
critical for energy metabolism and the maintenance of a normal 
intracellular environment. Extracellular magnesium is crucial for 
normal neuromuscular excitability and nerve conduction, and 
many of the clinical consequences of magnesium deficiency or 
excess reflect abnormalities in this sphere.
The levels of extracellular calcium and phosphate are regu­
lated in a coordinated way that reflects the roles of calcium and 
phosphate in

at reflects the roles of calcium and 
phosphate in mineralization of bone. The concentrations of these 
ions in body fluids are together close to the concentrations that 
could lead to spontaneous precipitation in soft tissues. In fact, 
elaborate mechanisms, most poorly understood, have evolved 
to prevent calcium phosphate precipitation in tissues and yet 
to allow the controlled deposition of calcium and phosphate in 
Functions
Concentration   
   total, in serum
free
Calcium io

um and phosphate. Note the 
dramatic differences between intracellular and extracellular concentra­
tions of calcium ion and the dramatically different functions of calcium and 
phosphate inside cells.
1198
SECTION VII    Mineral Metabolism
bone.2 The importance of the mineral ions for normal cellular 
physiology as well as skeletal integrity is reflected in the power­
ful endocrine control mechanisms that have evolved to maintain 
their extracellular concentrations within relatively narrow limi

lular concentrations within relatively narrow limits. 
The following sections describe the structures, secretory controls, 
actions, and interactions of parathyroid hormone, calcitonin, 
1,25-dihydroxyvitamin D (1,25[OH]2D3 or calcitriol), and fibro­
blast growth factor 23 (FGF23)—the major hormones involved 
in mineral ion homeostasis. Subsequent sections cover the wide 
variety of clinical disorders that accompany abnormalities in this 
hormonal network. 
Parathyroid Hormone
Parathyroid hormon

l network. 
Parathyroid hormone (PTH) is the peptide hormone that controls 
the minute-to-minute level of ionized calcium in the blood and 
extracellular fluids. PTH binds to cell surface receptors in bone and 
kidney, thereby triggering responses that increase blood calcium 
(Fig. 29.2). PTH also increases renal synthesis of 1,25(OH)2D3, 
the hormonally active form of vitamin D, which then acts on the 
intestine to augment absorption of dietary calcium, in addition to 
promo

sorption of dietary calcium, in addition to 
promoting calcium fluxes into blood from bone and kidney. The 
resulting increase in blood calcium (and in 1,25[OH]2D3) feeds 
back on the parathyroid glands to decrease the secretion of PTH. 
The parathyroid glands, bones, kidney, and gut are thus the crucial 
organs that participate in PTH-mediated calcium homeostasis.
Parathyroid Gland Biology
Parathyroid glands first appeared in evolutionary history with the 
exit of amphibians from the sea and a 

y with the 
exit of amphibians from the sea and a switch from dependence 
on gills to sole dependence on bone, intestine, and kidney to 
maintain extracellular calcium homeostasis. Reptiles, birds, and 
mammals all have parathyroid glands that develop as epithelial 
specializations from the endoderm of the pharyngeal pouches. 
Though fish have no discrete parathyroid glands, they do have 
several PTH-related genes, including two or more that can influ­
ence calcium homeostasis.3
Parathyroid chie

influ­
Parathyroid chief cells have three properties vital to their 
homeostatic function: First, they rapidly secrete PTH in response 
to changes in blood calcium. Second, they can synthesize, process, 
and store large amounts of PTH in a regulated manner. Third, 
parathyroid cells replicate when chronically stimulated. These 
functional attributes allow for short-term, intermediate-term, 
and long-term adaptation, respectively, to changes in calcium 
availability.
Pa

pectively, to changes in calcium 
Parathyroid Hormone Biosynthesis
PTH, a protein of 84 amino acids in mammals, is synthesized as 
a larger precursor, pre-proparathyroid hormone (pre-pro-PTH); 
with the explosion of genome sequencing in the past decade, 
the gene has been sequenced in a large number of species (see 
sequences in Genbank: http://www.ncbi.nlm.nih.gov/gene/5741) 
from fish to humans. Fig. 29.3 illustrates representative pre-pro-
PTH sequences. These pre-pro-PTH sequen

e pre-pro-
PTH sequences. These pre-pro-PTH sequences share a 25-residue 
“pre” or signal sequence and a 6-residue “pro” sequence. The signal 
sequence, along with the short pro sequence, functions to direct 
the protein into the secretory pathway (Fig. 29.4). During tran­
sit across the membrane of the endoplasmic reticulum, the signal 
sequence is cleaved off and rapidly degraded. The importance of 
the signal sequence for normal secretion of PTH is illustrated by 
the hypoparathyroidism inher

TH is illustrated by 
the hypoparathyroidism inherited in families carrying mutations 
in the signal sequence of pre-pro-PTH.4–6
The role of the short pro sequence is not completely under­
stood; it may help the signal sequence work efficiently and 
ensure accurate cleavage of the precursor.7 After cleavage of the 
pro sequence, the mature PTH(1-84) is concentrated in secre­
tory vesicles and granules. One morphologically distinct subtype 
of granule contains both PTH and the proteases cathepsin

nule contains both PTH and the proteases cathepsin B and 
cathepsin H. This co-localization of proteases and PTH in secre­
tory granules probably explains the observation that a portion of 
the PTH secreted from parathyroid glands consists of carboxy-
terminal PTH fragments. No amino-terminal fragments of PTH 
are secreted. Although the possible functions of carboxy-termi­
nal fragments of PTH are still poorly characterized, these frag­
ments do not activate the PTH/PTHrP (PTH-related protein) 

not activate the PTH/PTHrP (PTH-related protein) 
receptor and may even block bone resorption8 (see later discus­
sion). The intracellular degradation of newly synthesized PTH 
thus may provide an important regulatory mechanism. Under 
conditions of hypercalcemia, the secretion of PTH is substan­
tially decreased, and most of what is secreted consists of carboxy-
terminal fragments.9 
Parathyroid Hormone Secretion
Although catecholamines, magnesium, and other stimuli can 
affect PTH secretion, t

um, and other stimuli can 
affect PTH secretion, the major regulator of PTH secretion is the 
concentration of ionized calcium in blood. Increased serum ion­
ized calcium leads to a decrease in PTH secretion (Fig. 29.5A). 
The shape of the dose-response curve is sigmoid. Properties of the 
parathyroid cell determine the conformation of the sigmoid curve 
but do not alone determine the point on the curve that represents 
a physiologic steady state for an individual. This point, usually 
between t

 for an individual. This point, usually 
between the midpoint and the bottom of the curve, is determined 
by how vigorously target organs respond to PTH.10 Fig. 29.5C 
(solid line) shows how an individual’s calcium level rises in response 
to increases in PTH; the parathyroid gland’s sigmoid curve is the 
dotted line. In the steady state, an individual’s blood levels of PTH 
and calcium represent the intersection of the two lines.
The sigmoid curve reveals several important physiologic prop­
ert

+, positive effect.
1199
but not zero. The maximal secretory rate represents the reserve of 
the parathyroid’s capacity to respond to hypocalcemia. Because 
values from normal persons in the steady state are located in the 
lower portion of the sigmoid curve, the system seems designed to 
respond more dramatically to hypocalcemia than to hypercalcemia.
Physiologic studies in humans confirmed this sigmoid relation­
ship and have revealed th

d this sigmoid relation­
ship and have revealed that the parathyroid cell responds both 
to the absolute level of blood calcium and to the rate of fall of 
calcium level. Thus PTH levels briefly rise higher during a sudden 
drop in blood calcium than they do during a more gradual fall in 
calcium. This property of the parathyroid cell offers an additional 
protection against sudden hypocalcemia.
The biochemical and cellular determinants of the parathy­
roid gland’s sigmoid response curve are beg

rathy­
roid gland’s sigmoid response curve are beginning to be defined. 
A parathyroid calcium-sensing receptor (CaSR) on the parathy­
roid cell surface is a member of the G protein–coupled family of 
receptors.11 The sequence of the receptor suggests that it spans 
the plasma membrane seven times, like other receptors in the G 
protein–linked receptor family (Fig. 29.6). A large extracellular 
domain resembles similar domains in brain metabotropic gluta­
mate receptors as well as bacterial peri

ic gluta­
mate receptors as well as bacterial periplasmic proteins designed 
to bind small ligands, including cations. This domain has been 
crystalized, and binding sites for calcium, phosphate, and other 
ligands have been identified.12 The receptor has been expressed 
in a number of cell types and has been shown to activate phos­
pholipase C (PLC) and to block stimulation of cyclic adenosine 
monophosphate (cAMP) production, just as it does in normal 
parathyroid cells.
The most convincing pr

 normal 
The most convincing proof of the identity of the parathyroid 
CaSR has been the observation that mutations in the receptor 
gene cause characteristic human diseases. Inactivating muta­
tions cause familial hypocalciuric hypercalcemia (FHH), a dis­
ease of defective calcium sensing (see later discussion),13 whereas 
activating mutations cause familial hypoparathyroidism with 
hypercalciuria.14 Furthermore, mice genetically engineered to 
have only one functioning copy 

lly engineered to 
have only one functioning copy of the CASR gene also have the 
expected defects in parathyroid calcium sensing.15 Of impor­
tance, calcimimetic compounds that activate the cloned CaSR 
have been shown to inhibit PTH secretion in humans and are 
useful in the treatment of secondary hyperparathyroidism.16,17 
Despite the enormous increase in understanding how extracel­
lular calcium activates the parathyroid CaSR, the mechanism 
whereby this activation leads to a decrease in PTH

whereby this activation leads to a decrease in PTH secretion is 
poorly understood.
The CaSR is expressed widely. Expression in the renal tubules 
and calcitonin-producing cells of the thyroid contributes to cal­
cium homeostasis, whereas expression in organs such as the brain 
points to multiple roles for calcium signaling. Knockout of the 
CaSR in osteoblasts of mice shows that this receptor regulates 
osteoblast differentiation and mineralization.18 The observation 
that the CaSR also respond

ion.18 The observation 
that the CaSR also responds to physiologic levels of certain amino 
• Fig. 29.3  Sequences of pre-proparathyroid hormone from six species. Completely conserved residues 
are in boldface. Arrows indicate the sites of signal sequence (“pre”) and “pro” sequence cleavage. Num­
bers start at residue +1 of mature parathyroid hormone (PTH); because of gaps, the numbers correspond 
only to the mammalian and not to the chicken sequence. Amino acids are indicated by the single-lett

ence. Amino acids are indicated by the single-letter 
code: A, Ala; R, Arg; N, Asn; D, Asp; C, Cys; Q, Gln; E, Glu; G, Gly; H, His; I, Ile, L, Leu; K, Lys; M, Met; F, 
Phe; P, Pro; S, Ser; T, Thr; W, Trp; Y, Tyr; V, Val.
Rough ER
Pre
Pro-PTH
Pro-PTH
PTH
PTH
PTH
Golgi
Secretory
granule
• Fig. 29.4  Intracellular processing of pre-proparathyroid hormone (pre-
pro-PTH). Diagonal arrows indicate sites of cleavage by enzymes that gen­
erate pro-PTH in the rough endoplasmic reticulum (ER), PTH in the 

 the rough endoplasmic reticulum (ER), PTH in the Golgi, 
and carboxy-terminal fragments of PTH in the secretory granule.
1200
acids19 suggests that the expression of the CaSR in the gut, para­
thyroid, and other sites may facilitate the assimilation of multiple 
nutrients. 
Regulation of the Parathyroid Hormone Gene
The minute-to-minute regulation of PTH blood levels can be 
explained by the two mechanisms already discussed—regulation 
of PTH secretion by the C

dy discussed—regulation 
of PTH secretion by the CaSR and amplification of this regula­
tion by intracellular degradation of stored hormone. Over a longer 
time frame, the parathyroid cell regulates the expression of the 
PTH gene as well.
Although 1,25(OH)2D3—the active form of vitamin D—
has no direct effect on PTH secretion, it dramatically suppresses 
PTH gene transcription.20 This suppression of transcription does 
not occur when 1,25(OH)2D3 is administered to chronically 
hypocalcemic anim

 is administered to chronically 
hypocalcemic animals, however, perhaps because hypocalcemia 
leads to a fall in parathyroid cell vitamin D receptors (VDRs) 
or because hypocalcemia increases the expression of calreticulin 
PTH secretion rate (ng/kg/min)
Plasma calcium (mg/dL)
[Ca2+] (mM)
13
5
10
5
17
7
17
7
9
6
27
9
28
10
16
10
14
9
16
7
9
5
28
10
19
1140
11
0
1
2
3
4
5
1.0
2.0
3.0
2.0
1.5
1.0
0.5
0.0
6
100
80
60
40
20
0
5
4
3
2
1
0
41
1241
11
26
10
5
PTH secretion, % of maximal
Plasma free cal

6
10
5
Plasma free calcium
(mmol/L)
Plasma PTH (arbitrary units)
Ca = f(PTH)
PTH = f(Ca)
A
B
C
B
A
C
D
6
7
14 15 16 17
8
9
10 11 12 13
• Fig. 29.5  Parathyroid hormone (PTH) secretion. (A) Secretory response of bovine parathyroid glands 
to induced alterations of plasma calcium concentration. Calves were infused with calcium chloride or 
ethylenediaminetetraacetic acid (EDTA), and PTH secretion was assessed by measuring PTH levels in 
the parathyroid venous effluent. 

g PTH levels in 
the parathyroid venous effluent. The symbols and vertical bars indicate the secretory rate (mean ± SE) in 
calcium concentration ranges of 1 or 0.5 mg/100 mL. The number of calves and samples are indicated, 
respectively, by numbers below and above the bars. (B) Sigmoidal curve generated by the equation Y = 
([A − D]/[1 + (X/C)B]) + D. Such a curve can be defined by four parameters: the maximal secretory rate 
(A), the slope of the curve at its midpoint (B), the level of calcium

he curve at its midpoint (B), the level of calcium at the midpoint (often called the set-
point) (C), and the minimal secretory rate (D); the significance of A, B, C, and D is described in the text. 
(C) Relationships between calcium and PTH levels when each in turn is treated as an independent vari­
able. The dashed line represents the sigmoidal relationship between calcium and PTH, when calcium is 
the independent variable. This curve is the same as that in (A) and (B), but it is turned on its

me as that in (A) and (B), but it is turned on its side because 
the axes are reversed. The solid line represents the relationship between calcium and PTH when PTH is 
considered the independent variable; values for this curve result from measurements made during PTH 
infusion into parathyroidectomized animals. Actual data are limited, thus the curves should be viewed as 
illustrative. (A, from Mayer GP, Hurst JG. Sigmoidal relationship between parathyroid hormone secretion 
rate and plasma calc

arathyroid hormone secretion 
rate and plasma calcium concentration in calves. Endocrinology. 1978;10:1037–1042; B, modified from 
Brown EM. Four-parameter model of the sigmoidal relationship between parathyroid hormone release and 
extracellular calcium concentration in normal and abnormal parathyroid tissue. J Clin Endocrinol Metab. 
1983;56:572–581; C, from Parfitt AM. Calcium homeostasis. In Mundy GR, Martin TJ, eds. Physiology and 
Pharmacology of Bone. Berlin: Springer-Verlag; 1993.)
1201

ogy of Bone. Berlin: Springer-Verlag; 1993.)
1201
in the parathyroid.21 The capability of hypocalcemia to override 
the effects of high levels of 1,25(OH)2D3 represents an impor­
tant defense because it provides a way for the parathyroid cell to 
synthesize large amounts of PTH and 1,25(OH)2D3 at the same 
time, when both are needed.
Calcium also regulates the biosynthesis of PTH. In vivo stud­
ies show that acute hypocalcemia in rats lead

tud­
ies show that acute hypocalcemia in rats leads, within an hour, 
to an increase in PTH messenger RNA (mRNA). In contrast, 
hypercalcemia leads to little or no change in PTH mRNA. Thus 
under normal conditions the inhibition by calcium of PTH 
biosynthesis already is nearly maximal, just as it is for PTH 
secretion. The parathyroid gland is poised to respond to a fall 
in calcium much more readily than to a rise. The mechanism 
for the increase in PTH mRNA in response to hypocalcemia 
is unc

se in PTH mRNA in response to hypocalcemia 
is uncertain; differing experimental paradigms suggest regula­
tion at the levels of gene transcription, mRNA translation, and 
mRNA stability. The latter mechanism is the one best under­
stood at the molecular level.22 In the parathyroid cell, peptidyl-
prolyl isomerase Pin1 binds to and leads to the activation of 
K-homology splicing regulator protein (KSRP), an RNA-bind­
ing protein that destabilizes PTH mRNA. Mice without Pin1 
have high PTH and PT

 PTH mRNA. Mice without Pin1 
have high PTH and PTH mRNA levels, and hypocalcemic rats 
and rats with chronic kidney disease have low Pin1 levels. Thus 
Pin1 and KSRP regulate PTH mRNA levels in normal physiol­
ogy and disease and mediate the effects of calcium and phos­
phate on PTH mRNA stability.23
For decades it has been known that phosphate elevation 
stimulates PTH secretion largely by lowering blood calcium and 
1,25(OH)2D3 levels. Furthermore, a series of studies in vitro24,25 
and in  v

more, a series of studies in vitro24,25 
and in  vivo26 have demonstrated that phosphate can increase 
PTH secretion directly, independent of effects on blood calcium 
and 1,25(OH)2D3. Phosphate increases PTH secretion acutely 
only after a delay and probably works largely through regulation 
of PTH mRNA levels. The mechanisms that parathyroid cells use 
to sense phosphate are unknown, but the effect of hypophospha­
temia and hyperphosphatemia on PTH mRNA occurs through 
regulation of PTH mRNA s

 PTH mRNA occurs through 
regulation of PTH mRNA stability through changes in protein 
binding to the 3′ noncoding region of the mRNA by mechanisms 
that closely parallel those used by calcium.27 FGF23, an impor­
tant phosphate-regulating hormone (see later), activates FGF 
receptor 1 and its coreceptor, Klotho, on parathyroid cells and 
thereby suppresses PTH synthesis,28 though Klotho-independent 
signaling by FGF23 may be important as well.29 In renal failure, 
both FGF23 levels and PTH level

In renal failure, 
both FGF23 levels and PTH levels are elevated. This paradoxic 
observation (given the role of FGF23 to suppress PTH secretion) 
may be explained by the downregulation of parathyroid cell FGF 
receptor 1 and Klotho in renal failure.30,31 
Regulation of Parathyroid Cell Number
Parathyroid cells divide during the growth of young animals but 
replicate little in adulthood.32 Parathyroid cell number can dra­
matically increase, however, in the setting of hypocalcemia, low 
levels o

ver, in the setting of hypocalcemia, low 
levels of 1,25(OH)2D3, hyperphosphatemia, or uremia, or during 
neoplastic growth.
Calcium, acting through the parathyroid CaSR, restrains 
parathyroid proliferation. The behavior of the parathyroids in 
patients who lack both copies of the CASR gene is consistent 
with this hypothesis. These neonates exhibit severe primary 
Amyloid- protein
Ionic strength, pH
Calcimimetics
Polyamines
Nucleus
Internal

 cAMP, 
cyclic adenosine monophosphate; cPLA2, cytosolic phospholipase A2; DAG, diacylglycerol; ERK, extracel­
lular signal–regulated kinase; Giα and Gqα, α-subunits of the i-type and q-type heterotrimeric G proteins, 
respectively; Ins(1,4,5)P3, inositol-1,4,5-trisphosphate; Ins(1,4,5)P3R, inositol-1,4,5-trisphosphate recep­
tor; JNK, Jun amino-terminal kinase; MAPK, mitogen-activated protein kinase; MEK, MAPK kinase; PI4K, 
phosphatidylinositol 4-kinase; PKC, protein kinase C; PLC, phospholipa

 4-kinase; PKC, protein kinase C; PLC, phospholipase C; PtdIns(4,5)P2, phosphati­
dylinositol-4, 5-bisphosphate. (From Hofer AM, Brown EM. Extracellular calcium sensing and signaling. 
Nat Rev Mol Cell Biol. 2003;4:530–538.)
1202
hyperparathyroidism with large, diffusely hyperplastic glands 
that presumably have developed because of insufficient activation 
of the parathyroid CaSR by extracellular calcium. Furthermore, 
administration of the calcimimetic compoun

rmore, 
administration of the calcimimetic compound NPS R-568, which 
activates the CaSR directly, prevents parathyroid cell proliferation 
in experimental uremia.
The role of 1,25(OH)2D3, independent of blood calcium, in 
regulating parathyroid cell proliferation is less well established 
than that of calcium. That 1,25(OH)2D3 can dramatically affect 
parathyroid cell number has been shown in  vivo in many set­
tings, but such studies cannot rigorously eliminate effects of tran­
sient changes i

orously eliminate effects of tran­
sient changes in blood calcium. The suppression of proliferation 
of cultured parathyroid cells by 1,25(OH)2D333 suggests that 
1,25(OH)2D3 can directly inhibit parathyroid cell replication. 
In experimental renal failure, the action of 1,25(OH)2D3 to sup­
press the increase in parathyroid cell transforming growth factor-α 
(TGFα) and epidermal growth factor (EGF) receptor expression 
that otherwise would occur may partly explain the dampening of 
parathyroid c

may partly explain the dampening of 
parathyroid cell proliferation.34 Nevertheless, vitamin D action 
through the VDR is not essential for control of parathyroid 
cell number, because calcium alone can prevent parathyroid cell 
hyperplasia in mice engineered to lack VDRs.35 Hypocalcemia 
and experimental renal failure activate the mTORC1 complex, 
and this increased signaling by mTOR is required for the parathy­
roid cell proliferation observed.36
Although the ability to increase parathyroid ce

36
Although the ability to increase parathyroid cell number in 
response to physiologic challenge represents an important defense 
against hypocalcemia, it is a slow response that is not easily revers­
ible. When the need for an increased number of parathyroid cells 
disappears (e.g., after renal transplantation for uremia), persis­
tent hyperparathyroidism can cause vexing clinical problems for 
months and years thereafter. The mechanisms for decreasing para­
thyroid cell number are poorly unde

creasing para­
thyroid cell number are poorly understood. 
Parathyroid Gland Development
Genes involved in making parathyroid cells during development 
may also regulate PTH synthesis and parathyroid cell number 
throughout life and may be mutated in human inherited hypo­
parathyroidism37; thus an understanding of parathyroid cell 
development may have broad clinical implications. Although the 
genetic mechanisms used to generate parathyroid chief cells dur­
ing development are largely unknown, 

f cells dur­
ing development are largely unknown, the importance of several 
specific genes has become clear. Studies of gene knockout mice 
have shown that the hoxa3,38 pax1,39 pax9,40 and Eya141 tran­
scription factors are needed to form parathyroid glands as well 
as many other pharyngeal pouch derivatives, such as the thymus, 
as reviewed in Liu and associates.42 Another transcription fac­
tor, Tbx1, regulated by the developmental paracrine factor, sonic 
hedgehog, is expressed early in para

actor, sonic 
hedgehog, is expressed early in parathyroid development and is 
essential for parathyroid cell development. In humans and mice, 
haploinsufficiency for the transcription factor Tbx1 is likely to 
be responsible for many of the abnormalities found in DiGeorge 
syndrome, including hypoparathyroidism.43 Even though these 
transcription factors together are essential for the early generation 
of parathyroid cells, another transcription factor, gcm2 in mice 
and GCMB in humans, is neede

factor, gcm2 in mice 
and GCMB in humans, is needed for the continued survival of 
parathyroid cells.42 Furthermore, mice or humans44 missing the 
gcm2 and GCMB genes, respectively, have no parathyroid glands. 
In both species, the deletion of gcm2 or GCMB (the human 
equivalent) is very specific for controlling parathyroid develop­
ment because no abnormalities in other tissues have been noted. 
Studies of human hypoparathyroidism have led to the discovery 
of the likely roles of other transcri

e discovery 
of the likely roles of other transcription factors in parathyroid 
development. Another transcription factor, MafB, is depen­
dent on GcmB expression and is also important for parathyroid 
function.45 Deletion of one copy of MafB in the mouse leads to 
ectopic parathyroid glands and, in adulthood, suppression of sec­
ondary hyperparathyroidism in a renal failure model.46 Sox3 is a 
transcription factor expressed in the pharyngeal pouches that give 
rise to parathyroid cells. Humans 

ches that give 
rise to parathyroid cells. Humans with X-linked hypoparathyroid­
ism manifest a deletion-insertion near the end of the SOX3 gene, 
a finding that suggests an important role for Sox3 in parathyroid 
development.47 People with mutations in the gene encoding the 
transcription factor GATA3 exhibit a syndrome of hypoparathy­
roidism, sensorineural deafness, and renal anomalies when only 
one copy of the gene is mutated.48 GATA3 synergizes with GcmB 
and MafB in stimulating PTH gene e

izes with GcmB 
and MafB in stimulating PTH gene expression.49 
Metabolism of Parathyroid Hormone
The earliest radioimmunoassays for PTH demonstrated that the 
molecular forms of PTH in the circulation differ from those in 
the parathyroid gland. Characterization of the metabolism of 
PTH and its fragments has clarified the origins and significance 
of immunoreactive PTH (iPTH) molecules in the bloodstream.50 
As noted previously, both PTH(1-84) and carboxy-terminal frag­
ments of PTH are secret

and carboxy-terminal frag­
ments of PTH are secreted from the parathyroid gland; the ratio 
of inactive PTH to active PTH secretion increases with increasing 
blood calcium. Secreted intact PTH(1-84) is extensively metabo­
lized by liver (70%) and kidney (20%) and disappears from the 
circulation with a half-life of 2 minutes. This rapid peripheral 
metabolism of PTH is unaffected by widely varying levels of blood 
calcium or 1,25(OH)2D3. Less than 1% of the secreted hormone 
finds its way to PT

an 1% of the secreted hormone 
finds its way to PTH receptors on physiologic target organs. These 
features of PTH metabolism ensure that the blood level of PTH 
is determined principally by the activity of the parathyroid glands 
and that the PTH level can respond rapidly to small changes in 
the rate of secretion of the hormone.
In the liver, a small amount of PTH binds to physiologically 
relevant PTH receptors, but most of the intact PTH is cleaved, 
initially after residues 33 and 36, proba

leaved, 
initially after residues 33 and 36, probably by cathepsins. In the 
kidney, a small amount of intact PTH binds to physiologic PTH 
receptors, but most of the intact PTH is filtered at the glomerulus 
and subsequently bound by a large, membrane-bound luminal 
protein, megalin51; this binding leads to internalization and deg­
radation of PTH by the tubules.52 Carboxy-terminal fragments 
are also cleared efficiently by glomerular filtration. In fact, the 
kidney is the only known site of c

. In fact, the 
kidney is the only known site of clearance of carboxy-terminal 
PTH fragments; these fragments thus accumulate dramatically 
when the glomerular filtration rate (GFR) falls. Even in the pres­
ence of normal renal function, the half-life of carboxy-terminal 
fragments of PTH exceeds that of PTH(1-84) by several-fold. 
Consequently, the concentration of carboxy-terminal fragments 
in the circulation exceeds that of intact PTH, even though intact 
PTH usually is the major form of PT

though intact 
PTH usually is the major form of PTH secreted from the para­
thyroid gland.
Careful analysis of PTH fragments using high-performance 
liquid chromatography (HPLC) and immunologic methods has 
revealed almost full-length PTH fragments missing the first sev­
eral amino acids of the hormone, but containing most or all of 
the remaining hormone sequence.53 These still incompletely char­
acterized fragments are both secreted from the parathyroid gland 
and generated by peripheral metab

rathyroid gland 
and generated by peripheral metabolism of the hormone. Because 
1203
they are missing the amino-terminal portion of PTH, they cannot 
stimulate cAMP production by the PTH/PTHrP receptor, and 
except in renal failure, they circulate in small amounts. Neverthe­
less, the possible biologic activity of these and other PTH frag­
ments, possibly through novel receptors, remains an area of active 
investigation. Experiments with 

n area of active 
investigation. Experiments with PTH(7-84) suggest that such 
extended carboxy-terminal fragments may exert potent effects 
in vivo, opposing those of intact PTH (see later discussion).8,54,55 
Actions of Parathyroid Hormone
Actions of Parathyroid Hormone on the Kidney
Stimulation of Calcium Reabsorption
Almost all of the calcium in the initial glomerular filtrate is reab­
sorbed by the renal tubules. Sixty-five percent or more is reab­
sorbed by the proximal convoluted and stra

s reab­
sorbed by the proximal convoluted and straight tubules via a 
passive, paracellular route.56 Changes in the transepithelial voltage 
gradient, determined largely by the rate of sodium reabsorption, 
control the rate of calcium transport in the proximal tubule, and 
PTH does little to affect calcium flux in this region. The remain­
ing calcium is largely reabsorbed more distally—20% of the ini­
tial filtrate in the cortical thick ascending limb (cTAL) of Henle 
loop and 10% in the distal 

 limb (cTAL) of Henle 
loop and 10% in the distal convoluted and connecting tubules. 
In the cTAL, calcium reabsorption also is mainly passive and 
paracellular, although some transcellular, active calcium transport 
may occur as well. Efficient paracellular calcium and magnesium 
movement requires expression of a unique tight-junction protein, 
paracellin-1, also called claudin-16; mutant paracellin-1 genes 
underlie a rare renal calcium-wasting and magnesium-wasting 
disorder.57 Because parace

 and magnesium-wasting 
disorder.57 Because paracellular cation transport in the cTAL is 
driven by the lumen-positive transepithelial voltage gradient that 
is established by active Na-K-Cl2 reabsorption, calcium reabsorp­
tion there is strongly inhibited by loop diuretics such as furose­
mide. The CaSR, initially characterized in the parathyroid, also 
is expressed in the cTAL. When activated by high blood calcium 
or magnesium, this receptor inhibits Na-K-Cl2 reabsorption in 
the cTAL and, th

nhibits Na-K-Cl2 reabsorption in 
the cTAL and, thereby, paracellular calcium reabsorption as well. 
This provides a parathyroid-independent mechanism for control­
ling renal calcium handling in direct response to changes in blood 
calcium concentration.
Although PTH modestly stimulates paracellular calcium reab­
sorption in the cTAL, the primary site for hormonal regulation of 
renal calcium reabsorption is the distal nephron, which normally 
reabsorbs nearly all of the remaining 10% of filtere

absorbs nearly all of the remaining 10% of filtered calcium by 
a unique transcellular active transport mechanism. As depicted in 
Fig. 29.1, the intracellular level of calcium is extremely low, about 
150 nM, compared with the millimolar levels in the glomerular 
filtrate and the blood. Calcium enters distal tubular cells from 
the tubular lumen down a highly favorable electrochemical gradi­
ent via selective channels (TRPV5 and TRPV6) present on the 
apical membrane of cells in the distal conv

n the 
apical membrane of cells in the distal convoluted tubule (DCT) 
and connecting tubule (CNT). Intracellular calcium inhibits 
the activity of these channels, but this is minimized by the avid 
binding of calcium to calbindin-D28K, which effectively buffers 
cytosolic calcium and transports it to the basolateral membrane. 
There, calcium is ejected via active processes involving mainly the 
sodium-calcium exchanger NCX1 and an ATP-driven calcium 
pump (PMCA).58 PTH stimulates DCT and CNT ac

cium 
pump (PMCA).58 PTH stimulates DCT and CNT active calcium 
transport by upregulating several of these components, including 
TRPV5, calbindin-D28K, and NCX1, both directly and indi­
rectly, via increased synthesis of 1,25(OH)2D.58 PTH acutely 
increases the flux of calcium through TRPV5 by leading to protein 
kinase A (PKA) phosphorylation of a residue of TRPV5 that oth­
erwise would bind calmodulin and thereby close the channel.59
The amount of calcium in the final urine reflects all of th

t of calcium in the final urine reflects all of the tubu­
lar reabsorption processes just enumerated but also depends cru­
cially on the initial filtered load of calcium. All of PTH’s actions 
serve to raise the blood calcium level so that the filtered load 
of calcium is high in states of PTH excess. In that setting, even 
though the rate of distal tubular calcium reabsorption is increased 
by PTH, the total amount of calcium in the final urine is likely to 
be high, because of the high initial

is likely to 
be high, because of the high initial filtered load. 
Inhibition of Phosphate Transport
Phosphate reabsorption occurs mainly in the proximal renal 
tubules, which reclaim roughly 80% of the filtered load. Some 
additional phosphate (8–10%) is reabsorbed in the distal tubule 
(but not in Henle loop), leaving about 10% to 12% for excretion 
in the urine. The normal overall fractional tubular reabsorption of 
phosphate (TRP), therefore, is about 90%, although a more reli­
able measure 

 is about 90%, although a more reli­
able measure of renal phosphate handling is the phosphate thresh­
old (TmP/GFR), which can be derived from the TRP through the 
use of a nomogram (Fig. 29.7) based on studies of experimental 
phosphate infusions in healthy persons and in patients with a vari­
ety of diseases that affect phosphate excretion.60
Phosphate reabsorption in both proximal and distal tubules 
is strongly inhibited by PTH, although the proximal effect is 
quantitatively most important

 proximal effect is 
quantitatively most important. Phosphate is reabsorbed by a 
transepithelial route. Transport from the glomerular filtrate into 
the cell is mediated by specific sodium phosphate (NaPi) cotrans­
porters, several types of which have been cloned and extensively 
0.0
1.0
2.0
3.0
4.0
5.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
0.0
1.0
2.0
3.0
4.0
5.0
0.0
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Actual plasma phosphate conc. [PO4]
0.01
0.05
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80

0.01
0.05
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
0.99
0.00
1.00
0.95
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
1.00
0.00
TRP
CPO4/Ccreat
Renal threshold phosphate conc. (TmPO4/GFR)
• Fig. 29.7  Nomogram for determining renal threshold phosphate concen­
tration (TmPO4/GFR) from the plasma phosphate concentration and the 
fractional reabsorption of filtered phosphate (TRP) or fractional excretion 
of filtered phosphate (1 − TRP, or CPO4/Ccreat). Because the blood level of 
phosphate influ

reat). Because the blood level of 
phosphate influences the renal handling of phosphate, the renal thresh­
old phosphate concentration best separates normal from abnormal renal 
phosphate handling. C, clearance; creat, creatinine; GFR, glomerular filtra­
tion rate; TRP, tubular resorption of phosphate. (From Walton RJ, Bijvoet 
OLM. Nomogram of derivation of renal threshold phosphate concentra­
tion. Lancet. 1975;2:309–310.)
1204
characterized.61 The low level o

ineral Metabolism
characterized.61 The low level of sodium within the cell drives 
the cotransport of sodium and phosphate, even though the phos­
phate travels up an electrochemical gradient. In response to PTH, 
the maximum velocity (Vmax) for sodium phosphate cotransport 
decreases because NaPi cotransporters (both NaPi-IIa and NaPi-
IIc) are rapidly (in 15 minutes) sequestered within subapical 
endocytic vesicles, after which they are delivered to lysosomes and 
undergo proteolysis.62 cAMP an

d to lysosomes and 
undergo proteolysis.62 cAMP and PKA mediate the rapid decrease 
in phosphate transport in response to PTH, though PLC activa­
tion by the PTH receptor is also required for prolonged suppres­
sion of phosphate transport by PTH.63 This response to PTH is 
dependent upon Na+/H+ exchange regulatory factors (NHERFs), 
which physically interact with both the PTH/PTHrP receptor and 
the NaPi-II transporters and control the pattern of PTH receptor 
signaling.64,65 Conversely, in hypo

 PTH receptor 
signaling.64,65 Conversely, in hypoparathyroidism expression of 
NaPi protein and mRNA is strongly upregulated.
Dietary intake of phosphate also reciprocally regulates the 
expression and activity of NaPi cotransporters and thus the proxi­
mal tubular absorption of phosphate by a mechanism that is inde­
pendent of PTH. Dietary deprivation of phosphate, for example, 
leads to a stimulation of phosphate reabsorption that can override 
the effects of PTH on the proximal tubule. It is

the effects of PTH on the proximal tubule. It is likely that this 
dietary regulation of NaPi expression is mediated by FGF2366 (see 
Other Renal Effects of Parathyroid Hormone
PTH stimulates the synthesis of 1,25(OH)2D in the proximal 
tubule by rapidly inducing transcription of the 25-hydroxyvita­
min D (25[OH]D) 1α-hydroxylase gene, an effect that can be 
overridden by hypercalcemia or by 1,25(OH)2D. The interac­
tions of 1,25(OH)2D and PTH in regulating the 25(OH)D 
1α-

 1,25(OH)2D and PTH in regulating the 25(OH)D 
1α-hydroxylase gene involve both PKA-mediated phosphoryla­
tion of an activating transcription factor and protein kinase C–
mediated demethylation of DNA upstream of the 25(OH)D 
1α-hydroxylase gene.67 PTH inhibits proximal tubular transcrip­
tion of the 25(OH)D 24-hydroxylase gene and antagonizes the 
upregulation of 24-hydroxylase activity by 1,25(OH)2D. (See dis­
cussion under “Metabolism of Vitamin D.”) PTH inhibits proxi­
mal tubular sodium, wa

in D.”) PTH inhibits proxi­
mal tubular sodium, water, and bicarbonate reabsorption, mainly 
via inhibition of the apical Na+/H+ exchanger (NHE3) and the 
basolateral Na+/K+-ATPase. PTH also stimulates proximal tubu­
lar gluconeogenesis and acts directly on glomerular podocytes to 
decrease both single nephron and whole kidney GFR. 
Actions of Parathyroid Hormone on Bone
The actions of PTH on bone are complicated because PTH acts 
on a number of cell types both directly and indirectly. For years

cell types both directly and indirectly. For years, 
the release of calcium from bone through stimulation of bone 
resorption has been considered to be the major action of PTH on 
bone. This is only part of the story, however. In fact, PTH admin­
istration by any route increases both bone resorption by increasing 
osteoclast number and bone formation by increasing osteoblast 
number. Which action dominates depends on the dose of PTH 
and the route of administration. When PTH is administered con­

e of administration. When PTH is administered con­
tinuously, the effect of PTH on bone resorption dominates, and 
the net result is release of calcium from bone and a decrease in 
bone mass. This action of PTH thus contributes to the increase 
in blood calcium seen in the disease, primary hyperparathyroid­
ism. In similar fashion, administration of a soluble form of recep­
tor activator for nuclear factor κB (RANK) ligand (RANKL), a 
major mediator of PTH’s increase in osteoclastic bone resorp­

tor of PTH’s increase in osteoclastic bone resorp­
tion (Fig. 29.8), also increases osteoclast number. But RANKL 
administration causes even lower bone mass than PTH because 
continuously administered PTH increases osteoblast number 
more vigorously than RANKL.68 This finding suggests that one 
teleologic reason for PTH to increase bone formation is that this 
separate action of PTH might help preserve bone mass during 
prolonged requirements for PTH action.
PTH Increases Bone Formation
Administ

 PTH action.
Administration of low doses of PTH or active amino-terminal 
fragments of PTH by once-daily subcutaneous injection leads to a 
net increase in bone mass, with only transient effects on blood cal­
cium. Mechanisms for these divergent effects of PTH are incom­
pletely understood but certainly reflect the variety of cell types in 
bone that respond directly to PTH, the varying time courses of 
these responses, and the indirect effects of PTH caused by auto­

s, and the indirect effects of PTH caused by auto­
crine and paracrine responses to PTH.69
Fig. 29.9 illustrates the cells of the osteoblast lineage (see also 
Chapter 30). Osteoblasts are probably derived from pluripotent 
mesenchymal stem cells that can differentiate into chondrocytes, 
adipocytes, osteoblasts, and other cell types.70,71 Perivascular cells 
within human bone can reconstitute bone that supports hemato­
poiesis after subcutaneous transplantation in mice72; these cells 
may well 

 transplantation in mice72; these cells 
may well represent one group of osteoblast stem cells in  vivo. 
Within the osteoblast lineage, committed osteoprogenitor cells 
divide, become preosteoblastic stromal cells (which can divide 
further), and then become osteoblasts. Osteoblasts no longer 
divide and are cuboidal cells found on the bone surface actively 
laying down new bone. These cells can surround themselves with 
newly formed bone matrix and extend large numbers of dendritic 
processes,

 and extend large numbers of dendritic 
processes, becoming osteocytes. Alternatively, osteoblasts can stop 
synthesizing matrix and remain on the bone surface as bone lin­
ing cells. Not all preosteoblasts and osteoblasts mature; a variable 
number die by apoptotic, programmed cell death.73
PTH administration can influence movement of cells through 
the osteoblast lineage.69 PTH administration in  vivo, whether 
administered continuously or intermittently, increases osteoblast 
surface and numb

rmittently, increases osteoblast 
surface and number and bone formation rate. PTH adminis­
tration increases the number and rate of differentiation of early 
osteoblast precursors in a murine lineage-tracing model.74 With 
Stromal/osteoblast
M-CSF
RANK
ligand
RANK
RANK
OPG
PTH
Osteoclast
precursor
• Fig. 29.8  Osteoblast lineage cell control of osteoclastogenesis and 
osteoclast activity. Parathyroid hormone (PTH) acts on PTH/PTH-related 
protein (PTHrP) receptors on precursors of osteoblasts to

 (PTHrP) receptors on precursors of osteoblasts to increase the 
production of macrophage colony-stimulating factor (M-CSF) and recep­
tor activator for nuclear factor κB (RANK) ligand and to decrease the pro­
duction of osteoprotegerin (OPG). M-CSF and RANK ligand stimulate the 
production of osteoclasts and increase the activity of mature osteoclasts 
by binding to the receptor RANK. OPG blocks the interaction of RANK 
ligand and RANK.
1205
Chapter 29  Hormones and Disorders of Mineral Metabol

pter 29  Hormones and Disorders of Mineral Metabolism
intermittent PTH administration to rats, the number of bone lin­
ing cells decreases as the number of active osteoblasts increases75: 
Subsequent studies using genetically marked mice showed that 
PTH converts bone lining cells to active osteoblasts.76 Further, 
the decreased rate of osteoblast apoptosis after intermittent PTH 
administration leads to an increase in the number of osteoblasts.73
When PTH is administered continuously to rats, m

3
When PTH is administered continuously to rats, mimicking 
the findings in primary hyperparathyroidism, large numbers of 
proliferating, alkaline phosphatase–positive fibroblastic cells accu­
mulate in the marrow, perhaps mimicking the findings of osteitis 
fibrosa in primary hyperparathyroidism. When the infusion of 
PTH was stopped, the fibroblastic cells disappeared and recently 
proliferated osteoblasts appeared, suggesting that many of the 
fibroblastic cells were osteoblast precursors.77

In addition to changing osteoblast numbers, PTH changes the 
activity of mature osteoblasts by a variety of mechanisms. When 
PTH is added to calvariae in vitro, the osteoblasts decrease their 
synthesis of collagen I and other matrix proteins. This action may 
reflect, in part, the action of PTH to steer the essential osteoblast 
transcription factor Runx2 toward proteosomal destruction.78 
In vivo, however, the most obvious effects of PTH are t

vo, however, the most obvious effects of PTH are to increase 
bone formation by osteoblasts, probably by indirect actions of 
PTH on autocrine and paracrine pathways. PTH stimulation of 
osteoblastic cells leads to release of growth factors such as IGF1, 
FGF2, and amphiregulin from these cells.79 PTH also decreases 
the synthesis of dickkopf-180 and sclerostin,81 inhibitors of Wnt 
signaling82; these actions are expected to increase the anabolic 
actions of Wnt proteins on osteoblasts. Sclerost

actions of Wnt proteins on osteoblasts. Sclerostin is particu­
larly synthesized by osteocytes, thus these cells respond to PTH 
to regulate the activity of nearby osteoblasts. Further, because 
bone matrix is a rich source of osteoblast growth factors, the 
release of these growth factors from this matrix following PTH-
induced bone resorption may increase bone formation and bring 
osteoblastic cells to sites of bone formation.83 Thus a variety of 
both direct and indirect actions of PTH can 

y of 
both direct and indirect actions of PTH can lead to the increased 
production of bone. 
PTH Increases Bone Resorption
Surprisingly, osteoclasts, the bone-resorbing cells derived from 
hematopoietic precursors, have no PTH receptors on their sur­
faces. Instead, cells of the osteoblast lineage, including preosteo­
blasts, osteoblasts, and osteocytes, signal to osteoclast precursors 
to cause them to fuse and form mature osteoclasts. This signaling 
also serves to stimulate mature osteoclast

naling 
also serves to stimulate mature osteoclasts to resorb bone and to 
avoid apoptosis (see Fig. 29.8). Two cell surface proteins, mac­
rophage colony-stimulating factor (M-CSF) and RANKL, are 
essential for stimulation of osteoclastogenesis,84 and RANKL is 
essential for the activation of mature osteoclasts. The growth factor 
M-CSF (or CSF1) is expressed both as a secreted protein and as a 
cell surface protein; the production of both forms is stimulated by 
PTH.85 RANKL—also named osteopr

 is stimulated by 
PTH.85 RANKL—also named osteoprotegerin ligand (OPGL), 
osteoclast-differentiating factor (ODF), and TRANCE—is a 
membrane-bound member of the tumor necrosis factor (TNF) 
family; its synthesis is also increased by PTH. RANKL binds to its 
receptor, RANK, a member of the TNF receptor family. RANK 
is found both on osteoclast precursors and on mature osteoclasts. 
The binding of RANKL to RANK can be blocked by osteopro­
tegerin (OPG), another member of the TNF receptor family. 

(OPG), another member of the TNF receptor family. 
OPG (also called OCIF and TR1) circulates and is also secreted 
by cells of the osteoblastic lineage. PTH decreases the synthesis 
and secretion of OPG from these cells. Thus PTH, by increasing 
RANK and decreasing OPG locally in bone, serves to increase 
bone resorption.
Activation of PTH receptors also causes release of calcium from 
bone by a less understood mechanism called osteocytic osteolysis. 
Osteocytes can directly release mineral from

sis. 
Osteocytes can directly release mineral from the matrix immedi­
ately surrounding them. These cells produce proteins associated 
with bone resorption by osteoclasts, including carbonic anhydrase 
2, cathepsin K, and tartrate-resistant acid phosphatase.86 For 
example, during lactation in mice, lacunae surrounding osteo­
cytes release calcium; this release depends upon those osteocytes 
expressing PTH/PTHrP receptors.87 Little is known about the 
quantitative importance of osteocytic osteol

 the 
quantitative importance of osteocytic osteolysis versus osteoclastic 
bone resorption in various settings. 
Molecular Basis of Parathyroid Hormone Action
Ever since the discovery that PTH stimulates the secretion of 
cAMP into the urine,88 PTH has been thought to act by trig­
gering a cascade of intracellular second messengers. This guiding 
hypothesis, in its current form, postulates that all of the actions of 
PTH result from the binding of the hormone to a receptor on the 
plasma membra

of the hormone to a receptor on the 
plasma membrane of target tissues. This receptor is a member of 
a large family of G protein–linked receptors that span the plasma 
membrane seven times (Fig. 29.10). The binding of hormone on 
the outside of the membrane causes conformational changes in 
the disposition of the seven transmembrane helices that activate 
the receptor’s ability to release guanosine diphosphate (GDP) 
from the α-subunit of a G protein bound to the receptor. The 
G protein then b

otein bound to the receptor. The 
G protein then binds guanosine triphosphate (GTP) in place of 
GDP. The GTP-binding α-subunit of the G protein then sepa­
rates from the βγ-subunits, and the separate subunits of the G 
protein then modulate the activity of enzymes and channels. The 
activity of these enzymes and channels then affects proteins far­
ther downstream, eventually leading to the physiologic responses 
of bone and kidney cells.
Mesenchymal
stem cell
Adipocyte
progenitor
Osteoprogenito

ymal
stem cell
Adipocyte
Osteoprogenitor
Osteocyte
Lining cell
Pre-osteoblast
Osteoblast
• Fig. 29.9  Osteoblast lineage. All precursors of osteoblasts can prolif­
erate; osteoblasts are transformed to osteocytes and lining cells without 
further proliferation. Lining cells can revert to osteoblast function after 
parathyroid hormone stimulation. At each stage in the lineage, apoptotic 
cell death is probably an alternative fate.
1206
SECTION VII    Mineral Meta

alternative fate.
1206
Parathyroid Hormone and Parathyroid Hormone–Related 
Protein Receptors
DNA encoding a PTH/PTHrP receptor has been isolated from 
tissues of many species, including rat, opossum, human, pig, 
Xenopus (toad), and zebrafish,89 and even some insects.90 The pre­
dicted amino acid sequence of the receptor and direct mapping of 
inserted epitopes suggest that the receptor spans the plasma mem­
brane seven times, but the sequence does not closely 

ne seven times, but the sequence does not closely resemble the 
sequences of most known G protein–linked receptors. Instead it 
is a member of a distinct subfamily of closely related receptors 
called family B. Most of these receptors bind peptides of 30 to 40 
amino acids in length. Known members include receptors for the 
secretin family of peptides (secretin, vasoactive intestinal peptide 
[VIP], glucagon, glucagon-like peptide, growth hormone–releas­
ing hormone [GHRH], pituitary adenylate c

–releas­
ing hormone [GHRH], pituitary adenylate cyclase–activating 
peptide, gastric inhibitory peptide), corticotropin-releasing hor­
mone (CRH), calcitonin, and insect diuretic hormones related to 
CRH. The PTH/PTHrP receptor most closely resembles recep­
tors of the secretin group. The gene encoding the PTH/PTHrP 
receptor has a complicated structure, with 13 introns interrupting 
the coding sequence.
The PTH/PTHrP receptor mediates many of the actions of 
both PTH and PTHrP. The cloned PTH/

he actions of 
both PTH and PTHrP. The cloned PTH/PTHrP receptor binds 
amino-terminal fragments of PTH and PTHrP with equal affin­
ity. The receptor is expressed at high levels in kidney and in osteo­
blasts of bone but is also expressed in a wide variety of tissues, such 
as smooth muscle, brain, and a variety of fetal tissues, which are 
thought to be target tissues more for PTHrP than for PTH. When 
the PTH/PTHrP receptor is ablated in mice, the mutant mice 
show defects both in calcium home

the mutant mice 
show defects both in calcium homeostasis and in local actions 
of PTHrP on the growth plates of bone, for example.91 Muta­
tions in the receptor found in humans are also consistent with 
this idea (see upcoming discussion). Nevertheless, the scheme of 
PTH action illustrated in Fig. 29.10 should be considered a sim­
plified outline. It is unlikely that all of the actions of PTH can be 
explained by interactions with the cloned PTH/PTHrP receptor: 
Fragments of PTH that seem not 

H/PTHrP receptor: 
Fragments of PTH that seem not to bind the receptor may be 
biologically active.92 Furthermore, the carboxy-terminal portion 
of PTH(1-84) binds a cell surface protein distinct from the PTH/
PTHrP receptor.
A second PTH receptor, which can be activated by PTH but 
not by PTHrP, called the PTH2 receptor (PTH2R), has been 
cloned. This receptor is expressed in multiple tissues, including 
brain, vascular endothelium and smooth muscle, endocrine cells 
of the gastrointestinal tra

scle, endocrine cells 
of the gastrointestinal tract, and sperm. Expression is not seen in 
osteoblasts or renal tubules, however. Although PTH activates the 
human PTH2R well, PTH only poorly activates the PTH2R in 
rats and other species. Furthermore, a novel ligand called TIP39 
(tubero-infundibular peptide of 39 residues) has been character­
ized and shown to be a potent activator of PTH2R. TIP39 bears 
only a weak resemblance to PTH or PTHrP and is likely to be 
a physiologically relevant a

 and is likely to be 
a physiologically relevant activator of the PTH2. The functional 
role of the PTH2R is unknown, but it appears to mediate many 
actions of TIP39 in the brain and testis; studies of knockout mice 
implicate TIP39 in regulation of responses to stress93 and germ 
cell development.94 The two cloned PTHRs, as well as distinct 
receptors for fragments of PTHrP (see later), probably are part of 
a complex network of ligands and receptors (Fig. 29.11). 
Functional Implications of P

eptors (Fig. 29.11). 
Functional Implications of Parathyroid Hormone Structure
Two important advances have helped organize thinking about 
how PTH activates the PTH/PTHrP receptor. First, it is now 
clear that all G protein–coupled receptors have structural flex­
ibility that allows them to interact with ligands through the use 
of multiple conformations.95 These multiple conformations can 
bind closely related ligands differently and can lead to differing 
signaling consequences. Second, high-r

 differing 
signaling consequences. Second, high-resolution structures of 
several family B receptors are now available, in the presence of 
agonists, antagonists, and G proteins. These structures are con­
sistent with much prior functional work and serve to rationalize 
many results.96 They all exhibit a large amino-terminal extracel­
lular domain that binds the carboxy-terminal ends of their respec­
tive ligands. The crystal structure of this extracellular domain of 
the PTH/PTHrP receptor bin

bound to G protein (Gs) α-subunits. Gs α-subunits bound to GTP 
are released from the receptor and from the βγ-subunits and then activate 
effectors. Gsα activates adenylate cyclase (AC), leading to the formation 
of cyclic adenosine monophosphate (cAMP), which then activates pro­
tein kinase A (PKA). Gqα and related α-subunits activate phospholipase C 
(PLC). PLC hydrolyzes phosphatidylinositol 1,4,5-trisphosphate to gener­
ate diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3). The DA

AG) and inositol 1,4,5-trisphosphate (IP3). The DAG 
then activates protein kinase C (PKC), and the IP3 activates a receptor 
on microsomal vesicles that directs the movement of calcium (CAi) from 
microsomal vesicles into the cytosol.
R
R
R
TIP 39
Actions on
placenta
Actions on
osteoclasts
PTH 2
Receptor
PTH/PTHrP
Receptor
PTH
PTHrP
• Fig. 29.11  Network of parathyroid hormone (PTH) ligands and receptors 
(R). PTH and PTH-related protein (PTHrP) closely resemble each other at 
the amino-termina

 closely resemble each other at 
the amino-terminal region; TIP 39 (tubero-infundibular peptide of 39 resi­
dues) is more distantly related. Although only the PTH/PTHrP receptor and 
the PTH2 receptors have been cloned, biologic actions suggest that there 
are receptors specific for the carboxy-terminal portion of PTH, as well as 
distinct receptors for the midregion of PTHrP and for a more distal region 
of PTHrP. PTHrP is also found in nuclei and may act directly there.
1207
Chapter 29  Hormon

nd may act directly there.
1207
The structures of the characterized family B receptors show how 
this binding of the carboxy-terminal portion of the ligands directs 
the amino-terminal portions of the ligands to interact with the 
region of the receptor that consists of seven transmembrane 
domains and associated extracellular loops (Fig. 29.12). The 
amino-terminal portion of the ligand changes the conformation of 
the sixth transmembrane

anges the conformation of 
the sixth transmembrane domain to allow interaction of the recep­
tor with G proteins to be activated. Amino-terminal fragments 
of PTH as short as PTH(1-34) have potency at least as great as 
that of the full-length PTH(1-84).89 The first several residues of 
PTH are particularly important for triggering the conformational 
change in the receptor that results in activation of Gs and adenyl­
ate cyclase. Sequences responsible for transmembrane activation 
of Gs make up

nsible for transmembrane activation 
of Gs make up most of the first 13 residues of PTH; these are the 
residues that are highly conserved between PTH and PTHrP. At 
high concentrations, PTH(1-14) by itself can activate the PTH/
PTHrP receptor. This activation domain interacts with the recep­
tor’s transmembrane domains and extracellular loops. When the 
first nine residues of PTH are covalently linked to the receptor’s 
transmembrane domains and extracellular loops, they can activate 
the recep

 extracellular loops, they can activate 
the receptor. An analogue of PTH(1-14) can also trigger Gq acti­
vation, thus activating PLC.99 These data, plus the observation 
that a PTH analogue modified at position 1 selectively loses its 
ability to activate PLC,100 demonstrate that the amino-terminal 
portion of PTH is essential for activation of both Gs and Gq. More 
distal regions of PTH(1-34) can activate protein kinase C and 
can raise intracellular calcium levels by mechanisms that have not 

llular calcium levels by mechanisms that have not 
been fully clarified. 
Activation of Second Messengers
Precisely how binding of PTH to the extracellular domains of 
the PTH/PTHrP receptor leads to activation of G proteins is not 
understood. The crystal structures of the transmembrane portions 
of other family B receptors resemble those of the more extensively 
studied rhodopsin family A receptors, but differ in that the poten­
tial pocket expected to bind the peptide ligands is larger, form­

ected to bind the peptide ligands is larger, form­
ing a V when seen from the side, with transmembrane domains 
(TMDs) 1, 6, and 7 forming one arm of the V and TMDs 2 
to 5 forming the other. This proposed structure, with TMD 1 
adjacent to TMDs 6 and 7, is consistent with the behavior of 
certain mutant PTH/PTHrP receptors.101,102 Presumably, bind­
ing of PTH to several different regions of the receptor changes 
the relationships of the transmembrane domains103 such that the 
receptor’s three i

brane domains103 such that the 
receptor’s three intracellular loops and carboxy-terminal tail inter­
act with G proteins in an altered way.
Receptors with certain point mutations in the second, sixth, 
and seventh transmembrane domains can activate Gs even with­
out stimulation by hormone. These mutant receptors were dis­
covered by analyzing the PTH/PTHrP receptors in patients 
with Jansen metaphyseal chondrodystrophy.104 Patients with 
this disorder have signs of parathyroid overactivity (hyp

sorder have signs of parathyroid overactivity (hypercalce­
mia, hypophosphatemia, and high levels of 1,25[OH]2D3 and 
urinary cAMP) but low PTH and PTHrP levels. These patients 
have growth abnormalities, presumably reflecting inappropriately 
vigorous PTHrP-like actions on growth cartilage. The mutations, 
all near the bottom of the predicted V-shaped structure of the 
transmembrane domains,105 must change the conformation of the 
intracellular portion of the receptor in a way that resembles th

portion of the receptor in a way that resembles the 
effect of binding of PTH to the normal receptor. The observation 
that inappropriate activation of the PTH/PTHrP receptor in Jan­
sen chondrodystrophy leads to all of the metabolic abnormalities 
found in primary hyperparathyroidism is one of the most per­
suasive pieces of evidence that the cloned PTH/PTHrP receptor 
does, in fact, mediate the actions of PTH in bone and kidney in 
humans in vivo.
PTH and PTHrP can both bind to and activate th

vo.
PTH and PTHrP can both bind to and activate the PTH/
PTHrP receptor on the surface of target cells. The fate of the 
ligand-receptor complexes differs dramatically subsequently, how­
ever.106–108 PTH, but not PTHrP, is internalized in vesicles, along 
with the PTH/PTHrP receptor, β-arrestin, Gαs, and adenylate 
cyclase. Remarkably, within these vesicles, PTH can signal for 
longer than PTHrP, which loses its signaling properties when it 
is internalized. Certain PTH analogs can signal for ho

nternalized. Certain PTH analogs can signal for hours because 
of their intracellular stabilization and signaling. The tight and 
acid-resistant binding of PTH to its receptor probably explains 
the differing properties of PTH and PTHrP, but this distinction 
and its full functional significance are current topics of active 
investigation. 
Second Messengers and Distal Effects of Parathyroid 
Hormone
The activation of multiple G proteins by PTH raises questions 
about the individual roles of eac

aises questions 
about the individual roles of each second messenger and their 
possible interactions. The importance of cAMP as a mediator of 
the physiologic actions of PTH has been demonstrated by studies 
in vivo88 and in vitro.109 Furthermore, patients with pseudohypo­
parathyroidism type I, who cannot increase urinary cAMP levels 
in response to PTH, show clear renal resistance to PTH (see later).
Activation of PLC, with concomitant activation of protein 
kinase C and synthesis of IP3, may

ion of protein 
kinase C and synthesis of IP3, may contribute to physiologic actions 
of PTH as well, such as inhibition of sodium-phosphate cotrans­
port110 and stimulation of the renal 25(OH)D 1α-hydroxylase.111 
Mice with mutant PTH/PTHrP receptors that cannot activate 
PLC have a mild delay in bone development112; growth plate 
abnormalities manifest when Gs is also defective in the growth 
plates,113 and abnormalities of phosphate handling by the kidney 
N
N
J
• Fig. 29.12  Binding of PTH(1

y the kidney 
N
N
J
• Fig. 29.12  Binding of PTH(1-34) peptide of the parathyroid hormone 
(PTH) to the PTH/PTH-related protein (PTHrP) receptor. The amino-termi­
nal (N) extracellular domain of the receptor (in green) binds rapidly to the 
carboxy-terminal portion of the ligand. The J domain of the receptor, con­
taining the transmembrane domains and associated loops, binds to the 
amino-terminal domain of the ligand (red cylinder marked N). This binding 
is slower and may require conformationa

s binding 
is slower and may require conformational changes in both the ligand and 
receptor. These conformational changes then trigger G protein activation, 
receptor internalization, and other actions. (Courtesy Tom Gardella.)
1208
occur when challenged with a low-calcium diet.114 When these 
mutant mice are challenged with infusions of PTH peptides, their 
phosphate response is normal at first, but a defect in phosphate 
handling occurs during prolonged infus

 phosphate 
handling occurs during prolonged infusion.63 When these same 
mutant mice are infused with high levels of PTH, they also exhibit 
a defect in generation of the expected accumulation of stromal 
cells in the marrow (osteitis fibrosa) that normal mice exhibit after 
prolonged PTH infusion.115 Thus the actions of PTH on bone 
and kidney that require PLC activation are most clearly seen with 
high levels of PTH sustained for some time.
The stimulation of one G protein or another by the P

e stimulation of one G protein or another by the PTH/
PTHrP receptor can vary in different types of cells and even in dif­
fering regions of the same cell.110 In some settings, this choice may 
be influenced by the interactions of the PTH/PTHrP receptor with 
intracellular scaffolding proteins, such as NHERF1 and NHERF2 
(Na+/H+ exchanger regulatory factor). Binding of the PTH/PTHrP 
receptor to NHERFs is directed by the last four amino acids in the 
receptor sequence. This binding, particularly

the 
receptor sequence. This binding, particularly prominent at the api­
cal surface of the proximal tubular cells of the kidney, for example, 
may change the G protein activated by the PTH/PTHrP receptor 
from predominantly Gs to predominantly Gi.116 
Target Cell Responsiveness to Parathyroid Hormone
Physiologic responses to PTH depend not only on the concentra­
tion of PTH in blood but also on the responsiveness of target cells 
to PTH. This responsiveness can be modified by previous expo­
sur

sponsiveness can be modified by previous expo­
sure to PTH or by exposure to a variety of other hormones and 
paracrine factors. Responsiveness can be changed by alterations at 
virtually every step in the cellular response to PTH.
Major regulators of PTH/PTHrP receptor gene expression 
include, not surprisingly, PTH and 1,25(OH)2D3, both of 
which can decrease PTH/PTHrP receptor mRNA in certain 
target cells. In some settings, PTH decreases the amount of 
immunoreactive and functional receptor 

amount of 
immunoreactive and functional receptor on the cell surface 
without changing the levels of PTH/PTHrP mRNA. This 
decrease reflects ligand-induced internalization and degra­
dation of receptors. Internalization of receptor is stimulated 
by PTH binding, which leads to phosphorylation of specific 
serines found in the receptor’s cytoplasmic tail and subse­
quent internalization directed by binding of β-arrestin to the 
receptor.117,118 
Parathyroid Hormone–Related Protein
PTHrP was disc

PTHrP was discovered because the secretion of PTHrP by a 
wide variety of tumors contributes to the humoral hypercalcemia 
of malignancy. For this reason, the initial studies of PTHrP in 
humans and animals stressed the PTH-like structure and proper­
ties of the molecule. Subsequent studies soon showed, however, 
that PTHrP, unlike PTH, is made by a wide variety of tissues, in 
which it acts locally in ways that may have little relevance to the 
control of blo

t may have little relevance to the 
control of blood calcium.
Gene and Protein Structure
PTHrP sequences in several species, ranging from fish to humans, 
have been identified119,120 (Fig. 29.13). In humans, alternative 
RNA splicing yields transcripts that encode three distinct proteins 
of 139, 141, and 173 residues that differ only after residue 139.
• Fig. 29.13  Sequences of parathyroid hormone–related protein (PTHrP) from five species. Completely 
conserved residues are in boldface; note t

letely 
conserved residues are in boldface; note the high level of conservation through residue 111. Arrows indi­
cate sites of internal cleavage after residues 37 and 95, which lead to generation of PTHrP(38-94) amide 
and PTHrP(38-95). Another site of cleavage, generating PTHrP(38-101) and, perhaps, PTHrP(107-139), 
is not shown.120 The three human sequences represent proteins synthesized from alternatively spliced 
messenger RNAs and differ only after residue 139. Amino acids are indicated by

ly after residue 139. Amino acids are indicated by the single-letter code 
(see Fig. 29.3).
1209
Inspection of these sequences suggests that PTHrP has several 
functionally distinct domains. Eight or nine of the first 13 resi­
dues of PTHrP are identical to those in known mammalian PTH 
sequences. These sequences encompass the known “activation” 
domain of PTH that interacts with the receptor’s transmembrane 
domain region (see earlier) an

or’s transmembrane 
domain region (see earlier) and are instrumental in the ability of 
PTHrP to activate PTH/PTHrP receptors.
The sequences in PTHrP(14-34) are also highly conserved. 
Although these sequences little resemble the corresponding region 
of PTH, they can displace PTH from the PTH/PTHrP receptor 
and, in the crystal structure of the amino-terminal domain of the 
PTH/PTHrP receptor, bind to overlapping but distinct contact 
sites.98
The remaining portion of the PTHrP molecule bears n

he remaining portion of the PTHrP molecule bears no 
resemblance to corresponding sequences in PTH. Nevertheless, 
residues 35 to 111 of PTHrP are strikingly well conserved, with 
only nine residues differing between mammalian and chicken 
PTHrP sequences. This sequence conservation is considerably 
greater than that found in the carboxy-terminal portion of PTH, 
suggesting that this region of PTHrP has unique and important 
functions. This region also includes a nuclear localization signal 
tha

n also includes a nuclear localization signal 
that has been demonstrated to be functional in cultured cells.121 
After residue 111, the PTHrP sequences vary considerably from 
species to species.
Interspersed within the PTHrP sequences are multiple sites 
containing one or several basic residues that might serve as post-
translational cleavage sites (see Fig. 29.13). Extensive analysis of 
PTHrP fragments in tumors, cell lines, and transfected cells has 
shown that several of these sites are, i

ells has 
shown that several of these sites are, in fact, functional cleavage sig­
nals. PTHrP is cleaved after the arginine at residue 37; this cleav­
age, followed by carboxypeptidase cleavage, generates a PTH-like 
PTHrP(1-36) fragment as well as the fragments PTHrP(38-94) 
amide, PTHrP(38-95), and PTHrP(38-101).122 More carboxy-
terminal fragments of PTHrP have been detected in cells as well. 
Fragments such as PTHrP(107-139) and PTHrP(107-111) can 
trigger intracellular kinase cascades and 

1) can 
trigger intracellular kinase cascades and affect bone mass in vivo,123 
though the physiologic roles of any of the carboxy-terminal frag­
ments of PTHrP have not yet been established.
In the blood of patients with humoral hypercalcemia of malig­
nancy, multiple immunoreactive species of PTHrP have been 
found that may well correspond to the fragments of PTHrP in 
cells and tissue culture media, although precise characterization 
of these various immunoreactive species is incomplete (see 

various immunoreactive species is incomplete (see later). 
Full-length PTHrP may well not circulate, because an amino-
terminal–specific immunoaffinity column was unable to extract 
carboxy-terminal immunoreactivity from the serum of patients 
with malignant hypercalcemia.124 
Functions of Parathyroid Hormone–Related Protein
The first actions of PTHrP to be defined were the PTH-like 
actions associated with the humoral hypercalcemia of malignancy. 
In this pathologic entity, PTHrP acts as a horm

. 
In this pathologic entity, PTHrP acts as a hormone; it is secreted 
from the tumor into the bloodstream and then acts on bone and 
kidney to raise calcium level (see discussion under “Hypercalce­
mia of Malignancy” later).125 Whether or not PTHrP circulates 
at high enough levels in normal adults to contribute to normal 
calcium homeostasis is an unanswered question. With metasta­
ses of breast cancer to bone, locally produced PTHrP can raise 
serum calcium without necessarily raising blood l

serum calcium without necessarily raising blood levels of PTHrP. 
PTHrP expression by tumors also contributes to the cachexia seen 
in cancer patients and stimulates the “browning” of adipocytes in 
tumor models.126
PTHrP acts as a calciotropic hormone during fetal life and 
in lactation. Fetal mice missing the PTHrP gene transport cal­
cium-45 (45Ca) across the placenta inefficiently. This action 
of PTHrP requires only the midregion of PTHrP and prob­
ably involves a receptor distinct from th

nd prob­
ably involves a receptor distinct from the PTH/PTHrP receptor. 
Amino-terminal portions of PTHrP and PTH may also be able 
to increase placental calcium transport, because PTH(1-84) can 
also increase placental calcium transport in mice missing the PTH 
gene.127
The second setting for humoral actions of PTHrP is lactation. 
In mice, secretion of PTHrP from the breast into the bloodstream 
leads to an increase in bone resorption.128 Calcium then activates 
the CaSR in breast tissue, incr

um then activates 
the CaSR in breast tissue, increases the movement of calcium into 
milk, and downregulates expression of PTHrP in the breast.129 
PTHrP therefore probably contributes to the dramatic but largely 
reversible bone loss during lactation in humans, which is only 
minimally affected by calcium supplementation.130 The bone loss 
is caused both by an increase in osteoclastic bone resorption and 
by activities of osteocytes to resorb the matrix that surrounds them 
that require activa

he matrix that surrounds them 
that require activation of the PTH/PTHrP receptor (osteocytic 
osteolysis).87 An exaggeration of this lactational role of PTHrP 
may explain the rare presentation of hypercalcemia and high 
PTHrP levels in pregnant and lactating women.131 Large amounts 
of PTHrP are also secreted into breast milk, although the role of 
PTHrP in milk is unknown.
Most of the actions of PTHrP are likely to be paracrine or 
autocrine.132 PTHrP is synthesized at one time or another dur­

2 PTHrP is synthesized at one time or another dur­
ing fetal life in virtually every tissue. Its role in the develop­
ment of fetal bone has been demonstrated through the striking 
abnormalities found in genetically engineered mice missing the 
PTHrP gene. These abnormalities suggest that PTHrP nor­
mally keeps chondrocytes proliferating in orderly columns, 
thereby delaying chondrocyte differentiation.133 PTHrP blocks 
the actions of Mef2 and Runx2 transcription factors to activate 
chondrocyte

nx2 transcription factors to activate 
chondrocyte differentiation by driving class IIa histone deacety­
lases into the nucleus.134 The role of PTHrP in many other fetal 
tissues may analogously involve regulation of proliferation and 
differentiation. The widespread expression of the PTHrP in fetal 
life probably underlies the expression of PTHrP in a wide variety 
of malignancies. As is often the case in malignancy, the expres­
sion of PTHrP represents the reinitiation of a fetal pattern of 
g

presents the reinitiation of a fetal pattern of 
PTHrP is synthesized by many adult tissues. In tissues such as 
skin, hair, and breast, it is likely that PTHrP regulates cell prolif­
eration and differentiation. PTHrP is also synthesized in response 
to stretch in the smooth muscle of blood vessels and of the gastro­
intestinal tract, uterus, and bladder and acts in an autocrine fash­
ion to relax the smooth muscle.135 PTHrP is also widely expressed 
in neurons of the central n

also widely expressed 
in neurons of the central nervous system. Its function in the brain 
is unknown, but it may protect neurons from excitotoxicity by 
decreasing flux through voltage-gated calcium channels. An analo­
gous mechanism may explain the role of PTHrP in smooth mus­
cle relaxation.
Many of the actions of PTHrP are mediated by the PTH/
PTHrP receptor. Others, such as the activation of placental cal­
cium transport, are probably mediated in part by a distinct recep­
tor, and other ac

ted in part by a distinct recep­
tor, and other actions on bone cells probably involve yet another 
receptor responsive to more distal portions of PTHrP. Increas­
ing evidence suggests, furthermore, that some actions of PTHrP 
involve direct nuclear actions of PTHrP.136 Thus both PTH and 
PTHrP are likely to use multiple mechanisms to stimulate cells 
(see Fig. 29.11). 
1210
Calcitonin has an important role in regulating blood calcium in 
fish and a d

 role in regulating blood calcium in 
fish and a demonstrable role in rodents; however, the importance 
of calcitonin in human calcium homeostasis remains uncertain.
The existence of a second calcium-regulating hormone, in 
addition to PTH, was first demonstrated during perfusion 
studies of the thyroid/parathyroid glands of dogs.137 High cal­
cium perfusion resulted in a rapid decrease in plasma calcium, 
even more rapid than after parathyroidectomy. This suggested 
that calcium had stimulated 

tomy. This suggested 
that calcium had stimulated the secretion of a hormone that 
lowered blood calcium. It was subsequently demonstrated that 
this missing hormone, named calcitonin for its role in regulat­
ing the “tone” or level of calcium, was elaborated by the thyroid 
gland, not the parathyroids. Calcitonin is found in the nonfol­
licular cells of the thyroid, called C cells, which originate from 
the neural crest.138 In fish, the location of the C cells in dis­
crete organs led to the ra

 of the C cells in dis­
crete organs led to the rapid isolation of calcitonin from these 
ultimobranchial bodies in dogfish, salmon, and several other 
species. The identification of the glandular origin of calcitonin 
enabled the isolation of sufficient quantities of calcitonin for 
sequence analysis139 and studies of its structure and biologic 
function.
Synthesis and Secretion
Calcitonin consists of a 32–amino acid polypeptide with an 
intrachain disulfide bond provided by the cysteines at po

ain disulfide bond provided by the cysteines at positions 
1 and 7 (Fig. 29.14). These two cysteine residues, along with the 
carboxy-terminal proline amide and six additional residues, are 
the only amino acids conserved among the calcitonins isolated 
from various species. The disulfide linkage and proline amide 
residues are important for the function of the molecule, although 
biologically active analogues lacking disulfide bonds have been 
developed. Interestingly, fish calcitonin is more p

eveloped. Interestingly, fish calcitonin is more potent in mam­
mals than is the mammalian hormone. The mature peptide is 
derived from the middle of a 136–amino acid precursor. The 
human calcitonin gene, located on the short arm of chromo­
some 11, contains 6 exons, which are alternatively spliced in a 
tissue-specific manner to yield the mRNAs encoding calcitonin 
or calcitonin gene-related peptide (CGRP) (see Fig. 29.14). The 
mRNA encoding calcitonin is derived by splicing together the 
fir

alcitonin is derived by splicing together the 
first four exons140 and represents over 95% of mature transcripts 
in the thyroid C cells. The splicing of the first three exons to 
exons 5 and 6 results in an mRNA that encodes the 37–amino 
acid αCGRP peptide. The mRNA encoding αCGRP is expressed 
in multiple tissues and is the only mature transcript of the cal­
citonin gene detected in neural tissue. A second CGRP gene 
encodes the closely related βCGRP. In humans, the predicted 
sequence of the

d βCGRP. In humans, the predicted 
sequence of the mature peptide differs from that of αCGRP by 
only three amino acids (see Fig. 29.14). The βCGRP gene is also 
found on chromosome 11; its tissue distribution is the same as 
that of αCGRP.
The synthesis and secretion of calcitonin are tightly regulated. 
Studies in a porcine model reveal a linear relationship between the 
secretion of calcitonin and ambient calcium levels.141 Cell culture 
studies with calcium ionophores and calcium channel blo

es with calcium ionophores and calcium channel blockers 
demonstrate that the calcium ion concentration within the C cell 
determines this secretion rate.142 The CaSR cloned from parathy­
roid cells is also expressed in C cells and contributes to the regu­
lation of calcitonin secretion.143 Other calcitonin secretagogues 
include glucocorticoids, CGRP, glucagon, enteroglucagon, gas­
trin, pentagastrin, pancreozymin, and β-adrenergic agents.144 The 
physiologic role of the gastrointestinal hormon

e 
physiologic role of the gastrointestinal hormones in regulating 
calcitonin remains unclear; however, they have been postulated 
to play a role in the regulation of postprandial hypercalcemia. 
The secretion of calcitonin is inhibited by somatostatin, which 
is also secreted by the thyroidal C cells. In vivo145 and in vitro146 
studies have demonstrated that 1,25(OH)2D3 decreases calcitonin 
mRNA levels by a transcriptional mechanism.
Calcitonin, when administered acutely, decreases tubular 

in, when administered acutely, decreases tubular 
resorption of calcium147 and impairs osteoclast-mediated bone 
resorption by a direct action on osteoclasts.148 In rodents, calcito­
nin has been shown to play a role in the regulation of postprandial 
hypercalcemia149 and the skeletal response to lactation.150 Stud­
ies in calcitonin knockout mice reveal a doubling of bone forma­
tion rate in the absence of hormone, accompanied by resistance 
to ovariectomy-induced bone loss.151 An increase in b

ovariectomy-induced bone loss.151 An increase in bone forma­
tion is also found in mice heterozygous152 or homozygous for cal­
citonin receptor ablation.153 This increase in bone formation is 
• Fig. 29.14  The amino acid sequences of calcitonin (CT), calcitonin gene–related peptide (CGRP), amy­
lin, adrenomedullin (ADM), intermedin (IMD), and calcitonin receptor–stimulating proteins (CSRP) from 
selected species. The bold Cs represent the cysteine residues that form the disulfide linkages criti

ne residues that form the disulfide linkages critical for 
the secondary structure of these peptides. The other conserved residues are indicated by a dashed line 
(see Fig. 29.3 for the single-letter amino acid codes).
1211
thought to be due to absence of calcitonin-mediated inhibition 
osteoclast secretion of sphingosine-1-phosphate, which induces 
bone formation.153
The physiologic role of calcitonin in humans, however, remains 
elusive.

f calcitonin in humans, however, remains 
elusive. The effect of calcitonin on bone density was examined in 
patients with long-term hypercalcitoninemia secondary to medul­
lary carcinoma of the thyroid (MCT) and in patients with subtotal 
thyroidectomy resulting in lack of calcitonin secretory reserve.154 
The abnormal calcitonin levels had no influence on the bone den­
sity at the lumbar spine and distal radius. Furthermore, long-term, 
high-dose administration of exogenous calcitonin caused n

se administration of exogenous calcitonin caused no phys­
iologic abnormalities.155
Many of the effects of calcitonin are mediated by a G pro­
tein–coupled cell surface receptor in the PTH/secretin receptor 
family.156,157 The mRNA encoding this receptor has been found 
in multiple tissues, including kidney, brain, and osteoclasts. The 
coupling of this receptor to different G proteins results in the acti­
vation of either adenylate cyclase or PLC; in some settings, this is 
cell cycle dependent

C; in some settings, this is 
cell cycle dependent.158 
Calcitonin Family: Calcitonin Gene-Related 
Peptide, Amylin, Adrenomedullin, Calcitonin 
Receptor–Stimulating Peptides, and Intermedin
CGRP, amylin, adrenomedullin, calcitonin receptor–stimulating 
peptide 1 (CRSP1), and intermedin have all been shown to have 
high-affinity binding sites on cell membranes, and displacement 
studies suggest that several receptor subtypes for these related 
ligands are present. However, cloning of specific re

gands are present. However, cloning of specific receptors for these 
ligands proved difficult, because the functional receptors consist 
of heterodimers between G protein–coupled receptors and single 
transmembrane proteins of the RAMP (receptor activity modify­
ing proteins) family.159,160 Interaction of the calcitonin receptor-
like receptor, a relative of the calcitonin receptor, with RAMP1 
results in a CGRP receptor, whereas RAMP2 and RAMP3 inter­
actions with the same calcitonin receptor-l

inter­
actions with the same calcitonin receptor-like receptor generate 
adrenomedullin receptors. Interaction of RAMP1 with the calci­
tonin receptor generates an amylin receptor.161 CGRP receptors 
have been shown to exhibit ligand-dependent and RAMP-depen­
dent signaling bias among the Gαs, Gαi, and Gαq/11 pathways.162
CGRP is thought to act as a neurotransmitter and vasodilator 
rather than as a hormone. In support of this hypothesis, inhibi­
tion of CGRP receptor signaling using monoclonal 

tion of CGRP receptor signaling using monoclonal antibodies 
decreases the frequency of migraines in predisposed individu­
als,163,164 and mice lacking αCGRP have been shown to have an 
increase in mean arterial pressure.165 Immunohistochemical stud­
ies of CGRP in the brain and peripheral nervous system suggest 
that this neuropeptide also plays an important role in sensory and 
integrative motor functions.
Three structurally related peptides have been isolated from por­
cine brain (see Fig. 2

ave been isolated from por­
cine brain (see Fig. 29.14). These calcitonin receptor-stimulating 
peptides (CRSPs) are also expressed in the thyroid gland. CRSP1, 
which is 60% homologous to αCGRP at the amino acid level, 
binds to the calcitonin receptor, dose-dependently stimulates 
cAMP production, and inhibits osteoclastogenesis.166 Consistent 
with this observation, administration of CRSP1, like that of cal­
citonin, results in a decrease in serum calcium. The receptors for 
CRSP2 and CRSP3 h

erum calcium. The receptors for 
CRSP2 and CRSP3 have not been identified.167
Amylin is highly homologous to CGRP and calcitonin (see 
Fig. 29.14). Although amylin has been shown to have skeletal 
actions, the presence of amylin in the pancreas of patients with 
type 2 diabetes mellitus suggests an etiologic role for this peptide 
in this disorder.168 Because amylin slows gastric emptying, pro­
moting satiety and attenuating the postprandial rise in glucagon, 
analogues of this peptide are curre

 in glucagon, 
analogues of this peptide are currently being used as therapeu­
tic agents for type 2 diabetes and are being investigated for the 
treatment of obesity. Amylin administration inhibits bone loss 
associated with ovariectomy and streptozotocin-induced diabetes 
mellitus in rats.169,170 Targeted ablation of amylin in mice results 
in low bone mass due to an increase in bone resorption.152 Amylin 
has also been shown to decrease food intake and inhibit gastric 
acid secretion, protect

ntake and inhibit gastric 
acid secretion, protecting against ulcer development in numerous 
models.171
Adrenomedullin (see Fig. 29.14) has vasodilatory effects similar 
to those of CGRP. In addition to activating CGRP receptors, adre­
nomedullin binds to specific receptors in the vascular system.172 
Mice lacking adrenomedullin die in midgestation.173 Haploinsuf­
ficiency of RAMP2, which regulates binding of adrenomedullin 
to the calcitonin receptor, results in hyperprolactinemia, delayed 
bon

eptor, results in hyperprolactinemia, delayed 
bone development, and decreased bone mineral density.174 Unlike 
other family members, adrenomedullin does not inhibit osteoclast 
activity or formation.166 The physiologic role of adrenomedullin 
in the skeleton remains to be clarified.
Intermedin (adrenomedullin-2), the newest member of this 
family (see Fig. 29.14), was identified by homology screening of 
expressed sequence tags. It is expressed primarily in the pituitary 
and the gastrointestin

primarily in the pituitary 
and the gastrointestinal tract. Intermedin is able to signal through 
CGRP receptors and competes with CGRP for receptor bind­
ing.175 However, unlike CGRP and adrenomedullin, intermedin 
is a nonselective agonist for the RAMP coreceptors. 
Calcitonin in Human Disease
Calcitonin is secreted by several endocrine malignancies and there­
fore can serve as a tumor marker. Basal and pentagastrin-stimu­
lated calcitonin levels have been used to identify and follow those 
at

ls have been used to identify and follow those 
at risk for, or affected by, medullary carcinoma of the thyroid (see 
Chapter 42), although abnormal basal and stimulated levels may 
be observed in patients on chronic hemodialysis.176 Calcitonin 
may also be ectopically secreted by other tumors, including insu­
linomas, VIPomas, and lung cancers. Severely ill patients, includ­
ing those with burn inhalation injury,177 toxic shock syndrome,178 
and pancreatitis,179 may also have elevated calcitoni

 pancreatitis,179 may also have elevated calcitonin levels. 
Therapeutic Uses
The observation that calcitonin inhibits osteoclastic bone resorp­
tion has led to its therapeutic use for the treatment of several 
disorders associated with excess bone resorption, including osteo­
porosis and Paget disease. Calcitonin has also been used for its 
analgesic effect, in the treatment of patients with vertebral crush 
fractures, osteolytic metastases, or phantom limb.180,181 Based on 
the more beneficial

phantom limb.180,181 Based on 
the more beneficial effects of other agents and potential increased 
risk of malignancy in those being treated with salmon calcitonin, 
as well as the marginal efficacy of the drug, it is no longer recom­
mended for treatment of osteoporosis.182 
Vitamin D
Metabolism of Vitamin D
Vitamin D is not a true vitamin, because nutritional supplementa­
tion is not required in humans who have adequate sun exposure. 
When exposed to ultraviolet irradiation, the cutaneous pre

osed to ultraviolet irradiation, the cutaneous precursor 
1212
of vitamin D, 7-dehydrocholesterol, undergoes photochemical 
cleavage of the carbon bond between carbons 9 and 10 of the 
steroid ring (Fig. 29.15). The resultant product, previtamin D, is 
thermally labile and over a period of 48 hours undergoes a temper­
ature-dependent molecular rearrangement that results in the pro­
duction of vitamin D. Alternatively, this thermally labile product 
can isomerize

vely, this thermally labile product 
can isomerize to two biologically inert products, luminosterol and 
tachysterol. This alternative photoisomerization prevents produc­
tion of excessive amounts of vitamin D with prolonged sun expo­
sure. The degree of skin pigmentation, which increases in response 
to solar exposure, also regulates the conversion of 7-dehydrocho­
lesterol to vitamin D by blocking the penetration of ultraviolet 
rays.
The alternative source of vitamin D is dietary. The elderly

native source of vitamin D is dietary. The elderly, 
the institutionalized, and those living in northern climates likely 
obtain most of their vitamin D from dietary sources. However, 
with increasing avoidance of sun exposure by the general popula­
tion, ensuring adequate dietary intake of vitamin D has become 
important for the population at large. Vitamin D deficiency is 
prevalent and has been shown to contribute significantly to osteo­
penia and fracture risk. The major dietary sources of v

 and fracture risk. The major dietary sources of vitamin D 
are fortified dairy products, although the lack of monitoring of 
this supplementation results in marked variation in the amount of 
vitamin D provided.183 Other dietary sources include egg yolks, 
fish oils, and fortified cereal products. Vitamin D provided by 
plant sources is in the form of vitamin D2, whereas that provided 
by animal sources is in the form of vitamin D3 (see Fig. 29.15). 
These two forms have equivalent biologic pot

15). 
These two forms have equivalent biologic potencies and are acti­
vated equally efficiently by the hydroxylases in humans. While 
vitamin D3 has been shown to be more effective at increasing 
25-hydroxyvitamin D levels,184 this effect is dependent upon vita­
min D–binding protein (VDBP) genotype and concentration.185
Vitamin D is absorbed into the lymphatics and enters the cir­
culation bound primarily to VDBP, although a fraction of vitamin 
D circulates bound to albumin. The human VDBP is

D circulates bound to albumin. The human VDBP is a 52-KDa 
α-globulin synthesized in the liver. The protein has a high affin­
ity for 25(OH)D but also binds vitamin D and 1,25(OH)2D. 
Approximately 88% of 25(OH)D circulates bound to the VDBP, 
0.03% is free, and the rest circulates bound to albumin. In con­
trast, 85% of the circulating 1,25(OH)2D3 binds to the VDBP, 
0.4% is free, and the rest binds to albumin. Mice lacking VDBP 
have increased susceptibility to 1,25(OH)2D3 toxicity as well a

d susceptibility to 1,25(OH)2D3 toxicity as well as 
to dietary vitamin D deficiency.186 Vitamin D–binding protein 
polymorphisms have been postulated to be the basis for difference 
in vitamin D levels in several groups, including African Americans 
and Finns.187 Measurements of free 25(OH) vitamin D across 
multiple populations, however, suggest that the monoclonal anti­
body used in the former study can bring misleading results.188
Studies in megalin-deficient patients and megalin null mice 

megalin-deficient patients and megalin null mice 
demonstrate that VDBP is filtered by the glomerulus and reab­
sorbed by a megalin-dependent pathway in the proximal renal 
tubule. Further investigations will be required to determine the 
importance of this pathway in vitamin D metabolism and the tis­
sues in which megalin-dependent endocytosis plays an important 
role.
In the liver, vitamin D undergoes 25-hydroxylation by a 
cytochrome P450–like enzyme present in the mitochondria and 
microsome

 enzyme present in the mitochondria and 
microsomes. The half-life of 25(OH)D is approximately 2 to 3 
weeks. The 25-hydroxylation of vitamin D is not tightly regulated, 
therefore the blood levels of 25(OH)D reflect the amount of vita­
min D entering the circulation. When levels of VDBP are low, 
such as in nephrotic syndrome, circulating levels of 25(OH)D are 
also reduced. The half-life of 25(OH)D is shortened by increases 
in levels of its active metabolite, 1,25(OH)2D3.
The final step in th

tive metabolite, 1,25(OH)2D3.
The final step in the production of the active hormone is 
the renal 1α-hydroxylation of 25(OH)D to 1,25(OH)2D3. The 
half-life of this hormone is approximately 6 to 8 hours. Like the 
25-hydroxylase, the 1α-hydroxylase in the proximal convoluted 
tubule is a cytochrome P450–like mixed function oxidase, but 
unlike the 25-hydroxylase, the 1α-hydroxylase is tightly regu­
lated. PTH and hypophosphatemia are the major inducers of this 
microsomal enzyme, whereas calciu

nducers of this 
microsomal enzyme, whereas calcium, 1,25(OH)2D, and FGF23 
repress it.189,190 Analogous to mice lacking FGF23, mice with 
inactivating mutations of the FGF23 coreceptor, α-Klotho, a type 
I membrane protein with homology to β-glycosidases, develop 
hypercalcemia as a result of increased levels of 1,25(OH)2D3. Like 
the FGF23 null mice, Klotho null mice have increased levels of the 
1α-hydroxylase,191 and their phenotype is ameliorated by impair­
ment of 1,25(OH)2D3 action. In an

rated by impair­
ment of 1,25(OH)2D3 action. In animal models and in  vitro 
studies, other hormones such as estrogen, calcitonin, growth hor­
mone, and prolactin have been shown to increase 1α-hydroxylase 
activity; however, the clinical importance of these observations has 
not been established. Ketoconazole has been shown to decrease 
levels of 1,25(OH)2D3 in a dose-dependent manner, presumably 
by interfering with 1α-hydroxylase activity.
HO
OH
A
B
C
D
R
HO
R
HO
R
HO
R
R
R
OH
OH
R
HO
R
Light

OH
A
B
C
D
R
HO
R
HO
R
HO
R
R
R
OH
OH
R
HO
R
Light
Lumisterol
Synthetic
reduction
21
22
24
20
23
25
27
26
1
Trans
Cis
2
3
4 5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
Previtamin D
Tachysterol
7-Dehydro-
cholesterol (RA)
or
Ergosterol (RB)
RA
D
Vitamin D3   Cholecalciferol (RA)
or
Vitamin D2   Ergocalciferol (RB)
Dihydrotachysterol
28
RB
• Fig. 29.15  Vitamin D precursors and alternative reaction products. The 
numbering system for vitamin D carbons and the distinct structures of vita­
min D2 (erg

s and the distinct structures of vita­
min D2 (ergocalciferol) and D3 (cholecalciferol) are noted, as is the struc­
ture of dihydrotachysterol, a synthetic product not produced in vivo. Note 
that the 3-hydroxyl group of dihydrotachysterol is in a pseudo-1-hydroxyl 
configuration. This may explain the relatively high potency of dihydrotachy­
sterol in conditions associated with low 1α-hydroxylase activity.
1213
The 1α-hydroxylase enzyme is

of Mineral Metabolism
The 1α-hydroxylase enzyme is also expressed in keratinocytes, 
the trophoblastic layer of the placenta, and in granulomas, includ­
ing sarcoid granulomas, among many other tissues. In granuloma­
tous tissue, the 1α-hydroxylase gene that is expressed is identical 
to that expressed in the kidney but is not regulated by PTH, 
phosphate, calcium, or vitamin D metabolites in these cells. Acti­
vation of macrophages with interferon-γ192 or with ligands that 
activate the heterod

on-γ192 or with ligands that 
activate the heterodimer of toll-like receptors 1 and 2,193 however, 
increases the expression of the 1α-hydroxylase in macrophages, 
whereas treatment of sarcoidosis-associated hypercalcemia with 
glucocorticoids, ketoconazole,194 or chloroquine195 has been 
shown to lower serum 1,25(OH)2D3 levels. Activation of the 
VDR in human macrophages induces the antimicrobial peptide 
cathelicidin and increases killing of intracellular Mycobacterium 
tuberculosis.193 Thus t

tracellular Mycobacterium 
tuberculosis.193 Thus the pathologic excess of 1,25(OH)2D3 in 
sarcoidosis may represent an exaggeration of a healthy paracrine 
response of tissue macrophages. This action can be viewed as a 
paradigm for many other actions of vitamin D that are mediated 
more by the local production of 1,25(OH)2D3 than by circulating 
1,25(OH)2D3.
25(OH)D and 1,25(OH)2D3 can also be hydroxylated by vita­
min D 24-hydroxylase, which is present in most tissues, including 
kidney, carti

 present in most tissues, including 
kidney, cartilage, and intestine. 1,25(OH)2D3 increases the activ­
ity of the 24-hydroxylase, thereby inducing its own metabolism. 
The 24-hydroxylated vitamin D metabolites, 24,25(OH)2D3 and 
1,24,25(OH)3D3, are not thought to play major biologic roles 
other than inactivation of 1,25(OH)2D3. Mice and humans 
with nonfunctional 24-hydroxylase genes demonstrate hypercal­
cemia, hypercalciuria, and nephrocalcinosis due to vitamin D 
toxicity.196,197

osis due to vitamin D 
1,25(OH)2D3 is also metabolized to several inactive products 
by 23-hydroxylation or 26-hydroxylation and side-chain oxida­
tion and cleavage. This latter side-chain cleavage, resulting in 
the formation of calcitroic acid, occurs in the liver and intestine, 
whereas inactivation of 1,25(OH)2D3 in a wide variety of target 
tissues occurs by 24-hydroxylation. In addition, polar metabolites 
of 1,25(OH)2D3 are excreted in the bile. Some of these metab­
olite

e excreted in the bile. Some of these metab­
olites are deconjugated in the intestine and reabsorbed into the 
enterohepatic circulation. 
Actions of Vitamin D
Vitamin D Receptors
1,25(OH)2D3 exerts its biologic functions by binding to a 
nuclear receptor, which then regulates transcription of DNA 
into RNA. Among the other nuclear receptors, the VDR 
most closely resembles the retinoic acid, triiodothyronine, 
and retinoid X receptors (RXRs). The affinity of the receptor 
for 1,25(OH)2D3 is app

e affinity of the receptor 
for 1,25(OH)2D3 is approximately three orders of magnitude 
higher than that for other vitamin D metabolites (Fig. 29.16). 
Although 25(OH)D3 is less potent on a molar basis, its con­
centration in the serum is approximately three orders of mag­
nitude higher than that of 1,25(OH)2D. However, its free 
concentration is only two orders of magnitude greater than 
that of 1,25(OH)2D3. Therefore, under normal circumstances, 
it is unlikely that 25(OH)D3 contributes import

, 
it is unlikely that 25(OH)D3 contributes importantly to cal­
cium homeostasis. Because the affinity of the vitamin D–bind­
ing protein for 25(OH)D3 is greater than for 1,25(OH)2D3, in 
states of vitamin D intoxication (with its associated high levels 
of 25[OH]D3), the free levels of 1,25(OH)2D3 increase because 
25(OH)D displaces it from the vitamin D–binding protein. 
25(OH)D3 may therefore play a role in the clinical syndrome 
of vitamin D intoxication both by its direct biologic effects, 

3
25-OH-
5,6-trans-D3 
25-OH-D3
24,25-OH2-D3
1--OH-D3
• Fig. 29.16  Relative potency of analogues of 1,25(OH)2D3 (1,25-dihydroxyvitamin D3) in competitive binding to 
vitamin D receptors of chick intestinal mucosa. Slopes are plotted for (left to right) 1,25-(OH)2-D3 (1,25-dihy­
droxyvitamin D3); 3 deoxy-1,25(OH)2D3 (3 deoxy-1,25-dihydroxyvitamin D3); 25-OH-DHT3 (25-hydroxydihy­
drotachysterol); 25-OH-5,6-trans-D3 (25-hydroxy-5,6-transvitamin D3); 25-OH-D3 (25-hydroxyvitamin D3); 
1-α-OH-D3 (1α

); 25-OH-D3 (25-hydroxyvitamin D3); 
1-α-OH-D3 (1α-hydroxyvitamin D3); 24,25-OH2-D3 (24,25-dihydroxyvitamin D3); 3-deoxy-1-α-OH-D3 (3-deoxy-
1α-hydroxyvitamin D3); D3 (vitamin D3); DHT3 (dihydrotachysterol). (From Proscal DA, Okamura WH, Norman 
AW. Structural requirements for the interaction of 1α,25-(OH)2-vitamin D3 with its chick intestinal system. J Biol 
Chem. 1975;250:8382–8388.)
1214
local activation of this prohormone may contribute to host 
immune respo

is prohormone may contribute to host 
immune response and barrier function.
The vitamin D receptor acts by forming a heterodimer with 
the retinoid-X receptor, binding to DNA elements, and recruiting 
coactivators in a ligand-dependent fashion. These coactivators link 
the receptor complex to the basal transcription apparatus, thereby 
regulating transcription of target genes. In most cases, the upregu­
latory response elements for vitamin D contain hexameric repeats 
separated by three bases (F

ain hexameric repeats 
separated by three bases (Fig. 29.17).198 The mechanism of tran­
scriptional repression by vitamin D varies. For example, VDR-
RXR heterodimers repress the 1α-hydroxylase and renin genes 
by blocking the function of other transcription factors,199,200 and 
interaction of the VDR with the Ku antigen, acting as a tran­
scription factor, is required for transcriptional repression of the 
hPTHrP gene.201
Glucocorticoids have been shown to decrease the expression of 
the VDR ge

en shown to decrease the expression of 
the VDR gene in osteosarcoma cell lines, whereas 1,25(OH)2D3 
increases its expression in many cell types. In the renal proximal 
convoluted tubule, however, 1,25(OH)2D3 decreases the levels of 
vitamin D receptors. This decrease has been postulated to lead to 
decreased activation of the renal 24-hydroxylase by 1,25(OH)2D3 
and thereby protect the newly synthesized 1,25(OH)2D from 
local inactivation.202
1,25(OH)2D3 also has some biologic effects that occ

,25(OH)2D3 also has some biologic effects that occur too 
rapidly for transcriptional mechanisms to be implicated. These 
so-called nongenomic actions, including a rapid increase in intra­
cellular calcium, activation of phospholipase C, and opening of 
calcium channels, are observed in several cell types within min­
utes of exposure to 1,25(OH)2D3. Additional data supporting 
the hypothesis that nongenomic actions are not dependent on 
the classic receptor include the identification of specific

ic receptor include the identification of specific bind­
ing sites for 1,25(OH)2D3 on the antiluminal surface of intes­
tinal cells203 and a disparity between the affinity of the various 
vitamin D analogues for the nuclear receptor and their potency 
in these nongenomic actions. However, both the rapid intracel­
lular accumulation of cGMP in association with the vitamin D 
receptor and the rapid increase in intracellular calcium in response 
to 1,25(OH)2D are dependent upon the presence of an i

1,25(OH)2D are dependent upon the presence of an intact 
nuclear receptor, because these effects are not observed in cells 
derived from patients and mice with VDR mutations.204 How­
ever, 1,25-dihydroxyvitamin D binding to the cell surface recep­
tor PDIA3/Erp57 causes a rapid increase in calcium uptake in 
enterocytes, which is abrogated by deletion of this receptor.205 The 
physiologic importance of the nongenomic actions of vitamin D 
metabolites206 has not yet been established.
The VDR is e

ites206 has not yet been established.
The VDR is expressed in most tissues and has been shown to 
regulate cellular differentiation and function in many cell types. 
Nevertheless, the most dramatic physiologic effects of vitamin 
D, acting through the VDR, involve regulation of intestinal cal­
cium transport. This is most clearly demonstrated by the pheno­
type of patients and mice with mutant VDRs (hereditary vitamin 
D–resistant rickets)35,207: Dramatic abnormalities in bone min­
eralization c

 Dramatic abnormalities in bone min­
eralization can be reversed by bypassing the defect in intestinal 
calcium absorption.208–210 
Intestinal Calcium Absorption
Under normal dietary conditions, calcium intake is in the range 
of 700 to 900 mg daily. Approximately 30% to 35% of this cal­
cium is absorbed; however, losses from intestinal secretion of 
calcium lead to a net daily uptake of approximately 200 mg. 
Though vitamin D is the major hormonal determinant of intes­
tinal calcium absorption,

al determinant of intes­
tinal calcium absorption, the bioavailability of mineral ions in 
the intestinal lumen may be affected by a number of local factors 
and dietary constituents. Absorption of calcium and magnesium 
is impaired by bile salt deficiency, unabsorbed free fatty acids in 
steatorrheic states, and high dietary content of fiber or phytate. 
Gastric acid is needed to promote dissociation of calcium from 
anionic components of food or therapeutic preparations of cal­
cium salts. Adm

r therapeutic preparations of cal­
cium salts. Administration of calcium salts with meals, especially 
in achlorhydrics, and use of divided doses or more soluble salts 
such as calcium citrate are commonly used strategies to increase 
calcium bioavailability.
Calcium is thought to be absorbed by two pathways: a saturable 
transcellular pathway and a nonsaturable paracellular route. The 
transcellular pathway is dependent on 1,25(OH)2D3. Although 
the necessity of vitamin D for paracellular calci

the necessity of vitamin D for paracellular calcium absorption 
remains controversial, substantial evidence exists that the hor­
mone enhances this pathway as well.211 Notably, 1,25(OH)2D3 
induces the expression of claudin-2 and claudin-12, which con­
tribute to intestinal calcium absorption and are thought to form 
paracellular channels between neighboring cells.212
The most extensively studied mechanism of intestinal cal­
cium absorption involves the transcellular route. This pathway is 
tho

lves the transcellular route. This pathway is 
thought to involve three steps: entry of calcium into the entero­
cyte (which is the rate-limiting step), transport across the cell, and 
extrusion across the basolateral membrane.213
Entry Into the Enterocyte
1,25(OH)2D3 induces synthesis of a number of brush border pro­
teins, including the intestinal membrane calcium-binding pro­
tein, brush border alkaline phosphatase, and low-affinity Ca2+/
Mg2+-ATPase. The activity of these proteins correlates

-ATPase. The activity of these proteins correlates with active 
calcium transport; however, a causal relationship remains to be 
established. Two calcium channels, TRPV5 and TRPV6, mem­
bers of the transient receptor potential vanilloid receptor subfam­
ily containing six membrane-spanning domains, are expressed in 
the duodenum, the jejunum, and the kidney as well as in other 
tissues. TRPV6 is thought to play a critical role in intestinal cal­
cium absorption, and 1,25(OH)2D3 increases its exp

cium absorption, and 1,25(OH)2D3 increases its expression, as 
well as that of TRP5.214 Studies in mice lacking TRPV5 demon­
strate that this channel is primarily responsible for renal calcium 
reabsorption, because mice lacking TRPV5 have enhanced, rather 
than impaired, intestinal calcium absorption due to their high cir­
culating levels of 1,25(OH)2D.215 Upon entering the enterocyte, 
calcium binds to components of the brush border complex subja­
cent to the plasma membrane. Calmodulin is red

ja­
cent to the plasma membrane. Calmodulin is redistributed to the 
brush border in response to 1,25(OH)2D3 and may play a role in 
this process, as may the 1,25(OH)2D3-inducible calcium-binding 
protein, calbindin-9K. 
RXR
VDR
GGGTGA ATG AGGACA
TATA
• Fig. 29.17  Transcriptional activation by 1,25-dihydroxyvitamin D3 
(1,25(OH)2D3). A heterodimer of retinoid X receptor (RXR) and vitamin D 
receptor (VDR) binds to a pair of hexameric sequences separated by three 
intervening bases (ATG). Arrows

eparated by three 
intervening bases (ATG). Arrows indicate that the hexamers found in the 
upregulated rat osteocalcin gene are variants of a consensus sequence, 
repeated here with identical orientations (direct repeats). Upon binding to 
DNA, the RXR-VDR heterodimer facilitates formation of a transcription ini­
tiation complex, which binds to DNA at and near the TATA sequence.
1215
Transcellular Transport
The best studied effect of vita

cellular Transport
The best studied effect of vitamin D on the enterocyte is the 
induction of synthesis of the intestinal calcium-binding protein, 
calbindin-9K. This protein has an EF hand structure that per­
mits the binding of two calcium ions per molecule. The affinity 
of calbindin for calcium is approximately four times that of the 
brush border calcium-binding components, so calcium is prefer­
entially transferred to calbindin. Calbindin serves to buffer the 
intracellular free calcium c

serves to buffer the 
intracellular free calcium concentration during calcium absorp­
tion. It associates with microtubules and may play a role in the 
transport of calcium across the enterocyte. However, studies in 
mice lacking both TRPV6 and calbindin-D9k maintain 40% of 
normal 1,25(OH)2D3-mediated calcium transport, demonstrat­
ing that other hormone-responsive genes may play a significant or 
compensatory role.216 Organelles such as the mitochondria, Golgi 
apparatus, and endoplasmic retic

ochondria, Golgi 
apparatus, and endoplasmic reticulum also serve as repositories for 
intracellular calcium. 
Exit From the Enterocyte
The transport of calcium across the antiluminal surface of the 
enterocyte, the final process involved in intestinal calcium absorp­
tion, is dependent on 1,25(OH)2D3. The main mechanism of 
calcium extrusion is the 1,25(OH)2D3-inducible ATP-dependent 
Ca2+ pump (PMCA1b). The affinity of the pump for calcium is 
approximately 2.5 times that of calbindin. With hi

approximately 2.5 times that of calbindin. With high calcium 
intake, a 1,25(OH)2D3-independent Na+/Ca2+ exchanger may 
play a role in the transfer of calcium across the basolateral mem­
brane as well. 
Actions on the Parathyroid Gland
1,25(OH)2D3 has been shown to regulate gene transcription and 
cell proliferation in the parathyroids. The hormone also inhib­
its the proliferation of dispersed parathyroid cells in culture, 
although the relative contribution of calcium and 1,25(OH)2D3 
in the r

 contribution of calcium and 1,25(OH)2D3 
in the regulation of parathyroid cell proliferation in vivo has not 
been established. Normocalcemic mice lacking functional vita­
min D receptors have normal serum PTH levels and normal-size 
parathyroid glands, demonstrating that the genomic actions of 
1,25(OH)2D3 are not essential for parathyroid cellular homeo­
stasis.210 1,25(OH)2D3 has, however, been shown to decrease the 
transcription of the PTH gene both in vivo and in vitro. This 
action has b

gene both in vivo and in vitro. This 
action has been exploited in the use of 1,25(OH)2D3 for the 
treatment of the secondary hyperparathyroidism associated with 
chronic renal failure (see discussions under “Parathyroid Hor­
mone Biosynthesis” and “Vitamin D Deficiency”). 
Actions on Bone
The effects of 1,25(OH)2D3 on bone are numerous. 1,25(OH)2D3 
is a major transcriptional regulator of the two most abundant bone 
matrix proteins: It represses the synthesis of type I collagen and 
induces the

 the synthesis of type I collagen and 
induces the synthesis of osteocalcin. 1,25(OH)2D3 promotes the 
differentiation of osteoclasts from monocyte-macrophage stem 
cell precursors in vitro and increases osteoclastic bone resorption 
in high doses in vivo by stimulating production of RANKL (also 
called osteoclast differentiating factor) by osteoblasts.217 Notably, 
osteoblast-specific knockout of the vitamin D receptor leads to 
increased bone mass associated with impaired bone resorption 
due 

ass associated with impaired bone resorption 
due to decreased RANKL expression.218 Consistent with this, and 
despite the multiple effects of 1,25(OH)2D3 on the biology of 
bone in vitro, in vivo studies in 1,25(OH)2D3-deficient rats and in 
mice lacking functional vitamin D receptors35,208 suggest that the 
major osseous consequences of hormone and receptor deficiency 
can be reversed when mineral ion homeostasis is normalized. In 
addition, parenteral calcium infusions have been shown to heal

renteral calcium infusions have been shown to heal 
the osteomalacic lesions in children with mutant VDRs.209 These 
observations suggest that the major role of 1,25(OH)2D3 in bone 
is to provide the proper microenvironment for bone mineraliza­
tion through stimulation of the intestinal absorption of calcium 
and phosphate. 
Other Actions of Vitamin D
The effects of 1,25(OH)2D3 on phosphate transport are less 
well studied than those on calcium transport. 1,25(OH)2D3 has 
been shown to promote t

ransport. 1,25(OH)2D3 has 
been shown to promote the already efficient intestinal phosphate 
absorption and to induce the expression of the phosphaturic hor­
mone FGF23.66
One of the striking clinical features of profound vitamin D 
deficiency that is poorly understood is the severe proximal myopa­
thy. While some studies demonstrate VDR expression in skeletal 
myofibers,219 both the expression and the role of this receptor in 
muscle remain controversial. Regardless, in vitro 1,25(OH)2D3 
treat

roversial. Regardless, in vitro 1,25(OH)2D3 
treatment increases 25(OH)D219 and amino acid uptake into 
muscle cells and alters phospholipid metabolism. Vitamin D 
administration has been shown to increase the concentration of 
troponin C, a calcium-binding protein in muscle that plays a role 
in excitation coupling and increases the rate of uptake of calcium 
by the sarcoplasmic reticulum. Vitamin D repletion of severely 
deficient adults has been shown to improve muscle strength, 
associated w

en shown to improve muscle strength, 
associated with an increase in mitochondrial function assessed 
by 31-phosphate nuclear magnetic resonance (NMR) spectros­
copy.220 Vitamin D receptor knockout mice demonstrate a delay 
in myoblast differentiation221; however, little is known regarding 
the direct role of vitamin D in normal muscle physiology. The 
myopathy that accompanies vitamin D deficiency is characterized 
by normal creatinine phosphokinase (CPK) levels, a myopathic 
electromyogram (EM

nase (CPK) levels, a myopathic 
electromyogram (EMG), and biopsy findings of loss of myofibrils, 
fatty infiltration, and interstitial fibrosis. The myopathy resolves 
within days to weeks of vitamin D replacement and is not related 
to normalization of mineral ion homeostasis; however, studies in 
vitamin D–deficient rats demonstrate that maintenance of nor­
mal mineral ion levels precludes the development of myopathy.222 
Vitamin D deficiency has also been implicated in statin-induced 
myopath

as also been implicated in statin-induced 
myopathy.223 In contrast to these studies, which suggest a benefi­
cial effect of vitamin D on muscle function, monthly administra­
tion of 60,000 IU vitamin D has been shown to be associated with 
an increased incidence of falls in those over the age of 70.224
While the vitamin D receptor is widely expressed, the clinical 
implications of these findings are not well understood. However, 
recent studies demonstrate that women with vitamin D levels 
abov

demonstrate that women with vitamin D levels 
above 30 ng/mL in early and late pregnancy have a lower risk 
of preeclampsia.225 Prenatal vitamin D supplementation has also 
been shown to reduce the risk of early childhood asthma.226 
Vitamin D Analogs
The recognition that 1,25(OH)2D3 promotes cellular differentia­
tion and inhibits cellular proliferation has led to efforts directed at 
producing new analogues that retain these effects but do not cause 
hypercalcemia. Several analogues have been 

cause 
hypercalcemia. Several analogues have been shown to have anti­
proliferative effects on normal cells as well as on malignant cells 
in vitro and in xenografts in immunosuppressed mice.227 In addi­
tion, analogues of vitamin D have been shown to synergize with 
cyclosporine in preventing rejection of transplanted islet cells in a 
murine model.228 Nonhypercalcemic analogs have been shown to 
1216
suppress PTH synthesis and secretion in rats at doses that s

TH synthesis and secretion in rats at doses that stimu­
late intestinal calcium absorption less than 1,25(OH)2D3. Such 
analogs are currently used for the prevention and treatment of 
hyperparathyroidism associated with chronic kidney disease.229 
The antiproliferative effects of vitamin D have been exploited 
clinically in the treatment of psoriasis.230 Although analogues with 
reduced calcemic activity are predominantly used, hypercalcemic 
crisis after excessive topical use of such compounds 

sis after excessive topical use of such compounds can occur.
The physiology underlying the differential biologic effects 
of these analogues is not completely understood. Altered affin­
ity for the vitamin D–binding protein, metabolism by target tis­
sues,231 and effects on recruitment of coactivators by the vitamin 
D receptor may contribute to the unique properties of vitamin D 
analogs.232 
Fibroblast Growth Factor-23
FGF23 in Human Disease
The search for the hormonal factor responsible for t

e search for the hormonal factor responsible for the hypo­
phosphatemia in patients with tumor-induced osteomalacia 
(TIO, see Chapter 31) was brought to a close with the identifi­
cation of the molecular basis for the human disorder, autosomal 
dominant hypophosphatemic rickets (ADHR).233 Linkage anal­
yses of affected kindreds identified mutation of the gene encod­
ing FGF23 as the basis for ADHR. The mutation in affected 
individuals abolishes an RXXR protease recognition motif that 
is thoug

 an RXXR protease recognition motif that 
is thought to be responsible for the cleavage and inactivation 
of FGF23.234,235 The cDNA encoding FGF23 predicts a pep­
tide of 251 amino acids, the first 24 of which comprise a signal 
peptide. Studies using recombinant FGF23 demonstrate that 
the full-length mature peptide is required for its biologic activ­
ity and that the cleavage site mutated in the ADHR patients is 
responsible for its inactivation. Cleavage of FGF23 is inhibited 
by furin inhibi

n. Cleavage of FGF23 is inhibited 
by furin inhibition, suggesting that the enzyme responsible is a 
subtilisin-like proprotein convertase.
Analyses of tumors isolated from patients with TIO reveal 
a dramatic increase in mRNA expression of FGF23.236 A sig­
nificant number of these tumors exhibit a fibronectin:FGFR1 
or fibronectin:FGF1 fusion thought to underlie the increase 
in FGF23 production.237 The elevated serum levels of FGF23 
in patients with TIO have been shown to normalize after 
rem

s with TIO have been shown to normalize after 
removal of the tumor, correlating with resolution of the hypo­
phosphatemia that characterizes this disorder.238,239 FGF23 is 
also responsible for the renal phosphate wasting observed in 
X-linked hypophosphatemia. Increased levels of FGF23 have 
been reported in disorders characterized by somatic moisaicism, 
including fibrous dysplasia and epidermal and large congenital 
melanocytic nevus syndrome. Exome sequencing in the latter 
individuals demo

. Exome sequencing in the latter 
individuals demonstrated somatic activating mutations in HRAS 
and NRAS, though how this leads to increased serum FGF23 
levels is not clear.240 Conversely, patients with the rare syndrome 
tumoral calcinosis present with hyperphosphatemia and soft 
tissue calcium-phosphate deposits. Some of these patients have 
point mutations in the FGF23 gene that cause abnormal pro­
cessing of the protein, with low levels of the active hormone 
in the blood and high levels o

the active hormone 
in the blood and high levels of inactive fragments.241–243 Other 
affected individuals have been shown to have mutations in the 
FGF23 coreceptor α-Klotho or in GALNT 3, which normally 
O-glycosylates FGF23.244,245 Thus human diseases of both 
increased and decreased FGF23 activity suggest that this factor 
is an important regulator of phosphate metabolism. 
Actions of FGF23
Evidence that FGF23 is a novel hormone that plays a key role in 
normal phosphate homeostasis has been

key role in 
normal phosphate homeostasis has been obtained in murine mod­
els of overexpression and ablation. Overexpression of FGF23 or 
administration of FGF23 to animals results in the development of 
hypophosphatemia235 and impaired 1α-hydroxylation of 25(OH)
D,190,246 thus recapitulating the findings observed in patients 
affected by TIO. Investigations in mice with targeted ablation of 
FGF23 have proven that endogenous production of this hormone 
is critical for normal phosphate homeosta

hormone 
is critical for normal phosphate homeostasis and the regulation 
of vitamin D metabolism.247,248 Absence of FGF23 results in 
impaired renal phosphate excretion, leading to the development 
of hyperphosphatemia within the first 2 weeks of life. Affected 
mice also develop hypercalcemia due to high levels of 1,25-dihy­
droxyvitamin D, a result of the lack of the normal suppressive 
effect of FGF23 on the renal 25(OH)D 1α-hydroxylase.190 Abla­
tion of FGF23 results in premature death asso

bla­
tion of FGF23 results in premature death associated with ecto­
pic mineralization of soft tissues, including the kidney. Impairing 
1,25-dihydroxyvitamin D action in these animals prevents the 
development of hypercalcemia and improves survival, suggest­
ing that the premature death is a direct consequence of impaired 
mineral ion homeostasis rather than a specific developmental or 
maturational effect of FGF23.249
FGF23 impairs Na-dependent phosphate transport in both 
intestinal and renal

 phosphate transport in both 
intestinal and renal brush border membrane vesicles.250 It has 
been shown to decrease the levels of the types IIa, IIb, and IIc 
Na-dependent phosphate transporters, thereby regulating both 
intestinal and renal phosphate transport.251–253 FGF23 decreases 
circulating levels of 1,25-dihydroxyvitamin D, both by decreasing 
mRNA levels for the renal 25(OH)D 1α-hydroxylase as well as 
by increasing expression of the 24-hydroxylase, the key enzyme 
involved in inactiva

-hydroxylase, the key enzyme 
involved in inactivation of 1,25-dihydroxyvitamin D.190 FGF23 
activates FGF receptor 1 in the presence of α-Klotho, a single-
pass transmembrane protein, which acts as a coreceptor.254 Klotho 
knockout mice exhibit the same hyperphosphatemia and high 
1,25(OH)2D3 levels seen in the FGF23 knockout mice,191 dem­
onstrating that it plays a critical role in mediating the actions of 
FGF23.
In chronic kidney disease, FGF23 levels correlate with 
increased left ventricul

23 levels correlate with 
increased left ventricular mass. While FGF23 has been shown to 
have direct effects on cardiomyocytes,255 it also enhances distal 
renal tubule sodium absorption contributing to hypertension.256 
High FGF23 levels in the setting of hypophosphatemia, such as 
is seen in humans257 and mice with X-linked hypophosphatemia 
and in mice with mutations of the DMP1 gene,257 do not lead 
to cardiac hypertrophy, suggesting that cooperative interactions 
between phosphate and FGF2

between phosphate and FGF23 may be required. 
Regulation of FGF23
Circulating FGF23 levels are increased by dietary phosphorus, 
serum phosphorus, and 1,25-dihydroxyvitamin D.66,258,259 Para­
thyroid hormone induces FGF23 mRNA levels, but also increases 
furin cleavage, thus limiting target organ effects.260 Iron deficiency 
has also been shown to increase expression of FGF23 in bone via 
an erythropoietin/hypoxia-inducible factor (HIF)-1α–dependent 
mechanism.261,262 Int

e factor (HIF)-1α–dependent 
mechanism.261,262 Interestingly, studies in mice with chondro­
cyte-specific ablation of the vitamin D receptor have increased 
circulating FGF23, suggesting that chondrocytes express a vita­
min D–regulated repressor of FGF23 production.263 In addition, 
studies in mice with mutations in the PHEX or DMP1 genes 
1217
demonstrate that these genes suppress FGF23 expression.248,264 It 
has been suggested that muta

xpression.248,264 It 
has been suggested that mutations in both PHEX and DMP1 alter 
the set-point for extracellular phosphate induction of FGF23.265 
Circulating FGF23 levels are dramatically increased in acute renal 
injury266 and in response to lipopolysaccharide (LPS)–induced 
inflammation. In the latter case, the spleen is a major source of 
circulating FGF23, demonstrating nonosseous contributions to 
circulating hormone levels.266
In patients with chronic renal failure, an increase in FGF

nts with chronic renal failure, an increase in FGF23 
levels has been shown to antedate the development of secondary 
hyperparathyroidism, and thus evaluation of FGF23 levels may 
be beneficial in predicting which individuals will develop this dis­
order.267 Treatment of dialysis patients with sevelamer hydrochlo­
ride and calcium carbonate decreases phosphate and FGF23 levels 
in parallel, implicating increases in serum phosphate or intestinal 
absorption as the pathophysiologic basis for the i

absorption as the pathophysiologic basis for the increased FGF23 
levels in this population.268 Based on the increase in serum lev­
els of FGF23 seen with iron deficiency and erythropoietin, care­
ful attention to the iron stores of CKD patients and limited use 
of erythropoietin may also serve to attenuate increases in serum 
FGF23. Thus FGF23 has emerged as an essential regulator of nor­
mal phosphate and 1,25(OH)2D3 homeostasis. Phosphate and 
1,25(OH)2D3 increase FGF23 levels; this FGF23 the

1,25(OH)2D3 increase FGF23 levels; this FGF23 then acts on the 
renal proximal tubule to suppress synthesis of 1,25(OH)2D3 and 
to decrease the reabsorption of phosphate. 
Calcium and Phosphate Homeostasis
The cytosolic concentrations of intracellular calcium, phosphorus, 
and magnesium differ markedly, as reviewed previously, and their 
physiologic roles within cells are diverse and largely unrelated (see 
Fig. 29.1). In contrast, the concentrations of these mineral ions in 
extracellular flui

tions of these mineral ions in 
extracellular fluid are quite comparable (i.e., 1–2 mmol/L), and 
it is here that they exert important interactions, both with cells 
and with one another, that are critical for bone mineralization, 
neuromuscular function, and normal mineral ion homeostasis. 
Extracellular calcium and phosphate, in particular, exist so close 
to the limits of their mutual solubility that stringent regulation of 
their concentrations is required to avoid diffuse precipitation of 

ns is required to avoid diffuse precipitation of 
calcium phosphate crystals in tissues.
Serum concentrations and total body balances of the mineral 
ions are maintained within narrow limits by powerful, interac­
tive homeostatic mechanisms. PTH, 1,25(OH)2D, and FGF23 
regulate mineral ion levels; mineral ion levels, in turn, regulate 
PTH, 1,25(OH)2D, and FGF23 secretion; and these hormones 
may regulate the production of one another. Calcium sensors in 
the parathyroid glands control PTH secre

nsors in 
the parathyroid glands control PTH secretion by monitoring the 
blood concentration of ionized calcium, and those in the kidney 
act to adjust tubular calcium reabsorption, independently of PTH 
or 1,25(OH)2D. In contrast, the mechanisms of the phosphate 
sensing needed for normal homeostasis are not understood. The 
operation of these homeostatic mechanisms can be appreciated by 
considering the following examples of how the organism adapts to 
changes in calcium loads (Fig. 29.18).
D

dapts to 
Dietary calcium restriction, for example, is followed by an 
increase in the efficiency of intestinal calcium absorption. This 
increased efficiency results from a sequence of homeostatic 
responses in which lowered blood ionized calcium activates secre­
tion of PTH, PTH augments synthesis of 1,25(OH)2D3 by the 
proximal tubules of the kidney, and 1,25(OH)2D3 then acts 
directly upon enterocytes to increase active transcellular transport 
of calci

 increase active transcellular transport 
of calcium. Enhanced intestinal calcium absorption is quantita­
tively the most important response to calcium deprivation, but a 
series of other homeostatic events also occur that limit the impact 
of this stress. Renal tubular calcium reabsorption is increased by 
PTH, an effect that is enhanced by increased 1,25(OH)2D3-stim­
ulated expression of calbindin-D28K in the distal tubules. Cal­
cium reabsorption is also enhanced directly by any tendency to 

w thickness indicates relative activity of trans­
port or secretory mechanisms, whereas amounts of hormones or transported ions are related to the size of 
their notations. Parentheses indicate an inhibitory regulation. Note that the extracellular calcium concentra­
tion is well maintained, although different underlying mechanisms are involved in the two circumstances 
(see text for details). 1,25 D, 1,25-dihydroxyvitamin D3; PTH, parathyroid hormone.
1218
in ne

mone.
1218
in net bone resorption causes release of phosphate as well as cal­
cium into the extracellular fluid. Intestinal phosphate absorption 
also is increased by 1,25(OH)2D3. These phosphate loads are 
problematic, in that phosphate directly lowers ionized calcium 
in extracellular fluid, suppresses renal synthesis of 1,25(OH)2D3, 
and directly inhibits bone resorption. These potentially negative 
effects of phosphate are obviated by the powerful phosphatur

 phosphate are obviated by the powerful phosphaturic 
action of PTH and of FGF23, the secretion of which is promoted 
by phosphate, calcium, and 1,25(OH)2D.
Finally, the possibility of unrestrained secretion of PTH, 
leading to excessive bone resorption and severe hypophospha­
temia, is prevented by the effects of calcium on PTH secretion 
and by the direct suppressive effect of 1,25(OH)2D3 on the 
synthesis of PTH and of PTH receptors. As a result of these 
homeostatic responses, calcium-depriv

lt of these 
homeostatic responses, calcium-deprived people maintain near-
normal serum calcium and phosphate concentrations but dis­
play increased intestinal calcium absorption, increased bone 
resorption and progressive osteopenia, increased renal tubular 
calcium reabsorption, decreased renal tubular phosphate reab­
sorption, low urinary calcium excretion, elevated urinary phos­
phate excretion, and high serum concentrations of PTH and 
Calcium loads induce an opposite series of

OH)2D3.
Calcium loads induce an opposite series of adaptations: para­
thyroid suppression, inhibition of renal 1,25(OH)2D3 synthesis, 
decreased intestinal active transport of calcium, increased renal 
excretion of calcium and decreased renal excretion of phosphate 
(secondary to functional hypoparathyroidism), and a decrease in 
bone resorption sufficient to allow positive skeletal calcium bal­
ance. The decline in intestinal calcium absorption is the major 
safeguard against calcium overload, 

is the major 
safeguard against calcium overload, although this mechanism may 
be overridden with extraordinarily high intakes of calcium because 
of the persistence of the passive, non-vitamin D–dependent mode 
of calcium absorption. Moreover, nonenteral sources of calcium, 
such as intravenous calcium infusion or excessive net bone resorp­
tion (as from immobilization or malignancy), may readily over­
whelm the limited homeostatic adaptations that remain once 
suppressed intestinal calcium abs

hat remain once 
suppressed intestinal calcium absorption is bypassed. In such situ­
ations, the kidney rather than the intestine becomes the principal 
defense against hypercalcemia, and calcium homeostasis becomes 
critically dependent on adequate renal function. If renal function 
is impaired in these settings, as frequently occurs clinically, severe 
hypercalcemia and pathologic calcium deposition in extraskeletal 
sites may ensue. 
Laboratory Assessment of Mineral 
Parathyroid Ho

y Assessment of Mineral 
The major challenges in the measurement of blood PTH have 
been the low levels of circulating PTH and the presence of inac­
tive PTH fragments in far greater abundance than for the intact, 
biologically active PTH molecule. The measurement of inactive 
fragments would not be a concern if the ratio of inactive to active 
PTH molecules remained constant. However, this ratio does 
change in response to changes in GFR and in parathyroid gland 

ponse to changes in GFR and in parathyroid gland 
secretory activity (see earlier discussions under “Parathyroid Hor­
mone Secretion” and “Metabolism of Parathyroid Hormone”). 
Consequently, radioimmunoassays of PTH have suffered from 
lack of sensitivity and from the inability to measure the biologi­
cally active hormone directly.
For these reasons, two-site assays that require the presence of 
amino-terminal and carboxy-terminal sequences of full-length 
PTH(1-84) on the same molecule have rep

ll-length 
PTH(1-84) on the same molecule have replaced older radioim­
munoassays.269 The assays are sensitive enough to detect PTH in 
all normal persons. The assays have demonstrated modest circa­
dian variation in PTH levels and some pulsatility in PTH secre­
tion, but these variations have not interfered with the diagnostic 
usefulness of randomly drawn PTH measurements. Some studies 
have reported modest increases of PTH levels with age, although 
others have not. Unlike older radioimmunoas

hough 
others have not. Unlike older radioimmunoassays, the two-site 
assays demonstrate virtually no overlap in PTH levels between 
patients with primary hyperparathyroidism and those with non­
parathyroid hypercalcemia (Fig. 29.19). Because this distinction 
represents the most important challenge in the clinical setting, the 
use of the two-site assay has dramatically facilitated the clinician’s 
task.
This straightforward picture has been complicated by the real­
ization that most two-site a

plicated by the real­
ization that most two-site assays detect small amounts of PTH 
fragments that are large but do not extend to the hormone’s amino-
terminus.270 These fragments accumulate in significant amounts 
in patients with renal failure. These observations have prompted 
the development of two-site PTH assays that use antibodies spe­
cific for the first four amino acids of PTH and thus do not detect 
large fragments of PTH. Although it seems plausible that such 
assays might prove part

seems plausible that such 
assays might prove particularly useful in some clinical situations, 
their role is presently unclear. They offer no advantage over older 
two-site assays, for example, in diagnosing primary hyperparathy­
roidism.271 These assays do detect a minor peak of PTH immu­
noreactivity that is also recognized by antibodies directed to the 
carboxy-terminus of PTH and yet is not recognized by the usual 
Parathyroid hormone(1-84) (pg/mL)
Normal
Primary
hyperpara-
thy

(pg/mL)
Normal
Primary
Hypopara-
Hyper-
calcemia of
malignancy
1000
500
100
90
80
70
60
50
40
30
20
10
1
1
• Fig. 29.19  Intact immunoreactive parathyroid hormone (PTH) determined 
using a two-site immunoradiometric assay in normal subjects and in 
three different patient groups. Note some overlap between normal people 
and patients with primary hyperparathyroidism, but no overlap between 
hypercalcemic patients with primary hyperparathyroidism and 

mic patients with primary hyperparathyroidism and those with 
hypercalcemia of malignancy. (From Segre GV. Advances in techniques 
for measurement of parathyroid hormone: current applications in clinical 
medicine and directions for future research. Trends Endocrinol Metab. 
1990;1:243–247.)
1219
“intact” PTH assays, perhaps because of a post-translational mod­
ification of PTH in the PTH(15-20) region.272 Perhaps because of 
detection of 

5-20) region.272 Perhaps because of 
detection of this unusual form of PTH, the PTH immunoreactiv­
ity recognized using the antibody to PTH(1-4) is particularly high 
in patients with parathyroid cancer when compared to patients 
with benign primary hyperparathyroidism.273 
The measurement of PTHrP in serum presents a series of chal­
lenges. The concentration of PTHrP in the bloodstream, even in 
some patients with PTHrP-mediated malignant hypercalcemia, 
is n

with PTHrP-mediated malignant hypercalcemia, 
is not high, and the molecular definition of circulating, biologi­
cally active fragments is incomplete. Despite these problems, 
several groups of investigators have developed assays for PTHrP 
that can be helpful in the evaluation of a subset of hypercalce­
mic patients. Radioimmunoassays for amino-terminal portions 
of PTHrP and two-site assays for amino-terminal and midre­
gion PTHrP124 separate healthy persons and patients with non­
malignant hy

ealthy persons and patients with non­
malignant hypercalcemia from most patients with the humoral 
hypercalcemia of malignancy (Fig. 29.20). When measured with 
the most recently developed assays, PTHrP levels are elevated in 
almost all patients with malignant hypercalcemia without bone 
metastases and in most patients with hypercalcemia and bone 
metastases.
In occasional patients, the PTHrP assay has helped distinguish 
an occult malignancy from other causes of non–PTH-dependent 
hypercalcemi

om other causes of non–PTH-dependent 
hypercalcemia. Nevertheless, because the diagnosis of malignancy 
as the cause of hypercalcemia is usually clinically obvious, and the 
PTH assay can be used to diagnose primary hyperparathyroidism, 
the role of PTHrP assays in clinical practice is limited.274 
Several assays for measuring serum calcitonin are commercially 
available. The measurements are based on single or double anti­
body radioimmunoassays or enzyme immunoassays, several of 
wh

mmunoassays or enzyme immunoassays, several of 
which are sufficiently sensitive to detect calcitonin deficiency.275 
Interference with calcitonin assays has been reported in individu­
als with a heterophile antibody.276 The calcitonin monomer is 
thought to be the biologically active molecule, therefore some 
investigators believe extraction of the multimeric forms prior to 
radioimmunoassay provides a more sensitive and specific measure­
ment of serum calcitonin levels. However, the double ant

f serum calcitonin levels. However, the double antibody 
assays are thought to provide the same information with less sam­
ple manipulation. The only clinical use of the calcitonin assay is as 
a tumor marker, primarily in medullary carcinoma of the thyroid. 
Vitamin D Metabolites
Several non-radioligand assays are available for determining the 
levels of vitamin D metabolites. These assays have shown consider­
able inter-assay variability, leading to misclassification of the vita­
min D status 

ng to misclassification of the vita­
min D status of individuals.277,278 Therefore mass spectrometry is 
being increasingly used for measuring 25(OH)D levels. Regardless 
of the method used, it has been recognized that a central reposi­
tory for uniform standards is required for validation of these assays 
and of the laboratories performing them. The National Institute 
PTHrP(1 to 74) (pmol/L)
Normal
subjects
(60)
Humoral
hyper-
calcemia
— cancer
(30)
Normo-
calcemia
— cancer
(23)
Hyper-
calcemi

(30)
Normo-
calcemia
— cancer
(23)
Hyper-
calcemia
— misc.
causes
(4)
Hyper-
parathy-
roidism
(13)
Hypo-
parathy-
roidism
(3)
Chronic
renal
failure
(15)
Milk
(7)
Local
osteolytic
hyper-
calcemia
— cancer
(8)
60,000
6,000
100
40
30
20
10
0
• Fig. 29.20  Plasma PTHrP(1-74) determined by two-site immunoradiometric assay in selected patient 
groups and normal subjects. Also shown are concentrations of PTHrP in human milk (filled circles) and in 
bovine milk (open circles). Two normocalcemic patient

ine milk (open circles). Two normocalcemic patients with cancer (filled triangles) subsequently became 
hypercalcemic. Shaded area denotes levels too low to detect with this assay. PTHrP, parathyroid hor­
mone–related protein. (Adapted from Burtis WJ, Brady TG, Orloff JJ, et al. Immunochemical characteriza­
tion of circulating parathyroid hormone–related protein in patients with humoral hypercalcemia of cancer. 
N Engl J Med. 1990;322:1106–1112.)
1220
of Standar

1220
of Standards and Technology in the United States has developed 
standard reference materials for this purpose.279
The levels of 25(OH)D correlate better with the clinical signs 
and symptoms of vitamin D deficiency than do the levels of 
1,25(OH)2D3. Because the 25-hydroxylation of vitamin D is not 
tightly regulated, measurements of 25(OH)D more accurately 
reflect body stores of vitamin D. Measurement of this metabo­
lite should therefore be performed wh

this metabo­
lite should therefore be performed when vitamin D deficiency is 
suspected.
Measurements of 1,25(OH)2D3 should be reserved for cases in 
which excessive or impaired 1α-hydroxylation is suspected. High 
1,25(OH)2D3 levels can be seen in sarcoidosis, lymphomas, Wil­
liams syndrome, and intoxication with 1α-hydroxylated metabo­
lites (see “Parathyroid-Independent Hypercalcemia”). Impaired 
1α-hydroxylation can contribute to the hypocalcemia of patients 
with renal dysfunction, oncogeni

emia of patients 
with renal dysfunction, oncogenic osteomalacia, and hereditary 
defects of vitamin D metabolism (see “Hypocalcemic Disorders”). 
Currently two types of immunoassays are available for the mea­
surement of serum FGF23 in humans. An assay using two poly­
clonal antibodies directed against C-terminal epitopes280 detects 
most, if not all, circulating forms of FGF23 but does not discrimi­
nate between the intact active hormone and the cleaved fragment, 
w

intact active hormone and the cleaved fragment, 
which is not thought to have biologic activity. Assays for the intact 
hormone are classic sandwich assays with antibodies directed 
against both the N-terminus and C-terminus of the hormone.235 
These latter assays have been shown to be more useful for study­
ing the effects of dietary phosphate on FGF23 levels in humans 
and thus are thought to provide a more precise determination of 
the biologically active levels of the hormone in the circulat

cally active levels of the hormone in the circulation.281 
Hypercalcemic Disorders
Parathyroid-Dependent Hypercalcemia
It is useful to delineate two categories of hypercalcemia: (1) that 
associated with dysfunction of the parathyroid cell and (2) that in 
which hypercalcemia occurs despite appropriate parathyroid sup­
pression. This distinction is particularly useful clinically because 
it emphasizes the centrality of the PTH assay in the diagnostic 
approach to the hypercalcemic patient. Abnor

stic 
approach to the hypercalcemic patient. Abnormal parathyroid 
glands are associated with hypercalcemia in three settings: (1) pri­
mary hyperparathyroidism, (2) familial hypocalciuric hypercalce­
mia, and (3) lithium-induced hypercalcemia.
Primary Hyperparathyroidism
In primary hyperparathyroidism, a primary abnormality of para­
thyroid tissue leads to inappropriate secretion of PTH. In con­
trast, increased secretion of PTH that is an appropriate response 
to hypocalcemia is called seconda

priate response 
to hypocalcemia is called secondary hyperparathyroidism. The 
inappropriately high serum concentration of PTH in primary 
hyperparathyroidism, in turn, sustains excessive renal calcium 
reabsorption, phosphaturia, and 1,25(OH)2D synthesis, as well 
as increased bone resorption. These actions of PTH produce 
the characteristic biochemical phenotype of hypercalcemia and 
hypophosphatemia, loss of cortical bone, hypercalciuria, and the 
various clinical sequelae of chronic hypercal

the 
various clinical sequelae of chronic hypercalcemia. Primary hyper­
parathyroidism results most often (75–80%) from the occurrence 
of one or more adenomas in previously normal parathyroid glands, 
although in 20% of cases diffuse hyperplasia of all parathyroid 
glands may be present or, rarely, parathyroid carcinoma may be 
found (<1–2%).282–285
Classic Primary Hyperparathyroidism
The bone disease “osteitis fibrosa cystica” first was described by 
von Recklinghausen in 1891, but the etiolog

ed by 
von Recklinghausen in 1891, but the etiologic link between this 
disease and parathyroid neoplasms was not established until 1925, 
when Mandl observed clinical improvement following removal 
of a parathyroid adenoma from a young male with severe bone 
disease. In early clinical descriptions of primary hyperparathy­
roidism, the disease emerged as a distinctly uncommon disorder 
with significant morbidity and mortality rates, in which nearly all 
affected patients manifested radiographica

ly all 
affected patients manifested radiographically significant or symp­
tomatic skeletal or renal involvement, or both.
The skeletal involvement in classic primary hyperparathyroid­
ism reflects a striking and generalized increase in osteoclastic 
bone resorption, which is accompanied by fibrovascular marrow 
replacement and increased osteoblastic activity. The radiographic 
appearance (Fig. 29.21) features generalized demineralization of 
bone, with coarsening of the trabecular pattern (due 

e, with coarsening of the trabecular pattern (due to osteoclas­
tic resorption of the smaller trabeculae); characteristic subperios­
teal resorption, often most evident in the phalanges of the hands, 
which gives an irregular, serrated appearance to the outer, subperi­
osteal cortex and may progress to extensive cortical resorption; 
bone cysts, usually multiple, which contain a brownish serous or 
• Fig. 29.21  Radiograph of the hand of a patient with severe primary 
hyperparathyroidism. Note t

t with severe primary 
hyperparathyroidism. Note the dramatic remodeling associated with the 
intense region of high bone turnover in the third metacarpal in addition to 
widespread evidence of subperiosteal, endosteal, and trabecular resorp­
tion. (Courtesy Fuller Albright Collection, Massachusetts General Hospital, 
Boston, MA.)
1221
mucoid fluid, and tend to occur in the central medullary portions 
of the shafts of the metacarpals, ribs

y portions 
of the shafts of the metacarpals, ribs, or pelvis and may expand 
into and disrupt the overlying cortex; osteoclastomas, or brown 
tumors, composed of numerous multinucleated osteoclasts (“giant 
cells”) admixed with stromal cells and matrix, which are found 
most often in trabecular portions of the jaw, long bones, and ribs; 
and pathologic fractures.
The skull may exhibit a finely mottled, “salt-and-pepper” 
radiographic appearance, with loss of definition of the inner and 
outer c

 with loss of definition of the inner and 
outer cortices. Dental radiographs typically show erosion or disap­
pearance of the lamina dura due to subperiosteal resorption, often 
with extension into the adjacent mandibular bone. The erosion 
and demineralization of cortical bone may lead to radiographic 
disappearance of some bones, most notably the tufts of the distal 
phalanges of the hands, the inferolateral cortex of the distal third 
of the clavicles, the distal ulna, the inferior margin of

clavicles, the distal ulna, the inferior margin of the femoral 
neck and pubis, and the medial aspect of the proximal tibia. The 
clinical correlates of these changes may include aching bone pain 
and tenderness, “bowing” of the shoulders, kyphosis and loss of 
height, and collapse of lateral ribs and pelvis with “pigeon breast” 
and triradiate deformities, respectively.
The renal manifestations of classic severe primary hyperpara­
thyroidism include recurrent calcium nephrolithiasis, nephro­
ca

lude recurrent calcium nephrolithiasis, nephro­
calcinosis, and renal functional abnormalities that range from 
impaired concentrating ability to end-stage renal failure. Associ­
ated signs and symptoms include recurrent flank pain, polyuria, 
and polydipsia. No unique features of the stone disease in pri­
mary hyperparathyroidism serve to distinguish it from that associ­
ated with other, more common causes of calcium kidney stones. 
The stone disease more often may be recurrent and severe, and 

sease more often may be recurrent and severe, and 
in some patients the stones may be composed entirely of calcium 
phosphate instead of the pure oxalate or mixtures of oxalate and 
phosphate more commonly encountered in other disorders. In 
patients diagnosed before 1965, the frequency with which neph­
rolithiasis complicated primary hyperparathyroidism was as high 
as 60% to 80% (the frequency is currently <25%), yet in studies 
of unselected patients conducted throughout the past 50 years, 
p

atients conducted throughout the past 50 years, 
primary hyperparathyroidism has accounted for fewer than 5% of 
all calcium kidney stones.
Other clinical features that have been reported in association 
with classic severe primary hyperparathyroidism are conjunctival 
calcifications, band keratopathy, hypertension (50%), gastrointes­
tinal signs and symptoms (anorexia, nausea, vomiting, constipa­
tion, or abdominal pain), peptic ulcer disease, and acute or chronic 
pancreatitis. The issue of wh

nd acute or chronic 
pancreatitis. The issue of whether primary hyperparathyroidism 
increases the risk for peptic ulcer disease and pancreatitis remains 
controversial. Although hyperparathyroidism is associated with a 
higher risk of hypertension, successful parathyroidectomy has not 
been shown to correct the hypertension.
Signs and symptoms in primary hyperparathyroidism may 
result from the involvement of bone (fracture, bone pain) or kid­
neys (renal colic, renal failure), peptic ulcer dis

eys (renal colic, renal failure), peptic ulcer disease, pancreatitis, 
or hypercalcemia per se (weakness, apathy, depression, polyuria, 
constipation, coma). The presence and severity of neuropsychi­
atric symptoms, in particular, correlate poorly with the serum 
calcium concentration, although few patients with severe hyper­
calcemia are entirely asymptomatic. Elderly persons are most likely 
to exhibit such symptoms. A peculiar neuromuscular syndrome, 
first described in 1949 but rarely encoun

ndrome, 
first described in 1949 but rarely encountered now, includes sym­
metric proximal weakness and gait disturbance, with muscle atro­
phy, characteristic electromyographic abnormalities, generalized 
hyperreflexia, and tongue fasciculations.286 
Contemporary Primary Hyperparathyroidism
The clinical spectrum of primary hyperparathyroidism was 
changed dramatically in the early 1970s by the introduction of rou­
tine multichannel serum chemistry screening, which unearthed a 
large population 

ry screening, which unearthed a 
large population of patients with previously unsuspected, asymp­
tomatic disease. In Rochester, Minnesota, for example, the annual 
incidence of the disease increased abruptly from 0.15 to 1.12 per 
1000 persons between the prescreening era (1965–1974) and 
1975, the year after routine screening was introduced.287 The peak 
incidence occurs in the sixth decade of life, and the disease rarely is 
encountered in patients below age 15. It is two to three times more 

ients below age 15. It is two to three times more 
common in women, who are slightly older at diagnosis than are 
men. Subsequently, the incidence of primary hyperparathyroidism 
has fallen; this decreased incidence may not simply be a residual 
effect of “sweeping the population,” because it remained low when 
serially checked in Rochester—most recently from 1992 to 2001, 
when the incidence was 0.21 per 1000.288
Ascertainment of mild or asymptomatic disease may decline 
even further in the fut

matic disease may decline 
even further in the future because of prevalent economic disincen­
tives to routine serum chemistry screening in the primary care set­
ting. On the other hand, insistence upon overt hypercalcemia as a 
diagnostic criterion may underestimate the true incidence of the 
disease. For example, when serum calcium and iPTH were mea­
sured in a large population of Swedish women undergoing routine 
mammographic screening, the prevalence of unsuspected primary 
hyperparathyroidi

evalence of unsuspected primary 
hyperparathyroidism, defined by criteria that included the combi­
nation of high-normal serum calcium plus elevated or high-normal 
iPTH, was 2.1%.289 Two-thirds of these women (72/109) were 
normocalcemic (10–10.4 mg/dL), yet bone density was reduced 
in the group as a whole, and the disease was confirmed histologi­
cally in 98% of the 61 who had surgery. Further, the widespread 
practice of evaluating PTH levels in patients with osteoporosis 
has led to the ide

 in patients with osteoporosis 
has led to the identification of patients with high PTH levels and 
normocalcemia.290 Many of these patients become hypercalcemic 
during follow-up.
Not surprisingly, given that primary hyperparathyroidism now 
usually is diagnosed incidentally, few patients are found to have 
overt signs or symptoms of the classic disease and thus are consid­
ered to be asymptomatic. For example, only 2% of patients with 
primary hyperparathyroidism residing in Olmsted County, Mi

hyperparathyroidism residing in Olmsted County, Min­
nesota, and only 17% of 121 patients studied at an academic refer­
ral center in New York City had classic disease symptoms.287,291 
In most of them, the relevant symptom was urolithiasis. Many 
clinicians argue, however, that most patients regarded as having 
asymptomatic primary hyperparathyroidism and only minimally 
elevated serum calcium actually suffer from various neuropsy­
chiatric or other symptoms that may improve following curative 

ther symptoms that may improve following curative 
surgery.292 These symptoms, however, which include fatigabil­
ity, weakness, forgetfulness, depression, somatization, polydip­
sia, polyuria, and bone and joint pain, are common in otherwise 
normal persons. In the small randomized studies of surgery for 
primary hyperparathyroidism (see later), the effects of surgery on 
measures of quality of life have been conflicting.290,293–295 This 
remains a critical issue, as the advent of less invasive 

 a critical issue, as the advent of less invasive operative 
approaches and concerns regarding fracture, cancer, and mortal­
ity risk have lowered the threshold for consideration of surgery in 
many patients with the disease (see later). Throughout this chap­
ter, asymptomatic primary hyperparathyroidism refers to patients 
who lack signs or symptoms of the classic disease, whether or not 
they experience any of the subtle symptoms mentioned earlier.
The natural history of untreated asymptomatic

ier.
The natural history of untreated asymptomatic primary hyper­
parathyroidism, as currently detected, remains incompletely 
1222
understood. Few patients seem to experience progression of dis­
ease, as measured by extreme elevations of serum or urinary cal­
cium, appearance of renal dysfunction or nephrocalcinosis, or 
worsening osteopenia, over many years of observation.291 On the 
other hand, late cortical bone loss observed at the femoral neck 
and distal 

one loss observed at the femoral neck 
and distal radius in a small number of patients followed with­
out surgery for 15 years points to the potential importance of 
continued monitoring in such patients.296 Also, an excess risk of 
mortality, mainly from cardiovascular disease, has been noted dur­
ing extended follow-up of large cohorts of patients with chronic 
hypercalcemia (and presumed primary hyperparathyroidism) 
identified by population health screening in Sweden,297 and simi­
lar observ

ealth screening in Sweden,297 and simi­
lar observations have been made during extended follow-up of 
postsurgical patients with hyperparathyroidism.292 Associations 
of hypertension, hyperuricemia, and glucose intolerance with 
primary hyperparathyroidism have been implicated, together 
with hypercalcemia per se, as contributors to this elevated risk.291 
Abnormal cardiac calcification and left ventricular hypertrophy 
(reversible by successful parathyroidectomy) have been reported 
in primary 

parathyroidectomy) have been reported 
in primary hyperparathyroidism as well.298 Increased cardiovas­
cular mortality risk may be a feature only of severe hyperpara­
thyroidism, as it was restricted to those in the highest quartile 
of serum calcium in the Olmsted County study, which otherwise 
showed an overall decreased risk of death.299 A 40% excess risk 
of malignancy also was reported among 4163 Swedish patients 
who had undergone surgery more than a year earlier for (presum­
ably symptoma

ore than a year earlier for (presum­
ably symptomatic) primary hyperparathyroidism.300 It has been 
argued that these increased risks for mortality and malignancy, 
even if confirmed, may apply only to those with primary hyper­
parathyroidism that is more severe than the asymptomatic version 
typically encountered today.291
Abnormalities of bone in modern, mild primary hyperpara­
thyroidism are far subtler than those associated with the classic 
disease. Histologically the rate at which new bone

disease. Histologically the rate at which new bone remodeling 
cycles are activated is increased. Because the phase of restorative 
bone formation at each remodeling site takes much more time 
than does the initial resorptive phase, such an increase in remodel­
ing rate inevitably increases the ambient volume of the remodeling 
space and thus the porosity of bone. Depending upon the rate 
and extent of the accompanying increase in osteoblastic activity 
and the resulting local balance between ne

tivity 
and the resulting local balance between net bone formation and 
resorption, mineralized bone volume may decrease further, remain 
stable, or even increase (despite an increased remodeling space). 
For reasons not yet understood, the balance achieved between 
increased resorption and formation of bone in primary hyperpara­
thyroidism depends not only upon the severity of the hyperpara­
thyroidism but also upon skeletal location. Thus net resorption 
of endosteal bone may predominate in co

esorption 
of endosteal bone may predominate in cortical sites, whereas net 
apposition of mineral may occur in trabecular bone, when mea­
sured through biopsy of the iliac crest301,302 (Fig. 29.22). Thus 
bone mineral density may be reduced, particularly at sites of 
predominantly cortical bone such as the midradius, by as much 
as 10% to 20%.303 In contrast, studies using dual-energy x-ray 
absorptiometry (DXA) demonstrate relative preservation of ver­
tebral bone mass in primary hyperparathyr

 of ver­
tebral bone mass in primary hyperparathyroidism.304 Despite this 
evidence of preserved trabecular bone mass in primary hyperpara­
thyroidism, the incidence of vertebral fracture is increased.305,306 
The explanation for this paradox appears to relate to abnormalities 
in trabecular (and cortical) microarchitecture that can be detected 
using high-resolution peripheral quantitative computed tomog­
raphy (CT) or by evaluating the trabecular bone score during 
DXA measurements.307,308 The

ar bone score during 
DXA measurements.307,308 These abnormalities reflect shifts from 
• Fig. 29.22  Iliac crest biopsy specimens from a patient with primary hyperparathyroidism (left) and a nor­
mal control subject (right), viewed by scanning electron microscopy. Note the thin cortices and contrasting 
maintenance of trabecular bone in the patient. (From Parisien M, Silverberg SJ, Shane E, et al. The histo­
morphometry of bone in primary hyperparathyroidism: preservation of cancellous bone str

arathyroidism: preservation of cancellous bone structure. J Clin 
Endocrinol Metab. 1990;70:930–938.)
1223
plate-like to rod-like microstructure that weaken the bone and pre­
sumably account for the elevated risk of vertebral fracture. These 
defects are improved following successful parathyroidectomy.309
Kidney stones now are reported in only 10% to 25% of patients 
with primary hyperparathyroidism, although some degree of renal 
dysfunct

hyroidism, although some degree of renal 
dysfunction, either a significant reduction in creatinine clearance 
or impaired concentrating or acidifying ability, may be found in 
up to one-third of those with asymptomatic disease. These renal 
abnormalities are not progressive in the majority of affected 
patients.291,310 No parameters of disease severity predict a lower 
estimated GFR.311 The association of kidney stones with primary 
hyperparathyroidism generally is viewed as an indication for 

roidism generally is viewed as an indication for 
parathyroidectomy, however, because successful surgery usually 
prevents further symptomatic stone disease.291,292 On the other 
hand, it is not possible at present to confidently predict, from 
biochemical measurements in blood or urine, which asymptom­
atic patients with hyperparathyroidism will go on to develop new 
stone disease. Stone-formers are more likely to be hypercalciuric 
than not, but less than one-third of hypercalciuric patients w

t less than one-third of hypercalciuric patients with 
hyperparathyroidism actually develop stones. 
Etiology and Pathogenesis
Parathyroid adenomas are caused by mutations in the DNA of 
parathyroid cells; these mutations confer a proliferative or survival 
advantage for affected cells over their normal neighbors.312,313 As 
a consequence of this advantage, the descendants of one particu­
lar parathyroid cell, a clone of cells, undergo clonal expansion to 
produce an adenoma.
Multiple chromosoma

ansion to 
Multiple chromosomal regions are missing in the parathyroid 
cells of individual parathyroid adenomas. These genetic deletions 
probably reflect the deletion of tumor suppressor genes. These 
chromosomal loci include portions of chromosome 1p–pter (in 
40% of adenomas), 6q (in 32% of adenomas), 15q (in 30% of 
adenomas), and 11q (in 25–30% of adenomas). Many of the 11q 
deletions are associated, in the undeleted chromosome 11, with 
mutations in the gene encoding t

omosome 11, with 
mutations in the gene encoding the transcription factor menin, 
the gene mutated in multiple endocrine neoplasia type 1 (MEN1). 
Thus this gene is also involved commonly in somatic mutations in 
patients with sporadic parathyroid adenomas. Somatic mutations 
have also been found in the mitochondrial genomes of a fraction of 
chief cell adenomas and have been found even more frequently in 
so-called oxyphil adenomas, known to exhibit mitochondria with 
abnormal morphologic appea

ibit mitochondria with 
abnormal morphologic appearance.314 The widespread presence of 
somatic mutations in sporadic parathyroid adenomas, which are 
detectable only because large numbers of cells in any one tumor 
contain the same deletion, constitutes the strongest evidence that 
parathyroid adenomas are clonal expansions of mutant cells.
Cyclin D1, called initially PRAD1, was the first noted recur­
rent parathyroid proto-oncogene.315 This gene was discovered 
at the breakpoint of an inversio

e was discovered 
at the breakpoint of an inversion on chromosome 11 in a para­
thyroid adenoma. This inversion led to the juxtaposition of the 
PTH gene’s regulatory region and the DNA encoding cyclin D1. 
As a consequence, the cyclin D1 gene was overexpressed. Cyclin 
D1 is an important regulator of the transition from the G1 phase 
of the cell cycle (which follows mitosis) to the S phase (associ­
ated with DNA synthesis) and is mutated or amplified in a wide 
variety of malignancies. Cyclin D

ified in a wide 
variety of malignancies. Cyclin D1 is overexpressed in about 
20% of parathyroid adenomas, though cyclin D1 gene rearrange­
ments have been documented in only 5% of adenomas. Over­
expression of cyclin D1 in the parathyroids of transgenic mice 
leads to formation of parathyroid adenomas and hypercalcemia 
over many months.316 The phenotype of these mice demonstrates 
that cyclin D1 overexpression can cause primary hyperparathy­
roidism. Cyclin D1 enables the functions of cyclin-

oidism. Cyclin D1 enables the functions of cyclin-dependent 
protein kinases. More recently, inhibitors of cyclin-dependent 
protein kinases (encoding the proteins p21, p15, and p18) have 
been found mutated in the germline and somatically in sporadic 
parathyroid adenomas, in a pattern that suggests these genes act 
as tumor suppressors.317 The gene mutated in multiple endocrine 
neoplasia, type 1 (encoding menin), is also mutated only in the 
tumors of a substantial fraction of sporadic parath

umors of a substantial fraction of sporadic parathyroid tumors 
(reviewed in Costa-Guda and Arnold318).
Improvements in DNA sequencing technology have allowed 
the sequencing of virtually all the exons in the human genome, 
and this sort of whole-exome sequencing has been performed by 
three groups.319–321 The most striking finding of these studies 
(sequencing 43 tumors between them) was that these tumors aver­
aged roughly 5 exome mutations per tumor (with the exception 
of one tumor with 110 

 tumor (with the exception 
of one tumor with 110 mutations in association with a mutation 
in the protection of telomeres 1 gene [POT1] known to increase 
genetic instability). This number is considerably lower than the 
number of mutations typically found in cancers or even benign 
tumors that have been sequenced. In both studies 35% of the 
tumors harbored mutations in the MEN1 gene, usually in associa­
tion with deletion of the second MEN1 gene. No other gene was 
mutated frequently in these

ne. No other gene was 
mutated frequently in these series, though mutation in a known 
oncogene, an activating mutation in EZH2, was found a second 
time when a larger series of tumors was tested for that mutation.319 
A recent whole-exome sequencing study of 24 parathyroid cancers 
demonstrated that these tumors harbor a much higher frequency 
of mutations than do parathyroid adenomas; the genes mutated in 
parathyroid cancer, so far, appear distinct from the genes mutated 
in parathyroid adeno

tinct from the genes mutated 
in parathyroid adenomas.322
As expected for a disease caused by mutations in DNA, parathy­
roid adenomas occur more frequently in patients who underwent 
neck irradiation decades earlier, with greater radiation exposure 
leading to higher risk. Most patients have no definite history of 
exposure to specific mutagens, however. An intriguing clue that 
abnormalities of vitamin D physiology may predispose to primary 
hyperparathyroidism has come from the observation th

perparathyroidism has come from the observation that patients 
with parathyroid adenomas are more likely than others to inherit 
a particular allele of the VDR gene.323 These patients have tumors 
with particularly low levels of mRNA encoding the VDR. Never­
theless, no mutations in the coding regions of the gene encoding 
the VDR have been found in parathyroid adenomas.324
The cause of sporadic primary parathyroid hyperplasia is 
unknown. The known stimulus for parathyroid cell prolifera­
tion—

own stimulus for parathyroid cell prolifera­
tion—low levels of blood calcium or 1,25(OH)2D3—is not pres­
ent in this disease. Presumably, some other stimulus outside the 
parathyroid glands or a genetic abnormality present in all four 
parathyroid glands leads to inappropriate cell proliferation. Such 
abnormalities have been found in several inherited forms of para­
thyroid hyperplasia (see later), but most cases of parathyroid 
hyperplasia are not found in familial clusters.
The theoretic dis

 not found in familial clusters.
The theoretic distinction between adenoma as a clonal pro­
liferation and hyperplasia as a polyclonal growth is clear cut. In 
some settings, however, clonal expansion can occur in the con­
text of preexisting nonclonal proliferation. The clearest example 
of this complication has been found in the large glands associated 
with severe renal failure. In many such glands removed surgically 
because of hypercalcemia or severe parathyroid-dependent bone 
disease, evi

or severe parathyroid-dependent bone 
disease, evidence for clonal proliferation complicating secondary 
hyperplasia has been found. Interestingly, the pattern of chro­
mosomal abnormalities in these clonal tumors differs from that 
1224
found in parathyroid adenomas in the absence of renal failure.325 
Analogous mechanisms may be operative in a number of settings 
associated with stimuli to parathyroid cell proliferation, such as 
X-linked hypophosphatemia and 

iferation, such as 
X-linked hypophosphatemia and long-term lithium therapy. Fur­
thermore, just as clonal tumors can arise in the setting of secondary 
parathyroid hyperplasia, they can also arise in the setting of spo­
radic primary parathyroid hyperplasia326 and in MEN1.327
The distinction between adenoma and hyperplasia is clini­
cally important, because removal of the one abnormal gland can 
be expected to cure a parathyroid adenoma, whereas removal of 
multiple glands is required to succes

 removal of 
multiple glands is required to successfully treat parathyroid hyper­
plasia. Unfortunately, differentiating adenoma from hyperplasia 
from normal parathyroid tissue at pathologic examination is not 
straightforward. Pathologists distinguish normal from abnormal 
parathyroid glands by the increase in size and the paucity of fat 
in abnormal glands. Attempts have been made to distinguish an 
adenoma from an individual hyperplastic gland on the basis of 
morphologic features, but no cr

d on the basis of 
morphologic features, but no criteria have proved completely reli­
able.328 The formation of clonal neoplasms in originally hyper­
plastic tumors may explain some of the difficulty in pathologic 
An increase in cell number is not the only abnormality in pri­
mary hyperparathyroidism. The ability of the normal parathyroid 
cell to suppress PTH secretion in response to hypercalcemia might 
be expected to protect the individual from sustained hypercalce­
mia, even if t

dividual from sustained hypercalce­
mia, even if the number of parathyroid cells increased moderately. 
Unfortunately, parathyroid cells in parathyroid adenomas usually 
demonstrate abnormalities in their responsiveness to calcium, 
with a shift in set-point to the right (Fig. 29.23). This set-point 
shift, combined with the nonsuppressible component of PTH 
secretion, leads to a new steady state in which both the PTH level 
and the blood calcium level are higher than normal. The molecu­
lar und

 level are higher than normal. The molecu­
lar underpinning of the abnormal parathyroid cell responsiveness 
is beginning to be understood. Parathyroid cells from adenomas 
respond to changes in extracellular calcium with smaller than nor­
mal increases in intracellular calcium, and the amount of CaSR 
protein on the cell surface is reduced.329 Perhaps surprisingly, no 
mutations in the gene encoding the CaSR have been found in 
parathyroid adenomas. In the experimental model in which over­
expr

mas. In the experimental model in which over­
expression of cyclin D1 results in primary hyperparathyroidism,316 
reduced expression of the CaSR occurs only after cell proliferation 
has been increased for some time. Thus the decreased expression 
of the CaSR in parathyroid adenomas is likely to be a secondary 
response that occurs during tumor formation. One demonstrated 
regulator of expression of the CASR gene in parathyroid cells is the 
developmental regulator, gcm2.330 
Inherited Primary H

lopmental regulator, gcm2.330 
Inherited Primary Hyperparathyroidism
Although uncommon, inherited forms of primary hyperparathy­
roidism are clinically important for several reasons. The manage­
ment of the parathyroid tumors found in familial parathyroid 
syndromes often differs from that of sporadic primary hyperpara­
thyroidism. Furthermore, extraparathyroidal manifestations of 
inherited syndromes may need treatment, and awareness of famil­
ial clustering should prompt systematic family scre

al clustering should prompt systematic family screening.
Multiple Endocrine Neoplasia Type 1 (see also Chapter 42). 
MEN1 is caused by inactivating mutations in the tumor suppres­
sor gene encoding menin.331 Menin is a ubiquitously expressed 
transcription factor that is part of a complex that targets histone 
H3 for methylation,332 and thereby leads to expression of cell 
cycle inhibitors in pancreatic islets and other tissues.333 Menin 
interacts with many nuclear proteins, and it is not known

ts with many nuclear proteins, and it is not known which 
interactions are central to the pathogenesis of MEN1.334 Rarely, 
mutations in genes encoding cyclin-dependent kinase inhibitors, 
such as p27, are found in MEN1 patients without menin muta­
tions335; some have termed this variant, which presents primarily 
with parathyroid and pituitary tumors, MEN4.336 These patients 
can also harbor pancreatic neuroendocrine tumors, gonadal, adre­
nal, renal, and thyroid tumors.337,338 Although MEN1 in

renal, and thyroid tumors.337,338 Although MEN1 includes 
tumors of the parathyroid, anterior pituitary, and pancreatic islets, 
the parathyroid tumors are far more prevalent than the others; 
95% of affected patients eventually develop hyperparathyroidism. 
Most of the parathyroid tumors harbor mutations in both cop­
ies of the menin gene; one mutation is inherited and the second 
occurs in the parathyroid cell whose progeny form the tumor.
The onset of hypercalcemia occurs in the second and th

onset of hypercalcemia occurs in the second and third 
decades of life, though occasional patients present in the first 
decade. Hypercalcemia never presents at birth or in infancy. The 
disease involves all four parathyroid glands, although the involve­
ment can be asymmetric and apparently asynchronous. Apart 
from the earlier age at diagnosis, the presenting clinical picture 
generally resembles that of sporadic primary hyperparathyroid­
ism, perhaps with somewhat greater loss of bone density

perhaps with somewhat greater loss of bone density.339 
One common complicating feature is that hypercalcemia can 
B
48
26
24
22
50
47
45
27
PTH release (% of maximal)
100
50
0
0
1.0
2.0 3.0
A
100
50
0
• Fig. 29.23  Abnormal patterns of parathyroid hormone (PTH) secretion 
from cells prepared from adenomatous glands and stimulated with varying 
levels of calcium in tissue culture. The shaded area shows the pattern of 
PTH release (±1 SD) from normal human parathyroid cells. Panel (A)

SD) from normal human parathyroid cells. Panel (A) illus­
trates the pattern from four patients with little suppression of PTH secre­
tion by calcium. Panel (B) illustrates the pattern from four patients with 
relatively intact mechanism of suppression of PTH secretion by calcium. 
Even in this group the set-point for calcium suppression is shifted to the 
right. (From Brown, EM. Calcium-regulated parathyroid hormone release 
in primary hyperparathyroidism: studies in vitro with dispersed parath

thyroidism: studies in vitro with dispersed parathyroid 
cells. Am J Med. 1979;66:923–931.)
1225
dramatically increase the gastrin levels and symptomatology of 
patients who also have gastrinomas. Treatment of the parathyroid 
disease in this setting can greatly simplify the management of the 
gastric hyperacidity. After parathyroid surgery, hypoparathyroid­
ism and recurrent hyperparathyroidism are more common than 
in other forms of hype

idism are more common than 
in other forms of hyperparathyroidism.340 The timing and type 
of surgery are therefore more complicated issues than in sporadic 
primary hyperparathyroidism. Most authorities agree that para­
thyroid disease recurs eventually, particularly if fewer than three 
glands are removed. Some surgeons prefer subtotal parathyroidec­
tomy, whereas others prefer total parathyroidectomy with forearm 
implantation of a small amount of parathyroid tissue. 
Multiple Endocrine Neopl

t of parathyroid tissue. 
Multiple Endocrine Neoplasia Type 2a (see also Chapter 42). 
Parathyroid disease is a usually late and infrequent (5–20%) 
occurrence in MEN2a, a disease defined by the clustering of med­
ullary carcinoma of the thyroid, pheochromocytoma, and hyper­
parathyroidism. The parathyroid disease is typically mild and can 
be asymptomatic.338 In some families, hyperparathyroidism is 
more common; however, these families have the same mutations 
in the RET gene as are found in f

 same mutations 
in the RET gene as are found in families without frequent hyper­
parathyroidism. Both parathyroid hyperplasia and adenoma have 
been noted at surgery. Because asymptomatic parathyroid hyper­
plasia has been noted at the time of thyroid surgery, a progres­
sion from hyperplasia to adenoma in MEN2a has been suggested. 
The approach to diagnosis and treatment of hyperparathyroid­
ism is similar to that in sporadic primary hyperparathyroidism, 
but hyperplasia is more frequently the

hyroidism, 
but hyperplasia is more frequently the underlying disorder. The 
pathogenesis of the hyperparathyroidism is uncertain, but the 
RET gene, mutated in virtually all cases of MEN2a, is expressed in 
parathyroid cells,341 so abnormal RET expression in parathyroid 
cells may directly cause parathyroid tumor formation. Hyperpara­
thyroidism does not occur in MEN2b, the variant associated with 
mucosal neuromas. 
Hyperparathyroidism–Jaw Tumor Syndrome. Patients with 
hereditary hyperparathy

r Syndrome. Patients with 
hereditary hyperparathyroidism–jaw tumor syndrome342 present 
with parathyroid adenomas that can be multiple and that are usu­
ally cystic. These tumors are often but not invariably associated 
with fibrous jaw tumors that are unrelated to the hyperparathy­
roidism. Importantly, the parathyroid tumors are frequently malig­
nant, in contrast to the findings in MEN1 and MEN2. Wilms 
tumor and polycystic renal disease also have occurred in affected 
families. The gene mut

 have occurred in affected 
families. The gene mutated in this syndrome, called CDC73 or 
HRPT2, encodes the nuclear protein, parafibromin.343 Parafibro­
min is part of the evolutionarily highly conserved PAF complex 
that binds RNA polymerase II, regulates chromatin structure, and 
regulates gene expression.344–346 Parafibromin binds β-catenin 
and can mediate Wnt pathway signaling and notch signaling, 
though whether these properties of parafibromin are related to 
its tumor suppressor functio

romin are related to 
its tumor suppressor function is unknown.347 Inactivating muta­
tions in parafibromin are found in a high number of patients 
with apparently sporadic parathyroid cancer.348 In a whole-exome 
sequencing study of sporadic parathyroid cancer, 8 of 17 patients 
harbored mutations in CDC73; half of the patients with CDC73 
mutations also manifest germline mutations, even though they 
presented as patients with apparently sporadic disease.322 These 
findings suggest that all pat

c disease.322 These 
findings suggest that all patients with parathyroid cancer should 
be screened for germline mutations in CDC73. 
Management of Primary Hyperparathyroidism
The strategy for management of primary hyperparathyroidism has 
evolved in parallel with the changing presentation of the disease. 
The only opportunity for permanent cure is surgical removal of 
the abnormal gland(s), an approach that clearly was appropriate 
for virtually all patients in whom the classic, severe form of 

 all patients in whom the classic, severe form of the 
disease was diagnosed many decades ago and which still is the 
treatment of choice for those patients who do present with recur­
rent kidney stones, nephrocalcinosis, clinically overt bone disease, 
or severe hypercalcemia.
In contrast, the choice of surgical versus medical management 
for patients with asymptomatic primary hyperparathyroidism 
remains an open and hotly debated question. Those who favor 
surgery point to the expected improve

e who favor 
surgery point to the expected improvement in bone mineral den­
sity (at the hip and spine) and left ventricular hypertrophy follow­
ing successful surgical intervention; evidence of increased risk for 
fracture, cardiovascular mortality, malignancy, and neuropsychiat­
ric symptoms associated with primary hyperparathyroidism; and 
the recent successful development of effective minimally invasive 
surgical procedures (see later). Those who favor an observational 
approach emphasize th

 who favor an observational 
approach emphasize the evidence for lack of disease progression 
in most asymptomatic patients; the small but finite risk of surgi­
cal failure and postoperative complications; the probability that 
excess mortality and cancer risks documented in patients with 
relatively severe disease may not apply to those with mild, asymp­
tomatic primary hyperparathyroidism; the difficulty in assigning 
vague neuropsychiatric symptoms to the parathyroid disorder; 
the lack of ev

ptoms to the parathyroid disorder; 
the lack of evidence (or negative evidence) that hypertension and 
increased risk of cancer, fracture, or cardiovascular mortality, even 
if present, are improved by successful parathyroidectomy; and the 
availability of sensitive techniques for monitoring disease status in 
nonoperated patients.291
Unfortunately no large prospective studies powered to com­
pare clinical outcomes in patients with asymptomatic primary 
hyperparathyroidism randomly assigned to s

rimary 
hyperparathyroidism randomly assigned to surgery versus medical 
management have been conducted. Nevertheless, three valuable 
smaller, randomized controlled trials of surgery versus observation 
have been conducted that allow some conclusions about surrogate 
markers of disease.293–295 All three trials demonstrated increases 
in bone density in the spine and hip in the surgical group; these 
increases were similar to those produced with bisphosphonate 
therapy in primary hyperparathyroi

bisphosphonate 
therapy in primary hyperparathyroidism (see later). Two of the 
three studies showed modest improvements in some quality-of-
life measures, though the unblinded nature of the studies limits 
interpretation of these findings. All of the findings reported so far 
from these studies have been after 2 years or less. As useful as these 
studies have been, their limitations have forced the field to tap 
observational studies to draw tentative recommendations based 
on limited data.
Suc

tative recommendations based 
Such provisional recommendations have emanated from a 
series of conferences: an NIH-sponsored Consensus Confer­
ence held in 1990, followed by more informal updates in 2002, 
2008, and 2014.349 The major conclusion of that group was that, 
although surgery is indicated for symptomatic hyperparathyroid­
ism and always should be considered an appropriate option in 
asymptomatic hyperparathyroidism, many patients with asymp­
tomatic disease can be saf

ny patients with asymp­
tomatic disease can be safely monitored without surgery for many 
years. Those suitable for medical observation should have no evi­
dence of significant compromise of skeletal integrity or renal func­
tion, should have no history of urolithiasis or gastrointestinal or 
neuropsychiatric symptoms, and should meet the criteria listed in 
Table 29.1.350 Such patients account for at least 50% of those who 
currently present with primary hyperparathyroidism.
On the other hand, 

h primary hyperparathyroidism.
On the other hand, surgery could be preferable if the patient 
desired surgery even when asymptomatic, if the probability of 
consistent monitoring seemed low, if concomitant illness seemed 
1226
likely to complicate management or obscure significant disease 
progression, or if the patient was relatively young (<50 years). The 
latter recommendation reflects the absence of reliable information 
about the natural history of the dise

information 
about the natural history of the disease over many decades of fol­
low-up; the cumulative cost of medical monitoring, which begins 
to exceed that of surgery by 5 to 10 years; and some data suggest­
ing that young people are more likely than others to have progres­
sive disease.351 On the other hand, age alone was not viewed as a 
contraindication to parathyroidectomy, as the procedure has been 
accomplished with excellent results, with a perioperative mortal­
ity rate of 1% to 3%, 

ith a perioperative mortal­
ity rate of 1% to 3%, in large numbers of appropriately selected 
patients over the age of 75 years. Because hypertension is not 
thought to be a feature of mild primary hyperparathyroidism, and 
because hypertension generally is not improved by parathyroidec­
tomy, hypertension was not viewed as an indication for surgery.
Although the Consensus Conference recommendations and 
subsequent modifications provide a useful framework for decision 
making, supporting data fr

framework for decision 
making, supporting data from large clinical trials are lacking. In a 
series of 52 asymptomatic patients selected for nonoperative man­
agement mainly on the basis of the 1990 Consensus Conference 
criteria and whose course was followed for 10 years, approximately 
25% developed one or more new indications for surgery.291 
Patients who do not meet the Consensus Conference criteria for 
surgery may nevertheless experience the same postsurgical increase 
in bone density as 

he same postsurgical increase 
in bone density as those who do.352 Some have emphasized that 
evidence of baseline vertebral osteopenia, an unusual finding in 
primary hyperparathyroidism, should be considered among the 
criteria for surgery353 and that surgery also should be considered 
for postmenopausal women who exhibit vertebral bone loss in the 
setting of primary hyperparathyroidism.291
A common dilemma is the inability to ascertain whether vague 
but troublesome symptoms such as fatigue,

r vague 
but troublesome symptoms such as fatigue, lethargy, weakness 
(without objective muscle weakness), and depression are due 
to hyperparathyroidism and thus qualify as “significant” in the 
context of considering the decision for surgery. Most clinicians do 
not routinely recommend parathyroidectomy on the basis of such 
symptoms alone, although dramatic responses to surgery are occa­
sionally seen. With the availability of improved, minimally inva­
sive surgical approaches, the threshold

ally inva­
sive surgical approaches, the threshold for considering surgery in 
patients who are significantly disabled by such symptoms clearly 
is lower now than in the past. Some have advocated, in selected 
cases, a limited trial of medical therapy to reduce serum calcium 
(i.e., calcimimetics—see later) and thereby attempt to predict the 
symptomatic response to surgical cure.
Medical Monitoring of Primary Hyperparathyroidism. The 
updated NIH Consensus Conference recommendations sug­
gest t

H Consensus Conference recommendations sug­
gest that patients not treated surgically should be followed 
carefully, with annual measurement of serum calcium and cal­
culated creatinine clearance and serial determination of bone 
mineral density at intervals of 1 to 2 years. The most appropri­
ate bone densitometric site is considered to be one that reflects 
mainly changes in cortical bone (i.e., distal forearm), although 
the importance of following vertebral bone density as well 
has been emp

owing vertebral bone density as well 
has been emphasized,291 and current criteria acknowledge the 
importance of significant bone loss at any site.350
Patients undergoing nonoperative medical management must 
be cautioned to maintain adequate hydration, to avoid diuretics 
and prolonged immobilization, and to seek prompt medical atten­
tion in the event of illnesses accompanied by significant vomiting 
or diarrhea. Dietary calcium should not be restricted.
The goal of an effective pharmacologic

restricted.
The goal of an effective pharmacologic therapy for primary 
hyperparathyroidism remains elusive, though studies of sex hor­
mones and selective estrogen receptor modulators, bisphospho­
nates, and calcimimetics continue. Estrogens and progestins may 
reduce serum calcium and phosphorus, urinary calcium and 
hydroxyproline, and histologic evidence of active bone resorption 
in women with primary hyperparathyroidism, although safety 
concerns have limited these therapeutic options in p

ncerns have limited these therapeutic options in postmeno­
Intravenous bisphosphonates have been used successfully in 
the urgent therapy of hypercalcemia due to primary hyperpara­
thyroidism, and several trials have shown that treatment with 
oral alendronate for 1 year or more improves bone density at the 
spine and hip, with only transient effects on serum calcium and 
PTH.354,355 The calcimimetics represent a new class of agents that, 
by sensitizing the CaSR to extracellular c

 that, 
by sensitizing the CaSR to extracellular calcium, can reduce PTH 
secretion. Cinacalcet, the first calcimimetic approved for control 
of secondary hyperparathyroidism in renal disease, was shown to 
lower serum calcium and PTH in primary hyperparathyroidism 
(and in some patients with parathyroid carcinoma), although 
improvement on bone density has not been documented in this 
population.356
Thus in patients for whom surgery for asymptomatic primary 
hyperparathyroidism is not an option

atic primary 
hyperparathyroidism is not an option, therapy with oral bisphos­
phonates can improve bone density without worsening other 
features of the disease, at least over 2 years of follow-up, and cina­
calcet can control blood calcium. Whether these agents or any 
other medical therapy offers a beneficial long-term alternative to 
surgery is unknown. 
Surgical Treatment of Primary Hyperparathyroidism. Parathy­
roidectomy is a safe and highly effective approach to definitive treat­
ment of

ly effective approach to definitive treat­
ment of primary hyperparathyroidism. The most serious potential 
complications of parathyroid surgery—vocal cord paralysis and 
permanent hypoparathyroidism—occur after fewer than 1% and 
4%, respectively, of procedures performed by highly skilled sur­
geons, although these rates can be much higher in less experienced 
  Indications for Surgery in Primary 
Overt clinical manifestations of disease
Kidney stones or nephrocalcinosis
Fr

ns of disease
Fractures or classic radiographic findings of osteitis fibrosa
Classic neuromuscular disease
Symptomatic or life-threatening hypercalcemia
Serum calcium >1 mg/dL above upper limit of normal
Creatinine clearance <60 mL/min, presence of stone(s) by radiograph, 
CT, or ultrasound
Urinary calcium >400 mg/day plus other urinary biochemical indices of 
stone risk
Bone mineral density low (T score ≤−2.5) at any sitea
Presence of vertebral fracture by radi

t any sitea
Presence of vertebral fracture by radiograph or by vertebral fracture 
analysis on DXA
History of fragility fracture
Young age (<50 years)
Uncertain prospects for adequate medical monitoring
aZ-score ≤−2.5 in premenopausal women and in men <50 years old.
CT, Computed tomography; DXA, dual-energy x-ray absorptiometry.
Modified from Bilezikian JP, Khan AA, Potts Jr JT, Third International Workshop on the Man­
agement of Asymptomatic Primary Hyperthyroidism. Guidelines for the managemen

mary Hyperthyroidism. Guidelines for the management of 
asymptomatic primary hyperparathyroidism: summary statement from the Third International 
Workshop. J Clin Endocrinol Metab. 2009;94:335–339. Based upon recommendations of 
the 2014 NIH-sponsored Workshop on the Management of Asymptomatic Primary Hyper­
parathyroidism.
   
TABLE 29.1
1227
hands. Such complications occur most often in patients who 
require subtotal parathyroid resectio

atients who 
require subtotal parathyroid resections for hyperplasia or resection 
of carcinoma. The surgical cure rate for primary hyperparathyroid­
ism in the best hands is at least 95%.357,358 Apart from operator 
inexperience, the usual cause of initial surgical failure (“persis­
tent disease”) is the presence of either unrecognized (often very 
asymmetric) parathyroid hyperplasia or ectopic parathyroid tissue 
(i.e., intrathyroidal, undescended, retroesophageal, or mediastinal 
glands)359 (

ded, retroesophageal, or mediastinal 
glands)359 (Fig. 29.24). Up to one in five parathyroid glands may 
be located ectopically, and this is especially true of supernumer­
ary glands. Recurrent disease, defined as that occurring after an 
interval of at least 6 to 12 months of normocalcemia, varies in 
incidence from 2% to 16%. Recurrent hyperparathyroidism usu­
ally arises in unresected hyperplastic glands, but rarely it may be 
due to parathyroid carcinoma, to a second adenoma, or to a mul­
ti

id carcinoma, to a second adenoma, or to a mul­
ticentric or miliary “parathyromatosis” engendered by inadvertent 
local seeding of parathyroid tissue (usually hyperplastic) into the 
neck during previous parathyroid surgery.282,360
In the past, there was broad agreement that the best approach 
is a bilateral neck exploration in which all four parathyroids are 
identified and all enlarged glands removed. With this procedure, 
preoperative parathyroid localization studies prior to initial cer­
vi

roid localization studies prior to initial cer­
vical exploration are superfluous, as the positive predictive value 
of even the best technique (99mTc-sestamibi scanning) falls well 
short of the success rate of experienced surgeons unaided by prior 
imaging.282,361
With the advent of preoperative 99mTc-sestamibi scanning, 
which can accurately localize 80% to 90% of the single adenomas 
that account for 75% to 85% of cases, there has been renewed 
interest in performance of directed unilateral 

d 
interest in performance of directed unilateral explorations, which 
reduce operative and recovery-room time, minimize the number 
of frozen sections required, are associated with significantly fewer 
postoperative complications, and can more readily be performed 
using minimally invasive techniques (including local anesthesia 
and intravenous sedation) that enable same-day discharge.362 
Sestamibi scanning also can identify the occasional mediastinal 
adenoma and thereby allow the avoidance o

astinal 
adenoma and thereby allow the avoidance of an unnecessary neck 
exploration. On the other hand, the sensitivity and positive pre­
dictive value of sestamibi scanning is poor (<50%) in the pres­
ence of multiglandular disease (hyperplasia or double adenomas), 
and thus the test may frequently miss the presence of bilateral 
disease.358 To reduce this failure rate, which is unacceptably high 
in comparison to bilateral exploration, supplemental preoperative 
ultrasonic imaging or high-res

ental preoperative 
ultrasonic imaging or high-resolution CT with dynamic contrast 
administration (four-dimensional CT [4D-CT])363 is often used 
(with or without needle biopsy), and rapid intraoperative PTH 
assays have been developed to verify successful excision.364 Because 
the half-life of intact PTH in blood is very short (<2 minutes), a 
decline of 50% or more from baseline within 10 minutes or so 
can signal successful removal of all hyperfunctioning parathyroid 
tissue. This approach h

erfunctioning parathyroid 
tissue. This approach has functioned well in patients with single 
adenomas, but can be misleading in those with multiglandular 
disease unless more stringent criteria for cure are applied (i.e., 
>90% decline, or even normalization, of iPTH).365
At present, preoperative imaging enables consideration of a 
minimally invasive unilateral parathyroidectomy in approximately 
70% of those patients thought preoperatively to have sporadic 
primary hyperparathyroidism due to a

ave sporadic 
primary hyperparathyroidism due to a solitary adenoma. Surgi­
cal cure rates in appropriately selected patients are comparable to 
those after bilateral neck exploration (i.e., 95–97%),358 although 
a recent study in which all patients selected for minimally invasive 
surgery were also subjected to immediate bilateral neck dissection 
demonstrated a failure to recognize multiglandular disease in 16% 
of these subjects.366 Patients known or suspected to have multi­
glandular disease

nown or suspected to have multi­
glandular disease, such as those with MEN1 and those younger 
than 30 years of age, should undergo bilateral neck exploration.367 
Options for patients with hyperplasia include resection limited to 
visibly abnormal glands, subtotal parathyroidectomy with cryo­
preservation of tissue, and total parathyroidectomy with imme­
diate autotransplantation (i.e., in the forearm) of some excised 
tissue. In patients with MEN1, considerations of recurrence rates 
(30–50% o

EN1, considerations of recurrence rates 
(30–50% or higher with long-term follow-up) and the timing 
thereof versus the potential morbidity of surgical hypoparathy­
roidism tend to favor subtotal parathyroidectomy as the preferred 
approach at present.
The incidence of parathyroid carcinoma in primary hyper­
parathyroidism is less than 1%,285 but this possibility should be 
strongly considered in patients with unusually severe hyperpara­
thyroidism, a palpable neck mass, hoarseness, evidence of 

sm, a palpable neck mass, hoarseness, evidence of local 
invasion at surgery, or recurrent hypercalcemia.368 Even so, para­
thyroid carcinoma rarely is suspected preoperatively and often 
eludes diagnosis at the time of initial surgery. When the disease 
is recognized, vigorous attempts should be made to remove the 
tumor en bloc. The incidence of local recurrence approaches 
50%, however, and distant metastases, particularly to lung, may 
be heralded by recurrent, severe hyperparathyroidism.369

alded by recurrent, severe hyperparathyroidism.369 Because 
apparently sporadic and isolated parathyroid cancer can occur in 
families with parafibromin mutations, a search for such muta­
tions in all patients with parathyroid cancer can facilitate family 
counseling.348
The immediate postoperative management of parathyroidec­
tomy focuses on establishing the success of the surgery and moni­
toring the patient closely for symptomatic hypocalcemia and for 
uncommon but potentially serious acute c

 and for 
uncommon but potentially serious acute complications such as 
C3
C4
C5
C6
C7
T1
T2
T3
T4
1
1
5
10
13
19
21
34
• Fig. 29.24  Sites of ectopic location of 104 parathyroid glands found at 
reoperation for primary hyperparathyroidism. (From Wang C-A. A clinical 
and pathological study of 112 cases. Ann Surg. 1977;186:140–145.)
1228
bleeding, vocal cord paralysis, or laryngospasm. After success­
ful resection of a parathyroid adenoma, serum intact PTH level

n of a parathyroid adenoma, serum intact PTH levels 
decline rapidly, often to undetectable concentrations, with a dis­
appearance half-time of about 2 minutes, whereas serum calcium 
typically reaches a nadir between 24 and 36 hours. Serum PTH 
returns to the normal range within 30 hours, although measure­
ments of the parathyroid secretory response to hypocalcemia sug­
gest that it does not fully normalize for at least several weeks.370
In the past, patients generally were maintained on a low-

past, patients generally were maintained on a low-
calcium diet until normalization of serum calcium was clearly 
documented, ampoules of injectable calcium and other seizure 
precautions were maintained at the bedside, serum calcium was 
measured at least every 12 hours until stable, and symptomatic 
hypocalcemia was promptly treated with calcium, either intrave­
nously (90-mg bolus, 50–100 mg/hour) or orally (1.5–3 g/day). 
This approach is no longer appropriate for most patients, who are 
dis

longer appropriate for most patients, who are 
discharged within a few hours after limited surgery. Instead, oral 
calcium supplements routinely are provided as soon as oral intake 
is reestablished, and moderate doses of 1,25(OH)2D (0.5–1 μg 
daily) are added for those with large adenomas and severe hyper­
parathyroidism or for those in whom alkaline phosphatase had 
been elevated properatively—that is, patients in whom an impres­
sive calcium requirement can be anticipated, often for many week

equirement can be anticipated, often for many weeks 
postoperatively, as they remineralize their skeletons. This “hungry 
bone” syndrome is associated with hypocalcemia, hypophosphate­
mia, and low urinary calcium excretion.
Serum calcium should be checked at intervals of several days 
initially to guide adjustment of calcium and vitamin D therapy 
as needed to achieve a stable result. In those in whom hypocal­
cemia persists for more than several days, serum PTH should be 
measured to exclude t

l days, serum PTH should be 
measured to exclude the possibility of postoperative hypoparathy­
roidism. Given evidence that bone mineral density continues to 
increase for at least 1 year after successful parathyroidectomy,291 it 
is prudent to continue calcium supplementation for at least that 
long.
The approach to patients with persistent or recurrent hyper­
parathyroidism is informed by the recognition that parathyroid 
hyperplasia or carcinoma, ectopic or supernumerary parathyroid 
tissue, 

ma, ectopic or supernumerary parathyroid 
tissue, and postoperative hypoparathyroidism and other compli­
cations of further surgery all are more common in this popula­
tion.359,361 The first issue to address is whether surgery is indicated. 
When a presumed adenoma had not been identified initially, the 
original indications for surgery generally still exist, although some 
patients may not be suitable candidates for more extensive surgery, 
such as a median sternotomy, because of concurrent med

 as a median sternotomy, because of concurrent medical ill­
ness. Patients with parathyroid hyperplasia may have experienced 
significant clinical improvement, even after incomplete parathy­
roidectomy, although those with MEN1 are very likely to experi­
ence further progression of their disease.
Preoperative localization studies are recommended for patients 
with persistent or recurrent disease after a first operation. Scanning 
with 99mTc-sestamibi offers the highest sensitivity and accuracy, 

mibi offers the highest sensitivity and accuracy, 
although other studies (ultrasonography, CT, magnetic resonance 
imaging [MRI]) may provide additional or confirmatory infor­
mation.371 Sestamibi does localize to thyroid nodules, which 
may accompany parathyroid disease in 20% to 40% of patients, 
although it tends to wash out of thyroid tissue much more rapidly 
than from parathyroids. 99mTc-sestamibi can be combined with 
123I scanning to improve distinction of parathyroids from thyroid 
nod

rove distinction of parathyroids from thyroid 
nodules or with single-photon emission CT (SPECT) imaging to 
achieve accuracy in localization not possible with planar imag­
ing (Fig. 29.25). On the other hand, sestamibi scanning may fail 
to reveal small glands (uptake is related to gland size and PTH 
levels372) or to demonstrate multiple abnormal glands in cases of 
parathyroid hyperplasia, the most common cause of persistent 
postoperative hyperparathyroidism.358,373 Use of 4D-CT with 
synchr

erparathyroidism.358,373 Use of 4D-CT with 
synchronous contrast-enhanced multiplanar anatomic reconstruc­
tion has been shown to provide sensitivity superior to sestimibi 
scanning alone for localizing functioning parathyroid tissue in 
candidates for reoperation.374
More invasive techniques have been used as well, including 
angiography and selective venous sampling for measurement of 
PTH.375,376 Ultrasound-guided or CT-guided fine-needle aspira­
tion of suspected parathyroid tissue may be us

ra­
tion of suspected parathyroid tissue may be used to obtain cytologic 
or immunochemical confirmation prior to surgery, and intraop­
erative ultrasonography has been useful in some cases to locate 
cervical or intrathyroidal glands.361 Success with video-assisted 
thoracoscopic resection of documented mediastinal lesions348,377 
offers a less invasive alternative to median sternotomy for this rela­
tively common cause of persistent hyperparathyroidism.
The need for these procedures depends on

yroidism.
The need for these procedures depends on the experience of the 
original surgeon and the confidence that the neck was adequately 
explored initially. For example, among reoperations at one center, 
over half of the “missed” hyperplastic parathyroid glands in those 
cases previously explored by a highly experienced parathyroid sur­
geon were found in the mediastinum or another ectopic location, 
whereas over 90% of those referred by less experienced surgeons 
were discovered in a normal

 experienced surgeons 
were discovered in a normal anatomic location in the neck.282
Following successful surgery for primary hyperparathyroidism, 
bone mass generally improves by as much as 5% to 10% in the 
• Fig. 29.25  Technetium-99m (99mTc) sestamibi, iodine-123 (123I) subtrac­
tion scanning of a patient with persistent hyperparathyroidism after two 
previous unsuccessful operations. Arrow points to parathyroid adenoma, 
shown as increased tracer uptake in the aortopulmonary window. (From 

racer uptake in the aortopulmonary window. (From 
Thule P, Thakore K, Vansant J, et al. Preoperative localization of parathy­
roid tissue with technetium-99m sestamibi123I subtraction scanning. J 
Clin Endocrinol Metab. 1994;78:77–82.)
1229
first year at sites rich in trabecular bone (spine, femoral neck), 
whereas improvement at cortical sites (distal radius) is less predict­
able.378,379 Increases at trabecular sites may continue for sev

Increases at trabecular sites may continue for several 
years, to as much as 12% to 15% after 10 years, although normal 
bone mineral density may not be achieved. This improvement, 
which is most apparent in those with the greatest preoperative 
reductions in bone mass, may be related in part to rapid reminer­
alization of the previously enlarged bone remodeling volume,352 
but the continued improvement over years suggests a more sus­
tained increase in net bone formation and total bone volume a

ease in net bone formation and total bone volume as 
well.291 
Familial Hypocalciuric Hypercalcemia
FHH, also appropriately called familial benign hypercalcemia, is, 
in most families, a disorder of autosomal dominant inheritance 
caused most often by mutations of the CASR gene found in para­
thyroid glands, kidney, and other organs (see earlier discussion 
of calcium sensing). The mutations, which cause complete or 
partial loss of function of the CaSR, lead to a shift in the para­
thyroid cell

he CaSR, lead to a shift in the para­
thyroid cell’s set-point for calcium.380 As a consequence, higher 
than normal levels of blood calcium are needed to suppress PTH 
secretion. Furthermore, abnormal function of the CaSR in the 
renal thick ascending limb leads to increased PTH-independent 
calcium reabsorption and consequent hypocalciuria. In a minor­
ity of patients, mutations in two other genes in the signaling 
pathway activated by the CaSR have been found. One encodes 
the α-subunit of th

 have been found. One encodes 
the α-subunit of the G11 heterotrimeric G protein381 known to 
participate in responding to the activation of the CaSR, and the 
other is in the AP2S1 gene that encodes the α-subunit of adapter 
protein complex 2, a scaffolding protein needed for formation of 
clathrin-coated pits needed to internalize the CaSR.382 Mutations 
in AP2S1 are considerably more common than mutations in Gα11 
in FHH383,384; patients with AP2S1 mutations have particularly 
severe hypercal

AP2S1 mutations have particularly 
severe hypercalcemia and can present with developmental delay 
and cognitive dysfunction.383
The presence of one normal calcium-sensing receptor gene 
with the abnormal one usually leads to a very mild clinical dis­
order, although the receptor functions as a dimer, and certain 
mutations can worsen the function of the normal allele. Rare 
patients who inherit mutant CASR genes from both parents 
present at birth with severe, life-threatening, primary hyperpara

h with severe, life-threatening, primary hyperpara­
thyroidism and almost always require immediate parathyroid 
surgery. In another genetic variation, a familial form of CaSR-
dependent hypercalcemia has been described in association with 
other autoimmune disorders such as Hashimoto hypothyroid­
ism and celiac sprue, in which autoantibodies directed against 
the sensor apparently antagonize calcium recognition by the 
parathyroids and renal tubules.385,386
FHH is manifested at birth by hypercal

les.385,386
FHH is manifested at birth by hypercalcemia. Although some 
controversy exists, most observers note that the condition associ­
ated with calcium-sensing receptor mutations is usually asymp­
tomatic and that apparent symptoms represent ascertainment bias. 
Possible exceptions include the occurrence of chondrocalcinosis 
and perhaps pancreatitis. The blood calcium level is usually less 
than 12 mg/dL but can be higher. Phosphate measurements are 
low, as in primary hyperparathyroidism.

ments are 
low, as in primary hyperparathyroidism. Blood magnesium levels 
are high-normal or slightly elevated. PTH levels are inappropri­
ately normal for the degree of hypercalcemia and are occasionally 
modestly elevated. Urine calcium is usually low, though one novel 
mutation in the receptor’s intracellular tail has been associated 
with hypercalciuria, possibly because of only mild dysfunction in 
the kidney.387
When patients present as adults, the distinction from mild 
primary hyperpara

ults, the distinction from mild 
primary hyperparathyroidism can be difficult. The distinction 
between FHH and primary hyperparathyroidism is a crucial 
one, however. Young patients with primary hyperparathyroid­
ism are usually treated surgically and cured. In contrast, hyper­
calcemia always recurs after surgery for FHH, unless the patient 
is rendered hypoparathyroid by the removal of all parathyroid 
tissue. Therefore surgery is contraindicated as therapy for FHH, 
except in the very rare p

ted as therapy for FHH, 
except in the very rare patient with severe, symptomatic hyper­
calcemia. No blood or urine measurements are completely reli­
able for distinguishing between the two conditions, though the 
ratio of calcium clearance to creatinine clearance distinguishes 
most patients with FHH from those with primary hyperpara­
thyroidism.388 Fig. 29.26 shows, however, that this ratio sepa­
rates the two groups, with modest overlap between the groups. 
However, because primary hyperpara

en the groups. 
However, because primary hyperparathyroidism is much more 
common than FHH, most patients with values near the cut­
off value of 0.01 for the ratio of calcium clearance to creati­
nine clearance will have primary hyperparathyroidism and not 
FHH.389 Consequently, a case can be made390 that many of such 
patients, particularly before parathyroid surgery, should undergo 
sequencing of their CASR gene, now available commercially. The 
most helpful diagnostic information is the prese

most helpful diagnostic information is the presence of hypercal­
cemia in an infant relative; such early hypercalcemia does not 
occur in MEN1. Furthermore, a past history of clearly normal 
blood calcium, considerably lower than current measurements, 
makes FHH unlikely if no other reason, such as severe vitamin 
D deficiency, for a change in blood calcium exists. In the occa­
sionally symptomatic patient, cinacalcet, a calcimimetic drug, 
can usually lower the blood level of calcium.391 
Cal

 usually lower the blood level of calcium.391 
Calcium clearance/creatinine clearance
Creatinine clearance
(mL/[1.73 m2  min])
0.1
0.05
0.02
0.01
0.005
0.002
0.001
0
40
80
120
160
200
• Fig. 29.26  Index of urinary excretion rate for calcium as a function of cre­
atinine clearance. Each point represents the mean of multiple determina­
tions for a hypercalcemic patient with familial hypocalciuric hypercalcemia 
(filled circles) or with typical primary hyperparathyroidism (open circles). 
The dat

imary hyperparathyroidism (open circles). 
The data are based on average 24-hour urinary excretion values and aver­
age fasting serum samples. (From Marx SJ, Attie MF, Levine M, et al. The 
hypocalciuric or benign variant of familial hypercalcemia: clinical and bio­
chemical features in fifteen kindreds. Medicine. 1981;60:397–412.)
1230
Lithium Toxicity
Treatment of bipolar affective disorders with lithium commonly 
leads to mild, persistent increases in blood c

ly 
leads to mild, persistent increases in blood calcium.392,393 Mea­
surement of ionized calcium has shown that ionized calcium is a 
more sensitive index of lithium effect and was elevated in 24% of 
consecutive patients in a cross-sectional study.394 After several years 
of therapy, clear elevations of PTH levels and modest increases in 
parathyroid gland size, detected by ultrasonography, often occur. 
Usually, when lithium therapy is stopped, the blood calcium and 
PTH normalize within seve

, the blood calcium and 
PTH normalize within several months. Uncommonly, substantial 
hypercalcemia and clear hyperparathyroidism ensue. At surgery, 
both single-gland and multigland disease are found, with a higher 
fraction of multigland disease than found in primary hyperpara­
thyroidism not associated with lithium therapy.395,396
The management of patients with mild lithium-induced hyper­
calcemia is somewhat complicated. Like patients with mild pri­
mary hyperparathyroidism, patients takin

mild pri­
mary hyperparathyroidism, patients taking lithium usually tolerate 
mild hypercalcemia without obvious symptoms. These patients 
can be monitored with protocols similar to those for patients with 
asymptomatic primary hyperparathyroidism. Close attention 
must be paid to urine-concentrating ability in these patients, how­
ever, because the nephrogenic diabetes insipidus associated with 
lithium therapy can lead to dehydration and sudden worsening 
of hypercalcemia. Substantial hypercal

 worsening 
of hypercalcemia. Substantial hypercalcemia should lead to with­
drawal of lithium therapy, if possible, with substitution of newer 
psychopharmacologic agents. If hypercalcemia persists after with­
drawal of lithium, decisions about surgery follow the same guide­
lines as those for patients with primary hyperparathyroidism.
Lithium increases the set-point for PTH secretion when it 
is added to isolated parathyroid cells in vitro. The set-point for 
PTH secretion in vivo is shifted t

 set-point for 
PTH secretion in vivo is shifted to the right in patients who have 
received lithium for several years as well. A corresponding shift in 
the concentration of extracellular calcium needed to raise intra­
cellular calcium levels397 suggests that lithium interferes with the 
action of the parathyroid CaSR. 
Parathyroid-Independent Hypercalcemia
In parathyroid-independent hypercalcemia, PTH secretion is 
appropriately suppressed. PTH levels, measured using two-site 
assays, are inva

 levels, measured using two-site 
assays, are invariably lower than 25 pg/mL and are usually lower 
than normal or undetectable. Most affected patients have malig­
nant hypercalcemia, although parathyroid-independent hypercal­
cemia occurs in a number of other settings as well.398
Hypercalcemia of Malignancy
The diagnosis of malignant hypercalcemia is seldom a subtle 
one.125 Most malignancies produce hypercalcemia only when 
they are far advanced; the diagnosis becomes evident after routine 
st

d; the diagnosis becomes evident after routine 
studies, guided by the history and physical examination. Patients 
with malignant hypercalcemia usually die 1 to 2 months after 
hypercalcemia is discovered. Patients present with the classic signs 
and symptoms of hypercalcemia: confusion, polydipsia, poly­
uria, constipation, nausea, and vomiting. Perhaps because of the 
acuteness of the hypercalcemia and the elderly patient population 
involved, dramatic changes in mental status, culminating in 

dramatic changes in mental status, culminating in coma, 
are relatively common. The diagnosis can be missed because the 
manifestations often overlap those of the underlying malignancy 
and because low blood albumin may lead to an apparently normal 
total blood calcium, despite an elevated blood ionized calcium. 
Even though the overall prognosis is grim, the diagnosis of malig­
nant hypercalcemia is important to make.
Treatment is usually simple and effective in the short term; 
such treatment 

 and effective in the short term; 
such treatment can importantly reverse the patient’s symptoms 
for several weeks and even provide time for a fundamental attack 
on the underlying tumor, if it is treatable. Treatment consists of 
restoration of volume, followed by intravenous bisphosphonate or 
denosumab (see “Management of Severe Hypercalcemia”). Only 
effective treatment of the underlying neoplasm can significantly 
influence the long-term prognosis for patients with malignant 
hypercalcemia

ognosis for patients with malignant 
hypercalcemia.
Although mechanisms in a given patient may be multiple, it 
is still useful to distinguish hypercalcemia associated with local 
involvement of bone from that caused by humoral mecha­
nisms. In all cases, resorption of bone plays a pivotal role in the 
pathogenesis.
Local Osteolytic Hypercalcemia
Hypercalcemia resulting from tumors invading bone occurs most 
clearly in multiple myeloma and some patients with breast cancer. 
There is little evide

atients with breast cancer. 
There is little evidence that the tumor cells themselves resorb bone. 
Instead, active osteoclasts found near the tumor cells are thought 
to be the proximate mediators of bone resorption.399 Myeloma 
cells and marrow cells associated with myeloma cells secrete 
numerous cytokines and chemokines capable of stimulating 
bone resorption, including macrophage inflammatory protein 1 
(MIP1), lymphotoxin (tumor necrosis factor-β), and interleukins 
1β, 3, and 6. These fac

ctor-β), and interleukins 
1β, 3, and 6. These factors lead to increased expression of RANKL 
(see Fig. 29.8) on the surface of marrow stromal cells and stimula­
tion of osteoclast formation and activity. RANKL is also found on 
the surface of myeloma cells, and therefore these cells may directly 
stimulate the production and activity of osteoclasts. The increased 
bone resorption not only releases calcium into the circulation but 
also weakens the bone structurally. Bone is further weakened by 

he bone structurally. Bone is further weakened by 
the suppression of bone formation by the secretion of dickkopf-1, 
an inhibitor of Wnt signaling, by myeloma cells.400–402 In patients 
with myeloma, treatment with intermittent intravenous bisphos­
phonates or denosumab inhibits bone resorption and reduces the 
incidence of bone pain, fracture, and hypercalcemia.403
The pathogenesis of hypercalcemia in breast cancer is not com­
pletely understood. Extensive metastases to bone are detected in 
m

d. Extensive metastases to bone are detected in 
most patients with hypercalcemia and breast cancer; this finding 
suggests that factors produced in bone by the metastatic tumor 
cells may be important. Breast cancer cells make a host of cyto­
kines capable of stimulating bone resorption by osteoclasts.404 
The role of tumor-produced PTHrP may be particularly impor­
tant.401 A majority of breast cancer patients with hypercalcemia 
have elevated blood levels of PTHrP. This circulating PTHrP, as 

lood levels of PTHrP. This circulating PTHrP, as 
well as PTHrP produced in bone by metastatic tumor cells, may 
generate the hypercalcemia. Primary breast tumors that stain for 
PTHrP are more likely to result in bone metastases than are those 
that do not stain for PTHrP; this PTHrP may be instrumental 
in the establishment of lytic metastases. Animal models indicate 
that transforming growth factor-β (TGFβ), released from bone 
matrix by PTHrP-stimulated osteoclastic resorption, may further 

-stimulated osteoclastic resorption, may further 
augment PTHrP secretion by the tumor cells. The latter may be 
further promoted by estrogen, which may explain the occasional 
occurrence of hypercalcemia following institution of estrogen or 
tamoxifen therapy in this disease.405 
Humoral Hypercalcemia of Malignancy
Albright, in 1941, was the first to propose that a PTH-like 
humoral factor caused the hypercalcemia in patients with malig­
nancy but few or no bone metastases.406 Four decades late

ut few or no bone metastases.406 Four decades later 
1231
biochemical analysis demonstrated that such patients have high 
blood calcium levels, low blood phosphate levels, and high uri­
nary cAMP levels like those found in primary hyperparathy­
roidism, but no elevation in iPTH levels.407 The stimulation 
of cAMP production was used as an assay to eventually purify 
PTHrP from human tumors associated with the humoral hyper­
calcemia of mal

associated with the humoral hyper­
calcemia of malignancy.408
The evidence that PTHrP mediates the humoral hypercalce­
mia of malignancy in most patients is substantial. As noted pre­
viously, PTHrP binds to the PTH/PTHrP receptor and mimics 
all of the actions of amino-terminal fragments of PTH. Blood 
levels of PTHrP are elevated in most patients with solid tumors 
and hypercalcemia. In animal models of the humoral hypercal­
cemia of malignancy, antibodies against PTHrP can reverse the 
hyperc

, antibodies against PTHrP can reverse the 
hypercalcemia.409
The acute actions of PTHrP cannot explain all of the findings in 
patients with the hypercalcemia of malignancy, however. Acutely 
administered PTHrP increases blood levels of 1,25(OH)2D3 by 
stimulating the renal 1α-hydroxylase, though the stimulation is 
less than that induced by PTH.410 Nevertheless, patients with the 
humoral hypercalcemia of malignancy usually have low levels of 
1,25(OH)2D3.407 This finding is particularly puzzl

1,25(OH)2D3.407 This finding is particularly puzzling, because 
human tumors from patients with elevated calcium and PTHrP 
but with low 1,25(OH)2D3 levels stimulate 1,25(OH)2D3 synthe­
sis after they are transplanted into nude mice.411 Possible expla­
nations for the low 1,25(OH)2D3 levels in patients include the 
weak activation of the renal 1α-hydroxylase in humans by PTHrP, 
combined with the inhibition of the 1α-hydroxylase by hypercal­
cemia412 or by tumor products.
A second disparity betw

a412 or by tumor products.
A second disparity between the acute actions of PTHrP and 
the findings in patients with malignant hypercalcemia involves 
the rate of bone formation. Acutely, PTHrP infusions in rats, 
like PTH, leads to increased bone formation.413 Nevertheless, in 
patients with malignant hypercalcemia, bone formation is mark­
edly lower than normal. The explanation for this effect may well 
lie in the action of other cytokines, immobilization, or particular 
fragments of PTHrP with

bilization, or particular 
fragments of PTHrP with novel properties.
The tumors most commonly associated with humoral hyper­
calcemia include squamous cell cancers of the lung, head and 
neck, esophagus, cervix, vulva, and skin; breast cancer; renal cell 
cancer; and bladder cancer. Benign or malignant pheochromo­
cytomas, islet cell tumors, and carcinoids can also overproduce 
PTHrP, causing hypercalcemia. The aggressive T-cell lymphoma 
associated with human T-cell lymphotropic virus type 1 (H

ted with human T-cell lymphotropic virus type 1 (HTLV1) 
infection is the only hematologic malignancy commonly associ­
ated with PTHrP overproduction and hypercalcemia.
Cachexia often accompanies the hypercalcemia of malignancy. 
In experimental models, administration of a monoclonal antibody 
to PTHrP ameliorates the cachexia, not simply by lowering blood 
calcium but also by blocking the ability of PTHrP to convert 
white adipose tissue to brown, heat-generating adipose tissue.126
It is unlike

n, heat-generating adipose tissue.126
It is unlikely that PTHrP is the sole cause of the humoral 
hypercalcemia of malignancy. As noted previously, many cyto­
kines produced by tumors can stimulate bone resorption. The 
actions of these cytokines have been shown to synergize with 
those of PTHrP in a number of experimental models. Further­
more, in hypercalcemic patients with non-Hodgkin lymphoma, 
blood levels of 1,25(OH)2D3 were found to be higher than oth­
erwise expected,414 and such patient

her than oth­
erwise expected,414 and such patients show abnormal sensitivity 
to 25-hydroxyvitamin D administration.415 In these hypercalce­
mic patients, the relative importance of 1,25(OH)2D3 cytokines, 
PTHrP, and immobilization needs to be clarified. Only a minority 
of patients with non-Hodgkin lymphoma have clear elevations of 
either PTHrP or 1,25(OH)2D3.416
In a few reported cases, malignant tumors secrete PTH and 
not PTHrP.125,417 Although this phenomenon has now been 
well documented

ough this phenomenon has now been 
well documented, it should be stressed that in almost all patients 
with cancer and high PTH levels, concurrent primary hyper­
parathyroidism, not ectopic PTH production, is the cause of the 
hyperparathyroidism. 
Vitamin D Intoxication
Because the synthesis of 1,25(OH)2D3 is so tightly regulated, 
extremely large doses of vitamin D, on the order of 100,000 
units per day, are required to cause hypercalcemia. Such doses 
are available in the United States only 

ch doses 
are available in the United States only by prescription, therefore 
most cases of vitamin D intoxication are iatrogenic. Occasionally, 
inadvertent ingestion occurs. All over the world, including in the 
United States, particularly in the era of the internet and wide­
spread enthusiasm about the potential properties of vitamin D, 
increasing reports of vitamin D intoxication caused by prepara­
tions of vitamin D containing much larger amounts of vitamin 
D than claimed have been publis

mounts of vitamin 
D than claimed have been published.418 Patients present with 
nausea, vomiting, weakness, and altered level of consciousness. 
Hypercalcemia can be severe and prolonged, because of the stor­
age of vitamin D in fat. As expected, PTH levels are suppressed, 
and levels of 25(OH)D, which are poorly regulated and reflect 
levels of ingested vitamin D, are dramatically elevated. In contrast, 
the levels of 1,25(OH)2D3 are only modestly elevated or can be 
normal or even low. The mo

tly elevated or can be 
normal or even low. The modest changes in 1,25(OH)2D3 levels 
result from the downregulation of the renal 1α-hydroxylase by low 
levels of PTH and high levels of phosphate, calcium, FGF23, and 
1,25(OH)2D3 itself. The cause of the hypercalcemia, when it occurs 
in the face of normal levels of 1,25(OH)2D3, is uncertain but may 
reflect the direct action of 25(OH)D and possibly other vitamin 
D metabolites, which are capable of binding the 1,25(OH)2D3 
receptor weakly or wh

 of binding the 1,25(OH)2D3 
receptor weakly or which may be locally 1α-hydroxylated by non­
renal 1α-hydroxylases. Of note, vitamin D causes hypercalcemia 
in mice missing the 1α-hydroxylase gene,419 arguing that 25(OH)
D and other metabolites, when present in large amounts, are suffi­
cient to cause hypercalcemia. Also, the weaker vitamin D metabo­
lites may displace 1,25(OH)2D3 from the circulating VDBP and 
increase the concentration of active, free 1,25(OH)2D3.420,421
The hypercalcemia of v

e, free 1,25(OH)2D3.420,421
The hypercalcemia of vitamin D intoxication results both from 
increased intestinal absorption of calcium and from the direct 
effect of 1,25(OH)2D3 to increase resorption of bone. Therefore 
in severe cases bisphosphonate therapy can be usefully added to 
the therapeutic regimen of hydration and omission of dietary 
calcium. 
Sarcoidosis and Other Granulomatous Diseases
Sarcoidosis may be associated with hypercalcemia and, even more 
commonly, hypercalciuria.422 Hype

 and, even more 
commonly, hypercalciuria.422 Hypercalcemic patients have high 
levels of 1,25(OH)2D3; the high level of 1,25(OH)2D3 prob­
ably causes the hypercalcemia, although overproduction of bone-
resorbing cytokines and PTHrP may contribute in some patients. 
As expected in 1,25(OH)2D3-dependent hypercalcemia, intestinal 
absorption of calcium is increased and PTH levels are suppressed. 
Furthermore, the hypercalcemia and high levels of 1,25(OH)2D3 
fall upon treatment with glucocorticoid

25(OH)2D3 
fall upon treatment with glucocorticoids. The unregulated synthe­
sis of 1,25(OH)2D3, found even in an anephric patient, occurs 
not in the kidney but in the sarcoid granulomas. Removal of a 
large amount of granulomatous tissue can reverse hypercalce­
mia. Furthermore, isolated sarcoid macrophages can synthesize 
1232
1,25(OH)2D3 from 25(OH)D, as can normal macrophages 
stimulated with interferon-γ or after activation of toll-like recep­
tors. Such m

 after activation of toll-like recep­
tors. Such macrophages express the gene encoding the identical 
25(OH)D 1α-hydroxylase found in the kidney. The local synthe­
sis of 1,25(OH)2D3 by activated macrophages and activation of 
VDRs in those cells is a paracrine system that activates antibacte­
rial mechanisms as part of the normal action of macrophages.193
The unusual increase in the numbers of activated macrophages 
in sarcoidosis leads, however, to elevations of blood calcium in 
these patient

, to elevations of blood calcium in 
these patients. These patients have unusual sensitivity to vitamin 
D and can become hypercalcemic in response to ultraviolet radia­
tion or oral vitamin D intake. Abnormalities in calcium metabo­
lism are usually found only in patients with active disease and 
large, clinically obvious total-body burdens of granulomas. Nev­
ertheless, hypercalcemia can present in patients without obvious 
pulmonary disease. Furthermore, subtle abnormalities of vitamin 
D met

urthermore, subtle abnormalities of vitamin 
D metabolism can be demonstrated even in patients with mildly 
active sarcoidosis.
Hypercalcemia is also associated with other granulomatous dis­
eases, such as tuberculosis, fungal infections, and berylliosis and 
has been reported in Wegener granulomatosis, in acquired immu­
nodeficiency syndrome (AIDS)–related Pneumocystis jirovecii 
infection, fat necrosis of the newborn,423 and even in association 
with extensive granulomatous foreign-body reacti

with extensive granulomatous foreign-body reactions.398 Patients 
with Crohn disease occasionally have hypercalcemia with eleva­
tions of 1,25(OH)2D3 levels but often have elevated 1,25(OH)2D3 
levels with normal calcium levels and low bone mass, associated 
with increased production of 1,25(OH)2D3 in intestinal macro­
phages.424 In an analysis of 101 patients with hypercalcemia and 
high levels of 1,25(OH)2D3 in Australia, half the patients had 
sarcoidosis and the others had hematologic mali

d 
sarcoidosis and the others had hematologic malignancies, other 
infections, or other causes.425 
CYP24A1 Loss of Function
Cyp24A1 encodes the vitamin D-24-hydroxylase, which is an 
important regulated inactivator of 1,25-(OH)2D. Homozygous 
loss of this gene leads to idiopathic infantile hypercalcemia, usu­
ally a severe disease presenting in early childhood. However, the 
spectrum of presentation is broad, and some patients present in 
late childhood and even adulthood, with hypercalciuria, 

hildhood and even adulthood, with hypercalciuria, kidney 
stones, and nephrocalcinosis.426 
Mild hypercalcemia can result from thyrotoxicosis.427 Blood 
calcium levels seldom exceed 11 mg/dL, but mild elevations are 
found in a quarter of patients. Patients have low PTH levels, 
low 1,25(OH)2D3 levels, and hypercalciuria. The hypercalce­
mia is caused by a direct action of thyroid hormone to stimulate 
bone resorption.428 β-Adrenergic blocking agents can reverse the 

rgic blocking agents can reverse the 
hypercalcemia.429 
Vitamin A Intoxication
Excess ingestion of vitamin A (retinol) results in a syndrome of dry 
skin, pruritus, headache from pseudotumor cerebri, bone pain, 
and occasional hypercalcemia. Hypercalcemia occurs only with 
the ingestion of 10 times the Recommended Dietary Allowance 
(RDA) (5000 IU/day). The identical syndrome can result from 
ingestion of the vitamin A derivatives isotretinoin (13-cis-retinoic 
acid [Accutane]) and tretinoin (a

13-cis-retinoic 
acid [Accutane]) and tretinoin (all-trans-retinoic acid [Retin-A]), 
used to treat acne and acute promyelocytic leukemia.430,431 Bones 
can show characteristic periosteal calcification on radiographs. 
The hypercalcemia is probably caused by the action of retinoids to 
stimulate bone resorption. The diagnosis is made by the association 
of a history of excess ingestion of retinoids with the characteristic 
syndrome and abnormal results of liver function tests; elevated 
vitamin 

esults of liver function tests; elevated 
vitamin A levels confirm the diagnosis. Treatment involves hydra­
tion and, if necessary, glucocorticoids or bisphosphonates. 
Hypercalcemia occurs in the setting of adrenal insufficiency. 
Blood calcium is elevated partly as a result of hemoconcentration 
and increased albumin levels, but the level of ionized calcium can 
be increased as well.432 PTH and 1,25(OH)2D3 levels are low to 
low-normal.433 The hypercalcemia in this study 

o 
low-normal.433 The hypercalcemia in this study resulted from a 
combination of influx of calcium into the vascular space, probably 
from bone, combined with low renal clearance. 
Thiazide Diuretics
Thiazide diuretics do not cause hypercalcemia by themselves, but 
they can exacerbate the hypercalcemia of primary hyperparathy­
roidism or any other cause of increased input of calcium into the 
bloodstream that is not suppressed by hypercalcemia. The mecha­
nism of the hypercalcemia may involve t

The mecha­
nism of the hypercalcemia may involve the action of thiazide 
diuretics to increase proximal tubular calcium reabsorption as a 
secondary consequence of direct action of thiazides on the distal 
tubule.434,435 Decreased renal clearance of calcium alone would be 
expected to raise blood calcium in the normal human only tran­
siently because the transient hypercalcemia would be expected to 
suppress PTH secretion and lead to return of the blood calcium 
to normal. However, in the presen

e blood calcium 
to normal. However, in the presence of primary hyperparathyroid­
ism, sarcoidosis, excess calcium intake, or any other cause of high, 
fixed calcium load, thiazide administration will increase the level 
of calcium in blood.
As predicted by this model, thiazide administration leads to 
chronic hypercalcemia in patients with abnormal parathyroid 
physiology but not in normal subjects.436 In primary hyperpara­
thyroidism, thiazide administration exacerbates the hypercalcemia, 
and

administration exacerbates the hypercalcemia, 
and in hypoparathyroidism, thiazide administration facilitates the 
maintenance of normocalcemia when given in conjunction with 
1,25(OH)2D3 and calcium. 
Milk-Alkali Syndrome
The triad of hypercalcemia, metabolic alkalosis, and renal fail­
ure can be the consequence of massive ingestion of calcium and 
absorbable alkali. This syndrome was first described when milk 
and sodium bicarbonate were used in large amounts to treat peptic 
ulcer disease. Wi

n large amounts to treat peptic 
ulcer disease. With the change in ulcer treatment to nonabsorbable 
antacids and suppression of acid secretion, milk-alkali syndrome 
became rare. In the last several years, however, the increased use of 
calcium carbonate to treat dyspepsia and osteoporosis has led to 
the reappearance of milk-alkali syndrome.437,438 In most cases, a 
history of ingestion of several grams per day of calcium in the form 
of calcium carbonate can be elicited. Some reports suggest 

m carbonate can be elicited. Some reports suggest that less 
than 4 g of calcium daily can cause milk-alkali syndrome, fewer 
than previously estimated, perhaps because of greater ingestion of 
vitamin D than in the past.439 The pathogenesis of the syndrome 
is not understood in detail but may well involve a vicious circle in 
which alkalosis decreases renal calcium clearance and hypercalce­
mia helps maintain alkalosis. Nephrocalcinosis, nephrogenic dia­
betes insipidus, decrease in GFR associa

enic dia­
betes insipidus, decrease in GFR associated with hypercalcemia, 
and hypovolemia from vomiting all lead to renal failure, which 
can be severe. PTH levels, measured with currently available two-
site assays, are invariably low in hypercalcemic patients, as are lev­
els of 1,25(OH)2D3. After clearance of the calcium by hydration 
1233
or dialysis, if necessary, renal function generally returns to normal, 
unless the disorder has b

ally returns to normal, 
unless the disorder has been severe and longstanding. 
Immobilization
Immobilization can lead to bone resorption sufficient to cause 
hypercalcemia. The immobilization is usually caused by spinal cord 
injury or extensive casting after fractures, though it can occur in 
settings such as Parkinson disease.434 Hypercalcemia after trauma 
requiring immobilization is common, when studied prospectively, 
and is usually asymptomatic.440 Hypercalcemia of immobiliza­
tion occurs

matic.440 Hypercalcemia of immobiliza­
tion occurs predominantly in the young or in patients with other 
reasons for a high rate of bone turnover, such as Paget disease or 
extensive fractures. Hypercalciuria and substantial bone loss are 
more common than hypercalcemia. After spinal cord injury, the 
hypercalciuria is maximal at 4 months and can persist for more 
than 1 year. PTH and 1,25(OH)2D3 levels are suppressed; bone 
biopsies show increased resorption and decreased formation of 
bone. Bi

ed resorption and decreased formation of 
bone. Bisphosphonates and denosumab have been used to reverse 
the hypercalcemia and hypercalciuria of immobilization. 
Following rhabdomyolysis, during the oliguric phase of acute renal 
failure, severe hypocalcemia can result from acute hyperphospha­
temia and calcium deposition in muscle.441 PTH levels are high, 
in response to the hypocalcemia. In the diuretic phase that fol­
lows, hypercalcemia can occur. The hypercalcemia results from

calcemia can occur. The hypercalcemia results from the 
mobilization of the calcium deposits and, in a fraction of patients, 
from associated high 1,25(OH)2D3 levels sometimes seen.442
In chronic renal failure, hypercalcemia can result from tertiary 
hyperparathyroidism or may appear during therapy of aplastic 
bone disease associated with low PTH levels and sometimes with 
aluminum toxicity. 
Williams Syndrome
Williams syndrome is a developmental disorder in which supraval­
vular aortic stenosi

l disorder in which supraval­
vular aortic stenosis is associated with elfin facies and mental retar­
dation.443 Hypercalcemia can occur transiently in the first 4 years 
of life, though, as a group, Williams patients have higher than 
normal calcium levels, usually at the top of the normal range.444 
Affected hypercalcemic infants have been found to have increased 
intestinal absorption of calcium and associated elevations of 
1,25(OH)2D3 that fall to normal as the blood calcium normal­
izes.44

all to normal as the blood calcium normal­
izes.445 Levels of 25(OH)D are normal. The hypercalcemia can 
generally be controlled by dietary manipulation and, if needed, 
bisphosphonates.446
Molecular analysis has clarified the origin of the connective tis­
sue component of Williams syndrome. Williams syndrome is a 
contiguous gene syndrome with deletions of one or several genes. 
Isolated supravalvular aortic stenosis is associated with deletion 
or translocation of the distal portion of the ela

or translocation of the distal portion of the elastin gene. Williams 
syndrome, with more protean connective tissue abnormalities and 
mental retardation, is associated with large deletions that include 
the elastin gene and a gene encoding the protein kinase LIM-
kinase 1. A gene within the Williams syndrome deletion region 
encodes a nuclear protein, Williams syndrome transcription fac­
tor, that is part of a large chromatin remodeling complex that can 
bind the VDR and influence the transcri

 that can 
bind the VDR and influence the transcription of VDR-responsive 
genes.447 For this reason, this gene is a strong candidate for the 
gene associated with transient hypercalcemia in this disorder. 
Genetic proof that this gene is responsible for the hypercalcemia 
and a connection between the gene and hypercalcemia, however, 
is lacking. 
Jansen Metaphyseal Chondrodysplasia
Jansen metaphyseal chondrodysplasia is a rare disease in which 
affected persons present in childhood with short s

affected persons present in childhood with short stature and 
hypercalcemia (Fig. 29.27). Blood chemistry studies suggest 
hyperparathyroidism, with high calcium, low-normal phosphate, 
high 1,25(OH)2D3, high alkaline phosphatase, and high uri­
nary hydroxyproline levels, but PTH levels are suppressed.448 A 
generalized defect in endochondral bone formation results from 
abnormally organized chondrocytes in growth plates. Metaphy­
ses appear disordered and rachitic on radiographs. The bones may 

rdered and rachitic on radiographs. The bones may 
show signs of osteitis fibrosa cystica. Constitutive activation of the 
PTH/PTHrP receptor, caused by five different point mutations in 
three different amino acid residues in the transmembrane domains 
of the receptor, explains the findings in this disorder.104,449,450 The 
abnormalities on serum chemistry studies result from PTH-like 
actions of the receptor in bone and kidney; one patient has been 
noted to have increased blood levels of FGF2

been 
noted to have increased blood levels of FGF23.451 The growth 
plate disorder results from PTHrP-like actions of the receptor on 
the growth plate. 
Approach to the Hypercalcemic Patient
The diagnostic approach to the hypercalcemic patient is strongly 
influenced by the clinical setting and the knowledge that primary 
hyperparathyroidism is at least twice as common as all other causes 
combined (Table 29.2). These considerations are particularly sig­
nificant in the patient who seems otherw

arly sig­
nificant in the patient who seems otherwise well and in whom the 
hypercalcemia is detected incidentally or is mild, stable, or known 
to be of long duration (i.e., years). Among outpatients referred 
to endocrinologists for evaluation of hypercalcemia, for example, 
more than 90% are found to have primary hyperparathyroidism. 
In ill or hospitalized patients, malignant disease is the cause in 
more than 50% of cases. The differential diagnosis is seldom com­
plicated, however, because

iagnosis is seldom com­
plicated, however, because malignant hypercalcemia usually pres­
ents in the context of advanced, clinically obvious disease.
• Fig. 29.27  A patient with Jansen metaphyseal chondrodysplasia at ages 
5 years and 22 years. Note the short stature, characteristic facies, and 
misshapen metaphyseal region of long bones. (From Frame B, Poznanski 
AK. Conditions that may be confused with rickets. In DeLuca HF, Anas­
tas CS, eds. Pediatric Diseases Related to Calcium. New York: 

 Pediatric Diseases Related to Calcium. New York: Elsevier; 
1980:269–289.)
1234
Because hypercalcemia usually is first detected as an elevation 
of total serum calcium, it is important to distinguish hemocon­
centration or rare instances of calcium-binding paraproteinemia 
or thrombocythemia-associated hypercalcemia (due to release of 
intracellular calcium in vitro) from a true increase in serum ion­
ized calcium (Fig. 29.28). The presence of hypercalcemia sho

um (Fig. 29.28). The presence of hypercalcemia should 
be confirmed by direct measurement of ionized calcium, and 
total calcium should be repeated, together with albumin, globu­
lin electrolytes, blood urea nitrogen, creatinine, and phosphate. 
Especially when hypercalcemia is mild, it is prudent to repeat the 
serum total or ionized calcium measurement at least twice, prefer­
ably fasting and without venous occlusion, before proceeding with 
more costly studies directed at its cause.
A careful

re costly studies directed at its cause.
A careful history and physical examination, combined with 
efforts to assess chronicity by seeking prior results of routine mul­
tichannel serum chemistry determinations, most often will point 
to the likely diagnosis. Serum phosphate often is low in hyper­
parathyroidism, but as this often is true also of PTHrP-secreting 
malignancies, the presence of hypophosphatemia is not helpful 
in distinguishing these possibilities. When serum phosphate is 
normal 

se possibilities. When serum phosphate is 
normal or high despite correction of dehydration, the possibil­
ity of PTH-independent or PTHrP-independent hypercalcemia 
should be considered more strongly, however. Elevations in serum 
chloride and alkaline phosphatase, often observed in primary 
hyperparathyroidism, cannot be reliably used in the differential 
diagnosis of hypercalcemia. Important elements of the medical 
history of hypercalcemic patients include inquiries about kid­
ney stones or 

tients include inquiries about kid­
ney stones or fractures; weight loss; back or bone pain; fatigue or 
weakness; cough or dyspnea; ulcer disease or pancreatitis; inges­
tion of vitamins, calcium preparations, lithium, or thiazides; dates 
of most recent mammograms and chest radiographs; and a family 
history of hypercalcemia, kidney stones, ulcer disease, endocri­
nopathy, or tumors of the head or neck. Because malignancy is 
a common cause of hypercalcemia and may occur concomitantly 
with pr

hypercalcemia and may occur concomitantly 
with primary hyperparathyroidism, clinical findings strongly sug­
gestive of malignancy should be acted upon by proceeding directly 
to a search for an underlying tumor, regardless of serum PTH.
The single most important test in the differential diagnosis of 
hypercalcemia is the measurement of serum PTH, preferably in 
a two-site assay specific for the intact, biologically active mole­
cule (see Fig. 29.19). New PTH assays have been introduced that 
ig

.19). New PTH assays have been introduced that 
ignore long circulating fragments of the hormone, which lack the 
amino-terminal residues required for activity at the PTH/PTHrP 
receptor, but whether these new assays will be more useful than 
standard “intact PTH” assays remains to be established.452,453 A 
consistently elevated serum PTH in the presence of true hypercal­
cemia always is abnormal and almost always indicates the presence 
of primary hyperparathyroidism. The exceptions that also c

ry hyperparathyroidism. The exceptions that also can be 
associated with elevated PTH levels are FHH, autonomous para­
thyroid secretion complicating secondary hyperparathyroidism 
(tertiary hyperparathyroidism), lithium-associated hyperparathy­
roidism, and, very rarely, ectopic PTH secretion by a malignant 
neoplasm or antagonizing autoantibodies directed against the 
CaSR in patients with other autoimmune disease(s)—an acquired 
condition that mimics FHH.385,386
Diagnosis of primary hyperpara

 mimics FHH.385,386
Diagnosis of primary hyperparathyroidism is complicated, 
however, by the fact that some patients fail to exhibit both hyper­
calcemia and elevated iPTH. In up to 10% of patients with hyper­
calcemia and primary hyperparathyroidism, PTH levels may fall 
within the normal range (high normal) with current PTH assays. 
Such PTH levels are inappropriate in the face of hypercalcemia, 
however, and support the diagnosis of PTH-dependent hypercal­
cemia. In fact, many such patients 

dent hypercal­
cemia. In fact, many such patients will manifest frankly elevated 
serum PTH if retested, especially if dietary calcium is restricted 
beforehand. As noted earlier, some patients may present with 
serum calcium in the high-normal range (>10 mg/dL) together 
with an elevated or high-normal PTH. This may be discovered 
incidentally in an otherwise asymptomatic subject or in the course 
of evaluating recurrent urolithiasis or osteopenia. Those with 
persistently high-normal serum cal

ia. Those with 
persistently high-normal serum calcium and high-normal iPTH 
should be retested at intervals and, meanwhile, given a provisional 
diagnosis of hyperparathyroidism and evaluated accordingly.
In patients with PTH-dependent hypercalcemia (Fig. 29.29), 
calcium and creatinine should be measured in a 24-hour urine col­
lection and simultaneous serum sample to measure total urinary 
calcium output (mg/day) and the clearance ratio of calcium (Ca)/
creatinine (Creat) in urine (U) and ser

cium (Ca)/
creatinine (Creat) in urine (U) and serum (S): UCa/SCa × SCreat/
UCreat). A daily calcium excretion of less than 100 mg per day, 
or a clearance ratio less than 0.01, should prompt consideration 
of FHH, especially in patients younger than 40 years, or those 
with a family history of FHH, or patients whose serum iPTH 
levels are within the normal range. A urinary calcium excretion 
greater than 4 mg/kg per day or clearance ratio greater than 
0.02 effectively excludes FHH. In FHH, ser

r than 
0.02 effectively excludes FHH. In FHH, serum phosphate is 
normal or slightly low, serum magnesium may be slightly high, 
and serum 1,25(OH)2D3 is normal or low (unlike in primary 
hyperparathyroidism).
A definite diagnosis of FHH, as in the MEN syndromes, may 
be provided by confirming the presence of mutations in the rel­
evant genes, although such studies are not invariably informative 
  Causes of Hypercalcemia
Primary hyperparathyroidism
Tertiary

Hypercalcemia
Tertiary hyperparathyroidism
Lithium-associated hypercalcemia
Antagonistic autoantibodies to the calcium-sensing receptor
Neoplasms
PTHrP dependent
Other humoral syndromes
Local osteolytic disease (including metastases)
PTHrP excess (nonneoplastic)
Excess vitamin D action
Ingestion of excess vitamin D or vitamin D analogues
Topical vitamin D analogues
Granulomatous disease
William

 vitamin D analogues
Williams syndrome
Acute renal failure
Chronic renal failure with aplastic bone disease
Jansen disease
Drugs
Vitamin A intoxication
Milk-alkali syndrome
Thiazide diuretics
Theophylline
PTHrP, Parathyroid hormone–related protein.
   
TABLE 29.2
1235
(presumably because of mutations in introns and other unchecked 
regions) and usually 

introns and other unchecked 
regions) and usually are unnecessary. The identification of RET 
gene mutations is now an essential part of the management of 
families with MEN2 because this information most effectively 
guides the decision for preventive thyroidectomy to prevent med­
ullary cancer of the thyroid. In contrast, the identification of 
MENIN gene mutations has not yet led to any effective preven­
tive strategies, thus genetic analysis may be useful only for genetic 
counseling in fami

may be useful only for genetic 
counseling in families with MEN1. Even for this purpose, the 
incomplete ascertainment of mutations limits the effectiveness of 
such analysis.
In patients with suspected lithium-induced hyperparathy­
roidism, a trial off lithium, if feasible clinically, may confirm the 
diagnosis or indicate the presence of persistent primary hyperpara­
thyroidism. Patients with primary hyperparathyroidism should 
undergo bone densitometry, preferably at sites rich in cortical an

sitometry, preferably at sites rich in cortical and 
trabecular bone (i.e., forearm or hip and lumbar spine, respec­
tively) to assist in the decision about surgery. Those younger than 
40 years of age or having a family history of hypercalcemia (or 
other MEN manifestations) should be evaluated for these syn­
dromes as well. Patients not meeting criteria for parathyroidec­
tomy should be followed medically, as should those with FHH. In 
rare patients with CaSR-blocking autoantibodies, hypercalc

ients with CaSR-blocking autoantibodies, hypercalcemia 
may respond to glucocorticoids.386
A low or undetectable serum PTH level signifies the presence 
of nonparathyroid hypercalcemia and should prompt a detailed 
evaluation for malignancy or other causes of PTH-independent 
hypercalcemia (see Table 29.2). Breast and lung cancers alone 
account for over 50% of all malignancy-associated hypercalce­
mias. Mammography, chest radiography with or without CT, 
abdominal CT, and serum and urinary immu

hout CT, 
abdominal CT, and serum and urinary immunoelectrophoresis 
are among the more useful tests for detecting the cause of non­
parathyroid hypercalcemia. Although humoral mechanisms, 
especially secretion of PTHrP, are implicated in the pathogenesis 
of most cancer-associated hypercalcemias, bone metastases are 
common, particularly in breast cancer. Technetium-99m bone 
scanning therefore generally is useful for detecting this syndrome 
and identifying bones vulnerable to fracture. The ut

d identifying bones vulnerable to fracture. The utility of serum 
PTHrP measurements probably is limited to the unusual situation 
in which serum PTH is suppressed but an underlying malignancy 
cannot readily be demonstrated. PTHrP-associated hypercalce­
mia can occur rarely during pregnancy and lactation, via secretion 
from benign neoplasms, or in association with lymphoid hyper­
plasia in lupus erythematosus or human immunodeficiency virus 
(HIV).398
In the absence of evident malignancy, unus

IV).398
In the absence of evident malignancy, unusual causes of hyper­
calcemia should be sought.398 Vitamin D and vitamin A intoxica­
tion can be excluded by measurement of serum 25(OH)D and 
retinoids, respectively. Elevated 1,25(OH)2D3 and hypercalcemia 
may occur in several settings, including sarcoidosis and other 
granulomatous diseases, B-cell and T-cell lymphomas (includ­
ing AIDS-associated lymphomas), and uncommonly, in Crohn 
disease, in neonatal subcutaneous fat necrosis syndrome, or

in neonatal subcutaneous fat necrosis syndrome, or in 
epithelial neoplasms such as lung cancer. Very rarely, patients 
HYPERCALCEMIA
Malignancy-associated
Measure serum total
and ionized calcium
History, physical examination
Measure electrolytes, BUN, creatinine,
    phosphate, alkaline phosphatase
Clinical evaluation
Measure serum PTH
Proceed to algorithm for
diagnosis of PTH-dependent
(see Fig. 29.29)
Normal
or high
Low
PTH-dependent
hypercalcemia 
PTH-independent

Low
Evaluate for other causes
of PTH-independent
(see Table 29.2)
Signs or symptoms
of malignancy
No
Yes
Select appropriate
therapy for malignancy;
consider
bisphosphonate
Chest radiograph
Serum/urine IEP
Mammogram
Abdominal/chest CT
Search for occult malignancy
Hemoconcentration or
serum protein abnormality
Normal
Elevated
• Fig. 29.28  Approach to the management of the hypercalcemic patient. BUN, blood urea nitrogen; CT,

percalcemic patient. BUN, blood urea nitrogen; CT, 
computed tomography; IEP, immunoelectrophoresis; PTH, parathyroid hormone.
1236
with severe idiopathic hypercalciuria and excessive absorption 
of dietary calcium may manifest mild, dietary-dependent hyper­
calcemia. Overtreatment of hypoparathyroidism or other condi­
tions with oral 1,25(OH)2D3 or topical use of analogues of the 
active metabolite in psoriasis should be obvious from the history. 
Because hyper

should be obvious from the history. 
Because hypercalcemia and hypercalciuria are observed in up to 
10% and 30%, respectively, of patients with thyrotoxicosis, mea­
surement of serum thyroid-stimulating hormone (TSH) may be 
helpful, especially in older patients who may be less overtly symp­
tomatic. Adrenal insufficiency and pheochromocytoma usually 
are accompanied by characteristic clinical features, but a definite 
diagnosis may be sought with appropriate studies. Granulomatous 
diseases ar

th appropriate studies. Granulomatous 
diseases are among the more common disorders that underlie ini­
tially unexplained hypercalcemia. 
Causes of Severe Hypercalcemia
The need for urgent therapy of acute, severe hypercalcemia, usu­
ally defined as a serum calcium concentration greater than 14 
mg/dL (>3.5 mmol/L), is unusual. This is because most patients 
with hypercalcemia have primary hyperparathyroidism, in which 
hypercalcemia is typically chronic and mild. Episodes of acute, 
severe hype

 chronic and mild. Episodes of acute, 
severe hypercalcemia may occur occasionally in primary hyper­
parathyroidism (parathyroid crisis), usually in patients with large 
parathyroid adenomas and very high PTH levels. The severe 
hypercalcemia in this setting may be triggered by dehydration 
due to diarrheal illness, protracted vomiting or diuretic therapy, 
recovery from major surgery, immobilization, ingestion of large 
amounts of oral calcium salts, hemorrhage or rupture of a cystic 
parathyro

alts, hemorrhage or rupture of a cystic 
parathyroid neoplasm, or parathyroid carcinoma.
Most often, acute, severe hypercalcemia is encountered in 
patients with underlying malignancy, in whom accelerated bone 
resorption dramatically increases the filtered load of calcium. The 
ensuing profound hypercalciuria impairs renal tubular sodium 
reabsorption, which induces progressive extracellular volume 
depletion, reduces GFR, impairs renal calcium clearance, and fur­
ther aggravates the hypercalce

clearance, and fur­
ther aggravates the hypercalcemia. In many such patients, elevated 
circulating levels of PTHrP compound the problem by mimicking 
the action of PTH to enhance distal tubular calcium reabsorption. 
Clinical Features of Severe Hypercalcemia
The indications for urgent therapy of hypercalcemia usually 
relate more to the presence of clinical symptoms of hypercalce­
mia than to the absolute level of serum calcium, although few 
clinicians would hesitate to treat patients in whom 

inicians would hesitate to treat patients in whom total serum 
calcium exceeded 14 mg/dL (>3.5 mmol/L). Many patients with 
previously mild hypercalcemia become symptomatic when serum 
calcium concentrations exceed 12 mg/dL (>3 mmol/L). It is 
important to remember that hypoalbuminemia may mask signifi­
cant elevations of ionized calcium. The most common symptoms 
of severe hypercalcemia are referable to disturbances of nervous 
system and gastrointestinal function—fatigue, weakness, lethargy, 

intestinal function—fatigue, weakness, lethargy, 
confusion, coma (rarely), anorexia, nausea, abdominal pain (rarely 
PTH
normal
PTH
high
Presumptive FHH;
consider family screening
No
No
Yes
Yes
Yes
No
Primary
hyperparathyroidism
Li therapy
Consider stopping Li therapy
Li-induced
Review criteria for
surgery (see Table 29.1)
Parathyroidectomy
Medical
observation
Urine calcium 100 mg/day,
or ClCa/ClCr 0.01
24-hr urine calcium
and creatinine
Perfor

Cr 0.01
Perform bone densitometry
Calcium
normal
Calcium
high
Age 40, or Fam. Hx.
• Fig. 29.29  Approach to the management of the hypercalcemic patient with parathyroid hormone–depen­
dent hypercalcemia. Cl, clearance; Fam. Hx., family history; FHH, familial hypocalciuric hypercalcemia; Li, 
lithium; PTH, parathyroid hormone.
1237
due to pancreatitis), and constipation. Polyuria, nocturia, and 
polydips

nd constipation. Polyuria, nocturia, and 
polydipsia commonly are present also.
Bone pain is often present but is usually due to underlying 
metastatic disease. Cardiac arrhythmias may occur, particularly 
bradyarrhythmias or heart block; digitalis toxicity may be poten­
tiated; and ST-segment elevation responsive to treatment of the 
hypercalcemia may be seen. Patients who suffer a fatal outcome 
from acute severe hypercalcemia may manifest coma, hypoten­
sion, acute pancreatitis, acute renal f

, hypoten­
sion, acute pancreatitis, acute renal failure, widespread soft tissue 
calcification, heart failure, or venous thrombosis, particularly of 
the renal veins. 
Management of Severe Hypercalcemia
The first decision to be made in the management of acute, severe 
hypercalcemia is whether or not to treat the problem at all. This 
may become an issue for the patient with an untreatable, widely 
disseminated malignancy, when all other approaches to control­
ling the neoplasm have been exhaust

es to control­
ling the neoplasm have been exhausted, and the patient has cho­
sen not to have complications treated. Otherwise, as noted earlier, 
patients who are symptomatic or have serum calcium levels above 
14 mg/dL ordinarily should be treated aggressively. Treatment 
most often entails rehydration and administration of a bisphos­
phonate intravenously (Table 29.3). Calcitonin can be useful as a 
temporary measure early in therapy, and glucocorticoids or dialy­
sis may be indicated in som

cocorticoids or dialy­
sis may be indicated in some patients.125
Volume Repletion
When treatment is indicated, the first priority is to correct the 
extracellular volume depletion that almost invariably is present, 
usually by infusing isotonic saline at a rate of 2 to 4 L/day. The 
aggressiveness with which the individual patient is rehydrated must 
be considered in relation to both the patient’s volume status and 
the risk of precipitating or aggravating congestive heart failure or 
ascites. D

ggravating congestive heart failure or 
ascites. Diuretics, particularly thiazides, should be discontinued. 
The use of furosemide or other potent “loop” diuretics to promote 
calciuresis may exacerbate extracellular volume depletion if used 
too early in the course of treatment. In light of the availability of 
highly effective alternatives for the therapy of hypercalcemia, such 
drugs probably are best avoided, except in circumstances in which 
vigorous rehydration fails to improve severe hype

vigorous rehydration fails to improve severe hypercalcemia or 
might precipitate congestive heart failure. In any case, prolonged 
use of saline-induced calciuresis without the early introduction of 
an effective antiresorptive agent is ill advised and ultimately futile. 
Bisphosphonates
Intravenous bisphosphonates rapidly inhibit bone resorption and 
currently are the agents of first choice in managing severe hyper­
calcemia that is known or suspected to be driven mainly by osteo­
clastic bone

spected to be driven mainly by osteo­
clastic bone resorption.125 Bisphosphonates should not be used in 
patients with milk-alkali syndrome, for example, in whom they 
are likely to induce posttreatment hypocalcemia.454 Pamidronate 
and zoledronate are approved by the Food and Drug Adminis­
tration (FDA) for treatment of hypercalcemia of malignancy and 
are the most widely used agents in the United States, although 
ibandronate and clodronate have been successfully used elsewhere. 
These drugs g

e been successfully used elsewhere. 
These drugs generally are well tolerated, although local pain or 
swelling at the infusion site, low-grade fever 1 to 2 days after the 
infusion, transient lymphopenia, and mild hypophosphatemia 
or hypomagnesemia may occur. Serum calcium usually declines 
within 24 hours and reaches a nadir within 1 week following a 
single infusion, at which point calcium levels may be normal in 
70% to 90% of treated patients. Intravenous bisphosphonates 
may be nephrotoxi

ts. Intravenous bisphosphonates 
may be nephrotoxic, but clinical data to guide their use in patients 
with renal insufficiency are not yet available. Most clinicians will 
use the standard dose (see Table 29.3)—perhaps at half or less 
of the usual rate of administration—in patients with moderate 
renal insufficiency (GFR >30 mL/minute), which is common in 
the setting of severe hypercalcemia. In patients with more severe 
renal insufficiency, bisphosphonates probably are best avoided 
and dial

isphosphonates probably are best avoided 
and dialysis may be a more appropriate alternative (see later). The 
duration of the response to intravenous bisphosphonate treatment 
is quite variable, ranging from 1 or 2 weeks to several months. 
Depending on clinical circumstances, repeated courses of therapy 
may be indicated and effective. 
Denosumab
Denosumab, a monoclonal antibody directed against RANKL, 
has been shown to be effective in managing malignancy-associ­
ated hypercalcemia, including

g malignancy-associ­
ated hypercalcemia, including patients refractory to bisphos­
phonates.455 Denosumab offers an alternative to intravenous 
bisphosphonates in patients with renal failure, but like bisphos­
phonates, denosumab carries a small risk of osteonecrosis of the 
jaw. Typical dosage is 60 mg given subcutaneously every week for 
a month, followed by 60 mg per month. 
Calcitonin, which directly inhibits osteoclast function, may be 
used with other antiresorptive agents to ac

ay be 
used with other antiresorptive agents to achieve more rapid con­
trol of severe hypercalcemia. Calcitonin rarely produces a decline 
in serum calcium of more than 1 to 2 mg/dL, and its efficacy typi­
cally is limited to a few days at most, possibly because of recep­
tor downregulation in target cells of bone and kidney. Its major 
advantages are a more rapid onset of action than bisphosphonates 
(several hours) and its potential to augment renal calcium excre­
tion directly. Calcitonin ge

 renal calcium excre­
tion directly. Calcitonin generally is well tolerated, although tran­
sient nausea, vomiting, abdominal cramps, flushing, and local 
skin reactions may occur. 
Other Approaches to Treatment of Severe Hypercalcemia
Because of their potential toxicity, other antiresorptives such as 
gallium nitrate, plicamycin (mithramycin), and intravenous 
  Treatment of Severe Hypercalcemia
Type of Therapy
Usual Dose
Frequency
Rehydration
2–4 L/day of 0.9% NaCl IV
qd × 1–5 days
Furosemide

20–40 mg IV (after 
rehydration)
q12–24 hr
Pamidronate
60–90 mg IV over 2–4 hr
Once
Zoledronate
4 mg IV over 15–30 min
Once
Denosumab
60 mg SC
Weekly
4–8 IU/kg SC
q12–24 hr
Gallium nitrate
200 mg/m2 IV over 24 hr
qd × 5 days
200–300 mg hydrocortisone IV
40–60 mg prednisone PO
qd × 3–5 days
Dialysis
IV, Intravenous; PO, orally; q, every; qd, every day; SC, subcutaneous.
   
TABLE 29.3
1238
SECTION VII    M

subcutaneous.
   
TABLE 29.3
1238
phosphate (in patients with severe hypophosphatemia) have 
largely been abandoned in the treatment of severe hypercalcemia, 
although a randomized trial demonstrated that gallium nitrate 
may be more effective than bisphosphonate in controlling hyper­
calcemia of malignancy.456 Oral or enteral phosphate repletion is 
appropriate for patients with significant hypophosphatemia (<2.5 
mg/dL), provided that serum phosphate and renal

5 
mg/dL), provided that serum phosphate and renal function are 
closely monitored. Intravenous or oral glucocorticoids should be 
considered early in patients with suspected vitamin D–dependent 
hypercalcemia, including those with lymphoma or granulomatous 
disease. The response to glucocorticoids may be more delayed than 
that to bisphosphonates. Successful treatment of hypercalcemia in 
Crohn disease with infliximab has been reported.457
In patients with severe renal insufficiency, with or wi

tients with severe renal insufficiency, with or without 
complicating heart disease, in whom saline rehydration and associ­
ated calciuresis may not be feasible and bisphosphonates probably 
are best avoided, dialytic therapy against a low-calcium or zero-
calcium dialysate may be the most appropriate tactic. In patients 
with known primary hyperparathyroidism and intercurrent severe 
hypercalcemia (parathyroid storm), urgent parathyroidectomy 
(following initial medical stabilization) should be

following initial medical stabilization) should be considered.
Novel approaches to the treatment of severe hypercalcemia cur­
rently are in development. One available therapy for parathyroid 
carcinoma is the calcimimetic cinacalcet, which may be effective 
in some patients,458 and monoclonal antibodies directed against 
PTHrP could prove useful in controlling PTHrP-dependent 
hypercalcemia.459 
Hypocalcemic Disorders
The predominant clinical symptoms and signs of hypocalce

edominant clinical symptoms and signs of hypocalcemia are 
those of neuromuscular irritability, including perioral paresthesias, 
tingling of the fingers and toes, and spontaneous or latent tetany. 
Tetany can be elicited by percussion of the facial nerve below the 
zygoma, resulting in ipsilateral contractions of the facial muscle 
(Chvostek sign) or by 3 minutes of occlusive pressure with a blood 
pressure cuff resulting in carpal spasm, which, on occasion, can be 
very painful (Trousseau sign

 on occasion, can be 
very painful (Trousseau sign) (Fig. 29.30). The usefulness of these 
signs in diagnosing hypocalcemia and in following therapeutic 
responses cannot be overemphasized.
Electrocardiographic abnormalities also result from hypocalce­
mia, including prolonged QT intervals and marked QRS com­
plex and ST-segment changes that may mimic acute myocardial 
infarction or conduction abnormalities. Ventricular arrhythmias 
are a rare complication of hypocalcemia, although congestive 
h

mplication of hypocalcemia, although congestive 
heart failure, corrected by normalization of serum calcium, has 
been reported. In profound hypocalcemia or during acute falls in 
serum calcium, grand mal seizures or laryngospasm also may be 
observed. Chronic hypocalcemia is associated with milder symp­
toms and signs of neuromuscular irritability and may even be 
asymptomatic. Longstanding hypocalcemia associated with hyper­
phosphatemia (observed with PTH deficiency or resistance) may 
lead t

ved with PTH deficiency or resistance) may 
lead to calcification of the basal ganglia460–463 and occasionally, 
extrapyramidal disorders. In addition, mineral ion deposits in the 
lens may lead to cataract formation. Chronic hypocalcemia, when 
associated with hypophosphatemia, as in vitamin D deficiency, 
is associated with growth plate abnormalities in children (rick­
ets) and defects in the mineralization of new bone (osteomalacia) 
(see Chapter 31). These findings are not seen in the hypoca

ter 31). These findings are not seen in the hypocalcemia 
of hypoparathyroidism. Severe symptomatic hypocalcemia is an 
emergency that requires immediate attention to prevent seizures 
and death from laryngospasm or cardiac causes.
Total calcium in serum includes both the free (biologically 
active) and protein-bound components; the major binding pro­
tein is albumin (discussed earlier). Therefore measurements of 
total calcium cannot be interpreted without concurrent measure­
ment of albumin. S

ted without concurrent measure­
ment of albumin. Studies of hypoalbuminemic patients with cir­
rhosis have led to a formula for correction of total calcium based 
on concurrent albumin levels (a decrease in calcium of 0.8 mg/
dL for every 1-g/dL decrease in albumin). No formula has proved 
to be accurate, however, for assessment of calcium in acutely ill 
patients. This probably relates to the variety of factors that may 
increase protein binding and decrease the fraction of total calcium 
prese

 and decrease the fraction of total calcium 
present as the free ion, including alkalosis, elevated circulating free 
fatty acids, and lipid infusions. Consequently, ionized calcium 
should be measured when the diagnosis of hypocalcemia is con­
sidered in the setting of acute illness and severe hypoalbuminemia.
Chronic hypocalcemia is most often due to deficiency of PTH 
or 1,25(OH)2D3 or to resistance to the biologic effects of these 
calcium-regulating hormones (Table 29.4). 
Parathyroid-Relat

gulating hormones (Table 29.4). 
Parathyroid-Related Disorders
Hypocalcemia associated with parathyroid dysfunction can be dif­
ferentiated from other causes of hypocalcemia by routine laboratory 
tests. Serum calcium is low owing to lack of PTH-mediated bone 
resorption and urinary calcium reabsorption. Serum phosphate is 
increased owing to impaired renal clearance. Serum 1,25(OH)2D3 
is low because PTH and hypophosphatemia stimulate the renal 
25(OH)D 1α-hydroxylase. Consequently, 1,25(OH)2D3

25(OH)D 1α-hydroxylase. Consequently, 1,25(OH)2D3-mediated 
intestinal calcium absorption is markedly decreased, further exacer­
bating the hypocalcemia. PTH levels measured using sensitive two-
site PTH assays (see Fig. 29.20) are usually low or undetectable but 
may be inappropriately normal if some degree of PTH production 
is preserved. Elevated levels of PTH are found in syndromes associ­
ated with resistance to the biologic effects of PTH.
Congenital or Inherited Parathyroid Disorders
Sev

Several rare syndromes associated with congenital or inher­
ited hypoparathyroidism appear sporadically or in a variety of 
• Fig. 29.30  Trousseau sign. (From Burnside JW, McGlynn TJ. Physical 
Diagnosis. 17th ed. Baltimore: Williams & Wilkins; 1987:63.)
1239
inheritance patterns, suggesting multiple causes. Mutation of a 
parathyroid-specific transcription factor, glial cells missing homo­
l

 transcription factor, glial cells missing homo­
log B (GCMB) (chromosome 6p23), which is expressed in the 
PTH-secreting cells of the developing parathyroids, has been 
shown to be a cause of familial hypoparathyroidism in humans and 
mice.44 The genetic abnormality responsible for X-linked, recessive 
hypoparathyroidism has been identified as a deletion/insertion of 
DNA near the SOX3 gene at Xq26-Xq27.47 Analyses of a second 
kindred implicated mutations in the FHL1 (four and a half LIM 
doma

d mutations in the FHL1 (four and a half LIM 
domains 1) gene as the molecular basis for the phenotype.464
In a number of diseases, hypoparathyroidism is associated with 
multiple abnormalities in embryonic development in the neck/chest 
region. DiGeorge syndrome occurs sporadically and is associated 
with an embryologic defect in the formation of the third, fourth, 
and fifth branchial pouches, resulting in the absence of parathyroid 
glands. DiGeorge syndrome may in fact be a neurocrestopathy,

George syndrome may in fact be a neurocrestopathy, since 
ablation of the premigratory cephalic neural crest in chick embryos 
produces the same phenotype.465 The contribution of homeobox 
genes to parathyroid development and their potential relation­
ship to DiGeorge syndrome also has been demonstrated by the 
absence of thymic and parathyroid tissue, accompanied by cardiac 
and craniofacial abnormalities, in mice lacking the homeobox gene 
hoxa3.466 DiGeorge syndrome is often associated with o

3.466 DiGeorge syndrome is often associated with other con­
genital abnormalities in a syndrome referred to by the acronym 
CATCH 22 (cardiac defect, abnormal facies, thymic hypoplasia, 
cleft palate, hypocalcemia, and 22q11 deletions).467 Microdeletion 
of 22q11.21–q11.23 and a t(2;22)(q14;q11) balanced transloca­
tion suggest that a gene at chromosome 22q11 may be pathoge­
netic in this syndrome. Recent studies identified the TBX1 gene as 
being the locus affected since point mutations in this

g the locus affected since point mutations in this gene result 
in DiGeorge syndrome.463 A number of cases of DiGeorge and 
velocardiofacial syndromes have been shown to have no detectable 
abnormality at 22q11, but rather terminal 10p deletions or intersti­
tial 10p13/10p14 deletions, suggesting that two loci may be critical 
for development of branchial pouch structures. Terminal deletions 
of 10p accompanied by hypoparathyroidism can be further subdi­
vided into DiGeorge critical region II (1

r subdi­
vided into DiGeorge critical region II (10p13-14) and a more telo­
meric region (10p14-10pter), wherein mutation of the transcription 
factor GATA3 causes the syndrome of hypoparathyroidism, senso­
rineural deafness, and renal anomaly (HDR).468 Studies in murine 
models have demonstrated that GATA3 regulates the expression of 
GCM2 (the murine homolog of GCMB) and is essential for the 
  Causes of Hypocalcemia
TABLE 29.4
Absence of the Parathyroid Glands o

ated Disorders
Absence of the Parathyroid Glands or of PTH
DiGeorge syndrome
X-linked or autosomally inherited hypoparathyroidism
Autoimmune polyglandular syndrome type I
PTH gene mutations
Postsurgical hypoparathyroidism
Infiltrative disorders
Wilson disease
Hypoparathyroidism post radioactive iodine thyroid ablation
Impaired Secretion of PTH
Hypomagnesemia
Respiratory alkalosis
Activating mutations of the calcium sensor or GNA11
Target Organ Resistance
Hyp

alcium sensor or GNA11
Pseudohypoparathyroidism
Type I
Type II
Vitamin D-Related Disorders
Vitamin D deficiency
Dietary absence
Malabsorption
Accelerated loss
Impaired enterohepatic recirculation
Anticonvulsant medications
CYP3A4 mutation
Impaired 25 hydroxylation
Isoniazid
CYP2R1 mutation
Impaired 1α-hydroxylation
Vitamin D–dependent rickets, type I
Oncogenic osteomalacia
Target organ resistance
Vitamin D–dependent rickets, type

organ resistance
Vitamin D–dependent rickets, type II
Phenytoin
Excessive deposition into the skeleton
Osteoblastic malignancies
Hungry bone syndrome
Impaired bone resorption
RANKL inhibition
Chelation
Foscarnet
Phosphate infusion
Infusion of citrated blood products
Infusion of EDTA containing contrast reagents
Fluoride
Neonatal hypocalcemia
Prematurity
Asphyxia
Diabetic mother
Hyperparathyroid mother
Vitamin D–deficient mother
Infantile malignan

ther
Infantile malignant osteopetrosis
HIV
Drug therapy
Impaired PTH responsiveness
Critical illness
Pancreatitis
Toxic shock syndrome
Intensive care unit patients
CYP2R1, Cytochrome P450, family 2, subfamily R, member 1; CYP3A4, cytochrome P450, family 3, subfamily A, member 4; EDTA, ethylenediaminetetraacetic acid; GNA11, G protein subunit alpha 11; 
PTH, parathyroid hormone; RANKL, receptor activator of nuclear factor κB ligand; X

 receptor activator of nuclear factor κB ligand; X, X chromosome.
1240
development of parathyroid glands.469 The genetic basis for Kenny-
Caffey syndrome type 1, characterized by hypoparathyroidism, 
extreme short stature, mental retardation, and dysmorphism, is 
mutations in the chaperone protein, TBCE, which is required for 
the proper folding of α tubulin and the formation of α-β tubulin 
heterodimers.470 In contrast, autosomal dominant Kenny-Caffey 
syndrome

ontrast, autosomal dominant Kenny-Caffey 
syndrome type 2, characterized by short stature, hypoparathyroid­
ism, and osteocraniostenosis, is caused by mutations in FAM111A, 
a gene of unknown function with protease motifs.461
Familial hypoparathyroidism is seen in conjunction with 
mucocutaneous candidiasis, Addison disease, and other immune 
disorders in autosomal recessive autoimmune polyglandular syn­
drome, type I, caused by mutations in the autoimmune regulatory 
gene (AIRE)471,472 (see Cha

autoimmune regulatory 
gene (AIRE)471,472 (see Chapter 43). NALP5 has been identified 
as a parathyroid-specific antigen in affected patients.473 Hypo­
parathyroidism may also be observed in association with mito­
chondrial myopathies such as mitochondrial trifunctional protein 
deficiency474 and the Kearns-Sayre syndrome.475 Other inherited 
forms of hypoparathyroidism may be observed as an isolated 
defect or may present with other features such as lymphedema, 
dysmorphism, and renal and cardi

h as lymphedema, 
dysmorphism, and renal and cardiac abnormalities. 
Abnormalities in the PTH Gene
Specific defects have been found in the PTH gene in a small number 
of kindreds affected by congenital hypoparathyroidism. These include 
point mutations in the signal peptide4,5 and in an intron border, lead­
ing to aberrant splicing476 as well as a homozygous mutation in exon 
2 of the PTH gene, leading to premature termination of the tran­
script.477 A homozygous mutation in amino acid 25 of PTH

.477 A homozygous mutation in amino acid 25 of PTH(1-84) 
has also been described in familial hypoparathyroidism.478 
Destruction of the Parathyroid Glands
The most common cause of chronic hypocalcemia is postsurgical 
hypoparathyroidism. This may occur after removal of all parathy­
roid tissue during thyroidectomy and radical neck dissection for 
malignancies or after inadvertent interruption of the blood sup­
ply to the parathyroid glands during head and neck surgery. Tran­
sient hypoparathyro

g head and neck surgery. Tran­
sient hypoparathyroidism, attributed to reversible damage to the 
remaining normal glands, is common after parathyroidectomy; 
permanent hypoparathyroidism may occur after vascular or surgi­
cal injury or inadvertent removal of all parathyroid tissue. Rarely, 
transient hypoparathyroidism may follow spontaneous infarction 
of autonomous tissue in primary hyperparathyroidism.479 Hypo­
parathyroidism is a rare complication of radioactive iodine abla­
tion of the thyr

ation of radioactive iodine abla­
tion of the thyroid gland for Graves disease.480
Hypoparathyroidism also can occur as a result of infiltrative 
diseases of the parathyroids. This is seen in diseases of iron over­
load such as hemochromatosis and in patients with thalassemia 
major who have been heavily transfused.481 Copper deposition in 
Wilson disease482 may also cause parathyroid dysfunction. Meta­
static disease to the parathyroids can cause hypoparathyroidism, 
but rarely, presumably beca

e hypoparathyroidism, 
but rarely, presumably because of the need for four-gland involve­
ment before significant hypoparathyroidism is observed. 
Impaired PTH Secretion
Impaired secretion of PTH from the parathyroid glands can lead to 
functional hypoparathyroidism. This is commonly seen in profound 
hypomagnesemia,483 in which target organ resistance to PTH can 
also occur. Both of these abnormalities are reversible upon magne­
sium repletion484,485 (see “Disorders of Magnesium Metabolism”).
C

4,485 (see “Disorders of Magnesium Metabolism”).
Chronic respiratory alkalosis leads to hyperphosphatemia and 
decreased ionized calcium levels accompanied by impaired renal 
calcium resorption and inappropriately normal PTH levels.486 
This biochemical phenotype suggests both an abnormality of 
PTH secretion and renal resistance to PTH. Acute alkalosis in 
dogs also suppresses PTH secretion.487
Activating mutations in the CaSR cause autosomal dominant 
hypocalcemia type 1 (ADH1) associated with

minant 
hypocalcemia type 1 (ADH1) associated with inappropriately nor­
mal PTH levels. This syndrome can also be seen in patients with 
activating antibodies directed against this receptor.488 ADH2 has 
been shown to be caused by gain-of-function mutations of the G 
protein subunit α11 (GNA11).381 The clinical syndrome is vari­
able; patients present with hypocalcemia and seizures, whereas their 
affected relatives may be only subsequently diagnosed with asymp­
tomatic hypocalcemia.14 Unlike pa

osed with asymp­
tomatic hypocalcemia.14 Unlike patients with inactivating muta­
tions of the calcium sensor, homozygously affected individuals with 
ADH1 do not appear to have a more severe phenotype. The pres­
ence of hypercalciuria in these patients makes medical management 
uniquely challenging. Treatment with vitamin D metabolites often 
results in a marked increase in renal calcium excretion, associated 
with renal calcification and resultant renal impairment. Based on 
these observations,

nt renal impairment. Based on 
these observations, it has been suggested that asymptomatic indi­
viduals be left untreated and that the goal of therapy in individuals 
with symptomatic hypocalcemia be solely to relieve symptoms, not 
to achieve normocalcemia. Treatment with calcium and vitamin D 
metabolites should be accompanied by the use of thiazide diuretics 
to decrease urinary calcium excretion as well as ensuring adequate 
urinary volume to decrease urinary calcium concentration. 
Pseudoh

o decrease urinary calcium concentration. 
The idiopathic and inherited forms of PTH resistance are referred 
to as pseudohypoparathyroidism (PHP). There are two main types 
of PHP: type I, which is characterized by impaired cAMP and 
phosphaturic responses to PTH, and type II, where the cAMP 
response is preserved but the phosphaturic response is not. The 
first cases of documented PTH resistance were described by 
Albright in 1942. The patients were hypocalcemic, hyper

ght in 1942. The patients were hypocalcemic, hyperphos­
phatemic, and exhibited a number of features now called Albright 
hereditary osteodystrophy (AHO). These features include short 
stature, rounded face, foreshortened fourth and other metacar­
pals, obesity, and subcutaneous ossifications (Figs. 29.31 and 
29.32). PTH administration to these patients failed to provoke 
a phosphate diuresis or an increase in serum calcium. It was sub­
sequently demonstrated that hypocalcemic patients with fea

y demonstrated that hypocalcemic patients with features 
of AHO had elevated PTH levels and that PTH infusions failed 
to stimulate renal production of cAMP. Failure of stimulation of 
cAMP production suggested a defect in the PTH receptor or in its 
cAMP-mediated signal transduction. The measurement of cAMP 
in the urine following an infusion of synthetic PTH(1-34) can be 
used to establish the diagnosis of PTH resistance.489
The variable presence of AHO and renal resistance to PTH 
has led to 

ce of AHO and renal resistance to PTH 
has led to the subclassification of pseudohypoparathyroidism 
(Table 29.5). Type IA is characterized by AHO and diminished 
Gαs activity (∼50% of normal). The diminished Gαs activity has 
been demonstrated in several tissues, including kidney, fibroblasts, 
transformed lymphocytes, platelets, and erythrocytes. Decreased 
amounts of Gαs mRNA (50%) are present in fibroblasts of many 
patients with PHP1A, and inactivating mutations involving exons 
1 to 13 of 

nactivating mutations involving exons 
1 to 13 of GNAS, the gene encoding Gαs, have been identified in 
numerous kindreds and individual patients. Epigenetic changes 
like those seen in PHP1B (to come) play an etiologic role in a sub­
set of patients with PHP1A.490 Mutations in PRKAR1A, PDE3A, 
and PDE4D have also been found in subjects with acrodysostosis 
and clinical findings similar to those in patients with AHO with­
out GNAS abnormalities; however, most of these patients have no 
or only m

however, most of these patients have no 
or only mild PTH resistance.491
1241
Impaired cognition is seen in approximately half of the patients 
with PHP1A and appears to be related to the Gαs deficiency rather 
than to chronic hypocalcemia, since patients with other forms of 
hypocalcemia do not have cognitive impairment. The Gαs defi­
ciency in PHP1A may be associated not only with PTH resistance 
but also with resistance to other hormone

istance 
but also with resistance to other hormones such as TSH, gluca­
gon, and gonadotropins, resulting in thyroidal and gonadal dys­
function. Paradoxically, two unrelated males with both PHP1A 
and gonadotropin-independent precocious puberty have been 
described. The Gαs point mutation found in these individuals is 
thought to lead to a protein that is unstable at 37°C and therefore 
confers renal resistance to PTH. At the lower temperature of the 
testes, however, the protein is not degrade

f the 
testes, however, the protein is not degraded. In this setting, the 
stable but mutated protein is constitutively active and stimulates 
the Leydig cell in a manner similar to the skeletal effects of the 
Gs mutations in McCune-Albright syndrome (see Chapter 26).492
Individuals with PHP1A also exhibit a decrease in resting 
energy expenditure493 and a decrease in insulin sensitivity,494 
which may contribute to the development of diabetes.
Pseudo-pseudohypoparathyroidism (pseudo-PHP) is a 

Pseudo-pseudohypoparathyroidism (pseudo-PHP) is a term 
used to refer to individuals with the phenotype of AHO but 
with normal biochemical parameters. Patients with pseudo-PHP 
often are found in the same kindreds as those with PHP1A, 
but never in the same sibship, and they invariably inherit the 
same GNAS mutations involving the Gαs-encoding exons found 
in their PTH-resistant relatives.495 When patients inherit the 
GNAS mutation from their father, they exhibit pseudo-pseudo­
hypoparathyroi

father, they exhibit pseudo-pseudo­
hypoparathyroidism; when they inherit the mutant allele from 
their mother, they exhibit PHP1A.496,497 This pattern, in which 
the phenotype depends on the parent of origin, is termed genetic 
imprinting; mice with targeted ablation of Gnas also display 
such imprinting.498 In addition to the AHO phenotype, inheri­
tance of paternal mutations involving GNAS exons 1 to 13 leads 
to severe intrauterine growth retardation suggesting a role for 
an mRNA transcribe

ardation suggesting a role for 
an mRNA transcribed from the paternal GNAS locus in fetal 
growth and development.499
• Fig. 29.31  Daughter (left) and mother (right) with pseudohypoparathyroidism and Albright hereditary 
osteodystrophy.
• Fig. 29.32  Radiograph of the hand from a patient with pseudohypopara­
thyroidism and Albright hereditary osteodystrophy. Note the shortened 
fourth metacarpal.
1242
The observation of a phenotype in a heterozygous loss of fun

ation of a phenotype in a heterozygous loss of function 
mutation affecting Gαs is in contrast to the findings in mice with 
targeted deletions of the other Gα genes (Gαi2, Gαo, Gαq, Gα11, 
Gα13), in which a phenotype is observed only in the homozygous 
state.500 The fact that the GNAS gene is imprinted in certain tissues 
has partly resolved the dilemma of this dominant phenotype. Nota­
bly, mice with targeted ablation of one GNAS exon 1 fail to express 
Gαs mRNA in the renal cortex when the mu

 express 
Gαs mRNA in the renal cortex when the mutant gene is inherited 
from the mother but have normal expression in the cortex when the 
mutant gene is inherited from the father. No such imprinting pat­
tern is seen in the inner medulla; this correlates with the mice (and 
patients) exhibiting PTH, but not vasopressin, resistance.498
Pseudohypoparathyroidism type IB (PHP1B) presents with 
hypocalcemia, high PTH levels, and failure of PTH infusions to 
increase urinary cAMP production. Howeve

sions to 
increase urinary cAMP production. However, in most patients it 
is neither accompanied by any of the clinical features of AHO nor 
associated with abnormal Gαs levels in fibroblasts. Although mild 
TSH resistance is frequent,501–503 renal resistance to PTH is the 
most prominent feature of PHP1B; therefore several investigators 
had postulated that this syndrome was due to an isolated abnormal­
ity of the PTH receptor. However, a search for mutations in the 
coding exons of the recepto

 for mutations in the 
coding exons of the receptor gene failed to reveal a functional recep­
tor abnormality in those affected.449 Subsequently, a family with a 
homozygous mutation in the PTH receptor coding sequence was 
identified, with some manifestations of PTH resistance and cogni­
tive difficulties, but without functional testing of the mutant recep­
tor.504 Further studies are needed to clarify the role of the receptor 
mutation in this family’s presentation. The target organ manifesta­

family’s presentation. The target organ manifesta­
tions of PHP1B are variable, with some affected individuals having 
manifestations of PTH overactivity in bone and PTH resistance 
in the proximal tubule of the kidney. Cultured osteoblast-like cells 
from a patient with this disorder demonstrated normal cAMP 
responsiveness to PTH, despite the lack of renal responsiveness.505
The locus responsible for the familial, autosomal dominant 
forms of PHP1B resides on chromosome 20q13.3,506 the same 
r

HP1B resides on chromosome 20q13.3,506 the same 
region that contains the GNAS gene encoding Gαs. The methyla­
tion defect found most consistently in these PHP1B patients is 
a loss of methylation at exon A/B, which leads to decreased Gαs 
expression in the proximal renal tubule leading to the hormone 
resistance observed.507 While the familial forms (15–20%) exhibit 
autosomal dominant inheritance,508,509 the disease is otherwise 
inherited with the imprinting characteristic of PHP1A. Autoso­
m

 the imprinting characteristic of PHP1A. Autoso­
mal dominant PHP1B is caused in most cases by heterozygous 
maternal deletions within STX16 or GNAS, which are associated 
with loss-of-methylation (LOM) at exon A/B either alone or loss 
of methylation at three maternal GNAS promoters, respectively. 
The disease-causing mutations for the sporadic form of PHP1B 
remains unknown (except for a few cases with paternal uniparen­
tal disomy of 20q501), but it is the most frequent form of the dis­
ease 

but it is the most frequent form of the dis­
ease and is also characterized by imprinting abnormalities of the 
GNAS locus.502,507,510 The GNAS locus gives rise to multiple tran­
scripts (Fig. 29.33), including Gαs, which is biallelically expressed 
in most tissues, but only the maternal transcript is expressed in 
the renal proximal tubules, thyroid, gonads, pituitary, and pos­
sibly a few other tissues. In contrast, the XLαs, A/B, and antisense 
(A/S) transcripts are paternally expressed, wher

(A/S) transcripts are paternally expressed, whereas the NESP55 
transcript is maternally expressed. Linkage analyses in multiple 
kindreds had pointed to deletion of a 3-kb region, approximately 
  Types of Pseudohypoparathyroidism
Disorder
Urinary cAMP 
Response to PTH
Urinary PO4 
Other Hormonal 
Resistance
AHO
Pseudohypoparathyroidism IA
Decreased
Decreased
Yes
Yes
Gαs mutation or imprinting 
Pseudo-pseudohypoparathyroidism
Normal
Normal
No
Yes
G

do-pseudohypoparathyroidism
Normal
Normal
No
Yes
Gαs mutation
Pseudohypoparathyroidism IB
Decreased
Decreased
Rare
No
GNAS1 locus imprinting 
Pseudohypoparathyroidism IC
Decreased
Decreased
Yes
Yes
Gαs function normal, 
imprinting abnormality
Pseudohypoparathyroidism II
Normal
Decreased
No
Rare
Vitamin D deficiency or 
myotonic dystrophy
AHO, Albright hereditary osteodystrophy; cAMP, cyclic adenosine monophosphate; GαS, G protein alpha subunit S; GNAS1, guanine nucleotide-binding pro

a subunit S; GNAS1, guanine nucleotide-binding protein, alpha-stimulating activity polypeptide 1; 
PO4, phosphate; PTH, parathyroid hormone.
   
TABLE 29.5
P
M
STX16
NESP
A/S
XL
A/B
Gsα
• Fig. 29.33  The GNAS locus. A schematic representation of the GNAS locus is shown, with the black 
boxes indicating exons for STX16, NESP55 (NESP), the antisense (A/S) NESP55 transcript, XLαs (XL), 
A/B and Gαs (Gsα). The start site of and direction (sense vs antisense) of transcription is indicated by 
the arr

tisense) of transcription is indicated by 
the arrows. Genes that are maternally transcribed (M) are indicated by arrows above the relevant genes, 
whereas those that are paternally transcribed (P) are indicated below. The expression of XLαs, A/B, and 
A/S is from the paternal allele, whereas the maternal NESP55 transcript is expressed. Deletions from exon 
4 to 6 or, less commonly, from exon 2 to 4, along with associated introns, in the region containing the 
STX16 gene are associated with impr

ontaining the 
STX16 gene are associated with imprinting abnormalities in GNAS. The arrowhead below the Gsα locus 
indicates that only the maternal allele is expressed in the renal tubules.
1243
220 kb centromeric of GNAS, in most families, in addition to 
an overlapping deletion in another family.511,512 These deletions 
remove several STX16 exons, which encode syntaxin-16, a protein 
that plays a role in intracellular trafficking, as wel

 plays a role in intracellular trafficking, as well as associated 
introns. Although the development of PHP1B correlates with 
maternal inheritance of this gene defect and loss of methylation 
at the distant GNAS exon A/B, STX16 itself is not imprinted.512 
Other deletions in this region, including that of DNA encoding 
NESP55, a chromogranin-like neurosecretory peptide, and of two 
noncoding GNAS antisense exons have also been found in autoso­
mal dominant PHP1B.512–514 Maternal inheritance of 

al dominant PHP1B.512–514 Maternal inheritance of these dele­
tions causes abnormal imprinting in this locus.
While most sporadic cases of PHP1B are not associated with 
STX16 or NESP55/AS deletions, they are uniformly associated 
with impaired methylation of GNAS exon A/B. In addition to 
the methylation defects in the A/B region, gain of methylation at 
NESP55 and loss of methylation of XL and of the promoter of 
the A/S transcript are found. Some sporadic PHP1B cases share 
the same maternal 

ome sporadic PHP1B cases share 
the same maternal GNAS haplotype with an unaffected sibling 
and an unaffected daughter, thus excluding the GNAS region as 
the disease-causing locus.515 It is likely that an additional genetic 
locus underlies the methylation abnormalities in both sporadic 
and some familial PHP1B cases. Whether a common locus will 
be identified as the genetic basis for the majority of these PHP1B 
cases remains to be determined.
Several patients with AHO and PTH resistance have

Several patients with AHO and PTH resistance have been 
found to have normal Gαs activity; this subgroup has been desig­
nated PHP1C. However, these patients have mutations involving 
the carboxy-terminal region of Gαs that lead to normal levels of 
Gαs activity when assayed in erythrocytes but defective activation 
by receptors.516 Additional analyses of methylation patterns of A/B, 
XL, A/S, and NESP55 in 26 patients with normal Gαs activity dem­
onstrated abnormal methylation patterns in at 

em­
onstrated abnormal methylation patterns in at least one of these dif­
ferentially methylated GNAS regions in six individuals, raising the 
possibility that these individuals represent sporadic PHP1B cases.517
In PHP2, PTH infusions increase urinary cAMP normally; how­
ever, PTH does not elicit a phosphaturic response.518 This syndrome, 
like PHP1B, lacks signs of AHO, and no familial cases have been 
described. The age of onset of patients with this disorder is variable, 
ranging from infanc

th this disorder is variable, 
ranging from infancy to senescence, suggesting that it is an acquired 
defect or that the biochemical phenotype may be unmasked by 
intercurrent abnormalities. A subset of patients with myotonic dys­
trophy display the biochemical features of PHP2, the degree of PTH 
resistance correlating with the degree of expansion of the pathogenic 
CTG repeats in the myotonin protein kinase gene. A similar bio­
chemical phenotype can also be observed in vitamin D deficiency.51

pe can also be observed in vitamin D deficiency.519 
Minagawa and colleagues reported cases of three neonates with no 
signs of rickets and with normal levels of vitamin D who presented 
with transient PHP2 that resolved at about 6 months of age.520 They 
postulated that PTH responsiveness is subject to maturation dur­
ing fetal and neonatal development. Several unrelated patients with 
acrodysostosis, displaying a skeletal phenotype similar to AHO and 
a biochemical phenotype consistent with PH

HO and 
a biochemical phenotype consistent with PHP2, were found to have 
heterozygous mutations in PRKAR1A, the gene encoding the regu­
latory subunit of PKA. Mutation in this gene impairs the protein 
kinase A response to cAMP stimulation, consistent with the nor­
mal urinary cAMP response to PTH observed in PHP2.521 PHP2 
therefore seems to reflect a heterogeneous clinical disorder associated 
with defects in PTH responsiveness distal to cAMP or involving a 
separate signal transduction pathw

or involving a 
separate signal transduction pathway.522
Unlike the proximal tubules, which show resistance to PTH 
in PHP1A and PHP1B, bone-forming osteoblasts isolated from 
patients with PHP1A and PHP1B505 show a normal cAMP 
response to PTH. This suggests that the hypocalcemia in PHP 
is not secondary to skeletal resistance but is a consequence 
of the renal resistance to PTH and the associated low levels of 
1,25(OH)2D. Thus hypocalcemia occurs in the face of relatively 
normal bone and ren

urs in the face of relatively 
normal bone and renal distal (hypocalciuric) tubular responses to 
PTH.523 The lack of activation of vitamin D results in diminished 
intestinal calcium absorption and osteomalacia, both of which 
further exacerbate the hypocalcemia. Deficiency of 1,25(OH)2D 
and the resultant hypocalcemia can, in turn, impair the phospha­
turic but not the urinary cAMP responses to PTH524; therefore 
it is imperative that studies to confirm the diagnosis of PHP2 be 
performed in n

o confirm the diagnosis of PHP2 be 
performed in normocalcemic patients who have normal vitamin 
D status. Recent investigations have demonstrated that, despite 
the presence of secondary hyperparathyroidism, region-specific 
bone density is not reduced in PHP1A, and total body bone min­
eral density is greater than normal.525 Patients with PHP1B have 
been reported to have skeletal changes consistent with hyperpara­
thyroid bone disease that resolve with treatment.526,527 Addition­
ally, two br

olve with treatment.526,527 Addition­
ally, two brothers with PHP1B due a STX16 deletion were found 
to have osteosclerosis with lumbar Z-scores of +5.4 and +4.9 asso­
ciated with increased cortical width, and bone formation rate on 
trabecular and endocortical surfaces on iliac crest biopsy.528 
Vitamin D–Related Disorders
Hypocalcemia secondary to vitamin D deficiency or resistance to 
the biologic effects of 1,25(OH)2D3 is easily differentiated from 
the hypocalcemia of hypoparathyroidism by 

d from 
the hypocalcemia of hypoparathyroidism by routine clinical and 
laboratory evaluation. The primary cause of hypocalcemia in vita­
min D deficiency is decreased intestinal absorption of calcium. In 
the setting of normal renal function, the hypocalcemia of vitamin 
D deficiency, unlike that of hypoparathyroidism, is accompanied 
by hypophosphatemia and increased renal phosphate clearance. 
This increase in phosphate clearance is a direct result of compensa­
tory (secondary) hyperparathyro

esult of compensa­
tory (secondary) hyperparathyroidism. The hyperparathyroidism 
is a consequence of the hypocalcemic stimulus to PTH secretion, 
as well as the stimulation of PTH gene expression and parathy­
roid cell proliferation caused by hypocalcemia (see “Parathyroid 
Hormone Biosynthesis”). Therefore measurement of serum phos­
phate and PTH are very useful in distinguishing these disorders 
from hypoparathyroidism. The secondary hyperparathyroid­
ism results in increased calcium mobiliza

thyroid­
ism results in increased calcium mobilization from the skeleton, 
increased renal reabsorption of calcium, and increased renal 
1α-hydroxylation of 25(OH)D. In severe vitamin D deficiency, 
the increased levels of PTH no longer lead to increased bone 
resorption, perhaps because osteoclasts do not resorb unmineral­
ized osteoid.
In profound vitamin D deficiency the level of 1,25(OH)2D3 is 
usually low, but in moderate vitamin D deficiency the stimulation 
of the renal 1α-hydroxylase by 

y the stimulation 
of the renal 1α-hydroxylase by PTH can result in a normal or even 
elevated 1,25(OH)2D3 level. These high levels of 1,25(OH)2D3 
reflect the action of PTH on the renal 1α-hydroxylase. The inef­
fectiveness of the high levels of total 1,25(OH)2D3 to normal­
ize serum calcium may be explained by increased binding of 
this metabolite to vitamin D binding protein when the levels of 
25(OH)D are very low or may be a consequence of impaired local 
activation of 25(OH)D.
Vitamin D De

mpaired local 
Vitamin D Deficiency
Because the two sources of vitamin D are the diet and cutaneous 
synthesis after ultraviolet (UV) irradiation, lack of solar irradiation 
1244
and decreased intake or impaired absorption of vitamin D can lead 
to vitamin D deficiency. As the population has become increas­
ingly educated about the risks of skin cancer from solar irradia­
tion, the avoidance of long periods of intense sun exposure and the 

e of long periods of intense sun exposure and the 
use of high solar protective factor (SPF) sunblocks have resulted 
in increased reliance on dietary sources of vitamin D. Based on 
the Institute of Medicine (USA) 2010 report, the recommended 
daily allowance for vitamin D is 600 IU from 1 to 70 years of age 
and 800 IU for those over 70. However, increased supplementa­
tion may be required to maintain vitamin D sufficiency during 
pregnancy,529 in those with underlying medical conditions, as w

 in those with underlying medical conditions, as well 
as the elderly and the obese.530,531 Genetic variants in vitamin D 
metabolizing enzymes, including CYP2R1 and CYP24A1, in the 
VDR and in vitamin D–binding protein also impact the vitamin 
D intake required to achieve normal levels.532,533 Vitamin D is 
present in many food sources, both vegetable and animal. In addi­
tion, many prepared foods, especially cereal products, are fortified 
with vitamin D. Although dairy products have been fort

 vitamin D. Although dairy products have been fortified with 
vitamin D as well, the actual amount of vitamin D provided does 
not correlate well with the purported content.183 The vitamin D 
derived from vegetable sources is vitamin D2 and that from animal 
sources is vitamin D3. These two forms of vitamin D are metabo­
lized identically and are used to fortify foods.
The Institute of Medicine has defined vitamin D “sufficiency” 
as a level greater than 50 nmol/L (>20 ng/mL); however, higher 
l

ter than 50 nmol/L (>20 ng/mL); however, higher 
levels may be required to optimize intestinal calcium absorption 
in those with disorders such as intestinal disease, short bowel 
syndrome, intestinal bypass, and obesity as well as in the elderly. 
Although elderly, homebound individuals are at high risk, several 
studies have demonstrated that vitamin D deficiency is prevalent 
in the general population (as reviewed in Thomas and Demay534). 
The clinical relevance of this vitamin D deficiency h

 clinical relevance of this vitamin D deficiency has been con­
firmed by a study demonstrating that vitamin D administration 
(800 IU/day) to an ambulatory elderly population decreases serum 
PTH levels as well as the incidence of hip fracture.535 Similarly, 
a meta-analysis of randomized clinical trials performed between 
2011 and 2015 supports the use of calcium and vitamin D for the 
prevention of fractures.536 Malabsorption also remains an impor­
tant cause of vitamin D deficiency in all age

por­
tant cause of vitamin D deficiency in all age groups. Because vita­
min D is a fat-soluble vitamin, its absorption is dependent upon 
emulsification by bile acids. Any cause of fat malabsorption or 
short bowel syndrome (including bypass surgery for morbid obe­
sity) can result in vitamin D deficiency, therefore malabsorption 
should be ruled out in patients with low 25(OH)D levels. 
Accelerated Loss or Inactivation of Vitamin D
25(OH)D and 1,25(OH)2D3 are secreted with bile salts and 
unde

1,25(OH)2D3 are secreted with bile salts and 
undergo enterohepatic circulation, therefore intestinal disease 
may also result in vitamin D deficiency due to excessive losses. 
Increased metabolism of vitamin D, leading to low blood levels of 
25(OH)D, is seen in individuals given anticonvulsant medications 
and antituberculous therapy. Phenobarbital, primidone, phenyt­
oin,537 rifampin, and glutethimide538 have all been reported to 
accelerate the hepatic inactivation of vitamin D.
Two unrelate

he hepatic inactivation of vitamin D.
Two unrelated subjects with early-onset rickets and impaired 
responsiveness to vitamin D metabolites were found to have a 
mutation in CYP3A4. This mutation dramatically increased the 
ability of CYP3A4 to inactivate vitamin D metabolites.539 
Impaired 25-Hydroxylation of Vitamin D
The vitamin D that is absorbed undergoes 25-hydroxylation in 
the liver, therefore severe hepatic parenchymal damage can result 
in 25(OH)D deficiency. Clinically, severe vitamin

in 25(OH)D deficiency. Clinically, severe vitamin D deficiency as 
a consequence of liver disease is rare, since the degree of hepatic 
destruction necessary to impair 25-hydroxylation is incompatible 
with long-term survival. However, isoniazid has been shown to 
decrease the 25-hydroxylation of vitamin D.540
Studies in individuals with mutations in CYP2R1 have iden­
tified this gene as the principal 25-hydroxylase in humans. The 
clinical and biochemical presentations and therapeutic response

biochemical presentations and therapeutic responses 
support an inherited 25-hydroxylation defect. 
Impaired 1α-Hydroxylation of 25-Hydroxyvitamin D
The final step in the activation of vitamin D is the hydroxylation 
of 25(OH)D by the renal 1α-hydroxylase to yield 1,25(OH)2D3. 
Renal parenchymal damage therefore can result in deficiency of 
the active metabolite of vitamin D. Impaired 1α-hydroxylation 
is observed once creatinine clearance decreases to approximately 
30 to 40 mL per minute. Unli

ses to approximately 
30 to 40 mL per minute. Unlike with liver failure, with renal 
failure, dialysis permits long-term survival, therefore deficiency 
of 1,25(OH)2D3 as a result of impaired renal 1α-hydroxylation 
is a common and important clinical entity. The metabolic con­
sequences of chronic renal failure on the parathyroid glands 
and the skeleton are complex. Impaired renal 1α-hydroxylation 
leads to decreased intestinal absorption of calcium, resulting in 
hypocalcemia. The diminished p

cium, resulting in 
hypocalcemia. The diminished phosphate clearance associated 
with renal failure leads to elevated levels of blood phosphate 
and consequently increases in circulating FGF23; this, in turn, 
further lowers levels of 1,25(OH)2D3 and calcium. The resul­
tant secondary hyperparathyroidism increases release of calcium 
and phosphate from bone; however, because of the renal insuf­
ficiency, PTH does not have a phosphaturic effect. As a result, 
serum phosphate rises further. Oral p

s a result, 
serum phosphate rises further. Oral phosphate binders are used 
to lower blood phosphate, including calcium-containing antac­
ids and the phosphate-binding exchange resin, sevelamer. Cal­
cium administration also attenuates the hypocalcemic stimulus 
to parathyroid secretion. 1,25(OH)2D3 therapy is critical for 
the absorption of this calcium and should be administered 
early in the course of renal failure (when the creatinine clear­
ance falls below 30–40 mL/minute) to avoid the de

ance falls below 30–40 mL/minute) to avoid the development 
of secondary hyperparathyroidism, with careful monitoring to 
avoid hypercalcemia. Once secondary hyperparathyroidism has 
developed, pharmacologic doses of 1,25(OH)2D3, delivered 
intravenously or orally, or calcimimetics541 may be required 
to suppress PTH gene transcription and parathyroid cellular 
Decreased levels of 1,25(OH)2D3 may also be observed in 
patients taking ketoconazole and in ADHR, X-linked hypophos­
ph

ng ketoconazole and in ADHR, X-linked hypophos­
phatemia, and TIO, diseases associated with high FGF23 levels 
(see Chapter 31).190
A rare heritable defect of vitamin D activation has been 
described in several kindreds. Biochemically, pseudo-vitamin 
D deficiency rickets (PDDR) is characterized by hypocalce­
mia and secondary hyperparathyroidism. The only meta­
bolic abnormalities that differentiate it from dietary vitamin 
D deficiency are the presence of normal or elevated levels 
of vitamin 

presence of normal or elevated levels 
of vitamin D and 25(OH)D accompanied by low levels of 
1,25(OH)2D3.542,543 The disease is inherited in an autosomal 
recessive fashion and presents in infancy with rickets, osteo­
malacia, and seizures.544 Cloning of the 1α-hydroxylase gene 
confirmed that mutation of this gene is the molecular basis for 
the disorder and, as expected, physiologic replacement doses 
of 1α-hydroxylated metabolites of vitamin D result in clinical 
remission.545 
1245
Chapter 

D result in clinical 
1245
Target Organ Resistance to 1,25(OH)2D3
A second rare inherited disorder, characterized by resistance to the 
biologic actions of 1,25(OH)2D3, has been described in several 
kindreds. This disorder, referred to as hereditary vitamin D–resis­
tant rickets (HVDRR) is also characterized by autosomal reces­
sive inheritance. Its biochemical presentation, with hypocalcemia, 
hypophosphatemia, and seconda

 with hypocalcemia, 
hypophosphatemia, and secondary hyperparathyroidism, resem­
bles that of vitamin D deficiency, but it is accompanied by ele­
vated levels of 1,25(OH)2D3. The molecular basis for this disease 
is mutation of the vitamin D receptor gene, resulting in impaired 
target organ responsiveness. Most of the mutations that have 
been described involve the DNA-binding domain of the receptor. 
These mutations result in a decreased affinity of the receptor for its 
response elements on t

ty of the receptor for its 
response elements on target genes leading to impaired regulation 
of these genes. Mutations in the hormone-binding and nuclear 
receptor coactivator–binding domains of the receptor have also 
been described in kindreds with HVDRR.546
The clinical presentation of HVDRR is variable; however, most 
patients present in infancy with rickets, hypophosphatemia, and 
seizures, although presentation in late adolescence has also been 
described. Alopecia totalis, developing in 

 been 
described. Alopecia totalis, developing in the first 2 years of life, 
is present in some kindreds. The finding of alopecia in mice with 
VDR mutations35 confirms that alopecia is due to disruption of 
the VDR gene.
Because of target organ resistance to the active metabolite 
of vitamin D, there is no ideal treatment for HVDRR. Phar­
macologic doses of vitamin D—25(OH)D, 24,25(OH)D, and 
1,25(OH)2D3—have been administered in an attempt to over­
come this target organ resistance547 with va

over­
come this target organ resistance547 with variable effects. In those 
patients in whom the hypocalcemia and osteomalacia are resis­
tant to such therapeutic interventions, parenteral calcium infu­
sions have been used to heal osteomalacic lesions.209 Studies in 
VDR-ablated mice have demonstrated that maintenance of nor­
mal mineral ion homeostasis prevents all the complications of 
VDR ablation except alopecia.208,210 Based on these observations, 
patients with VDR mutations should be tre

ations, 
patients with VDR mutations should be treated early and aggres­
sively to prevent skeletal abnormalities and parathyroid hyperpla­
sia. Lifelong therapy is usually required, although spontaneous 
remissions off therapy have been described. The pathophysiology 
of the spontaneous remissions is not well understood since the 
underlying genetic defect still exists. It is likely that these so-called 
remissions reflect compensated calcium homeostasis once the 
needs of the growing skeleton 

meostasis once the 
needs of the growing skeleton are met. In support of this hypoth­
esis is a report of a relapse in a pregnant woman, followed by a 
remission postpartum.548
Phenytoin causes target organ resistance to the biologic 
effects of 1,25(OH)2D3, in addition to its acceleration of the 
hepatic catabolism of vitamin D metabolites. Phenytoin has been 
shown to impair intestinal calcium absorption in vivo in rats549 
and impair PTH and 1,25(OH)2D3-mediated bone resorption 
in  vitro. Co

,25(OH)2D3-mediated bone resorption 
in  vitro. Combination chemotherapy with 5-fluorouracil and 
low-dose leucovorin has been reported to cause hypocalcemia 
in 65% of patients, associated with an acute decrease in plasma 
1,25(OH)2D3 levels.550 
Other Causes of Hypocalcemia
Excessive Deposition Into the Skeleton
Excessive deposition of calcium into the skeleton can occur in 
association with osteoblastic metastases, with chondrosarcomas,551 
or in the hungry bone syndrome. This syndrome presen

 in the hungry bone syndrome. This syndrome presents as pro­
longed hypocalcemia, hypocalciuria, and hypophosphatemia post 
parathyroidectomy for primary hyperparathyroidism (see “Primary 
Hyperparathyroidism”). The hypocalcemia is a consequence of rem­
ineralization of a skeleton that has been subjected to the bone resorb­
ing effects of PTH over a prolonged period. Hungry bone syndrome 
can also be observed after treatment of other diseases that are associ­
ated with excessive bone resorption.

t are associ­
ated with excessive bone resorption. It has been described following 
radioactive iodine treatment of a patient with Graves disease.552 
Impaired Resorption
Hypocalcemia can be observed as a consequence of administration 
of drugs that impair osteoclast-mediated bone resorption, includ­
ing bisphosphonates and denosumab. Vitamin D deficiency is 
thought to be a significant risk factor for hypocalcemia associated 
with antiresorptive therapy. 
Chelation
Decreases in ionized calcium 

 therapy. 
Chelation
Decreases in ionized calcium have been reported with foscarnet, a 
pyrophosphate analog that is used as an antiviral agent,553 perhaps 
because of complex formation between ionized calcium and the 
drug.
Hyperphosphatemia, due to phosphate administration or rapid 
destruction of soft tissue (i.e., rhabdomyolysis, chemotherapy of 
hematologic malignancies), may produce profound hypocalcemia 
by directly complexing and precipitating calcium in bone or soft 
tissues, by inhibit

ating calcium in bone or soft 
tissues, by inhibiting bone resorption, and by blocking renal syn­
thesis of 1,25(OH)2D3 (see “Hyperphosphatemia”).
Massive infusions of citrated blood products may cause hypo­
calcemia, presumably because citrate complexes calcium in the 
recipient’s plasma.554 Large doses of ethylenediaminetetraacetic 
acid (EDTA)–containing radiographic contrast dyes have also 
been reported to cause hypocalcemia. Hypocalcemia, due to com­
plexes of calcium and fluoride, has bee

ue to com­
plexes of calcium and fluoride, has been reported with hydroflu­
oric acid burns555 or ingestion.556 
Neonatal Hypocalcemia
Neonatal hypocalcemia is seen in infants of hyperparathyroid 
mothers, infants of diabetic mothers, in premature infants, and in 
infants with birth asphyxia. The cause of hypocalcemia in infants 
of diabetic mothers is likely multifactorial. Prematurity per se does 
not account for the higher incidence.557 The response of prema­
ture infants and infants of diabe

sponse of prema­
ture infants and infants of diabetic mothers to exogenous PTH 
suggests that functional hypoparathyroidism may, in part, account 
for the increased hypocalcemia in these two populations.557,558 
The hypocalcemia in infants of hyperparathyroid mothers is pre­
sumably secondary to the maternal hypercalcemia that, in turn, 
suppresses fetal parathyroid function.559 Neonatal hypocalcemia 
is also seen in offspring of vitamin D–deficient mothers and in 
infantile malignant osteopetro

ent mothers and in 
infantile malignant osteopetrosis.560,561 
HIV
Hypocalcemia is sixfold more prevalent in HIV-infected patients 
than in the general population.562 Although hypocalcemia is often 
a consequence of antiretroviral and antibiotic/antimycotic ther­
apy, vitamin D deficiency and hypomagnesemia are also common 
in patients with AIDS. Impaired parathyroid responsiveness to 
hypocalcemia has also been documented (see Chapter 44). 
Critical Illness
Hypocalcemia is commonly seen in crit

ical Illness
Hypocalcemia is commonly seen in critically ill patients and is 
thought to be a reflection of parathyroid gland suppression, failure 
1246
to activate vitamin D, calcium chelation or sequestration, and/or 
hypomagnesemia. However, an increased basal level and secretory 
response of PTH to lowering of serum calcium has been observed 
in some septic and nonseptic intensive care unit (ICU) patients, 
emphasizing the multifactorial origin of the hypoca

mphasizing the multifactorial origin of the hypocalcemia.563 
There was a correlation between cytokine levels and hypocalce­
mia in this and other studies, suggesting that these inflamma­
tory agents may play a role in redistribution of calcium to the 
intracellular or other pools. Interleukin 1 (IL1) has been shown 
to increase the expression of the calcium-sensing receptor on para­
thyroid cells and lower PTH secretion and blood calcium in rats 
injected with the cytokine.564 Severe acute panc

injected with the cytokine.564 Severe acute pancreatitis is often 
associated with hypocalcemia and is a negative prognostic indica­
tor. The hypocalcemia occurs shortly after the onset of pancreati­
tis and is associated with an increase in PTH levels, suggesting 
that parathyroid function is normal. It has long been thought that 
this hypocalcemia is secondary to deposition of “calcium soaps” 
consisting of calcium and fatty acids. Supporting this hypothesis, 
studies in a patient with a pan

 this hypothesis, 
studies in a patient with a pancreatic fistula have demonstrated 
hypocalcemia (4.3 mg/dL) in the setting of high levels of calcium 
(26 mg/dL) and fatty acids in ascitic fluid.565 Subsequent stud­
ies in a rat model have supported this finding and demonstrated 
that oleate has a high binding capacity for calcium.566 However, 
other investigations in a porcine model of experimental pancre­
atitis have demonstrated that hypocalcemia does not occur if the 
animals are subjected 

emia does not occur if the 
animals are subjected to thyroidectomy prior to the induction of 
pancreatitis.567 This finding suggests a role for calcitonin in the 
development of hypocalcemia with acute pancreatitis, although 
several clinical studies have documented normal calcitonin levels 
in hypocalcemic individuals with pancreatitis.568 Severe hypocal­
cemia with hypercalcitoninemia and hypophosphatemia has been 
reported in the toxic shock syndrome sepsis and in critically ill 
patients.569

yndrome sepsis and in critically ill 
patients.569 As in acute pancreatitis, this hypocalcemia is usually 
accompanied by increases in serum levels of PTH, and the degree 
of hypocalcemia is a negative prognostic indicator. The mecha­
nism of hypocalcemia in these patients is likely to be heteroge­
neous and has not been clearly defined. 
Treatment of Hypocalcemia
Acute hypocalcemia is an emergency that requires prompt atten­
tion. If symptoms of neuromuscular irritability are present and 
carpo

 neuromuscular irritability are present and 
carpopedal spasm is elicited on physical examination, or if elec­
trocardiogram (EKG) changes are present, treatment with intra­
venous calcium is indicated until the signs and symptoms of 
hypocalcemia subside. Approximately 100 mg of elemental cal­
cium should be infused over a period of 10 to 20 minutes (Table 
29.6). If this is not sufficient to alleviate the clinical findings of 
hypocalcemia, an infusion of 100 mg per hour can be given to 
adult

infusion of 100 mg per hour can be given to 
adults for several hours with close monitoring of calcium levels. 
In hypocalcemia associated with hypomagnesemia, magnesium 
replacement also is required. Magnesium should be given intrave­
nously, 100 mEq over 24 hours in the acute setting. Because most 
of the parenteral magnesium is excreted in the urine, oral mag­
nesium oxide should be instituted as soon as possible to replete 
body stores. Special caution and reduced doses are necessary when 
a

al caution and reduced doses are necessary when 
administering magnesium to patients in renal failure (see “Mag­
nesium Disorders”).
The treatment of hypocalcemia should be directed at the 
underlying disorder. In all cases, replacement with exogenous cal­
cium (1–3 g elemental calcium daily, from food sources or oral 
supplements) should be instituted. Calcium carbonate is the least 
expensive formulation, but it requires acidification for efficient 
absorption. This becomes important in patien

ient 
absorption. This becomes important in patients with achlorhydria 
and those in whom gastric acid production is being suppressed 
with pharmacologic agents. Notable in this respect is the acid buff­
ering capacity of calcium carbonate. Because of this, it is recom­
mended that patients take their calcium carbonate supplements in 
divided doses of 1 g or less. In these cases, the calcium should be 
taken with food or citrus drinks to promote maximal absorption.
In cases of vitamin D deficien

maximal absorption.
In cases of vitamin D deficiency or resistance, the metabolite of 
vitamin D chosen depends on the underlying disorder. If impaired 
renal 1α-hydroxylation is present, such as in renal failure, hypo­
parathyroidism (or PTH resistance), or the vitamin D–dependent 
rickets syndromes, metabolites that do not require this modifica­
tion should be administered (calcitriol 0.25–1 μg/day or doxer­
calciferol 2.5–5 mg/day). If decreased intake or increased losses 
are the problem, vi

ed intake or increased losses 
are the problem, vitamin D should be administered and the treat­
ment directed at the underlying disorder. Initial repletion of stores 
can be undertaken with 50,000 IU of vitamin D daily for 1 to 
3 weeks, followed by a daily maintenance dose that significantly 
exceeds the RDA. In patients with resistance to vitamin D, such 
as those on phenytoin, 5000 IU daily or more may be required as 
maintenance therapy. The use of sublingual vitamin D has been 
shown to eff

use of sublingual vitamin D has been 
shown to effectively replete levels in a patient with malabsorp­
tion due to Crohn disease.570 In those without chronic conditions, 
once treatment of the underlying disorder and repletion of body 
stores have been addressed, 800 to 1200 IU should provide suf­
ficient maintenance therapy.
Patients should be monitored closely to assess both response 
to therapy and to prevent therapeutic complications. Serum cal­
cium should be monitored frequently (daily in 

al­
cium should be monitored frequently (daily in profound hypo­
calcemia, weekly in moderate hypocalcemia) for the first week to 
month of therapy. Concomitant with resolution of hypocalcemia, 
one should observe a decline in the serum PTH level as second­
ary hyperparathyroidism resolves. Measurement of serum PTH 
and assessment of 24-hour urinary calcium excretion should be 
performed within 2 to 4 weeks of institution of therapy. The uri­
nary calcium measurement reflects the effect of thera

y calcium measurement reflects the effect of therapy on the 
patient’s ability to absorb calcium and the net uptake of calcium 
by bone. A low urine calcium indicates poor adherence to a regi­
men, poor absorption of calcium, or increased uptake by bone. In 
addition, the urine calcium provides important information on 
which to base therapeutic modifications to avoid nephrolithiasis. 
Once normalization of serum and urinary calcium and a decrease 
in PTH levels are observed, a transition from a

e 
in PTH levels are observed, a transition from aggressive replace­
ment therapy to maintenance therapy should be undertaken to 
prevent hypercalcemia and nephrolithiasis. These same param­
eters should be monitored 1 and 3 months after a dose change to 
assess the effect of the therapeutic intervention. Monitoring of the 
alkaline phosphatase can also be performed at this time. Alkaline 
phosphatase levels may actually increase soon after starting treat­
ment because of healing of the osteomal

ing treat­
ment because of healing of the osteomalacic lesions; however, by 
3 to 4 months after institution of therapy, a clear downward trend 
should be observed. Alkaline phosphatase and PTH values may 
remain elevated for 6 to 12 months after institution of therapy; 
they should not be a cause for alarm provided they are declining 
and the other parameters suggest therapy is effective.
The treatment of hypoparathyroidism is similar to that of 
vitamin D deficiency with the exception that the

vitamin D deficiency with the exception that these patients have 
impaired renal 1α-hydroxylation of 25(OH)D and therefore 
require treatment with 1α-hydroxylated metabolites. PTH(1-84) 
has also been approved for therapy. This therapy controls hypo­
calcemia with lower urine calcium excretion than with calcium 
1247
and calcitriol therapy, but it is expensive and requires parenteral 
administration. Oral calcium and 1α-hydroxylated vita

inistration. Oral calcium and 1α-hydroxylated vitamin D 
metabolites therefore remain the mainstay of therapy. Monitoring 
of serum and urinary calcium should be performed as in the treat­
ment of vitamin D deficiency. Therapy in these patients is lifelong, 
therefore careful monitoring is required to avoid renal or hyper­
calcemic complications. The aim of therapy should be to maintain 
serum calcium in the low-normal range without causing frank 
hypercalciuria. Because PTH plays an important r

hypercalciuria. Because PTH plays an important role in renal 
calcium reabsorption, difficulties are often encountered attain­
ing these therapeutic goals. In such cases, renal calcium losses can 
be minimized by the addition of a thiazide diuretic to the treat­
ment regime. Replacement therapy with PTH(1-34) has also been 
shown to be effective in treatment of hypoparathyroidism.571
One of the frustrations often encountered in treating 
patients with hypoparathyroidism is the fluctuating resp

ts with hypoparathyroidism is the fluctuating response to 
a seemingly stable therapeutic regimen. Episodes of hypercal­
cemia are occasionally observed without any discernible cause. 
Because of this, serum calcium should be monitored every 3 
months to permit temporary withdrawal of 1,25(OH)2D3, 
should a hypercalcemic trend be observed. Fortunately the half-
life of this metabolite is short, so that discontinuation for a 
few days to a week with resumption of a lower dose is usually 
efficaci

th resumption of a lower dose is usually 
efficacious.
All patients receiving vitamin D metabolites and calcium need 
to be aware of potential therapeutic complications. Importantly, 
the mild symptoms of hypercalcemia should be emphasized to the 
patient. It is essential that these patients be aware that their cal­
cium should be monitored more frequently during intercurrent 
illnesses that may affect the absorption of calcium or their hydra­
tion status, to prevent the development of hypocalce

on status, to prevent the development of hypocalcemia or severe 
hypercalcemia. 
  Therapeutic Mineral Ion Preparations
Compound
MWa
MINERAL ION 
CONTENT
AVAILABLE FORMULATIONS
ORAL PREPARATIONS
PARENTERAL PREPARATIONS
Compound
MINERAL ION CONTENT
Compound
mg/g
mmol/g
mg/g
mmol/g
mg/g
mmol/g
Calcium
Ca carbonate
100
400
10.0
1250 mgb
500 mg
12.5 mmol
Ca phosphate
310
383
9.6
1565 mg
600 mg
15.0 mmol
Ca acetate
158
253
6.3
668 mgb
167 mg
4.2 mmol
Ca citrate
498
210
6.0
950 mg

 mgb
167 mg
4.2 mmol
Ca citrate
498
210
6.0
950 mgb
200 mg
5.0 mmol
Ca lactate
218
130
4.6
650 mgb
84 mg
2.1 mmol
Ca glubionate
64
1.7
5 mL
115 mg
2 mmol
Ca gluconate
430
93
2.3
1000 mgb
93 mg
2.3 mmol
10% soln
93 mg/10 mL
2.3 mmol/10 mL
Ca gluceptate
488
82
2
22% soln
90 mg/5 mL
Ca chloride
147
273
6.8
10% soln
273 mg/10 mL
11.2 mmol/mL
Magnesium
Mg oxide
40
603
24.8
400 mgb
241 mg
9.9 mmol
Mg gluconate
450
54
2.2
500 mg
27 mg
1.1 mmol
Mg chloride
203
120
4.9
535 mg
64 mg
2.6 mmo

 mmol
203
120
4.9
535 mg
64 mg
2.6 mmol
20% soln
24 mg/mL
1 mmol/mL
Mg sulfate
246
99
4.1
50% solnb
49 mg/mL
2 mmol/mL
Phosphorusc
Na/K phosphate 
(neutral)
Capsule
250 mg
8.1 mmol
K phosphate 
(neutral)
Capsule
250 mg
8.1 mmol
soln
94 mg/mL
3 mmol/mL
Na phosphate 
(neutral)
soln
94 mg/mL
3 mmol/mL
aMolecular weights (MW) shown are for the usual chemical form, including water molecules (e.g., MgSO4 • 7 H2O).
bOther formulations exist. Those shown are among those approved in the Unite

 Those shown are among those approved in the United States.
cPhosphate preparations contain buffered mixtures of monobasic (H2PO4−) and dibasic (HPO4−) ions; the phosphorus content therefore is specified in millimoles. Oral phosphates contain 7 mEq sodium 
and potassium per capsule (Na/K form) or 14 mEq potassium per capsule (K form). Parenteral solutions typically contain 4 mEq of sodium or potassium per milliliter.
MW, Molecular weight; soln, solution.
Data from Drug Facts and Comparisons. St.

olution.
Data from Drug Facts and Comparisons. St. Louis: Facts and Comparisons; 1995.
   
TABLE 29.6
1248
Disorders of Phosphate Metabolism
Hyperphosphatemia
Serum phosphate levels are controlled primarily by the rate of 
proximal renal tubular phosphate reabsorption, which is due, in 
turn, to the integrated activity of the major sodium-dependent 
cotransporters (NaPi-IIa and NaPi-IIc). The latter are strongly 
downregulated by parathyroid hormone and FGF23, b

downregulated by parathyroid hormone and FGF23, both of 
which are stimulated by phosphate. Thus absent extraordinary fil­
tered loads of phosphate, the capacity of normal kidneys to excrete 
phosphate is not easily exceeded. Consequently, the occurrence 
of hyperphosphatemia usually signifies impaired renal functon, 
hypoparathyroidism, defective FGF23 action, a huge flux of phos­
phate into the extracellular fluid, or some combination of these 
factors (Table 29.7).
The most common cause of h

The most common cause of hyperphosphatemia is acute or 
chronic renal failure in which GFR is so reduced that the usual 
daily load of phosphate cannot be excreted at a normal level of 
serum phosphate, despite maximal inhibition of phosphate reab­
sorption in the remaining functional nephrons. In hypoparathy­
roidism (or PHP), serum phosphate may rise to levels as high as 6 
to 8 mg/dL because of loss of the tonic inhibitory effect of PTH 
on phosphate reabsorption, alth

ory effect of PTH 
on phosphate reabsorption, although elevated FGF23 levels may 
prevent even further increases in serum phosphate.572 The hyper­
phosphatemia of hypoparathyroidism is only partly due to the 
absence of PTH per se. Hypocalcemia may further impair phos­
phate clearance in this setting, and correction of hypocalcemia 
by treatment with vitamin D metabolites and oral calcium may 
reduce serum phosphate, for example, even though PTH levels 
remain low.573
Other circumstances in whic

levels 
Other circumstances in which renal tubular phosphate excre­
tion is decreased, in the absence of renal failure, include acro­
megaly,574 chronic therapy with heparin, and familial tumoral 
calcinosis.575 Familial tumoral calcinosis can result from inactivat­
ing mutations in either FGF23 or the O-linked glycosyl transferase 
GalNAc-T3, which glycosylates FGF23 at its cleavage site tar­
geted by furin-like proteases, thereby suppressing this cleavage.576 
In the absence of 

 suppressing this cleavage.576 
In the absence of GalNAc-T3, FGF23 is cleaved at an acceler­
ated rate.577–579 The choice of FGF23 assay is important therefore 
for diagnosing tumoral calcinosis. The responsible FGF23 and 
GalNac-T3 mutations may render the molecule more susceptible 
to proteolyic degradation, such that the blood levels of (inactive) 
carboxyl fragments may be quite high in contrast to low levels of 
(bioactive) intact FGF23.578 Affected patients may display focal 
hyperostosis;

Affected patients may display focal 
hyperostosis; large, lobulated periarticular ectopic calcifications, 
especially around shoulders or hips; hyperphosphatemia due to 
increased renal tubular reabsorption of phosphate; increased serum 
1,25(OH)2D despite normal or low serum PTH; and increased 
intestinal calcium absorption, consistent with the elevated serum 
1,25(OH)2D concentration. The disorder may present in child­
hood or adulthood, is more common in African Americans, and is 
lifelong, w

e common in African Americans, and is 
lifelong, with a tendency for the tumoral calcifications to progress 
at affected sites. In contrast to the elevated serum 1,25(OH)2D, 
hyperphosphatemia is not a constant feature of tumoral calcino­
sis, although it tends to be most severe in those with prominent 
calcifications. Despite their chronic hyperphosphatemia, second­
ary hyperparathyroidism does not develop in these patients, pre­
sumably because of the high 1,25(OH)2D levels and intestinal 
hyp

 of the high 1,25(OH)2D levels and intestinal 
hyperabsorption of calcium. Treatment is problematic, although 
some success has been reported with phosphate-binding antacids, 
calcium deprivation, calcitonin, and acetazolamide therapy.580
Hyperphosphatemia may result from overly rapid adminis­
tration of therapeutic phosphate preparations or phosphate-rich 
drugs (fosphenytoin, liposomal amphotericin B), especially if renal 
function is compromised581 or from rapid shifts of phosphate out 
of ce

ed581 or from rapid shifts of phosphate out 
of cells, most often provoked by mechanical injury or metabolic 
insult. Most cases of hyperphosphatemia associated with intestinal 
phosphate loads have involved children who received phosphate-
containing laxatives or enemas or older adults with impaired renal 
function receiving phosphate-based cathartics in preparation for 
colonoscopy.582 Hyperphosphatemia due to cytolytic release of 
intracellular phosphate can be quite dramatic, with serum phos

r phosphate can be quite dramatic, with serum phos­
phate concentrations up to or exceeding 20 mg/dL. This disor­
der was described initially as a complication of rapid induction 
chemotherapy for certain hematologic malignancies (tumor lysis 
syndrome), although it also may occur from cellular injury associ­
ated with trauma, hyperthermia, overwhelming infection, hemo­
lysis, rhabdomyolysis, or metabolic acidosis.583 Rarely, apparent 
hyperphosphatemia may reflect measurement artifact caused by

hatemia may reflect measurement artifact caused by 
paraproteins in myeloma.584
Most often, hyperphosphatemia is mild and asymptomatic, 
although chronic hyperphosphatemia is an important factor in 
the development of secondary hyperparathyroidism in progressive 
renal failure. The clinical manifestations of acute, severe hyper­
phosphatemia are related mainly to those of the accompanying 
hypocalcemia, caused by formation of insoluble calcium phos­
phate precipitates. Thus tetany, muscle cramps

os­
phate precipitates. Thus tetany, muscle cramps, paresthesias, and 
seizures may occur, and they may be compounded by other meta­
bolic disturbances (hyperkalemia, acidosis, hyperuricemia) that 
frequently coexist. Generalized precipitation of calcium phos­
phate into soft tissues may produce organ dysfunction, notably 
renal failure.582
  Causes of Hyperphosphatemia
Impaired Renal Phosphate Excretion
Familial tumoral calcinosis
Endocrinopathies
Hypoparathyroid

inosis
Heparin
Increased Extracellular Phosphate
Rapid Administration of Phosphate (Intravenous, Oral, Rectal)
Phosphate salts
Fosphenytoin
Liposomal amphotericin B
Rapid Cellular Catabolism or Lysis
Catabolic states
Tissue injury
Crush injuries
Fulminant hepatitis
Cellular lysis
Hemolytic anemia
Rhabdomyolysis
Tumor lysis syndrome
Transcellular Shifts of Phosphate
Metabolic acidosis
Respiratory acidosis
TABLE 2

te
TABLE 29.7
1249
Therapeutic options for hyperphosphatemia are limited. Vol­
ume expansion may be helpful to improve GFR in acute syn­
dromes. Identification and removal of any exogenous sources 
of phosphate are important, and phosphate-binding aluminum 
hydroxide antacids may be useful in limiting intestinal phosphate 
absorption and chelating phosphate secreted into the intestine. 
Hemodialysis 

sphate secreted into the intestine. 
Hemodialysis is the most effective approach and should be con­
sidered early in severe hyperphosphatemia, especially in the tumor 
lysis syndrome and particularly if symptomatic hypocalcemia can­
not be adequately treated for fear of inducing widespread soft tis­
sue calcification. 
Hypophosphatemia
Etiology
Hypophosphatemia may result from one or more of three gen­
eral mechanisms (Table 29.8): increased urinary losses due 
to decreased net renal tubular pho

ary losses due 
to decreased net renal tubular phosphate reabsorption; rapid 
shifts of phosphate from extracellular fluid into the intracel­
lular space or the mineral phase of bone; or rarely, severe and 
selective deprivation of dietary phosphate, as may occur with 
chronic ingestion of large amounts of nonabsorbable alumi­
num-containing antacids. Fasting or starvation does not lead 
directly to hypophosphatemia, apparently because phosphate 
is mobilized from catabolized bone and soft tissu

is mobilized from catabolized bone and soft tissue in amounts 
sufficient to maintain serum phosphate, even during prolonged 
caloric deprivation.585 Starvation does induce phosphate defi­
ciency and therefore predisposes to subsequent hypophospha­
temia upon refeeding.586
Chronic hypophosphatemia usually can be traced to ongoing 
renal phosphate wasting. Elevation of serum PTH for any rea­
son (other than renal failure), as in primary hyperparathyroidism 
or secondary hyperparathyroidism due t

or secondary hyperparathyroidism due to vitamin D or calcium 
deficiency, results in inhibition of tubular phosphate reabsorp­
tion and fasting hypophosphatemia. Phosphate clearance also 
is increased in PTHrP-associated hypercalcemia of malignancy, 
although when such patients develop severe hypercalcemia, 
hypophosphatemia may be masked initially by underlying vol­
ume depletion and compromised GFR. Therapy with the tyro­
sine kinase inhibitors imatinib and nilotinib appears to cau

e inhibitors imatinib and nilotinib appears to cause 
hypophosphatemia, at least in part by inhibiting both osteoblast 
and osteoclast formation, lowering serum calcium, and stimulat­
ing secondary hyperparathyroidism.587–589 When PTH secretion 
is compromised by severe hypomagnesemia, rapid intravenous 
administration of magnesium alone, without concurrent atten­
tion to coexisting hypocalcemia, can provoke massive phos­
phaturia and hypophosphatemia in patients with underlying 
phosphate deple

temia in patients with underlying 
phosphate depletion.
The discovery that gain-of-function mutations in FGF23 
cause autosomal dominant hypophosphatemic rickets inau­
gurated a new era in understanding of phosphate homeosta­
sis.590–592 Elevated FGF23 also occurs in autosomal recessive 
hypophosphatemia (ARHP), which is caused by mutations in the 
genes encoding dentin matrix protein-1 (DMP1) and FAM20C. 
DMP1 is expressed in osteocytes and presumably regulates local 
production of FGF23.593 FA

umably regulates local 
production of FGF23.593 FAM20C is a secreted kinase that phos­
phorylates FGF23 near the furin cleavage site; this phosphoryla­
tion is required for inactivating cleavage by a furin-like protease. 
Immunoassays for FGF23594 have pointed to elevated circulating 
FGF23 as at least one “phosphatonin” responsible for reduction 
of phosphate reabsorption and serum 1,25(OH)2D levels in the 
more common disorder, X-linked hypophosphatemic rickets 
(XLH), in the rare but distinct

sphatemic rickets 
(XLH), in the rare but distinctive TIO and epidermal nevus syn­
dromes, and in the approximately 50% of patients with McCune-
Albright syndrome (fibrous dysplasia of bone) who manifest 
hypophosphatemia.590–592,595–597 These disorders share a com­
mon biochemical phenotype, which may include a more general­
ized proximal tubular dysfunction, with modest proteinuria and 
aminoaciduria. Serum calcium usually is normal or low-normal, 
urinary calcium often is low, PTH is normal o

al, 
urinary calcium often is low, PTH is normal or only slightly ele­
vated, and 1,25(OH)2D is inappropriately normal. The clinical 
picture is dominated by weakness, bone pain, and other features 
attributable to the associated rickets or osteomalacia (see Chap­
ter 31). Increased FGF23 also may play a role in impaired phos­
phate reabsorption seen in the 20% or so of patients with calcium 
kidney stones and idiopathic hypercalciuria who exhibit fasting 
hypophosphatemia,598 although a few suc

t fasting 
hypophosphatemia,598 although a few such patients may harbor 
mutations in the NaPi-IIa sodium phosphate cotransporter.599
Renal phosphate clearance may be impaired in the context 
of a more generalized renal tubular disorder such as Fanconi 
syndrome(s) or others associated with systemic diseases such as 
amyloidosis, Wilson disease, or cystinosis (see Table 29.8). In 
addition to NaPi-IIa mutations, inactivating mutations in the 
NaPi-IIc cotransporter, also expressed in the proxima

i-IIc cotransporter, also expressed in the proximal tubule 
and known to be regulated by both PTH and FGF23, have 
been shown to account for the rare disorder known as hereditary 
hypophosphatemic rickets with hypercalciuria, in which primary 
  Causes of Hypophosphatemia
Reduced Renal Tubular Phosphate Reabsorption
Excess PTH or PTHrP
PTHrP-dependent hypercalcemia of malignancy
Secondary hyperparathyroidism
Vitamin D deficiency/resistance
Calcium starvation or malab

 deficiency/resistance
Calcium starvation or malabsorption
Imatinib
Rapid, selective correction of severe hypomagnesemia
Excess FGF23 or Other “Phosphatonins”
Familial hypophosphatemic rickets (XLH)
Autosomal dominant hypophosphatemic rickets (ADHR)
Autosomal recessive hypophosphatemia (ARHP)
Tumor-induced osteomalacia syndrome (TIO)
McCune-Albright syndrome (fibrous dysplasia)
Epidermal nevus syndrome
Following renal or hepatic transplantation
Idiopathic hypercalciuria
Intrinsic Renal Disease
F

diopathic hypercalciuria
Fanconi syndrome(s), other renal tubular disorders
Cystinosis
Amyloidosis
Multiple myeloma
Hemolytic uremic syndrome
Heavy metal toxicity
Magnesium deficiency
Rewarming or hyperthermia
NaPi-IIa mutations
NaPi-II2c mutations (HHRH)
Other
Poorly controlled diabetes, alcoholism
Hyperaldosteronism
Following partial hepatectomy
Following renal transplantation
TABLE 29.8
Continued
1250
renal tubular phosp

TION VII    Mineral Metabolism
renal tubular phosphate wasting causes appropriate elevation of 
serum 1,25(OH)2D and resulting hypercalciuria.600,601 Other 
causes of impaired renal tubular phosphate reabsorption include 
the osmotic diuresis associated with poorly controlled diabetes, 
alcoholism, hyperaldosteronism, and exposure to any of a wide 
variety of drugs or toxins (see Table 29.8). The pathogenesis of 
phosphate wasting that often follows partial hepatectomy or renal 
transplantation 

ows partial hepatectomy or renal 
transplantation remains unclear, but humoral mechanisms seem 
to be involved.602,603
Rapid egress of extracellular phosphate into cells is the cause 
of hypophosphatemia that develops acutely during administra­
tion of intravenous glucose, insulin therapy for hyperglycemia, 
administration of catecholamines (pressors or bronchodilators), 
thyrotoxic periodic paralysis, profound respiratory alkalosis, 
refeeding syndrome in the wake of severe acidosis or starva­

yndrome in the wake of severe acidosis or starva­
tion, recovery from acute hepatic failure (in which hypophos­
phatemia is a recognized favorable prognostic factor604), or 
other circumstances involving rapid cellular proliferation such 
as leukemic blast crisis or responsiveness to hematopoietic 
growth factors. Hypophosphatemia in these situations is most 
pronounced when there is underlying phosphate depletion, as 
in hyperparathyroidism or vitamin D deficiency, or following 
prolonged malnu

itamin D deficiency, or following 
prolonged malnutrition, alcoholism, or glycosuria. Accelerated 
uptake of phosphate into cells is particularly common in post­
surgical, burn, or trauma patients, in whom it may be promoted 
by high levels of circulating catecholamines and exacerbated by 
concurrent respiratory alkalosis, fever, volume expansion, sepsis, 
and hypokalemia. Situations of greatly accelerated net bone for­
mation, such as hungry bone syndrome occurring immediately 
following parath

e syndrome occurring immediately 
following parathyroidectomy for primary or tertiary hyperpara­
thyroidism, during initial treatment of severe vitamin D defi­
ciency or Paget disease, or in occasional patients with extensive 
osteoblastic bone metastases, may manifest hypophosphatemia 
as well as hypocalcemia. 
The clinical significance of hypophosphatemia depends on the 
presence and severity of underlying phosphate depletion. Unfor­
tunately the status of the total-body phos

 Unfor­
tunately the status of the total-body phosphorus pool, and more 
particularly the critical intracellular pool, is reflected only indi­
rectly by the concentration of phosphate in the extracellular 
fluid, which contains less than 0.5% of body phosphorus. Thus, 
although serum phosphate concentrations generally are used to 
characterize hypophosphatemia as severe (<1–1.5 mg/dL, <0.3–
0.5 mmol/L), moderate (1.5–2.2 mg/dL, 0.5–0.7 mmol/L), or 
mild (2.2–3 mg/dL, 0.75–1 mmol/L), serum phosph

r 
mild (2.2–3 mg/dL, 0.75–1 mmol/L), serum phosphate may be 
normal or even high (depending on renal function) in the pres­
ence of profound intracellular phosphate deficiency. Conversely, 
it may be low when intracellular phosphate is relatively normal, 
such as following a sudden movement of extracellular phosphate 
into cells.
The prevalence of severe hypophosphatemia among hospi­
talized patients overall is less than 1%, whereas mild or mod­
erate hypophosphatemia may be detected in 2% to 5

erate hypophosphatemia may be detected in 2% to 5%.605 
Hypophosphatemia is recognized most often in critically ill 
patients, alcoholics or other malnourished individuals, decom­
pensated diabetics, and those with acute infectious or pulmo­
nary disorders.605
The clinical manifestations of severe hypophosphatemia are 
protean. Among the most common are various neuromuscular 
symptoms, ranging from progressive lethargy, muscle weakness, 
and paresthesias to paralysis, coma, and even death, depe

esthesias to paralysis, coma, and even death, depending 
on the severity of the phosphate depletion. Confusion, profound 
weakness, paralysis, seizures, and other major sequelae generally 
are limited to those with serum phosphate concentrations below 
0.8 to 1 mg/dL.606 Biochemical evidence of muscle injury is 
observed within 1 to 2 days in over one-third of patients whose 
serum phosphate concentrations fall to less than 2 mg/dL.607 
Overt rhabdomyolysis also may occur, especially in the sett

bdomyolysis also may occur, especially in the setting of 
chronic alcoholism with underlying malnutrition and phosphate 
depletion.608,609 However, by the time this is recognized the serum 
phosphate often has been raised by the large amounts of cellular 
phosphate released from damaged muscle. Reversible respiratory 
failure due to respiratory muscle weakness may preclude successful 
weaning from ventilatory support.610,611 Left ventricular dysfunc­
tion, heart failure, and ventricular arrhythm

unc­
tion, heart failure, and ventricular arrhythmias may result from 
profound hypophosphatemia but may not be significant if serum 
phosphate is greater than 1.5 mg/dL.612 Correction of moderate 
hypophosphatemia (<2 mg/dL) in patients with septic shock led 
to a significant increase in blood pressure as well as left ventricu­
lar function and arterial pH.612 Hematologic sequelae of severe 
hypophosphatemia include hemolysis, platelet dysfunction with 
bleeding, and impaired leukocyte function

on with 
bleeding, and impaired leukocyte function (phagocytosis and 
killing).613 Erythrocytes demonstrate increased fragility; altered 
membrane composition, rigidity, and microspherocytosis; and 
reduced levels of ATP and 2,3-diphosphoglycerate (2,3-DPG).613 
The reduction in erythrocyte 2,3-DPG impairs oxyhemoglobin 
  Causes of Hypophosphatemia—cont’d
TABLE 29.8
Drugs or toxins
Ethanol
High-dose estrogens
Acetazolamide, other diuretics
Ifosfamide
High-dose glucocorticoids
Cisplatin
Bicarbo

famide
Cisplatin
Bicarbonate
Suramin
Toluene
Foscarnet
Heavy metals (Pb, Cd)
N-methylformamide
Tenofovir
Paraquat
Shifts of Extracellular Phosphate Into Cells or Bone
Acute Intracellular Shifts
Intravenous glucose, fructose, glycerol
Insulin therapy for hyperglycemia, diabetic ketoacidosis
Catecholamines (epinephrine, albuterol, terbutaline, dopamine)
Thyrotoxic periodic paralysis
Acute respiratory alkalosis, salicylate intoxication, acute gou

tory alkalosis, salicylate intoxication, acute gout
Gram-negative sepsis, toxic shock syndrome
Recovery from acidosis, starvation, anorexia nervosa, hepatic failure
Rapid cellular proliferation
Leukemic blast crisis
Intensive erythropoietin, G-CSF therapy
Accelerated Net Bone Formation
Following parathyroidectomy
Osteoblastic metastases
Treatment of vitamin D deficiency
Antiresorptive therapy of severe Paget disease
Impaired Intestinal Phosphate Absorption
Aluminum-containing antacids
FGF23, Fib

Absorption
FGF23, Fibroblast growth factor 23; G-CSF, granulocyte colony-stimulating factor; HHRH, 
hereditary hypophosphatemic rickets with hypercalciuria; PTH, parathyroid hormone; PTHrP, 
parathyroid hormone–related hormone.
   
1251
dissociation and thereby may reduce oxygen delivery to tissues. 
This problem, together with accelerated hemolysis, may provoke 
a substantial increase in cardiac output. The bl

a substantial increase in cardiac output. The blockade in cellu­
lar glycolysis becomes demonstrable at levels of serum phosphate 
between 1 and 2 mg/dL.614 Glucose intolerance and insulin resis­
tance also have been demonstrable in these patients.615 
Treatment
Hypophosphatemia appears most often in acutely or critically ill 
individuals. Accordingly, it often is difficult to discern whether 
hypophosphatemia is responsible for features of the multiple 
organ dysfunction commonly encountered 

 multiple 
organ dysfunction commonly encountered in this population. For 
example, although depression of intracellular high-energy organo­
phosphates has been demonstrated during treatment of diabetic 
ketoacidosis and phosphate repletion leads to more rapid recovery 
of erythrocyte 2,3-DPG concentrations, opinion is divided as to 
whether phosphate therapy in this setting hastens recovery, pre­
vents complications, or reduces mortality rate.616,617 Nevertheless, 
because severe hypophosphatem

6,617 Nevertheless, 
because severe hypophosphatemia has been associated, in a variety 
of clinical settings, with serious neuromuscular, cardiovascular, 
and hematologic dysfunction that is at least partially reversible 
with phosphate repletion, most now agree that one should adopt 
a relatively low threshold for treatment.612
The decision to correct hypophosphatemia urgently should be 
guided by the estimated severity of the cellular phosphate deficit, 
the presence of signs or symptoms sugge

 deficit, 
the presence of signs or symptoms suggestive of phosphate deple­
tion, and the overall clinical status of the patient. The presence 
of renal insufficiency (a risk for iatrogenic hyperphosphatemia), 
concomitant administration of intravenous glucose (alone or as a 
component of hyperalimentation solutions), and the potential for 
aggravating coexistent hypocalcemia also should be considered.
Limited data are available from clinical trials to predict the 
appropriate dose and rate of p

als to predict the 
appropriate dose and rate of phosphate administration. In patients 
without severe renal insufficiency or hypocalcemia, administra­
tion of intravenous phosphate at rates of 2 to 8 mmol/hour of 
elemental phosphorus over 4 to 8 hours frequently corrects hypo­
phosphatemia without provoking hyperphosphatemia or hypocal­
cemia.605,618–620 Suggested guidelines based on serum phosphate 
are shown in Table 29.9. It is essential that serum calcium and 
phosphate be monitored every 

t serum calcium and 
phosphate be monitored every 6 to 12 hours during and after 
phosphate therapy, both to detect untoward consequences and 
because many patients require additional infusions for recurrent 
hypophosphatemia within 24 to 48 hours of apparently success­
ful repletion.619 Less acute or severe hypophosphatemia should be 
managed with oral (or enteral) phosphate supplements if possible, 
generally given as a total of 1 to 2 g a day (as elemental phosphate) 
of neutral sodium or pot

(as elemental phosphate) 
of neutral sodium or potassium phosphate in divided doses three 
to four times a day (see Table 29.6). In many patients, however, 
oral phosphate therapy is limited by gastrointestinal symptoms 
such as nausea or diarrhea. 
Disorders of Magnesium Metabolism
The fourth most abundant extracellular cation, magnesium, like 
calcium, plays a critical physiologic role, particularly in neuro­
muscular function but also as a component of the mineral phase 
of bone. Intracellula

ponent of the mineral phase 
of bone. Intracellular magnesium is crucial for normal energy 
metabolism, as a cofactor for ATP and numerous enzymes and 
transporters, which is reflected in the rather global clinical effects 
that accompany disorders of magnesium homeostasis. Hypomag­
nesemia and hypermagnesemia are among the most common 
electrolyte disturbances; one or the other of these abnormalities 
is observed in as many as 20% of hospitalized patients and even 
more frequently (i.e., 30–40%

d patients and even 
more frequently (i.e., 30–40%) among those admitted to ICUs.621
Hypermagnesemia
Magnesium homeostasis is achieved mainly through highly effi­
cient regulation of tubular magnesium reabsorption in the loop 
of Henle.1 Because normal kidneys can readily excrete even large 
amounts of magnesium (i.e., 500 mEq/day), high filtered loads 
of magnesium rarely cause hypermagnesemia except in patients 
with significant renal insufficiency.622 Increased magnesium loads 
in such cases 

ency.622 Increased magnesium loads 
in such cases may arise from ingestion of large amounts of oral 
magnesium salts, typically given as cathartics or antacids, or from 
extensive soft tissue ischemia or necrosis in patients with trauma, 
sepsis, cardiopulmonary arrest, burns, or shock622 (Table 29.10). 
Hypermagnesemia may result from parenteral administration of 
magnesium salts, such as when magnesium is used to treat pre­
eclampsia or as a tocolytic.623 The infants of such hypermagne­
semic 

colytic.623 The infants of such hypermagne­
semic mothers may manifest transient hypermagnesemia as well, 
along with parathyroid suppression and neurobehavioral symp­
toms.624,625 The use of oral magnesium preparations as laxatives 
may lead to hypermagnesemia if absorption is increased by intes­
tinal ileus, obstruction, or perforation.626
The most prominent clinical manifestations of hypermagnese­
mia are vasodilatation and neuromuscular blockade, which may 
involve both presynaptic and posts

ade, which may 
involve both presynaptic and postsynaptic inhibition of neuro­
muscular transmission.627 Signs and symptoms generally do not 
appear unless the serum magnesium exceeds 4 mEq/L.622 Hypo­
tension, often refractory to pressors and volume expansion, may 
be one of the earliest signs of progressive hypermagnesemia.627,628 
Lethargy, nausea, and weakness, accompanied by reduction or 
loss of deep tendon reflexes, may progress to stupor or coma with 
respiratory insufficiency or quadrip

or coma with 
respiratory insufficiency or quadriparesis at serum concentrations 
in excess of 8 to 10 mEq/L. Gastrointestinal hypomotility or 
ileus is common. Facial flushing and pupillary dilatation may be 
observed. Hypotension may be complicated by a paradoxic relative 
bradycardia, and other cardiac effects may be evident, including 
  Urgent Therapy of Hypophosphatemiaa
Factors to Consider
Severity of hypophosphatemia
Likelihood of underlying phosphate depletion
Clinical condition of the

ying phosphate depletion
Clinical condition of the patient
Concurrent parenteral therapy (glucose, hyperalimentation)
GUIDELINES
Serum PO4
Rate of Infusion
Duration
Total PO4
(mg/dL)
(mmol/hr)
(hr)
(mmol)
<2.5
2
6
12
<1.5
4
6
24
<1
8
6
48
aRates shown are normalized for a 70-kg person. Most formulations available in the United 
States provide 3 mmol/mL of sodium or potassium phosphate.
PO4, Phosphate.
   
TABLE 29.9
1252
prolongation

252
prolongation of the PR, QRS, and QTc intervals, appearance 
of heart block, and ultimately asystole as serum concentrations 
approach 20 mEq/L.
Hypermagnesemia activates CaSRs in the parathyroids, thereby 
suppressing PTH secretion,629 and in the renal distal tubules, 
thereby reducing tubular calcium and magnesium reabsorption. 
Severe hypocalcemia opposes the effect of hypermagnesemia on 
PTH secretion so that serum PTH may remain within the normal 
range 

hat serum PTH may remain within the normal 
range but still inappropriate for the level of serum calcium.630
Successful treatment of hypermagnesemia requires identifica­
tion and interruption of the source of magnesium, together with 
measures to increase clearance of magnesium from the extracellu­
lar fluid. Use of magnesium-free cathartics or enemas to accelerate 
clearance of ingested magnesium from the gastrointestinal tract, 
together with vigorous intravenous hydration, generally have been

igorous intravenous hydration, generally have been 
successful in reversing hypermagnesemia. Refractory cases, espe­
cially those with advanced renal insufficiency, may require hemo­
dialysis. Intravenous calcium (100–200 mg) infusions have been 
advocated as an effective antidote to hypermagnesemia, and there 
are examples in which this approach has apparently been success­
ful, at least temporarily.622,627,631 
Hypomagnesemia may occur because of impaired intestinal magne­
sium 

 occur because of impaired intestinal magne­
sium absorption or, more commonly, because of excessive gastroin­
testinal losses due to diarrhea, preprocedural bowel preparation, or 
prolonged drainage. Most often, hypomagnesemia reflects defective 
renal tubular reabsorption of magnesium, although rapid shifts into 
cells, other extrarenal losses, or incorporation into new bone may 
occur (Table 29.11). Because only 1% of the body’s magnesium 
content is present in extracellular fluid, measuremen

tent is present in extracellular fluid, measurements of serum 
total or ionized magnesium concentration may not adequately 
reflect total-body magnesium or the magnesium status of the intra­
cellular compartment in critical tissues such as muscle.632 Thus 
patients with deficiency of tissue magnesium may fail to manifest 
overt hypomagnesemia633 but may exhibit abnormal retention (i.e., 
>50% in 24 hours) of infused magnesium, a maneuver that may be 
used to assess magnesium status.634
Etiology

 be 
Etiology
Intestinal Causes of Hypomagnesemia
Selective dietary magnesium deficiency does not occur, and it is 
remarkably difficult, in fact, to induce magnesium depletion experi­
mentally by feeding magnesium-deficient diets, probably because 
renal magnesium conservation is so efficient. Large amounts of mag­
nesium may be lost in chronic diarrheal states (this fluid may con­
tain >10 mEq/L magnesium) or via intestinal fistulas or prolonged 
gastrointes

 via intestinal fistulas or prolonged 
gastrointestinal drainage.635 More commonly, magnesium becomes 
trapped within fatty acid soaps in disorders associated with chronic 
malabsorption.636 Investigation of a rare autosomal recessive disor­
der, hypomagnesmia with secondary hypocalcemia (HSH), led to 
the identification of the transient receptor potential channel protein 
TRPM6 in the form of a hetero-oligomer, with the closely related 
channel protein TRPM7 as a key molecular mediator of intes

protein TRPM7 as a key molecular mediator of intestinal 
(and renal tubular) transepithelial magnesium transport.637 
Renal Causes of Hypomagnesemia
Roughly 60% of renal magnesium reabsorption occurs in the 
thick ascending limb of Henle loop, and another 5% to 10% is 
reabsorbed in the distal tubules.1 Investigation of the pathogen­
esis of several genetic disorders associated with renal magnesium 
wasting has identified key pathways of magnesium reabsorption 
at these sites (see Table 29.11). 

m reabsorption 
at these sites (see Table 29.11). Thus in familial hypomagnesemia 
with hypercalciuria and nephrocalcinosis, loss-of-function muta­
tions in the claudin 16 gene encoding the paracellin-1 protein (or 
in the related gene, claudin 19), a component of the tight junc­
tions between adjacent epithelial cells, selectively impair paracel­
lular magnesium (and calcium) reabsorption in response to the 
(lumen-positive) transepithelial voltage gradient.57,638
In Bartter syndrome(s), inacti

age gradient.57,638
In Bartter syndrome(s), inactivating mutations in any of sev­
eral transporters involved in sodium chloride reabsorption in the 
ascending limb cause salt wasting, compromise the voltage gra­
dient, and similarly impair paracellular magnesium and calcium 
reabsorption.639–641 In autosomal dominant hypocalcemia, muta­
tions causing increased sensitivity of CaSRs to cationic agonists 
cause hypomagnesemia, as well as hypocalcemia, through inap­
propriate CaSR-dependent suppress

a, through inap­
propriate CaSR-dependent suppression of PTH secretion and of 
renal tubular cation reabsorption.642
In Gitelman syndrome, inactivating mutations in the luminal 
thiazide-sensitive NaCl cotransporter (NCC) expressed in the dis­
tal convoluted tubules lead to sodium chloride and magnesium 
wasting, in this case with hypocalciuria.639–641,643 The manner 
whereby impaired NCC activity compromises (transcellular) mag­
nesium reabsorption in this segment is unclear, although NCC-
knoc

ion in this segment is unclear, although NCC-
knockout mice (or normal mice treated with thiazides) display 
reduced distal tubular expression of the TRPM6 channel pro­
tein required for normal magnesium transport across the apical 
membrane.434
Mutations in the FXYD2 γ-subunit of the distal tubular baso­
lateral Na+/K+-ATPase similarly impair salt and magnesium reab­
sorption at that site and account for some, but not all, cases of 
isolated renal magnesium wasting.644,645 An autosomal recessiv

al magnesium wasting.644,645 An autosomal recessive 
form of renal hypomagnesemia is caused by inactivating muta­
tions in the EGF gene, which likely also explains why hypomag­
nesemia may complicate use of cetuximab, a monoclonal antibody 
directed against the EGF receptor.646 Another genetic syndrome 
featuring renal magnesium wasting and hypocalciuria (as in 
Gitelman syndrome), and thus presumably involving a defect in 
distal tubular function as well, in association with hypertension 
and h

n as well, in association with hypertension 
and hypercholesterolemia, is linked to a mutation in mitochon­
drial tRNA DNA.647 Other rare genetic causes of distal tubular 
  Causes of Hypermagnesemia
Excessive Magnesium Intake
Cathartics, antacids, enemas
Dead Sea drowning
Parenteral magnesium administration
Magnesium-rich urologic irrigants
Intestinal obstruction or perforation following magnesium ingestion
Rapid Mobilization From Soft Tissues
Trauma
Shock, sepsis
Cardiac arrest
Burns
Impaired

Trauma
Burns
Impaired Magnesium Excretion
Other
TABLE 29.10
1253
magnesium wasting involve mutations in K channels (Kv1.1 or 
Kir4.1) that disrupt voltage gradients needed for efficient magne­
sium reabsorption.648
Most often, renal magnesium wasting is attributable to an 
acquired abnormality in tubular magnesiu

le to an 
acquired abnormality in tubular magnesium reabsorption. In 
normal subjects, magnesium reabsorption is virtually complete 
within several days of instituting experimental dietary magnesium 
deficiency, even before serum magnesium has declined substan­
tially.649 Thus the finding of more than 1 mEq per day of uri­
nary magnesium in a frankly hypomagnesemic patient indicates 
a defect in renal tubular magnesium reabsorption. The causes of 
acquired primary tubular magnesium wasting inclu

acquired primary tubular magnesium wasting include various 
tubulointerstitial disorders, recovery from acute tubular necrosis 
or obstruction, renal transplantation, various endocrinopathies, 
alcoholism, and exposure to certain drugs (see Table 29.11).
Hypomagnesemia or magnesium depletion due to subnor­
mal renal reabsorption may complicate a variety of endocri­
nopathies, including hyperaldosteronism, hyperthyroidism, 
and disorders associated with hypercalcemia, hypercalciuria, 
or phosph

ted with hypercalcemia, hypercalciuria, 
or phosphate depletion.634 In primary hyperparathyroidism, 
PTH stimulates increased tubular magnesium reabsorption, 
but this action is opposed by a direct tubular effect of hyper­
calcemia. As a result, serum magnesium in primary hyperpara­
thyroidism generally is normal or only slightly reduced.650 In 
hypoparathyroidism, serum and urinary magnesium are low. 
The magnesium depletion in hypoparathyroidism is consistent 
with loss of both the magnesium-r

m is consistent 
with loss of both the magnesium-retaining renal action of PTH 
and the stimulatory effect of 1,25(OH)2D on intestinal mag­
nesium absorption.651
Diabetes is among the most common disorders associated with 
hypomagnesemia.652,653 The severity of the hypomagnesemia in 
diabetics correlates with indices of glycosuria and poor glycemic 
control,654 suggesting that urinary losses of magnesium on the 
basis of glycosuria may partly explain the magnesium depletion. 
Rapid correction of

lain the magnesium depletion. 
Rapid correction of hyperglycemia with insulin therapy causes 
magnesium to enter cells and may further lower the extracellular 
magnesium concentration during treatment.
Alcoholism is another very common clinical setting in which 
hypomagnesemia occurs. Magnesium depletion in alcoholism 
may result in part from nutritional deficiency of magnesium, 
overall caloric starvation and ketosis, and gastrointestinal losses 
due to vomiting or diarrhea, but an acute magnes

due to vomiting or diarrhea, but an acute magnesuric effect of 
alcohol ingestion likely plays the major role.655 This effect of 
alcohol is most evident during the rising limb of the blood etha­
nol curve and may be related to transient suppression of PTH 
secretion.655 Other factors that may contribute to hypomagne­
semia in alcoholism include pancreatitis, malabsorption, sec­
ondary hyperaldosteronism, respiratory alkalosis, and elevated 
plasma catecholamines, which increase intracellular 

asma catecholamines, which increase intracellular sequestra­
tion of magnesium.634
  Causes of Hypomagnesemia
TABLE 29.11
Impaired Intestinal Magnesium Absorption
Hypomagnesemia with secondary hypocalcemia
Malabsorption syndromes
Increased Intestinal Magnesium Losses
Protracted vomiting or diarrhea
Bowel preparation (procedures, surgery)
Intestinal drainage or fistulas
Impaired Renal Tubular Magnesium Reabsorption
Genetic Magnesium-Wasting Syndromes
Bartter syndrome(s)
Familial hypomagnesemia w

omes
Familial hypomagnesemia with hypercalciuria and nephrocalcinosis
Autosomal dominant hypocalcemia
Gitelman syndrome
Isolated renal magnesium wasting
Hypomagnesemia with hypertension and hypercholesterolemia
Acquired Renal Disease
Tubulointerstitial disease
Postobstruction, acute tubular necrosis (diuretic phase)
Renal transplantation
Drugs and Toxins
Ethanol
Digoxin
Diuretics (loop, thiazide, osmotic)
cis-Platinum
Cyclosporine
Ta

p, thiazide, osmotic)
Tacrolimus
Cetuximab
Interleukin 2
Pentamidine
Aminoglycosides
Foscarnet
Amphotericin B
Endocrine and Metabolic Abnormalities
Extracellular fluid volume expansion
Hyperaldosteronism (primary, secondary)
Inappropriate antidiuretic hormone secretion
Phosphate depletion
Other
Sézary syndrome
Acute brain injury
Hydrogen fluoride burns
Rapid Shifts of Magnesium Out of Extrace

ide burns
Rapid Shifts of Magnesium Out of Extracellular Fluid
Intracellular Redistribution
Recovery from diabetic ketoacidosis
Refeeding syndrome
Correction of respiratory acidosis
Calcitonin therapy
Other Losses
Blood transfusions
Extensive burns
Excessive sweating
Pregnancy (third trimester) and lactation
1254
SECTION VI

cy (third trimester) and lactation
1254
A number of drugs have been identified as causes of defective 
renal tubular magnesium reabsorption and hypomagnesemia.634 
These drugs include diuretics (especially loop diuretics), digoxin, 
cisplatin, cetuximab, pentamidine, cyclosporine, tacrolimus, inter­
leukin 2, aminoglycosides, foscarnet, and amphotericin B. Most 
often, drug-induced hypomagnesemia is mild and reversible, par­
ticularly when it is associated with 

rsible, par­
ticularly when it is associated with diuretic therapy. In over half 
of patients treated with cisplatin, hypomagnesemia occurs within 
days or weeks, and roughly half of those who develop it exhibit 
persistent hypomagnesemia many months or even years later. The 
median duration of hypomagnesemia in cisplatin-treated patients 
is about 2 months, but recovery has been observed for up to 2 
years after treatment.656 Cisplatin may induce a more global 
nephropathy and azotemic renal fa

e a more global 
nephropathy and azotemic renal failure, but the magnesium wast­
ing appears to be an isolated functional abnormality. 
Other Causes of Hypomagnesemia
Magnesium, like phosphate, is a major intracellular ion, and sig­
nificant shifts of magnesium from the extracellular compartment 
therefore may occur during recovery from chronic respiratory aci­
dosis or acute ketoacidosis, after refeeding, following administra­
tion of hyperalimentation solutions, and in response to elevations 

ntation solutions, and in response to elevations 
of circulating catecholamines.634 Other rapid losses of extracellu­
lar magnesium may occur during periods of greatly accelerated net 
bone formation (after parathyroidectomy, during recovery from 
vitamin D deficiency, with osteoblastic metastases) or with large 
losses due to pancreatitis, cardiopulmonary bypass surgery,657 
massive transfusion,658 extensive burns, excessive sweating, preg­
nancy, or lactation. 
Consequences of Hypomagnesemia
M

y, or lactation. 
Most of the signs and symptoms of hypomagnesemia reflect alter­
ations in neuromuscular function: tetany, hyperreflexia, positive 
Chvostek and Trousseau signs, tremors, fasciculations, seizures, 
ataxia, nystagmus, vertigo, choreoathetosis, muscle weakness, apa­
thy, depression, irritability, delirium, and psychosis.634 Patients 
usually are not symptomatic unless serum magnesium falls below 
1 mEq/L, although occurrence of symptoms, such as intr

q/L, although occurrence of symptoms, such as intracellu­
lar magnesium, may not correlate well with serum magnesium. 
Atrial or ventricular arrhythmias may occur, as may various elec­
trocardiographic abnormalities—prolonged PR or QT intervals, 
T-wave flattening or inversion, or ST-segment straightening.634 
Hypomagnesemia also increases myocardial sensitivity to digitalis 
intoxication.659
Hypomagnesemia evokes important alterations in mineral ion 
and potassium homeostasis that frequently ag

 ion 
and potassium homeostasis that frequently aggravate the clini­
cal syndrome. Magnesium-deprived humans or animals develop 
hypocalcemia, hypocalciuria, hypokalemia (due to impaired tubu­
lar reabsorption of potassium), and positive calcium and sodium 
balance.649,660 Sustained correction of hypocalcemia or hypoka­
lemia cannot be achieved by administration of calcium or potas­
sium alone, respectively, whereas both abnormalities respond to 
administration of magnesium.636,661
The mechanism

The mechanism of hypocalcemia in this setting may be mul­
tifactorial. Inappropriately normal or low serum PTH, despite 
hypocalcemia, is common and indicates a defect in PTH secre­
tion,662 which is due to augmented signaling by CaSR-associated 
G proteins, normally inhibited by magnesium, within the para­
thyroid cell.663 Other evidence indicates that hypomagnesemia 
also may impair PTH action on target cells in bone and kidney, 
although some have observed

s in bone and kidney, 
although some have observed normal responsiveness; the issue 
remains controversial.483,484,661,662,664
Vitamin D resistance also is a feature of hypomagnesemic 
states.665,666 This disorder appears to be due mainly to impaired 
renal 1α-hydroxylation of 25(OH)D, although tissue resistance to 
1,25(OH)2D also may play a role.651,667 The serum 1,25(OH)2D 
concentration usually is low during hypomagnesemia, which 
may result from magnesium depletion per se, parathyroid insuf

from magnesium depletion per se, parathyroid insuf­
ficiency, or coexistent vitamin D deficiency.668–670 Deficiency 
of 1,25(OH)2D probably is not the main cause of hypocalce­
mia in these patients, however, because hypocalcemia can be 
rapidly corrected (within hours to days) by magnesium therapy 
alone, well in advance of any increase in the serum 1,25(OH)2D 
concentration.668,669 
Therapy of Hypomagnesemia
Patients with mild, asymptomatic hypomagnesemia may be 
treated with oral magnesium sal

magnesemia may be 
treated with oral magnesium salts (i.e., MgCl2, MgO, Mg[OH]2), 
usually given in divided doses totaling 40 to 60 mEq (480–720 
mg) per day (see Table 29.6). Diarrhea sometimes occurs with 
larger doses but generally is not a problem. The gluconate form 
(54 mg magnesium per gram) is said to cause less diarrhea.634 
Patients with malabsorption or ongoing urinary magnesium losses 
may require chronic oral therapy to avoid recurrent magnesium 
depletion. Although intestinal magne

nt magnesium 
depletion. Although intestinal magnesium absorption is severely 
impaired in renal failure,671 oral magnesium must be administered 
with great caution in this setting, especially in patients receiving 
concomitant therapy with 1,25(OH)2D.
Symptomatic or severe (<1 mEq/L) hypomagnesemia, espe­
cially if complicated by hypocalcemia, usually signifies magnesium 
deficits of at least 1 to 2 mEq/kg and is best treated promptly with 
parenteral magnesium salts. The use of intramuscular M

nteral magnesium salts. The use of intramuscular MgSO4 is to 
be discouraged, as the injections are painful and provide relatively 
little magnesium (2 mL of 50% MgSO4 supplies only 8 mEq of 
magnesium, compared with typical magnesium deficits in excess 
of 100 mEq). Moreover, because unretained sulfate ions also may 
increase urinary calcium excretion, intravenous magnesium chlo­
ride or gluconate probably is the most logical approach to initial 
parenteral therapy for patients who also may be 

parenteral therapy for patients who also may be hypocalcemic. 
In adult hypomagnesemic patients with normal renal function, 
rates of infusion of 2 to 4 mEq per hour (i.e., 50–100 mEq/day) 
generally are needed to maintain serum magnesium in the range 
of 2 to 3 mEq/L.630,662,666 Up to 100 mEq per day for 2 days can 
be safely administered without elevating serum magnesium above 
4 mEq/L, whereas doses of 200 mEq per day may increase serum 
magnesium to 4.5 to 5.5 mEq/L and thus are excessive.

nesium to 4.5 to 5.5 mEq/L and thus are excessive.672 In 
patients with active seizures or other urgent indications, the infu­
sion may be preceded by a slowly administered bolus of 10 to 20 
mEq, followed by a higher rate of infusion (i.e., 10–15 mEq/hour) 
for the first 1 to 2 hours only. Patients with normal renal function 
can readily excrete over 400 mEq per day of magnesium in the 
urine without becoming hypermagnesemic, but even mild renal 
failure may greatly limit magnesium excretion. T

failure may greatly limit magnesium excretion. Therefore doses of 
magnesium supplements should be reduced twofold to threefold, 
and careful serial monitoring of serum magnesium should be per­
formed in patients with compromised renal function.
It is important to appreciate that a large fraction of parenter­
ally administered magnesium may be excreted in the urine, even 
in patients with profound magnesium deficiency. Many such 
patients will excrete as much as 50% to 75% of infused magne­
si

excrete as much as 50% to 75% of infused magne­
sium, whereas in normal subjects this approaches 100%.636 More­
over, because equilibration of the intracellular and extracellular 
magnesium pools is relatively slow, it is generally necessary to 
continue magnesium therapy for 3 to 5 days to achieve adequate 
1255
repletion of the typical deficit of 1 to 2 mEq/kg. Because serum 
magnesium may become normal well before tissue stores are 
rep

y become normal well before tissue stores are 
repleted, monitoring of urinary magnesium excretion is a more 
reliable measure of the approach to full repletion, especially after 
patients are switched to oral therapy.
The need for calcium, potassium, and phosphate supplementa­
tion should be considered in the usual clinical setting of hypo­
magnesemia. Vitamin D deficiency also frequently coexists and 
should be treated with oral or parenteral vitamin D or 25(OH)
D. Use of 1,25(OH)2D is not nec

itamin D or 25(OH)
D. Use of 1,25(OH)2D is not necessary, does not hasten recovery, 
and may actually worsen hypomagnesemia by suppressing PTH 
secretion and thereby promoting renal magnesium excretion.673 
Initial parenteral magnesium therapy in hypocalcemic patients 
may produce dramatic hypophosphatemia via the rapid stimula­
tion of PTH secretion. This is most likely to be problematic in 
those with underlying phosphate depletion (malabsorption, alco­
holism, diabetes), in whom it may provok

on, alco­
holism, diabetes), in whom it may provoke acute neuromuscular 
dysfunction, and it may be avoided by concomitant intravenous 
calcium therapy.
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care setting. Am J Med. 1988;84(2):209–214.
	570.	 McCullough P, Heaney R. Correction of vitamin D deficiency 
using sublingually administered vitamin D2 in a Crohn’s disease 
patient with mal-absorption and a new ileostomy. J Steroid Biochem 
Mol Biol. 2017;173:211–214.
	571.	 Winer KK, Zhang B, Shrader JA, et al. Synthetic human parathyroid 
hormone 1-34 replaceme

ynthetic human parathyroid 
hormone 1-34 replacement therapy: a randomized crossover trial 
comparing pump versus injections in the treatment of chronic hypo­
parathyroidism. J Clin Endocrinol Metab. 2012;97(2):391–399.
	572.	 Gupta A, Winer K, Econs MJ, et al. FGF-23 is elevated by chronic 
hyperphosphatemia. J Clin Endocrinol Metab. 2004;89(9):4489–
4492.
	573.	 Okano K, Furukawa Y, Morii H, Fujita T. Comparative efficacy of 
various vitamin D metabolites in the treatment of various types of

phosphatemic familial 
tumoral calcinosis: response to acetazolamide and postulated mech­
anisms. Am J Med Genet. 2014;164a(6):1545–1549.
	581.	 McBryde KD, Wilcox J, Kher KK. Hyperphosphatemia due 
to fosphenytoin in a pediatric ESRD patient. Pediatr Nephrol. 
2005;20(8):1182–1185.
	582.	 Markowitz GS, Stokes MB, Radhakrishnan J, D’Agati VD. Acute 
phosphate nephropathy following oral sodium phosphate bowel 
purgative: an underrecognized cause of chronic renal failure. J Am 
Soc Nephrol. 2005

. Comparison of two assays 
for fibroblast growth factor (FGF)-23. J Bone Miner Metab. 
2005;23(6):435–440.
	595.	 Yamamoto T, Imanishi Y, Kinoshita E, et al. The role of fibroblast 
growth factor 23 for hypophosphatemia and abnormal regulation 
of vitamin D metabolism in patients with McCune-Albright syn­
drome. J Bone Miner Metab. 2005;23(3):231–237.
	596.	 Riminucci M, Collins MT, Fedarko NS, et al. FGF-23 in fibrous 
dysplasia of bone and its relationship to renal phosphate wasting. J 
Cli

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Metab. 2006;91(3):959–963.
	599.	 Prie D, Huart V, Bakouh N, et al. Nephrolithiasis and osteopo­
rosis associated with hypophosphatemia caused by mutations 
in the type 2a sodium-phosphate cotransporter. N Engl J Med. 
2002;347(13):983–991.
	600.	 Bergwitz C, Roslin NM, Tieder M, et al. SLC34A3 mutations in 
patients with hereditary hypophosphatemic rickets with hypercal­
ciuria predict a key role for the sodium-phosphate cotransporter 
NaPi-IIc in maintaining phosphate homeostasis. Am J Hu

-IIc in maintaining phosphate homeostasis. Am J Hum Genet. 
2006;78(2):179–192.
	601.	 Lorenz-Depiereux B, Benet-Pages A, Eckstein G, et al. Hereditary 
hypophosphatemic rickets with hypercalciuria is caused by muta­
tions in the sodium-phosphate cotransporter gene SLC34A3. Am J 
Hum Genet. 2006;78(2):193–201.
	602.	 Green J, Debby H, Lederer E, et al. Evidence for a PTH-indepen­
dent humoral mechanism in post-transplant hypophosphatemia 
and phosphaturia. Kidney Int. 2001;60(3):1182–1196.
	60

pha­
temia on diaphragmatic contractility in patients with acute respira­
tory failure. N Engl J Med. 1985;313(7):420–424.
	612.	 Bollaert PE, Levy B, Nace L, et al. Hemodynamic and metabolic 
effects of rapid correction of hypophosphatemia in patients with 
septic shock. Chest. 1995;107(6):1698–1701.
	613.	 Lichtman MA, Miller DR, Cohen J, Waterhouse C. Reduced red 
cell glycolysis, 2,3-diphosphoglycerate and adenosine triphosphate 
concentration, and increased hemoglobin-oxygen affinity caus

ion, and increased hemoglobin-oxygen affinity caused by 
hypophosphatemia. Ann Intern Med. 1971;74(4):562–568.
	614.	 Travis SF, Sugerman HJ, Ruberg RL, et al. Alterations of red-cell 
glycolytic intermediates and oxygen transport as a consequence of 
hypophosphatemia in patients receiving intravenous hyperalimen­
tation. N Engl J Med. 1971;285(14):763–768.
	615.	 DeFronzo RA, Lang R. Hypophosphatemia and glucose intoler­
ance: evidence for tissue insensitivity to insulin. N Engl J Med. 
1980;

repletion regimen for critically ill patients with moderate 
hypophosphatemia. Crit Care Med. 1995;23(7):1204–1210.
	619.	 Charron T, Bernard F, Skrobik Y, et al. Intravenous phosphate in the 
intensive care unit: more aggressive repletion regimens for moderate and 
severe hypophosphatemia. Intensive Care Med. 2003;29(8):1273–1278.
1255.e16 References
	620.	 Taylor BE, Huey WY, Buchman TG, et al. Treatment of hypo­
phosphatemia using a protocol based on patient weight and serum 
phosphorus lev

gmann KP, Gudermann T. Insights into the 
molecular nature of magnesium homeostasis. Am J Physiol Renal 
Physiol. 2004;286(4):F599–F605.
	641.	 Schlingmann KP, Konrad M, Seyberth HW. Genetics of heredi­
tary disorders of magnesium homeostasis. Pediatr Nephrol. 
2004;19(1):13–25.
	642.	 Nagase T, Murakami T, Tsukada T, et al. A family of autosomal 
dominant hypocalcemia with a positive correlation between serum 
calcium and magnesium: identification of a novel gain of function 
mutation (Ser(82

tion of a novel gain of function 
mutation (Ser(820)Phe) in the calcium-sensing receptor. J Clin 
Endocrinol Metab. 2002;87(6):2681–2687.
	643.	 Simon DB, Nelson-Williams C, Bia MJ, et al. Gitelman’s variant of 
Bartter’s syndrome, inherited hypokalaemic alkalosis, is caused by 
mutations in the thiazide-sensitive Na-Cl cotransporter. Nat Genet. 
1996;12(1):24–30.
	644.	 Meij IC, Koenderink JB, De Jong JC, et al. Dominant isolated 
renal magnesium loss is caused by misrouting of the Na+,K+-
AT

e RK, Adams JS, Ryzen E, et al. Low serum concentrations of 
1,25-dihydroxyvitamin D in human magnesium deficiency. J Clin 
Endocrinol Metab. 1985;61(5):933–940.
	669.	 Fuss M, Cogan E, Gillet C, et  al. Magnesium administration 
reverses the hypocalcaemia secondary to hypomagnesaemia despite 
low circulating levels of 25-hydroxyvitamin D and 1,25-dihydroxy 
vitamin D. Clin Endocrinol. 1985;22(6):807–815.
	670.	 Fuss M, Bergmann P, Bergans A, et al. Correction of low circulating 
levels of 1,2

t al. Correction of low circulating 
levels of 1,25-dihydroxyvitamin D by 25-hydroxyvitamin D during 
reversal of hypomagnesaemia. Clin Endocrinol. 1989;31(1):31–38.
	671.	 Brannan PG, Vergne-Marini P, Pak CY, et al. Magnesium absorp­
tion in the human small intestine. Results in normal subjects, 
patients with chronic renal disease, and patients with absorptive 
hypercalciuria. J Clin Invest. 1976;57(6):1412–1418.
	672.	 Flink EB. Therapy of magnesium deficiency. Ann N Y Acad Sci. 
1969;162(2

agnesium deficiency. Ann N Y Acad Sci. 
1969;162(2):901–905.
	673.	 Sutton RA, Walker VR, Halabe A, et  al. Chronic hypomag­
nesemia caused by cisplatin: effect of calcitriol. J Lab Clin Med. 
1991;117(1):40–43.
1256
30
Osteoporosis: Basic and Clinical Aspects
FRANCISCO J.A. DE PAULA, DENNIS M. BLACK, AND CLIFFORD J. ROSEN
Historical Context
Osteoporosis is a disorder characterized by reduced bone mass, 
impaired bone quality, and a propensity to fracture in both men 
and women. Previously, thi

o fracture in both men 
and women. Previously, this disease was considered within the 
context of a syndrome identified by back pain, vertebral fractures, 
and reduced mineralization on plain radiographs. In the past, the 
approach to patients with these features focused on identifying 
secondary causes of low bone mass and treating fractures with 
orthopedic intervention and pain management.1 However, with 
the advent of bone density measurements, the development of 
new treatment options, and 

s, the development of 
new treatment options, and greater public awareness, osteoporo­
sis emerged as a primary disease manifested in multiple ways and 
suitable to be managed by aggressive prevention and intervention. 
During this period, significant progress was also made in under­
standing the complex pathogenesis of the disease.2,3 Furthermore, 
there have been tremendous advances in delineating the role of 
bone remodeling in normal physiology, particularly in defining the 
process of peak 

gy, particularly in defining the 
process of peak bone acquisition.4 Recently, the skeleton was also 
found to be an endocrine organ regulating metabolic homeostasis 
through the release of bone-specific peptides that modulate glu­
cose transport, phosphate balance, and muscle function.
In addition to these developments, a consensus began to 
emerge concerning the strength of the association between low 
bone mineral density (BMD) and fracture risk and about the 
importance of qualitative aspect

sk and about the 
importance of qualitative aspects of the skeleton as additional risk 
determinants for fractures.5,6 Newer imaging technology has pro­
vided a window into the microstructural world of bone, and these 
advances now allow investigators to better understand the many 
diseases and drugs associated with skeletal fragility.7,8 Great strides 
have also been accomplished in understanding the epidemiology 
of this disease and the socioeconomic impact of fractures on both 
patients and s

onomic impact of fractures on both 
patients and society. The picture that has emerged is that osteopo­
rosis is a disease with a significant degree of morbidity and mortal­
ity risks.9,10 Advances in the field have led to the development of 
drugs capable of increasing bone mass and improving bone quality 
that have proven to be effective in reducing the incidence of frac­
tures. Since 1995, when US Food and Drug Administration (FDA) 
approval for new antiosteoporotic drugs first began, there h

or new antiosteoporotic drugs first began, there has been 
a significant decline in the rate of fracture in the US population. 
Historical Context, 1256
Skeletal Biology, 1257
Bone Remodeling and Its Regulation, 1267
Epidemiology of Osteoporosis and Fractures, 1275
Pathogenesis of Osteoporosis, 1283
Approach to Management of Osteoporosis, 1291
KEY POINTS
 • 
The functional significance of bone has been reassessed in light 
of recent discoveries. In addition to the well-known mec

ent discoveries. In addition to the well-known mechani­
cal support and storage of mineral, mineralized mesenchymal 
tissue also exports peptides critical for regulation of circulating 
phosphate and whole-body energy metabolism. We now have a 
more complete picture of the skeleton and its role in maintain­
ing mineral and metabolic homeostasis.
 • 
Osteoporosis has a great impact on quality of life and mortali­
ty because of its negative impact on bone strength. In the past 
three decades, ris

 on bone strength. In the past 
three decades, risk factors for fractures have been exhaustively 
scrutinized. Online algorithms are available to help predict 
long-term fracture risk and are a readily available and impor­
tant tool for evaluating patients. In parallel, biochemical mark­
ers of bone remodeling and imaging techniques to evaluate 
bone metabolism and structure, respectively, are employed for 
early recognition of fracture susceptibility.
 • 
Advances in our knowledge of mechanism

ility.
 • 
Advances in our knowledge of mechanisms driving bone loss 
have accelerated development of cost-effective drugs for the 
prevention of fractures. Moreover, ongoing studies point to 
additional and more effective drugs for osteoporosis. Curiously, 
clinical awareness for diagnosing and treating osteoporosis 
remains underdeveloped; unfortunately, few individuals benefit 
from the level of knowledge and technology available to diag­
nose and manage the disorder.
 • 
Unlike available t

e and manage the disorder.
 • 
Unlike available therapeutic options for other chronic disorders, 
osteoporosis therapy is unique in that weekly, monthly, bian­
nual, or even annual dosing may be sufficient to accomplish 
successful treatment.
1257
Chapter 30  Osteoporosis: Basic and Clinical Aspects
Unfortunately, in the past few years, this auspicious trend has 
reversed, attracting great interest in what has been coined the “cri­
sis in osteoporosis treatment.”11 Although the incidence of adv

orosis treatment.”11 Although the incidence of adverse 
events are rare, their significance has served to understate the vast 
fracture prevention benefits enjoyed by the majority of patients, 
leading to physician underprescription and patient reluctance to 
use the medication. Thus, a systematic approach to understand­
ing and treating this disease lies in appreciating the physiology of 
skeletal remodeling and the processes by which the basic skeletal 
unit becomes disordered, as well as on t

skeletal 
unit becomes disordered, as well as on the appropriate use of the 
tools available for diagnosis and management. 
Skeletal Biology
Structure and Function of the Skeleton
The skeleton is one of the largest organ systems in the body, 
consisting of a mineralized matrix and a highly active cellular 
remodeling unit, composed of osteoblasts, osteoclasts, osteocytes, 
adipocytes, and lining cells. The most apparent function of the 
skeleton is to provide structural integrity for the organis

is to provide structural integrity for the organism that 
allows for a range of locomotor activity while concomitantly 
keeping additional body weight to a minimum. In addition to 
its structural function, the skeleton also serves as a mineral depot 
ensuring the maintenance of serum calcium and phosphate levels 
through normal remodeling processes and by secretion of bone-
specific factors, such as fibroblast growth factor 23 (FGF23). Just 
as important, the skeleton is the home for hematopoies

mportant, the skeleton is the home for hematopoiesis, main­
taining a niche within trabecular bone elements that consists of 
osteoblasts, adipocytes, reticuloendothelial cells, sinusoids, and 
mesenchymal stromal and stem cells in a hypoxic environment. 
That niche provides progenitors that can respond to injuries at 
any site in the body for critical repair processes. Remarkably, the 
adult skeleton also harbors a huge adipose depot, composing 
10% to 15% of all fat tissues in the body. Thus, 

10% to 15% of all fat tissues in the body. Thus, it is clear that 
alterations in either the structural or metabolic functions of the 
skeleton have tremendous implications for the overall health of 
the organism.12 
Embryology and Anatomy
Skeletal development begins early in embryonic life. The process 
starts with the condensation of mesenchymal cells that differ­
entiate into a cartilaginous structure. Bone formation can then 
take place through one of two mechanisms: endochondral (i.e., 
th

ugh one of two mechanisms: endochondral (i.e., 
through use of a cartilage framework and involving osteoblasts 
laying down true bone matrix on top of the cartilaginous matrix, 
followed by osteoclast-driven turnover of matrix and cells) or 
intramembranous bone formation, in which the mesenchymal pre­
cursors differentiate into bone-forming osteoblasts and lay down 
bone matrix without a cartilage template.13 The growth of long 
bones and vertebrae involves endochondral bone formation. The 
car

rae involves endochondral bone formation. The 
cartilage cells in the growth plate proliferate and undergo hyper­
trophy; the hypertrophic chondrocytes then direct the mineral­
ization of their matrix and, along with the action of osteoclasts, 
partly degrade their matrix. The cartilage is invaded by vessels, 
and the spicules of mineralized cartilage are covered by osteoblasts 
to form a cancellous or trabecular bone often called the primary 
spongiosa. These structures are resorbed and replace

ongiosa. These structures are resorbed and replaced by trabecular 
plates made up entirely of bone, called secondary spongiosa (Fig. 
30.1). This process occurs at the ends of the long bones and in the 
bodies of the vertebrae.
Intramembranous bone formation occurs next to the cartilage 
template in flat bones, such as the skull, scapula, and ileum, and 
on the outer surfaces of long bones, leading to periosteal apposi­
tion and expansion. Woven bone, with disordered fibrils of col­
lagen I and 

bone, with disordered fibrils of col­
lagen I and a disorganized osteocyte network, is formed during 
the early stages of intramembranous acquisition but then becomes 
more organized as lamellar bone is produced by oriented layers 
of osteoblasts. As noted previously, the main difference between 
endochondral and intramembranous bone formation is that 
the latter does not use calcified cartilage as a direct template for 
osteoblasts.
Cortical bone is dense bone found in the shafts of long bones.

e is dense bone found in the shafts of long bones. 
It makes up 80% of the mass of the skeleton, determines its shape, 
and provides much of its strength. During longitudinal skeletal 
growth, endochondral and periosteal appositional bone forma­
tion determine the length and width of the bones.4 New cortical 
bone is shaped by a process called modeling, in which osteoblast 
activity occurs uncoupled to osteoclastic bone resorption. Model­
ing leads to skeletal shape changes, which are critical f

ds to skeletal shape changes, which are critical for defin­
ing the strength of bone. Importantly, modeling is influenced by 
mechanical forces and is increased during the adolescent growth 
spurt.4 As bones elongate, the wide cortex formed just below the 
growth plate must be sculpted by the modeling/resorption process 
to allow these bones to elongate while maintaining and extending 
the narrow tubular structure of the diaphysis.
Bone remodeling is an essential element of skeletal activ­
ity t

g is an essential element of skeletal activ­
ity that provides skeletal stability and elasticity. It is the pro­
cess that defines adult bone mass and maintenance (Fig. 30.2). 
(1st cycle)
(2nd cycle)
R(CC)
F(WB)
R(WB)
F(LB)
Rev
Rev
Calcified cartilage
Osteoclasts
Osteoblasts
Woven bone
Lamellar bone
Vascular invasion
Reversal
Reversal
• Fig. 30.1  Steps in endochondral bone formation. CC, calcified cartilage; 
F, formation; LB, lamellar

, calcified cartilage; 
F, formation; LB, lamellar bone; R, resorption; Rev, reversal; WB, woven 
bone. (Redrawn from Baron R. Anatomy and ultrastructure of bone. In: 
Favus MJ, ed. Primer on the Metabolic Bone Diseases and Disorders of 
Mineral Metabolism. 2nd ed. New York, NY: Lippincott-Raven; 1993:3–9. 
Copyright 1993, American Society for Bone and Mineral Research.)
1258
Remodeling is temporally orchestrated to maintain a balance 
between the amount of bone

 to maintain a balance 
between the amount of bone formed and the amount resorbed. 
Basic multicellular units (BMUs) carry out bone remodeling and 
consist of osteoblasts, osteoclasts, bone lining cells, and osteocytes. 
Remodeling is more active in cancellous or trabecular bone than 
in cortical bone.14 In smaller animals, such as rodents, cortical 
bone can remain lamellar. In large animals and humans, lamellar 
cortical bone is gradually replaced through haversian remodeling 
to form cylindri

ced through haversian remodeling 
to form cylindrical osteons. The initiation of bone remodeling is 
directed by endocrine, paracrine, and autocrine factors. The osteo­
cyte, communicating by release of factors through tiny canaliculi, 
is thought to initiate the remodeling process, providing signals 
to both the lining cell and the osteoblast.15 These cells can then 
signal to attract the osteoclast to the remodeling site where bone 
resorption occurs. This is followed by the release of matrix 

occurs. This is followed by the release of matrix pro­
teins that, in combination with osteoclast-derived factors, direct 
osteoblast differentiation, collagen synthesis, and ultimately 
matrix mineralization.
Bone Matrix and Mineral
The bone matrix consists of fibers of type I collagen laid down 
in layers that have various orientations, a portion of which in 
the mammalian skeleton may be disordered but still adds to the 
strength of the matrix (Figs. 30.3 and 30.4). The matrix contains 
sever

 (Figs. 30.3 and 30.4). The matrix contains 
several additional proteins, including other collagen types that 
may be important in the interaction of type I collagen with non­
collagen proteins within the matrix. The noncollagen proteins, 
such as osteocalcin and several proteoglycans, represent about 
10% of the total protein in bone and may direct the formation 
of fibers, mineralize bone, regulate the attachment of bone cells 
to its matrix, and play a role in the function of bone-forming and

nd play a role in the function of bone-forming and 
resorbing cells.
Protein composition of the matrix may vary, particularly 
between woven and lamellar bone.16 The proteins range from 
the large cell-attachment proteins (e.g., thrombospondin, 
fibronectin), which have molecular masses higher than 400 kDa, 
to the small, vitamin K–dependent γ-carboxylated proteins (e.g., 
matrix Gla protein and osteocalcin), which are 6-kDa calcium-
binding proteins. Osteocalcin can be incompletely or totally c

eins. Osteocalcin can be incompletely or totally car­
boxylated depending on the number of glutamic acid sites within 
the molecule that are changed to γ-carboxylated glutamic acid by 
vitamin K–dependent enzymes; incomplete γ-carboxylation may 
represent the action of inhibitors such as warfarin or the action 
of decarboxylating processes. Undercarboxylated osteocalcin 
(GLU13-OCN) is released from the skeletal matrix during bone 
resorption (see later discussion). Some noncollagen proteins (e.

e later discussion). Some noncollagen proteins (e.g., 
biglycan, decorin, bone sialoprotein, osteopontin, osteoadherin) 
are highly acidic and play an important role in signaling and in 
the matrix of the hematopoietic niche. In addition to cell-attach­
ment sequences, these proteins contain various amounts of carbo­
hydrate and are called glycoproteins or proteoglycans. Noncollagen 
proteins of bone are often highly phosphorylated, which enables 
them to bind calcium, and thus may regulate mine

them to bind calcium, and thus may regulate mineralization. 
Genetic manipulations in experimental mouse models have pro­
vided important information on the function of noncollagenous 
proteins. For example, null mutations of the osteonectin gene lead 
to osteopenia in some studies, indicating that this matrix protein 
may be important for the maintenance of a normal bone struc­
ture.17 However, deletion of the osteocalcin gene has been shown 
to increase bone mass.18
Osteocalcin null mice als

Osteocalcin null mice also have a striking body composition 
and insulin sensitivity phenotype.19 Studies principally from 
the Karsenty laboratory, but now validated by other groups, have 
shown that GLU13-OCN is released from the skeletal matrix 
and can bind to a G protein–coupled receptor on the surface of 
beta cells and adipocytes.20 This results in enhanced insulin pro­
duction and greater glucose transport in adipocytes. Moreover, 
insulin itself can stimulate re

ocytes. Moreover, 
insulin itself can stimulate release of matrix GLU13-OCN. This 
activity requires the co-participation of osteoclasts, integrating 
bone remodeling in the regulation of insulin sensitivity. Insulin 
Time
0
1
2
3
4
5
6
OCL.
MON.
POB.
OB.
OB.
Lining cells
• Fig. 30.2  Three-dimensional reconstruction of the remodeling sequence in human trabecular bone. 1, 
Early bone resorption with osteoclasts (OCL). 2, Late bone resorption with mononuclear cells (MON). 3, 
Reversal phase with 

 mononuclear cells (MON). 3, 
Reversal phase with pre-osteoblasts (POB). 4, Early matrix formation by osteoblasts (OB). 5, Late bone 
formation with mineralization. 6, Completed remodeling cycle with reversion to lining cells. (From Eriksen 
EF. Normal and pathological remodeling of human trabecular bone: three dimensional reconstruction of the 
remodeling sequence in normals and in metabolic bone disease. Endocr Rev. 1986;7:379–408. Copyright 
1986 by The Endocrine Society.)
1259
Chapter 30  Os

986 by The Endocrine Society.)
1259
signaling in osteoblasts downregulates osteoprotegerin (OPG) 
via the forkhead box protein O1 (FOXO1), thereby enhancing 
osteoclastogenesis and ultimately greater bone resorption. The 
resultant increased osteoclastic activity creates the conditions nec­
essary for acid-mediated decarboxylation of γ-carboxyglutamate 
residues21,22 (Fig. 30.5). This was the first of several remarkable 
studies demonstrating 

irst of several remarkable 
studies demonstrating the endocrine nature of the skeleton, in this 
case due to release of matrix proteins. Importantly, this finding led 
to even greater insights into the role of the skeleton in modulating 
energy metabolism. 
Collagen Synthesis
Type I collagen is the most abundant protein of the bone matrix. 
It is a rigid, rod-like, insoluble molecule composed of two α1 
chains and one α2 chain23,24 (see Fig. 30.4). Collagen chains 
consist of repeating triplets 

). Collagen chains 
consist of repeating triplets of amino acids, with glycine in every 
third position and a high content of proline and lysine. The two 
α1 and the α2 collagen chains form a triple helix that is sta­
bilized by the hydroxylation of proline and lysine residues and 
requires ascorbic acid. Collagen is synthesized as a soluble pro­
protein with large nonhelical extensions at the carboxy (C)- and 
amino (N)-terminal ends. Procollagen also contains C-terminal 
interchain disulfide b

n also contains C-terminal 
interchain disulfide bonds that help to initiate formation of the 
triple helical structure. Procollagen is released into the cister­
nae of the rough endoplasmic reticulum, packaged in the Golgi 
vesicles, and secreted extracellularly. The procollagen peptide 
ends are then removed by specific peptidases to produce mature 
insoluble collagen molecules, which are further stabilized by 
intramolecular and intermolecular cross-links. The major col­
lagen cross-links are

 cross-links. The major col­
lagen cross-links are formed by lysine and hydroxylysine resi­
dues that ultimately form pyridinium ring structures (Fig. 30.6). 
In addition, nonenzymatic cross-links are formed as a result of 
reactions that create advanced glycation end products (AGEs), 
such as pentosidine. With advancing age, and particularly in the 
presence of diabetes mellitus, this process is more pronounced, 
compromising the structure and functional role of type I col­
lagen in bone.25 
Mi

unctional role of type I col­
Mineralization
Bone mineral is formed by small, imperfect hydroxyapatite crys­
tals, which contain carbonate, magnesium, sodium, and potas­
sium in addition to calcium and phosphate. Mineralization 
occurs by two distinct mechanisms, one outside the cell and 
catalyzed by alkaline phosphatase, and one in matrix vesicles that 
is accelerated by the enzyme phospho1.26 Both enzymes are criti­
cal for adequate mineralization, the former acting to enhan

dequate mineralization, the former acting to enhance the 
breakdown of pyrophosphate, an inhibitor of calcium-phosphate 
precipitation, and the latter to accelerate phosphate availability 
within the matrix by acting on phosphocholine and phospho­
ethanolamine. The initial mineralization of calcified cartilage and 
woven bone probably occurs by means of matrix vesicles.26 These 
membrane-bound bodies are released from chondrocytes and 
osteoblasts, contain alkaline phosphatase, and can form a ni

s, contain alkaline phosphatase, and can form a nidus 
for crystallization in the presence of adequate phosphate. In con­
trast, in lamellar bone, the collagen fibers are tightly packed, and 
matrix vesicles are rarely seen. Mineralization does not occur 
immediately after collagen deposition, and there is a layer of 
10 to 100 μm of unmineralized osteoid between the mineral­
ization front and the osteoblast. Changes in the packing of the 
fibrils and in the composition of the noncollagen protei

s and in the composition of the noncollagen proteins may 
be required for mineralization. A group of proteins, named SIB­
LINGs (small, integrin-binding ligand, N-linked glycoprotein), 
synthesized in osteocytes (i.e., osteopontin, dentin matrix pro­
tein 1 [DMP1], bone sialoprotein, and matrix extracellular phos­
phoglycoprotein [MEPE]), have a crucial role in the deposition 
of calcium into bone matrix. SIBLINGs combined with several 
endopeptidases, including phosphate-regulating endopeptidas

dases, including phosphate-regulating endopeptidase 
homolog, X-linked (PHEX) modulate phosphate metabolism 
through the regulation of FGF23 synthesis, but also affect osteo­
clastogenesis and energy metabolism.27,28 Mineralization of col­
lagen fibrils begins in the hole zones of the calcium fibrils, where 
there is more room for inorganic ions to accumulate (see Fig. 
30.3). Mineralization requires calcium, phosphate, and alkaline 
A
B
C
Gap zone
Type I collagen monomeric and fibrillar structu

ne
Type I collagen monomeric and fibrillar structure
Overlap zone
N
e d
c
b
a
d c
b
a
e
C
• Fig. 30.3  Type I collagen monomeric and fibrillar structure.  (A) By rotary 
shadowing electron microscopy, procollagen (≅300 nm long) appears as a 
rope-like triple helix to which globular carboxy (C)-terminal (to the right) and 
amino (N)-terminal domains are attached. (B) Chapman’s model of the col­
lagen fibril. Diagram showing the arrangement of tropocollagen monomers 
within the collagen fibril, re

ocollagen monomers 
within the collagen fibril, relative to the location of the overlap and gap zone 
fibril staining pattern. Tropocollagen molecules are shown as horizontal 
rods, and the polarity of all monomers in the fibril is indicated by N (NH2 
terminus) and C (COOH terminus) markings on one monomer. (C) Elec­
tron micrograph of glutaraldehyde-fixed, heparin-gold type I collagen fibril 
complexes, in fibrils visualized by uranyl acetate staining. Letters below the 
micrograph show positi

taining. Letters below the 
micrograph show positions of positively stained fibril bands, following the 
accepted notation. Dotted lines between the molecular model in (B) and 
the electron micrograph show corresponding overlap and gap zones. The 
location of heparin-gold particles relative to the molecular structure of the 
fibril can be measured within each 67-nm period, beginning at the center 
of the left border of the overlap zone (origin, arrow), and extending to the 
center of the right b

rrow), and extending to the 
center of the right border of the gap zone. Heparin-gold particles appear 
as circular dark objects present mainly in the “a” bands region of the fibrils. 
(From San Antonio JD, Lander AD, Karnovsky MJ, Slayter HS. Mapping 
the heparin-binding sites on type I collagen monomers and fibrils. J Cell 
Biol. 1994;125:1179–1188 by copyright permission of The Rockefeller 
1260
phosphatase. This process is impaired in circ

lism
phosphatase. This process is impaired in circumstances of vita­
min D deficiency, very low calcium intake, hypophosphatemia, 
mutation in the gene encoding alkaline phosphatase, and impor­
tantly by the mineralization inhibitor pyrophosphate.29 The 
characteristic feature of undermineralization on bone biopsy is 
greater osteoid (i.e., newly synthesized, nonmineralized collagen 
at the bone surface), and the most dramatic undermineralization 
occurs in the syndrome of hypophosphatasia, whic

occurs in the syndrome of hypophosphatasia, which is due to an 
absence of total nonspecific alkaline phosphatase. Recent work 
using a recombinant form of alkaline phosphatase has shown 
promising results by promoting complete mineralization of the 
skeleton and improvement in quality of life of patients with 
hypophosphatasia.30 
Collagen Degradation by Osteoblasts and Osteocytes
As part of the bone remodeling process, collagen is cleaved and 
degraded by a group of proteases called collagen

degraded by a group of proteases called collagenases. They are matrix 
metalloproteases (MMPs) that can initiate cleavage of collagen 
fibrils at neutral pH, and they are central to the process of colla­
gen degradation, matrix breakdown, and bone remodeling. Three 
collagenases in bone have been described: collagenase 1 (MMP1), 
2 (MMP8), and 3 (MMP13).31 Human osteoblasts express the 
collagenase 1 and 3 genes (MMP1 and MMP13). Resting osteo­
blasts secrete limited amounts of collagenase, an

blasts secrete limited amounts of collagenase, and changes in the 
synthesis of collagenase correlate with changes in bone resorp­
tion. Collagenase plays a critical function in bone remodeling. 
Mice with deletions of the collagenase 3 gene or mutations of the 
Lysyl and prolyl residues
• Hydroxylation
• Hydroxylysyl glycosylation
Secretion
Secretory
vesicles
Triple helix formation
Procollagen
Propeptide
cleavage
Fibril
formation
Lysyl
oxidase
Fibrils
Cross-li

ge
Fibril
formation
Lysyl
oxidase
Fibrils
Cross-linked
fibrils
N-proteinase
C-proteinase
N-telopeptides
C-telopeptides
C-propeptides
N-propeptides
• Fig. 30.4  Schematic diagram showing the different post-translational modifications and assembly of 
type I collagen into fibrils. (Modified from Myllyharju J, Kivirikko KI. Collagens, modifying enzymes and 
their mutations in humans, flies and worms. Trends Genet. 2004;20:33–43 and Viguet-Carrin S, Garnero 
P, Delmas PD. The

43 and Viguet-Carrin S, Garnero 
P, Delmas PD. The role of collagen in bone strength. Osteoporos Int. 2006;17:319–336, with permission.)
1261
α132 type I collagen gene have resistance to collagenase 3 cleavage 
and fail to resorb bone after exposure to parathyroid hormone 
(PTH).33 The synthesis of collagenase by osteoblasts is regulated 
by hormones and by cytokines in the bone microenvironment 
that act by transcriptional and post-transcript

t 
that act by transcriptional and post-transcriptional mechanisms.34 
It should be noted that although cleavage by collagenase is an 
early and obligatory event, most degradation of collagen during 
remodeling is accomplished by osteoclasts that secrete protons and 
enzymes (e.g., cathepsin K) that solubilize mineral and degrade 
the matrix. Cleavage of collagen fragments leads to their excretion 
in the urine, in which sensitive assays can detect the N-terminal or 
C-terminal fragments (see la

ct the N-terminal or 
C-terminal fragments (see later discussion).
Recent evidence points to the participation of osteocytes in 
aspects of remodeling. Osteocytes are terminally differentiated 
osteoblasts that are buried within the matrix of the skeleton but 
possess receptors for PTH and have metabolic activity. These cells 
are arranged in a functional unit termed the osteon that includes 
an elaborate dendritic network that communicates with the bone 
surface and probably participates in mec

the bone 
surface and probably participates in mechanical sensing. Osteo­
cytes also are a rich source of receptor activator for nuclear factor 
κB ligand (RANKL), which enhances osteoclast differentiation, 
particularly during states of increased calcium demand, such as 
lactation, or immediately following estrogen deficiency. Moreover, 
osteocytes can secrete enzymes that directly degrade bone matrix 
in a process called osteocytic osteolysis, likely through elaboration 
of acid phosphatase an

likely through elaboration 
of acid phosphatase and collagenases. Whether osteocytes also 
secrete alkaline phosphatase and participate in normal skeletal 
remodeling per se is still open to debate. 
Bone Lining Cells, Osteoblasts, and Osteocytes
Bone is formed by osteoblasts, which are terminally differentiated 
cells that do not undergo mitosis and have many unique features 
(Fig. 30.7). Osteoblasts are derived from mesenchymal cells in the 
skeletal microenvironment.35 Some osteogenic precurs

eletal microenvironment.35 Some osteogenic precursors may 
appear in the circulation, particularly during growth or following 
injury; they originate from skeletal tissue, and their contribution 
to bone formation is not certain. Osteoprogenitor cells, or pre-
osteoblasts, replicate and differentiate into active osteoblasts that 
exhibit various phenotypic characteristics.36 For example, osteo­
blasts in early development and during repair produce woven 
bone, whereas more mature osteoblasts pro

e woven 
bone, whereas more mature osteoblasts produce lamellar bone. 
Osteoblast activity varies during bone formation. Some cells are 
tall and closely packed and produce a large amount of matrix 
in a small area; others are flatter and produce matrix at a slower 
rate over a larger area. Nevertheless, all differentiated osteoblasts 
share certain features. They are connected by gap junctions and 
contain a dense network of rough endoplasmic reticulum and a 
large Golgi complex, and they secre

ticulum and a 
large Golgi complex, and they secrete collagen and noncollagen 
Pancreas
Insulin sensitivity
Adipose
tissue
Activated
osteoclast
RANK
RANKL
Runx2
P
Twist2
OPG
IR
Insulin
:hydroxyapatite
Mineralized bone
Acidic resorption pit
Undercarboxylated
osteocalcin
Esp
• Fig. 30.5  Energy regulation and bone turnover by the insulin/osteocalcin Axis. A putative feed-forward 
regulatory loop ties bone turnover to energy regulation as proposed by Ferron and associates22 a

egulation as proposed by Ferron and associates22 and Ful­
zele and colleagues.21 Insulin activates skeletal remodeling (i.e., increases bone formation by osteoblasts 
and resorption by osteoclasts), which in turn releases uncarboxylated osteocalcin from the skeletal matrix 
into the circulation. This enhances insulin secretion and increases the insulin sensitivity of adipocytes. A 
tyrosine phosphatase OST-PTP, which is encoded by the Esp gene, binds the insulin receptor (IR) and sup­
presses it

inds the insulin receptor (IR) and sup­
presses its activation through dephosphorylation. The transcription factor Twist2 is a critical downstream 
suppressor of osteoblast differentiation. Osteoprotegerin (OPG) is an osteoblast-specific inhibitor of recep­
tor activator for nuclear factor κB ligand (RANKL), acting as a decoy receptor to block bone resorption. 
Hydroxyapatite is the mineral component of bone. (From Rosen CJ, Motyl KJ. No bones about it: insulin 
modulates skeletal remodeling. Ce

out it: insulin 
modulates skeletal remodeling. Cell. 2010;142[2]:198–200.)
1262
proteins in an oriented fashion. Some products, such as osteocal­
cin, are synthesized almost uniquely by osteoblasts and osteocytes. 
A large proportion of the osteocalcin originating from osteo­
blasts is deposited in the matrix and subsequently released during 
bone remodeling. Hence, changes in serum levels of osteocalcin 
reflect bone turnover rather than bone formation per se.

t bone turnover rather than bone formation per se. As noted 
previously, GLU13-OCN may serve as an endocrine hormone 
prompting insulin secretion and enhancing insulin sensitivity in 
peripheral tissues.
Mature osteoblasts have a finite capacity to produce matrix, 
and bone formation is sustained by the arrival of new popula­
tions of cells at the bone surface. The number and function of 
osteoblasts are determined by hormones, local growth factors, and 
cytokines. Some act as classic cell mitog

rs, and 
cytokines. Some act as classic cell mitogens and increase the popu­
lation of pre-osteoblastic cells, some determine their differentia­
tion into mature osteoblasts, and others modify the function of 
mature cells or enhance osteocytic formation.37 The ultimate fate 
of mature osteoblasts varies. They may die by apoptosis; they may 
become embedded in the matrix and become osteocytes; or they 
may be converted to flattened lining cells, which synthesize little 
protein and cover a large

which synthesize little 
protein and cover a large percentage of the surface of bone with 
a thin cytoplasmic layer (i.e., the bone lining cell) (see Fig. 30.7).
The bone lining cells are flattened and have a fibroblastic-like 
appearance. Recent work has begun to clarify the role of these 
lining cells as more active participants in the remodeling unit, in 
a manner similar to the osteocyte rather than purely quiescent 
cells. For example, bone lining cells express osterix (Sp7), which 
is a ma

lining cells express osterix (Sp7), which 
is a major transcription factor for osteoblast differentiation. In 
addition, these cells are in communication through small cana­
liculi with osteocytes buried within the matrix and express similar 
markers of differentiation. In response to PTH, which is a bone 
remodeling stimulant, bone lining cells differentiate into osteo­
blasts and thus become a pool of reserve cells necessary during 
accelerated turnover within the remodeling unit.38 Bone linin

 turnover within the remodeling unit.38 Bone lining 
cells also express stem-like genes that potentially indicate that 
there could be dedifferentiation of osteoblasts or that these cells 
are another reservoir for skeletal or even adipogenic progenitors 
Bone marrow stroma contain pluripotent cells that can dif­
ferentiate into diverse cells of mesenchymal lineage, including 
osteoblasts, chondrocytes, and adipocytes35 (Fig. 30.8). The ulti­
mate cellular phenotype depen

ig. 30.8). The ulti­
mate cellular phenotype depends on factors present in the cellular 
microenvironment, the degree of hypoxia, the biochemical and 
metabolic signature of these cells, and their effects on intracellular 
signals and gene expression. The types and numbers of transcrip­
tion factors are nuclear proteins that bind to DNA to regulate 
gene transcription. Some can determine the fate of undifferen­
tiated cells, although the process is complex and includes meta­
bolic determinants s

is complex and includes meta­
bolic determinants such as sufficient mitochondrial and glycolytic 
machinery.39
CCAAT/enhancer-binding proteins β and δ and peroxisome 
proliferator-activated receptor γ (PPARγ) play an essential role in 
the differentiation of cells toward adipocytes, whereas runt-related 
transcription factor 2 (RUNX2) plays a central role in the dif­
ferentiation of cells toward osteoblasts.40,41 Targeted disruption of 
the RUNX2 gene results in disorganized chondrocyte maturati

 gene results in disorganized chondrocyte maturation 
and a complete lack of bone formation due to an arrest of osteo­
blast development.42
Osterix (Sp7) is another transcription factor that is required for 
endochondral and intramembranous bone formation. It is regu­
lated by RUNX2 and thought to be the next stage in osteoblast 
differentiation. Sp7 null mice fail to develop a mineralized skel­
eton because of an arrest of late stages of osteoblast differentia­
tion. Interactions between nuclea

ast differentia­
tion. Interactions between nuclear factors are common steps in the 
regulation of transcription and differentiation.43 Osterix associ­
ates and acts cooperatively with a nuclear factor of activated T cells 
(NFAT), a transcription factor that regulates osteoblastogenesis 
and osteoclastogenesis.44 CCAAT/enhancer-binding proteins can 
interact with RUNX2 and with the activating transcription factor 
(ATF)/cyclic adenosine monophosphate response element–bind­
ing protein family of

phate response element–bind­
ing protein family of proteins. ATF4 plays a central role in osteo­
blastic function, and its activity is regulated by a nuclear matrix 
attachment region binding protein, SATB2, which interacts with 
ATF4 and RUNX2 to regulate osteoblast differentiation.45
The conversion of osteoblasts to osteocytes involves a change 
in metabolic activity and the development of an extensive net­
work of dendrite-like processes that communicate with those on 
adjacent osteocytes and

communicate with those on 
adjacent osteocytes and cells of the bone surface (Fig. 30.9). Both 
osteoblasts and bone lining cells contain cell processes that are 
connected to underlying osteocytes through small canaliculi. After 
mineralization is complete and the osteocyte is encased in miner­
alized bone, these processes maintain connections among osteo­
cytes and allow for at least two important features that are essential 
for the multicellular unit. First, the extended syncytium with its 

lar unit. First, the extended syncytium with its 
extensive canalicular network that allows rapid diffusion of small 
molecules from the marrow space, along with cell-cell junctions 
that allow transport from the cytoplasm of one osteocyte directly 
to that of another, is important for supporting the viability of the 
osteocytes. Second, it allows a constant exchange of information 
A
B 
Pyridinoline (Pyr)
Deoxypyridinoline (DPD)
NTX-1
α2
α1
α1
α1
CTX-1
ICTP or CTX-MMP
Secretion
Prop

1
CTX-1
Secretion
cleavage
Fibril
formation
N
C
C
N
P1CP
P1NP
• Fig. 30.6  (A) During bone formation, the type I collagen molecule is syn­
thesized as procollagen, and then amino (N)-terminal and carboxy (C)-
terminal propeptides (P1NP and P1CP, respectively) are cleaved. The 
central part of the molecule, triple helix of collagen, is inco

of the molecule, triple helix of collagen, is incorporated into 
bone matrix. (B) During bone resorption, different products of the break­
down of type I collagen are produced: cross-linked molecules (pyridinoline 
[Pyr], deoxypyridinoline [DPD]), C-terminal and N-terminal cross-linked 
telopeptides generated by cathepsin K (CTX-I and NTX-I, respectively), 
and C-terminal telopeptide generated by metalloproteinases (ICTP or 
CTX-MMP). (Redrawn from Szulc P, Kaufman JM, Delmas PD. Biochemi­
cal a

om Szulc P, Kaufman JM, Delmas PD. Biochemi­
cal assessment of bone turnover and bone fragility in men. Osteoporos Int. 
2007;18:1451–1461.)
1263
in the form of secreted factors between the endocortical surface 
and the matrix to regulate remodeling, as well as potential recruit­
ment of precursors, such as the bone lining cells (Fig. 30.10; see 
Fig. 30.9).
Initially, osteocytes may continue to synthesize collagen and 
play a role in minerali

o synthesize collagen and 
play a role in mineralization. Later, the major role of the osteo­
cyte-osteoblast syncytium may be to sense mechanical forces.46 
Osteocytes probably sense bone deformation, as well as fluid 
shifts, and provide signals for the adaptive remodeling of bone size 
and shape.46 One hypothesis is that small strains produce fluid 
shear stress in the canaliculi between osteocytes. This effect may 
result in intracellular signaling through changes in ion channels 
or in the 

naling through changes in ion channels 
or in the production of biologically active molecules. Regions of 
bone microdamage contain apoptotic osteocytes, which may pro­
vide signals for the initiation of bone remodeling by osteoclasts 
and the consequent removal of damaged bone.23
Cells of the osteoblastic lineage are important for forming bone 
and for initiating bone resorption. Both mature osteoblasts and 
osteocytes may play a role in activating resorption. Most of the 
hormonal factors that

ing resorption. Most of the 
hormonal factors that stimulate bone resorption act on cells of 
the osteoblastic lineage. They stimulate the synthesis and perhaps 
release of RANKL and colony-stimulating factor 1 (CSF1), which 
are essential for osteoclastogenesis.46,47 Osteoblasts also produce 
additional factors that regulate bone resorption, including cyto­
kines, prostaglandins, and local growth factors. In cell culture, 
contact between osteoblastic cells and hematopoietic cells appears 
to b

lastic cells and hematopoietic cells appears 
to be necessary for osteoclast formation (Fig. 30.11). Osteoblasts, 
as noted earlier, may also play a role in initiating bone resorption by 
releasing collagenases, other metalloproteinases, and plasminogen 
activator. These enzymes may remove the surface proteins of bone, 
which prevent the access of osteoclasts to the mineralized matrix. 
Osteoblasts also influence the development and maintenance of 
the marrow through their production of growth f

f 
the marrow through their production of growth factors, cyto­
kines, and chemokines that regulate the growth and development 
of hematopoietic cells. 
Osteoclast Differentiation and Function
Osteoclasts are derived from hematopoietic progenitors and are 
myeloid in origin. Hematopoietic stem cells under the direction 
of cytokines and possibly cell-cell interactions express transcrip­
tion factors that define their commitment to the osteoclast lineage 
(Fig. 30.12). CSF1 (i.e., macrophage CSF 

lineage 
(Fig. 30.12). CSF1 (i.e., macrophage CSF [M-CSF]) is the major 
cytokine regulating the replication and development in bone mar­
row of progenitor cells that are capable of differentiating into 
osteoclasts. Expression of the transcription factor SPI1 (formerly 
called PU.1) is also necessary for the osteoclast precursor cell to 
develop.48 In bone marrow, the osteoclast precursor cell is multipo­
tent and can differentiate into monocyte-macrophages, dendritic 
cells, or pre-osteoclasts

-macrophages, dendritic 
cells, or pre-osteoclasts.49 The latter fuse to form highly differenti­
ated, multinucleated osteoclasts that resorb bone. Osteoclasts also 
exhibit other remarkable characteristics, including a high number 
of mitochondria, the capacity to function in both oxygenated and 
hypoxic environments, and susceptibility to apoptosis.50 Progres­
sion through the osteoclast pathway is influenced by multiple local 
and systemic hormones that may include 1,25-dihydroxyvitamin 
D [1

mones that may include 1,25-dihydroxyvitamin 
D [1,25(OH)2D], prostaglandins, and the cytokines interleukin 1 
(IL1), IL6, and tumor necrosis factor (TNF). Osteomacs are mac­
rophages in the bone remodeling unit and may be important for 
Osteocytes
Empty
lacuna
Osteoid
Bone-
lining
cells
• Fig. 30.7  Microstructure of an actively remodeling trabecular bone surface. The osteoclast initiates the 
remodeling cycle by resorbing an area of bone matrix, immediat

ycle by resorbing an area of bone matrix, immediately followed by osteoblast differentiation 
and osteoid (unmineralized bone matrix) production to replace the resorbed bone. During this process, a 
small fraction of osteoblasts differentiate further to become osteocytes, encasing themselves within the 
mineralizing bone matrix and joining the osteocyte network. Mature bone surfaces are populated with 
bone lining cells, whose origin and function remain unclear. (From DiGirolamo DJ, Clemens TL, 

 remain unclear. (From DiGirolamo DJ, Clemens TL, Kousteni S. 
The skeleton as an endocrine organ. Nat Rev Rheumatol. 2012;8[11]:674–683.)
1264
development of a canopy over the remodeling unit and clearance 
of degraded proteins, as well as antigen presentation. Additionally, 
osteomacs may participate in the removal of “old” osteoblasts.
The nature of the osteoblast-lineage cell products, which 
directly regulate osteoclast formation and function, has been clar

e osteoclast formation and function, has been clari­
fied.47 The principal stimulator of osteoclast formation is RANKL, 
a member of the TNF protein superfamily. This protein was origi­
nally identified as a product of activated T lymphocytes, but it 
is also recognized as a critical stimulator of osteoclastogenesis 
from mesenchymal stromal cells, pre-osteoblasts, osteocytes, and 
hypertrophic chondrocytes. Production of RANKL in osteoblast-
lineage cells is stimulated by essentially all agents

eage cells is stimulated by essentially all agents that enhance 
osteoclast formation, including PTH, 1,25(OH)2D, prostaglan­
dins, and many cytokines. Mice that are deficient in RANKL do 
not form osteoclasts and have osteopetrosis. In contrast, injection 
of RANKL into mice stimulates osteoclast formation and bone 
resorption but may also be associated with increases in marrow 
adiposity. RANKL is produced as a membrane protein; in acti­
vated T lymphocytes, RANKL is cleaved from the cell memb

T lymphocytes, RANKL is cleaved from the cell membrane 
and is released as a soluble factor.51 It is unclear whether similar 
events occur in osteoblast-lineage cells, although there is some 
evidence that cleavage and release of soluble RANKL occurs in 
malignant cells that metastasize to bone.
OPG is a native inhibitor of osteoclastogenesis; it is a soluble 
receptor for RANKL that binds this ligand and prevents interac­
tion of RANKL with its cognate receptor, RANK. OPG is pro­
duced widely. 

cognate receptor, RANK. OPG is pro­
duced widely. In bone marrow cultures, a number of stimulators 
of resorption, including PTH, 1,25(OH)2D, and prostaglandin 
E2 (PGE2), inhibit OPG production. For these factors, there 
is a reciprocal relationship between RANKL stimulation and 
OPG inhibition that causes activation of osteoclastogenesis and 
enhanced resorption. Mice that are deficient in OPG have osteo­
porosis, whereas mice that overexpress OPG have increased bone 
mass. These results, toge

OPG have increased bone 
mass. These results, together with those for RANKL-deficient 
mice and mice injected with RANKL, demonstrate that osteo­
clast-mediated bone resorption is tightly regulated by the com­
bined actions of RANKL and OPG.
The active receptor for RANKL is RANK, a member of the 
TNF receptor superfamily. Osteoclasts and their immediate 
precursor cells express RANK, and this expression is induced 
muscle cell
fibroblast
stem cell
(MSC)
?
P

ast
stem cell
(MSC)
?
Pre-adipocyte
OPG
M-CSF, IL6
Dlx5, Msx1, Runx2
C/EBP, PPARγ
Pre-osteoclasts
Hematopoetic
stem cells
RANK
RANKL
• Fig. 30.8  For a mesenchymal stem cell to become an osteoblast, activation of several key factors, 
such as Runx2, BMP 2, TGFβ, and transcription factor Sp7 (osterix), are necessary, although the precise 
sequence of events in this cascade has not been fully clarified. In contrast, to achieve full adipocytic 
diffe

ed. In contrast, to achieve full adipocytic 
differentiation, there are two groups of critical factors already present in mesenchymal stem cells that 
need to be activated: C/EPBs α, β, and δ, and PPARs α, γ2, and δ. PPARγ2 activation by endogenous 
(e.g., prostaglandin J2, long-chain and oxidized fatty acids) or exogenous (e.g., rosiglitazone) ligands dra­
matically shifts allocation of mesenchymal stem cells toward the adipocytic pathway and away from the 
osteoblast lineage. In vitro, this sh

ay from the 
osteoblast lineage. In vitro, this shift is characterized as an either/or allocation: either the cell becomes a 
fat cell or it becomes a bone cell, but not both. Inflammatory cytokines can be released from adipocytes, 
and circulating hormones such as leptin, adipsin, adiponectin, and resistin are also produced by fat cells. 
The solid arrows represent confirmed networks for regulation, and the dashed arrows represent potential 
regulatory pathways. BMP, bone morphogenetic protein;

ulatory pathways. BMP, bone morphogenetic protein; C/EBP, CCAAT/enhancer-binding proteins; Dlx5, 
distal-less homeobox 5; HSC, hematopoietic stem cell; IGF, insulin-like growth factor; IGFR, insulin-like 
growth factor receptor; IL, interleukin; M-CSF, macrophage colony-stimulating factor; Msx1, MSH homeo­
box homolog 1; OPG, osteoprotegerin (tumor necrosis factor ligand superfamily, member 11); PPAR, 
peroxisome proliferator-activated receptor; RANK, receptor activator of nuclear transcription 

RANK, receptor activator of nuclear transcription factor κB 
(tumor necrosis factor receptor superfamily, member 11a); RANKL, RANK ligand (tumor necrosis factor 
receptor superfamily, member 11); Runx2, runt-related transcription factor 2; TGF, transforming growth 
factor. (From Rosen CJ, Bouxsein ML. Mechanisms of disease: is osteoporosis the obesity of bone? Nat 
Clin Pract Rheumatol. 2006;2[1]:35–43.)
1265
by M-CSF.52 Binding of RANKL to RA

linical Aspects
by M-CSF.52 Binding of RANKL to RANK activates a series of 
intracellular pathways that activate NF-κB and mitogen-acti­
vated protein kinases, as well as NFAT and the activator protein 
1 (AP1) family of transcription factors. The TNF receptor-asso­
ciated factors (TRAFs), and particularly TRAF6, are ubiquitin 
E3 ligases that bind RANK intracellularly and are involved in 
RANK responses. Mice deficient in TRAF6, like those deficient in 
RANK, develop osteopetrosis. In addition 

ient in 
RANK, develop osteopetrosis. In addition to its effects on bone, 
the RANKL/RANK system is involved in lymphocyte function, 
barrier functions in the skin, and breast and lymph node develop­
ment. Mature osteoclasts express RANK, and treatment of these 
cells with RANKL inhibits apoptosis and stimulates resorptive 
activity.53
In addition to RANKL, M-CSF is essential for osteoclast for­
mation. Mice that are deficient in M-CSF have osteopetrosis and 
few osteoclasts.54 In cultures of is

petrosis and 
few osteoclasts.54 In cultures of isolated osteoclast precursor cells, 
both M-CSF and RANKL must be present to form mature osteo­
clasts. M-CSF enhances RANK production in osteoclast precur­
sors and inhibits apoptosis of osteoclast precursors and mature 
osteoclasts. The receptor for M-CSF, CSF1 receptor (CSF1R) 
(formerly designated C-FMS), is present on osteoclast precursors 
and mature osteoclasts.52 Binding of CSF1R by M-CSF activates 
tyrosine kinase activity in the receptor

ctivates 
tyrosine kinase activity in the receptor, which initiates a series of 
intracellular downstream events.
A series of coactivator molecules is critical for osteoclast devel­
opment. These molecules include members of the cytoplasmic 
immunoreceptor tyrosine-based activation motif (ITAM) family, 
namely Fc-receptor γ-subunit (FcRγ) and DNAX activation pro­
tein 12 (DAP12). ITAM proteins interact with receptor proteins 
in the cell membrane of osteoclast precursor cells. The search for 
re

 of osteoclast precursor cells. The search for 
receptors associated with these ITAM adaptors in myeloid cells has 
identified at least two candidates that associate with FcRγ (osteo­
clast-associated receptor [OSCAR] and paired immunoglobu­
lin-like receptor [PIR] and two that associate with DAP12 (the 
triggering receptor expressed by myeloid cells 2 [TREM2] and the 
signal regulatory protein β1 [SIRPB1]). Mice that are deficient in 
FcRγ and DAP12 have osteopetrosis and deficient osteoclast f

AP12 have osteopetrosis and deficient osteoclast for­
mation despite their ability to express RANKL, RANK, M-CSF, 
and CSF1R.44,55 This signaling pathway responds to ligands that 
have not yet been established genetically; together with RANK, 
it stimulates the accumulation of intracellular calcium, which is 
required for dephosphorylation of NFAT; this allows the transport 
of NFAT into the nucleus, where it acts as a transcription factor.
The formation of multinucleated osteoclast-like cells i

ormation of multinucleated osteoclast-like cells in vitro 
requires hematopoietic precursors and cells of the mesenchymal/
osteoblast lineage present within the niche.56 In vivo and in cul­
tures with devitalized bone, mononuclear pre-osteoclasts attach to 
the bone surface and form multinucleated osteoclasts by fusion.57 
The accumulation of additional nuclei into osteoclasts by fusion 
probably continues while the cell is actively resorbing. The life span 
of the osteoclast is limited. As oste

e life span 
of the osteoclast is limited. As osteoclasts become inactive, they die 
by apoptosis. Hormones that enhance bone resorption may delay 
Stro 1
CD29
CD105
CD166
Cbfa1
Osterix
Alk Phos
Casein kinase II
PHEX
MEPE
E11/gp38
Destrin
MT1-MMP
DMP1
CapG
Osteoid
Mineral
Sclerostin
FGF23
ORP150
Pre-
Osteoid
osteocyte
Mineralizing
osteocyte
Mature
osteocyte
• Fig. 30.9  Expression of markers during osteoblast-to-osteocyte ontogeny. The osteocyte appears to be 
t

osteocyte ontogeny. The osteocyte appears to be 
the descendant of the matrix-producing osteoblast, which is a descendant of the mesenchymal stem cell 
known to express markers such as Stro1, CD29, CD105, and CD166. Matrix-producing osteoblasts express 
Cbfa1 and osterix, necessary for osteoblast differentiation, followed by alkaline phosphatase and collagen, 
necessary for the production of osteoid. Osteocalcin is produced by the late osteoblast and continues to 
be expressed by the osteocyte. 

 and continues to 
be expressed by the osteocyte. By some unknown mechanism, some designated cells begin to embed in 
osteoid and begin to extend dendritic projections, keeping connections with already embedded cells and 
cells on the bone surface. Molecules such as E11/gp38 and MT1-MMP appear to play a role in dendrite/
canaliculi formation, whereas molecules such as destrin and CapG regulate the cytoskeleton. Phosphate-
regulating endopeptidase homolog, X-linked (PHEX), matrix extracellular ph

 homolog, X-linked (PHEX), matrix extracellular phosphoglycoprotein (MEPE), and 
dentin matrix protein 1 (DMP1) regulate biomineralization and mineral metabolism, and fibroblast growth 
factor 23 (FGF23) regulates renal phosphate excretion. FGF23 is elevated not only in osteocytes from hypo­
phosphatemic animals but also in those of normal rats. Sclerostin is a marker of the mature osteocyte and is 
a negative regulator of bone formation. ORP150 may preserve viability of this cell in a hypoxic e

may preserve viability of this cell in a hypoxic environment. 
(Redrawn from Bonewald LF. The amazing osteocyte. J Bone Miner Res. 2011;26[2]:229–238.)
1266
apoptosis, and inhibitors of resorption probably accelerate it. The 
mechanisms that limit the extent of osteoclastic resorption are 
incompletely understood and may involve inhibition by calcium 
ions, which accumulate under the osteoclast resorbing surface, or 
by local inhibitory factors, such as transfor

 or 
by local inhibitory factors, such as transforming growth factor β 
(TGFβ), which are released and activated during resorption.
The mature osteoclast is a unique and highly specialized cell 
(Fig. 30.13). It usually contains 10 to 20 nuclei, but giant osteo­
clasts with up to 100 nuclei can be seen in Paget disease and in 
giant cell tumors of bone. The large size of osteoclasts is probably 
essential for their resorptive function. The best evidence for this 
comes from studies of dendritic 

vidence for this 
comes from studies of dendritic cell–specific transmembrane pro­
tein (DC-STAMP), because inhibition of this protein or its com­
plete deficiency in mouse models results in the generation of only 
mononuclear osteoclasts that have impaired resorptive activity.58
The capacity of osteoclasts to resorb bone depends on their 
ability to isolate a region of the bone surface from the extracel­
lular fluid and produce a local environment that can dissolve bone 
mineral and degrade mat

nt that can dissolve bone 
mineral and degrade matrix. The osteoclast must polarize and pro­
duce a basolateral membrane opposite the resorption space, which 
anti-E11
Acc.V
5.00 kV 4.0
2000x
SE   11.7 · UMKC-OB
SpotMagn
Det. WD
10 m
Alexa488-Phalloidin
Merge
A
B
• Fig. 30.10  (A) Visualization of early, embedding osteocytes. Using anti-E11 immunostaining and visu­
alization of the actin cytoskeleton by alexa488 staining for phalloidin, one can visualize the embedding 
osteocyte and the early o

visualize the embedding 
osteocyte and the early osteocyte in 12-day murine calvaria. The merged image shows that the majority 
of the E11 is on the cell surface and along the dendritic processes. In addition, if one looks closely, the 
dendrites that end on the cell surface have a bulbous tip of unknown function. This structure must inter­
face with the cells on the bone surface. (B) An acid-etched resin-embedded murine sample showing an 
osteocyte lacuna sending canaliculi to the bone surface.

yte lacuna sending canaliculi to the bone surface. Note the rough surfaces of canaliculi toward the 
bone surface and the smooth surface of canaliculi that project away from the bone surface, suggesting a 
difference between forming and formed canaliculi. Both sets of images demonstrate the complexity of this 
network and the interface of osteocytes with the bone surface. (The images in A are provided by Dr. Sarah 
Dallas, University of Missouri at Kansas City, MO; B, from Bonewald LF. The amazi

at Kansas City, MO; B, from Bonewald LF. The amazing osteocyte. J Bone 
Miner Res. 2011;26[2]:229–238.)
1267
facilitates the excretion of resorption products (see Fig. 30.13). 
The resorbing apparatus consists of a central ruffled border area, 
which secretes hydrogen ions and proteolytic enzymes, surrounded 
by a clear or sealing zone in a structure called the podosome. The 
podosome contains filamentous actin linked to αvβ3 integrin, and 
it

filamentous actin linked to αvβ3 integrin, and 
it anchors the cell to the bone surface. The osteoclast attaches to 
bone through the interaction of integrins in the podosome with 
noncollagenous proteins such as vitronectin and osteopontin in 
the matrix.
Acidification of the resorption space adjacent to the ruffled 
border membrane requires that osteoclasts have a vacuolar pro­
ton pump (H+-ATPase [adenosine triphosphatase]) and a chloride 
channel that is charge coupled to H+ secretion across

nnel that is charge coupled to H+ secretion across the ruffled 
membrane to preserve electron neutrality.59,60 These osteoclast 
H+-ATPase pumps are similar to the vacuolar proton pumps that 
acidify intracellular organelles, but in the osteoclast they are exteri­
orized to increase the extracellular hydrogen ion concentration in 
the resorption space (see Fig. 30.13). The hydrogen ions dissociate 
from carbonic acid, which is synthesized by carbonic anhydrase II; 
the bicarbonate generated by t

onic anhydrase II; 
the bicarbonate generated by this dissociation is removed from the 
cell by chloride-bicarbonate exchange at the basolateral membrane 
of the osteoclast. Ion pumps can transport the dissolved calcium 
from the bone surface through the cell to the extracellular fluid.
However, calcium can also reach the extracellular fluid directly 
if the sealing zone is disrupted. The proteolytic enzymes produced 
by the osteoclast include lysosomal enzymes and metalloprotein­
ases.61 Lysoso

sosomal enzymes and metalloprotein­
ases.61 Lysosomal proteases can degrade collagen at the low pH 
present in the ruffled border area. Cathepsin K is probably the 
most important of these.62 Metalloproteinases, which are active 
at neutral pH, have also been detected at the resorption site.63 
The products of resorption are transported across the ruffled bor­
der membrane and excreted through the basolateral membrane 
of the osteoclast by a process called transcytosis.64 In trabecular 
bone, os

ess called transcytosis.64 In trabecular 
bone, osteoclasts characteristically resorb to a limited depth and 
then move laterally to produce irregular, plate-like resorption 
areas called Howship lacunae. In cortical remodeling, the path of 
directed resorption is longer, possibly because of renewal of osteo­
clasts from hematopoietic cells brought to the site through the 
haversian canal. 
Bone Remodeling and Its Regulation
Overview of Remodeling
Adult bone mass is determined by two processes: 

g
Adult bone mass is determined by two processes: acquisition of 
peak bone mass during adolescence and the subsequent bone loss 
OC
precursor
OC
Pericyte
MSC
Canopy
T-cell
HSC
1
2
4
5
3
• Fig. 30.11  Intercellular communication pathways within the basic multicellular unit that comprise all steps 
of the bone remodeling process. 1, Stimulatory and inhibitory signals from osteocytes to osteoblasts (e.g., 
Oncostatin M [

m osteocytes to osteoblasts (e.g., 
Oncostatin M [OSM], parathyroid hormone–related peptide, and sclerostin). 2, Stimulatory and inhibitory sig­
nals from osteoclasts to osteoblasts (e.g., matrix-derived transforming growth factor β and insulin-like growth 
factor 1, secreted CT-1, Sema4D, and S1P). 3, Signaling within the osteoblast lineage (e.g., ephrinB2 and 
EphB4, Sema3a, parathyroid hormone–related peptide, OSM). 4, Stimulatory and inhibitory signals between 
the osteoblast and osteoclast 

ry signals between 
the osteoblast and osteoclast lineages (e.g., receptor activator for nuclear factor κB ligand, Sema3B, Wnt5a, 
and osteoprotegerin). 5, Marrow cell signals to osteoblasts (e.g., macrophage-derived OSM, T cell–derived 
interleukins, and receptor activator for nuclear factor κB ligand). HSC, hematopoietic stem cell; MSC, mes­
enchymal stem cell; OC, osteoclast. (From Sims NA, Martin TJ. Coupling the activities of bone formation and 
resorption: a multitude of signals within the

and 
resorption: a multitude of signals within the basic multicellular unit. Bonekey Rep. 2014;3:481.)
1268
after maturity. Changes in bone mass result from physiologic and 
pathophysiologic processes in the bone remodeling cycle, and 
ultimately this can lead to skeletal fragility.65 The most vulner­
able periods in women are during accelerated linear growth in 
adolescence (ages 10–16 years) and later in life, usually imme­
diately after menopause (ages 45–60 

usually imme­
diately after menopause (ages 45–60 years). Male bone loss is 
much more gradual but is also determined by peak acquisition 
and age-related loss.
The bone remodeling cycle is a tightly coupled process whereby 
bone is resorbed at approximately the same rate as new bone is 
formed. BMUs comprise the remodeling unit of bone and, as 
noted previously, include osteoclasts, osteoblasts, osteocytes, and 
bone lining cells.66 The bone marrow niche includes these cells plus 
hematopoietic

row niche includes these cells plus 
hematopoietic elements, adipocytes, reticuloendothelial cells lin­
ing the sinusoids, and mesenchymal stromal cells (see Fig. 30.11). 
Activation of the remodeling cycle serves two functions in the 
adult skeleton: (1) to supply calcium acutely, as well as chronically, 
to the extracellular space, and (2) to provide elasticity and strength 
to the skeleton. When the remodeling process is uncoupled so that 
resorption exceeds formation, bone is lost. However, 

orption exceeds formation, bone is lost. However, during peak 
bone acquisition, formation exceeds resorption, resulting in a net 
gain of bone. Remodeling is more pronounced in the trabecular 
skeleton (e.g., spine, calcaneus, and proximal femur), the most 
metabolically active compartment of bone. The trabecular ele­
ments are likely surrounded by a canopy that encloses the BMU 
and contains a capillary network for nutrient supply, as well as 
cells that can be identified as osteomacs. Althoug

cells that can be identified as osteomacs. Although trabecular bone 
remodels more frequently than cortical bone, it is also extremely 
vulnerable to perturbations by local or systemic factors that can 
cause significant imbalances in bone turnover.
The bone remodeling cycle begins with activation of resting 
osteoblasts on the surface of bone, as well as the bone lining cells 
(see Fig. 30.11). The initial signal for remodeling has been actively 
debated, as has been the source of that signal (

debated, as has been the source of that signal (i.e., systemic or 
local). Microdamage or force changes sensed by the osteocyte may 
initiate the remodeling signal, although it is likely that local fac­
tors secreted by osteoblasts, osteoclasts, or lining cells could also 
begin the process. That initiating signal is followed by a series of 
secretory products originating from activated osteoblasts to osteo­
clasts and their precursors. These intercellular signals recruit and 
differentiate mu

roids, and prostaglandins in osteoclast formation. Under the 
influence of other cytokines (data not shown), hematopoietic stem cells (HSCs) commit to the myeloid 
lineage, express c-Fms and receptor activator for nuclear factor κB (RANK), the receptors for macrophage 
colony-stimulating factor (M-CSF) and RANK ligand (RANKL), respectively, and differentiate into osteo­
clasts. Mesenchymal cells in the marrow respond to a range of stimuli, secreting a mixture of pro- and 
antiosteoclastogenic pr

ing a mixture of pro- and 
antiosteoclastogenic proteins, the latter primarily osteoprotegerin (OPG). Glucocorticoids (GCs) suppress 
bone resorption indirectly but possibly also target osteoclasts and their precursors. Estrogen (E2), by a 
complex mechanism, inhibits activation of T cells, decreasing their secretion of RANKL and tumor necro­
sis factor α (TNFα); the sex steroid also inhibits osteoblast and osteoclast differentiation and life span. A 
key factor regulating bone resorption is the

n. A 
key factor regulating bone resorption is the RANKL/OPG ratio. IFN, interferon; ILs, interleukins; M-CSF, 
macrophage colony-stimulating factor; TGF, transforming growth factor. (From Rosen CJ, ed. Primer on 
the Metabolic Bone Diseases and Disorders of Mineral Metabolism. 8th ed. Ames, IA: Wiley-Blackwell; 
2013:2–33, Copyright 2013, American Society for Bone and Mineral Research.)
1269
such as TGFβ and insulin-like growth factor 1 (IGF1

uch as TGFβ and insulin-like growth factor 1 (IGF1), as well 
as collagen, osteocalcin, calcium, and other protein and mineral 
components, are released into the microenvironment. Growth 
factors released by resorption contribute to the recruitment of 
new osteoblasts to the bone surface, which begin the process of 
collagen synthesis and biomineralization. In addition, cytokines 
released by activated osteoclasts such as bone morphogenetic pro­
tein (BMP) 7, Wnt 10b, and sphingosine 1-phosphate

tein (BMP) 7, Wnt 10b, and sphingosine 1-phosphate can stimu­
late osteoblastogenesis in reverse sequence.68 Ephrins serving as 
both ligands and receptors also contribute to this bidirectional 
signaling mechanism69 (Fig. 30.14). In healthy adults, as many 
as 2 million remodeling sites may be active at any given time, 
and it is estimated that nearly one fourth of all trabecular bone 
is remodeled each year. In general, resorption takes only 10 to 13 
days, whereas formation is much more delib

 to 13 
days, whereas formation is much more deliberate and can take 
upward of 3 months (see Fig. 30.2). Under ideal circumstances, 
by the end of the cycle, the amount of bone resorbed equals the 
amount reformed.
Although 80% of skeletal mass is cortical bone, the surface area 
of cortical bone is only about one fifth that of cancellous bone. 
Moreover, more osteoclast precursor cells are available in cancel­
lous bone and on the endosteal surfaces of cortical bone. Conse­
quently, turnover i

faces of cortical bone. Conse­
quently, turnover is greater on these surfaces than on periosteal 
bone, which normally undergoes little remodeling. However, sub­
periosteal resorption can be activated in hyperparathyroidism, and 
the periosteal surface contains pre-osteoblasts that may become 
active late in life and cause an age-related increase in the periosteal 
diameter of long bones.70 This periosteal expansion may maintain 
bone strength and compensate for losses at the endosteal surfaces 

d compensate for losses at the endosteal surfaces 
and in cancellous bone.71
Several key components of the remodeling cycle are sus­
ceptible to systemic and local alterations, which can lead to 
deleterious changes in bone mass. In particular, activation of 
remodeling via the osteoblast and recruitment of osteoclasts rep­
resent the two most vulnerable sites in the cycle. Often, with 
systemic changes in circulating hormones, such as estrogen defi­
ciency, chronic PTH excess, or hyperthyroidis

efi­
ciency, chronic PTH excess, or hyperthyroidism, the remodeling 
cycle is stressed such that coupling of the two processes cannot 
be maintained. This failure is due to the difference in timing 
between resorption, which generally occurs within a BMU over 
about 2 weeks, and formation, which can take upward of 12 
weeks to fully mineralize newly synthesized matrix. Not sur­
prisingly, a third cell altered in disease states is the osteocyte. 
Remodeling of the skeleton implies coupling of res

Remodeling of the skeleton implies coupling of resorption to 
formation, and therefore no net change in bone mass. Because 
the osteocyte responds to mechanical loading or stress by com­
municating with bone lining cells and osteoblasts, these cells 
could also become damaged72 (see Fig. 30.11). Indeed, osteo­
cyte apoptosis may contribute to age-related osteoporosis either 
directly or through the elaboration of systemic peptides. Inter­
estingly, remodeling may end with the osteocyte as well b

• Fig. 30.13  Schematic representation of a resorbing osteoclast. The osteoclast acidifies the resorption 
lacunae by secreting H+ and Cl− ions for demineralization, and lysosomal cathepsin K for degradation of 
type I collagen. ADP, adenosine diphosphate; ATP, adenosine triphosphate; ATPase, adenosine triphos­
phatase; CLC-7, chloride channel 7; RANK, receptor activator of nuclear transcription factor κB; RANKL, 
RANK ligand. (From Rodan SB, Duong LT. Cathepsin K—a new molecular target for ost

ong LT. Cathepsin K—a new molecular target for osteoporosis. IBMS 
Bonekey. 2008;5:16–24.)
1270
that bind to sclerostin have been developed, and at least one is 
currently in phase III trials as a means of reducing the inhibi­
tory effect of this protein on osteoblast differentiation, thereby 
increasing bone mass.73,74
As noted, uncoupled remodeling occurs during menopause 
because of estrogen deprivation or in response to endogenous 
PTH fluxes, cytokine stimu

response to endogenous 
PTH fluxes, cytokine stimulation, growth hormone surges, glu­
cocorticoid excess, or changes in serum calcium. For the most 
part, estrogen deprivation remains one of the most common 
and critical elements in shifting resorption rates to a higher 
set-point. Although bone formation initially can “catch up,” 
the length of time for each component of the remodeling cycle 
clearly favors resorption over formation, as the process of laying 
down new bone requires the interact

ess of laying 
down new bone requires the interaction of several processes72 
(see Fig. 30.2). However, it is still unclear why falling estrogen 
levels, which is a universal event during the menopausal years, 
causes rapid bone loss in a relatively small percentage of women, 
although insidious bone loss has been clearly demonstrated for 
most, if not all, women who enter menopause.75 Other fac­
tors such as peripheral conversion of testosterone to estradiol, 
which occurs to a greater degree i

 to estradiol, 
which occurs to a greater degree in individuals who are obese, 
higher adrenal androgen production, and/or genetic determi­
nants, as well as other local signals, may be important. Although 
the genetic causes of rapid bone loss have not been identified 
in humans, mice have strong heritable determinants that affect 
the rate of age-related and estrogen-deficient bone loss.76,77 With 
advancing age, regardless of gender, a negative balance in bone 
remodeling is a natural occurre

e balance in bone 
remodeling is a natural occurrence when the number of senes­
cent cells within bone increases. Remarkable changes are found 
in the chromatin and secretome profile of senescent cells. Their 
presence within the tissue microenvironment can be recognized 
by detecting increased expression of the biomarker p16Ink4a. 
Proinflammatory molecules are part of the repertory of paracrine 
and endocrine peptides secreted by senescent cells, producing a 
negative environment both locally 

s, producing a 
negative environment both locally and systemically. Osteocytes, 
the most abundant cells within bone tissue, can acquire the 
senescent phenotype. In experimental studies, these cells were 
removed by using the INK-ATTAC “suicide” transgene encod­
ing an inducible caspase 8 expressed specifically in senescent 
cells. The investigators observed improvements in bone mass 
and microstructure, as well as in the biomechanical properties 
of bone. These results were reinforced by pharm

s 
of bone. These results were reinforced by pharmacologic elimi­
nation of senescent cells, using a “senolytic compound,” which 
also reversed bone deterioration in aged mice. Moreover, these 
results are in line with previous data suggesting that the removal 
of senescent cells potentially has beneficial effects on the cardio­
vascular system, insulin sensitivity, and frailty, opening the door 
for the development of new opportunities for preventing age-
associated degenerative diseases.78
The

nting age-
The nature of osteocyte-osteoblast-osteoclast interaction has 
been one of the most active areas of investigation (see Fig. 30.11). 
External signals (e.g., PTH, growth hormone, IL1, estrogen depri­
vation) to resting osteoblasts and stromal cells cause these cells to 
release cytokines (i.e., interleukins [e.g., IL1, IL6, and IL11], as 
well as M-CSF, TNF, and TGFβ) that enhance the recruitment 
and differentiated function of multinucleated giant c

 differentiated function of multinucleated giant cells destined 
to become bone-resorbing cells.79 However, one of the most criti­
cal pathways in the osteoblast-osteoclast interaction scheme is the 
RANKL-OPG relationship.47
The OPG, RANKL, and RANK system, which affects osteo­
clast differentiation, in addition to the effects of M-CSF on osteo­
clast proliferation, provides the critical link between osteoclasts 
Bone resorption
ephrinB2
ephrinB2
EphB4
EphB4
PDZ
protein?
Kinase doma

inB2
ephrinB2
EphB4
EphB4
PDZ
protein?
RhoA
Fos, NFATc1
?
Bone formation
Bidirectional ephrin signaling
Blocks
Stimulates
• Fig. 30.14  Ephrin-Eph signaling and bone homeostasis. Osteoclasts are responsible for the resorption 
of a localized packet of bone, and when they cease their activity, a team of osteoblasts is attracted to the 
resorption site, where they proliferate, differentiate, and then re-f

ere they proliferate, differentiate, and then re-form the packet of bone. Inset, Ephrin-Eph 
forward signaling from osteoclasts to osteoblasts may be responsible for driving the formation of the new 
bone packet, and ephrin-Eph reverse signaling may be responsible for the cessation of continued bone 
resorption by osteoclasts. (Redrawn from Mundy GR, Elefteriou F. Boning up on ephrin signaling. Cell. 
2006;126[3]:441–443.)
1271
and osteoblasts

orosis: Basic and Clinical Aspects
and osteoblasts. It has also led to the synthesis of RANKL anti­
bodies that, after successful human trials, have become therapeu­
tic agents to inhibit bone resorption. Denosumab (Prolia) is the 
first monoclonal antibody against RANKL and was approved by 
the FDA, European regulatory agencies, and the Endocrinologic and 
Metabolic Drugs Advisory Committee for the treatment of post­
menopausal osteoporosis, as well as for metastatic bone diseases, 
owing to it

well as for metastatic bone diseases, 
owing to its strong efficacy in reducing spine and hip fractures.80 
It is administered once every 6 months and suppresses bone 
resorption by 80% to 90%. Long-term follow-up of the original 
phase III trials reveals significant improvement in bone mass at 
both the spine and hip out to 10 years, with maintenance of anti­
fracture efficacy and few, if any, adverse events.81 After 10 years 
of treatment, the gain in bone mass was 21.7% in the lumbar 
spine, 

gain in bone mass was 21.7% in the lumbar 
spine, 9.2% in the total hip, and 9.0% in the femoral neck, with 
a continuous increase in BMD throughout therapy.81 However, 
therapy withdrawal leads to a rapid inflexion in the BMD curve, 
implicating significant bone loss. The magnitude of reduction in 
BMD after a 17-month therapy gap was 8.1% and 8.4%, respec­
tively, in the lumbar spine and the total hip, which corresponds to 
35.5% of the total gain in the spine and 103.3% in the total hip.82 
D

ain in the spine and 103.3% in the total hip.82 
Denosumab has also been approved for the treatment of women 
with breast cancer and skeletal metastases and is the only agent 
shown to reduce fractures in men undergoing androgen depriva­
tion therapy for prostate cancer.83,84
The osteoblast functions not only to signal osteoclasts during 
remodeling but also to lay down collagen and orchestrate mineral­
ization of previously resorbed lacunae in the skeletal matrix. These 
complex functions are t

he skeletal matrix. These 
complex functions are tied to differentiation of mesenchymal stro­
mal cells and bone lining cells, which become osteoblasts and rest 
on the surface of the remodeling space.85 Recruitment of stromal 
cells into osteoblasts, rather than adipocytes, is a critical step in bone 
formation and requires a series of factors that enhance differentia­
tion (see Fig. 30.8). One of the most important drivers of this pro­
cess is RUNX2, a transcription factor that is essential fo

RUNX2, a transcription factor that is essential for the early 
differentiative pathway of stromal cells toward bone and away from 
adipogenesis.86,87 Regulation of RUNX2 and its downstream effects 
has become a major focus of work as investigators have begun to 
consider novel ways to enhance bone formation and reduce marrow 
adipogenesis. However, within that context, it is clear that pre-adi­
pocytes or pre-osteoblasts may have significant plasticity such that 
the older paradigm illustrated i

ticity such that 
the older paradigm illustrated in Fig. 30.8 may not truly represent 
the status of mesenchymal cells within the marrow niche.
During activation of resting osteoblasts, the mature cells syn­
thesize collagen I; several minor collagens; the noncollagenous 
matrix proteins; and a series of growth factors, such as IGF1, IGF2, 
and TGFβ. These, in turn, are necessary for further recruitment 
of bone-forming cells.88 In addition, osteoblasts deposit growth 
factors in the skeletal ma

oblasts deposit growth 
factors in the skeletal matrix, where they are stored in latent forms 
(e.g., TGFβ, IGF1, IGF2) and released during subsequent remod­
eling cycles. Osteoblast fate is determined within the remodeling 
sequence by several different systemic and local factors. Osteo­
blasts can further differentiate into osteocytes, become quiescent 
lining cells on the bone surface, or die through apoptosis.85 As 
noted earlier, osteocytes may participate in bone resorption of the 
cortex 

may participate in bone resorption of the 
cortex by secreting RANKL to stimulate osteoclastogenesis and 
can also participate in osteocytic osteolysis, which may occur dur­
ing lactation and other states of high calcium demand.89
The Wnt/β-catenin signaling pathway has emerged as a major 
regulator of bone formation and a potential mediator of the mech­
anostat. Wnts belong to a large family of proteins that bind to 
frizzled receptors and activate multiple pathways within the cell. 
When Wnts 

ate multiple pathways within the cell. 
When Wnts also bind to several coreceptors LRP5, LRP6, and 
probably LRP4, a so-called canonical signaling pathway is trig­
gered.90 Activation of this pathway occurs through a complex 
intracellular signaling pathway mediated by β-catenin, which 
translocates to the nucleus and stimulates the transcription of 
genes through cooperative interactions with TCF/Lef transcrip­
tion factors. Sclerostin (the product of the SOST gene), the osteo­
cyte-specific pr

uct of the SOST gene), the osteo­
cyte-specific protein that inhibits bone formation and another 
important Wnt inhibitor, dickkopf 1 (Dkk1), work by binding to 
LRP5 and LRP6, thereby blocking Wnt signaling.91 
Local Regulators of Remodeling
Characterization of local regulators produced within the bone 
itself represents a major advance in bone biology.92,93 These local 
factors can be synthesized by bone cells or by adjacent hemato­
poietic cells and can interact with each other and with syste

ls and can interact with each other and with systemic 
hormones. They are critical in the repair of skeletal damage and in 
the response to mechanical forces.
Cytokines
The proinflammatory cytokines IL1α, IL1β, TNFα, and TNFβ 
are potent stimulators of bone resorption and inhibitors of bone 
formation; thus, they may play a role in the bone loss after estro­
gen withdrawal.79 IL6 increases osteoclastogenesis in cell cultures 
and may mediate some of the resorbing activity of PTH. IL6 is 
synthes

 of the resorbing activity of PTH. IL6 is 
synthesized by osteoblasts and immune cells, and its production is 
stimulated by PTH,94 PGE2, and other factors that increase bone 
resorption. There is also evidence suggesting that IL6 may modu­
late osteocyte communication toward osteoclasts.95
IL11, another member of the IL6 cytokine family, also stimu­
lates resorption. LIF, also a member of the IL6 family, increases 
proliferation of osteoblast precursors but then slows their sub­
sequent differe

ecursors but then slows their sub­
sequent differentiation into mineralizing osteoblasts.96 Inflam­
matory cytokines such as IL7, IL15, and IL17 stimulate bone 
resorption, whereas IL4, IL10, IL13, and IL18 inhibit bone 
resorption.51,97–99 Interferon-β and interferon-γ inhibit resorp­
tion by blocking RANK signaling pathways.100 IL10 is an inhibi­
tor of osteoclastogenesis and bone resorption.101 IL15 and IL17 
stimulate resorption, whereas IL18 is inhibitory through its 
ability to increase pr

 is inhibitory through its 
ability to increase production of granulocyte-macrophage CSF 
(GM-CSF).102 Interferon-β and interferon-γ inhibit resorption 
by blocking RANK signaling pathways.100 In addition to direct 
effects, responses to cytokines can be blocked by inhibitors such as 
the IL1 receptor antagonist and the soluble TNF receptor, or they 
can be enhanced by activators such as the soluble IL6 receptor. 
TGFα and Epidermal Growth Factor
These peptides stimulate bone resorption through 

These peptides stimulate bone resorption through the same 
receptor and act by prostaglandin-dependent and -independent 
pathways. TGFα and epidermal growth factor (EGF) are potent 
mitogens in bone that probably act on mesenchymal and hema­
topoietic precursors.103,104 The EGF family of peptides regulates 
cell migration and adhesion, playing an important role in the 
early recruitment of osteoblast progenitors to the remodeling 
site. Signaling in pre-osteoblasts by the EGFs is via the Cdc42/

g in pre-osteoblasts by the EGFs is via the Cdc42/
Rac network.105 Studies in vitro, using immortalized and primary 
bone marrow stromal cells, reported that EGF receptor signal­
ing enhances mechanotransduction, suggesting that the EGF 
system may act as a mechanosensitizer in bone marrow stromal 
cells.106 TGFα is produced by neoplasms and may play a role in 
the increased bone resorption that occurs in certain malignancies. 
1272

  Mineral Metabolism
Prostaglandins are potent regulators of bone cell metabolism and 
are synthesized by many cell types in the skeleton.107 Prostaglan­
din production in bone is regulated by effects of local and systemic 
hormones and mechanical forces on the inducible cyclooxygen­
ase 2. Increased prostaglandin production may contribute to the 
increase in bone resorption that occurs with immobilization, the 
enhanced bone formation seen with impact loading, and the bone 
loss 

tion seen with impact loading, and the bone 
loss after estrogen withdrawal. Many of the hormones, cytokines, 
and growth factors that stimulate bone resorption also increase 
prostaglandin production. Prostaglandins have biphasic effects on 
bone formation. Stimulation of bone formation is seen in vivo, 
and inhibition of collagen synthesis occurs in osteoblast cultures. 
Bone cells produce PGE2, PGF2α, prostacyclin, and lipoxygenase 
products (e.g., leukotriene B4), which may also stimulate bo

e.g., leukotriene B4), which may also stimulate bone 
resorption. 
Peptide Growth Factors
Skeletal cells synthesize a variety of growth factors that regulate 
the replication, differentiation, and function of bone cells. These 
growth factors are not synthesized only by skeletal cells, and some 
are present in the systemic circulation and can act as local and sys­
temic regulators of bone remodeling. Skeletal cells also synthesize 
growth factor binding proteins, which regulate the activity and 

binding proteins, which regulate the activity and 
storage of specific factors and their interactions with other pro­
teins in the extracellular matrix.92
Fibroblast Growth Factors
FGFs form a large family of polypeptides characterized by their 
affinity to glycosaminoglycan heparin-binding sites.108 FGF1 
and FGF2, which have been studied extensively, have mitogenic 
properties for cells of the osteoblastic lineage, although eventual 
differentiation into mature osteoblasts does not occur in th

ation into mature osteoblasts does not occur in the pres­
ence of the FGF family of peptides.109 In fact, FGF2 inhibits Wnt 
signaling and the synthesis of IGF1, which results in a decrease 
in osteoblastogenesis and in osteoblastic function.110,111 In vivo 
experiments have confirmed this action of FGF, and mice over­
expressing FGF2 are osteopenic.112 However, studies in Fgf2 null 
mice indicate that FGF is necessary for osteoblast formation, 
possibly because of its early effects on cell repl

possibly because of its early effects on cell replication.113 FGF 
can stimulate bone resorption by prostaglandin-dependent and 
-independent pathways.114 The FGFs bind to a series of four FGF 
receptors that have intrinsic tyrosine kinase activity. In vertebrates, 
the 22 members of the FGF family range in molecular mass from 
17 to 34 kDa and share 13% to 71% amino acid identity. Among 
vertebrate species, FGFs are highly conserved in both gene struc­
ture and amino acid sequence. FGFs have a 

 struc­
ture and amino acid sequence. FGFs have a high affinity for hepa­
ran sulfate proteoglycans and require heparan sulfate to activate 
one of four cell-surface FGF receptors. During embryonic devel­
opment, FGFs have diverse roles in regulating cell proliferation, 
migration, and differentiation. In the adult organism, FGFs are 
homeostatic factors and function in tissue repair and response to 
injury. When inappropriately expressed, some FGFs can contrib­
ute to the pathogenesis of cancer

GFs can contrib­
ute to the pathogenesis of cancer.
FGF19, FGF21, and FGF23 are unique in that they bind to 
FGF receptors that heterodimerize with α- or β-Klotho. FGF23 is 
secreted by osteocytes and has been shown to regulate phosphate 
homeostasis by inhibiting vitamin D 1α-hydroxylase activity in 
the kidney, thereby suppressing the production of 1,25(OH)2D. 
In addition, FGF23 promotes phosphate loss in the renal tubule, 
acting as an endocrine hormone produced by bone cells. Disorders 
in 

ine hormone produced by bone cells. Disorders 
in FGF23, including excess production or impaired degradation, 
result in several syndromes that are characterized by hypophospha­
temia and rickets/osteomalacia. FGF21 is produced in the liver 
and in adipocytes. It is an important counterregulatory hormone 
that stimulates PPARα and oxidation of fatty acids.115 FGF21 is 
high during calorie restriction, malnutrition, and lactation. It can 
potentially cause browning of white adipocytes, but in mic

lly cause browning of white adipocytes, but in mice it has 
been shown to stimulate bone resorption. Recently, FGF21 has 
been shown to induce significant bone resorption, and the FGF21 
null mice have very high bone mass.116 Increased production of 
IGFBP1 may mediate bone resorption, at least in part, by increas­
ing the production of FGF21.117,118 
Platelet-Derived Growth Factors, Vascular Endothelial 
Growth Factors, Hypoxia-Inducible Factors, and 
Reactive Oxygen Species
Platelet-derived gr

, and 
Platelet-derived growth factor (PDGF) was originally isolated 
from human platelets, and four members of the PDGF gene family 
have been identified: PDGFA, PDGFB, PDGFC, and PDGFD.119 
These peptides signal through the PDGFR family of receptors. 
Vascular endothelial growth factor (VEGF) shares a high degree 
of sequence homology with PDGF, and these factors are often 
referred to as members of the PDGF/VEGF family.120
PDGFs must form homodimers or heterodimers to 

120
PDGFs must form homodimers or heterodimers to exhibit 
activity. PDGF-AA, -AB, and -BB are the isoforms studied more 
extensively in skeletal cells, and they exert similar biologic actions. 
The primary function of PDGF in bone is the stimulation of cell 
replication, and PDGF impairs osteoblast differentiation and 
function.121 PDGF also stimulates bone resorption. In mice, null 
mutations of Pdfga or Pdfgb and their receptors cause embryonic 
lethality or perinatal death, not allowing the 

c 
lethality or perinatal death, not allowing the study of the func­
tion of PDGF in the postnatal skeleton.122 Although skeletal 
cells express products of the Pdfga, Pdfgb, and Pdfgc genes, the 
major source of PDGF is the systemic circulation, and skeletal 
cells become exposed to PDGF after platelet aggregation. PDGF 
receptor 1a (PDGFR1a) is expressed on numerous progenitor 
cells in the hematopoietic niche, including early osteoblasts and 
adipocytes.
VEGFA is essential for angiogenesis, a

VEGFA is essential for angiogenesis, and VEGFA and VEGF 
receptor genes are expressed by chondrocytes and osteoblasts.123 
VEGFA is essential for blood vessel formation and vessel invasion 
into cartilage during the process of endochondral bone forma­
tion and for chondrocyte survival during skeletal development.124 
Importantly, VEGFA is also required for intramembranous bone 
formation and osteoblastic maturation.125 Osteoblast expression 
of PDGF and VEGF is regulated by other loc

ession 
of PDGF and VEGF is regulated by other locally derived growth 
factors.
Hypoxia-inducible factors (HIFs) are produced locally in 
response to low oxygen tension in the hematopoietic niche and 
are transcription factors that mediate pathways that lead to angio­
genesis, including production of VEGF. HIFs also induce genes 
essential for glycolysis, an essential biochemical pathway for ATP 
generation in the niche. During fracture repair, it has been dem­
onstrated that more bone is produc

t has been dem­
onstrated that more bone is produced in a mouse model with con­
stitutively active HIF1α in osteoblasts. Conversely, mice deficient 
in HIF1α in osteoblasts had defective bone healing.126
Reactive oxygen species (ROS), which are free radicals that 
include peroxides, are products of cell metabolism, particularly 
oxidative phosphorylation. These products are inactivated by sev­
eral enzymes that serve to prevent mitochondrial and cell damage. 
In several mouse models of aging, ch

cell damage. 
In several mouse models of aging, changes that occur in bone turn­
over and cell metabolism were associated with increases in ROS in 
bone cells.127 Suppressing ROS through antioxidant therapy can 
1273
reverse some of the effects on bone from the loss of sex steroids.128 
However, ROS generation may be important to maintain a certain 
level of metabolic activity, and differences in ROS production may 
be critical to determining 

in ROS production may 
be critical to determining the fate switch between osteoblasts and 
adipocytes. 
IGFs are mitogens that also can increase the differentiated func­
tion of the osteoblast, supporting mineralization and bone for­
mation.129 Animal models have demonstrated that both systemic 
and locally synthesized IGF1 contribute to bone formation.130,131 
Transgenic mice overexpressing IGF1 have increased bone mass, 
whereas Igf1 null mice exhibit decreased bone

ss, 
whereas Igf1 null mice exhibit decreased bone formation, reduced 
mineralization, and decreased cortical bone.132 IGF1 stimulates 
bone turnover by increasing osteoclastogenesis and bone remodel­
ing.133 Both IGF1 and IGF2 are synthesized by bone cells and are 
stored in the bone matrix, but IGF1 is a more potent stimulator 
of osteoblastic function.129 Six IGF-binding proteins have been 
identified in bone and in the circulation (e.g., IGFBP1 through 
IGFBP6). IGFBPs can inhibit or enhance

P1 through 
IGFBP6). IGFBPs can inhibit or enhance IGF responses, depend­
ing on their local concentration relative to IGFs and the presence 
of proteases that can cleave the IGFBPs. IGFBP2 has been shown 
to be synergistic in its actions on osteoblasts with IGF1 via signal­
ing through the pleiotropic receptor that phosphorylates PTEN 
(phosphatase and tensin homolog).134 PTH and PGE2 are major 
inducers of skeletal IGF1 and IGFBP2 synthesis, and glucocor­
ticoids suppress IGF1 transcription.12

d glucocor­
ticoids suppress IGF1 transcription.129 Thus, IGFs can mediate 
selected effects of these hormones on bone formation. 
Transforming Growth Factor β
TGFβ belongs to a family of closely related polypeptides with 
various degrees of structural homology and important effects on 
cell function that are only expressed in mammals.135 Skeletal cells 
express TGFβ1, TGFβ2, and TGFβ3. TGFβ has complex and 
somewhat contradictory actions in bone cells. TGFβ can stimu­
late osteoblastic cell rep

 cells. TGFβ can stimu­
late osteoblastic cell replication and bone formation, but it does 
not favor osteoblastic differentiation.136,137 The effects of TGFβ 
depend on the target cell and experimental conditions. The actions 
of TGFβ on bone resorption have been a source of controversy. 
TGFβ has a biphasic effect on osteoclastogenesis, but it decreases 
bone resorption.138 Targeted disruption of the mouse Tgfb1 gene 
is lethal but does not result in abnormal skeletal development.139 
TGFβ is 

ult in abnormal skeletal development.139 
TGFβ is secreted as a latent high-molecular-weight complex con­
sisting of the C-terminal remnant of the TGFβ precursor and a 
TGFβ-binding protein.140 The biologically active levels of TGFβ 
depend on changes in its synthesis and in its activation from its 
latent form. 
BMPs and Wnt Proteins
BMPs are members of the TGFβ superfamily of polypeptides, and 
they were originally identified because of their ability to induce 
endochondral bone formation. BMP

bility to induce 
endochondral bone formation. BMPs are expressed by osteoblasts 
and play an autocrine role in osteoblastic differentiation and func­
tion.92 The fundamental function of BMPs is the induction of 
osteoblastic cell differentiation, endochondral ossification, and 
chondrogenesis.92,141 The genesis and differentiation of osteo­
blasts and osteoclasts are coordinated events, and BMPs also 
induce osteoclastogenesis and osteoclast survival.142
BMP activity is regulated by a large gro

vival.142
BMP activity is regulated by a large group of secreted polypep­
tides that bind and limit BMP action. These extracellular BMP 
antagonists prevent BMP signaling. Extracellular BMP antagonists 
include noggin, follistatin, myostatin, twisted gastrulation, the 
chordin family, and the Dan/Cerberus family of proteins.92 These 
molecules can also bind to one or two activin receptors, ACVR2A 
and ACVR2B. Myostatin, a TGFβ superfamily member, is a nega­
tive regulator of muscle growth acting

 is a nega­
tive regulator of muscle growth acting through ACVR2B. Inhibi­
tors of these signaling peptides and their receptors may result in 
enhanced muscle mass and in some cases increased bone mass.143
The Wnt family of secreted cysteine-rich glycoproteins, like 
BMPs, plays a critical role in directing osteoblastogenesis. In skel­
etal cells, many Wnt family members use the canonical Wnt/β-
catenin signaling pathway.144 In the absence of Wnt, the proteins 
axin, adenomatous polyposis coli (

, the proteins 
axin, adenomatous polyposis coli (APC), and β-catenin form a 
complex that facilitates the phosphorylation and degradation of 
β-catenin. Binding of Wnt family proteins to specific frizzled 
membrane receptors and their coreceptors (LRP5 and LRP6) 
leads to the stabilization of β-catenin. This allows β-catenin to be 
translocated to the nucleus, where it can regulate the transcription 
of target genes. The Wnt/β-catenin signaling pathway is central 
to osteoblastogenesis and bone

pathway is central 
to osteoblastogenesis and bone formation, and Wnt and BMPs 
act in concert to regulate cell differentiation. Deletions of Wnt 
or β-catenin genes result in the absence of osteogenesis and of 
skeletal tissue, and inactivating mutations of Wnt coreceptors 
result in osteopenia.145 For instance, either the loss-of-function 
or gain-of-function mutation in LRP5 results in skeletal diseases 
respectively characterized by either low bone mass (osteoporosis 
pseudoglioma)146 or hig

w bone mass (osteoporosis 
pseudoglioma)146 or high bone mass.147 Wnt/β-catenin signaling 
induces OPG, and through this mechanism, Wnts are negative 
regulators of osteoclastogenesis.148,149 Wnt activity, like that of 
BMPs, is controlled by extracellular antagonists and intracellular 
signaling proteins.93 Extracellular antagonists such as sclerostin 
and Dkk1 prevent interactions between Wnt family members and 
coreceptors, although others, like soluble frizzled-related proteins, 
bind Wnts d

ke soluble frizzled-related proteins, 
bind Wnts directly and block their actions. These antagonists 
limit Wnt signaling, which decreases osteoblast function and bone 
mass.144,150 Conversely, deletion of sclerostin in mice produces a 
high bone mass phenotype.151 Sclerostin, made predominantly by 
osteocytes, is regulated by systemic factors such as PTH and by 
mechanical forces on bone. Precisely how circulating versus local 
sclerostin regulates bone formation and resorption is an important 

tes bone formation and resorption is an important 
unanswered question, particularly because circulating sclerostin 
levels have been shown to have a negative or positive relation­
ship to bone mass and fractures in normal individuals and patients 
with type 2 diabetes mellitus (T2DM).152,153 
Systemic Hormones and Bone Remodeling
Remodeling is activated by both systemic and local factors. 
Changes in mechanical force can activate remodeling to improve 
skeletal strength, and remodeling removes 

mprove 
skeletal strength, and remodeling removes and repairs bone that 
has undergone microdamage. This occurs particularly in cortical 
bone and may explain the fact that remodeling is sustained in the 
aging skeleton.154 However, loss of osteocytes with age may impair 
this response.155 Systemic hormones influence bone remodeling 
to regulate the movement of mineral from bone to the extracel­
lular fluid to maintain serum calcium levels and to sustain linear 
growth. During pubertal growth, b

 sustain linear 
growth. During pubertal growth, bone modeling and remodel­
ing intensify and correlate with serum levels of IGF1, support­
ing the role of growth hormone as a major mediator of skeletal 
formation.156 In addition to the growth hormone/IGF1 axis, it 
is now apparent that testosterone and estradiol must be present 
during peak acquisition to attain the highest possible BMD. Min­
eralization of newly formed bone is assured by activation of vita­
min D 1α-hydroxylase in the kidney l

tion of vita­
min D 1α-hydroxylase in the kidney leading to greater calcium 
1274
absorption and higher 1,25(OH)2D levels. Hence, modeling and 
epiphyseal closure are both modulated by systemic hormones that 
are both calcium regulating and growth centric.
Calcium-Regulating Hormones
PTH acts on bone to stimulate resorption but does not act on 
osteoclasts in the absence of cells of the osteoblastic lineage. PTH 
receptors are abundant on ost

lastic lineage. PTH 
receptors are abundant on osteoblasts, bone lining cells, and 
osteocytes, but not on osteoclasts.157 PTH acts on osteoblasts to 
cause cell contraction; to induce immediate-early response genes, 
including c-Fos and the inducible form of prostaglandin G/H 
synthase (i.e., cyclooxygenase), and to increase the synthesis of 
local mediators, IGF1 and IL6.157,158 High concentrations of 
PTH in vitro inhibit the expression of type I collagen, but inter­
mittent administration of

e I collagen, but inter­
mittent administration of PTH in vivo or in vitro can stimulate 
bone formation.159 PTH induces the production of RANKL and 
inhibits the production of OPG by cells of the osteoblast lineage, 
thereby increasing osteoclastogenesis and thus bone resorption. 
In some circumstances, PTH increases proliferation of cells of 
the osteoblast lineage and decreases their death by apoptosis.157 
PTH also induces 1α-hydroxylase activity in the kidney, thereby 
enhancing the active 

vity in the kidney, thereby 
enhancing the active form of vitamin D (see later discussion). 
In osteocytes, PTH not only stimulates production of RANKL 
but also inhibits production of sclerostin and increases syn­
thesis of FGF23. In this way, PTH uses osteocytes to regulate 
bone remodeling and systemic calcium/phosphate homeostasis. 
Genetic deletion of the PTH/PTHrP receptor (PTH1R) gene 
in mesenchymal stem cells of mice has a great impact on bone 
phenotypes (i.e., low bone formation, incr

n bone 
phenotypes (i.e., low bone formation, increased bone resorption, 
and high bone marrow adipose tissue [MAT]).160 In addition, 
the same study showed that intermittent PTH administration 
to control group mice reduced bone MAT significantly.160 These 
results suggest that PTH also drives progenitor cells differentia­
tion into osteoblasts. 
Vitamin D
The hormonal form of vitamin D, 1,25(OH)2D3, is necessary 
for intestinal calcium and phosphorus absorption and therefore 
for mineralizatio

phorus absorption and therefore 
for mineralization. This form of vitamin D also has effects on 
the skeleton, but its physiologic role in bone remodeling is not 
clear.161 By increasing RANKL production on osteoblasts or 
osteoblast progenitor cells, vitamin D is a potent stimulator of 
osteoclast formation in cell culture. High concentrations increase 
osteocalcin synthesis by osteoblasts and inhibit collagen synthe­
sis and mineralization in  vitro.162 Lower concentrations may 
increase bone 

vitro.162 Lower concentrations may 
increase bone formation, although not to the extent seen with 
intermittent administration of PTH. Recent studies using con­
ditional deletions of the vitamin D receptor (VDR) in bone and 
intestine provide further insight into the physiologic role of vita­
min D on the bone remodeling unit. Intestinal deletion of the 
VDR mimics a low-calcium diet and is associated with bone loss 
related to increased bone resorption and decreased bone forma­
tion. However, s

orption and decreased bone forma­
tion. However, serum calcium is preserved.162 But in the osteo­
cyte and mature osteoblast-specific Vdr null mice, there is no 
alteration in circulatory levels of calcium, phosphorus, PTH, and 
1,25(OH)2D, as well as no bone phenotype. Moreover, mice miss­
ing the VDR only in the intestine have a delay in mineralization 
through the actions of high levels of 1,25(OH)2D3 on osteoblas­
tic cells.162 These data provide a strong mechanistic rationale for 
active vi

vide a strong mechanistic rationale for 
active vitamin D action on remodeling: high 1,25(OH)2D levels 
due to increased 1,25(OH)2D bind to the VDR on osteoblasts 
and stimulate RANKL production leading to increased bone 
resorption as a way to protect the body against low serum cal­
cium. It further slows entry of calcium into the skeleton by sup­
pressing mineralization in vitro and in vivo, thereby preserving 
essential functions of the mammal. 
Calcitonin inhibits bone resorption 

. 
Calcitonin inhibits bone resorption by acting directly on the 
osteoclast, but it appears to play a smaller role in the regulation 
of bone turnover in adults.163 Bone mass is not greatly altered 
in patients with medullary thyroid carcinoma, who have an 
excess of calcitonin production, or in athyreotic patients receiv­
ing adequate thyroid hormone replacement, who have low cal­
citonin levels.164,165 Bone turnover is increased in patients with 
medullary thyroid carcinoma.166 Mic

medullary thyroid carcinoma.166 Mice with a deletion of the gene 
for calcitonin/calcitonin-related polypeptide α (CALCA), which 
is responsible for the production of calcitonin and its alternate 
transcript calcitonin gene–related peptide, have increased bone 
mass and enhanced rates of bone formation.167 In contrast, mice 
with a deletion of only the calcitonin gene–related peptide have 
decreased bone mass.168 These results imply that calcitonin influ­
ences both bone formation

y that calcitonin influ­
ences both bone formation and bone resorption. However, the 
mechanisms by which calcitonin affects bone formation remain 
unknown.169 
Other Systemic Hormones That Influence Remodeling
Deficiency and excess of growth hormone have marked effects 
on skeletal growth, as noted previously.170,171 Growth hormone 
increases circulating and local levels of IGF1, which mediates 
many of the skeletal effects of growth hormone. Exogenous 
growth hormone and IGF1 in

wth hormone. Exogenous 
growth hormone and IGF1 increase bone remodeling presumably 
by directly acting on osteoblasts, although IGF1 is also present 
on osteoclasts, and recombinant IGF1 stimulates bone resorption. 
Osteocytes also possess IGF1,172 and one can speculate that it 
might play an important role in regulating phosphate homeostasis 
via FGF23 during critical growth phases. Growth hormone stimu­
lates cartilage growth, probably through an increase in local and 
systemic IGF1 productio

 an increase in local and 
systemic IGF1 production and possibly by direct stimulation of 
cartilage cell proliferation because low levels of growth hormone 
receptors are present in chondrocytes. 
Glucocorticoids exert profound effects on bone remodeling.173 
Glucocorticoids decrease the intestinal absorption of calcium and 
have the potential to induce osteoclastogenesis and bone resorp­
tion because they increase the expression of RANKL and CSF1 in 
osteoblasts.174 However, th

 of RANKL and CSF1 in 
osteoblasts.174 However, the most significant effect of glucocorti­
coids is their ability to suppress bone remodeling through deple­
tion of the osteoblastic cell population.175,176 Glucocorticoids 
inhibit the replication of osteoblast precursors and their differen­
tiation into mature osteoblasts. This effect occurs in part because 
they suppress Wnt signaling and factors necessary for osteoblastic 
differentiation.177 In a recent study, it was observed in humans 
that 

n a recent study, it was observed in humans 
that glucocorticoids have diverse effects in the serum levels of two 
Wnt signaling antagonists: a progressive decrease in Dkk1 with 
a significant increase in circulatory levels of sclerostin.178 Gluco­
corticoids induce the apoptosis of osteoblasts and osteocytes, con­
tributing to the decrease in bone-forming cells.179 Glucocorticoids 
inhibit the differentiated function of the osteoblast and bone for­
mation. This results from direct effects of gl

or­
mation. This results from direct effects of glucocorticoids on the 
osteoblast and suppression of IGF1 transcription.180 
1275
Thyroid hormone signaling is mediated by thyroid hormone 
receptor (TR), a ligand-dependent transcription regulator mol­
ecule. TR is encoded by the thyroid hormone receptor α (THRA 
or c-erbAα) and β (THRB or c-erbAβ) genes, which have different 
isoforms for which distribution is tissue- and age 

soforms for which distribution is tissue- and age dependent.181,182 
The TRβ1 and TRα1 isoforms are expressed in bone, but previous 
studies suggested that TRα1 is the major mediator of thyroid hor­
mone effects in the skeleton.183 However, the net effect of thyroid 
hormone is complex and depends on the circumstances.184,185 In 
children, hyperthyroidism is associated with increased bone min­
eralization and epiphyseal maturation, and hypothyroidism results 
in decreased growth.186 But in adult

dism results 
in decreased growth.186 But in adults, hyperthyroidism is associ­
ated with bone loss. Thyroid hormones are crucial for cartilage 
growth and differentiation and enhance the response to growth 
hormone. Thyroid hormones increase bone resorption and turn­
over, although their effects on bone formation are less clear.187 
Coupled with their effects on bone resorption, thyroid hormones 
increase the transcription of collagenase and gelatinase by osteo­
blasts.188 As thyroid hormones i

atinase by osteo­
blasts.188 As thyroid hormones increase bone remodeling, thyrox­
ine (T4) may also increase bone formation. Thyroid hormones also 
have indirect effects on skeletal metabolism by suppressing the 
synthesis of thyroid-stimulating hormone (or thyrotropin), which 
can inhibit osteoclast formation and survival and as a consequence 
suppressed bone resorption.187,189,190 However, it is still debated 
whether thyroid-stimulating hormone (or thyrotropin) has direct 
effects on osteobl

ne (or thyrotropin) has direct 
effects on osteoblasts and osteoclasts. 
Insulin
Normal skeletal growth depends on an adequate amount of insu­
lin.191 Excess insulin production by the fetuses of mothers with 
uncontrolled diabetes results in excessive growth of the skeleton 
and other tissues, possibly through its actions on IGF1. Poorly 
controlled diabetes mellitus leads to impaired skeletal growth and 
mineralization. Children and adolescents with type 1 diabetes 
mellitus (T1DM) are at incre

with type 1 diabetes 
mellitus (T1DM) are at increased risk for decreased bone min­
eral acquisition and accumulation of AGEs.192 T2DM is asso­
ciated with normal bone mass but increased skeletal fragility in 
part due to enhanced cortical porosity and increased AGEs in the 
matrix. In vitro, insulin at physiologic concentrations selectively 
stimulates osteoblastic collagen synthesis by a pretranslational 
mechanism. Insulin can mimic the effects of IGF1, although 
only at supraphysiologic leve

s of IGF1, although 
only at supraphysiologic levels.109 Mice deficient in insulin recep­
tor substrate 1, a major substrate of insulin and IGF1 receptor 
tyrosine kinases, exhibit impaired differentiative osteoblast func­
tion, low-turnover osteopenia, and an impaired response to PTH, 
documenting the central role of insulin and IGF1 signaling in the 
maintenance of bone remodeling.193 More recently, insulin has 
been shown to stimulate osteoblastic function and bone resorp­
tion leading to gre

stic function and bone resorp­
tion leading to greater release of GLU13-OCN (see Fig. 30.5), 
which in turn causes greater insulin sensitivity and enhanced 
insulin synthesis in the islet cells. The effect of insulin on glucose 
transport in osteoblasts is still controversial, although deletion of 
the insulin receptor in osteoblasts, using the osteocalcin Cre pro­
moter, results in low bone mass, obesity, and insulin resistance. 
In humans, the ultimate effect of insulin resistance on bone stil

ultimate effect of insulin resistance on bone still 
is to be elucidated, but there is much clinical evidence suggest­
ing that it does not have detrimental effects on bone mass. Both 
pandemic (i.e., obesity) and rare (i.e., generalized congenital lipo­
dystrophy) conditions linked to insulin resistance are associated 
with high bone mass.194,195 In addition, there is no consensus in 
the literature about the association between visceral adipose tissue 
and bone mass.196 In commonality, insulin

 tissue 
and bone mass.196 In commonality, insulin deficiency and insulin 
resistance are associated with excessive formation of AGEs, which 
are involved in the development of classic diabetic micro- and 
macroangiopathy. AGEs may also impact bone microarchitecture 
through effects on collagen cross-linking, major post-translational 
modifications of collagen that influence bone strength. Collagen 
cross-links can be formed by lysyl hydroxylase and lysyl oxidase–
mediated enzymatic immature div

and lysyl oxidase–
mediated enzymatic immature divalent cross-links and mature 
trivalent pyridinoline and pyrrole cross-links, and cross-links can 
also be produced by glycation- or oxidation-induced nonenzy­
matic cross-links (AGEs), such as glucosepane and pentosidine. 
The last may impair mineralization and the capacity of bone to 
repair itself from microdamage. This mechanism can be a com­
mon link to bone fragility, which is common in both T1DM and 
T2DM patients.197,198 
Gonadal Hormones

 T1DM and 
Estrogens and androgens are critical for skeletal development and 
maintenance. Bone cells contain estrogen and androgen receptors, 
but it has been difficult to demonstrate direct effects of gonadal 
steroids on bone formation or resorption in cell and organ cul­
ture. Gonadal hormones are crucial for the pubertal growth spurt, 
and estrogen is necessary for epiphyseal closure.199 In addition, 
gonadal steroids contribute to skeletal gender dif

gonadal steroids contribute to skeletal gender differences. Men 
exhibit larger bones due to androgen stimulation of periosteal for­
mation, whereas estrogen has an inhibitory effect.200 Skeletal gen­
der dimorphism impacts on bone biomechanical properties and, 
at least in part, explains the higher incidence of osteoporotic frac­
ture in women.201 Deficiency of estrogen or androgen increases 
bone resorption in vivo, partly by increasing the local synthesis or 
sensitivity to cytokines, such as

al synthesis or 
sensitivity to cytokines, such as IL1 and IL6 or TNFα; to pros­
taglandins; and through direct actions on estrogen receptors in 
osteoclasts. Androgens can increase bone formation in  vivo.202 
The effect of estrogens on bone formation is less clear, depending 
on the animal model and the dose of estrogen. The absolute rate of 
bone formation is increased in estrogen deficiency states because 
of an increase in bone remodeling. However, estrogen deficiency 
causes bone loss, imp

owever, estrogen deficiency 
causes bone loss, implying a relative deficiency in bone formation. 
In other words, in the condition of hypoestrogenism, the increase 
in bone formation is not of the same magnitude as the enhance­
ment in bone resorption. 
Epidemiology of Osteoporosis and Fractures
Over the past 25 years, our understanding of fractures and their 
causes has significantly evolved and along with it, the definition of 
the disease osteoporosis. The current definition of osteoporosis w

oporosis. The current definition of osteoporosis was 
developed by an international consensus development committee 
in 1993 as “a disease characterized by low bone mass and micro­
architectural deterioration of bone tissue, leading to enhanced 
bone fragility and a consequent increase in fracture risk.”203 This 
definition reflects our understanding of the link between low 
bone quantity, compromised bone quality, and increased fracture 
risk. Although hip, vertebral, and wrist fractures are mo

lthough hip, vertebral, and wrist fractures are most com­
monly associated with osteoporosis, risk of other fractures is also 
increased.
Fractures
Hip Fractures
Proximal femur fractures are a major cause of morbidity and fatal­
ity and occur much more frequently in older people, increasing 
1276
the costs of osteoporosis. Most commonly, these fractures occur 
in the femoral neck or intertrochanteric regions and require sur­
gical repair. Increased risk of falls

require sur­
gical repair. Increased risk of falls together with decreases in bone 
strength account for the increased risk with increasing age. Morbid­
ity and mortality rates associated with hip fractures are substantial. 
The mortality rate within 1 year of the fracture is between 5% and 
20%.204 They are a major source of disability and loss of indepen­
dence. For example, it is estimated that of those living indepen­
dently before a hip fracture, only 50% are able to do so 1 year after 
the

ture, only 50% are able to do so 1 year after 
the hip fracture.205 After a hip fracture, there is also a substantial risk 
of other fractures, including a second hip fracture, and it is therefore 
important that further assessment and possible treatment be con­
sidered in patients with fractures at the hip (and other sites).206,207 
Vertebral Fractures
Vertebral fractures are the most common manifestation of osteo­
porosis.208 These fractures occur in a range of severities. Mild 
fractures, onl

cur in a range of severities. Mild 
fractures, only some of which may be symptomatic, may only be 
apparent via radiographic or other types of imaging but are still, 
in aggregate, associated with clinical symptoms.209,210 However, 
severe vertebral fractures can be accompanied by significant acute 
back pain that may persist for many weeks or indefinitely. Long-
term consequences of vertebral fractures, particularly accumulation 
of multiple vertebral fractures, are substantial, leading to heig

tebral fractures, are substantial, leading to height 
loss, kyphosis, increased disability, decreased pulmonary func­
tion, substantial chronic back pain, and diminished overall quality 
of life.211 There is also increased fatality associated with vertebral 
fractures.212 Severity of these consequences increases with increas­
ing severity and number of these fractures. Although they can be 
caused by trauma, they are often associated with little or no trauma.
Vertebral fractures have often been 

or no trauma.
Vertebral fractures have often been referred to as the hallmarks 
of osteoporosis and tend to occur at younger ages than other frac­
tures. Vertebral fractures increase the future risk of additional ver­
tebral fractures by 5 to 10 times and are associated with a much 
higher risk of nonvertebral fractures, including hip fractures.213,214 
Therefore, like hip fractures, they should lead to further assessment 
(e.g., dual-energy x-ray absorptiometry [DXA]) and consideration 
of oste

y absorptiometry [DXA]) and consideration 
of osteoporosis prevention and treatment. This is particularly true 
of incidental findings noted on chest and lumbar radiographs for 
other reasons. Often, vertebral fractures reported on radiographs 
are asymptomatic, and a history of trauma cannot be ascertained. 
This does not diminish the importance of the fracture with respect 
to future risk or associated morbidity. 
Wrist Fractures
Fractures of the distal radius are common in postmenopausal 
wom

he distal radius are common in postmenopausal 
women and are associated with osteoporosis. However, in general, 
they have less morbidity and fewer costs than hip or spine frac­
tures. Their incidence rises in women at menopausal age but then 
rises no further with increasing age. This pattern may be explained 
by different reactions to falling with age. Men have a much lower 
incidence of wrist fracture. 
Other Types of Fractures
In addition to hip, spine, and wrist fractures, a large number of

hip, spine, and wrist fractures, a large number of 
other fracture sites, such as the arm, lower leg, humerus, and ribs, 
are common and have been shown to have significant morbid­
ity and mortality risks.215 Furthermore, there is increasing evi­
dence that the risk of most, if not all, of these fracture types are 
increased in those with low BMD or osteoporosis216 and can be 
reduced by osteoporosis treatment.207,217–219 A recently published 
study evaluated the relationship of mortality within

udy evaluated the relationship of mortality within a group of 
individuals with nonhip, nonvertebral fractures with or without 
comorbidities.9 The mortality rate attributable to any fracture 
without comorbidity was 9.2% in women and 5.3% in men. 
Moreover, comorbidity independently worsened the mortality 
rate; for instance, a woman showing both humeral fracture and 
one comorbidity had a similarly reduced 5-year survival to that of 
a woman with a hip fracture and no comorbidities.9 
Clinical

th a hip fracture and no comorbidities.9 
Clinical Assessments of Osteoporosis
Dual-Energy X-Ray Absorptiometry
DXA is the standard method for assessing the presence and extent 
of osteoporosis. DXA is most often performed at the spine and 
hip, although other sites (whole body, distal radius) can also be 
assessed. Although not a true density, it provides an areal density 
of mass per area in units of grams per square centimeter. The Inter­
national Osteoporosis Foundation and other organizatio

onal Osteoporosis Foundation and other organizations have 
developed a classification system for DXA values called T-scores 
that classify the measurement (Table 30.1). T-scores are calculated 
by comparing a specific value with a normative reference range for 
patients of the same gender and ethnicity. The T-score is the num­
ber of standard deviations (SD) below young normal values. A 
value below −2.5 SD is usually considered osteoporotic. Clinically, 
usually three regions are used for diagn

nically, 
usually three regions are used for diagnosis: BMD at the total hip, 
the femoral neck, and the lumbar spine.
DXA values at the hip generally peak at about age 30 to 40 years 
and then begin to decline (Fig. 30.15). The decline is somewhat 
accelerated at menopause for a few years and then becomes accel­
erated again after about age 65 or 70 years in women.220 Spine 
BMD measurements can be useful; however, in older women and 
especially men, they can be confounded by osteophytes, aorti

 men, they can be confounded by osteophytes, aortic cal­
cification, degenerative disease, and other conditions that increase 
the apparent spinal BMD and therefore are less reliable in those 
older than 65 to 70 years.
A large number of studies (many of them prospective) have 
consistently shown that DXA BMD is strongly predictive of future 
risk of fracture. Hip BMD is predictive of various types of both 
nonvertebral and vertebral fracture but is particularly strongly pre­
dictive of hip frac

 is particularly strongly pre­
dictive of hip fractures. Although the risk of fracture increases sig­
nificantly with decreasing BMD, several studies have verified that 
   Diagnostic Categories for Osteoporosis 
Based on Measurements of BMD and 
Bone Mineral Content
Category
Normal
BMD ±1 SD of the young adult 
­reference mean
Low bone mass (osteopenia)
BMD >1 SD and <2.5 SD lower than 
the young adult mean
BMD >2.5 SD lower than the young 
adult mean
Severe osteoporosi

ower than the young 
adult mean
Severe osteoporosis 
­(established osteoporosis)
adult mean in the presence of one 
or more fragility fractures
BMD, Bone mineral density; SD, standard deviation.
   
TABLE 30.1
1277
osteoporotic fractures occur across a wide spectrum of BMDs. 
Most likely, these events are related to bone quality, a component 
not captured by DXA measurements. The World Health Organi­
zation de

A measurements. The World Health Organi­
zation definition is a guideline for interpreting DXA results, not a 
definitive way of diagnosing osteoporosis. Therefore, DXA is the 
primary imaging tool used in diagnosing and treating osteoporo­
sis. It can be used in conjunction with other risk assessment tools 
that combine other risk factors with DXA (see later discussion). 
DXA measurements have been somewhat standardized across dif­
ferent types of scanners, although follow-up measures to assess

of scanners, although follow-up measures to assess 
change should be performed on the same scanner if at all possible. 
Trabecular Bone Score
Approved by the FDA in 2012, the trabecular bone score (TBS) 
is now incorporated into clinical practice for the estimation of 
fracture risk. The TBS is an optional tool added to the menu 
of DXA equipment, which provides gray-scale textural analysis 
of spine DXA images, utilizing variograms of two-dimensional 
projection images. The TBS is a surrogate m

ional 
projection images. The TBS is a surrogate measurement of tra­
becular microarchitecture, and there is evidence showing corre­
lation between the several parameters of quantitative computed 
tomography (QCT) and TBS values. The strongest association is 
observed at lumbar spine trabecular volumetric BMD, whereas 
in other sites the relationship between TBS with bone microar­
chitectural parameters has more variation. Several studies show 
convergent results that the TBS is an independent p

onvergent results that the TBS is an independent predictor of 
fracture in osteoporotic postmenopausal women, as well as in 
men. A large prospective study performed in Manitoba, Canada, 
which included more than 29,000 postmenopausal women fol­
lowed for almost 5 years, reported the accuracy of the TBS in 
predicting fracture. Moreover, it demonstrated that the combina­
tion of BMD with TBS assessment provides a better estimate of 
fracture risk than either evaluation by itself.221 Similar resu

 than either evaluation by itself.221 Similar results were 
obtained in smaller samples in the United States (Osteoporotic 
Fractures in Men Study [MrOs]) and in Japanese populations who 
were followed for 10 years.222 The TBS has also been shown to 
enhance the power of fracture prediction obtained by the use of 
the algorithm FRAX, which was designed to estimate a 10-year 
probability of fracture.223 
Quantitative Computed Tomography
Other imaging modalities have been studied and may be used 

ing modalities have been studied and may be used 
for osteoporosis assessment, although DXA remains the pre­
ferred method, when available. QCT can be performed at the 
hip and spine, and studies have shown that it can predict fracture 
risk.219,224,225 Several versions of software226,227 are commercially 
available for analysis of QCT scans. One advantage of QCT is 
that it can provide specific measurements of trabecular and corti­
cal BMD. However, QCT has the disadvantages (compared with 
DXA

ver, QCT has the disadvantages (compared with 
DXA) of higher radiation exposure, less standardization across 
computed tomography scanners, and higher cost. Peripheral QCT 
(pQCT) and high-resolution pQCT have been developed and 
can currently measure bone density and bone microarchitecture 
(in the case of high-resolution pQCT) in the radius and tibia. 
Although these measurements can be helpful in the research con­
text to help understand the impact of diseases such as diabetes and 
medicatio

impact of diseases such as diabetes and 
medications such as bisphosphonates on bone microarchitecture 
and quality, there is little evidence that they have advantages in 
clinical practice for predicting fracture risk compared with DXA 
(examples of such measurements are shown in Fig. 30.16).
Other measurement tools to assess risk include ultrasound of 
the fingers, calcaneus, and distal extremities and magnetic reso­
nance imaging of the radius. Ultrasound has predictive value 
for fracture ri

. Ultrasound has predictive value 
for fracture risk, but longitudinal assessments have not proved 
helpful clinically.228 Magnetic resonance imaging provides sig­
nificant insight into trabecular and cortical microstructure but 
is basically a research tool because of expense and time on the 
machines. 
Bone Turnover Markers
The balance between bone resorption and formation is an impor­
tant contributor to bone loss and osteoporosis. Bone turnover 
markers (BTMs) can provide an assessment of re

er 
markers (BTMs) can provide an assessment of resorption and 
formation and might therefore be useful in understanding these 
imbalances within individual patients, prediction of fracture risk, 
and the effect of treatment. Bone formation can be assessed by sev­
eral biochemical markers, including bone-specific alkaline phos­
phatase (BSAP), total procollagen type 1 N-terminal propeptide 
(P1NP), and osteocalcin. Bone resorption has traditionally been 
Peak
bone mass
“Steady-
state”
Cortical
b

ly been 
Peak
bone mass
“Steady-
state”
Cortical
bone
CTX
Bone
5th
decade
4th
decade
Menopause
2nd
decade
R
A
N
K
L
O
P
G
R
A
N
K
Premenopausal
A
B
Years postmenopausal
Total body calcium (g)
750
800
850
900
950
1000
1050
1100
1150
25 30
35
40
45
50
0
5
10
15
20
25
30
35
bone
bone
• Fig. 30.15  (A) Bone mass development, maintenance, and loss: trabecular bone loss starts earlier and is 
more intense than in cortical bone. (B) Th

and is 
more intense than in cortical bone. (B) The decline of total body calcium with years since menopause. CTX, 
cross-linked telopeptide of type I collagen; OPG, osteoprotegerin; RANK, receptor activator of nuclear 
transcription factor κB. (Redrawn from Tella SH, Gallagher JC. Prevention and treatment of postmeno­
pausal osteoporosis. J Steroid Biochem Mol Biol. 2014;142:155–170.)
1278
assessed with urinary N-terminal cross-linked telopeptide (NTX) 
but is 

N-terminal cross-linked telopeptide (NTX) 
but is now more commonly assessed by serum C-terminal cross-
linked telopeptide (CTX), both being collagen cross-linked pep­
tides assessed with antibodies (Table 30.2).
There are several potential clinical applications of BTMs. 
However, all of them must take into account the high analytic 
variability (both within individual and between assays and 
laboratories) for these markers.229 Important sources of varia­
tion within individuals include circadia

of varia­
tion within individuals include circadian variation, food intake, 
exercise, seasonal variation, diseases such as renal impairment, 
and recent fractures.230 For predicting fracture risk, there is con­
troversy about whether BTMs predict fracture independently of 
BMD and other risk factors, with some studies showing a posi­
tive relationship; however, others have not shown that BTMs 
independently predict fracture.231–233 Because BTMs change 
dramatically and quickly with both antires

change 
dramatically and quickly with both antiresorptive and anabolic 
treatment, it has been suggested that they may be of some use in 
monitoring therapy. Although the value of this application is con­
troversial, they could have some value for a clinician with patients 
whose self-reported compliance may be unreliable.234–236 They 
are more commonly used in specialty practices where they can 
be helpful in defining secondary causes of osteoporosis. BTMs 
are particularly useful when measured

rosis. BTMs 
are particularly useful when measured in groups of patients dur­
ing the development of new medications. In the research setting, 
these measurements can provide early assessments of potency of 
drugs, their effects over time, and assist in determining optimal 
doses.237,238 
Bone Biopsy
Bone biopsy is used clinically to assess dynamic and static indi­
ces related to bone remodeling. It is rarely used clinically to 
diagnose osteoporosis but can be useful to assess the degree of 
mi

osis but can be useful to assess the degree of 
mineralization (e.g., to exclude osteomalacia), rates of bone for­
mation or resorption, and the overall status of remodeling (e.g., 
to exclude adynamic renal bone disease). Universal criteria can 
then be applied to the indices obtained after labeling bone by 
timed tetracycline administration.239 Transiliac biopsies can be 
performed by physicians who specialize in osteoporosis medicine 
or by orthopedic surgeons and requires local anesthetic fo

thopedic surgeons and requires local anesthetic for a core 
biopsy, which generally requires a 7.5-mm trephine core for bone 
histologic diagnosis and histomorphometry. Mineral apposition 
rate, mineralizing surface, bone formation rate, eroded surfaces, 
number of osteoblasts and osteoclasts, and osteoid volume/bone 
volume all can be determined from a single biopsy, but only after 
serial tetracycline labeling, which is required to measure the dis­
tance between two mineralization fronts. Labe

dis­
tance between two mineralization fronts. Labeling intervals vary 
somewhat but generally are for 3 days at the beginning (days 1–3) 
and for 3 days 21 days later, using 200 mg of demeclocycline three 
times per day. Several commercial laboratories provide analysis 
of biopsies, although turnaround time can vary from 1 to several 
months. Not infrequently, however, bone biopsies in age-related 
or postmenopausal osteoporosis are normal, and hence the diag­
nostic specificity is low. 
Fractur

ence the diag­
Fracture Epidemiology
Some of the central features of the epidemiology of fractures for 
the three primary osteoporotic fractures are illustrated in Fig. 
30.17. These data show very clearly the exponential increase in 
women in hip and spine fractures. For example, an average Cau­
casian woman at age 50 years has about a 15% to 20% annual 
risk of hip fracture that increases still further beyond age 80 years. 
Wrist fractures show a different pattern i

years. 
Wrist fractures show a different pattern in women with an increase 
at the time of menopause but then no further increase with time. 
In men, the exponential increase in hip and spine fractures with 
age is parallel to that for women. However, importantly, the age-
specific risk of hip and spine fractures in men is much lower than 
that in women (approximately 50%), highlighting the key role of 
gender in the epidemiology of osteoporotic fractures. Wrist frac­
ture incidence in men is mu

fractures. Wrist frac­
ture incidence in men is much lower than in women at all ages.
Co
Ultradistal
Distal
Fx
DM
DMFx
• Fig. 30.16  Representative high-resolution peripheral quantitative computed tomography images of the 
ultradistal (above) and distal (below) tibia: shown are the mid-stack tomograms for the Co (left), Fx (left-
center), DM (right-center), and DMFx (right) groups. Major cortical porosity can be seen in DMFx (right). Co, 
controls; Fx, nondiabetic fracture patients; DM, diabetic

s; Fx, nondiabetic fracture patients; DM, diabetic patients without fractures; DMFx, diabetic patients 
with fractures. (From Patsch JM, Burghardt AJ, Yap SP, et al. Increased cortical porosity in type 2 diabetic 
postmenopausal women with fragility fractures. J Bone Miner Res. 2013;28[2]:313–324.)
1279
   Markers of Bone Turnover
Marker
Tissue of Origin
Specimen
Analytic Method
Remarks
Bone Formation
Bone-specific alkaline 
­phosphatase (BAP

ormation
­phosphatase (BAP)
Bone
Serum
Electrophoresis, 
precipitation, 
IRMA, EIA, ECMA
Specific product of osteoblasts; some assays show up to 
20% cross-reactivity with the liver isoenzyme (LAP)
Osteocalcin (OC)
Bone, platelets
Serum
RIA, IRMA, EIA
Specific product of osteoblasts; many immunoreactive 
forms in blood; some may be derived from bone 
resorption
C-terminal propeptide of type I 
procollagen (PICP)
Bone, soft tissue, 
skin
Serum
RIA, EIA
Specific product of 

 tissue, 
skin
Serum
RIA, EIA
Specific product of proliferating osteoblasts and 
N-terminal propeptide of type I 
procollagen (PINP)
skin
Serum
RIA, EIA
Specific product of proliferating osteoblast and fibroblasts; 
partly incorporated into bone extracellular matrix
Markers of Bone Resorption
Collagen-Related Markers
Hydroxyproline, total and 
dialyzable (Hyp)
Bone, cartilage, soft 
tissue, skin
Urine
Colorimetry HPLC
Present in all fibrillar collagens and partly 

PLC
Present in all fibrillar collagens and partly in collag­
enous proteins, including C1q and elastin
Present in newly synthesized and mature collagen 
(i.e., both collagen synthesis and tissue breakdown 
contribute to urinary hydroxyproline)
Hydroxylysine-glycosides
skin, serum 
complement
Urine, serum
HPLC, EIA
Hydroxylysine in collagen is glycosylated to varying 
degrees, depending on tissue type
Glucosylgalactosyl-OH-Lys in high proportion in 
collagens of soft tissues, 

in high proportion in 
collagens of soft tissues, and C1q
Galactosyl-OH-Lys in high proportion in skeletal 
collagens
Pyridinoline (PYD)
Bone, tendon, 
cartilage, blood 
vessels
HPLC, EIA
Collagens, with highest concentrations in cartilage 
and bone; absent from skin; present in mature 
collagen only
Bone, dentin
HPLC, EIA
Collagens, with highest concentration in bone; 
absent from cartilage or skin; present in mature 
C-terminal cr

in; present in mature 
C-terminal cross-linked 
­telopeptide of type I collagen 
(ICTP, CTX-MMP)
Bone, skin
Serum
RIA, EIA
Collagen type I, with highest contribution probably 
from bone; may be derived from newly synthesized 
collagen
­telopeptide of type I 
­collagen (CTX-I)
All tissues containing 
type I ­collagen
Urine (α/β), 
serum (β 
only)
EIA, RIA
Collagen type I, with highest contribution probably from 
bone; isomerization of aspartyl to β-aspartyl 

om 
bone; isomerization of aspartyl to β-aspartyl occurs 
with aging of collagen molecule
N-terminal cross-linked 
­collagen (NTX-I)
EIA, RIA, CLIA
Collagen type I, with highest contribution from bone
Collagen I alpha 1 helicoidal 
peptide (HELP)
Urine
EIA
Degradation fragment derived from the helical part of 
type I collagen, α1 chain, AA (620-633); correlates 
highly with othe

 chain, AA (620-633); correlates 
highly with other markers of collagen degradation, 
no specific advantage or difference in regard to 
clinical outcomes
Noncollagenous Proteins
Bone sialoprotein (BSP)
Bone, dentin, 
­hypertrophic 
cartilage
Serum
RIA, EIA
Acidic, phosphorylated glycoprotein, synthesized by 
osteoblasts and osteoclast-like cells, laid down in 
bone extracellular matrix; appears to be associated 
with osteoclast function
OC fragments (uf-OC, 
U-Mid-OC, U-Long-OC)
Bone
Urine
EIA
C

nts (uf-OC, 
Bone
Urine
EIA
Certain age-modified fragments are released during 
osteoclastic bone resorption and may be considered 
an index of bone resorption
TABLE 30.2
Continued
1280
Ethnicity and culture also play a key role in the epidemiology 
of osteoporosis (Fig. 30.18). For hip fractures, those of Caucasian 
ethnicity are at higher risk, Hispanics and Asians are at medium 
risk, and African Americans are at lowest risk. However, the

African Americans are at lowest risk. However, these 
general relationships can often be more subtle. For example, in 
Asians, there is evidence that rates are very low in areas with tradi­
tional lifestyles and that they increase substantially with increasing 
urbanization and adoption of Western lifestyles.240 Rates of spine 
fractures are more challenging to assess, but many studies sug­
gest that these rates vary less geographically than do rates of hip 
fracture.
Another key epidemiologic f

ates of hip 
fracture.
Another key epidemiologic factor that interacts with age, 
gender, and ethnicity is weight or body mass index (BMI). There 
are studies showing that low lean mass is an independent predic­
tor of incident fractures in a cohort of 65-year-old community 
dwellers.241 In addition to the obvious mechanical connection 
between muscles and bones, other factors are important deter­
minants of this association, including molecular, genetic, and 
endocrine interaction.242 In genera

 genetic, and 
endocrine interaction.242 In general, higher BMI individuals are 
at lower risk of hip and spine fractures, owing to several fac­
tors, including a larger number of fat cells that produce greater 
amounts of estrogen, which is protective, and the higher amount 
of padding in higher BMI patients, which produces a larger dis­
tribution of the forces in the event of a fall. Although some data 
suggest that individuals who are obese have a higher risk of frac­
tures, most studies supp

ve a higher risk of frac­
tures, most studies support the tenet that a higher BMI is pro­
tective for most types of fractures, the exception possibly being 
lower limb fractures. Very importantly, those at low BMI are at 
particularly high risk of hip fracture for several reasons, one of 
which may be that low BMI, particularly in the elderly, may be 
a sign of frailty.243,244
Clinical Risk Factors and Their Combination With BMD
Several other clinical risk factors (in addition to BMD, age, 
gend

nical risk factors (in addition to BMD, age, 
gender, and race) have been consistently associated with frac­
ture risk (Table 30.3; Fig. 30.19). Most important of these is a 
history of fracture since age 50 years or a mother’s (or father’s) 
history of hip fracture.245 Other risk factors, including cigarette 
smoking, excessive alcohol consumption (more than two drinks 
per day), rheumatoid arthritis, and glucocorticoid use, also have 
some association with risk.244,246 Compromised neurologic o

ciation with risk.244,246 Compromised neurologic or 
muscular function is a strong risk factor, probably through an 
increased likelihood of falls. There is also a growing awareness 
that diabetics, despite a generally high BMI, are at higher risk 
of fracture and that this risk at the same BMD is higher than 
that for nondiabetics.247 Surprisingly, some recent studies have 
suggested that obesity may be a risk factor for fracture in some 
settings: in the MrOS study, it was suggested that at th

gs: in the MrOS study, it was suggested that at the same 
BMD, elderly men who were obese were at higher risk than 
those who were not obese.248
There is a growing appreciation that fracture risk, like heart 
disease risk, is multifactorial and that these risk factors work in 
combination to increase risk.244,246 More recently, risk assess­
ment tools predicting 5- or 10-year risk of fracture from combi­
nations of BMD and risk factors together have been developed. 
The most widely used and avai

ave been developed. 
The most widely used and available is the FRAX fracture risk 
Marker
Specimen
Remarks
Osteoclast Enzymes
Tartrate-resistant acid 
­phosphatase (TRAcP)
Bone
Blood plasma
Serum colorimetry, 
RIA, ELISA
Six isoenzymes found in human tissues (osteoclasts, 
platelets, erythrocytes); band 5b predominant in 
bone (osteoclasts)
Cathepsins (e.g., K, L)
K: Primarily in 
osteoclasts;
L: macrophage, 
Plasma, serum
ELISA
Cathepsin K, a cystein

oclasts
ELISA
Cathepsin K, a cysteine protease, plays an essential role 
in osteoclast-mediated bone matrix degradation by 
cleaving helical and telopeptide regions of collagen 
type I; cathepsin K and L cleave the loop domain of 
tartrate-resistant acid phosphatase and activate the 
latent enzyme; cathepsin L has a similar function in 
macrophages; tests for measurement of cathepsins 
in blood are presently under evaluation
BAP, Serum bone alkaline phosphatase; BMD, bone mineral d

rum bone alkaline phosphatase; BMD, bone mineral density; β-CTX-I, epitope of C-terminal cross-linked telopeptide of collagen type I; CLIA, chemiluminescent immunoassay; C-terminal, carboxy-
terminal; CTX-MMP, C-terminal cross-linking telopeptide of type I collagen generated by matrix metalloproteinases; DPD, deoxypyridinoline; ECMA, electrometric immunoassay; EIA, enzyme immunoassay; 
HPLC, high-performance liquid chromatography; ICTP, carboxyterminal type I collagen telopeptide; IRMA, immunora

rminal type I collagen telopeptide; IRMA, immunoradiometric assay; N-terminal, amino-terminal; PICP, C-terminal propeptide of procollagen 
type I; PINP, N-terminal propeptide of procollagen type I; PTH, parathyroid hormone; PTHrP, parathyroid hormone–related peptide; PYD, pyridinoline; RIA, radioimmunoassay; TAP, serum total alkaline 
phosphatase; ufOC, urinary fragments of osteocalcin; U-Long-OC, long N-terminal fragment; U-Mid-OC, midmolecule epitope of the OC.
Adapted from Seibel MJ. Biochemi

pitope of the OC.
Adapted from Seibel MJ. Biochemical markers of bone turnover: part I: biochemistry and variability. Clin Biochem Rev. 2005;26(4):97–122.
   
   Markers of Bone Turnover—cont’d
TABLE 30.2
0
1,000
2,000
3,000
4,000
35-39
≥85
35-39
≥85
Incidence/100,000 person-yr
Age group (yr) 
Men
Hip
Hip
Vertebrae
Vertebrae
Colles
Colles
Women
• Fig. 30.17  Age-specific incidence rates for hip, vertebral, and Colles frac­
tures in Rochester, Minnesota. (From Cooper C, Melton LJ. Epidemiology 

nnesota. (From Cooper C, Melton LJ. Epidemiology 
of osteoporosis. Trends Endocrinol Metab. 1992;3:224. Copyright 1992 
by Elsevier Science, Inc.)
1281
Age-standardized hip fracture incidence (per 100,000 person-years)
Nigeria
South Africa
New Zealand
Australia
Ecuador
Colombia
Venezuela
Brazil
Chile
Mexico
Argentina
Tunisia
Morocco
Saudi Arabia
Jordan
Kuwait
Israel
Oman
Lebanon
Iran
Philippines
India
Indonesia
China
Thailand
Russia
Japan
Mala

s
India
Indonesia
China
Thailand
Russia
Japan
Malaysia
Hong Kong
South Korea
Singapore
Taiwan
Canada
USA
Poland
Croatia
Spain
Serbia
Netherlands
Romania
France
Finland
Ireland
Lithuania
UK
Portugal
Belgium
Germany
Greece
Malta
Slovakia
Italy
Slovenia
Turkey
Hungary
Czech Republic
Iceland
Austria
Switzerland
Sweden
Denmark
Norway
0
0
200
200
400
400
600
600
Men
Women
• Fig. 30.18  Age-standardized hip fracture incidence rates in women and men according to country. Coun­
tries are organized by con

rding to country. Coun­
tries are organized by continent or geographic region: Europe (dark pink), North America (green), Asia (light 
blue), Middle East (yellow), South America (purple), Oceania (dark blue), and Africa (red). UK, United King­
dom; USA, United States of America. (Redrawn from Cauley JA, Chalhoub D, Kassem AM, Fuleihan Gel-
H. Geographic and ethnic disparities in osteoporotic fractures. Nat Rev Endocrinol. 2014;10[6]:338–351.)
1282
   Different 

2
   Different Combinations of Relevant Fracture Risk Factors Used to Calculate an Individual’s Probability 
of Fracture in 10 Years
FRAX
Q-Fracture Score
Garvan
Age
Age
Age
Sex
Sex
Sex
Weight
Weight
Personal history of fracture (vertebral inclusive)
Personal history of 
fracture
Parental history of hip fracture
Current smoking
Current smoking (graded)
Glucocorticoid (prednisolone equivalent 5 mg 
for ≥3 months)
At least two pre

e equivalent 5 mg 
At least two prescriptions for systemic corticosteroids in the 6 months before 
baseline
Rheumatoid arthritis
Secondary osteoporosis, type 1 diabetes mellitus, 
osteogenesis imperfecta in adults, hyperthyroid­
ism, hypogonadism, chronic malnutrition, and 
liver disease
Secondary osteoporosis, cardiovascular disease, type 2 diabetes mellitus, 
asthma, chronic liver disease, gastrointestinal conditions likely to result 
in malabsorption (e.g.,

nditions likely to result 
in malabsorption (e.g., Crohn disease, ulcerative colitis, celiac disease, 
steatorrhea, blind loop syndrome), thyrotoxicosis, primary or secondary 
hyperparathyroidism, Cushing syndrome
Alcohol ingestion (>3 units/day)
At least two prescriptions for tricyclic antidepressants in the 6 months before 
baseline
At least two prescriptions for hormone replacement therapy (in women) in the 
6 months before baseline
History of falls before bas

months before baseline
History of falls before baseline
History of falls
Bone mineral density (optional)
TABLE 30.3
No. of risk factors
Calcaneal bone
density
Risk of hip fracture
(per 1000 woman-years)
0
≥5
3-4
0-2
Lowest
third
Middle
third
Highest
third
30
10
20
2.6
4.0
27.3
1.1
5.6
14.1
1.1
1.9
9.4
2.6
4.0
27.3
1.1
5.6
14.1
1.1
1.9
9.4
• Fig. 30.19  Annual risk of hip fracture according to the number of risk factors and the age-specific cal­
caneal bone density. (Redrawn 

e age-specific cal­
caneal bone density. (Redrawn from Cummings SR, Nevitt MC, Browner WS, et al. Risk factors for hip 
fracture in white women. Study of Osteoporotic Fractures Research Group. N Engl J Med. 1995;332:767–
773. Copyright © 1995 Massachusetts Medical Society. Reprinted with permission.)
assessment tool, which includes the risk factors in Table 30.3 
and has been implemented with country-specific versions249 for 
predicting risk of hip and major osteoporotic fractures. Other 
risk a

ip and major osteoporotic fractures. Other 
risk assessment tools have also been proposed and include dif­
ferent or fewer risk factors.250 Clinical guidelines for treatment 
have been developed incorporating these tools (discussed later; 
see Table 30.3). 
Prevalence of Osteoporosis and Incidence of 
Fractures in the Population
Estimates of the prevalence of osteoporosis have depended on the 
working definition of this disease and the appropriate diagnos­
tic criteria, and they have been based 

e diagnos­
tic criteria, and they have been based on either the prevalence of 
low BMD or fracture incidence.251–253 Basing a definition solely 
on BMD became increasingly possible after the development and 
1283
clinical availability of DXA. In 1992, the World Health Organi­
zation set a cutoff point of 2.5 SD below a young normal mean 
value for BMD as a tool for estimating osteoporosis prevalence 
and comparing across populations254 (see Ta

lence 
and comparing across populations254 (see Table 30.1). The use of 
DXA in larger epidemiologic studies in women (e.g., the Study of 
Osteoporotic Fractures) and men (the MrOs study), and particu­
larly in population-based samples (e.g., the National Health and 
Nutrition Examination Study), has provided estimates of the pro­
portion of people with osteoporosis by BMD.255–257 Estimates of 
overall prevalence of osteoporosis vary greatly but place the num­
ber close to 40 million Americans.2

place the num­
ber close to 40 million Americans.251,258
Based on fracture incidence, currently most estimates suggest 
that there are approximately 0.3 million hip fractures per annum 
in the United States and 0.6 million hip fractures in Europe.252,253 
Best estimates are that more than a million American postmeno­
pausal women will suffer a spine fracture in the course of a sin­
gle year.251,258 Considering the importance of fracture from an 
individual’s point of view, a 50-year-old Caucasia

individual’s point of view, a 50-year-old Caucasian woman has 
approximately a 15% to 20% lifetime risk of hip fracture, a similar 
lifetime risk of wrist fracture, about a 16% lifetime risk of clinical 
spine fracture, and approximately a 50% risk of at least one osteo­
porotic fracture.259–261 Men have about one third the lifetime risk 
of hip and spine fractures as women but a much lower risk of 
wrist fracture. Combining osteoporosis prevalence based on either 
the presence of osteoporotic f

ce based on either 
the presence of osteoporotic fractures (i.e., vertebral compressions, 
wrist fractures, hip fractures, or humerus/tibial fractures) or low 
BMD, it is estimated that more than 40 million Americans are 
afflicted with osteoporosis.262 Worldwide, there are approximately 
9 million fractures per year.263
Although numbers of fractures are likely to increase in the 
United States and Europe, this increase will be much more striking 
in the developing world, particularly in Asia an

in the developing world, particularly in Asia and Latin America, 
because of an intersection of several trends including increas­
ing population size, increasing life span (into the high-fracture-
risk ages), and adoption of a more Western lifestyle (particularly 
decreasing activity in daily living).264,265 This increase will result 
in a consequent increase in need for health care resources. Interest­
ingly, although hip fractures are the most costly individual frac­
tures, the overall costs

t costly individual frac­
tures, the overall costs of other types of fractures together may be 
greater than that for hip fractures.266 
Pathogenesis of Osteoporosis
Osteoporosis is a complex disease with a multifactorial origin (Fig. 
30.20). The cardinal feature of this disease is fractures usually 
accompanied by enhanced skeletal fragility. Although the primary 
focus of this chapter is on the endocrine and metabolic aspects of 
this disease, it should be noted that falls cause fractures and

 it should be noted that falls cause fractures and that 
one of the primary causes of osteoporotic fractures is falling. Falls 
are also multifactorial, particularly in older individuals, because 
B
Normal
A
C
1
2
3
4
• Fig. 30.20  Osteoporosis at a glance. Osteoporosis is a systemic skeletal disease in which bone resorp­
tion exceeds bone formation and results in microarchitectural changes. (A) Fragility fractures typically 
involve the wrist, vertebrae, and h

res typically 
involve the wrist, vertebrae, and hip. (B) Microcomputed tomography shows marked trabecular thinning of 
osteoporotic bone compared with normal bone. (C) Microscopic views of bone-resorbing osteoclasts and 
bone-forming osteoblasts: 1, osteoclast with its distinctive morphologic appearance; 2, tartrate-resistant 
acidic phosphatase staining of multinucleated osteoclasts; 3, multiple osteoblasts on mineralized matrix; 
4, alkaline phosphatase staining of osteoblasts. (From Rachner 

hosphatase staining of osteoblasts. (From Rachner TD, Khosla S, Hofbauer LC. Osteoporosis: 
now and the future. Lancet. 2011;377[9773]:1276–1287.)
1284
primary muscle weakness, neurologic disorders, drugs, vitamin D 
deficiency, balance problems, and cardiovascular events such as 
syncope all can cause falls. Hence, primary prevention of osteopo­
rosis also mandates specific steps to reduce the chance of any fall 
for whatever reason. Noted in this section are c

for whatever reason. Noted in this section are causes of imbalance 
in the bone remodeling unit that lead to secondary bone loss and 
subsequent reduction in BMD, enhancing skeletal fragility.
Gonadal Deficiency
Estrogen
Altered bone remodeling is at the heart of the osteoporosis syn­
drome and can take many forms. Historically, the importance of 
estrogen in maintaining calcium homeostasis via coupled remod­
eling in the postmenopausal woman was first established by 
Fuller Albright in 1947.2

as first established by 
Fuller Albright in 1947.267 Since that time, much evidence has 
accumulated from randomized intervention trials demonstrating 
that gonadal steroid replacement (estrogen with or without pro­
gesterone) reduces bone turnover and increases bone mass, thus 
protecting the skeleton from fractures.202,268 However, these data 
provide only indirect evidence that estrogen levels are impor­
tant as pathogenic components of the osteoporosis syndrome. 
More recent studies provide 

teoporosis syndrome. 
More recent studies provide stronger evidence of the association 
between low estradiol concentrations and low bone mass. Several 
investigators have demonstrated that the lowest estradiol levels in 
postmenopausal women (i.e., <5 pg/mL) are associated with the 
lowest BMD and the greatest likelihood of fracture.269 In addi­
tion, several studies have shown that males with osteoporosis have 
lower serum levels of estradiol than do age-matched men who do 
not have low bone m

han do age-matched men who do 
not have low bone mass.270 Moreover, there are now several case 
reports describing mutations in genes encoding either aromatase or 
the estrogen receptor, producing a phenotype of severe osteoporo­
sis in men.271,272 In the former case, estrogen replacement therapy 
in those men resulted in a marked increase in spine and hip BMD. 
In both situations, the lack of functional estrogen, despite normal 
to high levels of testosterone, resulted in severely low BMD.199
A

f testosterone, resulted in severely low BMD.199
Although declining estradiol levels contribute to the osteoporo­
sis syndrome, the precise molecular events or sequences that result 
from changes in ambient hormonal concentrations are not clear. 
In some animal models, estrogen deprivation is associated with a 
marked increase in IL6 synthesis from stromal and osteoblastic 
cells. This is consistent with experimental findings demonstrat­
ing that estrogen regulates the transcriptional activity o

 estrogen regulates the transcriptional activity of the IL6 
promoter.273 However, results in other studies are conflicting. In 
other experimental paradigms, changes in TNF, IL11, and IL1 can 
be associated with increased bone resorption.274–276 RANKL has 
been identified as a major regulator of osteoclast differentiation, 
and increases (both locally and systemically) likely contribute to 
the rapid increase in osteoclastogenesis after estrogen withdrawal. 
Thus, it seems likely that several c

 withdrawal. 
Thus, it seems likely that several cytokines, working in concert 
with RANKL, are active during estrogen deprivation, and each 
can accelerate the process of bone resorption. Osteoclasts express 
estrogen receptors, and some evidence suggests that direct actions 
of estrogen on osteoclasts are important as well. Enhanced bone 
resorption eventually leads to bone loss from estrogen deprivation 
because bone formation rates cannot keep up with the high rates 
of bone resorption277 (s

p up with the high rates 
of bone resorption277 (see Fig. 30.15). 
Androgens
In contrast to the plethora of studies on uncoupled bone remodel­
ing and bone loss with low estradiol levels, there are fewer stud­
ies relating androgen deprivation to bone loss in both men and 
women. Androgen receptors are present on osteoblasts, and testos­
terone and dihydroxytestosterone both stimulate osteoblast differ­
entiation.278 Testosterone may also increase skeletal and circulating 
IGF1. However, both in

e skeletal and circulating 
IGF1. However, both in vitro and in vivo studies in men have 
yielded conflicting results with respect to bone resorption. Like 
estrogen, androgens can regulate the IL6 promoter, and in experi­
mental animals, orchiectomy has been associated with increased 
IL6 production and bone loss.279 Men who undergo androgen 
deprivation therapy after prostate cancer lose bone rapidly as a 
result of increased bone resorption. Similarly, hypogonadal men, 
either due to primary 

imilarly, hypogonadal men, 
either due to primary or secondary insufficiency, have lower bone 
density values than control men. Because, unlike estradiol, testos­
terone can stimulate bone formation, this may be an additional 
factor that contributes to bone loss when absent in men.280
Low testosterone levels that are found in postmenopausal 
women have not been causally associated with rapid bone loss, but 
women with greater body weight have greater capacity to enzy­
matically convert testoste

eater capacity to enzy­
matically convert testosterone to estradiol, which may contribute 
to the protective effect of obesity on bone mass, at least in women. 
Notwithstanding, because in men with chronically low andro­
gen levels have been associated with low bone mass, testosterone 
replacement can be used to both enhance BMD and increase lean 
body mass.280 However, as noted, estradiol levels in men may be 
a more important risk factor for fracture than androgen levels. In 
sum, the combinat

acture than androgen levels. In 
sum, the combination of both estrogen and androgens probably 
determines peak bone mass and maintenance of BMD in both 
men and women. In elderly men, estradiol levels may be essential 
for maintaining trabecular bone mass. To underline that tenet, in 
one randomized trial using a gonadotropin-releasing hormone 
analogue (to suppress gonadotropins) with and without an aro­
matase inhibitor in men, changing estrogen levels were associated 
with protection of the t

n levels were associated 
with protection of the trabecular skeleton from bone resorption, 
whereas circulating androgen levels were more related to markers 
of bone formation than those of resorption.202 In another study, 
Almeida and colleagues281 found that in rodents, protection of 
cortical bone mass by estrogens is mediated via estrogen receptor 
α. In contrast, the androgen receptor of mature osteoblasts was 
indispensable for the maintenance of trabecular bone mass in male 
mammals but w

nce of trabecular bone mass in male 
mammals but was not required for the anabolic effects of andro­
gens on cortical bone.278,282
In men, conditions other than hypogonadism can contribute 
to bone loss and fractures. These states include chronic alcohol­
ism, glucocorticoid excess, smoking, and idiopathic hypercal­
ciuria. In the first two cases, low testosterone levels probably 
contribute to the pathogenic features of osteoporosis syndrome, 
whereas hypercalciuria due to renal loss probably c

hereas hypercalciuria due to renal loss probably causes bone 
loss through secondary hyperparathyroidism. Less frequent but 
still important secondary causes of osteoporosis in men must also 
be considered independent of androgen levels, and these include 
gluten enteropathy, primary hyperparathyroidism, thyrotoxicosis, 
multiple myeloma, lymphomas, or granulomatous diseases, all of 
which can present with multiple fractures and low bone mass283 
(see Table 30.3). 
Age-Related Bone Loss
In women

In women, bone loss is accelerated immediately after menopause. 
However, recent studies demonstrate that markers of bone resorp­
tion are also very high later in life. In particular, women in their 
80s and 90s have been noted to lose bone at a rate of greater than 
1% per year from the spine and hip220 (see Fig. 30.15). Contrary 
to earlier studies, it is now evident that the older woman who is not 
1285
Chapter 30  Osteoporosis: Basic and Clinical Asp

5
as physically active, and is not taking estrogen, is at extremely high 
risk of bone loss and subsequent fractures. The pathogenesis of 
this process is multifactorial, although dietary calcium deficiency, 
leading to secondary hyperparathyroidism, certainly plays some 
role. The average calcium intake of women in their eighth and 
ninth decades of life is now estimated to be between 800 and 1000 
mg/day. If vitamin D intake is also suboptim

1000 
mg/day. If vitamin D intake is also suboptimal and serum levels of 
25-hydroxyvitamin D are less than 20 ng/mL, or 50 nmol/L, sec­
ondary hyperparathyroidism may occur, although there are other 
causes in the elderly for increases in PTH, including a low glo­
merular filtration rate and low calcium intake.284 PTH stimulates 
osteoblasts and provokes the remodeling sequence including the 
elaboration of several cytokines that accelerate bone resorption. 
Unfortunately, in most elders, bone 

 resorption. 
Unfortunately, in most elders, bone formation is not enhanced, 
although the reasons for this are not entirely clear. Overall, the 
secondary hyperparathyroidism coupled with high 1,25(OH)2D 
(see later discussion in the Vitamin D section) leads to further 
uncoupling in the bone remodeling cycle and significant bone 
loss. Furthermore, among elders with poor calcium intake who 
live in northern latitudes, seasonal changes in vitamin D levels, 
lowering levels below 20 ng/mL, might

n D levels, 
lowering levels below 20 ng/mL, might aggravate bone loss.284,285 
Whether increased bone loss is an independent risk factor for 
future fractures in the elderly remains somewhat controversial, 
necessitating further studies to define such a risk.
Many older individuals already have established osteoporosis, 
so coincidental vitamin D deficiency due to poor intake, absent 
sunlight exposure, or impaired conversion of vitamin D to its 
active metabolite can result in osteomalacia and

active metabolite can result in osteomalacia and aggravate preex­
isting osteoporosis.286 Osteomalacia can lead to dramatic changes 
in skeletal microarchitecture, as was recently demonstrated in a 
cohort from Germany.287 Trabecular and cortical changes seen in 
that study led to microcracks and greater skeletal fragility. Priemel 
and associates287 reported that more than 50% of elders who pre­
sented with a hip fracture were vitamin D deficient. Combining 
vitamin D deficiency with inadequa

ent. Combining 
vitamin D deficiency with inadequate calcium intake enhances 
the likelihood of rapid bone loss in a very susceptible population. 
Despite these findings, elevated PTH levels in older women have 
been associated with bone loss in some studies but not in other 
studies. In elderly individuals, it has been reported that PTH levels 
are closely correlated with increased synthesis of IGFBP4, which 
suppresses IGF action on bone cells and may increase sclerostin 
secretion.288,289 Bec

and may increase sclerostin 
secretion.288,289 Because IGF1 is an important growth factor for 
osteoblasts, it is conceivable that PTH downregulates IGF activ­
ity during states of relative calcium or vitamin D deficiency. This 
would shift the remodeling balance toward preserving intravascu­
lar calcium concentrations while inhibiting new calcium incor­
poration into the skeletal matrix. This response makes teleologic 
sense, although further studies are needed to assess whether serum 
IGFBP4 i

udies are needed to assess whether serum 
IGFBP4 is a reliable marker of calcium deficiency in older indi­
viduals.289 In sum, there is little doubt that calcium and vitamin 
D insufficiency can cause accelerated bone loss in the elderly, 
although the threshold for low vitamin D and poor calcium 
intake is still actively debated. Other concomitants of aging, such 
as accumulation of ROS (seen in aged mice) and other causes of 
damage to bone cells, appear likely to contribute to age-associated 

ls, appear likely to contribute to age-associated 
osteoporosis as well.290 
Secondary Osteoporosis
The division of osteoporosis into primary and secondary forms is 
somewhat arbitrary. For example, patients with diseases that lead 
to hypogonadism early in life are considered to have secondary 
osteoporosis, whereas osteoporosis in women with natural meno­
pause and older men with low sex hormone levels is called primary 
osteoporosis. Moreover, patients may have a combination of primary 
and s

 patients may have a combination of primary 
and secondary forms. Although most postmenopausal women and 
older men do not have a definable secondary cause, those who do 
can be treated more effectively. This possibility should be consid­
ered for every patient. There are many causes of secondary osteo­
porosis (Table 30.4), only a few of which are discussed here.
Glucocorticoid-Induced Bone Loss
The most common cause of secondary osteoporosis is glucocorti­
coid-induced bone loss, which is ofte

 glucocorti­
coid-induced bone loss, which is often a result of pharmacologic 
doses of steroids used to treat inflammatory or autoimmune disor­
ders. Generally, it is considered that glucocorticoids have a dose-
dependent effect on the skeleton such that longer duration and 
higher doses of steroids are most likely to cause bone loss and 
fractures. However, there clearly are subsets of individuals who are 
more or less sensitive to the skeletal effects of high doses of gluco­
corticoids. As a 

 effects of high doses of gluco­
corticoids. As a clinical rule, those individuals with a cushingoid 
appearance and fat redistribution phenotypes almost always have 
low bone mass and fractures. Notwithstanding, BMD measure­
ments are indicated in patients on long-term glucocorticoids for 
both preventive approaches and treatment decisions.291,292
As noted previously, high circulating levels of glucocorticoids 
have a significant impact on bone acquisition and maintenance. 
In 1932, Harvey Cush

acquisition and maintenance. 
In 1932, Harvey Cushing recognized the syndrome of endog­
enous steroid excess, which included marked osteopenia and 
fractures.293,294 Long-term exposure to pharmacologic doses of 
glucocorticoids results in significant bone loss and enhanced mar­
row adipogenesis as marrow stromal cells differentiate down the 
fat lineage. In addition to having direct effects on the osteoclast 
and osteoblast, glucocorticoids also induce secondary hypogonad­
ism and hyperparathyro

induce secondary hypogonad­
ism and hyperparathyroidism, impaired vitamin D metabolism, 
muscle atrophy, and hypercalciuria. All of these factors contribute 
to a rapid and sustained loss of bone during the first few months of 
steroid therapy.295,296 The addition of other immunosuppressants, 
such as cyclosporine, has been shown to aggravate bone loss by 
further increasing bone resorption. Because the number of organ 
transplants has increased exponentially over the past decade, the 
prevalenc

exponentially over the past decade, the 
prevalence of post-transplantation osteoporosis has risen substan­
tially. Steroid-induced osteoporosis is now considered the second 
most common cause of low bone mass in the general population 
and one of the most common causes of osteoporotic fractures.297
The effects of glucocorticoids on the skeleton are multifac­
eted and are particularly devastating because these agents cause 
uncoupling in the remodeling unit. Besides the indirect suppres­
sive ef

deling unit. Besides the indirect suppres­
sive effects of glucocorticoids on the hypothalamic-gonadal axis, 
and inhibition of calcium absorption in the gut due to impaired 
1,25(OH)2D production, high doses of steroids can stimulate 
osteoclastogenesis, increase RANKL production, and decrease 
OPG. This situation results in higher rates of bone resorp­
tion. Additionally, glucocorticoids also have a strong negative 
effect on bone formation by suppressing expression of IGF1 in 
bone cells and 

suppressing expression of IGF1 in 
bone cells and by shifting marrow stromal cells into the fat lin­
eage rather than down the osteoblast differentiation pathway.292 
It is presumed that just as fat redistribution is a clinical hallmark 
of Cushing syndrome in the supraclavicular and mediastinal area, 
enhanced adiposity in the bone marrow is a characteristic fea­
ture of steroid-induced bone disease, almost certainly as a func­
tion of increased stromal cell differentiation into adipocytes.295 

 stromal cell differentiation into adipocytes.295 
Bone strength is markedly compromised by dramatic uncoupling 
in remodeling, and bone loss can be rapid over a short period, 
1286
particularly with high doses of glucocorticoids. Although there is 
no true dose-dependent effect on bone resorption, it is thought 
that prednisone doses as low as 5 mg/day may increase fracture 
risk.298 Indeed, in this syndrome, baseline BMD is not predictive 
of fractures and can

seline BMD is not predictive 
of fractures and can often be normal even in the presence of ongo­
ing resorption and recurrent fractures.299 Trabecular bone suffers 
the most in this syndrome, and spine DXA is the most sensitive 
indicator of bone loss. Markers of bone turnover are not helpful in 
the management of these patients.
Therapy for steroid-induced bone loss centers on treating the 
underlying disease and reducing the dose of glucocorticoids to the 
lowest possible regimen. Barring that

oids to the 
lowest possible regimen. Barring that, several interventions have 
been shown to retard bone loss and prevent fractures. Adequate 
calcium and vitamin D intake is critical for every patient receiv­
ing glucocorticoids. However, these measures alone are not suf­
ficient. Regulatory approval for the prevention and treatment of 
glucocorticoid-induced osteoporosis has been consistent for the 
group of bisphosphonates, including alendronate, risedronate, 
and zoledronic acid.300–303 The

ate, risedronate, 
and zoledronic acid.300–303 These drugs are administered either 
weekly or cyclically to prevent bone loss and reduce the risk of 
fractures. Some anecdotal data support the use of gonadal steroids 
in this condition, but clearly bisphosphonates are superior. In a 
randomized trial, PTH (20 μg of teriparatide subcutaneously daily 
for 18 months) significantly increased hip and spine bone mass 
density and reduced the number of new vertebral fractures (7.2% 
to 3.4%) but not ot

new vertebral fractures (7.2% 
to 3.4%) but not other fractures compared with alendronate.304 
More studies are needed to establish its long-term efficacy in this 
disease, particularly because secondary hyperparathyroidism is a 
frequent accompaniment of steroid-induced osteoporosis. 
Osteoporosis Associated With Diabetes Mellitus
Diabetes mellitus includes a group of heterogeneous metabolic 
disorders that in common show hyperglycemia. The association 
between diabetes mellitus and osteoporosi

ciation 
between diabetes mellitus and osteoporosis underpins several rel­
evant aspects. First, energy metabolism integrates the whole body; 
as such, severe disorders affecting energy metabolism do not spare 
any single system or tissue. Second, the endocrine regulation of 
bone and energy metabolism is not restricted to classical glands 
and neural regulation. Au contraire, there is an intricate mutual 
regulation between energy metabolism and mesenchymal tissues, 
including bone.305 Last, bo

 mesenchymal tissues, 
including bone.305 Last, bone fragility in diabetes mellitus rep­
resents another aspect of the heterogeneity of diabetes mellitus; 
AIDS, Acquired immunodeficiency syndrome; HIV, human immunodeficiency virus.
   
  Causes of Secondary Osteoporosis
Menstrual irregularity (even athletes)
Low testosterone and estradiol levels in men

ne and estradiol levels in men
Pregnancy and lactation
Autoimmune Disorders
Inflammatory bowel disease
Lupus erythematosus
Multiple sclerosis
Ankylosing spondylitis
Digestive and Gastrointestinal Disorders
Weight loss surgery
Gastrectomy
Hematologic/Blood Disorders
Blood and bone marrow disorders
Plasma cell dyscrasias: multiple myeloma and macroglobulinemi

 dyscrasias: multiple myeloma and macroglobulinemia
Myeloproliferative disorders: polycythemia
Thalassemia
Neurologic/Nervous System Disorders
Stroke, Parkinson disease, and multiple sclerosis
Spinal cord injuries
Mental Illness
Eating disorders
Cancer
Breast
Prostate
Connective Tissue Disorders
Osteogenesis imperfecta
Ehlers-Danlos syndrome
Menkes syndrome
Drug-Induced Disorders
Heparin
Methotrexate, cyclosporine
Luteinizing hormone–rel

Luteinizing hormone–releasing hormone agonist or antagonist 
therapy
Proton pump inhibitors
Other Diseases and Conditions
AIDS/HIV
Chronic obstructive pulmonary disease
Female athlete triad
Kidney disease
Organ transplant
Poliomyelitis and post-polio syndrome
Poor diet, including malnutrition
Lipidoses: Gaucher disease
Scurvy
TABLE 30.4
1287
striking differences 

: Basic and Clinical Aspects
striking differences exist between bone disorders in T1DM and 
T2DM. T1DM is associated with bone loss and was rapidly rec­
ognized as a risk factor for osteoporosis and bone fractures. How­
ever, bone mass is preserved or increased in T2DM, meaning that 
bone quality instead of quantity is the primordial mechanism 
affecting bone strength in this disorder.
Findings from epidemiologic studies in recent decades indi­
cating dramatic increases in the prevalence of T2DM

ating dramatic increases in the prevalence of T2DM worldwide 
mirror the evolving numbers of US residents diagnosed with 
this metabolic disorder. The “Diabesity” (obesity and T2DM) 
epidemic has been referred to as the biggest epidemic in human 
history.306 T2DM accounts for approximately 90% of all cases 
of diabetes. Globally, the number of people with diabetes is esti­
mated to be 415 million, affecting more than one third of the US 
population. Therefore, the magnitude of the importance of 

on. Therefore, the magnitude of the importance of osteo­
porosis in diabetes mellitus has only recently been fully recognized 
when it was demonstrated that the risk of fracture is increased 
not only in T1DM but also in T2DM. The delayed inclusion of 
T2DM among the risk factors of fracture was certainly influenced 
by consistent documentation that T2DM does not affect bone 
mass. Although there are studies suggesting that the rate of bone 
loss is increased in T2DM, in all probability bone qua

 is increased in T2DM, in all probability bone quality, rather 
than quantity, is primarily affected by this disorder. At the same 
level of BMD, individuals with T2DM have increased fracture risk 
when compared with normoglycemic individuals247 (Fig. 30.21).
Several findings support the increased bone mass observed in 
T2DM. First of all, obesity, which is the central determinant for 
the emergence of insulin resistance and hyperglycemia, has a posi­
tive effect on bone mass.307 Bone remodeling

posi­
tive effect on bone mass.307 Bone remodeling is typically reduced 
in diabetes mellitus.308 Moreover, a clinical study described a 
negative association between bone mass and the serum levels of 
osteocalcin in T2DM. Most likely, these results mean that bone 
turnover is not only reduced, but it also exerts a protective effect 
on bone mass in T2DM.194 Hyperinsulinemia is another element 
involved in the protection of bone mass in T2DM. The strik­
ing difference in the secretion of growth 

 strik­
ing difference in the secretion of growth factors and adipokines 
between T1DM and T2DM certainly is another point to be taken 
into consideration. T1DM is usually associated with low serum 
levels of IGF1, whereas this is not part of the hormonal alterations 
in T2DM. However, T2DM predominantly exhibits an inflam­
matory profile in adipokine secretion by the white adipose tissue 
that is represented by increased levels of leptin, chemerin, resistin, 
TNF, and interleukins, whereas adip

in, resistin, 
TNF, and interleukins, whereas adiponectin is reduced. Leptin 
and adiponectin have complex effects on bone, and there are still 
no conclusive results about their ultimate effects on bone.305
The paradoxical increase in fracture risk in T2DM has respec­
tively challenged researchers and clinicians to unveil mechanisms 
determining bone fragility and expedite the identification of indi­
viduals at risk, as well as the best therapeutic options to prevent 
fracture in this populatio

tic options to prevent 
fracture in this population. Using an animal model mimicking 
T2DM, structural alterations in collagen and deterioration in 
bone strength were found coincidently with the onset of hyper­
glycemia.309 The analysis showed an increase in AGEs, whereas a 
decrease in immature enzymatic collagen cross-links occurred in 
diabetic animals.309 Under clinical investigation, it was observed 
that the urinary excretion of pentosidine, an AGE, was associ­
ated with vertebral and hip

e, an AGE, was associ­
ated with vertebral and hip fracture in postmenopausal women.310 
Advances in imaging techniques allowed the detection of altera­
tions not previously captured by conventional methods. Long 
bone cortical porosity was described in individuals diagnosed with 
T2DM.311,312 Although this finding was not confirmed in other 
studies, cortical porosity remains a potential alteration deter­
mining fracture susceptibility in individuals with T2DM.313,314 
Magnetic resonance has be

duals with T2DM.313,314 
Magnetic resonance has been successfully used to quantify MAT 
and the composition of lipids within marrow adipocytes305 (Fig. 
30.22A). In several conditions, such as anorexia nervosa, glucocor­
ticoid therapy, and aging, there is an inverse relationship between 
bone mass and MAT (see Fig. 30.22B). The amount of MAT was 
similar in normal weight/normoglycemic, obese/normoglycemic 
and obese/diabetic individuals.194 However, another study found 
that both fractures and 

ver, another study found 
that both fractures and T2DM are associated with alterations in 
the composition of lipids in MAT, namely increased saturated and 
decreased unsaturated lipids.315 In accordance, individuals with 
diabetes who are diagnosed with fractures exhibit remarkable 
Women
-3
-4
-2
-1
0
40
30
20
10
0
Femoral Neck T-score
10-Year Hip Fracture Risk (%)
A
Men
-3
B
-2
-1
0
40
30
20
10
0
DM, no insulin
DM, insulin
Non-DM
• Fig. 30.21

(%)
Non-DM
• Fig. 30.21  Femoral neck bone mineral density T-score and 10-year fracture risk at age 75 years by dia­
betes status and insulin use. Estimated 10-year cumulative fracture risk at age 75 years in women (A) and 
men (B), calculated using the Cox proportional hazards regression model baseline survival function raised 
to the power of the relative hazard for each combination of diabetes group and T-score. DM, diabetes 
mellitus. (From Schwartz AV, Vittinghoff

diabetes 
mellitus. (From Schwartz AV, Vittinghoff E, Bauer DC, et al. Association of BMD and FRAX score with risk 
of fracture in older adults with type 2 diabetes. JAMA. 2011;305[21]:2184–2192.)
1288
accentuation of saturated lipids within MAT.315 More recently, 
a clinical investigation, using an in vivo microindentation device, 
observed an alteration in a bone material property, namely the 
bone material strength index (BMSi). The studies also highlighted 

ength index (BMSi). The studies also highlighted 
a negative association between BMSi and long-standing poor 
metabolic control, indicating a connection between bone disease 
and chronic diabetes complications.313 Reinforcing these results, 
there was also an inverse association between BMSi and AGEs, as 
determined by skin autofluorescence.313
Altogether, BMD underestimates fracture risk in individuals 
with T2DM (see Fig. 30.21). Objectively, for a similar hip frac­
ture risk, it was estimated

or a similar hip frac­
ture risk, it was estimated that T2DM women and men have 
higher T-scores of 0.59 (95% confidence interval, 0.31–0.87) and 
0.38 (95% confidence interval, 0.09–0.66), respectively, than nor­
moglycemic individuals.247 Awareness regarding the implication 
of osteoporotic fractures in individuals with diabetes has to be 
highlighted due to their increased morbidity and mortality rate, as 
well as the worse fracture outcomes in this population.
The intricate relationship betw

n this population.
The intricate relationship between bone and energy metabo­
lism aggregates issues concerning the potential repercussions of 
osteoporosis therapy on glucose metabolism, on one side, and the 
effects of diabetes management on bone mass and fracture occur­
rence, on the other. Osteoblasts express insulin receptor, and the 
experimental silencing of insulin receptor specifically in osteoblasts 
results in both decreased bone mass and impairment in glucose 
tolerance. Two research

and impairment in glucose 
tolerance. Two research groups independently collected evidence 
80
UL
0
8.0
7.0
6.0
5.0
Chemical shift (ppm)
4.0
3.0
2.0
1.0
0
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
UL
RL
W
SL
MALE
W
RL
FEMALE
SL
70
60
50
r=0.75
p < 0.0001 
Female
Male
40
30
20
10
0
0
60
80
40
20
MAT (%)
B
A
• Fig. 30.22  (A) 1H magnetic resonance spectroscopy allows the estimation of lipids composition in MAT. 
Recent studies called attention for the clinical relevance of t

s called attention for the clinical relevance of this parameter, such as the association of saturated 
lipids with fracture in diabetic individuals. The top figure exhibits differences in unsaturated fat between male 
and female (female > male). (B) Age-related increase in marrow adipose tissue in a group of healthy controls: 
53 individuals, 28 females and 25 males; age, female: 38.8 ± 13.2 years and male: 39.7 ± 15.6 years; weight, 
female: 60.4 ± 6.4 kg and male: 66.0 ± 8.0 kg; height, female

4 ± 6.4 kg and male: 66.0 ± 8.0 kg; height, female: 1.65 ± 0.08 m and male: 1.71 ± 0.06 m; and 
body mass index, female: 22.1 ± 1.8 kg/m2 and male: 22.3 ± 1.9 kg/m2). H, hydrogen; MAT, marrow adipose 
tissue; RL, residual lipids; SL, saturated lipids; UL, unsaturated lipids, W, water. (Redrawn from de Paula FJ and 
Rosen CJ. Structure and function of bone marrow adipocytes. Compr Physiol. 2017;8[1]:315–349.)
1289
that uncarboxylated osteocalci

nd Clinical Aspects
that uncarboxylated osteocalcin is an endocrine molecule originat­
ing in osteoblasts that modulates insulin secretion and sensitiv­
ity.21,22 Initially produced in a carboxylated form, osteocalcin is 
the most abundant noncollagenous protein in the bone matrix and 
has a role in the mineralization process. The acidic environment 
created by osteoclasts to elicit bone resorption promotes osteo­
calcin decarboxylation. GPRC6A, a G-protein coupled receptor, 
has been identified

a G-protein coupled receptor, 
has been identified as the pancreatic effector of uncarboxylated 
osteocalcin promoting beta-cell hyperplasia and insulin secretion, 
as verified in several experimental models.316 Moreover, uncarbox­
ylated osteocalcin seems to increase insulin sensitivity by stimu­
lating adiponectin secretion in white adipose tissue.317 However, 
inconclusive findings have been reported in clinical investigations 
related to the association between osteocalcin and insulin sensit

association between osteocalcin and insulin sensitiv­
ity, as well as glucose tolerance. For instance, individuals showing 
decreased circulatory levels of osteocalcin due to bisphosphonate 
and denosumab therapy do not exhibit increased incidence of 
diabetes.318,319 Moreover, patients diagnosed with primary hyper­
parathyroidism exhibit insulin resistance despite an increased rate 
of bone remodeling and increased serum levels of osteocalcin.320
Antidiabetic drugs have diverse effects on bone,

0
Antidiabetic drugs have diverse effects on bone, from benefi­
cial to neutral or even detrimental effects321 (Table 30.5). Thus, 
implementation of an appropriate strategy is necessary to preclude 
undesirable increase in fracture risk in these individuals. Thiazoli­
dinediones are a group of drugs associated with adverse effects on 
bone mass and increased risk of fracture. Thiazolidinediones are 
exogenous agonists of PPARs that drive bone marrow mesenchy­
mal stromal cells to differentiate 

rrow mesenchy­
mal stromal cells to differentiate into adipocytes, inhibiting osteo­
blastogenesis via decreasing Runx2 transcription factor, as well as 
IGF1 and Wnt signaling pathways. Insulin positively affects bone 
mass; notwithstanding, insulin therapy is associated with bone 
fracture. The mechanism determining bone fragility and whether 
it is mediated through hypoglycemia and increased risk of falls are 
still to be clarified. Metformin and sulfonylureas, however, seem 
to have neutral 

and sulfonylureas, however, seem 
to have neutral or beneficial effects on bone. Among the sodium-
dependent glucose transporter 2 inhibitors, issues have been raised 
regarding the bone safety of canagliflozin.322 There are several 
potential determinants of detrimental effect of sodium-dependent 
glucose transporter 2 inhibitors on bone, including hyponatre­
mia-induced bone resorption.323 In addition elevation in the 
serum levels of phosphate can stimulate the secretion of PTH and 
possibly 

 can stimulate the secretion of PTH and 
possibly FGF23, thereby leading to bone loss.324 Experimental 
evidence indicates that incretins-based therapy (DDP4 inhibi­
tors) and GLP1 receptor agonists (GLP1RA) have positive effects 
on bone. Osteopenia, increased osteoclast activity/numbers, and 
reduced bone strength are components of the bone phenotype in 
GLP1 receptor (GLP1R) knockout mice. Moreover, administra­
tion of GLP1RA reversed the bone loss in rats subjected to an 
experimental model 

 loss in rats subjected to an 
experimental model of disuse osteoporosis.325 Notwithstanding, 
there is limited clinical data about the bone effects of DPP4 inhib­
itors and GLP1RA; additional studies are needed to clarify the 
effects of the incretins-based therapy on fracture risk. 
Factors That Impair Peak Bone Acquisition
Peak bone mass is acquired between the ages of 10 to 16 years. It 
is the zenith of bone acquisition and represents the sum of several 
processes including a marked increas

m of several 
processes including a marked increase in bone formation.326 Boys 
tend to peak 2 years later than girls, and their BMD is higher 
than women at all skeletal sites. In part, this difference relates to 
a greater cross-sectional bone area in males than females.326 Peak 
bone mass results from linear growth and consolidation of corti­
cal and trabecular components. Acquisition is most rapid during 
the latter stages of puberty and coincides with maximum growth 
hormone secretion, high

cides with maximum growth 
hormone secretion, high serum IGF1 levels, and rising levels of 
estradiol and testosterone. In addition, calcium absorption is 
maximum and skeletal accretion is optimal owing to higher levels 
of 1,25(OH)2D. These processes coalesce over a relatively short 
period to produce a bone mass that subsequently plateaus and 
then falls during later life. It is estimated that more than 60% 
of adult bone mass can be related to peak acquisition.327 Hence, 
understanding the m

o peak acquisition.327 Hence, 
understanding the mechanisms responsible for low bone mass 
must include perturbations in peak bone acquisition.
There are several hormonal, environmental, and heritable 
determinants of peak bone mass, including estrogen/testosterone, 
growth hormone/IGF1, calcium/vitamin D, and unknown genetic 
factors. If any is perturbed, dramatic alterations in peak bone mass 
may occur, setting the stage for low bone density throughout life. 
Not surprisingly, gonadal steroid

hroughout life. 
Not surprisingly, gonadal steroids are essential not only to bone 
maintenance but also to acquisition. During puberty, estrogen 
and testosterone levels rise and contribute to consolidation of 
bone mass. Estrogen is also necessary for epiphyseal closure. Stud­
ies of males with an estrogen receptor mutation and men with an 
aromatase deficiency have established that estradiol is critical for 
  Effects of Antidiabetic Agents on Bone Based on Clinical Investigation
BONE BIOMAR

n Bone Based on Clinical Investigation
BONE BIOMARKERS
BMD
Fracture
Agent
Bone Resorption
Metformin
↓/=
↓/=
=/↑
↓/=
Sulfonylureas
↑/=
↓/=
ND
↓/=/↑
Thiazolidinediones
↓↓/=/↑
↑↑/=
↓↓/=
↑↑/=
Incretin (GLP1 analogue)
=
↓↓
↑/=
=
Incretin (DPP4i)
↓/=
=
–
↓/=
SGLT2 inhibitor
=
=/↑
=
=/↑
Insulin
=
=
=
↑
↑, increased; ↓, decreased; =, unchanged; DPP4i, Dipeptidyl peptidase inhibitor; GLP1, glucagon-like peptide 1; ND, no data; SGLT2, sodium-dependent glucose cotransporter 2.
Adapted from P

-dependent glucose cotransporter 2.
Adapted from Palermo A, D’Onofrio L, Eastell R, et al. Oral anti-diabetic drugs and fracture risk, cut to the bone: safe or dangerous? A narrative review. Osteoporos Int. 2015;26(8):2073–2089.
   
TABLE 30.5
1290
bone acquisition.199,271,272 These young men share several pheno­
typic characteristics, including tall stature, unfused epiphysis, and 
very low bone mass. Hence, there must be a threshold effect for 
estradiol in me

re must be a threshold effect for 
estradiol in men, and this effect must be time dependent. Similar 
conclusions can be drawn from studies in women. Acquired defi­
ciencies in estrogen, such as occur with anorexia nervosa or che­
motherapy-induced ovarian dysfunction, result in low peak bone 
mass and lead to subsequent risk for osteoporosis.328,329 Similar 
findings have been noted in patients with untreated Turner syn­
drome and in men with Klinefelter syndrome.330,331
The timing of gonadal s

inefelter syndrome.330,331
The timing of gonadal steroid surges is critical for bone acquisi­
tion because there is a relatively short window of time in which bone 
formation is favored and matrix synthesis is markedly enhanced.332 
That time window is likely to be less than 3 years and earlier in girls 
than boys. Probably the best study that addressed this issue comes 
from a retrospective analysis of men in their 30s who underwent 
late onset of puberty (i.e., at the age of 17 or 18 years) bu

of puberty (i.e., at the age of 17 or 18 years) but were 
otherwise normal by full endocrine testing.333 These men had sig­
nificantly lower BMD than age-matched men who went through 
puberty at the normal time. These data suggest that both timing 
and quantity of gonadal steroids are critical for bone acquisition.
Pubertal surges of estrogen and androgens are also important 
for priming the growth hormone/IGF1 axis. Rising levels of both 
contribute to growth hormone surges that lead to increas

bute to growth hormone surges that lead to increases in cir­
culating and tissue expression of IGF1, an essential growth factor 
for chondrocyte hypertrophy and expansion. IGF1 may also be 
critical in defining the cross-sectional size of bone, a potentially 
important determinant of bone strength.334 Once again, stud­
ies in individuals who are growth hormone deficient or resistant 
have established that low levels of circulating IGF1, especially 
during puberty, are associated with reduced bon

y 
during puberty, are associated with reduced bone mass.170,335 In 
addition, recombinant human growth hormone replacement has 
been shown to restore linear growth and improve peak bone mass 
acquisition. Several studies in experimental animals, including 
inbred strains of mice, have established that IGF1 is important for 
bone acquisition and that the timing of IGF1 peaks coincides with 
maximal rates of bone formation.336 Impairment in production of 
IGF1 due to acquired disorders (e.g., ano

tion of 
IGF1 due to acquired disorders (e.g., anorexia nervosa), malnutri­
tion, delayed puberty, or diabetes mellitus can also impede peak 
bone acquisition.337
Hormonal abnormalities not only enhance bone resorption in 
older individuals but also may blunt the capacity of bone cells 
to maximize bone formation during adolescence. Clearly, hypo­
gonadal boys and girls have impaired peak bone mass, resulting 
in low adult BMD.330,331 One form of contraception, medroxy­
progesterone, may reduce 

 contraception, medroxy­
progesterone, may reduce estrogen concentrations enough in the 
teenage girl to reduce her capacity to acquire peak bone mass.338 
Similarly, it seems likely, although not yet proved, that smoking 
during the teen years could impair osteoblast activity and flatten 
projected trajectories for peak bone acquisition.
To mineralize newly synthesized bone, calcium must become 
bioavailable to the skeletal matrix. In experimental studies in 
rodents and humans, it is clear tha

al studies in 
rodents and humans, it is clear that the several pools of available 
calcium are markedly enhanced during puberty. These sources 
include calcium efflux from the gastrointestinal tract and the 
calcium pool available for incorporation in the matrix. It is no 
coincidence that growth hormone surges not only increase IGF1 
(thereby enhancing skeletal growth and matrix biosynthesis) but 
also result in increases in 1,25(OH)2D (possibly via IGF1 induc­
tion of 1α-hydroxylase activity)

y via IGF1 induc­
tion of 1α-hydroxylase activity), the active metabolite of vitamin 
D, which markedly enhances calcium absorption from the gut.339–
341 Although there are no longitudinal studies in pubertal indi­
viduals with prolonged calcium deficiency, several randomized, 
placebo-controlled trials (RPCTs) in pubertal and prepubertal 
girls and boys have established that supplemental calcium can 
enhance BMD.342,343 In a twin study in which one twin received 
calcium supplementation and one

one twin received 
calcium supplementation and one received placebo, radial BMD 
increased by as much as 5% after 3 years in the twins given cal­
cium when compared with those given placebo.343 This study sug­
gests that there is significant gene-environmental interaction, and 
that even in those individuals with heritable determinants of low 
peak bone mass, calcium supplementation may provide an impor­
tant and relatively simple means of protecting individuals from 
future osteoporotic fractur

ting individuals from 
future osteoporotic fractures. 
Genetic Factors That Determine Peak Bone Mass
Probably the most important determinant of peak bone mass, 
albeit one that has lacked clear definition, is the genetic contri­
bution. As noted earlier, low peak bone mass may be the most 
important pathogenic factor in the osteoporosis syndrome of later 
life. Further, it appears that at least 50% of peak bone mass is 
determined by genetic factors.344–346 Efforts to define heritable 
determina

ors.344–346 Efforts to define heritable 
determinants of peak bone mass have been plagued by several 
issues that are also common to other complex diseases. They 
include the following: (1) a quantifiable phenotype, (2) heteroge­
neity within a given population under study, and (3) the polygenic 
nature of the disorder. In the postgenomic era, it has become clear 
that complex diseases, such as osteoporosis, cannot be settled by 
single nucleotide polymorphisms (SNPs). Instead, they are origi­
n

e polymorphisms (SNPs). Instead, they are origi­
nated by a confluence of multiple genetic variations. Naturally, the 
interest has been driven to assess epistasis to unveil the complex 
network involved within multiple SNP interactions, as well as the 
influence of environmental factor into gene expression. Notwith­
standing these barriers, it is now clear that BMD is an accept­
able phenotype for defining heritable determinants. In addition, 
BMD is fully quantifiable and therefore is amenable

MD is fully quantifiable and therefore is amenable to complex 
trait analysis. Moreover, BMD in the population is distributed in a 
gaussian manner, thereby allowing analyses at the extremes (<−2.0 
SD or >2.0 SD) of the density distribution. Large homogeneous 
and heterogeneous populations are now being studied to ascertain 
genetic determinants of bone density in humans. Candidate genes 
identified by whole genome studies include RANKL, OPG, the 
VDR, collagen IA1, the estrogen receptor IL1, I

e 
VDR, collagen IA1, the estrogen receptor IL1, IGF1, and oth­
ers. Depending on the cohort, the phenotype, and the number of 
individuals studied, there are likely to be hundreds of genes that 
contribute to individual variation in bone mass.347 Indeed, in most 
large genome-wide association studies using validation cohorts to 
confirm candidate genes, the effect size of most noncoding SNPs 
is at best 1%. Notwithstanding, there has been a major effort to 
consolidate studies from around the w

r effort to 
consolidate studies from around the world to increase power and 
detect rare variants that might provide even greater insight into 
genetic determinants, and importantly shed greater light on the 
biology of low bone mass. Twin studies examining discordant or 
concordant phenotypes are also helpful, as are sibling-pair studies, 
although the results have been disappointing.76,347,348
Originally, Johnson and colleagues349 identified an extended 
family with very high bone density and

n extended 
family with very high bone density and fine mapped the locus to 
a region in chromosome 11. After several years of intense high-
throughput analysis, the high-bone-density gene that was mutated 
in this family was identified as LRP5.350 This member of the lipo­
protein receptor family is important for binding Wnts, ligands 
critical for cell differentiation in several organisms. One year 
earlier, Gong and colleagues351 identified mutations in the LRP5 
gene in several children with 

ations in the LRP5 
gene in several children with osteoporosis-pseudoglioma syn­
drome. The potential pathways that direct osteoblast function and 
1291
mineralization through LRP5, a coreceptor for the Wnt receptor, 
frizzled, have opened up new areas of investigation (Fig. 30.23). 
Moreover, natural antagonists to the Wnt/LRP5 signaling system, 
including sclerostin and Dkk1, have been studied using genetic 
engineering in mice. In the past 

d using genetic 
engineering in mice. In the past 5 years, LRP5 has been stud­
ied extensively both in its function and its allelic effects through 
genome-wide association studies, as well as in translational bench 
work. Importantly, LRP5 polymorphisms have been strongly asso­
ciated with BMD and fractures in large cohort studies, consistent 
with the central role of LRP5 in human skeletal biology. Hence, 
this pathway must be important in defining peak bone mass. How­
ever, it is also clear t

ning peak bone mass. How­
ever, it is also clear that because BMD is a polygenic trait, other 
genes are soon to be discovered.
In addition to the search for osteoporosis genes, intervention 
studies in adolescents have provided insight into the environ­
mental impact on genetic determinants.347 In another twin study 
from the Indiana group, the investigators found that as long as 
calcium supplementation continued during puberty, young boys 
could enhance their peak bone mass.352 In a Swiss stu

ld enhance their peak bone mass.352 In a Swiss study, younger 
prepubertal girls supplemented with a protein product had a sig­
nificant increase in spine bone density, as did a cohort of pubertal 
girls receiving a milk powder in England.353 Remarkably, in the 
latter cohort, serum IGF1 levels also rose dramatically, providing 
further indirect evidence of a link between pubertal status, bone 
mass, and the growth hormone/IGF1 axis.353 
Approach to Management of Osteoporosis
A comprehensive man

 to Management of Osteoporosis
A comprehensive management plan for osteoporosis includes diag­
nosing those at highest risk, excluding secondary causes of low BMD, 
and selecting appropriate treatment. Screening by DXA to assess 
BMD at the hip and spine has been commonly recommended in 
most women older than 65 years and men older than 70 years,354,355 
and many, but not all, have this assessment. In addition, there is a 
growing appreciation that occurrence of a fracture, particularly one 
due

at occurrence of a fracture, particularly one 
due to minimal trauma, should be a signal for more comprehen­
sive risk assessment including BMD and has led to the explosion in 
development of fracture liaison services over the past 5 years and the 
emphasis on their importance by many groups.263,354,356
With increasing recognition of the importance of risk factors 
in addition to BMD, an assessment of risk should include both 
BMD and risk factors. These factors can be easily combined in the 
FR

s. These factors can be easily combined in the 
FRAX algorithm to attain an overall risk of fracture, which may be 
helpful, in addition to BMD alone, in making clinical decisions. 
FRAX scores are available online and have now been implemented 
within the software for most DXA scanners so that primary care 
physicians can easily obtain relevant diagnostic information in 
one report.
One commonly used algorithm in the United States is within 
the guidelines developed by the National Osteoporosis

. The calcium-sensing receptor is 
antagonized by MK-5442 and triggers short bursts of PTH secretion. Binding of PTH to its receptor 
enhances osteoblast functions and bone formation. Presence of Wnt antagonists Dkk1 and sclerostin 
inhibits Wnt signaling. Dkk1 needs to form a complex with Kremen to bind LRP5/6, whereas scleros­
tin binds LRP5/6 directly. BHQ-880 and AMG-785 are antibodies for Dkk1 and sclerostin, respectively. 
After neutralizing Dkk1 and sclerostin, Wnt can bind to LRP5/6, whi

g Dkk1 and sclerostin, Wnt can bind to LRP5/6, which results in degradation GSK3β. As 
a consequence, β-catenin is stabilized, accumulates, and translocates into the nucleus, where it reg­
ulates transcription of osteoblastic genes. APC, adenomatosis polyposis coli; cAMP, cyclic adenosine 
monophosphate; CaSR, calcium-sensing receptor; Dkk1, dickkopf 1; GSK, glycogen synthase kinase 
3; LRP, lipoprotein receptor–related protein; PKA, protein kinase A; PTH, parathyroid hormone; PTH1R, 
PTH 1 rece

se A; PTH, parathyroid hormone; PTH1R, 
PTH 1 receptor. (From Rachner TD, Khosla S, Hofbauer LC. Osteoporosis: now and the future. Lancet. 
2011;377[9773]:1276–1287.)
1292
with a BMD T-score below −2.5 or a history of hip or spine 
fracture be considered for treatment. In these guidelines, FRAX 
is used for patients with BMD that is low but not in the osteo­
porotic range. In other countries, treatment decisions are based 
solely on risk, but consideration of tr

are based 
solely on risk, but consideration of treatment of those with BMD 
T-scores below −2.5, regardless of other risk, is well justified by 
many clinical trial results.357–359
Clinical decision making regarding pharmacologic therapy 
should consider several caveats. First, osteoporosis therapy can 
reduce fracture risk by as much as 50%, but this means that some 
people will continue to have fractures despite treatment. Second, 
lifestyle and pharmacologic interventions may be long-term co

nd pharmacologic interventions may be long-term com­
mitments, and therefore cost, compliance, and safety must be fac­
tored into therapeutic decisions. Studies suggest that with weekly 
or monthly oral bisphosphonate therapy, more than 40% of indi­
viduals treated will not continue therapy beyond 1 year. Third, 
it is not uncommon for women with T-scores higher than −2.5 
to have fragility fractures and therefore have osteoporosis by the 
consensus conference definition mentioned earlier.198 In

sus conference definition mentioned earlier.198 In fact, in 
the National Osteoporosis Risk Assessment cohort of more than 
140,000 postmenopausal women in the United States, almost one 
third of the women who had fractures also had BMD scores in the 
low BMD (T-score between −1 and −2.5) range.
Diet
Calcium
Calcium supplementation should be an adjunct to drug treat­
ments for women with established osteoporosis and must be part 
of any prevention strategy to ameliorate bone los

of any prevention strategy to ameliorate bone loss. Increased cal­
cium intake reduces the secondary hyperparathyroidism often 
seen with advancing age and can enhance mineralization of newly 
formed bone. Evidence that calcium and vitamin D together or 
individually reduce fracture risk in the osteoporotic individual 
remains somewhat controversial. However, a recent meta-analysis 
of calcium and vitamin D intervention trials demonstrated a con­
sistent, albeit small, increase in BMD and a red

­
sistent, albeit small, increase in BMD and a reduction in non­
vertebral fractures when 1200 mg of calcium is combined with 
more than 800 units of vitamin D.360 Calcium supplementation 
alone has not been shown to reduce the incidence of nonverte­
bral fractures in high-risk women. Recently, a very large calcium 
intervention trial from the Women’s Health Initiative (WHI) did 
not demonstrate hip fracture reduction with daily calcium supple­
ments and 400 IU of vitamin D for all postmenopausa

ents and 400 IU of vitamin D for all postmenopausal women, 
but for those older than 60 years, the risk reduction was statisti­
cally significant.361 Interestingly, calcium supplementation in this 
cohort was associated with a 17% greater risk of kidney stones. 
Currently, an average total calcium intake of 1200 mg/day is still 
recommended by the Institute of Medicine (IOM) for all post­
menopausal women. With intakes greater than 2000 mg/day, the 
risk of nephrocalcinosis certainly increases. 

he 
Vitamin D
Vitamin D is essential for skeletal maintenance and for enhance­
ment of calcium absorption. Insufficiency of this vitamin is a 
growing problem; as many as two thirds of all patients who have 
hip fractures are classified as vitamin D deficient. Elderly people in 
chronic care living situations are particularly vulnerable. One large 
RPCT demonstrated a 43% reduction in hip fractures for nursing 
home residents, 84 years of age, who r

or nursing 
home residents, 84 years of age, who received calcium and vitamin 
D compared with those receiving placebo.362 However, those sub­
jects had significantly suppressed 25(OH)D levels and hence were 
likely to also have osteomalacia. In a large population-based study 
with calcium and vitamin D, supplementation had no effect on 
nonvertebral fractures,361 although compliance and assessment of 
vitamin D levels were not sufficiently well documented to exclude 
an effect. At least one met

documented to exclude 
an effect. At least one meta-analysis suggests that 800 IU/day of 
vitamin D plus 1200 mg of calcium per day is needed to reduce 
hip fractures by about 10%.360 The IOM recommends an aver­
age intake of 600 IU/day, except for men and women older than 
70 years, for which the IOM recommends 800 IU of vitamin D 
per day. The tolerable upper limit of vitamin D supplementation 
noted by the IOM was 4000 IU/day.
It should be noted that the US Preventive Services Task Force 
met

ed that the US Preventive Services Task Force 
meta-analysis failed to show an effect of calcium and vitamin D 
on fracture risk.363 And three systematic reviews showed that vita­
min D alone did not affect BMD.364–366 Thus, if there is an effect 
of vitamin D on bone mass and fracture, it probably is limited 
to older individuals who are at high risk and have low calcium 
intake. Besides the potentially positive effects of vitamin D sup­
plementation on the skeleton, particularly in older women

ation on the skeleton, particularly in older women, vita­
min D may reduce the risk of falling, although there continues to 
be significant controversy over the effect size, as noted in a recent 
meta-analysis. Moreover, it is uncertain whether the VDR expres­
sion level in adult muscle tissue is sufficient to mediate any direct 
effects. Therefore, for most individuals with osteoporosis, 800 
IU/day of vitamin D is sufficient to maintain adequate levels of 
25-hydroxyvitamin D. However, in thos

e levels of 
25-hydroxyvitamin D. However, in those patients with low bone 
mass and insufficient or deficient 25-hydroxyvitamin D levels (i.e., 
<20 ng/mL), administration of 50,000 IU of ergocalciferol (vita­
min D2) or cholecalciferol (vitamin D3) given once weekly is a safe 
and effective way to restore vitamin D levels to the normal range. 
Moreover, high-dose bolus vitamin D supplementation to prevent 
fractures, falls, and or/cardiovascular disease should be avoided 
due to inefficacy (10

r disease should be avoided 
due to inefficacy (100,000 IU of cholecalciferol monthly),367,368 
or even an increased risk of falls and fractures (annual dose of 
500,000 IU of cholecalciferol administered orally).369 Upper lev­
els of vitamin D are currently being reviewed to determine if there 
is toxicity at higher doses. The current upper limit of tolerability 
is set at 4000 IU per day. The US Preventive Services Task Force 
did not recommend screening of normal individuals for adequacy 
of 

 screening of normal individuals for adequacy 
of vitamin D levels; however, among osteoporotic patients, mea­
surement of at least one serum 25(OH)D level is considered to 
be standard of care, particularly for those individuals starting 
bisphosphonate therapy.
Vitamin D analogues have been used in the treatment of osteo­
porosis since the early 1980s. However, this remains a contro­
versial area. High doses of 1,25(OH)2D increase bone mass, but 
many patients develop hypercalciuria or hyperca

t 
many patients develop hypercalciuria or hypercalcemia. At doses 
of 0.5 μg/day, 1,25(OH)2D reduced the rate of both vertebral and 
nonvertebral fractures and increased bone density, but this find­
ing was made in a very small randomized trial. Other studies have 
found little benefit with a narrow therapeutic window, particu­
larly in relation to renal function and hypercalcemia. Currently, 
vitamin D analogues are not recommended for the routine treat­
ment of osteoporosis. A subset of patie

ine treat­
ment of osteoporosis. A subset of patients with renal insufficiency 
(chronic kidney disease ≥3 and high PTH levels) may benefit 
from supplementation with calcitriol, with very careful monitor­
ing of serum PTH. 
Physical Activity
Bed rest or immobility, particularly in elderly persons, can result 
in rapid bone loss. Moreover, the number of falls increases with 
age, and the number of falls that result in fractures also rises. A 
meta-analysis by the Cochrane Review Group demonstrat

a-analysis by the Cochrane Review Group demonstrated that 
1293
muscle strengthening, balance retraining, home hazard assess­
ment, withdrawal of psychotropic medications, and use of a 
multidisciplinary risk factor assessment program are beneficial in 
protecting against falls.370 An alternative approach is to reduce 
loads applied to the hip during a fall by padding. Hip protectors 
have been shown to reduce the risk of hip fractures in at l

 shown to reduce the risk of hip fractures in at least one 
population, although compliance is generally poor. A more recent 
study failed to demonstrate the efficacy of these devices in older 
women in an assisted living facility.371 Regular physical activity, 
including aerobic, weight-bearing, and resistant exercises, is effec­
tive in increasing spine BMD and in strengthening muscle mass 
in postmenopausal women, but no large-scale studies have estab­
lished whether these interventions reduc

ve estab­
lished whether these interventions reduce fracture risk. 
Lifestyle
Other interventions, such as smoking cessation and reduction 
of alcohol intake, should be considered within the framework 
of an individual’s preventive health strategy. However, studies to 
date have been inconclusive with respect to understanding how 
changes in these lifestyles affect overall fracture risk. Notwithstand­
ing, some promising data support the use of Tai Chi to enhance 
balance and reduce falls and fr

ai Chi to enhance 
balance and reduce falls and fractures.370 The data on cigarette 
smoking and fractures are somewhat conflicted, although most 
investigators support the notion that smoking may impair peak 
bone acquisition in adolescence and could contribute to post­
menopausal bone loss directly or via the development of chronic 
obstructive lung disease with hypoxia and hypercarbia.372 Lifestyle 
also includes tools to manage activities of daily life with a particu­
lar focus on avoiding f

daily life with a particu­
lar focus on avoiding falls and eliminating barriers to ambulation. 
Pharmacologic Approaches to the Treatment 
of Osteoporosis
Abundant evidence indicates that an aggressive intervention program 
can be successful in reducing fracture risk and in improving quality 
of life among postmenopausal women with preexisting osteoporosis. 
Several pharmacologic options are available, and they can be classified 
by their mechanism of action. The two major classes of osteoporosi

sm of action. The two major classes of osteoporosis 
drugs are (1) antiresorptives (i.e., agents that block bone resorption 
by inhibiting osteoclasts) and (2) anabolics (i.e., drugs that stimulate 
bone formation by primarily acting on osteoblasts).
Antiresorptive Agents
Antiresorptives inhibit bone resorption by suppressing osteoclast 
activity. Slowing the remodeling cycle allows bone formation to 
catch up to resorption, thereby enhancing matrix mineralization 
and stabilizing trabecular mic

rix mineralization 
and stabilizing trabecular microarchitecture. The antiresorptives 
increase BMD and reduce fracture risk, but their efficacy varies.
Estrogen
Estrogen replacement therapy was long considered the cornerstone 
of therapy for postmenopausal women with osteoporosis. Stud­
ies in vitro and in vivo have supported the hypothesis that this 
hormone works by slowing bone resorption through inhibition of 
cytokine signaling from the osteoblast to the osteoclast, thereby 
increasing BMD

eoblast to the osteoclast, thereby 
increasing BMD. However, there is also compelling evidence from 
at least two groups that osteoclasts have estrogen receptors and 
that estrogen blocks apoptosis of osteoclasts.373,374
Estrogen treatment inhibits both cortical and trabecular bone 
loss, and BMD generally increases by 3% to 5% after 3 years.375 
There does not appear to be an additive effect from progester­
one on bone mass in women also receiving estrogen. Conversely, 
progesterone is a necess

ng estrogen. Conversely, 
progesterone is a necessary part of hormone replacement therapy 
in women with a uterus because it prevents the development of 
endometrial hyperplasia and carcinoma. In the WHI, estrogen 
and progesterone lowered hip fracture risk by one third.268 Low-
dose conjugated estrogens (0.3 or 0.45 mg/day) and ultralow-dose 
estradiol increase BMD and have been approved for the preven­
tion of bone loss, but antifracture efficacy for these preparations 
has not been establishe

cy for these preparations 
has not been established. Discontinuation of estrogen results in 
measurable bone loss (3–5% in the first year), although contro­
versy exists as to whether that translates into a greater fracture risk.
Significant concern has been noted about the nonskeletal risks 
associated with long-term estrogen and estrogen in combination 
with progesterone. Particularly troublesome is the increased risk 
of breast cancer with the long-term use of estrogen and proges­
terone. In 

 long-term use of estrogen and proges­
terone. In the WHI, there was a 26% increase in risk of invasive 
breast cancer over a 5.2-year period of follow-up.268 Hence, estro­
gen replacement is contraindicated in any woman with a history 
of breast cancer; yearly mammograms are indicated in all women 
receiving hormone replacement therapy. Previous case-control and 
retrospective studies suggested that estrogen could reduce the risk 
of coronary artery disease; however, in the WHI, the risk of myo

tery disease; however, in the WHI, the risk of myo­
cardial infarction or death from coronary artery disease was 29% 
higher in women receiving combination therapy.268 Thromboem­
bolic disease is also increased more than threefold by hormone 
replacement therapy.268 Hence, the use of estrogen or estrogen 
in combination with progestins for the prevention and treatment 
of osteoporosis has fallen dramatically. Moreover, the availability 
of newer and effective antiresorptive drugs for the treatme

and effective antiresorptive drugs for the treatment of 
osteoporosis has lessened enthusiasm for primary hormonal ther­
apy in osteoporotic women. 
Selective estrogen receptor modulators, such as tamoxifen and ralox­
ifene, also inhibit bone resorption by the same mechanisms used by 
estradiol. Both have been shown to reduce bone loss in postmeno­
pausal women with breast cancer, but only raloxifene is approved by 
the FDA for the prevention and treatment 

oved by 
the FDA for the prevention and treatment of osteoporosis.
Both of these agents block the actions of estrogen on the breast 
but act like an estrogen agonist in bone; tamoxifen, but not raloxi­
fene, has estrogen agonistic properties on the uterus and is associ­
ated with a greater risk of endometrial carcinoma with long-term 
use.376 Both agents have been associated with a reduction in new 
cases of breast cancer when they are administered as prophylaxis 
for high-risk patients.377 Low-

ed as prophylaxis 
for high-risk patients.377 Low-density lipoprotein cholesterol lev­
els are also reduced in patients receiving these selective estrogen 
receptor modulators. Raloxifene increases spine BMD slightly (as 
does tamoxifen) and lowers the risk of vertebral fracture by 40%, 
although it has no effect on nonvertebral fracture risk.378 Hot 
flashes, leg cramps, and a greater risk of deep venous thrombosis 
can occur with raloxifene therapy. The recommended dose of ral­
oxifene is 60 m

rapy. The recommended dose of ral­
oxifene is 60 mg once daily.
Tissue selectivity with these selective estrogen receptor modu­
lators and others being investigated is a subject of great scientific 
interest. Raloxifene and estradiol both bind to the same region of 
the estrogen receptor, but they induce different conformational 
changes in that receptor. Differing coactivating and corepressing 
proteins are recruited to these receptor-ligand complexes, and it is 
thought that these coactivators

plexes, and it is 
thought that these coactivators and corepressors ultimately deter­
mine the activity of the nuclear complexes. Because recruitment 
also depends on cell type, it is highly likely that significant tis­
sue selectivity exists for these partners. Newer agents have been 
designed to facilitate particular complexes and rearrangements 
within the nucleus; they are being studied at both preclinical and 
clinical levels. 
1294
Bisphosp

VII    Mineral Metabolism
Bisphosphonates are the most widely prescribed antiresorptives and 
are often considered first-line therapy for the treatment of severe 
postmenopausal osteoporosis. These drugs are carbon-substituted 
analogues of pyrophosphate that bind tightly to hydroxyapatite 
crystals. It is thought that these agents directly suppress resorption 
by inhibiting osteoclast attachment and enhancing programmed 
cell death. The first-generation bisphosphonates include e

th. The first-generation bisphosphonates include etidronate 
and clodronate. Neither is approved for the treatment of osteopo­
rosis in the United States, although etidronate is used off label and 
in Europe. The dose of etidronate is 400 mg/day for 2 weeks every 
3 months. The drug has few gastrointestinal side effects, and verte­
bral fracture risk reduction is significant with this agent. Alendro­
nate and risedronate, two second-generation nitrogen-containing 
bisphosphonates, are effective 

trogen-containing 
bisphosphonates, are effective in suppressing bone resorption and 
increasing BMD. In RPCTs of postmenopausal women with estab­
lished osteoporosis, alendronate and risedronate reduced vertebral, 
hip, and nonvertebral fractures by nearly 50%, particularly dur­
ing the first year of treatment.219,379 As with other antiresorptive 
drugs, increases in BMD with alendronate or risedronate account 
for a small fraction of their antifracture efficacy. Hence, follow-up 
DXA measureme

fracture efficacy. Hence, follow-up 
DXA measurements may significantly underestimate fracture risk 
reduction. Recent clinical trials have shown that these drugs can be 
safely administered for at least 5 years without adversely affecting 
bone strength. Moreover, discontinuation of alendronate after 5 
years results in minimal bone loss over the ensuing 5 years without 
a significant increase in fracture risk.380 Both drugs have excellent 
safety profiles, although erosive esophagitis is a ser

ty profiles, although erosive esophagitis is a serious complica­
tion of all oral nitrogen-containing bisphosphonates. Once-weekly 
administration of alendronate has been shown to reduce the preva­
lence of drug-induced esophagitis, and currently both alendronate 
and risedronate are marketed as once-weekly treatments.
Two other bisphosphonates have been approved by the FDA 
and have reached the market since 2007: ibandronate and zole­
dronate. The former is given orally in a single monthly dose

he former is given orally in a single monthly dose (150 
mg) or intravenously every 3 months (3 mg).381,382 Ibandronate 
suppresses bone resorption and reduces the rate of spine fractures 
by nearly 50%,358,383 but its efficacy in nonspine fractures is some­
what less than that of alendronate or risedronate. Compliance with 
the once-monthly regimen is higher than with the weekly dosing, 
although long-term data are not encouraging that this effect per­
sists. First-dose hypersensitivity can occ

ct per­
sists. First-dose hypersensitivity can occur with ibandronate, and 
because it is a nitrogen-containing bisphosphonate, it is also asso­
ciated with esophageal reflux. Zoledronate is also approved for the 
prevention and treatment of osteoporosis. It is administered as a 
single intravenous infusion over 15 minutes (5 mg) once yearly. 
Large randomized controlled trials have unequivocally established 
antifracture efficacy for hip, spine, and other nonspine fractures.384 
Recently, the F

 and other nonspine fractures.384 
Recently, the FDA approved the use of zoledronate for prevention 
of osteoporosis by administration of the drug once every 2 years. 
Both newer bisphosphonates can cause side effects with the first 
dose, including joint pain, stiffness, and low-grade fevers. Gener­
ally, these do not persist with recurrent administration of the drug. 
However, the FDA has cautioned that zoledronic acid should be 
administered over an hour rather than over 15 minutes to lessen 

ver an hour rather than over 15 minutes to lessen 
any risk, albeit small, of renal damage. In addition, it should not 
be used in patients with a reduced glomerular filtration rate (<30 
mL/minute) and should be used with caution in the elderly. Intra­
venous zoledronate has been approved for the treatment of malig­
nant hypercalcemia, multiple myeloma, and skeletal metastases.
Other bisphosphonates are available for off-label use or are 
being studied for the treatment of osteoporosis. Intrave

studied for the treatment of osteoporosis. Intravenous pami­
dronate has been available since the mid-1990s for the treatment 
of Paget disease and malignant hypercalcemia. It is currently 
also used to treat osteoporotic women who cannot tolerate oral 
bisphosphonates, although it has not been formally approved by 
the FDA, and its antifracture efficacy has not been established. The 
dose ranges from 30 to 90 mg given every 3 to 9 months. Acute 
and delayed-type hypersensitivity reactions can o

and delayed-type hypersensitivity reactions can occur with this 
drug, and its use is contraindicated in patients who are vitamin 
D deficient because it can precipitously drop serum calcium—a 
concern that also applies to use of zoledronic acid and denosumab.
With widespread use of bisphosphonates for both prevention 
and treatment, two uncommon but serious adverse events have 
been associated with administration of this class of agents: subtro­
chanteric, or atypical, femur fracture and osteo

­
chanteric, or atypical, femur fracture and osteonecrosis of the jaw 
(ONJ).385,386 With respect to the former, atypical femur fractures 
were found in some but not all studies to be associated with dura­
tion of bisphosphonate use and generally were noted in younger 
individuals relative to those affected by other types of hip frac­
tures. Prodromal symptoms of hip or thigh pain and associated 
cortical thickening or beaking in the shaft of the proximal femur 
are risk indicators of these frac

 proximal femur 
are risk indicators of these fractures, which with minimal trauma 
can have devastating consequences in terms of quality of life and 
mobility. Some guidelines recommend prophylactic rod place­
ment to prevent fractures in high-risk individuals in the ipsilateral 
and contralateral femora. Currently, the prevalence of this fracture 
is not well established, although meta-analyses suggest that there 
is a causal association with bisphosphonate use.
The prevalence of ONJ is very l

isphosphonate use.
The prevalence of ONJ is very low when bisphosphonates are 
used in the doses used to treat osteoporosis. When much higher 
doses are used to prevent the skeletal complications of cancer, 
ONJ is a substantial concern. However, in osteoporosis patients, 
the prevalence is estimated to be less than 1 in 100,000 patients 
exposed to oral or intravenous bisphosphonates who are other­
wise healthy. ONJ has devastating effects on the mandible and can 
require prolonged antibiotic t

e mandible and can 
require prolonged antibiotic treatment and local oral care. Patients 
with dental procedures that invade bone, such as tooth implan­
tation and tooth extraction, are at increased risk. Concomitant 
treatment with glucocorticoids likely enhances the risk, and infec­
tion often accompanies the necrosis. 
Calcitonin is a 32–amino acid peptide normally produced by the 
thyroid C cells. Osteoclasts have calcitonin receptors, and calcito­
nin can rapidly inhibit bone res

ors, and calcito­
nin can rapidly inhibit bone resorption. Salmon calcitonin is more 
potent than human and is the preferred treatment choice. Nasal 
and subcutaneous calcitonin are both approved for the treatment 
of postmenopausal osteoporosis. However, the evidence favoring 
a strong effect from this hormone on either bone loss or fracture 
efficacy is lacking. In an RPCT of women with postmenopausal 
osteoporosis, 200 IU/day of nasal calcitonin reduced vertebral frac­
ture incidence by one t

in reduced vertebral frac­
ture incidence by one third.387 However, methodologic flaws in 
that trial have limited enthusiasm for this agent as a primary treat­
ment for osteoporosis. In at least one placebo-controlled study, 
nasal calcitonin reduced the pain associated with new spine frac­
tures. The recommended dose of nasal calcitonin is 200 IU/day, 
and that of subcutaneous calcitonin is 100 IU/day. Side effects 
are uncommon with intranasal calcitonin and include nasal stuffi­
ness and flu

 calcitonin and include nasal stuffi­
ness and flushing. With subcutaneous administration, nausea is 
not infrequent. 
Strontium Ranelate
Strontium ranelate is orally administered and stimulates calcium 
uptake in bone while it inhibits bone resorption. It is thought 
to have some anabolic activity, although the precise mechanism 
1295
of action in the skeleton, where it is incorporated, is not known. 
In an RPCT of postmenopausal women with e

 known. 
In an RPCT of postmenopausal women with established dis­
ease, daily strontium reduced the risk of vertebral fractures by 
40%.388 However, a statistically significant effect on nonverte­
bral fractures was limited to a small subset of women in a post 
hoc analysis.389 Recent data raised the safety issue of an increase 
in cardiovascular events among strontium users.390 This drug 
was originally approved by European regulatory agencies but 
not by the FDA.
Strontium ranelate has been as

ut 
Strontium ranelate has been associated with increased risk of 
severe allergic skin reactions, venous thromboembolism, stroke, 
and heart ischemia. The European Medicine Agency has decided 
to restrict the use of strontium ranelate only to patients with severe 
osteoporosis who cannot be treated with other drugs approved for 
osteoporosis. The agency has also warned about the necessity of 
regular medical care and advised the discontinuation of therapy in 
those patients who 

discontinuation of therapy in 
those patients who develop heart or circulatory problems, includ­
ing high blood pressure or angina. As expected, it has maintained 
the recommendation against using strontium ranelate in individu­
als with antecedent of cardiovascular disorders. 
Denosumab
Denosumab is a fully human monoclonal antibody to the RANKL, 
the essential osteoclast-differentiating factor. The antibody inhibits 
osteoclast formation, decreases bone resorption, increases BMD, 
and reduces 

ases bone resorption, increases BMD, 
and reduces the risk of fracture. As noted previously, RANKL is 
a member of the TNF superfamily of ligands that is essential for 
the function of bone-resorbing osteoclasts. RANKL interacts with 
its receptor (RANK) on both osteoclast precursors and mature 
osteoclasts, and the RANKL-RANK interaction results in activa­
tion, migration, differentiation, and fusion of hematopoietic cells 
of the osteoclast lineage to begin the process of bone resorption. 
Den

eage to begin the process of bone resorption. 
Denosumab blocks that activation by binding directly to RANKL. 
Unlike bisphosphonates, denosumab does not persist in the skel­
eton and hence needs to be administered once every 6 months to 
maintain its efficacy. In fact, discontinuation of denosumab can 
lead to a rebound increase in bone resorption but no increase in 
fractures.391
Clinical trials with denosumab, 60 mg once every 6 months, 
included the very large (>7000 postmenopausal women wit

ded the very large (>7000 postmenopausal women with low 
bone mass and fractures) FREEDOM (Fracture Reduction Evalu­
ation of Denosumab in Osteoporosis Every 6 Months) trial. This 
trial, which was used for registration of the agent with the FDA, 
demonstrated that after 3 years, denosumab improved BMD of 
the spine and hip compared with placebo (9.2% vs. 0% and 4.0% 
vs. −2.0%, respectively).80 In addition, biochemical markers of 
bone turnover were significantly reduced in patients taking deno

were significantly reduced in patients taking deno­
sumab. Importantly, fractures of spine were reduced by 70%, and 
nonvertebral fractures, including hip fractures, were also signifi­
cantly reduced. The long-term extension of the FREEDOM trial 
revealed that at 6 years, BMD continued to improve and fracture 
risk remained low.392 In males undergoing androgen deprivation 
therapy, denosumab reduced bone loss and prevented fractures. 
Denosumab was the first biologic approved by the FDA for the 

as the first biologic approved by the FDA for the 
treatment of osteoporosis in both men and women. Surpris­
ingly, long-term studies (in 2015, out to 8 years in the extension 
trial) have failed to show significant adverse events for this agent, 
although atypical femoral fractures occur. 
Cathepsin K Inhibitors
Cathepsin K is a proteinase that is secreted by osteoclasts and 
results in bone degradation, primarily of type I collagen (see 
Fig. 30.4). In addition, it is also produced by cancer c

0.4). In addition, it is also produced by cancer cells that 
metastasize to bone. Odanacatib, one cathepsin K inhibitor, has 
been tested in postmenopausal women with osteoporosis and is 
shown to be an effective suppressor of bone resorption. In addi­
tion, women receiving this agent in a large phase III registration 
trial showed significant increases in BMD of the spine and hip, as 
well as a reduction in spine and nonvertebral fractures, prompt­
ing premature discontinuation of the trial due

pt­
ing premature discontinuation of the trial due to enhanced effi­
cacy.393 However, the safety profile for odanacatib is still not 
clearly defined, and the FDA has not yet approved this agent 
for use. Interestingly, women treated with odanacatib have sup­
pressed bone resorption but no change or a slight increase in bone 
formation. This may be due to the finding that this agent blocks 
the breakdown of collagen but does not kill osteoclasts; hence, 
signals from osteoclasts to osteoblasts 

; hence, 
signals from osteoclasts to osteoblasts may be maintained, thereby 
preserving bone formation. If correct, this is one of the first drugs 
for osteoporosis that can uncouple remodeling in a positive man­
ner. A correlative study examining the effect of odanacatib in the 
setting of bony metastases from breast cancer demonstrated that 
it suppressed a biochemical marker of bone resorption, N-ter­
minal telopeptide, in much the way that zoledronic acid did.394 
In 2015, Merck released pr

oledronic acid did.394 
In 2015, Merck released promising bone results from the LOFT 
(Long-Term Odanacatib Fracture Trial) study.395 In comparison 
to individuals receiving placebo, patients treated with odanacatib 
showed the following reductions in relative risk rates: (1) 54% of 
new and worsening morphometric vertebral fractures, (2) 47% 
of clinical hip fractures, (3) 23% of clinical nonvertebral frac­
tures, and (4) 72% of clinical vertebral fractures (all p < 0.001). 
In addition, the co

al fractures (all p < 0.001). 
In addition, the company recognized that adjudicated events of 
morphea-like skin lesions and atypical femoral fractures were a 
more frequent occurrence in the odanacatib group than in the 
placebo group.396 Moreover, the number of adjudicated stroke 
events was more common in the odanacatib than in the control 
group. The sponsor, Merck, decided to remove odanacatib from 
consideration by the FDA. 
Anabolic Agents
A new class of antiosteoporosis drugs was introdu

A new class of antiosteoporosis drugs was introduced in 2002. 
These so-called anabolic agents stimulate bone formation more 
than bone resorption. As such, these agents can enhance bone 
remodeling and contrast sharply with the antiresorptives, which 
slow bone turnover. PTH(1-34) (teriparatide) was the first of this 
class of drugs to be approved by the FDA. Previously, the proto­
typical anabolic drug was sodium fluoride, which saw widespread 
use in the 1970s and 1980s because of its abilit

use in the 1970s and 1980s because of its ability to stimulate new 
bone formation. However, an RPCT in 1990 established that 
although there were dramatic increases in BMD, nonvertebral 
fracture risk actually increased.
It has been known for several decades that PTH, when given 
intermittently, increases bone formation and bone mass in mam­
mals.397 The increase in bone turnover begins with a dramatic 
increase in bone formation that is followed later by a marked rise 
in

mation that is followed later by a marked rise 
in bone resorption, maintaining the coupling in the multicellular 
unit. However, chronically high levels of PTH cause bone loss 
in the cortical compartment as noted for primary and secondary 
hyperparathyroidism.398 The development of PTH as an anabolic 
agent centered on defining the intermittent nature of administra­
tion, which for teriparatide (human PTH[1-34]) was by daily sub­
cutaneous injections. Teriparatide has been approved in virtuall

ctions. Teriparatide has been approved in virtually 
every country for the treatment of postmenopausal osteoporosis 
because it not only increases bone mass but also reduces fractures. 
In the largest RPCT using teriparatide in postmenopausal women 
1296
with severe osteoporosis, 20 μg/day of PTH, administered sub­
cutaneously, reduced spinal and nonvertebral fractures by more 
than 50% while it substantially increased (i.e., 8%/year) lumbar 
BMD.399 Similar fin

reased (i.e., 8%/year) lumbar 
BMD.399 Similar findings were noted in men with osteoporosis 
who were treated for 11 months.400 Unfortunately, the PTH trial 
in postmenopausal women was stopped after 20 months because 
of concerns related to the development of osteosarcoma in rats 
treated with high doses of PTH(1-34). However, retrospective 
studies have found no association between osteosarcoma and 
primary or secondary hyperparathyroidism, and only one case 
of osteosarcoma in PTH-treated pat

 only one case 
of osteosarcoma in PTH-treated patients has been reported from 
more than a million users. More recently, recombinant human 
PTH(1-84) has shown similar benefits and is approved for the 
treatment of hypoparathyroidism in the United States.401 In 
Europe and Asia, but not in the United States, PTH(1-84) is 
approved for the treatment of postmenopausal osteoporosis. Cur­
rently, it is recommended that PTH therapy be limited to those 
individuals with moderate to severe osteoporosi

e 
individuals with moderate to severe osteoporosis and then given 
for only 2 years, based on a long-term toxicity study in rats dem­
onstrating an increased risk of osteosarcoma.
Despite the appeal of using an anabolic with an antiresorp­
tive, most evidence indicates that combinations of classes of drugs 
are not additive or synergistic. PTH plus bisphosphonates initi­
ated together do not raise BMD more than PTH alone in either 
men or women.402,403 However, PTH plus denosumab has been 
show

402,403 However, PTH plus denosumab has been 
shown to increase BMD in the spine to 13% after just 1 year, 
a greater increase than that seen with other drugs.404 That dif­
ference is maintained even after 2 years of combination therapy. 
Unlike with discontinuation of bisphosphonates, discontinuation 
of PTH can result in bone loss of 3% to 4% in the first year after 
PTH cessation. This posttreatment effect is prevented by adding 
an antiresorptive agent once PTH is stopped. In general, PTH is

tive agent once PTH is stopped. In general, PTH is 
well tolerated, although nausea, flushing, hypotension, and mild 
but asymptomatic hypercalcemia (i.e., serum calcium <11 mg/dL) 
can occur. Cost and compliance have been limiting factors. 
PTH-Related Protein
Since the approval of teriparatide in 2002, it took 15 years for 
the launching of a new bone anabolic drug in the US market. 
PTH-related protein (PTHrP) has been shown to work via the 
PTH receptor on osteoblasts to stimulate new bone f

TH receptor on osteoblasts to stimulate new bone formation in 
a manner analogous to PTH. PTHrP acts primarily as a paracrine 
growth/differentiation factor but also functions as a hormone 
during normal lactation and in the hypercalcemia of malignancy. 
However, when administered intermittently, PTHrP(1-36), an 
analogue of native PTHrP, increases BMD without significant 
hypercalcemia, similar to PTH.405 Phase III trials with PTHrP(1-
36) have been completed in Europe and show a significant re

been completed in Europe and show a significant reduc­
tion in both vertebral and nonvertebral fractures (including hip 
fractures). In April 2017, the N-terminal fragment of PTH-
related peptide analogue (abaloparatide) received approval from 
the FDA for subcutaneous application in postmenopausal women 
with osteoporosis at high risk for fracture. Abaloparatide is appro­
priately indicated for those postmenopausal women with history 
of osteoporotic fracture, those harboring multiple risk fact

rotic fracture, those harboring multiple risk factors 
for fracture, or patients showing failure or intolerance to other 
therapy options. The ACTIVE (Abaloparatide Comparator Trial 
in Vertebral Endpoints) study was a phase III, double-blind, ran­
domized controlled trial developed in 10 countries. In comparison 
to placebo, after 18 months of therapy, abaloparatide significantly 
increased BMD in the lumbar spine (11.20% vs. 0.63%), femo­
ral neck (3.60% vs. −0.43%), and total hip (4.18% vs. −

eck (3.60% vs. −0.43%), and total hip (4.18% vs. −0.10%). 
More importantly, the number of new morphometric vertebral 
fractures was significantly lower in the abaloparatide group (0.58% 
[n = 4] vs. 4.22% [n = 30] in the placebo group). In addition, the 
ACTIVE study reported a similar 80% reduction in vertebral frac­
ture rate with abaloparatide compared with teriparatide. However, 
abaloparatide was associated with a significant reduction in non­
vertebral fractures when compared with placebo

on­
vertebral fractures when compared with placebo—an effect not 
observed in teriparatide therapy. The occurrence of hypercalcemia 
was lower in women using abaloparatide (3.4%) than teriparatide 
(6.1%).406 
Future Anabolic Agents
There is a significant appeal for the use of anabolic agents for the 
treatment of low bone mass and fractures in severely affected indi­
viduals (see Fig. 30.22). Bone remodeling is a highly regulated 
activity encompassing bone resorption and formation that dictate

passing bone resorption and formation that dictates 
bone mass maintenance. Skeletal anabolic drugs stimulate bone 
formation, enhancing the number and function of bone-forming 
cells. Advances in knowledge of bone biology have allowed the 
recognition of paracrine peptides that represent local modulators 
of the differentiation of progenitor cells into osteoblasts. Wnt sig­
naling is a positive driver of osteoblast differentiation, which is 
cautiously regulated by both intra- and extracellular

tiously regulated by both intra- and extracellular mechanisms. 
In parallel, Wnt signaling inhibits osteoclastogenesis. Osteocytes, 
the terminally differentiated osteoblasts, have an important role 
in maintaining a constitutive inhibition of the Wnt/β-catenin 
canonical signaling pathway by secreting sclerostin. Mechanical 
loading on bone and PTH therapy stimulates bone formation by 
downregulating sclerostin. Osteopetrosis (i.e., sclerosteosis and 
van Buchem disease) can emerge due to a los

s and 
van Buchem disease) can emerge due to a loss of function muta­
tion in the SOST gene, leading to high bone mass. Those studies 
encouraged the development of monoclonal humanized antibody 
to sclerostin. 
Monoclonal Antibodies to Sclerostin
As noted previously, sclerostin is produced by osteocytes and 
inhibits bone formation by blocking canonical Wnt signaling. 
Sost null mice have increased bone formation and high bone mass. 
It follows, therefore, that inhibition of sclerostin should e

 therefore, that inhibition of sclerostin should enhance 
osteoblast function and improve bone mass. In animal models and 
in a phase I trial in healthy adults, administration of a scleros­
tin monoclonal antibody does increase bone mass. Similarly, in a 
phase II trial in postmenopausal women, all doses of a monoclonal 
antisclerostin antibody (romosozumab) increased bone density at 
the lumbar spine, total hip, and femoral neck.73 In this 1-year 
trial, 419 postmenopausal women with low bone m

r 
trial, 419 postmenopausal women with low bone mass (T-score 
between −2.0 and −3.5 at the lumbar spine, total hip, or femo­
ral neck) were randomly assigned to subcutaneous romosozumab 
(variable dosing once monthly or once every 3 months), an active 
comparator, versus alendronate (70 mg weekly), or subcutane­
ous teriparatide (20 μg daily), or placebo injections. The greatest 
increase in BMD was seen in the group receiving romosozumab 
(210 mg monthly; 11.3% compared with 4.1% and 7.1% in 

 mg monthly; 11.3% compared with 4.1% and 7.1% in the 
alendronate and teriparatide groups, respectively).73 Interestingly, 
there was a transient increase in bone formation markers and a 
more sustained decrease in bone resorption markers, a pattern 
that has not been seen among available osteoporosis therapies, 
again suggesting that it is possible to uncouple remodeling toward 
a more favorable balance. The suppression in bone resorption is 
most likely a function of reduced RANKL production 

ost likely a function of reduced RANKL production due to inhi­
bition of the Wnt/LRP5/6 signaling pathway. QCT examinations 
detailed that the positive effects of romosozumab occurred in 
1297
trabecular bone of the vertebrae and total hip, as well as in the cor­
tical compartment of the vertebrae.407 In accordance to the previ­
ous results, finite element analysis showed improved vertebrae and 
total hip strength in postmenopausal women treat

total hip strength in postmenopausal women treated with romo­
sozumab.408 The reported data suggested that romosozumab has 
beneficial effects in both trabecular and cortical compartments of 
bone. The head-to-head comparison between romosozumab and 
other agents showed that romosozumab has better performance in 
several aspects. Romosozumab therapy induced greater increase 
in BMD than alendronate (70 mg weekly) and teriparatide (20 
μg daily). The improvement in bone strength, as assessed by

. The improvement in bone strength, as assessed by infi­
nite element analysis, was higher in osteoporotic women receiving 
romosozumab than teriparatide. Moreover, romosozumab poten­
tially seems to have greater capability to enhance bone mass and 
volume in patients previously receiving a bisphosphonate.409
Romosozumab entered in a phase III trial but is not yet 
approved by the FDA.348 Actually, the efficacy of romosozumab 
to alleviate fracture occurrences has been tested in two clinical tri

re occurrences has been tested in two clinical tri­
als. In the first, romosozumab (210 mg subcutaneously monthly) 
was compared with placebo for 1 year, then both arms changed 
to open-label denosumab for a year (60 mg biannually). In the 
whole period of 24 months, in 75% of patients, romosozumab/
denosumab reduced the incidence of new vertebral fractures and 
had a statistically insignificant reduction in nonvertebral frac­
tures.410 In the second clinical trial, the authors defined the fol­

cond clinical trial, the authors defined the fol­
lowing design: monthly subcutaneous romosozumab (210 mg) or 
weekly oral alendronate (70 mg) administered in a blinded fash­
ion for 12 months, followed by open-label alendronate in both 
groups. The romosozumab/alendronate group showed a 48% 
and 19% reduction, respectively, in incident vertebral and non­
vertebral fractures compared with the alendronate-only group.411 
Necrosis of the jaw and atypical fractures have rarely been reported 
with r

ypical fractures have rarely been reported 
with romosozumab. In addition, there are concerns about serious 
cardiovascular events. During year 1, positively adjudicated seri­
ous cardiovascular adverse events were observed more often with 
romosozumab than with alendronate (50 of 2040 patients [2.5%] 
vs. 38 of 2014 patients [1.9%]). Safety studies and completion of 
the phase III trial will determine the place of this agent in treating 
osteoporosis.
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macodynamics of subcutaneous recombinant parathyroid hormone 
(1-84) in patients with hypoparathyroidism: an open-label, single-
dose, phase I study. Clin Ther. 2014;36:722–736.
	402.	 Finkelstein JS, Hayes A, Hunzelman JL, et al. The effects of para­
thyroid hormone, alendronate, or both in men with osteoporosis. 
N En

Med. 
2016;375:1532–1543.
	411.	 Saag KG, Petersen J, Brandi ML, et al. Romosozumab or alendro­
nate for fracture prevention in women with osteoporosis. N Engl J 
Med. 2017;377:1417–1427.
1298
Introduction, 1298
Historical Perspective, 1299
Epidemiology and Demographics, 1300
Bone Remodeling and Mineralization, 1300
Pathogenesis of Rickets and Osteomalacia, 1301
Clinical Manifestations of Classical Rickets and Osteomalacia, 1305
Treatment of Nutritional Rickets and Osteomalacia,

Treatment of Nutritional Rickets and Osteomalacia, 1308
Rickets Due to Genetic Disorders of Vitamin D Metabolism, 1309
Tumor-Induced Osteomalacia, 1312
Drug-Induced Osteomalacia, 1314
Conditions That Resemble Rickets and Osteomalacia, 1315
Concluding Remarks, 1317
KEY POINTS
 • 
Rickets is a specific bone disorder of growing children and ado­
lescents and is associated with characteristic skeletal deformi­
ties.
 • 
Osteomalacia is a generalized softening of the bones regardless 
of age or cau

d softening of the bones regardless 
of age or cause and thus occurs both in children and adults.
 • 
Worldwide nutritional vitamin D deficiency is the most common 
cause of rickets and osteomalacia. However, in countries where 
routine fortification of dairy and food products is implemented, 
genetic and acquired forms of rickets and osteomalacia are 
relatively more common.
 • 
Calcium-deficiency rickets is a specific type of rickets that is 
mostly confined to certain geographic regions of 

mostly confined to certain geographic regions of the world. Cal­
cium-deficiency osteomalacia in adults has not been reported.
 • 
There are several types of genetic rickets and osteomalacia 
resulting in excess renal loss of phosphate due to inhibition of 
renal tubular reabsorption of phosphate.
 • 
Several acquired forms of rickets and osteomalacia occur due 
to ectopic production of fibroblast growth factor 23, mostly by 
benign mesenchymal or skeletal tumors—the removal of which 
cures t

l or skeletal tumors—the removal of which 
cures the bone disease.
 • 
Several drugs, such as etidronate, sodium fluoride, and alu­
minum-containing antacids, as well as iron overload, directly 
inhibit the bone mineralization process, producing unusual 
types of histologic osteomalacia.
 • 
The antiviral drugs tenofovir and adefovir, which are now widely 
used for the treatment of human immunodeficiency virus and 
hepatitis, cause renal tubular damage resulting in acquired 
forms of hypophosp

r damage resulting in acquired 
forms of hypophosphatemic rickets and osteomalacia.
 • 
Many rare bone disorders resemble either rickets on x-rays or 
osteomalacia on bone biopsy but do not respond to conven­
tional doses of vitamin D therapy.
31
Rickets and Osteomalacia
ARTI BHAN, AJAY D. RAO, SANJAY K. BHADADA, AND SUDHAKER D. RAO
Rickets is a specific bone disorder of the growing skeleton, thus occur­
ring only in children and adolescents before the epiphyseal fusion has 
occurr

olescents before the epiphyseal fusion has 
occurred, and is associated with characteristic skeletal deformities,1 
whereas osteomalacia is a generalized softening of the bones regard­
less of age or cause and therefore occurs in both children and adults.2 
Rickets is due to defective mineralization of both preosseous carti­
laginous and mature osseous matrix resulting in subnormal linear 
growth, a consequence of the involvement of growth plates. In con­
trast, osteomalacia is due to defective 

. In con­
trast, osteomalacia is due to defective mineralization of the mature 
lamellar bone. Of the four major metabolic bone diseases, osteopo­
rosis is by far the most common (see Chapter 30), whereas rickets 
and osteomalacia combined are a distant second, followed by osteitis 
deformans (also known as Paget disease of bone; see Chapter 30). 
Osteitis fibrosa cystica, the typical bone disease of severe primary, sec­
ondary, or tertiary hyperparathyroidism, is the least common, and is 
rarel

parathyroidism, is the least common, and is 
rarely seen in the United States but still prevalent in other parts of the 
world where vitamin D deficiency is endemic3,4 (see Chapter 29). 
The key contrasting features of these four common metabolic bone 
diseases are summarized in Table 31.1.
Soon after the discovery of vitamin D, rickets and osteomalacia 
became synonymous with any condition that could be cured by 
vitamin D therapy. However, with the recent discovery of fibroblast 
growth factor

 the recent discovery of fibroblast 
growth factor 23 (FGF23), and the understanding of its critical role 
in phosphate homeostasis and vitamin D metabolism5,6 (see Chap­
ter 29), several types of both genetic and acquired forms of rickets 
and osteomalacia are now better characterized with specific geno­
typic and phenotypic features5–9 (see the specific sections that fol­
low). These unusual FGF23-mediated rickets and osteomalacia do 
1299
Chapter 31  Rickets and Osteomalacia
  Contrasting Fe

ter 31  Rickets and Osteomalacia
  Contrasting Features of Four Major Metabolic Bone Diseases
Variable
Osteitis Fibrosa
Osteitis Deformans
Basic abnormality
Defective mineralization of 
cartilage and bone
Insufficient replacement of 
normal lamellar bone
Replacement by woven bone 
and fibrous tissue
Abnormal woven bone
Second most common
Most common
Uncommon
Not uncommon
Normal/Low
Normal
High/Very high
Normal
Serum phosphate
Normal

ormal
Normal
Normal/Low/Very low
Normal
Frequently low
Normal
High
Normal/High
High
Parathyroid hormone
High
Normal
25-Hydroxyvitamin D
Low/Normal
Cortical thinning
Yes, except in X-linked 
hypophosphatemia
Sometimes
Yes
No
Vertebral deformities
Biconcave or cod fish like
Wedged/Compressed
Variable
Enlarged size
Long bone deformities
Bowing and pseudofractures
None, 

bone deformities
None, except fragility 
fractures
Brown tumors
Bowing and stress fractures
TABLE 31.1
not respond to the conventional doses of vitamin D that cure nutri­
tional vitamin D deficiency rickets and osteomalacia, and indeed 
many are resistant to even 10- to 100-fold the therapeutic doses of 
vitamin D used to treat vitamin D–deficiency rickets and osteoma­
lacia. Furthermore, a variety of rare genetic bone disorders either 
resemble rickets on x-rays10 or 

disorders either 
resemble rickets on x-rays10 or osteomalacia on bone biopsy,11–14 
but their pathogenesis is distinct from that of vitamin D–related or 
hypophosphatemic rickets and osteomalacia with variable or incon­
sistent response to therapy with vitamin D or its analogues.12,13,15–17 
In addition, a few rare acquired forms of osteomalacia are the result 
of adverse effects of a variety of drugs that either impair bone min­
eralization or inhibit renal tubular reabsorption phosphate and 

inhibit renal tubular reabsorption phosphate and 
respond to the withdrawal of the offending drugs.
The following sections begin with a historical perspective of 
rickets and osteomalacia that is both instructive and informative, 
followed by epidemiology and demographics, process of normal 
and abnormal bone mineralization, pathogenesis, and clinical 
manifestations. The treatment strategies will be discussed in detail 
for nutritional-deficiency rickets and osteomalacia, but specific 
aspects 

y rickets and osteomalacia, but specific 
aspects as they relate to different types and forms of rickets and 
osteomalacia will be dealt within the specific sections. Where 
appropriate, reference will be made as it applies to management in 
children and adults. 
Rickets
The earliest reports of rickets date back to the 17th century, with 
the first detailed contemporaneous descriptions by William Glis­
son and Daniel Whistler in 1645.18 Glisson was also the first to 
recog

er in 1645.18 Glisson was also the first to 
recognize rickets as a separate entity from infantile scurvy, which 
often coexisted with rickets during that time. Even more inter­
esting is that Glisson believed that rickets was neither congenital 
nor inherited,18 although it is now clear that both types of rickets 
forms can occur. As early as in the middle of the 17th century, 
rickets was attributed to the industrial revolution and grow­
ing urbanization (presumably less sunlight exposure),19 

banization (presumably less sunlight exposure),19 as well 
as to breastfeeding (breast milk has poor vitamin D content).20 
The temporal relationship between rickets and sunshine was not 
appreciated until the 19th century,21 and the “proof of concept” 
that sunshine can cure rickets did not occur until the beginning of 
the 20th century.18,19 Early 20th-century reports on rickets were 
exclusively from the regions of the world where vitamin D defi­
ciency was endemic, numerically the most commo

fi­
ciency was endemic, numerically the most common, and almost 
all cases were due to nutritional vitamin D deficiency.22–33 With 
the current policies of routine fortification of milk and other food 
products with vitamin D, nutritional vitamin D–deficiency rick­
ets and osteomalacia all but disappeared from developed coun­
tries.34–36 Consequently, genetic37–40 and acquired8,9,41–49 forms 
of rickets and osteomalacia are more prevalent in nonendemic 
regions of the world.8,9,11,12,41–46,50 
O

ndemic 
Osteomalacia
The term osteomalacia originally referred to the generalized soften­
ing of the bones resulting in crippling deformities, and the most 
comprehensive report on its etiology and treatment dates back to 
1896.51 In earlier publications, various descriptive terms, such as 
mollities ossium,52 rheumatic, syphilitic, senile, and even neurotic 
osteomalacia, were used to describe osteomalacia, implying that 
the bone disease was due to eith

a, implying that 
the bone disease was due to either infection or inflammation.53 
Because of the grotesque deformities involving long bones, even 
more descriptive terms, such as boomerang bones or osteomala­
cia sclerotica, were used to describe a peculiar disease seen rather 
exclusively in Australian Aborigines and in Sudanese popula­
tions.54 Since osteoporosis was such a frequent accompaniment 
of osteomalacia, the term osteoporomalacia was often used in the 
20th century.55,56 The distinc

 often used in the 
20th century.55,56 The distinction between rickets that specifically 
involved metaphyses, and to a lesser extent epiphyses, and osteo­
malacia, which affected all of the bones, was recognized early in 
the 18th century,51–53 and was reemphasized by Fuller Albright in 
his classic monograph.57
The histologic differentiation of osteomalacia from both osteo­
porosis and osteitis fibrosa was first made by Pommer in the late 
19th century by examining cadaveric bones58 and later 

 century by examining cadaveric bones58 and later confirmed 
1300
in humans by Fuller Albright with his elegant balance studies.57 The 
first detailed tetracycline-based bone histomorphometric measure­
ments were made by Harold Frost in 1966.59 Although the original 
descriptions defined rickets and osteomalacia as distinct entities, the 
two terms are often used (and probably can be used, except that rick­
ets occurs only in children) interchangeably. However, 

ccurs only in children) interchangeably. However, and perhaps 
more confusing, is that the descriptive terms have been extended to 
include various other conditions that resembled radiologic or bone 
histologic phenotypes of vitamin D–related disorders.12–14,60 This is 
both unfortunate and unwarranted, as it may erroneously be mis­
construed that conditions resembling rickets and osteomalacia will 
respond to vitamin D therapy; a recent review on the subject under­
scores this cautionary approa

n the subject under­
scores this cautionary approach.61 
Epidemiology and Demographics
Worldwide, nutritional deficiency of vitamin D and calcium is 
still the most common cause and type of rickets and osteomala­
cia.15,34,40,62–73 Unlike the known prevalence, incidence, epidemi­
ology, and demographics of osteoporosis (see Chapter 30), osteitis 
fibrosa (see Chapter 29), and osteitis deformans (see Chapter 30), 
precise estimates for the prevalence of rickets and osteomalacia are 
lacking. Rece

nce of rickets and osteomalacia are 
lacking. Recent reports suggest a rising trend in the prevalence of 
nutritional rickets worldwide, including the regions where rou­
tine fortification of dairy and other food products with vitamin 
D is practiced.34,64–68,74–76 The most recent estimates of vitamin 
D–deficiency rickets ranged from 2.2 in 100,000 people in 1980 
to 24 in 100,000 people in 2000.64 Similarly, the prevalence of 
osteomalacia in adults due to vitamin D depletion may be ris­
ing b

dults due to vitamin D depletion may be ris­
ing because of the increasing rates of bariatric surgery for morbid 
obesity, which results in malabsorption of both vitamin D and 
calcium.4,62,77 Although precise estimates on the prevalence or 
incidence of osteomalacia following bariatric surgery are lacking, 
it was the most common cause of osteomalacia at our institution 
in Detroit, Michigan.4,77 A similar trend was observed following 
earlier, but now abandoned, intestinal bypass surgeries for

but now abandoned, intestinal bypass surgeries for obe­
sity4,77–79 and after gastrectomy (partial or total) for peptic ulcer 
disease, which is now performed rarely.80–83 Rarity of vitamin D 
deficiency rickets and osteomalacia in developed countries84–87 
explains the relatively higher prevalence of genetic and acquired 
forms of rickets and osteomalacia in the Western world.6–8,88,89 
It is estimated that the most common genetic form of rickets and 
osteomalacia occurs 1 in 20,000 live births

s and 
osteomalacia occurs 1 in 20,000 live births.90 In the past decade, 
there has been a growing number of reports of osteomalacia 
related to the expanding use of the antiretroviral drugs tenofo­
vir9 and adefovir.91 Although rickets in children due to antiviral 
drug therapy has not yet been reported, it is most likely because 
of the low prevalence of their use in the pediatric population. In 
addition, the incidence of rickets and osteomalacia due to anti­
convulsants, antacids, and alumi

acia due to anti­
convulsants, antacids, and aluminum toxicity remains very low. 
Thus, rickets and osteomalacia not related to nutrition and drugs 
are most prevalent in the developed countries, whereas rickets and 
osteomalacia related to vitamin D and calcium deficiency are most 
common in developing countries.
Certain individuals and ethnic groups are particularly suscep­
tible to the development of nutritional rickets and osteomalacia. 
Both secular and nonsecular tendencies of the populati

 secular and nonsecular tendencies of the populations, geo­
graphic locations, prevailing sunlight, and local dietary habits all 
contribute to the development of rickets and osteomalacia related 
to deficiency of vitamin D and calcium. Immigrants, particularly 
those with darkly pigmented skin or specific dietary habits (veg­
etarians, high phytate intake) moving to temperate zones with 
limited or reduced sunlight exposure are at risk of developing 
rickets and osteomalacia—so-called immigrant

ping 
rickets and osteomalacia—so-called immigrant osteomalacia.15 
Because of the rarity of rickets and osteomalacia due to vitamin 
D deficiency in developed countries,64,67,68,74,75 the diagnosis is 
often missed or delayed.4,77 
Bone Remodeling and Mineralization
Bone remodeling is a necessary mechanism by which old bone is 
replaced by new bone throughout the life span of an individual 
and is sufficient to maintain integrity of the skeleton, both as an 
organ to serve its metabolic functio

, both as an 
organ to serve its metabolic functions and as a structure for loco­
motion (see Chapters 29 and 30). In the course of normal bone 
remodeling, a moiety of old bone is removed and replaced by the 
same amount of normal lamellar bone in young adults, but in 
aging and disease, the replacement mechanism is not as efficient 
as it is in the young. A lesser amount of normal lamellar bone is 
replaced in osteoporosis (see Chapter 30), by a mixture of woven 
bone and ﬁbrous tissue in oste

a mixture of woven 
bone and ﬁbrous tissue in osteitis ﬁbrosa due to hyperparathyroid­
ism (see Chapter 29), by an abnormal local production of woven 
bone in osteitis deformans, and by an unmineralized bone matrix 
(or osteoid tissue) in osteomalacia.92 This fundamental difference 
in the nature of the replaced bone distinguishes osteomalacia from 
the most common metabolic bone diseases (see Table 31.1), as 
well as from rare bone disorders such as hypophosphatasia, fibro­
genesis imperfecta o

h as hypophosphatasia, fibro­
genesis imperfecta ossium, and axial osteomalacia that resemble 
classical osteomalacia in one respect or another (see the Condi­
tions That Resemble Rickets and Osteomalacia section).
Despite significant advances in our understanding of bone 
biology, very little is known about mechanisms that control or 
regulate the process of mineralization in bone. Why does biologic 
mineralization normally occur only in certain types of connec­
tive tissues but not in others, 

 types of connec­
tive tissues but not in others, and how is the temporal and spa­
tial regulation of matrix production and mineralization process 
accomplished? For proper and optimal mineralization of bone, at 
a minimum, two principal requirements must be met: synthesis 
of mature lamellar bone matrix by osteoblasts (see Chapter 30) 
and exposure of this newly synthesized lamellar bone matrix to 
optimal calcium × phosphate product provided by the mineral 
homeostatic system regulated by para

 the mineral 
homeostatic system regulated by parathyroid hormone (PTH) 
and vitamin D (see Chapter 29). Any abnormality in either com­
ponent will result in defective mineralization. In classical osteo­
malacia, deficiency of minerals, however produced, results in the 
accumulation of unmineralized bone matrix or osteoid. In con­
trast, in all other bone disorders that resemble osteomalacia (or 
“osteomalacia like”), the osteoid accumulation is a consequence 
of abnormalities outside of these t

a consequence 
of abnormalities outside of these two principal components. In 
hypophosphatasia, it is the enzyme deficiency, whereas in Paget 
disease of bone, fibrous dysplasia, fibrogenesis imperfecta ossium, 
and possibly osteogenesis imperfecta, it is the abnormal bone 
matrix, and in certain drug-induced osteomalacia (etidronate, 
fluoride, aluminum, and iron), it is the toxic effects of the drugs 
inhibiting matrix mineralization. Understanding the differences 
in the mechanism of osteoid

nding the differences 
in the mechanism of osteoid accumulation in different diseases is 
critical for clinical management because disorders that resemble 
osteomalacia do not respond to vitamin D therapy as the term 
osteomalacia might imply.
Normal mineralization of bone matrix occurs in two stages. 
In the rapid phase, termed primary mineralization, 75% to 80% 
of the maximal mineral content is deposited within a few days to 
weeks. In the second and much slower phase, termed secondary 
miner

ond and much slower phase, termed secondary 
mineralization, the mineral content of the bone increases further 
1301
to reach about 90% to 95% over a period of months. The remain­
ing 5% to 10% represents the bone matrix that is newly formed 
but not yet mineralized. Accordingly, an osteoid surface greater 
than 15% of the bone surfaces, sometimes referred to as hyper­
osteoidosis, can be seen in conditions with high rates of bone 
turnover, such as immediate

th high rates of bone 
turnover, such as immediately after estrogen depletion in post­
menopausal women, hyperparathyroidism (primary or second­
ary), hyperthyroidism, and osteitis deformans. However, strictly 
speaking, the bone histologic features and osteoid indices in these 
high bone turnover diseases do not conform to the classical defini­
tion of osteomalacia (see the next section).
Definition and Histologic Evolution of 
Classical vitamin D deficiency osteomalacia, irrespect

sical vitamin D deficiency osteomalacia, irrespective of its 
cause, evolves in three stages. The first stage is characterized by 
an increased bone remodeling due to secondary hyperparathy­
roidism (2°HPT), associated with increased osteoid surface and 
osteoid volume, but not the thickness of osteoid, and normal min­
eralization of bone. This represents the earliest bone histologic 
phenotype of vitamin D deficiency, designated as hypovitaminosis 
D osteopathy stage I (HVO I) or preosteomalaci

is 
D osteopathy stage I (HVO I) or preosteomalacia87 (Fig. 31.1). 
Similar bone histologic features can also be seen in patients with 
calcium malabsorption, but without vitamin D deficiency, des­
ignated as 2°HPT. However, the osteoid indices are much lower 
than in HVO I with some overlap (see Fig. 31.1). In both HVO 
I and 2°HPT, there is evidence of mainly cortical bone loss due to 
excess PTH secretion,93 and the patients are usually asymptomatic 
at this stage but may present with fragili

omatic 
at this stage but may present with fragility fractures. Serum lev­
els of calcium and phosphate are normal, and the serum level of 
alkaline phosphatase is usually, but not always, elevated.62,77 The 
serum 25-hydroxyvitamin D level is low (<10 ng/mL), and serum 
levels of PTH and 1,25-dihydroxyvitamin D are elevated. The 
increases in serum levels of alkaline phosphatase and 1,25-dihy­
droxyvitamin D respectively are related to increased bone turnover 
and increased 1α-hydroxylase activ

 bone turnover 
and increased 1α-hydroxylase activity in the kidney (see Chapter 
29) as a consequence of 2°HPT. In addition, an irreversible PTH-
mediated cortical bone loss may have already occurred.93
In the second stage, designated as HVO II, there is further 
accumulation of osteoid with increases in osteoid surface, osteoid 
volume, and osteoid thickness but with preservation of some min­
eralization as assessed by tetracycline uptake at the mineralization 
front.87 Both serum PTH and alka

e mineralization 
front.87 Both serum PTH and alkaline phosphatase levels increase 
further, but serum 1,25-dihydroxyvitamin D levels may return to 
normal or low depending on the degree of vitamin D deficiency 
(as assessed by its substrate, 25-hydroxyvitamin D) and PTH ele­
vation. The serum calcium level usually declines at this stage with 
low normal or frank hypocalcemia, and serum phosphate levels 
are usually low. Patients may be symptomatic with bone pain, 
muscle weakness, and fragility

ic with bone pain, 
muscle weakness, and fragility or pseudofractures.
In the third stage, designated as HVO III, the mineralization of 
bone matrix ceases and osteoid accumulation continues to cover 
more than 90% of the bone surfaces. It is at this stage that hypo­
calcemia is invariable, as the osteoid covered bone is resistant to 
osteoclastic bone resorption, which is a necessary mechanism to 
maintain normal serum calcium levels. The extensive coverage of 
the bone surfaces with osteoid is

ive coverage of 
the bone surfaces with osteoid is perhaps a “protective mechanism” 
to prevent complete dissolution of bone. However, peritrabecular 
and bone marrow fibrosis, a feature of more severe hyperparathy­
roidism, occurs only in HVO III and can be demonstrated on 
bone biopsy.4 Patients are almost always symptomatic at this stage, 
with diffuse bone pain, muscle weakness, and pseudofractures 
(Fig. 31.2), although an occasional patient may present primarily 
with muscle weakness witho

 may present primarily 
with muscle weakness without bone pain.94 Osteomalacia defined 
by an osteoid thickness greater than 12.5 μm and a mineralization 
lag time of more than 100 days (see Fig. 31.1) conforms to the 
conventional clinical and radiologic descriptions of osteomalacia. 
Regrettably, no such data exist on the evolution of hereditary or 
acquired forms of hypophosphatemic osteomalacia, as most of 
those patients present clinically late in the course of their disease, 
corresponding

ate in the course of their disease, 
corresponding to the third stage (HVO III) of vitamin D–defi­
ciency osteomalacia.87,92 
Pathogenesis of Rickets and Osteomalacia
The three principal mechanisms by which rickets and osteomalacia 
develop are vitamin D depletion or deficiency, phosphate deple­
tion or deficiency, and calcium deficiency—in that order of fre­
quency.62,95,96 Hypophosphatemia due to nutritional phosphate 
deficiency is a very rare cause of rickets or osteomalacia, although 
occas

 cause of rickets or osteomalacia, although 
occasional cases have been reported in patients on prolonged total 
parenteral nutrition. Vitamin D deficiency or depletion, however 
produced (Table 31.2), if prolonged or left untreated, ultimately 
leads to rickets and osteomalacia.97 Phosphate depletion, how­
ever caused (genetic, tumor induced, or acquired), is the second 
most common cause of rickets and osteomalacia, and the most 
prevalent type in parts of the world where vitamin D deficiency 

 in parts of the world where vitamin D deficiency 
is not endemic. The two most common causes of hypophospha­
temic rickets and osteomalacia are hereditary hypophosphatemic 
syndromes5,7 and FGF23-secreting tumors.8 Other less frequent 
causes of hypophosphatemic rickets and osteomalacia include pro­
longed use of phosphate-binding antacids,41,98 as well as various 
genetic and acquired renal tubular defects99 (Table 31.3). Even 
more uncommon, but histologically distinct from the vitamin D–
def

ut histologically distinct from the vitamin D–
deficiency and hypophosphatemic rickets and osteomalacia syn­
dromes, are the atypical and focal osteomalacia caused by the toxic 
effects of drugs such as sodium fluoride,100,101 etidronate,102 alu­
minum, and iron, which directly inhibit bone mineralization49,103 
(Table 31.4; see Fig. 31.1).
Vitamin D deficiency can be extrinsic or intrinsic. Extrinsic 
vitamin D deficiency is due to deficient endogenous cutaneous 
production of vitamin D3 or poo

ogenous cutaneous 
production of vitamin D3 or poor dietary intake (see Table 31.2). 
Inadequate exposure or avoidance of sunlight,21,104 use of sun-
protective lotions or sunscreens,105,106 darkly pigmented skin,107 
excessive covering of the body with clothing for cultural rea­
sons,108,109 and aging104 all contribute to the decreased production 
of vitamin D3 or cholecalciferol from its precursor 7-dehydrocho­
lesterol (see Chapter 29). Although inadequate intake of vitamin 
D as a cause of r

gh inadequate intake of vitamin 
D as a cause of rickets and osteomalacia is rare in the United 
States, occasional cases or case series have been reported.64,84,86
Rickets and osteomalacia due to intrinsic vitamin D depletion 
(the descriptive term depletion for all intrinsic causes of vitamin 
D deficiency is probably more appropriate) is most commonly 
caused by impaired gastrointestinal absorption of vitamin D (and 
calcium)110,111 as a result of intestinal disease, resection, or gastric 
by

t of intestinal disease, resection, or gastric 
bypass surgery (see Table 31.2). Vitamin D deficiency can also 
result from genetic or acquired causes of impaired or defective 
vitamin D 25-hydroxylase in the liver or impaired or deficient 
25-hydroxyvitamin D 1α-hydroxylase in the kidney and other tar­
get tissues, the necessary enzymes to convert the parent compound 
vitamin D to its biologically active metabolites,112–118 or increased 
catabolism of vitamin D and its active metabolites to ine

ism of vitamin D and its active metabolites to inert com­
pounds.119,120 Strictly speaking, rickets and osteomalacia due to 
1302
20
0
Osteoid surface (% bone surface)
Mean osteoid seam thickness (µm)
Adjusted apposition rate (µm/day)
0
10
20
30
40
50
60
A
B
40
2°HPT
HVO I
2°HPT
HVO II
FOM
AOM
HVO III
60
80
100
0.1
0
0.2
MLT-100 days
0.3
0.4
0.5
HVO I
LTO
LTO
• Fig. 31.1  Topographic depiction of stages of hypovitaminosis D osteopathy (HVO I, II, and III), atypi

taminosis D osteopathy (HVO I, II, and III), atypi­
cal osteomalacia (AOM), and focal osteomalacia (FOM). For comparison, secondary hyperparathyroidism 
(2°HPT) without mineralization defect and low turnover osteoporosis (LTO) are shown. In (A), the location 
of the seven types of bone lesions is based on the relationship between osteoid thickness (y-axis) and the 
extent of bone surface covered by osteoid (x-axis). In (B), the location is based on the relationship between 
osteoid thickness (y-

on the relationship between 
osteoid thickness (y-axis) and adjusted appositional rate (x-axis) as determined by tetracycline uptake 
(x-axis). In normal subjects, and in patients with 2°HPT, HVO I, and LTO, there is no relationship between 
osteoid thickness and osteoid surface until the osteoid surface exceeds greater than 50% to 60% of the 
bone surface (straight solid horizontal lines), after which the relationship becomes hyperbolic (interrupted 
curvilinear lines). By contrast, there is a 

pted 
curvilinear lines). By contrast, there is a positive relationship between osteoid thickness and the adjusted 
mineral apposition rate (straight interrupted lines) in normal subjects and in patients with 2°HPT, HVO I, 
and LTO (B). The oblique interrupted line indicates the reversal of this relationship in patients with more 
severe osteomalacia (HVO II and III), the cardinal feature of osteomalacia unlike all other conditions (2°HPT, 
LTO, AOM, and FOM). The solid straight line represents 

AOM, and FOM). The solid straight line represents a mineralization lag time of 100 days (MLT, the 
time delay between matrix deposition osteoblasts and subsequent mineralization) that separates patients 
with and without osteomalacia. Locations are shown for clarity and simplicity. Note a significant overlap of 
2°HPT, HVO I, and LTO. (Modified from Bhan A, Qiu S, Rao SD. Bone histomorphometry in the evaluation 
of osteomalacia. Bone Rep. 2018;8:125–134.)
A
C
D
B
• Fig. 31.2  Radiologic manifest

125–134.)
A
C
D
B
• Fig. 31.2  Radiologic manifestations of osteomalacia. (A) Pseudofracture in the scapula and a brown 
tumor in the proximal humerus. (B) Pseudofractures in both superior pubic rami. (C) Triradiate pelvis due to 
softening of the pelvic bones. (D) Bilateral protrusio acetabuli and deformed pelvis.
1303
target organ resistance to vitamin D action37,38,121–123 are neither 
a vitamin D deficient state nor a genetic defect in the vitamin D 
acti

 state nor a genetic defect in the vitamin D 
activation pathway (see Tables 31.2 and 31.3). Although excessive 
intestinal loss of 25-hydroxyvitamin D via enterohepatic circula­
tion or renal loss of vitamin D and its metabolites due to heavy 
proteinuria (as in nephrotic syndrome) as a cause of vitamin D 
depletion has been proposed, the magnitude and scope of these 
mechanisms contributing to vitamin D depletion are not well 
characterized.124–126
Of all the intrinsic causes of vitamin D depl

–126
Of all the intrinsic causes of vitamin D depletion, malabsorp­
tion of vitamin D is by far the most common cause of osteoma­
lacia.110 Both gluten enteropathy and Crohn disease have been 
associated with osteomalacia due to vitamin D depletion.62,127–131 
Although decreased bone mineral density (BMD), increased frac­
ture risk, and growth retardation (possibly related to rickets) have 
been associated with inflammatory bowel disease,132–134 osteo­
malacia due solely to inflammatory bowel di

osteo­
malacia due solely to inflammatory bowel disease has not been 
reported. Severe calcium malabsorption, malnutrition, or both, 
as seen in patients with inflammatory bowel disease, may lead to 
2°HPT with the consequent cortical bone loss and increased risk of 
fragility fractures (see Chapter 30) but without vitamin D deple­
tion severe enough to cause osteomalacia.78,93,135 Total and par­
tial gastrectomy, vagotomy and pyloroplasty,80,81,136–139 intestinal 
resection,125,140 and gastric 

136–139 intestinal 
resection,125,140 and gastric or intestinal bypass surgery for morbid 
obesity62,80,128,138,141,142 have all been associated with vitamin D 
depletion and osteomalacia (see Table 31.2). Because of the mal­
absorption of multiple nutrients, including calcium and vitamin 
D, the bone phenotype varies from simple osteopenia detected 
by BMD testing to osteoporosis with increased fracture risk, to 
frank osteomalacia on bone histomorphometry. The relative fre­
quency of osteomala

morphometry. The relative fre­
quency of osteomalacia in patients with various gastrointestinal 
disorders or surgeries is not clearly established but may be as high 
as 50%.4,77,128,135,143,144 Prolonged 2°HPT can occasionally lead 
to bone marrow fibrosis and hypercalcemic 2°HPT (or tertiary 
hyperparathyroidism; see Chapter 29).4,130,145–147
By contrast, hepatobiliary and pancreatic disorders are relatively 
less common causes of rickets and osteomalacia, although osteopo­
rosis is very commo

steomalacia, although osteopo­
rosis is very common in both kinds of disorders (see Chapter 30). 
Because of the large functional reserve, by inference adequate avail­
ability of vitamin D 25-hydroxylase enzyme activity, vitamin D 
depletion sufficient to cause rickets and osteomalacia is unlikely in 
patients with the usual types of parenchymal liver diseases.148–152 
Most often, additional factors, such as poor dietary vitamin D 
intake, antiviral drug therapy for hepatitis153–156 (see the Dru

ral drug therapy for hepatitis153–156 (see the Drug-
Induced Osteomalacia section), and coexistent primary biliary 
cirrhosis,124,152,157–159 contribute to severe vitamin D depletion 
and osteomalacia. Immaturity and neonatal hepatitis are also rare 
causes of rickets in children2,114 and osteomalacia in adults,160 most 
likely related to defective or insufficient vitamin D 25-hydroxylase 
enzyme, although not conclusively documented. Despite a signifi­
cant fat malabsorption and steatorrhea in 

ignifi­
cant fat malabsorption and steatorrhea in patients with exocrine 
pancreatic insufficiency, rickets and osteomalacia are uncommon, 
but both rickets and osteomalacia have been reported in patients 
with cystic fibrosis.161–165 Drugs that interfere with the 25-hydrox­
ylation step in the vitamin D activation pathway are discussed in the 
section on drug-induced rickets and osteomalacia (see Table 31.6).
Calcium-Deficiency Rickets
Unlike nutritional vitamin D and phosphate deficiency, whic

tritional vitamin D and phosphate deficiency, which cause 
both rickets and osteomalacia, only rickets has been convincingly 
documented as resulting from nutritional calcium deficiency with­
out associated vitamin D deficiency as assessed by normal or low 
normal serum 25-hydroxyvitamin D levels.1,96 Although much has 
been learned on the role of calcium nutrition in the pathogenesis of 
  Causes of Vitamin D–Deficiency Rickets or 
Osteomalacia, or Both
Extrinsic
Inadequate dietary intake of v

, or Both
Extrinsic
Inadequate dietary intake of vitamin D
Decreased exposure or avoidance of sunlight
Use of sunscreens (especially a solar protective factor >8)
Excess clothing (veil/hijab)
?Increased or dark skin pigmentation
Intrinsic
Advancing age with decreased cutaneous production of vitamin D
Malabsorption due to various gastrointestinal disorders
Gastrectomy (partial, total, or bypass procedure)
Small intestinal disease, resection, or bypass
Gluten enteropathy (Celiac sprue)
Biliary cir

pass
Biliary cirrhosis (uncommon)
Pancreatic insufficiency including cystic fibrosis (uncommon)
Impaired or genetically defective vitamin D 25-hydroxylase enzyme
?Immaturity
?Neonatal hepatitis
?Cirrhosis of the liver
Genetic defect (vitamin D–dependent rickets type 1B)
Impaired or genetically defective 25-hydroxyvitamin D 1α-
hydroxylase enzyme
Genetic defect (vitamin D–dependent rickets type 1A)
“Acquired” vitamin D deficiency
Increased c

ailure
Increased catabolism due to microsomal enzyme induction
Calcium deficiency with secondary hyperparathyroidism
TABLE 31.2
  Causes of Hypophosphatemic Rickets or 
Genetic
Autosomal dominant rickets
Autosomal recessive rickets
X-linked dominant rickets (X-linked hypophosphatemia)
Hypophosphatemic hypercalciuric hereditary rickets (or Dent disease)
Neurofibromatosis
Genetic Fanconi syn

fibromatosis
Genetic Fanconi syndrome (may have renal failure)
Acquired
Tumor-induced osteomalacia (most common acquired cause)
Renal tubular damage or nonfamilial Fanconi syndrome
Paraproteinemia
Tyrosinemia
Glycogenosis
Tenofovir and adefovir (second most common acquired cause)
Phosphate-binding antacids
Cadmium poisoning
?Use of outdated tetracycline
?Amphotericin
TABLE 31.3
1304
osteoporosis (see Cha

ON VII    Mineral Metabolism
osteoporosis (see Chapter 30), no case of osteomalacia in an adult 
due to calcium deficiency alone has been reported; the reasons for 
this discordant effect of calcium nutrition on the skeleton in chil­
dren and adults is perplexing. A more severe 2°HPT over a relatively 
short period due to severe calcium malnutrition in a growing child 
may produce radiologic features similar to rickets—the so-called 
short-latency disease.166 However, a milder 2°HPT over a longe

y disease.166 However, a milder 2°HPT over a longer 
period of time caused cortical bone loss, osteoporosis, and increased 
fracture risk—the so-called long-latency disease.166 Further studies 
are needed to clarify this apparent paradox: why two different nutri­
ent (calcium and vitamin D) deficiencies independently produce 
the same disease in children (rickets) but different diseases in adults 
(osteoporosis and osteomalacia).
Calcium malnutrition as a cause of rickets was first suggested 
in

tion as a cause of rickets was first suggested 
in a child from San Francisco, California, who responded to cal­
cium infusion, but the child also had aminoaciduria, and vitamin 
D deficiency was not conclusively excluded.167 A similar case of 
rickets in an Italian child from Toronto, Canada, in whom both 
vitamin D deficiency and resistance were excluded with appro­
priate biochemical testing, probably represents the first unam­
biguous human case of true calcium-deficiency rickets.168 A 
more

ase of true calcium-deficiency rickets.168 A 
more comprehensive study on calcium-deficiency rickets in chil­
dren from rural South Africa further characterized the condition 
as a separate entity,169 with additional reports from Nigeria,96,170 
India,171 Mongolia,172 and Europe,173 confirming the concept, 
but many children also had some degree of vitamin D insuf­
ficiency. A daily calcium intake of greater than 200 mg appears 
to be the lowest threshold for the risk of developing calcium-
defi

threshold for the risk of developing calcium-
deficiency rickets independent of vitamin D nutritional status1; 
“wet-nursing” and prolonged breastfeeding, practices that are 
prevalent in some cultures, are other risk factors for calcium-
deficiency rickets.174,175 Rickets due to calcium deficiency tends 
to occur later in life than that due to vitamin D deficiency, with 
an average age at presentation of 4 years in Nigeria, but ranging 
from 4 to 16 years in other series.1 Interestingly, calciu

o 16 years in other series.1 Interestingly, calcium-defi­
ciency rickets appears to be particularly prevalent in parts of the 
world with abundant sunshine.96,170,171,176 Clinically, calcium-
deficiency rickets differs from other forms of rickets, especially 
in adolescents who may have significant genu valgum without 
many end-plate deformities.177 The rarity of calcium-deficiency 
rickets in developed countries may be related to a much higher 
dietary calcium intake and less prolonged breastfe

dietary calcium intake and less prolonged breastfeeding.176,178 
Nevertheless, when a child is encountered with rickets and if 
the serum level of 25-hydroxyvitamin D is normal, think of 
calcium-deficiency rickets, particularly if serum calcium is low 
and PTH is elevated. 
  Relevant Abnormalities in Various Types of Vitamin D–Related Rickets and Osteomalacia
Variable
Vitamin D 
Dependency Type 
1B
Vitamin D 
1A
Vitamin D 
Dependency 
Type 2
Hypophosphate

pe 
1A
Vitamin D 
Type 2
Hypophosphatemic 
Vitamin D Resistanta
Basic defect
Nutritional/ 
malabsorption
25-Hydroxylase 
defect
1α-Hydroxylase 
defect
VDR defect
Excess FGF23
Gene locus
Not applicable
Chromosome 
11p15.2
12q13.1
12q12-q14
Xp22.11
Enzyme defect
CYP2R1
CYP27B1
Receptor defect
PHEX gene defect
Low/normal
Low
Low
Low
Normal/low
Very low
Alkaline phospha­
tase

mal/Low
Very low
tase
High
High
High
High
High
Parathyroid hor­
mone
High
High
High
High
Normal/high
25-Hydroxyvita­
min D
Low
Low/very low
Normal
Normal
Normal
1,25-Dihydroxyvi­
tamin D
Variable
Low/low normal
Low
High
Low
Urine calcium
Low
Low
Low
Low
Vitamin D dose 
required to heal/
cure ricketsb
1000–2000 IU daily
Few weeks to months
10,000 IU daily
Lifelong
Lifelong
100,000 units daily
Lifelong
Lifelong
Calcitriol

y
Lifelong
Lifelong
Calcitriol dose 
cure rickets
0.04 μg/kg per day
Lifelong
Lifelong
1–2 μg/day
Lifelong
1–2 μg/day and oral 
phosphate
Lifelong
aPreviously referred to as vitamin D–resistant rickets because of the large doses of vitamin D required to heal rickets; the current name is X-linked hypophosphatemic rickets and osteomalacia.
bShown only to illustrate comparative effective doses. Both calc

 illustrate comparative effective doses. Both calcitriol and alphacalcidol are now widely available and used to treat all vitamin D–dependent rickets and osteomalacia
CYP, Cytochrome P450; FGF23, fibroblast growth factor 23; PHEX, phosphate-regulating endopeptidase homolog, X-linked; VDR, vitamin D receptor.
   
TABLE 31.4
1305
Phosphate-Deficiency/Depletion Rickets and 
Nutritional phosphate deficiency is a very rare cause of rick­
ets and osteo

ciency is a very rare cause of rick­
ets and osteomalacia, as there is abundant phosphate in foods, 
fruits, vegetables, and dairy products. Since intestinal absorption 
of phosphate is mostly passive and quite efficient, it is very diffi­
cult to produce true nutritional phosphate deficiency in an other­
wise healthy individual. The serum phosphate level is maintained 
within a narrow range by the kidney under the control of PTH and 
FGF23.179,180 Hypophosphatemia, however, is not uncommon in 

80 Hypophosphatemia, however, is not uncommon in 
hospitalized patients,181 in patients with iron deficiency,182 and in 
those receiving phosphate binders and antacids known to deplete 
phosphorus in the body.41,43,45,98 Such hypophosphatemic states 
usually do not last long enough to produce rickets and osteoma­
lacia. Consequently, almost all hypophosphatemic rickets and 
osteomalacia are either genetic or acquired (see later discussion). 
Clinical Manifestations of Classical Rickets 
and Oste

ical Manifestations of Classical Rickets 
and Osteomalacia
The symptoms and signs of rickets and osteomalacia are primarily 
related to the musculoskeletal system. With few exceptions (dis­
cussed later in detail in the relevant sections), the clinical mani­
festations are similar regardless of their pathogenesis—nutrient 
deficiencies, genetic or acquired causes, tumor induced, or drug 
induced. Because rickets involves the growth plates affecting linear 
growth, short stature is common. When n

ng linear 
growth, short stature is common. When nutritional deficiencies 
begin in childhood, and if left untreated for prolonged periods, 
as usually occurs in vitamin D and calcium-deficient endemic 
regions, deformities of the skeleton are common. In long bones, 
rickets affects diaphysis (bowing), metaphysis (widening, fraying, 
and cupping), and epiphysis (irregular margins), whereas osteo­
malacia involves only the diaphysis of the long bones. If osteo­
malacia develops later in life with

nes. If osteo­
malacia develops later in life without a history of rickets during 
infancy and childhood, as is common in clinical practice in devel­
oped countries, the clinical manifestations are subtle and resemble 
those of age-related osteoporosis (see Chapter 30). In general, the 
later the onset of osteomalacia, the more easily its clinical clues 
are missed and the more likely the symptoms are dismissed as 
aches and pains of aging. The most common presenting clinical 
symptoms are bone 

ost common presenting clinical 
symptoms are bone pain, muscle weakness and difficulty in walk­
ing, skeletal deformities, and fractures. Carpal and pedal spasms, 
muscle cramps, and seizures due to hypocalcemia are uncommon 
but mostly seen in children with rickets than in adults with osteo­
malacia. Triradiate pelvis, a rare complication of osteomalacia due 
to softening of the pelvic bones (see Fig. 31.2), may cause difficult 
or obstructed labor in childbearing women; however, it is far less

bor in childbearing women; however, it is far less 
common today, and its presence suggests severe disease.
Bone Pain
Bone pain in osteomalacia is diffuse, nondescript, dull aching, 
deep seated, and poorly localized, and at times can be debilitat­
ing.85,183–185 It is felt more in the bones than in the joints and 
often is bilaterally symmetric. Because of its vague nature, bone 
pain is often misdiagnosed as tension headache (so-called osteo­
malacic cephalalgia), “angina” (chest pain due to p

alacic cephalalgia), “angina” (chest pain due to pseudofractures 
in the ribs), rheumatism, and fibromyalgia.186–188 The pain is per­
sistent and gnawing, is aggravated by weight bearing or muscle 
contractions during attempted walking, and is rarely relieved by 
rest. The pain usually begins in the lower back and spreads to the 
pelvis, hips, thighs, upper back, and ribs but is nonradiating and 
is rarely felt below the knees unless fragility or pseudofractures are 
present in tibiae and fibula

 pseudofractures are 
present in tibiae and fibulae. Bone tenderness can be elicited by 
pressure or percussion over the shin bones, squeezing of the fore­
arm with a fist, lateral compression of the pelvis and rib cage, and 
posterior compression of the sternum. The propensity of pain to 
localize to the axial skeleton is probably related to an earlier and 
a greater accumulation of osteoid in the cancellous bone, whereas 
the appendicular skeleton, rich in cortical bone, is more subject 
to fr

ton, rich in cortical bone, is more subject 
to fragility fractures. The mechanism for bone pain is believed to 
be related to the stretching of the periosteum by the overhydrated 
unmineralized bone matrix. Bone pain almost never occurs in 
patients with osteoporosis in the absence of a fracture, but in osteo­
malacia, bone pain occurs with or without a fracture. A significant 
minority of the patients can be completely asymptomatic.94 
Muscle Weakness
Proximal muscle weakness, especially in th

eakness
Proximal muscle weakness, especially in the lower extremities, is the 
most common muscular manifestation in osteomalacia, although 
weakness in the upper arms can also be demonstrated by physi­
cal examination. The severity of muscle weakness varies from sub­
tle abnormalities detectable only on careful examination to severe 
debilitating disabilities mimicking more serious muscle disorders 
and rarely can cause total paralysis. In mild cases, the muscle weak­
ness must be distinguished

cases, the muscle weak­
ness must be distinguished from the patient’s reluctance to stand or 
walk for fear of aggravating the bone pain. Difficulty in rising from a 
sitting position or going up and down stairs without using the arms 
is quite specific. In advanced cases, classical waddling gait (walk­
ing like a duck), the result of a combination of muscle weakness 
and bone pain, is observed. With prolonged vitamin D depletion 
of increasing severity, as seen in endemic regions that are vitam

everity, as seen in endemic regions that are vitamin D 
and calcium deficient, a patient may become completely immobi­
lized and bed bound because of profound weakness and excruciat­
ing bone pain, sometimes masquerading as a terminal illness.
Despite profound muscle weakness, muscle atrophy is uncom­
mon, although mild muscle wasting with atrophy of the type II 
fibers has been reported occasionally. Hypotonia can be present, 
but fasciculation and clonus are absent. Deep tendon reflexes are 
n

and clonus are absent. Deep tendon reflexes are 
normal or increased—all helpful clinical signs to distinguish the 
muscle weakness of rickets and osteomalacia from other types of 
muscle diseases and myopathies. Rarely, dilated cardiomyopathy 
that responds to vitamin D repletion has been reported in severe 
rickets.189 Other muscular symptoms such as muscle cramps and 
spasms, tingling and numbness, and seizures (usually in children) 
occur when the serum calcium level falls below 6.0 mg/dL. B

n the serum calcium level falls below 6.0 mg/dL. Both 
the muscle weakness and atrophy are commonly attributed to 
hypocalcemia and 2°HPT, but both muscle abnormalities are also 
seen in patients with primary and tertiary hyperparathyroidism in 
whom the serum calcium level is elevated (see Chapter 29). Since 
muscle weakness is also a manifestation of hypophosphatemic 
osteomalacia in the absence of hyperparathyroidism or hypocal­
cemia, the relative contributions of hypocalcemia, hypophospha­

ative contributions of hypocalcemia, hypophospha­
temia, and hyperparathyroidism to muscular manifestations of 
rickets and osteomalacia is not entirely clear. In general, muscle 
weakness is more prominent in hypophosphatemic rickets and 
osteomalacia, whereas bone pain is more common in vitamin D–
deficiency osteomalacia. Interestingly, despite a significant muscle 
weakness in patients with X-linked hypophosphatemia (XLH), 
bone mass, bone size, and estimated bone strength are normal or 
incr

e, and estimated bone strength are normal or 
increased.190,191 
1306
Skeletal Deformities and Fractures
Skeletal deformities are common in children with rickets, vary 
with the age of presentation, and may remain permanent, 
whereas bone deformities are uncommon in adult-onset osteo­
malacia unless fractures have occurred. Infants present with 
open fontanelles, dolichocephaly, frontal bossing, rachitic rosary 
(due to consecutive pseudofractures of multiple ri

(due to consecutive pseudofractures of multiple ribs often bilat­
erally symmetric), Harrison sulcus (a visible horizontal line of 
depression at the level of the diaphragm due to weakness of the 
chest muscles), swollen wrist and ankle joints (due to widened 
metaphysis), and double malleoli. Once the child starts walk­
ing, bowing of the long bones, genu valgum, genu varum, and 
windswept deformity are seen. The skeletal deformities are usu­
ally more severe in genetic hypophosphatemic rickets

ly more severe in genetic hypophosphatemic rickets and osteo­
malacia, and they predominantly involve lower limbs, resulting 
in a disproportionate short stature.191 Fragility fractures are not 
uncommon, but Looser zones or pseudofractures, the diagnos­
tic radiologic abnormalities (see Fig. 31.2), are more common 
in patients with rickets and osteomalacia. Pseudofractures are 
linear radiolucent bands perpendicular to the long axis of the 
bones, and are stress fractures that can extend to a c

s, and are stress fractures that can extend to a complete 
fracture with separation or displacement with the pseudofrac­
tures that can progress to a complete fracture, usually in the 
subtrochanteric region of the femur or metatarsals—the great­
est load-bearing bones. Rib fractures also commonly occur. The 
osteomalacic bones in children with rickets are soft rather than 
brittle, so fractures are less common. Conversely, when osteo­
malacia begins later in adult life, the usual types of fract

gins later in adult life, the usual types of fractures are 
more common. 
Biochemical Changes
Relevant biochemical abnormalities at presentation of various 
forms of rickets and osteomalacia are summarized in Tables 31.4 
and 31.5. Several excellent reviews have been published,62,87 and 
only the salient abnormalities within each type and contrasting 
features between the types are briefly discussed in the following. 
In all types of rickets and osteomalacia, elevated serum alkaline 

osteomalacia, elevated serum alkaline 
phosphatase is the most frequent (∼80–90%) and the earliest 
biochemical abnormality,77 although an occasional patient may 
have normal alkaline phosphatase with histologically confirmed 
osteomalacia.94 In general, hypocalcemia is a late biochemical 
manifestation, but it occurs earlier in the course of development 
of rickets in children than during the evolution of osteomalacia 
in adults. Mild to moderate hypocalcemia (serum calcium level 
of 7.0–8.5 mg

e hypocalcemia (serum calcium level 
of 7.0–8.5 mg/dL) is often asymptomatic unless it falls below the 
threshold for symptoms (usually <6.0 mg/dL). Like other types 
of biochemical abnormalities in clinical practice, it is the rate of 
change rather than the absolute value that determines the devel­
opment of relevant symptoms. Most importantly, serum calcium 
is always normal in all forms of hypophosphatemic rickets and 
osteomalacia, except in certain types of renal tubular defects. The 
seru

 certain types of renal tubular defects. The 
serum phosphate level is quite variable, lacks specificity, and is 
subject to diurnal variation, meal ingestion (and hence should 
be measured in the fasting state in the morning), renal function, 
and degree of serum PTH elevation—a known regulator of the 
serum phosphate level179; thus, serum phosphate in nutritional 
rickets and osteomalacia can be normal, low, and occasionally 
high, particularly in patients with more severe hypocalcemia.94 
By 

 in patients with more severe hypocalcemia.94 
By definition, the serum phosphate level is less than 2.5 mg/
dL in all forms of hypophosphatemic rickets and osteomalacia. 
Although all patients with nutritional rickets and osteomalacia, 
by definition, have low 25-hydroxyvitamin D levels (usually <10 
ng/mL), not all patients with low 25-hydroxyvitamin D levels 
necessarily develop rickets or osteomalacia.62,87 This distinction 
is important in planning therapy, underscores the need not to 
equa

lanning therapy, underscores the need not to 
equate low serum 25-hydroxyvitamin D level to osteomalacia, 
and avoids the often misused phrase “biochemical osteomala­
cia.” In calcium-deficiency rickets, serum 25-hydroxyvitamin D 
is either normal or slightly reduced, although not to the same 
extent as in vitamin D–deficiency rickets and osteomalacia. 
An accelerated catabolism of 25-hydroxyvitamin D to its more 
polar and biologically active 1,25-dihydroxy vitamin D or to 
its inert metabolite

25-dihydroxy vitamin D or to 
its inert metabolites contributes to the lower levels of serum 
25-hydroxyvitamin D, sometimes referred to as conditional or 
obligatory vitamin D insufficiency. Serum levels of 1,25-dihy­
droxyvitamin depend on the stage in the evolution of nutri­
tional rickets and osteomalacia, the availability of its precursor 
(25-hydroxyvitamin D), and the degree of PTH elevation; thus, 
the serum levels of 1,25-dihydroxyvitamin D can be high, nor­
mal, or low.62,77,87,143 By 

n D can be high, nor­
mal, or low.62,77,87,143 By contrast, serum 1,25-dihydroxyvitamin 
D levels are low, although not invariably, in hypophosphatemic 
rickets and osteomalacia.8,88,89 Serum levels of PTH are always 
elevated in nutritional-deficiency (both vitamin D and calcium) 
rickets and osteomalacia, and the levels are normal in hypophos­
phatemic disorders regardless of the pathogenesis, unless vitamin 
D deficiency also exists. However, serum PTH levels rise pro­
gressively over time in

serum PTH levels rise pro­
gressively over time in patients with hypophosphatemic rickets 
and osteomalacia treated with long-term oral phosphate supple­
ments and in some patients may even progress to hypercalcemic 
hyperparathyroidism that requires surgery.192 A suggested clini­
cal algorithm based on biochemical abnormalities in the evalu­
ation of patients suspected of having rickets and osteomalacia is 
presented in Fig. 31.3. 
Radiologic Imaging Features
The major radiologic manifestations

aging Features
The major radiologic manifestations of nutritional rickets and 
osteomalacia are bone structural changes discernible on routine 
x-rays (now better defined on digital images), generalized decrease 
in apparent bone density on x-rays, vertebral deformities (Fig. 
31.4), and pseudofractures (or Looser zones; see Fig. 31.2). Gener­
alized thinning of cortices in the long bones is probably the earliest 
radiologic manifestation due to PTH-mediated endocortical bone 
resorption. Subper

PTH-mediated endocortical bone 
resorption. Subperiosteal bone resorption (best seen on the radial 
aspect of the middle phalanges, metacarpals, and metatarsals) and 
brown tumors (osteitis fibrosa cystica; see Fig. 31.2) are seen in 
more advanced cases with severe hyperparathyroidism193,194 (see 
Chapter 29). Although these latter bone abnormalities may resolve 
following vitamin D therapy, cortical thinning remains permanent 
and increases the fracture risk for the remainder of the patient’s 

 fracture risk for the remainder of the patient’s 
life.93,195 Symmetrical biconcavity of vertebrae, referred to as cod 
fish vertebrae because they resemble vertebrae in cod fish, involves 
almost all vertebrae. “Fish-mouth” appearance of the intervertebral 
space, another structural change in the vertebral column, is the 
result of yielding of the soft vertebral bone to the pressure of the 
intervertebral discs (see Fig. 31.4A). This characteristic appearance 
of the vertebral column in osteom

stic appearance 
of the vertebral column in osteomalacia is in stark contrast to the 
random anteriorly wedged or compressed asymmetrical vertebral 
deformities seen in patients with osteoporosis (see Fig. 31.4B; also 
see Chapter 30); when present, the cod fish vertebrae–like appear­
ance of the spine is virtually diagnostic of osteomalacia (see Fig. 
31.4A). The generalized apparent decrease in density of the bones 
on radiographs is manifested as decreased BMD by dual energy 
x-ray absorptiom

 as decreased BMD by dual energy 
x-ray absorptiometry. Looser zones or pseudofractures are lucent 
1307
bands (2–5 mm in width) perpendicular to the long axis of the 
bone or periosteum, often bilaterally symmetrical with sclerotic 
borders (Milkman syndrome); they occur more commonly in the 
ribs, pubic rami, and outer border of the scapulae (see Fig. 31.2), 
and less commonly at the inferomedial region of the proximal 
femurs and medial aspect of the shaft

he proximal 
femurs and medial aspect of the shafts of the long bones. How­
ever, insufficiency fractures, sometimes inappropriately referred 
to as pseudofracture, can be seen in Paget disease of bone, hypo­
phosphatasia, fibrous dysplasia, and atypical femur fractures due 
to long-term bisphosphonate therapy but usually occur (or begin) 
on the lateral cortex of long bones. Both insufficiency fractures 
and pseudofractures can progress across the shaft of the bone to a 
complete fracture. Loos

he shaft of the bone to a 
complete fracture. Looser zones or pseudofractures are caused by 
erosion of bone by nutrient arterial pulsation, which explains their 
specific medial cortical location, and represent the unhealed insuf­
ficiency type of stress fracture.
The generalized increase in radionuclide uptake throughout the 
skeleton, referred to as super scan, is specific for conditions associ­
ated with increased bone turnover but is more common in osteo­
malacia. Typically, there are no di

mmon in osteo­
malacia. Typically, there are no discrete focal abnormalities in the 
absence of pseudofractures, and the radionuclide uptake in the kid­
neys, in the absence of renal dysfunction, is either faint or absent, as 
most of the isotope is retained in the skeleton and very little is avail­
able for renal excretion (Fig. 31.5). When present, Looser zones are 
seen as “hot spots” on nuclear imaging (see Fig. 31.5). With a few 
exceptions, most radiologic features are similar among the va

 most radiologic features are similar among the various 
type of rickets and osteomalacia. Cortical thinning in long bones is 
not seen in XLH, and in fact, thick cortices are the rule rather than 
the exception. Similarly, enthesopathy is seen exclusively in XLH, 
but the underlying mechanisms for its development is not known. 
In most acquired forms of hypophosphatemic osteomalacia (tumor- 
and drug-induced or renal tubular defects), cortical thinning and 
decreased BMD may be seen, most likel

hinning and 
decreased BMD may be seen, most likely the result of age-related 
bone loss considering that 2°HPT is not a feature of the genetic or 
acquired forms of hypophosphatemic disorders. Horizontal bands of 
endplate sclerosis of the vertebrae with relative lucency in the middle 
portion of the vertebral bodies (Rugger-Jersey spine, so named as 
the appearance resembles jerseys of rugby players) are sometimes 
observed in osteomalacia due to renal tubular acidosis but is a more 
character

to renal tubular acidosis but is a more 
characteristic feature of renal osteodystrophy (see Chapter 29). 
PTH High
P-normal; Ca-low/normal
25-D-normal/low
SUSPECTED RICKETS/OSTEOMALACIA
CLINICAL/RADIOLOGICAL FEATURES
Hypophosphatasia
Low
Alkaline Phosphatase
High
NORMAL
Transient increase in bone remodeling
Consider various skeletal dysplasia
TmP/GFR & TRP
Urine AA/ HCO3/GLUCOSE
High
N/High
Low
lα-hydroxylase
VDDR-1A
Calcium
or
Recently
VDD
VDDR-2
or
HVO I
Ca, P, 25-D & PT

y
or
Recently
VDD
VDDR-2
or
HVO I
Ca, P, 25-D & PTH
Hypophosphatemia (<2.5 mg/dl)
Normal: Ca, PTH & 25-D
Calciopenic
25-D-Low
VDD or
VDDR-1B
1,25-D
25-D-Normal
Phosphopenic
TmP/GFR-Low
Urine AA/HCO3/Glu: Normal or
Urine AA/HCO3/Glu
Normal
Urine AA/HCO3/Glu:High or 
High
TmP/GFR-High
Normal
Renal Tubular Defects
1,25-D & Urine Ca
FGF-23 Mediated
XLH & TIO
Non FGF-23 Mediated
HHRH
Nutritional
Phosphate
Depletion
NORMAL/HI

ted
HHRH
Phosphate
Depletion
NORMAL/HIGH
• Fig. 31.3  Suggested clinical algorithm based on biochemical abnormalities in evaluating patients sus­
pected of having rickets, osteomalacia, or both. Analytes: Ca, serum calcium; P, serum phosphate; 25-D, 
serum 25-hydroxyvitamin D; 1,25-D, 1,25-dihydroxyvitamin D; PTH, serum parathyroid hormone; TmP/
GFR, tubular maximum for phosphate reabsorption/glomerular filtration rate; HVO I, hypovitaminosis 
D osteopathy stage I; TRP, tubular reabs

aminosis 
D osteopathy stage I; TRP, tubular reabsorption of phosphate; AA, urine amino acid analysis; HCO3, 
urine bicarbonate excretion; Glu, urine glucose. Conditions: VDD, vitamin D deficiency; VDDR, vitamin 
D–dependent rickets; XLH, X-linked hypophosphatemic rickets and osteomalacia; TIO, tumor-induced 
osteomalacia; HHRH, hereditary hypophosphatemic rickets with hypercalciuria.
1308
Bone Mineral Density
BMD as assessed by dual energy x-ray absorptiometry 

D as assessed by dual energy x-ray absorptiometry is 
reduced at all of the relevant sites (lumbar spine, proximal 
hip, and forearm), usually with a greater deficit at the site of 
rich cortical bones in the forearms. By contrast, BMD is either 
normal or even increased at the lumbar spine in adults with 
XLH osteomalacia (see later discussion). A lower BMD in 
tumor-induced osteomalacia (TIO) is more likely age related. 
Although the patterns of BMD deficits are different in differ­
ent types 

f BMD deficits are different in differ­
ent types of osteomalacia, the findings are neither sensitive nor 
specific to differentiate osteomalacia from osteoporosis. Accord­
ingly, in vivo tetracycline-labeled bone histomorphometry is the 
gold standard method to conclusively establish the diagnosis of 
osteomalacia in suspected cases.87 
Treatment of Nutritional Rickets and 
Treatment of rickets and osteomalacia must be based on their 
pathogenesis. Since therapeutic approaches to t

r 
pathogenesis. Since therapeutic approaches to treat and manage 
various types of rickets and osteomalacia (nutritional, genetic, and 
acquired) differ, the treatments that substantially differ will be dis­
cussed in the relevant sections in this chapter. In general, there is 
no “fixed-dose” or “one-size-fits-all” regimen to treat all varieties 
of rickets and osteomalacia. Several points deserve emphasis in the 
management of rickets and osteomalacia. First, most recommen­
dations are largel

omalacia. First, most recommen­
dations are largely based on personal preferences, clinical experi­
ence, and availability of suitable vitamin D preparations. Second, 
moderate to severe rickets and osteomalacia related to vitamin D 
and calcium deficiency, but not the hypophosphatemic variety, are 
frequently associated with hypocalcemia and thus require much 
higher doses of vitamin D and calcium supplements initially. 
A
B
• Fig. 31.4  (A) Typical symmetrical biconcave deformities of all the 

ical symmetrical biconcave deformities of all the vertebrae, sometimes referred to as cod 
fish vertebrae because they resemble the vertebrae of the fish. When present, it is diagnostic of osteomala­
cia of all types. (B) Typical random osteoporotic wedge and compression deformities in a postmenopausal 
woman with osteoporosis.
• Fig. 31.5  Characteristic bone scan in a patient with osteomalacia. Note 
increased nuclide uptake throughout the skeleton with very little or no 
uptake in the kidneys

eton with very little or no 
uptake in the kidneys as the radionuclide is all retained in the high remodel­
ing skeleton. Also see multiple bilateral symmetrical uptake in the ribs (left) 
due to pseudofractures, often confused with metastatic disease.
1309
Third, it must be kept in mind that patient symptom relief is 
much faster (a few weeks to a few months) than the biochemi­
cal, radiologic, or histologic improvements, which may take a few 
months to year

improvements, which may take a few 
months to years. Fourth, even after apparent “cure” of clinical, 
biochemical, radiologic, and bone histologic abnormalities, many 
patients remain at risk for fractures because of irreversible cortical 
bone loss. In some patients with long-standing vitamin D–defi­
ciency rickets and osteomalacia, 2°HPT may persist196 with con­
tinued bone loss for years and may even progress to hypercalcemic 
2°HPT (or tertiary hyperparathyroidism) requiring parathyroid­
ect

ry hyperparathyroidism) requiring parathyroid­
ectomy.146,197,198 This is analogous to the development of tertiary 
hyperparathyroidism after long-term oral phosphate therapy in 
patients with hypophosphatemic (genetic or acquired) osteoma­
lacia,192 and in patients on chronic maintenance dialysis199 or 
after kidney transplantation.200 Accordingly, the goals of therapy 
are not only to simply relieve symptoms but also to restore bone 
strength by mineralizing the excess osteoid and prevent bone

y mineralizing the excess osteoid and prevent bone loss 
by correcting 2°HPT.
Several dose regimens have been suggested, each with some 
merit,201 but there is no perfect method. Whichever method is 
chosen, it must achieve the therapeutic goals, and which method 
to choose depends heavily on the urgency with which vitamin 
D repletion is needed, local standard of practice, availability 
of appropriate vitamin D preparations, cost, and potential for 
lapses in patient adherence. In symptomatic p

for 
lapses in patient adherence. In symptomatic patients with mod­
erate to severe rickets and osteomalacia, we recommend 50,000 
IU of either ergocalciferol (vitamin D2) or cholecalciferol (vita­
min D3) weekly for 8 to 12 weeks followed by a maintenance 
dose of 1000 to 2000 units daily. Despite the contradictory 
claims,202,203 there is not much difference between vitamin D2 
and vitamin D3 in replenishing depleted vitamin D stores,203 
and both vitamin D preparations are equally effective i

oth vitamin D preparations are equally effective in treating 
rickets and osteomalacia. During follow-up, adjustments to the 
vitamin D dose should be made based on serum and urine lev­
els of calcium, alkaline phosphatase, PTH, and achieved serum 
25-hydroxyvitamin D levels, with target levels of 25-hydroxyvi­
tamin D greater than 30 ng/mL and PTH in the reference range. 
Once achieved, a maintenance dose of 1000 to 2000 IU/day 
is recommended. Several vitamin D formulations (oral, sublin­
gual

Several vitamin D formulations (oral, sublin­
gual, and injectable) are available, and most have comparable 
therapeutic efficacy, although no formal studies have been con­
ducted. In addition, certain vitamin D metabolites (calcidiol, 
calcitriol, and alphacalcidol) are available in other countries, but 
only calcitriol is available in the United States. Perhaps calcidiol 
(25-hydroxyvitamin D3), if available, is preferred because it has a 
shorter half-life (∼2 weeks), which is an advantage sh

ter half-life (∼2 weeks), which is an advantage should hyper­
calcemia develop during therapy, it can be measured directly to 
monitor treatment, and the dose can be adjusted based on serum 
levels. The other active metabolites (calcitriol and alphacalcidol) 
are not preferred in treating vitamin D–deficiency rickets and 
osteomalacia but have been used. The active vitamin D metab­
olites are a bit more expensive and do not provide any major 
advantage except for more rapid and effective suppres

antage except for more rapid and effective suppression of 
PTH. Therefore, the use of calcitriol along with vitamin D is 
suggested in patients with more severe 2°HPT (PTH levels >500 
pg/mL), in some patients with significant malabsorption of cal­
cium due to celiac sprue or gastric bypass surgery, in patients 
with documented bone marrow fibrosis,4 or in those with com­
promised kidney function. However, calcitriol should not be 
either the first-line or sole therapy for vitamin D–deficiency 

st-line or sole therapy for vitamin D–deficiency 
rickets and osteomalacia. Although the therapeutic efficacy of all 
formulations is comparable, the frequency with which they are 
administered differs.
In malabsorptive states, particularly in patients with small intes­
tinal resection or gastric bypass surgery, higher doses of vitamin D 
(10,000–50,000 IU/day) may be required to replete vitamin D 
stores. Variable strengths of injectable preparations of cholecalcif­
erol (300,000–600,000 units)

tions of cholecalcif­
erol (300,000–600,000 units) are available in some countries but 
not widely in the United States and may be preferable in specific 
situations, such as in patients with significant malabsorption or 
poor compliance or intolerance to oral preparations. Compared 
with parenteral administration, the rise in serum 25-hydroxyvi­
tamin D levels is rapid with oral preparations. Careful close fol­
low-up is essential during the first few months (1–3 months) of 
treatment to monito

st few months (1–3 months) of 
treatment to monitor treatment-related adverse events, such as 
hypercalcemia, hypercalciuria, and renal dysfunction. Occasion­
ally, high-dose vitamin D supplementation may unmask underly­
ing primary hyperparathyroidism (see Chapter 29).
Vitamin D deficiency of sufficient severity to produce osteo­
malacia is invariably associated with decreased intestinal calcium 
absorption and negative calcium balance. Accordingly, oral cal­
cium supplements in the form of cal

gly, oral cal­
cium supplements in the form of calcium carbonate (or citrate) 
1000 to 1500 mg/day in divided doses must be prescribed with 
vitamin D administration to accomplish not only clinical and 
biochemical but also radiologic and bone histologic responses. 
With effective therapy, symptoms of osteomalacia start improving 
within a few weeks, but complete disappearance of symptoms usu­
ally takes a few months and sometimes years.
Although nutritional rickets is mostly curable, osteomalac

 nutritional rickets is mostly curable, osteomalacia 
is only treatable but not curable and requires long-term main­
tenance therapy largely dictated by the clinical and biochemical 
responses. Complete healing of osteomalacia and resolution of 
2°HPT must be ensured before commencing antifracture ther­
apy for the associated osteoporosis (see Chapter 30). Almost all 
patients with vitamin D deficiency due to gastrointestinal disor­
ders, resection, or bypass require lifelong vitamin D supplemen

on, or bypass require lifelong vitamin D supplements/
therapy and regular follow-up to prevent recurrence of osteoma­
lacia due to lapses in therapy. Nonadherence to long-term ther­
apy is not uncommon because both the patient and sometimes 
the treating physician might assume that the condition is “cured” 
based on clinical symptoms, and therefore there is no need for 
continued therapy. 
Rickets Due to Genetic Disorders of Vitamin 
D Metabolism
Vitamin D–Dependent Rickets Types 
1A, 1B, and 2

There are three unique forms of rickets due to genetic disorders, 
resulting in the loss of function of the enzymes required for vita­
min D biologic activation.204 A unique case of osteomalacia resis­
tant to high-dose vitamin D therapy was reported in 1979, perhaps 
the first documented case of osteomalacia due to possible defective 
vitamin D 25-hydroxylase,160 although evidence presented was less 
than convincing, and no additional similar ca

ess 
than convincing, and no additional similar cases of osteomalacia in 
adults have been reported since. Severe rickets despite adequate vita­
min D intake that responded only to high doses of vitamin D2 in 
two siblings was probably the first example of rickets due to a pos­
sible genetic defect in the 25-hydroxylase enzyme.114 Recent genetic 
studies suggest that mutations in the CYP2R1 gene,112,113 the 
principal 25-hydroxylase in humans, are responsible for the severe 
atypical form of vit

e responsible for the severe 
atypical form of vitamin D deficiency and rickets, although no 
mutations were found in another child with a similar phenotype.118 
1310
The genetic transmission appears to be both autosomal dominant 
and recessive, and this rare atypical form rickets is now designated 
as vitamin D–dependent rickets type 1B (VDDR1B).205 The other 
two types—vitamin D–dependent rickets type 1A (VDDR1A) and 
vitamin D–dependent rickets type 2 (VDDR2)

A) and 
vitamin D–dependent rickets type 2 (VDDR2)—are due either to a 
defective 25-hydroxyvitamin D 1α-hydroxylase, the critical enzyme 
required in the final step of vitamin D biologic activation,204 or to 
an end-organ resistance to vitamin D action due to loss of vitamin 
D receptors,206 respectively. Impairment of 1α-hydroxylase enzyme 
activity can also be acquired, as in chronic kidney disease, various 
renal tubular disorders,99 or excess or ectopic production of FGF23, 
known to inhibi

s or ectopic production of FGF23, 
known to inhibit 1α-hydroxylase enzyme activity.207 Genetic defect 
in the 25-hydroxyvitamin D 1α-hydroxylase enzyme, VDDR1A, 
or pseudo–vitamin D deficiency, is a rare autosomal recessive 
disorder due to mutations in the cytochrome P450 (CYP27B1) 
gene for the 1α-hydroxylase enzyme located on the chromosome 
12q13.3, resulting in decreased serum levels of 1,25-dihydroxyvi­
tamin D.208,209 Rickets develops during the first year of life, and 
clinical features 

ng the first year of life, and 
clinical features include growth failure, hypotonia, weakness, con­
vulsions, tetany, open fontanels, and pathologic fractures, as well 
as oral and dental manifestations.209 As can be predicted, the con­
dition responds to the physiologic doses (0.04 μg/kg per day) of 
active vitamin D metabolite, 1,25-dihydroxyvitamin D or calcitriol, 
but requires much higher doses of the parent compound vitamin D 
(∼10,000 units/day) to heal rickets—hence the name vitamin D–
d

s/day) to heal rickets—hence the name vitamin D–
dependent rickets. Similar to VDDR1A, but even more resistant to 
vitamin D therapy, is due to defects in the vitamin D receptor, des­
ignated as VDDR2.206 Children with VDDR2 have alopecia, a very 
unique feature that distinguishes VDDR2 from both VDDR1A 
and VDDR1B. However, the prevalence of alopecia is variable as 
is its extent of involvement ranging from alopecia of the head to 
alopecia of the entire body—“alopecia universalis.” Interesting

he entire body—“alopecia universalis.” Interestingly, 
although the biochemical and radiologic abnormalities in VDDR2 
respond to high-dose vitamin D or calcitriol and phosphate therapy, 
alopecia does not. Vitamin D receptors are lacking in fibroblasts of 
the skin.210
Biochemical and radiologic features of vitamin D–dependent 
rickets are similar to those seen in nutritional rickets, except 
serum 25-hydroxyvitamin D levels are normal in VDDR1A and 
VDDR2, and low or very low in VDDR1B (see Ta

 and 
VDDR2, and low or very low in VDDR1B (see Table 31.4). 
Because of the low or very low serum 25-hydroxyvitamin D 
levels, the biochemical findings in VDDR1B resemble those of 
nutritional vitamin D–deficiency rickets (see Table 31.4) and thus 
may be misdiagnosed as nutritional rickets. Family history, ade­
quate vitamin D intake, lack of clinical and biochemical response 
to standard-dose vitamin D therapy, and lack of rise in serum 
25-hydroxyvitamin D levels with vitamin D therapy shoul

droxyvitamin D levels with vitamin D therapy should raise 
suspicion for the possibility of this very rare inborn error of vita­
min D metabolism due to mutations in the CYP27B1 gene.205,211
Both VDDR1A (due to 25-hydroxyvitamin D 1α-hydroxylase 
deficiency) and VDDR1B (due to vitamin D 25-hydroxylase 
deficiency) respond to physiologic replacement doses of calcitriol 
(0.04 μg/kg per day), just as nutritional rickets.208 However, 
patients with VDDR2 require much higher doses of vitamin D or 
c

VDDR2 require much higher doses of vitamin D or 
calcitriol because of end-organ resistance as a result of vitamin D 
receptor defects.206 All children with vitamin D–dependent rick­
ets regardless of the type require adequate calcium and phosphate 
supplements during therapy with calcitriol to achieve healing of 
the rickets and suppress 2°HPT. Hypercalciuria and hypercalce­
mia are potential complications during long-term therapy. All 
patients must be monitored closely, regularly, and through

 must be monitored closely, regularly, and throughout 
their lifetime. 
Hereditary Hypophosphatemic Rickets and 
There are several types of hereditary rickets that in the past were 
collectively referred to as vitamin D–resistant rickets because of the 
very high doses of vitamin D required to cure rickets.50,95,190,212 
These rare types of rickets can be distinguished from the other 
types of rickets by the pattern of their inheritance, underlying 
genetic defects, and hypophosphat

nce, underlying 
genetic defects, and hypophosphatemia with normal serum cal­
cium level, and if they are FGF23 dependent or independent.6,7,95 
The most common type of rickets and osteomalacia among them 
and clinically most relevant is XLH rickets and osteomalacia.
Autosomal Dominant and Recessive Rickets
Autosomal dominant hypophosphatemic rickets (ADHR) is 
caused by mutations in the FGF23 gene,212 whereas inactivating 
mutations in the dentin matrix protein (DMP1) and the ecto­
nucleotide p

n matrix protein (DMP1) and the ecto­
nucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) genes 
are responsible for autosomal recessive hypophosphatemic rickets 
(ARHR) type 1 and type 2, respectively.212,213 In patients with 
ADHR, there are mutations in several amino acids (Arg176 or 
Arg179) resulting in resistance to proteolytic processing. In addi­
tion, the degree of hypophosphatemia correlates with the intact 
FGF23 levels.95 In patients with FGF23-dependent hypophos­
phatemia due to 

ts with FGF23-dependent hypophos­
phatemia due to mutations in the DMP1 gene, there are defor­
mities of the legs and short stature in childhood, similar to the 
clinical phenotype of nutritional rickets.7,95 ARHR typically man­
ifests during childhood with characteristic clinical features of rick­
ets7; however, in adulthood, ARHR patients may manifest with 
bone pain, fatigue, muscle weakness, and repeated bone fractures. 
X-Linked Recessive Hypophosphatemic Rickets
X-linked recessive hypophos

pophosphatemic Rickets
X-linked recessive hypophosphatemic rickets—a rare genetic form 
of hypophosphatemic rickets also known as the Dent disease com­
plex—comprises of a group of heterogeneous inherited disorders 
characterized by a proximal renal tubular reabsorptive disorder 
of the Fanconi type.50 It is caused by missense, nonsense, frame­
shift, and splicing mutations in genes located on the chromosome 
Xp11.22 and X25.50,214,215 Two subtypes are known to exist: type 
1 (∼50–60% of cases),

es are known to exist: type 
1 (∼50–60% of cases), caused by an inactivating mutations in the 
chloride channel 5 (CLCN5) gene that codes for a chloride-proton 
exchanger, and type 2 (∼15% of cases), by inactivating mutations 
in the oculocerebrorenal syndrome (OCRL) gene located on an X 
chromosome that codes for inositol polyphosphate 5-phosphatase 
OCRL-1.50 Similar to other types of genetic or acquired forms of 
Fanconi syndrome,9,156,216 Dent disease is associated with hyper­
calciuria with

t disease is associated with hyper­
calciuria with variable other proximal renal tubular dysfunction, 
nephrocalcinosis or nephrolithiasis, low-molecular-weight pro­
teinuria, and progressive renal insufficiency, but only a minority 
of patients manifest rickets.214 In addition to hypercalciuria and 
proteinuria, the disease may be associated with defective renal 
tubular reabsorption of one or more of the following solutes: glu­
cose, phosphate, uric acid, potassium, bicarbonate, and amino 
aci

 uric acid, potassium, bicarbonate, and amino 
acids. However, unlike all other forms of hereditary rickets, Dent 
disease is the only type of genetic rickets associated with nephro­
calcinosis and nephrolithiasis, and it is the least common of all 
types of rickets. 
X-Linked Hypophosphatemic Rickets and Osteomalacia
XLH (OMIM #307800) is an X-linked dominantly inher­
ited disorder with an estimated prevalence of about 1 in 
20,000 live births.7 It is the most common form of hereditary 
1311
Ch

 It is the most common form of hereditary 
1311
hypophosphatemic rickets, caused by an inactivating mutation in 
the phosphate-regulating gene with homologies to endopeptidases 
on the X chromosome (PHEX).6,7,95,212 PHEX is expressed on 
the cell surface of bones and teeth. It is the most common form 
of FGF23-mediated hypophosphatemic rickets, but the substrate 
for PHEX remains largely unknown.217 More than 300 types of 
mutations in the PHEX gene have been

300 types of 
mutations in the PHEX gene have been reported in patients with 
XLH, and the mutations may be de novo in some patients. The 
prevailing hypothesis is that the mutations found in the PHEX 
gene in patients with XLH are likely to enhance the production of 
FGF23 in bones, but the pathogenic implications to the clinical 
expression of the disorder are unclear.7,95,212,217 Nevertheless, the 
unifying hypothesis proposed by Jan de Beur and Levine is most 
convincing and probably the mos

nd Levine is most 
convincing and probably the most clinically relevant.5 It is worth­
while to restate the concept: Although TIO, XLH, and ADHR all 
have overlapping phenotypical features, all three types of rickets 
share a common biochemical phenotype—hypophosphatemia as a 
result of defective renal tubular reabsorption of phosphate. Cur­
rent evidence supports the notion that under normal physiologic 
conditions, the concentration of FGF23 in serum (and possibly 
other tissues) is regulated 

 serum (and possibly 
other tissues) is regulated by PHEX-dependent proteolysis. How­
ever, conditions with excess circulating FGF23 concentrations or 
activity are associated with a marked depression in the proximal 
renal tubular reabsorption of phosphate and hypophosphatemia. 
When PHEX is inactive, as in patients with XLH, FGF23 is not 
degraded and accumulates in the circulation. In ADHR, missense 
mutations replace the key amino acids in FGF23 and render the 
protein resistant to proteolys

F23 and render the 
protein resistant to proteolysis, thereby leading to increased circu­
lating FGF23 concentrations or action. Similarly, ectopic overpro­
duction of FGF23 by tumors causing TIO (see the following) may 
saturate the capacity of endogenous proteolytic enzymes, such as 
PHEX, to degrade FGF23. It is clear that FGF23 plays a central 
role in the pathogenesis of these disorders; however, the role of 
FGF23 in normal physiology and the mechanism(s) by which it 
regulates renal phosp

he mechanism(s) by which it 
regulates renal phosphate handling or calcitriol synthesis is not 
fully understood.
The clinical features of XLH are variable, with most patients 
presenting with rickets during childhood (Fig. 31.6). In child­
hood-onset XLH, skeletal deformities such as bowed legs and 
short stature are common. In adults, XLH may be discovered 
during a routine biochemical work-up revealing hypophospha­
temia. Symptoms in adults resemble those seen in osteomalacia, 
such as bone p

semble those seen in osteomalacia, 
such as bone pain, insufficiency fractures, muscle weakness that 
is often disabling, neurologic complications related to enthesopa­
thy, and ectopic calcifications. In addition, dental disease, such as 
root abscesses, often develop due to a defect in dentin and enamel 
microdefects.218 
Radiologic and Biochemical Findings
The radiologic features of all hereditary rickets are similar to those 
seen in nutritional rickets, except metaphyseal involvement is 
sl

nal rickets, except metaphyseal involvement is 
slightly asymmetrical and bowing is slightly more common (see 
Fig. 31.6). As adults, most patients are obese and manifest a dis­
proportionate short stature with greater shortening of the lower 
extremities. However, none of these findings is specific enough 
to distinguish the type of rickets, but enthesopathy occurs almost 
exclusively in XLH and is almost never seen in other types of rick­
ets and osteomalacia.
The most common and consistent bi

nd osteomalacia.
The most common and consistent biochemical findings are 
hypophosphatemia, renal phosphate wasting as assessed by tubu­
lar reabsorption of phosphate or tubular maximum for phosphate 
A
B
• Fig. 31.6  Nutritional vitamin D–deficiency (A) and X-linked hypophosphatemic (B) rickets in 5-year-old 
black and 5-year-old white children. Typical metaphyseal widening, fraying, and cupping at the ends of 
all of the long bones can be seen. Note the similarities, but a few subtle differenc

 Note the similarities, but a few subtle differences can be appreciated. 
Bowing is more pronounced and cortices are thicker in X-linked hypophosphatemic rickets (B) than in 
nutritional rickets (A).
1312
reabsorption adjusted for glomerular filtration rate (GFR; TmP/
dlGFR), and elevated serum alkaline phosphatase levels. Serum 
calcium, 25-hydroxyvitamin D, and PTH levels are characteristi­
cally normal in the untreated state. Serum FGF23 levels are inap­
prop

untreated state. Serum FGF23 levels are inap­
propriately elevated in the context of chronic hypophosphatemia.
Although the constellation of clinical and biochemical find­
ings is unmistakably diagnostic, the differential diagnosis of 
XLH includes nutritional rickets, metaphyseal dysplasia, physi­
ologic bowing, and other forms of renal phosphate wasting dis­
orders. In a patient with rickets with chronic hypophosphatemia 
and inappropriately elevated serum FGF23 levels, a diagnosis 
of FGF23-r

evated serum FGF23 levels, a diagnosis 
of FGF23-related hypophosphatemia is highly likely. Consider­
ing the possibility of de novo mutations in the PHEX gene, an 
absence of family history of rickets does not necessarily exclude 
XLH. Genetic testing is not a prerequisite for a clinical diagnosis 
of XLH. 
Treatment of Hereditary Hypophosphatemic 
Standard Treatments
The standard of practice for the treatment of XLH in children 
includes a combination of active vitamin

children 
includes a combination of active vitamin D metabolites (calcitriol 
or alphacalcidol) and oral phosphate supplementation.7 Skeletal 
deformities and growth retardation may improve with treatment 
but do not completely resolve. In adults, these same medications 
are used for symptom management and to improve impaired bone 
mineralization. In one observational study, treatment of adult 
patients with XLH reduced pain symptoms and improved frac­
ture healing following orthopedic surgery b

 frac­
ture healing following orthopedic surgery but did not prevent or 
reverse enthesopathy.219 In one prospective study, 6 years of treat­
ment showed increases in bone resorption markers albeit without 
changes in spine and hip areal BMD.220 
Novel Treatments
Recently, the use of burosumab, a recombinant human immu­
noglobulin G1 monoclonal antibody that binds to the FGF23 
receptor and inhibits its activity, was evaluated in a phase 1 
double-blind, placebo-controlled randomized clinical tr

e-blind, placebo-controlled randomized clinical trial.221,222 
With a single dose, burosumab increased the TmP/GFR, and 
serum levels of phosphate and 1,25-dihydroxyvitamin D.221,222 
The peak serum phosphate concentration achieved was between 8 
and 15 days and returned to the baseline within 50 days after the 
initial subcutaneous injection. In a subsequent phase 1/2 open-
label, dose-escalation study, monthly burosumab resulted in a 
sustained improvement in TmP/GFR, and in serum phosphate 
a

 improvement in TmP/GFR, and in serum phosphate 
and 1,25-dihydroxyvitamin D levels, with a favorable safety pro­
file.221,222 However, at present, it is not clear if these early clini­
cal trials will have long-lasting effects on clinical and radiologic 
resolution of rickets and osteomalacia. Several clinical trials with 
burosumab in patients with both XLH and TIO are currently 
under way. 
Long-Term Management
Treatment with active vitamin D metabolite, calcitriol, and oral 
phosphate supple

metabolite, calcitriol, and oral 
phosphate supplements has the potential for adverse effects over 
the long term. Hypercalciuria with or without hypercalcemia may 
develop and may lead to nephrolithiasis, nephrocalcinosis, and 
impaired renal function. Use of oral phosphate supplements causes 
diarrhea and abdominal pain, which in turn leads to poor medica­
tion adherence. An unintended consequence of long-term (usually 
years) oral phosphate therapy is the development of 2°HPT, which 
may evol

erapy is the development of 2°HPT, which 
may evolve into hypercalcemic 2°HPT (also referred to as tertiary 
hyperparathyroidism).223–225 Several treatment strategies for ter­
tiary hyperparathyroidism have been suggested depending on the 
degree of hypercalcemia, severity of hyperparathyroidism, and 
clinical status.192,224 Cinacalcet, which produces a relative hypo­
parathyroidism, radiofrequency ablation of the tumor to reduce 
ectopic production of FGF23, and deliberate total parathyroidec­

ion of FGF23, and deliberate total parathyroidec­
tomy have all been tried with variable success.192 
Tumor-Induced Osteomalacia
TIO was first reported by Robert McCance in 1947,226 but the 
first “proof of concept” was provided by Andrea Prader in 1959, 
who postulated the production of a “rachitogenic substance” by a 
“giant cell reparative granuloma of bone,” the removal of which 
cured the young patient’s rickets.227 Interestingly, the discovery of 
FGF23 and characterization of its function

ery of 
FGF23 and characterization of its function as a major phosphate-
regulating hormone did not occur until a few decades later.228 
The condition is conventionally referred to as osteomalacia, as 
most cases are detected in adults, but rickets in children has been 
reported.226,227 Nearly 500 cases of TIO have been reported in 
the literature; the majority are adults with a mean age of 45 years 
at the time of diagnosis and a wide age range, and there does not 
appear to be any gender predi

 and there does not 
appear to be any gender predilection.8
TIO is a rare paraneoplastic syndrome that clinically manifests 
with diffuse nonspecific bone pain, profound muscle weakness, 
and fractures.8,88,89,229 The pathogenesis is related to the ectopic 
production of FGF23—a phosphatidic hormone secreted by small 
mesenchymal tumors.228 The biochemical hallmark of TIO is the 
triad of hypophosphatemia due to renal phosphate wasting, inap­
propriately low or normal serum 1,25-dihydroxyvitamin

priately low or normal serum 1,25-dihydroxyvitamin D level, 
and elevated or inappropriately normal serum FGF23 level.8,207
FGF23, produced exclusively by osteocytes and osteoblasts,6,230 
exerts its effects on the kidney and parathyroid glands to maintain 
normal phosphate homeostasis and regulate 1,25-dihydroxyvi­
tamin D synthesis and metabolism.6,95,207 The kidney is the 
principal target for FGF23, where it regulates renal tubular reab­
sorption of phosphate and production of 1,25-dihydroxy

tion of phosphate and production of 1,25-dihydroxyvitamin 
D.6,95,179,207 FGF23 inhibits both sodium-dependent phosphate 
reabsorption and 1α-hydroxylase activity in the proximal tubule, 
leading to hypophosphatemia and aberrant production and inap­
propriately low levels of 1,25-dihydroxyvitamin D. The combina­
tion of hypophosphatemia and “low” 1,25-dihydroxyvitamin D 
causes muscle weakness and mineralization defect, respectively.
TIO is usually caused by tumors of mesenchymal origin, 
referr

ly caused by tumors of mesenchymal origin, 
referred to as phosphaturic mesenchymal tumor of mixed con­
nective tissue variant, and rarely by other types of tumors, such 
as osteosarcoma, giant cell tumor, glomus tumor, small cell carci­
noma of the lung, and adenocarcinoma of the colon.8,88,89 These 
tumors tend to be small (often escaping clinical detection), slow 
growing,231 and difficult to localize; about half of the tumors 
are located in the skeleton, and the remaining are located in sof

the skeleton, and the remaining are located in soft 
tissues. Histology of the tumors reveals neoplastic spindled and 
stellate-shape cells with low nuclear and mitotic activity that 
stain for FGF23.229 Although FGF23 is the predominant secre­
tory product of the tumors and the most common phosphatonin, 
other phosphatonins such as frizzled-related protein 4, FGF7, and 
matrix extracellular phosphoglycoprotein have been described.
Patients with TIO usually present with long-standing and pro­
gr

TIO usually present with long-standing and pro­
gressive debilitating symptoms that go undiagnosed for years, but 
an occasional patient may cruise along without necessarily suf­
fering, except for the progressive slow growth of the tumor over 
1313
decades causing minimal disability.231 Symptoms can be nonspe­
cific, and the most common complaints are bone pain, fatigue, 
muscle weakness, and multiple fractures. Many patients with TIO 
receive various errone

es. Many patients with TIO 
receive various erroneous diagnoses (rheumatologic, malignant, 
and even psychiatric) and, consequently, inappropriate treatments 
for years before the tumor causing osteomalacia is found.88
Laboratory evaluation of a patient suspected of having TIO 
should begin with measurement of serum phosphate, preferably 
in the fasting state. Since hypophosphatemia is uncommon in 
the ambulatory nonhospitalized patient, a serum phosphate level 
less than 2.5 mg/dL should raise 

phosphate level 
less than 2.5 mg/dL should raise suspicion in the right clinical 
setting. Elevated serum alkaline phosphatase supports the clini­
cal suspicion. Other relevant biochemical abnormalities are sum­
marized in Table 31.5. Once hypophosphatemia is detected and 
confirmed, further assessment of the renal tubular handling of 
phosphate (tubular reabsorption of phosphate and TmP/GFR: 
tubular maximum for phosphate reabsorption per unit of GFR; 
also see Chapter 29) should be performed,

of GFR; 
also see Chapter 29) should be performed, and a low value for 
either measurement clinches the diagnosis of a phosphate-wasting 
condition.
As in genetic hypophosphatemic disorders, serum levels of cal­
cium, 25-hydroxyvitamin D, and PTH are normal in TIO, but 
similar to genetic disorders, 1,25-dihydroxyvitamin D levels are 
either inappropriately “normal” or low (see Table 31.5). Interest­
ingly, serum levels of alkaline phosphatase are elevated in all vari­
eties of hypophosphatemic 

e elevated in all vari­
eties of hypophosphatemic rickets and osteomalacia despite low 
or absent bone remodeling, and the reason for this biochemical 
abnormality is not well understood.
Serum FGF23 levels are elevated in the majority of, but not 
all, patients with TIO and depend on the type of assay used 
(intact vs. C-terminal fragment).232 Although an elevated level 
of FGF23 supports diagnostic suspicion, a normal serum FGF23 
level does not exclude the diagnosis. A positive family history

t exclude the diagnosis. A positive family history of 
rickets or osteomalacia, or presence of metabolic acidosis, makes 
TIO unlikely. In addition, serum FGF23 levels do not distinguish 
different types of hypophosphatemic rickets and osteomalacia. 
Similarly, tumors with demonstrable FGF23 expression by immu­
nohistochemistry may not necessarily cause osteomalacia and are 
referred to as nonphosphaturic variants. Whether they represent 
a separate entity or simply the preclinical stage of the 

ate entity or simply the preclinical stage of the disease is 
unknown, because preoperative serum phosphate and FGF23 lev­
els have not been reported in any of the so-called nonphosphaturic 
tumors.229
Radiologic and radionuclide manifestations of TIO are similar 
to those seen in other types of rickets and osteomalacia (see the 
following) with some exceptions: unlike genetic hypophospha­
temic but similar to nutritional deficiency disorders, BMD is low, 
most likely the result of some combinat

D is low, 
most likely the result of some combination of age-related bone 
loss and impaired bone mineralization due to hypophosphatemia. 
In addition, unlike XLH but similar to nutritional rickets and 
osteomalacia, enthesopathy is not seen in TIO, but fractures are 
common. Except for higher osteoid volume, which may remain 
high even after clinical “cure,” bone histologic features are similar 
to other types of osteomalacia.62,84,87
Localization of the tumor can be quite challenging because 

on of the tumor can be quite challenging because 
most are small tumors, often found in obscure locations. A 
detailed physical examination followed by imaging is necessary. 
Since somatostatin receptors are expressed in many phosphatu­
ric mesenchymal tumors of mixed connective tissue variant, an 
octreotide scan can help with tumor localization in about 50% 
of cases, especially when the extremities and skull are involved. 
18F-fluorodeoxyglucose positron emission tomography is quite 
sensitiv

se positron emission tomography is quite 
sensitive in localizing tumors but can lead to false-positive results. 
Gallium-DOTATATE positron emission tomography is an 
emerging imaging modality for tumors producing TIO, is now 
more widely available, and may be the imaging method of choice. 
Functional imaging can be supported by selective venous sampling 
with measurement of FGF23 to confirm the location of the tumor 
in cases in which the imaging studies suggest multiple lesions or 
the locatio

g studies suggest multiple lesions or 
the location of the single lesion poses a significant surgical risk.8,89
The treatment of choice for TIO is resection of the tumor, 
which results in clinical, biochemical, radiologic, and bone 
  Key Abnormalities in Various Types of Rickets and Osteomalacia
Variable
Vitamin D–Resistant 
Hypophosphatemic
Tumor-Induced 
Drug-Induced 
Nutritional/malabsorption
Defective catabolism 

labsorption
Defective catabolism of 
FGF23
Ectopic production of 
FGF23
Renal tubular damage
Direct effect on bone 
mineralization
FGF23 levels
Usually high
Almost always high
High/variable
Normal
Variable
Very low
Variable
High
High
High
Variable
High
Normal
Variable
Low
Normal
Normal
1,25-Dihydroxyvitami

Low
Normal
Normal
1,25-Dihydroxyvitamin D
Variable
Low
Low
Low/variable
Low
Often low
Often low
Therapeutic strategy
Vitamin D + Calcium
Calcitriol and phosphate
Burosumab
Resection of the tumor
Other therapies (see text)
Discontinuation of offending 
drug
FGF23, Fibroblast growth factor 23; PHEX, phosphate-regulating endopeptidase homolog, X-linked.
   
TABLE 31.5
1314
histologic improvements. Wide surgical resection

m
histologic improvements. Wide surgical resection is essential to 
avoid tumor recurrence. Levels of serum phosphate and FGF23, 
which has a half-life of ∼45 minutes, return to normal rapidly, 
often within 24 hours, after tumor resection, but healing of osteo­
malacia may take several months. Rarely, recurrences with metas­
tasis can occur. If the tumors are not readily identifiable, or not 
amenable to surgical removal, lifelong medical therapy with oral 
phosphate and calcitriol is required.

y with oral 
phosphate and calcitriol is required. Oral phosphate in three to 
four divided doses with meals and calcitriol 0.5 to 1.0 μg/day in 
divided doses to maintain serum phosphates level at the lower end 
of the age-appropriate reference range is recommended. However, 
long-term oral phosphate therapy may lead to secondary and 
occasionally hypercalcemic 2°HPT (tertiary hyperparathyroid­
ism) that requires surgical intervention.192,225,233,234 Other poten­
tial treatment options in situa

3,234 Other poten­
tial treatment options in situations where the tumor cannot be 
surgically removed or localized, use of calcium-sensing receptor 
agonists, radiofrequency ablation of the tumor, or deliberate total 
parathyroidectomy can be considered.192,235–237 
Drug-Induced Osteomalacia
Several drugs have been implicated in the pathogenesis of rick­
ets and osteomalacia, and the most relevant drugs are listed in 
Table 31.6, along with the proposed mechanisms by which vari­
ous drugs cause 

roposed mechanisms by which vari­
ous drugs cause mineralization defects. Among them, nucleoside 
reverse transcriptase inhibitors (NRTIs), tenofovir and adefovir, 
are currently the most common causes of drug-induced rickets 
and osteomalacia.9,156 Other drugs that cause rickets or osteoma­
lacia, although less common, include anticonvulsants, aluminum-
containing antacids, non-nitrogen containing bisphosphonates, 
and sodium fluoride.
Tenofovir and adefovir, adenosine analogue NRTIs, are now 

 and adefovir, adenosine analogue NRTIs, are now 
widely used in the treatment of human immunodeficiency virus 
and viral hepatitis infections. Sporadic Fanconi syndrome, renal 
failure, and osteomalacia have been reported in patients treated 
with this class of drugs.9,156 Fanconi syndrome is a generalized 
transport defect in the proximal tubules, resulting in losses of 
glucose, phosphate, calcium, uric acid, amino acids, bicarbon­
ate, and other organic compounds leading to hypophosphatemia,

her organic compounds leading to hypophosphatemia, 
metabolic acidosis, and glycosuria.216 Phosphate depletion due 
to renal phosphate wasting results in hypophosphatemic rickets 
and osteomalacia similar to the genetic and tumor-induced hypo­
phosphatemic disorders (see later discussion). However, unlike 
other hypophosphatemic syndromes, serum FGF23 levels are 
characteristically normal in NRTI-related hypophosphatemia.238 
Studies have shown that a time interval between initiation of 
NRTI tr

hat a time interval between initiation of 
NRTI treatment and the development of Fanconi syndrome var­
ies from 1 to 26 months, and the prevalence of osteomalacia is 
about 0.5% in patients receiving the drugs.9,156 Concomitant use 
of ritonavir-boosted protease inhibitor increases NTRI toxicity 
because protease inhibitors increase intracellular concentrations 
of the drugs.9,156,216 The exact mechanism of tenofovir-induced 
nephrotoxicity is not well understood, but it appears to be due to 
mi

t well understood, but it appears to be due to 
mitochondrial damage and alterations in human organic anion 
transporter 1.216 Proximal renal tubular mitochondrial injury due 
to the inhibition of mitochondrial DNA polymerase results in 
decreased mitochondrial DNA replication and impairs molecu­
lar transport, vitamin D activation, and urinary acidification.216 
There is a modest decline in renal function that can become rel­
evant in patients with preexisting renal dysfunction.216 Discontin­
u

ith preexisting renal dysfunction.216 Discontin­
uation of the causative drug promptly corrects hypophosphatemia 
and renal dysfunction if detected early, but additional therapy 
with oral phosphate and vitamin D or its metabolites is required 
for patients with osteomalacia.9,156
Anticonvulsants, phenytoin, primidone, phenobarbital, and 
rifampin induce the hepatic cytochrome P450 oxidase enzyme sys­
tem, which increases the conversion of vitamin D to polar inactive 
metabolites in the liver, r

n D to polar inactive 
metabolites in the liver, reducing the bioavailable 25-hydroxyvi­
tamin D.239,240 The resulting conditional vitamin D deficiency, if 
prolonged and severe, ultimately leads to rickets and osteomala­
cia.46,239,241,242 The clinical manifestations, biochemical changes, 
and radiologic and bone histologic features are similar to those 
seen in patients with nutritional vitamin D deficiency and defec­
tive genetic 25-hydroxylase or VDDR1B (see later discussion). 
Treatment wit

e or VDDR1B (see later discussion). 
Treatment with vitamin D and calcium in the recommended 
doses for nutritional vitamin D deficiency is usually effective (see 
later discussion). Unlike in the case of NRTIs, there is no need to 
discontinue the drugs that caused the problem in the first place, 
but lifelong monitoring for vitamin D deficiency is required. 
Although both isoniazid and ketoconazole inhibit 1α-hydroxylase 
enzyme in the kidney, resulting in conditional vitamin D defi­
ciency, c

resulting in conditional vitamin D defi­
ciency, cases of rickets and osteomalacia have not been reported.
In earlier times, a unique form of “vitamin D–resistant” osteo­
malacia related to the use of tap water in dialysate solution and 
aluminum-containing phosphate binders was seen exclusively in 
patients on maintenance hemodialysis.243–245 Aluminum is pref­
erentially deposited at the interface of mineralized and unmin­
eralized (osteoid) bone, and uncouples matrix synthesis and its 
subsequ

e, and uncouples matrix synthesis and its 
subsequent mineralization, resulting in excess osteoid accumu­
lation.246 With the use of deionized water, reverse osmosis, and 
abandonment of aluminum-containing phosphate binders, alu­
minum-induced osteomalacia has all but disappeared.243,244,246 A 
similar form of osteomalacia has been reported with the use of 
sucralfate, another aluminum-containing antacid, particularly if 
used in conjunction with aluminum-containing phosphate bind­
ers or antid

iron infusions develop FGF23-mediated hypophosphatemic 
osteomalacia.247,248
Drug-induced osteomalacia has rarely been reported with the 
use of etidronate and sodium fluoride, usually in high doses and 
over long periods, but no currently approved second-generation 
nitrogen-containing bisphosphonates have been reported to cause 
osteomalacia.87 Neither etidronate nor sodium fluoride is approved 
for the prevention or treatment of osteoporosis (see Chapter 30) 

steoporosis (see Chapter 30) 
in the United States, but both drugs are approved for management 
of osteoporosis in other countries, including Canada. In addition, 
etidronate is used in high doses (up to 20 mg/kg per day) to treat 
rare bone and mineral disorders such as fibrous dysplasia, hetero­
topic ossification, and myositis ossificans with some success. Since 
the etidronate dose used in these conditions is much higher than 
the recommended dose (5 mg/kg per day) for osteoporosis and 
Page

 dose (5 mg/kg per day) for osteoporosis and 
Paget disease of bone, the risk of osteomalacia exists; the exact 
scope of this complication is currently unknown.
Bone histologic features of drug-induced mineralization 
defects due to aluminum, iron, etidronate, and sodium fluoride 
differ substantially from the osteomalacia as defined earlier due 
to vitamin D and phosphate deficiency (see the earlier Defini­
tion and Histologic Evolution of Osteomalacia section). Defec­
tive bone mineralization

omalacia section). Defec­
tive bone mineralization due to aluminum overload, the osteoid 
accumulation is generalized, as in vitamin D deficiency and hypo­
phosphatemic osteomalacia, but osteoid thickness is not increased 
and may even be thinner. This type of bone histologic abnormal­
ity is designated as atypical osteomalacia87 (see Fig. 31.2). Bone 
turnover is extremely low, staining for aluminum is positive at the 
osteoid-mineralized bone interface establishing the diagnosis, and 
the bone

nterface establishing the diagnosis, and 
the bone lesion does not respond to vitamin D therapy. Osteoid 
accumulation in etidronate- and sodium fluoride–related osteo­
malacia is patchy with very thick osteoid seams randomly dis­
tributed throughout bone (both on bone surfaces and within the 
interstitial bone), which is designated as focal osteomalacia (see 
Fig. 31.2). A full exposition on the evolution of such bone lesions 
has recently been reviewed.87 
Conditions That Resemble Rickets and 

reviewed.87 
Several other bone diseases, some rather poorly characterized, 
resemble rickets and osteomalacia in one respect or another, but 
all have unique distinguishing features that separate them from 
classical rickets on x-rays or osteomalacia on bone histology. Much 
of the confusion has arisen because of the extension of the terms 
rickets and osteomalacia to include these other disorders without 
regard to the mechanisms by which the 

rs without 
regard to the mechanisms by which the abnormalities occur. 
By not conforming to the strict definitions for rickets and osteo­
malacia, the terms lose their clinical relevance and might give a 
false impression that the conditions might respond to vitamin 
D administration. In fact, many of these conditions were once 
treated with large doses of vitamin D without much improvement 
in clinical, radiologic, or bone histologic features.
Pathogenesis of many of these rare bone disorders 

Pathogenesis of many of these rare bone disorders is currently 
poorly understood. The mechanism for the development of radio­
logic abnormalities and mineralization defect is different from 
that of classical rickets and osteomalacia. In most, but not all, con­
ditions listed in Table 31.6, the abnormalities are either due to the 
effects of PTH excess on bone or to the defects in bone collagen 
matrix structure that does not mineralize normally.
In children with primary hyperparathyroidism, me

.
In children with primary hyperparathyroidism, metaphyseal 
abnormalities resemble rickets or a child might have rickets due 
to both vitamin D deficiency and masked primary hyperparathy­
roidism. The radiologic abnormalities respond to parathyroidec­
tomy. Similarly, in severe 2°HPT in children with end-stage renal 
disease, both “rickets” and “osteomalacia” have been noted. Any 
condition that increases bone remodeling inevitably increases the 
extent of osteoid surface (usually <50% of the b

extent of osteoid surface (usually <50% of the bone surface) and 
by extension osteoid volume (usually >3–5% of bone volume), 
but osteoid thickness, the hallmark of mineralization defect in 
traditional osteomalacia, is always normal (<12 μm). Some have 
used the term hyperosteoidosis to describe this type of histologic 
abnormality, which is seen in conditions associated with increased 
bone remodeling, such as renal osteodystrophy, hyperthyroid­
ism, primary hyperparathyroidism, and osteiti

oid­
ism, primary hyperparathyroidism, and osteitis deformans (Paget 
disease of bone). However, the use of such a descriptive term as 
hyperosteoidosis does not serve any useful purpose and potentially 
confuses our understanding of these various disorders.
In certain very rare disorders, such as fibrogenesis imperfecta 
ossium and axial osteomalacia, various degrees of defective miner­
alization are seen, but they are due to abnormal collagen structure. 
Bone histomorphometric measurements in 

tructure. 
Bone histomorphometric measurements in these disorders differ 
substantially from those of classical osteomalacia as defined earlier.
One rare bone disorder deserves special mention in the context 
of rickets and osteomalacia. Hypophosphatasia is now reasonably 
well characterized, and enzyme replacement therapy with asfotase 
alfa was recently approved for childhood onset of the disease. It is 
due to “loss of function” mutations in the gene that codes for the 
tissue nonspecific alk

he gene that codes for the 
tissue nonspecific alkaline phosphatase. It is an autosomally inher­
ited disorder with more than 300 different gene defects reported 
so far. Although radiologic and bone histologic findings may 
resemble rickets and osteomalacia, the condition is easily distin­
guished by the low serum alkaline phosphatase levels (<40 IU/L). 
Age-appropriate references ranges must be used for children and 
adolescents, and many laboratories now provide a lower limit 
for serum alkal

atories now provide a lower limit 
for serum alkaline phosphatase ranges. As noted previously, the 
serum alkaline phosphatase is almost always elevated in vitamin 
D– and phosphate-deficiency rickets and osteomalacia. Bowing 
of long bones with insufficiency fractures resemble radiologic fea­
tures of osteomalacia in adults, and metaphyseal and epiphyseal 
changes resemble those of rickets in children (Fig. 31.7A and B). 
The tongue-like lucent in the metaphyses of the long bones, espe­
cially 

in the metaphyses of the long bones, espe­
cially in the distal femur, is characteristic of childhood hypophos­
phatasia (see Fig. 31.7B). However, cortical thinning, a feature 
of vitamin D–deficiency osteomalacia, is not seen in hypophos­
phatasia. In addition, pseudofractures are usually located on the 
lateral cortex of long bones, unlike the medial location in osteo­
malacia related to vitamin D and phosphate deficiency, but this 
feature lacks diagnostic specificity because it can sometime

cks diagnostic specificity because it can sometimes be 
seen on the medial cortex (see Fig. 31.7C). Many adult patients 
with a milder form of the disease may present as having age-
related osteoporosis, and the prevalence of hypophosphatasia in 
unselected patients with osteopenia and osteoporosis is unknown. 
Some have speculated that the pathogenesis of atypical femur frac­
tures after long-term bisphosphonate therapy may be related to 
unrecognized hypophosphatasia, but convincing evidence i

gnized hypophosphatasia, but convincing evidence is lack­
ing. Nevertheless, it is clear that patients with hypophosphatasia 
should not receive bisphosphonate therapy for their “osteoporo­
sis.” The diagnosis is supported by high serum levels of vitamin 
B6, pyridoxal phosphate, and inorganic pyrophosphate. Genetic 
testing for mutations is not mandatory, but any gene abnormali­
ties should be interpreted with caution because not all carriers 
1316
A
C
B
• Fig.

316
A
C
B
• Fig. 31.7  (A) Radiologic features of lower extremities in a child with hypophosphatasia. Note abnormalities 
in the distal ends (arrows) resembling nutritional and X-linked hypophosphatemia rickets (see Fig. 31.4). 
(B) Tongue-like lucent area (arrow) in the metaphysis of the distal femur, characteristic radiologic sign of 
hypophosphatasia. In addition, note abnormal metaphyseal regions resembling mild rickets (see Fig. 31.4). 
(C) Pseudofracture (

ild rickets (see Fig. 31.4). 
(C) Pseudofracture (arrow) in the tibia. Note the similarity of pseudofractures seen in vitamin D–deficiency 
osteomalacia (see Fig. 31.2).
1317
manifest the disease. Enzyme replacement therapy with asfotase 
alfa is now available for childhood-onset hypophosphatasia. It is 
unclear how many patients with adult-onset hypophosphatasia 
are missed or misdiagnosed as “usual age-related” osteoporosis 
because the relevant reference r

ed” osteoporosis 
because the relevant reference ranges for the alkaline phospha­
tase have not yet been widely adopted by all clinical chemistry 
laboratories. One clinically relevant aspect of unrecognized adult 
hypophosphatasia is that it may contribute to the development of 
atypical femur fractures after long-term bisphosphonate therapy, 
although proof of this speculation is still lacking. Nevertheless, 
attention should be paid to lower alkaline phosphatase levels (<40 
IU/L) in women wi

lkaline phosphatase levels (<40 
IU/L) in women with postmenopausal osteoporosis in whom 
antifracture therapy with a bisphosphonate or denosumab is con­
sidered (see Chapter 30). 
Concluding Remarks
Rickets and osteomalacia are a group of disorders due to varied 
pathogenic mechanisms, but they all respond to administration of 
vitamin D or its metabolites, calcium, or phosphate supplements 
as appropriate, removal of a tumor producing ectopic FGF23, or 
administration of an antibody to FGF23. 

GF23, or 
administration of an antibody to FGF23. The resolution of clini­
cal, biochemical, radiologic, and bone histologic abnormalities is 
usually complete in most cases, but lifelong therapy is needed in 
some forms of rickets and osteomalacia. In nutritional rickets and 
osteomalacia, 2°HPT may persist for months or years despite clini­
cal improvement and confers an increased fracture risk due to irre­
versible cortical bone loss that has already occurred by the time of 
diagnosis. In a f

already occurred by the time of 
diagnosis. In a few patients, particularly those with hypophospha­
temic rickets and osteomalacia, long-term oral phosphate therapy 
leads to hypercalcemic 2°HPT (or tertiary hyperparathyroidism) 
requiring parathyroidectomy. In most patients, the clinical 
response is excellent and gratifying, both to the patients and car­
ing physicians. However, one must not lose sight of the fact that 
genetic rickets and osteomalacia, as well as TIOs, require lifelong 
follo

omalacia, as well as TIOs, require lifelong 
follow-up for the development of therapy-related complications 
or malignant transformation of the tumors causing TIO.
The authors thank Ms. Stephanie Stebens, MLIS, Sladen Library, 
for literature search and citation help; Sarah Whitehouse, senior 
medical writer, for editing and proofreading; and Wendy Gill for 
the preparation of all illustrations. The chapter is dedicated to the 
memory of the late A. Michael Parﬁtt, who taught me 

mory of the late A. Michael Parﬁtt, who taught me (DSR), one 
on one, everything I know about bone and mineral disorders from 
1974 to 2015, and to the late Harold Frost, who first described 
the detailed tetracycline-based bone histomorphometry in osteo­
malacia from the Henry Ford Hospital, Detroit, Michigan. The 
senior author (SDR) still continues work in the same laboratory 
created by Dr. Frost in 1954. One of Dr. Parﬁtt’s last desires was to 
report in more detail on the histologic evolut

to 
report in more detail on the histologic evolution of osteomalacia 
and its response to vitamin D, calcidiol, and calcitriol, but this 
never materialized.
This work was partly supported by National Institutes of Health 
grant AR062103; Fund for Henry Ford Hospital, Fred White­
house Chair in Endocrinology, Diabetes, and Bone & Mineral 
Disorders; and the Indian Society of Bone and Mineral Research.
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2018;33:540–542.
1318
32
Kidney Stones
DAVID A. BUSHINSKY
N
ephrolithiasis is a common disorder with an incidence 
greater than one case per 1000 patients per year. In the 
year 2000, this resulted in nearly 2 million physician 
office visits with an estimated annual cost between $2.0 and $5.5 
billion in the United States alone.1–3 The prevalence in industrial­
ized nations is approximately 7% in women and 11% in men and 
appears to be rising over time.4,5 The in

men and 
appears to be rising over time.4,5 The incidence of nephrolithiasis 
peaks when patients are in their 30s and 40s, and the prevalence 
increases with age until about the seventh decade of life.5,6 The 
prevalence of nephrolithiasis in the United States increased from 
nearly 3% in the late 1970s to nearly 5% in the 1990s, which is 
far faster than can be accounted for by alterations in our genome 
indicating that changes in our diet and/or environment are 
responsible, at least in part,

or environment are 
responsible, at least in part, for the increasing prevalence.6
Stones may be composed of calcium oxalate, calcium phos­
phate, uric acid, magnesium ammonium phosphate (struvite), or 
cystine, alone or in combination. A variety of pathogenic mecha­
nisms determine the type of stone formed. Symptomatic stones 
tend to localize in the renal tubules and collecting system but 
are also commonly found within the ureters and bladder.7,8 The 
recurrence rate of calcium oxalate stones

7,8 The 
recurrence rate of calcium oxalate stones is about 10% to 30% at 
5 years and 50% at 10 years and higher for cystine, uric acid, and 
struvite stones.9–11
Kidney stones result in substantial morbidity. The severe pain 
of renal colic can lead to frequent hospitalization, shock wave lith­
otripsy, or invasive surgical procedures. Although rarely a cause of 
end-stage renal disease (ESRD), nephrolithiasis has been associated 
with chronic kidney disease (CKD) in various populations.12–16 

kidney disease (CKD) in various populations.12–16 
Patients with kidney stones have an increase in aortic calcification, 
and even mild CKD is associated with significant adverse cardio­
vascular events,17–20 especially in men, and an increase in stroke, 
especially in women.21 There is a significant decrease in bone min­
eral density leading to fracture in patients with nephrolithiasis.22
Insight into the mechanisms involved in stone formation can 
help direct appropriate therapy, which is know

an 
help direct appropriate therapy, which is known to significantly 
decrease the incidence of stone formation and its associated 
morbidity.
Epidemiology of Stone Formation
Numerous factors determine the prevalence of stones, including 
sex, age, race, obesity, and geographic distribution. Men are more 
likely to get nephrolithiasis than women.5,6,23 In the United States, 
blacks, Hispanics, and Asian Americans are much less likely to have 
stones than whites.5 Obesity is correlated with the r

nes than whites.5 Obesity is correlated with the risk for kidney 
stone formation.24–26 Those weighing more than 220 pounds or 
having a body mass index (BMI) greater than 30 are more likely 
to form stones than those weighing less than 150 pounds or having 
a BMI between 21.0 and 22.9.27 Geography also appears to influ­
ence stone formation in the United States, with a decreasing preva­
lence from south to north and, to some degree, from east to west.23 
The greater exposure to sunlight in the 

 west.23 
The greater exposure to sunlight in the southeastern United States 
may be responsible for the increased rates of nephrolithiasis in that 
area. Sun exposure can lead to more concentrated urine by increas­
ing insensible fluid losses due to sweating.28,29 Although increased 
sun exposure should increase levels of serum 25-hydroxyvitamin D 
Epidemiology of Stone Formation, 1318
Pathogenesis of Stone Formation, 1319
Clinical Presentation and Evaluation, 1324
Therapy, 1329

one, 
and gut, or calcium sensing at the calcium-sensing receptor 
on the parathyroid glands and renal tubular cells, can lead to 
hypercalciuria and stone formation.
 • 
All patients, even those with a single stone, should undergo at 
least a basic evaluation to rule out a systemic etiology of stone 
formation.
 • 
Increasing fluid intake is a simple measure that has considerable 
impact on reducing stone growth and new stone formation.
1319
Chapter 32  Kidney Stones
[25(OH)D], there is no ev

hapter 32  Kidney Stones
[25(OH)D], there is no evidence for a subsequent increase in the 
level of 1,25-dihydroxyvitamin D [1,25(OH)2D], nor is there any 
evidence that hypercalciuria will worsen.
Along with geography, genetic predisposition can influence the 
type of stone formed.28,30 For example, uric acid stones are seen in 
up to 75% of all cases in Mediterranean and Middle Eastern coun­
tries but constitute fewer than 10% of cases in the United States. 
By contrast, more than 70% of stone

nited States. 
By contrast, more than 70% of stones formed in the United States 
are calcium based.31 Less common are magnesium ammonium 
phosphate (struvite or infection) stones, which account for about 
10% to 25% of stones formed, and cystine stones, which are due 
to an autosomal recessive disorder and constitute only about 2% 
of all stones formed6,30,32 (Fig. 32.1).
Diet and pharmacologic agents can also significantly impact stone 
formation. In a dramatic example, an outbreak of nephrolit

n. In a dramatic example, an outbreak of nephrolithiasis in 
Chinese infants was attributed to ingestion of melamine in infant 
formulas and milk powder. Melamine, intentionally added to raise 
the apparent protein content of the concentrates, led to formation of 
large particles in the kidney and resulted in many cases of nephroli­
thiasis and renal failure due to obstructive uropathy.33–35 A variety of 
other dietary factors can have a significant impact on both formation 
and prevention of ki

ant impact on both formation 
and prevention of kidney stones (see the following discussion). 
Pathogenesis of Stone Formation
Kidney stones form when urine becomes supersaturated with respect 
to the specific components of the stone.7,8 Saturation is dependent 
on chemical free ion activities of the stone constituents. Factors that 
affect chemical free ion activity include urinary ion concentration, 
pH, and the combination of the constituent with other substances. 
For example, an 

nstituent with other substances. 
For example, an increase in the urinary calcium concentration or a 
decrease in urine volume increases the free ion activity of calcium 
ions in the urine. Urinary pH can also modify chemical free ion 
activity. A low urinary pH increases the free ion activity of uric 
acid ions but decreases the activity of calcium and phosphate ions. 
Citrate combines with calcium ions to form soluble complexes 
and will decrease the free ion activity of unbound citrate and ca

se the free ion activity of unbound citrate and cal­
cium. When the chemical free ion activities are increased, the urine 
becomes supersaturated (also termed oversaturated). In this setting, 
new stones may form and established stones may grow. In the event 
of decreased free ion activity, urine becomes undersaturated, and 
stones do not grow and can even dissolve. The equilibrium solubil­
ity product is the chemical free ion activity of stone components in 
a solution at which the stone neithe

omponents in 
a solution at which the stone neither grows nor dissolves.
Stones form through the processes of homogeneous or het­
erogeneous nucleation. In homogeneous nucleation, progressive 
supersaturation eventually results in formation of small clusters 
secondary to the aggregation of identical molecules. These clusters 
grow to form a permanent solid phase, or crystals. Heterogeneous 
nucleation refers to crystal formation on the surface of a differ­
ent crystal type or on other dissimila

f a differ­
ent crystal type or on other dissimilar substances, such as cells. 
In vivo, this type of nucleation is more common than homoge­
neous nucleation because crystals form at a lower level of super­
saturation in the presence of a solid phase.
The small crystals may then aggregate into larger clinically sig­
nificant stones. Crystals generally anchor to renal tubular epithe­
lium; this allows more time for growth. This anchoring of crystals 
occurs at the renal papillae, over areas of in

ls 
occurs at the renal papillae, over areas of interstitial calcium phos­
phate present in the form of apatite termed Randall plaques36–40 
(Fig. 32.2). The apatite crystals appear to originate at the basement 
membrane of tubular cells in the thin loop of Henle and extend 
into the interstitium without damaging the cells themselves or 
filling the tubular lumens. A combination of apatite crystal and 
organic material extends from the loop of Henle tubular basement 
membrane to the papillary ur

nle tubular basement 
membrane to the papillary uroepithelial surface, where calcium 
oxalate crystals or other crystals can adhere and form stones. If 
the stone breaks this anchor to the urothelial surface, it will then 
be carried by the urine through the ureter and into the bladder. If 
the stone is small (generally <5 mm in diameter), it may pass with 
minor discomfort; however, if it has grown sufficiently, this migra­
tion may be extremely painful and, if the stone is of sufficient size, 

 painful and, if the stone is of sufficient size, 
it may even completely obstruct the ureter leading to nonfunction 
of the unilateral kidney.
An important factor in the development of kidney stones 
may be the absence of adequate levels or activity of crystalliza­
tion inhibitors in the urine.8 Uropontin, pyrophosphate, citrate, 
and nephrocalcin are endogenously produced substances that have 
been shown to inhibit calcium crystallization. Differences in the 
Struvite, 15%–22%
Uric acid, 5%–10

erences in the 
Uric acid, 5%–10% 
Calcium
phosphate,
5%–10%
Calcium oxalate
and calcium
35%–37% 
Calcium
oxalate,
25%–35%
Cystine,
1%–2% 
• Fig. 32.1  Frequency of different types of kidney stones.
• Fig. 32.2  An attached stone (paired arrows) is seen resting on a region 
of white plaque (single arrows) and intermixed with small areas of white 
(single arrow) and yellow plaques (arrowheads). (From Evan AP, Linge­
man JE, Worcester EM, et al. Renal histopathology an

n JE, Worcester EM, et al. Renal histopathology and crystal deposits 
in patients with small bowel resection and calcium oxalate stone disease. 
Kidney Int. 2010;78:310–317.)
1320
amount or activity of inhibitors are thought to account for the 
variability in stone formation among people with similar degrees 
of urinary supersaturation.32
Clinically, most physicians evaluate the lithogenic potential of 
the urine from stone formers by measuring the rate of excre

 from stone formers by measuring the rate of excretion of 
the principal stone-forming elements in mass per unit time (e.g., 
milligrams or millimoles per 24 hours). It is clear, however, that 
the lithogenic potential of urine is better determined by the degree 
of supersaturation. Computer programs that calculate saturation 
from concentrations of various elements in the urine and the uri­
nary pH are now available (e.g., Quest Diagnostics, Mayo Clinic, 
and Litholink are laboratories that mea

o Clinic, 
and Litholink are laboratories that measure urine ion excretion 
and calculate supersaturation) and more accurately determine 
the risk of stone formation. Any calculation of mean saturation 
underestimates the maximum supersaturation that may drive 
stone formation because of hourly variations in water and solute 
excretion throughout the day. 
Diet
Dietary factors have a great influence on the concentration of 
excreted ions. Simply instructing patients to increase fluid intake 
app

instructing patients to increase fluid intake 
appears to have a considerable impact on reducing stone growth 
and formation.41–44 Renal calcium excretion is augmented by 
increased sodium excretion,45–47 and hypercalciuric patients tend 
to have a greater calciuric response to a sodium load than control 
subjects.48 Dietary sodium restriction with the consequent decrease 
in urinary sodium excretion reduces calcium excretion and lowers 
supersaturation with respect to calcium-containing kidney 

uration with respect to calcium-containing kidney stones. 
Patients are counseled to limit their daily sodium intake to a maxi­
mum of 3000 mg (∼130 mEq) to reduce hypercalciuria.6,7,49
A moderate reduction in animal protein (∼1.0 mg/kg per day) 
is known to be beneficial in patients with nephrolithiasis. Ani­
mal protein contributes to stone formation via multiple mecha­
nisms.49,50 A mild metabolic acidosis develops when animal 
proteins are metabolized. To buffer the excess hydrogen ions, cal

tabolized. To buffer the excess hydrogen ions, cal­
cium is resorbed from bone, which leads to an increased filtered 
load of calcium.51 Metabolic acidosis also directly decreases renal 
tubular calcium reabsorption, which further enhances hypercalci­
uria.51 In addition, metabolism of amino acids contained in ani­
mal protein generates sulfate ions, which couple with calcium ions 
to form insoluble complexes.51,52 Citrate, a base, acts as a urinary 
inhibitor of stone formation. Citrate forms s

ary 
inhibitor of stone formation. Citrate forms soluble complexes 
with calcium and lowers calcium oxalate and calcium phosphate 
supersaturation. During metabolic acidosis, citrate is reabsorbed 
proximally, reducing the amount excreted in the urine.52,53 Hypo­
kalemia can also lead to reduced citrate excretion. An animal 
protein–induced reduction in urinary citrate can promote forma­
tion of both calcium oxalate and uric acid stones.48,54 Indeed, the 
Dietary Approaches to Stop Hypertension 

eed, the 
Dietary Approaches to Stop Hypertension (DASH) diet appears 
to reduce the risk of forming a kidney stone.55–57
Fructose has become a ubiquitous sweetener in American pro­
cessed foods. In large food questionnaire studies, this sugar has 
been associated with a significant risk of developing nephrolithia­
sis. Although the mechanism is not known, fructose is the only 
carbohydrate that can increase uric acid production, and fructose 
metabolism may increase stone formation.58
Several s

tabolism may increase stone formation.58
Several studies have demonstrated the benefits of a diet contain­
ing an age- and gender- appropriate amount of calcium in patients 
with kidney stones.42,49,50,59 Ingested calcium binds intestinal oxa­
late, reducing its absorption and consequent renal excretion.42 In 
a long-term prospective trial, Borghi and colleagues49 randomized 
hypercalciuric male stone formers to either a low-calcium diet or 
to a diet with a normal amount of calcium but low in s

 diet with a normal amount of calcium but low in sodium and 
animal protein. Both groups of men were instructed to restrict 
oxalate intake and drink 2 to 3 L of water daily. The group of 
men on a normal-calcium, low-sodium, and low–animal protein 
diet had a significantly lower recurrence of nephrolithiasis and a 
greater reduction in oxalate excretion and calcium oxalate super­
saturation compared with the men on the low-calcium diet.49
Thus, patients should be maintained on an age- and gende

patients should be maintained on an age- and gender-
appropriate intake of calcium.50 Dietary calcium restriction 
should be strongly discouraged because it not only increases risk 
of stone formation but also engenders a significant risk of bone 
demineralization and development of osteoporosis.22,60,61 Note 
that although dietary calcium intake has been associated with a 
reduced incidence of kidney stones, calcium intake in the form of 
supplements can exacerbate stone formation in older wome

ments can exacerbate stone formation in older women. The 
recommended dietary intake for men and women is 1000 mg of 
elemental calcium from ages 19 through 50 years and 1200 mg 
of calcium thereafter.62 Teenagers should consume 1300 mg of 
calcium per day. Excess calcium should be avoided, because the 
combination of calcium and vitamin D supplementation has been 
shown to significantly increase the risk of kidney stones in post­
menopausal women.63 
Pathogenesis of Idiopathic Hypercalciuria
Id

n.63 
Idiopathic hypercalciuria (IH) is defined as excessive urinary cal­
cium excretion in the setting of normocalcemia and the absence of 
secondary causes of hypercalciuria. IH is the most common cause 
of calcium-containing kidney stones. The disorder is familial; it 
was initially thought to exhibit an autosomal dominant pattern of 
inheritance but is almost certainly polygenic.64–67
The mechanism by which IH leads to hypercalciuria is not 
known. I

 which IH leads to hypercalciuria is not 
known. It has been postulated that IH comprises three distinct 
disorders: excessive intestinal calcium absorption, decreased renal 
tubular calcium reabsorption, and enhanced bone demineral­
ization. In a genetic strain of hypercalciuric stone-forming rats, 
hypercalciuria appears to be due to an excessive number of enteric 
vitamin D receptors leading to a generalized disorder of calcium 
transport at all sites of calcium transport including the kidney

ll sites of calcium transport including the kidney, 
intestine, and bone.67,68 In humans, recent observations also sug­
gest that IH may be a systemic disorder of calcium homeostasis 
with dysregulation of calcium transport. An understanding of 
calcium homeostasis helps elucidate the potential mechanisms 
involved in IH.
Urinary calcium homeostasis is regulated in the gastrointestinal 
(GI) tract, kidneys, and bone by the hormones parathyroid hor­
mone (PTH) and 1,25(OH)2D (

mones parathyroid hor­
mone (PTH) and 1,25(OH)2D (also see Chapter 29 for addi­
tional discussion on calcium homeostasis). Approximately 99% 
of the calcium in the body is contained within the bone mineral. 
Daily bone resorption and bone formation, which in healthy, 
nonpregnant, nonosteoporotic adults should be equal, allow less 
than 1% of bone calcium to be exchanged with that in the extra­
cellular fluid.
Both PTH and 1,25(OH)2D at high concentrations stimulate 
release of calcium from the 

entrations stimulate 
release of calcium from the bone mineral through osteoclast-medi­
ated bone resorption. Net calcium influx into the extracellular 
fluid is achieved by absorption from the GI tract, which occurs 
through 1,25(OH)2D-dependent and -independent mechanisms. 
Although PTH appears to have no direct effect on GI calcium 
1321
absorption, increased levels of the hormone stimulate production 
of 1,25(OH)2D, which in turn leads to enhanced absorption. 
Incre

which in turn leads to enhanced absorption. 
Increased serum levels of calcium and 1,25(OH)2D provide nega­
tive feedback to the parathyroid glands, resulting in reduced PTH 
The roughly 60% of calcium in the extracellular fluid is 
not protein bound and is freely filtered by the renal glomeruli. 
Approximately 80% to 85% of this amount is passively reab­
sorbed in the proximal tubule. Most of the remaining calcium 
is reabsorbed in the thick ascending limb (TAL) of Henle and 
distal 

e thick ascending limb (TAL) of Henle and 
distal cortical tubules under PTH stimulation. Ultimately, these 
reabsorptive mechanisms result in a urinary calcium excretion that 
is less than 2% of the daily filtered load of calcium.69 Except dur­
ing pregnancy and lactation, in healthy, nonosteoporotic adults, 
urinary calcium excretion (and any calcium lost in sweat) precisely 
equals net intestinal calcium absorption. 
Potential Mechanisms for the Development of IH
Dysregulation of calcium tran

he Development of IH
Dysregulation of calcium transport in the intestine, kidney, or 
bone can lead to hypercalciuria. For example, excessive calcium 
absorption by the GI tract leads to a transient increase in the 
serum calcium. This increase in serum calcium suppresses secre­
tion of PTH that, along with the increased filtered load of calcium 
to the kidneys, results in hypercalciuria. Excessive 1,25(OH)2D 
has a similar effect of increasing intestinal calcium absorption 
but also results in 

ntestinal calcium absorption 
but also results in an influx of calcium into the extracellular fluid 
because of enhanced bone resorption. The result is hypercalciuria 
even in the setting of a low-calcium diet or an overnight fast. The 
excess 1,25(OH)2D also suppresses PTH secretion, thereby fur­
ther reducing renal tubular reabsorption of calcium.
If a primary defect in renal calcium reabsorption leads to 
hypercalciuria, there is a fall in the serum calcium concentration 
that stimulates synt

 serum calcium concentration 
that stimulates synthesis of PTH and 1,25(OH)2D. Increased 
1,25(OH)2D results in enhanced intestinal calcium absorption 
and bone resorption. The renal loss of calcium persists even with a 
low-calcium diet or an overnight fast.
Hypercalciuria can also develop as a result of a defect in renal 
phosphate reabsorption. The resultant hypophosphatemia leads 
to enhanced 1,25(OH)2D production, which stimulates intesti­
nal absorption of phosphorus and calcium. The incre

al absorption of phosphorus and calcium. The increased serum 
calcium and 1,25(OH)2D suppresses PTH synthesis and release. 
The increased filtered load of calcium in the setting of suppressed 
PTH leads to hypercalciuria. Enhanced bone resorption due to 
excessive 1,25(OH)2D increases the serum calcium concentration, 
which in turn suppresses PTH production further. The increase in 
the filtered load of calcium in this setting results in hypercalciuria.
Thus, there are several potential mechanis

ciuria.
Thus, there are several potential mechanisms for hypercalci­
uria. Do human or animal data support one mechanism above all 
others? From a clinical therapeutic standpoint, is it worth differ­
entiating among the various potential mechanisms in each patient 
with suspected IH? 
Human Data
Lemann70 compiled the results of numerous calcium balance stud­
ies on patients with IH and normocalciuric control subjects and 
normalized the results for calcium intake. He found that intestinal 
calci

or calcium intake. He found that intestinal 
calcium absorption was significantly higher in the subjects with 
IH.
Bushinsky and associates8 and Coe and colleagues71 also col­
lected data from published metabolic balance studies, and com­
pared net intestinal calcium absorption and urinary calcium 
excretion in hypercalciuric and normocalciuric adults. They also 
noted an increase in intestinal calcium absorption in subjects with 
IH but found that urinary excretion of calcium was increased to 

at urinary excretion of calcium was increased to 
an even greater degree, thus placing many of these patients in 
net negative calcium balance. Although these data confirm that 
enhanced intestinal absorption of calcium probably plays a role in 
the pathogenesis of IH, the investigators could not clarify whether 
this is the primary defect or if it is secondary to another lesion, 
such as a primary dysregulation of renal tubular calcium reab­
sorption. Others suggested that the increase in intes

ption. Others suggested that the increase in intestinal calcium 
absorption, in combination with a decrease in renal calcium reab­
sorption, indicated a more generalized defect in calcium homeo­
stasis. Nonetheless, the finding of enhanced calcium absorption 
makes enhanced bone resorption an unlikely primary mechanism 
of IH, because the increase in serum calcium concentration result­
ing from bone resorption would suppress 1,25(OH)2D-mediated 
intestinal calcium absorption.
In most published s

In most published studies, patients with IH have higher serum 
levels of 1,25(OH)2D than normocalciuric control subjects.72–74 
Kaplan and associates74 determined that 1,25(OH)2D levels were 
higher than control values in approximately one third of patients 
with IH and that intestinal calcium absorption was inappropri­
ately high for the level of 1,25(OH)2D. These studies support 
either 1,25(OH)2D-mediated intestinal calcium absorption or a 
primary defect in ren

nal calcium absorption or a 
primary defect in renal tubular calcium reabsorption as a primary 
mechanism of hypercalciuria in IH.
PTH levels in patients with IH have been reported as normal 
or slightly lower than those in controls.54,60 This finding argues 
against a reduction in renal tubular calcium reabsorption as the 
primary defect in IH, because with this mechanism, the hypercal­
ciuria would lead to low serum calcium levels and stimulation of 
PTH secretion. This finding also does not s

on of 
PTH secretion. This finding also does not support the hypothesis 
that elevated levels of PTH are the stimulus for the increased levels 
of serum 1,25(OH)2D observed in many studies. It is, however, 
consistent with the other potential mechanisms for IH.
Bone mass in patients with IH has been assessed by several 
methods, including radiologic densitometry, quantitative com­
puted tomography (CT), dual-energy x-ray absorptiometry, and 
single-photon absorptiometry, among others. Studies of

le-photon absorptiometry, among others. Studies of patients 
with IH have generally shown a reduction of bone mineral den­
sity compared with values in controls.22,54,60,75 The studies were 
unable to reveal a unifying mechanism for the mild reduction in 
bone mineral density. Altered 1,25(OH)2D regulation would be 
consistent with this finding because the effects of 1,25(OH)2D 
on bone resorption would be mitigated by the increased intestinal 
calcium absorption stimulated by the hormone.
Previ

alcium absorption stimulated by the hormone.
Previously, it was considered important to determine whether 
a patient with IH tended to have excessive GI calcium absorp­
tion (absorptive hypercalciuria) or excessive renal excretion (renal 
leak).76,77 Patients with excessive renal calcium excretion were pre­
scribed thiazide diuretics, and those thought to have a predomi­
nantly absorptive defect were prescribed a low-calcium diet. Coe 
and associates60 undermined the validity of this approach in

ates60 undermined the validity of this approach in a 
study in which 24 patients with IH and nine control subjects were 
given a low-calcium diet (2 mg/kg per day) for more than 1 week. 
Urine and blood tests revealed normal serum calcium levels, a 
mild decrease in PTH levels in the patients with IH, and no differ­
ence in 1,25(OH)2D levels. The striking finding was that whereas 
all of the normocalciuric subjects excreted less calcium than they 
ingested on the low-calcium diet, 16 of the 24 s

ingested on the low-calcium diet, 16 of the 24 subjects with IH 
had urinary calcium excretion that exceeded their intake. Thus, 
most of the patients with IH receiving a low-calcium diet were 
in net negative calcium balance. There was no clear demarcation 
1322
between the patients who excreted excessive amounts of calcium 
and those who did not. Instead, there was a smooth continuum 
of urinary calcium excretion among patients with and without IH 
that appe

tion among patients with and without IH 
that appeared not to be influenced by calcemic hormones. From 
a therapeutic standpoint, these findings have rendered obsolete 
both the need to clinically distinguish IH mechanisms in humans 
and also the prescription of a low-calcium diet in any of these 
patients. This approach to diet is important because a low-calcium 
diet can result in a dangerous reduction in bone mineral den­
sity, especially in women.22,61,75,78 As mentioned earlier (see the 
Pa

omen.22,61,75,78 As mentioned earlier (see the 
Pathogenesis of Stone Formation section), a low-calcium diet also 
appears to increase recurrent stone formation.42,49,59 Thus, there 
is no benefit, and additionally, there are several well-documented 
risks to advising a low-calcium diet to prevent recurrent stone 
formation in patients with IH. 
Genetic Hypercalciuric Stone-Forming Rats
To more fully explain the mechanism of IH in humans, we have 
developed an animal model of this disorder.67,79

developed an animal model of this disorder.67,79–85 Through more 
than 100 generations of successive inbreeding of the most hyper­
calciuric progeny of hypercalciuric Sprague-Dawley rats, we have 
established a strain of rats that excrete more than 10 times as much 
urinary calcium as control Sprague-Dawley rats (Fig. 32.3).
Compared with control Sprague-Dawley rats, the genetic 
hypercalciuric rats absorb far more calcium at lower dietary levels 
of 1,25(OH)2D.86,87 When these hypercalciuric 

ls 
of 1,25(OH)2D.86,87 When these hypercalciuric rats were fed a 
diet very low in calcium, their urinary calcium excretion remained 
elevated compared with that of similarly treated control rats, 
indicating a defect in renal calcium reabsorption or an increase 
in bone resorption, or both,88 again similar to observations in 
humans.60,89 Bone from these hypercalciuric rats released more 
calcium than the bone of control rats when exposed to increas­
ing amounts of 1,25(OH)2D,90 and their bone

creas­
ing amounts of 1,25(OH)2D,90 and their bone mineral densities 
were lower than control rats.91 The administration of a bisphos­
phonate to the genetic hypercalciuric rats fed a low-calcium diet 
significantly reduced urinary calcium excretion.92 In addition, a 
primary defect in renal calcium reabsorption was observed during 
clearance studies.93 We have shown that besides the intestine, both 
the bone and kidney of the hypercalciuric rats have an increased 
number of vitamin D receptors 

 have an increased 
number of vitamin D receptors and calcium receptors.90,94,95
Studies suggest the hypothesis that an increased number of 
vitamin D and/or calcium receptors may be the underlying mech­
anism for hypercalciuria in the rats.96 Since the hypercalciuric 
rats have normal levels of serum 1,25(OH)2D, the higher levels 
of vitamin D receptors suggest that these receptors are under­
saturated with 1,25(OH)2D. We tested the hypothesis that 
1,25(OH)2D would induce a greater increase in

hat 
1,25(OH)2D would induce a greater increase in urine calcium 
in the hypercalciuric rats.82 After injecting 1,25(OH)2D, urine 
calcium in both the controls and hypercalciuric rats increased; 
however, the increased urine calcium with 1,25(OH)2D in the 
hypercalciuric rats exceeded that of controls, indicating that the 
increased number of vitamin D receptors in the hypercalciuric rats 
induces a greater biologic response. This increase in urine calcium 
must come from the intestine and/or bo

ne calcium 
must come from the intestine and/or bone. To determine whether 
the excess urine calcium was derived primarily from diet or from 
increased bone resorption, we asked whether 1,25(OH)2D would 
increase urine calcium in genetic hypercalciuric stone-forming 
(GHS) rats fed a low-calcium diet.83 On this low-calcium diet 
with 1,25(OH)2D, urine calcium in controls increased; however, 
urine calcium again increased significantly more in the hypercal­
ciuric rats. In the hypercalciuric rats

 hypercal­
ciuric rats. In the hypercalciuric rats on the low-calcium diet with 
or without 1,25(OH)2D, urine calcium far exceeded daily calcium 
intake, implying a loss of bone mineral. To determine the role of 
bone resorption in the increased urine calcium in these hyper­
calciuric rats, we tested the hypothesis that a low-calcium diet, 
coupled with inhibition of bone resorption by the bisphospho­
nate alendronate, would eliminate the enhanced 1,25(OH)2D-
induced hypercalciuria in GHS rats.8

d 1,25(OH)2D-
induced hypercalciuria in GHS rats.81 Alendronate eliminated 
the 1,25(OH)2D-induced increase in urine calcium in controls. 
However, in the hypercalciuric rats, alendronate decreased, but 
did not eliminate, the 1,25(OH)2D-induced hypercalciuria, sug­
gesting that maximal alendronate cannot completely prevent the 
1,25D-induced bone resorption in the GHS rats. These results 
confirm the role of increased bone resorption in the hypercalci­
uria of GHS rats. To study the effect of t

rcalci­
uria of GHS rats. To study the effect of the increased number 
of vitamin D receptors on the osseous response to 1,25(OH)2D, 
we fed hypercalciuric and control rats ample calcium and injected 
1,25(OH)2D or vehicle daily.80 With 1,25(OH)2D, there was a 
mineralization defect and a loss of bone mineral density in the 
hypercalciuric rats that exceeded changes in controls and contrib­
uted to increased hypercalciuria, suggesting that these bones would 
be more prone to fracture. The enhanc

bones would 
be more prone to fracture. The enhanced effect of 1,25(OH)2D in 
the hypercalciuric rats indicates that the increased number of vita­
min D receptors are biologically active and supports our hypoth­
esis that in these hypercalciuric rats at baseline, their increased 
number of vitamin D receptors remain available for stimulation 
by exogenous 1,25(OH)2D.
These studies suggest that an increased number of vitamin D 
receptors in the hypercalciuric rats may be the underlying mecha­
nis

percalciuric rats may be the underlying mecha­
nism for their hypercalciuria, and perhaps this occurs in humans 
as well.68,97,98 Circulating monocytes from humans with IH have 
been shown to have an increased number of vitamin D receptors.99
Thus, hypercalciuric rats appear to have a systemic abnormal­
ity in calcium homeostasis. They absorb more intestinal calcium, 
resorb more bone, and do not adequately reabsorb filtered cal­
cium. Because every one of the hypercalciuric rats forms renal 
st

ery one of the hypercalciuric rats forms renal 
stones, we have described them as GHS rats.68,97 
Genetics of IH in Humans
The difficulty in ascertaining the genetics of IH arises, in part, 
from the numerous other factors that influence stone formation, 
such as diet, environment, and gender. Because half of patients 
with IH report a family history of stones and male patients often 
have fathers or sons with the disorder, inheritance is not believed to 
be recessive or X-linked.100 A multitude

ieved to 
be recessive or X-linked.100 A multitude of monogenic hereditary 
GHS generation
Urine calcium (mg/24 h)
12
8
4
0
100
90
80
60
70
50
40
30
All data from published studies.
20
10
0
• Fig. 32.3  Hypercalciuria in subsequent generations of genetic hypercal­
ciuric stone-forming (GHS) rats. All data are from published studies.
1323
disorders (see later discussion) can lead to hypercalciuria because 
they are caused by a variety of mutations resulting in changes 
i

 by a variety of mutations resulting in changes 
in calcium handling at the level of kidney, bone, gut, and the 
calcium-sensing receptor in the kidneys and parathyroid glands. 
Given the evidence that IH is a complex trait with multiple path­
ways for developing the hypercalciuria phenotype, it is most likely 
a polygenic disorder, with heterogeneity of loci and possibly poly­
genic modifiers.65,101 Lieske and associates66 found that genetic 
factors appear to explain 20% to 36% of the interind

ctors appear to explain 20% to 36% of the interindividual varia­
tion in excretion of the ions critical to stone formation. Although 
attempts to diagnose the exact cause of IH in a particular patient 
might not be critical from a therapeutic standpoint, determining 
the etiology of IH in a particular family is essential for researchers 
attempting to clarify the genetics of IH.8,65,66,85,102 Genome-wide 
association studies have found that genes associated with phos­
phate carrier NPT2a and NPT

 associated with phos­
phate carrier NPT2a and NPT2c, claudin 14, aquaporin I, and 
others are associated with calcium nephrolithiasis.65 
Other Genetic Causes of Stones and Nephrocalcinosis
Numerous monogenic disorders cause hypercalciuria leading to 
nephrolithiasis or nephrocalcinosis.8,65,66,85,102 Disorders that 
lead to hypercalciuria by augmenting bone resorption include 
osteogenesis imperfecta type 1, multiple endocrine neoplasia type 
1 syndrome with hyperparathyroidism, McCune-Albrigh

 syndrome with hyperparathyroidism, McCune-Albright syn­
drome, and infantile hypophosphatemia. Disorders that result in 
hypercalciuria because of excessive intestinal absorption of calcium 
include hypophosphatemia, Down syndrome, and congenital lac­
tate deficiency. Others include autosomal dominant hypocalcemia 
(which is caused by an activating mutation of the calcium-sensing 
receptor), Lowe oculocerebrorenal syndrome, and Wilson disease. 
Next, we describe in more detail several disorders

Next, we describe in more detail several disorders that result in 
hypercalciuria via their effect on genes expressed in the kidney. 
X-Linked Hypercalciuric Nephrolithiasis (Dent Disease 
and Others)
Several families around the world have an X-linked disorder of 
proximal renal tubular dysfunction that is manifested by a variable 
combination of disorders including hypercalciuria, low-molecular-
weight proteinuria, nephrocalcinosis or stones, hypophosphatemic 
rickets, and renal failure.103–107

pophosphatemic 
rickets, and renal failure.103–107 Some affected persons demonstrate 
evidence of defects in proximal tubular reabsorption of amino acids, 
glucose, or phosphate. PTH tends to be quite low and 1,25(OH)2D 
high in the majority of patients. The abnormalities completely 
resolve in the patients who receive renal transplants, a finding that 
suggests a renal tubular disorder rather than a systemic process. In 
all families, the pattern of inheritance is consistent with an X-linked 
r

n of inheritance is consistent with an X-linked 
recessive disorder, with male patients affected to a greater extent 
than female patients. The latter are often minimally affected but 
transmit the disorder to half of their male offspring. Over time, the 
various disorders—X-linked recessive nephrolithiasis in the United 
States, Dent disease in the United Kingdom, X-linked recessive 
hypophosphatemic rickets in Italy, and low-molecular-weight pro­
teinuria with hypercalciuria and nephrocalcinos

o­
teinuria with hypercalciuria and nephrocalcinosis in Japan—have 
all been linked to mutations affecting the genes encoding the Cl–/
H+ exchanger ClCN5 in the majority of patients and/or inositol 
polyphosphate 5-phosphatase (OCRL1), both on the X chromo­
some.103–105 How defects in these genes lead to the array of these 
proximal tubular disorders is not yet understood. 
Bartter Syndrome
Bartter syndrome is caused by at least five genetic mutations, 
predominantly autosomal recessive, that le

tions, 
predominantly autosomal recessive, that lead to sodium chloride 
wasting at the TAL of the loop of Henle.37,38,64,83,105 Bartter syn­
drome is an autosomal disease caused by mutations of bumetanide-
sensitive NKCC2 (the Na-K-2Cl cotransporter), ROMK (the renal 
outer medullary potassium channel), CaSR (the calcium-sensing 
receptor), and CLC-Kb (the voltage-gated chloride channel) or 
the CLC-Kb beta subunit barttin. These genes, all expressed in the 
TAL, cause a defect in sodium transp

essed in the 
TAL, cause a defect in sodium transport that leads to a reduction 
in the transtubular potential difference, resulting in a decrease in 
paracellular calcium reabsorption in the TAL. The ensuing reduc­
tion in intravascular volume also induces an aldosterone-mediated 
metabolic alkalosis. Bartter syndrome therefore resembles high-
dose furosemide administration (that targets NKCC2) and differs 
from Gitelman syndrome in that hypercalciuria, nephrocalcinosis, 
and nephrolithiasis ar

lciuria, nephrocalcinosis, 
and nephrolithiasis are seen with Bartter but not with Gitelman. 
An autosomal dominant form of Bartter results from a gain-of-
function mutation in the calcium sensing receptor in renal tubular 
cells. This mutation leads to reduced calcium reabsorption and 
also hypocalcemia caused by low PTH levels. Therapy with vita­
min D and calcium supplementation can exacerbate stone disease 
in this disorder. 
Familial Hypomagnesemia With Hypercalciuria and 
Nephrocalcinosis

nesemia With Hypercalciuria and 
Familial hypomagnesemia with hypercalciuria and nephrocal­
cinosis is an autosomal recessive disorder that results in hypo­
magnesemia, hypercalciuria, nephrolithiasis, and distal renal 
tubular acidosis (dRTA). Polyuria and severe nephrocalcinosis also 
ensue, and progressive kidney failure is common by late child­
hood.37,108–110 The genetic disorder results in defective production 
of either of the tight junction proteins, claudin-16 and claud

 the tight junction proteins, claudin-16 and claudin-19, 
that bind together to facilitate paracellular calcium and magne­
sium transport in the TAL, as well as renal sodium reabsorption. 
Currently, there is no effective treatment for the hypercalciuria or 
progression to kidney failure. 
Distal Renal Tubular Acidosis
dRTA is caused by dysfunctional α-intercalated cells, resulting in 
defective acid excretion.37,38,111–115 This inability to adequately 
acidify the urine results in metabolic aci

uately 
acidify the urine results in metabolic acidosis, hypocitratu­
ria, hypokalemia, hypercalciuria, nephrocalcinosis, and stones. 
The metabolic acidosis leads to resorption of both calcium and 
phosphate from bone. The increased filtered load of calcium and 
phosphate, along with the elevated urine pH and hypocitratu­
ria, results in favorable conditions for calcium phosphate stone 
formation. Although there are secondary causes of dRTA, such 
as Sjögren syndrome and use of carbonic anhydra

h 
as Sjögren syndrome and use of carbonic anhydrase inhibitors 
(e.g., acetazolamide), there are also several hereditary causes of 
dRTA.116,117 Some are autosomal recessive and can also result in 
hearing loss; others are autosomal dominant. One form of dRTA 
that targets carbonic anhydrase II results in osteopetrosis and 
brain calcifications.117 Patients with dRTA fail to lower their urine 
pH below 5.5 following ingestion of an acid load. Their urine 
citrate is extremely low despite mildly

eir urine 
citrate is extremely low despite mildly reduced or even normal 
serum bicarbonate levels. 
Hereditary Hypophosphatemic Rickets With 
Hypercalciuria
Hereditary hypophosphatemic rickets with hypercalciuria is an 
autosomal form of hypophosphatemic rickets that is manifested 
clinically by hypophosphatemia secondary to renal phosphate 
wasting.118–121 This disease is caused by mutations in solute carrier 
family 34, member 3 (SLC34A3), which is the gene that encodes 
1324
SECTION VII    

ich is the gene that encodes 
1324
the sodium Na+-dependent phosphate cotransporter 2c (NPT2c). 
These patients have hypophosphatemia-induced increase in lev­
els of 1,25(OH)2D, which leads to increased intestinal calcium 
absorption and results in hypercalciuria. The bone pain, muscle 
weakness, limb deformities, and rickets resolve with administra­
tion of oral phosphate. 
Primary Hyperoxaluria and Cystinuria
Primary hyperoxaluria and cystinuria are each discu

rimary hyperoxaluria and cystinuria are each discussed in the 
Therapy section. 
Clinical Presentation and Evaluation
Kidney stones vary in clinical presentation, from those discovered 
asymptomatically on routine imaging to their painful passage 
through the ureters to large and obstructing staghorn calculi that 
can significantly impair renal function and even lead to ESRD.6,7 
The severity of stone disease depends on the pathogenic factors 
contributing to the rate of stone formation, as well

ntributing to the rate of stone formation, as well as the stone 
type, size, and location.
In its most classic form, nephrolithiasis manifests as renal colic. 
This discomfort of abrupt onset intensifies over time into an excru­
ciating, severe flank pain that resolves only with stone passage or 
removal. The pain often migrates anteriorly along the abdomen 
and inferiorly to the groin, testicles, or labia majora as the stone 
moves toward the ureterovesical junction. Gross hematuria, uri­
nary 

eterovesical junction. Gross hematuria, uri­
nary urgency and frequency, nausea, and vomiting may be present. 
Nephrolithiasis can also result in a dull, poorly localizing abdominal 
pain. The probability of passing a kidney stone without intervention 
depends on its size and varies from about 70% for kidney stones 5 
mm or less in size, whereas stones between 5 and 7 mm have a 60% 
chance, stones between 7 and 9 mm have a 48% chance, and stones 
9 mm or larger have a 25% chance of passing spont

9 mm or larger have a 25% chance of passing spontaneously.122
Several studies, including a large meta-analysis, demonstrate 
that nephrolithiasis is associated with an increase in CKD that is 
independent of other risk factors, such as diabetes and hyperten­
sion found in stone formers; however, at least one other study did 
not find an increased risk of CKD.123–127 A French study esti­
mated the incidence rate of ESRD because of nephrolithiasis to 
be about 3.1 cases per million population per

 to 
be about 3.1 cases per million population per year,128 whereas a 
Canadian study demonstrated that although only 0.8% of patients 
with ESRD had nephrolithiasis, any stone episode previously was 
associated with an increased risk of ESRD (hazard ratio 2.16).129 
The common reasons for loss of a single kidney in stone formers 
were staghorn calculi, high stone burden, infection, and ureteral 
obstruction.130 However, no association between nephrolithiasis 
and incident CKD could be found in 

phrolithiasis 
and incident CKD could be found in a recent study of more than 
10,000 patients.123
The development of CKD from stone disease is thought to 
be from ureteral obstruction leading to parenchymal damage.125 
Most data are derived from animal models and suggest that unilat­
eral ureteral obstruction causes intense renal vasoconstriction that 
reduces renal blood flow and glomerular filtration rate (GFR).131 
Brushite (CaHPO4) stone formers have an increased risk of cor­
tical fibrosis

mers have an increased risk of cor­
tical fibrosis,132 and the formation of Randall plaques in such 
patients was associated with duct plugging, collecting duct cell 
death, and inflammation.133 Renal biopsy specimens in patients 
with staghorn calculi demonstrate extensive inflammation and 
macrophage infiltration.134 Other stone-forming diseases like 
primary hyperoxaluria, cystinuria, and Dent disease have all been 
associated with crystal formation in the renal parenchyma that 
presumably tr

mation in the renal parenchyma that 
presumably triggers subsequent inflammation and CKD.133
Certain disorders can lead to diffuse renal parenchymal cal­
cifications termed nephrocalcinosis.6,38,115,135 The calcifications, 
usually calcium phosphate or calcium oxalate, may be present in 
the cortex or medulla. Among the most common causes of stone-
related nephrocalcinosis are primary hyperoxaluria and medullary 
sponge kidney.
Metabolic Evaluation of Stone Formers
Although it is uniformly accep

on of Stone Formers
Although it is uniformly accepted that patients with multiple 
stones merit a thorough investigation into the cause of nephroli­
thiasis, the need for evaluation of the patient with a single stone is 
controversial. This is probably due to the difficulty in determining 
the cost-to-benefit ratio of stone evaluations and wide differences 
in reported rates of stone recurrence.
The National Institutes of Health has convened several con­
sensus conferences to resolve such issues

ral con­
sensus conferences to resolve such issues related to the preven­
tion and treatment of kidney stones.136 These panels determined 
that all patients, even those with a single stone, should undergo at 
least a basic evaluation to rule out a systemic etiologic mechanism. 
Patients with an increase in number or size of stones (metabolically 
active stones), all children, all non–calcium oxalate stone formers, 
and those in demographic groups not typically susceptible to stone 
formation war

 not typically susceptible to stone 
formation warrant a more complete metabolic evaluation.136 
The Basic Evaluation
Elements of the basic evaluation are listed in Table 32.1.6,7
History
In addition to the medical history typically obtained from new 
patients, the evaluation of the stone former includes a stone his­
tory and a thorough review of diet, fluid intake, and lifestyle. Spe­
cific laboratory studies and radiographic tests are also required.
Stone History
The stone history begins with 

ired.
The stone history begins with a chronology of stone events: age 
of incidence of first stone, size and number of stones formed, fre­
quency of passage, stone type (if known), and whether the stones 
occur equally in both kidneys or unilaterally. Also helpful is a 
report of the patient’s symptoms with each episode, as well as the 
need for and response to surgical intervention.
This information is useful to judge not only the severity of the 
stone disease but also clues to t

severity of the 
stone disease but also clues to the origin of the patient’s neph­
rolithiasis. For example, nephrolithiasis that begins at a young 
age may be attributable to an inherited metabolic disorder such 
as primary hyperoxaluria or cystinuria. Large staghorn calculi 
that are difficult to eradicate and tend to recur despite frequent 
surgical intervention are more likely to be composed of struvite 
(NH4MgPO4.6H2O) instead of calcium oxalate. Cystine stones 
are not disintegrated thorou

late. Cystine stones 
are not disintegrated thoroughly with the use of lithotripsy, and 
alternative surgical modalities are generally required for stone 
removal. In patients who tend to form stones in only one kidney, 
the possibility of congenital abnormalities of that kidney, such as 
megacalyx or medullary sponge kidney, should be explored. 
Medical History
Systemic disorders that can contribute to nephrolithiasis are 
sought in the medical history. For example, any disorder that can 
resul

history. For example, any disorder that can 
result in hypercalcemia, such as sarcoidosis or certain malignan­
cies, may also lead to hypercalciuria. A variety of GI disorders 
associated with malabsorption (e.g., sprue, Crohn disease) can 
1325
cause calcium oxalate nephrolithiasis on the basis of enteric hyper­
oxaluria. Patients with gout or insulin resistance are more likely to 
have uric acid stones30,137 (Tables 32.2 and 32.3). 
Family History
As noted earlier, se

.2 and 32.3). 
As noted earlier, several stone disorders are inherited, making the 
family history an important component of the basic evaluation. 
IH appears to be a familial disorder. Although the exact chro­
mosomes and genes have not yet been identified, the pattern of 
inheritance is almost certainly polygenic.
Stones arising in childhood or young adulthood can be related 
to autosomal recessive disorders such as cystinuria and primary 
oxaluria. These genetic disorders are r

d primary 
oxaluria. These genetic disorders are reviewed in the later sections 
on treatment of cystine and primary hyperoxaluria.
The high prevalence of uric acid stones in certain areas of the 
world is suggestive of both genetic and environmental risk factors. 
Genes that cause either excessively acidic urine or hyperuricosuria 
have been implicated.30,37,65,66,78,135,138,139 
Medications can contribute to stone formation in several ways. 
Calcium-containing supplements, for exam

al ways. 
Calcium-containing supplements, for example, can increase the 
amount of calcium absorbed and subsequently excreted.63 Loop 
diuretics can directly promote renal tubular excretion of calcium 
and are associated with nephrocalcinosis in neonates who have 
received the drug.140,141 Acetazolamide, a weak diuretic, induces 
a mild metabolic acidosis and alkaline urine, which are favorable 
conditions for the development of calcium phosphate stones. 
Other uricosuric medications, such as sa

 stones. 
Other uricosuric medications, such as salicylates and probenecid, 
have been implicated in uric acid lithiasis.142
Certain crystals or stones can consist completely of precipitated 
medication. Such medications include intravenously administered 
acyclovir, triamterene, indinavir, and various sulfonamides, such 
as sulfadiazine. Oxalate is a metabolic end product of vitamin C, 
and large doses increase oxalate excretion and may predispose to 
stone formation143,144 (Table 32.4). 
Lifes

se to 
Lifestyle and Diet
Occupation and lifestyle are aspects of the social history that can 
be relevant to stone formation. Surgeons and traveling salespeo­
ple, for example, tend to minimize fluid intake to avoid frequent 
micturition throughout the day. Insensible losses of fluid can also 
exacerbate nephrolithiasis and may be related to employment 
(e.g., construction work) or hobbies (running, gardening).
The evaluation proceeds with a thorough review

g).
The evaluation proceeds with a thorough review of the patient’s 
diet and fluid intake. Patients are asked to review what they eat 
at all meals and snacks. Particular attention is paid to ingestion 
of foods high in sodium (fast foods, canned foods, added salt or 
soy sauce) and the quantity of animal protein consumed (see later 
discussion). Patients are also asked to list four or five favorite foods 
or snacks to assess whether they may be consuming foods high in 
oxalate or purine as wel

 consuming foods high in 
oxalate or purine as well. Many patients are erroneously counseled 
by physicians to avoid calcium-containing foods. As noted ear­
lier, doing so is associated with increased risk of stone formation 
		
The Basic Evaluation of Stone Formers
History
Stone history
Medical history
Family history
Occupation and lifestyle
Diet and fluid intake
Physical examination
Laboratory tests
Urinalysis
Urine culture and sensitivity
Cystine screening
Blood tests
Sodium, pota

itivity
Sodium, potassium, chloride, bicarbonate
Calcium, phosphorus, uric acid, creatinine
Intact parathyroid hormone if calcium is elevated or at the upper 
limit of normal
Tetrahydrodeoxycortisol, urinary free cortisol, and 25-hydroxyvitamin 
D levels as appropriate
Stone analysis
Radiology (choose appropriate study as indicated; see text)
Unenhanced helical (spiral) computed tomography
Kidneys, ureter, and bladder examination
Intravenous pyelography
Ultrasonogra

r examination
Data from Monk RD, Bushinsky DA. Nephrolithiasis and nephrocalcinosis. In: Johnson RJ, 
Frehally J, Floege J, eds. Comprehensive Clinical Nephrology, 5th ed. Philadelphia, PA: Else­
vier; 2015:688–702.
   
TABLE 32.1
	
Causes of Calcium Stone Formation
Granulomatous diseases
Hypercalcemic disorders
Sarcoid
Thyro

syndrome
Sarcoid
Medications (see Table 32.4)
Hyperoxaluria
Biliary obstruction
Chronic pancreatitis
Crohn disease
Dietary hyperoxaluria (urine oxalate secretion 40–60 mg/day)
Enteric oxaluria (urine oxalate 60–100 mg/day)
Jejunoileal bypass
Malabsorptive disorders
Primary hyperoxaluria types 1 and 2 (oxalate 80–300 mg/day)
Sprue (celiac disease)
Hyperuricosuria (see Table 32.3)
Hypocitraturia
Androgens
Exercise
Hypokalemia
Infection
Meta

Exercise
Infection
Renal tubular acidosis (distal, type 1)
Anatomic genitourinary tract abnormalities
Congenital megacalyx
Medullary sponge kidney
Tubular ectasia
   
TABLE 32.2
1326
and may also result in b

and may also result in bone demineralization, a grave concern in 
women with stones.42,49,59 
For most patients with nephrolithiasis, physical findings are nor­
mal. In some patients, however, the findings may reveal a sys­
temic disorder related to the stone disease. An enterocutaneous 
fistula, for example, may be associated with Crohn disease, a com­
mon cause of enteric hyperoxaluria. A paraplegic patient with an 
indwelling catheter may be susc

c patient with an 
indwelling catheter may be susceptible to frequent urinary tract 
infections with urease-producing organisms and consequent stru­
vite stone formation. Hyperuricosuria and uric acid stone forma­
tion may be seen in patients with tophi related to gout.6,7,73 
Although valuable information is gleaned from the history 
and physical examination, it is often difficult to determine the 
metabolic cause of a patient’s nephrolithiasis without laboratory 
data. The uri

 nephrolithiasis without laboratory 
data. The urinalysis is an easy and inexpensive test that provides a 
great deal of information. Often, the presence of different kinds of 
crystals can suggest the type of underlying stone (Fig. 32.4). Uric 
acid and calcium oxalate stones, for example, grow more favorably 
at an acidic pH, and a consistently high urinary pH may suggest 
calcium phosphate or struvite nephrolithiasis. The specific gravity, 
if high, can confirm suspicions of inadequate fluid 

 high, can confirm suspicions of inadequate fluid intake.
Hematuria is often present in active stone disease. Microscopic 
examination of the urine in this case might reveal characteristic crys­
tals. Bacteria and pyuria noted in conjunction with a high urinary 
pH (∼6.5) are characteristic of struvite stone disease. Urine specimens 
for culture should be obtained in this setting. Because enough urease 
may be produced to form struvite stones even when colony counts 
are low (∼50,000 colony-form

n when colony counts 
are low (∼50,000 colony-forming units), the microbiology laboratory 
should be instructed specifically to identify the organism and check 
for urease-producing bacteria despite low colony counts.145
Qualitative cystine screening should be performed on a urine speci­
men. Urine turns purple-red when sodium nitroprusside is added to a 
specimen containing cystine at a concentration greater than 75 mg/L.138
Recommended blood tests in the basic evaluation include 
electrolytes 

sts in the basic evaluation include 
electrolytes (sodium, potassium, chloride, bicarbonate) uric acid, 
calcium, phosphorus, and serum creatinine to determine renal 
function.73,78,111 If the serum calcium level is elevated or at the 
upper limit of normal, or if the serum phosphorus level is reduced 
or at the lower limit of normal, a serum intact PTH level is also 
determined to rule out primary hyperparathyroidism. Low serum 
bicarbonate levels suggest a hypocitraturic disorder such as renal

ls suggest a hypocitraturic disorder such as renal 
tubular acidosis (RTA) or acetazolamide ingestion. 
Stone Analysis
Stone analysis should be performed, whenever possible, in 
patients with a new history of nephrolithiasis or in patients with 
	
Factors Associated With Noncalcium 
Stone Formation
Uric Acid Stones
Diarrhea
Diet high in animal protein
Excessive dietary purine
Excessive insensible losses
Genetic predisposition
Glucose-6-phosphatase deficiency
Gout
Hemolytic anem

ucose-6-phosphatase deficiency
Gout
Hyperuricemia
Hyperuricosuria
Inadequate fluid intake
Inborn errors of metabolism
Intracellular to extracellular uric acid shift
Lesch-Nyhan syndrome
Low urine pH (<5.5)
Low urine volume
Metabolic syndrome
Myeloproliferative disorders
Obesity
Tumor lysis
Struvite Stones
Urease-producing bacteria
Proteus, Pseudomonas, Haemophilus, Yersinia, Ureaplasma, Klebsiella, 
Coryneba

hilus, Yersinia, Ureaplasma, Klebsiella, 
Corynebacterium, Serratia, Citrobacter, Staphylococcus, and others
Never Escherichia coli—not a urease producer
High urine pH (∼6.5)
Indwelling urinary catheter
Neurogenic bladder
Cystine Stones
Autosomal recessive trait
Excessive excretion of cystine, ornithine, lysine, and arginine
Low solubility of cystine (<250 mg/L)
Frehally J, Floege J, eds. Comprehensive Clinic

, 
   
TABLE 32.3
	
Medications Associated With Renal 
Lithiasis and Nephrocalcinosis
Medications That Promote Calcium Stone Formation
Acetazolamide
Antacids (calcium and noncalcium antacids)
Calcium supplements
Loop diuretics
Vitamin C
Vitamin D
Medications That Promote Uric Acid Lithiasis
Allopurinol (associated with xanthene stones)
Pro

Probenecid
Salicylates
Medications That Can Precipitate Into Stones or Crystals
Acyclovir (when infused rapidly intravenously)
Indinavir
Nelfinavir
Sulfonamides
Triamterene
   
TABLE 32.4
1327
long-standing stone disease who 

32  Kidney Stones
long-standing stone disease who note a difference in clinical pre­
sentation or in the color, shape, or texture of any stone passed. 
Knowing the constituents of a stone can help the physician target 
certain elements of the medical history and specific urine studies. 
In most cases, the stone must be sent to an outside laboratory 
for examination. X-ray diffraction crystallography and infrared 
spectroscopy are currently the most accurate methods available for 
stone analysis.

st accurate methods available for 
stone analysis.32 
Radiologic Evaluation
Various radiologic tests can help determine the location and extent 
of the stone burden and might elucidate genitourinary abnor­
malities contributing to stone formation (Fig. 32.5). For acute 
renal colic, spiral (or helical) CT without contrast (unenhanced) 
has replaced intravenous pyelogram (IVP) as the optimal test 
for detection and localization of kidney stones. Helical CT has 
proved to be more sensitive (95%) a

elical CT has 
proved to be more sensitive (95%) and specific (98%) than IVP 
in detecting stones of all types in both the kidneys and ureters.146 
In addition, it can more accurately reveal causes of flank pain and 
hematuria not related to stones, and it circumvents the use of 
intravenous contrast material. Radiation exposure is a disadvan­
tage of both CT and IVP, and the exposure to patients undergoing 
helical CT may be triple that of IVP. As such, it should be used 
judiciously, especiall

As such, it should be used 
judiciously, especially in young patients with frequent episodes 
of renal colic. Helical CT takes less time to perform, which is a 
potential advantage in an emergency department setting, but it is 
more expensive.147–150 Some institutions are exploring the use of 
low-dose CT, which has about one third of the radiation exposure 
of conventional CT with a similar sensitivity and specificity.146
CT should be followed by a plain film (radiograph) of the 
abdomen that i

y a plain film (radiograph) of the 
abdomen that includes the kidneys, ureter, and bladder (KUB). 
Plain films can assist in determining stone composition. Stones 
composed of calcium, cystine, and struvite are radiopaque and vis­
ible on KUB, whereas radiolucent stones, such as those composed 
of uric acid and xanthine, are not.
Renal ultrasound is a useful test for patients who must avoid 
exposure to radiation or contrast, such as pregnant women and 
children. It is almost as sensitive (84%) 

en and 
children. It is almost as sensitive (84%) but not as specific (53%) 
as spiral CT for detecting kidney stones.151 Ureteral stones are 
very difficult to visualize on ultrasound. Emergency department 
patients with suspected nephrolithiasis who initially have ultraso­
nography have been shown to have a lower total radiation exposure 
than those initially evaluated with CT without any difference in 
serious adverse events, missed important diagnoses, pain scores, or 
subsequent emergency d

diagnoses, pain scores, or 
subsequent emergency department visits or hospitalizations.152,153
IVPs are useful in detecting certain genitourinary abnormalities 
that can predispose to nephrolithiasis, such as medullary sponge kid­
ney and caliceal abnormalities. Another advantage of IVP is that the 
osmotic diuresis generated by the administered contrast agent may 
aid in excretion of the offending stone during an episode of acute 
A
B
C
D
• Fig. 32.4  Crystals seen in urine of stone formers. (A

 32.4  Crystals seen in urine of stone formers. (A) Calcium oxalate. (B) Urate. (C) Cystine. (D) Struvite.
1328
renal colic. A major disadvantage of IVP is exposure to radiographic 
contrast material. Administration of contrast should be avoided in 
patients who are at high risk for developing nephrotoxicity from 
the contrast, such as the elderly; those with diabetes mellitus, pro­
teinuria, or preexisting kidney disease; and patients with significant 
intravas

y disease; and patients with significant 
intravascular volume depletion. With the numerous advantages of 
CT and ultrasound, IVPs are rarely indicated.
Once a patient is known to have a certain type of stone, specific 
tests may be used in follow-up. For example, a patient known to 
have asymptomatic calcium stones can have a KUB test 6 to 12 
months later to assess for any increase in stone size or number.6,7,78 
However, because of the radiation exposure, reimaging should be 
limited to patie

on exposure, reimaging should be 
limited to patients in whom the results of the test will alter treat­
ment. Little is gained in asymptomatic patients by checking for 
stone growth or movement if maximal dietary and pharmacologic 
therapy is already being prescribed. 
The Complete Evaluation
The complete evaluation comprises the entire basic examination as 
well as a 24-hour urine collection to determine volume and levels 
of calcium, oxalate, citrate, sodium, urate, phosphorus, creatinine, 
an

itrate, sodium, urate, phosphorus, creatinine, 
and the urinary supersaturation with respect to the common solid 
phase components6,7,111 (Table 32.5). Creatinine is used to assess the 
adequacy of the collection: men should excrete approximately 15 to 
20 mg/kg of creatinine per day, whereas women should excrete 10 to 
15 mg/kg of creatinine per day. Cystine should also be measured in 
patients known to have cystine stones or in whom prior urine studies 
have not determined if there is excessiv

 studies 
have not determined if there is excessive excretion of this amino acid.
Patients should be instructed to collect their urine on a day 
when they perform usual activities and have their typical fluid and 
dietary intake. The first morning’s urine specimen is discarded; 
following this, all urine for the next 24 hours (including the next 
morning’s specimen) is collected in the container. The ideal 24-hour 
urine collection includes measurement and reporting of the daily 
excretion of th

rement and reporting of the daily 
excretion of the constituents listed in Table 32.5 and also reports 
supersaturation of calcium oxalate, calcium phosphate, and uric 
acid. Patients should be instructed to discontinue multivitamins 
approximately 5 days before the collection to prevent any antioxi­
dant effect of the vitamins on the urine sample. In most cases, an 
acid or antibiotic is included in the collection container or added 
with the first urine sample as a preservative. Certain labora

rst urine sample as a preservative. Certain laboratories 
require various preservatives for the different factors measured. 
Physicians should ask their laboratory how many 24-hour urine 
	
Optimal 24-Hour Urine Values in 
Patients With Nephrolithiasis
Parameter
Value
Volume
>2.0–2.5 L
pH
>5.5, <7.0 (24-hour specimen not 
required)
Calcium
<300 mg or <3.5–4.0 mg/kg in men
<250 mg or <3.5–4.0 mg/kg in women
Oxalate
<40 mg
Sodium
<3000 mg or <130 mEq
Uric acid
<800 mg in men
<750 mg in women
Phos

mEq
Uric acid
Phosphorus
<1100 mg
Citrate
>320 mg
Creatinine
∼15 mg/kg in men
∼10 mg/kg in women to ensure 
adequacy of collection
Supersaturation of calcium 
oxalate
<5
phosphate
0.5–2.0a (ideally <1.0)
Supersaturation of uric acid
0–1a
aIdeal values can vary among laboratories that perform supersaturation analysis.
   
TABLE 32.5
A
B
• Fig. 32.5  Renal stones on an abdominal radiograph and a computed tomography scan. (A) Radiolucent 

 and a computed tomography scan. (A) Radiolucent 
kidney stone can be seen on the kidneys, ureter, and bladder radiograph at the ureterovesical junction. (B) 
A large stone is seen in the renal pelvis of the right kidney.
1329
collections and which preservatives are required for the complete 
evaluation. Simplifying this process to a single urine collection in 
which all of the measurements are performed and resultant super­
saturation calculated is available from sever

per­
saturation calculated is available from several national laboratories 
and almost certainly improves adherence and perhaps accuracy of 
the calculation of supersaturation.78,154
Patients who require the complete evaluation are as follows: all 
children, nonwhite patients (demographic groups not typically prone 
to nephrolithiasis in the United States), non–calcium stone formers, 
and patients with metabolically active stone disease (metabolically 
active stones are those that grow in size o

cally 
active stones are those that grow in size or number within 1 year).6,7 
Therapy
Surgical Treatment
Treatment of an acute episode of renal colic often involves surgi­
cal management for large stones that do not pass spontaneously. 
Stones less than 5 mm in size have a 68% chance of passing 
spontaneously, whereas those greater than 5 mm but less than 
10 mm have a spontaneous stone passage rate less than 50%.155 
Most stones greater than 10 mm, and many greater than 5 mm in 
size, require 

 mm, and many greater than 5 mm in 
size, require surgical intervention for relief of renal colic, ureteral 
obstruction, or other symptoms of clinically active stone disease. 
With the advent of newer, less invasive urologic therapies, open 
surgical stone extraction is rarely performed. Current urologic 
therapy includes percutaneous nephrolithotomy (PNL), extracor­
poreal shock wave lithotripsy (ESWL), and ureteroscopic removal 
of stones (URS). The exact procedure used varies according to 
s

). The exact procedure used varies according to 
stone location in the kidney or ureter, size, composition, various 
patient factors, and surgical expertise.155–158
PNL involves placement of a large needle through the flank into 
the renal collecting system. The tract is dilated, and instruments 
are used to rupture and remove the stone. Although more invasive 
than URS and ESWL, PNL is more effective in removing large (>2 
cm) or staghorn calculi, as well as stones that do not fragment well 
wi

i, as well as stones that do not fragment well 
with lithotripsy. Large, infected stones, such as struvite stones, in 
which complete removal is desired, are best treated with PNL.157,158 
ESWL involves focusing sound waves from a lithotripter outside the 
body onto the kidney stone. The impulses fragment the stone into 
smaller stones, or “gravel,” that can more easily be passed spontane­
ously. Newer-generation lithotripters do not require a water bath 
and often require less analgesia.159 Kid

ter bath 
and often require less analgesia.159 Kidney stones less than approxi­
mately 15 mm, proximal ureteral stones, upper and middle pole 
kidney stones, and those not composed of cystine or calcium oxalate 
monohydrate respond best to ESWL.155,157–159 Because fluoroscopy 
is typically used to visualize radiopaque stones during the proce­
dure, ESWL may be more complicated in patients with uric acid 
lithiasis. ESWL is relatively contraindicated in patients with coagu­
lopathy and in pregnan

ted in patients with coagu­
lopathy and in pregnant women. It may be less successful in patients 
with a higher BMI, as a low skin to stone distance is necessary to 
achieve optimal success.158,159
Recent meta-analyses found that PNL is superior to ESWL for 
most ureteral stones because it results in longer stone-free rates 
and decreases the need for retreatment. However, it is associated 
with more, although generally minor, complications and longer 
hospitalizations.160,161
Ureteroscopy, the 

onger 
Ureteroscopy, the passage of a semirigid or flexible scope through 
the bladder and into the ureter, is a mainstay of surgical stone extrac­
tion for many ureteral stones, especially distal ureteral stones, and larger 
proximal stones. Endocorporeal lithotripsy can be added to URS to 
directly fragment visualized stones. One of the more commonly used 
devices is the holmium:YAG (yttrium-aluminum-garnet) laser litho­
tripter, which combines both pneumatic and ultra

­
tripter, which combines both pneumatic and ultrasound lithotripsy to 
fragment stones. This results in high stone-free rates following the pro­
cedure.156,162 Complications of ESWL and URS include urinary tract 
infection; sepsis; ureteral stricture; ureteral injury; and steinstrasse, or 
“stone street,” the linear accumulation of small stones blocking a ureter 
following fragmentation of a larger stone. 
Medical Expulsive Therapy
Another form of therapy that has been shown to reduce the time 

of therapy that has been shown to reduce the time 
to stone passage is medical expulsive therapy. Medical expulsive 
therapy may be utilized with ureteral stones that are less than 10 
mm in diameter for up to 6 weeks if kidney function is normal, 
there is no evidence of infection or significant obstruction, and pain 
can be controlled. Several medications have shown benefit in reduc­
ing the time to stone passage and in assisting with passage of larger 
stones.163 A recent trial found that tam

f larger 
stones.163 A recent trial found that tamsulosin (0.4 mg/day) and 
nifedipine (30 mg/day) were not effective at decreasing the need for 
further treatment to achieve stone clearance in 4 weeks.164 How­
ever, meta-analyses have demonstrated a benefit for α-adrenergic 
blockers such as tamsulosin, terazosin (2–5 mg/day), or doxazosin 
(4 mg/day).165,166 These agents act by reducing spasm of the ureteral 
smooth muscle and allowing ureteral peristalsis to more effectively 
move the stone t

 peristalsis to more effectively 
move the stone through. The addition of corticosteroids may also 
assist with stone passage by reducing ureteral inflammation and 
swelling in the ureter where the stone is lodged. In a controlled 
study, these therapies were compared with control groups involving 
placebo167 or standard therapy, such as antispasmodic therapy, non­
steroidal anti-inflammatory agents, or analgesics. In studies com­
paring α-adrenergic receptor blockers to calcium channel blockers

rgic receptor blockers to calcium channel blockers, 
the α-adrenergic blocker tamsulosin was found in a few studies to 
result in higher stone passage rates and more rapid stone expulsion 
than nifedipine.163,168,169 Both agents were generally well tolerated, 
although perhaps with less hypotension in the tamsulosin groups. 
The selective α-1A receptor blocker silodosin (8 mg/day) provided 
a significantly higher expulsion rate than tamsulosin for distal ure­
teral stones.170 Frequent follow-up 

r distal ure­
teral stones.170 Frequent follow-up and occasional imaging with 
ultrasound is recommended to ensure that the patient remains free 
of complications while awaiting stone passage. 
Medical Preventative Therapy
Medical preventative therapy is the mainstay of medical manage­
ment, and the remainder of the chapter will focus on prevention 
of stone recurrence.
Nonspecific Therapy
Most patients, irrespective of stone type, are given general advice 
about fluid and dietary modification t

ral advice 
about fluid and dietary modification to prevent further stone forma­
tion.6,7,111 These nonpharmacologic interventions, which include an 
increase in fluid intake, as well as restriction of dietary sodium and 
animal protein, can reduce the incidence of stone formation—a result 
termed the stone clinic effect.42,171–173 In one study, such interventions 
resulted in a 40% decrease in stone recurrence over 5 years.41
The mainstay of nonspecific therapy involves dietary measures 
(see t

specific therapy involves dietary measures 
(see the Diet discussion in the Pathogenesis of Stone Formation 
section): increased fluid intake to raise urine volume to approxi­
mately 2 to 2.5 L, a reduction in sodium intake to less than 3000 
mg/day (130 mEq), moderate reduction in animal protein inges­
tion to approximately 1.0 mg/kg per day, and perhaps eating cer­
tain fruits or juices high in citrate.42,45,78,171,172,174,175
1330
Dietary calcium restriction 

   Mineral Metabolism
Dietary calcium restriction is no longer recommended, because 
it not only can lead to a reduction in bone mineral content but 
also increases the rate of stone recurrence presumably by decreas­
ing intestinal calcium oxalate absorption and increasing urinary 
oxalate excretion.42 In retrospective studies of dietary intake, both 
women and men have been found to have reduced stone forma­
tion with increased dietary calcium ingestion. Calcium supple­
ments, however, were ass

ngestion. Calcium supple­
ments, however, were associated with an increased risk of stones 
in women. Patients should therefore be advised to maintain an 
age- and gender-appropriate intake of dietary calcium, preferably 
without supplements.42,49,59,63,176 
Specific Therapy Matched to Specific Pathogenesis
The optimal therapy for a patient with metabolically active stone 
disease is directed at the patient’s particular metabolic abnormal­
ity.8,177 Before medications for nephrolithiasis are pre

177 Before medications for nephrolithiasis are prescribed, all 
patients should be treated with the nonspecific measures noted 
earlier. Prior to any therapeutic intervention, some clinicians assess 
the patient’s existing stone burden with a radiologic examination 
(KUB, spiral CT, IVP, or ultrasonography). If stones are present, 
any subsequent passage of stones would not necessarily indicate 
therapeutic failure, because the patient may simply be passing the 
stones formed prior to therapy. H

 be passing the 
stones formed prior to therapy. However, this approach must be 
weighed against the expense and the radiation exposure associ­
ated with the radiologic examination. Our personal opinion is 
to not obtain baseline or follow-up radiographs in asymptomatic 
patients unless the results will alter subsequent therapy. The basic 
and complete evaluations help direct the clinician to the specific 
treatments discussed here.
Calcium Stones
Most kidney stones (∼70%) contain calcium (see F

s
Most kidney stones (∼70%) contain calcium (see Fig. 32.1). More 
than one third of these are composed of calcium oxalate alone, 
and another 7% are composed of calcium phosphate alone. The 
rest are composed of a combination of calcium oxalate with either 
urate or calcium phosphate. The stones tend to be gray, brown, or 
tan and rarely grow larger than 1 to 2 cm.78,178,179
The main causes of calcium stone formation are hypercalci­
uria (excessive urinary calcium excretion), hyperoxaluria (exc

ive urinary calcium excretion), hyperoxaluria (excessive 
oxalate excretion), hyperuricosuria (excessive uric acid excretion), 
hypocitraturia (insufficient citrate excretion), RTA, congenital 
abnormalities of the genitourinary tract, and certain medications 
(see Tables 32.2 and 32.3).
Hypercalciuria. Patients with persistent hypercalciuria often 
benefit from a thiazide diuretic. This class of drugs is inexpen­
sive and extremely effective at reducing urinary calcium excretion 
and stone form

reducing urinary calcium excretion 
and stone formation.44,78,180 To maximize the efficacy of thiazides, 
patients must consume a sodium-restricted diet. Urinary cal­
cium excretion parallels sodium excretion (Fig. 32.6), and reduc­
ing sodium consumption is essential to reducing hypercalciuria. 
Whereas hydrochlorothiazide is commonly used for hypertension, 
chlorthalidone is favored for treating hypercalciuria because it has 
a longer half-life and requires only once-daily dosing. The starting

 and requires only once-daily dosing. The starting 
dose is 25 mg and can be increased to 50 mg. In petite patients 
or those with low blood pressure, therapy can be initiated with 
12.5 mg.
Side effects of thiazides include an increase in serum lipid lev­
els and hyperglycemia. For patients in whom this is a concern, 
such as those with hypercholesterolemia, other cardiac risk factors, 
or elevated blood glucose levels, indapamide (1.25–2.5 mg) is an 
effective alternative.181 This agent has le

is an 
effective alternative.181 This agent has less of an effect on serum 
lipids and blood sugar than thiazides.
Hypokalemia is another common side effect of thiazide therapy. 
Patients should be advised to increase their dietary intake of potas­
sium-rich foods, and the potassium level should be checked 7 to 
10 days after starting the medication. Hypokalemia can result not 
only in cardiac and neuromuscular problems but also in hypoci­
traturia—another risk factor for stone formation. The su

ia—another risk factor for stone formation. The supplement 
of choice is potassium with a base, such as citrate or bicarbonate, 
as the accompanying anion. Potassium citrate is available as a liq­
uid or as a wax-matrix tablet. The wax-matrix form is preferable 
because many patients find the liquid unpalatable. Patients with 
malabsorption disorders, however, may absorb potassium citrate 
better in the liquid form. Potassium citrate in the wax-matrix for­
mulation is available as 5- and 10-mEq 

atrix for­
mulation is available as 5- and 10-mEq tablets; 20 to 40 mEq/day 
in single or divided doses is usually adequate supplementation. 
Determination of follow-up potassium and bicarbonate levels 
may be required for further dose adjustment. Because citrate is 
a base, metabolic alkalosis can result with this medication, espe­
cially when given with a thiazide diuretic. In such a situation, an 
alternative potassium supplement (e.g., potassium chloride) may 
be necessary. If hypokalemia pe

ium chloride) may 
be necessary. If hypokalemia persists or if large doses of supple­
mental medication are required, the patient might benefit from 
the addition of a potassium-sparing diuretic. Triamterene is gener­
ally avoided because it can precipitate into stones. Amiloride may 
be initiated at a starting dose of 5 mg or in a combination tablet 
with thiazide.
After 4 weeks of therapy with the new medication, the 24-hour 
urine collection should be repeated to assess the efficacy of ther­

hould be repeated to assess the efficacy of ther­
apy in reducing calcium levels. The 24-hour urinary sodium and 
citrate levels should also be measured. The thiazide dose may need 
to be increased to decrease calcium excretion to less than 3 to 4 
mg/kg per day. If sodium excretion remains high in conjunction 
with elevated urinary calcium excretion, further dietary counsel­
ing aimed at reducing dietary sodium may be required. Additional 
No clearance (mL/min)
Urine pH 7.3–7.8
Urine pH 6.9–7.2

earance (mL/min)
Urine pH 5.4–6.8
Urine pH not measured
Ca clearance (mL/min)
6
5
3
1
0
2
4
12
6
8
10
4
2
0
• Fig. 32.6  Relationship between urinary calcium and sodium excre­
tion. This study shows how urinary calcium excretion parallels urinary 
sodium excretion. There is almost a linear relationship between the urinary 
excretion of calcium and sodium, making sodium restriction in the diet 
imperative in the treatment of hypercalciuria. (From Walser M. Calciu

reatment of hypercalciuria. (From Walser M. Calcium 
clearance as a function of sodium clearance in the dog. Am J Physiol. 
1961;200:1099–1104.)
1331
potassium citrate may be required if urinary citrate or serum 
potassium levels remain low.78,182 
Hyperoxaluria. Oxalate is produced predominantly by endog­
enous metabolism of glyoxylate and, to a lesser extent, by ascorbic 
acid. Some urinary oxalate is derived from dietary sources, such 
as rhubarb, cocoa, nuts, tea, a

ary sources, such 
as rhubarb, cocoa, nuts, tea, and certain leafy green vegetables. 
Absorbed oxalate is excreted unchanged in the urine and raises uri­
nary supersaturation with respect to calcium oxalate.72,134,183,184 
Hyperoxaluria, as the sole metabolic abnormality, accounts for the 
formation of only approximately 5% of all calcium-based stones, 
even though it is often present with other urinary abnormalities 
that lead to an increase in supersaturation.37,185,186
The three main causes o

supersaturation.37,185,186
The three main causes of hyperoxaluria are excessive oxalate 
ingestion (dietary oxaluria), malabsorptive GI disorders (enteric 
oxaluria), and excessive endogenous production of oxalate related 
to a hepatic enzyme deficiency (primary hyperoxaluria).
Because ethylene glycol (used as antifreeze in automobiles) is 
metabolized to oxalate, nephrolithiasis, in conjunction with severe 
metabolic acidosis and renal failure, is often observed in patients 
after ingestion of 

is often observed in patients 
after ingestion of ethylene glycol.8
Dietary Oxaluria. Dietary oxaluria results in urinary oxalate 
levels that are mildly elevated (40–60 mg/day). Many high-oxalate 
foods are fruits, vegetables, and nuts that are generally considered 
beneficial in most diets. In a retrospective analysis, patients con­
suming diets similar to the DASH diet have fewer stones than 
those who consume diets that are markedly different despite the 
DASH diet being high in oxalate.187 

 despite the 
DASH diet being high in oxalate.187 The diet is also high in potas­
sium and calcium and low in sodium, which are factors that may 
be more preventive in stone formation than oxalate is detrimen­
tal. Patients with dietary hyperoxaluria should be provided with 
a detailed list of high-oxalate foods to review (Table 32.6). How 
restrictive patients need to be with regard to the list is guided by 
urinary supersaturation and common sense, especially because 
many patients with stones

nse, especially because 
many patients with stones may also have hypertension and diabe­
tes mellitus and benefit from a diet high in fruits and vegetables. 
Patients should be instructed to ingest calcium-containing foods, 
such as a glass of milk, when eating foods high in oxalate. The 
calcium in milk binds the dietary oxalate and may prevent its 
absorption.37,42 In patients who have severe dietary hyperoxaluria 
with active stone disease, two or three calcium carbonate tablets 
(500–650 mg/

o or three calcium carbonate tablets 
(500–650 mg/tablet) may be prescribed with high-oxalate meals. 
However, this should be done cautiously given the association 
between calcium supplements and kidney stones in women in the 
general population (see the earlier Nonspecific Therapy section). 
Enteric Oxaluria. Enteric oxaluria results in higher urinary oxa­
late levels (60–100 mg/day) than dietary hyperoxaluria. GI malab­
sorptive conditions associated with normal colonic function, such 
as Cro

ociated with normal colonic function, such 
as Crohn disease, celiac sprue, jejunoileal bypass, chronic pancre­
atitis, and biliary obstruction, can lead to enteric oxaluria. In these 
disorders, malabsorbed fatty acids bind calcium in the intestinal 
lumen, making more “free” oxalate available for absorption in the 
colon. In addition, the colonic mucosa becomes more permeable 
to oxalate as a result of exposure to malabsorbed bile salts.188–190
The mainstay of treatment, whenever possible, is 

The mainstay of treatment, whenever possible, is therapy for the 
underlying disorder. A gluten-free diet, for example, can significantly 
reduce hyperoxaluria associated with sprue. For other conditions 
(e.g., surgical short bowel syndrome), no specific therapy is feasible. 
In such cases, reduction of malabsorption and oxalate absorption 
may be achieved by instituting general therapy for steatorrhea, such 
as a low-fat diet, cholestyramine, and medium-chain triglycerides. 
As in patients wi

and medium-chain triglycerides. 
As in patients with dietary oxaluria, an oxalate-restricted diet and 
calcium carbonate with meals should be prescribed.42,191 Because 
of chronic diarrhea, these patients are at substantial risk for low 
urine volumes, hypocitraturia, hypokalemia, and hypomagnesuria. 
The acidic, concentrated urine also predisposes to development of 
uric acid stones.192,193 Additional fluid intake must be stressed, and 
potassium citrate (the liquid form is generally better abs

m citrate (the liquid form is generally better absorbed but 
poorly tolerated in these patients) and magnesium supplementation 
are often prescribed. Magnesium appears to be an inhibitor of stone 
formation and is supplied as magnesium oxide at 400 mg by mouth 
two times a day or magnesium gluconate at 0.5 to 1.0 g by mouth 
three times a day.194 
Primary Hyperoxaluria. Primary hyperoxaluria (PHO) leads 
to nephrolithiasis because of hepatic enzyme deficiencies that lead 
to massive endogenous o

me deficiencies that lead 
to massive endogenous oxalate production and excretion in these 
patients.135,195–198 PHO results not only in severe hyperoxaluria 
(80–300 mg/day) but also in widespread deposition of oxalate 
in numerous organs and tissues such as the heart, bone marrow, 
muscle, and renal parenchyma at a young age. Cardiomyopathy, 
bone marrow suppression, and renal failure can ensue. In type 
1 PHO (80% of cases), the deficient hepatic enzyme is alanine 
glyoxylate aminotransferase

tic enzyme is alanine 
glyoxylate aminotransferase (AGT), and deficiency is caused by 
one of several mutations found in the AGT gene, AGXT. In some 
patients with type 1 PHO, pyridoxine (vitamin B6) can increase 
enzyme activity, thereby reducing oxalate production. In type 2 
PHO (10% of cases) patients lack D-glycerate reductase and gly­
oxylate reductase due to mutations in the GRHPR gene. PHO 
type 3 is due to a defect in the HOGA1 gene that encodes for 
mitochondrial 4-hydroxy-2-oxoglutara

 encodes for 
mitochondrial 4-hydroxy-2-oxoglutarate aldolase and makes up 
the remaining cases of PHO.199
All patients with PHO should be treated with measures that 
reduce calcium oxalate supersaturation, such as ample fluid sup­
plementation, potassium citrate, magnesium, and orthophos­
phate.197 Orthophosphate is an effective inhibitor of calcium 
oxalate crystallization but should be avoided in patients with a 
GFR less than 50 mL/minute. The combination of pyridoxine 
and orthophosphate im

e combination of pyridoxine 
and orthophosphate improved renal survival at 20 years from 20% 
	
Foods High in Oxalate
Beans (green and dried)
Beer (draft, stout, lager, pilsner)
Beets
Berries (blackberries, blueberries, raspberries, strawberries, juice made 
from berries)
Black tea
Black pepper
Celery
Chocolate, cocoa
Eggplant
Figs, dried
Greens (collard greens, dandelion greens, endive, escarole, kale, leeks, 
mustard greens, parsley, sorrel, spinach, Swiss chard, watercress)
Green peppers
Lem

pinach, Swiss chard, watercress)
Lemon, lime, and orange peel
Nuts
Pecans, peanuts, peanut butter
Okra
Rhubarb
Sweet potato
Tofu
   
TABLE 32.6
1332
to 74% in patients with PHO I and II.200,201 Oxalobacter formin­
genes is an enteric bacteria that re

acter formin­
genes is an enteric bacteria that relies on oxalate for its metabo­
lism.144,202 In small studies, probiotic supplementation of these 
bacteria to patients with PHO type 1 resulted in a slight reduc­
tion in urinary oxalate excretion.203,204 Provision of these bacteria, 
if they become commercially available, may provide additional 
therapy in the treatment of PHO. PHO patients with renal fail­
ure might benefit from renal transplantation because dialysis is 
not as effective as a 

tation because dialysis is 
not as effective as a functioning kidney in oxalate removal. The 
general measures to treat PHO should be continued after renal 
transplantation to prevent rapid loss of the allograft caused by 
calcium oxalate deposition. Ultimately, for patients with type 1 
PHO, liver transplantation can supply the missing AGT and is 
curative, especially if it is performed before the development of 
end-stage renal failure. Some patients require combined liver and 
kidney transpla

tients require combined liver and 
kidney transplantation.135,195,205 
Hyperuricosuria. Up to 15% of patients with hyperuricosuria 
have calcium stones. In contrast to patients with pure calcium 
oxalate stones, these patients typically have elevated urinary uric 
acid levels but normal urinary calcium and oxalate levels.206–208 
They also differ from patients with pure uric acid stones in that 
they tend to have a higher urinary pH.
The mechanism by which uric acid promotes calcium stone 
forma

m by which uric acid promotes calcium stone 
formation is unclear. The terms heterogeneous nucleation or epitaxy 
are used to describe the preferential formation of calcium oxa­
late crystals around a lattice of uric acid crystals present in the 
urine.72,209,210 More recently, this mechanism has come into ques­
tion. Grover and associates211,212 have shown that the addition of 
sodium urate to urine or similar solutions increases calcium oxalate 
crystallization, with denser, more aggregated de

crystallization, with denser, more aggregated deposits, without the 
presence of urate crystals, and with no increase in calcium oxalate 
supersaturation. They attribute this to “salting out”—a process 
in which the solubility of electrolytes (or salts) in a solution is 
reduced (or the ion activity increased) by the addition of differ­
ent electrolytes/salts. As such, the activity coefficient of calcium 
and oxalate would be increased not only by the concentrations of 
calcium and oxalate in 

 by the concentrations of 
calcium and oxalate in the urine but also the urate concentration. 
This theory would explain why allopurinol is often an effective 
therapy for recalcitrant calcium oxalate nephrolithiasis, even in 
the absence of hyperuricosuria.213,214 Another potential mecha­
nism (but not borne out by studies) is that urate may reduce the 
concentration or the activity of urinary stone inhibitors.215–217
Whatever the mechanism, uric acid in the form of sodium urate 
is important i

c acid in the form of sodium urate 
is important in calcium oxalate crystal formation. Therapy has 
generally consisted of dietary purine restriction and increased fluid 
intake. If urinary uric acid levels remain uncontrolled with these 
measures, allopurinol, 100 to 300 mg/day, may be added.206,207 
Hypocitraturia. Citrate, by combining with calcium to form a 
soluble complex, reduces calcium oxalate and calcium phosphate 
precipitation, thus acting as the most important inhibitor of cal­
cium

cting as the most important inhibitor of cal­
cium crystallization in urine.8,72,177 In some patients, hypocitratu­
ria is the principal metabolic abnormality found in the 24-hour 
urine collection. Risk factors for hypocitraturia include high pro­
tein intake, hypokalemia, metabolic acidosis, exercise, infection, 
starvation, and therapy with androgens or acetazolamide. Men 
tend to have lower urinary citrate concentrations than women, 
which may be responsible for the higher incidence of stone

y be responsible for the higher incidence of stone for­
mation in men. Furthermore, women with nephrolithiasis have 
lower urinary citrate concentrations than non–stone-forming 
women.218 Although citrate excretion below 320 mg/L per day of 
urine is defined as hypocitraturia, the risk of nephrolithiasis is a 
continuous function of urinary citrate concentration.219
Along with therapy for the underlying condition, such as 
moderating dietary protein intake, potassium citrate or potas­
sium-magne

ein intake, potassium citrate or potas­
sium-magnesium citrate is prescribed, and both formulations are 
effective in preventing calcium stones, even in patients who are 
not hypocitraturic.220 The potassium salt is preferable to sodium 
citrate because sodium excretion promotes calcium excretion in 
parallel and leads to hypercalciuria. Again, potassium citrate in 
the wax-matrix formulation is preferred to the liquid preparation 
because of increased palatability. Large amounts may be required

reased palatability. Large amounts may be required 
(30–75 mEq/day) in divided doses to raise the urinary citrate con­
centration to more than 320 mg/day. Potassium and bicarbon­
ate levels should be closely monitored, especially in patients with 
CKD. If metabolic alkalosis or hyperkalemia ensues, reduction of 
the dose may be necessary.221,222 
Renal Tubular Acidosis. dRTA (type 1) is a disorder in which 
distal tubular hydrogen ion excretion is impaired, resulting in 
a non–anion gap metaboli

s impaired, resulting in 
a non–anion gap metabolic acidosis and a persistently alkaline 
urine.223,224 The acidosis leads to calcium and phosphate release 
from bone, as well as enhanced proximal tubular reabsorption of 
citrate and diminished tubular reabsorption of calcium.8 The net 
result is an increased filtered load and excretion of calcium and 
phosphate, severe hypocitraturia, and an elevated urinary pH, all 
of which promote calcium phosphate precipitation. Nephrocalci­
nosis, or renal

sphate precipitation. Nephrocalci­
nosis, or renal parenchymal calcification, is frequently seen in this 
setting.6
The 24-hour urinary citrate levels are commonly lower than 
100 mg in patients with dRTA. Therapy consists of potassium 
citrate or potassium bicarbonate supplementation to treat both 
the metabolic acidosis and hypocitraturia. Large doses of these 
medications are often required: 1 to 3 mEq/kg per day in two or 
three divided doses.178,195,206 
Nephrocalcinosis. Nephrocalcinosis i

.178,195,206 
Nephrocalcinosis. Nephrocalcinosis is a process in which cal­
cium is deposited in the renal parenchyma.6 There are two forms: 
dystrophic calcification and metastatic calcification.
In dystrophic calcification, calcium deposition arises from tis­
sue necrosis secondary to neoplasm, infarction, or infection. It 
may be seen in the setting of renal transplant rejection, renal cor­
tical necrosis, chronic glomerulonephritis, ethylene glycol toxic­
ity, acquired immunodeficiency syndr

glycol toxic­
ity, acquired immunodeficiency syndrome–related infections, and 
Alport syndrome. In general, in dystrophic calcification, serum 
calcium and phosphorus levels are normal, and calcium phosphate 
deposition predominantly occurs in the renal cortex.
In metastatic calcification, patients often have elevated serum 
calcium and phosphate levels or an elevated urinary pH. Calcifica­
tion in this setting occurs more commonly in the renal medulla. 
Common causes include RTA; primary hyperp

edulla. 
Common causes include RTA; primary hyperparathyroidism (or 
any disorder resulting in elevated serum calcium levels); medullary 
sponge kidney; papillary necrosis; PHO; hereditary hypophospha­
temic rickets; and administration of acetazolamide, amphotericin 
B, oral sodium phosphate bowel preparations, and triamterene. 
PHO can result in both medullary and cortical calcifications.
Both medullary and cortical parenchymal calcifications are 
easily noted with ultrasonography and CT scanni

e 
easily noted with ultrasonography and CT scanning, even before 
they can be detected on plain radiographs. Therapy consists of 
treating the underlying disorder whenever possible. Otherwise, 
measures aimed at reducing hypercalcemia, oxalosis, and hyper­
phosphatemia should be attempted.38,114,225 
Uric acid lithiasis is far more common in Mediterranean coun­
tries than in the United States. However, the incidence of uric 
acid stones in the United States appears to be rising

d stones in the United States appears to be rising in parallel 
1333
with the epidemic of obesity. Obesity and the metabolic syndrome 
are associated with insulin resistance, which results in a very low 
urine pH.30,226–228 Uric acid stones tend to be round, smooth, and 
yellow-orange. Because they are radiolucent, they are not visible 
on plain films but can be detected by ultrasonography or CT or as 
filling defects on IVP. Uric acid is a purine metabolite and is also

 IVP. Uric acid is a purine metabolite and is also 
found in large quantities within cells. Most patients with uric acid 
stones have a reduced urinary pH, whereas other less common 
causes are low urine volume or elevated urinary uric acid levels. 
Factors associated with uric acid stones are listed in Table 32.3.
A low urine pH is the major cause of uric acid nephropathy. 
The solubility of uric acid increases sixfold with an increase in 
urine pH from 5.3 to 6.5207,226 (Fig. 32.7). Thus, cond

 pH from 5.3 to 6.5207,226 (Fig. 32.7). Thus, conditions that 
lower urine pH tend to predispose patients to uric acid lithiasis. 
Loss of bicarbonate during chronic diarrheal diseases and the 
higher acid load of diets rich in animal protein lead to acidemia 
and contribute to an acidic urinary pH. In four studies, every 
patient with uric acid stones had a urine pH less than 6.0.229–232 A 
low urinary pH leads to the formation of poorly soluble uric acid 
as opposed to the more soluble urate a

 uric acid 
as opposed to the more soluble urate anion, thus predisposing to 
uric acid lithiasis, even when the total amount of uric acid being 
excreted is not above normal.233
Uric acid stone formers often have greater body weight and 
a higher incidence of insulin resistance and type 2 diabetes mel­
litus. Most of these patients have significantly lower urinary pH 
compared with non–uric acid stone formers. It is thought that 
insulin resistance leads to impaired ammoniagenesis and ammo­
niu

nce leads to impaired ammoniagenesis and ammo­
nium excretion, resulting in excretion of more urinary hydrogen 
ions with anions other than ammonia and at a lower urinary 
pH.29,30,226,230
Hyperuricosuria may be evident in patients who ingest large 
quantities of dietary purine or animal protein. Foods high in 
purine include organ meats, shellfish, certain fish, meat extracts, 
yeast, gravy, and stock (Table 32.7). Hyperuricemic disorders such 
as gout, myeloproliferative disorders, tumor lysis

as gout, myeloproliferative disorders, tumor lysis syndrome, and 
certain inborn errors of metabolism (e.g., glucose-6-phosphatase 
deficiency, Lesch-Nyhan syndrome) can also contribute to an 
increased urinary filtered load of uric acid. Certain medications 
such as salicylates and probenecid are hyperuricosuric as well and 
can predispose patients to uric acid lithiasis.6,7,30,143,225
Therapy for patients with uric acid stones begins with nonspe­
cific measures such as increasing fluid intake 

e­
cific measures such as increasing fluid intake to maintain urine 
volume at about 3 L/day. A lowered animal protein diet is gener­
ally beneficial because the decreased endogenous acid production 
raises urinary pH.51 Ideally, the urinary pH should be elevated to 
approximately 6.5 to 7.0, a level that can dissolve existing crystals 
and stones. However, care should be taken to prevent the urine 
pH from rising above 7.0 to minimize the risk of calcium phos­
phate lithiasis. A low-fructose di

f calcium phos­
phate lithiasis. A low-fructose diet may also be beneficial in reduc­
ing uric acid levels and hyperuricosuria (see the discussion in the 
Nonspecific Therapy section).58
Potassium citrate at doses of 30 mEq by mouth two times a 
day or more may be required to raise the urinary pH sufficiently to 
decrease supersaturation with respect to uric acid (see “Hypercalci­
uria” and “Hypocitraturia” on available potassium citrate prepara­
tions). If the urinary pH cannot be raised adequa

tions). If the urinary pH cannot be raised adequately despite high 
doses of potassium citrate or if the dose prescribed results in hyper­
kalemia, the carbonic anhydrase inhibitor acetazolamide may be 
initiated. Use of this medication results in an alkaline urine and 
mild systemic metabolic acidosis, a pattern similar to that in type 1 
RTA. The urinary pH should be maintained at less than 7.0 to avoid 
calcium phosphate precipitation.206 Prescription of nitrazine paper 
allows patients to m

scription of nitrazine paper 
allows patients to monitor the urinary pH at various times of day 
and adjust their potassium citrate intake accordingly.
Patients with hyperuricemia are prescribed a low-purine diet to 
decrease uric acid production. Despite dietary intervention, hyper­
uricemia often persists, especially in patients with disorders of cel­
lular metabolism. In this setting, allopurinol should be prescribed 
at a starting dose of 100 mg/day, increasing to 300 mg/day as 
needed.72,23

 mg/day, increasing to 300 mg/day as 
needed.72,234 Although sodium bicarbonate can effectively alkalin­
ize the urine, it should be avoided because the additional sodium 
excretion encourages sodium urate formation, which may result in 
further crystal formation (see the earlier Hyperuricosuria section). 
Struvite stones are also called triple phosphate stones, magnesium 
ammonium phosphate stones, and infection stones.8 Although they 
make up only about 10% to 15% of all stones

 they 
make up only about 10% to 15% of all stones formed, most stag­
horn calculi (large stones that extend beyond a single renal calyx) are 
composed of struvite. The propensity of these stones to grow rapidly 
to a large size, to recur despite therapy, and to result in significant 
1400
1200
1000
800
600
400
200
200
0
Total uric acid (mg/L)
Urinary pH
400
600
800
1000
1200
0
4.5 5.0 5.5 6.0 6.5 7.0 7.5
Undissociated uric acid (mg/L)
• Fig. 32.7  The pH and solubility of uric acid. The relatio

7  The pH and solubility of uric acid. The relationship between 
undissociated uric acid, total uric acid, and urinary pH is presented. The 
limit of solubility of undissociated uric acid is depicted by the colored line 
(∼100 mg/L). Two hypothetical urine pHs are considered (two arrows). At 
low urinary pH (e.g., 5.0), even a modest amount of total urinary uric acid 
will exceed its solubility. At high urine pH (e.g., 6.5), even massive hyper­
uricosuria is well tolerated. (From Maalouf NM, Cam

ricosuria is well tolerated. (From Maalouf NM, Cameron MA, Moe OW, 
Sakhaee K. Novel insights into the pathogenesis of uric acid nephrolithia­
sis. Curr Opin Nephrol Hypertens. 2004;13:181–189.)
	
Foods High in Purine
Organ meats: brain, heart, kidney, liver, sweetbreads
Meat extracts: bouillon, consommé, stock, gravy
Meat: beef, chicken, goose, lamb, pork
Shellfish: clams, mussels, scallops, shrimp, oysters
Fish: anchovies, fish roe, herring, mackerel, sardines, and others
Certain vegetables: 

ackerel, sardines, and others
Certain vegetables: asparagus, cauliflower, kidney beans, lentils, lima 
beans, mushrooms, peas, spinach
   
TABLE 32.7
1334
morbidity (and potential mortality) has also led to the appellation 
“stone cancer.” Infection with urease-pr

ellation 
“stone cancer.” Infection with urease-producing bacteria must be 
present for these stones to form, and therefore severe renal infec­
tions, as well as sepsis and loss of renal function, can develop. Fac­
tors associated with struvite stones are listed in Table 32.3.
In contrast to other stone types, struvite stones occur with a 
higher incidence in women than in men, largely because of the 
increased susceptibility of women to urinary tract infections.235 
Other groups at risk for dev

tract infections.235 
Other groups at risk for development of struvite stones because 
of urinary stasis or infection include elderly people and patients 
with neurogenic bladders, indwelling urinary catheters, spinal 
cord lesions, or genitourinary abnormalities. Even without a stone 
analysis, struvite stones should be suspected in patients with large 
stones, an alkaline urinary pH (∼7.0), and the presence of urease-
producing urinary bacteria. Early detection and therapy are essen­
tial to p

. Early detection and therapy are essen­
tial to prevent great potential morbidity.145
Urease-Producing Bacteria. The formation of struvite stones 
depends on the presence of both ammonium ions and an alkaline 
urinary pH—conditions met clinically only through the actions 
of urease-producing bacteria. Ammonium, magnesium, and car­
bonate apatite [Ca10(PO4)6CO3] in the urine combine with phos­
phate, which is present in its trivalent form in this setting.8
Numerous bacteria, both gram negative a

 setting.8
Numerous bacteria, both gram negative and gram positive, 
as well as Mycoplasma and yeast species, have been implicated in 
urease production. Bacteria species in which urease is frequently 
isolated include Proteus, Haemophilus, Corynebacterium, and Urea­
plasma. Escherichia coli, despite its frequent role as a urinary tract 
pathogen, has not been shown to produce urease and thus is not 
implicated in the genesis of struvite stones. Urease production 
adequate to stimulate stone for

Urease production 
adequate to stimulate stone formation may be present despite low 
bacterial colony counts. For this reason, the microbiology labora­
tory should be asked specifically to perform identification of bac­
teria and determine sensitivities even if colony counts are lower 
than 100,000 colony-forming units. If no bacteria are isolated but 
a urease producer is suspected, special cultures should be ordered 
for Ureaplasma urealyticum, which is a mycobacterium that tends 
to be a fast

 which is a mycobacterium that tends 
to be a fastidious grower on regular culture media.236 
Therapy for Struvite Stones. To eradicate struvite stones, early 
and aggressive medical and urologic management is required.237 
Appropriate antibiotic therapy is essential but must be combined 
with long-term bacterial suppression and complete surgical or 
medical stone removal. ESWL is often adequate for fragmentation 
of stones smaller than 2 cm, but percutaneous nephrostolithot­
omy, or a combinati

 percutaneous nephrostolithot­
omy, or a combination of the two procedures, is usually required 
for larger stones.238 Antibiotics should be continued on the basis 
of cultures of any stone fragments retrieved. After approximately 
2 weeks of antibiotic therapy, when the urine culture is sterile, the 
dose of antibiotic should be halved. Suppressive antibiotics should 
continue at this dose until monthly surveillance cultures remain 
sterile for 3 consecutive months. At this point, antibiotics m

3 consecutive months. At this point, antibiotics may be 
discontinued as long as surveillance urine cultures are obtained 
monthly for 1 year.236,239
In addition to antimicrobial therapy, medical treatment may 
involve urease inhibition and chemolysis. In chemolysis, the kid­
ney is irrigated with an acidic solution through a nephrostomy 
tube or ureteral catheter. Although rarely used today with the 
advent of less invasive surgical techniques, this procedure may be 
useful in the dissolution o

this procedure may be 
useful in the dissolution of residual stone fragments. Ten percent 
hemiacidrin, the solution most commonly used, is composed of 
carbonic acid, citric acid, d-gluconic acid, and magnesium at a 
pH of 3.9. The use of chemolysis has been controversial because 
high mortality rates have been reported in the past.240,241 The 
morbidity and mortality were mainly due to sepsis from instru­
mentation, local bacterial or fungal infections, and uroepithelial 
irritation rather tha

fections, and uroepithelial 
irritation rather than toxicity from the agents. When used as an 
adjuvant to surgical removal, chemolysis leads to lowered stone 
and infection recurrence rates.240,242,243 The safety of the proce­
dure remains in question due to the variety of techniques, stone 
burden, and comorbidities reported in older literature, but with 
close monitoring of serum magnesium levels, intrapelvic pres­
sures, infection, and obstruction to flow, it may have a supporting 
role in t

ction to flow, it may have a supporting 
role in the treatment of large struvite stones.236,240,244
Urease inhibition has been shown to retard stone growth and 
prevent new stone formation.245,246 It does not decrease bacterial 
counts and cannot eradicate existing stones. Combined with anti­
microbial therapy, it serves primarily as palliative care for patients 
who cannot undergo definitive surgical management. The agent 
most commonly used is acetohydroxamic acid (AHA). These 
medications req

acetohydroxamic acid (AHA). These 
medications require adequate renal clearance for therapeutic effi­
cacy and are contraindicated in patients with a GFR less than 60. 
CKD increases the incidence of side effects of these medications, 
which are numerous and limit their use. Side effects that result 
in discontinuation of the drug include neurologic symptoms, GI 
upset, hair loss, hemolytic anemia, and rash. Fortunately, the side 
effects all resolve with discontinuation of the drug. AHA is also

olve with discontinuation of the drug. AHA is also 
teratogenic. The starting dose of AHA is 250 mg by mouth two 
times a day. If it is well tolerated for about 1 month, the dose is 
increased to 250 mg by mouth three times a day.236 
Cystinuria, which is not to be confused with the more serious and 
debilitating disorder cystinosis that results in extensive intracel­
lular cystine accumulation, is an autosomal disorder that may be 
recessive or dominant with incomplete penetrance

recessive or dominant with incomplete penetrance.247 The disor­
der is due to mutations of the SL3A1 gene on chromosome 2 or to 
mutations of the SCLC7A9 gene on chromosome 19, both result­
ing in decreased renal tubular reabsorption and excessive urinary 
excretion of the dibasic amino acids cystine, ornithine, lysine, and 
arginine.248 The genetic defect would probably go unnoticed were 
it not for the low solubility of cystine of approximately 300 mg/L. 
Factors associated with cystine ston

ly 300 mg/L. 
Factors associated with cystine stones are listed in Table 32.3.
People with no tubular defect in cystine transport excrete 
approximately 30 to 50 mg of cystine per day that dissolves in 
the normal daily urine volume. Patients heterozygous for this 
condition excrete about 400 mg/day, whereas homozygotes often 
excrete more than 600 mg/day.138 Even though the solubility of 
cystine increases significantly when urine pH is greater than 6.5, 
the excessive amounts produced in this 

than 6.5, 
the excessive amounts produced in this condition leads to cystine 
crystals precipitating and aggregating as cystine stones.
Stones usually develop in patients within the second or third 
decade of life. The stones can grow to a large size and can appear as 
staghorn calculi or multiple stones. They are radiopaque because 
of the sulfur content of cystine. The disease should be suspected 
in any patient with stone onset in childhood, frequent recurrence 
of nephrolithiasis, and a stro

requent recurrence 
of nephrolithiasis, and a strong family history of the disease. The 
presence of the classic hexagonal cystine crystals in the urine can 
verify the diagnosis. Because these crystals might not be evident 
in dilute or alkaline urine, qualitative screening with the sodium 
nitroprusside test better confirms the presence of cystinuria at a 
concentration greater than 75 mg/L. Quantitative cystine mea­
sures with a 24-hour urine sample should follow to determine the 
risk of sto

sample should follow to determine the 
risk of stone formation and to guide therapy.
Therapy for Cystine Stones. The aim of treatment is to lower 
the urinary cystine concentration below the limits of solubility 
1335
(∼300 mg/L). Patients are advised to drink large quantities of flu­
ids. A patient with a cystine excretion of 750 mg/day, for example, 
should ideally drink enough fluid to increase urine output to more 
than 3 L/day. Large quantities of milk should be av

han 3 L/day. Large quantities of milk should be avoided because 
dairy products and foods high in protein contain large amounts 
of methionine, an essential amino acid that is a precursor of cys­
tine.249 Because cystine is more soluble at a higher pH, juices are 
encouraged because they tend to alkalinize the urine. Potassium 
citrate (see the Hypercalciuria and Hypocitraturia sections for 
details) is also prescribed to maintain the urinary pH between 6.5 
and 7.0.250
Approximately 50% of cyst

between 6.5 
Approximately 50% of cystine stones are mixed stones. Patients 
with cystinuria often have other metabolic defects such as hyper­
calciuria, hypocitraturia, and hyperuricosuria. Therefore, a com­
plete 24-hour urine collection for all stone-forming elements is 
necessary to treat nephrolithiasis fully in this setting. Restricting 
dietary sodium is also beneficial.251
If these measures are inadequate in controlling stone forma­
tion or if the urinary cystine concentratio

forma­
tion or if the urinary cystine concentration is too high to make 
adequate fluid intake practical, chelating agents called cystine-
binding thiol drugs (CBTDs) may be added. Increasing urine pH 
increases the efficacy of thiol drugs.250 d-Penicillamine is a CBTD 
that reduces the cystine concentration by forming a more soluble 
compound with cystine.252 However, this medication is associ­
ated with numerous serious side effects that limit its use. Other 
CBTDs, such as tiopronin, α-mercap

its use. Other 
CBTDs, such as tiopronin, α-mercaptoproprionyl glycine, and 
bucillamine, are now available that reduce the cystine concentra­
tion with fewer side effects.138,253–255
         
1336.e1
	 1.	 Brener ZZ, Winchester JF, Salman H, Bergman M. Nephrolithia­
sis: evaluation and management. South Med J. 2011;104:133–139.
	 2.	 Saigal CS, Joyce G, Timilsina

ces in idiopathic calcium nephrolithiasis: a 5-year random­
ized prospective study. J Urol. 1996;155:839–843.
	 42.	 Lemann Jr J, Pleuss JA, Worcester EA, et al. Urinary oxalate excre­
tion increases with body size and decreases with increasing dietary 
calcium intake among healthy adults. Kidney Int. 1996;49:200–
208 (Erratum: Kidney Int 50:341,1996).
	 43.	 Xu C, Zhang C, Wang X-L, et al. Self-fluid management in preven­
tion of kidney stones: a PRISMA-compliant systematic review and 
dose–r

 Snyder W. The hypercalciurias: 
causes, parathyroid functions, and diagnostic criteria. J Clin Invest. 
1974;54:387–400.
	 78.	 Monk RD, Bushinsky DA. Pathogenesis of idiopathic hypercalci­
uria. In: Coe F, Favus M, Pak C, Parks J, Preminger G, eds. Kidney 
Stones: Medical and Surgical Management. Philadelphia: Lippincott-
Raven; 1996:759–772.
	 79.	 Krieger NS, Asplin JR, Frick KK, et al. Effect of potassium citrate 
on calcium phosphate stones in a model of hypercalciuria. J Am Soc 
Nephrol

es in a model of hypercalciuria. J Am Soc 
Nephrol. 2015;26:3001–3008.
	 80.	 Ng AH, Frick KK, Krieger NS, et al. 1,25(OH)2D3 induces a min­
eralization defect and loss of bone mineral density in genetic hyper­
calciuric stone-forming rats. Calcif Tissue Int. 2014;94:531–543.
	 81.	 Frick KK, Asplin JR, Culbertson CD, et  al. Persistence of 
1,25(OH)2D3-induced hypercalciuria in alendronate-treated 
genetic hypercalciuric stone-forming rats fed a low-calcium diet. 
Am J Physiol. 2014;306:F1081

al. Isolation and confirma­
tion of a calcium excretion quantitative trait locus on chromosome 
1 in genetic hypercalciuric stone-forming congenic rats. J Am Soc 
Nephrol. 2006;17:1292–1304.
	 96.	 Li XQ, Tembe V, Horwitz GM, et al. Increased intestinal vitamin 
D receptor in genetic hypercalciuric rats. A cause of intestinal cal­
cium hyperabsorption. J Clin Invest. 1993;91:661–667.
	 97.	 Bushinsky DA, Parker WR, Asplin JR. Calcium phosphate super­
saturation regulates stone formation in gen

super­
saturation regulates stone formation in genetic hypercalciuric 
stone-forming rats. Kidney Int. 2000;57:550–560.
	 98.	 Karnauskas AJ, van Leeuwen JP, van den Bemd GJ, et al. Mechanism 
and function of high vitamin D receptor levels in genetic hypercal­
ciuric stone-forming rats. J Bone Miner Res. 2005;20:447–454.
	 99.	 Favus MJ, Karnauskas AJ, Parks JH, Coe FL. Peripheral blood mono­
cyte vitamin D receptor levels are elevated in patients with idiopathic 
hypercalciuria. J Clin Endocr

tionship of spontaneous 
passage of ureteral calculi to stone size and location as revealed by 
unenhanced helical CT. Am J Roentgenol. 2002;178:101–103.
	123.	 Kummer AE, Grams M, Lutsey P, et al. Nephrolithiasis as a risk 
factor for CKD: the atherosclerosis risk in communities study. Clin 
J Am Soc Nephrol. 2015;10:2023–2029.
	124.	 Zhe M, Hang Z. Nephrolithiasis as a risk factor of chronic kidney 
disease: a meta-analysis of cohort studies with 4,770,691 partici­
pants. Urolithiasis. 2017;

thic hypercalciuria. Kidney 
Int. 1988;33:1140–1146.
	181.	 Borghi L, Meschi T, Guerra A, Novarini A. Randomized prospec­
tive study of a nonthiazide diuretic, indapamide, in preventing cal­
cium stone recurrences. J Cardiovasc Pharmacol. 1993;22:S78–S86.
	182.	 Pak C, Peterson R, Sakhaee K, et al. Correction of hypocitratu­
ria and prevention of stone formation by combined thiazide and 
potassium citrate therapy in thiazide-unresponsive hypercalciuric 
nephrolithiasis. Am J Med. 1985;79:28428

ton S-A, et al. Efficacy and safety of 
Oxalobacter formigenes to reduce urinary oxalate in primary hyp­
eroxaluria. Nephrol Dial Transplant. 2011;26:3609–3615.
	204.	 Hoppe B, Niaudet P, Salomon R, et al. A randomised Phase I/
II trial to evaluate the efficacy and safety of orally administered 
Oxalobacter formigenes to treat primary hyperoxaluria. Pediatr 
Nephrol. 2017;32:781–790.
	205.	 Watts RWE, Danpure CJ, de Pauw L, et al. Combined liver-kidney 
and isolated liver transplantations for 

ysiology, diagnosis and treat­
ment of inherited distal renal tubular acidosis. J Nephrol. 
2018;31(4):511–522.
	224.	 Alonso-Varela M, Gil-Peña H, Coto E, et al. Distal renal tubular 
acidosis. Clinical manifestations in patients with different underly­
ing gene mutations. Pediatr Nephrol. 2018;33(9):1523–1529.
	225.	 Ramchandani P, Pollack HM. Radiologic evaluation of patients 
with urolithiasis. In: Coe FL, Favus MJ, Pak CYC, et al., eds. Kid­
ney Stones: Medical and Surigical Management. P

 Lee H, et al. Randomized, double-blind 
trial of lithostat (acetohydroxamic acid) in the palliative treatment 
of infection-induced urinary calculi. Eur Urol. 1991;20:243–247.
	246.	 Williams JJ, Rodman JS, Peterson CM. A randomized double-
blind study of acetohydroxamic acid in struvite nephrolithiasis. N 
Engl J Med. 1984;311:760–764.
	247.	 Font-Llitjos M, Jimenez-Vidal M, Bisceglia L, et al. New insights 
into cystinuria: 40 new mutations, genotype-phenotype correla­
tion, and digenic inh

In: Resnick MI, Pak CYC, eds. Uroli­
thiasis: A Medical and Surgical Reference. Philadelphia: WB Saun­
ders; 1990:133–143.
	254.	 Pak CYC. Prevention of recurrent nephrolithiasis. In: Pak CYC, ed. 
Renal Stone Disease: Pathogenesis, Prevention, and Treatment. Bos­
ton: Martinus-Njhoff; 1987:165–199.
	255.	 Lindell A, Denneberg T, Hellgren E, et  al. Clinical course and 
cystine stone formation during tiopronin treatment. Urol Res. 
1995;23:111–117.
1338
33
Physiology of Insulin Secretion
ELE F

117.
1338
33
ELE FERRANNINI AND ANDREA MARI
Introduction, 1338
Neural Regulation of Beta-Cell Function, 1339
Beta-Cell Mass, 1339
Beta-Cell Insulin Content, 1339
Insulin Secretion Versus Plasma Insulin, 1340
Characteristics of Insulin Secretion In Vivo, 1342
Modes of Beta-Cell Response, 1342
Insulin Secretion in the Fasting State, 1343
Insulin Secretory Response to Intravenous Glucose, 1343
Slow Beta-Cell Response Modes and Adaptation Mechanisms, 1

a-Cell Response Modes and Adaptation Mechanisms, 1345
Insulin Secretory Response to Oral Stimuli, 1345
Insulin Secretion and Insulin Sensitivity, 1346
Genetic Influences on Insulin Secretion, 1347
Insulin Secretion, Insulin Action, and Glucose Homeostasis, 1347
KEY POINTS
 • 
Beta-cell function is central for glucose homeostasis: insulin 
secretion is finely regulated by multiple mechanisms to ensure 
appropriate hormone delivery under changing short-term con­
ditions (e.g., variable nutrient i

short-term con­
ditions (e.g., variable nutrient intake) and longer-term circum­
stances (e.g., obesity, insulin resistance, pregnancy).
 • 
At a cellular level, the current model features different popula­
tions of cytoplasmic insulin granules based on their ability 
to undergo rapid exocytosis. An immediately releasable pool 
underlies the fast insulin secretion phenomena, and its exo­
cytosis is triggered by membrane depolarization and calcium 
influx consequent to glucose metabolism. The am

m 
influx consequent to glucose metabolism. The amplifying role 
of glucose contributes to sustained insulin secretion and likely 
involves changes in granule status.
 • 
The insulin secretion response is polymorphic, and its in vivo 
study requires different tests to assess different response modes.
 • 
The main stimulus for insulin secretion is glucose. Measure­
ment of plasma C-peptide concentrations during an oral (OGTT 
or mixed meal) or intravenous (hyperglycemic clamp, and 
graded gluco

ntravenous (hyperglycemic clamp, and 
graded glucose infusion) glucose tolerance test coupled with 
mathematical modeling allow recapitulating the main insulin 
secretion characteristics that are relevant to both normal and 
impaired glucose tolerance.
 • 
In an oral glucose tolerance test, the insulin secretion excur­
sions parallel those of glucose concentration, thereby defining 
a dose response relating the two variables, which is the most 
important beta-cell response mode. Additional mode

important beta-cell response mode. Additional modes include 
an anticipatory secretion phase at rising glucose and potentia­
tion over time.
 • 
An important longer-term insulin secretion adaptative response 
is the insulin secretion increase with insulin resistance.
 • 
Reduced beta-cell glucose sensitivity and increased absolute 
insulin secretion characterize the early stages of glucose intoler­
ance and predict progression to overt diabetes.
The endocrine pancreas is made up o

.
The endocrine pancreas is made up of roughly 3 million islets scat­
tered through the exocrine tissue, for a total weight of about 1 g 
(under 3% of the pancreas volume).1 The human islet is popu­
lated by beta cells intermingled with α cells, δ cells, γ cells, and 
ε cells, secreting glucagon, somatostatin, pancreatic polypeptide, 
and ghrelin, respectively. Beta cells are the most abundant cell type 
(composing about 60% of islet cells) and represent a higher pro­
portion of all

 cells) and represent a higher pro­
portion of all endocrine cells in smaller, compared with larger, 
islets. Small islets also have a higher insulin content and are in 
closer contact with blood vessels, thereby representing a func­
tionally distinct subpopulation. The spatial arrangement of cells 
within the islet is also important for islet function. Although beta 
cells are irregularly distributed in human islets, when stimulated 
by glucose, membrane potentials and cytoplasmic Ca2+ concen­

 membrane potentials and cytoplasmic Ca2+ concen­
tration ([Ca2+]c) show a complex pattern of oscillations in both 
single beta cells and whole islets.2 Furthermore, gap junctions pro­
vide connectivity, which may be devolved to small clusters of cells 
with pacemaker properties.3 The origin and regulation of intrinsic 
electrical oscillations and hyperspecialized beta-cell subsets are still 
under investigation, but the heterogeneity of islets, and of beta-
cell subsets within them, adds levels

and of beta-
cell subsets within them, adds levels of complexity to well-known 
intra-islet interactions among different cell types (e.g., paracrine 
inhibition of glucagon secretion by insulin, inhibition of glucagon 
and insulin secretion by somatostatin). Moreover, endogenous 
1339
Chapter 33  Physiology of Insulin Secretion
islet regeneration may occur not only by neogenesis from ductal 
cells and beta-cell replication but also by transdifferentiation of 
α cells4 or δ cells5 following extre

entiation of 
α cells4 or δ cells5 following extreme beta-cell loss.6 Therefore, a 
novel concept emerging in beta-cell science is that function is not 
only the result of mass and quality of beta cells but of the architec­
ture and structural integrity of the islet as an organ.
Beta cells express a large number of transporters and receptors 
on their plasma membrane; their simultaneous or sequential ligand 
engagement following a variety of stimuli is eventually integrated 
into a given secreti

uli is eventually integrated 
into a given secretion rate.7 Like neurons, whose response to mul­
tiple synaptic inputs is an action potential, the beta cell modulates 
its secretory response in phase and amplitude (i.e., in time course 
and amount) to precisely fit the stimulus.8 The overwhelming 
complexity of stimulatory and inhibitory signals and their intra­
cellular transduction and modulation can be operationally reduced 
to two main physiologic domains: a triggering pathway, starting 
at 

logic domains: a triggering pathway, starting 
at the cell membrane level, and an amplifying pathway, which is 
mostly intracellular.9 Thus, glucose enters beta cells via the isoform 
2 of the glucose transporter (GLUT2) and is metabolized through 
the glycolytic pathway and, subsequently, in mitochondria. The 
first rate-limiting step in this process is the phosphorylation of 
glucose to glucose-6-phosphate. This reaction is mediated by the 
enzyme glucokinase, which, by determining the rate of

yme glucokinase, which, by determining the rate of glycoly­
sis, functions as the glucose sensor.10 Glucose metabolism raises 
adenosine triphosphate (ATP) production, leading to closure of 
the ATP-sensitive potassium (KATP) channel and membrane depo­
larization. This causes Ca2+ entry through the voltage-dependent 
Ca2+ channel and elevation of [Ca2+]c, which initiates exocytosis of 
the hormone from readily releasable granules. The increase in ATP 
production is likely due both to nicotinamid

 ATP 
production is likely due both to nicotinamide adenine dinucleo­
tide production at the glycolytic step of glyceraldehyde-3-phos­
phate oxidation and to pyruvate oxidation in the tricarboxylic acid 
cycle in mitochondria. The increase in ATP not only closes KATP 
channels but also serves as a major permissive factor for movement 
of insulin granules and for priming of exocytosis.
The biology of KATP channels is relevant for beta-cell physi­
ology. These channels include sulfonylurea recepto

ology. These channels include sulfonylurea receptors (SURs) and 
potassium inward rectifiers (Kir6.1 and Kir6.2), which assemble 
to form a large octameric channel. In the beta cell, the SUR1/
Kir6.2 pairs constitute the KATP channel, which controls the flux 
of potassium ions. Importantly, the opening of these channels can 
reset the resting membrane potential below the threshold for acti­
vation of voltage-gated Ca2+ channels, thereby aborting insulin 
secretory bursts. This occurs when plasma

insulin 
secretory bursts. This occurs when plasma glucose levels are low 
or on insulin stimulation, in the latter case, installing an autocrine 
control of insulin secretion.11 Mutations in both components of 
the beta-cell KATP—SUR1 (encoded by the ABCC 8 gene) and 
Kir6.2—have been shown to lead to hypersecretion of insulin, 
resulting clinically in either a recessive form of familial hyperinsu­
linemia or persistent hyperinsulinemic hypoglycemia of infancy.
The amplifying pathway also requi

emia of infancy.
The amplifying pathway also requires glucose metabolism but is 
KATP channel independent. In signal amplification, cyclic adenosine 
monophosphate (cAMP) plays an important role. This second mes­
senger is generated at the plasma membrane from ATP and potenti­
ates glucose-stimulated insulin secretion, particularly in response to 
glucagon, glucagon-like peptide 1 (GLP1), and glucose-dependent 
insulinotropic peptide (GIP). The cAMP–dependent pathways 
appear to be particularly 

AMP–dependent pathways 
appear to be particularly important in the exocytotic machinery.
More recently, additional molecules have been shown to act as 
glucose sensors independent of glucose metabolism.12 In rodent 
systems, these are cell-surface receptors identified by blocking glu­
cose utilization following the application of a glucose stimulus. 
They also work by changing the synarchic messengers cAMP and 
[Ca2+]c. These additional glucose sensors have tentatively been 
defined as heterodim

ensors have tentatively been 
defined as heterodimers of an atypical sweet taste receptor (T1R3) 
and a calcium-sensitive receptor (CaSR). Their role in human beta 
cells and in vivo insulin secretion is still under investigation. 
Neural Regulation of Beta-Cell Function
The brain provides supplemental control of insulin and glucagon 
secretion via efferent nerves penetrating the pancreatic islet.13 Sym­
pathetic stimulation (through α2 receptor–mediated norepineph­
rine release) inhibits insuli

mediated norepineph­
rine release) inhibits insulin secretion and potentiates glucagon 
secretion,14 whereas parasympathetic stimulation (through M3 mus­
carinic-mediated acetylcholine release) enhances insulin and gluca­
gon release.15 These autonomic circuits emanate from hypothalamic 
regions that differentially control hormone release. In particular, 
experimentally lowering glucose sensing in ­glucokinase-expressing 
neurons of the arcuate nucleus results in deficient glucose-stimulated 
in

ucleus results in deficient glucose-stimulated 
insulin secretion and glucose intolerance, whereas the same manipu­
lation in the lateral hypothalamic area results in enhanced glucose 
sensitivity and glucagon response to hypoglycemia.16 The relative 
contribution of the brain/islet axis to whole body glucose homeostasis 
and counterregulation is difficult to quantitate in humans; however, 
it is interesting that in rodents, islet grafts under the kidney capsule17 
or in the anterior chamber of 

e kidney capsule17 
or in the anterior chamber of the eye18 undergo timed peri-islet and 
intra-islet reinnervation, a striking example of neuroplasticity. 
Beta-Cell Mass
Beta cell mass is the net balance between positive factors, including 
islet neogenesis, beta-cell proliferation, and beta-cell hyperplasia, 
and negative factors such as beta-cell apoptosis and dedifferentia­
tion.6 Despite being a terminally differentiated cell type, β cells are 
not entirely postmitotic. In humans, most bet

are 
not entirely postmitotic. In humans, most beta-cell neogenesis from 
ductal epithelial progenitor cells occurs preterm; together with a neo­
natal burst of β-cell replication (from pancreatic progenitor and/or 
ductal precursor cells), the full complement of beta-cell mass is estab­
lished within the first 5 years of life. Thereafter, beta-cell proliferation 
is very low, with the average lifetime of beta cells extending to about 
25 years.19 However, at the islet periphery, a specialized m

9 However, at the islet periphery, a specialized microenvi­
ronment or neogenic niche harbors a population of transcriptionally 
immature (virgin) beta cells, which constitute a lifelong reservoir of 
new beta cells.20 Such cells have an intermediate phenotype between 
α-cells and mature beta cells and can transdifferentiate to one or 
the other depending on prevailing signals, such as increased insulin 
demand (as in obesity, extreme beta-cell loss or pregnancy) or stressors 
(e.g., hyperglycem

oss or pregnancy) or stressors 
(e.g., hyperglycemia, oxidative overload as in the transition from nor­
mal glucose tolerance to type 2 diabetes [T2D]).4-6 Of note, replicat­
ing beta cells upregulate hundreds of proliferation-related genes, but 
not genes involved in beta-cell function (glucose sensing and insu­
lin secretion), suggesting that the quiescence-proliferation transition 
involves global amplification of gene expression, except for a subset of 
tissue-specific genes.21 Efforts to ca

 subset of 
tissue-specific genes.21 Efforts to catalog the transcriptional program 
of replicating beta cells are key not only to account for the physiologic 
plasticity of islet cell types but also to identify molecular pathways of 
beta-cell generation for potential transplant-based therapies. 
Beta-Cell Insulin Content
Despite its small size, the endocrine pancreas has a large functional 
reserve, as pancreatic insulin content has been estimated to be in 
the range of 200 to 250 units (a 10-

ted to be in 
the range of 200 to 250 units (a 10-day supply for a healthy lean 
1340
SECTION VIII    Disorders of Carbohydrate and Fat Metabolism
adult). Within the beta-cell cytoplasm, insulin is packaged in 5000 
to 8000 secretory granules spatially distributed in age-distinct 
populations.22 As shown in Fig. 33.1, each granule (300–350 nm 
in diameter) contains an electron-dense core composed of tightly 
packed crystals of insulin hexamers stabilized by one calcium and 
two zinc ions.23 Gran

abilized by one calcium and 
two zinc ions.23 Granules are not just delivery depots but dynamic 
structures containing many proteins, small molecules, and ions in 
the lumen, as well as several transmembrane proteins, channels, 
and membrane-associated proteins. Granule turnover and traffick­
ing is a highly regulated process involving relationships with the 
cytoskeleton, intracellular insulin degradation, mobility (dynamic, 
restricted, or nearly none), docking, and fusion with the plasma 
mem

ly none), docking, and fusion with the plasma 
membrane (mediated by soluble ­N-ethylmaleimide–sensitive factor 
attachment protein receptor [SNARE] proteins).22,23 Functionally, 
the established facts are that (1) only a small fraction (much less 
than 1%) of granule insulin is secreted in response to acute in vitro 
glucose stimulation; (2) granule half-life is less than 5 days, with 
intracellular degradation starting already within about 3 days; and 
(3) younger granules are fewer but more m

ys; and 
(3) younger granules are fewer but more mobile than older granules 
even if they come from deep in the cytoplasm and therefore form a 
readily releasable pool. Which features of secretory granule dynam­
ics—age, position in the cell, and molecular signature—are critical 
for which aspect of in vivo insulin release dynamics and which steps 
may be altered in the diabetic state remain to be determined.
Insulin Secretion Versus Plasma Insulin
Circulating insulin concentrations are a functi

in
Circulating insulin concentrations are a function of secretion 
and clearance, which is complicated by the appearance of the 
hormone first into the portal circulation and by substantial 
hepatic clearance. Insulin secretion normally cannot be mea­
sured directly due to the inaccessibility of the portal circulation. 
Thus, when assessing insulin secretion in vivo, methods based on 
the measurement of C-peptide currently are state of the art. The 
principles of this method are that (1) C-pepti

he 
principles of this method are that (1) C-peptide is co-secreted 
with insulin in equimolar amounts as a consequence of proin­
sulin cleavage, (2) C-peptide is not extracted by the liver, and 
(3) C-peptide clearance—half of which occurs through the kid­
ney24—is approximately constant in any given individual. By 
this approach, insulin secretion is calculated using a mathemati­
cal procedure called deconvolution,25 which reconstructs the pan­
creatic insulin secretion rate (in pmol/min) as i

creatic insulin secretion rate (in pmol/min) as it occurs before 
hepatic insulin degradation,26,27 and can be reliably estimated 
from individual anthropometric data.28 Thus, transducing 
plasma C-peptide measures into plasma insulin concentrations 
involves two catabolic processes (and computational steps) in 
series with one another (Fig. 33.2). Considering that about 15% 
of renal C-peptide uptake is excreted intact into the urine (the 
remainder being degraded),24 measurements of the ratio

inder being degraded),24 measurements of the ratio of uri­
nary C-peptide to creatinine have been shown to be reasonably 
well correlated with postprandial plasma C-peptide levels29 and 
may therefore be used as indicators of residual beta-cell function 
in patients with type 1 diabetes.30
Because plasma insulin kinetics are fast, insulin secretion rates 
are quickly reflected in plasma insulin concentrations, which 
therefore are the prime and most widely used proxy for insulin 
secretion. Howe

ost widely used proxy for insulin 
secretion. However, insulin clearance may vary during stimulation 
of insulin secretion and may be lower or higher depending on the 
metabolic status. Hence, insulin concentrations may distort the 
actual time course of insulin secretion. Insulin clearance occurs 
through two principal routes depending on the site of entry of 
the hormone into the circulation: (1) peripheral (or exogenous) 
insulin clearance (pMCRI), which can be determined experimen­
tally dur

CRI), which can be determined experimen­
tally during a euglycemic hyperinsulinemic clamp as the ratio of 
exogenous insulin infusion rate to arterial plasma insulin concen­
tration at steady state, and (2) prehepatic (or endogenous) insulin 
A
B
• Fig. 33.1  (A) Electron micrograph of a human beta cell. The electron-dense spherical bodies in the cyto­
plasm are insulin granules, some in close proximity of the plasma membrane. Mitochondria and endoplasmic 
reticulum are also visible. (B) An insu

doplasmic 
reticulum are also visible. (B) An insulin granule is blown up to schematically highlight its complex structure: 
in addition to insulin hexamers, receptors, ion channels, transporters, and other proteins are located in the 
vesicle membrane and within the granule. Also indicated is the maturation of the insulin prohormone proin­
sulin to insulin (A and B chain) and C-peptide catalyzed by carboxypeptidase E (CPE) and proconvertases 
(PCsk1 and PCsk2). 5-HT, ­5-Hydroxytryptamine (serot

Csk1 and PCsk2). 5-HT, ­5-Hydroxytryptamine (serotonin); GABA, γ–aminobutyric acid; IAPP, islet amyloid 
polypeptide; IGF2, insulin-like growth factor 2; INS, insulin; MIF, macrophage migration inhibitory factor; 
PACAP, pituitary adenylate cyclase-activating polypeptide. (From Suckale J, Solimena M. The insulin secre­
tory granule as a signaling hub. Trends Endocrinol Metab. 2010;21[10]:599–609.)
1341
clearance (eMCRI). Direct experimental determinati

clearance (eMCRI). Direct experimental determination of eMCRI 
is hampered by the difficulty of gaining access to the portal vein 
where pancreatic insulin is secreted. However, endogenous insulin 
clearance can be calculated as the ratio between endogenous insu­
lin secretion—reconstructed from C-peptide deconvolution—
and arterial plasma insulin concentration at steady state.
If both eMCRI and pMCRI are known from separate experi­
ments, their fractional difference—for instance, (eMCRI – 
pMCR

fractional difference—for instance, (eMCRI – 
pMCRI)/eMCRI)—is an estimate of fractional hepatic insulin 
extraction. The fraction of portal insulin that is removed by the 
liver in its first pass is about 65%, ranging between 50% and 
70%.31,32 Once in the systemic circulation, insulin recirculates to, 
and is further cleared by, the liver and, to a lesser extent, by skel­
etal muscle and the kidneys. The overall contribution of the liver 
(first pass plus recirculation) is therefore dominant (

t pass plus recirculation) is therefore dominant (approximately 
80%). The physiologic characteristics of exogenous insulin clear­
ance (pMCRI from insulin clamp studies) include large interin­
dividual variability; saturation kinetics31; a substantial influence 
of genetic factors33,34; and, to a lesser extent, a negative effect of 
abdominal obesity and liver fat.35,36
By using the C-peptide method to calculate eMCRI and an 
estimate of hepatic plasma flow, one can reconstruct prehepatic 
(70%

 plasma flow, one can reconstruct prehepatic 
(70% portal, 30% hepatic arterial) plasma insulin concentra­
tions. As shown in Fig. 33.3, in individuals with normal glucose 
tolerance, fasting prehepatic insulin concentrations are related to 
peripheral insulin levels in an approximately linear fashion, with 
an average ratio of 4:1. In the fed state (e.g., after an oral glucose 
load), eMCRI is lower than in the fasting state as a consequence 
of saturation of liver extraction. The contribution 

 saturation of liver extraction. The contribution of muscle is 
reduced,37 whereas that of the kidney is increased,31 although 
together they still contribute no more than 20% to overall insulin 
clearance. As a consequence, the ratio of prehepatic to peripheral 
insulin is progressively lower as pancreatic insulin release increases 
(see Fig. 33.3).
Multiple mechanisms appear to modulate insulin degradation. 
In mice lacking zinc-transporter 8 (ZnT8), which enriches the 
insulin vesicles in zin

ZnT8), which enriches the 
insulin vesicles in zinc, the clathrin-dependent insulin endocytosis 
in the hepatocyte is reduced; individuals with a single nucleotide 
polymorphism in the gene encoding for this transporter also dis­
play an increased insulin clearance during an oral glucose toler­
ance test (OGTT).38 Neural inputs39 and cellular redox state40 
may also influence insulin degradation, whereas gut hormones do 
not affect this process.41 Many drugs may interfere with insulin 
degradati

1 Many drugs may interfere with insulin 
degradation, especially at doses causing a degree of hepatotoxicity. 
Among antidiabetic agents, sulphonylureas are known to reduce 
0.30
0.25
0.20
0.15
0.10
0.05
0.35
0
0
30
Time (min)
Deconvolution
60
0.20
0.15
0.10
0.05
0.25
0
0
30
Time (min)
Insulin clearance
60
Insulin secretion
(pmol/min)
Plasma C-peptide
(pmol/L)
Plasma insulin
(pmol/L)
• Fig. 33.2  The pancreatic insulin secretion rate is reconstructed by decon­
volution of plasma C-peptide concen

cted by decon­
volution of plasma C-peptide concentrations by the C-peptide disappear­
ance rate (green curve). Secreted insulin then undergoes fast clearance 
(red curve), eventually yielding plasma insulin concentrations.
400
300
200
100
500
0
0
120
100
80
60
40
20
140
Prehepatic insulin (pmol/L)
Peripheral insulin (pmol/L)
Fasting
1600
1200
800
400
2000
2400
0
0
1000
800
600
400
200
1200
OGTT
• Fig. 33.3  Relationship between calculated 

OGTT
• Fig. 33.3  Relationship between calculated prehepatic and measured 
peripheral plasma insulin concentrations in 1123 normoglycemic subjects 
(mean fasting plasma glucose of 90 mg/dL [5.0 mmol/L]) after an overnight 
fast (fasting) and during the absorption of a 75-g oral glucose load (OGTT). 
Note the linear relationship between prehepatic and peripheral insulin lev­
els in the fasting state and the curvilinear relationship during the OGTT, 
indicating saturation of insulin degradation. T

, 
indicating saturation of insulin degradation. The fasting range is plotted on 
the OGTT range (dashed lines). OGTT, oral glucose tolerance test. (Data 
from Ferrannini E, Balkau B, Coppack SW, et al. Insulin resistance, insulin 
response, and obesity as indicators of metabolic risk. J Clin Endocrinol 
Metab. 2007;92[8]:2885–2892.)
1342
insulin clearance.42 Whether this effect is a direct one on the 
degrading machinery or is seconda

rect one on the 
degrading machinery or is secondary to saturation due to insulin 
hypersecretion is not known.
The role of insulin clearance in the maintenance of glucose 
homeostasis is complex. A reduction in insulin degradation is 
commonly interpreted as compensatory, with lower values being 
observed in individuals who are more insulin resistant and 
would take advantage of increased insulin levels. On the other 
hand, experimental animals in which the insulin-degrading 
enzyme is knocked 

in which the insulin-degrading 
enzyme is knocked out develop insulin resistance and diabetes.43 
Similarly, fasting hyperinsulinemia and mild degrees of insulin 
resistance develop in pancreas-transplanted patients with organ 
venous drainage into the systemic circulation as compared with 
those with portal drainage.44 However, insulin clearance can 
influence glucose tolerance independently of other factors. For 
instance, an acute 30% increase in insulin clearance, produced 
by systemic nitri

 in insulin clearance, produced 
by systemic nitric oxide inhibition, worsens glucose tolerance in 
normal glucose-tolerant persons.45 Thus, it is difficult to cleanly 
dissect out saturation from a bidirectional association of insu­
lin clearance and insulin action. The bulk of evidence, however, 
indicates that even after accounting for saturation of degrading 
capacity, insulin clearance is positively associated with insulin 
sensitivity regardless of the temporal sequence of changes in 
thes

dless of the temporal sequence of changes in 
these two functions. 
Characteristics of Insulin Secretion In Vivo
Insulin acts on several target metabolic pathways, such as lipids 
and amino acids, which also influence its secretion; however, the 
tightest physiologic feedback is with plasma glucose concentra­
tion. In vivo, beta-cell function must supply insulin to body tis­
sues in amounts and time course apt to maintain plasma glucose 
within a narrow concentration range on a minute-by-minute 

 narrow concentration range on a minute-by-minute 
basis. To this end, insulin output must handle acute challenges, 
such as size, composition, and appearance rate of meals, and adapt 
to long-term settings, such as changes in target tissue sensitivity to 
insulin. In comparison, insulin action, if equally complex at the 
cellular level, is functionally relatively stable within any given indi­
vidual. In fact, when measured in vivo by a direct technique (the 
euglycemic hyperinsulinemic clamp), 

chnique (the 
euglycemic hyperinsulinemic clamp), insulin sensitivity has been 
shown to vary by 30% to 80% during 24 hours of free living.46 
In most cases, insulin sensitivity can at best double with physi­
ologic or pharmacologic intervention. In contrast, insulin secre­
tion can vary manyfold in the same person in a matter of minutes, 
as occurs with a large mixed meal, or over years, as happens with 
weight gain.47 For example, a lean, insulin-sensitive adult may 
need as little as 0.5 unit

in-sensitive adult may 
need as little as 0.5 units of insulin to dispose of an oral load of 
75 g of glucose over 2 hours, whereas an obese, insulin-resistant, 
glucose-intolerant person may require 45 units to perform the 
same task—an approximate 100-fold span.48
Given the highly polymorphic nature of beta-cell responses, 
absolute insulin secretion is a meaningful index of beta-cell func­
tion only in the context of a standardized stimulus (mainly glucose). 
For this reason, typical protocol

ainly glucose). 
For this reason, typical protocols employ a controlled stimulus, 
such as a hyperglycemic clamp (described in the following). When 
the stimulus is not standardized, the secretory response can be 
empirically normalized by calculating ratios between insulin (or 
insulin secretion) and glucose (absolute or incremental values or 
areas under the response curves). For instance, the time-honored 
insulinogenic index uses the ratio of 30-minute increments from 
baseline of insulin an

 30-minute increments from 
baseline of insulin and glucose concentrations from an OGTT.
More elaborate approaches rely on mathematical models, 
which have been widely used to assess beta-cell function from clin­
ical tests—in which glucose levels are not controlled—and also to 
gain insight into the mechanisms governing insulin secretion.49 In 
general, these models relate insulin secretion rates to concomitant 
plasma glucose concentrations on the basis of a formal descrip­
tion of the secreto

the basis of a formal descrip­
tion of the secretory machinery. Requisites for such models are a 
simplified description of the system—to allow robust mathemati­
cal identifiability of its parameters—and a realistic congruity with 
known features of beta-cell function as emerging from cellular 
physiology. 
Modes of Beta-Cell Response
There are multiple in  vivo tests of beta-cell function (described 
later). Each in vivo test of insulin secretion reveals some aspect 
of beta-cell function; howe

n reveals some aspect 
of beta-cell function; however, alone, none recapitulates the poly­
morphic nature of insulin’s secretory response. The main modes 
of beta-cell response include (1) first-phase or acute insulin secre­
tion, (2) glucose sensitivity, and (3) potentiation of insulin secre­
tion. These three main features of insulin secretory response were 
already identified in isolated perfused rat islets more than four 
decades ago50 and can be resolved by modern mathematical mod­
eling fr

n be resolved by modern mathematical mod­
eling from multiple types of stimulation.49,51,52 First-phase insu­
lin release is the sharp and short-lived peak of insulin secretion 
elicited by a brisk rise in glucose levels. Its magnitude depends on 
the size of the glucose stimulus and can be also represented as a 
function of the glucose rate of change (also called rate sensitivity, 
anticipation, or derivative component). First-phase insulin secretion 
is rarely observed under free-living condit

etion 
is rarely observed under free-living conditions, although a biphasic 
response can be occasionally detected during an OGTT. Despite 
much investigation, the physiologic interpretation of the biphasic 
secretory response to a glucose step is still somewhat uncertain. 
Single-cell electrophysiologic studies have shown that acute expo­
sure to high glucose enhances electrical activity synchronously with 
spikes in cytosolic calcium concentrations, which parallel insulin 
release both in term

ions, which parallel insulin 
release both in terms of glucose dependency and time course.53 
Morphometric studies have shown that the beta cells harbors dif­
ferent populations of secretory granules, in different stages of mat­
uration and in variable spatial array between the trans-Golgi and 
the plasma membrane, constituting a chain of secretory pools in 
dynamic exchange with one another.54 First-phase insulin secretion 
contributes an estimated one tenth (about 3 nmol/m2) of supra­
basal se

ted one tenth (about 3 nmol/m2) of supra­
basal secretion during a 2-hour OGTT; nevertheless, a prompt 
stimulation of insulin release as soon as glucose starts to rise is cru­
cial to suppress endogenous glucose production and to prime tissue 
glucose uptake, thereby curbing subsequent glycemic excursions.55
Beta-cell glucose sensitivity measures the increase in secretion 
rate for any concomitant increase in plasma glucose concentra­
tion. This key mode of response determines the amount of ins

 key mode of response determines the amount of insulin 
secreted during a meal and therefore is a major control of glucose 
tolerance. In fact, decreased glucose sensitivity is a hallmark of all 
forms of glucose intolerance.
Potentiation of insulin secretion is an intrinsic feature of beta-
cell function. Potentiation occurs when the dose-response rela­
tionship between glucose level and insulin secretion is enhanced, 
as when prior exposure to glucose leads to a greater insulin secre­
tion on 

glucose leads to a greater insulin secre­
tion on subsequent exposure. Potentiation may be generated by a 
glucose “memory,” incretins, or factors such as glucagon, cholin­
ergic stimuli, other nutrients such as fructose, or drugs including 
sulfonylureas.56
1343
From this description, it is clear that maintenance of glucose 
homeostasis depends not only on the absolute amount of insulin 
release but also on the time dynamics of the secretory response.

so on the time dynamics of the secretory response.
Other clinical tests reveal additional details of beta-cell func­
tion. For example, an arginine bolus superimposed on ramping 
glycemic plateaus explores the effect of an amino acid separately 
from that of glucose and has been used to estimate maximal secre­
tory capacity.57 Furthermore, many studies have gauged the influ­
ence on insulin secretion of different amino acids,58 endogenous59 
or exogenous free fatty acids (FFAs),60 incretin hormo

xogenous free fatty acids (FFAs),60 incretin hormones61 and 
neural stimuli.62 These influences depend on stimulus qual­
ity, dose, route of delivery, duration (acute or chronic), combi­
nation with other stimuli, and simultaneous impact on insulin 
sensitivity. Ultimately, however, beta cells integrate all inputs and 
respond in a manner that can be analyzed in quantifiable terms of 
glucose sensitivity, potentiation, and rate sensitivity. Table 33.1 
reports normative values of basic parameter

 33.1 
reports normative values of basic parameters of beta-cell function 
as measured in a large group of lean (body mass index ≤25 kg/m2) 
healthy volunteers with normal glucose tolerance.
Insulin Secretion in the Fasting State
In healthy normoglycemic adults after an overnight (10–14 hours) 
fast, insulin secretion rates range widely, in rough proportion to 
the degree of obesity (and insulin resistance, see later discussion) 
and equally in women and men (Fig. 33.4). In addition to obesity, 

omen and men (Fig. 33.4). In addition to obesity, 
fasting insulin secretion values are also physiologically dependent 
on fasting plasma glucose levels. Thus, in hyperglycemic patients 
(e.g., individuals with impaired glucose tolerance [IGT] or overt 
T2D), fasting insulin secretion is generally higher than in normo­
glycemic subjects at each level of obesity.
When frequently sampled from portal vein blood, insu­
lin concentrations oscillate, with detectable pulses at 5- to 
­14-minute interva

ith detectable pulses at 5- to 
­14-minute intervals.63,64 The pulsatility of insulin secretion 
is intrinsic to the islet and may reflect coupling between slow 
oscillations of glycolysis and faster oscillations involving Ca2+ 
fluxes.65 Over 24 hours, slower ultradian cycles occur at 80- 
to 180-minute intervals; these can be entrained by small sub­
threshold changes in plasma glucose concentrations.66 Pulsatile 
insulin secretion is disrupted in hyperglycemic states,66,67 but 
its impact on i

in hyperglycemic states,66,67 but 
its impact on insulin action on target organs (liver and periph­
eral tissues) is uncertain.
The ratio of proinsulin to insulin concentration (or the absolute 
proinsulin concentration adjusted for insulin) in a fasting plasma 
sample has been proposed as a marker of beta-cell function.68 In 
epidemiologic studies, a higher ratio of proinsulin to insulin or 
proinsulin to C-peptide has been related to incident diabetes.69 
Insulin Secretory Response to Intraven

iabetes.69 
Insulin Secretory Response to Intravenous 
Glucose
Various formats of intravenous glucose administration have been 
used to gauge beta-cell response independently of gastrointestinal 
influences. 
Hyperglycemic Clamp and Biphasic Insulin 
Secretion
The hyperglycemic clamp protocol has been used in vivo, in the 
perfused pancreas, and in islet cultures. The rationale of this test 
is to expose beta cells to a square wave of hyperglycemia, whereby 
glucose concentration is abruptly rai

ia, whereby 
glucose concentration is abruptly raised and kept constant at 
a preset suprabasal level by means of a controlled variable glu­
cose infusion. The insulin secretory response to this challenge 
  Insulin Secretion Parameters in Lean 
Persons With Normal Glucose Tolerance
Mean
Median
25–75%
Fasting plasma glucose (mmol/L)
4.81
4.90
4.60–5.10
2-hour plasma glucose (mmol/L)
5.18
5.10
4.33–6.00
Fasting plasma insulin (pmol/L)
26
23
17–32
2-hour plasma insulin (pmol/L)
139
119
75–177
Fas

139
119
75–177
Fasting insulin secretion rate 
(pmol.min−1.m−2)
61
58
46–72
Insulin secretion at 5 mM ­glucose 
86
68
48–93
Total insulin output (2 hours) 
(nmol.m−2)
37
36
29–44
Beta-cell glucose sensitivity 
(pmol.min−1.m−2.mM−1)
150
125
91–181
Rate sensitivity (nmol.m−2.mM−1)
1.06
0.69
0.01–1.43
Potentiation (ratio)
2.34
1.83
1.26–2.73
Insulin sensitivity 
(μmol.min−1.kgFFM.nM−1)
165
155
117–199
Data from 620 participants in the RISC study.11
 
TABLE 33.1
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
≥ 35
< 35
< 30
≤ 25
Fasting insulin release
(U.kg–1.day–1)
Fasting insulin
(pmol/L)
Insulin secretion in the fasting state
0
80
60
40
20
100
Hyperglycemic subjects
Normoglycemic subjects
• Fig. 33.4  Fasting plasma insulin concentrations and corresponding fast­
ing insulin secretion rates (extrapolated to 24 hours) in 1123 normoglyce­
mic subjects (mean fasting plasma glucose

moglyce­
mic subjects (mean fasting plasma glucose of 90 mg/dL [5.0 mmol/L]) by 
conventional category of obesity (lean, overweight, moderate, and severe 
obese). Plots are mean ± 1 standard deviation. Stars indicate mean val­
ues of fasting insulin secretion (corresponding insulin concentrations not 
indicated) in 289 subjects with impaired glucose tolerance or overt diabe­
tes. (Data from Ferrannini E, Balkau B, Coppack SW, et al. Insulin resis­
tance, insulin response, and obesity as indicato

­
tance, insulin response, and obesity as indicators of metabolic risk. J Clin 
Endocrinol Metab. 2007;92[8]:2885–2892 and Ferrannini E, Gastaldelli A, 
Miyazaki Y, et al. Beta-cell function in subjects spanning from normal glu­
cose tolerance to overt diabetes: a new analysis. J Clin Endocrinol Metab. 
2005;90[1]:493–500.)
1344
typically is biphasic, with an initial sharp insulin secretory burst 
lasting about 5 to 8 minutes (first-ph

tory burst 
lasting about 5 to 8 minutes (first-phase secretion), followed by 
a transient decrease and then a progressive slow increase, which 
continues as long as hyperglycemia is maintained (second-phase 
secretion) (Fig. 33.5).
The amount of insulin secreted during the first phase (also 
referred to as acute insulin response [AIR]) is dependent on the mag­
nitude of the glucose rise; in a typical + 126 mg/dL (+7 mmol/L) 
hyperglycemic clamp, it is approximately 4 nmol per square meter 
of b

 it is approximately 4 nmol per square meter 
of body surface area (∼1 unit in a 70-kg adult), representing 10% 
to 15% of what is secreted per hour during the second phase. 
Although small, the amount of insulin secreted in the first phase is 
relevant for at least two reasons. First, although the secretory burst is 
apparent with a rapid elevation in glucose concentration, the under­
lying mechanisms appear to operate even for a more gradual rise 
in glucose concentration. These mechanisms are

se 
in glucose concentration. These mechanisms are likely responsible 
for a response that is anticipated compared with what would be 
predicted solely on the basis of the dose response. As discussed later, 
this anticipation has relevant physiologic implications for glucose 
homeostasis. Second, attenuated first-phase insulin secretion is a 
very sensitive marker of early beta-cell dysfunction.70 Impairment 
of first-phase secretion is already present in individuals at risk of 
developing diabe

resent in individuals at risk of 
developing diabetes71 and is predictive of diabetes onset.72 Because 
of these relevant characteristics, assessment of first-phase secretion 
has been widely used.
Higher glycemic plateaus elicit larger secretory responses. 
However, if multiple glucose steps are applied in sequence, the 
first-phase response is progressively attenuated while second-phase 
secretion increases in proportion to the height of the glycemic pla­
teaus.73 This paradoxical behavior may

ycemic pla­
teaus.73 This paradoxical behavior may reflect an inhibitory influ­
ence of antecedent glycemia on the subsequent first-phase response 
or depletion of the pool of readily releasable insulin granules.51,74
 Intravenous Glucose Tolerance Test
Another clinical test yielding the biphasic insulin response is the 
intravenous glucose tolerance test (IVGTT). Here, a glucose bolus 
standardized to body size is injected intravenously, and glucose, 
insulin and, possibly, C-peptide concentrat

cose, 
insulin and, possibly, C-peptide concentrations are measured. 
In contrast to the hyperglycemic clamp, glucose concentrations 
fall rapidly after the initial peak, and second-phase secretion is 
not sustained but transient with a multiphasic pattern.25 Thus, 
the assessment of first-phase secretion is very similar between 
the hyperglycemic clamp and the IVGTT, whereas second-phase 
secretion on the IVGTT occurs at variable glucose and is thus 
less representative. Like the hyperglycemic 

thus 
less representative. Like the hyperglycemic clamp, the IVGTT has 
demonstrated the relevance of first-phase secretion as a hallmark 
of beta-cell function. 
Graded Glucose Infusion Test and Beta-Cell Dose 
Response
In the graded glucose infusion test, glucose is infused at ascending 
rates to stimulate a progressive increase in insulin secretion. The 
plot of insulin secretion rates against plasma glucose concentra­
tions represents the beta-cell dose response, whose slope quantifies 
the 

a-cell dose response, whose slope quantifies 
the sensitivity of the beta cell to glucose (Fig. 33.6). In healthy per­
sons, basal insulin secretion increases by five- to sixfold up to 180 
1000
9 mM
5 mM
first phase
second phase
Plasma
glucose
800
600
400
200
0
100
–40
80
60
40
20
0
–20
120
Insulin secretion rate
(pmol / min)
Time (min)
The hyperglycemic clamp
• Fig. 33.5  Biphasic insulin response to a step increase in plasma glucose 
concentrations from 90 to 162 mg/dL (5–9 mmol/L). Plots are

tions from 90 to 162 mg/dL (5–9 mmol/L). Plots are mean ± 
standard error of the mean. (Personal data.)
Graded glucose infusion test
8
6
10
4
0
120
180
60
240
Plasma glucose
(mmol/L)
Time (min)
Stimulus
400
200
300
100
500
0
0
120
180
60
240
Time (min)
Response
400
450
200
250
300
350
100
150
500
50
4
7
8
6
5
9
10
Plasma glucose (mmol/L)
Dose-response
y/x=sensitivity
• Fig. 33.6  The slope (y/x) of the dose-response curve measures beta-ce

 (y/x) of the dose-response curve measures beta-cell 
glucose sensitivity to plasma glucose. Typical data in a normoglycemic 
subject. (Modified from Byrne MM, Sturis J, Polonsky KS. Insulin secretion 
and clearance during low-dose graded glucose infusion. Am J Physiol. 
1995;268[1 Pt 1]:E21–E27.)
1345
mg/dL (10 mmol/L) glucose; correspondingly, beta-cell glucose 
sensitivity is about 13 mU.min-1mM-1 (80 pmol min-1mM-1). The 
relationship is fairly lin

0 pmol min-1mM-1). The 
relationship is fairly linear within this glucose concentration range 
and up to at least 360 mg/dL (20 mmol/L).75 
Slow Beta-Cell Response Modes and 
Adaptation Mechanisms
Under sustained, constant glucose stimulation, insulin secre­
tion continues to rise slightly. This time-dependent potentiation 
of insulin secretion can be detected in the hyperglycemic clamp 
(see Fig. 33.5). Similarly, prolonged (2–4 days) exposure to mild 
hyperglycemia markedly enhances insulin se

o mild 
hyperglycemia markedly enhances insulin secretion, with a steep­
ening of the dose-response curve, as shown in hyperglycemic 
clamp76 and graded glucose infusion studies.75 Thus, the healthy 
beta cell is constitutively capable of potentiating its response 
when a chronic condition of mild hyperglycemia is induced by 
intravenous glucose administration.
Insulin Secretory Response to Oral Stimuli
When glucose is ingested, the biphasic pattern of insulin 
response seen with intravenous glu

ern of insulin 
response seen with intravenous glucose is substantially spread 
out over the time of glucose absorption, with plasma glucose 
levels typically peaking at 0.5 to 1.0 hour and returning to 
baseline by 2 hours post ingestion. Insulin secretion rates rise 
and fall in close synchrony with plasma glucose levels. When 
a mixed meal containing the same amount of glucose (75 g 
as in a conventional OGTT) is ingested, glycemic excursions 
are smoothed out, but insulin secretion is enhanc

are smoothed out, but insulin secretion is enhanced due to the 
direct or indirect insulin-stimulating effect of protein and fat 
(Fig. 33.7). When four successive mixed meals with a high car­
bohydrate content are fed over a 14-hour period, insulin output 
appears to max out at 70 to 85 units in nondiabetic insulin-
sensitive persons and at 140 to 170 units in insulin-resistant 
individuals, which highlights the large functional secretory 
reserve of the endocrine pancreas.77 With consecutive 

rve of the endocrine pancreas.77 With consecutive nutri­
ent loads, the plasma glucose and insulin secretory responses 
are attenuated during the second, as compared with the first, 
meal (Staub-Traugott effect); this phenomenon, which is also 
observed in patients with diabetes, is due to persistent suppres­
sion of endogenous glucose production by the hyperglycemia 
and hyperinsulinemia induced by the first load and to enhanced 
potentiation of insulin release.78
Notably, circulating insulin l

 insulin release.78
Notably, circulating insulin levels are reduced during starvation 
but quickly rebound on refeeding.79 This transition, which is key 
to survival with the paleolithic hunter-gatherer lifestyle, requires 
a major, rapid adaptation of islet function. In rats subjected to 
prolonged (72 hours) fast, beta cells show marked degranula­
tion and blunted in vivo insulin secretion. In contrast, autopha­
golysomal and lysosomal organelles are upregulated, the Golgi 
apparatus is expand

es are upregulated, the Golgi 
apparatus is expanded, and (pro)insulin biosynthetic capacity is 
enhanced. Within 4 to 6 hours of refeeding, proinsulin biosynthe­
sis, cellular ultrastructure, in vivo insulin secretion, and glucose 
tolerance all recover, indicating a rapid replenishment of insulin 
secretory capacity.80 Likewise, in obese nondiabetic individuals, 
plasma insulin concentrations drop by about 40% (and plasma 
glucagon levels rise) following a 14-day fast along with a 50% 
reducti

 following a 14-day fast along with a 50% 
reduction in insulin sensitivity; 10 days of refeeding return all of 
these changes to baseline.81 Thus, the beta cell protects against 
hypoglycemia during a prolonged fast but retains the potential 
to efficiently increase insulin production on refeeding. With the 
present-day lifestyle, the fasting-feeding cycle is much shorter, but 
the coordinate regulation of beta-cell function and insulin sensi­
tivity during the transition phase is the same. Thi

ivity during the transition phase is the same. This is the basis for 
the recommendation that oral glucose (or mixed meal) tolerance 
tests be carried out in settings of an adequate carbohydrate load to 
prevent spurious glucose intolerance.
It has long been known that if glucose is ingested rather than 
infused, insulin secretion is higher at the same glucose levels.82 
GIP and GLP1, which are released by the intestinal epithelium 
in response to nutrient ingestion, are mainly responsible for t

o nutrient ingestion, are mainly responsible for the 
higher insulin release and have therefore been collectively termed 
incretins. The classical test to unveil and quantify the incretin 
effect is based on a standard OGTT and a separate test in the 
same subject, in which the time course of glucose concentration 
observed with the OGTT is reproduced by a controlled intrave­
nous glucose infusion (also called isoglycemic glucose infusion).83 As 
illustrated in Fig. 33.8, insulin secretion is co

illustrated in Fig. 33.8, insulin secretion is considerably potenti­
ated by the oral route of glucose entry, thereby resulting in an 
increase in slope of the beta-cell dose response, as estimated by 
modeling analysis.56 The relative role of GIP and GLP1 in this 
OGTT vs mixed meal
8
6
10
4
0
2
OGTT
Mixed meal
3
1
4
5
(mmol/L)
300
600
0
0
2
3
1
4
5
(pmol.min–1.m–2)
600
500
700
0
4.5
6.5
7.5
5.5
8.5
9
6
7
5
8
9.5
(pmo

5
7.5
5.5
8.5
9
6
7
5
8
9.5
400
300
200
100
• Fig. 33.7  Plasma glucose concentrations and insulin secretion rates in 
response to a standard (75-g) OGTT and to a mixed meal test (75 g of glu­
cose plus 40 g of parmesan cheese and one 50-g egg, for a total of 500 
kcal) in 22 normoglycemic subjects. The bottom panel shows the insulin 
secretion dose response to glucose: note the upward shift with the mixed 
meal versus oral glucose alone

ift with the mixed 
meal versus oral glucose alone. Plots are mean ± standard error of the 
mean. OGTT, oral glucose tolerance test. (Personal data.)
1346
potentiation during physiologic stimulation, the interrelation­
ships of the two hormones, and their relation to a host of other 
hormones secreted by islets (glucagon, somatostatin, ghrelin), 
as well as extrapancreatic tissues (leptin, obestatin), form a net­
work whose nodes and l

in, obestatin), form a net­
work whose nodes and links are difficult to dissect out in human 
experiments. The features of the in vivo incretin effect that are 
reasonably well established are the following: (1) GLP1 and GIP 
are released in phase with insulin and glucose concentrations; 
(2) when gastric emptying is accelerated (e.g., following gastric 
bypass surgery47), both the glucose and insulin peaks are antici­
pated and GLP1 levels are much higher if still synchronous with 
insulin; (3)

uch higher if still synchronous with 
insulin; (3) glucose-induced and incretin-induced potentiation of 
insulin secretion—as resolved by modeling of isoglycemic experi­
ments56—have different time courses and relation to glucose tol­
erance; and (4) strength of potentiation depends on the stimulus 
and the quality of beta-cell function. 
Insulin Secretion and Insulin Sensitivity
A separate important aspect of beta-cell function is its relation 
to insulin sensitivity. In insulin-resistant state

to insulin sensitivity. In insulin-resistant states, more insulin 
needs to be secreted at the same glucose levels to maintain glu­
cose tolerance. Evidence for this phenomenon dates back to 
the infancy of the insulin assay.84 More systematic investiga­
tions have been carried out using the IVGTT, as this test allows 
the simultaneous assessment of the AIR and insulin sensitivity. 
The reciprocal relationship between AIR and insulin sensitivity 
likely reflects adaptation of the beta cell to im

likely reflects adaptation of the beta cell to impaired insulin 
action, which sets in as a steep increment in secretion as insulin 
sensitivity declines (Fig. 33.9). By fitting equilateral hyperbola 
on these IVGTT data, it has been proposed that a more appro­
priate index of intrinsic beta-cell function is the product of the 
insulin secretion and sensitivity indices, or disposition index.85 
The paradigm is commonly assumed to apply to other indices 
of insulin secretion, such as those obtai

indices 
of insulin secretion, such as those obtained from an OGTT.86 
This, however, is only partly the case, as a similar relation of 
insulin sensitivity exists with postprandial insulin output (or 
fasting insulin secretion) but not with beta-cell glucose sen­
sitivity.87 Furthermore, in individuals with IGT, the curve is 
displaced as shown in Fig. 33.9, indicating that compensation 
is reduced until insulin sensitivity drops below approximately 
50 μmol.min-1.kgFFM.nM-1, at which level AIR

tely 
50 μmol.min-1.kgFFM.nM-1, at which level AIR rises back to 
normal, very probably as a consequence of the mild hypergly­
cemia of these individuals.88,89 The physiologic interpretation 
of these interrelationships is that insulin resistance raises the 
setpoint of beta-cell function, whereby absolute measures of 
insulin secretion (fasting, AIR, and postglucose) are chronically 
upregulated. However, the secretory mechanisms that control 
postprandial glucose excursions are less influenced

ostprandial glucose excursions are less influenced by this form 
of adaptation. The anatomic counterpart of this phenomenon 
is the expansion of beta-cell mass in obese individuals,90 in 
many of whom beta-cell glucose sensitivity and glucose toler­
ance are normal. 
The isoglycemic test
8
6
10
4
–30
150
120
90
60
30
0
180
(mmol/L)
Time (min)
400
200
600
0
0
150
120
90
60
30
180
Time (min)
Oral glucose
Incretin effect
i.v. glucose
600
300
500
200

ucose
600
300
500
200
400
100
700
800
0
4
9
8
7
6
5
10
oral glucose
• Fig. 33.8  Matching the plasma glucose response to oral glucose with 
a controlled i.v. glucose infusion reveals the incretin effect, namely the 
potentiation of insulin secretory response due to the oral route of glucose 
entry. The corresponding dose-response curves highlight the enhanced 
beta-cell 

response curves highlight the enhanced 
beta-cell sensitivity to oral, as compared with i.v., glucose. i.v., intrave­
nous. (Personal data.)
Insulin release vs insulin sensitivity
160
100
180
0
0
200
NGT
300
100
150
250
50
350
400
(µmol.min–1.kgFFM
–1.nM–1)
Acute insulin response
(pmol.min–1.m–2.mM–1)
IGT
140
60
120
20
80
40
• Fig. 33.9  Nonlinear relationship between the acute insulin response to 
intravenous glucose (during an intravenous glucose tolerance test) and 
insuli

an intravenous glucose tolerance test) and 
insulin sensitivity (separately measured by a euglycemic hyperinsulinemic 
clamp) in subjects with NGT (n = 1123) and individuals with IGT (n = 156). 
IGT, impaired glucose tolerance; NGT, normal glucose tolerance. (Modified 
from Mari A, Tura A, Natali A, et al. Impaired beta cell glucose sensitivity 
rather than inadequate compensation for insulin resistance is the domina;nt 
defect in glucose intolerance. Diabetologia. 2010;53[4]:749-756.)
1347
Chap

ance. Diabetologia. 2010;53[4]:749-756.)
1347
Genetic Influences on Insulin Secretion
The large interindividual variability of beta-cell secretory outputs 
even in a very selected clinical phenotype (see Table 33.1) per se 
suggests that multiple influences are at play. Most of the more 
than 150 polymorphisms shown by genome-wide scans to be 
associated with diabetes or glycemic traits have been linked with 
defects in insulin secretion.91 Even when t

 with 
defects in insulin secretion.91 Even when tested in mendelian ran­
domization studies,92 the effect size of several of these risk variants 
is small, and they explain only a small proportion of the total heri­
tability of T2D. Nevertheless, some interesting mechanistic infor­
mation is emerging. For example, genetics variants of TCF7L2, a 
transcription factor that mediates the wingless/integrated (Wnt) 
pathway,93 have been associated with impaired incretin-induced 
insulin secretion.94 

h impaired incretin-induced 
insulin secretion.94 Other gene variants (GIPR, WFS1, KCNQ1) 
have also been shown to affect GIP- or GLP1-mediated insulin 
secretion.95 In the future, it is likely that more and more knock-
out and knock-down experiments will add cellular and molecular 
granularity to these associations.
More detailed information on insulin secretion pattern is avail­
able in monogenic forms of diabetes, particularly in maturity-onset 
diabetes of the young (MODY). Mutations in one 

t 
diabetes of the young (MODY). Mutations in one of at least six 
different genes (inherited by an autosomal dominant mode) result 
in hyperglycemia of variable severity in young, nonobese subjects 
(typically younger than 25 years) with a multigenerational family 
history of diabetes. As all these genes are expressed in beta cells, 
their mutation causes beta-cell dysfunction. In MODY 2, a hetero­
zygous private mutation of the gene (GCK) encoding glucokinase 
results in partial enzyme deficie

ing glucokinase 
results in partial enzyme deficiency and a loss of beta-cell glucose 
sensitivity, evidenced by a rightward shift in the dose response on 
a graded glucose infusion test.96 Coexpression of the transcription 
factors (hepatic nuclear factor) HNF1α, HNF1β, and HNF4α 
controls gene expression during embryonic development and 
adulthood; in beta cells, they regulate the expression of the insu­
lin gene and genes encoding proteins involved in glucose trans­
port and metabolism.97,98 

olved in glucose trans­
port and metabolism.97,98 Mutations in HNF4α (MODY 1) or 
HNF1α (MODY 3) also cause loss of beta-cell glucose sensitivity 
on a graded glucose infusion test96 and rapidly progressing hyper­
glycemia. Interestingly, associated abnormalities include a deficient 
insulin secretory response to arginine and reduced glucagon release 
but not insulin resistance.99 Mutations in other MODY-related 
genes (insulin promoter factor 1 [or MODY 4], HNF1β [or MODY 
5], and neurogenic di

[or MODY 4], HNF1β [or MODY 
5], and neurogenic differentiation 1 transcription factor [BETA2, or 
MODY 6]) expressed in beta cells likewise cause diabetes related to 
beta-cell dysfunction. Because these transcription factors are also 
expressed in other tissues (liver and kidney), the mutations produce 
a clinical phenotype in which islet dysfunction is associated with 
other abnormalities, particularly microvascular complications. 
Insulin Secretion, Insulin Action, and Glucose 
T

retion, Insulin Action, and Glucose 
The information derived from clinical tests carried out across dif­
ferent conditions can be assembled into a simplified resume of 
beta-cell physiology (Fig. 33.10). Under most circumstances, the 
primary feedback is between glucose concentrations and insu­
lin secretion rates: insulin decreases glucose and glucose increases 
insulin secretion. Several major processes modulate this feedback. 
Insulin secretion is potentiated by nonglucose substra

lin secretion is potentiated by nonglucose substrates (FFAs and 
amino acids); incretin hormones; and, to a lesser extent, neurotrans­
mitters. Insulin clearance—predominantly hepatic—translates 
secretion rates into circulating insulin concentrations; this step is 
modulated by an intrinsic saturation of liver-degrading capacity and 
by insulin resistance. Plasma insulin lowers plasma glucose by pro­
moting tissue glucose uptake; this step is gated by insulin resistance. 
Plasma insulin also re

ted by insulin resistance. 
Plasma insulin also restrains lipolysis100 and protein breakdown,101 
thereby lowering circulating FFAs and amino acids; this closes a 
secondary feedback loop modulating insulin secretion. Endog­
enous glucose production—enhanced by insulin resistance—and 
dietary carbohydrate independently contribute to plasma glucose 
levels, whereas glycosuria dampens them when the renal threshold 
for glucose reabsorption is exceeded. The cycle is completed by the 
stimulatory ef

ded. The cycle is completed by the 
stimulatory effect of glucose on insulin secretion, with a chronic 
effect of insulin resistance to raise the beta-cell set-point.
Diabetes and obesity can also be represented within this sche­
matic: the presence and severity of insulin resistance affect insulin 
secretion by increasing baseline secretory activity and by incre­
menting the stimulatory signals (i.e., glucose, FFAs, and amino 
Glucose and insulin secretion: the integrated feedback
Insulin secre

n secretion: the integrated feedback
[Glucose]
Glycosuria
Diet
[Insulin]
FFA
AA
Liver
Kidney
Liver
Liver (kidney)
Muscle,
fat
Neural
inputs
Incretin
hormones
• Fig. 33.10  Integrated control of in  vivo insulin secretion. The square 
brackets stand for plasma concentration, the purple line encircles the pri­
mary feedback, the green arrows indicate stimulation, and the red arrows 
indicate inhibition. AA, amino acids; FFA,

arrows 
indicate inhibition. AA, amino acids; FFA, free fatty acid.
50
45
40
35
30
25
20
15
10
5
55
0
120
insulin output
glucose sensitivity
100
80
60
40
20
0
140
60
300
260
220
180
140
100
340
380
Glucose-induced insulin output
(nmol.m–2)
β-Cell glucose sensitivity
2-hour plasma glucose (mg / dL)
NGT
IGT
T2D
• Fig. 33.11  Median values of total insulin output over the 2 hours of a standard 
oral glucose tolerance test (blue squares) and beta-cell glucose sensitivity 
(gree

 squares) and beta-cell glucose sensitivity 
(green squares) are plotted against the 2-hour plasma glucose concentrations 
to highlight the different behavior of absolute insulin release and beta-cell func­
tion across progressive stages of glucose intolerance. NGT, normal glucose 
tolerance; IGT, impaired glucose tolerance; T2D, type 2 diabetes. (Data from 
Mari A, Tura A, Natali A, et al. Influence of hyperinsulinemia and insulin resis­
tance on in vivo β-cell function: their role in human β-c

n in vivo β-cell function: their role in human β-cell dysfunction. Diabe­
tes. 2011;60[12]:3141–3147 and Ferrannini E, Gastaldelli A, Miyazaki Y, et al. 
Beta-cell function in subjects spanning from normal glucose tolerance to overt 
diabetes: a new analysis. J Clin Endocrinol Metab. 2005;90[1]:493–500.)
1348
  Insulin Secretion Parameters in Individuals With NGT, IFG, IGT, and T2D
NGT
(n = 1189)
IFG
(n = 33)
IGT
(n = 140)
T2D.1
(n = 

(n = 1189)
IFG
(n = 33)
IGT
(n = 140)
T2D.1
(n = 56)
T2D.2
(n = 56)
Fasting plasma glucose (mg/dL)
91
113
95
143
220
2-hour plasma glucose (mg/dL)
97
113
152
264
364
Fasting insulin secretion rate (pmol.min-1.m-2)
68
86
96
117
104
Total insulin output (2 hours) (nmol.m-2.h)
39
44
50
33
21
Beta-cell glucose sensitivity (pmol.min-1.m-2.mM-1)
118
83
64
26
10
Rate sensitivity (nmol.m-2.mM-1)
0.83
0.47
0.81
0.34
0.14
1.72
1.94
1.34
1.10
1.00
Insulin sensitivity (μmol.min-1.kgFFM

34
1.10
1.00
Insulin sensitivity (μmol.min-1.kgFFM.nM-1)
131
126
77
38
32
IFG, Impaired fasting glycemia; IGT, impaired glucose tolerance; NGT, normal glucose tolerance; T2D, type 2 diabetes.
Entries are median values. The T2D group is split at the median fasting plasma glucose concentration (T2D.1 and T2D.2).
   
TABLE 33.2
acids). Genetic makeup (and epigenetic modifications), chronic 
hyperglycemia, and other acquired insults impair beta-cell secre­
tory dynamics (glucose sensitivity and pote

secre­
tory dynamics (glucose sensitivity and potentiation) and compro­
mise islet structure and cellular phenotype through the expression 
of risk gene variants; glucose- and lipotoxicity; and, quite possibly, 
insulin resistance itself.102-104
An important concept is the opposite behavior of absolute 
insulin release and the glucose sensitivity of the beta cell across 
stages of glucose intolerance as reconstructed by physiologic mod­
eling of OGTT data (Fig. 33.11). In the conventional diagno

OGTT data (Fig. 33.11). In the conventional diagnostic 
categories of impaired fasting glycemia and IGT, insulin release is 
increased (to compensate for the insulin resistance), whereas beta-
cell glucose sensitivity is already markedly decreased and continues 
to decline monotonically throughout progressive hyperglycemia in 
overt T2D. In impaired fasting glycemia, for example, insulin out­
put is increased by 15%, insulin sensitivity (on an insulin clamp) 
is only slightly impaired, but gluco

sulin clamp) 
is only slightly impaired, but glucose sensitivity is 30% lower105 
(Table 33.2). Concordant with these cross-sectional data is the 
finding that beta-cell glucose sensitivity is a powerful negative pre­
dictor of incident T2D in nondiabetic cohorts above and beyond 
the impact of conventional risk factors (sex, age, body mass index, 
family history, etc.). In these models, insulin output typically fea­
tures as an independent positive predictor106 (it is reduced only in 
long-stan

ive predictor106 (it is reduced only in 
long-standing, severely hyperglycemic T2D patients). Also of note 
is that the model-derived potentiation factor picks up the incretin 
defect that is consistently observed in patients with T2D and, to a 
lesser extent, in IGT individuals. Thus, the pathogenesis and natu­
ral history of human diabetes can be construed as a recapitulation 
of the physiology of insulin secretion.

 insulin secretion in the dog. Endocrinology. 
1986;118:1551–1557.
	16.	 Rosario W, Singh I, Wautlet A, et al. The brain-to-pancreatic islet 
neuronal map reveals differential glucose regulation from distinct 
hypothalamic regions. Diabetes. 2016;65:2711–2723.
	17.	 Juang JH, Pengh SJ, Kuo CH, et al. Three-dimensional islet graft 
histology: panoramic imaging of neural plasticity in sympathetic 
innervation of transplanted islets under the kidney capsule. Am J 
Physiol Endocrinol Metab. 2014;3

rrannini E, Pilo A. Pattern of insulin delivery after intravenous 
glucose injection in man and its relation to plasma glucose disap­
pearance. J Clin Invest. 1979;64:243–254.
	27.	 Byrne MM, Sturis J, Polonsky KS. Insulin secretion and clear­
ance during low-dose graded glucose infusion. Am J Physiol. 
1995;268:E21–E27.
	28.	 Van Cauter E, Mestrez F, Sturis J, et  al. Estimation of insulin 
secretion rates from C-peptide levels. Comparison of individual 
and standard kinetic parameters for C-

ip of 
insulin sensitivity, insulin secretion, and adiposity with insulin 
clearance in a multiethnic population: the insulin resistance athero­
sclerosis study. Diabetes Care. 2013;36:101–103.
	37.	 Eggleston EM, Jahn LA, Barrett EJ. Hyperinsulinemia rapidly 
increases human muscle microvascular perfusion but fails to 
increase muscle insulin clearance: evidence that a saturable process 
mediates muscle insulin uptake. Diabetes. 2007;56:2958–2963.
	38.	 Tamaki M, Fujitani Y, Hara A, et al. Th

ion of nitric oxide 
synthesis in non-diabetic individuals produces a significant deteriora­
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 Cavaghan MK, Toffolo G, et al. Oral glucose tolerance 
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 Influence of endog­
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1349
34
Pathophysiology of Type 2 
C. RONALD KAHN, HEATHER A. FERRIS, AND BRIAN T. O’NEILL
KEY POINTS
 • 
Type 2 diabetes mellitus (T2DM) is one of the most common 
health problems facing mankind and is a major public health 
problem. The International Diabetes Federation estimated in 

ronmental influences.
 • 
Insulin receptors are nearly ubiquitously expressed, yet the 
physiologic actions of insulin are highly tissue specific, pro­
moting calorie storage in the postprandial state via pleotropic 
effects on growth, metabolism, and cell survival.
Type 2 diabetes mellitus (T2DM) is the predominant form of dia­
betes worldwide, accounting for 90% to 95% of cases globally. An 
epidemic of T2DM is under way in both developed and developing 
countries. The Internatio

eveloped and developing 
countries. The International Diabetes Federation estimated in 2017 
that 425 million people have diabetes worldwide and that by 2045 
this number will rise to 629 million. Of those with diabetes cur­
rently, 79% live in low- and middle-income countries. The largest 
percentage of increases in diabetes cases are predicted for Africa, 
the Middle East, and Southeast Asia, where the greatest numbers 
of people are moving from low to middle income.1 In the United 
States, th

m low to middle income.1 In the United 
States, the Centers for Disease Control and Prevention estimated 
in 2017 that 30.3 million people, or 9.4% of the population, had 
diabetes and that 7.2 million of them (23.8%) were undiagnosed. 
Based on fasting glucose or hemoglobin A1c levels, they also esti­
mated that 84 million people (34% of adults older than 20 years) 
had prediabetes and thus were at high risk of developing diabetes.2
The economic burden of diabetes is enormous. The International

 burden of diabetes is enormous. The International 
Diabetes Federation estimated that in 2017, diabetes-related health 
expenditures amounted to $727 billion.1 The American Diabetes 
Association estimated that in the United States alone in 2017, diabe­
tes accounted for $237 billion in direct health care expenditures and 
an additional $90 billion in lost productivity. Average expenditures 
per person, adjusted for age and gender, were 2.3-fold higher com­
pared with the nondiabetic US populati

higher com­
pared with the nondiabetic US population. The increased spending 
was driven by both the increased prevalence of the disease and the 
costs associated with care.3 In addition, the high cost of diabetes 
medications, including insulin, leaves many patients in the United 
States unable to afford the medications they are prescribed.
Epidemiology, 1349
Pathogenesis, 1350
Insulin Signaling, 1354
Insulin Resistance and the Risk of Type 2 Diabetes Mellitus, 1360
Special Cond

isk of Type 2 Diabetes Mellitus, 1360
Special Conditions That Induce Insulin Resistance, 1367
1350
Considerable information is available on the factors that are 
responsible for the development of T2DM, and these determi­
nants are summarized in Table 34.1.4,5 T2DM is thought to occur 
in genetically predisposed persons who are exposed to a series of 
environmental influences that precipitate the onset of clinical 
disease. Although T2

pitate the onset of clinical 
disease. Although T2DM is strongly associated with a high body 
mass index (BMI), there are clear differences in fat distribution 
and the degree of excess fat that is required to drive susceptibility 
to T2DM.6 For example, individuals with high levels of visceral 
fat (central obesity) are clearly more insulin resistant and at higher 
risk for T2DM than those with high levels of peripheral, mainly 
subcutaneous, fat. Likewise, individuals from Southeast Asia are 

t. Likewise, individuals from Southeast Asia are 
at high risk of developing T2DM at lower BMIs than other eth­
nicities due to greater proportional visceral adipose stores. Histori­
cally, when ethnicities prone to developing T2DM at lower BMIs 
migrate to the United States, they see weight gain and a rapid 
rise in the rate of T2DM with the change to a Western diet.7,8 
Not surprisingly, as many people in Asia adopt a more Western 
diet, the rates of T2DM increase even within those countries w

tes of T2DM increase even within those countries with 
previously low incidence rates, and there is no longer a migration 
effect because the diet had already changed.9 In the United States, 
the prevalence of T2DM varies by race and ethnicity, with Native 
American populations having twice the rate of diabetes at 15.1% 
than non-Hispanic whites at 7.4%2 (Fig. 34.1).
Historically, T2DM was viewed as a disorder of aging. 
Although this remains true today, the prevalence of obesity and 
T2DM in ch

e today, the prevalence of obesity and 
T2DM in children has risen dramatically over the past decade. 
Thus, in the past, the overwhelming majority of children with dia­
betes had type 1 diabetes mellitus (T1DM), with only 1% to 2% 
of diabetic children considered to have T2DM or other rare forms 
of diabetes. Recent reports suggest that as many as 20% to 25% 
of children in the United States with newly diagnosed diabetes 
have non-T1DM, i.e., non–immune-mediated, forms of the dis­
ease. Most of

n–immune-mediated, forms of the dis­
ease. Most of these children have T2DM, but the rarer monogenic 
types are also being increasingly identified. Nevertheless, T2DM 
in children remains relatively rare, with an estimated prevalence of 
5 cases per 10,000 children.2,10 
The pathogenesis of T2DM is complex and involves the interac­
tion of genetic and environmental factors. Several environmental 
factors have been shown to play a critical role in the development 
of the disease, par

tical role in the development 
of the disease, particularly excessive caloric intake leading to obe­
sity and a sedentary lifestyle (Fig. 34.2). Other environmental 
factors that impact obesity and T2DM development, including 
epigenetics, medications, inflammation, circadian rhythm disrup­
tions, and the microbiome, also need to be considered. The clini­
cal presentation of T2DM is heterogeneous, with a wide range 
in age at onset, severity of associated hyperglycemia, and degree 
of obesity. F

ed glucose production by the liver leading to hypergly­
cemia in the fasting state.
In addition, people with T2DM may have hyperglucagonemia, 
alterations in incretin hormone secretion or action, accelerated lipoly­
sis in the fat cell, increased renal tubular reabsorption, and abnormali­
ties in central nervous system (CNS) regulation of metabolism.11
From a pathophysiologic standpoint, many argue that it is the 
inability of the pancreatic beta cell to adapt to the reductions in insu­
lin sens

 cell to adapt to the reductions in insu­
lin sensitivity that precipitates the clinical onset of T2DM. What is 
clear is that the earliest detectable abnormality in those predisposed 
to T2DM is insulin resistance. Indeed, insulin resistance may pre­
cede T2DM by many years.12 The most common factors that place 
an increased secretory burden on the beta cell are additional insulin 
resistance factors such as puberty, pregnancy, a sedentary lifestyle, and 
overeating leading to weight gain. An u

tyle, and 
overeating leading to weight gain. An underlying genetic predisposi­
tion appears to be a factor in determining the frequency with which 
beta-cell failure occurs, although like T2DM itself, no single genetic 
alternation has been identified as the driver of beta-cell failure.
Genetic Factors in the Development 
of Type 2 Diabetes Mellitus
The precise way in which genetic, environmental, and pathophysi­
ologic factors interact to lead to the clinical onset of T2DM is com­
plex and may

to the clinical onset of T2DM is com­
plex and may vary from individual to individual. The common forms 
of T2DM are polygenic in nature and are caused by an interaction 
between these genes and multiple environmental factors (known and 
unknown), as well as epigenetic factors. Therefore, it is not surprising 
that the individual genes involved in the common polygenic forms 
of T2DM have been difficult to identify and their contribution to 
disease development modest. The monogenic forms, althou

se development modest. The monogenic forms, although rela­
tively uncommon, are important to recognize because they have led 
to important insights about normal physiology; if identified early, 
they may be important in selecting appropriate therapy. Several genes 
involved in monogenic forms have been identified and characterized. 
Monogenic Forms of Diabetes Associated With 
Insulin Resistance
In the monogenic forms of diabetes, the gene involved is both 
necessary and sufficient to cause dise

ed is both 
necessary and sufficient to cause disease. In other words, envi­
ronmental factors play little or no role in determining whether a 
  Epidemiologic Determinants of and Risk 
Factors for Type 2 Diabetes Mellitus
Genetic markers
Demographic Characteristics
Age
Ethnicity
Behavioral and Lifestyle-Related Risk Factors
Obesity (including distribution of obesity and duration)
Physical inactivity
Diet
Stress
Westernization, urbanization, modernization
Medicati

sternization, urbanization, modernization
Shift work
Metabolic Determinants and Intermediate-Risk Categories 
of Type 2 Diabetes
Gestational diabetes
Offspring of women with diabetes during pregnancy
Intrauterine malnutrition or overnutrition
Microbiome composition
TABLE 34.1
1351
Chapter 34  Pathophysiology of Type 2  Diabetes Mellitus
genetically predisposed person develops clinical diabetes, although 
there is some variation in penetra

etes, although 
there is some variation in penetrance of disease among individuals 
with the same genetic defect. The monogenic forms of diabetes 
are usually diagnosed in younger patients, often in the first two to 
three decades of life; however, if only mild, asymptomatic eleva­
tions in blood glucose occur, the diagnosis may be missed until 
later in life. The monogenic forms of diabetes can be divided into 
those in which the mechanism is a defect in insulin secretion and 
those that involv

defect in insulin secretion and 
those that involve defective responses to insulin or insulin resis­
tance. In this chapter, we will discuss the forms associated with 
insulin resistance.
Mutations in the Insulin Receptor
Numerous mutations have been identified in the insulin recep­
tor gene in various insulin-resistant patients. Although there is a 
spectrum of disorders, at least three clinical syndromes caused by 
mutations in the insulin receptor gene have been described. Type 
A insulin res

ptor gene have been described. Type 
A insulin resistance is defined by the presence of insulin resistance, 
acanthosis nigricans, and hyperandrogenism. Most often, these 
patients are identified as adolescents or young adults, often based 
on the acanthosis nigricans and/or signs of hyperandrogenism, 
rather than by their disturbances in glucose metabolism, even 
though insulin resistance can be very severe and accompanied 
by marked hyperinsulinemia.13 Patients with Donohue syndrome 
(formerly

nemia.13 Patients with Donohue syndrome 
(formerly called leprechaunism), however, have multiple early 
abnormalities, including severe intrauterine growth retarda­
tion, abnormal facies leading to the name of the syndrome, and 
death within the first 1 to 2 years of life.14-16 Interestingly, the 
patients may have episodes of hypoglycemia despite their severe 
insulin resistance. Rabson-Mendenhall syndrome is associated with 
short stature, protuberant abdomen, and abnormalities of teeth 
and n

uberant abdomen, and abnormalities of teeth 
and nails; pineal hyperplasia was a characteristic in the original 
description of this syndrome.17
These mutations impair receptor function by several different 
mechanisms. The majority of mutations associated with type A 
insulin resistance are in the intracellular tyrosine kinase domain, 
whereas in Donohue and Rabson-Mendenhall syndromes, the 
mutations are more frequently found in the extracellular domain, 
causing impairment in ligand binding, 

ar domain, 
causing impairment in ligand binding, or the FnIII domains, 
which are key for receptor folding.18 As noted earlier, the insulin 
resistance that is associated with these insulin receptor mutations 
is usually severe, manifesting in the neonatal period (e.g., Dono­
hue and Rabson-Mendenhall syndromes), or it can occur in a 
milder forms in adulthood, and is often most easily detected bio­
chemically by marked hyperinsulinemia and clinically by acan­
thosis nigricans often in the abse

ically by acan­
thosis nigricans often in the absence of obesity. Some individuals 
with insulin receptor mutations are able to remain normoglyce­
mic due to massive elevations of endogenous insulin secretion, 
whereas others have presented with hyperglycemia that fails to 
respond to insulin therapy sometimes in doses exceeding 10,000 
units/day.13 
Lipodystrophic Diabetes
Lipodystrophic diabetes syndromes, which can either be genetic 
or acquired, are syndromes of severe insulin resistance ass

ed, are syndromes of severe insulin resistance associated 
with lipoatrophy (loss of fat) and lipodystrophy (loss and mald­
istribution of fat). These forms of diabetes are characterized by a 
paucity of fat, insulin resistance, and hypertriglyceridemia. The 
genetic forms can be divided into generalized lipodystrophies or 
partial lipodystrophies; however, within each of these categories, 
mutations in many different genes have been identified. The gene 
0
10
5
15
Percentage
Race/Ethnicity
AI/A

The gene 
0
10
5
15
AI/AN
14.9% 15.3%
Hispanic
12.6% 11.7%
Black,
non-Hispanic
12.2%
13.2%
Asian
9.0%
7.3%
White,
8.1%
6.8%
Men
Women
• Fig. 34.1  Estimated prevalence of diabetes among US adults 18 years or older, age-adjusted data for 
2013–2015. Error bars represent upper and lower bounds of the 95% confidence interval. AI/AN, Ameri­
can Indian/Alaska Native. (From Centers for Disease Control and Prevention. National Diabetes Statistics 
Report, 2017. Av

on. National Diabetes Statistics 
Report, 2017. Available at https://www.cdc.gov/diabetes/data/statistics/statistics-report.html. Accessed 
October 3, 2018.)
Genetics
Obesity,
Insulin
& Diabetes
Gut
Microbiome
Diet
• Fig. 34.2  Genetics, the environment, diet, and the gut microbiome are 
among many interacting factors influencing the development of obesity, 
insulin resistance, and diabetes.
1352
mutations that c

f Carbohydrate and Fat Metabolism
mutations that cause lipodystrophy control a variety of functions 
including insulin signaling (AKT2, PIK3R1), caveolins (CAV1, 
PTRF), phospholipid biosynthesis (AGPAT2, PCYT1A), lipid 
droplet morphology (LMNA, BSCL2, ZMPSTE24), adipogen­
esis (CIDEC, PPARG), and lipolysis (LIPE, PLIN1).19 Depend­
ing on the gene mutation and the particular mutation within 
the gene, the associated syndrome may be confined to loss of fat 
and the associated metabolic abnormali

oss of fat 
and the associated metabolic abnormalities or may cause a syn­
drome with a wider array of phenotypes. For example, mutation 
in the lamin A gene (LMNA) can cause either the face-sparing 
familial partial lipodystrophy commonly referred to as Dunnigan 
syndrome or through a different mutation in the same gene can 
cause mandibuloacral dysplasia syndrome, resulting in a partial 
lipodystrophy, accompanied by postnatal growth retardation, 
and craniofacial and skeletal malformations.20

n, 
and craniofacial and skeletal malformations.20,21 The generalized 
lipodystrophies are quite rare but easily diagnosed by the loss of 
subcutaneous fat throughout the entire body. These syndromes 
cause severe metabolic abnormalities, including severe fatty liver 
disease, sometimes leading to ascites and esophageal varices. Gen­
eralized lipodystrophy is also associated with very low levels of 
the adipose tissue–derived hormones leptin and adiponectin.22 
Leptin-replacement therapy improve

adiponectin.22 
Leptin-replacement therapy improves glycemic control, decreases 
fatty liver disease, and decreases circulating triglyceride levels in 
patients with lipodystrophy and leptin deficiency.23
In contrast, the partial lipodystrophies are frequently missed, 
and thus the incidence is unknown. The phenotypes of the partial 
lipodystrophies vary significantly by mutation and gender. Leptin 
levels may be in the normal range or slightly below the normal 
range but do not show the elevati

elow the normal 
range but do not show the elevations generally seen with obesity.24 
Diagnosis is made by clinical phenotype, with genetic testing gen­
erally only performed in research settings.
Acquired generalized lipodystrophy (also known as Seip-Law­
rence syndrome) is a rare condition that may appear during child­
hood, adolescence, or young adulthood, characterized by fat loss 
affecting large areas of the body, initially usually the face, arms, 
and legs. Acquired generalized lipodystro

, arms, 
and legs. Acquired generalized lipodystrophy is thought to be an 
autoimmune disorder with secondary destruction of the adipose 
organ, but definitive proof of autoimmunity is lacking. 
Genetics of the Polygenic Forms of Type 2 
The common polygenic form of T2DM has complex pathophysi­
ology, with genetic and environmental factors playing major roles. 
The phenotypic manifestations of the disease are also complex 
and include resistance to the action of insulin in the 

nclude resistance to the action of insulin in the muscle, fat, 
and liver and defects in insulin secretory responses from the pan­
creatic beta cell. Together, these lead to reduced glucose uptake 
and increased hepatic glucose production. However, the primary 
defect or defects responsible for the development of the syn­
drome remain elusive. Indeed, with more than 100 genes linked 
to T2DM, the role of these genes and the nature of the gene-
environment interactions that are ultimately respons

vironment interactions that are ultimately responsible for devel­
opment of the disorder in predisposed persons remain unknown.
Insulin resistance is present in persons predisposed to T2DM 
long before the onset of hyperglycemia, and this finding has been 
interpreted by some to indicate that insulin resistance is the pri­
mary abnormality responsible for the development of T2DM. 
Since some patients with severe insulin resistance can mount 
extreme hyperinsulinemic responses, it seems likely th

eme hyperinsulinemic responses, it seems likely that in typical 
T2DM with less profound insulin resistance, defective beta-cell 
function is also present before the onset of T2DM, as impaired 
glucose tolerance (IGT) develops. In fact, altered insulin secretion 
has been reported in first-degree relatives of persons with T2DM 
who have normal plasma glucose concentrations.12 Therefore, 
although there is still controversy about whether insulin resistance 
or abnormal insulin secretion represent

esistance 
or abnormal insulin secretion represents the primary defect in 
T2DM, there is general consensus that both defects are present 
in essentially all subjects with the disorder at clinical presentation.
Over the past decade, there have been dramatic advances in under­
standing the role of genes in T2DM. Earlier genetic studies relied 
either on the candidate gene approach, in which the search for diabe­
tes genes is dictated by the prevailing understanding of the pathways 
involved in gl

ling understanding of the pathways 
involved in glucose regulation, or on linkage studies, which involve 
identifying regions of chromosomal DNA that are shared to excess by 
affected family members. Although these approaches did allow iden­
tification of important diabetes genes, especially in monogenic forms 
of the disease, more recent studies have focused on the application 
of genome-wide associations (GWAS). These GWAS use an unbi­
ased interrogation of the entire genome comparing cases an

errogation of the entire genome comparing cases and controls 
to determine which single-nucleotide polymorphisms (SNPs) are 
associated with disease. The location of the SNP can suggest associ­
ated genes in the region, but often SNPs are in noncoding intragenic 
regions, making the causative gene unclear. The genes that have been 
implicated in the pathogenesis of T2DM are depicted in Fig. 34.3. 
The following sections provide a summary of some of the genes that 
have strong evidence in the pat

of the genes that 
have strong evidence in the pathogenesis of T2DM.
Insulin Receptor Substrate 1 Gene
The first polymorphism identified for T2DM was Gly972Arg in 
IRS1, a key protein in the canonical insulin signaling pathway. This 
common polymorphism was detected by direct sequencing of 86 
patients with T2DM and 76 controls, with three times as many 
cases present in cases than controls.25 Subsequent studies demon­
strated that this polymorphism results in impaired insulin-stimulated 
phosph

ism results in impaired insulin-stimulated 
phosphatidylinositol 3-kinase signaling.26 As genomic information 
expanded, it became clear that this polymorphism plays only a minor 
role in overall T2DM risk; however, this initial finding provided the 
first evidence of the genetic association with T2DM. 
Transcription Factor 7–Like 2 Gene
Grant and colleagues27 genotyped 228 microsatellite markers in 
Icelandic patients with T2DM and in control subjects. A microsat­
ellite, DG10S478, in intron 3 

bjects. A microsat­
ellite, DG10S478, in intron 3 of the transcription factor 7–like 2 
gene (TCF7L2; formerly TCF4) was associated with T2DM. This 
was replicated in a Danish cohort and in a US cohort. Compared 
with noncarriers, heterozygous and homozygous carriers of the at-
risk alleles (38% and 7% of the population, respectively) have rela­
tive risks of 1.45 and 2.41 for development of T2DM. Follow-up 
studies showed that specific polymorphisms in TCF7L2 increase the 
risk of progression f

isms in TCF7L2 increase the 
risk of progression from IGT to T2DM, and this effect is associ­
ated with a reduction of glucose-induced insulin secretion.28 The 
TCF7L2 gene product is a high-mobility group box containing tran­
scription factor previously implicated in colon cancer. In T2DM, it 
has been suggested to act through regulation of proglucagon gene 
expression in enteroendocrine cells by impacting on the Wnt signal­
ing pathway. Of the common variants that determine diabetes risk, 
var

common variants that determine diabetes risk, 
variants at the TCF7L2 locus have the greatest impact. However, 
the diabetes-associated polymorphism marks an intronic variant, 
and how or if this affects expression or function of the TCF7L2 
mRNA or protein remains unknown. 
KATP Channel Genes: KCNJ11 and ABCC8
The beta-cell KATP channel is composed of two subunits, Kir6.2 
(KCNJ11) and SUR1 (ABCC8). Kir6.2 is the pore-forming 
1353
compon

athophysiology of Type 2  Diabetes Mellitus
component of the potassium channel, whereas SUR1 is the sulfo­
nylurea receptor that regulates opening of the channel. Opening 
of the channel results in potassium efflux from the beta cell, caus­
ing insulin secretion. Mutations in these genes are the most com­
mon cause of neonatal diabetes. Identification of these patients is 
important because these forms of neonatal diabetes are generally 
treatable with sulfonylureas rather than insulin.29 The tw

e with sulfonylureas rather than insulin.29 The two genes 
are located adjacent to each other on chromosome 11, and SNPs 
within both genes are associated with an increased risk of diabe­
tes.30 A missense mutation Glu23Lys (E23K) in KCNJ11 has also 
been associated with increased risk of typical adult-onset T2DM 
by an average of 13%, and the KK homozygote is at greatest risk 
(relative risk, 1.28).31 A recent study suggests that, like neonatal 
diabetes caused by mutations in KCNJ11 and ABCC8,

diabetes caused by mutations in KCNJ11 and ABCC8, polymor­
phisms in these genes, which increase the risk for T2DM, may be 
more readily treated with sulfonylureas.32 
Peroxisome Proliferator-Activated Receptor γ Gene
PPARγ is a member of the PPAR subfamily of nuclear receptors. 
It is an important regulator of lipid and glucose homeostasis and 
cellular differentiation. Although PPARγ is most abundantly 
expressed in adipose tissue, it is also expressed in muscle and in the 
pancreatic beta ce

expressed in muscle and in the 
pancreatic beta cell. Targeted elimination of the receptor in mus­
cle alters muscle fatty acid oxidation,33 and knockout in the beta 
cell leads to a blunting of the normal increase in beta-cell mass 
that occurs on a high-fat diet.34 Meta-analyses show that the com­
mon missense mutation Pro12Ala (P12A) in PPARG (the gene 
encoding PPARγ2) is associated with decreased risk for T2DM 
(estimated risk ratio for the alanine allele, 0.79).35 People homo­
zygous for t

alanine allele, 0.79).35 People homo­
zygous for the Pro12 allele are more insulin resistant than those 
with one Ala12 allele and have a 1.25-fold increased risk of diabe­
tes. A second polymorphism, C161 → T, has been linked to insulin 
resistance in Hispanic and non-Hispanic white women.36 Loss of 
function mutations in PPAR cause familial partial lipodystrophy 
type 3 (FPLD3), and these patients may show dramatic improve­
ment when treated with the thiazolidinedione PPAR agonists.37 
Hepatoc

th the thiazolidinedione PPAR agonists.37 
Hepatocyte Nuclear Factor 4α Gene
Despite its name, HNF4A was the first discovered MODY (Matu­
rity Onset Diabetes of the Young) gene, where it leads to abnormali­
ties in insulin secretion.38 HNF4α is also active in the liver, where 
it is upregulated by an alternative promoter from the pancreas and 
is important for gluconeogenesis. Patients with the risk alleles have 
increased hepatic glucose production, not impaired insulin secre­
tion, confirming 

ion, not impaired insulin secre­
tion, confirming the liver-specific impact of the polymorphisms 
in the alternative hepatic promoter.39 Several clinical studies have 
demonstrated that genetic variation in this liver-specific promoter 
for the HNF4A gene is associated with increased risk of T2DM.39-42 
BMI
WHR
Fasting
insulin 
FTO
MC4R
GCKR
IRS1 PPARG
ANKRD55
ARL15
ANK1 AP3S2 BCAR1
BCL11A BCL2 CCND2 CDC123 CENTD2 CHCHD9
CILP2 DUSP9 FAF1 FAM58A GATAD2A GPSM1
GRK5 HCCA2 HHEX HMG20

P9 FAF1 FAM58A GATAD2A GPSM1
GRK5 HCCA2 HHEX HMG20A HMGA2 HNF1A
HNF1B HNF4A INS JAZF1 KCNJ11 KCNK16 KCNQ1
KLHDC5 LAMA1 LEP LPP MACF1 MAEA
MPHOSPH9 NOTCH2 PAM PAX4 PEPD POU5F1
PRC1 PSMD6 PTPRD RASGRP1
RBM43 RBMS1 SGCG SLC16A13 SPRY2 SREBF1
SRR ST6GAL1 THADA TLE1 TLE4 TMEM154
TMEM163 TP53INP1 TSPAN8 UBE2E2 VPS26A
WFS1 ZFAND3 ZFAND6 ZMIZ1
BDNF CADM2 CDKAL1 ETV5
FAIM2 FANCL FLJ35779
GALNT10 GNPDA2 GP2
GPRC5B KCTD15 KLF9
LRP1B LRRN6C MAP2K5
MIR148A MTCH2 MTIF3
NEGR1 NUDT3 PCSK1
PRKD1 PTBP2 QPCTL RBJ

CH2 MTIF3
RPL27A SEC16B SH2B1
SLC39A8 TMEM160
TMEM18 TNNI3K ZNF608
CHEBRF
PPP1R3B
Waist circumference
NRXN3
TFAP2B
MSRA
QPCTL/
GIPR
ARSPO3
LYPLAL1
ADAMTS9
Fasting
glucose
GRB14/
 COBLL1
KLF14 
FAM13A1 HIP1
IGF1 PDGFC
PEPD SC4MOL
TET2 UHRF1BP1
YSK4
WARS2 DNM3 NISCH CPEB4
LY86 BTNL2 VEGFA NFE2L3
ZEB1 ITPR2 HOXC13 ZNRF3
ADCY5 C2CD4A CDKAL1
CDKN2A DGKB GCK GLIS3
IGF2BP2 MTNR1B PROX1
RREB1 SLC30A8 TCF7L2
ZBED3
ADRA2A AMT ARAP1 C12orf51
CAMK2B CRY2 DNLZ DPYSL5 F

DRA2A AMT ARAP1 C12orf51
CAMK2B CRY2 DNLZ DPYSL5 FADS1
FAM148B FOXA2 G6PC2 GLS2 GRB10
IKBKAP KL MADD MRPL33 OR4S1
P2RX2 PCSK1 PCSK1 PDX1 SIX3
SLC2A2 TOP1 WARS
• Fig. 34.3  Venn diagram of the intersection between loci associated at genome-wide significance with 
type 2 diabetes mellitus, measures of adiposity, and glucose homeostasis. Genome-wide significant asso­
ciations for six metabolic traits are shown. Gene symbols shown in the plot are by convention the closest 
gene and not necessarily t

convention the closest 
gene and not necessarily the functional gene. BMI, body mass index; WHR, waist-to-hip ratio. (Adapted 
from Grarup N, Sandholt CH, Hansen T, Pedersen O. Genetic susceptibility to type 2 diabetes and obesity: 
from genome wide association studies to rare variants and beyond. Diabetologia. 2014;57:1528–1541.)
1354
Kruppel-like Factor 14 (KLF14)
Kruppel-like factor 14 (KLF14) is a maternally imprinted tran­
scripti

14 (KLF14) is a maternally imprinted tran­
scription factor. Noncoding polymorphisms in the KLF14 gene 
are associated with increased fasting insulin, waist-to-hip ratio, 
and T2DM.43,44 Regulatory variants of KLF14 appear to influ­
ence T2DM risk by causing a redistribution of fat from gynoid 
to visceral depots and increasing adipocyte size.43 Accompanying 
in vitro experiments demonstrated that these changes are medi­
ated, in part, by an increase in preadipocyte proliferation but 
impairment

ease in preadipocyte proliferation but 
impairment in lipogenesis. Interestingly, these effects only occur 
when the polymorphism is inherited from mother to daughter. 
Diabetes Genes Identified by Genome-Wide 
Association Studies
GWAS continue to identify variants that determine genetic risk of 
T2DM and to define the genetic architecture of the disease. Fig. 
34.3 depicts the intersection between loci associated at genome-wide 
significance with T2DM and five metabolic traits strongly associ­

h T2DM and five metabolic traits strongly associ­
ated with T2DM, including BMI, waist circumference, waist-to-hip 
ratio, fasting insulin, and fasting glucose. Note that there is substan­
tial, but not complete, overlap in the loci significantly associated 
with these traits. For example, a variant in the gene CREBRF, found 
almost exclusively in the Samoan population, is associated with a 
large (1.4 kg/m2) increase in BMI per risk allele and is also associated 
with a paradoxical decrease in 

s also associated 
with a paradoxical decrease in odds ratio of 0.6 for the development 
of T2DM.45 The mechanisms driving the increase in BMI but pro­
tection from T2DM are still to be elucidated but suggest that the 
increase in adipose tissue is confined to metabolically healthy subcu­
taneous tissues rather than pro-inflammatory visceral adipose tissue.
Based on these advances, the following general comments can 
be made regarding the genetics of T2DM:
	1.	 The genes identified to date indi

substantial proportion 
of the genetic variants associated with increased risk for diabetes 
were in genes that might alter insulin secretion, in recent studies 
the number of SNPs associated with decreased insulin sensitivity 
have increased, and at least some of these effects on insulin sensi­
tivity appear to be independent of obesity.43,46,47
	4.	 The majority of the SNPs associated with risk of T2DM appear 
to reside in noncoding regions of the chromatin. Studies have 
shown that some of t

 the chromatin. Studies have 
shown that some of these allelic variations exist in open chroma­
tin regions alternatively called stretch enhancers,48 which appear 
to be bound by proteins. Stretch enhancers across the genome 
are cell type specific and are located near and associated with 
increased expression of genes involved in cell-specification, as 
opposed to housekeeping, processes.
	5.	 A large number of genes are associated with increased suscepti­
bility to this disease. Whereas the t

ed suscepti­
bility to this disease. Whereas the total number of loci linked to 
T2DM is large, they account for a small proportion (estimated 
at no more than 5–10%) of the total genetic risk for diabetes 
in the population. In the search for this missing heritability, 
increasing attention is being given to the rare variant hypoth­
esis, which states that common diseases are due to multiple, 
but rare, variants with large effects. Alternatively, there is new 
evidence that T2DM can be divided 

, there is new 
evidence that T2DM can be divided into multiple phenotypic 
subgroups with distinct genetic signatures that were difficult to 
identify when all of these subtypes were grouped together in 
previous studies.49,50 Finally, the high familial risk of T2DM 
may be due to some element of shared environment or epigen­
etic effects (see the following discussion). 
Epigenetic Risk of Diabetes
In addition to variations in the sequence of genes conferring risk of 
T2DM, epigenetic marks, wh

nes conferring risk of 
T2DM, epigenetic marks, which influence the expression of genes, 
can also influence disease risk. Detrimental epigenetic marks can 
be created in the developing fetus by both over- and undernutri­
tion.51,52 The idea that perinatal nutrition could influence the risk 
for obesity in offspring came from the observation of children born 
of pregnancies during the Dutch Hunger Winter, a period of starva­
tion in the Netherlands during World War II. During this period, 
all D

ds during World War II. During this period, 
all Dutch citizens, including pregnant mothers, were restricted to 
as little as 400 to 800 calories per day. It was subsequently noted 
that the offspring of these pregnancies were more likely to become 
obese than same sex siblings conceived during normal food avail­
ability. This is attributed to epigenetic marks that persisted 60 years 
beyond the neonatal exposure.53 Evidence of the same phenomena 
has been documented following several other fami

has been documented following several other famines around the 
world. These human observations have since been replicated in 
mouse models of undernutrition, which not only demonstrate the 
passage of obesity risk from mother to offspring, but that risk can 
be perpetuated for an additional generation by male offspring of 
undernourished mothers due to epigenetic changes to primordial 
sperm.54 Similar to undernutrition, evidence also exists that overnu­
trition during pregnancy can cause epi

hat overnu­
trition during pregnancy can cause epigenetic changes that predis­
pose offspring to obesity and T2DM for multiple generations.55,56
Insulin Signaling
Insulin signaling is initiated through binding and activation of 
its tyrosine kinase cell-surface receptor. This initiates a cascade 
of phosphorylation and dephosphorylation events, generation of 
second messengers, and protein-protein interactions that result in 
diverse metabolic events in almost every tissue. Insulin receptors 
ar

ents in almost every tissue. Insulin receptors 
are ubiquitously expressed, yet insulin action is highly tissue spe­
cific due to the varied composition of downstream targets of insu­
lin signaling. The insulin receptor consists of two insulin-binding 
α-subunits and two catalytically active β-subunits. Like insulin 
itself, the α- and β-subunits are derived from a single chain precur­
sor or prereceptor. Once the subunits are formed, they are disul­
fide linked into an α2β2 heterotetrameric com

sul­
fide linked into an α2β2 heterotetrameric complex. Insulin binds 
to the extracellular α-subunits, activating the intracellular tyrosine 
kinase domains of the β-subunits that phosphorylate intracellular 
partners on specific tyrosine residues.57 On phosphorylation of 
the receptor, multiple classes of adapter proteins including insu­
lin receptor substrates (IRSs), Src homology 2 domain containing 
(Src), growth factor receptor–bound protein (Grb), and others58 
can associate with the rece

n (Grb), and others58 
can associate with the receptor and propagate the downstream sig­
nal resulting in modulation of cellular metabolism, stimulation of 
mitogenesis,59 and receptor internalization.60 
Downstream Events After Insulin Receptor 
IRS, phosphoinositide 3-kinase (PI3K), and Akt represent three 
critical nodes of insulin signaling that regulate a majority of the 
metabolic and transcriptional effects downstream of insulin activa­
tion (Fig. 34.4). The IRS proteins a

sulin activa­
tion (Fig. 34.4). The IRS proteins are critical adaptors that mediate 
1355
many of the key metabolic actions of insulin across multiple tis­
sues. IRSs have multiple functional domains, including pleckstrin 
homology (PH), phosphotyrosine binding (PTB), and Src homol­
ogy (SH) domains that allow for interaction with phosphorylated 
tyrosine residues on the insulin receptor and the closely related insu­
lin-like growth factor

d the closely related insu­
lin-like growth factor 1 (IGF1) receptor, allowing for docking with 
downstream effectors, such as PI3K.61 There are four IRS isoforms 
(IRS1-IRS4), with IRS1 and IRS2 being ubiquitously expressed. 
Although there is evidence for functional overlap, IRS1 and IRS2 
do display unique roles in growth and metabolism across tissues. 
Disruption of IRS1 in mice resulted in mild insulin resistance and 
growth retardation, whereas disruption of IRS2 resulted in beta-cell 
fai

reas disruption of IRS2 resulted in beta-cell 
failure and secondary insulin resistance.62
PI3K generates phosphatidylinositol 3,4,5-triphosphate (PIP3), 
which activates several PIP3-dependent serine-threonine kinases, 
such as PI-dependent protein kinases 1 and 2 (PDPK1 and PDPK2), 
which in turn activate Akt isoforms, atypical PKCs, wortmannin-
sensitive and insulin-stimulated serine kinase, and others.
Akt kinase (also known as protein kinase B) exists as three dis­
tinct isoforms that are a

 B) exists as three dis­
tinct isoforms that are activated by phosphorylation on specific 
threonine and serine residues.63,64 Full activation of Akt requires 
PDPK1-directed phosphorylation at threonine 308 (T308) in 
the kinase domain (KD) followed by phosphorylation at serine 
473 (S473) in the hydrophobic motif (HM) of Akt by the rictor-
containing complex, mTOR complex 2 (mTORC2).64 Activated 
Akt has the ability to phosphorylate proteins that regulate lipid 
synthesis, glycogen synthesis, 

at regulate lipid 
synthesis, glycogen synthesis, protein synthesis, and apoptosis. 
Disruption of Akt1 results in growth retardation,65 whereas dis­
ruption of Akt2 results in insulin resistance and diabetes in mice.66 
Several investigators have examined the role of PI3K and Akt in 
persons with insulin resistance. Studies have shown a decrease in 
IRS-associated PI3K67 and Akt68 activity in insulin-resistant skel­
etal muscle; however, in some patients with reduced PI3K activity, 
there was n

 patients with reduced PI3K activity, 
there was normal activation of Akt.69 
Tissue-Specific Actions of Insulin
Mechanisms of Insulin-Mediated Glucose Uptake in 
Muscle and Fat
A primary effect of insulin in skeletal muscle and adipose tissue 
is to stimulate glucose uptake by translocation of GLUT4 from 
an intracellular pool to the surface of cells70 (Fig. 34.5A). Indeed, 
Node 1
Node 2
Node 3
p110β
p110γ
p110α
p55α
p50α
p85α
p55γ
p85β
Insulin
Insulin
receptor
IGF1
IGF1
receptor
Plasma membra

Insulin
receptor
IGF1
IGF1
receptor
Ras
Erk1
Erk2
Ras-MAPK
pathway
PI 3-kinase pathway
P
P
P
P
P
P
P
P
P
P
IRS1
IRS2
IRS3
IRS4
Cell growth,
Catalytic
Akt1
Akt2
Akt3
PIP3
Glycogen
synthesis
Glucose
uptake
Lipid
synthesis
aPKC
PDPK1,2
Shc
PI 3-kinase
Metabolic effects
Growth effects
• Fig. 34.4  Insulin signaling activates three critical nodes of regulation to mediate most of its metabolic actions 
across tissues. Node 1 contains the insu

 actions 
across tissues. Node 1 contains the insulin receptor substrate (IRS) isoforms 1 through 4. Node 2 is phosphati­
dylinositol-3-kinase (PI3K), which is comprised of a p85 or p55 regulatory subunit and a p110 catalytic subunit. 
Node 3 contains three isoforms of Akt. These three signaling nodes regulate lipid and glucose metabolism, 
as well as cell growth and differentiation, and run parallel to the Ras-mitogen–activated protein kinase (MAPK) 
pathway with some crosstalk. aPKC, atypical 

APK) 
pathway with some crosstalk. aPKC, atypical protein kinase C; ERK, extracellular signal–regulated kinase; IGF1, 
insulin-like growth factor 1; MAPK, mitogen-activated protein kinases (MEK, MAPK/ERK); PDPK, phosphoino­
sitol-3-phosphate–dependent kinase; Ras, rat sarcoma oncogene; Shc, SH3-containing protein.
PKA
A  Muscle
B  Adipose
C  Liver
Glucose
uptake
Insulin
receptor
Gene regulation
Suppression of atrophy/autophagy
Protein synthesis/growth
Glucose uptake/metabolism
Glut4
Vesicle
Insu

rowth
Glut4
Vesicle
Insulin
Exercise
AMPK
TBC1D1
AS160
GSK
MAPK
SOS
Grb2
Shc
Akt
PI3K
mTORC1
FoxOs
FoxOs
β
β
α
α
IRS
Glucose
Glucose
Insulin
receptor
Suppression of glucose production
Lipogenesis
Insulin
aPKC
SREBPs
SREBPs
Akt
PI3K
mTORC1
FoxOs
G6Pase
FoxOs
CREB
β
β
α α
IRS
Glucose
uptake
Insulin
receptor
Suppression of lipolysis
Glucose uptake
Glut4
Vesicle
Insulin
TC10
CAP
Cbl
AS160
Glucose
G3P + FA
TG
FA
TG
HSL
PKA
AC
Akt
PI

AS160
Glucose
G3P + FA
TG
FA
TG
HSL
PKA
AC
Akt
PI3K
mTORC1
ChREBP
β
β
α
α
IRS
PDE-3B
cAMP
ATGL
receptor signaling
Glucagon
signaling
Glut2
PEPCK
AC
cAMP
• Fig. 34.5  Tissue-specific insulin signaling. Insulin binds to the insulin receptor, allowing for docking and 
activation of multiple signaling molecules, most notably insulin receptor substrates (IRS), phosphatidylinositol-
3-kinase (PI3K) and Akt, which can mediate different physi

 (PI3K) and Akt, which can mediate different physiologic effects, depending on the tissue. (A) In 
muscle, insulin action increases glucose uptake/metabolism, promotes protein synthesis and suppresses 
atrophy, some of which can also be mediated by exercise. (B) In adipose, insulin increases glucose uptake 
and lipogenesis while suppressing lipolysis. (C) In liver, insulin promotes lipogenesis and counters the actions 
of glucagon, thereby suppressing glucose production. AC, adenylate cyclase; A

ssing glucose production. AC, adenylate cyclase; AMPK, adenosine monophos­
phate (AMP)–activated kinase; aPKC, atypical protein kinase C; AS160, Akt substrate of 160 kilodaltons (also 
TBC1D4); ATGL, adipose triglyceride lipase; CAP, Cbl-associated protein; Cbl, Cas-Br-M (murine) ectopic 
retroviral transforming sequence; ChREBP, carbohydrate response element–binding protein; CREB, cyclic 
AMP (cAMP) response element–binding protein; FA, fatty acid; FoxO, forkhead box O; G3P, glycerol-3-phos­
ph

id; FoxO, forkhead box O; G3P, glycerol-3-phos­
phate; G6Pase, glucose-6-phosphatase; Glut, glucose transporter; Grb2, growth factor receptor–bound 
protein 2; GSK3, glycogen synthase kinase 3; HSL, hormone-sensitive lipase; MAPK, mitogen-activated 
protein kinases (MEK, MAPK/ERK kinases); mTORC1, mammalian target of rapamycin complex 1; PEPCK, 
phosphoenolpyruvate carboxykinase; PDE-3B, phosphodiesterase 3B; PKA, protein kinase A; Shc, SH3-
containing protein; SOS, son of sevenless; SREBP, ster

aining protein; SOS, son of sevenless; SREBP, sterol responsive element–binding protein; TBC1D1, 
tre-1/USP6 BUB2 cdc16 domain family member 1; TC10, small GTP-binding protein TC10; TG, triglyceride.
1357
disruption of GLUT4 in muscle or adipose tissue causes an insulin-
resistant phenotype.71,72 Akt substrate of 160 kDa (AS160, also 
known as TBC1D4) and TBC1D1 are paralog Rab family gua­
nosine triphosphatase (GTPase)-activating (Rab GAP

nosine triphosphatase (GTPase)-activating (Rab GAP) proteins 
that have been proposed to inhibit the translocation of GLUT4 to 
the plasma membrane through interaction with insulin-responsive 
aminopeptidase (IRAP).73 AS160 activity is suppressed by Akt 
phosphorylation in response to insulin stimulation and mediates 
translocation of GLUT4-containing vesicles to the plasma mem­
brane to increase glucose uptake in muscle and adipose tissue.73,74 
TBC1D1, however, is phosphorylated by adenosine m

TBC1D1, however, is phosphorylated by adenosine monophos­
phate–activated kinase (AMPK) and appears to play a key role in 
exercise-regulated increases in muscle glucose uptake, which remain 
intact in patients with T2DM.75-77 On phosphorylation of the Rab 
GAPs, the inhibition of GLUT4 translocation by AS160 and/or 
TBC1D1 is relieved, contributing to the increased glucose uptake. 
Although the composition and signaling pathways that converge 
on the intracellular GLUT4-containing vesicles to 

on the intracellular GLUT4-containing vesicles to cause GLUT4 
translocation are still not fully understood, it appears that the num­
ber of glucose transporters in skeletal muscle of insulin-resistant 
persons is not changed, but the ability of insulin to effect GLUT4 
translocation is disrupted.78-80 In adipose tissue, PI3K-independent 
mechanisms also play a role in mediating glucose uptake. Insulin 
receptors can associate with Cbl-associated protein (CAP) and the 
E3 ubiquitin-protein ligas

 protein (CAP) and the 
E3 ubiquitin-protein ligase Cbl to activate TC10, an actin remodel­
ing enzyme.81 This interaction occurs in lipid rafts on the plasma 
membrane, contributing to GLUT4 vesicle translocation in the 
absence of IRS-PI3K-Akt activation. A similar mechanism of actin 
remodeling to regulate GLUT4 trafficking occurs in striated mus­
cle, but this is via Rac proteins and is not dependent on TC10.82
In addition to its regulation of glucose metabolism, insulin also 
has profound e

f glucose metabolism, insulin also 
has profound effects on protein homeostasis (proteostasis).83 In 
muscle, insulin promotes hypertrophy by both enhancing protein 
synthesis and suppressing protein degradation pathways includ­
ing proteasome and autophagy (see Fig. 34.5A). Insulin, via Akt, 
activates mTOR complex 1 (mTORC1) to promote protein syn­
thesis and cellular growth.84 At the same time, insulin suppresses 
forkhead box O (FoxO) transcription factor activation, a master 
regulator of p

iption factor activation, a master 
regulator of protein degradation pathways.83,85 Mouse models 
with muscle-specific deletion of insulin receptor, or both insulin 
receptor and IGF1 receptor, show profound muscle atrophy,86 but 
the atrophy is completely rescued by deletion of FoxOs.87 This 
increased protein degradation is also seen in rodent models of 
T1DM88 and in patients with T1DM when insulin is withdrawn 
for as little as 8 hours.89,90 Thus, insulin coordinates muscle pro­
tein turnove

Thus, insulin coordinates muscle pro­
tein turnover, and this regulation is dependent on suppression of 
FoxO-regulated protein degradation.
Insulin Regulation of Lipolysis and Lipogenesis
Insulin plays a critical role in regulating lipid storage. The liver 
is the primary site for de novo lipogenesis of fatty acids and tri­
glycerides, whereas adipose tissue serves as the primary storage 
depot for triglycerides. Insulin signaling enhances lipid storage 
in adipose by both stimulating triglycer

 storage 
in adipose by both stimulating triglyceride synthesis and sup­
pressing lipolytic triglyceride breakdown (see Fig. 34.5B). First, 
insulin stimulates rapid and potent increases in adipose tis­
sue glucose uptake that is then converted into glycerol-3-phos­
phate, used to synthesize triglycerides from fatty acids. Fatty 
acid transport into adipose is also increased by insulin, which 
likely involves translocation of fatty acid transporters, includ­
ing FATP1 and CD36,91 and/or increase

ers, includ­
ing FATP1 and CD36,91 and/or increases in lipoprotein lipase 
expression and activity.92-94 In addition to providing substrates 
for triglyceride synthesis, glucose also activates carbohydrate 
response element–binding proteins to upregulate glycolytic and 
lipogenic genes in adipocytes.95 This pathway to enhance lipid 
storage is critical for metabolic homeostasis, as overexpression 
of GLUT4 in adipose protects against insulin resistance caused 
by diet-induced obesity, but deleti

stance caused 
by diet-induced obesity, but deletion of carbohydrate response 
element–binding protein prevents this effect.95,96
Insulin potently suppresses adipose tissue lipolysis, which, 
when combined with the enhanced lipid uptake in adipose, can 
significantly reduce circulating fatty acid content.97 Insulin regu­
lates lipolysis at multiple levels (see Fig. 34.5B). β-adrenergic 
signaling enhances cyclic adenosine monophosphate (cAMP)-
induced protein kinase A activation, which in turn a

duced protein kinase A activation, which in turn activates two 
of the principal lipases in fat cells: adipose tissue triglyceride lipase 
(ATGL) and hormone-sensitive lipase. Insulin activates PDE-3B, 
which reduces cAMP levels and inhibits protein kinase A–medi­
ated activation of ATGL and hormone-sensitive lipase.98,99 Two 
further mechanisms by which insulin suppresses lipolysis are by 
increases in ATGL mRNA and by increased expression of FSP27, 
a lipid droplet stabilizing protein.100-102 

P27, 
a lipid droplet stabilizing protein.100-102 These actions of insulin to 
suppress lipolysis are clinically important in T1DM because newly 
diagnosed T1DM patients show a rapid gain of fat during the first 
year of insulin therapy.103,104
Regulation of fat distribution by insulin is also important in 
metabolic health. A recent genomic analysis found strong asso­
ciations between reduced adiposity in peripheral compartments, 
severe insulin resistance, and SNPs in 53 genes, among which 
in

 resistance, and SNPs in 53 genes, among which 
insulin receptor (INSR), IRS1, and a regulatory subunit of PI3K 
(PIK3R1) were identified.105 The 53 genes also correlated highly 
with familial partial lipodystrophy type 1 (FPLD1). These stud­
ies agree with data from rodent models that indicate deletion of 
insulin receptors in adipocytes leads to rapid decreases in adipose 
tissue mass, redistribution of body fat to liver, and lipodystrophic 
diabetes.106-108 Thus, insulin regulation of lipolys

abetes.106-108 Thus, insulin regulation of lipolysis and lipogen­
esis in adipose plays a critical role in storage of lipids, maintaining 
whole-body insulin sensitivity, and preventing metabolic disease.
Insulin also regulates hepatic lipogenesis, the primary site of de 
novo lipogenesis in the body.97 SREBP transcription factors are mas­
ter regulators of lipogenic genes including fatty acid synthase, lipo­
protein lipase, acetyl coenzyme A carboxylase (ACC), and stearoyl 
coenzyme A (CoA) des

boxylase (ACC), and stearoyl 
coenzyme A (CoA) desaturase isoforms, and are regulated by insu­
lin at multiple levels in liver (see Fig. 34.5C). Through activation of 
atypical PKC isoforms λ/ζ (aPKC), expression of sterol regulatory 
element-binding transcription factor 1c (SREBP1c) is increased by 
insulin in liver.109,110 Furthermore, SREBP1c requires proteolytic 
cleavage to enter the nucleus and activate transcription of lipogenic 
genes. This processing of SREBP1c is regulated by mTORC1 si

is processing of SREBP1c is regulated by mTORC1 sig­
naling downstream of the IRS-PI3K-Akt pathway,111 and mTORC1 
activation can also promote mRNA stability of SREBP1 target 
genes.112 Studies in mice and humans with insulin receptor muta­
tions show that reduced insulin signaling in the liver protects against 
hepatic steatosis.113,114 However, patients with T2DM and metabolic 
syndrome have a paradoxical upregulation of hepatic lipogenesis and 
dyslipidemia that is observed in the context of 

dyslipidemia that is observed in the context of muscle and adipose 
insulin resistance, altered CNS signaling to the liver, and increased 
nutrient supply. Although the exact mechanisms have not been elu­
cidated, the upregulation of hepatic lipogenesis in insulin-resistant 
states is proposed to be due to either a selective postreceptor insulin 
resistance113-115 or a combination of insulin resistance with altered 
substrate supply and extrahepatic signaling.116-118 
Insulin Regulation of Hep

patic signaling.116-118 
Insulin Regulation of Hepatic Carbohydrate Metabolism
The production of glucose by the liver is regulated primarily by a bal­
ance between the actions of insulin to suppress glucose production 
1358
and glucagon to activate glucose production, with insulin being 
dominant over glucagon (see Fig. 34.5C). The sympathetic ner­
vous system119 and glucose autoregulation also play roles in hepatic 
glucose production

ion also play roles in hepatic 
glucose production but are probably less important.120 The ability 
of insulin to reduce hepatic glucose output is an important mecha­
nism for maintaining normal fasting blood glucose and normal 
glucose tolerance.121,122 Insulin also suppresses glucose release from 
the liver by inhibiting glycogenolysis.123 As with gluconeogenesis, 
the effect of insulin to suppress hepatic glycogenolysis is dominant 
over glucagon.124 Glucagon increases glycogenolysis by activ

gon.124 Glucagon increases glycogenolysis by activation 
of the classic protein kinase A cascade involving cAMP and also 
increases gluconeogenesis in part by increasing transcription of 
phosphoenolpyruvate carboxykinase (PEPCK) by means of cAMP 
response element–binding protein (CREB).122,125
Data suggest that the regulatory mechanisms triggered by 
cAMP are much more complex, with the CREB transcriptional 
coactivator, TORC2, playing an important role. TORC2 is specif­
ically dephosphorylated

ant role. TORC2 is specif­
ically dephosphorylated in response to cAMP; this results in trans­
location of the TORC2 protein to the nucleus, allowing activation 
of CREB-dependent transcription of gluconeogenic enzymes.126 
In addition, CREB may increase transcription of PPARγ coacti­
vator 1α (PGC1α), which serves as a critical coactivator of the 
transcription factor forkhead box protein O1 (FoxO1). FoxO1 
plays a role in the transcriptional activation of various genes 
associated with glucone

tivation of various genes 
associated with gluconeogenesis, including PEPCK and glucose-
6-phosphorylase.125 Indeed, deletion of FoxO isoforms in the liver 
can restore glucose levels to normal in mice with insulin receptor 
knockout.127,128 Deacetylation of transcription factors and coacti­
vators can also regulate gluconeogenesis. The sirtuin, SIRT1, 
deacetylates PGC1α129 and histone deacetylases and sirtuins can 
deacetylate FOXOs,130,131 increasing their nuclear localization and 
interactio

reasing their nuclear localization and 
interaction with HNF4A. The PGC1α/FOXO1/HNF4A com­
plex is a potent activator of gluconeogenic gene transcription.125
Insulin decreases endogenous glucose production by direct and 
indirect mechanisms.132 In its direct action, portal insulin suppresses 
glucose production by inhibiting glycogenolysis and gluconeogenesis 
by insulin receptor activation (see Fig. 34.5C). The indirect or periph­
eral effect of insulin in controlling glucose production by the 

 insulin in controlling glucose production by the liver is 
twofold. First, insulin profoundly decreases glucagon secretion by the 
alpha cell of the pancreas through systemic and paracrine effects.133,134 
The decrease in glucagon secretion decreases the activation of glycoge­
nolysis and gluconeogenesis. The second important peripheral action 
of insulin is to decrease substrates for gluconeogenesis, such as alanine 
from muscle protein degradation,83 and glycerol and free fatty acid 
levels, 

tion,83 and glycerol and free fatty acid 
levels, by suppressing lipolysis in adipose tissue135 (see Fig. 34.5A,B). 
When the reduction in plasma fatty acids during a hyperinsulinemic 
clamp is prevented by infusion of triglyceride emulsions with hepa­
rin (which produces increased fatty acid levels through activation of 
lipoprotein lipase), insulin-mediated suppression of hepatic glucose 
output is reduced.136 The suppression of glucagon secretion and the 
decrease in gluconeogenic substrate d

ion and the 
decrease in gluconeogenic substrate delivery to the liver are additive in 
reducing liver glucose production.137
Clinically, hepatic insulin resistance plays an important role in 
the hyperglycemia of T2DM,138,139 and the impaired suppression 
of hepatic glucose output appears to be quantitatively similar to, or 
even larger than, the defect in stimulation of peripheral glucose dis­
posal.140 Defects in the direct effect of insulin to suppress hepatic 
glucose production in humans a

o suppress hepatic 
glucose production in humans appear to be caused by a large right­
ward shift in the steep dose-response curve for insulin’s inhibition 
of glycogenolysis.141 There is a direct relationship between increased 
hepatic glucose output and fasting hyperglycemia142 (Fig. 34.6). 
Insulin-mediated suppression of hepatic glucose output is impaired 
at both low and high plasma insulin levels in T2DM140,143; hepatic 
glucose production is elevated early in the course of the disease144,

is elevated early in the course of the disease144,145 
but may be normal in lean, relatively insulin-sensitive T2DM.146 
Treatment of patients with metformin, which suppresses hepatic 
glucose production, results in improved glucose tolerance.145 
Expanding Collection of Insulin-Sensitive Tissues
Although much of our understanding of insulin action on metab­
olism involves the insulin-sensitive metabolic tissues of muscle, 
liver, and adipose, insulin receptors are ubiquitously expressed and 
in

sulin receptors are ubiquitously expressed and 
insulin action on other “nonclassic” tissues plays an important role 
in health (Fig. 34.7). Insulin signaling in beta cells is critical for 
adaptive survival and proliferation of these cells. Insulin secretion 
and beta-cell mass both increase in response to chronic mild hyper­
glycemia, and the ability to upregulate beta-cell mass influences 
the development of T2DM. Interestingly, when insulin receptors 
are deleted from beta cells, first-phase

eceptors 
are deleted from beta cells, first-phase insulin secretion is lost and 
compensatory growth of beta cells is impaired in response to diet-
induced obesity and hepatic insulin resistance,147,148 indicating 
an unexpected autocrine loop. Furthermore, FoxO transcription 
factors, which are downstream targets of insulin action, are impor­
tant for maintenance of beta-cell function and identity,149 and 
beta cells appear to de-differentiate when FoxOs are deleted.150 
Evidence that beta-cel

when FoxOs are deleted.150 
Evidence that beta-cell dedifferentiation occurs in insulin-resistant 
states in humans are emerging,151 but the degree to which this 
phenomenon accounts for beta-cell failure in T2DM and whether 
this process can be intervened upon is still under investigation.
Insulin action on glucagon-producing alpha cells suppresses 
glucagon secretion and maintains glucose homeostasis. This was 
demonstrated by deletion of insulin receptors on alpha cells of 
mice, which leads 

in receptors on alpha cells of 
mice, which leads to mild glucose intolerance, increased glucagon 
secretion, and progressive hyperinsulinemia,134 all of which are 
features of metabolic syndrome and T2DM. Thus, insulin signal­
ing on alpha and beta cells in islets is important for metabolic 
health and glucose homeostasis.
Vascular complications are a major contributor to morbidity 
and mortality in diabetes. Thus, it may not be surprising that 
vascular endothelium and cardiomyocytes are exqui

vascular endothelium and cardiomyocytes are exquisitely insulin 
120
90
30
60
0
0
100
200
300
400
Basal hepatic glucose production
(mg/m2 per minute)
Fasting plasma glucose level
(mg/dL)
• Fig. 34.6  The relationship between fasting hepatic glucose output and 
fasting plasma glucose levels. Open squares represent nondiabetic control 
subjects; closed squares represent diabetic subjects. (From Maggs DG, 
Buchanan TA, Burant CF, et al. Metabolic effects of troglitazone mono­
therapy in type 2 dia

ffects of troglitazone mono­
therapy in type 2 diabetes mellitus: a randomized, double-blind, placebo-
controlled trial. Ann Intern Med. 1998;128:176–185. © American College 
of Physicians, used with permission.)
1359
responsive. In endothelial cells, insulin action promotes vaso­
dilation via nitric oxide production, which is impaired in insu­
lin resistant states.152-154 Endothelial insulin sensitivity is also 
important for transendothe

in sensitivity is also 
important for transendothelial transport of insulin to peripheral 
tissues,155,156 and the relative permeability of the endothelial bar­
rier in various tissues plays an important role in timing of insulin 
action in those tissues after a bolus of insulin.157 Endothelial 
insulin signaling also plays a major role in the development of 
atherosclerosis. Insulin receptor knockout in endothelial cells 
caused a two- to threefold increase in atherosclerotic lesions in 
an ath

old increase in atherosclerotic lesions in 
an atherogenic mouse model.158 Conversely, enhancing down­
stream activation of insulin signaling by either overexpression of 
IRS or inhibition of FoxOs improves vascular endothelial func­
tion and can prevent atherosclerosis.159,160 In cardiomyocytes, 
the insulin signaling cascade controls growth and metabolism 
in both physiologic and pathologic conditions.161,162 The heart 
has a remarkable ability to utilize a wide variety of substrates 
for ener

 to utilize a wide variety of substrates 
for energy production, but under fasted states, fatty acids are 
preferentially metabolized. On insulin stimulation, glucose oxi­
dation is increased and fatty acid oxidation is suppressed, but 
in insulin-resistant or diabetic states, this metabolic flexibility is 
lost.163 Insulin also plays an important role in postnatal cardiac 
growth by suppression of autophagy.164 These signals in the car­
diovascular system integrate with insulin action in other 

lar system integrate with insulin action in other tissues 
and metabolic flux to coordinate cardiovascular health. Disrup­
tion of the tissue cross talk during obesity and insulin resistance 
results in features of metabolic syndrome, which increase cardio­
vascular risk.165
Insulin can also affect cardiovascular risk by its action on 
immune cells, particularly macrophages. Macrophages play an 
important role in the development of atherosclerosis by formation 
of foam cells within atherosclerot

s by formation 
of foam cells within atherosclerotic plaques, enhanced inflamma­
tion, and apoptosis leading to a necrotic core that is susceptible to 
rupture.166 Mouse models with loss of insulin signaling in mac­
rophages show mild protective effects on atherosclerotic lesion 
size167 but also show worsening of the necrotic core, likely due 
to enhanced macrophage apoptosis from endoplasmic reticulum 
(ER) stress.168 This observation also extends to obesity and aging 
models of atherosclerosi

nds to obesity and aging 
models of atherosclerosis169,170 indicating that insulin resistance 
in macrophages contributes to plaque progression in the context 
of metabolic syndrome.171 
Insulin Signaling in the Central Nervous System
Although previously considered insulin independent, it is now 
clear that the brain is an important site for insulin signaling. Insu­
lin action is not required for glucose uptake in the brain because 
it predominantly expresses the insulin-independent glucose 
tra

tly expresses the insulin-independent glucose 
transporters Glut1 and Glut3. The insulin receptor is expressed 
throughout the brain. Brain-specific knockout of the receptor in 
mice results in a variety of abnormalities including increased body 
weight, hypothalamic hypogonadism, increased hepatic glucose 
production, and a depression-like phenotype.172 The impact of 
insulin signaling in the brain is cell-type specific. Although neu­
ronal insulin receptor appears to be key for regulation of h

lin receptor appears to be key for regulation of hepatic 
glucose production,173 astrocyte insulin receptor signaling is 
vital for insulin-mediated effects on mood.174 Activation of the 
brain insulin receptor can lead to improved synaptic transmission 
through cholesterol synthesis; proliferation through mitogen-
activated protein kinase; axon growth and neuroplasticity through 
glycogen synthase kinase 3β (GSK3β); gene transcription and 
neuronal polarity through FOXO; and autophagy, protein 

nal polarity through FOXO; and autophagy, protein syn­
thesis, and neuronal plasticity through mTORC1.175,176 Insulin 
entry into the brain shows regional differences and in some areas 
is dependent on a receptor-mediated transcytosis process.157,177 
Systemic insulin resistance results in lower than expected cerebro­
spinal fluid insulin levels due to decreased transport across the 
blood-brain barrier.178 Brain-specific resistance to both insulin 
and IGF1 signaling in humans is associated wit

in 
and IGF1 signaling in humans is associated with Alzheimer dis­
ease, but the significance of these findings to disease progression 
remains uncertain.179
Glucagon
Survival/proliferation
First-phase secretion
Insulin
Satiety,
Mood
stabilization,
synthesis
ER stress
Apoptosis
Vasodilation
NO
Glucose oxidation
Hypertrophy
Cytokine
release
Fatty acid oxidation
Autophagy
Brain/neuron
Cardiomyocyte
Endothelium
αcell
βcell
• Fig. 34.7  The expanding collection of 

ll
βcell
• Fig. 34.7  The expanding collection of insulin-sensitive tissues. Insulin receptors are ubiquitously 
expressed, and thus insulin controls pleotropic effects on numerous “nonclassical” tissues that play an 
important role in health. ER, endoplasmic reticulum.
1360
The insulin receptor has two isoforms: IR-A is missing exon 11, 
whereas IR-B includes exon 11. Isoform A has a higher affinity for 
insulin-like growth factor 2 b

igher affinity for 
insulin-like growth factor 2 binding than isoform B. In contrast to 
most other tissues in the body, the brain predominantly expresses 
isoform A. Other unique aspects of brain insulin signaling include 
a unique glycosylation pattern of the brain insulin receptor and 
the propensity for insulin receptor to form heterodimers with the 
more abundant IGF1 receptor, which appears to favor IGF1 signal­
ing over insulin signaling.180 Greater understanding of the relative 
impacts 

80 Greater understanding of the relative 
impacts of insulin and IGF1 signaling on the various cell types of 
the brain may prove critical as clinical trials are under way in test­
ing the efficacy of insulin delivered to the brain intranasally to treat 
such diverse disorders as Alzheimer disease, depression, and obesity. 
Insulin Resistance and the Risk of Type 2 
The term insulin resistance indicates the presence of an impaired 
biologic response to either

esence of an impaired 
biologic response to either exogenously administered or endog­
enously secreted insulin. Insulin resistance is primarily manifested 
by decreased insulin-stimulated glucose transport and metabolism 
in skeletal muscle, impaired insulin suppression of adipocyte lipoly­
sis, and impaired ability of insulin to suppress hepatic glucose out­
put. However, given the pleiotropic actions of insulin, it is clear 
that insulin resistance could lead to disorders of multiple metabolic

ance could lead to disorders of multiple metabolic 
pathways involving amino acids, glucose, and lipid metabolism.
Insulin resistance can be measured using a variety of techniques. 
The gold standard for determining insulin sensitivity/resistance 
in an individual is a hyperinsulinemic euglycemic clamp. In this 
technique, developed by DeFronzo, a patient is given a constant 
infusion of insulin to produce hyperinsulinemia. A second infu­
sion containing glucose is given concurrently and adjuste

ntaining glucose is given concurrently and adjusted to pro­
duce euglycemia. Since the individual is in steady state, the glucose 
infusion rate represents the rate of glucose uptake/disposal into 
muscle, fat, and other tissues under hyperinsulinemic conditions 
(i.e., the insulin sensitivity of the patient).181 When conducted with 
tracers, one can also measure the hepatic glucose production rate 
as a measure of liver insulin sensitivity. Because this technique is 
costly, invasive, and time 

use this technique is 
costly, invasive, and time consuming, alternative approaches have 
been developed to estimate insulin resistance, such as the steady-
state plasma glucose method of Reaven182 or the Bergman mini­
mal model of glucose disposal assessed using a frequently sampled 
intravenous glucose tolerance test.183 When coupled with insulin 
levels, one can also obtain the Disposition Index, which is a com­
bined measure of insulin sensitivity and insulin secretion.184,185 For 
large coh

ivity and insulin secretion.184,185 For 
large cohort studies or a simple measure in an outpatient setting, 
HOMA-IR (homeostatic model assessment of insulin resistance) 
can be calculated using just fasting glucose and insulin levels. It cor­
relates favorably with a hyperinsulinemic euglycemic clamp with an 
Rs of 0.88.186 Others favor the use of QUICKI (quantitative insu­
lin-sensitivity check index), as an alternative mathematical model, 
also based on the reciprocal of the logarithm of HOMA

o based on the reciprocal of the logarithm of HOMA-IR.187
Insulin sensitivity is influenced by several factors, including age, 
weight, ethnicity, body fat (especially intra-abdominal obesity), 
physical activity, dietary intake, gut microbiota, and medications. 
Substantial data indicate that insulin resistance plays a major role in 
the development of IGT and diabetes. Insulin resistance is a consis­
tent finding in patients with T2DM, and resistance is present years 
before the onset of diabe

stance is present years 
before the onset of diabetes.188-191 Prospective studies have shown 
that insulin resistance predicts the onset of diabetes.189,190 There is 
also a strong influence of environmental factors on the genetic pre­
disposition to insulin resistance and therefore to diabetes.192,193
The molecular mechanisms of insulin resistance are initiated by 
various pathophysiologic processes that feed back on the insulin 
receptor and its adapter proteins (Fig. 34.8). The insulin recep­

s adapter proteins (Fig. 34.8). The insulin recep­
tor β-subunit has been shown to undergo serine/threonine phos­
phorylation, which might decrease the ability of the receptor to 
autophosphorylate. The activities of several serine/threonine pro­
tein kinases that catalyze inhibitory phosphorylation of the insulin 
receptor or IRS proteins are elevated in animal models of insulin 
resistance and in insulin-resistant humans.194,195 Interventions 
that decrease serine phosphorylation (inhibitory s

that decrease serine phosphorylation (inhibitory signal) of the 
insulin receptor result in increased insulin signaling.196 Further­
more, enhanced tyrosine dephosphorylation by protein tyrosine 
phosphatases can attenuate insulin signaling. Two protein tyrosine 
phosphatases that have been shown to negatively regulate insulin 
signaling, PTP1B and LAR (leukocyte antigen related), have been 
reported to be elevated in insulin-resistant patients.197,198 Con­
versely, disruption of PTB1B in mice r

97,198 Con­
versely, disruption of PTB1B in mice resulted in a marked increase 
in insulin sensitivity and resistance to diet-induced obesity.199
As mentioned previously, mutations in the insulin receptor 
are associated with rare forms of insulin resistance. In addition, 
individuals may develop autoantibodies to the insulin receptor, 
which induce severe insulin resistance (type B insulin resistance 
and acanthosis nigricans).13,200 
Obesity and Type 2 Diabetes Mellitus
The association of obes

d Type 2 Diabetes Mellitus
The association of obesity with T2DM has been recognized for 
decades (Fig. 34.9). Central (also referred to as intra-abdominal or 
visceral) adiposity is more strongly linked to insulin resistance and 
to several important metabolic variables, including elevated plasma 
glucose, insulin, total plasma cholesterol and triglyceride concen­
trations, and decreased plasma high-density lipoprotein cholesterol 
concentration, than is total adiposity.201-207 Indeed, some stud

 than is total adiposity.201-207 Indeed, some studies 
have suggested that subcutaneous fat may actually be protective 
against insulin resistance.208 As a result, the association between 
abdominal fat and glucose tolerance is independent of total adipos­
ity.209,210 The reason for the relationship between intra-abdominal 
fat and abnormal metabolism is not clearly defined, but several 
hypotheses have been proposed and may interact in this associa­
tion. Abdominal fat is more lipolytically act

cia­
tion. Abdominal fat is more lipolytically active than subcutane­
ous fat, perhaps because of its greater complement of adrenergic 
receptors.211 In addition, the abdominal adipose store is resistant to 
the antilipolytic effects of insulin,212 including alterations in lipo­
protein lipase activity; this leads to increased lipase activity and a 
greater flux of fatty acids into the circulation, with the portal circu­
lation receiving the greatest fatty acid load. Conversely, subcutane­
ous f

test fatty acid load. Conversely, subcutane­
ous fat makes and releases more adiponectin, which is a beneficial 
adipokine. Finally, the high levels of 11β-hydroxysteroid dehydro­
genase type 1 (HSD11B1) in intra-abdominal fat could result in 
enhanced conversion of inactive cortisone to active cortisol, result­
ing in increased local cortisol production. This might change adi­
pocytes to increase lipolysis and alter the production of adipokines, 
which may directly modulate glucose metabolism. 

Hyperinsulinemia and Insulin Resistance
Hyperinsulinemia can cause insulin resistance. Elevated concen­
trations of insulin downregulates insulin receptors and desensitizes 
postreceptor pathways.213 Del Prato and colleagues214 showed 
that 24 and 72 hours of sustained physiologic hyperinsulinemia 
1361
in normal persons specifically inhibited the ability of insulin to 
increase nonoxidati

ted the ability of insulin to 
increase nonoxidative glucose disposal in association with an 
impaired ability of insulin to stimulate glycogen synthase activity. 
Suppression of insulin secretion in obese, insulin-resistant persons 
results in increased insulin sensitivity.215,216
In cell systems, serine phosphorylation of IRS proteins inhib­
its insulin signaling and is mediated by a variety of kinases (see 
Fig. 34.8), including mammalian target of rapamycin (mTOR)/
S6 kinase (S6K), which is 

et of rapamycin (mTOR)/
S6 kinase (S6K), which is downstream of the PI3K-Akt pathway. 
Feedback serine phosphorylation of IRSs on appropriate residues 
might increase ubiquitination and degradation IRS isoforms, 
which would result in decreased insulin signaling.217 However, 
the mechanisms appear much more complex in vivo, because a 
mouse model with a serine to alanine mutation at 302 on IRS1, 
the prime target of mTOR-induced disruption of IRS signal­
ing, did not prevent insulin resistance f

 signal­
ing, did not prevent insulin resistance from mTOR activation in 
the liver.218 Signaling via mTOR can also increase growth factor 
receptor bound protein 10 (Grb10) levels by phosphorylation.219 
Grb10 and Grb14 are negative regulators of insulin signaling, 
which act in key metabolic tissues including muscle and adi­
pose,220 and contain polymorphisms in humans that are linked to 
development of T2DM.221 Extracellular signal–regulated kinase 
Fat cells
ER stress/UPR
Mitochondrial
dysfu

inase 
Fat cells
Ectopic
lipid
Nucleus
UPR
SOCS
Stat
Jak
PTP1B
Grb10/14
Muscle or
liver cell
Insulin action
Microbiome/
metabolite
alterations
Adipose tissue inflammation
Cytokine
receptor
Toll-like
receptor
Insulin
receptor
TNFR
JNK
PKC
ROS
Toxic lipids
DAG/cerimides
IKK
P-Ser- IRS
PI3K
S6K
mTORC1
PERK/IRE1
XPB-1
Glycation/
O-GlcNAc
Adipose
tissue
macrophage
TNFα
IL6
TNFα
IL6
FAs
PAI-1
RBP4
Adiponectin
Resistin
TNFα
IL-6
FAs

FAs
PAI-1
RBP4
Resistin
TNFα
IL-6
FAs
PAI-1
RBP4
Resistin
Obesity
LPS
FA
• Fig. 34.8  Cellular mechanisms of insulin resistance in overnutrition. Numerous environmental and patho­
logic insults can combine to impair cellular insulin signaling leading to insulin resistance. Overnutrition 
leads to adipose tissue inflammation, hyperinsulinemia, hyperglycemia, and microbiome/metabolite altera­
tions in the body. These are associated with ectopic lipid accumulation, mitochon

sociated with ectopic lipid accumulation, mitochondrial dysfunction, and 
endoplasmic reticulum (ER) stress/unfolded protein response (UPR) in insulin-sensitive tissues. A com­
mon mechanism of insulin resistance is activation of serine/threonine kinases (shown in red font) that 
phosphorylate insulin receptor and IRS proteins leading to feedback inhibition of these proximal signaling 
intermediates in the insulin cascade. Another mechanism is protein-protein interactions that decrease 
insulin 

otein-protein interactions that decrease 
insulin receptor phosphorylation or inhibit its interaction with IRS proteins. DAG, diacylglycerol; ERK, 
extracellular signal–regulated kinase; FA, fatty acid; Grb, growth factor receptor-bound protein; IKK, IκB 
kinase; IRS, insulin receptor substrates; IL6, interleukin 6; Jak, janus kinase; JNK, c-Jun N-terminal kinase; 
LPS, lipopolysaccharide; mTORC1, mammalian target of rapamycin complex 1; O-GlcNAc, O-linked β-N-
acetylglucosamine; PI3K, phosphati

, O-linked β-N-
acetylglucosamine; PI3K, phosphatidylinositol-3-kinase; PKC, protein kinase C; PTP1B, protein tyrosine 
phosphatase 1B; ROS, reactive oxygen species; S6K, ribosomal protein S6 kinase; Shc, SH3-containing 
protein; SOCS, suppressor of cytokine signaling; SOS, son of sevenless; Stat, Signal transducer and acti­
vator of transcription; TNFα, tumor necrosis factor α.
1362
350
300
250
200
SSPG (mg/dL)
Body Mass Index (kg/m2)

0
300
250
200
Waist Cirumference (cm)
150
100
50
0
15
20
25
30
35
40
45 50
60
70
80
90 100 110 120 130 140
p < 0.001
p < 0.001
r = 0.58
r = 0.57
8
10
6
4
2
0
Insulin sensitivity index
(× 10–5 min–1/pM)
20
22
24
26
Intra-abdominal fat area (cm2)
28
30
32
r = .59
p < 0.05
B
A
• Fig. 34.9  Relationships between body mass index, waist circumference, and insulin resistance measured 
by steady-state plasma glucose (SSPG) (A) or intra-abdominal fat and insulin sensi

SSPG) (A) or intra-abdominal fat and insulin sensitivity (B). (A from Farin 
MF, Fahim A, Reaven GM. Body mass index and waist circumference both contribute to differences in 
insulin-mediated glucose disposal in nondiabetic adults. Am J Clin Nutr. 2006;83:47–51; B from Kahn SE, 
Prigeon RL, McCulloch DK, et al. Quantification of the relationship between insulin sensitivity and beta-cell 
function in human subjects: evidence for a hyperbolic function. Diabetes. 1993;42:1663–1672.)
(ERK) has also

tion. Diabetes. 1993;42:1663–1672.)
(ERK) has also been shown to feed back and inhibit IRS function 
and is downstream of insulin-mediated mitogen-activated pro­
tein kinase activation. Last, FoxO transcription factors regulate 
transcription of the insulin receptor and many inhibitors of Akt 
action, including Tribbles 3 (Trb3) and protein phosphatase 2A 
(PP2A).222 Thus, activation of the insulin signaling cascade feeds 
back at the level of IRS and other insulin targets, with chronic 
activat

S and other insulin targets, with chronic 
activation from hyperinsulinemia itself impairing insulin signal­
ing by chronic feedback inhibition.
Nutrient Overload and Insulin Resistance
Cells have developed several ways to sense incoming nutrients, 
including direct and indirect activation of transcription factors 
and protein kinases. Amino acids stimulate growth by direct acti­
vation of mTOR complex 1.84 Fatty acids are sensed by several 
mechanisms, including cell surface receptors, such as 

anisms, including cell surface receptors, such as toll-like 
receptors (TLRs), and by transcription factors, including PPARα 
and PPARγ.223 Glucose itself can be converted to O-linked β-N-
acetylglucosamine (O-GlcNAC) post-translational modifications 
that modify enzymes on Ser/Thr residues, acting similar to or 
opposing phosphorylation.224 Many of these signals provide feed­
back inhibition on the insulin receptor cascade through Ser/Thr 
kinase activation (including S6K, c-Jun N-terminal kina

e activation (including S6K, c-Jun N-terminal kinase (JNK), 
and inhibitor of κB kinase (IKK).217,225-227 However, chronic 
overnutrition can alter these normal cellular signals, causing insu­
lin resistance (see Fig. 34.8). In a sense, the reaction of different 
tissues to obesity may be a relatively normal physiologic response 
to excess nutrient delivery, with prolonged activation leading to 
unintended and pathologic states that result in insulin resistance, 
inflammation, and even cell deat

ulin resistance, 
inflammation, and even cell death.
Adipose Tissue and Insulin Resistance
To maintain metabolic homeostasis when nutrient intake exceeds 
energy expenditure, the excess calories must be used to increase 
cellular mass or it must be stored. Most excess nutrients, whether 
carbohydrate, protein, or lipid, are ultimately stored as triglyceride 
in white adipose tissue. If the storage capacity of adipose tissue is 
exceeded, lipids and other nutrients enter nonstorage tissues. This 

nd other nutrients enter nonstorage tissues. This 
ectopic lipid accumulation occurs in myocytes, hepatocytes, vas­
cular cells, and beta cells, and can produce toxic lipid metabolites 
(e.g., diacylglycerol or ceramides) that trigger activation of PKC 
isoforms among other adaptive and maladaptive cellular responses 
that lead to insulin resistance.228 Indeed, some data suggest that 
the inability to expand fat mass in response to overeating may be 
an important factor in the development of ins

 be 
an important factor in the development of insulin resistance.229
Adipocytes are more than storage cells. They regulate the 
uptake and release of fatty acids; participate in the glycerol–fatty 
acid cycle; release leptin and other hormones that signal the energy 
1363
status of the body; and secrete an ever-expanding number of cyto­
kines that have hormonal, paracrine, and autocrine actions.230 
The adipocyte itself can be adversely a

tions.230 
The adipocyte itself can be adversely affected by accumulation of 
excess nutrients, leading to events that can have adverse conse­
quences on the body. As adipocyte surface area increases in obe­
sity, there is increased expression of leptin, interleukin 6 (IL6), 
IL8, monocyte chemoattractant protein 1 (MCP1), and granulo­
cyte colony-stimulating factor. These and possibly other cytokines 
attract pro-inflammatory macrophages (M1 type), which release 
factors such as tumor necrosis 

e), which release 
factors such as tumor necrosis factor α (TNFα) that may have 
local and systemic inflammatory effects.231
In addition to energy-storing white adipose tissue, humans and 
other mammals have energy-burning brown adipose tissue. Brown 
adipose tissue is a thermogenic tissue that contains unique adipocytes 
with distinct morphology including multiloculated lipid droplets, 
increased mitochondrial content, and expression of the uncoupling 
protein 1 (UCP1) to uncouple electron tran

upling 
protein 1 (UCP1) to uncouple electron transport and generate heat. 
Brown adipose tissue is activated by the sympathetic nervous system, 
which increases the mobilization and oxidation of fatty acids.232 The 
content of brown adipose tissue in humans is negatively correlated 
with age and obesity, which may be a consequence or a contributor 
to insulin resistance in these conditions.233 In humans, prolonged or 
repeated cold exposure can increase the mass and activity of brown 
adipose t

increase the mass and activity of brown 
adipose tissue in the neck and supraclavicular regions, as defined 
by the uptake of glucose, and can improve glucose homeostasis.234 
It has been recognized that rodents and humans also have beige or 
brite (brown in white) adipocytes,235 which appear mixed into some 
white adipose tissue depots following cold or hormonal stimuli. Like 
brown adipocytes, beige adipocytes express UCP1; however, the 
developmental origin of these cells appears to be distin

pmental origin of these cells appears to be distinct from brown 
adipose tissue. The degree to which humans can increase beige adi­
pocytes and how it may impact metabolism remains unknown but 
presents a potential therapeutic target. 
Ectopic Lipid Accumulation
When the storage capacity of adipose tissue is exceeded, lipids accu­
mulate in tissues such as muscle and the liver that are not well adapted 
to lipid storage, a process known as ectopic lipid accumulation. This 
leads to metabolic dys

c lipid accumulation. This 
leads to metabolic dysfunction in these tissues. In muscle, insulin-
stimulated glucose uptake is inversely related to the amount of intra­
muscular triglycerides measured by biopsy, computed tomography, 
and magnetic resonance imaging , which can distinguish intramyo­
cellular from extramyocellular fat.236 First-degree relatives of T2DM 
patients have an increase in intramyocellular fat, and in this group 
there is also a correlation with insulin resistance.237
The m

so a correlation with insulin resistance.237
The mechanism for accumulation of triglyceride in the skeletal 
muscle of obese and insulin-resistant persons is probably related 
to mismatching of fatty acid uptake and oxidation. During rest­
ing postabsorptive conditions, about 30% of fatty acid flux in the 
plasma pool is accounted for by oxidation, and the remaining 
70% of flux is recycled into triglyceride, indicating a physiologic 
reserve that exceeds immediate tissue needs for oxidative sub

t exceeds immediate tissue needs for oxidative sub­
strates. The uptake, transport, and metabolism of fatty acids are 
highly regulated processes, and alteration of the balance between 
uptake and oxidation in skeletal muscle leads to increased intra­
myocellular triglycerides. The increased lipolysis associated with 
obesity provides an increased amount of fatty acids presented to 
muscle, which again appears to activate PKC isoforms to feedback 
on insulin signaling intermediates.238,239
Howev

However, increased muscle triglyceride content is not invari­
ably linked to insulin resistance. Indeed, exercise training is asso­
ciated with increased muscle triglyceride content,240 and chronic 
exercise increases both insulin sensitivity and the capacity for fatty 
acid oxidation.241-243 The reason for this dissociation is not com­
pletely understood, but recent studies show that perilipin proteins 
PLIN2, PLIN3, and PLIN5 rapidly associate with 

s 
PLIN2, PLIN3, and PLIN5 rapidly associate with lipid droplets 
in muscle from trained individuals but not sedentary individu­
als.244 These data suggest that exercise training allows for improved 
lipid storage in muscle, thereby sequestering lipid intermediates 
and preventing fatty acid–induced insulin resistance, which is also 
seen with acute exercise.245
Lipid accumulation in the liver is a common feature of insulin 
resistance and T2DM often referred to as nonalcoholic fatty liver 
dise

ften referred to as nonalcoholic fatty liver 
disease (NAFLD).246 Enhanced substrate delivery to the liver in 
insulin-resistant states likely plays a key role in this lipid accumula­
tion.117,228 Enhanced fatty acid delivery from adipose tissue lipolysis 
and increased postprandial glucose concentrations due to muscle 
insulin resistance provide excess substrates for the liver that result 
in accumulation of acetyl-CoA and toxic lipid intermediates.247 
Humans with lipodystrophy and mouse model

tes.247 
Humans with lipodystrophy and mouse models of lipodystrophy 
develop NAFLD and severe insulin resistance indicating that with­
out the capacity to store lipids in adipose tissue, the liver becomes 
metabolically dysfunctional.107,248,249 However, excess lipid intake 
is not the only way to develop NAFLD. Indeed, feeding mice excess 
glucose or fructose induces metabolic pathways in the liver that lead 
to NAFLD.250,251 These data indicate that a combination of excess 
macronutrients and

e that a combination of excess 
macronutrients and decreased adipose tissue storage promote lipid 
accumulation in the liver that is concurrent with insulin resistance. 
Endoplasmic Reticulum Stress/Unfolded Protein 
Response
The ER functions in the post-translational processing of protein, 
including protein folding, maturation, quality control, and traf­
ficking to other cellular compartments. As part of its quality con­
trol machinery, when the ER accumulates excess levels of unfolded 
or mis

e ER accumulates excess levels of unfolded 
or misfolded proteins, a distinct series of reactions occurs that 
slows overall protein synthesis while increasing the production of 
chaperones and other proteins that increase the fidelity of protein 
processing. Three pathways are commonly activated to mediate 
the unfolded protein response (UPR) and alleviate ER workload: 
RNA-dependent protein kinase–like ER kinase (PERK), inositol-
requiring enzyme 1 (IRE1), and activating transcription factor 6

me 1 (IRE1), and activating transcription factor 6 
(ATF6). PERK phosphorylates eukaryotic translation initiation 
factor 2 (eIF2α), resulting in inhibition of most protein synthe­
sis. IRE1 induces cleavage of X-box–binding protein 1 (XBP1) 
mRNA, which results in production of the active XBP1 tran­
scription factor. XBP1, in combination with ATF6α, activates 
transcription to produce chaperones and proteins involved in ER 
biogenesis, phospholipid synthesis, and secretion.252
In states of over

pid synthesis, and secretion.252
In states of overfeeding and obesity, there can be activation of 
the UPR in the liver, adipose tissue, pancreatic beta cells, muscle, 
and other tissues. The activation of UPR in response to overnu­
trition is thought to have several effects, including activation of 
the JNK and nuclear factor–κB (NF-κB)/inhibitor of κB kinase 
(IKK) pathways leading to decreased IRS1 activity; increased levels 
of endogenous inflammatory mediators; alteration in SREBP1-
mediate

lammatory mediators; alteration in SREBP1-
mediated transcription; reduction in hepatic gluconeogenesis; and, 
after prolonged activation, cellular dysfunction and apoptosis.253 
Innate Immunity
The innate immune system was originally thought to be a cellular 
system that allowed discrimination of self and nonself to adapt cel­
lular metabolism to fight microbial pathogens. However, this system 
is now recognized as a general response to cellular stress that activates 
1364
SECTION VIII    Disor

 stress that activates 
1364
the inflammation and cellular repair systems. The innate immune 
system contains a series of pattern-recognition receptors to detect 
microbial motifs. These proteins, including TLRs and C-type lec­
tins, are expressed on a variety of cell types, including macrophages, 
monocytes, dendritic cells, neutrophils, epithelial cells, and cells of the 
adaptive immune system. The pattern-recognition receptors dete

une system. The pattern-recognition receptors detect 
extracellular and intracellular pathogen-related molecules, including 
lipids and nucleic acids, and initiate a stereotypical response, includ­
ing NF-κB activation and activator protein 1 (AP1) transcription, 
which increases expression of cytokines and chemokines.
Activation of the innate pathway also increases the production 
of so-called inflammasomes—large, multisubunit protein com­
plexes that are important in the control of caspase 1–m

s that are important in the control of caspase 1–mediated, 
post-translational maturation and secretion of interleukins, pri­
marily IL1β, that have potent pro-inflammatory responses and are 
a risk factor for development of T2DM, perhaps through disrup­
tion of beta-cell function.254
Activation of the innate immune system in acute infection and obe­
sity is associated with significant insulin resistance. In obesity, this 
is mediated in part by elevated levels of fatty acids. TLRs, especially 

elevated levels of fatty acids. TLRs, especially 
TLR2 and TLR4, which normally respond to bacterial cell wall lip­
ids to induce the innate inflammatory response, are also activated 
by circulating saturated fatty acids. Conversely, polyunsaturated fatty 
acids inhibit TLR signaling. Activation of TLRs in cells results in 
insulin resistance, whereas genetic disruption of the TLR4 receptor 
in mice is protective against fatty acid–induced insulin resistance.227 
Mitochondrial Abnormalities
A de

n resistance.227 
A decrease in oxidative capacity is seen in both humans and ani­
mals with insulin resistance, obesity, and T2DM.255,256 Studies 
have suggested that increases in intramyocellular fat content in 
skeletal muscle associated with insulin resistance may be caused 
by alterations in mitochondrial mass. In one study, young insulin-
resistant offspring of parents with T2DM demonstrated a 60% 
reduction in insulin-stimulated skeletal muscle glucose uptake 

nsulin-stimulated skeletal muscle glucose uptake 
compared with control subjects, and this reduction correlated 
with an increase of approximately 80% in intramyocellular lipid 
content.257 The elevated intramyocellular lipid content was attrib­
utable to the 30% reduction in mitochondrial oxidative capacity, 
which may be due to the lower ratio of the mitochondria-rich 
oxidative type I fibers to glycolytic type II muscle fibers.
Decreased expression of nuclear-encoded genes that regulate 
mito

ssion of nuclear-encoded genes that regulate 
mitochondrial biogenesis, such as PPARγ coactivators 1α and 
1β (PGC1α and PGC1β, respectively), have been shown to be 
important for mitochondrial biogenesis and for fiber type selec­
tion during development.258 PGC1α and PGC1β, and their 
target genes, were found to be downregulated in obese patients 
with IGT and T2DM.259-261 The activity of the electron transport 
chain is reduced and intramyofibrillar mitochondria are smaller in 
patients with T

illar mitochondria are smaller in 
patients with T2DM, and both of these measures correlate with 
severity of insulin resistance.262 A study in which rats were bred 
for differences in oxidative capacity (determined by their intrinsic 
ability to run) suggests that the defects found in humans could 
have a genetic basis.263 The group with poor aerobic capacity had 
several significant abnormalities, including obesity, insulin resis­
tance, hypertension, and dyslipidemia. The metabolic decline al

ension, and dyslipidemia. The metabolic decline also 
correlated with a reduction in mitochondrial gene expression and 
PGC1α in skeletal muscle, similar to that seen in humans. These 
data lead to the hypothesis that reversing the observed mitochon­
drial mass decline in muscle of T2DM patients may be a way to 
improve their metabolism. However, the potential beneficial effect 
of increasing mitochondrial mass by activation of PGC1α activity 
has been dampened by the finding that overexpression

s been dampened by the finding that overexpression of PGC1α 
in mouse skeletal muscle does not improve metabolic status fol­
lowing a high-fat diet.264
Some studies have questioned the cause-effect relationship 
between alterations in mitochondrial mass/function and skeletal 
muscle insulin resistance. Rather than mitochondrial insufficiency 
being an inherited trait, these observed changes could be acquired. 
First, insulin itself can upregulate mitochondrial biogenesis, and 
withdrawal of insu

 mitochondrial biogenesis, and 
withdrawal of insulin in patients with T1DM leads to reductions 
in mitochondrial gene transcription and adenosine triphosphate 
(ATP) synthesis.265 Second, the observed reduction in ATP synthe­
sis and tricarboxylic acid (TCA) cycle activity could be explained 
by reduced turnover of ATP in relatively sedentary individuals. In 
the absence of energy demand (e.g., exercise), the reduction in the 
adenosine diphosphate/ATP ratio would impair electron trans­
port in

ate/ATP ratio would impair electron trans­
port in the mitochondria, increasing levels of the reduced form of 
nicotinamide adenine dinucleotide (NADH), which would impair 
TCA cycle activity. This would result in the generation of reac­
tive oxygen species due to elevated reducing pressure in the mito­
chondria.266 Indicative of impaired oxidation in the mitochondria, 
mice fed a high-fat diet show a reduction in TCA cycle interme­
diates and an impairment of adequate β-oxidation of fatty acids

 impairment of adequate β-oxidation of fatty acids, 
marked by increases in even-chained acyl-carnitine levels. Increases 
in plasma acyl-carnitines are found in obese, insulin-resistant indi­
viduals, and it appears that they are markers of resistance.267,268
Post-translational modification of mitochondrial proteins by 
acetylation (as well as succinylation and malonylation) of lysine 
residues on each protein has a marked effect on their activity and 
provides a potentially important mechanism

ty and 
provides a potentially important mechanism for control of mito­
chondrial flux and insulin resistance.269 In general, the acetyla­
tion events reduce mitochondrial enzyme activity. No enzymes 
promoting acetylation have been found. Rather, these appear 
to occur through pH-dependent, nonenzymatic chemical reac­
tions wherein acetyl-CoA or other acyl-CoAs act as donors for 
exposed lysine residues. An increase in flux of nutrients in the 
mitochondria is expected to increase acetylation p

mitochondria is expected to increase acetylation proportional to 
the steady-state levels of acetylation. An imbalance of formation 
to utilization, which occurs in overnutrition, would therefore be 
expected to inhibit further fuel utilization. The mitochondrial 
sirtuin, Sirt3, is the primary deacetylase of mitochondria270 that 
is activated by nicotinamide adenine dinucleotide (NAD+) and 
can deacetylate critical metabolic enzymes, including long-chain 
acyl-CoA dehydrogenase (LCAD) in the li

ong-chain 
acyl-CoA dehydrogenase (LCAD) in the liver271 and pyruvate 
dehydrogenase complex in muscle.272 Similarly, the NAD+-
dependent Sirt5 leads to desuccinylation and demalonylation of 
mitochondrial enzymes.273,274 As ATP use increases, such as with 
exercise,275 increased electron flow from NADH to support oxida­
tive phosphorylation will result in increased intramitochondrial 
NAD+, leading to Sirt3- and Sirt5-mediated protein deacylation 
and increased mitochondrial capacity for substr

n 
and increased mitochondrial capacity for substrate utilization. 
Skeletal Muscle Insulin Resistance
The primary site of glucose disposal after a meal is skeletal mus­
cle, and the primary mechanism of glucose storage is through 
its conversion to glycogen.276 In obesity, insulin resistance in 
skeletal muscle is manifested before abnormalities in insulin sig­
naling in adipose tissue and the liver, possibly reflecting the rela­
tively limited nutrient storage capacity of skeletal muscle. Stud

nutrient storage capacity of skeletal muscle. Studies 
using the hyperinsulinemic-euglycemic clamp technique have 
demonstrated that in insulin-resistant people (with and with­
out T2DM), there is a deficiency in the nonoxidative disposal of 
1365
glucose related primarily to a defect in glycogen synthesis, which 
itself is secondary to a decrease in insulin-stimulated glucose 
uptake277,278 (Fig. 34.10).
Elevated fatty acids predict the p

8 (Fig. 34.10).
Elevated fatty acids predict the progression from IGT to diabe­
tes,279,280 although peripheral fatty acids might not be markedly ele­
vated because of efficient extraction by the liver and skeletal muscle. 
Increased fatty acid flux to skeletal muscle related to increased vis­
ceral lipolysis has been implicated in the inhibition of muscle glu­
cose uptake. The glucose–fatty acid cycle was originally proposed to 
account for the ability of fatty acids to inhibit muscle glucose u

ability of fatty acids to inhibit muscle glucose utiliza­
tion. Randle’s group281 hypothesized that fatty acids compete with 
glucose for substrate oxidation in isolated muscle. They found that 
increased fatty acid metabolism leads to an increase in the ratio of 
intramitochondrial acetyl-CoA to CoA and reduced nicotinamide 
adenine dinucleotide (NADH/NAD+) ratios, which leads to inhibi­
tion of pyruvate dehydrogenase. The resulting increased intracellular 
mitochondrial (and cytosolic) citrate

tracellular 
mitochondrial (and cytosolic) citrate concentrations result in allo­
steric inhibition of phosphofructokinase, the key rate-controlling 
enzyme in glycolysis. Subsequent accumulation of glucose 6-phos­
phate inhibits hexokinase II activity, resulting in an increase in intra­
cellular glucose concentrations and decreased glucose uptake.
More recent in vivo studies in humans suggest that the primary 
effect of fatty acids, at least in the presence of elevated insulin lev­
els, is a de

the presence of elevated insulin lev­
els, is a decrease in glucose transport, as measured by a reduction 
in the rate of accumulation of intracellular glucose and glycogen 
using 13C and 31P nuclear magnetic resonance spectroscopy. In 
normal subjects, elevated fatty acids, achieved by infusion of tri­
glyceride emulsions and heparin (which activates lipoprotein lipase 
resulting to release fatty acids into the circulation), resulted in a 
fall in intracellular glucose and glucose 6-phosphate c

in intracellular glucose and glucose 6-phosphate concentra­
tions that precede the fall in glycogen accumulation.282,283 These 
results challenge the Randle hypothesis (which predicts a rise in 
intracellular glucose 6-phosphate concentrations) as the basis of 
the reduction in insulin sensitivity seen with elevated fatty acids. 
Similar decreases in glucose transport have been seen in patients 
with T2DM and in lean, normoglycemic, insulin-resistant off­
spring of T2DM parents.284 These studies

tant off­
spring of T2DM parents.284 These studies also find decreased 
activity of PI3K and increased activity of novel forms of protein 
kinase C, such as PKCθ, PKCδ, and others, that might, in part, 
mediate the effect of elevated fatty acids.238,239
Studies also suggest that PKC-mediated serine phosphory­
lation of the IKKβ subunit, leading to its degradation, and the 
unregulated translocation of NF-κB into the nucleus, might also 
be important to fatty acid–induced insulin resistance.285 T

ant to fatty acid–induced insulin resistance.285 This is 
the mechanism by which high-dose aspirin and salsalate improve 
glucose metabolism in T2DM.285,286 Disruption of the IKKβ 
inflammatory pathway by high-dose salicylate therapy in a small 
human trial resulted in improvements in insulin sensitivity.
Fatty Acid Metabolism in Skeletal Muscle
The uptake of fatty acid from the serum, where it is mostly bound to 
albumin, is mediated by at least three families of proteins: fatty acid 
transloca

 three families of proteins: fatty acid 
translocase, plasma membrane fatty acid–binding proteins (FABPs), 
and fatty acid transport proteins.287-289 The levels of the putative trans­
port proteins are regulated by exercise,290 correlated with body weight 
(at least in women), and can be modulated by insulin infusion.291
FABPs are capable of binding multiple hydrophobic ligands, 
including fatty acids, eicosanoids, and retinoids, with high affin­
ity.292 FABPs are thought to facilitate uptake of

ity.292 FABPs are thought to facilitate uptake of fatty acids and to 
promote subsequent intracellular transport to subcellular organ­
elles.293 There is a direct correlation between heart-type FABP 
content and oxidative capacity observed during development and 
among different muscle types.294,295 In mice that have a disrup­
tion of the heart isoform296 or the adipocyte isoform297 of FABP, 
plasma fatty acid concentrations are significantly elevated and 
plasma glucose is decreased, suggestin

evated and 
plasma glucose is decreased, suggesting a key role in normal regu­
lation of fatty acid oxidation. Some,297 but not all,298 studies have 
shown a decrease in heart-type FABP in insulin-resistant humans.
Carnitine palmitoyltransferase 1 (CPT1) has been the subject 
of intense scrutiny for many years because of its central role in the 
balance between mitochondrial glucose and fatty acid metabolism, 
primarily because malonyl-CoA inhibits mitochondrial fatty acid 
uptake by inhibiting 

ts mitochondrial fatty acid 
uptake by inhibiting CPT1 in the liver.299,300 A specific isoform 
contributes 97% of the CPT1 in muscle and has 100-fold lower 
sensitivity to inhibition by malonyl-CoA.301 This lower sensitiv­
ity to malonyl-CoA inhibition suggests that the levels of CPT1 
itself, as well as malonyl-CoA, may be important in the balance of 
uptake and oxidation of fatty acids. Evidence for this in skeletal 
muscle stems from the finding that overexpression of CPT1 in 
rodent muscle 

ing that overexpression of CPT1 in 
rodent muscle increased fatty acid oxidation and reduced intra­
myocellular triglyceride.302 Furthermore, muscle CPT1 mRNA, 
as with other fatty acid–oxidizing enzymes, is regulated by PPARα 
activators, fat feeding, and exercise in rodents and is inversely cor­
related with obesity in humans.303-306
Long-chain fatty acids, after passing through the inner mito­
chondrial membrane as acylcarnitines, are metabolized at the 
surface of the inner mitochondrial mem

zed at the 
surface of the inner mitochondrial membrane by CPT2 and the 
long chain–specific oxidation system consisting of very long chain 
acyl-CoA dehydrogenase (VLCAD) and the trifunctional protein 
oxidation complex (Fig. 34.11). Transfer of the acyl chain from 
carnitine to CoA, catalyzed by CPT2, is followed by one cycle 
of oxidation. This is catalyzed by VLCAD and trifunctional pro­
tein to yield a chain-shortened acyl-CoA that can recycle through 
the same oxidation system.307 In  vivo

le through 
the same oxidation system.307 In  vivo, four different acyl-CoA 
dehydrogenase enzymes catalyze the initial dehydrogenation of 
straight-chain fatty acids in mitochondria. Three of them—short-
chain acyl-CoA dehydrogenase (SCAD), medium-chain acyl-
CoA dehydrogenase (MCAD), and LCAD—are soluble enzymes 
located in the mitochondrial matrix as homotetramers. A fourth, 
VLCAD, is attached to the inner membrane as a homodimer. 
Their names derive from the length of the fatty acids that t

s derive from the length of the fatty acids that they 
process. The enzymes can act in tandem. VLCAD and LCAD 
shorten the long-chain fatty acids into medium-chain fatty acids 
that can then be processed by MCAD and SCAD.308 The SCAD, 
MCAD, and LCAD monomers share a high degree of homology 
7
6
5
4
3
2
1
0
Glucose uptake (mg/kg/min)
Controls
Type 2
diabetics
Splanchnic
Adipose
Muscle
Brain
• Fig. 34.10  Tissue uptake of glucose in nondiabetic and insulin-resistant 
diabetic subjects during a hy

d insulin-resistant 
diabetic subjects during a hyperinsulinemic-euglycemic clamp. (From 
DeFronzo RA. The triumvirate: beta-cell, muscle, liver—a collusion respon­
sible for NIDDM. Lilly Lecture 1987. Diabetes. 1988;37:667–687.)
1366
but do not share homology with VLCAD. At least some of these 
enzymes can be regulated in humans by exercise training.309
UCP1 mediates the uncoupling of oxidative phosphorylation in 
brown adipose tissue

oxidative phosphorylation in 
brown adipose tissue for thermogenesis.310 UCP2 and UCP3 have 
structural similarities to UCP1, but it is not clear that they are actu­
ally uncouplers of oxidative phosphorylation under physiologic con­
ditions.311,312 UCP3 mRNA is found primarily in skeletal muscle and 
in brown adipose tissue. UCP2 has a ubiquitous tissue distribution 
and may be involved in reactive oxygen species detoxification and 
CNS regulation of metabolism.313 UCP2 and UCP3 mRNA levels 
ha

on of metabolism.313 UCP2 and UCP3 mRNA levels 
have been correlated with different physiologic states, and numerous 
studies indicate that expression of UCP2 and UCP3 are stimulated by 
thyroid hormones and in the presence of high levels of fatty acids.314 
In humans, the levels of UCP2 and UCP3 mRNAs were upregulated 
by a high-fat diet, and the upregulation was more pronounced in 
humans with high percentages of type IIA muscle fibers.315 In a small 
study, exercise training in humans increas

 small 
study, exercise training in humans increased mitochondrial oxida­
tive capacity but did not change UCP2 or UCP3 levels.316 Obesity 
itself was shown to be positively correlated with a splice isoform of 
UCP3.317 A unique polymorphism in the promoter region of UCP3 
correlated with expression of UCP3 in skeletal muscle.318 
Glucose Influence on Skeletal Muscle Fatty 
Acid Metabolism
A concept that can help explain glucose elevations leading to the 
intramyocellular triglyceride accumulati

g to the 
intramyocellular triglyceride accumulation associated with impaired 
insulin action is the central role of malonyl-CoA in regulating fatty 
acid and glucose oxidation319 (see Fig. 34.11). Malonyl-CoA is an 
allosteric inhibitor of CPT1, the enzyme that controls the transfer 
of long-chain fatty acyl-CoAs into the mitochondria.299,300,320 In 
the presence of elevated glucose and insulin levels, the TCA cycle is 
activated, resulting in an increase in citrate in the cytoplasm through 
in

n increase in citrate in the cytoplasm through 
increased malate cycling in the mitochondria. The increased citrate 
is converted to acetyl-CoA through citrate lyase and thus provides 
an indirect substrate for ACC. Citrate also allosterically activates 
ACC and makes ACC a better substrate for phosphatases that acti­
vate the enzyme.321,322 Even in insulin-resistant skeletal muscle, 
glucose uptake into the skeletal muscle is higher than normal, 
especially at the elevated levels of glucose fou

especially at the elevated levels of glucose found in T2DM.323,324 
The glucose is shunted toward the glycolytic pathway, generating 
acetyl-CoA that can be converted to malonyl-CoA in the cytoplasm 
by the action of the highly regulated ACC. This has been demon­
strated in humans where an infusion of insulin and glucose at a 
high rate leads to increases in the concentration of malonyl-CoA 
in skeletal muscle and to decreases in whole-body and, presumably, 
muscle fatty acid oxidation.325
ACC

, presumably, 
ACC is also regulated by AMPK-mediated phosphorylation, 
which inhibits ACC basal activity and reduces ACC activation 
by citrate.326 ACC then generates malonyl-CoA, which in turn 
allosterically inhibits CPT1 residing on the outer mitochondrial 
membrane, inhibiting uptake of acyl-CoA. The resulting buildup 
of long-chain acyl-CoAs and diacylglycerols is proposed to acti­
vate one or more PKC isoforms or other lipid-activated proteins, 
resulting i

ms or other lipid-activated proteins, 
resulting in insulin resistance.319 In support of this hypothesis, 
humans with obesity or T2DM show elevated intracellular mal­
onyl-CoA levels associated with reduced ACC phosphorylation, 
impaired fatty acid oxidation, and accumulation of intramyocel­
lular acyl-CoA and triglyceride, all of which can be reversed with 
a 3-month treatment with the insulin-sensitizer rosiglitazone.327
Circadian Rhythms, Obesity, and Insulin 
Almost all mammals h

sity, and Insulin 
Almost all mammals have a well-developed circadian cycle that is 
controlled by a complex, integrated network of transcription-trans­
lation feedback loops that work in a 24-hour cycle. A defined set 
Glucose
Citrate
Citrate
Malonyl-CoA
Triglyceride
Fatty acid
Diacylglycerols
Kinase activation?
Glycolysis
Acetyl-CoA
Acyl-CoA
synthase
Fatty acyl-CoA
β-Oxidation
CPTII
CPTI
–
+
ATP: citrate lyase
carboxylase
TCA
c

+
TCA
cycle
• Fig. 34.11  Glucose effect on triglyceride metabolism. Increased uptake of glucose results in increased 
production of acetyl coenzyme A (acetyl-CoA) as a product of glycolysis. The increased tricarboxylic acid 
(TCA) cycle activity associated with oxidation of triglycerides and glucose increases the production of 
citrate, which is shuttled to the cytoplasm, activates the enzyme acetyl-CoA carboxylase (ACC) by alloste­
ric mechanisms, and

 carboxylase (ACC) by alloste­
ric mechanisms, and increases the susceptibility of ACC to phosphatases. This leads to increased ACC 
activity, converting acetyl-CoA to malonyl-CoA. Malonyl-CoA is a potent inhibitor of carnitine palmitoyl­
transferase (CPT) I on the outer mitochondrial membrane, which leads to accumulation of fatty acyl-CoAs 
in the cytoplasm. This may result in the production of signaling molecules that can increase the activity of 
kinases and other enzymes and lead to insulin 

of 
kinases and other enzymes and lead to insulin resistance. ATP, adenosine triphosphate.
1367
of genes establishes the circadian cycle, which sets behavioral and 
physiologic functions, including sleep-wake cycles, feeding behav­
iors, hormone secretion, and metabolism. The core circadian clock 
genes are present not only in the suprachiasmatic nucleus of the 
hypothalamus but also in nearly every cell of the body, where they 
activate s

rly every cell of the body, where they 
activate specific sets of genes.328 In mammals, the clock is driven 
by two transcription factors, CLOCK (circadian locomotor out­
put cycles kaput) and BMAL1 (brain and muscle aryl hydrocarbon 
receptor nuclear translocator (ARNT)-like 1).329 CLOCK and 
BMAL1 dimerize and bind the promoter region of target genes, 
including period (PER) and cryptochrome (CRY). The increased 
levels of these proteins promote heterodimerization and translo­
cation to the nu

e heterodimerization and translo­
cation to the nucleus, creating a negative feedback loop by sup­
pressing transcriptional activity of CLOCK-BMAL1. In a second 
negative feedback loop, CLOCK-BMAL1 induce REV-ERBα 
(encoded by the nuclear receptor subfamily 1, group D, member 
1 gene, NR1D1), which in turn suppresses BMAL1 transcription. 
Additional modulation of the activity and stability of these pro­
teins is provided by post-translational modifications such as phos­
phorylation and ubiquitin

ifications such as phos­
phorylation and ubiquitination.
There are significant epidemiologic associations in humans 
between reduction of sleep and increased rates of obesity and 
other metabolic disturbances, including T2DM.330 Experimental 
sleep disruption can directly impair insulin action, alter secretion 
of leptin and ghrelin resulting in stimulation of appetite, increase 
inflammatory cytokine production, and create alterations in other 
cardiovascular risk factors. Alterations in normal

cardiovascular risk factors. Alterations in normal feeding patterns 
that are attuned to circadian metabolism can change the relation­
ship between nutrient appearance and nutrient-metabolizing 
enzymes. For instance, alterations in fatty acid appearance and 
lipoprotein lipase activity can lead to altered partitioning of lipids 
to vulnerable tissues, leading to lipotoxicity and decreased secre­
tion of leptin, increasing appetite.331 Obstructive sleep apnea, 
which combines sleep fragmentation

e sleep apnea, 
which combines sleep fragmentation and hypoxemia, is also a risk 
factor for insulin resistance and diabetes. There is growing evi­
dence that glucose control in patients with T2DM patients can 
be improved by treating sleep apnea, although poor compliance to 
therapy is a major barrier to success.332 
Role of the Gut Microbiome and Metabolome 
in Diabetes and Insulin Resistance
There is growing evidence that a major mediator of the effect of 
diet and other environmental factors

he effect of 
diet and other environmental factors in diabetes is the gut micro­
biome—or, more specifically, the bacteria that reside in the gas­
trointestinal tract.333 In mammals, the gut microbiome is seeded 
at birth; it is reformed during infancy and then becomes relatively 
stable in adulthood, although it can be remodeled by diet, anti­
biotics, and a variety of disease states. Most gut microbiota are 
viewed as commensal (nonharmful) or mutualistic (providing ben­
efit) to the host. The

mutualistic (providing ben­
efit) to the host. These microbes play important roles in nutrient, 
xenobiotic, and drug metabolism, maintenance of gut mucosal 
integrity, protection against pathogens, and immunomodulation, 
and, through these basic functions, affect many aspects of normal 
physiologic function. Over the past decade, it has become clear 
that the gut microbiome can also contribute to obesity, diabetes, 
metabolic syndrome, and insulin resistance in both rodents334-336 
and humans,3

in resistance in both rodents334-336 
and humans,337-340 and act as an integrator and mediator of some 
of the effects of genetics, diet, and bariatric surgery.341-344 In mice, 
it has been shown that administration of low-dose antibiotics early 
in life may predispose to obesity and glucose intolerance by per­
turbing the development of a normal microbiome.345
The mechanisms by which gut microbiota may affect patho­
genesis of diabetes, obesity, and insulin resistance are multiple 
(Fig. 34.12)

 and insulin resistance are multiple 
(Fig. 34.12). Gut microbiota have major effects on intestinal 
barrier function, breakdown of otherwise indigestible dietary 
components, modification of bile acids and other substances, 
development of the gut, and education of the immune sys­
tem.333,346 These effects can lead to a release of bacterial proteins, 
endotoxins, and cytokines into the bloodstream,347,348 as well as 
produce changes in hundreds of metabolic products, including 
bile acids, shor

of metabolic products, including 
bile acids, short-chained fatty acids, amino acids, and many other 
classes of molecules.343,349-354 Together, these lead to tissue-spe­
cific metabolic dysregulation and immune activation, leading to 
insulin resistance and progression of diabetes pathogenesis. Several 
metabolites have been shown to correlate with insulin resistance 
in both mice355 and humans,356 and some, such as 2-aminoadi­
pate, α-hydroxybutyrate, and N-acetylglycine, are suggested to be 

tyrate, and N-acetylglycine, are suggested to be 
biomarkers of T2DM.357-359 Further studies will be needed to 
determine exactly how gut microbiota affect insulin sensitivity 
and diabetes progression, and whether therapies that change gut 
microbiota can be used to treat or prevent T2DM. In this regard, 
several options are possible, ranging from administration of prebi­
otics (nutrients that change the composition of gut microbiota), 
probiotics (mixtures of bacteria themselves), and even tra

cs (mixtures of bacteria themselves), and even transplant 
of a healthy microbiome by fecal transfer. 
Special Conditions That Induce Insulin 
Gestational Diabetes
Diabetes diagnosed during pregnancy is categorized as either pre­
existing T2DM if diagnosed during the first trimester of pregnancy 
or gestational diabetes if diagnosed during the second or third tri­
mester.5 Although gestational diabetes generally resolves with the 
end of the pregnancy, it is considered a significant r

of the pregnancy, it is considered a significant risk factor for the 
future development of T2DM.360,361 Insulin resistance is a normal 
consequence of pregnancy. As the pregnancy progresses and insulin 
resistance builds, maternal insulin secretion increases by as much as 
250% to compensate. When this compensation is inadequate, ges­
tational diabetes develops.362 Pre-pregnancy obesity is a major risk 
factor for the development of gestational diabetes.
The driver of insulin resistance in preg

diabetes.
The driver of insulin resistance in pregnancy is not simply the 
weight gain of pregnancy, as euglycemia generally returns within 
days of delivery. Rather, placenta-derived factors are believed to 
drive both the insulin resistance of pregnancy and the beta-cell 
expansion of pregnancy. Exactly what placental factors contribute 
remains debated because there are significant differences in beta-
cell physiology between rodents and humans, and intervention 
studies of pregnant patients 

s, and intervention 
studies of pregnant patients are challenging.363 Placenta-derived 
growth hormone is one hormone that contributes to maternal insu­
lin resistance but can account for only a portion of the change.364 
Other candidates that may contribute include placental lacto­
gen365,366 and placenta-derived pro-inflammatory exosomes.367 
Drugs and Stress-Induced Insulin Resistance
In addition to the genetic and epigenetic risk factors discussed 
previously, there are many disease states a

ussed 
previously, there are many disease states and medications that can 
induce insulin resistance or a combination of insulin resistance 
and decreased insulin secretion leading to glucose intolerance 
(Table 34.2).
1368
Glucocorticoid-Induced Insulin Resistance
Cushing syndrome and exogenous glucocorticoid treatment 
have long been known to induce significant insulin resistance in 
humans. More than 80% of nondiabetic individuals w

humans. More than 80% of nondiabetic individuals with rheu­
matoid arthritis treated with greater than 30 mg/day of predni­
sone have an increase in their hemoglobin A1c. The mechanism 
is multifactorial. There are rapid effects related to increasing 
hepatic glucose production, which occurs via increased gluco­
neogenesis, as well as acute insulin resistance in skeletal muscle, 
in part due to downregulation of IRS1.368 Glucocorticoids also 
have tissue-specific effects in  vivo, because 24-hou

e tissue-specific effects in  vivo, because 24-hour treatment 
of humans with hydrocortisone enhances adipose tissue insu­
lin sensitivity while whole-body insulin sensitivity declines.369 
Tissue crosstalk is important in glucocorticoid-induced insulin 
resistance: a mouse model lacking glucocorticoid receptors spe­
cifically in adipose tissue retains some muscle insulin sensitivity 
in response to chronic dexamethasone.370 Long-term glucocor­
ticoid treatment also results in redistribution of 

icoid treatment also results in redistribution of fat from the 
periphery to the central compartment, increased lipolysis and 
elevations in triglyceride and fatty acid, and a reduction in insu­
lin secretion. There is also increased muscle protein breakdown 
leading to muscle atrophy and alterations in skin integrity. These 
effects are due to a combination of modulation of transcriptional 
events by glucocorticoids and through inhibition of insulin and 
growth factor signaling. 
Post-Transplan

ulin and 
Post-Transplant Diabetes Mellitus
Patients receiving organ transplants frequently develop IGT or 
post-transplant diabetes mellitus (PTDM) following the trans­
plant, related to the immunosuppressive regimens. Most patients 
become transiently hyperglycemic immediately post-transplant 
due to the stress of surgery and high doses of corticosteroids; 
thus, diagnosis of PTDM should not be made until the patient 
has been discharged from the hospital and is on a 

has been discharged from the hospital and is on a stable immu­
nosuppressant dose.371 Consensus recommendations discour­
age the use of hemoglobin A1c for PTDM screening during the 
first year following transplant due to significant changes in red 
blood cell turnover, which can result in missed cases. The doses 
and drug combinations used in transplant vary by organ but fre­
quently include chronic use of medications that impair glucose 
metabolism, such as glucocorticoids, mTOR inhibitors (sir

ism, such as glucocorticoids, mTOR inhibitors (siroli­
mus and everolimus), and calcineurin inhibitors (tacrolimus and 
cyclosporine).
The primary mechanism for glucocorticoids and mTOR 
inhibitors is via induction of insulin resistance. Glucocorticoids 
Changes in food/
bile acid metabolism
Changes in metabolites
and metabolite signaling
Metabolic regulation
Immune activation
Diabetes and
metabolic syndrome
Endotoxins/cytokines
Bacterial proteins
Exercise
Other
Prebiotics
Probiotics

s
Exercise
Other
Diet
Genes
Changes in gut barrier
Portal
circulation
Changes in gut
microbiome
• Fig. 34.12  The gut microbiome is impacted by our genes and environment. Changes to the gut micro­
biome may result in altered gut barrier function and food and bile acid metabolism, leading to immune 
activation and insulin resistance.
  Drugs and Stressors Associated With 
Drugs
Human immunodeficiency viru

Drugs
Human immunodeficiency virus medications (all cause varying degrees 
of metabolic abnormalities)
Calcineurin inhibitors
Mammalian target of rapamycin (mTOR) inhibitors
Phosphoinositide 3-kinase inhibitors
Statins
Stressors
Pregnancy
Glucotoxicity
Surgery
Inflammation (from obesity or infection)
Overnutrition
TABLE 34.2
1369
are discussed earlier. mTOR inhibitors induce insulin resistance 
via inhibition of the mTOR c

e insulin resistance 
via inhibition of the mTOR complex downstream of the insulin 
receptor (see Fig. 34.8). It seems that this inhibition is not enough 
to cause diabetes in low-risk patients but can induce diabetes in 
those with risk factors, particularly if used in combination with cal­
cineurin inhibitors.372 Calcineurin inhibitors appear to contribute 
to diabetes predominantly by decreasing insulin secretion.373 This 
is a result of decreased insulin synthesis and reduced glucokinase 
ac

ased insulin synthesis and reduced glucokinase 
activity.374,375 There is also some evidence for increased insulin 
resistance in patients treated with calcineurin inhibitors; however, 
these data are mixed, possibly reflecting increased insulin resis­
tance only in those with underlying risk factors.372 
Inflammation and Inflammatory Cytokines
Obesity is associated with increased inflammation in adipose 
tissue, including macrophage infiltration and increased cyto­
kine expression. Inflammation

 and increased cyto­
kine expression. Inflammation and cytokines drive insulin 
resistance.376 This occurs through multiple mechanisms. Stud­
ies in humans and in animal models of obesity have identified 
decreases in the number and the kinase activity of insulin recep­
tors and impairment in the activation of IRS1, PI3K, and Akt. 
These changes are driven, in part, by increased levels of cyto­
kines, such as TNFα, which is made and secreted by adipocytes, 
as well as the infiltrating macrophage

dipocytes, 
as well as the infiltrating macrophages. TNFα administration 
reduces the kinase activity of the insulin receptor in rats377 or 
in 3T3L1 adipocytes in vitro,378 possibly by increased activity 
of JNKs and IKKs, and increased serine phosphorylation of the 
insulin receptor and IRS1.379 High fat–fed mice with genetic 
ablation of TNFα production and rats treated with neutralizing 
antisera or soluble TNF receptors have increased kinase activity 
of the insulin receptor,380,381 indicat

 activity 
of the insulin receptor,380,381 indicating increased insulin sensi­
tivity. However, thus far, interventions to decrease TNFα action 
in humans have had little or no effect in reducing the insulin 
resistance of obesity or T2DM.382,383 
Human Immunodeficiency Virus Infection
With the advent of effective medications to manage the infection, 
human immunodeficiency virus (HIV) infection has become a 
chronic disease. However, these medications are associated with 
significant metabolic 

ations are associated with 
significant metabolic risk. In a cohort of more than 3800 patients, 
there was nearly a doubling in the prevalence of T2DM in HIV 
versus non-HIV patients. A significant portion of this increase 
may be related to protease inhibitors used to treat chronic HIV. 
HIV-infected patients also have a 75% increase in the risk for 
acute myocardial infarction compared with uninfected patients, 
again related to protease inhibitor use. Other contributing fac­
tors to cardiomet

tor use. Other contributing fac­
tors to cardiometabolic risk were male sex, diagnosis of acquired 
immunodeficiency syndrome, responsiveness to antiretroviral 
treatment, and increases in CD4 T-cell counts.384
Both lipoatrophy and lipohypertrophy are seen in HIV-infected 
patients on antiretroviral therapy. Lipoatrophy is primarily asso­
ciated with the use of the older thymidine analogue nucleoside 
reverse transcriptase inhibitors, whereas the contributors to lipo­
hypertrophy are not as well

 contributors to lipo­
hypertrophy are not as well understood.385 In the setting of multi-
drug HIV treatment regimens and frequent changes to therapy, 
it is difficult to determine the relative contributions of individual 
drugs to metabolic syndrome or distinguish the drug effect from 
the metabolic consequences of adipose tissue redistribution.
There appear to be multiple mechanisms contributing to 
metabolic syndrome in HIV. The effects of the major HIV treat­
ments are reviewed in detail by

e major HIV treat­
ments are reviewed in detail by Srinivasa and Grinspoon384 In 
brief, several protease inhibitors can inhibit glucose transport 
in vitro and in vivo, possibly through interaction with the insulin-
responsive glucose transporter GLUT4 that could inhibit glucose 
uptake specifically in insulin-responsive tissue.386 Mitochondrial 
abnormalities have been described in adipose tissue biopsy speci­
mens obtained from HIV-infected patients with lipodystrophy, 
including reductions i

tients with lipodystrophy, 
including reductions in mitochondrial number and oxidative 
function in adipocytes from patients treated with highly active 
antiretroviral therapy.387,388 A direct effect of protease inhibitors 
on differentiation of adipocytes has also been described.389,390 In 
adipose tissue of treated HIV patients, especially those with lipo­
dystrophy, there is downregulation of the microRNA-processing 
enzyme Dicer. This is associated with decreased levels of circulat­
ing exos

ciated with decreased levels of circulat­
ing exosomal microRNAs, which may also contribute to insulin 
resistance.391,392
Treatment of HIV-associated lipodystrophy continues to be 
inadequate, although the incidence may be less with use of newer 
protease inhibitors. Switching from thymidine analogues has been 
shown to improve lipoatrophy in some patients.393 Treatment with 
insulin-sensitizing thiazolidinediones has shown mixed results, with 
improvement in insulin sensitivity but little alte

improvement in insulin sensitivity but little alteration in fat distri­
bution.394 Because of the increased risk of cardiovascular disease, 
treatment of hyperlipidemia is essential in these patients. Treatment 
guidelines do not differ from the general population; however, 
current cardiac risk predictors underestimate risk in HIV-infected 
patients.395 Tesamorelin, a growth hormone–releasing hormone 
analogue, is approved to treat abnormal lipids associated with HIV 
lipodystrophy. The medicat

ds associated with HIV 
lipodystrophy. The medication causes a modest redistribution of 
visceral fat with subsequent improvement in the lipid profile.396 
Statins
The medications and disease states discussed earlier may have 
profound effects on insulin resistance and glycemia; however, 
there are other factors that will have much smaller, but poten­
tially significant, impacts on a population basis. One example 
is the statin class of cholesterol-lowering drugs. These medica­
tions do not caus

ol-lowering drugs. These medica­
tions do not cause an appreciable change in blood glucose in 
individual patients, but large clinical trials have demonstrated 
an increased risk for developing T2DM in patients treated with 
a statin.406 Given the multifactorial nature of insulin resistance 
and T2DM, it is likely that other medications will also increase 
the risk of T2DM in the right setting of genetic predisposition 
and environmental factors. 
Glucotoxicity, Glucosamine
Hyperglycemia itself 

Hyperglycemia itself can cause insulin resistance, as well as 
decreased beta-cell function. In Pima Indians, the level of fast­
ing glycemia is strongly correlated with insulin sensitivity.397 This 
defect is primarily in skeletal muscle,398 but its exact mechanism 
remains controversial.
On entry into cells, glucose is converted to glucose 6-phosphate, 
which has multiple metabolic fates. The hexosamine pathway is a 
relatively minor branch of the glycolytic pathwa

relatively minor branch of the glycolytic pathway, encompassing less 
than 3% of total glucose used. Hexosamines, such as glucosamine, 
when incubated with adipose tissue, induce insulin resistance in fat 
cells399 and in skeletal muscle.400 Infusion of glucosamine into rats 
resulted in a dose-dependent increase in insulin resistance of skeletal 
muscle,400 and transgenic mice that overexpress the rate-limiting 
enzyme glutamine–fructose-6-phosphate aminotransferase specifi­
cally in skeletal

sphate aminotransferase specifi­
cally in skeletal muscle acquired severe insulin resistance.401 By a 
pathway that is unclear, glucosamine overproduction resulted in a 
disruption of the ability of insulin to cause translocation of GLUT4 
to the cell surface.402 Through its anti-insulin action, the hexosamine 
1370
pathway has been hypothesized to be a glucose sensor that allows the 
cell to sense and adapt to the prevailing level of 

ell to sense and adapt to the prevailing level of glucose.398 
Postoperative Hyperglycemia
Surgery is a significant stress to the body. This is driven by the 
surgical wound, anesthesia, and pain, all of which can stimulate 
the release of stress hormones, including cortisol, epinephrine, 
and norepinephrine. In addition, particularly following coronary 
bypass surgery, the use of pressors is common and can cause severe 
hyperglycemia. Postoperative hyperglycemia predicts mortality 
in nondiabet

ive hyperglycemia predicts mortality 
in nondiabetic patients following coronary bypass or general sur­
gery.403,404 Aggressive treatment of postoperative hyperglycemia, 
with a target blood glucose of between 140 and 180 mg/dL for 
both diabetic and nondiabetic patients, has been shown to reduce 
both wound infections and mortality following surgery.5
Epinephrine is one of the key mediators of postoperative 
hyperglycemia. This can be both endogenously derived and deliv­
ered through infusions.

enously derived and deliv­
ered through infusions. Epinephrine acts to increase blood glu­
cose through several mechanisms. These include decreased insulin 
secretion and increased insulin resistance both in muscle and in 
the liver, where it leads to a failure to suppress hepatic glucose pro­
duction.405 When epinephrine infusions are used in an acute care 
setting, adjustments in the insulin infusion rate are generally nec­
essary due to the rapid and substantial impact of this medication.
Ref

pid and substantial impact of this medication.
1370.e1
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approach. Diabetes. 2013;62(12):4270–4276.
	360.	 Noctor E, Crowe C, Carmody LA, et al. Abnormal glucose tol­
erance post-gestational diabetes mellitus as defined by the Inter­
national Association of Diabetes and Pre

cemia 
in prolactin receptor mutant, but not prolactin mutant, mice and 
feeding-responsive regulation of placental lactogen genes implies 
placental control of maternal glucose homeostasis. Biol Reprod. 
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	366.	 Billestrup N, Nielsen JH. The stimulatory effect of growth hor­
mone, prolactin, and placental lactogen on beta-cell proliferation 
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1991;129(2):883–888.
	367.	 Salomon C, Scholz-Romero K, Sarker S, et al. G

fluence of 
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mus (FK506) on human insulin gene expression, insulin mRNA 
levels, and insulin secretion in HIT-T15 cells. J Clin Invest. 
1996;98(12):2786–2793.
	375.	 Radu RG, Fujimoto S, Mukai E, et  al. Tacrolimus suppresses 
glucose-induced insulin release from pancreatic islets by reduc­
ing

or necrosis factor 
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 CM, et  al. Practical review of 
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ic characteristics that predict the development of non-insu­
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1371
35
Therapeutics of Type 2 Diabetes Mellitus
MATTHEW C. RIDDLE AND ANDREW J. AHMANN
Type 2 diabetes mellitus (T2DM) is the predominant form of 
diabetes worldwide, accounting for 90% of cases. An epidemic 
of T2DM is underway in both developed and developing coun­
tries, although the brunt of the disorder is fel

­
tries, although the brunt of the disorder is felt disproportion­
ately in non-European populations. In the Pacific island of 
Nauru, diabetes was virtually unknown 50 years ago, but the 
prevalence among adults rose to nearly 40% before a recent 
slight decline. The highest prevalence rates remain in Micro­
nesia at 23% to 35%.1,2 It is projected that South Asia will see 
a 150% increase in adult diabetes from 2000 to 2025.3 The 
International Diabetes Federation (IDF) projects that global 
pr

Diabetes Federation (IDF) projects that global 
prevalence will increase from an estimated 150 million in 2000 
and 415 million in 2015 to nearly 600 million by 2035.2,4 
Along with this increase of prevalence, the economic burden 
of diabetes is expected to rise as well.5 In the United States, the 
Centers for Disease Control and Prevention (CDC) has esti­
mated that in 2016 14% of the adult population had diabetes, 
and 30.7% of those with diabetes were undiagnosed.6 As shown 
in Fig. 35.1, am

etes were undiagnosed.6 As shown 
in Fig. 35.1, among those with diagnosed diabetes, prevalence 
varied by ethnicity from 8.5% in non-Hispanic white to 10.2% 
in non-Hispanic Asian, 13.6% in Hispanic, and 13.9% in non-
Hispanic black subpopulations.6 Moreover, 33.9% of US adults 
(84.1 million) were estimated to have prediabetes and thus be 
at high risk for developing diabetes.7 Based on epidemiologic 
Epidemiology, 1371
General Approaches to Management, 1373
Glucose-Lowering Ph

Approaches to Management, 1373
Glucose-Lowering Pharmacotherapy, 1385
Practical Aspects of Treatment, 1395
Considerations in Personalizing Therapy, 1397
Special Situations in Clinical Management, 1401
Preventing Type 2 Diabetes Mellitus, 1402
Future Directions, 1402
KEY POINTS
 • 
Type 2 diabetes mellitus (T2DM) is a major public health 
problem. The World Health Organization estimated in 2014 that 
422 million people had diabetes worldwide, and a continuing 
increase is projected.
 • 
T2DM is

ion effectively lowers glu­
cose with essentially no risk of hypoglycemia.
 • 
If the response to metformin is inadequate initially or subse­
quently, one or more other agents can be added. There are 
six recommended second-line therapies: thiazolidinedione, 
sulfonylurea, dipeptidyl peptidase-4 (DPP4) inhibitor, sodium-
glucose cotransporter (SGLT) inhibitor, glucagon-like peptide-1 
(GLP1) receptor agonist, and basal insulin. Each has its advan­
tages and disadvantages.
 • 
A team approach t

n­
 • 
A team approach to management greatly improves success, 
especially for patients with diabetes longer than 10 years and 
therefore requiring greater complexity and personalization of 
therapy.
1372
trends, for those born in the United States in 2000, the esti­
mated lifetime risk of developing diabetes is 32.8% for men and 
38.5% in women.8
The metabolic abnormalities of T2DM lead to illness, disabil­
i

 abnormalities of T2DM lead to illness, disabil­
ity, and early mortality. In the United States, diabetes is the lead­
ing cause of blindness and accounts for at least 40% of end-stage 
renal disease. The risk of heart disease and stroke is commonly 
stated to be two to four times higher, and that of lower-extremity 
amputation is approximately 20 times higher for people with dia­
betes than for those without diabetes. Life expectancy is reduced 
by approximately 10 years in people with diabetes

by approximately 10 years in people with diabetes, and the hazard 
ratio for death is most pronounced in those having diabetes under 
age 55 years.8–10 Although diabetes is considered the seventh lead­
ing cause of death in the United States, this figure is surely an 
underestimate because many illnesses identified as causes of death 
are in fact complications of diabetes. Only about 35% to 40% 
of those who die with diabetes have the disease listed anywhere 
on the death certificate, and 10% t

sted anywhere 
on the death certificate, and 10% to 15% have it listed as the 
underlying cause of death. Rates of morbidity and mortality differ 
between countries for various reasons, but the overall patterns are 
very similar.
Thus, mainly through its long-term complications, T2DM 
causes a serious burden of illness, much of which might be 
delayed or prevented by treatment. There is evidence that rates 
of some complications of diabetes have indeed declined recently. 
Data from the United St

indeed declined recently. 
Data from the United States suggest a reduction by 50% or more 
between 1990 and 2010 for myocardial infarction (MI), stroke, 
amputations, and death from hyperglycemic crisis, and end-stage 
renal disease declined approximately 30%.11 Amputations expe­
rienced the greatest relative reduction. However, there are rea­
sons for concern. Although incidence rates have decreased in the 
United States in the last decade, prevalence rates will continue to 
be high. More indiv

alence rates will continue to 
be high. More individuals are being diagnosed with T2DM at a 
younger age, and the complications of diabetes will be occurring 
more often in middle-aged adults with a prevalence that will cause 
a significant personal and societal burden.10
The economic effect of diabetes is enormous. Recent estimates 
suggest the worldwide cost of diabetes to be $1.31 trillion, with 
anticipated increases from 1.8% of the global gross domestic 
product to 2.2% by 2030, with total

ross domestic 
product to 2.2% by 2030, with total costs of $2.2 trillion.5 Costs 
directly attributed to diabetes in the United States in 2017 were 
$327 billion with average expenditures per person, adjusted for age 
and gender, 2.3-fold higher than in the nondiabetic population.12 
These costs increased by 26% between 2012 and 2017. They result 
from the complications of diabetes, comorbid conditions, costs of 
medications, and the frequency of visits, as well as from missed 
work, reduced pr

f visits, as well as from missed 
work, reduced productivity, and premature mortality.12
Considerable information is available on factors underlying the 
development of T2DM.13–18 Overt diabetes is thought to occur 
mainly in genetically predisposed persons who are exposed to vari­
ous environmental influences that precipitate the onset of clini­
cal disease. The syndrome consists of monogenic and polygenic 
forms that can be differentiated both on clinical grounds and in 
terms of the genes inv

on clinical grounds and in 
terms of the genes involved in pathogenesis. Greater adiposity 
19.8
20
10
15
5
30
25
0
Men
Women
Gender
Age range
(years)
All
14.0
15.9
12.2
Undiagnosed
Diagnosed
Percent
20
10
15
5
30
25
0
40–59
60 and over
20–39
3.5
16.3
28.2
Diagnosed
20
10
15
5
30
25
0
Overweight
Obese
Weight range
Race and Hispanic origin 
Underweight
or normal
weight
6.2
11.8
20.7
Diagnosed
Percent
20
10
15
5
30
25
0
Non-Hispanic
white
Asian
bla

Hispanic
white
Asian
black
Hispanic
12.4
15.3
17.9
6.2
13.6
4.3
9.7
5.1
10.8
3.4
8.8
1.7
1.8
5.2
11.1
7.2
21.0
4.3
11.8
3.2
8.6
20.7
6.8
14.0
3.9
8.5
5.1
10.2
4.0
13.9
1.9
• Fig. 35.1  Demographic factors, including gender, age, weight, and race, affecting prevalence of diabe­
tes in adults in the United States in 2013–2016. (Data from Centers for Disease Control and Prevention. 
National Diabetes Statistics Report, 2017. https://www.cdc.gov/diabetes/pdfs/data/statistic

. https://www.cdc.gov/diabetes/pdfs/data/statistics/national-
diabetes-statistics-report.pdf.)
1373
Chapter 35  Therapeutics of Type 2 Diabetes Mellitus
and restricted physical activity are important factors in determin­
ing the risk of developing T2DM. The disorder is more common 
in men, and the increased prevalence in certain racial and ethnic 
groups has already been mentioned.6 As in the past the risk of 
onset of T2DM increases steadily with age, but in recent years the 
prevalence in chil

h age, but in recent years the 
prevalence in children has risen dramatically, presumably due to 
increasing obesity. Previously, an overwhelming majority of chil­
dren with diabetes had type 1 diabetes mellitus (T1DM), and less 
than 2% were considered to have T2DM or other rare forms of 
diabetes. More recent reports suggest as many as 20% to 25% of 
children in the United States with newly diagnosed diabetes have 
T2DM, with the proportions highest among children of non-
European ancestry dia

ghest among children of non-
European ancestry diagnosed between age 10 and 19 years.19
The diagnosis of diabetes rests on demonstration of hyperglycemia. 
Because plasma glucose concentrations range as a continuum, the 
criteria are based on estimated thresholds for the complications 
of diabetes. The primary endpoint used to evaluate the relation­
ship between glucose levels and complications is retinopathy. 
All three commonly used tests—fasting plasma glucose (FPG), 
2-ho

nly used tests—fasting plasma glucose (FPG), 
2-hour plasma glucose after a 75-g oral glucose load (2-hour PG), 
and glycated hemoglobin A1c (HbA1c)—predict the presence of 
retinopathy and, by inference, define the glucose levels that are 
diagnostic of diabetes.20 There is a relationship between elevated 
levels of all three markers and nephropathy, neuropathy, and car­
diovascular disease as well as retinopathy.
Current criteria for the diagnosis of prediabetes and diabetes 
are shown in Tabl

sis of prediabetes and diabetes 
are shown in Table 35.1.21
When the present fasting glucose and glucose tolerance test 
parameters were defined for diagnosing diabetes in 1997 accord­
ing to the levels associated with an increase in retinopathy, mea­
surement of HbA1c was recognized as a viable alternative but not 
recommended for diagnostic purposes because assays were not 
reliably standardized.22 Current HbA1c assays are standardized 
and have several technical advantages over the other curr

e several technical advantages over the other currently 
used laboratory measurements of glucose. Furthermore, measures 
of fasting and post-challenge glucose concentrations in the same 
individual over time are less reproducible than those for HbA1c. In 
one study, the coefficient of variation for measurements repeated 
in single individuals was 6.4% for fasting plasma glucose and 
16.7% for 2-hour plasma glucose values, and less than 2% for 
HbA1c.20 Use of HbA1c has the added advantage of sim

A1c.20 Use of HbA1c has the added advantage of simplicity in 
not requiring timing of collection, and it is generally a more stable 
measure of average glucose control. It has also been shown to be 
a good indicator of prediabetes with an ability to predict develop­
ment of T2DM.23,24 Nevertheless, the HbA1c level is misleading 
in some situations, and discrepancies must be evaluated.25–27
Although the oral glucose tolerance test (OGTT) is very useful 
for research, it is not recommended for rou

seful 
for research, it is not recommended for routine use in diagnos­
ing diabetes. It is generally inconvenient, and in most cases the 
diagnosis can be made by either an elevated fasting plasma glucose 
concentration, an elevated random glucose determination in the 
presence of hyperglycemic symptoms, or confirmed elevation of 
HbA1c. The various glycemic parameters do not always correlate 
to make the diagnosis. It is possible to see a diagnostic elevation 
of plasma glucose while the HbA1c 

stic elevation 
of plasma glucose while the HbA1c is below 6.5% or vice versa. 
A confirmatory test by the same or a different method is usually 
required to confirm the diagnosis.21 
General Approaches to Management
Over the past 20 years a transformation in population-based man­
agement of T2DM has occurred. Understanding of the lifetime 
trajectory of this illness has improved, and there is now greater 
urgency to identify individuals at risk, make the diagnosis in a 
timely way, and interven

 make the diagnosis in a 
timely way, and intervene systematically over time to improve 
medical outcomes while minimizing risks associated with therapy. 
The hyperglycemia that defines diabetes is accepted as a modifi­
able risk factor for microvascular (eye, kidney, and nerve) com­
plications and, to a lesser degree, cardiovascular outcomes.28–32 
The variable but sometimes long delay between appearance of 
the metabolic abnormalities of diabetes and these complications 
is now more fully appr

es and these complications 
is now more fully appreciated. Many additions to the available 
array of pharmacologic agents and monitoring devices have made 
it possible to lower glucose levels safely to nearly normal ranges in 
a majority of patients.
In this chapter we address the main features of the diagnostic 
strategies, treatment guidelines, lifestyle interventions, and phar­
macotherapeutic agents before discussing clinical decisions and 
special considerations in management. The Clinical 

pecial considerations in management. The Clinical Practice Rec­
ommendations of the American Diabetes Association (ADA) is an 
excellent source of additional information on these matters. It is 
published each January as a supplement to the journal Diabetes 
Care and is available in a periodically updated version online.33
Fig. 35.2 illustrates the extended temporal path of T2DM 
and identifies some milestones that present opportunities for 
  Criteria for Diagnosis of Prediabetes and Diabetes

riteria for Diagnosis of Prediabetes and Diabetes
Units
Diagnostic of Prediabetes
Diagnostic of Diabetesa
Comments
Fasting plasma glucose (FPG)
mg/dL
mmol/L
100–125
5.6–6.9
≥126
≥7
After no calorie intake at least 8 hours
2-hour plasma glucose (2-hour 
PG)
mg/dL
mmol/L
140–199
7.8–11
≥200
≥11.1
2 hours after 75 g 
Random plasma glucose
mg/dL
mmol/L
≥200
≥11.1
Without oral glucose but with classic 
symptoms
HbA1c
% 
mmol/mol
5.7–6.4
39–47
≥6.5
≥48
Measured 

HbA1c
% 
mmol/mol
5.7–6.4
39–47
≥6.5
≥48
Measured in certified laboratory
aIn the absence of unequivocal hyperglycemia, results should be confirmed by repeat testing.
Data from American Diabetes Association. Classification and diagnosis of diabetes. Standards of medical care in diabetes, 2018. Diabetes Care. 2018;41:S13–S27.
   
TABLE 35.1
1374
assessment and intervention.34 This evolution of the disorder over 
time suggests division o

ion of the disorder over 
time suggests division of interventions into three categories that 
are most relevant at specific intervals. 
Public Health Measures
For the interval prior to diagnosis, public health interventions are 
possible. These may include administrative or regulatory efforts 
to educate families about risks of obesity and diabetes, promote 
healthful lifestyles, and restrain marketing and distribution of 
unhealthful foods. Efforts can be made to promote desirable 
lifestyle ch

rts can be made to promote desirable 
lifestyle choices for at-risk individuals in the setting of primary 
medical care. A public health approach also includes screening to 
identify emergence of hyperglycemia as soon as possible.35 
Primary Care Management
Individuals known to have prediabetes do not routinely require 
glucose-lowering pharmacotherapy, but attention to other modifi­
able cardiovascular risk factors is always appropriate.36 (See the 
upcoming section regarding diabetes preventio

the 
upcoming section regarding diabetes prevention in patients with 
prediabetes.) More specific medical interventions are needed at 
the time of diagnosis of overt T2DM.37 Because the development 
of tissue complications is related to both the severity and duration 
of hyperglycemic exposure, the goal of therapy is to maintain gly­
cemic control as close to normal levels as is safely possible from the 
start. Initial therapy includes enhanced lifestyle efforts together, in 
most cases, with ph

ifestyle efforts together, in 
most cases, with pharmacotherapy.33,38 Drug treatment can ini­
tially be given in a standardized and relatively simple way, based 
on medical evidence for both benefits and risks. This approach is 
most effective in the first 10 years after diagnosis. 
Complex Diabetes Care
Progression of the pathophysiologic abnormalities underly­
ing T2DM typically leads to increasing difficulty in controlling 
hyperglycemia over time. At the same time, an increasing burden 
of m

time. At the same time, an increasing burden 
of medical illness, both diabetes related and from other causes, 
adds to potential risks and may limit potential benefits of intensive 
therapy. By 10 years after diagnosis, progression of the disorder 
commonly leads to a need for multiple and often injected thera­
pies, more personalized choices of therapies, and greater reliance 
on treatments with less well-understood long-term benefits versus 
risks. How to provide these more complex and time-c

isks. How to provide these more complex and time-consuming 
services is one of the dilemmas of health system management. 
In general, the best results are obtained by access to a specialized 
team approach. 
Screening
As noted, the prevalence of T2DM in the United States and else­
where continues to increase. The increasing burden of diabetes is 
driven by aging of the population; by population growth, particu­
larly among ethnic groups with greater susceptibility to the disease; 
and by increas

ter susceptibility to the disease; 
and by increasing obesity related to increasingly sedentary lifestyles 
and greater consumption of simple sugars, fats, and highly pro­
cessed calorie-dense foods. Opportunistic screening in high-risk 
populations is recommended by professional societies and many 
insurers. In the United States the proportion of people with dia­
betes who have been diagnosed has increased from approximately 
50% in the 1990s to about 70% today.6 Nonetheless, undiag­
nosed T2DM

 about 70% today.6 Nonetheless, undiag­
nosed T2DM is still common. People with undiagnosed T2DM 
have approximately twofold greater risk for coronary heart disease, 
        Diagnosis
     Identify LADA,
single-gene disorders
Public health measures
          Education
           Nutrition
       Physical activity
           Screening
   High risk due to
IGT/IFG, prior GDM,
     family history
    Common problems
            Injuries
          Infections
Psychosocial adjustments
    Common probl

fections
Early neuropathy, retinopathy,
          nephropathy
  Subclinical CV disease
    Potential problems
  Amputation, blindness,
           renal failure
    MI,stroke, CV death
     diabetes
   Early diabetes
    (<10 years?)
 Long-duration diabetes
         (>10 years?)
HbA1cgoal <7%
(53 mmol/mol)
HbA1cgoal 7–8%
(53–64 mmol/mol)
Evidence-based standard therapy
                  Lifestyle
                Metformin
             Sulfo

style
                Metformin
             Sulfonylureas
              Basal insulin
        Long-acting GLP-1 agonists
            Other agents as needed
           Individualized therapy
    Standard therapies when effective
New glucose-lowering agents as needed
                 SGLT2 inhibitors
         Other CV protective agents
    Reassessment
Evaluate CV disease,
  glycemic variability
• Fig. 35.2  Overview of the natural history of type 2 diabetes (T2D) and opportunities for assessment

 2 diabetes (T2D) and opportunities for assessment and 
intervention. A schematic depiction of three stages of management of the natural history of T2D, noting 
several opportunities for improvement of management. CV, cardiovascular; GDM, gestational diabetes 
mellitus; IFG, impaired fasting glucose; IGT, impaired glucose tolerance; LADA, latent autoimmune diabe­
tes of adulthood; MI, myocardial infarction; SGLT2, sodium glucose transporter 2. (From Zinman B, Skyler 
JS, Riddle MC, et al. Diabet

rom Zinman B, Skyler 
JS, Riddle MC, et al. Diabetes research and care through the ages. Diabetes Care. 2017;40:1302–1313.)
1375
stroke, and peripheral vascular disease. Delay in the diagnosis of 
T2DM allows hyperglycemia to remain untreated for a period of 
time, resulting in an increase in microvascular and macrovascu­
lar disease. Affected individuals also have a greater likelihood of 
having dyslipidemia, hypertension, and obesity, all of

ng dyslipidemia, hypertension, and obesity, all of which are 
modifiable risk factors for cardiovascular disease. Therefore it is 
desirable to screen individuals who demonstrate major risk factors 
for diabetes, as summarized in Table 35.2.
Recent modeling studies based on the US population sug­
gest that universal screening coupled with guideline-based ther­
apy for T2DM is cost effective when initiated between the ages 
of 30 and 45 and subsequently conducted every 3 to 5 years.39 
In a syste

uently conducted every 3 to 5 years.39 
In a systematic review of 16 studies, people with an HbA1c of 
6% to 6.5% have a 25% to 50% likelihood of developing dia­
betes within 5 years, a 20-fold excess risk compared to those 
with an HbA1c of 5%.40 A more recent study of American Indi­
ans found similar predictive value for HbA1c in children and 
adolescents.23 
Interventional Study Results
Prospective, randomized clinical trials have documented reduced 
rates of microvascular complications in pa

educed 
rates of microvascular complications in patients with T2DM 
treated to lower glycemic targets. In the United Kingdom Pro­
spective Diabetes Study (UKPDS),28,41 patients with new-onset 
diabetes were treated with diet and exercise for 3 months, with 
an average reduction in HbA1c from approximately 9% to 7%, 
still somewhat above the upper limit of the normal range. Patients 
with fasting plasma glucose remaining greater than 108 mg/dL 
(6 mmol/L) were then randomly assigned to two treatm

 mmol/L) were then randomly assigned to two treatment poli­
cies. In the “conventional” intervention, participants continued 
lifestyle therapy alone. Pharmacologic therapy was initiated only 
if fasting plasma glucose reached 270 mg/dL (15 mmol/L) or the 
patient became symptomatic. In the “intensive” treatment arm, 
all patients were assigned to treatment with either a sulfonylurea 
or basal insulin as initial therapy, with the dose increased with 
the aim of maintaining fasting plasma glucose

ith 
the aim of maintaining fasting plasma glucose less than 108 mg/
dL. Combinations of agents were used only if the patient became 
symptomatic or fasting plasma glucose rose to greater than 270 
mg/dL. In a secondary study within the UKPDS a similar com­
parison was made between lifestyle alone and metformin as the 
intensive intervention for participants who were overweight or 
obese.41
As a consequence of the design, which aimed to test the effects 
of therapy with only one class of glucose

effects 
of therapy with only one class of glucose-lowering agents, HbA1c 
fell initially to about 6% but gradually rose to approximately 8% 
over 10 years of randomized treatment. During that time the 
median HbA1c in the conventional treatment group was approxi­
mately 1 percentage point higher than with the active treatments. 
Hypoglycemia needing assistance in the group treated with a sul­
fonylurea or insulin occurred between 1% and 5% per year in the 
first few years, with the highest rate

ear in the 
first few years, with the highest rates seen with glyburide. Weight 
gain was modest in all groups but higher in patients assigned to 
sulfonylurea or insulin and lower in those receiving metformin.41 
Associated with this improvement in glycemic control, there was 
a significant 25% reduction in the risk of microvascular compli­
cations (retinopathy, nephropathy, and neuropathy) in the group 
assigned treatment with a sulfonylurea or insulin (Table 35.3). 
Although there was also a 

 insulin (Table 35.3). 
Although there was also a trend toward a reduced rate of MI 
with sulfonylurea or insulin, it did not reach statistical signifi­
cance (p = 0.052).28 In the metformin part of the study, more 
consistent improvements were found (Table 35.4). Metformin 
treatment was associated with a nonsignificant 29% reduction 
  Criteria for Testing for Prediabetes or 
Diabetes in Asymptomatic Adults
	1.	 Testing should be considered in overweight or obese adults (body 
mass index ≥25

from American Diabetes Association. Classification and diagnosis of diabetes. Stan­
dards of medical care in diabetes, 2018. Diabetes Care. 2018;41:S13–S27.
   
TABLE 35.2
  Treatment Effects of Sulfonylureas or Insulin in the UKPDS
Aggregate Endpoints
YEAR 1997
END OF RANDOMIZED TREATMENT
YEAR 2007
AFTER 10 YEARS OF FURTHER OBSERVATION
Relative Risk Reduction
p Value
p Value
Any diabetes-related endpoint
12%
0.029
9%
0.040
Microvascular disease
25%
0.0099
24%
0.001
Myoc

40
25%
0.0099
24%
0.001
Myocardial infarction
16%
0.052
15%
0.014
6%
0.44
13%
0.007
Data from UK Prospective Diabetes Study (UKPDS) Group. Intensive blood-glucose control with sulphonylureas or insulin compared with conventional treatment and risk of complications in patients with 
type 2 diabetes (UKPDS 33). Lancet. 1998;352:837–853; Holman RR, Paul SK, Bethel MA, et al. 10-Year follow-up of intensive glucose control in type 2 diabetes. N Engl J Med. 20

ucose control in type 2 diabetes. N Engl J Med. 2008;359:1577–1589.
   
TABLE 35.3
1376
of microvascular disease, but there were statistically and clinically 
significant reductions of any diabetes-related endpoint (32%), MI 
(39%), and all-cause mortality (36%).
Similar reductions in microvascular events were observed in the 
Kumamoto study, a trial of somewhat different design and much 
smaller size. Japanese patients of normal weigh

h 
smaller size. Japanese patients of normal weight with T2DM 
receiving insulin were randomly assigned to standard treatment 
or an intensive program of insulin therapy designed to achieve 
normal glycemia. The control group maintained HbA1c values at 
approximately 9%, whereas the HbA1c in the intensive therapy 
group was reduced to approximately 7%, and the separation was 
maintained for 6 years. Again, there was a modestly increased risk 
of hypoglycemia and weight gain, a reduction in micro

hypoglycemia and weight gain, a reduction in microvascular 
complications, and a (not statistically significant) trend toward 
reduced rates of cardiovascular events.42
In 2008, three studies examining the effects of two lev­
els of glycemic control on cardiovascular endpoints in T2DM 
were reported. Action to Control Cardiovascular Risk in Dia­
betes (ACCORD),43 Action in Diabetes and Vascular Dis­
ease—Preterax and Diamicron Modified Release Controlled 
Evaluation (ADVANCE),44 and the Veterans

ntrolled 
Evaluation (ADVANCE),44 and the Veterans Affairs Diabetes 
Trial (VADT)45 each randomized middle-aged and older indi­
viduals who were selected for having high risk for cardiovas­
cular events. Participants randomized to intensive treatment 
in ACCORD and VADT aimed for an HbA1c target of less 
than 6% using complex combinations of oral agents and insu­
lin. In ADVANCE the HbA1c target was 6.5% or less using a 
somewhat less intensive approach based on the addition of the 
sulfonylurea

pproach based on the addition of the 
sulfonylurea gliclazide. None of the trials demonstrated a statis­
tically significant benefit in combined cardiovascular endpoints. 
In ACCORD a 22% increase in total mortality accompanied 
intensive therapy. Modest improvements in some microvascular 
endpoints were documented in all three trials. Some secondary 
analyses of these trials suggested that people without clinical 
cardiovascular disease and with shorter duration of disease and 
lower baseline H

 shorter duration of disease and 
lower baseline HbA1c had better outcomes with the more inten­
sive glucose-lowering strategies. Meta-analysis of the randomized 
trials confirmed statistically and clinically significant microvas­
cular benefits from intensive treatment. The same analysis sug­
gested that an average HbA1c reduction of 0.9% correlates with a 
17% reduction in nonfatal MI and a 15% reduction in coronary 
heart disease without significant effects on stroke or all-cause 
mortality r

ficant effects on stroke or all-cause 
mortality rate. However, there was significant heterogeneity in 
the results with respect to mortality rates across trials, the cause 
of which is still unknown.46
Continued follow-up of the UKPDS cohort, during which the 
difference in glycemic control was not maintained, demonstrated 
that the relative benefits of more intensive management of glucose 
noted at the end of 10 years of randomized treatment persisted 
during 10 more years of observation29 (se

rsisted 
during 10 more years of observation29 (see Tables 35.3 and 35.4). 
By 20 years after enrollment, sulfonylurea or insulin treatment 
still showed a significant 24% relative risk reduction for micro­
vascular outcomes, compared with the conventional intervention 
with lifestyle alone. For MI and all-cause mortality the relative risk 
reductions with sulfonylurea or insulin had become statistically 
significant, 15% (p = 0.014) and 13% (p = 0.007), respectively. 
Similarly, the risk reduct

 0.007), respectively. 
Similarly, the risk reductions for MI and all-cause mortality in the 
metformin part of the UKPDS remained highly significant after 
20 years, 33% (p = 0.005) and 27% (p = 0.002), respectively. This 
pattern suggests a “legacy effect” of initially good glycemic con­
trol, likely based on persisting changes of tissue structure in blood 
vessels and elsewhere.
Finally, two post hoc analyses of data from ACCORD have 
added further insights into the risks associated with inte

rther insights into the risks associated with intensive 
management in a high-risk population. First, an analysis of on-
treatment HbA1c showed that the individuals who accounted 
for the excess mortality in the intensive treatment group were 
not those who attained low HbA1c, levels, but rather those 
whose average HbA1c remained greater than 7% after starting 
treatment at a higher level.32 Thus, aiming for an HbA1c of less 
than 6% and achieving a target of less than 7% was not associ­
ated w

ng a target of less than 7% was not associ­
ated with excess mortality.32 On the other hand, those indi­
viduals who were seeking the 6% target but failed to attain even 
7% after a year of intensive treatment had clearly increased risk. 
The cause for increased risk in this subgroup remains unknown, 
but might be related to more advanced physiologic abnormali­
ties or limitations in adherence. In another analysis, an index 
was calculated based on relationships between baseline HbA1c 
and fasti

on relationships between baseline HbA1c 
and fasting glucose. The ACCORD participants in the top 
tertile of this index (with higher HbA1c than would be pre­
dicted based on fasting glucose) had increased mortality and 
no improvement of the primary cardiovascular endpoint. On 
the other hand, those in the lower and middle tertiles had no 
increased risk of mortality and an approximately 25% benefit 
by the primary endpoint. Whether observations using this index 
reflect an altered rate of glyca

using this index 
reflect an altered rate of glycation in tissues or disproportionate 
  Treatment Effects of Metformin in the UKPDS
YEAR 1997
END OF 10 YEARS OF RANDOMIZED TREATMENT
YEAR 2007
AFTER 10 YEARS OF FURTHER
OBSERVATION
 Relative Risk Reduction
p Value
p Value
32%
0.0023
21%
0.013
29%
0.19
16%
0.31
39%
0.010
33%
0.005
36%
0.011
27%
0.002
Data from 

36%
0.011
27%
0.002
Data from UK Prospective Diabetes Study (UKPDS) Group. Intensive blood-glucose control with sulphonylureas or insulin compared with conventional treatment and risk of complications in 
patients with type 2 diabetes (UKPDS 33). Lancet. 1998;352:837–853; Holman RR, Paul SK, Bethel MA, et al. 10-Year follow-up of intensive glucose control in type 2 diabetes. N Engl J Med. 
2008;359:1577–1589.
   
TABLE 35.4
1377
Chapter 35  Therapeutics of Type 2 Diabetes Mel

77
daytime hyperglycemia, the analysis further suggests that ben­
efits and risks of intensive treatment may differ between sub­
populations of T2DM.47 
Glycemic Treatment Targets
Whereas the glucose levels used for the diagnosis of diabetes were 
determined by their epidemiologic associations with compli­
cations of diabetes, the glycemic targets judged appropriate for 
treatment meant to reduce or prevent these complications have 
been deriv

ce or prevent these complications have 
been derived from the results of the intervention studies described 
earlier in the chapter. All of these studies obtained HbA1c levels 
averaging close to or below 7% during randomized assignment to 
intensive therapy.
Glycemic targets suggested by the ADA are presented in 
Table 35.5.48 These guidelines suggest that the goal of treatment 
should generally be an HbA1c value of less than 7%. Further­
more, the ADA suggests that lower targets may be pursued

the ADA suggests that lower targets may be pursued in 
selected patients, such as those with recent-onset disease, long life 
expectancy, and no significant cardiovascular disease, if they can 
be achieved without significant hypoglycemia or other adverse 
effects of treatment. The guidelines also recommend that a less 
stringent HbA1c target, such as 7% to 8% or even higher in some 
cases, may be appropriate for patients with evidence suggesting 
high risk, including a history of severe hypogly

high risk, including a history of severe hypoglycemia, limited life 
expectancy, advanced complications, or extensive comorbid con­
ditions, as well as inability to achieve HbA1c less than 7% despite 
the usual initial therapeutic efforts.48 The American College of 
Endocrinology (ACE) has recommended an HbA1c goal that is 
less than or equal to 6.5%, again with language suggesting indi­
vidualization of targets.38 It is agreed that treatment targets and 
tactics should be adapted to clinical 

argets and 
tactics should be adapted to clinical circumstances and personal­
ized to each patient’s needs and preferences, to the extent possible.
With respect to fasting, premeal, or postprandial glucose, 
there is little experimental support for any particular targets for 
these measurements in the management of T2DM. The ADA 
recommendation to seek fasting and premeal plasma glucose 
levels between 80 and 130 mg/dL (4.4–7.2 mmol/L) was based 
on an estimate of the range of average glucose va

on an estimate of the range of average glucose values that would 
be associated with a low risk of hypoglycemia and an HbA1c less 
than 7%. The ACE fasting glucose target of less than 110 mg/
dL (<6.1 mmol/L) is an effort to achieve a normal level. How­
ever, fasting and premeal glucose values consistently lower than 
110 mg/dL would be expected to be associated with an HbA1c 
of approximately 5.5%, and such levels are not safely attained by 
most patients as diabetes progresses.49
No published

st patients as diabetes progresses.49
No published studies have demonstrated safety or improved 
clinical outcomes for a particular targeted level of postprandial 
blood glucose. The ADA treatment target for peak postprandial 
glucose levels is less than 180 mg/dL (<10 mmol/L) in part because 
such levels would be associated with an HbA1c close to 7% and 
also because people without diabetes who consume a large evening 
meal can experience transitory elevations of glucose to that level.50 
The A

tory elevations of glucose to that level.50 
The ACE has recommended a 2-hour postprandial glucose target 
of less than 140 mg/dL (<7.8 mmol/L) in an effort to achieve 
near-normal glycemia.51 Consistent postprandial glucose values 
lower than 140 mg/dL would be associated with an average HbA1c 
of approximately 5% and are very difficult to attain with current 
treatments except quite early in the course of T2DM.49 
Monitoring of Glucose During Treatment
Hemoglobin A1c
Measurement of HbA1c provi

reatment
Measurement of HbA1c provides an estimate of the average plasma 
glucose level over a period of 2 to 3 months preceding the test.52 
Use of this indicator of glycemic control not only facilitates clinical 
trials but also assists routine clinical management. When treatment 
is started or intensified, measurement at approximately 3-month 
intervals will reveal the success of the intervention. When treat­
ment is established and glycemic control appears stable, testing 
one

 and glycemic control appears stable, testing 
one or two times a year is usually sufficient.48 However, measure­
ment of HbA1c has limitations. Because HbA1c reflects glycemia 
over a 3-month period, current glucose levels do not always cor­
relate well with current HbA1c measurements. Furthermore, indi­
vidual patients differ somewhat in the relationship of HbA1c to 
mean glucose concentrations, due to variations in erythrocyte sur­
vival or red cell turnover, hemoglobin variants, late kidney 

d cell turnover, hemoglobin variants, late kidney disease, 
or other factors.26,27,53,54 Such a mismatch between glucose and 
HbA1c values has sometimes been termed a “glycation gap.” This 
glycation gap can be due to recent changes in glycemia and may 
warrant changes in therapy. Lower HbA1c than expected based on 
blood glucose levels may occur with increased red cell turnover 
seen in a variety of circumstances, such as occult blood loss or 
iron treatment of iron deficiency anemia. The inapp

ron treatment of iron deficiency anemia. The inappropriately low 
HbA1c could be misconstrued as adequate control when intensifi­
cation of therapy is indicated. Alternatively, low red cell turnover 
such as with untreated iron deficiency anemia could lead to the 
opposite conclusion. In many cases, however, the higher HbA1c 
  American Diabetes Association Recommendations for Typical Target Ranges for Glycemic Control for 
Many Nonpregnant Adults
Units
For Healthy Individuals
For h

ts
Units
For high-Risk Individuals
Comments
Preprandial plasma 
glucose
mg/dL
mmol/L
80–130
4.4–7.2
Fasting, before lunch, and before dinner
Peak postprandial 
glucose
mg/dL
mmol/L
<180
<10
1 to 2 hours after any meal
Mean plasma glucose
mg/dL
mmol/L
<154
<8.6
Computed from glucose profile values
HbA1c
% 
mmol/mol
<7
<53
7–8
53–64
May need adjustment when mismatch between 
glucose and HbA1c occurs
Data from American

tween 
Data from American Diabetes Association. Glycemic targets: standards of medical care in diabetes, 2018. Diabetes Care. 2018;41:S55–S64.
   
TABLE 35.5
1378
level relative to the self-monitored blood glucose results is due to 
unrecognized high glucose levels such as postprandial or noctur­
nal times when patients typically are not testing. Comparison of 
HbA1c values with other measures of glucose is adv

HbA1c values with other measures of glucose is advisable at some 
stage of management, so that individuals with differences of 0.3% 
to 0.4% or greater from predicted levels can be identified.25 More­
over, HbA1c does not assess daily patterns or day-to-day variations 
of glycemic control. 
Self-Monitoring of Blood Glucose
Self-monitoring of capillary blood glucose (SMBG) with glucose-
oxidase strips provides that information. Since introduction of 
SMBG 40 years ago, the convenience and reliabi

of 
SMBG 40 years ago, the convenience and reliability of test strips 
and meters for use in measuring point-in-time glucose values 
have improved greatly, and all currently available personal glucose 
meters are generally accurate.55 Even so, recent studies have shown 
variable performance under conditions of daily use, highlighting 
the need for expertise and care in use of the information thus 
obtained.56 At the present time SMBG is widely recommended 
to monitor the progress of therapy and 

commended 
to monitor the progress of therapy and to identify specific glucose 
patterns in T2DM. It is especially helpful for patients with T1DM 
and those with T2DM who use intensive insulin therapy. Testing 
before-breakfast glucose levels is essential for initiating and guid­
ing titration of basal insulin when it is added to oral agents, and 
premeal testing is necessary to guide adjustment of further injec­
tions of rapid-acting insulin before meals.57–59 Although clinical 
trials have not

ore meals.57–59 Although clinical 
trials have not consistently shown SMBG can change outcomes in 
other groups of patients with T2DM when used in isolation,60,61 
diabetes self-management programs that include SMBG can be 
beneficial in controlling glucose and reducing complications.62,63 
Potential benefits of SMBG in non–insulin-treated patients 
include supporting educational concepts of nutrition and exercise, 
determining a daily glucose pattern to select the best pharmaco­
logic tactics, 

ttern to select the best pharmaco­
logic tactics, documenting early responses to pharmacotherapy, 
and serving generally as a motivational tool.48,62,64 Use of SMBG 
is important for all patients with a new diagnosis of diabetes for 
purposes of education, and it subsequently should be advised 
selectively according to the duration of diabetes, risks of hypogly­
cemia, and other factors.65,66
Use of SMBG is particularly recommended for patients with 
T2DM who are taking insulin or sulfonylureas 

ith 
T2DM who are taking insulin or sulfonylureas because it can 
identify minimal or asymptomatic episodes of hypoglycemia, 
which may predict increased risk of more severe events. Severe 
hypoglycemia is relatively rare in T2DM, but it can have devastat­
ing consequences such as falls and other trauma, or a change in the 
ability of a patient to continue to live independently due to risk of 
confusion or loss of consciousness. Recognition of minor hypogly­
cemia is also important to avoid undu

nor hypogly­
cemia is also important to avoid undue concern about nonspecific 
symptoms that may be due to other causes. Monitoring studies 
show that most symptoms in patients with T2DM are not related 
to hypoglycemia, but some patients are sufficiently concerned that 
they may consume extra calories in response to sweating, anxiety, 
or emotional distress without documenting low glucose levels by 
SMBG. Fear of hypoglycemia may also lead to lack of adherence 
to medications or inappropriate r

ck of adherence 
to medications or inappropriate reductions of dose, and the ability 
to document actual glucose values can be reassuring.
The timing of SMBG that is most useful depends on various 
factors. One basic principle is that patients should periodically 
vary the time of day at which glucose is tested. For some patients 
glucose values are highest in the morning, whereas for others glu­
cose is highest in the evening. Times of highest risk of hypoglyce­
mia can vary as well. When gluco

isk of hypoglyce­
mia can vary as well. When glucose control is poor, concentrating 
on premeal glucose levels is adequate. Once premeal glucose levels 
are reduced to 120 to 130 mg/dL (6.7–7.2 mmol/L), adding some 
measurements 1 to 2 hours after meals is appropriate. This calls 
attention to the effect of diet and enables patients to understand 
how changes in meals, activity, and medications have significant 
influences on glycemic control. Postprandial testing also becomes 
important when th

stprandial testing also becomes 
important when the HbA1c remains high despite attainment of 
reasonably good premeal glucose levels. In such circumstances, 
postprandial glucose increments can be responsible for the higher-
than-expected HbA1c and may require a change in therapy. In 
recently diagnosed diabetes and gestational diabetes, monitoring 
only 1 to 2 hours after meals can allow patients to assess the effect 
of lifestyle and pharmacologic efforts on postprandial glucose 
levels, which

gic efforts on postprandial glucose 
levels, which may be the leading glycemic abnormality in those 
settings.
Clinical trials have confirmed the benefits of structured use of 
SMBG in patients who have been educated on how to use the 
information.67–69 For example, the Structured Testing Program 
(STeP) trial found use of 7-point daily glucose testing for 3 days 
each quarter increased the frequency of treatment adjustments and 
improved HbA1c levels.70 It is likely that the benefits of SMBG 
s

evels.70 It is likely that the benefits of SMBG 
stem from assisting patients in controlling of their own therapy. 
If patients are aware of the glycemic targets associated with the 
outcomes they seek to achieve, SMBG enables them to evaluate 
their responses to therapy and actively participate in reaching 
their goals. Present meters are downloadable, and clinics should 
have software that allows reproducing a summary of experience 
since the last visit, a task once falling to the patients the

ast visit, a task once falling to the patients themselves. In 
some cases it is useful for patients periodically to keep a personal 
daily diary of SMBG, making them more aware of their results. 
Unfortunately, patients often fail to bring their meter or their 
handwritten log to the visit.71 The importance of this informa­
tion must be emphasized by the treating clinic. Providers should 
examine records of SMBG carefully at the time of visit, as it is 
frustrating for patients to put extensive 

 it is 
frustrating for patients to put extensive effort into collection of 
data that are not taken seriously by their health advisors. In fact, it 
appears that the more attention that is given to the blood glucose 
results at a visit, the more frequent the monitoring by the patient 
and the lower the HbA1c.72 In some circumstances, such as recent 
changes in therapy, rapid deterioration of control, or worrisome 
hypoglycemia, SMBG should be communicated to a member of 
the health care team by

mmunicated to a member of 
the health care team by telephone, fax, or privacy-protected email 
at shortened intervals to facilitate appropriate correction of the 
problem. Providing adequate financial support for such services 
may be challenging for health systems, but this is necessary to 
limit the burden placed upon providers. Similarly, it may also be 
challenging for patients and providers to keep current on the array 
of available equipment and supplies for glucose testing and han­
dling 

t and supplies for glucose testing and han­
dling of the resulting data. A useful resource in this regard is the 
annual Consumer Guide, which is published as the January issue 
of Diabetes Forecast, a magazine for laypeople with diabetes and 
their families.73 
Continuous Glucose Monitoring
Recently, development of continuous glucose monitoring (CGM) 
systems has added a new dimension to tracking glycemic control. 
These devices sample subcutaneous interstitial fluids at frequent 
intervals and

ous interstitial fluids at frequent 
intervals and can report the results either in real time or upon 
demand. Their value in T1DM is well documented, and CGM 
is now frequently recommended for patients with T2DM who 
embrace newer technologies with the aim of reducing glycemic 
variability and hypoglycemia while seeking improved HbA1c.74 
1379
The real-time CGM devices include audible alerts to inform the 
patient of higher trends as well as 

o inform the 
patient of higher trends as well as low or impending low glucose 
values. A smartphone may serve as the receiver, and appropriate 
apps will give real-time glucose information, updating every 5 
minutes. Trend arrows are shown, and information can be shared 
with providers as well as family members or other supporting 
individuals. The intermittently scanned or “flash” CGM moni­
tors only record glucose results when the reader is passed over the 
sensor that is in the interstitial 

ssed over the 
sensor that is in the interstitial space. However, once scanned, the 
reader will display the present glucose value as well as a rate of 
change arrow and a recording of the glucose trend over the last 8 
hours. If the sensor is scanned every 8 hours, a continuous chart of 
the glucose trends can be viewed. Favorable effects of using CGM 
for patients with T2DM who are treated with intensive insulin 
regimens have also been validated.75,76 Medicare covers CGM for 
use by people wi

ed.75,76 Medicare covers CGM for 
use by people with T2DM on intensive insulin therapy. A strong 
rationale for this decision is that protection against hypoglycemia 
is of heightened importance in older patients.77,78 Patients with 
T2DM requiring insulin may also benefit from CGM through 
promotion of desirable diet and exercise patterns, but the cost 
effectiveness of this application has not yet been fully assessed. 
Finally, CGM can provide accurate measurement of mean glucose 
levels, ther

accurate measurement of mean glucose 
levels, thereby validating HbA1c values or, alternatively, identify­
ing patients with a significant glycation gap.25 Fig. 35.3 shows one 
CGM      Data point
Very High
Above 250
mg/dL
Coefficient
of Variation
SD
mg/dL
% Time CGM
Active
High Alert
Above 180
mg/dL
Low Alert
Below 70
mg/dL
Very Low
Below 54
mg/dL
Glycemic
Estimate
7.0%
Avg Glucose
mg/dL
156
Level 2
Level 1
Level 1
46.3%
72
70.6%
Level 2
GLUCOSE EXPOSURE
GLUCOSE EXPOSURE CLOSE-UP
Ambulatory glu

 EXPOSURE
Ambulatory glucose profile
VARIABILITY CLOSE-UP
Avg Min. Per Day
Mean Episodes/Day
Mean Duration (Min)
An episode ≥15 
continuous min.
<54
00
0.0
00
<70
00
0.0
00
>140
00
0.0
00
>180
00
0.0
00
>250
00
0.0
00
50%-Median
EPISODES CLOSE-UP
Glucose Statistics
Wake
5 AM to 12AM
000
000
000
000
00
MODD
00
0.0
LBGI
0.0
AUC
Hourly
(mg/dL)*h
Sleep
12 AM to 6 AM
24 Hours
IQR
mg/dL
MAGE
HBGI
2AM
4AM
6AM
8AM
10AM
12PM
2PM
4PM
6PM
8PM
10PM
12AM
1

4AM
6AM
8AM
10AM
12PM
2PM
4PM
6PM
8PM
10PM
12AM
12AM
90%
400
mg/dL
Level 1 = Needs attention. Level 2 = Immediate action.
Curves/plots represent glucose frequency distributions by time regardless of date.
350
300
250
200
150
100
50
0
75%
50%
25%
10%
GLUCOSE RANGES
GLUCOSE VARIABILITY
DATA
SUFFICIENCY
In Target Range
70–180
mg/dL
10/90%
25/75%-IQR
Target range
First Name_____________ Last Name_________________
Dates covered by this report_________________
4.4%
10.1%
54.5%
35.4%
11.3%
• Fig. 35.3

_________
4.4%
10.1%
54.5%
35.4%
11.3%
• Fig. 35.3  An electronic ambulatory glucose profile report that visualizes the key continuous glucose 
monitoring metrics: (1) mean glucose; (2) hypoglycemia: clinically significant/immediate action required; (3) 
hypoglycemia alert/low/monitor; (4) target range; (5) hyperglycemia: alert/elevated/monitor; (6) hypergly­
cemia: clinically significant/very elevated/immediate action required; (7) glycemic variability; (8) estimated 
HbA1c; (9) time blocks; (1

ability; (8) estimated 
HbA1c; (9) time blocks; (10) collection period; (11) percentage of expected readings; (12) hypoglycemia/
hyperglycemia episodes; (13) area under the curve; (14) hypoglycemia/hyperglycemia risk; and (15) stan­
dardized continuous visualization. AUC, area under the curve; Avg, average; IQR, interquartile range; 
MAGE, mean amplitude of glucose excursions; MODD, mean of daily differences; SD, standard deviation. 
(Modified from Danne T, Nimri R, Battelino T, et al. Internati

om Danne T, Nimri R, Battelino T, et al. International consensus on use of continuous glucose 
monitoring. Diabetes Care. 2017;40:1631–1640.)
1380
form of display that can be used to summarize and communicate a 
variety of kinds of clinically useful information provided by CGM 
systems.79 
Among the most important purposes of measuring glycemic pat­
terns during treatment of T2DM, as with T1DM, is to identify 
hypoglycemia

f T2DM, as with T1DM, is to identify 
hypoglycemia.80,81 The definitions of hypoglycemia have recently 
been a topic of debate. Table 35.6 summarizes the ADA’s most 
recent statements on this point.82 Division of hypoglycemia into 
three levels is based on three kinds of observations. Under normal 
circumstances a decline of glucose concentrations below 70 mg/
dL (<3.9 mmol/L) triggers physiologic responses of the so-called 
counterregulatory hormones: glucagon, epinephrine and norepi­
nephrine,

mones: glucagon, epinephrine and norepi­
nephrine, cortisol, and growth hormone. Symptoms of hypogly­
cemia may or may not result from these responses, but declines 
below this threshold, termed level 1 hypoglycemia, are considered 
important as alerts to allow protection against more worrisome 
declines of glucose. Level 2 hypoglycemia is defined as glucose 
documented at or lower than 54 mg/dL (3 mmol/L). Repeated 
declines to this range are considered clinically significant because 
they can 

nsidered clinically significant because 
they can cause blunting of compensatory hormone responses and 
loss of warning symptoms (“hypoglycemia unawareness”), and 
thereby increase the risk of more dangerous lows. Level 3 is con­
sidered severe hypoglycemia, defined as requiring assistance by 
another person, whether a friend or family member or a medical 
provider. This level of hypoglycemia in T2DM is strongly associ­
ated with risk of physical injury, cardiovascular events, and death. 
Althou

 injury, cardiovascular events, and death. 
Although hypoglycemia can directly cause such events through 
falls and cardiac arrhythmias, this relationship with serious medi­
cal outcomes is thought to be partly related to an association of 
hypoglycemia with other causes of risk, including concurrent ill­
ness, inconsistent eating patterns, malnutrition and weight loss, 
physical frailty, and cognitive impairment. Nonetheless, level 3 
severe events are markers of risk and call for both conserva

nts are markers of risk and call for both conservative 
glycemic management and renewed attention to other aspects of 
care. 
Lifestyle Interventions
The leading lifestyle interventions for management of T2DM 
are medical nutrition counseling and exercise recommendations. 
Beyond these traditional pillars of lifestyle management there is 
growing attention to psychosocial concerns as well as interest in 
various forms of stress reduction, relaxation, and sleep manage­
ment. The evidence base sup

ion, and sleep manage­
ment. The evidence base supporting such interventions has become 
much stronger recently, and reimbursement for these efforts has 
become more frequently available. They are most effective when 
presented as parts of a comprehensive plan of self-management 
with the goal of moving the focus of care from the provider to the 
individual with diabetes.83–85 
Diabetes Self-Management Education and 
Support
Diabetes is a chronic, ever-present disease that places the patient 
in

, ever-present disease that places the patient 
in a unique position of self-care, requiring daily decisions and the 
ability to solve problems when they arise. The provider has little 
connection with day-to-day care, thus the patient needs the tools, 
confidence, and a plan for self-management. Professional services 
promoting this process are now described as diabetes self-manage­
ment education and support (DSMES). The evidence for benefit of 
DSME is now quite strong.86,87 The ADA together 

DSME is now quite strong.86,87 The ADA together with the Amer­
ican Association of Diabetes Educators (AADE) and the Academy 
of Nutrition and Dietetics recommend that all individuals with 
diabetes should receive DSMES.85 As illustrated in Table 35.7, it 
is recommended that DSMES be emphasized at four critical set­
tings in the evolution of T2DM. These include the time of diag­
nosis, annual reassessments, when complicating factors arise, and 
the time of critical transitions in medical care

the time of critical transitions in medical care or life circumstances. 
The importance of ongoing contact and reassessment has proven to 
be significant.88 There is also consensus on a core group of consid­
erations, methods, and barriers to address.85 Unfortunately, there is 
evidence that DSMES services are underutilized. A review of a large 
commercial insurance database reported that only 6.8% of patients 
received DSMES within the first 12 months of diagnosis.89 When 
subgroups were cons

2 months of diagnosis.89 When 
subgroups were considered by ethnicity, socioeconomic groups, 
age, and other factors, the highest utilization subgroup received 
DSMES only 15% of the time. Why utilization is so low is not 
clear, but providers’ unawareness of the significance and availabil­
ity of this important resource are thought to be important factors. 
The AADE (telephone 800-TEAM-UP4) and the ADA (telephone 
800-DIABETES) can provide information regarding diabetes edu­
cators and educatio

mation regarding diabetes edu­
cators and education programs in local areas of the United States.
DSMES refers to all activities and individuals that help patients 
turn knowledge into action. A summary of the necessary components 
of an effective DSMES program is provided in Table 35.8.90,91 Health 
care providers are seldom adequately trained and often do not have 
the patient contact time to deal with all these issues alone. The need 
for a team approach has long been recognized in diabetes c

am approach has long been recognized in diabetes care and 
remains a critical ingredient for success. The providers (physicians 
and advanced practice providers) enrich their care with the help of 
nurses, dietitians, exercise specialists, behavioral specialists, pharma­
cists, podiatrists, and other medical and surgical specialists. Among 
  American Diabetes Association Classification of Hypoglycemia
Levels of Hypoglycemia
Glucose Units
Criteria
Comments
Level 1 (Alert value)
mg/dL
mmol/L
<70

ia
Comments
mg/dL
mmol/L
<70 and ≥54
<3.9 and ≥3
Important independent of symptoms, 
adjustment of therapy may be 
needed
Level 2 (Clinically significant)
mg/dL
mmol/L
≤54
≤3
Requires immediate action
Level 3 (Severe)
mg/dL
mmol/L
Altered mental and/or physical status requiring assistance
Calls for reevaluation of regimen and/
or targets
Modified from American Diabetes Association. Glycemic targets: standards of medical care in diabetes, 2018. Diabetes Care. 2018;41:S55–S64

 in diabetes, 2018. Diabetes Care. 2018;41:S55–S64.
   
TABLE 35.6
1381
the commonly overlooked contributors to ineffective care of T2DM 
are the barriers created by psychiatric, neurocognitive function, and 
adjustment disorders, which are largely responsive to psychosocial 
therapies.92 Modern diabetes management principles emphasize that 
the patients are at the center of the team and that the management plan 
is not dictated by the provide

he management plan 
is not dictated by the provider but ultimately determined by informed 
patients who may include personal health beliefs as well as cultural 
influences and other factors in their approaches to self-care.83,85,93
People other than medical professionals can contribute to 
DSMES. Group sessions provide added support as do programs 
that involve peer groups and health coaches without a traditional 
medical professional directly involved.94–96 In the last decade we 
have also seen

volved.94–96 In the last decade we 
have also seen growing implementation of digital health and tele­
health tools that improve communication and otherwise support 
patients in self-management.97–102 Finally, the potential role of 
the community in which the patient lives and works in the diabe­
tes self-care process is enormous; family, friends, employers, and 
health insurance providers may all be involved.103 Peer support 
has proven benefits.
Medical Nutrition Therapy
Medical nutrition thera

Medical nutrition therapy (MNT) is an essential component of 
DSMES. Ideally all patients should have access to a registered 
dietitian-nutritionist (RDN) to facilitate it.83,104–106 Recent 
guidelines recommend three to six MNT sessions in the first 
6 months after diagnosis. Medical nutrition therapy has been 
shown to decrease HbA1c by 0.3% to 2%. In a typically obese 
and older T2DM population the basic goals of nutrition therapy 
include weight reduction or mainte

rition therapy 
include weight reduction or maintenance, adequate glucose con­
trol, a nutrient-sufficient diet, and nutritional efforts to reduce 
cardiovascular risk by controlling blood pressure and lipid levels. 
Nutritional strategies, of course, require individualization based 
on the patient’s clinical profile, personal and cultural factors, 
health literacy, motivation, and economic limitations. A summary 
of general nutritional principles for people with diabetes is shown 
in Table 35.9

s for people with diabetes is shown 
in Table 35.9.83
Beyond the general principles, there are specific features of 
eating patterns that are relevant to the medical management of 
individual patients, and some of them are directly related to the 
success of treatment with insulin. It is very helpful to know how 
many meals the patient eats in a day, and at what times. Does 
this pattern differ from day to day? Are meals often missed, and 
if so why? What does the patient view as the primary cha

why? What does the patient view as the primary challenge 
in healthful eating? Do family members eat with the patient and 
  Diabetes Self-Management Education and Support (DSMES) for Adults With Type 2 Diabetes: 
Algorithm of Care
Emotional
Health
Mental health
professional if
needed
Education
DSMES
Nutrition
Registered
Dietitian
for MNT
FOUR CRITICAL TIMES TO ASSESS, PROVIDE, AND ADJUST DSMES
At Diagnosis
Annual Assessment
When Complicating Factors Arise
When Transitions in Care Occur
When pr

ctors Arise
When primary care or specialist provider should consider referral
Initial evaluation at diagnosis 
and to ensure both nutrition 
and emotional health are 
addressed
Needs review of knowledge, skills, 
behaviors
Long-duration T2DM
Change of regimen
HbA1c out of target range
Unexplained hypoglycemia or hyper­
glycemia
Planning pregnancy or pregnant
For support in changing life situations
For weight or nutritional concerns
Complex medication regimen
Change

itional concerns
Change in health conditions such as 
renal disease, stroke, glucocorticoid 
use, etc.
Physical limitations such as visual 
impairment, dexterity issues
Emotional factors such as anxiety, 
Basic living needs such as food access, 
housing, financial status
Rehabilitation, assisted living, living 
alone
New medical care team
Change of insurance
Age-related changes affecting 
cognition, self-care, etc.
MNT, Medical nutritional therapy.
Modified 

n.
From Davies MJ, D’Alessio DA, Fradkin J, et al. Management of hyperglycemia in type 2 dia­
betes, 2018. A consensus report by the American Diabetes Association (ADA) and the Euro­
pean Association for the Study of Diabetes (EASD). Diabetes Care. 2018. [Epub ahead of print.]
   
TABLE 35.8
1382
support the recommended nutrition goals? What beverages does 
the patient drink? How many alcoholic drinks does the patient 
take daily? This

lcoholic drinks does the patient 
take daily? This information should be reassessed periodically, 
with specific suggestions for changes that are achievable.
Problematic habits may become apparent. Intake of caloric bev­
erages (e.g., juices) or very high glycemic index foods (e.g., cold 
cereal) might be perceived as healthful but can significantly affect 
glycemic control.107,108 Finding foods that the patient may view as 
healthful due to being “low carb” while ignoring the high fat con­
tent

g “low carb” while ignoring the high fat con­
tent or, conversely, foods chosen for being low in fat but having high 
carbohydrate content can provide opportunities for improvement. 
A pattern of snacking from dinner until bedtime or consistently 
overeating on weekends can create difficulties. Portion control tech­
niques, variation in food preparation (e.g., fried vs. grilled), appro­
priate meal spacing, and mindful eating are important concepts to 
reinforce for the patient and the family, i

ts to 
reinforce for the patient and the family, including a potential ben­
efit for family members who are at risk of developing diabetes.109 
It has also become apparent that diabetes providers should consider 
food insecurity that is becoming increasingly recognized and relates 
to high HbA1c.110 Finally, it is important to track the success of 
nutrition goals determined in previous visits, setting new goals and 
potentially offering new resources where appropriate.111
  Major Nutritional R

e considered for those with type 2 diabetes who are not taking insulin, who have limited health literacy or numeracy, or who are 
older and prone to hypoglycemia.
Energy balance
 • Weight loss (>5%) achievable by the combination of reduction of calorie intake and lifestyle modification benefits overweight or 
obese adults with type 2 diabetes and also those with prediabetes. Intervention programs to facilitate weight loss are recom­
mended.
Eating patterns and 
macronutrient 
distribution
 • T

 fruits, legumes, whole grains, and dairy products, with an emphasis on foods higher in fiber 
and lower in glycemic load, is preferred over other sources, especially those containing added sugars.
 • For people with type 1 diabetes and those with type 2 diabetes who are prescribed a flexible insulin therapy program, education 
on how to use carbohydrate counting and in some cases fat and protein gram estimation to determine mealtime insulin dosing is 
recommended to improve glycemic control.
 
ing is 
 • For individuals whose daily insulin dosing is fixed, a consistent pattern of carbohydrate intake with respect to time and amount 
may be recommended to improve glycemic control and reduce the risk of hypoglycemia.
 • People with diabetes and those at risk should avoid sugar-sweetened beverages to control weight and reduce their risk for CVD 
and fatty liver and should minimize the consumption of foods with added sugar that have the capacity t

of foods with added sugar that have the capacity to displace healthier, more 
nutrient-dense food choices. 
Protein
 • In individuals with type 2 diabetes, ingested protein appears to increase insulin response without increasing plasma glucose 
concentrations. Therefore carbohydrate sources high in protein should be avoided when trying to treat or prevent hypoglycemia.
Dietary fat
 • Data on the ideal total dietary fat content for people with diabetes are inconclusive, so an eating plan emphas

iabetes are inconclusive, so an eating plan emphasizing elements of a 
Mediterranean-style diet rich in monounsaturated and polyunsaturated fats may be considered to improve glucose metabolism and 
lower CVD risk and can be an effective alternative to a diet low in total fat but relatively high in carbohydrates.
 • Eating foods rich in long-chain n-3 fatty acids, such as fatty fish (EPA and DHA) and nuts and seeds (ALA), is recommended to 
prevent or treat CVD; however, evidence does not suppor

nt or treat CVD; however, evidence does not support a beneficial role for the routine use of n-3 dietary supplements. 
Micronutrients and 
herbal supple­
ments
 • There is no clear evidence that dietary supplementation with vitamins, minerals, herbs, or spices can improve outcomes in people 
with diabetes who do not have underlying deficiencies, and are not generally recommended. There may be safety concerns regard­
ing the long-term use of antioxidant supplements such as vitamins E and C and c

of delayed hypoglycemia are warranted.
Sodium
 • As for the general population, people with diabetes should limit sodium consumption to <2300 mg/day, although further restriction 
may be indicated for those with both diabetes and hypertension.
Nonnutritive sweet­
eners
 • The use of nonnutritive sweeteners may have the potential to reduce overall calorie and carbohydrate intake if substituted for 
caloric (sugar) sweeteners and without compensation by intake of additional calories from other f

tion by intake of additional calories from other food sources. Nonnutritive sweet­
eners are generally safe to use within the defined acceptable daily intake levels.
ALA, Alpha-lipoic acid; CVD, cardiovascular disease; DHA, docosahexaenoic acid; EPA, eicosapentaenoic acid; MNT, medical nutrition therapy.
Modified from American Diabetes Association. Lifestyle management: standards of medical care in diabetes, 2018. Diabetes Care. 2018;41:S38–S50.
   
TABLE 35.9
1383
Chapter 35  Therapeutics of Ty

TABLE 35.9
1383
Structured lifestyle programs can produce long-term weight 
loss of 5% to 7% of starting weight and112 reduce the risk of devel­
oping diabetes.113 Such systems emphasize education, reduced 
energy intake (especially fat), conscious decisions regarding 
carbohydrate intake, regular physical activity, and ongoing 
support from professionals or peers. Achieving a 5% weight 
loss more consistently yields significant metabolic 

ss more consistently yields significant metabolic benefits than 
lower levels of weight loss.114 Focusing on a specific macro­
nutrient distribution appears to be less important than reduc­
ing overall caloric intake and facilitating maintenance of the 
weight loss.115,116
As an example, it has been demonstrated in a population with 
T2DM of less than 6 years’ duration that consistent application 
of a specific intervention program, in a primary care setting, can 
result in significant reduction

care setting, can 
result in significant reduction of weight and HbA1c over a 1-year 
period.117 The same report showed glycemic improvement was 
proportional to the reduction of weight. Patients who lost less than 
5% of body weight had insignificant HbA1c reductions, similar to 
that in the control group. Of those in the intervention group who 
lost 5% to 10 % of body weight, 34% ended with HbA1c below 
6.5%. Among those losing more than 15%, 86% achieved this 
level of control. The reduction 

6% achieved this 
level of control. The reduction of HbA1c was often accompanied 
by less use of pharmacologic agents as well. For more information 
on therapeutic weight loss, see Chapter 40.
Much controversy has surrounded the macronutrient makeup 
of the diet of people with T2DM. In general, the critical nutrient 
for glycemic consistency is carbohydrate. Glucose levels after meals 
are strongly influenced by the carbohydrate content of the diet. 
For patients with T2DM requiring prandial ins

et. 
For patients with T2DM requiring prandial insulin, the carbohy­
drate-counting technique is used to facilitate consistent carbohy­
drate intake or to allow adjustment of insulin doses in response 
to changes in carbohydrates consumed.118 However, evidence for 
the benefit of calculating insulin:carbohydrate ratios in T2DM 
over other methods of dose adjustment is less compelling than in 
T1DM.118,119 For patients not using insulin, carbohydrates still 
affect postprandial glucose excursions

ates still 
affect postprandial glucose excursions, and consistency of carbo­
hydrate intake is still relevant. However, other methods can be 
used to assess and modulate carbohydrate intake. These include 
the “plate method,” carbohydrate exchange lists, and even simple 
portion control tactics.119,120 Whereas the beta cell in T2DM has 
usually lost its immediate response to glucose, the second phase of 
insulin secretion is largely spared in T2DM and is in part driven 
by amino acids and fatty

DM and is in part driven 
by amino acids and fatty acids. Therefore including some protein 
and fat in each meal and snack is useful, and these macronutri­
ent groups along with fiber content can attenuate the effect of 
carbohydrate on postprandial elevations. Although the carbohy­
drate content of typical nutrition plans for those with T2DM has 
generally decreased in the last decade, the optimal proportion of 
macronutrients is uncertain. Low carbohydrate diets, including 
ketogenic diets, ha

carbohydrate diets, including 
ketogenic diets, have grown in popularity and are associated with 
decreased postprandial glucose and a reduction in HbA1c.121–123 
Even so, the benefit in reducing HbA1c may come more from the 
frequently associated weight loss as the evidence indicates car­
bohydrate content, independent of weight loss, does not signifi­
cantly influence HbA1c. In many cases the postprandial glucose 
decreases with lower carbohydrate intake, but fasting glucose may 
be higher. Lo

ate intake, but fasting glucose may 
be higher. Long-term adherence to very low carbohydrate diets is 
difficult. The glycemic index refers to the glucose response to equal 
amounts of carbohydrates in various foods. This index has proven 
somewhat difficult to use for meal planning, and studies evalu­
ating the benefit of lowering glycemic index have yielded mixed 
results.107,108,124 As a practical measure, identifying high glycemic 
index foods in a patient’s diet could help reduce high postp

s in a patient’s diet could help reduce high postprandial 
blood glucose patterns. Nutrition practice guidelines also recog­
nize the benefit of SMBG in confirming the glucose response to 
food intake and success of nutritional interventions.83,85
Dietary fat is the nutrient that is most closely associated in epi­
demiologic studies with the risk of developing T2DM. Although 
dietary fats clearly have an impact on total caloric intake (related 
to their caloric density) and on circulating lipids

o their caloric density) and on circulating lipids, they have little 
effect on glycemia. Fat intake is a contributor to obesity and is the 
critical nutrient for cardiovascular risk management. It is recom­
mended that people with diabetes (and everyone in general) con­
sume a diet that is modestly restricted in calories (if overweight) 
and contains less than 10% of total calories as saturated fat and less 
than 10% as polyunsaturated fat.85 Some advocate substituting 
foods high in monounsatu

me advocate substituting 
foods high in monounsaturated fatty acids (i.e., seeds, nuts, avo­
cado, olives, olive oil, and canola oil) for carbohydrate, but most 
patients do not find adequate variety in the monounsaturated fatty 
acid category and often overeat these high-calorie foods. Higher 
carbohydrate diets can raise postprandial glucose and triglycerides 
but are much less calorically dense than higher fat diets and have a 
higher thermic effect, both of which tend to promote weight loss.

effect, both of which tend to promote weight loss.
Dietary protein similarly has little impact on glucose levels, 
although amino acids do promote insulin secretion. Metabolism 
of protein results in the formation of acids and nitrogenous waste, 
which can lead to bone demineralization and glomerular hyperfil­
tration. At least 0.8 g of high-quality dietary protein per kilogram 
of body weight is generally recommended. Restriction of pro­
tein intake to 10% to 20% of total calories minimizes pot

take to 10% to 20% of total calories minimizes potential 
adverse long-term effects of high protein intake. However, recent 
guidelines do not support the notion that dietary protein need be 
reduced in those with chronic kidney disease.105 Likewise, there 
is no evidence for a distinction between the effects of vegetable-
based versus animal-based protein sources in kidney function.
The evidence base for benefit of higher fiber intake is not very con­
vincing. Still, there may be some benefit o

y con­
vincing. Still, there may be some benefit on HbA1c and lipid profiles 
for those ingesting at least 15 g of fiber per 1000 kcal compared to 
lower fiber diets. Alcohol in moderation (up to one drink per day for 
adult women or two drinks per day in adult men) is not specifically 
recommended but is considered acceptable. An added risk associated 
with excessive intake of alcohol by people with T2DM, especially 
those who use sulfonylureas or insulin, is delayed hypoglycemia, 
which typica

or insulin, is delayed hypoglycemia, 
which typically occurs at night when ability to recognize hypoglyce­
mia can be impaired. Moderate red wine intake may result in mild 
improvement in some lipid parameters but seems to have little effect 
on glucose control. Nonnutritive sweeteners such as aspartame, ace-
K, saccharin, stevia, or sucralose do not appear to impact lipid param­
eters, insulin secretion, or blood pressure independent of weight loss. 
They are deemed safe for use in T2DM if cons

oss. 
They are deemed safe for use in T2DM if consumed within the FDA 
recommended daily intake amounts. Sodium restriction to less than 
2300 mg per day is recommended, and lower sodium diets may have 
additional benefit in some individuals with hypertension.85
The roles of vitamins, trace minerals, and nutritional supple­
ments in the treatment of diabetes are poorly understood. Some 
clinicians are convinced of the utility of magnesium, chromium, 
zinc, folic acid, pyridoxine, cyanocobalamin,

um, 
zinc, folic acid, pyridoxine, cyanocobalamin, vitamin A, vitamin 
C, vitamin E, vanadium, selenium, garlic, and other micronu­
trients. Clinical trial data regarding their safety and efficacy are 
inconclusive at best. Many patients are convinced that nutritional 
supplementation is healthful, and it is often counterproductive 
to engage in scholarly discussion of the nature of the evidence 
base for their decision. At a minimum, discussion should include 
the documented efficacy of more cl

should include 
the documented efficacy of more classic lifestyle and pharmaco­
logic interventions and the idea that these efforts should not be 
ignored.125
1384
Although various specific dietary strategies have their propo­
nents, few data from long-term outcome studies are available to 
support routine use. For example, ketogenic diets or low carbo­
hydrate diets have limited long-term evidence of benefit or risk 
at the present ti

erm evidence of benefit or risk 
at the present time. However, if a patient or practitioner wants to 
use a lower carbohydrate higher protein/fat reduced calorie diet, 
that choice may be associated with short-term improvements in 
glycemia, cardiovascular risk markers, and weight. The most vali­
dated meal plans in T2DM include the Mediterranean diet, the 
Dietary Approaches to Stop Hypertension (DASH), and plant-
based diets.115,126–128 A Mediterranean diet emphasizing use of 
monounsaturated 

erranean diet emphasizing use of 
monounsaturated and polyunsaturated fats may have the best 
experiment support for glycemic control, cardiovascular protec­
tion, and perhaps other outcomes.127 
Physical Activity and Exercise
There is a substantial body of literature supporting exercise as 
a modality of treatment in T2DM. Exercise is associated with 
improved glycemic control, insulin sensitivity, cardiovascular fit­
ness, and quality of life.83,129,132 Aerobic exercise and resistance 
(streng

3,129,132 Aerobic exercise and resistance 
(strength) training both have positive impact on glucose control.130 
Self-directed structured exercise programs have been associated 
with mean HbA1c reductions of 0.4% to 0.9%, with the larg­
est reduction seen in those using combined aerobic and resis­
tance exercise.131,133 Supervised exercise is also effective but too 
expensive for many and less practical. In many cases the benefits 
in HbA1c are not accompanied by weight loss. With this inde­
pen

ot accompanied by weight loss. With this inde­
pendent benefit it is clear why the physician can refer to exercise 
as a therapeutic modality equivalent to pharmacologic therapies. 
Improvements in glycemic control are usually apparent immedi­
ately, but the improvement in insulin resistance may not last more 
than 48 to 72 hours. It is often recommended that individuals 
strive for 150 minutes of moderate-intensity physical activity (50–
70% of maximum heart rate) or 75 minutes per week of vigo

maximum heart rate) or 75 minutes per week of vigorous 
exercise (>70% of maximum heart rate) on at least 3 days per week 
with no more than 2 consecutive days without exercise. Both aero­
bic and resistance exercise are effective and recommended.83,132,133
The key concept is to promote an increase in activity using an 
approach similar to the one discussed for diet. Goals, methods, 
intensity, and frequency must be negotiated with patients with 
great sensitivity to recognizing barriers and hel

great sensitivity to recognizing barriers and helping patients dis­
cover solutions. The role of educators, exercise specialists, physical 
therapists, and social supports in this process is critical. The major 
role for the physician is to screen for complications (neuropathy, 
nephropathy, retinopathy, vascular disease) and discover ways for 
patients to be able to exercise safely.132 It is also important for the 
physician to inquire about exercise patterns and potential barri­
ers and consi

ercise patterns and potential barri­
ers and consider implications of exercise on hypoglycemia at each 
clinic visit.
Although it has not been recommended to have all patients 
formally evaluated for ischemic heart disease with a stress test, 
conducting a careful history of potential signs and a clinical risk 
assessment is desirable to identify patients who may warrant addi­
tional testing or cautions. Patients at high risk for coronary artery 
disease should start with short periods of low-in

disease should start with short periods of low-intensity exercise 
and increase the intensity and duration slowly as tolerated.132 
Development of symptoms of coronary ischemia, including 
dyspnea out of proportion with activity, calls for further evalua­
tion and treatment. It is important to encourage patients not to 
overexert and to recognize exertional chest, jaw, or arm discom­
fort as well as palpitations and dyspnea as symptoms of cardiac 
For the average patient with T2DM 

c 
For the average patient with T2DM starting an exercise pro­
gram it is best to start with low-level activity such as walking at a 
pace of 2 miles per hour. Initially it may be advantageous to start 
with shorter duration exercise sessions and build up the frequency 
and duration. Caution is important to avoid overuse injuries that 
then interrupt the exercise progression. Many different exercise 
programs are beneficial, and individualization is appropriate. For 
example, interv

vidualization is appropriate. For 
example, interval training appears to be effective but is not appro­
priate for all.134 Aquatic exercise is beneficial and may be particu­
larly useful for those with joint pain.135 Walking after dinner can 
have a significant benefit on postprandial hyperglycemia.136 Con­
sideration of diabetes complications is imperative. For example, 
in the setting of proliferative diabetic retinopathy intense aerobic 
and resistance exercise should be avoided. With signifi

esistance exercise should be avoided. With significant periph­
eral neuropathy, careful attention must be given to footwear, and 
weightbearing exercise should be limited. Daily foot inspection 
for foot ulcers is critical. Autonomic neuropathy requires particu­
lar cardiac cautions. Exercise does not appear to accelerate kid­
ney disease but will increase albuminuria acutely and could cause 
false-positive albumin:creatinine ratios temporarily.137
In addition to scheduled exercise, all individu

37
In addition to scheduled exercise, all individuals should be 
encouraged to reduce sedentary time, particularly by breaking up 
extended amounts of time spent sitting.137,138 It is recommended 
to have some activity break at least every 30 minutes to include 
brief walks, resistance exercise, or at least standing. This may also 
help prevent diabetes. Balance training and exercise improving 
flexibility may be helpful as patients get older, with the aim of 
decreasing falls and increasing mob

th the aim of 
decreasing falls and increasing mobility.139 
Addressing Psychosocial Needs
Many psychosocial factors influence the ability of a patient with 
T2DM to practice effective self-management.92 There is a com­
plex interaction of factors such as economic stability, food insecu­
rity, family support or conflict, intellectual abilities and learning 
skills, and the incessant need to consider the implications of tim­
ing and content of food intake, emotional stress, physical activity, 
an

d intake, emotional stress, physical activity, 
and adherence to medications each day. Not surprisingly, patients 
often experience diabetes distress and depression, two separate but 
related diagnoses.140 The incidence of diabetes distress over an 
18-month period has been reported to range from 38% to 48%.141 
Diabetes distress screening tools have been developed and utilized 
in revealing studies where the level of distress impacts glycemic 
control, adherence to medication regimens, exercise

ontrol, adherence to medication regimens, exercise behaviors, 
self-efficacy, and diet. There are also depression screening tools for 
use in clinical practice. When clinical depression is diagnosed, the 
patient should be referred to a mental health provider, preferably 
one with an understanding of diabetes and its treatment.92 It is 
recommended that patients with diabetes be screened at least once 
yearly as depression is present in about 25% of screened patients, 
and appropriate treatment 

 of screened patients, 
and appropriate treatment can greatly facilitate diabetes self-man­
agement and quality of life. Another psychologic comorbidity to 
be considered in T2DM is chronic anxiety, which often appears 
as fear of hyperglycemia, hypoglycemia, chronic complications, 
or injections. On occasion, management may be influenced by 
obsessive compulsive disorder resulting in inadequate nutrition or 
overtreatment of hypoglycemia. A few patients on intensive insu­
lin may experience a p

patients on intensive insu­
lin may experience a posttraumatic stress disorder or panic disor­
der related to a severe episode of hypoglycemia.
The psychosocial aspects of diabetes care are relevant for the 
entire treatment team, emphasizing the importance of patient-
centered care and a collaborative approach.92 Therefore, in addi­
tion to a psychiatric history, the office visit should include basic 
1385
questions on stress levels, family s

etes Mellitus
questions on stress levels, family support structures, general mood 
and energy, and perceived barriers to self-management success. 
Recognizing that diabetes distress is so prevalent, the practice can 
facilitate stress reduction through one-on-one discussion and by 
providing resource lists or directly providing clinic-based support 
groups, exercise groups with support elements, and mindfulness 
training. When screening suggests a psychologic barrier beyond 
the capacity of the 

a psychologic barrier beyond 
the capacity of the clinical practice, referral to a mental health 
specialist should be proposed to the patient in a sensitive, profes­
sional manner. 
Glucose-Lowering Pharmacotherapy
A revolution in the treatment of T2DM since 1995 in the United 
States has been driven by the release of several new classes of drugs 
that independently address different pathophysiologic mecha­
nisms that contribute to the development of diabetes. Antihy­
perglycemic agents have co

nt of diabetes. Antihy­
perglycemic agents have commonly been divided into those that 
improve the sensitivity of tissues to insulin, those that augment 
insulin availability, and those with other mechanisms of action. 
This classification has become less helpful as additional classes of 
agents have been introduced, some of them with multiple phar­
macologic and clinical effects. Here we will present each class of 
oral therapy separately, and then describe the currently available 
agents that 

hen describe the currently available 
agents that require injection or other means of delivery. Some 
important properties of various classes of drugs available for the 
management of T2DM are summarized in Table 35.10. This area 
has been the subject of extensive reviews.33,49–51,78,118,125,142–148 In 
the following discussion, the principles outlined in these reviews 
are summarized, and a limited number of additional references are 
provided. In addition, when questions arise regarding the de

In addition, when questions arise regarding the details 
of handling, dosing, and administering any of the agents available, 
it is advisable to consult the product information approved by 
regulatory authorities.
Oral Agents
Some clinically relevant features of the main currently available 
oral antihyperglycemic agents are summarized in Table 35.11.
Biguanides
Metformin is the only biguanide available in the United States. 
Phenformin was removed from the market in the 1970s because of 
deaths

ed from the market in the 1970s because of 
deaths associated with lactic acidosis. Phenformin and buformin 
remain available in some countries, however. The precise mecha­
nism of action of metformin is still not well understood.149,150 Some 
studies suggest it activates adenosine monophosphate-activated 
protein kinase (AMPK), an intracellular signal of depleted cellular 
energy stores that has been implicated in stimulation of skeletal 
muscle glucose uptake and inhibition of hepatic gluconeo

 glucose uptake and inhibition of hepatic gluconeogenesis, 
whereas more recent studies implicate inhibition of mitochon­
drial glycerophosphate dehydrogenase. Additional new evidence 
suggests much of metformin’s effect is mediated by direct contact 
with the intestinal mucosa, provoking neural or hormonal signals 
to the brain or other tissues.151 There is some evidence that it can 
influence the microbiome, and it increases glucagon-like peptide-1 
(GLP1) release. It has been suggested the ma

de-1 
(GLP1) release. It has been suggested the main clinical effect of 
metformin is to reduce hepatic gluconeogenesis and glucose pro­
duction, but this does not account for all of its effects.150 It less 
consistently improves insulin sensitivity in peripheral tissues. 
Because of its limited duration of action, it is usually taken at least 
twice daily, although a sustained-release formulation is available.
  Classes of Antihyperglycemic Agents for Type 2 Diabetes
Classes of Agents
Route of

nts for Type 2 Diabetes
Route of 
Delivery
Mechanism of Effect on Glucose
Basal 
Glucose 
Control
Prandial 
Glucose 
Control
Weight 
Control
BP 
Control
Short-Term CV 
Risk Reduction
Biguanide
Oral
Decrease hepatic glucose production
+++
+
++
+
+
Secretagogue
Oral
Increase insulin secretion
+++
++
Weight 
increase
↔
↔
Thiazolidinedione
Oral
Decrease insulin resistance
+++
++
Weight 
increase
+
?
DPP4 inhibitor
Oral
Increase insulin, decrease glucagon
++
+
↔
↔
↔
α-Glucosidase in

lin, decrease glucagon
++
+
↔
↔
↔
α-Glucosidase inhibitor
Oral
Delay carbohydrate absorption
+
+++
++
↔
+
SGLT inhibitor
Oral
Increase renal clearance of glucose, 
sodium
++
+
+++
++
+++
Bile-acid sequestrant
Oral
Delay carbohydrate absorption?
+
+
↔
↔
?
Oral
+
+
+
+
?
Insulin
Injection
Increase insulin availability
+++
+++
Weight 
increase
↔
↔
GLP1 receptor agonist
Injection
Increase insulin, decrease glucagon, 
slow gastric emptying
+++
+++
+++
++
+

 glucagon, 
+++
+++
+++
++
++
Amylin receptor agonist
Injection
Decrease glucagon, slow gastric 
emptying
+
+++
+++
↔
?
CV, Cardiovascular; DPP4, dipeptidyl peptidase-4; GLP1, glucagon-like peptide-1; SGLT, sodium-glucose cotransporter.
   
TABLE 35.10
1386
Because biguanides do not increase insulin levels, they are not 
associated with a significant risk of hypoglycemia. The most com­
mon adverse events are gastr

ycemia. The most com­
mon adverse events are gastrointestinal: nausea, abdominal pain or 
bloating, and diarrhea. Up to a third of patients have some gastro­
intestinal distress, especially early in their course of treatment. This 
can be minimized by starting with a low dose once daily with meals 
and titrating upward over several weeks to fully effective doses. 
Sustained-release metformin is associated with less frequent and 
less severe upper gastrointestinal symptoms, but it can increase th

 gastrointestinal symptoms, but it can increase the 
frequency of diarrhea, which is overall less common. Most patients 
tolerate metformin adequately during long-term use. Perhaps 
related to clinical or subclinical gastrointestinal effects, metformin 
is associated with less weight gain than other antihyperglycemic 
agents and can be associated with a modest weight loss.
Metformin has been said to cause lactic acidosis, which is quite 
rare and occurs almost exclusively in patients who are at 

 occurs almost exclusively in patients who are at high risk 
for development of the condition independent of metformin ther­
apy.152 Metformin is not metabolized and is cleared only through 
the kidney. Prior to April 2016 the package insert stated that it was 
contraindicated in patients with renal insufficiency in an effort 
to avoid lactic acidosis. The revised package insert recommends 
obtaining an estimated glomerular filtration rate (eGFR) before 
  Clinical Features of Commonly Used Ora

 before 
  Clinical Features of Commonly Used Oral Antihyperglycemic Agents
Classes and Specific Agents 
(Commercial Names)
Commonly Used 
Dosages
Contraindications
%HbA1c Reduction as 
First or Second Therapy
Biguanide
Metformin (Glucophage)
Metformin-ER
500–1000 mg BID
T1D, DKA 
eGFR <30
Severe cardiac, hepatic disease
Nausea, diarrhea, 
abdominal pain
Vitamin B12 deficiency
1 to 2
Glipizide (Glucotrol)
Glipizide-ER
Gliclazide (Diamicron)
Gliclazide-M

)
Gliclazide-MR
Glimepiride (Amaryl)
Glyburide (Micronase et al)
Repaglinide (Prandin)
Nateglinide (Starlix)
5–20 mg BID
2.5–10 mg QD
80–160 BID
30–120 QD
0.5–4 mg QD
2.5–10 mg BID
0.5–2 mg TID
60–120 mg TID
T1D, DKA
Weight gain
1 to 2
Pioglitazone (Actos)
Rosiglitazone (Avandia)
15–30 mg QD
4–8 mg QD
T1D, DKA
Symptomatic heart failure
Edema
Fractures
0.75 to 1.5
DPP4 inhibitora
Sitagliptin (Januvia)
Vildagliptin (Gal

inhibitora
Vildagliptin (Galvus)
Saxagliptin (Onglyza)
Linagliptin (Trajenta)
Alogliptin (Nesina)
25–100 mg QD
50 mg QD or BID
2.5–5 mg QD
5 mg QD
6.25–25 mg QD
T1D, DKA
Hypersensitivity
0.5 to 1
α-Glucosidase inhibitorb
Acarbose (Precose)
Miglitol (Glyset)
25–50 mg TID
T1D, DKA
Flatulence, 
diarrhea,
abdominal discomfort
0.5 to 1
Canagliflozin (Invokana)
Dapagliflozin (Farxiga)
Empagliflozin (Jardiance)
Ertugliflozin (Steglatro)
100–300 mg QD
5–

diance)
5–10 mg QD
10–25 mg QD
5–15 mg QD
TID, DKA 
eGFR <30
Urinary frequency
Urogenital infections
0.5 to 1
Colesevelam (Welchol)
Six 625-mg tabs QD
T1D, DKA
Pancreatitis, 
intestinal disease, 
­hypertriglyceridemia
0.5 to 1
Bromocriptine (Cycloset)
1.6–4.8 mg QD
T1D, DKA
Somnolence, dizziness, 
hypotension
0.5 to 1
aThe DPP4 inhibitors listed here are approved in the United

 inhibitors listed here are approved in the United States and/or the European Union. Other DPP4 inhibitors are available in certain countries, including anagliptin (Suiny), evogliptin (Suganon), 
gemigliptin (Zemiglo), gosogliptin (SatRx), omarigliptin (Marizef), teneligliptin (Tenelia), trelagliptin (Zafatek), and vildagliptin (Galvus).
bAcarbose and miglitol are available in the United States and the European Union. Voglibose (Basen et al) is available in other countries.
BID, Twice daily; DKA

vailable in other countries.
BID, Twice daily; DKA, diabetic ketoacidosis; eGFR, estimated glomerular filtration rate; ER, extended release; MR, modified release; QD, once daily; TID, three times daily.
   
TABLE 35.11
1387
starting metformin and at least annually thereafter. Initiation is 
not recommended if the eGFR is below 45 mL/minute/1.73 m2 
and is contraindicated if the eGFR is below 30 mL/minute/1.73 
m2. Once treatment is established

 mL/minute/1.73 
m2. Once treatment is established, one should consider a dose 
reduction if the eGFR is under 45 mL per minute, and metformin 
should definitely be stopped if the eGFR is less than 30 mL per 
minute.153 Metformin remains contraindicated in patients with 
hepatic insufficiency and in the setting of alcohol abuse. Because 
some patients taking metformin develop significant vitamin B12 
deficiency that might exacerbate peripheral neuropathy, supple­
mentation with vitamin B12 (e.g.

uropathy, supple­
mentation with vitamin B12 (e.g., 1000 μg daily) or intermittent 
monitoring may sometimes be prudent.154–156
Both the glucose-lowering efficacy and the frequency of gas­
trointestinal side effects of metformin increase in the dose range 
from 500 to 2000 mg per day. The maximal daily dose of 2550 
mg does not generally provide additional benefit beyond that seen 
at 2000 mg daily.157 To minimize gastrointestinal side effects, dos­
age should usually be started with 500 mg once

os­
age should usually be started with 500 mg once or twice daily. 
Newer formulations of metformin combined with other classes of 
oral antihyperglycemic agents have been developed to maximize 
glucose-lowering effectiveness with a single prescription through 
the synergy of two classes of agents with different actions.
Arguably, metformin has the best record among oral antihyper­
glycemic agents in medical outcome studies. As previously noted 
(see Table 35.4) overweight participants in the UK

(see Table 35.4) overweight participants in the UKPDS who were 
randomly assigned to metformin had reduced rates of myocardial 
infarction and all-cause mortality that persisted long after cessa­
tion of randomized therapy.41 While the number of participants 
using metformin in the UKPDS was relatively small, other stud­
ies have supported its protective effects.158 A beneficial effect of 
metformin on cardiovascular complications through mechanisms 
independent of glycemic control is certainly 

sms 
independent of glycemic control is certainly plausible and is con­
sistent with metformin-associated modest reductions in LDL, tri­
glycerides, blood pressure, and procoagulant factors.
Based on its good tolerability, safety, and effectiveness together 
with evidence for medical benefits in controlled trials, it is gen­
erally recommended that metformin therapy be started in all 
patients with T2DM at or near the time of diagnosis of diabetes, 
provided no contraindications are present.33,3

s, 
provided no contraindications are present.33,38,159 
Insulin Secretagogues
Along with biguanides, this class of oral agents has a long history 
of clinical use, and mechanisms have been carefully studied. Some 
characteristics of the most widely used insulin secretagogues are 
summarized in Table 35.11. Currently available secretagogues all 
bind to the sulfonylurea receptor-1 (SUR1), a subunit of the KATP 
potassium channel on the plasma membrane of pancreatic beta 
cells. The SUR1 subunit 

brane of pancreatic beta 
cells. The SUR1 subunit regulates the activity of the channel and 
also binds adenosine triphosphate (ATP) and adenosine diphos­
phate (ADP), effectively functioning as a glucose sensor and trig­
ger for insulin secretion. Binding leads to closing of the channel, 
as do increases in intracellular ATP and decreases in ADP resulting 
from fuel metabolism. The membrane depolarization that ensues 
causes the opening of voltage-dependent L-type calcium channels. 
Subsequent 

ge-dependent L-type calcium channels. 
Subsequent calcium influx results in an increase in intracellular 
calcium, which leads to insulin secretion. A direct effect on insulin 
secretion, independent of ambient glucose levels, is seen with the 
first dose of any secretagogue. However, this direct effect wanes 
during continued occupancy of SUR1, while potentiation of the 
effect of current glucose levels continues.
Differences in pharmacokinetic and binding properties of the 
various agents cont

and binding properties of the 
various agents contribute to their differing clinical effects, as do 
differences in their formulations and mechanisms of clearance 
from circulation. These properties are apparent clinically as differ­
ences in duration of action and likelihood of causing hypoglyce­
mia. The best uses and also the relative risks versus benefits of the 
various secretagogues differ considerably. 
The sulfonylureas have been available since the 1950s. They are reli­
ab

 been available since the 1950s. They are reli­
ably effective in lowering glucose levels early in the natural history 
of T2DM when considerable beta-cell function is present.160,161 
Glimepiride and extended-release glipizide and gliclazide are gener­
ally the most widely used. These currently preferred sulfonylureas 
share several pharmacologic features. Each has a long duration of 
action after a single dose, allowing once-daily administration for 
most patients. Due to these longer profiles

n for 
most patients. Due to these longer profiles, continued SUR1 occu­
pancy limits glucose-independent insulin release accompanying each 
subsequent dose, while continuing to potentiate glucose-dependent 
basal insulin secretion. The clinical effect of these agents is to lower 
fasting glucose levels with little effect on postprandial glucose lev­
els, and relatively low risk of hypoglycemia compared with other 
secretagogues. Because none of these agents is strongly dependent 
on renal excre

these agents is strongly dependent 
on renal excretion, declining renal function has less direct effect on 
risk of hypoglycemia during their use. In contrast, chlorpropamide 
is a long-acting sulfonylurea that is cleared only by the kidney. Thus 
it can cause severe hypoglycemia when renal function becomes 
impaired, and largely for this reason it is now rarely used.
The shorter acting sulfonylureas include standard formula­
tions of glipizide and glyburide. Taken twice daily they can be 
as ef

nd glyburide. Taken twice daily they can be 
as effective as the longer-acting agents in overall glucose control. 
However, they have a greater effect on postprandial hyperglycemia 
and can cause daytime hypoglycemia when food intake is reduced. 
Glyburide has an active metabolite that can be cleared only by the 
kidney, and for this and other reasons it is associated with greater 
risk of hypoglycemia than other currently used sulfonylureas.162
There has been long-standing concern that sulfonyl

There has been long-standing concern that sulfonylureas might 
cause increased arrhythmic cardiovascular events in patients with 
diabetes as a result of an effect on SUR2 subunits of the KATP 
complex in vascular and cardiac cells. Binding to SUR2 can blunt 
ischemic preconditioning, a normal cardioprotective mecha­
nism. Epidemiologic studies assessing whether this effect confers 
increased cardiovascular risk in clinical practice differ in their con­
clusions, but more recent ones, including 

ir con­
clusions, but more recent ones, including the newer sulfonylureas 
and accounting for confounding factors, are generally unable to 
confirm that risks of sulfonylureas outweigh potential benefits.162 
The ADVANCE trial, which used extended-release gliclazide as its 
dominant glucose-lowering strategy, did not show any evidence of 
cardiovascular toxicity.44 The UKPDS similarly demonstrated long-
term safety relative to lifestyle intervention for glyburide, glipizide, 
and chlorpropamide,

ion for glyburide, glipizide, 
and chlorpropamide, which were all used in its main (sulfonylurea 
or insulin) treatment arm.28 However, the individual sulfonylureas 
differ in pharmacologic studies of ischemic preconditioning, with 
this adverse effect confirmed for tolbutamide and glyburide and 
less apparent with glimepiride, glipizide, and gliclazide. Concern 
about cardiovascular risk is especially relevant to glyburide, an 
older sulfonylurea that continues to be prescribed despite its effe

a that continues to be prescribed despite its effects 
on ischemic preconditioning and tendency to cause hypoglycemia.
The maximum approved dose for the sulfonylureas is two to four 
times higher than the maximum effective dose. Initiating treatment 
with no more than a quarter of the maximal approved dose pro­
vides significant glucose lowering while limiting costs and adverse 
events. Small doses of the longer-acting modern sulfonylureas (e.g., 
0.5–1 mg of glimepiride or 2.5 mg of extended-re

0.5–1 mg of glimepiride or 2.5 mg of extended-release glipizide) 
are often effective, particularly in patients receiving metformin 
concomitantly, and are almost always well tolerated. The leading 
1388
disadvantages of sulfonylureas are their tendency to cause hypo­
glycemia and usually modest weight gain. These unwanted effects 
may contribute to their more rapid loss of effectiveness than other 
agents, another observation best d

ess than other 
agents, another observation best documented with glyburide.163 
Glinides
Repaglinide is a member of the meglitinide family of insulin secre­
tagogues, distinct from the sulfonylureas. It has a short half-life 
and a distinct SUR1 binding site. As a result of rapid absorption, 
it produces a faster and briefer stimulus to insulin secretion than 
the shorter acting sulfonylureas. It is typically taken with each 
meal and provides better postprandial control and generally less 
hypo

tter postprandial control and generally less 
hypoglycemia and weight gain than glyburide. Repaglinide has 
a long residence time on the SUR1 and therefore some effect on 
fasting glucose, even though its pharmacologic half-life is quite 
short. Repaglinide is available in 0.5-mg, 1-mg, and 2-mg tablets. 
The maximum dose is 4 mg with each meal. As with the sulfonyl­
ureas, there is only a modest glucose-lowering advantage of high 
compared with moderate doses of repaglinide.
Nateglinide is a de

moderate doses of repaglinide.
Nateglinide is a derivative of phenylalanine and is structur­
ally distinct from both sulfonylureas and the meglitinides. It has 
a quicker onset and a shorter duration of action than repaglinide. 
Its interaction with SUR1 is fleeting. As a result, its effect in low­
ering postprandial glucose is quite specific, and it has little effect 
on fasting glucose. This provides both advantages (less overnight 
hypoglycemia) and disadvantages (less overall glucose-lowerin

a) and disadvantages (less overall glucose-lowering 
effectiveness). Nateglinide is most appropriately used when glu­
cose levels are modestly elevated in early diabetes or in combina­
tion with agents that control overnight glucose levels. It is available 
as 120-mg tablets and is taken with each meal. A 60-mg tablet is 
available but is not generally used except in patients with minimal 
hyperglycemia.
The rationale for using repaglinide or nateglinide to stimulate 
insulin secretion at mealti

eglinide to stimulate 
insulin secretion at mealtime to improve postprandial control 
without increasing the risk of overnight hypoglycemia is attrac­
tive. Furthermore, these newer agents demonstrate little binding 
to the vascular smooth muscle and cardiac SUR2 receptors. How­
ever, use of them has been modest, probably because of the need 
for multiple daily doses, higher cost than sulfonylureas, and lack 
of head-to-head comparative studies that demonstrate superiority 
over the newer sulfon

hat demonstrate superiority 
over the newer sulfonylureas. 
The thiazolidinedione class of drugs (TZDs, or glitazones) has 
engendered great interest and controversy since the first agent, 
troglitazone, was approved in 1997. Rare fatal hepatotoxicity 
was associated with troglitazone, and it was withdrawn from the 
US market in 2000. The currently available TZDs (pioglitazone 
and rosiglitazone) are considered safer. These agents bind to and 
modulate of the activity of a fam

nts bind to and 
modulate of the activity of a family of nuclear transcription factors 
termed peroxisome proliferator-activated receptors (PPARs), thereby 
initiating many downstream effects. They are associated with slow 
improvement in glycemic control over weeks to months in parallel 
with an improvement in insulin sensitivity and a reduction in free 
fatty acid levels.
Pioglitazone and rosiglitazone are equally effective in improv­
ing glycemic control and provide equivalent improvements in

mic control and provide equivalent improvements in 
markers of insulin resistance and inflammation. No substantial 
evidence has linked the newer TZDs to hepatotoxicity, and a 
record of safety has been established in appropriate patients. The 
package insert nevertheless calls for liver function tests before 
beginning TZD therapy, and these agents are contraindicated in 
patients with active hepatocellular disease or unexplained serum 
alanine aminotransferase (ALT) levels greater than 2.5 tim

aminotransferase (ALT) levels greater than 2.5 times the 
upper limit of normal. Nonetheless, recent studies show a favor­
able effect of pioglitazone on nonalcoholic steatohepatitis, a com­
mon and otherwise difficult to treat condition.
Pioglitazone and rosiglitazone differ with respect to lipid effects. 
In a head-to-head study among dyslipidemic patients, piogli­
tazone reduced triglycerides by approximately 20%, whereas rosi­
glitazone increased triglycerides on average by 5%. Pioglitazone 

ased triglycerides on average by 5%. Pioglitazone is 
associated with a modestly greater improvement in high-density 
lipoprotein (HDL) particle number and size and an improvement 
in low-density lipoprotein (LDL) particle size and number. Rosi­
glitazone was associated with an increase in LDL particle number 
and improved LDL particle size.164
Early epidemiologic evidence suggested that the ability of TZDs 
to improve insulin-sensitivity might lead to cardiovascular protec­
tive effects. This h

ead to cardiovascular protec­
tive effects. This hypothesis was supported by a series of physi­
ologic associations with TZD use: reduced carotid intimal/medial 
thickness, improvement of vascular endothelial function, improve­
ments in dyslipidemia, lower blood pressure, and improved fibri­
nolytic and coagulation parameters. The hypothesis was tested in 
the PROactive (PROspective pioglitAzone Clinical Trial In mac­
roVascular Events) study, a randomized, double-blind, placebo-
controlled tria

randomized, double-blind, placebo-
controlled trial in 5238 patients with T2DM and documented 
macrovascular disease. Subjects were randomized to placebo or to 
45 mg per day of pioglitazone and otherwise treated according to 
guidelines for hyperglycemia and major cardiovascular risk fac­
tors. The primary endpoint was the time from randomization to 
a broad set of macrovascular outcomes. Although this endpoint 
showed no significant improvement with pioglitazone, there was a 
marginally signif

 with pioglitazone, there was a 
marginally significant 16% reduction in a secondary, more restric­
tive cardiovascular composite endpoint. After extensive discussion 
of this controversial trial, it appears that pioglitazone therapy may 
be associated with some reduction of cardiovascular risk related to 
improvements in control of glucose, lipids, and blood pressure, 
but that these benefits are attenuated by an increased incidence of 
weight gain and heart failure.165 Notably, in a recent pla

ain and heart failure.165 Notably, in a recent placebo con­
trolled study in nondiabetic patients with a recent ischemic stroke 
or transient ischemic attack, pioglitazone reduced the subsequent 
occurrence of stroke or myocardial infarction.166
The Rosiglitazone Evaluated for Cardiovascular Outcomes 
in Oral Agent Combination Therapy for Type 2 Diabetes 
(RECORD) trial was an open-label study that compared the effect 
of adding rosiglitazone versus either metformin or sulfonylurea 
to patients 

sus either metformin or sulfonylurea 
to patients who had T2DM inadequately controlled with sulfo­
nylurea or metformin. There was no difference in cardiovascular 
hospitalizations or death.167 Subsequently, a controversy based on 
epidemiologic analyses developed as to whether rosiglitazone was 
associated with increased risk of myocardial infarction.168 This 
dispute remains unresolved, but it caused a shift away from use 
of rosiglitazone.
Another potentially important attribute of the TZDs i

ther potentially important attribute of the TZDs is an 
improvement in insulin secretory dynamics. Several trials have 
demonstrated that TZDs can delay the progression of dysglycemia 
to diabetes.33,142 In addition, ADOPT (A Diabetes Outcome Pro­
gression Trial) found that patients with early diabetes had a slower 
rate of secondary glycemic failure when they were treated with 
rosiglitazone compared with metformin and glyburide. These out­
comes were correlated with improved insulin secretion,

s were correlated with improved insulin secretion, although 
it remains unclear whether the improvement resulted mainly from 
direct effects on the beta cell as opposed to persisting reduction of 
insulin resistance.163
1389
Despite these desirable effects, several adverse effects of the 
TZDs have raised concerns. These include weight gain, fluid reten­
tion, increased risk of bone fractures, and anemia. The weight gain 
has been shown to res

and anemia. The weight gain 
has been shown to result both from extracellular fluid accumula­
tion and an increase of subcutaneous but not visceral fat.165 There 
are, in fact, desirable reductions in visceral, hepatic, and intramyo­
cellular fat. The weight gain resulting from TZDs may not have 
the negative metabolic consequences that are generally ascribed 
to overweight and obesity, but it is nevertheless viewed negatively 
by most patients and practitioners. All patients prescribed TZDs 
sh

nd practitioners. All patients prescribed TZDs 
should be counseled to enhance lifestyle efforts to limit weight 
gain.
With appropriate caution, relatively few patients will need to 
withdraw use of a TZD because of fluid retention. The patients 
most likely to experience edema are those using insulin and those 
with preexisting edema.169,170 It is prudent to teach patients 
with preexisting edema how to assess pitting pretibial edema at 
home and to suggest they make a habit of checking nightl

nd to suggest they make a habit of checking nightly. If 
weight accompanied by increasing edema is noted, patients can 
be instructed to restrict sodium intake, to start a diuretic, or to 
increase their diuretic dosage as needed.
It is usually prudent to begin therapy with the lowest available 
dose, which in the case of pioglitazone is 15 mg daily. If after 3 
months the glycemic response has been inadequate and signifi­
cant edema has not developed, an increase to 30 mg may be con­
sidered. T

loped, an increase to 30 mg may be con­
sidered. The highest approved dose, 45 mg, is less well tolerated. 
Many patients with mild edema respond to a thiazide diuretic or 
spironolactone. and combination therapy with a moderate-dose 
loop diuretic may be considered.171 Fluid retention to the point 
of clinically apparent heart failure can occur, in which case the 
TZD should be discontinued. In the PROactive and RECORD 
studies, an excess of approximately 2% of patients treated with 
high-dose 

roximately 2% of patients treated with 
high-dose TZD required hospitalization for heart failure. There 
have been conflicting reports of worsened macular edema with 
thiazolidinediones.172,173
A more recent safety concern regarding TZDs is bone health. 
An excess of fractures has been reported during TZD use in both 
pharmacoepidemiologic studies and randomized trials, mainly in 
older women.174,175 Whereas distal sites were primarily affected in 
these studies, loss of bone density in the lumb

n 
these studies, loss of bone density in the lumbar spine may occur 
as well. Preclinical studies suggest that activation of PPARγ inhib­
its bone formation by diverting stem cells from the osteogenic to 
the adipocytic lineage. To minimize this risk, an assessment of risk 
factors and measurement of bone density should be considered 
before prescribing a TZD, especially for women.176
Finally, pioglitazone has been suspected of causing bladder can­
cer as a result of inconsistent preclinical an

an­
cer as a result of inconsistent preclinical and clinical observations. 
Large, long-term studies have not confirmed this association.177 If 
pioglitazone is associated with bladder cancer, the absolute risk to 
an individual is low. However, current recommendations to avoid 
its use in patients with a history of bladder cancer seem prudent. 
DPP4 Inhibitors
Oral glucose has a greater stimulatory effect on insulin secretion 
than does intravenous glucose at the same circulating glucose con­
c

ous glucose at the same circulating glucose con­
centration. In humans this effect is mediated mainly by gluca­
gon-like peptide-1, which is secreted in response to nutrients by 
intestinal L cells. Its binding to GLP1 receptors stimulates insulin 
secretion in a glucose-dependent fashion, inhibits inappropriate 
glucagon secretion, slows gastric emptying, reduces appetite, and 
improves satiety. Secreted GLP1 is very rapidly (1–2 minutes) 
inactivated in plasma by the enzyme dipeptidyl peptidas

ivated in plasma by the enzyme dipeptidyl peptidase-4. A 
variety of pharmacologic techniques have been developed to har­
ness the potential of GLP1 signaling to treat diabetes.178,179 Sev­
eral injectable drugs that are agonists for GLP1 receptors will be 
discussed later. Another approach is inhibition of DPP4, resulting 
in increased blood levels of endogenously produced GLP1 and 
other circulating peptides.
Five DPP4 inhibitors—sitagliptin, vildagliptin, saxagliptin, 
linagliptin, and alogli

ildagliptin, saxagliptin, 
linagliptin, and alogliptin—are approved for clinical use in the 
United States and/or the European Union, and others are available 
in other countries. Some of their clinical characteristics are shown 
in Table 35.11. These agents increase fasting and postprandial lev­
els of GLP1 about twofold. This elevation of GLP1 potentiates 
glucose-dependent insulin secretion and suppresses basal and post­
prandial glucagon. However, neither satiety nor the rate of gastric 
emp

ever, neither satiety nor the rate of gastric 
emptying is affected. The main clinical effect of DPP4 inhibitors 
is on fasting glucose levels, leading to HbA1c reductions of 0.5% 
to 1%. They have no consistent effect on weight and no tendency 
to cause hypoglycemia when used alone or with metformin.180
All agents in this class are remarkably well tolerated, with an 
adverse effect profile similar to that of placebo. Postmarketing 
cases of pancreatitis have been reported, and DPP4 inhibitors a

reatitis have been reported, and DPP4 inhibitors are 
contraindicated for use by patients with a prior history. Speci­
ficity for DPP4 appears to be crucial, as inhibitors that are less 
specific for this enzyme been shown to have effects on immune 
function and cancer growth in animal studies. Further, the bio­
logic effects of all the DPP4 inhibitors are complex, owing to their 
potential effects on clearance of other peptides that are substrates 
for this enzyme.181 In cardiovascular outcome 

es 
for this enzyme.181 In cardiovascular outcome trials, there was an 
increased risk for heart failure hospitalization with saxagliptin and 
a numerical imbalance for alogliptin,182–184 but no increased risk 
was demonstrated for sitagliptin.185 It is not clear whether these 
findings represent an important clinical distinction, the play of 
chance, or differences in trial populations or conduct. Serious 
hypersensitivity reactions have been reported, but causality has 
not been substantiated 

ported, but causality has 
not been substantiated because of the rarity of events.
Linagliptin is available in only one tablet size. For the other 
DPP4 inhibitors in current use, which are partially cleared by the 
kidney, the usual dose is the maximum marketed dose, and smaller 
doses are recommended in the setting of stage 3 or greater chronic 
kidney disease. The weight neutrality, lack of hypoglycemia, broad 
applicability, tolerability, and ease of use of DPP4 inhibitors are 
appealing pro

 ease of use of DPP4 inhibitors are 
appealing properties. Consequently, they have been widely used 
despite their relatively weak glucose-lowering power that is partly 
dependent on retained beta-cell function. 
α-Glucosidase Inhibitors
α-Glucosidase inhibitors (AGIs) slow the terminal step of carbo­
hydrate digestion at the brush border of the intestinal epithelium. 
As a result, carbohydrate absorption is shifted more distally in the 
intestine and is delayed, allowing the sluggish insulin se

e and is delayed, allowing the sluggish insulin secretory 
dynamics characteristic of T2DM to catch up with carbohydrate 
The two agents currently available in the United States are acar­
bose and miglitol. Voglibose is available in other countries. Use of 
AGIs in the United States has been limited by a number of factors, 
including the need to administer the medication at the beginning 
of each meal, flatulence as a common side effect, and only modest 
reductions in blood glucose l

ct, and only modest 
reductions in blood glucose levels. These factors should be bal­
anced against the ability of AGIs to lower postprandial glucose 
without increasing weight or hypoglycemic risk.186 There is also 
some evidence suggesting that acarbose improves cardiovascular 
outcomes better than most antihyperglycemic agents.187
1390
To maximize the potential for these agents to be well tolerated, 
the starting dose should be low 

 well tolerated, 
the starting dose should be low (e.g., one-fourth of the maximum 
dose) and taken just once daily, with slow increases in dosage and 
frequency of administration over a period of weeks or months. 
Sodium-Glucose Transporter Inhibitors
This is the newest class of glucose-lowering medications, first mar­
keted for the treatment of T2DM in 2013.188–190 These agents 
depend upon the kidney’s central role in the regulation of glucose 
dynamics and fluid balance. Under normal conditi

dynamics and fluid balance. Under normal conditions about 180 g 
of glucose is filtered from plasma at the glomerulus and reabsorbed 
in the proximal tubule. That quantity is equal to a fairly standard 
daily dietary intake of carbohydrate. In the setting of diabetes, 
glucose appearance in the proximal tubule increases, and glucose 
reabsorption by SGLT2 increases as well. This high-capacity low-
affinity transporter is responsible for 90% of renal tubular glucose 
reabsorption. There is a se

renal tubular glucose 
reabsorption. There is a second active transport molecule, SGLT1, 
which is a low-capacity high-affinity transporter responsible for 
approximately 10% of glucose reabsorption in the kidney but 
critically important for glucose absorption from the gut. Naturally 
occurring mutations in the SGLT2 gene cause a rare familial renal 
glycosuria syndrome associated with up to 170 g glucose lost in the 
urine daily, lesser rates of obesity and diabetes, and no evidence of 
long-t

f obesity and diabetes, and no evidence of 
long-term renal consequences. When diabetes is poorly controlled, 
more glucose enters the renal tubules than can be reabsorbed, 
and glucose appears in the urine. Because increased reabsorption 
of glucose by SGLT2 is accompanied by increased reabsorption 
of sodium, sodium balance may also be disrupted in poorly con­
trolled diabetes. Given these renal dynamics, it is not surprising 
that drugs blocking SGLT2 have important physiologic and clini­
cal

ng SGLT2 have important physiologic and clini­
cal effects. They reduce both fasting and postprandial glucose levels 
and promote modest weight loss through loss of calories as glucos­
uria. They also reduce blood pressure, at least in part by increasing 
sodium clearance and reducing extracellular fluid volume.
Four SGLT inhibitors are currently available in the United 
States for clinical use: canagliflozin, dapagliflozin, empagliflozin, 
and ertugliflozin. All are relatively specific for inhi

rtugliflozin. All are relatively specific for inhibiting SGLT2, 
but others in development also have some effect on SGLT1. The 
available agents have been studied in T2DM as monotherapy, in 
combination with other oral antihyperglycemic agents, and with 
insulin, with results that are quite similar between agents.191 Stud­
ies comparing SGLT inhibitors with metformin, sulfonylurea, and 
DPP4 inhibitors suggest that these agents are about as effective in 
HbA1c reduction as the other oral agents 

tive in 
HbA1c reduction as the other oral agents when used in combina­
tion regimens. In 26-week studies these agents lead to approxi­
mately 2 to 3 kg more weight loss than placebo.
Beyond these desirable effects that were expected based on 
known mechanisms of action, this class of drugs also provides 
cardiovascular benefits. The Empagliflozin Cardiovascular Out­
come Event Trial in Type 2 Diabetes Mellitus Patients—Remov­
ing Excess Glucose (EMPA-REG OUTCOME) trial was a large 
randomized c

(EMPA-REG OUTCOME) trial was a large 
randomized cardiovascular outcome study of empagliflozin. It 
showed unexpectedly good effects of this agent in T2DM patients 
who were selected for having high cardiovascular risk. Treatment 
with empagliflozin for a median time of about 3 years was associ­
ated with a 32% relative risk reduction of all-cause mortality, 38% 
reduction of cardiovascular mortality, and 35% reduction of hos­
pitalization for heart failure. Favorable effects of empagliflozin on

art failure. Favorable effects of empagliflozin on 
established renal disease were also found in this study.192 A subse­
quent report of similar studies of canagliflozin described generally 
similar, although quantitatively less prominent, cardiovascular and 
renal results.193 These findings are summarized in Table 35.12.
Demonstration of strong short-term cardioprotective effects of 
this class of drugs has generated much enthusiasm for their use, 
supported by relatively consistent findings, i

e, 
supported by relatively consistent findings, including real-world 
studies.194–199 It has also led to follow-up studies that should pro­
vide more information regarding whether all drugs in this class 
have similar effects, and to what extent these observations are 
applicable to T2DM patients without established cardiovascular 
or renal disease. There is particular interest in the potential value 
of SGLT inhibitors for patients with heart failure, a cardiovascular 
complication of diabetes

ailure, a cardiovascular 
complication of diabetes that is increasingly common.199,200 The 
mechanisms underlying these cardiovascular and renal effects are 
not well understood. At present it seems likely that several factors 
contribute, including improvement of glycemic control, blood 
pressure, and weight control, together with improved regulation 
of fluid balance and/or increased availability of ketones as fuel for 
injured cardiac or renal tissues.
  Treatment Effects of Empagliflozin an

 tissues.
  Treatment Effects of Empagliflozin and Canagliflozin in the EMPA-REG OUTCOME and CANVAS 
Program Studies
EMPAGLIFLOZIN IN EMPA-REG OUTCOME
CANAGLIFLOZIN IN THE CANVAS PROGRAM
Endpoints
Hazard Ratio (95% CI)
p Value
p Value
Primary cardiovascular 
composite
Cardiovascular death, nonfatal 
myocardial infarction, or 
nonfatal stroke
0.86 
(0.74–0.99)
0.04
0.86 (0.75–0.97)
0.02
Cardiovascular death
0.62 (0.49–0.77)
<0.001
0.87 (0.72–1.06)
NA
All cause death
0.68 (0

<0.001
NA
0.68 (0.57–0.82)
<0.001
0.87 (0.74–1.01)
NA
Hospitalization for heart failure
0.65 (0.50–0.85)
0.002
0.67 (0.52–0.87)
NA
Progression of albuminuria
0.62 (0.54–0.72)
<0.001
0.73 (0.67–0.79
NA
CI, Confidence interval; NA, not available.
Data from Zinman B, Wanner C, Lachin JM, et al. Empagliflozin, cardiovascular outcomes, and mortality in type 2 diabetes. N Engl J Med. 2015;373:2117–2128; Neal B, Perkovic V, Mahaffey KW, et al. 
Canagliflozin and cardiov

 V, Mahaffey KW, et al. 
Canagliflozin and cardiovascular and renal events in type 2 diabetes. N Engl J Med. 2017;377:644–657.
   
TABLE 35.12
1391
The most common side effects of this class are related to glycos­
uria and include urinary frequency, genital infections, and relatively 
rare episodes of lower urinary tract infections as well as dehydration 
and its consequences. Genital infections are usually related to yeast, 
occur in approxim

s are usually related to yeast, 
occur in approximately 10% to 15% of women (about a fivefold 
increase of risk), and may recur.201,202 Genital mycotic infections are 
much less common in men and predominantly are found in those 
who have not been circumcised. A cardiovascular outcome trial 
found increased lower extremity amputations with canagliflozin.195 
There is still uncertainty about this risk and whether it could involve 
other agents in the class.195,196,203 Theoretic questions related 

the class.195,196,203 Theoretic questions related to 
bone metabolism have been raised, but an increased risk of fractures 
has not been found in most completed clinical trials.190,204
Good renal function is essential for full efficacy of SGLT inhibi­
tors. Initiation is not recommended when eGFR is below 45 mL/
minute per 1.73 m2, and use should be discontinued when eGFR 
is lower than 45 mL/minute per 1.73 m2. Although acute renal 
insufficiency has been reported, presumably due to changes in 

y has been reported, presumably due to changes in 
renal perfusion, this class of medications shows potential for long-
term renal protection as well.205–207 An unexplained imbalance in 
bladder cancer without a change in total cancer or cancer fatality 
was reported in early development of dapagliflozin.208,209 Dapa­
gliflozin is the only agent in the class with a warning against use of 
the drug in patients with bladder cancer.
As a final concern, there have been reports of diabetic keto­
acid

rn, there have been reports of diabetic keto­
acidosis occurring without markedly elevated glucose levels (eug­
lycemic diabetic ketoacidosis) during treatment of presumed 
T2DM with an SGLT inhibitor. Although some of the individu­
als affected certainly had T2DM, some likely had unrecognized 
adult-onset T1DM. In many cases the event was preceded by a 
surgical procedure, an acute illness, or an inappropriate reduction 
of insulin dosage. The mechanisms underlying this adverse effect 
are only

echanisms underlying this adverse effect 
are only partly understood, but may include a tendency toward 
dehydration, elevation of circulating glucagon levels, and acceler­
ated gluconeogenesis due to glucosuria during fasting.190,210 Any 
patient treated with an SGLT inhibitor who experiences persistent 
or recurrent nausea, vomiting, or malaise, or who develops a meta­
bolic acidosis, should be evaluated for the presence of urine and 
serum ketones, even if glucose levels are near normal.211 

ones, even if glucose levels are near normal.211 
Colesevelam
Colesevelam is a second-generation bile acid sequestrant. It was 
observed to mediate modest reductions in glucose during the 
clinical development program. An expanded program in patients 
with T2DM resulted in approval for marketing of this drug as an 
adjunct for the treatment of diabetes. It provides an HbA1c reduc­
tion of about 0.5% in addition to approximately 15% improve­
ment in LDL.212 HDL changes tend to be trivial. Triglyc

in LDL.212 HDL changes tend to be trivial. Triglycerides can 
increase by 5% to 20%. Gastrointestinal side effects affect 10% or 
more of patients but seldom lead to withdrawals. The mechanisms 
for the glycemic effects are not definitively known.213 
A quick-release formulation of bromocriptine administered 
within 2 hours of rising in the morning has been developed and is 
approved for marketing in the United States. It is suggested that 
creating a circadian peak in central dopa

ed that 
creating a circadian peak in central dopaminergic tone improves 
insulin sensitivity. Nausea is the most common adverse effect, 
occurring in about 30% of patients and leading to discontinuation 
in about 10% at highest doses. Lower doses are better tolerated. 
HbA1c reductions are generally modest but have been reported to 
be as high as 1.2%. In a 1-year safety study, broad cardiovascular 
outcomes were improved 40% compared with placebo.214 Recent 
studies suggest substantial efficac

bo.214 Recent 
studies suggest substantial efficacy in the setting of inadequate gly­
cemic control on high-dose insulin therapy.215,216 
Glucose-Lowering Agents Requiring Injection
In addition to the glucose-lowering therapies that are orally 
administered small molecules, there is a group of relatively larger 
peptide agents that must be administered by other means. At pres­
ent, these are mostly delivered by subcutaneous injection, but 
delivery by inhalation, sublingual absorption, nasal spr

ry by inhalation, sublingual absorption, nasal spray, or other 
means may become more common in the future. At present, one 
formulation of inhalable insulin is available for use, and other 
products that can be delivered by means other than injection are 
under study. Because injected therapies require more active par­
ticipation by the patient, or assistance by others, more education 
and support in their use is needed than with oral therapies.
Insulins
Insulin has been commercially available 

Insulins
Insulin has been commercially available since the early 1920s and is 
arguably the best studied form of therapy for diabetes. It is essential 
for all patients with T1DM and a mainstay of treatment for T2DM. 
Subcutaneous injection of insulin can be used to supplement endog­
enous production of insulin both in the basal state, to modulate 
hepatic glucose production, and in the postprandial state, in which 
a surge in insulin release normally further suppresses hepatic glucose 
product

rmally further suppresses hepatic glucose 
production and facilitates glucose uptake by muscle and fat. Almost 
all insulins currently used worldwide are human insulins synthesized 
by recombinant technology or synthetic analogues of the human 
molecule.217,218 The various formulations differ largely in their phar­
macokinetics, as shown in Table 35.13. As patents on insulin ana­
logues expire, equivalent products that are manufactured by different 
companies are becoming available. These “biosi

nt 
companies are becoming available. These “biosimilar” insulins appear 
to have effects very like those of the originally patented versions.219,220
Insulins are grouped into shorter-acting versus longer-acting 
formulations. This distinction has practical importance, as these 
two categories of insulin are used quite differently. Both short-
acting and long-acting insulins have profiles of action that vary by 
the size of the dose injected. With higher doses the onset and time 
to peak effect 

h higher doses the onset and time 
to peak effect become more delayed and the total duration longer.
Long-Acting Insulins
Long-acting insulins are usually initiated before short-acting ones. 
In the past, formulations of human insulin were modified to 
prolong its action by slowing absorption. The most widely used 
formulations were ultralente insulin and protamine zinc insulin 
(PZI), both of which were very long acting, and lente insulin and 
neutral protamine Hagedorn (NPH), both considered i

eutral protamine Hagedorn (NPH), both considered intermedi­
ate acting. Ultralente, lente, and PZI are no longer available, but 
NPH human insulin is still commonly used. The time to onset, 
peak effect, and full duration of action of NPH are about twofold 
greater than those of regular human insulin, with an onset in 1 
to 2 hours, a peak usually at 4 to 8 hours, and a duration of 12 
to 16 hours. Insulin detemir is a long-acting analogue in which a 
fatty acid side chain is covalently bound to

ch a 
fatty acid side chain is covalently bound to the insulin molecule, 
resulting in slowing of both absorption from subcutaneous tis­
sue and clearance from circulation. Detemir’s time of onset and 
duration of action are quite similar to those of NPH, but more 
consistent from injection to injection.221 To reliably provide a 
sustained between-meal and overnight (basal) effect, both NPH 
and detemir must be taken twice daily, usually before breakfast 
and either before dinner or at bedtime. 

reakfast 
and either before dinner or at bedtime. However, for patients with 
1392
T2D who add detemir as basal insulin while continuing noninsu­
lin agents to cover the day, once daily dosing can be sufficient for 
at least 40% of patients.58
Insulin glargine is a long-acting analogue that is soluble at 
acid pH but precipitates when neutralized in tissues after injec­
tion. The resulting gradual absorption leads to a duration of acti

ing gradual absorption leads to a duration of action 
that, at doses commonly needed in T2DM, usually provides more 
than 24 hours of basal insulin effect with relatively little peak-to-
trough variation. This action profile results in consistently less risk 
of hypoglycemia than occurs with NPH insulin, and has led to use 
of insulin glargine taken once daily, either before breakfast or at 
bedtime, as a reliable way to initiate insulin therapy.
Insulin degludec and a more concentrated formulat

Insulin degludec and a more concentrated formulation (300 
units/mL) of insulin glargine have recently been introduced, and 
both provide even longer and flatter profiles of action than detemir 
and glargine.222,223 Further prolongation of action of degludec is 
due both to slower absorption after injection and to delayed clear­
ance, whereas that of glargine 300 units/mL results entirely from 
an effect on absorption. The pharmacokinetics of these newer 
long-acting insulins result in even low

ese newer 
long-acting insulins result in even lower risk of hypoglycemia than 
with glargine, but their advantage over glargine in this respect 
appears smaller than that with glargine versus NPH.224 
Short-Acting Insulins
Regular human insulin and the rapid-acting analogues (insulins 
lispro, aspart, and glulisine) are all administered by injection with 
meals to assist in control of postprandial hyperglycemia.217,218 
Regular insulin has an onset of action about 30 minutes after 
injection, a

set of action about 30 minutes after 
injection, a peak at 2 to 4 hours, and duration of 6 to 8 hours 
or longer at high doses. The three rapid-acting analogues all have 
an onset in 5 to 15 minutes, peak activity in approximately 1 
hour, and duration of approximately 4 hours. A more rapidly 
absorbed formulation of insulin aspart with onset of action within 
2.5 minutes has recently become available.225 Other very rapid-
acting insulin formulations and delivery technologies are being 
develope

ions and delivery technologies are being 
developed. Regular human insulin administered intravenously is 
immediately effective, with a half-life on the order of 10 minutes. 
Lispro, aspart and glulisine are approved in the United States for 
intravenous use but offer no advantage over regular insulin for this 
purpose and cost much more.
U-500 regular insulin has been available for decades, but its 
use has increased in the setting of severe insulin resistance.226 This 
concentrated insulin has

ulin resistance.226 This 
concentrated insulin has a delayed and prolonged action profile 
compared to U-100 regular.227 It has been administered twice 
daily, three times daily, and by insulin pump with good efficacy.228 
Pen devices are now available for ease and safety.
One inhaled formulation of human insulin is available.229,230 
The device by which it is delivered is quite convenient to carry 
and use. The insulin-loaded microparticles produce rapid onset 
of effect with peak activity in 3

uce rapid onset 
of effect with peak activity in 30 minutes (the tmax is about 10 
minutes), much earlier than with lispro.231 The time to peak is 
extended with higher doses, and the duration also varies from 90 
minutes at lower doses to 180 minutes with 12 units. Inhaled 
insulin may be preferred by patients who are very averse to taking 
injections, and some patients prefer its more rapid action to help 
with postprandial glucose control, but the shorter duration can be 
problematic. The cur

 the shorter duration can be 
problematic. The current formulation requires spirometry before 
initiation, at 6 months, and annually thereafter. This preparation 
is contraindicated in patients with asthma or chronic obstructive 
pulmonary disease, and there are advisory precautions for patients 
with active lung cancer and smokers. Its use is associated with 
cough or throat irritation in addition to the usual hypoglycemic 
risk, and is at present limited to 4-unit, 8-unit, and 12-unit doses 
r

nt limited to 4-unit, 8-unit, and 12-unit doses 
resulting in limitations of dose adjustments. 
  Clinical Features of Commonly Used Insulins
Types and Generic Names (Commercial Names)
Onset of Action 
(Minutes)
Time to Peak (Hours)
Duration (Hours)
Rapid Acting
Aspart (Fiasp)
Aspart (Novolog)
Lispro (Humalog)
Glulisine (Apidra)
<5
10–20
10–20
10–20
0.5–1.5
0.5–1.5
0.5–1.5
0.5–1.5
3–5
3–5
3–5
3–5
Just before or just after meals
0–15 minutes before or just 
after meals
Short Actin

15 minutes before or just 
Short Acting
Regular human (Humulin R, Novolin R)
30–45
2–4
4–8
15-30 minutes before meals
Intermediate Acting
NPH (Humulin N, Novolin N)
60–120
4–8
12–20
Once or twice daily
Long Acting
Detemir (Levemir)
Glargine (Lantus, Basaglar)
Degludec (Tresiba)
60–120
60–120
60–120
6–10
No pronounced peak
16–24
∼24
Up to 72
Usually once daily
Premixed
70/30 NPH/R (Humulin 70/30, Novolin 70/30)
75/25 Protamine-lispro/lispro (Humalog Mix 70/30)
70/30

 Protamine-lispro/lispro (Humalog Mix 70/30)
70/30 Protamine-aspart/aspart (Novolog Mix 70/30)
50/50 Protamine-lispro/lispro (Humalog Mix 50/50)
50/50 Protamine-aspart/aspart (Novolog Mix 50/50)
30–40
10–20
10–20
10–20
15–60
4–8
4–8
4–8
4–8
4–8
12–20
12–20
12–20
12–20
12–20
Usually twice daily, 0–30 
minutes before meals
Concentrated
U-500 human regular (Humulin U-500)
U-200 degludec (Tresiba U-200)
U-300 glargine (Toujeo 300 U/mL)
30–45
60–120
60–120
6–12
1

0–120
6–12
12–24
>24
Up to 72
Once daily
Once daily
TABLE 35.13
1393
Premixed Insulins
Premixed insulin formulations provide greater convenience and 
accuracy of dosing than mixing of two formulations in a syringe 
by the patient at the time of administration, but at the expense 
of reduced flexibility of dosing. Premixed formulations available 
in the United States are 70/30 and 50/50 mixtures

in the United States are 70/30 and 50/50 mixtures of NPH and 
regular insulin, a 75/25 and a 50/50 mixture of lispro insulin in its 
NPH-like formulation with insulin lispro, and a 70/30 mixture of 
insulin aspart with its NPH-like congener. Premixed insulins pro­
vide a single-peak action at about 4 to 6 hours after each injection, 
as expected from summation of the activities of their components. 
In a very instructive study, use of 70/30 insulin in T2DM showed 
greater improvement in glucose

lin in T2DM showed 
greater improvement in glucose control but at the cost of signifi­
cantly more hypoglycemia compared to basal insulin.232 
Human Versus Analogue Insulins
Insulin analogues are clearly helpful in managing T1DM, which 
requires complete replacement of endogenous insulin and accu­
rate matching of the profile of action to daily needs. The degree to 
which both long-acting and rapid-acting analogues offer advan­
tages over NPH and regular human insulin for T2DM is less well 
esta

 regular human insulin for T2DM is less well 
established and a focus of debate. Among the issues is that both 
the list price and the direct cost to patients can be much higher for 
the analogues.142,233,234 Both long-acting and rapid-acting insulin 
analogues have been shown in large studies to cause less frequent 
hypoglycemia than human insulins, but for some patients little 
difference may be found. The absolute risk of severe hypoglyce­
mia in patients with T2DM is relatively small, approx

 in patients with T2DM is relatively small, approximately 
one-third to one-tenth as high as in similarly treated patients with 
T1DM. This risk can be further minimized with appropriate edu­
cation of patients and expectant monitoring of glucose at times 
when hypoglycemia is most likely to occur, such as at night or dur­
ing unplanned or strenuous activity. Allergies to currently avail­
able insulins are rare, as are chronic skin reactions, which include 
lipoatrophy and lipohypertrophy. 
Insu

ch include 
Insulin Delivery Devices
Newer insulin needles cause less discomfort than those previ­
ously available because of a finer gauge, shorter length, sharper 
points, and smoother surfaces. Improved insulin pen design has 
made teaching a patient to take insulin much easier and provides 
greater convenience and accuracy of dosing. Pen injectors capable 
of injecting larger doses, up to 160 units, are now available for 
U-300 glargine and U-200 degludec. I

vailable for 
U-300 glargine and U-200 degludec. Insulin pump therapy has 
been used in patients with T2DM but is not widely accepted as 
cost effective in routine practice.235–237 Insulin pump delivery of 
U-500 regular insulin has been proposed for highly insulin-resis­
tant T2DM but is still a relatively uncommon application.226 
GLP1 Receptor Agonists
GLP1 agonists are homologs or analogs of human GLP1 that have 
significantly changed the landscape of T2DM therapy.179,238,239 
As with insuli

dscape of T2DM therapy.179,238,239 
As with insulins, GLP1 agonists divide logically into two groups: 
those that are short acting and those with longer duration of 
effects. Important features of the available agents in this class are 
shown in Table 35.14.
Short-Acting GLP1 Agonists
The first to be developed was exendin-4, a short-acting agent. It 
is a naturally occurring component of the saliva of the Gila mon­
ster (Heloderma suspectum) and shares 53% sequence identity with 
GLP1 but is rel

shares 53% sequence identity with 
GLP1 but is relatively resistant to degradation by DPP4. Exena­
tide is synthetic exendin-4 and was the first GLP1-based thera­
peutic agent to be approved for human use.240,241 When injected 
subcutaneously, it has a peak of action at about 2 hours and total 
duration of no more than 6 hours. Like GLP1 itself, it poten­
tiates insulin secretion in response to rising glucose levels, sup­
presses postprandial glucagon secretion, delays gastric emptying, 
and pro

cagon secretion, delays gastric emptying, 
and promotes satiety. With twice-daily injection before breakfast 
and before dinner it produces a reduction of approximately 1% in 
HbA1c, largely through a reduction of postprandial glucose along 
  Clinical Features of Commonly Used Injectable Agents Other Than Insulin
Types and Generic Names 
Main Effects
Short-acting GLP1 agonist
Exenatide (Byetta)
Lixisenatide (Adlyxin)
5–10 μg BID 

atide (Byetta)
5–10 μg BID before breakfast 
and dinner
10–20 μg QD before breakfast
Postprandial glucose control 
and weight loss
T1D
DKA
History medullary 
carcinoma
Pancreatitis?
Long-acting GLP1 agonist
Liraglutide (Victoza)
Dulaglutide (Trulicity)
Extended-release exenatide (Bydureon)
Semaglutide (Ozempic)
0.6–1.8 mg daily
0.75 or 1.5 mg weekly
2 mg weekly
0.5 or 1 mg weekly
Basal glucose control and 
weight loss
T1D
DKA

ly
T1D
DKA
carcinoma
Fixed-dose GLP1/insulin combination
Liraglutide/degludec (Xultophy)
Lixisenatide/glargine (Soliqua)
Daily, titrated
Daily before breakfast, titrated
Glucose and weight control
T1D
DKA
carcinoma
Amylin agonist
Pramlintide (Symlin)
TID before meals in T1D or T2D 
requi

ide (Symlin)
requiring prandial insulin
Confirmed gastro­
paresis
Nausea
Abdominal pain
TABLE 35.14
1394
with modest weight loss (2–4 kg). With prolonged use, these 
effects can be associated with modest improvements in blood pres­
sure and lipids. The most common adverse effect is nausea, which 
occurs in up to 50% of patients, is usually mild t

occurs in up to 50% of patients, is usually mild to moderate in 
intensity, occurs early in the course of therapy, and typically wanes 
over time. Nausea leads to withdrawal of therapy in about 5% of 
patients.
Lixisenatide is a more recently introduced agent that has prop­
erties similar to those of exenatide except for a slightly longer 
duration of action.242 It has been approved for administration 
once daily before breakfast. When given in this way it reduces 
postprandial increments of glu

his way it reduces 
postprandial increments of glucose during the daytime but has 
much less effect overnight. Both exenatide and lixisenatide are rec­
ommended to be started at the lower available dose and advanced 
to full dosage after several weeks to limit side effects on initiating 
therapy. 
Long-Acting GLP1 Agonists
Newer, longer-acting GLP1 receptor agonists include liraglutide, 
an extended-release formulation of exenatide, dulaglutide, and 
semaglutide.179,239,243 Liraglutide is admini

and 
semaglutide.179,239,243 Liraglutide is administered once daily with­
out any restriction as to timing or relation to meals. The other 
long-acting agents are to be given once weekly. In general, they are 
associated with greater HbA1c-lowering efficacy than short-acting 
exenatide and lixisenatide, owing to their greater effects on over­
night fasting glucose levels.179,239 However, they have much less 
effect on postprandial increments of glucose than the short-acting 
agents. They also ar

lucose than the short-acting 
agents. They also are associated with a lower frequency of gastro­
intestinal side effects. All the various long-acting agents appear to 
be quite effective in lowering HbA1c, although some differences 
are shown in early head-to-head trials.244–247 Some evidence sug­
gests that semaglutide can cause superior reduction in HbA1c but 
also more nausea.245,246,248 Studies under conditions of clinical 
practice have shown some differences from randomized clinical 
trial

n some differences from randomized clinical 
trials, suggesting that results of treatment can be influenced by 
differences in adherence to therapy.249 There are some differences 
in the pen injectors used for delivering doses of the various agents, 
and instruction in their use is needed to optimize dosing and 
adherence.250
Weight loss is consistently seen and similar among the vari­
ous GLP1 receptor agonists, although only liraglutide is currently 
approved for weight loss in patients with o

ently 
approved for weight loss in patients with or without diabetes (see 
Chapter 40). Semaglutide appears to have slightly more weight 
loss potential when used at doses approved for diabetes in the 
United States.
Recently, large randomized trials examining cardiovascular 
safety of two long-acting GLP1 agonists—liraglutide and sema­
glutide—have shown significant cardiovascular benefit in popula­
tions selected for having high cardiovascular risk.251–255 Details 
of these studies are shown i

risk.251–255 Details 
of these studies are shown in Table 35.15. The Liraglutide Effect 
and Action in Diabetes: Evaluation of Cardiovascular Outcome 
Results (LEADER) study followed patients randomized to treat­
ment with liraglutide or placebo for a median of more than 3 
years. Significant reductions of risk were found for the primary 
composite cardiovascular endpoint (13%), cardiovascular death 
(22%), all-cause mortality (15%), and progression of albumin­
uria (26%).252 The Trial to Evalua

ion of albumin­
uria (26%).252 The Trial to Evaluate Cardiovascular and Other 
Long-term Outcomes with Semaglutide in Subjects with Type 2 
Diabetes (SUSTAIN-6) had a generally similar design but stud­
ied patients starting with somewhat worse glycemic control who 
received randomized treatment for about 2 years.251 This study 
demonstrated somewhat greater reduction of risk for its pri­
mary cardiovascular composite endpoint (26%) compared with 
LEADER and showed a reduction of renal endpoints.

LEADER and showed a reduction of renal endpoints. However, 
no reduction of mortality was found. Neither LEADER nor SUS­
TAIN-6 showed a reduction of hospitalizations for heart failure. 
Up to now, these favorable outcomes have not been found with 
short-acting GLP1 agonists and there has been inconsistency in 
the specific benefits seen in trials of the other long-acting agents. 
Many of the variations could relate to the study designs and popu­
lations selected in those trials.
Hypoglycemia i

­
Hypoglycemia is not a direct effect of GLP1 receptor ago­
nists but can occur when they are added to other agents. When 
one is added to prior therapy with a secretagogue, it is pru­
dent to reduce to the minimum dose of secretagogue, at least 
initially. When a GLP1 receptor agonist is added to insulin, 
a 20% reduction of insulin dosage is advised unless HbA1c is 
greater than 8%.
Concern about pancreatitis has emerged from postmarketing 
reports of GLP1 rec

as emerged from postmarketing 
reports of GLP1 receptor agonists. A direct causal link has not 
been proved, although passage of gallstones accompanying weight 
loss remains a possible mechanism. It is recommended that drugs 
in this class be avoided in patients with a history of pancreatitis.256 
Exenatide is renally cleared and is contraindicated in the setting 
  Treatment Effects of Liraglutide and Semaglutide in the LEADER and SUSTAIN-6 Studies
Endpoints
LIRAGLUTIDE IN LEADER
SEMAGLUTIDE I

dies
Endpoints
SEMAGLUTIDE IN SUSTAIN-6
p Value
p Value
Primary cardiovascular composite
Cardiovascular death, nonfatal myocardial 
infarction, or nonfatal stroke
0.87 (0.78–0.97)
0.01
0.74 (0.58–0.94)
0.02
0.78 (0.66–0.93)
0.007
0.98 (0.65–1.48)
0.92
0.85 (0.74–0.97)
0.02
1.05 (0.74–1.5)
0.79
0.87 (0.73–1.05)
0.14
1.11 (0.77–1.61)
0.57
Progression of albuminur

.14
0.57
0.74 (0.60–0.91)
0.004
NA
NA
Data from Marso SP, Daniels GH, Brown-Frandsen K, et al. Liraglutide and cardiovascular outcomes in type 2 diabetes. N Engl J Med. 2016;375:311–322; Marso SP, Bain SC, Consoli A, et al. Semaglutide 
and cardiovascular outcomes in patients with type 2 diabetes. N Engl J Med. 2016;375:1834–1844.
   
TABLE 35.15
1395
of ad

35  Therapeutics of Type 2 Diabetes Mellitus
of advanced kidney disease (eGFR <30 mL/minute/1.73 m2). The 
other GLP1 receptor agonists do not share this feature with exena­
tide. Nevertheless, cases of acute renal failure have been reported 
in association with GLP1 receptor agonist therapy, usually in 
patients with chronic renal insufficiency who develop superim­
posed prerenal azotemia in the context of prolonged anorexia, 
nausea, and vomiting. To mitigate the risk of pancreatitis and 
rena

ng. To mitigate the risk of pancreatitis and 
renal failure, it is prudent to instruct patients treated with GLP1 
receptor agonists to stop their medication if they develop nausea, 
vomiting, or abdominal pain of more than a few hours’ duration 
and to seek medical attention if they are unable to keep down 
fluids after 4 hours.
Another safety concern is medullary thyroid cancer. No signal 
exists for this problem with GLP1-based therapy in humans, but 
a clear increase in the incidence of thes

ns, but 
a clear increase in the incidence of these tumors was observed in 
preclinical testing in rodents, although not in other animal mod­
els. It is suggested that these agents be avoided in those with a 
personal or family history of medullary thyroid cancer. 
Amylin Receptor Agonists
Amylin is a neuroendocrine hormone that is cosecreted with insu­
lin by pancreatic beta cells. A deficiency of amylin is therefore 
evident in patients with T1DM or T2DM, in parallel with insu­
lin deficiency.

DM or T2DM, in parallel with insu­
lin deficiency. Amylin and insulin have complementary actions 
in regulating plasma glucose. Amylin binds to brain nuclei and 
promotes satiety and reduces appetite. Through efferent neural 
pathways it mediates a decrease in the rate of gastric emptying and 
limits inappropriate glucagon secretion in a glucose-dependent 
fashion. By these means it regulates the rate of plasma glucose 
appearance from the gastrointestinal tract and the liver. Insulin 
concurren

intestinal tract and the liver. Insulin 
concurrently regulates the rate of glucose appearance from the 
liver and stimulates glucose uptake in muscle and fat.
Amylin is relatively insoluble in aqueous solution and aggre­
gates on plastic and glass. Pramlintide was developed as a soluble, 
nonaggregating amylin analogue, with similar clinical effects, 
that is administered by injection before meals. It is approved for 
use by patients with T1DM or T2DM requiring both basal and 
mealtime insulin.

r T2DM requiring both basal and 
mealtime insulin.257 Most of the effect on glycemic patterns is a 
reduction of postprandial increments, with little effect on over­
night control. When pramlintide is added to insulin therapy with 
or without metformin or a sulfonylurea in T2DM, HbA1c can be 
reduced by about 0.5 to 0.7 percentage points, and weight is vari­
ably reduced as well.258
Mild or moderate nausea, which wanes with continued ther­
apy, is pramlintide’s most common adverse effect. It is 

s pramlintide’s most common adverse effect. It is minimized 
by initiating therapy with 60 μg before meals, with an increase 
to the full 120 μg dose over 1 to 2 weeks as tolerated. Hypogly­
cemia is less frequent in patients with T2DM than in patients 
with T1DM and can be limited by reducing dosage of mealtime 
insulin by about 50% when initiating therapy, with the option of 
subsequently returning to higher doses as needed.257 Oral medi­
cations that require rapid absorption for effectiveness

ns that require rapid absorption for effectiveness should be 
administered either 1 hour before or 2 hours after injection of 
pramlintide. Coformulations of pramlintide with regular human 
insulin are being evaluated. 
Practical Aspects of Treatment
A Team Approach
Considering the wide array of interventions available, and the 
need to adapt to differences between patients as well as changing 
needs over time, it is obvious that many treatment decisions will 
be needed for every patient. It is 

ecisions will 
be needed for every patient. It is also clear that various kinds of 
providers with differing training and experience can contribute 
to making and implementing these decisions. No one provider 
can do it all, and the concept of a team approach to manage­
ment of diabetes is well established. Teams may include nurses, 
nutritionists, pharmacists, educators, and physicians, all with spe­
cific interest and experience in diabetes. Although the previous 
discussion of DSMES describes

though the previous 
discussion of DSMES describes in detail the services and processes 
required for care, it does not define specifically the kinds of pro­
viders needed. This is because different health systems use differ­
ent models for providing DSMES, with different configurations 
of providers. However, in each setting a locally available group of 
providers should be identified who can function as a team with 
each individual patient. Providers specialized in other aspects of 
care commo

viders specialized in other aspects of 
care commonly needed by people with T2DM—such as cardiol­
ogy, nephrology, ophthalmology, and podiatry—also can contrib­
ute to the team effort. The overall goal of this collaboration is to 
ensure optimal management of overall metabolic control, other 
risk factors for complications of diabetes, and the complications 
themselves. Making the team concept a reality is a major challenge 
for health systems today.
The specialized team can assist at various po

day.
The specialized team can assist at various points in the natural 
history of T2DM. As noted in Fig. 35.2 and Table 35.7, these 
include the time of diagnosis, a later time when injected therapies 
or use of devices such as CGM systems are needed, or whenever 
diabetes-related problems such as severe hypoglycemia, ketoacido­
sis, or repeated hospitalization occur or reoccur. 
Identifying Pathophysiologic Subgroups
The time of diagnosis presents an opportunity to favorably influ­
ence the ent

ts an opportunity to favorably influ­
ence the entire future experience of the patient. It is typically 
a teachable moment at which the patient can be introduced to 
DSMES and develop a long-term plan of management. It is also 
an opportunity to identify those individuals who do not have the 
most typical form of T2DM and adjust the plan accordingly. Sev­
eral of the more common atypical subgroups are discussed in this 
section.
Latent Autoimmune Diabetes of Adulthood
The largest subgroup is th

e Diabetes of Adulthood
The largest subgroup is that with diabetes of autoimmune 
origin, commonly called latent autoimmune diabetes of adult­
hood (LADA) or adult-onset type 1 diabetes. This group com­
prises up to 12% of patients diagnosed with diabetes after age 
20.259,260 Such individuals may have rapid deterioration of gly­
cemic control prior to diagnosis, sometimes accompanied by 
prominent symptoms of hyperglycemia and weight loss, but 
these tend to progress less rapidly than in youth-

these tend to progress less rapidly than in youth-onset T1DM. 
They frequently are not obese and may have other autoimmune 
disorders or a family history of these. Laboratory support can 
be provided by detection of anti–glutamic acid decarboxylase 
(GAD) antibodies or anti-islet cell antibodies, although lack of 
antibodies does not exclude an autoimmune process. Both fam­
ily history and genetic markers are likely to be more consistent 
with T1DM, but there is heterogeneity and uncertainty in 

DM, but there is heterogeneity and uncertainty in clas­
sifying individual cases.261,262 Insulin has been considered the 
main therapy from the outset, but other therapies may be pos­
sible in those with slow progression. There is some evidence that 
early use of insulin may prolong beta-cell function and prevent 
a period of sustained hyperglycemia that could initiate chronic 
complications. 
1396
Pancreatic Diabetes
Another relativel

t Metabolism
Another relatively common subgroup comprises patients with 
other disorders affecting the pancreas. Pancreatic diabetes has been 
referred to as type 3c diabetes and is underdiagnosed.263,264 The 
leading causes include acute or chronic pancreatitis, pancreatec­
tomy for cancer or pancreatitis, hemochromatosis, and cystic 
fibrosis. Despite widely differing pathogenesis, patients with pan­
creatic diabetes share a tendency to have significant deficiency of 
insul

 tendency to have significant deficiency of 
insulin secretion and prominent postprandial hyperglycemia, and 
may respond less well than expected to oral therapies.264 In addi­
tion to insulin deficiency, there are significant decreases in gluca­
gon secretion and in the effects of incretin hormones. As in the 
case of LADA, patients in this subgroup commonly need insulin 
at or relatively soon after diagnosis and quite frequently require 
both basal and prandial insulin. In some cases, due to t

asal and prandial insulin. In some cases, due to the mul­
tihormonal loss, there is more glucose variability than is seen in 
T2DM or T1DM.265 
Monogenic Diabetes
A smaller but increasingly recognized subgroup is that with mono­
genic diabetes. This is a heterogeneous group of heritable disorders, 
which are often diagnosed in childhood but sometimes present as 
diabetes diagnosed in adulthood.266–268 A subset of this group of 
disorders has been commonly described as maturity-onset diabe­
tes o

 commonly described as maturity-onset diabe­
tes of the young (MODY). The proportion of newly diagnosed 
adults with one of these conditions is estimated to be between 
1% and 2%. The diagnosis should be considered in atypical cases 
that might otherwise be diagnosed as T1DM or T2DM. A strong 
family history consistent with autosomal dominant inheritance 
with high penetrance is typical but not always present. A defini­
tive diagnosis depends on genetic testing that can identify specific 
subtyp

genetic testing that can identify specific 
subtypes and thus assist in planning treatment. This approach to 
monogenic diabetes is one of the best present-day examples of 
precision medicine.
Two of these disorders deserve specific comment. One is a heri­
table deficiency of glucokinase (GCK-MODY) that alters glucose 
sensing in the beta cell.269 It presents typically in youth as mild 
fasting hyperglycemia (e.g., 100–140 mg/dL) that is not progres­
sive and causes little or no tissue damage ov

res­
sive and causes little or no tissue damage over time. The HbA1c 
may range from 5.6% to 7.8%. In many cases this disorder is 
not recognized until routine chemistry tests indicate elevated 
fasting glucose levels typical of prediabetes. It is often recognized 
in women during testing for gestational diabetes. Recognition of 
GCK-MODY is important mainly to avoid unnecessary treat­
ment of the patient and other identified members of the family. 
Current evidence indicates that those with a G

y. 
Current evidence indicates that those with a GCK mutation will 
not develop chronic complications of diabetes, and treatment is 
not likely to render significant changes in glucose control. How­
ever, longer follow-up of larger numbers or individuals with this 
disease will be needed to be certain.270
In contrast, the most common form of monogenic diabetes, 
hepatic nucleocyte factor-1A (HNF1A), has direct clinical rel­
evance.271 It can present in adulthood with progressive hyper­
glycemia,

ent in adulthood with progressive hyper­
glycemia, more postprandial than fasting, that responds very 
well to sulfonylureas. Other monogenic forms of diabetes, such 
as HNF4α mutations, may also respond to sulfonylureas, which 
bind to the molecular complex on the surface of beta cells that 
is genetically affected. Successful responses to sulfonylureas may 
persist for extended periods of time. The possibility of sulfonyl­
urea-responsive monogenic diabetes can be suspected when there 
is a pr

enic diabetes can be suspected when there 
is a prominent history of adult-onset but quite stable and slowly 
progressive diabetes on one side of the patient’s family, often in the 
absence of obesity. Laboratory screening for these disorders is now 
more available than in the past but not always covered by insur­
ance. Identification of affected families can avoid unnecessary or 
inappropriate therapy for many family members. 
Standardized Versus Personalized Tactics
The remaining majority of p

s Personalized Tactics
The remaining majority of patients with diabetes newly diagnosed 
in adulthood can be considered to have T2DM. Although some 
progress is being made in identifying subgroups based on clini­
cal characteristics,272 at the present time it is simplest to begin 
with relatively standardized, algorithmic approaches that will be 
described later. Moreover, even after separation of subgroups, 
T2DM remains physiologically heterogeneous and patients also 
differ markedly in their 

neous and patients also 
differ markedly in their daily habits, preferences, and capacities 
for self-management. The metabolic defects leading to diabe­
tes also tend to worsen, and the burden of diabetes-related and 
other infirmities increases over time. For all these reasons, what­
ever approach is first used will need to be personalized eventually. 
This expectation can from the start be shared with the patient, 
who should be enlisted to participate actively in making decisions 
beyond any

rticipate actively in making decisions 
beyond any initial algorithm. 
A Standardized Initial Therapeutic Approach
The most critical part of long-term glycemic management is con­
tinuing reassessment of glucose patterns and HbA1c values to 
guide refinement of interventions. The goal is to maintain opti­
mal control with the lowest doses of the fewest medications. In 
general, initial drug therapy should be considered from the time 
of diagnosis, along with lifestyle intervention, and by 5 years

 along with lifestyle intervention, and by 5 years after 
diagnosis most patients require two or more drugs to maintain 
selected targets. Early in therapy it is usually desirable to add addi­
tional agents to prior therapies, rather than replacing one agent 
with another. Guidance from randomized trials, when available, 
is desirable in making therapeutic choices. A general approach to 
be considered in the absence of any patient-specific factors is sug­
gested here.142,159
Metformin
Metformin 

s is sug­
Metformin
Metformin is considered the default choice for initial drug therapy. 
Used in combination with diet, exercise, and a DSMES program 
it can usually provide impressive lowering of glucose with essen­
tially no risk of hypoglycemia.273 However, approximately 5% of 
patients will experience gastrointestinal symptoms with even low 
doses of metformin, and others should be prescribed metformin 
only with caution due to impaired renal function or serious car­
dia

due to impaired renal function or serious car­
diac, liver, or intestinal disease. If metformin is contraindicated or 
poorly tolerated, or when the response to it is inadequate after a 
trial of 3 to 6 months, various agents can be substituted or added. 
There are six recommended second-line therapies: sulfonylureas, 
TZDs, DPP4 inhibitors, SGLT inhibitors, GLP1 receptor ago­
nists, and basal insulins. Each has advantages and disadvantages. 
Stepwise Combination Therapy
Because of more than 60 

pwise Combination Therapy
Because of more than 60 years of experience, widespread current 
use, and clear evidence of benefit relative to risk from long-term 
randomized interventional studies, sulfonylureas and basal insu­
lins are still appropriate considerations for second-line therapy. 
Addition of a sulfonylurea to metformin may serve as an example 
of the value of combination regimens that is relevant to many 
other combinations of agents in different classes. Because most 
classes act thr

n different classes. Because most 
classes act through distinctive mechanisms, their glycemic effects 
1397
are usually additive. Moreover, combining agents generally allows 
greater glucose lowering without increased side effects. In the 
case of metformin and the preferred sulfonylureas (glimepiride, 
long-acting glipizide, and gliclazide), more than half the maximal 
therapeutic effect is obtained at half maximal dosage. At the same 
time, 

tained at half maximal dosage. At the same 
time, the leading side effects—gastrointestinal symptoms with 
metformin and hypoglycemia with sulfonylureas—are more likely 
at higher doses. As a specific example, combining metformin 500 
mg twice daily with glimepiride 1 mg daily is likely to provide 
greater therapeutic power and fewer side effects than full dos­
age of either alone. However, if metformin has previously been 
titrated to the maximally effective dose of 2000 mg daily and is 
well t

lly effective dose of 2000 mg daily and is 
well tolerated, there is no reason to decrease the metformin dose 
when adding a second agent, including sulfonylureas, unless renal 
function is impaired. Continuing metformin and a sulfonylurea 
when basal insulin is added similarly may allow continuation of 
good glycemic control while keeping insulin dosage and risk of 
hypoglycemia low.
The effectiveness of combination therapy approaches using 
these agents in the Outcome Reduction with an Initial

se agents in the Outcome Reduction with an Initial Glargine 
Intervention (ORIGIN) trial is shown in Fig. 35.4.274 This trial 
enrolled participants who had either dysglycemia (impaired glu­
cose tolerance and/or impaired fasting glucose) or T2DM treated 
with diet and no more than one oral agent. Participants were 
selected for having high cardiovascular risk. They were randomly 
assigned to one of two regimens, each seeking fasting glucose no 
higher than 95 mg/dL (5.3 mmol/L). One regimen was

higher than 95 mg/dL (5.3 mmol/L). One regimen was based 
on glargine as basal insulin with continuation of any prior oral 
therapy, the other on conventional stepwise use of oral therapies 
with insulin added only if necessary. Participants were followed an 
average of about 5 years. Both regimens maintained HbA1c levels 
close to 6.5% during the study, which for those with T2DM at 
entry was up to a total of 10 years after diagnosis.275 Most partici­
pants in both randomized arms used only met

rtici­
pants in both randomized arms used only metformin, a sulfonyl­
urea, or basal insulin, confirming the ability of these older agents 
to provide good glycemic control for many patients up to 10 years 
after diagnosis. No differences in medical outcomes between the 
two arms of the trial were observed other than more hypoglycemia 
in the group assigned to initial glargine therapy and less progres­
sion from dysglycemia to overt diabetes in that group. 
Considerations in Personalizing Therap

hat group. 
Considerations in Personalizing Therapy
As the array of glucose-lowering therapies continues to grow, 
promising results of clinical trials using newer agents are provid­
ing additional guidance for therapy. As a result, personalizing drug 
therapy beyond traditional use of metformin, sulfonylureas, and 
basal insulin is increasingly common and very appropriate. Any of 
the classes of agents other than metformin may be considered as 
second-line therapy when there are specific reason

second-line therapy when there are specific reasons to do so.276 In 
many cases, specific short-term clinical advantages may be judged 
to outweigh greater cost and lack of long-term experience. The 
rationale for the choice of therapy should be discussed with the 
patient, especially in the case of the newest agents.
Oral Agents and Injected Therapies Other than 
Insulin
The DPP4 inhibitors, the TZD pioglitazone, and long-acting 
GLP1 agonists are often used in combination with metformin. The 

re often used in combination with metformin. The 
glucose-lowering effect of DPP4 inhibitors is often stated to be less 
than that of sulfonylureas, but they are unlikely to cause hypogly­
cemia and have few symptomatic side effects.277 Furthermore, it 
has been argued that some of the apparent difference in efficacy 
is due to considering older sulfonylurea studies where baseline 
HbA1c was higher.278,279 Pioglitazone is variably effective in con­
trolling glucose and often causes weight gain a

n­
trolling glucose and often causes weight gain and edema, among 
other side effects, but it improves peripheral insulin sensitivity and 
can reduce hepatic fat content in patients with nonalcoholic ste­
atohepatitis (NASH).280,281 It is most often prescribed in lower 
doses now when treating T2DM, although many of the original 
efficacy studies and the NASH studies have been performed with 
the 45-mg full dose. Fixed-dose combination formulations of met­
formin with a DPP4 inhibitor or pioglit

ns of met­
formin with a DPP4 inhibitor or pioglitazone can facilitate dosing.
The injectable GLP1 receptor agonists are increasingly pro­
posed as similarly effective alternatives to basal insulin for patients 
who are known to have high cardiovascular risk because of the 
cardiovascular protection shown with liraglutide and semaglu­
tide.251,252 However, they often cause gastrointestinal symptoms 
requiring thoughtful titration. Use of the long-acting GLP1 
agents with once-weekly dosing may i

-acting GLP1 
agents with once-weekly dosing may improve convenience and 
reduce symptoms, but even so gastrointestinal side effects can 
adversely affect adherence. Fixed-dose combinations of basal 
insulin with GLP1 agonists have been proposed as initial inject­
able therapy, and while they are reliably effective in reducing glu­
cose the selection of proper doses and the titration process itself 
require specialized skills.282,283 Because the mechanism of action 
of GLP1 agonists is similar t

mechanism of action 
of GLP1 agonists is similar to that of DPP4 inhibitors, there is no 
advantage to continuing the latter agents when an injected GLP1 
agonist is started.
8.0
7.5
7.0
6.5
Median A1c %
6.0
5.5
5.0
Std
Glar
717
731
683
692
664
674
632
634
604
614
577
581
419
436
92
109
578
571
8.0
7.5
7.0
6.5
6.0
5.5
5.0
0
1
2
3
4
Diabetes
No diabetes
5
6
7
End
Std
Glar
5476
5451
5239
5149
5089
4990
4800
4757
4593
4565
4353
4359
2945
2995
626
632
4155
4158
B
A
• 

4565
4353
4359
2945
2995
626
632
4155
4158
B
A
• Fig. 35.4  Long-term control of HbA1c in the ORIGIN trial. Participants (n = 
12,537) with dysglycemia (impaired glucose tolerance or impaired fasting 
glucose) or diagnosed T2DM on diet with or without one oral agent were 
randomized to treatment based on insulin glargine (closed circles, solid 
lines) or to sequential use of oral therapies (open circles, dashed lines), 
seeking fasting plasma glucose 95 mg/dL (5.3 mmol/L) or less. Both regi­
me

ucose 95 mg/dL (5.3 mmol/L) or less. Both regi­
mens maintained HbA1c levels close to 6.5% for the duration of the trial, 
shown for study participants with dysglycemia at study entry in (A) and 
for those with diabetes at study entry in (B). (Modified from the ORIGIN 
Trial Investigators. Characteristics associated with maintenance of mean 
A1C<6.5% in people with dysglycemia in the ORIGIN trial. Diabetes Care. 
2013;36:2915–2922.)
1398
SECTION VIII    Disorders of Carbohydrate and Fat Metaboli

VIII    Disorders of Carbohydrate and Fat Metabolism
Following the studies showing cardiovascular benefits with 
empagliflozin and canagliflozin, early use of an SGLT inhibitor 
has been proposed for consideration for patients with established 
cardiovascular disease, especially when heart failure has been diag­
nosed or suspected. However, the glucose-lowering effect of these 
agents is only moderate, and because they are new the magnitude of 
risk from several side effects has not been fully e

isk from several side effects has not been fully explored.198,284,285
Basal Insulin
Uniquely among the classes of glucose-lowering therapies, use of 
insulin must always be personalized. The need to personalize dos­
ing of insulin is driven by both physiologic and behavioral factors. 
The strategy of adding basal insulin when agents other than insu­
lin are no longer successful is well established.286,287 Some prac­
titioners prefer to stop oral glucose-lowering agents when insulin 
is started, 

glucose-lowering agents when insulin 
is started, especially when using twice-daily premixed insulin, but 
most continue oral therapy while adding NPH or a long-acting 
analogue as basal insulin. NPH is usually given as a single injec­
tion at bedtime or twice daily, before breakfast and at bedtime. 
The long-acting analogues can be usually given once daily either 
in the evening or before breakfast, depending on the patient’s 
preference. They are significantly less likely to cause hypoglycemia

re significantly less likely to cause hypoglycemia 
overall and especially at night, compared with human NPH insu­
lin. The newest long-acting analogues, degludec and the U-300 
formulation of glargine, offer a modest further reduction of risk 
of hypoglycemia compared with U-100 glargine, especially for 
patients using lower doses and those with prior difficulties with 
hypoglycemia.221,223,288–291 There is evidence that hypoglycemia 
during initiation of basal insulin is associated with lower 

tiation of basal insulin is associated with lower adher­
ence and higher costs of care.292 Delays in the initiation of basal 
insulin as well as its appropriate titration over time and the pro­
gression beyond basal insulin have all been characterized as clinical 
inertia.293,294
Personalization of insulin applies particularly to the dose 
required. The necessary daily dosage of basal insulin ranges between 
10 and 200 units, and a given patient’s requirement must be deter­
mined by titrating to

s requirement must be deter­
mined by titrating to a fasting glucose target. Treatment can be 
started either with a fixed daily dose of 10 units or with a dose 
calculated as 0.1 to 0.2 units per kilogram body weight.159 Dosage 
should be increased at regular intervals, guided by SMBG before 
breakfast. Various titration schemes can be used with equal suc­
cess.295 Once-weekly titration by 2 to 4 units is commonly used, as 
is daily titration by a single unit every day until a target level of f

 a single unit every day until a target level of fast­
ing glucose is attained. For patients continuing metformin alone 
or with one additional oral agent, a typical dose of basal insulin 
required to approach the fasting glucose target is 0.4 to 0.5 units 
per kilogram, but with wide variation between patients. In many 
clinical trials the titration target has been set at 100 mg/dL (5.5 
mmol/L) or less, and early in the natural history of T2DM this is 
often attainable with little or no risk o

this is 
often attainable with little or no risk of hypoglycemia. However, 
when insulin is started after HbA1c levels have increased to above 
8%, an initial target of less than 120 mg/dL (<6.7 mmol/L) is safer 
and more realistic. The glucose goals and the importance of titration 
must be emphasized at each visit, as it has been shown that patients 
often do not remember the initial instructions in basal insulin dos­
ing.296 In most cases a patient-directed titration is safe and effective, 
ev

ient-directed titration is safe and effective, 
even to fasting goals below 100 mg/dL.297 Fig. 35.5 illustrates the 
responses of fasting glucose and HbA1c to this approach to starting 
basal insulin with use of human NPH versus insulin glargine.57 A 
stable level of fasting glucose can often be attained after 12 weeks 
of careful titration, with HbA1c levels lagging slightly behind as 
expected. Failure to maintain HbA1c close to 7% despite use of a 
regimen that includes properly titrated basa

of a 
regimen that includes properly titrated basal insulin calls for atten­
tion to postprandial hyperglycemia. 
Treating Postprandial Hyperglycemia
The greatest challenge to personalization is presented by therapy of 
postprandial hyperglycemia. Hyperglycemic exposure in untreated 
T2DM is quantitatively due more to overnight and between-meal 
(basal) hyperglycemia than to further elevations after meals. The 
predominance of basal hyperglycemia is more pronounced when 
overall control is poor 

 is more pronounced when 
overall control is poor and HbA1c is above 8%.298 Fortunately 
most glucose-lowering therapies are more effective in controlling 
basal hyperglycemia than limiting postprandial increments. Spe­
cifically, metformin, the long-acting secretagogues, basal insulin, 
DPP4 inhibitors, long-acting GLP1 agonists, and SGLT inhibitors 
have greater basal than postprandial effects. After initial therapy 
with one or several of these agents, most of the residual glycemic 
exposure 

e agents, most of the residual glycemic 
exposure is postprandial.299 As beta-cell function declines over 
time, postprandial hyperglycemia becomes more prominent, and 
maintaining HbA1c in a target range becomes more difficult. After 
150
200
A
B
100
0
4
8
12
16
20
24
FPG (mg/dL)
7.5
7
6.5
9
8.5
8
6
0
4
8
12
16
20
24
A1c (%)
NPH
Glargine
NPH
Glargine
• Fig. 35.5  Responses of fasting plasma glucose (A) and HbA1c (B) after 
initiation and titration of neutra

bA1c (B) after 
initiation and titration of neutral protamine Hagedorn (NPH) or glargine as 
basal insulin with continuation of prior oral therapies. (From Riddle MC, 
Rosenstock J, Gerich J. Insulin glargine 4002 study investigators. The 
treat-to-target trial: randomized addition of glargine or human NPH insulin 
to oral therapy of type 2 diabetic patients. Diabetes Care. 2003;26:3080–
3086.)
1399
therapy of basal glucose has been optimized,

litus
therapy of basal glucose has been optimized, the postprandial glu­
cose excursion is generally highest after breakfast, and the highest 
postprandial glucose is after dinner.300 At this stage of T2DM, 
which commonly is reached by 10 years after diagnosis, further 
therapy targeting postprandial control is needed. Use of prandial 
therapies demands consideration of the patient’s eating habits, and 
these can differ markedly between individuals and in each patient 
from day to day.
In addit

als and in each patient 
In addition to dietary strategies—more frequent but smaller 
meals and selection of foods that are absorbed less rapidly and 
have less tendency to increase glucose—several drugs can be help­
ful. The short-acting secretagogues modestly limit postprandial 
increments of glucose when taken at low dosage two or three 
times a day with meals.301,302 The alpha-glucosidase inhibitors 
reduce postprandial hyperglycemia significantly and have favor­
able effect

glycemia significantly and have favor­
able effects on weight, but their use is often limited by gastroin­
testinal side effects.
Short-acting insulins are more effective and are the most widely 
used approach. Traditionally, a direct change from basal insulin 
alone to full basal-bolus therapy with injections prior to two or 
three meals has been recommended. Recent studies suggest that 
adding a single injection of short-acting insulin (at the morning 
meal or at the largest meal) is just as e

morning 
meal or at the largest meal) is just as effective as basal-bolus ther­
apy at this stage and is less likely to cause weight gain or hypogly­
cemia.289,303,304 Initial dosage for the first prandial injection can 
be 4 to 6 units, with systematic titration upward by 1 to 2 units 
weekly, aiming for glucose prior to the next meal (or at bedtime if 
the dose is prior to dinner) approaching 120 mg/dL. When pran­
dial insulin is added to basal insulin with continuation of one or 
more oral ag

 insulin with continuation of one or 
more oral agents, optimal control often requires total daily insulin 
doses greater than 1 unit per kilogram of body weight daily. In the 
typical patient with insulin-resistant T2DM, there is usually little 
advantage to splitting basal insulin into two injections. If a second 
injection is needed it usually should be prandial insulin. As with 
basal-bolus insulin therapy in T1DM, close monitoring and effec­
tive DSMES are needed.
While adding prandial insu

While adding prandial insulin has been a common prac­
tice when basal insulin is not adequately effective in T2DM, an 
increasingly popular new option is the addition of a GLP1 recep­
tor agonist while continuing basal insulin. The shorter acting 
drugs in this class have the greater postprandial effect, but the lon­
ger acting ones—despite having less ability to control postprandial 
glucose excursions—may improve overall control sufficiently to 
equal the effect of adde

 control sufficiently to 
equal the effect of added prandial insulin. Compared with addi­
tion of prandial insulin, addition of a GLP1 agonist has several 
advantages: Less SMBG is required, hypoglycemia is less likely, 
and weight gain will be avoided.305 
Examples of Personalized Choices of Therapy
Fig. 35.6 illustrates four specific ways in which personalized 
choices of therapy may be applied.
Minimizing Cardiovascular Risks
Evidence supporting initial use of metformin for reducing car­
diov

ng initial use of metformin for reducing car­
diovascular risk has established this drug as the fundamental 
therapy for T2DM.29,306 Recently, the quest for further therapy 
for patients with high cardiovascular risk has favored early use 
of GLP1 agonists and SGLT inhibitors in some settings. An 
expert group has recently suggested that GLP1 agonists with 
evidence of cardiovascular benefit should be preferred over 
insulin as the first injected therapy for T2DM in the setting 
of established a

 therapy for T2DM in the setting 
of established atherosclerotic cardiovascular disease, provided 
they are locally available and affordable. Similarly, the expert 
group proposed use of an SGLT inhibitor with evidence for 
cardiovascular benefit as second-line therapy after metformin 
for patients with known or suspected heart failure or chronic 
kidney disease provided eGFR is not seriously reduced.90 It is 
still unknown whether benefits of these two classes of drugs for 
these conditions can

ese two classes of drugs for 
these conditions can be extrapolated to all drugs in each class 
and whether primary protection against cardiovascular or renal 
events will be found in the larger numbers of T2DM patients 
with relatively low cardiovascular risk. More specific informa­
tion as to which patients will derive the greatest cardiovascular 
benefit from these newer agents is needed and likely to become 
available soon.
Sulfonylureas and insulin have been suspected of increasing 
cardiova

nsulin have been suspected of increasing 
cardiovascular risk. Although direct evidence supporting this 
contention is weak, the availability of other agents providing 
glycemic control allows these classes to be moved down the pri­
ority list for patients with known cardiovascular disease. It seems 
especially desirable to avoid use of prandial insulin, which is 
much more likely to cause weight gain and hypoglycemia than 
basal insulin. Pioglitazone increases the risk of heart failure 
in susc

azone increases the risk of heart failure 
in susceptible patients and thus is also less appropriate in this 
setting. 
Minimizing Hypoglycemia
Hypoglycemia is of concern for reasons beyond its possible 
contribution to cardiovascular events. Older patients with a 
long duration of diabetes, especially those who have cognitive 
impairment or nephropathy, have greater risk of hypoglycemia-
related falls or other injuries.307,308 Fear of hypoglycemia can 
limit attainment of even conservative glyc

ia can 
limit attainment of even conservative glycemic treatment goals, 
either due to therapeutic inertia on the part of the provider or 
incomplete adherence by the patient. A prior history of hypo­
glycemia requiring assistance by another person is a strong risk 
factor for future hypoglycemia. When these considerations 
apply, therapeutic tactics should be adjusted to avoid hypoglyce­
mia. Metformin does not cause hypoglycemia when used alone, 
and drug choices after metformin with little or

, 
and drug choices after metformin with little or no additional 
risk include DPP4 inhibitors, GLP1 agonists, SGLT inhibitors, 
and TZDs.90,309 Combination therapy using these agents could 
provide considerable glucose-lowering power with little risk of 
hypoglycemia. Should insulin eventually become necessary, the 
newer long-acting insulins will cause the lowest risk of hypogly­
cemia. Finally, prior occurrence of severe hypoglycemia may be 
sufficient reason to increase the HbA1c target rang

ufficient reason to increase the HbA1c target range to 7% to 8% 
or even higher. 
Minimizing Weight Gain
Weight gain associated with the treatment of diabetes is of con­
cern to most clinicians and can be an overriding issue for some 
patients. A strategy to minimize weight gain would emphasize diet 
and exercise and use metformin as initial therapy. Both GLP1 ago­
nists and SGLT inhibitors are options as second-line therapy when 
weight is a major consideration. Surgical procedures that lead to

or consideration. Surgical procedures that lead to 
both weight loss and improved glycemic control are beyond the 
scope of this chapter, but are increasingly considered when weight 
control is a primary concern in T2DM.310–313 Because of their 
tendency to cause weight gain, sulfonylureas, pioglitazone, and 
prandial insulin are less desirable. Basal insulin is less likely to 
cause weight gain than prandial insulin and can be included in the 
regimen when necessary. 
1400 SECTION VIII    Disor

egimen when necessary. 
1400 SECTION VIII    Disorders of Carbohydrate and Fat Metabolism
• Fig. 35.6  Glucose-lowering medication in type 2 diabetes: overall approach. ASCVD, atherosclerotic cardiovascular disease; CKD, chronic kidney 
disease; CVD, cardiovascular disease; DPP-4i, dipeptidyl peptidase-4 inhibitor; eGFR, estimated glomerular filtration rate; GLP-1 RA, glucagon-like 
peptide-1 receptor agonist; HF, heart failure; NPH, neutral protamine Hagedorn; SGLT2i, sodium glucose transporter

amine Hagedorn; SGLT2i, sodium glucose transporter-2 inhibitor; SU, sulfonylurea; 
TZD, thiazolidinedione. (From Davies MJ, D’Alessio DA, Fradkin J, et al. Management of hyperglycemia in type 2 diabetes, 2018. A consensus report 
by the American Diabetes Association (ADA) and the European Association for the Study of Diabetes (EASD). Diabetes Care. 2018;41(12):2669–
2701.)
1401
Minimizing Costs
For many patients, including large populations in

For many patients, including large populations in developing 
countries and individuals with inadequate insurance coverage in 
developed countries, the cost of drugs limits management of dia­
betes. Despite the effectiveness of newer agents in clinical trials 
where they are provided free of charge, no therapy is successful 
when it is unaffordable. Fortunately, diet and exercise can be effec­
tive and are almost cost free. The least expensive drugs for the 
treatment of diabetes are metformin,

rugs for the 
treatment of diabetes are metformin, sulfonylureas, pioglitazone, 
alpha-glucosidase inhibitors, and human insulin. There can be 
marked differences in price between regions and between pharma­
cies, and websites are available to find the best prices.314 In the 
case of insulin, there is a name brand of human regular, NPH, and 
70/30 premixed human insulin (ReliOn) that is generally avail­
able in the United States for approximately 10% of the list price 
of analogue insulins. Huma

 10% of the list price 
of analogue insulins. Human insulin remains an effective option 
for treatment of T2DM, although its use may require twice-daily 
dosing of NPH, more consistent timing of meals, and slightly 
higher HbA1c targets (e.g., 7.5–8%) to limit hypoglycemia. A 
recent statement by the World Health Organization (WHO) has 
noted and endorsed these considerations for treatment of T2DM 
in low-resource settings.315 
Special Situations in Clinical Management
In addition to the princip

 in Clinical Management
In addition to the principles of treatment described for most 
T2DM patients in the usual ambulatory setting, some specific 
therapeutic challenges deserve further comment.
Severe Hyperglycemia at Diagnosis
Some patients will come to diagnosis of T2DM with very poor 
glycemic control, even ketoacidosis.316 Those who are especially 
ill, with serious dehydration, acidosis, impaired mental status, or 
serious concomitant illness will need hospitalization to receive 
intrave

ness will need hospitalization to receive 
intravenous fluids and insulin. Some will recover remarkably after 
in-hospital treatment, with substantial recovery of insulin secre­
tion and tissue responsiveness to insulin. Those with the best 
metabolic recovery may maintain good glycemic control with life­
style management and little or no oral therapy. How such patients 
differ from those who continue to need routine oral therapy or 
insulin is not entirely clear, but they have been described as

ot entirely clear, but they have been described as having 
ketosis-prone diabetes.317
Many other patients will be diagnosed when they have poor 
glycemic control, with HbA1c above 9%, but are not acutely ill. 
Several studies have suggested that intensive glycemic manage­
ment in this setting may also lead to a substantial remission of 
diabetes by reversal of glucolipotoxicity in a significant number of 
patients.318,319 After 2 to 4 weeks of treatment with oral combina­
tion therapy, basal ins

eatment with oral combina­
tion therapy, basal insulin, or basal plus prandial insulin seeking 
nearly normal glucose levels, HbA1c levels below 6.5% or 7% may 
persist for up to 1 year, even if pharmacotherapy is completely 
withdrawn. These proof-of-principle studies have not led to rec­
ommendations for this approach routinely in newly diagnosed 
T2DM but they do suggest that temporary use of insulin to attain 
excellent glycemic control soon after diagnosis is appropriate for 
some patients.

after diagnosis is appropriate for 
some patients. It has also been proposed that those with very high 
HbA1c at diagnosis, especially if accompanied by symptoms, could 
also be treated immediately with dual or triple therapy that may 
include basal insulin.38 Multiple drug combinations without insu­
lin are less well validated but may reduce HbA1c to target levels 
more consistently than a usual single-agent approach. However, 
independent of the regimen used, rapid reduction of HbA1c in 
patie

e regimen used, rapid reduction of HbA1c in 
patients who have had many months of poor control has potential 
risks. Aside from the risk of hypoglycemia from use of insulin 
by relatively inexperienced patients, there is potential for wors­
ening of retinopathy or neuropathy accompanying overly rapid 
reduction of glucose. As a general rule, patients diagnosed with 
HbA1c above 9% should seek a gradual improvement of control 
with HbA1c below 7% within 6 months, with continuation of at 
least me

within 6 months, with continuation of at 
least metformin after that. 
Youth-Onset T2DM
The worldwide increase of obesity has led to diagnosis of T2DM 
in susceptible individuals at earlier ages than in the past. One con­
sequence of this is an alarming increase of onset below age 20.320 
In some regions, the incidence of T2DM in children is approach­
ing that of T1DM. Recent studies of T2DM in adolescence have 
demonstrated severe insulin resistance and initially high levels of 
insulin secreti

ance and initially high levels of 
insulin secretion but a relatively rapid decline of beta-cell function 
that is not slowed by metformin therapy.321,322 At present there is 
little evidence as to how treatment of youths with T2DM can be 
improved, but it is evident that much greater effort to improve 
dietary and exercise patterns is needed in this population for both 
prevention and treatment.323 While the ability of weekly GLP1 
receptor agonists to conveniently control both glucose and weig

ists to conveniently control both glucose and weight 
suggests these agents may come to be important, their use in this 
setting is at present off label, and objective data are limited. 
Pregnancy
Management of diabetes during pregnancy is beyond the scope 
of this chapter. However, because it is centrally important in the 
natural history of T2DM, some important features must be men­
tioned. Women with risk factors for diabetes often first develop 
overt hyperglycemia during pregnancy, otherwis

op 
overt hyperglycemia during pregnancy, otherwise known as gesta­
tional diabetes mellitus (GDM). After delivery, some continue to 
have sufficient hyperglycemia to justify the diagnosis of T2DM, 
but more revert to having normal glucose levels. For the lat­
ter group, the risk of reappearance of hyperglycemia consistent 
with the diagnosis of T2DM in the absence of pregnancy is 50% 
to 70% after 15 to 25 years, a risk that is increased 10-fold to 
18-fold.324,325 Consequently, periodic follow

 to 
18-fold.324,325 Consequently, periodic follow-up with testing of 
glucose or HbA1c is indicated every 1 to 3 years to allow early detec­
tion. In addition to signaling a high risk of future development of 
diabetes, GDM poses more immediate risks to both mother and 
child. The likelihood of complicated delivery or neonatal compli­
cations is greatly increased by GDM. Furthermore, children with 
exposure to hyperglycemia in utero have increased risk of later 
obesity and/or T2DM.326 This ris

ed risk of later 
obesity and/or T2DM.326 This risk appears to be attenuated by 
good maternal glycemic control during gestation. Preconception 
counseling of women at high risk, routine screening for GDM 
during pregnancy, careful management of hyperglycemia during 
pregnancy, and systematic follow-up after delivery should be part 
of routine clinical practice. For women of childbearing age who 
already have diabetes, preconception planning and gestational 
management are even more urgently nee

gestational 
management are even more urgently needed, especially to limit 
the risk of fetal malformations resulting from poor glycemic con­
trol in the first 10 weeks of pregnancy. Glucose targets during ges­
tation are lower than advised in routine management of T2DM. 
The recommended target ranges are less than 95 mg/dL (<5.3 
mmol/L) for fasting glucose, less than 140 mg/dL (<7.8 mmol/L) 
for 1-hour postprandial glucose, and less than 120 mg/dL (<6.7 
1402
SECTION VIII    Disorders of Carbo

/dL (<6.7 
1402
mmol/L) for 2-hour postprandial glucose.327 Careful nutritional 
therapy is essential, and additional treatment with insulin is often 
needed. Metformin or the sulfonylurea glyburide are sometimes 
used instead of insulin, but there are concerns about their safety, 
particularly with sulfonylureas.327,328 For women with preexisting 
T2DM, insulin therapy is generally recommended during gesta­
tion. Whenever possible, an

ommended during gesta­
tion. Whenever possible, an experienced specialized team should 
assist in management of diabetes during pregnancy. 
Preventing Type 2 Diabetes Mellitus
The possibility that T2DM can be prevented in high-risk per­
sons has been formally tested in a series of clinical trials reviewed 
elsewhere. Lifestyle intervention can provide a reduction of 30% 
to 60% in progression to diabetes over a period of 3 to 5 years. 
This is sustained and correlates with weight loss. However, 

stained and correlates with weight loss. However, the 
average sustained weight loss in these trials was modest, on the 
order of 5%.329–331 Intervention with metformin was associated 
with a somewhat smaller reduction in progression to diabetes, 
although the benefit was similar to that of lifestyle in persons 
younger than 45 years of age, those with body mass index (BMI) 
greater than 35 kg/m2, and those with an fasting plasma glucose 
level greater than 110 mg/dL (>6.1 mmol/L). Metformin had

reater than 110 mg/dL (>6.1 mmol/L). Metformin had 
less effect in patients older than 60 years of age, those with a 
BMI of less than 30 kg/m2, and those with a fasting plasma glu­
cose of less than 100 mg/dL (<5.6 mmol/L). Metformin’s ability 
to reduce progression to diabetes has been shown to be durable 
and cost effective.332
Acarbose has also been shown to reduce progression to diabetes 
without evidence of diminished efficacy in different subgroups. 
The TZDs provide sustained improvement

subgroups. 
The TZDs provide sustained improvement of insulin sensitivity 
and can delay onset of diabetes. However, concern regarding pos­
sible long-term effects on weight, heart, and bones has limited 
the use of this class for prevention. GLP1 receptor agonists have 
also shown promise, but the expense and initial adverse effects 
preclude their promotion for this indication at present. Bariatric 
surgery has also been shown to reduce incident diabetes in more 
severely obese patients.
The s

t diabetes in more 
The success of the lifestyle interventions is impressive, showing 
that by using various techniques at-risk individuals can achieve phys­
iologically relevant changes in body weight. It is unknown whether 
lifestyle plus medications provides even greater benefit. It seems rea­
sonable to screen on the basis of the recommendations outlined ear­
lier, while recognizing that patients with prediabetes (HbA1c >5.7% 
and particularly ≥6%, fasting plasma glu

1c >5.7% 
and particularly ≥6%, fasting plasma glucose ≥100 mg/dL [≥5.6 
mmol/L] and particularly ≥110 mg/dL [≥6.1 mmol/L], or impaired 
glucose tolerance) are the best candidates for preventive strategies. 
High-risk persons should be counseled about nutritional approaches 
to weight loss, instructed to increase physical activity, and monitored 
for progression of dysglycemia. Translation of the evidence found 
in the DPP study to practical expansion across the United States 
has been evolving 

nsion across the United States 
has been evolving through the CDC National Diabetes Prevention 
Program. The program includes a structured curriculum of 22 ses­
sions over 1 year in a group setting or online programs delivered 
through a diverse group of public and private organizations. Each 
local program is certified through the CDC Diabetes Prevention 
Recognition Program to maintain standards, promote utilization, 
and document outcomes. Providers must increase identification of 
those with

viders must increase identification of 
those with prediabetes or other high-risk characteristics and direct 
patients to the programs available in the community.333,334 The sta­
tus of drug therapy, notably with metformin, is less clear, in part 
because significant diabetes complications are unlikely to develop 
in the relatively short window of time during which fasting glucose 
levels increase from 100 to 126 mg/dL (5.6–7 mmol/L). On the 
other hand, metformin therapy brings little risk and 

er hand, metformin therapy brings little risk and offers sub­
stantial benefit for some individuals, notably those with BMI of 35 
kg/m2 or more, those under age 60 years, and women with previ­
ous gestational diabetes.36 Therefore on balance, consideration of 
metformin therapy in patients who are at particularly high risk is 
recommended.36 
Future Directions
Present-day management of T2DM is significantly more effec­
tive and easier for patients than in the past, and it promises to 
improve f

ts than in the past, and it promises to 
improve further. Development of new drugs and smart devices for 
tracking glucose levels and administering therapies continues and 
challenges our ability to evaluate and assimilate these new tools. 
Notable unmet needs concern controlling glucose after meals, 
maintaining metabolic control during acute illness and hospital­
ization, and limiting weight gain. However, it can be argued that 
the main limitation of therapy at present is not a lack of tools 

tion of therapy at present is not a lack of tools and 
treatments, but a variety of economic, psychologic, and organiza­
tional barriers to using them to full advantage.335 Practical, cost-
effective public health approaches are needed to slow the increase 
in incidence and prevalence of T2DM.336 Screening for diabetes or 
prediabetes may be cost saving and should be made more efficient. 
Globally, there continues to be inequality in access to medical care 
for all conditions, including diabetes

dical care 
for all conditions, including diabetes. Although the prognosis for 
people with diabetes has never been better, the greatest challenges 
they face relate to the complexity and cost of care. If both the 
means of managing diabetes and the organizational structure of 
health systems are improved simultaneously, the morbidity and 
mortality of T2DM may be greatly reduced in the near future.
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1403
36
Type 1 Diabetes Mellitus
MARK A. ATKINSON, DAYNA E. MCGILL, EYAL DASSAU, AND LORI LAFFEL
W
here to begin to tell the story of the 

I LAFFEL
W
here to begin to tell the story of the disorder we now 
refer to as type 1 diabetes mellitus (T1DM) represents 
somewhat of a literary challenge. Does one begin with 
the earliest writings scripted thousands of years ago conveying 
the disease’s symptoms, or is it more fitting to describe exciting 
works performed in the 1800s and early 1900s that defined both 
the anatomy of and physiologic roles for the pancreas and insu­
lin-secreting beta cells?1 Other pieces of literature initiat

ng beta cells?1 Other pieces of literature initiate their 
narrative on T1DM by sharing an account of the fast-paced and 
ultimately Nobel Prize–winning efforts in the 1920s by Banting 
and Best,2 when the ability to purify insulin from animal pan­
creata brought forward a means to sustain life to those who, in the 
absence of such an intervention, met an early demise.
We have elected to begin this chapter by building on the story 
of a German pathologist named Martin Schmidt,3 who, in 1902, 
no

thologist named Martin Schmidt,3 who, in 1902, 
noted a small peri-islet cellular infiltrate on microscopic evalua­
tion of the pancreas obtained at autopsy from a 10-year-old child 
with diabetes. This effort represented a key early investigation 
seeking to better define the pathogenesis of the disorder, a pro­
cess that was continued by many over the ensuing century. Works 
shortly thereafter by Shields Warren4 in the 1920s drew attention 
to the relationship between this infiltrate and the a

the relationship between this infiltrate and the age of diabe­
tes onset. The term used to define this pancreatic feature, insu­
litis, was coined in 1940 by yet another pathologist, Hanns von 
Meyenburg.5 Studies of pancreatic disease over ensuing decades 
taught us much regarding this inflammatory lesion, including its 
Diagnosis, 1404
Animal Models, 1404
Histopathology, 1405
Natural History of T1DM—Historical Concepts, 1407
Genetics, 1408
Environmental Factors, 1416
Natural Hi

etics, 1408
Natural History of T1DM—Emerging Concepts, 1417
Immunotherapy for the Prevention and Reversal of T1DM, 1419
Pancreas and Islet Cell Transplantation, 1422
Disorders Associated With Immunity to Insulin/Insulin Receptor, 1423
Clinical Presentation, 1424
Treatment, 1424
Acute Diabetic Emergencies, 1435
KEY POINTS
 • 
Type 1 diabetes mellitus (T1DM) is a disorder resulting from 
a chronic autoimmune destruction of the insulin-producing 
pancreatic beta cells.

s an islet immune 
infiltrate derived from a variety of immunologic phenotypes, is 
reduced in size and weight, and is subject to unusual exocrine 
features.
 • 
Significant advances in terms of disease management, afforded 
by technological innovations related to insulin analogues, ad­
vanced diabetes technologies, and more, set the foundation for 
marked reductions in hemoglobin A1c, enhanced diabetes care, 
and improved biomedical and psychosocial outcomes.
 • 
Although there have been many

ocial outcomes.
 • 
Although there have been many advances in the management 
of T1DM, this disease remains associated with many acute- and 
chronic complications.
 • 
Numerous metrics (costs, patient outcomes, etc.) demonstrate 
that specialists (i.e., endocrinologists) provide more effective 
care for those with T1DM than nonspecialists, with best results 
arising from multidisciplinary, individualized care.
1404
relative infrequen

Carbohydrate and Fat Metabolism
relative infrequency in older individuals diagnosed with diabetes, 
its association with reduction in beta-cell mass, the identification 
of pseudoatrophic islets (i.e., islets devoid of insulin-containing 
cells), a preferential targeting of insulitis for beta cells containing 
insulin, and many other findings eventually deemed seminal.6,7 
These conditions, when joined together with immunologic and 
genetic studies of living patients in the 1970s, noting the pre

es of living patients in the 1970s, noting the presence 
of self-reactive antibodies (i.e., autoantibodies) against islet cells 
in persons with T1DM and that disease susceptibility was asso­
ciated with major histocompatibility complex (MHC) molecules 
known to influence immune system functions, formed three lines 
of evidence pointing to an autoimmune basis for T1DM.8 Built 
on this foundation, this chapter will share current views on the 
autoimmune nature of T1DM (including its natural histo

immune nature of T1DM (including its natural history and 
pathogenesis); emerging views on genetics, disease biomarkers, 
and epidemiology; increased information on the contribution of 
beta cells to their own demise; and perhaps most importantly, how 
treatments are advancing for this disease—all as we consider how 
this information is vital to the optimal diagnosis and optimal care 
of individuals with T1DM.
Diagnosis
A diagnosis of diabetes has historically included an elevated fast­
ing bloo

s historically included an elevated fast­
ing blood glucose level, any glucose value higher than 200 mg/dL 
(11 mmol/L) with symptoms of hyperglycemia, or an abnormal 
2-hour oral glucose tolerance test (OGTT).9 American Diabetes 
Association (ADA) guidelines for the diagnosis of diabetes were 
modified in 2009 to include a hemoglobin A1c (HbA1c) value 
greater than 6.5%.10 Under certain settings (e.g., obesity, racial sta­
tus other than Caucasian) and particularly among adults, the diag­
nosis

an) and particularly among adults, the diag­
nosis of T1DM versus type 2 diabetes mellitus (T2DM) can prove 
quite challenging. At present, the best criterion for separating the 
two disorders resides in laboratory identification of any one of a 
number of islet cell autoantibodies (also known as T1DM-associ­
ated autoantibodies; i.e., anti-insulin autoantibodies [IAA], anti–
glutamic acid decarboxylase [GADA], anti–insulinoma-associated 
antigen 2 [IA2A], or anti-zinc-transporter 8 [ZnT8A]). Li

 2 [IA2A], or anti-zinc-transporter 8 [ZnT8A]). Literally 
hundreds of studies over the past three decades have suggested 
that the presence of these autoantibodies provides high sensitivity 
for diagnosing persons with T1DM.11 Indeed, more than 90% of 
Caucasian children presenting with diabetes express at least one of 
these four T1DM-associated autoantibodies.12 In terms of disease 
specificity, T1DM-associated autoantibodies are typically positive 
in less than 1% to 2% of unaffected (i.e., 

itive 
in less than 1% to 2% of unaffected (i.e., non-T1DM) individu­
als, further validating their diagnostic utility. However, among 
African-American and Latino children and adolescents in the 
United States diagnosed with diabetes, almost one half lack any 
T1DM-associated autoantibody. Many patients from these ethnic 
minorities in the United States present clinically as if they have 
early-onset T2DM (e.g., mild ketosis, slow symptomatic onset), 
some have attendant risk factors such as ob

set), 
some have attendant risk factors such as obesity, and many lack 
human leukocyte antigen (HLA) alleles associated with T1DM. 
Increasingly diverse genetic admixtures, due to geographic migra­
tion and social changes (e.g., multiracial offspring), further con­
tribute to diagnostic complexity.
In childhood and adolescence, two peaks of T1DM presen­
tation occur: a smaller peak between 5 and 7 years of age and 
a larger peak at or near puberty.13 Although most autoimmune 
disorders dispropo

ty.13 Although most autoimmune 
disorders disproportionately target females, T1DM affects males 
slightly more than females. The incidence of T1DM varies with 
seasonal changes and birth month. Incidence of T1DM diagnosis 
is higher in autumn and winter, whereas being born in the spring 
is associated with an increased likelihood for T1DM.14 Interest­
ingly, the development of T1DM-associated autoimmunity (i.e., 
the formation of islet autoantibodies) in the months to years prior 
to the onset o

dies) in the months to years prior 
to the onset of symptomatic T1DM also shows a degree of sea­
sonal synchronization.
The measurement and presence of T1DM-associated autoan­
tibodies have also fueled much debate regarding the percentage of 
T1DM cases that are errantly misclassified as T2DM. Indeed, it is 
conceivable that 5% to 15% of adults diagnosed with T2DM may 
have T1DM, given the frequency of T1DM-associated autoanti­
bodies in populations diagnosed with T2DM.15 This is, in effect, 
a 

ons diagnosed with T2DM.15 This is, in effect, 
a problem in health care provider recognition of the potential 
for T1DM disease in adult populations combined with a lack of 
widespread screening for such autoantibodies in settings in which 
screening would seem warranted. If this assertion is correct, given 
the vastly greater number of persons diagnosed with T2DM (rela­
tive to that of T1DM), the number of actual T1DM cases in a 
given population may be massively underestimated. This is thera­

on may be massively underestimated. This is thera­
peutically unfortunate because an accurate diagnosis of T1DM is 
vital; persons misdiagnosed as having T2DM when they indeed 
have T1DM experience higher HbA1c, have risk of diabetic ketoac­
idosis (DKA) due to use of noninsulin therapies, and have acceler­
ated progression toward micro- and macrovascular complications..
Efforts to differentiate adult-onset T1DM from T2DM (Table 
36.1) have also resulted in a series of proposed new disease clas­

resulted in a series of proposed new disease clas­
sifications, most notably ketosis prone diabetes and latent auto­
immune disease of adults.16 The lack of firm diagnostic criteria 
dampens enthusiasm for adopting these presumed new disease 
entities as novel categories for diabetes. It is also important to 
note that T1DM patients can manifest insulin resistance, a feature 
most often associated with T2DM. Specifically, T1DM patients 
(including those who are autoantibody-positive) may present

g those who are autoantibody-positive) may present with 
diabetes characterized by high serum levels of fasting insulin or 
C-peptide but loss of stimulated insulin secretion.
As noted previously, some Caucasian children (∼10%) are devoid 
of a T1DM-associated autoantibody at disease diagnosis (i.e., time of 
hyperglycemic onset), raising questions as to whether such persons 
lost their autoantibody expression (or at least the ability to detect 
it with laboratory testing) by the time of diagnos

it with laboratory testing) by the time of diagnosis, or whether a 
diagnosis of T1DM is accurate. The notion of autoantibody loss 
versus the presence of a different form of diabetes is also difficult to 
ascertain because many have HLA alleles associated with suscepti­
bility for T1DM; are not insulin resistant; present with DKA; and, 
with time, lose their C-peptide secretion. With appropriate labora­
tory testing, notation of such persons as T1DM patients negative for 
T1DM-associated autoan

T1DM patients negative for 
T1DM-associated autoantibodies can be an appropriate diagnosis. 
This said, advances in understanding the heretofore underappreci­
ated disease classifications of monogenic diabetes, including those 
of maturity onset of diabetes in youth, warrant genetic testing of 
diabetes cases diagnosed in the very young (i.e., <1 year of age).17 
Nonobese Diabetic Mouse Model
The nonobese diabetic (NOD) mouse is, without question, the 
most intensively studied anim

thout question, the 
most intensively studied animal model for T1DM.18 The frequency 
of diabetes among different colonies ranges from 30% to 100% 
among female NOD mice, suggesting that housing conditions 
(i.e., environment) represent an important variable. Unlike human 
T1DM that has a relatively equal distribution among genders, a 
1405
Chapter 36  Type 1 Diabetes Mellitus
strong female bias with respect to disease exists in NOD mice. As 
with T1DM of humans, diabetes in the NOD mouse is pol

h T1DM of humans, diabetes in the NOD mouse is polygenic 
with specific MHC class I and II molecules being central for dis­
ease pathogenesis19 and many other genetic loci bearing relatively 
small contributions neither necessary nor sufficient for disease.20 
NOD mice, like humans, produce IAA before developing diabetes, 
although T cells, not autoantibodies, appear to mediate the beta-
cell destruction. The insulitis lesion, the hallmark facet of T1DM, 
appears more intense and quantitatively 

of T1DM, 
appears more intense and quantitatively obvious in NOD mice as 
compared with humans. Interestingly, a relative overrepresentation 
of T cells bears a receptor specific for the beta-cell antigen, insulin. 
Although a majority of studies have provided evidence for a central 
role of T-cell autoimmunity directed at insulin,21 to be clear, there 
is considerable debate whether any given beta-cell antigen is pri­
mary (i.e., the first, major driver of disease development in an auto­
immune

r driver of disease development in an auto­
immune response). Studies of islet beta-cell mass indicate that both 
destruction and replication/regeneration are present months before 
the onset of T1DM, with an acceleration of beta-cell destruction 
at disease onset.22 A seeming plethora of published studies high­
light the therapeutic potential from beta-cell replication/regenera­
tion in NOD mice or other rodent strains.23–25 However, recent 
convincing evidence suggests that in terms of beta-ce

vincing evidence suggests that in terms of beta-cell replication, 
rodent models differ dramatically from human physiology, with 
beta-cell replication being a rare event in humans from adolescence 
through adulthood.26 One of the primary concerns for the NOD 
mouse relates to the relative ease by which the disease can be pre­
vented or reversed.18 One can observe clear therapeutic benefits in 
the NOD mouse (e.g., preserved C-peptide secretion), yet only a 
limited number these findings have be

 yet only a 
limited number these findings have been effectively translated to 
humans. Animal models (unfortunately) represent somewhat of an 
idealized situation characterized by features that are not reflective 
of the sociologic and lifestyle settings in humans, such as limited 
genetic diversity through selective inbreeding, housing in highly 
controlled environments devoid of pathogens, uniform diets, and 
several other facets. It remains unclear which of these or other dif­
ferences under

nclear which of these or other dif­
ferences underlie the generally disappointing translation. 
Induced Models of T1DM
Diabetes, and even insulitis, can be induced in several strains of 
rodents with drugs that induce islet cell destruction and broadly 
activate immune responses, including those directed against beta-
cell antigens. For example, the drug streptozotocin is directly toxic 
to islet beta cells. In high doses, it rapidly induces diabetes (i.e., 
hyperglycemia). In low doses, a more 

etes (i.e., 
hyperglycemia). In low doses, a more chronic diabetes develops 
that is likely to have some immunologic derivation, meaning 
pathogenesis tied to immune responses against beta cells.27 Other 
models are used less frequently. 
Histopathology
As noted earlier, T1DM in humans is characterized by selective 
destruction of beta cells within the pancreatic islets7 (Fig. 36.1). 
Although certain exceptions exist, an overall assimilation of the 
  Characteristic Comparison of Type 1 Versus

the 
  Characteristic Comparison of Type 1 Versus Type 2 Diabetes Mellitus
Type 1
Type 2
Nature
Very different
Autoimmune disorder marked by destruction of 
insulin-producing beta cells and loss of insulin 
A disorder of insulin deficiency involving an interplay 
between both pancreatic and extrapancreatic contribu­
tions to disease
Symptoms
Partial overlap
Rapid onset; very high to extremely high blood 
glucose levels; polyphagia; polydipsia, polyuria; 
ketoacidosis
M

 polyphagia; polydipsia, polyuria; 
Mild to moderate onset; modest to high elevations in blood 
glucose; mild polydipsia/polyuria; fatigue; visual changes/
headache
Onset
Sudden (symptoms for days to weeks)
Slower onset (symptoms for months to years)
Typically different but overlap
Family history of autoimmune disease but particularly 
type 1 diabetes mellitus (10-fold increased risk vs. 
general population)
Overweight/obese; poor diet; sedentary lifestyl

n)
Overweight/obese; poor diet; sedentary lifestyle; ethnicity 
(higher in African Americans, Hispanics); family history of 
type 2 diabetes mellitus; history of gestational diabetes
Onset age
Typically early life through adolescence but can occur 
at any age
Typically adults but trending toward earlier age of onset
Treatment strategy
Typically different
Absolute requirement for insulin (multiple daily injec­
tions or insulin pump); self-management lifestyle 
modi

 or insulin pump); self-management lifestyle 
modification (monitor food types, exercise, etc.)
Dietary modifications and exercise alongside oral agents (for 
most); increasingly greater percentage of patients require 
insulin over time
Can it be prevented?
Not at present (subject of major research efforts); 
future cases can be predicted by autoantibodies 
and genetics
Yes, for more than half of potential cases, with dietary 
modifications and exercise
Can it be reversed?
Very di

fications and exercise
Not at present (subject of major research efforts)
No, but for a limited few; patients can see disease managed 
and risk for complications reduced through diet modifica­
tions, exercise; growing evidence for disease improve­
ments through combination therapies
Complications
Mostly similar, but some variation
Acute emergencies of hypoglycemia and ketoacidosis 
leading to hypoglycemic unawareness; chronic 
effects of hyperglycemia can lead 

eness; chronic 
effects of hyperglycemia can lead to retinopathy, 
nephropathy, neuropathy, cardiovascular disease, etc.
leading to hypoglycemic unawareness; chronic effects 
of hyperglycemia can lead to retinopathy, nephropathy, 
neuropathy, cardiovascular disease, etc.
TABLE 36.1
1406
literature supports that T1DM patients with a disease onset at age 
0 to 14 years and within 1 year

ease onset at age 
0 to 14 years and within 1 year of diagnosis show more inflamed 
islets (68%) and fewer islets with residual beta cells (39%) than in 
patients with onset at 15 to 39 years of age.28 This finding suggests 
that a more vigorous autoimmune response occurs when disease 
develops in young children.
The immunotype of insulitic lesions shows a predominance of 
CD8+ T lymphocytes and macrophages, but cells of other phe­
notypes are also observed (e.g., CD4+, CD20+, and CD68+).29,30 

so observed (e.g., CD4+, CD20+, and CD68+).29,30 
More recent studies have evaluated immunotype as a function 
of age, with younger age of onset associated with higher levels of 
CD20+ B cells, CD45+ cells, and CD8+ T cells in insulitis lesions, 
alongside fewer insulin-positive islets.28,31 Conversely, infiltrates 
with fewer CD20+ cells are observed in T1DM patients who are 
older at onset and associated with lower levels of CD45+ cells and 
CD8+ T cells, as well as more insulin-positive islet

8+ T cells, as well as more insulin-positive islets. These same 
studies also show that islet CD8+ T cells express T-cell receptors 
that bind MHC class I tetramers loaded with the beta-cell auto­
antigen G6P2 and other target peptides in recent-onset T1DM 
patients.31 These studies are also in agreement with recent findings 
that the T cells invading pancreatic islets are, in fact, directed at 
beta-cell antigens.30 In contrast to long-standing dogma, the pres­
ence of insulitis in human T1DM i

dogma, the pres­
ence of insulitis in human T1DM is quantitatively limited, with an 
expert panel establishing a standardized definition for this lesion of 
three islets, containing more than 15 CD45+ cells, in a pancreas.6
Somewhat surprisingly, inflammation is also present in pan­
creatic exocrine tissue in T1DM. Specifically, chronic inflamma­
tion, including enhanced CD8+ T-cell infiltration (and, to a lesser 
degree, CD4+ and CD11c+ cells), is present in the exocrine pan­
creas in T1DM subj

is present in the exocrine pan­
creas in T1DM subjects.32 Other studies report a similar propen­
sity for neutrophil invasion of the pancreas (along with decreased 
peripheral neutrophil counts) in T1DM.33 The propensity of this 
organ for inflammation/pancreatitis induced by multiple factors 
(e.g., hypertriglyceridemia, virus infection, drugs) could be a result 
of a susceptibility gene that affects tissue-based inflammation or 
other facets that have, in the past, been considered “leakiness.”

at have, in the past, been considered “leakiness.” 
Importantly, the inflammatory process in the exocrine pancreas 
appears to be subclinical, as most new-onset T1DM patients do 
not present with symptoms of pancreatic inflammation.
Recent studies assessing the persistence of insulin production 
in patients with long-standing T1DM (i.e., stimulated C-pep­
tide production) using ultrasensitive assays have overturned the 
long-standing dogma that residual C-peptide secretion is rare.34 
Emerging h

esidual C-peptide secretion is rare.34 
Emerging histopathologic evidence supports this notion through 
identification of insulin-positive islets, albeit extremely lim­
ited in number, in patients with long-standing T1DM.35 The 
non–­insulin-producing cells of the islets (i.e., alpha, delta, pan­
creatic polypeptide; see Chapter 33) remain in patients with long-­
standing T1DM, and these remaining islets, lacking insulitis and 
beta cells, are termed pseudoatrophic. This information sets the 
st

rmed pseudoatrophic. This information sets the 
stage for another remarkable feature of the pancreas of patients 
with new-onset T1DM, namely heterogeneity of islet lesions. His­
topathologic examination of the same section of pancreas showed 
that a normal islet with no immune infiltrate can coexist with 
an islet containing beta cells with intense infiltration, as well as a 
pseudoatrophic islet that has no infiltrate (Fig. 36.2). This spotti­
ness of the pathologic process is reminiscent of t

ness of the pathologic process is reminiscent of the destruction of 
areas of the skin in patients with vitiligo, in which melanocytes are 
destroyed in patches. Such heterogeneity of lesions may underlie 
the variable natural history of T1DM, in which the time period 
between initial beta-cell autoimmunity and overt symptoms can 
be one of months, years, or even many decades. Additionally, pan­
creata from non-T1DM but autoantibody-positive organ donors 
often show limited to no insulitis but h

n donors 
often show limited to no insulitis but have shown distinct histo­
logic abnormalities, such as an increased proinsulin to insulin area 
ratio,36 that match functional alterations. Such information has 
been considered to support the notion that T1DM is a relapsing/
remitting disease,37 with waves of immunologic destruction.
The islets of T1DM and T1DM autoantibody-positive nondia­
betic patients overexpress class I HLA antigens, interferon-alpha, 
and potentially Fas molecules.38,39 Th

ron-alpha, 
and potentially Fas molecules.38,39 The specific means by which the 
immune system destroys beta cells is not known. Molecules such 
as Fas may be important because T cells expressing Fas ligand can 
induce apoptosis of beta cells. This said, more enthusiasm surrounds 
the notion that cytokines (e.g., interleukin 1 [IL1]) and CD8+ cyto­
toxic T lymphocytes are likely to be major contributors to beta-
cell destruction. With respect to what might induce these features, 
searches for vi

what might induce these features, 
searches for viral particles and viral RNA within islets of patients 
with new-onset diabetes have largely been unrewarding. However, 
newer technologies and concepts should facilitate additional stud­
ies, and it is likely that there will be heterogeneity among patients.
Finally, reduced pancreatic weight has been demonstrated at 
autopsy or in organ donors with recent-onset T1DM, as well as 
in those with long-standing disease.40 Reduced pancreatic volume 
ha

-standing disease.40 Reduced pancreatic volume 
has also been shown in living patients with T1DM through nonin­
vasive imaging techniques.41 Mechanisms underlying loss of pan­
creatic size or weight in T1DM are not well understood but have 
largely been hypothesized to result from loss of insulinotrophic 
Islet cells in type 1 diabetes 
Pancreas in type 1 diabetes 
• Insulitis (mixed mononuclear, adjacent
or within islet)
• Loss of  cells (increase with disease
duration)
• Hyper-expression of c

tics of the pancreas in T1DM. (A) Islet infiltrate (i.e., insulitis) seen in a 
patient with recent-onset T1DM. Immunohistochemistry shows the intra-islet presence of CD3-positive 
cells (brown) and glucagon-producing alpha cells (pink). Histologic features of islet cells (B) and gross 
pathologic characteristics of the pancreas (C) associated with the natural history of T1DM (i.e., preonset, 
onset, post onset). MHC, major histocompatibility complex; T1DM, type 1 diabetes mellitus. (From Atkin­

plex; T1DM, type 1 diabetes mellitus. (From Atkin­
son MA, Eisenbarth GS, Michels AW. Type 1 diabetes. Lancet. 2014;383:69–82, used with permission.)
1407
effects on the exocrine pancreas, as islets constitute only 1% to 
2% of the entire pancreatic volume (see the Pancreatic Exocrine 
Abnormalities in T1DM section).
Mechanisms of Beta-Cell Death in T1DM
The mechanism(s) by which beta cells die in T1DM remains some­
what controversial. A majority of studies s

 some­
what controversial. A majority of studies support necrotic cell death, 
mediated by cytotoxic T cells in response to self-antigens, as one 
likely mechanism. Evidence from studies of human T1DM pancreas 
noting the presence of CD8+ T cells and macrophages is consistent 
with this concept.7 T cells can induce necrosis following release of 
cytolytic granules containing granzymes and perforin, which act on 
the beta-cell membrane. Ischemia resulting from impaired vascular­
ity, a process th

esulting from impaired vascular­
ity, a process that may result from hyperexpression of extracellular 
matrix materials (e.g., hyaluronan, hyaluronan-binding proteins) 
may also lead to necrotic cell death.42 Additional mediators of 
necrotic cell death finding support in T1DM studies include reac­
tive oxygen species, which are capable of inducing mitochondrial 
injury and loss of cell membrane integrity through oxidative DNA 
damage, lipid peroxidation, and protein damage.43 Finally, necrosis 

xidation, and protein damage.43 Finally, necrosis 
of beta cells may release factors, including post-translationally modi­
fied antigenic peptides44 or modifications of “normal” antigens (i.e., 
hybrid peptides45) that further stimulate the immune response and 
thereby perpetuate beta-cell death by necrosis.
Beyond necrosis, apoptosis has also been implicated for its role 
in beta-cell death in T1DM through activities largely linked to 
the janus kinases (JAK1, JAK2) and nonreceptor tyrosine-pro

 kinases (JAK1, JAK2) and nonreceptor tyrosine-protein 
kinase (TYK2) pathways.46 Immunity finds a link here, with apop­
tosis likely driven by cytokines, including tumor necrosis factor 
(TNF), that activate Fas/FasL, TNFR1, and TNFR2. Apopto­
sis can also be initiated through the B-cell lymphoma 2 (BCL2) 
pathway via cytokine deprivation or endoplasmic reticulum stress, 
leading to caspase activation. Apoptosis due to the activity of cas­
pases leads to apoptotic cell morphology, nuclear fragm

 leads to apoptotic cell morphology, nuclear fragmentation, 
and chromatin condensation—facets that have been observed in 
human T1DM pancreas. The notion of the so-called unfolded 
protein response (i.e., how a cell responds to the accumulation 
of misfolded proteins in the endoplasmic reticulum) and how it 
may modulate beta-cell inflammation and death in T1DM is also 
gaining interest in T2DM.47
In reality, beta-cell death in T1DM likely sees contributions 
from both of these mechanisms (i.e.

contributions 
from both of these mechanisms (i.e., necrosis and apoptosis). The 
dynamic glycemic fluctuations, ultimately including overt hypergly­
cemia seen in the period of pre-T1DM (see the Stages in the Natural 
History of T1DM section), support the notion of ever-increasing 
func­tional changes in the beta cells, possibly as a result of immuno­
logic stressors or increased metabolic demand for residual beta cells. 
Natural History of T1DM—Historical Concepts
Perhaps the most helpful and 

—Historical Concepts
Perhaps the most helpful and beneficial guide to studies pertain­
ing to the natural history of T1DM was the model put forward 
in 1986 by a previous author of this chapter, the late Dr. George 
Eisenbarth.48 In this model (Fig. 36.3), persons destined to develop 
T1DM were assumed to begin life with a full cadre of beta cells. 
A triggering insult, likely environmental, was proposed to initiate 
a process involving recruitment of antigen-presenting cells. The 
antigen-prese

nt of antigen-presenting cells. The 
antigen-presenting cells would sequester self-antigens released by 
injured beta cells, followed by their transport to pancreatic lymph 
Ki67
C,D
A
C
D
E
F
B
E,F
INS
Ki67 INS
Ki67 INS
CD3 GCG
CD3 GCG
CD3 GCG
• Fig. 36.2  Lobular variability in insulin+ Islets and insulitis. Islets were imaged from a 13-year-old patient 
with type 1 diabetes mellitus for 5 years (nPOD 6243). Serial paraffin sections were stained for Ki67 
plus insulin (INS) (A, C, and E) and C

d for Ki67 
plus insulin (INS) (A, C, and E) and CD3 plus glucagon (GCG) (B, D, and F), and islets were subtyped as 
described in research design and methods. Five insulin+ islets (A) are seen in a lobule adjacent to a lobule 
with two insulin− islets in the patient with type 1 diabetes mellitus (A and B, blue arrows). Three insulin+ 
islets had insulitis (B, black arrows), and both insulin− islets did not have insulitis. One of the insulin+ islets 
with insulitis is shown at higher magnificatio

ts 
with insulitis is shown at higher magnification (C and D), as well as an insulin− insulitis− islet (E and F). Few 
islet cells were Ki67+ (A and C), indicating no effect of insulitis on proliferating cell numbers. Scale bars: A 
and B, 500 μm; C–F, 50 μm. (From Campbell-Thompson M, Fu A, Kaddis JS, et al. Insulitis and β-cell mass 
in the natural history of type 1 diabetes. Diabetes. 2016;65[3]:719–731.)
1408
nodes, where they are 

ohydrate and Fat Metabolism
nodes, where they are subsequently presented to autoreactive T 
cells. These T cells, rogue constituents brought to life because of 
genetically driven failures of thymic deletion (i.e., central toler­
ance) combined with defects in mechanisms designed to induce 
peripheral immune tolerance, would come into play.49 This toxic 
duo imparting lack of tolerance formation, again in the context of 
genetic susceptibility, would allow for migration of self-reactive T 
cells

ould allow for migration of self-reactive T 
cells to islets, mediating beta-cell killing and promoting further 
inflammation.50 Symptoms of the disease were thought to appear 
when 85% to 90% of pancreatic beta cells had met their demise. 
In the final stage of the model, the autoimmune process ends with 
the complete elimination of beta cells.
Although this model faithfully served as a guide for multiple 
decades, and certainly the majority of its aspects hold true to 
this day, some constitue

its aspects hold true to 
this day, some constituents are under states of correction (e.g., 
the aforementioned notion that C-peptide production is lost in 
T1DM at onset) or are still subject to intense investigation—fac­
ets that will be discussed throughout this chapter (see the “Natural 
History of T1DM-Emerging Concepts” section). 
Natural History of T1DM From Long-Term 
Family and Population Studies
The major longitudinal clinical epidemiologic studies of T1DM 
have included BABYDIAB in Ge

ogic studies of T1DM 
have included BABYDIAB in Germany51; DIPP (T1DM Predic­
tion and Prevention) in Finland52; DAISY (Diabetes Autoimmunity 
Study in the Young) in Denver, Colorado53; TEDDY (The Environ­
mental Determinants of Diabetes in the Young) with US sites in the 
states of Colorado, Georgia, Florida, and Washington, as well as 
European sites in Germany, Sweden, and Finland54; T1DM Trial­
Net Pathway to Prevention, with more than 200 locations across the 
United States, Europe, Canada,

cations across the 
United States, Europe, Canada, Australia and New Zealand55; and 
SEARCH (the SEARCH for Diabetes in Youth), with study centers 
in the US states of South Carolina, Ohio, Colorado, California, and 
Washington.56 These efforts, together with cross-sectional investiga­
tion of human pancreas tissues obtained through biobanks, such as 
the Network for Pancreatic Organ donors with Diabetes (nPOD),57 
have reshaped our understanding of the natural history of T1DM 
and human pancrea

 of the natural history of T1DM 
and human pancreas pathology with some of the seminal findings 
reviewed in Table 36.2.
A pivotal discovery was made from combined analysis of data 
generated in the BABYDIAB, DAISY, and DIPP studies. Namely, 
once two or more anti-islet autoantibodies are present in a given 
individual, the individual’s 10-year risk for developing diabetes 
ranges from 60% in older seroconverters (>3 years) to 75% in 
those who seroconvert before 3 years of age.11 Recent and ong

eroconvert before 3 years of age.11 Recent and ongo­
ing analyses of longitudinal data generated from the DIPP and 
TEDDY cohorts are unveiling potential environmental influences 
(e.g., birth by cesarean section, gut microbiome, dietary fiber, 
early introduction of cow’s milk or gluten to an infant’s diet, vita­
min D, antibiotic use in early life, vaccination, infections)58–64 
and novel biomarkers (e.g., gut microbiome, metabolomics, lipi­
domic and proteomic profiles) of eventual T1DM progr

mic and proteomic profiles) of eventual T1DM progression (see 
the Emerging Biomarkers of T1DM Natural History and Diag­
nosis section)65 that might ultimately enable identification of the 
disease early in its pathogenesis. The hope is that prevention or 
delay of T1DM onset may one day be possible through early life­
style adjustments or tailored intervention. 
Genetics
It has long been recognized that diabetes is a heterogeneous group 
of metabolic disorders, with hyperglycemia representing a

bolic disorders, with hyperglycemia representing a com­
mon feature. More recently, it has become apparent that T1DM is 
also heterogeneous as a disease entity. This conclusion is based on 
a series of observations including the age at disease onset, form of 
Beta cell mass 
(? Precipitating
event)
Genetic
predisposition
Overt
immunologic
Normal insulin
release
Progressive
loss insulin
release 
Glucose
normal 
Overt
diabetes 
C-peptide
present 
No
C-peptide 
1. Precipi

betes 
C-peptide
present 
No
C-peptide 
1. Precipitating
events may occur
in utero 
2. Genetic 
predisposition likely
the key driver or
linkage to immune
abnormalities 
3. Beyond “precipitating,”
environment may
influence entire
natural history  
5. Multiple islet
autoantibodies (2)
may represent
“Asymptomatic” T1D  
6. Increasing glucose
excursions as
one approaches
symptomatic onset  
7. Some T1D
patients produce low-
level C-peptide long
after onset 
8. -cell mass not
always zero in
long-st

r onset 
long-standing patients 
4. While overall loss
of  cells is potentially
linear, may be
relapsing/remitting  
• Fig. 36.3  The natural history of type 1 diabetes mellitus—a 25-year-old concept revisited. A re-creation of 
the model as originally proposed in 1986 is tracked by the black line. Several additions and conjectures can 
be made to this model based on recent knowledge gains (lavender line). T1D, type 1 diabetes. (Redrawn from 
Atkinson MA, Eisen

but 
still detectable in the human T1DM pancreas
7, 39, 40, 57, 
399–401
GADA, Glutamic acid decarboxylase autoantibodies; HLA, human leukocyte antigen; IAA, insulin autoantibodies; N/A, not applicable; NOD, nonobese diabetic; OGTT, oral glucose tolerance test; T1DM, 
   
TABLE 36.2
symptomatic presentation (i.e., rapid and ketotic vs. mild without 
ketosis), pancreatic pathologic changes, type and number of auto­
antibodies, and certainly the genetics.
Indeed, there ar

dies, and certainly the genetics.
Indeed, there are likely many genetic forms of T1DM, influ­
enced to a major degree by the presence of specific forms of HLA 
class II molecules.66 Beyond HLA, the form of T1DM is also 
influenced by the presence of abnormalities (immunologic, beta 
cell, etc.) fostered by the quantity and combinations of genes that 
influence these aberrancies. The number of specific genes that con­
tribute to T1DM remains in flux, to some degree, as studies of 
loci validation

ux, to some degree, as studies of 
loci validation in varying global populations are conducted. How­
ever, current estimates provided by genome-wide association stud­
ies (GWAS) support that more than 50 such loci exist, with the 
majority thought to influence immune responses.67 Many of the 
1410
HLA and non-HLA genes underlying T1DM susceptibility are 
similar in diverse countries, although specific alleles of those genes 
differ in 

though specific alleles of those genes 
differ in their frequency. It is generally thought that specific com­
binations of HLA and non-HLA genes contribute to a loss of toler­
ance to self-antigens in beta cells, ultimately resulting in T1DM.
Beyond the polymorphisms seen in these HLA and non-
HLA genes identified by GWAS, genetics studies have also been 
extremely helpful in identification and characterization of sev­
eral monogenic forms of T1DM. Although these disorders often 
clinically cont

M. Although these disorders often 
clinically continue to be classified as T1DM, such diseases should 
strictly be identified as monogenic diabetes. Genetic risk scores 
(GRSs) accounting for HLA and non-HLA risk alleles can discrim­
inate T1DM from other forms of diabetes, including monogenic 
diabetes and T2DM68–70 (see The Genetic Risk Score section).
Overview of Disease Prevalence
In the United States, the risk of childhood diabetes is approxi­
mately 1 in 300.71 This risk is 15 times lower 

i­
mately 1 in 300.71 This risk is 15 times lower than the diabetes risk 
for a first-degree relative of a patient with T1DM (Table 36.3), 
which, in turn, is 150 times lower than the risk for a monozygotic 
twin of a patient with T1DM.72 Although the risk of diabetes is 
much greater for relatives of patients with T1DM, it is important 
to realize that most persons (>85%) in whom T1DM develops do 
not have a first-degree relative with the disease. The incidence of 
sporadic cases results, in pa

e. The incidence of 
sporadic cases results, in part, because almost 40% of persons in 
the general population carry high-risk HLA alleles for T1DM (see 
The Major Histocompatibility Complex section).
The highest known incidence of T1DM is observed in Finland 
and Sardinia. Finland now has an annual incidence approaching 
60 per 100,000 children.73 Since the 1950s, the incidence has 
increased almost fivefold, suggesting a dramatic environmental 
change (either an increase of causative factors o

change (either an increase of causative factors or a decrease of 
protective factors), and it is doubling in many Western countries, 
including the United States, every 10 to 15 years (Fig. 36.4). Of 
note, the increased T1DM incidence in many countries has not 
been linear but rather fluctuating over the past several decades. 
In contrast, the disorder is quite uncommon in China, India, and 
Venezuela (∼0.1 per 100,000 per year). 
Twin Studies
Twin studies of diabetes have made impressive cont

Twin studies of diabetes have made impressive contributions to our 
understanding of the disease. The study of monozygotic twins of 
patients with diabetes by Barnett and coworkers74 contributed to 
the recognition of distinct forms of diabetes, initially termed juve­
nile onset and adult onset, later insulin dependent and non–insulin 
dependent, and now T1DM and T2DM. The concordance rates for 
monozygotic and dizygotic twins provide important information 
regarding genetic factors contributing

nformation 
regarding genetic factors contributing to a given disease, because 
monozygotic twins share all germline-inherited polymorphisms or 
mutations, whereas dizygotic twins are similar to nontwin siblings 
of patients with a disease and have only half of their genes in com­
mon. For a locus that contributes to disease in a recessive man­
ner, only one quarter of dizygotic twins would be homozygous 
to a sibling with diabetes at that locus, but all monozygotic twins 
would be homozygous fo

 but all monozygotic twins 
would be homozygous for all recessive loci of their diabetic twin. 
As overall concordance rates of monozygotic twins for T1DM vary 
across studies, it is likely that T1DM is heterogeneous and that 
groups of monozygotic twins have different genetic causes for their 
diabetes. With such genetic heterogeneity, one would expect dif­
ferent concordance rates for different genetic causes.
Redondo and coworkers75 analyzed prospective follow-up data 
from a large series of 

rospective follow-up data 
from a large series of initially discordant monozygotic twins from 
the United Kingdom combined with a series from the United 
States. Progression to diabetes was identical for both series of 
twins. There was no length of time of discordance beyond which a 
monozygotic twin of a patient with T1DM did not have a risk of 
developing the disease. Nevertheless, the hazard rate for develop­
ment of diabetes decreased as the period of discordance increased. 
There was also 

 period of discordance increased. 
There was also a marked variation in the risk of diabetes relative to 
the age at which the disorder developed in the index twin. With 
long-term follow-up, the overall rate of concordance for mono­
zygotic twins exceeds 50%.72 However, if T1DM developed in 
the index twin after age 25 years, the concordance rate by life 
table analysis in the study of Redondo and coworkers75 was less 
than 10%. If diabetes developed in the index twin before age 5 
years, the c

loped in the index twin before age 5 
years, the concordance rate was 70% after 40 years of follow-
up. Therefore, environmental factors, random factors, and non–
germline-inherited variations (e.g., imprinting, T-cell receptor 
polymorphisms, somatic mutations) likely contribute to lifetime 
diabetes risk. Interestingly, studies of dizygotic twins suggest that 
their risk of diabetes may not differ from that of nontwin siblings 
or, at most, may be increased by a factor of two as compared with 

 be increased by a factor of two as compared with 
the 10-fold increase for monozygotic twins.76
Genetic factors influence not only the development of diabetes 
but also the expression of anti-islet autoantibodies. For identical 
twins, the expression of anti-islet autoantibodies is tightly linked 
to the eventual progression to overt diabetes, and, true to form, 
monozygotic twins have a highly concordant prevalence for the 
presence or absence of anti-islet autoantibodies. Dizygotic twins 
muc

of anti-islet autoantibodies. Dizygotic twins 
much less often exhibit concordant positivity for anti-islet autoan­
tibodies, and the prevalence is similar to that of nontwin siblings.77 
The Major Histocompatibility Complex
The most important loci determining the risk of T1DM reside 
within the MHC on chromosome 6p21 and, in particular, the 
HLA class II molecules (DR, DQ, and DP).66 The major deter­
minants of T1DM susceptibility are DR and DQ molecules, and 
specific alleles of both HLA-DR an

molecules, and 
specific alleles of both HLA-DR and -DQ can either increase or 
decrease the risk of diabetes. Table 36.4 summarizes the diabetes 
risk associated with several DR and DQ haplotypes.78 In addition, 
HLA class I loci (HLA-A, -B, and -C) influence disease, and it is 
likely that additional loci within or linked to the MHC influence 
immune function and contribute to risk.79 The nomenclature for 
alleles of this region is somewhat daunting, but with definitions of 
  Risk of Type 1 

nting, but with definitions of 
  Risk of Type 1 Diabetes Mellitus
Group
Childhood Annual Incidence
US general population
0.3% (15–25/100,000)
Offspring
1%
Sibling
3.2% (through adolescence); 6% lifetime
Dizygotic twin
6%
Mother
2%
Father
4.6%
Both parents
∼10%
Monozygotic twin
50%, but incidence varies with age of index twin
TABLE 36.3
1411
several terms and a description of the basis for classification, it is 
comprehensible and quite important to understa

is 
comprehensible and quite important to understand.
As noted previously, the function of HLA molecules is to pres­
ent peptides to T lymphocytes. Each HLA molecule is made up 
of two chains, and each chain is encoded by a separate gene. These 
molecules are extremely polymorphic in amino acid sequence. Each 
polymorphic variant of each chain is designated with a gene locus 
name (e.g., HLA-DRB1) followed by an asterisk (*), followed by 
two digits referring to the serologic specificity (from t

its referring to the serologic specificity (from the time in 
diagnostic history when typing was performed with antibodies), 
followed by two digits for the specific allele (now determined with 
DNA-based typing), followed by a single digit to distinguish silent 
nucleotide polymorphisms (nucleotide differences that do not 
change the amino acid sequence). For example, the designated allele 
HLA-DRB1*04:05 has DR4 serologic specificity and is associated 
with high T1DM risk. There are hundreds o

ociated 
with high T1DM risk. There are hundreds of known alleles of HLA-
DRB1. Each person inherits two HLA-DRB1 alleles, one from each 
parent. For HLA-DR alleles, usually only the DRB chain is specified, 
because the DRA chain is not polymorphic. Likewise, for the class I 
molecules (A, B, and C), only a single chain is specified, because the 
other chain, β2-microglobulin, is minimally polymorphic.
Because HLA gene loci are in close proximity to each other on 
the sixth chromosome, a group o

 to each other on 
the sixth chromosome, a group of alleles is usually inherited as a unit, 
which is termed a haplotype. For example, the alleles HLA-A*01:01, 
A
B
0.1-9.4
9.5-18.6
18.7-31.4
31.5-43.1
43.2-57.6
No data
1950
1960
1970
1980
1990
2000
T1D incidence per 100,000/year in children 0-14
0
70
10
20
30
40
50
60
Finland
Sweden
Colorado
Germany
• Fig. 36.4  Incidence of T1D in children age 0 to 14 years by geographic region and over time.  (A) Esti­
mated global incidence of TID, by region

(A) Esti­
mated global incidence of TID, by region, in 2011. (B) Time-based trends for the incidence of TID in 
children ages 0 to 14 years in areas with high or high-intermediate rates of disease. T1D, type 1 diabetes. 
(Redrawn from Atkinson MA, Eisenbarth GS, Michels AW. Type 1 diabetes. Lancet. 2014;383:69–82, used 
with permission.)
1412
HLA-B*08:01, and HLA-DRB1*0301-DQA1*0501-DQB1*0201 
constitute a common haplotype associated w

1*0201 
constitute a common haplotype associated with diabetes risk prob­
ably related to the presence of HLA-DRB1*0301. When specific 
alleles of different genes are nonrandomly associated with each other 
on a haplotype (e.g., A1, B8, and DR3), the alleles are said to be 
in linkage disequilibrium. Linkage disequilibrium is not the same 
as linkage, although to have linkage disequilibrium, genes must be 
linked. Genes are linked when they are close together on the same 
chromosome and thus tra

lose together on the same 
chromosome and thus transmitted from parent to child as a hap­
lotype group. If alleles of linked genes are nonrandomly associated 
with each other in a population, they are in linkage disequilibrium.
Two MHC haplotypes, one inherited from each parent, consti­
tute an individual’s MHC genotype. This genotype ultimately deter­
mines the MHC-encoded risk of T1DM. For HLA-DQ molecules, 
both of the chains (DQA and DQB) are polymorphic. This adds an 
important level of div

 polymorphic. This adds an 
important level of diversity in that the protein chains encoded by 
the alleles of one haplotype can combine with the chains encoded 
by the other haplotype. For example, persons with the highest-
risk genotype HLA-DRB1*03:01-DQA1*05:01-DQB1*02:01 
and HLA-DRB1*04:05-DQA1*03:01-DQB1*03:02 can pro­
duce four different DQ molecules: the expected DQA1*05:01-
DQB1*02:01 
and 
DQA1*03:01-DQB1*03:02 
but 
also 
DQA1*05:01-DQB1*03:02 and DQA1*03:01-DQB1*02:01. 
Studies sugge

B1*03:02 and DQA1*03:01-DQB1*02:01. 
Studies suggest that the DQA1*05:01-DQB1*03:02 combination 
determines enhanced diabetes risk for DR3/4 individuals. The 
DQ molecule DQA1*05:01-DQB1*02:01 is also called DQ2, 
and DQA1*03:01-DQB1*03:02 is called DQ8.
A common DQ molecule, HLA-DQA1*01:02-DQB1*06:02, 
provides dominant protection from T1DM and is termed DQ6. 
A DR allele, HLA-DRB1*14:01, also appears to provide domi­
nant protection.80 The protective haplotype HLA-DRB1*15:01-
DQA1*01:02-DQB1*0

ective haplotype HLA-DRB1*15:01-
DQA1*01:02-DQB1*06:02 is present in 20% of the general 
population and in fewer than 3% of patients with T1DM.
In several, children from the general population or rela­
tives of patients with T1DM have been HLA typed.81,82 In the 
United States, 2.4% of newborns have the highest risk HLA-DR-
DQ genotype for T1DM, namely DR3-DQ2 with DR4-DQ8 
(DR3/4-DQ2/8 heterozygotes). Fifty percent of children younger 
than 10 years and approximately 30% of older children who 

ears and approximately 30% of older children who 
develop T1DM have this highest-risk genotype. Approximately 
1 of 16 children with the highest-risk HLA genotype from the 
general population progresses to T1DM (compared with a popu­
lation risk of 1 per 300). Alternatively, 15 of 16 children from 
the general population who are DQ2/DQ8 heterozygotes do not 
develop T1DM. Studies indicate that newborn siblings of patients 
with T1DM who have DQ2 and DQ8 have a risk of expressing 
islet autoantib

and DQ8 have a risk of expressing 
islet autoantibodies exceeding 40% by age 6 years, and 50% of 
these newborns will develop diabetes by age 10 years. Ninety-five 
percent of persons who develop T1DM have either DR3-DQ2 or 
DR4-DQ8, as does approximately 40% of the general population.
There are additional high-risk haplotypes that are not com­
mon, such as HLA-DQA1*04:01-DQB1*04:02. It has been pro­
posed as a simple rule that the presence of aspartic acid at position 
57 of the DQβ chain and a

partic acid at position 
57 of the DQβ chain and arginine at DQα 52 is associated with 
T1DM risk.83 There are many exceptions to this rule, and knowl­
edge of the complete sequences (allele) rather than dependence on 
this rule is essential. 
Other Loci
In impressive collaborative form, more than a decade of interna­
tional GWAS efforts led to the definition of many loci that con­
tribute to small but varying degrees toward the development of 
T1DM.84 For example, polymorphisms of the cytotoxic

1DM.84 For example, polymorphisms of the cytotoxic T-lym­
phocyte–associated protein 4 gene contribute to Graves disease and 
apparently to T1DM in some, but not all, populations, with rela­
tive risks less than 1.3.85 A locus associated with the IL2 receptor 
has a statistical association based on analysis of thousands of per­
sons.86 Sequencing in one region has also identified rare mutations 
of a gene influencing interferon-alpha induction.87 A curated list 
of T1DM risk loci can be found at

 A curated list 
of T1DM risk loci can be found at http://www.immunobase.org.
Although much of the attention toward T1DM genetics has 
resided on HLA, other loci have drawn attention for their poten­
tial contributions to disease risk, including those related to insu­
lin. In 1984, Bell and colleagues88 published their discovery that 
variations in the number of nucleotide repeat elements located 
upstream (also known as 5′) of the insulin gene were associated 
with the development of T1DM. The 

ere associated 
with the development of T1DM. The longest group of repeats 
was associated with decreased diabetes risk.89 In a mechanism 
potentially related to the role of autoimmune regulator (AIRE) in 
the thymus, the protective insulin gene polymorphism is associ­
ated with greater insulin messenger RNA expression within the 
thymus.90 Indeed, for both the AIRE mutation of autoimmune 
polyendocrine syndrome type 1 (APS1) and the effect of insulin 
gene mutations, the level of expression of 

sulin 
gene mutations, the level of expression of insulin in the thymus 
may be critical.
Additionally, PTPN22, a gene encoding a lymphoid-specific 
phosphatase that influences T-cell receptor signaling, is the third 
confirmed gene (vs. mere loci) influencing T1DM risk.91 The 
polymorphism associated with T1DM (Trp for Arg) blocks bind­
ing to a signaling kinase molecule, C-terminal Src kinase (CSK). 
Nevertheless, the relative risk associated with this polymorphism 
for T1DM and other autoimmu

ith this polymorphism 
for T1DM and other autoimmune disorders, such as rheumatoid 
arthritis, is only 1.7. The variant associated with disease risk results 
in gain of function and decreased T-cell receptor signaling.92 
  Diabetes Risk of Representative HLA-DR 
and DQ Haplotypes
DRB1
DQA1
DQB1
High Risk
0401 or 0403 or 0405
0301
0302 (DQ8)
0301
0501
0201 (DQ2)
Moderate Risk
0801
0401
0402
0404
0301
0302
0101
0101
0501
0901
0301
0303
Moderate Protection
0403
0301
0302
0701
0201
0201
1101
0501

rotection
0403
0301
0302
0701
0201
0201
1101
0501
0301
Strong Protection
1501
0102
0602 (DQ6)
1401
0101
0503
0701
0201
0303
TABLE 36.4
1413
Genetic and Immunologic Heterogeneity 
by Age of Onset
T1DM can develop at any age, from the neonatal period to the 
last decades of life. Given that identical twins can become con­
cordant more than 40 years after the first twin develops T1DM, 
not all age heterogeneity can be ascribed to different genetic syn­
dromes.77

an be ascribed to different genetic syn­
dromes.77 Nevertheless, there is an overall correlation between 
the age at which diabetes develops in one twin or sibling and the 
age of development of diabetes in his or her relative. Children in 
whom T1DM develops at an early age more often are DR3/4-
DQ2/8 heterozygotes. In addition, there is evidence that class I 
HLA alleles (or other non–class II genes within the HLA region) 
can influence the age at diabetes onset; an example is the A24 
allele.

 at diabetes onset; an example is the A24 
allele.93 At the other end of the age spectrum, there is evidence 
that the protective HLA allele DQA1*01:02-DQB1*06:02 is not 
as protective for young adults as it is for children.94
The most characteristic difference related to age at diabe­
tes onset is the presence of higher levels of IAA in children who 
develop the disease at an early age (e.g., <5 years). The high levels 
and frequent positivity of IAA make measurement of these auto­
antibodies t

f IAA make measurement of these auto­
antibodies the best single marker for diabetes development in 
young children.95 In those children in whom autoantibodies arise 
during the first 3 years of life, IAA often appear first. In contrast, 
GADA are more often positive in adults who develop T1DM. 
Seroconversion to IAA or GADA as the first anti-islet autoanti­
body has also been linked to HLA.96 The correlation of levels of 
IAA and age at diabetes onset may be related to faster progression 
to di

 onset may be related to faster progression 
to diabetes in children with higher levels. However, such rapid 
progression occurs only if IAA are present with another anti-islet 
autoantibody (discussed in the Stages in the Natural History of 
T1DM section). 
Monogenic Forms of Diabetes
The most common forms of diabetes, T1DM and T2DM, are poly­
genic, meaning that the risk of developing these disorders is related 
to multiple genes. Environmental factors (discussed later) also play 
a part in th

 factors (discussed later) also play 
a part in the development of polygenic forms of diabetes. However, 
additional and rare forms of diabetes, termed monogenic, result from 
mutations in a single gene. Monogenic forms of diabetes account 
for about 1% to 5% of all cases of diabetes in young individuals. So 
far, more than 20 genes have been linked to monogenic diabetes. In 
most cases of monogenic diabetes, the gene mutation is inherited; 
in the remaining cases, the gene mutation develops spo

he remaining cases, the gene mutation develops spontaneously. 
Neonatal diabetes (ND) and maturity-onset diabetes of the young 
(MODY) are the two main forms of monogenic diabetes (Table 
36.5), with ND far less common than MODY.
Genetic testing can diagnose most forms of monogenic diabe­
tes. If genetic testing is not performed, people with monogenic 
diabetes may appear to have one of the polygenic forms of diabe­
tes (i.e., T1DM or T2DM). When hyperglycemia is first detected 
in adulthood, mo

 hyperglycemia is first detected 
in adulthood, monogenic diabetes is often misdiagnosed as gen­
eral T2DM. Importantly, some monogenic forms of diabetes can 
be treated with oral diabetes medications, whereas other forms 
require insulin injections. A correct diagnosis that allows the 
proper treatment to be selected should lead to better glucose con­
trol and improved health in the long term. 
ND is a monogenic form of diabetes that occurs in newborns 
through the first 6 mon

s that occurs in newborns 
through the first 6 months of life. It is a rare condition occurring 
in only 1 in 100,000 to 500,000 live births. Owing to lack of pro­
fessional awareness (although at decreasing frequency) or lack of 
screening, ND can be mistaken for T1DM, a disorder that usually 
occurs later than the first 6 months of life. For approximately one 
half of those with ND, the condition is lifelong, yet for the remain­
der, the condition disappears during infancy but can reappear lat

ion disappears during infancy but can reappear later 
in life (i.e., transient ND). Symptoms of ND mimic those of T1DM 
and include thirst, frequent urination, and dehydration. In severe 
cases, insulin deficiency can lead to DKA. In addition, fetuses with 
ND may not see normal growth in utero, with newborns being 
small for gestational age (i.e., intrauterine growth restriction). Fol­
lowing, some infants demonstrate protracted growth and weight 
gain, but appropriate therapy has the ability t

t 
gain, but appropriate therapy has the ability to improve and may 
in fact normalize growth and development. 
Maturity-Onset Diabetes of the Young
Maturity-onset diabetes of the young (MODY) is a monogenic 
form of diabetes that usually first occurs during adolescence or 
early adulthood97 but may remain undiagnosed until later in life, as 
MODY patients often are asymptomatic or demonstrate only mild 
hyperglycemia. Several different gene mutations have been shown 
to cause MODY98 (see Table 

tions have been shown 
to cause MODY98 (see Table 36.5). MODY variants have been esti­
mated to account for about 1% to 5% of all cases of diabetes in 
the United States.99 Relatives of those with MODY are at increased 
risk for the disorder (e.g., children of a MODY parent have a 50% 
chance of inheriting the disease). MODY patients are generally not 
overweight. Although both T2DM and MODY can run in families, 
MODY pedigrees typically portray a family history of diabetes in 
multiple successi

a family history of diabetes in 
multiple successive generations. 
Autoimmune Polyendocrine Syndrome Type I (AIRE 
Gene Mutations)
Autoimmune polyendocrine syndrome type 1 (APS1)  is rare (<500 
cases worldwide), with an increased incidence in Finland and Sar­
dinia and among Iranian Jews, but it has a worldwide occurrence. 
The disorders of the syndrome, such as T1DM, mucocutaneous 
candidiasis, hypoparathyroidism, Addison disease, and hepatitis (see 
Chapter 43 for a more detailed discussion b

 (see 
Chapter 43 for a more detailed discussion beyond genetics herein), 
identify a unique syndrome, and patients with this group of disorders 
almost always have mutations (usually autosomal recessive) of the 
AIRE gene on chromosome 21, a gene that encodes a DNA-binding 
protein. This gene may play an important role in maintaining self-
tolerance, as well as influence the expression of what immunologists 
term peripheral antigens (e.g., insulin) in the thymus. This notion 
becomes relevant i

in) in the thymus. This notion 
becomes relevant in that it has been hypothesized that greater thymic 
expression of insulin and other tissue-specific antigens leads to T-cell 
tolerance and disease suppression. At the same time, the influence on 
insulin is not specific, as damage to the adrenal glands, parathyroid 
glands, and other organs underlie the major features of APS1 and are 
why candidiasis infections predominate. 
X-Linked Polyendocrinopathy, Immune Dysfunction, and 
Diarrhea (Scurfy

nopathy, Immune Dysfunction, and 
Diarrhea (Scurfy Gene)
The syndrome of X-linked polyendocrinopathy, immune dysfunc­
tion, and diarrhea (XPID, also termed IPEX) is associated with 
overwhelming neonatal autoimmunity, and most children die in 
the first few days of life or in infancy.100 In this syndrome, lym­
phocytes invade multiple organs. It is associated with insulitis and 
beta-cell destruction, as well as lymphocytic intestinal inflamma­
tion with flattened villi and severe malabsorption.

ion with flattened villi and severe malabsorption. It is inherited as 
an X-linked recessive disease affecting only boys, with a frequent 
clinical history of lack of male births.
1414
  Characteristics of Monogenic Diabetes
Type of 
Diabetes
Gene or 
Syndrome
Affected Protein
How Common?
Typical 
Age at Onset
Type of Inheritance 
or Mutation
Causes Intrauterine 
Growth Restriction?
Transient or 
Permanent?
Treatment

ansient or 
Treatment
Neonatal Diabetes Mellitus (NDM) (Rare; Occurs in ∼1 of Every 100,000–500,000 Live Births)
Permanent Neonatal Diabetes Mellitus (PNDM) (50% of All Cases of NDM)
PNDM
KCNJ11
Kir6.2
Most common 
type of PNDM
3–6 months
Autosomal dominant 
(10%)
Spontaneous
Yes
Permanent (this gene 
also causes a 
transient form of 
NDM)
Treated with insulin in the past 
but often can be treated 
with oral sulfonylureas
PNDM
ABCC8
SUR1-sulfonylurea 
receptor 1
Rare
1–3 months
Autoso

R1-sulfonylurea 
receptor 1
Rare
1–3 months
(12% of NDM cases)
No
NDM)
PNDM
GCK
Glucokinase
Rare
1 week
Autosomal ­recessive
Yes
Permanent
Insulin
PNDM
IPF1; also known 
as PDX1
Insulin promoter 
factor 1
Rare
1 week
Yes
Permanent
Treat to replace endocrine and 
exocrine pancreas functions
PNDM
PTF1A
Pancr

 and 
PNDM
PTF1A
Pancreas transcription 
factor 1 A
Rare
At birth
Yes
Permanent
PNDM
FOXP3, IPEX 
syndrome
Forkhead box P3
Rare
Sometimes 
present at 
birth
X-linked
Yes
Permanent
Insulin
PNDM
EIF2AK3, Wol­
cott-Rallison 
syndrome
Eukaryotic translation 
initiation factor 
2-alpha kinase 3
Rare
3 months
Yes
Permanent
Insulin and treatment for 
associated conditions
Tr

nsulin and treatment for 
Transient Neonatal Diabetes Mellitus (TNDM) (50% of All Cases of NDM)
TNDM
ZAC/HYMAI
ZAC: pleomorphic 
adenoma gene-like 
1 or PLAG1
HYMAI: hydatidiform 
mole–associated 
and imprinted 
transcript
form of NDM
Birth to 3 
months
Yes
Transient
Initially, treat with insulin; 
reduce dosage as needed; 
when diabetes recurs, treat 
with diet modification and 
physical activity; may also 
require insulin
TNDM
A

hysical activity; may also 
TNDM
ABCC8
SUR1 (sulfonylurea 
receptor 1)
Rare
Birth to 6 
months
Varies
Transient (this gene 
permanent form 
of NDM)
Oral sulfonylureas
TNDM
KCNJ11
Kir6.2
Uncommon cause 
of TNDM but 
most common 
cause of PNDM
Birth to 6 
months
Yes
of NDM)
TNDM
HNF1 β (beta); 
also known as 
HNF1B
Hepatocyte nuclear 

(beta); 
HNF1B
factor 1B
Rare
Birth to 6 
months
(60%)
Yes
Transient
Insulin
TABLE 36.5
         
1415
Maturity-Onset Diabetes of the Young (MODY; 1–5% of All Cases of Diabetes in the United States)
MODY 1
HNF4A
factor 4α (alpha)
Rare
Adolescence 
or early 
adulthood
No
Permanent
For most, oral sulfonylureas; 
some patients may need 
insulin
MODY 2
GCK
Gl

as; 
insulin
MODY 2
GCK
MODY 2 and 
MODY 3 
account for 
about two 
thirds of all 
cases of MODY
MODY 2 is the 
second most 
common form 
of MODY
Mild hypergly­
cemia may 
be present 
at birth; 
otherwise, 
early child­
hood
Lower than 
normal birth 
weight can 
occur
Permanent
Diet modification and physical 
activity; medications usually 
not required; some patients 
do not require any treatment 
during childhood
MODY 3
TCF1
Hepatic nuclear

ment 
MODY 3
TCF1
Hepatic nuclear fac­
tor 1α (alpha) or 
HNF1α (alpha) or 
HNF1A
MODY 3 is the 
form of MODY
or early 
adulthood
No
Permanent
Initially, treat with diet modifi­
cation; can be treated with 
oral sulfonylureas; some 
patients may need insulin
MODY 4
as PDX1
factor 1
Rare
Early adult­
hood; can 
present 
later
No
Permanent
Oral sulfonylureas; some 
patients may need

manent
MODY 5
TCF2
Hepatic nuclear factor 
1β (beta) or HNF1B
Rare
or early 
adulthood
No
Permanent
Insulin; patients also may need 
treatment for related condi­
tions such as kidney failure 
or cysts
MODY 6
NeuroD1, or 
BETA2
Neurogenic differen­
tiation factor 1
Rare
In the fourth 
decade of 
life
No
Permanent
Insulin
         
1416

II    Disorders of Carbohydrate and Fat Metabolism
The disease results from mutations of the gene encoding fork­
head box P3 (FOXP3), which functions as a transcription factor and 
master switch for regulatory (suppressor) CD4+CD25+CD127–/low 
T lymphocytes.101 Lack of such regulatory T cells leads to over­
whelming autoimmunity. This is an important syndrome to rec­
ognize early, as bone marrow transplantation, affording restoration 
of functional T-regulatory cells (even with partial chimerism

al T-regulatory cells (even with partial chimerism), has 
proved to be therapeutic. 
The Genetic Risk Score
A recent advance in the ability to stratify patients with high risk 
for developing T1DM has occurred through construction of cumu­
lative genetic risk models utilizing logistic regression algorithms that 
incorporate HLA and non-HLA alleles associated with T1DM.68–70 
Although HLA predominates risk, the inclusion of non-HLA loci 
increases model accuracy for classifying individuals as T1D

 model accuracy for classifying individuals as T1DM 
patients versus those at low risk. Although not currently used in 
standard clinical care, the GRS is being applied in the research 
space to improve subject selection and stratification. Genetic risk 
models enable prediction of T1DM progression,102 differentiation 
of T1DM from T2DM and monogenic forms of diabetes,68,69 and 
determination of T1DM onset prevalence in individuals older 
than 30 years. In the prospective TEDDY study, a GRS mode

 years. In the prospective TEDDY study, a GRS model 
improved the ability to identify infants with high risk of develop­
ing anti-islet autoantibodies by 6 years of age.103 One caveat is due 
to inherent biases resulting from the predominance of GWAS and 
genotype/phenotype studies being conducted in majority Cauca­
sian populations from Western Europe and the United States. The 
need for studies characterizing the T1DM genetic risk profile and 
the development of new GRS models in non-Caucasian

the development of new GRS models in non-Caucasian popula­
tions represents a clear limitation to clinical introduction of cur­
rent GRS modeling.70 
Environmental Factors
Environmental factors have long been considered to influence 
the pathogenesis of T1DM. Regional differences in disease rates 
based on geography, seasonality in its diagnosis, rising incidence 
trends, and variance among twins are consistent with this conclu­
sion. However, despite decades of research, no single environmen­
t

spite decades of research, no single environmen­
tal agent has been identified that would universally explain these 
observations. Identification of any such agent is especially difficult 
due to facets of the disorder’s pathogenesis (e.g., the long prodro­
mal/prediabetic phase that often precedes T1DM, that 85% of new 
cases arise from the general population rather than having a family 
history of T1DM, the limited disease frequency of 1 in 300).
The fivefold increase in T1DM rates over the pa

).
The fivefold increase in T1DM rates over the past half-century 
has been considered strong evidence that environmental factors 
related to diabetes risk have changed since the 1960s. Factors that 
increase diabetes risk may be increasing, or just as likely, factors 
that suppress the development of T1DM may be decreasing. It is 
interesting that the rising incidence as a function of age at onset is 
occurring globally at different rates. The increase in Finland is high­
est for children in wh

ncrease in Finland is high­
est for children in whom the disease develops before age 5 years, yet 
in the United States, the recent Search for Diabetes in Youth Study 
(SEARCH) showed the most rapid rise in teenagers.56,104
Models to Explain the Influence of Environment
Although the mechanisms by which environment influences 
T1DM are not known, numerous hypothetical models have been 
proposed. The accelerator and overload hypotheses suggest that 
childhood obesity increases insulin demand, over

childhood obesity increases insulin demand, overloading the islet 
cells and accelerating beta-cell autoimmune damage.105,106 In the 
Copenhagen model, beta-cell destruction is proposed to result 
from interactions between the environment, immune system, 
and the beta cells themselves in genetically susceptible individu­
als.107 The hygiene hypothesis attributes the rising incidence of 
autoimmune disorders to the decline in infections with reduced 
stimulation of the immune system.108 Convers

uced 
stimulation of the immune system.108 Conversely, the fertile field 
hypothesis proposes that microbial infection induces a temporary 
state in which other antigens can more easily react to yield autore­
active T cells.109 Also implicating the gut, the old friends hypoth­
esis implicates dietary exposure as a possible direct regulator of the 
immune system and of self-tolerance by altering gut microbiota 
and intestinal permeability.110 Finally, the threshold hypothesis 
suggests that the e

lly, the threshold hypothesis 
suggests that the etiologic influences of genetics and environment, 
when evaluated as intersecting and reciprocal odds-ratio–based 
trend lines, result in a method to define the attributable risk for 
T1DM.111
A popular model of T1DM involves an environmental “trig­
ger” presumed to initiate disease. Whereas it is true that the 
underlying genetic susceptibility to T1DM might allow for a trig­
gering of anti–beta-cell immunity by an environmental event, it 
is inc

ell immunity by an environmental event, it 
is increasingly accepted that environmental agents may contrib­
ute far beyond an initial triggering and rather act throughout the 
natural history of T1DM development, such as by modulating 
the ongoing autoimmune process (e.g., genes controlling immune 
regulation). There is no shortage of potential environmental can­
didates (e.g., breastfeeding, antibiotics, infant and childhood diet, 
viruses) that, either alone or in combination, may contribute t

, either alone or in combination, may contribute to 
the disorder’s pathogenesis. However, most fall into the groups 
involving infection (particularly viruses), vaccination, and diet. 
Candidate Environmental Factors
Infection
The best evidence, if not the only strong evidence, for a spe­
cific environmental agent to contribute to T1DM pathogenesis 
involves congenital rubella infection, which, in contrast to non­
congenital infection, greatly increases development of T1DM.112 
The mechanism(s)

ncreases development of T1DM.112 
The mechanism(s) by which this congenital infection increases 
diabetes development is currently unknown, with hypotheses 
ranging from molecular mimicry to long-term alteration in T-cell 
function secondary to the congenital insult.
Enteroviruses are the most often discussed agents associated 
with T1DM. Enteroviruses are small RNA viruses that frequently 
infect young children. Coxsackie virus (a form of enterovirus) 
infections were implicated after initial a

virus) 
infections were implicated after initial anecdotal reports involving 
children who had severe infections and who died at T1DM onset. 
These case studies preceded the recognition that T1DM is not an 
acute disease but one involving a chronic prediabetic period of 
autoimmunity. At the time of presentation with diabetes, almost 
all children have a rising elevation in HbA1c, reflecting what is 
likely months of hyperglycemia preceding diagnosis.113 Thus, it 
is likely that viral infection 

osis.113 Thus, it 
is likely that viral infection at the onset of T1DM is most often 
incidental.
The potential importance of enteroviral infection was evaluated 
in studies from Scandinavia, with enteroviral infection (detected 
by changes in antiviral antibodies or detection of enteroviral 
RNA by molecular techniques) assessed during pregnancy and in 
infancy.114 Some studies report an increased incidence of entero­
viral infection during pregnancy among mothers whose children 
later develope

nancy among mothers whose children 
later developed T1DM, but others have not.114,115 Considering 
that infants with a genetic risk for development of diabetes are 
1417
followed from birth, it becomes possible to prospectively analyze 
expression of enterovirus RNA. Enteroviral infections have been 
associated with appearance of anti-islet autoantibodies in some 
regions (Finland)114 but not in others (Colorado),115 a finding 
that may be related to differin

do),115 a finding 
that may be related to differing frequency and/or timing of entero­
viral infections in these two populations.
Rotavirus infection, another virus that commonly infects 
young children, has been associated with increased anti-islet auto­
antibodies in one study from Australia,116 but an increase in rota­
virus infection was not found in children with T1DM compared 
with controls. Other viruses are also being evaluated for associa­
tion with the triggering of autoimmunity. 
Vacc

a­
Vaccines
A series of studies have been performed addressing the concern that 
childhood vaccine timing might influence development of T1DM, 
yet none has provided any evidence that childhood vaccinations 
influence the development of diabetes.117 This is an important 
health concern if parents alter their family’s childhood vaccination 
schedule because of concern about development of T1DM. 
Dietary Factors
Dietary factors as environmental T1DM trigg

actors
Dietary factors as environmental T1DM triggers have also been 
extensively investigated. Early introduction of bovine milk in 
the form of infant formula has been hypothesized to increase 
T1DM, based primarily on retrospective studies associating early 
or increased bovine milk ingestion (or less breastfeeding) with 
an increased risk of disease. Several prospective studies in which 
infants were observed until the development of anti-islet autoan­
tibodies failed to find any association

et autoan­
tibodies failed to find any association or found only a weak asso­
ciation with either breastfeeding or bovine milk ingestion. Pilot 
studies of an infant formula lacking bovine milk proteins were 
initiated in Finland and suggest that weaning to insulin-free cow’s 
milk formula might reduce the development of anti-islet autoanti­
bodies in the first 3 years of life.118 In contrast, the TEDDY study 
found no association between choice of first infant formula and 
risk of developing an

of first infant formula and 
risk of developing anti-islet autoantibodies.119
Studies from Germany and Denver, Colorado, in the United 
States provided evidence that early (<3 months) introduction of 
cereals may increase development of islet autoimmunity.120 Both 
low vitamin D and ω-3 fatty acids, which can influence immune 
function, have also been associated with risk of T1DM.121 Dietary 
interventions with these factors to date have not been shown to be 
therapeutic. 
Natural History of T1D

 shown to be 
Natural History of T1DM—Emerging 
Concepts
Beta-Cell Mass Is Not Equal in All Individuals
Most historical models of T1DM have assumed a normal beta-cell 
mass at birth that declines once the autoimmune attack occurs 
(see Fig. 36.3). However, recent studies of cadaveric pancreata 
have shown beta-cell mass in normal, nondiabetic humans var­
ies threefold to fivefold, independent of adult age or body mass 
index, with beta-cell mass likely mostly determined in the firs

eta-cell mass likely mostly determined in the first 
two decades of life.122 This has important implications when one 
considers the starting point for declining beta-cell mass during 
autoimmune beta-cell destruction. An individual’s timeline to dia­
betes onset could be determined not by the severity of the auto­
immune attack but the starting point for beta-cell mass (see Fig. 
36.3). The reasons for variation in beta-cell mass are unknown 
but could include the in utero environment, events d

t could include the in utero environment, events during the 
first decade of life, and yet unknown genetic or environmental 
determinants. Given the aforementioned observations of a smaller 
total pancreatic mass in individuals with new-onset T1DM,40,41 
both pancreatic mass and beta-cell mass might be impacted, 
because endocrine islet cells and exocrine cells share a common 
embryologic heritage. Together, these structural changes empha­
size the need to better understand the timeline and dete

he need to better understand the timeline and determinants 
of human beta-cell mass. 
Pancreatic Exocrine Abnormalities in T1DM
Autopsy studies first revealed reduced pancreas weights in 
deceased persons with T1DM. More recently, studies of higher-
quality organ donor tissues reveal that this phenomenon affects 
not only subjects with long-standing T1DM but also recent-onset 
T1DM and nondiabetic persons with anti-islet autoantibodies.40 
Noninvasive imaging methods permit studies of pancreas v

asive imaging methods permit studies of pancreas volume 
in living subjects, which confirm that the pancreas in generally 
smaller in T1DM patients than in those without disease.41 Islets 
constitute only 1% to 2% of the entire pancreatic volume; hence, 
beta-cell loss alone cannot account for the reduction in total pan­
creas size. Insulinopenic effects were initially thought to cause loss 
of pancreatic size or weight following T1DM diagnosis. However, 
detection of low pancreas weight at T1DM

However, 
detection of low pancreas weight at T1DM onset and in anti-
islet autoantibody–positive organ donors without diabetes,40,41 
together with certain histopathologic findings in the T1DM 
exocrine pancreas (e.g., invasion of exocrine pancreatic tissue by 
CD8+ T cells, CD4+ T cells, CD11c+ dendritic cells, and neu­
trophils32,33 and C4d complement deposition123), suggest early 
loss of pancreas mass prior to diagnosis. Moreover, serum levels of 
trypsinogen, which are indicative of exocri

ls of 
trypsinogen, which are indicative of exocrine pancreas function, 
are reduced in patients with long-standing T1DM, in those with 
recent-onset T1DM, and in nondiabetic individuals with anti-
islet autoantibodies124 implying that disease-related effects within 
the exocrine pancreas may be present prior to T1DM diagnosis. 
Metabolic Progression Before Hyperglycemia
The intravenous glucose tolerance test aids in evaluating the time to 
onset of diabetes among persons expressing anti-islet a

 of diabetes among persons expressing anti-islet autoantibod­
ies. Glucose is given at 0.5 g/kg over 5 minutes (maximum 35 g, 25 
g/dL), and insulin levels are measured before and 1 and 3 minutes 
after the glucose infusion.125 Most persons who are within 1 year 
of developing overt T1DM have no first-phase insulin secretion 
after intravenous glucose administration. A simpler measurement 
in someone progressing to T1DM is the HbA1c, which in most 
prediabetic individuals increases progressively

t 
prediabetic individuals increases progressively (although in the 
normal range) 1 or 2 years before overt hyperglycemia develops.
The diagnosis of T1DM usually relies on the presence of fast­
ing hyperglycemia (≥126 mg/dL, 7.0 mmol/L), but with pro­
spective evaluation, many persons have diabetes determined by 
OGTT according to the 120-minute criterion (≥200 mg/dL, 11.1 
mmol/L), with a nondiagnostic fasting glucose level. Impaired 
fasting glucose (100–125 mg/dL, 5.6–6.9 mmol/L) or impaired

lucose (100–125 mg/dL, 5.6–6.9 mmol/L) or impaired 
glucose tolerance (glucose at 120 minutes on OGTT, 140–199 
mg/dL, 7.8–11.0 mmol/L) is usually present within 6 months 
before the onset of overt T1DM. 
C-Peptide Loss After Hyperglycemia
After diabetes is diagnosed, levels of C-peptide in serum can be 
used to assess remaining beta-cell function, because C-peptide is 
1418
released from the beta cell in equimolar concentrations with 

om the beta cell in equimolar concentrations with insu­
lin but does not undergo hepatic extraction and has a longer half-
life in circulation. C-peptide levels are usually measured in the 
fasting state, after intravenous glucagon, or with a standard liquid 
meal. Such measurements are primarily of importance for trials 
of therapies to alleviate loss of insulin secretion after diagnosis. 
Determination of the C-peptide concentration provides the best 
current measure for assessing the impact o

e best 
current measure for assessing the impact of new therapies. A small 
amount of remaining C-peptide is associated with impressive met­
abolic benefit, as shown in the Diabetes Control and Complica­
tions Trial (DCCT). 
Transient Hyperglycemia
A significant number of children are evaluated by endocrinolo­
gists for transient hyperglycemia. The usual history is one of 
severe stress associated with hyperglycemia that resolves within 
days to 1 month. Such children may be in the honeymoon pha

1 month. Such children may be in the honeymoon phase 
of T1DM, or they may truly have a transient episode of hyper­
glycemia. Rarely, diabetes in children is misdiagnosed. Children 
without severe stress who have transient hyperglycemia or who 
have a relative with T1DM are more likely to have early T1DM. 
Absence of anti-islet autoantibodies and a normal first-phase insu­
lin secretion during an intravenous glucose tolerance test strongly 
indicate transient hyperglycemia and not T1DM. It is no

ate transient hyperglycemia and not T1DM. It is not known 
whether children with transient hyperglycemia are at increased 
risk for T2DM later in life. 
Emerging Biomarkers of T1DM Natural History 
and Diagnosis
As mentioned throughout this chapter, seroconversion to anti-islet 
autoantibody positivity is the most reliable predictive biomarker 
of T1DM identified to date,11 but as noted, as many as 10% of 
T1DM patients are negative for anti-islet autoantibodies at the 
time of diagnosis.126 Ear

t autoantibodies at the 
time of diagnosis.126 Early diagnosis and treatment with exoge­
nous insulin has been associated with improved disease manage­
ment and fewer complications. Moreover, there are notions that 
early intervention prior to the first signs of autoimmunity might 
offer the greatest chance to successfully prevent T1DM progres­
sion and onset in high-risk individuals. Hence, a substantial effort 
has been directed at the discovery and characterization of new bio­
markers to impr

y and characterization of new bio­
markers to improve our understanding of the disease pathogen­
esis, as well as our ability to predict and stage disease progression, 
facilitating early diagnosis.
The majority of biomarkers identified signify metabolic defects 
across T1DM progression. Specifically, so-called beta-cell death 
assays, proinsulin/C-peptide ratio, and the T1D Diagnostic 
Index60 (Index60) monitor the decline in functional beta-cell 
mass related to autoimmune pathology and/or bet

l 
mass related to autoimmune pathology and/or beta-cell stress in 
T1DM. Specifically, beta-cell death assays involve the measure­
ment of cell-free unmethylated insulin (INS) DNA from blood or 
serum.127 Cell-specific methylation patterns can be used to iden­
tify INS DNA released from necrotic beta cells as opposed to other 
cell types. The short half-life of cell-free DNA in circulation is a 
notable limitation, but as a single example, cell-free unmethyl­
ated INS DNA assays were able to de

free unmethyl­
ated INS DNA assays were able to detect a decline in beta-cell 
death consistent with preserved beta-cell function (measured by 
C-peptide) following treatment with teplizumab (anti-CD3) in a 
clinical trial setting.128 Similarly, an elevated serum proinsulin/C-
peptide ratio measured under fasting conditions implies impaired 
processing of proinsulin to insulin and C-peptide within the beta 
cell, suggesting duress related to metabolic, oxidative, or immu­
nologic stress.129 Thes

bolic, oxidative, or immu­
nologic stress.129 These assays are expected to have the greatest 
utility in the research setting, but the ability to evaluate changes in 
beta-cell death and dysfunction in real time represents a massive 
step forward. Meanwhile, the Index60 has been introduced as a 
clinical/diagnostic biomarker measuring functional beta-cell mass. 
The test requires an OGTT with calculations incorporating the 
log-adjusted fasting C-peptide level, 60-minute C-peptide level, 
and 60

-peptide level, 60-minute C-peptide level, 
and 60-minute blood glucose level.130 An Index60 value greater 
than 2 in an individual with anti-islet autoantibodies is consid­
ered indicative of T1DM onset, but the need for standard clinical 
C-peptide assays has limited its widespread implementation.
Alterations to the microbiome have been suggested as yet another 
biomarker of T1DM. Although less conclusive than the metabolic 
markers discussed previously, prospective studies of infants and 
chi

reviously, prospective studies of infants and 
children with high genetic risk for T1DM have revealed the gut 
microbiome as an early indicator of disease progression, even prior 
to seroconversion. Extensive profiling of the gut microbial com­
munity is generally accomplished by sequencing the microbial 16S 
ribosomal RNA gene from fecal samples. Overall, a high Bacte­
roides to Firmicutes ratio has been associated with anti-islet auto­
immunity, whereas the presence of butyrate-producing bacte

, whereas the presence of butyrate-producing bacteria 
may protect against T1DM by promoting the synthesis of mucin 
and reducing intestinal leakiness.131 In addition, low diversity 
and poor stability of the gut microbial community are associated 
with T1DM,132 although a cause-and-effect relationship has not 
been determined. Yet these T1DM-related microbiome biomark­
ers from high-risk infants enrolled in the TEDDY study have also 
shown geographically distinct patterns with the relative abun

aphically distinct patterns with the relative abundance 
of the most prominent strains and overall microbial diversity dif­
fering across six study sites (the US states of Colorado, Georgia/
Florida, and Washington, as well as Finland, Germany, and Swe­
den).58 At present, it is unknown what factors contribute toward 
these shifts in the pre-T1DM gut microbiome, but they have been 
hypothesized to include diet, antibiotics, infections, and/or house­
hold environment (e.g., exposure to pets, alle

se­
hold environment (e.g., exposure to pets, allergens, pollutants).
Stages in the Natural History of T1DM
Whereas the etiology and precise mechanisms leading to T1DM 
remain somewhat elusive, studies over the past three to four decades 
have generated, without question, a wealth of information—be it 
immunologic, serologic, metabolic, or genetics based—regarding 
the natural history of pre-T1DM through the time of its formal 
diagnosis.133 As but one example and as noted earlier, anti-islet 
a

ut one example and as noted earlier, anti-islet 
autoimmunity in the form of anti-islet autoantibodies, in the 
context of genetic susceptibility and metabolic assessments, have 
clearly been identified as predictive tools for progression to overt 
disease.11 Large international cohorts (e.g., TEDDY, Diabetes 
Prevention Trial Type 1 [DPT-1], DAISY, BabyDiab) have been 
studied in both relatives of patients with T1DM and in the gen­
eral population who are at high genetic risk; this, to better u

on who are at high genetic risk; this, to better under­
stand the disease, reduced rates of DKA, decrease mortality, and 
set the stage for research-based efforts seeking T1DM prevention. 
Indeed, these natural history studies often led to the earlier diag­
nosis of T1DM and a reduced frequency of DKA at onset in those 
followed prospectively.134 Several recent studies have also demon­
strated long-term glycemic benefits seen for those patients who 
did not experience the severe metabolic derang

ho 
did not experience the severe metabolic derangement of DKA at 
formal diagnosis.135
Although the diagnosis of diabetes has traditionally been based 
on ADA criteria,136 it is increasingly clear that the actual onset of 
1419
the disease, per se, often occurs months to years before the onset of 
symptoms. Thus, T1DM is a unique physiologic state where auto­
immunity is present and progression to metabolic derangement 
and clinical onset can be predicted, e

erangement 
and clinical onset can be predicted, especially in younger children 
and adolescents. As such, the ADA, JDRF, and Endocrine Society 
released a joint position statement calling for the staging of pre-
T1DM137 (Fig. 36.5). Stage 1 is defined by the presence of two 
or more anti-islet autoantibodies with normoglycemia (normal 
glucose tolerance on 2-hour OGTT). Stage 2 shows progression 
to dysglycemia (impaired glucose tolerance) in the setting of two 
or more anti-islet autoantibodie

e setting of two 
or more anti-islet autoantibodies. Stage 3 occurs when a patient 
meets ADA criteria for the diagnosis of diabetes.
To be clear, there are no evidence-based recommendations for prac­
titioners caring for anti-islet autoantibody–positive patients, or 
for performing formal “T1DM staging,” other than to encourage 
enrollment in research studies. Nonetheless, close monitoring of 
high-risk patients in natural history studies markedly reduces DKA 
rates at diagnosis, and when combi

ly reduces DKA 
rates at diagnosis, and when combined with the notion that partak­
ing in research is critical to finding a means of preventing T1DM, 
research participation in family members of those with T1DM 
should be encouraged. Yet only when an effective preventative strat­
egy for T1DM is identified will the notion of universal risk screen­
ing and T1DM staging for all children be considered justifiable. 
Immunotherapy for the Prevention 
and Reversal of T1DM
The intersection of two indep

The intersection of two independent events in the 1970s—the gen­
eralized acknowledgment that T1DM represents an autoimmune 
disease, alongside the development of immunosuppressive 
agents—led to decades of efforts directed toward immunotherapy 
as a means to either prevent T1DM (i.e., avert symptomatic onset) 
or, once diagnosed, reverse it (commonly termed intervention). To 
be clear, studies attempting to prevent the development of T1DM 
involve treatment of persons at va

opment of T1DM 
involve treatment of persons at varying stages of disease risk based 
on a combination of genetic susceptibility, the number of T1DM-
associated autoantibodies, and the degree to which glucose levels 
are elevated.138 Although the populations subject to such proce­
dures would differ, the common goal is to prevent further loss of 
beta-cell function.
At present, in a setting of either disease prevention or inter­
vention, there is no proven safe and effective immunotherapy for 
u

 no proven safe and effective immunotherapy for 
use in a general public health care setting that prevents T1DM or 
reverses the disease, as judged by preservation of C-peptide. How­
ever, in research settings, potentially promising candidates do exist 
for disease prevention (e.g., oral insulin, GAD vaccine) or inter­
vention (e.g., anti-CD3, antithymocyte globulin [ATG], ATG 
plus granulocyte colony stimulating factor, CTLA4-Ig).139–143 
Indeed, dozens of clinical trials, ranging from small to

, dozens of clinical trials, ranging from small to large, have 
been completed in both settings; a limited number are discussed 
later based on their proposed method of action (Fig. 36.6).
The earliest studies of therapies to prevent beta-cell destruction 
used immunosuppressive agents. Large intervention trials of 
cyclosporine indicated that this agent prevented further loss of 
C-peptide secretion and improved metabolic function when it was 
administered in recent-onset T1DM

ion when it was 
administered in recent-onset T1DM cases. Cyclosporine therapy 
100%
Functional β-Cell Mass
Time
0%
Variable
Genetic &
Risk for Type 1
Diabetes
Presymptomatic
Type 1 Diabetes
Symptomatic
Phenotypic Characteristics
β-Cell
Autoimmunity
Normoglycemia
β-Cell
Dysglycemia
β-Cell
Proposed nomenclature
Stage 1
Stage 2
Stage 3
Phase in Natural History
• Fig. 36.5  Staging of pre–t

e in Natural History
• Fig. 36.5  Staging of pre–type 1 diabetes mellitus. (Redrawn from Insel RA, Dunne JL, Atkinson MA, et al. 
Staging presymptomatic type 1 diabetes: a scientific statement of JDRF, the Endocrine Society, and the 
American Diabetes Association. Diabetes Care. 2015;38[10]:1964–1974.)
1420
did not, however, maintain a nondiabetic state when therapy was 
instituted after the onset of diabetes, and with discontinuation 

r the onset of diabetes, and with discontinuation of 
the drug, patients rapidly lost C-peptide reserve. The combination 
of inability to cure diabetes and toxicities associated with cyclospo­
rine (particularly nephrotoxicity and concern about increased risk 
of malignancy) ruled out its acceptance for use. Other immuno­
suppressive agents used in subsequent studies, such as prednisone 
and azathioprine, demonstrated relatively little effect in disease 
intervention. In the time since, a litany

disease 
intervention. In the time since, a litany of agents (e.g., anti-CD20, 
ATG, mycophenolate plus anti-IL2 receptor α-monoclonal anti­
body) thought to act at least in part via immune depletion, a 
form of immune suppression, have demonstrated mixed results 
in T1DM intervention efforts (see Fig. 36.6A). Some provide 
no therapeutic benefit (i.e., preservation of C-peptide, reduced 
Aldesleukin
Treg
Sitagliptin
Incretin
Delay & AbATE—
Teplizumab
DEFEND 1 & DEFEND 2—
Otelixizumab
Autologous

zumab
Non-myeloablative
Hematopoietic Stem
Cell Transplant with
ATG +
Cyclophosphamide
Conditioning
A
1-alpha-
hydroxyvitamin D3
1alpha, 25-
dihydroxyvitamin D3
Hematopoietic
Stem Cell
Transplant
ChAglyCD3
Protégé—
DIATOR—
Atorvastatin
Mycophenolate
mofetil +
Daclizumab
BCG
Vaccination
Bone Marrow
Stem Cells
Wharton’s
jelly-derived
Stem Cells
Anakinra
DILT1D—IL-2
CD3 CD4 CD25 CD127
Polyclonal Tre

—IL-2
Polyclonal Treg cell
therapy
REPAIR-
T1D—
Sitagliptin +
Lansopnazole
START – ATG
ATG + G-CSF
G-CSF
Umbilical Cord
Blood
Cord
Blood + Oral
Docosahexaenoic
acid + Vitamin D
Exenatide +/–
Stem Cells
T cells
+/–
Incretin
Vitamin D
T & B
cells
T cells
T cells
UCB
Incre-
tin
& H
Thymocytes +/–
Immunomobili-
zation
Treg
IL-1
IL-1
T cell-
APC
Stem
Cells
Stem
Cells
Hygiene
Hypoth-
esis
2007
2008
B cells
Cytokines
Global Immun

oth-
esis
2007
2008
B cells
Cytokines
Global Immune
UCB
DCs
Vitamin D
Treg +/– β cell
T cell co-
Innate
Immunity
Vitamin
D
Stem Cells
Innate
Immunit
Alpha-1
antitrypsin
Alpha-1
Tolerogenic
Dendritic
Cells
ENBREL
(Etanercept)
Oral IFN-α
Rituximab
Umbilical
Cord Blood
Transfusion
Cholecalciferol
Cyclosporin +
Methotrexate
Rapamycin
+ IL-2
DF-IL2—
IL-2
TIDAL—Alefacept
Abatacept
Immunoablation (ATG + Cyclophosphamid

lecalciferol
Immunoablation (ATG + Cyclophosphamide) +
Autologous Hematopoietic Stem Cell Transplant
+ G-CSF
Umbilical Cord Mesenchymal Stromal Cell +
Autologous Bone Marrow Mononuclear Cell
Stem Cell Transplant
Adipose-derived Insulin-secreting Mesenchymal
Stromal Cells + Bone Marrow-derived
Hematopoietic Stem Cell Infusion
Stem Cell Educator Therapy
Autologous Mesenchymal Stromal Cells
Stem Cells
Chemotherapeutics
Alpha1-Antitrypsin – 2017
PGD2 receptor antagonist MK-1092 – 201

psin – 2017
PGD2 receptor antagonist MK-1092 – 2018
Autologous Immunoregulatory Dendritic Cells – 2019
Mesenchymal Stem Cells – 2017
Stem Cell Educator Therapy – 2019
aLD-SCs/CB-MSCs + G-CSF – 2019
Allogeneic Mesenchymal Stem Cells – 2020
Allogeneic Adipose Mesenchymal Stem Cells with
Bone Marrow Mononuclear Cells – 2020
Imatinib – 2019
Neuropeptide
Substance P – 2017
CXCR1/2 Inhibitor
Ladarixin – 2019
Incretin
Exenatide – 2017
Saxagliptin – 2017
Albiglutide – 2017
Vildagliptin – 2017
Rapamycin 

Rapamycin + Vildagliptin – 2019
Liraglutide – 2020
Vitamin D
Ergocalciferol – 2020
Probiotics – 2018
Prebiotic Fiber – 2019
GNbAC1 –2019
Enterovirus vaccination – 2022
BCG vaccination – 2023
 cell Stress
Tauroursodeoxycholic Acid (TUDCA) – 2019
Verapamil – 2019
Alpha difluoromethylornithine (DFMO) – 2019
Thymocyte Depletion + Immunomobilization
Cytokines
T cells
Tregs
Costimulation
ATG + G-CSF + Rapamycin + Islet Transplant – 2018
Al

G + G-CSF + Rapamycin + Islet Transplant – 2018
Alemtuzumab + Anakinra + Etanercept + Liraglutide +
Plerixafor – 2021
ATG + G-CSF + Low dose IL-2 + Etanercept +
Exenatide – 2021
Anti-IL-6 (Siltuximab) – 2019
Anti-IL21 (NNC0114-0006) + Liraglutide – 2019
Anti-IL-6 Receptor (Tocilizumab) – 2020
Anti-TNFα (Golimumab) – 2019 + 2021
Anti-IL12/IL-23 (Ustekinumab) + INGAP – 2022
Anti-IL12/IL-23 (Ustekinumab) – 2023
Otelixizumab – 2018
IMCY-0098 – 2019
Teplizumab – 2022
Abatacept (CTLA4-lg) – 2020
IL-2 

eplizumab – 2022
IL-2 – 2017
Low dose rhIL-2 – 2018
Umbilical Cord Blood Tregs + Liraglitude – 2019
Umblilical Cord Blood Tregs – 2019
CLBS03 (Autologous Ex Vivo Expanded Polyclonal
Regulatory T-cells) – 2020
Tregs + anti-CD20 antibody – N/A
Low dose Proleukin (IL-2) – 2023
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
ATG + G-CSF
Ongoing
Canakinumab
AIDA—
Anakinra
Immunomobilization
CD4 CD127
CD25  Polyclonal Tregs + IL-2 – 2021
• Fig. 36.6  (

D25  Polyclonal Tregs + IL-2 – 2021
• Fig. 36.6  (A) Shifts in T1DM clinical research emphasis over 10 years from 2007 to 2017. Therapeutic 
agents and target mechanisms/objectives are noted for T1DM clinical trials over the past decade, organized 
by their year of first publication. Current ongoing studies are noted with their expected year of completion. 
AIDA, Anti-Interleukin-1 in Diabetes Action trial; APC, antigen-presenting cell; ATG, antithymocyte globulin; 
BCG, bacillus Calmette-Guérin

thymocyte globulin; 
BCG, bacillus Calmette-Guérin; DCs, dendritic cells; DF, dose finding; G-CSF, granulocyte colony-stimu­
lating factor; IL, interleukin; T1DM, type 1 diabetes mellitus; UCB, umbilical cord blood. (A, modified from 
Atkinson MA, Posgai A, Wheeler DCS, Peakman M. The challenge of modulating β-cell autoimmunity in 
type 1 diabetes. Lancet Diabetes Endocrinol. 2019;7[1]:52–64. 
1421
insulin needs, reduction in hypoglycemic events), whereas oth

ds, reduction in hypoglycemic events), whereas oth­
ers have achieved at least one of these beneficial parameters, usu­
ally C-peptide preservation. However, even when efficacious, for 
most subjects, the beneficial effect was not durable (i.e., 6 months 
to 1 year of benefit).141,143 It is possible that the treatment effect 
may persist for a longer term with repeat dosing, although this 
approach has not been tested. Therefore, at present, although 
T1DM is an immune-mediated disorder, it is n

ough 
T1DM is an immune-mediated disorder, it is not treated with 
immunotherapeutic agents in clinical practice. This is not to say 
that hope is lost for such being the case in the future. Indeed, in 
recent times (e.g., year 2000 to the present), studies related to 
two forms of modified antibodies against the CD3 molecule have, 
without question, generated the most research interest and offer 
some degree of eventual optimism.
Throughout several phase I and phase II intervention studies, a 

ral phase I and phase II intervention studies, a 
single course of anti-CD3 therapy decreased the loss of C-peptide 
secretion over a 12- to 24-month period in new-onset T1DM 
patients.141 However, phase III trials were not considered of thera­
peutic benefit to the extent that commercialization of these agents 
was to some degree disrupted. Concerns related to the activation 
of Epstein-Barr virus infection, the duration of C-peptide preser­
vation, and the protocol by which the drugs are admin

ion, and the protocol by which the drugs are administered have 
further limited their development. Thus, limited efforts continue 
to move anti-CD3 back into the therapeutic setting. A recent 
study demonstrated that a short course of the anti-CD3 monoclo­
nal antibody, teplizumab, delayed progression to clinical T1DM 
in high-risk patients.144 
Immunologic Vaccination
In animal models (especially the NOD mouse), it is relatively easy 
to prevent T1DM. Indeed, although somewhat dated, a widely 

 T1DM. Indeed, although somewhat dated, a widely 
recognized effort notes more than 200 such means.145 Yet animal 
models have permitted development of the most exciting poten­
tial prevention or intervention modality of immunologic vaccina­
tion. Excitement is derived from the specificity and the relatively 
low risk of these therapies compared with immunosuppression. 
The basic concept behind immunologic vaccination is the induc­
tion of regulatory T lymphocytes that target a given beta-cell a

tory T lymphocytes that target a given beta-cell anti­
gen and, on encountering the target antigen (e.g., insulin, GAD) 
in the context of the vaccine formula or route of administration, 
produce cytokines and cell-mediated effects that suppress autoim­
munity and tissue destruction.146
Induction of a protective immune response may depend on the 
route of administration of the given antigen (e.g., oral, nasal, intra­
dermal) or on the use of an altered antigen (e.g., altered peptide 
ligands). F

ltered antigen (e.g., altered peptide 
ligands). For example, insulin given either orally or by subcutane­
ous injection prevents diabetes in NOD mice.147,148 Intact insulin 
is not necessary, because the insulin B chain and an immunodomi­
nant B(9-23) peptide of insulin were also effective.149 The latter 
molecules have no insulin-like metabolic effect but are able to 
activate regulatory T lymphocytes that target insulin.
In terms of translation, DPT-1 studied both oral insulin and 
parenteral

n, DPT-1 studied both oral insulin and 
parenteral injections of low doses of insulin (see Fig. 36.6B). The 
results of the parenteral trial did not demonstrate a reduction in 
MultiPepT1De 
Insulin
Insulin
B-chain
insulin
Proinsulin
C19-A3
peptide
INIT:
Insulin
DNA
plasmid
Pre-POINT:
Oral Insulin
Insulin
Insulin
GAD-alum
GAD-alum
GAD-alum
GAD-alum
GAD or MultiPep
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
Ongoing
C19-A3
peptide
DPT-1:

2017
2018
Ongoing
C19-A3
peptide
DPT-1:
DiaPep277 in
European adults
European children
Israeli adults
Israeli chlidren
Belgian adults
B
DiAPREV-IT:
GAD-alum
D-Sense: PlpepToIDC – 2018
PINIT Study: Intranasal Insulin – 2019
DIAGNODE: GAD-alum + Vitamin D – 2019
EE-ASI-1: C19-A3 GNP – 2020
GABA-GAD: GAD-alum + GABA – 2020
Fr1da Insulin Intervention: Oral Insulin – 2021
DiAPREV-IT2: GAD-alum + Vitamin D – 2022
INITII: Intra

REV-IT2: GAD-alum + Vitamin D – 2022
INITII: Intranasal Insulin – 2024
DIABGAD:
GAD-alum +
Vitamin D +
Ibuprofen
TrialNet:
Oral
Insulin
Pre-
POINT-
Early:
Oral
Insulin
Fig. 36.6, cont’d  (B) Timeline of completed and ongoing antigen-specific trials for T1DM. B, modified from 
Roep BO, Wheeler DCS, Peakman M. Antigen-based immune modulation therapy for type 1 diabetes in 
the era of precision medicine. Lancet Diabetes Endocrinol. 2019;7[1]:65–74.)
1422
SECTION VIII    Disorders of Carbohydrate an

1422
the risk of developing diabetes. The oral trial did not document 
an overall benefit of insulin, but in the subgroup with higher lev­
els of IAA at entry, a statistically significant delay in progression 
to diabetes was observed.150 Although the National Institutes of 
Health TrialNet conducted a follow-up study testing the efficacy 
of oral insulin to prevent T1DM in those having certain criteria 
related to age and IAA level, 

g certain criteria 
related to age and IAA level, no benefit of oral insulin for delay or 
prevention of T1DM was seen.151 Nevertheless, a pilot trial (Pre-
POINT) led by investigators in Germany involving the adminis­
tration of high-dose oral insulin in children at high genetic risk 
for T1DM with a family history of the disease reported that this 
form of therapy was capable of altering the anti-insulin immune 
response without inducing hypoglycemia.152 Thus, potential value 
of oral insulin 

ycemia.152 Thus, potential value 
of oral insulin remains uncertain. 
Therapeutic Targeting for Mechanisms 
of Beta-Cell Death
As noted, the most prominent cell found in human islets in set­
tings of T1DM is the CD8+ cytotoxic T cell, which likely partici­
pates in beta-cell killing because of its ability to recognize targets 
via antigen in the context of MHC class I, which is elevated in 
many islets in those with the disease.39 Therapeutically, lympho­
cytes and memory lymphocytes of the adap

, lympho­
cytes and memory lymphocytes of the adaptive immune response 
can be targeted by anti–T-cell drugs directed at CD3 (teplizumab), 
CD2 (LFA3-Ig), and certain co-stimulatory blockers.141,153,154 
Indeed, partial success of such compounds in recently diagnosed 
T1DM, defined by preservation of C-peptide production over 
several months to years, speaks toward an important role for such 
autoreactive lymphocytes in beta-cell destruction, at least late in 
the pathogenesis of this disease. I

least late in 
the pathogenesis of this disease. Inflammatory cytokines are also 
known to harm islets, and therefore anti-inflammatory therapies 
targeting cytokines may hold promise, as observed in a trial block­
ing TNF.155 These observations provide further support for the 
concept of combination therapies.
With the growing concept that multiple mechanisms likely 
contribute to the pathogenesis of T1DM, enthusiasm has 
increased for utilizing combination therapies to prevent or reverse 
the 

 combination therapies to prevent or reverse 
the disease. Examples of combinations with respect to immune 
intervention would include an induction component using drugs 
targeting inflammation and T-/B-cell memory and a maintenance 
component that could involve antigens to induce tolerance to beta 
cells. At the same time, emerging data support an increasing role 
for beta-cell stress and loss of function as potential contributing 
factors for T1DM development; therefore, combination therapy 
i

1DM development; therefore, combination therapy 
involving the addition of drugs that stabilize and maintain beta-
cell survival and function should be considered.156 
Pancreas and Islet Cell Transplantation
Pancreatic transplantation for patients requiring a kidney 
transplant has become an accepted clinical procedure, but one 
that for a variety of reasons, particularly the shortage of organ 
donors, remains relatively rare. Patients with a kidney trans­
plant receive immunosuppressive drugs t

ney trans­
plant receive immunosuppressive drugs that, given the nature 
of clinical monitoring for renal transplant rejection (e.g., cre­
atinine, albumin), allow for careful (i.e., tailored) drug delivery 
for optimal care. Several studies suggest that improved results 
for pancreatic transplantation occur in the setting of simulta­
neous pancreas and kidney transplantation versus transplanta­
tion of pancreas alone.157 Indeed, there is considerable debate 
concerning pancreas transplantation 

rable debate 
concerning pancreas transplantation without transplantation 
of a kidney. With a successful pancreas transplant, hypergly­
cemia is immediately reversed for most, and there is growing 
evidence of improved long-term outcomes. Nevertheless, the 
surgery is extensive, and there are many potential complica­
tions associated with the transplant. Perhaps for these reasons, 
along with improvements in diabetes management (and thus 
reducing long-term complications, including kidney failu

ng long-term complications, including kidney failure), it 
appears that the procedure peaked in the mid- to late 2000s, 
with declines in the number of procedures performed in the 
United States thereafter.158
Beyond therapeutic interest in pancreatic transplantation, 
additional research has focused on the mechanisms of pancreas 
rejection for what it might reveal regarding the immunologic 
mechanisms of T1DM. In that regard, in T1DM patients pro­
vided with a pancreatic transplant, diabetes (i

o­
vided with a pancreatic transplant, diabetes (i.e., a repeated need 
for exogenous insulin replacement therapy) can recur as a result of 
mechanisms considered attributable to either recurrent autoim­
munity (as typified by the recurrence of T1DM-associated auto­
antibodies once lost) or, more often, allograft rejection.159 It is 
difficult to monitor and determine which of these two forms of 
islet destruction are occurring with the development of hypergly­
cemia, and currently there are no 

nt of hypergly­
cemia, and currently there are no specific means in terms of treat­
ment that would separate the two, although at least one study has 
provided evidence for recurrent autoimmunity manifested by the 
induction of anti-islet autoantibodies prior to graft rejection.160
With respect to islet (vs. pancreas) transplant, therapeutic 
hopes for this procedure have been high since the late 1960s 
when early pioneering approaches were developed for islet isola­
tion from whole pancreata. I

oped for islet isola­
tion from whole pancreata. Indeed, with islet autotransplantation 
in patients with pancreatitis, most patients become insulin inde­
pendent and remain so.161 Yet it was not until the early 2000s that 
a group from Edmonton, Canada, used meticulous islet isolation 
techniques, transplantation of islets from multiple pancreata, 
avoided the use of steroids, and utilized an immunosuppressive 
regimen involving the drug rapamycin, resulting in improved 
outcomes for patients w

in, resulting in improved 
outcomes for patients with T1DM. This so-called Edmonton 
protocol has subsequently been tested in a series of specialized 
centers throughout North America and Europe. It is clear that 
in many centers, albeit with varying degrees of success, islet cell 
transplant can prove effective (i.e., ∼60% insulin independent at 
1 year), especially for patients with severe hypoglycemia when the 
goal is long-term prevention of severe hypoglycemic episodes. For 
most of the pat

severe hypoglycemic episodes. For 
most of the patients who achieve insulin independence, resump­
tion of use of low doses of insulin is necessary within 2 years, and 
by 5 years the benefits of the form of therapy wane.162 Even with 
these positive results, the number of islets available from cadav­
eric donors for transplantation is quite limited, and the toxicities 
of the drugs used probably exceed the level of benefit achieved, 
except for those most subject to disease-associated complicati

hose most subject to disease-associated complications 
(e.g., severe, recurrent hypoglycemia). Further research to achieve 
tolerance without long-term immunosuppression is essential, as 
is continued development of systems allowing for xenogeneic 
transplantation.
Of immense promise for the future are therapies involving 
the use of surrogate beta-like cells (i.e., insulin producing, glu­
cose responsive) produced from stem cells, either embryonic 
or induced from pluripotent cells originating 

ic 
or induced from pluripotent cells originating from a variety 
of tissues. In addition, use of encapsulation or biosynthetic 
membranes to protect transplanted islets, combined with the 
increasing use of long-term continuous glucose monitoring 
(CGM), combined with improvements in modes of insulin 
delivery (e.g., analogues, pumps) or use of secondary agents in 
1423
patients with T1DM, will almost certainly raise the bar for con­
sideration of the risks 

ly raise the bar for con­
sideration of the risks and benefits of both islet and pancreatic 
Stem Cell or Xenogeneic Islet Cell 
Transplantation
As noted, improved metabolic control averting the need for 
exogenous insulin replacement can be achieved with beta-cell 
replacement in those with T1DM, either through whole-pan­
creas transplantation or via pancreatic islet cell transplantation. 
Therefore, although whole-pancreas and islet cell transplanta­
tion have both become high

islet cell transplanta­
tion have both become highly operationalized and often effective 
procedures, the quantitative paucity of suitable organ donor tis­
sues, taken together with complications associated with immune 
suppression, have severely limited their widespread therapeutic 
application.163 As a result, numerous research efforts have been 
extended over the past decade toward the development of sus­
tainable sources of “surrogate” insulin-producing cells (e.g., 
xenogeneic islets, induc

n-producing cells (e.g., 
xenogeneic islets, induced pluripotent stem cells, fetal stem cells) 
alongside additional methodological investigations, all designed 
to improve therapeutic outcomes utilizing such cells (e.g., genetic 
modification, encapsulation).164,165 Importantly, engineering 
and/or genetic modification–based systems have been proposed to 
avert aberrant and deleterious immune responses associated with 
T1DM and/or the use of allogenic cells.166
Indeed, remarkable progress has b

ogenic cells.166
Indeed, remarkable progress has been made over a relatively 
short period of time with respect to generating beta-cell equiv­
alents (i.e., glucose-responsive insulin-producing cells) from 
human stem cell populations, be they derived from induced plu­
ripotent stem cells or embryonic stem cells.167 Xenogeneic efforts 
(e.g., porcine islets), for a variety of reasons, although still a poten­
tial therapeutic option, are currently less favored than stem cells 
due to a variety of

 less favored than stem cells 
due to a variety of technical challenges, as well as concerns over 
safety.168
Stem cell therapy–based efforts as a therapy for T1DM largely 
arose from a strategy designed in laboratory-based settings to 
recapitulate the islet cell developmental pathway while generating 
single hormone-positive beta cells capable of glucose-stimulated 
insulin secretion.169 Progress has been remarkable, with clinical 
trials already planned involving the use of stem cell–derived 

dy planned involving the use of stem cell–derived pan­
creatic progenitors capable of developing into functional beta and 
islet cells.170
Although the future of stem cells as a clinical therapy is promis­
ing, a series of concerns/limitations remain. First, beta cells, the pri­
mary cell sought for stem cell development, in their natural setting 
do not exist in isolation (i.e., beta cells alone) but rather exist in a 
heterogenous three-dimensional structure comprised of the islets of 
Langerh

onal structure comprised of the islets of 
Langerhans. Hence, the notion of generating a cell therapy in the 
absence of a full complement of islet cells (i.e., alpha, beta, delta, 
pancreatic polypeptide; see Chapter 33) must be considered moving 
forward, especially in the context of hormonal counterregulation. 
In addition strong consideration must continue to be given toward 
averting what likely would be an assault on the target tissue—both 
autoimmune and alloimmune, depending on the setti

autoimmune and alloimmune, depending on the setting. To this 
end, effort for any so-called protective devices should encompass 
a need to (1) provide biocompatibility; (2) see ample blood supply 
to sustain cellular survival and functional beta-cell mass capable of 
imparting normoglycemia; (3) prevent sensitization and be immune 
protective; (4) trap potentially oncogenic cells; and (5) allow for 
rapid response of insulin, in terms of both activating and suppress­
ing insulin secretion, to c

tivating and suppress­
ing insulin secretion, to changing glucose concentrations.166 
Disorders Associated With Immunity 
to Insulin/Insulin Receptor
Insulin Autoimmune Syndrome
The insulin autoimmune syndrome, also termed Hirata syn­
drome, is rare and is typically associated with hypoglycemia.171 
These patients have extremely high concentrations of autoanti­
bodies reacting with human insulin in the absence of exogenous 
insulin therapy. It is thought that inappropriate (i.e., nonregu­
lated 

 thought that inappropriate (i.e., nonregu­
lated by the overall blood glucose level) release of autoantibody-
bound insulin produces the hypoglycemia. Interestingly, and for 
reasons unknown, the disease occurs most commonly in persons 
of Asian descent.172 Among 50 Japanese patients with the syn­
drome and possessing the typical polyclonal anti-IAA, 96% had 
an HLA-DR4 allele, and 84% possessed a DRB1*04:06 allele. In 
contrast, patients with monoclonal anti-IAA do not possess such 
a profound

onoclonal anti-IAA do not possess such 
a profound and specific HLA association.173 Most patients with 
this disorder develop the disease in association with treatment of 
sulfhydryl-containing medications, particularly methimazole, as 
well as alpha-lipoic acid.174 Treatment usually consists of stopping 
these medications, and for more than 75% of the patients, the 
disease remits. 
Insulin Allergy
Mild immune reactivity against exogenously administered insu­
lin is not an uncommon feature in T

nistered insu­
lin is not an uncommon feature in T1DM management. Indeed, 
essentially all patients treated with recombinant human insu­
lin produce anti-insulin antibodies.175 The levels of these anti­
bodies are relatively low, and they do not appear to interfere 
with insulin therapy for the majority of individuals. Most stud­
ies show no relationship between the presence of anti-insulin 
antibodies and complications associated with T1DM (e.g., reti­
nopathy, neuropathy).175 However, there ar

, reti­
nopathy, neuropathy).175 However, there are reports correlating 
insulin antibodies with macrosomia.176 With the introduc­
tion of recombinant human insulin replacing animal insulins, 
symptomatic immune responses to insulin such as immediate 
hypersensitivity, delayed hypersensitivity, lipoatrophy, and lipo­
hypertrophy have decreased.
Allergic reactions can occur with insulin analogues, modifica­
tions of recombinant human insulin, although this is uncommon. 
More common features inclu

ough this is uncommon. 
More common features include allergies to affiliated lubricants; 
preservatives; and plastics in bottles, stoppers, syringes, and nee­
dles. The usual mode of therapy for such situations consists of 
substituting the type or formulation of insulin and administration 
of oral antihistamines or topical mast cell stabilizers for immuno­
globulin E–mediated local reactions, followed by insulin desen­
sitization or addition of small amounts of glucocorticoids to the 
insulin i

small amounts of glucocorticoids to the 
insulin injected for local delayed hypersensitivity reactions.177,178 
Anti-Insulin Receptor Autoantibodies
Anti-insulin receptor autoantibodies (i.e., type B insulin resis­
tance) are associated with hypoglycemia, hypercatabolism, severe 
acanthosis nigricans, and insulin resistance. It appears that anti-
insulin receptor autoantibodies can act as either antagonists or 
agonists for this disease. This syndrome is quite rare, is often asso­
ciated with no

drome is quite rare, is often asso­
ciated with non–organ-specific autoimmunity, and treatments 
usually involve various forms of immunosuppression (e.g., ritux­
imab (anti-CD20), steroids, intravenous immune globulin, cyclo­
phosphamide), albeit with mixed success.179 
1424
The peak age for presentation of T1DM is at or during puberty, 
with a smaller peak in children between 5 and 7 years of age. 
The symptoms a

dren between 5 and 7 years of age. 
The symptoms and signs are related to the presence of hypergly­
cemia and the resulting effects on fluid and electrolyte balance, 
which include polyuria, polydipsia, polyphagia, weight loss, 
and blurred vision.9 Because infection may have precipitated or 
preceded the initial presentation, symptoms of infection may 
also be present, such as fever, sore throat, cough, or dysuria. 
Particularly in children, the onset of symptoms can occur over 
a brief period,

 onset of symptoms can occur over 
a brief period, and families may be able to date the onset with 
considerable accuracy. In older persons with T1DM, the onset 
of symptoms may be insidious over months, and many are mis­
takenly diagnosed as having T2DM by screening during this 
asymptomatic period.
If onset of T1DM is associated with DKA, which is not 
uncommon, additional symptoms related to this acute metabolic 
complication of diabetes are also present. These symptoms can 
include abdominal

lso present. These symptoms can 
include abdominal pain, nausea, and vomiting. Variable effects on 
mental status may be seen, ranging from slight drowsiness to pro­
found lethargy and even coma if the condition has been untreated 
for a significant period.
Laboratory Findings at Presentation
Plasma glucose concentrations at presentation of T1DM are 
elevated, usually in the range of 300 to 500 mg/dL (16.7–27.8 
mmol/L). If the presentation is uncomplicated, the remainder of 
the fluid and elect

complicated, the remainder of 
the fluid and electrolyte measurements may be completely nor­
mal. However, if DKA is present, the measurements will reflect 
the presence of an acidosis, as well as more severe dehydration. 
DKA is discussed more fully later in this chapter.
At presentation, the C-peptide level (a surrogate marker 
for insulin secretion) is generally in the low-normal range and 
declines over time. However, residual C-peptide may be detected 
throughout the natural history of diab

e detected 
throughout the natural history of diabetes. In reference laborato­
ries, pancreatic autoantibodies are present in approximately 98% 
of individuals at diagnosis, but most commercial laboratories 
do not provide either the full spectrum of assays or equivalently 
sensitive or specific assays, resulting in both false-negative and 
-positive assays (discussed previously). Furthermore, antibody 
titers diminish over time and may be less prevalent in certain 
ethnicities.180 
Treatment
Pr

revalent in certain 
Treatment
Principles of T1DM Management
Despite advances in understanding the autoimmune nature of 
T1DM, this disease remains incurable yet manageable. Man­
agement requires orchestrated efforts on the part of the person 
with diabetes, family members, and the multidisciplinary health 
care team. Important stakeholders may vary, depending on the 
age of onset of the disease, comorbid conditions, stages of tran­
sition throughout the life span, from early ch

an­
sition throughout the life span, from early childhood through 
adolescence and young adulthood to adulthood and the middle-
age years, then finally culminating in the advancing years of the 
geriatric population. Although diabetes diagnosed in childhood 
is predominantly due to T1DM,181 there is increasing recogni­
tion that T1DM is diagnosed in adulthood.182 The likelihood for 
residual beta-cell function is greater with an older age of onset of 
disease.183 Nonetheless, a diagnosis of T1DM

t of 
disease.183 Nonetheless, a diagnosis of T1DM at any age requires 
exogenous insulin therapy given by injection or continuous sub­
cutaneous insulin infusion (CSII); careful attention to dietary 
intake, especially with regard to carbohydrates, the major contrib­
utor to glycemic excursions; frequent or continuous monitoring of 
glucose levels with a blood glucose meter or CGM, respectively; 
and an exercise program that is as much therapeutic as recreational 
in nature. These management ac

ic as recreational 
in nature. These management activities require constant vigilance, 
with attention to treatment needs 24 hours per day, 7 days a week, 
every year.
Such a rigorous management program requires input of a mul­
tidisciplinary treatment team, composed of an endocrinologist or 
physician with specific training and interest in diabetes, a certified 
diabetes educator, a dietician, and a mental health professional 
(either a social worker or psychologist) with diabetes behavioral 
h

orker or psychologist) with diabetes behavioral 
health expertise. Other team members can include an exercise 
physiologist, an ophthalmologist or optometrist with diabetes 
expertise, a podiatrist, a nephrologist, and other specialists as indi­
cated. Most importantly, persons with T1DM of all ages require 
continuous access to a health care provider through an emergency 
contact number in case of emergence of acute complications, such 
as severe hypoglycemia or DKA.
Visit frequency for diabete

e hypoglycemia or DKA.
Visit frequency for diabetes care depends on the age at diagno­
sis, duration of disease, and presence of comorbidities. At T1DM 
diagnosis, initiation of insulin therapy is amenable to outpatient 
management when electrolytes are stable and in the absence of 
acidosis. Initial visits usually entail 2 to 3 sequential days of mul­
tidisciplinary visits, followed by visits in 2 to 3 weeks, then 1 to 2 
months later. Thereafter, diabetes follow-up care generally occurs 
quart

r, diabetes follow-up care generally occurs 
quarterly, especially in childhood when there is need to monitor 
growth and development. Both weight and height, along with 
vital signs, should be obtained at every quarterly visit, and Tan­
ner staging should be performed at least annually for pediatric 
patients. Similarly, weights and vital signs should be obtained at 
each visit along with timely screening for complications and car­
diovascular risk factors.
In childhood, initial management educ

isk factors.
In childhood, initial management education is directed to the 
parents/guardians, whereas a family-based approach to care con­
tinues throughout adolescence.184,185 As children get older, diabe­
tes management tasks and the delivery of medical care gradually 
shift onto the person with diabetes, although T1DM at any age 
is not a do-it-yourself condition, given the many daily treatment 
demands and nuances of advanced diabetes technologies. In adult­
hood, following the visit freque

logies. In adult­
hood, following the visit frequency required for all persons newly 
diagnosed with T1DM, follow-up visits may occur every 3 to 6 
months, depending on the achievement of glycemic goals and the 
presence of comorbidities.180,186,187
Diabetes education for all stakeholders occurs in a develop­
mentally appropriate manner, geared to ensure comprehension of 
all involved in care. Diabetes management is also tailored to the 
needs and desires of the person with diabetes, creating an

d desires of the person with diabetes, creating an indi­
vidualized care approach. Thus, a collaborative care model allows 
for optimal selection of one of the various approaches to insulin 
delivery, from the simpler two-injection-per-day regimen to the 
more complex semiautomated approach to insulin delivery using 
CSII and CGM. It remains important to reassess management 
approaches during the various developmental stages, especially 
during puberty, when physiologic insulin resistance couple

uberty, when physiologic insulin resistance coupled with 
psychosocial turmoil can lead to increased insulin needs and dete­
riorating glycemic control.188,189 Similarly, adult needs change 
and require equal attention by the provider to the individual’s 
preferences. 
1425
Glycemic Targets
The fundamental approach of individualized care is based on the 
treatment goal of achieving glycemic targets. The DCCT was the 
randomized clinical trial that laid the fo

as the 
randomized clinical trial that laid the foundation of intensive 
insulin therapy as the standard of care.190 In the DCCT, intensive 
insulin was compared with conventional insulin therapy in 1441 
persons, 13 to 39 years old, with established T1DM. Intensive 
insulin therapy consisted of physiologic insulin replacement with 
either basal bolus therapy or CSII; conventional therapy included 
one to two injections of insulin daily. Notably, at the time of that 
clinical trial, there were n

 at the time of that 
clinical trial, there were no rapid-acting or long-acting insulin ana­
logues; only short- and intermediate-acting insulin preparations 
were available (see insulin regimens in the following).
Intensive insulin therapy yielded superior glycemic control to 
conventional insulin therapy, with HbA1c levels that were approxi­
mately 2% lower in the intensively treated group than in the con­
ventionally treated group, medians of 7% and 9%, respectively.190 
The intensively treat

7% and 9%, respectively.190 
The intensively treated group experienced a 35% to 76% reduc­
tion in the occurrence of microvascular complications, including 
retinopathy, nephropathy, and neuropathy compared than in the 
conventionally treated group. During the subsequent 20 to 25 
years of the DCCT observational follow-up study, the Epidemiol­
ogy of Diabetes Interventions and Complications (EDIC) study, 
the intensively treated group compared with the conventionally 
treated group continued to 

th the conventionally 
treated group continued to demonstrate a reduced risk of both 
micro- and macrovascular complications and overall mortality, 
mainly attributable to the difference in glycemic control during 
the DCCT.191–196 These findings help derive glycemic targets in 
practice today.
Glycemic targets are based on HbA1c levels, especially given 
that the explanation for complication risk in the DCCT and EDIC 
studies was attributable to glycemia.197 Various national and inter­
national

 glycemia.197 Various national and inter­
national professional organizations have created HbA1c goals. The 
ADA recommends HbA1c levels of less than 7% for adults and 
less than 7.5% for younger children and adolescents; additionally, 
HbA1c levels should be routinely monitored quarterly.180,186,187
Prior to 2014, the ADA’s pediatric HbA1c goals were higher 
for younger children at less than 8.5% for those younger than 6 
years and less than 8% for those 6 to 12 years old, whereas the goal 
was

for those 6 to 12 years old, whereas the goal 
was less than 7.5% for those 13 to 19 years old, due to previous 
concerns regarding neurocognitive damage from severe, recurrent 
hypoglycemia in young children.198,199 The earlier data suggesting 
neurocognitive damage from hypoglycemia occurred during the 
era prior to modern insulins and physiologic insulin replacement. 
Recent data demonstrate that intensive insulin therapy confers no 
additional risk to neurocognitive function in young childre

l risk to neurocognitive function in young children with 
T1DM.200,201 Additional research suggests that hyperglycemia 
and glycemic variability may incur adverse short-term outcomes 
on neurocognitive function and risk on central nervous system 
development based on magnetic resonance imaging changes of 
white matter.202–205
In consideration of emerging data on risk of hyperglycemia 
for the central nervous system, along with available management 
tools to minimize risk of hypoglycemia in young

t 
tools to minimize risk of hypoglycemia in young persons with 
T1DM, HbA1c targets have been lowered by some professional 
organizations, including the International Society for Pediatric 
and Adolescent Diabetes (ISPAD) and the National Institute for 
Health and Clinical Excellence Guidelines in the United King­
dom. ISPAD now recommends HbA1c targets of less than 7%, 
and the National Institute for Health and Clinical Excellence rec­
ommends targets HbA1c of 6.5% or less.206 Similarly, for a

targets HbA1c of 6.5% or less.206 Similarly, for adults, 
the American Association of Clinical Endocrinologists recom­
mends an HbA1c target of 6.5% or less.207
Although HbA1c target recommendations may have become 
more rigorous over recent years, the ADA and other organizations 
emphasize that glycemic targets need to be individualized. The tar­
get HbA1c goal for any single person with diabetes needs to reflect 
his or her risk of severe hypoglycemia, as well as hyperglycemia, 
considering th

lycemia, as well as hyperglycemia, 
considering the presence of comorbidities that may interfere with 
survival. For example, now the ADA offers higher HbA1c goals of 
less than 8% for persons with a history of severe hypoglycemia, 
limited life expectancy, or advanced complications, as well as more 
stringent HbA1c goals of less than 6.5% for those without hypogly­
cemia risk.187 ISPAD also considers less stringent goals for regions 
of the world where advanced diabetes therapies and technologi

d where advanced diabetes therapies and technologies 
may be unavailable to reduce risk of severe hypoglycemia.206 Thus, 
personalized glycemic goals are tailored to individual needs and 
consideration of risk of severe hypoglycemia (see Chapter 38). 
Lifestyle Management: Nutrition and Exercise
Lifestyle management is a major component of the care of persons 
with T1DM. Attention to both nutrition and exercise is funda­
mental in glycemic management, as well for overall health and 
cardiovascul

ment, as well for overall health and 
cardiovascular disease prevention. There is need to provide educa­
tion on these lifestyle issues to the persons with diabetes and to the 
family members of young children and adolescents with diabetes, 
especially given the global epidemic of childhood and adult over­
weight/obesity.208,209 Overall, all diet and exercise recommenda­
tions are individualized to meet the patient’s and family’s needs. 
These knowledge areas are addressed in diabetes self-manag

owledge areas are addressed in diabetes self-management 
education and support and should begin at diagnosis and progress 
as disease duration increases and the needs of the person with dia­
betes change. 
Nutrition Therapy
Medical nutrition therapy addresses diabetes-specific issues, such 
as those related to the carbohydrate content of foods, to limit post­
prandial glycemic excursions, as well as ways to maintain normal 
growth in the pediatric population and to avoid excessive weight 
gain a

c population and to avoid excessive weight 
gain across the life span. Nutrition education, generally provided 
by a registered dietitian annually following the initial education 
at diagnosis, includes both personal and family dietary choices, 
food preference changes over time, challenges regarding access to 
food, meal schedules, patterns of exercise, growth and develop­
ment, weight status, cardiovascular risk factors, and consideration 
of disordered eating behaviors.
For the growing child 

isordered eating behaviors.
For the growing child with diabetes, provision of adequate calo­
ries to maintain normal growth and development is fundamental. 
Underweight is a rare occurrence today in youth with T1DM. In 
fact, one third of pediatric patients with T1DM are overweight or 
obese.210–212 Diet prescriptions for adults with T1DM should also 
account for weight status, given the rise in overweight and obesity 
among those with T1DM, affecting up to 50%,213 and the asso­
ciation of incre

cting up to 50%,213 and the asso­
ciation of increased weight with cardiovascular risk in T1DM.214
Nutrition education addresses the way foods impact glucose 
excursions, how diet can help achieve glycemic goals, and dietary 
approaches to avoiding hypo- and hyperglycemia with exercise. 
Nutrition education addresses numeracy, literacy, and self-man­
agement abilities of those with diabetes and their family members. 
Overall, adherence to nutrition management is associated with 
better glycemic 

on management is associated with 
better glycemic control in youth with T1DM.215
1426
Principles of general nutrition include a focus on whole foods 
and the standard composition of macronutrients with approxi­
mately 50% of energy intake as carbohydrates, 20% as protein, 
and 30% as fat. Fat intake should include less than 10% of energy 
intake as saturated fat, less than 10% as polyunsaturated fat, and 
greater than 10% as monounsatu

saturated fat, and 
greater than 10% as monounsaturated fat. One should try to eat 
five portions of fruits and vegetables daily. Often, young persons 
with diabetes, with the support of their parents, may choose to 
eat less healthy food choices, like prepackaged foods that include 
the carbohydrate content on the label, instead of fruits or veg­
etables.216 Nutrition education follows published guidelines by 
professional organizations, such as the ADA187 and ISPAD.217 
Very low carbohydrates 

 the ADA187 and ISPAD.217 
Very low carbohydrates should be avoided, as such diets can lead 
to elevations in lipid levels and there can be risk for ketosis, espe­
cially with insulin dose reductions, disordered eating behaviors, or 
use of the sodium-glucose cotransporter 2 (SGLT2) class of oral 
agents, currently unapproved for use in persons with T1DM (see 
discussion of adjunctive therapies in the following).
The major focus of nutrition education is on carbohydrate 
counting, because carboh

ation is on carbohydrate 
counting, because carbohydrates have the largest impact on glyce­
mic excursions. So-called carbohydrate counting can provide flexi­
ble diet management by allowing for a wide range of food choices. 
It can also offer a way to select insulin doses based on food intake. 
When numeracy skills are deficient, experienced estimation of 
carbohydrate intake suffices. In fact, previous studies have shown 
that exactitude in carbohydrate counting is unnecessary, because 
under-

bohydrate counting is unnecessary, because 
under- or overestimating the carbohydrate content by 5 to 7 g, or 
by +/–15%, does not produce significant hypo- or hyperglycemia, 
respectively.218,219 Furthermore, it is more important to be con­
sistent rather than exact with carbohydrate counting, because the 
former is associated with better glycemic control.220
Additional studies have addressed the need to consider pro­
tein and fat intake on glycemic excursions.221–223 After one has 
mastered ca

emic excursions.221–223 After one has 
mastered carbohydrate counting, additional advanced nutrition 
education can include issues related to meal composition, car­
bohydrate quality (the glycemic index), and the contributions of 
protein and fat to glycemic excursions. Considering that meals 
containing substantial protein and fat can delay gastric empty­
ing and the subsequent postprandial glycemic excursion, nutrition 
education can address advanced approaches to insulin bolusing. 
For exampl

vanced approaches to insulin bolusing. 
For example, one can use a square (or extended) wave bolus or 
dual (or combination) wave bolus with CSII, or one can divide 
the injected bolus doses into two separate injections separated in 
time by 60 to 90 minutes. 
Physical activity is important for persons with and without dia­
betes. Exercise offers physical fitness, strength building, weight 
management, socialization, enhancement of self-esteem, and car­
diovascular

, enhancement of self-esteem, and car­
diovascular risk factor management. Exercise is a key component 
of diabetes management. It is important to recognize how exercise 
affects glycemic excursions, as exercise can lead to both hypogly­
cemia and hyperglycemia. Recommendations for youth include 
60 minutes or more of daily physical activity, which should 
include muscle- and bone-strengthening activities on 3 or more 
days each week.224 Recommendations for adults include at least 
150 minutes o

dations for adults include at least 
150 minutes of moderate-intensity aerobic activity or 75 min­
utes of vigorous-intensity aerobic activity each week plus muscle-
strengthening activities on 2 or more days each week.225 Adults 
with diabetes, and occasionally pediatric patients with diabetes, 
should receive medical clearance before embarking on an exercise 
program to evaluate the presence of any comorbid conditions or 
diabetes complications that could limit exercise participation, 
such as

 that could limit exercise participation, 
such as previously undiagnosed coronary heart disease.
Exercise impacts fuel metabolism through the recruitment of 
various hormones (insulin, glucagon, catecholamines, and glu­
cocorticoids), and the impact of exercise on metabolism varies 
by the type, intensity, and duration of the activity.226,227 With 
intact islet cells, including fully functioning beta cells, the body 
maintains euglycemia during exercise by balancing glucose uptake 
with hepatic

exercise by balancing glucose uptake 
with hepatic glucose production. In T1DM, it is not possible to 
reduce insulin production following administration of an insulin 
dose; therefore, the circulating insulin can lead to hypoglycemia 
by both inhibiting hepatic glucose production and promoting 
exercise-related glucose uptake. Alternatively, in T1DM, high-
intensity activities, such as sprints or resistance training, can lead 
to hyperglycemia when there is not enough circulating insulin due 
t

hen there is not enough circulating insulin due 
to the intense exercise’s stimulation of counterregulatory hormone 
release, which, in turn, will increase hepatic glucose production 
and inhibit glucose uptake into skeletal muscle.
In general, most persons with T1DM, as well as their family 
members, worry more about exercise-induced hypoglycemia. Fur­
ther, they are often frustrated and puzzled by the appearance of 
hyperglycemia with exercise. Hypoglycemia can occur during and 
immediately fo

 Hypoglycemia can occur during and 
immediately following the activity, and again 7 to 11 hours after 
the exercise, corresponding to the so-called lag effect of exercise. 
The latter phenomenon was shown in insulin clamp studies228 and 
likely results from enhanced insulin sensitivity, blunted counter­
regulatory hormone release, and increased glucose uptake by the 
liver and skeletal muscles to replenish glycogen stores after exer­
cise. The late risk of hypoglycemia often occurs overnight whe

te risk of hypoglycemia often occurs overnight when 
persons with diabetes exercise in the late afternoon or evening, 
causing particular concern for patients and family members. For 
some persons with T1DM, there can be additional impairment 
of counterregulatory hormone responses due to blunting during 
sleep, antecedent hypoglycemia, and autonomic failure.229–231 
Thus, exercise-related hypoglycemia, along with fear of hypo­
glycemia, can curtail exercise participation. Fortunately, there 
ar

ail exercise participation. Fortunately, there 
are ways to limit the risk of exercise-related hypoglycemia and 
hyperglycemia while managing glycemia with exercise. Manage­
ment approaches follow; for more in-depth discussion, there are 
recently published exercise management guidelines.226,227,232
To prevent hypoglycemia, one should begin exercise with a glu­
cose of 100 mg/dL (5.6 mmol/L) or higher and plan to ingest car­
bohydrates during and/or after the exercise according to its duration 

/or after the exercise according to its duration 
and intensity. One can consider providing 0.25 g to up to 1.00 g 
of carbohydrate per minute of exercise when the activity lasts 40 
minutes or longer, individualized based on the person’s size (i.e., 
toddler or child), his or her needs, and past experience. The addi­
tional carbohydrates can be provided by increasing food intake. 
Alternatively, one can reduce bolus insulin doses, by approximately 
50% as a starting point, for any meal or snack

ly 
50% as a starting point, for any meal or snack within 2 hours of 
the planned activity, which may be preferable to increasing car­
bohydrate ingestion for weight management. For those receiving 
CSII, basal rates can also be lowered by approximately 50% or 
even suspended for periods of 1 to 2 hours for exercise based on 
the patient’s needs.233 To manage the lag effect or late hypoglyce­
mia risk after exercise, basal rates can be reduced by approximately 
20% for up to 6 hours at bedtime f

 approximately 
20% for up to 6 hours at bedtime for those treated with CSII, or 
long-acting insulin doses can be reduced by approximately 20% 
at bedtime.234 Despite these insulin-dosing strategies, it remains 
important to have carbohydrates available for ingestion and to 
ensure continuous or frequent glucose monitoring for safe exercise.
1427
To prevent significant hyperglycemia with exercise and induc­
tion of ketosis, it is important not to exercise wh

ion of ketosis, it is important not to exercise when one is in an 
insulin-deficient state. The person with diabetes should exercise 
only when feeling well and in the absence of significant keto­
sis, defined as more than small urinary ketones or a blood beta-
hydroxybutyrate level of 1.5 mmol/L or more.226,227,232 One 
should also postpone intense exercise with significant hypergly­
cemia, defined as a glucose level of 350 mg/dL (19.4 mmol/L) or 
more, until corrected with insulin.
Exercise ed

r 
Exercise education can help limit the hypo- and hyperglycemic 
effects of exercise. When available, an exercise physiologist can 
add training with respect to exercise management, which can be 
especially important for the competitive athlete. 
Insulin Therapy
Background
Normal physiologic insulin secretion results in low levels of insu­
lin in the fasting and overnight period, and rapid increases in 
insulin following food intake, stress, and certain medic

n following food intake, stress, and certain medications. Low 
levels of basal insulin secretion are necessary for covering basal 
metabolic needs, suppressing lipolysis, and balancing glucose uti­
lization by organs such as the brain with hepatic glucose produc­
tion.235 Increases in insulin in response to rising blood glucose 
levels facilitate cellular glucose uptake and suppress hepatic glu­
cose production. Normally, this is achieved through a substantial 
initial first-phase release, follo

 a substantial 
initial first-phase release, followed by an extended second-phase 
secretion into the portal circulation. The goal of insulin therapy 
in persons with T1DM is to mirror pancreatic insulin patterns, 
and this biphasic insulin release is particularly difficult to repli­
cate.235 Fortunately, incremental improvements in the pharma­
cology and delivery of insulin have made it increasingly possible 
to achieve reasonable insulin levels in persons with diabetes. The 
discovery and init

 in persons with diabetes. The 
discovery and initial isolation of animal insulin in 1921 first made 
it possible to treat persons with T1DM. In the late 1940s, regular 
animal insulin was combined with protamine (neutral protamine 
Hagedorn insulin, NPH) to delay absorption and increase dura­
tion of action. In 1977, recombinant DNA technology allowed 
purification of insulin and production of synthetic “human” insu­
lin. This major breakthrough generated the production of large 
amounts of ins

 generated the production of large 
amounts of insulin and also resulted in a reduction of insulin anti­
bodies and insulin allergies. This development was followed by the 
biochemical modification of the insulin molecule to vary insulin 
action, giving rise to the so-called insulin analogues.
In 1993, the DCCT established a reduced risk of the develop­
ment and progression of both micro- and macrovascular compli­
cations and all-cause mortality among those treated with intensive 
insulin therap

among those treated with intensive 
insulin therapy with the goal of achieving near-normal blood 
glucose levels.190,191 In the DCCT, intensive insulin therapy was 
associated with a high rate of severe hypoglycemia (62 episodes 
per 100 patient-years of intensive insulin therapy); this related, 
in part, to the utilization of older nonanalogue insulin formula­
tions.190 The treatment of diabetes was further improved by subse­
quent development of rapid-acting insulin analogues in the 1990s 
and

f rapid-acting insulin analogues in the 1990s 
and long-acting (basal) insulin analogues in the early 2000s, which 
has led to lower rates of hypoglycemia, along with less weight gain 
and improved glycemic control, in persons with T1DM.236–238 
Insulin Preparations
Examples of commercially available insulin preparations are 
shown in Table 36.6 and vary in time to onset, peak, and duration 
of action. The typical insulin concentration is U-100 (100 units 
of insulin per 1 mL); however, more con

100 units 
of insulin per 1 mL); however, more concentrated preparations are 
commercially available, including U-200 insulin degludec (Novo 
Nordisk Tresiba pen), U-300 insulin glargine (Sanofi Toujeo pen), 
U-200 lispro (Humalog U-200 KwikPen), and U-500 regular 
human insulin (Lilly Humulin pen or vial). These concentrated 
insulin preparations are most commonly used in patients with 
T2DM, but they are beneficial for some individuals with T1DM 
who have significant insulin resistance and hig

M 
who have significant insulin resistance and high insulin require­
ments.239 In very young children requiring small doses of insulin 
(e.g., <0.5 unit bolus doses), rapid-acting insulin diluted to a tenth 
  Insulin Types and Action Profiles
Product
Onset of Action
Peak Action
Duration
Rapid acting
Aspart (Novolog [Novo Nordisk, Princeton, NJ])
Lispro (Humalog U-100 [Eli Lilly, Indianapoils, IN], Humalog U-200 
[Eli Lilly, Indianapoils, IN], Admelog [Sanofi, Bridgewater, NJ])
Glulisine (Apidr

dmelog [Sanofi, Bridgewater, NJ])
Glulisine (Apidra [Sanofi, Bridgewater, NJ])
10–30 min
30–180 min
3–5 h
Aspart (Fiasp [Novo Nordisk, Princeton, NJ])
2.5 min
40–50 min
Insulin human (Afrezza [MannKind, Westlake Village, CA]) inhalation powder
12 min
35–45 min
1.5–3 h
Short acting
Regular U-100 (Humulin R U-100 [Eli Lilly, Indianapolis, IN])
Regular U-100 (Novolin R [Novo Nordisk, Princeton, NJ])
Regular U-500 (Humulin U-500)
30–60 min
2–4 h
U-100: Up to 10 h
U-500: Up to 24 h
Acti

0: Up to 10 h
Acting
NPH (Humulin N)
NPH (Novolin N)
2–4 h
4–8 h
12–18 h
Detemir (Levemir [Novo Nordisk, Princeton, NJ])
Glargine (Lantus [Sanofi, Bridgewater, NJ], Basaglar [Eli Lilly, Indianapoils, IN], 
Toujeo U-300 [Sanofi, Bridgewater, NJ])
Degludec (Tresiba U-100 [Novo Nordisk, Princeton, NJ], Tresiba U-200)
2–4 h
Minimal
Detemir: 12–24 h
Glargine: up to 24 h
Degludec: up to 48 h
Premixed
70/30 (NPH/Aspart) (Novolog 70/30)
70/30 NPH/Regular (Humu

PH/Aspart) (Novolog 70/30)
70/30 NPH/Regular (Humulin 70/30)
75/25 (NPH/Lispro) (Humalog 75/25)
50/50 (NPH/Lispro) (Humalog 50/50) [Other combinations may be available in 
Europe.]
5–60 min
Dual
12–18 h
TABLE 36.6
1428
the usual concentration (10 units/mL, often called U-10) or other 
concentrations (U-50, U-25) may be used, after education by the 
child’s diabetes health care team; diluted insulin may be prepared 
by a compounding pha

uted insulin may be prepared 
by a compounding pharmacy or by the young child’s family using 
a diluent provided by the insulin manufacturer.
Insulin can be dispensed in vials and administered via syringes 
or continuous infusion pumps or dispensed and administered in 
pen form. Rapid-acting inhaled insulin is also available, and it has 
been demonstrated to be noninferior when compared with insulin 
aspart for mean change in HbA1c, although more patients treated 
with insulin aspart achieved an

 patients treated 
with insulin aspart achieved an HbA1c of less than 7% compared 
with patients treated with inhaled insulin (31% vs. 18%)240; avail­
ability of limited dosing increments has been one barrier to treat­
ment with inhaled insulin.
In light of results from the DCCT, intensive insulin therapy is 
the standard of care for treatment of T1DM and can be achieved 
with multiple daily injections (MDI) basal-bolus therapy or insulin 
pump therapy. In both MDI and pump therapy, rapid-acting

herapy. In both MDI and pump therapy, rapid-acting insu­
lin is given as a bolus at meals and snacks and periodically to correct 
hyperglycemia (no more frequently than every 2–3 hours) and as 
basal insulin given once or twice a day with long-acting insulin (for 
MDI) or as a basal rate (for insulin pumps). A small proportion of 
patients and families require simplified insulin regimens rather than 
intensive insulin therapy, which will be discussed in the following. 
Initial Management of Newl

ssed in the following. 
Initial Management of Newly Diagnosed T1DM
In pediatric patients with newly diagnosed diabetes, initial manage­
ment includes resuscitation with oral or intravenous fluids to rehy­
drate and correct electrolyte imbalances, administration of insulin 
to halt lipolysis and reverse hepatic gluconeogenesis and ketogen­
esis, and commencement of diabetes education. Depending on fac­
tors such as the age of the individual patient and severity of his or 
her clinical presentatio

t and severity of his or 
her clinical presentation, family factors, and availability of diabetes-
related resources, initial management may occur in an intensive care 
unit, inpatient hospital setting, or outpatient unit. Initial presenta­
tion of T1DM is often DKA. Please refer to the discussion on DKA. 
Initiation of Insulin Therapy
For those individuals who do not initially present in DKA, treat­
ment with subcutaneous insulin should be started at diagnosis of 
diabetes. For those who presen

ed at diagnosis of 
diabetes. For those who present in DKA, subcutaneous insulin 
should begin with resolution of DKA (pH ≥7.3, CO2 ≥18 mEq/L 
[18 mmol/L], and normalization of the anion gap) in conjunction 
with the first meal being eaten post-DKA. Determination of the 
appropriate starting insulin dose must consider the patient’s weight, 
age, pubertal status, and history of DKA. For young children and 
postpubertal adults who are not obese and do not present in DKA, 
a typical starting dose i

 do not present in DKA, 
a typical starting dose is 0.3 to 0.5 units/kg per day. Higher doses 
will be needed for individuals who are obese, children in puberty, or 
following presentation with DKA, such that a pubertal child with 
obesity and resolved DKA may receive a starting dose of 1 unit/kg 
per day. This total daily dose (TDD) of insulin is typically given in a 
divided program of MDI with about 50% of the TDD delivered as 
a once-daily dose of basal insulin and 50% given as boluses. 
Ins

se of basal insulin and 50% given as boluses. 
Insulin Dose Determination
Treatment with basal-bolus therapy requires the calculation of the 
appropriate insulin bolus at each meal and snack based on the pre­
meal blood glucose level and the planned carbohydrate intake. To 
calculate a bolus, the person will first calculate the proper carbohy­
drate coverage for the meal using his or her individualized insulin-
to-carbohydrate ratio. The carbohydrate ratio is the number of 
grams of carbohydrate

rate ratio is the number of 
grams of carbohydrate covered by 1 unit of insulin (roughly 450 
divided by the total daily insulin dose, TDD). Next, the person 
will determine the amount of insulin required to correct for the 
premeal blood glucose level, using his or her individualized cor­
rection factor (also sometimes called sensitivity factor or sensitivity 
index). The correction factor is the expected decrease in blood glu­
cose resulting from 1 unit of insulin (approximately 1650 divided 

om 1 unit of insulin (approximately 1650 divided 
by the total daily insulin dose, TDD). The bolus delivered with 
the meal or snack is the sum of the calculated carbohydrate cover­
age and the calculated blood glucose correction. Most patients do 
these calculations by hand, mental math, or calculators, or with 
the help of apps or pump pen bolus wizards, but some families 
require sliding scales or charts for determination of bolus dosing. 
Rapid-acting insulin boluses should be given 10 to 15

id-acting insulin boluses should be given 10 to 15 minutes 
before the start of a meal or snack because onset of action is about 
15 minutes. In cases where food intake is less predictable (e.g., 
very young children, gastroenteritis), postprandial administra­
tion of a rapid-acting insulin bolus is safe and effective.241 Since 
rapid-acting insulin has a 3-hour action time, a correction factor 
is usually not given any more frequently than every 3 hours to 
avoid hypoglycemia from “stacking” of

 3 hours to 
avoid hypoglycemia from “stacking” of insulin action. Anticipated 
activity (i.e., recent or planned exercise) may factor into insulin 
bolus calculations, with less insulin required before, during, or 
after certain types of exercise. Additionally, a blood glucose trend 
based on continuous glucose monitor arrows may also factor into 
the insulin bolus calculation at mealtime.242,243 
Other Insulin Regimen Options
In individualizing each person’s insulin regimen, clinicians must 
c

 each person’s insulin regimen, clinicians must 
consider the frequency of blood glucose monitoring, number of 
daily injections, the need for flexibility in meal planning, and fam­
ily schedule. In a minority of patients with T1DM, treatment with 
intensive insulin therapy and mealtime bolus calculations may not 
be feasible or appropriate. In these cases, intermediate-acting insu­
lin (NPH) may be prescribed, often in conjunction with rapid- 
or short-acting insulin, and sometimes in premixed 

r short-acting insulin, and sometimes in premixed form (70/30, 
75/25, 50/50). To avoid hypoglycemia in such regimens, meals are 
typically fixed with regard to timing and amount of carbohydrates 
consumed. Table 36.7 provides examples of insulin regimens. 
Blood Glucose Monitoring
Self-management of diabetes requires the routine monitoring of 
glucose levels to select and adjust insulin doses, to understand the 
impact of various foods on glycemic excursions, to reduce the risk 
of exercise rel

ic excursions, to reduce the risk 
of exercise related hypo- or hyperglycemia, and to react to periods 
of illness or stress that can lead to increased glucose variability 
and hyperglycemia with ketosis along with risk for ketoacidosis. 
There are numerous glucose meters available that utilize a small 
volume of blood (<1 mL), provide results in less than 5 seconds, 
offer alternative site use beyond fingertips, and meet recent stan­
dards of accuracy. Accuracy standards include the need for at

curacy. Accuracy standards include the need for at least 
95% of blood glucose meter results to fall within ±15 mg/dL (0.8 
mmol/L) for blood glucose <100 mg/dL (5.6 mmol/L) and within 
±15% for blood glucose ≥100 mg/dL (5.6 mmol/L) performed 
with a reference method,244 with recommendations allowing for 
ongoing assessment of meter performance.245 Each year, the ADA 
provides an update of available meters, along with a cataloging 
of their various features.246 Although there may be substantial 

us features.246 Although there may be substantial 
choice in the selection of glucose meters, one must mainly con­
sider insurance coverage and the need to link the meter with a 
pump for CSII.
1429
Daily blood glucose monitoring frequency has been repeat­
edly linked to lower HbA1c levels.247–249 For persons with T1DM, 
blood glucose monitoring frequency is generally recommended at 
least four times daily and up to 10 to 12 times daily. Monitoring 
generally

 up to 10 to 12 times daily. Monitoring 
generally occurs before meals/snacks and at bedtime, with monitor­
ing at other salient times, such as postprandially, overnight, before/
during/after exercise, and during periods of illness or stress. More 
frequent monitoring is often recommended for persons with hypo­
glycemic unawareness (i.e., prior to driving) and for those unable 
to communicate symptoms, such as the very young or the elderly. 
It is important to download the glucose meter at routi

s important to download the glucose meter at routine appoint­
ments to review glycemic patterns with patients and families and 
to utilize the data to encourage self-management for insulin dose 
adjustments. Modern CGM technologies, especially those with 
nonadjunctive claims for insulin management, have supplanted the 
need for frequent blood glucose monitoring (see the next section). 
Real-time CGM is increasingly utilized for routine diabetes care in 
adults, chi

utilized for routine diabetes care in 
adults, children, and adolescents with T1DM. CGM is a portable 
device that allows for measuring and visualizing the glucose con­
centration continuously in real time for several days to weeks.250 
CGM devices are composed of three main units: a sensor, a trans­
mitter, and a receiver. The sensing electrode is placed in the subcu­
taneous space and measures glucose in the interstitial fluid, which 
is correlated to blood glucose via a calibration procedure.

ated to blood glucose via a calibration procedure. The sens­
ing electrode is inserted by the user or a caregiver for most devices 
or implanted by a clinical team for the new long-term sensor. Each 
sensor can last between 5 and 14 days for the minimally invasive 
CGMs, and from 90 to 180 days for the implantable one. Infor­
mation from the sensing electrode is transmitted to a dedicated 
receiver or to a smart device like a phone or an insulin pump. The 
CGM transmitter is attached on top of t

pump. The 
CGM transmitter is attached on top of the sensor and can last 3 
to 6 months of use or is rechargeable in some devices. The trans­
mitter unit utilizes either a proprietary radio or Bluetooth Low 
Energy (BTLE) communication to connect to the receiver unit or 
mobile device. The dedicated receiver, an app on a smart phone 
or an insulin pump, receives the glucose readings, which then can 
be used to visualize glucose trajectories and provide hypoglycemia 
and hyperglycemia alerts and 

rovide hypoglycemia 
and hyperglycemia alerts and alarms to the user and followers. The 
CGM is calibrated using self-monitoring blood glucose (SMBG) 
devices. Calibration for some of the CGMs occurs as often as 
every 12 hours. Calibration values can be entered on the mobile 
device, CGM receiver, or insulin pump. Recent CGM technology 
enables a calibration-free option or factory calibration without 
loss of accuracy, which liberates users from capillary glucose mea­
surements, although calibr

 capillary glucose mea­
surements, although calibration is still optional.
The first modern CGM devices were introduced by Medtronic 
Diabetes (Northridge, CA) as a diagnostic tool providing 72 hours 
of retrospective data when calibrated with blood glucose meter val­
ues; the latter were to be used for insulin dosing or other therapeu­
tic adjustments. New devices added the use of trend arrows and 
predictions, and with each newer generation, the sensor use time 
increased and the accuracy meas

e sensor use time 
increased and the accuracy measured as mean absolute relative dif­
ference.251–257 CGM technology, based on electrochemical detec­
tion of glucose using enzymatic chemistry that employs a glucose 
oxidase reaction, has improved dramatically in the past decade, as 
depicted in Fig. 36.7.258,259 Modern CGM technology has reached 
the accuracy of SMBG with the new Dexcom G6 (San Diego, 
CA), which is factory calibrated and approved for insulin dosing, 
as well as the new Abbott F

d for insulin dosing, 
as well as the new Abbott FreeStyle Libre (Alameda, CA), which 
has reduced the warm-up time to 1 hour and can run for 14 days 
(Fig. 36.8). The Senseonics (Germantown, MD) CGM is a new 
implantable device based on fluorescent chemistry that functions 
from 90 to 180 days using a removable rechargeable receiver and 
transmits glucose to a mobile app260–264 (see Fig. 36.8).
Innovations are ongoing with smaller devices and longer use 
time between sensor insertions, integrat

nger use 
time between sensor insertions, integration of CGM devices with 
insulin pumps and mobile apps together enhancing the usability 
and have opened the door for automated insulin delivery (AID) 
or artificial pancreas.259
Benefits of CGM devices have accrued with improved perfor­
mance of the devices over time.265–267 Consistent CGM use of 6 
or more days per week has been associated with improvements in 
glycemic control, reflected in lower HbA1c levels, more patients 
achieving HbA1c ta

er HbA1c levels, more patients 
achieving HbA1c targets, and reduction in time in the hypoglyce­
mic range of less than 70 mg/dL (<3.9 mmol/L) or less than 54 
mg/dL (3 mmol/L).268–271 Similarly, more frequent scanning of 
the Freestyle Libre has also been associated with improved glyce­
mic control, noted by an increase of glucose time in range, with 
reductions of time in hypoglycemia and hyperglycemia; mean 
scanning frequency was about 16 times per day in one study.272 
Finally, recent publi

es per day in one study.272 
Finally, recent publications from the Endocrine Society have 
advocated the use of CGM trend arrows to fine tune insulin bolus 
doses at mealtimes in pediatric and adult patients to avoid extreme 
postprandial glucose excursions, especially given the recognition 
that the glucose trajectory and the magnitude of the glucose level 
impact the response to the insulin bolus and ingested meal.242,273 
Continuous Subcutaneous Insulin Infusion
CSII is a minimally invasive f

us Insulin Infusion
CSII is a minimally invasive form of insulin delivery that enables 
intensive insulin therapy. One of the advantages of the use of 
  Examples of Insulin Regimens Used in 
Intensive Insulin Regimens
Multiple Daily Injections
Long-acting insulin given once daily, and rapid-acting insulin given at 
meals, snacks, and periodically to correct high blood sugars
Continuous Subcutaneous Insulin Infusion via Insulin Pump
Rapid-acting insulin given at basal rate (rate

ump
Rapid-acting insulin given at basal rate (rate can vary throughout the 
day) and as a bolus at meals, snacks, and as needed to correct high 
blood sugars
Rapid-acting insulin given via pump according to automated insulin 
delivery algorithm and based on continuous glucose monitoring
Simplified Insulin Regimens
Long-acting insulin given once daily, NPH given at breakfast, and rapid-
acting insulin given at breakfast and dinner (may be used when 
lunchtime insulin dos

nd dinner (may be used when 
lunchtime insulin dosing is logistically difficult; lunch carbohydrate 
intake must be consistent)
NPH given twice daily (breakfast and bedtime), and rapid-acting insulin 
given at meals
NPH and rapid- or short-acting insulin given twice daily (breakfast and 
dinner)
Premixed insulin (70/30, 75/25, 50/50) given twice daily (breakfast and 
dinner)
TABLE 36.7
1430
CSII, as its name implies, is its ability to 

lism
CSII, as its name implies, is its ability to continuously deliver 
insulin and allow the users to respond more rapidly to any changes 
in their physiologic insulin requirements. Second, most modern 
CSII pumps enable the user to program and modify key param­
eters that effect insulin delivery, such as (1) the basal rate profile, 
in which several basal rates with multiple insulin segments can 
be entered to address changes in insulin requirements between 
workdays and weekends, active days 

ments between 
workdays and weekends, active days or sick days, and more; (2) 
the storing of multiple sensitivity factors, as well as carbohydrate 
to insulin ratio; (3) alerts and alarms; (4) bolus calculators and 
insulin-on-board estimators, to prevent overstacking of insulin.
CSII therapy has been associated with improved glycemic 
control, decreased hypoglycemia, and better quality of life.274–276 
Still, there are several disadvantages of this delivery route. Using 
28%
26%
24%
22%
20%
18

this delivery route. Using 
28%
26%
24%
22%
20%
18%
16%
14%
12%
10%
8%
6%
30%
Accuracy (MARD) %
Years
SMBG
4%
2018
2016
2014
2012
2010
2008
2006
2004
2020
Medtronic
Abbott
Dexcom
Senseonics
8.8%
9.0%
9.4%
10.5%
9.0%
9.7%
9.0%
13.0%
13.8%
15.9%
17.0%
26.0%
15.8%
12.8%
19.7%
• Fig. 36.7  Accuracy of current continuous glucose monitoring devices in the past decade. MARD, mean 
absolute relative difference; SMBG, self-monitoring blood glucose. (Modified from Facchinetti A. Con­
tinuous glucose monit

ied from Facchinetti A. Con­
tinuous glucose monitoring sensors: past, present and future algorithmic challenges. Sensors (Basel). 
2016;16[12]:E2093.)
RF link between
the transmitter
and the receiver
or the mobile app 
Transmitter/skin cross-section
Receiver
• Glucose
• Glucose trend
• Glucose chart
• Alert
Mobile app
interface
• Glucose
• Alert
Transmitter
Skin
Glucose
sensor
fluid
Cell
Glucose
Blood
vessel
• Fig. 36.8  The components of a continuou

vessel
• Fig. 36.8  The components of a continuous glucose monitoring system, which include the glucose sen­
sor that resides in the subcutaneous tissue along with the transmitter that wirelessly transmits the glucose 
data to a separate receiver that can be a stand-alone device or an application on a mobile device. Wireless 
transmission occurs via radio frequency (RF) and Bluetooth. Both glucose concentrations and trends can 
be displayed. Alerts can notify the user when glucose levels are ri

rts can notify the user when glucose levels are rising or falling.
1431
CSII means that the user is attached to an external device, which 
may be inconvenient in daily life. Additionally, it is recommended 
to change the infusion set every 3 days to reduce problems with 
site irritation and irregularity in insulin delivery caused by blocked 
catheters, insulin leakage, or cannula dislodgement.277 A failure of 
the infusion site can lead to hyperglycemia, and 

the infusion site can lead to hyperglycemia, and potentially DKA, 
if the problem is not detected in time. Pump technology has pro­
gressed to support smarter devices that integrate CGM function­
ality, as well as algorithms that enable insulin suspension based 
on fix threshold or predictive design, to minimize hypoglycemia 
and improve glycemic outcomes. The latest leap forward in pump 
technology is the development of closed-loop insulin delivery 
devices that build on top of the current sma

very 
devices that build on top of the current small and smart devices, 
and add a control algorithm that is tasked with calculating the safe 
dose of insulin, in most cases, or a combination of insulin and glu­
cagon. As can be seen in Table 36.8, several pumps are available to 
people with T1DM, with different features, sizes, and technology 
to support their individual needs and requirements.
The first advantage of connected care between CGM and the 
insulin pump delivery system is the introd

nd the 
insulin pump delivery system is the introduction of integrated or 
linked systems that enable automated insulin suspension. Suspend­
ing basal insulin delivery for low sensor glucose levels has been 
shown to distinctly reduce hypoglycemia without worsening glyce­
mia.278 Sensor-augmented pumps that preemptively suspend insu­
lin delivery when sensor glucose levels are predicted to be low show 
promise in minimizing hypoglycemia.279–282 The greatest potential 
for improved glycemic contr

he greatest potential 
for improved glycemic control is the dynamic regulation of insulin 
delivery for both high and low glucose levels. “Hybrid” closed-loop 
systems that modulate basal insulin delivery by sensor glucose levels 
increase time spent within target glucose ranges, reduce hyper- and 
hypoglycemia exposure, lower HbA1C levels, and improve measures 
of quality of life in both adult and adolescent subjects.283–287 Trans­
lation of AID from research to clinical care will require educa

 from research to clinical care will require educating 
users and providers to optimize outcomes and understand the new 
technology and the best way to use it.288,289 Users of current hybrid 
closed-loop systems must still count carbohydrates and bolus 
manually for meals. Systems that reduce reliance on carbohydrate 
counting, and systems that administer glucagon under automated 
control to mitigate the risk of hypoglycemia, remain in develop­
ment.290 A recent systematic review and meta-analys

ent.290 A recent systematic review and meta-analysis of ran­
domized controlled trials suggest that artificial pancreas systems 
uniformly improve glucose control in outpatient settings despite 
heterogenous technical and clinical factors.291 
AID: “The Artificial Pancreas”
The combination of continuous glucose technology with insulin 
pumps has enabled the development of AID systems (closed-
loop or artificial pancreas devices). A controller, an algorithm, 
adjusts insulin delivery based on CGM

 algorithm, 
adjusts insulin delivery based on CGM data to a glucose target or 
range that is preset and can be changed based on user lifestyle or 
need.292–294 These feedback control algorithms achieve safe and 
effective glucose regulation for people with T1DM, maximize 
time spent within the euglycemic safe range of 70 to 180 mg/dL, 
minimize hypoglycemic events, and prevent postprandial hyper­
glycemia with little user intervention.
A closed-loop glucose control or AID system is composed of 

loop glucose control or AID system is composed of 
several basic elements, as depicted in Fig. 36.9. The source of 
information is the sensor module that is based on a CGM. Addi­
tional sensors that monitor other physiologic parameters (e.g., 
heart rate, accelerometry, skin temperature) are being integrated 
in new AID systems.295 The delivery module is based on an insu­
lin pump, while a combination of insulin and other hormones or 
medications are also being explored. The core module, the con

 are also being explored. The core module, the control 
algorithm, processes the sensor measurements with or without 
additional input provided by the user, such as meal or exercise 
information, to achieve the desired glucose set-point target or 
range. The control signal is sent to the user’s pump to manipu­
late insulin delivery and other drugs, such as glucagon or pram­
lintide, in a variable rate or microdose. Current artificial pancreas 
systems are based on subcutaneous sensing and delive

stems are based on subcutaneous sensing and delivery; however, 
the intraperitoneal space has been investigated as a potential alter­
native site to insulin delivery and glucose sensing, one that will 
be closer to physiologic insulin secretion.296,297 The core of AID 
  Modern Insulin Pumps in the United States
Product
Reservoir
Basal Range
Bolus Range
Main Features
Omnipod
INSULET CORP. 
(Acton, MA)
200 U
0.05–30 U/h 
in 0.05-U 
increments
0.05–30 U; Incre­
ments of 0.05, 
0.1, 0.5, or 1 U
 •

ry for up to 2 hours if the glucose level reaches 
a preset low limit and the user does not react to a low-glucose 
alarm.
MiniMed 670G System
MEDTRONIC DIABETES
300 U
0.025–35 U/h 
in 0.025-U 
0.025–25 U; 
Increments of 
0.1 U
The MiniMed 670G, a hybrid closed-loop pump, uses SmartGuard 
technology to allow users to choose from increasing levels of auto­
mation that best fit their diabetes management needs.
T:slim X2 Pump
TANDEM DIABETES
CARE (San Diego, CA)
300 U
0.1–15 U/h 
in 0.00

TES
300 U
0.1–15 U/h 
in 0.001-U 
0.05–25 U; 
0.01 U
The T:slim X2 uses the Tandem Device Updater to remotely update 
software from a computer without requiring purchase of a new 
device. Integrated with Dexcom’s G5 CGM.
New Basal-IQ pump supports the Dexcom G6 and has a PLGS feature.
BTLE, Bluetooth low energy; CGM, continuous glucose monitoring; PLGS, predictive low-glucose suspend.
Modified from Diabetes Forecast: Consumer Guide. 2018. diabetesfo

iabetes Forecast: Consumer Guide. 2018. diabetesforecast.org. Available at http://main.diabetes.org/dforg/pdfs/2018/2018-cg-insulin-pumps.pdf.
   
TABLE 36.8
1432
systems is the control algorithm. Many control techniques are 
being explored in academic studies, and some have translated to 
industrial development and implementation as part of new AID 
systems. The lead feedback control algorithms for automated glu­
cose regulation are p

lgorithms for automated glu­
cose regulation are proportional integral derivative (PID) control, 
fuzzy logic control, and model predictive control (MPC). 
Proportional Integral Derivative AID
The PID controller is the most common strategy of implement­
ing feedback control.298 PID control consists of the summation 
of three distinct terms or components: the proportional term that 
captures the current tracking error (the difference between the 
current control variable and the desired set-point

current control variable and the desired set-point), the integral 
term that captures historical accumulation of the error, and the 
derivative term that captures anticipated change of the error.298 
Steil and colleagues299 have pioneered the development of PID-
based AID systems as part of Medtronic Diabetes. PID control 
algorithms have been used in several automated closed-loop 
insulin delivery systems, including the first US Food and Drug 
Administration (FDA)-approved hybrid closed-loop sy

dministration (FDA)-approved hybrid closed-loop system—the 
Medtronic 670G.299–302 
Fuzzy Logic AID
Closed-loop algorithms using fuzzy logic follow specially designed 
dosing rules to incorporate expert clinical knowledge in diabetes 
decision making.303,304 A fuzzy logic–based controller has three 
main stages: (1) input fuzzification, mapping the glucose and/or 
its rate of change using membership functions; (2) fuzzy infer­
ence, combining the fuzzy inputs from the first stage and a set of 
f

 fuzzy inputs from the first stage and a set of 
fuzzy rules to determine the degree of activation of output mem­
bership functions; and (3) output defuzzification, where the final 
output—insulin dose—is calculated. One fuzzy logic–based artifi­
cial pancreas system is known as the MD-Logic Artificial Pancreas, 
which has been developed by researchers at Schneider Children’s 
Medical Center of Israel. The system has been used in multiple 
clinical studies and is proven to regulate glucose level

al studies and is proven to regulate glucose levels across age 
groups.305 The MD-Logic closed-loop algorithm consists of two 
control modules: a control to range module that aims to keep 
glucose in the 80 to 120 mg/dL (4.4–6.7 mmol/L) range and a 
control to target module. 
Model Predictive Control AID
MPC is one of the most widely used approaches in advanced con­
trol system design of AID devices due to its ability to directly handle 
practical constraints and multiple inputs and outputs, for

l constraints and multiple inputs and outputs, formulate 
control objectives into a constrained optimization problem, and 
find the optimal control inputs by solving the optimization prob­
lem in a receding-horizon fashion. MPC is a model-based control 
strategy using models in two ways: (1) utilizing a reliable model to 
predict the effect of past control moves on future outputs, assum­
ing no future moves, and (2) using the same model to compute 
optimal controller moves, implementing the firs

e 
optimal controller moves, implementing the first move and repeat 
procedure. So far, this approach has been successfully utilized in 
the development of AID systems by multiple research groups and 
industry translation of the technology.284,285,287,290,306,307
AID systems can be divided into two main classes: insulin-only 
and bihormonal systems that use insulin and glucagon or other 
drugs to improve care. In the past decade, multiple clinical stud­
ies have been launched to evaluate the saf

l stud­
ies have been launched to evaluate the safety and efficacy of AID 
systems in both groups. Different control algorithms, CGMs, and 
delivery devices were evaluated in these studies. AID systems have 
been tested with multiple challenges that people with T1DM are 
facing, such as nocturnal glucose control and both announced and 
Glucose and other
physiologic
signals
Other
Glucagon
Delivery Device
Insulin Delivery
Device
Other Delivery
Device
CGM
Other Sensors
Food
Exercise
User Action
Pum

ce
CGM
Food
Exercise
Pump
Sensors
Control
Algorithm(s)
Patient
Cloud Data
Storage and
Adaptation
Desired
Glucose
Target
• Fig. 36.9  Feedback control loop for automated glucose management. CGM, continuous glucose 
­monitoring.
1433
unannounced meals and physical activity. These studies started 
in a tight, supervised environment and gradually transitioned to 
hotel and at-home clinical studies that ranged from short visits to 
prolon

l studies that ranged from short visits to 
prolonged multiweek studies.284,290,295,300,305,307–317 As depicted 
in Fig. 36.10, both insulin-only and bihormonal AID have dem­
onstrated safe and effective glucose regulation with minimal time, 
less than 1.5% spent under 70 mg/dL (3.9 mmol/L) and approxi­
mately 75% in the 70 to 180 mg/dL (3.9–10.0 mmol/L) range. 
Additional work with new technology, as well as added wearable 
sensors, and/or drugs may improve these remarkable outcomes. 
Adjunctiv

 may improve these remarkable outcomes. 
Adjunctive therapies to treat T1DM such as pramlintide and glu­
cagon-like peptide 1 (GLP1) receptor agonists (e.g., liraglutide, 
exenatide) or SGLT2 inhibitors that are used in clinical care are 
yet to be fully evaluated with AID systems, and only limited clini­
cal studies have been done to evaluate how to integrate them with 
AID systems.318,319 
Decision Support Systems
An emerging field in diabetes care is decision support that uses 
connected care

care is decision support that uses 
connected care devices such as CGM, smart insulin pens, SMBG, 
and insulin pumps to provide guidance and advice to people with 
T1DM via smart apps or cloud-based algorithms. This evolving 
field can improve care of both MDI and CSII users by provid­
ing tools to adjust basal boluses, as well as meal and correction 
boluses, based on glucose input and peoples’ lifestyle. These tools 
can provide additional insight to the clinical team with enhanced 

ght to the clinical team with enhanced 
visualization and advice on insulin management.320–323 
Use of Adjunctive Drugs in T1DM
Advances in treatment for T1DM have primarily targeted the 
development of insulin analogues and devices that optimize insu­
lin delivery and mimic normal physiologic serum insulin levels. 
Even with these advances, insulin therapy has been associated 
with hypoglycemia and weight gain. To offset some of these side 
effects of insulin therapy, several drugs that are cur

cts of insulin therapy, several drugs that are currently used in 
T2DM have been investigated for adjunctive use in T1DM, to 
assess potential benefits related to improved glycemic control, less 
hypoglycemia, reduced glucose variability, and less weight gain. 
To date, pramlintide is the only drug that is approved by the FDA 
for adjunctive use in adults with T1DM; no adjunctive treatments 
have been FDA approved for use in children with T1DM.
Pramlintide
Pramlintide, an injectable amylin analo

ramlintide
Pramlintide, an injectable amylin analogue, slows gastric emptying, 
suppresses postprandial glucagon secretion, and enhances satiety. 
Several large, randomized, placebo-controlled, double-blinded tri­
als have shown that the addition of pramlintide to insulin at meal­
time in adults with T1DM improved glycemic control, including 
modest reductions in HbA1c by 0.3% to 0.6%, and decreased post­
prandial glucose excursions.324,325 Pramlintide was also associated 
with modest weight los

intide was also associated 
with modest weight loss (mean, 0.4–1.3 kg) and reduced insulin 
dose; nausea was the most frequent side effect. In a recent closed-
loop insulin delivery study, the addition of mealtime pramlintide 
was associated with a reduction in postprandial hyperglycemia 
compared with closed loop alone.326 Pramlintide is approved for 
adjunctive use in adults with T1DM but not in children. 
Metformin
Several clinical trials have studied the addition of metformin 
to target insu

 studied the addition of metformin 
to target insulin resistance, particularly in individuals who are 
obese and during puberty. Clinical trials have failed to demon­
strate improvement in HbA1c with adjunctive use of metformin, 
although some studies in youth have shown decreases in weight 
and insulin requirement.327,328 In a recent randomized controlled 
trial of adjunctive metformin in adults with T1DM at risk for car­
diovascular disease, the metformin group exhibited improvements 
in body 

e metformin group exhibited improvements 
in body weight, low-density lipoprotein cholesterol, and estimated 
glomerular filtration rate, but metformin did not significantly 
improve carotid artery intima-media thickness (the primary study 
outcome) or glycemic control.329 Metformin is not FDA approved 
for treatment of T1DM. 
Incretin-Based Therapies
Incretin-based therapies include GLP1 receptor agonists and 
dipeptidyl peptidase 4 (DPP4) inhibitors. GLP1 is an incre­
tin hormone produced in t

itors. GLP1 is an incre­
tin hormone produced in the gut that has been shown to boost 
glucose-dependent insulin secretion, suppress glucagon secre­
tion, delay gastric emptying, and promote satiety. Individuals 
with T1DM appear to have normal GLP1 production but dys­
regulation between the pancreas and the gut, leading to ineffective 
glucagon suppression.330 A recent 24-week randomized, double-
blind, placebo-controlled trial of overweight adults with T1DM 
and HbA1c greater than 8% showed th

lts with T1DM 
and HbA1c greater than 8% showed that the addition of the once-
daily injectable GLP1 receptor agonist liraglutide was associated 
with weight loss and decreased insulin dose, but change in HbA1c 
from baseline did not differ between groups.331 In a recent closed-
loop insulin delivery study, the addition of once-daily liraglutide 
was associated with fewer postprandial increases in blood glucose, 
weight loss, and decreased insulin compared with closed-loop 
control.326 Consideri

n compared with closed-loop 
control.326 Considering that endogenous incretins such as GLP1 
are rapidly degraded by the enzyme DPP4, treatment with DPP4 
inhibitors has been shown to have many of the same glycemic 
benefits as GLP1 receptor agonists in the treatment of T2DM. 
Trials of DPP4 inhibitors in T1DM to date have not yet shown 
significant clinical benefit.332 Incretin-based therapies are not 
FDA approved for use in T1DM. 
0
0.5
1
1.5
2
2.5
% Time <70 mg/dL
3
3.5
4
4.5
5
5.5
50
60
70

2
2.5
3
3.5
4
4.5
5
5.5
50
60
70
80
90
100
% Time in 70–180 mg/dL
2015
2016
2017
2018
• Fig. 36.10  Clinical performance of insulin-only (disks) and insulin and 
glucagon (squares) automated insulin delivery systems, where the size 
of the shape depicts the number of subjects in the study and the color 
depicts the publication year. (Modified from Dassau E. The path to a medi­
cal internet of things (IoT) and human-centric design of an artificial pan­
creas. Presented at the Sym

 of an artificial pan­
creas. Presented at the Symposium on Artificial Pancreas and Decision 
Support Approaches at the American Diabetes Association 78th Scientific 
Sessions, Orlando, FL, 2018.)
1434
SGTL1 and SGLT2 Inhibitors
Oral SGLTs reduce postprandial hyperglycemia, reducing glu­
cose reabsorption in the proximal renal tubule (SGLT2) and in 
some agents decreasing and slowing intestinal glucose absorption 
(SGLT1).333 In those 

testinal glucose absorption 
(SGLT1).333 In those with T2DM, they have been associated with 
improvement of glycemic control, weight loss, and blood pres­
sure reduction,334–337 lower rates of cardiovascular morbidity and 
mortality in those at high cardiovascular risk,338 and decreased 
albuminuria.339 Two recent large, double-blinded, phase III ran­
domized control trials studied the efficacy and safety of SGLTs 
in adults with T1DM combined with optimized insulin ther­
apy.340,341 The inTande

th optimized insulin ther­
apy.340,341 The inTandem1 trial (n = 793) was a 1-year study of the 
dual SGLT1/SGLT2 inhibitor sotagliflozin, 200 or 400 mg once 
daily before breakfast combined with optimized insulin therapy.341 
The DEPICT-2 trial (n = 813) was a 24-week study of the SGLT2 
inhibitor dapagliflozin, 5 or 10 mg once daily.340 Both studies 
showed that, compared with placebo, adjunctive use of an SGLT 
inhibitor was associated with modest but durable dose-dependent 
improvements in Hb

est but durable dose-dependent 
improvements in HbA1c, reductions in insulin dose, and weight 
loss. Unfortunately, those treated with SGLT inhibitors also had 
higher rates of genital mycotic infections and DKA (in Tandem1 
trial participants with ≥1 DKA event in the 1-year study: 0.4% in 
the placebo group, 3.4% in the 200-mg sotagliflozin group, 4.2% 
in the 400-mg sotagliflozin group; DEPICT-2 trial participants 
with a definite DKA event over the 24-week study: 0 in the placebo 
group, 2.6%

r the 24-week study: 0 in the placebo 
group, 2.6% in the 5-mg dapagliflozin group, 2.2% in the 10-mg 
dapagliflozin group). Similar large phase III studies of once-daily 
oral doses of empagliflozin adjunctive to insulin therapy over 52 
weeks in patients with T1DM (EASE-2 and EASE-3) have pre­
sented preliminary data showing similar benefits. Risk of DKA 
with SGLT2 inhibitors might be higher in patients with a history 
of DKA, individuals on a low-carbohydrate diet, and in those who 
do not c

 low-carbohydrate diet, and in those who 
do not check ketones in the setting of potential symptoms. SGLT 
inhibitors are yet not FDA approved for use in T1DM.
In considering the addition of adjunctive drugs in a person 
with T1DM, any potential benefits must be carefully weighed 
against risks and costs of the medications. 
Outcomes Beyond HbA1c
CGM offers a new opportunity to assess glycemic control beyond 
HbA1c, which assesses average glycemic control over the previous 
2 to 3 months, althou

c control over the previous 
2 to 3 months, although it is more heavily weighted toward the 
most recent month. Yet there are challenges with the use of HbA1c 
as a measure of glycemia, as it can reflect a wide range of mean 
glucose levels and can vary in persons of different races, in those 
with different rates of glycation, and according to red cell survival 
time.342–346
CGM data reflect overall glycemia with the collection of just 2 
weeks of data.347 Recent consensus conferences, reflecti

of data.347 Recent consensus conferences, reflecting major 
national and international professional organizations, have pub­
lished a series of CGM metrics important in the assessment of 
glycemia in persons with diabetes,348,349 based on the recognition 
that there can be an improved understanding of a person’s glycemia 
beyond the HbA1c. At present, there is a series of glucose metrics 
reported on CGM data downloads, along with an effort to stan­
dardize the CGM metrics and to include glucose

an­
dardize the CGM metrics and to include glucose data collected 
over a 2-week period, with the goal that regulators will also accept 
these metrics as important glycemic outcomes.350 The metrics 
include measures of overall glucose levels and glucose variability. 
The major measures include glucose mean, glucose standard devi­
ation, coefficient of variation ([standard deviation/mean] × 100 
as a percentage), glucose time in range (70–180 mg/dL [3.9–10.0 
mmol/L]), time in hyperglycemia (>180

L [3.9–10.0 
mmol/L]), time in hyperglycemia (>180 mg/dL [10.0 mmol/L] 
and >250 mg/dL [13.9 mmol/L]), and time in hypoglycemia (<70 
mg/dL [3.9 mmol/L] and <54 mg/dL [3.0 mmol/L]). Glucose 
time in range, as well as time in hyperglycemia and hypoglycemia, 
should be reported per unit time, as in the percentage of the values 
in the specified range of 24 hours/day. A clinically meaningful 
episode of hypoglycemia has been defined as 15 minutes or more 
of consecutive CGM readings less than 54 mg

 more 
of consecutive CGM readings less than 54 mg/dL (3.0 mmol/L). 
There is no doubt that CGM has heralded in a new era to enhance 
our understanding of outcomes beyond HbA1c.
In addition to glycemic metrics beyond HbA1c, there are 
numerous patient-reported outcomes that warrant attention. 
These include quality of life, well-being, fear of hypoglycemia, 
diabetes distress or burden, treatment satisfaction, self-efficacy, 
disordered eating behaviors, anxiety, depression, diabetes family 
con

haviors, anxiety, depression, diabetes family 
conflict, sleep deprivation, and others, many of which have been 
discussed in a recent position statement by the ADA.351 Several 
validated surveys to assess such patient-reported outcomes are 
catalogued in this publication. With the continued development 
of adjunctive therapies and advanced diabetes technologies, there 
will be a continued need to consider the patient- and family-
reported outcomes in addition to metrics of glycemic control. 
Tr

es in addition to metrics of glycemic control. 
Transition in Care From Pediatric to Adult 
Health Care Settings and Beyond
The delivery of pediatric diabetes care focuses on the young per­
son with diabetes, as well as family members who assist with 
diabetes management tasks and provide support and guidance. 
Adult care generally tends to follow a medical model with a focus 
almost exclusively on the person with diabetes. This variation in 
approaches can produce a disconnect for the young per

roaches can produce a disconnect for the young person with 
T1DM who transitions between health care settings. Additionally, 
transition generally occurs between the ages of 18 and 22 years, 
although it can occur at younger ages (down to age 14–16 years) 
or older ages (up to 26 years), depending on the health care sys­
tem. Nonetheless, transition often occurs during the fragile devel­
opment stage of young adulthood, between the ages of 18 and 25 
years, when the young person faces many compe

 25 
years, when the young person faces many competing challenges 
related myriad choices concerning education, occupation, social­
ization, intimacy, economic self-sufficiency, and potential geo­
graphic separation from the family unit, among other issues.352
Transition is considered as the process that begins during ado­
lescence when the teen accepts more diabetes self-care responsibil­
ities and attends part of the diabetes encounter without parents or 
guardians. During transition, the teen

parents or 
guardians. During transition, the teen or young adult is expected 
to master self-care skills, as well as learn how to make follow-up 
appointments, seek on-call help, and obtain prescriptions, for 
example. At the end of transition, there is the actual transfer to 
adult care, the final step that entails the need for an introduction 
to a different health care delivery model.
There has been increased interest in the study of transition from 
pediatric to adult diabetes care due to t

on from 
pediatric to adult diabetes care due to the recognition that young 
adults experience many adverse outcomes, including achieving 
the highest mean HbA1c level of 9.2% at the age of 19 years.353 
Many young adults with T1DM often have substantial gaps in 
care exceeding 6 to 12 months between visits, resulting in dete­
riorating glycemic control, the need for emergency room use, and 
the potential emergence of diabetes complications that go unde­
tected and untreated in a timely manner.3

go unde­
tected and untreated in a timely manner.354–356 The ADA, along 
with other major national and international societies, created a 
1435
joint consensus statement on transition, based mainly on expert 
consensus, which has helped lay the groundwork for continued 
research in this area.185,356,357 There are few published controlled 
trials, although there are studies evaluating the use of care coordi­
nator, often called a patient navigator358 or care a

tor, often called a patient navigator358 or care ambassador,359–361 
at time of transfer from the pediatric to adult team.362,363
In general, growing teens should begin to learn about the pro­
cess of transition to adult heath care starting in early to mid-ado­
lescence from the pediatric diabetes health care team. In the final 
year of pediatric care, the young adult should receive names and 
contact information for adult diabetes care providers. Then, both 
the pediatric and adult providers ca

. Then, both 
the pediatric and adult providers can help support the transfer and 
ensure the appropriate transmission of important diabetes medical 
information. Numerous online resources have been developed to 
assist in the transition process.364,365 Finally, although the topic of 
transition from pediatric to adult care continues to generate sub­
stantial clinical and research interest, another transition period, 
specifically the one from adulthood to a more mature older adult 
group, also 

ulthood to a more mature older adult 
group, also requires ongoing research. This is especially important 
given that there are substantial numbers of persons with T1DM 
living many decades with diabetes and surviving well into the geri­
atric years, indicating a need to reassess the cognitive abilities and 
hypoglycemia risk in the older person with diabetes.366,367 
Acute Diabetic Emergencies
Diabetic Ketoacidosis
DKA is a significant, expensive, and possibly lethal and potentially 
avoidable 

e, and possibly lethal and potentially 
avoidable acute complication of T1DM.
Pathophysiology of DKA
DKA is defined by hyperglycemia (blood glucose >200 mg/
dL, ∼11 mmol/L), ketonemia (blood beta-hydroxybutyrate ≥3 
mmol/L or moderate/large urine ketones), and acidosis (venous 
pH <7.3 or serum bicarbonate <15 mmol/L).368 It is the result of 
relative or absolute insulin deficiency causing an increase in gluca­
gon concentrations and accompanying increases in epinephrine, 
norepinephrine, cortis

 increases in epinephrine, 
norepinephrine, cortisol, and growth hormone levels.369 These 
hormone changes lead to a catabolic state in the periphery, mobi­
lizing substrates that the liver then uses to increase production 
of glucose and ketone bodies. When serum glucose concentra­
tions exceed the renal threshold of approximately 180 mg/dL (10 
mmol/L), osmotic diuresis leads to dehydration and loss of elec­
trolytes. Vomiting, which commonly occurs with severe ketosis, 
can further contribute

ccurs with severe ketosis, 
can further contribute to dehydration and electrolyte abnormali­
ties. The vicious cycle continues with additional stress hormones 
production, which leads to worsening insulin resistance, hypergly­
cemia, and ketosis.368 
Epidemiology and Risk Factors for DKA
DKA is a common presentation in new-onset T1DM. The esti­
mated proportion of children with new-onset T1DM who pres­
ent with DKA ranges from 15% to 70% in different studies and 
in different countries.368,370–3

rent studies and 
in different countries.368,370–372 Individuals who are young (<5 
years), do not have a first-degree relative with T1DM, and with 
lower socioeconomic status are at a greater risk of DKA at diagno­
sis.372 In a review of international DKA rates, higher DKA inci­
dence at first presentation of T1DM was associated with lower 
gross domestic product, lower latitude, and lower background 
incidence of T1DM.371 Most cases of DKA, in fact, occur in those 
with established diabetes, w

fact, occur in those 
with established diabetes, where the risk of DKA is between 1 
and 10 per 100 person-years, and is higher in patients with poor 
metabolic control and/or history of insulin omission; a history 
of a prior episode of DKA; gastrointestinal illness; psychiatric 
disorders, including eating disorders; low socioeconomic status; 
and, for unclear reasons, females in adolescence.353,368,373 Insulin 
pump use is actually associated with lower rates of DKA compared 
with MDI,374,375

with lower rates of DKA compared 
with MDI,374,375 although insulin pump failure is an important 
cause of hyperglycemia, ketonemia, and DKA. 
Prevention of DKA /Sick Day Management
Appropriate management of sick days in individuals with T1DM 
can usually be achieved in the outpatient setting with ongoing 
education and support. Guidelines are available for management 
of sick days in children and adolescents with diabetes.376 The pre­
vention of progression to DKA and avoidance of hypoglycemia 

 progression to DKA and avoidance of hypoglycemia 
rely on modifying doses of but never omitting insulin, frequent 
monitoring of blood glucose and ketone levels every 1 to 3 hours, 
preventing/treating dehydration and hypoglycemia, treating the 
underlying illness, and maintaining frequent contact with the 
diabetes team. Monitoring of ketones using blood ketone meters 
that measure beta-hydroxybutyrate is preferred over monitoring 
of urine ketone strips, due to easier sampling, lower rates of

one strips, due to easier sampling, lower rates of hos­
pitalization/emergency department visits, and the potential for 
earlier and more accurate identification of clinical worsening or 
improvement.376–378
Insulin Dosing During Sick Days
The degree of hyperglycemia and ketosis, along with the rate of 
change of these parameters, helps guide insulin dose adjustments. 
Supplemental rapid-acting insulin boluses (correction doses or 
5–20% of the total daily insulin dose, TDD) should be given ever

otal daily insulin dose, TDD) should be given every 
2 to 3 hours while ketones persist; see Table 36.9 for bolus dos­
ing suggestions. For ketones with hypoglycemia, consider reduc­
ing basal rates for a short period of time for patients on insulin 
pumps. For ketones, hypoglycemia, and decreased oral intake over 
a prolonged period, consider reducing the insulin glargine dose 
for patients on MDI and basal rates for those on insulin pumps. 
Prevention/Treatment of Dehydration During Sick Days

vention/Treatment of Dehydration During Sick Days
Persons with T1DM and ketonemia require extra fluid due to risk 
of dehydration. A reasonable target oral fluid intake is 0.5 to 1.0 
ounce per year of age per hour (maximum of 8 ounces/hour). The 
type of drink should be adjusted according to blood glucose levels 
such that when blood glucose is 250 mg/dL (∼14 mmol/L) or less, 
rehydrate with sugar-containing clear liquids (e.g., sports drinks, 
soda, popsicles, oral electrolyte solution), and w

soda, popsicles, oral electrolyte solution), and when blood glucose 
is greater than 250 mg/dL (14 mmol/L), give sugar-free clear liq­
uids (e.g., water, diet soda, seltzer, diet juices). 
Mini-Dose Glucagon
When blood sugars remain low and ketones remain elevated, small 
doses of glucagon (mini-dose glucagon) may be given. As long as 
there are adequate hepatic glycogen stores, mini-dose glucagon 
can help with resolution of hypoglycemia in the setting of pro­
longed vomiting or fasting.379 Glu

setting of pro­
longed vomiting or fasting.379 Glucagon should be reconstituted 
with the diluent in the glucagon kit, then the desired mini-dose 
glucagon is drawn up and administered with an insulin syringe. 
Table 36.10 provides information on dosing.380 
Education
The ADA recommends yearly education on sick day management 
and DKA prevention for persons with T1DM and their caregivers, 
1436
including the importance of insulin admin

tabolism
including the importance of insulin administration, as well as glu­
cose and ketone monitoring.381 Patients and families should be 
aware that they should seek immediate medical attention for signs 
of dehydration, prolonged vomiting for more than a few hours, 
persistent hyperglycemia (>250–300 mg/dL, >14–16.7 mmol/L), 
ketones that do not improve after 12 hours, or symptoms of DKA 
(abdominal pain, nausea, vomiting, fruity-smelling breath, hyper­
ventilation, or altered mental status)

ath, hyper­
ventilation, or altered mental status). 
Treatment of DKA
Detailed guidelines for the treatment of DKA are available.368 
Immediate initial treatment of DKA begins with fluid replace­
ment, typically with one to two intravenous fluid boluses with 
normal saline, 10 mL/kg each. Rehydration should begin before 
starting insulin therapy, and initial fluid resuscitation is then 
followed by continued cautious rehydration with the goal of 
replacing the estimated fluid deficit over the fi

replacing the estimated fluid deficit over the first 24 to 48 hours. 
Initiation of an insulin drip at a rate of 0.05 to 0.1 units/kg per 
hour, without an insulin bolus, should begin at least 1 hour after 
fluid replacement is started. Patients must be monitored very 
closely for clinical and biochemical response to treatment. Fluid 
content and rate, and occasionally insulin rate, should be titrated 
so that blood glucose falls gradually at a rate of 50 to 75 mg/dL 
per hour (2.8–4.2 mmol/L p

rate of 50 to 75 mg/dL 
per hour (2.8–4.2 mmol/L per hour). Fluid should typically con­
tain saline with potassium, phosphate, and acetate. Bicarbonate 
is not typically recommended in the treatment of DKA, except 
in unusual circumstances related to severe acidosis or hyperkale­
mia. Potassium replacement is usually required due to total-body 
potassium depletion, but potassium supplementation should be 
deferred until urine output is documented.
The Pediatric Emergency Care Applied Research Ne

.
The Pediatric Emergency Care Applied Research Network 
(PECARN) Fluid Therapies Under Investigation in DKA 
(FLUID) study was a multisite prospective randomized factorial 
trial examining long- and short-term neurologic outcomes asso­
ciated with the rate of intravenous fluid administration (rapid 
or slow) and the sodium chloride content of intravenous fluids 
(0.9% or 0.45%).382 In the 1389 episodes of DKA reported in 
1255 children, the primary outcome of a decline in mental status 
(two co

ary outcome of a decline in mental status 
(two consecutive Glasgow Coma Scale scores <14; possible scores 
range from 3–15) was met in 48 episodes of DKA (3.5%) and was 
not associated with either the rate of fluid administration or the 
sodium chloride content of the fluids. Secondary neurologic out­
comes including clinically apparent brain injury during treatment 
for DKA, short-term memory, and memory and IQ measured 2 to 
6 months after recovery from DKA were also not associated with 
flui

overy from DKA were also not associated with 
fluid administration rate or content.
One important cause of morbidity and mortality in DKA is 
cerebral edema. Risk factors for cerebral edema include young 
age (<5 years), new-onset T1DM, longer duration of symptoms, 
lower initial pH, higher initial blood urea nitrogen, treatment 
with bicarbonate, and serum sodium concentrations increasing 
during initial treatment of DKA. In patients at high risk of cere­
bral edema, or suspected of having cere

k of cere­
bral edema, or suspected of having cerebral edema, timely inter­
vention with mannitol or hypertonic saline is important. 
Other Complications
Hypoglycemia is the other major acute complication of T1DM 
and is discussed in Chapter 38. Chronic complications are dis­
cussed in Chapter 37. 
Comorbidities
There is a higher prevalence of additional autoimmune conditions 
among persons with T1DM compared with the general popula­
tion. In an analysis of more than 25,000 participants in the T

analysis of more than 25,000 participants in the T1D 
Exchange Registry, 27% of children and adults with T1DM were 
diagnosed with one or more additional autoimmune diseases, 
and prevalence was higher in those of older age, female sex, and 
non-Hispanic white race.383 The most common comorbidities are 
thyroid dysfunction and celiac disease, but autoimmune primary 
adrenal insufficiency (Addison disease), Graves disease, inflamma­
tory bowel disease, collagen vascular disease, skin disease, and

ease, collagen vascular disease, skin disease, and oth­
ers can occur as well. Regular assessment for signs and symptoms 
  Glucagon Mini-Dosing
Glucagon 
Dose (μg)
Glucagon 
Dose (mg)
Unit Markings 
on Insulin 
Syringe
Age <2 years
20
0.02
2
2–15 years
10 per year of 
age (range, 
20–150)
0.01 per year of 
0.02–0.15)
1 per year of 
1–15)
Age >15 years
150
0.15
15
TABLE 36.10
  Guidelines for Insulin Adjustment With Ketones
BG <90 mg/dL
(BG <5 mmol/L)
BG 90–179 mg/dL

tones
(BG 5–9.9 mmol/L)
BG 180–249 mg/dL
(BG 10–13.9 mmol/L)
BG >250 mg/dL
(BG >14 mmol/L)
Ketones <0.6 mmol/L
(negative/trace)
Ordinary bolus for carbohydrates
Ordinary bolus
Increase bolus by 5%
Ketones 0.6–1.4 mmol/L
(small)
Increase bolus by 10%
Ketones 1.5–2.9 mmol/L
(moderate)
Increase bol

ase bolus by 5%
Increase bolus by 15%
Ketones ≥3 mmol/L
(large)
Increase bolus by 20%
BG, Blood glucose.
   
TABLE 36.9
1437
of other autoimmune diseases and periodic screening in asymp­
tomatic individuals is recommended.
Thyroid Disease
Approximately 20% of individuals with T1DM have hypothy­
roidism or hyperthyroidism, making thyroid disease the most 
common 

yroidism, making thyroid disease the most 
common autoimmune disorder associated with T1DM.383 The 
ADA and ISPAD recommend measuring thyroid antibodies and 
thyroid-stimulating hormone (TSH) levels soon after diagnosis of 
T1DM.381,384 One large study of children with T1DM found that 
about 25% of children had positive thyroid peroxidase autoanti­
bodies at the time of T1DM diagnosis.385 Thyroid function tests 
may be slightly abnormal at the time of T1DM diagnosis due to 
sick euthyroid syndro

me of T1DM diagnosis due to 
sick euthyroid syndrome; if this is the case, thyroid function tests 
should be reassessed once glycemic control improves. If antibodies 
are negative and TSH levels are normal soon after diagnosis, regu­
lar measurement of TSH concentrations is recommended every 1 
to 2 years, and sooner if the patient develops any new symptoms 
or signs concerning for thyroid dysfunction, including thyro­
megaly, abnormal growth pattern, or unexplained fluctuations in 
blood glucos

tern, or unexplained fluctuations in 
blood glucose level. There is some evidence that subclinical hypo­
thyroidism is associated with decreased linear growth velocity and 
increased risk of symptomatic hypoglycemia.386 
Celiac Disease
Celiac disease is the second most common coexistent autoim­
mune disorder in patients with T1DM, is diagnosed in 2% to 8% 
of youth with T1DM and is more prevalent in those who were 
younger at T1DM diagnosis and those of white race.383,387,388 
Introduction of gl

hose of white race.383,387,388 
Introduction of gluten-containing foods between 4 and 6 months 
of age (compared with introduction in the first 3 months of life 
or in the seventh month or later) in infants at increased risk for 
celiac disease and T1DM appears to mitigate the risk of celiac dis­
ease.389 Individuals with T1DM and celiac disease may experience 
increased glycemic variability and unexplained hypoglycemia.390 
The ADA and ISPAD recommend screening for celiac disease in 
children w

ommend screening for celiac disease in 
children with T1DM soon after diagnosis and at regular intervals 
after diagnosis (ADA: rescreen within 2 years and again after 5 
years; ISPAD: screen every 1–2 years).381,384 Screening labs include 
immunoglobulin A (IgA) tissue transglutaminase antibodies, with 
documentation of normal total serum IgA levels; if an individual is 
found to be IgA deficient, send immunoglobulin G–specific anti­
body tests. More frequent screening may be necessary if sympt

 More frequent screening may be necessary if symptoms 
develop or in children who have a first-degree relative with celiac 
disease. If screening tests are positive, referral to gastroenterology 
is recommended before prescribing a gluten-free diet. Typically, a 
biopsy is done to confirm the diagnosis of celiac disease, especially 
in asymptomatic children.391 In symptomatic individuals with 
high antibody titers, some providers may prescribe dietary changes 
without a confirmatory biopsy. Once

etary changes 
without a confirmatory biopsy. Once the diagnosis is confirmed, 
children with celiac disease should be placed on a gluten-free diet 
and be referred to a dietitian experienced in managing both dia­
betes and celiac disease, because the nutritional management of a 
person with both T1DM and celiac disease can be complex and 
burdensome. A gluten-free diet may reduce gastrointestinal com­
plaints, metabolic symptoms, and hypoglycemia.392
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	392.	 Abid N, McGlone O, Cardwell C, et al. Clinical and metabolic 
effects of gluten free diet in children with type 1 diabetes and coe­
liac disease. Pediatr Diabetes. 2011;12(4 Pt 1):322–325.
	393.	 Kimpimäki T, Kulmala P, Savola K, et al. Natural history of beta-
cell autoimmunity in young children with increased genetic sus­
ceptibility to type 1 diabetes rec

ed genetic sus­
ceptibility to type 1 diabetes recruited from the general population. 
J Clin Endocrinol Metab. 2002;87(10):4572–4579.
	394.	 Hoppu S, Härkönen T, Ronkainen MS, et al. IA-2 antibody iso­
types and epitope specificity during the prediabetic process in chil­
dren with HLA-conferred susceptibility to type I diabetes. Clin Exp 
Immunol. 2006;144(1):59–66.
	395.	 Bonifacio E, Beyerlein A, Hippich M, et al. Genetic scores to stratify 
risk of developing multiple islet autoantibodies 

risk of developing multiple islet autoantibodies and type 1 diabetes: 
a prospective study in children. PLoS Med. 2018;15:e1002548.
	396.	 Battaglia M, Anderson MS, Buckner JH, et al. Understanding and 
preventing type 1 diabetes through the unique working model of 
TrialNet. Diabetologia. 2017;60(11):2139–2147.
	397.	 Fouts A, Pyle L, Yu L, et al. Do electrochemiluminescence assays 
improve prediction of time to type 1 diabetes in autoantibody-pos­
itive TrialNet subjects? Diabetes Care. 201

 Pugliese A, Vendrame F, Reijonen H, et al. New insight on human 
type 1 diabetes biology: nPOD and nPOD-transplantation. Curr 
Diabetes Rep. 2014;14(10):530.
	401.	 Pugliese A, Yang MD, Kusmarteva I, et al. The juvenile diabetes 
research foundation network for pancreatic organ donors with dia­
betes (nPOD) program: goals, operational model and emerging 
findings. Pediatr Diabetes. 2014;15(1):1–9.
1438
37
Complications of Diabetes Mellitus
MICHAEL BROWNLEE, LLOYD P. AIELLO, JENNIFER K. SUN, M

HAEL BROWNLEE, LLOYD P. AIELLO, JENNIFER K. SUN, MARK E. COOPER, 
EVA L. FELDMAN, JORGE PLUTZKY, AND ANDREW J.M. BOULTON
Biochemistry and Molecular Cell Biology
Clinical Overview
All forms of diabetes mellitus are characterized by insufficient or 
absolute lack of insulin secretion, insulin resistance, altered fuel 
metabolism, and development of diabetes-specific complica­
tions in the eye, kidney, and peripheral nerve. Diabetes is also 
associated with accelerated atherosclerosis affecting art

ted with accelerated atherosclerosis affecting arteries of the 
heart, brain, and lower extremities. In coronary arteries, diabetic 
atherosclerosis is more diffuse, with greater inflammatory infiltrate 
and larger necrotic core size.1 Diabetes increases heart failure risk 
fourfold, and doubles both early and late post–myocardial infarc­
tion fatality rates.2 As a consequence of these complications, dia­
betes is the leading cause of new blindness in people 20 to 74 years 
old,3 the leading cau

ss in people 20 to 74 years 
old,3 the leading cause of end-stage renal disease (ESRD), and the 
KEY POINTS
 • 
Chronic hyperglycemia causes retinopathy and nephropathy. The 
effects of former high HbA1c levels can persist for years of subse­
quent lower HbA1c values. Both chronic hyperglycemia and the 
consequences of insulin resistance cause neuropathy, atheroscle­
rosis, and cardiomyopathy. In some patients, chronic unresolved 
inflammation is as great a cardiovascular disease risk factor as

s as great a cardiovascular disease risk factor as 
high blood pressure and cholesterol. Mitochondrial dysfunction is 
a component of many mechanisms underlying complications.
 • 
Proliferative diabetic retinopathy and diabetic macular edema can 
both cause severe visual loss. Treatment of diabetic macular edema 
with intraocular injection of vascular endothelial growth factor 
(VEGF) inhibitors—medications that inhibit vascular endothelial 
growth factor—reduces diabetes-induced abnormal retin

wth factor—reduces diabetes-induced abnormal retinal thick­
ening and improves long-term visual acuity outcomes. Treatment 
with panretinal photocoagulation reduces blindness from prolifera­
tive diabetic retinopathy by over 90%. Intraocular anti-VEGF therapy 
can now also effectively treat proliferative diabetic retinopathy.
 • 
Chronic diabetic kidney disease increases the risk of cardiovas­
cular disease and death. Optimal management includes early 
control of blood pressure using renin-angi

 early 
control of blood pressure using renin-angiotensin-aldosterone 
system blockade and other agents, control of hyperglycemia 
(glucagon-like peptide-1 [GLP1]) analogs and sodium glucose 
cotransporter-2 (SGLT2) inhibitors that may be more renopro­
tective, and control of dyslipidemia.
 • 
Diabetic neuropathies include distal symmetric polyneuropathy, 
mononeuropathies, and a variety of autonomic neuropathies. 
Loss of heart rate variability due to autonomic neuropathy 
increases the risk o

 due to autonomic neuropathy 
increases the risk of cardiac events more than fourfold. The cor­
nerstone of treatment is blood glucose control. Specific antide­
pressants, anticonvulsants, and the γ-aminobutyric acid (GABA) 
analogue pregabalin are used to treat painful neuropathy.
 • 
Prediabetes and diabetes both substantially increase risk of 
coronary disease, early and late myocardial infarction fatality rates, 
and risk of congestive heart failure, even after adjustment for other 
cardiov

 failure, even after adjustment for other 
cardiovascular risk factors. Hyperglycemia and insulin resistance in­
teract synergistically with hypertension and dyslipidemia. Optimal 
treatment includes intensive lowering of low-density lipoprotein 
(LDL) cholesterol with early use of statins or other agents, blood 
pressure control, and use of glucose-lowering agents that have 
proven efficacy to reduce cardiovascular events, including met­
formin, GLP1 receptor agonists, and SGLT2 inhibitors. In 

 GLP1 receptor agonists, and SGLT2 inhibitors. In general, 
coronary artery bypass grafting may provide better outcomes than 
coronary stenting in patients with diabetes. Angiotensin-convert­
ing enzyme (ACE) inhibitors and angiotensin II receptor blockers 
(ARBs) reduce post–myocardial infarction mortality risks.
 • 
Diabetic foot ulcers are the major cause of nontraumatic leg am­
putation. Risk factors are sensory neuropathic loss of propriocep­
tion, motor neuropathic foot deformity, and per

p­
tion, motor neuropathic foot deformity, and peripheral vascular 
disease. Simple clinical interventions reduce amputation by up 
to 80%. Management involves pressure off-loading, parenteral 
antibiotics for infection, and ensuring adequate arterial inflow.
Biochemistry and Molecular Cell Biology, 1438
 Insulin Resistance Increases Fatty Acid Oxidation, Causing 
Mitochondrial Overproduction of ROS, 1451
Diabetic Retinopathy and Other Ocular Complications of 
Diabetes, 1463
Diab

Other Ocular Complications of 
Diabetic Nephropathy, 1478
Diabetic Neuropathies, 1488
Diabetic Heart Disease, 1503
The Diabetic Foot, 1519
1439
Chapter 37  Complications of Diabetes Mellitus
leading cause of both neuropathy and nontraumatic lower extrem­
ity amputations. Life expectancy for patients with diabetic ESRD 
is under 4 years. Neuropathy affects more than 60% of patients 
with diabetes. Diabetic neuropathy includes distal symmetric 
polyneuropathy, mononeuropathies, and 

 symmetric 
polyneuropathy, mononeuropathies, and a variety of autonomic 
neuropathies causing cardiac arrhythmias, hypotension, erectile 
dysfunction, urinary incontinence, gastroparesis, and nocturnal 
diarrhea.
Overall life expectancy is about 11 to 13 years shorter for 
people with type 1 diabetes mellitus (T1DM), and 7 to 10 years 
shorter for people with type 2 diabetes mellitus (T2DM) com­
pared to people without diabetes.4–6 Accelerated cardiovascular 
disease is the leading cause of dea

ardiovascular 
disease is the leading cause of death in people with both T1DM 
and T2DM diabetes. T1DM patients diagnosed before the age of 
10 years have a 30-fold increased risk of coronary heart disease and 
acute myocardial infarction occurring in their early adult years.7 
While the incidence of diabetes-related ESRD, acute myocardial 
infarction, and amputation has declined over the past 25 years, the 
rates are still 10-fold higher than for the overall adult population.8
A strong relation

r the overall adult population.8
A strong relationship between hyperglycemia and diabetic 
retinopathy and nephropathy has been demonstrated in both 
T1DM and T2DM large prospective clinical studies—the Dia­
betes Control and Complications Trial (DCCT) and the United 
Kingdom Prospective Diabetes Study (UKPDS), respectively.9,10 
There is a continuous (although not linear) relationship between 
the level of glycemia and the risk of development and progres­
sion of these microvascular complicatio

d progres­
sion of these microvascular complications (Fig. 37.1). In T1DM, 
hyperglycemia is similarly associated with neuropathy, while in 
T2DM, it is more closely associated with the consequences of 
insulin resistance/hyperinsulinemia.11,12 In contrast to the benefi­
cial effects of glycemic reduction on microvascular complications, 
a meta-analysis of randomized controlled trials shows reduction in 
myocardial infarction rates but limited benefits of intensive glu­
cose-lowering treatment o

nefits of intensive glu­
cose-lowering treatment on all-cause mortality and deaths from 
cardiovascular causes.13 While consequences of insulin resistance/
hyperinsulinemia and chronic hyperglycemia both play a role in 
the pathobiology of diabetic cardiovascular complications, the 
consequences of insulin resistance/hyperinsulinemia may play 
a larger role.14,15 In the San Antonio Heart Study, for example, 
individuals with normal glucose tolerance and higher insulin resis­
tance had twice the 

nce and higher insulin resis­
tance had twice the 8-year risk of adverse cardiovascular outcomes 
than those with lower insulin resistance, even after adjustment for 
11 risk factors.16,17
Chronic, unresolved systemic inflammation is a cardiovascular 
risk factor independent of traditional lipid and nonlipid risk fac­
tors. In healthy men followed prospectively as part of the Physician’s 
Health Study, those with the highest quartile concentrations of 
high sensitivity C-reactive protein (hsCRP)

ns of 
high sensitivity C-reactive protein (hsCRP) had a 2.9-fold greater 
risk of myocardial infarction than those in the lowest quartile, a 
magnitude of independent risk as great as hypertension and cho­
lesterol.18 Directly reducing inﬂammation with canakinumab, an 
interleukin 1β (IL1β) neutralizing monoclonal antibody, reduced 
cardiovascular event rates in a population receiving lipid-lowering 
therapy at baseline. The greatest reduction occurred among those 
achieving hsCRP concentration

ccurred among those 
achieving hsCRP concentrations below 2 mg/L19 (Fig. 37.2). 
Individuals with and without diabetes had similar magnitude 
of benefit from canakinumab suggesting the IL1β pathway is 
not unique to diabetes. Chronic inflammation also contributes 
to chronic kidney disease, mediated in part by activation of the 
NLRP3 (nucleotide-binding oligomerization domain-like recep­
tors [NLR] family pyrin domain containing three) inflammasome 
in renal cells. Inhibition of IL1β with canak

ome 
in renal cells. Inhibition of IL1β with canakinumab also reduced 
major cardiovascular event rates among high-risk patients with 
chronic kidney disease.20 Recent studies suggest defective resolu­
tion of chronic inflammation is also a general feature of most 
diabetic complications.
For each of the complications of diabetes, the effects of for­
mer high HbA1c levels can persist for years after HbA1c values 
Relative risk
HbA1c(%)
20
15
10
5
0
6
Sustained progression
of retinopathy
Progress

0
6
Progression to albuminuria
>300 mg/dL
Progression to clinical
neuropathy
Progession to microalbuminuria
>40 mg/dL
7
8
9
10
11
12
• Fig. 37.1  Relative risks for the development of diabetic complications at 
different mean levels of glycosylated hemoglobin (HbA1c) in patients with 
type 1 diabetes obtained from the Diabetes Control and Complications 
Trial. (Adapted from Skyler J. Diabetic complications: the importance of 
glucose control. Endocrinol Meta

he importance of 
glucose control. Endocrinol Metab Clin North Am. 1996;25:243–254.)
0.00
0.05
0.10
Cumulative Incidence
0.15
0.20
Placebo
Canakinumab, hsCRP ≥2mg/L
Canakinumab, hsCRP <2mg/L
1.0
0.99
0.69
HR
(referent)
(0.82-1.21)
(0.56-0.85)
(95%CI)
P
0.95
0.0004
CANTOS - Cardiovascular Mortality
0
1
2
Years
3
4
5
• Fig. 37.2  Cumulative incidence and hazard ratios of cardiovascular 
mortality among CANTOS participants allocated to either placebo or 
canakinumab according to whether 

ther placebo or 
canakinumab according to whether postrandomization on-treatment 
hsCRP levels were above or below 2 mg/L. Hazard ratios are adjusted 
for age, sex, smoking status, hypertension, diabetes, body mass index, 
baseline concentration of hsCRP, and baseline concentration of LDL-C. 
CI, Conﬁdence interval; HR, hazard ratio. (From Aday AW, Ridker PM. Anti­
inflammatory therapy in clinical care: the CANTOS trial and beyond. Front 
Cardiovasc Med. 2018;5:62.)
1440
SECTION VIII    Disorder

asc Med. 2018;5:62.)
1440
have been lowered (called “metabolic memory” by the DCCT 
and long-term observational follow-up Epidemiology of Diabe­
tes Interventions and Complications [EDIC] study investigators 
and “a legacy effect” by the UKPDS investigators). Data from 
DCCT/EDIC studies proved effects of former intensive and con­
ventional therapy persist for at least 18 years.21,22 In the conven­
tional therapy group, effects of prev

 the conven­
tional therapy group, effects of previous high HbA1c on poststudy 
retinopathy, nephropathy, and major adverse cardiovascular events 
(MACE; comprised of nonfatal myocardial infarction or stroke, 
or cardiovascular death) persisted as if there had been no improve­
ment in HbA1c at all (Fig. 37.3). Atherosclerotic changes not even 
present at the end of the DCCT appeared subsequently in the 
previously higher HbA1c group, followed by a twofold increase in 
heart attacks, strokes, and

a twofold increase in 
heart attacks, strokes, and cardiovascular death, even though the 
HbA1c level in these patients since the end of the DCCT was iden­
tical to that of the formerly intensive-control group during the 
entire time that these arterial changes developed.23 On the other 
hand, the beneficial effects of previous lower HbA1c persisted in 
the intensive treatment group after their HbA1c increased after the 
DCCT intervention ended, as if there had been no deterioration 
in their Hb

as if there had been no deterioration 
in their HbA1c.
Thus T1DM patients in the DCCT with long-term exposure 
to a higher level of hyperglycemia during the study became more 
susceptible to damage despite subsequent lower levels of hyper­
glycemia than when they first started the trial. In contrast, lower 
levels of hyperglycemia during the trial made patients more resis­
tant to damage from subsequent higher levels. Data from long-
term follow-up studies from the UKPDS confirm a similar effect

up studies from the UKPDS confirm a similar effect 
for T2DM patients associated with intensive glucose control, long 
after the cessation of randomized intervention.24
Much current knowledge about diabetic complication mecha­
nisms comes from studies in murine models and of cultured cells. 
Murine models are valuable tools for defining the pathogenesis 
of diabetic complications. However, it is important to recognize 
the limits of murine models. No diabetic animal model, regard­
less of geneti

. No diabetic animal model, regard­
less of genetic background, develops the advanced stages of 
human diabetic complications, such as proliferative retinopathy, 
end-stage renal failure, significant nerve fiber loss, or complex 
atherosclerotic plaques and plaque rupture, nor do they develop 
the degree of fibrosis seen in human hearts with late-stage diabe­
tes–associated heart failure. Also, since much of the mechanistic 
data currently available from studies of cell and mouse models 
reflect

ble from studies of cell and mouse models 
reflect the earliest stages of each complication, mechanisms domi­
nant in the pathogenesis of later stages of each complication are 
likely different from those dominant in the early stages. Recent 
proteomic, genomic, and molecular studies of tissue from patients 
with longer duration of diabetes have contributed to enormous 
recent progress in understanding the biochemical, molecular, and 
cellular mechanisms involved in the pathogenesis of diabetic 

chanisms involved in the pathogenesis of diabetic com­
plications. Many of the multiple mechanisms discussed in this 
section share a common element: prolonged increases in reactive 
oxygen species (ROS) production in cells critical for the develop­
ment of diabetic complications in the retina, kidney, peripheral 
nerve, arteries, and heart. In diabetic mouse models, transgenic 
overexpression of mitochondrial superoxide dismutases or catalase 
prevents retinopathy,25 nephropathy,26,27 neuropath

revents retinopathy,25 nephropathy,26,27 neuropathy,28 athero­
sclerosis,29 and cardiomyopathy.30,31 
Physiologic Reactive Oxygen Species 
Production Is Essential for Normal Intracellular 
Signaling and Cellular Homeostasis
In normal physiology, ROS production is coupled to metabolic 
networks and circadian clocks, and ROS species (hydrogen 
peroxide, H2O2) function as signaling molecules essential for 
normal cellular homeostasis.32–34 Physiologic ROS (H2O2) sig­
naling is essential for normal 

ic ROS (H2O2) sig­
naling is essential for normal intracellular communication, cell 
differentiation, autophagy, response to insulin and growth fac­
tor stimulation, and the generation of physiologic inflammatory 
responses. ROS regulate protein stability, augment protein func­
tion, inactivate function, alter subcellular localization, and alter 
protein-protein interactions (Fig. 37.4). In normal cardiovascu­
lar function, for example, enhanced production of H2O2 from a 
mitochondrial source of

production of H2O2 from a 
mitochondrial source of superoxide leads to increased flow rates 
in human coronary resistance vessels. H2O2 hyperpolarizes and 
dilates human coronary arterioles through opening of Ca2+-acti­
vated K+ channels. Catalase, a scavenger of H2O2, greatly inhibits 
this flow-induced dilation.35 Similarly, H2O2 exerts a beneficial 
effect on vasodilator function and reduces blood pressure in 
transgenic mice with endothelium-targeted nicotinamide ade­
nine dinucleotide phosp

targeted nicotinamide ade­
nine dinucleotide phosphate (NADPH) oxidase 4 (Nox4) over­
expression.36 In the heart, low levels of hydrogen peroxide induce 
proliferation of mouse embryonic stem cells as well as neonatal 
cardiomyocytes, and ROS induce expression of cardiac-specific 
genes, transcription factors, and growth factors in embryonic 
stem cells. These effects are dampened by free radical scavengers. 
ROS also act as transducers of mechanical strain–induced cardio­
vascular differentiati

ical strain–induced cardio­
vascular differentiation of embryonic stem cells.37 ROS-depen­
dent activation of integrins and subsequent induction of PI3-K/
Akt (phosphatidylinositol-4,5-bisphosphate 3-kinase/RACα/β 
serine/threonine-protein kinase) signaling are also involved in 
cyclic strain–mediated cardiomyogenesis.38 Localized ROS pro­
duction contributes to stretch-induced augmentation of cardiac 
contractile activity as well.39 Physiologic excitation–contrac­
tion coupling in heart muscle 

excitation–contrac­
tion coupling in heart muscle may also involve ROS signaling 
as physiologic stretch rapidly activates NOX2 located on sarco­
lemma and T tubule membranes in cardiomyocytes.39 The local 
ROS produced sensitizes ryanodine receptors in the sarcoplasmic 
reticulum. This triggers a burst of Ca2+ sparks, thereby increasing 
Ca2+ signaling sensitivity in healthy cardiomyocytes. Thus ROS 
Cumulative incidence
of retinopathy progression (%)
Conventional therapy
Intensive therapy
24
2

on (%)
24
20
16
12
8
4
0
EDIC year
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
• Fig. 37.3  Cumulative incidence of further progression of retinopathy 4 
years after the end of the Diabetes Control and Complications Trial. The 
median glycosylated hemoglobin level was 8.2% for the conventional ther­
apy group and 7.9% for the intensive therapy group. EDIC, Epidemiology 
of Diabetes, Interventions, and Complications Research Group. (Modified 
from Diabetes Control and

search Group. (Modified 
from Diabetes Control and Complications Trial/Epidemiology of Diabetes 
Interventions and Complications Research Group, Lachin JM, Genuth S, 
et al. Retinopathy and nephropathy in patients with type 1 diabetes four 
years after a trial of intensive therapy. N Engl J Med. 2000;342:381–389.)
1441
production at the right time, place, level, and duration plays a 
crucial role in physiologic homeostasis.40 However, ROS produc­
ti

physiologic homeostasis.40 However, ROS produc­
tion at too high a level, for too long, or at an inappropriate sub­
cellular location, leads to impaired cellular function and diabetic 
tissue pathology.
Recent data suggest the site at which ROS are generated is as 
important as the amount of ROS produced.41 ROS generated 
by mitochondrial complex III are released into the mitochon­
drial matrix, intermembrane space, outer membrane, and cyto­
sol. In contrast, ROS generated by mitochondrial compl

 In contrast, ROS generated by mitochondrial complex I are 
released into the matrix. ROS from these two sites reacted with 
distinct subsets of proteins involved in different physiologic path­
ways.42 Importantly, ROS from mitochondrial complex I caused 
a high percentage of irreversible overoxidations of cysteine thi­
ols, consistent with persistent oxidative damage. They also cause 
nuclear deoxyribonucleic acid (DNA) double-strand breaks and 
oxidize mitochondrial DNA, both of which have adv

oxidize mitochondrial DNA, both of which have adverse con­
sequences.43–45 In contrast, ROS from mitochondrial complex 
III caused reversible oxidation of cysteine thiols, consistent with 
their function as redox switches in critical signaling pathways. Use 
of indiscriminate antioxidants, which block both the damaging 
and the physiologic effects of mitochondrial ROS, likely explain 
the negative findings of clinical trials with selected antioxidant 
compounds. 
Mechanisms of Hyperglycemia-Ind

idant 
Mechanisms of Hyperglycemia-Induced Damage
Increased Aldose Reductase Substrate Conversion
Aldose reductase (AKR1B1), a member of the large aldo-keto 
reductase superfamily, catalyzes reduction of a wide variety of 
hydrophobic and hydrophilic carbonyl-containing compounds—
including glucose and several glycolytic intermediates—to their 
corresponding alcohols. This enzyme is cytosolic and requires 
NADPH as a cofactor. In some cell types, where glucose is con­
verted to the s

 cell types, where glucose is con­
verted to the sugar alcohol sorbitol, sorbitol is then converted to 
fructose by another enzyme, sorbitol dehydrogenase, using NAD+ 
(nicotinamide adenine dinucleotide) as a cofactor. This series 
of reactions, termed the polyol pathway, has been implicated in 
the pathogenesis of several diabetic complications. However, the 
amount of substrate converted to product per second (Kcat) of 
human aldose reductase for glucose is 0.15 [s−1]. Kcat values for 
most en

or glucose is 0.15 [s−1]. Kcat values for 
most enzymes are between 1 and 104. Moreover, because the intra­
cellular glucose concentration in capillary retinal endothelial cells 
incubated in 25 mM glucose is approximately 0.15 mM,46 while 
the Km of aldose reductase for glucose reported by Bohren and 
coworkers is 100 mM and the Kcat/Km is 1.3 [s−1 M−1], the pre­
dicted rate of aldose reductase reduction of glucose to sorbitol in 
microvascular endothelial cells would be expected to be rather l

endothelial cells would be expected to be rather low. 
However, aldose reductase has high affinity and enzyme activity 
for a variety of other substrates, including several glycolytic inter­
mediates such as glyceraldehyde 3-phosphate, and its degradation 
product, methylglyoxal.47,48 AKR1B1 also efficiently reduces oxi­
dative stress-generated lipid aldehydes such as 4-hydroxynonenal 
with a Km in the micromolar range (10–30 μM) as compared to a 
Km of 50–100 mM for glucose49 (Fig. 37.5).
In vi

In vivo studies of aldose reductase inhibition in a 5-year study 
in dogs showed that diabetic neuropathy was prevented, but aldose 
reductase inhibition failed to prevent retinopathy or capillary base­
ment membrane thickening in the retina, kidney, or muscle.50 In 
response to this preclinical result with neuropathy, 32 clinical tri­
als were completed in the 1990s and early 2000s.51 None of these 
trials showed clinical efficacy. In diabetic apolip

rials showed clinical efficacy. In diabetic apolipoprotein E (ApoE) 
knockout mice, overexpression of human aldose reductase acceler­
ated atherosclerosis, and pharmacologic inhibition of the enzyme 
Regulates
protein stability
For example,
oxidation of the
NEDD8-conjugating
enzyme UBC12
Inactivates
function
oxidation
of PTPs
Augments
function
redox-dependent
modification of EGFR
after EGFR-activated
ROS production
Alters
subcellular
translocat

s
translocation
of NRF2 to
the nucleus
Alters
protein-protein
oxidation
of NRX to
release DVL
SH
Cys
Redox-sensitive
target protein
SOH
Cys
Oxidation
ROS
• Fig. 37.4  The multiple ways in which oxidant signaling can affect cellular function. Oxidation of a reactive 
cysteine residue SH (thiol) to SOH (sulphenic acid) leads to an alteration in the reduction-oxidation (redox)–
sensitive target that can then have myriad effects on prote

 target that can then have myriad effects on protein stability, activity, localization, or protein-protein 
interaction. Cys, Cysteine; DVL, dishevelled; EGFR, epidermal growth factor receptor; NEDD8, neural 
precursor cell expressed developmentally down regulated-8; NRF2, nuclear related factor-2; NRX, nucleo­
redoxin; PTP, protein tyrosine phosphatase; ROS, reactive oxygen species. (From Holmström KM, Finkel 
T. Cellular mechanisms and physiological consequences of redox-dependent signaling; U

gical consequences of redox-dependent signaling; UBC12, ubiquitin 
carrier protein 12. Nat Rev Mol Cell Biol. 2014;15:411–421.)
1442
prevented this.52 In contrast, in mice expressing physiologic levels 
of aldose reductase, knockout or pharmacologic inhibition of the 
enzyme unexpectedly caused increased early atherosclerotic lesion 
size in both diabetic and nondiabetic mice.53 Differences in total 
enzyme activity, cofactor levels, a

nces in total 
enzyme activity, cofactor levels, and levels of alternative intracellu­
lar substrates, coupled with known differences in enzyme kinetics 
for different substrates, likely explain these seemingly paradoxic 
observations. 
Increased Intracellular Formation of the Major Advanced 
Glycation End Products—Precursor Methylglyoxal
The post-translational modifications of proteins called advanced 
glycation end products (AGEs) are formed by glucose-derived 
dicarbonyls reacting with amino 

 glucose-derived 
dicarbonyls reacting with amino groups of unprotonated lysine 
and arginine residues of proteins. Methylglyoxal, formed by the 
nonenzymatic fragmentation of the glycolytic intermediate triose 
phosphate, accounts for the majority of hyperglycemia-induced 
increase in AGE adducts in diabetic tissues.54 Intracellular meth­
ylglyoxal is detoxified by the glyoxalase system.55 The enzyme gly­
oxalase I, together with glyoxalase II and a catalytic amount of 
glutathione, reduces thi

nd a catalytic amount of 
glutathione, reduces this highly reactive α-oxoaldehyde to D-lac­
tate (Fig. 37.6). In cells, methylglyoxal reacts with unprotonated 
arginine residues to form the major methylglyoxal-derived epitope 
MG-H1 (methylglyoxal hydroimidazolone 1). Intracellular pro­
duction of AGE precursors damages target cells by three general 
mechanisms (Fig. 37.7). First, AGE modification of intracellu­
lar proteins changes their function. Second, AGE modification 
of extracellular matr

n. Second, AGE modification 
of extracellular matrix components alters their interaction with 
other matrix components and with integrin matrix receptors. 
Third, intracellular methylglyoxal increases expression of both 
the pattern recognition receptor for AGEs (RAGEs) and its major 
endogenous ligands, the proinflammatory S100 calgranulins.56 
Ligation of these ligands with RAGE causes cooperative interac­
tion with the innate immune system signaling molecule toll-like 
receptor 4 (TLR4).57 Ex

naling molecule toll-like 
receptor 4 (TLR4).57 Expression of RAGE, S100A8, S100A12, 
and high mobility group box 1 (HMGB1) are all increased by 
high glucose concentrations in cell culture and in diabetic ani­
mals. This hyperglycemia-induced overexpression is mediated by 
ROS-induced increases in methylglyoxal, which increase binding 
of the transcription factors nuclear factor-κB (NFκB) and activa­
tor protein 1 (AP1) to the promoters of RAGE and of RAGE 
ligands, respectively.56
Diabetes inc

and of RAGE 
Diabetes increases levels of the major methylglyoxal-derived 
adduct in retina, renal glomerulus, and sciatic nerve of rats.58,59 In 
the retina, diabetic mice with knockouts of four transient receptor 
potential cation channels (Trpc1/4/5/6 mice) have increased lev­
els of glyoxalase 1 protein and activity, which protects them from 
the initial phenotype of diabetic retinopathy, pericyte dropout, 
and acellular capillary formation.60 In the kidneys of nondi

lar capillary formation.60 In the kidneys of nondiabetic 
mice, knockdown of glyoxalase I (GLO1) increases to diabetic lev­
els both methylglyoxal modiﬁcation of glomerular proteins and 
oxidative stress, causing alterations in kidney morphology indis­
tinguishable from those caused by diabetes, while in kidneys of 
diabetic mice, GLO1 overexpression completely prevents diabe­
tes-induced increases in methylgloxal modiﬁcation of glomeru­
lar proteins, increased oxidative stress, and the developm

eins, increased oxidative stress, and the development of 
diabetic kidney pathology, despite unchanged levels of diabetic 
hyperglycemia.61
Increased levels of methylglyoxal may contribute to pain­
ful diabetic neuropathy by altering the function of the voltage-
gated sodium channel Nav1.8. Nav1.8 is expressed exclusively 
ROS
Toxic
aldehydes
Increased glucose
NADPH
GSSG
NADP+
GSH
Glutathione reductase
Inactive alcohols
Sorbitol
SDH
NAD+
NADH
Fructose
Aldose
reductase
• Fig. 37.5  Aldose reducta

ctose
Aldose
reductase
• Fig. 37.5  Aldose reductase (AKR1B1) catalyzes reduction of the ROS-
generated and the polyol pathway. Aldose reductase reduces reactive 
oxygen species (ROS)–generated toxic aldehydes such as lipid-derived 
4-hydroxynonenal to inactive alcohols, and in some cell types, glucose to 
sorbitol, using triphosphopyridine nucleotide (NADPH), the reduced form 
of nicotinamide adenine dinucleotide phosphate (NADP+), as a cofactor. 
It also reduces glyceraldehyde 3-phosphate and 

. 
It also reduces glyceraldehyde 3-phosphate and its reactive degradation 
product methylglyoxal. GSH, Reduced glutathione; GSSG, oxidized gluta­
thione; SDH, sorbitol dehydrogenase.
Glucose
The Glyoxylase
System
Glc-6-P
Fru-6-P
Fru-1,6-bP
D-Lactate
GA3P
DHAP
Methylglyoxal
GSH
S-D-Lactoyl-
glutathione
Glyoxalase II
Glyoxalase I
• Fig. 37.6  Intracellular methylglyoxal, the major advanced glycation end 
product (AGE) precursor, is detoxified by the glyoxalase system. The 
enzyme glyoxalase I, to

he glyoxalase system. The 
enzyme glyoxalase I, together with glyoxalase II and a catalytic amount 
of glutathione, reduces this highly reactive α-oxoaldehyde to D-lactate. 
DHAP, Dihydroxyacetone phosphate; GA3P, glyceraldehyde 3-phosphate; 
GSH, glutathione; Glc-6-P, glucose-6-phosphate; Fru-6-P, fructose-
6-phosphate; Fru-1,6-bP, fructose-1,6-bisphosphate. (From Schmoch T, 
Uhle F, Siegler BH, et al. The glyoxalase system and methylglyoxal-derived 
carbonyl stress in sepsis: glycotoxic aspect

ived 
carbonyl stress in sepsis: glycotoxic aspects of sepsis pathophysiology. 
Int J Mol Sci. 2017;18.)
1443
in unmyelinated, small-diameter sensory neurons called C 
fibers. Post-translational modification of Nav1.8 by methylgly­
oxal depolarizes sensory neurons, facilitating firing of these pain 
pathway neurons. In streptozotocin-induced and genetic mouse 
models of diabetes but not in Nav1.8 knockout (Scn10−/−) dia­
betic mice,62 post-translati

ckout (Scn10−/−) dia­
betic mice,62 post-translation modification of Nav1.8 by meth­
ylglyoxal increased electrical excitability and facilitated firing of 
pain pathway neurons, which in turn facilitated neurosecretion 
of calcitonin gene-related peptide, increasing cyclooxygenase-2 
(COX2) expression. Methylglyoxal treatment also evoked thermal 
and mechanical hyperalgesia, reflected by increased blood flow in 
brain regions that are involved in pain processing. Plasma meth­
ylglyoxal concentra

 pain processing. Plasma meth­
ylglyoxal concentrations above 600 nM discriminated between 
diabetes-affected individuals with painful neuropathy and those 
without pain.
Increased methylglyoxal is also an important element in the 
pathogenesis of both diabetic atherosclerosis and diabetic car­
diomyopathy. In nondiabetic ApoE null mice, increasing plasma 
methylglyoxal levels to diabetic levels using a glyoxalase-1 inhibi­
tor caused endothelial inflammation and atherogenesis similar 
to that i

 inflammation and atherogenesis similar 
to that induced by diabetes mellitus.63 In human atherosclerotic 
plaques, MG-H1 (methylglyoxal hydroimidazolone 1) levels were 
associated with rupture-prone plaques having increased levels of 
the inflammatory mediators IL8 and monocyte chemotactic pro­
tein-1 (MCP1), and higher matrix metalloproteinase-9 (MMP9) 
activity. MG-H1 was primarily found in lesion macrophages sur­
rounding the necrotic core, and co-localized with cleaved cas­
pase-3.64 In the

nd co-localized with cleaved cas­
pase-3.64 In the diabetic heart, methylglyoxal preferentially reacts 
with both ryanodine receptor 2, the major myocardial intracellu­
lar mediator of calcium-induced calcium release, and with sarco­
plasmic reticulum Ca2+ ATPase (SERCA2a), which is responsible 
for the synchronized reuptake of released intracellular calcium.65 
This coordinated process of calcium cycling is critical for effi­
cient cardiac contractions, and diabetes-induced defects caused 
by i

actions, and diabetes-induced defects caused 
by increased methylglyoxal adduct formation likely contribute to 
impaired systolic function.
Increased methylglyoxal production also seems to be respon­
sible for poor cardiac stem cell–mediated repair and angiogenic 
capacity.66 Cardiac stem cells from biopsies of hearts from 
human diabetics were less able to repair postinfarction damage in 
immunodeficient mice than cardiac stem cells from nondiabetic 
patients, and conditioned medium from these 

etic 
patients, and conditioned medium from these cells had less angio­
genic capacity. Culture of nondiabetic murine cardiac stem cells 
in high glucose induced the same cardiac repair and angiogenesis 
defects seen in human diabetic cells. In both human and mouse 
cells, overexpression of GLO1 restored the angiogenic defects.66 
In diabetic mice with defective postischemia hindlimb revascular­
ization, overexpression of the methylglyoxal-metabolizing enzyme 
GLO1 exclusively in bone marrow cel

lizing enzyme 
GLO1 exclusively in bone marrow cells was sufficient to restore 
bone marrow cell function and neovascularization of ischemic tis­
sue in diabetes mellitus.67
Increased methylglyoxal also activates the unfolded protein 
response in cardiomyocytes.68 Although transient activation of the 
unfolded protein response relieves endoplasmic reticulum (ER) 
stress, prolonged activation of the unfolded protein response in 
cardiovascular disease triggers apoptosis, mediated by the down­
str

ease triggers apoptosis, mediated by the down­
stream effector C/EBP-homologous protein (CHOP). CHOP 
plays a critical role in macrophage apoptosis, a process involved in 
plaque necrosis in advanced atheromata. In Chop−/−Apoe−/− mice, 
lesion area plaque necrosis was reduced by 50%. In high fat–fed 
ApoE−/− and low-density lipoprotein (LDL) receptor−/− mice, 
CHOP promoted plaque growth, apoptosis, and plaque necro­
sis.69 In cardiomyocytes, methylglyoxal also induces apoptosis via 
CHOP.68 Inf

hylglyoxal also induces apoptosis via 
CHOP.68 Infusion of methylglyoxal in nondiabetic mice induced 
Glucose
Matrix
Integrins
Glucose
Intracellular Protein Glycation
AGE Precursors
Intracellular Proteins
Transcription Factors
DNA
RNA
RAGE
Ligands
Hormones
Cytokines
RAGE
ROS
NFκB
Endothelial Cell
• Fig. 37.7  Potential mechanisms by which intracellular production of advanced glycation end product 
(AGE) precursors damage vascular cells. First, intracellular protein modif

vascular cells. First, intracellular protein modification alters protein function. Sec­
ond, extracellular matrix modified by AGE precursors has abnormal functional properties. Third, intracel­
lular methylglyoxal increases expression of both the receptor for AGEs (RAGE) and its major endogenous 
ligands. Ligand binding to RAGE induces receptor-mediated production of deleterious gene products such 
as cytokines. NFκB, Nuclear factor-κB; ROS, reactive oxygen species. (Adapted from Brownlee M. Lil

tive oxygen species. (Adapted from Brownlee M. Lilly 
lecture 1993: glycation and diabetic complications. Diabetes. 1994;43:836–841.)
1444
cardiomyocyte apoptosis, inflammation, and a significant reduc­
tion in left ventricular fractional shortening and left ventricular 
ejection fraction. Each of these adverse effects was prevented in 
CHOP−/− mice. In the hearts of diabetic mice, overexpression of 
GLO1 in the vasculature70 prevented

expression of 
GLO1 in the vasculature70 prevented diabetes mellitus–induced 
reduction of myocardial capillary density, increased apopto­
sis, and loss of cardiac function. Neuregulin production, which 
transduces signals between the heart’s microvasculature and car­
diomyocytes,71 endothelial nitric oxide synthase (eNOS) dimer­
ization, and expression of the antiapoptotic protein BCL-2 were 
also maintained in the diabetic GLO1 transgenic hearts. 
Activation of Protein Kinase C β, δ, and θ
Pro

s. 
Protein kinase C (PKC) is a family of protein kinase enzymes 
with 15 isoforms that are involved in the regulation of protein 
function. Nine of these 15 PKC isoforms are activated by a lipid 
second messenger, diacylglycerol. Elevated intracellular glucose 
increases diacylglycerol levels in a variety of diabetic target tissues, 
including arterial smooth muscle cells and cardiomyocytes, by de 
novo synthesis.72–74 Hyperglycemia primarily activates

 synthesis.72–74 Hyperglycemia primarily activates the β and δ 
isoforms of PKC, but increases in activity of several other isoforms 
have also been found (Fig. 37.8). These PKC isoforms can also 
be activated by intracellular ROS in the absence of diacylglycerol 
or Ca2+. The regulatory domain of these PKC isoforms contains 
two pairs of zinc fingers with six cysteine residues and two zinc 
atoms, which can be oxidized by intracellular ROS. Oxidation 
alters zinc-finger conformation and activat

ation 
alters zinc-finger conformation and activates PKC.75 Intracellular 
hyperglycemia activates PKC in retina and glomeruli of diabetic 
animals.72,76 In contrast to other complication-prone tissues, total 
PKC activity is reduced in the peripheral nerve of diabetic ani­
mals. Schwann cell PKCα activity is reduced, while vascular wall 
PKCβII activity is increased.77
In early experimental diabetes, activation of PKCβ isoforms 
mediates retinal and renal blood flow abnormalities,78 perhaps by 

 and renal blood flow abnormalities,78 perhaps by 
depressing nitric oxide (NO) production and increasing endo­
thelin-1 activity. In the diabetic retina, hyperglycemia persis­
tently activates PKC and p38a mitogen-activated protein kinase 
(MAPK) to increase expression of a previously unknown target 
of PKC signaling, Src homology-2 domain-containing phospha­
tase-1 (SHP1), a protein tyrosine phosphatase. This signaling cas­
cade leads to platelet-derived growth factor (PDGF) receptor-β 
dephos

et-derived growth factor (PDGF) receptor-β 
dephosphorylation and a reduction in downstream signaling from 
this receptor, resulting in pericyte apoptosis.79 Abnormal activa­
tion of PKC also has been implicated in the decreased glomerular 
production of NO induced by experimental diabetes80 and in the 
decreased smooth muscle cell NO production induced by hyper­
glycemia.81 PKC activation also mediates glucose-enhanced extra­
cellular matrix accumulation in rat glomerular mesangial cells.82 
Hy

umulation in rat glomerular mesangial cells.82 
Hyperglycemia increases endothelin 1–stimulated MAPK activ­
ity in glomerular mesangial cells by activating PKC isoforms.83 
The increased endothelial cell permeability induced by high glu­
cose concentrations in cultured cells is mediated by activation of 
PKCα.84 Activation of PKC by elevated glucose levels also induces 
expression of the permeability-enhancing vascular endothelial 
growth factor (VEGF) in vascular smooth muscle cells.85 In addi­

(VEGF) in vascular smooth muscle cells.85 In addi­
tion to affecting hyperglycemia-induced abnormalities of blood 
flow and permeability, activation of PKC contributes to increased 
matrix protein accumulation, inducing expression of transforming 
growth factor beta-1 (TGFβ1), fibronectin, and α1 type IV col­
lagen in glomeruli of diabetic rats.80 Many cellular abnormalities 
involved in diabetic cardiovascular disease have also been linked to 
PKC activation. These include endothelial dysfuncti

KC activation. These include endothelial dysfunction, increased 
vascular permeability, impaired angiogenesis, and increased apop­
tosis. Molecular mechanisms affected by diabetes-induced PKC 
  DAG
  Intracellular glucose
  eNOS
  ET1
  VEGF
  TGFβ
  Collagen
  Fibronectin
  PAI1
  NFκB
  NAD(P)H
oxidases
Blood-flow
Vascular
permeability
Capillary
occlusion
Vascular
occlusion
Proinflammatory
gene expression
Multiple
effects
  Fibrinolysis
  ROS
NFAT
 PKC
(α,β,δ, and θ

fects
  ROS
NFAT
 PKC
(α,β,δ, and θ isoforms)
T-cell
• Fig. 37.8  Potential consequences of hyperglycemia-induced protein kinase C (PKC) activation. Hyper­
glycemia increases diacylglycerol (DAG) content, which activates several PKC isoforms. Activated PKC 
has a number of pathogenic consequences. eNOS, Endothelial nitric oxide synthase; ET1, endothelin 1; 
NAD(P)H, nicotinamide adenine dinucleotide phosphate; NFAT, nuclear factor of activated T cells; NFκB, 
nuclear fa

ear factor of activated T cells; NFκB, 
nuclear factor-κB; PAI, plasminogen activator inhibitor; ROS, reactive oxygen species; TGF, transforming 
growth factor; VEGF, vascular endothelial growth factor. (Adapted from Koya D, Jirousek MR, Lin YW, 
et al. Characterization of protein kinase C beta isoform activation on the gene expression of transforming 
growth factor-beta, extracellular matrix components, and prostanoids in the glomeruli of diabetic rats. J 
Clin Invest. 1997;100:115–126.)
1445
C

ic rats. J 
1445
activation include alterations in functionally significant enzymatic 
activities, such as mitogen-activated protein kinase, cytosolic 
phospholipase A2, and Na+-K+-ATPase, and alterations in several 
transcription factors.86
Hyperglycemia-induced activation of PKCβ promotes vascular 
inflammation and acceleration of atherosclerosis in diabetic ApoE 
null mice by augmenting expression of inflammatory m

ll mice by augmenting expression of inflammatory mediators. 
In addition, it increases macrophage expression of cluster of dif­
ferentiation 11c (CD11c), chemokines (C-C motif ligand 2), and 
interleukin-1β, via increased extracellular signal-regulated kinase 
1 and 2 (ERK1 and ERK2), and c-Jun-N terminus kinase (JNK)–
mitogen-activated kinase.87 In this same diabetic model, PKCβ 
activation increased transcription of the proinflammatory cyto­
kine IL18 and inhibited transcription of IL18-bindin

ne IL18 and inhibited transcription of IL18-binding protein 
in the aorta. Diabetic mice showed increased plaque formation, 
cholesteryl ester content, and macrophage infiltration. Treatment 
with a PKCβ inhibitor prevented these.88 PKCβ2 in endothe­
lial cells from transgenic ApoE null mice overexpressing PKCβ 
decreased insulin- stimulated Akt/eNOS (endothelial nitric oxide 
synthase) activation and increased basal and angiotensin-induced 
expression of the vasoconstrictor endothelin-1. These 

ession of the vasoconstrictor endothelin-1. These dual effects 
increased endothelial dysfunction and accelerated atherosclerosis 
in this model compared with ApoE−/− mice.89
PKC activity has been linked to myocardial dysfunction, caus­
ing cardiomyopathy and cardiac failure. Selective overexpression 
of PKCβ2 in the myocardium of diabetic mice increased expres­
sion of connective tissue growth factor and transforming growth 
factor-β1, cardiomyopathy, and cardiac fibrosis.90 More recently, 
act

pathy, and cardiac fibrosis.90 More recently, 
activation of PKCα/β in diabetic hearts has been shown to medi­
ate reactivation of fetal splicing programs by phosphorylation 
and upregulation of the RNA-binding proteins CUGBP elav-like 
family member 1 (CELF1) and RNA binding fox-1 homolog 2 
(Rbfox2).91 Chronic activation of PKC isozymes α, β, and δ pro­
motes diastolic and systolic dysfunction, fibrosis, cardiomyocyte 
hypertrophy, and apoptosis92 Another PKC isoform, PKCθ, plays 
crucial role

s92 Another PKC isoform, PKCθ, plays 
crucial roles in the proliferation, differentiation, and activation 
of mature T cells via activation of several transcription factors 
in the nuclei of T cells, including nuclear factor of activated T 
cells (NFAT), c-Jun, c-Fos, and AP1. Diabetes-induced cardiac 
interstitial fibrosis, reduced contractility, reduced expression of the 
tight junction maintaining protein ZO1, and T-cell infiltration 
were all improved by treatment with an isoform-specific PK

 improved by treatment with an isoform-specific PKCθ 
inhibitor.93 
Increased Protein Modification by O-GlcNAc
The hexosamine pathway causes reversible post-translational 
modification of intracellular protein serine and threonine resi­
dues by N-acetylglucosamine. O-GlcNAc modifies proteins that 
regulate gene expression, translation, protein degradation, signal 
transduction, protein localization, epigenetics, and mitochon­
drial bioenergetics.94 In cells damaged by hyperglycemia, excess 
intr

94 In cells damaged by hyperglycemia, excess 
intracellular glucose provides increased fructose-6-phosphate, 
which is converted to glucosamine-6-phosphate by the rate-lim­
iting enzyme glutamine:fructose-6-phosphate amidotransferase. 
Glucosamine-6-phosphate is further converted to N-acetyl­
glucosamine-6-phosphate (GlcNAc-6-P) and finally to UDP-
N-acetylglucosamine (UDP-GlcNAc). The enzyme O-GlcNAc 
transferase (OGT) uses UDP-GlcNAc to transfer N-acetylglu­
cosamine to a variety of proteins, 

r N-acetylglu­
cosamine to a variety of proteins, resulting in increased pro­
tein modification by N-acetylglucosamine. Another enzyme, 
N-acetylglucosaminidase (O-GlcNAcase [OGA]), removes this 
protein modification (Fig. 37.9). Alternative splicing of the 
genes encoding the O-linked GlcNAc cycling enzymes OGT and 
OGA yields isoforms targeted to discrete sites in the nucleus, 
OH
OGA
OGT
Protein
O – GlcNAc
Protein
Glycolytic Pathway
Glucose
Glucose
Glc-6-P
Gln
Glu
Fruc-6-P
Glucosamine-6-P
UDP

ucose
Glc-6-P
Gln
Glu
Fruc-6-P
UDP-GlcNAc
GFAT
• Fig. 37.9  Schematic representation of the hexosamine biosynthetic pathway (HBP). The glycolytic inter­
mediate fructose 6-phosphate (Fruc-6-P) is converted to glucosamine 6-phosphate (GlcN-6-P) by the 
enzyme glutamine:fructose-6-phosphate amidotransferase (GFAT). Increased donation of N-acetylglucos­
amine moieties from UDP-GlcNAc to serine and threonine residues of proteins by O-GlcNAc transferase 
(OGT) and their removal by O-G

GlcNAc transferase 
(OGT) and their removal by O-GlcNAcase (OGA) affect proteins that regulate gene expression, translation, 
protein degradation, signal transduction, protein localization, epigenetics, and mitochondrial bioenerget­
ics. Glc, Glucose, GlcNAc, N-acetylglucosamine; OGT, O-linked N-acetylglucosamine (GlcNAc) transfer­
ase; UDP, uridine diphosphate. (Modified from Slawson C, Copeland RJ, Hart GW. O-GlcNAc signaling: a 
metabolic link between diabetes and cancer? Trends Biochem Sci. 

 between diabetes and cancer? Trends Biochem Sci. 2010;35:547–555.)
1446
cytoplasm, and mitochondria. O-GlcNAc serves as a nutrient/
stress sensor regulating cellular homeostasis by altering signal­
ing, transcription, metabolism, organelle biogenesis, cytoskeletal 
dynamics, and apoptosis.95,96 Aberrant O-GlcNAc processing 
reduced mitochondrial protein expression and respiration, mito­
chondrial proteins involved in the respiratory c

­
chondrial proteins involved in the respiratory chain and the tri­
carboxylic acid (TCA) cycle were decreased, and mitochondrial 
morphology was altered.97
In mouse coronary endothelial cells isolated from diabetic 
mice, O-GlcNAcase protein expression was significantly decreased 
compared with coronary endothelial cells from control mice. 
In contrast, OGT protein expression was markedly increased.98 
The resultant increased protein modification by O-GlcNAc was 
responsible for decreased endot

n by O-GlcNAc was 
responsible for decreased endothelium-dependent relaxation of 
the coronary arteries and reduced capillary density in the left 
ventricle. Both of these defects were restored by overexpression 
of O-GlcNAcase. Decreased endothelium-dependent relaxation 
of the coronary arteries and reduced capillary density both reflect 
inhibition of eNOS, which is required for endothelium-depen­
dent arterial relaxation and for mobilization of stem and progeni­
tor cells from the bone marrow

f stem and progeni­
tor cells from the bone marrow compartment.99 In human arterial 
endothelial cells, activation of eNOS by phosphorylation at serine 
1177 is inhibited directly by hyperglycemia-induced O-GlcNAc­
ylation at this site.100 eNOS activity is also affected by several 
other post-translational modifications, but the effect of diabetes 
on these has not yet been determined.101 Carotid plaques from 
diabetic patients have a marked increase of O-GlcNAcylation in 
both cytoplasm and nuc

ease of O-GlcNAcylation in 
both cytoplasm and nuclear compartments of endothelial cells 
compared with nondiabetic subjects.102 Increased O-GlcNAcyl­
ation may also contribute to diabetes-accelerated atherosclerosis 
by increasing ubiquitination and proteasomal degradation of the 
anti-inflammatory NFκB inhibitory protein A20 in coronary 
endothelial and smooth muscle cells.103
Chronically elevated O-GlcNAc levels also adversely affect 
myocardial function. Cardiac effects of increased O-GlcNAc

al function. Cardiac effects of increased O-GlcNAcyla­
tion include decreased mitochondrial function, decreased autoph­
agic signaling, and decreased contractile function.104 Ventricular 
contraction and relaxation are controlled mainly by release and 
uptake of Ca2+ by the SERCA2 pump. In hypertrophied and fail­
ing myocardium, SERCA2 protein level and its Ca2+ uptake func­
tion are depressed. Overexpression of OGT significantly reduced 
transcription of SERCA2, causing decreased calcium reupta

iption of SERCA2, causing decreased calcium reuptake 
and impaired diastolic relaxation.105 High glucose also increased 
O-GlcNAc modification of the calcium2+/calmodulin-dependent 
protein kinase IIδ (CaMKIIδ), an enzyme critical for Ca2+ homeo­
stasis and reuptake in cardiomyocytes. O-GlcNAc-modified CaM­
KII at serine 279 is increased in the heart of diabetic humans and 
rats,106 causing autonomous activation of CaMKII. Thus CaM­
KII remains activated even after intracellular Ca2+ declines. T

ctivated even after intracellular Ca2+ declines. This 
contributes to decreased cardiac contractility and potentially fatal 
arrhythmias, such as premature ventricular complexes and delayed 
afterdepolarizations (Fig. 37.10). Delayed afterdepolarizations 
are associated with the initiation of long QT-interval arrhyth­
mias such as torsade de pointes. Overexpression of GlcNAcase or 
Glc-6-P
Frc-6-P
GlcN-6-P
Glucose
SERCA2A transcription
factors
O-GlcNAc
O-GlcNAc
RyR
Ca2+

factors
O-GlcNAc
O-GlcNAc
RyR
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
Ca2+
CaMKII
O-GlcNAc
CaMKII
SERCA2A
Delayed afterdepolarizations
OGT
PLB
• Fig. 37.10  Hyperglycemia-induced myocardial protein modification by β-linked N-acetylglucosamine 
(O-GlcNAc) causes increased intracellular Ca2+ and delayed afterpolarizations. Increased O-GlcNAc modifi­
cation of calcium/calmodulin-dependent protein kinase IIδ (CaMKII) causes autonomous CaMKII activation. 
CaMKII increases intracel

mous CaMKII activation. 
CaMKII increases intracellular Ca2+ by phosphorylating ryanodine receptor 2 (RyR). O-linked N-acetylglu­
cosamine (GlcNAc) transferase (OGT) also modifies transcription complex factors regulating expression 
of sarcoplasmic reticulum Ca2+-ATPase (SERCA2a), reducing SERCA2A expression and contributing to 
increased intracellular Ca2+. Increased O-GlcNAc modification of these proteins causes delayed afterde­
polarizations in cardiomyocytes. PLB, Phospholamban. (Redrawn fro

n cardiomyocytes. PLB, Phospholamban. (Redrawn from Shah M, Brownlee M. Molecular 
and cellular mechanisms of cardiovascular disorders in diabetes. Circ Res. 2016;118:1808–1829.)
1447
inhibition of GlcNAc modification increased expression of SER­
CA2a, ablated sarcoplasmic reticulum Ca2+ leak, improved car­
diac contractility, and reduced arrhythmic events. Increased levels 
of ROS also cause autonomous activation of CaMKII by oxida­
tion of adjacen

ous activation of CaMKII by oxida­
tion of adjacent methionine residues in its regulatory domain.107 
Activation of this mitochondrial ROS–oxidized CaMKII pathway 
increased mortality after myocardial infarction in diabetic mouse 
models.108 Mitochondrial OGT is increased in diabetic cardiac 
mitochondria, whereas O-GlcNAcase (OGA) is reduced, causing 
increased O-GlcNAcylation of cardiac mitochondrial proteins. 
Inhibition of OGA and the resulting increased mitochondrial 
protein modification b

ng increased mitochondrial 
protein modification by O-GlcNAc increases oxygen consump­
tion and reduces reserve capacity.109 Reduced bioenergetic reserve 
capacity makes cells more sensitive to stress and cell death. 
Increased Soluble Epoxide Hydrolase
Soluble epoxide hydrolase (sEH) is the dominant member of 
the epoxide hydrolase family in humans. It binds to specific 
epoxides such as epoxyeicosatrienoic acids (EETs) and rap­
idly converts them to less active or inactive dihydrodiols, the 
d

em to less active or inactive dihydrodiols, the 
dihydroxy-eicosatrienoic acids (diHETrEs). EETs are signaling 
molecules formed from arachidonic acid by cytochrome P450 
enzymes such as CYP2J2. In cell and animal models EETs have 
major anti-inflammatory activity.110 They reduce activation of 
NFκB, causing transcriptional downregulation of the proinflam­
matory enzymes inducible nitric oxide synthase (iNOS), lipoxy­
genase 5 (LOX5), cyclooxygenase 2 (COX2), and of several 
pro-inflammatory cyt

ase 2 (COX2), and of several 
pro-inflammatory cytokines (Fig. 37.11). EETs also prevent the 
signaling cascade activated by the three major unfolded protein 
response/ER stress pathway sensors: inositol requiring protein 
1α (IRE1α), protein kinase RNA-like endoplasmic reticulum 
kinase (PERK), and activating transcription factor 6 (ATF6).
Expression of sEH was increased in diabetic mouse retinas and 
in retinas and vitreous humor of patients with diabetes.111 As a 
consequence, levels of the s

th diabetes.111 As a 
consequence, levels of the sEH diol product 19,20-dihydroxydo­
cosapentaenoic acid (19,20-DHDP) were increased. Nonprolif­
erative diabetic retinopathy is characterized by pericyte loss and 
endothelial cell dysfunction. 19,20-DHDP prevented the associa­
tion of presenilin 1 with N-cadherin and VE-cadherin in retinal 
vessels, compromising pericyte-endothelial cell interactions and 
interendothelial cell junctions.111 Inhibition of sEH prevented 
the early nonproliferative 

tion of sEH prevented 
the early nonproliferative stage of diabetic retinopathy in animal 
models. Similarly, sEH in glomerular podocytes is a significant 
contributor to hyperglycemia-induced renal injury. Podocyte 
sEH deficiency was associated with attenuated hyperglycemia-
induced renal endoplasmic reticulum stress, inflammation and 
fibrosis, and enhanced autophagy. These effects were recapitulated 
in immortalized murine podocytes treated with a selective sEH 
pharmacologic inhibitor.112 E

ith a selective sEH 
pharmacologic inhibitor.112 EETs and other epoxides also reduce 
pain caused by diabetic peripheral neuropathy.113
In the arterial wall, anti-inﬂammatory actions of EETs include 
attenuation of cytokine-induced endothelial activation and leu­
kocyte adhesion, prevention of endothelium-dependent vascu­
lar remodeling, and improvement of vascular inﬂammation and 
endothelial dysfunction. EETs also inhibit platelet aggregation 
and promote fibrinolysis, and may inhibit vascular

and promote fibrinolysis, and may inhibit vascular smooth muscle 
cell proliferation. In human studies, two single nucleotide poly­
morphisms in the sEH coding region of the EPHX2 gene are asso­
ciated with increased risk of developing coronary artery disease in 
different populations. Caucasians homozygous for the nonsynony­
mous K55R allele, with higher apparent soluble epoxide hydrolase 
activity in vivo, have a 3.5-fold increased risk of developing coro­
nary heart disease.114 Low levels of 

loping coro­
nary heart disease.114 Low levels of EET and high levels of soluble 
epoxide hydrolase are also associated with cardiac hypertrophy. 
PLA2s
ATP CoA
PLA2s
ATP CoA
Free fatty acid pool
LA, ARA, EPA, DHA…
Dihydroxy-fatty acids
Prostacyclins
Thromboxanes
Leukotrienes
Lipoxins
HETEs
20-HETE
DiHOMEs, DHETs,
DiHETEs, DiHDPEs
Anti-inflammatory
EpOMEs, EETs,
EpETEs, EpDPEs…
COX
LOX
CYP450s
sEH
Epoxy-fatty acids
HO OH
OH
O
O
OH
O
O
OH
O
NFκB action
UPR/ER stress
• 

OH
OH
O
O
OH
O
O
OH
O
• Fig. 37.11  Overview of the three proinflammatory branches of the arachidonic acid cascade generated 
by cyclooxygenase (COX), lipoxygenase (LOX), and cytochrome P450 (CYP450s). Opposing these are the 
anti-inflammatory epoxy fatty acids generated by P450 enzymes such as CYP2J2. These anti-inﬂammatory 
epoxides are reduced to their corresponding 1,2-diols by the enzyme soluble epoxide hydrolase. ARA, 
Arachidonic acid; DHA, docosahexaenoic acid; 

RA, 
Arachidonic acid; DHA, docosahexaenoic acid; EETs, epoxyeicosatrienoic acids; EPA, eicosapentaenoic 
acid; EpDPEs, DHA epoxygenase metabolites; EpETEs, epoxyeicosatetraenoic acids; EpOMEs, cis epoxy 
fatty acids; ER, endoplasmic reticulum; HETE, hydroxyeicosatetraenoic acid; LA, linoleic acid; NFκB, nuclear 
factor kappa-light-chain-enhancer of activated B cells;  PLA2, phospholipase A2; sEH, soluble epoxide 
hydrolase, UPR, unfolded protein response. (Modified from Morisseau C, Hammock BD.

 response. (Modified from Morisseau C, Hammock BD. Impact of soluble 
epoxide hydrolase and epoxyeicosanoids on human health. Annu Rev Pharmacol Toxicol. 2013;53:37–58.)
1448
Increasing EETs by inhibition of soluble epoxide hydrolase lowers 
fetal gene markers of cardiac hypertrophy, reduces heart size, and 
decreases cardiac ﬁbrosis in animal models.115–118 EETs appear to 
act in part by reducing inﬂammation in cardiomyocytes.
Upregul

by reducing inﬂammation in cardiomyocytes.
Upregulation of sEH decreases levels of the arachidonic 
acid-derived inflammation stop signal lipoxin A4, while inhibi­
tion of sEH increased lipoxin A4 by increasing its EET precur­
sor.119 Lipoxins are one of four classes of proresolving mediators 
of inflammation.120,121 Lipoxin A4 is a polymorphonuclear cell 
(PMN) stop signal that limits further recruitment. It also stim­
ulates macrophage efferocytosis, the phagocytosis of apoptotic 
PMNs and deb

tosis, the phagocytosis of apoptotic 
PMNs and debris. A failure to resolve arterial inflammation, which 
normally involves the suppression of inflammatory cell influx, 
effective clearance of apoptotic cells, and promotion of inflamma­
tory cell egress, promotes the progression of atherosclerotic lesions 
into unstable plaques, which can trigger atherothrombotic vascu­
lar events.122,123 
Reduced Pyruvate Kinase M2 Activity
The less active isoform of the glycolytic enzyme, pyruvate kinase 
M2 (

rm of the glycolytic enzyme, pyruvate kinase 
M2 (PKM2), is only active as a homotetramer. Its activity can be 
further inhibited by tyrosine kinases, acetylation, O-GlycNAcyla­
tion,124 and oxidation of cysteine residue 358.125 Although PKM2 
is expressed in normal differentiated, nonproliferating cells, the 
increased expression of this less active isoform in most tumors has 
focused attention on the potential of PKM2 to divert glycolytic 
intermediates toward biosynthetic pathways in tumor ce

rmediates toward biosynthetic pathways in tumor cells. Inde­
pendent of its enzymatic activity, PKM2 can also regulate aerobic 
glycolysis by acting as a dual-speciﬁcity protein kinase, increasing 
transcription of itself, the glucose transporter GLUT1, and lactate 
dehydrogenase A (LDLA).126–128 In postmortem kidneys from 
people with T1DM for more than 50 years, an unbiased proteomic 
analysis identified expression of glomerular PKM2 as a critical 
determinant of susceptibility or resistance t

cal 
determinant of susceptibility or resistance to diabetic nephropathy 
(Fig. 37.12).129 Individuals with no or mild nephropathy showed 
significant upregulation of mitochondrial encoded cytochrome C 
oxidase II, a subunit of mitochondrial electron transport chain 
complex IV, the two methylglyoxal detoxifying enzymes glyoxa­
lase 1 and AKRB1 (aldo/keto reductase family 1 member 1B), 
and the antioxidant proteins superoxide dismutase 1 (SOD1) and 
thioredoxin. Interestingly, PKM2 protein level

nd 
thioredoxin. Interestingly, PKM2 protein levels in the nephropa­
thy susceptible group were identical to nondiabetics, while protein 
levels in the protected group were 40-fold higher than in nondia­
betics. PK enzymatic activity in the protected group were not dif­
ferent from PK activity in nondiabetics, while PK activity in the 
nonprotected group was significantly lower. The median level of 
sulfenylated and oxidized PKM2 was fourfold higher in the non­
protected group, consistent with t

her in the non­
protected group, consistent with the observed relative differences 
in PKM2 protein level versus activity. Mitochondrial cytochrome 
C oxidase II protein levels in glomeruli from protected diabetic 
individuals were also nearly 30-fold higher than in both nonpro­
tected individuals and nondiabetic controls, and protein levels of 
the methylglyoxal degrading enzymes glyoxalase 1 and AKR1B1 
were 10-fold higher. 
Different Hyperglycemia-Induced Pathogenic Mecha­
nisms Reflect a Sin

emia-Induced Pathogenic Mecha­
nisms Reflect a Single Upstream Process: Mitochondrial 
Overproduction of ROS
A single upstream hyperglycemia–induced process—mitochon­
drial overproduction of superoxide—activates all the pathogenic 
mechanisms described in the previous sections.130,131 Enhanced 
intracellular glucose transport and oxidation leads to mitochondrial 
overproduction of superoxide.43,131 This can, in turn, activate other 
superoxide production pathways that may amplify the original da

oduction pathways that may amplify the original dam­
aging effect of hyperglycemia. Examples of amplification mecha­
nisms include increased mitochondrial fission mediated by the 
rho-associated protein kinase 1 (ROCK1),132,133 and ROS-medi­
ated uncoupling of eNOS dimers to superoxide-producing eNOS 
monomers in endothelial cells.134 Although NADPH oxidase 4 
(Nox4) has been hypothesized to be a direct source of increased 
ROS in the diabetic kidney,135,136 it appears to act indirectly by 
incr

dney,135,136 it appears to act indirectly by 
increasing mitochondrial ROS production. Nox4 is constitutively 
active and associates with mitochondria, inhibiting mitochondrial 
biogenesis and function.137 The resultant decrease in maximal res­
piration suggests oxidative damage to enzymes of the tricarboxylic 
acid (TCA) cycle, the electron transport chain (ETC), or inner 
mitochondrial membrane lipids has occurred.138,139 Knockout of 
Nox4 increases mitochondrial biogenesis and maximal respira

eases mitochondrial biogenesis and maximal respira­
tory capacity dramatically, which would prevent substrate-driven 
increased mitochondrial ROS production.137 Mitochondrial ROS 
cause activation of other NOX isoforms.140
The initiating role of mitochondrial ROS is suggested by the 
observation that cells lacking mitochondrial electron transport 
chain function (ρ0 cells)43 fail to increase ROS production in 
response to high glucose.
Dugan and associates141 have suggested diabetic nephropa­
th

associates141 have suggested diabetic nephropa­
thy is caused by reduced, rather than increased, mitochondrial 
ROS production, based on the observation that AMPK activity 
is decreased in the diabetic kidney. Protein levels of the master 
regulator of mitochondrial biogenesis and function, PGC1α, and 
mitochondrial density were also decreased. These authors pro­
posed a model in which these observations reflect a feed-forward 
cycle initiated and maintained by decreased mitochondrial ROS. 
Howe

d maintained by decreased mitochondrial ROS. 
However, in human endothelial cells, silencing of AMPK causes 
increased, not decreased, ROS.142 An alternate model of a feed-
forward cycle involving decreased AMPK activity and decreased 
mitochondrial biogenesis caused by increased mitochondrial 
ROS is more consistent with these and other observations. In this 
model, increased mitochondrial superoxide causes the release of 
Fe++ from ferritin and iron sulfur cluster–containing proteins. 
Interac

 iron sulfur cluster–containing proteins. 
Interaction of this released free iron in the nucleus with mito­
chondrial superoxide–derived hydrogen peroxide forms hydroxyl 
radicals, the only ROS species capable of cleaving bonds in mac­
romolecules (Fig. 37.13).143 This results in ROS-mediated DNA 
double-strand breaks in the nucleus. DNA double-strand breaks 
activate DNA repair mechanisms, including the enzyme poly-
ADP-ribose polymerase 1 (PARP1). Activation of PARP1 inhibits 
the key glycolyt

1). Activation of PARP1 inhibits 
the key glycolytic enzyme glyceraldehyde 3-phosphate dehydroge­
nase (GAPDH) by poly-ADP-ribosylation, and depletes intracel­
lular NAD+ by degrading it to synthesize ADP-ribose. Inhibition 
of GAPDH activity causes upstream accumulation of early gly­
colytic intermediates, which are diverted into pathogenic signal­
ing pathways.43 Diversion of glucose increases polyol pathway 
flux, whereas diversion of fructose-6-phosphate increases hexos­
amine pathway activi

-6-phosphate increases hexos­
amine pathway activity. Diversion of glyceraldehyde-3-phosphate 
to α-glycerol phosphate activates PKC, and reduced activity of 
GAPDH diverts accumulated triose phosphates toward methylg­
lyoxal formation. Methylglyoxal increases expression of the recep­
tor for advance glycation end products (RAGE) and its activating 
ligand S100A8/9. Hyperglycemia-induced ROS reduce intra­
cellular NAD+ content by 50%.144 Reduced content of NAD+ 
inhibits the activity of the NAD+

content of NAD+ 
inhibits the activity of the NAD+-dependent protein deacetylase 
sirtuin 1 (SIRT1), which normally deacetylates and activates 
both the master regulator of mitochondrial biogenesis, PGC1α, 
1449
and of liver kinase B1 (LKB1), the kinase that activates AMPK. 
Thus decreased SIRT1 activity would decrease activity of LKB1, 
PGC1α, and AMPK, as observed by Nishikawa and cowork­
ers, but causing increased, not decreased, mitochondrial RO

causing increased, not decreased, mitochondrial ROS 
production.145
SIRT1 also deacetylates and thereby inactivates the p65 sub­
unit of NFκB, and skews macrophages to their reparative polar­
ity.146 Consistent with this model, diabetic db/db mice with a 
conditional deletion of SIRT1 in podocytes developed more pro­
teinuria and kidney injury compared with db/db control mice,147 
TCA cycle
Fructose
Fructose-6-P
DHAP
GLO1 ↑ 2.0
HAGH ↑ 2.2
Lactate
LDHB ↑ 2.5
5-lactoylglutathione
ALD

LDHB ↑ 2.5
ALDO A ↑ 2.5
ALDO B ↑ 3.4
Fructose-1, 6-BisP
FBP
HK
PFK
Glyceraldehyde-3-P
TPI1 ↑ 2.3
GAPDH ↑ 2.5
PGK
1-3-bisphosphoglycerate
3-phosphoglycerate
PGM1 ↑ 2.2
ENO1 ↑ 2.8
PKM2 ↑ 2.7
2-phosphoglycerate
Phosphoenolpyruvate
Pyruvate
Nucleus
PDH
NDUFB9 (NADH: ubiquinone oxidoreductase: complex I) ↑ 1.6
MT-CO2 (Cytochrome c oxidase: complex IV) ↑ 2.1
HK
AR ↑ 1.8
Glucose
Glucose-6-P
G6PC
Sorbitol
SORD ↑ 3.2
Superoxide dismutase (SOD)
Thioredoxin (T

RD ↑ 3.2
Thioredoxin (TXN)
Glyoxalase 1 (GLO1)
Aldose reductase (AR)
• Fig. 37.12  Schema illustrating the significant alterations of glucose metabolism and glycolysis path­
way proteins from individuals with T1DM 50 years or more protected versus nonprotected from diabetic 
nephropathy. aldo A/B, aldolase A and B; AR, aldose reductase; DAG, diglyceride; DHAP, dihydroxyac­
etone phosphate; ENO1, enolase 1; FBP, fructose-1,6-diphosphatase; G6PC, glucose-6-phosphatase, 

-1,6-diphosphatase; G6PC, glucose-6-phosphatase, 
catalytic subunit; GAPDH, glyceraldehyde 3-phosphate dehydrogenase; GLO1, glyoxalase 1; GPI, glu­
cose phosphate isomerase; HAGH, hydroxyacyl glutathione hydrolase; HK, hexokinase; LDHB, lactate 
dehydrogenase; PDH, pyruvate dehydrogenase; PFK, phosphofructokinase; PGK, phosphoglycerate 
kinase; PGM1, phosphoglucomutase-1; PHGDH, 3-phosphoglycerate dehydrogenase; PKM, pyruvate 
kinase isoenzyme type M2; SORD, sorbitol dehydrogenase; TPI1, triosep

pe M2; SORD, sorbitol dehydrogenase; TPI1, triosephosphate isomerase 1. (Modified 
from Qi W, Keenan HA, Li Q, et al. Pyruvate kinase M2 activation may protect against the progression of 
diabetic glomerular pathology and mitochondrial dysfunction. Nat Med. 2017;23:753–765.)
1450
and PGC1α knockout diabetic mice developed a more severe neu­
ropathy, with mitochondrial degeneration and an increase in oxi­
dative modification of intracel

n increase in oxi­
dative modification of intracellular proteins.148 Recently, real-time 
in vivo studies using a mitochondrial matrix localized GFP-based 
redox biosensor in db/db mice confirmed increased mitochondrial 
ROS production in diabetic nephropathy.149
Together, these diversions and pathway activations lead to cel­
lular dysfunction, inflammation, apoptosis, and fibrosis in cells 
exposed to high intracellular glucose. The central importance of 
ROS in initiating each of these process

rtance of 
ROS in initiating each of these processes is illustrated by the fact 
that each can be prevented when hyperglycemia-mediated ROS 
generation is curtailed by transgenic expression of the enzymes 
superoxide dismutase or catalase.
Intracellular formation of ROS also upregulates soluble epoxide 
hydrolase mRNA and protein in mouse endothelial cells.150 Essen­
tial mitochondrial genes are downregulated in glomeruli of the 
diabetic nephropathy–susceptible mouse strain DBA/2J, but not 
in 

athy–susceptible mouse strain DBA/2J, but not 
in the diabetic nephropathy–resistant strain C57BL/6J. Diabetic 
nephropathy–susceptible DBA/2J mice had increased tissue levels 
and urinary excretion of 8-oxo-deoxyguanosine, a major product 
of DNA oxidation. In humans with diabetic kidney disease, biop­
sies showed increased mitochondrial DNA damage, and increased 
urinary 8-oxo-deoxyguanosine.151 Glomerular podocyte-specific 
knockout of PKM2 in diabetic nephropathy–susceptible mice 
had reduce

 diabetic nephropathy–susceptible mice 
had reduced expression of PGC1α, reduced mitochondrial mass, 
increased mitochondrial fission, and increased ROS. Increased ROS 
caused DNA strand breaks, increased activated PKCδ, and increased 
methylglyoxal levels. Activities of PARP and GAPDH were not 
reported. These changes were associated with accelerated podocyte 
apoptosis and glomerular pathology. Analysis of cellular respiration 
using Seahorse Extracellular Flux (XF) Analyzers showed that high 

xtracellular Flux (XF) Analyzers showed that high 
glucose decreased mitochondrial maximal respiration by about 36%, 
suggesting that oxidative damage to enzymes of the tricarboxylic 
acid cycle, the electron transport chain, or the inner mitochondrial 
membrane lipids had occurred.138,139 In cultured podocytes and in 
diabetic mice, activation of PKM2 with a small molecular weight 
NADPH
oxidases
eNOS
eNOS
eNOS
1,3-Diphosphoglycerate
DNA damage
Increased AKR1B1
substrate conversion
Increased
O-

Increased
O-GlcNAcylation
Increased AGE formation
Activation of
protein kinase C
PARP
Glucose
ROS
ROS
Sorbitol
Fructose
NADPH
NADP+
Gln
GFAT
Glu
NADH
NAD+
NADH
NAD+
DHAP
α-Glycerol-P
AGEs
RAGE and
RAGE ligands
DAG
PKC
NFκB
ROS
NAD+
NAD+
SIRT1
LKB1
AMPK
PGC1α
biogenesis
Diabetes
GAPDH
• Fig. 37.13  Four hyperglycemia-induced pathogenic mechanisms are activated b

emia-induced pathogenic mechanisms are activated by overproduction of reac­
tive oxygen species (ROS). Increased intracellular glucose flux causes mitochondrial overproduction of ROS, 
which can be further amplified by activating NADPH oxidases and uncoupling endothelial nitric oxide syn­
thase (eNOS). Stable ROS species (hydrogen peroxide, H2O2) diffuse into the nucleus, where they cause 
DNA damage, which activates poly(ADPribose) polymerase (PARP). Poly-ADPribosylation of glyceraldehyde-
3-de

ARP). Poly-ADPribosylation of glyceraldehyde-
3-dehydrogenase (GAPDH) by PARP reduces GAPDH activity, causing upstream accumulation of early gly­
colytic intermediates, which are diverted into four pathogenic signaling pathways. PARP activity depletes 
intracellular NAD+ by degrading it to synthesize ADPribose. This reduces activity of the NAD+-dependent 
deacetylase sirtuin 1 (SIRT1), which reduces activity of liver kinase B1 (LKB1), the enzyme that activates 
AMP-activated protein kinase (AMPK

that activates 
AMP-activated protein kinase (AMPK), which activates peroxisome proliferator-activated receptor gamma 
coactivator 1α (PGC1α) by phosphorylation, and of PGC1α itself (which requires deacetylation for activation). 
This results in decreased mitochondrial biogenesis. AGEs, Advanced glycation end products; AKR1B1, 
aldose reductase; DAG, diacylglycerol; DHAP, dihydroxyacetone phosphate; eNOS, endothelial nitric oxide 
synthase; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; GFAT, 

, glyceraldehyde-3-phosphate dehydrogenase; GFAT, glutamine fructose-6-phosphate 
amidotransferase; Gln, glucosamine; Glu, glucose; NAD, nicotinamide adenine dinucleotide; NADPH, nico­
tinamide adenine dinucleotide phosphate; NFκB, nuclear factor-κB; PKC, protein kinase C; RAGE, receptor 
for AGEs; UDP-GlcNAc, uridine diphosphate N-acetylglucosamine. (Modified from Shah M, Brownlee M. 
Molecular and cellular mechanisms of cardiovascular disorders in diabetes. Circ Res. 2016;118:1808–1829.)
1451

 in diabetes. Circ Res. 2016;118:1808–1829.)
1451
activator increased mitochondrial biogenesis, normalized mitochon­
drial maximal respiration, prevented high glucose–induced elevated 
ROS, and restored levels of PKCδ and methylglyoxal to low glucose 
levels. These effects are consistent with increased shunting of glucose 
to lactate, thereby reducing nonproductive increased flux through 
the mitochondrial electron transport chain.
 Insulin Resistan

ndrial electron transport chain.
 Insulin Resistance Increases Fatty Acid 
Oxidation, Causing Mitochondrial 
Insulin resistance occurs in most patients with T2DM. To isolate 
the effects of insulin resistance from those of hyperglycemia and 
diabetes, insulin resistance was evaluated in persons without diabe­
tes or impaired glucose tolerance. Those in the highest quintile of 
insulin resistance had a 2.5-fold increase in cardiovascular disease 
risk compared with those in 

rdiovascular disease 
risk compared with those in the lowest quintile, after correction for 
11 known cardiovascular risk factors, including LDL, triglycerides, 
systolic blood pressure, and smoking.17 This observation suggests 
that insulin resistance itself is a major cause of cardiovascular dis­
ease in T2DM. Adipocyte insulin resistance increases lipolysis and 
circulating free fatty acids (FFAs). Hydrolysis of triglyceride-rich 
lipoproteins by cell surface lipoprotein lipases floods the co

 by cell surface lipoprotein lipases floods the coronary 
arteries, heart, and liver with triglyceride-derived fatty acids. In the 
liver, this drives hepatic lipoprotein synthesis and secretion, and sup­
presses hepatic glucose production.152–154 Patients with T1DM are 
also insulin resistant, with insulin sensitivity reduced by about 50%. 
T1DM patients have significant insulin resistance in adipose tissue, 
as well as in liver and skeletal muscle.155,156 In T1DM patients, insu­
lin resistance

cle.155,156 In T1DM patients, insu­
lin resistance predicts the extent of coronary artery calcification and 
likely contributes to their increased risk of cardiovascular disease.157 
The role of insulin itself remains controversial. In mice fed a high-fat 
diet, genetically induced reduction of hyperinsulinemia decreased 
adipose tissue inflammation and increased insulin responsiveness.153
Insulin resistance/hyperinsulinemia increases mitochondrial 
fatty acid oxidation in arterial endothelial c

al 
fatty acid oxidation in arterial endothelial cells, macrophages, 
and cardiomyocytes, causing excessive ROS production. The 
major site of electron leakage from increased fatty acid oxida­
tion is electron transfer flavoprotein, which receives electrons 
from the FADH2 formed during the first oxidation step of 
β-oxidation158 (Fig. 37.14). These fatty acid oxidation-derived 
ROS induce expression of the noncoding RNA gadd7, which 
amplifies oxidative stress and its induction of the endoplasm

xidative stress and its induction of the endoplasmic 
reticulum stress response in a feed-forward loop.159 In two insu­
lin-resistant nondiabetic animal models, inhibition of either FFA 
release from adipocytes or FFA oxidation in arterial endothelium 
Mitochondria-associated membrane of endoplasmic reticulum
C:18 CL
Cyt C
2
DHA CL
Outer membrane
QH2
Q
QH2
2QH2
Fe-S
Fe-S
Cyt C1
Q0 site
Q1 site
O2
O•–
2
O2
O•–
2
bL
bH
Q
FADH2
ETF-QOR
FADH2
FADH2
MnSOD
Acetyl CoA
β-keto
acyl-CoA
Succinate Fumarate

nSOD
Acetyl CoA
β-keto
acyl-CoA
FFA
H2O2
H2O2
FADH2
FAD•
H+ + NADH
NAD+ + 2H+
FAD•
FAD
FADH2
FAD
2Q
QH•
Q
Inner
membrane
Inner
membrane
Complex I
Complex II
ETF-QOR
ETF
ACAD
Complex III
ALCAT1
• Fig. 37.14  Increased fatty acid oxidation, reactive oxygen species (ROS) formation, and cardiolipin (CL) 
remodeling. Insulin resistance–induced increased β oxidation of free fatty acids (FFAs) causes greater 
H2O2 production than does increased glucose oxidation because of increased 

 increased glucose oxidation because of increased electron leakage from the 
electron transfer flavoprotein (ETF) complex. These ROS activate acyl-CoA:lysocardiolipin acyltransferase 
1 (ALCAT1) transcription. ALCAT1, located in the mitochondrial-associated membrane of the endoplasmic 
reticulum, causes pathologic remodeling of cardiolipin from tetra 18:2 cardiolipin to cardiolipin with highly 
unsaturated fatty acid side chains and cardiolipin deficiency because of oxidative damage. This reduce

eficiency because of oxidative damage. This reduces 
ETC (electron transport chain) electron flux, ATP synthesis, and further increases ROS. ACAD, acyl-CoA 
dehydrogenase. (Redrawn from Shah M, Brownlee M. Molecular and cellular mechanisms of cardiovascu­
lar disorders in diabetes. Circ Res. 2016;118:1808–1829.)
1452
prevented the increased production of ROS and its damaging 
effects.160 In human arterial endothelial cells, this FFA-in

0 In human arterial endothelial cells, this FFA-induced 
increase in ROS activates the same damaging pathways seen with 
high glucose. FFA-induced overproduction of superoxide also 
inactivates two important antiatherogenic enzymes: prostacyclin 
synthase and eNOS.160
In vitro studies suggest that at the level of the vessel wall, insulin 
has both antiatherogenic and proatherogenic effects (Fig. 37.15).161 
One major antiatherogenic effect is the stimulation of endothe­
lial NO production. NO re

 stimulation of endothe­
lial NO production. NO released from endothelial cells is a potent 
inhibitor of platelet aggregation and adhesion to the vascular wall. 
Endothelial NO also controls the expression of genes involved in 
atherogenesis. It decreases expression of monocyte chemoattractant 
protein 1 (MCP1) and of surface adhesion molecules such as CD11/
CD18, P-selectin, vascular cell adhesion molecule 1 (VCAM1), and 
intercellular adhesion molecule 1 (ICAM1). Endothelial cell NO also 
red

 molecule 1 (ICAM1). Endothelial cell NO also 
reduces vascular permeability and decreases the rate of oxidation of 
LDL to its more proatherogenic form. Finally, endothelial cell NO 
inhibits proliferation of vascular smooth muscle cells (VSMCs).162 
However, in insulin resistance, overproduction of ROS leads to oxida­
tion of tetrahydrobiopterin (BH4), the essential cofactor of endothe­
lial nitric oxide synthase. Decreased BH4 uncouples oxygen reduction 
from NO synthesis, thereby converting 

 reduction 
from NO synthesis, thereby converting eNOS to a superoxide-pro­
ducing enzyme.163 Although this important antiatherogenic effect of 
insulin is blocked by insulin resistance–induced ROS, two major pro­
atherogenic effects of insulin are not. Insulin both potentiates plate­
let-derived growth factor (PDGF)–induced VSMC proliferation and 
stimulates endothelial and VSMC production of the thrombolysis-
inhibitor plasminogen activator inhibitor 1 (PAI1).164–166
Macrophages are a central 

ibitor 1 (PAI1).164–166
Macrophages are a central participant in atherogenesis. While 
most atherosclerotic lesions are stable, insulin resistance increases 
macrophage ROS, which drive chronic inflammation and accel­
erate their progression to unstable rupture-prone plaques. These 
plaques have a greater inflammatory infiltrate, a larger pro­
thrombotic necrotic core, and increased production of matrix 
metaloproteinases that weaken the fibrous cap. Insulin resis­
tance in macrophages induces m

cap. Insulin resis­
tance in macrophages induces mitochondrial ROS production 
by activating the CHOP branch of the stress unfolded protein 
response (UPR).167,168 Plaque necrosis is caused by a combina­
tion of increased macrophage apoptosis and decreased phagocytic 
clearance of the apoptotic macrophages by a process called effe­
rocytosis, which results in post–apoptotic necrosis of apoptotic 
cells and inflammation. Both of these processes are promoted by 
macrophage insulin resistance.169 A

e promoted by 
macrophage insulin resistance.169 Apoptotic cells can inappropri­
ately express the ROS-TNFα-NFκB-inducible “don’t eat me” cell 
surface signal CD47,170 and efferocyte receptors on phagocytes 
such as MER tyrosine kinase (MerTK) can be cleaved and inac­
tivated by the mitochondrial ROS-activated protease ADAM17 
(a disintegrin and metalloproteinase).171 Blocking antibodies to 
CD47 prevent progression of established lesions, protect against 
plaque rupture, and induce regression o

t against 
plaque rupture, and induce regression of the necrotic core in sev­
eral mouse models.123,170,172,173
Fatty acid oxidation appears to be the source of mitochondrial 
ROS production in macrophages, directly or indirectly via altered 
mitochondrial cardiolipin fatty acid composition.174,175 When 
fatty acids are oxidized instead of glucose, the ratio of NADH to 
FADH2 changes from 5:1 to 2:1. This causes an overreduction of 
the mitochondrial electron transport chain coenzyme Q (CoQ) 
po

rial electron transport chain coenzyme Q (CoQ) 
pool, and an increase in reverse electron transport (RET) and 
ROS production by complex I. ROS from mitochondrial electron 
transport chain complex I cause a high percentage of irreversible 
overoxidation of cysteine thiols.41,42 Reverse electron transport to 
complex I, driven by increased complex II oxidation of the TCA 
cycle intermediate succinate, drives the polarization of macro­
phages to a proinflammatory phenotype.176
In diabetic heart, i

proinflammatory phenotype.176
In diabetic heart, increased myocardial ROS production occurs 
early, before accumulation of triglycerides and subsequent syn­
thesis of C16:0 ceramides are evident.177,178 This likely reflects 
increased fatty acid oxidation and resultant oxidative damage to 
mitochondrial cardiolipin,179 the specific phospholipid of mito­
chondrial membranes. Cardiolipin is important for efficient 
electron flux, ATP synthesis, and reduced ROS formation. In 
addition, cardiolipin 

 reduced ROS formation. In 
addition, cardiolipin is involved in mitochondrial-mediated apop­
tosis, and it plays a critical role in regulating mitochondrial fission 
and fusion.180–182 In nondiabetic heart, the major species of car­
diolipin contains four linoleic acids (tetra 18:2 cardiolipin). This 
unique acyl composition is not derived from de novo synthesis of 
cardiolipin, but rather from a remodeling process that involves 
phospholipases and acyltransferase-transacylases. In diabetic myo

and acyltransferase-transacylases. In diabetic myo­
cardium from murine models with insulin resistance (ob/ob, db/
Insulin
MAPKK
PAI1
ET1
VCAM-1
VEGF
HO1
MAP-K
In insulin resistance
Proatherogenic action
increased
PI 3-Kinase
AKT
Antiatherogenic action
decreased
IR
IRS1, 2
Vasoconstriction
Migration
Monocyte Adhesion
Oxidative Stress
1176
NO
S
eNOS
• Fig. 37.15  Selective insulin resistance in vascular cells. Selective insulin 
resistance in vascular cells

s. Selective insulin 
resistance in vascular cells occurs when angiotensin II, elevated free fatty 
acids (FFAs) and glucose levels, and proinﬂammatory cytokines induced 
by diabetes and insulin resistance inhibit only the IRS/PI3K/Akt pathway. 
In contrast, insulin’s stimulation of the SOS/Grb2/MAPK pathway is unaf­
fected or even enhanced. The selective loss of insulin’s actions via the 
IRS/PI3K/Akt pathway causes the reduction of insulin’s antiatherosclerotic 
action and contributes to the a

tiatherosclerotic 
action and contributes to the acceleration of atherosclerosis and other 
cardiovascular pathologies in diabetes. AKT, protein kinase B; eNOS, 
endothelial nitric oxide synthase; ET1, endothelin-1; HO1, heme oxygen­
ase-1; ICAM, intracellular adhesion molecule; IR, insulin receptor; IRS1,2, 
insulin receptor substrate 1,2; MAP-K, mitogen-activated protein kinase; 
MAPKK, MAPK kinase; NO, nitric oxide; PAI, plasminogen activator inhibi­
tor; PI, phosphatidylinositol; TNF, tumor 

inhibi­
tor; PI, phosphatidylinositol; TNF, tumor necrosis factor; VCAM1, vascular 
cell adhesion molecule-1; VEGF, vascular endothelial growth factor; VSMC, 
vascular smooth muscle cell. (Data from King G, Brownlee M. The cellular 
and molecular mechanisms of diabetic complications. Endocrinol Metab 
Clin North Am. 1996;25:255–270; King GL, Park K, Li Q. Selective insulin 
resistance and the development of cardiovascular diseases in diabetes: 
the 2015 Edwin Bierman Award Lecture. Diabetes. 201

he 2015 Edwin Bierman Award Lecture. Diabetes. 2016;65:1462–1471.)
1453
db, and high-fat diet) and in models of severe insulin deficient 
type 1 diabetes, the fatty acyl content of the more saturated 18:2 
cardiolipin is dramatically reduced, whereas the content of longer 
chain, more unsaturated fatty acyl cardiolipin such as 20:4 is sub­
stantially increased.183 Because of this increase in highly unsatu­
rated side chains, diabetic heart cardiolip

nsatu­
rated side chains, diabetic heart cardiolipin is more vulnerable to 
oxidative damage. Cardiac overexpression of cardiolipin synthase 
increases tetra 18:2 cardiolipin in diabetic mice and prevents dia­
betes-induced changes in cardiolipin lipid remodeling. The cardio­
lipin deficiency and profound remodeling caused by diabetes and 
insulin resistance is caused by ROS-induced transcription of acyl-
CoA:lysocardiolipin acyltransferase 1 (ALCAT1) (see Fig. 37.14). 
ALCAT1 catalyzes the tran

CAT1) (see Fig. 37.14). 
ALCAT1 catalyzes the transfer of linoleoyl-CoA onto monoly­
socardiolipin or dilysocardiolipin. Overexpression of ALCAT1 
caused cardiolipin deficiency and fatty acid compositional changes 
similar to diabetes and obesity, with increased production of ROS, 
whereas ALCAT1 deficiency increased levels of tetra 18:2 cardio­
lipin in mouse heart and reduced ROS production.184
High levels of ROS are also a major proximal activator of the 
FOXO family of transcription factors.

ator of the 
FOXO family of transcription factors. Increased ROS stimulate 
FOXO translocation from the cytosol into the nucleus by increas­
ing FOXO GlcNAcylation, Jun-N-terminus kinase signaling, 
and CaMKII activation.185,186 Cysteine oxidation also increases 
FOXO transcriptional output. In the hearts of diabetic mice and 
of mice with high-fat diet–induced insulin resistance, FOXO 
proteins were persistently activated.187 This persistent activation 
was associated with downregulation of ins

ivation 
was associated with downregulation of insulin receptor substrate 
1 (IRS1), reduced activity of IRS1 and its downstream target Akt, 
and the development of cardiomyopathy. In cardiomyocyte-spe­
cific FOXO1 knockout mice fed a high-fat diet, neither insulin 
resistance nor cardiomyopathy occurred. 
Diabetes Reduces Activity of Nuclear Erythroid-
Related Factor 2, the Master Regulator of 
Antioxidant Gene Expression
The number of recognized ROS-regulating enzymes has increased 
substantia

d ROS-regulating enzymes has increased 
substantially over the past 15 years.40 Examples include superoxide 
dismutases, catalases, glutathione peroxidases, glutathione reduc­
tase, thioredoxins, thioredoxin reductases, methionine sulfoxide 
reductases, and peroxiredoxins. The activity of these enzymes is 
largely determined by ROS-induced changes in their transcrip­
tion. Increased transcription of many of these antioxidant enzymes 
is mediated by the transcription factor nuclear erythroid–rela

by the transcription factor nuclear erythroid–related 
factor 2 (Nrf2), a member of the cap ‘n’ collar subfamily of basic 
region leucine zipper transcription factors.188 By regulating oxi­
dant levels and oxidant signaling, Nrf2 participates in the control 
of the unfolded protein response, apoptosis, mitochondrial bio­
genesis, and stem cell regulation. Nrf2 also increases transcription 
of GLO1, the rate-limiting enzyme of the glyoxalase system, which 
prevents post-translational modification

m, which 
prevents post-translational modification of proteins and histones 
by methylglyoxal, the major AGE precursor.189,190 It also increases 
transcription of the rate-controlling enzyme in the nonoxidative 
branch of the pentose phosphate pathway, transketolase. Activa­
tion of transketolase inhibits three of the major hyperglycemia-
driven pathways implicated in the pathogenesis of diabetic vascular 
damage (the diacylglycerol-PKC pathway, the methylglyoxal-AGE 
formation pathway, and the 

the methylglyoxal-AGE 
formation pathway, and the hexosamine pathway) and inhibits 
hyperglycemia-induced NFκB activation.58 Preclinical studies 
using Nrf2 activators or Nrf2-deficient diabetic mice established 
that Nrf2 is a crucial endogenous modulator of ROS and protects 
from experimental diabetic nephropathy.191–193 In the kidneys of 
db/db mice, treatment with the tetracycline antibiotic minocycline 
increased Nrf2 protein levels, reduced glomerular oxidative stress 
markers, and amelior

 glomerular oxidative stress 
markers, and ameliorated diabetic nephropathy.194
Nrf2 is expressed constitutively, and its intranuclear levels are 
controlled post-translationally. In the absence of inducers, Nrf2 
associates with the redox-sensitive protein Kelch-like erythroid 
cell–derived protein with cap ‘n’ collar homology–associated pro­
tein 1 (Keap1), where it is rapidly polyubiquinated by Keap1-asso­
ciated cullin-3–RING E2 ubiquitin ligase proteins and degraded 
by proteasomes. Nrf2 bo

ase proteins and degraded 
by proteasomes. Nrf2 bound to Keap1 is released by ROS oxida­
tion of critical cysteine thiols of Keap1, or by the reaction of these 
thiols with ROS-generated electrophiles such as glycolysis-derived 
methylgloxal and unsaturated fatty acid peroxidation-derived 
4-hydroxynonenal. Phosphorylation of Nrf2 by protein kinases 
such as casein kinase 2 (CK2) may help target Nrf2 to the nucleus. 
After forming heterodimers with small Maf proteins, Nrf2 binds 
to the antioxid

th small Maf proteins, Nrf2 binds 
to the antioxidant response element (ARE) to induce transcription 
of its target genes. Export of Nrf2 from the nucleus is controlled 
by phosphorylation. Src family members such as Fyn phosphory­
late Nrf2 at Tyr568, causing export from the nucleus and degrada­
tion.195 Reduction of Nrf2 protein in the cytosolic compartment 
is mediated by β-transducin repeat–containing protein, a substrate 
adaptor for the S-phase kinase-associated protein 1−Cul1−F-box 
prote

hase kinase-associated protein 1−Cul1−F-box 
protein E3 ubiquitin ligase, which targets GSK3β-phosphorylated 
Nrf2 to the proteosome196 (Fig. 37.16).
Modification of critical Keap1 cysteine thiols by sulfora­
phane, a dietary isothiocyanate found in cruciferous vegetables, 
also releases Nrf2.199 In endothelial cells, sulforaphane prevented 
hyperglycemia-induced activation of the hexosamine and PKC 
pathways and prevented increased cellular accumulation and 
excretion of the major AGE precursor

mulation and 
excretion of the major AGE precursor methylglyoxal.197 In the 
aortae of diabetic mice, sulforaphane treatment restored aortic 
levels of Nrf2 and Nrf2-dependent antioxidant gene expression, 
preventing diabetes-induced increases in wall thickness, fibrosis, 
inflammation (tumor necrosis factor-α [TNFα] and vascular cell 
adhesion molecule-1 expression), apoptosis, and increased cell 
proliferation.198 Diabetic cardiomyopathy was also prevented in 
mouse models by sulforaphane trea

so prevented in 
mouse models by sulforaphane treatment.199 Normalized Nrf2 
activity also prevented diabetes-associated cardiac inflammation, 
fibrosis, lipid accumulation, and impaired autophagy.
In hearts from diabetic patients, Nrf2 protein is significantly 
reduced. In mice, cardiac Nrf2 protein was similarly reduced after 5 
months of diabetes mellitus.200 Hyperglycemia-induced mitochon­
drial overproduction of ROS has been shown to increase glycogen syn­
thase kinase 3 beta (GSK3β) activi

e glycogen syn­
thase kinase 3 beta (GSK3β) activity by inhibiting Akt1-dependent 
phosphorylation of GSK3β at serine 9.143 Activated GSK3β targets 
cytoplasmic Nrf2 for increased proteasomal degradation mediated by 
the β transducin repeat–containing protein/S-phase kinase-associated 
protein 1−Cul1−F-box protein E3 ubiquitin ligase.196 
Diabetes Activates the NLR Family Pyrin Domain 
Containing 3  Inflammasome
Dysregulated NLRP3 inflammasome activity underlies many 
chronic inflammatory states

tivity underlies many 
chronic inflammatory states. The NLRP3 inflammasome is 
formed by oligomerization of inactive NLRP3, associated with 
apoptosis-associated speck–like protein (ASC), and procaspase-1. 
This complex, in turn, catalyzes the conversion of procaspase-1 to 
caspase-1, which contributes to the production and secretion of 
mature proinflammatory IL1β and IL18.201 Activation requires 
two steps. The first, called priming, activates the transcription fac­
tor NFκB, which promotes tr

the transcription fac­
tor NFκB, which promotes transcription of NLRP3, pro-IL1β, 
1454
ARE
Cul1
Proteosomal
Cellular protection and cell survival
Antioxidant proteins
Antioxidant gene
expression
Nrf2
Nrf2
Nrf2
Nrf2
Other
proteins
sMaf
CBP/
p300
SH SH
E3
Ub
Cul3
Nrf2
E2
Ub
ROS
βTrCP
Keap1
P
Akt
GSK3β
P
• Fig. 37.16  Diabetes mellitus reduces nuclear erythroid–related factor 2 (Nrf2) protein in diabetic kidney 
and heart. Nr

2 (Nrf2) protein in diabetic kidney 
and heart. Nrf2, the master regulator of antioxidant gene expression, associates with the redox-sensitive 
protein Kelch-like erythroid cell–derived protein with cap ‘n’ collar homology–associated protein 1 (Keap1), 
where it is rapidly polyubiquitinated by Keap1-associated cullin-3 (Cul3)–RING E2 ubiquitin ligase proteins 
and degraded by proteasomes. Reactive oxygen species (ROS) oxidation of critical cysteine thiols of Keap1 
causes release of bound Nrf2 p

ne thiols of Keap1 
causes release of bound Nrf2 protein. Phosphorylation of Nrf2 by protein kinases such as CK2 help target 
Nrf2 to the nucleus. Nrf2 forms heterodimers with small Maf proteins, which bind to the antioxidant response 
element (ARE) in its target gene promoters. After export of Nrf2 from the nucleus, cytosolic Nrf2 is phos­
phorylated by GSK3β. This phosphorylated Nrf2 is recognized by β-transducin repeat–containing protein 
(βTrCP), a substrate adaptor for the S-phase kinase–as

CP), a substrate adaptor for the S-phase kinase–associated protein-1−Cul1−F-box protein E3 ubiquitin 
ligase, which targets Nrf2 phosphorylated by GSK3β to the proteasome. Akt, protein kinase B; CBP/p300, 
CREB-binding protein and its homologue p300; CK2, casein kinase II; E2, ubiquitin-­conjugating enzymes; 
GSK3β, glycogen synthase kinase-3β; NRF2, nuclear factor (erythroid-derived 2)–like 2; sMaf, small mus­
culoaponeurotic fibrosarcoma proteins; Ub, ubiquitin. (Redrawn from Shah M, Brownlee 

ns; Ub, ubiquitin. (Redrawn from Shah M, Brownlee M. Molecular and 
cellular mechanisms of cardiovascular disorders in diabetes. Circ Res. 2016;118:1808–1829.)
and pro-IL18.202 In mononuclear cells from diabetic patients and 
in vascular endothelial cells from diabetic rats, NFκB is chronically 
active.203 This likely results from TLR4 or TLR4/RAGE heterodi­
mer signaling stimulated by binding of ROS-induced S100A8/9 
calgranulins. A second signal activates the NLRP3 inflammasome 
by facilitatin

l activates the NLRP3 inflammasome 
by facilitating the oligomerization of inactive NLRP3, apoptosis-
associated speck–like protein, and procaspase-1. Although diverse 
molecular cues can trigger this step, several studies had suggested 
that these might all act through a mitochondrial pathway asso­
ciated with high levels of mitochondrial ROS. Blocking ROS 
generation by mitochondria has been shown to abolish NLRP3 
inflammasome activation, whereas artificial induction of mito­
chondrial ROS ca

eas artificial induction of mito­
chondrial ROS can spontaneously induce NLRP3-mediated IL1β 
secretion.204,205 Mitochondrial ROS stimulate calcium influx via 
the TRPM2 channel (transient receptor potential cation channel 
subfamily M member 2), and macrophages deficient in TRPM2 
have drastically impaired NLRP3 inflammasome activation and 
IL1β secretion.206 Synthesis and oxidation of mitochondrial DNA 
also drive this activation step. TLR4 signaling increases levels of 
the human mitochondria

naling increases levels of 
the human mitochondrial CMP kinase enzyme CMPK2, which 
increases nucleotide cytidine triphosphate, causing synthesis of 
mitochondrial DNA. Oxidized DNA fragments then exit the 
mitochondria, bind to the NLRP3 inflammasome, and activate 
it (Fig. 37.17).
Increased expression of the NLRP3 inflammasome compo­
nents Nlrp3 and apoptosis-associated speck–like protein was 
found in monocytes from new, untreated patients with T2DM. 
Along with increased expression, there wa

h T2DM. 
Along with increased expression, there was increased inflamma­
some activation. Consistent with this, drug-naive T2DM patients 
had significantly higher serum levels of the proinflammatory cyto­
kines IL1β and IL18 than did healthy individuals.207 In kidneys 
from diabetic humans and in murine models of both T2DM and 
T1DM, inflammasome activation was detected in glomerular 
endothelial cells and podocytes.208 Nlrp3 deficiency protected 
these mice from experimental diabetic nephropathy

these mice from experimental diabetic nephropathy. Analysis of 
gene expression data in the Nephromine database revealed persis­
tently elevated glomerular expression of inflammasome markers 
in patients with diabetic nephropathy and in murine models.209 
NLRP3 inflammasome activation also occurs in diabetic renal 
tubulointerstitial cells. Tubulointerstitial fibrosis is the final com­
mon pathway for loss of renal function in diabetic nephropathy, 
and tubuloepithelial to mesenchymal transdiff

hy, 
and tubuloepithelial to mesenchymal transdifferentiation is one 
1455
source of renal interstitial fibrogenic myofibroblasts. In diabetic 
kidney disease, the degree of tubulointerstitial fibrosis is probably 
a stronger predictor of GFR decline than glomerular changes.210 A 
critical mediator of renal fibrosis is the ROS-activated redox-sen­
sitive kinase apoptosis signal-regulating kinase 1 (ASK1), which 
activates p38 and JNK downstream sign

SK1), which 
activates p38 and JNK downstream signaling pathways. In renal 
biopsies from patients with diabetic kidney disease, ASK1 acti­
vation was associated with myofibroblasts, and in db/db eNOS-
/- diabetic mice, inhibition of ASK1 arrested progressive GFR 
decline, glomerulosclerosis, and proteinuria.211
In a T2DM rat model, excessive activation of NLRP3 was 
associated with cardiac inflammation, cell death, disorganized 
ultrastructure, and fibrosis. NLRP3 gene silencing ameliorated 
ca

and fibrosis. NLRP3 gene silencing ameliorated 
cardiac inflammation, apoptosis, fibrosis, and left ventricular 
cardiac dysfunction.212 In a well-established porcine model of 
diabetic atherosclerosis, which develops complex atherosclerotic 
plaques resembling those of humans, the cleavage processing of 
the sterol regulatory element-binding protein transcription fac­
tors (SREBP1 and SREBP2) and expression of their target genes, 
which increase fatty acid synthesis and cholesterol biosynthesis

 fatty acid synthesis and cholesterol biosynthesis, 
were increased in endothelial cells and infiltrating macrophages 
of both fatty streaks and advanced lesions with fibrous caps, 
necrotic cores, and cholesterol cores. SIRT1 and AMPK activity 
were also reduced.213 Increased SREBP1a in macrophages directly 
upregulates transcription of NLRP3,214 and in porcine diabetic 
atherosclerosis, increased NLRP3 was found both in macrophages 
of advanced lesions and in endothelial cells and smooth muscl

 lesions and in endothelial cells and smooth muscle 
cells. The changes found in porcine diabetic atherosclerosis were 
also present in coronary atherosclerosis samples from diabetic 
humans. 
Diabetes Activates the Transcription Factor NFAT
The transcription factor NFAT plays a role in the development of 
diabetic retinopathy, nephropathy, atherosclerosis, and cardiomy­
opathy. In resting cells, NFAT proteins are phosphorylated and 
are located in the cytoplasm. In diabetes, intracellular calci

in the cytoplasm. In diabetes, intracellular calcium is 
increased by increased ROS. Increased intracellular calcium then 
activates NFATc1-c4 by increasing NFAT dephosphorylation by 
the Ca2+/calmodulin-dependent serine/threonine phosphatase cal­
cineurin, which facilitates NFAT translocation into the nucleus. 
Once in the nucleus, NFAT interacts with coregulators to achieve 
optimal NFAT activation.215 In the nucleus, ADP-ribosylation 
mediated by PARP1 acts as a molecular switch to positively

 by PARP1 acts as a molecular switch to positively regulate 
NFAT-dependent cytokine gene transcription216 (Fig. 37.18). In 
diabetes, nuclear PARP1 is activated by intracellular ROS-induced 
DNA strand breaks (see Fig. 37.13).
In diabetic retinopathy, upregulation of proinflammatory 
cytokines such as TNFα and upregulation of retinal microvascu­
lar endothelial cell adhesion proteins are important components 
NFκB
Pro-IL1β
Pro-IL18
IL1β
IL18
ROS
Inflammasome
SIGNAL 1
SIGNAL 2
Priming
ASC
ASC
Pr

SIGNAL 1
SIGNAL 2
Priming
ASC
ASC
Procaspase-1
Active caspase-1
Inactive NLRP3
NLRP3
RAGE
S100A8/12
TLR4
Glucose/fatty acids
Ca2+ influx
Newly made oxidized
mitochondrial DNA
Ca2+
• Fig. 37.17  NOD-like receptor family, pyrin domain-containing 3 (NRLP3) inflammasome activation in dia­
betic monocytes, kidney, coronary arteries, and heart. Hyperglycemia and increased fatty acids induce 
reactive oxygen species (ROS). These increase receptor for advanced glycation end pr

se increase receptor for advanced glycation end products (RAGE) 
expression, which heterodimerizes with toll-like receptor 4 (TLR4). Signaling from this complex causes 
nuclear factor-κB (NFκB)–mediated transcription of inactive NRLP3, prointerleukin 1β (Pro-IL1β), and Pro-
IL18. Increased intracellular Ca2+ and/or newly made oxidized mitochondrial DNA triggers oligomerization 
of inactive NLRP3, associated with apoptosis-associated speck–like protein (ASC), and procaspase-1. 
This activated inf

otein (ASC), and procaspase-1. 
This activated inflammasome complex catalyzes the conversion of procaspase-1 to caspase-1, and of 
pro-IL1β and proIL-18 to mature IL1β and IL18. S100A8/12, Calgranulin A/B heterodimer ligand for RAGE. 
(Data from Shah M, Brownlee M. Molecular and cellular mechanisms of cardiovascular disorders in dia­
betes. Circ Res. 2016;118:1808–1829; Murphy MP. Newly made mitochondrial DNA drives inflammation. 
Nature. 2018;560:176–177.)
1456
SECTION VIII    Disorders of Carb

0:176–177.)
1456
of chronic inflammation.217 In human retinal microvascular 
endothelial cells, TNFα activates NFAT signaling. Activated 
NFAT specifically upregulated expression of the adhesion protein 
VCAM1, which increases adhesion of lymphocytes and macro­
phages to endothelium, the inflammatory cytokine CX3CL1 
(C-X3-C motif chemokine ligand 1), which promotes leukocyte 
adhesion, the neutrophil chemotactic cytokine CXCL6, and th

 the neutrophil chemotactic cytokine CXCL6, and the 
activated T-cell chemotactic cytokine CXC11.218 In human renal 
glomerular podocytes exposed to sera from diabetic kidney disease 
patients, increased TNFα production caused NFATc1-mediated 
repression of the cholesterol efflux regulator ATP-binding cas­
sette transporter A1 (ABCA1), and reduced activity of sterol-O-
acyltransferase 1 (SOAT1), resulting in free cholesterol-mediated 
podocyte injury.219
In arteries of diabetic mice, activated N

jury.219
In arteries of diabetic mice, activated NFATc3 induces arte­
rial smooth muscle cell expression of the proinflammatory matrix 
protein osteopontin, a cytokine that promotes atherosclerosis and 
diabetic vascular disease. NFAT inhibition effectively reduced 
arterial wall osteopontin, IL6, MCP1, intercellular adhesion mol­
ecule-1, CD68, and tissue factor expression, and lowered plasma 
IL6 in diabetic mice.220 In diabetic ApoE−/−mice, inhibition of 
NFAT-signaling completely suppressed 

hibition of 
NFAT-signaling completely suppressed a 2.2-fold increase in ath­
erosclerotic plaque area. Inhibition of NFAT also reduced lipid 
content in the plaque of diabetic mice independent of plasma 
glucose and lipid levels.221 NFATc3 activated by increased mito­
chondrial ROS production also increases arterial vasoconstrictor 
reactivity to endothelin-1.222 NFAT activation also appears to play 
a role in cardiac hypertrophy, fibrosis, and cardiomyocyte apop­
tosis.223 In the diabetic hear

cardiomyocyte apop­
tosis.223 In the diabetic heart, NFAT is activated by the calcium-
activated neutral cysteine protease calpain, which in turn activates 
calcineurin. In cardiomyocytes, increased calpain activity activates 
NFAT-dependent cardiac hypertrophy and heart failure.224 In 
two mouse models of diabetes, cardiac-specific deletion of calpain 
reduced myocardial hypertrophy and fibrosis, leading to improve­
ment of myocardial function. Calpain activation correlated with 
increased acti

Calpain activation correlated with 
increased activity of NFAT and NFκB, consistent with calpain’s 
role in activation of calcineurin and degradation of the cytosolic 
NFκB inhibitor, NFκB inhibitor α.225,226 
Diabetes Increases Neutrophil Extracellular 
Traps
In atherosclerotic plaques, neutrophils prime macrophages for 
proinflammatory responses.227 This priming is mediated by extra­
cellular webs of DNA bound to cytotoxic histones, which are 
released by activated neutrophils, called neutroph

released by activated neutrophils, called neutrophil extracellular 
traps (NETs).228 This process follows a coordinated multistep pro­
cess: histone citrullination, chromatin decondensation, migration 
of elastase and other granule enzymes into the nucleus, disinte­
gration of the nuclear membrane, and release of DNA, histones, 
and granule proteins into the extracellular space.229 The release of 
NETs primes macrophages to produce pro-IL1β, which is cleaved 
to mature proinflammatory IL1β by ca

h is cleaved 
to mature proinflammatory IL1β by caspase-1. Caspase-1, in 
turn, is secreted by macrophages in response to activation of the 
Ca2+/CaM
Calcineurin
Calpain
Glucose/free fatty acids
L-type calcium channel
Calcium
Increases osteopontin,
MCP1, ICAM1, IL6,
and atherosclerosis
hypertrophy,
fibrosis, and
heart failure
ROS
P
NFAT
NFAT
R
Nuclear transcription
PARP
RyR
Ca2+
• Fig. 37.18  Increased mitochondrial oxidation of glucose or fatty acids activates nuclear factor of acti­

e or fatty acids activates nuclear factor of acti­
vated T cells (NFAT)–mediated transcription of genes promoting diabetic retinopathy, nephropathy, ath­
erosclerosis, and cardiomyopathy. Mitochondrial overproduction of reactive oxygen species (ROS) causes 
increased intracellular Ca2+, which activates the calcium-activated neutral cysteine protease calpain. Cal­
pain then activates the Ca2+/calmodulin-dependent (CaM; Ca2+) serine/threonine phosphatase calcineu­
rin. Dephosphorylation facilitate

hatase calcineu­
rin. Dephosphorylation facilitates nuclear translocation of the transcription factor NFAT. In the nucleus, 
NFAT interacts with poly-ADPribose polymerase (PARP), which increases NFAT transcriptional activity via 
NFAT poly-ADPribosylation. ICAM1, Intercellular adhesion molecule-1; IL6, interleukin-6; MCP1, monocyte 
chemoattractant protein-1; RyR, ryanodine receptor. (Modified from Shah M, Brownlee M. Molecular and 
cellular mechanisms of cardiovascular disorders in diabetes. Ci

anisms of cardiovascular disorders in diabetes. Circ Res. 2016;118:1808–1829.)
1457
NLRP3 inflammasome.227 Both ROS and increased intracellular 
Ca2+ activate NET formation (Fig. 37.19).230 Atherosclerosis-
prone Apoe−/− mice with deletions of two serine proteases that 
localize to NETs developed dramatically smaller atherosclerotic 
lesions compared with Apoe−/− control animals, despite similar 
lipid concentrations and leukocyte counts in blood. T

id concentrations and leukocyte counts in blood. These triple 
mutant mice had no NETs, lower systemic IL1β concentration, 
and fewer lesional IL17-producing T cells.231 Quenching mito­
chondrial ROS in a mouse model of atherosclerosis (aged myeloid 
cell mCAT→LDL receptor null [Ldlr−/−] chimeric mice) decreased 
lesional NETs compared with age-matched Ldlr−/− mice, and also 
decreased total macrophage content in atherosclerotic lesions.232 
Oxidized LDL (oxLDL) and 7-keto-cholesterol, the most 

ized LDL (oxLDL) and 7-keto-cholesterol, the most abun­
dant oxysterol in human oxLDL, induced mitochondrial ROS 
production and highly proinflammatory oxidized mitochondrial 
DNA-bound NETs.233 Neutrophils from both type 1 and type 
2 diabetic humans have elevated expression of peptidyl arginine 
deiminase 4, the enzyme critical for histone citrullination-medi­
ated chromatin decondensation and NET formation.234 
Nonresolving Inflammation in Metabolic 
Syndrome, Diabetes, and Atherosclerosis
Ch

abolic 
Chronic low-grade inflammation is associated with meta­
bolic syndrome, diabetes, and atherosclerosis.235–240 In normal 
physiology, inflammation has two temporal phases: an initia­
tion phase of acute inflammation and a resolution phase. The 
initiation phase involves recruitment of neutrophils followed 
by monocytes, which differentiate into proinflammatory mac­
rophages. These cells engulf the inflammatory stimulus and 
remove damaged tissue deb

flammatory stimulus and 
remove damaged tissue debris. Resolution of inflammation is an 
active process involving a switch from synthesis of arachidonic 
acid–derived proinflammatory prostaglandins and leukotri­
enes to synthesis of at least four families of specialized prore­
solving mediators (SPMs). These SPM families are arachidonic 
acid–derived lipoxins, and omega-3 polyunsaturated fatty acid–
derived resolvins, protectins, and maresins.121 SPMs decrease 
further recruitment of PMNs, stimu

 SPMs decrease 
further recruitment of PMNs, stimulate macrophage uptake 
and clearance of apoptotic cells (efferocytosis), shift macrophage 
phenotype from proinflammatory to proresolution, and initiate 
tissue repair processes. Obesity and diabetes delay PMN apop­
tosis and impair macrophage efferocytosis. In human arteries, 
high levels of resolvin D1 and low levels of the proinflammatory 
leukotriene B4 were associated with plaques having less necrosis 
and thicker fibrous caps. In contrast,

s necrosis 
and thicker fibrous caps. In contrast, high levels of leukotriene 
B4 and low levels of resolvin D1 were associated with plaques 
having large necrotic cores and thin fibrous caps.241
Current evidence suggests diabetes causes defective SPM bio­
synthesis downstream of their fatty acid precursors.238,239 The 
resultant nonresolving inflammation causes continuous leuko­
cyte influx into atherosclerotic lesions, inflammatory macrophage 
polarization, and defective efferocytosis. These d

polarization, and defective efferocytosis. These drive the progres­
sion of atherosclerosis to the development of clinically dangerous 
lesions with large necrotic, procoagulant cores and thin fibrous 
caps. SPMs limit plaque progression by suppressing inflammation, 
enhancing efferocytosis, and promoting an increase in thickness 
Activation of neutrophil PAD4, histone
citrullination and chromatin decondensation
1
Release of nuclear
material and
enzymes (NETs)
2
Priming of macrophage
3
Secretion

2
3
Secretion of
Pro-IL1β and
cleavage by
inflammasome
caspase-1
4
IL1β
Propagation of inflammation
5
Cit
H4R3
H4cit3
ROS
PAD4
• Fig. 37.19  Diabetes increases neutrophil extracellular traps (NETs), priming macrophages for inflammation. 
Increased reactive oxygen species (ROS) increase transcription and activation of peptidylarginine deimi­
nase 4 (PAD4), the enzyme that initiates formation and release of NETs by citrullination (Cit) 

ation and release of NETs by citrullination (Cit) of histones. 
Released NETs prime macrophages to produce pro-interleukin 1β (Pro-IL1β), which is cleaved to mature 
proinflammatory IL1β by caspase-1 secreted by macrophages in response to NOD-like receptor family, 
pyrin domain-containing 3 inflammasome activation. H4cit3, Histone 4 with arginine residue 3 converted to 
citrulline; H4R3, histone 4 arginine 3; ROS, reactive oxygen species. (Modified from Shah M, Brownlee M. 
Molecular and cellula

ed from Shah M, Brownlee M. 
1458
of the fibrous cap (Fig. 37.20). Enhanced efferocytosis promotes 
macrophage production of several SPMs, including lipoxin A4, 
resolvin E1, and protectin D1. Lipoxins also recruit monocytes as 
macrophages further enhance efferocytosis in the resolving lesions. 
Efferocytosis requires specific pha

ving lesions. 
Efferocytosis requires specific phagocytotic receptors, which bind 
to apoptotic cells. Deletion of one of these receptors, Mer tyrosine 
kinase (MerTK), caused impaired efferocytosis, accumulation of 
apoptotic cells, and large necrotic cores in murine atherosclerotic 
lesions. It also caused impaired SPM production by macrophages, 
because MerTK signaling decreases proinflammatory leukotriene 
synthesis and increases synthesis of proresolving lipoxin A4.239
MerTK can be inactiva

 proresolving lipoxin A4.239
MerTK can be inactivated by the metalloproteinase ADAM17, 
which, when activated by macrophage overproduction of mito­
chondrial ROS, cleaves the receptor.171 High levels of soluble 
MerTK are found in advanced human atherosclerotic lesions, 
suggesting that this is one mechanism impairing efferocytosis. 
Increased levels of ROS cause autonomous activation of CaM­
KII by oxidation of adjacent methionine residues in its regulatory 
domain.107 In arteries, activated Ca

s regulatory 
domain.107 In arteries, activated CaMKII suppresses efferocytosis 
in macrophages and promotes atherosclerotic plaque necrosis.242 
Diabetes Alters Mitochondrial Dynamics
Mitochondria dynamics are the continuous processes of mitochon­
drial fusion, fission, biogenesis, and mitophagy, which together 
maintain optimal cellular bioenergetics and ROS homeostasis.243 
In normal physiology, fission helps facilitate mitophagy, which 
degrades and recycles damaged mitochondria and mitochon

des and recycles damaged mitochondria and mitochondrial 
fragments, although fission also increases ROS production due 
to incomplete transfer of electrons through the electron transport 
chain. Mitochondrial fission is a critical process that enables mac­
rophages to clear multiple apoptotic cells. When fission is disabled, 
phagosome formation around secondarily encountered apoptotic 
cells is impaired.244 The primary regulators of outer mitochondrial 
fusion are dynamin-related guanosine trip

ondrial 
fusion are dynamin-related guanosine triphosphate hydrolases 
(GTPases), termed mitofusin 1 (Mfn1) and mitofusin 2 (Mfn2), 
while inner mitochondrial fusion and cristae stabilization involve 
optic atrophy protein 1 (Opa1). The primary regulator of mamma­
lian mitochondrial fission is the GTPase dynamin-related protein 
1 (Drp1), which is recruited from the cytosol to the mitochondrial 
outer membrane where it binds to four Drp1 receptors: mitochon­
drial fission factor (MFF), mitochond

s: mitochon­
drial fission factor (MFF), mitochondrial dynamics protein of 49 
kDa (miD49) and 51 kDa (miD51), and mitochondrial fission 1 
protein (FIS1).245 Oligomerization of Drp1 is thought to provide 
the mechanical force to constrict mitochondrial membranes and to 
fragment the organelle. Mitochondrial fission occurs at mitochon­
dria-endoplasmic reticulum contact sites, with involvement of the 
mitochondrial phospholipid cardiolipin and calcium transfer.246
Increased mitochondrial fission

lcium transfer.246
Increased mitochondrial fission caused by increased mitochon­
drial recruitment of Drp1 is associated with key features of diabetic 
nephropathy in diabetic mouse models133 (Fig. 37.21). In diabetic 
mice lacking Drp1, decreased fission protected against progres­
sion of diabetic nephropathy.247 Increased mitochondrial fission 
also occurs in mitochondria from mouse coronary endothelial 
cells, where the level of the fusion protein Opa1 is decreased and 
the level of Drp1 is i

tein Opa1 is decreased and 
the level of Drp1 is increased.248 This response may represent an 
important proresolving response in atherosclerotic lesions, since 
mice lacking myeloid Drp1 have defective efferocytosis.235,244 
Persistent increased fission is also associated with myocardial dys­
function in humans with type 2 diabetes, due in part to decreased 
expression of the fusion protein mitofusin 1.249 Drp1 recruitment 
to mitochondria is regulated by its phosphorylation at serine 616 
by e

gulated by its phosphorylation at serine 616 
by either CaMKII or ROCK1. In the diabetic heart and kidney, 
ROS can activate both of these kinases.108,133 
Resolving inflammation
Nonresolving inflammation
Smooth muscle cells
polarization
Impaired
efferocytosis
Resolving
Increased
Impaired
Necrotic
core
Thin
fibrous cap
monocyte
influx
Platelet
aggregation
No platelet
aggre

cyte
influx
Platelet
creased
rocytosis
Smaller
necrotic
core
Thick
Decreased
monocyte
influx
A
B
Advanced
plaque
SPMs
LTs
Stable
plaque
SPMs
LTs
• Fig. 37.20  Nonresolving inflammation in atherosclerosis. Reduced specialized proresolving mediators 
(SPMs) and increased leukotrienes (LTs) promotes instability of atherosclerotic plaques. When the SPM:LT 
ratio is low (A), resolution of inflammation is impaired, leading to sust

ution of inflammation is impaired, leading to sustained inflammatory monocyte influx, 
platelet aggregation, proinflammatory macrophage polarization, impaired efferocytosis, large necrotic 
cores, and thin fibrous caps. Conversely, when the SPM:LT ratio is high (B), resolution of inflammation is 
characterized by decreased monocyte influx, proresolving macrophage polarization, enhanced efferocy­
tosis, decreased necrotic core formation, and thicker fibrous caps. (Redrawn from Kasikara C, Doran A

er fibrous caps. (Redrawn from Kasikara C, Doran A, 
Cai B, et al. The role of non-resolving inflammation in atherosclerosis. J Clin Invest. 2018;128:2713–2723.)
1459
Genetic Determinants of Susceptibility to 
Microvascular Complications
Clinicians have long observed that different patients with simi­
lar duration and degree of hyperglycemia differ markedly in their 
susceptibility to microvascular complications. Such observations 
suggested that ge

omplications. Such observations 
suggested that genetic differences exist that affect the pathways 
by which hyperglycemia damages microvascular cells. The leveling 
of risk of overt proteinuria after 30 years’ duration of T1DM at 
27% is evidence that only a subset of patients are susceptible to 
development of diabetic nephropathy.250 A role for genetic deter­
minants of susceptibility to diabetic nephropathy is more directly 
supported by familial clustering, with an estimated heritability of

lial clustering, with an estimated heritability of 
at least 40%. In two studies of families in which two or more sib­
lings had T1DM, the risk of nephropathy in a diabetic sibling was 
83% or 72% if the proband diabetic sibling had advanced diabetic 
nephropathy, but only 17% or 22% if the index patient did not 
have diabetic nephropathy (Fig. 37.22).251,252 Familial clustering 
for the risk of severe diabetic retinopathy was also reported by the 
DCCT. Likewise, familial aggregation of coronar

e 
DCCT. Likewise, familial aggregation of coronary artery calcifi­
cation occurs in families with T2DM.253 Numerous associations 
have been made between various candidate gene polymorphisms 
and the risk of diabetic complications. However, in a meta-analy­
sis of three large cohorts from the Genetics of Nephropathy—an 
International Effort (GENIE), no associations remained signifi­
cant after correction for multiple testing or application of strin­
gent significance thresholds.254
While genome-

in­
While genome-wide association studies (GWAS) performed in 
large, well-characterized study populations have identified a few 
susceptibility loci for diabetic retinopathy, nephropathy, and car­
diovascular disease,255 the cumulative effects of disease-associated 
Ca2+/CaM
Calpain
Calcium
ROS
P
DRP1
DRP1
RyR
Ca2+
ROS
DRP1
FIS1
DRP1
MFF
DRP1
MID49
DRP1
MID51
• Fig. 37.21  Diabetes induces increased mitochondrial fission in k

betes induces increased mitochondrial fission in kidney, coronary artery, and myocardium. 
Increased reactive oxygen species (ROS) from either excess glucose or fatty acids increases intracel­
lular Ca2+. Increased Ca2+ activates the calcium-activated neutral cysteine protease calpain, which acti­
vates the Ca2+/calmodulin-dependent (CaM; Ca2+) serine/threonine phosphatase calcineurin. Calcineurin 
dephosphorylates the GTPase dynamin–related protein 1 (DRP1), which is then recruited from the cyt

ein 1 (DRP1), which is then recruited from the cytosol 
to the mitochondrial outer membrane where it binds to four DRP1 receptors—mitochondrial fission fac­
tor (MFF), mitochondrial dynamics protein of 49 kDa (MID49), and 51 kDa (MID51), and mitochondrial 
fission 1 protein (FIS1). Drp1 oligomerization provides the mechanical force to constrict mitochondrial 
membranes and fragment the organelle (mitochondrial fission). Increased fission causes further increases 
in ROS production and mitochondr

urther increases 
in ROS production and mitochondrial dysfunction. (Modified from Shah M, Brownlee M. Molecular and 
Prevalence of diabetic
nephropathy (%)
Siblings of
probands with
diabetic nephropathy
probands without
80
60
40
20
0
Study 1
Study 2
• Fig. 37.22  Familial clustering of diabetic nephropathy. Prevalence of dia­
betic nephropathy in two studies of diabetic si

a­
betic nephropathy in two studies of diabetic siblings of probands with or 
without diabetic nephropathy. (Adapted from Seaquist ER, Goetz FC, Rich 
S, et al. Familial clustering of diabetic kidney disease: evidence for genetic 
susceptibility to diabetic nephropathy. N Engl J Med. 1989;320:1161–
1165; Quinn M, Angelico MC, Warram JH, et al. Familial factors determine 
the development of diabetic nephropathy in patients with IDDM. Diabeto­
logia. 1996;39:940–945.)
1460

a. 1996;39:940–945.)
1460
single nucleotide polymorphisms (SNPs) have failed to account 
for the majority of complex-trait heritability for a large number 
of diseases, including diabetes and its complications (the “missing 
heritability” problem). Ultimately, the central unanswered ques­
tion is: How is coordinated gene expression regulatory informa­
tion conveyed—the information that determines when, where, at 
what level, in what co

determines when, where, at 
what level, in what combinations, and for how long, genes are 
expressed? With protein coding sequences representing approxi­
mately 2% of the genome, research in the post-GWAS era is 
currently focused on noncoding RNAs, genetic-epigenetic inter­
actions, and long-range chromatin 3D looping architecture.256,257 
Noncoding RNAs and Diabetic Complications
Although piwi-interacting RNAs (piRNAs), endogenous small 
interfering RNAs (siRNAs), intron-derived microRNAs (miR

ering RNAs (siRNAs), intron-derived microRNAs (miR­
NAs), and a host of long noncoding RNAs all have regulatory 
roles, the best understood noncoding RNAs with respect to 
diabetic complications are the miRNAs, which regulate several 
key biologic pathways and cellular functions involved in dia­
betic complications. Most individual miRNAs target hundreds 
of specific mRNAs,258 thereby coordinately regulating complex 
gene networks. In the retina, diabetes increased levels of several 
microRNA me

 diabetes increased levels of several 
microRNA mediators of inflammation (miR146, miR155, and 
miR132),259 while decreasing levels of the anti-inflammation 
mediator miR146a. Levels of miR146a are also reduced in sci­
atic nerve of db/db mice. Treatment with a miR146a mimetic 
reduced macrophage activation and improved nerve conduction 
velocity, intraepidermal nerve fiber density, and axonal degen­
eration. miR146a was also decreased in kidneys and hearts of 
type 1 and type 2 diabetic animals

 and hearts of 
type 1 and type 2 diabetic animals.260 In diabetic mouse kid­
ney cortex, levels of the profibrotic miR192 are also increased, 
and miR192 knockdown prevented diabetes-induced protein­
uria and renal fibrosis.261 Diabetes also increased miR29c in the 
kidney, which induces cell apoptosis and increases extracellular 
matrix protein accumulation. Knockdown of miR29c prevented 
high glucose–induced cell apoptosis and significantly reduced 
albuminuria and kidney mesangial matrix acc

duced 
albuminuria and kidney mesangial matrix accumulation in db/
db mice.262
miR33 regulates cellular functions associated with cardiovas­
cular disease. It increases macrophage activation, inhibits mito­
chondrial biogenesis, and represses autophagy.263 In mice placed 
on an atherogenic diet for 16 weeks and then made diabetic by 
streptozotocin injection, anti-miR33 treatment decreased ath­
erosclerotic plaque macrophage content and inflammatory gene 
expression. The decreased macrophage con

ory gene 
expression. The decreased macrophage content in anti-miR33–
treated diabetic mice was associated with a blunting of hypergly­
cemia-induced monocytosis and reduced monocyte recruitment 
to the plaque.264 Diabetes increases circulating numbers of neu­
trophils and monocytes through an ROS-mediated mechanism 
and impairs regression of early atherosclerosis by increasing mac­
rophage entry into the lesion in mouse models.265,266
MicroRNAs may also play a role in the pathogenesis of diabet

may also play a role in the pathogenesis of diabetic 
cardiomyopathy. miR499, miR133a, and miR373 are downregu­
lated in diabetic cardiomyocytes.267 Downregulation of miR133a 
in a normal adult genetic background was sufficient to induce car­
diac hypertrophy, and its downregulation is a prerequisite for the 
development of apoptosis, fibrosis, and prolongation of the QT 
interval in animal models.268 The protective capacity of cardiac 
progenitor cells after myocardial infarction is also enhanc

r cells after myocardial infarction is also enhanced by 
miR133a.269
In patients with over 50 years of T1DM, expression levels of 
miR200 differentiated those with no complications from those 
with complications.44 Induced pluripotent stem cells (iPSCs) from 
fibroblasts of patients with no complications had low miR200 lev­
els, while iPSCs from fibroblasts of patients with complications 
(retinopathy, nephropathy, neuropathy, and cardiovascular disease) 
had high miR200 levels. miR200 was found

disease) 
had high miR200 levels. miR200 was found to inhibit DNA dam­
age checkpoint protein expression, causing accumulation of DNA 
double-strand breaks, inflammation, and apoptosis (Fig. 37.23). 
Knockdown of miR200 in fibroblasts from patients with compli­
cations rescued checkpoint protein expression and reduced DNA 
damage. In differentiated neurons derived from all clinical back­
grounds, exogenous overexpression of miR200 produced the DNA 
damage mark γH2AX phosphorylation (pH2AX). Thes

A 
damage mark γH2AX phosphorylation (pH2AX). These data are 
consistent with the model of ROS-mediated DNA damage activat­
ing multiple hyperglycemia/IR-induced pathogenic mechanisms 
discussed in previous sections. 
Molecular Basis for Metabolic Memory
Stable cellular phenotypes are maintained by a multitude of inter­
connected multilevel positive and negative feedback loops,270,271 
interacting with combinatorial extracellular signals from adja­
cent cells, from receptor ligands secreted by c

a­
cent cells, from receptor ligands secreted by cells in other tissues, 
and from changes in metabolite flux and concentration. Famil­
iar examples of stable alterations in cellular phenotype are the 
induction of pluripotency in terminally differentiated cells, and 
the differentiation of these iPS cells into a variety of cell types.272 
In preclinical models of T1DM and T2DM, diabetes irrevers­
ibly depletes two bone marrow–derived mesenchymal progenitor 
subpopulations having provasculogenic

 progenitor 
subpopulations having provasculogenic expression profiles, and 
this deficit in vascular progenitor cells is not correctable by restor­
ing glucose homeostasis in vitro or in vivo.273 These data suggest 
that diabetes causes permanent modifications in subpopulations 
of mesenchymal progenitor cells that are passed to daughter cells 
with each division.
Similarly, when fibroblasts from patients with T1DM for more 
than 50 years with complications were used to make iPS cells, 
the miR

omplications were used to make iPS cells, 
the miR200 family was upregulated in fibroblasts from patients 
with complications, in the reprogrammed iPS cells from these 
fibroblasts, and in neurons differentiated from iPS cells from the 
cohort with complications. These neurons had increased suscep­
tibility to genotoxic stressors, including exposure to high glucose 
and various sources of increased ROS. The miR200 family target 
transcripts encoding DNA damage checkpoint proteins, and there 
was

ing DNA damage checkpoint proteins, and there 
was a concomitant increase in DNA damage and a loss of DNA 
damage checkpoint proteins.44 These findings were confirmed in 
aortic tissue from patients with complications and were reversed 
in fibroblasts and iPS cells by knockdown of miR200. In fibro­
blasts from patients with complications, there was also a striking 
impairment in reprogramming efficiency, and the induced iPCs 
had impaired cellular growth and differentiation. In tissue from 
pati

r growth and differentiation. In tissue from 
patients with complications, there was increased inflammation due 
to macrophage infiltration. There was also increased apoptosis due 
to elevated levels of DNA damage, which was not compensated 
by differentiation of progenitor cells. Taken together, the obser­
vations from both these studies could reflect underlying genetic 
or epigenetic causes. However, since the DCCT study patients 
were randomly assigned to one of two different treatment groups

 assigned to one of two different treatment groups, 
treatment-related epigenetic causes are a more likely explanation 
for the long-term metabolic memory observed subsequently in the 
EDIC study.
Epigenetic causes are those that change DNA or RNA struc­
ture and function, but do not change DNA-RNA sequence. Along 
with the activities of microRNAs and other noncoding RNAs, a 
1461
Co-regulators
MDC1
DNA double strand break
Controls or Medalists
With

Without Complications
Fixing of DNA errors leading to
normal cell cycle and growth
Accumulation of DNA errors,
inflammation leading to apoptosis
Medalists
With Complications
Genotoxic stress
Low miR200
High DNA repair
machinery 
High miR200
Low DNA repair
machinery 
ATM
DNA-Pk
Chromatin
Chromatin
modifiers
MED15/17
MRE11
RAD50
pHA2X
NBS1
PARP1
POL II
Ac
Ac
Me
Me
Ub
Ub
Ub
ATR
EZH2, SUZ12,
DNMTs, HDACs, etc
MDC1
ATM
DNA-Pk
T

MDC1
ATM
DNA-Pk
Chromatin
Chromatin
modifiers
MED15/17
MRE11
RAD50
NBS1
PARP1
POL II
Ac
Ac
Me
Me
Ub
Ub
Ub
ATR
pHA2X
• Fig. 37.23  Expression levels of microRNA (miR) miR200 from patients with over 50 years of T1DM differ­
entiate those with no complications from those with complications. Induced pluripotent stem cells (iPSCs) 
from fibroblasts of patients with no complications had low miR200 levels, while iPSCs from fi

cations had low miR200 levels, while iPSCs from fibroblasts 
of patients with complications (retinopathy, nephropathy, neuropathy, and cardiovascular disease) had 
high miR200 levels. miR200 was found to inhibit DNA damage checkpoint protein expression, caus­
ing accumulation of DNA double-strand breaks, inflammation, and apoptosis. ATM, Ataxia telangiectasia 
mutated; ATR, ataxia telangiectasia and Rad3‐related protein; DNA-PK, DNA-dependent protein kinase; 
DNMTs, DNA methyltransferases; EZH2,

tein kinase; 
DNMTs, DNA methyltransferases; EZH2, enhancer of zeste homolog 2; HDACs, histone deacetylases; 
MDC1, mediator of DNA damage checkpoint 1; Me, methyl; MED 15/17, mediator of RNA polymerase II 
transcription subunits 15 and 17; MRE11, MRE11 homolog, double-strand break repair nuclease; NBS1, 
Nijmegen breakage syndrome protein 1; PARP1, poly-ADPribose polymerase 1; pH2AX, phosphorylated 
H2A histone family member X; POL II, RNA polymerase II; RAD 50, DNA repair protein RAD50; SUZ12,

erase II; RAD 50, DNA repair protein RAD50; SUZ12, 
suppressor of zeste 12 protein homolog; Ub, ubiquitin. (Modified from Bhatt S, Gupta MK, Khamaisi M, 
et al. Preserved DNA damage checkpoint pathway protects against complications in long-standing type 1 
diabetes. Cell Metab. 2015;22:239–252.)
variety of enzymes modify chromatin by adding and removing 
chemical groups to DNA (primarily cytosine methylation) and to 
the protruding N-terminal tails of nucleosome histone proteins 
(monomethylatio

ls of nucleosome histone proteins 
(monomethylation, dimethylation, and trimethylation of lysine 
and arginine residues, acetylation of lysine residues, and modifica­
tion of arginines by methylglyoxal and of serines and threonines 
by O-GlcNAc).190,274 In conjunction with remodeling complexes, 
these modifications mediate the availability or nonavailability of 
DNA sequences for transcription (Fig. 37.24). Increased ROS 
modulate several epigenetic mechanisms in different cell types and 
contex

tic mechanisms in different cell types and 
contexts.275–277
DNA methylation can repress or activate transcription, 
depending on its location near promoters or near enhancers in 
intergenic regions.275 Similarly, methylation of histone tails can 
repress or activate transcription, depending on the position of 
the modified residue in a specific histone tail, and on the degree 
of methylation. Acetylation of histone tails generally facilitates 
transcription.
In whole blood and isolated monocyte

ranscription.
In whole blood and isolated monocyte samples from DCCT/
EDIC type 1 diabetic patients at the EDIC study baseline and 
in samples from the same patients obtained during EDIC years 
16 to 17, DNA methylation profiling revealed strong connec­
tions with networks associated with diabetic complications, even 
though the post-DCCT HbA1c values for both intensive and 
standard treatment groups became identical at the end of the 
DCCT (∼8%), and remained identical for the duration of the 

, and remained identical for the duration of the 
EDIC study. The most significant persistent DNA methylation 
change was hypomethylation of thioredoxin-interacting protein 
(TXNIP).278 TXNIP binds to and inhibits thioredoxin, increas­
ing ROS production and inhibiting renal tubular autophagy.279 
The importance of this DNA methylation profiling study is that 
it shows, for the first time, that differential methylation at several 
loci persists for more than 16 to 17 years in circulating leukocy

or more than 16 to 17 years in circulating leukocytes 
from the same diabetic patient cohort. Every diabetic complica­
tion is a complex heterocellular process, however, and each dif­
ferent cell type involved has a distinct epigenome regulating gene 
expression and cell type–specific pathologies despite sharing a 
common genetic sequence.275 A similar but technically more lim­
ited study of selected histone modifications in monocytes showed 
enrichment of activating histone 3 lysine 9 acetylati

ichment of activating histone 3 lysine 9 acetylation (H3K9Ac) 
in promoters of more than 15 genes related to the NFκB inflam­
matory pathway.280
Transient hyperglycemia, at a level sufficient to increase mito­
chondrial ROS production, induces long-lasting activating epi­
genetic changes (increased monomethylation of histone 3 lysine 
4) in the proximal promoter of the NFκB subunit p65 in vitro in 
1462
human aortic endothelial cells (

nd Fat Metabolism
human aortic endothelial cells (16-hour exposure), and in vivo in 
aortic cells in nondiabetic mice (6-hour exposure). These epigen­
etic changes caused sustained increases in p65 gene expression and 
in the expression of p65-dependent proinflammatory genes. Both 
the epigenetic changes and the gene expression changes persist for 
at least 6 days of subsequent normal glycemia in cultured cells and 
for months in previously diabetic mice whose beta-cell function 
had recovered.2

tic mice whose beta-cell function 
had recovered.281,282 Hyperglycemia-induced epigenetic changes 
and increased p65 expression are prevented by normalizing mito­
chondrial superoxide production or superoxide-induced methylg­
lyoxal. Demethylation of another histone lysine residue, histone 
3 lysine 9, is also induced by hyperglycemia-induced overpro­
duction of ROS. This reduces inhibition of p65 gene expression, 
and therefore acts synergistically with the activating methylation 
of histone 3 

lly with the activating methylation 
of histone 3 lysine 4. The specific H3K4m1 enrichment is medi­
ated by the lysine methyltransferase Set7, which is recruited to the 
nucleus by high glucose–induced mechanisms.283 Genome-wide 
analysis of Set7 function in human vascular endothelial cells con­
firmed its role in regulating NFκB-dependent pathways, affecting 
a large number of genes associated with vascular function by both 
histone lysine methylation and lysine methylation on nonhistone 
subst

lation and lysine methylation on nonhistone 
substrates such as transcription factors.284,285 The demethylation 
of histone 3 lysine 9 is mediated by the histone demethylase LSD1 
(lysine-specific histone demethylase 1) (Fig. 37.25).
Another component of epigenetic regulation is the action of 
epigenetic reader proteins that bind to modified histone residues 
and facilitate the formation of transcriptional complexes. In endo­
thelial cells, proinflammatory activation of NFκB led to rapid 
recrui

flammatory activation of NFκB led to rapid 
recruitment of NFκB to newly formed superenhancers, and rapid 
large-scale redistribution of the epigenetic reader bromodomain 
and extraterminal domain 4 (abbreviated BET4 or BRD4) from 
decommissioned preexisting basal enhancers to the newly formed 
superenhancers.286 This recruitment caused increased levels of 
H3K27ac and immediate transcription of inflammatory genes. 
Blocking the association of BRD4 with acetylated lysines in LDL 
receptor–defici

D4 with acetylated lysines in LDL 
receptor–deficient hypercholesterolemic mice attenuated both 
early and late atherosclerosis development.
BRD4 and other members of the BET family are also critical 
effectors of pathologic cardiac remodeling and heart failure via 
their ability to coactivate multiple master transcription factors 
known to initiate and promote heart failure, including NFκB 
and NFAT.287 BET inhibition suppresses pathologic cardiac 
gene expression programs and arrests pathologi

ac 
gene expression programs and arrests pathologic hypertrophy 
and heart failure in vivo. BRD4 (most potently its splice isoform 
BRD4B) also functions as an endogenous inhibitor of DNA dam­
age response signaling, which enhances radiation ROS-induced 
cell death. In contrast, knockdown of this isoform caused rapid 
cell-cycle checkpoint recovery and enhanced cell survival.288 
Much has been learned since the original clinical description 
of metabolic memory, but much more research remains to

etabolic memory, but much more research remains to be 
done before the molecular basis for metabolic memory is clearly 
understood. 
Reactive oxygen species
DNA
modification
5hmC
5mC
8OG
Histone
noncoding RNAs
(miRNA, IncRNA)
ATP-dependent
chromatin remodelling
Epigenetic landscape
Gene expression (nuclear/mitochondrial)
Ac
Met
ADP
Diabetic complications
Ribo
MGO
O-GlcNAc
• Fig. 37.24  Increased metabolite-generated reactive oxygen species (ROS) modulate the epig

ed reactive oxygen species (ROS) modulate the epigenetic landscape, 
altering histone modiﬁcations, DNA modiﬁcations, expression of noncoding RNAs, and ATP-dependent chro­
matin remodeling. These changes subsequently affect gene expression patterns implicated in the pathogenesis 
of diabetic complications. DNA modiﬁcations include cytosine methylation (5mC), hydroxymethylation (5hmC), 
or 8-oxo-2’-deoxyguanosine (8OG) formation. Histone modiﬁcations include methylation (Met), acetylation 
(Ac), 

ons include methylation (Met), acetylation 
(Ac), ADP-ribosylation (ADP-Ribo), phosphorylation (P), glycation by methylglyoxal (MGO), and glycosylation by 
O-linked N-acetylglucosamine (O-GlcNAc). Noncoding RNAs include microRNAs (miRNA) and long noncoding 
RNAs (lncRNA). ATP-dependent chromatin remodeling includes moving and adding/removing nucleosomes by 
ATPase-containing complexes. (Modified from Kietzmann T, Petry A, Shvetsova A, et al. The epigenetic landscape 
related to reactive oxygen s

epigenetic landscape 
related to reactive oxygen species formation in the cardiovascular system. Br J Pharmacol. 2017;174:1533–1554.)
1463
Diabetic Retinopathy and Other Ocular 
Complications of Diabetesa
Diabetic retinopathy is a well-characterized sight-threatening 
chronic microvascular complication that eventually afflicts vir­
tually all patients with diabetes mellitus.289 Diabetic retinopa­
thy is characterized by gradually progressive alterat

 is characterized by gradually progressive alterations in the 
retinal microvasculature, leading to areas of retinal nonperfusion, 
increased vascular permeability, and pathologic intraocular pro­
liferation of retinal vessels. The complications associated with 
retinal neovascularization, termed proliferative diabetic retinopathy 
(PDR), and increased vascular permeability, termed diabetic mac­
ular edema (DME), can result in severe and permanent vision loss.
Early-stage diabetic retinopathy ev

t vision loss.
Early-stage diabetic retinopathy eventually occurs in nearly 
all patients, although early intensive treatment of hyperglycemia 
delays its onset and progression in T1DM. In selected patients 
with T2DM, systemic treatment with fenofibrate, ACE inhibi­
tors, and angiotensin II receptor blockers (ARBs) may affect the 
development and progression of diabetic retinopathy. Multiple 
phase 3 clinical trials over the past 5 years have now established 
intravitreously delivered anti-VEGF

w established 
intravitreously delivered anti-VEGF therapy as the new standard 
a Portions of this section draw on, among others, Aiello LM, Cavallerano JD, 
Aiello LP. Diagnosis, management, and treatment of nonproliferative diabetic 
retinopathy and diabetic macular edema. In: Albert DM, Jakobiec FA, eds. 
Principles and Practice of Ophthalmology. 2nd ed. Philadelphia: WB Saunders; 
2000:1900–1914; Aiello LP, Cavallerano J, Klein R. Diabetic eye disease. In 
DeGroot LJ, James JL, eds. Endocrin

e disease. In 
DeGroot LJ, James JL, eds. Endocrinology, 5th ed. Philadelphia: WB Saun­
ders; 2005:1305–1317; Aiello LP, Gardner TW, King GL, et  al. Diabetic 
retinopathy: technical review. American Diabetes Association. Diabetes Care. 
1998;21:143–156; Aiello LP, Cavallerano J. Diabetic retinopathy. In John­
stone MT, Veves A, eds. Contemporary Cardiology: Diabetes and Cardiovascular 
Disease. Totowa, NJ: Humana Press; 2001:385–398.
of care for most eyes with visual impairment from center-invo

 most eyes with visual impairment from center-involved 
DME.290,291 Currently, more than 90% of severe vision loss result­
ing from PDR can be prevented, and approximately 50% of eyes 
with visual loss from DME can resolve their retinal thickening 
and/or recover vision of 20/20 or better with appropriate medi­
cal and ophthalmologic care.292,292a Thus the primary clinical care 
emphasis for the prevention of vision loss in the diabetic patient 
is on early identification, accurate classificatio

is on early identification, accurate classification, and timely treat­
ment of retinopathy through lifelong routine ophthalmologic fol­
low-up appointments and optimized treatment of hyperglycemia, 
hypertension, and hyperlipidemia.
Epidemiology and Impact of Proliferative 
Diabetic Retinopathy and Diabetic Macular 
Edema
There is a higher risk of more frequent and severe ocular com­
plications in T1DM than T2DM.293 Approximately 25% of 
patients with T1DM have retinopathy after 5 years, and thi

 with T1DM have retinopathy after 5 years, and this 
figure increases to 60% and 80% after 10 and 15 years, respec­
tively. The most visually threatening form of retinopathy (PDR) 
is present in about 67% of T1DM patients who have had dia­
betes for 35 years.294 However, because T2DM accounts for 
90% to 95% of the diabetic population in the United States, 
type 2 disease accounts for a higher fraction of patients with 
vision loss.
In the United States, an estimated 700,000 people have PDR, 
13

 States, an estimated 700,000 people have PDR, 
130,000 of whom have high-risk PDR. Among people with 
diabetes, 500,000 have macular edema, of whom 325,000 have 
clinically significant macular edema (CSME).295–299 An estimated 
63,000 cases of PDR, 29,000 cases of high-risk PDR, 80,000 cases 
of macular edema, 56,000 cases of CSME, and 12,000 to 24,000 
new cases of legal blindness occur each year as a result of diabetic 
retinopathy.295,296,300 Blindness is estimated to be 25 times more 
commo

 Blindness is estimated to be 25 times more 
common in persons with diabetes than in those without the dis­
ease.301,302 Rates of progression to PDR or severe visual loss have 
declined over the last four decades in developed countries due to 
improvements in medical management, earlier identification, and 
treatment of diabetic retinopathy with laser photocoagulation.303 
However, given the dramatically increasing prevalence of diabetes 
worldwide, with 629 million individuals expected to be af

de, with 629 million individuals expected to be affected 
by diabetes by the year 2045,304 a proportionally larger number 
of individuals will be at risk for vision loss from diabetic eye com­
plications over the next few decades. Threefold increases in vision-
threatening diabetic retinopathy will affect 3.4 million Americans 
aged 40 and older by 2050, as estimated by the Centers for Dis­
ease Control and Prevention (CDC).306
Many individuals with diabetes and vision-threatening dia­
betic ret

ith diabetes and vision-threatening dia­
betic retinopathy are not aware of the presence or the severity of 
their disease. The 2010 National Health and Nutrition Exami­
nation Survey revealed that only 45% of Americans with DME 
were aware that diabetes had affected their eyes and nearly 60% of 
these individuals had not had a dilated eye examination within the 
past year. These findings reflect both a lack of awareness among 
patients at risk for vision loss from diabetic eye complications and

or vision loss from diabetic eye complications and 
insufficient evaluation for many patients with vision-threatening 
retinopathy.306 
Pathophysiology of Diabetic Retinopathy
A detailed discussion of the pathophysiologic mechanisms under­
lying diabetic retinopathy and other diabetes-related compli­
cations has been presented earlier in this chapter. The earliest 
A R
T
T
Q
m
Set 7
Histone methyl
transferase
LSD1
Histone
demethylase
m
K
K
A R
T
T
Q
m
K
K
• Fig. 37.25  Transient hyperglycemia in

T
Q
m
K
K
• Fig. 37.25  Transient hyperglycemia induces long-lasting activating epi­
genetic changes in the proximal promoter of the NFκB subunit p65 in 
endothelial cells. Hyperglycemia-induced reactive oxygen species (ROS) 
and methylglyoxal cause activating modifications of histone 3 lysine 4 
(monomethylation) and derepressing modifications of histone 3 lysine 9 
(removal of two methyl groups) at the NFκB p65 proximal promoter. K is 
the symbol for the amino acid lysine. The chains of circle

ol for the amino acid lysine. The chains of circled letters are the 
N-terminal tails of histone H3. LSD1, lysine-specific histone demethylase 
1A; Set 7, SET domain-containing protein 7. (From Brasacchio D, Okabe J, 
Tikellis C, et al. Hyperglycemia induces a dynamic cooperativity of histone 
methylase and demethylase enzymes associated with gene-activating 
epigenetic marks that co-exist on the lysine tail. Diabetes. 2009;58:1229–
1236; El-Osta A, Brasacchio D, Yao D, et al. Transient high glu

 A, Brasacchio D, Yao D, et al. Transient high glucose causes 
persistent epigenetic changes and altered gene expression during subse­
quent normoglycemia. J Exp Med. 2008;205:2409–2417.)
1464
histologic effects of diabetes mellitus in the eye include loss of 
retinal vascular pericytes (supporting cells for retinal endothelial 
cells), thickening of vascular endothelium basement membrane, 
and alterations in retinal blood flow (Fig. 3

ne, 
and alterations in retinal blood flow (Fig. 37.26).307–310 With 
increasing loss of retinal pericytes, the retinal vessel wall develops 
outpouchings (microaneurysms) and becomes fragile.
Clinically, microaneurysms and small retinal hemorrhages 
might not always be readily distinguishable from each other 
and are usually evaluated together as “hemorrhages and micro­
aneurysms” (Fig. 37.27A). Rheologic changes occur in diabetic 
retinopathy and result from increased platelet aggregation, int

nd result from increased platelet aggregation, integ­
rin-mediated leukocyte adhesion, and endothelial damage.311–313 
Disruption of the blood-retina barrier can ensue, characterized 
by increased vascular permeability.314,315 Subsequent leakage of 
blood and serum from the retinal vessels results in retinal hemor­
rhages, retinal edema, and hard exudates (see Fig. 37.27A and C). 
Vision loss can follow if the fovea is affected by the leakage.316
With time, increasing sclerosis and endothelial c

With time, increasing sclerosis and endothelial cell loss lead to 
narrowing of the retinal vessels, which decreases vascular perfusion 
and can ultimately lead to obliteration of the capillaries and small 
vessels (see Fig. 37.27B). The resulting retinal ischemia is a potent 
inducer of angiogenic growth factors. Several angiogenic growth 
factors have been isolated from eyes with diabetic retinopathy, 
including insulin-like growth factors, basic fibroblast growth fac­
tor (bFGF), and VEGF.31

sic fibroblast growth fac­
tor (bFGF), and VEGF.317,318 These factors promote the develop­
ment of new vessel growth and retinal vascular permeability.319,320 
Indeed, inhibition of angiogenic molecules such as VEGF and 
their signaling pathways can suppress the development of retinal 
Diabetes
High glucose
Pericyte loss
   thickening
Retinal blood flow change
Microaneurysm formation
Retinal hemorrhages
Capillary dropout
Retinal ischemia
Increased growth factors (e.g., VEGF)
Re

 ischemia
Retinal vascular
leakage
Macular edema
(may occur at any stage)
Moderate visual loss
Iris neovascularization
Retinal neovascularization
Vascular fibrosis
Vitreous hemorrhage
Mild to severe
visual loss
Severe
Retinal traction
Retinal detachment
Severe
Neovascular glaucoma
No
diabetic
retinopathy
Mild
nonproliferative
diabetic
(NPDR)
diabetic
Severe
NPDR
Moderate
NPDR
• Fig. 37.26  Pathoge

hy
Severe
NPDR
Moderate
NPDR
• Fig. 37.26  Pathogenesis of diabetic retinopathy. This schematic flow chart represents the major pre­
clinical and clinical findings associated with the full spectrum of diabetic retinopathy and macular edema. 
VEGF, Vascular endothelial growth factor.
1465
neovascularization and retinal vascular permeability.321,322 VEGF-
independent endogenous inhibitors of angiogenesis and vascular 
permeability such as pigment epit

is and vascular 
permeability such as pigment epithelial-derived factor (PEDF), 
plasma kallikrein, and erythropoietin have also been found in the 
eye, and these have therapeutic potential.323,324
New vessels tend to grow in regions of strong vitreous adhe­
sion to the retina, such as at the optic disc and major vascular 
arcades (see Fig. 37.27D and E). The posterior vitreous face also 
serves as a scaffold for pathologic neovascularization, and the 
new vessels commonly arise at the junctions

d the 
new vessels commonly arise at the junctions between perfused 
and nonperfused retina. When the retina is severely ischemic, the 
concentration of angiogenic growth factors can reach sufficient 
concentration in the anterior chamber to cause abnormal new ves­
sel proliferation on the iris and the anterior chamber angle.318,325 
Uncontrolled anterior segment neovascularization can result in 
neovascular glaucoma because the fibrovascular proliferation in 
the angle of the eye causes blockag

liferation in 
the angle of the eye causes blockage of aqueous outflow through 
the trabecular meshwork.305
Proliferating new vessels in diabetic retinopathy are fragile and 
have a tendency to bleed, which results in preretinal and vitreous 
hemorrhages (see Fig. 37.27E and F). Although the presence of a 
large amount of blood in the preretinal space or vitreous cavity is 
not damaging to the retina, these intraocular hemorrhages often 
cause vision loss by blocking the visual axis. Vitreous he

sion loss by blocking the visual axis. Vitreous hemorrhage 
Fibrous>
tissue
Fibrous
tissue
<Fibrous tissue
Extensive
NVD
NVE
VH
VH
<Fovea
Fovea>
Fovea>
Hard exudate>
IRMA
^
Ma^
Ma^
VB^
VB^
IRMA^
Non-perfusion
Macular
edema
<Hard exudate
^Optic disc
Optic disc>
<Fovea
<Optic disc
NVD>
Vitreous
hemorrhage
A
B
C
D
E
F
G
H
I
• Fig. 37.27  Clinical features of diabetic retinopathy: Some typical findings in human diabetic retinopathy. 
(A) Findings in severe n

an diabetic retinopathy. 
(A) Findings in severe nonproliferative diabetic retinopathy, including microaneurysms (Ma), venous bead­
ing (VB), and intraretinal microvascular abnormalities (IRMA). (B) Fluorescein angiogram showing marked 
capillary nonperfusion. (C) Clinically significant macular edema with retinal thickening and hard exudates 
involving the fovea. (D) Extensive neovascularization of the optic disc (NVD), illustrating high-risk prolifera­
tive diabetic retinopathy. (E) Neovascular

lifera­
tive diabetic retinopathy. (E) Neovascularization elsewhere (NVE) and two small vitreous hemorrhages (VH), 
also illustrating high-risk proliferative diabetic retinopathy. (F) Extensive vitreous hemorrhage arising from 
severe neovascularization of the disc (NVD). (G) Severe fibrovascular proliferation surrounding the fovea. 
(H) Traction retinal detachment from extensive fibrovascular proliferation. (I) Scars from scatter (panreti­
nal) laser photocoagulation. The macula, fovea, and opt

laser photocoagulation. The macula, fovea, and optic disc are not treated to preserve central vision. 
Laser burns are evident as white retinal lesions. (Adapted from Aiello LP. Eye complications of diabetes. In 
Korenman SG, Kahn CR, eds. Atlas of Clinical Endocrinology. Vol 2: Diabetes. Philadelphia, PA: Blackwell 
Scientific; 1999.)
1466
can clear spontaneously without intervention, but eyes in which 
hemorrhage is nonclearing may n

but eyes in which 
hemorrhage is nonclearing may need vitrectomy surgery to restore 
vision. Vitreous hemorrhage can also decrease the ability to visual­
ize the retina and thereby limit the ability to adequately diagnose 
and treat other retinal disease. Membranes on the retinal surface 
can be induced by blood and result in wrinkling and traction on 
the retina. Although all retinal neovascularization, given sufficient 
time, eventually becomes quiescent, as with most scarring pro­
cesses ther

 quiescent, as with most scarring pro­
cesses there is progressive fibrosis of the new vessel complexes that 
is associated with contraction. In the eye, such forces can exert 
traction on the retina, leading to tractional retinal detachment and 
retinal tears that can cause severe and permanent vision loss if left 
untreated (see Fig. 37.27G and H).
In short, causes of vision loss from complications of diabetes 
mellitus include retinal ischemia involving the fovea, macular 
edema at or near th

 involving the fovea, macular 
edema at or near the fovea, preretinal or vitreous hemorrhage, 
retinal detachment, and neovascular glaucoma. Vision loss can 
also result from more indirect effects of vasculopathy in diabetic 
patients, such as retinal vessel occlusion, accelerated atheroscle­
rotic disease, and embolic phenomena. 
Clinical Features of Diabetic Retinopathy
Duration of diabetes is closely associated with the onset and sever­
ity of diabetic retinopathy. Nearly all pat

sever­
ity of diabetic retinopathy. Nearly all patients with T1DM develop 
some degree of retinopathy after 20 years.293,296,326 In the United 
States approximately 20% of patients with newly diagnosed 
T2DM have retinopathy at the time of diagnosis,326 and more 
than 60% develop some degree of retinopathy over subsequent 
decades. In the UKPDS study of T2DM, 35% of female subjects 
and 39% of male subjects had some level of diabetic retinopathy 
at the time of diabetes diagnosis.327
Age of onse

Age of onset of diabetes is another risk factor. Diabetic reti­
nopathy is rare in prepubescent patients with T1DM.87 In the 
Wisconsin Epidemiologic Study of Diabetic Retinopathy, approx­
imately 4% of patients younger than 30 years of age at diagnosis 
and nearly 2% of patients older than 30 years of age at diagnosis 
were legally blind. In the younger-onset group, 86% of blindness 
was attributable to diabetic retinopathy. In the older-onset group, 
in w

betic retinopathy. In the older-onset group, 
in which other eye diseases were also common, 33% of the cases of 
legal blindness were due to diabetic retinopathy.294,326,328
Lack of appropriate glycemic control is the most significant 
known risk factor for the onset and progression of diabetic reti­
nopathy. The DCCT demonstrated a clear relationship between 
hyperglycemia and higher risk of diabetic microvascular complica­
tions, including retinopathy, in 1441 patients with T1DM.9,329–332 
Int

nopathy, in 1441 patients with T1DM.9,329–332 
Intensive insulin therapy reduced or prevented the development 
of retinopathy by 27% as compared with conventional therapy in 
patients monitored for 4 to 9 years in the DCCT. Additionally, 
intensive insulin therapy reduced the progression of diabetic reti­
nopathy by 34%, to 76%, and had a substantial beneficial effect 
over the entire range of retinopathy severity. These results under­
score the fact that although intensive therapy might not pre

fact that although intensive therapy might not prevent 
retinopathy completely, it reduces the risk of retinopathy onset 
and progression.
Interestingly, the effect of reducing the HbA1c in this group, 
from 9.1% for conventional treatment to the 7.3% for intensive 
treatment, resulted in a benefit maintained through 18 years of 
subsequent follow-up in the EDIC study, even though the dif­
ference in mean HbA1c levels of the two former randomized 
treatment groups was only 0.4% at 1 year (p < 0.

treatment groups was only 0.4% at 1 year (p < 0.001), continued 
to narrow, and became statistically nonsignificant from 5 years 
onward (8.1% vs. 8.2%, p = 0.09). The rate of further progression 
of complications from their levels at the end of the DCCT remains 
less in the former intensive treatment group. Thus the benefits of 
6.5 years of intensive treatment continue to accrue well beyond the 
period of initial HbA1c difference.9,22,330,333–335 Beyond retinopa­
thy progression itself, the 

–335 Beyond retinopa­
thy progression itself, the risk for diabetes-related ocular surgery 
was reduced 48% in the intensive therapy group336 over a median 
follow-up of 23 years. Applying DCCT intensive insulin therapy 
to all persons in the United States with T1DM would result in 
a gain of 920,000 person-years of sight,331 although the costs of 
intensive therapy are three times that of conventional therapy.337
Presence of renal disease, as manifested by microalbuminuria 
and proteinuria, is 

nifested by microalbuminuria 
and proteinuria, is yet another significant risk factor for onset and 
progression of diabetic retinopathy.338,339 Hypertension is associ­
ated with PDR and is an established risk factor for the develop­
ment of macular edema.340 Independent of renal disease, elevated 
serum lipid levels are associated with extravasated lipid in the 
retina (hard exudates) and vision loss.341 
Clinical findings associated with early and progressing diabetic reti­
n

ciated with early and progressing diabetic reti­
nopathy include microaneurysms, hemorrhages, cotton-wool spots, 
hard exudates, intraretinal microvascular abnormalities, and venous 
abnormalities such as venous loops, venous tortuosity, and venous 
beading (see Fig. 37.27A and C). Microaneurysms are outpouch­
ings of the capillary walls that can leak fluid and result in intrareti­
nal edema and hemorrhages. The intraretinal hemorrhages can be 
flame shaped or dot-blot–like in appearance, reflec

lame shaped or dot-blot–like in appearance, reflecting the archi­
tecture of the retinal layer in which they occur. Flame-shaped hem­
orrhages occur in inner retina closer to the vitreous, and dot-blot 
hemorrhages occur deeper in the retina. Intraretinal microvascular 
abnormalities are either new vessel growth within the retinal tissue 
itself or shunt vessels through areas of poor vascular perfusion. It is 
common for intraretinal microvascular abnormalities to be located 
adjacent to cotton-

r abnormalities to be located 
adjacent to cotton-wool spots. Cotton-wool spots are caused by 
microinfarcts in the nerve fiber layer of the retina. Venous caliber 
abnormalities, also known as venous beading, are generally a sign of 
severe retinal hypoxia. In some cases of extensive vascular loss, how­
ever, the retina may actually appear free of nonproliferative lesions 
on clinical examination. Such areas are termed featureless retina and 
are a sign of severe retinal hypoxia.
Vision loss fr

e a sign of severe retinal hypoxia.
Vision loss from diabetic retinopathy generally results from per­
sistent nonclearing vitreous hemorrhage, traction retinal detach­
ment, retinal nonperfusion, or DME (see Figs. 37.26 and 37.27). 
Neovascularization with fibrous tissue contraction can distort the 
retina and lead to traction retinal detachment. The new vessels can 
bleed, causing preretinal or vitreous hemorrhage. The most com­
mon cause of vision loss from diabetes, however, is macular diseas

ion loss from diabetes, however, is macular disease 
and macular edema. Macular edema is more likely to occur in 
patients with T2DM. In diabetic macular disease, macular edema 
involving the fovea or nonperfusion of the capillaries in the central 
macula is responsible for the loss of vision. 
Classification Systems
Classification of Diabetic Retinopathy
Diabetic retinopathy is broadly classified into nonproliferative 
diabetic retinopathy (NPDR) and proliferative diabetic retinopathy 
categori

) and proliferative diabetic retinopathy 
categories.342,343 Macular edema can be present in eyes with non­
proliferative or proliferative disease and is not used in the clas­
sification of level of retinopathy. The historic terms background 
retinopathy and preproliferative diabetic retinopathy have been 
1467
replaced to reflect the specific characteristics and risk stratification 
of the prognostically important subgroups in NPDR (Table 37.1).
Ge

cally important subgroups in NPDR (Table 37.1).
Generally, diabetic retinopathy progresses from no retinopa­
thy through mild, moderate, severe, and very severe NPDR and 
eventually to PDR. The level of NPDR is determined by the 
extent and location of clinical manifestations of retinopathy. Mild 
NPDR is characterized by limited microvascular abnormalities 
such as hemorrhages or microaneurysms, cotton-wool spots, and 
increased vascular permeability. Moderate and severe NPDR is 
characterized 

ility. Moderate and severe NPDR is 
characterized by increasing severity of hemorrhages or microan­
eurysms, venous caliber abnormalities, intraretinal microvascular 
abnormalities, and vascular closure. The level of NPDR establishes 
the risk of progression to sight-threatening retinopathy and dic­
tates appropriate clinical management and follow-up.
  Glossary and Abbreviations Pertinent to Diabetic Eye Disease
Term
Antivascular endothelial growth 
factor (anti-VEGF) therapies; 
VE

othelial growth 
VEGF inhibitors
For the purposes of this chapter, these refer to VEGF inhibitors (including aflibercept, bevacizumab, and ranibizumab) 
that are given intravitreously for the treatment of diabetic macular edema and proliferative diabetic retinopathy.
Background diabetic retinopathy 
(BDR)
An outdated term referring to some stages of NPDR; not closely associated with disease progression; replaced by the 
various levels of NPDR.
Center-involved (or c

the 
Center-involved (or central-
involved) diabetic macular 
edema (ciDME)
Abnormal thickening of the central retina (usually 1 mm in diameter central retinal subfield) due to increased vascular 
permeability. Because central involvement is more likely to cause visual impairment, it is commonly used as a 
threshold for treatment.
Clinically significant macular 
edema (CSME)
Thickening of the retina in the macular region of sufficient extent and location to threaten centr

f sufficient extent and location to threaten central visual function.
Cotton-wool spot
A gray or white area lesion in the nerve fiber layer of the retina resulting from stasis of axoplasmic flow caused by 
microinfarcts of the retinal nerve fiber layer.
Diabetes Control and Complications 
Trial (DCCT)
A multicenter, randomized clinical trial designed to address whether intensive insulin therapy could prevent or slow 
the progression of systemic complications of diabetes mellitus.
Diabetic retino

omplications of diabetes mellitus.
Diabetic retinopathy (DR)
Retinal damage related to the underlying systemic disease of diabetes mellitus.
Diabetic Retinopathy Study (DRS)
The first multicenter, randomized clinical trial to demonstrate the value of scatter (panretinal) photocoagulation in 
reducing the risk of vision loss among patients with all levels of diabetic retinopathy.
Diabetic Retinopathy Vitrectomy 
Study (DRVS)
A multicenter clinical trial evaluating early vitrectomy for patients wi

 trial evaluating early vitrectomy for patients with very advanced diabetic retinopathy or nonre­
solving vitreous hemorrhage.
Early Treatment Diabetic Retinopathy 
Study (ETDRS)
A multicenter, randomized clinical trial that addressed at what stage of retinopathy scatter (panretinal) photocoagula­
tion was indicated, whether focal photocoagulation was effective for preventing moderate vision loss due to clini­
cally significant macular edema, and whether aspirin therapy altered the progression o

 whether aspirin therapy altered the progression of diabetic retinopathy.
Focal or grid laser photocoagulation
A type of laser treatment whose main goal is to reduce vascular leakage, either by focal treatment of leaking retinal 
microaneurysms or by application of therapy in a gridlike pattern for patients with clinically significant macular 
edema.
Hard exudate
Lipid accumulation within the retina as a result of increased vasopermeability.
High-risk characteristic proliferative 
diabetic retin

-risk characteristic proliferative 
diabetic retinopathy (HRC-PDR)
Proliferative diabetic retinopathy of defined extent, location, or clinical findings that is particularly associated with 
severe vision loss.
Microaneurysm
An early vascular abnormality consisting of an outpouching of the retinal microvasculature.
Neovascular glaucoma (NVG)
Elevation of intraocular pressure caused by the development of neovascularization in the anterior segment of the eye.
Neovascularization at the disc (NVD)
Re

f the eye.
Retinal neovascularization occurring at or within 1500 μm of the optic disc.
Neovascularization elsewhere (NVE)
Retinal neovascularization that is located more than 1500 μm away from the optic disc.
Neovascularization of the iris (NVI)
Neovascularization occurring on the iris (rubeosis iridis), usually as a result of extensive retinal ischemia.
No light perception (NLP)
The inability to perceive light.
Nonproliferative diabetic retinopathy 
(NPDR)

ht.
(NPDR)
Severities (mild, moderate, severe) of clinically evident diabetic retinopathy that precede the development of PDR.
Preproliferative diabetic retinopathy 
(PPDR)
An outdated term referring to more advanced levels of NPDR; not closely associated with disease progression; 
replaced by the various levels of NPDR.
Proliferative diabetic retinopathy 
(PDR)
An advanced level of diabetic retinopathy in which proliferation of new vessels or fibrous tissu

hich proliferation of new vessels or fibrous tissue occurs on or within 
the retina.
Rubeosis iridis
Neovascularization of the iris.
TABLE 37.1
1468
PDR is characterized by vasoproliferation of the retina and its 
complications, including new vessels on the optic disc (NVD), 
new vessels elsewhere on the retina (NVE), preretinal hemor­
rhage (PRH), vitreous hemorrhage, and fibrous tissue prolifera­
tion (FP). On the basis of the extent

e prolifera­
tion (FP). On the basis of the extent and location of these lesions, 
PDR is classified as early PDR or high-risk PDR. Larger areas of 
these complications as well as new vessels that are near the optic 
disc are associated with greater risks of vision loss. 
Classification of Diabetic Macular Edema
DME can be present at any severity of diabetic retinopathy. His­
torically, eyes with DME were categorized as having nonclini­
cally significant or clinically significant macular edema. 

nificant or clinically significant macular edema. The term 
CSME was first introduced in the Early Treatment Diabetic Reti­
nopathy Study (ETDRS) to indicate an increased risk for moder­
ate visual loss and was used as a threshold to determine need for 
laser treatment. CSME exists if there is retinal thickening at or 
within 500 μm of the fovea, hard exudates at or within 500 μm of 
the fovea with adjacent retinal thickening, or an area or areas of 
retinal thickening one disc area or more in s

 of 
retinal thickening one disc area or more in size, any part of which 
is within 1500 μm of the fovea.342,344,345 CSME is a clinical diag­
nosis that is not dependent on visual acuity or results of ancillary 
testing such as fluorescein angiography and can be present even 
when vision is 20/20 or better.
In the recent era, with the advent of ocular coherence tomogra­
phy (OCT) and its objective quantitative measurement of retinal 
thickness, clinical care and clinical trial endpoints have shi

linical care and clinical trial endpoints have shifted to 
evaluating whether the center of the macula is involved or not. 
Data from the ETDRS evaluating eyes with macular edema have 
shown the presence or absence of thickening involving the center 
of the macula, now termed center-involved DME, which is highly 
associated with short-term and long-term visual acuity outcomes. 
In the ETDRS, eyes with center-involved DME had nearly a 
10-fold greater risk for developing moderate visual loss comp

ater risk for developing moderate visual loss compared 
to eyes without center involvement. The identification of center-
involved DME is critical because it will generally indicate how 
likely the DME is to cause visual impairment and is a key crite­
ria for determining the need for treatment. Noncenter-involved 
DME does not typically cause visual loss or symptoms, and fre­
quently does not progress to center-involved DME. Thus if the 
center of the macula is not involved there is often not a 

f the macula is not involved there is often not a compel­
ling reason to treat. As a result, recent clinical studies and guide­
lines for clinical care have generally used the presence or absence 
of thickening of the central 1-mm diameter of retina, along with 
visual status, as the defining criteria to consider initiation of treat­
ment for eyes with DME. 
International Classification of Diabetic Retinopathy and 
Diabetic Macular Edema
The American Academy of Ophthalmology initiated a project 

ican Academy of Ophthalmology initiated a project to 
establish a consensus International Classification of Diabetic Reti­
nopathy and Diabetic Macular Edema in an effort to simplify clas­
sification and standardize communication among diabetes health 
care providers.346,347 This international classification describes five 
clinical levels of diabetic retinopathy: no apparent retinopathy 
(no abnormalities), mild NPDR (microaneurysms only), moder­
ate NPDR (more than microaneurysms only but less

­
ate NPDR (more than microaneurysms only but less than severe 
NPDR), severe NPDR (any of the following: >20 intraretinal 
hemorrhages in each of four retinal quadrants, definite venous 
beading in two or more retinal quadrants, prominent intraretinal 
microvascular abnormalities in one or more retinal quadrants, and 
no PDR), and PDR (one or more of retinal neovascularization, 
vitreous hemorrhage, or preretinal hemorrhage). Table 37.2 com­
pares levels of retinopathy in the international clas

es levels of retinopathy in the international classification to 
those defined by the landmark ETDRS.
In regard to DME, the international classification identifies two 
broad categories: macular edema apparently absent (no apparent 
retinal thickening or hard exudates in the posterior pole) and macu­
lar edema apparently present (some apparent retinal thickening or 
hard exudates in the posterior pole). Macular edema is subclassified 
as mild (some retinal thickening or hard exudates in the post

me retinal thickening or hard exudates in the posterior 
pole but distant from the center of the macula), moderate (retinal 
thickening or hard exudates approaching the center of the macula 
but not involving the center), or severe (retinal thickening or hard 
exudates involving the center of the macula). Table 37.3 compares 
levels of DME in the international classification to ETDRS levels 
of DME.
As compared with ETDRS retinopathy grading, the Interna­
tional Classification of Diabetic Retino

 Interna­
tional Classification of Diabetic Retinopathy and Diabetic Macu­
lar Edema reduces the number of levels of diabetic retinopathy, 
simplifies descriptions of the categories, and describes the levels 
  Levels of Diabetic Retinopathy
International 
Classification Level
ETDRS Level
No apparent retinopathy
Level 10: DR absent
Mild NPDR
Level 20: very mild NPDR
Moderate NPDR
Levels 35, 43, and 47: moderate NPDR
Severe NPDR
Levels 53A–E: severe to very severe NPDR
PDR
Levels 61, 65, 71, 75,

ere to very severe NPDR
PDR
Levels 61, 65, 71, 75, 81, 85: PDR, high-risk 
PDR, very severe or advanced PDR
DR, Diabetic retinopathy; ETDRS, Early Treatment Diabetic Retinopathy Study; NPDR, nonpro­
liferative diabetic retinopathy; PDR, proliferative diabetic retinopathy.
From Grading diabetic retinopathy from stereoscopic color fundus photographs—an exten­
sion of the modified Airlie House classification. ETDRS report number 10. Early Treatment 
Diabetic Retinopathy Study Research Group. Ophtha

Diabetic Retinopathy Study Research Group. Ophthalmology. 1991;98:S786–S806.
   
TABLE 37.2
  International Classification of Diabetic 
Macular Edema (DME)
Disease 
Severity Level
Ophthalmic Findings (Retinal 
Thickening or Hard Exudates 
in Posterior Pole)
ETDRS Scale 
Equivalent
DME apparently 
absent
None apparent
present
Some apparent
Mild DME
Some findings present but distant 
from center of the macula
DME but not 
CSME
Moderate DME
Findings approaching the center 
but not

erate DME
but not involving the center
CSME
Severe DME
Findings involving the center of the 
macula
CSME
CSME, Clinically significant macular edema; ETDRS, Early Treatment Diabetic Retinopathy Study.
   
TABLE 37.3
1469
without relying on reference to the standard photographs of the 
Airlie House Classification of diabetic retinopathy. This approach 
makes clinical use easier and more uniform among practitioners 
not

e easier and more uniform among practitioners 
not versed in the complexities of the ETDRS grading system. 
However, because of this simplification, the International Clas­
sification of Diabetic Retinopathy and Diabetic Macular Edema 
is not a replacement for ETDRS levels of diabetic retinopathy in 
large-scale clinical trials or studies for which precise retinopathy 
classification is required. 
Other Ocular Manifestations of Diabetes
All structures of the eye are susceptible to complications 

tures of the eye are susceptible to complications of dia­
betes. The consequence of these changes can range from being 
unnoticed by both patient and physician, to symptomatic but not 
sight threatening, to requiring evaluation to rule out potentially 
life-threatening underlying causes other than diabetes.
Mononeuropathies of the third, fourth, or sixth cranial nerves 
can arise in association with diabetes; mononeuropathy of the 
fourth cranial nerve is least likely associated with diabetes an

 nerve is least likely associated with diabetes and 
warrants workup for other causes.348–350 Nerve palsies present a 
significant diagnostic challenge because misdiagnosis can result in 
a life-threatening lesion remaining untreated. In one review of cra­
nial nerve palsies treated in a diabetic patient population in 1967, 
42% of mononeuropathies were not diabetic in origin.349 This 
finding underscores the danger of routinely attributing mononeu­
ropathies to the diabetic condition itself wit

eu­
ropathies to the diabetic condition itself without carefully ruling 
out other potential causes. The percentage of all extraocular mus­
cle palsies attributable to diabetes mellitus is estimated at 4.5% to 
6%.350 Mononeuropathies may be the initial presenting sign of 
new-onset diabetes, and diabetes should therefore be considered 
in the differential diagnosis of any mononeuropathy affecting the 
extraocular muscles, even in patients who do not claim a history of 
diabetes. Diabetes-induce

 not claim a history of 
diabetes. Diabetes-induced third-nerve, fourth-nerve, and sixth-
nerve palsies are usually self-limited and should resolve spontane­
ously in 2 to 6 months. Palsies can recur or subsequently develop 
in the contralateral eye.
The optic disc can be affected by diabetes in a variety of ways 
other than vasoproliferation. Diabetic papillopathy is a diagnosis 
of exclusion and must be distinguished from other causes of disc 
swelling such as true papilledema from increased i

swelling such as true papilledema from increased intracranial pres­
sure, pseudopapilledema such as optic nerve head drusen, toxic 
optic neuropathies, neoplasms of the optic nerve, and hyperten­
sion.351 Optic disc pallor can occur following spontaneous remis­
sion of proliferative retinopathy or remission following scatter 
(panretinal) laser photocoagulation (see Fig. 37.27I). Because dia­
betes poses an increased risk for developing open-angle glaucoma, 
the disc pallor following remission o

e glaucoma, 
the disc pallor following remission of retinopathy or laser photo­
coagulation must be considered when evaluating the optic nerve 
head for glaucoma.
A diabetic ocular complication with potentially serious con­
sequences is neovascularization of the iris. Neovascularization of 
the iris occurs in 4% to 7% of diabetic eyes and may be present in 
40% to 60% of eyes with proliferative retinopathy. Usually new 
iris vessels are first observed at the pupillary border, followed by 
a fine

erved at the pupillary border, followed by 
a fine network of vessels over the iris tissue progressing into the 
filtration angle of the eye. Closure of the angle by the fibrovascular 
network results in neovascular glaucoma, which can lead to irre­
versible optic nerve damage and vision loss due to the rise in intra­
ocular pressure.354 Neovascular glaucoma is difficult to manage 
and requires aggressive treatment. Diabetes is the second leading 
cause of neovascular glaucoma, accounting for 32

cause of neovascular glaucoma, accounting for 32% of cases.355 
When possible, scatter (panretinal) laser photocoagulation is the 
primary therapy for neovascular glaucoma and provides durable 
regression of neovascularization of the iris and angle for most 
eyes. Other approaches such as goniophotocoagulation, topical or 
systemic antiglaucoma medications, and antiglaucomatous filtra­
tion surgery are available when needed.356–358 Intravitreal admin­
istration of VEGF inhibitors is now freque

 admin­
istration of VEGF inhibitors is now frequently utilized in eyes 
with acute neovascular glaucoma and results in remarkably rapid 
regression of the neovascularization with resulting normalization 
of the intraocular pressure.359 However, the effects of anti-VEGF 
agents are often transient, and there is frequently recurrence of 
neovascularization unless injections are given on a monthly basis. 
Thus a more durable therapy such as scatter laser photocoagula­
tion is often performed.
The 

 laser photocoagula­
The cornea of the diabetic person is more susceptible to 
injury and slower to heal after injury than is the nondiabetic 
cornea.359a,360 The diabetic cornea is also more prone to infec­
tious corneal ulcers, which can lead to rapid loss of vision, 
need for corneal transplant, or loss of the eye if it is not treated 
aggressively. Consequently, diabetic patients using contact lenses 
should exercise caution to avoid contact lens overwear and to 
mai

caution to avoid contact lens overwear and to 
maintain careful monitoring.
Open-angle glaucoma is 1.4 times more common in the dia­
betic population than in the nondiabetic population.361 The prev­
alence of glaucoma increases with age and duration of diabetes, 
but medical therapy for open-angle glaucoma is generally effective. 
In a study of 76,318 women enrolled in the Nurses’ Health Study, 
Pasquale and coworkers found that T2DM is associated with an 
increased risk of primary open-angle gl

d with an 
increased risk of primary open-angle glaucoma in women.362
Diabetes effects on the crystalline lens can result in transient 
refractive changes, alterations in accommodative ability,363 and 
cataracts. Refractive change can be significant and is related to 
fluctuation of blood glucose levels with osmotic lens swelling.365 
Cataracts can occur earlier in life and progress more rapidly in the 
presence of diabetes.364,365 Cataracts are 1.6 times more common 
in people with diabetes tha

1.6 times more common 
in people with diabetes than in those without diabetes.364,365 In 
patients with earlier onset diabetes, duration of diabetes, retinopa­
thy status, diuretic use, and HbA1c levels are risk factors.366 In 
patients with later onset diabetes, age of the patient, lower intraoc­
ular pressure, smoking, and lower diastolic BP may be additional 
risk factors.367,368 Diabetic patients undergoing simultaneous 
kidney and pancreas transplantation are at an increased risk of 
develo

ransplantation are at an increased risk of 
developing all types of cataracts, independent of the use of corti­
costeroids after transplantation.369 Both phacoemulsification and 
extracapsular cataract extraction with intraocular lens implanta­
tion are appropriate surgical therapies. The principal determinant 
of postoperative vision and progression of retinopathy is related 
to the preoperative presence of DME and the severity of diabetic 
retinopathy.370,371
Other findings with higher inciden

nopathy.370,371
Other findings with higher incidence among patients with 
diabetes include xanthelasma,348 microaneurysms of the bulbar 
conjunctiva,372 posterior vitreous detachment,373 and the rare but 
often fatal orbital fungal infection Mucorales phycomycosis.352,353 
Prompt diagnosis and treatment of phycomycosis caused by 
Mucor species are crucial, although the survival rate remains at 
only 57%.353,374 
Monitoring and Treatment of Diabetic 
Retinopathy
Appropriate clinical management of

ic 
Appropriate clinical management of diabetic retinopathy has been 
defined by results of major randomized, multicenter clinical trials 
(Fig. 37.28): the Diabetic Retinopathy Clinical Research Network 
1470
Protocols I, S, and T,290,291,375 Diabetic Retinopathy Study 
(DRS),376 ETDRS,316 Diabetic Retinopathy Vitrectomy Study 
(DRVS),377 DCCT,378 and UKPDS.379 These studies have elu­
cidated the progression rates of each 

s have elu­
cidated the progression rates of each level of diabetic retinopathy, 
guided follow-up intervals, and demonstrated the proper delivery, 
timing, and resulting effectiveness of glycemic control, laser pho­
tocoagulation, and intravitreous anti-VEGF therapy (Figs. 37.29, 
37.30, 37.31, and 37.32). They have also established recommen­
dations for vitrectomy surgery for diabetic eye complications.
Comprehensive Eye Examination
An accurate ocular examination detailing the extent and loca­

 ocular examination detailing the extent and loca­
tion of retinopathy-associated findings is critical for determining 
monitoring and treatment decisions in patients with diabetic reti­
nopathy. As detailed later, most of the blindness associated with 
advanced stages of retinopathy can be averted with appropriate 
and timely diagnosis and therapy. Unfortunately, many diabetic 
patients do not receive adequate eye care at an appropriate stage 
of their disease.380,381 In fact, 11% of T1DM and 7

f their disease.380,381 In fact, 11% of T1DM and 7% of T2DM 
patients with high-risk PDR necessitating prompt treatment 
had not been examined by an ophthalmologist within the past 
2 years.381 In one study, 55% of patients with high-risk PDR or 
CSME had never had laser photocoagulation.380
The comprehensive eye examination is the mainstay of such 
evaluation and is necessary on a repetitive, lifelong basis for patients 
with diabetes.342,382 Such an evaluation has four major compo­
nents: hist

ch an evaluation has four major compo­
nents: history, examination, diagnosis, and treatment as needed. 
Annual retinal evaluation to assess the presence and level of dia­
betic retinopathy and DME is essential to guide patient care. The 
fundamentals of a comprehensive eye examination for the non­
diabetic patient have been detailed by the American Academy of 
Ophthalmology382 and the American Optometric Association.383 
No diabetic
DCCT/UKPDS
Sorbinil retinopathy trial
ETDRS
DRS
DR

ETDRS
DRS
DRVS
Mild
Nonproliferative diabetic retinopathy
Moderate
Severe
Very severe
Early
PDR
High risk
PDR
Severe
PDR
• Fig. 37.28  Schematic representation of the major multicenter clinical trials of diabetic retinopathy and the 
severity levels of diabetic retinopathy that they primarily addressed. DCCT, Diabetes Control and Com­
plications Trial; DRS, Diabetic Retinopathy Study; DRVS, Diabetic Retinopathy Vitrectomy Study; ETDRS, 
Early Treatment Diabe

hy Vitrectomy Study; ETDRS, 
Early Treatment Diabetic Retinopathy Study; PDR, proliferative diabetic retinopathy; UKPDS, United King­
dom Prospective Diabetes Study.
Diabetes
Yes
Ophthalmic
follow-up per
patient’s diabetic
retinopathy level
Has patient
had eye exam
since diagnosis
of diabetes?
No
Diabetes
type
Type 2
Refer to eye
care provider for
comprehensive
eye exam
Age
≥10 years
Yes
Is patient
pregnant?
No
Yes
Is diabetes
duration
≥5 years?
No
No
Yes
Type I
Initial
ophthalmic
exam not yet
r

No
No
Yes
Type I
Initial
required
• Fig. 37.29  Schematic flow chart of major principles involved in determining the timing of initial ophthalmic 
examination after a diagnosis of diabetes mellitus. These are minimal recommended times. Ocular symp­
toms, complaints, or other associated medical issues can necessitate earlier evaluation. Guidelines are 
regularly reevaluated based on new study results.
1471
The examination of t

ications of Diabetes Mellitus
The examination of the patient with diabetes should be similar, 
with additional emphasis on portions of the examination that 
relate to problems particularly relevant to diabetes.
Dilated ophthalmic examination is superior to undilated evalu­
ation because only 50% of eyes are correctly classified as to pres­
ence and severity of retinopathy through undilated pupils.384,385 
Appropriate ophthalmic evaluation entails pupillary dilation, slit-
lamp biomicroscopy, exa

 pupillary dilation, slit-
lamp biomicroscopy, examination of the retinal periphery with 
indirect ophthalmoscopy or mirrored contact lens, and sometimes 
gonioscopy.382,383 Because of the complexities of the diagnosis 
and treatment of PDR and CSME, ophthalmologists with spe­
cialized knowledge and experience in the management of diabetic 
retinopathy are required to determine and provide appropriate 
surgical intervention.386 Thus it is recommended that all patients 
with diabetes should have 

nded that all patients 
with diabetes should have dilated ocular examinations by an expe­
rienced eye care provider (ophthalmologist or optometrist), and 
diabetic patients should be under the direct or consulting care of 
an ophthalmologist experienced in the management of diabetic 
retinopathy at least by the time moderate diabetic retinopathy or 
DME is present. Retinal imaging that has demonstrated equiva­
lency to dilated retinal fundus examination or the accepted stan­
dard of ETDRS seven-

ination or the accepted stan­
dard of ETDRS seven-standard-field stereoscopic retinal imaging 
when interpreted by a trained eye care provider can also be appro­
priate for retinal evaluation.387,388 Furthermore, ocular telehealth 
programs for diabetic retinopathy that utilize validated means of 
retinal imaging have the potential to expand access to highly effec­
tive evidence-based diabetes eye care and provide cost-effective 
alternative methods of care.389 
Initial Ophthalmic Evaluation
The

ods of care.389 
The recommendation for initial ocular examination in persons with 
diabetes is based on the prevalence rates of retinopathy and the inci­
dence of subsequent vision threatening diabetic eye complications (see 
Fig. 37.29). Approximately 80% of T1DM patients have retinopathy 
after 15 years of disease, but only about 25% have any retinopathy 
after 5 years of diabetes.385 The prevalence of PDR is less than 2% at 
5 years and 25% by 15 years. For T2DM

 than 2% at 
5 years and 25% by 15 years. For T2DM, the onset date of diabetes 
is usually unknown, and severe retinal disease can be observed even at 
the time of diabetes diagnosis. Up to 3% of patients whose diabetes 
is first diagnosed after age 30 years (T2DM) have CSME or high-risk 
PDR at the time of initial diagnosis of diabetes.390 Thus in patients 
older than 10 years, initial ophthalmic examination is recommended 
beginning 5 years after the diagnosis of T1DM and immediately after 
di

er the diagnosis of T1DM and immediately after 
diagnosis of T2DM (see Fig. 37.29).342,391
Diabetes
Is DME
present?
No
No
Yes
Yes
Panretinal
photocoagulation
probably
indicated
Yes
Is HRC PDR
present?
Is severe NPDR or
worse present?
Is moderate
NPDR present?
Is mild
Follow-up
every 12 months
No
No
Yes
No
No
No
Yes
Yes
Yes
Is CSME
present
Minimum follow-up
every 3 months;
sooner if indicated
by DR
Consider panretinal
Follow-up every
4–8 months
8–12 

n
4–8 months
8–12 months
Consider
focal/grid laser
Follow-up in 4
months or less
Is vision
excellent?
Focal/grid laser
probably
indicated
• Fig. 37.30  Diabetic retinopathy and macular edema examination and treatment flow chart: nonpregnant 
patients. The schematic flow chart presents major principles involved in determining routine ophthalmic 
follow-up and indications for treatment in nonpregnant patients with diabetes. These in

nt in nonpregnant patients with diabetes. These intervals are only 
general, minimal recommended frequencies. Ocular symptoms, complaints, or other associated ophthal­
mic or medical issues can necessitate earlier evaluation or an altered approach. Guidelines are regularly 
reevaluated based on new study results. CSME, Clinically significant macular edema; DME, diabetic macu­
lar edema; DR, diabetic retinopathy; HRC PDR, high-risk characteristic proliferative diabetic retinopathy; 
NPDR, nonprol

proliferative diabetic retinopathy; 
NPDR, nonproliferative diabetic retinopathy; PDR, proliferative diabetic retinopathy.
1472
Puberty and pregnancy can accelerate retinopathy progression. 
The onset of vision-threatening retinopathy is rare in children 
prior to puberty, regardless of the duration of diabetes391,392; how­
ever, significant retinopathy can arise within 6 years of disease if 
diabetes is diagnosed between the ages of 1

se if 
diabetes is diagnosed between the ages of 10 and 30 years.295 Dia­
betic retinopathy can become particularly aggressive during preg­
nancy in patients with diabetes.393,394 In the past, the prognosis 
for pregnancy in the diabetic patient with microvascular complica­
tions was so poor that pregnant diabetic patients were commonly 
advised to avoid or terminate pregnancies.395 With recognition of 
the importance of glycemic control, diabetic patients in the child­
bearing age now can exper

c patients in the child­
bearing age now can experience safe pregnancy and childbirth 
with minimal risk to both the mother and the baby.396
Ideally, patients with diabetes who are planning pregnancy 
should have a comprehensive eye examination within 1 year prior 
to conception (see Fig. 37.31). Treatment may be indicated prior 
to conception for patients who are at risk for vision loss if their dia­
betic retinopathy were to worsen during pregnancy. Patients who 
become pregnant should have a 

ancy. Patients who 
become pregnant should have a comprehensive eye examination 
in the first trimester of pregnancy. Close follow-up throughout 
pregnancy is indicated, with subsequent examinations determined 
by the findings present at the first-trimester examination.342 This 
recommendation does not apply to women who develop gesta­
tional diabetes because they are not at increased risk of developing 
diabetic retinopathy. 
Follow-Up Ophthalmic Examination
Follow-up ocular examination is dete

c Examination
Follow-up ocular examination is determined from the risk of 
disease progression at any particular retinopathy severity (see 
Fig. 37.30). NPDR is clinically categorized into four levels 
of severity based on clinical findings compared to stereo fun­
dus photographic standards: mild, moderate, severe, and very 
severe.397 Progression of nonproliferative retinopathy to the visu­
ally threatening level of high-risk PDR is closely correlated with 
baseline NPDR severity (Table 37.4). 

elated with 
baseline NPDR severity (Table 37.4). Progression rates from 
each individual NPDR level to any other retinopathy level are 
also known. These are used to define standard minimal follow-
up intervals as detailed in Fig. 37.30 and Table 37.5. Because 
significant sight-threatening retinopathy can initially occur with 
no or minimal visual symptoms, patients with no clinically evi­
dent diabetic retinopathy and no known ocular problems still 
require annual comprehensive ophthalmic exa

still 
require annual comprehensive ophthalmic examinations even if 
they are totally asymptomatic.
Evaluation and Treatment of Proliferative Diabetic 
The extent and location of neovascularization determine the level 
of PDR.398,399 PDR is best evaluated by dilated fundus exami­
nation using slit-lamp biomicroscopy combined with indirect 
ophthalmoscopy or stereo fundus photography. Fluorescein angi­
ography and optical coherence tomography angiography can also 
Diabetes
Yes
Yes
Yes

ography angiography can also 
Diabetes
Yes
Yes
Yes
Yes
Yes
No
No
No
No
No
Is patient
pregnant?
No
No
Is patient
planning
pregnancy?
Standard eye evaluation
protocol for patient’s level
of diabetic retinopathy
Yes
Yes
Refer to eye care
provider for comprehensive
ophthalmic evaluation 
Follow-up per
eye care provider
or every 3 months
Does patient
have DR or DME?
Is CSME
present?
Is DME
present?
probably indicated
Is HRC PDR
present?
Consider
photocoagu

 PDR
present?
Consider
Consider
panretinal
Is severe
NPDR or worse
present?
Eye care provider
follow-up every 3
• Fig. 37.31  Diabetic retinopathy and macular edema examination and treatment flow chart: pregnant 
patients. The schematic flow chart shows major principles involved in determining routine ophthalmic 
follow-up and indications for treatment in pregnant patients with 

dications for treatment in pregnant patients with diabetes. These intervals are only gen­
eral, minimal recommended frequencies. Ocular symptoms, complaints, or other associated ophthalmic 
or medical issues can necessitate earlier evaluation or an altered approach. Because retinopathy can 
progress rapidly in pregnant patients with diabetes, careful and more frequent evaluation is often indicated. 
Guidelines are regularly reevaluated based on new study results. CSME, Clinically significant mac

ew study results. CSME, Clinically significant macular 
edema; DME, diabetic macular edema; DR, diabetic retinopathy; HRC PDR, high-risk characteristic prolif­
erative diabetic retinopathy; NPDR, nonproliferative diabetic retinopathy.
1473
aid in identifying small, subtle patches of retinal neovasculariza­
tion. However, these imaging modalities are usually not required 
for the diagnosis of PDR because these findings are clinically evi­
dent in mos

use these findings are clinically evi­
dent in most cases (Fig. 37.33).
The presence of PDR in an eye substantially increases the 
risk for severe vision loss. Severe visual loss is defined as best 
corrected acuity of 5/200 or worse on two consecutive visits 4 
months apart. This represents vision loss substantially worse than 
the 20/200 or worse limit defined as legal blindness. Without 
treatment, eyes with high-risk PDR have a 28% risk of severe 
vision loss within 2 years. This risk compar

vere 
vision loss within 2 years. This risk compares with a 7% risk of 
severe vision loss after 2 years for eyes with PDR but without 
high-risk characteristics.398
Until this decade, scatter (panretinal) laser photocoagulation 
was the sole therapy shown to be effective against PDR. The DRS 
demonstrated that panretinal laser photocoagulation was effec­
tive in reducing the risk of severe vision loss from PDR by 50% 
or more. The ETDRS further demonstrated that panretinal laser 
photocoagulati

demonstrated that panretinal laser 
photocoagulation applied when an eye approaches or just reaches 
high-risk PDR reduces the risk of severe vision loss to less than 
4%. Based on these results, prompt scatter photocoagulation is 
considered appropriate for all patients with high-risk PDR, usu­
ally appropriate for patients with PDR less than high risk, and 
may be advisable for patients with severe or very severe NPDR, 
especially in the setting of T2DM (see Fig. 37.30).316,398-401
Recent prog

g of T2DM (see Fig. 37.30).316,398-401
Recent progression of eye disease, status of the fellow eye, com­
pliance with follow-up, concurrent health concerns such as hyper­
tension or kidney disease, and other factors must be considered in 
determining if laser surgery should be performed in these patients. 
In particular, patients with T2DM should be considered for scat­
ter photocoagulation before high-risk PDR develops because the 
risk of severe vision loss and the need for pars plana vitrecto

e vision loss and the need for pars plana vitrectomy 
(PPV) can be reduced by 50% in these patients, especially when 
macular edema is present.401
In scatter photocoagulation, 1200 to 1800 laser burns are 
applied to the peripheral retinal tissue, actually focally destroy­
ing the outer photoreceptor and retinal pigment epithelium of the 
retina (see Fig. 37.27I). Large vessels are avoided, as are areas of 
Follow-up
dependent on
ocular status
Is treatment for
diabetic retinopathy or
macular ede

 treatment for
macular edema indicated?
Follow-up in 3 months
or as indicated by
Focal/grid
laser for DME
Yes
No
Yes
Yes
Yes
Yes
Yes
Is treatment of
macular edema
indicated?
No
Is further laser
apparently
futile?
Consider
novel
therapies
Consider
novel
therapies
PRP for DR
Follow-up in
3-4 months
DR indicated?
still active or
progressive?
No
No
No
Is treatment
of DME still
Is there
retina
left to treat?
Re-treat
DME
Re

ed?
Is there
retina
Re-treat
DME
Re-treat
PRP
Follow-up as
indicated by
• Fig. 37.32  Photocoagulation flow chart. This schematic flow chart details general photocoagulation treat­
ment approaches in patients with diabetic retinopathy or diabetic macular edema. These are only general 
guidelines, and actual treatment choices can be affected by numerous other factors, including findings in 
the same eye or in the contralateral eye and systemic issues. DME, Diabetic ma

alateral eye and systemic issues. DME, Diabetic macular edema; DR, diabetic 
retinopathy; PRP, scatter (panretinal) photocoagulation.
  Progression to Proliferative Diabetic 
Retinopathy by Level of Nonproliferative 
Diabetic Retinopathy
Retinopathy Level
CHANCE OF HIGH-RISK PDR (%)
1 Year
5 Years
Mild NPDR
1
16
3–8
27–39
15
56
Very severe NPDR
45
71
PDR with fewer high-risk 
22–46
64–75
NPDR, Nonproliferative diabetic retinopathy; PDR, proliferative di

rative diabetic retinopathy; PDR, proliferative diabetic retinopathy.
From Aiello LP, Gardner TW, King GL, et al. Diabetic retinopathy. Diabetes Care. 1998;21:143–156.
   
TABLE 37.4
1474
  Recommended General Management of Diabetic Retinopathy
Level of DR
RISK OF PROGRESSION (%)
EVALUATION
TREATMENT
Follow-Up 
(mo)
To PDR (1 yr)
To High-Risk 
PDR (5 yr)
Color 
Photo
OCT
FA
Scatter Laser 
(PRP)
Focal 
Laser
Intravitreal 
VEGF Inhibito

er 
(PRP)
Focal 
Laser
VEGF Inhibitors
Mild NPDR
All
5
15
No DME
No
No
No
No
No
No
12
Non-ciDME
Yes
Yes
Occ
No
Occ
Occ
4–6
ciDME
Yes
Yes
Yes
No
Occ
Yes
1–4
All
12–27
33
No DME
Yes
No
No
No
No
No
6–8
Non-ciDME
Yes
Yes
Occ
No
Occ
Occ
4–6
ciDME
Yes
Yes
Yes
No
Yes
Yes
1–4
All
52
60
No DME
Yes
No
No
Rarely
No
No
3–4
Non-ciDME
Yes
Yes
Occ
Occ after DME 
treatment
Occ
Occ
2–3
ciDME
Yes
Yes
Yes
treatment
Yes
Yes
1–3
Very Severe NPDR
All
75
75
No DME

ment
Yes
Yes
1–3
All
75
75
No DME
Yes
No
No
Occ
No
No
2–3
Non-ciDME
Yes
Yes
Occ
treatment
Occ
Occ
2–3
ciDME
Yes
Yes
Yes
treatment
Yes
Yes
1–3
PDR < High Risk
All
—
75
No DME
Yes
No
No
Occ
No
Occ
2–3
Non-ciDME
Yes
Yes
Occ
treatment
Occ
Occ
2–3
ciDME
Yes
Yes
Yes
treatment
Yes
Yes
1–3
PDR With High-Risk Characteristics
All
—
—
No DME
Yes
No
No
Yes
No
Occ
2–3
Non-ciDME
Yes
Yes
Occ
Yes
Usually
Occ
1–2
ciDME
Yes
Yes
Yes
Yes
Y

es
Occ
Yes
Usually
Occ
1–2
ciDME
Yes
Yes
Yes
Yes
Yes
Yes
1–2
ciDME, Center-involved diabetic macular edema; DME, diabetic macular edema; FA, fluorescein angiography; ME, macular edema; NPDR, nonproliferative diabetic retinopathy; Occ, occasionally; OCT, 
optical coherence tomography; PDR, proliferative diabetic retinopathy; VEGF, vascular endothelial growth factor.
   
TABLE 37.5
1475
preretinal hemorrhage. The treatment is thought to exert its effe

rrhage. The treatment is thought to exert its effect 
by increasing oxygen delivery to the inner retina, decreasing viable 
hypoxic growth factor–producing cells, and increasing the rela­
tive perfusion per area of viable retina. The total treatment is usu­
ally applied over one to three sessions, spaced 1 to 2 weeks apart. 
Follow-up evaluation usually occurs at 3 months.
The response to scatter photocoagulation varies. The most 
desirable effect is to see a regression of the new vessels, altho

t is to see a regression of the new vessels, although 
stabilization of the neovascularization with no further growth can 
result. This latter situation requires careful clinical monitoring. In 
some cases, new vessels continue to proliferate, requiring addi­
tional scatter photocoagulation or adjuvant treatment with intra­
vitreous anti-VEGF agents (see Fig. 37.32).
Recent studies have shown that anti-VEGF treatment with 
aflibercept or ranibizumab is an effective first-line alternative 
to sca

mab is an effective first-line alternative 
to scatter photocoagulation in carefully selected patients with 
PDR375,402,403 Neovascular processes are exquisitely sensitive to 
anti-VEGF agents, and eyes with severe neovascularization of 
the retina or anterior segment have demonstrated dramatic and 
rapid improvement with anti-VEGF therapy.322,404 The Diabetic 
Retinopathy Clinical Research (DRCR) Network Protocol S 
demonstrated that visual acuity outcomes at 2 and 5 years were 
noninferior in 

ty outcomes at 2 and 5 years were 
noninferior in eyes treated with intravitreal ranibizumab to those 
obtained with scatter photocoagulation. Anti-VEGF treatment 
with ranibizumab was associated with several advantages over laser 
therapy. There was greater average visual gain over the course of 
the first 2 years, reduced rates of visual impairment from DME 
onset, less loss of peripheral visual field, and decreased need for 
vitrectomy surgery at both 2 and 5 years in the ranibizumab group.
W

 at both 2 and 5 years in the ranibizumab group.
When delivered by a standardized retreatment algorithm such 
as that developed for Protocol S (Fig. 37.34), anti-VEGF therapy 
is highly effective at reducing retinal neovascularization. In Pro­
tocol S, 38% of the eyes treated with ranibizumab were eligible 
to defer monthly injections due to sustained stability of neovas­
cularization at least once over the first 2 years of follow-up, and 
retinal neovascularization was completely resolved at 44

l neovascularization was completely resolved at 44% of visits 
through 2 years.405 Nonetheless, eyes randomized to ranibizumab 
treatment in Protocol S continued to need a median of three injec­
tions per year through 2 to 5 years. It should also be realized that 
patients with PDR often have systemic comorbidities that may 
contribute to missed or rescheduled office visits. Thus patient 
compliance with follow-up and treatment recommendations is a 
critical aspect of ensuring successful outcome

 a 
critical aspect of ensuring successful outcomes with anti-VEGF 
therapy for PDR. If compliance is expected to be a problem for 
a specific patient, then treatment with laser panretinal photoco­
agulation alone, or in combination with a VEGF inhibitor, is the 
preferred therapeutic approach.
Surgical intervention with PPV is usually reserved for eyes 
with nonclearing vitreous hemorrhage or traction detachment 
from PDR. The DRVS, completed in 1989, demonstrated that 
early PPV in persons wit

 1989, demonstrated that 
early PPV in persons with severe fibrovascular proliferation was 
more likely to result in better vision and less likely to result in 
poor vision, particularly in patients with T1DM.377 PPV aims 
primarily to remove abnormal fibrovascular tissue, alleviate retinal 
traction, allow the retina to obtain a more anatomically normal 
position, and remove vitreous opacities such as vitreous hemor­
rhage. The actual outcome data from this study is not entirely 
applicable tod

ta from this study is not entirely 
applicable today due to dramatic advances in surgical techniques 
and the advent of laser endophotocoagulation that have occurred 
in the intervening years. Nevertheless, it is clear that PPV can save 
and restore vision in many cases of severe retinal disease that are 
not amenable, or not responsive, to laser photocoagulation. 
Diabetes
No
No
No
have macular edema?
Will patient
treatment be altered
by angiogram
results (rare)?
Is focal/grid lase

ed
Is focal/grid laser
planned, if possible?
Yes
No
Yes
Yes
Yes
Fluorescein
angiogram usually
not indicated
angiogram
often indicated
Is patient pregnant
or allergic to
fluorescein dye?
• Fig. 37.33  Fluorescein angiogram flow chart. The schematic flow chart 
details a general algorithm for appropriate use of fluorescein angiogra­
phy in the ocular evaluation of patients with diabetes mellitus. In unusual 
cases, confounding factors can alter the appropri

cases, confounding factors can alter the appropriate approach.
Assessment 1 Month
After Initial
Injections(s)
No Injection and
Return in 1 Month
Yes
Yes
NV Stable or
Resolved?
Re-inject and Return
in 1 Month
NV Worsens
or Recurs
Double Follow-Up
Intervals Up to 4
Months
No
No
• Fig. 37.34  Principles of the Diabetic Retinopathy Clinical Research 
Network (DRCR.net) antivascular endothelial growth factor treatment 
algorithm for proliferative diabetic retinopathy. NV, Neovascularization. 
(Redra

betic retinopathy. NV, Neovascularization. 
(Redrawn from Sun JK, Glassman AR, Beaulieu WT, et al. Rationale and 
application of the protocol S anti-vascular endothelial growth factor algo­
rithm for proliferative diabetic retinopathy. Ophthalmol. 2018;125.)
1476
Treatment of Nonproliferative Diabetic Retinopathy
Studies of intravitreous therapy with VEGF inhibitors and ste­
roids have demonstrated beneficial effects of these medicatio

demonstrated beneficial effects of these medications on 
DR severity and rates of progression to PDR-related complica­
tions such as vitreous hemorrhage and need for vitrectomy. Nearly 
40% of eyes achieve two or more step regression of diabetic reti­
nopathy severity after 3 years of monthly continuous anti-VEGF 
therapy for DME.406–408 Decreased rates of PDR onset and need 
for PDR treatment are also observed after ranibizumab therapy as 
compared to sham treatment.407 After an average of 4.4 

red to sham treatment.407 After an average of 4.4 injections 
of aflibercept over 6 months, 58% of eyes experienced a two or 
more step improvement in diabetic retinopathy severity as com­
pared to 6% of sham-treated eyes.408 Steroid therapy, whether 
delivered by the peribulbar or intravitreal route, also resulted in 
decreased risk of diabetic retinopathy worsening. Over 2 years, 
intravitreal steroid administration resulted in a 32% relative risk 
reduction for retinopathy progression as comp

isk 
reduction for retinopathy progression as compared to sham treat­
ment.407 Although these results are promising, there is no treat­
ment mandate at this time for eyes with nonproliferative diabetic 
retinopathy since long-term visual outcomes have not been defini­
tively shown to be superior after treatment with any of these intra­
ocular therapeutic approaches versus medical care alone. 
Treatment of Diabetic Macular Edema
Untreated CSME is associated with an approximately 25% chance 
of mo

associated with an approximately 25% chance 
of moderate vision loss after 3 years (defined as at least doubling 
the visual angle; e.g., 20/40 reduced to 20/80).316 Macular edema 
is best evaluated clinically by dilated examination using slit-lamp 
biomicroscopy or stereo fundus photography. The newer diagnos­
tic ophthalmic imaging technique of OCT has provided a means 
to objectively quantify retinal thickening and is currently the 
objective method of choice. When used in conjunction with vi

method of choice. When used in conjunction with visual 
acuity measurement, OCT has been used to monitor response to 
treatment and help determine timing of intervention.409,410 As 
discussed earlier, the current standard for macular edema evalu­
ation focuses on determining the presence or absence of edema 
involving the center of the macula.
Current first-line therapy for most eyes with center-involved 
DME and DME-related visual impairment of 20/32 or worse is 
intravitreous injections of VEG

20/32 or worse is 
intravitreous injections of VEGF inhibitors following a defined 
protocol such as that described by the DRCR Network291 (Fig. 
37.35). Typically, injections are performed monthly with a load­
ing dose of at least four to six injections. On average, eight to nine 
injections are performed in the first year of treatment. The average 
number of injections required to maintain the beneficial visual 
acuity gains declines substantially to three or four in the second 
year, one or t

lly to three or four in the second 
year, one or two in the third year, and zero to one in years 4 and 5.
Results from an initial multicenter randomized controlled 
clinical trial evaluating intravitreal administration of ranibizumab 
either with immediate laser or with deferred laser show a marked 
benefit of the anti-VEGF agent compared to laser alone. After 1 
year, ranibizumab as applied in the trial resulted in a nine-letter 
mean gain (p < 0.001) when combined either with prompt laser 
or 

0.001) when combined either with prompt laser 
or deferral of laser for at least 24 weeks. This was more effective 
than prompt laser alone (three-letter gain) for the treatment of 
center-involving DME in eyes with central thickening and vision 
reduced to 20/32 to 20/320. The number of eyes gaining two or 
more lines of vision almost doubled in the anti-VEGF groups as 
compared with laser alone. Conversely, eyes losing two or more 
lines of vision were about one-third as many in the anti-VEGF 

ion were about one-third as many in the anti-VEGF 
groups as compared to laser alone. Results were sustained over 
5 years despite a substantially decreased number of injections 
throughout years 2 to 5.411,412 No increased systemic events or 
serious ocular adverse events were attributed to the treatment 
other than the known small risk of endophthalmitis (1 per 1000 
injections) associated with intravitreal injections themselves.
A head-to-head comparative effectiveness trial demonstrated 
ove

 comparative effectiveness trial demonstrated 
overall efficacy of all three currently commercially available intra­
ocular VEGF inhibitors (aflibercept, bevacizumab, and ranibi­
zumab) in improving visual acuity and reducing retinal thickening 
in eyes with center-involved DME.291 Eyes with only mild visual 
impairment from DME and baseline vision of 20/32 to 20/40 
had similar visual acuity outcomes on average regardless of thera­
peutic group assignment. However, in eyes with baseline vision 

assignment. However, in eyes with baseline vision of 
20/50 or worse, aflibercept provided visual gains superior to those 
achieved with bevacizumab at 1 and 2 years, and vision outcomes 
were superior to ranibizumab at 1 but not 2 years.
Although VEGF inhibitors are highly successful in many eyes, 
approximately 40% to 50% of eyes still have persistent DME 
or visual impairment despite chronic monthly therapy. Thus 
there is a continued unmet need for novel therapies that address 
VEGF-independ

ed for novel therapies that address 
VEGF-independent mechanisms for DME onset, persistence, and 
worsening.
Focal laser photocoagulation may still be indicated for some 
patients with DME (see Figs. 37.27C and 37.30) without center 
involvement or in eyes with good vision or in patients who cannot 
tolerate a regimen of intravitreal injections. The ETDRS demon­
strated focal laser photocoagulation for CSME reduced the 5-year 
risk of moderate vision loss by 50%, but 15% of patients con­
tinue t

sion loss by 50%, but 15% of patients con­
tinue to experience vision loss.345 In focal laser photocoagulation, 
lesions from 500 to 3000 μm from the center of the macula that 
are contributing to thickening of the macular area are generally 
Injection(s)
No
Yes
DME
Improving
in 1 Month
DME Worsens
or Recurs
Double Follow-up
Interval Up to 4
Months
Yes
No
• Fig. 37.35  Diagram showing the Diabetic Retinopath

ig. 37.35  Diagram showing the Diabetic Retinopathy Clinical Research 
Network (DRCR.net) rationale for treatment and follow-up of center-
involved diabetic macular edema (DME) with antivascular endothelial 
growth factor therapy. (Modified from Aiello LP, Beck RW, Bressler NM, 
et  al. Rationale for the Diabetic Retinopathy Clinical Research Network 
intravitreal anti-VEGF treatment and follow-up protocol for center-involved 
diabetic macular edema. Ophthalmol. 2011;118:e5–e14.)
1477
Chapter 37

ema. Ophthalmol. 2011;118:e5–e14.)
1477
directly photocoagulated. These lesions are identified clinically or 
by fluorescein angiography and consist primarily of leaking micro­
aneurysms. When leakage is diffuse or microaneurysms are exten­
sive, photocoagulation may be applied to the macula in a grid 
configuration, avoiding the fovea region.
If macular laser is planned, fluorescein angiography can be a 
useful test for guiding the focal treatment 

be a 
useful test for guiding the focal treatment of microaneurysm that 
are leaking fluid into the retina and identifying areas of macular 
capillary nonperfusion that might benefit from grid laser treat­
ment (see Fig. 37.33). Because there are risks associated with fluo­
rescein angiography, including nausea, urticaria, hives, and rarely 
death (1 in 222,000 patients) or severe medical sequelae (1 in 2000 
patients),413–415 fluorescein angiography is not otherwise part of 
the routine examina

aphy is not otherwise part of 
the routine examination of a diabetic patient without diabetic reti­
nopathy, and the procedure is usually contraindicated in patients 
with known allergy to fluorescein dye or during pregnancy.
Follow-up evaluation after focal laser treatment generally 
occurs after 3 months (see Fig. 37.30). In cases in which mac­
ular edema persists, further treatment may be necessary. In the 
presence of macular edema, patients with PDR or severe NPDR 
should be considered for 

with PDR or severe NPDR 
should be considered for anti-VEGF or laser treatment of macu­
lar edema whether or not the macular edema is center involved 
because they are likely to require scatter laser photocoagulation in 
the near future and because scatter photocoagulation, while ben­
eficial for PDR, can exacerbate existing macular edema.
The ophthalmic use of corticosteroids administered either through 
the periocular or intravitreal routes for the treatment of DME gained 
widespread use due t

 the treatment of DME gained 
widespread use due to early case reports and uncontrolled clini­
cal trials documenting its rapid and often dramatic effect on retinal 
thickening. Two multicenter randomized prospective clinical trials 
were undertaken to address both the effectiveness and safety of both 
routes of steroid administration. In the first, peribulbar steroid injec­
tions were found to have no significant benefit for the treatment of 
DME.416 The 3-year results of a second multicenter r

E.416 The 3-year results of a second multicenter randomized con­
trolled trial comparing intravitreal steroids to focal laser therapy have 
shown that despite an initial rapid reduction in retinal thickness and 
improvement in vision with the intravitreal steroid injection, by 1 
year the results were no better than laser photocoagulation, and at 
2 years through 3 years, steroid was inferior to the laser treatment 
in both visual outcome and retinal thickness improvement.417,418 
Intravitreal s

inal thickness improvement.417,418 
Intravitreal steroid injections were associated with an approximately 
fourfold increase in the rate of intraocular pressure complications and 
fourfold increase in need for cataract surgery compared to laser treat­
ment. The development of cataract was likely a large contributor to 
declining visual acuity in the steroid-treated group after the first 6 
months. This hypothesis is supported by the fact that patients who 
had already undergone cataract surgery 

ients who 
had already undergone cataract surgery prior to initiating intravitreal 
steroid treatment demonstrated vision improvement comparable to 
that seen in the anti-VEGF-treated group.375
Intravitreous steroid therapy has also been investigated as a 
potential additive therapy to intravitreous VEGF inhibitors in 
patients who have persistent retinal thickening and visual impair­
ment despite at least six injections of anti-VEGF. Although retinal 
thickening is significantly reduced in eyes

tinal 
thickening is significantly reduced in eyes after combined steroid 
and anti-VEGF therapy compared to those receiving anti-VEGF 
treatment alone, visual acuity outcomes do not appear to be 
improved.419 Thus, currently, intravitreal steroid alone is not the 
preferred primary therapy for DME or a recommended adjuvant 
therapy in eyes that have not responded successfully to anti-VEGF 
therapy. Intraocular steroid treatment may have a role, however, 
for treatment of patients with DME who c

however, 
for treatment of patients with DME who cannot receive anti-
VEGF agents or who are pseudophakic before treatment.
The pathogenesis of DME is highly complex, and a variable 
response to treatment modalities has been observed in many 
patients. Investigations of varying drug dosages to identify the 
optimal treatment concentration and development of sustained 
drug delivery devices to limit risks, costs, and inconvenience 
associated with repeated intraocular injections are underway. 
Mu

 repeated intraocular injections are underway. 
Multiple potential therapeutics are also being tested in clinical 
trials for the treatment of DME, including a bispecific antibody 
against VEGF and angiotensin II, plasma kallikrein inhibition, 
photobiomodulation therapy, and other novel approaches. 
Control of Systemic Disorders and Effect of Systemic 
In addition to the importance of intensive glycemic control in 
reducing the onset and progression of diabetic retinopathy as dis­
c

 and progression of diabetic retinopathy as dis­
cussed earlier, it is critical for the optimal ocular health of diabetic 
patients that several other systemic considerations be optimized.
Elevated blood pressure exacerbates the development and 
progression of diabetic retinopathy. Concomitant hypertension 
is common in diabetes. Patients with T1DM have a 17% preva­
lence of hypertension at baseline and a 25% incidence after 10 
years.420 There is a 38% to 68% prevalence in T2DM.421–423 In 
most

s a 38% to 68% prevalence in T2DM.421–423 In 
most studies, hypertension correlates with other retinopathy risk 
factors, including duration of diabetes, higher HbA1c level, pres­
ence of proteinuria, and male gender. The risk of PDR is associ­
ated with the presence of hypertension at the baseline visit, higher 
HbA1c levels, and presence of more severe levels of retinopathy 
at the initial visit.424 Patients with hypertension are more likely 
to develop retinopathy, diffuse macular edema, and 

o develop retinopathy, diffuse macular edema, and more severe 
levels of retinopathy425–427 and have more rapid progression of 
retinopathy when compared with diabetic patients who do not 
have hypertension.427–429
The large randomized, prospective UKPDS in 1148 patients 
with T2DM demonstrated a 34% (p = 0.0004) and 47% (p < 
0.004) reduction in risk of diabetic retinopathy progression and 
moderate visual acuity loss, respectively, in patients assigned to 
intensive blood control.430 Effects w

assigned to 
intensive blood control.430 Effects were independent of glycemic 
control, and risk reductions were similar regardless of whether 
the hypertension was controlled with an angiotensin-converting 
enzyme (ACE) inhibitor (captopril) or a beta blocker (atenolol). 
Overall, hypertension appears to be a significant risk factor in the 
development and progression of diabetic retinopathy and should 
be rigorously controlled. Until the results of specific trials inves­
tigating the blood pre

s of specific trials inves­
tigating the blood pressure levels required to minimize end-organ 
damage in patients with diabetes are known,431 target blood pres­
sure should most likely be maintained as low as safely possible.
Associations between renal and retinal angiopathy are numer­
ous. Both microalbuminuria and proteinuria are associated with 
retinopathy.432–435 The presence and severity of diabetic retinopa­
thy are indicators of the risk of gross proteinuria,436,437 and con­
versely, pro

of gross proteinuria,436,437 and con­
versely, proteinuria predicts PDR.433,438,439 Half of all T1DM 
patients with PDR and 10 or more years of diabetes have concom­
itant proteinuria.432 In T1DM, the prevalence of PDR increases 
from 7% at onset of microalbuminuria to 29% 4 years after onset 
of albuminuria, compared with 3% and 8%, respectively, in 
patients without persistent microalbuminuria.440 The Appropriate 
Blood Pressure Control in Diabetes (ABCD) trial found both the 
severity and pro

etes (ABCD) trial found both the 
severity and progression of retinopathy were associated with overt 
albuminuria.441–443 The presence of gross proteinuria at baseline is 
associated with 95% increased risk of developing macular edema 
among patients with T1DM,424 and dialysis can improve macular 
edema in diabetic patients with renal failure.
1478
Despite these associations, a causal relationship between diabetic 
kidney disease and d

elationship between diabetic 
kidney disease and diabetic retinopathy has not been established. 
The frequent coexistence of retinal and renal microangiopathies 
with factors that affect both complications, such as associated 
hypertension and disease duration, make it difficult to establish.444 
Overall, it is important to carefully consider the renal status of any 
patient with diabetes and to ensure that the patient is receiving opti­
mal care in this regard. In addition, rapidly progressive 

 in this regard. In addition, rapidly progressive retinopathy, 
especially in a patient with a long history of diabetes where retinop­
athy previously has been stable, should suggest the need for renal 
Low hematocrit was an independent risk factor in the 
ETDRS analysis of baseline risk factors for development of 
high-risk PDR and severe vision loss.445 A cross-sectional study 
involving 1691 patients revealed a twofold increased risk of any 
retinopathy in patients with a hemoglob

sk of any 
retinopathy in patients with a hemoglobin level less than 12 g/
dL compared to those with a higher hemoglobin concentration, 
using multivariate analyses controlling for serum creatinine, 
proteinuria, and other factors.446 In patients with retinopathy, 
those with low hemoglobin levels have a fivefold increased risk 
of severe retinopathy compared with those with higher hemo­
globin levels. There have been limited reports of resolution of 
macular edema and hard exudate with improvem

n of 
macular edema and hard exudate with improvement or stabiliza­
tion of visual acuity in erythropoietin-treated patients after an 
increase in mean hematocrit.447 In view of the potential associa­
tion of low hematocrit and diabetic retinopathy, it is important 
to ensure that patients with diabetic retinopathy and anemia are 
receiving appropriate management.
In summary, diabetes is clearly a multisystem disease requir­
ing a comprehensive medical team approach. Even with regard to 
ocular 

edical team approach. Even with regard to 
ocular health, this necessitates the involvement of multiple health 
care specialists for optimal patient care. 
Diabetic Nephropathy
Diabetic nephropathy remains a major cause of morbidity and 
death for persons with either T1DM or T2DM. In Western 
countries, diabetes is the leading single cause of end-stage renal 
disease.210 Indeed, in many countries such as the United States, 
more than 50% of patients in renal replacement therapy programs 
have di

nts in renal replacement therapy programs 
have diabetes as the major cause of their renal failure. However, 
the full impact of diabetic nephropathy is far greater.448 Glob­
ally most patients with diabetes are in developing countries that 
do not have the resources or health infrastructure to provide uni­
versal renal replacement therapy.449 Even in developed countries, 
for every 20 patients with diabetes and chronic kidney disease, 
less than one will survive to end-stage renal disease, succ

 one will survive to end-stage renal disease, succumbing 
instead to atherosclerotic cardiovascular disease, heart failure, or 
infection. Furthermore, the presence and severity of diabetic renal 
disease significantly contributes to the risk of these conditions. 
For example, almost all of the excess in cardiovascular deaths in 
persons with diabetes younger than 50 years can be attributed to 
nephropathy.450 Indeed, in T1DM subjects without nephropathy 
there may be a lower risk of premature d

hropathy 
there may be a lower risk of premature death,451 although a recent 
review of the FinnDiane cohort indicates that in nonalbuminuric 
T1DM subjects there is still evidence of increased mortality.452 In 
patients with T2DM, microalbuminuria is associated with a two­
fold to fourfold increase in the risk of death. In patients with overt 
proteinuria and hypertension, the risk is even higher.453 Conse­
quently, the goal to reduce ESRD in patients with diabetes is only 
one component as par

tients with diabetes is only 
one component as part of the overall benefit in preventing diabetic 
kidney disease.
It is estimated that 25% to 40% of patients with T1DM and 
5% to 40% of patients with T2DM ultimately develop some fea­
tures of diabetic kidney disease.454,455 Over 20% of patients with 
T2DM already have diabetic kidney disease when they are diag­
nosed with diabetes,456 and a further 20% to 40% develop dia­
betic nephropathy, mostly within 10 years of diagnosis. Although 
nephrop

ly within 10 years of diagnosis. Although 
nephropathy appears be more common in T1DM, because of the 
large and increasing number of persons with T2DM,449,457 more 
than 80% of diabetic patients in renal replacement programs have 
T2DM.
Natural History of Nephropathy in Type 1 
Diabetes
Nephropathy and specifically proteinuria in the setting of diabetes 
have been known for more than 100 years, and the classic struc­
tural features of glomerulosclerosis were described more than 70 
years ago.45

clerosis were described more than 70 
years ago.458 However, it is only since the 1980s that the natu­
ral history of this condition has been extensively delineated. This 
is partly because significantly more patients are surviving to see 
the full presentation of this condition. For example, in 1971, the 
median survival time of patients with T1DM and overt nephropa­
thy was 5 years, with less than 10% surviving more than 10 years. 
Consequently, few patients were able to survive the course of 

, few patients were able to survive the course of their 
renal disease. By comparison, in 1996, the median survival in 
an equivalent population was more than 17 years. Not surpris­
ingly, nearly 10 times more patients with T1DM are now entering 
ESRD programs.
Diabetic nephropathy is characterized clinically as a triad of 
hypertension, proteinuria, and ultimately renal impairment.459 
The classic five stages of nephropathy as described by Mogensen 
and colleagues,460 although not totally accur

sen 
and colleagues,460 although not totally accurate, remain the best 
way of describing this condition (Fig. 37.36). This description 
relies on functional evaluation of the renal disease and is based 
on serial measurements of glomerular filtration rate (GFR) and 
albuminuria.
Stage 1: Hyperfiltration
The initial phase has been termed the hyperfiltration phase. It is 
associated with an elevation of glomerular filtration rate460 and 
presumably an increase in capillary glomerular pressure. Al

y an increase in capillary glomerular pressure. Although 
invariably present in animal models of T1DM,461 an elevation of 
GFR occurs in up to 40% of type 1 diabetic patients. Hyper­
filtration is considered to occur as a result of concomitant renal 
hypertrophy462 and partly due to a range of intrarenal hemody­
namic abnormalities that occur in the diabetic milieu that con­
tribute to glomerular hypertension.461 The pathophysiology of 
Hyperfiltration
Silent
Microalbuminuria
Macroalbuminuria
Re

ration
Silent
Renal impairment
Glomerular
filtration rate
=/
=/
/
/
Blood
pressure
• Fig. 37.36  The phases (natural history) of diabetic nephropathy.
1479
renal hypertrophy associated with diabetes remains unexplained, 
although specific growth factors such as the growth hormone 
(GH)/insulin-like growth hormone (IGF1) system and transform­
ing growth factor β (TGFβ) have been implicated.463,464 Notably, 
there is 

) have been implicated.463,464 Notably, 
there is not only glomerular but also tubular hypertrophy. Indeed, 
the tubular hypertrophy explains the increased kidney weight in 
diabetes because tubules make up more than 90% of the kidney 
weight.465 In addition, increased salt reabsorption associated with 
proximal tubular hypertrophy can also contribute to glomerular 
hyperfiltration via tubuloglomerular feedback.463
The second explanation for the increase in GFR associated 
with diabetes relates 

increase in GFR associated 
with diabetes relates to hemodynamic changes within the kidney. 
Although not directly tested in humans, micropuncture studies 
in rodents, particularly by Brenner’s group in the 1980s, revealed 
that experimental diabetes was associated with a range of intrare­
nal hemodynamic changes.461 Alongside hyperfiltration, there is 
an increase in effective renal plasma flow, and thus some investi­
gators call this the hyperperfusion-hyperfiltration phase of diabetic 
nephro

erfusion-hyperfiltration phase of diabetic 
nephropathy. At the same time, intraglomerular capillary pressure 
is increased, reflecting relative efferent versus afferent arteriolar 
vasoconstriction461 with activation of the intrarenal renin-angio­
tensin system (RAS) and reduced synthesis of the vasodilator 
nitric oxide.
The importance of this hyperfiltration phase in predict­
ing and leading to diabetic nephropathy remains controversial. 
Several groups have confirmed the initial relationship

ral groups have confirmed the initial relationship between 
elevated GFR and later development of proteinuria as described 
by Mogensen and Christensen.466 However, this has not been a 
universal finding. Nevertheless, subsequent studies with anti­
hypertensive agents, and in particular agents that interrupt the 
renin-angiotensin system, have shown attenuation of some of 
these glomerular hemodynamic abnormalities.467 This provides 
justification to consider that at least some of these intraren

n to consider that at least some of these intrarenal 
hemodynamic changes in diabetes play a role in the development 
and progression of nephropathy. However, in insulin-dependent 
diabetic patients the ACE inhibitor captopril had the smallest 
effect (17%) in reducing the risk of doubling serum creatinine in 
patients with serum creatinine of 1 mg/dL and the largest effect 
(76%) in patients whose serum creatinine was 2 mg/dL. These 
data do not seem to support a primary role for early hyperfil

 seem to support a primary role for early hyperfiltra­
tion in the development of early diabetic nephropathy. Results 
suggest that in patients with type 2 diabetes with advanced renal 
disease, a link to hyperfiltration was not apparent.468,469
This field has recently been reactivated with the advent of 
sodium glucose cotransporter-2 (SGLT2) inhibitors, which influ­
ence tubuloglomerular feedback and have been shown to reduce 
intraglomerular pressure via effects on afferent arteriolar dila­
t

ressure via effects on afferent arteriolar dila­
tion.470 Indeed, these agents have impressive effects in attenuating 
diabetes-associated hyperfiltration, as reported initially in T1DM 
but subsequently in T2DM.471,472 Whether these hemodynamic 
benefits explain, at least in part, the renoprotection seen with 
these agents in large clinical studies, including Empagliflozin 
Cardiovascular Outcome Event Trial in Type 2 Diabetes Mellitus 
Patients–Removing Excess Glucose (EMPA-REG OUTCOME) 
and C

–Removing Excess Glucose (EMPA-REG OUTCOME) 
and Canagliflozin Cardiovascular Assessment Study (CANVAS), 
remains to be determined.473,474 
Stage 2: The Silent Stage
The next stage is known as the silent stage, where, from a clini­
cal point of view, there is no overt evidence of any form of renal 
dysfunction. Patients usually have normal GFR with no evidence 
of albuminuria. However, this phase is associated with significant 
structural changes, including basement membrane thickening and 
mesa

, including basement membrane thickening and 
mesangial expansion. More recently, loss of the glomerular cells 
known as podocytes has also been described as a relatively early 
structural change in the diabetic kidney. Indeed, by performing 
detailed quantitative studies of renal morphology it is often possible 
to detect those who will develop renal damage.475 This is a very 
important phase clinically because it is hoped that investigators will 
be able to develop new tests, such as biomarker

l 
be able to develop new tests, such as biomarkers in plasma or urine 
or sophisticated assessments from renal biopsy material, to identify 
which patients will progress to more advanced renal disease. Because 
overall less than 40% of T1DM subjects will progress, it is critical 
that we detect those who are the potential progressors and could 
be candidates for early prevention and treatment strategies to avoid 
end-stage renal disease. As yet, no reliable clinically translatable sur­
rogate m

 no reliable clinically translatable sur­
rogate markers or predictors have been identified as worth pursuing 
in clinical practice during this silent phase of the disease.
Extensive studies have tested various plasma markers, such 
as prorenin,476 tumor necrosis factor α (TNFα) receptor,477 
advanced glycation end products,478 and precursors such as meth­
ylglyoxal,479 but the predictive value has not been conclusive. 
Although diabetic kidney disease probably has at least in part a 
genetic ba

isease probably has at least in part a 
genetic basis, few potentially relevant gene polymorphisms such as 
the angiotensin-converting enzyme gene have been identified.480 
Recent genome-wide association studies have not generally been 
very helpful.481,482 The measurement of albumin fragments (ghost 
albumin) in the urine of patients with diabetes may be another, 
albeit unproven, approach.483 Serial prospective ambulatory 
blood pressure monitoring studies have also demonstrated mod­
est rises

ring studies have also demonstrated mod­
est rises in blood pressure in patients in this silent phase up to 
5 years before urinary albumin excretion begins to increase.484 
However, none of these markers has been proved to be sensitive 
or specific enough on further clinical evaluation for widespread 
clinical application. 
Stage 3: Microalbuminuria
The third phase is known as microalbuminuria or the stage of 
incipient nephropathy. At this stage, often 5 to 15 years after the 
initial diagnosi

e, often 5 to 15 years after the 
initial diagnosis of T1DM, the urinary albumin excretion rate has 
increased into the microalbuminuric range of 20 to 200 μg per 
minute or 30 to 300 mg per 24 hours.485 In the past, microal­
buminuria was considered to be a predictor rather than a mani­
festation of diabetic kidney disease. Increasingly it has been 
appreciated, particularly when based on interpretation of renal 
morphologic studies, that in the microalbuminuric phase there is 
often but not al

 microalbuminuric phase there is 
often but not always widespread evidence of advanced glomerular 
structural changes.486,487 Concomitant with these changes, sys­
tolic and diastolic blood pressures are increased. Furthermore, the 
nocturnal dip in blood pressure seen in normal persons is often 
lost with the development of microalbuminuria.488 Renal func­
tion during this phase may be increased, normal, or reduced.
The best approach to screen for microalbuminuria remains 
controversial. The ori

r microalbuminuria remains 
controversial. The original studies used 24-hour or overnight urine 
sampling methods. However, a spot urine albumin:creatinine ratio 
in an early-morning urine specimen has been validated and appears 
to be a practical option for routine clinical practice.210,485 Because 
the onset of persistent microalbuminuria, if left untreated, is often 
a reliable harbinger of overt nephropathy,466 it is incumbent on 
clinicians to perform serial measurements of this parameter a

to perform serial measurements of this parameter and to 
repeat the measurement if there is an isolated elevation in urinary 
albumin excretion.
Studies suggest that in many patients with T1DM, microalbu­
minuria can be transient and can reverse to normoalbuminuria.489 
1480
Thus the onset of microalbuminuria does not irrevocably seal the 
fate of the patient. A study of 386 patients with persistent micro­
albuminuria at study entry sh

th persistent micro­
albuminuria at study entry showed that regression of microalbu­
minuria occurred in 58% of patients,489 although other groups 
have reported much lower rates of this phenomenon.490 Notably, 
in that study, microalbuminuria of short duration, optimal levels 
of HbA1c (<8%), low systolic blood pressure (<115 mm Hg), and 
low levels of both cholesterol and triglycerides were independently 
associated with the regression of microalbuminuria. Therefore 
screening of diabetic pati

albuminuria. Therefore 
screening of diabetic patients for nephropathy is now recom­
mended to include at least twice-annual measurements of urinary 
albumin concentrations in T1DM patients. 
Stage 4: Macroalbuminuria
The next stage is the macroalbuminuria phase or overt nephropathy. 
This stage represents the phase that has been previously described 
as diabetic nephropathy and is highly predictive of subsequent 
renal failure if left untreated. It is characterized by a urinary albu­
min excret

 It is characterized by a urinary albu­
min excretion rate greater than 300 mg per 24 hours (200 μg/
minute). This phase usually occurs after 10 to 15 years of diabetes, 
but the risk of overt renal disease never truly disappears and can 
appear after 40 or 50 years of T1DM.
There are at least two peaks of incidence of overt nephropathy, 
and this has been termed by some investigators as representing 
slow and fast trackers.491 The key contributors to this marked 
variation in the timing of onse

rs to this marked 
variation in the timing of onset of proteinuria, independent of 
glycemic or blood pressure control, remain elusive, although 
a range of genetic, molecular, and environmental factors have 
been proposed. In association with this increase in proteinuria, 
more than two-thirds of patients have overt systemic hyper­
tension.492 During this phase, if left untreated, blood pressure 
continues to rise, accelerating the decline in GFR, which pro­
motes a further rise in blood pressu

R, which pro­
motes a further rise in blood pressure, creating a vicious cycle of 
progressive renal impairment that ultimately leads to end-stage 
renal disease. 
Stage 5: Uremia
The final uremic phase, which historically occurred in up to 40% 
of T1DM subjects,454 requires the institution of renal replacement 
therapy. As recently as the 1970s, patients with diabetes were not 
considered candidates for renal replacement therapy because of 
their abysmal prognosis. However, improvements in the 

r abysmal prognosis. However, improvements in the manage­
ment of cardiovascular disease and renal replacement options have 
seen the survival on dialysis approach that of patients with renal 
disease from other causes. Many patients with diabetes and end-
stage renal disease are also now considered candidates for renal 
transplantation, which is associated with better outcomes than 
remaining on dialysis. However, there is evidence that the renal 
lesions of diabetes often recur in the transpla

l 
lesions of diabetes often recur in the transplanted kidney, though 
the lead time to develop ESRD means that few kidneys are lost 
through recurrent disease.
Increasingly, single pancreas-kidney (SPK) and pancreas-after-
kidney (PAK) transplantation have become therapeutic options 
for patients with T1DM and end-stage renal disease, and these 
newer options appear to offer advantages over kidney-alone trans­
plantation. In particular there is some evidence that maintaining 
euglycemia followi

some evidence that maintaining 
euglycemia following pancreas transplantation can lead to reso­
lution of many diabetes-related renal lesions such as mesangial 
expansion.493 This reversal is often not apparent until after 10 
years of euglycemia, emphasizing the slow turnover of matrix and 
the potential long-term effects of hyperglycemic memory on the 
kidney. 
Natural History of Nephropathy in Type 2 
Diabetes
The natural history of diabetic nephropathy in patients with 
T2DM is less well und

ephropathy in patients with 
T2DM is less well understood than in patients with T1DM. 
This partly reflects the fact that T2DM is largely a disease of an 
older population, with associated obesity, hypertension, dyslip­
idemia, and high rates of cardiovascular disease, which are all 
factors associated with increased risk of renal disease. In addi­
tion, approximately 7% of patients with T2DM already have 
microalbuminuria at the time of diagnosis. This may be partly 
related to the fact that mo

s. This may be partly 
related to the fact that most of these patients have had untreated 
diabetes for 5 to 10 years (on average) before diagnosis. Within 
5 years of a diagnosis of T2DM, up to 18% of patients have 
microalbuminuria, especially those with poor metabolic control 
and high blood pressure. This has led some investigators to sug­
gest that nephropathy in T2DM is different from that seen in 
patients with T1DM.
However, the natural history of nephropathy in T2DM has 
more similariti

istory of nephropathy in T2DM has 
more similarities than differences from that seen in T1DM. 
Hyperfiltration does occur in T2DM,494 although it has been 
reported to be less commonly observed than in T1DM. This 
observation must be interpreted with caution because GFR 
normally declines with age, and hyperfiltration can still exist, 
although the GFR remains in the normal adult range. Micro­
albuminuria also occurs in T2DM. However, the finding of 
microalbuminuria in T2DM might not be as spec

 of 
microalbuminuria in T2DM might not be as specific for diabetic 
renal disease as described in the seminal studies in T1DM. In 
the context of a very high prevalence of cardiovascular disease, 
microalbuminuria may be more closely associated with nonre­
nal events such as stroke and myocardial infarction.439 Further­
more, incipient or overt cardiac failure, urinary tract infection, 
and urinary obstruction (e.g., enlarged prostate) can also lead to 
microalbuminuria.
Many patients with T2DM

lead to 
Many patients with T2DM and microalbuminuria also 
progress to overt proteinuria. However, it is increasingly 
appreciated that the situation has become more complex, and 
many groups have now described subjects with T1DM495 as 
well as T2DM496 who develop renal impairment with progres­
sive decline in GFR in the absence of significant proteinuria. 
The exact explanation for this phenomenon is unknown, and 
ongoing studies are exploring if these patients have different

ies are exploring if these patients have different 
renal morphologic changes from those with the more clas­
sic syndrome of diabetic nephropathy: overt proteinuria and 
declining GFR. Preliminary studies suggest a prominent vascu­
lar component for this form of nonproteinuric renal dysfunc­
tion. Nonetheless, it appears that the risk of end-stage renal 
disease in patients with T2DM and renal impairment is simi­
lar in the presence or absence of microalbuminuria, underlin­
ing the importance of

 microalbuminuria, underlin­
ing the importance of an estimated GFR in the management 
of patients with T2DM. This has led to many of the national 
and international guidelines now recommending the inclusion 
of regular measurements of serum creatinine and determina­
tion of estimated GFR using a variety of different formulas. 
The frequency of these measurements differs among the vari­
ous guidelines, but at a minimum these measurements should 
be performed on a yearly basis. Importantly, in T2

be performed on a yearly basis. Importantly, in T2DM, many 
subjects will develop a progressive decline in GFR without the 
development of albuminuria. Indeed, recent renal biopsy stud­
ies although not conclusive suggest that there may be a dif­
ference in renal morphologic lesions between albuminuric and 
nonalbuminuric forms of diabetic kidney disease.497 
1481
Pathogenesis of Diabetic Nephropathy
It is likely that many of the mechanisms implicat

It is likely that many of the mechanisms implicated in dia­
betic microvascular complications, in general, play a central 
role in the development and progression of diabetic nephropa­
thy (Fig. 37.37).43 It is evident that hyperglycemia is neces­
sary for the initiation of renal injury, because patients without 
diabetes do not develop this type of nephropathy. Moreover, 
intensive therapy designed to achieve improved glycemic con­
trol is able to attenuate the development of nephropathy, as 

 to attenuate the development of nephropathy, as 
assessed by urinary albumin excretion, although it is not fully 
prevented.334 However, it is now clear that other factors must 
also be involved because continuous florid hyperglycemia is 
not necessarily required for diabetic hyperfiltration and kidney 
growth to occur. Indeed, glomerular hyperfiltration and tubu­
lar hypertrophy can persist in patients with T1DM even after 
euglycemia is achieved through aggressive insulin therapy.498
Other pa

ed through aggressive insulin therapy.498
Other pathways that may be involved in diabetic nephropa­
thy include generation of mitochondrial ROS, accumulation of 
AGEs, and activation of intracellular signaling molecules such as 
PKC.131,499 Many of the seminal studies performed in endothe­
lial cells demonstrating a central role for mitochondrial ROS in 
activating pathways implicated in diabetic vascular complications 
have been reproduced in mesangial cells.500 Advanced glycation, 
which occur

sangial cells.500 Advanced glycation, 
which occurs at an accelerated rate in diabetic patients, is a promi­
nent phenomenon in the kidney. Not only is the kidney the major 
site for excretion of AGEs, but also many of the proteins with a 
long life, such as collagen, are extensively glycated in patients with 
diabetes.501 Furthermore, various AGE receptors (e.g., RAGE) 
have been described in the kidney, which appear to play a role in 
mediating some of the deleterious effects of AGEs, such as 

 some of the deleterious effects of AGEs, such as stim­
ulation of growth factor expression and induction of important 
phenotypic changes within certain renal cell populations to pro­
mote scarring.56 Preliminary studies have used various approaches 
to inhibit renal AGE accumulation and action, including a form 
of soluble RAGE (sRAGE). A range of pharmacologic agents has 
shown promising results, but clinical translation of these findings 
remains to be fully defined.501 Selective PKC isoform

ains to be fully defined.501 Selective PKC isoform inhibitors 
have been evaluated in small clinical trials, but their role in renal 
disease remains controversial.502 Some exciting pilot studies evalu­
ating a number of cytosolic sources of oxidative stress, such as nic­
otinamide adenine dinucleotide phosphate oxidase, suggest that 
certain NADPH oxidase isoforms such as Nox4 may be excellent 
targets for new renoprotective therapies.503 This is strengthened 
by the advent of orally bioavailab

s strengthened 
by the advent of orally bioavailable NADPH oxidase inhibitors,136 
which are now under clinical investigation.
In addition to the mechanisms described here, the diabetic 
kidney appears to be readily modulated by a range of vaso­
active hormones. Indeed, it is increasingly appreciated that 
there may be important interactions between metabolic path­
ways and various hemodynamic factors, including vasoactive 
hormones such as angiotensin II in mediating renal injury 
in diabetes (

tensin II in mediating renal injury 
in diabetes (see Fig. 37.37).504,505 Although many drugs that 
modulate hormone levels or action might not be specific for 
diabetic kidney disease, interruption of the renin-angiotensin 
system appears to be an excellent approach not only for reduc­
ing blood pressure but also for correcting many of the cellular, 
biochemical, hemodynamic, and structural abnormalities seen 
in the diabetic kidney. These agents appear to be very powerful 
antiproteinuric agen

s appear to be very powerful 
antiproteinuric agents at all stages of renal disease, including 
microalbuminuria and macroalbuminuria, although the exact 
mechanism of action remains to be fully defined. Furthermore, 
these agents influence renal structure although these findings 
are primarily deduced from preclinical studies. In addition, as 
seen in the Reduction in Endpoints in Noninsulin-Dependent 
Diabetes Mellitus with the Angiotensin II Antagonist Losartan 
(RENAAL) study,506 they also a

ntagonist Losartan 
(RENAAL) study,506 they also attenuate decline in renal func­
tion in type 2 subjects with advanced renal disease. Based on 
the discovery in the late 1990s that proteinuria in a range of 
nephropathies could occur as a result of molecular and struc­
tural abnormalities in a highly specialized structure known as 
the slit diaphragm within the glomerular epithelial cell (podo­
cyte), a number of experimental studies, subsequently con­
firmed in humans, showed that depletion of

ly con­
firmed in humans, showed that depletion of one of these slit 
pore proteins, nephrin, could be attenuated or prevented by 
agents that interrupt the RAS.507
In addition to promoting glomerular nephrin depletion, angio­
tensin II also appears to have other actions that promote the devel­
opment of proteinuria, including trophic effects on the kidney 
and increasing glomerular membrane pore size.508 Although many 
investigators have focused on the RAS and, in particular, the vaso­
constric

 on the RAS and, in particular, the vaso­
constrictor angiotensin II, it is increasingly appreciated that other 
vasoconstrictors may be important. These include endothelin and 
a number of vasodilators, such as nitric oxide, bradykinin, atrial 
natriuretic peptide, and vasodilatory angiotensins such as angio­
tensin 1-7.509 This exploration of the role of vasoactive hormones 
and their respective receptors in the diabetic kidney is critical for 
designing new treatments for this condition, beca

designing new treatments for this condition, because these path­
ways are ideal targets for drug development. This point has already 
been demonstrated for agents that interrupt the renin-angiotensin 
system, including ACE inhibitors and ARBs. However, the use 
of dual RAS inhibition, specifically the combination of an ACE 
inhibitor and an ARB, despite positive effects on albuminuria,510 
is not recommended due to increased risks of hyperkalemia and 
acute kidney injury, as was also seen with 

a and 
acute kidney injury, as was also seen with RAS blockade with a 
combination of a renin inhibitor and an ARB.511 
Pathology of Diabetic Renal Disease
Diabetic renal disease was originally described as a glomeru­
lopathy associated with diffuse or nodular glomerulosclerosis.458 
Subsequent studies using electron microscopy have revealed 
that glomerular basement membrane thickening and mesangial 
expansion are prominent glomerular abnormalities in diabe­
tes475 (Fig. 37.38). Indeed, prospec

ies in diabe­
tes475 (Fig. 37.38). Indeed, prospective studies have shown that 
these changes predict to a certain degree the development of 
overt renal disease in patients with T1DM. However, fewer than 
one-third of diabetic patients with microalbuminuria have the 
Metabolic
Hemodynamic
Growth factors and cytokines
Intracellular signaling molecules
Glucose
Advanced glycation
Oxidative stress
Flow/pressure
Renin angiotensin
• Fig. 37.37  Interactions between metabolic an

in
• Fig. 37.37  Interactions between metabolic and hemodynamic factors in 
promoting diabetic complications, including nephropathy.
1482
typical glomerulopathy described by Kimmelsteil and Wilson 
in 1936.458,512 Although initial studies emphasized the mesan­
gial cell changes in the glomerulus, glomerular epithelial cell 
abnormalities represent new areas of active research.507 Podo­
cyte dysfunction and subsequent apoptosis, ultimat

cyte dysfunction and subsequent apoptosis, ultimately leading 
to depletion of podocytes within the glomerulus appears to play 
a pivotal role in the development of proteinuria in diabetes.513
Although most of the focus has been on glomerular changes in 
the diabetic kidney, more recent studies have identified important 
changes in the other sites within the kidney, including the tubules, 
interstitium, medulla, and papilla.465 Diabetic tubulopathy is 
characterized by a variety of structural an

hy is 
characterized by a variety of structural and functional changes, 
including tubuloepithelial cell hypertrophy, tubular basement 
membrane thickening, epithelial-mesenchymal transition,514 and 
the accumulation of glycogen (see Fig. 37.38). There is also an 
expansion of the interstitial space with infiltration of various cell 
types, including myofibroblasts and macrophages.
These tubular changes represent more than just the aftermath 
of diabetic nephropathy. The dysregulation of tubular

diabetic nephropathy. The dysregulation of tubular functions 
in diabetes can precede or at least accompany the changes in 
the renal glomerulus and the onset of albuminuria.512 Indeed, 
the functional and structural changes in the proximal tubule 
may be key to contributing to the development and progression 
of diabetic nephropathy.465 For example, it has been suggested 
that tubuloglomerular feedback mechanisms can drive hyper­
filtration associated with diabetes462 and that tubular dysfunc­

ciated with diabetes462 and that tubular dysfunc­
tion can contribute to albuminuria due to defective uptake and 
lysosomal processing.483 Indeed, renal function and prognosis 
correlate better with structural lesions in the tubules and corti­
cal interstitium than with classic glomerular changes of diabetic 
nephropathy. 
Other Renal Manifestations of Diabetes
Renal Artery Stenosis
Because diabetic patients have, in general, an increased burden of 
atherosclerosis, they appear to have a higher 

of 
atherosclerosis, they appear to have a higher risk of renal artery ste­
nosis. However, although angiographic studies have demonstrated 
a high prevalence of renal artery stenosis in diabetic patients, these 
lesions are often of no hemodynamic significance. Nevertheless, a 
small subgroup will have a hemodynamically significant stenosis, 
enhancing hypertension, increasing the risk of acute pulmonary 
edema, and inducing progressive renal impairment.515 In such 
subjects, specific intervent

pairment.515 In such 
subjects, specific interventions such as surgery or angioplasty need 
to be considered.516 Furthermore, some patients have bilateral 
renal artery stenosis that, on commencement of an agent such as 
an ACE inhibitor, can lead to acute renal failure.517 Fortunately, 
in most patients, if the renal failure is diagnosed early, cessation 
of the ACE inhibitor leads to rapid restoration of renal function 
in this situation. 
Renal Papillary Necrosis
Renal papillary necrosis invo

l Papillary Necrosis
Renal papillary necrosis involves a severe destructive process, 
presumably as a result of ischemia to the medulla and papilla.518 
Beethoven’s final illness might have been papillary necrosis 
in the context of diabetes.519 The papilla is very sensitive to 
these ischemic changes because even in the normal setting it 
is exposed to a relatively hypoxic environment. Concomitant 
exacerbating factors include urinary tract infection and analge­
sic abuse. The importance of isc

ction and analge­
sic abuse. The importance of ischemia and possibly angiotensin 
II in this disorder has been suggested in experimental stud­
ies in transgenic rats that overexpress renin and angiotensin 
II in their kidney after induction of diabetes.520 In these rats, 
Glomerulopathy
Mesangial expansion
Glomerular hypertension
Diffuse thickening of the GBM
Broadening of foot processes
Podocyte loss
Reduced slit pore proteins
Glomerulomegaly
Kimmelstiel-Wilson lesion
Adhesions to Bowman’s caps

mmelstiel-Wilson lesion
Adhesions to Bowman’s capsule
Neovascularization
Nodular and diffuse
glomerulosclerosis
Tubulopathy
Tubular hyperplasia and hypertrophy
Progressive and cumulative atrophy
Thickening of the TBM
Epithelial mesenchymal transition
Accumulation of lysosomal bodies
Armani-Ebstein lesion
Reduced tubular brush border
Increased tubular salt reabsorption
Increased Na+/H+ antiporter activity
Impaired tubular acidification
Abnormal tubuloglomerular feedback
Decreased endocytosis of p

buloglomerular feedback
Decreased endocytosis of protein
Abnormal lysosomal processing
Impaired uptake of organic ions
• Fig. 37.38  Glomerular and tubular manifestations of diabetic nephropathy. GBM, Glomerular basement 
membrane; TBM, tubular basement membrane.
1483
diabetes was associated with development of papillary necrosis, 
which was prevented by blockade of the renin-angiotensin sys­
tem. Clinically, papillary necrosis often manifests as fl

inically, papillary necrosis often manifests as flank pain, 
hematuria, and fever. Urinalysis reveals red and white blood 
cells, bacteria, and papillary fragments. Ureteric obstruction 
can occur as a result of these fragments and must be addressed 
as an emergency. 
Renal Tubular Acidosis
A well-known functional abnormality associated with diabetic 
tubulopathy is renal tubular acidosis (type 4), manifesting as hyper­
kalemia and hyperchloremic metabolic acidosis.521 This is thought 
to be a m

 metabolic acidosis.521 This is thought 
to be a manifestation of hyporeninemic hypoaldosteronism asso­
ciated with diabetes, resulting in proximal tubule ammonia pro­
duction being reduced to levels inadequate to buffer acid in the 
distal nephron. The precise cause of this abnormality remains 
to be established. In some patients, there appears to be a defect 
in the conversion of prorenin to active renin.522 It has also been 
suggested that damage to the tubular cells of the juxtaglomeru­
lar 

age to the tubular cells of the juxtaglomeru­
lar apparatus associated with diabetes can contribute to impaired 
renin release, possibly due to reduced renal prostaglandin produc­
tion and elevated vasopressin levels.523
A major risk associated with hyporeninemic hypoaldosteron­
ism is the development of life-threatening hyperkalemia. This is 
an increasingly important issue with the widespread use of ACE 
inhibitors and ARBs, often in combination, in this population. 
This is further exacerbate

n, in this population. 
This is further exacerbated by the use of potassium-sparing diuret­
ics (such as spironolactone) and beta blockers. 
Contrast-Induced Nephropathy
Because many diabetic patients have impaired renal function, they 
are at high risk of increased renal impairment from certain neph­
rotoxic agents. One of the most important risks relates to radio­
contrast dyes.524 Where possible, patients with diabetes and renal 
impairment should avoid imaging studies that involve contrast 

ould avoid imaging studies that involve contrast 
and, in particular, multiple studies performed in rapid succession. 
Where intravenous contrast forms an indispensable tool to man­
agement, low-osmolality, nonionic, or gadolinium-based contrast 
media may be less nephrotoxic in patients with renal failure.525,526 
It has been suggested that patients who require such procedures 
should be well hydrated before, during, and after the procedure. 
However, a recent report suggests that this approach

wever, a recent report suggests that this approach may not be as 
effective as previously presumed.527 The role of N-acetylcysteine, 
a thiol-containing antioxidant, shows promise to protect against 
contrast-induced nephropathy.528 The oral hypoglycemic drug 
metformin should also be discontinued before contrast procedures 
to prevent life-threatening lactic acidosis. 
Management of Diabetic Kidney Disease
Blood pressure and glycemic control represent the major corner­
stones for preventing and

resent the major corner­
stones for preventing and treating diabetic nephropathy (Figs. 
37.39 and 37.40). In the early 1980s, a number of Scandinavian 
researchers found that aggressive blood pressure reduction reduces 
the rate of progression of diabetic nephropathy,529,530 and in the 
1990s other researchers found that intensified glycemic control has 
a similar benefit in both T1DM (DCCT)9 and T2DM (UKPDS) 
Microalbuminuria: 31–299 mg/day
• BP <135/75 mm Hg (ACE inhibitor)
• Hemoglobin A1c <

P <135/75 mm Hg (ACE inhibitor)
• Hemoglobin A1c <7%
• LDL cholesterol <100 mg/dL (statin)
Treatment targets
• Urinary protein
• Creatinine clearance
• Retinopathy (cataracts)
• Cardiac integrity
• Bone density
• Peripheral perfusion
• Neurologic stability
• Psychosocial adjustment
Monitoring
• Persistent angina
• Congestive heart failure, cardiomyopathy
• Respiratory disease
• Autonomic neuropathy: gastroparesis, obstipation,
     diarrhea, cystopathy, orthostatic hypotension
• Neurologic: cere

opathy, orthostatic hypotension
• Neurologic: cerebrovascular accident or stroke residual
• Musculoskeletal disorders, renal bone disease
• Infections: HIV, hepatitis, indolent ulcers
• Hematologic problems other than anemia
• Vision impairment (decreased acuity to blindness) loss
Assess comorbid conditions
• Electrocardiogram
• Echocardiography
• Dobutamine stress test
• Urine culture
• Fluorescein angiography
• Doppler limb flow
Baseline/periodic
• Treat hypertension
• Strive for euglycemia
• 

ic
• Reduce hyperlipidemia
Immediate
• Fig. 37.39  Flow chart illustrating the management of diabetic nephropathy before the onset of renal fail­
ure. ACE, Angiotensin-converting enzyme; BP, blood pressure; HIV, human immunodeficiency virus; LDL, 
1484
diabetic subjects. These findings have led to the view that optimi­
zation of blood pressure and plasma glucose leve

­
zation of blood pressure and plasma glucose levels should be the 
mainstay of therapy for diabetic nephropathy.
Glycemic Control in Diabetic Kidney Disease
The importance of glucose as a factor in the progression of dia­
betic kidney disease, as initially suggested from epidemiologic 
and preclinical studies, was clearly demonstrated in the DCCT 
study in patients with T1DM.9 In both the primary and sec­
ondary prevention arm of the study, any decrease in HbA1c was 
strongly associated with a 

decrease in HbA1c was 
strongly associated with a reduction in the risk of development 
of microalbuminuria as well as a decrease in the risk of progres­
sion to overt nephropathy. The follow-up EDIC study has con­
firmed long-lasting benefits of this therapeutic approach.531 The 
UKPDS clearly demonstrated a role for intensified glycemic con­
trol in newly diagnosed T2DM subjects when treatment led to a 
reduction in HbA1c from 7.9% to 7%. The Action in Diabetes 
and Vascular Disease: Preterax 

ction in Diabetes 
and Vascular Disease: Preterax and Diamicron MR Controlled 
Evaluation (ADVANCE) study has demonstrated that a further 
reduction of HbA1c to an average of 6.5% was associated with a 
further reduction in renal events, as assessed by the development 
and progression of microalbuminuria.532 A subsequent evaluation 
of this study533 demonstrated intensified glycemic control that 
reduced the development of end-stage renal disease, emphasizing 
the fact that tighter glycemic cont

, emphasizing 
the fact that tighter glycemic control continues to confer renal 
benefits even in the setting of more advanced renal disease. This 
renal benefit persisted, as seen in the follow-up study known as 
ADVANCE-ON.534 Thus despite the ongoing controversy as to 
the appropriate HbA1c target to reduce macrovascular disease as a 
result of the findings from the Action to Control Cardiovascular 
Risk in Diabetes (ACCORD) study,535 no such controversy as to a 
possible deleterious effect o

controversy as to a 
possible deleterious effect of intensified glycemic control has been 
reported with respect to nephropathy.
It remains to be determined how useful the intensification of 
glycemic control is in the setting of overt nephropathy as a last-
ditch strategy to delay the onset of end-stage renal disease. Aggres­
sive management of hypertension and lipid lowering are clearly 
more important than glycemic control in reducing cardiovascular 
events and slowing renal disease progressi

scular 
events and slowing renal disease progression at this stage of rela­
tively advanced disease, although some studies suggest that poor 
glycemic control can accelerate the loss of renal function in dia­
betic nephropathy.536 However, a number of large studies have 
failed to show any evidence that strict glycemic control per se 
retards renal progression once overt nephropathy is present.537 In 
fact, in the Veterans Administration Diabetes Trial (VADT) tight 
glycemic control reduced albu

 Trial (VADT) tight 
glycemic control reduced albuminuria but failed to demonstrate 
any effect on GFR.538 In addition, as renal function fails, tight 
glycemic control becomes more hazardous, with an increased risk 
of hypoglycemia. Nonetheless, because there is sufficient evidence 
that glycemic control can reduce both macrovascular events and 
microvascular complications of diabetes at other sites, it is reason­
able to suggest that optimization of metabolic control in patients 
with overt ne

on of metabolic control in patients 
with overt nephropathy remains worthwhile.
The choice of agent, however, remains controversial. Certainly, 
several types of drugs are able to improve glycemic control in 
patients with T2DM539; however, the particular advantages of one 
class over another for preventing and treating diabetic nephropa­
thy remain to be established. Nevertheless, this issue appears to 
have become more clinically relevant with the widespread use of 
newer classes of glucose-lo

the widespread use of 
newer classes of glucose-lowering drugs, including dipeptidyl pep­
tidase 4 (DDP4) inhibitors, GLP1 analogs, and SGLT2 inhibi­
tors.539 With respect to DPP4 inhibitors, modest renal benefits 
have been suggested, particularly with saxagliptin in the Saxa­
gliptin Assessment of Vascular Outcomes Recorded in Patients 
with Diabetes Mellitus (SAVOR) study540 and in pooled analy­
ses with linagliptin.541 Pending results from studies such as the 
• Urinary pro

udies such as the 
Immediate
• Attain dry weight: diuretics
• BP <135/75 mm Hg: ACE inhibitors,
     multiple drugs
• Hematocrit >35%: erythropoietin
• Hemoglobin A1c <7%: metabolic control
• LDL cholesterol <100 mg/dL: diet + control
• Educate patient re: regimen appli

diet + control
• Educate patient re: regimen applied
• Extract edema
• Correct anemia
Proteinuria: >299 mg/day
Nephrosis: >3.5 g/day
• In

Musculoskeletal disorders, renal bone disease
• Vision impairment (decreased acuity to blindness, loss)
• Fig. 37.40  Flow chart illustrating the management of diabetic nephropathy after the onset of clinical pro­
teinuria. ACE, Angiotensin-converting enzyme; BP, blood pressure; HIV, human immunodeficiency virus; 
LDL, low-density lipoprotein.
1485
Cardiovascular 

Cardiovascular and Renal Microvascular Outcome Study With 
Linagliptin in Patients With Type 2 Diabetes Mellitus (CARME­
LINA)542 will further clarify the potential renoprotective role of 
these agents. Recent studies have been performed with GLP1 ana­
logues, including liraglutide and semaglutide.543–545 Both agents, 
in addition to conferring cardiovascular protection, afforded renal 
benefits. In the Liraglutide Effect and Action in Diabetes: Evalu­
ation of

ide Effect and Action in Diabetes: Evalu­
ation of Cardiovascular Outcome Results (LEADER) trial, lira­
glutide-treated patients had a 22% lower incidence of new onset 
of persistent macroalbuminuria and a slower decline in the eGFR 
over time, particularly in the subgroup of patients who had evi­
dence of kidney damage at baseline.545 In the Trial to Evaluate 
Cardiovascular and Other Long-term Outcomes with Semaglu­
tide in Subjects with Type 2 Diabetes (SUSTAIN 6), semaglu­
tide-treated patie

 Diabetes (SUSTAIN 6), semaglu­
tide-treated patients had a similar reduction in renal outcomes. 
However, semaglutide-treated patients also had increased risk of 
serious diabetic retinopathy, including 1.91-fold increased risk of 
retinal photocoagulation and a 2.29-fold increased risk of vitre­
ous hemorrhage,543,544 which may be due to more severe retinal 
disease at baseline and rapid glycemic improvements. Finally, 
SGLT2 inhibitors have been shown to have strong renoprotec­
tive effects. 

en shown to have strong renoprotec­
tive effects. For example, the EMPA-REG OUTCOME study 
demonstrated an approximately 50% decrease in composite renal 
outcomes, including benefits on reducing hard renal endpoints 
such as end-stage renal failure.474 Similar benefits have also been 
reported in the CANVAS study using the SGLT2 inhibitor cana­
gliflozin.473 Of major interest is the finding that renoprotection 
is observed even in the absence of glucose lowering with these 
agents as seen in sub

glucose lowering with these 
agents as seen in subjects with eGFR below 45 mL per minute.546 
This suggests that these agents confer renoprotection at least in 
part in a glucose-independent manner, but the mechanism(s) are 
unknown. Changes in sodium balance, intrarenal hemodynam­
ics, plasma volume, decreased insulin resistance, and triglyceride-
rich lipoprotein production and modest actions in reducing blood 
pressure, body weight, and uric acid have all been suggested as 
possible mechanism

cid have all been suggested as 
possible mechanisms.
A number of differences in the side effect profiles should influ­
ence prescribing habits. In patients with renal impairment, particular 
care must be exercised in selecting and dosing oral diabetic therapy 
because an accumulation of either the drug or active metabolites 
can lead to hypoglycemia (e.g., with glyburide) and other serious 
adverse effects such as lactic acidosis (with metformin). Thiazolidin­
ediones (TZDs), such as pioglitazon

. Thiazolidin­
ediones (TZDs), such as pioglitazone and rosiglitazone, should be 
used with caution in patients with advanced nephropathy who have 
or are at risk of heart failure, although renal impairment per se is not 
a contraindication for this class of drugs. 
Blood Pressure Control in Diabetic Kidney Disease
A sustained reduction in blood pressure appears to be one of the 
most important interventions to prevent progressive nephropathy 
in T1DM and T2DM. For example, in the UKPDS, a reduc

 T1DM and T2DM. For example, in the UKPDS, a reduction 
in blood pressure from 154 to 144 mm Hg was associated with 
a 30% reduction in microalbuminuria. All national and interna­
tional guidelines now emphasize the importance of blood pressure 
reduction in the diabetic patient. Although many guidelines sug­
gest that specific targets should be achieved, no such threshold 
appears to exist for any renal endpoint in patients with diabetes. 
In particular, the risk of progressive diabetic nephrop

rticular, the risk of progressive diabetic nephropathy contin­
ues to decrease, with blood pressure reductions into the normal 
range and below, meaning that the lowest achievable blood pres­
sure is associated with the best clinical outcomes. This is particu­
larly important in those with the greatest risk of renal damage, 
patients with overt nephropathy. In these patients, it has been sug­
gested that optimal blood pressure control is less than 125/75 mm 
Hg.547 Indeed, in a subanalysis from 

n 125/75 mm 
Hg.547 Indeed, in a subanalysis from the blood pressure arm of 
the ADVANCE study, no blood pressure threshold was detected. 
Specifically, in those subjects in whom achieved blood pressure 
was reduced to levels even lower than currently recommended in 
national and international guidelines, there was a further decrease 
in renal events.548 Thus it is possible that if individuals can toler­
ate lower blood pressures without major side effects such as diz­
ziness and syncope it may 

de effects such as diz­
ziness and syncope it may be worth considering treating some 
T2DM subjects to blood pressure levels below those currently rec­
ommended. However, syncope is often a side effect limiting the 
options to intensify blood pressure–lowering therapy, particularly 
in those diabetic subjects with systolic hypertension as a result of 
diabetes-associated vascular stiffness.
There is good evidence that tight blood pressure control, no mat­
ter how it was achieved, is associated w

, no mat­
ter how it was achieved, is associated with a significant reduction 
in the risk of microalbuminuria (primary prevention). Although 
blood pressure reduction appears paramount, there is also evidence 
that ACE inhibitors549 have renoprotective actions beyond their 
antihypertensive effects for primary prevention.550,551 However, if 
treatment should commence in the normoalbuminuric stage, such 
a strategy would involve treating the majority of patients who are 
not at risk of nephropat

rity of patients who are 
not at risk of nephropathy. Ideally, it would be useful to be able 
to identify patients, still normoalbuminuric, whose likelihood of 
progression is increased. As of yet, no such markers of predisposi­
tion to renal disease are available, although serum prorenin476 and 
modest elevations in urinary albumin excretion albeit still within 
the normal range (borderline microalbuminuria)489 might ulti­
mately be examples of such markers.
The issue of primary prevention has 

such markers.
The issue of primary prevention has been readdressed in two 
studies in which subjects with normoalbuminuria were treated 
with agents that interrupt the RAS. In the first smaller trial, which 
included the performance of sequential renal biopsies, no benefit 
of early institution of either the ACE enalapril or the angiotensin 
II receptor antagonist losartan was observed, as defined not only 
as a lack of effect on albuminuria but also no significant retarda­
tion in progression o

also no significant retarda­
tion in progression of renal morphologic injury despite benefits 
on retinopathy.552 In the second, much larger trial known as the 
Diabetic Retinopathy Candesartan Trials (DIRECT) study, the 
angiotensin II antagonist candesartan, despite some modest reti­
noprotective effects, had no major impact on reducing the new 
onset of microalbuminuria.553
In secondary prevention studies, the additional benefits achieved 
from blocking the renin-angiotensin system are cleare

m blocking the renin-angiotensin system are clearer. A meta-anal­
ysis incorporating the findings of more than 10 studies in patients 
with microalbuminuria has demonstrated the ability of ACE inhibi­
tors not only to retard the development of overt proteinuria but 
also to decrease urinary albumin excretion by more than 30%. In 
some patients with microalbuminuria, ACE inhibition can reduce 
urinary albumin excretion into the normoalbuminuric range.550 In 
patients with T1DM and overt proteinur

nge.550 In 
patients with T1DM and overt proteinuria, aggressive blood pres­
sure reduction reduced proteinuria by up to 50% and retarded the 
rate of decline in renal function.529,530
Similar studies have been performed in T2DM patients. Two 
landmark trials, RENAAL and Irbesartan in Diabetic Nephropa­
thy Trial (IDNT), examined the renoprotective effects of the 
ARBs losartan and irbesartan, respectively.506,469 In both stud­
ies, when compared to various alternative antihypertensive agents 
s

 to various alternative antihypertensive agents 
such as calcium antagonists (but not ACE inhibitors), ARB treat­
ment was associated with a reduction in end-stage renal failure, a 
greater than 30% decrease in proteinuria, and a major reduction 
1486
in hospitalization for heart failure. As a result of these studies, 
ARBs are recommended as first-line treatment for blood pressure 
reduction in T2DM patients with overt proteinuria.554

uction in T2DM patients with overt proteinuria.554
Although ACE inhibitors have not been as extensively stud­
ied in this population, the Diabetics Exposed to Telmisartan and 
Enalapril (DETAIL) trial suggested similar renoprotective actions 
for both drug classes.555 Similar findings comparing the angioten­
sin II antagonist telmisartan to the ACE inhibitor, ramipril, are 
also reported in the much larger ONTARGET study, albeit that 
trial was not performed exclusively in diabetic subjects.556 

ot performed exclusively in diabetic subjects.556 Thus 
from a clinical perspective no clear difference between these two 
drug classes has been identified. The one exception is cough, which 
occurs in 5% to 30% of patients taking ACE inhibitors, depend­
ing on ethnicity, which is higher in Asian subjects. In microal­
buminuric T2DM subjects, ARBs have also been demonstrated 
to have a role. For example, in the Irbesartan Microalbuminuria 
Type 2 (IRMA2) trial, irbesartan dose-dependently reduce

 (IRMA2) trial, irbesartan dose-dependently reduced the 
risk of development of macroproteinuria,557 confirming the find­
ings seen predominantly with ACE inhibitors in microalbumin­
uric T1DM subjects.550
Another approach to inhibit the renin-angiotensin system has 
involved the use of the recently introduced renin inhibitors such 
as aliskiren. For example, in the Safety and Efficacy of Aliskiren 
When Added to Standardized Losartan and Optimal Antihyper­
tensive Therapy in Patients With Hyper

 Antihyper­
tensive Therapy in Patients With Hypertension, Type 2 Diabe­
tes and Proteinuria (AVOID) study in T2DM subjects,558 this 
agent appeared to have an additional effect on albuminuria when 
administered with the angiotensin II antagonist losartan. Unfortu­
nately, as reported in the larger subsequent Aliskiren Trial in Type 
2 Diabetes Using Cardiorenal Endpoints (ALTITUDE) study,511 
aliskiren has failed to demonstrate superior cardiovascular or renal 
protection, and thus this approac

scular or renal 
protection, and thus this approach of add-on renin inhibition is 
not recommended. Other approaches focusing on blood pressure 
reduction continue to be examined in these populations. Pilot 
studies with various mineralocorticoid (MC) antagonists, albeit 
not studied extensively, have revealed that spironolactone and 
eplerenone reduce albuminuria.559,560 Unfortunately, these agents 
are associated with hyperkalemia, particularly in subjects with renal 
impairment and/or type 4 

 in subjects with renal 
impairment and/or type 4 renal tubular acidosis. However, the 
advent of a newer MC antagonist, finerenone, which has a lower 
risk of hyperkalemia, has led to renewed interest in this treatment 
approach. Indeed, in the mineralocorticoid Receptor Antagonist 
Tolerability Study—Diabetic Nephropathy (ARTS-DN) trial, a 
dose-dependent effect of finerenone was associated with a progres­
sive decrease in albuminuria.561 These positive findings have led to 
larger trials with

 positive findings have led to 
larger trials with this drug that are currently in progress. 
Evaluation of Additional Approaches to the 
Low-protein diets (0.75 g/kg/day) have been shown to retard the 
progression of renal disease, although the data are not totally con­
vincing for diabetic nephropathy, per se. A meta-analysis of five 
studies in T1DM subjects supported a minor renoprotective role 
for these diets,562 but this has not been a universal findi

 diets,562 but this has not been a universal finding.563 There 
are even fewer data in T2DM subjects with overt nephropathy.564 
However, the expected benefits that can be achieved through pro­
tein restriction in patients with diabetic nephropathy are at best 
modest in comparison with adequate blood pressure control and 
blockade of the renin-angiotensin system. Moreover, the nutri­
tional impact of such interventions must be carefully considered, 
particularly in patients with brittle glycemi

ed, 
particularly in patients with brittle glycemic control.
The role of lipid-lowering agents as renoprotective drugs 
remains controversial. Although in rodents a large body of evi­
dence suggests that lipids promote renal injury and that various 
lipid-lowering drugs reduce nephropathy, even in the setting of no 
or minimal effect on lipids,565 the data in humans are variable.566 
However, in a study of fenofibrate in T2DM, known as the Feno­
fibrate Intervention and Event Lowering in Diabete

fibrate Intervention and Event Lowering in Diabetes (FIELD) 
study, there was an impressive reduction in albuminuria.567,568 
Furthermore, in the Heart Protection Study, simvastatin appeared 
to retard the decline in renal function, although this analysis was 
not confined to the diabetic subgroup.569 Another group has also 
reported a potential renoprotective effect of a statin,570 although 
this effect has not been observed in all studies. Indeed, several 
meta-analyses have generally reported

ed, several 
meta-analyses have generally reported at best modest benefits of 
statins on renal disease.571–573 Nevertheless, because cardiovascu­
lar disease is so prominent in diabetic patients, particularly those 
with incipient or overt renal disease, lipid-lowering treatment 
should be considered in most patients independent of its putative 
renoprotective actions.574
Other approaches to consider include correction of anemia 
with agents such as erythropoietin.575 The role of these agents a

h as erythropoietin.575 The role of these agents as 
renoprotective drugs remains to be clarified,576 but the potential 
benefits on general patient well-being and in reducing left ven­
tricular hypertrophy577 provide a rationale for using such agents 
judiciously in diabetic patients. However, the Trial to Reduce 
Cardiovascular Events With Aranesp Therapy (TREAT) study, 
although focusing on cardiovascular events and mortality using 
the erythropoietin analogue darbepoetin, did not demonstrate

opoietin analogue darbepoetin, did not demonstrate 
that darbepoetin is renoprotective. Furthermore, the drug was 
unfortunately associated with a twofold increase in cerebrovascu­
lar events.578 With the advent of new hemoglobin-raising drugs 
such as the hypoxia-inducible factor prolyl hydroxylase inhibi­
tors,579 there is likely to be a new approach for correcting anemia 
in diabetes-associated chronic kidney disease.
Over the past decade, several clinical trials targeting diabetic 
nephropat

eral clinical trials targeting diabetic 
nephropathy with novel agents have yielded disappointing results. 
For example, PKCβ inhibition with ruboxistaurin, which had 
renal benefits in experimental diabetes, failed to show any major 
benefits on albuminuria in T2DM subjects.580 Another promising 
agent, sulodexide, postulated to restore the glomerular charge by 
repleting the loss of glycosaminoglycans581 and thereby act as an 
antiproteinuric and ultimately renoprotective drug, also failed to 

nd ultimately renoprotective drug, also failed to 
demonstrate any evidence of renoprotection in several large trials. 
An endothelin antagonist, avosentan, was assessed in the Assess 
the Effect of the Endothelin Receptor Antagonist Avosentan on 
Time to Doubling of Serum Creatinine, End Stage Renal Disease 
or Death in Patients With Type 2 Diabetes Mellitus and Diabetic 
Nephropathy (ASCEND) trial.582 Although this drug was asso­
ciated with impressive reductions in albuminuria, the associated

pressive reductions in albuminuria, the associated 
side effect of fluid retention reduced the enthusiasm for this agent. 
However, another endothelin antagonist, atrasentan, with fewer 
side effects and being antiproteinuric,583 was developed. However, 
this endothelin antagonist also unfortunately failed at a relatively 
advanced stage of clinical development. Finally, bardoxolone, an 
agonist of the transcription factor Nrf2, which appears to act as 
an antioxidant, has been investigated in T

act as 
an antioxidant, has been investigated in T2DM subjects with 
impaired renal function. In the initial Bardoxolone Methyl Treat­
ment: Renal Function in CKD/Type 2 Diabetes (BEAM) trial584 
an improvement in renal function was reported. However, a subse­
quent larger study in T2DM subjects with stage 4 chronic kidney 
disease known as the Bardoxolone Methyl Evaluation in Patients 
with Chronic Kidney Disease and Type 2 Diabetes Mellitus: the 
1487
Chapter 37  Complications of Diabetes Mell

e 
1487
Occurrence of Renal Events (BEACON) study was prematurely 
terminated because of increased cardiovascular events.585 Never­
theless, this drug continues to be evaluated,586 including in new 
studies, albeit with a lower dose, such as in the Phase II Study 
of Bardoxolone Methyl in Patients With Chronic Kidney Disease 
and Type 2 Diabetes (TSUBAKI) study where positive findings 
have been reported. 
Treatment of the Diabetic Uremic Patient
Re

rted. 
Renal impairment in a patient with diabetes necessitates changes 
in therapy. Often, blood glucose control becomes more brittle 
because the half-life of insulin is prolonged and the renal response 
to hypoglycemia is impaired. High swinging blood glucose levels 
in a patient with nephropathy can often mistakenly lead to an 
increase in oral therapy. However, in patients with renal impair­
ment, particular care must be exercised in the selection an

ticular care must be exercised in the selection and dosing 
of oral hypoglycemic therapy. Fortunately, DPP4 inhibitors can 
be used safely in such individuals, albeit often the dose needs to be 
reduced in subjects with low GFR. Importantly, one DPP4 inhibi­
tor, linagliptin, which is not excreted by the kidney, can be given 
without dose reduction, even in subjects on dialysis.587 Nonste­
roidal anti-inflammatory drugs (NSAIDs) and COX2 inhibitors 
should be avoided when possible because their 

rs 
should be avoided when possible because their use is associ­
ated with inadequate blood pressure control, often as a result of 
reduced efficacy of antihypertensive drug therapy. Patients at high 
risk for progressive deterioration in their renal function should 
be considered for an early referral to a nephrology service for 
management of renal failure (Fig. 37.41). This facilitates access to 
erythropoietin, control of calcium phosphate balance, and plan­
ning for renal replacement therap

lance, and plan­
ning for renal replacement therapy with the preemptive placement 
of access catheters and lines. Delay in referral can result in a more 
precipitous start to renal replacement and usually a bad prognostic 
outcome.588
Many options are now available for the diabetic patient requir­
ing renal replacement therapy.589 These include home or facility 
hemodialysis, including overnight dialysis, peritoneal dialysis, 
renal transplantation (cadaveric or living related), or combined 
pan

on (cadaveric or living related), or combined 
pancreas-kidney transplantation. Most patients choose hemodi­
alysis rather than peritoneal dialysis, although data are conflicting 
regarding which approach leads to better survival (Table 37.6). 
Some patients opt for withdrawal of treatment because their qual­
ity of life, with advanced cardiovascular disease, visual impair­
ment, and amputations, is poor. 
The Burden of Nephropathy
One must never consider renal disease in a diabetic patient in 

 consider renal disease in a diabetic patient in 
isolation. Proteinuria, per se, is strongly associated with other 
complications such as macrovascular disease, heart failure, and ret­
inopathy. Furthermore, treatments directed toward one complica­
tion may be useful for the other complications. Indeed, intensified 
glycemic control has been shown to be particularly useful for other 
microvascular complications.9 In addition, the various antihyper­
tensive regimens, particularly those using age

er­
tensive regimens, particularly those using agents that interrupt the 
renin-angiotensin system as well as glucose and blood pressure 
lowering with SGLT2 inhibitors, also confer important cardio­
vascular benefits such as reducing heart failure.469,506,590 Recent 
data also support use of angiotensin receptor neprilysin inhibi­
tors (ARNi). A post hoc analysis of the Prospective Comparison 
of ARNI With ACEI to Determine Impact on Global Mortal­
ity and Morbidity in Heart Failure (PARADIGM-H

al­
ity and Morbidity in Heart Failure (PARADIGM-HF) trial using 
Treatment objectives
Asses comorbid conditions
Immediate
• Aut

ailure, cardiomyopathy
• Hematocrit >3

CE inhibitors,
• LDL cholesterol <100 mg/dL: diet + statin
• Prepare patient for uremia regimen
• Inventory potential kidney donors
• Create access for hemodialysis or PD
• Tissue type
• Consider dietary protein restriction
Azotemia: Serum creatinine >2.0 mg/dL
• Fig. 37.41  Flow chart illustrating the management of diabetic nephropathy after onset of renal failure. 
ACE, Angiotensin-converting enzyme; 

nal failure. 
ACE, Angiotensin-converting enzyme; BP, blood pressure; HIV, human immunodeficiency virus; LDL, low-
density lipoprotein; PD, peritoneal dialysis.
1488
sacubitril/valsartan, a combination of neprilysin inhibition and 
angiotensin II receptor blockade, was reported to slow the rate of 
reduction of eGFR in patients with stage 2 to 4 heart failure with 
reduced ejection fraction, a finding which was more prominent in 
the p

tion, a finding which was more prominent in 
the participants with T2DM at study entry.591 Interestingly this 
agent is also associated with modest glucose-lowering effects.592
Thus as clearly expounded in the Intensified Multifactorial 
Intervention in Patients With Type 2 Diabetes and Microalbu­
minuria (Steno-2) study, the multifactorial approach in microal­
buminuric subjects will not only lead to renal benefits as confirmed 
in a follow-up study,593 but will also confer other advantages to 

tudy,593 but will also confer other advantages to 
the diabetic patient, including reducing all-cause mortality.594 
Because those with renal disease have the greatest risk for nonrenal 
complications, it stands to reason that they also are likely to have 
the greatest absolute benefit from risk-reduction strategies. 
Diabetic Neuropathies
Diabetic neuropathies are a group of clinical syndromes with dis­
tinct presentations secondary to different underlying pathogenetic 
mechanisms. These syndro

 underlying pathogenetic 
mechanisms. These syndromes are most commonly classified into 
two broad groups: diffuse versus focal neuropathies. Among the 
diffuse diabetic neuropathies, distal symmetric polyneuropathy 
(DSPN) is the most common, followed by the family of autonomic 
neuropathies, including cardiac autonomic neuropathy (CAN), 
and autonomic neuropathy of the gastrointenstinal, urogenital, 
and sudomotor systems. Focal neuropathies are much less common 
than diffuse neuropathies and 

e much less common 
than diffuse neuropathies and include isolated mononeuropathies 
of one or more peripheral nerves; even rarer are focal neuropathies 
of one or more nerve roots, classified as radiculopathy or polyra­
diculopathy, respectively (Tables 37.7 and 37.8).
Epidemiology and Impact of Diabetic 
Neuropathies
Diabetic neuropathies are the most common complications of 
T1DM and T2DM. Distal symmetric polyneuropathy repre­
sents the most commonly diagnosed and highly morbid of the 
diabe

commonly diagnosed and highly morbid of the 
diabetic neuropathies. While prevalence estimates vary depending 
on how the diagnosis is made, DSPN occurs in at least half if 
not more of all diabetic patients over time.595 Prevalence as deter­
mined by clinical examination was approximately 45% after 25 
years of diabetes in the classic study by Pirat following a cohort 
of 4400 diabetic patients between 1947 and 1973.596 However, 
when more contemporary quantitative sensory testing or nerve 
con

mporary quantitative sensory testing or nerve 
conduction studies are used as part of the diagnostic criteria, 
the prevalence of DSPN increases from 60%597 to greater than 
75%.598 Multiple studies now confirm that DPSN also occurs in 
  Options in Therapy for End-Stage Renal Disease in Diabetic Patients
Variable
Peritoneal Dialysis
Hemodialysis
Kidney Transplantation
Extensive extrarenal disease
No limitation
No limitation except for hypotension
Excluded in cardiovascular insufficiency
Geriat

on
Geriatric patients
Limited by frailty, cognitive 
dysfunction; patient treatment 
satisfaction greater with assisted 
peritoneal dialysis
Arbitrary exclusion as determined by 
program
Complete rehabilitation
Rare, if ever
Very few patients
Common as long as graft functions
Death rate
Much higher than for nondiabetic 
patients
patients
About the same as for nondiabetic 
patients
First-year survival rate
∼7

 nondiabetic 
patients
∼75%
∼75%
>90%
Morbidity during first year
∼15 days in hospital
∼12 days in hospital
Weeks to months hospitalized
Survival to second decade
Almost never
<5%
∼1 in 5
Progression of complications
Usual and unremitting; hyperglyce­
mia and hyperlipidemia
Usual and unremitting; might benefit 
from metabolic control
Interdicted by functioning pancreas plus 
kidney; partially ameliorated by correc­
tion of azotemia
Special advantage
Can be self-performed

f azotemia
Can be self-performed; avoids 
swings in solute and level of 
intravascular volume
Can be self-performed; efficient 
extraction of solute and water 
in hours
Cures uremia; freedom to travel
Peritonitis; hyperinsulinemia; 
hyperglycemia, hyperlipidemia; 
long hours of treatment; more 
days hospitalized than with either 
hemodialysis or transplantation
Blood access a hazard for clotting, 
hemorrhage, and infection; cyclic 
hypotension, weakness, aluminum 

nfection; cyclic 
toxicity, amyloidosis
Cosmetic disfigurement, hypertension, 
personal expense for cytotoxic drugs; 
induced malignancy; HIV (human 
immunodeficiency virus) transmission
Patient acceptance
Variable, usual compliance with 
passive tolerance for regimen
Variable; often noncompliant with 
dietary, metabolic, or antihyper­
tensive components of regimen
Enthusiastic during periods of good 
renal allograft function; exalted when 
pancreas proffers eugl

aft function; exalted when 
pancreas proffers euglycemia
Relative cost
Most expensive over long run
Less expensive than kidney trans­
plantation in the first year; subse­
quent years more expensive
Pancreas plus kidney engraftment most 
expensive uremia therapy for diabetics; 
after first year, kidney transplantation 
alone is lowest cost option
TABLE 37.6
1489
patients with prediabetes and the metabolic syndrome599–601 and 
while higher in those me

olic syndrome599–601 and 
while higher in those metabolic syndrome patients with hypergly­
cemia, ranging from prediabetic to T2DM levels, DSPN can also 
occur independent of glycemic status.601 With the alarming rise of 
prediabetes, T2DM, and the metabolic syndrome, estimates range 
from 15 million to 30 million individuals in the United States are 
living with DSPN.602
DSPN leads to a poor quality of life, limiting a patient’s func­
tion, especially DSPN associated with pain. Up to 70% of DSP

cially DSPN associated with pain. Up to 70% of DSPN 
patients experience some form of disrupted sleep, depression, 
anxiety, and poor work productivity.603 Impairment of physical 
functioning in DSPN is associated with a 15-fold increase in the 
likelihood of falling and fractures, particularly in older diabetics.604 
Foot ulceration leading to amputation is one of the most serious 
and adverse effects of DSPN. Foot ulcers are a strong predictor of 
early mortality in diabetic patients605 and pr

of 
early mortality in diabetic patients605 and precede nearly all ampu­
tations.606 Of patients with new foot ulcers, 5% die within 1 year 
of their first ulcer and 42.2% of people with existing ulcers die 
within 5 years, as shown in a recent study of 414,523 patients with 
diabetes.607 A 2017 systematic review reported that the global foot 
ulcer prevalence is 6.3%, with the highest prevalence, 13%, occur­
ring in North America.608 Worldwide DSPN accounts for more 
hospitalizations than all t

SPN accounts for more 
hospitalizations than all the other diabetic complications com­
bined and is the root cause of 50% to 75% of nontraumatic ampu­
tations in North America.609,610 Though rates of lower extremity 
amputation among Medicare-enrolled diabetics have decreased by 
28.8% between 2000 and 2010,611 the cost of caring for DSPN 
complications has not decreased because of delay in diagnosis.612
Like DSPN, autonomic neuropathies adversely affect function 
in patients with T1DM and T2DM,

y affect function 
in patients with T1DM and T2DM, affecting a wide variety of 
organ systems, including cardiovascular, gastrointestinal, urogeni­
tal, and sudomotor/thermoregulatory systems. Of these, cardiac 
autonomic neuropathy is the best studied. Multiple studies con­
firm early mortality and sudden death in diabetic CAN patients, 
with mortality estimates up to 56% over a 10-year period.613 One-
third of patients with autonomic neuropathy have impaired qual­
ity of life,614 with erectile

 have impaired qual­
ity of life,614 with erectile dysfunction predicting not only poor 
quality of life in the affected patient615 but also in the patient’s 
sexual partner.616
  Mononeuritis and Entrapment Syndromes
Feature
Mononeuritis
Entrapment
Onset
Sudden
Gradual
Nerves
Usually single but may 
be multiple
Single nerves exposed to 
trauma
Common 
nerves
C3, C6, C7, ulnar, 
median, peroneal
Median, ulnar, peroneal, 
medial and lateral plantar
Not progressive; resol­
ves spontan

ar
ves spontaneously
Treatment
Rest, splints, diuretics, 
steroid injections, surgery 
for paralysis
C3, C6, C7, Cervical spinal nerves 3, 6, and 7.
Modified from Vinik AI, Mehrabyan A. Diabetic neuropathies. Med Clin North Am. 
2004;88:947–999.
   
TABLE 37.8
From Pop-Busui R, Boulton AJM, Feldman EL, et al. Diabetic neuropathies: a position statement by the American Diabetes Association. Diabetes Care. 2017;40:136–154. Copyright and a

wareness
Abnormal pupillary function
	B.	 Mononeuropathy (mononeuritis multiplex) (atypical forms)
Isolated cranial or peripheral nerve (e.g., CN III, ulnar, median, 
femoral, peroneal)
Mononeuritis multiplex (if confluent may resemble 
polyneuropathy)
	C.	 Radiculopathy or polyradiculopathy (atypical forms)
Radiculoplexus neuropathy (a.k.a. lumbosacral polyradiculopathy, 
proximal motor amyotrophy)
Thoracic radiculopathy
Nondiabetic neuropathies common in diabetes
  Pressure palsies
  Chronic

es common in diabetes
  Chronic inflammatory demyelinating polyneuropathy
Radiculoplexus neuropathy
Acute painful small-fiber neuropathies (treatment induced)
DSPN, Distal symmetric polyneuropathy.
1490
While cost estimates vary, the total annual cost of DSPN is 
estimated to be greater than $25 billion,617 with total annual med­
ical costs for a diabetic patient with painless DSPN estimated at 
$12,492 compared to $

 painless DSPN estimated at 
$12,492 compared to $6632 for a diabetic patient without DSPN. 
Costs are even greater with painful DSPN, from $27,931 up to 
$30,755 per patient, depending on the severity of the pain617 and 
the use of generic versus nongeneric medications.618 Importantly, 
little money is spent in the United States on patient education or 
preventative foot care, an investment that has been proven to be 
effective in decreasing the incidence of DSPN complications.619 
Pathophysiol

 incidence of DSPN complications.619 
Pathophysiology of Diabetic Neuropathies
While the pathophysiology of diabetic neuropathy has been dis­
cussed earlier in this chapter, an understanding of nerve structure 
(Fig. 37.42) provides additional insight into neuropathy patho­
genesis and clinical presentation. In the peripheral nervous system, 
afferent axons from sensory receptors send information to the dor­
sal root ganglia of the central nervous system (CNS), while effer­
ent axons relay CNS s

s system (CNS), while effer­
ent axons relay CNS signals to muscles and joints. Schwann cells 
envelope and myelinate large and midsize sensory axons, knowns 
as large fibers. In contrast, the cytoplasm from one Schwann cell 
can envelope 30 or more unmyelinated small axons, known as 
small fibers. These large and small fibers carry specific informa­
tion: Large fibers relay vibratory, proprioception, and tactile sensa­
tion; small fibers relay thermal sensation and pain, and they also 
regulate

hermal sensation and pain, and they also 
regulate microvascular blood flow. Small fiber damage usually pre­
cedes large fiber damage in diabetes620 and frequently presents as 
burning pain, although patients also report tingling and prickly 
sensations.621 With time, large fiber involvement occurs, with cor­
responding numbness and poor position sense.
How diabetes changes the interaction between Schwann cells 
and axons is not yet understood, and it remains unclear if the 
primary insult in DS

nd it remains unclear if the 
primary insult in DSPN lies in the sensory neuron, axon, or 
Schwann cell; logically, it is likely that all three components are 
adversely affected, in both distinct and overlapping ways. In the 
diabetic microenvironment, glycolytic intermediates are depleted 
in Schwann cells622 while fatty acid oxidation is increased,623 pro­
ducing a state of cellular oxidative stress and excess cytosolic bio­
active lipids.624 Emerging evidence reveals that axon-Schwann cell 

Emerging evidence reveals that axon-Schwann cell 
communication, via myelinic channels between the two tissues, 
allows for the two-way transport of these glycolytic intermedi­
ates and bioactive lipids.602,625 The concept of crosstalk between 
Schwann cells and axons is further supported by a recent finding 
that delivery of exosomes from Schwann cells cultured in high 
glucose to sciatic nerves of mice resulted in murine DSPN.626 
Finally, the idea of axon-Schwann cell crosstalk and recipro­
c

idea of axon-Schwann cell crosstalk and recipro­
cal supply of critical intermediary metabolites is supported by 
Schwann cell–specific knockout experiments of a serine-threonine 
kinase required for Schwann cell energy regulation. This knockout 
caused degeneration of sensory greater than motor axons, espe­
cially in the small unmyelinated fibers.627
Clinical trials have yielded insights into potential differences 
underlying the pathogenesis of DSPN in T1DM and T2DM. 
Glucose control has a sig

 DSPN in T1DM and T2DM. 
Glucose control has a significant effect on DSPN onset and pro­
gression in T1DM but no effect or a modest 3% to 5% effect at 
Dorsal root
ganglion
Motor
neurons
Remak
bundle
C fibers
(unmyelinated)
Schwann cells/
myelin
Axon
Schwann
cell-axon
transport
Metabolites,
trophic factors,
signaling molecules
Myelin
channels
• Fig. 37.42  The peripheral nervous system. The peripheral nervous system (PNS) is comprised of both 
neurons and Schwann cells (SCs), and the structure, 

urons and Schwann cells (SCs), and the structure, location, and interaction of these components have 
important implications for PNS function. Efferent axons of motor neurons, whose cell bodies are located 
in the ventral horn of the spinal cord, carry signals from the central nervous system (CNS) to muscles 
and glands, whereas afferent axons of sensory neurons, whose cell bodies are located in the dorsal root 
ganglia, relay information from peripheral sensory receptors to the CNS. Thin and un

ripheral sensory receptors to the CNS. Thin and unmyelinated sensory 
axons, also known as C fibers or small fibers, are associated with nonmyelinating Schwann cells and are 
grouped as Remak bundles. These represent a large portion of the PNS neurons. Myelinated sensory 
axons, on the other hand, are surrounded by myelin sheaths made by Schwann cells that form distinct 
nodal domains important for saltatory conduction. They also form a tubular network of myelinic channels 
that connect the SC c

etwork of myelinic channels 
that connect the SC cytoplasm with the periaxonal space, providing substrate for energy generation to the 
axonal compartment. (Redrawn from Feldman EL, Nave KA, Jensen TS, et al. New horizons in diabetic 
neuropathy: mechanisms, bioenergetics, and pain. Neuron. 2017;93:1296–1313.)
1491
best on DSPN in T2DM.628 The idea that DSPN is actually two 
distinct disorders, DSPN T1DM and DSPN T2DM, is a source 
of active investi

T1DM and DSPN T2DM, is a source 
of active investigation,11 with the emerging concept that neurons 
can develop insulin resistance629 that contributes to DSPN in 
T2DM (Fig. 37.43).630 
Clinical Features of Diabetic Distal Symmetrical 
Polyneuropathy
Clinical Symptoms
DSPN is the most common and widely recognized form of dia­
betic neuropathy. An international consensus meeting defined 
DSPN as “the presence of symptoms and/or signs of peripheral 
nerve dysfunction in people with diabetes after 

nerve dysfunction in people with diabetes after the exclusion of 
other causes.”631,632 Alternate causes of DSPN include vitamin 
B12 deficiency, multiple myeloma, and hereditary neuropathies. 
The onset of DSPN is usually insidious but can be acute, as dis­
cussed later. DSPN is primarily a sensory neuropathy and can 
involve small fibers, large fibers, or both.633 Motor involvement is 
only seen late in the disease, if at all.
Symptoms of DSPN secondary to diabetes depend on the 
affected fi

N secondary to diabetes depend on the 
affected fiber types. Early in the course of the disease, small fibers 
are preferentially involved, leading to symptoms of pain, hyper­
algesia and allodynia in the lower limbs, followed over time by a 
loss of thermal sensitivity and pain perception.621 These same early 
symptoms of DSPN are also the main constellation of presenting 
Hyperglycaemia
Dyslipidaemia
Triglycerides
LDL
HDL
FFA
Glucose
Protein
AGEs
RAGE
Hexosamine
pathway
Polyol
pathway
Glycolys

Es
RAGE
pathway
Polyol
pathway
LOX1
TLR4
PI3K
Akt
Oxidised
LDL
AGE-LDL
Both
A
B
Mechanisms of cell damage
Cell damage       nerve dysfunction
Insulin
Insulin + C-peptide
Insulin
Insulin
signalling
Loss of
neurotrophic
signals
complex
Glucose
Apoptosis
Inflammatory signals
Vascular endothelial cells
Neurons
Glial cells
ER stress
DNA damage
ROS (*)
Osmotic
stress *
Electron
transport
o

mage
ROS (*)
Osmotic
stress *
Electron
transport
overload *
NADPH oxidase *
FFAs
• Fig. 37.43  Mechanisms of diabetic neuropathy. Factors linked to type 1 diabetes (red), type 2 diabetes 
(blue), and both (green) cause DNA damage, endoplasmic reticulum stress, mitochondrial complex dys­
function, apoptosis, and loss of neurotrophic signaling (A). These mechanisms can damage neurons, glial 
cells, and vascular endothelial cells and can trigger macrophage activation. Damage t

ls and can trigger macrophage activation. Damage to each of these cell 
types can lead to nerve dysfunction and neuropathy (B). The relative importance of the pathways in this 
network will vary with cell type, disease profile, and time. AGE, Advanced glycation end products; ER, 
endoplasmic reticulum; FFA, free fatty acids; HDL, high-density lipoprotein; LDL, low-density lipoprotein; 
LOX1, oxidized LDL receptor 1; PI3K, phosphatidylinositol-3-kinase; RAGE, receptor for advanced gly­
cation end

inase; RAGE, receptor for advanced gly­
cation end products; ROS, reactive oxygen species (red star); TLR4, toll-like receptor 4. (Redrawn from 
Callaghan BC, Cheng HT, Stables CL, et al. Diabetic neuropathy: clinical manifestations and current treat­
ments. Lancet Neurol. 2012;11:521–534.)
1492
symptoms in prediabetic neuropathy.600 Symptoms often are exac­
erbated at night and are manifested in the feet more than the hands. 
Spontane

ifested in the feet more than the hands. 
Spontaneous episodes of pain can be severely disabling. The pain 
varies in intensity and character and has been variably described by 
patients as lancinating, stabbing, or sharp. Paresthesias or episodes 
of distorted sensation, such as pins and needles, tingling, coldness, 
numbness, or burning, often accompany the pain.633 As a whole, 
these multiple symptoms are classified as positive symptoms, as the 
affected diabetic patient actively experiences 

e 
affected diabetic patient actively experiences discomfort.
In contrast, the symptoms of large fiber neuropathy are most 
commonly denoted as negative symptoms, with the patient pre­
senting with a numb, insensate foot. Diabetic patients with large 
fiber predominant DSPN are frequently unaware of their neurop­
athy, and are surprised when a physical examination reveals absent 
sensation in their feet. As large fibers mediate position sense, poor 
balance with frank falling is a common present

or 
balance with frank falling is a common presenting symptom in 
diabetic patients with a large fiber predominant DSPN. While less 
common, these patients can also experience a deep-seated, dull, 
toothache-like sensation in their feet and, in very severe cases, will 
experience symptoms of distal weakness of the toes and ankles, 
secondary to late-onset large motor fiber involvement.
While the distinction is classically made between small and 
large fiber predominant DSPN, a common presentatio

large fiber predominant DSPN, a common presentation of DSPN 
in diabetic patients is a combination of symptoms from both fiber 
types. A few clinical “pearls” can assist the clinician in identify­
ing symptoms that are atypical for DSPN secondary to diabetes. 
These include motor greater than sensory symptoms and a report­
able asymmetry. If the physical examination confirms these symp­
toms, an alternative diagnosis to DSPN must be pursued. 
Clinical Signs of Diabetic Neuropathy
A simple examin

nical Signs of Diabetic Neuropathy
A simple examination can distinguish between small and large 
fiber impairment. Inspection of the skin is an essential first part of 
the clinical examination because small fibers also subserve sudo­
motor function. The foot is inspected for dry skin, fissures, and 
callouses. Small fiber function is then assessed using a simple dis­
posable safety pin, a cotton wisp, or by assessing the ability of a 
patient to distinguish temperatures between a proximal locat

 distinguish temperatures between a proximal location 
(the face) and a distal location (the dorsum of the great toe). A 
simple bedside approach is to determine if the patient can dif­
ferentiate between the sharp and dull end of the safety pin on 
the dorsum of the toe, and if he or she can perceive a cotton 
wisp in the same anatomic location. To assess temperature in the 
most simple manner, a cool tuning fork is placed on the patient’s 
cheek, then the same instrument is placed on the great

k, then the same instrument is placed on the great toe, and 
the patient is asked to report on perceiving the same cool tem­
perature in both anatomic locations or not. Large fiber function is 
initially assessed by measuring the time in seconds that a patient 
can perceive vibration sensation on the great toe after a health 
care provider places a 128-Hz vibrating tuning fork on the dor­
sum of the patient’s great toe. Proprioception is assessed by minor 
movements of the first metatarsophalang

 by minor 
movements of the first metatarsophalangeal joint of the great toe. 
Ankle reflexes are an essential part of assessing large fiber function 
and are best produced with the examiner mildly dorsiflexing the 
patient’s foot and striking the Achilles tendon with a Tromner or 
Queens Square reflex hammer.
Given that the glove and stocking distribution of the DSPN 
reflects nerve fiber length, when sensory loss reaches the mid­
calves, patients begin to perceive symmetric sensory loss of the

ts begin to perceive symmetric sensory loss of the 
fingertips. Examination of the upper extremity is most commonly 
performed as outlined earlier for the lower extremity, except the 
dorsum of the index finger is used by the examiner.
The Semmes-Weinstein 10-g filament as an assessment of touch 
pressure, a large fiber function, can be used as a screening tool for 
DSPN.634 The filament is bent to a C-shape applying pressure to 
the plantar surface of the great toe. Because of the variability i

ace of the great toe. Because of the variability in 
the sensitivity and specificity of the 10-g filament,635 it is recom­
mended that a three-site test be done involving the plantar aspects 
of the great toe, the third metatarsal, and the fifth metatarsal. The 
ability to sense the 10-gram filament on the soles of the feet is 
also frequently used by podiatrists as a screening tool for DSPN, 
although it is generally held that this is not sufficiently sensitive to 
diagnose DSPN.634
In advanced

iently sensitive to 
In advanced cases of DSPN, examination will reveal wasting of 
the small muscles of the feet, with hammertoes and weakness of 
toe extension; in extreme cases, there is weakness of the anterior 
tibialis and the intrinsic hand muscles. 
Differential Diagnosis of Diabetic Neuropathy
The American Academy of Neurology suggests checking a serum 
protein electrophoresis with immunofixation, a B12 level, and thy­
roid function tests in all patients suspected of h

roid function tests in all patients suspected of having DSPN. If 
these are normal, and there are no atypical signs, the most likely 
diagnosis is prediabetic neuropathy or DSPN.631 The importance 
of performing these screening tests was confirmed in the Roches­
ter Diabetic Neuropathy Study, in which up to 10% of peripheral 
neuropathy in diabetic patients was determined to not be second­
ary to diabetes.636
Importantly, the symptoms and signs of DSPN are sensory and 
symmetrical, and any convi

f DSPN are sensory and 
symmetrical, and any convincing asymmetry evidence of motor 
greater than sensory involvement, or a rapid and aggressive clinical 
course, should lead the clinician to suspect an alternative etiology 
for the patient’s clinical presentation. In these atypical cases, it is 
essential to exclude other causes of neuropathy in patients with 
diabetes (Table 37.9).637
Classification of Diabetic DSPN
The Toronto Consensus panel convened in 2009 and redefined the 
minimal criter

convened in 2009 and redefined the 
minimal criteria for diagnosis of typical diabetic neuropathy638:
	1.	 Possible DSPN. The presence of symptoms or signs of DSPN 
may include the following: symptoms—decreased sensation, 
positive neuropathic sensory symptoms (e.g., “asleep numb­
ness,” prickling or stabbing, burning or aching pain) predomi­
nantly in the toes, feet, or legs; or signs—symmetric decrease 
of distal sensation or unequivocally decreased or absent ankle 
reflexes.
	2.	 Probable D

reased or absent ankle 
reflexes.
	2.	 Probable DSPN. The presence of a combination of symptoms 
and signs of neuropathy, including any two or more of the fol­
lowing: neuropathic symptoms, decreased distal sensation, or 
unequivocally decreased or absent ankle reflexes.
	3.	 Confirmed DSPN: The presence of an abnormality of nerve 
conduction and a symptom(s) or sign(s) of neuropathy con­
firm DSPN. If nerve conduction is normal, a validated mea­
sure of small fiber neuropathy (with Class 1 ev

a­
sure of small fiber neuropathy (with Class 1 evidence) may be 
used. To assess for the severity of DSPN, several graded clinical 
scores can be used that include various continuous measures of 
sum scores of neurologic signs, symptoms, or composite clini­
cal and electrophysiologic scores; scores of function of activities 
of daily living; or scores of predetermined tasks or of disability.
	4.	 Subclinical DSPN. The presence of no signs or symptoms of 
neuropathy are confirmed with abnormal 

mptoms of 
neuropathy are confirmed with abnormal nerve conduction or 
a validated measure of small fiber neuropathy (with Class 1 evi­
dence). Definitions 1, 2, or 3 can be used for clinical practice, 
and definitions 3 or 4 can be used for research studies.
1493
	5.	 Small fiber neuropathy (SFN). SFN should be graded as fol­
lows: (1) possible: the presence of length-dependent symptoms 
and/or clinical signs of small fiber damage; (2) probable: t

nical signs of small fiber damage; (2) probable: the 
presence of length-dependent symptoms, clinical signs of small 
fiber damage, and normal sural nerve conduction; and (3) 
definite: the presence of length-dependent symptoms, clinical 
signs of small fiber damage, normal sural nerve conduction, 
and altered intraepidermal nerve fiber (IENF) density at the 
ankle and/or abnormal thermal thresholds at the foot (both of 
the later provide Class 1 evidence). 
Graded Scores for the Classification 

 evidence). 
Graded Scores for the Classification of DSPN
While DSPN is diagnosed at the bedside, as outlined earlier, there 
are several scores that provide structure to the bedside examina­
tion and range from simple assessments of large and small fibers 
to more complex assessments requiring quantitative thermal test­
ing, electrophysiology, or skin biopsies. These scores include the 
Neuropathy Impairment Score of the lower limb (NIS-LL),639 
Michigan Diabetic Neuropathy Score (MDNS),640 Mod

Michigan Diabetic Neuropathy Score (MDNS),640 Modified 
Toronto Clinical Neuropathy Score (mTCNS),641 Total Neuropa­
thy Score–Clinical (TNS-C),642 the Utah Early Neuropathy Score 
(UENS),643 and the Neuropathy Disability Score (NDS).644 The 
sensitivities and specificities are listed in Table 37.10. Electrophys­
iologic testing or nerve conduction studies along with a referral to 
a neurologist is normally not needed, unless the patient has either 
atypical symptoms or signs.
The importance of

her 
The importance of the skin biopsy as a diagnostic tool for 
DSPN is increasingly recognized when no clear objective evidence 
of small or large fiber dysfunction is present on examination.621 
This technique quantitates small IENFs through antibody stain­
ing of the neuronal antigen protein gene product (PGP) 9.5. It is 
minimally invasive (3-mm-diameter punch biopsies) but enables 
direct study of small fibers that cannot be evaluated by standard 
nerve conducti

at cannot be evaluated by standard 
nerve conduction studies. Prediabetic neuropathy and DSPN are 
accompanied by loss of these small fibers in a distal-to-proximal 
gradient. 
Treatment of DSPN
Treatment of DSPN is commonly divided into either direct dis­
ease-modifying therapies or treatment of pain, a common occur­
rence in patients with DSPN. Accumulating evidence supports 
the idea that disease-modifying therapies are distinct for DSPN, 
depending if the patient has T1DM or T2DM.645 In cont

ending if the patient has T1DM or T2DM.645 In contrast, 
treatment of pain is not personalized to the type of diabetes, 
  Common Causes of Rare Subtypes of Peripheral Neuropathy
Condition
Diffuse, nonlength-dependent 
neuropathy; demyelinating 
sensory motor
AIDP; CIDP; CIDP variants: POEMS syndrome,
IgM anti-MAG neuropathy, Waldenstrom acroglobulinemia, and MGUS; diphtheria; and toxic exposures (hexane, arsenic, and 
Demyelinating sensory
Sensory CIDP or AIDP; and DAD

emyelinating sensory
Sensory CIDP or AIDP; and DADS (IgM anti-MAG neuropathy)
Demyelinating motor
MMN
Axonal sensory motor
Toxic exposures; ASMAN; and AIP
Axonal sensory
Paraneoplastic (Hu, CRMP5, and amphyphisin); Sjögren syndrome; chemotherapy (platinum based, bortezomib); vitamin 
B6 toxicity; idiopathic; HIV, HTLV; autoimmune hepatitis; celiac disease; HSAN; Friedrich ataxia; CANVAS; SANDO; and 
CANOMAD
Axonal motor
ALS; PMA; postpolio syndrome; HIV, HTLV, WNV, enterovirus D68; MMN without c

me; HIV, HTLV, WNV, enterovirus D68; MMN without conduction block; radiation; monomelic 
amyotrophy; HMN; SMA (including Kennedy syndrome); and complicated HSP
Multiple mononeuropathies
Systemic vasculitic neuropathy: microscopic polyangiitis, Wegener granulomatosis, polyarteritis nodosa, Churg-Strauss syn­
drome, cryoglobulinemia, Sjögren syndrome, rheumatoid arthritis, and SLE; nonsystemic vasculitic neuropathy; neoplasm 
(malignant and benign); HNPP; sarcoidosis; amyloidosis; MMN; and MADSAM

; HNPP; sarcoidosis; amyloidosis; MMN; and MADSAM
Polyradiculopathy
Compressive: disc herniation/spondylosis, osteomyelitis, and neoplasm; noncompressive: infection (CMV, VZV, Lyme, and 
tuberculosis), inflammatory (sarcoidosis), neoplastic (leukemia and lymphoma), and radiation
Plexopathy
Compressive: neoplasm and hemorrhage;
noncompressive: infection (VZV, HSV, CMV, and Lyme), inflammatory (sarcoidosis), neoplastic (leukemia, lymphoma), and 
radiation
Diabetic lumbar 

diation
Diabetic lumbar (diabetic amyotrophy); diabetic cervical; postsurgical inflammatory; nondiabetic lumbar or cervical; infection 
(VZV, HSV, CMV, and Lyme); inflammatory (sarcoidosis); neoplastic (leukemia and lymphoma); and radiation
AIDP, Acute inflammatory demyelinating polyneuropathy; AIP, acute intermittent porphyria; ALS, amyotrophic lateral sclerosis; ASMAN, acute sensory motor axonal neuropathy; CANOMAD, chronic ataxic 
neuropathy, ophthalmoplegia, monoclo

ronic ataxic 
neuropathy, ophthalmoplegia, monoclonal IgM protein, cold agglutinins, disialosyl antibodies; CANVAS, cerebellar ataxia neuropathy vestibular areflexia syndrome; CIDP, chronic inflammatory demyelinat­
ing polyneuropathy; CMV, cytomegalovirus; CRMP5, collapsing response mediator protein 5; DADS, distal-acquired demyelinating syndrome; HIV, human immunodeficiency virus; HMN, hereditary motor 
neuropathy; HNPP, hereditary neuropathy with liability to pressure palsies; HSAN, hereditary

th liability to pressure palsies; HSAN, hereditary sensory autonomic neuropathy; HSP, hereditary spastic paraplegia; HSV, herpes simplex virus; HTLV, human 
T-lymphotrophic virus; MADSAM, multifocal acquired demyelinating sensory and motor neuropathy; MGUS, monoclonal gammopathy of unclear significance; MMN, multifocal motor neuropathy; PMA, 
progressive muscular atrophy; POEMS, polyneuropathy, organomegaly, endocrinopathy, monoclonal gammopathy, and skin changes; SANDO, sensory ataxia neuropath

 and skin changes; SANDO, sensory ataxia neuropathy dysarthria ophthalmoplegia; SLE, 
systemic lupus erythematosus; SMA, spinal muscular atrophy; VZV, varicella zoster virus; WNV, West Nile virus.
Reproduced with permission from Callaghan BC, Price RS, Chen KS, et al. The importance of rare subtypes in diagnosis and treatment of peripheral neuropathy: A review. JAMA Neurol. 2015; 72:1510–
1518. Copyright ©2015 American Medical Association. All rights reserved.
   
TABLE 37.9
1494
SECTION VIII   

ghts reserved.
   
TABLE 37.9
1494
although emerging research indicates there may be differences in 
the pain phenotypes among the different types of diabetes.602
Disease-Modifying Therapy in DSPN
There are no US Food and Drug Administration (FDA)–approved 
therapies that directly address DSPN as a disease entity. In the 
absence of pharmaceuticals, dietary and lifestyle interventions 
are recommended to modify the course of the illnes

are recommended to modify the course of the illness. A 2012 
Cochrane review reported that glucose control has only modest 
effects, if any at all, on DSPN in T2DM, while it is an impor­
tant factor in determining the onset and progression of DSPN 
in T1DM.645 In both prediabetes and T2DM, the components 
of the metabolic syndrome are collectively drivers of neuropathy. 
Multiple studies confirm that dyslipidemia, hypertension, obesity, 
and low hip:waist ratios are present in patients with pred

hip:waist ratios are present in patients with prediabetic 
neuropathy and T2DM patients with DSPN. Modification of 
these factors, beyond glucose control alone, is required to effect 
a clinically meaningful change in neuropathy in both patient 
groups. Of interest, obese patients with the metabolic syndrome 
but normoglycemia also have neuropathy, further confirming that 
glucose control alone is insufficient to address neuropathy patho­
genesis in the prediabetic or T2DM patient.600
The 2017 A

 in the prediabetic or T2DM patient.600
The 2017 American Diabetes Association (ADA) recommen­
dations are outlined in Table 37.11.646 
Treatment of Painful Neuropathy
Pain is a common presentation of prediabetic neuropathy and 
present in approximately 20% of patients with DSPN.647 There 
are no compelling studies that show glucose control is helpful in 
the treatment of pain. In contrast, aggressive glucose control over 
a short period of time can result in debilitating pain syndromes in 
both

can result in debilitating pain syndromes in 
both T1DM and T2DM patients.648 These pain syndromes are 
known as treatment-induced neuropathies. It is now well estab­
lished that the degree of a patient’s impairment correlates with 
the rate of change of the HbA1c (Fig. 37.44),649 cautioning the 
clinician to achieve glycemic control at a rate of less than 1% per 
month HbA1c.649
There are three FDA-approved drugs for the treatment of pain­
ful DSPN, with better evidence for duloxetine and prega

SPN, with better evidence for duloxetine and pregabalin. 
The opioid tapentadol also is FDA approved, but with weaker 
evidence. In light of the current opioid epidemic in both North 
America and worldwide, opioid use is discouraged in the treat­
ment of painful DSPN.
Both the American Academy of Neurology (AAN) and the 
European Federation of Neurological Societies (EFNS) have 
reviewed the Class 1 and 2 studies for the treatment of painful 
DSPN (Table 37.12). In general, three classes of ther

N (Table 37.12). In general, three classes of therapies appear 
effective in the treatment of DSPN: the voltage-gated α2δ ligands 
(pregabalin, gabapentin), the serotonin-norepinephrine reuptake 
inhibitors (duloxetine, venlafaxine), and the secondary amine tri­
cyclic antidepressants (amytriptyline, nortriptyline, desipramine). 
Fig. 37.45 provides a management algorithm for the patient with 
painful DSPN; Table 37.13 outlines the commonly used doses 
and range of side effects. A simple rule is

doses 
and range of side effects. A simple rule is to begin with one drug 
from one of the three classes, and maximize the dose of this drug 
to either side effects or known therapeutic dosage; if there is some 
clinical benefit, although not complete, then a second agent from 
a different drug class should be added for more symptom relief. 
This agent is again titrated slowly to either side effects or maxi­
mal known therapeutic dose. While figures vary, this approach 
generally provides relief

res vary, this approach 
generally provides relief for at least two-thirds, if not more, of 
patients618; there is also an emerging literature that coupling pain 
treatment with an exercise regimen provides even greater symptom 
  Sensitivity and Specificity for Tested Neuropathy Scales
Cutoff
PPV
NPV
LRP
LRN
Modified Toronto Clinical Neuropathy Score (mTCNS)
3.00
98.00
97.00
0.99
0.94
31.20
0.03
Total Neuropathy Score (Clinical) (TNSc)
5.00
81.00
97.00
0.99
0.66
25.90
0

linical) (TNSc)
5.00
81.00
97.00
0.99
0.66
25.90
0.20
Utah Early Neuropathy Scale (UENS)
3.00
85.00
97.00
0.99
0.72
27.20
0.15
Early Neuropathy Score (ENS)
5.00
83.00
97.00
0.99
0.67
26.67
0.17
Michigan Diabetic Neuropathy Score (MDNS)
5.00
80.00
100.00
1.00
0.65
—
0.20
Neuropathy Impairment Score of the Lower Limb (NISLL)
3.00
83.00
97.00
0.98
0.69
26.47
0.18
Neuropathy Disability Score (NDS)
4.00
89.00
100.00
1.00
0.78
—
0.11
LRN, Likelihood ratio negative; LRP, likelihood ratio positive; NPV,

tio negative; LRP, likelihood ratio positive; NPV, negative predictive value; PPV, positive predictive value.
Modified from Zilliox LA, Ruby SK, Singh S, et al. Clinical neuropathy scales in neuropathy associated with impaired glucose tolerance. J Diabetes Complications. 2015;29:372–377.
   
TABLE 37.10
  American Diabetes Association 
Recommendations for Distal Symmetric 
Polyneuropathy (DSPN)
 • Tight glucose control targeting near-normal glycemia in patients 
with type 1 diabetes dramatical

cemia in patients 
with type 1 diabetes dramatically reduces the incidence of distal 
symmetric polyneuropathy and is recommended for distal symmetric 
polyneuropathy prevention in type 1 diabetes. A
 • In patients with type 2 diabetes with more advanced disease and 
multiple risk factors and comorbidities, intensive glucose control 
alone is modestly effective in preventing distal symmetric polyneu­
ropathy, and patient-centered goals should be targeted. B
 • Lifestyle interventions are recom

 targeted. B
 • Lifestyle interventions are recommended for distal symmetric 
polyneuropathy prevention in patients with prediabetes/metabolic 
syndrome and type 2 diabetes. B
ADA recommendations are assigned ratings of A, B, or C depending on the quality of ­evidence.
From Pop-Busui R, Boulton AJM, Feldman EL, et al. Diabetic neuropathies: a position state­
ment by the American Diabetes Association. Diabetes Care. 2017;40:136–154. Copyright 
and all rights reserved. Material from this publicat

d all rights reserved. Material from this publication has been used with the permission of 
the American Diabetes Association.
   
TABLE 37.11
1495
relief.650 This approach to pain is recommended in the 2011 AAN 
guidelines and the 2017 ADA position statement.
While not part of the ADA position statement, the AAN 
guidelines discuss the addition of selected therapies, which are 
briefly described later, for patients who do not achieve pain 
control 

er, for patients who do not achieve pain 
control with metabolic control, combination therapy, and exer­
cise.651 Alpha-lipoic acid is an antioxidant that is used in Europe 
at a dose of 600 mg per day as an adjunct to the treatment of 
painful DSPN.652,653 Capsaicin is a natural substance that locally 
depletes substance P, resulting in a loss of small fibers. Capsaicin 
cream applied topically (0.075% four times/day) to the feet in 
patients with painful DSPN was reported to provide a modest 

th painful DSPN was reported to provide a modest 
improvement in pain.654 Similarly local lidocaine applied to 
painful areas has been reported to improve pain in selected small 
studies,655 although a Cochrane systematic review found no role 
for lidocaine in the treatment of painful neuropathy.656 Studies 
on the use of transelectric stimulation (TENS) units are limited 
for painful DSPN, but do support its use as an adjuvant therapy 
in treatment-resistant patients.657–659
There is no role fo

ent-resistant patients.657–659
There is no role for opioids in the treatment of chronic pain­
ful DSPN, and limited use, if at all, in acute painful DSPN, 
possibly for the pain experienced with treatment-induced neu­
ropathy. If pain management is not achieved without opioids, 
then referral to a pain clinic for the patient is the next sug­
gested step. 
Other Diabetic Somatic Neuropathies: Clinical 
Features and Treatment Paradigms
As presented in Table 37.7, there are other diabetic neuropa­

 in Table 37.7, there are other diabetic neuropa­
thies. Cranial mononeuropathies occur primarily in the older 
population. Onset is usually acute and associated with pain, 
and the course is self-limited, resolving within 6 to 8 weeks. 
It is generally believed these neuropathies are secondary to 
vascular insufficiency.660 The most common cranial mononeu­
ropathy secondary to diabetes is a mononeuropathy of the third 
cranial nerve, presenting as a complete third nerve palsy with 
pupillary sp

as a complete third nerve palsy with 
pupillary sparing. Less common are mononeuropathies of cra­
nial nerves 4, 6, and 7. Because it is essential to distinguish 
these mononeuropathies from a more serious central etiology, 
including cranial aneurysms and stroke, neuroimaging is fre­
quently required to confirm that the symptoms and signs are 
due soley to diabetes.
Mononeuropathies must be distinguished from entrapment 
syndromes, which start slowly, progress, and persist without 
intervention

lowly, progress, and persist without 
intervention (see Table 37.8). The most common entrapment 
site in diabetic patients is the median nerve, although patients 
also experience ulnar, lateral cutaneous femoral, and peroneal 
nerve entrapment. Median nerve entrapment, commonly known 
as the carpal tunnel syndrome, occurs three times as often in per­
sons with diabetes than in a normal, healthy population,661,662 
and its increased prevalence in diabetes may be related to dia­
betic cheiroarthro

diabetes may be related to dia­
betic cheiroarthropathy,663 repeated undetected trauma (due to 
repetitious use of the hands), metabolic changes, or accumula­
tion of fluid or edema within the confined space of the carpal 
tunnel.664 Diagnosis can be confirmed by nerve conduction 
studies or ultrasound.665 The mainstays of nonsurgical treat­
ment are resting of the wrist, aided by the placement of a wrist 
splint in a neutral position for day and night use, and the addi­
tion of anti-inflammator

 night use, and the addi­
tion of anti-inflammatory medications. Surgical treatment con­
sists of sectioning the volar carpal ligament.666 The decision to 
proceed with surgery should be based on several considerations, 
A
Months
HbA1c
–15 –10 –5
15
10
5
15
20
10
5
0
Pain scores
8
10
6
4
2
0
B
Months
HbA1c
–15 –10 –5
15
10
5
15
20
10
5
0
8
10
6
4
2
0
C
D
E
F
Months
HbA1c
–15 –10 –5
15
10
5
15
20
10
5
0
8
10
6
4
2
0
• Fig. 37.44  Complications and risks associated with tre

37.44  Complications and risks associated with treatment-induced 
neuropathy of diabetes (TIND). (A–F) The 104 individuals with treatment-
induced neuropathy of diabetes are grouped by change in glycosylated 
HbA1c scores. (A) The 27 individuals with a decrease in HbA1c of 2% to 
3.9% are shown. (B) The 52 individuals with a decrease in HbA1c of 4% 
to 7% are shown. (C) The 25 individuals with a decrease in HbA1c of more 
than 7% are shown. (A–C) The lower portion of the graph (left y-axis) reve

 The lower portion of the graph (left y-axis) reveals 
the glycosylated haemoglobin (HbA1c) scores over time. The mean value is 
shown in blue and the standard deviation in light purple. The upper portion 
of the graph (right y-axis) reveals neuropathic pain scores during the same 
time frame. The mean value is shown in black and the standard deviation 
in dark purple. The representative distribution of neuropathic is shown in 
(D–F), with the area in dark purple representing pain common to all 

ea in dark purple representing pain common to all individu­
als, and the area in light purple common to many individuals. (D) The least 
widespread pain distribution in the individuals with the smallest change in 
HbA1c (corresponding to A). (E) The pain distribution in the individuals with 
moderate decreases in HbA1c (B). (F) The group with the largest decrease 
in HbA1c. (C) has widespread neuropathic pain. (Redrawn from Gibbons 
CH, Freeman R. Treatment-induced neuropathy of diabetes: an acu

. Treatment-induced neuropathy of diabetes: an acute, iat­
rogenic complication of diabetes (part 1). Brain. 2015;138:43–52.)
1496
  Class I and Class II Randomized Controlled Trials From the American Academy of Neurology and European Federation of Neurologic 
Societies Guidelines on the Treatment of Painful Diabetic Distal Symmetric Polyneuropathy
Sourcec
Treatment per Day
Evidence 
Classa
Study 
Duration (wk)
No. Receiving 
Treatmen

lassa
Study 
Treatment/ Total 
Sampleb
Mean Pain Reduction 
on 0-10 Rating Scale 
vs. Placebo (95% Cl)
PATIENTS WITH >50% PAIN 
REDUCTION
Common Adverse Effects
Treatment 
Effect (%)
Placebo 
Effect (%)
Lesser et al., 2004
Pregabalin, 300 mg
5
81/337
−1.26 (−1.86 to −0.65)
46
18
Dizziness, somnolence, 
peripheral edema, confu­
sion, blurry vision
Rosenstock et al., 2004
I
8
76/146
−1.47 (−2.19 to −0.75)
40
14.5
Lesser et al., 2004
Pregabalin, 600 m

.75)
40
14.5
Pregabalin, 600 mg
I
5
82/337
−1.45 (−2.06 to −0.85)
48
18
Richter et al., 2005
I
6
72/223
−1.26 (−1.89 to −0.64)
39
15
Freynhagen et al., 2005
Pregabalin, 
300–600 mg
II
12
82/209
Approximately −1.4 to 
1.6 (p = 0.002)
48-52
24
Backonja et al., 1998
Gabapentin, 
900–3600 mg
I
8
70/135
−1.2 (−1.9 to −0.6)
Not reported; 60% treated with 
gabapentin had at least moder­
ate improvement (>30%) vs 
33% treated with placebo
co

3% treated with placebo
confusion
Gorson et al., 1999
Gabapentin, 900 mg
II
6
19/30
No difference
Not reported; 42.5% treated with 
gabapentin reported moderate 
or excellent pain relief vs 22.5% 
treated with placebo
Simpson, 2001
900-3600 mg
II
8
27/54
−1.9 (Not reported; 
p < 0.01)
Not reported; 55.5% treated with 
gabapentin reported much to 
moderate improvement vs 25.9% 
Vrethem et al., 1997
Amitriptyline, 75 mg
I
4
33/99
−1.8 (Not 

l., 1997
I
4
33/99
−1.8 (Not reported; 
p < 0.001)
Not reported; 63% of patients 
treated with amitriptyline had at 
least 20% improvement vs 22% 
Dry mouth, sedation, vertigo
Max et al., 1987
Amitriptyline, 
25–150 mg
II
6
29 (Crossover)
Not reported
Not reported; 65.5% treated with 
amitriptyline reported moderate 
to complete improvement vs 
3.5% treated with placebo
Raskin et al., 2005
Duloxetine, 60 mg
I
12
116/348
−0.9 (−1.39 to −0.42)
50
30
Nausea

mg
I
12
116/348
50
30
Nausea, somnolence, hyper­
hidrosis, anorexia
TABLE 37.12
1497
Goldstein et al., 2005
II
12
86/344
−1.17 (−1.84 to −0.5)
49
26
Wernicke et al., 2006
II
12
85/248
−1.32 (−1.95 to −0.69)
43
27
Duloxetine, 120 mg
I
12
116/348
−0.87 (−1.36 to −0.39)
39
30
II
12
80/344
−1.45 (−2.13 to −0.78)
52
26
Wernicke et al., 

344
52
26
II
12
78/248
−1.44 (−2.08 to −0.81)
53
27
Rowbotham et al., 2004
Venlafaxine, 
150–225 mg
I
6
82/242
−0.7 (Not reported; 
p < .001)
56
34
Nausea, dyspepsia, sweating, 
somnolence, insomnia, 
blood pressure and cardia 
rhythm changes
aClass I randomized controlled trials must have allocation concealment, clearly defined primary outcomes, and inclusion and exclusion criteria with greater than 80% of patients completing the s

with greater than 80% of patients completing the study. Class II randomized controlled trials lack one or more of the require­
ments listed for class I studies.
bNumber of participants receiving the dosage in column 2 out of the total number of participants in the trial. Many trials had multiple intervention groups.
cRefer to source article for full reference listings.
Modified with permission from Callaghan BC, Price RS, Chen KS, et al. Distal symmetric polyneuropathy: a review. JAMA. 2015;14:2

ymmetric polyneuropathy: a review. JAMA. 2015;14:2172–2181. Copyright ©2015 American Medical Association. All rights reserved.
   
1498
including severity of symptoms, appearance of motor weakness, 
and failure of nonsurgical treatment.667
Another distinct class of diabetic neuropathies includes the 
diabetic polyradiculoneuropathies. This disorder is also known by 
many other names, including proximal motor neuropathy, diabetic 
amyot

cluding proximal motor neuropathy, diabetic 
amyotrophy, diabetic femoral neuropathy, and diabetic radiculo­
plexus neuropathy. It primarily affects older T2DM male patients. 
The onset of the disease can be gradual but is more commonly 
abrupt, presenting with excruciating pain in the thighs and hips or 
buttocks, followed by significant weakness of the proximal muscles 
of the lower limbs. The neuropathy begins unilaterally, but fre­
quently spreads bilaterally, and coexists with DSPN. Electro

reads bilaterally, and coexists with DSPN. Electrophysi­
ologic evaluation reveals lumbosacral plexopathy most commonly 
superimposed on DSPN.668 Patients usually clinically resolve with 
time, and the general course of treatment is supportive. Use of intra­
venous gammaglobulin is reported to accelerate the resolution of 
symptoms,669 but must be used with caution in diabetic patients 
because of the greater risk of inducing renal failure.670 
Clinical Features of Diabetic Autonomic 
Neuropathy

linical Features of Diabetic Autonomic 
Diabetic autonomic neuropathy is secondary to dysfunction 
in the sympathetic and parasympathetic nervous systems. The 
pathogenesis underlying diabetic autonomic neuropathy is gener­
ally considered to be similar to DSPN, with a distal-to-proximal 
dying back of axons due to multiple metabolic insults.671 Because 
of the widespread innervation pattern of the autonomic nervous 
system to critical organs, the symptoms and signs of autonomic 
dysf

 organs, the symptoms and signs of autonomic 
dysfunction are varied, dependent on the organ system. These are 
summarized in Table 37.14.646
Cardiac autonomic neuropathy is clinically the  most impor­
tant of the autonomic neuropathies because of its association with 
early cardiac arrhythmia, silent myocardial infarctions, and early 
mortality.672 CAN may present as lightheadedness upon stand­
ing, palpitations, frank syncope, and a generalized sense of weak­
ness. In very early stages, CAN ma

 sense of weak­
ness. In very early stages, CAN may be asymptomatic and only 
Is pain due to DSPN confirmed?
No/not sure
No clinically meaningful effect
Refer to pain clinic
Assess comorbidities, potential for AEs,
(Table 37.13), drug interactions, costs to select
initial therapy from the 3 choices below
a. Switch to another agent
from above
c. May add tramadol or
tapentadol
if a and b fail
No clinically meaningful
effect/not tolerated
b. Try combining agents
from above
Refer to
neurology/pain
c

bining agents
from above
Refer to
clinic
*Voltage gated α2-δ ligand
(pregabalin, gabapentin)
Titrate dose as shown in
Table 37.13
#Secondary amine tricyclic
antidepressant (nortriptyline
desipramine)
**Serotonin-norepinephrine
reuptake inhibitor
(duloxetine, venlafaxine)
Yes
• Fig. 37.45  Algorithm for management of the patient with pain because of distal symmetric polyneuropa­
thy (DSPN). AE, Adverse events

metric polyneuropa­
thy (DSPN). AE, Adverse events. *Pregabalin is FDA approved for painful DSPN, whereas gabapentin is 
not. Pharmacokinetic profile, spectrum of AEs, drug interactions, comorbidities, and costs to be consid­
ered in selecting the agent of choice. **Duloxetine is FDA approved for painful DSPN, whereas venlafax­
ine is not. Pharmacokinetic profile, spectrum of AEs, drug interactions, comorbidities, and costs should 
be considered in selecting the agent of choice. #None is FDA app

in selecting the agent of choice. #None is FDA approved for painful DSPN. Spectrum of 
AEs, drug interactions, and comorbidities need be considered if selecting these agents. (Modified from 
Pop-Busui R, Boulton AJ, Feldman EL, et al. Diabetic neuropathy: a position statement by the American 
Diabetes Association. Diabetes Care. 2017;40:136–154.)
1499
  Treatment for Pain Associated With Distal Symmetric Polyneuropathy
Drug Class
Agenta
DOSE
NNT Ra

etric Polyneuropathy
Drug Class
Agenta
DOSE
NNT Range 30–50% 
Improvementc
Common 
Adverse Events
Major Adverse Events
Initial
Effective
Pregabalinb
25–75 mg, 
1–3×/day
300–600 mg/day
3.3–8.3
Dizziness
Peripheral edema
Headache
Ataxia
Fatigue
Xerostomia
Angioedema
Hepatotoxicity
Suicidal thoughts and behavior
Seizures after rapid 
­discontinuation
Thrombocytopenia
Gabapentin
100–300 mg, 
1–3×/day
900–3600 mg/day
3.3–7.2
Dizziness
A

day
3.3–7.2
Dizziness
Ataxia
Fatigue
Stevens-Johnson syndrome
Serotonin-­
norepinephrine 
reuptake 
­inhibitors
Duloxetineb
20–30 mg/day
60–120 mg/day
3.8–11
Nausea
Dizziness
Dyspepsia
Diarrhea
Anorexia
Headache
Insomnia
Fatigue
Hypertensive crisis
Gastrointestinal hemorrhage
Deliri

rtensive crisis
Delirium
Glaucoma
Shift to mania in patients with 
bipolar disorder
Seizures
Severe hyponatremia
Fragility bone fractures
Serotonin syndrome
Neuroleptic malignant syndrome
Venlafaxine
37.5 mg/day
75–225 mg/day
5.2–8.4
Nausea
Dizziness
Dyspepsia
Diarrhea
Anorexia
Headache
Insomnia
Fatigue
Same as duloxetine
TABLE 37.13

ue
identified by abnormal heart rate variability, either with deep 
breathing or upon standing. Later signs include resting tachycar­
dia with a heart rate above 100 beats per minute and orthostatic 
hypotension defined as a reduction of more than 20 mm Hg in 
systolic or 10 mm Hg in diastolic pressure when going from a 
lying to a standing position.
The 2017 ADA guidelines suggest screening for CAN in 
all diabetic patients with DSPN or other micr

 in 
all diabetic patients with DSPN or other microvascular com­
plications of diabetes (Table 37.15).646 As with DSPN, there 
are no disease-modifying therapies, and T1DM patients are 
instructed to monitor glucose control, while T2DM patients 
are instructed to focus more on general lifestyle parameters and 
controlling all aspects of the metabolic syndrome. These rec­
ommendations are presented in Table 37.16 and parallel those 
for DSPN (see Table 37.11).
Diabetic autonomic neuropathy can in

Table 37.11).
Diabetic autonomic neuropathy can involve the gastrointes­
tinal tract, resulting in esophageal dysmotility, delayed gastric 
emptying, and both constipation and diarrhea with frank fecal 
incontinence. Among these disorders, the best studied is diabetic-
mediated delayed gastric emptying, also known as gastroparesis. 
While estimates vary, the disorder is more prevalent in T1DM 
than T2DM and has a prevalence of approximately 5% in T2DM 
patients.673 The 2017 ADA recommendations f

 T2DM 
patients.673 The 2017 ADA recommendations for the screen­
ing and diagnosis of gastroparesis is presented in Table 37.17.646 
Continued
1500
Treatment of gastroparesis is multifaceted. Patients are instructed 
to eat multiple small meals with lower fat and fiber content and 
to discontinue any drugs that decrease gastric motility, such as 
tricyclic antidepressants or opioids. If these interventions do not 
provide sufficient sy

 these interventions do not 
provide sufficient symptomatic relief, patients can begin meto­
clopramide, the only FDA-approved drug for the treatment of 
gastroparesis. Patients should not take the drug for more than 5 
consecutive days, secondary to the risk of developing extrapyrami­
dal symptoms, and even this short regimen should only be under­
taken when all other interventions have failed.646
Sexual and bladder function are also under the control of the 
autonomic nervous system, and dysfu

ontrol of the 
autonomic nervous system, and dysfunction occurs commonly in 
both with long-term diabetes. Erectile dysfunction is common in 
men and may occur in up to 50% of men over the age of 40. 
Men are more likely to experience erectile dysfunction if they have 
T1DM or T2DM, with an increase in dysfunction with a longer 
duration of diabetes. Erectile dysfunction is further exacerbated 
by obesity, smoking, and hypertension, along with excessive alco­
hol use and selected medications.674

cessive alco­
hol use and selected medications.674 While there is likely a com­
ponent of autonomic dysfunction in most affected individuals, it 
is essential to address the modifiable risks factors that are outlined 
earlier. The phosphodiesterase type 5 inhibitors provide a first line 
of therapy for erectile dysfunction; these drugs include sildenafil, 
tadalafil, and vardenafil.675 Additional therapeutic approaches 
Drug Class
Agenta
DOSE
Common 

 Range 30–50% 
Common 
Initial
Effective
Tricyclic antide­
pressants
Amitriptyline
10–25 mg/day
25–100 mg/day
2.1–4.2
Fatigue
Headache
Dizziness
Insomnia
Orthostatic 
­hypotension
Anorexia
Nausea
Urinary retention
Blurred vision
Accommodation
Disturbance
Mydriasis
Delirium
Conduction abnormalities
Heart failure exacerbation
Stroke
Seizures
Bone 

 exacerbation
Stroke
Seizures
Bone marrow suppression
Shift to mania in bipolar 
disorder
Desipramine
Same as above
Nortriptyline
Opioids
Tramadol
50 mg 1–2×/
day
210 mg/day
3.1–6.4
Nausea
Vomiting
Lightheadedness
Dizziness
Headache
Confusion
Seizures
Hypersen

Seizures
Hypersensitivity reactions
Tapentad­
olb
Immediate 
release: 
50–100 mg 
4–6×/day
Immediate release: 
Day 1: 700 mg; 
after day 1, 
60 mg/day
N/A
Nausea
Respiratory depression
Extended 
release: 50 
mg 2×/day
Extended release: 
50 mg 2×/day
Vomiting
Dizziness
Seizures
Neonatal opioid withdrawal 
syndrome
aRefer to source article for specific studies referenced for each agent

cle for specific studies referenced for each agent.
bFDA-approved.
cFDA considers 30% to 50% improvement to be significant.
NNT, Number needed to treat.
Modified from Pop-Busui R, Boulton AJM, Feldman EL, et al. Diabetic neuropathies: a position statement by the American Diabetes Association. Diabetes Care. 2017;40:136–154. Copyright and all rights 
reserved. Material from this publication has been used with the permission of the American Diabetes Association.
   
  Treatment for Pain Associate

s Association.
   
  Treatment for Pain Associated With Distal Symmetric Polyneuropathy—cont’d
1501
include intracavernosal injections, vacuum devices, psychosexual 
counseling, and cognitive behavioral therapy.675 Generally, a com­
bination of therapies provides the best therapeutic responses.
Bladder and lower urinary tract dysfunction can occur in men 
and women with diabetes. Symptoms include frequent urination 
with urinary urgency

s include frequent urination 
with urinary urgency, nocturia, and a weak urinary stream. In 
older men, these symptoms can be attributed to benign prostatic 
hyperplasia (BPH), but are frequently exacerbated by diabetes. 
While estimates vary, studies suggest up to a twofold increase in 
lower urinary tract symptoms in men with diabetes.676 Urinary 
incontinence occurs in 50% of middle-aged women with dia­
betes; in several studies, the presence of diabetes is reported to 
significantly increase

other comorbidities or drug effects/
interactions that could mimic cardiovascular autonomic neuropathy 
should be performed. E
 • Consider assessing symptoms and signs of cardiovascular auto­
nomic neuropathy in patients with hypoglycemia unawareness. C
ADA recommendations are assigned ratings of A, B, or C depending on the quality of evi­
dence. Expert opinion E is a separate category for recommendations in which there is no 
evidence from clinical trials, in which clinical trials may be impra

ical trials, in which clinical trials may be impractical, or in which there is 
conflicting evidence.
and all rights reserved. Material from this publication has been used with the permission of 
   
TABLE 37.15
Recommendations for Cardiac Autonomic 
Neu

tonomic 
neuropathy in patients with prediabetes. C
dence.
   
TABLE 37.16
1502
SECTION VIII    Disorders 

n.
   
1502
link bladder and lower urinary tract dysfunction to autonomic 
neuropathy are listed in Table 37.18676 and suggest an association 
with bladder and lower urinary tract dysfunction to both CAN 
and DSPN. In both men and women, bladder and lower urinary 
tract dysfunction increase the risk of urinary tract infections.677 
As with all forms of diabetic autonomic neuropathy, patients are 
counseled to achieve glucos

ropathy, patients are 
counseled to achieve glucose control and address other comor­
bidities, including obesity, hypertension, and hyperlipidemia, that 
may contribute to the disorder.678
Hypoglycemic unawareness is another form of diabetic auto­
nomic neuropathy with significant patient morbidity. Hypogly­
cemia is defined as a plasma glucose level of less than 70 mg/dL 
(3.9 mmol/L) without autonomic warning signs.679,680 In most 
patients, two categories of symptoms occur with hypoglycemia: 

o categories of symptoms occur with hypoglycemia: 
neuroglycopenic and autonomic symptoms. Neuroglycopenic 
symptoms are directly correlated with lack of available glucose for 
brain function, including hunger, altered mental status, confusion, 
perioral paresthesias, difficulty speaking, diffuse weakness, and 
syncope. In more severe cases, patients develop seizures, become 
comatose, and (in extreme cases) die. Autonomic symptoms 
Recommendations for Gastroint

iabetes Association 
Recommendations for Gastrointestinal 
 • Evaluate for gastroparesis in people with diabetic neuropathy, 
retinopathy, and/or nephropathy by assessing for symptoms of 
unexpected glycemic variability, early satiety, bloating, nausea, and 
vomiting. C
 • Exclusion of other causes documented to alter gastric emptying, 
such as use of opioids or glucagon-like peptide 1 receptor agonists 
and organic gastric outlet obstruction, is needed before performing 
speciali

obstruction, is needed before performing 
specialized testing for gastroparesis. C
 • To test for gastroparesis, either measure gastric emptying 
with scintigraphy of digestible solids at 15-min intervals for 
4 h after food intake or use a 13C-octanoic acid breath 
test. B
ment by the American Diabetes Associati

ion state­
   
TABLE 37.17
  Studies of Autonomic Neuropathy and Bladder Dysfunction in Diabetes
Authors (Year)b
Overall Population 
(Diabetes Type)
Definition of Autonomic Neuropathy
Definition of Bladder 
Findings
Male + Female
Ueda et al., 1997
63 diabetesa

 Female
Sympathetic skin response
Volume at first desire 
to void
Max bladder capacity
Bladder pressure
Residual urine
Mean vol. at first desire to void, 
max bladder capacity lower for 
sympathetic skin response absent. 
Mean bladder pressure and residual 
urine greater for sympathetic skin 
response absent
Low et al., 2004
231 T1DM/T2DM
Autonomic Symptom Profile (ASP)
Composite Autonomic Severity Score 
(CASS)
ASP urinary domain: 
bladder dysfunction, 
sexual dys

 urinary domain: 
sexual dysfunction 
(males only)
Significant correlations between ASP 
urinary domain and overall CASS 
and domain scores
Kebapci et al., 
2007
54 T2DM
27 males
27 females
CAN: deep breathing, Valsalva, stand test
LUTS: IPSS, urinary 
incontinence, uro­
dynamic studies
QT prolongation associated with 
increased postvoid residual urine 
OR 2.33 (0.16–34.89)
Pavy-Le Traon 
et al., 2010
684 T1DM
CAN severity Ewing Score (0–5): deep 
breathing, Valsalva, stand

wing Score (0–5): deep 
breathing, Valsalva, stand test, HRV, SBS
Bladder dysfunction 
symptoms
Bladder dysfunction independently 
associated with CAN
Males
Pop-Busui et al., 
2015
635 T1DM DCCT/
EDIC study
CAN: R-R variation <15, or R-R varia­
tion 15–19.9 plus Valsalva ratio ≤1.5, 
10 mmHg drop in DBP
LUTS: AUASI 8–35
LUTS prevalence: 158 (25 %)
Odds of ED + LUTS: 2.65 (1.47–4.79)
Bansal et al., 
2011
52 diabetesa
Sympathetic skin response: Medtronic 
electromyographic system
LUTS: IPSS 8–35 u

tronic 
LUTS: IPSS 8–35 uro­
Diabetic cystopathy correlated with 
abnormal motor and sensory nerve 
conduction velocity studies and 
abnormal sympathetic skin responses
Females
Hotaling et al., 
2016
571 T1DM DCCT/
EDIC study
10 mm Hg drop in DBP
UI: Sandvik Severity 
Index 3–12
UI prevalence: 172 (30 %)
aType of diabetes not indicated.
bRefer to source article for full referenc

icated.
bRefer to source article for full reference listing of each study.
AUASI, American Urological Association Symptom Index; CAN, cardiovascular autonomic neuropathy; DBP, diastolic blood pressure; DCCT/EDIC, Diabetes Control and Complications Trial/Epidemiology 
of Diabetes Interventions and Complications; ED, erectile dysfunction; HRV, heart rate variability; IIEF, International Index of Erectile Dysfunction; IPPS, International Prostate Symptom Score; LUTS, lower 
urinary tract symptoms; 

mptom Score; LUTS, lower 
urinary tract symptoms; SBS, spontaneous baroreflex slope; T1DM, type 1 diabetes mellitus; T2DM, type 2 diabetes mellitus; UI, urinary incontinence.
Modified from Braffett BH, Wessells H, Sarma AV. Urogenital autonomic dysfunction in diabetes. Curr Diabetes Rep. 2016;16:119.
   
TABLE 37.18
1503
include diaphoresis, tremulousness, pallor, palpitations, and gen­
eralized anxiety. Hypoglycemic unawareness is defined as the pr

ety. Hypoglycemic unawareness is defined as the pres­
ence of neuroglycopenic symptoms prior to or in the absence of 
autonomic warning symptoms.681
Counterregulatory responses under the control of the auto­
nomic nervous system are required to restore normal glycemia in 
response to hypoglycemia. These essential responses include stimu­
lation of catecholamines (both norepinephrine and epinephrine), 
glucagon, cortisol, and growth hormone, while inhibiting insulin 
secretion.682 These counterre

 inhibiting insulin 
secretion.682 These counterregulatory responses stimulate hepatic 
glucose production while decreasing glucose utilization in the 
periphery, and collectively restore euglycemia (Fig. 37.46). Recur­
rent hypoglycemic episodes decrease the plasma glucose level that 
serves as the setpoint for activating the counterregulatory response 
in both T1DM and T2DM patients.682 Severe hypoglycemia with 
life-threatening neuroglycopenic symptoms is more common in 
T1DM patients with hy

 symptoms is more common in 
T1DM patients with hypoglycemic unawareness and correlates 
with longer duration of diabetes, intensive control, and advanc­
ing age.683,684 While a commonly cited statistic is that 40% of 
T1DM patients experience one or more episodes of hypoglyce­
mic unawareness,682 a recent multinational study using a patient 
self-reported assessment tool, the global Hypoglycemia Assess­
ment Tool (HAT), reported the global incidence of hypoglycemic 
unawareness in T1DM and T2DM

ence of hypoglycemic 
unawareness in T1DM and T2DM patients on insulin therapy as 
97.4% and 95.3%, respectively, with approximately 6.9 events per 
month in T1DM patients and 2.4 events in T2DM patients.685
Regardless of the exact percentages, it is clear that hypoglyce­
mic unawareness is common and a source of significant patient 
morbidity. There is a strong association between recurrent hypo­
glycemia and an increase in fatal cardiac arrhythmias.686 An analy­
sis of unrecognized hypoglycemi

mias.686 An analy­
sis of unrecognized hypoglycemia in 10,096 participants from the 
ACCORD trial revealed a relationship between the number of 
episodes of hypoglycemic unawareness and death, leading to early 
discontinuation of the trial.687 While hypoglycemic unawareness 
does not result in significant permanent cognitive impairment 
in adults,688,689 it represents a major and potentially disabling 
event in children and adolescents.690 Recurrent episodes in young 
children can result in beha

ent episodes in young 
children can result in behavioral problems and neurocognitive 
dysfunction691,692 associated with changes in brain structure.693 
In adults who drive, it can result in potentially fatal automobile 
accidents.
Management of hypoglycemic unawareness begins with pre­
vention, with careful blood glucose monitoring and reasonable 
blood glucose targets personalized to each patient,694 in parallel 
with patient education.695 This approach can be augmented with 
β2-adrenergic age

 approach can be augmented with 
β2-adrenergic agents, caffeine, and serotonin reuptake inhibitors, 
although the effects are small and use of these agents is controver­
sial.682 Most therapeutic efforts focus on glycemic control. Con­
tinuous glucose monitoring (CGM) to identify hypoglycemia can 
decrease the number and severity of adverse hypoglycemic events. 
In a group of T1DM patients with hypoglycemic unawareness, 
CGM reduced severe hypoglycemic events from 8.1 to 0.6 events 
per year.696

ycemic events from 8.1 to 0.6 events 
per year.696 Pharmacologic treatment options begin with optimiz­
ing insulin regimens; for example, rapid-acting insulin analogs 
afford more flexibility for patients with exercise regimens and 
varying mealtimes,697 while long-acting insulin analogs provide 
a flatter, less variable glycemic profile that decreases the rate of 
overall and nocturnal hypoglycemia.698 The use of continuous 
subcutaneous insulin infusion (insulin pump) is very effective in 
dec

 infusion (insulin pump) is very effective in 
decreasing the number of hypoglycemic events,699 with reported 
improvements in patient-reported quality of life assessments.700 
Regardless of the insulin regimen chosen by the patient, person­
alized insulin treatment with appropriate glycemic targets is the 
cornerstone of treatment.682,701 
Diabetic Heart Disease
Coronary Artery Disease
In recent decades, cardiovascular disease mortality rates have 
declined substantially in the general populati

ve 
declined substantially in the general population in the United 
States. However, improvement in cardiovascular mortality rates 
has been significantly less in people with diabetes.8 Diabetes 
remains associated with excess morbidity and mortality rates even 
after adjustment for traditional risk factors. Most studies of car­
diovascular risk in diabetes have been in the T2DM population, 
who represent more than 90% of all patients with diabetes.
Effect of Diabetes on Risk of Coronary Heart D

es.
Effect of Diabetes on Risk of Coronary Heart Disease 
(CHD)
The Framingham Heart Study showed a twofold to threefold eleva­
tion in risk of clinically evident atherosclerotic disease in patients 
with T2DM compared to those without diabetes.702 Diabetic 
Autonomic nervous system
n Glucagon
Liver
Target
organs
Counter-
regulatory
hormones
Effector
organs
Sensor
p Insulin
n Epinephrine
n Cortisol
Adrenal
gland
Adrenal
gland
Brain
Brain
Brain glucose
sensing
brain stem

rain
sensing
brain stem
p Glucose utilization
n Glucose output
D-cell
E-cell
Pancreas
Pancreas
Muscle
Muscle
• Fig. 37.46  Counterregulatory response to hypoglycemia in type 2 dia­
betes. (From Martín-Timón I, del Cañizo-Gómez FJ. Mechanisms of 
hypoglycemia unawareness and implications in diabetic patients. World J 
Diabetes. 2015;6:912–926.)
1504
men in the Multiple Risk Factor Intervention Trial (MRFIT) ha

Multiple Risk Factor Intervention Trial (MRFIT) had 
an absolute risk of cardiovascular death more than three times 
higher than that of the nondiabetic cohort, even after adjustment 
for established risk factors.703 Seminal work from Finland showed 
that patients with T2DM without a previous myocardial infarc­
tion have a risk of myocardial infarction over 7 years as high as that 
of nondiabetic patients with a history of a myocardial infarct.704 
In this study, the case fatality rate after myo

4 
In this study, the case fatality rate after myocardial infarction was 
also substantially higher in patients with diabetes. In women, dia­
betes eliminates the cardioprotective effects of the premenopausal 
period, and women with diabetes have a cardiovascular mortality 
rate as high as that of diabetic men. The follow-up of the Diabe­
tes UK cohort, a group of 23,751 subjects with insulin-treated 
diabetes diagnosed before the age of 30 years, also showed similar 
mortality rates for men and

, also showed similar 
mortality rates for men and women, and the size of this cohort 
permitted robust gender-specific estimates of standardized mortal­
ity ratios.705 For example, among those aged 30 to 39 years, the 
standardized mortality ratios for ischemic heart disease mortal­
ity were 8 and 41.6, respectively. Other forms of cardiovascular 
diseases, such as hypertension, valvular disease, cardiomyopathy, 
heart failure, and stroke, are also increased.
The risks of cardiovascular events 

lso increased.
The risks of cardiovascular events and death conferred by 
T2DM have been examined in several prospective and observa­
tional trials with various populations of patients. The adjusted 
relative risk is generally increased by 1.3 to 1.6 for total mortality, 
mortality attributed to cardiovascular events, nonfatal myocardial 
infarction or stroke, and congestive heart failure.706–709 Many epi­
demiologic studies observe the hazard ratio for cardiovascular mor­
tality risk in patient

tio for cardiovascular mor­
tality risk in patients with diabetes is similar to that in nondiabetic 
patients who had experienced a previous myocardial infarction. 
Whether or not the diagnosis of diabetes itself is a risk equivalent 
to having a history of prior myocardial infarction remains con­
troversial.709,710 However, all studies demonstrate diabetes results 
in a significantly increased risk of cardiovascular disease. Thus 
the strategy of considering diabetes as a CHD risk equivalent is

f considering diabetes as a CHD risk equivalent is 
appropriate for purposes of assessing risk and defining an aggres­
sive multirisk treatment regimen. With more intensive manage­
ment of cholesterol, blood pressure, and decreased smoking, rates 
of myocardial infarction, stroke, amputation, and mortality are 
decreasing among people with diabetes, especially in high-income 
countries. Yet the global burden of cardiovascular disease attrib­
utable to diabetes is not declining due to major incre

le to diabetes is not declining due to major increases in the 
number of people developing diabetes.711
Coronary heart disease risk has also been evaluated in smaller 
numbers of patients with T1DM. In the Framingham study, the 
cumulative cardiovascular mortality rate in patients with T1DM 
was approximately four times that of nondiabetic patients by age 
55 years.710 Patients with T1DM and a HbA1c level of 6.9% or 
lower, considered at goal by most management guidelines, had a 
risk of death f

most management guidelines, had a 
risk of death from any cause or from cardiovascular causes that was 
twice as high as the risk for matched controls.712 As in patients 
with T2DM, the early deaths related to coronary heart disease in 
patients with T1DM may occur by the fourth decade of life, and 
the cumulative mortality rate increases at a similar rate in both 
T1DM and T2DM in the subsequent decades. The increased car­
diovascular mortality rate with age in patients with T1DM is sub­
stanti

rate with age in patients with T1DM is sub­
stantially even higher in those patients who also have nephropathy, 
with gradations of cardiovascular risk evident across the spectrum 
from microalbuminuria to macroalbumuria to end-stage renal dis­
ease, with a 2.8-fold, 9.2-fold, and 18.3-fold increase in mortality 
compared to the general population across these groups.451 There­
fore persistent proteinuria is a strong predictor of the development 
of coronary heart disease in this population. Stu

of coronary heart disease in this population. Studies suggest that 
proteinuria is a marker of generalized vascular damage that predis­
poses to atherosclerosis and coronary events.
It is unclear whether there has been any recent decline in mor­
tality or morbidity rates from coronary artery disease associated 
with T1DM. The Pittsburgh Epidemiology of Diabetes Compli­
cations (EDC) study reported no difference in the cumulative inci­
dence of cardiovascular disease with 20, 25, or 30 years of 

ardiovascular disease with 20, 25, or 30 years of disease 
duration according to year of diagnosis (1950 through 1980).713 
The benefits of improved treatment of hyperglycemia and con­
comitant hypertension and other comorbid conditions with stan­
dard care therefore do not appear to have reduced cardiovascular 
mortality rates associated with T1DM, in contrast to dramatic 
reductions realized for diabetic retinopathy and nephropathy. 
Aggregation of Traditional CHD Risk Factors in Diabetes
Mult

n of Traditional CHD Risk Factors in Diabetes
Multiple traditional cardiovascular risk factors (e.g., hyperten­
sion, dyslipidemia, obesity, insulin resistance) occur together in 
patients with diabetes.714 Approximately 50% of patients with 
T2DM have hypertension, and more than 30% have hypercho­
lesterolemia at the time of diagnosis. As in nondiabetic patients, 
these risk factors independently predict cardiovascular mortal­
ity.703 However, even in the presence of one or more concomitant 
ri

ven in the presence of one or more concomitant 
risk factors, diabetes further increases the cardiovascular death rate 
(Fig. 37.47). It also appears that diabetes interacts synergistically 
with other risk factors to more sharply increase risk as the number 
of total risk factors increases.
The UKPDS further confirmed the importance of risk factor 
aggregation in T2DM as well as the relationship between specific 
risk factors and future cardiovascular events. In this large popula­
tion of patie

scular events. In this large popula­
tion of patients with newly diagnosed T2DM, higher LDL choles­
terol levels, lower HDL cholesterol concentrations, higher HbA1c 
levels and systolic blood pressure measurements, and a history of 
smoking measured at baseline were all associated with a risk of 
future cardiovascular disease.10
Cardiovascular risk associated with T2DM may be a conse­
quence of insulin resistance and its associated abnormalities that 
are often present in the prediabetic state.7

that 
are often present in the prediabetic state.715 In patients spanning 
140
120
100
80
60
40
20
0
Age-adjusted CVD death rate
per 10,000 person-years
None
One only
Two only
All three
Diabetes
*Serum cholesterol >200 mg/dL,
 smoking, SBP >120 mm Hg
Influence of multiple risk factors* on CVD
death rates in diabetic and nondiabetic men
• Fig. 37.47  Age-adjusted cardiovascular disease (CVD) death rates by 
number of risk factors for men with and without diabetes at baseline 
screened

en with and without diabetes at baseline 
screened for the Multiple Risk Factor Intervention Trial. In the presence 
of diabetes, the cardiovascular death rate steeply rises at any level of 
concomitant risk factors. SBP, Systolic blood pressure. (From Stamler J, 
Vaccaro O, Neaton JD, et  al. Diabetes, other risk factors, and 12-year 
cardiovascular mortality for men screened in the Multiple Risk Factor Inter­
vention Trial. Diabetes Care. 1993;16:434–444.)
1505
Chapter 37  Complications of Dia

16:434–444.)
1505
the spectrum of glucose tolerance, from normal to impaired glu­
cose tolerance to frank diabetes, insulin resistance positively cor­
relates with atherosclerosis as assessed by carotid intimal medial 
thickening.716 The San Antonio Heart Study, a population-based 
study of diabetes and cardiovascular disease in Mexican Americans 
and non–Latin American whites, showed those who converted to 
diabetes from a prediabetic state and who

rted to 
diabetes from a prediabetic state and who were more insulin resis­
tant had higher blood pressure, higher triglyceride levels, and lower 
HDL cholesterol levels.17 Strikingly, individuals with normal glu­
cose tolerance and high insulin resistance had twice the 8-year 
risk of adverse cardiovascular outcomes compared to those with 
low insulin resistance, even after adjustment for 11 risk factors. 
Although it is not possible to fully distinguish hyperinsulinemia 
from hyperglycemia, hy

stinguish hyperinsulinemia 
from hyperglycemia, hyperinsulinemia has been hypothesized to 
contribute mechanistically to the relationship between hypergly­
cemia and coronary heart disease in patients with T2DM,717 and 
a number of studies have shown hyperinsulinemia to indepen­
dently predict cardiovascular risk. These results suggest treatment 
strategies that increase insulin sensitivity in these patients can 
reduce cardiovascular risk. However, no significant difference in 
the rates of dea

er, no significant difference in 
the rates of death and major cardiovascular events were observed 
using insulin provisional compared to insulin sparing therapeutic 
strategies in patients with both T2DM and heart disease in the 
Bypass Angioplasty Revascularization Investigation 2 Diabetes 
Trial (BARI 2D).718 Furthermore, when insulin therapies were 
used prospectively to lower HbA1C in either patients with T2DM 
or prediabetes, they have not been shown to worsen cardiovascular 
outcomes. Thi

been shown to worsen cardiovascular 
outcomes. This suggests that hyperinsulinemia is a marker of insu­
lin resistance but not a cause of the adverse cardiovascular effects 
of insulin resistance.
Given the multifactorial nature of atherogenic risk in patients 
with T2DM, it is reasonable to conclude that an aggressive mul­
tifactorial intervention could significantly reduce cardiovascu­
lar risk. The value of such a treatment regimen was tested in the 
Steno-2 study,595 in which 160 patients wi

in the 
Steno-2 study,595 in which 160 patients with T2DM and microal­
buminuria were randomized to receive either conventional treat­
ment in accordance with national guidelines or intensive therapy 
that included behavior modification and targeted pharmacologic 
therapy for hyperglycemia, hypertension, dyslipidemia, and 
microalbuminuria along with secondary prevention of cardio­
vascular disease with aspirin. Over a mean follow-up period of 
7.8 years, patients who received intensive treatmen

.8 years, patients who received intensive treatment had greater 
improvements in HbA1c, blood pressure, fasting serum choles­
terol and triglyceride values, and urinary albumin excretion than 
did patients receiving conventional therapy. The greater degree 
of improvement in risk factors with intensive therapy was also 
reflected in outcomes. Patients receiving intensive therapy had 
an approximately 50% reduction in events comprising a multiple 
cardiovascular endpoint, and in retinopathy, neph

cardiovascular endpoint, and in retinopathy, nephropathy, and 
autonomic neuropathy.594 
Insulin Resistance, Plasma Glucose, and Excess CHD Risk 
in Diabetes
Insulin resistance and its consequences increase cardiovascular 
risk in people with normal glucose tolerance. Yet diabetes confers 
increased risk that is not explained by the known risk factors asso­
ciated with insulin resistance. Newer proatherogenic consequences 
of pathway-specific insulin resistance are discussed in the first sec­
t

sulin resistance are discussed in the first sec­
tion of this chapter. Although a meta-analysis of randomized con­
trolled trials showed limited benefits of intensive glucose-lowering 
treatment on all-cause mortality and deaths from cardiovascular 
causes, using HbA1c as an indicator,13 effects of hyperglycemia not 
captured by HbA1c may also explain excess cardiovascular risk in 
prediabetes as well as in diabetes. Hyperglycemia increases insulin 
resistance, and circadian variation in insulin

in 
resistance, and circadian variation in insulin resistance causes fast­
ing hyperglycemia. One study showed that mortality rates from 
all causes, cardiovascular mortality, and ischemic heart disease 
increased progressively across quintiles of fasting blood glucose 
levels in patients with T2DM (Fig. 37.48).719 Other data sup­
port a dose-response relationship between fasting hyperglycemia 
and cardiovascular mortality rate in diabetes, with patients with 
the highest levels of fasting blood

the highest levels of fasting blood glucose having a cardiovascular 
mortality rate almost five times higher than patients with the two 
lowest levels combined.720
Prediabetes refers to the presence of impaired fasting glucose 
(≥100 mg/dL [≥5.5 mmol/L] and <120 mg/dL [<6.7 mmol/L]), 
and impaired glucose tolerance, defined as a 2-hour post–75-g oral 
glucose load value of 140 to 199 mg/dL (7.8–11 mmol/L) with 
the fasting level below 100 mg/dL (<5.5 mmol/L) and/or HbA1c 
between 

low 100 mg/dL (<5.5 mmol/L) and/or HbA1c 
between 5.7% and 6.5%. Impaired glucose tolerance (IGT)—a 
prediabetic state associated with insulin resistance—is also a risk 
factor for mortality. In a large study of the relationship between 
2-hour postload blood glucose and cardiovascular fatality, 17,869 
male civil servants enrolled in the Whitehall Study from 1967 to 
1969 were monitored, and outcomes were correlated with base­
line measurements of the 2-hour postload blood glucose (2hBG) 
level

of the 2-hour postload blood glucose (2hBG) 
level after a 50-g oral glucose load.721 The hazard ratio for car­
diovascular mortality increased as a linear function of 2hBG for 
all values of 2hBG greater than 83 mg/dL (>4.6 mmol/L). With 
2hBG values between 83 and 200 mg/dL (4.6 and 11.1 mmol/L), 
the age-adjusted hazard ratio for cardiovascular disease was 3.62 
(95% CI, 2.3–5.6).
Tominaga and colleagues examined survival rates in a cohort 
of participants in a diabetes prevalence trial in Ja

 participants in a diabetes prevalence trial in Japan722 and 
concluded the risk of cardiovascular fatality is associated with 
impaired glucose tolerance rather than impaired fasting glucose. 
Further substantiation of the role of impaired glucose tolerance 
in cardiovascular mortality risk was provided by an analysis of 
50
40
30
20
10
0
Mortality (%)
4.7–<6.7
6.7–<7.8
7.8–<8.8
Quintiles of average FBG values
8.8–9.9
=>10.0
Ischemic heart disease mortality (%)
Cardiovascular mortality (%)
All-

se mortality (%)
• Fig. 37.48  All-cause mortality, cardiovascular mortality, and ischemic 
heart disease mortality rates in patients with type 2 diabetes mellitus by 
quintiles of average fasting blood glucose (FBG) values. Cardiovascular 
mortality and all-cause mortality rates increase throughout the range of 
fasting plasma glucose in a graded fashion. (From Andersson DK, Svärd­
sudd K. Long-term glycemic control relates to mortality in type I

rm glycemic control relates to mortality in type II diabetes. 
Diabetes Care. 1995;18:1534–1543.)
1506
data from the Diabetes Epidemiology: Collaborative Analysis of 
Diagnostic Criteria in Europe (DECODE) study.723 In this study, 
more than 25,000 men and women were monitored for a mean 
of 7.3 years, and outcomes were correlated with measurements of 
fasting glucose and 2hBG after a 75-mg glucose load at baseline. 
The results indica

5-mg glucose load at baseline. 
The results indicated that the oral glucose tolerance test provides 
the best index of risk of mortality associated with impaired glucose 
tolerance.
The Nurses’ Health Study also implicated the prediabetic state 
as a cardiovascular risk factor.724 In this large cohort of women, 
5894 developed diabetes over a 20-year follow-up. The age-
adjusted relative rate for myocardial infarction was 3.75 (95% CI, 
3.10–4.53) in the period before diagnosis of diabetes and 4

) in the period before diagnosis of diabetes and 4.57 
(95% CI, 3.87–5.39) after diagnosis, compared with women who 
did not develop diabetes, even after adjustment for other cardio­
vascular risk factors. The risk of stroke was also increased before 
onset of diabetes.
The continuum of cardiovascular risk with rising glucose levels 
has also been identified in patients with T1DM725 and in people 
without clinically overt diabetes but with varying levels of glucose 
intolerance. 

g levels of glucose 
Cardiovascular Disease in Patients With 
Definitions and Diagnosis
Metabolic syndrome is a term referring to the frequent clustering 
of the cardiovascular risk factors of hypertension, obesity, insulin 
resistance, dyslipidemia, and dysglycemia in a single patient. Sepa­
rate diagnostic criteria for different risk components have been put 
forth by the National Cholesterol Education Program (NCEP) 
Adult Treatment Panel III (ATP III),726 the

NCEP) 
Adult Treatment Panel III (ATP III),726 the World Health Orga­
nization (WHO),727 and the International Diabetes Federation 
(IDF),728 with clinical overlap among these definitions and the 
designation of prediabetes.729
According to prior NCEP guidelines,729a the metabolic syn­
drome is based on the presence of three of the following five risk 
factors726:
 • Abdominal obesity (waist circumference >40 inches (>101.6 
cm) in men and >35 inches (>88.9 cm) in women)
 • Plasma triglyceride

probably less stringent than would be used 
to identify a categoric risk factor, reflecting the fact that many 
marginal risk factors can result in a significant risk for CHD. The 
NCEP criteria do not require explicit demonstration of insulin 
resistance for diagnosis of the metabolic syndrome, and patients 
with diabetes are not excluded from the diagnosis.727 There are 
modest differences between the NCEP criteria and those devel­
oped by the WHO727 and the IDF.728 One intriguing aspect of 
t

WHO727 and the IDF.728 One intriguing aspect of 
the IDF criteria is the recognition that individuals of Asian origin 
may have increased visceral adiposity with lower waist circumfer­
ence than what is observed in Western countries and hence the 
difference in waist circumference criteria for such individuals.
Studies support the concept that insulin resistance during 
the prediabetic state contributes to atherogenic risk.730 Pre­
diabetic subjects who were insulin resistant had higher levels o

cts who were insulin resistant had higher levels of 
inflammatory markers (C-reactive protein, PAI1, and fibrinogen) 
than converters to diabetes with predominantly low insulin secre­
tion or nonconverters. Thus insulin resistance is associated with a 
proinflammatory state that can contribute to the atherogenic risk 
profile in prediabetic patients. Many aspects of insulin resistance 
and the proatherogenic and proinflammatory state of prediabetes 
are associated with increased visceral fat.731

es 
By 2012, more than one-third of all US adults met the defi­
nition and criteria for metabolic syndrome agreed to jointly by 
several international organizations.732 Although there continues 
to be debate about the validity and significance of identifying the 
metabolic syndrome or prediabetes in patients, clinicians, includ­
ing primary care physicians, often find the construct of value in 
understanding the relevant underlying issues and explai

standing the relevant underlying issues and explaining these 
to patients. Behind some of the reluctance to identify metabolic 
syndrome/prediabetic states in patients is the clinical question of 
what to do after such a diagnosis is made. Perhaps new data show­
ing cardiovascular risk reduction with newer glucose-lowering 
therapies that do not cause hypoglycemia (discussed later) may 
shift this perspective. These new drug options, along with effective 
medications for reducing cholesterol and

ffective 
medications for reducing cholesterol and hypertension, support 
the clinical utility of continuing to identify prediabetes and the 
metabolic syndrome. 
Cardiovascular Consequences of Metabolic Syndrome
Metabolic syndrome, prediabetes, and other overlapping defini­
tions from the NCEP, WHO, and others have been evaluated 
in multiple studies regarding the incidence and mortality risk of 
CHD.733–735 In general, age-adjusted cardiovascular mortality 
and all-cause mortality increase two

ar mortality 
and all-cause mortality increase twofold to threefold in patients 
with metabolic syndrome, regardless of the diagnostic criteria used 
to define the syndrome. Increased risk is not seen in every study. 
Differences among these studies exist and involve when and how 
analyses were done and the metabolic syndrome definitions used, 
including key issues such as whether frank diabetes is included or 
excluded from metabolic syndrome definition and the hypergly­
cemia threshold used.
I

finition and the hypergly­
In a definitive meta-analysis of 87 studies involving 951,083 
patients, the metabolic syndrome was found to be associated with 
an increased risk of cardiovascular disease (relative risk [RR]: 2.35; 
95% CI: 2.02–2.73), cardiovascular mortality (RR: 2.40; 95% 
CI: 1.87–3.08), all-cause mortality (RR: 1.58; 95% CI: 1.39–
1.78), myocardial infarction (RR: 1.99; 95% CI: 1.61–2.46), and 
stroke (RR: 2.27; 95% CI: 1.80–2.85); metabolic syndrome in 
th

.27; 95% CI: 1.80–2.85); metabolic syndrome in 
the absence of diabetes was still associated with increased cardio­
vascular risk,732 although it is based on fewer studies.
Another approach to assessing cardiovascular risk of metabolic 
syndrome involves consideration of this cohort in clinical tri­
als. For example, in the West of Scotland Coronary Prevention 
Study,735 those with the metabolic syndrome had an increased 
cardiovascular risk, along with elevated C-reactive protein levels 
versus

ng with elevated C-reactive protein levels 
versus the rest of the cohort, which added prognostic value for 
both cardiovascular disease and diabetes.
Controversies over metabolic syndrome and other prediabetic 
states exist.736 One issue has been whether the cardiovascular risk 
associated with the metabolic syndrome differs from the additive 
risk conferred by the presence of each metabolic syndrome com­
ponent. Another has been what therapeutic intervention should 
ensue once a diagnosis of m

c intervention should 
ensue once a diagnosis of metabolic syndrome is made. These and 
other issues have limited the extent to which this issue has been 
studied in trials or further considered by guidelines.737 Evidence 
continues to emerge regarding the pathologic issues associated 
1507
with prediabetes, including intimal medial thickening and plaque 
in carotid arteries, and adverse functional cardiac parameters such 
as reduced diastolic filli

ardiac parameters such 
as reduced diastolic filling (now referred to as heart failure with 
preserved ejection fraction).738 For now, specific relatively subtle dif­
ferences among diagnostic criteria of prediabetic states may be less 
relevant for the clinician than the recognition of the increased risk 
of cardiovascular disease and diabetes associated with metabolic 
syndrome, the commonly encountered clustering of these risk fac­
tors, and the impact lifestyle and modest weight reduction ca

he impact lifestyle and modest weight reduction can 
have of many component risk factors. New data for cardiovascular 
event reduction in patients with diabetes with agents that do not 
cause hypoglycemia as well as weight loss agents may stimulate 
reexamination of these issues. 
The Role of Glycemic Control in Improving 
Cardiovascular Outcomes
The UKPDS confirmed the positive association between plasma 
glucose levels and CHD risk for HbA1c levels greater than 6.2% 
in patients with diabetes.

vels greater than 6.2% 
in patients with diabetes.717 Each percentage point elevation in 
HbA1c increased cardiovascular risk by 11%. Multiple other lines 
of evidence, as noted, confirm the relationship between hypergly­
cemia, diabetes, and increased cardiovascular risk. Despite this, 
it has been difficult to establish that glucose control in diabetes 
could decrease future macrovascular events, including trials inves­
tigating intensive versus standard diabetes treatment and different 
strat

s standard diabetes treatment and different 
strategies of achieving glycemic control. Although meta-analyses 
of randomized controlled trials of diabetes treatments revealed 
improved glucose control was associated with reductions in non­
fatal myocardial infarction rates, there has been no evidence for 
decreases in either cardiovascular death or all-cause mortality.13 
Given the clear increased cardiovascular risk with diabetes, the 
inability to reduce cardiovascular events in response to tr

 to reduce cardiovascular events in response to treating 
diabetes represented a clinical paradox, a contrast to contempora­
neous studies showing the increased cardiovascular risk of hyper­
cholesterolemia could be reversed by lowering LDL with statin 
therapy, including in patients with diabetes. Various hypotheses 
have been raised regarding this inability of diabetes treatment to 
show improved cardiovascular outcomes, including trial issues 
such as study design, sample size, duration, and 

such as study design, sample size, duration, and poorly selected 
primary endpoints, and disease issues such as disease duration or 
treatment-specific factors (e.g., drugs used).
In 2008, the FDA mandated that novel glucose-lowering thera­
pies seeking approval must demonstrate cardiovascular safety. The 
resulting data, involving over nine cardiovascular outcome trials 
reported to date involving 200,000 patients, have led to a striking 
pivot for the fields of both diabetes and cardiology wi

 for the fields of both diabetes and cardiology with data dem­
onstrating not only cardiovascular safety but also cardiovascular 
benefit for glucagon-like peptide 1 receptor agonists (GLP1RA) 
and sodium glucose transporter 2 inhibitors (SGLT2i). Other 
studies established cardiovascular safety but also identified poten­
tial concerns over an increased risk for inducing heart failure, as 
seen with some dipeptidyl-peptidase 4 inhibitors (DPP4i). Given 
the landmark nature of these trials, their

 Given 
the landmark nature of these trials, their impact on clinical man­
agement of diabetes, and the new questions posed by these find­
ings, a review of the prior studies with older antidiabetic agents 
warrants review and provides context before the results with these 
newer agents are considered.
Earlier studies, such as the DCCT in T1DM and the smaller 
Veterans Affairs Diabetes Feasibility (VA) trial in T2DM, did 
not show a reduction in cardiovascular endpoints with intensive 
metabolic

cardiovascular endpoints with intensive 
metabolic control. These well-conducted studies had inherent 
limitations. Although relatively large (N = 1441), the DCCT fol­
lowed a relatively young (mean age 27 years) population of patients 
with T1DM for approximately 6 years. At the end of follow-up, 
few events had occurred.9 Intensive therapy reduced the risk of 
coronary artery and peripheral vascular disease by 41% compared 
with conventional therapy, but the difference was not statistically 
s

erapy, but the difference was not statistically 
significant. Similarly, in the VA trial, intensive blood glucose con­
trol in patients with T2DM trended to but did not significantly 
reduce cardiovascular endpoints.739 Both studies lacked adequate 
power to detect a difference in macrovascular events between 
treatment groups because of the small number of events in each 
group, small patient populations, and relatively short follow-up.
In contrast, the 17-year follow-up of the 1441 patients fr

ast, the 17-year follow-up of the 1441 patients from the 
DCCT trial (EDIC) demonstrated the benefit of early intensive 
glycemic control in T1DM on cardiovascular risk in the subsequent 
post-DCCT, EDIC observational setting.740 This study and its 
findings are discussed in the sections on biochemistry and molecu­
lar cell biology and retinopathy earlier in the chapter. During fol­
low-up, those who had previously been randomized to intensive 
treatment had an emergent reduction in cardiovascul

reatment had an emergent reduction in cardiovascular risk of 42% 
(p = 0.02) and a reduction in the composite risk of nonfatal myo­
cardial infarction, nonfatal stroke, or cardiovascular death of 57% 
(p = 0.02). Of note, this separation was evident even though after 
11 years of follow-up, HbA1c, as well as blood pressure and lipid risk 
factors, were essentially identical between the two arms. Patients 
in the conventional treatment group had more albuminuria and 
microalbuminuria than intensi

ore albuminuria and 
microalbuminuria than intensively treated patients, but the differ­
ences in risk remained significant after adjustment for these factors. 
These findings indicate early intensive glycemic control reduced 
the long-term risk of cardiovascular disease in patients with T1DM 
while also raising intriguing mechanistic questions regarding meta­
bolic memory (discussed in the first section of this chapter).
For T2DM, the UKPDS was larger and adequately powered to 
detect differenc

larger and adequately powered to 
detect differences between groups in terms of macrovascular events 
in regular versus intensive glycemic control.379 Those receiving 
intensive glycemic control showed a trend toward lower myocardial 
infarction rates compared to conventional treatment, which was not 
statistically significant (p = 0.052).379 Despite the lack of overall 
efficacy of intensive treatment for management of macrovascular 
complications of diabetes in the UKPDS, there were indication

ns of diabetes in the UKPDS, there were indications 
that metformin as a specific therapy may be effective in reducing 
cardiovascular events.379,741 In a retrospective analysis of an over­
weight subset (n = 342) of the UKPDS cohort who were random­
ized to metformin, significant reductions in the occurrence of any 
diabetes-related endpoint (32%), diabetes-related death (42%), and 
all-cause mortality (36%) were seen compared with conventionally 
treated patients.741 Although this is subset da

y 
treated patients.741 Although this is subset data in a limited num­
ber of patients, these results, along with registry data suggesting 
decreased mortality with metformin use,742 promoted acceptance 
of metformin having potential cardiovascular benefit.
Subsequent trials examined whether lowering the target for 
glycemic control (to HbA1c <7%) would reduce cardiovascular 
events. The ACCORD trial randomized patients with T2DM who 
had established cardiovascular disease or multiple risk facto

shed cardiovascular disease or multiple risk factors to 
a treatment-directed HbA1c value lower than 7% or to standard 
therapy with an HbA1c target value between 7% and 7.9%. Over 
3.5 years of follow-up, total mortality rate increased in the inten­
sively treated group without any significant reduction in cardiovas­
cular events.535 The VADT, which randomized patients to either 
intensive treatment (median HbA1c 6.9%) or standard therapy 
(median HbA1c 8.4%) also failed to demonstrate cardiova

an HbA1c 8.4%) also failed to demonstrate cardiovascular 
benefit through tighter HbA1c control.538 The ADVANCE trial, 
1508
which randomly assigned patients to an intensively treated group, 
who attained HbA1c of 6.5% versus a standard control group who 
had HbA1c values of 7.3%, demonstrated a relative reduction of 
10% in the combined outcome of major macrovascular and micro­
vascular events that was not statistically significant.53

r events that was not statistically significant.532 Of note, 
this statistically insignificant 10% relative decrease in cardiovascu­
lar events was driven largely by a 21% reduction in nephropathy. 
Taken together, these three studies do not support HbA1c reduction 
beyond 7%, improving cardiovascular outcomes in patients with 
established coronary heart disease and treated with these agents.
An additional perspective on the prior data showing limited if 
any cardiovascular benefit through older

mited if 
any cardiovascular benefit through older glucose-lowering treat­
ments is that HbA1c represents a flawed surrogate marker for 
identifying risk of future macrovascular outcomes responsive to 
therapy, contrasting with the ability of HbA1c to track efficacy 
in predicting microvascular responses to glucose-lowering treat­
ment. Given that HbA1c is used to define the disease of diabetes, 
enrolling patients based on HbA1c may not consistently enroll 
those more likely to benefit in terms

ntly enroll 
those more likely to benefit in terms of macrovascular outcomes 
in response to the treatment under study.
Overall, many studies provide strong epidemiologic data show­
ing that diabetes is associated with a large increase in cardiovascular 
risk. In contrast to studies of microvascular disease, where the asso­
ciated risk is high per unit change in HbA1c, the associated cardio­
vascular risk per unit change in HbA1c is lower, once corrected for 
triglycerides, LDL cholesterol, hype

orrected for 
triglycerides, LDL cholesterol, hypertension, and smoking.
While the benefit of glucose lowering with metformin to 
reduce myocardial infarction was shown in the substudy of the 
UKPDS,741 as previously mentioned, little data was available to 
support other therapeutic approaches or specific agents. 
Studies Using Insulin-Sensitizer Medications
The BARI 2D trial randomized 2368 patients with T2DM and 
coronary heart disease to either prompt revascularization or inten­
sive medical 

r prompt revascularization or inten­
sive medical therapy alone and to either insulin sensitization or 
insulin provision diabetes therapy. Randomization was stratified 
by the proposed revascularization method. The 5-year survival 
and major cardiovascular event rates were similar in all study 
subgroups except for those patients undergoing coronary artery 
bypass grafting who had fewer major cardiovascular events after 
revascularization. There was less hypoglycemia and weight gain 
and greate

 was less hypoglycemia and weight gain 
and greater apparent benefit from coronary artery bypass grafting 
in the insulin sensitization group.
To evaluate insulin versus other therapies for lowering glucose 
as a strategy to decrease cardiovascular events, in the Outcome 
Reduction With Initial Glargine Intervention (ORIGIN) trial, 
insulin or noninsulin therapies were used in patients with either 
prediabetes or early T2DM to achieve a fasting plasma glucose of 
95 mg/dL versus 123 mg/dL (5.3 m

lasma glucose of 
95 mg/dL versus 123 mg/dL (5.3 mmol/L vs 6.8 mmol/L) with 
other therapies.743 No difference in cardiovascular outcomes was 
evident in the more aggressively treated arm.
Thiazolidinediones are direct insulin sensitizers, improving 
insulin sensitivity rather than increasing insulin levels. TZDs bind 
to and activate the nuclear receptors known as peroxisomal pro­
liferator-activated receptor gamma (PPARγ) and to mitoNEET, 
a mitochondrial protein-regulating energy metabolism.7

tochondrial protein-regulating energy metabolism.744 TZD 
effects in vascular and inflammatory cells limit atherosclerosis and 
inflammatory responses. In the PROactive study (i.e., Prospec­
tive Pioglitazone Clinical Trial in Macrovascular Events), 5238 
patients with T2DM and evidence of macrovascular disease were 
randomized to either pioglitazone or usual care with the objective 
of similar glucose control in both arms.745 After a mean of 34.5 
months, there was no significant difference in 

5 
months, there was no significant difference in the two treatment 
groups in the study’s primary endpoint—a seven-component com­
posite of all-cause mortality, nonfatal myocardial infarction, stroke, 
acute coronary syndrome, surgical intervention in the leg or coro­
nary arteries, and amputation above the ankle. This broad primary 
endpoint, which extended beyond the more standard objective car­
diovascular primary endpoint used in cardiovascular trials, was used 
to increase the prospect of 

lar trials, was used 
to increase the prospect of demonstrating cardiovascular benefit, 
under the assumption that peripheral arterial disease behaves like 
coronary disease. However, improvements in peripheral arterial dis­
ease endpoints have been difficult to demonstrate, even with potent 
cardiovascular risk-reducing agents such as statins. Patients treated 
with pioglitazone had significantly lower risk for the prespecified 
secondary endpoint in this trial, which was more consistent with 

nt in this trial, which was more consistent with 
the standard primary cardiovascular trial endpoint of combined all-
cause mortality, nonfatal myocardial infarction, and nonfatal stroke 
(HR: 0.84; 95% CI: 0.80–1.02; p = 0.027).
Two issues have complicated the use of TZDs. A meta-analysis 
of rosiglitazone effects on cardiovascular outcomes examining short-
term trials of glycemic control suggested rosiglitazone was associ­
ated with an increased risk of myocardial infarction. Total event 
rate

d risk of myocardial infarction. Total event 
rates were low in the included studies and they did not examine car­
diovascular events as primary endpoints nor were the cardiovascular 
events adjudicated.746 In contrast, other meta-analyses, including 
ones using only randomized clinical trials with prespecified cardio­
vascular endpoints and/or adjudication, found no increase in car­
diovascular events or cardiovascular death with either rosiglitazone 
or pioglitazone.747 This conclusion was als

azone 
or pioglitazone.747 This conclusion was also supported by other 
subsequent studies and an FDA consensus review panel.748 Differ­
ences do exist between pioglitazone and rosiglitazone. A second fac­
tor limiting use of these agents has been their known side effects, 
including fluid retention that may increase heart failure (although 
without myocardial changes or increased mortality), increased bone 
fractures, significant increases in weight among a subset of patients, 
and increases in

ight among a subset of patients, 
and increases in bladder cancer.749 Whether these untoward effects 
are offset by clinical cardiovascular benefit may require further con­
sideration, especially given more recent positive clinical trial data 
discussed later in the chapter.
Prior inability to conclusively demonstrate cardiovascular 
event reduction with specific diabetes approaches or therapies 
could have been due to not having the right agent or targeting the 
most impactful mechanism rather 

or targeting the 
most impactful mechanism rather than other explanations such as 
patient characteristics or study design.750,751
As a result of the 2008 FDA-instituted guidance that new 
antidiabetic drugs must provide cardiovascular safety data in their 
approval process, a large amount of new data regarding cardiovas­
cular effects of newer antidiabetic agents has emerged and con­
tinues to do so, including new data showing cardiovascular risk 
reduction for patients treated with GLP1RA or S

k 
reduction for patients treated with GLP1RA or SGLT2 inhibi­
tors. These results have focused thinking on specific agents and 
potential mechanisms that are likely more complex than simply 
lowering blood glucose concentration. 
Studies Using Newer Antidiabetic Medications: A 
New Era?
Dipeptidyl Peptidase 4 Inhibitors and Cardiovascular 
Disease
DPP4i lower glucose by inhibiting degradation of the incre­
tin GLP1. The SAVOR–Thrombolysis in Myocardial Infarction 
(TIMI) 53 (SAVOR-TIMI 53) tria

cardial Infarction 
(TIMI) 53 (SAVOR-TIMI 53) trial established safety for this 
1509
agent with no difference in major adverse cardiovascular events 
(MACE) (the composite of nonfatal myocardial infarction, non­
fatal stroke, or cardiovascular death); however, a 27% increase in 
hospital admissions for heart failure was seen.752 The Examination 
of Cardiovascular Outcomes With Alogliptin Versus Standard of 
Care (EXAMINE) similarly established card

dard of 
Care (EXAMINE) similarly established cardiovascular safety for 
the MACE outcome with this DPP4i.753 Although EXAMINE 
investigators reported that alogliptin had no impact on heart fail­
ure,754 differences in the design and endpoint measurements in 
this trial as compared to SAVOR-TIMI 53 may limit this conclu­
sion.755 In the Trial to Evaluate Cardiovascular Outcomes After 
Treatment With Sitagliptin (TECOS), sitagliptin showed no 
cardiovascular benefit but also no adverse effects, i

diovascular benefit but also no adverse effects, including no 
increased admissions for heart failure, all as compared to usual 
care. Other approaches to the issue of DPP4i and heart failure, 
like observational studies and meta-analyses, have varied in con­
clusion about whether specific agents differ in heart failure risk. 
The FDA has included heart failure warnings in DPP4i labeling. 
While none of these agents increase mortality, it is prudent to 
examine DPP4i-treated patients with diabet

ent to 
examine DPP4i-treated patients with diabetes for signs and symp­
toms of heart failure. Notably, prior to these outcome studies, less 
rigorous forms of data had suggested cardiovascular benefits with 
DPP4i, underscoring the primacy of randomized, prospective, 
blinded clinical trial data.
In the CARMELINA study,756,756a the effect of linagliptin (5 
mg once daily) was compared to placebo (both added to standard 
of care) on cardiovascular outcomes in 6979 adults with T2DM 
and high car

ar outcomes in 6979 adults with T2DM 
and high cardiovascular risk, the majority of whom also had kid­
ney disease. Linagliptin demonstrated a similar cardiovascular 
safety profile compared to placebo when added to standard of care. 
There was a similar kidney safety profile and no increase in heart 
failure compared to placebo.756,756a The companion Cardiovascu­
lar Outcome Study of Linagliptin Versus Glimepiride (CARO­
LINA)757 is comparing linagliptin to the sulfonylurea glimepiride 
in pati

nagliptin to the sulfonylurea glimepiride 
in patients with T2DM and higher cardiovascular risk (known 
cardiovascular disease or T2DM-associated end-organ damage or 
older age or two or more cardiovascular risk factors). 
GLP1RAs and Cardiovascular Disease
In contrast to the DPP4 inhibitor data, studies with both GLP1RAs 
and SGLT2 inhibitors provided landmark data showing decreased 
cardiovascular events with glucose-lowering therapy. Not surpris­
ingly, these positive findings have prompted i

is­
ingly, these positive findings have prompted intense scrutiny and 
further questioning regarding mechanisms explaining these results. 
Nevertheless, the findings stand. The fact that two different agents 
within each drug class have shown a positive cardiovascular pri­
mary endpoint reinforces the general validity of the data. Fur­
thermore, both GLP1RAs and SGLT2 inhibitors improved renal 
outcomes (see “Glycemic Control in Diabetic Kidney Disease”).
GLP1RAs have GLP1 receptor-dependent and

isease”).
GLP1RAs have GLP1 receptor-dependent and receptor-inde­
pendent effects on the cardiovascular system.758,759 GLP1RAs can 
induce satiety, promote weight loss, lower blood pressure, and 
have other effects in preclinical and smaller translational studies 
that might promote cardiovascular benefit.760 In the LEADER 
trial543 (Table 37.19), 9340 patients with T2DM and increased 
cardiovascular risk received random assignment to either lira­
glutide (titrated to 1.8 mg once daily) or place

­
glutide (titrated to 1.8 mg once daily) or placebo, with median 
follow-up of 3.8 years. The primary time-to-event analysis, which 
was the combined results of first occurrence of cardiovascular 
death, nonfatal myocardial infarction, or nonfatal stroke, was 
significantly decreased among liraglutide (overall incidence 13%) 
versus placebo group (14.9%, p < 0.001 for noninferiority; p = 
0.01 for superiority).543
Semaglutide, a GLP1RA administered once a week, was stud­
ied in the SUSTAIN 6 tr

red once a week, was stud­
ied in the SUSTAIN 6 trial, which enrolled 3297 patients with 
T2D, of whom the majority had established cardiovascular dis­
ease, chronic kidney disease, or both. Participants received either 
semaglutide (0.5 or 1 mg) or placebo once weekly for 2 years. The 
primary composite outcome, which was the first occurrence of the 
same 3-point major adverse cardiovascular event, was significantly 
reduced in those on semaglutide (p < 0.001 for noninferiority), 
with nonsigni

ide (p < 0.001 for noninferiority), 
with nonsignificantly decreased nonfatal myocardial infarction 
and significantly decreased nonfatal stroke (see Table 37.19).544 
In further support of GLP1RAs having cardiovascular benefit, the 
Harmony Outcomes trial demonstrated that 9463 patients with 
T2DM and cardiovascular disease receiving albiglutide 30 to 50 
mg based on glycemia and tolerability had a 22% reduction in 
cardiovascular events compared to those receiving placebo in con­
junction with

d to those receiving placebo in con­
junction with usual care761 over a median follow-up of 1.5 years.
In contrast, other GLP1RAs have shown cardiovascular safety 
but no benefit in outcome trials. In the Evaluation of Lixisenatide 
in Acute Coronary Syndrome (ELIXA) trial, 6068 patients with 
T2DM with recent (<180 days) myocardial infarction or unsta­
ble angina hospitalization received lixisenatide 10 to 20 μg once 
daily or placebo762 with usual care with median follow-up of 25 
months. The 

ual care with median follow-up of 25 
months. The primary composite endpoint of cardiovascular death, 
  Key Findings From GLP1 Receptor Agonist Cardiovascular Outcome Trials
Trial
GLP1RA
Sample 
Size
Follow-Up 
Years
Primary CV 
Composite 
Outcomea
Myocardial 
Infarction
Nonfatal 
Stroke
CV Mortality
All-Cause 
Mortality
Hospitalization 
for Heart 
Failure
LEADER
Liraglutide
9340
3.5
0.87 
(0.78-0.97); 
p = 0.01
↓14% p = 0.046
↓11%
p = 0.30
NS
↓22% 
p = 0.007
↓15% 
p = 0.02
↓13%
p = 0.14
NS
SU

↓22% 
p = 0.007
↓15% 
p = 0.02
↓13%
p = 0.14
NS
SUSTAIN-6
Semaglutide
3297
2.0
0.74 (0.58-0.95); 
p = 0.02
↓15%
p = 0.38
NS
↓39%
p = 0.04
→
p = 0.92
NS
→
p = 0.79
NS
→
p = 0.57
NS
Results are shown as hazard ratio or odds ratio with (95% confidence intervals), or as percent change.
Full trial names and references are given in the text.
aCV death, myocardial infarction, or stroke.
CV, Cardiovascular; GLP1RAs, glucagon-like peptide-1 receptor agonists.
   
TABLE 37.19
1510

sts.
   
1510
myocardial infarction, stroke, or unstable angina hospitalization 
showed no superiority, while also establishing safety given non­
inferiority versus placebo. The Exenatide Study of Cardiovascular 
Event Lowering (EXSCEL) trial, with extended-release exenatide 
also showed safety but no benefit when studied in 14,752 patients 
with T2DM treated with either extended-release exenatide 2 mg 
weekly or placebo ov

ended-release exenatide 2 mg 
weekly or placebo over 3.2 years; no difference was seen versus 
placebo on the primary composite outcome of first occurrence of 
cardiovascular death, nonfatal myocardial infarction, or nonfatal 
stroke (HR: 0.91; 95% CI: 0.83–1.00; p < 0.001 for noninferior­
ity; p = 0.06 for superiority).763
The basis for positive cardiovascular outcomes with liraglutide, 
semaglutide, and albiglutide but not lixisenatide or extended-
release exenatide remains obscure. Difference

ded-
release exenatide remains obscure. Differences between clinical 
trials such as enrollment criteria, subtle differences in patient pop­
ulations, study locations, trial duration, and concomitant medi­
cation use may be underlying factors. It remains possible some 
members of the same drug class may have more potent effects on 
discrete cardiovascular risk parameters, such as blood pressure 
or weight loss. Given the prior literature, differences in glucose 
control itself seem unlikely to e

nces in glucose 
control itself seem unlikely to explain variable outcome benefits 
within drug classes or between older and newer agents. Pharma­
cologic differences among the agents is difficult to exclude (e.g., 
short-acting vs long-acting activity with long-acting exenatide 
nearly reaching statistical significance).
Initial concerns about increased risk of pancreatitis, pancreatic 
cancer, and thyroid cancer with GLP1RAs have not been borne 
out in clinical trials, although a prior history

out in clinical trials, although a prior history of such issues is 
viewed as a potential contraindication to use. Given that GLP1 
provides a gut-derived satiety signal to the brain, the gastrointes­
tinal side effects seen with these drugs, mainly nausea, are not 
surprising, may contribute to beneficial weight loss, and are not 
a barrier for most patients. GLP1RAs do not pose concerns for 
patients with chronic kidney disease, within the limits of the 
populations enrolled. A decrease in p

mits of the 
populations enrolled. A decrease in progression of chronic kidney 
disease has been noted. For some patients, the injectable nature 
of GLP1RAs may be an issue. These agents on their own do not 
cause hypoglycemia, although caution may be needed when com­
bined with insulin provisional diabetes treatments (e.g., sulfonyl­
ureas or insulin). 
SGLT2 Inhibitors and Cardiovascular Disease
By blocking glucose absorption in the proximal tubule, SGLT2 
inhibitors lower glucose levels, whil

bule, SGLT2 
inhibitors lower glucose levels, while also wasting an energy sub­
strate and having a diuretic effect, with resulting weight loss and 
blood pressure lowering.764
In the EMPA-REG OUTCOME trial765 (Table 37.20), 
7020 patients with T2DM and established cardiovascular dis­
ease received empagliflozin, 10 mg or 25 mg, or placebo once 
daily in addition to usual care. After a median duration of 3.1 
years, the primary major cardiovascular event composite outcome 
among those receiving 

ar event composite outcome 
among those receiving empagliflozin was significantly decreased. 
Especially impressive was that this positive outcome was driven 
primarily by a major decrease in death, which was seen in both 
cardiovascular (38%) and all-cause mortality (32%). Hospitaliza­
tion for heart failure was significantly reduced in those on empa­
gliflozin (−35%, p = 0.002), as evident in both those with and 
without a history of heart failure, as possibly expected for an agent 
with diure

ure, as possibly expected for an agent 
with diuretic effects. An antihypertensive effect was also evident. 
However, a similar magnitude of reduction in hospitalization for 
heart failure has not been seen in clinical trials of diuretics or other 
antihypertensive drugs, making this explanation less likely. Given 
that most participants did not have a heart failure history, this 
effect highlights the extent to which subclinical heart failure as 
well as risk for heart failure is a major issue 

well as risk for heart failure is a major issue in T2DM while also 
suggesting new directions for SGLT2 inhibitor use now being 
investigated. The rapid separation of curves in terms of mortality 
and heart failure, both occurring after approximately 6 months, 
was also noteworthy.
Canagliflozin, another SGLT2 inhibitor, was studied in the 
CANVAS473 program (see Table 37.20), which involved two car­
diovascular trials, including CANVAS-R that focused on patients 
with renal disease. These stu

focused on patients 
with renal disease. These studies involved 10,142 subjects with 
T2DM and either a history of cardiovascular disease (approxi­
mately two-thirds) or just an additional cardiovascular risk factor 
(one-third). After a mean of 3.6 years (median 2.4 years), the pri­
mary major adverse cardiovascular endpoint (myocardial infarc­
tion, stroke, cardiovascular mortality) was significantly reduced. 
Although each of these components trended toward benefit, none 
of them were signifi

trended toward benefit, none 
of them were significant on their own, including cardiovascular 
mortality or total mortality, in contrast to the EMPA-REG OUT­
COME trial. A signal for an increased risk of amputations was 
seen in CANVAS, without clear evidence for similar issues in the 
EMPA-REG OUTCOME trial. Other analyses have also raised 
concerns regarding possible increased risk for amputations with 
canagliflozin. The extent to which this finding is valid involves 
other SGLT2 inhibitors, 

inding is valid involves 
other SGLT2 inhibitors, and the potential underlying mechanism 
for this putative issue remains unresolved and awaiting further 
data, although the significant reduction in cardiovascular events 
is evident. In Dapagliflozin Effect on Cardiovascular Events 
(DECLARE-TIMI 58), the effects of dapagliflozin in 17,150 
patients with inadequately controlled diabetes and either known 
cardiovascular disease (secondary prevention cohort) or at least two 
risk factors for cardi

on cohort) or at least two 
risk factors for cardiovascular disease (primary prevention cohort) 
was studied. In announced but not yet published topline results, 
dapagliflozin demonstrated cardiovascular safety and a reduction 
in a composite of cardiovascular death or hospitalization for heart 
failure but without a significant difference in the major adverse 
composite outcome of combined cardiovascular death, nonfatal 
myocardial infarction, or ischemic stroke.
Other large cardiovascular out

or ischemic stroke.
Other large cardiovascular outcome trials are being done with 
other SGLT2 inhibitors. Ertugliflozin is also being investigated in 
a large outcome cardiovascular trial. Future studies in other specific 
populations will provide additional insight into SGLT2 inhibi­
tor effects. Other real-world data analyses, performed outside of 
a randomized clinical trial setting, have supported cardiovascular 
benefits with SGLT2 inhibitors. For example, CVD-REAL766 (see 
Table 37.20), w

rs. For example, CVD-REAL766 (see 
Table 37.20), which reviewed medical claims, hospital records, 
and registries from 309,056 patients in six countries taking SGLT2 
inhibitors, found SGLT2 inhibitor use was associated with a 39% 
lower risk of heart failure hospitalization (p < 0.001) and a 51% 
reduction in total death (p < 0.001) independent of country.
The mechanism for cardiovascular benefits with SGLT2 inhibi­
tors remains to be defined. The multitude of prior negative trials 
with glucos

he multitude of prior negative trials 
with glucose-lowering argues against the modest improvement in 
HbA1c as underlying the observed benefit. Reductions in blood 
pressure, weight and volume, with concomitant increases in hema­
tocrit as well as unique aspects to tissue-based volume shifts, have 
been hypothesized to contribute to the decrease in cardiovascular 
risk. However, the relative small changes in these parameters and 
the lack of similar effects in trials with diuretics or other ant

ilar effects in trials with diuretics or other antihy­
pertensives suggest the involvement of other factors. Serum uric 
acid levels, previously linked to cardiovascular risk, are decreased 
with SGLT2 inhibition, but the rapidity with which the curves 
1511
  Key Findings From SGLT2 Inhibitor Cardiovascular Outcome and Observational Studies
Studies
SGLT2 Inhibitor 
Daily Dose vs 
Comparator
SGLT2i vs 
Placebo or 
Comparator, N
History 
of CVD 
Pat

vs 
Placebo or 
History 
of CVD 
Patients, %
Median 
Follow-Up 
Years
Primary CV 
Composite 
Outcomea
Infarction (Fatal 
or Nonfatal)
Stroke (Fatal or 
Nonfatal)
All-Cause 
Mortality
for Heart 
Failure
Randomized Controlled Trials vs Placebo
EMPA-REG 
OUTCOME1
Empagliflozin 10 
or 25 mg vs 
placebo
4687 vs 2333
99
3.1
0.86 (0.74-0.99; 
p = 0.04
0.87 (0.70-1.09); 
p = 0.23
1.18 (0.89-1.56); 
p = 0.26
0.62 (0.49-0.77); 
p < 0.001
0.68 (0.57-0

p = 0.26
p < 0.001
0.68 (0.57-0.82); 
p < 0.001
0.65 (0.50-0.85); 
p = 0.002
CANVAS2
Canagliflozin 
100–300 mg 
vs placebo
5795 vs 4347
65
2.4
0.86 (0.75-0.97); 
p = 0.02
0.85 (0.69-1.05); 
NS
0.90 (0.71-1.15); 
NS
0.87 (0.72-1.06); 
NS
0.87 (0.74-1.01); 
NS
0.67 (0.52-0.87)b
Observational Studies vs Active Comparator
CVD-REAL3
SGLT2i (dapa­
gliflozin 42%, 
canagliflozin 
53%)
154528 vs 
154528 
(propensity 
matching)
13
0.6–0.7
NA
0.85 (0.72-1.00); 
p = 0.05
0.83 (0.71-0.97);

7
NA
p = 0.05
0.83 (0.71-0.97); 
p = 0.02
NA
0.49 (0.41-0.57); 
p < 0.001
0.61 (0.51-0.73); 
p < 0.001
Results are shown as hazard ratio or odds ratio (95% confidence intervals). p values are provided as available.
aCardiovascular death, nonfatal myocardial infarction, or nonfatal strokes.
bHospitalization for heart failure was prespecified exploratory cardiovascular outcome.
NA, Not available; NS, nonsignificant; SGLT2, sodium glucose cotransporter type 2.
Data from 1Zinman B

 glucose cotransporter type 2.
Data from 1Zinman B, Wanner C, Lachin JM, et al. Empagliflozin, cardiovascular outcomes, and mortality in type 2 diabetes. N Engl J Med. 2015;373:2117–2128; 2Neal B, Perkovic V, Mahaffey KW, et al. Canagliflozin and cardiovascular and renal events in type 
2 diabetes. N Engl J Med. 2017;377:644–657; 3Kosiborod M, Cavender MA, Fu AZ, et al. Lower risk of heart failure and death in patients initiated on sodium-glucose cotransporter-2 inhibitors versus other glucose-l

 cotransporter-2 inhibitors versus other glucose-lowering drugs: the CVD-REAL study (comparative 
effectiveness of cardiovascular outcomes in new users of sodium-glucose cotransporter-2 inhibitors). Circulation. 2017;136:249–259.
   
TABLE 37.20
1512
separate in the SGLT2 trials make this explanation less likely. 
Other proposed mechanisms include more efficient fuel switching 
in patients with T2DM and ischemia, through increased plas

nts with T2DM and ischemia, through increased plasma 
β-hydroxybutyrate as a better energy substrate, changes in dys­
regulated neurohormonal signaling, and alterations in sodium-
hydrogen exchange, among others.
Untoward effects with SGLT2 inhibition align in part with its 
known mechanism of action. Induced glucosuria can promote 
myocotic genital infections, which can be offset by good hygienic 
practices, with consideration of avoiding these agents in patients 
with a history of these issues

agents in patients 
with a history of these issues. Increased diabetic ketoacidosis and 
euglycemic ketoacidosis have been reported with SGLT2 inhibi­
tors although not seen in meta-analyses or the randomized trials 
discussed earlier. The question of increased amputation risk with 
canagliflozin was noted previously. Hypoglycemia is not an issue 
with this class of drugs, unless added to a regimen that includes 
insulin provisional therapies.
The positive cardiovascular outcome trials with both

e positive cardiovascular outcome trials with both GLP1RAs 
and SGLT2 inhibitors (Fig. 37.49) raise fundamental questions 
regarding optimal T2DM management: How should these data 
be incorporated into guidelines, including the choice between 
GLP1RAs and SGLT2 inhibitors? Most guidelines have adopted 
an approach of recommending these agents in ways that align with 
the trials, for use mainly in patients with inadequately controlled 
HbA1c and established or high risk for cardiovascular disease

stablished or high risk for cardiovascular disease. This 
leaves unresolved the question of changing therapies for patients 
who have reasonable glycemic control but use antidiabetic drugs 
with no proven cardiovascular benefit. Separately, should cardiolo­
gists prescribe glucose-lowering agents, and how should previously 
encountered barriers to cardiologist engagement with diabetes 
treatment be overcome? Greater insight into the mechanisms 
underlying cardiovascular benefit from these newer 

nderlying cardiovascular benefit from these newer agents may 
yield further refinement in their use, while also advancing con­
tinued drug development. Finally, the positive clinical trial data 
in T2DM patients with established cardiovascular disease or high 
cardiovascular risk have prompted further consideration of how 
the benefits seen with these therapies might be extended to other 
indications and patient groups, particularly T2DM patients with­
out current cardiovascular disease. Clinica

 with­
out current cardiovascular disease. Clinical trials of these agents in 
people with T2DM and hypertension or heart failure are already 
under study. These agents, which do not cause hypoglycemia, may 
be especially appealing for reducing cardiovascular risk in those 
with prediabetes. 
Characteristic Features and Treatment of 
Dyslipidemia in Diabetic Patients
Dyslipidemia has been well characterized as a risk factor that 
increases atherosclerosis in patients with T2DM. Various pat­
tern

sclerosis in patients with T2DM. Various pat­
terns of dyslipidemia are often encountered in diabetes and may 
Hazard ratio (95% CI)
Favors treatment
Favors placebo
SGLT2i
GLP1RAs
Differences in CV risk reduction within
4 CVOTs demonstrating MACE superiority
EMPA-REGc
CANVASd
MACE
CV death
Nonfatal stroke
Nonfatal MI
MACE
CV death
LEADERa
SUSTAIN 6b
0.

LEADERa
SUSTAIN 6b
0.3
2.0
0.3
2.0
0.3
2.0
0.3
2.0
• Fig. 37.49  Components of positive primary endpoint responses in landmark glucagon-like peptide 1 
receptor agonist (GLP1RA) and sodium-glucose cotransporter-2 (SGLT2) inhibitor trials. Data shown for 
GLP1RAs (LEADER/liraglutide, SUSTAIN 6/semaglutide) and SGLT2 inhibitor (EMPA-REG/empagliflozin, 
CANVAS/canafliglozin) cardiovascular outcome trials (CVOT). CI, Confidence

rdiovascular outcome trials (CVOT). CI, Confidence interval; MACE, major 
adverse cardiovascular events; MI, myocardial infarction. Full study names are noted in text. aMarso SP, 
Daniels GH, Brown-Frandsen K, et al. Liraglutide and cardiovascular outcomes in type 2 diabetes. N 
Engl J Med. 2016;375:311–322; bMarso SP, Bain SC, Consoli A, et al. Semaglutide and cardiovascular 
outcomes in patients with type 2 diabetes. N Engl J Med. 2016;375:1834–1844; cZinman B, Wanner C, 
Lachin JM, et al. Emp

–1844; cZinman B, Wanner C, 
Lachin JM, et al. Empagliflozin, cardiovascular outcomes, and mortality in type 2 diabetes. N Engl J Med. 
2015;373:2117–2128; dNeal B, Perkovic V, Mahaffey KW, et al. Canagliflozin and cardiovascular and renal 
events in type 2 diabetes. N Engl J Med. 2017;377:644–657.
1513
increase atherogenic risk (also see Chapter 41). Although patients 
with diabetes tend not to have marked elevations in plasma LDL 
cholesterol leve

 marked elevations in plasma LDL 
cholesterol levels, their LDL cholesterol levels remain predictive 
of cardiovascular risk. LDL particles in diabetes are smaller and 
more dense than normal LDL. These small, dense triglyceride-
rich LDL particles are especially pathologic. Small dense LDL par­
ticles are more susceptible to oxidation, particularly in the setting 
of poor glucose control. Other evidence suggests that glycation 
of LDL may be enhanced in diabetes, impairing recognition of 
the l

anced in diabetes, impairing recognition of 
the lipoprotein by its hepatic receptor and extending its half-life. 
Conversely, HDL cholesterol levels are decreased in patients with 
T2D.767
A key feature of diabetic dyslipidemia is an increase in hepatic 
very low-density lipoprotein (VLDL) production in response to 
elevations in FFA flux often seen in diabetes with insulin-resis­
tant adipocytes. Although insulin and lipases such as lipoprotein 
lipase mediate the uptake of triglyceride-derive

lipase mediate the uptake of triglyceride-derived FFAs by striated 
muscle, reducing the levels presented to the liver, insulin resistance 
results in the opposite effect, increasing the levels of FFAs pre­
sented to the liver. The metabolic syndrome, with its characteristic 
abdominal obesity and insulin resistance, also increases FFA deliv­
ery to the liver. In addition, reduced lipoprotein lipase activity in 
T2DM leads to an accumulation of triglyceride-rich lipoproteins 
in the plasma of 

 triglyceride-rich lipoproteins 
in the plasma of these patients and may also result in decreased 
physiologic delivery of lipolytically derived, biologically active 
molecules to cells. Such models align with genetic evidence in the 
general population. Gain-of-function variants with increased lipo­
protein lipase activity, as well as those with genetic loss of function 
of inhibitors of the lipoprotein lipase pathway, appear to have less 
cardiovascular risk. Triglyceride-rich lipoproteins als

ovascular risk. Triglyceride-rich lipoproteins also play a role 
in the reduced levels of HDL cholesterol by increasing the transfer 
of cholesterol from these particles.
A number of landmark trials have established that lowering 
LDL cholesterol levels produces major clinical benefits in terms of 
reducing cardiovascular events in patients with and without a his­
tory of cardiovascular disease at baseline. These findings have now 
been extended to the population of subjects with T2DM and dys­
l

to the population of subjects with T2DM and dys­
lipidemia. Although LDL levels are often within the average range 
in patients with diabetes, treatment with 3-hydroxy-3-methylglu­
taryl coenzyme A (HMG-CoA) reductase inhibitors (statins) has 
consistently shown improved outcomes among both those with 
and without diabetes, including those with and without a prior 
history of heart disease. Statins are now incorporated into cur­
rent diabetes management guidelines, discussed further later in 
th

agement guidelines, discussed further later in 
the chapter.
In the Cholesterol and Recurrent Events (CARE) trial, T2DM 
patients with a prior cardiovascular event receiving pravastatin had 
a 25% reduction in the incidence of cardiovascular death, nonfatal 
myocardial infarction, coronary artery bypass graft, and revascu­
larization procedures compared to placebo.768 In the Long-Term 
Intervention With Pravastatin in Ischemic Disease (LIPID) study, 
patients with diabetes had a 19% reduction in

dy, 
patients with diabetes had a 19% reduction in major coronary 
heart disease events (fatal cardiovascular disease and nonfatal myo­
cardial infarction).769 In a post hoc subgroup analysis of secondary 
prevention in a large cohort of patients with diabetes, impaired 
glucose tolerance, or normal glucose tolerance, simvastatin nor­
malized associated elevations in total cholesterol and triglycerides 
across the range of glucose values.770 Treatment also significantly 
reduced major coronary e

tment also significantly 
reduced major coronary events and revascularizations in patients 
with diabetes and reduced major coronary events, revasculariza­
tions, and total and coronary mortality in patients with impaired 
glucose tolerance.
Several studies have further validated the use of statins in 
patients with diabetes and related conditions.567,771–774 In the 
Heart Protection Study, a large (N = 20,536), randomized, pla­
cebo-controlled trial of the use of simvastatin 40 mg in high-risk 

rial of the use of simvastatin 40 mg in high-risk 
patients, roughly 29% of the study participants had T2DM.775 
Over the 5-year course of the study, treatment with simvastatin 
resulted in a significant reduction in the occurrence of major vas­
cular events in patients with T2DM who had established coronary 
heart disease (33.4% vs 37.8% in simvastatin-treated and placebo-
treated patients, respectively), in T2DM with no prior coronary 
heart disease (13.8% vs 18.6%), and in both categories com

sease (13.8% vs 18.6%), and in both categories combined 
(20.2% vs 25.1%). Overall, the study also demonstrated a highly 
significant 12% relative risk reduction in all-cause mortality and 
an 18% reduction in coronary mortality among all subjects treated 
with simvastatin.
The Collaborative Atorvastatin Diabetes Study (CARDS) had 
a fundamental impact on the management of diabetes because of 
the magnitude of beneficial effects seen in the statin-treated group. 
CARDS was a randomized, placebo-

n-treated group. 
CARDS was a randomized, placebo-controlled trial that tested 
the effect of starting low dose atorvastatin (10 mg/day) in T2DM 
patients with no prior history of heart disease, with plasma LDL 
levels lower than 160 mg/dL (<4.1 mmol/L) (N = 2838) on pre­
venting acute coronary heart events, coronary revascularization, 
or stroke.574 The trial was terminated 2 years early because the 
prespecified efficacy criteria were met. After a median of 3.9 years 
follow-up, patients treat

r a median of 3.9 years 
follow-up, patients treated with atorvastatin had a relative risk 
reduction for first cardiovascular event of 37% (95% CI: 52–17% 
reduction; p = 0.001), compared with placebo-treated patients. 
Assessed separately, acute coronary heart disease events, coronary 
revascularizations, and stroke were significantly reduced, by 36%, 
31%, and 48%, respectively. The CARDS trial showed that in 
patients with T2DM, a threshold LDL cholesterol level should 
not be the sole deter

DL cholesterol level should 
not be the sole determinant of whether a statin is prescribed. Of 
note, stroke was reduced in CARDS, addressing prior questions 
about whether LDL lowering with statins might or might not 
reduce stroke to the same extent as they lower coronary heart dis­
ease. Currently, it is generally accepted that statins are effective in 
reducing cerebrovascular disease.776 As a result of CARDS and 
other studies discussed, most if not all guidelines have endorsed 
statin use 

t if not all guidelines have endorsed 
statin use in most appropriately aged individuals with diabetes.
The Treating to New Targets (TNT) study compared the 
effects of atorvastatin 10 mg or 80 mg daily for a median follow-
up period of 4.9 years in patients with clinically evident coro­
nary heart disease who also met the NCEP criteria for diagnosis 
of the metabolic syndrome.772 The study included 778 patients 
with T2DM, who constituted 22% of the study population. 
Treatment with atorvastati

 the study population. 
Treatment with atorvastatin 80 mg reduced major cardiovascular 
events 29% more than atorvastatin 10 mg, presumably due to the 
greater reduction in LDL cholesterol seen with higher dosing of 
atorvastatin.
Pooling of data from the multiple statin trials further sup­
ports statin benefits in those with diabetes. In the Cholesterol 
Treatment Trialists’ (CTT) Collaboration,777 analysis of 18,646 
patients with diabetes in 14 different studies revealed a 9% pro­
portional r

4 different studies revealed a 9% pro­
portional reduction in all-cause mortality per mmol/L reduction 
in LDL cholesterol (RR: 0.91, 99% CI: 0.82–1.01; p = 0.02) as 
compared to a similar 13% reduction in those without diabe­
tes (RR: 0.87, 99% CI: 0.82–0.92; p < 0.0001).778 A clinically 
and statistically significant 21% proportional reduction in major 
vascular events per mmol/L reduction in LDL cholesterol was 
observed in people with diabetes (RR: 0.79, 99% CI: 0.72–0.86; 
p < 0.0001).
1514

s (RR: 0.79, 99% CI: 0.72–0.86; 
1514
In 2013, revised cholesterol treatment guidelines were pro­
vided by the American College of Cardiology (ACC) and the 
American Heart Association (AHA).779 These recommendations 
depart from prior guidelines by focusing initiation of lipid-low­
ering therapy on four clinically defined patient groups who war­
ranted either high-intensity, moderate-intensity, or low-intensity 
statin int

, moderate-intensity, or low-intensity 
statin intervention while also abandoning use of LDL targets as 
a means of guiding therapy. Patients with diabetes were one of 
these four treatment groups, as supported by the data from pri­
mary and secondary prevention trials noted previously. A con­
troversial aspect of these guidelines was their abandonment of 
specific LDL targets. Both TNT and the subsequent Improved 
Reduction of Outcomes: Vytorin Efficacy International Trial 
(IMPROVE-IT), which 

Efficacy International Trial 
(IMPROVE-IT), which showed that adding the nonstatin 
LDL-lowering agent ezetimibe to stable statin therapy further 
decreased cardiovascular events in patients with coronary heart 
disease,780,781 support the clinical utility of using specific LDL 
targets. The 2013 guidelines might be best viewed as a scientific 
statement given their precise focus on what had been proven in 
clinical trials, which explains the specific age cutoff points for 
statin treatment of p

cific age cutoff points for 
statin treatment of patients with diabetes.
Clinical trials have also revealed statins are associated with an 
increase in new-onset diabetes among a small subset of patients 
at high risk for development of T2DM. In the Justification for 
the Use of Statins in Prevention: An Intervention Trial Evaluating 
Rosuvastatin (JUPITER) study, which investigated rosuvastatin 
20 mg versus placebo in 17,603 patients with no history of ath­
erosclerotic events, baseline mean L

story of ath­
erosclerotic events, baseline mean LDL levels of 108 mg/dL, and 
an elevated C-reactive protein of 4.2 mg/L, physician-reported 
new-onset diabetes was found in 270 patients on rosuvastatin 
versus 216 on placebo.782 In the JUPITER population 41% of 
participants had metabolic syndrome at study entry. New-onset 
diabetes was found in 28% in patients with one or more risk fac­
tors for developing diabetes, who at the same time benefited by 
a 39% reduction in cardiovascular events a

ted by 
a 39% reduction in cardiovascular events and a 17% reduction 
in total mortality. Among those with no risk factors for diabe­
tes, there was no increase in new diagnoses of diabetes.783 Thus 
the modest increase in diabetes in response to statin treatment is 
markedly offset by major decreases in cardiovascular events. For 
patients in whom statin treatment is appropriate, concerns over 
increased conversion to diabetes should not be a factor in the deci­
sion to use statins. While incre

ctor in the deci­
sion to use statins. While increases in new diabetes has been seen 
with multiple statins, the mechanism underlying this statin effect 
remains obscure.
Additional data are now available regarding LDL cholesterol 
levels and cardiovascular risk associated with using additional new 
LDL-lowering therapies. In population studies, individuals with 
lifelong low LDL cholesterol levels were found to have genetic loss 
of function in the Proprotein Convertase Subtilisin/Kexin Type 
9

the Proprotein Convertase Subtilisin/Kexin Type 
9 (PCSK9) gene, which was associated with decreased atheroscle­
rosis and cardiovascular events but no other untoward effects. 
Further supporting PCSK9 as a drug target, genetic variants with 
increased PCSK9 function resulted in a lipid and clinical picture 
that replicated familial hypercholesterolemia (FH). PCSK9 action 
targets the LDL receptor for proteosomal degradation rather than 
allowing it to return to the cell surface for another cycl

 it to return to the cell surface for another cycle of LDL 
clearance. In an impressive glimpse into future drug development 
prospects, in less than 10 years, PCSK9 inhibitors in the form 
of monoclonal antibodies were being tested for their clinical effi­
cacy in cardiovascular event reduction. In FOURIER, adding the 
PCSK9 inhibitor evolocumab (Repatha) to statin therapy versus 
statin alone in 27,546 patients with established coronary disease 
reduced LDL levels by 59% (92 mg/dL to 30 mg/dL 

reduced LDL levels by 59% (92 mg/dL to 30 mg/dL [2.4 mmol/L 
to 0.78 mmol/L]) and was associated with a statistically significant 
reduction in cardiovascular events of 15%.784 The ODYSSEY trial 
with alirocumab (Praluent) in patients with recent acute coronary 
syndrome also showed a statistically significant reduction in car­
diovascular events, as presented but not yet published.785 Patients 
with diabetes are receiving attention in terms of PCSK9 inhibitor 
use as a function of their incre

 PCSK9 inhibitor 
use as a function of their increased risk for cardiovascular events. 
Post hoc studies on outcomes in those with diabetes was a pre­
specified analysis in FOURIER and demonstrated similar benefit 
and no adverse issues among these patients (40%) compared to 
those who did not have diabetes at baseline (60%). In ODYS­
SEY, 29% of subjects had diabetes, indicating that there should 
be forthcoming data about this group. Other studies indicate no 
issues with alirocumab use in tho

ies indicate no 
issues with alirocumab use in those with diabetes, with presenta­
tions suggesting an augmented impact on event reduction in acute 
coronary syndrome patients. An ongoing scientific question is evi­
dence suggesting PCSK9 loss of function may be associated with 
increased risk of hyperglycemia and perhaps diabetes,786 although 
no increased signal for new diabetes has been seen in the clinical 
trials published to date.
Outcomes of treatment with other nonstatin therapies 
have 

of treatment with other nonstatin therapies 
have been variable. With niacin, data have been disappoint­
ing, with no additional benefit seen when added to statins in 
several trials.773,787,788 Although the lower HDL levels found in 
patients with diabetes might make niacin an appealing treat­
ment option, niacin’s lack of proven efficacy combined with its 
side effects of increasing insulin resistance and relatively poor 
tolerability have limited its use. Recent genetic studies using 
Mendeli

ted its use. Recent genetic studies using 
Mendelian randomization have demonstrated that variants 
associated with higher HDL levels often do not protect against 
CV events.789 Current attention is focused on the concept that 
the key element in the inverse relationship between HDL and 
cardiovascular disease is HDL functionality, which may not be 
reflected in total HDL levels.790
Fibric acid derivatives, which lower high triglycerides and 
raise HDL, have also been proposed as a treatment in 

se HDL, have also been proposed as a treatment in people 
with diabetes given the usual nature of diabetic dyslipidemia. In 
the VA High-Density Lipoprotein Cholesterol Intervention Trial 
(VA-HIT), men given gemfibrozil had a significant decrease in 
coronary events and strokes in the absence of statin use.774 Post 
hoc analyses suggesting these benefits were driven by the diabetes 
subgroup focused further attention on fibrates. The FIELD study 
assessed the effect of long-term fenofibrate the

assessed the effect of long-term fenofibrate therapy on cardiovas­
cular events in patients with T2DM.567 Patients were random­
ized to receive either micronized fenofibrate 200 mg per day (n = 
4895) or placebo (n = 4900). During 5-year follow-up, 5.9% of 
the placebo-treated patients versus 5.2% of the fenofibrate-treated 
patients had a major coronary event (coronary heart disease death 
or nonfatal myocardial infarction), the primary endpoint of the 
trial, a difference that was not statis

nt of the 
trial, a difference that was not statistically significant. Fenofibrate 
therapy significantly reduced some secondary endpoints, includ­
ing total cardiovascular events (HR: 0.89, 95% CI: 0.75–1.05, 
p = 0.035), coronary revascularization, progression of albumin­
uria, and the need for laser treatment of retinopathy. Other sec­
ondary endpoints such as cardiovascular mortality were higher, 
although not statistically significant, with fenofibrate. Statistical 
significance for the pri

fenofibrate. Statistical 
significance for the primary endpoint of the study might have 
been missed because a greater percentage of patients in the placebo 
group began statin therapy during the study period compared to 
the fenofibrate group, thus modulating their risk and potentially 
masking the treatment effect.
1515
The ACCORD trial addressed the use of fibrates in combina­
tion with statins in patients with T2DM. This trial investigated 
whet

 patients with T2DM. This trial investigated 
whether combination therapy with simvastatin plus fenofibrate, 
compared with simvastatin alone, would reduce cardiovascular 
events in diabetic patients at high risk for cardiovascular disease. 
In this randomized clinical trial comparing fenofibrate to placebo 
in statin-treated patients, combination therapy did not reduce the 
rate of cardiovascular mortality, nonfatal myocardial infarction, or 
nonfatal stroke, although other secondary parameters

nfatal stroke, although other secondary parameters such as first 
myocardial infarction were improved. In a prespecified subgroup 
with a high triglyceride ratio (triglycerides ≥204 mg/dL [≥5.3 
mmol/L] and HDL ≤34 mg/dL [≤0.88 mmol/L]), fenofibrate did 
reduce the primary outcome as compared to the placebo group. A 
pattern has emerged in fibrate trials of greater benefit in those with 
significantly elevated triglycerides and lower HDL—the popula­
tion in which fibrates would be commonly used.

la­
tion in which fibrates would be commonly used. At this time, no 
formal recommendation exists that patients with significant dys­
lipidemia receive combination statin-fibrate therapy for further 
risk reduction on the basis of T2DM alone.791 In those with a 
significant history of CVD, elevated triglycerides, and low HDL, 
expert opinion might endorse fibrates as an adjunct to statin ther­
apy once LDL levels are appropriately controlled as a reasonable 
if not definitively established optio

 reasonable 
if not definitively established option. Importantly, improved gly­
cemic control and decreased insulin resistance, whether through 
drug therapy or improved lifestyle interventions and modest 
weight loss, can improve diabetic dyslipidemia. Topline results 
from the Reduction of Cardiovascular Events With EPA—Inter­
vention Trial (REDUCE-IT),792 using a specific eicosapentaenoic 
acid only from fish oil in patients with established cardiovascular 
disease or diabetes mellitus and at

disease or diabetes mellitus and at least one other cardiovascu­
lar risk factor indicate a 25% reduction in the primary endpoint 
composite of the first occurrence of major adverse cardiovascu­
lar events, including cardiovascular death, nonfatal myocardial 
infarction, nonfatal stroke, coronary revascularization, or unstable 
angina requiring hospitalization.
Genetic variants linked to low triglyceride levels, like apoli­
poprotein CIII (apoCIII) loss of function, have been repo

in CIII (apoCIII) loss of function, have been reported to 
predict future cardiovascular events.793 ApoCIII loss of function 
reduces plasma triglyceride, and the atherogenicity of chylomi­
crons, VLDL, LDL, and HDL particles. These findings provide a 
rationale for pursuing ApoCIII inhibition and genetic knockdown 
as a therapeutic strategy, which continues to be pursued despite 
initial FDA rejection of volanesorsen, an apoCIII antisense ther­
apy, out of concerns for thrombocytopenia.794
Sour

Sources for cholesterol guidelines have transitioned from the 
prior NCEP Adult Treatment Panel recommendations, the last 
being the NCEP ATPIII, to joint guidance from the American 
College of Cardiology and the American Heart Association. Con­
sidering both the ATPIII recommendations and the most recent 
ACC/AHA approaches is valuable and provides additional per­
spective. The importance of dyslipidemia as a contributor to car­
diovascular risk in 

emia as a contributor to car­
diovascular risk in patients with diabetes was incorporated in the 
NCEP ATPIII guidelines, which identified diabetes as being equal 
in cardiovascular risk to having had prior coronary heart events.726 
According to the NCEP ATPIII guidelines, patients with T2DM 
would receive cholesterol-lowering therapy if the LDL cholesterol 
level were higher than 130 mg/dL (>3.36 mmol/L), with the 
goal of reducing LDL cholesterol to less than 100 mg/dL (<2.57 
mmol/L).726 In 

rol to less than 100 mg/dL (<2.57 
mmol/L).726 In practice, more aggressive approaches were often 
used, instituting drug treatment if LDL cholesterol level were 
higher than 100 mg/dL (>2.57 mmol/L). Subsequent guideline 
updates endorsed an LDL cholesterol goal of less than 70 mg/
dL (<1.81 mg/dL) in higher risk patients even when the high-risk 
patient had a baseline LDL cholesterol level lower than 100 mg/
dL (<2.57 mmol/L).795 The most recent ACC/AHA cholesterol 
guidelines took a completel

t ACC/AHA cholesterol 
guidelines took a completely different approach. The guidelines 
abandon specific LDL targets, instead identifying four specific 
patient groups in whom clinical trial data supported statin use. 
Since clinical trials had not specifically targeted more-intensive 
versus less-intensive LDL goals with a single statin, the recom­
mendations also focused on the appropriate statin dose intensity 
for given patient groups. The presence of diabetes was one of these 
four patient 

esence of diabetes was one of these 
four patient groups, with the recommendation that designated 
patients with diabetes (age 40–75 years), as defined by subjects 
enrolled in trials, receive a high-intensity statin (atorvastatin 40 or 
80 mg, rosuvastatin 20 or 40 mg) if their 10-year calculated risk 
was greater than or equal to 7.5%, and a moderate-intensity statin 
(these same statins at their other lower doses and remaining other 
statins at their mid to highest doses) if calculated 10-yea

t their mid to highest doses) if calculated 10-year risk was 
less than 7.5%. This approach has been controversial, especially 
the abandonment of target LDL levels. However, data from mul­
tiple clinical trials, including those with ezetimibe, supported 
benefits from lower LDL levels. The subsequent PCSK9 inhibitor 
data has further strengthened evidence for additional cardiovas­
cular benefit from achieving lower LDL levels as mentioned in 
further guideline updates and will likely be incorpo

rther guideline updates and will likely be incorporated in an 
upcoming AHA/ACC guideline release. 
Hypertension in Diabetic Patients
Approximately 50% of patients with newly diagnosed diabetes 
also have hypertension. As with dyslipidemia, hypertension inter­
acts with diabetes to amplify the risk of cardiac mortality (see Fig. 
37.47). Although the cause of hypertension is multifactorial, the 
insulin-resistant state is one factor postulated to predisp

esistant state is one factor postulated to predispose patients 
to develop hypertension. In addition to its negative effects on the 
cardiovascular system, high blood pressure is a key contributor to 
the development of microvascular disease in diabetes. Based on the 
guidelines of the Joint National Committee on Prevention, Detec­
tion, Evaluation, and Treatment of High Blood Pressure (JNC 7), 
blood pressure should be reduced to less than 130/85 mm Hg in 
patients with diabetes.547 A longer du

5 mm Hg in 
patients with diabetes.547 A longer duration of diabetes has been 
linked to increased arterial stiffness, decreased vasomotor function, 
and changes in pulse pressure, which can contribute to patient 
symptoms in response to blood pressure treatment. Patients with 
diabetes may also lose the usual nocturnal blood pressure dip.
Results of the most recent clinical trials underscore the benefits 
of aggressive treatment of hypertension in patients with diabetes, 
although achieving mea

in patients with diabetes, 
although achieving mean blood pressure reductions to currently 
recommended targets is challenging both in clinical trials and in 
real-world settings. This may also help explain disconcordant trial 
results. Data have continued to support achieving a systolic blood 
pressure between 120 and 140 mm Hg in those with diabetes, 
while additional benefit is less clear with systolic blood pressure 
less than 120 mm Hg.796
Use of a long-acting dihydropyridine calcium channe

se of a long-acting dihydropyridine calcium channel blocker 
in the Systolic Hypertension in Europe (Syst-Eur) study resulted 
in substantial reductions in rates of total mortality (55%), cardio­
vascular mortality (76%), and cardiovascular events (69%) in the 
diabetic subgroup, greater benefits than were seen in the subgroup 
without diabetes. In the Heart Outcomes Prevention Evaluation 
(HOPE) study, in which almost 40% of patients had diabetes and 
one other cardiovascular risk factor, ramip

s and 
one other cardiovascular risk factor, ramipril reduced the primary 
1516
outcome by 24% and total mortality risk by 25%.797 Even in nor­
motensive patients with diabetes, some benefit was seen with a 
drop in blood pressure (2–4 mm Hg) with ACE inhibitor therapy, 
raising questions about what mechanisms were responsible for the 
benefit seen with even modest blood pressure changes. Other rig­
orously designed studies, such as th

s. Other rig­
orously designed studies, such as the UKPDS798 and the Hyper­
tension Optimal Treatment (HOT) study,799 suggested that tight 
blood pressure control in patients with diabetes caused cardiovas­
cular benefits of even greater magnitude. In the ADVANCE trial, 
over 11,000 type 2 diabetes patients received either a fixed-dose 
combination of perindopril and indapamide or placebo along 
with usual care.800 Those on perindopril/indapamide had on aver­
age 5.6–mm Hg lower systolic blood p

 had on aver­
age 5.6–mm Hg lower systolic blood pressures and 2.2–mm Hg 
diastolic blood pressures, with a 9% decrease in the relative risk 
of a major macrovascular or microvascular event as compared to 
16.8% for those on placebo, with a hazard ratio 0.91. The mac­
rovascular and microvascular events responded similarly but not 
independently, highlighting the pathogenic role of hypertension 
in both large and small vessel disease.
In the Losartan Intervention For Endpoint Reduction in 
Hyper

rtan Intervention For Endpoint Reduction in 
Hypertension (LIFE) study, patients with diabetes, hypertension, 
and signs of left ventricular hypertrophy were randomly assigned 
to treatment with losartan-based (n = 586) or atenolol-based 
(n = 609) treatment for hypertension.800a Despite similar blood 
pressure reductions, losartan was more effective than atenolol for 
reducing rates of cardiovascular morbidity and mortality while 
also demonstrating fewer conversions to new diabetes. The abilit

ting fewer conversions to new diabetes. The ability 
of losartan to reduce events more effectively than atenolol may be 
related to the ability of angiotensin receptor blockers to reverse left 
ventricular hypertrophy more effectively than beta blockers.
Although beta blockers are thought to worsen glycemic control 
in patients with diabetes, it is not clear whether this is a property 
of all members of this drug class or whether this property persists 
if beta blockers are given in combination 

rsists 
if beta blockers are given in combination with renin-angiotensin 
system inhibitors that are known to increase insulin sensitivity. In 
the Glycemic Effects in Diabetes Mellitus: Carvedilol-Metoprolol 
Comparison in Hypertensives (GENINI) trial, patients with doc­
umented T2DM and hypertension who were taking a stable dose 
of either an ARB or an ACE inhibitor were randomized to receive 
either carvedilol, a nonselective beta blocker, which also blocks 
α1-adrenergic receptors, or metopr

ch also blocks 
α1-adrenergic receptors, or metoprolol, a β1-selective adrenergic 
blocker. Although the degree of blood pressure control was similar 
with both beta blockers, HbA1c and insulin resistance increased 
significantly with metoprolol but not with carvedilol. Therefore 
carvedilol appears not to cause the adverse effects that metopro­
lol does on glucose levels when used in combination with RAS 
inhibitors, although this conclusion needs to be confirmed in a 
longer term outcome trial

ds to be confirmed in a 
longer term outcome trial.
The investigators of the Antihypertensive and Lipid-Lowering 
Treatment to Prevent Heart Attack Trial (ALLHAT) compared 
outcomes during first-step treatment of hypertension in 31,512 
patients with T2DM, impaired fasting glucose (IFG) levels, or 
normoglycemia with a calcium channel blocker (amlodipine 2.5–
10 mg/day) or ACE inhibitor (lisinopril 10–40 mg/day) compared 
with a thiazide-type diuretic (chlorthalidone 12.5–25 mg/day).801 
There w

retic (chlorthalidone 12.5–25 mg/day).801 
There was no significant difference in the occurrence of the pri­
mary outcome (fatal cardiovascular disease or nonfatal myocardial 
infarction) in patients with T2DM treated with a calcium chan­
nel blocker or an ACE inhibitor compared with chlorthalidone. 
Patients with impaired fasting glucose treated with a calcium 
channel blocker had a significantly higher relative risk for the pri­
mary outcome than patients receiving chlorthalidone.
A major unre

an patients receiving chlorthalidone.
A major unresolved question has been whether treatment of 
hypertension to lower targets than currently recommended would 
further reduce cardiovascular risk in patients with type 2 diabetes. 
Most trials establishing the remarkable benefit of blood pressure 
lowering, regardless of the medications used, have studied people 
whose systolic blood pressure was higher than 140 mm Hg. In 
the ACCORD trial, the effect of lowering systolic blood pressure 
to two d

fect of lowering systolic blood pressure 
to two different levels on cardiovascular risk was assessed.802 One 
group was randomized to a systolic blood pressure lower than 
120 mm Hg (intensive therapy); the other group was treated to 
a systolic pressure lower than 140 mm Hg (standard therapy). 
Targeting the lower blood pressure level did not reduce the rate 
of fatal and nonfatal cardiovascular events. Based on this result, 
there is no recommendation to treat patients with T2DM and 
hyperten

endation to treat patients with T2DM and 
hypertension to a systolic blood pressure lower than the currently 
recommended target of 130 mm Hg to decrease cardiovascular 
events. However, further blood pressure reduction may reduce the 
incidence of diabetic nephropathy.548,802 In contrast, in the Sys­
tolic Blood Pressure Intervention Trial (SPRINT), which did not 
enroll patients with diabetes, more aggressive blood pressure low­
ering was beneficial,803 prompting subsequent discussions about 

icial,803 prompting subsequent discussions about 
potential discrepancies between SPRINT and these other studies, 
including differences in hypertension management in the presence 
or absence of diabetes.804
The HOT trial also studied intensive versus standard control 
in 18,790 participants, including 1501 subjects who had diabe­
tes, with a focus on enrollment and targeted blood pressure based 
on diastolic measurements. While more intensive blood pressure 
control did not make a difference in

ood pressure 
control did not make a difference in the primary cardiovascular 
outcome in the study as a whole, the subgroup with diabetes expe­
rienced a 51% decrease in events, with optimal cardiovascular risk 
reduction found in those achieving a diastolic blood pressure of 
82.6 mm Hg and systolic blood pressure of 138.5 mm Hg.799 
Acute Coronary Syndromes in Diabetes Mellitus
The case fatality rate from myocardial infarction is almost twice 
as high in patients with diabetes as in nondiabet

as high in patients with diabetes as in nondiabetic patients. This 
excess risk is seen both during the acute phase and in the early and 
late postinfarction periods. A number of mechanisms have been 
hypothesized to be responsible for worse outcomes in patients 
with diabetes, including the following:
 • Increased risk of heart failure due to maladaptive remodeling of 
the left ventricle805–807
 • Increased risk of sudden death due to sympathovagal imbal­
ance as a consequence of autonomic n

t a variety of treat­
ment modalities can improve outcomes from myocardial infarc­
tion in patients with diabetes. In terms of interventions, patients 
with diabetes experiencing an acute infarct respond as favorably 
to fibrinolytic therapy as do nondiabetic patients.814,815 Glycemic 
control is an essential component of overall management. Glucose 
levels at hospital admission have been independently correlated 
with early and late fatality after myocardial infarct in patients with 
and withou

er myocardial infarct in patients with 
and without diabetes mellitus.820–823
1517
Studies such as the Diabetes and Insulin-Glucose Infusion 
in Acute Myocardial Infarction (DIGAMI) study have assessed 
the impact of intensive glycemic control in patients with diabe­
tes during the acute phase of myocardial infarction. Patients in 
this study were randomized to either intensive insulin therapy 
(insulin-glucose infusion for 24 hours, followed by sub

lin-glucose infusion for 24 hours, followed by subcutaneous 
insulin injection for 3 months) or standard glycemic control.824 
The intensive insulin regimen lowered blood glucose level dur­
ing the first hour after admission and at discharge compared with 
conventional therapy. The 1-year mortality rate was reduced with 
the insulin infusion group compared with the control group, a dif­
ference that was maintained after 3.4 years of follow-up.
Although the mechanisms responsible for the potentia

though the mechanisms responsible for the potential ben­
efit shown in the original DIGAMI study are not entirely clear, 
experimental data suggest that strict glycemic control can improve 
myocardial cell metabolism by increasing the availability of glu­
cose as a substrate for ATP generation and reducing the formation 
of FFAs, thereby shifting cardiac metabolism from FFA oxidation 
to glycolysis and glucose oxidation. Intensive glycemic control 
can also reverse the impaired fibrinolysis that

l 
can also reverse the impaired fibrinolysis that is typically seen in 
patients with diabetes.
However, DIGAMI 2, a prospective, randomized, open-label 
trial that followed up the DIGAMI trial, comparing outcomes in 
patients with either T1DM or T2DM, failed to corroborate the 
earlier reported improvement in outcomes with intensive insulin 
treatment.825 The lack of effect of long-term insulin treatment on 
outcomes may have been perhaps partially influenced by protocol 
violations, including

ally influenced by protocol 
violations, including the patient group assigned to standard meta­
bolic management, without insulin or glucose infusions, having 
such interventions, with 14% receiving insulin-glucose infusions 
and as many as 41% receiving extra insulin injections. As a result, 
the blood glucose levels in all three groups were not significantly 
different after treatment.
Results from the CREATE-ECLA (Clinical Trial of Revipa­
rin and Metabolic Modulation in Acute Myocardial Infa

 and Metabolic Modulation in Acute Myocardial Infarction 
Treatment Evaluation—Estudios Cardiologicos Latin America), 
a randomized, controlled trial of 20,201 patients who presented 
with ST-segment elevation myocardial infarction within 12 hours 
after onset of symptoms, are consistent with DIGAMI 2 findings. 
Patients were randomized to either high-dose glucose-insulin-
potassium (GIK) infusion (i.e., 25% glucose, 50 U/L regular 
insulin, and 80 mEq KCl) administered over 24 hours or to usual

80 mEq KCl) administered over 24 hours or to usual 
care.826 Roughly 18% of the patients in both treatment arms had 
T2DM. After 30 days, there were no differences in the rate of 
occurrence of mortality, cardiac arrest, cardiogenic shock, or rein­
farction in the two treatment groups.
Sulfonylureas have been implicated as increasing cardiovascular 
mortality rate, particularly in patients undergoing revasculariza­
tion for acute myocardial infarction.827 The UKPDS did not show 
a deleterious ef

ction.827 The UKPDS did not show 
a deleterious effect of these agents on the incidence of sudden 
death or myocardial infarction379 or over 10 years of follow-up.24 
The sulfonylureas act through the sulfonylurea receptor compo­
nent of ATP-sensitive potassium channels in the pancreatic beta 
cell. In the heart, ATP-sensitive potassium channels are involved 
in ischemic preconditioning and coronary vasodilation.828–830 It 
is not clear whether the sulfonylureas modulate these channels in 
the h

he sulfonylureas modulate these channels in 
the heart or vascular system or whether they significantly increase 
risk in patients with diabetes who are experiencing an acute myo­
cardial infarct.
ACE inhibitors dramatically reduce the mortality rate after 
a myocardial infarction in patients with diabetes, ostensibly 
through their effects to reduce infarct size and limit ventricular 
remodeling. In addition to these hemodynamic benefits, ACE 
inhibitors can also improve outcomes in diabetes by

nhibitors can also improve outcomes in diabetes by improving 
endothelial function,831 improving fibrinolysis,832 and decreasing 
insulin resistance.833
In a retrospective analysis of the GISSI-3 (Gruppo Italiano per 
lo Studio della Sopravvivenza nell’Infarto Miocardico) study,834 
lisinopril administration within 24 hours after hospital admis­
sion for acute myocardial infarction reduced both 6-week and 
6-month mortality rates more in patients with diabetes than in 
patients without diabetes.

 with diabetes than in 
Similarly, in the Trandolapril Cardiac Evaluation Study 
(TRACE), which investigated trandolapril use in patients with 
left ventricular dysfunction after prior anterior myocardial infarct, 
a subgroup analysis of diabetes patients revealed trandolapril treat­
ment greatly improved outcomes, with a 36% decrease in mortal­
ity from any cause and a 62% reduction in progression to severe 
heart failure as compared to placebo in those with diabetes,

ure as compared to placebo in those with diabetes, ben­
efits that were more pronounced than the effects seen in TRACE 
subjects without diabetes (18% reduction in mortality, no effect 
on progression to severe CHF).834a
Beta blockers are now widely accepted for the treatment of 
acute coronary syndrome in patients with diabetes. Older, noncar­
dioselective beta blockers might have adversely affected the lipid 
profile and inhibited the metabolic response to hypoglycemia, but 
more recent data w

 response to hypoglycemia, but 
more recent data with cardioselective beta blockers suggest that 
these agents have less negative effect on metabolic indices, perhaps 
because they increase peripheral blood flow and improve glucose 
delivery.835,836 Clinical trial data confirm that beta blockers reduce 
the rates of mortality and reinfarction in patients with myocar­
dial infarction in the presence of diabetes. In fact, the magnitude 
of their effects in patients with diabetes appear to exceed t

fects in patients with diabetes appear to exceed those 
seen in nondiabetic patients. A large review of data from more 
than 45,000 patients, 26% of whom had diabetes, showed beta 
blocker therapy was associated with a lower 1-year mortality rate 
in patients with diabetes than in those without diabetes, with no 
evidence of an increase in diabetes-related complications.837
Postulated mechanisms for the benefit of beta blockers in 
patients with diabetes include dampening of the sympathetic ner­

diabetes include dampening of the sympathetic ner­
vous system overactivity that arises as a consequence of autonomic 
neuropathy. Beta blockers can also reduce FFA levels and thereby 
reduce myocardial oxygen requirements. Carvedilol, although not 
a cardioselective beta blocker, has unique α1-adrenergic recep­
tor blocking properties along with purported antioxidant effects. 
These actions may underlie the decreased insulin resistance and 
greater blood pressure effects of this agent, which ma

ter blood pressure effects of this agent, which may be of par­
ticular benefit in patients with type 2 diabetes.838
Aspirin has been a cornerstone of therapy for the primary or 
secondary prevention of acute coronary syndrome in patients 
with T1DM and T2DM who do not have contraindications to its 
use. Aspirin significantly lowers the risk of myocardial infarction 
without increasing the risk of vitreous or retinal bleeding, even 
in patients with retinopathy.839 Use of aspirin therapy (75–162 

th retinopathy.839 Use of aspirin therapy (75–162 
mg/day) as a secondary prevention strategy in those with dia­
betes and a history of atherosclerotic cardiovascular is currently 
recommended by the ADA,840 with presumed benefits through 
modulation of the enhanced platelet aggregation seen with both 
T1DM and T2DM.841 Despite this, controversy has arisen about 
the ideal dose of aspirin, if enteric coating might interfere with 
aspirin’s effects,842 and more recently, if aspirin benefits in pr

s,842 and more recently, if aspirin benefits in pri­
mary prevention are offset by the risk of bleeding. The Aspirin 
in Reducing Events in the Elderly (ASPREE) trial studied aspirin 
100 mg versus placebo in 19,114 subjects 70 years of age or older 
1518
with no known cardiovascular disease, including about 10% of 
participants who had diabetes, and found no benefit on cardio­
vascular outcomes, including the diabetes subgroup, but a 

 outcomes, including the diabetes subgroup, but a signifi­
cant increase in bleeding.843 In contrast, in ASCEND, in which 
15,480 participants with diabetes received either aspirin 100 mg 
or placebo for a mean of 7.4 years, the aspirin group had fewer 
serious vascular events (658 participants [8.5%] vs 743 [9.6%]; 
RR: 0.88, 95% CI: 0.79–0.97, p = 0.01) than placebo, but at 
the expense of more major gastrointestinal or other extracranial 
bleeding (4.1% on aspirin group vs 3.2% on placebo gro

ding (4.1% on aspirin group vs 3.2% on placebo group, RR: 
1.29, p = 0.003).844 Aspirin use in patients with diabetes must be 
tailored to their particular overall cardiovascular and bleeding risk.
Antiplatelet therapy with the antagonist of the P2Y12 subclass 
of ADP receptor clopidogrel also benefits patients with diabetes. 
The Clopidogrel Versus Aspirin in Patients at Risk of Ischaemic 
Events (CAPRIE) trial compared outcomes in patients with non–
ST-segment elevation myocardial infarction t

 non–
ST-segment elevation myocardial infarction treated with aspirin 
or with clopidogrel and included 3866 patients with diabetes.845 
Although the event rate was higher among the diabetic patients 
than in the overall study population, the response to treatment 
was also better. The event rate for the primary endpoint (vascular 
death, ischemic stroke, myocardial infarction, or rehospitalization 
for ischemia or bleeding) was 17.7% for diabetic patients treated 
with aspirin and 15.6% for tho

c patients treated 
with aspirin and 15.6% for those randomized to clopidogrel, a 
significant RR of 12.5%. Newer antiplatelet therapies are also 
now available, including P2Y12 receptor antagonists with greater 
potency. In the Trial to Assess Improvement in Therapeutic Out­
comes by Optimizing Platelet Inhibition With Prasugrel-Throm­
bolysis In Myocardial Infarction 38 (TRITON-TIMI 38) study, 
participants with diabetes had a 30% relative risk reduction in 
the primary endpoint (cardiovascula

 reduction in 
the primary endpoint (cardiovascular death, nonfatal myocardial 
infarct, or nonfatal stroke) without evidence for increases in major 
bleeding.846 In the Platelet Inhibition and Patient Outcomes 
(PLATO) trial, ticagrelor equally reduced the rate of ischemic 
events in acute coronary syndrome patients both with and with­
out diabetes; in the diabetes subgroup, reductions were seen in the 
primary composite endpoint (18%), all-cause mortality (18%), 
and stent thrombosis (35%) wit

e mortality (18%), 
and stent thrombosis (35%) with no increase in major bleeding. 
Patients with diabetes are felt to have an increased risk for com­
plications with percutaneous interventions like stent thrombo­
sis, and may warrant more intensive oral antiplatelet therapy in 
appropriate patients.
Newer adjunct therapies, such as the platelet glycoprotein IIb/
IIIa receptor antagonists that antagonize platelet action, have 
also been assessed in diabetic patients who present with unstable 
an

in diabetic patients who present with unstable 
angina or non–Q-wave infarction. Overall, these agents appear to 
work equally well, or perhaps slightly better, in patients with dia­
betes compared with nondiabetic patients. In the Platelet Recep­
tor Inhibition in Ischemic Syndrome Management in Patients 
Limited by Unstable Signs and Symptoms (PRISM-PLUS) study, 
the addition of tirofiban to heparin therapy reduced the 7-day 
composite endpoint, compared with heparin alone. This effect 
was gr

, compared with heparin alone. This effect 
was greater in patients with diabetes than in patients without 
diabetes.847
In one study of patients undergoing percutaneous transluminal 
coronary angioplasty, glycoprotein IIb/IIIa antagonist therapy was 
associated with fewer acute events but a higher rate of long-term 
target-vessel revascularization in the diabetic cohort compared 
with the nondiabetic cohort.848 However, in another trial, in 
which stents were used, the rate of target vessel rev

ch stents were used, the rate of target vessel revascularization 
at 6 months was significantly decreased with the addition of a gly­
coprotein IIb/IIIa antagonist compared with placebo.849
Results of the BARI study showed that coronary artery bypass 
grafting provides better outcomes than percutaneous transluminal 
coronary angioplasty in patients with diabetes, possibly because 
it addresses the extensive coronary vascular disease in these 
patients.850 This study did not employ stents or glyc

ients.850 This study did not employ stents or glycoprotein IIb/
IIIa inhibitors, two modalities that, when used together, appear 
to improve outcomes after percutaneous transluminal coronary 
angioplasty in patients with diabetes.
In terms of revascularization strategies, the Future Revasculariza­
tion Evaluation in Patients With Diabetes Mellitus: Optimal Man­
agement of Multivessel Disease (FREEDOM) trial had a major 
impact on management decisions. This trial was an appropriately 
powered, ra

ions. This trial was an appropriately 
powered, randomized comparison of percutaneous coronary inter­
ventions (with drug-eluting stents) and coronary artery bypass graft 
(using arterial grafting) in patients with diabetes and multivessel 
coronary artery disease on concomitant optimal medical therapy. It 
provided strong evidence that that coronary bypass resulted in fewer 
major cardiovascular events, and reduced 5-year mortality compared 
to percutaneous coronary interventions.851 Continued 

percutaneous coronary interventions.851 Continued progress in 
stent technology and adjunctive therapies, as noted, continues to 
make this a challenging decision. Patients with diabetes with less 
extensive coronary disease for whom invasive intervention is war­
ranted can certainly be managed with percutaneous interventions.
Cardiomyopathy in Patients With Diabetes 
Mellitus
Diabetes is associated with a fourfold increase in the risk of conges­
tive heart failure, even after adjustment for oth

tive heart failure, even after adjustment for other cardiovascular risk 
factors such as age, blood pressure, cholesterol level, obesity, and 
history of coronary heart disease.852 Contributing factors include 
diabetes-induced cardiomyocyte dysfunction, impaired microvas­
cular perfusion because of defective endothelial function, increased 
collagen deposition with fibrosis, and maladaptive remodeling after 
myocardial infarction, leading to both diastolic and systolic heart 
failure (also dis

th diastolic and systolic heart 
failure (also discussed in “Biochemistry and Molecular Cell Biol­
ogy”). Current nomenclature853 organizes heart failure into two 
broad categories: with preserved ejection fraction (HFpEF) and 
with reduction ejection fraction (HFrEF).854 Patients with diabetes, 
who commonly present with either form, also experience higher 
rates of heart failure than do nondiabetic patients after an acute 
myocardial infarction, regardless of the size of the infarct zone.806,8

, regardless of the size of the infarct zone.806,855 
These findings suggest that diabetes itself causes deleterious effects 
on the myocardium, leading to poorer outcomes.
A number of key structural, functional, and metabolic factors 
in diabetes have been implicated in the increased risk of maladap­
tive remodeling that leads to heart failure. For example, evidence of 
silent myocardial infarction is found in up to 40% of patients with 
diabetes who present with a clinically apparent infarct a

s who present with a clinically apparent infarct and can lead 
to unrecognized regional and global ventricular dysfunction.834,856 
As many as 50% of patients with diabetes and coronary heart disease 
have cardiac autonomic neuropathy, which is known to contribute 
to both systolic and diastolic dysfunction.857 Like hypertension, 
diabetes can cause fibrosis of the myocardium and increased col­
lagen deposition.858,859 These effects are even more pronounced in 
patients with coexisting hypertens

 pronounced in 
patients with coexisting hypertension and diabetes and may con­
tribute to the finding of diastolic dysfunction common in patients 
with diabetes.860 Enhanced myocardial endothelial dysfunction in 
diabetes has also been described as a pathophysiologic pathway to 
impaired microvascular perfusion and ischemia.861,862
On a cellular level, both hyperglycemia and insulin resistance 
have direct negative effects on myocardial metabolism. Depression 
1519
Chapter 37  Complications of 

sm. Depression 
1519
of myocardial GLUT4 levels in the setting of diabetes inhibits 
glucose entry and glycolysis in the heart. As a result, intracellu­
lar metabolism shifts from glycolysis to FFA oxidation, thereby 
reducing glycolytic ATP generation, a major source of energy 
under anaerobic (i.e., ischemic) conditions.816 The production of 
oxygen free radicals can also be enhanced in this situation, fur­
ther depressing myocardial contractile f

ion, fur­
ther depressing myocardial contractile function.862 Mitochondrial 
dysfunction is a feature of ischemia-reperfusion injury, leading to 
generation of reactive oxygen species. In experimental models of 
ischemia-reperfusion, mitochondrial reactive oxygen species gen­
eration is mediated by reversal of succinate dehydrogenase, lead­
ing to succinate accumulation. On reperfusion, restoration of 
succinate dehydrogenase activity leads to rapid succinate oxida­
tion, causing reactive oxygen

pid succinate oxida­
tion, causing reactive oxygen species generation by reverse electron 
transport at mitochondrial complex I.863 Increased myocardial 
production of succinate is a feature of acute ST-segment–eleva­
tion myocardial infarction in humans.864
A major advance in the heart failure field in general was the find­
ing in PARADIGM-HF that the combination of an angiotensin 
receptor blocker (valsartan) and a neprilysin inhibitor (sacubitril) 
proved superior to the ACE inhibitor enalapr

ril) 
proved superior to the ACE inhibitor enalapril in patients with 
reduced ejection fraction (≤40%), significantly reducing cardiovas­
cular death or hospitalization for heart failure, and doing so against 
a medicine previously shown to have a mortality benefit.865 Pro­
gression of heart failure and renal dysfunction were also decreased. 
Of note, among the nearly 45% of patients in this study who had 
diabetes, there was evidence of greater decreases in HbA1c and less 
progression to insul

 decreases in HbA1c and less 
progression to insulin use, raising intriguing mechanistic questions, 
while also potentially supporting the use of sacubitril/valsartan in 
patients with diabetes and reduced ejection fractions.592
Knowledge about the pathophysiology underlying increased 
coronary artery disease in people with prediabetes and type 2 dia­
betes has increased enormously in recent years. That knowledge 
has directed seminal clinical studies, which in turn have enlarged 
our understand

udies, which in turn have enlarged 
our understanding of pathophysiology. These seminal studies 
provide the rationale for an optimized multifactorial approach to 
the prevention and treatment of diabetic cardiovascular disease, 
including choice of drugs to manage hyperglycemia, dyslipidemia, 
and hypertension, and postinfarction sequelae. Data from ongoing 
clinical trials and new pharmaceutical developments will further 
improve the cardiovascular prognosis for people with diabetes. 
The Diab

ular prognosis for people with diabetes. 
The Diabetic Foot
Foot complications of ulceration and amputation are associated 
with high mortality, yet diabetic foot ulcers are the most preventable 
late adverse outcome of diabetes.866–869 Elliott Joslin correctly noted 
in 1934 “diabetic gangrene is not heaven-sent, but earth-born.” The 
neuropathic foot does not spontaneously ulcerate; insensitivity, 
deformity, and unperceived trauma (e.g., inappropriate footwear) 
promote skin breakdown. Most a

ropriate footwear) 
promote skin breakdown. Most amputations are preceded by foot 
ulcers. Increased appreciation of the complex pathogenesis of foot 
ulcers permits design of screening programs for risk identification, 
preventive education, and design of multidisciplinary foot care pro­
grams. Much progress has been made, but suboptimal adherence to 
current professional guidelines has prevented translation of this into 
a universal decline in amputation rates.
Epidemiology of Diabetic Foot Ul

amputation rates.
Epidemiology of Diabetic Foot Ulceration
Foot ulceration is common and occurs in both T1DM and 
T2DM. The annual incidence of foot ulceration is about 2% 
to 2.5% in Western countries: Higher incident rates are found 
in select diabetic populations, including US Medicare recipi­
ents (6%), US veterans (5%), and the global diabetes popula­
tion (6.3%).870 Approximately 5% to 10% of diabetic patients 
have had past or present foot ulceration, and 1% have under­
gone amputation.86

 ulceration, and 1% have under­
gone amputation.866 Although between 1990 and 2010, a sig­
nificant reduction in certain diabetes-related complications was 
reported from the United States,8 unfortunately there was no 
reduction in rates of amputations reported in this time frame. 
A large community-based study in the United Kingdom showed 
an annual incidence of ulceration of approximately 2%; this 
rose to 7% with known diabetic neuropathy and to as high as 
50% with a past history of ulcerati

to as high as 
50% with a past history of ulceration.871 The lifetime risk for 
development of a foot ulcer in a diabetic patient is estimated to 
be as high as 25%.868,872 Multiple factors likely contribute to 
different rates of foot ulceration, including diabetes duration, 
smoking, and other neurologic and cardiovascular risk factors. 
Foot ulceration is most common in patients with a history of 
similar problems. In experienced diabetic foot clinics, more 
than 50% of patients with new foot

 clinics, more 
than 50% of patients with new foot ulcers give a past ulcer his­
tory. Patients with other complications of diabetes, including 
retinopathy and renal dysfunction, are at increased risk for foot 
ulceration. Patients on dialysis are among those at the very high­
est risk for ulceration.872a
More than 80% of amputations are preceded by foot ulcers. 
Diabetes is the most frequent cause of nontraumatic lower limb 
amputation in the United States, and rates are 15 times greater 
than

nited States, and rates are 15 times greater 
than those in the nondiabetic population. Reduced amputation 
rates occur following implementation of foot screening and edu­
cation programs.873,874 Implementation of strategies to reduce 
ulceration and amputation are necessary and may benefit from 
additional education on behavioral aspects of diabetic foot care, 
including awareness of ulceration risk, compliance with daily foot 
inspection, and the importance of nonweightbearing with early 
foot

he importance of nonweightbearing with early 
foot infections.875 Treatments to reduce cardiovascular and renal 
risk likely further reduce amputation rates, but this has been less 
clearly established. 
Pathogenesis of Foot Ulceration
Foot ulceration results from multiple interacting component 
causes, none of which alone is sufficient to cause ulceration but 
when combined may lead to skin breakdown. A combination 
of two or more risk factors promotes diabetic foot ulceration, 
and the triad o

romotes diabetic foot ulceration, 
and the triad of component causes—neuropathy, deformity, 
and trauma—is present in 63% of incident ulcers. Edema and 
ischemia are also common component causes. These risk fac­
tors were demonstrated in a North American/United Kingdom 
collaborative study of more than 150 consecutive foot ulcer 
cases.876
Diabetic Neuropathy and the Foot Ulcer
All three components of neuropathy—sensory, motor, and 
autonomic—can contribute to foot ulceration. The risk of foot 

 contribute to foot ulceration. The risk of foot 
ulceration in patients with neuropathy is sevenfold higher than 
in those without this complication of diabetes.871 Chronic sen­
sorimotor neuropathy is common, affecting at least one-third of 
older diabetic patients in Western countries. Its onset is gradual 
and insidious, and symptoms may be so minimal that they go 
unnoticed. Although uncomfortable, painful, and paresthetic 
symptoms predominate in most patients, some never experience 
sympt

ate in most patients, some never experience 
symptoms. Clinical examination usually reveals a sensory deficit 
1520
in a glove-and-stocking distribution, with signs of motor dys­
function such as small muscle wasting in the feet, and absent 
ankle reflexes. Although a history of typical symptoms strongly 
suggests a diagnosis of neuropathy, absence of symptoms does 
not exclude the diagnosis and must never be equated with a lack 
of fo

nosis and must never be equated with a lack 
of foot ulcer risk. Therefore assessment of foot ulcer risk must 
always include a careful foot examination, including assessment 
of proprioception (i.e., pressure perception using a 10-g mono­
filament) regardless of history. Loss of sensation and propriocep­
tion reduce capillary perfusion and increase ischemia at sites of 
increased pressure load.
Sympathetic autonomic neuropathy affecting the lower limbs 
results in reduced sweating, dry skin, an

r limbs 
results in reduced sweating, dry skin, and development of cracks 
and fissures. In the absence of large-vessel arterial disease, there 
may be increased blood flow to the foot, with arteriovenous shunt­
ing leading to the warm but at-risk foot. 
Callus, Deformity, and High Foot Pressures
Motor neuropathy, with imbalance of the flexor and extensor 
muscles in the foot, commonly results in foot deformity, with 
prominent metatarsal heads and clawing of the toes (Fig. 37.50). 
The combinat

nd clawing of the toes (Fig. 37.50). 
The combination of proprioceptive loss due to sensory neuropathy 
and the prominence of metatarsal heads lead to increased pressures 
and loads under the diabetic foot. Neuropathy is only one cause of 
high foot pressures. High pressures, together with dry skin, often 
result in the formation of callus under weightbearing areas of the 
metatarsal heads. Presence of plantar callus is a highly significant 
marker for foot ulcer risk. Conversely, removal of pla

er for foot ulcer risk. Conversely, removal of plantar callus is 
associated with reduced foot pressures and thereby a reduced foot 
ulcer risk.877 
Peripheral Vascular Disease, Diabetic Foot Ulcers, 
and Amputation Risk
Peripheral vascular disease in isolation rarely causes ulceration. 
However, the common combination of vascular disease with minor 
trauma can lead to ulceration. Minor injury and subsequent infec­
tion increase the demand for blood supply beyond the circulatory 
capacity, and i

ood supply beyond the circulatory 
capacity, and ischemic ulceration and risk of amputation develop. 
Early identification of patients at risk for peripheral vascular dis­
ease is essential, and appropriate investigation involving noninvasive 
Doppler studies, together with arteriography, often leads to revascu­
larization procedures to improve lower extremity blood flow. Pres­
ence or absence of a dorsalis pedis or posterior tibial pulse may be 
the simplest and most reliable indicator of signi

the simplest and most reliable indicator of significant ischemia that 
can be elicited at the bedside.878 However, Doppler-derived ankle 
pressure can be misleadingly high in patients with long-standing 
diabetes, so the use of the toe-brachial index together with Doppler 
waveforms of distal arteries can be useful in deciding which patients 
need further diagnostic intervention.879
Distal bypass surgery or endovascular interventions have 
good short-term but mixed long-term results in terms of

short-term but mixed long-term results in terms of limb 
salvage.878,880 Advances in techniques and devices for both dis­
tal arterial bypass surgery and endovascular interventions have 
been made, and it is unclear which approach is preferred. A 
recent systematic review was unable to conclude any particu­
lar method was superior to any other.881 However, a recent 
study in all patients with diabetic foot ulcers demonstrates that 
prompt vascular evaluation and revascularization within 2 weeks 

r evaluation and revascularization within 2 weeks 
reduces the rate of amputations to the rate of nondiabetics with 
peripheral artery disease, supporting a change in clinical disease 
management.882
A
B
• Fig. 37.50  The high-risk neuropathic foot. (A, B) Two lateral views of a patient with typical signs of a 
high-risk neuropathic foot. Notice the small-muscle wasting, clawing of the toes, and marked prominence 
of the metatarsal heads. At presentation with type 2 diabetes mellitus, this patie

entation with type 2 diabetes mellitus, this patient had severe neuropathy 
with foot ulceration on both the right foot (shown here) and the left foot. (From Andersson DK, Svärdsudd 
K. Long-term glycemic control relates to mortality in type II diabetes. Diabetes Care. 1995;18:1534–1543.)
1521
Prevention of Foot Ulceration and Amputation
Relatively simple interventions can reduce amputations by up to 
80%.873,874 Therefore strategies for early ident

80%.873,874 Therefore strategies for early identification of patients 
at potential risk for ulceration are required, and education pro­
grams that can be adapted for widespread application need to 
be developed. Because foot ulcers precede most amputations, 
are among the most common causes of hospital admission for 
patients with diabetes, and account for much morbidity and even 
fatality, the widespread application of preventive foot care strate­
gies is urgently required.
Patients with any

rate­
Patients with any type of diabetes require regular foot exami­
nation for evidence of risk factors for ulceration, irrespective of 
disease duration. Neuropathy, vascular disease, and even foot 
ulceration may be the presenting feature of T2DM, so there can be 
no exception to the rule of screening (Fig. 37.51). At a minimum, 
such screening should be carried out annually. Of all the long-term 
complications of diabetes, foot problems and their risk factors are 

abetes, foot problems and their risk factors are 
probably the easiest to detect. No expensive equipment is required, 
and feet can be examined for evidence of neuropathic and vascular 
deficits in the office setting using simple equipment.871
In 2008, a task force of the ADA published a report on the 
examination components that should be included in the annual 
Comprehensive Diabetic Foot Examination (CDFE) (Table 
37.21).884 The most important message is for practitioners to have 
patients re

 message is for practitioners to have 
patients remove shoes and socks to examine feet for the presence 
of callus, deformity, muscle wasting, and dry skin, all of which 
are clearly visible on inspection. A simple neurologic examination 
includes assessment of pressure perception using a 10-g mono­
filament. Sensory neuropathy with clinical evidence of peripheral 
arterial disease are the strongest predictors of future foot ulcer­
ation. Absence of the ability to perceive pressure from a 10-g 

 of the ability to perceive pressure from a 10-g 
monofilament, inability to perceive a vibrating 128-Hz tuning 
fork over the hallux, and absent ankle reflexes all have been shown 
to be predictors of foot ulceration.868,869 
The Diabetic Foot Care Team
Patients identified as being at high risk for foot ulceration should be 
managed by a team of specialists with expertise in the diabetic foot. 
The podiatrist usually takes responsibility for care of the skin and 
nails and, together with the sp

e of the skin and 
nails and, together with the specialist nurse or diabetes educator, 
provides foot care education. The orthotist, or shoe fitter, is invalu­
able to advise about and sometimes design footwear to protect high-
risk feet. Team members should work closely with the diabetologist 
and the vascular and orthopedic surgeons. Patients with risk factors 
for ulceration require preventive foot care education, including self-
foot care at home, frequent review by a foot surgeon or podiatr

home, frequent review by a foot surgeon or podiatrist, 
good foot wear with orthotics when indicated, and avoidance of 
potentially dangerous over-the-counter foot products.874,877 
Classification of Foot Ulcers
Many different classification systems for grading diabetic foot 
ulcers have been reported.868,877 One developed by Wagner885 
(Table 37.22) is widely used and accepted. More recently, the 
University of Texas (UT) group developed an alternative classifica­
tion system that, in addition 

rnative classifica­
tion system that, in addition to ulcer depth (as in the Wagner sys­
tem), accounts for presence or absence of infection and ischemia 
(Table 37.23). A prospective study from 2001 compared these two 
wound classification systems and concluded that the UT scheme 
is a better predictor of outcome than the older Wagner system.885 
Management of Diabetic Foot Ulcers
Basic principles of wound healing apply equally to diabetic foot 
ulcers as to wounds in any other site or condition

ulcers as to wounds in any other site or condition. In general, a 
Risk factors for ulceration?
• Peripheral neuropathy
• Peripheral vascular disease
• Foot deformity
• Edema
• Past ulcer history
• Other complications
Diabetic patient
Provide general advice
• Nail care, hygiene
• Footwear
• Podiatry
No risk factor
Review risk status
at least annually
• Foot care education
• Regular podiatry
• Possible special 
   footwear, hosiery, etc.
Yes risk factor
Much more frequent review,
always inspectin

factor
always inspecting feet
• Fig. 37.51  Simple algorithm for risk screening in the diabetic foot.
  Key Components of the Comprehensive 
Diabetic Foot Examination
Dermatologic
Skin status: color, thickness, dryness, cracking
Sweating
Infection: check between toes for fungal infection
Ulceration
Calluses/blistering: hemorrhage into callus?
Deformity (e.g., claw toes, prominent metatarsal heads, Charcot joint)
Muscle wasting (guttering between metata

ot joint)
Muscle wasting (guttering between metatarsals)
Assess whether shoes are appropriate for the feet (e.g., size, width)
Neurologic
Ability to perceive pressure from a 10-g monofilament plus one of the 
following:
Vibration using 128-Hz tuning fork
Pinprick sensation
Ankle reflexes
Vibratory perception threshold
Vascular
Foot pulses
Ankle-brachial index, if indicated
Adapted from Boulton AJ, Armstrong DG, Albert SF, et al. Comprehensive foot examination and 
risk assessment: a report of th

t examination and 
risk assessment: a report of the Task Force of the Foot Care Interest Group of the American 
Diabetes Association, with endorsement by the American Association of Clinical Endocrinolo­
gists. Diabetes Care. 2008;31:1679–1685.
   
TABLE 37.21
1522
diabetic foot ulcer has high likelihood of healing if the following 
three conditions are satisfied:
 • Arterial inflow is adequate.
 • Infection is treated appropriately.

 adequate.
 • Pressure is removed from the wound and the immediate sur­
rounding area.
Although this approach appears simplistic, failure of diabetic 
foot ulcers to heal is usually a result of failure to pay sufficient 
attention to one or more contributing conditions, including 
pressure on the wound, infection, ischemia, and inadequate 
debridement.
The most common cause of nonhealing neuropathic foot ulcers 
is failure to remove pressure from the woun

lcers 
is failure to remove pressure from the wound and immediate sur­
rounding area. Patients who are advised not to put pressure over 
an ulcer find it especially difficult to adhere to such advice when 
peripheral sensation is lost or reduced. Pain results in protection of 
an injured area; the lack of pain permits pressure to be put directly 
onto the ulcer and results in nonhealing. A patient with normal 
sensation and a foot wound will limp to avoid putting pressure on 
the wound because d

 to avoid putting pressure on 
the wound because doing so is painful. A patient who walks on a 
plantar wound without limping must have neuropathy.
The effect of pressure relief on the histopathologic features of 
neuropathic ulcers was assessed in a randomized study886 with 
biopsy either at presentation or after 20 days of off-loading in a 
total-contact cast (TCC). Histologic features of chronic inflam­
mation, with mononuclear infiltration, cellular debris, and scarce 
evidence of angiogenes

ellular debris, and scarce 
evidence of angiogenesis or granulation, were seen in patients who 
underwent biopsy at presentation; whereas granulation, neoangio­
genesis, and a predominance of fibroblasts were seen in patients 
treated with TCC before biopsy. These important observations 
support that repetitive pressure on a neuropathic wound contrib­
utes to the chronicity of the wound, whereas pressure relief results 
in a reparative phase.
The next most common error is inappropriate managemen

 next most common error is inappropriate management 
of infection. Topical applications are usually unhelpful, and if 
clinical infection is present, then it must be treated urgently with 
antibiotics. Most infections are polymicrobial, with gram-positive 
cocci, gram-negative rods, and anaerobes, with or without multi­
drug-resistant organisms (see later discussion).
Another common management error is failure to appreciate 
ischemic symptoms that are atypical due to altered pain sensation 
as a

t are atypical due to altered pain sensation 
as a result of neuropathy. The most difficult ulcer to heal is the 
neuroischemic ulcer. Symptoms and even signs of ischemia may 
be altered in the diabetic state. Therefore appropriate noninvasive 
investigation and arteriography are indicated for patients with a 
nonhealing diabetic foot ulcer if there is any question about the 
vascular status.
Inappropriate wound debridement contributes to slow healing 
or nonhealing of a diabetic foot ulcer. App

aling 
or nonhealing of a diabetic foot ulcer. Appropriate debridement 
and removal of all callus, dead, and macerated tissue is essential 
local treatment of a diabetic foot ulcer, which results in more rapid 
healing compared with inadequately debrided wounds.
The principles of management of neuropathic and neuroisch­
emic foot ulcers are considered in the following sections referenc­
ing both the UT and the Wagner grading systems.
Neuropathic Foot Ulcer Without Osteomyelitis (Wagner 
Grades 1

Foot Ulcer Without Osteomyelitis (Wagner 
Grades 1, 2; University of Texas Grades 1a, 1b, 2a, 2b)
Provision of adequate pressure relief is the most important feature 
in the management of neuropathic foot ulcers that occur under 
weightbearing areas such as the metatarsal heads and great toe. 
This is usually achieved by a TCC or a removable Scotch cast 
boot.868,877
The TCC has long been recognized as the gold standard for 
off-loading a foot wound. The TCC was associated with the short­
est he

und. The TCC was associated with the short­
est healing time in a randomized, controlled trial conducted by 
Anderson and colleagues comparing three off-loading techniques 
demonstrated.887 When any cast device is used, regular removal of 
the cast is essential, because regular debridement of the wound by 
a specialist is essential, and any casting device could injure insensi­
tive skin, especially over bony prominences. As the TCC requires 
a specially trained casting technician to apply it, an

ecially trained casting technician to apply it, and frequent 
removal is required for wound assessment, recent research has 
focused on alternative reusable devices that can only be removed 
in the specialist’s office.
The removable cast walker (RCW) resulted in slower heal­
ing than the TCC, in the aforementioned trial by Armstrong 
  Wagner Diabetic Foot Ulcer Classification 
System
Grade
0
No ulcer, but high-risk foot (e.g., deformity, callus, insensitivity)
1
Superficial full-th

mity, callus, insensitivity)
1
Superficial full-thickness ulcer
2
Deeper ulcer, penetrating tendons, no bone involvement
3
Deeper ulcer with bone involvement, osteitis
4
Partial gangrene (e.g., toes, forefoot)
5
Gangrene of whole foot
Modified from Oyibo S, Jude EB, Tarawneh I, et al. A comparison of two diabetic foot ulcer 
classification systems: the Wagner and the University of Texas wound classification systems. 
Diabetes Care. 2001;24:84–88.
   
TABLE 37.22
  University of Texas Wound Clas

  University of Texas Wound Classification System
Stage
Grade 0
Grade 1
Grade 2
Grade 3
A
Preulcer or postulcer lesion; no skin break
Superficial ulcer
Deep ulcer to tendon or capsule
Wound penetrating bone or joint
B
+ Infection
C
+ Ischemia
+ Ischemia
+ Ischemia
+ Ischemia
D
+ Infection and ischemia
Modified from Armstrong DG, Lavery LA, Harkless LB. Validation of a 

trong DG, Lavery LA, Harkless LB. Validation of a diabetic wound classification system. The contribution of depth, infection, and ischemia to risk of amputation Diabetes Care. 
1998;12:855–859.
   
TABLE 37.23
1523
et al,887 even though prior gait laboratory studies suggested they 
are equally efficacious at off-loading. The reason for this dispar­
ity may be explained by the observation that although patients 
were instructed to wear the RCW at all

h patients 
were instructed to wear the RCW at all times, patients used these 
devices for only 28% of all footsteps during a 24-hour period. To 
overcome this limitation the RCW, which can be applied by any 
clinic personnel and does not require specialist training, can be 
rendered irremovable by wrapping it in casting material. A con­
trolled trial showed the irremovable RCW was as effective at heal­
ing neuropathic foot wounds as the TCC.888
Theoretically, complete healing of all superficial

Theoretically, complete healing of all superficial and neuro­
pathic ulcers should be possible without the need for amputation. 
In the treatment of neuropathic ulcers with a good peripheral 
circulation, antibiotics are not indicated unless there are clear 
clinical signs of infection, including prominent discharge, local 
erythema, and cellulitis. The presence of any infective features in 
Wagner grade 1 or 2 ulcers would warrant reclassification in the 
UT system from 1a or 2a to 1b or 2b. In

on in the 
UT system from 1a or 2a to 1b or 2b. In such cases, deep wound 
specimens should be taken and broad-spectrum oral antibiotic 
treatment should be started with, for example, either an amoxi­
cillin–clavulanic acid combination (Augmentin) or clindamy­
cin. The antibiotic may need to be altered after sensitivity results 
become available.889 
Neuroischemic Ulcers (Wagner Grades 1, 2; University of 
Texas Grades 1c, 1d)
The principles of management of neuroischemic Wagner grade 1 
and 2 u

anagement of neuroischemic Wagner grade 1 
and 2 ulcers are similar to those for neuropathic ulcers, with the fol­
lowing important differences. TCCs are not usually recommended 
for management of neuroischemic ulcers, although removable casts 
and pneumatic cast boots (Aircast) may be used in cases without 
infection. Antibiotic therapy is required for most neuroischemic 
ulcers. Investigation of the circulation is indicated, including non­
invasive assessment and, if required, arteriography wi

sive assessment and, if required, arteriography with appropriate 
subsequent surgical management or angioplasty.878 
Osteomyelitis (Wagner Grade 3; University of Texas 
Grades 3b, 3d)
Wagner or UT grade 3 ulcers are deeper and involve underlying 
bone, often with abscess formation. Osteomyelitis is a serious 
complication of foot ulceration and may be present in as many 
as 50% of diabetic patients with moderate to severe foot infec­
tions.868 If the physician can probe down to bone in a deep ul

 the physician can probe down to bone in a deep ulcer, 
the presence of osteomyelitis is strongly suggested. Plain radio­
graphs are indicated for any nonhealing foot ulcer and are use­
ful in the diagnosis of osteomyelitis in more than two-thirds of 
patients, although radiologic changes may be delayed. In difficult 
cases, further investigation, such as magnetic resonance imaging 
(MRI), bone scans, or an indium-111 (111In)–labeled white blood 
cell scan can be useful in diagnosing bone infect

cell scan can be useful in diagnosing bone infection.890
Although osteomyelitis treatment is traditionally surgical and 
involves resection of the infected bone, successful long-term treat­
ment can occasionally be effective with antibiotics. Staphylococcus 
aureus is the most common infective bacteria. Therefore agents 
such as clindamycin (which penetrates bone well) or flucloxacillin 
are often used. Antibiotic therapy for 90 days was equally effica­
cious when compared to local surgery for 

 effica­
cious when compared to local surgery for diabetic foot osteomy­
elitis in a recent randomized trial.891 
Gangrene (Wagner Grades 4, 5)
The presence of gangrene is always a serious sign in the diabetic 
foot. However, localized areas of gangrene, especially in the toes, 
that are without cellulitis, spreading infection, or discharge can 
occasionally be left to spontaneously auto-amputate. The presence 
of more extensive gangrene requires urgent hospital admission; 
treatment of infectio

 urgent hospital admission; 
treatment of infection, often with multiple antibiotics; glycemic 
control, usually with intravenous insulin; and detailed vascular 
assessment. It is in this situation that the team approach is most 
important, with close collaboration among the diabetes specialist, 
the vascular surgeon, and the radiologist. 
Charcot Neuroarthropathy
Charcot neuroarthropathy is a disabling condition affecting the 
joints and bones of the foot in the general diabetic population. It 

f the foot in the general diabetic population. It 
is much more frequent (10–13%) in high-risk diabetic patients. 
Permissive features for development of this condition include the 
presence of severe peripheral neuropathy and autonomic dysfunc­
tion with increased blood flow to the foot. The peripheral circula­
tion is usually intact. In the Western world, diabetes is the most 
common cause of a Charcot foot, and increased awareness of this 
condition can enable earlier diagnosis and treatment 

dition can enable earlier diagnosis and treatment to prevent 
severe deformity and disability.
The pathogenesis of the Charcot process remains poorly 
understood; however, the patient with peripheral insensitivity and 
autonomic dysfunction, with increased blood flow reflecting the 
autonomic dysfunction, to the foot is vulnerable to trauma, which 
the patient may not recall. Repetitive trauma results in increased 
blood flow through the bone, increased osteoclastic activity, and 
bone remodelin

creased osteoclastic activity, and 
bone remodeling. In some cases, patients walk on a fracture, 
which leads to continuing bone and joint destruction in that area. 
Recent evidence suggests acute Charcot neuropathy may be trig­
gered in the susceptible (i.e., neuropathic) individual by any event 
that leads to localized inflammation in the affected foot, triggering 
a vicious cycle in which there is increasing inflammation, increas­
ing expression of receptor activator of nuclear factor-κΒ liga

on of receptor activator of nuclear factor-κΒ ligand 
(RANKL), a member of the tumor necrosis factor superfamily, 
and increasing bone breakdown.892 Targeting activation of the 
RANKL/osteoprotegerin pathway might lead to novel, future 
treatments.
Charcot neuropathy can be difficult to distinguish from 
osteomyelitis or an inflammatory arthropathy.890 A unilateral 
swollen, hot foot without an ulcer in a patient with neuropathy 
must be considered to be a Charcot foot until proved otherwise. 
C

ed to be a Charcot foot until proved otherwise. 
Charcot arthropathy can be diagnosed in most patients by plain 
radiography and a high index of suspicion (Fig. 37.52). Radio­
graphs reveal bone and joint destruction, fragmentation, and 
remodeling, although radiographic findings may be normal early 
in disease. In such cases, three-phase bisphosphonate bone scans 
show increased bone uptake, while 111In-labeled bone scans will 
be negative in the absence of infection. In patients where osteo­
m

e absence of infection. In patients where osteo­
myelitis is suspected, 18F-fluorodeoxyglucose–positron emission 
tomography–computed tomography (FDG-PET-CT) has the 
highest sensitivity for distinguishing Charcot arthropathy from 
osteomyelitis.
After diagnosis, management of the acute phase involves 
immobilization, usually in a TCC.893 There is little evidence 
to support use of any pharmacologic treatment in the manage­
ment of this condition. Although Charcot neuroarthropathy is 
rare, it s

. Although Charcot neuroarthropathy is 
rare, it should be suspected in any patient with unexplained 
swelling and heat in a neuropathic foot. Early intervention with 
immobilization may halt progression that, in the untreated 
state, can lead to marked foot deformity and require local or 
major amputations. 
1524
Adjunct Treatments for Foot Ulcers
Tissue-Engineered Skin and Platelet-Derived Growth 
Factors
Genetically derived growth f

rived Growth 
Factors
Genetically derived growth factors and novel bioengineered skin 
substitutes have been developed as adjunctive treatments for dia­
betic foot ulcers, including Apligraf, a bilayered living human 
skin equivalent; Dermagraft, a human fibroblast-derived dermal 
substitute; and human platelet-derived growth factors. Although 
there are some benefits for each of these agents, they are costly894 
and should be reserved for ulcers that fail to respond to standard 
treatments. Any

 that fail to respond to standard 
treatments. Any newer treatments should be considered an addi­
tion to good wound care, which must always include adequate 
off-loading and regular debridement. There is little objective evi­
dence to date in support of routine use of these therapies, as well 
as others under study, including stem cell therapy and hyperbaric 
oxygen.895 
Negative-Pressure Wound Therapy (NPWT)
NPWT, also known as vacuum-assisted closure, is increas­
ingly used to treat large and

closure, is increas­
ingly used to treat large and complex diabetic foot wounds. The 
treatment may stimulate development of granulation tissue in 
previously nonhealing wounds and can also be helpful in the post­
operative management of diabetic foot wounds, as supported by 
randomized, controlled trials in complex nonhealing diabetic foot 
ulcers896 and postoperative cases.895 Use of NPWT remains con­
troversial, as discussed in a systematic review. The strongest evi­
dence for efficacy is wit

view. The strongest evi­
dence for efficacy is with postoperative diabetic foot wounds.895 
Achilles Tendon Lengthening Procedures
Although some centers advocate and perform tendo-Achilles 
lengthening as an adjunct treatment to prevent recurrent diabetic 
foot ulcers posthealing, there are no adequate data supporting use 
of this procedure in the diabetic foot. Given the high risk of any 
foot surgery in the diabetic foot, the nonsurgical approach of pre­
scribing footwear designed to reduce pr

ch of pre­
scribing footwear designed to reduce pressure on the metatarsal 
heads is preferable unless and until adequately powered prospec­
tive randomized clinical trials of Achilles tendon lengthening pro­
cedures demonstrate superior outcomes. 
SGLT2 Inhibitors and Diabetic Foot Disease
An increased incidence of distal lower extremity amputations, 
mainly at the toe and metatarsal levels, was observed in the CAN­
VAS trial473 designed to evaluate the cardiovascular safety of cana­
gliflozin.

uate the cardiovascular safety of cana­
gliflozin. Numerous potential mechanisms have been suggested, 
but the cause of this finding remains enigmatic. However, a recent 
pharmacovigilance analysis confirmed that use of canagliflozin, 
but not dapagliflozin or empagliflozin, might be associated with 
an increased risk of amputations. The use of canagliflozin should 
probably be avoided in patients with distal neuropathy or periph­
eral arterial disease or with a history of foot disease. Although

isease or with a history of foot disease. Although this 
does not appear to be a class effect,897 use of other SGLT2 inhibi­
tors should be used with caution in the abovementioned groups.
No signs
of trauma
Gapping
Shortened
first ray (first
metatarsal bone)
Bony
consolidation,
osteosclerosis,
fusion of joints
Pain, swelling,
redness in
the midfoot
Bony
fragmentation
Normal
Charcot neuroarthopathy
• Fig. 37.52  

tion
Normal
• Fig. 37.52  Some of the key features of Charcot neuroarthropathy, an often overlooked complication of 
diabetes. (Redrawn from Botek G, Anderson MA, Taylor R. Charcot neuroarthropathy: an often overlooked 
complication of diabetes. Cleve Clin J Med. 2010;77:593–599.)
1524.e1
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hypertensive Agents. Arch Intern Med. 1999;159(6):551–558.
	836.	 Shorr RI, Ray WA, Daugherty JR, Griffin MR. Antihypertensives 
and the risk of serious hypoglycemia in older persons using insulin 
or sulfonylureas. J Am Med Assoc. 1997;278(1):40–43.
	837.	 Chen J, Marciniak TA, Radford MJ, Wang Y, Krumholz HM. Beta-
blocker therapy for se

 Wang Y, Krumholz HM. Beta-
blocker therapy for secondary prevention of myocardial infarction 
in elderly diabetic patients. Results from the National Cooperative 
Cardiovascular Project. J Am Coll Cardiol. 1999;34(5):1388–1394.
	838.	 Giugliano D, Acampora R, Marfella R, et al. Metabolic and cardio­
vascular effects of carvedilol and atenolol in non-insulin-dependent 
diabetes mellitus and hypertension. A randomized, controlled trial. 
Ann Intern Med. 1997;126(12):955–959.
	839.	 Aspirin effe

gl J Med. 1998;338(21):1488–1497.
	848.	 EPILOG Investigators. Platelet glycoprotein IIb/IIIa receptor 
blockade and low-dose heparin during percutaneous coronary 
revascularization. N Engl J Med. 1997;336(24):1689–1696.
	849.	 Lincoff AM, Califf RM, Moliterno DJ, et al. Complementary clin­
ical benefits of coronary-artery stenting and blockade of platelet 
glycoprotein IIb/IIIa receptors. Evaluation of Platelet IIb/IIIa Inhi­
bition in Stenting Investigators. N Engl J Med. 1999;341(5):319–
32

Aronson D, Rayfield EJ, Chesebro JH. Mechanisms determining 
course and outcome of diabetic patients who have had acute myo­
cardial infarction. Ann Intern Med. 1997;126(4):296–306.
	856.	 Cabin HS, Roberts WC. Quantitative comparison of extent of 
coronary narrowing and size of healed myocardial infarct in 33 nec­
ropsy patients with clinically recognized and in 28 with clinically 
unrecognized (“silent”) previous acute myocardial infarction. Am J 
Cardiol. 1982;50(4):677–681.
	857.	 Vinik AI

imens 
obtained from patients with diabetes mellitus with and without 
hypertension. Heart Vessel. 1997;12(6):267–274.
	860.	 Paulus WJ, Tschope C. A novel paradigm for heart failure with pre­
served ejection fraction: comorbidities drive myocardial dysfunc­
tion and remodeling through coronary microvascular endothelial 
inflammation. J Am Coll Cardiol. 2013;62(4):263–271.
	861.	 Nahser PJ Jr, Brown RE, Oskarsson H, Winniford MD, Ros­
sen JD. Maximal coronary flow reserve and metabolic coro­
n

 and 
non-invasive screening tests in the diagnosis of peripheral artery 
disease in those with diabetes related foot ulceration. Diabet Med. 
2018;35(7):902.
	880.	 Albayati MA, Shearman CP. Peripheral arterial disease and 
bypass surgery in the diabetic lower limb. Med Clin North Am. 
2013;97(5):821–834.
	881.	 Hinchliffe RJ, Brownrigg JR, Andros G, et  al. Effectiveness of 
revascularization of the ulcerated foot in patients with diabetes and 
peripheral artery disease: a systematic review.

ion and risk assessment: a report of the task force of the 
foot care interest group of the American Diabetes Association, with 
endorsement by the American Association of Clinical Endocrinolo­
gists. Diabetes Care. 2008;31(8):1679–1685.
	885.	 Oyibo SO, Jude EB, Tarawneh I, Nguyen HC, Harkless LB, Boul­
ton AJ. A comparison of two diabetic foot ulcer classification sys­
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1525
38
PHILIP E. CRYER AND ANA MARIA ARBELÁEZ
I
atrogenic hypoglycemia is the limiting factor in glycemic 
management of diabetes mellitus with insulin provisional 
therapies: insulin, a sulfonylurea, or a glinide.1 Hypoglyce­
mia is a distinctly uncommon clinical disorder in persons without 
diabetes.2
Glucose is an obligate metabolic fuel for the brain under physi­
ologic conditions.3 Because the brain cannot sy

i­
ologic conditions.3 Because the brain cannot synthesize glucose, 
store more than a few minutes supply as glycogen, or utilize physi­
ologic concentrations of other circulating fuels effectively, survival 
of the brain, and therefore the individual, requires a virtually con­
tinuous supply of glucose from the circulation. Blood-to-brain 
glucose transport is a direct function of the arterial plasma glucose 
concentration and requires the plasma glucose concentration be 
maintained within, or 

a glucose concentration be 
maintained within, or above, the physiologic range. Hypoglyce­
mia causes functional brain failure, which is typically corrected 
after the plasma glucose concentration is raised (Table 38.1). 
Rarely, it causes a fatal cardiac arrhythmia or, if it is profound and 
prolonged, brain damage and death.
Physiology of Defense Against Hypoglycemia, 1526
Clinical Hypoglycemia, 1530
Hypoglycemia in Persons With Diabetes, 1531
Hypoglycemia in Persons Without Di

 Diabetes, 1531
Hypoglycemia in Persons Without Diabetes, 1540
Hypoglycemia in Infancy and Childhood, 1545
KEY POINTS
 • 
Hypoglycemia—a plasma glucose concentration low enough 
to cause symptoms or signs—is a rare occurrence in individuals 
without diabetes but is common in sulfonylurea-, glinide-, or 
insulin-treated diabetes.
 • 
The plasma glucose concentration is normally maintained in a 
relatively narrow range, 72 to 144 mg/dL (4.0–8.0 mmol/L), owing 
to a fine balance between glucose i

mol/L), owing 
to a fine balance between glucose influx (exogenous glucose 
delivery and endogenous glucose production) and glucose efflux 
(glucose utilization by insulin-sensitive tissues, such as the skel­
etal muscle, and insulin-insensitive tissues, particularly the brain).
 • 
Hypoglycemia results from an imbalance between glucose in­
flux and glucose efflux due to either excessive glucose removal 
from the circulation, deficient glucose delivery into the circula­
tion, or both.
 • 
Hypo

elivery into the circula­
 • 
Hypoglycemia in diabetes is typically the result of the interplay 
of therapeutic hyperinsulinemia and compromised defenses 
against falling glucose levels resulting in hypoglycemia-asso­
ciated autonomic failure (HAAF), including defective glucose 
counterregulation and impaired awareness of hypoglycemia.
 • 
An attenuated sympathoadrenal response to falling glucose 
levels, the key feature of HAAF, is induced by recent antecedent 
hypoglycemia, sl

 is induced by recent antecedent 
hypoglycemia, sleep, or prior exercise, and reversible by short-
term scrupulous avoidance of hypoglycemia.
 • 
Iatrogenic hypoglycemia is associated with both morbidity and 
fatality in type 1 and type 2 diabetes mellitus.
 • 
The practice of hypoglycemic risk reduction in persons with 
diabetes at risk of hypoglycemia includes addressing the issue, 
applying the principles of aggressive glycemic therapy, and 
considering the conventional risk factors and tho

considering the conventional risk factors and those indicative 
of HAAF.
 • 
In the absence of diabetes, hypoglycemia in ill or medicated 
individuals can be caused by many drugs, critical illnesses, en­
docrine deficiencies, or non–islet cell tumors; in seemingly well 
individuals, it may be caused by endogenous hyperinsulinism or 
by various accidental, surreptitious, or malicious mechanisms. 
In children, it may also be the result of abnormalities in enzyme 
deficiencies that are key for th

lities in enzyme 
deficiencies that are key for the metabolism of fuel substrates 
that will contribute to abnormalities in glucose production and 
utilization.
 • 
The decision to evaluate a given nondiabetic person sys­
tematically—seeking evidence of a metabolic disorder or an 
endogenous insulin excess during hypoglycemia—is recom­
mended only for persons in whom Whipple triad (symptoms, 
signs, or both consistent with hypoglycemia; a low reliably 
measured plasma glucose; and resolution of

liably 
measured plasma glucose; and resolution of those symptoms 
or signs after the glucose concentrations is raised) can be 
documented.
 • 
Short-term treatment of hypoglycemia includes oral carbohy­
drates, or parenteral glucagon or glucose; long-term treatment 
requires correction of the hypoglycemic mechanism.
1526
Given the survival requirement to maintain the plasma glucose 
concentration, it is not surprising that physiologi

oncentration, it is not surprising that physiologic and behavioral 
mechanisms that prevent or rapidly correct hypoglycemia have 
evolved.3 These mechanisms are so effective that hypoglycemia is 
an uncommon clinical event, except in persons who use drugs that 
lower the plasma glucose concentration, such as insulin, sulfonyl­
ureas, or alcohol.
Glucoregulatory failure resulting in hyperglycemia (diabetes 
mellitus) is discussed in Chapters 33 through 37; that resulting in 
hypoglycemia is discu

rough 37; that resulting in 
hypoglycemia is discussed in the paragraphs that follow.
Physiology of Defense Against 
Glucose Metabolism
Glucose is derived from three sources: intestinal absorption fol­
lowing digestion of dietary carbohydrates; glycogenolysis, which is 
the breakdown of glycogen, the polymerized storage form of glu­
cose; and gluconeogenesis, which is the formation of glucose from 
precursors including lactate (and pyruvate), amino acids (espe­
cially alanine and gl

yruvate), amino acids (espe­
cially alanine and glutamine), and, to a lesser extent, glycerol. 
There are multiple fates of glucose transported into cells (exter­
nal losses are normally negligible) (Fig. 38.1). Glucose may be 
stored as glycogen, or may undergo glycolysis to pyruvate, which 
can be reduced to lactate, transaminated to form alanine, or con­
verted to acetyl coenzyme A (CoA). Acetyl CoA, in turn, may 
be oxidized to carbon dioxide and water through the tricarbox­
ylic acid cycle,

 and water through the tricarbox­
ylic acid cycle, converted to fatty acids that can be incorporated 
into triglycerides, oxidized, or utilized for synthesis of ketone 
bodies (acetoacetate, β-hydroxybutyrate) or cholesterol. Finally, 
glucose may be released into the circulation. Only liver and kid­
neys express glucose-6-phosphatase, the enzyme necessary for 
release of glucose into the circulation, at levels sufficient to per­
mit substantial contributions to the systemic glucose pool. Many 

contributions to the systemic glucose pool. Many 
tissues express the enzymes required to synthesize and hydrolyze 
glycogen (glycogen synthase and phosphorylase, respectively). 
The liver and kidneys also express the enzymes necessary for 
gluconeogenesis, including the critical gluconeogenic enzymes 
pyruvate carboxylase, phosphoenolpyruvate carboxykinase, and 
fructose-1,6-bisphosphatase.
The liver is the major source of net endogenous glucose produc­
tion (through glycogenolysis and gluconeo

 produc­
tion (through glycogenolysis and gluconeogenesis). Conversely, 
the liver can be an organ of net glucose uptake and glycogen syn­
thesis. The kidneys also produce glucose (through gluconeogen­
esis) and utilize glucose.
Muscle can take up and store glucose as glycogen, or metabolize 
glucose (through glycolysis) to pyruvate, which, among other fates, 
can be reduced to lactate or transaminated to form alanine. Lactate 
(and pyruvate) released from muscle can be transported to the liver 

eased from muscle can be transported to the liver 
and the kidneys, to serve as a gluconeogenic precursor (the Cori or 
glucose-lactate cycle). Alanine, glutamine, and other amino acids 
can also flow from muscle to liver and kidneys, where they also 
serve as gluconeogenic precursors. These constitute the glucose-ala­
nine and glucose-glutamine cycles, in which new glucose formation 
is from precursors (e.g., amino acids) where the carbons are not 
derived from glucose. Although quantitatively 

ot 
derived from glucose. Although quantitatively less important than 
muscle, fat can also take up and metabolize glucose.
Glucose is essentially the sole metabolic fuel for the brain 
under physiologic conditions.3 Glucose largely undergoes termi­
nal oxidation in the brain. Although the adult human brain con­
stitutes only about 2.5% of body weight, its oxidative metabolism 
accounts for approximately 25% of the basal metabolic rate and 
more than 50% of whole-body glucose utilization. The br

than 50% of whole-body glucose utilization. The brain can 
utilize alternative fuels if their circulating levels rise high enough 
for them to enter the brain in quantity. For example, during 
extended fasting, markedly elevated circulating ketone levels can 
support the majority of the energy needs of the brain and reduce 
its utilization of glucose. Notably, ketogenesis is suppressed during 
episodes of insulin-mediated hypoglycemia such that again, the 
  Whipple Triad
Whipple triad is: 
sym

gain, the 
symptoms, signs, or both consistent with hypoglycemia, a low reliably 
measured plasma glucose concentration, and resolution of those 
symptoms or signs after the plasma glucose concentration is raised.a
Clinically, neuroglycopenic symptoms are more compelling than neuro­
genic symptoms.
The definition of “low” is judgmental, but a plasma glucose concentra­
tion of less than 55 mg/dL (3.0 mmol/L) is unequivocally low.
Documentation of Whipple triad 

unequivocally low.
Documentation of Whipple triad is particularly important for the diagno­
sis of hypoglycemia in patients who do not have insulin-, sulfonyl­
urea-, or glinide-treated diabetes because hypoglycemic disorders 
are uncommon in such individuals.
aModified from Whipple AO. The surgical therapy of hyperinsulinism. J Int Chir 1938; 3:237–
276.
   
TABLE 38.1
1
Glucose
e
s
o
c
u
l
G
Lactate
Alanine
Glucose-6-P 
Fructose-6-P 
Fructose-1,6-P 
Triose-P 
Pyruvate
Oxalo

,6-P 
Triose-P 
Pyruvate
Oxaloacetate
TCA 
cycle
Glutamine
Glucose-1-P 
Glycogen
Glycerol
Citrate
Acetyl CoA 
Fatty acids 
Ketones
2
5
8
9
3
7
4
6
5
4
3
2
1 Hexokinase/glucokinase
Glycogen synthase 
Phosphofructokinase
Pyruvate kinase 
Pyruvate carboxylase 
9
8
7
6 Phosphoenolpyruvate carboxykinase
Fructose-1,6-bisphosphatase
Phosphorylase
• Fig. 38.1  Schematic representation of glucose metabolism. CoA, coen­
zyme A; P, pho

n of glucose metabolism. CoA, coen­
zyme A; P, phosphate; TCA, tricarboxylic acid.
1527
Chapter 38  Hypoglycemia
brain is critically dependent on a virtually continuous supply of 
glucose from the circulation.3 
Systemic Glucose Balance
Normally, rates of endogenous glucose influx into the circulation 
and those of glucose efflux out of the circulation into tissues other 
than the brain are coordinately regulated—largely by the plasma 
glucose–lowering (regulatory) hormone insulin and the plasma

wering (regulatory) hormone insulin and the plasma 
glucose–raising (counterregulatory) hormones glucagon and 
epinephrine—such that systemic glucose balance is maintained, 
hypoglycemia (as well as hyperglycemia) is prevented, and a con­
tinuous supply of glucose to the brain is ensured (Table 38.2). 
This is accomplished despite wide variations in exogenous glucose 
influx (e.g., after meals vs. during fasting) and in glucose efflux 
(e.g., during exercise vs. rest). Hypoglycemia occurs when r

ing exercise vs. rest). Hypoglycemia occurs when rates 
of glucose appearance in the circulation (the sum of endogenous 
glucose production and exogenous glucose delivery from ingested 
carbohydrates) fail to keep pace with rates of glucose disappear­
ance from the circulation (the sum of ongoing glucose metabolism 
largely by the brain and of variable glucose utilization by tissues 
including muscle, fat, liver, and kidneys).
In healthy adults, the physiologic postabsorptive (fasting) 
plasma g

the physiologic postabsorptive (fasting) 
plasma glucose concentration ranges from approximately 70 mg/
dL (3.9 mmol/L) to 110 mg/dL (6.1 mmol/L), with a mean of 
about 90 mg/dL (5.0 mmol/L).3 In the postabsorptive steady 
state, rates of glucose production and utilization average approxi­
mately 2.2 mg/kg per minute (12 μmol/kg per minute), with a 
range of 1.8 to 2.6 mg/kg per minute (10–14 μmol/kg per min­
ute). These rates are as much as threefold higher in infants, at least 
in part because

efold higher in infants, at least 
in part because of their greater brain mass relative to body weight.
The liver is the predominant source of endogenous glucose 
production in the postabsorptive state; the kidneys, which both 
utilize and produce glucose, contribute little to net glucose pro­
duction. However, as in the liver, glucose production in the kidney 
is regulated; it is suppressed by insulin and stimulated by epineph­
rine but not by glucagon. As a result, net renal glucose production

lucagon. As a result, net renal glucose production 
occurs under some conditions, including hypoglycemia.4 There­
fore, endogenous glucose production cannot be equated solely 
with hepatic glucose production.
Gluconeogenesis and glycogenolysis are important for mainte­
nance of the plasma glucose concentration.5 The glucose pool—
namely, free glucose in the extracellular fluids and in the cells 
of certain tissues (primarily the liver)—is only about 15 to 20 g 
(83–111 mmol). Glycogen that can b

out 15 to 20 g 
(83–111 mmol). Glycogen that can be mobilized to provide cir­
culating glucose (e.g., hepatic glycogen) contains approximately 
70 g (390 mmol) of glucose, with a range of about 25 to 130 
g (135–722 mmol). Therefore, in an adult of average size, pre­
formed glucose can provide less than an 8-hour supply of energy, 
even at the diminished rate of glucose utilization that occurs in the 
postabsorptive state.
If fasting is prolonged to 24 to 48 hours, the plasma glucose 
concentrat

 to 24 to 48 hours, the plasma glucose 
concentration declines and then stabilizes; hepatic glycogen con­
tent falls to less than 10 g (55 mmol), and gluconeogenesis becomes 
the sole source of glucose production. Because amino acids are the 
main gluconeogenic precursors that result in net glucose forma­
tion, muscle protein is degraded. Glucose utilization by muscle 
and fat virtually ceases. As lipolysis and ketogenesis accelerate 
and circulating ketone levels rise, ketones become a major fu

ting ketone levels rise, ketones become a major fuel for 
the brain. Glucose utilization by the brain declines by about half, 
which reduces the rate of gluconeogenesis required to maintain the 
plasma glucose concentration and hence decreases protein wasting.
After a meal, glucose absorption into the circulation increases 
to more than twice the rate of postabsorptive endogenous glucose 
production, depending on the carbohydrate content of the meal, 
the rate of gastric transit, and the rate of

eal, 
the rate of gastric transit, and the rate of digestion and absorption. 
As glucose is absorbed, endogenous glucose production is sup­
pressed, and glucose utilization by muscle, fat, and liver accelerates. 
The exogenous glucose is assimilated and, after a small rise, the 
plasma glucose concentration returns to the postabsorptive level.
Exercise increases glucose utilization (by muscle) to rates that 
can be several times greater than those of the postabsorptive state. 
Endogenous glucose

e of the postabsorptive state. 
Endogenous glucose production normally accelerates to match 
use so that the plasma glucose concentration is maintained.
In summary, the plasma glucose concentration is normally 
maintained within a relatively narrow range despite wide varia­
tions in glucose flux and thus maintains the systemic glucose 
balance. This remarkable homeostatic feat is accomplished by an 
array of hormonal, neural, and substrate glucoregulatory factors. 
Responses to Hypoglycemia
Fall

egulatory factors. 
Falling plasma glucose concentrations cause a sequence of responses, 
with defined glycemic thresholds, in healthy individuals1,6–9 (Table 
38.3). The first response is a decrease in insulin secretion. This 
decrease occurs as glucose levels decline within the physiologic 
range. Increased secretion of glucose counterregulatory hormones, 
including glucagon and epinephrine, occurs as glucose levels fall just 
below the physiologic range. Lower plasma

ll just 
below the physiologic range. Lower plasma glucose concentrations 
cause a more intense sympathoadrenal (sympathetic neural as well 
as adrenomedullary) response and symptoms. Even lower glucose 
levels cause cognitive dysfunction and additional manifestations of 
functional brain failure including seizure or coma. 
Clinical Manifestations of Hypoglycemia
The symptoms and signs of hypoglycemia are nonspecific.9 Clinical 
hypoglycemia—that sufficient to cause symptoms and signs2—is 
most 

 sufficient to cause symptoms and signs2—is 
most convincingly documented by Whipple triad (see Table 38.1).
  Systemic Glucose Balancea and Effects of 
Circulating Hormones on Endogenous 
Production and Use of Glucose
Source of glucose influx 
or efflux
HORMONAL EFFECTS
Insulin
Glucagon Epinephrine
Glucose Influx Into the Circulation
Exogenous glucose delivery
Endogenous glucose delivery
  In liver: glycogenolysis and 
gluconeogenesis
↓
↑
↑
  In kidneys: gluconeogenesis
↓
↑
Glucose Efflux Out 

n kidneys: gluconeogenesis
↓
↑
Glucose Efflux Out of the Circulation
Ongoing brain glucose utilization
Variable glucose utilization by 
other tissues (e.g., muscle 
fat, liver, kidneys)
↑
↓
aTotal glucose influx = total glucose efflux.
   
TABLE 38.2
1528
Neuroglycopenic symptoms are a direct result of brain glu­
cose deprivation. They include cognitive impairments; behavioral 
changes; psychomotor abnormalities; and, at lower glucose 

 psychomotor abnormalities; and, at lower glucose lev­
els, seizure and coma.1,9 Neurogenic (or autonomic) symptoms 
are largely the result of the perception of physiologic changes 
caused by the sympathoadrenal (particularly the sympathetic neu­
ral10) discharge triggered by hypoglycemia. They include adren­
ergic (catecholamine-mediated) symptoms such as palpitations 
and tremor, and anxiety/arousal and cholinergic (acetylcholine-
mediated) symptoms such as sweating, hunger, and paresthesias. 

ptoms such as sweating, hunger, and paresthesias. 
Central mechanisms may also be involved in the generation of 
some of these symptoms (e.g., hunger).11 Subjective awareness of 
hypoglycemia is largely the result of the perception of neurogenic 
symptoms9 (Fig. 38.2).
The mechanism(s) of the signaling of the glucose regulatory 
and counterregulatory hormone and sympathetic responses to 
hypoglycemia are not known. However, those responses are not 
mediated by a decrease in brain glucose metabol

t 
mediated by a decrease in brain glucose metabolism because the 
glycemic threshold for a decrease in the cerebral metabolic role of 
glucose, measured with [11C]-glucose positron emission tomog­
raphy, is at a lower plasma glucose concentration than the glyce­
mic thresholds for the hormonal and sympathetic responses12 (see 
Table 38.3).
Signs of hypoglycemia include pallor and diaphoresis, which 
result from adrenergic cutaneous vasoconstriction and cholinergic 
activation of sweat glands, r

ion and cholinergic 
activation of sweat glands, respectively.2,3 Heart rates and systolic 
blood pressures are raised, but often not greatly. Neuroglycopenic 
manifestations are often observable. 
Maintenance of Systemic Glucose Balance
Although obligatory glucose utilization, largely by the brain, is 
continuous, the delivery of exogenous glucose from dietary carbo­
hydrates is intermittent. Systemic glucose balance (see Table 38.2) 
is normally maintained, and hypoglycemia and hyperglycemia a

y maintained, and hypoglycemia and hyperglycemia are 
prevented, by dynamic, minute-to-minute regulation of endoge­
nous glucose production from the liver and kidneys and of glucose 
utilization by tissues outside the central nervous system (CNS), 
such as muscle.1,3 This regulation is exerted primarily by insulin, 
glucagon, and epinephrine1–3 (Fig. 38.3; see also Table 38.3), 
although an array of hormones, neurotransmitters, and substrates 
is involved.13
The key physiologic defenses against 

 involved.13
The key physiologic defenses against falling plasma glucose 
concentrations are (1) a decrease in insulin, (2) an increase in glu­
cagon, and, in the absence of the latter, (3) an increase in epineph­
rine.3 The behavioral defense is carbohydrate ingestion prompted 
by symptoms that are largely sympathetic neural in origin1,3,10 (see 
Table 38.3 and Fig. 38.3).
The first physiologic defense against hypoglycemia is a decrease 
in insulin secretion by the pancreatic islet beta cells. 

lin secretion by the pancreatic islet beta cells. Signaled pri­
marily by declining glucose levels at the beta cells, this response 
  Physiologic Responses to Decreasing Plasma Glucose Concentrations
Response
Glycemic Thresholda 
(mmol/L [mg/dL])
Physiologic Effects
Role in Prevention or Correction of 
Hypoglycemia (Glucose Counterregulation)
↓ Insulin
4.4–4.7 (80–85)
↑ Ra (↓ Rd)
Primary glucose regulatory factor, first defense 
against hypoglycemia
↑ Glucagon
3.6–3.9 (65–70)
↑ Ra
Primary gluc

cemia
↑ Glucagon
↑ Ra
Primary glucose counterregulatory factor, second 
defense against hypoglycemia
↑ Epinephrine
↑ Ra, ↓ Rc
Involved, critical when glucagon is deficient, third 
↑ Cortisol and growth hormone
↑ Ra, ↓ Rc
Involved, not critical
Symptoms
2.8–3.1 (50–55)
↑ Exogenous glucose
Prompt behavioral defense (food ingestion)
↓ Cognition
<2.8 (50)
—
(Compromises behavioral defense)
↓ Brain glucose metabolism
<2.8 (5

vioral defense)
<2.8 (50)
—
—
aArterialized venous, not venous, plasma glucose concentrations.
Ra, Rate of glucose appearance, glucose production by the liver and kidneys; Rc, rate of glucose clearance by insulin-sensitive tissues; Rd, rate of glucose disappearance, glucose utilization by insulin-
sensitive tissues such as skeletal muscle (no direct effect on central nervous system glucose utilization).
   
TABLE 38.3
Neurogenic
Sweaty
Hungry
Tingling 
Shaky/tremulous

eurogenic
Sweaty
Hungry
Tingling 
Heart pounding 
Nervous/anxious
Neuroglycopenic
Warm 
Weak 
Difficulty thinking/confused 
Tired/drowsy 
Faint
Dizzy
Difficulty speaking 
Blurred vision 
EU
Hypo
3
2
1
0
ADB PAB 
Blood sugar low 
(p < 0.001)
• Fig. 38.2  Neurogenic (autonomic) and neuroglycopenic symptoms of 
hypoglycemia in healthy humans. Among the neurogenic symptoms, 
“sweaty,” “hungry,” and “tingling” are cholinergic, and “shaky/tremulous,” 
“heart pounding,” and “nervous/anx

ky/tremulous,” 
“heart pounding,” and “nervous/anxious” are adrenergic. See the text for 
discussion. Mean subject scores (± standard error) for awareness of hypo­
glycemia (low blood sugar) are shown during clamped euglycemia (EU) 
and during three conditions of hypoglycemia (Hypo): alone, with combined 
α- and β-adrenergic blockade by infused phentolamine and propranolol 
(ADB), and with combined α- and β-adrenergic blockade plus muscarinic 
cholinergic blockade by atropine (panautonomic block

olinergic blockade by atropine (panautonomic blockade, PAB). (From 
Towler DA, Havlin CE, Craft S, Cryer P. Mechanism of awareness of hypo­
glycemia: perception of neurogenic [predominantly cholinergic] rather than 
neuroglycopenic symptoms. Diabetes Care. 1993;42:1791–1798, used 
with permission of the American Diabetes Association.)
1529
occurs as plasma glucose concentrations decline within the physi­
ologic range3 (see Table 38.3) and favors increased hepatic and 
re

e Table 38.3) and favors increased hepatic and 
renal glucose production with virtual cessation of glucose utiliza­
tion by insulin-sensitive tissues such as muscle (see Fig. 38.3).
The second physiologic defense against hypoglycemia is an 
increase in glucagon secretion by pancreatic islet alpha cells. This 
increase occurs as plasma glucose concentrations fall just below the 
physiologic range (see Table 38.3) and stimulates hepatic glucose 
production, largely by stimulating glycogenolysis (s

oduction, largely by stimulating glycogenolysis (see Fig. 38.3). 
This response is signaled primarily by a decrease in intraislet insulin, 
perhaps among other beta-cell secretory products, in the setting of 
low alpha-cell glucose concentrations14–18 (see Fig. 38.3) and other 
signals that may include a decrease in somatostatin among others 
as yet to be determined mechanisms14–18 and only secondarily by 
increased autonomic nervous system (sympathetic, parasympa­
thetic, adrenomedullary) input

athetic, parasympa­
thetic, adrenomedullary) inputs.19
The third physiologic defense against hypoglycemia, which 
becomes critical when glucagon is deficient, is an increase in adre­
nomedullary epinephrine secretion. Signaled via the CNS, it also 
occurs as plasma glucose concentrations fall just below the physi­
ologic range1–3 (see Table 38.3) and raises plasma glucose con­
centrations largely by β2-adrenergic stimulation of hepatic and 
renal glucose production (see Fig. 38.3). However, the 

 glucose production (see Fig. 38.3). However, the plasma 
glucose–raising actions of epinephrine also involve limitation of 
glucose clearance by insulin-sensitive tissues, mobilization of glu­
coneogenic precursors such as lactate and amino acids from mus­
cle and glycerol from fat, and α2-adrenergic limitation of insulin 
secretion20 (Fig. 38.4). Indeed, the adrenergic actions on beta-cell 
insulin secretion normally play an important role in the glycemic 
actions of epinephrine. α2-Adrenergic

he glycemic 
actions of epinephrine. α2-Adrenergic limitation of insulin secre­
tion permits the glycemic response; β2-adrenergic stimulation 
alone has little effect because it also stimulates insulin secretion. 
However, some increase in insulin secretion—due to the rising 
glucose level, β2-adrenergic stimulation, or both—limits the mag­
nitude of the glycemic response to epinephrine. These physiologic 
interactions explain why glycemic sensitivity to epinephrine is 
increased in patients who

ivity to epinephrine is 
increased in patients who cannot increase insulin secretion (e.g., 
those with type 1 diabetes mellitus [T1DM]).20 Circulating epi­
nephrine is derived almost exclusively from the adrenal medulla 
in adults.10 Whereas circulating norepinephrine is derived largely 
from sympathetic nerve terminals under resting conditions and 
in many stimulated states (e.g., exercise), the plasma norepineph­
rine response to hypoglycemia is derived largely from the adrenal 
medulla.10
Pe

is derived largely from the adrenal 
sensors
Decreased
glucose
CNS
(SNS)
(PNS)
Pancreas
Liver
Adrenal
medulla
Muscle
Kidney
Fat
Increased sympathoadrenal outflow
Increased NE
(palpitations,
tremor, arousal)
Increased ACh
(sweating, hunger)
Decreased
insulin
Increased
Decreased
glucose
clearance
Increased
glucose
Increased
lactate,
amino acids,
glycerol
Increased
neurogenic symptoms
Increased
ingestion of carbohydrates
(Often attenuated in T1DM)
Increased

arbohydrates
Increased
epinephrine
Increased
glucagon
(Lost in T1DM)
Decreased
insulin
Increased glycogenolysis
+
Increased gluconeogenesis
β cell
α cell
• Fig. 38.3  Physiologic and behavioral defenses against hypoglycemia in humans. ACh, acetylcholine; α 
cell, pancreatic islet alpha cell; β cell, pancreatic islet beta cell; CNS, central nervous system; NE, norepi­
nephrine; PNS, parasympathetic nervous system; SNS, sympathet

NS, parasympathetic nervous system; SNS, sympathetic nervous system; T1DM, type 1 dia­
betes mellitus. (From Cryer PE. Mechanisms of sympathoadrenal failure and hypoglycemia in diabetes. J 
Clin Invest. 2006;116:1470–1473, used with permission of the American Society for Clinical Investigation.)
1530
These physiologic defenses against hypoglycemia typically abort 
episodes of declining plasma glucose concentrations and prevent 
clinica

plasma glucose concentrations and prevent 
clinical (i.e., symptomatic) hypoglycemia. If they do not, the lower 
plasma glucose concentration causes a more intense sympathoad­
renal response, resulting in symptoms1,3 (see Table 38.3). These 
symptoms, particularly the neurogenic symptoms, cause awareness 
of hypoglycemia that prompts the behavioral defense against hypo­
glycemia—ingestion of carbohydrates3 (see Fig. 38.3).
The integrated physiology of glucose counterregulation1–3 is 
further ill

gy of glucose counterregulation1–3 is 
further illustrated in Fig. 38.5. Falling glucose levels within the 
pancreatic islet cells signal a decrease in insulin secretion and an 
increase in glucagon secretion. Falling glucose levels sensed in 
the periphery and in the CNS, acting through the hypothala­
mus, signal an increase in sympathoadrenal activity that results 
in an increase in adrenomedullary epinephrine secretion and in 
neurogenic symptoms, the latter largely resulting from increased 

oms, the latter largely resulting from increased 
sympathetic neural activity. Fig. 38.5 includes a putative cerebral 
network that may modulate the hypothalamic response. 
Clinical Hypoglycemia
Definition and Diagnosis
Clinical hypoglycemia is, by definition, a plasma glucose concen­
tration low enough to cause symptoms or signs, including impair­
ment of brain function. The glycemic thresholds for symptoms and 
signs of hypoglycemia are dynamic; for example, they shift to lower 

; for example, they shift to lower 
plasma glucose concentrations in patients with recurrent hypo­
glycemia21–23 and to higher concentrations in those with poorly 
controlled diabetes. Therefore, it is not possible to state a single 
plasma glucose concentration that categorically defines hypoglyce­
mia. Furthermore, the symptoms and signs of hypoglycemia are 
nonspecific, and a low measured plasma glucose concentration can 
be artifactual. For all of these reasons, hypoglycemia is most con­
vin

ll of these reasons, hypoglycemia is most con­
vincingly documented by Whipple triad1,2 (see Table 38.1).
Documentation of Whipple triad is particularly important 
when hypoglycemia is suspected in a person who does not have 
diabetes mellitus, because hypoglycemic disorders are rare. In the 
absence of diabetes, a thorough diagnostic evaluation is recom­
mended only for patients in whom Whipple triad can be doc­
umented.2 Ideally, patients with diabetes being treated with an 
insulin secretagog

diabetes being treated with an 
insulin secretagogue or insulin should monitor their plasma glu­
cose concentration whenever they suspect hypoglycemia. None­
theless, the likelihood that a symptomatic episode is the result 
Liver 
Pancreatic 
islets
Muscle 
Fat 
  Lactate 
and Alanine 
Glycolysis   Glucose 
transport
Lipolysis
  Glucose 
Glucose
  Glucose 
utilization
Glycerol
NEFA 
β2
α2
β 
Insulin 
Glucagon 
β2
β1, β2
(?β3)
• Fig. 38.4  Sch

nsulin 
Glucagon 
β2
β1, β2
(?β3)
• Fig. 38.4  Schematic representation of the mechanisms of the hypergly­
cemic effect of epinephrine, mediated by α- and β-adrenergic stimulation. 
NEFA, nonesterified fatty acid. (From Cryer PE. Catecholamines, pheo­
chromocytoma and diabetes. Diabetes Rev. 1993;1:309–317, used with 
permission of the American Diabetes Association.)
Medial prefrontal
cortex
Orbital prefrontal
cortex
Dorsal midline
thalamus
Brain stem
Amygdala
  Sympathoad

lamus
Brain stem
Amygdala
  Sympathoadrenal
activity
+
  Insulin +   Glucagon
Pancreatic islets
β-cells
Insulin
Glucose
α-cells
Glucagon
• Fig. 38.5  Schematic representation of the integrated mechanisms of the physiologic responses to hypo­
glycemia in humans: decrements in insulin secretion, increments in glucagon secretion, and increments 
in sympathoadrenal (adrenomedullary and sympathetic neural) activity. α-cells, pancreatic islet alpha cells; 
β-cells, pancreatic 

ancreatic islet alpha cells; 
β-cells, pancreatic islet beta cells. (From Cryer PE. Hypoglycemia in Diabetes: Pathophysiology, Preva­
lence and Prevention. Alexandria, VA: American Diabetes Association; 2009, used with permission of the 
American Diabetes Association.)
1531
of hypoglycemia is high, because hypoglycemia is common in 
patients treated with insulin, a sulfonylurea, or a glinide. It is also 
important to recognize that glucose concentrations measured in 
who

gnize that glucose concentrations measured in 
whole blood are approximately 15% lower than those in plasma 
and may be further reduced if the hematocrit is high. 
Clinical Classification of Hypoglycemia
Causes of hypoglycemia2 are outlined in Table 38.4. Drugs are, by 
far, the most common cause of hypoglycemia. Such drugs include 
insulin secretagogues and insulin used to treat diabetes. Although 
persons with diabetes can suffer from the same hypoglycemic dis­
orders as those without diabetes

hypoglycemic dis­
orders as those without diabetes, their hypoglycemic episodes are 
usually the result of treatment of their diabetes. Furthermore, the 
pathophysiology of hypoglycemia in diabetes is unique, and the 
diagnostic and management approaches are different from those 
for individuals without diabetes. Therefore, hypoglycemia in per­
sons with diabetes and hypoglycemia in those without diabetes are 
discussed separately in this chapter. 
Hypoglycemia in Persons With Diabetes
The Clini

. 
The Clinical Problem of Hypoglycemia in 
Diabetes
Iatrogenic hypoglycemia is the limiting factor in the glycemic 
management of diabetes with insulin, a sulfonylurea, or a glinide.1 
It causes recurrent morbidity in most persons with T1DM and in 
many of those with advanced type 2 diabetes mellitus (T2DM), 
and it is sometimes fatal. The barrier of hypoglycemia generally 
precludes maintenance of euglycemia over a lifetime of diabetes 
and thus full reali

a over a lifetime of diabetes 
and thus full realization of the vascular benefits of glycemic con­
trol. Furthermore, hypoglycemia compromises physiologic and 
behavioral defenses against subsequent falling plasma glucose con­
centrations, resulting in a vicious cycle of recurrent hypoglycemia.
Hypoglycemia in diabetes is caused by pharmacokinetically 
imperfect treatment with insulin or an insulin secretagogue (e.g., 
a sulfonylurea or a glinide) that results in episodes of therapeutic 
hyperin

) that results in episodes of therapeutic 
hyperinsulinemia. Thus, it is fundamentally iatrogenic. Episodes 
of substantial absolute therapeutic hyperinsulinemia can cause iso­
lated episodes of hypoglycemia. However, as developed later in this 
chapter, recurrent hypoglycemia in diabetes is typically the result 
of the interplay of relative or mild to moderate absolute therapeu­
tic hyperinsulinemia and compromised physiologic and behav­
ioral defenses against falling plasma glucose concentrati

efenses against falling plasma glucose concentrations.1,2
Frequency of Hypoglycemia in Diabetes
Hypoglycemia is unfortunately a fact of life for people with 
T1DM.24–26 The average patient suffers untold numbers of epi­
sodes of asymptomatic hypoglycemia (which are not benign, 
because they impair defenses against subsequent hypoglycemia), 
approximately two episodes of symptomatic hypoglycemia per 
week (thousands of such episodes over a lifetime of diabetes), and 
roughly one episode per year 

e of diabetes), and 
roughly one episode per year of severe, at least temporarily dis­
abling hypoglycemia, often with seizure or coma. This problem 
has not abated since it was highlighted by the report of the Diabe­
tes Control and Complications Trial (DCCT) in 1993.24 In 2007, 
the United Kingdom Hypoglycaemia Study Group reported 
an incidence of severe hypoglycemia twice that reported in the 
DCCT among patients who had had T1DM for less than 5 years 
and five times the DCCT incidence among

n 5 years 
and five times the DCCT incidence among those with T1DM for 
more than 15 years in their population-based study.27
The overall incidence of hypoglycemia during treatment of 
T2DM with an insulin secretagogue, or even with insulin, is 
lower than that in patients with T1DM.26–34 However, for patho­
physiologic reasons discussed later, the incidence of hypoglyce­
mia increases progressively over time as patients approach the 
absolute endogenous insulin-deficient end of the spectrum of 

dogenous insulin-deficient end of the spectrum of 
T2DM.27,29,30 Indeed, the incidence of hypoglycemia has been 
reported to be similar in patients with T2DM and in those with 
T1DM matched for duration of insulin therapy.30 The United 
Kingdom Hypoglycaemia Study Group found a prevalence of 
severe hypoglycemia of 7% and an incidence of 10 episodes per 
100 patient-years among patients with T2DM treated with insu­
lin for less than 2 years; among those treated for longer than 5 
years, these fi

g those treated for longer than 5 
years, these figures rose to 25% prevalence and 70 episodes per 
100 patient-years.27 The patterns for self-treated hypoglycemia 
were similar. Therefore, at least with current, less than euglycemic 
treatment goals, the frequency of iatrogenic hypoglycemia is rela­
tively low during the first few years of treatment of T2DM with 
insulin but increases substantially in advanced T2DM, approach­
ing the frequency among patients with T1DM. As discussed later 
in th

mong patients with T1DM. As discussed later 
in this chapter, new technologies, particularly continuous glucose 
monitoring (CGM) and drugs for the treatment of T2DM that do 
not cause hypoglycemia, will reduce the frequency of hypoglyce­
mia for those who are able to use them.
Estimates of the incidence and prevalence of hypoglycemia in 
diabetes are generally underestimates because of the challenge of 
ascertainment.1,2 Asymptomatic episodes will be missed unless 
  Causes of Hypoglycemia in 

ll be missed unless 
  Causes of Hypoglycemia in Adults
Ill or Medicated Individual
Drugs
Insulin or insulin secretagogue
Alcohol
Others (see Table 38.8)
Critical Illnesses
Hepatic, renal, or cardiac failure
Sepsis
Inanition
Hormonal Deficiency
Cortisol
Glucagon and epinephrine (in insulin-deficient diabetes mellitus)
Non–Islet Cell Tumor
Seemingly Well Individual
Endogenous Hyperinsulinism
Insulinoma
Functional beta-cell disorders (nesidioblastosis)
Noninsulinoma pancreatogenous hypoglycemia
P

osis)
Post–gastric bypass hypoglycemia
Autoimmune hypoglycemia
Antibody to insulin
Antibody to insulin receptor
Insulin secretagogue
Other
Accidental, Surreptitious, or Malicious Hypoglycemia
From Cryer PE, Axelrod L, Grossman AB, et al. Evaluation and management of adult hypogly­
cemic disorders: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 
2009;94:709–728, used with permission of The Endocrine Society.
   
TABLE 38.4
153

ssion of The Endocrine Society.
   
TABLE 38.4
1532
they are incidentally detected by self-monitoring of blood glucose 
or by CGM. Symptomatic episodes may not be recognized to be 
the result of hypoglycemia, because the symptoms of hypoglyce­
mia are nonspecific. Even if they are recognized, they may not be 
remembered35,36 and reported at periodic patient contacts. Because 
they are dramatic events that are more likely to be reported

ramatic events that are more likely to be reported (by the 
patient or an associate), estimates of the frequency of severe hypo­
glycemia (that requiring the assistance of another person) are more 
reliable, although they represent only a small fraction of the total 
hypoglycemic burden. Prospective, population-based studies with 
a focus on hypoglycemia should provide the most reliable data.
The prospective, population-based data of Donnelly and col­
leagues26 indicate that the overall incidenc

 col­
leagues26 indicate that the overall incidences of any episode of 
hypoglycemia and of severe hypoglycemia in insulin-treated 
T2DM are about one-third of those in T1DM. Two other popu­
lation-based studies reported the incidence of severe hypoglycemia 
requiring emergency treatment in insulin-treated T2DM to be 
40%31 and 100%32 of that in T1DM. Taken together, and con­
sidering that the prevalence of T2DM is approximately 20-fold 
greater than that of T1DM and that most patients with T2DM

than that of T1DM and that most patients with T2DM 
ultimately require treatment with insulin, these data suggest that 
most episodes of iatrogenic hypoglycemia, including severe hypo­
glycemia, occur in persons with T2DM. 
Impact of Hypoglycemia in Diabetes
Iatrogenic hypoglycemia causes recurrent physical and psychoso­
cial morbidity and impairs glycemic defenses against subsequent 
hypoglycemia in many patients with diabetes.1 The barrier of 
hypoglycemia generally precludes maintenance of eu

hypoglycemia generally precludes maintenance of euglycemia over 
a lifetime of diabetes. Hypoglycemia often causes functional brain 
failure that is typically reversed after the plasma glucose concen­
tration is raised. Rarely, it causes sudden death, presumably the 
result of cardiac arrhythmia, or, if it is profound and prolonged, 
permanent brain dysfunction and death.
At a minimum, an episode of symptomatic hypoglycemia is a 
nuisance and a distraction. It can impair judgment, behavior, and 

istraction. It can impair judgment, behavior, and 
performance of physical tasks such as driving. It can cause a seizure 
or loss of consciousness. Transient neurologic deficits sometimes 
occur, but permanent neurologic damage is rare. Systematic long-
term follow-up of the DCCT patients suggests that recurrent iatro­
genic hypoglycemia does not cause chronic cognitive impairments 
in young adults,37 but the possibility that it does so in young chil­
dren38,39 and the elderly40 remains. Among o

chil­
dren38,39 and the elderly40 remains. Among other psychological dis­
orders, fear of hypoglycemia can be a barrier to glycemic control.41
Iatrogenic hypoglycemia can be fatal.42–53 That hypoglycemia 
can kill has been known since the discovery of insulin.42 There are 
epidemiologic associations of hypoglycemia with death,43 reports 
of excessive mortality rates during intensive glycemic therapy in 
randomized controlled trials of patients with T2DM43,44 and 
intensive care unit patients,45,

th T2DM43,44 and 
intensive care unit patients,45,46 and reports of hypoglycemia mor­
tality rates in series of patients with diabetes.47–51 Early reports 
indicated that 2% to 4% of patients with T1DM died of hypo­
glycemia.47–49 More recent reports indicate that 4%,49,50 7%,51 
8%,52 and 10% or greater53 of deaths of persons with T1DM 
were caused by hypoglycemia. Indeed, hypoglycemia at the time 
of death has been documented by CGM.54 Death rates of up to 
10% of patients who suffer severe su

tes of up to 
10% of patients who suffer severe sulfonylurea-induced hypogly­
cemia have been reported.55
The finding of Currie and colleagues56 that lower glycosylated 
hemoglobin (HbA1c) levels are associated with increased mortality 
in patients with T2DM at higher risk of hypoglycemia is entirely 
consistent with this body of evidence that iatrogenic hypoglycemia 
is a risk factor for death in diabetes. Thus, there are iatrogenic 
mortality rates in both T1DM and T2DM.
Although prolonged, pr

ates in both T1DM and T2DM.
Although prolonged, profound hypoglycemia can cause brain 
death, most instances of sudden hypoglycemic death are thought 
to be the result of cardiac arrhythmias triggered by an intense sym­
pathoadrenal response to hypoglycemia.44,57,58 These are mediated 
through β-adrenergic receptors.58 Sudden death has been associ­
ated with QT interval prolongation and reduced baroreflex sensi­
tivity in patients with classic diabetic autonomic neuropathy.59–61 
Adler and colle

abetic autonomic neuropathy.59–61 
Adler and colleagues62 demonstrated, in studies of nondiabetic 
individuals, that recent antecedent hypoglycemia causes reduced 
baroreflex sensitivity the following day. The induction of func­
tional sympathetic failure by recent antecedent hypoglycemia as 
an additional, potentially fatal, feature of the concept of hypogly­
cemia-associated autonomic failure (HAAF) in diabetes1 is dis­
cussed later in this chapter. 
Clinical Definition and Classification of H

pter. 
Clinical Definition and Classification of Hypoglycemia in 
Diabetes
The American Diabetes Association/Endocrine Society Workgroup 
on Hypoglycemia63 (ADA/ES Workgroup) defined hypoglycemia 
in diabetes as “all episodes of abnormally low plasma glucose con­
centration that expose the individual to potential harm.” That is 
broader than the recommended definition of clinical hypoglycemia 
in persons without diabetes (i.e., a plasma glucose concentration 
low enough to cause symptoms or sign

oncentration 
low enough to cause symptoms or signs1,2), because it includes 
asymptomatic episodes. The latter are not benign in persons with 
diabetes; they compromise defenses against subsequent hypoglyce­
mia,1,2 and they identify increased risk of imminent severe iatro­
genic hypoglycemia.64 Again, it is not possible to define a plasma 
glucose concentration that categorically defines hypoglycemia, 
because the glycemic thresholds for responses to hypoglycemia 
are dynamic. They shift to lo

ses to hypoglycemia 
are dynamic. They shift to lower plasma glucose concentrations in 
patients with recurrent hypoglycemia21,22 and to higher plasma glu­
cose concentrations in those with poorly controlled diabetes.20,21
The ADA/ES Workgroup recommends that people with diabe­
tes (implicitly those treated with an insulin secretagogue or insulin) 
become concerned about the possibility of developing hypoglyce­
mia at a self-monitored plasma glucose concentration of 70 mg/
dL (3.9 mmol/L) or les

ose concentration of 70 mg/
dL (3.9 mmol/L) or less.63 Within the error of self-monitoring 
(or CGM), that conservative alert value approximates the lower 
limit of the nondiabetic postabsorptive plasma glucose range1,2 
and the normal glycemic thresholds for activation of physiologic 
glucose counterregulatory systems1,2 (see Table 38.3); it is low 
enough to reduce glycemic defenses against subsequent hypogly­
cemia in nondiabetic persons. Therefore, the recommended glu­
cose alert level of 70

efore, the recommended glu­
cose alert level of 70 mg/dL (3.9 mmol/L) or less is data driven; it 
usually gives the patient time to take action to prevent a clinical 
hypoglycemic episode, and it provides some margin for the lim­
ited accuracy of glucose monitoring devices at low plasma glucose 
concentrations. It has been implicitly endorsed by the US Food 
and Drug Administration and the European Medicines Agency.
The recommended alert value does not, of course, mean that 
persons with diabete

es not, of course, mean that 
persons with diabetes should always self-treat for hypoglycemia 
at an estimated plasma glucose concentration of 70 mg/dL (3.9 
mmol/L) or less. Rather, they should consider actions ranging 
from repeating the measurement in the near term, through behav­
ioral actions such as avoiding exercise or driving until the glucose 
level is raised, to carbohydrate ingestion and subsequent adjust­
ments of the therapeutic regimen.
The ADA/ES Workgroup63 also recommended a cli

t 
 • Level 3: Severe hypoglycemia as defined by the American Dia­
betes Association.
These levels were endorsed by the American Diabetes Associa­
tion and the European Association for the Study of Diabetes.65
Based on the fact that the HbA1c level provides useful infor­
mation about long-term glycemic control but no information 
about short-term glycemic control, it has been suggested66 that 
the classification of severe hypoglycemia (see Table 38.5) should be 
expanded to include

ia (see Table 38.5) should be 
expanded to include a distinctly low plasma glucose concentration, 
perhaps less than 50 mg/dL (2.8 mmol/L) or less than 54 mg/dL 
(3.0 mmol/L),65 detected by self-monitoring of plasma glucose, 
CGM, or a laboratory determination. This would be particularly 
useful if there were no symptoms at the time of the detection of 
the distinctly low glucose concentration, because it would docu­
ment impaired awareness of hypoglycemia, a treatable condition. 
Pathophysiolog

poglycemia, a treatable condition. 
Pathophysiology of Glucose Counterregulation 
The pathophysiology of glucose counterregulation and its relation­
ship to clinical hypoglycemia in diabetes1,2 are summarized in the 
paragraphs that follow (see earlier discussion and Figs. 38.3 and 
38.5). Again, the key physiologic defenses against falling plasma 
glucose concentrations are a decrease in insulin, an increase in 
glucagon, and, in the absence of the latter, an increase in epineph­
ri

 absence of the latter, an increase in epineph­
rine. The behavioral defense is carbohydrate ingestion prompted 
by symptoms9 that are largely sympathetic neural in origin.10
Insulin Excess
Episodes of therapeutic hyperinsulinemia, produced by treat­
ment with an insulin secretagogue (a sulfonylurea or a glinide) or 
with insulin, are a prerequisite for the development of iatrogenic 
hypoglycemia. Marked absolute insulin excess can cause isolated 
episodes of hypoglycemia. However, iatrogenic hy

episodes of hypoglycemia. However, iatrogenic hypoglycemia is 
typically the result of the interplay of relative or mild to moderate 
absolute therapeutic hyperinsulinemia and compromised physi­
ologic and behavioral defenses against falling plasma glucose con­
centrations1,2 (Fig. 38.6). 
Defective Glucose Counterregulation and Hypoglycemia 
Unawareness
In established (absolute deficiency of endogenous insulin) T1DM, 
circulating (exogenous) insulin concentrations do not decrease 
as plasma g

nsulin concentrations do not decrease 
as plasma glucose concentrations fall in response to therapeutic 
hyperinsulinemia. That is the result of absolute beta-cell failure 
with no regulated endogenous insulin secretion. Therefore, the 
first physiologic defense against hypoglycemia3 is lost. Further­
more, despite the presence of functional alpha cells, there is no 
increase in glucagon secretion1,2,18 (Fig. 38.7). That also is largely 
the result of beta-cell failure,14–18 which causes loss of

lt of beta-cell failure,14–18 which causes loss of the decre­
ment in insulin that normally signals an increase in alpha-cell glu­
cagon secretion during hypoglycemia1,18 (see Figs. 38.3 and 38.5). 
Therefore, the second physiologic defense against hypoglycemia3 
is lost as well.
In addition, the increase in epinephrine secretion, the third 
physiologic defense against hypoglycemia,3 is typically attenu­
ated1,2,67 (see Fig. 38.7). In the setting of absent insulin and 
glucagon responses, the at

 of absent insulin and 
glucagon responses, the attenuated epinephrine response causes 
the clinical syndrome of defective glucose counterregulation67–69 
(see Fig. 38.6), which is associated with a 25-fold69 or greater70 
increased risk of severe hypoglycemia in T1DM. The attenuated 
epinephrine response is a marker of attenuation of the sympa­
thoadrenal response (including the sympathetic neural response) 
that normally causes neurogenic symptoms that largely prompt 
the behavioral response l

oms that largely prompt 
the behavioral response leading to carbohydrate ingestion (see 
Figs. 38.3 and 38.5). This attenuated response (largely the attenu­
ated sympathetic neural response) causes the clinical syndrome of 
hypoglycemia unawareness1,2 (see Fig. 38.6).
Although the term hypoglycemia unawareness is used widely, 
“impaired awareness of hypoglycemia”71 is more precise because 
there is a spectrum ranging from partial to complete loss of symp­
toms. Attenuated sympathoadrenal respons

 of symp­
toms. Attenuated sympathoadrenal responses to falling plasma 
glucose concentrations can be caused by recent antecedent hypo­
glycemia72 (Figs. 38.8 and 38.9), prior exercise,73 or sleep,74,75 
but the precise mechanism is not known. (See later discussion on 
HAAF.)
Impaired awareness of hypoglycemia is largely the result of 
reduced release of the sympathetic neurotransmitters norepi­
nephrine and acetylcholine.10 There is evidence of decreased 
β-adrenergic sensitivity, specifically 

decreased 
β-adrenergic sensitivity, specifically reduced cardiac chronotropic 
sensitivity to isoproterenol, in affected patients.76,77 However, 
vascular sensitivity to a β-adrenergic agonist was not found to be 
reduced in unaware patients.78 Reduced sensitivity to β-adrenergic 
  Classification of Hypoglycemia in Diabetes
Clinical Classification
Severe hypoglycemia
An event requiring the assistance of another 
person to actively administer carbohydrate, 
glucagon, or other resusc

dminister carbohydrate, 
glucagon, or other resuscitative actions. 
Plasma glucose measurements may not 
be available during such an event, but 
neurologic recovery attributable to the 
restoration of plasma glucose to a normal 
level is considered sufficient evidence that 
the event was induced by a low plasma 
glucose concentration.
Documented 
symptomatic 
An event during which typical symptoms 
of hypoglycemia are accompanied by a 
measured plasma glucose concentration of 
≤70 m

a 
≤70 mg/dL (3.9 mmol/L).
Asymptomatic 
An event not accompanied by typical symp­
toms of hypoglycemia but with a measured 
plasma glucose concentration of ≤70 mg/
dL (3.9 mmol/L).
Probable symptomatic 
An event during which symptoms typical of 
hypoglycemia are not accompanied by a 
plasma glucose determination but were 
presumably caused by a plasma glucose 
concentration of ≤70 mg/dL (3.9 mmol/L).
Pseudohypoglycemia
An event

70 mg/dL (3.9 mmol/L).
An event during which the person with diabetes 
reports any of the typical symptoms of hypo­
glycemia and interprets those as indicative 
of hypoglycemia, with a measured plasma 
glucose concentration that is >70 mg/dL 
(3.9 mmol/L) but is approaching that level.
TABLE 38.5
Early T2DM
(relative β-cell failure)
Advanced T2DM and T1DM
(absolute β-cell failure)
Attenuated sympathoadrenal
responses to hypoglycemia (HAAF)
  Adrenomedullary
epinephrine respons

cemia (HAAF)
epinephrine responses
  Sympathetic
neural responses
Defective glucose
counterregulation
Impaired awareness
 of hypoglycemia   
Recurrent hypoglycemia
Isolated episodes of hypoglycemia
β-cell failure        No   insulin and no   glucagon
Episodes of hypoglycemia
Sleep
Exercise
Marked absolute therapeutic
hyperinsulinemia
Falling glucose levels
Relative or mild-moderate absolute
therapeutic hyperinsulinemia
Hypoglycemia-Associated Autonomic Fa

lucose levels
Hypoglycemia-Associated Autonomic Failure
• Fig. 38.6  Schematic representation of the concept of hypoglycemia-associated autonomic failure (HAAF) 
in diabetes and the pathogenesis of iatrogenic hypoglycemia including the clinical syndromes of defec­
tive glucose counterregulation and impaired awareness of hypoglycemia. β-cell, pancreatic islet beta cell; 
T1DM, type 1 diabetes mellitus; T2DM, type 2 diabetes mellitus.
Nominal glucose (mmol/L)
5.0
4.4
3.9
3.3
2.8
120
80
40
0
0
Gluc

se (mmol/L)
5.0
4.4
3.9
3.3
2.8
120
80
40
0
0
Glucose
Nondiabetic
IDDM c CDAN
IDDM s CDAN
IDDM with CDAN
IDDM without CDAN
60
120
180
240
300
5.6
5.0
4.4
3.9
3.3
2.8
pmol/L
750
500
250
0
Insulin
Time (min)
µU/mL
160
120
80
40
0
60
120
180
240
300
5.6
5.0
4.4
3.9
3.3
2.8
160
120
80
0
pmol/L
ng/L
Glucagon
pg/mL
120
60
80
100
40
20
0
60
120
180
240
300
mmol/L
6.0
5.0
4.0
3.0
2.0
mg/dL
800
600
400
200
0
0
60
120
180
240
300
5.6
5.0
4.4
3.9
3.3
2.8
5000
4000
3000
2000
1000
0
p

6
5.0
4.4
3.9
3.3
2.8
5000
4000
3000
2000
1000
0
pg/mL
• Fig. 38.7  Mean (± standard error) plasma glucose, insulin, epinephrine, and glucagon concentrations 
during hyperinsulinemic stepped hypoglycemic glucose clamps in three groups of subjects: nondiabetic 
subjects (open squares and columns), people with type 1 diabetes (IDDM) who have classic diabetic 
autonomic neuropathy (CDAN; open triangles and crosshatched columns), and people with IDDM without 
CDAN (closed circles and columns). IDDM,

 without 
CDAN (closed circles and columns). IDDM, insulin-dependent diabetes mellitus. (From Dagogo-Jack SE, 
Craft S, Cryer PE. Hypoglycemia-associated autonomic failure in insulin-dependent diabetes mellitus. J 
Clin Invest. 1993;91:819–828, used with permission of the American Society for Clinical Investigation.)
1535
Nominal glucose (mmol/L) 
5.0 
4.4 
3.9 
3.3 
2.8 
120
80
40
0
0
Glucose
60 
120 
180 
240 
300 
5.6 
5.0 
4.4 
3.9 
3.3 
2.8 
pmol/L 
750
500
250
0
In

5.0 
4.4 
3.9 
3.3 
2.8 
pmol/L 
750
500
250
0
Insulin
Time (min) 
µU/mL 
120
80
40
0 
60 
120 
180 
240 
300 
5.6 
5.0 
4.4 
3.9 
3.3 
2.8 
120
80
0
pmol/L 
ng/L 
Glucagon
pg/mL 
120
60
80
100
40
0 
60 
120 
180 
240 
300 
mmol/L 
6.0
5.0
4.0
3.0
mg/dL 
400
300
200
100
0
0 
60 
120 
180 
240 
300 
5.6 
5.0 
4.4 
3.9 
3.3 
2.8 
2500
2000
1500
1000
500
0
pg/mL 
After hyper. 
After hypo. 
After hyper.
After hypo.
• Fig. 38.8  Mean (± standard error) plasma glucose, insulin, epinephrin

tandard error) plasma glucose, insulin, epinephrine, and glucagon concentrations 
during hyperinsulinemic stepped hypoglycemic glucose clamps in patients with type 1 (insulin-dependent) 
diabetes mellitus without classical diabetic autonomic neuropathy on mornings after afternoon hyperglyce­
mia (After hyper.; closed circles and columns) and on mornings after afternoon hypoglycemia (After hypo.; 
open circles and columns). (From Dagogo-Jack SE, Craft S, Cryer PE. Hypoglycemia-associated auto­
no

raft S, Cryer PE. Hypoglycemia-associated auto­
nomic failure in insulin-dependent diabetes mellitus. J Clin Invest. 1993;91:819–828, used with permission 
of the American Society for Clinical Investigation.)
50
10
0 
60 
120 
180 
240 
300 
5.6 
5.0 
4.4 
3.9 
3.3 
2.8 
50
40
40
30
30
20
20
10
Time (min) 
30
0 
60 
120 
180 
240 
300 
5.6 
5.0 
4.4 
3.9 
3.3 
2.8 
30
20
20
10
0
10
0
30
0
0 
60 
120 
180 
240 
300 
5.6 
5.0 
4.4 
3.9 
3.3 
2.8 
30
20
20
10
10
0
After hy

5.0 
4.4 
3.9 
3.3 
2.8 
30
20
20
10
10
0
Total symptoms 
symptoms
symptoms
• Fig. 38.9  Mean (± standard error) total, neurogenic, and neuroglycopenic symptom scores during hyper­
insulinemic stepped hypoglycemic glucose clamps in patients with type 1 (insulin-dependent) diabetes 
mellitus without classical diabetic autonomic neuropathy on mornings after afternoon hyperglycemia (After 
hyper., closed columns) and on mornings after afternoon hy

closed columns) and on mornings after afternoon hypoglycemia (After hypo., open columns). (From 
Dagogo-Jack SE, Craft S, Cryer PE. Hypoglycemia-associated autonomic failure in insulin-dependent dia­
betes mellitus. J Clin Invest. 1993;91:819–828, used with permission of the American Society for Clinical 
Investigation.)
1536
signaling of neurogenic symptoms remains to be demonstrated in 
patients with unawareness, and it would be nece

n 
patients with unawareness, and it would be necessary to also pos­
tulate decreased cholinergic sensitivity to explain reduced cholin­
ergic symptoms such as sweating.
In contrast to these compromised defenses in T1DM, defenses 
against hypoglycemia are intact early in the course of T2DM. 
However, they become compromised over time.1,79 In advanced 
T2DM (i.e., with absolutely deficient endogenous insulin), insu­
lin and glucagon responses to falling plasma glucose concentra­
tions are lost an

alling plasma glucose concentra­
tions are lost and sympathoadrenal responses to hypoglycemia 
are reduced by recent antecedent hypoglycemia, as is observed in 
T1DM.79
In summary, the pathophysiology of glucose counterregula­
tion is the same in T1DM and T2DM but with different time 
courses.67,79,80 The pathogenesis of an episode of iatrogenic hypo­
glycemia involves therapeutic hyperinsulinemia resulting in falling 
plasma glucose concentrations. With the absence of appropriate 
decrements in

ns. With the absence of appropriate 
decrements in insulin and of increments in glucagon, hypogly­
cemia occurs. That, in turn, causes attenuated sympathoadrenal 
responses to subsequent falling glucose levels and recurrent epi­
sodes of hypoglycemia (see Fig. 38.6). Because beta-cell failure, 
which causes loss of both insulin and glucagon responses, occurs 
rapidly in T1DM but slowly in T2DM, the syndromes of defec­
tive glucose counterregulation and impaired awareness of hypogly­
cemia develo

on and impaired awareness of hypogly­
cemia develop early in T1DM but later in T2DM. That temporal 
pattern of compromised glycemic defenses explains why iatrogenic 
hypoglycemia becomes progressively more frequent as patients 
approach the insulin-deficient end of the spectrum of T2DM.27 
HAAF in Diabetes
The concept of HAAF1,2,67,68,81 in diabetes developed in humans 
posits that in patients with absolute endogenous insulin deficient 
diabetes (T1DM or advanced T2DM), necessarily imperfect ins

(T1DM or advanced T2DM), necessarily imperfect insu­
lin replacement results in falling plasma glucose concentrations, 
but no decrease in insulin secretion and no increase in glucagon 
secretion, and thus recurrent episodes of hypoglycemia. Those 
episodes72 (as well as sleep74,75 or prior exercise73) attenuate 
adrenomedullary epinephrine secretion and sympathetic neural 
activation in response to subsequent hypoglycemia. In the set­
ting of absent insulin and glucagon responses, the attenuate

sent insulin and glucagon responses, the attenuated 
epinephrine responses cause the clinical syndrome of defective 
glucose counterregulation, which is associated with a 25-fold69 
or greater70 increased risk of severe iatrogenic hypoglycemia dur­
ing intensive glycemic therapy. The attenuated sympathetic neural 
responses cause the clinical syndrome of impaired awareness of 
hypoglycemia, which is associated with at least a sixfold increased 
risk of severe iatrogenic hypoglycemia during inten

isk of severe iatrogenic hypoglycemia during intensive glycemic 
therapy.71 The resulting recurrent hypoglycemia further attenuates 
the sympathoadrenal response to falling plasma glucose concen­
trations (see Fig. 38.6). HAAF is a functional form of autonomic 
failure, distinct from classic diabetic autonomic neuropathy.82 
HAAF has cardiovascular implications because, like autonomic 
neuropathy, it reduces baroreflex sensitivity62 and may predispose 
patients to cardiac arrhythmias.
Recent ant

spose 
Recent antecedent hypoglycemia, even asymptomatic noc­
turnal hypoglycemia, reduces epinephrine and symptomatic 
responses to a given level of subsequent hypoglycemia,83 reduces 
detection of hypoglycemia in the clinical setting,84 and reduces 
glycemic defense against hyperinsulinemia80 in T1DM. Per­
haps the most compelling support for the clinical relevance of 
HAAF is the finding, originally from three independent labora­
tories,85–88 that as little as

independent labora­
tories,85–88 that as little as 2 to 3 weeks of scrupulous avoidance 
of hypoglycemia reverses impaired awareness of hypoglycemia 
(Fig. 38.10) and improves the attenuated epinephrine component 
of defective glucose counterregulation in most affected patients. 
HAAF also occurs in advanced T2DM.79
There are three recognized causes of a reversibly attenuated 
sympathoadrenal response to hypoglycemia and, therefore, three 
forms of HAAF.1,2 Antecedent hypoglycemia–related HAAF l

of HAAF.1,2 Antecedent hypoglycemia–related HAAF led 
to the concept.72,80 Exercise-related HAAF73 is exemplified by late 
postexercise hypoglycemia, which typically occurs 6 to 15 hours 
after strenuous exercise and is often nocturnal.89,90 Sleep-related 
HAAF74,75 is the result of further attenuation of the sympatho­
adrenal response to hypoglycemia during sleep. Sleeping patients 
with T1DM are much less likely to be awakened by hypoglycemia 
than nondiabetic individuals,75 probably because o

than nondiabetic individuals,75 probably because of their attenu­
ated sympathoadrenal responses. There may well be additional, 
unrecognized causes of HAAF, and there may also be a structural 
(neuropathic) component.1,2
The integrated pathophysiologic mechanisms of HAAF are 
illustrated in Fig. 38.11. Loss of insulin and glucagon responses to 
falling plasma glucose concentrations caused by therapeutic hyper­
insulinemia is the result of beta-cell failure in T1DM and advanced 
T2DM.1,2,67 Neit

ell failure in T1DM and advanced 
T2DM.1,2,67 Neither can be attributed to loss of islet autonomic 
innervation, because low glucose concentrations decrease insulin 
secretion and increase glucagon secretion in patients with a trans­
planted (i.e., denervated) pancreas,91 in dogs with a denervated 
pancreas,92 and in isolated perfused pancreas and perifused islets. 
The mechanism of the attenuated sympathoadrenal response is 
symptom scores 
Time (min) 
20
20

cores 
Time (min) 
20
20
16
16
12
12
8
8
4
4
0
0
0 days 
3 days 
3–4 weeks 
3 months 
IDDM, unaware
5.6 
5.0 
4.4 
3.9 
3.3 
2.2 
2.8
0 
60 
120 
180 
240 
300 
360 
12
12
8
8
4
4
0
0
5.6 
5.0 
4.4 
3.9 
3.3 
2.2 
2.8
0 
60 
120 
180 
240 
300 
360 
• Fig. 38.10  Mean (± standard error) neurogenic and neuroglycopenic 
symptom scores during hyperinsulinemic stepped hypoglycemic glucose 
clamps in nondiabetic subjects (rectangle

glucose 
clamps in nondiabetic subjects (rectangles) and in people with type 1 dia­
betes mellitus (IDDM) selected for clinical impaired awareness of hypo­
glycemia studied at various time points during scrupulous avoidance of 
iatrogenic hypoglycemia: at baseline (0 days), after 3 days, after 3 to 4 
weeks, and after 3 months (see the key in the figure). IDDM, insulin-depen­
dent diabetes mellitus. (From Dagogo-Jack S, Rattarasarn C, Cryer PE. 
Reversal of hypoglycemia unawareness, but not defe

Reversal of hypoglycemia unawareness, but not defective glucose coun­
terregulation, in IDDM. Diabetes. 1994;43:1426–1434, used with permis­
sion of the American Diabetes Association.)
1537
not known, but it must involve the brain or the afferent or effer­
ent components of the sympathoadrenal system.67 The proposed 
mechanisms include the systemic mediator, brain fuel transport, 
brain metabolism, and cerebral network hypotheses.93–97 There 
is considerable evidence aga

potheses.93–97 There 
is considerable evidence against the first two of these proposed 
mechanisms.67 Among many potential mechanisms, an effect of 
lactate on brain metabolism has been studied98–100 and includes 
evidence of reduced glutamate release within the ventromedial 
nucleus of the rat hypothalamus.101
Much of the fundamental research into the mechanism of 
HAAF has focused on the hypothalamus, the central integrator of 
the sympathoadrenal response to hypoglycemia. It is conceivable, 

nal response to hypoglycemia. It is conceivable, 
however, that the changes in the hypothalamic/sympathoadrenal 
response reflect modulation by higher brain centers.67,95–97 For 
example, measurements of regional cerebral blood flow using oxy­
gen-15–labeled water and positron emission tomography indicate 
that hypoglycemia increases synaptic activity in widespread but 
interconnected brain regions in humans95 and that recent ante­
cedent hypoglycemia both reduces sympathoadrenal and symp­
tomat

cemia both reduces sympathoadrenal and symp­
tomatic responses (a model of HAAF) and increases synaptic 
activation in the dorsal midline thalamus (and only in that brain 
region) during subsequent hypoglycemia.96,97 Therefore, it has 
been suggested that there may be a cerebral network that results in 
thalamic inhibition of hypothalamic activity in HAAF.96,97 Such 
a putative cerebral network is included in Fig. 38.11, although it 
remains theoretical. 
Risk Factors for Hypoglycemia in Diabete

retical. 
Risk Factors for Hypoglycemia in Diabetes
The risk factors for iatrogenic hypoglycemia102 (Table 38.6) follow 
directly from the pathophysiology of glucose counterregulation 
in diabetes and are based on the tenet that iatrogenic hypogly­
cemia is typically the result of the interplay of relative or abso­
lute therapeutic insulin excess and compromised physiologic and 
behavioral defenses against falling plasma glucose concentrations 
(i.e., HAAF) in T1DM and advanced T2DM, as discusse

i.e., HAAF) in T1DM and advanced T2DM, as discussed earlier.
Absolute or Relative Insulin Excess
The conventional risk factors for hypoglycemia in diabetes1,102 
(see Table 38.6) are based on the premise that absolute or relative 
insulin excess is the sole determinant of risk. Absolute therapeutic 
insulin excess occurs when doses of insulin or insulin secretagogue 
are excessive, ill timed, or of the wrong type, or when the clearance 
Pathophysiology in HAAF
cortex
Orbital pr

iology in HAAF
cortex
cortex
thalamus
Brain stem
Amygdala
  Attenuated   Sympatho-
adrenal activity
+
No   Insulin + No   Glucagon
β-cells
Insulin
Glucose
α-cells
Glucagon
• Fig. 38.11  Schematic representation of the integrated mechanisms of hypoglycemia-associated auto­
nomic failure (HAAF) in diabetes. Compare with Fig. 38.5; see the text for discussion. α-cells, pancreatic 
islet alpha cells; β-cel

ion. α-cells, pancreatic 
islet alpha cells; β-cells, pancreatic islet beta cells. (From Cryer PE. The barrier of hypoglycemia in diabetes. 
Diabetes. 2008;57:3169–3176 and Cryer PE. Hypoglycemia in Diabetes: Pathophysiology, Prevalence and 
Prevention. Alexandria, VA: American Diabetes Association; 2009, used with permission of the American 
Diabetes Association.)
  Risk Factors for Hypoglycemia in Diabetes
Conventional Risk Factors: Absolute or Relative Insulin 
Excess
Insulin or insulin secr

r Relative Insulin 
Excess
Insulin or insulin secretagogue doses are excessive, ill timed, or of the 
wrong type
Exogenous glucose delivery is decreased (e.g., after missed meals, dur­
ing the overnight fast)
Glucose utilization is increased (e.g., during exercise)
Endogenous glucose production is decreased (e.g., after alcohol ingestion)
Sensitivity to insulin is increased (e.g., after weight loss, with improved 
fitness or improved glycemic control, in the middle of the night)
Insulin clearanc

trol, in the middle of the night)
Insulin clearance is decreased (e.g., with renal failure)
Risk Factors for Hypoglycemia-Associated Autonomic 
Failure
Absolute endogenous insulin deficiency
A history of severe hypoglycemia, impaired awareness of hypoglycemia, 
or both, and recent antecedent hypoglycemia, prior exercise, or sleep
Aggressive glycemic therapy per se (lower HbA1c levels, lower glycemic 
goals, or both)
HbA1c, Glycosylated hemoglobin.
   
TABLE 38.6
1538
SECTION VIII    Disorders of

.
   
TABLE 38.6
1538
of insulin is decreased (as in renal failure). Relative insulin excess 
occurs when exogenous glucose delivery is reduced, glucose utili­
zation is increased, endogenous glucose production is decreased, 
or sensitivity to insulin is increased. Persons with diabetes and 
their caregivers must consider these risk factors, including each of 
the examples listed in Table 38.6, when iatrogenic hypoglycemia 
is recogniz

le 38.6, when iatrogenic hypoglycemia 
is recognized to be a problem. However, these factors explain only 
a minority of episodes.103
Clinical risk factors for hypoglycemia include the presence of 
HAAF (including both defective glucose counterregulation and 
impaired awareness of hypoglycemia), a history of severe hypo­
glycemia, chronic kidney disease, a long duration of diabetes, and 
malnutrition.1 
Risk Factors for HAAF
The risk factors for HAAF102,104–109 (see Table 38-6) include the 
foll

re determines 
the extent to which insulin levels will not decrease and glucagon 
levels will not increase as glucose levels fall in response to thera­
peutic hyperinsulinemia. The importance of loss of a decrease in 
insulin as glucose levels decline in response to therapeutic hyper­
insulinemia is underscored by the fact that patients with T1DM 
and some degree of preserved insulin secretion have a lower inci­
dence of hypoglycemia.110 A history of severe hypoglycemia indi­
cates (and that of 

y of severe hypoglycemia indi­
cates (and that of impaired awareness of hypoglycemia implies) 
recent antecedent hypoglycemia. Like prior exercise or sleep, 
recent antecedent hypoglycemia causes attenuated sympathoadre­
nal and symptomatic responses to subsequent hypoglycemia, the 
key feature of HAAF. Aggressive glycemic therapy is a risk factor 
for hypoglycemia. That does not mean, however, that one cannot 
both improve glycemic control and minimize the risk of hypogly­
cemia in many patient

inimize the risk of hypogly­
cemia in many patients102 (see later discussion). 
Prevention of Hypoglycemia in Diabetes
Hypoglycemia risk factor reduction102,111 (Table 38.7) is an 
empiric approach to minimizing the risk of iatrogenic hypoglyce­
mia while maintaining or improving glycemic control in persons 
with diabetes. It involves four steps: acknowledge the problem, 
apply the principles of aggressive glycemic therapy,102 consider the 
conventional risk factors, and consider the risk factor

ntional risk factors, and consider the risk factors for HAAF 
(see Table 38.6).
Acknowledge the Problem
The problem of hypoglycemia should be addressed, at least in per­
sons treated with an insulin secretagogue or with insulin, in every 
patient contact. Acknowledging the issue allows the caregiver to 
move on if hypoglycemia is not a problem, or to deal with it and 
keep it in perspective if hypoglycemia is a problem. Some patients 
are reluctant to raise the issue, but their concerns about th

nt to raise the issue, but their concerns about the real­
ity, or even the possibility, of hypoglycemia can be a barrier to 
glycemic control.112–114 If possible, close associates of the patient 
should be asked whether they have observed clues to episodes 
not recognized by the patient. Even if no concerns are expressed, 
review of the record of self-monitoring of blood glucose (or CGM 
data) may disclose that hypoglycemia is a problem. 
Apply the Principles of Aggressive Glycemic Therapy
These

he Principles of Aggressive Glycemic Therapy
These principles111 include diabetes self-management by a well-
informed patient, frequent self-monitoring of blood glucose levels 
(and in some instances CGM), flexible and appropriate insulin 
and other drug regimens, individualized glycemic goals, and 
ongoing professional guidance and support (see Table 38.7).
As the therapeutic regimen becomes progressively more com­
plex—early in T1DM and later in T2DM—successful glycemic 
management becomes pro

n T2DM—successful glycemic 
management becomes progressively more dependent on the many 
decisions and the skills of a well-informed person with diabetes. 
Therefore, patient education and empowerment are fundamen­
tally important. Patients treated with insulin secretagogues or 
insulin need to know the common symptoms of hypoglycemia, 
their individual most meaningful symptoms, and how to treat 
(and not overtreat) an episode. They need to know the relevant 
conventional risk factors for hypogl

the relevant 
conventional risk factors for hypoglycemia, including the tem­
poral patterns of the glucose-lowering actions of their individual 
secretagogue or insulin preparations and the effects of missed 
meals, the overnight fast, exercise, and alcohol ingestion. They 
also need to know that increasing episodes of hypoglycemia signal 
an increased likelihood of future, often more severe, hypoglyce­
mia.105–107 Close associates also need to know how to recognize 
hypoglycemia, to understand 

now how to recognize 
hypoglycemia, to understand why a neuroglycopenic patient may 
become uncooperative, and to know when and how to administer 
glucagon. Finally, patients need to learn to apply the data from 
their self-monitoring of blood glucose concentrations (or from 
CGM) toward the goal of minimizing hypoglycemia (as well as 
hyperglycemia). Structured education focused on avoidance of 
hypoglycemia can restore awareness of hypoglycemia and reduce 
the incidence of severe hypoglycemia 

 and reduce 
the incidence of severe hypoglycemia without deterioration of gly­
cemic control.115–117
Frequent self-monitoring also becomes vital to diabetes self-
management as the therapeutic regimen becomes more complex, 
early in T1DM and later in T2DM. Ideally, patients should moni­
tor their glucose level whenever they suspect hypoglycemia. That 
would confirm or refute an episode and help the patients learn 
their individual key symptoms and might lead to regimen adjust­
ments. It is impo

nd might lead to regimen adjust­
ments. It is important for patients with impaired awareness of 
hypoglycemia to monitor glucose levels before performing critical 
tasks such as driving. Unfortunately, conventional blood glucose 
monitoring, in which a measurement is obtained at one point in 
time, does not indicate whether the glucose level is falling, stable, 
or rising. That shortcoming is being addressed by evolving tech­
nologies for CGM.118,119
  Hypoglycemia Risk Factor Reductiona
	1.	 

al risk factors for hypoglycemia.a
	4.	 Consider the risk factors for HAAF.a
aSee Table 38.6.
HAAF, Hypoglycemia-associated autonomic failure in diabetes.
   
TABLE 38.7
1539
Despite considerable enthusiasm for continuous subcutaneous 
insulin infusion (CSII) over multiple daily injections (MDI) of 
insulin,120 one meta-analysis121 disclosed no significant difference 
in minor or severe hypoglycemia events with CSII or MDI, and 
a randomized trial122 that specifically i

DI, and 
a randomized trial122 that specifically included structured patient 
education in both groups revealed no difference in severe hypogly­
cemia with CSII or MDI.
There is convincing evidence that insulin analogues reduce the 
incidence of severe hypoglycemia in patients with diabetes who 
are at high risk for hypoglycemia123 that includes both rapid-
acting prandial insulins (lispro, aspart) and the long-acting basal 
insulins degludec124 and glargine U-300,125 both compared with 
glargin

and glargine U-300,125 both compared with 
glargine U-100 once daily.
There is increasing evidence that hybrid closed-loop insulin 
replacement systems, specifically those that temporarily suspend 
insulin infusion when a low glucose concentration is predicted, 
reduce hypoglycemia in T1DM.126 Pancreatic islet transplanta­
tion prevents hypoglycemia, as long as the transplant is functional, 
and improves glucose counterregulation.127
Therapeutic hyperinsulinemia, a prerequisite for iatrogenic 
h

hyperinsulinemia, a prerequisite for iatrogenic 
hypoglycemia, can occur during treatment with an insulin secreta­
gogue (e.g., a sulfonylurea or a glinide) or with insulin. Early in its 
course, T2DM may respond to drugs that do not raise insulin lev­
els at normal or low plasma glucose concentrations and therefore 
should not, and probably do not,1,128 cause hypoglycemia. Such 
drugs include the biguanide metformin, sodium-glucose cotrans­
porter 2 inhibitors, thiazolidinediones, α-glucosidase

er 2 inhibitors, thiazolidinediones, α-glucosidase inhibitors, 
glucagon-like peptide 1 (GLP1) receptor agonists, and dipeptidyl 
peptidase 4 inhibitors. All of these drugs require endogenous insu­
lin secretion to lower plasma glucose concentrations, and insulin 
secretion declines appropriately as glucose levels fall into the nor­
mal range. That is true even for the GLP1 receptor agonists and 
dipeptidyl peptidase 4 inhibitors, which enhance glucose-stimu­
lated insulin secretion (among other

lucose-stimu­
lated insulin secretion (among other actions). Nonetheless, all six 
categories of drugs increase the risk of hypoglycemia if used with 
an insulin secretagogue or insulin.
Among the commonly prescribed sulfonylureas, hypogly­
cemia is more often associated with the longer-acting glyburide 
(glibenclamide) than with the shorter-acting glimepiride.129 
Despite some advantages,123–125 insulin analogues can cause 
hypoglycemia.130,131
As reviewed in detail elsewhere,132 there is compe

As reviewed in detail elsewhere,132 there is compelling evidence 
from randomized controlled trials that tight glycemic control par­
tially prevents or delays microvascular complications (retinopathy, 
nephropathy, neuropathy) in T1DM and T2DM. As reviewed 
there and developed earlier in this chapter, there is also compel­
ling evidence that tight glycemic control increases hypoglycemic 
morbidity and mortality risks. Thus, it follows that the selection 
of a glycemic goal should be linked to th

lection 
of a glycemic goal should be linked to the risk of hypoglycemia.132
Given the premise that glycemic goals in diabetes are a trade-
off between glycemic control and iatrogenic hypoglycemia,132 it 
has been suggested that a reasonable individualized glycemic goal 
is the lowest HbA1c that does not cause severe hypoglycemia and 
preserves awareness of hypoglycemia, preferably with little or no 
symptomatic or even asymptomatic hypoglycemia, at a given stage 
in the evolution of the individ

 at a given stage 
in the evolution of the individual’s diabetes. Parenthetically, the 
substantial relationship between a lower HbA1c level and a higher 
incidence of severe hypoglycemia has been consistently docu­
mented in randomized control clinical trials in both T1DM28,133 
and T2DM.13,43,52,134,135 In these trials, when patients with diabe­
tes were randomly assigned to intensive glycemic therapy, largely 
with insulin, and shown to have lower HbA1c levels or to more 
conventional glycemi

ower HbA1c levels or to more 
conventional glycemic goals and shown to have higher HbA1c lev­
els, the incidence of severe hypoglycemia was two- to threefold 
higher in each of the groups with the lower HbA1c levels. The fre­
quency of hypoglycemia was inversely related to the HbA1c level 
in both the original DCCT and in the follow-up Epidemiology 
of Diabetes Intervention and Complications (EDIC) phase,28,133 
although the slope was less steep in the EDIC phase. The extent 
to which the latter

in the EDIC phase. The extent 
to which the latter is the result of insulin analogues, improved 
insulin delivery, glucose monitoring, patient education, patient or 
caregiver skill, or some other fact is not known.
There is increasing evidence that hypoglycemia is proarrhyth­
mogenic.136–138 Holter monitoring during CGM detected epi­
sodes of iatrogenic hypoglycemia has detected runs of cardiac 
arrhythmias ranging from ventricular tachycardia136 to brachycar­
dia,137 and repolarization abnorma

6 to brachycar­
dia,137 and repolarization abnormalities have been identified in 
persons with diabetes.138 Thus, fatal cardiac arrhythmias almost 
assuredly underlie hypoglycemic cardiovascular mortality in 
diabetes.
In an extensive review, with the exception of a 15% reduc­
tion of nonfatal myocardial infarction, Rodriguez-Gutierrez and 
Montori139 found no significant impact of tight glycemic control 
of T2DM on outcomes important to patients—end stage renal 
disease/dialysis, renal death, b

—end stage renal 
disease/dialysis, renal death, blindness, clinical neuropathy, car­
diovascular or all-cause mortality, stroke, amputation, or periph­
eral vascular disease. They did find a two- to threefold increase in 
severe hypoglycemia during intensive therapy. The authors con­
cluded that the overwhelming consensus in favor of tight glycemic 
control to prevent complications needs to be recalibrated. There 
is evidence of an association between mortality and substantially 
elevated HbA1c

etween mortality and substantially 
elevated HbA1c levels in T1DM.140,141 In a 27-year follow-up of 
DCCT patients, a rise in mortality above that of the US general 
population began only with a HbA1c level greater than 9% (75 
mmol/mol).140 Analysis of a much larger dataset141 disclosed the 
same finding. Notably, 8% of all of the deaths of DCCT/EDIC 
patients attributed to hypoglycemia and hypoglycemia with sei­
zure, coma, or both were associated with mortality.52 At 30 years 
of follow-up, p

ted with mortality.52 At 30 years 
of follow-up, previous intensive glycemic therapy (i.e., that during 
the DCCT) did not reduce major cardiovascular events (nonfatal 
myocardial infarctions, stroke, or cardiovascular death) signifi­
cantly, although the trend was in that direction.142 Given these 
data,52,139–142 it would seem that the overwhelming consensus in 
favor of tight glycemic control for the prevention of macrovascu­
lar complications,143 like that for neurovascular complications,139

,143 like that for neurovascular complications,139 
is stronger than the evidence to support it. However, these tri­
als antedated T2DM drugs that do not cause hypoglycemia, such 
as GLP1 receptor agonists and sodium-glucose cotransporter 
2 inhibitors, and newer technologies, including CGM, which 
undoubtedly will reduce the risk of intensive glycemic therapy in 
patients who can use them.
Clearly, one should strive to balance benefits and harms in the 
selection of a glycemic goal for a person

 in the 
selection of a glycemic goal for a person with diabetes. Many per­
sons with diabetes—those with HAAF, a history of severe hypo­
glycemia, chronic kidney disease, a long duration of diabetes, or 
malnutrition—are at risk of harm from iatrogenic hypoglycemia. 
Those with chronic vascular complications, serious comorbidities, 
or a short life expectancy have little likelihood of benefit. In both 
of these groups, a less stringent glycemic goal is indicated,144 per­
haps an HbA1c less than

goal is indicated,144 per­
haps an HbA1c less than 8.5% (69 mmol/mol).145
Finally, because the glycemic management of diabetes is 
empiric, caregivers should work with each patient to find the most 
effective and safest regimen at a given point in the course of that 
patient’s diabetes. 
1540
Consider the Conventional Risk Factors
The conventional risk factors are those that result in relative, as 
well as absolute, insulin excess (see

elative, as 
well as absolute, insulin excess (see Table 38.6). In addition to 
insulin secretagogue doses, timing, and type, they include condi­
tions in which exogenous glucose delivery or endogenous glucose 
production is decreased, glucose utilization or sensitivity to insu­
lin is increased, or insulin clearance is decreased. 
Consider the Risk Factors for HAAF
The risk factors indicative of HAAF (see Table 38.6) include the 
degree of endogenous insulin deficiency; a history of severe hypo

enous insulin deficiency; a history of severe hypo­
glycemia, impaired awareness of hypoglycemia, or both, and recent 
antecedent hypoglycemia, prior exercise, or sleep; and aggressive 
glycemic therapy per se. An episode of severe hypoglycemia is a 
clinical red flag. Unless the cause is easily explainable, it should 
prompt consideration of a fundamental change in the therapeutic 
regimen. Without such a change, the risk of a subsequent episode 
of severe hypoglycemia is high.105–107 In a pati

of severe hypoglycemia is high.105–107 In a patient with impaired 
awareness of hypoglycemia, patient reeducation115–117 and a 2- to 
3-week period of scrupulous avoidance of hypoglycemia is advisable 
and can be expected to restore awareness.85–88 This approach usu­
ally requires somewhat higher glycemic goals in the short term. A 
history of late postexercise hypoglycemia, nocturnal hypoglycemia, 
or both should prompt appropriately timed regimen adjustments 
to provide more carbohydrate int

imen adjustments 
to provide more carbohydrate intake, less insulin action, or both. 
Treatment of Hypoglycemia in Diabetes
Most episodes of symptomatic hypoglycemia or of asymptomatic 
hypoglycemia detected by self-monitoring or CGM are effectively 
self-treated by ingestion of glucose tablets or carbohydrates.1 A 
reasonable dose is 20 g of glucose.1 A meta-analysis indicated 
that glucose tablets result in a higher rate of relief of symptomatic 
hypoglycemia than dietary sugar.146 Clinical im

c 
hypoglycemia than dietary sugar.146 Clinical improvement should 
occur in 15 to 20 minutes. The temptation to overtreat is under­
standable but should be avoided. With ongoing hyperinsulinemia, 
the glycemic response to oral glucose is transient, typically lasting 
less than 2 hours (Fig. 38.12). Therefore, ingestion of a snack or 
meal shortly after the glucose level is raised is usually advisable.
In a patient who is unable or unwilling (because of neurogly­
copenia) to take carbohydrate or

ause of neurogly­
copenia) to take carbohydrate orally, parenteral therapy is neces­
sary. Glucagon, in a dose of 1.0 mg in adults, can be injected 
subcutaneously or intramuscularly by an associate of the patient. 
Administration of glucagon can be lifesaving, but it often causes 
substantial, albeit transient, hyperglycemia (see Fig. 38.12), and 
it can cause nausea and even vomiting. Smaller doses of glucagon 
(e.g., 150 μg), repeated if necessary, have been found to be effec­
tive without si

sary, have been found to be effec­
tive without side effects.147 For the prompt treatment of episodes 
of severe iatrogenic hypoglycemia and particularly for infusion, 
with insulin, in a bihormonal artificial pancreas, a glucagon ana­
logue that is stable in solution and that retains the rapid plasma 
glucose–raising action has been developed. That preparation147 
and a nasal glucagon preparation148 represent major improvement 
for the treatment of hypoglycemia with glucagon because they 
simpl

 of hypoglycemia with glucagon because they 
simplify treatment.
Because it acts by stimulating glycogenolysis, glucagon is inef­
fective in glycogen-depleted individuals (e.g., after a binge of alco­
hol ingestion). Although it is not an issue in T1DM or advanced 
T2DM, glucagon stimulates insulin secretion. Indeed, glucagon 
has been reported to cause hypoglycemia in nondiabetic individu­
als (see later discussion). Glucagon can be administered intrave­
nously by medical personnel; however, in

 intrave­
nously by medical personnel; however, in that setting, intravenous 
(IV) glucose (dextrose) is the standard parenteral therapy. A com­
mon initial dose is 25 g, but lower doses are generally advisable to 
avoid posthypoglycemic hyperglycemia.13
The glycemic response to IV glucose is, of course, transient in 
the setting of ongoing hyperinsulinemia. Therefore, IV glucose 
administration often needs to be followed by glucose infusion 
and, once it is practical, by carbohydrate feeding.
T

, once it is practical, by carbohydrate feeding.
The duration of an episode of severe hypoglycemia is a func­
tion of its cause. An episode caused by a rapid-acting insulin secre­
tagogue or insulin analogue will be relatively brief. That caused by 
a long-acting insulin secretagogue or insulin analogue can result in 
prolonged hypoglycemia requiring hospitalization.
Finally, research on the development of glucose-responsive insu­
lin formulations that would not cause hypoglycemia continues.149 

s that would not cause hypoglycemia continues.149 
Hypoglycemia in Persons Without Diabetes
The Decision to Evaluate for Hypoglycemia
Although hypoglycemia is common in persons with diabetes,1 it is 
a distinctly uncommon clinical event in persons who do not have 
diabetes2,150 because of the effectiveness of the normal physiologic 
and behavioral defenses against falling plasma glucose concentra­
tions. Therefore, in the absence of diabetes, a thorough evalua­
tion for hypoglycemia is recommend

horough evalua­
tion for hypoglycemia is recommended only for patients in whom 
Whipple triad (see Table 38.1) can be documented. In the absence 
of such documentation, evaluation for hypoglycemia may expose 
the patient to unnecessary evaluation, costs, and potential harm 
without expectation of benefit.2
Plasma glucose concentrations used to document Whipple 
triad must be measured with a reliable laboratory method and not 
with a blood glucose self-monitor. A reliably measured low plasma 
glu

 self-monitor. A reliably measured low plasma 
glucose concentration obtained in the absence of recognized 
mmol/L 
13.0
12.0
11.0 
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
mg/dL 
240
220
200
180
160
140
120
100
80
60
40
Glucose
10 g PO
20 g PO
Glucose
1.0 mg SC
Glucagon
Insulin
0
Time (hours) 
1 
2 
3 
4 
5 
6 
7 
8 
IDDM
• Fig. 38.12  Mean (± standard error) plasma glucose concentrations during 
hypoglycemia produced by subcutaneous insulin injection in people with 
type 1 diabetes mellitus in respons

n people with 
type 1 diabetes mellitus in response to 10 g (circles) or 20 g (squares) of 
oral (PO) glucose or 1.0 mg of subcutaneous (SC) glucagon (triangles) 
compared with placebo (shaded area). IDDM, insulin-dependent diabetes 
mellitus. (From Wiethop BV, Cryer PE. Alanine and terbutaline in treatment 
of hypoglycemia in IDDM. Diabetes Care. 1993;16:1131–1136, used with 
1541
symptoms or signs should not be ignored.

oglycemia
symptoms or signs should not be ignored. However, such a finding 
raises the possibility of “pseudohypoglycemia,” an artifact of con­
tinued glucose metabolism by the formed elements of the blood 
after the blood sample is drawn.2 This may occur if the sample 
is collected in a container that does not include an inhibitor of 
glycolysis, such as sodium fluoride or ethylenediaminetetraacetic 
acid-citrate (EDTA), and separation of the plasma or serum from 
the formed elements is delayed

asma or serum from 
the formed elements is delayed, particularly in the setting of eryth­
rocytosis, leukocytosis, or thrombocytosis.
Venous sampling is standard in the clinical setting, but it is the 
arterial plasma glucose concentration that fuels the brain. Arte­
riovenous plasma glucose concentration differences are negligible 
in the postabsorptive state, but antecubital venous glucose levels 
are as much as one third lower than arterial glucose levels when 
insulin secretion is increased 

ucose levels when 
insulin secretion is increased (e.g., after an oral glucose load) stim­
ulating glucose extraction across the forearm.151 Because of the 
provision of alternative fuels (specifically ketones) to the brain, 
plasma glucose concentrations lower than the overnight fasted 
physiologic range occur in healthy individuals, especially women 
and children, during extended fasting.3 Finally, the glycemic 
thresholds of responses to hypoglycemia shift to lower plasma glu­
cose concentrat

lycemia shift to lower plasma glu­
cose concentrations in patients with recurrent hypoglycemia.21,22 
For all of these reasons, it is important to document Whipple triad 
before concluding that a hypoglycemic disorder exists in a per­
son without diabetes.2 However, a reliably measured, unequivo­
cally normal plasma glucose concentration (e.g., >70 mg/dL [3.9 
mmol/L]) during a symptomatic episode provides strong evidence 
that the symptoms were not the result of hypoglycemia.2 
Clinical Classif

not the result of hypoglycemia.2 
Clinical Classification of Hypoglycemic Disorders
The traditional classification of hypoglycemic disorders in non­
diabetic persons, as either postabsorptive (fasting) or postprandial 
(reactive) hypoglycemias, has been supplanted by a clinical cat­
egorization. This distinguishes a patient who has a relevant disease 
or treatment from a patient who is otherwise seemingly well2,152 
(see Table 38.4). The presence of postprandial symptoms without 
Whipple triad (

 of postprandial symptoms without 
Whipple triad (see Table 38.1), previously called reactive hypogly­
cemia, is now considered a functional disorder in which symptoms 
are not due to hypoglycemia and for which an oral glucose toler­
ance test is not indicated.2
Drugs are the most common cause of hypoglycemia.153–159 In addi­
tion to insulin secretagogues and insulin (discussed earlier), offend­
ing drugs include alcohol, among many others154–158 (Table 38.8). 
Drugs,

ol, among many others154–158 (Table 38.8). 
Drugs, often in the setting of critical illnesses including renal fail­
ure, are the most common cause of hypoglycemia in hospitals.153 
Again, insulin or insulin secretagogues are common offending 
drugs,153,160 particularly if they are administered when enteral or 
parenteral nutrition is interrupted.
Ethanol inhibits gluconeogenesis. Clinical alcohol-induced 
hypoglycemia typically follows a binge of alcohol consumption 
during which the person eats

 alcohol consumption 
during which the person eats little food (i.e., in the setting of gly­
cogen depletion).155 Alcohol-induced hypoglycemia can be fatal, 
but with restoration of euglycemia and supportive care, recovery 
is the rule. Ethanol is usually measurable in blood at the time of 
presentation.
Hypoglycemia sometimes occurs in patients with critical ill­
nesses2 (see Table 38.4). Hepatogenous hypoglycemia occurs most 
commonly when destruction of the liver is rapid and massive (e.g., 

ruction of the liver is rapid and massive (e.g., 
in toxic hepatitis). It is unusual in common forms of cirrhosis or 
hepatitis, although glucose metabolism is measurably altered in 
uncomplicated viral hepatitis.161 Hypoglycemia is also unusual in 
metastatic liver disease despite extensive hepatic replacement.162 
The pathogenesis of hypoglycemia in some patients with renal fail­
ure is unknown and likely multifactorial; it has been attributed to 
drugs, sepsis, or inanition.153,163–165 Reduce

to 
drugs, sepsis, or inanition.153,163–165 Reduced renal clearance of 
insulin and reduced renal glucose production might be relevant 
factors. However, renal transplantation did not correct hypoglyce­
mia in patients with glucose-6-phosphatase deficiency.166
The pathogenesis of the hypoglycemia occasionally seen in 
patients with severe cardiac failure is also not understood. The 
finding of elevated blood lactate levels associated with hypogly­
cemia167 raises the possibility of inhibited glu

­
cemia167 raises the possibility of inhibited gluconeogenesis. Sepsis 
is a relatively common cause of hypoglycemia.153,154,168 Increased 
glucose utilization (by skeletal muscle and by macrophage-rich tis­
sues, e.g., liver, spleen, and lung), which is thought to be cytokine 
mediated and is matched initially by increased glucose produc­
tion, characterizes experimental sepsis.169–171 The later decline 
in glucose production, which in the setting of persistently high 
glucose utilization resul

ng of persistently high 
glucose utilization results in hypoglycemia, is not the result of 
glucose counterregulatory failure; rather, it is caused by decreased 
responsiveness to appropriate glucose counterregulatory signals 
(i.e., low insulin and high glucagon and epinephrine levels).172 
  Drugs, Other Than Antihyperglycemic 
Agents and Alcohol, Reported to Cause 
Moderate Quality of Evidence
Cibenzoline
Gatifloxacin
Quinine
Indomethacin
Glucagon (during endoscopy)

Quinine
Low Quality of Evidence
Chloroquinoxaline sulfonamide
Artesunate/artemisinin/artemether
Lithium
Propoxyphene/dextropropoxyphene
Very Low Quality of Evidence
>25 Cases Identified
Angiotensin-converting enzyme inhibitors
Angiotensin receptor antagonists
β-Adrenergic receptor antagonists
Levofloxacin
Mifepristone
Disopyramide
Trimethoprim-sulfamethoxazole
Heparin
6-Mercaptopurine
<25 Cases Identified
See Murad et al

ercaptopurine
See Murad et al.,157 2009
2009;94:709–728; based on Murad MH, Coto-Yglesias F, Wang AT, et al. Drug-induced hypo­
glycemia: a systematic review. J Clin Endocrinol Metab. 2009;94:741–745, with permission 
of the Endocrine Society.
   
TABLE 38.8
1542
SECTION VIII    Disorders of Car

TABLE 38.8
1542
Hypoglycemia can be caused by inanition.173 A plausible sugges­
tion is that glucose becomes the sole oxidative fuel in the setting 
of total body fat depletion, and the resulting high rates of glucose 
utilization exceed the capacity to produce glucose because of a 
limited supply of gluconeogenic precursors (e.g., amino acids). 
Postabsorptive hypoglycemia (with low circulating alanine con­
centrations) has been rep

circulating alanine con­
centrations) has been reported in patients with profound muscle 
atrophy174,175; hypoglycemia is presumably the result of substrate 
limitation of gluconeogenesis in such patients.
With the notable exception of HAAF in patients with T1DM 
and advanced T2DM (discussed earlier), hormone deficiencies 
resulting in hypoglycemia are not common. Postabsorptive hypo­
glycemia, typically after a period of caloric deprivation caused by 
an intercurrent illness, can occur in patie

ed by 
an intercurrent illness, can occur in patients with deficient secre­
tion of cortisol, growth hormone, or both, particularly infants and 
young children.176–178 Glycemic intolerance of fasting is largely 
corrected by glucocorticoid replacement; growth hormone replace­
ment has a lesser effect.177,178 Because cortisol normally supports 
gluconeogenesis by increasing gluconeogenic enzyme activities 
and mobilizing gluconeogenic precursors,177,179 the hypoglyce­
mic mechanism is thought to 

77,179 the hypoglyce­
mic mechanism is thought to be reduced glucose production in 
the setting of glycogen deficiency. Nonetheless, most adults with 
deficiencies of these hormones do not experience hypoglycemia. 
Indeed, plasma glucose concentrations and rates of endogenous 
glucose production after overnight fasting have been reported to 
be indistinguishable from normal values in short-term glucocorti­
coid-withdrawn patients with hypopituitarism never treated with 
growth hormone.180 Hypogl

rism never treated with 
growth hormone.180 Hypoglycemia has been reported in adreno­
corticotropic hormone–deficient adults when glucose utilization 
or loss was increased (as during exercise or pregnancy, respec­
tively)181 or when glucose production is impaired (as after alcohol 
ingestion).182
Non–islet cell tumor hypoglycemia (NICTH) is rare. The 
tumors are usually, but not invariably, large, clinically apparent, 
and mesenchymal in origin. NICTH is often the result of over­
production of 

 NICTH is often the result of over­
production of incompletely processed pro–insulin-like growth 
factor 2 (pro-IGF2),183–186 but hypoglycemia attributed to over­
production of insulin-like growth factor 1 (IGF1) has also been 
reported.187 The pro-IGF2 binds poorly to its binding proteins 
and therefore more freely enters tissue spaces, where its insulin-
like actions cause hypoglycemia. Concentrations of plasma free 
IGF2 (or IGF1187) are elevated.184 Because of suppression of 
growth hormone 

ted.184 Because of suppression of 
growth hormone secretion and the resulting low IGF1 levels, the 
ratio of plasma IGF2 to IGF1 is elevated in pro-IGF2–mediated 
hypoglycemia. Plasma total IGF2 levels may be within the nor­
mal range, but the ratio of pro-IGF2 to IGF2 may be elevated.185 
Endogenous insulin secretion is suppressed appropriately during 
hypoglycemia in NICTH. Treatment of the tumor is seldom cura­
tive but may alleviate hypoglycemia. Treatment with a glucocorti­
coid, growth hor

mia. Treatment with a glucocorti­
coid, growth hormone, or both is sometimes effective. 
In seemingly well individuals with no evidence of drug, critical 
illness, hormone deficiency, or non–islet cell tumor as a cause of 
their hypoglycemia, the differential diagnosis narrows to two cat­
egories: (1) accidental, surreptitious, or even malicious hypoglyce­
mia and (2) endogenous hyperinsulinism2,150,152,188–190 (see Table 
38.4). Consideration of the former possibility 

e 
38.4). Consideration of the former possibility should precede a 
systematic assessment of the latter. Medical, pharmacy, and hospi­
tal errors can result in hypoglycemia and do occur. Surreptitious 
hypoglycemia188–192 is more common in people with knowl­
edge of, and access to, glucose-lowering medications. Malicious 
hypoglycemia188,189 can be accomplished by administration of an 
insulin secretagogue or insulin.
Insulinomas (insulin-secreting pancreatic beta-cell tumors) are 
the prototypi

ng pancreatic beta-cell tumors) are 
the prototypical, but not the only, cause of endogenous hyperin­
sulinemic hypoglycemia.2,150,193–197 Patients with an insulinoma 
typically present with a history of episodes of neuroglycopenia 
occurring in the postabsorptive (fasting) state. However, an appre­
ciable subset of patients (6% in one series193) report symptoms 
exclusively in the postprandial state. Insulinomas are rare; an 
incidence of 1 in 250,000 patient-years has been reported. Less 
than

50,000 patient-years has been reported. Less 
than 10% of the patients have malignant insulinomas, multiple 
insulinomas, or multiple endocrine neoplasia type 1 (MEN1) syn­
drome.197 Long-term survival is the rule after successful surgical 
removal of an insulinoma.197
Some patients (4% of one series197) with fasting endogenous 
hyperinsulinemic hypoglycemia do not have an insulinoma but 
have diffuse islet involvement with islet hypertrophy, sometimes 
with hyperplasia, and enlarged and hyperch

etimes 
with hyperplasia, and enlarged and hyperchromatic beta-cell 
nuclei. This condition is often termed nesidioblastosis, although 
the histologic finding of islets budding from pancreatic ducts 
is not invariably present. Such patients are clinically indistin­
guishable from those with an insulinoma.198–202 Other patients 
have noninsulinoma pancreatogenous hypoglycemia syndrome 
(NIPHS)202–205 or post–gastric bypass hypoglycemia.206–216
NIPHS202–205 is characterized by spells of neuroglyco

HS202–205 is characterized by spells of neuroglycopenia 
caused by endogenous hyperinsulinemic hypoglycemia occurring 
typically, but not invariably, after a meal. NIPHS is less common 
than insulinoma.2,150 Because the syndrome is diffuse, anatomic 
tumor imaging studies are uniformly negative. Given documented 
postprandial hyperinsulinemic hypoglycemia, documentation of 
diffuse beta-cell hyperfunction depends on a positive selective 
arterial calcium stimulation test. The findings from that 

 calcium stimulation test. The findings from that test can 
be used to guide partial pancreatectomy if empiric medical ther­
apy (e.g., diet, an α-glucosidase inhibitor, diazoxide, octreotide) 
fails. In one relatively large surgical series, most patients improved; 
however, recurrence of symptoms was the rule, and some patients 
were not helped after partial pancreatectomy.204
Some patients who have undergone Roux-en-Y gastric bypass 
develop postprandial endogenous hyperinsulinemic hypoglycemi

stprandial endogenous hyperinsulinemic hypoglycemia 
several months to years after the surgery.206–214,217,218 Affected 
patients have accelerated absorption of ingested glucose that trig­
gers a large insulin secretory response that is mediated, at least in 
part, by a robust increase in GLP1.209–213 Post–gastric bypass hypo­
glycemia is rare, reported to occur in 0.2% of operated patients.214 
GLP1 receptor blockade has reported to eliminate hypoglyce­
mia,215 although this remains experimenta

poglyce­
mia,215 although this remains experimental. Suggested treatments 
include a low-carbohydrate diet, acarbose, somatostatin analogues, 
diazoxide, and possibly feeding into the bypassed stomach.208
Autoimmune hypoglycemia caused by an antibody to insulin 
is rare.216,219 Affected individuals often have a history of other 
autoimmune disorders. Hypoglycemia occurs in the late postpran­
dial period as insulin, which is secreted in response to the meal 
and then bound to the circulating anti

o the meal 
and then bound to the circulating antibody, dissociates from the 
antibody in an unregulated fashion. A clue to the diagnosis is the 
finding of very high measured plasma insulin levels during hypo­
glycemia. The diagnosis is made by the finding of high-titer serum 
insulin antibodies. There is no consistently effective therapy. A 
similar disorder has been reported in patients with a high-capacity 
insulin-binding monoclonal paraprotein.220
Accidental or surreptitious ingestion of a

ein.220
Accidental or surreptitious ingestion of an insulin secretagogue 
causes endogenous hyperinsulinemic hypoglycemia indistinguish­
able from that caused by an insulinoma aside from the presence 
1543
of a measurable oral hypoglycemic agent in the circulation at the 
time of hypoglycemia.2,150
Very rare causes of insulin-related hypoglycemia have been 
linked to a mutation of the insulin receptor,221 to exercise-induced 
hyperinsulinemia,222 or to an agonist antibod

ced 
hyperinsulinemia,222 or to an agonist antibody to the insulin 
receptor.223 In the latter case, endogenous insulin secretion is 
suppressed appropriately, and inappropriately high insulin levels 
are thought to result from blockade of receptor-mediated insulin 
clearance by the antibody. Finally, although seemingly convincing 
cases of ectopic insulin secretion have been reported (e.g., by Seckl 
and colleagues224), the condition must be extraordinarily rare. 
Diagnostic Approach
Patients w

raordinarily rare. 
Patients with a hypoglycemic disorder may present in several differ­
ent ways, including with a history of symptomatic episodes com­
patible with hypoglycemia, a serendipitously measured low plasma 
glucose concentration, or a familial syndrome that includes a hypo­
glycemic disorder (e.g., MEN1).2,152,188 A careful history of any 
spells—including the specific symptoms, their timing in relation 
to meals, their duration, and any factors that aggravate or 

their duration, and any factors that aggravate or alleviate 
them—is important for the formulation of a diagnostic plan. A 
history that includes neuroglycopenia is particularly compelling.2 
Again, documentation of Whipple triad establishes that a hypo­
glycemic disorder exists.2 The diagnostic strategy recommended in 
the Endocrine Society clinical practice guideline2 is described here.
First, review the history, physical findings, and all available 
laboratory data, seeking clues to specific 

lable 
laboratory data, seeking clues to specific disorders such as drugs, 
critical illnesses, hormone deficiencies, or non–islet cell tumors 
(see Table 38.4), then pursue those.2 This approach will identify 
the cause of hypoglycemia in most instances. Again, drugs2 are, 
by far, the most common cause of hypoglycemia (see Table 38.8).
If the cause of a hypoglycemic disorder is not evident (i.e., in a 
seemingly healthy individual), measure plasma glucose, insulin, 
C-peptide, proinsulin, and 

sma glucose, insulin, 
C-peptide, proinsulin, and β-hydroxybutyrate concentrations and 
screen for oral hypoglycemic agents during an episode of spontaneous 
hypoglycemia, and observe the plasma glucose response to IV injec­
tion of 1.0 mg of glucagon.2 In addition, measure insulin antibodies.2
Failure of insulin secretion to fall to very low rates as plasma 
glucose concentrations fall to hypoglycemic levels is the key patho­
physiologic feature of endogenous hyperinsulinism. Hypoglyce­
mia is 

 of endogenous hyperinsulinism. Hypoglyce­
mia is the result of low rates of glucose production rather than 
high rates of glucose utilization.225 Plasma insulin, C-peptide, and 
proinsulin concentrations may not always be high relative to nor­
mal values obtained under euglycemic conditions, but they are 
essentially always inappropriately high relative to the low plasma 
glucose concentrations.2,152,188 The traditional critical diagnos­
tic criteria (assuming Whipple triad is documented) are p

teria (assuming Whipple triad is documented) are plasma 
insulin concentrations of 3 μU/mL (18 pmol/L) or higher, plasma 
C-peptide concentrations of 0.6 ng/mL (0.2 nmol/L) or higher, 
and plasma proinsulin concentrations of 5.0 pmol/L or higher 
when plasma glucose concentrations are less than 55 mg/dL (3.0 
mmol/L) (Table 38.9). These data, first published in 1995,152 reas­
sessed in 2009,193,196 and incorporated into the Endocrine Soci­
ety clinical practice guideline on adult hypoglycemic di

inical practice guideline on adult hypoglycemic disorders,2 
were independently reassessed in 2013.196 Notably, however, the 
extent to which the unaffected subjects in the latter study had low 
plasma glucose concentrations at the time of insulin, C-peptide, 
and proinsulin sampling is not made clear.196 The insulin criterion 
of 18 pmol/L (3 μU/mL) or higher was generally supported: 93% 
sensitivity with 95% to 100% specificity193 and 98% sensitivity 
with 60% specificity.196 The C-peptide cri

tivity 
with 60% specificity.196 The C-peptide criterion of 0.6 ng/mL 
(0.2 nmol/L) or higher was not supported: sensitivity at 100% 
or lower with specificity of 60% to 78%193 versus sensitivity at 
  Patterns of Findings During Fasting or After a Mixed Meal in Normal Individualsa and in Individuals 
With Hyperinsulinemic (or IGF-Mediated) Hypoglycemia or Hypoglycemia Caused by Other 
Mechanisms
Symptoms, 
Signs, or 
Both
Glucose 
(mg/dL)
Insulin 
(μU/mL)
C-Peptide 
(nmol/L)
Proinsulin 
(pmol/

in 
(μU/mL)
C-Peptide 
(nmol/L)
(pmol/L)
β-Hydroxybutyrate 
(mmol/L)
Glucose Increase 
After Glucagon 
(mg/dL)
Circulating 
Oral 
Hypoglycemic 
Agent
Antibody 
to Insulin
No
<55
<3
<0.2
<5
>2.7
<25
No
No
Normal
Yes
<55
≫3
<0.2
<5
≤2.7
>25
No
Neg (Pos)
Exogenous insulin
Yes
<55
≥3
≥0.2
≥5
≤2.7
>25
No
Neg
Insulinoma, NIPHS, 
PGBH
Yes
<55
≥3
≥0.2
≥5
≤2.7
>25
Yes
Neg
Oral hypoglycemic 
agent
Yes
<55
≫3
≫0.2b
≫5b
≤2.7
>25
No
Pos
Insulin autoimmune
Yes
<55
<3
<0.

≤2.7
>25
No
Pos
Yes
<55
<3
<0.2
<5
≤2.7
>25
No
Neg
IGFc
Yes
<55
<3
<0.2
<5
>2.7
<25
No
Neg
Not insulin- or IGF 
mediated
aNormal individuals are those with no symptoms or signs despite relatively low plasma glucose concentrations (i.e., those in whom Whipple triad is not documented).
bConcentrations of free C-peptide and proinsulin are low.
cIncreased pro-IGF2, free IGF2, and IGF2/IGF1 ratio.
IGF, Insulin-like growth factor; NIPHS, noninsulinoma pancreatogenous hypoglycemia; 

IPHS, noninsulinoma pancreatogenous hypoglycemia; PGBH, post–gastric bypass hypoglycemia.
From Cryer PE, Axelrod L, Grossman AB, et al. Evaluation and management of adult hypoglycemic disorders: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2009;94:709–728, 
used with permission of the Endocrine Society. Data from Service152 and Placzkowski and associates.193 See discussion of Guettier and associates196 for independent data.
   
TABLE 38.9
1544
SECTION VIII    Disord

t data.
   
TABLE 38.9
1544
100% but specificity only 10%196; raising the criterion to 2.3 ng/
mL (0.8 nmol/L) or higher lowered sensitivity to 84% but raised 
specificity to 76%.196 The proinsulin criterion of 5 pmol/L or 
higher provided 100% sensitivity in both series but with speci­
ficity of 68% to 78%193 and 41%196; raising the criterion to 27 
pmol/L or higher still provided 100% sensitivity but with 100% 
specificity.196 Plasma

 sensitivity but with 100% 
specificity.196 Plasma β-hydroxybutyrate concentrations of 2.7 
mmol/L or lower and an increase in the plasma glucose concentra­
tion of more than 25 mg/dL (1.4 mmol/L) over the low value over 
30 minutes after glucagon injection provide evidence of biologic 
actions of inappropriately high insulin (or IGF) levels, with sup­
pression of lipolysis and ketogenesis and preservation of hepatic 
glycogen stores, respectively. The patterns of findings in patients 
with a hy

y. The patterns of findings in patients 
with a hypoglycemic disorder and in those with hyperinsulinemic 
(or IGF-mediated) hypoglycemia are summarized in Table 38.9. 
Occasionally, a patient with an insulinoma may not fulfill these 
criteria even during a 72-hour fast,2,226 and a few have plasma 
insulin levels lower than 3 μU/mL (18 pmol/L) during hypogly­
cemia; however, at least in some series,227 plasma C-peptide and 
proinsulin levels are elevated in such patients.
If Whipple triad has not

levated in such patients.
If Whipple triad has not been documented and the measure­
ments described have not been obtained during an episode of 
spontaneous hypoglycemia, an attempt should be made to re-create 
the circumstances in which symptomatic hypoglycemia is likely to 
occur.2 This can be accomplished by withholding food in a patient 
with a history suggestive of fasting hypoglycemia or by providing a 
mixed meal likely to cause a symptomatic episode in a patient with 
a history suggestiv

tic episode in a patient with 
a history suggestive of postprandial hypoglycemia. Failing these 
relatively informal procedures, a patient with a history suggestive 
of fasting hypoglycemia should undergo a prolonged supervised 
fast.2,226 The fast should be continued until Whipple triad is docu­
mented (or until a plasma glucose concentration of less than 55 mg/
dL [<3.0 mmol/L] is reached if Whipple triad was unequivocally 
documented previously226) unless a progressive increase in plasma 
β-h

y226) unless a progressive increase in plasma 
β-hydroxybutyrate levels signals a negative fast. Serial plasma glu­
cose concentrations should be measured with a precise method, 
not with a point-of-care glucose monitor. About two thirds of 
patients with an insulinoma meet the diagnostic criteria during a 
fast of less than 24 hours; most, but not all, do so in less than 48 
hours.226 Therefore, the diagnostic fast can be initiated, and often 
completed, in the outpatient setting and continued 

ompleted, in the outpatient setting and continued in the inpatient 
setting if necessary. However, a patient with a history suggestive of 
postprandial hypoglycemia should undergo a mixed meal test con­
ducted over 5 hours. Standards for interpretation of the findings of 
the mixed meal test have not been established; current usage2 is to 
apply the criteria developed under fasting conditions.152 Detailed 
suggestions for performance of a prolonged supervised fast and of 
a mixed meal test have 

ed supervised fast and of 
a mixed meal test have been published.2
An insulinoma may well be present in a patient with docu­
mented Whipple triad; inappropriately high levels of insulin, 
C-peptide, and proinsulin along with no detectable circulating 
oral hypoglycemic agent; suppressed β-hydroxybutyrate levels; a 
brisk glycemic response to IV glucagon during fasting (or even 
postprandial193) hypoglycemia; and no circulating insulin anti­
body. However, as noted earlier, there are other causes

 However, as noted earlier, there are other causes of hyperin­
sulinemic hypoglycemia (see Tables 38.4 and 38.9). Therefore, the 
next step is to attempt to localize an insulinoma.2
Computed tomography, magnetic resonance imaging, and 
transabdominal ultrasonography detect approximately 75% of 
insulinomas.193,228,229 They also detect metastases in the minority 
of patients who have a malignant insulinoma. Older somatostatin 
receptor scintigraphy is somewhat less sensitive.230 Endoscopic 
pancr

y is somewhat less sensitive.230 Endoscopic 
pancreatic ultrasonography, with the option of fine-needle aspira­
tion of a detected tumor, has a sensitivity greater than 90%.230,231 
With the combination of noninvasive imaging and, if necessary, 
endoscopic pancreatic ultrasonography, preoperative localization 
of insulinomas has become the rule.193 Given the promise of posi­
tron emission tomography with radiotracers such as 68Gallium 
DOTA-(Tyr3)-octreotate,232 noninvasive localization of insul

)-octreotate,232 noninvasive localization of insulino­
mas may become the preferred approach. If anatomic localization 
of insulinoma is negative or equivocal, selective pancreatic arterial 
calcium injections, with an end point of at least a twofold232,233 
increase (or perhaps a greater than fivefold increase with contem­
porary assays234) in hepatic venous insulin levels over baseline, 
regionalize insulinomas with high sensitivity.233,235 Although this 
invasive procedure is seldom necessary

hough this 
invasive procedure is seldom necessary in patients with an insuli­
noma, it is the procedure of choice for confirming NIPHS216,219 
and hypoglycemia occurring after Roux-en-Y gastric bypass206–214 
(Table 38.10). Finally, intraoperative pancreatic ultrasonography 
almost invariably localizes tumors that are not apparent even to 
the experienced pancreatic surgeon. 
Treatment of Hypoglycemia Disorders
Prevention of ongoing or recurrent hypoglycemia requires treat­
ment that corrects o

 hypoglycemia requires treat­
ment that corrects or circumvents the hypoglycemic mechanism.2 
Obviously, treatment should be tailored to the specific hypogly­
cemic disorder identified. Offending drugs can be discontinued 
or their dosage reduced. Critical illnesses can often be treated. 
Deficient hormones, such as cortisol, can be replaced. Reduc­
tion of non–islet cell tumor mass with surgery, irradiation, or 
chemotherapy may alleviate hypoglycemia even if the tumor can­
not be cured. Treatm

ycemia even if the tumor can­
not be cured. Treatment with a glucocorticoid, growth hormone, 
or even octreotide may alleviate hypoglycemia in such patients. 
Surgical resection of a benign insulinoma typically is curative. 
  Causes of Hypoglycemia Unique to, or 
Typically With Onset in, Infancy and 
Childhood
Intolerance of Fasting
Preterm or small-for-gestational-age infants
Hypopituitarism, adrenal hypoplasia, congenital adrenal hyperplasia
Ketotic hypoglycemia of childhood
Hyperinsulinism

Infant of a diabetic mother
Maternal drugs (sulfonylurea, β2-adrenergic agonist)
Congenital hyperinsulinism
Perinatal stress, small for gestational age
Others: Rh incompatibility, Beckwith-Wiedemann syndrome, exchange 
transfusions, perinatal stress, insulin-secreting tumors of pancreatic 
islet cells (insulinomas)
Inborn Errors of Metabolism (Enzyme Defects)
Carbohydrate metabolism: Glycogen storage disease types I, III, and 
VI; glycogen syntha

age disease types I, III, and 
VI; glycogen synthase deficiency; fructose-1,6-bisphosphatase 
deficiency; fructose-1-phosphate aldolase deficiency; galactose-
1-phosphate uridyltransferase deficiency
Protein metabolism: Branched-chain α-keto acid dehydrogenase com­
plex deficiency
Fat metabolism: Fatty acid oxidation defects including deficiencies in the 
carnitine cycle, the β-oxidation spiral, the electron transport system, 
and the ketogenesis sequence
TABLE 38.10
1545
Chapter 38  Hypoglycemi

 sequence
1545
In unresectable disease, empirical treatments (diet, diazoxide, 
octreotide) can be tried; there has been progress with chemo­
therapy (e.g., everolimus).236 Diet, including frequent feedings, 
an α-glucosidase inhibitor, diazoxide, or octreotide, can be tried 
in patients with NIPHS or post–gastric bypass hypoglycemia. 
Treatment of autoimmune hypoglycemia (with a glucocorticoid 
or another immunosuppressive medication) is problematic, but 
th

unosuppressive medication) is problematic, but 
the disorder is sometimes self-limited. Failing these treatments, 
frequent feedings during the day and bedtime administration of 
large doses of uncooked cornstarch or even overnight intragas­
tric glucose infusion may be necessary. Interestingly, a missense 
MAFA mutation leading to familial insulinomas or diabetes mel­
litus has been identified in two families.237 
Hypoglycemia in Infancy and Childhood
The fetus receives a continuous supply of g

ldhood
The fetus receives a continuous supply of glucose from the mater­
nal circulation across the placenta to meet a substantial propor­
tion of its energy needs, and its glucose levels are mostly reflective 
of the maternal glycemic level. Hepatic glucose production and 
gluconeogenesis are absent during fetal life, due to the absence or 
very low activity of pyruvate carboxylase, phosphoenolpyruvate 
carboxykinase, glucose-6-phosphatase, and fructose 1,6 diphos­
phatase.238 These rate-limiti

fructose 1,6 diphos­
phatase.238 These rate-limiting enzymes of gluconeogenesis rap­
idly increase within the first few hours of life. Even though the 
enzymes needed for glycogen synthesis and glycogenolysis are 
present in the fetal liver, hepatic glycogen stores do not increase 
much until the third trimester, accounting for about 5% of the 
total liver weight at birth.239
Plasma glucose concentrations decline after birth as placental 
blood flow is interrupted, and they reach their nadir usu

low is interrupted, and they reach their nadir usually in the 
first 2 hours of life.240 This is accompanied by a decrease in insu­
lin and activation of glucose counterregulatory systems that favors 
not only mobilization of glycogen stores and gluconeogenesis but 
also lipolysis and eventually ketogenesis in the neonate. Because 
mobilizable glycogen stores are limited and quickly depleted, 
and feeding is intermittent, the newborn is forced to increase its 
endogenous glucose production and t

 increase its 
endogenous glucose production and thus become largely depen­
dent on gluconeogenesis initially. In most infants, the plasma glu­
cose concentration is usually steady or is increasing by 4 to 6 hours 
of life. As feedings are spaced and the fasting period is prolonged, 
lipolysis and ketogenesis increase and ketones become a significant 
fuel source, particularly for the brain.
The brain relies almost exclusively on a constant supply of glu­
cose to meet its metabolic demands and f

y of glu­
cose to meet its metabolic demands and fuel brain growth and 
key maturational changes. The brain cannot do gluconeogenesis 
or use free fatty acids, because they are not transported across 
the blood-brain barrier. However, under special circumstances, it 
can use ketone bodies (beta-hydroxybutyric acid and acetoacetic 
acid) or lactate as transient surrogate fuels if levels rise significantly 
enough. In the first years of life, the human brain accounts for 
nearly 50% of the body’s 

uman brain accounts for 
nearly 50% of the body’s glucose consumption due to the pro­
portionally larger brain to body mass ratio,238,241 with the peak 
of cerebral glucose consumption occurring during middle child­
hood (∼5–10 years of age).242 Likewise, this metabolic peak is 
mirrored by increases in synaptic density and synaptic pruning, 
which occurs well beyond the periods of greatest linear brain 
growth and differs across brain regions243 (Fig. 38.13). The rate 
of total-body and brain g

 (Fig. 38.13). The rate 
of total-body and brain glucose metabolism then falls to normal 
adult rates. Overall, the rates of glucose flux (production and 
utilization) are nearly three times higher in infants and children 
than in adults. Throughout the lifespan, this glucose flux is finely 
regulated to maintain euglycemia by insulin, counterregulatory 
hormones (glucagon, catecholamines, cortisol, and growth hor­
mone), incretins, other gut hormones (ghrelin and leptin), and 
neuronal inputs.2

mones (ghrelin and leptin), and 
neuronal inputs.244 However, transient or persistent hypoglycemia 
may occur when glucose supplies are low and ketone body pro­
duction is small or impaired, as in children with hyperinsulinism 
0
10
20
30
40
-2
0
2
4
6
3
2
1
0
-1
-2
-3
Human development
CMRglu
Spine Density
Brain Growth Rate
Linear  Growth Rate
Standardized value
Standardized growth rate
• Fig. 38.13  Brain (green) and linear (blue) growth rates, dendritic spine density (purple), and

rowth rates, dendritic spine density (purple), and cerebral 
metabolic rate of glucose (red) values across the life span during human development. CMRglu, cerebral 
metabolic rate of glucose. (Data from Goyal MS, Venkatesh S, Milbrandt J, et al. Feeding the brain and 
nurturing the mind: linking nutrition and the gut microbiota to brain development. Proc Natl Acad Sci U S 
A. 2015;112[46]:14105–14112.)
1546
or inborn errors of metaboli

te and Fat Metabolism
or inborn errors of metabolism, or when there is impairment of 
the glucoregulatory mechanisms.245,246 Hypoglycemia, if severe, 
may cause neurologic compromise or death. The extent of brain 
injury seems to be influenced by multiple factors, such as duration 
and degree of hypoglycemia.
It is key to recognize that transient low blood glucose concen­
trations are common in healthy infants in the first 24 to 48 hours 
of life as the glucose supply changes from a continuous t

 as the glucose supply changes from a continuous transpla­
cental supply to an intermittent supply from feeds, resulting in a 
quick depletion of the glycogen stores. Furthermore, newborns 
in these first hours of life are relatively hyperinsulinemic com­
pared with older children due to a lower glycemic threshold for 
insulin secretion shortly after birth—55 to 65 mg/dL (3.0–3.6 
mmol/L) compared with 80 to 85 mg/dL (4.4–4.7 mmol/L) in 
older infants, children, and adults.247 Therefore, if the 

fants, children, and adults.247 Therefore, if the first feed­
ings after birth are delayed, a considerable amount of normal term 
newborns cannot maintain a plasma glucose concentration above 
40 mg/dL (2.2 mmol/L).240 But as feeds are established, main­
tenance of plasma glucose concentrations is no longer primarily 
dependent on gluconeogenesis, and hypoglycemia becomes less 
common. Thus, it is important to distinguish this normal physi­
ologic transitional response from disorders that result

c transitional response from disorders that result in persistent 
or recurrent hypoglycemia beyond the first 3 days of life. Because 
of potential difficulty in distinguishing a persistent hypoglycemia 
disorder from transitional neonatal glucose concentrations during 
the first 48 hours of life, the Pediatric Endocrine Society suggests 
delaying diagnostic evaluations for hypoglycemia until 2 to 3 days 
after birth. However, in infants who are at risk for hypoglycemia, 
glucose screening should

t risk for hypoglycemia, 
glucose screening should be performed after the first feed, which 
should occur within 1 hour after birth. Plasma glucose concentra­
tions should continue to be measured every 3 to 6 hours before 
feeds for the first 24 to 48 hours of life, because many at-risk new­
borns present with their first documented low glucose concentra­
tions during this period.247
As in adults,2 the diagnosis of clinical hypoglycemia should be 
considered in pediatric patients that have Whipp

considered in pediatric patients that have Whipple triad (see Table 
38.1). Signs and symptoms of hypoglycemia are quite nonspe­
cific in a neonate or children and may be difficult to recognize. 
Some of these may include jitteriness/tremors, hypotonia, changes 
in the level of consciousness, apnea/bradycardia, cyanosis, tachy­
pnea, poor suck or feeding, hypothermia, and/or seizures. In addi­
tion, as in adults, it is not possible to state a single low plasma 
glucose concentration that categ

ingle low plasma 
glucose concentration that categorically defines hypoglycemia in 
infants and children; its precise definition continues to be contro­
versial. This is because (1) normal distributions of glucose values 
will vary depending on age; (2) glucose values will vary depend­
ing on conditions of feeding and fasting; (3) there are potential 
artifacts that may lead to inaccuracies in glucose determination; 
(4) the thresholds for specific responses to hypoglycemia can be 
altered by re

ic responses to hypoglycemia can be 
altered by recent antecedent hypoglycemia; and (5) there is lack of 
outcome data, as there is not a single glucose value that has been 
associated with brain injury.245–247 However, most clinical guide­
lines postulate arbitrary glucose thresholds to provide guidance on 
when to initiate a workup to identify children with an underlying 
disorder and when to initiate an intervention to reduce potential 
harm from hypoglycemia and minimize possible overtreatme

rom hypoglycemia and minimize possible overtreatment.
The American Academy of Pediatrics in 2011, followed by 
the Pediatric Endocrine Society in 2015, wrote clinical reports 
and guidelines on screening and subsequent management of 
neonatal hypoglycemia.247,248 Although these guidelines do not 
coincide on the glycemic thresholds for the diagnosis of hypogly­
cemia, they both endorse that hypoglycemia persisting beyond 
48 hours is not likely to be transitional, and they emphasize 
the need fo

o be transitional, and they emphasize 
the need for early identification of the infant at risk for severe 
hypoglycemia during the first 48 hours after delivery and for 
persistent hypoglycemia beyond 48 hours of life to determine 
the need for screening. These include not only the infant of a 
diabetic mother or the child with a family history of a genetic 
form of hypoglycemia or a congenital syndrome associated with 
hypoglycemia, such as congenital hyperinsulinism, Beckwith-
Wiedemann syndro

enital hyperinsulinism, Beckwith-
Wiedemann syndrome, or hypopituitarism, but also those with 
relatively more common perinatal stress hyperinsulinism: birth 
asphyxia, intrauterine growth restriction, or toxemia, or infants 
receiving total parenteral nutrition.
The Pediatric Endocrine Society suggests a plasma glucose con­
centration of 50 mg/dL (2.8 mmol/L) or less as an appropriate 
threshold to trigger further diagnostic testing in a child less than 
48 hours old and 60 mg/dL (3.3 mmol/L) o

ess than 
48 hours old and 60 mg/dL (3.3 mmol/L) or less after 48 hours 
of age.247 Nevertheless, many will criticize that it is a relatively 
conservative threshold, whereas others will say that it may lead 
to overtreatment of many neonates, but the neonatal threshold is 
intended to avoid discharging newborns who may be at risk for 
recurrent and severe hypoglycemia.247,249 The diagnostic evalua­
tion for neonatal hypoglycemia should include a plasma confir­
mation of a low blood glucose valu

 plasma confir­
mation of a low blood glucose value and simultaneous measure 
of carbon dioxide to determine if there is an associated acidosis, 
insulin level and ketone bodies to determine if there is evidence of 
hyperinsulinism and measure of the counterregulatory hormones 
(cortisol for cortisol deficiency and growth hormone for growth 
hormone deficiency), free fatty acids to see if there is a defect on 
fatty acid oxidation, and a lactate level. If outside of the newborn 
period, or if th

level. If outside of the newborn 
period, or if there are concerns for exogenous hyperinsulinism, 
C-peptide, which is a by-product of the metabolism of insulin, 
should also be also measured. In the case of a child with hypogly­
cemia of unknown cause, a glucagon stimulation test at the time 
of hypoglycemia can provide very useful diagnostic information 
about glycogen stores and possible hyperinsulinism. In such a test, 
0.03 mg/kg of glucagon is given intravenously or by intramuscular 
injec

 is given intravenously or by intramuscular 
injection when the child is hypoglycemic. Plasma glucose levels 
are measured prior to the injection and 10, 20, and 30 minutes 
after. If plasma glucose rises by more than 30 mg/dL (1.7 mmol/L) 
within the first 30 minutes after glucagon administration, it sug­
gests that the child has hyperinsulinemia, because during hyperin­
sulinism there is inappropriate buildup of hepatic glycogen stores 
that are released in response to glucagon administration.

e released in response to glucagon administration.
If the patient persists in being hypoglycemic or becomes hypo­
glycemic after obtaining the critical laboratory samples and doing 
the glucagon stimulation test, it is key to treat the low plasma 
glucose to prevent adverse outcomes. If the patient is conscious 
and able to drink and swallow safely, then he or she can be treated 
with administration of 10 to 20 g of rapidly absorbed carbohy­
drates by mouth or by a gastric tube, if available. Th

es by mouth or by a gastric tube, if available. This process may 
be repeated after 15 minutes, but if the hypoglycemia does not 
improve within 30 minutes, parenteral glucose is recommended. 
In infants and children with altered consciousness, IV dextrose 
should be given. A slow bolus of 2 mL/kg of 10% dextrose solu­
tion can be given, followed by a continuous infusion of dextrose 
at 6 to 9 mg/kg per minute. If IV access is not readily available 
and the patient has documented hyperinsulinism

le 
and the patient has documented hyperinsulinism, then intramus­
cular glucagon (0.03 mg/kg up to a maximum of 1 mg) should 
be given.
Close monitoring for further evaluation and treatment is 
needed in children found to have low plasma glucose. In neonates, 
monitoring should continue until plasma glucose concentrations 
1547
can be maintained with regular feedings at above 50 mg/dL (2.8 
mmol/L) in newborns less than 48 hours old and above 60 mg/dL 
(3.3 mmol/L) in n

48 hours old and above 60 mg/dL 
(3.3 mmol/L) in newborns more than 48 hours old. However, the 
mean plasma glucose concentration in the postabsorptive state of 
children and neonates after 48 hours of age does not differ from 
that in adults (70–100 mg/dL [3.9–5.5 mmol/L]). Thus, in chil­
dren with the diagnosis of a disorder causing persistent hypoglyce­
mia or a known risk of a persistent hypoglycemic disorder, the goal 
of treatment is to maintain a plasma glucose concentration above 
70 mg/

ntain a plasma glucose concentration above 
70 mg/dL (3.89 mmol/L). Therefore, a safety fast (6-hour fast), in 
which a term neonate over 3 days of life is asked to skip a single 
feed, should always be done before discharge to ensure that plasma 
glucose concentrations can be maintained above that range.
Even though hypoglycemia in children can be caused by the 
same mechanisms as in adults2 (see Table 38.4), including drugs 
and critical illnesses, there are several hypoglycemic disorders 
uni

ses, there are several hypoglycemic disorders 
unique to infancy or childhood that can be classified as intoler­
ance of fasting, hyperinsulinism, or are due to inborn errors of 
metabolism245,246 (see Table 38.10). The presentation of these dif­
ferent conditions may vary, as well as the age of onset and the 
tolerance of the fasting period (Fig. 38.14). Thus, a good clinical 
history, physical examination, and critical laboratory samples are 
key to determine the diagnosis.
Intolerance of Fast

ey to determine the diagnosis.
Infants and toddlers may have a plasma glucose concentration 
less than 70 mg/dL (3.9 mmol/L) and hyperketonemia after over­
night fasting because of limited fasting tolerance.250–252 It is par­
ticularly common in preterm or small-for-gestational-age infants 
-Glycogen Storage Disease
• Glucose-6-phosphatase
   deficiency (type 1 GSD)
-Defects in Glycogenolysis
(type 3, 6, 9 GSD)
• Debrancher
• Liver phosphorylase
• Phosphorylase
kinase defi

kinase deficiencies
-Defects in Hepatic
• Fructose-1.6-
diphosphatase deficiency
• Pyruvate carboxylase and
• Phosphoenolpyruvate
-Normal neonate
-Ethanol ingestion
-Normal Child
-Ketotic Hypoglycemia
-Growth hormone or
cortisol deficiency
-IDDM or Insulin Therapy
-Oral Hypoglycemic Agents
-SGA & birth asphyxia
-Neonatal Hypopituitarism
-Infants of diabetic mothers
-Congenital Hyperinsulinism
-Genetic defects in fatty acid
oxidat

erinsulinism
oxidation
&
Ketogenesis
Hypoglycemia: PG≤50 mg/dL
Maternal & Infant
History, Physical Exam
Acidosis
High Lactate
Ketosis
No Ketosis & No FFA
No Ketones, High FFA
Short Fasting Period
Moderate Fasting Period
Short to Moderate Fasting Period
Prolonged Fasting Period
No Acidosis
• Fig. 38.14  Diagnostic algorithm for determining the etiology of hypoglycemia in children. FFA, free fatty 
acid; GSD, glycogen storage disease; PG, plasma glucose. (Modified fr

 storage disease; PG, plasma glucose. (Modified from Sprague JE, Arbeláez AM. 
Glucose counterregulatory responses to hypoglycemia. Pediatr Endocrinol Rev. 2011;9[1]:463–475.)
1548
and is thought to result partially from incomplete development 
of gluconeogenic mechanisms.2 When hypoglycemia occurs, gly­
cogenolysis is the first mechanism that occurs, followed by glu­
coneogenesis, to try to raise systemic blood sugars. However, if 
fa

ry to raise systemic blood sugars. However, if 
fasting is prolonged or feeding is interrupted, as during an inter­
current illness in children who are least tolerant of fasting, there is 
lack of increased renal gluconeogenesis, and the endogenous sub­
strate availability from muscles or liver is low253,254 and ketotic 
hypoglycemia occurs. The syndrome of ketotic hypoglycemia of 
childhood typically occurs in 2- to 5-year-old children and remits 
spontaneously before age 10 years, when the rat

s 
spontaneously before age 10 years, when the ratio of brain to body 
weight changes. This should be a diagnosis of exclusion after other 
conditions that can present with ketosis, such as growth hormone 
deficiency, hypopituitarism, adrenocorticotropic hormone unre­
sponsiveness, and glycogen synthase deficiency, are ruled out. 
Cortisol and growth hormone deficiency reduce gluconeogenesis 
and hepatic glucose production. Moreover, there are low plasma 
concentrations of gluconeogenic substrat

w plasma 
concentrations of gluconeogenic substrate, and there is a blunted 
glucagon response.255 
Hyperinsulinism can be suspected when the plasma insulin con­
centration is inappropriately normal or elevated for the level of 
hypoglycemia, and plasma or urine ketones levels, as well as free 
fatty acids, are low. In addition, this condition should also be 
suspected when there is a glycemic response to glucagon at the 
time of hypoglycemia. In the neonatal period, these childr

hypoglycemia. In the neonatal period, these children are 
typically large for gestational age, and hypoglycemia occurs after a 
short to moderate fasting period.
Maternal diabetes is a common cause of neonatal hypoglyce­
mia due to transient hyperinsulinism in the offspring.246 Infants 
of diabetic mothers have been hyperglycemic in utero due to the 
mother’s hyperglycemia, which causes chronic stimulation of fetal 
insulin secretion in utero. Therefore, shortly after birth, there is 
failure of

erefore, shortly after birth, there is 
failure of insulin to fall normally as glucose levels decline and 
transient neonatal hypoglycemia develops. Transient hyperin­
sulinemia also causes transient hypoglycemia and can occur due 
to other conditions listed in Table 38.10. Infants with perinatal 
stress, prematurity, or small for gestational age can develop tran­
sient hypoglycemia secondary to hyperinsulinism, which is usu­
ally responsive to diazoxide and resolves by 6 months of age. In 
chil

iazoxide and resolves by 6 months of age. In 
children, postprandial hypoglycemia analogous to that occurring 
after gastric bypass can follow Nissen fundoplication. Insulin-
secreting tumors of pancreatic islet cells are rare in children but 
have been reported in association with MEN1 and occur outside 
of the neonatal period. As in adults, hyperinsulinemic hypogly­
cemia can be accidental, surreptitious, or even malicious. When 
suspecting those cases, a C-peptide test can be very informative

se cases, a C-peptide test can be very informative.
Congenital hyperinsulinism256–258 is the most common cause 
of nontransient neonatal hypoglycemia, although it occurs in only 
1 of every 30,000 to 50,000 live births. However, it can be as 
common as 1 in 2,500 live births in highly consanguineous com­
munities.259 Hypoglycemia may persist from the neonatal period 
or become apparent during the first year of life. These conditions 
usually have a significant associated risk of hypoglycemic sei

 a significant associated risk of hypoglycemic seizures 
and developmental delays. The need for very high glucose infusion 
rates is a diagnostic clue.
Glucose-stimulated insulin secretion normally involves increased 
glucose transport into beta cells, glucokinase-mediated phosphory­
lation of glucose, and glucose metabolism via the glycolytic pathway, 
resulting in an increase in the ratio of adenosine triphosphate (ATP) 
to adenosine diphosphate. This leads to closure of membrane ATP-
gated po

e. This leads to closure of membrane ATP-
gated potassium (KATP) channels and their sulfonylurea receptor 
1 (SUR1) and potassium inward rectifying channel (Kir6.2) sub­
units, membrane depolarization, calcium influx, and exocytosis of 
insulin. Insulin secretion is usually stimulated by glucose oxidation 
via glucokinase, but it can also occur due to leucine stimulation of 
glutamate oxidation via glutamate dehydrogenase. Several inherited 
abnormalities of these different cellular mechanisms (

rmalities of these different cellular mechanisms (Fig. 38.15) 
are now known to cause congenital hyperinsulinism and hypogly­
cemia in neonates and infants.256–258,260,261
ABCC8
KCNJ11
  (Kir6.2)
Glucose
ATP
(SUR1)
Ca2+
Ca++
Glucose
Glutamate
Leucine
Pyruvate
Amino Acids
Insulin Secretion
Pyruvate
GDH1
NH4
GK
UCP2
SCHAD
ATP-sensitive
    K+ channel
Voltage-Gated
  Ca2+ channel
GLUT2
MCT1
ATP/ADP
Diazoxide
• Fig. 38.15  Cellular mechanisms affecting beta-cell

ig. 38.15  Cellular mechanisms affecting beta-cell insulin secretion in congenital hyperinsulinism and 
therapeutic targets. Medications used to treat congenital hyperinsulinism are in green. ADP, adenosine 
diphosphate; ATP, adenosine triphosphate; GDH1, glutamate dehydrogenase 1; GK, glucokinase; GLUT2, 
glucose transporter 2; MCT1, monocarboxylate transporter 1; SUR1; sulfonylurea receptor 1; UCP2, 
uncoupling protein 2. (Courtesy Dr. Stephen Stone.)
1549
Congenital h

Stone.)
1549
Congenital hyperinsulinism has been associated with muta­
tions of an increasing array of genes,256–258,260,261 including SUR1 
(encoded by ABCC8), Kir6.2 (encoded by KCNJ11), glucoki­
nase (GK, encoded by GCK), glutamate dehydrogenase (GDH, 
encoded by GLUD1), short-chain 3-hydroxyacyl-CoA dehy­
drogenase (SCHAD encoded by HADH), ectopic expression on 
beta-cell plasma membranes of SLD16A1 (which encodes mono­
carboxylate transporter 1 [MCT1]), and mutation

o­
carboxylate transporter 1 [MCT1]), and mutations of HNF4A 
and HNF1A. Many patients with hyperinsulinemic hypoglyce­
mia respond to the KATP channel opener diazoxide262 and oth­
ers to octreotide,263 whereas some do not respond to medical 
Identification of the genetic mutation in patients with congen­
ital hyperinsulinism is key, as the management and prognosis will 
vary depending on the gene that is affected. Inactivating SUR1 or 
Kir6.2 mutations are the most common and most s

r 
Kir6.2 mutations are the most common and most severe causes of 
congenital hyperinsulinism. They result in reduced KATP channel 
activity and, consequently, increased constitutive insulin secre­
tion. Therefore, most affected patients do not respond to treat­
ment with the KATP channel opener diazoxide, which normally 
suppresses insulin secretion. In contrast, patients with dominantly 
inherited mutations do retain responsiveness to diazoxide. Homo­
zygous or compound heterozygous recessive 

. Homo­
zygous or compound heterozygous recessive mutations in either 
the ABCC8 or KCNJ11 gene typically result in diffuse congeni­
tal hyperinsulinism. However, patients with paternally recessively 
inherited mutations of the ABCC8 or KCNJ11 gene and a spe­
cific loss of maternal alleles of the imprinted chromosome region 
11p15 develop focal congenital hyperinsulinism, in which only 
a small portion of the pancreas is affected with somatic loss of 
heterozygosity and paternal isodisomy of the

s of 
heterozygosity and paternal isodisomy of the chromosome 11p15 
region that contains the ABCC8 and KCNJ11 genes in addition 
to several imprinted genes that increase beta-cell proliferation.264 
For those who do not have a sustained response to medical therapy 
(e.g., frequent feedings, diazoxide, octreotide), near-total pancre­
atectomy is required. Those with a focal lesion do not respond to 
diazoxide and are usually cured by surgical resection of the lesion. 
Focal lesions can be detect

ection of the lesion. 
Focal lesions can be detected noninvasively with [18F]-dihydroxy­
phenylalanine positron emission tomography265 with a sensitivity 
of 85%, a specificity of 96%, and a 96% positive predictive value 
for diagnosis of focal hyperinsulinism.266 These children should be 
evaluated by clinicians experienced with congenital hyperinsulin­
ism and the multiple subspecialty services that are necessary for 
the management of the disorder.267
Activating, dominantly inherited mutation

rder.267
Activating, dominantly inherited mutations of the glutamate 
dehydrogenase gene cause hyperinsulinemia and hyperammo­
nemia syndrome, the second most common form of congenital 
hyperinsulinism. Hypoglycemia typically develops after several 
months of life and is responsive to diazoxide. Activating, domi­
nantly inherited mutations of the glucokinase gene cause vary­
ing degrees of hypoglycemia that may respond to diazoxide but 
may require pancreatectomy. Recessively inherited mutations

re pancreatectomy. Recessively inherited mutations 
of the SCHAD gene result in hypoglycemia that is generally 
responsive to diazoxide. In addition to hyperinsulinemic hypo­
glycemia, biochemical markers include increased levels of plasma 
3-hydroxybutyrylcarnitine and increased urinary 3-hydroxyglu­
tarate. Dominantly inherited mutations of the monocarboxylate 
transporter 1 gene (SLC16A1), resulting in increased pyruvate 
transport into beta cells and hyperinsulinism, have been associ­
ated w

ells and hyperinsulinism, have been associ­
ated with exercise-induced hypoglycemia.
Clinical manifestations, including low plasma glucose con­
centrations and inappropriately high levels of plasma insulin and 
C-peptide coupled with low plasma β-hydroxybutyrate levels and 
a brisk glycemic response to administered glucagon—all analo­
gous to hyperinsulinemic hypoglycemia in adults2—characterize 
congenital hyperinsulinism in neonates and infants.256–258,260,261 
As in adults with endogenous hyp

.256–258,260,261 
As in adults with endogenous hyperinsulinism,2 plasma insulin 
concentrations are not invariably 3 μU/mL (18 pmol/L) or higher 
during hypoglycemia in patients with congenital hyperinsulin­
ism,257 although hyperinsulinism is often documented by serial 
insulin measurements and by inclusion of C-peptide measure­
ments during hypoglycemia. Genetic testing for many mutations 
is available commercially. Finally, an association of hypertro­
phic cardiomyopathy and congenital hyperi

ypertro­
phic cardiomyopathy and congenital hyperinsulinism has been 
recognized.268
Activating mutations in the postreceptor insulin signaling 
pathway (e.g., in the RAC-beta serine/threonine-protein kinase 
[AKT2] gene269,270) results in hypoinsulinemic hypoglycemia that 
resembles hyperinsulinemic hypoglycemia aside from evidence of 
reduced insulin secretion.
Given the array of potential causes of hypoglycemia in neo­
nates and infants, the differential diagnosis is facilitated by an 
array 

ifferential diagnosis is facilitated by an 
array of measurements during hypoglycemia when the precise 
hypoglycemic mechanism is obscure.271 In addition to glucose, 
insulin, C-peptide, and β-hydroxybutyrate levels (and the glyce­
mic response to glucagon), such measurements should include 
plasma bicarbonate, lactate, nonesterified fatty acid (NEFA), 
growth hormone, and cortisol levels. When laboratory findings 
are not consistent with hyperinsulinism, other causes of hypogly­
cemia should th

sulinism, other causes of hypogly­
cemia should then be explored. In those instances, a plasma acyl­
carnitine profile and measurement of ammonia and urine organic 
acids are also needed, but they do not have to be obtained during 
hypoglycemia. 
Inborn Errors of Metabolism (Enzyme 
Deficiencies)
Hypoglycemia that develops in infancy and persists into adult life 
with effective therapy can be caused by enzymatic defects in car­
bohydrate, protein, or fat metabolism245 (see Table 38.10). Hypo­
gl

, or fat metabolism245 (see Table 38.10). Hypo­
glycemia usually becomes apparent later in infancy as the intervals 
between feedings become longer.
Abnormalities in the metabolism of carbohydrates are usually 
due to enzymatic deficiencies in the synthesis or metabolism of 
glycogen, gluconeogenesis, or metabolism of galactose or fructose. 
Glycogen storage diseases (GSDs) are due to different enzyme 
deficiencies that present early in childhood and are usually charac­
terized by hypoglycemia a

 and are usually charac­
terized by hypoglycemia after short fasting periods, that may have 
mild to moderate ketosis, and may present with or without hepa­
tomegaly and have no response to glucagon stimulation. GSD 
type 0 is caused by glycogen synthase deficiency, which results 
from mutations in GYS2 and does not cause hepatomegaly, but is 
characterized by preprandial ketotic hypoglycemia and postpran­
dial hyperglycemia and lactic acidemia. GSD type Ia (von Gierke 
disease) is caused by mut

GSD type Ia (von Gierke 
disease) is caused by mutations in G6PC, the gene that encodes 
glucose-6-phosphatase hydrolase activity. It occurs in approxi­
mately 1 of every 100,000 live births272 and accounts for 80% of 
GSD type I cases. Given that glucose-6-phosphatase is the final 
enzyme in the hepatic release of glucose from gluconeogenic and 
glycogenolytic pathways, absence of its activity results in low rates 
of endogenous glucose production and severe fasting hypoglyce­
mia272 with no gl

on and severe fasting hypoglyce­
mia272 with no glycemic response to administered glucagon. Clin­
ical findings include failure to thrive, hepatomegaly (due to both 
glycogen and fat accumulation), hypertriglyceridemia, accelerated 
lipolysis and ketogenesis, hyperuricemia, platelet dysfunction, and 
1550
marked lactic acidosis (from the metabolism of glucose-6-phos­
phate). With the exception of hepatomegaly, these abnormali­
ties can

ception of hepatomegaly, these abnormali­
ties can be reversed by effective prevention of hypoglycemia with 
frequent feedings during waking hours and continuous intragas­
tric glucose infusion during sleep or bedtime administration of 
large doses of uncooked cornstarch. Liver transplantation corrects 
hypoglycemia and the associated metabolic abnormalities. Late 
complications include progressive renal disease due to glycogen 
accumulation in the kidneys and hepatic adenomas. GSD type Ib 
is c

he kidneys and hepatic adenomas. GSD type Ib 
is caused by mutations in G6PT1, the glucose-6-phosphate micro­
somal transporter. Their clinical presentation and biochemical 
findings are identical to GSD type Ia, but these patients also have 
chronic or intermittent neutropenia and neutrophil dysfunction, 
and are susceptible to recurrent infections. The diagnosis of GSD 
type Ia and type Ib is confirmed by mutation analysis of G6PC and 
G6PT1. Hypoglycemia is less prominent in GSD type III (hep

ypoglycemia is less prominent in GSD type III (hepatic 
amylo-1,6-glucosidase deficiency due to mutations in AGL), GSD 
type VI (hepatic glycogen phosphorylase deficiency due to muta­
tions in PYGL), and GSD type IX (hepatic phosphorylase kinase 
deficiency due to mutations in PHKA2) because hepatic gluco­
neogenesis is preserved and these defects in glycogenolysis are 
rarely complete. These forms of GSD are rare and are treated with 
avoidance of hypoglycemia by frequent administration of freq

of hypoglycemia by frequent administration of frequent 
high-carbohydrate feedings and uncooked cornstarch, particularly 
at bedtime; in GSD type III, a high-protein diet can be of benefit.
Hypoglycemia can also be caused by enzymatic defects in gluco­
neogenesis including fructose-1,6-bisphosphatase, phosphoenol­
pyruvate carboxykinase, and pyruvate carboxylase deficiencies.245 
Glucose-6-phosphatase deficiency is considered by some as a glu­
coneogenesis defect; however, it was discussed earli

neogenesis defect; however, it was discussed earlier because it is 
also involved in glycogenolysis. The disorders of gluconeogenesis 
are characterized by hypoglycemia after moderate fasting peri­
ods that occur when hepatic glycogen stores are depleted. Thus, 
they present with lactic acidemia, ketosis, hyperlipidemia, and no 
response to glucagon stimulation, except in fructose-1,6-bispho­
sphatase deficiency, which may have a response in the fed state. 
Fructose-1,6, bisphosphatase deficienc

fed state. 
Fructose-1,6, bisphosphatase deficiency presents similar to GSD 
type I, except hepatic glycogen accumulation does not occur, and 
hepatomegaly, which is the result of lipid accumulation instead of 
glycogen, is usually mild and liver function is normal. This con­
dition is usually treated with a high-carbohydrate diet and fre­
quent feeding. PEPCK deficiency and pyruvate carboxylase are 
quite rare. In patients with a mutation in the glucose transporter 
1 (GLUT1) gene, plasma gluco

 glucose transporter 
1 (GLUT1) gene, plasma glucose concentrations are normal but 
brain glucose levels are low, causing neuroglycopenia. Hypoglyce­
mia has been attributed to GLUT2 deficiency in Fanconi-Bickel 
syndrome. Postprandial, rather than postabsorptive, hypoglyce­
mia occurs in galactosemia and in hereditary fructose intolerance 
(fructose-1-phosphate aldolase deficiency). Other disorders of 
gluconeogenesis may be due to lack of some gluconeogenic sub­
strates such as galactose, fruc

gluconeogenic sub­
strates such as galactose, fructose, or amino acids. In galactosemia, 
there is a deficiency of galactose-1-phosphate uridyl transferase 
(GALT), in which infants are unable to metabolize galactose to 
glucose, causing hepatic accumulation of galactose-1-phosphate, 
which inhibits enzymes of glycogenolysis. This can be a big issue 
in very young children considering that galactose is a by-product 
of lactose hydrolyzation and lactose is the major dietary carbohy­
drate for inf

actose is the major dietary carbohy­
drate for infants. These children may present with hypoglycemia, 
vomiting after ingestion of lactose or galactose, failure to thrive, 
or sepsis. Hypoglycemia in hereditary fructose intolerance, caused 
by a deficiency of the enzyme aldolase B, occurs after ingestion of 
fructose or sucrose (a disaccharide that is hydrolyzed to glucose and 
fructose). The main goal of long-term treatment of classic galacto­
semia is to minimize dietary galactose intake, and 

emia is to minimize dietary galactose intake, and in patients with 
hereditary fructose intolerance, complete elimination of fructose 
and sucrose from the diet is an effective treatment for most people, 
although this can be challenging. Impaired gluconeogenesis may 
also occur due to alcohol intoxication and salicylate poisoning.
Deficiencies of enzymes involved in protein metabolism 
(see Table 38.10) that can cause fasting hypoglycemia include 
branched-chain ketoaciduria (maple syrup urine 

e 
branched-chain ketoaciduria (maple syrup urine disease and 
tyrosinemia). They present with profound acidosis and failure to 
thrive. The pathogenesis of hypoglycemia is unclear but includes 
defective gluconeogenesis due to liver disease.
Several defects that ultimately impair fatty acid oxidation result 
in hypoglycemia with hypoketonemia during extended fasting.269 
Mitochondrial fatty acid oxidation and ketogenesis require trans­
port of fatty acids across the plasma membrane, formation o

atty acids across the plasma membrane, formation of fatty 
acyl-CoA derivatives, and transport of those derivatives into mito­
chondria. Because the inner mitochondrial membranes are not 
permeable to long-chain (as opposed to medium-chain and short-
chain) fatty acyl-CoA esters, the long-chain fatty acyl-CoA esters 
are transesterified to fatty acylcarnitines at the outer surface of the 
membranes (by carnitine palmitoyltransferase 1, CPT1), trans­
ported across the membranes (by a translocase)

ns­
ported across the membranes (by a translocase), and reconverted 
to the fatty acyl-CoA esters (by carnitine palmitoyltransferase 2, 
CPT2) at the inner surface of the membranes. Then, they can be 
oxidized or converted to ketones. Insulin decreases fat oxidation 
and ketogenesis by decreasing lipolysis and by increasing lipogen­
esis and the formation of malonyl CoA, which inhibits CPT1. 
Conversely, low insulin levels favor fatty acid oxidation and keto­
genesis. High glucagon levels do so 

ion and keto­
genesis. High glucagon levels do so by decreasing malonyl CoA; 
catecholamines do so largely by stimulating lipolysis. Any defect in 
this complex sequence (see Table 38.10) decreases fatty acid oxida­
tion (and ketogenesis) and reciprocally increases glucose oxidation, 
resulting in hypoketonemic postabsorptive hypoglycemia. Reduced 
plasma carnitine levels (20–50% of normal) are the rule in these dis­
orders, but extremely low carnitine levels characterize the carnitine 
transpor

nitine levels characterize the carnitine 
transport defect—a true carnitine deficiency state that is responsive 
to carnitine supplementation.270 The diagnosis of specific fatty acid 
oxidation defects is typically accomplished by blood acylcarnitine 
profiling,271 although molecular diagnosis is increasingly possible.
There are many fatty acid oxidation disorders that result in hypo­
ketonemic hypoglycemia.a The most common is medium-chain 
acyl-CoA dehydrogenase deficiency. Because affected pa

-CoA dehydrogenase deficiency. Because affected patients can 
become symptomatic—fatigue, vomiting, seizure, and coma—
before becoming hypoglycemic, a normal acylcarnitine profile 
should be documented before a diagnostic fast is performed in such 
infants. Other fatty oxidation disorders include very long-, long-, 
and short-chain acyl-CoA dehydrogenase defects, as well as defects 
in electron transfer (glutaric acidemia type 2), 3-hydroxy-3-meth­
ylglutaryl CoA (HMG-CoA) synthase deficiency, a

h­
ylglutaryl CoA (HMG-CoA) synthase deficiency, and HMG-CoA 
lyase deficiency. Defects in carnitine transport and in the carnitine 
cycle that cause hypoketonemic hypoglycemia include primary car­
nitine deficiency due to autosomal recessive mutations in the carni­
tine transporter (OCTN2). Others are CPT1 deficiency, carnitine 
acylcarnitine translocase (CACT) deficiency, and CPT2 deficiency. 
Treatment includes frequent feedings and a low-fat diet rich in 
medium-chain triglycerides in CPT1 d

diet rich in 
medium-chain triglycerides in CPT1 deficiency276 and carnitine 
supplementation in primary carnitine deficiency.
In summary, neonatal hypoglycemia may present with or with­
out acidosis. If acidosis is present, this may be associated with lactic 
aReferences 245, 246, 253, 256, 257, 260–263, 265, 268, 273–275.
1551
acidemia or ketosis. If the patient has no acidosis and suppressed 
levels of NEFAs and β-hydroxybutyrate, it suggests hyperinsulin­
ism; if the

oxybutyrate, it suggests hyperinsulin­
ism; if the patient has high NEFA but low β-hydroxybutyrate, 
it suggests a defect in fatty acid oxidation or ketogenesis. Hypo­
glycemia with acidosis and high lactate levels suggests a defect in 
gluconeogenesis or glucose release. If the patient has high concen­
trations of both NEFA and β-hydroxybutyrate, it suggests a defect 
in glucose production or release including deficiency of cortisol, 
although NEFA and ketone levels need not be elevated in pati

EFA and ketone levels need not be elevated in patients 
with hypopituitarism (see Fig. 38.14). 
The senior author’s original work cited in this chapter was sup­
ported, in part, by US Public Health Service, National Institutes 
of Health grants R37 DK27085, M01 RR00036 (now UL1 
RR24992), P60 DK20579, and T32 DK07120 and fellowship 
and grant awards from the American Diabetes Association. The 
author is grateful for the contributions of postdoctoral fellows 
and the skilled nursi

ons of postdoctoral fellows 
and the skilled nursing, technical, dietary, and data management 
statistical assistance of the staff of the Washington University Gen­
eral Clinical Research Center. The authors acknowledge the assis­
tance of Alex Shimony in the preparation of this chapter.
This chapter was written after the senior author chaired a panel 
that developed Evaluation and Management of Adult Hypoglycemic 
Disorders, an Endocrine Society clinical practice guideline,2 and 
shortly after 

 clinical practice guideline,2 and 
shortly after publication of the third edition of the author’s book, 
Hypoglycemia in Diabetes: Pathophysiology, Prevalence and Preven­
tion.1 Therefore, much of the conceptual and interpretive content 
here is the same, as is no small part of the phraseology.
Disclosures
The senior author has served as a consultant to Novo Nordisk A/S 
in recent years.
1551.e1
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	178.	 Wolfsdorf JI, Sadeghi-Nejad A, Senior B. Hypoketonemia and 
age-related fasting hypoglycemia in growth hormone deficiency. 
Metabol

d glucose handling in a monocentric series of 
insulinomas. J Endocrinol Invest. 2013;36:753–758.
	195.	 Sakurai A, Yamazaki M, Suzuki S, et al. Clinical features of insuli­
noma in patients with multiple endocrine neoplasia type 1: analy­
sis of the database of the MEN Consortium of Japan. Endocr J. 
2012;59:859–866.
	196.	 Guettier JM, Lungu A, Goodling A, et al. The role of proinsulin 
and insulin in the diagnosis of insulinoma: a critical evaluation of 
the Endocrine Society clinical pract

valuation of 
the Endocrine Society clinical practice guideline. J Clin Endocrinol 
Metab. 2013;98:4752–4758.
	197.	 Service FJ, McMahon MM, O’Brien PC, Ballard DJ. Function­
ing insulinoma—incidence, recurrence, and long-term survival of 
patients: a 60-year study. Mayo Clin Proc. 1991;66:711–719.
	198.	 Anlauf M, Wieben D, Perren A, et al. Persistent hyperinsulinemic 
hypoglycemia in 15 adults with diffuse nesidioblastosis: diagnostic 
criteria, incidence, and characterization of beta-cell c

 Adult-onset nesidioblastosis 
causing hypoglycemia: an important clinical entity and continuing 
treatment dilemma. Arch Surg. 2001;136:656–663.
	202.	 Service FJ, Natt N, Thompson GB, et  al. Noninsulinoma pan­
creatogenous hypoglycemia: a novel syndrome of hyperinsulinemic 
hypoglycemia in adults independent of mutations in Kir6.2 and 
SUR1 genes. J Clin Endocrinol Metab. 1999;84:1582–1589.
	203.	 Starke A, Saddig C, Kirch B, et al. Islet hyperplasia in adults: chal­
lenge to preoperatively

perplasia in adults: chal­
lenge to preoperatively diagnose non-insulinoma pancreatogenic 
hypoglycemia syndrome. World J Surg. 2006;30:670–679.
	204.	 Vanderveen KA, Grant CS, Thompson GB, et al. Outcomes and 
quality of life after partial pancreatectomy for noninsulinoma pan­
creatogenous hypoglycemia from diffuse islet cell disease. Surgery. 
2010;148:1237–1245; discussion 1245–1236.
	205.	 Won JG, Tseng HS, Yang AH, et al. Clinical features and mor­
phological characterization of 10 patien

 and mor­
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creatogenous hypoglycaemia syndrome (NIPHS). Clin Endocrinol. 
2006;65:566–578.
	206.	 Service GJ, Thompson GB, Service FJ, et  al. Hyperinsulinemic 
hypoglycemia with nesidioblastosis after gastric-bypass surgery. N 
Engl J Med. 2005;353:249–254.
	207.	 Patti ME, McMahon G, Mun EC, et  al. Severe hypoglycaemia 
post-gastric bypass requiring partial pancreatectomy: evidence for 
inappropriate insulin secretion and pan

dence for 
inappropriate insulin secretion and pancreatic islet hyperplasia. 
Diabetologia. 2005;48:2236–2240.
	208.	 Goldfine AB, Mun E, Patti ME. Hyperinsulinemic hypoglycemia 
following gastric bypass surgery for obesity. Curr Opin Endocrinol 
Diabetes Obes. 2006;13.
	209.	 Goldfine AB, Mun EC, Devine E, et al. Patients with neurogly­
copenia after gastric bypass surgery have exaggerated incretin and 
insulin secretory responses to a mixed meal. J Clin Endocrinol 
Metab. 2007;92:4678–4685.

P, Hanaire H. Post-bypass hypoglycaemia: a review of current 
findings. Diabetes Metab. 2011;37:274–281.
	213.	 Salehi M, Gastaldelli A, D’Alessio DA. Blockade of glucagon-like 
peptide 1 receptor corrects postprandial hypoglycemia after gastric 
bypass. Gastroenterology. 2014;146:669–680.e662.
	214.	 Marsk R, Jonas E, Rasmussen F, Naslund E. Nationwide cohort 
study of post-gastric bypass hypoglycaemia including 5,040 
patients undergoing surgery for obesity in 1986-2006 in Sweden. 
Diabetolo

 al. Hypoglycemia due to an insu­
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Clin Endocrinol Metab. 2007;92:2013–2016.
	221.	 Hojlund K, Hansen T, Lajer M, et al. A novel syndrome of autoso­
mal-dominant hyperinsulinemic hypoglycemia linked to a mutation 
in the human insulin receptor gene. Diabetes. 2004;53:1592–1598.
	222.	 Meissner T, Friedmann B, Okun JG, et al. Massive insulin secre­
tion in response to anaerobic exercise in exercise-induced hyperin­
sulinism. Horm Me

, Doppman JL, et al. Intraarterial calcium 
stimulation and intraoperative ultrasonography in the localization 
and resection of insulinomas. Surgery. 1997;122:1189–1193; dis­
cussion 1193–1184.
	234.	 Wiesli P, Brandle M, Schmid C, et al. Selective arterial calcium 
stimulation and hepatic venous sampling in the evaluation of 
hyperinsulinemic hypoglycemia: potential and limitations. J Vasc 
Interv Radiol. 2004;15:1251–1256.
	235.	 Doppman JL, Miller DL, Chang R, et al. Insulinomas: localiza­

 Miller DL, Chang R, et al. Insulinomas: localiza­
tion with selective intraarterial injection of calcium. Radiology. 
1991;178:237–241.
	236.	 Liu E, Marincola P, Oberg K. Everolimus in the treatment of 
patients with advanced pancreatic neuroendocrine tumors: latest 
findings and interpretations. Therap Adv Gastroenterol. 2013;6:
412–419.
	237.	 Iacovazzo D, Flanagan SE, Walker E, et al. MAFA missense muta­
tion causes familial insulinomatosis and diabetes mellitus. Proc Natl 
Acad Sci. 2018

sulinism with long-acting release 
octreotide. Eur J Endocrinol. 2012;166:333–339.
	264.	 Ackermann AM, Palladino AA. Managing congenital hyperinsulin­
ism: improving outcomes with a multidisciplinary approach. Res 
Rep Endocr Disord. 2015;5:103–117.
	265.	 Blomberg BA, Moghbel MC, Saboury B, et al. The value of radio­
logic interventions and (18)F-DOPA PET in diagnosing and 
localizing focal congenital hyperinsulinism: systematic review and 
meta-analysis. Mol Imaging Biol. 2013;15:97–105.
	2

Hypoglycemia and the ABC’S (sugar) of pediatric resus­
citation. Ann Emerg Med. 2000;35:43–46.
	275.	 Haymond MW, Karl IE, Pagliara AS. Increased gluconeogenic 
substrates in the small-for-gestational-age infant. N Engl J Med. 
1974;291:322–328.
	276.	 Bougneres PF, Saudubray JM, Marsac C, et al. Fasting hypogly­
cemia resulting from hepatic carnitine palmitoyl transferase defi­
ciency. J Pediatr. 1981;98:742–746.
1552
39
Neuroendocrine Control of Energy 
Stores
MARTIN G. MYERS, JR., AND DAVID

 of Energy 
Stores
MARTIN G. MYERS, JR., AND DAVID P. OLSON
The Biologic Control of Energy Balance
The law of conservation of energy dictates that body energy 
stores reflect the difference between energy taken in and energy 
expended. Consuming more calories than one expends leads to 
the storage of excess energy (generally, in adipose tissue); chroni­
cally, such positive energy balance causes obesity. Conversely, 
when expenditure exceeds intake, energy/fat stores decline.
While often thought

ke, energy/fat stores decline.
While often thought of as a cosmetic problem, obesity represents 
a major health concern that contributes to the development of dia­
betes, cardiovascular disease, and cancer (among other illnesses).1 
In addition to their human toll, obesity and its complications 
cost $147 billion each year in the United States alone.1 Further­
more, while common wisdom holds that voluntary behaviors (i.e., 
choosing to eat too much) dictate energy balance and underlie the 
devel

ch) dictate energy balance and underlie the 
development of obesity, several lines of evidence demonstrate the 
biologic (involuntary) basis for the regulation of body weight and 
adiposity.2
Humans and other animals exhibit a remarkable degree of 
weight stability over the long term because the depletion of body 
energy stores activates compensatory physiologic systems to coun­
teract weight deviations and return energy stores to their previ­
ous levels or set-point.2 Energy deficit causes hype

s levels or set-point.2 Energy deficit causes hyperphagia and 
decreased energy expenditure, while forced overfeeding causes 
anorexia with increased energy expenditure, such that body weight 
ultimately returns to previous levels upon ad libitum feeding. Such 
observations prompted Gordon Kennedy3 to propose a model of 
body weight regulation in which a signal proportional to energy 
The Biologic Control of Energy Balance, 1552
The Hypothalamic Melanocortin System, 1554
Hypothal

he Hypothalamic Melanocortin System, 1554
Hypothalamic Systems and Signals That Regulate Energy 
Balance, 1556
Central Nervous System Control of Thermogenesis, 1561
Control of Glucose Homeostasis by the Brain, 1561
Intersection of Energy Balance and Reward Circuits, 1562
Brainstem Circuits That Regulate Energy Balance, 1563
KEY POINTS
	•	
The body’s physiologic systems maintain stable energy stores 
and body weight by matching long-term food intake to energy 
expenditure.
	•	
The study of mono

ake to energy 
	•	
The study of monogenetic obesity syndromes in rodents and 
humans and the identification of human genetic variants that 
predispose to obesity have revealed key gene products and 
brain systems (such as the adipose-derived hormone, leptin, 
and the hypothalamic melanocortin system) that participate in 
the regulation of energy balance.
	•	
Hypothalamic circuits mediate the long-term balance between 
energy intake and expenditure; hypothalamic nuclei that play 
i

 and expenditure; hypothalamic nuclei that play 
important roles in energy homeostasis include the arcuate 
nucleus, the ventromedial nucleus, the dorsomedial nucleus, 
the paraventricular nucleus, and the lateral hypothalamic area.
	•	
Brain regions such as the ventromedial nucleus, along with 
melanocortin circuits, also play a role in the central control of 
glucose homeostasis.
	•	
Food and drug rewards share some common neural substrates, 
and the signals and systems that control food int

and the signals and systems that control food intake and body 
weight modulate the brain circuits that control reward. Under­
standing the molecular and neural mechanisms that modulate 
rewarding aspects of feeding may reveal sites for therapeutic 
intervention in obesity.
	•	
The brainstem has traditionally been considered to control 
short-term feeding by mediating the effects of gut-derived sati­
ety signals, but the brainstem represents the likely site of action 
for several antiobesity me

 likely site of action 
for several antiobesity medications that decrease food intake 
and body weight over the long term. Brainstem nuclei that con­
tribute to the control of food intake include the area postrema, 
the nucleus of the solitary tract, the dorsal motor nucleus of the 
vagus, and the lateral parabrachial nucleus.
1553
Chapter 39  Neuroendocrine Control of Energy Stores
stores elicited compensatory changes in food intake and energy 
expenditure to maintain adipose mass at a presumed

expenditure to maintain adipose mass at a presumed set-point.
It has long been understood that organic lesions within specific 
regions of the brain can cause obesity. For example, at the end of the 
19th century clinicians, including Alfred Fröhlich, described a con­
dition characterized by pituitary tumors associated with obesity and 
hypogonadism.4,5 While several groups (including Harvey Cushing 
and his colleagues) argued that the syndrome was due to disrup­
tion of the pituitary gland,6–8 

as due to disrup­
tion of the pituitary gland,6–8 removing the pituitary gland with­
out damage to the overlying hypothalamus did not result in obesity 
in dogs.9 Indeed, it was subsequently shown that destruction of 
the medial basal hypothalamus (which contains the arcuate and 
ventromedial nuclei) without damage to the pituitary gland caused 
morbid obesity and neuroendocrine derangements similar to those 
described by Frölich.10 Conversely, lesions in other brain regions 
(such as the latera

esions in other brain regions 
(such as the lateral hypothalamic area) suppressed feeding, result­
ing in leanness.11 Thus the hypothalamus plays crucial roles in the 
control of energy balance and neuroendocrine function with early 
formulations suggesting that the lateral hypothalamic area contains 
a putative feeding center, while the medial basal hypothalamus con­
tains a presumptive satiety center.11
The past several decades have expanded our understanding of 
the systems that contribute to

r understanding of 
the systems that contribute to the control of energy balance.2 In 
addition to the originally described hypothalamic centers, we have 
Behavioral outputs
Autonomic
Outputs
Pituitary
Fat
Brainstem
Liver
Stomach
Pancreas
Muscle,
Other Tissues
Nutrients
Autonomic
Outputs
Autonomic
Outputs
Autonomic
Outputs
Insulin
Ghrelin
PYY
CCK
GLP1
Amylin
Nutrients
Endocrine
Outputs
Autonomic
Afferents
Leptin
• Fig. 39.1  Circuits and hormones that contribute to the control of fo

 and hormones that contribute to the control of food intake and energy homeostasis by 
the brain-gut-adipose axis. CCK, cholecystokinin; GLP1, glucagon-like peptide 1; PYY, peptide YY.
1554
learned a great deal about brainstem systems that control feeding, 
the circuits that modulate the incentive value of food, and the 
peripheral signals that modulate them (Fig. 39.1).
Leptin Signals the Repletion of Adipose Stores
Parabiosis (joinin

the Repletion of Adipose Stores
Parabiosis (joining the circulatory systems of two animals to permit 
the exchange of hormones) between obese rats with medial basal 
hypothalamus lesions and nonlesioned rats led to starvation and 
weight loss in the latter, while parabiosis between two medial basal 
hypothalamus-lesioned rats did not alter energy balance in either 
animal.12–14 Thus medial basal hypothalamus-lesioned obese rats 
must produce a circulating factor that inhibits feeding in normal 

rculating factor that inhibits feeding in normal 
animals and acts via a medial basal hypothalamus satiety center.11 
An important series of experiments by Douglas Coleman almost 
50 years ago provided the first insights into a potential mediator of 
this effect. Coleman used mice homozygous for ob, a recessive allele 
that causes hyperphagia, decreased energy expenditure, endocrine 
dysfunction, and obesity, and for db, which lies at a different locus 
but produces a phenotype similar to that o

 locus 
but produces a phenotype similar to that of ob. Parabiosis of lean 
(wild-type) mice with ob/ob mice suppressed weight gain in the ob/
ob mice, whereas parabiosis of wild-type and db/db mice caused 
profound hypophagia and weight loss in the wild-type mice.15–17 
Based upon these results, Coleman predicted that the ob locus pro­
duces a circulating satiety factor, while the db locus encodes a com­
ponent required for the response to the presumptive ob hormone.
Cloning of the causative ge

resumptive ob hormone.
Cloning of the causative gene mutations in ob and db strains 
confirmed the predictions of these parabiosis studies: The gene 
mutated in ob encodes a hormone of the type 1 cytokine fam­
ily (subsequently named leptin [from the Greek leptos, meaning 
“thin”]), while db affects the gene that encodes the leptin recep­
tor (LepR), a member of the type 1 cytokine receptor family.18–20 
Treatment with leptin decreases feeding, adipose mass, and body 
weight in leptin-deficient 

dipose mass, and body 
weight in leptin-deficient ob/ob (Lepob/ob) mice and in lean normal 
mice, but fails to alter db/db (Leprdb/db) mice.21–23
Adipose tissue produces leptin in approximate proportion to 
triglyceride stores, serving as a signal of the repletion of adipose 
energy stores to the central nervous system to control energy bal­
ance. Decreased leptin following caloric restriction initiates the 
neuroendocrine starvation response, increasing food-seeking and 
appetite and suppressin

creasing food-seeking and 
appetite and suppressing the expenditure of energy by neuroendo­
crine systems (resulting in infertility, decreased sympathetic ner­
vous system tone, thyroid function, etc.).24,25 Exogenous leptin 
reverses the neuroendocrine manifestations of starvation as well 
as the neuroendocrine dysfunction of Lepob/ob mice. Leptin also 
reverses the hyperphagia, obesity, and neuroendocrine dysfunc­
tion of rare human patients with congenital leptin deficiency.26–28
Similarly, h

th congenital leptin deficiency.26–28
Similarly, human patients and transgenic animals that lack adipose 
tissue (lipodystrophy) exhibit hyperphagia along with a predisposi­
tion to insulin resistance and other endocrine and metabolic abnor­
malities that are not corrected with caloric restriction.29,30 Due to the 
dearth of adipose tissue in lipodystrophy, this syndrome causes low 
leptin concentrations, and leptin treatment improves hunger indices 
and endocrine/metabolic abnormalities. Indeed

ces 
and endocrine/metabolic abnormalities. Indeed, leptin was recently 
approved for the treatment of lipodystrophy syndromes in humans.31
In contrast to the dramatic ability of leptin to reverse the hyper­
phagia and neuroendocrine abnormalities associated with congeni­
tal leptin deficiency and lipodystrophy, exogenous leptin modestly 
and briefly blunts food intake and body weight in normal-weight 
animals and minimally alters energy balance in obese humans or 
animals that have elevated end

in obese humans or 
animals that have elevated endogenous leptin due to increased fat 
mass.32 Indeed, serum leptin levels in humans are generally propor­
tional to adipose mass,33,34 thus the vast majority of obese humans 
are not leptin deficient, but rather fail to respond to elevated leptin 
concentrations with weight loss. Hence the absence of leptin sends 
a more powerful physiologic signal than does its excess. Indeed, 
while low leptin clearly represents the crucial signal of energy defi

early represents the crucial signal of energy deficit, 
leptin does not mediate the anorexia that follows forced overfeed­
ing,35 suggesting the existence of a separate signal important for the 
suppression of food intake during nutritional surfeit. 
The Hypothalamic Melanocortin System
In 1902, French geneticist Lucien Cuenot described the obese 
agouti mouse (Ay/a; also termed lethal yellow because homozygotes 
for the allele die prenatally) that had been bred for decades by Euro­
pean mouse f

at had been bred for decades by Euro­
pean mouse fanciers.36 The Ay/a strain is notable for the dominant 
inheritance of an obesity phenotype proportional to the intensity 
of its yellow coat color.36 The increased body weight of Ay/a mice 
results mainly from hyperphagia and reflects both increased fat 
mass and lean body mass (with increased body length).37
The Ay allele results from a genetic rearrangement that deletes 
the Raly gene and fuses the Asip gene to the Raly promoter, result­
ing i

 the Asip gene to the Raly promoter, result­
ing in continuous expression of agouti signaling protein (ASIP) 
throughout the body.38,39 ASIP is a secreted peptide that binds 
to and inhibits melanocortin receptors. The yellow coat color of 
the Ay/a mouse results from overexpression of ASIP in the skin, 
which blocks α-melanocyte stimulating hormone (αMSH) signal­
ing at MC1 receptors (MC1R) in the hair follicle.38 Since MC1R 
promotes the synthesis of eumelanin (black pigment) instead of 
pheom

sis of eumelanin (black pigment) instead of 
pheomelanin (yellow pigment) in melanocytes, ASIP-mediated 
antagonism of hair follicle MC1R results in yellow coat color.
The brain contains two predominant melanocortin receptor 
isoforms: MC3R and MC4R,40 both of which are activated by 
αMSH and inhibited by ASIP. Intracerebroventricular administra­
tion of αMSH or other melanocortin receptor agonists decreases 
food intake and body weight; overexpression of ASIP in the brain 
antagonizes the anore

ession of ASIP in the brain 
antagonizes the anorectic action of αMSH signaling and blunts 
endogenous melanocortin receptor activity resulting in hyperpha­
gia. Thus the hypothalamic melanocortin system is crucial for the 
control of food intake and energy balance.
At its core, the hypothalamic melanocortin system includes arcu­
ate nucleus neurons that express pro-opiomelanocortin (POMC), 
the precursor peptide for melanocortin receptor agonists. POMC 
in the arcuate nucleus is processed to pr

s. POMC 
in the arcuate nucleus is processed to produce αMSH (and at times, 
β-endorphin), although other POMC-expressing cells in the pitu­
itary and elsewhere produce other POMC-derived peptides41 (Fig. 
39.2). Mice null for Pomc throughout the body or specifically in the 
arcuate nucleus are hyperphagic and weigh approximately twice as 
much as control animals in adulthood.42 These animals also exhibit 
adrenal insufficiency due to the lack of POMC-derived adrenocor­
ticotropic hormone (ACTH)

 POMC-derived adrenocor­
ticotropic hormone (ACTH) in the anterior pituitary.
In addition to POMC neurons, the arcuate nucleus con­
tains distinct neurons that express the ASIP homolog, agouti-
related peptide (AgRP).38 AgRP (like ASIP) binds and inhibits 
melanocortin receptors (especially MC3 and MC4 receptors, 
which are mainly expressed in the brain), and intracerebroven­
tricular administration of AgRP elicits a long-lasting hyperphagic 
response in rodents.38 These neurons also contain the

ponse in rodents.38 These neurons also contain the inhibitory 
transmitters neuropeptide Y (NPY, which, like AgRP, promotes 
feeding when administered intracerebroventricularly) and gamma 
amino butyric acid (GABA). Negative energy balance (e.g., caloric 
restriction) activates these arcuate nucleus NPY/AgRP–containing 
neurons and increases the expression of Agrp and Npy.43
Mice null for Mc4r display substantial hyperphagia and increased 
adiposity body weight, and display increased linear grow

ity body weight, and display increased linear growth, as is 
1555
characteristic of Ay/a mice.44 Mice null for Mc3r display a subtler 
obesity phenotype, as noted later in the chapter. Thus MC4R rep­
resents the major melanocortin receptor that mediates the control 
of food intake and body weight. MC4R is expressed in hypotha­
lamic sites critical for feeding control, including the paraventricular 
nucleus, dorsomedial nucleus, ventromedial nuc

ar 
nucleus, dorsomedial nucleus, ventromedial nucleus, and lateral 
hypothalamic area39 as well as in the brainstem and other brain 
areas. While some other sites play roles in the control of energy 
balance, Mc4r expression in the paraventricular nucleus is necessary 
and sufficient for the control of feeding.45,46 Thus the hypothalamic 
melanocortin system suppresses food intake by acting on Mc4r-
expressing paraventricular nucleus neurons (Fig. 39.3).
Extrahypothalamic MC4Rs also contribute 

g. 39.3).
Extrahypothalamic MC4Rs also contribute to melanocortin 
action on energy balance. For example, MC4R agonist injec­
tion into the fourth ventricle or the dorsal motor nucleus of the 
vagus decreases food intake, while injection of MC4R antagonists 
increases food intake.47,48 Also, deleting MC4Rs in autonomic 
(cholinergic) neurons lowers energy expenditure and increases 
adiposity, especially in high-fat diet–fed animals.49,50 Deleting 
MC4Rs in parasympathetic preganglionic neurons (

MC4Rs in parasympathetic preganglionic neurons (vagal motor 
neurons) causes hyperinsulinemia and insulin resistance but does 
not significantly impact energy balance, suggesting an important 
role for MC4R in presympathetic cells for the control of energy 
expenditure.49,50
MC3R, which is distributed in central nervous system sites 
largely distinct from those that contain MC4R, also plays a role 
in energy homeostasis. In the mouse, this receptor is most highly 
expressed in arcuate nucleus 

ptor is most highly 
expressed in arcuate nucleus POMC and NPY/AGRP neurons, 
the ventromedial nucleus, the ventral tegmental area, and the 
medial habenula.51,52 Within the arcuate nucleus, MC3R is also 
expressed presynaptically on NPY/AGRP terminals and acts as 
an inhibitory autoreceptor on the POMC-MC4R circuitry.53 
Although Mc3r haploinsufficiency does not produce a phenotype, 
homozygous Mc3r deletion in mice causes increased adipose mass, 
decreased lean mass, reduced fast-induced refee

, 
decreased lean mass, reduced fast-induced refeeding,54–56 elevated 
basal and fasting-induced corticosterone,54 and defects in circa­
dian rhythms and meal entrainment.57
The first evidence for a human melanocortin obesity syndrome 
resulted from the astute recognition of a rare agouti mouse–like 
syndrome in two families, resulting from null mutations in the 
POMC gene58 (Fig. 39.4). These patients have ACTH insuffi­
ciency, red hair, and obesity, resulting from the lack of ACTH 
peptide in 

sity, resulting from the lack of ACTH 
peptide in the serum and a lack of melanocortin peptides in skin 
and brain, respectively. This obesity syndrome demonstrated that 
the central melanocortin circuitry subserves energy homeostasis 
in humans as it does in the mouse. Shortly after the description 
of POMC mutations in humans, heterozygous frameshift muta­
tions in the human MC4R locus associated with nonsyndromic 
obesity in two separate families were reported.59,60 Additional 
reports61–63 p

ies were reported.59,60 Additional 
reports61–63 provide a clearer picture of the frequency and diver­
sity of MC4R mutations and revealed that MC4R haploinsuffi­
ciency in humans represents the most common monogenic cause 
of severe obesity, accounting for up to 5% of cases. Remarkably, 
the associated physical findings are virtually identical to those 
reported for the mouse,44 with increased adipose mass, increased 
linear growth and lean mass, hyperinsulinemia greater than that 
seen in matc

, hyperinsulinemia greater than that 
seen in matched obese control subjects, and severe hyperphagia. 
MC4R haploinsufficient adults also exhibit reduced sympathetic 
tone and mild hypotension.64 Initial pharmacologic approaches 
to melanocortin obesity syndromes were complicated by blood 
pressure effects of MC4R agonist therapy. Recently, however, the 
MC4R agonist, setmelanotide (currently under clinical develop­
ment), has been shown to promote significant weight loss without 
associated blo

te significant weight loss without 
associated blood pressure problems in patients with either POMC 
deficiency or leptin receptor deficiency.65,66 Thus central nervous 
system circuits that regulate energy balance may be targeted for 
therapeutic benefit.
While a syndrome resulting from MC3R mutations in humans 
has not yet been definitively identified, several other monogenetic 
obesity syndromes in mice and humans likely result from alter­
ations in melanocortin signaling. The products of man

ons in melanocortin signaling. The products of many of the 
affected genes play important roles in peptide processing. POMC 
is cleaved by prohormone convertase 1 (PC1, also called PCSK1 
or PC1/3) and PC2 (PCSK2) to generate a precursor peptide that 
Signal peptidase
PC1/PC3
PC1/PC3
& PC2
Preprohormone
N
C
Pituitary anterior lobe
N-terminal
ACTH
β-LPH
γ-MSH
(51-62)
γ-LPH
(1-58)
α-MSH
(1-13)
β-Endorphin
(61-91)
CLIP
(18-39)
Pituitary intermediate
lobe and arcuate nucleus
of hypothalam

ntermediate
of hypothalamus
• Fig. 39.2  Organization of pro-opiomelanocortin (POMC), the precursor hormone of corticotropin (ACTH, 
adrenocorticotropic hormone), β-lipoprotein (βLPH), and related peptides. The precursor protein contains 
a leader sequence (signal peptide), followed by a long fragment that includes sequence 51-62, corre­
sponding to γ-melanocyte-stimulating hormone (γMSH). This fragment is cleaved at Lys-Arg bonds to form 
corticotropin 1-39, which in tu

Arg bonds to form 
corticotropin 1-39, which in turn includes the sequences for αMSH (corticotropin 1-13) and corticotropin-
like intermediate lobe peptide (CLIP; corticotropin 18-39) and a sequence corresponding to βLPH (1-91) 
that includes γLPH (1-58) and β-endorphin (61-91). The β-endorphin sequence also includes a sequence 
corresponding to met-enkephalin. The precursor molecule in the anterior lobe of the pituitary is processed 
predominantly to corticotropin and βLPH. In the intermediate 

ly to corticotropin and βLPH. In the intermediate pituitary lobe (in the rat), corticotropin and 
βLPH are further processed to αMSH and a β-endorphin–like material. In all extrapituitary tissues, post-
translational processing of the prohormone resembles that in the intermediate lobe. Hypothalamic pro­
cessing is similar but not identical to that in the intermediate lobe. In the latter, β-endorphin and αMSH are 
present predominantly in their acetylated forms. C, carboxy-terminal; PC, prohormon

etylated forms. C, carboxy-terminal; PC, prohormone-converting enzyme. 
(Courtesy Dr. Malcolm Low, University of Michigan, Ann Arbor, MI.)
1556
is further cleaved by carboxypeptidase E (CPE) to generate active 
αMSH67 (see Fig. 39.2). Prolyl carboxypeptidase (PRCP) acts on 
substrate proteins (such as αMSH) where the penultimate amino 
acid is a proline residue, cleaving the COOH-terminal amino acid 
and inactivating the peptide.68
The

al amino acid 
The fat mutation, which causes diabetes and obesity in mice, 
results from a point mutation in the Cpe gene69; miscleavage of 
POMC presumably underlies the obesity in Cpefat mice. Similarly, 
mice and humans null for Pcsk1 are obese with adrenal insuffi­
ciency (in addition to displaying other endocrine defects) due to 
impaired processing of POMC and other peptides.70,71 Mice null 
for Pcsk2 are not obese, however,72 presumably due to the par­
tia

t obese, however,72 presumably due to the par­
tial activity of the POMC PC1 product on MC3/4R, even in the 
absence of PC2-mediated processing. Conversely, mice null for 
Prcp are lean due to prolonged activity of αMSH.68
Hypothalamic Systems and Signals That 
Regulate Energy Balance
The hypothalamus, a highly conserved brain region, plays cru­
cial roles in the maintenance of homeostasis; destruction of the 
hypothalamus is not compatible with life.73 The hypothalamus 
receives sensory inputs 

life.73 The hypothalamus 
receives sensory inputs from the external environment and infor­
mation regarding the internal environment. In addition, several 
hormones crucial for the regulation of food intake and metabolism 
(e.g., leptin, ghrelin, insulin, estrogen) directly act on neurons in 
the hypothalamus. The hypothalamus integrates these inputs to 
modulate key outputs (including the pituitary gland, the cerebral 
cortex, premotor and motor neurons in the brainstem and spinal 
cord, and au

 neurons in the brainstem and spinal 
cord, and autonomic [parasympathetic and sympathetic] pregangli­
onic neurons) to coordinate endocrine, behavioral, and autonomic 
responses that maintain homeostasis in several physiologic systems, 
including energy balance (see Fig. 39.1). Several hypothalamic sites 
(including the arcuate nucleus, ventromedial nucleus, dorsomedial 
nucleus, paraventricular nucleus, and lateral hypothalamic area) 
play key roles in coordinating food intake and other parame

roles in coordinating food intake and other parameters of 
energy homeostasis. Information flow for the hypothalamic circuits 
generally begins with the arcuate nucleus, the entry point for many 
hormonal signals (such as insulin and leptin) (Fig. 39.5). The para­
ventricular nucleus mediates important outflow from the hypothal­
amus to brainstem centers that control food intake. In between, the 
dorsomedial nucleus integrates other signals and relays information 
to and from the arcuate and par

elays information 
to and from the arcuate and paraventricular nuclei.
MC3R
Nutrient partitioning
ARC
VMH
LHA
VTA
Satiety
Energy expenditure
homeostasis
Lipid mobilization
Linear growth
Glucose
Autonomic output
PBN
DMV/NTS
SC
LHA
others
Glucose
Lean mass accrual
Locomotor Activity
Circadian Entrainment
MC4R
Melanocortins
(POMC or AgRP)
PVH
• Fig. 39.3  Distribution of melanocortin action in the central nervous system. Pro-opiomelanocortin neu­
rons (P

 nervous system. Pro-opiomelanocortin neu­
rons (POMC) in the arcuate nucleus (ARC) produce melanocortin-receptor agonists, whereas agouti-
related peptide (AgRP)–producing neurons antagonize melanocortin action. MC3R and MC4R are the 
predominant melanocortin receptors in the brain and have relatively distinct physiologic roles. MC3R is 
expressed at high levels in the ARC and the ventromedial nucleus (VMN), where it mainly controls nutrient 
partitioning, conversion of food to fat, and lean ma

rtitioning, conversion of food to fat, and lean mass accrual. MC3R in the lateral hypothalamic area (LHA) 
and ventral tegmental area (VTA) likely control motivation and activity. MC4R is expressed in many brain 
regions and plays a critical role in energy balance. MC4Rs in the paraventricular hypothalamic nucleus 
(PVH) profoundly affect food intake through projections to the parabrachial nucleus (PBN). MC4Rs in other 
brain regions, including the lateral hypothalamus (LHA), dorsal motor nucleu

he lateral hypothalamus (LHA), dorsal motor nucleus of the vagus (DMV), nucleus 
of the solitary tract (NTS), and spinal cord (SC) are involved in energy expenditure, autonomic output, and 
glucose regulation. MC4R action in the DMV regulates insulin secretion, and MC4Rs regulate sympathetic 
output (and energy expenditure) through the intermediolateral cell column of the spinal cord (IML). The 
sites mediating melanocortin action on linear growth, lipid handling, and blood pressure are not defi

h, lipid handling, and blood pressure are not defined.
1557
Orthologs of many of the genes that encode crucial media­
tors of energy balance and/or that underlie rodent obesity models 
also cause or contribute to obesity in humans. In the past several 
decades, causative mutations in multiple genes have been found in 
human obesity disorders, including LEP, LEPR, POMC, MC4R, 
PCSK1, GNAS, SH2B1, BDNF, TRKB, and SIM1.74 These men­
delian disease

 BDNF, TRKB, and SIM1.74 These men­
delian disease genes appear to act primarily in the hypothalamus. 
Furthermore, genome-wide association studies have identified 
over 100 single nucleotide polymorphisms significantly associated 
with body mass index, and these are predominantly located in or 
near genes thought to act primarily in the hypothalamus.75,76
Leptin-Regulated Hypothalamic Circuits
Alternative splicing of the Lepr transcript produces multiple LepR 
isoforms77; Leprdb produces an mRN

 multiple LepR 
isoforms77; Leprdb produces an mRNA splicing error that inserts the 
final exon of one of the short LepR isoforms (LepRa) into the mRNA, 
replacing the long LepRb isoform (which uniquely contains all intra­
cellular cytokine receptor signaling motifs) with LepRa throughout 
the body.78 Because the Leprdb allele synthesizes all LepR isoforms 
except LepRb, this isoform plays a crucial role in energy homeostasis.79
Consistent with leptin’s behavioral effects (e.g., on feeding) 
and

eptin’s behavioral effects (e.g., on feeding) 
and its effects on the neuroendocrine and autonomic systems, 
most LepRb-expressing cells lie in the brain,19,80,81 suggesting the 
potential importance of the central nervous system as a site of 
leptin action. Indeed, transgenic overexpression of LepRb in the 
central nervous system substantially corrects the obesity syndrome 
Hindbrain
Circulating Hormones,
Nutrients
ARC
LepRb
LepRb
DMH
PVH
Spinal Cord
SNS
Pituitary
endocrine
MC4R
(and
Other)
Sup

 Cord
SNS
Pituitary
endocrine
MC4R
(and
Other)
of Feeding
Melanocortin
system
(some LepRb)
5HT
PVH
ARC
A
B
DMH
• Fig. 39.5  Flow of information into and out of the hypothalamus. (A) Cartoon diagram of circuits. ARC, arcuate 
nucleus; DMH, dorsomedial nucleus; PVH, paraventricular nucleus; LepRb, leptin receptor; SNS, sympathetic 
nervous system. Core “homeostatic” hypothalamic circuits are shown in red, brainstem circuits are in dark 
blue, peripheral signals are green, leptin recept

blue, peripheral signals are green, leptin receptor pathways are in orange, and melanocortin pathways are in 
light blue. (B) Images of hypothalamic regions in question, using LepRb-GFP (left panel) reporter mice showing 
LepRb neurons in areas, including the ARC and DMH and LepRb-enhanced green fluorescent protein (EGFP) 
(right panel) reporter mice, showing projections from LepRb neurons into regions, including the PVH.
• Fig. 39.4  A monogenic neuroendocrine obesity syndrome of adrenocor­
ti

c neuroendocrine obesity syndrome of adrenocor­
ticotropic hormone insufficiency, obesity, and red hair resulting from a null 
mutation in the pro-opiomelanocortin gene. (Photograph courtesy Dr. A. 
Gruters, Charité-Universitätsmedzin, Berlin, Germany.)
1558
of Leprdb-3J mice (which lack all LepR isoforms).82 Similarly, abla­
tion of LepRb specifically in the central nervous system promotes 
hyperphagia, neuroendocrine failure, and obe

otes 
hyperphagia, neuroendocrine failure, and obesity.83
Within the brain, the majority of LepRb-expressing neurons 
reside within the hypothalamus and brainstem, consistent with 
the known roles for these structures in the control of feeding and 
endocrine and autonomic function.80,81,84 In the brainstem, LepRb 
knockdown in the nucleus of the solitary tract and surround­
ing regions alters the control of meal size and slightly increases 
long-term food intake,85 while lateral parabrachial nuc

term food intake,85 while lateral parabrachial nucleus LepRb 
ablation alters the response to hypoglycemia and other metabolic 
emergencies but does not impact energy balance.86,87 In contrast, 
pan-hypothalamic ablation of LepRb promotes a phenotype very 
similar in quality and magnitude to that of whole-body null 
Leprdb/db animals, suggesting that hypothalamic LepRb neurons 
mediate the majority of leptin action on energy balance.88
Within the hypothalamus, multiple nuclei contain popula­
tio

 hypothalamus, multiple nuclei contain popula­
tions of LepRb neurons, including the arcuate nucleus, ventro­
medial nucleus, dorsomedial nucleus, lateral hypothalamic area, 
and ventral premammillary nucleus.80,81 Substantial subpopula­
tions of arcuate nucleus POMC and NPY/AGRP neurons contain 
LepRb, although additional LepRb-expressing arcuate nucleus 
populations also exist.89–91 While it was initially assumed that 
the absence of LepRb from POMC and/or NPY/AGRP neu­
rons would recapitulate

 POMC and/or NPY/AGRP neu­
rons would recapitulate the obesity phenotype of Leprdb/db mice 
(which results from defects that exist in the early embryo), the 
early embryonic ablation of LepRb from these cell populations 
only minimally alters energy balance, suggesting important roles 
for LepRb in distinct hypothalamic population(s).92–94
Importantly, however, undefined compensatory processes 
attenuate phenotypes that result from the early developmental 
alteration of NPY/AGRP cells, and ablat

lopmental 
alteration of NPY/AGRP cells, and ablating LepRb from NPY/
AGRP cells in adult mice produces a dramatic hyperphagic obe­
sity.95 Thus while LepRb in NPY/AGRP neurons contributes only 
minimally to the Leprdb/db phenotype, the control of NPY/AGRP 
neurons by LepRb plays a crucial role in energy balance in adult 
animals.
Roles in leptin action have been examined for other cir­
cumscribed sets of hypothalamic LepRb neurons as well. Early 
embryonic ablation of LepRb in the ventromedial 

embryonic ablation of LepRb in the ventromedial nucleus blunts 
energy expenditure and thereby increases adiposity,96 while dele­
tion of LepRb in neurotensin-expressing lateral hypothalamic 
area neurons diminishes motor activity to blunt energy expen­
diture and increase adiposity.96,97 LepRb in the ventral premam­
millary nucleus contributes to the control of reproduction98 but 
not energy balance; LepRb in arcuate nucleus growth hormone–
releasing hormone (Ghrh)–expressing cells does not c

leasing hormone (Ghrh)–expressing cells does not contribute 
detectably to the control of growth or energy balance.94
In contrast to the minimal phenotypes observed following the 
early embryonic ablation of LepRb in the circumscribed sets of 
hypothalamic LepRb neurons tested to date, ablation of LepRb 
from broadly distributed hypothalamic vesicular GABA transporter 
(vGat)–expressing or nitric oxide synthase-1 (NOS1)–express­
ing neurons promotes dramatic hyperphagia and obesity.99,100 
While

tes dramatic hyperphagia and obesity.99,100 
While this might suggest that the total number of LepRb neurons 
affected (rather than the specific cell types) matters most for the 
control of energy balance, deletion of LepRb across broad popu­
lations of hypothalamic serotonin 2c receptor (Ht2cr)–expressing 
neurons largely distinct from vGat and NOS1 cells minimally 
impacts energy balance,94 suggesting that specific LepRb cell 
types do play specialized roles in energy balance. The dorsome­
dia

cialized roles in energy balance. The dorsome­
dial nucleus contains substantial populations of both vGat and 
NOS1-expressing LepRb neurons, suggesting that important sets 
of food intake–controlling LepRb neurons may lie in the dorso­
medial nucleus. Although a subpopulation of prolactin-releasing 
hormone (Prlh)–expressing dorsomedial nucleus LepRb neurons 
contributes only to the control of energy expenditure,101 ablation 
of dorsomedial nucleus LepRb in adults increases food intake and 
pro

eus LepRb in adults increases food intake and 
promotes dramatic obesity,102 suggesting an important role for 
non-Prlh dorsomedial nucleus LepRb neurons in the control of 
food intake and energy balance. The vGat-containing dorsome­
dial nucleus LepRb neurons that play important roles in the acute 
regulation of NPY/AGRP neurons103 may represent at least some 
of these crucial energy balance–controlling LepRb neurons. 
Roles for NPY/AGRP Neurons in Energy Balance
Not only does activating NPY/AG

 in Energy Balance
Not only does activating NPY/AGRP neurons increase food 
intake and body weight,104 but ablating NPY/AGRP neurons in 
adult animals results in anorexia.105–107 Thus NPY/AGRP neu­
rons play important roles in the control of food intake and energy 
balance. Indeed, while leptin inhibits NPY/AGRP neurons, these 
cells also respond to other hormonal modulators of energy balance 
(including ghrelin, which activates NPY/AGRP cells to promote 
food intake [see upcoming discussion]), 

 promote 
food intake [see upcoming discussion]), and nutrient signals (such 
as glucose, which inhibits NPY/AGRP cells).108
While early models proposed that the endogenous MC4R ago­
nist/antagonist pair (αMSH and AgRP, respectively) might simply 
act competitively to determine the relative amount of activation 
of the MC4R, recent findings have demonstrated additional roles 
for AgRP.109 AgRP not only acts via MC4R to control the activ­
ity of the G protein, GαS, but also alters membrane potent

he G protein, GαS, but also alters membrane potential in 
target neurons by controlling potassium channels independently 
of G proteins.
Furthermore, NPY/AGRP neurons control feeding via several 
neuronal mechanisms. While NPY/AGRP cells contain the orexi­
genic peptides AgRP and NPY, early developmental ablation of 
these peptides minimally alters the ability of NPY/AGRP neu­
rons to promote food intake or to control energy balance.110 In 
contrast, ablation of GABA signaling by NPY/AGRP cells 

ast, ablation of GABA signaling by NPY/AGRP cells abro­
gates their orexigenic action.111 GABA signaling by NPY/AGRP 
cells not only provides direct inhibitory input to arcuate nucleus 
POMC neurons but also is required to promote food intake 
by the paraventricular nucleus and to inhibit anorexigenic and 
aversive lateral parabrachial nucleus cells that express calcitonin 
gene–related peptide (CGRP) (see hindbrain section later in the 
chapter).105,111,112
Ghrelin
Ghrelin (an acylated 28–amino

.105,111,112
Ghrelin
Ghrelin (an acylated 28–amino acid peptide that potently stimu­
lates food intake) acts via the ghrelin receptor (known as the growth 
hormone secretagogue receptor [GHSR]).113–117 Predominantly 
secreted by the stomach, ghrelin levels are markedly reduced with 
meal ingestion in both rodents and humans but rebound to base­
line before the next meal or increase after an overnight fast115–117 
(Fig. 39.6). Ghrelin is unique in that it requires the addition of an 
eight-carbon

 that it requires the addition of an 
eight-carbon fatty acid (octanoate) side chain to have agonist activ­
ity at the ghrelin receptor.118 The enzyme ghrelin O-acyltransferase 
catalyzes the addition of the octanoyl group.118 Unlike other signals 
involved with energy homeostasis, ghrelin secretion is inhibited in 
response to meals and stimulates appetite, leading to the notion 
that ghrelin participates in meal initiation.117 Fasting ghrelin levels 
are inversely proportional to body weight11

evels 
are inversely proportional to body weight115 and are higher than 
normal in underweight subjects with anorexia nervosa and cardiac 
1559
cachexia compared to control subjects.114,119 Both of these changes 
presumably reflect the regulation of ghrelin by nutritional status 
rather than any role for ghrelin in the pathogenesis of these diseases.
Peripheral administration of ghrelin activates arcuate nucleus 
NPY/AGRP neurons,120 and ablati

s arcuate nucleus 
NPY/AGRP neurons,120 and ablation of the arcuate nucleus 
blocks the actions of the ghrelin administration on feeding.121 
Furthermore, re-expression of the ghrelin receptor only in NPY/
AGRP neurons in ghrelin receptor-null mice suffices to restore the 
orexigenic effect of exogenous ghrelin.122
Despite the significant amount of data suggesting that 
ghrelin increases food intake, mice null for ghrelin or ghrelin 
receptor demonstrate little alteration of food intake or energ

onstrate little alteration of food intake or energy 
balance. Similarly, neither genetic deletion of ghrelin O-acyl­
transferase123 nor the ablation of ghrelin-producing cells124 
results in major changes in food intake and body weight. These 
findings have brought into question the physiologic relevance of 
ghrelin. They may also reflect developmental compensation of 
NPY/AGRP neurons to the lack of ghrelin action. 
Roles for the Paraventricular Nucleus in Energy Balance
The metabolic consequen

 Nucleus in Energy Balance
The metabolic consequences that result from abnormal para­
ventricular nucleus development or the interference with para­
ventricular nucleus function underscore the importance of the 
paraventricular nucleus in energy balance and autonomic func­
tion. Destruction of the paraventricular nucleus by electrolytic 
lesion causes hyperphagia and obesity.125 Similarly, loss of one 
copy of single-minded homolog-1 (Sim1), a key transcription 
factor regulating paraventricular

y transcription 
factor regulating paraventricular nucleus development, disrupts 
paraventricular nucleus development and function, resulting 
in hyperphagic obesity with associated glucose dysregulation in 
both rodents and humans.126–128 In addition, the paraventricular 
nucleus serves as an important regulatory output center for stimuli 
that modulate food intake, including leptin, melanocortins, and 
dehydration.129,130 Melanocortin agonists engage and stimulate 
melanocortin receptor–bearin

engage and stimulate 
melanocortin receptor–bearing neurons in the paraventricular 
nucleus to activate effector pathways that inhibit food intake and 
stimulate energy expenditure (see Fig. 39.5). Melanocortin action 
in paraventricular nucleus neurons plays crucial roles in the con­
trol of food intake, energy balance, and glucose homeostasis.45,131 
While the role for the paraventricular nucleus in regulating energy 
balance is clear, much less is known about the identity of the neu­
rons and

s is known about the identity of the neu­
rons and specific circuitry mediating these effects, however.
The paraventricular nucleus is composed of a heterogeneous 
group of glutamatergic neurons that have been classically described 
as parvocellular or magnocellular based on cell size and axonal 
projection patterns. Paraventricular nucleus parvocellular cells are 
neurochemically diverse and send projections within the central 
nervous system to three main areas: (1) the median eminence, where 

 three main areas: (1) the median eminence, where 
secreted factors (e.g., thyroid-releasing hormone [TRH] and cor­
ticotrophin-releasing hormone [CRH]) enter the portal hypophy­
seal circulation and affect pituitary function; (2) the brainstem, 
including the dorsal vagal complex (comprised of the dorsal motor 
nucleus of the vagus and the nucleus of the solitary tract) and the 
lateral parabrachial nucleus, both of which have been implicated in 
feeding105,132; and (3) the preganglionic sympat

feeding105,132; and (3) the preganglionic sympathetic output cen­
ters, such as the intermediolateral column of the spinal cord.133,134 
Hypothalamic parvocellular paraventricular nucleus neurons that 
respond to the satiety signals are thought to regulate feeding by 
modulating hindbrain responses to ascending signals from the gut 
and periphery.135–137 Magnocellular neurons (expressing oxytocin 
or vasopressin [AVP]) send their axonal projections to the posterior 
pituitary to release their 

ions to the posterior 
pituitary to release their neuropeptides and transmitters directly 
into the systemic circulation. Paraventricular nucleus-mediated 
energy balance control is thought to be regulated predominantly 
by parvocellular paraventricular nucleus neurons.
Specific Roles of Paraventricular Nucleus Neuronal 
Subsets in Metabolic Homeostasis
Oxytocin and CRH neurons within the paraventricular nucleus 
have garnered significant attention as potential regulators of 
energy balance. Bot

on as potential regulators of 
energy balance. Both oxytocin and CRH peptides alter feeding 
when delivered to the brain, and oxytocin and CRH paraventricu­
lar nucleus neurons project to other brain regions that regulate 
feeding. Rodents lacking oxytocin or CRH have near normal 
energy balance under basal conditions, however.138,139
Moreover, direct activation of paraventricular nucleus oxyto­
cin neurons does not alter feeding, and ablation of paraventricular 
nucleus oxytocin has little effe

 paraventricular 
nucleus oxytocin has little effect on body weight unless animals 
are challenged with a high-fat diet.140,141 In addition, mice lack­
ing oxytocin neurons demonstrate normal anorectic responses to 
A
Ghrelin (pg/mL)
800
700
600
500
400
300
200
B
Insulin (pmol/L)
600
500
400
300
200
100
0
C
Time of day
B
L
D
Leptin (ng/mL)
25
20
15
0400
0200
2400
2200
2000
1800
1600
1400
1200
1000
0800
0600
• Fig. 39.6  Average plasma ghrelin (A), insulin (B), and leptin (C) concentra­
tions dur

, insulin (B), and leptin (C) concentra­
tions during a 24-hour period in 10 human subjects consuming breakfast 
(B), lunch (L), and dinner (D) at the times indicated (0800, 1200, and 1730 
hours, respectively). (From Cummings DE, Purnell JQ, Frayo RS, et al. A 
preprandial rise in plasma ghrelin levels suggests a role in meal initiation in 
humans. Diabetes. 2001;50:1714–1719.)
1560
melanocortin agonists. Thus oxytocin neurons (and th

lanocortin agonists. Thus oxytocin neurons (and their contents) 
are not required for anorectic melanocortin action, and other (non­
oxytocin) paraventricular nucleus neurons play crucial roles in the 
control of feeding (including in response to melanocortin action).
Paraventricular nucleus MC4R neurons play a critical role in 
energy balance control. Deletion of paraventricular nucleus Mc4r 
results in hyperphagic obesity, whereas re-expression of Mc4r 
selectively in the paraventricular nucle

 of Mc4r 
selectively in the paraventricular nucleus in an otherwise null 
Mc4r background normalizes feeding and dramatically attenuates 
the obesity phenotype of these mice.45,131 Selective manipulation 
of paraventricular nucleus MC4R-expressing neurons revealed 
that these neurons directly regulate feeding but not energy expen­
diture.45 Indeed, the paraventricular nucleus MC4R → lateral para­
brachial nucleus projection pathway is critical for these feeding 
effects.46 Other paraventricular

or these feeding 
effects.46 Other paraventricular nucleus cell types shown to be 
involved in energy balance regulation include brain-derived neuro­
trophic factor-expressing neurons,142 Nos1-expressing neurons,140 
AVP neurons,143 and nesfatin-expressing cells.144 Whether these 
neurochemical markers identify unique cell populations, or target 
a common neural circuitry for anorexia, remains unclear. 
Role for the Ventromedial Nucleus in Energy 
Balance
As noted earlier, medial basal hypothala

Balance
As noted earlier, medial basal hypothalamus lesion studies demon­
strated that combined ablation of the arcuate nucleus and ventro­
medial nucleus produce obesity. In addition to the roles played by 
arcuate nucleus (including POMC and NPY/AGRP neurons), the 
ventromedial nucleus contributes to the control of energy balance 
as well. Indeed, animals null for the transcription factor steroido­
genic factor-1 (SF1), which is expressed in most if not all ventro­
medial nucleus neurons,145

most if not all ventro­
medial nucleus neurons,145 exhibit ventromedial nucleus dysgenesis 
and adrenal agenesis and develop late-onset obesity.97,146,147 Fur­
thermore, deletion of LepRb from SF1 neurons produces obesity 
due to decreased energy expenditure (especially in animals exposed 
to high-fat diet),96 but without increasing food intake, suggesting 
a role for ventromedial nucleus neurons in the control of energy 
expenditure. Indeed, the ventromedial nucleus represents an impor­
tant si

 ventromedial nucleus represents an impor­
tant site for the modulation of sympathetic nervous system outflow. 
Serotonergic Control of Energy Balance
The central serotonin (5-hydroxytryptamine [5HT]) system rep­
resents another key regulator of energy balance. This system con­
sists of 5HT containing cell bodies that lie in several midbrain 
raphe nuclei and that project widely through the central nervous 
system; 5HT acts via 14 different 5HT receptors (HTRs).148,149 
Note also that 5HT repres

eceptors (HTRs).148,149 
Note also that 5HT represents a widely used and important neu­
rotransmitter in the enteric nervous system of the gut. A variety 
of compounds that promote HTR signaling, including fenflura­
mine, promote weight loss.150 In combination with phentermine 
(Fen/Phen), the clinical use of fenfluramine successfully reduced 
food intake and body weight in humans, prior to its removal from 
the market as a result of heart valve disorders. Subsequent work 
identified central ner

disorders. Subsequent work 
identified central nervous system HT2CRs151–153 as crucial media­
tors of 5HT-mediated weight loss: The anorexic properties of fen­
fluramine depend on the presence of Htr2c, and mice null for 
Htr2c demonstrate hyperphagic obesity,154 demonstrating both a 
physiologic and pharmacologic role for 5HT and HT2CR in the 
control of feeding and body weight. In 2012 the US Food and 
Drug Administration (FDA) approved the use of the HT2CR ago­
nist lorcaserin for the treatme

 of the HT2CR ago­
nist lorcaserin for the treatment of obesity.150
The hypothalamic melanocortin pathway mediates the sup­
pression of feeding by 5HT and HT2CR.155,156 Arcuate nucleus 
POMC neurons express 5HT2CRs,155 and 5HT directly activates 
POMC neurons via HT2CR-mediated mechanisms.155,157,158 
Indeed, mice in which expression of the HT2CR is disrupted glob­
ally but the receptor is re-expressed only in POMC neurons do 
not develop the hyperphagia and obesity characteristic of global 
5HT

erphagia and obesity characteristic of global 
5HT2CR deficiency and respond to HT2CR agonists.156,159
Interestingly, while POMC neurons express LepRb and 
HT2CR, the HT2CR-expressing POMC neurons are distinct from 
those that express LepRb,157 suggesting that the two popula­
tions of POMC cells may suppress feeding in response to distinct 
stimuli. Furthermore, the regulation of the central nervous system 
5HT system by feeding-relevant stimuli has only recently begun 
to be explored,160 and th

has only recently begun 
to be explored,160 and the specific physiologic role played by the 
5HT system in the control of food intake remains unclear: While 
the 5HT system is well known for its role in responses to a vari­
ety of psychologic (e.g., restraint, social defeat) and physiologic 
stressors (e.g., fasting), whether this system mediates the anorectic 
response to stress has not been defined. 
The Role of Insulin and Glucose in the Regulation 
of Energy Homeostasis
In addition to the we

ation 
In addition to the well-known role for insulin action on peripheral 
tissues to control glucose homeostasis, insulin acts in the central 
nervous system to contribute to the control of energy balance.161 
Insulin injection into the brain can reduce food intake,161,162 and 
pan-neuronal deletion of insulin receptors causes mild obesity.163 
Glucose may also act in the brain to contribute to the control of 
energy balance. Several distinct populations of neurons in the

ce. Several distinct populations of neurons in the brain 
sense glucose164: some neurons are activated by rising concentrations 
of glucose (glucose-excited cells), while other classes of neurons are 
inhibited by rising glucose (glucose-inhibited cells). While the chem­
ical identity of many glucose-sensing cells remain to be determined, 
glucose-inhibited neurons include arcuate nucleus NPY/AGRP cells, 
lateral hypothalamic area neurons that contain orexin (OX),165,166 
lateral parabrachial nu

ntain orexin (OX),165,166 
lateral parabrachial nucleus LepRb cells,86 and other cell types in the 
hindbrain and thalamus that likely play roles in energy homeostasis. 
Estrogen
In addition to their primary reproductive effects, gonadal ste­
roids also play a key role in regulating energy balance and glucose 
homeostasis.167,168 Estrogens exert antiobesity and antidiabetic 
effects, and lower levels of estrogens in postmenopausal women 
are associated with an increased risk for developing obesi

ciated with an increased risk for developing obesity.169–171 
Ovariectomy reduces estrogen, causing obesity.172–174 Although 
ovariectomy increases food intake, hyperphagia does not account 
for all of the ovariectomy-associated obesity.174 Indeed, ovariec­
tomy causes rats to gain weight to a similar extent when they are 
pair-fed to estradiol-treated rats,175,176 suggesting that endogenous 
estrogens regulate body weight homeostasis primarily by modu­
lating energy expenditure. Estradiol repla

y modu­
lating energy expenditure. Estradiol replacement decreases food 
intake and increases energy expenditure in rodents, however,177 
indicating that exogenous estrogens may influence both energy 
intake and energy expenditure.
The alpha isoform of the estrogen receptor (ERα) mediates 
many of estrogen’s effects on body weight homeostasis. For exam­
ple, female mice with a targeted deletion (knockout [KO]) in the 
ERα gene (ERαKO) develop obesity and hyperadiposity primar­
ily because of dec

sity and hyperadiposity primar­
ily because of decreased energy expenditure.178–180 ERα is clearly 
1561
required to mediate the normal satiation process because estradiol-
induced hypophagia and cholecystokinin (CCK)–induced satia­
tion in wild-type mice are blocked in ERαKO mice.180
ERα is expressed in brain regions implicated in the regulation 
of energy balance, including the arcuate nucleus, paraventricular 
nucleus, and ventromedial nucle

, paraventricular 
nucleus, and ventromedial nucleus in the hypothalamus, as well as 
key brainstem sites such as the nucleus of the solitary tract.181,182 
Consistent with the role for the ventromedial nucleus in control­
ling sympathetic nervous system outflow and energy expenditure, 
knockdown or knockout of ERα in the ventromedial nucleus 
produces mice that are less sensitive to estradiol-induced weight 
loss and develop increased visceral adipose tissue167,183,184 due to 
decreased physica

dipose tissue167,183,184 due to 
decreased physical activity and impaired diet-induced thermogen­
esis; food intake of these animals is not directly affected.183 Col­
lectively, these results support the hypothesis that ERα signaling 
in ventromedial nucleus neurons plays an important role in regu­
lating energy expenditure rather than food intake.
Arcuate nucleus POMC neurons express ERα,167,185,186 and 
estrogens regulate the excitability of POMC neurons. The num­
ber of excitatory inputs to a

MC neurons. The num­
ber of excitatory inputs to arcuate POMC neurons rises as female 
mice enter proestrus, when estrogen levels are high.177,187 Fur­
ther, central estradiol administration rapidly increases excitatory 
inputs to POMC neurons.177,187 The synaptic changes in POMC 
neurons are tightly paralleled with the effects of estradiol on food 
intake and body weight.177 Consistently, female mice lacking ERα 
only in POMC neurons develop hyperphagia.167 These observa­
tions suggest that est

erphagia.167 These observa­
tions suggest that estrogen signals in POMC neurons are physi­
ologically relevant in the regulation of food intake.167 In contrast, 
although estradiol inhibits the activity of NPY/AGRP neurons, 
NPY/AGRP cells do not express ERα, suggesting that these effects 
are mediated indirectly.188 
Central Nervous System Control of 
The systems that coordinate energy homeostasis balance energy 
intake with energy expenditure. Energy expenditure is often 
grouped

 expenditure. Energy expenditure is often 
grouped into three categories: energy required for basal metabo­
lism, energy required for physical activity, and the thermic effect 
of food.189 The latter, often referred to as diet-induced thermo­
genesis, is estimated at 8% to 10% of total expenditure and is 
defined as the increased energy expenditure in response to energy 
intake.189 This process is under the control of the sympathetic ner­
vous system, and energy expenditure is increased by stimu

stem, and energy expenditure is increased by stimulation 
of β-adrenergic receptors. The importance of sympathetic output 
in energy balance is revealed by the profound obesity of mice lack­
ing all β-adrenergic receptors when exposed to a high-fat diet.190 
In rodents, the major tissue mediating the energetic response to 
sympathetic output is brown adipose tissue, which contains adi­
pocytes with dense collections of mitochondria.191 In addition, 
brown adipocytes express uncoupling protein 1 

n, 
brown adipocytes express uncoupling protein 1 (UCP1), which 
uncouples mitochondrial respiration and thus induces energy 
expenditure and heat. In humans the key tissue mediating energy 
expenditure in response to changing energy intake is likely skel­
etal muscle, although brown adipose tissue has been identified in 
humans and may play a role in energy balance control.191,192 It is 
clear that the sympathetic nervous system is required for coordi­
nated control of energy expenditure and re

coordi­
nated control of energy expenditure and resistance to diet-induced 
obesity, however.
Key mediators of central nervous system autonomic output 
are the parasympathetic and sympathetic preganglionic neurons 
in the brainstem and spinal cord.73,193 Sympathetic pregangli­
onic neurons are found within the interomediolateral cell col­
umn of the spinal cord and extend from the upper thoracic to 
the upper lumbar cord segments. Different rostral-caudal levels 
of the intermediolateral column 

al-caudal levels 
of the intermediolateral column provide innervation to different 
target organs and thus mediate distinct autonomic responses. For 
example, sympathetic preganglionic neurons in the upper thoracic 
levels of the intermediolateral column are thought to be important 
for control of the heart and cardiovascular system. Additionally, 
sympathetic preganglionic neurons in thoracic levels T6 to T12 of 
the intermediolateral column provide innervation of the adrenal 
gland and endocri

vide innervation of the adrenal 
gland and endocrine pancreas.194–196
Descending central nervous system input to sympathetic 
preganglionic neurons from key regulatory groups in the hypo­
thalamus and brainstem play a dominant role in autonomic 
control.73,197,198 The largest projection to the sympathetic pregan­
glionic neurons in the spinal cord is composed of inputs from 
the arcuate nucleus, the paraventricular nucleus, and the lateral 
hypothalamic area,194,198,199 although other important 

halamic area,194,198,199 although other important projections 
also arise from the brainstem.200–203
As noted earlier, the hypothalamic melanocortin system regu­
lates energy expenditure (in addition to controlling food intake). 
For example, MC4R blockade in mice prevents diet-induced 
thermogenesis204 and blocks the upregulation of brown adipose 
tissue activity.205 In addition, blocking MC4R signaling prevents 
the normal induction of thermogenic proteins by high-fat diet 
feeding.206 Paraven

nic proteins by high-fat diet 
feeding.206 Paraventricular nucleus MC4Rs do not mediate this 
affect, however, because although activation of the paraventricular 
nucleus neurons increases energy expenditure, selective activation 
of MC4R paraventricular nucleus neurons does not.46
Sympathetic preganglionic neurons express MC4Rs207 and 
receive direct inputs from leptin-responsive POMC neurons, sug­
gesting a potential role for melanocortin action on these cells.208 
Indeed, selective deletion o

n on these cells.208 
Indeed, selective deletion or re-expression of MC4R expression 
in cholinergic neurons produces high-fat diet sensitivity, failure 
to increase adipose tissue UCP1, an impairment of diet-induced 
thermogenesis, and increased cold tolerance.49,209,210 Thus direct 
melanocortin input to sympathetic preganglionic neurons medi­
ates important aspects of diet-induced thermogenesis and energy 
expenditure. 
Control of Glucose Homeostasis by the Brain
While peripheral hormones (e.

ostasis by the Brain
While peripheral hormones (e.g., insulin) are crucial for glucose 
homeostasis, autonomic outputs from the central nervous system 
control important aspects of metabolism.211–213 The concept that 
the central nervous system plays a primary role in the control of 
insulin action and glucose homeostasis originates from physiolo­
gist Claude Bernard’s 1849 observation that physical lesions to 
the floor of the fourth ventricle of rabbits dramatically increased 
blood glucose, r

f rabbits dramatically increased 
blood glucose, resulting in glucose spillover into the urine.7,214 
Bernard concluded that this effect was mediated by the stimula­
tion of autonomic drive to increase hepatic glucose output. While 
the neuroanatomy and neural mechanism(s) by which this early 
brainstem lesion altered blood glucose remain somewhat obscure, 
several hypothalamic systems clearly play important roles in the 
control of glucose homeostasis.
A variety of nutrient and hormonal cues (e

stasis.
A variety of nutrient and hormonal cues (e.g., glucose and 
leptin) act in the hypothalamus to modulate glucose homeosta­
sis.213,215,216 For instance, as part of the counterregulatory response 
(CRR) to hypoglycemia, the central nervous system activates 
the sympathetic nervous system to alter hormone secretion and 
1562
promote hepatic glucose output to restore normoglycemia.217,218 
Conversely, other central nervous brain sy

17,218 
Conversely, other central nervous brain systems control glucose 
disposal and utilization.
Leptin plays a role in glucose homeostasis as well as energy 
balance. For instance, Lepob/ob mice exhibit hyperglycemia despite 
constitutively elevated insulin production. Similarly, lipodys­
trophic mice and humans exhibit severe insulin resistance that 
is corrected by leptin replacement.29,30,219 Leptin action in the 
arcuate nucleus mediates part of this effect, since restoration of 
arcuate 

art of this effect, since restoration of 
arcuate nucleus LepRb expression in otherwise LepRb-null mice 
improves glucose homeostasis.220–222 Leptin mediates at least part 
of its effects on glucose homeostasis by promoting glucose disposal 
and utilization: Direct central nervous system infusion of leptin 
increases glucose disposal and rapidly normalizes blood glucose in 
Lepob/ob mice.223,224 Central nervous system infusion of leptin also 
increases glucose disposal into muscle and brown adip

reases glucose disposal into muscle and brown adipose tissue 
and normalizes blood glucose in insulin-deficient rodents.225–227
Ventromedial Nucleus Control of Glucose and 
Energy Homeostasis
The ventromedial nucleus plays important roles in the regulation 
of the sympathetic nervous system and contributes to both hepatic 
glucose output and glucose disposal.228–231 Low glucose activates 
glucose-inhibited ventromedial nucleus neurons that presumably 
participate in augmenting hepatic glucose ou

ably 
participate in augmenting hepatic glucose output (e.g., to increase 
blood glucose during the CRR), while glucose-excited ventrome­
dial nucleus neurons are postulated to promote glucose disposal. 
Since the ventromedial nucleus contains both glucose-inhibited 
and glucose-excited neurons, pan-ventromedial nucleus manipu­
lations reveal the aggregate function of all cell types within the 
ventromedial nucleus. For instance, activating all ventromedial 
nucleus neurons mimics the CRR232 bec

entromedial 
nucleus neurons mimics the CRR232 because unrestrained hepatic 
glucose output overwhelms the ability of peripheral tissues to dis­
pose of glucose. Conversely, under fed conditions the neurons that 
promote glucose utilization play a dominant role in glycemic con­
trol; thus disrupting overall ventromedial nucleus function causes 
glucose intolerance and blunts energy expenditure to promote 
weight gain.233 Similarly, disruption of glutamatergic neurotrans­
mission within the ventr

glutamatergic neurotrans­
mission within the ventromedial nucleus (a predominantly glu­
tamatergic nucleus) impairs glucose tolerance and increases body 
weight at baseline but blunts the CRR to glucoprivation.234
Ventromedial nucleus LepRb neurons likely contribute to the 
control of glucose disposal (as well as energy expenditure), since 
intraventromedial nucleus leptin increases glucose uptake into 
muscle and brown adipose tissue, and ablation of LepRb in the 
ventromedial nucleus decreases

on of LepRb in the 
ventromedial nucleus decreases glucose tolerance.96,227 Activation 
of ventromedial nucleus LepRb neurons (unlike the activation of 
all ventromedial nucleus neurons) does not mimic the CRR, how­
ever,232 suggesting that non-LepRb ventromedial nucleus neurons 
mediate the CRR and other responses that increase hepatic glucose 
output. The ventromedial nucleus neurons that promote hepatic 
glucose output and mediate the CRR have yet to be definitively 
identified. 
POMC Neurons

 yet to be definitively 
POMC Neurons Sense Changes in Glucose 
The hypothalamic melanocortin system, including arcuate nucleus 
POMC neurons, also contributes to the control of glucose homeo­
stasis.215,220,221 For example, mice lacking MC4Rs are hyperinsulin­
emic prior to the onset of obesity,44,235 while intracerebroventricular 
MC4R agonists decrease plasma insulin levels in lean and obese 
mice.235 Moreover, MC4R-deficient humans display greater hyper­
insulinemi

deficient humans display greater hyper­
insulinemia than would be expected from their degree of obesity 
alone, and children as young as 12 months old are hyperinsulin­
emic.236,237 Additionally, although ablation of LepRb from arcuate 
nucleus POMC cells minimally impacts body weight, it promotes 
glucose intolerance. Conversely, restoration of POMC neuron 
LepRb expression on a LepRb-null background improves glucose 
homeostasis without substantially altering body weight.
Glucose-Inhibited Neu

tially altering body weight.
Glucose-Inhibited Neurons of the Arcuate Nucleus and 
Lateral Hypothalamic Area
While independent effects of blood glucose, such as food intake 
and body weight, are less well established for arcuate nucleus 
NPY/AGRP cells than for POMC neurons, decreased glucose 
concentrations activate NPY/AGRP cells, suggesting a potential 
role for these cells in the response to low blood sugar (perhaps by 
promoting food intake during hypoglycemia). Furthermore, low 
glucose ac

during hypoglycemia). Furthermore, low 
glucose activates orexin-producing neurons in the lateral hypotha­
lamic area.165,166 Since orexin neurons play key roles in the control 
of arousal (see upcoming discussion), these cells could promote 
hypoglycemia awareness. 
Intersection of Energy Balance and Reward 
Circuits
Behaviorally, two systems govern eating: the circuits that control 
the incentive and reward values (wanting and liking) of food and 
the satiety system, which promotes meal termin

nd 
the satiety system, which promotes meal termination associated 
with the sensation of fullness.238 Although physiologically inte­
grated, they are often studied and described as distinct entities to 
reduce complexity.
Overlapping central nervous systems mediate motivation 
for natural and artificial rewards (e.g., food and drugs of abuse, 
respectively). The mesolimbic dopamine system, composed of 
dopaminergic neurons in the ventral tegmental area that proj­
ect to the nucleus accumbens an

al area that proj­
ect to the nucleus accumbens and other places, play important 
roles in motivation and reward.239 For example, mice lacking 
the ability to produce dopamine normally die of starvation but 
will resume feeding after reintroduction of dopamine into the 
nucleus accumbens and nearby structures.240,241 Also, lesions of 
the nucleus accumbens reduce food intake242 and the desire to 
work for food.243
The neurohormonal systems that control food intake and 
metabolism also modulate t

ontrol food intake and 
metabolism also modulate the mesolimbic dopamine system and 
parameters of reward. Indeed, leptin suppresses the motivation 
of animals to perform rewarding brain self-stimulation,244 and 
subpopulations of ventral tegmental area neurons express LepRb 
or ghrelin receptor.245,246 Furthermore, ablating ventral tegmen­
tal area LepRb slightly increases the intake of rewarding foods. 
Ventral tegmental area LepRb neurons may also modulate anx­
iety-related behaviors.247,248 

also modulate anx­
iety-related behaviors.247,248 Similarly, ghrelin administration 
increases the reward value of food, and ablation of ghrelin recep­
tor in the ventral tegmental area increases food intake, while 
restoration of ventral tegmental area ghrelin receptor in ghrelin 
receptor-null mice increases feeding as well as partially restoring 
the orexigenic effects of ghrelin.249 Taken together, these find­
ings suggest that signals such as leptin and ghrelin not only affect 

s leptin and ghrelin not only affect 
hypothalamic pathways but also control reward behaviors—in 
part by direct action within the central nervous system reward 
circuitry.
1563
The Lateral Hypothalamic Area Links Food Intake 
Control and Arousal
Ablation of the lateral hypothalamic area abrogates the motivation 
to eat (as well as the motivation for other rewards),250 and many 
types of lateral hypothalamic area neurons innervate components 
o

 hypothalamic area neurons innervate components 
of the mesolimbic dopamine system. Melanin-concentrating hor­
mone and the orexin peptides (orexin-A and orexin-B, also known 
as hypocretins) are uniquely expressed in distinct populations of 
lateral hypothalamic area neurons and contribute distinctly to the 
control of the mesolimbic dopamine system and feeding.251–253 
Both populations project widely throughout the brain254,255; the 
expression patterns of the receptors for these peptides are 

 patterns of the receptors for these peptides are also 
widespread (Fig. 39.7).256–258
Injection of melanin-concentrating hormone into the brain 
increases food intake.251 Mice lacking melanin-concentrating hor­
mone are hypophagic and lean, while mice that overexpress mel­
anin-concentrating hormone become obese.259,260 In addition, 
mice lacking both melanin-concentrating hormone and leptin are 
leaner compared to mice that lack leptin but express melanin-con­
centrating hormone.261 Melanin-co

ess melanin-con­
centrating hormone.261 Melanin-concentrating hormone modu­
lates the incentive value of food and drugs of abuse, at least in 
part via direct action in the nucleus accumbens.262 Thus melanin-
concentrating hormone contributes to the control of food intake 
and energy balance by modulating the reward-encoding systems 
of the brain.
Orexin contributes to the control of energy balance, but its role 
is more complex.263,264 While initially thought to promote feeding 
based upon the 

tially thought to promote feeding 
based upon the finding that central injection of orexin peptides 
increases feeding and body weight,252,265 orexin-null mice, dogs, 
and humans exhibit narcolepsy (a cataplectic sleep syndrome), 
and orexin promotes hypervigilance. Hence the control of wake­
fulness and vigilance represents a major function for orexin.266,267 
Indeed, mice null for orexin or its receptors exhibit obesity due to 
decreased activity and energy expenditure rather than changes in 

ty and energy expenditure rather than changes in 
food intake. Thus the likely mechanism by which orexin injection 
promotes food intake is by the increase in wakefulness during the 
normal sleep cycle. Orexin neurons express ghrelin receptor, and 
ghrelin directly activates these cells.
While neither orexin nor melanin-concentrating hormone 
neurons express LepRb, leptin acts directly on a distinct set of lat­
eral hypothalamic area LepRb neurons.268 These lateral hypotha­
lamic area LepRb neur

s.268 These lateral hypotha­
lamic area LepRb neurons inhibit orexin neurons and also project 
directly to the ventral tegmental area and a few other midbrain 
sites and modulate the function of the mesolimbic dopamine sys­
tem.269–271 Disruption of LepRb in the neurotensin-expressing 
subpopulation of lateral hypothalamic area LepRb neurons alters 
mesolimbic dopamine function to decrease locomotor activity 
and promote obesity. Thus leptin action in the lateral hypotha­
lamic area controls ore

on in the lateral hypotha­
lamic area controls orexin cells and ventral tegmental area cells 
to modulate the mesolimbic dopamine system. Leptin likely con­
trols melanin-concentrating hormone neurons via its action on 
the hypothalamic melanocortin system.272 
Brainstem Circuits That Regulate Energy 
Balance
Circuits in the brainstem play important roles in the control of 
food intake, as well.273–276 The brainstem represents a crucial entry 
point for a wide variety of signals from visceral or

int for a wide variety of signals from visceral organs, including 
the gastrointestinal tract, which mediate parasympathetic reflex 
arcs to control gut physiology and which convey signals further 
into the brain to promote meal termination and to modulate emo­
tional valence relevant to gut status (e.g., nutrients in the gut pro­
mote positive valence, while toxins in the gut promote aversion) 
(Fig. 39.8). These gut signals converge in the dorsal vagal com­
plex, which contains the nucleus of 

al vagal com­
plex, which contains the nucleus of the solitary tract, the dorsal 
motor nucleus of the vagus, and the area postrema.
The area postrema is a circumventricular organ (and thus lies 
outside the blood-brain barrier, facilitating the sensing of circu­
lating factors) that lies directly above the nucleus of the solitary 
tract.277 Circulating signals of gut status (including a variety of 
gut-derived peptides) activate area postrema neurons, which relay 
those signals into the nucleus

urons, which relay 
those signals into the nucleus of the solitary tract and the lateral 
parabrachial nucleus.275
Sensory afferent signals carried by the glossopharyngeal and 
vagus nerves (e.g., taste, gastric stretch, nutrients in the gut and 
portal vein) innervate the nucleus of the solitary tract, which rep­
resents the primary site for innervation by vagal afferents from the 
gut.278–280 The cell bodies of vagal afferent nerves lie in the nodose 
ganglia; those deriving input from the upp

nodose 
ganglia; those deriving input from the upper gastrointestinal tract 
respond to three basic stimuli: gastric and duodenal distention 
or contraction, chemical contents of the lumen, and gut peptides 
and neurotransmitters released from the stomach and duodenum 
in response to nutrients.278 Recent work demonstrates vagal affer­
ent neurons that express the receptor for glucagon-like peptide-1 
(GLP1; GLP1R) and convey signals of stomach distension, while 
those that express G protein–coup

stension, while 
those that express G protein–coupled receptor 65 (GPR65) sense 
nutrients in the gut.281
The activation of nucleus of the solitary tract neurons that 
receive feeding-related information from the gut suppresses food 
intake, consistent with a role for certain nucleus of the solitary 
tract neurons in the control of meal termination.282,283 While 
the nucleus of the solitary tract innervates a variety of hypotha­
lamic structures (e.g., the paraventricular nucleus and dorsome­
di

(e.g., the paraventricular nucleus and dorsome­
dial nucleus), other important nucleus of the solitary tract targets 
include brainstem sites, including the dorsal motor nucleus of 
the vagus and the lateral parabrachial nucleus. The dorsal motor 
nucleus of the vagus contains parasympathetic preganglionic neu­
rons, which innervate and provide parasympathetic input to the 
DMH,
PVH, ARC
NAc
Wanting,
Liking
Amygdala
Aversion, Fear
Hindbrain
Arousal
DA
VTA
OX
(GHSR)
GHSR
MCH
LepRb
LepRb
Melanocor

al
DA
VTA
OX
(GHSR)
GHSR
MCH
LepRb
LepRb
system
LHA
DRIVE TO FEED
• Fig. 39.7  Reward circuits involved in feeding control. Core components 
of the mesolimbic dopamine system are shown in light purple, brainstem 
circuits are in dark blue, leptin receptor pathways are in orange, and mela­
nocortin pathways are in light gray. ARC, arcuate nucleus; DA, dopamine; 
DMH, dorsomedial hypothalamic nucleus; GHSR, ghrelin receptor; LepRb, 
leptin receptor; MCH, melanin-concentrating hormone;

ptin receptor; MCH, melanin-concentrating hormone; NAc, nucleus 
accumbens; OX, orexin; PVH, paraventricular hypothalamic nucleus.
1564
entire gastrointestinal tract. Meal-activated nucleus of the solitary 
tract cells project to and modulate the activity of dorsal motor 
nucleus of the vagus neurons that alter parasympathetic tone to 
the stomach, decreasing the rate of gastric emptying as part of a 
meal-activated reflex arc.
Major p

g as part of a 
Major projections of the lateral parabrachial nucleus, which 
receives input from the area postrema and the nucleus of the 
solitary tract, include the thalamus, the amygdala, and several 
hypothalamic sites.284 Recent findings identified a set of calcito­
nin gene–related peptide (CGRP)–expressing lateral parabrachial 
nucleus neurons required for the suppression of food intake in 
response to aversive gut stimuli.105,285,286 Stimuli that promote 
gut 

gut stimuli.105,285,286 Stimuli that promote 
gut malaise (such as lithium chloride [LiCl] injection in rodents 
and some chemotherapeutic agents) activate these neurons. Fur­
thermore, lateral parabrachial nucleus CGRP cells act via their 
projections to the amygdala to mediate both the suppression of 
food intake and the aversive response to such stimuli. Arcuate 
nucleus NPY/AGRP neurons also innervate lateral parabrachial 
nucleus CGRP neurons, inhibiting these cells to promote the con­
sump

s, inhibiting these cells to promote the con­
sumption of disfavored foods when NPY/AGRP neuron activity 
is high (e.g., during prolonged fasting). The withdrawal of NPY/
AGRP neuron-mediated inhibition of lateral parabrachial nucleus 
CGRP cells activates their amygdala projection to suppress food 
intake.45,105,137,287–289 Other lateral parabrachial nucleus neurons 
appear to mediate the nonaversive suppression of food intake in 
response to distinct signals,46 but the molecular identity of th

stinct signals,46 but the molecular identity of this 
putative second set of lateral parabrachial nucleus cells has yet to 
be determined.
Gut Peptides Involved in Satiety and Hunger
In addition to signals from gut distention, gut peptides stimulated 
by meal intake mediate satiety via centers in the brainstem. These 
gut peptide–derived brainstem signals can be modified by signals 
that mediate long-term energy balance, such as leptin, which acts 
in part via nucleus of the solitary tract LepRb

s 
in part via nucleus of the solitary tract LepRb neurons to increase 
the meal-terminating response to a number of gut peptides.290–292 
While gut-peptide signaling and brainstem signals (in general) are 
often considered to modulate short-term effects on feeding (e.g., 
decreasing meal size, which is compensated by increased meal fre­
quency), gut peptides also interact with hypothalamic circuits that 
mediate long-term energy balance. Furthermore, pharmacologic 
agonists for many gut peptide

more, pharmacologic 
agonists for many gut peptide receptors suppress food intake and 
can promote chronic weight loss in humans as well as in preclini­
cal models.293
GLP1 Action in the Central Nervous System
Current strategies for the treatment of type 2 diabetes mellitus 
are not optimally effective, and even multiple drug combinations 
often fail to normalize glycemia in a sustained manner in many 
patients. Hence there remains intense interest in new therapies 
that safely and effectively l

st in new therapies 
that safely and effectively lower blood glucose in diabetes. Over 
the past decade, drugs that enhance the action of incretins have 
seen increasing use for the treatment of type 2 diabetes and obe­
sity.294,295 Incretins are hormones released by the gut in response 
to oral ingestion of nutrients that act to increase insulin secretion. 
The prototypical incretin hormone is GLP1, which is derived from 
proglucagon; upon secretion, GLP1 undergoes rapid degradation 
by dipepti

tion, GLP1 undergoes rapid degradation 
by dipeptidyl peptidase-4 (DPP4) in the circulation.296 While 
pancreatic alpha cells mainly generate glucagon from progluca­
gon in pancreatic alpha cells, the L cells of the intestine and the 
proglucagon-expressing cells in the brain mainly produce GLP1 
and GLP2.294,295
GLP1 receptor (GLP1R) agonists induce multiple desirable 
antidiabetic and antiobesity actions, and protease-resistant long-
acting GLP1 analogues are currently available for the treatm

1 analogues are currently available for the treatment 
of type 2 diabetes and/or obesity.294,295 In addition to enhanc­
ing glucose-dependent insulin secretion, slowing gastric empty­
ing, and inhibiting gastric acid secretion, these compounds reduce 
food intake.
Given the increasing use of GLP1R analogues and DPP4 
inhibitors (which block GLP1 degradation) for the treatment 
of diabetes and the FDA approval in 2014 of liraglutide for the 
treatment of obesity, understanding the central actions

ment of obesity, understanding the central actions of GLP1 
is important for predicting the biologic consequences of sustained 
GLP1R agonist administration. The effects of GLP1R agonists on 
Amygdala
PBN
NTS
AP
Gut,
food
Systemic
DMV
CGRP
GDF15
Vagus
Vagus
Autonomic reflexes,
Gut Physiology
CCK, GLP-1,
Amylin, PYY
• Fig. 39.8  Brainstem circuits involved in feeding control. Core hypothalamic circuits are shown in red, core 
components of the mesolimbic d

shown in red, core 
components of the mesolimbic dopamine system are shown in light purple, brainstem circuits are in dark 
blue, peripheral signals are green. AP, area postrema; CCK, cholecystokinin; DMV, dorsal motor nucleus 
of the vagus; GCRP, calcitonin gene-related peptide; GDF15, growth and differentiation factor-15; GLP1, 
glucagon-like peptide-1; NTS, nucleus of the solitary tract; PBN, parabrachial nucleus; PYY, peptide YY.
1565
appet

 39  Neuroendocrine Control of Energy Stores
appetite may be mediated in part via inhibition of gastric emp­
tying and may also reflect direct effects of GLP1R agonists on 
satiety, including the induction of nausea and other aversive gut 
symptoms.297–299
Initial interest in the central nervous system actions of 
GLP1 stemmed from the observation that GLP1 inhibits food 
intake.300,301 In humans, peripheral GLP1 administration to 
normal and diabetic subjects induces satiety and reduces food 
i

betic subjects induces satiety and reduces food 
intake in short-term studies.302–305 Chronic continuous GLP1 
administration to human diabetic subjects is associated with 
weight loss.306,307 The central nervous system expression of 
GLP1 is restricted to a population of caudal nucleus of the soli­
tary tract neurons that receive and process viscerosensory infor­
mation from the gastrointestinal tract. Thus these nucleus of the 
solitary tract GLP1 neurons are in a prime position to rapidly 
mo

LP1 neurons are in a prime position to rapidly 
modify ingestive behavior in direct response to gut signals; these 
cells also express LepRb and respond to leptin. Indeed, activation 
of GLP1 neurons in the nucleus of the solitary tract suppresses 
food intake and decreases metabolic rate, and chronic activation 
in the setting of high-fat diet–induced obesity leads to weight 
loss consistent with the antiobesity effects of chronic GLP1R 
agonist treatment.308
Numerous brain areas that participa

 treatment.308
Numerous brain areas that participate in the control of food 
intake receive projections from nucleus of the solitary tract GLP1 
neurons and express Glp1r mRNA, including the area postrema, 
lateral parabrachial nucleus, paraventricular nucleus, dorsomedial 
nucleus, and arcuate nucleus.281,309,310 Infusion of GLP1R ago­
nists directly into several of these brain regions suppresses feeding 
consistent with the idea that GLP1R agonists exert their beneficial 
effects through sever

ists exert their beneficial 
effects through several brain regions. Deleting Glp1r from specific 
cell types in the mouse demonstrated that the feeding effects but 
not the glucose-lowering effects of GLP1R agonists are located in 
the central nervous system.311 Several groups have ablated Glp1r 
from POMC, paraventricular nucleus, or ventromedial nucleus 
cells312 without affecting anorexic response to GLP1R agonists. 
Broader deletion of GLP1Rs from glutamatergic neurons dra­
matically attenua

 from glutamatergic neurons dra­
matically attenuates the response to GLP1R agonists, however, 
suggesting that a crucial population of GLP1R cells may lie in 
the hindbrain (e.g., in the area postrema [the main site where 
exogenous GLP1R agonists activate GLP1R-expressing neurons]) 
(Fig. 39.9).310 
Peptide YY (PYY)
Endocrine cells in the ileum and colon313 release both a full-length 
(i.e., 36–amino acid) and a 34–amino acid (PYY[3-36]) form of 
PYY following a meal.314 PYY is a potent agonis

f 
PYY following a meal.314 PYY is a potent agonist of both Y1 and Y2 
receptors, whereas PYY(3-36) is a Y2-specific agonist, with approxi­
mately a 1000 times greater affinity for the Y2 versus Y1 receptor.315 
Y1-preferring and Y2-preferring binding sites are located in the area 
postrema, nucleus of the solitary tract, and dorsal motor nucleus 
of the vagus. Peripheral administration of PYY in the rat activates 
neurons in the area postrema, as well as in the nucleus of the solitary 
tract.31

s well as in the nucleus of the solitary 
tract.316 Infusion of PYY within the physiologic range has numer­
ous effects, including inhibition of gastric emptying,317 gastric acid 
secretion,318 and pancreatic exocrine secretion.318 These actions of 
PYY appear to be mediated by PYY action directly on the nucleus of 
the solitary tract and dorsal motor nucleus of the vagus as well as on 
gastric mucosal enterochromaffin-like cells (for review see Yang319). 
Both Y1 and Y2 receptors are found with

Yang319). 
Both Y1 and Y2 receptors are found within the nucleus of the soli­
tary tract and dorsal motor nucleus of the vagus.320 For example, 
PYY appears to inhibit gastric acid secretion primarily through vagal 
innervation of the gastric fundus.321 The ability of low-dose PYY 
and PYY(3-36) to inhibit the activity of dorsal motor nucleus of 
the vagus efferents appears to be Y2 mediated, whereas Y1 agonists 
appear to stimulate these cells.319
Peripheral administration of PYY(3-36) in pharm

19
Peripheral administration of PYY(3-36) in pharmacologic 
doses suppresses feeding in rodents as well as humans,276,322,323 
thus suggesting a potential function as a satiety factor. The mecha­
nisms underlying the action of PYY(3-36) in reduction of food 
intake have not been fully elucidated. Intraperitoneal administra­
tion of PYY(3-36) activates a small number of arcuate POMC neu­
rons.322,324 However, vagotomy also blocks PYY(3-36)-induced 
inhibition of feeding,325 and the inhibition of 

inhibition of feeding,325 and the inhibition of feeding by PYY(3-
36) persists in the MC4R knockout mouse326 and in obese Ay/a 
mice.327 Thus the release of melanocortin peptides derived from 
POMC and their subsequent activation of the MC4R does not 
appear to be required for the inhibition of feeding by PYY(3-36). 
Furthermore, PYY appears to induce conditioned taste aversion in 
Saline
AP
AP
NTS
NTS
• Fig. 39.9  Activation of area postrema/nucleus of the solitary tract glucagon-

a postrema/nucleus of the solitary tract glucagon-like peptide-1 receptor (GLP1R) 
neurons by peripheral administration of GLP1R agonist. Glp1rcre mice on a green fluorescent protein (GFP) 
reporter background labels GLP1R expressing cells with GFP (green). Neuron activation can be assayed 
by increased expression of the immediate early gene FOS. Treatment of fasted mice with the long-acting 
GLP1R agonist, liraglutide, stimulates FOS expression in GLP1R neurons (green) in the area postrema (AP)

in GLP1R neurons (green) in the area postrema (AP) 
and nucleus of the solitary tract (NTS) 2 hours after administration. Arrows mark colocalization of FOS and 
GLP1R expression (orange). (Images courtesy Jessica Adams, PhD, University of Michigan.)
1566
rodents and nausea in some human studies, suggesting an aversive 
effect of the peptide likely to involve brainstem sites such as the 
area postrema. Interestingly, however, PYY also c

area postrema. Interestingly, however, PYY also contributes to the 
inactivation of NPY/AgRP neurons during feeding.328
Despite the short-acting anorexic effects of the peptide in most 
experimental models, two knockout studies demonstrate that 
removal of the gene encoding PYY produces obese hyperinsu­
linemic mice,329,330 suggesting that the peptide may also play an 
important role in the regulation of long-term energy stores. One 
study shows the peptide may be specifically involved in the s

 the peptide may be specifically involved in the satiat­
ing effects of protein in the diet.329 
Cholecystokinin (CKK)
Produced by the gastrointestinal tract in response to meal ingestion, 
the diverse actions of CKK include stimulation of pancreatic enzyme 
secretion, inhibition of gastric motility, activation of intestinal motility, 
and the acute suppression of feeding. Early experiments administering 
CCK peripherally supported a role for increased CCK levels in the 
early termination of a m

reased CCK levels in the 
early termination of a meal.331,332 The finding that repeated injections 
of CCK lead to reduced meal size without a change in body weight, 
due to a compensatory increase in meal frequency, argued against 
CCK acting as a signal regulating long-term energy stores.333,334
Two subtypes of the CCK receptor belonging to the G pro­
tein–coupled family of receptors have been described: CCKA and 
CCKB. Studies using CCK receptor–specific antagonists as well 
as surgical or ch

or–specific antagonists as well 
as surgical or chemical vagotomy demonstrated that the satiety 
effects of CCK are specifically mediated via CCKA receptors on 
afferent vagal nerves.335–337 
Amylin
Amylin, or islet amyloid polypeptide (IAPP), is a 37–amino 
acid polypeptide that is cosecreted with insulin from pancre­
atic beta cells in response to nutrient intake and insulin secre­
tagogues.338,339 In humans, pancreatic IAPP can form amyloid 
fibrils that are hypothesized by some to play a rol

ibrils that are hypothesized by some to play a role in the decline 
in islet cell function that accompanies type 2 diabetes.340,341 In 
addition, amylin has been shown to impair gastric motility and 
have effects independent of insulin on energy homeostasis.
Both peripheral and intracerebroventricular infusions of amylin 
inhibit food intake acutely and, during chronic infusion, lead to a 
sustained reduction in body weight.342–344 Amylin readily enters the 
brain, and high-affinity amylin-bindi

 enters the 
brain, and high-affinity amylin-binding sites have been found in sev­
eral brain regions, including the hypothalamus and brainstem (includ­
ing the area postrema and nucleus of the solitary tract).342,345,346 The 
exact site(s) of anorectic amylin action remain unclear. 
GDF15
Also known as MIC1, growth and differentiation factor-15 (GDF15) 
is a distant relative of the transforming growth factor-β family, most 
closely related to the glial-derived neurotrophic factors (GDNF).347,34

e glial-derived neurotrophic factors (GDNF).347,348 
While GDF15 is not technically a gut-derived peptide, it acts via the 
central nervous system to control food intake. Most tissues produce 
GDF15 at low levels, but a variety of cellular stressors (including 
hypoxia, ER stress, mitochondrial dysfunction, etc.) increase GDF15; 
GDF15 is not controlled by fasting, feeding, or most other nutri­
tional interventions. Hence circulating GDF15 levels increase in dis­
ease processes that produce cell

 increase in dis­
ease processes that produce cellular stress, including in malignancies 
(most tumor cells produce GDF15), heart failure, kidney failure, and 
systemic infection. Exogenous GDF15 suppresses feeding (including 
at levels observed in the aforementioned disease states), and interfer­
ence with GDF15 action provides protection from the anorexia and 
weight loss associated with these disease states.
GDF15 acts via the GDNF receptor alpha-like receptor 
(GFRAL), which is expressed onl

pha-like receptor 
(GFRAL), which is expressed only in the area postrema (and to 
a lesser extent in the nearby nucleus of the solitary tract).349–352 
Mice null for Gfral (or Gdf15) display mildly increased body 
weight, especially on a high-fat diet, and GFRAL is required for 
the anorectic effects of GDF15 and (like GDF15) for weight loss 
in response to a variety of pathophysiologic insults. Thus GDF15 
appears to act via area postrema GFRAL to limit food intake in 
response to pathologic co

to limit food intake in 
response to pathologic conditions that cause cellular stress rather 
than in response to alterations in nutritional or metabolic status. 
Bariatric Surgery
In contrast to most pharmacologic therapies, certain types of bar­
iatric surgeries, such as the Roux-en-Y gastric bypass and verti­
cal sleeve gastrectomy not only cause significant weight loss but 
also mediate the maintenance of reduced body weight for many 
years (Fig. 39.10). Furthermore, improvement in diabetes 

Fig. 39.10). Furthermore, improvement in diabetes follow­
ing these procedures often occurs before significant weight loss 
and is sustained in a significant percentage of patients. Both these 
findings imply that the procedures mediate a profound impact 
on the neuroendocrine control of long-term energy stores. That 
bariatric surgery appears capable of creating a new stable weight 
set-point suggests that hormonal or vagal and nutritional signals 
from the gut may indeed have a more profound i

ls 
from the gut may indeed have a more profound impact on long-
term energy homeostasis than previously thought.
Many researchers are attempting to identify the peripheral and 
central changes that produce weight loss and improve glucose con­
trol following these procedures, with the ultimate goal being to 
reproduce the response to surgery using a nonsurgical approach. 
Circulating GLP1 is elevated following sleeve gastrectomy353 
and gastric bypass,353–355 as are bile acids. GLP1R deletion fa

ypass,353–355 as are bile acids. GLP1R deletion fails 
to modify weight loss or glucose responsiveness in a sleeve gas­
trectomy in the mouse however,356 although alteration of the bile 
acid receptor (FXR) blunts the response to sleeve gastrectomy.357 
Presumably the alterations in feeding and body weight following 
sleeve gastrectomy result from increased activity of neurons within 
the area postrema and/or the dorsal vagal complex.

Years of follow-up
Weight change (%)
5
0
−5
−10
−15
−20
−25
−30
−35
−40
−45
10.0
8.0
Control
Banding
Vertical banded gastroplasty
Gastric
bypass
6.0
4.0
3.0
2.0
1.0
0.5
0.0
• Fig. 39.10  Apparent alteration of the adipostatic set-point after bariatric sur­
gery. (Modified from Sjostrom L, Lindroos AK, Peltonen M, et al. Swedish obese 
subjects study scientific group. Lifestyle, diabetes, and cardiovascular risk fac­
tors 1

ptors 
reciprocally regulate sympathetic and parasympathetic pregangli­
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1567
40
Obesity
ELEFTHERIA MARATOS-FLIER
Obesity is a chronic disease and a significant risk factor for multiple 
serious medical problems. Obesity is also associated with impaired 
quality of life, and considerable economic and social burden due 
to increased health care costs and loss of productivity.1,2 This chap­
ter addresses key clinical and pathophysiologic issues in obesity.
Obesity is not a new phenomenon. It was described by

esity is not a new phenomenon. It was described by Hip­
pocrates, who made observations on the importance of diet in 
individuals with excess weight, also noting that the Scythians were 
obese because they were sedentary, and describing them as flabby 
and torpid.3 Galen, writing in the second century of the common 
era, described obesity as an illness for which he prescribed diet and 
exercise and used the term polysarkia to refer to morbid obesity.4 
Obesity as a recognizable medical condition

ity.4 
Obesity as a recognizable medical condition persists throughout 
history usually as a problem of the wealthy and sedentary, such as 
Henry VIII of England, who weighed more than 300 pounds (136 
kg) and likely suffered from type 2 diabetes and gout at his death.
In the United States, rates of obesity became worrisome in the 
1990s as overweight and obesity increased in the population. Until 
the past two decades, rates of both were higher in the United States 
than in other countries. Rec

in the United States 
than in other countries. Recently other nations have caught up, and 
prevalence is higher now in Mexico and countries in the Middle East 
than in the United States. Considering that the genetics of the popu­
lation has not changed over the interval during which the dramatic 
rise in rates of obesity have occurred, the increase must result from 
a complex interplay of genes and environment. Many individuals 
have a predisposition toward eating and storing calories when they 

tion toward eating and storing calories when they 
are available. The tendency to store excess fat is common in many 
mammals, as seen in the incidence of obesity in pets in developed 
countries. This may be a consequence of selection over time for effec­
tive storage, as starvation is an acute threat to survival, whereas the 
comorbidities of obesity take years to decades to become problematic.
Mammals have evolved to acquire food and store it as fat against 
times of food scarcity. Until the m

s fat against 
times of food scarcity. Until the modern era, food scarcity had been 
a substantial threat to existence over excess calories. A pound of fat, 
containing 3500 calories, may provide 2 to 4 days of survival for an 
individual without any food supply. In a very calorie restricted envi­
ronment, 50 pounds of extra fat would make a substantial survival 
difference. Lean persons will die after only approximately 60 days 
of starvation, when more than 35% of body weight is lost. Obese 
p

hen more than 35% of body weight is lost. Obese 
persons can tolerate longer fasts, even for more than 1 year, without 
major adverse effects. In the longest reported fast, a 456-pound (207-
kg) man ingested only acaloric fluids, vitamins, and minerals for 382 
days and lost 278 pounds (126 kg), or 61% of his initial weight.
Assuming genetic traits and a physiology favoring eating and 
storing excess calories dating back to antiquity, the modern obesity 
epidemic is more likely explained by chan

obesity 
epidemic is more likely explained by changes in the food environ­
ment. As a consequence of modern agriculture and ease of trans­
portation of food to the consumer, food is cheaper, more readily 
available, and more varied than ever before, and together with 
genetic predisposition much of the world population is now fight­
ing obesity. Alternative explanations for obesity have also been 
proposed. For example, one posits that mammals can risk excess 
body weight when they are not subje

n risk excess 
body weight when they are not subject to predation and that food 
uncertainty does not play a role in driving excess weight gain.5
Definition of Obesity, 1568
Physiology of Energy Balance, 1569
Energy Expenditure, 1569
Pathogenesis of Obesity: Genes and Environment, 1571
Adipose Tissue as an Endocrine and Immune Organ, 1572
Metabolically Normal Obesity, 1574
Adverse Consequences of Obesity, 1574
KEY POINTS
	•	
Body mass index permits stratification of risk for obe

 gene mutations 
are responsible for a relatively small number of cases of obesity.
	•	
Currently available weight-loss treatments include dietary 
intervention, increased physical activity, behavior modification, 
pharmacotherapy, and surgical approaches.
1568
Definition of Obesity
Body Mass Index
Obesity represents an unhealthy excess in body fat mass. The cur­
rent practical definition of obesity is determined by calculating 
body 

ion of obesity is determined by calculating 
body mass index (BMI), by dividing an individual’s weight in 
kilograms by the square of height in meters (BMI = weight [kg]/
height [m2]). Underweight, normal weight, overweight, and obe­
sity are defined on the bases of increased risk for mortality and 
predisposition of comorbidity. Table 40.1 summarizes the classifi­
cation of weight status by BMI proposed by the major national and 
international health organizations. However, BMI is an imperfect 

ealth organizations. However, BMI is an imperfect 
index of excess fat. BMI calculations do not factor muscle mass so 
that at any given weight, an individual with greater muscle mass 
will be less liable to comorbidities. Furthermore, individuals clas­
sified as obese may be healthy without any comorbidity. Other as 
yet unidentified factors predispose some individuals to metabolic 
consequences early in life, whereas in some, comorbidities evolve 
later, after decades of obesity. The relations

ve 
later, after decades of obesity. The relationship between BMI 
and comorbidity is visualized in Fig. 40.1. Increased mortality 
at lower BMI may represent the contribution of smoking; eating 
disorders; and chronic disease states such as heart failure, cancer 
or malabsorption. As BMI rises, mortality increases, as increased 
fat increases risk for diabetes, cardiovascular disease, liver disease, 
cancer, sleep apnea, arthritis, and other diseases.
The precise magnitude of excess weight asso

eases.
The precise magnitude of excess weight associated with 
increased mortality risk remains somewhat unclear. Overweight 
and even class I obesity was not associated with increased mortal­
ity in a report on the National Health and Nutrition Examination 
Survey data collected between 1971 and 2000.6 However, stud­
ies concur that risk is increased for class II (BMI >35 kg/m2) or 
higher-magnitude obesity. 
Body Fat Distribution
The relationship between BMI and health risk is not absolute but

ip between BMI and health risk is not absolute but 
is influenced by body fat distribution, age, concomitant medical 
illness, weight gain, aerobic fitness, and ethnicity.
Excess body fat is not distributed equally in all people. Some 
obese persons develop excess visceral obesity and will be at higher 
risk of comorbidities than those with excess subcutaneous fat. 
Waist circumference correlates with abdominal fat mass and is a 
surrogate marker for visceral obesity, and thus is a predictor of 

 for visceral obesity, and thus is a predictor of 
health outcomes in adult men and women regardless of age or eth­
nicity. The relationship between waist circumference and comor­
bidities is strongest for diabetes risk, and waist circumference is 
an independent and better predictor of diabetes than BMI. Waist 
circumference thresholds for increased cardiometabolic risk are 
40 inches (102 cm) in Caucasian men and 35 inches (88 cm) in 
women; these cutoff values were derived from waist circumfe

ese cutoff values were derived from waist circumference 
values that correlated with a BMI of 30 kg/m2 or greater, based on 
populations of European origin.
BMI-associated health risk is influenced by ethnicity. Asian-
Pacific populations are at increased risk for the development of 
diabetes and cardiovascular disease at lower BMI ranges than other 
populations. Accordingly, in these populations, the World Health 
Organization proposed lower BMI cutoffs for public health action7 
(see Table 40.

 cutoffs for public health action7 
(see Table 40.1). Asian populations also tend to have more visceral 
obesity at lower BMI compared with other ethnic groups, and, 
further, at any given waist circumference, the relative risk of mor­
tality is higher. World Health Organization guidelines define waist 
circumference thresholds in the Asian population as 90 cm for 
men and 80 cm for women. Different levels have been suggested 
in Japan and China, with cutoff values of 85 cm for men and 80 
cm fo

 with cutoff values of 85 cm for men and 80 
cm for women, and slightly lower values have been suggested in 
India. Black individuals also appear to have increase disease risk 
at lower BMI thresholds than individuals of European descent—
for example, the equivalent risk BMI for diabetes is 30 kg/m2 in 
Caucasian women and 26 kg/m2 in black women.8 There is also a 
significant sex difference in comorbid disease risk, with men hav­
ing higher risk than women across the range of ethnicities.9
The 

 than women across the range of ethnicities.9
The risk of developing obesity-associated diabetes or cardiovas­
cular disease can be modified by aerobic fitness. The incidences of 
diabetes and cardiovascular fatality are lower in those who are physi­
cally fit, as defined by maximal ability to consume oxygen during 
exercise, compared with those who are unfit across a range of body 
adiposity. For example, in a population of middle-age men, low aer­
obic capacity was associated with increased mo

er­
obic capacity was associated with increased mortality rates, indepen­
dent of traditional risk factors, including smoking, blood pressure, 
and serum cholesterol, during more than 40 years of follow-up.10 
  Weight Classification by BMI
Weight 
Obesity 
Class
Risk of Obesity-
Related
Europeans
Asians
Diseases
<18.5
<17.5
Increased
Normal weight
18.5–24.9
17.5–22.9 Normal
25.0–29.9
23.0–27.4 Increased
Obesity
I
30.0–34.9
27.5–32.4 High
II
35.

ncreased
Obesity
I
30.0–34.9
II
35.0–39.9
32.5–37.5 Very high
Extreme obesity 
(polysarkia)
III
≥40.0
≥37.5
Extremely high
BMI, Body mass index.
   
TABLE 40.1
Mortality ratio
Very low
Low
High
Moderate
0
20
25
30
35
40
0
1.0
1.5
2.0
2.5
• Fig. 40.1  Relative risk for morbidity and mortality in obese European pop­
ulations. The thresholds for defining obesity are based on risk. Although risk 
rises even at fairly low body mass index, the inflection threshol

airly low body mass index, the inflection thresholds are seen 
in the body mass index range greater than 35 kg/m2, especially in men.
1569
Chapter 40  Obesity
Physiology of Energy Balance
Energy homeostasis, defined as the balance between energy intake 
and energy expenditure, is regulated by complex molecular and 
physiologic processes. Control of energy homeostasis requires 
physiologic integration of biologic signals from different organs 
including fat, muscle, liver, gut, and brain; nutrien

luding fat, muscle, liver, gut, and brain; nutrient-related sig­
nals; and postprandial neural and hormonal influences.
Regulation of energy intake is complicated because it includes 
both homeostatic and hedonic feeding. Homeostatic feeding 
is the component responding to signals aimed at maintaining 
weight, such as hunger occurring with long intermeal intervals, 
whereas hedonic feeding is defined as excess eating in the satiated 
state based on availability of highly palatable food or social

on availability of highly palatable food or social cues 
that encourage eating. Eating can also be influenced by emotional 
moods including stress or depression, which lead to overeating in 
some individuals while decreasing food consumption in others.
Central Nervous System Regulation of Appetite
The hypothalamus is an important area for integrating complex 
signals that govern food ingestion. Pro-orexigenic neurons in the 
arcuate nucleus, which express neuropeptide Y (NPY) and agouti-
related

h express neuropeptide Y (NPY) and agouti-
related protein (AgRP), and appetite inhibiting neurons, which 
express the proopiomelanocortin (POMC) gene and process its 
product to α-melanocyte-stimulating hormone (αMSH), are 
two well-studied hypothalamic networks that inversely regulate 
weight. Key issues are summarized in the following (for additional 
details, see Chapter 39).
Administration of both NPY and AgRP peptides into rodent 
brain markedly increased food intake, even in satiated rats

kedly increased food intake, even in satiated rats. Direct 
stimulation of these neurons also induces feeding. However, 
eliminating expression of NPY, AgRP, or both peptides in trans­
genic mice is associated with at best minimal feeding phenotypes. 
Full ablation of the neuronal population is required to observe 
decreased feeding, suggesting that the neurotransmitter expressed 
by theses neurons, γ-aminobutyric acid, is more important than 
either peptide in regulating appetite. POMC neurons,

ther peptide in regulating appetite. POMC neurons, a discreet 
population localized to the arcuate nucleus, play a role to inhibit 
eating. The precursor protein POMC is processed to αMSH, 
which suppresses feeding. Elimination of POMC expression leads 
to obesity in both rodents and humans. Furthermore, a down­
stream receptor for αMSH in the brain, the melanocortin 4 recep­
tor (MC4R) is critical for maintaining normal energy balance. 
MC4R is regulated by both αMSH and agouti, and the output 

regulated by both αMSH and agouti, and the output of 
MC4R neurons in the paraventricular nucleus and other locations 
represents a balance of AgRP and MSH signaling. The finding 
that MC4R deletion in mice and mutations of MC4R in humans 
are associated with obesity confirms the central role of these neu­
rons in energy balance. Indeed, in humans, MC4R mutations are 
the most common cause of monogenic obesity, accounting for 
about 5% of early-onset familial obesity.
Other areas in the hypothal

nset familial obesity.
Other areas in the hypothalamus contribute to feeding and 
weight. For example, the lateral hypothalamus contains popula­
tions of neurons expressing the neuropeptide melanin-concen­
trating hormone (MCH), and a distinct population expresses 
orexin. In rodents, MCH administration into the brain leads to 
an acute, robust increase in feeding, whereas genetic deletion of 
the neuropeptide leads to leanness.11 MCH antagonists inhibit 
feeding in several species; however, the

 inhibit 
feeding in several species; however, they are frequently associated 
with undesirable side effects complicating the development of an 
effective therapeutic.12 MCH neurons receive inputs from the 
arcuate nucleus, and MCH appears to be important in the action 
of leptin. However, human biology is complicated by the presence 
of two receptors (MCH-R1 and MCH-R2) in higher mammals. 
Orexin is another lateral hypothalamic peptide implicated in feed­
ing but more important in generalized a

d in feed­
ing but more important in generalized arousal, as rodents, dogs, 
and humans without orexin have narcolepsy.
The endocannabinoid system is also involved in the regulation 
of food intake, particularly the cannabinoid 1 (CB1) receptors 
(encoded by CNR1) and their endogenous ligands, anandamide 
(N-­arachidonoyl-ethanolamine) and 2-arachidonoylglycerol. Absence 
of CB1 receptors in mice with a disrupted CB1 gene causes hypo­
phagia and leanness. Administration of cannabinoids increases

leanness. Administration of cannabinoids increases food 
intake and promotes body-weight gain, and treatment with selective 
CB1 receptor antagonists decreases food intake and body weight in 
obese mice. Although randomized controlled trials in obese individ­
uals with a CB1 receptor antagonist found decreased body weight, 
administration of the antagonist was associated with depression and 
anhedonia,13 which complicated attempts to target the system for 
weight loss. 
Signals From the Peripher

ystem for 
Signals From the Periphery Regulating Appetite
Multiple peripheral signals regulate energy homeostasis. Leptin, 
the product of the ob gene, is critically important for weight main­
tenance. Leptin absence, resulting from gene mutations, is associ­
ated with absent appetite control resulting in morbid obesity in 
mice, rats, and humans. Although leptin is necessary for weight 
maintenance, it is not sufficient. As rodents become obese, adi­
pose tissue synthesizes increa

 become obese, adi­
pose tissue synthesizes increased leptin, causing increased levels in 
the circulation. However, the increased levels fail to decrease food 
intake. Likewise, most overweight and obese humans have high 
circulating leptin concentrations. Thus, most obesity is associated 
with resistance to leptin action.
Most other peripheral signals that participate in energy bal­
ance are derived from the gut. Of these, ghrelin is an orexigenic 
factor derived from the stomach that increase

nic 
factor derived from the stomach that increases before meals and 
decreases after feeding. Most other gut peptides, secreted by 
enteroendocrine cells, inhibit appetite. Of these, glucagon-like 
peptide 1 (GLP1), derived from pre-proglucagon, is secreted by 
the L cells of the intestine that also secrete peptide YY. GLP1 and 
peptide YY are co-secreted after a meal and induce satiety,14 and 
GLP1, when infused directly into the rat ventricle, opposes the 
actions of orexigenic peptides inclu

 opposes the 
actions of orexigenic peptides including MCH and NPY.15 Cho­
lecystokinin, another gut peptide, is distributed throughout the 
gastrointestinal tract. It stimulates bile release from the gallbladder 
after a meal but likely also contributes to satiety. 
Energy Expenditure
Energy expenditure is a key component critical for weight homeo­
stasis. Several factors contribute to total daily energy expenditure 
(Fig. 40.2). The largest component is basal or resting energy expen­
diture, d

ponent is basal or resting energy expen­
diture, defined as the energy required in the basal state for normal 
cellular and organ function; it has also been termed basal metabolic 
rate. Resting energy expenditure accounts for approximately 70% 
of total energy expenditure. A second significant contributor is the 
energy expended by physical activity, which represents a smaller 
contribution, about 20%, of the daily total. Voluntary activities 
such as exercise, and involuntary activities import

uch as exercise, and involuntary activities important in maintain­
ing posture and fidgeting, contribute to this component of total 
energy expenditure. Digestion of ingested nutrients also contrib­
utes to energy expenditure and is termed the thermic effect of food, 
1570
accounting for about 10% of the daily total and representing the 
energy expended in chewing, digestion, absorption, and sympa­
thetic nervous system activation afte

, and sympa­
thetic nervous system activation after a meal.
Resting energy expenditure is regulated by several mechanisms, 
particularly by thyroid hormone.16 An excess of thyroid hormone 
as seen in thyrotoxicosis leads to increased resting energy expendi­
ture that in many individuals will lead to significant weight loss, 
unless they experience compensatory hyperphagia. Indeed, weight 
loss can be a presenting symptom of hyperthyroidism. Given the 
effects of thyroid hormone to reduce weight,

 the 
effects of thyroid hormone to reduce weight, in the mid-20th cen­
tury it was transiently used therapeutically in obesity. Although 
effective, serious side effects curtailed use.
In contrast to the clear effects of thyroid hormone to regulate 
resting energy expenditure, the role of the sympathetic nervous 
system is less clear. Epinephrine and norepinephrine are critical 
regulators of metabolism across multiple organs activating what 
is commonly referred to as the fight-or-flight respo

 commonly referred to as the fight-or-flight response, but a pre­
cise role in regulating resting energy expenditure remains hard to 
define. Sympathetic activity increases after meals and contributes 
to the thermic effect of food. In addition, patients with tumors 
secreting epinephrine and norepinephrine (pheochromocytoma) 
manifest downstream effects of increased sympathetic activity, such 
as tachycardia and hypertension, but rarely present with weight 
loss. Although it is possible that in

with weight 
loss. Although it is possible that individuals prone to obesity have 
lower sympathetic activity and therefore resting energy expendi­
ture than those who maintain lower body weights, the role of sym­
pathetic activity in energy homeostasis in humans is uncertain.
Effects of resting energy expenditure on weight gain also remain 
unclear. In a longitudinal study of 92 Pima Indians, the highest 
cumulative incidence of a 22-pound (10-kg) weight gain after 1 
to 4 years was seen in tho

g) weight gain after 1 
to 4 years was seen in those in the lowest tertile of resting energy 
expenditure at baseline.17 In contrast, no relationship between ini­
tial resting energy expenditure and weight change was found in 
the Baltimore Longitudinal Study on Aging, which followed 775 
men for 10 years.18 However, currently available research technol­
ogy is unable to detect small but chronic reductions in energy 
metabolism that may be clinically important over time. In addi­
tion, it is dif

ally important over time. In addi­
tion, it is difficult to establish a causal relationship between energy 
expenditure and the development of obesity, because energy 
metabolism measurements capture only a brief point in time and 
therefore may not reveal abnormalities that emerge during specific 
life stages. Genetic or environmental differences may also impact 
the relationship between resting energy expenditure and weight 
change.
The two other components of energy expenditure—voluntary 
act

er components of energy expenditure—voluntary 
activity (exercise) and nonexercise activities—contribute to obe­
sity. Increased food consumption is required for energy homeosta­
sis in the setting of increased voluntary activity, as seen in athletes, 
people with regular exercise, or physically demanding jobs. When 
these activities end, as for young adults previously participating 
in college sports, resulting weight gain can be substantial. Like­
wise, nonexercise activity thermogenesis, such

ke­
wise, nonexercise activity thermogenesis, such as fidgeting, may 
contribute to weight maintenance, as individuals with higher 
nonexercise activity thermogenesis tend to have some resistance 
to weight gain.
Body Weight Set-Point
Many individuals will maintain a constant weight over years and 
even decades, supporting the theory that there is a set-point reg­
ulating food intake and energy expenditure so that weight stays 
constant.19 The set-point theory is that a complex set of physi­
olo

t-point theory is that a complex set of physi­
ologic adaptations maintains body weight. According to the set-
point theory, body weight is predetermined such that weight loss 
promotes a decrease while weight gain promotes an increase in 
metabolic rate that acts to restore body weight to a preset level.20 
The effectiveness of the set-point has been called into question 
because many individuals will gain weight over time.21
Hypocaloric feeding reduces energy expenditure by an average 
of 8 kc

 reduces energy expenditure by an average 
of 8 kcal/kg of lean body mass in obese individuals but only 6 
kcal/kg of lean body mass in normal-weight individuals, amount­
ing to a 15% to 20% daily decrease.22 This reduction cannot be 
completely accounted for by the accompanying decrease in body 
size, or lean or adipose body mass, and is considered a normal part 
of the physiologic adaptation to energy restriction.
The decrease in energy metabolism with weight loss is largely 
appropriate for c

ism with weight loss is largely 
appropriate for concomitant changes in body composition, and 
a persistent decrease might promote weight regain. However, the 
reduction in resting energy expenditure that occurs during nega­
tive energy balance is transient and does not persist during main­
tenance of a lower body weight. Long-term maintenance of weight 
loss is not associated with an abnormal decrease in either resting or 
total energy expenditure when adjustments are made for changes 
in body 

re when adjustments are made for changes 
in body composition,23 although this remains controversial.24
When energy intake exceeds energy expenditure, weight gain 
occurs, but the amount of weight gained varies among individu­
als. Genetic factors can influence the amount of weight gained 
with overfeeding. Data from a study that fed monozygotic twins 
an extra 1000 kcal/day for 84 days found considerable variability 
Physical
activity
Resting
metabolic
rate
NEAT
Arousal
Sleep
Purposeful
Thermic

tabolic
rate
NEAT
Arousal
Sleep
Thermic effect
of food
• Fig. 40.2  Components of energy expenditure.  The largest component 
is heat generated by biochemical reactions and physiologic processes 
occurring at rest that are required for systemic homeostasis, including 
respiration, circulation, and excretion. The process of arousal increases 
resting energy expenditure as posture needs to be supported. Resting 
energy expenditure is regulated by hormones, output from the central ner­
v

ulated by hormones, output from the central ner­
vous system, and sympathetic activity. Eating also requires energy, and 
postprandial processes involved in digestion and absorption and distribu­
tion of nutrients contribute to the thermic effect of food. Physical activity 
is a smaller component of energy expenditure and includes the energy 
expended toward purposeful activities and nonexercise activity thermo­
genesis (NEAT), such as fidgeting.
1571
in weight gained among d

1571
in weight gained among different twin pairs but that members of 
each twin pair gained similar amounts of weight while overfed 
and then lost similar amounts during the postoverfeeding recov­
ery period.25 The amount of weight gained may also reflect the 
extent of the increase in energy expenditure that is a consequence 
of overfeeding.22 In another study, increase in body fat after 8 
weeks of overfeeding was inversely related to changes in nonvoli­
tional energy expe

 related to changes in nonvoli­
tional energy expenditure (e.g., fidgeting), aspects of which may 
be regulated.26 
Pathogenesis of Obesity: Genes and 
It is challenging to identify causal factors of obesity because there 
are few monogenetic forms of obesity, and many single nucleotide 
polymorphisms (SNPs) associated with excess weight are in non­
coding regions of the genome. In addition, there are substantial 
social and environmental determinants of weight. Food environ­
ments a

ntal determinants of weight. Food environ­
ments are complex and include not just food availability but food 
cost, cultural perspectives of weight, and an individual’s social 
network.
Environmental Effects in High-Risk Populations
A striking example of the effects of modern diet on obesity is seen in 
Pima Indians living in Arizona. A combination of diet and lifestyle 
changes starting in the 1950s resulted in an epidemic of both obe­
sity and diabetes. The modern diet is much higher in fat (5

diabetes. The modern diet is much higher in fat (50% of 
energy as fat) than the traditional Pima diet (15% of energy as fat). 
In addition, Pima Indians who became urbanized are more seden­
tary, especially when compared with the Pima Indian population 
still living in the Sierra Madre Mountains of northern Mexico. 
These rural Pima Indians eat a traditional diet and are physically 
active as farmers and sawmill workers; they have a much lower 
incidence of obesity and diabetes than their Arizo

incidence of obesity and diabetes than their Arizona kindred.27 
Likewise, Aborigines of northern Australia are another high-risk 
population whose weight and health status has been compromised 
by exposure to a modern environment. Urbanized Aborigines are 
heavier than and have a high prevalence of type 2 diabetes mellitus 
compared with rural kindred, who are usually very lean (BMI <20 
kg/m2). The traditional Aboriginal diet included a low-fat, low-
calorie diet of wild game, fish, and plants

, low-
calorie diet of wild game, fish, and plants, as well as a high level 
of physical activity. Short-term (7 weeks) reexposure to the tradi­
tional lifestyle can result in weight loss and significant improve­
ments or normalization of glucose tolerance and fasting glucose, 
insulin, and triglyceride concentrations in urbanized Aborigines 
with type 2 diabetes mellitus and hypertriglyceridemia.28
Influences of Childhood and Parental Obesity
Childhood obesity increases the risk of becoming an 

ildhood obesity increases the risk of becoming an obese adult, 
as does having at least one obese parent. The risk of adult obesity 
increases with increasing age and with the severity of obesity in 
childhood. For example, the risk of being obese at 21 to 29 years 
of age ranged from 8% for persons who were obese at 1 to 2 years 
of age and had nonobese parents to 79% for persons who were 
obese at 10 to 14 years of age and had at least one obese parent, 
with multiple permutations between thes

e parent, 
with multiple permutations between these extremes. It remains 
unclear how much of this is due to the genetic risk from paren­
tal obesity, including epigenetic factors, or the environmental and 
behavioral traits that may be shared between parent and offspring. 
Genetics and Obesity
Monogenic Causes of Obesity
Only a small percentage of obese people have an identified single 
gene mutation that leads to obesity. The most common identi­
fied mutation associated with obesity involves M

­
fied mutation associated with obesity involves MC4R, which is 
expressed in the central nervous system, in neurons downstream 
of those emanating in the arcuate nucleus. Action of αMSH from 
the arcuate nucleus on MC4R inhibits appetite, so a function­
ing MC4R is essential to maintain normal body weight. Frame­
shift mutations in MC4R in individuals with childhood obesity, 
reported by multiple groups,29,30 may account for about 5% of 
early-onset obesity. These are usually heterozygous, and 

nset obesity. These are usually heterozygous, and a gene 
dosage effect is seen; individuals homozygous for the same muta­
tion or double heterozygotes for different mutations will have 
more severe obesity. The range of mutations is very diverse.31 Indi­
viduals with MC4R mutations will respond to bariatric surgery, 
although the magnitude of response may vary depending on the 
specific mutation.32. In addition, MC4R agonists are currently in 
clinical development, as it may be possible to resc

linical development, as it may be possible to rescue melanocortin 
signaling using MC4R agonists, depending on the specific MC4R 
mutation involved.33
Other, much less common, mutations in the melanocortin sig­
naling pathway involve those in the POMC gene, which leads to 
loss of all of the product peptides including adrenocorticotropic 
hormone and αMSH. The presentation of patients with POMC 
mutations is typically secondary to adrenal insufficiency, usually 
observed in early life. Consisten

ciency, usually 
observed in early life. Consistent with absent melanocortin signal­
ing, individuals also have red hair.
Although leptin is critical for the normal maintenance of 
energy balance, very few leptin-deficient humans have been iden­
tified, and leptin mutations are extremely rare.34 The first humans 
with a leptin mutation,35 which resulted in a protein that was not 
secreted, was found in a pair of consanguineous cousins. Shortly 
thereafter, three members in an obese family were r

hereafter, three members in an obese family were reported with 
leptin receptor mutations.36 There is remarkable consistency in 
the phenotype of mice and humans with absolute deficiency in 
leptin signaling, whether as a result of absent leptin or a mutation 
in the leptin receptor. Both mice and humans demonstrate mas­
sive early-onset obesity, hyperphagia, exaggerated food-seeking 
behavior, and infertility. Normal pubertal development does not 
occur in the absence of leptin. The handful of 

t 
occur in the absence of leptin. The handful of individuals with 
leptin deficiency can be treated with exogenous leptin, which nor­
malizes weight and food intake, permits puberty,37,38 and reestab­
lishes insulin sensitivity.39 As would be expected, leptin is of no 
use in individuals with leptin receptor deficiency.40
Prohormone Convertase 1 Gene Mutation
Prohormone convertase 1 (also known as proprotein convertase 
subtilisin/kexin type 1 [PCSK1]) cleaves POMC and is involved 
in processin

PCSK1]) cleaves POMC and is involved 
in processing of peptides in enteroendocrine cells in the gut. A few 
individuals with mutations in PCSK1 and obesity have been iden­
tified.41 Although the initial report involved a mutation in PCSK1 
in an adult obese woman with a history of severe childhood obe­
sity, a significant number of individuals with this mutation pres­
ent with diarrhea and failure to thrive. In some individuals, this 
then evolves into obesity, although the mechanisms promoting 

s into obesity, although the mechanisms promoting 
the transition remain poorly understood. 
Mutation of the Neurotrophin Receptor TrkB
The neurotrophin brain-derived neurotrophic factor (BDNF) 
acts through a receptor kinase, TrkB, to potentiate synaptic 
1572
transmission. Heterozygous deletion of BDNF in mice leads to 
a syndrome of hyperphagia obesity and aggression. In humans, 
mutations in this system are very rare but include a 

ations in this system are very rare but include a report of a 
chromosomal deletion encompassing the BDNF gene in a mother 
and child pair.42 At least one case of a mutation in the TrkB signal­
ing pathway associated with developmental delays and obesity has 
been reported in an 8-year-old boy. The mutation was associated 
with impaired receptor autophosphorylation.43 
Single-Minded Homolog 1 (SIM1) Gene Mutation
A de novo balanced translocation between chromosomes 1 and 6 
was found in a severe

between chromosomes 1 and 6 
was found in a severely obese girl who weighed 104 pounds (47 
kg) at 5½ years of age.44 The mutation caused a disruption in 
SIM1, the human homolog of the Drosophila single-minded (sim) 
gene that regulates neurogenesis. SIM1 encodes a transcription 
factor involved in the formation of the paraventricular and supra­
optic nuclei. It is likely that this mutation altered energy balance 
in this patient by stimulating food intake, because measured rest­
ing energy exp

food intake, because measured rest­
ing energy expenditure was normal. In addition, several chromo­
somal deletions involving 6q14-q21, a region that includes SIM1, 
have been identified in obese patients presenting with develop­
mental delay and a Prader-Willi–like syndrome,45 although the 
specific role of SIM1 haploinsufficiency has not been definitively 
established. 
Src Homology 2B 1 (SH2B1) Deficiency
Src homology 2B adapter protein (SH2B1) interacts with sev­
eral receptors to modulate s

) interacts with sev­
eral receptors to modulate signaling of ligands including leptin, 
insulin, and growth hormone. In mice, deletion of this gene 
results in obesity and insulin resistance when mice are exposed 
to a high-fat diet. In humans, this deletion is associated with a 
range of abnormalities including early-onset obesity and hyper­
phagia. Consistent with a role in insulin signaling, problems with 
insulin sensitivity appear disproportionate to the severity of obe­
sity. Behavioral a

tionate to the severity of obe­
sity. Behavioral abnormalities have also been reported.46 Interest­
ingly, the result of deletions that include the SH2B1 gene at the 
chromosomal 16p11.2 site may be pleiotropic and not include 
obesity.47
Obesity Syndromes
Prader-Willi syndrome was initially described more than 60 years 
ago as an association of morbid obesity, short stature, hypogo­
nadism, and cognitive deficiency.48 Prader-Willi results from a 
chromosomal abnormality in

r-Willi results from a 
chromosomal abnormality in which the paternal segment of chro­
mosome 15q11.2-q12 is either deleted or absent. Interestingly, 
maternal deletion of the same segment is associated with a distinct 
phenotype known as Angelman syndrome, which is an autistic 
spectrum disorder.49 Prader-Willi has a prevalence of 1 in 15,000 
to 30,000 births. The molecular etiology has been linked to a dele­
tion in the melanoma antigen gene (MAGE) family member L2 
(MAGEL2), a regulator of u

MAGE) family member L2 
(MAGEL2), a regulator of ubiquitin kinase.50 
Bardet-Biedl Syndrome
Bardet-Biedl syndrome is rare, with a prevalence of less than 1 in 
100,000, mostly seen in consanguineous populations. The disor­
der is clinically heterogeneous and includes obesity, hypogonad­
ism, and abnormalities that include dysmorphic extremities, renal 
impairments, and retinopathies. Severe vision loss is not uncom­
mon. Bardet-Biedl syndrome maps to multiple genes that lead to 
abnormal ciliary

s to multiple genes that lead to 
abnormal ciliary function51,52; how these cause obesity remains 
unknown. 
Polygenic Causes of Obesity
Obesity results from a complex interaction of genetic predisposi­
tion and a nutrient-rich environment. More than 100 polymor­
phisms have been identified as potentially contributing to the 
predisposition to obesity.53 However, most of these loci individu­
ally make only small weight contributions to obesity. Among hun­
dreds of genes, the best association is 

mong hun­
dreds of genes, the best association is with FTO (the fat mass and 
obesity associated gene).54
FTO was originally identified as a gene playing a role in 
programed cell death. In 2007, three studies demonstrated an 
association between variations in FTO and body mass. Several 
polymorphisms have been identified. The 16% of individuals 
homozygous for the risk allele SNP rs9939609 weigh about 3 kg 
more and have 1.67-fold increased odds of obesity when compared 
with those not inheriti

 of obesity when compared 
with those not inheriting a risk allele. This association reflects a 
specific increase in fat mass and can be seen in those 7 years and 
older.55 A similar association with common FTO variants was 
reported in a population from Sardinia.56 These reports were fol­
lowed by numerous reports of FTO and excess body weight in mul­
tiple human populations. However, the mechanistic link between 
FTO and obesity has been difficult to make. The phenotype of the 
SNPs is diffic

cult to make. The phenotype of the 
SNPs is difficult to identify, with some reporting increased caloric 
intake and some reporting decreased physical activity. Mouse stud­
ies point to a role of FTO in regulating body composition.57 FTO 
has also been implicated in reward systems and methylation of 
proteins involved in synaptic transmission and cell signaling,58 as 
FTO encodes a 2-oxoglutarate–dependent nucleic acid demethyl­
ase.59 However, the causal link between FTO and obesity remains 
el

he causal link between FTO and obesity remains 
elusive and raises significant questions as to how to best investigate 
causality of human genetic variants in disease.60
Most individuals, perhaps up to BMIs in the high 30 range, are 
likely physiologically normal and become obese as they respond 
to the temptations of modern environments rich in high-calorie 
food. Given the polygenic nature of obesity, where many variants 
in the same or different genes contribute only modestly to body 
weight 

nt genes contribute only modestly to body 
weight but may act synergistically to quantitatively affect weight, 
it is likely that polygenic risk scores may better estimate the overall 
genetic risk of obesity from common variants rather than indi­
vidual risk variants considered alone.61 Epigenetic factors may also 
contribute by regulating expression of obesity-associated genes. A 
better understanding of factors contributing to the pathogenesis 
of obesity might be obtained if we use the BMI r

of obesity might be obtained if we use the BMI range to identify 
the relative risk of comorbidities rather than as a guide for discov­
ering pathology. The search for genetic variants of disease might 
be more fruitful if the comparisons focused on extreme leanness 
and extreme obesity. 
Adipose Tissue as an Endocrine and Immune 
Organ
Until fairly recently, the traditional view of adipocytes was as a pas­
sive, efficient storage depot for triglycerides that would be released 
with fasting to

glycerides that would be released 
with fasting to provide fuel. This view changed as discoveries indi­
cated adipocytes secrete multiple factors, termed adipokines, with 
both metabolic and immune functions62 (Table 40.2). One of the 
first secreted factors was adipsin, a serine protease in the comple­
ment family63 whose expression was impaired in both genetic and 
acquired obesity.64 Although more than 30 years have elapsed since 
this initial observation, the role of adipsin in adipocyte and

 observation, the role of adipsin in adipocyte and sys­
temic biology remains relatively obscure and serves as an example 
of some of the difficulties encountered in understanding adipocyte 
1573
biology. Understanding immune responses in obesity is further 
complicated by findings of increased infiltration of immune cells 
in fat. Initially considered pathologic, recent findings suggest that 
they may also play a positive role in adipose tissue metabolism by 
clearing lipids

e in adipose tissue metabolism by 
clearing lipids and senescent fat cells.65 Different types of infil­
trating immune cells may play different roles within the adipose 
tissue. Multiple hormones and cytokines are synthesized in fat, a 
selection of which are considered here.
Leptin
Potentially the most critical product of the adipocytes is the pro­
tein leptin, which is synthesized in the adipocyte and released into 
the circulation. Leptin is required for normal energy balance, as 
genetic del

equired for normal energy balance, as 
genetic deletion in both rodents and humans is associated with 
morbid obesity, as noted earlier.66 Leptin synthesis is regulated, 
and levels fall dramatically with starvation, leading to many func­
tional consequences including a decrease in fertility, which can 
be corrected by exogenous leptin in both mice and humans.67,68 
Leptin levels also rise with acute overfeeding.69
Although leptin deficiency leads to morbid obesity, most obese 
humans have high 

s to morbid obesity, most obese 
humans have high levels of leptin, and indeed adiposity correlates 
with circulating serum leptin levels in both rodents and humans, 
leading to the concept of leptin resistance,70–72 which is supported 
by attenuated induction of downstream mediators of leptin action 
in obese rodents. The existence of leptin resistance, which has 
recently been linked to the stability of the leptin receptor,73 also 
suggests that leptin does not serve as a biologic “adipostat” 

t leptin does not serve as a biologic “adipostat” and 
that its primary function is to signal starvation.74 
Resistin
Resistin is another signaling polypeptide secreted by adipocytes. 
Resistin concentrations are increased in mice with diet-induced 
and genetic forms of obesity and insulin resistance. Administra­
tion of recombinant resistin to normal mice leads to impaired 
glucose tolerance and attenuated insulin action. Based on these 
findings, it has been proposed that resistin is a hormone

s, it has been proposed that resistin is a hormone that links 
obesity to diabetes by inducing insulin resistance.75 
Adiponectin is a key signaling peptide produced by adipocytes; it 
is also the most abundant.76 Plasma adiponectin concentrations 
are decreased in obesity and insulin resistance in both rodents 
and humans. Adiponectin acts to increase insulin sensitivity,77 
and expression increases with improved insulin sensitivity, such as 
when animals are treated with thiazolidi

 such as 
when animals are treated with thiazolidinediones.78 Weight loss79 
independent of the type of diet is also associated with increases in 
circulating adiponectin. However, adiponectin structure is quite 
complex, being composed of protein dimers and tetramers. The 
ratio of high-molecular-weight isoforms to low-molecular-weight 
isoforms is more important than total adiponectin in determin­
ing insulin sensitivity80 and weight loss. Additionally, there are 
two receptors mediating its a

tionally, there are 
two receptors mediating its action. Administration of exogenous 
adiponectin lowers glucose in obese rodent models through sup­
pression of hepatic gluconeogenesis, as well as promoting liver 
ceramide breakdown.76 Unfortunately, it has not been possible to 
develop adiponectin-based therapies due to the complex structure. 
Several SNPs of adiponectin in human populations have been 
reported and may be linked to cardiovascular disease; however, 
the possible mechanism(s) rem

r disease; however, 
the possible mechanism(s) remain uncertain.81 
Estrogens
Adipose tissue contributes to total serum estrogen derived from 
androgens through the action of aromatase, which catalyzes for­
mation of estrone from androstenedione. The conversion rate of 
androstenedione into estrone increases with age and obesity. In 
women, after menopause, adipose tissue becomes a significant 
source of estrogen biosynthesis. Increased estrogen has been impli­
cated in the risk of breast cancer

has been impli­
cated in the risk of breast cancer in obese women.82 Adipose tis­
sue also expresses estrogen receptors ERα and ERβ. Differential 
expression of estrogen receptors in different depots and gender 
differences in estrogen concentrations may explain, in part, dif­
ferences in lipid accumulation in men and women.83 Postmeno­
pausal women who lose weight have lower breast cancer risk 
than those with stable weight or weight gain. It is possible that 
this effect is modulated through a

 possible that 
this effect is modulated through adipose estrogens, although this 
remains speculative.84 
Selected Cytokines
Expansion of adipose tissue leads to a low-grade inflammatory 
state in fat.65 Increased expression of cytokines from adipose tis­
sue results in recruitment of macrophages, which may serve to 
aggravate inflammation and insulin resistance.85 The precise con­
sequences of macrophage recruitment are unclear, because while 
promoting inflammation, they may also have benefic

promoting inflammation, they may also have beneficial effects 
related to buffering of released lipids (efferocytosis) and clear­
ance of necrotic cells.86 Several dozen cytokines are synthesized 
in adipose tissue and have been reviewed extensively.87 Two key 
examples are discussed here.
Tumor Necrosis Factor α
Tumor necrosis factor α (TNFα) is an inflammatory cytokine that 
plays a role in many disease processes. It is synthesized by mul­
tiple cell types, including adipocytes, macrophages, m

e cell types, including adipocytes, macrophages, monocytes, 
and neutrophils. TNFα regulates expression of other cytokines, 
growth factors, and transcription factors. In adipocytes, TNFα 
expression increases with obesity and leads to localized inflamma­
tion that is associated with decreased insulin sensitivity. Likewise, 
  Adipocyte-Secreted Factors
Category
Protein
Hormones
Leptin, resistin, angiotensinogen, adiponectin, estro­
gens, visfatin, glucocorticoids, angiopoietin 4, apelin
Cytoki

in, glucocorticoids, angiopoietin 4, apelin
Cytokines
Interleukins 1, 6, 8, 10; monocyte chemoattractant 
protein 1; interferon-γ; tumor necrosis factor α
Extracellular 
matrix 
proteins
Various subtypes of collagen α1, various metallopro­
teinases, fibronectin, osteonectin, laminin, entactin, 
thrombospondin 1 and 2
Complement 
factors
Adipsin, complement C3, factor B
Enzymes
Cholesterol ester transfer protein, lipoprotein lipase
Acute phase 
response 
proteins
Alpha-1-acid glycoprotein, haptog

sponse 
proteins
Alpha-1-acid glycoprotein, haptoglobin
Other
Fatty acids, plasminogen activator inhibitor 1, prosta­
cyclin, vascular endothelial growth factor
TABLE 40.2
1574
blocking TNFα action improves insulin sensitivity.88 A positive 
correlation exists between circulating TNFα concentrations and 
obesity; however, it seems likely that the major effects of TNFα are 
local.89 Notably, despite widespread use of anti-TNFα treatment

bly, despite widespread use of anti-TNFα treatments 
for rheumatologic diseases,90,91 improvements of insulin sensitiv­
ity and glycemia in patients with type 2 diabetes mellitus are not 
clinically evident. 
Interleukin 6
Another proinflammatory cytokine increased in obesity is inter­
leukin 6 (IL6), which may contribute to systemic inflammation 
and insulin resistance. Insulin sensitivity is inversely related to 
plasma IL6 concentrations, and IL6 directly impairs insulin sig­
naling.92 Admini

IL6 directly impairs insulin sig­
naling.92 Administration of IL6 induces dose-dependent increases 
in fasting blood glucose in humans, probably by stimulating 
release of glucagon and other counterregulatory hormones, by 
inducing peripheral resistance to insulin action, or both. IL6 and 
other proinflammatory cytokines may also play a direct role in 
regulating adipocyte metabolism and vascular health.93 
Brown Adipose Tissue
Brown adipose tissue (BAT) is a distinct fat depot that is struc­
tu

ue (BAT) is a distinct fat depot that is struc­
turally and functionally different from white adipose tissue; it 
contains multilocular fat vacuoles and large mitochondria and is 
intensively innervated by sympathetic nerves.94 BAT contributes 
to energy homeostasis in rodents, where it is part of the adaptation 
to cold exposure95 but also plays a role in the adaptation to very 
low carbohydrate diets.96 Under these conditions, BAT is activated; 
as a result, there is an increase in the levels 

as a result, there is an increase in the levels of the factor uncou­
pling protein 1 (UCP1), which generates a mitochondrial proton 
leak that results in less production of adenosine triphosphate and 
energy wastage through the generation of heat. Recently, BAT 
depots have been identified in humans, which increased interest 
in BAT biology. Human BAT can be activated by cold exposure97 
and increased adrenergic activity.98 However, human BAT depots 
are relatively small, and it is unlikely th

epots 
are relatively small, and it is unlikely that BAT plays a significant 
role in energy expenditure in otherwise healthy humans. 
Metabolically Normal Obesity
Obesity is commonly associated with impaired metabolic func­
tion—namely, insulin resistance, diabetes, dyslipidemia (increased 
serum triglyceride and decreased serum high-density lipoprotein 
cholesterol), and increased blood pressure. Nevertheless, some 
individuals with increased weight remain metabolically normal 
based on measur

eight remain metabolically normal 
based on measurements of lipids, blood pressure, and insulin resis­
tance. Metabolically healthy obese individuals do not appear to 
have increased risk for cardiovascular mortality.99 The concept of 
“metabolically healthy obese” is difficult to evaluate, and assess­
ment of such individuals may be confounded by multiple factors, 
such as the presence of comorbidities, selection bias, or other fac­
tors.100 For example, individuals most susceptible to cardiova

 example, individuals most susceptible to cardiovascular 
disease may die earlier. It seems logical that obesity will have dif­
ferential impact on individuals based on as yet undefined genetic 
or behavioral predispositions. It would be expected that some indi­
viduals with mild to moderate obesity would remain healthy.
Although, in general, increased BMI is associated with 
increased disease risk, in certain patient populations, being over­
weight or obese is associated with lower mortality ra

ght or obese is associated with lower mortality rates than for 
similar patients with normal BMI values. This BMI paradox is 
associated with cardiovascular disease101 (myocardial infarc­
tion, congestive heart failure, hypertension, and coronary heart 
disease), coronary artery bypass graft surgery, and end-stage renal 
disease.102 Although this may represent an intrinsic paradox, it 
may also reflect that chronic disease is associated with sarcope­
nia, and individuals with higher BMI may be h

ope­
nia, and individuals with higher BMI may be healthier as a con­
sequence. It is also plausible that treatment of comorbidities with 
statins, angiotensin-converting enzyme inhibitors or angiotensin-
receptor blockers, and others can provide some degree of protec­
tion when excess weight is not extreme, with differential use in 
the setting of excess weight compared with the same value in the 
normal-weight patient. 
Adverse Consequences of Obesity
Obesity as a Disease Risk Factor
Metabolic 

besity
Metabolic syndrome refers to the common co-occurrence of mul­
tiple diseases with obesity, particularly obesity with a visceral dis­
tribution. Most of these obesity-related disorders are associated 
with increased cardiovascular disease risk. Metabolic syndrome 
has multiple definitions. National Cholesterol Education Program 
Adult Treatment Panel III includes any three of the following five 
clinical measures: increased waist circumfe

 five 
clinical measures: increased waist circumference defined on the 
basis of ethnic specific criteria, elevated triglyceride levels, low 
high-density lipoprotein cholesterol levels, increased blood pres­
sure, or high fasting glucose. World Health Organization criteria 
for metabolic syndrome includes presence of insulin resistance 
along with the identification of at least two additional risk fac­
tors.103 Based on meta-analysis of studies that included more than 
900,000 people, metabolic

that included more than 
900,000 people, metabolic syndrome led to a 2-fold increased risk 
of cardiovascular disease and a 1.5-fold risk of increased mortality 
from all causes.104
Obesity predisposes to cardiovascular disease through multiple 
mechanisms, as it increases the likelihood of type 2 diabetes melli­
tus, dyslipidemia, and hypertension, all of which are independent 
risk factors for cardiovascular morbidities. Factors secreted from 
the adipocyte can also promote a proinflammatory, 

the adipocyte can also promote a proinflammatory, prothrom­
botic state. 
Type 2 diabetes mellitus is a common consequence of excess 
adiposity.105 Indeed, more than 90% of individuals with type 2 
diabetes mellitus are obese.106 Rising prevalence rates of type 2 
diabetes mellitus coincide with rising rates of obesity. Rates of dia­
betes across the continental United States range from a low of 7% 
in Colorado to a high of 13.6% in Mississippi, and rates in the US 
territories o

in Mississippi, and rates in the US 
territories of Guam and Puerto Rico are even higher.107 Diabetes 
prevalence parallels the prevalence of obesity, which is lowest in 
Colorado, where rates range between 20% and 25%, and highest 
in the southeastern states, including Mississippi, with rates higher 
than 35%.
Risk of diabetes increases with earlier onset and more severe 
obesity. For example, the likelihood of developing diabetes is 70% 
for women with a BMI greater than 35 at age 18 years, wi

men with a BMI greater than 35 at age 18 years, with an 
expected diagnosis of diabetes by the sixth decade.108 An increased 
waist to hip ratio, reflective of increased visceral obesity, is also 
associated with increased diabetes risk.109 
Dyslipidemia
Primary dysfunction in the adipocyte is associated with abnormal 
lipid metabolism, increasing the risk of dyslipidemia. Abnormal 
lipid metabolism in association with genetic factors results in dys­
lipidemia including hypertriglyceridemia; red

dys­
lipidemia including hypertriglyceridemia; reduced high-density 
1575
lipoprotein cholesterol levels; and an increased fraction of small, 
dense low-density lipoprotein particles. This association is espe­
cially strong in persons with abdominal obesity.110 
There is a linear relationship between hypertension and BMI.111 
Prevalence rates are more than twice as high in obese men and 
women compared with lean men and women, and risk of hyperten­
sion increases

en and women, and risk of hyperten­
sion increases with both age and weight gain. Approximately 70% 
of hypertension in adults is attributable to excess adiposity, espe­
cially visceral adiposity. Obesity-related hypertension has distinct 
genetic determinants compared with hypertension in the absence 
of obesity. Physiologic mechanisms of obesity-related hypertension 
include insulin resistance, sodium retention, increased sympathetic 
nervous system activity, activation of renin-angiotensin-al

ystem activity, activation of renin-angiotensin-aldoste­
rone, and altered vascular function. Weight loss results in blood 
pressure reductions112,113; however, the effects may not be durable. 
Obesity is associated with a significantly increased lifetime risk 
of cardiovascular disease, including coronary artery disease, heart 
failure, and cerebrovascular disease. Risks are higher for men and 
for those with visceral obesity.114 Absolute thresholds for weight-

ity.114 Absolute thresholds for weight-
associated cardiovascular disease are hard to determine and can 
depend on the type of event being assessed, the subpopulation, 
and other confounding factors. In middle-age men, the risk of a 
cardiovascular event increases with increased BMI, and the risk of 
cardiovascular death is twofold for individuals with a BMI greater 
than 40 kg/m2 compared with normal-range BMI. A similar 
increase in risk in cardiovascular events is seen in women, although 
ris

diovascular events is seen in women, although 
risk of cardiovascular death does not significantly correlate with 
BMI. Overweight individuals have similar longevity to those with 
normal BMI; however, the increased risk in cardiovascular events 
leads to living with chronic cardiovascular morbidity.115
In a recent analysis of data from both the Nurses’ Health Study 
and the Health Professionals study, no increased cardiovascular 
risk was seen in the overweight category. When this was adjusted 

n the overweight category. When this was adjusted 
for lifetime weight history, a small increase was noted in the over­
weight BMI range,116 indicating that at lower excess weights, 
assessment of risk is complicated.
Increasing BMI also increases the risk of ischemic cerebrovas­
cular events in both men and women. The risk of fatal and non­
fatal ischemic stroke is approximately twice as great in individuals 
with a BMI greater than 35 kg/m2. In intermediate BMI ranges of 
greater than 25 kg/m2

 intermediate BMI ranges of 
greater than 25 kg/m2 and up to 32 kg/m2, it is difficult to assess 
the relative increased risk in otherwise healthy individuals. At least 
one study reports similar survival following an ischemic event in 
normal-weight and overweight subjects.117
Obesity is also associated with an increased risk of thromboem­
bolic disease, and risk increases with higher waist circumference.118 
Liver Disease
As a result of the increasing prevalence of obesity, nonalcoholic 
fatty

reasing prevalence of obesity, nonalcoholic 
fatty liver disease (NAFLD) is now the most common cause of 
chronic liver disease in both developed and developing countries. 
In the United States, NAFLD affects 30% of the obese popula­
tion and 53% of obese children.119 Risk increases with additional 
excess weight such that prevalence increases to 90% in morbidly 
obese populations.120,121 Hepatosteatosis is diagnosed when 
5% of all cells contain lipid droplets by histology or when mag­
netic re

 lipid droplets by histology or when mag­
netic resonance imaging reveals more than 5% fat in the liver. 
Diagnosis of NAFLD requires exclusion of other causes of liver 
pathology, including alcohol abuse, viral infections, and biliary or 
autoimmune disease. Although fatty liver itself is a relative benign 
state, it can progress unpredictably to steatosis, cirrhosis, and hepa­
tocellular carcinoma. Progression to nonalcoholic steatohepatitis, 
characterized by hepatocyte apoptosis, inflammatio

characterized by hepatocyte apoptosis, inflammation, and fibrosis, 
is seen in 10% to 20% of NAFLD patients and poses a high risk 
for further progression to cirrhosis and hepatocellular carcinoma. 
Unfortunately, at present, it is not possible to identify individuals 
who will progress from NAFLD to nonalcoholic steatohepatitis 
and hepatocellular carcinoma. Effective treatments are limited to 
weight loss, which is problematic because few interventions lead 
to meaningful sustained weight loss

ventions lead 
to meaningful sustained weight loss. Thus, understanding the 
molecular mechanisms underlying the progression from hepatic 
steatosis to frank steatohepatitis is of critical importance for clini­
cal assessments and for pharmacologic treatment. 
Obstructive Sleep Apnea
Overall effects of obesity on lung function are relatively benign. 
However, respiratory function can be significantly affected by 
obstructive sleep apnea that can be severe. Obstructive sleep 
apnea occurs when th

be severe. Obstructive sleep 
apnea occurs when there is either complete or partial upper air­
way obstruction due to mechanical pressure in the neck. Daytime 
sleepiness is common. Sleep apnea is also an independent risk fac­
tor for hypertension and may predispose to type 2 diabetes mel­
litus. Weight loss is effective in improving indices of apnea.122 
Musculoskeletal Disease
Because overall body weight contributes to mechanical pressure at 
the hip and knee, obesity is a risk factor for oste

he hip and knee, obesity is a risk factor for osteoarthritis. Over­
weight increases the risk of knee osteoarthritis by almost twofold, 
whereas frank obesity is associated with additional risk. Overweight 
and obesity are more significant factors in new-onset knee pain than 
a previous knee injury. The increasing prevalence of overweight and 
obesity is also associated with increasing total knee and hip arthro­
plasty.123 Hyperuricemia and gout are also associated with obesity.124 
The risk of 

are also associated with obesity.124 
The risk of gout also increases with body weight, and the relative risk 
in individuals with a BMI greater than 35 kg/m2 is almost threefold 
higher compared with normal-weight persons.125,126 
Cancer
Overweight and obesity increase the risk of certain cancers. Based 
on data from a prospective study in more than 900,000 adults 
in the United States, it was estimated that overweight and obe­
sity could account for 14% of all deaths from cancer in men and 
20

t for 14% of all deaths from cancer in men and 
20% of such deaths in women. Obesity is associated with higher 
rates of death due to cancers of the gastrointestinal tract (liver, 
pancreas, stomach, esophagus, colon and rectum, and gallbladder) 
and kidney, multiple myeloma, and non-Hodgkin lymphoma, as 
well as prostate cancer in men and uterine, cervical, ovarian, and 
postmenopausal breast cancer in women. The risks of breast and 
endometrial cancer fatality increase with both obesity and we

 cancer fatality increase with both obesity and weight 
gain after age 18 years. The risk of breast cancer increases with 
increasing BMI in postmenopausal women; in premenopausal 
women, increased BMI may actually protect against breast cancer. 
Genetic, hormonal, and metabolic factors associating adiposity to 
cancers remains incompletely understood. 
Diagnostic and Therapeutic Approach
Height and weight measurements are intrinsic to the physical 
examination; most electronic medical records a

al 
examination; most electronic medical records automatically calcu­
late BMI, which can also be calculated manually by dividing the 
weight in kilograms by the height in meters squared or by using 
the online calculator provided by the Centers for Disease Control 
and Prevention. Once the current BMI is known, an essential part 
1576
of the medical history is to determine an individual’s weight trend 
over time, often by obtaining we

l’s weight trend 
over time, often by obtaining weight at age 18 years. For normal-
weight and modestly overweight individuals (i.e., BMI <27 kg/m2 
without comorbidities for those with European heritage or 25 kg/
m2 for Asian heritage), counseling with no additional intervention 
other than monitoring weight over time is warranted.
Beyond weight, little additional evaluation is warranted for 
obesity per se. In those reporting early obesity, presenting during 
childhood or adolescence, who also

senting during 
childhood or adolescence, who also have a family history, genetic 
testing for a mutation in MC4R might be considered. This is the 
only commercially available test, and it may be helpful in a small 
number of individuals who test positive. However, at present, 
there are no specific therapies that would be more effective if the 
mutation were present, so pursuing the test is dependent on clini­
cal suspicion of physician and patient preference.
Management of overweight and obesi

ent preference.
Management of overweight and obesity can be difficult.127 A 
conservative clinical approach is provided in Table 40.3. First, an 
individual must acknowledge a potential problem. At the time of 
annual physical examination, weight should be noted, BMI calcu­
lated, and a brief overview of BMI and risk provided. In general, 
mildly overweight patients need to be advised of the advantages 
of modest weight loss and warned against additional weight gain. 
For individuals with a BMI 

ditional weight gain. 
For individuals with a BMI greater than 27 kg/m2 (25 kg/m2 
for Asians) or those with comorbidities, the risk of excess weight 
should be addressed and patients should be encouraged to be 
attentive to their weight, offered access to a nutritionist, provided 
with dietary advice, and given an approach to behavioral mod­
ification. Using a home scale for scheduled weighing time and 
recording the result alone may be ineffective for achieving weight 
loss, but it may help an

ive for achieving weight 
loss, but it may help an individual be attentive to risk and may 
prevent or slow down further weight gain.
Individuals with a BMI greater than 35 kg/m2 should be 
advised of risks, encouraged to obtain a consultation from a nutri­
tionist, and be counseled in behavioral modifications, and a repeat 
visit should be scheduled within 3 to 4 months of the initial visit. 
If the interval has passed without weight loss, pharmacotherapy 
should be considered. Although GLP1 re

acotherapy 
should be considered. Although GLP1 receptor agonists (see later 
discussion) are highly effective, few individuals are willing to con­
sider an injectable therapy at the outset; additionally, many insur­
ance companies insist on a trial of available oral therapies to start. 
There is no reliable way of predicting a response to the various 
medications available; the initial choice needs to be individually 
tailored on the basis of the likelihood of compliance and risk of 
side effec

e likelihood of compliance and risk of 
side effects. Metabolic surgery options should be discussed.
Once therapy is prescribed, it is critically important to evaluate 
weight loss within a short interval to determine effectiveness. An 
official weight, in a medical environment, should be recorded at 
8 weeks. If no weight loss is observed, weight should be checked 
at 12 to 16 weeks. Weight gain should signal cessation of therapy. 
If weight loss is absent, or less than 5% or 3.3 pounds (1.5 kg

s is absent, or less than 5% or 3.3 pounds (1.5 kg), 
therapeutic dose adjustments may be considered up to maximal 
approved levels or therapy should be discontinued between 12 and 
16 weeks. If weight loss exceeds 3.3 pounds (1.5 kg), individuals 
should be recalled for weight checks every 8 weeks. If weight loss 
is ongoing, the decision to continue medical therapy over the long 
term needs to be evaluated on an individualized basis that balances 
the risk of medication with the benefit of red

es 
the risk of medication with the benefit of reduced body weight. 
Weight regain is common when pharmacotherapy is stopped. 
Benefits of Intentional Weight Loss
Weight loss resulting from any therapeutic modality will lead to 
improvements in the complications of obesity and significantly 
lower the risk for diabetes and other comorbidities,128 although there 
may be unintended risk associated with pharmacologic and surgical 
approaches. Diabetes remission with weight loss can be of several 
y

es remission with weight loss can be of several 
years duration but appears to depend on residual beta-cell function 
and additional potential for beta cells to recover.129,130 However, it is 
interesting the lifestyle interventions promoting weight loss through 
decreased caloric intake and increased physical activity do not 
improve cardiovascular event rates in individuals with type 2 diabetes 
mellitus,131 perhaps due to increased use of lipid-lowering and blood 
pressure medications in pati

d-lowering and blood 
pressure medications in patients not engaged in the intervention.
Modalities that can be used to address obesity include lifestyle 
interventions such as diet and exercise, behavioral therapy, phar­
macotherapy, bariatric surgery, and some various combinations. 
Dietary Intervention
Caloric Restriction
Dietary intervention may involve decreasing food intake without 
making major changes in the composition of food consumed. A 
pound of fat contains 3500 calories, so an indiv

pound of fat contains 3500 calories, so an individual aiming to lose 
1 pound per week needs a net daily calorie deficit of 500 calories. 
A 55-year-old, moderately active, woman weighing 300 pounds 
(136 kg), will need approximately 2600 calories for weight main­
tenance. For her, a net 500 calorie deficit will be approximately a 
20% reduction in her daily food intake. If maintained, this would 
lead to about 50 pounds (22.5 kg) of weight loss in a year, which 
can seem discouragingly slow f

s in a year, which 
can seem discouragingly slow for someone aiming for ideal body 
weight. It is important to set realistic expectations to foster success 
and continued efforts.
To increase the rate of weight loss, many individuals consider 
restricting daily food intake by pursuing very low calorie diets using 
meal replacements. Very low calorie diets that limit to 800 calories 
per day can also be effective for weight loss, especially if coupled 
with either a commercial provider or physici

pled 
with either a commercial provider or physician or nutritionist sup­
port. Many commercial options providing 200 to 300 calories of 
meal substitutes replete with protein are available, such as Opti­
fast, HMR, and Medifast. These are available over the counter 
  Suggested Weight-Loss Treatment 
Options Based on BMI and Risk Factors
BMI 
(kg/m2)
Advice, 
Nutrition, 
Exercise
Pharmacotherapya
Surgeryb
25.0–26.9
Yes
No
No
27.0–29.9
Yes
Consider if there are risk 
factors and weight 
gain tr

der if there are risk 
gain trajectory
No
30.0–34.9
Yes
Strongly consider with 
risk factors and weight 
No
35.0–39.9
Yes
Yes
If drugs fail, with 
comorbidity
≥40
Yes
Yes
If drugs fail
Ethnic adjustments to BMI ranges should be considered.
aPharmacotherapy should be considered in patients who are unable to achieve adequate 
weight loss with available conventional therapy and who do not have any absolute contrain­
dications for drug therapy.
bBariatric surgery 

n­
bBariatric surgery should be considered in patients who are unable to lose weight with avail­
able conventional therapy and who do not have any absolute contraindications for surgery.
   
TABLE 40.3
1577
or may be prescribed and overseen by a physician, but they may 
be considered relatively expensive by the patient. Diets that limit 
calories to 800 calories daily are typically associated with 2 pounds 
of weight loss week

ally associated with 2 pounds 
of weight loss weekly. Some approaches involve a period of sev­
eral weeks of meal replacements only, limiting food intake to 800 
calories (3–4 meal supplements daily), whereas other approaches 
include a single meal of conventional food combined with two 
daily supplements. When initiated solely as meal replacements, 
very low calorie diets typically progress to modified fasts wherein 
a single relatively low calorie meal is consumed. A meta-analysis 
comparing c

rie meal is consumed. A meta-analysis 
comparing commercial programs with simple nutritional counsel­
ing and calorie restriction reports that commercial programs fare 
better with more weight loss between 6 and 12 months. In gen­
eral, weight loss is greater with these programs than with simple 
counseling, but long-term weight loss is only sustained in a rela­
tively small number of individuals.132
Additional recent approaches involve limited fasting to 2 to 
3 days each week. A current popula

asting to 2 to 
3 days each week. A current popular approach is the “5/2 diet,” 
which involves limiting food intake to less than 400 calories 2 days 
a week and eating “normally” the other 5 days. Other approaches 
involve eating only during a limited number of hours daily and 
maintaining at least 16 hours per day without any food intake. 
Macronutrient Composition
Significant attention has been given to macronutrient composi­
tion of diets in attempts to determine what proportion of nutri­
en

attempts to determine what proportion of nutri­
ents might lead to greater and more sustainable weight loss. There 
is significant debate between those that favor low-fat diets in com­
parison to low-carbohydrate, Atkins-style diets. Most controlled 
studies show that weight loss is similar regardless of diet composi­
tion, although very low carbohydrate diets have a small early advan­
tage.127,133 For individuals who self-select Atkins-style diets and 
who also utilize a support system, weight 

ts and 
who also utilize a support system, weight loss can be dramatic and 
substantial improvements in glucose control have been observed.134
As there is little support for any specific type of diet, it is best 
to inform individuals regarding different options, recommend 
based on their preference, and follow closely to confirm efficacy 
and safety.135 A potential exception is recommendation of Atkins-
style diets in individuals with type 2 diabetes mellitus. Although 
still somewhat controver

betes mellitus. Although 
still somewhat controversial, increasing evidence suggests that 
significant limitation of carbohydrates improves glucose control, 
decreases dependence on medications, and facilitates weight loss.134 
In the past half century, sedentary behavior has increased with 
physical labor progressively mechanized, both commercially, such 
as in farming operations, and in households where machines such as 
vacuum cleaners and dishwashers lead to less daily ener

m cleaners and dishwashers lead to less daily energy expended. 
Reduced energy expended on these activities is estimated to rep­
resent a 140 kcal/day decline and may contribute to population 
weight gain.136 However, increasing physical activity through dis­
cretionary exercise is not particularly effective for weight loss. In 
a study of exercise alone, postmenopausal women were advised 
to exercise at moderate intensity for 45 minutes 5 days per week. 
Weight loss was minimal, averaging 2.8 p

er week. 
Weight loss was minimal, averaging 2.8 pounds (1.3 kg) over the 
course of a year.137 When coupled with diet, exercise has a modest 
effect to improve weight loss in men but only prevented weight gain 
in women.138 The failure of exercise to have a significant impact on 
weight loss may reflect compensatory overeating. 
Behavior Modification
Behavior modification is an unspecific term that covers approaches 
to weight loss and or limiting weight gain that include exercise, 
diet, food 

ng weight gain that include exercise, 
diet, food shopping habits, and food pantry stocking.139 Behav­
ioral modification strategies and guidelines should be sensitively 
discussed with obese and overweight individuals, especially those 
who are gaining weight over time. Raising awareness may slow 
down weight gain; prevent additional weight gain; and, under 
some circumstances, encourage weight loss. Recognizing and 
learning to avoid behaviors such as stressful eating and setting real­
istic a

such as stressful eating and setting real­
istic and achievable goals are both important. Successful weight 
loss has been reported with combinations of exercise and intensive 
lifestyle modification that includes frequent monitoring and sup­
port. In the Look AHEAD study of individuals with type 2 diabe­
tes mellitus and obesity, those randomized to the intensive lifestyle 
modification limb lost up to 17.6 pounds (8 kg) in the first year. 
Although, in general, individuals regained weight over

ough, in general, individuals regained weight over time, 
weight loss of about 9 pounds (4 kg) persisted out to 4 years.140 
Pharmacotherapy
Most medications currently available to treat obesity are associated 
with relatively modest weight loss (Table 40.4). Furthermore, non­
responder rates can be high so that for any agent, only a minority 
of individuals who initiate therapy in a standard outpatient setting 
will lose weight. It is uncertain if patients who fail one therapy are 
more likely 

if patients who fail one therapy are 
more likely to succeed with other class agents.
To date, the most tolerated and effective class of medications 
for weight loss are GLP1 receptor agonists. These were developed 
for the treatment of type 2 diabetes mellitus and fortuitously were 
found to have meaningful effects in reducing appetite and body 
weight. Effects on appetite with continuous infusion were reported 
early, and when the first agonist, exenetide, came to market for 
glucose control i

, exenetide, came to market for 
glucose control in type 2 diabetes mellitus, modest weight loss was 
noted in many patients. Subsequent studies found that weight loss 
effects could be extended to obese individuals without diabetes 
and, further, that it was possible to stratify subjects as responders or 
nonresponders based on weight loss after 8 weeks of treatment.141 
The GLP1-R agonist liraglutide, also used in type 2 diabetes mel­
litus (marketed as Victoza), and in a higher-dose formulati

rketed as Victoza), and in a higher-dose formulation 
(Saxenda) has now been approved for obesity management in the 
United States, Europe, and Japan. Liraglutide provides benefit in 
terms of reduced rates of nonfatal myocardial infarction and stroke 
and cardiovascular death in patients with diabetes and established 
or at high risk for cardiovascular disease.142 Semaglutide has also 
been recently approved for management of type 2 diabetes mel­
litus with once-weekly dosing. Semaglutide may a

­
litus with once-weekly dosing. Semaglutide may also reduce the 
rate of nonfatal myocardial infarction or stroke or cardiovascular 
death in patients with type 2 diabetes mellitus at high cardiovascu­
lar risk.143 Whether these agents will provide similar cardiovascu­
lar benefits in reducing cardiovascular risk in obese individuals is 
unclear. When administered at lower daily doses, it leads to weight 
loss in obese nondiabetic individuals. In a direct comparison of 
semaglutide and liraglut

n a direct comparison of 
semaglutide and liraglutide, weight loss at 0.3 mg of semaglutide 
daily resulted in mean weight loss of 12.3% compared with mean 
weight loss of 8.3% in those randomized to 3.0 mg of liraglu­
tide.144 Currently, most GLP1 receptor agonists are administered 
by subcutaneous injection. Headache and gastrointestinal events 
such as nausea, vomiting, diarrhea, or constipation represent the 
major side effects that limit use of GLP1 receptor agonists.
Phentermine is a sympa

 of GLP1 receptor agonists.
Phentermine is a sympathomimetic that was approved for obe­
sity treatment in 1959. It stimulates the release of norepinephrine, 
and to a lesser extent serotonin (5-hydroxytryptamine [5HT]) and 
dopamine. Weight loss with phentermine is approximately 5% at 
8 to 12 weeks, and typically is administered at 15 mg to 37.5 
mg orally three times daily with meals as a short-term adjunct (a 
few weeks) to behavioral weight loss approaches. A lower 8-mg 
1578

oss approaches. A lower 8-mg 
1578
dose for longer-term use is available, but durability has not been 
reported. The clinical effect of phentermine is to diminish appe­
tite, and it appears to work better in individuals who start with 
greater hunger and less cognitive restraint on their eating behav­
ior.145 Side effects of phentermine can include pulmonary hyper­
tension, valvular heart disease, palpitations, increased heart rate 
or

rt disease, palpitations, increased heart rate 
or blood pressure, insomnia, restlessness, dry mouth, diarrhea, 
constipation, and changes in sexual drive.
Phentermine has been combined with topiramate into an 
extended-release (ER) formulation marketed as Qsymia, which is 
available in four doses ranging from combinations of 3.75/23 to 
15/92 of phentermine to topiramate. Phentermine-topiramate ER 
has been evaluated in two large randomized clinical trials. Weight 
loss at 1 year demonstrated a

ical trials. Weight 
loss at 1 year demonstrated a dose-response relationship and was 
similar in both studies. In an intention-to-treat analysis after 1 year 
of therapy, the placebo-subtracted weight loss was approximately 9% 
for the top dose and approximately 6.5% for the recommended dose. 
Common adverse effects of phentermine-topiramate ER include dry 
mouth, dizziness, dysgeusia, constipation, insomnia, and paresthesia. 
Cognitive impairment (attention or memory deficits) is also reported

nt (attention or memory deficits) is also reported 
and, when it occurs, leads to discontinuation of the drug.
Orlistat inhibits pancreatic lipase and thereby reduces enteric 
absorption of fatty acids. In turn, this leads to some degree of fat 
malabsorption. Excretion of about 30% of ingested triglycerides, 
which is near the maximum plateau value, occurs at a dose of 360 
mg/day (120 mg three times daily with meals). A meta-analysis of 
multiple weight loss studies indicated that about 50% of

le weight loss studies indicated that about 50% of individu­
als taking orlistat will lose weight,146 although weight loss at 1 year 
is small, with an average loss of 11 pounds (5 kg).147 Use of Orli­
stat is limited by the common side effect of fatty diarrhea, which 
leads to reduced compliance. Systemic side effects directly related 
to the drug are uncommon because orlistat is not absorbed.
Lorcaserin (marketed in the United States as Belviq) is a selec­
tive 5HT2c receptor agonist that decr

 is a selec­
tive 5HT2c receptor agonist that decreases food intake leading to 
modest weight loss, which in a meta-analysis of several trials was 
just over 11 pounds (5 kg). In a high-risk population of overweight 
or obese patients, lorcaserin promoted sustained weight loss with­
out increasing rates of major cardiovascular events.148 Lorcaserin 
has greater binding affinity and activation of the 5HT2c receptor 
subtype over the 2a or 2b receptor subtypes, and given the tissue 
distribution o

tor subtypes, and given the tissue 
distribution of the receptor subtypes, it would be expected to have 
a reduced cardiac valve risk profile compared with earlier seroto­
nergic agents, such as fenfluramine. Valvulopathy has not been 
reported. The most frequent adverse effects of lorcaserin in these 
studies were headache, dry mouth, dizziness, and nausea.
The naltrexone slow-release (SR)/bupropion SR combination, 
marketed as Contrave, is a μ-opioid receptor antagonist combined 
with a norepi

opioid receptor antagonist combined 
with a norepinephrine and dopamine receptor inhibitor. Bupro­
pion has neuronal effects associated with reduced energy intake 
and increased energy expenditure, and naltrexone potentiates this 
effect such that the effects of the combination are greater than 
those with buproprion alone. The overall magnitude of weight loss 
is on the order of 5%, but in trials, more patients lose 5% of their 
body weight with treatment compared with placebo. There are 
impro

 treatment compared with placebo. There are 
improvements in lipid and glucose profiles and patient reported 
outcomes but small increases in heart rate and blood pressure.152a
Metformin is typically used to treat diabetes and is associ­
ated with weight loss. Data from most studies on the effect of 
metformin on body weight indicate that metformin does not 
cause weight gain, as single-agent therapy and concomitant treat­
ment with other medications may limit the expected weight 
gain.149,150 M

ions may limit the expected weight 
gain.149,150 Metformin has also been reported to lead to an aver­
age net 11-pound (5-kg) weight loss after 6 months of treatment 
over control obese individuals without diabetes.151 Although not 
approved for weight loss in nondiabetic individuals, metformin 
has been prescribed off label for weight loss, and because there are 
few side effects, therapy may be continued indefinitely if effective.
A summary of medications approved for weight loss, along 
with 

medications approved for weight loss, along 
with maximal placebo-adjusted maximum weight loss, is provided 
in Table 40.4. 
Endoscopic Gastric Therapies
Endoscopic bariatric and metabolic therapies are more recently 
available for the treatment of obesity.152 These include devices that 
are placed via flexible endoscopy and procedures that utilize flex­
ible endoscopy instruments for weight loss indications. Currently, 
three intragastric balloon devices have been approved for use in 
the Unite

n devices have been approved for use in 
the United States, including the ReShape Integrated Dual Bal­
loon System, the ORBERA Intragastric Balloon System, and the 
Obalon Balloon System; others are under development. A small 
number of deaths have occurred in patients using these devices. 
Aspiration therapy using the AspireAssist device permits food to 
be removed from the stomach after a meal. Electrical stimulation 
systems place an electrical stimulator in the abdomen to block 
nerve activi

l stimulator in the abdomen to block 
nerve activity between the brain and stomach, such as the Maestro 
System. Procedures include endoscopic sleeve gastrectomy, as well 
as POSE (Primary Obesity Surgery, Endoluminal), which is per­
formed via endoscopy to reduce the size of the stomach and dimin­
ish hunger cravings. Currently, there are limited data to compare 
safety effectiveness and durability of these devices and procedures. 
  Drugs Commonly Prescribed for Weight 
Loss146
Year 
Approved

only Prescribed for Weight 
Loss146
Year 
Approved
Generic Name
Trade Name
Placebo-
Corrected 
Anticipated 
Weight Loss 
(kg)
1959
Phentermine
Ionamin, Adipex-P, 
Fastin, Oby-Trim 
(approved only 
for short-term 
weight loss)
Approved for 
short-term 
use only
1999
Orlistat
Xenical, Ally (over 
the counter)
2.63
2010
Victoza (approved 
for type 2 dia­
betes mellitus)
0 to 3.7a
2012
Phentermine-­
topiramate 
extended release
Qsymia
8.80
2013
Lorcaserin
Belviq
3.25
2014
Sax

0
2013
Belviq
3.25
2014
Saxenda (approved 
for obesity)
5.24
2014
Bupropion-­
naltrexone
Contrave
4.95
TABLE 40.4
aDepending on comparator.
Note: Intensity of lifestyle interventions and maximum weight loss differs in studies. Repre­
sented values mean weight loss in excess of placebo.
1579
Attempts to treat obesity through surgical procedures were first 
attempted more than 60 years ago, with almost all initial proce­
dures involving 

o, with almost all initial proce­
dures involving shunts between the jejunum and the colon. Jejuno­
ileal bypass surgery was effective for weight loss but was associated 
with significant complications, including liver disease,153 liver fail­
ure resulting in death,154 and protein malnutrition.155 Thus, this 
sur­gical approach was largely abandoned. A decade later, a report 
on the success of Roux-en-Y-gastrojejunostomy in more than 600 
morbidly obese patients revealed that this surgical appro

y obese patients revealed that this surgical approach was 
effective with very little morbidity and mortality.156
Although many consider bariatric surgery as a draconian 
approach to obesity, weight loss is substantial and sustained, and 
associated with remission or improvement of type 2 diabetes 
mellitus, dyslipidemia, hypertension, and other weight-related 
comorbidities157 that may occur early following surgery and per­
sist for years. Not surprisingly, following the procedure, decreased 
l

urprisingly, following the procedure, decreased 
levels of inflammatory markers have also been reported.158 Surgi­
cal weight loss may reduce mortality by as much as 30% to 40% 
in the severely obese, although these studies were not random­
ized.159,160 There is some controversy regarding whether the sever­
ity of obesity is the optimal characteristic to use when considering 
a patient for metabolic surgery; however, the accepted indications 
for surgery are a BMI greater than 40 kg/m2 or a BMI 

 surgery are a BMI greater than 40 kg/m2 or a BMI between 35 
and 40 kg/m2 with an associated comorbidity.161
Four surgical procedures are offered for obesity therapy, 
including adjustable gastric banding, sleeve gastrectomy, Roux-
en-Y gastric bypass, and biliopancreatic diversion (Fig. 40.3). 
Although biliopancreatic diversion is effective, few procedures 
are currently performed because of the higher rates of complica­
tion. Adjustable gastric banding has also become less common, 
with Roux

ic banding has also become less common, 
with Roux-en-Y and gastric sleeve surgery now also routinely 
performed laparoscopically, providing more rapid recovery. In 
addition, gastric banding has significantly lower expected weight 
loss.
Roux-en-Y gastric bypass and sleeve gastrectomy are the two 
favored procedures. Similar weight loss is achieved, of almost 11 
pounds (5 kg) per month for the first 4 months, with weight nadir 
between 6 and 24 months and effects that can persist over years. 

 months and effects that can persist over years. 
Perioperative mortality with bariatric surgery is low and reported 
as 0.04% to 0.3%, similar to that of a cholecystectomy. Poten­
tially fatal risks include pulmonary emboli, sepsis, and bleeding. 
Because of the underlying increase in cardiovascular disease, the 
likelihood of cardiovascular risk is higher in the perioperative 
period. Of concern would be a leak at the anastomosis site, which 
can lead to sepsis and needs to be diagnosed and tr

an lead to sepsis and needs to be diagnosed and treated aggres­
sively. Overall, the incidence of adverse events in the initial 28-day 
period was less than 5%.
Hypoglycemia can occur as a long-term complication of 
weight loss surgery, which may occur to some degree in up to 
14% of individuals without a prior history. Severe hypoglyce­
mia is only reported in a small number of patients162; it appears 
A
B
C
D
E
• Fig. 40.3  Schematic diagrams of Roux-en-Y gastric bypass (A), laparoscopic adjus

f Roux-en-Y gastric bypass (A), laparoscopic adjustable gastric band­
ing (B), sleeve gastrectomy (C), biliopancreatic diversion (D), and biliopancreatic diversion with duodenal 
switch (E). (Modified from Bradley D, Magkos F, Klein S. Effects of bariatric surgery on glucose homeosta­
sis and type 2 diabetes. Gastroenterology. 2012;143[4]:897–912.)
1580
to be more common after the Roux-en-Y procedure163 and 
may be managed by continuou

en-Y procedure163 and 
may be managed by continuous blood glucose monitoring and 
carbohydrate support,164 although, on occasion, somatostatin 
analogues or diazoxide are necessary to reduce insulin secretion 
following meals.
Patients having Roux-en-Y gastric bypass should be followed 
for development of anemia and deficiencies of fat-soluble vita­
mins, because the absorption of iron and fat-soluble vitamins can 
be impaired, leading to low levels even in the setting of standard 
replacement t

els even in the setting of standard 
replacement therapy with oral multivitamins. Iron infusion ther­
apy may be needed in some patients. Bone health should be moni­
tored. Bariatric surgery may also exacerbate depression and may 
increase the likelihood of this comorbidity after surgery. Rates of 
depression are high in individuals with morbid obesity, which is 
also associated with increased suicide risk.165,166 A psychological 
evaluation for depression and emotional postsurgical support is a

depression and emotional postsurgical support is an 
essential part of the care of individuals undergoing surgery. 
Summary
Obesity results from complex interactions of individuals’ risk and 
environmental influences. The first-order advice regarding diet 
and exercise is rarely successful. In the past decade, there have 
been some significant improvements in medical therapy, as GLP1 
agonists have demonstrated effectiveness in at least 30% of indi­
viduals. Bariatric surgery has become safer an

di­
viduals. Bariatric surgery has become safer and more acceptable. 
However, the demand for therapies persists.
1580.e1
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	136.	 Church TS, Thomas DM, Tudor-Locke C, et  al. Trends over 5

S, Thomas DM, Tudor-Locke C, et  al. Trends over 5 
decades in U.S. occupation-related physical activity and their asso­
ciations with obesity. PLoS One. 2011;6(5):e19657.
	137.	 Irwin ML, Yasui Y, Ulrich CM, et al. Effect of exercise on total and 
intra-abdominal body fat in postmenopausal women: a random­
ized controlled trial. J Am Med Assoc. 2003;289(3):323–330.
	138.	 Donnelly JE, Kirk EP, Jacobsen DJ, et al. Effects of 16 mo of veri­
fied, supervised aerobic exercise on macronutrient int

mes in patients with type 2 diabetes. N Engl J Med. 
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	144.	 O’Neil PM, Birkenfeld AL, McGowan B, et  al. Efficacy and 
	145.	 Thomas EA, McNair B, Bechtell JL, et al. Greater hunger and less 
restraint predict weight loss success with phentermine t

s in Empowir (Enhance 
the Metabolic Profile of Women with Insulin Resistance): a dou­
ble blind, placebo-controlled, randomized trial of normoglycemic 
women with midlife weight gain. Endocr Pract. 2016;22(5):575–
586.
	151.	 Seifarth C, Schehler B, Schneider HJ. Effectiveness of metformin 
on weight loss in non-diabetic individuals with obesity. Exp Clin 
Endocrinol Diabetes. 2013;121(1):27–31.
	152.	 Sullivan S. Endoscopic medical devices for primary obesity treatment 
in patients with diab

r primary obesity treatment 
in patients with diabetes. Diabetes Spectr. 2017;30(4):258–264.
	152a.	Halpern B, Mancini MC. Safety assessment of combination thera­
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extended release and phentermine-topiramate extended release. 
Expert Opin Drug Saf. 2017;16(1):27–39.
	153.	 Peters RL, Gay T, Reynolds TB. Post-jejunoileal-bypass hepatic 
disease. Its similarity to alcoholic hepatic disease. Am J Clin Pathol. 
1975;63(3):318–331.
	15

effective therapy 
for adult-onset diabetes mellitus. Ann Surg. 1995;222(3):339–350; 
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ric surgery in the United Kingdom: a cohort study of weight 
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	158.	 Zhang C, Zhang J, Liu Z, Zhou Z. More than an anti-diabetic 
bariatric surgery, metabolic surgery alleviates systemic and local 
inflammation in obesity. Obes Surg. 2018;28(1

at Dis. 2017;13(5):888–896.
	165.	 Bjerkeset O, Romundstad P, Evans J, Gunnell D. Association of 
adult body mass index and height with anxiety, depression, and sui­
cide in the general population: the HUNT study. Am J Epidemiol. 
2008;167(2):193–202.
	166.	 Pines AR, Sacchet MD, Kullar M, et al. Multi-unit relations among 
neural, self-report, and behavioral correlates of emotion regulation 
in comorbid depression and obesity. Sci Rep. 2018;8(1):14032.
1581
41
Disorders of Lipid Metabolism
CL

1):14032.
1581
41
CLAY F. SEMENKOVICH AND IRA J. GOLDBERG
Lipid Biochemistry and Metabolism
There are multiple adverse health effects of abnormal serum or tis­
sue lipids. Endocrine disorders impact lipids, making mechanisms 
underlying primary and secondary disorders of lipid metabolism 
relevant to both clinicians and basic scientists. Primary genetic 
disorders of lipid metabolism, such as familial hypercholesterol­
emia (FH), are relatively common, occurring in 

l­
emia (FH), are relatively common, occurring in about 1 in 300 to 
500 people. The low-density lipoprotein (LDL) receptor pathway 
is functionally impaired in FH, with higher LDL contributing 
to a genetic predisposition to atherosclerotic heart disease. This 
pathway underlies the mechanism of action of statin drugs, which 
decrease the risk of vascular events and prolong life,1 and propro­
tein convertase subtilisin/kexin type 9 (PCSK9) inhibitors, which 
lower lipid levels and reduce cardio

itors, which 
lower lipid levels and reduce cardiovascular events even in the set­
ting of statin treatment.2 The quintessential secondary disorder of 
lipid metabolism is that seen in diabetes, a disease so frequently 
characterized by abnormalities of fat that lipids have been impli­
cated in its pathogenesis.
Lipids are required for life, constituting the physical bilayer 
allowing formation of cell membranes, which compartmentalize 
specialized organelles inside cells and regulate transport 

ed organelles inside cells and regulate transport between 
extracellular and intracellular environments. Lipids circulate 
in blood, with fatty acids and triglycerides providing an energy 
source to tissues such as heart and skeletal muscle, and non-
nutritive sterols providing substrates for hormone production by 
gonads and adrenals. Specialized functions include development 
of surfactant in lung to maintain patency of alveoli, formation 
of bile to facilitate excretion of a variety of metabo

ile to facilitate excretion of a variety of metabolites, and con­
stitution of myelin throughout the nervous system to ensure the 
fidelity of nerve transmission. Lipids are also signaling molecules, 
serving as targets of lipid kinases that perpetuate signaling cas­
cades, substrates for cyclooxygenases and related enzymes that 
generate prostaglandins, and ligands for nuclear receptors such 
as the peroxisome proliferator-activated receptors (PPARs). The 
broad spectrum of lipid functions resu

PARs). The 
broad spectrum of lipid functions results in part from their bio­
physical characteristics.
Simple and Complex Lipid Structure
The functional versatility of lipids is attributable to their hydro­
phobic structure. Due to the presence of fairly long carbon chains, 
lipids tend to associate with each other and have limited or no solu­
bility in water. Fatty acids and cholesterol are simple lipids, whereas 
triglycerides and phospholipids are complex lipids (Fig. 41.1).

s are complex lipids (Fig. 41.1).
Lipid Biochemistry and Metabolism, 1581
Integrative Physiology of Lipid Metabolism, 1595
Overview of Hyperlipidemia, Dyslipidemia, 
and Atherogenesis, 1597
Hypertriglyceridemia, 1599
Hypercholesterolemia Without Hypertriglyceridemia, 1603
Elevated Triglycerides and Cholesterol, 1605
Hypocholesterolemia, 1606
Treatment of Lipid Disorders, 1607
KEY POINTS
	•	
Abnormalities of lipid metabolism cause heart disease, pancre­
atitis, vitamin deficienci

m
Fatty Acids
The chemical structure of fatty acids is determined by the number 
of carbon atoms and number of double bonds (see Fig. 41.1A). 
For example, stearic acid has 18 carbon atoms and is saturated, 
meaning that it has no double bonds; it is designated using the 
abbreviation C18:0. The 18-carbon monounsaturated fatty acid 
oleic acid (C18:1) has one double bond, and the polyunsaturated 
fatty acid linoleic acid (C18:2) has two double bonds.
Linoleic acid and arachidonic acid (C20:4) ar

nds.
Linoleic acid and arachidonic acid (C20:4) are omega-6 fatty 
acids, meaning that a double bond is present at the sixth carbon 
from the end of the molecule farthest from the carboxy (COOH)-
terminus. Fish oils, which lower lipids, are omega-3 fatty acids, 
with a double bond present at the third carbon from the end 
opposite the COOH-terminus. Saturated fatty acids and some 
unsaturated fatty acids, such as oleic acid, are nonessential (i.e., 
they can be synthesized). Most omega-6 and ome

., 
they can be synthesized). Most omega-6 and omega-3 fatty acids 
are essential; they cannot be synthesized and are usually required 
for health, especially during development and in times of physi­
ologic stress. Table 41.1 shows major food sources of fatty acids. 
The structure of tristearin, a triglyceride with three molecules of 
stearic acid connected to a glycerol molecule by means of ester 
linkages, is shown in Fig. 41.1B. Other triglycerides have a simi­
lar structure wi

 Other triglycerides have a simi­
lar structure with alternative fatty acids esterified to the glycerol 
backbone.
Most adipose tissue mass is composed of triglycerides; tri­
glycerides that circulate in the blood primarily reflect the fatty 
acid composition of adipose tissue triglycerides, and both sources 
reflect dietary fatty acid composition. Butter, common in Western 
CH3
CH3
CH3
CH2
CH2
CH2
H
C
H
C
OH
3
A
D
C
B
H2C – O – C – (CH2)16 – CH3
O
O
O
O
O
O

2C – O – C – (CH2)16 – CH3
O
O
O
O
O
O
HC – O – C – (CH2)16 – CH3
Glycerol
Tristearin
Fatty acid
H2C – O – C – Fatty acid
HC – O – C – Fatty acid
H2C – O – P – O – CH2 – CH2 – N+ – CH3
CH3
CH3
Choline
Phosphatidylcholine
Stearic acid:
Oleic acid:
Linoleic acid:
CH3 – (CH2)16 – COOH
CH3 – (CH2)7 – CH = CH – (CH2)7 – COOH
CH3 – (CH2)4 – CH = CH – CH2 – CH = CH – (CH2)7 – COOH
O–
• Fig. 41.1  Structures of common lipids, exemplified by the stear

uctures of common lipids, exemplified by the stearic, oleic, and linoleic fatty acids (A); the 
triglyceride tristearin (B); the phospholipid phosphatidylcholine (C); and cholesterol (D).
  Major Fatty Acids
Chemical 
Designation
Common Name
Common Food
Saturated Fatty Acids (No Double Bonds)
C12:0
Lauric
Coconut oil
C14:0
Myristic
Coconut oil, butter fat
C16:0
Palmitic
Butter, cheese, meat
C18:0
Stearic
Beef, chocolate
Monounsaturated Fatty Acids (One Double Bond)
C18:1
Oleic
Olive and canola 

ds (One Double Bond)
C18:1
Oleic
Olive and canola oils
Polyunsaturated Fatty Acids (Two or More Double Bonds)
Omega-6 Fatty Acids
C18:2
Linoleic
Sunflower, corn, soy­
bean, and safflower 
oils
C20:4
Arachidonic
Chicken, eggs
Omega-3 Fatty Acids
C18:3
α-Linolenic
Canola, flaxseed, and 
soybean oils
C20:5
Eicosapentaenoic 
acid (EPA)
Salmon, cod, mackerel, 
tuna
C22:6
Docosahexaenoic 
acid (DHA)
tuna
TABLE 41.1
1583
Chapter 41  Disorders of Lipid Metabolism
diets, consists 

41  Disorders of Lipid Metabolism
diets, consists of similar amounts of palmitate and oleate with a 
lesser amount of stearate; adipose tissue and circulating triglycer­
ides in persons eating Western diets contain mostly palmitate and 
oleate. Olive oil, found in Mediterranean diets, is predominantly 
oleate with much less palmitate, so fat and circulating triglycerides 
in people eating a Mediterranean diet are enriched in oleic acid. 
Extremely high levels of triglycerides in the blood predis

y high levels of triglycerides in the blood predispose to 
pancreatitis. 
The chemical structure for a generic phosphatidylcholine, a type 
of phospholipid, is shown in Fig. 41.1C. As with triglycerides, 
phospholipids have a glycerol backbone to which fatty acids are 
esterified at the first two alcohols. The characteristics of these fatty 
acids are important for determining cell membrane shape and 
function.3,4 The third alcohol is esterified to a phosphate moi­
ety linked to an

is esterified to a phosphate moi­
ety linked to another molecule, such as choline, ethanolamine, 
or serine.
The presence of long-chain fatty acids comprising hydropho­
bic regions and the charged species at the end of the molecule 
make phospholipids ideal for generating cell membranes and 
lipoprotein surface components: the bilayer is oriented so that 
the hydrophobic regions point toward each other, and the hydro­
philic regions interact with the aqueous environment. Phospho­
lipids are dist

 the aqueous environment. Phospho­
lipids are distributed asymmetrically in cell membranes, with 
choline-containing lipids directed toward the outer surface and 
amine-containing lipids directed toward the cytoplasmic surface. 
Appearance of the aminophospholipid phosphatidylserine on the 
cell surface initiates blood clotting and marks apoptotic cells for 
phagocytosis. Several enzymes are involved in the dynamic process 
of remodeling cell membrane phospholipids, and some appear to 
be involv

brane phospholipids, and some appear to 
be involved in metabolic disease, such as 1-acylglycerol-3-phos­
phate-O-acyltransferase 5 (AGPAT5), a membrane protein that 
converts lysophosphatidic acid to phosphatidic acid as the second 
step in de novo phospholipid biosynthesis, and which mediates 
insulin resistance in mice.5 
The structure of cholesterol is shown in Fig. 41.1D. Cholesterol in 
the plasma membrane is critical for maintaining membrane fluid­
ity, probably by disrupting 

ining membrane fluid­
ity, probably by disrupting interactions between phosphatidylcho­
line and other molecules. It is also required for assembly of lipid 
rafts, which are ordered plasma membrane domains that mediate 
signal transduction.6 The concentration of cholesterol is enriched 
in the plasma membrane, with much lower levels detected in the 
membranes of most intracellular organelles. Cholesterol is nec­
essary for the synthesis of estrogen, progestins, androgens, aldo­
sterone, vitamin 

en, progestins, androgens, aldo­
sterone, vitamin D, glucocorticoids, and bile acids. Cholesterol 
deficiency is associated with severe developmental defects, as man­
ifested in the rare Smith-Lemli-Opitz syndrome, which is likely 
caused by disruption of the Hedgehog signal transduction path­
way.7 Cholesterol excess is associated with gallstones and vascular 
disease. 
Fatty Acid Metabolism
Fatty Acid Biosynthesis
In humans eating a typical Western diet, the overall contribution 
of de novo li

tern diet, the overall contribution 
of de novo lipogenesis to lipid metabolism is small because the 
ingestion of exogenous fat is sufficient to suppress the energy-
requiring process of synthesizing fats from carbohydrates. 
However, high-carbohydrate diets, especially those containing 
fructose,8 substantially increase lipogenesis in liver and adipose 
tissue of humans.
Fatty acid biosynthesis is carried out in most tissues at least 
minimally regardless of nutritional status. However, the li

 regardless of nutritional status. However, the liver is 
by far the major site of de novo fatty acid synthesis. Several key 
steps in fatty acid biosynthesis, presented in Fig. 41.2, also have 
major effects on systemic metabolism. Citrate derived from the 
tricarboxylic acid (TCA) cycle is converted to acetyl coenzyme 
A (acetyl CoA) in the cytoplasm by the action of adenosine tri­
phosphate (ATP) citrate lyase. Acetyl CoA is then converted to 
malonyl CoA by acetyl CoA carboxylase (ACC), whic

malonyl CoA by acetyl CoA carboxylase (ACC), which exists in 
two isoforms: ACC1 (encoded by the gene ACACA) is cytosolic 
and important in liver and fat for de novo lipogenesis, and ACC2 
(ACACB) is associated with mitochondria, also plays a role in liver 
metabolism, and is expressed at highest levels in muscle and heart. 
Antisense targeting of ACC isoforms has been shown to improve 
lipid metabolism and insulin sensitivity.9
Malonyl CoA inhibits carnitine palmitoyltransferase 1 (CPT1), 
whi

bits carnitine palmitoyltransferase 1 (CPT1), 
which transports fatty acids into mitochondria, and thereby malo­
nyl CoA prevents catabolism of fats under physiologic conditions 
in which energy is being stored as fat through fatty acid biosyn­
thesis. Malonyl CoA also serves as substrate for fatty acid synthase, 
which sequentially connects two carbon fragments to generate 
saturated fatty acids such as palmitate. Inhibition of fatty acid 
synthase in the hypothalamus suppresses appetite by dec

ase in the hypothalamus suppresses appetite by decreasing 
orexigenic peptides and increasing anorexigenic peptides in the 
arcuate nucleus, inducing weight loss and improving insulin sensi­
tivity.10 Pharmacologic inhibition of fatty acid synthase improves 
glucose metabolism in mice and primates,11 and the mechanism 
may involve suppression of inflammatory pathways in macro­
phages.12 Palmitate is converted to stearate through the action of a 
long-chain fatty acid elongase, which, when inacti

long-chain fatty acid elongase, which, when inactivated, promotes 
obesity but prevents insulin resistance.13 Stearate is subsequently 
converted to oleate by stearoyl-CoA desaturase 1, which, when 
inactivated, increases fatty acid oxidation and protects against 
diet-induced obesity and insulin resistance.14 
Fatty Acid Oxidation
Metabolism of fatty acids provides more energy per gram than 
metabolism of carbohydrates or proteins. Fatty acids undergo the 
process of β-oxidation in mitochondria

ndergo the 
process of β-oxidation in mitochondria (see Fig. 41.2). They are 
transported or diffuse across the plasma membrane, converted to 
acyl-CoA species by acyl-CoA synthase, then translocated to the 
mitochondrial matrix by CPT1 and CPT2. β-oxidation removes 
two carbon fragments through sequential actions of acyl-CoA 
dehydrogenases (e.g., medium-chain acyl-CoA dehydrogenase and 
very long chain acyl-CoA dehydrogenase), enoyl-CoA hydratase, 
hydroxy-CoA dehydrogenase, and thiolase. This

se, 
hydroxy-CoA dehydrogenase, and thiolase. This process generates 
reduced nicotinamide adenine dinucleotide and reduced flavin 
adenine dinucleotide, which participate in electron transport to 
yield ATP. After multiple cycles, acetyl CoA is produced, which 
is a substrate for the tricarboxylic acid cycle and for ketogenesis.
Ketogenesis, a process restricted to the liver, is necessary for 
life during times of nutritional deprivation, as it can spare the 
use of glucose for ATP generation b

can spare the 
use of glucose for ATP generation by the brain and other organs. 
Extreme production of ketones occurs in the setting of insulin defi­
ciency usually due to autoimmune destruction of pancreatic beta 
cells and represents a threat to life. 3-Hydroxy-3-methylglutaryl 
(HMG)-CoA synthase (rate limiting in mitochondria) converts 
acetyl CoA to hydroxymethylglutaryl CoA, which is converted to 
acetoacetate by HMG-CoA lyase. Acetoacetate is either reduced 
to β-hydroxybutyrate or conver

 is either reduced 
to β-hydroxybutyrate or converted to acetone.
1584
Defects in fatty acid oxidation are among the most common 
inborn errors of metabolism. Presentations include nonketotic 
hypoglycemia, liver dysfunction, and cardiomyopathy.15 
Triglyceride and Phospholipid Metabolism
Dietary fat, consisting of triglycerides and phospholipids, is 
digested in the stomach and proximal small intestine. Triglycer­
ides are broken down

l small intestine. Triglycer­
ides are broken down into component fatty acids in part through 
the action of pancreatic lipase, which is activated by bile acids. 
Bile salts form micelles that acquire fatty acids and interact with 
the unstirred water layer of the intestine, where fatty acids are 
absorbed. Long-chain fatty acids are taken up by enterocytes, 
re-esterified into triglycerides, and exported into the lymph as 
lipoproteins. Medium-chain (≤C10) fatty acids directly enter the 
portal

hain (≤C10) fatty acids directly enter the 
portal vein to access the liver.
Triglyceride Synthesis
Key steps in triglyceride synthesis have a major impact on systemic 
metabolism. Most triglycerides are synthesized through the glycerol 
phosphate pathway (Fig. 41.3, top portion) by a sequence of acyla­
tions. Another pathway, the monoacylglycerol pathway, is thought 
to be active only in the small intestine. Glycerol-3-phosphate is acted 
on by one of the glycerol-3-phosphate acyltransferases (

one of the glycerol-3-phosphate acyltransferases (GPATs) to 
generate lysophosphatidic acid. An important isoform is GPAT1, 
which competes with CPT1 for fatty acyl CoA molecules inside the 
cell, with GPAT1 prevailing when energy is to be stored and CPT1 
dominant when energy is required. The next acylation is mediated 
by acylglycerol-3-phosphate acyltransferases (AGPATs) and gener­
ates phosphatidic acid. Human mutations in AGPAT2 are respon­
sible for the disease called congenital generalize

sible for the disease called congenital generalized lipodystrophy.
Phosphatidic acid represents an important branch point in 
lipid metabolism. It serves as substrate for synthesis of either cyti­
dine diphosphate diacylglycerol (CDP-DAG, the precursor for 
molecules like phosphatidylinositol) or diacylglycerol (DAG). 
Synthesis of DAG requires a phosphatase activity provided by 
lipins.16 DAG can serve as a signaling molecule and as substrate 
for synthesis of either triglycerides or common pho

or synthesis of either triglycerides or common phospholipids. 
The acylation of DAG to form triglycerides is catalyzed by acyl-
CoA:diacylglycerol acyltransferases (DGATs). Inactivation of 
DGAT1 in mice prevents diet-induced obesity,17 but mutation 
of the gene in humans prevents normal fat absorption and causes 
steatorrhea in infants.18 
Phospholipid Synthesis
As shown in the bottom left portion of Fig. 41.3, phospholipid 
synthesis is intimately related to triglyceride synthesis. Generation 

ely related to triglyceride synthesis. Generation 
of the most common phospholipids, phosphatidylcholine and 
phosphatidylethanolamine, occurs mostly through the Kennedy 
pathway, which utilizes choline as an initial substrate and DAG at 
the final step. Mammalian liver can generate phosphatidylcholine 
from phosphatidylethanolamine through successive methylations. 
Phosphatidylserine can be converted to phosphatidylethanol­
amine, and other conversions between phospholipid species are 
possible

ersions between phospholipid species are 
possible. 
Lipolysis of Triglyceride Stores in Adipose Tissue
Most of the body’s triglyceride mass resides in adipose tissue, and 
turnover of energy stores from fat has important effects on lipid 
metabolism, normal physiology, and human health. Increased 
lipolysis in adipose tissue in obesity results in elevated circulat­
ing free fatty acids, which may cause dysfunction in pancreatic 
Acetyl CoA
Ketones
TCA cycle
ATP citrate lyase
Glycol

hondria
Ketones
TCA cycle
Pyruvate
CPT1
ACS
Acyl CoA
Glucose
Fructose
Citrate
Acetyl CoA
Fatty acid synthase
Palmitate
ELOVL6
Stearate
SCD1
Oleate
Acetyl CoA
Malonyl CoA
• Fig. 41.2  Fatty acid metabolism. Fatty acids are substrates for ACS, which generates CoA moieties that 
are transported into mitochondria by CPT1. This process is inhibited by malonyl CoA. In mitochondria, 
β-oxidation generates acetyl CoA, which can

dria, 
β-oxidation generates acetyl CoA, which can also be generated from glycolysis (bottom left). Acetyl CoA can 
be used to produce ketones, or it may enter the TCA cycle, leading to production of citrate; in the cytoplasm, 
citrate is a substrate for ATP citrate lyase, which produces acetyl CoA. The acetyl CoA serves as a substrate 
for de novo synthesis of fatty acids, as depicted on the right side of the figure. ACS, acyl-CoA synthase; ATP, 
adenosine triphosphate; CoA, coenzyme A; CPT1, c

adenosine triphosphate; CoA, coenzyme A; CPT1, carnitine palmitoyltransferase 1; ELOVL6, elongation of 
very long chain fatty acid protein 6; SCD1, stearoyl-CoA desaturase 1; TCA, tricarboxylic acid.
1585
beta cells, liver, skeletal muscle, and heart. Healthy offspring with 
parents who have type 2 diabetes mellitus have impaired insulin-
mediated suppression of circulating fatty acids,19 suggesting that 
an early defect in adipose tissue fatty acid me

t 
an early defect in adipose tissue fatty acid metabolism contributes 
to the evolution of diabetes.
Release of free fatty acids and glycerol from adipose tissue is 
controlled by a variety of hormones, many of which act through G 
protein–coupled receptors. The most robust mediators of fatty acid 
release are catecholamines, which bind to β-adrenergic receptors, 
activating stimulatory G proteins (Gs) that prompt an increase 
in the activity of cyclic adenosine monophosphate and protein 
kinas

 cyclic adenosine monophosphate and protein 
kinase A. Glucagon, adrenocorticotropic hormone, α-melanocyte-
stimulating hormone, and thyroid-stimulating hormone also 
induce lipolysis through Gs proteins. Adenosine suppresses lipoly­
sis by binding to receptors that activate inhibitory G proteins (Gi). 
Niacin, which also suppresses lipolysis, binds to the G protein 
receptor GPR109A, but this interaction does not mediate this 
vitamin’s effects on lipid metabolism.20 Insulin is a major lipolyti

on lipid metabolism.20 Insulin is a major lipolytic 
inhibitor, activating the insulin receptor–signaling cascade and sup­
pressing lipolysis at many steps, one of which includes a decrease in 
protein kinase A activity. At least three enzymes and two accessory 
proteins are required for the normal process of hormone-induced 
lipolysis in adipose tissue.21 Stored triglycerides are acted on by the 
enzyme, adipose triglyceride lipase (encoded by PNPLA2), which 
requires the coactivator protein co

NPLA2), which 
requires the coactivator protein comparative gene identification 58 
(CGI58). Diglycerides are hydrolyzed by hormone-sensitive lipase, 
yielding monoglycerides that are metabolized by monoglyceride 
lipase. This process cannot occur unless perilipin 1, a protein that 
coats small lipid droplets, is phosphorylated by protein kinase A. 
Defects in PLIN1, the gene encoding perilipin 1, are associated 
with familial partial lipodystrophy.22 Adipose tissue lipolysis is 
depicted schema

hy.22 Adipose tissue lipolysis is 
depicted schematically in Fig. 41.4. Human mutations in adipose 
triglyceride lipase or CGI58 are responsible for two variants of neu­
tral lipid storage disease that are characterized by hepatic steatosis; 
lipid accumulation in skeletal muscle; cardiomyopathy; neurologic 
problems; and, in one variant, skin defects. 
Lipoprotein Lipase
Most lipids are delivered to peripheral tissues such as muscle and 
fat through the activity of lipoprotein lipase (LPL). LPL

ough the activity of lipoprotein lipase (LPL). LPL is rate 
limiting for clearance of plasma triglycerides and essential for 
generation of high-density lipoprotein (HDL) particles,23 and 
hydrolyzes triglycerides (and, to a lesser extent, phospholipids) in 
circulating triglyceride-rich lipoproteins to allow peripheral sites 
access to preformed fatty acids. Much of this lipid flux is con­
trolled by insulin, which increases LPL in fat and decreases LPL 
in muscle. Exercise tends to have the op

ases LPL 
in muscle. Exercise tends to have the opposite effect,24 ensuring 
appropriate energy supplies to meet metabolic demands. LPL 
activity requires the presence of a coenzyme, apoC-II, which pri­
marily circulates as a component of very low density lipoprotein 
(VLDL) and HDL. In addition, the association of LPL with the 
luminal surface of endothelial cells requires the presence of its 
anchoring protein, glycosylphosphatidylinositol-anchored HDL-
binding protein 1 (GPIHBP1).
Glycerol-3-

ored HDL-
Glycerol-3-phosphate
GPATs
Lysophosphatidic acid
AGPATs
Phosphatidic acid
Phosphatidylinositol
Phosphatidylglycerol
Cardiolipin
Phosphatidylethanolamine
Phosphatidylserine
CDP-DAG
DAG
Lipins
DGATs
Choline
Phosphocholine
CDP-choline
+DAG
                 Ethanolamine
Phosphoethanolamine 
CDP-ethanolamine 
+DAG
Kennedy
pathway
• Fig. 41.3  Phospholipid and triglyceride synthesis. Glycerol-3-phosphate is converted by glycerol-

is. Glycerol-3-phosphate is converted by glycerol-
3-phosphate acyltransferases (GPATs) to lysophosphatidic acid, which is converted to phosphatidic acid 
by acyl­glycerol-phosphate acyltransferases (AGPATs). Phosphatidic acid can be converted to cytidine 
diphosphate diacylglycerol (CDP-DAG), to fuel one arm of phospholipid synthesis, or to diacylglycerol 
(DAG), which is a substrate for another arm of phospholipid synthesis and for acyl-CoA:diacylglycerol 
acyltransferases (DGATs), which gener

cylglycerol 
acyltransferases (DGATs), which generate triglycerides. The Kennedy pathway, which utilizes DAG at its 
final steps, is depicted at the bottom left of the figure.
1586
Free fatty acids released from lipoproteins by the action of 
LPL associated with the luminal surface of capillary endothelial 
cells must then cross the endothelial cell barrier to be captured by 
resident cells via conversion to acyl-CoA species. Tissue up

ells via conversion to acyl-CoA species. Tissue uptake of 
fatty acids at low concentrations appears mediated by cell surface 
transporters, whereas at high concentrations fatty acids may enter 
and transit the member in a nonregulated, nonsaturable process, 
sometimes referred to as flip-flop. Fatty acyl CoAs are then either 
stored as triglycerides or subjected to fatty acid oxidation. LPL 
is not synthesized in endothelial cells but is produced in adipo­
cytes, cardiac myocytes, and skeletal 

d in adipo­
cytes, cardiac myocytes, and skeletal myocytes and then secreted 
and targeted to the luminal surface of the endothelium. Both LPL 
and triglyceride-rich lipoproteins bind to endothelial GPIHBP1, 
which serves as a platform for lipolysis in the plasma.25 
Cholesterol Metabolism
In adults, dietary cholesterol is not required because many tissues 
are capable of cholesterol synthesis. Most diets include animal 
products, the source of cholesterol. Plants do not have cholesterol, 
but t

holesterol. Plants do not have cholesterol, 
but their membranes contain phytosterols, which are structurally 
similar to cholesterol and are useful in the dietary treatment of 
hypercholesterolemia because they compete with cholesterol for 
absorption. The liver and intestine are quantitatively the most 
important sites for cholesterol metabolism in humans, although 
a very small amount of cholesterol is also lost through the normal 
turnover of skin.
Cholesterol Absorption, Synthesis, and Excr

 skin.
Cholesterol Absorption, Synthesis, and Excretion
Cholesterol is absorbed through a process that requires formation 
of bile salt micelles. The efficiency of absorption varies widely in 
humans. There is a gradient of absorption through the intestine 
that is greatest in the proximal small intestine and least in the 
ileum. This gradient parallels the expression of Niemann-Pick C1–
like 1 (NPC1L1), a transmembrane protein with a sterol-sensing 
domain that is involved in cholesterol absorp

ing 
domain that is involved in cholesterol absorption.26 NPC1L1 is 
the target of ezetimibe, a drug that lowers cholesterol and has been 
shown to decrease heart disease. NPC1L1 also absorbs phytos­
terols such as sitosterol. The function of NPC1L1 in cholesterol 
absorption requires an interaction with LIM domain and actin 
binding 1 (LIMA1), which when mutated in humans is associ­
ated with low LDL cholesterol.27 Sterols are pumped out of the 
enterocyte and into the intestinal lumen by two A

enterocyte and into the intestinal lumen by two ATP-binding cas­
sette (ABC) transporters: ABCG5 and ABCG8. Human muta­
tions in these transporters cause the rare disorder sitosterolemia,28 
characterized by increased absorption and circulating levels of 
sitosterol and cholesterol, xanthomas, and heart disease (see later 
Fig. 41.5A illustrates cholesterol synthesis. Acetate is converted 
to HMG-CoA. The latter is a substrate for HMG-CoA reductase, 
the enzyme that is rate limitin

MG-CoA reductase, 
the enzyme that is rate limiting for cholesterol biosynthesis and 
is inhibited by statin drugs. Cells exquisitely regulate cholesterol 
acquisition.29 When levels are low, the transcription factor sterol 
response element–binding protein (SREBP) translocates to the 
nucleus to activate genes that increase cholesterol biosynthesis and 
import cholesterol from the extracellular environment. Statins, by 
lowering cholesterol and preventing cholesterol biosynthesis, work 
predomi

preventing cholesterol biosynthesis, work 
predominantly by increasing liver uptake of cholesterol from the 
plasma through the LDL receptor (see later discussion) and pro­
moting its excretion. Free cholesterol in cells is esterified to form 
cholesteryl esters for storage. This esterification reaction is carried 
out by acyl CoA:cholesterol acyltransferases (ACATs). These endo­
plasmic reticulum enzymes exist in two forms: ACAT1 is present 
in macrophages and has been implicated in atheroscler

macrophages and has been implicated in atherosclerosis, and 
ACAT2 is present in the liver and intestine and is implicated in 
cholesterol absorption. Nonspecific ACAT inhibition in humans 
does not affect serum lipids and does not have beneficial effects on 
atherosclerosis.30
Glycerol
Adipocyte
Diglycerides
Monoglycerides
Monoglyceride
lipase
Hormone-
sensitive
lipase
ATGL/
CGI58
• Fig. 41.4  Lipolysis in adipocytes. Stored triglycerides are metabolized to 
yield the 

tored triglycerides are metabolized to 
yield the fatty acids that circulate in plasma through the action of three dis­
tinct lipases with separate substrate specificities. Triglycerides are acted 
on by adipose triglyceride lipase (ATGL) in complex with the coactivator 
protein CGI58 to yield diglycerides, which are acted on by hormone-sensi­
tive lipase to yield monoglycerides. The monoglycerides, in turn, are acted 
on by monoglyceride lipase to yield glycerol. Lipid droplet proteins modu­
la

to yield glycerol. Lipid droplet proteins modu­
late this lipolytic process.
A
B
Acetate
HMG-CoA reductase
HMG-CoA
Mevalonic acid
Feedback
regulation
Cholesterol biosynthesis
Enterohepatic circulation of cholesterol and bile acids
50% Reabsorption
97% Reabsorption
Acetate
Bile acids
Bile acids
Bile
Liver
Intestine
Excretion
50%
3%
• Fig. 41.5  (A) Cholesterol biosynthesis. 3-Hydroxy-3-methylglutaryl 
coenzyme A (HMG-CoA) reductase is the rate-limit

coenzyme A (HMG-CoA) reductase is the rate-limiting enzyme regulating 
cholesterol biosynthesis. The enzyme is downregulated by excess choles­
terol in the cell. (B) Enterohepatic circulation of cholesterol and bile acids. 
Approximately 50% of cholesterol and 97% of bile acids are reabsorbed 
from the intestine and recirculated to the liver. (A, Modified from Brown MS, 
Goldstein JL. A receptor-mediated pathway for cholesterol homeostasis. 
Science. 1986;232:34–47.)
1587
Chapter 41  Disorders

ience. 1986;232:34–47.)
1587
Cholesterol, which is non-nutritive and cannot be catabolized 
to carbon dioxide and water, is either secreted into the bile as free 
cholesterol (about half of which is reabsorbed) or converted to bile 
acids for secretion into bile. Most bile acids are reabsorbed in the 
terminal ileum. This enterohepatic circulation of cholesterol and 
bile acids is shown in Fig. 41.5B. The rate-limiting enzyme for bile 
acid synthesis is 

 rate-limiting enzyme for bile 
acid synthesis is cholesterol 7α-hydroxylase, which is under feed­
back regulation by bile acids. Interruption of the enterohepatic 
circulation of bile acids through the use of bile acid sequestrants 
reduces the farnesoid X receptor (FXR)-mediated inhibition of 
bile acid synthesis. The subsequent induction of bile acid synthesis 
is associated with a reduction in plasma cholesterol and an increase 
in triglyceride production, explaining triglyceride elevations 

de production, explaining triglyceride elevations seen 
with bile acid sequestrant therapies. At least in mice, evidence sug­
gests that cholesterol can be excreted directly by enterocytes (inde­
pendent of the biliary system) through an active metabolic process 
called transintestinal cholesterol excretion.31
Nuclear Receptors and Lipid Metabolism
Nuclear receptors, usually transcription factors with both ligand- 
and DNA-binding domains, affect lipid metabolism. Classic hor­
mones that interac

 lipid metabolism. Classic hor­
mones that interact with nuclear receptors and have important 
lipid effects include thyroid hormone, glucocorticoids, estrogen, 
SREBPs are a family of transcription factors that control the 
metabolism of both cholesterol and triglycerides.32 In an inac­
tive state such as occurs with excess cellular cholesterol, they are 
associated with the endoplasmic reticulum membrane. Reduced 
endoplasmic reticulum cholesterol allows proteolytic processin

reticulum cholesterol allows proteolytic processing 
of SREBPs allowing translocation to the nucleus, where they regu­
late production of biosynthetic genes for cholesterol and the LDL 
receptor (SREBP2) and for triglyceride (SREBP1c) (both are dis­
cussed in more detail later).
Thyroid hormone reduces circulating cholesterol concentra­
tions via modulation of LDL receptor expression, or through 
reduced hepatic synthesis of apoB lipoproteins and greater excre­
tion of cholesterol into the bile.

 greater excre­
tion of cholesterol into the bile.33 Thus, lipid levels tend to be 
high in hypothyroid patients and low with hyperthyroidism. 
Glucocorticoids have robust effects on multiple aspects of lipid 
metabolism, inducing expression of HMG-CoA reductase to 
promote cholesterol synthesis, increasing expression of fatty acid 
synthase to promote fatty acid synthesis, and decreasing LPL to 
impair clearance of circulating lipids. Accordingly, hyperlipidemia 
is commonly seen in the setting

y, hyperlipidemia 
is commonly seen in the setting of glucocorticoid treatment, and 
insulin resistance induced by glucocorticoids amplifies the hyper­
lipidemia. Estrogens and selective estrogen receptor modulators 
such as raloxifene lower cholesterol34 by inducing LDL receptor 
activity; they tend to increase triglyceride levels, especially when 
higher oral doses are administered. Derivatives of cholesterol can 
serve as selective estrogen receptor modulators to affect the vascu­
lature.35 A

ceptor modulators to affect the vascu­
lature.35 Androgens, by activating the androgen receptor, decrease 
HDL.36 Changes in HDL with estrogens and androgens correlate 
with changes in hepatic triglyceride lipase, an enzyme that, like 
LPL, is released into the bloodstream after heparin injection.
Aside from classic hormones and their receptors, other nuclear 
receptors affect lipid metabolism after interacting with several 
types of metabolic by-products. These receptors include the 
PPARs, the

-products. These receptors include the 
PPARs, the liver X receptors (LXRs), and FXR. A schematic view 
of the roles of these receptors in lipid metabolism is presented in 
Fig. 41.6.
There are three known types of PPARs: α, γ, and δ. PPARα pro­
motes fatty acid oxidation, as well as ketogenesis, and is induced 
by starvation. PPARα expression is highest in tissues adapted to 
metabolize fats, such as liver and skeletal muscle, but is also pres­
ent in numerous other sites. In humans, pharmacolo

ent in numerous other sites. In humans, pharmacologic activation 
of PPARα with fibrates lowers triglycerides and increases HDL. 
Fatty acids interact with the receptor, but a phosphatidylcholine 
species was identified as an endogenous ligand for PPARα.37
Whereas PPARα facilitates energy utilization, PPARγ activates 
genes that promote energy storage. It is expressed at the highest 
levels in adipose tissue and is also found in macrophages, where 
it may help coordinate the complex relationship

e 
it may help coordinate the complex relationship between inflam­
mation and metabolism. Ether lipids, which are phospholipids 
generated in peroxisomes, appear to be endogenous ligands for 
PPARγ.38 Pharmacologic activation of PPARγ in humans with 
thiazolidinediones results in insulin sensitization and weight gain 
(see Chapter 35). The latter effect occurs because this nuclear 
receptor promotes both adipogenesis and fluid retention through 
effects on the kidney. Importantly, thiazolidinedi

effects on the kidney. Importantly, thiazolidinediones increase 
incident heart failure.39 Thiazolidinedione treatment in humans 
tends to lower triglycerides and increase HDL, probably by 
modulating insulin signaling. In mice, these agents reduce ath­
erosclerosis, and in people with metabolic syndrome with a recent 
stroke or transient ischemic attack, pioglitazone reduces risk of 
stroke and myocardial infarction.40 Dual agonists for PPARα and 
PPARγ lower hemoglobin A1c and serum lipids in 

d 
PPARγ lower hemoglobin A1c and serum lipids in humans but 
also increase the all-cause mortality rate.41
PPARδ promotes fatty acid oxidation in skeletal muscle, and 
activation of this receptor may mimic some aspects of exercise.
LXRs and FXR are also involved in lipid metabolism. LXRα 
and LXRβ are activated by oxysterols (derivatives of cholesterol) 
to increase conversion of cholesterol into bile acids, increase bile 
acid excretion, and decrease cholesterol absorption. LXR activa­
tion in

crease cholesterol absorption. LXR activa­
tion inhibits cholesterol uptake by inducing the degradation of 
the LDL receptor.42 LXRs also induce fatty acid and triglyceride 
synthesis. FXR is activated by bile acids to stimulate both bile acid 
secretion and reabsorption. Administration of the bile acid seques­
trant colesevelam to humans with diabetes lowers blood sugar, but 
the role of nuclear receptors in this effect is not defined.
Additional nuclear receptors also play important roles in l

l nuclear receptors also play important roles in lipo­
genesis—the process of converting carbohydrates to triglycerides 
rather than glycogen. Carbohydrate response element–binding 
LXR, FXR
Intestine
PPARs
PPARs
PPAR∝
LXR
FXR
Fatty
acids
Fatty
acids
Muscle
Adipose tissue
Liver
Bile acids
Bile acids
VLDL
Chylomicrons
• Fig. 41.6  Nuclear receptors in lipid metabolism. Peroxisome proliferator-
activated receptors (PPARs) are active in adipose tissue, which is a source 
of fatty acids that are tra

ue, which is a source 
of fatty acids that are transported to liver, where PPARα, the liver X recep­
tors (LXRs), and farnesoid X receptor (FXR) are active. Bile acids produced 
by the liver participate in an enterohepatic circulation with the intestine, 
another site of LXR and FXR expression. Very low density lipoprotein 
(VLDL), produced by the liver, and chylomicrons, from the intestine, are 
metabolized to release fatty acids that fuel muscle (another site of PPAR 
expression) and may be st

e (another site of PPAR 
expression) and may be stored by adipose tissue.
1588
protein (ChREBP) may control as much as 50% of this process. 
It responds to carbohydrate excess by transactivating a series of 
glycolytic and lipogenic genes.43 SREBP1c is also critical for this 
process.44 
Plasma Lipoproteins, Apolipoproteins, Receptors, and Other 
Proteins
Lipoproteins, which are spherical particles that circulate in the 
blood, transpo

al particles that circulate in the 
blood, transport vital non–water-soluble nutrients, vitamins, 
structural components, and proteins with specialized functions 
from the gut through the plasma compartment to remote tissues. 
Appropriate concentrations of lipoproteins are essential for health, 
but increased circulating concentrations of certain lipoproteins are 
associated with increased risk for cardiovascular disease. Absent 
or low levels of other lipoproteins may lead to vitamin deficiency

 other lipoproteins may lead to vitamin deficiency 
syndromes. The roles of the various lipoproteins are discussed in 
detail in the following sections. 
Major Lipoproteins
A prototypical lipoprotein is shown in Fig. 41.7. The fundamental 
structure of a lipoprotein exploits biochemical characteristics of 
its components. The surface consists of charged molecules that 
interact with the aqueous environment, such as phospholipids and 
free cholesterol. Amphipathic proteins (with both hydrophilic 

erol. Amphipathic proteins (with both hydrophilic and 
hydrophobic domains), called apolipoproteins (or simply apopro­
teins), are also present on the surface, with hydrophilic domains 
oriented toward the plasma and hydrophobic domains toward 
the core of the particle. Apolipoproteins regulate the lipoprotein 
interaction with metabolic enzymes and cellular receptors. The 
lipoprotein core consists of neutral (uncharged) lipids, such as tri­
glycerides and cholesteryl esters.
Lipoprotein moveme

cerides and cholesteryl esters.
Lipoprotein movement through the plasma compartment is 
dynamic. Humans spend most of their lives in the postprandial 
state. Eating is associated with generation of lipoproteins and the 
induction of enzymes that metabolize those lipoproteins. Within 
the circulation, lipolysis of triglyceride leads to rapid reduction 
in the size of chylomicrons. Along with loss of core triglyceride, 
excess surface components of shrinking particles are extruded, and 
both nones

 shrinking particles are extruded, and 
both nonesterified fatty acids and fat-soluble vitamins are deliv­
ered into tissues.
The major classes of lipoproteins are listed in Table 41.2. They 
were identified based on migration in an ultracentrifuge, and 
classes were defined based on density assessed using salt-contain­
ing solutions. An alternative original classification scheme, which 
is no longer useful, involved electrophoretic mobility in agarose 
gels. Chylomicrons, chylomicron remnants, 

garose 
gels. Chylomicrons, chylomicron remnants, and VLDLs are rich 
in triglycerides. Intermediate-density lipoproteins (IDLs), LDLs, 
and lipoprotein(a) (Lp[a]), are rich in cholesterol. HDLs are 
enriched in phospholipids. Triglyceride-rich lipoproteins such as 
chylomicrons are large and generally insoluble, which accounts for 
the cloudy appearance of plasma when it is obtained in nonfasting 
subjects or in fasting subjects with some types of hyperlipidemias. 
Apoprotein C
Apoprotein E
Apo

of hyperlipidemias. 
Apoprotein B100
Surface
Core
Triglyceride and
cholesteryl esters
Free
• Fig. 41.7  General structure of lipoproteins: schematic representation of a 
very low density lipoprotein particle.
  Major Classes of Plasma Lipoproteins
Type
Density (g/mL)
Origin
Major lipids
Major Apolipoproteins
Size (nm)
<0.95
Intestine
85% Triglyceride
B48, AI, AIV, E, CI, CII, CIII
∼100–500
Chylomicron remnants
<1.006
Derived from c

100–500
<1.006
Derived from chylomicrons
60% Triglyceride
20% Cholesterol
B48, E
∼80–125
VLDL
<1.006
Liver
55% Triglyceride
B100, E, CI, CII, CIII
30–80
IDL
1.006–1.019
Derived from VLDL
35% Cholesterol
25% Triglyceride
B100, E
25–35
LDL
1.019–1.063
Derived from IDL
60% Cholesterol
5% Triglyceride
B100
18–25
HDL
1.063–1.21
Liver, intestine, plasma
25% Phospholipid
AI, AII, CI, CII, CIII, E
5–12
Lp(a)
1.05–1.09
Liver
60% Cholest

II, CIII, E
5–12
Lp(a)
1.05–1.09
Liver
B100, apo(a)
∼30
apo, Apolipoprotein; IDL, intermediate-density lipoprotein; LDL, low-density lipoprotein; Lp(a), lipoprotein(a); VLDL, very low density lipoprotein.
   
TABLE 41.2
1589
Table 41.2 provides general ranges for particle size, which dif­
fer substantially among lipoproteins and within each class. The 
relative amount of protein versus less dense lipids correlates with 
th

otein versus less dense lipids correlates with 
the size and also the buoyancy of the lipoproteins—for example, 
large VLDLs contain much more lipid than the smaller HDLs. 
The subspecies of these particles similarly differ in size and pro­
tein/lipid content. One lipoprotein subtype, small dense LDL, 
is associated with cardiovascular disease, insulin resistance, and 
hypertriglyceridemia.
Chylomicrons originate in the gut. They are lighter than 
water and float to the top of a plasma sample. T

water and float to the top of a plasma sample. The particles 
are cleared rapidly (within minutes) after a meal and should 
be absent after an overnight fast. Their distinguishing apoli­
poprotein is apoB48, which is the only form of apolipoprotein 
B produced by intestinal cells in humans. B48 is so named 
because, as a splice variant, it is 48% of the entire B100 pro­
tein found in VLDL and LDL. Chylomicrons acquire apoC 
and apoE molecules by interacting with HDL particles, a pro­
cess that

y interacting with HDL particles, a pro­
cess that promotes chylomicron metabolism and conversion to 
chylomicron remnants. Chylomicron remnants, also character­
ized by the presence of apoB48, are cleared rapidly from the 
plasma. Remnant particles, as well as LDL, promote arterial 
atherosclerosis.
VLDL particles are of hepatic origin. Smaller than chylomi­
crons, their distinguishing apolipoprotein is apoB100, the form 
of apoB produced by the liver. VLDLs also carry apoC molecules 
that modu

 liver. VLDLs also carry apoC molecules 
that modulate the conversion of VLDLs to IDLs, which are VLDL 
remnants and are also atherogenic. IDL particles contain apoB100 
and apoE, and they are converted to LDL, which is characterized 
by carrying essentially only apoB100 as an apolipoprotein. LDL is 
the major carrier of cholesterol in most humans, and its measure­
ment forms the basis for coronary heart disease risk stratification 
and treatment goals. For most clinical laboratories, LDL result

 goals. For most clinical laboratories, LDL results 
represent both IDL and LDL particles.
HDL particles have a complex biology. They can be gener­
ated by the liver and intestine or assembled in the plasma as 
a consequence of the metabolism of other lipoproteins. They 
are arbitrarily divided into HDL2 (less dense, at 1.063–1.125 
g/mL), which typically contains both apoAI and apoCs, and 
HDL3 (more dense, at 1.125–1.21 g/mL), which typically con­
tains apoAI, apoAII, and apoCs. There is also 

on­
tains apoAI, apoAII, and apoCs. There is also a minor subclass 
known as HDL1 that carries a large percentage of plasma apoE. 
High HDL levels are associated with low cardiovascular risk in 
population studies, but it is not known if HDL plays a direct role 
in atherosclerosis. HDL can be assessed by its cholesterol con­
tent or more recently by a functional assay of cholesterol efflux 
capacity. This activity, reflecting the movement of labeled cho­
lesterol from a cultured macrophage cell 

led cho­
lesterol from a cultured macrophage cell line to apoB-depleted 
plasma, may be inversely associated with cardiovascular events 
(see later discussion).45 In part, the disparity between HDL 
concentration and function might explain why genetic markers 
associated with HDL levels do not associate with cardiovascular 
disease risk.46
Lp(a), produced by the liver, consists of an LDL particle in 
which the apolipoprotein apo(a) has been covalently linked to 
apoB100. Apo(a) has substantial p

ently linked to 
apoB100. Apo(a) has substantial protein homology to plasmino­
gen, which is required for the endogenous thrombolytic response, 
and it exists in isoforms based on Kringle repeats (named after a 
type of pastry). Isoforms with fewer repeats, and therefore lower 
mass, tend to circulate at higher concentrations. Higher levels 
increase the risk of myocardial infarction, aortic valve calcifica­
tion, and aortic stenosis.47,48 
The chromosomal location, size, s

 Apolipoproteins
The chromosomal location, size, sites of synthesis, and major func­
tions of important apolipoproteins are summarized in Table 41.3.
Apolipoproteins AI, AII, AIV, and AV
ApoAI is the most abundant apolipoprotein in HDL. It is syn­
thesized by the liver and intestine and activates the enzyme 
lecithin:cholesterol acyltransferase (LCAT), which transfers a fatty 
acid from lecithin to the free hydroxyl group on cholesterol to 
generate cholesteryl ester. Maturation of HDL particles

ate cholesteryl ester. Maturation of HDL particles begins as 
lipid-poor discs containing apoAI which then acquire free choles­
terol. Cholesterol within the nascent HDL is converted to cho­
lesteryl ester through the activity of LCAT leading to expansion 
into spheres. ApoAI mediates efflux of cholesterol from periph­
eral tissues, an important step in the process of reverse cholesterol 
transport.49 Human genetic mutations in apoAI cause low levels 
of HDL and corneal opacities. ApoAI is consi

vels 
of HDL and corneal opacities. ApoAI is considered to be an anti­
atherogenic protein, but genetic defects leading to low levels of 
apoAI are not consistently associated with coronary artery disease.
ApoAII is present with apoAI in some HDL particles. Synthe­
sized mostly in the liver, it has been implicated in the activation 
of hepatic lipase, an enzyme involved in HDL metabolism, and 
in the inhibition of LCAT. ApoAII disrupts the ability of HDL 
to promote reverse cholesterol transport

y of HDL 
to promote reverse cholesterol transport, but the genetic absence 
of this protein in humans does not seem to be associated with a 
phenotype.50
ApoAIV originates in the gut, and its secretion is induced by 
consumption of a high-fat meal. It may affect food intake in mice, 
but information in humans is not available.
ApoAV is encoded by a locus near the apoAIV gene in the 
apoAI/CIII/AIV/AV gene cluster on chromosome 11. It is pro­
duced by liver and circulates at low concentrations i

ed by liver and circulates at low concentrations in associa­
tion with VLDL particles in humans. ApoAV is involved in the 
hydrolysis of triglyceride-rich lipoproteins by LPL, its expression 
is inversely related to triglyceride levels, and it promotes lipopro­
tein clearance by hepatic proteoglycans.51 In humans, homozy­
gous mutations in apoAV increase risk for hyperchylomicronemia 
and pancreatitis.52 
Apolipoprotein B
There are two forms of this apolipoprotein, apoB100 and apoB48, 
which are

his apolipoprotein, apoB100 and apoB48, 
which are encoded by a single gene. A unique mechanism that 
involves RNA editing allows translation into these two forms of 
apoB (Fig. 41.8). In both liver and intestinal cells, the same mes­
senger RNA is transcribed, but an editing protein complex inter­
acts with the message only in the intestine (in humans) to change 
the cytosine at nucleotide position 6666 to uracil leading to pro­
duction of an intestinal protein that is approximately 48% of the 

testinal protein that is approximately 48% of the 
length of apoB100.
ApoB48 is important for the assembly of chylomicrons.53 
There are one or two apoB48 molecules on each chylomicron, 
where they provide structural support to the particle. The COOH-
terminus of apoB100, missing in apoB48, determines interaction 
with the LDL receptor, so apoB48 does not appear to be involved 
in the clearance of gut-derived lipoproteins.
ApoB100 originates in the liver, where it is cotranslation­
ally associat

he liver, where it is cotranslation­
ally associated with lipids to coordinate the formation of VLDL 
particles. VLDL assembly and export, which affect the levels of 
circulating atherogenic lipoproteins, are determined not by tran­
scriptional control of the apoB gene but by a mechanism involv­
ing stabilization of the apoB protein by lipid. VLDL production 
1590
is shown in Fig. 41.9. Assembly is thought to involve two dis­
tinct pro

 Assembly is thought to involve two dis­
tinct processes. First, as the apoB message is translated on the 
rough endoplasmic reticulum, it binds to lipids that are provided 
by microsomal triglyceride transfer protein (MTP; the target of 
a medication, see later discussion). This protein heterodimerizes 
with protein disulfide isomerase, which remodels the apoB protein 
by rearranging the positions of disulfide bonds in the molecule 
to accommodate incoming lipid. Most of this lipid originates i

te incoming lipid. Most of this lipid originates in 
adipose tissue, where triglyceride lipolysis releases free fatty acids 
that are transported to the liver. Phospholipids and cholesterol 
also associate with apoB at this step. If sufficient lipids are not 
available in the liver, apoB (which is constitutively produced) is 
ubiquitinated and degraded in the proteasome. Second, maturing 
VLDL particles fuse with additional lipid droplets in the Golgi 
apparatus, a process facilitated by apoE. T

Golgi 
apparatus, a process facilitated by apoE. The triglyceride-rich par­
ticles are then secreted into the space of Disse. The discrete fates of 
these two potential sources of VLDL may be complex, but VLDL 
particles ultimately gain access to the circulation, where the trans­
fer of apolipoproteins from other lipoproteins modifies VLDL 
structure to promote metabolism in the periphery.
Increased VLDL production, fueled by the increased avail­
ability of lipid, is predominantly responsible fo

ability of lipid, is predominantly responsible for the dyslipidemia 
seen with obesity and diabetes. There is one copy of apoB100 
on each VLDL particle, and this relationship is retained as these 
lipoproteins are metabolized to IDL and then to LDL. Therefore, 
measurements of apoB100 in the plasma reflect the particle num­
ber, and higher levels of apoB are associated with cardiovascular 
disease. The complete absence of apoB, which occurs in the rare 
human disorder abetalipoproteinemia, is 

the rare 
human disorder abetalipoproteinemia, is usually caused by muta­
tions in MTP.54 Patients with this disease can have severe neu­
rologic deficits, probably reflecting vitamin E deficiency, because 
triglyceride-rich lipoproteins transport this lipid-soluble vitamin. 
Very low, but not absent, apoB, which occurs in the human 
5′
mRNA editing 
Gln
CAA 
B100
1 
4536 
1 
2152 
B48
(Stop)
UAA 
ApoB mRNA 
H2N 
COOH H2N 
COOH 
ApoB protein
3′
ApoB gene 
• Fig. 41.8  Synthesis of apolipoprotein

poB gene 
• Fig. 41.8  Synthesis of apolipoprotein B100 and apoB48 by a messenger 
RNA (mRNA) editing mechanism. In the human intestine, a specific cyto­
sine (C) is changed to a uracil (U) in the apoB mRNA. This change results 
in a stop codon and the formation of apoB48, which contains only the first 
2152 amino acids of the full-length apoB100 (4536 amino acids). COOH, 
carboxy-terminus; Gln, glutamine; H2N, amino-terminus.
  Major Apolipoproteins
Apolipoprotein (Chromosome No.)
Molecular We

teins
Molecular Weight (kDa)
Synthesis
Functions
AI (11)
∼29
Liver, intestine
Structural protein (HDL)
Cofactor for LCAT
Crucial role in reverse cholesterol transport
Ligand for ABCA1 and SR-BI
AII (1)
∼17 (dimer)
Liver
Inhibits apoE binding to receptors
Activates hepatic lipase
Inhibits LCAT
AIV (11)
∼45
Intestine
Potential satiety factor
Activator of LCAT
Facilitates lipid secretion from intestine
AV (11)
39
Liver
Activator of LPL-mediated lipolysis
Might inhibi

r
Might inhibit hepatic VLDL synthesis
B100 (2)
∼500
Liver
Structural protein (VLDL and LDL)
Ligand for LDL receptor
B48 (2)
∼200
Intestine
Structural protein (chylomicrons)
CI (19)
6.6
Liver
Modulates remnant binding to receptors
Activates LCAT
CII (19)
8.9
Liver
Cofactor for LPL
CIII (11)
8.8
Liver
Inhibitor of LPL
E (19)
∼34
Liver, brain, skin, testes, 
spleen
Ligand for LDL and remnant receptors
Local lipid redistribu

r LDL and remnant receptors
Local lipid redistribution
Reverse cholesterol transport (HDL with apoE)
apo(a) (6)
∼400–800
Liver
Modulates thrombosis/fibrinolysis
ABCA1, Adenosine triphosphate–binding cassette transporter A1; apo, apolipoprotein; HDL, high-density lipoprotein; LCAT, lecithin:cholesterol acyltransferase; LDL, low-density lipoprotein; LPL, lipoprotein 
lipase; SR-B1, scavenger receptor class B type 1; VLDL, very low density lipoprotein.
   
TABLE 41.3
1591
Chapter 41  Disorders of L

in.
   
TABLE 41.3
1591
disorder hypobetalipoproteinemia, is usually caused by trunca­
tion mutations in apoB. These individuals present with low cho­
lesterol and triglycerides and appear to be healthy. Another cause 
of hypobetalipoproteinemia is a defect in angiopoietin-like pro­
tein 3 (ANGPTL3), which leads to reduced hepatic secretion of 
lipoproteins and increased clearance without causing fatty liver.55 
A mutation at amino acid residue 3500 of t

ver.55 
A mutation at amino acid residue 3500 of the apoB100 protein, 
within the COOH-terminal region of the molecule that mediates 
binding to the LDL receptor, causes familial defective apoB100. 
These individuals have high LDL cholesterol, mimicking the pre­
sentation of FH.56 
Apolipoproteins CI, CII, and CIII
These small apolipoproteins are encoded by loci residing at two 
different locations in the genome. ApoCI and apoCII are tran­
scribed from a site on chromosome 19 near the apoE gene.

d from a site on chromosome 19 near the apoE gene. The 
apoCIII gene is a component of the apoAI/CIII/AIV/AV cluster 
on chromosome 11. ApoCs, which can be exchanged freely among 
lipoprotein particles, are important for triglyceride metabolism 
because their presence either interferes with the recognition of 
apoE by lipoprotein receptors or displaces apoE from lipoproteins 
(both of which would increase triglycerides by impairing their 
clearance). The function of apoCII is more complex. High 

ce). The function of apoCII is more complex. High lev­
els in mice cause elevated triglycerides by displacing apoE, but 
normal levels of apoCII are required for normal lipid clearance 
because this apolipoprotein is a cofactor for the enzyme LPL. 
Mutations of apoCII in humans cause severe hypertriglyceride­
mia, mimicking LPL deficiency.
ApoCIII may be particularly relevant to human health. Lev­
els are increased in the setting of many dyslipidemias, and most 
lipid-lowering medications lower 

emias, and most 
lipid-lowering medications lower apoCIII levels. A mutation in 
the apoCIII gene causing lower apoCIII levels is associated with 
an improved lipid profile and less atherosclerosis,57 suggesting that 
therapies targeted at apoCIII might provide clinical benefit. In 
patients with extremely high triglycerides due to familial chylo­
micronemia syndrome (FCS; see later discussion), inhibiting the 
apoCIII mRNA results in substantial triglyceride lowering.58 
Apolipoprotein E
ApoE b

 triglyceride lowering.58 
ApoE biology is also complex. ApoE expression is highest in the 
liver, followed by the brain. In the brain, astrocytes and microglial 
cells synthesize apoE, which can also be produced by injured neu­
rons. Many other cell types synthesize the protein, including mac­
rophages. ApoE circulates in plasma within all lipoproteins, with 
the probable exception of LDL. Its principal function involves 
interactions with the two major receptors mediating the 

ctions with the two major receptors mediating the clearance 
of plasma lipoproteins: the LDL receptor and the LDL receptor–
related protein (LRP1, also known as the chylomicron remnant 
receptor). Therefore, apoE is primarily responsible for clearance 
of intestinal-derived lipoproteins after a meal and for clearance of 
VLDL and IDL particles before they are converted to LDL.
There are three major apoE isoforms: E2, E3, and E4. They 
are encoded, respectively, by alleles referred to as ε2, ε3, 

, respectively, by alleles referred to as ε2, ε3, and ε4, 
with charge differences caused by variations in amino acids at resi­
dues 112 and 158 in the protein. ApoE3 is considered the nor­
mal isoform; it has a cysteine at residue 112 and an arginine at 
158. ApoE2 has cysteines at both 112 and 158, and apoE4 has 
an arginine at both 112 and 158. These variations have structural 
and functional consequences (Fig. 41.10). The protein has two 
domains: an amino (NH2)-terminus interacts with lipop

ains: an amino (NH2)-terminus interacts with lipoprotein 
receptors and a COOH-terminus interacts with lipids (see Fig. 
41.10A). In apoE4, the isoform associated with disease, these 
domains interact, which does not occur with apoE3 (see Fig. 
41.10B).
Comprehensive data (more than 86,000 individuals for lipids 
and more than 37,000 for coronary events) link apoE allele and 
genotype frequencies, lipid levels, and coronary risk.59 Allele fre­
quencies in healthy adults are 7% for ε2, 82% for ε3

encies in healthy adults are 7% for ε2, 82% for ε3, and 11% for 
ε4. Genotype frequencies are 0.7% for ε2/ε2, 11.6% for ε2/ε3, 
2.2% for ε2/ε4, 62.3% for ε3/ε3 (the most abundant genotype), 
21.3% for ε3/ε4, and 1.9% for ε4/ε4. There is a linear relationship 
between the genotype and both LDL level and coronary risk, from 
least to most, as follows: ε2/ε2 < ε2/ε3 < ε2/ε4 < ε3/ε3 < ε3/ε4 < 
ε4/ε4. Compared with the reference group (ε3/ε3), the presence 
of the ε2 allele decreases coronary risk by

sence 
of the ε2 allele decreases coronary risk by about 20%, and the 
presence of the ε4 allele slightly increases risk. These observations 
are interesting for two reasons. First, ε2/ε2 individuals, although 
they are protected from coronary heart disease on a population 
basis, are at risk for dysbetalipoproteinemia, occurring in about 
5% of ε2/ε2 individuals and which is associated with aggressive 
vascular disease. Second, the E2 protein binds less well to the LDL 
receptor than E3 and E4.

nds less well to the LDL 
receptor than E3 and E4. This suggests that LDL cholesterol in 
patients with the E2 protein should be higher (because it is less 
Sinusoid
Space of Disse
Hepatocyte
Endothelial cells
VLDL
Golgi
SER
Secretory vesicle
VLDL
RER
Nucleus
Nascent
ApoB-containing
lipoproteins
• Fig. 41.9  Very low density lipoprotein (VLDL) biosynthesis by hepato­
cytes. The nascent apolipoprotein B (ApoB)-containing apolipoproteins 
synthesized by the rough endoplasmic reticulum (RER) are th

ed by the rough endoplasmic reticulum (RER) are thought to 
combine with lipids in the smooth endoplasmic reticulum (SER). The 
VLDLs are processed in the Golgi apparatus and accumulate in large 
secretory vesicles. They are then released into the space of Disse and 
enter the plasma. (Modified from Alexander CA, Hamilton RL, Havel RJ. 
Subcellular localization of B apoprotein of plasma lipoproteins in rat liver. 
J Cell Biol. 1976;69:241–263; by copyright permission of the Rockefeller 
Universi

 copyright permission of the Rockefeller 
1592
likely to be cleared by this receptor), yet the opposite is observed. 
These data suggest that other receptor-mediated processes, such as 
those mediated by heparan sulfate proteoglycans (HSPGs), may 
be critical for clearance of apoE-containing lipoproteins.60
ApoE is a well-characterized genetic marker for Alzheimer 
disease. Risk increases approximately 3-fold in thos

sease. Risk increases approximately 3-fold in those with one ε4 
allele and 12-fold in those with two ε4 alleles.61 The presence of 
an ε2 allele is protective. These relationships hold for both early- 
and late-onset Alzheimer disease. There are HDL-like lipoproteins 
in the central nervous system, and apoE-mediated delivery of cho­
lesterol is important for normal synaptic function. The relation of 
lipid metabolism to Alzheimer disease is incompletely understood, 
but evidence suggests that d

mpletely understood, 
but evidence suggests that deposition of amyloid-β (the major 
constituent of the plaques that characterize the disease) begins 
sooner in the brains of those with the E4 protein. Because peo­
ple with the ε4 allele are also more likely to have atherosclerosis, 
central nervous system vascular disease may help explain why this 
apoE variant is involved in neurodegeneration. 
Major Receptors Involved in Lipid Metabolism
LDL Receptor Gene Family
There are at least 10 members 

eceptor Gene Family
There are at least 10 members of the LDL receptor family; LDL 
receptor and LRP1 are the two most important ones for sys­
temic lipid metabolism. The LDL receptor recognizes apoB100 
and apoE, whereas LRP1 recognizes only apoE. Other core fam­
ily members (those that share considerable structural homol­
ogy) include the VLDL receptor, the apolipoprotein E receptor 
2 (apoER2 or LRP8), LRP4, LRP1B, and megalin (LRP2, also 
known as gp330 and as the major Heymann nephritis anti

n as gp330 and as the major Heymann nephritis antigen).
Three family members lack some of the structural features of 
the others. They are sortilin-related receptor L1 (LR11/SORL1), 
A
B
ApoE4
ApoE3
Glu-255
Glu-255
Arg-61
Arg-112
Arg-61
Cys-112
NH2
NH2
Arg
Cys
Arg
Cys
244
272
Major
lipid-binding
region
CO2H-terminal
domain
158
150
136
112
NH2-terminal
domain
domain
domain
domain
domain
Receptor
binding
region
• Fig. 41.10  (A) The amino (NH2)

r
binding
region
• Fig. 41.10  (A) The amino (NH2)-terminal domain of apoprotein E is composed of a four-helix bundle. A 
region of random structure encompassing residues 165 to 200 forms a connector or hinge region linked 
to the carboxy (CO2H)-terminal domain. There are two major functional regions. Residues 136 to 150 (yel­
low helix) encompass the receptor-binding region; residues 240 to 260 in the carboxy-terminal domain 
encompass the lipid-binding region. (B) ApoE4 displays the unique pro

-binding region. (B) ApoE4 displays the unique property of domain interaction that 
distinguishes it from apoE3 (Arg-61 in the amino-terminal domain interacts with Glu-255 in the carboxy-
terminal domain). Arg, arginine; Cys, cysteine; Glu, glutamic acid.
1593
LRP5, and LRP6. Along with the LDL receptor and LRP1, sor­
tilin receptors participate in brain development, synaptic func­
tion, and neuroprotection, making them relevant to Alzheimer 
disease.62 

on, making them relevant to Alzheimer 
disease.62 LRP5 and LRP6 are both coreceptors for a family of 
G protein–coupled receptors known as frizzled receptors. Frizzled 
receptors bind the Wnt molecule to induce an important signaling 
cascade upstream of the transcription factor β-catenin. Genetic 
variants in LRP5 are associated with obesity,63 and a human muta­
tion in LRP6 results in metabolic syndrome and coronary artery 
disease.64 Loss-of-function mutations in LRP5 or LRP6 cause 
osteoporo

unction mutations in LRP5 or LRP6 cause 
osteoporosis in humans, whereas gain of function in LRP5 causes 
osteopetrosis. These observations suggest that insulin resistance, 
coronary disease, and osteoporosis, which are common comorbid 
conditions in patients, may be related to abnormal Wnt signaling.
LDL Receptor
The LDL receptor is a large (160 kDa) glycoprotein expressed on 
most cells. Because it recognizes apoB100 as well as apoE, it is 
involved in the uptake of LDL, chylomicron remnants, 

olved in the uptake of LDL, chylomicron remnants, VLDL, 
and IDL. Most HDL particles do not have apoE and therefore 
do not interact with this receptor or the LRP. The discovery of 
this receptor in the 1970s by Brown and Goldstein65 explained 
a human disease (FH), the mechanism of action of drugs that 
lower cholesterol, and defined receptor-mediated endocytosis as 
a paradigm for providing cells with critical components from the 
external environment.
The LDL receptor shares functional domain

ronment.
The LDL receptor shares functional domains with other 
members of this receptor family, including the ligand-binding 
domain, the epidermal growth factor (EGF) precursor domain, 
the O-linked sugar domain at the cell surface, the membrane-
spanning domain, and the cytoplasmic domain at the COOH-
terminus (Fig. 41.11). The ligand-binding domain includes seven 
repeats of approximately 40 amino acids, each containing six cys­
teines that form three disulfide bonds within the repeat to sta

orm three disulfide bonds within the repeat to sta­
bilize the structure. The repeat also includes negatively charged 
amino acids that interact with positively charged residues on the 
ligands apoB and apoE and with calcium ions. The EGF precursor 
domain consists of three EGF-like repeats (see Fig. 41.11) with a 
structure known as a β-propeller located between repeats B and 
C. The O-linked sugar domain is the site at which carbohydrate 
moieties attach to the molecule, and this is followed b

ies attach to the molecule, and this is followed by a short 
membrane sequence. The cytoplasmic domain consists of 50 resi­
dues that include an NPXY (asparagine, proline, any amino acid, 
tyrosine) targeting sequence where adapter proteins dock, which 
leads to receptor clustering in coated pits.
Coated pits are specialized cell surface regions characterized by 
the presence of the protein complex clathrin. When LDL receptors 
bind lipoproteins, they migrate to coated pits, and clathrin directs

 they migrate to coated pits, and clathrin directs 
the complex to a cell membrane region that folds inward, creat­
ing an intracellular vesicle or endosome (Fig. 41.12). Endosomes 
become acidic, causing the lipoprotein to be displaced from the 
LDL receptor by the β-propeller of the EGF precursor domain.66 
The unoccupied receptor recycles back to the cell surface. In the 
presence of PCSK9 (see later discussion and Fig. 41.18), the LDL 
receptor conformation is altered, promoting degradation 

or conformation is altered, promoting degradation and pre­
venting recycling to the cell surface.67
Lipoproteins are degraded in lysosomes. Cholesterol is trans­
ported out of the lysosomes through the action of two pro­
teins, Niemann-Pick C1 and C2 (NPC1 and NPC2), which are 
mutated in the human disease Niemann-Pick type C, which is 
characterized by intracellular accumulation of cholesterol. Intra­
thecal administration of cyclodextrin may confer clinical ben­
efit.68 NPC2, which is soluble,

nfer clinical ben­
efit.68 NPC2, which is soluble, binds cholesterol after lipoprotein 
hydrolysis in the lysosome and moves this sterol to the membrane-
associated NPC1 for subsequent release to the cell, where it serves 
structural and regulatory functions.
One key regulatory function of cholesterol is control of LDL 
receptor expression. Intracellular sterol concentrations are sensed 
1
2 3
A
Cysteine
COOH
B
C
45 6 7
NH2
Domain
Ligand binding
EGF precursor
O-Linked sugar
Membrane spanning
Cyt

Cytoplasmic
• Fig. 41.11  Functional domains of the low-density lipoprotein receptor. 
Numbers 1 through 7 indicate repeats in the ligand-binding domain. A, B, 
and C are epidermal growth factor (EGF)-like repeats in the EGF precur­
sor domain. See the text for a complete description. COOH, carboxy-
terminus.
Coated
vesicle
Endosome
Coated pit
Receptor
recycling
LDL
receptors
Lipoprotein
Clathrin
Golgi
apparatus
Rough
endoplasmic
reticulum
Lysosome

gi
apparatus
Rough
reticulum
Lysosome
• Fig. 41.12  Low-density lipoprotein (LDL) receptor pathway. LDL interacts 
with receptors on the cell surface. The complex enters the coated pit and 
is internalized. The coated vesicle loses its clathrin coat and becomes an 
endosome, the site of lipoprotein and receptor dissociation. The receptors 
recycle to the cell surface, and the lipoproteins are degraded. Alterna­
tively, new receptors are synthesized in the rough endoplasmic reticulum 

re synthesized in the rough endoplasmic reticulum 
and transported to the cell surface. (Modified from Brown MS, Goldstein 
JL. A receptor-mediated pathway for cholesterol homeostasis. Science 
1986;232:34–47; and Myant NB. Cholesterol Metabolism, LDL, and the 
LDL Receptor. San Diego, CA: Academic Press; 1990.)
1594
by SREBP cleavage–activating protein (SCAP), which binds to 
SREBPs in the endoplasmic reticulum. SREBP2 is most impor­

 the endoplasmic reticulum. SREBP2 is most impor­
tant for LDL receptor transcription; its NH2-terminus contains 
a leucine zipper–type transcription factor structure that binds to 
a sterol regulatory element in the promoter of the LDL recep­
tor gene. When cells are sterol depleted (Fig. 41.13, left side), 
SCAP migrates to the Golgi apparatus, where sugar moieties 
attached to the protein are modified. This allows SCAP to trans­
port SREBPs to the site-1 protease (S1P) compartment. There, 
tw

 the site-1 protease (S1P) compartment. There, 
two proteases, S1P and site-2 protease (S2P), sequentially act on 
SREBPs to release their NH2-terminus, which migrates to the 
nucleus and binds to the sterol regulatory element in the pro­
moter region of lipid genes such as the LDL receptor and HMG-
CoA reductase, increasing transcription and subsequent levels 
of functional proteins. In the presence of cholesterol (see Fig. 
41.13, right side), SCAP does not cycle to the Golgi structure, 
it ca

SCAP does not cycle to the Golgi structure, 
it cannot move SREBPs to the S1P compartment, and SREBPs 
are not cleaved to allow their transcription factor to migrate to 
the nucleus. This disruption of SREBP activation in the pres­
ence of cholesterol protects cell from excessive accumulation of 
cholesterol. 
LDL Receptor–Related Protein 1
LRP1 is also known as the apoE receptor or the chylomicron 
remnant receptor. LRP1 is large, containing components of four 
LDL receptors with a multiplicity

ponents of four 
LDL receptors with a multiplicity of ligand-binding domains. 
Embryonic inactivation of LRP1 (but not of the LDL receptor) 
is lethal in mice. The major cell types expressing LRP1 include 
hepatocytes, neurons, and placental syncytiotrophoblasts. Mul­
tiple different ligands bind to LRP1 and participate in both nutri­
ent flow and signaling. These ligands include amyloid precursor 
protein (processed to form the amyloid-β of plaques in Alzheimer 
disease), bacterial by-products,

ues in Alzheimer 
disease), bacterial by-products, tissue plasminogen activator 
(which interacts with LRP1 to promote inflammation in the set­
ting of brain ischemia), plasminogen activator inhibitors, and 
α2-macroglobulin (which contributes to inflammation, in part by 
inactivating matrix metalloproteinases). LRP1 is associated with 
receptor-associated protein, a small protein that binds to several 
regions of LRP1 to chaperone this large protein during intracel­
lular processing.
LRP1 binds

tein during intracel­
LRP1 binds apoE but not apoB100. Therefore, it mediates the 
metabolism of the major apoE-containing lipoproteins, includ­
ing chylomicron remnants and IDL (VLDL remnants), but is 
not involved in LDL metabolism. The interaction between LRP1 
and lipoproteins is more complex than that between LDL and the 
LDL receptor. Multiple apoE molecules are required for LRP1 
binding, and this interaction requires an initial binding of the 
lipoprotein to proteoglyca

initial binding of the 
lipoprotein to proteoglycans on the cell surface. Other moieties 
on apoE-containing lipoproteins also facilitate the binding pro­
cess. LPL, which metabolizes chylomicrons and VLDL particles, 
adheres to particles after mediating the release of fatty acids and 
other components at the endothelium. Lipoprotein-bound LPL 
molecules (as well as hepatic lipase) are thought to interact with 
LRP1 and to facilitate the uptake of remnants by the liver. 
Pattern Recognition Rece

f remnants by the liver. 
Pattern Recognition Receptors
The delivery of excess lipids to the blood vessel leads to atheroscle­
rosis. The innate immune system and at least two broad types of 
receptors—scavenger receptors and toll-like receptors (TLRs)—
that preferentially recognize ligand patterns (e.g., those associated 
with carbohydrates, lipids, or nucleic acids) instead of discrete 
features (e.g., specific amino acid sequences) are involved in the 
atherosclerotic process and thought to b

d in the 
atherosclerotic process and thought to be activated by lipids.
Scavenger Receptors
The observation that macrophages can bind and internalize modi­
fied forms of LDL but not native LDL prompted the discovery of 
scavenger receptors. There are multiple classes of these receptors69 
that bind altered (e.g., oxidized, acetylated) LDL or other poly­
anionic ligands. Class A and class B receptors may be particularly 
Class A receptors include scavenger receptor A (SR-A types 1 
an

ors include scavenger receptor A (SR-A types 1 
and 2, which consists of alternative splice variants), macrophage 
receptor with collagenous structure (MARCO), scavenger recep­
tor A 5 (SCARA5), and scavenger receptor with C-type lectin 
domain (SRCL-I/II, also referred to as CL-P1). SR-A, the first to 
be discovered, binds a wide variety of ligands (including bacterial 
by-products), activates stress signaling pathways including mito­
gen-activated protein kinases, and is believed to be involve

ted protein kinases, and is believed to be involved in ath­
erosclerosis, the clearance of apoptotic cells, and Alzheimer disease.
Class B receptors include CD36 and scavenger receptor class B 
(SR-BI [called CLA1 in humans]). These receptors bind modified 
LDL, but unlike other scavenger receptors, they also bind VLDL, 
native LDL, and HDL. CD36 is expressed on a wide variety 
– Sterols
+ Sterols
Endo-
plasmic
reticulum
SREBP
SCAP
Golgi SCAP
cycling
Golgi
SCAP
cycling
COOH
COOH
COOH
COOH
S1P
SC

ling
Golgi
SCAP
cycling
COOH
COOH
COOH
COOH
S1P
SCAP
S1P
Release
of active
SREBP
No release
of active
SREBP
S2P
No SCAP
cycling
No SREBP
transport
SCAP
cycling
SREBP
transport
LDL receptors
Nucleus
NH2
NH2
NH2
NH2
NH2
SREBP
LDL
Receptor
gene
SRE
Sp1
Sp1 TATA
Nucleus
LDL
Receptor
gene
SRE
Sp1
Sp1 TATA
• Fig. 41.13  Low-density lipoprotein (LDL) receptor gene regulation. 
COOH, carboxy-terminus; S1P, site-1 protease; S2P, site-2 protease; 
SCAP, SREBP cleavage–activating 

site-2 protease; 
SCAP, SREBP cleavage–activating protein; SRE, sterol regulatory element; 
SREBP, sterol regulatory element–binding protein; TATA, the TATA box or 
core promoter sequence.
1595
of cell types, including monocytes, macrophages, adipocytes, 
platelets, endothelial cells, hepatocytes, microglial cells, and the 
tongue, where it detects dietary fat. In addition to lipoproteins, 
long-chain fatty acids are ligands for CD36.70 The tissue distri

ty acids are ligands for CD36.70 The tissue distribu­
tion of SR-BI is more limited, with expression on hepatocytes, 
monocyte/macrophages, and steroidogenic tissues. A genetic defi­
ciency of SR-BI in humans leads to increased circulating HDL but 
also increased risk for coronary heart disease.71
Toll-Like Receptors
The TLR family comprises key effectors of the innate immune 
system required for host defense mechanisms against pathogens. 
Their activation has been implicated in many chronic inf

activation has been implicated in many chronic inflamma­
tory diseases, including atherosclerosis, through signaling path­
ways that increase nuclear factor kappa-light-chain-enhancer of 
activated B cells (NF-κB), activator protein 1 (AP1), and other 
transcription factors that control inflammation. Some scavenger 
receptors, such as CD36, may be coreceptors for TLRs. TLRs are 
found on myeloid cells such as monocyte/macrophages and on the 
gut epithelium. TRL ligands include lipopolysaccharide

epithelium. TRL ligands include lipopolysaccharide (TLR4) 
and glycolipids found in bacteria (TLR2). TLR2 mediates mono­
cyte activation by apoCIII on triglyceride-rich lipoproteins.72 
Other Enzymes and Transfer Proteins Mediating 
Lipid Metabolism
Hepatic Lipase
Heparin lipase primarily is a phospholipase with some triglyceride 
lipase activity. It is synthesized in hepatocytes but found mostly on 
endothelial cells in the liver and on HSPG in the space of Disse. It 
is also found in steroidog

the space of Disse. It 
is also found in steroidogenic tissues but is not synthesized at those 
sites. Unlike LPL, which is mostly present at tissues remote from 
the liver, thereby ensuring the peripheral delivery of lipids and vita­
mins, hepatic lipase centrally coordinates lipoprotein metabolism. 
Functions include conversion of IDL to LDL, HDL2 to HDL3, and 
probably the final metabolism of chylomicron remnants to facilitate 
their uptake by LRP1. Unlike LPL, hepatic lipase does not require

 LRP1. Unlike LPL, hepatic lipase does not require a 
cofactor such as apoCII, but both enzymes are displaced from their 
endothelial sites of activity by injection of heparin (postheparin 
lipase activity). High levels of hepatic lipase decrease HDL concen­
trations, whereas high levels of LPL increase HDL. 
Endothelial Lipase
Evolutionarily related to hepatic lipase and LPL, endothelial lipase 
is a phospholipase with almost no triglyceride lipase activity. It is 
expressed at high levels in e

ase activity. It is 
expressed at high levels in embryonic endothelial cells, with expres­
sion declining during maturation. In adults, expression is highest 
in endothelial cells, thyroid, lung, liver, placenta, and gonads. In 
mice, overexpression decreases HDL and inactivation increases 
HDL. Endothelial lipase is expressed in the aorta, where it may 
increase with atherosclerosis. Human loss-of-function mutations 
are associated with increased HDL.73 Endothelial lipase does not 
appear to af

d HDL.73 Endothelial lipase does not 
appear to affect atherosclerotic risk. 
Proprotein Convertase Subtilisin/Kexin Type 9
PCSK9 is a secreted protease that promotes the degradation of 
the LDL receptor by interacting with the receptor so that it is 
targeted to the lysosome. PCSK9 catalytic activity is not required 
for receptor degradation. Highest expression levels are seen in the 
liver, intestine, and kidney. The importance of PCSK9 in lipid 
metabolism was discovered when missense mutatio

d 
metabolism was discovered when missense mutations within the 
gene (subsequently determined to be gain-of-function mutations) 
were associated with hypercholesterolemia and coronary artery 
disease.74 Overexpression of PCSK9 in mice decreases LDL recep­
tor protein. Human loss-of-function mutations in PCSK9 are 
associated with low LDL concentrations and decreased risk of vas­
cular disease.75,76 Some PCSK9 associates with circulating LDL,77 
which might assist with targeting this protein to 

which might assist with targeting this protein to LDL receptors. 
Antibodies to PCSK9 are useful for treatment in humans. 
Lipoprotein-Associated Phospholipase A2
Phospholipases hydrolyze the ester bond at the sn2 position of 
phospholipids, usually resulting in the release of a fatty acid and 
lysophosphatidylcholine, which can induce inflammation. Phos­
pholipases were originally identified as a component of snake 
venom, and many distinct classes have since been characterized. 
Membrane phosp

ses have since been characterized. 
Membrane phospholipids are the substrate for most phospholi­
pases. Lipoprotein-associated phospholipase A2 (Lp-PLA2) is an 
exception because it can hydrolyze substrate in the aqueous phase. 
Lp-PLA2 binds to both LDL and HDL lipoproteins and is a bio­
marker for coronary artery disease.78 In humans, inhibition of this 
enzyme decreases the expansion of the lipid core of atherosclerotic 
plaques79 but does not decrease cardiovascular end points.80 
Cholestery

 decrease cardiovascular end points.80 
Cholesteryl Ester Transfer Protein
Cholesteryl ester transfer protein (CETP) promotes the exchange 
between lipoproteins of two classes of neutral lipids: cholesteryl 
esters and triglycerides. HDL cholesteryl esters are transferred to 
VLDL, IDL, and chylomicron remnants; in return, triglycerides 
from VLDL, IDL, and remnants are transferred to HDL. Humans 
and other primates have CETP activity; the transfer of choles­
teryl ester from HDL to apoB-contain

er of choles­
teryl ester from HDL to apoB-containing lipoproteins ultimately 
leads to most of their cholesterol burden being carried by LDL, 
and this is thought to result in atherosclerosis. Rodents and dogs 
do not have CETP. Most of their cholesterol is carried in HDL; 
LDL is low, and these animals are resistant to atherosclerosis. Such 
observations led to the notion of inhibiting CETP activity as a 
treatment for atherosclerosis in humans. In clinical trials, different 
inhibitors have i

. In clinical trials, different 
inhibitors have increased HDL cholesterol and lowered LDL cho­
lesterol but also increased mortality,81 selectively increased HDL 
with no impact on cardiovascular events,82 and reduced events 
perhaps due to lower LDL cholesterol.83 
Lecithin:Cholesterol Acyltransferase
LCAT is an enzyme synthesized primarily in the liver; it circulates 
in the plasma associated with HDL particles and, to a lesser extent, 
with LDL particles. LCAT is activated by several apolipo

DL particles. LCAT is activated by several apolipoproteins 
(apoAI and others) and uses the phospholipid lecithin (phospha­
tidylcholine) and free cholesterol as substrates to generate lysoleci­
thin (lysophosphatidylcholine) and cholesteryl ester. Most of the 
cholesteryl esters in lipoproteins are derived from LCAT activity. 
Rare human mutations in LCAT result in low HDL levels. The 
role of LCAT in atherosclerosis is uncertain, and some reports sug­
gest that humans with loss-of-function LCA

ts sug­
gest that humans with loss-of-function LCAT mutations do not 
appear to have increased risk for atherosclerosis,84 perhaps because 
their residual HDL is functional in assays of reverse cholesterol 
transport. The other clinical manifestations of LCAT deficiency 
are described in the Lecithin:Cholesterol Acyltransferase Defi­
ciency section. 
Integrative Physiology of Lipid Metabolism
Lipid metabolism is characterized by a dynamic flux of multiple 
lipid species from the external environ

 multiple 
lipid species from the external environment to the liver, from the 
1596
liver to peripheral tissues, from peripheral tissues back to the liver, 
and eventually back to the external environment through the 
excretion of bile acids. Integrated views of the major pathways 
involved are shown in Figs. 41.14 and 41.15.
Exogenous Lipid Transport
Dietary fat and cholesterol (see Fig. 41.14, top left) absorbed by 
the duodenum and 

g. 41.14, top left) absorbed by 
the duodenum and proximal jejunum are used to generate chylo­
microns that are secreted at the lateral borders of enterocytes and 
enter mesenteric lymphatics. They access the plasma via the tho­
racic duct and are rapidly metabolized by LPL to yield chylomi­
cron remnants. These are taken up by remnant receptors (LRP1/
HSPG) and by LDL receptors in the liver. Free fatty acids liber­
ated by the action of LPL are available to adipose tissue for storage 
and to ot

available to adipose tissue for storage 
and to other tissues (e.g., skeletal muscle, heart) for use as energy 
substrates. 
Endogenous Lipid Transport
Lipid derived from remnants and from lipolysis of adipose tis­
sue is reassembled in the liver (see Fig. 41.14, bottom left) as 
VLDL particles, which are secreted into the plasma. Abnormal 
lipid metabolism in insulin resistance is mediated in large part 
by overproduction of VLDL, an event that occurs through dis­
ruption of signaling downstrea

occurs through dis­
ruption of signaling downstream of the insulin receptor and the 
insulin receptor substrate (IRS) adapter proteins. VLDL particles 
are metabolized by LPL to yield IDL particles, which are metabo­
lized by LPL and hepatic lipase to yield LDL particles. Thus, LDL 
is derived from VLDL, which helps explain why treatments that 
increase LPL actions to lower triglycerides (carried by VLDL) are 
frequently associated with at least transient increases in LDL. IDL 
Liver
Intestine
R

transient increases in LDL. IDL 
Liver
Intestine
Remnant
receptors
LDL
receptors
Bile acids
Chylomicron
remnants
Dietary fat and
LPL
VLDL
IDL
LDL
FFA
FFA
LPL
HL
ApoE mediated
ApoB mediated
tissues
Adipose
tissue
FFA
LPL
LPL
HL
• Fig. 41.14  General scheme summarizing the major pathways involved in the metabolism of chylomicrons 
synthesized by the intestine and VLDL synthesized by the liver. ApoB, apolipoprotein B; ApoE, apolipopro­
tein E; FFA, 

 apolipoprotein B; ApoE, apolipopro­
tein E; FFA, free fatty acid; HL, hepatic lipase; IDL, intermediate-density lipoprotein; LPL, lipoprotein lipase; 
VLDL, very low density lipoprotein. (Modified from Mahley RW. Biochemistry and physiology of lipid and 
lipoprotein metabolism. In: Becker KL, ed. Principles and Practice of Endocrinology and Metabolism. 2nd 
ed. Philadelphia, PA: JB Lippincott; 1995:1369–1378.)
tissues
Tg
CE
CE
SR-BI
LDLR
Bile
Liver
VLDL
IDL
remnants
excre

DLR
Bile
Liver
VLDL
IDL
remnants
excretion
Free cholesterol
LCAT
LCAT
Hepatic
lipase
HDL2
HDL3
HDL1
ApoE
CETP
delivery
• Fig. 41.15  Role of HDL in the redistribution of lipids from cells with excess 
cholesterol to cells requiring cholesterol or to the liver for excretion. The 
reverse cholesterol transport pathway is indicated by arrows (net transfer 
of cholesterol from cells to HDL, then to LDL and liver). ApoE, apolipo­
protein E; CE, cholesteryl ester; CETP, cholest

o­
protein E; CE, cholesteryl ester; CETP, cholesteryl ester transfer protein; 
HDL, high-density lipoprotein; IDL, intermediate-density lipoprotein; LCAT, 
lecithin:cholesterol acyltransferase; LDLR, low-density lipoprotein recep­
tor; SR-BI, scavenger receptor class B, type 1; Tg, triglyceride; VLDL, very 
low density lipoprotein.
1597
can be taken up by the liver through an apoE-dependent process, 
and LDL is taken up by the liver through the binding 

 LDL is taken up by the liver through the binding of apoB100 
to LDL receptors. Small VLDL particles, IDL particles, and LDL 
particles may be taken up by peripheral tissues to deliver nutrients, 
cholesterol, and fat-soluble vitamins. When present in excess, each 
of these lipoproteins may be atherogenic. 
Reverse Cholesterol Transport and Dysfunctional 
HDL
Cholesterol cannot be metabolized by peripheral tissues and 
must be returned to the liver for excretion. This process, called 
reverse ch

er for excretion. This process, called 
reverse cholesterol transport, is dependent on HDL and its precur­
sors and is depicted in Fig. 41.15. Excess cholesterol in tissues can 
be effluxed either to lipid-poor apoAI, mediated by the protein 
transporter ABCA1 (ATP-binding cassette transporter 1), or to 
nascent HDL particles, mediated by ABCG1. Efflux from cul­
tured cells to human plasma as a biomarker of cardiovascular risk 
is thought to mostly reflect the activity of ABCA1. There is also 
e

ly reflect the activity of ABCA1. There is also 
evidence that cholesterol can be acquired by HDL without the 
assistance of transporters by following a concentration gradient 
at the cell surface. LCAT esterifies HDL-associated cholesterol to 
form cholesteryl ester and induces the maturation of HDL. HDL 
particles have three pathways for transporting sterols to the liver. 
First, they can directly bind to SR-BI (CLA1) at the liver, which 
induces cholesteryl ester delivery through a mechanism 

es cholesteryl ester delivery through a mechanism involving 
lateral lipid transfer and not receptor internalization. Second, cho­
lesteryl esters can be transferred to apoB-containing lipoproteins 
by CETP, and these particles can deliver cholesterol to the liver 
through the LDL receptor. Third, a small portion of HDL can 
acquire apoE and bind to the liver LDL receptor. Once in the 
liver, cholesterol is converted to bile acids for excretion.
Cholesterol is the principal component of atherosc

Cholesterol is the principal component of atherosclerotic 
plaque, and thus atherosclerosis might be treated by promoting 
the efflux of cholesterol from lesions. HDL participates in this 
process, but static levels of HDL are poor predictors of reverse 
cholesterol transport. Dysfunctional HDL particles incapable 
of mediating cholesterol efflux could explain why some HDL-
elevating interventions have not been associated with decreased 
cardiovascular disease. Measurements of the rate of flux o

scular disease. Measurements of the rate of flux of cho­
lesterol from the periphery to the liver, which may be possible in 
humans, would represent a better predictor of beneficial therapies. 
In vitro cholesterol efflux capacity might be useful for assessing 
risk45 but does not appear to be universally inversely associated 
with disease.85,86
In addition to participating in reverse cholesterol transport, 
HDL has other properties that could be impaired by a variety 
of processes leading to a 

 impaired by a variety 
of processes leading to a dysfunctional particle. They include the 
induction of endothelial nitric oxide synthase, the transport of 
proteins involved in the acute phase response and inflammation, 
and the suppression of thrombosis through induction of prosta­
cyclin (which decreases thrombin production via the protein C 
pathway and decreases platelet activation). 
and Atherogenesis
Abnormal lipids can present as elevated trigl

esis
Abnormal lipids can present as elevated triglycerides, elevated cho­
lesterol, elevations of both triglycerides and cholesterol, and low 
HDL. A summary of the primary and secondary causes of each 
condition is presented in Table 41.4.
Plasma lipid levels are highly dependent on lifestyle. For exam­
ple, the high-fat, high-cholesterol diets of Western societies raise 
plasma cholesterol, and vigorous exercise lowers both atherogenic 
particles and triglycerides. For this reason, normal bloo

es and triglycerides. For this reason, normal blood con­
centrations—those that are within 2 standard deviations of the 
mean—vary among countries and over time. The overriding influ­
ences of diet and lifestyle on plasma cholesterol were illustrated 
by studies of ethnic Japanese populations showing that choles­
terol was markedly increased in Japanese-Americans consuming a 
Westernized diet.87 Because total cholesterol levels correlate with 
the risk for coronary heart disease over a broad ran

e risk for coronary heart disease over a broad range (Fig. 41.16), 
normal levels are preferably defined as those associated with mini­
mal cardiovascular risk rather than population averages, suggest­
ing that most people in the developed world have lipid levels that 
put them at risk for atherosclerotic cardiovascular disease. Epide­
miologic studies support that total cholesterol below 150 mg/dL 
(3.9 mmol/L) is associated with low risk of cardiovascular disease 
(see Fig. 41.16). Furthermore

diovascular disease 
(see Fig. 41.16). Furthermore, atherosclerosis regression, such as 
the reduction of the size and presumably inflammatory status of 
established lesions, can be seen with LDL below 70 to 80 mg/dL 
(<2.0 mmol/L) in patients with established atherosclerotic cardio­
vascular disease.88
Population, genetic, and therapeutic data support that ath­
erosclerosis is frequently caused by cholesterol deposition within 
the arterial wall. As described earlier, several genetic hyperlip­

. As described earlier, several genetic hyperlip­
idemic disorders are associated with premature atherosclerosis. 
Reductions of blood cholesterol by diet, statins, and other cho­
lesterol-lowering therapies, and even surgical ileal bypass, have 
convincingly shown that cholesterol reduction will reduce the 
incidence of major cardiovascular events, especially myocardial 
infarction, need for revascularization, and stroke. Most striking is 
the observation that the majority of patients with coro

bservation that the majority of patients with coronary artery 
disease will have reduced disease (i.e., regression) with statin ther­
apy.88 The cholesterol hypothesis is further supported by a wealth 
of animal data: altering blood lipid levels by diet or genetic modi­
fication causes atherosclerosis-free animals to develop disease, and 
reduction of cholesterol leads to regression.
Atherosclerosis and its clinical presentations as coronary artery 
disease, stroke, and peripheral vascular disea

ry 
disease, stroke, and peripheral vascular disease are likely the 
product of several pathophysiologic changes. Depending on the 
patient and setting, arterial disease results from a varying reaction 
to lipid infiltration, arterial damage, and macrophage inflamma­
tion. More than 100 years ago, pathologists identified cholesterol 
as a major component of atherosclerosis. This poorly metabolized 
lipid is found to be associated with both collagen and proteogly­
cans (key components of connecti

en and proteogly­
cans (key components of connective tissue consisting of proteins 
attached to carbohydrates), and within arterial cells. Macrophages 
and smooth muscle cells are converted into foam cells, so called 
because of their intracellular foamy lipid. In addition, there are 
often acellular lipid-rich areas, a variable amount of overlying col­
lagen-rich connective tissue, and regions where the atherosclerotic 
plaque has ruptured. The process begins with lipid infiltration into 
the a

process begins with lipid infiltration into 
the arterial wall.89 Unresolved pathogenic issues include (1) how 
lipid enters the artery, (2) determining which pathways lead to 
excess lipid uptake by foam cells, and (3) determining which pro­
cesses cause rupture and thrombosis.
Lipoproteins cross the arterial barrier and infiltrate the artery 
wall. Larger particles, such as chylomicrons, are likely to be 
excluded by the endothelial barrier. LDL can leave the circulation 
via channels between 

L can leave the circulation 
via channels between cells, along the continuous transendothe­
lial movement of cell-free plasma, or via interaction with specific 
receptors. Once in the subendothelial space, lipoproteins must 
1598
accumulate to cause disease. It is theorized that positive charges on 
apoB interact with negatively charged proteoglycans to promote 
lipoprotein retention. Both apoB100 in LDL and VLDL, apoB48 
in chylomicro

oth apoB100 in LDL and VLDL, apoB48 
in chylomicron remnants, and apoE in several classes of lipopro­
teins have proteoglycan-binding sequences.90 Another possibility 
is that within the artery, lipoproteins fuse to form large aggregates 
that are unable to disassociate and reenter the circulation. Lipo­
protein uptake into cells is usually well controlled because excess 
cellular cholesterol downregulates LDL receptors. Therefore, aber­
rant lipoprotein receptor regulation may be a factor, or L

oprotein receptor regulation may be a factor, or LDL (and 
remnants) may enter cells by non–cholesterol-regulated pattern 
recognition receptors (see earlier discussion).
Cholesterol-containing lipoproteins can become inflamma­
tory. Enzymes within the artery might induce alterations in the 
protein and lipid content of LDL that convert these particles into 
more inflammatory oxidized LDL. Although low-dose aspirin has 
some anti-inflammatory properties, its reduction of cardiovascu­
lar events 

perties, its reduction of cardiovascu­
lar events is most likely secondary to effects on platelet aggrega­
tion rather than direct effects on atherosclerosis. A recent clinical 
trial showed that blocking the interleukin 1β receptor reduced 
major coronary events in patients with high C-reactive protein 
and known vascular disease.91 This finding supports that at least 
Plasma cholesterol (mg/dL) 
CHD mortality rate per 1000
100 
150 
200 
250 
300 
2
4
6
8
10
12
14
16
0
• Fig. 41.16  Relation b

00 
2
4
6
8
10
12
14
16
0
• Fig. 41.16  Relation between plasma cholesterol levels and coronary heart 
disease (CHD) mortality rate in the Multiple Risk Factor Intervention Trial. 
(Modified from Stamler J, Wentworth D, Neaton JD. Is relationship between 
serum cholesterol and risk of premature death from coronary heart disease 
continuous and graded? Findings in 356,222 primary screenees of the 
Multiple Risk Factor Intervention Trial [MRFIT]. JAMA. 1986;256:2823–
2828; Copyright © 1986, by the

AMA. 1986;256:2823–
2828; Copyright © 1986, by the American Medical Association.)
  Differential Diagnosis of Hyperlipidemia and Dyslipidemia
Hypercholesterolemia
Increased Cholesterol and 
Low HDL
Primary Disorders
LPL deficiency
Familial hypercholesterolemia
Familial combined hyperlipidemia
Familial hypoalphalipoproteinemia
ApoCII deficiency
Familial defective apoB100
Dysbetalipoproteinemia
ApoAI mutations
Familial hypertriglyceridemia
Polygenic hypercholest

milial hypertriglyceridemia
Polygenic hypercholesterolemia
LCAT deficiency
Sitosterolemia
ABCA1 deficiency
Secondary Disorders
Anabolic steroids
Obstructive liver disease
Retinoids
High-carbohydrate diets
HIV infection
Thiazides
Immunosuppressives
Hepatitis C infection
Obesity/insulin resistance
Protease inhibitors
Estrogens
E

Estrogens
Ethanol
Lipodystrophies
Beta blockers
Retinoids
Bile acid–binding resins
Antipsychotics
Thiazides
ABCA1, Adenosine triphosphate–binding cassette transporter 1; apo, apolipoprotein; HDL, high-density lipoprotein; HIV, human immunodeficiency virus; LCAT, lecithin:cholesterol acyltransferase; LPL, 
lipoprotein lipase.
   
TABLE 41.4
1599
some therapies direc

some therapies directed to inflammation will be useful to prevent 
and treat residual risk for atherosclerosis even in the presence of 
lipid-lowering therapy.
Although the arterial lumen can be progressively narrowed by 
the accumulation of macrophages, the proliferation of smooth 
muscle cells, and the deposition of cholesterol, the truly danger­
ous lesion (the culprit lesion) may not cause marked luminal nar­
rowing.92 As atherosclerosis progresses, there is a c

ing.92 As atherosclerosis progresses, there is a compensatory 
expansion of the lumen that maintains an almost constant lumen 
size. As the lesion develops within the intima, the complication 
of rupture of the overlying intima or endothelial erosion leads to 
exposure of the lesional contents to platelets, initiating thrombo­
sis. It is the acute thrombosis that is responsible for infarctions in 
most patients. Rupture or erosion occurs where the fibrous cap 
covering the underlying thrombogeni

e fibrous cap 
covering the underlying thrombogenic lipid is thin.
The surfaces of complicated lesions can become thrombo­
genic as endothelial cells are lost or the fibrous cap ruptures and 
the subendothelial space is exposed. Platelets can adhere to this 
exposed surface, promoting thrombus formation. In these unsta­
ble plaques, blood actually dissects into the artery wall, leading to 
the formation of a large thrombus. Calcification is also a feature of 
late lesions. Advanced lesions can w

a feature of 
late lesions. Advanced lesions can weaken the elasticity and integ­
rity of the artery wall, potentially creating an aneurysm of the ves­
sel. Removal or reduction of the atherogenic stimulus can result 
in plaque regression and stabilization, leaving a remnant devoid 
of lipid that resembles a wound scar and is less likely to serve as a 
nidus for thrombus formation.
Observations first made in animals and now confirmed in 
humans indicate that the atherosclerotic process can be re

ndicate that the atherosclerotic process can be reversed 
if plasma cholesterol reduction is intensive.93 LDL reduction 
to levels below 70 mg/dL (1.8 mmol/L) reduced lesions size in 
about two thirds of subjects studied by intravascular ultrasound.88 
Even greater regression was found with lower LDL achieved using 
PCSK9 inhibitors.94
In humans, vascular disease is not universal, even among those 
individuals with marked hypercholesterolemia. Similarly, patients 
with low cholesterol are not as

milarly, patients 
with low cholesterol are not assured of being protected from the 
disease. Perhaps half of atherosclerosis is attributable to hyperlip­
idemia and other known cardiac risk factors.95
Despite the importance of lipids in atherosclerosis, there 
remains an unmet medical need to find additional approaches for 
residual risk in the lipid-lowering era. Several approaches to ath­
erosclerosis treatment directed toward nonlipid causes have not 
been successful. Use of vitamin E and an

have not 
been successful. Use of vitamin E and antibiotics has not altered 
cardiovascular event rates. Other inflammatory diseases (e.g., col­
lagen vascular diseases) associated with cardiac events have not yet 
been shown to achieve lower cardiac event rates with cholesterol-
lowering therapies. Statin-mediated cholesterol reduction appears 
to be ineffective in end-stage renal disease, although those patients 
usually do not have elevated LDL levels. In contrast, cardiovascu­
lar events in 

L levels. In contrast, cardiovascu­
lar events in patients with less severe renal disease are reduced by 
treatment with statin/ezetimibe.96 
Severe hypertriglyceridemia usually leads to pancreatitis if triglyc­
erides exceed 2000 mg/dL (22.6 mmol/L). At approximately 500 
mg/dL (5.6 mmol/L), LPL is saturated and a large bolus of fat dur­
ing a dietary indiscretion may not be cleared from the bloodstream, 
thereby promoting severe hypertriglyceridemia and pancreatitis. 
Trig

evere hypertriglyceridemia and pancreatitis. 
Triglyceride guidelines have focused on prevention of pancreatitis 
and not cardiovascular disease, as there is no level 1 evidence to 
date that directed therapies to reduce triglyceride will affect car­
diovascular events. Ongoing trials using fibrates and omega-3 fatty 
acids are likely to resolve this issue in the near future.
Fasting Hyperchylomicronemia
The most dramatic example of severe hypertriglyceridemia is that 
of fasting hyperchylomicro

riglyceridemia is that 
of fasting hyperchylomicronemia. This can result from a primary 
defect in chylomicron metabolism, or it can occur secondary to 
increased VLDL and saturation of LPL. LPL saturation occurs 
when triglyceride levels exceed about 500 mg/dL (5.6 mmol/L); 
this means that higher triglyceride levels exceed the capacity for 
the enzyme to act on its substrate, so dietary triglyceride is not 
metabolized and remains in the circulation. Therefore, familial 
hypertriglyceridemia, 

ation. Therefore, familial 
hypertriglyceridemia, familial combined hyperlipidemia, and 
dysbetalipoproteinemia can be associated with fasting hyper­
chylomicronemia. One common cause of such exacerbations is 
poorly controlled diabetes leading to increased adipose intracel­
lular lipolysis, return of fatty acids to the liver, greater secretion 
of VLDL triglyceride, and saturation of LPL. Several dietary and 
environmental factors also modulate triglyceride production. The 
most dramatic is alc

triglyceride production. The 
most dramatic is alcohol, a major substrate for triglyceride pro­
duction. In addition, diets that are rich in free carbohydrates, and 
especially simple sugars, induce triglyceride production. Fructose 
also increases de novo production of lipids in the liver but has less 
effect on circulating triglycerides.
Triglyceride levels above 500 mg/dL (5.6 mmol/L) and hyper­
triglyceridemic pancreatitis are relatively common; triglycerides 
above 1000 mg/dL (11.3 mmol/L) 

on; triglycerides 
above 1000 mg/dL (11.3 mmol/L) are found in 0.4% of a general 
medical population, and of these, 5.4% had a pancreatitis episode 
within 1 year.97 Genetic defects leading to this condition, called 
familial chylomicronemia syndrome (or FCS), are relatively rare and 
estimated to occur in approximately 1 in 250,000 people.98 Thus, 
the majority of patients are likely to be genetically predisposed 
to severe hypertriglyceridemia, with symptoms emerging due to 
environmental fact

 with symptoms emerging due to 
environmental factors such as alcohol use or another underlying 
conditions such as diabetes. Defects in LPL most commonly cause 
genetic FCS; the lack of normal LPL prevents chylomicron clear­
ance. LPL deficiency usually, but not always, manifests in child­
hood. The symptoms vary from difficulty feeding young infants to 
frank pancreatitis, which is sometimes mistaken as appendicitis. 
The plasma is often milky, and whole blood may have a pinkish, 
“cream of to

 and whole blood may have a pinkish, 
“cream of tomato soup” hue. The trigger level of triglyceride eleva­
tion leading to pancreatitis is variable; some patients have triglycer­
ides in excess of 10,000 mg/dL (113 mmol/L) with no symptoms, 
whereas others develop pancreatitis at much lower triglyceride lev­
els but usually in excess of 2000 mg/dL (22.6 mmol/L). However, 
patients presenting with pancreatitis have often avoided eating, 
and the first measured sample may not reflect the peak trig

irst measured sample may not reflect the peak triglycer­
ide. Several additional mutations in proteins required for normal 
LPL actions also cause FCS (see the LPL Deficiency section).
The pathophysiology of the relationship between hyperchy­
lomicronemia and pancreatitis is unknown. Lipid-rich blood 
may sludge, leading to pancreatic ischemia. The small number of 
lipases that normally leak from the acinar cells may lead to exu­
berant local lipolysis; creation of toxic local concentrations of 

olysis; creation of toxic local concentrations of free 
fatty acids and lysolecithin, a toxic lipid produced from phos­
phatidylcholine; and further acinar cell damage to adjacent cells. 
Insults to the acinar cells such as those provided by alcohol can 
accelerate this process.
Although most patients with severe hyperchylomicronemia 
who do not develop pancreatitis are asymptomatic, a few with 
1600 SECTION VIII    Disorders of Carbohydrate and Fat Metabolism
extreme levels exceeding 10,000 mg/

Fat Metabolism
extreme levels exceeding 10,000 mg/dL (113 mmol/L) develop 
the hyperchylomicronemia syndrome. These patients have dys­
pnea and confusion that may be indistinguishable from early 
dementia. Presumably, this is the result of reduced blood flow or 
defective oxygen delivery.
The marked increase in blood triglyceride concentration can 
lead to accumulation of triglycerides in several organs and can be 
observed in the blood. The latter is best appreciated by examin­
ing the blood di

er is best appreciated by examin­
ing the blood directly, allowing the red blood cells to settle, and 
observing a creamy layer on the plasma, or by noting the pinkish 
discoloration of the blood on funduscopic examination, known 
as lipemia retinalis (Fig. 41.17B). Eruptive xanthomas, as shown 
in Fig. 41.17G, are 2- to 5-mm papules with a yellow center sur­
rounded by erythema. They are caused by triglyceride-enriched 
skin macrophages. These lesions are sometimes confused with 
acne or follic

esions are sometimes confused with 
acne or folliculitis. For unclear reasons, eruptive xanthomas are 
most commonly found on the buttocks, extensor surfaces of the 
arms, and the back. Enlargement of the liver and spleen is not 
uncommon and is thought to be caused by triglyceride accumula­
tion in these organs.
Aside from severe hypertriglyceridemia, other laboratory indi­
ces are sometimes abnormal. Plasma sodium is reduced; liver 
transaminases are sometimes elevated. Despite the presence of

es are sometimes elevated. Despite the presence of 
pancreatitis, amylase may be normal due to an assay artifact; 
serum lipase is a more reliable indicator in this setting. Often the 
clinical laboratory will note the severe lipemia and fail to report 
measurements of routine chemistries due to the turbidity of the 
serum. If these other measurements are required, plasma can be 
centrifuged, the chylomicron layer removed, and the remaining 
plasma examined. 
LPL Deficiency
Almost every racial g

ma examined. 
Almost every racial group has been reported to have patients with 
genetic defects in LPL, and a founder mutation makes the defect 
especially common among French Canadians. At least half of the 
cases of severe genetic primary hypertriglyceridemia are the result 
of LPL defects. Most LPL enzyme deficiencies are caused by inac­
tive LPL protein. However, lack of protein production has also 
been reported, and because LPL might have receptor functions 
that do not req

LPL might have receptor functions 
that do not require catalytic function, patients with these defects 
may have a more severe phenotype.
Although genetic LPL deficiency has been reported to mani­
fest in adulthood, most cases of severe hyperchylomicronemia 
in adulthood are associated with partial LPL deficiency or other 
causes. In adults, the most important of these causes are type 2 
diabetes mellitus and obesity, because insulin resistance is associ­
ated with defective clearance of lipopro

s associ­
ated with defective clearance of lipoproteins. Postprandial lipemia 
is a prominent feature of diabetes. A thorough history of triglycer­
ide-raising medications should be taken (see the Secondary Causes 
of Hypertriglyceridemia section).
Regulation of LPL is complicated, and defects in its actions are 
associated with genetic or acquired abnormalities that are exclusive 
of genetic defects in the LPL molecule. Defective apoCII, the obli­
gate cofactor for LPL, leads to deficient LPL a

i­
gate cofactor for LPL, leads to deficient LPL activity. Two molecu­
lar defects initially found in mice occasionally cause severe human 
hypertriglyceridemia. GPIHBP1 (discussed earlier) is a molecule 
expressed by endothelial cells whose deficiency leads to defective 
association of LPL with its binding site on the capillary lumen and 
defective intravascular lipolysis. In one report, 20% of patients 
with FCS had GPIHBP1 mutations.99 Lipase maturation factor 1 
(LMF1) is an intracellular pr

maturation factor 1 
(LMF1) is an intracellular protein required for correct intracellular 
folding and activation of LPL.100 Mutations in LPL, GPIHBP1, 
and LMF1 all reduce postheparin LPL activity; mutated apoCII 
does not, but serum from these patients fails to maximally activate 
LPL. An additional genetic mutation associated with FCS involves 
glucokinase regulatory protein, which when dysfunctional allows 
excess de novo synthesis of fatty acids within the liver.101
Autoimmune conditions c

acids within the liver.101
Autoimmune conditions can be associated with defective tri­
glyceride catabolism due to inhibition of LPL, apoCII, or heparin. 
Antibodies against heparin are thought to prevent normal LPL 
association with the endothelial surface. Antibodies against GPI­
HBP1 may be the most common causes of autoimmune chylo­
micronemia, as the tertiary structure of this protein appears to 
make it especially immunogenic.102 Recognition of an autoim­
mune disorder such as concomitant 

n of an autoim­
mune disorder such as concomitant presence of systemic lupus 
or rheumatoid arthritis is especially important because treatment 
with glucocorticoids, avoided in other hyperchylomicronemia 
syndromes, is often indicated. In addition, patients with vascu­
lar disease or generalized intravascular reactions to transfusions or 
chemotherapy can occasionally develop defects in LPL. Transient 
episodes of fasting hyperchylomicronemia have been attributed 
to viral infections and to exc

ve been attributed 
to viral infections and to excessive fat/calorie intake after fasting. 
Postprandial Hyperlipidemia
Although plasma lipid levels are usually measured after an over­
night fast, chylomicron remnants are associated with vascular dis­
ease in several animal models and with genetic or dietary causes of 
hyperlipidemia. This has led to a widely accepted hypothesis that 
remnant lipoproteins are an overlooked cause of human vascular 
disease. Postprandial lipidemia, measured as tri

disease. Postprandial lipidemia, measured as triglyceride increase, 
is associated with greater risk of atherosclerotic cardiovascular 
disease.103 However, postprandial triglyceride elevations are also 
correlated with fasting triglycerides and reduced HDL, so the use 
of postprandial measurements in clinical practice is not currently 
recommended. Postprandial lipemia is a prominent feature of 
diabetes104 but does not impair fasting glucose or impair glucose 
tolerance.105 
Diagnostic Evalu

or impair glucose 
Diagnostic Evaluation of Severe 
Assessment of underlying medical conditions and consideration 
of age at onset are required. If possible, biochemical and genetic 
evaluation of LPL and its regulating genes will refine the diagno­
sis. Conditions that cause fasting hypertriglyceridemia (discussed 
later) can lead to severe hypertriglyceridemia when exacerbated 
by diet, drugs, or other conditions such as diabetes or pregnancy. 
Genetic LPL d

ions such as diabetes or pregnancy. 
Genetic LPL deficiency is diagnosed by the clinical setting and 
either biochemical deficiency of LPL activity in postheparin blood 
or a genetic defect in LPL. LPL deficiency is more typically associ­
ated with younger age at onset, especially onset in childhood. A 
family history of low HDL is the most common lipid abnormality 
in heterozygous carriers. LPL variants are also a determinant of 
HDL levels within the general population. A family history of 
Fr

in the general population. A family history of 
French Canadian ancestry is suggestive.
More than 100 mutations of the LPL gene have been described, 
and the diagnosis can be made through commercial DNA sequenc­
ing. Postheparin plasma can be collected for measurements of LPL 
activity in unusual presentations, but heparin administration is 
contraindicated in the setting of acute pancreatitis. This approach 
should be pursued in consultation with a research laboratory, because 
LPL activity is 

h a research laboratory, because 
LPL activity is not measured in clinical laboratories. Deficiency of 
1601
A
B
C
D
E
F
G
• Fig. 41.17  Physical examination findings associated with hyperlipidemia. (A) Xanthelasma. (B) Lipemia 
retinalis. (C) Achilles tendon xanthomas. Notice the marked thickening of the tendons. (D) Tendon xan­
thomas. (E) Tuberous xanthomas. (F) Palmar xanthomas. (G) Eruptive xanthomas. (A and B, courtesy Dr. 
Mark Dresner and Hospita

. (A and B, courtesy Dr. 
Mark Dresner and Hospital Practice [May 1990, p. 15]; C through F, courtesy Dr. Tom Bersot; G, courtesy 
Dr. Alan Chait.)
1602
apoCII, the LPL activator, and inhibitors of LPL such as antibod­
ies can be detected in research assays by mixing the patient’s serum 
with a standard human source of LPL and then assessing activity. 
Diagnosis in these rare cases can improve clinical management.106
Moderate Fasting H

improve clinical management.106
Moderate Fasting Hypertriglyceridemia Due to 
Elevated VLDL
Triglyceride levels of 150 to 500 mg/dL (1.7–5.6 mmol/L) are con­
sidered to be abnormal. These more moderately increased levels may 
confer risk of pancreatitis in addition to cardiovascular disease. A 
study from Copenhagen of 116,550 individuals identified nonfast­
ing triglycerides exceeding 177 mg/dL (2.0 mmol/L) as associated 
with a risk of acute pancreatitis that was greater than the risk for 
myo

ncreatitis that was greater than the risk for 
myocardial infarction.107 Genome-wide association studies108 and 
the observation that reduced triglycerides are associated with both 
lower atherosclerotic disease and genetic mutations in apoCIII,57 a 
protein that might inhibit LPL actions and reduce uptake of triglyc­
eride-rich lipoproteins by the liver, suggest an atherogenic role for 
triglycerides. Moreover, heterozygous LPL mutations also increase 
atherosclerosis risk,109 even though hyper

crease 
atherosclerosis risk,109 even though hypertriglyceridemia is often 
manifested only when other conditions are present.
Several different clinical conditions lead to fasting hypertri­
glyceridemia. Familial combined hyperlipidemia is associated 
with increased apoB production, and at different times and in dif­
ferent family members, hypertriglyceridemia (increased VLDL), 
increased cholesterol (LDL), or both occur. This disorder is associ­
ated with increased risk of cardiovascular disea

­
ated with increased risk of cardiovascular disease,110 but its specific 
role is clouded by its presence in patients with other risk factors 
associated with metabolic syndrome. The concomitant insulin 
resistance, obesity, and/or overt diabetes in many hypertriglyceri­
demic patients make it difficult to isolate one specific cause of this 
metabolic disturbance. Although families with isolated triglycer­
ide elevations and no vascular disease have been described, in the 
presence of metabolic

have been described, in the 
presence of metabolic syndrome, triglyceride elevations probably 
predispose to vascular disease through unclear mechanisms.
Some cases of isolated hypertriglyceridemia have been associ­
ated with hepatic overproduction of bile acids in the setting of 
impaired intestinal absorption of bile acids, analogous to the 
hypertriglyceridemia associated with use of bile acid–binding res­
ins. There is uncertainty as to whether the large, triglyceride-rich 
VLDLs in this set

er the large, triglyceride-rich 
VLDLs in this setting are atherogenic.
A homozygous mutation in apoE, apoE2/E2, leading to defec­
tive clearance of chylomicron and small VLDL remnants (see ear­
lier discussion) underlies dysbetalipoproteinemia. These patients, 
with a prevalence of approximately 1 of every 10,000 people in 
the general population, present with elevated triglyceride and 
cholesterol due to defective clearance of remnant lipoproteins. 
Patients with dysbetalipoproteinemia sometim

ins. 
Patients with dysbetalipoproteinemia sometimes have tuberous 
and palmar xanthomas and a propensity to peripheral vascular 
disease (see later discussion).
Genetic hypoalphalipoproteinemia syndromes are invari­
ably associated with moderate hypertriglyceridemia. These syn­
dromes include LCAT deficiency, Tangier disease, and apoAI 
Milano variant and are diagnosed as discussed later (see the 
Lecithin:Cholesterol Acyltransferase Deficiency, ABCA1 Defi­
ciency, and Apolipoprotein AI Mutatio

 ABCA1 Defi­
ciency, and Apolipoprotein AI Mutations sections). A reduced 
apoCII reservoir on HDL may be responsible. 
Secondary Causes of Hypertriglyceridemia
Diabetes mellitus is the most prominent cause of hypertriglyceri­
demia; in the Action to Control Cardiovascular Risk in Diabetes 
(ACCORD) trial, the average triglyceride was 162 mg/dL (1.8 
mmol/L),111 above the healthy ideal value. Insulin reduces fatty 
acid flux from adipose tissue, reduces liver apoB production, 

m adipose tissue, reduces liver apoB production, 
inhibits de novo triglyceride synthesis, and optimizes LPL pro­
duction. Kinetic studies in humans show that both increased 
triglyceride secretion and reduced clearance of triglyceride from 
the bloodstream often occur in concert in patients with diabe­
tes.112 Diabetes is also associated with increased postprandial 
lipemia. The most common diabetic dyslipidemia is moderate 
hypertriglyceridemia and low HDL. Reduced HDL results from 
greater ex

 and low HDL. Reduced HDL results from 
greater exchange of VLDL triglyceride for HDL cholesteryl ester, 
hydrolysis of the triglyceride-rich HDL by hepatic lipase, and 
more rapid clearance of the smaller HDL from the circulation. 
Defective lipolysis might also reduce the amount of cholesterol 
contributed to HDL from triglyceride-rich lipoproteins. CETP 
also enhances transfer of triglyceride to LDL, allowing these lipo­
proteins to be converted to smaller, denser forms that may be 
more athe

ed to smaller, denser forms that may be 
more atherogenic. This lipoprotein phenotype is also commonly 
referred to as diabetic dyslipidemia, although it is also found in 
nonhyperglycemic patients with metabolic syndrome. Although 
patients with diabetes do not have increased LDL compared with 
age- and sex-matched controls, improved diabetes control can be 
associated with reduced LDL. As stated earlier, diabetes is asso­
ciated with severe fasting chylomicronemia. Many such patients 
also hav

ting chylomicronemia. Many such patients 
also have an underlying dyslipidemia (e.g., due to heterozygous 
LPL deficiency), but others do not have a defined lipid disorder.
Patients with type 1 diabetes mellitus in poor control develop 
hypertriglyceridemia. However, because insulin stimulates HDL 
production, patients in good control sometimes develop high 
levels of HDL. Limited data suggest insulin-induced increases in 
HDL do not prevent cardiovascular disease. 
Renal failure i

iovascular disease. 
Renal failure is associated with hypertriglyceridemia and low HDL 
levels. The reasons for this are not clear but may reflect underlying 
insulin resistance and defects in lipolysis of plasma triglycerides. 
Hypertriglyceridemia in nephrotic syndrome has been linked to 
increased circulating levels of the LPL inhibitor angiopoietin-like 
protein 4.113 
Drugs
Diabetes, obesity, and renal disease are common causes of fasting 
hypertriglyceridemia, but various dru

 of fasting 
hypertriglyceridemia, but various drugs can also elevate triglyc­
erides. The most common drugs associated with hypertriglyceri­
demia are estrogens, thiazides, beta blockers, protease inhibitors, 
glucocorticoids, immunosuppressives, retinoids (isotretinoin, 
Accutane), bile acid–binding resins, and newer antipsychotic 
medications.
Oral estrogen therapy increases plasma triglyceride levels as a 
result of greater liver production of VLDL, but combined estro­
gen-progestin therapy 

f VLDL, but combined estro­
gen-progestin therapy does not raise triglycerides and is sometimes 
associated with reduced LDL.114 In the setting of an underlying 
hypertriglyceridemia, severe hyperchylomicronemia and pancreati­
tis can occur in patients taking oral estrogen alone or estrogen-con­
taining birth control pills, or during oocyte induction for fertility. 
For this reason, triglyceride levels should be measured in women 
before estrogen therapy or estrogen-inducing therapy is initiated

 therapy or estrogen-inducing therapy is initiated. 
Transdermal estrogen administration, which does not lead to high 
liver exposure, does not increase triglycerides.115 Tamoxifen, a 
selective estrogen-receptor modulator, can cause severe hypertri­
glyceridemia and pancreatitis,116 but raloxifene, another selective 
estrogen-receptor modulator, does not raise triglycerides.117 
1603
Diet and Alcohol
Diets can lead to marked changes in plasma triglyceri

ts can lead to marked changes in plasma triglyceride levels. 
Most lipoprotein profiles use fasting blood to avoid the postpran­
dial increase in triglycerides that represent both dietary fat and 
hepatic de novo triglyceride production. Liver triglyceride pro­
duction is especially robust after ingestion of simple sugars, such 
as those found in sweetened foods (especially beverages including 
high fructose corn syrup) and other carbohydrates (bread, pasta, 
rice, and potatoes). Excessive intak

read, pasta, 
rice, and potatoes). Excessive intake of simple carbohydrates usu­
ally leads to moderate hypertriglyceridemia but can also exacerbate 
underlying genetic hypertriglyceridemias. Fat intake, especially 
in the setting of triglyceride levels greater than 500 mg/dL (5.6 
mmol/L), can cause severe hyperchylomicronemia.
Alcohol is a major clinical cause of hypertriglyceridemia. Sen­
sitivity to the triglyceride-raising effects of alcohol is variable, 
but elimination of alcohol from the

 is variable, 
but elimination of alcohol from the diet of hypertriglyceridemic 
patients is often curative. Alcohol has many effects on lipid metabo­
lism, including inducing de novo fatty acid synthesis and inhibit­
ing fatty acid oxidation in the liver. A common clinical conundrum 
is deciding whether hypertriglyceridemia alone is responsible for 
pancreatitis in an alcohol-using patient. Patients with dysbetalipo­
proteinemia are particularly sensitive to the effects of alcohol con­
sumption

 sensitive to the effects of alcohol con­
sumption because of alcohol-induced overproduction of VLDL 
and subsequent production of remnant particles in the setting of 
impaired remnant clearance. Because alcohol also raises HDL, the 
presence of elevated triglycerides without reduced HDL is a clinical 
clue that an alcohol effect may be contributing to the lipid disorder. 
Diagnostic Evaluation of Moderate 
A search for associated disorders, review of medication use, and del

iated disorders, review of medication use, and delin­
eation of dietary choices are appropriate. If both triglycerides and cho­
lesterol are elevated, a search for the underlying lipoprotein disorder 
is sometimes useful. By ultracentrifugation, dysbetalipoproteinemia 
due to the presence of cholesterol-enriched VLDL can be differenti­
ated from that of familial combined hyperlipidemia: The usual ratio 
of VLDL triglyceride to cholesterol of approximately 5 is reduced to 3 
or less in dysbetalip

oximately 5 is reduced to 3 
or less in dysbetalipoproteinemia. ApoE genotyping will determine if 
the patient has the apoE2/E2 genotype, which can be associated with 
aggressive vascular disease. Cholesterol-enriched VLDL is also found in 
patients with hypothyroidism, renal failure, or hepatic lipase deficiency. 
Hypercholesterolemia Without 
Because all lipoproteins contain cholesterol, dramatic increases 
in triglycerides will invariably also lead to elevated blood chole

 will invariably also lead to elevated blood choles­
terol values. However, the ratio of triglyceride to cholesterol will 
be greater than 5. Disorders associated with primarily increased 
cholesterol are discussed in this section. The clinical presentation 
of hypercholesterolemia is limited. Although patients with severe 
disease occasionally present with cosmetic concerns or orthopedic 
issues associated with tendon xanthomas, hypercholesterolemia is 
usually clinically occult and uncovered b

emia is 
usually clinically occult and uncovered by blood testing during 
routine assessment or in the setting of vascular disease.
Polygenic Hypercholesterolemia
Most patients with elevated LDL cholesterol do not have FH, and 
even cholesterol levels above 300 mg/dL (7.8 mmol/L) are not 
usually associated with xanthomas or defects in the LDL receptor. 
If, as is conventional for other laboratory values, hypercholesterol­
emia is defined as a cholesterol value that exceeds the 95th percen­
tile

olesterol value that exceeds the 95th percen­
tile for the population, only 1 in 25 of these patients should have 
FH. Although diet and lifestyle influence LDL, the genetic and 
environmental factors associated with most elevated LDL levels 
are unknown, and therefore this type of hypercholesterolemia is 
referred to as polygenic. Nonetheless, the increased cholesterol is 
associated with a higher risk of coronary artery disease.
Increased LDL levels can be a result of defective LDL clearance 

evels can be a result of defective LDL clearance 
with normal LDL receptors, a more subtle regulation of the recep­
tor, or increased LDL production. This last type occurs in the setting 
of familial combined hyperlipoproteinemia (see later discussion), 
which can manifest as primary elevations of LDL. Greater LDL 
production due to greater absorption of gut cholesterol, abnormali­
ties in the regulation of lipid-regulated nuclear transcription factors, 
or gain-of-function abnormalities in chol

actors, 
or gain-of-function abnormalities in cholesterol and apoB lipopro­
tein assembly pathways are all possible but poorly defined. 
Genetic FH
The centrality of the LDL receptor to the understanding of choles­
terol metabolism was uncovered through investigations by Brown 
and Goldstein (see the LDL Receptor section) into the cause of het­
erozygous FH. This relatively common cause of severe hypercholes­
terolemia results from one of many defects in production of the LDL 
receptor leading t

fects in production of the LDL 
receptor leading to impaired function or production of the LDL 
receptor protein. Patients with FH (most with heterozygous forms 
of the disease) have cholesterol levels that exceed 300 mg/dL (7.8 
mmol/L). Homozygous presentations of the disease include choles­
terol levels that are approximately twice this value. In one health 
care system in the United States, the heterozygous form was detected 
with a prevalence of 1 in 256 in unselected participants and 1 in 

e of 1 in 256 in unselected participants and 1 in 118 
in cardiac catheterization laboratory participants.118 FH is a major 
cause of early-onset coronary artery disease. In addition, homozy­
gous and some other severe forms of LDL receptor deficiency are 
associated with aortic valve calcification and stenosis. Humans can 
have LDL receptor defects without hypercholesterolemia, but these 
individuals are still at increased risk of cardiovascular disease.119
Patients with heterozygous FH have ch

 disease.119
Patients with heterozygous FH have cholesterol deposition 
on tendons leading to tendon xanthomas like those seen in Fig. 
41.17C and D. They are most common on the Achilles tendon. 
On inspection, a loss of the usual bow-like shape of the tendon 
occurs or a bump or generalized thickening of the tendon is appar­
ent. The irregularity of the tendon can also be detected by palpa­
tion. This physical finding is sometimes similar to scarring that 
results from tendon rupture. If the te

rring that 
results from tendon rupture. If the tendon is abnormal, a history of 
athletic injury should be sought. Xanthomas also occur on exten­
sor tendons of the hands, but less frequently, and are best appreci­
ated on the knuckles of a clenched fist. Xanthomas of the Achilles 
tendon can cause recurrent episodes of Achilles tendinitis. Some 
xanthomas are subtle and are apparent only as a thickening of the 
tendon or a small bump at the insertion of the tendon into mus­
cle. Patients with 

ertion of the tendon into mus­
cle. Patients with FH also have xanthelasmas (see Fig. 41.17A) and 
premature corneal arcus (i.e., in persons younger than 40 years), 
but these findings can also occur in patients without FH. Many 
affected subjects have no physical findings. Premature coronary 
artery disease is common but variable. Some FH patients develop 
coronary disease in the third or fourth decade of life, especially 
if they also have reduced HDL or associated risk due to cigarette 
smoki

ced HDL or associated risk due to cigarette 
smoking. Even before the introduction of statin therapy, some 
FH patients (especially women) never develop clinical vascular 
1604
disease. Incomplete penetrance or protective factors perhaps due 
to lifestyle may contribute to their normal life span.
Homozygosity for FH is not common but is likely to occur more 
frequently than previously appreciated.120 These subjects come to 
clinical at

ppreciated.120 These subjects come to 
clinical attention early in life because of the appearance of tendon and 
planar xanthomas, as well as tuberous xanthomas (see Fig. 41.17E), 
marked hypercholesterolemia apparent at birth, premature coronary 
disease, or aortic valve disease. Typical plasma cholesterol concentra­
tions range from 600 mg/dL (15.5 mmol/L) to 1000 mg/dL (25.9 
mmol/L), and LDL concentrations range from 500 to 950 mg/dL 
(13.0 to 24.6 mmol/L). Symptomatic coronary disease can o

o 24.6 mmol/L). Symptomatic coronary disease can occur 
before age 10 years. If not treated, these homozygous persons usually 
die from myocardial infarction by age 20 years. Aortic valve disease in 
homozygotes can be valvular or supravalvular. The diagnosis of homo­
zygous FH should be suspected in any child with extremely high 
plasma cholesterol (typically >500 mg/dL [13.0 mmol/L]) or the xan­
thomas characteristic of FH. Both parents are obligate heterozygotes 
and should manifest the pheno

igate heterozygotes 
and should manifest the phenotype of heterozygous FH. These chil­
dren are treated with repetitive LDL apheresis or liver transplantation. 
Familial Defective Apolipoprotein B100
Population-based genotyping shows that defects in LDL bind­
ing to normal LDL receptors leads to elevated LDL cholesterol 
by 60 to 70 mg/dL (1.5–1.8 mmol/L).121 Familial defective 
apoB100 has a phenotype that is indistinguishable from that of 
FH, including increased susceptibility to coronary hea

including increased susceptibility to coronary heart disease. 
The substitution of glutamine for arginine at amino acid 3500, 
which reduces LDL binding to the LDL receptor, accounts for 
most cases of familial defective apoB100, although other defects 
have also been reported. Often the LDL elevations are less severe, 
such as with LDL below 200 mg/dL (5.2 mmol/L), a reflection of 
a partial defect in receptor binding attributed to the mutation or 
the continued ability of apoE to mediate lipop

or 
the continued ability of apoE to mediate lipoprotein uptake. 
Rare Mutations Associated With Elevated LDL 
Levels
Several rare, isolated causes of hypercholesterolemia have been 
reported. Mutations in LDLRAP1, the gene encoding a putative 
adaptor protein (ARH) required for internalization of LDL bound 
by the LDL receptor on the surface of hepatocytes, cause autoso­
mal recessive hypercholesterolemia. Mutations in ARH associated 
with autosomal recessive hypercholesterolemia have been repo

omal recessive hypercholesterolemia have been reported 
mostly in Italians (Sardinia) and Lebanese.
Autosomal dominant hypercholesterolemia caused by a muta­
tion in the gene encoding cholesterol 7α-hydroxylase has been 
reported.122 The hypercholesterolemia is caused by defective cho­
lesterol conversion to cholic acid.
Mutation of PCSK9 (see the LDL Receptor section) leads to 
alterations in LDL receptor expression because this enzyme mod­
ulates intracellular LDL degradation.123 Gain-of-funct

es intracellular LDL degradation.123 Gain-of-function muta­
tions reduce LDL receptor numbers and lead to defective LDL 
clearance by the liver. Inhibitors of this protein are an effective 
therapy for hypercholesterolemia.124 As noted earlier, loss-of-
function mutations in PSCK9 are associated with low total cho­
lesterol and LDL, and decreased risk of coronary artery disease. 
Elevated Plasma Lp(a)
Apo(a), a protein of unknown function that shares high 
sequence homology with plasminogen, ass

ares high 
sequence homology with plasminogen, associates with apoB to 
produce an LDL called Lp(a). Elevated levels of Lp(a) increase 
the risk of cardiovascular disease and calcific aortic valvular ste­
nosis125 and are found in approximately 20% of patients.126 
Patients with high Lp(a) are more likely to clot bypass grafts 
and stents. Unlike LDL, which confers risk as a continuous 
function of plasma level, risk is found only in individuals with 
the highest levels of Lp(a).
Plasma Lp(a) le

with 
Plasma Lp(a) levels are largely determined genetically. Lp(a) 
particles contain a variable number of a protein repeat known 
as a Kringle. Smaller Lp(a) particles, with fewer Kringle repeats, 
are usually produced at higher levels. For this reason, some sug­
gest Lp(a) size, rather than plasma level, that confers vascular 
risk. Renal failure leads to elevations of Lp(a). Niacin treatment, 
but not statin treatment, reduces Lp(a), whereas PCSK9 inhibi­
tors de

ment, reduces Lp(a), whereas PCSK9 inhibi­
tors decrease Lp(a) by increasing its plasma clearance.127 Specific 
therapies to reduce Lp(a) using antisense and silencing RNA 
approaches are in development. 
Lipoprotein(X)
Obstructive liver disease sometimes leads to a marked increase in 
plasma cholesterol. In part, this is the result of increased LDL, pre­
sumably due to a defect in LDL receptors. In addition, free choles­
terol circulates in association with albumin, a particle referred to 
as L

ciation with albumin, a particle referred to 
as Lp(X). This is caused by a deficiency in the cholesterol esterify­
ing enzyme, LCAT. The clinical setting suggests the diagnosis. In 
addition, an abnormal ratio of free to total cholesterol (or choles­
teryl ester) can be determined by the laboratory. The relationship 
of Lp(X) to vascular disease is unclear. 
In this rare disorder, dietary sitosterol and other plant sterols, 
which are not normally absorbed in significant quantiti

 are not normally absorbed in significant quantities in the 
intestine, are absorbed in large amounts, resulting in their accu­
mulation in the plasma and in peripheral tissues and causing pre­
mature atherosclerosis.128 The molecular cause is a mutation in 
the genes encoding ABCG8 and ABCG5, which are responsible 
for resecretion of absorbed plant sterols. Patients develop tendon 
xanthomas in childhood and have normal to high plasma LDL; 
the differential diagnosis includes FH and cerebrotend

differential diagnosis includes FH and cerebrotendinous xan­
thomatosis. The diagnosis can be confirmed by gas-liquid chroma­
tography of plasma lipids, which demonstrates the high levels of 
plant sterols. Therapy consists of restriction of dietary plant sterols 
and treatment with ezetimibe (see later discussion), which inhibits 
absorption of dietary sterols.129 
Cerebrotendinous Xanthomatosis
Cerebrotendinous xanthomatosis is a rare disorder of sterol 
metabolism associated with neurologic d

of sterol 
metabolism associated with neurologic disease, tendon xan­
thomas, and cataracts in young adults. Neurologic manifesta­
tions include cerebellar ataxia, dementia, spinal cord paresis, 
and subnormal intelligence. Premature atherosclerosis is com­
mon. Osteoporosis has been reported and is presumably caused 
by alterations in vitamin D metabolism. The disorder results 
from mutations that cause deficiencies of 27-hydroxylase, a key 
enzyme in cholesterol oxidation and bile acid synthes

yme in cholesterol oxidation and bile acid synthesis, leading 
to high plasma levels of cholesterol and cholestanol, with sub­
sequent accumulation of these sterols in tendons and in tissues 
of the nervous system. Chenodeoxycholic acid is indicated for 
1605
Hypothyroidism and Elevated Cholesterol
All patients with significant hyperlipidemia should be screened 
for hypothyroidism, because thyroid hormone deficiency causes 
hypercholesterolem

roid hormone deficiency causes 
hypercholesterolemia, and low levels of thyroid hormone pre­
dispose to statin-induced myositis.130 Although hypothyroidism 
usually increases LDL, it can also be associated with high plasma 
triglycerides. Levels of HDL are usually unchanged or slightly 
lower in hypothyroidism and may be reduced in hyperthyroidism. 
Subclinical hypothyroidism is a cause of hypercholesterolemia 
that sometimes responds to thyroid hormone replacement.131 
Diagnostic Evaluation of 

hormone replacement.131 
Diagnostic Evaluation of Isolated 
All adults and perhaps children should have a cholesterol level 
assessed at least every 5 years. In adults, cholesterol levels higher 
than 300 mg/dL (7.8 mmol/L) and LDL greater than 190 mg/dL 
(4.9 mmol/L) suggest FH. Heterozygous FH should be suspected 
in any person with premature atherosclerotic cardiovascular dis­
ease. The diagnosis of heterozygous FH can be confirmed clini­
cally by the presence of tendon x

confirmed clini­
cally by the presence of tendon xanthomas and a family history 
of premature coronary disease. The diagnosis of FH is primar­
ily clinical, but genetic testing is available and, although costly, 
may be helpful for assessing risk in family members and in genetic 
counseling. This is important because many patients with FH will 
not initially present with elevated LDL but have increased risk of 
cardiovascular disease, presumably due to a lifelong LDL that is 
higher than desirab

due to a lifelong LDL that is 
higher than desirable. 
Increased HDL
Occasionally, patients with hypercholesterolemia primarily have 
elevations of HDL with normal LDL. Usually, this pattern is 
found in families with little cardiovascular disease. However, 
genetic association studies fail to link genetic factors that elevate 
HDL with reduced vascular disease,132 and no pharmacologic 
interventions that raise HDL in the statin era appear to provide 
benefit.
There are subgroups of patients wit

vide 
benefit.
There are subgroups of patients with increased HDL and 
whose HDL does not confer a protection from cardiovascular 
disease. Isolation of HDL from such patients with cardiovascular 
disease and high levels of HDL led to the observation that these 
HDLs do not have anti-inflammatory properties.133 Atherogenic 
HDL has been detected in patients with systemic lupus erythema­
tosus.134 Therefore, there are clinical circumstances in which HDL 
may be dysfunctional (see the Reverse Chol

ch HDL 
may be dysfunctional (see the Reverse Cholesterol Transport and 
Dysfunctional HDL section).
Genetic Disorders Causing Increased HDL
CETP deficiency is a hereditary syndrome in which plasma HDL 
levels are increased because of diminished activity of plasma CETP. 
The disorder is not uncommon in the Japanese population. Its fea­
tures include marked elevations of plasma HDL in homozygotes 
(usually >100 mg/dL [2.6 mmol/L]). Despite the elevated HDL 
levels, the effect on coronary disease 

vated HDL 
levels, the effect on coronary disease risk of CETP gene muta­
tions is unclear. Heterozygotes have moderately elevated HDL. 
The lower activity of CETP results in diminished transfer of cho­
lesteryl esters from HDL to apoB-containing lipoproteins. As a 
result, more cholesteryl esters are found in HDL, and the ratio 
of total cholesterol to HDL is markedly reduced. CETP is most 
active in the setting of hypertriglyceridemia, so, in addition to 
reducing HDL, it provides a mechanism 

ddition to 
reducing HDL, it provides a mechanism for reduction of triglyc­
eride through HDL metabolism. Perhaps for this reason, studies 
in mice, which normally do not have significant plasma CETP 
activity, have shown reduced apoB-containing lipoproteins and 
reduced atherosclerosis with addition of a CETP transgene. As 
noted, CETP inhibitors do not appear to have benefit that can be 
attributed to increased HDL.83
Several other pharmacologic agents are associated with 
increased plasma HDL

c agents are associated with 
increased plasma HDL. The antiatherogenic capacity of HDL 
produced in these circumstances is unknown. HDL induction 
occurs with the use of oral estrogens, alcohol intake, phenytoin 
(Dilantin), phenobarbital, and insulin therapy in some patients 
with type 1 diabetes mellitus.
Endothelial lipase deficiency is caused by defects in the 
third member of the lipase gene family—endothelial lipase. As 
described earlier, loss-of-function mutations in this lipase are ass

 loss-of-function mutations in this lipase are asso­
ciated with increased HDL levels but not reduced risk of coronary 
heart disease. 
Elevated Triglycerides and Cholesterol
Elevated triglyceride and cholesterol levels can be caused by 
increased VLDL and LDL, combined hyperlipidemia, or the pres­
ence of increased circulating remnant lipoproteins (dysbetalipo­
proteinemia). Methods to diagnose dysbetalipoproteinemia are 
usually not fruitful unless patients present with premature vascu­
lar di

less patients present with premature vascu­
lar disease or palmar xanthomas.
Combined Hyperlipidemia
Combined hyperlipidemia is a common disorder associated 
with elevations of plasma cholesterol and triglyceride levels and 
increased susceptibility to coronary disease. Lipoprotein isolation 
reveals increased LDL and VLDL. Kinetic analysis has associated 
this pattern primarily with overproduction, rather than defective 
clearance, of apoB lipoproteins. When it occurs within families, it 
is te

roteins. When it occurs within families, it 
is termed familial combined hyperlipidemia. These individuals, who 
do not necessarily have other causes for lipid abnormalities, char­
acteristically come from families with various hyperlipidemias that 
include increased isolated triglycerides or increased isolated LDL. 
Moreover, the abnormal lipoprotein pattern (increased triglycer­
ides, cholesterol, or both) can vary over time in an individual.
Regulation of apoB production involves several step

egulation of apoB production involves several steps (see the 
Apolipoprotein B section), and understanding this physiology 
explains some of the association of combined hyperlipidemia 
with other diseases. Increased fatty acid return to the liver and 
reduced insulin action prevent degradation of newly synthesized 
apoB. Therefore, it is not surprising that both metabolic syndrome 
and type 2 diabetes mellitus are commonly found with combined 
hyperlipidemias. Although the genetics of this disor

perlipidemias. Although the genetics of this disorder have been 
investigated, the coexistence of this lipoprotein pattern with 
insulin resistance and obesity syndromes has been a confounder. 
Therefore, although alterations in several genes associated with 
lipid metabolism, such as LPL and apoCIII, have been observed, 
a firm genetic marker is not currently available. 
Familial Dysbetalipoproteinemia
Familial dysbetalipoproteinemia is an uncommon disorder of lipo­
protein metabolism that is c

mon disorder of lipo­
protein metabolism that is characterized by moderate to severe 
hypertriglyceridemia and hypercholesterolemia caused by the 
accumulation of cholesterol-rich remnant particles in the plasma. 
1606
Premature peripheral vascular disease and coronary artery disease 
are common. The cause is mutations in the apoE gene that result 
in defective binding of apoE to lipoprotein receptors. The disor­
der is associated with

otein receptors. The disor­
der is associated with the apoE2 isoform and in most instances is 
inherited as an autosomal recessive trait. Because the phenotypic 
expression of the disorder is limited to approximately 5% of the 
patients with the apoE2/E2 phenotype, other genetic or environ­
mental factors must also be operative. The hyperlipidemia is caused 
by a defect in clearance of remnant lipoproteins whose liver uptake 
requires apoE interaction with the LDL receptor, LRP1, and HSPG 
(see 

ction with the LDL receptor, LRP1, and HSPG 
(see the Apolipoprotein E section). The remnants that accumulate 
have lost much of their triglyceride through LPL-mediated triglyc­
eride hydrolysis and therefore are cholesterol rich. The predominant 
remnant particles, termed β-VLDL, can be identified by abnormal 
migration on gel electrophoresis or by abnormal lipid content.
Dysbetalipoproteinemia is usually diagnosed in adults and 
rarely detected in persons younger than 20 years. The disorder is

 in persons younger than 20 years. The disorder is 
more common in men than in women. Plasma triglyceride and 
cholesterol levels typically range from 300 to 400 mg/dL (3.4–
45.2 mmol/L). Concentrations of HDL are normal. Xanthomas 
are present in more than half of affected subjects. Palmar xantho­
mas, which are planar xanthomas in the palmar creases (see Fig. 
41.17F), are pathognomonic for this disorder. Tuberous or tubo­
eruptive xanthomas (see Fig. 41.17E) are also common but are less 
spec

ee Fig. 41.17E) are also common but are less 
specific for this disorder. Tendon xanthomas and xanthelasma may 
occur. Unlike FH, in which peripheral vascular disease is uncom­
mon, premature peripheral vascular disease occurs in addition 
to premature coronary artery disease in patients with dysbetali­
poproteinemia. Coexisting metabolic conditions that exacerbate 
the phenotype of dysbetalipoproteinemia, such as obesity, alcohol 
consumption, diabetes mellitus, and hypothyroidism, are often 
p

iabetes mellitus, and hypothyroidism, are often 
present.
In addition to homozygosity for the apoE2 isoform, some 
mutations in the apoE gene are known to lead to the dysbetalipo­
proteinemia phenotype in an autosomal dominant fashion. The 
phenotype manifests at an early age without exacerbating factors. 
Hepatic Lipase Deficiency
Hepatic lipase deficiency is a rare disorder associated with lack of 
heparin-releasable hepatic lipase activity in the plasma. Because 
this enzyme mediates the fina

the plasma. Because 
this enzyme mediates the final step of conversion of IDL to LDL 
and is involved in chylomicron remnant clearance, its deficiency 
causes a phenotype that is similar to that found with dysbetali­
poproteinemia—namely, elevated plasma cholesterol (250–1500 
mg/dL [6.5–38.9 mmol/L]) and triglycerides (395–8200 mg/
dL[4.5–92.7 mmol/L]). Patients also have palmar and tuboerup­
tive xanthomas, premature arcus corneae, and premature coronary 
artery disease. Because hepatic lipase

e coronary 
artery disease. Because hepatic lipase also mediates HDL metabo­
lism, HDL levels are not decreased. The demonstration of hepatic 
lipase deficiency requires in vitro assays of hepatic lipase activity in 
postheparin plasma or DNA analysis to identify mutations. 
Nephrotic Syndrome
Hyperlipidemia almost always accompanies nephrotic syndrome. 
Total cholesterol, VLDL, LDL, total triglycerides, and plasma 
apoB may all be elevated. Nephrotic syndrome increases liver 
production of apoB

rotic syndrome increases liver 
production of apoB-containing lipoproteins, leading to increased 
plasma LDL, VLDL, or both. This may be a response to hypoal­
buminemia and an associated generalized increase in liver protein 
Protease Inhibitor Use in Human 
Immunodeficiency Virus Infection
Treatments for human immunodeficiency virus infection often 
cause hyperlipidemia, lipodystrophy, and insulin resistance. 
Hypertriglyceridemia is the most common lipid abnormality, 
although incr

 the most common lipid abnormality, 
although increases in LDL are also found. These effects were ini­
tially thought to result from the use of protease inhibitors, but 
other agents, such as reverse transcriptase inhibitors, can also 
cause dyslipidemia. Metabolic syndrome is not uniformly induced 
by all drugs; older agents such as ritonavir are more likely to have 
metabolic side effects. The cause of this syndrome is unclear but 
may be related to greater liver production of apoB lipoprotein

ed to greater liver production of apoB lipoproteins 
and triglycerides. 
Immunosuppressive Regimens
Patients undergoing transplantation who require several medi­
cations commonly present with hypertriglyceridemia with or 
without hypercholesterolemia. Glucocorticoids more often raise 
triglycerides, cyclosporine raises cholesterol, and rapamycin 
increases both cholesterol and triglycerides. 
Diagnostic Evaluation of Elevated Triglycerides 
and Cholesterol
Mixed elevations of triglycerides and c

holesterol
Mixed elevations of triglycerides and cholesterol are common. 
Appropriate assessment usually requires that clinicians deter­
mine whether the hyperlipidemia is primarily a genetic disorder 
or is secondary to a systemic disorder or medications. Medica­
tions are perhaps the most common cause. Table 41.4 provides 
an overview of diagnostic considerations. In terms of lipid dis­
orders, familial combined hyperlipidemia requires both medical 
history and fasting lipid panels for family 

ical 
history and fasting lipid panels for family members. This disor­
der is common and associated with premature vascular disease. 
In some situations, an evaluation for dysbetalipoproteinemia, 
a rare disorder, is useful. In other cases, empiric treatment is 
reasonable (see the Combination Therapy for Other Hyperlip­
idemias section). 
Hypocholesterolemia
Secondary causes that can lead to very low levels of apoB-contain­
ing lipoproteins and low cholesterol levels include malabsorption, 
sep

low cholesterol levels include malabsorption, 
sepsis, liver failure, cachexia syndromes, and malnutrition. There 
are also genetic conditions that cause low cholesterol.
Familial Hypobetalipoproteinemia
Familial hypobetalipoproteinemia is defined as apoB and LDL 
levels below the 5th percentile. Hypobetalipoproteinemia is not 
associated with a phenotype and leads to a reduced risk for cardio­
vascular disease. Possible causes of this syndrome include muta­
tions leading to truncation of apoB a

nclude muta­
tions leading to truncation of apoB and mutations in PCSK9 
presumably leading to increased numbers of LDL receptors. 
PCSK9 loss-of-function mutations occur in as many as 2% of 
African Americans but are rare in persons of European descent. 
Other patients have a defect in angiopoietin-like protein 3.55 
Patients with hypobetalipoproteinemia, except those due to 
the angiopoietin-like protein 3 defect, have an increased risk of 
hepatic steatosis. 
1607
Chapter 41  Disorders of Lip

atic steatosis. 
1607
Abetalipoproteinemia
Abetalipoproteinemia is a rare autosomal recessive disorder caused 
by a deficiency in MTP (see the Apolipoprotein B section), which 
results in a virtual absence of apoB-containing lipoproteins in the 
plasma. Abetalipoproteinemia occurs in fewer than 1 in a million 
persons and has the same phenotype as homozygous hypobet­
alipoproteinemia, including malabsorption of fat and fat-soluble 
vitamins from the inte

ion of fat and fat-soluble 
vitamins from the intestine, which can lead to neurologic disease 
related to vitamin E deficiency. The disorder is usually detected 
in infancy because of fat malabsorption associated with marked 
decreases in plasma cholesterol and triglyceride levels. 
Chylomicron Retention Syndrome
Anderson disease, or chylomicron retention syndrome, is a rare 
condition that is phenotypically similar to abetalipoproteinemia. 
Subjects with Anderson disease cannot secrete chylomic

ects with Anderson disease cannot secrete chylomicrons from 
the intestine. Mutations in the SAR1B gene (formerly SARA2) 
have been linked to Anderson disease. This gene encodes SAR1B, 
a protein that is important in the transport of chylomicrons 
through the secretory pathway in enterocytes. 
Familial Hypoalphalipoproteinemia
Although low HDL in the general population is correlated with 
greater coronary artery disease, genetic disorders leading to very 
low plasma HDL have variable and sometim

 to very 
low plasma HDL have variable and sometimes undefined coro­
nary risk. In part, this might result from the rarity of some of 
these diseases. For this reason, the approach to the disorders is 
often unclear.
Apolipoprotein AI Mutations
Mutations in the apoAI gene can decrease HDL formation and 
result in low plasma HDL levels. ApoAI deficiency can be caused 
by point mutations in the apoAI gene or by deletions or gene 
rearrangements at the apoAI/CIII/AIV/AV gene locus. ApoAI 
deficienc

the apoAI/CIII/AIV/AV gene locus. ApoAI 
deficiency typically results in plasma HDL lower than 10 mg/dL 
(0.26 mmol/L). Some apoAI variants activate LCAT (see the next 
section) poorly. The molecular diagnosis is made by protein analy­
sis showing an altered apoAI size or by genetic sequencing. Some 
apoAI variants are associated with amyloidosis, but there are no 
defined specific clinical effects of these variants.
The apoAI Milano variant (apoAIMilano) is caused by a substi­
tution of cystein

AIMilano) is caused by a substi­
tution of cysteine for arginine at amino acid 173. This results in 
low plasma HDL levels without premature coronary heart disease. 
This HDL is hypothesized to be antiatherogenic, but administra­
tion of recombinant apoAIMilano does not appear to induce plaque 
regression in acute coronary syndrome in the setting of contem­
porary statin therapy.135 
Lecithin: Cholesterol Acyltransferase Deficiency
LCAT deficiency136 leads to low HDL as a consequence of defec­
t

y136 leads to low HDL as a consequence of defec­
tive conversion of cholesterol to cholesteryl ester. LCAT deficiency 
leads to striking corneal arcus (sometimes leading to marked visual 
impairment), normochromic anemia, and, occasionally, renal fail­
ure in young adults. Renal biopsies show characteristic foam cells. 
In a phenotypically appropriate patient, measurement of the ratio 
of free cholesterol to cholesteryl ester is diagnostic; normally, free 
cholesterol is approximately 30% of the

lly, free 
cholesterol is approximately 30% of the total, but this percentage 
increases to more than 90% in LCAT deficiency. LCAT activity 
measurements and sequencing can also be performed. Despite 
very low HDL levels, risk for coronary disease in patients with 
LCAT deficiency is not pronounced, which may reflect the low 
levels of LDL also found in many of these patients.
A rare variant of LCAT deficiency, called fish-eye disease, is also 
caused by mutations of the LCAT gene. The phenotype

aused by mutations of the LCAT gene. The phenotype is less 
severe than that seen in complete LCAT deficiency. Fish-eye dis­
ease is characterized by low plasma HDL and corneal opacities; 
anemia and renal disease do not occur. The phenotypic differences 
between LCAT deficiency and fish-eye disease have been attrib­
uted to whether mutations in the LCAT gene encode variants 
that fail to esterify cholesterol of both HDL and apoB-containing 
lipoproteins (LCAT deficiency) or of HDL only (fish-ey

proteins (LCAT deficiency) or of HDL only (fish-eye disease). 
ABCA1 Deficiency
The ABCA1 transporter is essential to complete synthesis of 
mature HDL by the liver and the small intestine. Several muta­
tions in this receptor are associated with hypoalphalipoprotein­
emia. Because the number of genetic mutations is great, assays 
often study fibroblast “unloading” of cholesterol to apoAI to show 
the defect.
Tangier disease is the most flagrant example of a defect in 
ABCA1. These patients, ori

example of a defect in 
ABCA1. These patients, originally from Tangier Island in the 
Chesapeake Bay, had hypocholesterolemia resulting from a marked 
decrease in plasma HDL, as well as low LDL and the striking 
physical finding of orange tonsils. The orange tonsils likely result 
from defective reverse cholesterol transport from macrophages of 
the reticuloendothelial system, with the color from carotenoids. 
The risk of cardiovascular disease with this syndrome is uncertain. 
Treatment of Lipi

ith this syndrome is uncertain. 
Treatment of Lipid Disorders
Evidence Supporting Treatment of Lipid 
Disorders: Cholesterol and Cardiovascular 
Disease
Despite a very significant decrease in the incidence of vascular dis­
ease, cardiovascular diseases, including coronary heart disease and 
cerebrovascular disease, remain the major causes of death in the 
United States for both men and women.137 The major risk fac­
tors are age, elevated LDL, reduced HDL, smoking, hypertension, 
insulin resistan

uced HDL, smoking, hypertension, 
insulin resistance with or without overt diabetes mellitus, and a 
family history of premature cardiovascular disease. Modifiable risk 
factors account for most of the excess cardiovascular disease risk. 
Although several new risk factors have been proposed to increase 
the accuracy of predicting risk of cardiovascular events, four con­
ditions—dyslipidemia, hypertension, cigarette smoking, and dia­
betes mellitus—account for increased cardiovascular disease ris

s—account for increased cardiovascular disease risk 
in most patients.
Statins reduce the chances of recurrent events (secondary 
prevention) in patients with previous cardiovascular events.1,138 
Meta-analyses of clinical trials demonstrate that statins reduce 
risk of initial cardiovascular events (primary prevention) in most 
groups of patients, including men and women. They also reduce 
cardiovascular events in high risk patients with diabetes, with 
hypertension, and regardless of baseline 

s, with 
hypertension, and regardless of baseline LDL cholesterol.
Evidence for the benefit of lowering triglycerides or raising 
HDL levels, or both, is not strong. Data are most significant in 
studies of the fibric acid gemfibrozil. The Helsinki Heart Study139 
and the Veterans Affairs–High-Density Lipoprotein Cholesterol 
Intervention Trial140 both used gemfibrozil, the former for pri­
mary prevention and the latter for secondary prevention, and in 
1608
SECTION VIII    Disorders of Carbohyd

and in 
1608
both studies the greatest benefit was in men with high triglycerides 
and low HDL. Studies with other fibrates, such as fenofibrate, 
have yielded equivocal results.141 In the lipid treatment arm of 
the ACCORD trial in type 2 diabetes, fenofibrate was added to 
statin therapy in patients with an average triglyceride of 162 mg/
dL (1.8 mmol/L). There was no overall benefit, although a sub­
group of patients with higher tri

 although a sub­
group of patients with higher triglycerides and reduced HDL had 
reduced cardiovascular events,142 and these observations persisted 
with extended follow-up.143
Although decreasing LDL reduces cardiovascular event rates 
by 30% to 50% in statin studies, there is still significant residual 
risk, especially in patients with established vascular disease, many of 
whom have events despite achieving LDL levels of 70 to 80 mg/dL 
(1.8–2.1 mmol/L), raising the issue of how to decrease

–2.1 mmol/L), raising the issue of how to decrease this residual 
risk. Other questions include the following: How early in life should 
therapy be started for primary prevention? Will longer duration of 
therapy lead to better outcomes? What is the benefit of lowering tri­
glycerides or raising HDL? What biomarkers are best to screen and 
monitor in at-risk patients? Can other mediators of atherosclerosis 
be manipulated? How can lifestyle changes be accomplished? 
Measurement of Plasma Lipids

es be accomplished? 
A complete plasma lipid profile (total cholesterol, calculated 
LDL, HDL, and triglycerides) should be measured in all adults 
20 years of age and older at least once every 5 years. Triglycer­
ides should be measured in all patients with pancreatitis. Ide­
ally, plasma lipids should be measured at least twice under fasting 
steady-state conditions before therapeutic decisions are made. 
Plasma lipids are usually measured after a 12-hour fast to p

ids are usually measured after a 12-hour fast to pre­
clude detection of significant elevations in atherogenic remnant 
lipoproteins. Because cholesterol is a minor component of chylo­
microns, total plasma cholesterol can be measured in either a fast­
ing or a nonfasting state. Plasma lipid measurements are usually 
reliable if done within the first 24 hours after an acute myocardial 
infarction.144
Most clinical laboratories measure plasma total cholesterol, 
total triglycerides, and HDL; the 

l cholesterol, 
total triglycerides, and HDL; the last analysis is performed after 
apoB-containing lipoproteins are removed from the plasma. For 
many years, the plasma LDL concentration has been calculated 
from these measurements using the Friedewald formula:
LDL = total cholesterol –HDL –VLDL
where VLDL is calculated as triglycerides divided by 5. This for­
mula assumes the cholesterol content of VLDL is about 20% of 
the plasma triglyceride level. It is reliable only when triglycerides 
are

level. It is reliable only when triglycerides 
are 400 mg/dL (4.5 mmol/L) or less. LDL concentrations calcu­
lated by this formula may be inaccurate in the presence of severe 
hypertriglyceridemia or when the triglyceride-to-cholesterol ratio 
of VLDL is altered as occurs in dysbetalipoproteinemia. Notably, 
the Friedewald formula may underestimate LDL levels in the low 
range that is now encountered with intensive statin and PCSK9 
therapy.
A novel formula is now being used by commercial labora

vel formula is now being used by commercial laborato­
ries. The Martin-Hopkins calculation uses an adjustable factor for 
the triglyceride-to-cholesterol ratio of VLDL based on an array 
of triglyceride and non-HDL cholesterol levels from more than 
1.3 million lipid samples in the United States.145 This approach 
is more accurate than the Friedewald formula at determining 
LDL levels in certain patients, especially those with triglycerides 
between 100 and 400 mg/dL (0.11–4.5 mmol/L).
Specializ

een 100 and 400 mg/dL (0.11–4.5 mmol/L).
Specialized laboratories can directly assay different lipoproteins 
by ultracentrifugation or nuclear magnetic resonance techniques. 
Direct measurement of LDL is also now available in many clinical 
laboratories. Value for direct measurements in clinical practice is 
uncertain, as they were not utilized in large clinical trials and are 
not incorporated into most guidelines. 
Treatment of High LDL
As noted, clinical trials clearly demonstrate the benefit

d, clinical trials clearly demonstrate the benefit of LDL reduc­
tion. Optimal risk factor management reduces clinical atheroscle­
rotic event rates. However, in the United States, the 5-year incidence 
of myocardial infarction or cardiac death after a first event remains 
high, depending on age, race, and gender and risk factor control.137
The initial evaluation for all patients consists of a history and 
physical examination, including assessment of diet, exercise, cardio­
vascular risk factor

nt of diet, exercise, cardio­
vascular risk factors, and measurement of plasma lipids. Exclusion 
of secondary causes of lipid disorders (see Table 41.4) is important. 
Obesity is an independent risk factor for cardiovascular disease, 
reflected in the waist circumference measurement that is used to 
define metabolic syndrome (see Table 41.5). Obesity aggravates 
dyslipidemia, hypertension, and insulin resistance, and is a target 
of therapy regardless of the severity of traditional cardiovascul

rdless of the severity of traditional cardiovascular risk 
factors (see Table 41.6). Particular emphasis should be placed on 
obtaining a detailed history of all first-degree relatives to identify 
cholesterol disorders or premature cardiovascular disease. In pri­
mary care, a child-parent screening strategy is effective at identify­
ing FH with high cardiovascular risk.146 The examination should 
emphasize the cardiovascular system, manifestations of hyperlipid­
emia, and disorders causing seco

ns of hyperlipid­
emia, and disorders causing secondary lipid abnormalities. 
Hyperchylomicronemia-Induced Pancreatitis
Markedly increased triglyceride levels are most often diagnosed by 
laboratory detection, including the observation of lipemia. A tri­
glyceride higher than 1000 mg/dL (11.3 mmol/L) usually signifies 
  Criteria for Diagnosis of Metabolic 
Syndrome
Measurea
Categorical Threshold
Whites, African Americans, Latin Americans: 
Men, ≥40 in.; women, ≥35 in.
Asian

tin Americans: 
Asians: Men, ≥35 in.; women, ≥32 in.
Elevated triglycerides
≥150 mg/dL
or On drug treatment for elevated triglycerides
Reduced HDL
Men, <40 mg/dL; women, <50 mg/dL
or On drug treatment for reduced HDL
Elevated blood 
pressure
≥130 mm Hg systolic or ≥85 mm Hg 
diastolic
or On antihypertensive drug treatment
Elevated fasting 
glucose
≥100 mg/dL
or On drug treatment for elevated glucose
aThree of the five measures are required for diagnosis.
HDL, High-de

 measures are required for diagnosis.
HDL, High-density lipoprotein.
Adapted from Grundy SM, Cleeman JI, Daniels SR, et al. Diagnosis and management of the 
metabolic syndrome: an American Heart Association/National Heart, Lung, and Blood Institute 
scientific statement. Circulation. 2005;112:2735–2752.
   
TABLE 41.5
1609
the presence of two or more abnormalities of lipid metabolism (e.g., 
estrogen therapy in the presence of underlying familial hypertr

apy in the presence of underlying familial hypertriglyc­
eridemia). Elevated plasma triglyceride levels can fluctuate markedly 
within an individual over short periods. Fluctuation occurs because 
the LPL-mediated clearance mechanisms for triglyceride-rich par­
ticles becomes saturated at plasma triglyceride concentrations of 
approximately 500 mg/dL (5.6 mmol/L). Therefore, triglyceride 
levels can rise precipitously as dietary fat intake increases and can 
fall rapidly with dietary fat restric

ses and can 
fall rapidly with dietary fat restriction. Very high carbohydrate diets, 
unless composed of primarily complex carbohydrate, may lead to 
poor glycemic control and increased triglyceride levels. Patients with 
the chylomicronemia syndrome may initially require a diet with less 
than 10% of calories from fat to decrease chylomicron production.
Visual inspection of plasma after it has been refrigerated over­
night can be helpful. A cream-like layer on the top signifies chy­
lomicrons.

am-like layer on the top signifies chy­
lomicrons. A turbid infranatant signifies high levels of VLDL. A 
cream-like top layer and turbid plasma indicates the presence of 
both chylomicrons and VLDL.
A history of diabetes, alcohol use, or triglyceride-elevating 
medications is common. Physical examination might be useful. 
Lipemia retinalis (see Fig. 41.17B), a condition in which lipemic 
blood causes opalescence of retinal arterioles, can be observed dur­
ing funduscopic examination. It is typi

erved dur­
ing funduscopic examination. It is typically seen only when tri­
glycerides are 2000 mg/dL (22.6 mmol/L) or higher.
Eruptive xanthomas (see Fig. 41.17G) appear as small, yellow­
ish, round papules that contain a pale center and an erythema­
tous base. Their distribution includes the abdominal wall, back, 
buttocks, and other pressure contact areas. They are caused by 
accumulation of triglyceride in dermal histiocytes and typically 
occur when the plasma triglyceride level is 1000 to 

cur when the plasma triglyceride level is 1000 to 2000 mg/mL 
(11.3–22.6 mmol/L) or higher. They can disappear rapidly with 
lowering of the plasma triglyceride concentration.
Severe hypertriglyceridemia (1000 mg/dL [>11.3 mmol/L]) 
should be treated aggressively because pancreatitis associated with 
these levels can be fatal. Lifestyle intervention includes exercise, 
diet (avoid high fat, simple sugars, and noncomplex carbohy­
drates), diabetes control, and elimination of triglyceride-raising 

 control, and elimination of triglyceride-raising 
medications if possible. Initial drug treatments are fibrates, fish 
oils, and/or niacin. Patients have also been treated with newer 
therapies that inhibit apoB production (mipomersen) or MTP 
(lomitapide), but increased liver accumulation of triglyceride is a 
concern. Antisense reduction of liver production of apoCIII effec­
tively reduces triglyceride levels in patients with and without LPL 
mutations.58 Additional approaches under developme

mutations.58 Additional approaches under development target the 
LPL inhibitory protein ANGPTL3.147
In the setting of acute pancreatitis, the approach to treat­
ment is based on clinical impression and our understanding of 
the pathophysiology rather than randomized clinical trials. In the 
setting of diabetes, insulin therapy is essential. In nonhypergly­
cemic patients, low-dose insulin (1–2 units per hour) is usually 
sufficient to block adipose tissue lipolysis, reduce circulating free 
fatt

se tissue lipolysis, reduce circulating free 
fatty acids levels, and theoretically reduce triglyceride production 
by the liver. LPL activity is also induced by insulin. Higher doses 
of insulin, such as those used to treat diabetic ketoacidosis, require 
large amounts of glucose infusion to prevent hypoglycemia and 
are likely to increase de novo triglyceride production in the liver. 
In most patients, triglycerides will decrease approximately 50% 
within 24 hours, so evidence for the lack of 

50% 
within 24 hours, so evidence for the lack of response requires 
accurate initial levels that should be obtained by plasma dilution. 
Plasmapheresis is rarely indicated and has not been proven to be 
effective,148 despite a marked reduction in circulating triglycer­
ide levels. For this reason, it might be most reasonable to con­
sider this therapy only in patients not responding to conventional 
methods, with associated organ dysfunction, with plasma levels 
that are extraordinarily elevate

th plasma levels 
that are extraordinarily elevated (more than 10,000 mg/dL [259 
mmol/L]), or during pregnancy when triglyceride increases are 
driven by estrogen induction of liver triglyceride production.
Screening for Secondary Disorders
The history and physical examination should be directed toward 
uncovering secondary disorders of lipid metabolism and identify­
ing agents including medications that could cause hyperlipidemia 
(see Table 41.4). Minimal studies should include fasting blood 

.4). Minimal studies should include fasting blood 
glucose, glycosylated hemoglobin, renal and hepatic function 
tests, urinary protein, and thyroid-stimulating hormone. 
Patient Selection and Treatment Goals
Many professional groups have issued and updated guidelines for 
management of hyperlipidemia over decades. As guidelines for 
treatment of lipoprotein disorders and approaches to reduce car­
diovascular risk have evolved, they have engendered controversy. 
Early recommendations based on ep

ed controversy. 
Early recommendations based on epidemiology, preclinical data, 
and limited randomized clinical data were shown to be prescient 
by subsequent clinical trials. Although it is widely accepted that 
LDL cholesterol reduction prevents atherosclerotic cardiovascular 
disease and reduces clinically important cardiovascular events in 
patients with established disease or at increased risk, who should 
be treated and to what lipid target has been disputed. The National 
Cholesterol Edu

t has been disputed. The National 
Cholesterol Education Program, established by the National Insti­
tutes of Health in the late 1980s, created a standard for choles­
terol levels and pioneered a practical approach to treatment by 
dividing the population according to cardiac risk based on lipid 
and other cardiac risk factors. This approach has persisted. More 
recent clinical trials have shown greater reductions of plaque with 
aggressive and safe LDL reduction. Although, in general, desirable

afe LDL reduction. Although, in general, desirable 
levels of LDL for primary prevention of disease are less than 100 
mg/dL (0.26 mmol/L) and for secondary prevention of disease 
are less than 70 mg/dL (1.8 mmol/L), others have recommended 
even lower targets (<55 mg/dL [1.4 mmol/L]) in the setting of 
documented atherosclerosis, very high risk, or evidence of con­
tinued disease progression despite LDL less than 70 mg/dL (1.8 
mmol/L).149
The American College of Cardiology and American Heart 

bohydrate and Fat Metabolism
Canadian Cardiovascular Society and European groups have con­
tinued to include goals for treatment (Table 41.8). With evidence 
that all approaches to LDL reduction lead to a similar reduction in 
cardiovascular events and the advent of new potent therapies, such 
as PCSK9 inhibitors, recommended therapeutic goals will likely 
become more aggressive.
Risk calculators based on current ACC/AHA guidelines are 
available on the ACC and AHA websites. The Framingham risk 

 on the ACC and AHA websites. The Framingham risk 
calculator remains frequently used and includes age, total cho­
lesterol, HDL, systolic blood pressure, and smoking status, and 
many others are available and include additional well-known risk 
factors such as Lp(a), C-reactive protein, and coronary calcium 
scores. Different risk scoring systems may be appropriate for 
diverse populations and regions.151–153 In acknowledgement that 
lipoproteins other than LDL are also atherogenic, some guidel

s other than LDL are also atherogenic, some guidelines 
use non-HDL cholesterol (total cholesterol minus the HDL) as a 
secondary goal that is 30 mg/dL (0.8 mmol/L) higher, accounting 
for the likely atherogenic risk due to triglyceride-rich lipoproteins.
Most guidelines recommend treatment for any patient with 
LDL greater than 190 mg/dL (4.9 mmol/L). When lipid-lower­
ing therapy for primary prevention should be initiated is unclear. 
Biomarkers such as C-reactive protein, Lp(a), and imaging t

s such as C-reactive protein, Lp(a), and imaging tech­
niques such as coronary calcium scores are sometimes added to 
more precisely estimate individual risk. Guidelines for treatment 
of patients with type 2 diabetes mellitus consider their increased 
risk of cardiovascular events. Patients with established vascular dis­
ease and diabetes mellitus are considered to be at very high risk.154 
Cardiovascular disease accounts for a high percentage of deaths in 
the geriatric population, and there a

f deaths in 
the geriatric population, and there are survival benefits of treat­
ment in elderly patients up to the age of 85 years who have known 
coronary heart disease.
Some patients with LDL levels below previous treatment 
thresholds benefit from greater LDL cholesterol reduction.155 
Therefore, treat-to-target approaches may underestimate benefits 
that could accrue with greater LDL reduction.156,157 Cardiovas­
cular outcome trials using PCSK9 antibodies, which can reduce 
LDL to very low 

SK9 antibodies, which can reduce 
LDL to very low levels, may lead to recommendations for LDL 
levels below current goals. 
Specific Therapies
Specific therapies include lifestyle changes (diet, exercise, and 
weight management) and drug therapy.
Lifestyle Treatment
Lifestyle changes including dietary intervention, moderate exer­
cise, and weight loss are first-line therapies for hyperlipidemia 
(Table 41.9) and may be sufficient for mild dyslipidemias in 
low-risk patients. All other treatments

demias in 
low-risk patients. All other treatments should build on therapeu­
tic lifestyle changes.158 Hypertriglyceridemia often responds to 
decreased intake of fat, simple sugars, alcohol, and calories. Most 
patients will have a 10% to 15% decrease in LDL levels with 
diet, which may be adequate for primary prevention in low-risk 
patients. In patients with established coronary heart disease, drug 
therapy should be instituted along with diet therapy. Response to 
lipid-lowering drugs may be

 therapy. Response to 
lipid-lowering drugs may be disappointing in patients who do not 
follow dietary recommendations.
Daily moderate exercise, such as walking, may help reduce 
triglyceride levels. Exercise may be particularly useful in obese, 
insulin-resistant patients with high triglycerides, low HDL, and 
moderately elevated LDL. An exercise program combined with a 
moderately hypocaloric diet leading to modest weight loss may 
improve dyslipidemia, as well as glucose tolerance and blood 

lipidemia, as well as glucose tolerance and blood 
pressure, especially in patients with metabolic syndrome. Team 
management that includes nutritionists and dietitians is likely 
to enhance dietary therapies. Dietary changes are best when 
individualized and instituted gradually. Involvement of family 
members is important. The DASH (Dietary Approaches to Stop 
Hypertension) or Mediterranean diets are appropriate. Patients 
with the chylomicronemia syndrome may initially require a diet 
with le

mia syndrome may initially require a diet 
with less than 10% of calories from fat to decrease chylomicron 
production. The response to very low fat diets may be disap­
pointing in patients with impaired glucose tolerance unless the 
diet is hypocaloric.
Effects of various types of dietary fats have been studied exten­
sively.159 Current recommendations160 are to restrict saturated fats 
and trans fats and substitute with complex carbohydrates, poly­
unsaturated fats, and monounsaturated fats. H

oly­
unsaturated fats, and monounsaturated fats. High cholesterol and 
saturated fat intake elevate plasma cholesterol by decreasing recep­
tor-mediated clearance of LDL. High dietary cholesterol also pro­
motes increased LDL synthesis. Trans fats are unsaturated fatty acids 
with at least one trans double bond; they are produced when liquid 
vegetable oils are partially hydrogenated to produce semisolid fats 
used in margarines and shortening. Trans fats raise LDL and reduce 
HDL, and they have

rans fats raise LDL and reduce 
HDL, and they have been implicated in cardiovascular disease.
Fish oils are rich in eicosapentaenoic acid (EPA) or docosahexae­
noic acid (DHA) (see Table 41.1). Daily doses of 4 g of EPA plus 
DHA lower VLDL and treat elevated triglycerides. Observational 
data support that fish intake is associated with decreased cardiovas­
cular risk in those with and without known cardiovascular disease. 
Many experts recommend eating two servings of oily fish weekly 
or takin

 eating two servings of oily fish weekly 
or taking fish oil capsules that provide 1 g/day of EPA plus DHA 
daily as reasonable for those not interested in consuming fish. 
  American College of Cardiology/American 
Heart Association 2013 Statin Benefit 
Groups
Benefit Group
Clinical ASCVD
Age <75 years without contraindica­
tions: high-intensity statin
Age >75 years or with contraindica­
tions: moderate-intensity statin
LDL >190 mg/dL, age >21 
years
High-intensity statin
Can consid

L, age >21 
years
Can consider nonstatin drug if fur­
ther LDL reduction is desirable
Primary prevention—diabe­
tes: age 40–75 years, LDL 
70–189 mg/dL
Moderate-intensity statin
If 10-year ASCVD risk is ≥7.5%, 
consider high-intensity statin
Primary prevention—no diabe­
tesa: ≥7.5% 10-year ASCVD 
risk,b age 40–75 years, LDL 
Moderate- to high-intensity statin
aRequires risk discussion between clinician and patient before statin initiation.
bStatin therapy may b

nt before statin initiation.
bStatin therapy may be considered if risk decision is uncertain after use of the ASCVD risk 
calculator.
ASCVD, Atherosclerotic cardiovascular disease; LDL, low-density lipoprotein.
Data from Stone NJ, Robinson JG, Lichtenstein AH, et al. 2013 ACC/AHA guideline on the 
treatment of blood cholesterol to reduce atherosclerotic cardiovascular risk in adults: a report 
of the American College of Cardiology/American Heart Association Task Force on Practice 
Guidelines. J 

Association Task Force on Practice 
Guidelines. J Am Coll Cardiol. 2014;63:2889–2934.
   
TABLE 41.7
1611
It is not clear whether fish oil supplementation is effective for pre­
vention or treatment of atherosclerotic vascular disease,161,162 and 
Cochrane analysis of published reports suggests that increasing 
EPA and DHA has little or no effect on mortality or cardiovascu­
lar health.163 However, a recent study using 4 grams of icosapent 
ethyl in hyper

t study using 4 grams of icosapent 
ethyl in hypertriglyceridemic subjects with known atherosclerosis 
or high risk (such as diabetes) reduced coronary events.164
Other dietary components can influence plasma lipids. For 
example, soluble fibers such as psyllium or oat bran, which can 
bind bile acids in the gut and promote net cholesterol excretion, 
decrease LDL modestly (about 5–10%). Margarines made with 
sitostanol or sitosterol, which are plant sterols that inhibit choles­
terol absorption

lant sterols that inhibit choles­
terol absorption, reduce serum cholesterol by about 10%. Together, 
the combination of plant sterols, soluble fiber, and restriction of 
saturated fat and cholesterol can reduce LDL levels by about 30%. 
Drug Treatment
Table 41.10 lists drugs that inhibit cholesterol synthesis in cells 
(HMG-CoA reductase inhibitors), interfere with the degrada­
tion of the LDL receptor by inducing LDL receptor expression 
in hepatocytes (PCSK9 inhibitors), block cholesterol abs

atocytes (PCSK9 inhibitors), block cholesterol absorption 
from the gut (ezetimibe), or interfere with bile acid absorption 
from the gut (bile acid sequestrants). Fibrates, and omega-3 fatty 
acids, and niacin either inhibit VLDL production or enhance 
clearance of triglyceride-rich particles. Drugs that decrease 
hepatic VLDL production include the apolipoprotein B anti­
sense oligonucleotide, mipomersen, and the MTP inhibitor 
lomitapide.
HMG-CoA Reductase Inhibitors (Statins)
Statins inhibit

CoA Reductase Inhibitors (Statins)
Statins inhibit cholesterol biosynthesis, upregulate LDL receptors, 
enhance LDL clearance, reduce lipoprotein release from the liver, 
and may decrease triglycerides by enhancing VLDL clearance and 
decreasing lipoprotein production.
Statins are useful in all types of hyperlipidemias in which LDL 
is elevated. They are particularly useful for patients with vascular 
disease and those with very high LDL (e.g., FH, combined hyper­
lipidemia), and they are the dr

H, combined hyper­
lipidemia), and they are the drugs of choice for lowering LDL as 
primary or secondary prevention but are less effective in homozy­
gous LDL receptor deficiency. Several statins are approved for use 
in children and adolescents with FH, high LDL, or a significant 
family history of premature coronary artery disease.
  Comparison of Dyslipidemia Guidelines
Risk Category
TREATMENT INTENSITY OR GOALS
ACC/AHAa
CCSb
Eur Soc Card+c
Secondary preven­
tion/high or very 
high risk
Cli

high risk
Clinical ASCVD:
Lifestyle + high-intensity statin 
(LDL reduction ≥50%)
Clinical ASCVD, most DM (age ≥40 years, 
age ≥30 with 15-year duration, micro­
vascular disease), abdominal aortic 
aneurysm, chronic kidney disease
Lifestyle + statin to goal of LDL <77 mg/
dL (2.0 mmol/L) or >50% reduction or 
non-HDL <100 mg/dL (2.6 mmol/L)
Very high risk (documented CVD, DM with target 
organ damage, CKD with GFR <30 mL/
min/1.73 m2, calculated SCORE ≥10%)
L

 GFR <30 mL/
Lifestyle advice and concomitant drug treatment
Goal of LDL <70 mg/dL (1.8 mmol/L)
Primary prevention
LDL ≥190 mg/dL (rule out 
secondary causes): lifestyle + 
high-intensity statin (add non­
statin if insufficient response)
Framingham Risk Score 10% to 19% 
and LDL ≥135 mg/dL (3.5 mmol/L) or 
non-HDL ≥166 mg/dL (4.3 mmol/L) or 
one additional risk factor
non-HDL <100 

L (2.0 mmol/L) or >50% reduction or 
High risk: Cholesterol >310, most DM, moderate 
CKD with GFR 30 to 59 mL/min/1.73 m2, 
calculated SCORE ≥5% to <10%
Lifestyle advice with drug treatment if LDL >100
Goal of LDL <100 mg/dL (2.5 mmol/L)
DM without ASCVD, LDL <190 
mg/dL: perform risk calculation
Moderate risk calculated SCORE level 1% to 5%
Lifestyle advice, consider drug if LDL uncon­
trolled
Goal of LDL <115 mg/dL (3.0 mmol/L)

uncon­
trolled
ASCVD risk ≥7.5%
Risk discussion with patient
Other considerations
Evaluate triglycerides >500 mg/
dL
Ezetimibe first-line add-on and PCSK9 
inhibitors as second line; apoB levels 
for risk stratification
Triglycerides <150 desirable, physical activity 
and other lifestyle changes to increase HDL
aACC/AHA data from American College of Cardiology/American Heart Association, Stone NJ, Robinson JG, Lichtenstein AH

ssociation, Stone NJ, Robinson JG, Lichtenstein AH, et al. ACC/AHA guideline on the treatment of blood cholesterol to reduce athero­
sclerotic cardiovascular risk in adults: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines. J Am Coll Cardiol. 2014;63:2889–2934.
bCanadian Cardiovascular Society data from Anderson TJ, Grégoire J, Pearson GJ, et al. 2016 Canadian Cardiovascular Society guidelines for the management of dyslipidemia for the p

lines for the management of dyslipidemia for the prevention of cardio­
vascular disease in the adult. Can J Cardiol. 2016;32(11):1263–1282.
cEur Soc Card data from Piepoli MF, Hoes AW, Agewall S, et al. 2016 European guidelines on cardiovascular disease prevention in clinical practice: the Sixth Joint Task Force of the European Society of 
Cardiology and Other Societies on Cardiovascular Disease Prevention in Clinical Practice (constituted by representatives of 10 societies and by invited expert

resentatives of 10 societies and by invited experts) developed with the special contribution of 
the European Association for Cardiovascular Prevention & Rehabilitation (EACPR). Eur Heart J. 2016;37(29):2315–2381.
ACC/AHA, American College of Cardiology/American Heart Association; ASCVD, atherosclerotic cardiovascular disease; CCS, Canadian Cardiovascular Society, Eur Soc Card+, European Society of Car­
diology plus other societies; CVD, cardiovascular disease; CKD, chronic kidney disease; DM, d

scular disease; CKD, chronic kidney disease; DM, diabetes mellitus; GFR, glomerular filtration rate; HDL, high-density lipoprotein; LDL, low-density lipoprotein 
cholesterol; PCSK9, proprotein convertase subtilisin/kexin type 9; SCORE, Systematic Coronary Risk Estimation.
   
TABLE 41.8
1612
Table 41.11 provides the relative potency of various statins 
when used at different dosages, and Table 41.12 lists expected 
effects of various s

d Table 41.12 lists expected 
effects of various statins on LDL. They reduce LDL by 20% to 
60%, increase HDL by 2% to 16%, and reduce triglycerides by 
7% to 37%, depending on the drug; the dosage; and, in the case 
of triglycerides, baseline levels. Effects also vary among patients, 
with greater or lesser degrees of LDL reduction even at the same 
dose. For each statin, doubling of the dose typically produces 
an additional 6% reduction of LDL.165 LDL lowering is seen 
within 1 to 2 weeks aft

.165 LDL lowering is seen 
within 1 to 2 weeks after the start of therapy and is stable in 
about 4 to 6 weeks. Pitavastatin, atorvastatin, and rosuvastatin 
have long half-lives, about 12 hours, 14 hours, and 21 hours, 
respectively. The other statins have half-lives of about 2 to 3 
hours. First-generation statins with short half-lives should be 
taken in the evening. Atorvastatin and fluvastatin have minimal 
renal clearance and may be more suitable for patients with renal 
insufficiency. Sev

itable for patients with renal 
insufficiency. Several statins are available as generic drugs in the 
United States. Table 41.13 shows specific features of available 
statins.
The most common side effects of statins are abdominal pain, 
constipation, flatulence, nausea, headache, fatigue, diarrhea, and 
muscle complaints. Except for musculoskeletal symptoms, most 
side effects are infrequent.
Liver toxicity is not common with statin use. Hepatic ami­
notransferase elevation is usually mild and d

ami­
notransferase elevation is usually mild and does not require 
discontinuation of the statin. It may be dose dependent, as 
demonstrated in clinical trials that showed rates of persistent 
elevations of liver aminotransferase greater than three times the 
upper limit of normal occurring in 0.1% to 1.9% of patients, 
depending on the statin and the dose. The requirement for rou­
tine monitoring of hepatic transaminases was removed from 
statin labeling by the US Food and Drug Administration i

 labeling by the US Food and Drug Administration in 
2012. Hepatic transaminases should be obtained at baseline 
and during treatment only if there is a clinical indication for 
their measurement. Only about 1% of statin-using patients 
have aminotransferase increases to greater than three times 
the upper limit of normal, and the elevation often decreases 
even if patients continue on the statin.166 Statins can be used 
cautiously in the presence of liver disease that is not decom­
pensated, an

e of liver disease that is not decom­
pensated, and nonalcoholic hepatic steatosis is not a contraindi­
cation.167 If aminotransferases remain greater than three times 
the upper limit of normal, consider lowering the dose or chang­
ing to a different statin and identify other contributing condi­
tions or drugs. Irreversible liver damage resulting from statins 
is extremely rare, with a liver failure rate of 1 case per 1 million 
person-years of use.167
About 5% of patients complain of muscle si

use.167
About 5% of patients complain of muscle side effects, but 
recent randomized placebo-based trials have suggested that many 
of these issues are due to a nocebo effect, which is the anticipa­
tion of a side effect leading to altered perception.168 The adverse 
effects associated with statin use correlate with drug dose,169 but 
many large randomized, placebo-controlled clinical trials have not 
shown differences in rates of myalgias and creatine kinase eleva­
tions between statin-treated 

eatine kinase eleva­
tions between statin-treated and placebo groups. Regardless, mus­
cle complaints are a common reason for statin discontinuation 
by patients, perhaps related to misinformation about the risks of 
statin therapy promulgated through the internet.170 In the proper 
context, it is possible to carefully reintroduce statins after a pre­
sumed myalgia.
In patients with muscle complaints after statin administration, 
disorders that might influence skeletal muscle or present with 
mu

ight influence skeletal muscle or present with 
muscle symptoms must be excluded, such as hypothyroidism, vita­
min D deficiency, rheumatologic conditions, and perhaps depres­
sion. Inhibition of statin catabolism is associated with increased 
myopathy risk. Drugs metabolized through the cytochrome P450 
(CYP) system, such as ketoconazole, itraconazole, clarithromycin, 
and erythromycin, increase statin plasma levels especially with use 
of older statins that have a more limited hepatic disposal

 statins that have a more limited hepatic disposal. Other 
drugs that increase the risk of statin myopathy include gemfibro­
zil, cyclosporine, digoxin, verapamil, diltiazem, amiodarone, col­
chicine, and protease inhibitors.
The most serious potential side effect of statins is rhabdo­
myolysis leading to myoglobinuria and renal failure. Rhab­
domyolysis is rare and is more likely in patients with renal 
insufficiency, advanced age, other comorbid conditions, or 
polypharmacy and during perioper

d conditions, or 
polypharmacy and during perioperative periods. Routine 
surveillance of creatine kinase is not useful in most patients. 
Management of dyslipidemia in patients with muscle-related 
complaints is challenging and may include using a different 
statin, decreasing the dose, giving the medication less fre­
quently, or considering other LDL-lowering drugs. Higher 
doses of simvastatin should be avoided. The US Food and 
Drug Administration recommends that simvastatin 80 mg not 
be us

ation recommends that simvastatin 80 mg not 
be used, and simvastatin should be limited to 10 mg in patients 
taking amiodarone, verapamil, and diltiazem, and to 20 mg in 
those taking amlodipine and ranolazine.
Statin therapy is associated with increased incidence of type 
2 diabetes mellitus in patients with a predisposition to develop­
ing this disorder. This association was most evident with the use 
of the highest doses of atorvastatin (80 mg) and rosuvastatin 
  Lifestyle Recommendations 

g) and rosuvastatin 
  Lifestyle Recommendations to Reduce 
Cardiovascular Risk
Modality
Intervention
Diet
Follow a dietary pattern high in vegetables, fruits, 
whole grains, poultry, fish, low-fat dairy products, 
legumes, nontropical vegetable oils, and nuts
Limit red meat, sweets, and sugar-sweetened 
beverages
Limit saturated fat to 5–6% of total calories
Limit calories from trans fats
Physical activity
Engage in aerobic physical activity 3–4 sessions per 
week of moderate- to vigorous-inte

4 sessions per 
week of moderate- to vigorous-intensity physical 
activity averaging 40 minutes per session
Weight 
management
For obese patients (BMI ≥30) or overweight patients 
(BMI ≥25) who have additional risk factors, sus­
tained weight loss of 3–5% or greater reduces 
ASCVD risk
Consultation with a registered dietitian may be 
helpful to plan, start, and maintain a diet that 
promotes weight loss and restricts intake of 
saturated fats
ASCVD, Atherosclerotic cardiovascular disease; BMI, b

VD, Atherosclerotic cardiovascular disease; BMI, body mass index.
Data from Eckel RH, Jakicic JM, Ard JD, et al. 2013 AHA/ACC guideline on lifestyle manage­
ment to reduce cardiovascular risk: a report of the American College of Cardiology American/
Heart Association Task Force on Practice Guidelines. J Am Coll Cardiol. 2014;63:2960–2984; 
Jensen MD, Ryan DH, Apovian CM, et al. 2013 AHA/ACC/TOS guideline for the management 
of overweight and obesity in adults: a report of the American College of

ity in adults: a report of the American College of Cardiology/American 
Heart Association Task Force on Practice Guidelines and the Obesity Society. J Am Coll Car­
diol. 2014;63:2985–3023.
   
TABLE 41.9
1613
(40 mg).171 Genetic studies172,173 indicate that that variants in 
the LDL receptor, PCSK9, and HMG-CoA reductase (the tar­
get of statins) are associated with type 2 diabetes mellitus risk, 
suggesting that promoting uptake of LDL might affect diab

ing that promoting uptake of LDL might affect diabe­
tes pathogenesis. Nonetheless, the major adverse cardiovascular 
event reduction benefits of lowering LDL markedly outweigh 
the risks of increased glucose in all but those at lowest athero­
sclerotic risk. Attention to diet, exercise, and weight can mitigate 
the diabetes risk.
Patients have many complaints that might or might not be due 
to statins. Complaints about confusion or memory loss should be 
appropriately evaluated for causes of th

hould be 
appropriately evaluated for causes of these symptoms. There is no 
evidence that statins cause direct adverse effects on renal function 
beyond that due to rhabdomyolysis. Statins are contraindicated 
during pregnancy and nursing and in patients with significant 
hepatic dysfunction. 
Proprotein Convertase Subtilisin/Kexin Type 9 Inhibitors
This newer class of medications is fully humanized monoclo­
nal antibodies to PCSK9 that lower LDL levels by prevent­
ing degradation of the LDL re

L levels by prevent­
ing degradation of the LDL receptor, increasing recirculation 
of the receptor to the surface of hepatocytes, and consequent 
  Drugs used to Treat Hyperlipidemia
Class and Drugs Available
Dosage
Major Lipoprotein 
Decreased
Mechanism
HMG-CoA Reductase Inhibitors
Rosuvastatin
5–40 mg qd
LDL
Decrease cholesterol synthesis; increase 
LDL receptor–mediated removal of LDL
10–80 mg qd
Simvastatin
5–40 mg qd
Lovastatin
10–80 mg qd
Pravastatin
10–40 mg qd
Fluvastatin

n
10–80 mg qd
10–40 mg qd
20–80 mg qd
Pitavastatin
1–4 mg qd
PCSK9 Inhibitors
Evolocumab
140 mg sc q 2 weeks or 420 mg 
sc q month
LDL
Prevent degradation of the LDL receptor
Alirocumab
75–150 mg sc q 2 weeks
Intestinal Cholesterol Absorption Inhibitor
Ezetimibe
10 mg qd
LDL
Inhibits cholesterol absorption
Bile Acid Sequestrants
4–12 g bid
LDL
Increase sterol excretion and LDL clearance
Colestipol
5–15 g bid
3.75–4.375 g qd
Fibric Acid Derivativ

Fibric Acid Derivatives
Gemfibrozil
600 mg bid
VLDL (LDL)
Decrease VLDL production; enhance LPL 
action
Fenofibratea
30–200 mg qd
Lovaza (1-g capsule contains EPA and DHA)
4 g qd
VLDL
Inhibit VLDL production
Vascepa (1-g capsule contains EPA)
4 g qd
Epanova (1-g capsule contains EPA and DHA 
free fatty acids)
2–4 g qd
Nicotinic Acid
Niacin (crystalline)
1–3 g qd
VLDL (LDL)
action
Niaspan (extended-release nia

 enhance LPL 
action
Niaspan (extended-release niacin)
500–2000 mg qd
ApoB Antisense Oligonucleotide
Mipomersen
200 mg once a week sc injection
VLDL, LDL, Lp(a)
Inhibits synthesis of apolipoprotein B
Microsomal Triglyceride Transfer Protein Inhibitor
Lomitapide
5–60 mg qd
Inhibits microsomal triglyceride transfer 
protein
aThere are several different preparations of fenofibrate with different doses.
bid, Twice a day; EPA, highly concentrated ethyl esters of eicosapentaenoic acid

concentrated ethyl esters of eicosapentaenoic acid; DHA, docosahexaenoic acid; HMG-CoA, 3-hydroxy-3-methylglutaryl coenzyme A; LDL, low-density lipoproteins; Lp(a), 
lipoprotein(a); LPL, lipoprotein lipase; PCSK9, proprotein convertase subtilisin/kexin type 9; q, every; qd, every day; sc, subcutaneously; VLDL, very low density lipoprotein.
   
TABLE 41.10
1614
  High-, Moderate-, and Low-Intensity Statin Therapy
High Intensity
Moderat

ow-Intensity Statin Therapy
Moderate Intensity
Low Intensity
Daily dose lowers LDL on average by approxi­
mately ≥50%*:
Rosuvastatin 20–40 mg
Atorvastatin 40–80 mg
Daily dose lowers LDL on average, by approximately 
30% to <50%*:
Rosuvastatin 5–10 mg
Atorvastatin 10–20 mg
Simvastatin 20–40 mg
Lovastatin 40 mg
Pravastatin 40–80 mg
Fluvastatin 40 mg bid
Pitavastatin 2–4 mg
Daily dose lowers LDL, on average, by <30%a:
Simvastatin 10 mg
Lovastatin 20 mg
Pravastatin 10–20 mg
Fluvastati

g
Fluvastatin 20–40 mg
Pitavastatin 1 mg
aNote that individual responses vary.
bid, Twice a day; LDL, low-density lipoprotein cholesterol.
Data from Robinson JG, Lichtenstein AH, et al. ACC/AHA guideline on the treatment of blood cholesterol to reduce atherosclerotic cardiovascular risk in adults: a report of the American College of 
Cardiology/American Heart Association Task Force on Practice Guidelines. J Am Coll Cardiol. 2014;63:2889–2934.
   
TABLE 41.11

m Coll Cardiol. 2014;63:2889–2934.
   
  Typical LDL Reductions (% Change From Baseline) by Statin Dose
Treatment
5 mg
10 mg
20 mg
40 mg
80 mg
−40
−46
−52
−55
—
—
−37
−43
−48
−51
−26
−30
−38
−41
−47
—
−21
−27
−31
−40
—
−20
−24
−30
−36
—
—
−22
−25
−35
—
—
(1 mg) −32
(2 mg) −36
(4 mg) −43
Ezetimibe 10 mg plus variable 
simvastatin
—
−45
−52
−55
−60
LDL, Low-density lipoprotein cholesterol; —, data no

L, Low-density lipoprotein cholesterol; —, data not available.
Data from Hou R, Goldberg AC. Lowering low-density lipoprotein cholesterol: statins, ezetimibe, bile acid sequestrants, and combinations—comparative efficacy and safety. Endocrinol Metab Clin North 
Am. 2009;38:79–97; Livalo package insert. Available from http://www.kowapharma.com/documents/LIVALO_PI_CURRENT.pdf.
   
TABLE 41.12
  Features of Individual Statins
Drug
Pharmacologic Considerations
Safety Issues
Synthetic c

siderations
Synthetic compound; active metabolite is formed by CYP2C9
Can increase INR when used with warfarin—monitor INR; 
dose reduction in renal insufficiency, Asian patients, and 
Synthetic drug; <2% excreted in urine; half-life 14 hours
Interacts with CYP3A4 substrates; increases digoxin levels
Synthetic derivative of fermentation product of Aspergillus terreus; dose 
reduction in severe renal insufficiency
Interactions w

ction in severe renal insufficiency
Interactions with CYP3A4 substrates
First statin marketed in United States; isolated from a strain of A. terreus; 
food intake increases absorption
Derived from fermentation product of A. terreus; dose reduction in renal 
Drug interaction with cyclosporine
Synthetic drug; minimal renal excretion
Can interact with warfarin, phenytoin, glyburide, diclofenac, 
fluconazole, ketoco

ytoin, glyburide, diclofenac, 
fluconazole, ketoconazole
Dose reduction in severe renal failure
Drug interaction with cyclosporine, erythromycin, rifampin
CYP2C9 and CYP3A4, Cytochrome P450 isoenzymes 2C9 and 3A4; INR, international normalized ratio.
Am. 2009;38:79–97; Livalo package ins

 Clin North 
   
TABLE 41.13
1615
lowering of circulating LDL (Fig. 41.18). Alone or in con­
junction with statins, they increase removal of LDL from the 
circulation, leading to nearly 60% further reduction in circu­
lating LDL; a reduction in coronary heart disease events2; and, 
as noted earlier, atheroma regression in patients with know

earlier, atheroma regression in patients with known 
Clinical trials with both PCSK9 inhibitors marketed in 
the United States, alirocumab and evolocumab, demonstrate 
reductions in atherosclerotic cardiovascular disease events, 
particularly in patients with recent acute coronary syndrome, 
multivessel coronary artery disease, or peripheral arterial dis­
ease. Reductions in LDL to levels less than 25 mg/dL (0.65 
mmol/L), which occurred in more than a quarter of the 
patients i

occurred in more than a quarter of the 
patients in the clinical trials, are associated with even lower 
cardiovascular disease event rates, establishing no lower limit 
for LDL when using PCSK9 alone or in combination with 
a statin. PCSK9 inhibitors are recommended for high-risk 
patients with LDL levels of 70 mg/dL (1.8 mmol/L) or higher 
on maximally tolerated oral therapies including statins and/or 
ezetimibe. PCSK9 inhibitors are useful in homozygous FH, 
where carriers have a high prevale

homozygous FH, 
where carriers have a high prevalence of premature death and 
which can otherwise be challenging to treat. Unlike statins, 
PCSK9 inhibitors decrease Lp(a), although modestly. These 
drugs are injected every 2 to 4 weeks. As with many new bio­
logics, insurance restrictions and expense limit clinical avail­
ability. Aside from injection site reactions, side effects of this 
therapy thus far appear minimal. 
Ezetimibe
Ezetimibe inhibits cholesterol absorption by binding to 
NPCL1,

ibits cholesterol absorption by binding to 
NPCL1, the intestinal cholesterol absorption transporter. 
Ezetimibe lowers LDL by 14% to 25% when used alone or in 
combination with statins. Ezetimibe has been shown to fur­
ther reduce cardiovascular events when added to statin therapy; 
in this study, LDL was reduced from nearly 70 to 54 mg/dL 
(1.8–1.4 mmol/L).155 Ezetimibe can be useful in patients who 
are intolerant of statins. Ezetimibe undergoes glucuronidation, 
leading to extensive enterohe

es glucuronidation, 
leading to extensive enterohepatic circulation. Absorption is 
not affected by food.
Side effects are relatively uncommon but may include diarrhea 
and abnormal liver functions. Myopathy is rare and, like myalgias, 
is not clearly related to the medication.174 Ezetimibe can increase 
LDL
receptor
Golgi
PCSK9
PCSK9
No
PCSK9
LDL
Lysosomal
mAb to
PCSK9
LDL
LDL
LDL
LDL
LDL
LDL
LDL
LDL
LDL
LDL
LDL
LDL
• Fig. 41.18  Role of PCSK9 and its inhibitio

LDL
• Fig. 41.18  Role of PCSK9 and its inhibition in low-density lipoprotein (LDL) metabolism. Both the LDL 
receptor and proprotein convertase subtilisin/kexin type 9 (PCSK9) are processed by the Golgi apparatus 
with PCSK9 being secreted. In the absence of PCSK9 inhibition, this protein interacts with the LDL recep­
tor and promotes its retention in pathways that lead to lysosomal degradation of the LDL receptor protein. 
In the presence of PCSK9 inhibition, more of the LDL receptor recycles

CSK9 inhibition, more of the LDL receptor recycles to the cell surface, where it clears 
LDL particles from the circulation to lower LDL cholesterol and decrease cardiovascular risk.
1616
cyclosporine levels. Fibrates can increase the levels of ezetimibe, a 
finding with unknown clinical significance.175 Ezetimibe is con­
traindicated in pregnancy and in severe liver dysfunction. 
The bile acid sequestrants have 

Acid Sequestrants
The bile acid sequestrants have been used since the 1970s but now 
are primarily reserved for patients unable to take statins or who 
require additional cholesterol reduction. Clinical trials conducted 
prior to widespread use of statins showed that bile acid seques­
trants reduce cardiovascular events.176,177 They work by binding 
negatively charged bile acids and bile salts in the small intestine to 
interrupt the enterohepatic circulation of bile acids and increase 
the conv

c circulation of bile acids and increase 
the conversion of cholesterol into bile. Decreased hepatocyte cho­
lesterol content increases LDL receptors, leading to reduced cir­
culating LDL levels. Cholesterol synthesis also increases, which 
promotes VLDL secretion; for this reason, these drugs are contra­
indicated in patients with hypertriglyceridemia.
As monotherapy, sequestrants lower LDL by 5% to 30% in a 
dose-dependent manner. Cholestyramine, colestipol, and cole­
sevelam are available in 

e, colestipol, and cole­
sevelam are available in the United States. Colesevelam has greater 
bile acid–binding capacity and affinity than cholestyramine or 
colestipol and is used at lower doses. LDL reduction is typically 
15% at 3.8 g/day (six 625-mg tablets) and 18% at 4.3 g/day (seven 
625-mg tablets).178,179 Bile acid sequestrants lower fasting blood 
glucose and hemoglobin A1c levels in patients with diabetes mel­
litus.180 The mechanism underlying this effect is unclear but might 
relate

nderlying this effect is unclear but might 
relate to activation of a bile sensitivity receptor (TGR5) in the colon.
Gastrointestinal disturbances are common and include consti­
pation, nausea, bloating, abdominal pain, flatulence, and aggrava­
tion of hemorrhoids. Initiation with low doses, patient education, 
and use of stool softeners or psyllium can increase compliance. 
These drugs are not absorbed, so they can be used in pregnancy. 
Bile acid sequestrants affect absorption of a wide variet

id sequestrants affect absorption of a wide variety of drugs; 
other medications should be taken either 1 to 2 hours before or 4 
to 6 hours after the sequestrant. 
Fibrates
In the Helsinki Heart Study (primary prevention) and the Veterans 
Affairs–High-Density Lipoprotein Cholesterol Intervention Trial 
(secondary prevention), the use of the fibrate gemfibrozil reduced 
fatal and nonfatal cardiovascular events and did not increase mor­
tality from noncardiac causes.139,140 In the Fenofibrate In

om noncardiac causes.139,140 In the Fenofibrate Intervention 
and Event Lowering in Diabetes study, although fenofibrate did not 
reduce the composite cardiovascular outcome of cardiovascular death, 
nonfatal myocardial infarction, and stroke in patients with type 2 
diabetes mellitus, fewer nonfatal events were observed.141 The addi­
tion of fenofibrate to a statin may provide benefit to diabetes patients 
with high triglycerides and low HDL.142,143 Fenofibrate decreased 
the progression of ret

,143 Fenofibrate decreased 
the progression of retinopathy in patients with diabetes in the Feno­
fibrate Intervention and Event Lowering in Diabetes and ACCORD 
trials and is approved by the Australian Health Authorities for dia­
betic retinopathy.181,182 The underlying mechanism is unknown and 
does not appear to be related to effects on circulating lipids.
Fibrates activate PPARα (see earlier discussion), which increases 
fatty acid oxidation, increases LPL, and increases apoAI, as well 
as a

 increases LPL, and increases apoAI, as well 
as apoAII, while lowering triglycerides (by 30–50%), decreasing 
apoCIII, and raising HDL (by 10–20% in those with elevated tri­
glycerides). Fibrates may lower LDL modestly, but they are most 
commonly used for severe hypertriglyceridemia and combined 
hyperlipidemia. Fenofibrate may be taken once daily. Gemfibrozil 
is given twice a day with meals but is used less often due to a less 
favorable drug interaction profile.
Fibrates are contraindicated

 interaction profile.
Fibrates are contraindicated in patients with liver or gallblad­
der disease. The risk of gallstones may be increased. Liver trans­
aminases may increase, particularly with fenofibrate. Side effects 
may include gastrointestinal discomfort, rash, and pruritus. Feno­
fibrate should be avoided in patients with renal insufficiency, 
which predisposes to myopathy. The combination of gemfibrozil 
and most statins is associated with an increased risk of myopathy 
due to increased

th an increased risk of myopathy 
due to increased statin blood levels, whereas fenofibrate does not 
interfere with statin metabolism and is preferred in fibrate/statin 
combination regimens.183 Because of the effects on protein bind­
ing, warfarin doses may need to be adjusted when fibrate therapy 
is started. Gemfibrozil may be used beginning in the second tri­
mester in pregnant women with severely elevated triglycerides 
who are at risk for pancreatitis. 
Fish-derived om

Fish-derived omega-3 fatty acids (EPA and DHA) reduce circu­
lating triglycerides, but their effects on cardiovascular events are 
less clear. The Japan EPA Lipid Intervention Study showed that 
combined treatment with a statin plus EPA (1.8 g/day) in patients 
with coronary heart disease reduced major coronary events by 
19% compared with a statin alone.184 This effect was similar to 
that found in the recent REDUCE-IT trial.164 Other clinical out­
come studie

EDUCE-IT trial.164 Other clinical out­
come studies with fish oils and meta-analysis of trials with EPA 
and DPA do not show mortality or cardiovascular health bene­
fits.162,163,185 Whether the positive outcome of REDUCE-IT was 
due to its specific omega 3 formulation, subject selection, or dose 
is unclear.
Omega-3 fatty acids decrease triglyceride secretion from the 
liver through unclear mechanisms and are indicated for triglycer­
ides higher than 500 mg/dL (5.6 mmol/L). EPA and DHA lower 
t

 than 500 mg/dL (5.6 mmol/L). EPA and DHA lower 
triglycerides by 20% to 50% depending on the baseline levels. 
There is minimal effect on HDL. LDL may increase as VLDL is 
converted to LDL.
Approximately 3 to 4 g/day of EPA plus DHA is used to lower 
triglycerides. Over-the-counter preparations have variable quanti­
ties of EPA and DHA. Prescription formulations usually contain 
twice the content of omega-3 fatty acids than nonprescription 
preparations. Preparations contain EPA and DHA, uneste

arations. Preparations contain EPA and DHA, unesterified 
EPA and DHA, or only EPA. Unlike fibrates, EPA and DHA 
do not affect statin metabolism and do not increase the risk of 
myopathy. Side effects with omega-3 fatty acids include eructa­
tion, diarrhea, and abdominal discomfort. There is potential for 
increased bleeding, but this has not been seen in clinical trials. 
Niacin
Nicotinic acid, or niacin, is a B-complex vitamin that was found 
to lower plasma cholesterol in humans in 1955. The

to lower plasma cholesterol in humans in 1955. The mechanism 
underlying the effects of niacin are not understood beyond the 
physiologic observation of decreased VLDL secretion. Niacin low­
ers triglycerides by 10% to 30% and increases HDL by 10% to 
40%. Niacin also lowers Lp(a) by up to 25%. The niacin arm of 
the Coronary Drug Project showed a decrease in nonfatal myo­
cardial infarctions in men with coronary artery disease during the 
6-year trial and a decrease in total mortality rate in t

 trial and a decrease in total mortality rate in the 9 years after 
the study.186 However, more recent trials of slow-release niacin 
in subjects whose LDL was already markedly reduced by statins, 
the AIM-HIGH (Atherothrombosis Intervention in Metabolic 
Syndrome With Low HDL/High Triglycerides: Impact on Global 
Health Outcomes) and HPS2-THRIVE (Heart Protection Study 
2–Treatment of HDL to Reduce the Incidence of Vascular Events) 
studies, failed to show additional clinical benefit with niaci

ersion to bile acids. The combination 
can have additive effects.
	•	 This combination is useful for patients who can­
not take statins.
CHD, Coronary heart disease; LDL, low-density lipoprotein; PCSK9, proprotein convertase 
subtilisin/kexin type 9.
   
TABLE 41.14
despite improvements in HDL and triglycerides. This lack of 
proven efficacy and the introduction of new agents (ezetimibe and 
PCSK9 inhibitors) have reduced niacin use except in refractory 
hypercholesterolemia or hypertriglycerid

fractory 
hypercholesterolemia or hypertriglyceridemia.
Cutaneous flushing, most notable with the first doses, is the 
most common side effect. Tolerance can occur with repeated and 
consistent dosing. Flushing occurs 15 to 60 minutes after inges­
tion, lasts 15 to 30 minutes, and may be related to release of der­
mal prostaglandin D2. Ingestion with food and taking aspirin 
(preferably 325 mg) 30 to 60 minutes in advance of niacin mini­
mizes flushing,189 whereas alcoholic beverages and spicy f

ushing,189 whereas alcoholic beverages and spicy foods 
potentiate it. Prescription extended-release niacin (Niaspan) may 
be better tolerated than crystalline niacin. Starting low and gradu­
ally increasing the dose improves tolerability.
Niacin has additional adverse effects. Hepatotoxicity is the 
most serious and is seen more with crystalline niacin compared 
with prescription slow-release forms. Worsening of glucose toler­
ance and hyperuricemia may occur. Niacin is contraindicated in 
acti

emia may occur. Niacin is contraindicated in 
active peptic ulcer disease. Rare side effects include blurred vision 
and a reversible condition known as cystoid macular edema. 
Myopathy is rare with niacin alone or in combination with statins. 
Niacin is contraindicated during pregnancy. 
Combination Therapies
Combination therapy is indicated for patients with severe lipid 
elevations and those who have an insufficient response to mono­
therapy. Patients with FH or familial combined hyperlipidem

Patients with FH or familial combined hyperlipidemia 
are at particularly increased risk and may require LDL reductions 
that cannot be achieved with a single agent. LDL levels lower than 
70 mg/dL (1.8 mmol/L) can be difficult to achieve with a single 
drug. Higher statin doses may be associated with increased side 
effects. If the highest tolerated statin dose does not produce ade­
quate LDL reduction, adding an agent from a different class may 
produce the desired result. Statins, ezetimibe, 

produce the desired result. Statins, ezetimibe, PCSK9 inhibitors, 
and sequestrants work through different mechanisms and can be 
more effective in combination than when used alone. Data on 
clinical outcomes with combination therapy are limited. Table 
41.14 presents combination therapies for LDL.
Combination Therapy for Other Hyperlipidemias
Statin Plus Fibrate
The combination of a statin plus a fibrate can be used in patients 
requiring treatment for elevated triglycerides and elevated LDL,

tment for elevated triglycerides and elevated LDL, 
which may be useful for patients with metabolic syndrome, diabetes, 
or other forms of mixed dyslipidemia. Risk of myopathy, including 
rhabdomyolysis, is increased with the combination of most statins 
with gemfibrozil, because the latter drug interferes with the gluc­
uronidation of statins, leading to higher serum levels of the statin 
drug.190 Rhabdomyolysis is about 15 times less likely with fenofi­
brate combined with statins (0.58 per 1 

h fenofi­
brate combined with statins (0.58 per 1 million prescriptions) than 
with gemfibrozil plus statins (8.6 per 1 million prescriptions).183
Statin-fibrate combinations should be avoided in patients 
with renal insufficiency, congestive heart failure, severe debility, 
or other conditions that affect medication clearance. Side effects 
include mild gastrointestinal discomfort, rash, and pruritus. 
Other Combinations
Triple-drug therapy with a statin, ezetimibe, and fenofibrate may 
help to

 a statin, ezetimibe, and fenofibrate may 
help to obtain adequate reductions of both triglycerides and LDL 
without using very high statin doses or when PCSK9 inhibi­
tors are not covered by patient insurance. When triglycerides 
are decreased with a fibrate, LDL may increase. If statins are not 
tolerated and triglycerides well controlled, a bile acid sequestrant 
could be added. Ezetimibe may lower LDL in combination with 
a fibrate when triglyceride levels are not optimal. Adding omega-3 
fa

yceride levels are not optimal. Adding omega-3 
fatty acids can be helpful if triglycerides are not well controlled. 
For markedly elevated triglyceride levels, it may be necessary to 
combine a fibrate with niacin or with omega-3 fatty acids, or both. 
Treatment for Chylomicronemia Syndrome
1618
Patients with chylomicronemia syndrome should be treated with 
total fat restriction and other measures mentioned previously until 
the trigl

her measures mentioned previously until 
the triglycerides fall to less than 1000 mg/dL (11.3 mmol/L), after 
which a fat-restricted diet (e.g., <10% of calories) can be instituted. 
The goal is to maintain triglycerides under 1000 mg/dL (11.3 
mmol/L) and preferably lower if possible. Diet and modification of 
glycemia, alcohol consumption, or offending medications are use­
ful. Medium-chain triglyceride oils can be provided for cooking. A 
fibrate or niacin is usually required to control trigl

ate or niacin is usually required to control triglycerides. Therapy 
with orlistat to block fat absorption may be beneficial, because it 
mimics a low-fat diet.191 In the setting of pancreatitis risk, a low-fat/
low-carbohydrate liquid formula diet may be indicated.
Several novel therapies for hyperchylomicronemia and recur­
rent pancreatitis are on the horizon. A gene therapy treatment for 
LPL deficiency had been available in Europe using an adeno-asso­
ciated virus expressing LPLS447X, an act

eno-asso­
ciated virus expressing LPLS447X, an active truncated LPL. Effects 
were transient following multiple intramuscular injections. Addi­
tional therapies using methods to inhibit production of apoC-III 
or ANGPLT3 are in development. 
Treatment of FH
Guidelines for FH patients are available.192 Treatment of hetero­
zygous FH includes a diet low in total and saturated fats and cho­
lesterol, but effects on cholesterol are modest (5–15%). Adequate 
cholesterol lowering can occasionally be a

equate 
cholesterol lowering can occasionally be achieved with a single 
potent statin, but combinations of drugs are often needed. Many 
patients will achieve LDL levels below 100 mg/dL (2.59 mmol/L) 
with a high-intensity statin and ezetimibe. Addition of a PCSK9 
inhibitor to a statin or statin plus ezetimibe can lead to even greater 
LDL reduction.193 PCSK9 inhibitors have some effect in patients 
with mutated, but not absent, receptors.194 LDL apheresis or liver 
transplantation are other o

DL apheresis or liver 
transplantation are other options in these patients.195
The age at which drug treatment should begin in heterozygous 
FH is controversial, but starting treatment at younger ages may 
have a favorable impact during early stages of lesion development. 
Statins are approved for the treatment of children with heterozy­
gous FH who are 8 years of age (pravastatin) or 10 years (other 
agents) or older. Factors such as the age at onset of coronary dis­
ease in parents and grandpa

onset of coronary dis­
ease in parents and grandparents and the presence of other risk 
factors should be considered.
Several medical approaches for treatment of homozygous FH 
have been developed. Mipomersen is an apoB antisense oligonu­
cleotide that was approved but rarely used in the United States 
for the treatment of homozygous FH. It prevents apoB transla­
tion, which decreases apoB levels and LDL.196 It lowers LDL in 
patients with homozygous and heterozygous FH.197,198 Pyrexia, 
body ac

gous and heterozygous FH.197,198 Pyrexia, 
body aches, and reactions at the injection site are the most com­
mon side effects, but potential hepatotoxicity and adverse cardio­
vascular effects are the most serious. Lomitapide is an inhibitor of 
MTP and approved in the United States and European Union for 
the treatment of homozygous FH. MTP (see earlier discussion) is 
required for VLDL assembly. Its inhibition may decrease LDL by 
50% in homozygous FH.199 Side effects include diarrhea, elevate

gous FH.199 Side effects include diarrhea, elevated 
hepatic transaminases, and increased hepatic fat. In a long-term 
study in homozygous FH, lomitapide decreased LDL by 50% 
during the initial phase of the study and by 31% at 78 weeks of 
treatment.200
Treatment of familial defective apoB100 is similar to that of 
heterozygous FH and consists of a low-fat, low-cholesterol diet 
and a combination drug regimen. Family members at risk should 
also be screened for the dominant mutation. 
Treatment

 be screened for the dominant mutation. 
Treatment of Familial Combined Hyperlipidemia
Weight reduction and dietary treatment can help correct meta­
bolic abnormalities such as obesity and insulin resistance that con­
tribute to the hyperlipidemia. Drug therapy should be directed at 
the predominant lipid abnormality. Statins are most appropriate 
for most patients. Fibrates can lower triglycerides and raise HDL, 
and they reduce the incidence of coronary events in insulin-resis­
tant hypertrigl

 coronary events in insulin-resis­
tant hypertriglyceridemic patients with low HDL. Patients with 
low HDL should be treated with statins. Because familial com­
bined hyperlipidemia is associated with premature coronary heart 
disease, affected family members should be identified. 
Treatment of Metabolic Syndrome
Metabolic syndrome is an extremely common condition defined 
by the presence of at least three of the five features presented in 
Table 41.5: increased abdominal girth, hypertriglycerid

 41.5: increased abdominal girth, hypertriglyceridemia, low 
HDL, hypertension, and elevated fasting glucose. Metabolic syn­
drome is clearly associated with increased risk of vascular disease 
and development of type 2 diabetes mellitus, and it is equivalent 
to what was previously referred to as prediabetes. Because obe­
sity is a risk for coronary disease, as well as diabetes mellitus and 
dyslipidemia, a body mass index less than 25 kg/m2 in patients 
of European ancestry (lower for those fr

patients 
of European ancestry (lower for those from southern and eastern 
Asia) is a goal that may require combinations of dietary, exercise, 
pharmacologic, and surgical therapies (see Chapter 40). Because 
this goal is often not achievable, these patients may also need 
therapies directed at hypertension, smoking, diabetes, and dys­
lipidemia. All patients should be assessed according to existing 
guidelines for treatment of high triglycerides, low HDL, hyper­
tension, and hyperglycemia. 
Tre

, low HDL, hyper­
Treatment of Dysbetalipoproteinemia
Because dysbetalipoproteinemia is influenced by coexisting meta­
bolic conditions, a vigorous effort should be made to identify and 
treat obesity, diabetes mellitus, and hypothyroidism, and to reduce 
alcohol consumption. Lipid abnormalities can often be resolved 
without the use of drug therapies. Dysbetalipoproteinemia is asso­
ciated with hypothyroidism in particular and responds dramati­
cally to thyroid horm

icular and responds dramati­
cally to thyroid hormone replacement therapy. Dietary therapy 
should be aimed at restricting total fat; saturated fat; cholesterol; 
and, if appropriate, calories. If diet and treatment of coexisting 
metabolic conditions are unsatisfactory, drug therapy should be 
initiated using statins. Combination therapy may be required. 
Because the disorder is associated with premature vascular disease, 
first-degree relatives should be screened for the presence of apoE2 
(se

 should be screened for the presence of apoE2 
(see earlier discussion). 
Treatment for Elevated Plasma Lp(a)
There are no outcome trials of lowering Lp(a). Newer antisense 
and silencing RNA approaches are being studied in clinical tri­
als.201 Apheresis lowers Lp(a) and has been used in those with 
elevated Lp(a) and progressive coronary heart disease.202 Evo­
locumab, alirocumab, niacin, mipomersen, and lomitapide each 
lower Lp(a), although modestly, in addition to lowering LDL. 
Treatment f

odestly, in addition to lowering LDL. 
Treatment for Low Levels of HDL
Patients with familial hypoalphalipoproteinemia can have nor­
mal or modestly increased plasma cholesterol but very low HDL, 
resulting in a predisposition to coronary heart disease. Such 
patients can have high ratios of total cholesterol to HDL (e.g., 
>10) despite having normal plasma cholesterol levels. Statins 
lower total cholesterol and represent the most efficacious way to 
1619
Chapter 41  Disorders of Lipid Metaboli

y to 
1619
lower the ratio of total cholesterol to HDL. Statins decrease clini­
cal events in patients with low HDL.203 Fibrates do not increase 
HDL in patients with normal triglycerides. As noted earlier, phar­
macologic therapies for raising HDL levels do not provide clinical 
benefits. 
Summary of Treatment of Lipid Disorders
In the absence of clear demonstration of serious adverse effects, 
statins should be considered for most common lipid disorder

hould be considered for most common lipid disorders. 
Inappropriate false claims about harms associated with statins 
likely contribute to discontinuation and underuse of these agents. 
Abundant evidence indicates that any future findings about the 
effects of statins will not fundamentally alter the favorable balance 
of benefits over risks for this class of drugs.204
         
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ion of acute 
coronary events with lovastatin in men and women with aver­
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Texas Coronary Atherosclerosis Prevention Study. J Am Med Assoc. 
1998;279(20):1615–1622.
	204.	 Collins R, Reith C, Emberson J, et  al. Interpretation of the 
evidence for the efficacy and safety of statin therapy. Lancet. 
2016;388(10059):2532–2561.
1622
42
Multiple Endocrine Neoplasia
PAUL J. NEWEY AND RAJESH V. THAKKER
Introduction to Multiple Endocrine 
Ne

 V. THAKKER
Neoplasia Syndromes
Multiple endocrine neoplasia (MEN) is characterized by the occur­
rence of tumors involving two or more endocrine glands within a 
single patient.1–3 The disorder has previously been referred to as 
multiple endocrine adenopathy or pluriglandular syndrome. How­
ever, glandular hyperplasia and malignancy may also occur in some 
patients, and the term multiple endocrine neoplasia is now preferred. 
Four major forms of MEN—MEN type

s now preferred. 
Four major forms of MEN—MEN types 1 through 4—are recog­
nized, and each form is characterized by the development of tumors 
within specific endocrine glands1,4 (Table 42.1). All of these forms 
of MEN may be inherited as autosomal dominant disorders or may 
occur sporadically in the absence of a family history.1,3 However, 
this distinction between sporadic and familial cases may sometimes 
be difficult, because in some sporadic cases, a familial history may 
be absent because

c cases, a familial history may 
be absent because the patient with the disease may have died before 
symptoms developed. In addition to MEN types 1 through 4, six 
other syndromes, which are associated with tumors involving one 
or more of the endocrine glands as well as nonendocrine organs, 
have been reported.5–10 These include the hyperparathyroidism-jaw 
tumor syndrome, von Hippel–Lindau disease, Carney complex, 
neurofibromatosis type 1, Cowden syndrome, and McCune-
Albright syndrome; all 

wden syndrome, and McCune-
Albright syndrome; all of these may be inherited as autosomal 
dominant disorders, except McCune-Albright syndrome, which 
is due to a mosaic expression of a postzygotic somatic cell muta­
tion. This chapter will focus on describing the major clinical and 
molecular aspects of MEN type 1 through 4 syndromes. 
Introduction to Multiple Endocrine Neoplasia Syndromes, 1622
MEN Type 1, 1623
MEN Types 2 and 3, 1640
MEN Type 4, 1656
Future Directions and Concl

 1640
Future Directions and Concluding Remarks, 1657
KEY POINTS
	•	
Multiple endocrine neoplasia (MEN) syndromes, which may be 
inherited as autosomal dominant traits, are characterized by the 
occurrence of two or more tumors in a patient, and four major 
types (MEN1–4) are recognized.
	•	
MEN1 is characterized by occurrence of parathyroid, anterior 
pituitary, and duodenopancreatic neuroendocrine tumors, and 
occasionally also foregut carcinoid and adrenal tumors. MEN1 
is c

o foregut carcinoid and adrenal tumors. MEN1 
is caused by mutations of the MEN1 gene, which encodes the 
tumor suppressor protein menin.
	•	
MEN2 (also referred to as MEN2A) is characterized by occurrence 
of medullary thyroid carcinoma (MTC), pheochromocytomas, 
and parathyroid tumors. MEN2 includes the variants MEN2A 
with Hirschsprung disease, MEN2A with cutaneous lichen amy­
loidosis, and familial MTC-only.
	•	
MEN3 (also referred to as MEN2B) is characterized by occurrence 
of MTC and ph

N2B) is characterized by occurrence 
of MTC and pheochromocytomas in association with a mar­
fanoid habitus, mucosal neuromas, medullated corneal fibers, 
and intestinal ganglioneuromatosis.
	•	
MEN2 and MEN3 are due to rearranged during transfection (RET) 
proto-oncogene mutations that lead to constitutive activation 
of the encoded receptor tyrosine kinase.
	•	
MEN4 patients may develop parathyroid, anterior pituitary, and 
pancreatic neuroendocrine tumors with gonadal, adrenal, renal, 
and 

docrine tumors with gonadal, adrenal, renal, 
and thyroid tumors due to mutations of the cyclin-dependent 
kinase inhibitor p27Kip1 (CDKN1B) gene.
	•	
Genetic testing should be offered to MEN patients and their 
first-degree relatives, and individuals with a mutation, who are 
at risk of developing tumors, should have periodic clinical, bio­
chemical, and radiologic screening for the early detection and 
treatment of tumors.
	•	
Treatment of MEN patients, which aims to minimize the disease-
as

N patients, which aims to minimize the disease-
associated morbidity and mortality, while maintaining quality of 
life, requires a multidisciplinary approach.
1623
Chapter 42  Multiple Endocrine Neoplasia
  MEN Syndromes and Their Characteristic Tumors and Associated Genetic Abnormalities
Type (Chromosome Location)
Tumors (Estimated Penetrance)
Gene; Most Frequently Mutated Codons
Parathyroid adenoma (90%)
MEN1
Enteropancreatic tumor (30–70%)
83/84, 4-bp del (≈4%)
— Gastrinoma (40%

r (30–70%)
— Gastrinoma (40%)
119, 3-bp del (≈3%)
— Insulinoma (10%)
209-211, 4-bp del (≈8%)
— Nonfunctioning (20–55%)
418, 3-bp del (≈4%)
— Glucagonoma (<1%)
514-516, del or ins (≈7%)
— VIPoma (<1%)
Intron 4 ss (≈10%)
Pituitary adenoma (30–40%)
— Prolactinoma (20%)
— Somatotropinoma (10%)
— Corticotropinoma (<5%)
— Nonfunctioning (<5%)
Associated tumors
— Adrenal cortical tumor (20–40%)
— Pheochromocytoma (<1%)
— Bronchopulmonary NET (2%)
— Thymic NET (2%)
— Gastric NET (1

monary NET (2%)
— Gastric NET (10%)
— Lipomas (30%)
— Angiofibromas (85%)
— Collagenomas (70%)
— Meningiomas (8%)
MEN2b, also known as MEN2A
MTC (90%)
RET
(10 cen-10q11.2)
Pheochromocytoma (50%)
634, missense (e.g., Cys→Arg)
Parathyroid adenoma (20–30%)
MEN3, also known as MEN2B
MTC (>90%)
RET 918, Met→Thr
Pheochromocytoma (40–50%)
Associated abnormalities (40–50%)
Mucosal neuromas
Marfanoid habitus
Medullated corneal nerve fibers
Megacolon
MEN4 (12p13)
Parathy

orneal nerve fibers
Megacolon
Parathyroid adenomaa
CDKN1B,
no common mutations identified
Reproduction organ tumorsa (e.g., testicular cancer, neuroendocrine 
cervical carcinoma)
?Adrenal + renal tumorsa
aInsufficient numbers reported to provide prevalence information.
bMEN2 comprises variants that include familial MTC-only, MEN2A with cutaneous lichen amyloidosis, and MEN2A with Hirschsprung disease.
Autosomal-dominant inheritance of MEN1 syndrome has been establ

inant inheritance of MEN1 syndrome has been established.
del, Deletion; ins, insertion; MEN, multiple endocrine neoplasia; MEN1, MEN type 1; MEN2, MEN type 2; MEN3, MEN type 3; MEN4, MEN type 4; NET, neuroendocrine tumor; PPoma, pancreatic polypep­
tide–secreting tumor; VIPoma, vasoactive intestinal polypeptide–secreting tumor; MTC, medullary thyroid cancer.
Modified from Thakker RV, Newey PJ, Walls GV, et al. Clinical practice guidelines for multiple endocrine neoplasia type 1 (MEN1). J Clin En

tiple endocrine neoplasia type 1 (MEN1). J Clin Endocrinol Metab. 2012;97(9):2990–3011.
   
TABLE 42.1
MEN Type 1
MEN1, which has been referred to as Wermer syndrome, is an 
autosomal dominant disorder with an estimated prevalence of 1 
in 30,000 of the population. MEN1 is characterized by the com­
bined occurrence of parathyroid, pituitary, and duodenopancre­
atic neuroendocrine tumors (NETs). In addition, patients may 
also develop other endocrine tumors (e.g., adrenal cortical tumors, 
carcin

ine tumors (e.g., adrenal cortical tumors, 
carcinoids of the thymus and bronchus) and nonendocrine 
tumors (e.g., meningiomas, facial angiofibromas, collagenomas, 
and cutaneous lipomas) (see Table 42.1). The first description 
of MEN1 was reported by Erdheim11 in 1903, at autopsy in a 
patient with an anterior pituitary tumor and enlarged parathy­
roid glands. In the 1920s, the occurrence of pancreatic islet cell 
tumors in association with parathyroid and pituitary tumors was 
reported,12,13 

athyroid and pituitary tumors was 
reported,12,13 and throughout 1930 and 1960, the triad of para­
thyroid, pancreatic islet cell, and anterior pituitary tumors became 
recognized as characteristic of MEN1, together with the familial 
basis and autosomal dominant inheritance of the syndrome.14–16 
Studies throughout the 1980s and 1990s of MEN1 families and 
1624
SECTION IX    Polyendocrine and Neoplastic Disorders
MEN tumors led to the identification of the MEN1 gene, which is 
located on chromo

tion of the MEN1 gene, which is 
located on chromosome 11q13.17,18 Since then, the implementa­
tion of germline MEN1 genetic testing of affected individuals (and 
their relatives) has transformed the diagnosis and management of 
the disorder. In addition, somatic MEN1 mutations have been 
identified as major drivers of sporadic parathyroid and pancreatic 
NETS, and this has widened the biologic and clinical significance 
of the MEN1 gene and its encoded protein, menin, which con­
sists of 610 am

encoded protein, menin, which con­
sists of 610 amino acids and is a nuclear protein that acts as a 
tumor suppressor by interacting with other proteins in transcrip­
tion regulation, genome stability, cell division, proliferation, and 
epigenetic regulation.
Clinical Features and Management
The clinical manifestations of MEN1 are related to the sites of 
tumor development and/or consequences of hormone hyperse­
cretion. MEN1 is highly penetrant; virtually all patients develop 
clinical or bioch

 virtually all patients develop 
clinical or biochemical evidence of tumor development by age 
50 years. MEN1 tumors are unusual in early childhood (i.e., ≤5 
years of age) but thereafter demonstrate an increasing age-related 
penetrance—approximately 75% of patients will have one or 
more tumors by age 20 years.19 Parathyroid tumors are typi­
cally the first manifestation of disease in 75% to 90% of MEN1 
patients (see Table 42.1), although childhood presentations 
with pancreatic NETs (e.g., i

dhood presentations 
with pancreatic NETs (e.g., insulinoma) or pituitary tumors are 
not infrequent, whereas some patients may present with gastri­
noma, thymic carcinoid, or adrenal tumors. Overall, clinically 
relevant duodenopancreatic NETs, including both hormone-
secreting and nonsecreting tumors (Fig. 42.1), occur in 40% 
to 70% of patients, whereas anterior pituitary tumors occur 
in 30% to 40% of patients. The frequency of other endocrine 
tumors is variable: 20% to 55% of MEN1 patients

e 
tumors is variable: 20% to 55% of MEN1 patients have adrenal 
tumors, whereas fewer than 10% manifest thymic or bronchial 
tumors. The recognition and appropriate management of the 
MEN1-associated tumors is important because they are associ­
ated with high morbidity and mortality—approximately 30% 
to 70% of MEN1 patients will die of causes directly related to 
MEN1, with malignant duodenopancreatic NETs and thymic 
carcinoid tumors accounting for the greatest risk of premature 
death.20
A d

g for the greatest risk of premature 
death.20
A diagnosis of MEN1 may be established in an individual by 
one of three criteria1,21,22: on the basis of the occurrence of two 
or more primary MEN1-associated endocrine tumors (i.e., para­
thyroid adenoma, enteropancreatic tumor, and pituitary ade­
noma); the occurrence of one of the MEN1-associated tumors 
in a first-degree relative of a patient with a clinical diagnosis of 
MEN; and identification of a germline MEN1 mutation in an 
individual, w

n of a germline MEN1 mutation in an 
individual, who may be asymptomatic and has not yet devel­
oped serum biochemical or radiologic abnormalities indicative 
of tumor development.
The management of each of the respective MEN1-associated 
tumors is broadly similar to that of their sporadic counterparts, 
although there are several MEN1-specific factors that require 
consideration. Most importantly, MEN1-associated tumors are 
frequently multiple, resulting in a reduced likelihood of surgical 
cu

 resulting in a reduced likelihood of surgical 
cure. For example, MEN1 patients often develop multiple small 
submucosal duodenal gastrinomas, and achieving biochemical 
remission is difficult without extensive surgical resections. In this 
setting, disease control with proton pump inhibitor (PPI) therapy 
may be considered a suitable alternative and has been associated 
with improved long-term outcomes. Similarly, the occurrence of 
synchronous pancreatic NETs may make planning of therapeutic 

 pancreatic NETs may make planning of therapeutic 
interventions challenging (e.g., localizing functioning tumors for 
resection), and it is important to consider that any remnant pan­
creatic tissue post tumor resection will remain at risk of further 
tumor development. Thus, the goal of treatment in MEN1 should 
be to balance the potential risks and benefits of any intervention 
with the ultimate aim of minimizing disease-associated morbidity 
and mortality while preserving the patient’s quali

and mortality while preserving the patient’s quality of life. In this 
regard, it is important that MEN1 is managed by a multidisci­
plinary team, and that patients play an active role in the decision-
making process.
Parathyroid Tumors
Primary hyperparathyroidism (PHPT) is the most common 
feature of MEN1. PHPT occurs in approximately 95% of all 
patients1,23–25 and is the first manifestation of MEN1 in 75% 
to 90% of patients.19,23 Patients are frequently asymptomatic 
with o

19,23 Patients are frequently asymptomatic 
with only biochemical evidence of disease, although symptom­
atic presentations due to hypercalcemia (i.e., polyuria, polydip­
sia, constipation, malaise) or other manifestations, including 
nephrolithiasis, osteitis fibrosa cystica, or peptic ulceration, may 
occur.1,26 The diagnosis of PHPT is made by the demonstra­
tion of hypercalcemia in the presence of raised or inappropriately 
normal circulating parathyroid hormone (PTH) concentrations. 
The de

 parathyroid hormone (PTH) concentrations. 
The degree of hypercalcemia is usually mild, and severe hyper­
calcemia or parathyroid carcinoma is rare.1 PHPT in MEN1 
patients usually occurs in those older than 15 years, although 
the earliest symptomatic and asymptomatic presentations are 
reported in children of 8 and 4 years of age, respectively.19 Bio­
chemical evidence of PHPT has been reported in up to 75% 
of children and young adults with MEN1 (<21 years of age), 
although only a minority 

EN1 (<21 years of age), 
although only a minority of these patients will have clinical 
features (e.g., nephrolithiasis).19 In addition to the early age of 
onset, MEN1-associated PHPT has other important differences 
when compared with non-MEN1 PHPT, and these include an 
equal sex distribution (male to female, 1:1 vs. 1:3, respectively) 
and the synchronous or asynchronous involvement of all four 
parathyroid glands with tumors,1,19,25,27 resulting from the 
monoclonal expansion of one or more

ting from the 
monoclonal expansion of one or more population of cells within 
individuals glands due to biallelic inactivation of the MEN1 
gene.24,28,29 MEN1-associated PHPT is reported to be associ­
ated with a greater reduction in bone mineral density than that 
occurring in non-MEN1 PHPT, and therefore osteoporosis and 
osteopenia are common in MEN1 patients.30,31 The reduction 
in bone mineral density and bone demineralization in MEN1 
patients is particularly evident at the lumbar spine, 

ents is particularly evident at the lumbar spine, femoral neck, 
and distal radius when compared with those with equivalent spo­
radic PHPT.31,32 These observations may be due to earlier age of 
onset and chronicity of MEN1-associated PHPT or differences 
in disease pathogenesis. 
Treatment
Surgical removal of the overactive parathyroid glands is the treat­
ment of choice for MEN1-associated PHPT. However, several 
aspects of management remain controversial, including the indi­
cations for and t

ntroversial, including the indi­
cations for and timing of surgery and the extent of surgery.1 These 
uncertainties reflect a paucity of high-quality evidence to guide 
clinical recommendations.1 Currently, surgery is recommended 
for MEN1-associated PHPT in those with symptomatic disease, 
1625
severe hypercalcemia (i.e., >3.00 mmol/L), and/or evidence 
of end-organ damage (e.g., nephrolithiasis, hypercalciuria [>9 
mmol/L per 24 hours or 400 mg per 24 h

ciuria [>9 
mmol/L per 24 hours or 400 mg per 24 hours], creatinine clear­
ance <60 mL/minute, reduced bone mineral density [i.e., T-score 
<–2.5] and/or previous fragility fracture).1 Most centers will rec­
ommend subtotal parathyroidectomy (removal of 3–3.5 glands) 
or total parathyroidectomy with or without autotransplantation 
of cryopreserved parathyroid tissue.1,33–40 Concurrent transcervi­
cal thymectomy is also suggested at the time of neck surgery to 
remove parathyroid tumors that may 

ck surgery to 
remove parathyroid tumors that may be embedded in the thy­
mus.1 Minimally invasive selective parathyroidectomy, unilateral 
clearance, and less than subtotal parathyroidectomy (i.e., removal 
of <3–3.5 glands) are not recommended because all four para­
thyroid glands are usually affected with multiple adenomas or 
hyperplasia, although this histologic distinction may be difficult. 
The aims of parathyroid surgery in MEN1 are to maintain normo­
calcemia for as long as possible and

intain normo­
calcemia for as long as possible and to avoid iatrogenic complica­
tions of surgery, including laryngeal nerve damage and permanent 
A
0 cm
5
10
B
F
C
D
E
H&E
G
I
J
H&E
Insulin
Gastrin
Chromogranin A
MIB-1
Menin
Menin
H
• Fig. 42.1  Nonfunctioning pancreatic NET in a 14-year-old MEN1 patient.  The abdominal magnetic reso­
nance imaging scan demonstrates a low-intensity, larger than 2.0 cm (anteroposterior maximal diam­
eter) tumor within the neck of pancreas (indicated by the arrow

ithin the neck of pancreas (indicated by the arrow) (A). There was no evidence of invasion 
of adjacent structures or metastases. The pancreatic NET was removed by surgery, and macroscopic 
examination confirmed the location of the tumor in the neck of the pancreas (white interrupted circles) (B). 
H&E examination demonstrated a tumor that was largely well circumscribed (C), but focally the margin 
between tumor (paler cells) and normal pancreas was poorly defined (D). Immunostaining supported t

was poorly defined (D). Immunostaining supported the 
clinical and biochemical diagnosis of a nonfunctioning pancreatic NET because the tumor did not have 
significant expression of gastrointestinal peptides (result for gastrin and insulin shown [E and F]) but did 
contain chromogranin A (G). The proliferative index measured by MIB-1 (Ki-67) was low, consistent with 
a low-grade tumor (H). Loss of menin expression was demonstrated in the tumor; in the adjacent nontu­
morous pancreatic tissue, nu

n the adjacent nontu­
morous pancreatic tissue, nuclear menin expression is evident within pancreatic islets (I), whereas nuclear 
menin expression is lost within the tumor (J), consistent with biallelic inactivation of the MEN1 gene. H&E, 
hematoxylin and eosin; MEN1, multiple endocrine neoplasia type 1; NET, neuroendocrine tumor. (A and C 
to J, modified from Newey PJ, Jeyabalan J, Walls GV, et al. Asymptomatic children with multiple endocrine 
neoplasia type 1 mutations may harbor nonfunction

neoplasia type 1 mutations may harbor nonfunctioning pancreatic neuroendocrine tumors. J Clin Endocri­
nol Metab. 2009;94[10]:3640–3646.)
1626
hypoparathyroidism. The lowest risk of persistent or recurrent 
PHPT occurs with subtotal and total parathyroidectomy, with 
higher rates of recurrence occurring in those with less than subto­
tal parathyroidectomy.36–39 Long-term outcome data for unilateral 
or minimally invasive parathyroidectomy has

teral 
or minimally invasive parathyroidectomy has not been reported, 
although recurrent disease is inevitable in this setting.41,42 Total 
parathyroidectomy is reported to be associated with the highest 
risk of permanent hypoparathyroidism, which may occur in 13% 
to 67% of cases.37,39 The development of permanent hypopara­
thyroidism requires lifelong therapy with active vitamin D metab­
olites (i.e., calcitriol or alfacalcidol), and this may be associated 
with significant morbidity (e.g., 

 be associated 
with significant morbidity (e.g., due to development of inadver­
tent significant hypo- or hypercalcemia). Total parathyroidectomy 
with autotransplantation of either fresh or cryopreserved parathy­
roid tissue into the forearm has therefore been considered as an 
alternate approach.1,35,43 The use of cryopreserved tissue allows 
confirmation of hypoparathyroidism postoperatively but is associ­
ated with a higher graft failure rate due to reduced cell viability, as 
well as a hig

e due to reduced cell viability, as 
well as a higher rate of permanent hypoparathyroidism. Moreover, 
recurrent disease is frequently observed in the transplanted tis­
sue,36 which may necessitate surgical removal,37 and the reported 
finding in a MEN1 patient of a metastatic thymic carcinoma 
within the parathyroid autotransplanted tissue highlights the 
need for caution with this surgical approach.44 Thus, the majority 
of centers recommend either subtotal parathyroidectomy (of at 
least 3.5 

ther subtotal parathyroidectomy (of at 
least 3.5 glands) or total parathyroidectomy with long-term oral 
calcitriol or alfacalcidol treatment. It is recommended that the 
timing and extent of surgical intervention should be taken by a 
multidisciplinary team, which takes into account the local surgical 
expertise, the availability of vitamin D analogues for subsequent 
treatment of long-term hypoparathyroidism, and the preferences 
of the patient. Cinacalcet, a calcimimetic that is an allosteri

t. Cinacalcet, a calcimimetic that is an allosteric mod­
ulator of the calcium-sensing receptor, has been used to reduce or 
normalize plasma calcium and PTH levels in MEN1 patients with 
PHPT in whom surgery is contraindicated because of comorbidi­
ties or where surgery has failed to cure the PHPT.45
Preoperative imaging (e.g., ultrasound, technetium-99m sesta­
mibi, computed tomography [CT], magnetic resonance imaging 
(MRI), and fluorine-18 fluorocholine positron emission tomog­
raphy [PET]-C

uorocholine positron emission tomog­
raphy [PET]-CT) is of limited value because all parathyroid 
glands may be affected, thereby necessitating an open bilateral 
neck exploration, and reducing the rationale for such preopera­
tive localization studies. Indeed, preoperative imaging in MEN1 
patients has been reported to not alter the surgical approach in 
more than 90% of patients; to have limited value in identifying 
ectopic parathyroid glands, which were correctly identified in only 
approxim

 which were correctly identified in only 
approximately 38% of cases; and to only correctly localize the larg­
est parathyroid gland subsequently identified at surgery in 69% of 
cases while failing to identify enlarged contralateral glands in 86% 
of these cases.34,46 However, the use of an intraoperative PTH 
assay may help identify hyperfunctioning parathyroid tissue.34,47 
For example, a reduction in PTH concentration greater than 75% 
is associated with a high biochemical cure rate, althoug

ociated with a high biochemical cure rate, although it does 
not exclude the development of recurrent hyperparathyroidism at 
a later time.47 Bilateral neck exploration, together with the use 
of intraoperative PTH measurement, may also indicate the likely 
presence of ectopic and/or supernumerary hyperfunctioning para­
thyroid glands, which occur predominantly within the thymus, 
with other locations being the mediastinum or carotid sheath. 
Indeed, the potential for supernumerary glands within

eed, the potential for supernumerary glands within the thymus 
has led to the recommendation for bilateral transcervical thymec­
tomy at the time of parathyroid surgery.1,34,37
The optimal management of asymptomatic MEN1 patients, 
including children and young adults who manifest only mild bio­
chemical features, remains to be defined; at present, some centers 
advocate early treatment to minimize impacts on bone health, 
whereas others favor conservative treatment involving the regular 
assessm

servative treatment involving the regular 
assessment of patients for the onset of symptoms and/or associated 
complications.1,19 
Pancreatic NETs
Pancreatic NETs remain the leading cause of premature death 
in MEN1 patients. Clinically apparent pancreatic NETs are 
reported in 30% to 80% of MEN1 patients,1,19,48–51 although 
microscopic islet tumors are found in almost all MEN1 patients 
when evaluated histopathologically.52 Pancreatic NETs (e.g., 
gastrinoma, insulinoma, and glucagonoma) may s

., 
gastrinoma, insulinoma, and glucagonoma) may secrete excess 
hormones and result in relevant clinical features or may be 
nonsecreting (also referred to as nonfunctioning [NF]) tumors 
(see Fig. 42.1), and these include those producing pancreatic 
polypeptide (PP) that is not associated with manifestations of 
hormonal excess.1 A MEN1 patient may have more than one 
pancreatic NET—for example, 10% of patients may have gas­
trinomas and insulinomas, which may arise co-temporarily or at 
diffe

nomas, which may arise co-temporarily or at 
different times.23 The main goals of treatment for these MEN1-
associated pancreatic NETs are to reduce the morbidity and 
mortality associated with their occurrence (i.e., relief of symp­
toms and risk of malignancy). However, there are many different 
treatments available (Fig. 42.2), and the absence of high-quality 
evidence for their efficacy makes it challenging to decide on cor­
rect therapy. For example, the ideal treatment of a nonmeta­
static

 example, the ideal treatment of a nonmeta­
static, single pancreatic NET is surgical removal, as this offers 
the only potentially curative treatment. However, this situation 
rarely arises in MEN1 patients, who will have multiple pan­
creatic NETs—for example, gastrinomas and insulinomas are 
multiple in more than 95% and approximately 40% of MEN1 
patients, respectively, with sizes varying from microadenomas to 
larger than 4 cm. The clinical behavior of these pancreatic NETs 
also varies, an

behavior of these pancreatic NETs 
also varies, and generally all macroscopic pancreatic NETs are 
potentially malignant, although the aggressiveness of an indi­
vidual pancreatic NET cannot be accurately predicted by tumor 
size, radiologic features, or hormone production.53 However, 
studies have shown that most microadenomas are stable and 
infrequently increase in size, 30% to 40% of macroadenomas 
smaller than 2 cm will increase in size over 10 years,49 approxi­
mately 50% to 70% of pancrea

 10 years,49 approxi­
mately 50% to 70% of pancreatic NETs between 2 and 3 cm 
will be associated with lymph node metastases, and approxi­
mately 25% to 40% of pancreatic NETs larger than 4 cm will be 
associated with hepatic metastases.53 Survival in MEN1 patients 
correlates with nonmetastatic disease—for example, survival at 
15 years in MEN1 patients with gastrinomas smaller than 2.5 
cm in size that were associated with nonmetastatic or metastatic 
disease has been reported to be 100% and 5

static 
disease has been reported to be 100% and 50%, respectively. 
Thus, the occurrence of multiple pancreatic NETs and their var­
ied and unpredictable malignant potential in MEN1 patients 
pose major challenges in their management.1,53 The diagnosis 
and treatments for these MEN1-associated pancreatic NETs 
will be reviewed. 
Gastrinoma
Gastrin-secreting tumors are associated with a marked overpro­
duction of gastric acid, which results in recurrent peptic ulcer­
ation, a c

hich results in recurrent peptic ulcer­
ation, a combination referred to as Zollinger-Ellison syndrome 
1627
(ZES).54,55 Symptoms of ZES include those associated with pep­
tic ulceration (i.e., abdominal pain, heartburn), as well as weight 
loss, diarrhea, and steatorrhea.1 In addition, esophageal stricture 
and/or Barrett esophagus are also more common in patients with 
ZES, and acute presentations with small bowel perforation and/or 
hemorrhage secondar

mall bowel perforation and/or 
hemorrhage secondary to peptic ulceration contribute to the high 
morbidity associated with ZES.56 Symptomatic presentations are 
rare in childhood, although they have been reported in children 
younger than 10 years.19 Gastrinomas occur in approximately 
20% to 60% of MEN1 patients53,57–59 and are found more fre­
quently in adult males.27 Approximately 20% of patients with 
sporadic gastrinoma will have MEN1.1 MEN1-associated gastri­
nomas frequently occur as smal

-associated gastri­
nomas frequently occur as small (<5 mm in diameter), multiple 
nodular lesions deep within the duodenal mucosa and are only 
rarely observed in the pancreas,57,60 in contrast to sporadic gastri­
nomas, which typically occur as solitary tumors within the pan­
creas or duodenum. In addition, MEN1-associated gastrinomas 
often occur as microscopic tumors (i.e., <1 mm), which despite 
their small size frequently metastasize to local lymph nodes at an 
early stage in the disease c

al lymph nodes at an 
early stage in the disease course.57,60 Indeed, local lymph node 
metastases are found in 30% to 70% of cases at diagnosis,53,61–63 
although advanced presentations with hepatic metastases are rare 
in MEN1; however, when present, they are associated with a poor 
prognosis.64,65 Additional poor prognostic indicators include 
markedly elevated gastrin levels, ectopic Cushing syndrome, and 
occurrence of primary pancreatic tumors.66 Gastrinoma in MEN1 
Alkylating 
agent
Antim

ors.66 Gastrinoma in MEN1 
agent
Antimetabolite
Anti-microtubule
agent
Topoisomerase 
inhibitor
Chemotherapy agent
Antibody
SSTR
Biotherapy
Somatostatin 
analogue
α interferon
IFNAR
RTK
mTOR
mTOR 
RTK 
VEGFR
VEGFA
VEGFA 
antibody
Cytotoxic 
antibiotics
Antibiotic
Pancreatic Neuroendocrine 
Tumor Cell
• Fig. 42.2  Current and emerging medical therapies for pancreatic neuroendocrine tumors (NETs).  
Medical therapies for pancreatic NETs include drugs, biotherapies

es for pancreatic NETs include drugs, biotherapies, and antibodies that target different 
pathways in cancer cells. Somatostatin analogues (e.g., octreotide and lanreotide) are used widely in 
the treatment of pancreatic NETs. Somatostatin analogues target members of the somatostatin receptor 
family on the tumor cell surface to control excess hormone secretion and inhibit growth (i.e., antiprolif­
erative effects). Additional medical therapies include mammalian target of rapamycin (mTOR) inhibi

nclude mammalian target of rapamycin (mTOR) inhibitors 
(e.g., everolimus) and receptor tyrosine kinase (RTK) inhibitors (e.g., sunitinib and pazopanib), which 
have been shown to delay pancreatic NET tumor progression. Current clinical trials are investigating the 
use of these agents in combination or with other therapies including monoclonal antibodies targeting 
the vascular endothelial growth factor receptor (VEGFR) (e.g., bevacizumab). Interferon-α (IFNα), which 
targets the IFNα/β-recepto

rferon-α (IFNα), which 
targets the IFNα/β-receptor (IFNAR) may also be effective in symptom and tumor control. Chemothera­
peutic agents may also be effective in the treatment of metastatic pancreatic NETs and include alkylating 
agents (e.g., streptozocin, temozolomide, cisplatin, cyclophosphamide, procarbazine, dacarbazine, and 
oxabiplatin), antimicrotubule agents (e.g., docetaxel and etoposide), antimetabolites (e.g., 5-fluorouracil, 
capecitabine, and gemcitabine), topoisomerase inhibitors

tabine, and gemcitabine), topoisomerase inhibitors (e.g., doxorubicin, etoposide, and irinotexan), 
and cytotoxic antibiotics (e.g., actinomycin D, doxorubicin, mitomycin C, mitoxantrone). Combinations 
of chemopreventative agents that target different cellular pathways are more frequently used rather than 
as a monotherapy. VEGFA, vascular endothelial growth factor A. (Modified from Frost M, Lines KE, Thak­
ker RV. Current and emerging therapies for PNETs in patients with or without MEN1. Nat R

 for PNETs in patients with or without MEN1. Nat Rev Endocrinol. 
2018;14[4];216–227.)
1628
patients appears to occur rarely in the absence of PHPT,58,67 and 
successful treatment of PHPT with restoration of normocalcemia 
is reported to result in symptomatic and biochemical improve­
ments in approximately 20% of MEN1 patients with hypergas­
trinemia and ZES.68
The diagnosis of gastrinoma is made by demonstrating an 
increased fasting serum ga

de by demonstrating an 
increased fasting serum gastrin in association with increased basal 
gastric acid secretion.1,61,69–71 A raised fasting serum gastrin alone 
is insufficient to make the diagnosis, as this may occur in achlor­
hydria, antral G-cell hyperplasia, Helicobacter pylori infection, 
renal failure, hypercalcemia, and PPI therapy.69–71 An intrave­
nous provocation test with secretin or calcium, which in patients 
with gastrinoma will be associated with a marked increase in 
gastrin

l be associated with a marked increase in 
gastrin, may help in the diagnosis.1 MEN1 gastrinomas in the 
duodenum may be localized by endoscopic ultrasound. CT, MRI, 
selective angiography, and/or somatostatin receptor scintigraphy 
combined with selective arterial secretagogue injection (e.g., cal­
cium) and hepatic venous gastrin measurements may help localize 
the tumor.1 
Treatment
The lack of results from prospective randomized controlled 
trials of treatments in MEN1 patients with gastrino

rials of treatments in MEN1 patients with gastrinomas makes 
their management challenging and reliant on expert opin­
ion.1,53 The aims of treatment should be to ameliorate the 
symptoms and/or sequelae of the associated hypergastrinemia 
while reducing the likelihood of developing advanced meta­
static disease. The medical treatment of gastrinoma has been 
transformed following the introduction of PPI therapies (e.g., 
omeprazole and lansoprazole), which are highly efficacious 
at reducing basa

e), which are highly efficacious 
at reducing basal acid secretion to less than 10 mmol/L and 
reducing symptoms associated with gastrinoma.1 H2-receptor 
antagonists (e.g., ranitidine) may be added if symptoms remain 
uncontrolled on high-dose PPI therapy. These therapies rep­
resent the mainstay of treatment for controlling symptoms 
and have resulted in a marked reduction in the morbidity and 
mortality previously associated with ZES in MEN1 patients. 
However, the effects on tumor growth and

atients. 
However, the effects on tumor growth and/or risk of develop­
ing advanced disease with this treatment are not established. 
In addition, the role of somatostatin analogue therapy in 
MEN1 gastrinomas, which may express somatostatin recep­
tors, remains to be established.
The role of surgery in MEN1-associated gastrinoma remains 
controversial, which in part is due to not knowing the long-term 
natural course of disease in MEN1 patients.53,72 Overall, the 
prognosis of gastrinoma in MEN

53,72 Overall, the 
prognosis of gastrinoma in MEN1 patients is excellent with 5-, 
10-, and 20-year survival rates of 90% to 96%, 75% to 96%, 
and 58% to 90%, respectively,73 with the higher survival esti­
mates observed following the introduction of acid-suppressive 
therapies. However, a minority of patients develop aggres­
sive disease, and identifying these patients remains challeng­
ing. Poor prognostic features in MEN1-associated gastrinoma 
include large tumor size, a pancreatic location

a 
include large tumor size, a pancreatic location of the primary 
tumor, presence of liver metastases, early age of onset, and male 
gender.65,73 The risk of hepatic metastases is higher for a pan­
creatic gastrinoma and correlates with tumor size, and surgery 
is therefore recommended for all gastrinomas larger than 2 cm 
in MEN1 patients.1,73 In all other settings, the role of surgery 
remains controversial, with some centers advocating an initial 
medical approach and others pursuing an earl

tial 
medical approach and others pursuing an earlier surgical inter­
vention. Centers advocating a nonsurgical approach point to the 
excellent long-term prognosis associated with smaller tumors, 
even in the presence of lymph node metastases; the low surgical 
cure rates in the presence of multiple small duodenal tumors; 
the potential high morbidity associated with pancreaticoduode­
nal resections; the excellent symptom control achieved with PPI 
therapy; and the lack of evidence demonstratin

PI 
therapy; and the lack of evidence demonstrating improved sur­
vival in those undergoing surgical resections.1,53,73 In contrast, 
centers advocating early surgical intervention in all patients with 
MEN1-associated gastrinoma point to results that have achieved 
eugastrinemia in 30% to 75% of patients for 3 to 5 years.74,75 
Surgical approaches in these studies have included duodenotomy 
with excision of gastrinomas in the duodenal mucosa coupled 
with enucleation (if feasible) or resection 

pled 
with enucleation (if feasible) or resection of tumors in the pan­
creatic head, peripancreatic and lymph node removal, and corpo­
racaudal pancreatic resection; partial pancreaticoduodenectomy; 
pancreas-preserving total duodenectomy; and total pancreatico­
duodenectomy.1,53,74–77 Total pancreaticoduodenectomy (i.e., 
Whipple procedure), which is associated with a substantially 
greater risk of diabetes mellitus and malabsorption, is rarely per­
formed and is typically reserved for patient

 per­
formed and is typically reserved for patients with diffuse large 
pancreatic tumors.1 However, long-term remission and survival 
data are not known, and currently the benefits of medical thera­
pies over such surgery have resulted in many centers and guide­
lines recommending nonsurgical management for gastrinomas 
in MEN1 patients, with surgery being reserved for patients in 
whom medical therapy has failed.1,53
The management of advanced/disseminated gastrinomas is 
difficult and does no

disseminated gastrinomas is 
difficult and does not differ from that of sporadic disease. Che­
motherapy with streptozotocin and 5-fluorouracil, capecitabine 
and temozolomide, cisplatin and etoposide, hormonal therapy 
with octreotide or lanreotide (which are human somatostatin 
analogues) (see Fig. 42.2), selected internal radiation therapy, 
radio-frequency ablation, peptide receptor radionuclide ther­
apy, hepatic artery embolization, administration of human 
leukocyte interferon, and remova

tration of human 
leukocyte interferon, and removal of all resectable tumor and 
hepatic transplantation have each been employed with occa­
sional benefit.53,74 
Insulinomas, which arise from pancreatic islet beta cells, occur 
in approximately 10% to 30% of patients with MEN1, whereas 
5% to 10% of patients presenting with insulinoma will have 
MEN1.1,19,23,25 Insulinomas in MEN1 patients frequently occur, 
in contrast to non-MEN1 patients, before the age of 40 year

st to non-MEN1 patients, before the age of 40 years and 
may be the first manifestation of MEN1 in approximately 10% of 
cases. Indeed, a recent study of 160 MEN1 patients younger than 
21 years observed insulinomas to occur in 12% of the cohort, with 
the earliest presentation at 5 years of age.19
MEN1 patients with insulinomas typically present with symp­
toms of hypoglycemia (e.g., weakness, headaches, sweating, faint­
ness, anxiety, altered behavior, seizures, and loss of consciousness) 
tha

ehavior, seizures, and loss of consciousness) 
that develop after fasting or exercise and improve after glucose 
(food) intake.19 The diagnosis is most reliably made by a super­
vised 72-hour fast,1,69,78 in which hypoglycemia (i.e., glucose <2.2 
mmol/L [40 mg/dL]) is documented in the presence of inappro­
priate elevated concentrations of insulin (together with proinsulin 
and C-peptide).1 It is also important to demonstrate the absence 
of sulfonylureas in the plasma and urine samples obtaine

fonylureas in the plasma and urine samples obtained dur­
ing the investigation of hypoglycemia. Medical therapy, which 
consists of frequent carbohydrate meals, diazoxide, and soma­
tostatin analogues, is not always successful, and removal of the 
insulinoma by surgery is the optimal treatment. Surgical success 
1629
is improved by preoperative localization of the tumor. The major­
ity of insulinomas are single lesions, occurring in the body or tail 
of t

ingle lesions, occurring in the body or tail 
of the pancreas, although multiple or multicentric insulinomas 
may be observed in approximately 40% of MEN1 patients.79 
Furthermore, concurrent pancreatic NETs (e.g., NF tumors) are 
frequently present in MEN1 patients, thereby posing a challenge 
for the correct localization of the insulinoma. Imaging modalities 
routinely employed for preoperative localization include endo­
scopic ultrasound, CT, and MRI, whereas more specialized meth­
ods, inclu

and MRI, whereas more specialized meth­
ods, including celiac axis angiography and selective intra-arterial 
calcium stimulation combined with hepatic venous insulin mea­
surements may also be required.80–82 Somatostatin receptor scin­
tigraphy (SRS) with 111In-octreotide has been reported to be 
associated with low sensitivity in insulinoma (20–60%), although 
68Ga-DOTATATE PET-CT may have a higher sensitivity.83 More 
recently, scintigraphy based on glucagon-like peptide 1 has been 
reported t

ed on glucagon-like peptide 1 has been 
reported to be highly sensitive in localizing insulinomas when 
compared with conventional imaging modalities and may provide 
a useful diagnostic modality.83–85 The utility of 18F-fluorodeox­
yglucose (FDG)-PET/CT for insulinoma detection is typically 
limited to high-grade metastatic disease.83 Finally, direct intraop­
erative pancreatic ultrasound may be used at the time of surgery 
to identify the likely tumor.80,86,87 
Treatment
Surgery is the treatme

y tumor.80,86,87 
Treatment
Surgery is the treatment of choice for those with nonmetastatic 
disease. Several surgical procedures have been reported to pro­
vide long-term curative outcomes, including enucleation or 
excision of single or multiple tumors, distal or partial pancre­
atectomy, and pancreatoduodenectomy.79,88,89 In one large series 
of MEN1 patients undergoing surgery for insulinoma, distal 
pancreatectomy was associated with a lower risk of hypoglycemia 
recurrence than enucleation

 risk of hypoglycemia 
recurrence than enucleation but a higher risk of surgery-related 
morbidity.90 Minimally invasive approaches (e.g., laparoscopic, 
robot assisted) may be appropriate in selected MEN1 cases.88,91 
The surgical approach adopted will depend on the location 
and size of insulinoma, as well as the presence or absence of 
additional pancreatic tumors. In MEN1 patients, insulinomas 
typically occur as benign tumors, and surgical outcomes and 
long-term prognosis are excellent.90 

utcomes and 
long-term prognosis are excellent.90 For advanced and metastatic 
disease, chemotherapy (using streptozotocin, 5-fluorouracil, and 
doxorubicin), and locoregional approaches, including hepatic 
artery embolization, may be employed for disease and symptom 
control1,53,92 (see Fig. 42.2). 
Glucagonoma
Glucagonomas, which arise from the pancreatic islet α-cells and 
lead to excess glucagon secretion, occur in 1% to 2% of MEN1 
patients.93 However, the characteristic c

of MEN1 
patients.93 However, the characteristic clinical features of skin rash 
(necrolytic migratory erythema), stomatitis, weight loss, venous 
thrombosis, and anemia may be absent.93 Instead, glucagonomas 
may be detected in asymptomatic MEN1 patients following sur­
veillance pancreatic imaging or the finding of marked hypergluca­
gonemia (i.e., greater than two times the upper limit of normal) 
with or without glucose intolerance. It should be noted that a 
significant proportion of NF panc

be noted that a 
significant proportion of NF pancreatic NETs immunostain for 
glucagon (either in insolation or together with additional pancre­
atic hormones) in the absence of hyperglucagonemia,52,94 whereas 
a proportion of NF tumors will be associated with modest eleva­
tions in plasma glucagon (i.e., less than two times the upper limit 
of normal). 
Treatment
Glucagonomas most frequently present in the tail of the pan­
creas, where possible surgical resection, which can be curative, is 
th

 surgical resection, which can be curative, is 
the treatment of choice. However, curative surgery may not be 
feasible considering that 50% to 80% of patients may have large 
tumors with metastases.1 Medical treatment with somatostatin 
analogues (e.g., octreotide or lanreotide) or chemotherapy (with 
streptozotocin and 5-fluorouracil or dimethyltriazeno-imidazole 
carboxamide) (see Fig. 42.2) has been successful in some patients, 
and hepatic artery embolization has been used to treat metastat

rtery embolization has been used to treat metastatic 
disease.1,53,92 
VIPoma
Patients with vasoactive intestinal peptide (VIP)omas, which 
are VIP-secreting pancreatic tumors, develop watery diarrhea, 
hypokalemia, and achlorhydria.1,93 This clinical syndrome has 
been referred to as Verner-Morrison syndrome, WDHA (watery 
diarrhea, hypokalemia, achlorhydria) syndrome, or VIPoma syn­
drome.1 Only a few patients with MEN1 have been reported to 
have VIPomas.93 The diagnosis is 

een reported to 
have VIPomas.93 The diagnosis is established by excluding laxative 
and diuretic abuse and confirming a stool volume in excess of 0.5 
to 1.0 L/day during a fast, together with a markedly increased 
plasma VIP concentration. 
Treatment
Surgical management of VIPomas, which are mostly located in 
the tail of the pancreas, has been curative, although approximately 
50% of patients have metastases at diagnosis.95,96 In patients with 
unresectable disease, treatment with somatostati

unresectable disease, treatment with somatostatin analogues such 
as octreotide and lanreotide, streptozotocin with 5-fluorouracil, 
corticosteroids, indomethacin, metoclopramide, and lithium car­
bonate has proved beneficial, and hepatic artery embolization has 
been useful for the treatment of metastases1 (see Fig. 42.2). 
NF Pancreatic NETs
NF pancreatic NETs are islet-derived tumors that do not give rise 
to a clinical syndrome associated with excess hormone produc­
tion,

drome associated with excess hormone produc­
tion, and hence these tumors may have presentations with symp­
toms related to local mass effects (i.e., pain and/or compression of 
adjacent structures) or metastatic disease (e.g., cachexia, jaundice, 
hepatomegaly, and hepatic and bone pains). However, some NF 
pancreatic NETs may be associated with elevations in PP and/
or glucagon,1,97 although plasma PP and glucagon, together with 
chromogranin A, are reported to have low sensitivity and speci­

, are reported to have low sensitivity and speci­
ficity for pancreatic NET detection in MEN1 patients,98,99 and 
circulating biomarkers for detection of NF pancreatic NETs are 
not available at this time. As a consequence, pancreatic imaging 
remains the mainstay of diagnosis, although the optimal modality 
for detection has not been established and frequently depends on 
local availability and expertise related to different modalities. NF 
pancreatic NETs in MEN1 are typically considered as a 

reatic NETs in MEN1 are typically considered as a single entity 
(see Fig. 42.1), although they likely represent a heterogeneous 
group of tumors with distinct pathologic and/or molecular sub­
types. The implementation of surveillance imaging programs for 
patients with MEN1 has shown that NF pancreatic NETs are the 
most prevalent pancreatic NET in MEN1, with clinically appar­
ent tumors occurring in 15% to 55% of patients.49,51,53,97,100 For 
example, a prospective endoscopic ultrasound study 

xample, a prospective endoscopic ultrasound study (EUS) dem­
onstrated that approximately 55% of MEN1 patients had one or 
more NF NETs,48 and a histopathologic study reported that almost 
1630
all MEN1 patients have small microadenoma NF tumors.52,53 In 
addition, NF pancreatic NETs have been reported to occur in 
more than 5% to 40% of children and young adults, 12 to 20 
years of age, with MEN1.19,97,101 The current guidelines therefore 
re

EN1.19,97,101 The current guidelines therefore 
recommend surveillance imaging for NF pancreatic NETs from 
age 10 years,1 with the aims of detecting and monitoring clinically 
relevant tumors yet minimizing exposure to ionizing radiation 
and/or iatrogenic complications relating to the procedure. EUS 
has been reported to be the most sensitive method for detection 
of tumors smaller than 1 cm, and to provide accurate estimates of 
tumor growth rates, and also provides the potential opportunity 

ates, and also provides the potential opportunity 
to assess the mitotic count and/or Ki67 index of tumors obtained 
from EUS-guided fine-needle aspirates.53,102 However, EUS is an 
invasive and time-consuming procedure that is dependent on user 
expertise, and it has been reported to overestimate the size of NF 
pancreatic NETs in MEN1, which has important implications 
considering that decisions regarding surgery are frequently based 
on size criteria.103 Alternate methods of imaging include M

riteria.103 Alternate methods of imaging include MRI, 
CT, SRS (i.e., based on octreotide/68Gallium-DOTATATE PET), 
and FDG-PET, and each has been associated with high sensitiv­
ity and specificity in different series such that combinations of 
these modalities are often used to fully characterize tumors and for 
assessing the likelihood of metastatic risk.53,104,105
NF pancreatic NETs are the leading cause of premature mor­
tality in patients with MEN1, and they are associated with a worse 
pro

th MEN1, and they are associated with a worse 
prognosis than other MEN1-associated pancreatic NETs (e.g., 
insulinoma, gastrinoma).20,53,61,106 Premature morbidity and 
mortality typically results from the development of metastatic dis­
ease but may also arise due to complications of surgical interven­
tion.20,49,51,106,107 The risk of developing liver metastases correlates 
with primary tumor size, and synchronous liver metastases have 
been reported in approximately 43% of patients with NF NE

ported in approximately 43% of patients with NF NETs 
larger than 3 cm, 18% of patients with NF NETS 2 to 3 cm, 10% 
in those with NF NETS 1 to 2 cm, and only 4% of those with NF 
NETS smaller than 1 cm.100 However, tumor size is not univer­
sally correct at predicting metastatic risk in NF NETs, as a small 
percentage of patients with small tumors are reported to develop 
advanced disease.108 In addition, currently circulating biomarkers 
are not available for predicting metastatic risk in NF p

t available for predicting metastatic risk in NF pancreatic 
NETs, and circulating biomarkers based on microRNA, circulat­
ing tumor cells, multigene signatures, and tumor DNA, which 
are being evaluated, may hold future promise in predicting tumor 
behavior.104,109,110 
Treatment
The aims of treatment for MEN1-associated NF NETs are to 
minimize the risk of developing metastatic disease while avoid­
ing unnecessary surgical interventions that are known to result in 
significant early and late c

e known to result in 
significant early and late complications.1,53,107 However, surgical 
removal of NF pancreatic NETs is of benefit, as illustrated by a 
recent study in which only 6 of 16 (∼40%) MEN1 patients with 
NF pancreatic NETs larger than 3 cm and who had surgery, devel­
oped hepatic metastases, or died compared with 5 of 6 (∼80%) 
patients who did not have surgery.51 However, the same study 
reported that rates of metastasis in patients undergoing surgery 
for NF pancreatic NETs smal

ts undergoing surgery 
for NF pancreatic NETs smaller than 2 cm were not significantly 
different from those managed without surgery.51 Thus, the major­
ity of centers recommend surgery for NF pancreatic NETS larger 
than 2 cm, especially because smaller NF pancreatic NETS (<2 
cm) have been reported to have stable appearances over an average 
of 10 years.49,111 For example, in one study, 60% (28 of 46) of 
patients with NF pancreatic NETs smaller than 2 cm had stable 
disease, whereas in the re

r than 2 cm had stable 
disease, whereas in the remaining 40% of patients who had an 
increase in size or number of tumors, or developed hypersecretion 
syndromes, only 15% (7 of 46) required surgery and only 2% (1 of 
46) of patients died from metastatic disease.49 Moreover, another 
study reported that surgery in MEN1 patients with NF NETs 
smaller than 2 cm does not affect progression-free survival (PFS) 
when compared with those patients managed conservatively, as 
the majority of tumors dis

ged conservatively, as 
the majority of tumors displayed indolent behavior.111 However, 
surgery is recommended when there is rapid tumor growth (i.e., 
doubling of tumor size over a 3–6 month interval) (see Fig. 42.1), 
and some centers will consider surgery if pancreatic NETs are 1 
cm or larger in size. The decision to undertake surgery for NF 
pancreatic NETs in MEN1 should consider the potential pres­
ence of additional tumors within the pancreas (and elsewhere), the 
presence of occult met

creas (and elsewhere), the 
presence of occult metastatic disease either related to the tumor 
undergoing resection or from another source, and the fact that any 
remnant pancreatic tissue will remain a risk for the development 
of further tumors. Thus, all of these considerations highlight the 
importance of multidisciplinary working and the involvement of 
the patient in the decision-making process.106
Medical treatments for small (i.e., <2 cm) NF pancreatic NETs 
in MEN1 include long-acting o

 NF pancreatic NETs 
in MEN1 include long-acting octreotide, which has been reported 
to be associated with a tumor response in 10% of patients, sta­
ble disease in 80% of patients, and disease progression in 10% 
of patients over 12 to 15 months of treatment,112 with reduc­
tions in PP and glucagon in those with stable tumor size.113 For 
patients with advanced disease, treatments include locoregional 
and systemic approaches (see Fig. 42.2) and are similar to those 
employed for the other panc

 are similar to those 
employed for the other pancreatic NETs. However, the evidence 
for the use of such treatments arose from study of non-MEN1 
patients, and extrapolation of this to MEN1 patients requires 
some caution.53 Somatostatin analogue therapy is reported to be 
associated with a approximately 50% reduction in the risk of dis­
ease progression in non-MEN1 patients with well-differentiated 
advanced pancreatic NETs.114,115 The receptor tyrosine kinase 
(RTK) inhibitor sunitinib is ass

 tyrosine kinase 
(RTK) inhibitor sunitinib is associated with an increase in PFS 
from 5.5 to 11.4 months in non-MEN1 patients with advanced 
well-differentiated pancreatic NETs,116 and the mammalian tar­
get of rapamycin inhibitor everolimus increases PFS from 6 to 11 
months in non-MEN1 patients with low- or intermediate-grade 
advanced pancreatic NETs.117 Additional therapies for advanced 
NF pancreatic NETs include chemotherapy, peptide receptor 
radionuclide therapy, and locoregional thera

ptor 
radionuclide therapy, and locoregional therapies (e.g., radio-fre­
quency ablation, transarterial chemoembolization, and selective 
internal radiation therapy).53,92 
Somatostatinoma
Pancreatic tumors secreting somatostatin are associated with 
somatostatinoma syndrome, characterized by hyperglycemia, cho­
lelithiasis, low acid output, steatorrhea, diarrhea, abdominal pain, 
anemia, and weight loss. Although elevations in somatostatin are 
observed in a significant proportion of MEN1-assoc

observed in a significant proportion of MEN1-associated pancre­
atic NETs, somatostatinoma syndrome has not been reported.20,93 
Thus, this group of tumors is frequently considered part of the NF 
category. 
GHRHoma
Pancreatic islet tumors secreting growth hormone–releasing 
hormone (GHRH) have been reported in some patients with 
MEN1.1,118 Patients may present with features of acromegaly 
and are diagnosed by demonstrating elevations in serum growth 
hormone (GH), GHRH, and insulin-like growth

rowth 
hormone (GH), GHRH, and insulin-like growth factor 1. In the 
1631
context of MEN1, GHRHomas occur predominantly within the 
pancreas, although sporadic GHRHomas may arise in the lung or 
small intestine.118 Surgical removal is the treatment of choice for 
MEN1-associated pancreatic GHRHomas.118 
Pituitary Tumors
Anterior pituitary tumors occur in 30% to 50% of patients 
with MEN1, although the frequency of detection has increased

 although the frequency of detection has increased 
with the introduction of routine surveillance of MEN1 patients 
together with the improved sensitivity of imaging modali­
ties.1,23,119–121 Women are reported to be affected more frequently 
than men,27,121 and one study reported an intrafamilial correla­
tion and suggested occurrence of potential genetic modifying 
influences that are independent of the MEN1 mutation.122 Pitu­
itary tumors typically present in early adulthood at a mean age of 

cally present in early adulthood at a mean age of 
30 to 40 years.119,121 However, pituitary tumors may occur earlier; 
one study reported a prevalence of approximately 35% before 21 
years of age, with the majority of cases presenting between 15 and 
20 years of age,19 whereas the youngest presentation reported was 
in a 5-year-old boy.123 Pituitary tumors are the first manifestation 
of MEN1 in 10% to 20% of cases.19,124 Initial studies reported 
a high prevalence (>80%) of pituitary macroaden

d 
a high prevalence (>80%) of pituitary macroadenomas (i.e., >1 
cm) in MEN1 patients,121 although more recent series indicate 
that microadenomas (i.e., <1 cm) occur frequently, and these dif­
ferences are likely explained by the introduction of sensitive sur­
veillance imaging.119,124 Pituitary tumor subtypes are observed 
in MEN1 patients, with prolactinomas representing the most 
common type, accounting for 40% to 75% of MEN1-associated 
pituitary tumors.19,119,120,124 Other functioning pit

uitary tumors.19,119,120,124 Other functioning pituitary tumors 
include GH-secreting tumors (5–15%) and adrenocorticotropic 
hormone (ACTH)-secreting tumors (3–7%), whereas plurihor­
monal secretion (i.e., prolactin/GH and prolactin/ACTH) may 
be observed in a minority of tumors. The remaining tumors pre­
dominantly comprise NF pituitary tumors (15–40%),20,119,121,124 
although some of these are associated with secretion of glycopro­
tein subunits.1 One recent study reported that approximately 

ts.1 One recent study reported that approximately 
50% of pituitary tumors detected by screening in MEN1 patients 
(i.e., as opposed to clinical presentations) represented NF tumors, 
of which the majority were microadenomas.119 The clinical mani­
festations associated with MEN1-associated pituitary tumors are 
similar to those in patients with sporadic pituitary tumors. For 
example, patients with prolactinomas will have features associated 
with hyperprolactinemia (e.g., amenorrhea, galactorrh

h hyperprolactinemia (e.g., amenorrhea, galactorrhea, and 
infertility in females, and erectile dysfunction and loss of libido 
in males), whereas those with GH- and ACTH-secreting tumors 
will manifest symptoms and signs associated with acromegaly and 
Cushing disease, respectively. Pituitary macroadenomas typically 
present with local mass effects (e.g., headache, visual field defects) 
or features of hypopituitarism. Diagnosis and clinical investiga­
tions are similar to those for sporadic pi

estiga­
tions are similar to those for sporadic pituitary tumors and will 
include dynamic biochemical testing and imaging to characterize 
the nature of the tumor and to assess any compromise of pituitary 
function (e.g., hypopituitarism). Given the high risk of tumor 
development, MEN1 mutation carriers are recommended to 
undergo periodic biochemical evaluation for prolactin and insu­
lin-like growth factor 1, together with MRI of the pituitary fossa. 
Treatment
Overall, the prognosis for pat

y fossa. 
Treatment
Overall, the prognosis for patients with MEN1 pituitary tumors, 
the majority of which are benign neoplasms, is favorable.20 Pitu­
itary carcinoma is extremely rare in MEN1, with only a few cases 
reported.1,125 The treatment of pituitary tumors in patients with 
MEN1 is similar to that employed for their sporadic counterparts 
and comprises appropriate medical therapy (e.g., cabergoline for 
prolactinoma; somatostatin analogues and/or pegvisomant for 
somatotropinoma) or sel

es and/or pegvisomant for 
somatotropinoma) or selective transsphenoidal adenomectomy, 
if feasible, with radiotherapy reserved for those patients with 
residual and/or unresectable tumor tissue.125 However, pituitary 
tumors in MEN1 patients have been reported in some studies to 
be larger and more invasive than their sporadic counterparts, as 
well as less responsive to medical treatments.120,121,125 One study 
reported that pituitary tumors in young male MEN1 patients 
were larger than those 

 young male MEN1 patients 
were larger than those occurring in their female counterparts, with 
the majority occurring as macroadenomas with invasive imaging 
characteristics (i.e., Hardy grade 3 or 4).19 However, other studies 
of predominantly adult MEN1 patients have reported that the 
majority of pituitary tumors respond well to medical therapies 
and have a similar behavior to sporadic tumors (e.g., the response 
rate for prolactinomas is >90%), whereas NF tumors were fre­
quently small and

0%), whereas NF tumors were fre­
quently small and stable, not requiring surgical intervention.72,119 
Adrenal Tumors
The incidence of adrenocortical tumors in patients with MEN1 
is reported to be 20% to 55%,1,126 although a higher frequency 
of adrenal involvement (∼75%) has been reported when highly 
sensitive imaging modalities have been employed, including endo­
scopic ultrasound.127 Most affected patients are asymptomatic, as 
the majority of tumors, which may include cor

 as 
the majority of tumors, which may include cortical adenomas, 
hyperplasia, multiple adenomas, nodular hyperplasia, cysts, or 
carcinomas, are NF.1,126 Indeed, fewer than 10% of patients with 
enlarged adrenal glands have biochemical evidence of hormonal 
hypersecretion, and among these, primary hyperaldosteronism and 
ACTH-independent Cushing syndrome are the most commonly 
encountered.126 Occasionally, hyperandrogenemia may occur in 
association with adrenocortical carcinoma, whereas the o

ation with adrenocortical carcinoma, whereas the occur­
rence of pheochromocytoma in MEN1 patients is rare. Although 
adrenal involvement is most commonly observed in adults with 
MEN1 (with equal sex distribution), occasional presentations in 
childhood are reported. For example, adrenal carcinoma has been 
reported in a 4-year-old boy and 16-year-old girl, each having 
clinical and biochemical evidence of androgen excess.19 It is also 
reported that adrenal tumors demonstrate heritability in M

 that adrenal tumors demonstrate heritability in MEN1 
kindreds, highlighting a need for increased vigilance in those with 
other affected family members.122 Biochemical investigation (e.g., 
plasma renin and aldosterone concentrations, the low-dose dexa­
methasone suppression test, urinary catecholamines and/or meta­
nephrines) should be undertaken for those with symptoms or signs 
suggestive of functioning adrenal tumors, or for those with tumors 
larger than 1 cm. The incidence of adrenocorti

rs 
larger than 1 cm. The incidence of adrenocortical carcinoma is 
reported to be approximately 1% in MEN1 patients but is higher 
(at approximately 13%) in MEN1 patients with adrenal tumors 
larger than 1 cm.126 Thus, it is important that MEN1 patients 
with adrenal tumors are offered annual imaging and that those 
displaying atypical radiologic characteristics, significant growth, 
or are larger than 4 cm are considered for surgical removal.1,126 
Treatment
Consensus has not been reached abou

126 
Treatment
Consensus has not been reached about the management of 
MEN1-associated NF adrenal tumors, because the majority of 
these neoplasms are benign. However, the risk of malignancy is 
increased if the tumor has a diameter larger than 4 cm, although 
adrenocortical carcinomas have been identified in tumors smaller 
1632
than 4 cm in patients with MEN1.1,126 Surgery is recommended 
for adrenal tumors that are larger than 4 cm in diame

 adrenal tumors that are larger than 4 cm in diameter, have 
atypical or suspicious radiologic features (e.g., increased Houn­
sfield units on an unenhanced CT scan) and are 1 to 4 cm in 
diameter, or show significant measurable growth over a 6-month 
interval.1,127 The treatment of functioning (i.e., secreting) adrenal 
tumors in MEN1 patients is similar to that for tumors occurring 
in non-MEN1 patients. 
Carcinoid Tumors
Carcinoid tumors arising in the thymus, bronchi, or gastrointes­
tinal t

ng in the thymus, bronchi, or gastrointes­
tinal tract occur at varying frequencies in patients with MEN1. 
Thymic carcinoid tumors, although rare, present the most signifi­
cant clinical challenge because they are associated with an aggres­
sive disease course and are one of the leading causes of premature 
death in MEN1 patients.
Thymic Carcinoids
Thymic carcinoid tumors occur in 2% to 8% of MEN1 
patients1,128–131 and are observed predominantly in adult male 
patients, although gender differe

y in adult male 
patients, although gender differences appear specific to individ­
ual ethnic populations (e.g., European populations, 20 male to 1 
female; Japanese populations, 2 male to 1 female; Chinese popula­
tions, 1 male to 1 female).27,129,132 In MEN1 populations of Euro­
pean descent, smoking is an independent risk factor for tumor 
development.129 The median age at diagnosis is 40 to 45 years,129 
although much earlier presentations have been reported, includ­
ing a 16-year-old boy wh

ve been reported, includ­
ing a 16-year-old boy who died 49 months after diagnosis from 
local and distant metastatic disease.19 Although the overall fre­
quency of thymic carcinoid tumors is low in MEN1 patients, sev­
eral reports highlight clustering of cases within individual families, 
thereby suggesting a high heritability independent of the MEN1 
mutation.122,129 Thymic carcinoids are responsible for approxi­
mately 20% of premature deaths in MEN1 patients,20,61,129 with 
the highest hazar

in MEN1 patients,20,61,129 with 
the highest hazard ratio for increased mortality of all the MEN1-
associated tumors (hazard ratio, 4.64; 95% confidence interval, 
1.73–12.41).20 Once diagnosed, MEN1 patients with thymic 
carcinoid tumors have a median survival of 8 to 10 years.129,131 
Symptomatic presentations may include pain (e.g., arising from 
chest, shoulder, breast) or features of vena caval obstruction,131 
whereas the features of carcinoid syndrome (i.e., flushing, diar­
rhea) are not 

oid syndrome (i.e., flushing, diar­
rhea) are not typically observed.129,131 Indeed, the majority of 
patients are asymptomatic at diagnosis, and most tumors are 
detected by radiology.131 Biochemical markers (e.g., raised chro­
mogranin A, urinary 5-hydroxyindoleacetic acid) are not suffi­
ciently sensitive for tumor detection; as a consequence, diagnosis 
is reliant on radiologic imaging, although the optimal screening 
methods have not been established. CT is considered to be sensi­
tive for 

tablished. CT is considered to be sensi­
tive for tumor detection, but there is concern over the repeated 
exposure to ionizing radiation, particularly as the natural history 
of thymic carcinoid tumors in MEN1 is one of rapid development 
such that a frequent scanning interval (i.e., every 1–2 years) would 
be required.1,130 “Low-dose” CT or MRI may be optimized for 
tumor detection, but further studies are required to assess their 
utility in MEN1. Similarly, SRS is frequently positive in thym

EN1. Similarly, SRS is frequently positive in thymic 
carcinoids, although insufficient evidence is available to recom­
mend its use as a screening modality. FDG-PET may also be use­
ful in the evaluation of patients with thoracic lesions in MEN1.133 
The treatment of thymic carcinoids depends on the stage of presen­
tation. Surgical removal is recommended because it may be cura­
tive, although recurrence rates following surgery are high.129,131 
Older age of presentation, larger tumor diameter,

Older age of presentation, larger tumor diameter, and presence 
of metastases are each associated with worse clinical outcomes.129 
For those with advanced disease, chemotherapy (e.g., etoposide 
and cisplatin) and radiotherapy may be used.129,131 The value of 
somatostatin analogue therapy in this context is not known but 
may improve symptoms and/or result in tumor response. It should 
be noted that cervical thymectomy is recommended in MEN1 
patients undergoing parathyroidectomy, although th

patients undergoing parathyroidectomy, although there is little 
evidence that such prophylactic thymus removal reliably prevents 
thymic carcinoid development, and several cases of thymic car­
cinoid have been reported in patients who have undergone this 
procedure.131 Similarly, although current guidelines recommend 
regular screening for thymic carcinoid, there is no evidence that 
this approach, aimed at early tumor detection, results in improved 
outcomes.130 
Bronchial Carcinoids
Bronchial

oved 
Bronchial carcinoids, which are observed in 4% to 13% of adult 
patients with MEN1, are not typically associated with increased 
mortality,20,128,130,134 as they frequently run a benign course, 
although occasional patients have developed malignant disease.134 
The incidence of bronchial carcinoids appears to be approximately 
equal between sexes,128 although they were initially reported to 
occur predominantly in women. Bronchial carcinoids present in 
a

antly in women. Bronchial carcinoids present in 
adulthood at a median age of approximately 40 years, and child­
hood presentations do not appear to be reported.19,128,130 The 
majority of patients with bronchial carcinoids are asymptomatic, 
and features of carcinoid syndrome are typically absent.59,128 Diag­
nosis is made on imaging, most frequently with CT. However, it 
should be noted that pulmonary lesions in MEN1 patients require 
careful evaluation because they may represent metastatic le

valuation because they may represent metastatic lesions 
from other tumor sites.128,133 Surgery is typically performed for 
bronchial NETs, and although there is no direct evidence of 
improved outcomes employing this approach in MEN1 patients, 
in sporadic cases surgery is reported to be beneficial.130,134,135 
Gastric Carcinoids
Type II gastric carcinoids (also referred to as enterochromaffin-like 
cell carcinoids [ECLomas]) are observed in approximately 15% to 
70% of MEN1 patients with coexi

roximately 15% to 
70% of MEN1 patients with coexistent hypergastrinemia and are 
frequently detected incidentally at the time of upper gastrointesti­
nal endoscopy.1,136,137 The tumors are usually multiple and small 
(e.g., <1.5 cm), and SRS may demonstrate increased uptake in the 
stomach. The malignant potential of these tumors is uncertain, 
and, where feasible, surgical resection may be appropriate.136,137 
However, treatment with somatostatin analogue therapy has also 
been reported to res

in analogue therapy has also 
been reported to result in regression of these tumors.136–138 
Other Tumors
Central nervous system tumors including ependymomas, schwan­
nomas, and meningiomas have been reported in MEN1 patients.1 
Meningiomas are reported in approximately 8% of MEN1 patients. 
The majority of meningiomas were not associated with symptoms, 
and 60% did not enlarge.1 The treatment of MEN1-associated 
meningiomas is similar to that occurring in non-MEN1 

ingiomas is similar to that occurring in non-MEN1 patients. 
Lipomas
Subcutaneous lipomas may occur in 15% to 33% of patients with 
MEN1, and frequently they are multiple.1,23 In addition, visceral, 
pleural, or retroperitoneal lipomas may occur in patients with 
MEN1. Management is conservative. However, when surgically 
removed for cosmetic reasons, they typically do not recur. 
1633
Facial Angiofibromas and Collagenomas
Studies have revealed that multi

 and Collagenomas
Studies have revealed that multiple facial angiofibromas occur in 
22% to 88% and collagenomas in 0% to 72% of patients with 
MEN11 (Fig. 42.3). MEN1 angiofibromas are clinically and his­
tologically identical to those observed in patients with tuberous 
sclerosis, with the exception that in patients with MEN1, angio­
fibromas were also present on the upper lip and vermilion border 
of the lip, which are areas not involved in tuberous sclerosis. These 
cutaneous findings, which

berous sclerosis. These 
cutaneous findings, which occur with a higher frequency in patients 
with MEN1, may provide a useful means for possible presymptom­
atic diagnosis of MEN1 in the relatives of a patient with MEN1. 
Treatment for these cutaneous lesions is usually not required. 
Thyroid Tumors
Thyroid tumors consisting of adenomas, colloid goiters, and carcino­
mas have been reported to occur in more than 25% of patients with 
MEN1.1,23 However, the prevalence of thyroid disorders in the g

ever, the prevalence of thyroid disorders in the gen­
eral population is high, and it has been suggested that the association 
of thyroid abnormalities in patients with MEN1 may be incidental.1 
Female MEN1 patients have been reported to have an increased 
relative risk between 2.3 and 2.8 of developing breast cancer. The 
majority of breast tumors are of ductal type with mixed hormone 
receptor status (estrogen receptor [ER], progesterone receptor, 
and human epidermal growth fact

sterone receptor, 
and human epidermal growth factor receptor).139 Some of the 
breast cancers had reduced menin expression and loss of hetero­
zygosity of the MEN1 locus, although further studies are required 
to confirm the association.140 Based on these initial observations, 
together with subsequent studies reporting that the increased 
breast cancer is not related to other known risk factors,141 some 
centers have advocated the introduction of screening for breast 
cancer in female MEN1 pat

of screening for breast 
cancer in female MEN1 patients over the age of 40 years.141 
However, further evidence is required to support both the asso­
ciation of breast cancer with MEN1 and the value of undertaking 
such screening.85 
MEN1 Gene
The MEN1 gene is located on chromosome 11q13 and consists 
of 10 exons, which encode a 610 amino acid protein, menin, that 
regulates transcription, chromatin structure, genome stability, and 
cellular proliferation through direct associ

 and 
cellular proliferation through direct associations with interacting 
protein partners, or via the modulation of key cellular signaling 
pathways18,92,142–144 (Fig. 42.4). Patients with MEN1 harbor germ­
line heterozygous mutations of the MEN1 gene, which predisposes 
them to tumor development; however, tumorigenesis requires 
somatic inactivation of the wild-type MEN1 allele such that MEN1 
tumors demonstrate biallelic inactivation of the MEN1 gene. Most 
commonly, the inactivation of the 

EN1 gene. Most 
commonly, the inactivation of the wild-type allele occurs through 
a large somatic deletion (i.e., at the 11q13 locus), which manifests 
as loss of heterozygosity (LOH) of the tumor DNA, consistent 
with Knudson’s “two-hit” model of inherited tumorigenesis and a 
tumor suppressor function for menin in endocrine tissues. How­
ever, alternate mechanisms leading to inactivation of the wild-type 
MEN1 allele include point mutations (i.e., resulting in nonsense or 
missense amino acid

.e., resulting in nonsense or 
missense amino acid substitutions) or small insertions or deletions 
(indels), and in such cases LOH will not be apparent.1
The MEN1 gene spans 7.7 kb of genomic DNA, and at least 16 
different MEN1 transcripts have been identified. The main MEN1 
transcript is a 2.76-kb mRNA, which encodes the 610 amino acid 
isoform of menin, although the canonical transcript, which appears 
to be rare, is longer at 3.16 kb and is predicted to elongate the read­
ing frame by 5 am

 predicted to elongate the read­
ing frame by 5 amino acids at the exon2/intron2 junction, giving 
a protein isoform of 615 amino acids. The approximately 1400-bp 
region upstream of exon 2 is reported to display strong promoter 
activity, containing both the minimal promoter region and several 
regulatory regions. Expression of the MEN1 transcript is observed 
in all human tissues examined, although menin protein expression 
may not necessarily correlate with transcript levels. The MEN1 gene 
a

correlate with transcript levels. The MEN1 gene 
and menin protein are highly conserved in mammalian species (e.g., 
∼89% and ∼97% DNA and protein identity, respectively, with 
mouse and rat). Orthologues of menin are observed in evolutionary 
distant specifies, including zebrafish and Drosophila, although they 
are not present in yeast (e.g., Saccharomyces cerevisiae) or nematodes 
(e.g., Caenorhabditis elegans). Recent population-level genetic stud­
ies indicate that the coding region of MEN1 

stud­
ies indicate that the coding region of MEN1 demonstrates high lev­
els of constraint against missense and nonsense variation, suggesting 
that it remains under strong evolutionary selection pressure.145 
Transcription of the MEN1 gene is reported to be, in part, regulated 
by its own protein product, menin, whereby reductions in menin 
expression activate the MEN1 promoter activity, and menin over­
expression downregulates promoter activity.146 More recently, the 
MEN1 3′ untranslated regi

.146 More recently, the 
MEN1 3′ untranslated region has been observed to bind microRNA 
24-1, which is reported to suppress menin expression.147 
Germline MEN1 Mutations
To date, more than 1200 germline MEN1 mutations have been 
reported in patients with MEN1 or individuals with associated 
tumors, and of these, approximately 600 different germline mutations 
are observed.148–150 MEN1 mutations are most frequently inherited 
from an affected parent, although they arise de novo in approximately 

ent, although they arise de novo in approximately 
10% of cases. The majority of mutations (∼70%) are predicted to 
result in a loss of function through premature truncation of the menin 
protein (i.e., frameshift deletions or insertions [40–45%], nonsense 
mutations [14–20%], splice-site mutations [∼10%]), whereas the 
remainder occur as missense mutations (20–25%), in-frame deletions 
or insertions (∼5%), or as gross deletions involving all or part of the 
• Fig. 42.3  Multiple angiofibromas i

part of the 
• Fig. 42.3  Multiple angiofibromas in a patient with multiple endocrine neo­
plasia type 1.
1634
NUCLEUS
CYTOPLASM
K-Ras
B-Raf
MEK
ERK
RTK 
e.g. 
VEGFR
RASSF1A
FZD
β Catenin
Axin2 etc
WNT
β Catenin
MLL1
CDKN1B
CDKN2C
β Catenin
PRMT5
GAS1
PI3K
Akt
mTOR
X
TGFβR
Smad3
P
Smad3
P
X
X
SSTR
SST
TGFβ
JunD
P
Gastrin
SOS1
GF
Menin
Menin
Menin
Menin
Menin
Menin
Menin
Menin
Destruction
complex
• Fig. 42.4  Menin has nuclear and

ruction
complex
• Fig. 42.4  Menin has nuclear and cytoplasmic roles. Loss of menin expression (blue boxes) in endo­
crine tissues may result in increased cell proliferation by multiple pathways. In the nucleus, the loss of 
menin results in disruption of its interaction with transcription factors JUND and PRMT5 that lifts the 
transcriptional repression of target genes Gastrin and GAS1, respectively; binding to MLL1, MLL2, and 
SMAD3 (a TGFβ signaling component) to promote transcription of targ

naling component) to promote transcription of target genes; and ability to regulate 
the WNT pathway because β-catenin is no longer prevented from entering the nucleus by menin, thereby 
enabling transcription of WNT pathway target genes. Interactions with additional transcription factors 
and chromatin-modifying protein complexes may further modulate oncogenic signaling pathways. In the 
cytoplasm, loss of menin reduces its inhibitory actions on the mammalian target of rapamycin (mTOR) 
pathway

 the mammalian target of rapamycin (mTOR) 
pathway by binding to AKT (downstream of PI3K, part of the RTK signaling pathway) and preventing its 
translocation to the plasma membrane, and KRAS-induced proliferation (by possible inhibition of ERK-
dependent phosphorylation (P) and prevention of the interaction between SOS1 and KRAS). All pathways 
affect proliferation, which involves both nuclear and cytoplasmic mechanisms (shown in the cytoplasm 
only). Ligands are shown as green circles, and rec

only). Ligands are shown as green circles, and receptors are orange circles. Akt, protein kinase B; B-Raf; 
serine/threonine-protein kinase B-Raf; CDKN, cyclin-dependent kinase inhibitor; ERK, extra signal-related 
kinase; FZD, frizzled; GAS1, growth arrest specific 1; GF, growth fac­tor; MEK, mitogen-activated protein 
kinase kinase; MLL, mixed lineage leukemia; PI3K, phosphoinositide 3-kinase; PRMT5, protein arginine 
N-methytransferase 5; RASSF1A, Ras-associated domain family member 1 isoform

F1A, Ras-associated domain family member 1 isoform A; Smad3, moth­
ers against decapentaplegic hormone 3; SOS1, sons of sevenless 1; SST, somatostatin; SSTR, SST 
receptor;TGFβ, transforming growth factor β; TGFβR, TGFβ receptor; WNT, wingless-related integration 
site. (Modified from Frost M, Lines KE, Thakker RV. Current and emerging therapies for PNETs in patients 
with or without MEN1. Nat Rev Endocrinol. 2018;14[4];216–227.)
1635
MEN1 gene (1–2.5%).1

 Multiple Endocrine Neoplasia
MEN1 gene (1–2.5%).148,150 The mutations are observed throughout 
the entire coding region of the MEN1 gene, although nine individual 
mutations have been reported to account for more than 20% of all 
germline MEN1 mutations (e.g., c.249_252delGTCT, c.292C>T, 
c.358_360delAAG, c.628_631delACAG, c.784-9G>A, c.1243C>T, 
c.1378C>T, c.1546delC, c.1546_1547insC).148 In addition, 11 indi­
vidual codons (i.e., 45, 69, 70, 139, 156, 183, 220, 253, 418, 436, 
and 516) are re

39, 156, 183, 220, 253, 418, 436, 
and 516) are reported to be affected by five or more different MEN1 
mutations.150 Together, these studies indicate that specific regions 
of the MEN1 gene may be more susceptible to mutation, and it is 
notable that some of these mutations occur within repetitive DNA 
sequences, consistent with a replication-slippage model of mutagen­
esis. For example, the delC and insC (c.1546-1547) in codon 516 
involve a poly(C)7 tract, and a slipped-strand mispairing mode

ly(C)7 tract, and a slipped-strand mispairing model has 
been proposed to explain the high number of insertions or deletions 
at this site.148,151 An alternative proposed explanation for the occur­
rence of these recurrent mutations in unrelated kindreds is the pres­
ence of population-specific founder mutations, whose presence may 
be established by haplotype analysis.143,152,153
Correlations between MEN1 mutations and clinical manifesta­
tions of the disorder appear to be absent.4,148,151 For 

of the disorder appear to be absent.4,148,151 For example, stud­
ies of several large MEN1 kindreds, each harboring the same MEN1 
mutation, have shown that members of the respective families 
can develop a different range of tumors.4,154 However, it has been 
reported that mutations of codons 428 through 610, which disrupt 
menin interaction with the checkpoint kinase 1 (CHES1) protein, 
are associated with a higher frequency of aggressive pancreatic NETs 
and a higher mortality than other MEN1

eatic NETs 
and a higher mortality than other MEN1 mutations,155 and that 
mutations of the three menin domains (codons 1–40, 139–242, and 
323–428) that affect the interaction with JunD had a higher risk of 
premature death when compared with other MEN1 mutations.156 
Further studies are required to validate these findings. In addition, 
some kindreds with germline MEN1 mutations have been reported 
to not develop the full clinical phenotype of MEN1. For example, 
families with the Burin or pro

MEN1. For example, 
families with the Burin or prolactinoma variant of MEN1, who har­
bor specific nonsense mutations (i.e., Tyr312Ter, Arg460Ter), are 
reported to be characterized by a high occurrence of prolactinomas 
but a low occurrence of gastrinoma.157,158 Similarly, somatotropi­
nomas were not observed in a large kindred from Tasmania carry­
ing a splice site MEN1 mutation (c.446-3C>G).159 Other families 
with germline MEN1 mutations only develop parathyroid tumors, 
a condition referred

 develop parathyroid tumors, 
a condition referred to as familial isolated hyperparathyroidism 
(FIHP).148,150,160 These phenotypic variants may be due to the spe­
cific MEN1 mutation or genetic modifiers. For example, FIHP, when 
compared with MEN1, is associated with a high occurrence of mis­
sense MEN1 mutations (∼38% vs. 23%, p <.01), and several of the 
FIHP missense mutations, when compared with MEN1-associated 
mutations, have been reported to retain menin protein stability and 
biologic 

d to retain menin protein stability and 
biologic activity, consistent with the milder phenotype.161 However, 
some FIHP kindreds have the same protein-truncating mutations 
that occur in MEN1 families, thereby implicating a role for genetic 
modifiers. Germline MEN1 mutations have also been reported in 
patients with apparently sporadic pancreatic NETs, although it is 
unclear whether all such patients were systematically evaluated for 
the presence of other MEN1 manifestations.162 Independent 

ence of other MEN1 manifestations.162 Independent of 
MEN1 genotype, one study reported that the type O blood group 
was associated with an increased risk of NETs in MEN1 patients,163 
although a subsequent study found no such relationship.164
Finally, it should be noted that approximately 10% of patients 
with a clinical diagnosis of MEN1 do not harbor mutations in the 
MEN1 coding region,165 and these individuals may harbor muta­
tions in the promoter or untranslated regions of the gene, or re

romoter or untranslated regions of the gene, or represent 
phenocopies with mutations in other genes (see later discussion).1 In 
addition, some of these patients, when compared with patients with 
MEN1 mutations, have been reported to present with the first endo­
crine tumor at a later age, to very rarely develop a third MEN1 mani­
festation,165 and to have a greater life expectancy.72,165 Thus, it seems 
likely that such individuals may have two coincidental sporadic 
endocrine tumors rather t

o coincidental sporadic 
endocrine tumors rather than the hereditary MEN1 syndrome.165 
MEN1 Polymorphisms
At least 35 different common germline variants (i.e., polymorphisms 
with minor allele frequency [MAF] >0.5%) are observed in the cod­
ing and nonregions of the MEN1 gene, comprising 5 in the coding 
region (3 synonymous and 2 nonsynonymous [i.e., p.Arg176Gln and 
p.Ala546Thr in the canonical transcript]), 20 in the intronic regions, 
and 10 in the untranslated regions (http://phase3browser

 in the untranslated regions (http://phase3browser.1000
genomes.org/ and http://gnomad.broadinstitute.org/). It is important 
to recognize the occurrence of these polymorphisms, and especially 
the nonsynonymous coding region variants, as their finding in an 
apparently affected index case does not necessarily equate to pathoge­
nicity, and careful evaluation of any potential novel MEN1 mutation 
is required. For example, approximately 200 rare (MAF <0.5%) mis­
sense variants are observed in the

MAF <0.5%) mis­
sense variants are observed in the GnomAD dataset (http://gnomad.
broadinstitute.org/), and most of these will not be of clinical signifi­
cance. The recognition of such variants as benign is becoming easier 
owing to the availability of large population-based databases.145 
MEN1 Phenocopies and Mutations in Other Genes
Phenocopy refers to the development of disease manifestations, 
which are usually associated with a particular gene but have 
instead been caused by mutations in 

ene but have 
instead been caused by mutations in another gene or environmen­
tal factors. Phenocopies are reported in 5% to 10% of MEN1 
kindreds and may occur in different clinical settings.21,22,159,166 
For example, phenocopies have been reported in the context of 
familial MEN1, in which patients manifesting a MEN1-associ­
ated tumor (e.g., pituitary or parathyroid tumor) do not harbor 
the familial MEN1 mutation.21,22 Phenocopies may also occur in 
the context of patients or kindreds prese

ccur in 
the context of patients or kindreds presenting with an apparent 
clinical diagnosis of MEN1 (i.e., ≥2 MEN1-associated endocrine 
tumors) who do not harbor a MEN1 mutation but instead have 
mutations in another gene, which is more typically associated 
with a different disease.21,22 Such genes include CDC73, which 
encodes the tumor suppressor parafibromin, mutations of which 
result in hyperparathyroidism-jaw tumor syndrome167; CASR, 
which encodes the calcium-sensing receptor, mutation

ich encodes the calcium-sensing receptor, mutations of which 
result in familial benign hypocalciuric hypercalcemia type 1 and/
or FIHP168,169; and AIP, which encodes the aryl hydrocarbon 
receptor interacting protein, mutations of which are associated 
with familial isolated pituitary adenoma.170 Finally, a small per­
centage of patients manifesting clinical features of MEN1 in the 
absence of a MEN1 mutations may have a mutation in CDKN1B, 
which results in the associated disorder of MEN4171,1

ch results in the associated disorder of MEN4171,172 (see Table 
42.1). Thus, the possibility of a phenocopy or an alternate genetic 
diagnosis should be considered in those presenting with typical or 
atypical manifestations of MEN1 in whom a MEN1 mutation is 
not found. In addition, genetic testing should be undertaken in 
all kindred members where a familial MEN1 mutation is present, 
irrespective of the clinical disease status of the individual. 
Somatic MEN1 Mutations
More than 90% of tumor

al. 
More than 90% of tumors from MEN1 patients have LOH involv­
ing loci on chromosome 11q13, and this has generally been taken as 
evidence that the MEN1 gene acts as a tumor suppressor (i.e., as a 
result of biallelic MEN1 inactivation).24,29,173 The identification of the 
MEN1 gene and its encoded protein menin as a key determinant of 
1636
endocrine neoplasia has led to the investigation of its role in the devel­
op

 to the investigation of its role in the devel­
opment of sporadic endocrine tumors, which have been shown to have 
somatic inactivation of both MEN1 alleles, which most commonly 
results from an inactivating point mutation or small indel affecting 
one allele and a large-scale deletion of the other.148 Overall, somatic 
MEN1 mutations are observed in approximately 35% of parathyroid 
tumors,174,175 40% to 45% of NF pancreatic NETs,162,176 approxi­
mately 40% of gastrinomas,148 0% to 15% of insu

i­
mately 40% of gastrinomas,148 0% to 15% of insulinomas,177 3% 
to 5% of pituitary tumors,148,178 approximately 15% to 20% of pul­
monary carcinoids,179,180 less than 3% of small intestinal NETs,181,182 
less than 3% of adrenocortical tumors, 10% of angiofibromas, and 
approximately 30% of lipomas.148 The somatic mutations observed 
in these sporadic tumors are reported to occur throughout the MEN1 
coding region and comprise loss-of-function and missense mutations, 
similar to the germline mu

nd missense mutations, 
similar to the germline mutations.148 The identification of somatic 
MEN1 mutations in sporadic tumors currently has little clinical util­
ity, although it is possible that tumor genotyping may contribute to 
future personalized treatment approaches. In addition, such mutation 
profiling of tumors may provide prognostic information, and it has 
been reported that the presence of a somatic MEN1 mutation in spo­
radic pancreatic NETs is associated with an improved survival 

atic NETs is associated with an improved survival when 
compared with those without a MEN1 mutation.176 
Functions of the Menin Protein and Insights Into 
­Mechanisms of Tumorigenesis
Menin is a ubiquitously expressed protein that is located predom­
inantly in the nucleus, and it is reported to have at least three 
nuclear localization signals within its C-terminus.143,144,183 Menin 
is also reported to be found in the cytoplasm, where it is regulates 
key signaling pathways.184 Menin, which fun

lates 
key signaling pathways.184 Menin, which functions as a scaffold 
protein, has been reported to interact with more than 20 pro­
teins and molecules that facilitate its role in transcriptional and 
epigenetic regulation, genome stability, DNA repair, cell division, 
cell signaling, and cell motility143,144,183 (see Fig. 42.4). The roles 
of menin in transcription, in which it can act as an activator and 
repressor, and as a component in some canonical signaling path­
ways, will be briefly r

 canonical signaling path­
ways, will be briefly reviewed, as these illustrate menin’s ability in 
behaving as a tumor suppressor and oncogenic cofactor, depend­
ing on the cellular context.183 Thus, in the majority of endocrine 
tissues, menin acts as a tumor suppressor, whereas in other neo­
plasms including leukemia, pediatric glioma, and prostate, hepa­
tocellular, and breast cancer, it acts as an oncogene.143,144,183,185,186
Menin activates transcription by interacting with the mixed-lin­
e

transcription by interacting with the mixed-lin­
eage leukemia proteins 1 and 2 (MLL1 and MLL2) histone meth­
yltransferase complexes,187,188 which are responsible for histone H3 
lysine 4 residue trimethylation (H3K4Me3), which causes chromatin 
modification and is associated with activation of transcription187–189 
(see Fig. 42.4). Menin/MLL1 occupancy has been reported to occur 
at the promoter regions of thousands of genes, and to generally corre­
late with active transcription, although men

orre­
late with active transcription, although menin loss is associated with 
changes in transcription of only a small subset of genes.190 The menin-
MLL complex is a key activator of Hox gene expression187 and the 
p27 and p18 cyclin-dependent kinase inhibitors,191,192 and menin 
physically links MLL with the lens epithelium–derived growth fac­
tor (LEDGF), which is a chromatin-associated protein required for 
MLL-dependent transcription that has been implicated in leukemic 
transformation.193 

s been implicated in leukemic 
transformation.193 Analysis of the crystal structure of menin in com­
plex with MLL1 and LEDGF has demonstrated the key interacting 
domains, in which menin adopts a “curved left hand” structure, with 
the N-terminus resembling the thumb (a long β-hairpin), the cen­
tral portion the palm, and the C-terminus the curved fingers.144,194 
A deep pocket within the central “palm” region forms the MLL1-
binding site, whereas LEDGF is observed to simultaneously bind 
both 

as LEDGF is observed to simultaneously bind 
both menin and MLL1. Thus, menin acts as a molecular adaptor to 
link such proteins, and disruption to these interacting domains abro­
gates downstream transcriptional activity.144,194 However, the intact 
menin-MLL complex enhances transcriptional activation of genes 
(e.g., HOX genes, EZH2) involved in leukemogenesis,187,195,196 and 
small molecule inhibitors targeting this protumorigenic function of 
the menin-MLL interaction have been developed an

the menin-MLL interaction have been developed and are shown to 
be effective for the treatment of MLL-dependent leukemia in pre­
clinical in vitro and in vivo models.197,198 The menin-MLL complex 
is also reported to act as an oncogenic coactivator of androgen recep­
tor (AR) signaling in prostate cancer, which is effectively inhibited 
in vivo using small molecule menin-MLL inhibitors185; ESR1 signal­
ing in breast cancer, in which it recruits the transcriptional enhanc­
ers FOXA1 and GATA3 t

 the transcriptional enhanc­
ers FOXA1 and GATA3 to support expression of cancer-relevant 
genes183; and platelet-derived growth factor receptor A signaling 
in pediatric glioma.186 Transcriptional activation, independent of 
H3K4Me3 activity, has been reported to occur by a direct interac­
tion between menin and the oncogene MYC in fibrosarcoma and 
hepatocellular carcinoma (HCC) cell lines.199
Menin regulates transcriptional repression through several 
direct and indirect mechanisms. Thus, men

several 
direct and indirect mechanisms. Thus, menin directly interacts 
with JunD (see Fig. 42.4), a member of the activating protein 1 
(AP1) transcription factor family,143,144 and JunD utilizes the same 
menin binding pocket employed by MLL1 such that JunD and 
MLL1 likely compete for menin binding.194 The binding of menin 
to JunD facilitates the recruitment of the Sin3 transcription regu­
lator homolog (mSin3A) histone deacetylase (HDAC) complex 
to inhibit JunD-dependent transcription, an

mplex 
to inhibit JunD-dependent transcription, and the forkhead tran­
scription factor CHES1 associates with the menin-mSin3A tran­
scriptional repressor complex, to regulate an S-phase checkpoint 
pathway related to DNA damage.143,144,183 Menin binding to JunD 
blocks c-Jun N-terminal kinase (JNK)-mediated phosphorylation, 
thereby inhibiting post-translational modification (and subsequent 
activation) of JunD.194 In addition, menin represses expression of 
the PTN gene, which encodes the plei

xpression of 
the PTN gene, which encodes the pleiotrophin pro-proliferative 
receptor, and this increases the Polycomb gene enhancer of zeste 
homolog 2 (EZH2)-mediated histone H3 lysine 27 trimethylation 
(H3K27M3), which is a negative mark for gene transcription. Stud­
ies of HCC have revealed that elevated levels of menin, EZH2, and 
H3K27Me3 are associated with a poor prognosis, whereas inhib­
iting H3K27Me3 effectively blocked the aggressive phenotype of 
HCC cells.200 Menin also recruits 

e phenotype of 
HCC cells.200 Menin also recruits the transcription factor PRMT5 
(see Fig. 42.4) to the promoter of the growth arrest–specific pro­
tein 1 (GAS1) gene, which is an important cofactor required for 
the binding of the Sonic Hedgehog (Shh) ligand to its cell surface 
receptor, and thereby acts as a negative regulator of Hedgehog sig­
naling.201 Menin also represses transcription mediated by members 
of the NFκB family, and other menin regulatory functions include 
modulation of gen

in regulatory functions include 
modulation of gene expression through regulation of microRNA 
biosynthesis,147,202 regulation of expression of nuclear receptor 
target genes (e.g., direct interaction with ERα), and participation 
in the DNA damage response in which menin undergoes phos­
phorylation and influences the transcriptional response through an 
altered affinity for RNA polymerase II.203
Menin modulates several canonical signaling pathways involved 
in endocrine and nonendocrine tumorig

ys involved 
in endocrine and nonendocrine tumorigenesis,92,144 which are cell- 
and tissue specific. For example, menin has been reported to act as a 
negative regulator of the Wnt/β-catenin signaling pathway (see Fig. 
42.4) by regulating the phosphorylation and export of β-catenin 
out of the nucleus and into the cytoplasm, thereby diminishing 
Wnt target gene expression,184 and loss of menin expression in 
human and murine pancreatic NETs has been reported to result 
in activation of the Wnt

 been reported to result 
in activation of the Wnt signaling pathway. This has provided 
1637
support for the rationale of using small molecule Wnt inhibitors 
for the treatment of MEN1-associated pancreatic NETs in mutant 
mice.204 However, another study has reported that menin acts as a 
positive regulator of Wnt signaling in pancreatic endocrine cells, 
thereby highlighting apparent paradoxical activities dependent on 
the cellular environment.205 A si

es dependent on 
the cellular environment.205 A similar complex interplay is observed 
between menin and RAS signaling pathways, in which menin has 
been reported to determine the outcome of K-RAS signaling (see 
Fig. 42.4) in a cell-context specific manner within the pancreas.206 
K-RAS activation typically enhances cell proliferation, but in pan­
creatic endocrine cells, it suppresses proliferation; this activity is 
due to menin-dependent preferential activation of the antiprolif­
erative RAS

erential activation of the antiprolif­
erative RAS effector RASSF1A, with simultaneous suppression of 
the pro-proliferative RAF/MAPK pathway (see Fig. 42.4). Down­
regulation of menin in pancreatic endocrine cells then releases this 
inhibition and results in greater RAF/MAPK signaling and endo­
crine cell proliferation.206 Other signaling pathways also reported 
to be modulated by menin expression include transforming growth 
factor β (TGFβ), bone morphogenetic protein (BMP), Hedgehog, 
AKT, a

one morphogenetic protein (BMP), Hedgehog, 
AKT, and MYC pathways92,144,189,199 (see Fig. 42.4). The better 
understanding of these complexities of menin function, together 
with the identification of key interacting proteins and alterations in 
major signaling pathways, has facilitated the identification of sev­
eral potential novel therapeutic targets, which have been evaluated 
in preclinical models for both MEN1- and non–MEN1-related 
tumors, and these include menin-MLL inhibitors, other epi

 and these include menin-MLL inhibitors, other epigenetic-
modifying agents, and Wnt inhibitors (see Fig. 42.4). 
Menin is highly evolutionary conserved in mammalian species (i.e., 
90% and 97% DNA and protein identity, respectively, between 
and human and mouse), whereas more evolutionary distant organ­
isms harbor orthologues with lower degrees of similarity (e.g., 
zebrafish [Danio rerio], fly [Drosophila melanogaster]). Although 
some studies of menin function have employed the

some studies of menin function have employed these more dis­
tant model organisms (i.e., homozygote deletion of the MEN1 
orthologue Mnn1 in D. melanogaster results in viable offspring but 
increased sensitivity to DNA damage and other environmental 
stressors [e.g., heat shock, hypoxia]),207 the major insights into the 
in vivo function of menin have arisen from studies of conventional 
and conditional Men1 mouse knockout models.207
Conventional Men1 Mouse Knockout Models
Several conventional

al Men1 Mouse Knockout Models
Several conventional Men1 knockout models have been estab­
lished by the targeted deletion of different exonic regions of the 
Men1 gene including deletion of exons 1-2,208 exon 2 alone,209 
exon 3,210 and exons 3-8,211 resulting in either the absence of 
Men1 transcription (i.e., through the loss of the transcription start 
site) or the generation of severely truncated Men1 transcripts.207 
Despite some subtle differences in tumor spectrum, each of these 
models re

rences in tumor spectrum, each of these 
models recapitulates major features of the clinical MEN1 syn­
drome; thus, heterozygous (Men1+/−) mice are observed to develop 
multiple tumors affecting the pancreatic islets, anterior pituitary, 
parathyroid, and adrenal glands.208–211 Men1+/− mice also develop 
gonadal, thyroid, and prostate tumors that are not typically associ­
ated with MEN1 in patients.207,208,210,211 In several instances, bio­
chemical and/or immunohistochemical analysis have confi

cal and/or immunohistochemical analysis have confirmed 
features consistent with the respective human tumors, including 
raised plasma insulin levels and pancreatic NETs consistent with 
insulinoma, raised PTH and/or hypercalcemia and parathyroid 
hyperplasia consistent with PHPT, plurihormonal expressing 
pancreatic NETs, and somatostatin receptor type 2 (SSTR2) and 
vascular endothelial growth factor A (VEGF-A) expression in pan­
creatic NETs and pituitary tumors.208,210,211 In each of the Men

nd pituitary tumors.208,210,211 In each of the Men1+/− 
models, the various tumors emerged in a time-dependent fashion, 
typically commencing at approximately 9 months, and molecular 
analysis of the tumors confirmed LOH at the Men1 locus and loss 
of menin expression, consistent with biallelic Men1 inactivation 
for tumor development, and its role as a tumor suppressor.
In each of the conventional mouse models, homozygous Men1 
ablation (Men1−/−) is reported to result in embryonic lethality 
be

) is reported to result in embryonic lethality 
between embryonic day (E) 10.5 and 14.5 with craniofacial defects, 
hemorrhages, edema, and neural tube defects, as well as abnormali­
ties of early pancreatic endocrine development.212,213 Thus, in con­
trast to the situation in D. melanogaster, menin appears essential for 
viable mammalian development. However, the timing of embryonic 
death and the specific observed phenotypes are dependent on the 
background strain of mouse, indicating a possib

e 
background strain of mouse, indicating a possible role for genetic 
modifiers.212 Furthermore, it is possible that the differences in tumor 
phenotype observed in each of the conventional Men1+/− models may 
also reflect similar differences in genetic background and/or influ­
ences of genetic modifiers. The potential synergy between menin and 
other key tumor suppressor genes has also been investigated using 
Men1+/− mice. For example, an acceleration or augmentation of 
tumor development was

leration or augmentation of 
tumor development was not observed in Men1+/− mice that also had 
deletion of one allele of the retinoblastoma gene (Men1+/−/Rb+/−), 
thereby indicating that menin and the retinoblastoma protein (pRB) 
act in common tumorigenic pathways.214 This is supported by the 
observation that genetic ablation of the retinoblastoma binding pro­
tein 2 (RBP2; also called JARID1A or KDM5A), which is a histone 
demethylase that contributes to the tumor suppressor activity of 
pRB,

tributes to the tumor suppressor activity of 
pRB, decreases tumor formation, and prolongs survival of a beta-
cell specific Men1 knockout mouse model.215 In contrast, the com­
bined mutation of Men1 and the cyclin-dependent kinases (CDK) 
inhibitor Cdkn1b (Men1+/–/Cdkn1b–/–) was not observed to signifi­
cantly influence tumor expression, whereas disruption to Cdkn2c 
(encoding the p18 protein) accelerated endocrine tumor develop­
ment in Men1 mice (Men1+/–/Cdkn2c–/–).216 
Conditional Men1 Mouse

ce (Men1+/–/Cdkn2c–/–).216 
Conditional Men1 Mouse Knockout Models
The conventional Men1 mouse models have several potential 
limitations to study tumor biology, and these include the incom­
plete penetrance of organ-specific tumor formation and the 
latency period required for tumorigenesis (i.e., due to the need 
for a somatic “second hit”) such that it is difficult to study early 
events in tumor development. To address these deficiencies, sev­
eral tissue-specific conditional knockout models

­
eral tissue-specific conditional knockout models have been gener­
ated, including those under temporal control, thereby enabling 
the consequences of controlled biallelic Men1 inactivation to be 
evaluated in endocrine and nonendocrine tissues.207 For example, 
a parathyroid-specific conditional mouse model, which was gen­
erated using a Cre-recombinase under the control of the PTH 
promoter, was found to develop hyperparathyroidism at a higher 
penetrance than the respective conventional Men1

penetrance than the respective conventional Men1+/− model.217 
Pancreatic beta-cell specific Men1–/– mice, which have been gen­
erated exploiting different rat insulin promoter (RIP) Cre mouse 
lines, have been reported to have early-onset islet hyperplasia, 
followed by highly penetrant insulinoma formation, although 
some models also developed prolactinomas because of Rip-Cre 
expression in the pituitary.207 The targeted deletion of Men1 in 
both endocrine and exocrine pancreatic cells, achi

both endocrine and exocrine pancreatic cells, achieved by using 
a Pdx1-Cre promoter, resulted in increased endocrine cell prolif­
eration and pancreatic NET formation, but no exocrine tumor 
1638
manifestations.218 Pancreatic α-cell-specific Men1 knockout mod­
els surprisingly did not develop glucagonoma but instead had 
insulinomas, thereby suggesting the possibility of transdifferentia­
tion of α-cells to β-cells, or the presence of modulat

 of α-cells to β-cells, or the presence of modulatory paracrine 
effects.219,220 Temporal regulation of beta-cell Men1 inactivation 
has been achieved through the use of inducible conditional knock­
outs (i.e., combining established Rip-Cre Men1 models with 
transgenic mice harboring an ER-Cre, thereby facilitating Men1 
ablation on exposure to tamoxifen)207,221; studies of these mice 
have revealed a rapid onset of islet cell proliferation, following 
Men1 inactivation, and this mutant mouse ma

owing 
Men1 inactivation, and this mutant mouse may provide a model 
for investigation of early events in tumorigenesis and for assessing 
the effects of novel therapies on tumor development.221 
Nonendocrine Phenotypes in Men1 Mouse Knockout 
Models
Tissue-specific inactivation of Men1 in Pax3 or Wnt1 expressing 
neural crest cells resulted in mutant mice with cranial bone defects, 
cleft palate, and perinatal death,222 and menin inactivation in 
mature osteoblasts (using an osteocalcin-cre) re

mature osteoblasts (using an osteocalcin-cre) resulted in reduced 
bone mineral density, trabecular bone volume, and cortical bone 
thickness,223 thereby demonstrating a critical role for menin in 
bone development. Hepatocyte-specific Men1 deletion is reported 
to result in a high-fat diet-induced liver steatosis through a mecha­
nism involving histone deacetylation.224 Moreover, induction of 
hepatic cancers by a chemical carcinogen has been reported to be 
reduced in Men1–/– female mice, an

reported to be 
reduced in Men1–/– female mice, and studies of this model have 
indicated that menin plays an important epigenetic role involving 
H3K4Me3 in promoting liver tumorigenesis,225 
Preclinical Evaluation of Drug Therapies in Men1 Mouse 
Models
The Men1 knockout mouse models have been used to study several 
treatments, including gene therapy, new somatostatin analogues, 
epigenetic modulators, β-catenin antagonist, and MEK1/2 inhibi­
tors.92 These will be reviewed briefly.
In MEN1-ass

ors.92 These will be reviewed briefly.
In MEN1-associated tumors, biallelic inactivation of the MEN1 
gene is typically observed and consistent with a proposed tumor 
suppressor function for menin. Thus, MEN1 gene replacement 
therapy offers a potential therapeutic strategy to restore menin 
function. To establish proof of concept, a replication-incompetent 
adenoviral vector containing the mouse Men1 cDNA under a 
cytomegalovirus promoter was injected into the pituitary tumors 
of conventional 

jected into the pituitary tumors 
of conventional Men1+/− female mice, resulting in restoration of 
menin expression and decreased tumor proliferation, thereby indi­
cating the potential utility of such an approach.226
The efficacy of somatostatin analogue therapy has been assessed 
in different mouse models. Thus, pasireotide, a multiple-receptor-
ligand somatostatin analogue targeting SSTR1 through SSTR3 
and SSTR5, was observed to reduce proliferation and increase 
apoptosis of pancreatic NET

feration and increase 
apoptosis of pancreatic NETs and pituitary tumors in conven­
tional Men1+/− mice and pancreatic NETs in Pdx1-Cre- Men1 
conditional knockout mice.227,228 Systemic pasireotide therapy 
was also found to reduce pancreatic NET formation in Men1+/− 
mice,92,227 thereby indicating that it has chemopreventive actions. 
Use of a hybrid adeno-associated virus and phage displaying bio­
logically active octreotide for targeted delivery of a tumor necro­
sis factor transgene to pancr

ry of a tumor necro­
sis factor transgene to pancreatic NETs in the Pdx1-Cre-Men1 
model has shown that this can reduce tumor volume and improve 
survival.229
MEN1-associated tumors are reported to have alterations in 
epigenetic mechanisms such as histone modifications and DNA 
methylation, and menin interacts with several histone modify­
ing proteins, including MLL1 and PRMT5 (see Fig. 42.4), and 
deacetylase complexes (e.g., histone deacetylase complex subunit 
MSin3A). The use of epigenetic 

e complex subunit 
MSin3A). The use of epigenetic modulators to treat pancreatic 
NETs, in Men1+/− mice was assessed using JQ1, an inhibitor of 
the bromo and extraterminal domain (BET) family of proteins 
that bind to acetylene residues to promote gene transcription. 
JQ1 reduced proliferation and increased apoptosis of pancre­
atic and bronchial NET cell lines in vitro, and also in pancre­
atic NETs, in vivo, that developed in beta-cell–specific menin 
knockout (RIP-Cre-Men1–/–) mice.230 Thus,

c menin 
knockout (RIP-Cre-Men1–/–) mice.230 Thus, inhibitors that target 
epigenetic mechanisms may represent a novel class of anticancer 
drugs in MEN1.
Other drugs that have been effective in treating pancreatic 
NETs in Men1 knockout mice include WNT and K-RAS signal­
ing modulators (see Fig. 42.4). Thus, a small molecule β-catenin 
antagonist (PKF115-584) reduced proliferation of the pancreatic 
NETs in RIP-Cre-Men1–/– mice and ameliorated the excess in insu­
lin secretion.204 In addition, 

he excess in insu­
lin secretion.204 In addition, studies of pancreatic islet cells and 
NETs obtained from Ins2-Cre Men1–/– mice have demonstrated 
that in pancreatic endocrine beta cells, K-RAS activates oppos­
ing growth pathways, but the antiproliferative pathways dominate 
due to menin activity, which prevents the MAPK pathway from 
driving growth, while leaving the RASSF1A intact (see Fig. 42.4). 
Thus, loss of menin in the pancreatic NETs increases prolifera­
tion due to removal of the bl

increases prolifera­
tion due to removal of the blockage of MAPK-driven prolifera­
tion downstream of K-RAS, whereas K-RAS signaling increases 
proliferation by decreasing unopposed RASSF1A activity.206 The 
importance of the RAF/MEK/ERK pathway in driving inappro­
priate growth and survival of pancreatic beta cells with reduced 
menin activity was further demonstrated by the use of MEK1/2 
inhibitors (i.e., PD0325901 or GSK1120212) that modulate the 
KRAS pathway, and which revealed these inhib

 the 
KRAS pathway, and which revealed these inhibitors to have anti­
proliferative and cytotoxic actions. Thus, these may provide novel 
therapeutic approaches for menin-deficient tumors. 
Genetic Testing, Tumor Surveillance, and 
Organization of Care
Clinical Utility of MEN1 Mutational Analysis
MEN1 mutational analysis is helpful in clinical practice in several 
ways that include (1) confirmation of the clinical diagnosis, (2) 
identification of family members who harbor the MEN1 mutation 
and

f family members who harbor the MEN1 mutation 
and require screening for tumor detection and early treatment, 
and (3) identification of family members who do not harbor the 
familial germline MEN1 mutation and can therefore be reas­
sured1,21 (Fig. 42.5). Current guidelines recommend that MEN1 
mutational analysis should be undertaken in (1) an index case 
with two or more MEN1-associated tumors (i.e., parathyroid, 
pancreatic islet, or pituitary tumors); (2) all first-degree relatives 
of a kn

y tumors); (2) all first-degree relatives 
of a known MEN1 mutation carrier irrespective of whether they 
are asymptomatic or manifest associated clinical features (i.e., hav­
ing symptoms, signs, or biochemical and/or radiologic evidence of 
one or more MEN1-associated tumors); and (3) in patients with 
suspicion of MEN1 or atypical manifestations, which include 
those with a parathyroid adenoma below the age of 30 years and/
or multigland parathyroid disease, gastrinoma, or multiple pan­
creat

hyroid disease, gastrinoma, or multiple pan­
creatic NETs presenting at any age, or individuals who have two 
or more MEN1-associated tumors that are not exclusively limited 
to the classic triad of parathyroid, pancreatic islet, and parathyroid 
tumors (i.e., parathyroid plus adrenal tumor).1,21 These recom­
mendations are supported by results from several studies. Thus, a 
1639
Clinical diagnosis of MEN1
(i.e. ≥2 MEN1 associated tumors)
or high suspicio

or high suspicion of MEN1
Genetic testing for
MEN1 mutation
No MEN1 mutation identified
Consider occurrence of phenocopy
and mutation analysis of other
genes depending on clinical
features (e.g. CDC73, CASR, AIP,
CDKN1B)
Continue surveillance for additional
MEN1-associated tumors as per
MEN1 mutation positive individuals
Identify first-degree relatives for
clinical and biochemical evaluation
Asymptomatic first-degree relatives
should continue annual clinical
and

gree relatives
and biochemical screening
First-degree relatives with MEN1-
associated tumors should undergo
surveillance as per MEN1
mutation positive individuals
Clinical Evaluation: symptoms or signs
of MEN1-associated tumors
Biochemical Evaluation: Ca2+,, PTH,
PRL, IGF1, CgA, fasting gut hormones,
fasting glucose and insulin
Radiological Evaluation: MRI pituitary,
MRI (or CT or EUS) abdomen,
CT (or MRI) chest
No further investigation

 MEN1 mutation identified
required
Normal Result
Re-screen clinically
and biochemically
at 6-12 months
and radiologically
at 12-36 months
Proceed to
further
investigation
Abnormal Result
MEN1 mutation identified
Identify first-degree
relatives for genetic
counselling and MEN1
mutation testing
• Fig. 42.5  An approach to genetic testing and tumor screening in MEN1.  Index cases, or individuals in 
whom there is a high suspicion of clinical MEN1, should be offered genetic 

icion of clinical MEN1, should be offered genetic counseling and MEN1 mutation 
testing. The identification of a germline MEN1 mutation should prompt entry into a periodic clinical, bio­
chemical, and radiologic screening program. At the same time, all first-degree relatives should be identified 
and offered genetic counseling and MEN1 mutation testing irrespective of whether they express clinical 
features of MEN1 or are asymptomatic. Individuals who have inherited the MEN1 mutation should ente

s who have inherited the MEN1 mutation should enter 
periodic screening. First-degree relatives who have not inherited the MEN1 mutation require no further fol­
low-up and may be alleviated of the anxiety associated with the development of MEN1-associated tumors. 
For index cases in whom no MEN1 mutation is identified (including the exclusion of partial or whole gene 
deletions [e.g., by multiplex ligation-dependent probe amplification analysis]), additional genetic testing 
may be indicated dep

, additional genetic testing 
may be indicated depending on the specific clinical features. This may include examination for mutations 
in genes associated with familial parathyroid syndromes including CDC73 associated with the hyper­
parathyroidism-jaw tumor syndrome (HPT-JT) and the calcium-sensing receptor (CASR) associated with 
familial benign hypercalciuric hypercalcemia (FBHH), or cyclin-dependent kinase 1B (CDKN1B) and aryl 
hydrocarbon receptor interacting protein (AIP), which are rarel

eceptor interacting protein (AIP), which are rarely identified in those with clinical MEN1. Up 
to 10% of kindreds with clinical MEN1 may harbor phenocopies emphasizing the importance of accurate 
genetic evaluation. For MEN1 kindreds in whom no MEN1 mutation is identified, a pragmatic approach is 
to offer clinical, biochemical, and radiologic screening to those with clinical manifestations of disease and 
to offer annual clinical and biochemical screening to asymptomatic first-degree relatives

l screening to asymptomatic first-degree relatives. Ca2+, calcium; 
CgA, chromogranin A; CT, computed tomography; EUS, endoscopic ultrasound; IGF1, insulin-like growth 
factor 1; MEN1, multiple endocrine neoplasia type 1; MRI, magnetic resonance imaging; PRL, prolactin; 
PTH, parathyroid hormone. (Redrawn from Thakker RV, Newey PJ, Walls GV, et al. Clinical practice guide­
lines for multiple endocrine neoplasia type 1 (MEN1). J Clin Endocrinol Metab. 2012;97[9]:2990–3011.)
1640
SECTION IX    Pol

tab. 2012;97[9]:2990–3011.)
1640
study of 200 patients with endocrine tumors reported that MEN1 
mutations occurred in more than 70% of individuals with two 
or more of the major MEN1-associated endocrine tumors (e.g., 
parathyroid, pancreatic pituitary tumors) and a family history of 
these tumors; approximately 60% of individuals with at least one 
of the major endocrine tumors and a first-degree relative with a 
major endocrine tumor; and 6

gree relative with a 
major endocrine tumor; and 6% of patients with sporadic (i.e., 
nonfamilial) MEN1-associated endocrine tumors referred for test­
ing, although the MEN1 mutations were only observed in patients 
who had multiple endocrine tumors and/or were younger than 30 
years of age.231 In addition, another study has reported that the 
likelihood of finding a MEN1 mutation correlates with the pres­
ence of clinical features, with 80% of index cases presenting with 
three MEN1-related tum

ndex cases presenting with 
three MEN1-related tumors having a germline MEN1 mutation, 
which increased to more than 90% if family history revealed the 
presence of affected relatives, whereas only 15% of patients pre­
senting with a single MEN1-associated tumor had a MEN1 muta­
tion, and this decreased to 0% when the family history revealed 
the absence of affected relatives.232 A study of 205 patients with 
PHPT and a family history of PHPT revealed that approximately 
45% had a MEN1 mutation,

ealed that approximately 
45% had a MEN1 mutation, and that multigland disease, male 
sex, and age younger than 45 years were independent predictors 
for an associated germline mutation, with odds ratios of 14, 1.7, 
and 8, respectively.233 Finally, a study of 39 patients with atypical 
presentation (i.e., pituitary adenomas with pheochromocytoma/
paraganglioma) identified two patients with germline MEN1 
mutations.234 All of these studies indicate that failure to under­
take genetic testing may

te that failure to under­
take genetic testing may result in a missed opportunity for early 
diagnosis.1,235
Following the genetic diagnosis of MEN1, predictive genetic 
testing should be offered to all first-degree relatives (see Fig. 42.5), 
and current guidelines recommend that this should be undertaken 
at the earliest opportunity.1 Indeed, delays in the genetic diagno­
sis of first-degree relatives of an affected index case are reported 
to result in increased morbidity, highlighting the ne

result in increased morbidity, highlighting the need for pro­
active cascade testing within MEN1 kindreds.236 However, there 
are several ethical issues to consider, and pretest counseling and 
transparency are essential. Thus, patients may be concerned by 
the implications of finding a causative mutation on other family 
members, future reproductive decision making, and the potential 
for financial or societal discrimination. Indeed, a frequent concern 
of patients undergoing genetic testing is

concern 
of patients undergoing genetic testing is the potential implications 
on future employment or ability to get insurance, although many 
countries have legislation in place to protect individuals from 
such genetic discrimination. For example, in the United States, 
the Federal Genetic Information Nondiscrimination Act prohib­
its health insurance companies or employers from using genetic 
information to determine eligibility for health insurance cover­
age, or decisions in employment-rel

surance cover­
age, or decisions in employment-related matters (i.e., hiring, pro­
motion), respectively. With appropriate genetic counseling, most 
patients conclude that the potential benefits of genetic testing 
outweigh the potential harms. A further consideration involves 
the genetic testing of children, which is frequently undertaken 
with parental consent. Indeed, in view of the high penetrance of 
early-onset MEN1-associated tumors,19 it is recommended that 
at-risk asymptomatic childre

 is recommended that 
at-risk asymptomatic children (i.e., offspring of an affected par­
ent) undergo genetic testing in the first decade of life and ideally 
before 5 years of age.21,174
Preconception genetic counseling, where appropriate, should 
be provided to individuals at risk of transmitting MEN1 to their 
future offspring (i.e., symptomatic or asymptomatic MEN1 muta­
tion carriers), and advances in genetic testing combined with 
in vitro fertilization can now offer potential parents the 

fertilization can now offer potential parents the oppor­
tunity to have preimplantation genetic diagnosis. This may be 
achieved by the direct sequencing of DNA from a small num­
ber of cells obtained from the early blastocyst, with subsequent 
implantation of only those embryos identified to not carry the 
mutation, thereby markedly reducing the risk of transmitting the 
genetic disease to the child.
Finally, it is important to emphasize that all MEN1 genetic test­
ing should be undertaken in a

l MEN1 genetic test­
ing should be undertaken in accredited genetics laboratories using 
validated testing strategies that adequately sequence all exonic and 
splice-site regions of the MEN1 gene, thereby facilitating the reli­
able detection of all single nucleotide variants or small insertions 
or deletions (indels). In the absence of such mutations, multiplex 
ligation-dependent probe amplification should be employed to 
detect partial or whole gene deletions. 
Surveillance of “At-Risk” Indiv

e gene deletions. 
Surveillance of “At-Risk” Individuals
Current guidelines recommend that all individuals at high risk for 
MEN1 (i.e., mutant gene carriers) undergo periodic clinical, bio­
chemical, and radiologic screening to facilitate the early detection 
and treatment of tumors with the aim of reducing their associated 
morbidity and mortality1 (Table 42.2; see Fig. 42.5). Although it 
seems logical that such screening programs are likely to be ben­
eficial, high-quality evidence supportin

o be ben­
eficial, high-quality evidence supporting their effectiveness has 
not been established. However, most clinicians recommend the 
implementation of interval surveillance, although the frequency 
and scope of investigation is debated.19,72,85 Two particularly con­
troversial areas involve the detection of NF pancreatic NETs and 
thymic carcinoid tumors, which represent the two leading causes 
of premature MEN1-related death. In particular, the age to begin 
screening, the use of biochemi

, the age to begin 
screening, the use of biochemical tumor markers, and the opti­
mal radiologic approach are areas that are controversial and under 
debate.19,85,98,99,237 For example, current guidelines recommend 
commencing pancreatic imaging from the age of 10 years, and this 
is supported by several reports that indicate a high penetrance of 
NF-NETs in the second decade of life.19,97,101 However, some cen­
ters recommend that imaging should be delayed until at least 16 
years. Similarly, 

d be delayed until at least 16 
years. Similarly, for thymic carcinoid tumors, the rapid growth rates 
and aggressive disease course would favor at least annual screening 
with thoracic CT, but the high cumulative ionizing radiation dose 
associated with such a schedule is considered unacceptable, particu­
larly as such tumors occur in only a minority of MEN1 patients. 
Another controversial topic relates to breast screening in women 
with MEN1, with some groups advocating screening from age 40 

with some groups advocating screening from age 40 
years because of the reported increased relative risk of breast can­
cer.139,141 Thus, several uncertainties exist, and it is important to 
individualize these current recommendations to the patient, and to 
adapt them for the local availability of resources and wishes of the 
patient, who should be involved in the decision-making process. 
For example, a recent study highlights that the majority of MEN1 
patients are highly fearful of disease o

 of MEN1 
patients are highly fearful of disease occurrence, both for them­
selves and their relatives, which may be compounded by regular 
surveillance screening.238 Furthermore, this psychological distress is 
reported to be associated with a lower health-related quality of life, 
and these factors should be considered.238 
MEN Types 2 and 3
MEN2, which has been referred to as MEN2A and Sipple 
syndrome, is an autosomal dominant disorder with a reported 
incidence of 1 in 80,000 to 200,000 liv

 reported 
incidence of 1 in 80,000 to 200,000 live births, which is char­
acterized by the occurrence of medullary thyroid carcinoma 
1641
(MTC) in association with pheochromocytoma and parathyroid 
tumors.3,239,240 MEN2 also includes three variants, which are 
MEN2A with Hirschsprung disease (HSCR), MEN2A with 
cutaneous lichen amyloidosis (CLA), and familial MTC-only 
(FMTC),3 in which MTC is the sole manifestation of the syn­
drome. MEN3, also referre

anifestation of the syn­
drome. MEN3, also referred to as MEN2B, is characterized 
by the occurrence of MTC and pheochromocytoma, without 
PHPT, but in association with a marfanoid habitus, mucosal 
neuromas, medullated corneal fibers, and intestinal autonomic 
ganglion dysfunction leading to megacolon3 (Fig. 42.6). MEN2 
is more common than MEN3, with MEN2 accounting for 
greater than 90% and MEN3 between 5% and 10% of patients, 
although MTC in MEN3 usually arises in infancy and has a 
more ag

 MEN3 usually arises in infancy and has a 
more aggressive course.3
Historically, the association between thyroid carcinoma and 
pheochromocytoma was first reported by Sipple241 in 1961, with 
Cushman and Rochester reporting a family with autosomal domi­
nant inheritance of pheochromocytomas, MTC, and parathyroid 
adenoma,242 and Steiner and colleagues243 in 1968 proposing the 
term MEN2 to describe a family with MTC, pheochromocytoma, 
PHPT, and Cushing syndrome. The association of multiple neu

 Cushing syndrome. The association of multiple neu­
romas, pheochromocytoma, and MTC was reported by Williams 
and Pollock244 in 1966, and Schimke and colleagues245 in 1968, 
and Chong and colleagues246 in 1975, used the term MEN2B 
to describe this disorder. Interestingly, the first descriptions of 
MEN2A and pheochromocytoma, in 1886, were in an 18-year-
old woman who had bilateral adrenal tumors, and whose relatives, 
in the Black Forest in Germany, have subsequently been reported 
to have ph

rmany, have subsequently been reported 
to have pheochromocytoma and MTC due to a rearranged dur­
ing transfection (RET) mutation (Cys634Trp), indicating that 
this patient and family have MEN2A.247 The first descriptions of 
patients with likely MEN3 were in 1922 and 1923 by Wagen­
mann248 and Froboese,249 respectively.
Genetic studies of MEN2 and MEN3 families throughout 
the 1980s and 1990s led to the demonstration that mutations 
of the RET proto-oncogene, which is located on chromo­
some 10

roto-oncogene, which is located on chromo­
some 10q11.21 and encodes an RTK, caused the MEN2 and 
MEN3 syndromes. Approximately 95% of MEN2 RET muta­
tions involve the cysteine-rich extracellular domain (ECD), with 
mutations of Cys634 accounting for approximately 85% of all 
MEN2 mutations, whereas approximately 95% of MEN3 RET 
mutations are of Met918, which is located in the intracellular 
tyrosine kinase (TK) domain (Table 42.3 and Fig. 42.7; see 
Table 42.1). Moreover, the identification of

 see 
Table 42.1). Moreover, the identification of these RET muta­
tions, which reliably predict the risk of MTC with regard to its 
clinical expression and age of onset, have transformed the man­
agement of patients, as it has helped to determine the timing of 
prophylactic thyroid surgery that is highly effective in avoiding 
the morbidity and mortality associated with MTC. Indeed, pro­
phylactic thyroidectomy, with lifelong thyroxine replacement, 
has dramatically improved outcomes in the pat

nt, 
has dramatically improved outcomes in the patients with MEN2 
and MEN3 such that 90% of young patients with RET muta­
tions who had a prophylactic thyroidectomy had no evidence 
of persistent or recurrent MTC at 7 years after surgery.250 To 
facilitate management of MTC in MEN2 and MEN3 patients, 
the American Thyroid Association (ATA) has defined three cat­
egories for germline RET mutations that are based on their cor­
relations between genotype and phenotype (e.g., aggressiveness 
of MTC

notype and phenotype (e.g., aggressiveness 
of MTC), and these comprise highest risk (MEN3-associated 
Met918Thr mutation), high risk (Cys634 and Ala883Phe muta­
tions associated with MEN2 and MEN3, respectively), and 
moderate risk (all other MEN2 RET mutations)3 (Table 42.4; 
see Table 42.3). For asymptomatic individuals identified to har­
bor RET mutations, current recommendations for tumor sur­
veillance and treatment are based on these risk categories, with 
the aim of identifying RET mutat

categories, with 
the aim of identifying RET mutations at a sufficiently early age 
  Suggested Screening Guidelines for Individuals at Risk of MEN1
MEN1-Associated Tumor
Age to Begin 
Screening (Years)
Biochemical Screening Test 
(Annually)
Imaging Screening Test 
(Time Interval)
8
Calcium, PTH
None
  Gastrinoma
20
Fasting gastrin
None
  Insulinoma
5
Fasting glucose (± insulin)
None
  Other pancreatic NET 
(e.g., nonfunctioning)
10
Chromogranin A, gastrointestinal hormon

ioning)
10
Chromogranin A, gastrointestinal hormone profilea (e.g., glucagon, 
pancreatic polypeptide, vasoactive intestinal peptide)
MRI abdomen, EUS (annually)
Pituitary
  Prolactinoma
5
Prolactin
None
  Somatotropinoma
5
None
  Other pituitary adenoma 
(e.g., nonfunctioning NET)
10b
None, unless signs or symptoms of functioning tumor (e.g., 
corticotroph adenoma)
MRI pituitary (every 3 years)
Adrenocortical
<10
None, unless signs or symptoms of functioning tumor o

e, unless signs or symptoms of functioning tumor or tumor 
>1 cm on imaging
MRI abdomen (annually)
Thymic/bronchial carcinoid
15
None
CT or MRI chest (every 1–2 years)
aAlthough chromogranin A, pancreatic polypeptide, and glucagon concentrations can be elevated with nonfunctioning PNETs, they have low sensitivity and specificity such that their value is debated.
bAlthough pituitary tumors are reported in MEN1 patients as young as 5 years of age, in the absence of symptoms, signs, or biochemical 

in the absence of symptoms, signs, or biochemical evidence of a pituitary adenoma, pituitary imaging may be delayed 
until after 10 years of age to coincide with pancreatic imaging.
CT, Computed tomography; EUS, endoscopic ultrasound; MEN1, multiple endocrine neoplasia type 1; MRI, magnetic resonance imaging; NET, neuroendocrine tumor; PTH, parathyroid hormone.
Modified from Thakker RV, Newey PJ, Walls GV, et al. Clinical practice guidelines for multiple endocrine neoplasia type 1 (MEN1). J Clin

multiple endocrine neoplasia type 1 (MEN1). J Clin Endocrinol Metab. 2012;97(9):2990–3011.
   
TABLE 42.2
1642
A
B
C
D
• Fig. 42.6  Clinical features in a patient with multiple endocrine neoplasia type 3. The patient has evidence 
of a nodule in the left side of the neck representing metastases from medullary thyroid carcinoma (indicated 
by the upper arrow). Note the thyroidectomy scar (lower arrow) (A). Mucosal neuromas are evident on the 
t

arrow) (A). Mucosal neuromas are evident on the 
tongue and lips (B). Chest x-ray demonstrating bilateral lung metastases of medullary thyroid carcinoma 
(C). Radiograph of barium meal and follow through demonstrating multiple intestinal diverticula, which are 
secondary to autonomic ganglion dysfunction (D). The patient had a history of diarrhea and malabsorption. 
(Modified from Thakker RV. Multiple endocrine neoplasia. Medicine. 2013;41[10];562–565.)
  Clinical Relationships and MTC Risk Lev

2–565.)
  Clinical Relationships and MTC Risk Level Associated With Common RET Mutations in MEN2 and MEN3
Exon
Affected Codon/Mutation
ATA MTC Risk Level
Penetrance of 
Penetrance of PHPT
Additional Reported 
Associations
8
Gly533Cys
Moderate
c.10%
c.10%
-
10
Cys609Phe/Gly/Arg/Ser/Tyr
Moderate
c.10–20%
c.10%
HSCR
10
Cys611Phe/Gly/Ser/Tyr/Trp
Moderate
c.10–20%
c.10%
HSCR
10
Cys618Phe/Arg/Ser
Moderate
c.10–20%
c.10%
HSCR
10
Cys620Phe/Arg/Ser
Moderate
c.10–20%
c.10%
HSCR
11
Asp631

Phe/Arg/Ser
Moderate
c.10–20%
c.10%
HSCR
11
Asp631Tyr
Moderate
c.50%
—
—
11
Cys634 Phe/Gly/Arg/Ser/Trp/Tyr
High
c.50%
c.20–30%
CLA
11
Lys666Glu
Moderate
c.10%
—
—
13
Glu768Asp
Moderate
—
—
—
13
Leu790Phe
Moderate
c.10%
—
—
14
Val804Leu
Moderate
c.10%
c.10%
—
14
Val804Met
Moderate
c.10%
c.10%
CLA
15
Ala883Phe
High
c.50%
—
MEN3 clinical features
15
Ser891Ala
Moderate
c.10%
c.10%
—
16
Arg912Pro
Moderate
—
—
—
16
Met918Thr
Highest
c.50%
—
—, Not typically observed/associated; 

al features
—, Not typically observed/associated; ATA, American Thyroid Association; CLA, cutaneous lichen amyloidosis; HSCR, Hirschsprung disease; MEN2, multiple endocrine neoplasia type 2; MEN3, MEN type 
3; MTC, medullary thyroid carcinoma; PHPT, primary hyperparathyroidism.
Modified from Wells SA Jr, Asa SL, Dralle H, et al. Revised American Thyroid Association guidelines for the management of medullary thyroid carcinoma. Thyroid. 2015;25(6):567–610.
   
TABLE 42.3
1643
Chapter 42  Multiple 

567–610.
   
TABLE 42.3
1643
Cysteine-rich
 domain
Cadherin-like
 domains
Cys611/Phe/Gly/Ser/Trp/Tyr
Cys618Phe/Arg/Ser/Tyr/Gly
Cys620Phe/Arg/Ser/Gly/Tyr
Cys630Arg/Tyr
Asp631Tyr
Cys634Phe/Gly/Arg/Ser/Trp/Tyr
Leu790Phe
Ala883Phe (MEN3)
Tyrosine kinase 
domain
Val804Met/Leu
Met918Thr (MEN3)
TM
 Domain
Isoform-specific
A
Wild Type 
RET
GFRα1
GDNF
P
P
P
P
P
P
 
 RET
receptor 
P
P
P
P
P
P
MEN

P
P
P
 
 RET
receptor 
P
P
P
P
P
P
MEN2
EC Domain
mutations
S
S
MEN2
IC Domain
mutations
P
P
P
MEN3
Met918Thr
mutations
P
P
P
P
P
P
P
P
P
??
B
C
Ser891Ala
EC
Domain
IC
Domain
• Fig. 42.7  RET receptor structure highlighting the main functional domains and locations of common mul­
tiple endocrine neoplasia type 2 (MEN2)-associated RET mutations (A). MEN2-associated mutations arise 
most frequently in the cysteine-rich region of the extracellular (EC) domain, or in the intracellular (I

racellular (EC) domain, or in the intracellular (IC) tyro­
sine kinase domain, which are linked by the transmembrane domain (TM). The mutations shown represent 
those most commonly observed in MEN2 and MEN type 3 (MEN3), although additional RET mutations 
Continued
1644
(i.e., often in the first months or years of life) to provide a win­
dow of opportunity for “prophylactic” thyroidectomy. In addi­
tion, the identification of a MEN2-associated

ddi­
tion, the identification of a MEN2-associated RET mutation in 
an affected patient should prompt screening of all first-degree 
relatives, and the possibility of MEN2 or MEN3 should be con­
sidered in all patients presenting with MTC or pheochromocy­
toma, and germline RET genetic testing undertaken. However, it 
is important to note that de novo RET mutations are observed in 
5% to 10% and approximately 75% of individuals with MEN2 
and MEN3, respectively. These advances in our understandi

3, respectively. These advances in our understanding 
of RET and RTKs have also resulted in the use of RTK inhibi­
tors for the treatment of advanced and metastatic MTC. The 
treatment of patients presenting with one or more of the clinical 
manifestations of MEN2 and MEN3 require a multidisciplinary 
approach involving surgeons, oncologists, endocrinologists, and 
geneticists to ensure the optimal therapeutic approach.
Clinical Featur

gement
MTC is the most common and often the first manifestation of 
MEN2 and MEN3, and it occurs in almost all affected individuals 
(Fig. 42.8; see Fig. 42.6). MTC, which also represents the major 
cause of premature morbidity and mortality, frequently presents in 
children. Thus, early detection and treatment of MTC is impor­
tant, and widespread implementation of RET mutation testing 
has transformed the management of patients from MEN2 and 
MEN3 

ed the management of patients from MEN2 and 
MEN3 families in whom the causative RET mutation is known. 
Indeed, the identification of RET as the causative gene for MEN2 
and the subsequent ability to undertake genetic testing of “at-risk” 
first-degree relatives has resulted in a shift in clinical presentation 
from those presenting with a neck mass and advanced disease to 
Fig. 42.7, cont’d have been described in small numbers of kindreds. The American Thyroid Association risk 
category of RET

American Thyroid Association risk 
category of RET mutation is also indicated: “highest” risk, red; “high” risk, blue; “moderate” risk, green. 
Mutations associated with MEN3 are noted in parentheses. All other mutations are associated with MEN2. 
RET receptor activation occurs following binding of members of the glial cell–line derived neurotropic fac­
tor (GDNF) family of ligands (GFLs), which comprise GDNF (shown), neurturin, persephin, and artemin (B). 
However, binding is mediated by a core

temin (B). 
However, binding is mediated by a coreceptor, represented by members of the GDNF family receptor α 
group of proteins (GFRα1 shown). Thus, once formed, the GFL-GFRα complex engages RET, facilitating 
receptor dimerization and receptor activation, resulting in autophosphorylation of specific tyrosine residues 
within the tyrosine kinase domain, and subsequent recruitment and activation of downstream signaling 
complexes. Mutations associated with MEN2 are associated with ligand-indepe

ciated with MEN2 are associated with ligand-independent receptor activation 
but achieve this by different mechanisms (C). MEN2 mutations in the extracellular (EC) cysteine-rich domain 
result in ligand-independent receptor dimerization mediated by disulphide-bond (S) formation between 
unpaired cysteine residues, resulting in constitutive receptor activation. In contrast, MEN2-associated 
mutations affecting the intracellular (IC) tyrosine kinase domain result in receptor activation in monomeri

e domain result in receptor activation in monomeric 
form. The MEN3 Met918Thr mutation also results in monomeric ligand-independent receptor activation 
but potentially may be further enhanced by the presence of RET ligand, thereby facilitating receptor dimer­
ization and even higher levels of receptor signaling. However, the in vivo role of such ligand-enhanced 
activity in association with the Met918Thr mutation remains uncertain (represented by double question 
marks [??]). (A, modified from 

y double question 
marks [??]). (A, modified from Newey PJ. Multiple endocrine neoplasia. Medicine. 2017;45[9]:538–542.)
  Recommendations for Screening and Surgery in MEN2 and MEN3
RECOMMENDED AGE (YEARS) FOR SCREENING TEST/INTERVENTION
ATA Risk 
Categorya
Relevant RET Mutations
RET Mutational 
Analysis
First Serum 
Calcitonin 
and Neck US
Prophylactic 
Screening 
for PCCf
Screening 
for PHPT
Moderate
Validated pathogenic mutations excluding those 
in the high and highest categor

s excluding those 
in the high and highest categoriesb
<3–5
5
>5c
16
16
High
Ala883Phed
<3
3
5 or earliere
11
11
Highest
Met918Thrd
ASAP and by <1
ASAP and by 
<0.5–1
11
—
aATA risk category as defined in the Revised American Thyroid Association guidelines for the management of medullary thyroid carcinoma. Thyroid. 2015;25(6):567-561.
bRET mutations reported at ClinVar, ARUP database (arup.utah.edu/database/MEN2/MEN2_welcome.php).
cTiming of surgery t

abase/MEN2/MEN2_welcome.php).
cTiming of surgery to be based on elevation of serum calcitonin and/or the joint discussion of the pediatrician, surgeon, and parent/family. For example, later surgery may be appropriate if serum calcitonin 
and neck US are normal.
dRET mutation associated with MEN3.
eEarlier than 5 years based on elevation of serum calcitonin. The surgeon and pediatrician in consultation with the child’s parent should decide the optimal timing of surgery.
fIndividuals with MTC must

imal timing of surgery.
fIndividuals with MTC must have pheochromocytoma excluded prior to a surgical intervention and should be excluded in all at-risk individuals who are planning pregnancy or are pregnant.
—, Not required, as not part of MEN3; ASAP, as soon as possible; ATA, American Thyroid Association; MEN2, multiple endocrine neoplasia type 2; MEN3, MEN type 3; MTC, medullary thyroid carcinoma; 
PCC, pheochromocytoma; PHPT, primary hyperparathyroidism; US, ultrasound.
Data from American Th

athyroidism; US, ultrasound.
Data from American Thyroid Association Guidelines Taskforce, Kloos RT, Eng C, et al. Medullary thyroid cancer: management guidelines of the American Thyroid Association. Thyroid. 2009;19(6):565-612 
and from Wells SA Jr, Asa SL, Dralle H, et al. Revised American Thyroid Association guidelines for the management of medullary thyroid carcinoma. Thyroid. 2015;25(6):567–610.
   
TABLE 42.4
1645
otherwise asymptomatic RET mutation 

ine Neoplasia
otherwise asymptomatic RET mutation carriers, who are recom­
mended to have prophylactic thyroidectomy that substantially 
reduces their likelihood of developing advanced MTC.250,251 Fur­
thermore, widespread availability of RET genetic testing that iden­
tifies presymptomatic individuals who then undergo a prophylactic 
thyroidectomy has resulted in a significant fall in the proportion of 
MEN2 cases who present as index cases with MTC.252 However, 
in the absence of a relevant fa

 MTC.252 However, 
in the absence of a relevant family history (and/or prior knowl­
edge of RET genetic status), the presentation of MTC is typically 
with a palpable neck mass, which may be asymptomatic or associ­
ated with symptoms of pressure or dysphagia in more than 15% 
of patients, although it should be noted that only a small minor­
ity (0.3–1.4%) of the population presenting with thyroid nodules 
harbor an underlying MTC.3 For such index cases presenting with 
MTC (i.e., with a neck lum

 cases presenting with 
MTC (i.e., with a neck lump), regional and/or distant metastases 
are frequently evident at the point of diagnosis, and surgical cure 
is rarely achieved. MTC and pheochromocytoma may be pres­
ent synchronously, and this possibility should be considered prior 
to any thyroid surgical intervention, particularly if the results of 
RET genetic testing are unavailable. Additional symptoms associ­
ated with MTC include diarrhea, which is reported in approxi­
mately 30% of pati

, which is reported in approxi­
mately 30% of patients, and/or flushing, each reflecting high 
circulating concentrations of calcitonin or other tumor-secreted 
hormones (e.g., serotonin, prostaglandins). Diarrhea occurs most 
frequently in those with advanced disease and most often in the 
presence of liver metastases.3 The ectopic production of ACTH 
or corticotropin-releasing hormone from MTC may give rise to 
Cushing syndrome, and 1% to 3% of all cases of ectopic Cushing 
syndrome are due to

 all cases of ectopic Cushing 
syndrome are due to MTC.253 In those presenting with a neck 
mass, lymph node and distant metastases are frequently present at 
the time of diagnosis, and the extent of lymph node involvement 
provides important prognostic information.3,254 The cervical and 
mediastinal nodes are the most common sites of local metastases, 
whereas distant spread typically involves bone, liver, lung, or brain.
MTC occurs following the malignant transformation of para­
follicular C c

e malignant transformation of para­
follicular C cells, which are concentrated in the middle and upper 
regions of the thyroid gland. The embryologic origins of the para­
follicular C cells are considered to be of neuroectodermal origin, 
although there may be species-specific differences.255 In mammalian 
species, C cells are reported to be derived from a subpopulation of 
ectodermal-derived neural crest cells that migrate to the most infe­
rior pharyngeal arch (i.e., fourth arch), whereby they

 pharyngeal arch (i.e., fourth arch), whereby they merge with a 
pair of endoderm-derived ultimobranchial bodies and become part 
of the developing thyroid gland.255 C cells secrete several biogenic 
amines and peptides, including calcitonin, an evolutionary con­
served hormone that is involved in calcium homeostasis. However, 
the physiologic function of calcitonin in mammalian species is 
unknown, as an absence of the hormone either by genetic knock­
out of the calcitonin gene in mice, or thyr

knock­
out of the calcitonin gene in mice, or thyroid dysgenesis in humans, 
does not affect viability or bone development and indeed is associ­
ated with a normal development and a normal phenotype.255 The 
transformation of C cells to MTC occurs along a multistep path­
way, which initially involves C-cell hyperplasia (CCH), followed 
by noninvasive microscopic MTC (i.e., <1 cm), that progresses to 
invasive carcinoma with lymph node and distant metastases.239 The 
molecular basis of each of th

 metastases.239 The 
molecular basis of each of these stages remains ill defined, although 
aberrant RET signaling almost certainly represents the key initiat­
ing event in tumor formation. In the context of MEN2, MTC 
is frequently multifocal and bilateral, occurring in the upper two 
thirds of the thyroid corresponding to the highest concentration of 
C cells, and it is likely that the CCH observed early in the disease 
course represents monoclonal proliferation of transformed progen­
itor cel

onal proliferation of transformed progen­
itor cells rather than hyperplasia per se.3 The secretory capacity for 
calcitonin is usually retained throughout each stage of tumor devel­
opment, thereby providing a valuable tumor marker, with clinical 
utility for both MTC diagnosis and subsequent tumor surveillance 
(i.e., detecting residual, recurrent, or progressive disease).3 Thus, 
the diagnosis of MTC relies on the demonstration of high basal 
serum calcitonin concentrations together with supp

serum calcitonin concentrations together with supporting cyto­
logic/histopathologic evidence.3
Neck ultrasound with fine-needle aspiration (FNA) is the initial 
investigation of choice for individuals presenting with a solitary thy­
roid nodule, whereas radionuclide thyroid scans may reveal MTC 
tumors as “cold” nodules. MTC may display variable cytologic 
appearances and on occasion may be misdiagnosed as other tumor 
types.3 Typical cytologic appearances include the presence of disco­
hesive 

appearances include the presence of disco­
hesive or weakly cohesive cells, which may be spindle shaped or have 
plasmacytoid or epithelioid appearances.3 Important cytologic crite­
ria for MTC are reported to include a dispersed pattern of polygo­
nal or triangular cells, azurophilic cytoplasmic granules, extremely 
eccentrically placed nuclei with coarse granular chromatin, and 
amyloid.3 Some studies have reported a high diagnostic accuracy 
of FNA in MTC (i.e., >80%), but a meta-analysis of 

f FNA in MTC (i.e., >80%), but a meta-analysis of 15 studies 
and 641 MTC cases reported a detection rate of approximately 
55%, with several specimens that were initially classified as indeter­
minate or benign being subsequently found to be MTC; however, 
many specimens that were not identified as MTC were found to be 
malignancies, thereby indicating a need for surgery.3,256 For aspirate 
FDG uptake in
forearm consistent 
with skeletal 
metastates
Metastatic
MTC in
Liver
FDG avid MTC 
in Neck

ates
MTC in
Liver
in Neck
FDG avid left
pheochromocytoma 
• Fig. 42.8  18FDG-PET scan showing MTC with liver and skeletal metas­
tasis and PET-avid uptake by the left adrenal pheochromocytoma in a 
patient with MEN2 due to a Cys634Arg RET mutation. FDG, fluorode­
oxyglucose; MEN2, multiple endocrine neoplasia type 2; MTC, medullary 
thyroid carcinoma; PET, positron emission tomography (From Naziat A, 
Karavitaki N, Thakker R, et al. Confusing genes: a patient with MEN2A 

r R, et al. Confusing genes: a patient with MEN2A and 
Cushing’s disease. Clin Endocrinol. 2013;78[6]:966–968.)
1646
results that are suspicious for MTC or indeterminate, the diagnos­
tic accuracy may be improved by measuring calcitonin levels in the 
FNA washout fluid and/or undertaking additional immunohisto­
chemical evaluation for neuroendocrine markers including calcito­
nin, chromogranin, and carcinoembryonic antigen (CEA).3,257
Once MTC

 and carcinoembryonic antigen (CEA).3,257
Once MTC is suspected, the measurement of basal serum cal­
citonin levels should be undertaken. Modern commercial immu­
nochemiluminometric assays are highly sensitive and specific for 
monomeric calcitonin and do not typically display cross reactiv­
ity for procalcitonin or other calcitonin-related peptides. How­
ever, elevated serum calcitonin concentrations may be observed in 
other conditions, such as chronic renal failure, parathyroid disease, 
thyr

 chronic renal failure, parathyroid disease, 
thyroiditis, lung and prostate cancers, and other NETs, thereby 
diminishing the specificity of basal calcitonin levels for diagnosis 
of MTC.3 Elevated (or occasionally inappropriately low) calcito­
nin levels may also occur in the presence of heterophile antibod­
ies, whereas false-negative or inappropriately low calcitonin levels 
may occasionally be observed due to the “Hook effect,” in which 
very high levels of serum calcitonin saturate the bin

y high levels of serum calcitonin saturate the binding capacity 
of the antibody in the immunoassay, although the likelihood of 
this is reduced with modern immunochemiluminometric assays. 
Inappropriately low serum calcitonin concentrations (i.e., relative 
to disease stage) may also rarely occur in the setting of advanced 
MTC, in which tumor dedifferentiation results in reduced cal­
citonin secretion.3 Finally, occasional case reports of calcitonin-
negative MTC presenting with a solitary thy

tonin-
negative MTC presenting with a solitary thyroid nodule have been 
reported.258 Reference ranges for basal serum calcitonin levels are 
assay specific, which must also take into account age-related and 
sex-dependent factors. For example, serum calcitonin concentra­
tions are typically higher in males than females, reflecting a higher 
total C-cell mass. In addition, serum calcitonin levels are elevated 
in infants and young children, and age-specific reference ranges are 
required until 2

ge-specific reference ranges are 
required until 2 to 3 years of age, after which serum calcitonin con­
centrations are indistinguishable from those of adults.3,259 Estab­
lishing a serum calcitonin concentration below which the diagnosis 
of MTC can be confidently excluded in a patient presenting with 
a thyroid nodule remains challenging, as false positives occur fre­
quently.260 Thus, routine measurement of basal serum calcitonin 
concentrations in all patients presenting with thyroid nodules

ns in all patients presenting with thyroid nodules and 
before an FNA is controversial. Advocates of calcitonin measure­
ment pre-FNA cite the need for early diagnosis given the low cure 
rates observed once MTC spreads beyond the thyroid. However, 
the very low overall frequency of MTC in this setting (i.e., <1%) 
raises concerns over cost effectiveness, whereas the potential for 
iatrogenic morbidity in those with raised basal serum calcitonin 
levels, but no subsequent MTC, has not been fully

levels, but no subsequent MTC, has not been fully evaluated.261 
Thus, current guidelines recognize differences in clinical practice 
and do not make any clear recommendation.3 Previously, provoca­
tion tests with potent secretogogues including calcium and pen­
tagastrin, which were commonly used to improve the diagnostic 
value of serum calcitonin measurements, are now employed less 
frequently.3 In addition to calcitonin, other biomarkers that may 
be associated with MTC include CEA, although

 may 
be associated with MTC include CEA, although this is not specific 
for MTC and has little role in establishing the diagnosis but may 
be useful for monitoring disease progression,3 and others such as 
serum carbohydrate antigen 19.9, which may be associated with 
MTC but is not routinely employed for diagnosis.262
Once the diagnosis of MTC has been made by FNA of a thyroid 
nodule and demonstration of an elevated basal serum calcitonin 
concentration, further investigations are required to

centration, further investigations are required to determine the 
likely extent of disease. Thus, preoperative staging with neck ultra­
sound is mandatory, whereas cross-sectional imaging, using CT or 
MRI, should be undertaken in those in whom metastatic disease is 
suspected, which is usually based on extensive neck disease and/or 
very high serum calcitonin levels (e.g., >500 pg/mL). CT is used 
widely to detect lung and mediastinal lymph node involvement, 
whereas complementary additional mo

 involvement, 
whereas complementary additional modalities (e.g., MRI, bone 
scintigraphy) are employed to detect metastases at other locations. 
FDG-PET/CT or F-DOPA-PET/CT, which may be helpful (see 
Fig. 42.8), is not readily available in many centers. Finally, for the 
investigation of MTC, it is important to undertake RET germline 
genetic testing in all patients, and for those who have a mutation 
(or for those in whom there is likely to be a significant delay in 
testing), the presence of

 a significant delay in 
testing), the presence of pheochromocytoma and PHPT should be 
excluded prior to surgery.3
MTC is highly penetrant in MEN2, with 70% to 100% of 
affected individuals developing disease by the age of 70 years. There 
is a strong genotype-phenotype correlation such that the timing of 
MTC is predicted (at least in part) by the specific RET mutation, 
and the current ATA risk categorizations of germline RET muta­
tions are based on the potential “aggressiveness” of MTC, whi

ased on the potential “aggressiveness” of MTC, which 
in turn is based on the age of onset rather than tumor behav­
ior3,263 (see Table 42.4). Thus, the ATA highest risk category, rep­
resented by the Met918Thr mutation in association with MEN3, 
is invariably associated with MTC initiation during the first few 
years of life, and macroscopic disease may occur before the age of 
1 year. Indeed, the earliest reported case of MTC in MEN3 is 9 
weeks of age, and lymph node metastases have been iden

s of age, and lymph node metastases have been identified in 
the first year of life.264,265 However, the high frequency of de novo 
mutations in MEN3 patients frequently results in a delayed diag­
nosis of the associated MTC until the second decade of life (i.e., 
mean age of diagnosis ∼14 years), by which time spread beyond 
the thyroid is invariably present and the opportunity for cura­
tive treatment missed.264–266 Indeed, MTC is the leading cause 
of death in MEN3 and is associated with an a

ause 
of death in MEN3 and is associated with an apparent aggressive 
disease course such that it has a worse 10-year survival than the 
MTC occurring in MEN2, although this may, in part, reflect the 
earlier age of onset and typical later stage of diagnosis.265
MTC occurs in MEN2 and MEN3 patients with ATA high-
risk RET mutations (i.e., codon Cys634 mutations and the Ala­
883Phe mutation, respectively), with a median age of diagnosis 
of 20 to 25 years, although it has been reported in those y

25 years, although it has been reported in those younger 
than 5 years of age.263,267 However, in these high-risk RET muta­
tion carriers, lymph node metastases are unusual in those younger 
than 10 years of age,3,268 and different codon 634 mutations 
may be associated with subtle differences in MTC expression. 
For example, the penetrance of MTC is reported to be higher in 
carriers of the Cys634Arg mutation when compared with other 
codon 634 substitutions.269,270 MEN2 patients with ATA mod­

substitutions.269,270 MEN2 patients with ATA mod­
erate-risk RET mutations (i.e., all other RET mutations exclud­
ing Met918Thr, those of Cys634 and Ala883Phe) have a median 
age of MTC onset that is typically later than those with higher-
risk mutations, but this may be highly variable. For example, in 
a series of 127 patients with moderate-risk RET mutations, the 
median age of MTC diagnosis was 42 years, with a range of 6 to 
86 years.263 Furthermore, despite the later age of MTC diagnosis 

thermore, despite the later age of MTC diagnosis 
in moderate-risk compared with high-risk mutation carriers, no 
difference in clinical course was observed, suggesting that the RET 
mutations predominantly influence age of onset rather disease 
aggressiveness per se.263 In addition, recent studies indicate that 
it may be possible to further subdivide the ATA moderate-risk 
category to include moderate-high risk groups, such as mutations 
affecting codons Cys611, Cys618, and Cys620, and moderat

ing codons Cys611, Cys618, and Cys620, and moderate-low 
risk groups, such as mutations affecting codons Leu790, Val804, 
1647
and Ser891, according to the age-related progression of MTC.267 
The penetrance of MTC associated with certain moderate-risk 
RET mutations (e.g., Val804Met) is also likely to be significantly 
reduced, as indicated by a higher than anticipated frequency of 
individuals harboring the variant in the background popula­
tion.145,271 

he variant in the background popula­
tion.145,271 Thus, the risk and age-related penetrance of MTC is 
strongly determined by codon-specific RET mutations, although 
additional genetic and/or environmental modifying factors are 
likely to influence disease expression. For example, the risk of 
MTC (and other MEN2 clinical features) in individuals with a 
specific RET mutation may also be influenced by additional RET 
coding variants and/or the background RET haplotype.272,273 
Treatment
Surgery,

ckground RET haplotype.272,273 
Treatment
Surgery, comprising total thyroidectomy with dissection of cer­
vical lymph node compartments, is the recommended treatment 
for sporadic and hereditary MTC, as it offers the best chance of 
achieving cure. For recurrent or metastatic MTC, several differ­
ent treatment strategies (e.g., locoregional cytoreductive surgery, 
external beam radiotherapy, radio-frequency ablation, chemoem­
bolization, and systemic targeted therapies such as RTK inhibi­
tors) 

emic targeted therapies such as RTK inhibi­
tors) are available.
The majority of patients presenting with MTC in the context 
of a thyroid nodule will have evidence of lymph node metastases at 
diagnosis, and the extent of surgery remains controversial. In addi­
tion, index cases with MEN2 may have multifocal and/or bilateral 
disease, and the degree of cervical lymph node involvement and/
or presence of distant metastases will influence the extent of sur­
gery. Preoperative basal serum calciton

nt of sur­
gery. Preoperative basal serum calcitonin concentrations have been 
reported to be associated with the degree of cervical lymph node 
involvement and are used in some centers to make decisions on 
the extent of lymph node resection. For example, a study of 300 
treatment-naive MTC patients reported that the presence of lymph 
node metastases in the ipsilateral central and lateral neck, contra­
lateral central neck, contralateral lateral neck, and upper medias­
tinum were associated wi

l neck, and upper medias­
tinum were associated with basal serum calcitonin concentrations 
above 20, 50, 200, and 500 pg/mL, respectively,274 and that bilat­
eral compartment-oriented neck surgery could achieve biochemical 
cure in more than 50% of patients with pretreatment calcitonin 
levels of 1000 pg/mL or higher, whereas such outcomes were not 
achieved in patients with preoperative calcitonin levels of 10,000 
pg/mL or higher.274 Postoperative biochemical remission or cure is 
important, 

tive biochemical remission or cure is 
important, as it is associated with low recurrence rates and excellent 
long-term survival rates of 98% at 10 years.275,276 Prognostic infor­
mation on lymph node involvement and surgical cure rates has also 
been reported for CEA.277 For example, preoperative serum CEA 
concentrations greater than 30 ng/mL are reported to indicate cen­
tral and ipsilateral lateral lymph node metastases, whereas serum 
CEA concentrations greater than 100 ng/mL are reported 

oncentrations greater than 100 ng/mL are reported to sig­
nify the presence of contralateral lateral lymph node involvement 
and/or distant metastases.277 However, consensus has not been 
reached on the optimal surgical approach for MTC with regard 
to the extent of lymph node resection, and the current guidelines 
suggest total thyroidectomy and central compartment lymph node 
clearance in patients with MTC and no evidence of cervical lymph 
node metastases on ultrasound, whereas removal of lym

e metastases on ultrasound, whereas removal of lymph nodes 
in the lateral compartments may be considered based on calcitonin 
levels.3 If there is cervical lymph node involvement, then dissection 
of the central and involved lateral compartment is recommended, 
although if the lateral compartment on the opposite side to that of 
the MTC is observed to be free of disease, then decisions on lymph 
node removal in this area may be based on calcitonin levels.3
In patients with regional lymph node i

in levels.3
In patients with regional lymph node involvement, total thy­
roidectomy and bilateral lymph node dissection frequently fails 
to achieve biochemical cure. For example, in those with involve­
ment of 1 to 10 cervical lymph nodes involved, immediate post­
operative normalization of serum calcitonin is achieved in only 
31% to 57% of patients, whereas in those with involvement of 
more than 10 cervical nodes, biochemical remission is observed 
in 0% to 4%.278,279 Thus, the presence of l

erved 
in 0% to 4%.278,279 Thus, the presence of lymph node metastases 
is a poor prognostic factor, and the number of involved lymph 
nodes has also been reported to provide prognostic information 
on the likelihood of developing distant metastases.254,280 In addi­
tion, it has been reported that in patients undergoing thyroid­
ectomy and lymphadenectomy for MTC that a higher number 
of lymph nodes harvested at surgery (i.e., lymph node yield)—a 
surrogate for adequate staging—and the metastati

surrogate for adequate staging—and the metastatic lymph node 
ratio (i.e., number of metastatic lymph nodes/lymph node yield) 
correlated with a poorer survival.280 The American Joint Commit­
tee for Cancer tumor, node, metastasis staging system also pro­
vides prognostic utility, and increased recurrence rates are reported 
for a primary tumor that extends beyond the thyroid capsule to 
invade local structures (i.e., pT4 tumors).3 However, the Ameri­
can Joint Committee for Cancer system does

 Ameri­
can Joint Committee for Cancer system does not incorporate age, 
pre- and postoperative serum calcitonin levels, or quantification 
of the number of affected lymph nodes, which each also provide 
additional prognostic information.
Thyroidectomy together with lymph node clearance may be 
associated with several postoperative complications, includ­
ing lymphatic leakage, transient (and occasionally permanent) 
hypoparathyroidism, and transient or longer-term damage to the 
recurrent laryng

ent or longer-term damage to the 
recurrent laryngeal nerves and/or spinal accessory nerve. Follow­
ing surgery, basal or stimulated calcitonin levels should be ascer­
tained to assess disease status, although typically this is delayed by 
approximately 3 months to allow sufficient time for levels to reach 
a nadir.3,281 Following total thyroidectomy, all patents require life­
long thyroid hormone replacement and will also require monitor­
ing of plasma calcium concentrations with appropriate tr

 plasma calcium concentrations with appropriate treatment, 
where necessary. In patients with known advanced disease (i.e., 
extensive regional or metastatic disease), less aggressive surgery 
may be appropriate because cure will not be achieved and the aims 
in these patients will be to maintain the voice and swallowing, 
preserve the quality of life, and avoid local complications.3 
Prophylactic Thyroidectomy in MEN2
The ability to identify individuals at high-risk of developing 
hereditary MT

ividuals at high-risk of developing 
hereditary MTC through germline RET mutation testing pro­
vides a window of opportunity to undertake preventative or 
curative surgery in otherwise asymptomatic individuals.3,239,268 
Indeed, “prophylactic” thyroidectomy has become the mainstay 
of treatment for children at risk of hereditary MTC, and when 
undertaken at a sufficiently early stage, it has the potential to 
avoid the morbidity and mortality associated with MTC develop­
ment.3,239,250–252,268,2

ociated with MTC develop­
ment.3,239,250–252,268,282,283 For example, in 2005, a study reported 
that prophylactic thyroidectomy in 50 patients who were younger 
than 19 years of age with MEN2-associated RET mutations 
resulted in no evidence of residual or recurrent disease in approxi­
mately 90% of cases at a mean follow-up period of 7 years. More 
recently, a study reporting the outcomes of prophylactic thyroid­
ectomy in 167 children with MEN2- and MEN3-associated RET 
mutations reported no 

2- and MEN3-associated RET 
mutations reported no instances of recurrent or residual disease 
after a mean of 7 years in any of the 149 patients for whom follow-
up data were available. In addition, normalization of postopera­
tive serum calcitonin was observed in 99% of patients who had 
1648
raised preoperative calcitonin levels.251 However, it is important 
to note that the aim of such preventative surgery is not necessarily 
to remove the 

ntative surgery is not necessarily 
to remove the thyroid before any abnormality develops but rather 
to do so before there is a significant risk of metastatic disease. In 
this regard, the advent of highly sensitive serum calcitonin assays 
now provides additional information that may help inform surgi­
cal decisions, although it should be noted that serum calcitonin 
concentrations in young children, and especially in the first few 
months of life, are frequently high and do not accurately ref

ife, are frequently high and do not accurately reflect 
disease status. Thus, although prior recommendations on the tim­
ing of prophylactic thyroidectomy were based on the risk category 
of the RET mutation alone,2 more recent guidelines suggest that 
in some instances it is reasonable to take into account basal or 
stimulated serum calcitonin levels, which provide a reliable indica­
tor of MTC status and disease risk.3 This represents an important 
change and recognizes that there remains cons

ant 
change and recognizes that there remains considerable heteroge­
neity in the age of MTC onset in those carrying the same RET 
mutation who may be even from the same kindred. Furthermore, 
it suggests that although the risk category of RET mutation is the 
major determinant of the age of onset of CCH and subsequent 
transformation to MTC, additional genetic and/or environmental 
factors may influence disease expression.
Thyroidectomy in early childhood is associated with a higher 
complicati

 childhood is associated with a higher 
complication rate than that observed in older children or adults. 
Such surgery should only be undertaken at a center with appro­
priate experience. Complications associated with surgery include 
a higher likelihood of developing hypoparathyroidism, as fre­
quently the parathyroid glands are hard to identify in very young 
children. Complications in early childhood are also increased if 
central lymph node dissection is performed, and this includes 
the hi

dissection is performed, and this includes 
the higher risks of transient and permanent hypoparathyroidism, 
as well as the likelihood of transient recurrent laryngeal nerve 
palsy.268,284 As a consequence, the timing of prophylactic surgery 
aims to strike an important balance between the risks associated 
with early surgery and those associated with more extensive sur­
gical procedures, which may be necessary when intervention is 
delayed. Current guidelines attempt to address this balance, of

ent guidelines attempt to address this balance, offer­
ing some flexibility in the timing of prophylactic thyroidectomy 
in children with germline RET mutations associated with a later 
age of MTC onset while recommending early surgery in those 
deemed to be at highest risk (see Table 42.4). Thus, children 
identified to harbor the ATA highest-risk RET mutations (i.e., 
Met918Thr) are recommended to undergo total thyroidectomy 
in the first year of life, and the higher potential complication rat

of life, and the higher potential complication rate 
of hypoparathyroidism in this age group is considered acceptable 
given the potential for metastatic disease by delaying treatment. In 
children with ATA high-risk mutations (i.e., codon Cys634 and 
Ala883Phe mutations), prophylactic thyroidectomy is typically 
recommended before 5 years of age, with the exact timing based 
on annual clinical examination, neck ultrasound, and serum cal­
citonin levels starting from the age of 3 years. For thos

 levels starting from the age of 3 years. For those children 
with ATA moderate-risk RET mutations, the timing of prophy­
lactic thyroidectomy should be based on the findings of clinical 
examination, neck ultrasound, and serum calcitonin concentra­
tions commencing at age 5 years.3 Thyroidectomy is indicated 
once serum calcitonin concentrations are elevated, although it 
may also be appropriate in children with normal calcitonin levels 
in whom such long-term monitoring is not feasible or desi

 such long-term monitoring is not feasible or desired. The 
window of “safe” serum calcitonin concentrations in which cura­
tive surgery is feasible has not been precisely defined, although 
once basal serum calcitonin levels exceed the upper end of the 
normal range (e.g., ∼10 pg/mL), this may herald the early stages of 
MTC development and a suitable time for surgical intervention. 
Serum calcitonin concentrations between 10 and 30 pg/mL may 
represent an optimal window for intervention, as no

epresent an optimal window for intervention, as nodal metastases 
were not observed to occur in children with RET mutations who 
had serum calcitonin concentrations of 30 pg/mL or less.268,283 
Once calcitonin levels are greater than 30 pg/mL, the likelihood of 
nodal metastases increases, and this will often necessitate central 
node dissection, which is associated with increased operative mor­
bidity and a reduction in duration of long-term remission.268,285 
Postoperative Evaluation and Manag

ission.268,285 
Postoperative Evaluation and Management of Patients 
With MTC
Following surgery, the normalization of serum calcitonin levels 
is associated with very favorable long-term outcomes, although a 
small proportion of these patients may develop recurrent disease. 
For example, patients with postoperative basal serum calcitonin 
concentrations less than 10 pg/mL are reported to have 3- and 
5-year relapse-free survival of 95% and 90%, respectively,276 with 
only approximately 4% of pat

espectively,276 with 
only approximately 4% of patients with recurrent disease found to 
have had normal postoperative serum calcitonin levels.286 Thus, 
postoperative normalization of serum calcitonin levels cannot be 
considered curative in all patients and highlights the need for long-
term clinical and biochemical follow-up. Patients who have postop­
erative basal serum calcitonin levels above the reference range will 
require further investigation, and this will depend on the severity 
the 

gation, and this will depend on the severity 
the hypercalcitonemia. Thus, serum calcitonin levels that are greater 
than 10 pg/mL but less than 150 pg/mL indicate localized residual 
disease within the neck that should be evaluated by ultrasound, 
whereas serum calcitonin concentrations greater than 150 pg/mL 
indicate the possibility of more extensive disease that will require 
additional imaging (e.g., CT, MRI, bone scintigraphy). The extent 
and location of the residual/recurrent disease wit

and location of the residual/recurrent disease with the rate of tumor 
progression determined both by imaging and calcitonin and/or 
CEA doubling time will determine which of the different local or 
systemic treatment options (see the following discussion) should be 
pursued. Calcitonin doubling time is an important guide of prog­
nosis,281 and calcitonin doubling times of less than 6 months and 6 
to 24 months have been reported by one study to be associated with 
10-year survivals of approxima

be associated with 
10-year survivals of approximately 10% and approximately 40%, 
respectively, whereas all patients with a doubling time of greater 
than 24 months were alive at the end of the study.287 In another 
study, both calcitonin and CEA doubling times were also strongly 
related to disease progression, with approximately 95% of those 
with doubling times of 2 years or less observed to have progressive 
disease, whereas approximately 85% of those with doubling times 
greater than 2 yea

% of those with doubling times 
greater than 2 years had stable disease.288 
Management of Advanced Disease
The management of advanced MTC in patients with MEN2 
or MEN3 does not differ from that in patients with sporadic 
MTC.3 In MEN2 and MEN3 patients with significant elevations 
of serum calcitonin preoperatively (e.g., >500 pg/mL), postop­
eratively (e.g., >150 pg/mL), or during the subsequent follow-up 
periods, the possibility of locally advanced and/or metastatic dis­
ease should be inve

dvanced and/or metastatic dis­
ease should be investigated, as this will influence the choice of 
downstream therapeutic approach.3
In patients with residual or recurrent disease that is limited 
to the neck, reoperation with compartmental dissection of cen­
tral and/or lateral regions based on preoperative investigation 
(i.e., imaging and/or biopsy) may be appropriate, with the aim 
of normalizing or reducing serum calcitonin levels, although 
these potential benefits must be balanced against 

these potential benefits must be balanced against the increased 
1649
morbidity that may be associated with further neck surgery. Post­
operative radioiodine is not recommended for patients with MTC 
unless there is evidence that the recurrent MTC contains elements 
of papillary or follicular thyroid cancer. External beam radiother­
apy (EBRT) to the neck has been used widely for locoregional 
disease control, although there is little evidence of survival

rol, although there is little evidence of survival benefit 
when other prognostic factors are taken into account.289 Current 
guidelines suggest considering EBRT in those patients at high risk 
for local disease recurrence and those at risk of airway obstruction. 
However, the decision to use EBRT in these situations should take 
into account the potential acute and chronic toxicities associated 
with the treatment.3
Once distant metastases have developed, treatments are aimed 
at disease contro

developed, treatments are aimed 
at disease control and/or symptomatic benefit, as curative therapies 
are not available. Determining the optimal therapy will depend on 
several factors, including the site(s) and extent of metastases, the 
rate of tumor progression, the performance status of the patient, 
the treatment modalities available to the patient, and the wishes 
of the patient. MTC most commonly metastasizes to the liver and 
axial skeleton (see Fig. 42.8), and to a lesser extent the lu

ton (see Fig. 42.8), and to a lesser extent the lungs (see 
Fig. 42.6), brain, and skin, and in these settings, several treatment 
approaches may be employed to achieve symptom and/or disease 
control.3 Treatments for metastatic disease are broadly categorized 
into local and systemic therapies. Local treatments, which are typ­
ically used for disease or symptom control at specific sites related 
to tumor spread, include surgical resection/debulking, chemo­
embolization, radio-frequency ablation

ing, chemo­
embolization, radio-frequency ablation, and EBRT.3 Symptoms 
related to aberrant hormonal secretion may also require treatment. 
For example, antimotility agents (e.g., loperamide) and/or soma­
tostatin analogues may improve diarrhea in those with hormon­
ally active hepatic metastases and may be used prior to surgical 
debulking and/or selective arterial chemoembolization.3 Similarly, 
biochemical control of Cushing syndrome (i.e., due to the ectopic 
production of corticotropin-rel

ue to the ectopic 
production of corticotropin-releasing hormone or ACTH) may be 
required to minimize the associated morbidity (e.g., hyperglyce­
mia, osteoporosis, hypertension, hypokalemia, gastritis), and this 
may be achieved by several approaches, including medical thera­
pies (e.g., ketoconazole, metyrapone, mitotane), bilateral adrenal­
ectomy, or debulking of hepatic metastases. Remission of ectopic 
Cushing syndrome due to MTC has also been observed in case 
reports of patients receivi

been observed in case 
reports of patients receiving the RTK inhibitor vandetanib (see 
the following discussion).290
Systemic antitumor therapies are typically reserved for patients 
with metastatic MTC in whom there is evidence of significant 
disease burden and/or evidence of disease progression. The advent 
of targeted therapy with RTK inhibitors has provided a signifi­
cant advance in the field, and these drugs now represent first-line 
treatment for the majority of patients with advanced p

tment for the majority of patients with advanced progressive 
disease (see the following discussion). In contrast, conventional 
cytotoxic chemotherapeutic regimens, either as single agents or in 
combination, have been associated with low response rates and are 
not routinely recommended. Alternate systemic therapies, includ­
ing radionuclide therapies (e.g., [90Y-DOTA]-TOC), have also 
been employed in early-phase clinical trials, but these treatments 
require further evaluation to support the

atments 
require further evaluation to support their wider clinical use.3 
Targeted Therapies for MEN2-Associated MTC
Currently, two RTK inhibitors are approved for use in advanced 
progressive MTC: vandetanib, which targets RET, endothelial 
growth factor receptor (EGFR), and vascular EGFR (VEGFR) 
kinases, and cabozantinib, which targets RET, c-Met, and 
VEGFR kinases.3,266,291–293 The anti-RET activity of these drugs 
is considered to be the determinant of their clinical efficacy in 
MTC, but

eterminant of their clinical efficacy in 
MTC, but it is important to note that the targeting of alternate 
TKs, including VEGFR, by these drugs may also contribute sub­
stantially to their antitumor activity.
The safety and efficacy of vandetanib in advanced MTC was 
initially evaluated in clinical trials of MEN2 and MEN3 patients, 
which demonstrated partial response rates of 15% to 20% and 
stable disease persisting for 24 weeks or more in an additional 
approximately 50% of patients.294,295 

approximately 50% of patients.294,295 In a subsequent phase III 
trial of patients with advanced MTC due to sporadic of hereditary 
disease, vandetanib demonstrated increased PFS when compared 
with placebo (median PFS 30.5 months vs. 19.5 months, respec­
tively), whereas approximately 45% of patients in the treatment 
group demonstrated a partial tumor response.291 Vandetanib 
treatment in 28 patients with germline RET mutations revealed 
approximately 45% to have an objective tumor

aled 
approximately 45% to have an objective tumor response,291 and 
tumors harboring a somatic Met918Thr RET mutation also had an 
enhanced response.291 A subsequent study of vandetanib in chil­
dren and adolescents with MEN3 and locally advanced or meta­
static MTC reported an objective partial response rate of 47%, 
whereas all 15 MEN3 patients harboring the germline Met918Thr 
mutation experienced some reduction in tumor size.296
Cabozantinib, in a phase III clinical trial, resulted in a par

, in a phase III clinical trial, resulted in a partial 
tumor response in 28% of patients with locally advanced or meta­
static MTC compared with 0% in the placebo group, whereas 
PFS was increased in those receiving treatment compared with 
placebo (11.2 vs. 4.0 months, respectively).297 However, no over­
all survival benefit was demonstrated between treatment and 
placebo arms.297 Subsequent analysis of this trial reported an 
increased median PFS in patients with a RET or RAS mutation 
receiv

PFS in patients with a RET or RAS mutation 
receiving cabozantinib compared with placebo (13.8 months vs. 
4.6 months, and 10.8 months vs. 1.8 months, respectively). In 
contrast, no such benefit was observed in patients without RET or 
RAS mutations (median PFS in treatment and placebo groups of 
5.6 months vs. 5.3 months, respectively).298 In addition, patients 
with tumors harboring the Met918Thr RET mutation were also 
found to have the greatest median PFS benefit with cabozantinib 
treatmen

est median PFS benefit with cabozantinib 
treatment (14.1 months), and, importantly, these patients were 
the only group to have increased overall survival.298–300 More­
over, objective response rates of 32% and 22% were observed in 
patients with and without RET mutations, respectively, indicating 
some antitumor activity in patients without RET mutations.298,299
These studies reveal that treatment with RTK inhibitors 
of patients with advanced MTC resulted in objective tumor 
responses in more

MTC resulted in objective tumor 
responses in more than 50% of patients. In addition, the majority 
of patients developed drug resistance, thereby resulting in only 
short- to medium-term improvements in disease control, and 
long-term disease remission and major survival benefits were not 
demonstrated.266,299,300 Furthermore, the adverse effects asso­
ciated with these drugs, which included diarrhea, rash, fatigue, 
hypertension, abdominal pain, photosensitivity, prolonged QT 
interval, and ga

, photosensitivity, prolonged QT 
interval, and gastrointestinal fistulas, resulted in either drug dis­
continuation or the requirement for a dose reduction in 12% to 
16% and 35% to 79%, respectively.3 There was also marked het­
erogeneity in drug response even between individuals harboring 
similar RET mutations, and improved molecular understanding 
of these agents is required to establish their optimal use.266,296 For 
example, determining the clinical activities in the presence of dif­
fere

e clinical activities in the presence of dif­
ferent RET mutations and establishing the relative contribution of 
their different anti-TK activities, such as anti-VEGFR, would be 
important, especially because cabozantinib was demonstrated by 
using in vitro and in vivo tumor models to be active against several 
1650
mutated forms of RET, such as Met918Thr and Tyr791Phe, 
whereas other mutated forms of RET, including those harboring 
substitut

forms of RET, including those harboring 
substitutions at Val804, appeared to be resistant to certain RTK 
inhibitors, including vandetanib.301,302
Other multi-TK and RET inhibitors have also been evalu­
ated in early-phase clinical trials for advanced MTC, including 
sorafenib, which demonstrated partial responses in 6% to 25% 
of patients303,304; lenvatinib, which demonstrated objective tumor 
responses in approximately 35% of MTC patients305; and suni­
tinib, which demonstrated an objective r

and suni­
tinib, which demonstrated an objective response rate of 38%.306 
However, these and several additional TK inhibitors are not cur­
rently approved for use, and larger studies are required to deter­
mine the effectiveness of these treatments in MEN2 and MEN3 
patients with MTC. Preclinical studies have evaluated other RTK 
and RET inhibitors with activity against specific TK receptor muta­
tions. For example, alectinib and ponatinib demonstrated signifi­
cant anti-RET activity, including

strated signifi­
cant anti-RET activity, including effective targeting of the Val804 
“gatekeeper” mutants (e.g., Val804Met and Val804Leu),292,299,307 
and several RET-specific inhibitors are in development.292
Finally, it is likely that either sequential or combination sys­
temic therapy will be required to achieve longer-term treatment 
responses, and early phase studies are in progress.292 Furthermore, 
an improved understanding of the mechanisms of acquired resis­
tance is required to determ

sms of acquired resis­
tance is required to determine the optimal use of emerging agents. 
Pheochromocytomas are the second most frequent neoplasms in 
MEN2 and MEN3, with an overall penetrance of 40% to 50%. 
Pheochromocytoma may be the first manifestation in a minority 
(<10%) of cases and may also occur synchronously with MTC 
in approximately 35% of cases308–310 (see Fig. 42.8). Child­
hood presentations of pheochromocytoma are rare in MEN2, 
although the e

pheochromocytoma are rare in MEN2, 
although the earliest ages of onset related to codon Cys634 and 
Met918Thr mutations are 8 and 12 years, respectively.3,311 A key 
characteristic of MEN2-associated pheochromocytoma is that in 
50% of patients, it occurs as a bilateral disease, which can occur 
synchronously or metachronously.308,312–315 Such bilateral dis­
ease has a higher frequency in individuals with specific genotypes 
including the Cys634Arg RET mutation.269,270,315 The majority 
(>95%) 

Arg RET mutation.269,270,315 The majority 
(>95%) of MEN2- and MEN3-associated pheochromocytomas 
arise within the adrenal gland and occur on the background of 
adrenal medullary hyperplasia.239,316 However, more recent stud­
ies indicate that regions of adrenal medullary hyperplasia share 
molecular characteristics with pheochromocytoma and should be 
more appropriately considered as micropheochromocytomas.317 
The overwhelming majority (>95%) of MEN2- and MEN3-asso­
ciated pheochromocytomas ar

f MEN2- and MEN3-asso­
ciated pheochromocytomas are benign, and only 0% to 4% are 
reported to progress to malignancy in large series, although indi­
vidual cases reports highlight occasional instances of metastatic 
spread.308,315,318–320 Extra-adrenal pheochromocytoma/paragan­
glioma is extremely rare in MEN2 and MEN3.321
The clinical signs and symptoms of pheochromocytoma in 
MEN2 and MEN3 do not differ from those occurring in patients 
with nonsyndromic and nonfamilial pheochromocytomas. Thu

onsyndromic and nonfamilial pheochromocytomas. Thus, 
MEN2 and MEN3 patients with pheochromocytoma may pres­
ent with features associated with excess catecholamine secretion, 
and these include episodic headaches, sweating, palpitations, 
anxiety, and hypertension. However, 30% to 50% of MEN2 and 
MEN3 patients are asymptomatic, and the pheochromocytoma is 
diagnosed during a screening program.315,322 Thus, the possibil­
ity of pheochromocytoma due to MEN2 should be considered in 
all patients d

ue to MEN2 should be considered in 
all patients diagnosed with MTC prior to surgery (irrespective of 
signs or symptoms), as failure to diagnose and treat a concurrent 
pheochromocytoma may result in catastrophic outcomes due to 
an intraoperative adrenergic crisis.3 Furthermore, it is important 
to exclude pheochromocytoma in all “at-risk” MEN2 and MEN3 
patients prior to pregnancy.3
The diagnosis of pheochromocytoma is confirmed by dem­
onstrating elevated concentrations of plasma and/or urin

ting elevated concentrations of plasma and/or urinary free 
fractionated metanephrines. MEN2-associated pheochromocyto­
mas are typically adrenergic and are reported to secrete dispropor­
tionate amounts of epinephrine,239,315 and elevated epinephrine 
(and associated metanephrine) concentrations may help distin­
guish MEN2 from other hereditary pheochromocytoma/para­
ganglioma syndromes, such as von Hippel–Lindau disease and 
familial paraganglioma syndromes, in which norepinephrine or 
other m

oma syndromes, in which norepinephrine or 
other metabolites predominate. Preoperative tumor localization 
studies are essential because there is a high probability of bilat­
eral disease, and the imaging should evaluate both adrenal glands. 
Cross-sectional imaging with CT or MRI is recommended, and 
some centers will prefer CT because it provides a higher resolu­
tion with the pheochromocytoma appearing as a dense, hyper­
vascular mass.312,323 Several functional imaging modalities (e.g., 
123I

Several functional imaging modalities (e.g., 
123I-metaiodobenzylguanidine [123I-MIBG], 18F-FDOPA PET/
CT, 18F-FDG PET, 68Ga-DOTA) are also associated with high 
sensitivity and specificity for disease localization and detection 
of metastatic and extra-adrenal disease, although their useful­
ness in MEN2 and MEN3 may not be great considering that the 
majority of patients have benign disease limited to the adrenal 
medulla.324,325
The occurrence of pheochromocytomas in patients with 
MEN2 and M

 of pheochromocytomas in patients with 
MEN2 and MEN3 is dependent on genotype.239,268,270,312,314,322 
Thus, the highest prevalence of pheochromocytoma is observed in 
those with ATA high (codon Cys634 and Ala883Phe)- and high­
est (Met918Thr)-risk RET mutations, whereas a lower disease fre­
quency is observed in those with moderate-risk RET mutations (see 
Table 42.3 and Fig. 42.7). To date, most estimates of pheochro­
mocytoma penetrance are age related, and the absolute lifetime 
risk for se

ge related, and the absolute lifetime 
risk for several RET mutations is unknown. For example, 30% 
and 100% of patients with the Met918Thr mutation developed 
pheochromocytoma by the age of 27 and 56 years, respectively; 
25%, 30% to 60%, 52%, and 88% of codon Cys634 RET muta­
tion carriers developed disease by age 30, 35, 50, and 77 years, 
respectively; and fewer than 20% of patients carrying moderate-
risk exon 10 mutations (e.g., those affecting codons 609–620) 
had pheochromocytomas by 35 

ting codons 609–620) 
had pheochromocytomas by 35 years of age.268,270,309,312,314,322 
Different mutations affecting the same Cys634 residue have been 
reported to give rise to different tumor penetrances, with the 
Cys634Arg mutation being associated with the highest incidence 
of pheochromocytoma in MEN2 patients.269,270,310 In addition, 
recent studies also indicate the presence of potential genetic or 
environmental disease modifiers, and a different natural history 
of pheochromocytoma in 

different natural history 
of pheochromocytoma in MEN2 is reported in individuals with 
the same RET mutations from different geographic regions. For 
example, MEN2 patients from South America with exon 11 
mutations (e.g., affecting codon Cys634) appear to have a lower 
disease penetrance and/or later age of onset than those from Euro­
pean locations (i.e., Southern, Central, Western Europe).326 Fur­
ther support for the influence of genetic modifiers is provided by 
the observation that even w

ifiers is provided by 
the observation that even within the same kindred, the penetrance 
and expression of pheochromocytoma varies. Potential insights 
into such modifying influences include the observation that cer­
tain haplotypes containing RET mutations alongside additional 
1651
RET rare variants (e.g., Tyr791Phe) or polymorphisms (e.g., 
Leu769Leu, Ser836Ser, and Gly691Ser/Ser904Ser) result in an 
increased age-related risk of pheochromocytoma.273,

ncreased age-related risk of pheochromocytoma.273,327 
Treatment
Surgical removal of the pheochromocytoma(s) is the treatment of 
choice. However, pre- and perioperative α- and β-blockade must be 
instituted, and in patients having bilateral adrenalectomy, who are 
at risk of postoperative steroid and mineralocorticoid deficiency, 
perioperative steroids should be administered. For unilateral or 
bilateral adrenalectomy, open (i.e., retroperitoneoscopic) and 
laparoscopic approaches are appropri

oscopic) and 
laparoscopic approaches are appropriate and reported to be associ­
ated with similar outcomes.3 However, several additional factors 
should be considered specifically in MEN2 and MEN3 patients 
when compared with patients with nonsyndromic and nonfamilial 
pheochromocytomas in planning the surgical strategy. For exam­
ple, in patients with synchronous bilateral pheochromocytomas, 
the simultaneous removal of both adrenal glands results in post­
operative adrenal insufficiency, with

lts in post­
operative adrenal insufficiency, with all patients requiring lifelong 
glucocorticoid and mineralocorticoid replacement.268,322 For this 
reason, bilateral adrenalectomy is not recommended in those with 
unilateral disease despite the potential for tumor development in 
the contralateral gland.3 To reduce the risk of adrenal insufficiency 
in patients requiring bilateral adrenalectomy, some centers have 
advocated adrenal-sparing surgery (i.e., subtotal adrenalectomy) 
despite the p

gery (i.e., subtotal adrenalectomy) 
despite the potential risk for tumor recurrence in the remnant tis­
sue.268,312,322,328 Adrenal-sparing surgery involves removal of the 
pheochromocytoma while aiming to leave 10% to 30% residual 
adrenal cortical tissue to provide sufficient adrenal reserve for glu­
cocorticoid and mineralocorticoid function. Supporters of this 
approach cite the low risk of malignancy, a relatively low tumor 
recurrence rate with substantial disease-free intervals, and a lo

 with substantial disease-free intervals, and a low 
frequency of adrenal insufficiency associated with adrenal-sparing 
surgery. Several retrospective series have compared clinical out­
comes between conventional and adrenal-sparing adrenalectomy. 
For example, in one series of 552 MEN2 patients undergoing sur­
gery for pheochromocytoma, approximately 20% were treated by 
adrenal-sparing surgery and approximately 60% of those under­
going bilateral tumor resections were demonstrated to have pre

ral tumor resections were demonstrated to have pre­
served glucocorticoid production.322,328 Furthermore, following 
either unilateral or bilateral adrenal-sparing surgery, a low tumor 
recurrence rate of approximately 3% was observed in the rem­
nant operated gland after 10 years of follow-up.312,322 Although 
higher recurrence rates are reported in other series, and are likely 
to rise with increasingly lengths of follow-up, current guidelines 
suggest adrenal-sparing surgery as an alternative

suggest adrenal-sparing surgery as an alternative to adrenalectomy, 
although this approach may not be technically feasible in MEN2 
and MEN3 patients with multifocal disease.3,268,312,314,328–330 
Finally, in patients who have pheochromocytoma concurrently 
with MTC, the usual practice is to remove the adrenal tumor(s) 
prior to thyroidectomy.3 The decisions regarding surgical strat­
egy should balance the relative risks and benefits of the planned 
approach, as well as take into account the w

nned 
approach, as well as take into account the wishes of the patient. 
In addition, it is important to note that the improvements in the 
screening programs and treatment approaches have reduced the 
morbidity and mortality associated with MEN2- and MEN3-
associated pheochromocytoma, which have a very low frequency 
of malignant disease; thus, current outcomes generally are very 
favorable.312,322 Following surgery, lifelong follow-up is required 
for all patients to ensure appropriate complia

ed 
for all patients to ensure appropriate compliance and monitoring 
of glucocorticoid and mineralocorticoid replacement therapy in 
those with adrenal insufficiency, and to monitor for development 
of additional pheochromocytoma in the contralateral adrenal 
gland following unilateral resections, or in remnant adrenal tis­
sue following adrenal-sparing surgery, so that appropriate surgery 
(i.e., adrenalectomy) can be planned and provided. 
PHPT, w

mary Hyperparathyroidism
PHPT, which occurs in approximately 30% of patients with 
MEN2, typically presents in the third to fourth decade of 
life.3,268,270,312,314,331 The risk of PHPT is related to RET genotype, 
and patients with the Met918Thr RET mutation do not develop 
PHPT, whereas approximately 10% to 20% of patients with 
codon Cys634 mutations will develop PHPT by 35 to 40 years of 
age268,269,314; among these, carriers of the Cys634Arg RET muta­
tion are reported to 

s of the Cys634Arg RET muta­
tion are reported to be at the highest risk of having PHPT.269,270 
Patients with other RET mutations (e.g., of exon 10 involving 
codons Cys609, Cys611, Cys618, and Cys620) have less than 5% 
risk of having PHPT.3,332 However, in some MEN2 patients with 
codon Cys634 mutations, PHPT has been reported to occur as 
early as 2 years of age.3,333,334 In MEN2 patients with PHPT, mul­
tiple parathyroid glands may be affected with hyperplasia and/or 
adenoma formation, and

ed with hyperplasia and/or 
adenoma formation, and frequently this is asynchronous such that 
a dominant single-gland parathyroid tumor is often observed.312
The diagnosis of PHPT in MEN2 does not differ from patients 
with nonsyndromic and nonfamilial presentations and is reliant 
on demonstrating hypercalcemia with elevated or inappropriately 
normal plasma PTH concentrations. However, MEN2 patients 
with PHPT are frequently asymptomatic and only have mild 
hypercalcemia3,268 such that the dia

ly have mild 
hypercalcemia3,268 such that the diagnosis is often made following 
routine biochemical testing or during screening for other MEN2 
manifestations. Preoperative imaging using sestamibi, ultrasound, 
and four-dimensional CT scans to localize the parathyroids is use­
ful only in patients who have had prior neck surgery (i.e., thy­
roidectomy and/or central node neck dissection) for MTC, as this 
may help to guide the surgical approach. However, in patients in 
whom the diagnosis of P

. However, in patients in 
whom the diagnosis of PHPT is made concurrently with that of 
MTC, neck ultrasound or MRI for MTC staging may help iden­
tify enlarged parathyroid glands, but there is little added value 
of further parathyroid imaging because surgical exploration will 
allow evaluation of all four parathyroid glands. 
Treatment
The current recommendations for treatment of PHPT in MEN2 
favor the removal of only enlarged/diseased parathyroid glands 
rather than the more substantial ope

yroid glands 
rather than the more substantial operations recommended for 
MEN1.3,268 However, the surgical approach will depend on the 
timing of the diagnosis relative to MTC. Thus, in patients who 
have synchronous MTC and PHPT and are undergoing thyroid­
ectomy for the MTC, the recommendation is to remove only the 
enlarged parathyroid glands, with the success of parathyroidec­
tomy being monitored by intraoperative PTH assays.268 However, 
in patients with four-gland involvement, it is reco

n patients with four-gland involvement, it is recommended that 
subtotal parathyroidectomy that leaves a remnant in situ on a 
vascular pedicle or total parathyroidectomy with autotransplanta­
tion should be undertaken.3 In patients who have development 
of PHPT subsequent to thyroidectomy, the goals of parathyroid 
surgery should be tailored to the removal of only enlarged para­
thyroid glands, which may have been identified by preoperative 
imaging studies,3 and intra-operative PTH assays used

ging studies,3 and intra-operative PTH assays used to moni­
tor for successful removal of the enlarged glands, with the normal 
parathyroid glands being left in situ. Finally, in patients undergo­
ing thyroidectomy for MTC with normal calcium and PTH, it 
1652
recommended that prophylactic parathyroidectomy is not under­
taken, but that normal-appearing and viable parathyroid glands 
should be left in situ.331 
Additional Clinical Features Ass

left in situ.331 
Additional Clinical Features Associated With MEN2A 
­Variant Disorders
Familial MTC-Only
FMTC is characterized by MTC being the sole manifestation of 
MEN2. However, distinction between FMTC and MEN2A is 
difficult because of the low penetrance of pheochromocytoma in 
some patients, and FMTC should be considered only if there are 
at least four family members older than 50 years who are affected 
by MTC but not pheochromocytomas or PHPT.3,335,336 FMTC, 
when strict diagnostic c

 or PHPT.3,335,336 FMTC, 
when strict diagnostic criteria are applied, is found to be very rare. 
MEN2 With CLA
CLA has been reported to occur in up to 35% of MEN2 patients 
harboring codon Cys634 mutations337 (Fig. 42.9) but is very rarely 
observed in MEN2 patients with other RET mutations. CLA typi­
cally presents with intense pruritus and a rash in the interscapular 
area of the T2-6 dermatome region3 (see Fig. 42.9). The lesions 
improve with sunlight and may worsen at times of stress. CLA 

h sunlight and may worsen at times of stress. CLA 
may predate the other clinical manifestations of MEN2. Treat­
ment includes the use of topical creams including corticosteroids, 
systemic antihistamines, and/or phototherapy, which may provide 
partial symptom relief.3 
MEN2 With HSCR
Approximately 7% of patients with MEN2 manifest features of 
HSCR and typically present shortly after birth with the inability 
to pass stool and the development of megacolon, resulting from 
an absence of enteric

f megacolon, resulting from 
an absence of enteric ganglia along a variable length of the intes­
tine. The phenotype is only observed in MEN2 patients harboring 
activating RET mutations in exon 10 that affect cysteine residues at 
positions 609, 611, 618, and 620. Heterozygous loss-of-function 
RET mutations are observed in approximately 50% of patients 
with sporadic HSCR, and thus the co-occurrence of MEN2 with 
HSCR appears paradoxical (i.e., HSCR occurring in the context 
of both activating

 HSCR occurring in the context 
of both activating and loss-of-function RET mutations).338,339 
Knowledge of receptor structure/function (see the following dis­
cussion) has provided potential insights into the molecular basis 
of these paradoxical observations, with the hypothesis that these 
specific exon 10 mutations result in constitutive receptor activa­
tion sufficient for oncogenic signaling (i.e., resulting in the MEN2 
phenotype), but diminishing cell surface receptor expression 
(e.g.,

minishing cell surface receptor expression 
(e.g., through reduced stability), thereby resulting in inadequate 
ligand-dependent RET signaling during enteric nervous system 
development.340
Very occasionally, dermatologic features more typical of 
MEN3 have been reported in patients with MEN2 including der­
mal hyperneury (i.e., hypertrophic myelinated and unmyelinated 
nerve fibers in the skin). In addition, multiple sclerotic fibromas 
have also been reported, more reminiscent of changes obser

o been reported, more reminiscent of changes observed in 
the PTEN hamartoma tumor syndrome.341 
Additional Clinical Manifestations Associated With MEN3
Extraendocrine features occur in virtually all MEN3 patients, who 
typically have the endocrine manifestations of early-onset aggres­
sive MTC together with pheochromocytoma265. However, the 
penetrance and/or severity of these extraendocrine features may 
be variable, although some may be evident very early in life; when 
recognized, they may f

t very early in life; when 
recognized, they may facilitate early diagnosis. However, such 
features frequently are overlooked, resulting in delays in treat­
ment.342 These extra- (or non)endocrine features include a skel­
etal phenotype giving rise to a marfanoid habitus with tall stature, 
long limbs, narrow elongated face, high arched palate, chest wall 
abnormalities (e.g., pes excavatum, pes cavus), scoliosis, arach­
nodactyly, and increased risk of slipped capital femoral epiphy­
ses3,265;

 risk of slipped capital femoral epiphy­
ses3,265; multiple mucosal neuromas, typically presenting as soft 
papules in and around the oral cavity (e.g., affecting tongue, lips), 
as well as in the nasal and laryngeal mucosa (see Fig. 42.6); ocular 
manifestations such as alacrima, mild ptosis, conjunctival neuro­
mas, and prominent corneal nerve fibers265; upper gastrointesti­
nal symptoms such as swallowing difficulties and vomiting, likely 
due to esophageal abnormalities; lower gastrointestin

 to esophageal abnormalities; lower gastrointestinal symptoms 
such as altered bowel habit (most commonly constipation) and 
early feeding intolerance due to diffuse intestinal ganglioneuro­
matosis (see Fig. 42.6) and impaired colonic motility resulting in 
megacolon265,343; tooth malposition; and dermal hyperneury. 
The RET Proto-Oncogene
The human RET proto-oncogene, which encodes a single-pass 
transmembrane receptor of the TK family, is located in the pericen­
tromeric re

 TK family, is located in the pericen­
tromeric region of chromosome 10q11.2 and comprises 21 coding 
exons that span approximately 55 kb of genomic DNA.293,307,344–348 
The RET receptor, which is an RTK, consists of an ECD con­
taining four cadherin-like repeats and a cysteine-rich region, a 
transmembrane domain, and an intracellular domain (ICD) that 
comprises the TK domain and an isoform specific C-terminal (see 
Fig. 42.7). RET transcription is regulated by several DNA-binding 
factors tha

n is regulated by several DNA-binding 
factors that act on upstream promoter and enhancer elements, and 
the transcript levels are further controlled by regulatory elements 
within intronic and 3′ untranslated regions.307 Alternate splicing 
• Fig. 42.9  Cutaneous lichen amyloidosis in a patient with multiple endo­
crine neoplasia type 2 harboring a codon Cys634 mutation. (From Birla S, 
Singla R, Sharma A, Tandon N. Rare manifestation of multiple endocrine 
neoplasia type 2A & cutaneous lichen 

e endocrine 
neoplasia type 2A & cutaneous lichen amyloidosis in a family with RET 
gene mutation. Indian J Med Res. 2014;139[5];779–781.)
1653
of RET results in several highly conserved protein isoforms, and 
the two predominant isoforms are RET9 (“short”) and RET51 
(“long”), which differ only by 9 and 51 C-terminal amino acids, 
respectively, in their extreme C-terminal regions307 (see Fig. 42.7). 
Germline RET Mutations
More than 50 different germline

line RET Mutations
More than 50 different germline heterozygous RET mutations, 
which result in receptor activation, have been reported in asso­
ciation with MEN2 and MEN3.240,340 The majority of MEN2-
associated RET mutations involve heterozygous nonsynonymous 
amino acid substitutions of cysteine residues within the cyste­
ine-rich ECD of the receptor, or of noncysteine residues within 
the intracellular TK domain (see Fig. 42.7). However, several 
additional MEN2-associated RET mutations, the

ral 
additional MEN2-associated RET mutations, the majority of 
which affect a relatively small number of codons, are reported 
and include either nonsynonymous amino acid substitutions out­
side of these regions or small duplications, insertions, and dele­
tions (see Table 42.3). Thus, MEN2 is most frequently associated 
with amino acid substitutions of codon Cys634 in the cysteine-
rich ECD (see Fig. 42.7), with mutations of cysteine residues 
at codons 609, 611, 618, or 620 in this domain als

at codons 609, 611, 618, or 620 in this domain also account­
ing for a significant proportion of the remaining MEN2 muta­
tions.240,270,340 MEN2 mutations also occur in the intracellular 
TK domain, and these involve substitutions of Leucine 790 (i.e., 
Leu790Phe), Valine 804 (i.e., Val804Met, Val804Leu), and Ser­
ine 891 (i.e., Ser891Ala). Additional variants in the cysteine-rich 
and the TK domains have also been reported in some patients. 
However, some variants previously reported as pathoge

ever, some variants previously reported as pathogenic (i.e., 
mutations) may have been misclassified due to ascertainment 
and/or reporting biases, and these likely represent benign vari­
ants (e.g., Ser649Leu, Tyr791Phe, and Ile852Met).267,340,349 
Approximately 95% of MEN3 mutations involve a methionine 
to threonine substitution of codon 918 (Met918Thr) of the intra­
cellular TK domain, and fewer than 5% are represented by the 
Ala883Phe mutation, which is typically associated with a less 
ag

ion, which is typically associated with a less 
aggressive disease course.350
A paradigmatic feature of MEN2 and MEN3, which has 
transformed the clinical management of patients, is the strong 
genotype-phenotype correlation that predicts the potential disease 
spectrum and age of MTC onset associated with a specific RET 
mutation. However, the disease phenotype and its severity may 
vary between individuals harboring the same mutation, and even 
within the same kindred, thereby implicating a ro

within the same kindred, thereby implicating a role for genetic 
modifiers, which may include the co-occurrence of cis-acting RET 
coding region variants alongside the pathogenic mutations.307,340 
For example, several RET variants (e.g., Glu805Lys, Tyr806Cys, 
and Ser904Phe) have been reported in individuals harboring the 
Val804Met mutation, and these are associated with enhanced 
receptor signaling, which results in a more severe disease pheno­
type that resembles MEN3.351,352 In addition, s

o­
type that resembles MEN3.351,352 In addition, several noncoding 
and synonymous RET polymorphisms, identified by haplotype 
analysis, have been reported to modify disease expression and 
clinical course.272
The overall prevalence of MEN2-associated germline RET 
mutations in European populations has been estimated to be 
approximately 1 in 80 to 100,000,240 although recent evaluation 
of the Exome Aggregation Consortium cohort revealed a much 
higher frequency at approximately 1 in 2000.145 I

higher frequency at approximately 1 in 2000.145 In particular, a 
high frequency of the ATA moderate-risk Val804Met RET variant 
was observed, indicating that this mutation is likely associated with 
a lower disease penetrance than reported previously.145 This obser­
vation is supported by a further bioinformatic study in which the 
Val804Met was estimated to have a disease penetrance for MTC of 
less than 5%.271 Together, these studies highlight the need for fur­
ther unbiased population-based

t the need for fur­
ther unbiased population-based studies to refine current estimates 
of disease prevalence in RET variant carriers, as has been per­
formed for other hereditary endocrine tumor genes.353 However, it 
is also important to note that the incidence and spectrum of RET 
mutations may vary by geographic area, reflecting possible popu­
lation-specific factors and the presence of potential founder muta­
tions. For example, a study of approximately 500 MEN2 families 
from Germany, Ital

pproximately 500 MEN2 families 
from Germany, Italy, and France demonstrated that approximately 
34% of kindreds harbored codon Cys634 RET mutations, with 
approximately 17%, approximately 10%, and 7.6% having codon 
Val804, Met918, and Leu790 mutations, respectively.240 In con­
trast, another study from Denmark reported that codon Cys611 
RET mutations were the most prevalent, likely reflecting a founder 
mutation (e.g., Cys611Tyr) in this population.354
More than 50% of patients with FMTC have

ation.354
More than 50% of patients with FMTC have RET mutations 
of Cys618, with Cys618Arg being the most common. However, 
a small number of families with FMTC do not have RET muta­
tions, and recently a germline ESR2 mutation was reported to 
cosegregate with MTC in one family,355 thereby indicating the 
occurrence of genetic heterogeneity and roles for other genes in 
the etiology of MTC.
Germline loss of function RET mutations are the most com­
mon cause of isolated HSCR,356 and more than 2

om­
mon cause of isolated HSCR,356 and more than 200 different 
heterozygous RET mutations, which are predicted to result in 
diminished RET receptor signaling and a failure to transmit key 
developmental signals required for enteric nervous system devel­
opment, have been reported in HSCR patients. 
Somatic RET Mutations and Rearrangements
Somatic RET mutations are the most common recurrent muta­
tion in sporadic MTC.357 Overall, 40% to 50% of sporadic 
MTCs are reported to harbor somatic RET m

poradic 
MTCs are reported to harbor somatic RET mutations, although 
this frequency varies by tumor size and stage of disease.340,357,358 
Thus, the frequency of RET mutations is increased in tumors 
larger than 2 cm and may exceed 80% in those with advanced 
or metastatic presentations.358,359 The somatic RET mutations 
occurring in sporadic MTC typically affect the same residues as 
those disrupted in MEN2 and MEN3, and 60% to 80% are rep­
resented by the MEN3-associated Met918Thr substitutio

ented by the MEN3-associated Met918Thr substitution.358,359 
In MTC without RET mutations, somatic RAS mutations (pre­
dominantly of HRAS and KRAS) represent the second most com­
mon genetic abnormality and occur in approximately 10% to 
30% of sporadic MTCs, thereby supporting the role for activation 
of the RET-RAS-MAPK kinase pathway as a central feature of 
MTC development.357 Testing for somatic RET mutations may be 
of future clinical use, as a recent meta-analysis has reported that 
patie

as a recent meta-analysis has reported that 
patients with somatic RET mutations have an increased risk of 
lymph node metastases, distant metastases, tumor recurrence, and 
higher mortality,360 and another study has reported that advanced 
MTCs harboring double somatic RET mutations were associated 
with unfavorable outcomes.359
Somatic rearrangements of the RET locus, giving rise to chi­
meric fusion proteins with constitutive TK activity, have been 
reported in approximately 20% to 40% of spo

 been 
reported in approximately 20% to 40% of sporadic papillary 
thyroid carcinomas (PTCs), and these different RET fusion 
partners are collectively referred to as RET/PTC rearrange­
ments. Each RET/PTC rearrangement is characterized by the 
placement of the RET TK domain genetic locus adjacent to a 
ubiquitously expressed donor gene containing a coiled-coil dimer­
ization domain that facilitates constitutive receptor function and 
1654
act

N IX    Polyendocrine and Neoplastic Disorders
activation of downstream oncogenic signaling pathways.340 For 
example, the RET/PTC1 and RET/PTC3 rearrangements, which 
are most commonly observed, are the result of intrachromosomal 
inversions on the long arm of chromosome 10 that bring the 
RET TK domain adjacent to the coiled-coil domain containing 
6 (CCDC6) or nuclear receptor coactivator 4 (NCOA4) genes, 
respectively. The RET/PTC chimeric proteins have a higher prev­
alence in radiation-ind

oteins have a higher prev­
alence in radiation-induced PTC, indicating that ionizing radia­
tion likely is a risk factor for their occurrence, although they may 
also occur without exposure to radiation and are often observed in 
children with PTC.345 In addition, RET/PTC fusions have been 
reported in benign thyroid nodules and in patients with Hashi­
moto thyroiditis.340 Somatic RET rearrangements resulting in 
chimeric proteins have also been reported in approximately 1% 
of lung adenocarcino

reported in approximately 1% 
of lung adenocarcinomas (e.g., predominantly involving fusions 
with KIF5B) and in case reports of hematopoietic malignancies 
including chronic myelomonocytic leukemia.345 
RET Polymorphisms
Common polymorphisms and nonpathogenic rare sequence vari­
ants within the RET gene need to be distinguished from patho­
genic RET mutations. Common polymorphisms (global MAF 
>0.5%) within the coding region include 2 missense (p.Gly691Ser, 
p.Arg982Cys) and 6 synonymous varian

p.Gly691Ser, 
p.Arg982Cys) and 6 synonymous variants. However, many addi­
tional missense variants are observed at MAF ≥0.5% in specific 
ethnic groups (e.g., p.Leu56Met in the European population; 
p.Tyr791Phe in the Finnish European and Ashkenazi Jewish popu­
lations; p.Asp489Asn, pThr278Asn, p.Arg67His, p.Arg114His in 
East Asian populations; and p.Gly446Arg in African populations), 
although many of these are also observed at lower frequencies in 
other ethnic populations. At least 15 common

s in 
other ethnic populations. At least 15 common noncoding variants 
are observed in the RET untranslated regions, whereas the large 
RET intronic regions harbor more than 300 additional noncoding 
common polymorphisms (http://phase3browser.1000genomes.
org/ and http://gnomad.broadinstitute.org/). Thus, it is impor­
tant to recognize the occurrence of these polymorphisms, and cau­
tion is required when RET sequence variants are identified during 
genetic testing. This is particularly important

g 
genetic testing. This is particularly important for novel mis­
sense variants occurring in proximity to known functional RET 
domains, where ascribing pathogenicity may not be possible until 
there is clear evidence of variant segregation with the disease phe­
notype and/or abnormalities in cellular function established. The 
potential challenge of variant interpretation is highlighted by the 
observation that approximately 600 different rare (MAF <0.5%) 
missense RET variants have been repor

(MAF <0.5%) 
missense RET variants have been reported in the GnomAD data­
set (http://gnomad.broadinstitute.org/), whereas a recent study 
evaluating the Exome Aggregation Consortium cohort estimated 
that approximately 1 in every 35 individuals in the background 
population will harbor a rare (MAF <0.5%) nonsynonymous RET 
variant.145 In each setting, most individual RET coding variants 
will not be of clinical significance. 
RET Structure and Function
The RET receptor comprises several specifi

unction
The RET receptor comprises several specific regions: a large 
ECD containing four cadherin-like repeats and a cysteine-rich 
region, a hydrophobic single-pass transmembrane domain, a 
juxtamembrane segment, an intracellular TK domain, and an 
isoform-specific C-terminal tail2,307,344–346,348 (see Fig. 42.7). 
RET signaling is activated by the binding of members of the glial 
cell–line derived neurotropic factor (GDNF) family of ligands 
(GFLs), which comprise GDNF, neurturin, persephin, 

GFLs), which comprise GDNF, neurturin, persephin, and arte­
min344 (see Fig. 42.7). However, for receptor engagement, each 
of these ligands requires the presence of a ligand-binding core­
ceptor, which comprises members of the GDNF family recep­
tor α (GFRα) group of proteins.307,344,347 Four different GDFα 
coreceptors are described (GFRα1–4), with apparent preferen­
tial selectivity for each of the four GFLs. Although the precise 
sequence of events leading to receptor activation is debated, 

events leading to receptor activation is debated, the 
formation of the GFL-GFRα complex allows RET recruitment 
and the formation of heterodimers in which the RET-GFL-
GFRα complex has a 2:2:2 stoichiometry.344 The formation of 
this ternary complex results in activation of the TK domain and 
autophosphorylation of multiple intracellular tyrosine residues, 
which in turn form docking sites for signaling proteins such as 
those with SRC homology 2 (SH2) or phosphotyrosine-bind­
ing domains that 

2 (SH2) or phosphotyrosine-bind­
ing domains that ultimately facilitate downstream transmis­
sion of signals within the cell via a large number of effector 
pathways.293,344 Several conserved residues have been found to 
have key functions in catalytic activation and subsequent signal 
transduction.293,344 For example, autophosphorylated tyrosine 
residues at Tyr1062 and Tyr1096 in the RET51 isoform appear 
to form key signaling hubs that facilitate activation of multi­
ple distinct signaling pa

ate activation of multi­
ple distinct signaling pathways (e.g., RAS-MAPK and PI3K-
AKT),307,346 and autophosphorylation of the juxtamembrane 
residue Tyr687 has been shown to increase RET catalytic activity. 
Receptor function is also reported to be dependent on a phos­
phoserine residue (Ser909) within the activation loop indicating 
that RET acts as a dual-specific kinase.361
The activation of multiple downstream pathways (e.g., Ras/
MAPK, PI3K/Akt, phospholipase C-γ, JNK, JAK-STAT, FAK, 
and 

/Akt, phospholipase C-γ, JNK, JAK-STAT, FAK, 
and β-catenin/Wnt) modulates cellular processes that include dif­
ferentiation, proliferation, migration, and cytokine production.348 
The ability of RET to direct specific cellular effects is likely to 
be achieved by several mechanisms. For example, the binding of 
different RET ligands (i.e., GFLs) may result in differences in 
receptor activation,348 whereas the expression of different RET 
isoforms (e.g., RET9 and RET51) may result in the differ

ms (e.g., RET9 and RET51) may result in the differential 
deployment of individual intracellular signaling pathways.362 Fur­
thermore, the availability of each of the various intracellular sig­
naling partner complexes in different cells may also contribute to 
RET signaling specificity.
Under physiologic conditions, the RET protein is expressed at 
its highest levels during embryonic development, where it plays an 
essential role in the development of multiple tissues. For example, 
RET is esse

ent of multiple tissues. For example, 
RET is essential for the normal development of the kidneys and 
urogenital tract, where it plays a key role in modulating wolff­
ian duct patterning and branching morphogenesis of the ureteric 
bud.307,363,364 In addition, RET is predominantly expressed in cell 
lineages derived from the neural crest, including neuroendocrine 
and neuronal lineages, and it is therefore not surprising that sev­
eral of these relate to the diseases observed in patients with R

relate to the diseases observed in patients with RET 
mutations. Thus, RET is expressed in the thyroid parafollicular 
cells, adrenal chromaffin cells, and neurons, where it plays an 
essential role in the migration and establishment of the enteric 
nervous system.293,365 RET expression also occurs in neural crest–
derived peripheral and central neuronal populations, where it 
regulates axonal growth and cell survival.307 For example, RET 
is expressed in spinal cord motor neurons during early d

ressed in spinal cord motor neurons during early develop­
ment, where it mediates potent prosurvival activities, and similar 
protective effects have been reported in adult ventral midbrain 
dopaminergic neurons.347 Finally, RET is expressed at low levels 
in hematopoietic and immune cell precursors, and it is reported 
to be required for the formation of gut-associated lymphoid tissue 
and Peyer patches.366 
1655
RET Mutations and Receptor Function
Mecha

eoplasia
Mechanistic molecular insights into RET receptor function have 
been provided by studies of disease-associated RET mutations or 
translocations that result in abnormalities of receptor signaling. 
Thus, studies of heterozygous RET mutations, which cause loss 
of function and are associated with HSCR or gain of function 
(i.e., enhanced receptor signaling and are associated with MEN2 
and MEN3), have increased our understanding of the structure-
functi

creased our understanding of the structure-
function relationships of this RTK. For example, MEN2 muta­
tions involving substitutions of cysteine residues in the ECD 
(e.g., Cys609, Cys611, Cys18, Cys620, Cys630, Cys634) result 
in an unpaired cysteine residue that can form a disulfide bond 
with a similarly unpaired residue on a neighboring mutant RET 
receptor, thereby resulting in ligand-independent dimerization 
and constitutive receptor activation (see Fig. 42.7). Interest­
ingly, some of t

vation (see Fig. 42.7). Interest­
ingly, some of these mutations (e.g., affecting Cys611, Cys18, 
Cys620) can result in RET receptors with apparent paradoxi­
cal gain- and loss-of-function activity. This occurs because the 
mutant RET receptors, although having constitutive activity, 
have poor maturation that results in their decreased cell surface 
expression, which will have detrimental effects on the develop­
ment of tissues dependent on GFLs. Thus, the mutant RET 
receptors, although activa

. Thus, the mutant RET 
receptors, although activated, will be fewer, and this provides 
an explanation for the coexistence of MEN2 due to enhanced 
RET signaling, and HSCR, which is associated with RET hap­
loinsufficiency and loss-of-function mutations.293,307,338 The 
RET receptor mutations of the ECD may also alter downstream 
receptor signaling. For example, the ligand-independent dimer­
ization associated with cysteine-rich domain mutations may 
be associated with different cellular locali

 may 
be associated with different cellular localizations and result in 
altered access to downstream signaling complexes and prefer­
ential activation of individual pathways such as the PI3K/AKT 
pathway307,348 or otherwise altered ICD function that results in 
different patterns of receptor autophosphorylation and recruit­
ment of adaptor proteins. These different effects on RET sig­
naling may, in turn, contribute to the observed differences in 
phenotypes in patients.
The MEN2-associated RET

n 
The MEN2-associated RET mutations of the ICD (e.g., affect­
ing Leu790, Tyr791, Val804, Ser891) result in mutant receptors 
that are active as monomers in the absence of ligand, and this 
autonomous signaling is not further enhanced in the presence 
of ligand. This contrasts to the MEN3-associated RET muta­
tion, Met918Thr, which also resides in the ICD TK domain but 
results in a receptor conformation change that increases affinity 
for adenosine triphosphate and liga

ases affinity 
for adenosine triphosphate and ligand-independent kinase activ­
ity in both monomeric and dimeric forms, which is enhanced 
further by the presence of ligand.293,344 This enhanced function 
is more pronounced for the RET9 isoform when compared with 
RET51, and this greater signaling activity may contribute to the 
aggressive clinical phenotype observed in MEN3 patients. In 
addition, mutations in the RET ICD are reported to result in 
phosphorylation of substrates more commonly as

in 
phosphorylation of substrates more commonly associated with 
cytoplasmic TKs (e.g., SRC and FAK), and this may also contrib­
ute to enhanced tumorigenic activity and more aggressive tumor 
manifestations in patients.344 
The RET gene is highly evolutionary and conserved in mam­
malian and vertebrate species, with all higher organisms having 
a single copy of a RET orthologue. Furthermore, all verte­
brates have been observed to harbor GDNF family ligands and 
respective GFRα pr

harbor GDNF family ligands and 
respective GFRα proteins. RET orthologues are also observed in 
more evolutionary distant organisms, including D. melanogaster 
and Amphioxus.347 Although some insight into RET function 
has come from the study of nonmammalian models, includ­
ing zebrafish (D. rerio) and Drosophila, the predominant model 
organism for evaluating RET mutations that are relevant to 
MEN2 and MEN3 has been the mouse. Several mouse models 
have been generated by the transgenic express

els 
have been generated by the transgenic expression of mutant RET 
proteins, or knock-in methods in which specific mutations have 
been introduced into the Ret gene.367,368 Mice that are homozy­
gous for an inactivating Ret mutation die soon after birth and 
have renal agenesis and a lack of enteric neurons in the gastroin­
testinal tract.340,363
For MEN2, two transgenic models have been established in 
which the human Cys634Arg mutation is expressed in a tissue-
restricted manner. In one mode

ressed in a tissue-
restricted manner. In one model, expression of the human mutant 
RET9 isoform under the control of the rat calcitonin-gene related 
peptide/calcitonin promoter was reported to result in overt CCH 
followed by bilateral MTC with morphologic features similar to 
human MTC.369 In the other model, expression of the human 
mutant RET51 isoform under control of the human calcitonin 
promoter resulted in mice with early-onset CCH and subsequent 
bilateral MTC, and one founder line a

 subsequent 
bilateral MTC, and one founder line also developed tumors 
resembling papillary thyroid cancer, pancreatic cystadenomas, 
and cystoadenocarcinomas.368,370 In addition, a transgenic mouse 
expressing the human RET gene with the Cys634Arg mutation 
in multiple tissues (e.g., thyroid, heart, liver, colon, brain) devel­
oped CCH and/or MTC that was accompanied by high serum 
calcitonin concentrations, and the tumors, but not normal tis­
sues, from these mice had RET dimerization and com

sues, from these mice had RET dimerization and complex forma­
tion with Shc and Grb2 adaptor proteins.371 A knock-in mouse 
model with the Cys620Arg mutation has also been established 
for MEN2, and aged heterozygous (Ret+/620R) mice were found to 
develop CCH and adrenal gland hyperplasia, although no overt 
MTC or pheochromocytoma developed. However, homozygote 
mice (Ret620R/620R) were observed to die on the first postnatal day 
with renal agenesis and intestinal aganglionosis, consistent wit

nesis and intestinal aganglionosis, consistent with 
the phenotype observed in conventional Ret knockout mice, and 
these observations further demonstrate the apparent paradoxical 
activity of certain RET mutations that exhibit both loss- and gain-
of-function activity.372
For MEN3, transgenic and knock-in mouse models have 
been generated. Transgenic expression of the MEN3-associated 
Met918Thr mutation under the human calcitonin promoter in 
one mouse model resulted in the development of nodul

e mouse model resulted in the development of nodular CCH 
from 8 months of age and progression to MTC in 13% of mice 
after 11 months.373 Transgenic expression of the same mutation 
in another mouse model using the dopamine β-hydroxylase pro­
moter that facilitated expression in the developing sympathetic 
nervous system, enteric nervous system, and adrenal medulla 
resulted in neuroglial tumors, which were histologically indis­
tinct from human ganglioneuromas, in the adrenals and sympa­
thetic

ganglioneuromas, in the adrenals and sympa­
thetic nervous system.374 A knock-in mouse model harboring the 
murine equivalent of the Met918Thr mutation (i.e., Met919Thr) 
has been generated in which heterozygous (Ret+/MEN3) and homo­
zygous (RetMEN3/MEN3) mice develop CCH and adrenal chromaffin 
cell hyperplasia.375 Progression to pheochromocytoma and a more 
severe thyroid phenotype were observed in homozygote mice only, 
thereby indicating a potential dosage effect of the activating muta­
tion

tential dosage effect of the activating muta­
tion on disease expression. However, none of the mouse models 
developed mucosal neuromas or gastrointestinal ganglioneuro­
mas, which are found in MEN3 patients.367,368,375 
1656
Clinical Utility of Genetic Testing for MEN2 and MEN3
The widespread implementation of RET genetic testing has trans­
formed the management of patients with M

as trans­
formed the management of patients with MEN2 and MEN3, 
as it enables the identification of those individuals who are at a 
higher risk of developing the disease, and very often this is now 
in early childhood and before the onset of clinical features (i.e., 
presymptomatic diagnosis), and therefore appropriate monitoring 
and early treatment can be implemented. Indeed, the effective­
ness of RET genetic testing in MEN2 has been demonstrated in 
studies from Germany, which reveal a fall

ated in 
studies from Germany, which reveal a fall in the percentage of 
index cases in the population, owing to the increased detection of 
at-risk individuals through cascade genetic testing, together with 
a marked fall in MTC occurrence in nonindex RET mutation car­
riers, owing to prophylactic thyroidectomy.252 The situation for 
MEN3 is more difficult due to the high percentage (∼75%) of 
cases presenting with de novo mutations, and thus opportunities 
for early detection and prophylactic 

portunities 
for early detection and prophylactic treatment are often missed.342
The potential for improved clinical outcomes within kindreds 
following a genetic diagnosis of MEN2 highlights the need to iden­
tify potential index cases harboring germline RET mutations. Thus, 
genetic testing should be considered in several settings to maxi­
mize the likelihood of successfully establishing the diagnosis in an 
index case. Thus, currently, RET genetic testing is indicated in all 
patients present

etic testing is indicated in all 
patients presenting with a clinical diagnosis of MEN2 (e.g., MTC 
and pheochromocytoma), as well as those with clinical features 
of an associated MEN2 syndrome (e.g., FMTC, CLA, HSCR) 
or phenotypic manifestations of MEN3. Similarly, those present­
ing with MEN2-associated clinical features and a relevant family 
history require testing. In addition, all patients presenting with 
apparently sporadic MTC, or unilateral or bilateral adrenal pheo­
chromocytoma, sh

ateral or bilateral adrenal pheo­
chromocytoma, should undergo RET mutational analysis.3,340 For 
patients in whom the RET mutation status is unknown, either a 
prioritized DNA sequencing approach of the RET gene that incor­
porates recurrently affected exons (e.g., exons 10, 11, 13–16) or 
sequencing of the entire RET coding region should be undertaken. 
In patients in whom the clinical phenotype is highly suggestive of a 
specific genotype (e.g., typical MEN3 extraendocrine features with 
Met9

., typical MEN3 extraendocrine features with 
Met918Thr mutations), a more limited testing of the RET gene 
sequence may be appropriate. However, if RET testing is limited 
to specific exons, it is possible that additional sequence variants 
may be overlooked, and this may have clinical implications for the 
patient.3 RET sequence variants that have not been reported previ­
ously in MEN2 and MEN3 patients will need careful evaluation by 
the clinical genetics team for the likely pathogenicity of

ical genetics team for the likely pathogenicity of the variant, 
and decisions on the role of cascade genetic testing of family mem­
bers will have to be taken based on the clinical picture.
Once an established pathogenic RET mutation has been iden­
tified in an index case, genetic counseling and appropriate genetic 
testing for the specific sequence variant should be offered to all 
at-risk first-degree relatives. Cooperation of the patient will be 
required to contact the relatives, and occasi

 be 
required to contact the relatives, and occasionally complex eth­
ical issues may arise (e.g., if the patient is reluctant to disclose 
their health status to family members). In the management of 
children, similar ethical challenges may arise if an affected parent 
refuses to allow appropriate investigation and testing of an at-risk 
child. However, such situations are rare. RET genetic testing of 
at-risk individuals should be performed at an early time point, 
and in children who may har

 an early time point, 
and in children who may harbor the ATA highest- and high-risk 
categories of RET mutations, this should be in the first months 
or years of life, respectively, due to the early onset of disease in 
these groups. RET gene testing of the first-degree relatives of a 
presumed affected index case whose mutational status is unknown 
or unavailable is also indicated. Preconception genetic counseling, 
where appropriate, should be provided to all individuals at risk 
of transmitt

 provided to all individuals at risk 
of transmitting RET mutations to their future offspring. Prenatal 
genetic testing using invasive (e.g., amniocentesis) or noninvasive 
approaches (e.g., by evaluating cell-free fetal DNA from maternal 
blood) may inform prospective parents regarding the disease status 
of the developing fetus. Recent advances in genetic testing com­
bined with in vitro fertilization also offer prospective parents the 
opportunity to have a preimplantation genetic diagnosis.

unity to have a preimplantation genetic diagnosis. 
Screening and Intervention in RET Mutation Carriers
The ability to make a genetic diagnosis of MEN2 or MEN3 in 
at-risk individuals, often in early childhood and before the onset 
of clinical features, enables the appropriate monitoring and treat­
ment of asymptomatic individuals. Recommendations for the 
respective schedule of tumor surveillance and timing of potential 
prophylactic thyroidectomy is determined by the ATA risk cat­
egory of RET

my is determined by the ATA risk cat­
egory of RET mutation (see Table 42.4). Thus, for those with the 
highest- and high-risk categories of RET mutations, prophylactic 
thyroidectomy is typically recommended by the ages of 1 and 5 
years, respectively (although earlier in some instances based on 
clinical, biochemical, and/or radiologic features). The timing of 
prophylactic surgery in those with moderate-risk RET mutations 
is now based on the serial measurement of basal serum calcito­
nin lev

serial measurement of basal serum calcito­
nin levels in combination with neck imaging studies.3 Appropri­
ate annual screening for pheochromocytoma and PHPT, based 
on genotype-phenotype correlations, is also recommended and 
should commence at an age commensurate with the risk profile 
of RET mutation (see Table 42.4). In adult patients identified to 
harbor MEN2-associated RET mutations through cascade genetic 
testing, serum calcitonin concentrations and plasma/urinary free 
fractionated met

trations and plasma/urinary free 
fractionated metanephrines should be determined immediately. 
In individuals with normal serum calcitonin concentrations and/
or normal metanephrines, annual monitoring of serum calcitonin 
and plasma/urinary metanephrines should commence. This situa­
tion is most frequently observed for moderate-risk RET mutations 
that may result in late-onset disease or reduced tumor penetrance. 
In those identified to have elevated serum calcitonin concentra­
tions or elevat

evated serum calcitonin concentra­
tions or elevated plasma or urinary metanephrines, full evaluations 
to determine the stages of the MTC and/or pheochromocytoma 
should be performed prior to appropriate treatment. Furthermore, 
it is imperative that coexistent pathologies such as pheochromocy­
toma should be excluded prior to any intervention in all patients 
with a RET mutation or who has MTC, MEN2, or MEN3. 
MEN Type 4
MEN4, an autosomal dominant disorder due to mutation of the 
CDKN1B gene 

nant disorder due to mutation of the 
CDKN1B gene located on chromosome 12p13, has been reported 
in fewer than 20 index cases,172,376,377 and patients had clinical 
features that are similar to those patients with MEN1. Indeed, 
among the 5% to 10% of patients with a clinical diagnosis of 
MEN1 who do not have MEN1 gene mutations, approximately 
3% will have CDKN1B mutations.
MEN4 patients predominantly have parathyroid, pituitary, 
and pancreatic tumors in asso

thyroid, pituitary, 
and pancreatic tumors in association with other tumors, which 
1657
include gastrointestinal, bronchial, and cervical NETs; NF adrenal 
tumors; PTC; lipoma; and breast cancer.5,172,376–379 Parathyroid 
tumors associated with PHPT have been reported in approxi­
mately 80% of MEN4 cases, although PHPT is observed to occur 
at a later age than MEN1. Pituitary tumors (including somato­
tropinoma, corticotropinoma, prolactinoma, and NF ade

pinoma, corticotropinoma, prolactinoma, and NF adenoma) 
occur in approximately 40% of cases,172 and duodenopancreatic 
NETs occur in approximately 35% of cases. The investigation and 
treatments for these MEN4-associated tumors are similar to those 
for MEN1 and non-MEN1 tumors. 
Molecular Genetics and Animal Models
The identification of the gene responsible for MEN4 resulted 
from observations in a rat model of endocrine neoplasia (termed 
MENX),380 in which the disease locus was mapped to the

),380 in which the disease locus was mapped to the dis­
tal part of chromosome 4,381 a region that contained the Cdkn1b 
gene. Cdkn1b encodes the CDKI p27kip1, also referred to as 
p27.171,172,377,379 A homozygous frameshifting insertion of 8 bp 
in the rat Cdkn1b gene at codon 179, which resulted in a mis­
sense peptide and termination at codon 218 and a highly unstable 
mutant p27kip1 protein, was identified in the MENX rats.171,172 
Subsequent studies of a patient with a MEN1 phenotype (pitu­

 studies of a patient with a MEN1 phenotype (pitu­
itary and parathyroid disease) but absence of the MEN1 mutation 
identified a nonsense CDKN1B mutation.171 The identification 
of further patients harboring CDKN1B mutations followed, and 
MENX was renamed MEN4.172,382,383 To date, fewer than 20 
MEN4 index cases with CDKN1B mutations have been reported, 
and the majority of these mutations are associated with reduced 
levels of p27Kip1 or altered protein function, consistent with a 
tumor suppr

d protein function, consistent with a 
tumor suppressor role. The mutations reported include nonsense, 
frameshift, splice-site, and missense mutations.171,172,376,377,382–385 
Germline CDNK1B mutations have also been reported in patients 
with apparently sporadic (i.e., nonfamilial) PHPT, although fur­
ther studies are required to confirm the clinical significance of 
the variants and whether such individuals develop additional 
MEN-associated tumors.386,387 It is notable that that some rare 
m

umors.386,387 It is notable that that some rare 
missense variants in CDKN1B and other CDK inhibitor genes 
are observed at relative high frequency in the background popula­
tion,145 thereby suggesting that some CDKN1B variants that are 
benign polymorphisms may have been misclassified as mutations.
p27kip1 participates in cell cycle regulation through its interac­
tion with several cyclins and their respective CDKs. These include 
interactions with cyclin E/cdk2, cyclin A/cdk2, and cyclin D/
cd

ith cyclin E/cdk2, cyclin A/cdk2, and cyclin D/
cdk4.388–390 The major function of the p27kip1 protein appears to 
be the control of G1 to S phase during cell replication, where the 
binding of p27kip1 to cyclin E/cdk2 arrests cell cycle progression.388 
The cellular p27kip1 levels are tightly regulated at transcriptional, 
translational, and post-translational levels, and the transcription 
of CDKN1B is under the regulation of menin through its inter­
action with MLL.191,192 However, post-trans

inter­
action with MLL.191,192 However, post-translational regulation 
facilitates rapid changes in p27kip1 expression. For example, on 
receipt of a mitogenic signal, at least two pathways in the cyto­
plasm and nucleus are activated, which result in phosphorylation 
of specific residues on p27kip1 leading to proteosomal-mediated 
degradation and a derepression of cell cycle progression.388,391,392 
Furthermore, downregulation of p27kip1 is observed in many 
cancers, including those of the colo

rved in many 
cancers, including those of the colon, breast, stomach, and pros­
tate.388 The reported mechanisms for reduced levels of p27kip1 
protein may include decreased transcription, reduced translation, 
increased protein degradation, reduced binding to key interacting 
partners, or cellular mislocalization.376,382–384,388 
Genetic analysis for CDKN1B mutations is indicated in individu­
als who have clinical evidence of typica

individu­
als who have clinical evidence of typical or atypical MEN1 but 
without MEN1 mutations or those who are first-degree relatives of 
patients with a known CDKN1B mutation.1,172,376 The few num­
bers of patients reported with MEN4 make it difficult to provide 
guidelines specifically for the management of MEN4, and cur­
rently it would appear reasonable to follow a similar protocol of 
surveillance to that of MEN1.1,172,376,377 
Future Directions and Concluding Remarks
Significant progres

ections and Concluding Remarks
Significant progress has been made in the diagnosis and man­
agement of the MEN syndromes. In particular, the widespread 
implementation of MEN1, RET, and CDKN1B genetic testing 
has facilitated the identification of individuals at risk of develop­
ing MEN1, MEN2 and MEN3, and MEN4, respectively. This 
approach has facilitated the periodic screening of mutation carri­
ers, thereby enabling the early and/or presymptomatic detection of 
tumors, as well as leading to 

matic detection of 
tumors, as well as leading to treatments that aim to reduce disease-
associated morbidity and mortality. For example, the implemen­
tation of “prophylactic” thyroidectomy in MEN2 has resulted in 
significant improvements in clinical outcomes. Furthermore, an 
improved understanding of the natural history of the MEN syn­
dromes has led to the formulation of clinical guidelines, but these 
are often based on a consensus among experts rather than evidence 
from clinical trials, 

perts rather than evidence 
from clinical trials, which are required to address key issues such as 
the optimal treatment of MEN1-associated pancreatic NETs and 
parathyroid tumors. Reliable biomarkers and diagnostic tools that 
accurately predict the behavior of MEN1-associated pancreatic 
NETs and thymic carcinoids are also required, as is the evalua­
tion of novel therapeutic approaches that have emerged from an 
understanding of menin biology, which have been demonstrated 
to be effective in

, which have been demonstrated 
to be effective in preclinical models. In addition, the genetic and 
environmental factors that influence MEN2 disease expression 
and penetrance remain to be elucidated, and improved screening 
programs that enable the detection of de novo mutations associ­
ated with MEN3 are needed to reduce the number of children 
presenting with advanced MTC. More effective targeted treat­
ments for advanced MTC are also required, as well as establishing 
the optimal use of ex

ed, as well as establishing 
the optimal use of existing therapies, including RTK inhibitors. 
This is likely to be aided by further mechanistic insights into RET 
receptor structure/function and the development of personalized 
therapies targeting individual RET mutations. Finally, the natural 
history of MEN4 and its clinical expression needs to be character­
ized by the continued identification and careful clinical evaluation 
of individuals and kindreds with CDKN1B mutations. 

nd kindreds with CDKN1B mutations. 
PJN was supported by the Chief Scientist Office (Fellowship 
SCAF/15/01). RVT was supported by a Wellcome Trust Inves­
tigator Award (106995/2/15/2), the National Institute for 
Health Research (NIHR) Oxford Biomedical Research Cen­
tre Programme, and the NIHR Senior Investigator Award 
(NF-SI-0514-1091).
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1658
43
The Immunoendocrinopathy Syndromes
JENNIFER M. BARKER, PETER A. GOTTLIEB, AND MARK S. ANDERSON
Autoimmunity Primer, 1659
Natural History of Autoimmune Disorders, 1660
Autoimmune Polyendocrine Syndrome Type I, 1663
Autoimmune Polyendocrine Syndrome Type II, 1666
Other Polyendocrine Deficiency Autoimmune Syndromes, 1668
Conclusion, 1671
KEY POINTS

Since Addison’s initial description of primary adrenal insuffi­
ciency in a patient with two autoimmune disorders (vitiligo and 
the hyperpigmentation of Addison disease), the immunoendo­
crinopathy syndromes have contributed to the understanding of 
both endocrinology and immunology (Fig. 43.1). Understand­
ing the pathogenesis of the polyendocrine syndromes continues 
to expand. In particular, shared genetic loci underlying disease 
susceptibility, potential environmental factors, and organ-s

lity, potential environmental factors, and organ-specific 
autoantigens targeted by the immune system are being defined. 
Recent advances include the development of more reliable T-cell 
and other immunologic assays, further refinement in predictive 
models of disease, and continued unraveling of the genetic factors 
underlying disease susceptibility.
Most autoimmune endocrine disorders (e.g., type 1 diabetes, 
autoimmune thyroid disease) occur in isolation. Two distinct 
autoimmune polyendocrin

n isolation. Two distinct 
autoimmune polyendocrine syndromes with characteristic group­
ings of manifestations are readily recognized. Autoimmune polyen­
docrine syndrome type I (APS-I) is a rare disorder with autosomal 
recessive inheritance that is caused by defects in the autoimmune 
regulator (AIRE) gene. In contrast, autoimmune polyendocrine syn­
drome type II (APS-II) is more common but less well defined and 
includes overlapping groups of disorders. A unifying characteristic 
within APS-

 disorders. A unifying characteristic 
within APS-II is the strong association with polymorphic genes 
of the human leukocyte antigen (HLA) region located on the 
short arm of chromosome 6 (band 6p21.3). In addition to HLA, 
many other genetic loci are likely to contribute to susceptibility to 
APS-II. For purposes of simplicity in this chapter, APS-II encom­
passes what some clinicians divide into APS-II (Addison disease 
plus type 1 diabetes or thyroid autoimmunity), APS-III (thyroid 
autoimmu

 thyroid autoimmunity), APS-III (thyroid 
autoimmunity plus other autoimmune diseases, not Addison dis­
ease or type 1 diabetes), and APS-IV (two or more other organ-
specific autoimmune disorders).
APS-II has also been known by various other names, including 
Schmidt syndrome, polyglandular autoimmune disease, polyglan­
dular failure syndrome, organ-specific autoimmune disease, and 
polyendocrinopathy diabetes. Such diverse names reflect the large 
number of studies and case reports of this syn

ge 
number of studies and case reports of this syndrome and its his­
torical importance. Each of these other names has some shortcom­
ings, such as failure to include the fact that both hyperfunction 
and hypofunction of endocrine glands can occur, or failure to rec­
ognize that nonendocrine disorders such as pernicious anemia and 
celiac disease are parts of the syndrome. Studies of patients with 
APS-II were instrumental in identifying the autoimmune bases of 
several diseases and developing a

immune bases of 
several diseases and developing autoantibody assays such as those 
for type 1 diabetes and cytoplasmic islet cell antibodies.
Other rare autoimmune endocrine disorders have contributed to 
an understanding of the development of autoimmunity. For exam­
ple, the rare disorder called immunodysregulation polyendocrinopa­
thy enteropathy X-linked syndrome (IPEX) is caused by a mutation 
of the forkhead box P3 (FOXP3) gene. FOXP3 plays a central role 
in the development and function o

 a central role 
in the development and function of regulatory CD4+ T cells that 
function to maintain tolerance to self. It has become increasingly 
1659
Chapter 43  The Immunoendocrinopathy Syndromes
recognized that these T cells play a key role in the pathogenesis of 
many autoimmune diseases, and therapies targeting these cells will 
likely be developed and tested. A thorough understanding of these 
rare and often genetically simple disorders provides insight into 
the development of syndrom

 provides insight into 
the development of syndromes that are characterized by polygenic 
inheritance and that affect a larger group of patients.
Autoimmunity Primer
An understanding of the pathophysiology of autoimmune disease 
requires a basic knowledge of the immunologic mechanisms that 
underlie tolerance (the ability to differentiate self from nonself). 
Autoimmunity develops when the mechanisms of immune toler­
ance break down. It can occur centrally at the level of the generative 
organs 

 centrally at the level of the generative 
organs (e.g., thymus, bone marrow) or peripherally in the target 
organs or lymphoid tissues. T lymphocytes and autoantibodies 
produced by B cells are two arms of the immune system that differ 
fundamentally in their recognition of target antigens. Autoanti­
bodies react with intact molecules (including both soluble and cell 
surface molecules) and usually interact with conformational deter­
minants of the autoantigen. In contrast, T lymphocytes recogn

the autoantigen. In contrast, T lymphocytes recognize 
peptide fragments of autoantigens, often 8 to 12 amino acids in 
length, that are presented on the surface of another cell by mol­
ecules of the major histocompatibility complex (MHC).
Histocompatibility molecules interact with T-cell receptors 
when bound with an antigenic peptide. These molecules resemble 
a hot dog in a bun, with the antigenic peptide (the hot dog) bound 
in the groove of the histocompatibility molecule (the bun). His­
to

the histocompatibility molecule (the bun). His­
tocompatibility molecules are extremely polymorphic, with differ­
ent amino acids lining the peptide-binding groove. These variable 
amino acids determine which peptides are bound and presented 
to T lymphocytes.
T cells differ based on multiple cell surface molecules, and 
these molecules determine their function in the immune system. 
T cells interact with other cells within and outside the immune 
system. CD4+ T cells typically react with peptid

system. CD4+ T cells typically react with peptides that are derived 
from proteins in extracellular compartments that are bound and 
acquired by class II histocompatibility molecules (HLA-DP, HLA-
DQ, or HLA-DR in humans), expressed on antigen-presenting 
cells (APCs) such as macrophages, dendritic cells, and B lympho­
cytes. CD8+ T cells react with peptides bound by class I histo­
compatibility molecules (HLA-A, HLA-B, and HLA-C). Class I 
molecules are present on the surface of almost all nu

ecules are present on the surface of almost all nucleated cells. 
The antigen peptide in this case is derived from proteins expressed 
endogenously and is presented in a complex by class I HLA by the 
target cell itself. Recognition of the antigenic peptide by CD8+ T 
cells typically leads to the release of cytotoxic chemicals that kill 
the target cell.
The T-cell response depends on the context in which the anti­
gen is presented. The simple expression of histocompatibility mol­
ecules and rec

pression of histocompatibility mol­
ecules and recognition of antigen by a T cell are not sufficient for 
T-cell activation. This context is at least partially determined by 
the interaction of cell surface molecules on both the T cell and the 
APC. Interaction among the MHC, the peptide, and the T-cell 
receptor (signal one) is critical to the activation process; other 
co-stimulatory molecules then help to define the nature of the 
immune response (signal two). The context in which the antigen

nse (signal two). The context in which the antigens 
are presented is critical for the determination of this response. Cell 
surface molecules and receptors, cytokines, and chemokines form 
the context in which the antigen is presented. Based on this con­
text, the cell can become activated, tolerized, or anergic (immune 
unresponsive). For example, the APC cell surface molecule CD80 
or CD86 engages the CD28 receptor on the T cell and amplifies 
signal one, which leads to T-cell activation. Whe

signal one, which leads to T-cell activation. When a T cell recog­
nizes an antigen in the context of the MHC and does not receive 
the appropriate second signal, anergy results.
Tolerance induction is a staged process that begins in the thy­
mus during T-cell maturation. This process depends in part on 
the presence of peripheral antigens in the thymus. Peripheral anti­
gens are self-antigens (e.g., insulin) normally expressed in tissues 
outside the immune system that are expressed at low lev

de the immune system that are expressed at low levels in the 
thymus. Developing T cells that react strongly to these periph­
eral molecules in the context of the MHC are deleted in the thy­
mus and are thus removed from the T-cell repertoire in a process 
known as negative selection. Study of AIRE gene knockout mice 
has supported the importance of these phenomena in the develop­
ment of autoimmunity. These mice have low levels of expression 
of peripheral antigens in the thymus and develop lym

 peripheral antigens in the thymus and develop lymphocytic 
infiltrates in multiple organs (see later discussion).
Peripheral tolerance is an important mechanism for tolerance 
induction after T cells have matured in the thymus. Anergic and 
regulatory T cells are integral in the development of tolerance for 
naive T cells. A major population of T-regulatory cells carry the 
cell surface markers CD4 and CD25 and express FOXP3. The 
function of the population of CD4+/CD25high cells involves an 
a

e population of CD4+/CD25high cells involves an 
active suppressive activity and relies on the transcription factor 
FOXP3. Deletion of this transcription factor leads to fulmi­
nant autoimmunity in neonates (e.g., neonatal type 1 diabetes 
and enteropathy), often resulting in death within the first year 
of life (IPEX syndrome; see later discussion). Another set of key 
molecules that help control peripheral T-cell tolerance are cyto­
toxic T-lymphocyte antigen 4 (CTLA4) and programmed death 
1

mphocyte antigen 4 (CTLA4) and programmed death 
1 (PD1).1 CTLA4 is expressed in T cells and acts as a negative 
regulator of T-cell signaling by competing with the T-cell activator 
CD28 (described earlier). CTLA4 outcompetes CD28 for bind­
ing to its ligands CD80 and CD86 due to its higher affinity for 
ligand. In addition, CTLA4 is broadly expressed on the surface 
of FOXP3-expressing CD4-positive T-regulatory cells, where it 
likely plays a role in blocking CD28 interactions with CD80 and 
C

ole in blocking CD28 interactions with CD80 and 
CD86. PD1 is yet another co-stimulatory molecule that becomes 
upregulated on T cells that have been chronically activated, and 
it confers negative signals through inhibitory signaling domains 
in its intracytoplasmic tail. The importance of both PD1 and 
CTLA4 in peripheral tolerance is underscored in knockout mouse 
models that develop spontaneous multiorgan autoimmunity and 
in cancer patients treated with antibodies that block their activity 

treated with antibodies that block their activity 
where many of these patients develop autoimmune complications 
• Fig. 43.1  This illustration accompanied Addison’s initial description of 
primary adrenal insufficiency (Addison disease). (From Addison T. On the 
Constitutional and Local Effects of Disease of the Supra-renal Capsules. 
London, UK: Samuel Highley; 1855.)
1660
Cognate help is the process by which B cells

rders
Cognate help is the process by which B cells are activated by 
CD4+ T cells that are responding to the same antigen. CD4+ T 
cells activate B cells to produce the humoral immune response. 
This occurs after the CD4+ T cell engages an antigen in the con­
text of the MHC on the cell surface of a B cell. The cytokines 
(interleukin [IL] 4, IL5, and IL6) produced by the CD4+ T cells 
induce the maturation of a B cell. Depending on the cytokine 
milieu, the B cell will switch from producing imm

milieu, the B cell will switch from producing immunoglobulin M 
(IgM) to IgG, IgE, or IgA. The development of B-cell tolerance is 
partially dependent on this linked recognition: autoreactive B-cell 
clones that do not have a CD4+ T cell that can bind with the 
antigen in its MHC groove will not normally be activated. Thus, 
in most cases, the generation of autoantibodies by B cells is also 
linked to an autoreactive T cell specific for the same self-antigen. 
Growing evidence supports the role

 self-antigen. 
Growing evidence supports the role of autoreactive B cells as criti­
cal APCs to autoreactive T cells, creating a positive feedback loop 
in the expansion and maintenance of the autoimmune process. 
Natural History of Autoimmune Disorders
The natural history of autoimmune disorders can be divided into 
a series of stages beginning with genetic susceptibility, followed 
by triggering of autoimmunity (e.g., dietary gliadin exposure in 
celiac disease) and active autoimmunity preced

in 
celiac disease) and active autoimmunity preceding clinical mani­
festations (e.g., progressive glandular destruction), and, finally, 
overt disease. This is a theoretical construct that may be useful for 
understanding factors involved with the development of autoim­
munity and disease, but of necessity it is simplified and does not 
reflect the potential relapsing-remitting nature of autoimmunity 
(Fig. 43.2).
Genetic Associations
Although there is familial aggregation of APS-II and its com

here is familial aggregation of APS-II and its component 
disorders, there is no simple pattern of inheritance (Table 43.1). 
Susceptibility is probably determined by multiple genetic loci 
(with HLA having the strongest effect) interacting with environ­
mental factors. Autoimmune diseases share common genetic risk 
factors, including HLA, the MHC class I–related gene A (MICA), 
the gene for lymphoid tyrosine phosphatase (PTPN22), the cyto­
toxic T lymphocyte–associated antigen 4 (CTLA4), and th

 T lymphocyte–associated antigen 4 (CTLA4), and the gene 
for NACHT leucine-rich repeat protein 1 (NLRP1, or NALP1).2 
In addition, genetic susceptibility for some autoimmune diseases 
has been linked to polymorphisms that are organ specific—for 
example, polymorphisms in the variable nucleotide tandem repeat 
upstream from the insulin gene have been associated with risk for 
type 1 diabetes.3
Genes located on the MHC found on chromosome 6 have 
been implicated in the pathogenesis of organ-speci

been implicated in the pathogenesis of organ-specific autoim­
mune diseases. These genes are in strong linkage disequilibrium 
with each other and encode proteins that are important in the 
function of the immune system. Foremost in importance for the 
genetics of organ-specific autoimmune diseases are class I and 
class II HLA genes. Molecular HLA genotyping has revealed 
many subtypes of the older, serologically defined alleles, and the 
unique genetic sequence encoding each polymorphic chain 

 genetic sequence encoding each polymorphic chain of 
the histocompatibility molecules is now given a unique iden­
tifying number. A case in point is the DQ molecule, which is 
the histocompatibility molecule most strongly associated with 
endocrine autoimmunity. A number is assigned for each unique 
α- and β-chain sequence. Examples are DQA1*0501 for the α 
chain and DQB1*0201 for the β chain of the DQ molecule 
(also termed DQ2) commonly encoded on DR3 (DRB1*0301) 
haplotypes. A haplotype cons

d on DR3 (DRB1*0301) 
haplotypes. A haplotype consists of a series of alleles of differ­
ent genes on a contiguous region of a chromosome (e.g., DQA1 
and DQB1 alleles) that are inherited together. A genotype is 
the combination of the haplotypes of both chromosomes. Fine 
mapping of the HLA has shown remarkable conservation of 
the HLA-A1/B8/DR3 haplotype such that a region of approxi­
mately 2.9 megabases is invariable. Conservation of large areas 
PAE cell
PAE cell
T cell
T cell
B cell
HLA
AP

eas 
PAE cell
PAE cell
T cell
T cell
B cell
HLA
APC
Environmental activation
Innate immunity
Autoantibodies
Peptide of target organ
Th2 cell
Th1 cell
T cell
T cell
T cell
FOXP3 
IPEX
AIRE
APS1
Thymus
• Fig. 43.2  Model of the pathogenesis of autoimmunity in polyendocrine disorders. The development of 
autoimmune disease is determined by a group of T cells that recognize one or more organ-specific epito­
mes. Peptides are presented in the human l

 epito­
mes. Peptides are presented in the human leukocyte antigen (HLA) molecule and are recognized by the 
T-cell receptor. Recognition of self molecules depends on the maturation of the T cell, a process that 
begins in the thymus and continues in the periphery. The transcription factor FOXP3 stimulates the devel­
opment of CD4+/CD25+ regulatory T cells. B cells produce autoantibodies under the stimulation of T cells. 
AIRE, autoimmune regulator; APC, antigen-presenting cell; APS1, autoimmune

or; APC, antigen-presenting cell; APS1, autoimmune polyendocrine syndrome 
1; IPEX, immunodysregulation polyendocrinopathy enteropathy X-linked; PAE, peripheral antigen-express­
ing cell; Th1, type 1 helper T cell; Th2, type 2 helper T cell. (From Eisenbarth GS, Gottlieb PA. Autoimmune 
polyendocrine syndromes. N Engl J Med. 2004;350:2068–2079.)
1661
suggests that these areas of the genome have been inherited with­
out recombination and are in very 

inherited with­
out recombination and are in very tight linkage disequilibrium. 
This greatly complicates the ability to identify which, if any, of 
the genes within the area of conservation are associated with dis­
ease and must be accounted for when assessing susceptibility to 
disease in this region.
Part of the overlapping risk for autoimmune disease is related 
to shared genetic susceptibility, especially within the HLA. For 
example, the highest-risk HLA genotype for type 1 diabetes is 
DR

ghest-risk HLA genotype for type 1 diabetes is 
DR3-DQ2, DR4-DQ8 (DQ8 = DQA1*0301-DQB1*0302). 
The importance of this HLA genotype in the development of type 
1 diabetes is highlighted by the observation that children who 
inherited the same DR3-DQ2, DR4-DQ8 as a sibling with type 
1 diabetes are at greater than 75% risk for development of autoim­
munity by age 12 years and at greater than 50% risk of develop­
ing diabetes by 12 years.4 A specific DR4 subtype of this gene, 
DRB1*0404, shows a st

c DR4 subtype of this gene, 
DRB1*0404, shows a strong association with Addison disease.5,6 
The DR3-DQ2 haplotype is associated with celiac disease both 
in the presence7 and absence8 of type 1 diabetes. This haplotype 
has been associated with autoimmune thyroid disease,9 although 
conflicting reports exist.10
Whereas some HLA alleles increase disease risk, others are asso­
ciated with protection from disease. For example, the DQ alleles 
DQA1*0102-DQB1*0602 (usually associated with DR2) not 

0102-DQB1*0602 (usually associated with DR2) not 
only confer strong protection from type 1A diabetes in a dominant 
fashion11 but also confer susceptibility to another autoimmune 
disorder—multiple sclerosis. Furthermore, this protection is not 
general to endocrine autoimmunity, because no protection from 
Addison disease is afforded by DQB1*0602. DP is another gene 
within the MHC, and its 0402 polymorphism has been shown to 
be associated with a decreased risk for type 1 diabetes in subjects

h a decreased risk for type 1 diabetes in subjects 
with the highest-risk HLA genotype for type 1 diabetes (DR3/
DR4).12 Observations such as these suggest that as more is learned 
about the genetic influence of disease, researchers will be able to 
combine different genotypes and refine prediction of autoimmune 
disease.
MICA produces a protein that is expressed in the thymus and 
in naive CD8+ T cells. Polymorphisms of MICA have been asso­
ciated with type 1 diabetes,13 celiac disease,14 and A

ed with type 1 diabetes,13 celiac disease,14 and Addison dis­
ease.15 A particular polymorphism of MICA, denoted 5.1, results 
from the insertion of a single base pair. This insertion produces 
a premature stop codon and a truncated protein. This particular 
polymorphism has been shown to influence the risk for Addison 
disease in subjects with autoimmunity associated with Addison 
disease.
Genes outside the MHC have also been implicated in the 
pathogenesis of autoimmune disease. For example, t

pathogenesis of autoimmune disease. For example, the PTPN22 
gene encodes lymphoid tyrosine phosphatase (LYP) protein. 
LYP, through interactions with regulatory kinases such as CSK, 
appears to act as an inhibitor of the signal cascade downstream 
from the T-cell receptor. A specific polymorphism associated 
with a tryptophan substitution for arginine at position 620 
(R620W) blocks LYP’s interaction with CSK.16 Recent work has 
suggested that this allele may increase the development of auto­
r

his allele may increase the development of auto­
reactive B cells16,17 and affect intracellular signaling pathways 
in both T and B cells.18 This polymorphism has been associated 
  Genetic Associations With Autoimmune Disease
Gene
Proposed Mechanism of Action
Polymorphism/Mutation
Disease
HLA
Antigen presentation
DR3-DQ2/DR4-DQ8
DR3-DQ2
DR3-DQ2/DRB1*0404-DQ8
DR3 DR5
MICA
Priming 

raves disease
DR3 DR5
MICA
Priming of naive T cells
5, 5.1
4, 5.1
5.1
PTPN22
T-cell receptor signaling pathway through interaction 
with regulatory kinases
Tryptophan substitution for 
arginine at position 620
SLE
RA
Vitiligo
CTLA4
Receptor on activated CD4+ and CD8+ T cells; 
decreases T-cell activation
CT60
CT60; +49A/G
CT60

abetes
++49A/G
++49A/G
AIRE
“Peripheral” antigen presentation in the thymus
Multiple reported mutations
APS-I
FOXP3
Transcription factor important for maturation of CD4+/
CD25+ regulatory T cells
IPEX
X-linked
AIRE, Autoimmune regulator; APS-I, autoimmune polyendocrine syndrome type I; CTLA4, cytotoxic T lymphocyte–associated antigen 4; FOXP3, forkhead

T lymphocyte–associated antigen 4; FOXP3, forkhead box protein 3; HLA, human leukocyte antigen; 
IPEX, immunodysregulation polyendocrinopathy enteropathy X-linked; MICA, major histocompatibility complex class I–related gene A; PTPN22, the gene for lymphoid tyrosine phosphatase; RA, rheumatoid 
arthritis; SLE, systemic lupus erythematosus.
   
TABLE 43.1
1662
with type 1 diabetes,19 rheumatoid arthritis,20 systemic lupus 
erythematosus (SLE),21

rthritis,20 systemic lupus 
erythematosus (SLE),21 Graves disease,22 and vitiligo,23 and is 
weakly associated with Addison disease.24 Furthermore, this 
disease-associated allele has also been associated with SLE, rheu­
matoid arthritis, type 1 diabetes, and autoimmune hypothyroid­
ism in families with several members affected by more than one 
autoimmune disease.24
CTLA4 is expressed on activated CD4+ and CD8+ T cells, 
where it is hypothesized to act as a negative regulator as out­
lined earl

 to act as a negative regulator as out­
lined earlier.25 Several polymorphisms within the CTLA4 gene 
have been associated with autoimmune diseases. One polymor­
phism associated with AT repeats has been shown to reduce the 
inhibitory function of CTLA4 in subjects with Graves disease.26 
A single nucleotide polymorphism in the 3′ untranslated region, 
denoted CT60, has been associated with Graves disease27 and 
autoimmune hypothyroidism.28 An additional polymorphism, 
denoted +49A/G, has been a

ditional polymorphism, 
denoted +49A/G, has been associated with celiac disease in the 
Dutch population,29 with autoimmune thyroid disease,30 and 
with Addison disease.31
NALP1 regulates the innate immune system. After the initial 
observation that this gene was associated with the risk of vitiligo32 
and other related autoimmune diseases, it was associated with 
Addison disease and type 1 diabetes.33
Organ-specific genetic polymorphisms have been associ­
ated with the development of specific a

en associ­
ated with the development of specific autoimmune diseases. For 
example, polymorphisms of the variable number of tandem repeats 
upstream of the insulin gene have been associated with the devel­
opment of type 1 diabetes. Higher numbers of tandem repeats are 
associated with increased production of insulin in the thymus and 
protection from type 1 diabetes presumably due to improved nega­
tive selection of insulin-reactive T cells.3 Similarly, polymorphisms 
of the thyroglobulin gene 

milarly, polymorphisms 
of the thyroglobulin gene are associated with autoimmune thyroid 
disease.34
Single-gene defects such as AIRE and FOXP3 cause multi­
organ autoimmunity and are discussed in sections devoted to 
those topics. Analysis of mutations of the AIRE gene indicates 
that it generally does not play a role in APS-II or sporadic Addi­
son disease, with 1 (1.1%) in 90 patients with Addison disease 
(non–APS-I) and 1 (0.2%) in 576 control subjects having AIRE 
mutations.31 

ol subjects having AIRE 
Environmental Triggers
Although genetics is known to play an important role in the 
development of autoimmunity, it does not tell the whole story. 
For example, the highest-risk HLA genotype for type 1 diabe­
tes (DR3-DQ2, DR4-DQ8) is associated with a risk of 1 in 20 
for the development of diabetes.35 Although this is greater than 
the general population prevalence rate of 1 in 200 by the age of 
20 years, it is certainly not a 100% risk. Therefore, other

 it is certainly not a 100% risk. Therefore, other factors 
(genetic and environmental) must be involved in the initiation of 
autoimmunity. Some theorize that these factors may be environ­
mental triggers.
For one disease, celiac disease, the underlying environmental 
trigger has been identified: gluten. Through studies such as DAISY 
(the Diabetes Autoimmunity Study in the Young), BabyDiab, 
and CEDAR (Celiac Disease Autoimmunity Research), the tim­
ing of first exposure to cereal has been ide

 tim­
ing of first exposure to cereal has been identified as a risk factor 
for the development of diabetes and celiac disease autoimmunity. 
Infants exposed at a very young age to cereal developed diabetes 
and celiac-associated autoimmunity at a greater rate than those 
who had cereal introduced at a later date.36–38 In epidemiologic 
studies, cod liver oil consumption has been associated with a 
decreased risk for type 1 diabetes. Cod liver oil contains omega-3 
polyunsaturated fats and vitam

l contains omega-3 
polyunsaturated fats and vitamin D. In prospective studies, there 
is some suggestion that lower consumption of omega-3 poly­
unsaturated fats is associated with an increased risk for autoim­
munity associated with type 1 diabetes.39 Further investigation 
may identify additional environmental associations. Changes in 
our diet, our food composition, and use of medicines such as 
antibiotics have the potential to change our gut flora and micro­
biome. Animal studies suggest t

t flora and micro­
biome. Animal studies suggest that changes in the gut micro­
biome can affect disease in these models of autoimmunity.40,41 
Human evidence is being sought to determine whether the inter­
action between the microbiome and innate immunity is lead­
ing to increased inflammation and setting the stage for increased 
frequencies of all autoimmune disorders that have recently been 
observed.42,43
Virus and other infections have been considered as a cause of 
autoimmunity and have be

considered as a cause of 
autoimmunity and have been shown to cause the development 
of type 1 diabetes when infection occurs in utero for rubella, for 
example. Examination of pancreata from individuals with predia­
betes (autoantibodies) or those with diabetes has provided evi­
dence consistent with viral infection of the organ.44,45 Association 
is not causality, and further experiments are needed to determine 
whether this finding is an important component of how either 
autoimmunity is init

tant component of how either 
autoimmunity is initiated or propagated.
Targeted immunologic therapies are also associated with the 
induction of autoimmunity. A remarkable example is the treat­
ment of patients with multiple sclerosis with an anti-CD52 
monoclonal antibody. One-third of 27 patients given the mono­
clonal antibody developed antithyrotropin receptor autoanti­
bodies and hyperthyroidism.46 Interferon-α (IFNα) therapy for 
hepatitis has been associated with thyroid autoimmunity47 an

has been associated with thyroid autoimmunity47 and 
type 1 diabetes.48 Severe hypoglycemia associated with insulin 
autoantibodies in the absence of insulin administration, termed 
Hirata disease, is associated with methimazole treatment of Graves 
disease. The development of Hirata disease in these patients is 
associated with HLA-Bw62/Cw4/DR4 with a specific DRB1 
allele (DRB1*0406).49 Immune checkpoint blockade is now an 
approved treatment approach for several cancers, and these anti­
bodie

pproach for several cancers, and these anti­
bodies that block CTLA4 or PD1 on T cells have both been 
associated with the induction of autoimmune complications in 
a subset of patients. These autoimmune complications are broad 
but now include the induction of endocrine-related autoimmu­
nity including thyroiditis, type 1 diabetes, lymphocytic hypophy­
sitis, and adrenalitis.50 
Development of Organ-Specific Autoimmunity
Autoantibodies highly specific for a given disorder are present 
before di

ecific for a given disorder are present 
before disease onset. Each specific autoantibody reacts with only 
a single autoantigen, although autoantigens may be present in 
multiple tissues. The targets of autoantibodies appear to be unre­
lated; however, for organ-specific autoimmunity, they are usually 
expressed in specific cells and cellular sites. Anti-islet antibodies 
include antibodies to glutamic acid decarboxylase (GAD); islet 
cell antibody (ICA) 512 (also termed insulinoma antigen 2 [I

ody (ICA) 512 (also termed insulinoma antigen 2 [IA2]); 
insulin; and, the most recently discovered, ZnT8.51 Celiac dis­
ease is associated with antibodies against tissue transglutamin­
ase (tTG). Addison disease is associated with antibodies against 
21-hydroxylase.
1663
Given that the antibodies can be identified before the develop­
ment of organ dysfunction, they can be used to screen subjects 
who are at high risk for development of autoimmune d

o are at high risk for development of autoimmune disease to 
identify risk for additional autoimmune diseases. This approach 
has been employed in studies such as TrialNet for type 1 diabetes 
to screen first-degree relatives of patients with type 1 diabetes for 
diabetes-related autoantibodies. In this and other cohorts, risk for 
development of diabetes increases with the number of autoanti­
bodies and their persistence.
Organ-specific autoantibodies (identified with appropriate 
assays) are r

bodies (identified with appropriate 
assays) are rarely present (approximately 1 in 100) in the general 
population and identify a subset of people who are at greater risk 
for clinical disease. These autoantibodies may be expressed for years 
before the disease develops, and additional autoantibodies can 
develop over time. The pace at which disease develops is highly vari­
able. For example, children as young as 1 year can present with type 
1 diabetes. In contrast, a subset of subjects (5–10%

diabetes. In contrast, a subset of subjects (5–10%) with type 2 dia­
betes diagnosed in adulthood have autoimmunity as the underlying 
cause. This may be due in part to genetics, because subjects who 
develop autoimmune diabetes at an older age have a higher propor­
tion of the protective diabetes allele DQB1*0602, although even in 
adults, DQB1*0602 provides dramatic protection.52
In contrast, less is known concerning the specificity of patho­
genic T cells. Given the observation that cross-rea

enic T cells. Given the observation that cross-reactive recognition 
by pathogenic T-cell clones may be determined by as few as four 
properly spaced amino acids of a nonapeptide and the estimate 
that each T-cell receptor might react with a million different pep­
tides, there is considerable potential for patterns of autoimmunity 
to be determined by cross-reactive T cells. An important develop­
ment has been the discovery in the thymus and other lymphoid 
tissues of peripheral antigen-expressi

r lymphoid 
tissues of peripheral antigen-expressing cells that express autoan­
tigens such as insulin. Minute quantities of such molecules in the 
thymus can contribute to tolerance. Insulin messenger RNA in 
the thymus is regulated by genetic polymorphisms of the insu­
lin gene associated with diabetes risk.3 There is also evidence that 
stromal and lymphoid cells (CD11c+) in the spleen, lymph nodes, 
and circulation express multiple similar antigens.53 
Failure of Gland
Organ dysfunction deve

tigens.53 
Organ dysfunction develops over time and can include a period 
of intermediate function that may be characterized by increased 
levels of the stimulatory hormones (e.g., thyroid-stimulating 
hormone, corticotropin [ACTH]) with normal levels of certain 
hormones (triiodothyronine, thyroxine, and cortisol). Once a sig­
nificant portion of the gland has been destroyed, overt disease is 
then present. 
Autoimmune Polyendocrine Syndrome Type I
Table 43.2 

rine Syndrome Type I
Table 43.2 compares the features of APS-I with those of APS-II. 
Table 43.3 shows the clinical features and recommended follow-
up for patients with APS-I. Note some of the distinctions in the 
pattern of disease features of the two syndromes, particularly the 
propensity for hypoparathyroidism and candidiasis in APS-I, 
which is virtually absent in APS-II. Likewise, celiac disease is fre­
quently observed in APS-II but is not seen in APS-I.
APS-I (Mendelia

n APS-II but is not seen in APS-I.
APS-I (Mendelian Inheritance in Man [MIM] 240300), also 
known as autoimmune polyendocrinopathy-candidiasis-ectodermal 
dystrophy, or APECED, is characterized by the classic triad of 
mucocutaneous candidiasis, autoimmune hypoparathyroidism, 
and Addison disease, which form three of the most common 
components of the disorder. Patients with APS-I are at risk for 
the development of autoimmune diseases affecting almost every 
organ. Multiple patient series have 

almost every 
organ. Multiple patient series have been reported, including sub­
jects in two large series from both Finland54–56 and the United 
States.57,58
In a series of 89 Finnish patients described by Perheentupa,54 all 
had chronic candidiasis at some time, 86% had hypoparathyroidism, 
and 79% had Addison disease. Gonadal failure (72% in women, 
26% in men) and hypoplasia of the dental enamel (77% of patients) 
were also frequent findings. Other manifestations that occurred less 
often inc

Other manifestations that occurred less 
often included alopecia (40%), vitiligo (26%), intestinal malab­
sorption (18%), type 1 diabetes (23%), pernicious anemia (31%), 
chronic active hepatitis (17%), and hypothyroidism (18%). The inci­
dence rates for many of these disorders peak in the first or second 
decade of life, and the disease continues to develop over decades (Fig. 
43.3). Therefore, reported prevalence rates of component disorders 
are highly dependent on the age at which follow-up 

re highly dependent on the age at which follow-up ended.
APS-I is characteristically recognized in early childhood. Infants 
can present with chronic or recurrent mucocutaneous candidiasis 
in the first year of life, followed by hypoparathyroidism and Addi­
son disease, but new components can develop at any age. Decades 
can elapse between the diagnosis of one disorder and the onset of 
another in the same patient. Consequently, lifelong follow-up is 
important to allow early detection of additi

p is 
important to allow early detection of additional components.
Recurrent candidiasis commonly affects the mouth and nails 
and, less frequently, the skin and esophagus.54 Chronic oral can­
didiasis can result in atrophic disease with areas suggestive of 
leukoplakia. If this develops, the patient is at significant risk for 
carcinoma of the oral mucosa (with its high mortality rate).
Ectodermal dystrophy is another component of the syndrome 
(manifested by pitted nails, keratopathy, and enam

(manifested by pitted nails, keratopathy, and enamel hypoplasia) 
and cannot be attributed to hypoparathyroidism. Enamel hypo­
plasia can precede the onset of hypoparathyroidism and, despite 
adequate replacement therapy, can also affect teeth forming after 
the onset of hypoparathyroidism.59
  Contrasting Features of Autoimmune 
Polyendocrine Syndrome
Feature
APS-I
APS-II
Inheritance pattern
Autosomal reces­
sive (only sib­
lings affected)
Polygenic (multiple 
generations affected)

genic (multiple 
Associated gene
AIRE gene 
mutation
HLA-DR3 and DR4 
Gender association
Equal gender 
incidence
Female preponderance
Age at onset
Onset in infancy
Peak onset 20–60 
years
Clinical features
Mucocutaneous 
candidiasis
Autoimmune thyroid 
disease
Diagnostic antibodies
Anti-interferon
APS, Autoimmune polyendocrine syndrome.
   
TABLE 43.2
1664
SECTION IX    Polyendocrine and Neoplasti

3.2
1664
  Clinical Features and Recommended Follow-Up for APS-I and APS-II
Component Disease
Frequency at Age 40 Years (%)
Recommended Evaluation
79
Sodium, potassium, ACTH, cortisol, plasma renin activity, 
21-hydroxylase autoantibodies
Diarrhea
18
History
Ectodermal dysplasia
50-75
86
Serum calcium, phosphate, PTH
Hepatitis
17
Liver function te

ium, phosphate, PTH
Hepatitis
17
Liver function test
18
TSH; thyroid peroxidase and/or thyroglobulin autoantibodies
Male hypogonadism
26
FSH/LH
Mucocutaneous candidiasis
100
Obstipation
21
History
Ovarian failure
72
FSH/LH
31
CBC, vitamin B12 levels
Splenic atrophy
15
Blood smear for Howell-Jolly bodies; platelet count; ultra­
sound if positive
23
Glucose, hemoglobin A1c, diabetes-associated autoantibod­
ies (insulin, GAD65, I

tes-associated autoantibod­
ies (insulin, GAD65, IA2)
Autoimmune Polyendocrine Syndrome Type IIa
0.5
21-Hydroxylase autoantibodies
ACTH stimulation testing if positive
Alopecia
Autoimmune hypothyroidism
15–30
5–10
Transglutaminase autoantibodies; small intestine biopsy if 
positive
Cerebellar ataxia
Rareb
Dictated by signs and symptoms of disease
Chronic inflammatory demyelinating poly

of disease
Chronic inflammatory demyelinating polyneuropathy
Rareb
Rareb
Idiopathic heart block
Rareb
IgA deficiency
0.5
IgA level
Myasthenia gravis
Rareb
Myocarditis
Rareb
0.5–5
Anti–parietal cell autoantibodies
CBC, vitamin B12 levels if positive
Serositis

ies
Serositis
Rareb
Stiff-man syndrome
Rareb
Vitiligo
1–9
aIn the population with type 1 diabetes.
bRare reported disorders in subjects with APS-II.
ACTH, Adrenocorticotropic hormone; APS, autoimmune polyendocrine syndrome; CBC, complete blood count; FSH, follicle-stimulating hormone; GAD, glutamic acid decarboxylase; IA2, insulinoma antigen 
2; IgA, immuno

rboxylase; IA2, insulinoma antigen 
2; IgA, immunoglobulin A; LH, luteinizing hormone; PTH, parathyroid hormone; TSH, thyroid-stimulating hormone.
   
TABLE 43.3
1665
Friedman and colleagues60 reported the frequent occurrence 
of asplenism and cholelithiasis as additional features of APS-I. 
Splenic atrophy may also cause immune deficiency. Although 
the cause of this part of the disorder is unknown, it is relatively 
common: up to 15% of patients a

 it is relatively 
common: up to 15% of patients are asplenic.54 The presence 
of Howell-Jolly bodies on peripheral blood smear is sugges­
tive of asplenia. If asplenia is identified, immunization with 
polyvalent pneumococcal vaccine should be administered, and 
follow-up antibody titers should be obtained. If an adequate 
response is not produced, daily prophylactic antibiotics may 
be necessary.
Malabsorption with steatorrhea is of uncertain origin, is usually 
intermittent, and may be exacer

rigin, is usually 
intermittent, and may be exacerbated by hypocalcemia. Bereket 
and associates61 reported a case in which patchy intestinal lym­
phangiectasia was discovered by endoscopically directed biopsy. 
Pancreatic insufficiency has been treated with cyclosporine.62 
Autoantibodies (e.g., tryptophan hydroxylase, histidine decarbox­
ylase) reacting with intestinal endocrine cells (enterochromaffin, 
cholecystokinin, and enterochromaffin like) occur and are associ­
ated with loss of endocr

ke) occur and are associ­
ated with loss of endocrine cells on biopsy and with gastrointesti­
nal dysfunction.63,64 
Genetics
APS-I is inherited in a classic mendelian autosomal recessive fash­
ion and is caused by mutations in the AIRE gene, located on the 
short arm of chromosome 21 (near markers D21s49 and D21s171 
on 21p22.3).65 The gene encodes a transcriptional regulator 
protein that is highly expressed in antigen-presenting epithelial 
cells in the thymus and in a small subset of cells i

lls in the thymus and in a small subset of cells in lymphoid tis­
sues.66,67 It has been localized to the nucleus, and mutations have 
been demonstrated to be associated with decreased transcription 
of reporter products.68,69 A mouse model of APS-I has been gener­
ated, and Aire knockout mice also spontaneously develop autoim­
mune features. Detailed analyses of antigen-presenting epithelial 
cells in the thymus of Aire knockout mice has shown that these 
cells have decreased expression of peri

hat these 
cells have decreased expression of peripheral tissue-specific self-anti­
gens (TSAs) and that Aire promotes the expression of thousands 
of such self-antigens in these cells.66 Furthermore, autoreactive 
T cells with specificity to these TSAs can escape thymic deletion 
when Aire is defective and promote autoimmunity.70–72 Thus, 
Aire appears to act as a transcriptional activator that promotes the 
expression of a wide array of TSAs in the thymus to promote cen­
tral tolerance (Fig. 4

 the thymus to promote cen­
tral tolerance (Fig. 43.4).
Multiple mutations of AIRE have been identified in subjects 
with APS-I. The frequency of specific mutations varies in different 
populations. For example, in Sardinia, a deletion of amino acid 
257 is present in 90% of mutated alleles. A 136–base pair dele­
tion in exon 8 is present in 71% of British alleles and in 56% of 
alleles in the United States. Analysis of haplotypes indicates that 
this deletion has arisen on many occasions.
Most 

Most cases of APS-I are associated with autosomal recessive 
inheritance patterns; however, there are now reports of autosomal 
dominant inheritance. A rare kindred of patients from Italy have 
a mutation in the SAND domain of the AIRE protein (G228W) 
Age
Ovarian atrophy
Cases/patient year
0.25
0.2
0.15
0.1
0.05
0
40
30
20
10
0
• Fig. 43.3  Incidence of disease development

10
0
• Fig. 43.3  Incidence of disease development by age in patients with auto­
immune polyendocrine syndrome type I (APS-I). (From Perheentupa J. 
APS-I/APECED: the clinical disease and therapy. Endocrinol Metab Clin 
North Am. 2002;31:295–320.)
TSAs
Autoreactive
T cell
Thymic medullary epithelium
MHC
TCR
AIRE
Deletion
Tolerance
• Fig. 43.4  The autoimmune regulator (AIRE) gene functions to maintain tolerance by promoting the display 
of self-antigens in the thymus. An AIRE-expressing thymic m

ntigens in the thymus. An AIRE-expressing thymic medullary epithelial cell is shown (left). AIRE 
operates in the nucleus to promote the expression of thousands of different tissue-specific self-antigens 
(TSAs) that include many endocrine organ-specific proteins such as insulin. Peptide fragments of these 
TSAs are displayed on major histocompatibility complex (MHC) molecules to help promote the deletion of 
autoreactive T cells (right) that can be generated in the thymus from random gene rearr

 be generated in the thymus from random gene rearrangement of the 
T-cell receptor (TCR). Such T cells are deleted when they encounter their antigen-MHC in the thymus, and 
thus immune tolerance is maintained.
1666
that when present in the heterozygous state is associated with 
autoimmunity, thus fitting an autosomal dominant inheritance 
pattern.73 In a mouse model of this mutation, the expression of 
TSAs in the thymus was decreased compared

sion of 
TSAs in the thymus was decreased compared with that inthe con­
trol model, suggesting a mechanism for this observed inheritance 
pattern.74 In addition, point mutations in the PHD1 domain of 
AIRE have now been associated with a dominant inheritance pat­
tern.75 In all cases, the molecular explanation could be related to 
the observation that AIRE forms multimers through its N-termi­
nal domain, and thus mutations in other domains could still allow 
for multimerization with a wild-type 

still allow 
for multimerization with a wild-type copy of AIRE and a domi­
nant-negative effect in AIRE activity. The autoimmune phenotype 
in this group of patients appears to be milder with the develop­
ment of fewer autoimmune features or even a single disease feature 
that likely reflects partial AIRE activity in these patients. 
Diagnosis
The diagnosis of APS-I is highly likely when two or more of the 
primary component disorders (i.e., mucocutaneous candidiasis, 
hypoparathyroidism, and Ad

cutaneous candidiasis, 
hypoparathyroidism, and Addison disease) are present. Siblings 
of an affected patient should be considered affected even if only 
one of these disorders is present. Sequence analysis of AIRE for 
mutations may be helpful in identifying subjects with APS-I. Any 
patient with any of the component disorders deserves careful fol­
low-up to watch for the development of additional disease.
Autoantibodies against IFNα and IFNω have been identified 
in almost 100% of subjects wi

ave been identified 
in almost 100% of subjects with APS-I, regardless of age at screen­
ing.76 The autoantibody has been found in subjects with many 
different mutations of the AIRE gene; it is not present in other 
autoimmune diseases, and it has been proposed to use this autoan­
tibody to screen subjects with multiple autoimmune diseases for 
APS-I. It is unclear if antibodies to these type 1 IFNs are of any 
clinical consequence, as APS-I subjects do not have an increased 
propensity for vir

jects do not have an increased 
propensity for viral infections.54 In contrast, it has also been shown 
that many APS-I subjects also harbor autoantibodies to IL17A, 
IL17F, and IL22.77,78 These are key cytokines for the function of 
the Th17 T-cell subset, and recent work has demonstrated that 
loss of function of these cytokines, as in patients with mutations 
in the IL17 receptor, is associated with increased susceptibility to 
Candida albicans infections.79 Thus, it appears that the Candida 

s infections.79 Thus, it appears that the Candida 
susceptibility in APS-I may be due to an autoimmune response 
against this effector cytokine family. 
Therapy and Follow-Up
The treatment of adrenal insufficiency and hypoparathyroidism is 
the same as that discussed in other chapters with the caveat that 
malabsorption can complicate treatment. The therapy for muco­
cutaneous candidiasis has been improved with the use of orally 
active antifungal drugs such as fluconazole and ketoconazole. 
Inf

l drugs such as fluconazole and ketoconazole. 
Infection often recurs when the drug is discontinued or the dosage 
is decreased. Patients must be monitored carefully, because keto­
conazole can inhibit adrenal and gonadal steroid synthesis and can 
precipitate adrenal failure. It is also associated with transient eleva­
tion of liver enzyme levels and, occasionally, with hepatitis. Fluco­
nazole is associated with a lower frequency of hepatitis and does 
not inhibit steroidogenesis when given in

nd does 
not inhibit steroidogenesis when given in the recommended doses.
Autoantibodies associated with multiple autoimmune diseases 
have been reported. Antiparathyroid and antiadrenal antibodies 
have been reported. Whereas 21-hydroxylase appears to be the major 
autoantigen in isolated Addison disease and in Addison disease 
associated with APS-II, autoantibodies against 17α-hydroxylase and 
cytochrome P450 side-chain cleavage enzyme (CYP11A1) have also 
been reported in Addison disease asso

) have also 
been reported in Addison disease associated with APS-I.80 There 
have been reports of antibodies to tryptophan hydroxylase in intes­
tinal disease, tyrosine hydroxylase in alopecia areata, l-amino acid 
decarboxylase in hepatitis and vitiligo, and phenylalanine hydroxy­
lase,81,82 and antibodies reacting with hair follicles.83 Tuomi and 
coworkers84 originally observed that many more APS-I patients 
(41%) express anti-GAD65 autoantibodies than become diabetic, 
suggesting that react

odies than become diabetic, 
suggesting that reactivity to this single autoantigen has low predic­
tive value in this population. Recently, antibodies against NALP5 
were identified in 49% of patients with APS-I and hypoparathyroid­
ism compared with none of those with APS-I but no hypoparathy­
roidism.85 Screening for autoantibodies associated with additional 
autoimmune diseases may be useful in patients with APS-I.
Screening to allow the early detection of new disorders before 
overt symptoms

 detection of new disorders before 
overt symptoms and signs develop is recommended, including 
autoantibody studies, electrolytes, calcium and phosphorus levels, 
thyroid and liver function tests, blood smear, and plasma vitamin 
B12. Patients at risk for adrenal failure can be screened by measure­
ment of basal ACTH and supine plasma renin activity, followed 
by dynamic testing as appropriate. Evaluation for asplenism60 
with abdominal ultrasonography and blood smear examination 
for Howell-Jo

ography and blood smear examination 
for Howell-Jolly bodies is warranted, with pneumococcal vaccina­
tion and appropriate antibiotic coverage for affected patients.
There are case reports of severely affected patients who have 
benefited from immunosuppressive therapy. For example, Ward 
and colleagues62 treated a 13-year-old patient who had kerato­
conjunctivitis, hepatitis, and severe pancreatic insufficiency. Treat­
ment with cyclosporine was associated with normalization of stool 
fat (from

 associated with normalization of stool 
fat (from 31.5 to 2.5 g/day). 
Autoimmune Polyendocrine Syndrome Type II
APS-II (MIM 269200) is defined by the occurrence in the same 
patient of two or more of the following: primary adrenal insuffi­
ciency (Addison disease), Graves disease, autoimmune thyroiditis, 
type 1A diabetes, primary hypogonadism, myasthenia gravis, and 
celiac disease. Vitiligo, alopecia, serositis, and pernicious anemia 
also occur with increased frequency in 

us anemia 
also occur with increased frequency in patients with this syndrome 
and in their family members (see Table 43.3). APS-II is more 
common than APS-I. It occurs more often in female than in male 
patients, often has its onset in adulthood, and exhibits familial 
aggregation (see Table 43.2).
When one of the component disorders is present, an associated 
disorder occurs more commonly than in the general population. 
Furthermore, circulating organ-specific autoantibodies are often 
presen

ng organ-specific autoantibodies are often 
present even in the absence of overt clinical disease. For example, 
in subjects with type 1 diabetes, there is a 15% to 20% risk of 
hypothyroidism, a 5% to 10% risk of celiac-related autoimmu­
nity, and a 1.5% risk of adrenal autoimmunity. The risk of auto­
immunity is greater in relatives of patients with APS-II. In our 
assessment of APS-II families with Addison disease, up to 15% of 
relatives were found to have 21-hydroxylase autoantibodies (Addi

 found to have 21-hydroxylase autoantibodies (Addi­
son disease–associated autoantibodies), anti-islet autoantibodies, 
or tTG autoantibodies (celiac disease–associated autoantibod­
ies). The initial lesion and precipitating events that result in the 
syndrome are unknown, but immunogenetic and immunologic 
similarities are present with regard to both the time course and the 
pathogenesis of each of the component disorders.
1667
Because of the chron

mmunoendocrinopathy Syndromes
Because of the chronic development of organ-specific auto­
immunity, patients with the syndrome and their families should 
have repeated endocrinologic evaluations over time. In a family 
in which the syndrome has been documented, relatives should be 
advised of the early symptoms and signs of the principal compo­
nent diseases (a list is available from the Barbara Davis Center for 
Diabetes website86). Relatives of patients with multiple disorders 
should have a me

patients with multiple disorders 
should have a medical history, physical examination, and screening 
every 3 to 5 years, with measurement of anti-islet autoantibodies, 
a sensitive thyrotropin assay, and measurement of serum vitamin 
B12 levels. If there are any symptoms or signs, or if 21-hydroxylase 
autoantibodies are present, the patient should have annual assays 
of basal ACTH levels with Cortrosyn stimulation testing.
Among 224 patients with Addison disease and APS-II reported 
by Neufeld

th Addison disease and APS-II reported 
by Neufeld and colleagues,57 type 1 diabetes and autoimmune thy­
roid disease were the most common coexisting conditions (52% 
and 69% of patients, respectively). Other components were less 
common, including vitiligo (5%) and gonadal failure (4%). In an 
Italian cohort of more than 600 patients, the majority of patients 
with Addison disease (86.7%, after exclusion of subjects with 
APS-I) had additional autoimmunity diseases, most frequently 
thyroid dis

utoimmunity diseases, most frequently 
thyroid disease (56%) and diabetes (12%). A significant propor­
tion of subjects exhibited thyroid- (16%) and diabetes- (8.8%) 
related autoimmunity without yet developing disease over average 
follow-up of 10 years.87 Therefore, careful monitoring of patients 
with Addison disease for additional autoimmune diseases includ­
ing thyroid, type 1A diabetes, and premature ovarian insufficiency 
Among patients with type 1A diabetes, thyroid autoimm

ng patients with type 1A diabetes, thyroid autoimmu­
nity and celiac disease coexist with sufficient frequency to justify 
screening. Thyroid peroxidase autoantibodies are present in 10% 
to 20% of children with type 1 diabetes; this incidence is higher 
in female patients and increases in all patients with age and with 
diabetes duration. A significant fraction of patients with type 1 
diabetes and thyroid peroxidase autoantibodies develop thyroid 
disease. One study showed that after follow-up

id 
disease. One study showed that after follow-up for more than 15 
years, 80% of patients with thyroid peroxidase autoantibodies and 
type 1 diabetes became hypothyroid.88 However, several studies 
have shown that a subset of patients with negative autoantibodies 
develop thyroid disease. Therefore, patients with type 1 diabetes 
should be screened annually for thyrotropin levels, which is a cost-
effective approach.
With the identification of tTG as the major endomysial auto­
antigen of celia

tTG as the major endomysial auto­
antigen of celiac disease, radioimmunoassays were developed and 
demonstrated that 10% to 12% of patients with type 1 diabetes 
have tTG autoantibodies.7 The prevalence of tTG autoantibod­
ies was higher in diabetic patients with HLA-DQ2; one-third of 
DQ2-homozygous subjects were found to express anti-tTG anti­
body, reflective of the influence of HLA-DQ2 in the development 
of both type 1 diabetes and celiac disease. Seventy percent of those 
with high-titer a

sease. Seventy percent of those 
with high-titer antibody who underwent biopsy were subsequently 
found to have the disease.89 Celiac disease may be identified at 
onset of type 1 diabetes. However, it can develop in patients with 
long-standing diabetes as well and in at least one cohort has been 
shown to be increased in patients with adult-onset type 1 diabetes 
compared with childhood onset.90 If left untreated, symptomatic 
celiac disease is also associated with an increased risk of gastro­

 also associated with an increased risk of gastro­
intestinal malignancy, especially lymphoma. Survival analysis in 
patients with type 1 diabetes and celiac disease shows an increase 
in mortality rate in patients with type 1 diabetes and celiac dis­
ease duration of greater than 15 years.91 Frequency and method 
of screening remain areas of debate. At the very least, providers 
should be aware of the association and have a low threshold for 
screening with tTG antibodies. If the results are po

reening with tTG antibodies. If the results are positive and are 
confirmed on repeat assay, small bowel biopsy to document celiac 
disease is warranted, with institution of a gluten-free diet if the 
disease is present. Many patients have asymptomatic celiac dis­
ease that is nevertheless associated with osteopenia and impaired 
growth. 
Diagnosis
The diagnosis of APS-II requires an understanding of the risk for 
additional autoimmune diseases in patients with a single auto­
immune endocrine di

n patients with a single auto­
immune endocrine disorder. A thorough history and physical 
examination may identify symptoms or signs of an additional 
autoimmune disorder. Screening for disease can include screening 
for markers of the autoimmune diseases (organ-specific autoanti­
bodies) and assays of glandular function (e.g., thyrotropin levels).
Improved assays for several organ-specific autoantibodies have 
been developed since the cloning of specific autoantigens and 
the development of as

f specific autoantigens and 
the development of assays that use recombinant antigens. These 
radioimmunoassays are superior to assays based on immuno­
fluorescence with tissue sections, such as ICA testing. The most 
notable finding is the identification of multiple autoantigens tar­
geted even in single autoimmune disorders. Most of the endocrine 
autoantigens are hormones (e.g., insulin) or enzymes associated 
with differentiated endocrine function: thyroid peroxidase in thy­
roiditis; GAD, ca

tion: thyroid peroxidase in thy­
roiditis; GAD, carboxypeptidase H, and ICA 512/IA2 in type 1 
diabetes; 17α-hydroxylase and 21-hydroxylase in Addison disease; 
and the parietal cell enzyme H+/K+-adenosine triphosphatase in 
pernicious anemia.
In type 1 diabetes, the four most informative assays identify 
autoantibodies reacting with insulin, GAD65, ICA512/IA2, and 
ZnT8.51 Similarly, a radioassay for autoantibodies against the 
enzyme 21-hydroxylase in Addison disease has been developed 
and pr

lase in Addison disease has been developed 
and provides excellent disease specificity and sensitivity. Adre­
nal autoantibodies reacting with recombinant 21-hydroxylase 
usually precede the development of Addison disease. Screening 
with 21-hydroxylase autoantibodies may identify patients with 
antibodies but have normal production of cortisol in response to 
ACTH. Annual screening with a basal level of corticotropin with 
follow-up cosyntropin stimulation testing is an effective strategy 
for 

stimulation testing is an effective strategy 
for identifying adrenal insufficiency in patients with 21-hydroxy­
lase autoantibodies.
Typically, autoantibodies mark the presence of or risk for auto­
immune disease caused by T-cell–mediated glandular destruction. 
However, autoantibodies may also be pathogenic. A hallmark of 
pathogenic autoantibodies is the existence of a neonatal form of 
the disorder, secondary to transplacental passage of the autoan­
tibody. Examples include neonatal Graves d

autoan­
tibody. Examples include neonatal Graves disease (anti–thyro­
tropin-receptor autoantibodies) and neonatal myasthenia gravis 
(anti–acetylcholine-receptor autoantibodies). 
Therapy
Treatment of the individual diseases of the polyendocrine autoim­
mune syndrome is discussed in other chapters. Therapeutic con­
siderations related specifically to APS-II are discussed here.
Many of the component disorders of the syndrome have a long 
prodromal phase and are associated with the expression of 

l phase and are associated with the expression of auto­
antibodies before the manifestation of overt disease. Therefore, 
people at risk for autoimmune diseases can be identified prior 
to the clinical onset of disease, and if the right treatment were 
1668
identified, disease may be preventable. This is particularly impor­
tant for type 1A diabetes but is also likely to apply to Addison 
disease and hypogonadism.
Prevention strategies have be

se and hypogonadism.
Prevention strategies have been extensively evaluated in patients 
with type 1 diabetes. Prevention of disease can occur at multiple 
time points along the natural history of evolving beta-cell dys­
function. Treatments targeted at patients with genetic risk with­
out evidence for autoimmunity will expose people who will never 
get disease to an intervention (primary prevention). Therefore, 
any intervention must be safe and easily administered. Preven­
tion trials in patien

easily administered. Preven­
tion trials in patients with diabetes-related autoantibodies prior to 
abnormalities of glucose metabolism (secondary prevention) can 
employ agents with higher risks of adverse events. Tertiary pre­
vention of diabetes employs immune-modulating agents for the 
preservation of the beta-cell mass with the hopes of inducing a 
prolonged C-peptide production.
Primary prevention of type 1 diabetes targets infants at the 
highest risk for the development of type 1 diabete

highest risk for the development of type 1 diabetes on the basis of 
family history or specific HLA genotypes. Infants must be negative 
for diabetes-related autoantibodies. Interventions have focused on 
dietary manipulations including gluten-free diets, docosahexae­
noic acid (DHA) supplementation, and elemental formulas lack­
ing bovine insulin. To date, the interventions have not prevented 
the development of diabetes-related autoantibodies or diabetes in 
large-scale clinical trials.92
Seco

r diabetes in 
Secondary prevention of type 1 diabetes has been antigen 
specific (insulin and GAD65) and antigen nonspecific (nicotin­
amide). These studies have targeted subjects with positive diabe­
tes-related autoantibodies prior to the development of diabetes. 
Some studies have included patients with abnormalities of glu­
cose metabolism such as impaired first-phase insulin response and 
A large National Institutes of Health trial

rance.
A large National Institutes of Health trial—the Diabetes Pre­
vention Trial–Type 1 (DPT-1)—directly tested oral and parenteral 
insulin for prevention of diabetes. DPT-1 had two arms: intrave­
nous-subcutaneous for those at high risk (risk of diabetes >50% 
within 5 years) and oral insulin for those at moderate risk (risk of 
diabetes 25–50% within 5 years). Neither parenteral93 nor oral94 
insulin slowed progression to diabetes. However, in a subgroup 
analysis of subjects in the DPT-1 o

in a subgroup 
analysis of subjects in the DPT-1 oral trial, a treatment effect was 
noted for those who had the higher insulin autoantibody levels at 
diagnosis,94 and further trials are under way. In preclinical Addi­
son disease, a short course of glucocorticoids appeared to suppress 
the expression of adrenal autoantibodies and prevent progressive 
adrenal destruction.95 Studies of immune-modulating agents 
(abatacept and teplizumab) in patients with diabetes-related auto­
antibodies and abn

nts with diabetes-related auto­
antibodies and abnormalities of glucose metabolism are currently 
under way.
Because of the autoimmune nature of these disorders, sev­
eral studies have evaluated the use of immunosuppressive and 
immune-modulating drugs. Drugs such as cyclosporine have pre­
served some residual insulin secretion. However, because cyclospo­
rine is nephrotoxic and potentially oncogenic, more generalized 
use is precluded. Newer immunosuppressive agents (e.g., siroli­
mus) are bein

unosuppressive agents (e.g., siroli­
mus) are being studied, and biologics such as anti-CD20 anti­
body (rituximab), abatacept, and nonmitogenic CD3 antibodies 
have been shown to prolong C-peptide production and to result 
in a decreased insulin dose through the first year of diabetes when 
compared with control subjects.96–100
Thyroxine therapy can precipitate a life-threatening addiso­
nian crisis in a patient with untreated adrenal insufficiency and 
hypothyroidism. Therefore, it is necessar

ncy and 
hypothyroidism. Therefore, it is necessary to evaluate adrenal 
function in all hypothyroid patients in whom the syndrome is 
suspected before instituting such therapy. A decreasing insulin 
requirement in a patient with insulin-dependent diabetes can 
be one of the earliest indications of adrenal insufficiency, occur­
ring before the development of hyperpigmentation or electrolyte 
Other Polyendocrine Deficiency 
Autoimmune Syndromes
Rare polyendocrine syndromes are lis

une Syndromes
Rare polyendocrine syndromes are listed in Table 43.4.
Immunodysregulation Polyendocrinopathy 
Enteropathy X-Linked Syndrome
IPEX (MIM 340790, MIM 300292), first described in 1982, is a 
rare, X-linked recessive disorder that is characterized by immune 
dysregulation and results in multiple autoimmune diseases and 
early death (see Fig. 43.4). It is caused by mutations of the FOXP3 
gene. Clinical characteristics include very early onset type 1A 
diabetes, severe enteropathy result

onset type 1A 
diabetes, severe enteropathy resulting in failure to thrive, and 
dermatitis, generally resulting in death within the first couple of 
years of life unless definitive treatment is pursued. Other reported 
abnormalities include atopy, thrombocytopenia, hemolytic ane­
mia, hypothyroidism, lymphadenopathy, nephropathy, and alope­
cia (2012 review).101 Immunologic evaluation shows elevated IgE 
and eosinophilia in addition to the abnormalities associated with 
the component disorders.

malities associated with 
The identification of FOXP3 as the gene associated with IPEX 
was aided by the scurfy mouse model. The scurfy mouse exhib­
its characteristics similar to those seen in children affected with 
IPEX. The gene associated with this disorder in the scurfy mouse 
was found to be a DNA-binding protein with characteristics of 
a transcription factor.102 Clues about the function of this gene 
included the observations that implantation of thymus from a 

 observations that implantation of thymus from a 
scurfy mouse into immune-incompetent mice transfers the dis­
ease, but transplantation of thymus into immune-competent 
mice does not transfer disease, and injections of normal T cells 
can rescue the phenotype, suggesting that a regulatory cell is 
involved in the pathogenesis of this disorder. Linkage analysis 
demonstrated that a 17-centimorgan stretch of the X chromo­
some (Xp11.1-q13.3) is associated with IPEX, and mutations 
within the FOXP

sociated with IPEX, and mutations 
within the FOXP3 gene have been identified in most of the 
families studied thus far.103 FOXP3 encodes a protein that is 
a member of the forkhead class of winged helix transcription 
factors (forkhead box protein 3). FOXP3 has been shown to be 
expressed in CD4+/CD25+ regulatory T cells.104 These T cells 
can suppress activation of other T cells.104 Therefore, mutations 
in FOXP3 result in inability to generate regulatory T cells and 
the development of IPEX. 

 regulatory T cells and 
the development of IPEX. Patients with mutations throughout 
the FOXP3 gene have been described. In general, milder phe­
notypes are associated with point mutations or small deletions 
that do not result in a total lack of FOXP3. However, significant 
variability in phenotype within a single mutation suggests that 
other genes or environmental exposures may play an important 
role in the development of the disorder.
Treatment options are targeted to the underlying disord

ment options are targeted to the underlying disorders. 
At the time of diagnosis, infants may be so affected by the 
enteropathy that they require bowel rest and parenteral nutri­
tion. Diabetes is managed with insulin therapy. Immunosup­
pression with calcineurin inhibitors has been used in the initial, 
1669
stabilization phase of treatment. Definitive treatment is with 
hematopoietic stem cell transplantation (HSCT). HSCT results 
in reversal of 

nsplantation (HSCT). HSCT results 
in reversal of the enteropathy and other autoimmune compo­
nents.105 Generally, established type 1 diabetes and thyroid dis­
ease do not resolve. However, some reports document reversal of 
type 1 diabetes in patients who have received the conditioning 
immunotherapy and HSCT.106 An additional case report shows 
that reconstitution of the gut immune system may occur later 
than reconstitution of the bone marrow.107 Novel approaches 
to insert a functional FOXP3

107 Novel approaches 
to insert a functional FOXP3 gene into lymphocytes as gene 
therapy in addition to or eventually in place of HSCT are being 
developed as well.108,109
Over the past several years, disorders with similar clinical 
characteristics as IPEX but without mutations in FOXP3 have 
been identified. In these disorders, mutations in IL2RA,110,111 
STAT5b,112 STAT-1,113 and ITCH114 result in abnormalities of 
regulatory T-cell function, emphasizing the important role of 
these cell pop

 emphasizing the important role of 
these cell populations in the maintenance of immune tolerance. 
CTLA4, STAT3, and LRBA Mutations
With recent advances in whole genome sequencing, there has been 
rapid progress in associating rare genetic variants in isolated patients 
or kindreds with multiorgan autoimmunity that often involves 
endocrinopathy. Loss of function mutations in heterozygosity in 
the CTLA4 gene have now been associated with autoimmunity in 
a few isolated kindreds and appears to 

munity in 
a few isolated kindreds and appears to be associated with a defect in 
T-regulatory cell function.115,116 However, it should be noted that 
the autoimmunity associated with these mutations is much milder 
than IPEX. A similar syndrome has also been recently described in 
patients with heterozygous mutations in the LRBA gene.117 Here, 
the mechanism is less clear but may involve proper intracellular traf­
ficking of CTLA4 in T cells. Finally, dominant activating mutations 
in the STAT3

nally, dominant activating mutations 
in the STAT3 gene have now been associated with an autoimmune 
syndrome in isolated cases and several families.118,119 Here, patients 
frequently develop thyroiditis, cytopenias, and type 1 diabetes, 
along with other unusual features including short stature. The exact 
  Rare Polyendocrine Disorders
Disorder
Cause
Hirata disease (insulin resistance syndrome)
Insulin autoantibodies
Associated with methimazole
IPEX or 
CD25 def

dies
IPEX or 
CD25 deficiency
Mutations of FOXP3 for IPEX
Mutations of IL2RA for CD25-deficiency
Enteropathy
Autoimmune lymphoproliferative syndrome type V
Mutations of CTLA4 resulting in haploinsufficiency
Autoimmune cytopenias
Hypogammaglobulinemia
Infantile-onset autoimmune disease 1
Dominant activating mutations of STAT3 in heterozygosity
Short stature
Kearns-Sayre 

iditis
Kearns-Sayre syndrome
Primary gonadal failure
Deletions of mitochondrial DNA
Nonautoimmune diabetes
POEMS
Plasma cell dyscrasia with production of M protein and 
cytokines
Organomegaly
Diabetes
Thymic tumors
Thymomas
Red blood cell hypoglobulinemia
Autoimmune thyroid disease
Type B insulin resistance
Severe insulin resistance
Insulin receptor auto

ce
Insulin receptor autoantibodies
Wolfram syndrome
Mutations of WSF1, which encodes wolframin
Bilateral optic atrophy
Sensorineural deafness
IPEX, Immunodysregulation polyendocrinopathy enteropathy X-linked; POEMS, plasma cell dyscrasia with polyneuropathy, organomegaly, endocrinopathy, m protein, and skin changes.
   
TABLE 43.4
1670
mechanism of how these mutations lead to 

isorders
mechanism of how these mutations lead to autoimmunity in these 
subjects is not clear but could involve improper activation of T cells, 
as STAT3 is a known transducer of effector cytokines. 
Anti–Insulin Receptor Autoantibodies
In this rarely reported disorder (<100 patients), also called type B 
insulin resistance or acanthosis nigricans, insulin resistance is caused 
by the presence of anti–insulin receptor antibodies and anti-insu­
lin antibodies.120 Approximately one-third of patie

in antibodies.120 Approximately one-third of patients with these 
antibodies have an associated autoimmune illness such as SLE or 
Sjögren syndrome. Arthralgia, vitiligo, alopecia, autoimmune thy­
roid disease, secondary amenorrhea, and family history of auto­
immunity have also been reported. Autoimmune thyroid disease 
has been described in two such patients, one with hypothyroidism 
and the other with antithyroid antibodies. Antinuclear antibodies, 
an elevated erythrocyte sedimentation rate,

dies, 
an elevated erythrocyte sedimentation rate, hyperglobulinemia, 
leukopenia, and hypocomplementemia are common.121
The major clinical manifestations are related to the anti–insu­
lin receptor antibodies. Insulin resistance is profound, and up to 
175,000 U of insulin given intravenously per day may be ineffec­
tive in lowering the elevated glucose. Despite hyperglycemia and 
marked insulin resistance, ketoacidosis is uncommon. The course 
of the diabetes is variable, and several patients h

f the diabetes is variable, and several patients have had sponta­
neous remissions. Other patients have had severe hypoglycemia 
(perhaps related to the insulin-like effects of anti–insulin recep­
tor antibodies demonstrable in vitro).122 The acanthosis nigricans, 
which is caused by hypertrophy and folding of otherwise histo­
logically normal skin, appears to be related to the insulin-resistant 
state. Other forms of marked insulin resistance in the absence of 
antireceptor antibodies are also 

 the absence of 
antireceptor antibodies are also associated with acanthosis nigri­
cans. A treatment regimen developed at the National Institutes of 
Health that includes rituximab that targets B lymphocytes with 
cyclophosphamide and pulse corticosteroids has been used suc­
cessfully to treat this rare disorder.122,123 
POEMS Syndrome
The components of the multisystem disorder POEMS (plasma cell 
dyscrasia with polyneuropathy, organomegaly, endocrinopathy, 
monoclonal plasma cell disorder, and

ocrinopathy, 
monoclonal plasma cell disorder, and skin changes), also known as 
Crow-Fukase syndrome (MIM 192240), consist of diabetes mel­
litus (3–36% of patients), primary gonadal failure (55–89% of 
patients), plasma cell dyscrasia, sclerotic bone lesions, and neu­
ropathy.124 Patients usually present with severe progressive senso­
rimotor polyneuropathy, hepatosplenomegaly, lymphadenopathy, 
and hyperpigmentation. On evaluation, they are found to have 
plasma cell dyscrasia and sclerotic b

und to have 
plasma cell dyscrasia and sclerotic bone lesions. Patients present in 
the fifth to sixth decades of life and have a median survival time 
after diagnosis of 14 years.124,125
The pathophysiology of POEMS is poorly understood. There 
is evidence implicating cytokines such as IL1A, IL6, and TNFα in 
addition to the M protein in the pathogenesis of this disorder. In 
several studies, elevated levels of vascular endothelial growth fac­
tor (VEGF) correlated with the disease state, and t

or (VEGF) correlated with the disease state, and treatment with 
immunosuppressive agents reduced the symptoms of the disease 
and the levels of VEGF, suggesting that this growth factor plays a 
role in the disease.126,127 A therapeutic trial of an anti-VEGF anti­
body would provide more definitive evidence for this hypothesis.
Algorithms have been proposed for the treatment of 
POEMS.125 Important features of treatment include extensive 
baseline evaluation, ongoing monitoring for component dis

e evaluation, ongoing monitoring for component disor­
ders, systemic therapy for the plasma cell disorder, and radiation at 
the site of identified bone lesions. The diabetes mellitus responds 
to small, subcutaneous doses of insulin. 
Kearns-Sayre Syndrome
The rare Kearns-Sayre syndrome (MIM 530000), also known as 
oculocraniosomatic disease or oculocraniosomatic neuromuscu­
lar disease with ragged red fibers, is characterized by myopathic 
abnormalities leading to ophthalmoplegia and progressi

rmalities leading to ophthalmoplegia and progressive weak­
ness in association with several endocrine abnormalities, including 
hypoparathyroidism, primary gonadal failure, diabetes mellitus, 
and hypopituitarism.128 Crystalline mitochondrial inclusions 
are found in muscle biopsy specimens, and such inclusions have 
also been observed in the cerebellum. The relation between the 
mitochondrial disorders and endocrinologic abnormalities is not 
known. Antiparathyroid antibodies have not been desc

own. Antiparathyroid antibodies have not been described; how­
ever, antibodies to the anterior pituitary gland and striated muscle 
have been found, and the disease may have autoimmune compo­
nents. Other abnormalities include retinitis pigmentosa and heart 
block. Deletions in mitochondrial DNA have been associated 
with Kearns-Sayre syndrome.129 These mutations usually occur 
sporadically and are not associated with a familial syndrome. 
Thymic Tumors
The thymus is a complex tissue with a spec

 Tumors
The thymus is a complex tissue with a specialized endocrine epi­
thelium that synthesizes a variety of biologically active peptides 
involved in the control of T-cell maturation. This epithelium is 
derived from the neural crest and contains complex gangliosides 
that react with monoclonal antibody (A2B5) and tetanus toxin in 
a manner similar to that of pancreatic islets.
The illnesses associated with thymomas are similar to those seen in 
APS-II,130 although the incidence of specific d

n 
APS-II,130 although the incidence of specific disorders is different. In 
one review of patients with thymoma, myasthenia gravis was found to 
occur in 44% of the patients, red blood cell aplasia in approximately 
20%, hypoglobulinemia in 6%, autoimmune thyroid disease in 2%, 
and adrenal insufficiency in 1 of 423 patients (0.24%). The incidence 
of autoimmune thyroid disease reported in patients with thymoma 
is probably an underestimate, given the incidence of unsuspected 
thyroid disease i

en the incidence of unsuspected 
thyroid disease in patients with myasthenia gravis. Mucocutaneous 
candidiasis in adults is also associated with thymomas. Interestingly, 
recent work suggests that an alteration in the proper expression of 
AIRE and peripheral antigens may be part of the explanation of the 
autoimmunity that arises in these patients.131 
Wolfram Syndrome
Wolfram syndrome (MIM 222300, chromosome 4; MIM 598500, 
mitochondrial) is a rare autosomal recessive disease that is also cal

 rare autosomal recessive disease that is also called 
DIDMOAD (diabetes insipidus, diabetes mellitus, progressive bilat­
eral optic atrophy, and sensorineural deafness). In addition, neuro­
logic and psychiatric disturbances are prominent in most patients 
and can cause severe disability. Atrophic changes in the brain have 
been found on magnetic resonance imaging.132 Segregation analysis 
of the mutations found in familial and sporadic cases of Wolfram 
syndrome led to the identification of wo

 Wolfram 
syndrome led to the identification of wolframin, a 100-kDa trans­
membrane protein encoded by WFS1, a gene located at 4p16.1.116. 
Genotype and phenotype analyses have identified the severe pheno­
type (defined as the development of neurologic disease within the 
1671
first decade) in patients with truncated proteins and mutations in 
the carboxy-terminus of the protein.133
Wolframin has been localized to the endoplasmic reticulum134 
and 

en localized to the endoplasmic reticulum134 
and is found in neuronal and neuroendocrine tissue.135 Its expres­
sion induces ion channel activity with a resultant increase in 
intracellular calcium and may play an important role in intracel­
lular calcium homeostasis.136 Functional studies have shown that 
reported WFS1 mutations lead to decreased stability of the pro­
tein wolframin.137 Linkage to other loci in addition to WFS1 may 
explain the variability in phenotype seen in this disorder.
W

 variability in phenotype seen in this disorder.
Wolfram syndrome appears to be a slowly progressive neurodegen­
erative process, and there is also (nonautoimmune) selective destruc­
tion of the pancreatic beta cells, recently associated in a stem cell 
model with increased endoplasmic reticulum stress in the pancreatic 
beta cell.138 This association is probably a result of the expression pat­
tern of WFS1. Diabetes mellitus with an onset in childhood is usually 
the first manifestation. Diabet

ldhood is usually 
the first manifestation. Diabetes mellitus and optic atrophy are pres­
ent in all reported cases, but expression of the other features is variable. 
Duration of diabetes is linked to the development of microvascular 
complications.137 Additional endocrinologic diseases, such as ACTH 
deficiency and growth hormone deficiency, have been reported.137 In 
one case report, two related children with Wolfram syndrome had 
megaloblastic and sideroblastic anemia that responded to treat

c and sideroblastic anemia that responded to treatment 
with thiamine. Furthermore, thiamine treatment was associated with 
a marked decrease in insulin requirements.139 
Omenn Syndrome
Omenn syndrome (MIM 603554) is a primary immunodefi­
ciency syndrome with autoimmune manifestations affecting 
mainly the skin and gastrointestinal tract. Mutations associated 
with decreased recombination of the T-cell receptor have been 
described. One study showed that the levels of AIRE gene expres­
sion were

wed that the levels of AIRE gene expres­
sion were decreased in the thymus of two affected patients and 
that this was associated with decreased expression of peripheral 
antigens compared with control subjects.125 
Chromosomal Disorders
Down syndrome, or trisomy 21 (MIM 190685), is associated with 
the development of type 1 diabetes, thyroiditis, and celiac disease. 
Patients with Turner syndrome are at increased risk for the devel­
opment of thyroid disease and celiac disease. It is recommende

yroid disease and celiac disease. It is recommended to 
screen patients with trisomy 21 and Turner syndrome for associ­
ated autoimmune diseases on a regular basis. 
Through the study of rare disorders such as APS-I and IPEX, the 
processes of thymic expression of peripheral antigens and devel­
opment of regulatory T cells are beginning to be defined. This 
understanding provides invaluable insight into the development 
of the normal immune system and the mistakes that can occur and 

mmune system and the mistakes that can occur and 
lead to autoimmunity. Lessons learned from these rare diseases 
will help to better define the pathophysiology of more common 
autoimmune endocrine disorders, possibly leading to the develop­
ment of immunologic methods for the prevention and treatment 
of these disorders.
1671.e1
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	135.	 Hofmann S, Philbrook C, Gerbitz KD, Bauer MF. Wolfram 
syndrome: structural and functional analyses of mutant and 
wild-type wolframin, the WFS1 gene product. Hum Mol Genet. 
2003;12(16):2003–2012.
	136.	 Osman AA, Saito M, Makepeace C, Permutt MA, Schlesinger P, 
Mueckler M. Wolframin expression induces novel ion channel 

. Wolframin expression induces novel ion channel 
activity in endoplasmic reticulum membranes and increases intra­
cellular calcium. J Biol Chem. 2003;278(52):52755–52762.
	137.	 Medlej R, Wasson J, Baz P, et al. Diabetes mellitus and optic atro­
phy: a study of Wolfram syndrome in the Lebanese population. J 
Clin Endocrinol Metab. 2004;89(4):1656–1661.
	138.	 Shang L, Hua H, Foo K, et  al. beta-cell dysfunction due to 
increased ER stress in a stem cell model of Wolfram syndrome. 
Diabetes. 2

 stem cell model of Wolfram syndrome. 
Diabetes. 2014;63(3):923–933.
	139.	 Borgna-Pignatti C, Marradi P, Pinelli L, Monetti N, Patrini C. 
Thiamine-responsive anemia in DIDMOAD syndrome. J Pediatr. 
1989;114(3):405–410.
1672
44
Endocrinology of HIV/AIDS
TODD T. BROWN AND STEVEN K. GRINSPOON
H
uman immunodeficiency virus (HIV) disease affects up 
to 36.7 million patients worldwide and more than 1.1 
million in the United States. In many parts of the world, 
such as in sub-Saharan Africa, HIV re

 the world, 
such as in sub-Saharan Africa, HIV remains epidemic, with an 
estimated 25.6 million people infected.1 In addition, the number 
of HIV-infected patients is growing rapidly in Asia and other parts 
of the world. Endocrine dysfunction is common among HIV-
infected patients. Adrenal, gonadal, thyroid, bone, and metabolic 
abnormalities have all been reported. HIV itself, related infectious 
organisms, immune activation, cytokines, and antiretroviral medi­
cations may all affect endocri

ntiretroviral medi­
cations may all affect endocrine function. Endocrine disorders in 
HIV disease, such as hypogonadism, adrenal insufficiency, dia­
betes mellitus (DM), and bone loss, may cause significant mor­
bidity and are thus important to diagnose. Interactions between 
antiretroviral therapy (ART) and specific medication may also 
contribute to endocrine disturbances. Endocrine strategies may 
improve quality of life and long-term survival through effects on 
critical metabolic and body 

l through effects on 
critical metabolic and body composition parameters, including 
loss of muscle mass (sarcopenia) in acquired immunodeficiency 
syndrome (AIDS) wasting, fat redistribution with loss of periph­
eral and abdominal subcutaneous fat, and relative or absolute 
gains in central (visceral) adiposity among some patients. How­
ever, diagnosis and treatment may be difficult because of varying 
nutritional conditions and effects of the varied medications used 
to treat HIV disease. As H

aried medications used 
to treat HIV disease. As HIV patients live longer as a result of 
the success of antiretroviral medications, adverse effects resulting 
from these very medications and HIV-related immune dysfunc­
tion have resulted in increased cardiovascular risk and metabolic 
changes that require intervention and long-term management by 
the endocrine specialist. This chapter will review the prevalence, 
KEY POINTS
	•	
In patients infected with human immunodeficiency virus (HIV) 
adre

cted with human immunodeficiency virus (HIV) 
adrenal dysfunction is common. Specific protease inhibitors, 
including ritonavir, and the P450CYP3A4 inhibitor cobicistat 
may reduce metabolism of inhaled and injected steroids and 
increase systemic exposure.
	•	
Hypogonadism is often seen among HIV-infected patients, is 
frequently associated with low or normal gonadotropins, and 
should be assessed using a specific free testosterone assay, given 
increases in sex hormone–binding globulin in thi

increases in sex hormone–binding globulin in this population.
	•	
Fracture incidence is increased among HIV-infected patients 
and is multifactorial in nature, potentially related to increases 
in bone turnover, certain antiretroviral medications, immune 
activation, vitamin D deficiency, and gonadal dysfunction.
	•	
Cardiovascular disease is increased among HIV-infected patients 
and relates to increases in traditional risk factors and nontraditional 
risk factors including immunologic and 

ay be used to treat sarcopenia in ac­
quired immunodeficiency syndrome wasting (testosterone and 
growth hormone) or to reduce visceral fat (growth hormone–re­
leasing hormone) in HIV lipodystrophy.
	•	
Medications used in the treatment of HIV disease have numer­
ous endocrine and metabolic effects, including effects on 
steroid metabolism, gonadal function, vitamin D synthesis, renal 
phosphate excretion, glucose uptake, and very low density 
lipoprotein metabolism.
Adrenal Fun

ipoprotein metabolism.
Adrenal Function, 1673
Gonadal Function, 1675
Thyroid Function, 1676
Fluid Balance and Electrolytes, 1677
Calcium Homeostasis and Bone Changes, 1677
The GH/IGF1 Axis, 1680
Glucose Homeostasis and Pancreatic Function, 1681
Metabolic and Body Composition Changes in HIV-Infected 
Patients, 1681
CVD in HIV-Infected Patients, 1687
1673
Chapter 44  Endocrinology of HIV/AIDS
mechanisms, and optimal treatment strategies for endocrine 
abnormalities in HIV-infected 

gies for endocrine 
abnormalities in HIV-infected patients.
Adrenal Function
Adrenal dysfunction may be suspected in the patient with 
advanced HIV disease because of fatigue, hyponatremia, and 
other features of adrenal insufficiency. Although clinical adrenal 
dysfunction is relatively rare among patients with AIDS, subtle 
impairments in adrenal reserve may be seen in this population. 
Adrenal dysfunction most often is caused by destruction of adre­
nal tissue by cytomegalovirus (CMV) in pati

 adre­
nal tissue by cytomegalovirus (CMV) in patients with advanced 
HIV disease but may also be caused by medications, hypotha­
lamic/pituitary disease from opportunistic infection; idiopathic 
inflammation or tissue destruction; or, in rare cases, by cortisol 
resistance. In addition, some features of Cushing syndrome may 
be seen among HIV-infected patients with fat redistribution, but 
true Cushing disease is rare.
Biochemical evidence of adrenal insufficiency is relat

hemical evidence of adrenal insufficiency is relatively com­
mon among hospitalized AIDS patients, with 17% of 74 hospital­
ized AIDS patients screened by a cosyntropin test demonstrating 
inadequate adrenal stimulation (1-hour cortisol <18 μg/dL) in an 
early study. In contrast, fewer patients, 4%, demonstrate clinical 
symptoms of adrenal insufficiency.2 Among patients with clinical 
symptoms and signs of adrenal insufficiency, including hypona­
tremia, a higher percentage, up to 30%, may demo

­
tremia, a higher percentage, up to 30%, may demonstrate inad­
equate reserve using cosyntropin testing.3
Adrenal insufficiency occurring in the context of advanced 
HIV disease is most often caused by tissue destruction of the 
adrenal glands from opportunistic infections. CMV adrenalitis is 
the most common cause, seen in approximately 40% to 90% of 
patients with CMV infections at autopsy. However, adrenocortical 
destruction caused by CMV is usually less than 50% and therefore 
unlikely to 

 usually less than 50% and therefore 
unlikely to cause adrenal insufficiency,4 and CMV disease is rare 
with well-preserved immune function in patients on newer potent 
ARTs. Other organisms and processes that have been associated 
with adrenal destruction in HIV disease include Mycobacterium 
tuberculosis, Mycobacterium avium-intracellulare (MAI), Crypto­
coccus, and hemorrhage. Additionally, pituitary/hypothalamic 
destruction resulting in secondary adrenal insufficiency may be 
caused in rar

ondary adrenal insufficiency may be 
caused in rare instances by opportunistic infection (e.g., toxoplas­
mosis, Cryptococcus, and CMV). Idiopathic adenohypophyseal 
necrosis is also observed in a minority of patients, approximately 
10% at autopsy, and may be related to a direct effect of HIV.5 
Glucocorticoid Excess: Adrenal Shunting and 
Cortisol Resistance
Increased cortisol levels may also be seen in HIV-infected patients. 
More commonly, increased cortisol levels are seen as a stress 
resp

creased cortisol levels are seen as a stress 
response, in association with low weight or increasing degree of ill­
ness. Intra-adrenal shunting toward cortisol synthesis, potentially 
as a result of 17,20-lyase dysfunction, has been suggested by stud­
ies demonstrating a reduced dehydroepiandrosterone (DHEA) to 
cortisol ratio on cosyntropin testing.6 Cytokine modulation of 
the hypothalamic-pituitary-adrenal (HPA) axis may also contrib­
ute to increased cortisol levels. Interleukin 1 (IL1), pr

increased cortisol levels. Interleukin 1 (IL1), produced in 
the median eminence, has been shown to increase corticotropin-
releasing hormone and adrenocorticotropic hormone (ACTH) 
secretion in vitro and in animal studies. Increased IL1 secretion 
from infected monocytes in the median eminence is thus another 
possible cause of increased cortisol secretion in HIV-infected 
patients. In turn, higher cortisol levels and greater diurnal varia­
tion may reduce T-cell immune activation.7 Glucocortic

 may reduce T-cell immune activation.7 Glucocorticoid resis­
tance has also been shown in rare patients with advanced HIV 
disease, who demonstrate addisonian symptoms, including hyper­
pigmentation, in the setting of hypercortisolism and increased 
ACTH. 
Medication Effects
Medications may contribute to adrenal insufficiency in HIV 
patients (Table 44.1). Ketoconazole, an antifungal agent, inhib­
its side-chain cleavage enzyme and 11β-hydroxylase. These effects 
are not generally seen with fluc

e. These effects 
are not generally seen with fluconazole, itraconazole, and more 
recently introduced imidazole derivatives. Phenytoin, opiates, 
and rifampin, among other drugs, affect cortisol metabolism. For 
example, adrenal insufficiency may be precipitated by the use of 
rifampin for treatment of tuberculosis in patients with reduced 
adrenal reserve. Megestrol acetate, a potent synthetic progesta­
tional derivative used as an appetite stimulant, has glucocorticoid 
properties and decreas

mulant, has glucocorticoid 
properties and decreases ACTH. Abrupt withdrawal of megestrol 
acetate may precipitate adrenal insufficiency, and such patients 
should be tested for adrenal insufficiency and receive physiologic 
glucocorticoid administration as needed after megestrol with­
drawal. In addition, megestrol acetate can decrease gonadal func­
tion, which should also be monitored during and after therapy. 
Cases of Cushing syndrome have been described with the concom­
itant use of flutica

en described with the concom­
itant use of fluticasone and ritonavir, via inhibition of CYP3A4 
by ritonavir and resultant reduction in metabolism of flutica­
sone. This combination of medications can result in symptoms 
of severe cortisol excess and potential severe adrenal insufficiency 
with discontinuation of fluticasone.8 Such patients demonstrate 
very low measured cortisol and ACTH levels, despite symptoms 
of hypercortisolemia, due to suppression of the endogenous HPA 
axis by increased 

pression of the endogenous HPA 
axis by increased concentrations of circulating fluticasone. After 
discontinuation of the fluticasone, long-term physiologic steroid 
replacement is necessary until the HPA axis recovers. In addition, 
adrenal insufficiency has been reported in approximately 5% of 
patients receiving intra-articular steroids while on protease inhibi­
tors (PIs), particularly ritonavir, with increasing risk seen among 
those with more than two injections within the prior 6 months.

ore than two injections within the prior 6 months.9 
Similar interactions with intra-articular steroids have been seen 
with cobicistat, another potent CYP3A4 inhibitor used in combi­
nation with certain antiretroviral medications.10 
Clinical Assessment
HIV-infected patients with symptoms of adrenal insufficiency, 
and particularly those with hyponatremia and risk factors for 
adrenal insufficiency, such as known disseminated CMV or recent 
use of megestrol acetate, should be evaluated. Evaluat

of megestrol acetate, should be evaluated. Evaluation of the 
cortisol axis should proceed as in other patients with suspected 
adrenal dysfunction. Cosyntropin testing is usually an adequate 
first step, except in those patients in whom hypothalamic or pitu­
itary insufficiency of recent onset is suspected. In such patients, 
use of morning cortisol levels, metyrapone, or insulin tolerance 
testing may be necessary, if there are no contraindications. After 
adrenal insufficiency is documented, 

ions. After 
adrenal insufficiency is documented, ACTH testing and appro­
priate imaging are used to localize the defect. In patients with 
clinical symptoms of adrenal insufficiency and elevated cortisol 
1674
	
Major Endocrine and Metabolic Effects of Medications Used in the Treatment of HIV
Endocrine/Metabolic 
System
Interacting Medication
Mechanism
Effect
Adrenal
Ritonavir
Cobicistat
CYP3A4 inhibitor that decreases metabolism of 
fluticas

4 inhibitor that decreases metabolism of 
fluticasone and potentially other steroids
Cushing stigmata with reduced 
ACTH/cortisol
Ketoconazole
11β-Hydroxylase inhibition
Megestrol acetate
Synthetic progestational agent with glucocorti­
coid properties
Rifampin
Increased cortisol metabolism
Gonadal
Decreased gonadotropins
Inhibits side-chain cleavage enzyme
Protease inhibitor

in cleavage enzyme
Variable effects to cause hyperprolactinemia 
directly or via dopamine antagonism
Potential hypogonadism
Thyroid
Rifampin
Increased hepatic clearance of thyroid hormone
Reduced thyroxine levels
Interferon
Variable effects on thyroid function
Fluid and electrolytes
Trimethoprim
Structural similarities to amiloride with inhibi­
tion of tubular potassium secretion
Hyperk

inhibi­
Hyperkalemia
Tenofovir disoproxil fumarate
Fanconi-like syndrome, phosphate wasting
Calcium
Variable effects to inhibit 1α-hydroxylation of 
Hypocalcemia
Efavirenz
Increased catabolism of 25-hydroxyvitamin D 
through induction of CYP24A
Rifabutin
Induces cytochrome P450 and alters vitamin D 
Foscarnet
Complexes with calcium
Decrease

ficiency
Foscarnet
Decreased ionized calcium, mag­
nesium
Renal magnesium wasting
Inhibits 1,25-dihydroxyvitamin D synthesis
Bone
Tenofovir
Phosphate wasting and tubulopathy, possible 
direct effect on osteoclast function
Bone loss
Unclear
Bone loss
Glucose
Variable effects to inhibit GLUT4-mediated 
glucose transport
NRTIs
Variable effects to inhibit mitochondrial DNA 

TIs
polymerase gamma
Islet cell inflammation and insulin release
Hypoglycemia and subsequent hyper­
glycemia
Body composition
Variable effects to inhibit SREBP1 and PPARγ 
signaling in subcutaneous fat
Subcutaneous fat loss
NRTIs
polymerase gamma in subcutaneous fat
Lipids
Variable effects to decrease lipoprotein lip

itors
Variable effects to decrease lipoprotein lipase-
mediated clearance of VLDL and TG
Possible effects on hepatic lipase and LCAT
Low HDL
ACTH, Adrenocorticotropic hormone (corticotropin); GLUT4, glucose transporter type 4; HDL, high-density lipoprotein; HIV, human immunodeficiency virus; LCAT, lecithin:cholesterol acyltransferase; NRTIs, 
nucleoside reverse transcriptase inhibitors; PPAR, peroxisome proliferator-activated receptor; SREB1, sterol regulat

iferator-activated receptor; SREB1, sterol regulatory element–binding protein 1; TG, triglyceride; VLDL, very low density lipoprotein.
   
TABLE 44.1
1675
levels, cortisol resistance may be present and the diagnosis may 
be made by glucocorticoid receptor studies in blood monocytes. 
Conversely, in patients with symptoms of adrenal excess and low 
cortisol and ACTH levels, exogenous steroid use or interactions 
with ART should be suspected. 
Gonadal Function

s 
Male Gonadal Dysfunction
Gonadal dysfunction is common among HIV-infected men 
with weight loss and advanced illness. Initial studies indicated 
biochemical hypogonadism in approximately 50% of men with 
AIDS, in association with increased disease severity. Among HIV-
infected men with low weight, hypogonadism was seen in 20%.11 
More recent studies suggest a lower prevalence of hypogonadism 
of approximately 9% to 16%.12–15 The mechanisms of hy

approximately 9% to 16%.12–15 The mechanisms of hypogo­
nadism in HIV-infected patients may relate to severe illness or 
effects of undernutrition on gonadotropin secretion; medica­
tion effects; or, more rarely, tissue destruction from opportunis­
tic infections. Most often, hypogonadism is secondary in nature, 
with low or inappropriately normal gonadotropin levels, as seen in 
91% of patients with reduced free testosterone levels during initia­
tion of highly active antiretroviral therapy (HA

­
tion of highly active antiretroviral therapy (HAART) in the Swiss 
HIV cohort.16 Primary hypogonadism is seen less often and may 
be caused by cytokine effects on the testes, including effects of 
tumor necrosis factor (TNF) to inhibit steroidogenesis via effects 
on the side-chain cleavage enzyme and of IL1 to inhibit Leydig 
cell steroidogenesis and luteinizing hormone binding to the Ley­
dig cell. Indeed, among young men, median age 45 years, using an 
early morning total testosterone as a 

, using an 
early morning total testosterone as a test to define hypogonadism 
in a large Italian cohort, Rochira and associates13 demonstrated 
that gonadotropins were elevated in 16% of patients studied (Fig. 
44.1). In addition, opportunistic infections of the testes have 
rarely been reported, and up to 25% of HIV-infected patients 
with AIDS will demonstrate testicular involvement of widespread 
opportunistic infection or systemic neoplasms, including CMV, 
toxoplasmosis, Kaposi sarcoma (KS

 including CMV, 
toxoplasmosis, Kaposi sarcoma (KS), and testicular lymphoma, 
although there are few data to suggest that primary hypogonad­
ism develops in all such cases.
In addition, several medications may affect the hypothalamic-
pituitary-gonadal axis. Ketoconazole inhibits side-chain cleavage 
enzyme and other critical enzymes in testicular steroidogenesis. 
Megestrol acetate is used to increase appetite, but as a synthetic 
progestational agent it suppresses gonadotropin secretion and 

l agent it suppresses gonadotropin secretion and 
results in hypogonadism. Opiate therapy affects gonadotropin-
releasing hormone secretion and may result in hypogonadotropic 
More recently, increased prolactin levels and gynecomastia have 
been demonstrated among HIV-infected patients. In a case-con­
trol study, gynecomastia was seen in 1.8% of 2275 consecutively 
screened HIV-infected patients and was associated with hypogo­
nadism, hepatitis C, and degree of lipoatrophy (subcuta

m, hepatitis C, and degree of lipoatrophy (subcutaneous fat 
loss associated with potent ART). Thyroid-stimulating hormone 
(TSH, thyrotropin) levels were increased, although the proportion 
with hypothyroidism was not different.17 Of ART medications, 
efavirenz is most closely associated with gynecomastia,18 which is 
due to direct activation of the estrogen receptor.19 Hyperprolac­
tinemia has been reported in 21% of HIV-infected men with stable 
HIV disease and was significantly associated wi

e 
HIV disease and was significantly associated with opioid use and 
increased CD4 count but not with changes in body composition or 
gynecomastia.20 Increased prolactin levels in association with galac­
torrhea have also been described among patients treated with PIs. 
The mechanism of this effect is unclear and may relate to a direct 
stimulation of prolactin secretion by specific PIs or effects on the 
P450 system to potentiate the dopamine antagonistic effect of other 
drugs.21 Dopamine agon

tagonistic effect of other 
drugs.21 Dopamine agonists should be used cautiously among HIV-
infected men receiving PIs because of the potential for interactions.
Sex hormone–binding globulin levels are increased in 30% 
to 55% of HIV-infected patients. Therefore, use of bioavailable 
Total testosterone (ng/dL)
Threshold for normal serum
total testosterone 300 ng/dL
(10.4 nmol/L)
Compensated hypogonadism
Primary hypogonadism
Eugonadism
Secondary hypogonadism
8.9 mlU/mL
1.4 mlU/mL
1200
1000
800
60

ypogonadism
8.9 mlU/mL
1.4 mlU/mL
1200
1000
800
600
400
200
0
50
40
30
20
10
0
• Fig. 44.1  Gonadal status according to serum total testosterone threshold of 300 ng/dL and luteinizing 
hormone (LH) normal range. (From Rochira V, Zirilli L, Orlando G, et al. Premature decline of serum total 
testosterone in HIV-infected men in the HAART-era. PLoS One. 2011;6:e28512, Fig. 2.)
1676
or free testosterone is recommended to diagnose hypog

free testosterone is recommended to diagnose hypogonadism, 
because use of total testosterone assays may underestimate the 
prevalence of true hypogonadism in this population. As a caveat, 
analogue assays of free testosterone may not be independent of sex 
hormone–binding globulin and should not be used. In contrast, 
determination of free testosterone by equilibrium dialysis or calcu­
lation by an accepted equation are more useful strategies to detect 
hypogonadism among the HIV population. Fo

 detect 
hypogonadism among the HIV population. For example, Monroe 
and colleagues12 conducted a large study in the Multicenter AIDS 
Cohort Study, comparing normal-weight HIV-infected and well-
matched uninfected patients in the current era of ART. In this 
study, total testosterone levels were similar in those who were HIV 
infected versus those who were not HIV infected, but free testos­
terone calculated using the Vermeulen equation was significantly 
lower in the HIV-infected patients. Rel

ificantly 
lower in the HIV-infected patients. Reliance on total testosterone 
alone would have missed 33% of patients with hypogonadism.12 
Use of total testosterone or free testosterone analogue radioim­
munoassay to diagnose hypogonadism among HIV-infected 
patients is associated with poor sensitivities of 25% and 33%, 
respectively.14
In patients in whom acute and chronic illness may contrib­
ute to hypogonadism, retesting of gonadal function by measur­
ing an early morning free testosterone

 by measur­
ing an early morning free testosterone level is recommended on 
resolution of the illness, as endogenous function may return with 
improved health. In patients who remain hypogonadal, adminis­
tration of physiologic testosterone replacement after appropriate 
diagnostic workup for the cause of hypogonadism is appropriate. 
In contrast to the potential utility of specific anabolic steroids in 
the treatment of weight loss in AIDS wasting (see the Treatment 
of AIDS Wasting and Loss of

ng (see the Treatment 
of AIDS Wasting and Loss of Lean Body Mass section), no role 
has been demonstrated for the use of anabolic steroids alone or in 
combination with testosterone for the treatment of hypogonadism 
per se in HIV-infected men. 
Female Gonadal Dysfunction
Amenorrhea is seen in approximately 25% of HIV-infected women 
and may be caused by the reduction of gonadotropin production 
associated with the stress of illness. In contrast, anovulation may 
be seen in up to 50% of HIV-inf

, anovulation may 
be seen in up to 50% of HIV-infected women in association with 
reduced CD4 counts. Among anovulatory HIV-infected women, 
changes in menstrual function are three times as likely compared 
with normally ovulating patients. Early menopause has been 
reported in up to 8% of HIV-infected women.22
Androgen levels are often reduced in HIV-infected women. In 
one study, androgen levels, assessed with the use of a free testos­
terone assay, were reduced below the level seen in age-ma

assay, were reduced below the level seen in age-matched 
healthy women in more than 50% of HIV-infected women with 
significant weight loss and in more than one third of HIV-infected 
women without weight loss.23 The mechanisms of androgen defi­
ciency in HIV disease may be caused in part by intra-adrenal 
shunting toward cortisol production and away from androgen 
production, particularly in women with significant weight loss6 
(see the Adrenal Function section). 
Thyroid Function
Altered thyro

Function section). 
Altered thyroid function tests are common in HIV-infected 
patients. Thyroid-binding globulin (TBG) levels are increased in 
HIV-infected patients and correlate inversely with CD4 counts.24 
Abnormal thyroid function test results may be caused by the 
stress of illness in patients with advanced disease or concomitant 
disorders, as found in other patients with euthyroid sick syn­
drome. However, among adults, some studies have shown that 
reverse triiodothyro

some studies have shown that 
reverse triiodothyronine (rT3) levels do not rise in association with 
decreasing T3 levels, as one would expect in nonthyroidal illness.3 
Patients with progressive HIV disease therefore exhibit decreased 
T3 levels, increased TBG, and decreased rT3 levels with increasing 
illness.
In addition to the euthyroid sick syndrome, large screening 
studies have demonstrated an increased prevalence of primary 
hypothyroidism in HIV-infected patients. Recent studies have 
i

m in HIV-infected patients. Recent studies have 
investigated the prevalence of thyroid dysfunction in the current 
era of HAART. In one study of 1565 HIV-infected patients, the 
prevalence of overt hypothyroidism was 2.5% and of overt hyper­
thyroidism 1%. A higher percentage of patients demonstrated 
subclinical hypothyroidism (4%) and abnormal thyroid test 
results associated with nonthyroidal illness (17%). Conversely, 
76% of patients demonstrated normal thyroid function tests. HIV 
therapy

trated normal thyroid function tests. HIV 
therapy and specific antiretroviral medications were not associated 
with thyroid dysfunction.25 Among 2437 HIV-infected patients, 
Nelson and coworkers26 demonstrated prevalence rates of hyper­
thyroidism and hypothyroidism each to be 1%. Use of specific 
antiretroviral agents, including PIs and efavirenz, were associated 
with thyroid dysfunction, but this has not been a consistent find­
ing among studies.
Increased TSH has been demonstrated in young 

ies.
Increased TSH has been demonstrated in young children with 
an average age of 1.5 years and failure to thrive. Thyroxine (T4) 
levels were normal, but thyrotropin-releasing hormone testing 
showed exaggerated TSH responses, and growth rates increased 
in response to thyroid hormone.27 Fundaro and associates28 dem­
onstrated increased antithyroglobulin antibodies in 34% of symp­
tomatic HIV-infected children. Increased TSH levels were found 
in 28% of HIV-infected children, particularly thos

in 28% of HIV-infected children, particularly those with severe 
immunosuppression. In contrast, a larger study in perinatally 
infected children demonstrated reduced total T3, total T4, and free 
T4 and increased rT3, TBG, and TSH, with negative autoantibod­
ies, suggesting a euthyroid sick pattern, particularly in those with 
severe immunosuppression. HIV-infected children with failure to 
thrive should be screened for true hypothyroidism, but more often 
the thyroid function tests will reflec

more often 
the thyroid function tests will reflect nonthyroidal illness and the 
severity of immune compromise.29
Recently, thyroid dysfunction has been described with an 
immune reconstitution syndrome in which autoimmune thy­
roid disease occurs in association with use of potent ART and 
improved immune function, typically 12 to 36 months after ART 
is initiated.30 Graves disease is most often reported in this context. 
The estimated prevalence for immune reconstitution thyroid dis­
ease with

e for immune reconstitution thyroid dis­
ease with initiation of HAART was 3% for women and 0.2% for 
men.31 Graves disease has also been described after IL2 therapy in 
HIV-infected patients.32
In addition to autoimmune causes, thyroid disease related 
to anatomic replacement and infection of the thyroid has been 
reported in HIV-infected patients. Pneumocystis thyroiditis has 
been reported to cause a painful thyroiditis-like picture, with 
hyperthyroidism followed by hypothyroidism, decreased

erthyroidism followed by hypothyroidism, decreased uptake 
on scanning, and a firm but tender gland. Pneumocystis thyroiditis 
may result from the use of inhaled pentamidine, which is associ­
ated with extrapulmonary Pneumocystis infections.
CMV, MAI, Cryptococcus, and KS have been demonstrated 
in the thyroid on autopsy but have not been related to clinical 
thyroid disease among patients with AIDS. Clinically appar­
ent thyroidal abscesses from Aspergillus and Rhodococcus equi 
have been repor

 Aspergillus and Rhodococcus equi 
have been reported. Hypothalamic/pituitary replacement from 
1677
opportunistic infections, such as toxoplasmosis and CMV, has 
also been reported to cause secondary hypothyroidism.
Medications may affect thyroid function. Rifampin influ­
ences hepatic clearance of T4, and interferon is associated with an 
increased incidence of autoimmune hypothyroidism. 
Fluid Balance and Electrolytes
Disorders of fluid balance and electr

Disorders of fluid balance and electrolytes are common among 
patients with AIDS. Hyponatremia may be seen in upward of 
50% of patients and is most often related to the syndrome of inap­
propriate secretion of antidiuretic hormone (SIADH). Hyperkale­
mia is also frequently reported and may be seen in association with 
various drugs, such as trimethoprim. More rarely, hyperkalemia 
may be associated with adrenal insufficiency.
Sodium
Hyponatremia (sodium <130 mmol/L) is seen in 40% 

Hyponatremia (sodium <130 mmol/L) is seen in 40% to 60% of 
hospitalized patients with AIDS and 20% of outpatients. SIADH 
(volume-replete patients with low levels of serum sodium, and 
inappropriately elevated levels of urine osmolarity) is seen in 23% 
to 47% of hyponatremic patients. SIADH may be caused by vari­
ous infections and tumors and is treated with fluid restriction and 
hypertonic saline if severe. The vasopressin receptor 2 antagonist, 
tolvaptan, has also been used in the treatme

ist, 
tolvaptan, has also been used in the treatment of hyponatremia in 
patients with advanced HIV disease.33
Adrenal insufficiency is documented in 30% of volume-
depleted, hyponatremic HIV-infected patients.34 Volume deple­
tion (diarrhea, vomiting) with excessive free water and impaired 
water clearance (HIV nephropathy) may cause hyponatremia 
among ill HIV-infected patients, especially those in the hospital. 
Volume repletion is the treatment. Hyporeninemic hypoaldoste­
ronism,35 more typi

nt. Hyporeninemic hypoaldoste­
ronism,35 more typically associated with hyperkalemia, may be 
another cause of hyponatremia and is treated with mineralocorti­
coids. The use of medications such as vidarabine, miconazole, and 
pentamidine is associated with hyponatremia of unknown cause. 
Hypernatremia may be caused by foscarnet-induced nephrogenic 
diabetes insipidus. 
Potassium
Hyperkalemia occurs in 20% to 53% of AIDS patients on tri­
methoprim because of structural similarities to amiloride a

 because of structural similarities to amiloride and 
inhibition of tubular potassium excretion.36 Other potential 
causes include pentamidine-associated tubular nephropathy; HIV 
nephropathy (glomerular sclerosis); primary adrenal insufficiency; 
and, rarely, hyporeninemic hypoaldosteronism. Physiologic stud­
ies investigating potassium balance in HIV-infected patients also 
suggest an inadequate aldosterone response to hyperkalemia in 
HIV-infected patients. A Fanconi-like syndrome with tubula

cted patients. A Fanconi-like syndrome with tubular dys­
function, phosphate wasting, and hypokalemia has been described 
with the use of tenofovir disoproxil fumarate (TDF) and more 
rarely with adefovir, cidofovir, and didanosine.37 
Calcium Homeostasis and Bone Changes
Hypocalcemia is common in advanced HIV disease. Hypocalce­
mia, based on albumin-adjusted total calcium levels, was dem­
onstrated in 6.5% of a large cohort of HIV-infected patients 
with AIDS. Calcium level

of HIV-infected patients 
with AIDS. Calcium levels decreased progressively with stage of 
disease. Among patients with hypocalcemia, 48% were vitamin 
D deficient, and the expected increase in parathyroid hormone 
(PTH) levels was lacking in the majority.38 Jaeger and col­
leagues39 also demonstrated decreased PTH secretion in severely 
immunocompromised patients with AIDS, but the mechanism 
is unknown. In addition, decreased PTH levels may occur in 
the setting of hypomagnesemia during severe

ur in 
the setting of hypomagnesemia during severe illness or in asso­
ciation with renal magnesium wasting. Vitamin D deficiency 
may be caused by malabsorption from AIDS enteropathy or 
by specific effects of antiretroviral drugs (e.g., inhibition of 1 
α-hydroxylation of 25-hydroxyvitamin D by PIs or increased con­
version of 25-hydroxyvitamin D to 24,25-dihydroxyvitamin D by 
efavirenz).40–42
Severe vitamin D deficiency of nutritional origin has also been 
described in HIV-infected children.

has also been 
described in HIV-infected children. Osteomalacia has been asso­
ciated with use of rifabutin for MAI in an HIV-infected patient. 
Rifabutin induces cytochrome P450, which may affect vitamin 
D metabolism. Other drugs that induce P450 could have a 
similar effect. Several drugs may affect calcium homeostasis (see 
Table 44.1). Foscarnet complexes with calcium to decrease ion­
ized calcium levels and may also induce severe hypomagnesemia. 
Pentamidine therapy has been associated wit

emia. 
Pentamidine therapy has been associated with renal magnesium 
wasting and severe hypomagnesemia, which may, in turn, cause 
hypocalcemia, through decreased PTH release and resistance to 
circulating PTH. Ketoconazole inhibits 1,25-dihydroxyvitamin D 
synthesis.
Among patients with HIV disease, hypercalcemia can be 
caused by excessive 1,25-dihydroxyvitamin D production in the 
setting of granulomatous disease (tuberculosis) or lymphoma, by 
local osteoclastic bone resorption from dissemin

local osteoclastic bone resorption from disseminated CMV, or by 
human T-cell lymphotropic virus 1–related activation of PTH-
related protein. Immune reconstitution–associated hypercalcemia 
has been described in the setting of ART initiation among patients 
with known tuberculosis, in whom immune reconstitution results 
in increased granulomatous activity and 1,25-dihydroxyvitamin 
D production.43 
Bone Loss: Prevalence, Etiologic Factors, and 
Treatment Strategies
Studies performed recently 

Studies performed recently in the current era of ART give an esti­
mate of the prevalence and risk factors for progression of bone loss 
among HIV-infected patients. Bonjoch and associates44 investi­
gated 671 HIV-infected patients and demonstrated osteopenia in 
47.5% and osteoporosis in 23%. Progression rates to osteopenia 
and osteoporosis were 12.5% and 15.6% over 2.5 years of follow-
up and 18% and 29% over 5 years of follow-up. In this large 
cohort, factors associat

follow-up. In this large 
cohort, factors associated with progression of bone loss were age, 
male sex, lower body mass index (BMI), PI use, and TDF use.44 
Significant bone loss has also been shown among postmenopausal 
HIV-infected women, with time since menopause and traditional 
risk factors most significantly associated with bone loss. In a longi­
tudinal analysis of postmenopausal HIV-infected women, Yin and 
coworkers45 demonstrated increased markers of bone resorption 
and annualized bon

sed markers of bone resorption 
and annualized bone loss in HIV-infected women versus non–
HIV-infected control subjects.
Specific antiretroviral strategies may affect bone in the HIV 
population. Studies demonstrate that switching to a TDF-based 
regimen is associated with bone loss and increased bone turn­
over46,47 (Fig. 44.2). Strong evidence for an effect of TDF is shown 
in studies in which patients at risk for HIV are given single-agent 
1678
SECTION IX    Polyendocrine and Neoplastic Dis

678
tenofovir for preexposure prophylaxis. In these patients, bone loss 
averaged approximately 1% compared with control subjects over 
2 years of follow-up. Larger effects might be seen in less healthy 
HIV-infected patients with other comorbid contributors.48
Current recommendations suggest that TDF should be 
avoided for the initial or continued treatment of HIV-infected 
persons at risk for fracture.49 ART switching from TDF to abaca­
vir,

 fracture.49 ART switching from TDF to abaca­
vir, raltegravir, or the tenofovir prodrug, tenofovir alafenamide are 
associated with improvements in bone mineral density (BMD) in 
HIV-infected persons on effective ART.50–52 In addition, because 
of their negative effects on BMD, PIs should be avoided in HIV-
infected persons at risk for fracture.
Recent data suggest that immunologic factors including T-cell 
activation,53 low CD4 cell count,54 and coinfection with hepatitis 
B and C are strongly

d coinfection with hepatitis 
B and C are strongly related to reduced bone density, particularly 
among women.55 These data suggest the need to assess and follow 
bone density in HIV-infected patients with specific risk factors, 
such as older age, long-term ART use, low BMI, hypogonadism, 
hepatitis, low CD4 count, and TDF use.
Endocrine factors may contribute to reduced bone density in 
HIV-infected patients, including hypogonadism, relative growth 
hormone (GH) deficiency, and vitamin D defic

owth 
hormone (GH) deficiency, and vitamin D deficiency. In one study, 
GH pulse area determined from overnight frequent sampling was 
reduced in patients with central fat accumulation and correlated 
significantly with vertebral bone density.56 This relationship may 
explain, in part, the inverse association between increased vis­
ceral fat and reduced bone density in HIV-infected patients.57 
Vitamin D deficiency is common among HIV-infected patients. 
48
*
*
*
*
36
24
% change in BMD (%)
12
P

0.02
0.04
D
ABC/3TC arm
TDF/FTC arm
• Fig. 44.2  Changes from baseline in hip and lumbar spine BMD. Percentage and absolute mean changes 
from baseline in lumbar spine (A, C) and hip (B, D) bone mass density as measured by dual-energy x-ray 
absorptiometry. ABC/3TC, abacavir/lamivudine; BMD, bone mineral density; TDF/FTC, tenofovir/emtric­
itabine. Error bars show 95% confidence intervals. (From Rasmussen TA, Jensen D, Tolstrup M, et al. 
Comparison of bone and renal effects in HIV-infected ad

rison of bone and renal effects in HIV-infected adults switching to abacavir or tenofovir based 
therapy in a randomized trial. PLoS One. 2012;7:e32445, Fig. 2.)
1679
In a study of ambulatory clinic patients in London, the preva­
lence of 25-hydroxyvitamin D deficiency (<20 ng/mL) and severe 
25-hydroxyvitamin D deficiency (<10 ng/mL) was 58.5% and 
12.6%, respectively.58 In a large cohort of 2044 consecutively fol­
lowed HIV patients in Brussels, the preval

ly fol­
lowed HIV patients in Brussels, the prevalence of severe vitamin D 
deficiency using the 10-ng/mL cutoff was 32.4%.59 Among HIV-
infected patients in the study, advanced disease (CD4 <200 cells/
mm3) and current use of efavirenz were significantly associated 
with severe vitamin D deficiency.59 In carefully conducted case 
matching studies, the odds ratio for vitamin D deficiency in HIV 
versus non–HIV-infected patients tended to be increased at 1.46, 
although this did not reach statist

ased at 1.46, 
although this did not reach statistical significance.60 TDF may 
increase PTH and bone turnover, potentially related to effects on 
proximal tubule phosphate reabsorption or other effects. PIs may 
also inhibit 1α-hydroxylase and result in vitamin D deficiency.40
Studies in HIV-infected patients receiving HAART have inves­
tigated markers of bone resorption and formation and found 
evidence of increased bone turnover.61 Tebas and colleagues62 
evaluated serum and urine bone marker

olleagues62 
evaluated serum and urine bone markers in 73 HIV-positive 
patients receiving PI therapy. Increased serum bone alkaline phos­
phatase and urine N-telopeptide were found to be inversely cor­
related with BMD T- and Z-scores measured by dual-energy x-ray 
absorptiometry, suggesting an increased rate of bone turnover 
among HIV-infected patients receiving PI therapy.62 Recent data 
investigating the receptor activator for nuclear factor κB ligand 
(RANKL) system demonstrate reduced sol

 κB ligand 
(RANKL) system demonstrate reduced soluble RANKL in HIV-
infected patients,63,64 arguing against activation of this system as a 
mechanism for increased bone turnover in HIV-infected patients. 
In contrast, increased osteoprotegerin was shown among HIV-
infected women with low bone density, suggesting a compensa­
tory increase in the context of low bone density and high bone 
resorption rates.65
Several recent studies have examined fracture rates among 
HIV-infected patients. In a US

acture rates among 
HIV-infected patients. In a US-based data registry study, the 
overall fracture prevalence was 2.87 versus 1.77 per 100 persons 
for HIV versus non–HIV-infected patients.66 In a large cohort 
study performed in Europe, significantly increased adjusted haz­
ard ratios for hip and major fracture were 4.7 and 1.8, with an 
increased relative risk (RR) of fracture among older HIV-infected 
patients in the study67 (Fig. 44.3). In a Veterans Administration 
cohort of men, Womack an

 Veterans Administration 
cohort of men, Womack and coworkers68 demonstrated that 
increased fragility fractures were strongly associated with a higher 
frailty score, low BMI, alcohol-related diagnoses, white race, pro­
ton pump inhibitor use, and PI use. In a study limited to women, 
lifetime fragility fractures were significantly more common in 
HIV-infected versus non–HIV-infected women (odds ratio, 1.7; 
95% confidence interval, 1.1–2.6).69 Use of the FRAX (Fracture 
Risk Assessment Tool) s

Use of the FRAX (Fracture 
Risk Assessment Tool) score to predict fragility fracture has shown 
relative poor sensitivity but good specificity among HIV-infected 
patients.70 In partial contrast to the data on bone density, data 
from nonrandomized longitudinal cohorts demonstrate that over­
all use of PIs, nucleoside reverse transcriptase inhibitors (NRTIs), 
and non-NRTIs (NNRTIs) is associated with reduced fracture 
rates, whereas the effects of individual drugs vary. Low CD4 
count, hepatiti

of individual drugs vary. Low CD4 
count, hepatitis, and DM are independently related to increased 
fracture risk.71 These data suggest that overall effects of ART may 
be to improve bone through effects on improving immune func­
tion and virologic factors, but adverse effects of specific ART 
agents on bone may also be important.72
Limited data are available on treatment strategies for bone 
loss in HIV-infected patients. Among patients with idiopathic 
bone loss and high bone turnover, recent 

opathic 
bone loss and high bone turnover, recent studies suggest that 
alendronate is effective in increasing bone density among HIV-
infected patients, with studies demonstrating increases in lumbar 
spine bone density from 3.4% to 5.2% over 42 weeks and good 
safety tolerability, albeit in relatively small numbers of patients.73,74 
Use of longer acting bisphosphonates (e.g., zoledronate) showed 
even greater changes in spinal bone density of 8.9% over 2 years 
in a randomized, placebo-contro

8.9% over 2 years 
in a randomized, placebo-controlled study.75 Importantly, sig­
nificant differences persisted for bone density and bone turnover 
between groups 5 years after the final dose, suggesting potent, per­
sistent effects after short-term dosing.76 To date, there have been 
no studies examining the effect of teriparatide, abaloparatide, or 
denosumab in the HIV-infected population. Among men with 
AIDS wasting, testosterone at high doses (200 mg/week) has been 
shown to increase bone

ses (200 mg/week) has been 
shown to increase bone density.77
The first 2 years after the initiation of ART are associated with 
a 2% to 6% decrease in BMD, with the larger decreases in this 
range observed in those receiving TDF or PIs.78–81 Advanced 
HIV disease at the time of ART initiation has also been associated 
with greater loss of BMD in this period.82 In addition to avoiding 
TDF and PIs, as well as starting ART at a higher CD4 cell count, 
several strategies have been tested to mitiga

nt, 
several strategies have been tested to mitigate bone loss during 
ART initiation. In a randomized controlled trial, high-dose vita­
min D3 (4000 IU/day) and calcium (1000 mg/day) at the time 
of ART initiation with TDF/emtricitabine/efavirenz was shown 
to attenuate ART-initiation–associated bone loss by 50%.83 In 
another randomized trial, a single dose of zoledronic acid (5 mg) 
prior to ART initiation prevented bone loss over 96 weeks84; this 
may be an important strategy to maintain bon

this 
may be an important strategy to maintain bone health in those 
HIV-infected patients initiating ART who are at risk for fracture 
because of older age, lower BMD, or the presence of other fracture 
risk factors (Fig. 44.4). 
Bone Metabolism in HIV-Infected Children
Reduced bone density has also been reported in HIV-infected chil­
dren. O’Brien and associates85 demonstrated reduced total-body 
bone density in perinatally infected girls at age 9 years in asso­
ciation with increased N-telope

e 9 years in asso­
ciation with increased N-telopeptide and PTH levels. In a 2013 
study, HIV-infected children ages 12 to 13 years had a greater 
40–45 45–50 50–55 55–60 60–65 65–70 70–75 75–80
5
4.5
4
3.5
3
2
1.5
1
0.5
0
2.5
HIV uninfected
HIV infected
• Fig. 44.3  Age-specific fracture incidence rates (per 100 person-years) 
in human immunodeficiency virus–infected versus uninfected patients. 
(From Guerri-Fernandez R, Vestergaard P, Carbonell C, et al. HIV infection 
is strongly associated w

 C, et al. HIV infection 
is strongly associated with hip fracture risk, independently of age, gender, 
and comorbidities: a population-based cohort study. J Bone Miner Res. 
2013;28:1259–1263, Fig. 1, used with permission.)
1680
prevalence of low bone density than HIV-exposed but uninfected 
children, although these differences were attenuated after height 
and weight adjustment, suggesting that delays in growth may 
account for differences i

at delays in growth may 
account for differences in bone density.86 Among young HIV-
infected adults ages 20 to 25 years, Tanner stage 5, infected either 
perinatally or during adolescence, both volumetric bone density 
and cortical and trabecular thickness on high-resolution periph­
eral quantitative computed tomography was reduced, suggesting 
lower peak bone mass among HIV-infected patients.87 In contrast 
to increased markers of resorption, reduced osteocalcin levels have 
been reported in H

educed osteocalcin levels have 
been reported in HIV-infected children, suggesting reduced bone 
formation and a relative discrepancy between increased resorption 
and reduced formation in this group.88 Similar to data in adults, 
bone density is related to insulin-like growth factor 1 (IGF1) in 
HIV-infected children, suggesting a potential effect of low GH 
on bone.89 Mora and colleagues90 followed bone density longitu­
dinally over 1 year to compare changes in HIV-infected children 
and contr

ompare changes in HIV-infected children 
and control subjects and demonstrated relative reductions in total-
body but not spinal bone density accrual and relative increases 
in bone turnover. ART initiation with TDF has been associated 
with small decreases in BMD in children but does not appear to 
affect accrual of bone over 10 years in small studies.91,92 There is 
conflicting evidence whether TDF exposure affects bone health or 
linear growth in HIV-exposed uninfected infants.93,94
Vitamin D

 in HIV-exposed uninfected infants.93,94
Vitamin D supplementation has been tested as a strategy to 
improve BMD in HIV-infected children and adolescents. Among 
children, supplementation with vitamin D (100,000 IU of chole­
calciferol every 2 months and calcium [1 g/day]) did not increase 
bone density over 2 years.95 In another study of adolescents taking 
ART containing TDF, vitamin D3 (50,000 IU/monthly) was asso­
ciated with modest increases in lumbar spine BMD, which was 
not observed in t

 in lumbar spine BMD, which was 
not observed in the placebo comparison group.96 Another ran­
domized, placebo-controlled study over 24 months showed no 
differences in BMD change between vitamin D3 dosing of 18,000 
IU/month, 60,000 IU/month, and 120,000 IU/month.97 Taken 
together, these finding suggest that the effect of vitamin D supple­
mentation on BMD in children and adolescents may be minimal. 
Avascular Necrosis of Bone
Miller and coworkers98 demonstrated a 4.4% prevalence of avas­
cula

ers98 demonstrated a 4.4% prevalence of avas­
cular necrosis (AVN) among 339 asymptomatic HIV-infected 
individuals. A significant relationship was reported between AVN 
and prior use of systemic corticosteroids. Other potential factors 
significantly associated with an increased risk of AVN included 
the presence of anticardiolipin antibodies, as well as routine body­
building and its associated mechanical stress. Alcohol use may also 
be a factor associated with AVN among HIV-infected patients

or associated with AVN among HIV-infected patients.99 
In an analysis among HIV-infected patients, osteonecrosis of the 
bone was associated with exposure to one or more ART drugs 
and high triglyceride and cholesterol levels, as well as high serum 
immunoglobulin E.100 A role for altered coagulation and inflam­
mation was also suggested by the observation of increased d-dimer 
among patients with osteonecrosis101 and that past history of 
immunosuppression was a major risk factor for AVN incide

suppression was a major risk factor for AVN incidence.102 
The GH/IGF1 Axis
Significant abnormalities in the GH/IGF1 axis occur in HIV-
infected patients. Among patients with AIDS wasting and sig­
nificant weight loss, GH levels are increased in association with 
reduced IGF1 levels, a pattern typical of GH resistance seen with 
malnutrition. In contrast, in the setting of visceral fat accumula­
tion, frequent sampling of GH levels over 24 hours has suggested 
a different pattern.103 Mean overni

has suggested 
a different pattern.103 Mean overnight GH and GH pulse ampli­
tude were decreased in this setting, whereas pulse frequency was 
not different compared with age-matched and BMI-matched non­
lipodystrophic HIV- and non–HIV-infected patients. Reduced 
GH levels were strongly predicted by increased visceral fat in 
the patients. A role for suppression of GH release by free fatty 
acids (FFAs) was suggested by experiments in which acipimox, 
a nicotinic acid derivative that blocks peri

mox, 
a nicotinic acid derivative that blocks peripheral tissue lipolysis 
and lowers FFA levels, was administered. Peak GH response to 
GH–releasing hormone (GHRH) was increased in response to 
acipimox, in inverse association to the change in FFA.104 Physi­
ologic studies of GH in HIV-infected patients suggest a schema 
whereby increased somatostatin tone, reduced ghrelin, and 
increased lipolysis contribute to reduced GH secretion in viscer­
ally obese HIV-infected patients with lipodystrophi

lly obese HIV-infected patients with lipodystrophic changes in 
fat distribution.104
GH deficiency is a potential cause of growth failure in HIV-
infected children, and treatment with GH results in improved 
auxologic parameters in such children.105 Increased IGF-binding 
protein 3 (IGFBP3) proteolysis and reduced IGF1, IGFBP3, and 
acid-labile subunit of the IGFBP3 ternary complex are demon­
strated among HIV-infected children with failure to thrive.106 
IGF1 and IGFBP3 responses to GH may be i

rive.106 
IGF1 and IGFBP3 responses to GH may be impaired in HIV-
infected children, suggesting a degree of GH insensitivity in 
this population. These responses may improve with weight gain 
and improved immune function in response to HAART.107 
GH deficiency may also disrupt normal thymic development in 
–6
–4
–2
0
2
Zoledronic acid (ZOL)
Vehicle (PL)
0
12
24
48
Lumbar spine (% change from BL)
Weeks on study
26
23
32
29
ART+ZOL(n)
ART+PL(n)
27
24
25
23
24
23
• Fig. 44.4  Longitudinal percentag

24
25
23
24
23
• Fig. 44.4  Longitudinal percentage change over 48 weeks in lumbar spine 
bone mineral density (BMD) with initiation of antiretroviral therapy (ART) 
with concomitant zoledronic acid (5 mg intravenous; ZOL) or placebo (PL). 
BL, baseline. (Redrawn from Ofotokun I, Titanji K, Lahiri CD, et al. A single-
dose zoledronic acid infusion prevents antiretroviral therapy-induced bone 
loss in treatment-naive HIV-infected patients: a phase IIb trial. Clin Infect 
Dis. 2016;63[5]:663–671; 

 IIb trial. Clin Infect 
Dis. 2016;63[5]:663–671; Fig. 3B.)
1681
HIV-infected children. GH has been used to increase height in 
HIV-infected children with normal GH responses to stimulatory 
testing.105 Among HIV-infected children, reduced GH secretion 
is associated with excess visceral adiposity. Among adults, GH is 
approved by the US Food and Drug Administration (FDA) for 
the treatment of AIDS wasting. In addition, a GHRH analogue, 
tesamorelin, is FDA 

n addition, a GHRH analogue, 
tesamorelin, is FDA approved to reduce excess visceral fat accumu­
lation in HIV lipodystrophy (see the discussion in the Treatment 
of Metabolic and Body Composition Changes in HIV-Infected 
Patients section). 
Glucose Homeostasis and Pancreatic 
Function
Disorders of glucose homeostasis were relatively infrequent prior 
to the institution of potent ART but may result from use of 
specific antiretroviral agents and are commonly seen in associa­
tion with dyslipidem

are commonly seen in associa­
tion with dyslipidemia and fat redistribution in the current era 
of HAART (see the Metabolic and Body Composition Changes 
in HIV-Infected Patients section). The pancreas is a frequent tar­
get of opportunistic infections and malignancies in patients with 
advanced HIV disease. However, clinical endocrine dysfunction 
rarely results, except in cases of massive pancreatic replacement 
from lymphoma or KS. For example, opportunistic infections of 
the pancreas are se

, opportunistic infections of 
the pancreas are seen on postmortem examination but are rarely 
clinically relevant. More commonly, pancreatitis and hypoglyce­
mia follow use of certain drugs, such as pentamidine, didanosine, 
or zalcitabine. Hypoglycemia can result from pentamidine admin­
istration secondary to islet cell inflammation and insulin release, 
especially in the context of high-dose therapy and azotemia. Sub­
sequently, chronic hyperglycemia from pancreatic B-cell destruc­
tion may f

lycemia from pancreatic B-cell destruc­
tion may follow pentamidine use. Megestrol acetate use has been 
associated with new-onset DM because of its potent glucocorti­
coid action. Pancreatitis is common among patients with HIV 
and most often related to a drug effect (i.e., pentamidine, trime­
thoprim-sulfamethoxazole, didanosine, and zalcitabine). In a large 
study of almost 6000 patients followed for 23,460 person-years 
at the Johns Hopkins HIV Clinic, the incidence of acute pancre­
atitis w

IV Clinic, the incidence of acute pancre­
atitis was 5.1 in 1000 person-years between 2001 and 2006. Low 
CD4 count, aerosolized pentamidine, and female gender were 
associated with pancreatitis. In contrast, specific antiretroviral 
medications were not.108 In the EuroSIDA study, a lower inci­
dence of pancreatitis was seen since 2001—1.27 in 1000 person-
years—and, again, low CD4 count was predictive. Similar to the 
data from the Hopkins cohort, no association was seen between 
cumulative exp

t, no association was seen between 
cumulative exposure to ART and exposure to didanosine and 
stavudine—NRTIs that have been associated with pancreatitis.109 
Amylase levels may also be elevated in HIV-infected patients sec­
ondary to macroamylasemia and salivary amylase. 
Metabolic and Body Composition Changes in 
HIV-Infected Patients
HIV-infected patients demonstrate several changes in body com­
position and metabolism. These changes are present to varying 
degrees among HIV-infected patient

ent to varying 
degrees among HIV-infected patients and are multifactorial in 
nature, related, in part, to the HIV virus itself, inflammation, and 
specific antiretroviral drugs, as well as the interplay of these fac­
tors.110,111 Of importance, some of these changes may contribute 
to increased cardiovascular disease (CVD) and respond to changes 
in ART, lifestyle modification (LSM), and specific pharmacologic 
strategies (e.g., to improve lipids or insulin sensitivity).
The AIDS Wasting Syndr

ds or insulin sensitivity).
The AIDS Wasting Syndrome and Loss of Lean 
Body Mass
Wasting was initially a common feature of progressive HIV dis­
ease, which was known originally as “slim disease.” The AIDS 
wasting syndrome is currently defined by weight less than 90% 
ideal body weight or weight loss greater than 10% over 3 months. 
It is characterized by a disproportionate loss of lean body mass, 
with a relative sparing of body fat, particularly in men. In women, 
fat mass may be lost disprop

ly in men. In women, 
fat mass may be lost disproportionately with disease progres­
sion. The loss of lean body mass occurs early and may antedate 
weight loss. Muscle wasting, weakness, and increased resting 
energy expenditure of 8% to 9% are also features of this disease. 
Macallan and associates112 demonstrated that energy expenditure 
fell during periods of rapid weight loss but less than the decrease 
in caloric intake. Cytokines associated with severe illness may 
increase energy expendit

 with severe illness may 
increase energy expenditure and decrease appetite. In addition, 
chronic weight loss may be associated with gastrointestinal dis­
ease, including malabsorption. Weight loss is a significant predic­
tor of mortality rate in HIV infection, with BMI less than 18.4 
kg/m2 associated with a 2.2-fold increase in mortality rate and 
BMI less than 16.0 kg/m2 associated with a 4.4-fold increase in 
mortality rate.113 Hypogonadism is observed in 30% to 50% of 
men with AIDS wasti

 is observed in 30% to 50% of 
men with AIDS wasting and may contribute to loss of lean body 
mass114 (see the Gonadal Function section).
In contrast to the traditional paradigm of wasting, HIV dis­
ease may be associated with overweight and obesity, particularly 
in developed countries with emerging obesity epidemics. In one 
study, performed at a large urban clinic for HIV-infected patients 
located in the southern United States, the prevalence of under­
weight was less than 10% and the preval

 of under­
weight was less than 10% and the prevalence of overweight and 
obesity was 44% despite high viral load at ART initiation.115 
With ART, 20% of patients increased from normal to overweight 
or overweight to obese115 (Fig. 44.5). Subjects with more severe 
Obese
Normal
weight
**
**
**
*
60
50
40
30
Percentage of participants
20
10
0
• Fig. 44.5  Proportion of study sample by body mass index category at 
antiretroviral therapy initiation versus 2 years on therapy a

ral therapy initiation versus 2 years on therapy among treatment-
naive human immunodeficiency virus–infected patients at the University of 
Alabama at Birmingham 1917 HIV/AIDS Clinic, 2000-2008. Purple bars = 
therapy initiation; teal bars = 24 months. *, p < .05; **, p < .01. (From Tate 
T, Willig AL, Willig JH, et al. HIV infection and obesity: where did all the 
wasting go? Antivir Ther. 2012;17:1281–1289, Fig. 2A, used with permis­
sion.)
1682
SECTION IX    Polyendocrine and Neoplastic Diso

82
immune dysfunction and using boosted PIs tended to gain the 
most weight with ART. 
Changes in Fat Mass and Distribution
The most commonly seen change in body composition among 
HIV-infected patients is loss of abdominal and peripheral sub­
cutaneous fat, including loss of subcutaneous fat in the face.116 
Other changes that may also be seen include relative preservation 
of central fat, with relative and absolute accumulation of excess 
vi

h relative and absolute accumulation of excess 
visceral fat and excess upper trunk fat.116,117 In addition, ectopic 
fat collections can be seen, including in the dorsocervical area. 
Increased fat deposition in the liver and muscle is also seen and is 
associated with insulin resistance. In prospective studies of anti­
retroviral-naive patients beginning treatment in the early 2000s, 
including an NRTI and PI, initial gains in both peripheral subcu­
taneous and central fat depots are seen with

ubcu­
taneous and central fat depots are seen with reversal of the cata­
bolic wasting state in association with control of viral infection. 
These changes are followed by subsequent decreases in peripheral 
fat and relative preservation and even absolute gains in central fat. 
More recent data of ART-naive persons initiating treatment with 
more modern regimens suggest increase in both subcutaneous and 
visceral fat over 96 weeks.118 Data from the Study of Fat Redistri­
bution and Metabolic Cha

he Study of Fat Redistri­
bution and Metabolic Change in HIV Infection (FRAM study) 
demonstrate that increases in visceral fat and reductions in limb 
fat are independently associated with increased mortality rate in 
HIV-infected patients119 (Fig. 44.6).
The cause of relative central fat accumulation remains 
unknown. For example, it remains unknown whether this is a 
direct effect of specific antiretroviral drugs and whether abnormal 
nutrient partitioning to relatively preserved central adip

 partitioning to relatively preserved central adipose stores, 
less affected by NRTI administration and mitochondrial toxic­
ity, may contribute. In contrast, several mechanisms have been 
shown to contribute to the fat loss seen peripherally. NRTIs can 
inhibit mitochondrial DNA polymerase gamma and contribute 
to mitochondrial dysfunction. Use of specific NRTIs, including 
older NRTIs such as stavudine, has been associated with apopto­
sis of fat and reduced mitochondrial DNA in vitro and in v

at and reduced mitochondrial DNA in vitro and in vivo, 
as well as reduced expression of lipid metabolism genes, and 
clinically with reduced subcutaneous fat and lipoatrophy.120,121 
PIs may have direct effects on adipogenesis (via inhibition of 
nuclear localization of sterol regulatory element–binding protein 
1 [SREBP1] and reduction in peroxisome proliferator-activated 
receptor γ [PPARγ] expression).122 NRTIs and PIs have been asso­
ciated with increased lipolysis in vivo and in vitro.
Gen

with increased lipolysis in vivo and in vitro.
Genetic polymorphisms may predispose to changes in body 
composition and metabolic alterations in HIV-infected patients 
receiving ART. Such findings could indicate a gene-environment 
interaction contributing to such changes. For example, single-
nucleotide polymorphisms in the resistin gene were seen to pre­
dict the development of dyslipidemia, insulin resistance, and 
limb fat loss in response to a specific antiretroviral regimen.123 
In the sam

 a specific antiretroviral regimen.123 
In the same study, polymorphisms in the hemochromatosis gene 
and specific mitochondrial gene haplotypes were also associated 
with increased limb fat loss.124 Specific haplotypes of the Fas gene 
(APOC3), PPAR gene, and adrenergic receptor have also been 
associated with the development of lipoatrophy.125
Other studies suggest early molecular changes in the fat of 
patients with subsequent peripheral adipose tissue loss. Kratz and 
colleagues126 showed th

se tissue loss. Kratz and 
colleagues126 showed that reduced expression of messenger RNA 
(mRNA) encoding lipoprotein lipase and the transcription fac­
tors SREBP1, PPARγ, and CCAAT/enhancer binding protein C/
EBPα in thigh subcutaneous fat were associated with the loss of 
fat, before it became evident clinically, whereas increased levels 
of mRNA encoding hydroxysteroid 11β-dehydrogenase 1 and the 
transcription factor C/EBPβ were found in adipose tissue from 
patients with preservation of sub

-axis is on log10 scale. Estimates from multivariable adjusted models controlling for age, sex, 
race, traditional cardiovascular disease risk factors, HIV-related factors, C-reactive protein, fibrinogen, 
estimated glomerular filtration rate using cystatin C, albuminuria, arm SM, leg SM, and VAT. Reference 
category is tertile 1, those with the lowest amount of muscle or adipose tissue. CI, confidence interval; SM, 
skeletal muscle; VAT, visceral adipose tissue. (From Scherzer R, Heymsfield SB,

l adipose tissue. (From Scherzer R, Heymsfield SB, Lee D, et al. Decreased 
limb muscle and increased central adiposity are associated with 5-year all-cause mortality in HIV infection. 
AIDS. 2011;25[11]:1405–1414, Fig. 1, used with permission.)
1683
novel mechanisms for adipose dysfunction in HIV.127 In addition, 
studies suggest increased activation of the renin-angiotensin-aldo­
sterone system associated with visceral adipose accumulation in 
HIV. Recent 

ith visceral adipose accumulation in 
HIV. Recent data suggest that use of the mineralocorticoid recep­
tor antagonist eplerenone in HIV-infected patients may reduce 
immune activation and improve select metabolic indices.128
Changes in fat distribution have been most widely recognized 
since the introduction of HAART, but abnormal fat distribution 
can be seen in antiretroviral-naive patients, suggesting that viral 
factors also may contribute. In this regard, recent data suggest that 
adipose 

In this regard, recent data suggest that 
adipose tissue dysfunction may be related to a viral accessory pro­
tein, Vpr, which may simultaneously coactivate the glucocorticoid 
receptor and repress PPARγ.129
The changes in fat distribution seen among HIV-infected patients 
bear some similarities to those in Cushing syndrome, with dorso­
cervical fat accumulation and centripetal fat distribution. However, 
more specific stigmata of true Cushing syndrome, including proxi­
mal muscle weakness, faci

ndrome, including proxi­
mal muscle weakness, facial plethora, thin skin, bruising, and viola­
ceous striae, have not been seen, and thus these changes constitute 
a pseudo-Cushing syndrome.130 Miller and coworkers130 observed 
normal cortisol levels and adequate suppression in response to dexa­
methasone among HIV-infected patients with cushingoid features. 
Yanovski and associates131 compared HIV-infected patients with 
PI-associated lipodystrophic changes in fat to control patients and 
those

phic changes in fat to control patients and 
those with true Cushing syndrome. In contrast to patients with true 
Cushing syndrome, patients with PI-associated lipodystrophy dem­
onstrated normal diurnal variation in cortisol levels. The 24-hour 
urine free cortisol levels were reduced and 17-hydroxysteroid levels 
were increased compared with levels in control subjects. Among 
HIV-infected patients with changes in fat distribution, increased 
11β-hydroxysteroid dehydrogenase expression in subcu

β-hydroxysteroid dehydrogenase expression in subcutaneous 
adipose tissue (SAT) has been demonstrated in association with an 
increased ratio of urinary cortisol to cortisone metabolites and may 
also contribute to increased cortisol production.132 In addition, as 
noted earlier, viral proteins may activate the glucocorticoid receptor 
in specific fat depots.129
Other abnormalities of steroid metabolism have been noted 
in association with changes in fat distribution. In a longitudinal 
evaluati

s in fat distribution. In a longitudinal 
evaluation, the development of lipodystrophy was associated with 
reduced DHEA, increased cortisol/DHEA ratio, and increased 
interferon α.133 Increased cortisol regeneration from affected 
fat depots may contribute to insulin resistance and further fat 
redistribution. 
Lipid Abnormalities
Lipid abnormalities are highly prevalent among HIV-infected 
patients, particularly those with changes in fat distribution and 
increased visceral fat and upper trunk

bution and 
increased visceral fat and upper trunk fat. Hypertriglyceride­
mia has long been associated with HIV infection; was observed 
prior to the introduction of potent ART; and is related, in part, 
to increased very low density lipoprotein (VLDL) secretion and 
decreased clearance.134 The cause of these changes is not known 
but may relate to the effects of viral infection itself, microbial 
translocation via lipopolysaccharide,135 altered cytokines includ­
ing interferon α,136 or increas

 cytokines includ­
ing interferon α,136 or increased apolipoprotein E.137 In longitu­
dinal studies, reduction in high-density lipoprotein (HDL), total 
cholesterol, and LDL cholesterol has been observed with serocon­
version. With antiretroviral treatment, cholesterol and LDL rise to 
preinfection levels but low HDL levels persist.138
Among HIV-infected patients receiving combination ART includ­
ing a PI, hypercholesterolemia (>240 mg/dL), hypertriglyceridemia 
(>200 mg/dL), and low HDL (<35 mg

triglyceridemia 
(>200 mg/dL), and low HDL (<35 mg/dL) were reported in 27%, 
40%, and 27%, respectively, compared with corresponding per­
centages of 8%, 15%, and 26% in previously untreated patients.139 
Among patients with changes in fat distribution, 57% demonstrated 
hypertriglyceridemia and 46% low HDL in comparison to an age- 
and BMI-matched cohort from the Framingham Offspring Study.140 
A phenotype of atherogenic small, dense LDL particles among 
HIV-infected patients with lipodystroph

les among 
HIV-infected patients with lipodystrophy has been described.141 
In addition, investigators have demonstrated increased triglycer­
ide enrichment of LDL and impaired hepatic lipase, suggesting 
impaired lipoprotein processing, further contributing to atherogenic 
dyslipidemia.142
Dyslipidemia among HIV-infected patients may result from 
the effects of antiretroviral drugs, including specific PIs, which 
have been shown to increase triglyceride levels. Use of PIs may 
also be associate

lyceride levels. Use of PIs may 
also be associated with an atherogenic dyslipidemia and an 
increase in small, dense LDL143; increased apolipoprotein C-III 
and apolipoprotein E; and decreased proteasomal degradation of 
apolipoprotein B.144,145 Mulligan and colleagues146 demonstrated 
that changes in lipid levels occur within 3 months of PI therapy. 
Hypertriglyceridemia is most severe among patients treated with 
ritonavir or the ritonavir/saquinavir combination. Among the 
currently approved

uinavir combination. Among the 
currently approved PIs, atazanavir is least often associated with 
hyperlipidemia.147 Lipid changes appear to be less and improve 
with use of atazanavir in contrast to other PIs.148 Some NNRTIs, 
such as efavirenz, may raise total cholesterol and non-HDL cho­
lesterol but also raise HDL cholesterol.149 Newer NNRTIs, such 
as rilpivirine and etravirine, have less pronounced effects on the 
lipid profile than efavirenz.150–152 Integrase strand transfer inhibi­
tors

enz.150–152 Integrase strand transfer inhibi­
tors, such as raltegravir, dolutegravir, elvitegravir, and bictegravir, 
have a more favorable lipid profile than PIs.153–155 These agents 
are preferred as initial management of HIV infection in combina­
tion with NRTIs.156 
Hyperglycemia and Insulin Resistance
Insulin resistance and DM are relatively common among HIV-
infected patients. In an early longitudinal study, DM was 3.1 
times more likely to develop in HIV-infected patients receiving 
comb

o develop in HIV-infected patients receiving 
combination ART than in control subjects.157 More recent studies 
confirm increased risk of DM prior to 2000 but found equivalent 
rates among HIV-infected and non–HIV-infected patients in the 
era of modern ART, with agents less likely to contribute to glucose 
abnormalities.158 These estimates suggest a peak incidence of 23.2 
in 1000 patient-years immediately prior to 2000 that has decreased 
to less than 10 in 1000 patient-years, with no differen

ss than 10 in 1000 patient-years, with no difference between 
subjects naive to and those using ART159 (Fig. 44.7). However, a 
recent analysis comparing a large nationally representative popula­
tion of HIV-infected persons had an adjusted DM prevalence of 
11.8%, which was significantly higher than 8.0% DM prevalence 
in the National Health and Nutrition Examination Survey.160 
BMI, lipodystrophy, low CD4 counts, and exposure to specific, 
older ART medications including stavudine and indinavi

r ART medications including stavudine and indinavir are pre­
dictive of DM in the HIV population.158 HIV-infected patients 
with impaired glucose tolerance demonstrate hyperinsulinemia, 
suggesting that insulin resistance contributes to the development 
of impaired glucose homeostasis in this population.
Hemoglobin A1c has been shown to underestimate levels of 
blood glucose in HIV-infected patients, particularly those with 
increased mean corpuscular volume resulting from specific ART 
agents. 

cular volume resulting from specific ART 
agents. In one study, use of an A1c cutoff of 5.8% optimized 
the area under the curve for the diagnosis of DM and increased 
1684
sensitivity from 40.9% to 88.8% while simultaneously decreasing 
specificity from 97.5% to 77.5% compared with use of a cutoff 
of 6.5%.161
Insulin resistance among HIV-infected patients may be caused 
by the abnormal fat distribution itself (e.g., increased central adi­
po

stribution itself (e.g., increased central adi­
posity, loss of peripheral subcutaneous fat, and associated molecu­
lar changes). These changes include altered cytokines (e.g., low 
adiponectin, increased resistin, or elevated TNF) or changes in 
other processes such as mitochondrial dysfunction, increased 
lipolysis, increased proteolysis, increased expression of suppres­
sor of cytokine signaling 1 (SOCS1),162 and increased accumula­
tion of fat in the muscle and liver. Increased inflammation 

t in the muscle and liver. Increased inflammation after 
initiation of ART is associated with development of DM among 
HIV-infected patients,163 and metabolic changes may result from 
persistent microbial translocation across the gastrointestinal bar­
rier, resulting in increased levels of lipopolysaccharide in the 
blood despite viral suppression.164 Metabolic abnormalities in 
HIV have been shown to be associated with increased levels of 
retinol-binding protein 4 (RB4), which may further cont

ol-binding protein 4 (RB4), which may further contribute to 
insulin resistance.165 Rarely, insulin receptor autoantibodies may 
develop among HIV-infected patients experiencing an immune 
reconstitution syndrome during ART initiation.166 In addition, 
significant evidence suggests direct effects of specific antiretrovi­
rals to reduce insulin sensitivity. PIs have been shown to decrease 
glucose uptake by inhibiting the transport function of glucose 
transporter type 4 (GLUT4) in vitro167 and h

cose 
transporter type 4 (GLUT4) in vitro167 and have been shown to 
reduce insulin sensitivity in vivo.168 Effects to decrease beta-cell 
apoptosis and compromise insulin secretion have also been postu­
lated.169 NRTIs are associated with insulin resistance, which may 
be a direct effect, potentially related to mitochondrial toxicity170 
or through effects on subcutaneous fat.121 For more details on 
the metabolic effects of specific ART agents, see Table 44.1. The 
specific manifestations and 

 see Table 44.1. The 
specific manifestations and course of DM in HIV patients have 
not been well studied, but initial studies suggest an increased risk 
of albuminuria in HIV-infected patients, particularly those with 
increased viral load, independent of blood pressure.171 
Treatment of Metabolic and Body Composition 
Changes in HIV-Infected Patients
There are several options available for the treatment of body com­
position and metabolic abnormalities in HIV-infected patients.
Treatment of A

rmalities in HIV-infected patients.
Treatment of AIDS Wasting and Loss of Lean Body Mass
Testosterone has been used successfully to increase lean body 
mass in men with AIDS wasting. Randomized studies of intra­
muscular testosterone for hypogonadal men with AIDS wasting 
suggest a beneficial effect of testosterone administration on lean 
body mass (2.0 kg over 6 months) and improved quality of life.172 
Although a limited number of studies have shown a benefit of 
anabolic steroids in HIV-infec

shown a benefit of 
anabolic steroids in HIV-infected patients with wasting, these 
agents potently suppress endogenous gonadal function and may 
Incidence (per 1000 PYFU)
97–98
99–00
01–02
03–04
05–06 07–08–09
100
PI
89
59
50
53
55
9
PI/r
21
27
30
41
47
10
NNRTI
43
49
47
47
45
69
Stavudine
66
46
25
13
7
47
Indinavir
35
19
12
5
3
31
Didanosine
36
35
31
24
15
ART regimen (%)
0
5
10
15
20
25
30
35
40
45
50
A
97–98
99–00
01–02
03–04
100
88
61
55
62
63
11
23

–02
03–04
100
88
61
55
62
63
11
23
31
35
49
55
13
47
50
45
43
41
77
73
53
30
16
8
49
37
20
14
6
3
32
41
40
35
26
17
100
91
57
43
43
45
7
19
22
25
31
37
6
39
48
50
51
49
59
57
37
19
11
6
43
34
18
9
5
3
30
31
29
26
21
13
0
5
10
15
20
25
30
35
40
45
50
B
• Fig. 44.7  Calendar incidence of new-onset diabetes and antiretroviral exposure in the ANRS CO8 
APROCO-COPILOTE cohort. Diabetes incidence per 1000 PYFU over the period from 1997 to 2009 
is indicated for two groups: (A) all inclu

o 2009 
is indicated for two groups: (A) all included patients, and (B) patients who had never previously been 
treated with ART prior to PI initiation (ART-naive, teal line) or patients who had previously been treated 
with ART (non-naive, purple line). The percentages of patients receiving various drugs during follow-up are 
presented below the graphs (in graph B, gray lettering corresponds to ART-naive patients). Vertical lines 
indicate 95% confidence intervals. ART, antiretroviral therapy; 

onfidence intervals. ART, antiretroviral therapy; NNRTI, non-nucleoside analogue reverse 
transcriptase inhibitor; PI, protease inhibitor; PI/r, ritonavir-boosted PI; PYFU, person-years of follow-up. 
(From Capeau J, Bouteloup V, Katlama C, et al. Ten-year diabetes incidence in 1046 HIV-infected patients 
started on a combination antiretroviral treatment. AIDS. 2012;26:303–314, Fig. 1, used with permission.)
1685
thus cause hypogonadism. Methyltestosterone a

AIDS
thus cause hypogonadism. Methyltestosterone and anabolic ste­
roids may cause problems with the liver, including peliosis hepatis, 
worsening liver function, and potentially malignancy. In addition, 
high-dose oxandrolone suppresses endogenous testosterone levels, 
significantly increases transaminase levels, and increases LDLs.173 
Nandrolone (100 mg intramuscularly every other week) was 
shown to be effective in increasing weight and lean body mass in 
HIV-infected women with weight loss.

body mass in 
HIV-infected women with weight loss.174 Anabolic steroids are 
associated with decreased HDL and other side effects, including 
effects on the liver, and hold no advantage over natural testos­
terone in the treatment of hypogonadism associated with AIDS-
related weight loss. Short-term use of anabolic steroids may be 
considered in eugonadal patients with severe wasting, but they 
may be associated with adverse effects and should generally be 
avoided. DHEA may be used to improve m

nerally be 
avoided. DHEA may be used to improve mood and depression 
among HIV-infected patients with subsyndromal depression and 
dysthymia,175 but the use of DHEA has not been standardized 
particularly with respect to dose, duration of treatment, and clini­
cal end points, and thus it remains investigational.
Several studies have investigated androgen administration to 
HIV-infected women with low weight. These studies have inves­
tigated testosterone using a transdermal patch designed to de

tosterone using a transdermal patch designed to deliver 
low physiologic doses of 150 to 300 μg/day. Among studies using 
the 150-μg/day dose, functional capacity and strength signifi­
cantly improved with a trend toward increased lean body mass 
over 6 months. Hirsutism was not seen, and virilization did not 
occur. An 18-month randomized, placebo-controlled study con­
ducted among relatively androgen-deficient HIV-infected women 
demonstrated that 300 μg/day increased lean body mass and 
bone 

hat 300 μg/day increased lean body mass and 
bone density at the hip and improved depression indices without 
aggravation of lipid or glucose parameters.176 Preliminary stud­
ies have investigated the effects of DHEA on androgen levels in 
HIV-infected premenopausal women, but the clinical utility of 
this strategy in HIV-infected women remains unknown.
Megestrol acetate is a synthetic progestational agent with 
glucocorticoid-like properties. Randomized studies in the litera­
ture show that meg

ndomized studies in the litera­
ture show that megestrol acetate increases weight 3 to 4 kg over 
12 weeks with an increase in caloric intake.177,178 However, the 
change in weight is almost entirely fat mass without an increase 
in lean body mass. In addition, megestrol acetate, because of 
its glucocorticoid-like properties, is associated with several side 
effects, including hypogonadism and hyperglycemia, and abrupt 
withdrawal can precipitate adrenal crisis. In children, megestrol 
acetate 

e adrenal crisis. In children, megestrol 
acetate promotes weight gain without improving linear growth.179
Several other agents have been used in the setting of AIDS 
wasting. Thalidomide blocks the action of TNFα and decreases 
esophageal ulcers in AIDS patients. Clinical studies demonstrate 
a modest beneficial short-term effect of thalidomide on weight 
indices but significant associated adverse effects, including rash 
and fever. Human chorionic gonadotropin results in increased 
testosteron

nic gonadotropin results in increased 
testosterone levels and may have independent effects to inhibit 
KS. No data are available from randomized controlled studies to 
determine effects on wasting in humans.
In addition, patients with AIDS wasting may demonstrate a 
typical pattern of nutrition-related GH resistance (see The GH/
IGF1 Axis section). These patients exhibit elevated GH levels but 
decreased IGF1, the primary hormone mediating the action of 
GH on muscle, suggesting GH resistance.1

ction of 
GH on muscle, suggesting GH resistance.180,181 GH has been 
used in HIV-infected patients to increase lean body mass in sar­
copenic patients with AIDS wasting. Among patients with AIDS 
wasting, high-dose, supraphysiologic GH (0.1 mg/kg per day) 
has been investigated. Relatively small but significant effects on 
weight (1.6 kg) were seen over 3 months in a placebo-controlled 
study.182 GH administration at 0.1 mg/kg has also been shown 
to increase work output during treadmill exerci

n 
to increase work output during treadmill exercise182 and quality 
of life, as well as peripheral muscle oxygen extraction and utiliza­
tion, in patients with AIDS wasting.183 These studies suggest that 
large doses of GH may be necessary to significantly increase lean 
mass in patients with AIDS wasting and nutritionally mediated 
resistance to GH. However, high-dose GH is associated with side 
effects including hyperglycemia and fluid retention184 and is not 
well tolerated in the long term.

on184 and is not 
well tolerated in the long term. For these reasons, caution should 
be used with respect to long-term high doses of GH for treat­
ment of AIDS wasting, because this treatment may cause acute 
and chronic side effects of GH excess. At high doses, GH was 
shown to increase thymic mass and circulating CD4 cells in one 
study185 and to induce HIV1-specific T-cell responses at low doses 
(0.7 mg/day) in patients on effective ART,186 but further studies 
are needed to investigate the

but further studies 
are needed to investigate the effects of GH on the immune axis.
A multidisciplinary approach to AIDS wasting syndrome is 
most useful. Optimization of ART is paramount in conjunction 
with provision of adequate nutrition and protein intake. However, 
even in this context, weight and muscle loss may occur because 
of the highly catabolic nature of the disease. In such cases, endo­
crine evaluation should include assessment of gonadal function, 
which will often be reduced. GH

gonadal function, 
which will often be reduced. GH levels are increased in AIDS 
wasting, but supraphysiologic administration, as approved by the 
FDA, can further increase lean body mass. This strategy is best 
reserved for severe wasting refractory to other treatments. Other 
therapeutic strategies that increase weight by stimulating appetite, 
including megestrol acetate, are not associated with gain in lean 
body mass and may be associated with side effects. In the current 
era of HAART, lip

th side effects. In the current 
era of HAART, lipoatrophy, usually associated with antiretroviral 
use, should be distinguished from traditional wasting. Whereas 
true AIDS wasting involves sarcopenia and requires anabolic strat­
egies to increase muscle mass, the presence of severe lipoatrophy 
suggests the need for strategies to spare fat loss. 
Strategies for Treating Lipoatrophy and Subcutaneous 
Fat Loss
Antiretroviral switching to less toxic NRTIs or PIs may be useful 
for improving chang

ic NRTIs or PIs may be useful 
for improving changes in fat distribution and hyperlipidemia.187 
For example, one study demonstrated that switching off lopinavir/
ritonavir to atazanavir/ritonavir improved glucose trafficking into 
muscle, reduced triglyceride levels (by 182 mg/dL on average), 
and decreased visceral fat by 25%.148 Other studies have dem­
onstrated significant increases in SAT and reductions in visceral 
adipose tissue (VAT) after switching to a non-thymidine ana­
logue–based AR

r switching to a non-thymidine ana­
logue–based ART strategy.188 LSM and resistance training are 
unlikely to reverse the loss of subcutaneous fat often seen in HIV-
infected patients receiving ART. 
Treatments for Visceral Fat Accumulation
LSM, in combination with exercise, may improve lipid levels and 
visceral adiposity among HIV-infected patients.189 Progressive 
resistance training (PRT) may also significantly improve glucose 
homeostasis in HIV-infected patients through a reduction in mus­

 HIV-infected patients through a reduction in mus­
cle adiposity.190 Studies of LSM in HIV-infected patients modeled 
after the Diabetes Prevention Program demonstrate effects of LSM 
to improve cardiorespiratory fitness and raise levels of HDL and 
C-reactive protein (CRP)191 but do not show significant effects on 
body composition.
Limited data are available on the use of testosterone among 
relatively androgen-deficient HIV-infected men with abdominal 
1686
SECTION IX    Polyendocrine and Neo

bdominal 
1686
fat accumulation (waist-to-hip ratio >0.95 or waist circumfer­
ence >100 cm).192 Ten grams of topical testosterone daily did not 
improve visceral fat mass but reduced overall trunk fat by 15% 
relative to placebo over 24 weeks. Significant effects on glucose 
and insulin were not seen. Effects of high-dose testosterone and 
anabolic steroids on lipid parameters may limit utility of this class 
of drugs to reduce visceral fat in

 of this class 
of drugs to reduce visceral fat in HIV-infected patients. More­
over, studies among older, male, non–HIV-infected patients sug­
gest that testosterone use may be associated with increased CVD 
events.193,194 Use of testosterone among HIV-infected men should 
be limited to those with documented hypogonadism, among 
whom treatment may improve lean body mass and bone.
GH has also been used to reduce visceral fat in HIV-infected 
patients with central fat accumulation. Lo and coworke

ents with central fat accumulation. Lo and coworkers195 inves­
tigated low-dose, long-term GH over 18 months in HIV-infected 
patients with central fat accumulation and deficient responses to 
GHRH-arginine testing. Low-dose GH was shown to reduce VAT 
by 9% over 18 months, simultaneously lowering blood pressure 
and triglyceride level. Even with low-dose GH, however, 2-hour 
glucose increased, demonstrating that GH is potentially useful 
in decreasing central fat accumulation but is hard to tit

easing central fat accumulation but is hard to titrate in a 
population with significant insulin resistance. Largely because of 
its ability to aggravate glucose, GH was not approved by the FDA 
for HIV lipodystrophy.
In contrast, GH secretagogues have been used successfully 
to increase lean body mass and reduce visceral and truncal fat, 
without significant effects on glucose. In the combined analysis of 
two large, randomized, placebo-controlled phase III trials of more 
than 800 patients, GH

ed phase III trials of more 
than 800 patients, GHRH(1-44) (tesamorelin) reduced VAT by 
15.4% relative to placebo in HIV-infected patients with central 
fat accumulation. Improvements were noted in triglyceride level 
(equal to 43 mg/dL), total cholesterol, and cholesterol-to-HDL 
ratio. Further improvements in body composition were seen over 
12 months196 (Fig. 44.8). Interestingly, no clinically significant 
effect to reduce SAT was seen, and thus GHRH(1-44) was selec­
tive for VAT. Use of GH

thus GHRH(1-44) was selec­
tive for VAT. Use of GHRH(1-44) was associated with physiologic 
increases in IGF1 levels and was not associated with increased glu­
cose or insulin levels, in contrast to low-dose GH. Larger reduc­
tions in visceral fat were associated with improvements in lipid 
and glucose levels, as well as adiponectin, suggesting metabolic 
improvements with VAT reduction.197 Recently, GHRH(1-44) 
was also shown to improve liver fat in association with visceral 
fat, suggesting be

t in association with visceral 
fat, suggesting beneficial effects on other ectopic fat depots.198 
Discontinuation of both GH and GHRH resulted in reaccumula­
tion of visceral fat back to baseline levels, demonstrating that the 
effects of these agents do not outlast the treatment period.199–201 
GHRH(1-44) was approved by the FDA for the treatment of cen­
tral fat accumulation in HIV-infected patients in 2010. 
Insulin-Sensitizing Strategies
Significant insulin resistance occurs among HIV-infe

gnificant insulin resistance occurs among HIV-infected patients, 
in association with use of specific antiretroviral agents, changes 
in fat distribution, and other factors. Metformin is particularly 
appropriate for use in patients with significant truncal adiposity 
and increased FFA concentrations, in whom insulin resistance is, 
in part, attributable to increased hepatic glucose production. In 
addition, metformin has a modestly favorable effect on lipids. In 
patients with hyperlipidemia, m

ect on lipids. In 
patients with hyperlipidemia, metformin has been demonstrated 
to decrease triglyceride and LDL levels without adversely affecting 
other parameters. The modest 10% to 20% reduction in plasma 
triglyceride levels is thought to be caused by decreased hepatic 
VLDL production.
The effects of metformin have been investigated in HIV-
infected patients with central fat accumulation and insulin resis­
tance. Using a dose of 500 mg orally twice a day in a randomized, 
placebo-control

o for initial 26 weeks and tesamorelin for subsequent 26 weeks; SAT, subcutaneous adipose 
tissue; T-T, tesamorelin for initial 26 weeks and subsequent 26 weeks; T-P, tesamorelin for initial 26 weeks 
and placebo for subsequent 26 weeks; VAT, visceral adipose tissue. (From Falutz J, Mamputu JC, Potvin 
D, et al. Effects of tesamorelin (TH9507), a growth hormone-releasing factor analog, in human immunodefi­
ciency virus-infected patients with excess abdominal fat: a pooled analysis of two multice

ss abdominal fat: a pooled analysis of two multicenter, double-blind 
placebo-controlled phase 3 trials with safety extension data. J Clin Endocrinol Metab. 2010;95:4291–
4304, Fig. 2, used with permission.)
1687
demonstrated that administration of low-dose metformin over 
12 weeks significantly reduced insulin resistance, diastolic blood 
pressure, tissue plasminogen activator, and plasminogen activator 
inhibitor concentrations and tended to reduce abdomin

ibitor concentrations and tended to reduce abdominal vis­
ceral fat, thus improving the CVD risk profile of such patients. 
The effects of metformin were sustained over 9 months.204 Other 
studies of metformin in HIV-infected patients also demonstrate 
general improvements in markers of insulin sensitivity but less 
consistent effects on abdominal fat.205 In a randomized trial, the 
effects of metformin and PRT were compared with metformin 
alone, and the addition of PRT to metformin further imp

, and the addition of PRT to metformin further improved 
central adiposity, blood pressure, and insulin resistance.206 Com­
pared with LSM, metformin was shown to significantly reduce 
coronary artery calcium in HIV-infected patients.191
The loss of subcutaneous fat in HIV-infected patients may fur­
ther contribute to insulin resistance by limiting peripheral glu­
cose and triglyceride uptake. Therefore, attention has focused on 
insulin-sensitizing strategies that may act to increase subcutaneo

ing strategies that may act to increase subcutaneous 
adipogenesis. The thiazolidinediones (TZDs) have been shown to 
promote adipogenesis, primarily through activation of PPARγ. 
Although the TZDs have effects on both hepatic and peripheral 
insulin resistance, the dominant effect is to improve peripheral 
glucose uptake. The therapeutic efficacy of the TZDs has been 
questioned among non–HIV-infected patients, in whom rosigli­
tazone, but not pioglitazone, has been associated with an increased

ioglitazone, has been associated with an increased 
mortality rate.207 Weight has reported to increase in response to 
TZDs, in contrast to metformin, which is associated with weight 
loss. Nonetheless, based on the known effects of TZDs to stimu­
late insulin sensitivity through PPARγ and promote adipogenesis, 
this strategy has been investigated in HIV-infected patients with 
subcutaneous fat loss and insulin resistance.
In a 3-month randomized, placebo-controlled study involv­
ing 28 patients

, placebo-controlled study involv­
ing 28 patients selected based on insulin resistance, rosiglitazone 
was shown to improve insulin sensitivity, adiponectin, FFA, and 
subcutaneous fat mass.208 In contrast, an effect of rosiglitazone 
on subcutaneous fat was not shown over 48 weeks in patients 
with lipoatrophy, selected for fat loss but not for insulin resis­
tance.209 Adiponectin levels significantly increased and resisting 
levels decreased in response to rosiglitazone among HIV-infected 
pa

n response to rosiglitazone among HIV-infected 
patients. Rosiglitazone may be effective in increasing subcutane­
ous fat in selected subpopulations of HIV-infected patients, par­
ticularly those with insulin resistance,210 but significant adverse 
effects on lipid levels, particularly LDL, suggest that pioglitazone 
may be better in this regard. Slama and colleagues211 investi­
gated pioglitazone among HIV-infected patients with confirmed 
lipoatrophy in a large, randomized, placebo-controlled 

trophy in a large, randomized, placebo-controlled trial. Pio­
glitazone increased peripheral fat and improved HDL levels, sug­
gesting the potential utility among HIV-infected patients with 
insulin resistance and fat atrophy. Overall, the TZDs contrast 
with metformin in terms of effects on lipid and body composi­
tion but demonstrate similar effects to improve glucose in HIV-
infected patients.212
Mitochondrial function is significantly affected by NRTI 
therapy in HIV-infected patients. Mallo

d by NRTI 
therapy in HIV-infected patients. Mallon and associates213 have 
shown that simultaneous use of NRTIs may reduce effects of TZD 
agents to promote subcutaneous adipogenesis in HIV-infected 
patients with lipoatrophy by limiting effects of rosiglitazone to 
increase adipose expression of PPARγ. In addition, therapies 
for mitochondrial dysfunction, including increasing glutathione 
with cysteine and glycine supplementation, have been shown to 
improve mitochondrial fat and carbohydrate

own to 
improve mitochondrial fat and carbohydrate oxidation and insu­
lin sensitivity in small studies among HIV-infected patients.214 
l-Acetylcarnitine was shown to decrease intramyocellular lipid 
accumulation and to increase percentage of leg fat while decreas­
ing FFA in an open label study215 and then was shown to increase 
T-cell mitochondrial DNA but to have no effect on body com­
position and metabolic indices in a second randomized study.216 
Studies to date using strategies to improv

dy.216 
Studies to date using strategies to improve mitochondrial function 
are small, and further research is needed in this area. 
Leptin Treatment for Metabolic Dysregulation Among 
HIV-Infected Patients With Lipoatrophy
Leptin levels are low in association with fat loss and lipoatrophy 
in HIV-infected patients. Initial small studies of leptin administra­
tion to HIV-infected patients with lipoatrophy and reduced leptin 
levels demonstrated significant improvements in insulin sensitiv­
ity, 

ignificant improvements in insulin sensitiv­
ity, HDL, and triglyceride, as well as reduced truncal and visceral 
fat.217,218 Among patients with mixed lipoatrophy and lipohyper­
trophy but low leptin levels, leptin administration improved total 
cholesterol and non-HDL cholesterol, as well as glucose parameters, 
but not HDL or triglyceride or lipid kinetics.219 Because leptin 
reduces appetite, administration may be associated with weight loss. 
Metreleptin is FDA approved for treatment of com

Metreleptin is FDA approved for treatment of complications related 
to congenital and acquired lipodystrophy, not related to HIV infec­
tion or its treatment. Further studies are needed to understand the 
clinical potential of leptin to improve metabolic parameters and fat 
redistribution among HIV-infected patients. 
Lipid Management
See the Strategies for Hyperlipidemia Among HIV-Infected 
Patients section. 
CVD in HIV-Infected Patients
CVD is increased among HIV-infected patients. Tradition

s increased among HIV-infected patients. Traditional risk 
factors for CVD in HIV-infected patients include increased smok­
ing rates, insulin resistance, atherogenic dyslipidemia, truncal adi­
posity, hypertension (HTN), impaired fibrinolysis, and increased 
plasminogen activator inhibitor I and tissue plasminogen activator 
levels, as well as reduced adiponectin and increased CRP levels. 
DM is a risk equivalent in HIV-infected patients, conferring an 
increase in the RR of coronary heart dise

ring an 
increase in the RR of coronary heart disease equivalent to 2.41.220 
More than 40% of HIV-infected subjects meet the definition of 
the metabolic syndrome, and predicted myocardial infarction 
(MI) rates are increased in this subgroup.221
Abnormalities in surrogate markers, including carotid intima-
media thickness and endothelial function, suggest increased CVD 
in HIV-infected patients. Abnormal endothelial function correlates 
with dyslipidemia, including increased chylomicrons, VLDL

slipidemia, including increased chylomicrons, VLDLs, and 
intermediate-density lipoproteins and reduced HDL. Evidence for 
subclinical atherosclerosis has been demonstrated using coronary 
computed tomography in HIV-infected patients. Lo and cowork­
ers222 demonstrated an increased prevalence of coronary plaque 
among asymptomatic HIV-infected patients compared with well-
matched non–HIV-infected patients (59% vs. 34%). Follow-up 
studies confirmed that the plaque in HIV-infected patients is mor

ed that the plaque in HIV-infected patients is more 
often noncalcified,223 with vulnerable plaque features, including 
low-attenuation fatty lesions and eccentric positive remodeling.224 
Using fluorodeoxyglucose positron emission tomography to assess 
the degree of inflammation at the vascular surface, Subramanian 
and associates225 demonstrated increased arterial inflammation (Fig. 
44.9), even among those with minimal traditional cardiovascular 
risk indices. Increased immune activation indi

ar 
risk indices. Increased immune activation indices correlated with 
the arterial inflammation, independent of traditional risk factors, 
1688
suggesting an important role for nontraditional risk factors, includ­
ing immune activation, in the accelerated atherogenesis observed 
in HIV-infected patients. In particular, markers of monocyte acti­
vation, including soluble CD163 and CD14,226,227 and bacterial 
translocation228 have been associat

and bacterial 
translocation228 have been associated with atherosclerotic indices, 
including intima-media thickness and noncalcified coronary plaque 
among HIV-infected patients. In contrast, traditional risk factors 
cluster with coronary calcium among such patients226 and may 
underestimate atherosclerotic burden.229 Genetic polymorphisms 
and mRNA expression of chemokine receptor type 5 (CCR5) and 
oxidative stress markers may also be associated with increased ath­
erosclerosis among HIV-inf

ted with increased ath­
erosclerosis among HIV-infected patients.230,231 Immune activation 
may contribute to the increased prevalence of inflamed vulnerable 
plaque with high-risk morphologic findings, which in turn may 
contribute to the increased risk of sudden cardiac death among 
HIV-infected patients.232
Several studies suggest increased MI rates in HIV patients. Initial 
studies suggested dyslipidemia from ART was a major factor con­
tributing to increased CVD risk in HIV-infected patient

ting to increased CVD risk in HIV-infected patients. In a large 
prospective study of 23,468 patients, the covariate-adjusted risk was 
1.26 per each additional year of antiretroviral exposure. Other risk 
factors included male sex, DM, older age, previous MI, HTN, and 
dyslipidemia.233 Controlling for dyslipidemia significantly reduced 
the effects of HAART exposure, suggesting that dyslipidemia may 
contribute to excess CVD in HIV-infected patients.
In contrast, more contemporary studies sugge

ents.
In contrast, more contemporary studies suggest that other fac­
tors beyond traditional risks may contribute to increased CVD 
rates in HIV. In a large study of patients in a major US health 
care center, Triant and colleagues234 demonstrated an RR of 1.75 
(95% confidence interval, 1.51–2.02; p < .0001) for increased MI 
in HIV versus non–HIV-infected patients in a model accounting 
for age, gender, and race (Fig. 44.10). Increased rates of tradi­
tional risk factors, including DM (11.5% v

 tradi­
tional risk factors, including DM (11.5% vs. 6.6%, HIV infected 
vs. non–HIV infected), HTN (21.2% vs. 15.9%), and dyslipid­
emia (23.3% vs. 17.6%) were seen, and each contributed to the 
increase in MI rates (RRs of 1.62, 1.98, and 3.03, respectively, for 
each risk factor). However, regression modeling, accounting for 
HTN, DM, and dyslipidemia, demonstrated that these three risk 
factors contributed to only 25% of the excess risk in HIV- versus 
non–HIV-infected patients, suggesting t

V- versus 
non–HIV-infected patients, suggesting that other factors might 
contribute. Freiberg and associates235 demonstrated an increased 
hazard ratio of 1.48 for MI among a large cohort of HIV-infected 
Non-HIV FRS-matched control participant
(age 43 y, TBR  2.01)
CORONAL VIEW
Ascending
aorta
SVC
AXIAL VIEW
AXIAL VIEW
Participant with HIV
(age 42 y, TBR  3.42)
Ascending
aorta
SVC
• Fig. 44.9  Representative 18F-FDG-PET/CT imaging of the aorta. There is increased aortic PET-FDG

ng of the aorta. There is increased aortic PET-FDG uptake (red 
coloration) in a participant infected with HIV compared with a non-HIV FRS-matched control participant. Neither 
participant had known heart disease. For each participant, the FRS was low with a score of 2, and calcium was 
not present on the cardiac CT scan. Neither participant was receiving a statin. A indicates anterior-posterior orien­
tation, and F indicates foot-head orientation. CT, computed tomography; 18F-FDG-PET, [18F]fluo

n. CT, computed tomography; 18F-FDG-PET, [18F]fluoro-2-deoxy-d-glu­
cose positron emission tomography; FRS, Framingham risk score; HIV, human immunodeficiency virus; SVC, 
superior vena cava; TBR, target-to-background ratio. (From Subramanian S, Tawakol A, Burdo TH, et al. Arterial 
inflammation in patients with HIV. JAMA. 2012;308:379–386, Fig. 2, used with permission.)
1689
men in the Veterans Administration system, controlling for Fram­
ingham risk factor

n system, controlling for Fram­
ingham risk factors. Smoking rates are increased and contribute to 
increased CVD risk in HIV patients.
Inflammation may play an important role in increased CVD 
risk among HIV-infected patients. Increased CRP levels are 
observed among HIV-infected patients and predict increased MI 
rates.236 The SMART (Strategies for Management of Antiretroviral 
Therapy) Study Group demonstrated an increased rate of cardio­
vascular events among patients randomized to interrupt

ular events among patients randomized to interrupted therapy 
with less strict goals for control of immune function.237 Indeed, a 
2014 study suggests that low CD4 and nadir CD4 are important 
contributing factors to CVD among HIV-infected patients, sug­
gesting an important role for immune dysfunction.238
HIV-infected women are also at risk for increased CVD. In 
large cohort studies, the RR of MI and stroke is more increased in 
HIV-infected women compared with non–HIV-infected female 
control

men compared with non–HIV-infected female 
control subjects (RR 2.98 for acute MI) than among HIV-infected 
men compared with non–HIV-infected male control subjects (RR 
1.40 for acute MI).234 Increased waist-to-hip ratio, visceral adipos­
ity, CRP, IL6, triglyceride, and LDL, as well as reduced HDL and 
adiponectin, have been demonstrated in HIV-infected women 
compared with age- and BMI-matched control subjects. Central 
adiposity was significantly predictive of abnormal CVD risk indi­
ces, in

ntly predictive of abnormal CVD risk indi­
ces, including newer inflammatory indices.239 In addition, studies 
suggest a marked increase in immune activation indices among 
HIV-infected female patients, including markers of monocyte 
activation, in association with noncalcified plaque.240
Strategies for Hyperlipidemia Among HIV-
Infected Patients
Treatment for hyperlipidemia among HIV-infected patients is 
indicated to reduce cardiovascular risk and should proceed accord­
ing to current guidelin

and should proceed accord­
ing to current guidelines. It should be noted that CVD risk pre­
diction equations established in the general population, such as 
Framingham coronary heart disease or American College of Car­
diology/American Heart Association scores, perform poorly in 
HIV-infected persons and tend to underestimate true risk.241 (Fig. 
44.11). An HIV-specific risk prediction equation established in 
the Data Collection on Adverse Effects of Anti-HIV Drugs Study, 
which includes HIV-s

cts of Anti-HIV Drugs Study, 
which includes HIV-specific variables, may improve risk predic­
tion compared with the Framingham CHD score242 although this 
has not been seen in all populations.243
Severe hypertriglyceridemia should be treated to reduce the risk 
of pancreatitis. An initial option in patients with severe elevations 
in triglyceride levels is to switch treatment to a PI less likely to 
cause dyslipidemia. Triglyceride levels are reduced approximately 
20% by diet244 and 30% by exe

duced approximately 
20% by diet244 and 30% by exercise245 in HIV-infected patients. 
Neither diet nor exercise is likely to normalize triglyceride levels in 
the HIV population with severe dyslipidemia. Fenofibrate resulted 
in a 40% reduction in triglyceride levels and a 14% reduction 
in total cholesterol levels over 3 months in HIV-infected patients 
with hypertriglyceridemia.246 Lesser, but nonetheless beneficial, 
effects may be seen with gemfibrozil or other fibrate derivatives. 
Niacin a

emfibrozil or other fibrate derivatives. 
Niacin also significantly reduces the triglyceride level but may 
worsen glucose tolerance in HIV-infected patients, although such 
changes in glucose may only be transient.247 In addition, niacin 
may be difficult to use because of its associated flushing and poten­
tial liver abnormalities. In a large randomized 24-week controlled 
trial of diet and exercise with either fenofibrate, niacin, or both, 
maximal reduction in triglyceride (–52%) was seen wi

ximal reduction in triglyceride (–52%) was seen with combina­
tion drug therapy plus diet and exercise, whereas HDL increased 
most with niacin and diet and exercise. Flushing was reported by 
35% to 40% of niacin-treated subjects.248 Recent studies also sug­
gest the efficacy of omega-3 polyunsaturated fatty acids, which 
were shown to reduce triglyceride levels by 25.5% and to be well 
tolerated in a randomized, placebo-controlled study.249 In a study 
investigating combined therapy, triglycer

a study 
investigating combined therapy, triglyceride levels were reduced 
by 65.5% among hypertriglyceridemic HIV-infected patients 
receiving simultaneous fenofibrate and fish oil.250 Comparing fish 
oil with gemfibrozil, fibrates, and atorvastatin in hypertriglyceri­
demic HIV-infected patients, fish oil was shown to decrease tri­
glyceride by 45 mg/dL, whereas fibrates (–66 mg/dL), but not 
atorvastatin (–39 mg/dL), were more effective in this regard.251
The 3-hydroxy-3-methylglutaryl coenzy

s regard.251
The 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) 
reductase inhibitors are most useful for lowering cholesterol levels 
in HIV-infected patients but are less effective in lowering triglycer­
ide levels. For example, pravastatin combined with dietary advice 
100
80
60
40
20
0
Events per 1000 PYs
18–34
35–44
45–54
55–64
Age group (years)
65–74
• Fig. 44.10  Myocardial infarction rates by age group. Top line (squares) 
indicates patients diagnosed with human immunodeficiency virus d

ents diagnosed with human immunodeficiency virus disease. 
Bottom line (diamonds) indicates patients not diagnosed with human immu­
nodeficiency virus disease. Data shown include both genders. Rates represent 
number of events per 1000 person-years (PYs) as determined by International 
Classification of Diseases coding. (From Triant VA, Lee H, Hadigan C, Grin­
spoon SK. Increased acute myocardial infarction rates and cardiovascular risk 
factors among patients with human immunodeficiency virus d

among patients with human immunodeficiency virus disease. J Clin 
Endocrinol Metab. 2007;92:2506–2512, Fig. 1B, used with permission.)
Risk type
Predicted
Observed
0.00
0.05
0.10
0.15
1
2
3
Risk probability for ASCVD
Group
• Fig. 44.11  Observed and predicted 5-year risk of atherosclerotic cardio­
vascular disease (ASCVD) risk by predicted risk group in persons living 
with human immunodeficiency virus (group 1: <5%; group 2 5–7.5%; 
group 3: >7.5%). (Redrawn from Triant VA, Perez J, Regan S, et

5%). (Redrawn from Triant VA, Perez J, Regan S, et al. Car­
diovascular risk prediction functions underestimate risk in HIV infection. 
Circulation. 2018;137[21]:2203–2214.)
1690
reduced total cholesterol levels by 17% over 24 weeks without 
effects on triglyceride levels.252 The combination of gemfibrozil 
and atorvastatin resulted in a 30% reduction in cholesterol and 
60% reduction in triglyceride in HIV-infected patients244 and may 
be use

ceride in HIV-infected patients244 and may 
be useful in HIV-infected patients with combined hyperlipidemia. 
When using the combination of HMG-CoA reductase inhibitors 
and fibric acid derivatives, the risk of rhabdomyolysis increases. 
In addition, PIs can themselves affect metabolism of the HMG-
CoA reductase inhibitors. In this regard, ritonavir was shown to 
increase simvastatin levels by 3059% and atorvastatin levels by 
79%.253 Use of simvastatin should be avoided in patients receiving 
P

astatin should be avoided in patients receiving 
PI therapy. In an analysis of almost 900 HIV-infected patients in 
the Kaiser Permanente system, use of statins was shown to lower 
LDL by 25.6% versus 28.3% in HIV-versus non–HIV-infected 
patients, whereas use of gemfibrozil lowered triglyceride by 44.2% 
versus 59.3% in HIV- and non–HIV-infected patients. Effects 
of statins on LDL did not vary by antiretroviral treatment class, 
whereas effects of gemfibrozil were less in HIV-infected patients

 of gemfibrozil were less in HIV-infected patients 
receiving PIs and best among those patients receiving therapy with 
NNRTIs. Safety of the statins, including atorvastatin, pravas­
tatin, and lovastatin, were good, with few cases of myositis. The 
relative potency of each statin on LDL was generally similar, with 
reductions ranging from nearly 26.4% with atorvastatin to nearly 
23.6% with pravastatin.254 Rosuvastatin must be used at lower 
doses due to potential interaction with darunavir but

es due to potential interaction with darunavir but has been 
shown to lower LDL by 28% at 10 mg/day in placebo-controlled 
trials.255 New drugs, such as pitavastatin, may have the fewest 
interactions with ART, because they are glucuronidated and do 
not affect CYP3A metabolism. Ezetimibe is well tolerated in com­
bination with statins in patients with HIV and further lowers LDL 
(additional 14% reduction).256 Effects of statins on clinical events 
have not yet been reported in large randomized 

s 
have not yet been reported in large randomized controlled tri­
als, but cohort studies suggest a potential benefit on overall mor­
tality rate among HIV-infected patients treated with ART with 
virologic suppression,257 and preliminary studies suggest effects 
to improve atherosclerotic indices, such as carotid intima-media 
thickness and coronary plaque.258,259 A large, placebo-controlled 
randomized trial examining the effect of pitavastatin on clinical 
events, called REPRIEVE, is under wa

 on clinical 
events, called REPRIEVE, is under way in HIV-infected persons 
who do not have an indication for statin therapy.260 Inhibition of 
proprotein convertase subtilisin kexin 9 (PCSK9) offers an addi­
tional treatment strategy for HIV-infected persons at high CVD 
risk.261 PCSK9 levels were found to be higher in HIV-infected 
persons compared with matched HIV-uninfected controls and 
were related to markers of monocyte activation.262 
Anti-Inflammatory Strategies for CVD in HIV
Initial 

ti-Inflammatory Strategies for CVD in HIV
Initial studies are now investigating effects of anti-inflamma­
tory strategies to improve endothelial function and reduce arte­
rial inflammation. Such strategies include use of pentoxifylline, 
canakinumab, and methotrexate, but safety and efficacy of these 
agents in the HIV population remain unclear. Use of statins may 
also be beneficial as an anti-inflammatory strategy, as they may 
lower CRP and have pleiotropic effects on monocyte activation.
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persons: the Data-collection on Adverse Effects of Anti-HIV Drugs 
(D:A:D) study. Eur J Prev Cardiol. 2016;23(2):214–223.
	243.	 Thompson-Paul AM, Lichtenstein KA, Armon C, et al. Cardiovas­
cular disease risk prediction in the HIV outpati

8):1081–1089.
	248.	 Balasubramanyam A, Coraza I, Smith EO, et al. Combination of 
niacin and fenofibrate with lifestyle changes improves dyslipid­
emia and hypoadiponectinemia in HIV patients on antiretroviral 
therapy: results of “heart positive,” a randomized, controlled trial. J 
Clin Endocrinol Metab. 2011;96(7):2236–2247.
	249.	 De Truchis P, Kirstetter M, Perier A, et al. Reduction in triglyceride 
level with N-3 polyunsaturated fatty acids in HIV-infected patients 
taking potent antire

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taking potent antiretroviral therapy: a randomized prospective 
study. J Acquir Immune Defic Syndr. 2007;44(3):278–285.
	250.	 Gerber JG, Kitch DW, Fichtenbaum CJ, et al. Fish oil and feno­
fibrate for the treatment of hypertriglyceridemia in HIV-infected 
subjects on antiretroviral therapy: results of ACTG A5186. J Acquir 
Immune Defic Syndr. 2008;47(4):459–466.
	251.	 Munoz MA, Liu W, Delaney JA, et al. Comparative effectiveness of 
fish oil versus fenofibrate, 

ve effectiveness of 
fish oil versus fenofibrate, gemfibrozil, and atorvastatin on lowering 
triglyceride levels among HIV-infected patients in routine clinical 
care. J Acquir Immune Defic Syndr. 2013;64(3):254–260.
	252.	 Moyle GJ, Lloyd M, Reynolds B, et  al. Dietary advice with 
or without pravastatin for the management of hypercholes­
terolaemia associated with protease inhibitor therapy. AIDS. 
2001;15(12):1503–1508.
	253.	 Fichtenbaum CJ, Gerber JG. Interactions between antiretroviral 

, Gerber JG. Interactions between antiretroviral 
drugs and drugs used for the therapy of the metabolic complica­
tions encountered during HIV infection. Clin Pharmacokinet. 
2002;41(14):1195–1211.
	254.	 Silverberg MJ, Leyden W, Hurley L, et al. Response to newly pre­
scribed lipid-lowering therapy in patients with and without HIV 
infection. Ann Intern Med. 2009;150(5):301–313.
	255.	 Eckard AR, Jiang Y, Debanne SM, et  al. Effect of 24 weeks of 
statin therapy on systemic and vascular infla

 of 
statin therapy on systemic and vascular inflammation in HIV-
infected subjects receiving antiretroviral therapy. J Infect Dis. 
2014;209(8):1156–1164.
	256.	 Chow D, Chen H, Glesby MJ, et  al. Short-term ezetimibe is 
well tolerated and effective in combination with statin therapy to 
treat elevated LDL cholesterol in HIV-infected patients. AIDS. 
2009;23(16):2133–2141.
	257.	 Moore RD, Bartlett JG, Gallant JE. Association between use of 
HMG CoA reductase inhibitors and mortality in HIV-

HMG CoA reductase inhibitors and mortality in HIV-infected 
patients. PLoS One. 2011;6(7):e21843.
	258.	 Calza L, Manfredi R, Colangeli V, et al. Two-year treatment with 
rosuvastatin reduces carotid intima-media thickness in HIV type 
1-infected patients receiving highly active antiretroviral therapy 
with asymptomatic atherosclerosis and moderate cardiovascular 
risk. AIDS Res Hum Retrovir. 2013;29(3):547–556.
	259.	 Lo J, Lu MT, Ihenachor EJ, et al. Effects of statin therapy on coronary 
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, et al. Effects of statin therapy on coronary 
artery plaque volume and high-risk plaque morphology in HIV-infected 
patients with subclinical atherosclerosis: a randomised, double-blind, 
placebo-controlled trial. Lancet HIV. 2015;2(2):e52–e63.
	260.	 Gilbert JM, Fitch KV, Grinspoon SK. HIV-related cardiovas­
cular disease, statins, and the REPRIEVE trial. Top Antivir Med. 
2015;23(4):146–149.
	261.	 Kohli M, Patel K, MacMahon Z, et  al. Pro-protein subtilisin 
kexin-9 (PCSK9) inhibition in 

protein subtilisin 
kexin-9 (PCSK9) inhibition in practice: lipid clinic experience in 2 
contrasting UK centres. Int J Clin Pract. 2017;71(11).
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subtilisin/kexin 9 levels in relation to systemic immune activation 
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1691
Phylogenesis and Embryology, 1692
Molecular Genetics, 1692
Classification, 1692
Biochemistry, 1693
Clinical Prese

sification, 1692
Clinical Presentation, 1696
Diagnosis, 1700
Treatment, 1703
Prognosis, 1707
Other Flushing Disorders, 1708
KEY POINTS
	•	
The World Health Organization (WHO) classification system 
(2017) in neuroendocrine tumors (NET-G1, NET-G2, NET-G3) 
and neuroendocrine carcinoma (NEC-G3) is informative for the 
clinical management of neuroendocrine intestinal tumors (carci­
noids).
	•	
The most important circulating biomarkers are chromo­
granin A (general), urinary 5-h

arkers are chromo­
granin A (general), urinary 5-hydroxyindoleacetic acid 
(5HIAA), and neuron-specific enolase (NSE) for lung 
tumors.
	•	
The carcinoid syndrome includes flushing, diarrhea, right-sided 
heart fibrosis, and bronchial wheezing.
	•	
Molecular imaging 111In-DTPA-Phe octreotide (Octreoscan) and 
more recently 68Ga-DOTA-octreotate positron emission tomog­
raphy (PET) scanning are important procedures for staging of 
the disease as well determining possible therapy with peptide 
re

inical and histopathologic description of carci­
noid tumor was made by Otto Lubarsch in 1888.1 He was 
impressed by the multicentric origin of carcinoid tumors 
of the gastrointestinal (GI) tract, their lack of gland formation, 
and their lack of similarity with the usual adenocarcinoma of the 
alimentary system.
The term Karzinoide was introduced in 1907 by pathologist 
Siegfried Oberndorfer2 as a descriptive name for what he considered 
to be a benign type of neoplasm of the ileum, which coul

a benign type of neoplasm of the ileum, which could neverthe­
less behave like a carcinoma. This myth of benignity has survived to 
the present, even though in 1949 Pearson and Fitzgerald3 described 
a large series of metastasizing carcinoid tumors. The term carcinoid 
is nowadays less used and the preferred recommended term is neuro­
endocrine tumor. The typical midgut carcinoid is called small intesti­
nal neuroendocrine tumor (SI-NET). Lung-neuroendocrine tumors 
are still usually called carc

uroendocrine tumors 
are still usually called carcinoids. In this chapter, the term carcinoid 
will be used also for the gastrointestinal NETs.
Neuroendocrine tumors have subsequently been reported in a 
wide range of organs, but they most commonly involve the lungs 
and GI tract. Carcinoid tumors of the thymus, ovaries, testes, 
heart, and middle ear have also been described. The clinically well-
known carcinoid syndrome was described by Thorson and associ­
ates4 in 1954; 1 year earlier, Lembec

 and associ­
ates4 in 1954; 1 year earlier, Lembeck5 had extracted serotonin 
from a carcinoid tumor.
1692
Phylogenesis and Embryology
Carcinoid tumors are derived from neuroendocrine cells, and 
Gosset and Masson6 in 1914 were the first to point out the neuro­
endocrine properties of carcinoid tumors. Masson7 later described 
the remarkable affinity for silver salts displayed by intracytoplas­
mic granules in tumor cells and noted that carcin

mic granules in tumor cells and noted that carcinoid tumors origi­
nate from enterochromaffin cells, the Kulchitsky cells in the crypts 
of Lieberkühn in the intestinal epithelium. Furthermore, he sug­
gested that the tumors were of endocrine origin (Fig. 45.1).
The mammalian GI tract and pancreas contain 14 endocrine 
cell types, which initially were believed to originate from the neu­
roectoderm. This observation gave rise to the APUD (amine pre­
cursor uptake and decarboxylation) concept bec

re­
cursor uptake and decarboxylation) concept because of the ability 
of these cells to take up and decarboxylate amino acid precursors 
of biogenic amines such as serotonin and catecholamines.8 The 
APUD concept was later revised by others, who postulated that 
these endocrine cells might also be derived from mesoderm and 
endoderm.9 The neuronal phenotype is clearly seen when cultur­
ing carcinoid tumor cells in  vitro. The enterochromaffin cells, 
from which many carcinoid tumors derive, hav

lls, 
from which many carcinoid tumors derive, have the property of 
producing and secreting amines (such as serotonin) and polypep­
tides (such as neurokinin A and substance P) (Fig. 45.2).
Carcinoid tumors might also originate from other neuroen­
docrine cells, such as the enterochromaffin-like (ECL) cells of 
the gut and endocrine cells in the bronchi. The tumors derived 
from these cells can produce a wide range of hormones, such 
as gastrin, gastrin-releasing peptide (GRP), ghrelin, calcito

 gastrin-releasing peptide (GRP), ghrelin, calcitonin, 
pancreatic polypeptide, adrenocorticotropic hormone (ACTH), 
corticotropin-releasing hormone (CRH), and growth hormone–
releasing hormone (GHRH), as well as somatostatin, glucagon, 
and calcitonin gene–related peptide (CGRP).10 A common secre­
tory product from all types of carcinoid tumors is the glycopro­
tein chromogranin A (CgA)—the most important general tumor 
marker in these patients (see later discussion). 
Despit

(see later discussion). 
Despite advances in the diagnosis, localization, and treatment of 
carcinoid tumors, no etiologic factor associated with the develop­
ment of these tumors has been identified. Little is known about 
molecular genetic changes underlying tumorigenesis. Sporadic 
foregut carcinoids as well as the familial-type multiple endocrine 
neoplasia type 1 (MEN1) often display allelic losses at chromo­
some 11q13, and somatic MEN1 gene mutations have been 
reported

nd somatic MEN1 gene mutations have been 
reported in one-third of sporadic foregut tumors.11 In contrast 
with foregut carcinoids, molecular and cytogenetic data for mid­
gut carcinoids are quite limited, and these tumors are not included 
in MEN1 syndrome. Deletions of chromosomes 18q and 18p have 
been reported in 38% and 33%, respectively, of GI carcinoids.12
In one publication, deletions on chromosome 18 were found 
in 88% of midgut carcinoid tumors, but the SMAD4/DPC4 locus 
was not delete

d tumors, but the SMAD4/DPC4 locus 
was not deleted.13 In addition to the consistent finding of deletions 
on chromosome 18, multiple deletions on other chromosomes (4, 
5, 7, 9, 14, 20) were noticed in single tumors. The region telomeric 
to SMAD4/DPC4/DCC loci must be further explored for possible 
losses of a tumor suppressor gene in this area. Gene expression 
arrays in carcinoid tumors have demonstrated upregulation of 
the RET proto-oncogene, but no mutations have been detected 
so far. Re

e, but no mutations have been detected 
so far. Reports indicate that the Notch signaling pathway is a sig­
nificant regulator of neuroendocrine differentiation and serotonin 
production in GI carcinoid tumors.14,15 The Wnt signaling path­
way as well as transforming growth factor-β (TGFβ) signaling are 
upregulated in carcinoid tumors.16 CDKN1B, a tumor suppressor 
in small intestinal nets, encodes a cyclin-dependent kinase inhibi­
tor p27Kip1.17 Mutations in the MUTYH gene involved in DNA 
rep

7 Mutations in the MUTYH gene involved in DNA 
repair have recently been reported.18 
In 1963, Williams and Sandler reported a relationship between 
the embryonic origin of carcinoid tumors and the histologic, bio­
chemical, and (to some extent) clinical features of the tumors.19 
Three distinct groups were formed (Table 45.1): foregut carci­
noids (i.e., intrathoracic, gastric, and duodenal carcinoids), mid­
gut carcinoids (carcinoids of the small intestine, appendix, and 
proxim

oids of the small intestine, appendix, and 
proximal colon), and hindgut carcinoids (carcinoid tumors of the 
distal colon and rectum).
Although this original classification has been useful in the 
clinical assessment of patients with carcinoid tumors, it has dem­
onstrated significant shortcomings. As a result, a new classifica­
tion system (World Health Organization [WHO] classification 
of 2010) has emerged that takes into account not only the site 
of origin but also variations in the histop

 site 
of origin but also variations in the histopathologic characteristics 
of carcinoid tumors.20 In this revised system, typical tumors are 
classified as NET-G1 tumors, which show a characteristic growth 
pattern. Most recently, the WHO classification system has been 
updated (2017). This includes a new group NET-G3, which are 
well-differentiated tumors with higher proliferation (Ki67 >20%) 
but not as aggressive as NEC-G3.21 Typical carcinoid tumors are 
usually slow growing, with low prol

id tumors are 
usually slow growing, with low proliferation capacity (prolifera­
tion index <2%). They are usually confined to the mucosa and 
submucosa and are less than 1 to 2 cm in diameter (classical 
• Fig. 45.1  Normal human intestine stained with chromogranin A (Chrom. 
A) to delineate neuroendocrine cells. The cells are scattered in the intes­
tinal mucosa.
1693
CHAPTER 45  Neuroendocrine Tumors and Related Disorders
midgut carcinoid). NET-G2 tumors show widely invasive growth 
and a hig

T-G2 tumors show widely invasive growth 
and a high proliferation index (PI 2–20%). Poorly differentiated 
carcinomas (NEC-G3) are large tumors with metastases and a 
proliferation index greater than 20% (Fig. 45.3). The European 
Neuroendocrine Tumor Society (ENETS) has proposed a new 
tumor-node-metastasis (TNM) classification and grading system, 
which is widely used in the clinic.22
The neuroendocrine lung tumors are divided into typical car­
cinoids, atypical carcinoids, large cell neuroend

cinoids, atypical carcinoids, large cell neuroendocrine carcinomas, 
and small cell lung carcinomas. The difference between the typical 
and atypical carcinoid is based on histopathologic features, with 
higher proliferation and necrosis found in the atypical carcinoid23 
(Fig. 45.4). The incidence of carcinoid tumors is similar in West­
ern countries and is estimated to be 2.8 to 4.5 per 100,000 people 
(Surveillance, Epidemiology, and End Results [SEER] database).24 
Because many carcinoid tu

lts [SEER] database).24 
Because many carcinoid tumors are indolent, the true incidence 
may be higher. In particular, appendiceal carcinoids have not 
been included in many studies, but a higher incidence of 8.2 per 
100,000 was found in an autopsy study when appendiceal car­
cinoids were included.25 The incidence of patients with a carci­
noid syndrome is about 0.5 per 100,000.26 Data from the United 
States, based on results from the End Results Group and the Third 
National Cancer Survey, 19

ts Group and the Third 
National Cancer Survey, 1950 to 1969 and 1969 to 1971, respec­
tively, found that the stomach was the most common site of carci­
noid tumors, followed by the rectum, ileum, lungs, and bronchi.27
An analysis done in the SEER program of the National Cancer 
Institute between 1973 and 2000 reported an increase in the per­
centage of pulmonary and gastric carcinoids and a decrease in the 
percentage of appendiceal carcinoids.24 Age-specific incidence rates 
showed a peak betw

4 Age-specific incidence rates 
showed a peak between 65 and 75 years (7.5–9.5/100,000), with 
a male predominance. In persons younger than 50 years, a female 
predominance has been observed both for appendiceal and lung 
carcinoids.28 Analyses of the SEER database (SEER 18) show an 
incidence of 6.98/100,000 in 2012 and a prevalence of 48/100,000 
indicating a significant increase since 199324,27,29 (Fig. 45.5). 
Biochemistry
The production of hormones is a highly organized function of 
carcino

ormones is a highly organized function of 
carcinoid cells. In 1953, Lembeck isolated serotonin from a carci­
noid tumor; since then, the carcinoid syndrome has been related 
to serotonin overproduction.5 The biosynthesis of serotonin and 
its metabolic degradation are outlined in Fig. 45.6.
• Fig. 45.2  Histopathology of a classic well-differentiated midgut carcinoid 
tumor.
	
Classification of Carcinoid Tumors
Foregut
Midgut
Hindgut
Argyrophilic
Argentaffin positive

ogy
CgA positive
SVP2 positive
NSE positive
CgA positive, NSE positive
Chromosome 11q13 deletion
Chromosome 18q, 18p deletion
Unknown
Secretory Products
CgA, 5HT, 5HTP, histamine, ACTH, GHRH, CGRP, 
somatostatin, AVP, glucagon, gastrin, NKA, 
substance P, neurotensin, GRP
CgA, 5HT, NKA, substance P, prostaglandin E1 and 
F2, bradykinin
PP, YY, somatostatin
Carcinoid Syndrome
Present (30%)
Present (70%)
Ab

Absent
ACTH, Adrenocorticotropic hormone; AVP, arginine vasopressin; CgA, chromogranin A; CGRP, calcitonin gene–releasing peptide; GHRH, growth hormone–releasing hormone; GRP, gastrin-releasing pep­
tide; 5HT, 5-hydroxytryptamine; 5HTP, 5-hydroxytryptophan; NKA, neurokinin; NSE, neuron-specific enolase; PP, pancreatic peptide; YY, peptide YY; SVP2, synaptic vesicle protein 2.
   
TABLE 45.1
1694

TION IX    Polyendocrine and Neoplastic Disorders
GEP NEN
WD-NET
Ki67 <3%
Mitotic index <2
Ki67 3-20%
Mitotic index 2-20
Ki67 >20%
Mitotic index >20
Ki67 >20%
PD-NEC
G1
G2
G3
Small cell type
• The final grade is determined based on
  whichever index (Ki67 or mitotic) places WD-
  NET in the highest grade category*
Large cell type
G3
*Klöppel G, Couvelard A, Hruban RH et al. Neoplasms of the neuroendocrine pancreas.
In: Lloyd RV, Osamura RY, Klöppel G et al, eds. WHO classificat

 Osamura RY, Klöppel G et al, eds. WHO classification of tumours of
endocrine organs. 4th ed. Lyon, France: IARC Press, 2017:209–239.
• Fig. 45.3  Pathologic classification of gastroenteropancreatic neuroendocrine neoplasms, GEP-NEN 
(from WHO in 2017). Mitotic index is per 10 high-power fields. G, grade; GEP, gastroenteropancreatic; 
HPF, high-powered fields; NEC, neuroendocrine carcinoma; PD, poorly differentiated; WD, well differenti­
ated. (Data from de Mestier L, Cros J, Neuzillet C, et al.

ata from de Mestier L, Cros J, Neuzillet C, et al. Digestive system mixed neuroendocrine-non-
neuroendocrine neoplasms. Neuroendocrinology. 2017;105:412–425.)
Lung NEN
Low grade
grade
High grade
Ki67 up to 5%*
No necrosis
Ki67 up to 20%*
Mitotic index 2-10
Focal necrosis, if any
Ki67 50%-100%*
Mitotic index >10
Extensive necrosis 
Ki67 40%-80%*
TC
AC
SCLC
• A tumor with carcinoid-like morphology and mitotic rate >10 is likely to

-like morphology and mitotic rate >10 is likely to be aggressive and is
  best classified as LCNEC*
LCNEC
*Travis WD, Brambilla E, Burke AP et al, eds. WHO classification of tumours of the lung, pleura, thymus and heart. 4th ed. Lyon, 
France: International Agency for Research on Cancer, 2015.
• Fig. 45.4  Pathologic classification of lung neuroendocrine neoplasms, Lung-NEN (from WHO in 2015). 
Mitotic index is per 2 mm2. AC, atypical carcinoid; LCNEC, large cell neuroendocrine carcinoma; SCLC, 

LCNEC, large cell neuroendocrine carcinoma; SCLC, 
small cell lung carcinoma; TC, typical carcinoid. (Modified from Travis WD. Pathology and diagnosis of 
neuroendocrine tumors: lung neuroendocrine. Thorac Surg Clin. 2014;24:257–266.)
1695
Neuroendocrine tumors of the midgut and foregut regions 
with metastatic disease secrete serotonin and show elevated uri­
nary excretion of 5-hydroxyindoleacetic acid (5HIAA) in 76% and 
30%, respectively

eacetic acid (5HIAA) in 76% and 
30%, respectively.30 Carcinoid tumors arising from the foregut, 
however, commonly have low levels of l-amino acid decarboxyl­
ase, which converts 5-hydroxytryptophan (5HTP) to serotonin. 
Thus these tumors secrete primarily 5HTP.31,32
For many years, it was believed that the entire carcinoid syn­
drome could be explained by the secretion of these biologically 
active amines. However, further studies have indicated that sero­
tonin is mainly involved in the patho

d that sero­
tonin is mainly involved in the pathogenesis of diarrhea and that 
other biologically active substances play a more important part in 
the carcinoid flush and bronchoconstriction.
Oates and associates33 proposed that kallikrein, an enzyme 
found in carcinoid tumors, is released in association with flush 
and stimulates plasma kininogen to liberate lysyl-bradykinin and 
bradykinin. These biologically active substances cause vasodila­
tion, hypotension, tachycardia, and edema.33–35 Fu

tion, hypotension, tachycardia, and edema.33–35 Furthermore, 
prostaglandins (E1, E2, F1, F2) might also play a role in the car­
cinoid syndrome.36 Gastric carcinoids and lung carcinoids have 
been found to contain and secrete histamine, which might be 
responsible for the characteristic bright red flush seen in these 
patients.37–39 Metabolites of histamine are often present in high 
concentration in the urine from these patients. Dopamine and 
norepinephrine have also been found in carcinoid t

norepinephrine have also been found in carcinoid tumors.40
The occurrence of substance P in carcinoid tumors was first 
demonstrated by Håkansson and coworkers in 1977.41 Sub­
stance P belongs to a family of polypeptides that share the same 
carboxy-terminus and are called tachykinins (Fig. 45.7). A number 
of tachykinin-related peptides have been isolated from carcinoid 
tumors, such as neurokinin A, neuropeptide K, and eledoisin. Dur­
ing stimulation of flush in patients with midgut carcinoids

lation of flush in patients with midgut carcinoids, multiple 
forms of tachykinins are released to the circulation (Fig. 45.8).42–44
Many different polypeptides (e.g., insulin, gastrin, somatosta­
tin, S100 protein, polypeptide YY, pancreatic polypeptide, human 
chorionic gonadotropin α-subunit [hCGα], motilin, calcitonin, 
vasoactive intestinal polypeptide [VIP], and endorphins) have 
been demonstrated in carcinoid tumors by immunohistochemical 
staining and sometimes in tumor extracts.10 Ectop

staining and sometimes in tumor extracts.10 Ectopic ACTH or 
CRH production may be found in foregut carcinoids; in particu­
lar, patients with bronchial or thymic carcinoids seem susceptible 
to Cushing syndrome.45 Patients with carcinoid tumors of the 
foregut type might also present with acromegaly due to ectopic 
secretion of GHRH from the tumor.46 Duodenal carcinoids as 
part of von Recklinghausen disease can secrete somatostatin.47
The chromogranin/secretogranin family consists of CgA, CgB

mogranin/secretogranin family consists of CgA, CgB 
(sometimes called secretogranin I), secretogranin II (sometimes 
called CgC), and some other members. CgA was first isolated in 
1965 as a water-soluble protein present in chromaffin cells from 
bovine adrenal medulla.48 Its immunoreactivity has been found 
in all parts of the GI tract and pancreas and has also been isolated 
from all endocrine glands.49
CgA is an acidic glycoprotein of 439 amino acids with a molecu­
lar weight of 48 kDa. It ca

o acids with a molecu­
lar weight of 48 kDa. It can be spliced into smaller fragments at 
dibasic cleavage sites, generating multiple bioactive fragments such 
as vasostatins, chromostatins, and pancreastatins49–53 (Fig. 45.9).
Amines and hormones are stored intracellularly in two types of 
vesicles: large dense-core vesicles and small synaptic-like vesicles. 
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
Lung
Stomach
Cecum
Appendix
Colon
Rectum
Pancreas
0
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982

1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
Incidence per 100,000
Year
NETs by site
Small intestine
• Fig. 45.5  Age-adjusted incidence of gastroenteropancreatic neuroendocrine tumors by primary site 
(SEER 18). Notice the significant increase during the past decades. NET, neuroendocrine tumor; SEER, 
Surveillance, Epidemiology, and End Resu

r; SEER, 
Surveillance, Epidemiology, and End Results. (From Dasari A, Shen C, Halperin D, et al. Trends in the 
incidence, prevalence, and survival outcomes in patients with neuroendocrine tumors in the United States. 
JAMA Oncol. 2017;3:1335–1342.)
1696
These vesicles release amines and hormones on stimulation. Large 
dense-core vesicles contain the hormones and one or more mem­
bers of the chromogranin/secretogranin family of proteins.50,54

hromogranin/secretogranin family of proteins.50,54 Both 
amines and peptides are co-released (Fig. 45.10).
The physiologic function of CgA is not fully elucidated. Its 
ubiquitous presence in neuroendocrine tissues and its co-secretion 
with peptide hormones and amines indicate a storage role of the 
peptide within the secretory granule.49,50,54 It also acts as a pro­
hormone that can generate bioactive smaller fragments. CgA is 
an important tissue and serum marker for different types of neu­
r

sue and serum marker for different types of neu­
roendocrine tumors, including those of the foregut, midgut, and 
hindgut (see Table 45.1 and later discussion). 
The clinical presentation of neuroendocrine tumors depends 
on localization, hormone production, and extent of the dis­
ease. Usually a lung carcinoid is diagnosed incidentally on rou­
tine pulmonary radiography, whereas a midgut carcinoid may 
be identified as a bowel obstruction or as a cause of abdomi­
nal disco

wel obstruction or as a cause of abdomi­
nal discomfort or pain. Rectal carcinoids might cause bleed­
ing or obstruction. However, lung carcinoids can also manifest 
clinically with Cushing syndrome, due to secretion of CRH 
or ACTH, or with the carcinoid syndrome, due to production 
of serotonin, 5HTP, or histamine.55 A midgut carcinoid often 
manifests with the carcinoid syndrome due to production of 
serotonin and tachykinins.
The clinical manifestations at referral depend on the type 
of ref

ifestations at referral depend on the type 
of referral center. At our institution, which cares for patients 
with malignant tumors, 74% of the patients present with the 
carcinoid syndrome, 13% with abdominal pain, 12% with carci­
noid heart disease, and 2% with bronchial constriction.27 When 
unbiased material is analyzed, bowel obstruction is the most 
common problem leading to the diagnosis of ileal carcinoid 
tumor. The second most common symptom is abdominal pain. 
Flushing and diarrhea, w

mptom is abdominal pain. 
Flushing and diarrhea, which are components of the carcinoid 
CH2   CH   COOH
N
NH2
CH2   CH   COOH
N
NH2
HO
CH2   CH2   NH2
N
HO
CH2   CHO
N
HO
CH   COOH
N
HO
5-Hydroxytryptamine (serotonin)
5-Hydroxyindoleacetaldehyde
5-Hydroxyindoleacetic acid
(5-HIAA)
Aldehyde
Monoamine
oxidase
Aromatic-L-amino
acid decarboxylase
• Fig. 45.6  Biosynthesis and metabolism of 5-hydroxytryptamine (5HT) (serotonin).
• Fi

ism of 5-hydroxytryptamine (5HT) (serotonin).
• Fig. 45.7  The tachykinin family of peptides shares the same carboxy-terminus. Neuropeptide K is a pro­
hormone containing neurokinin A, which can be spliced off.
Before flush 
fmol/fr
Fraction no. 
TKLI(K12)
10
5
8
4
SPLI(SP2)
Acetonitrile % 
40
30
20
During flush 
TKLI(K12)
10
5
8
4
SPLI(SP2)
40
30
20
20
0
8
0
6
0
4
0
NKA 
NPK NKB 
ELE SP 
• Fig. 45.8  Chromatography samples of plasma from a patient with car­
cinoid before flush (

sma from a patient with car­
cinoid before flush (upper panel) and during flush (lower panel). Note the 
significant increase in eledoisin-like peptide as well as in neuropeptide K. 
ELE, eledoisin; NKA, neurokinin A; NKB, neurokinin B; NPK, neuropep­
tide K; SP, substance P; SPLI(SP2), substance P–like immunoreactivity; 
TKLI(K12), tachykinin-like immunoreactivity.
1697
syndrome, constitute only the third most common presenta­
tion.56–58 B

 only the third most common presenta­
tion.56–58 Because many patients have vague symptoms, how­
ever, diagnosis of the tumor may be delayed by approximately 
2 to 3 years.26
The Carcinoid Syndrome
In 1954, Thorson and coworkers for the first time described the 
carcinoid syndrome as having the following features: malignant 
carcinoid of the small intestine with metastases to the liver, val­
vular disease of the right side of the heart (pulmonary stenosis 
and tricuspid insufficiency without sep

 stenosis 
and tricuspid insufficiency without septal defect), peripheral vaso­
motor symptoms, bronchial constriction, and an unusual type 
of cyanosis.4 One year later, Dr. William Bean59 gave this color­
ful description of the carcinoid syndrome: “This witch’s brew of 
unlikely signs and symptoms, intriguing to the most fastidious 
connoisseur of clinical esoterica—the skin underwent rapid and 
extreme changes—resembling in clinical miniature the fecal phan­
tasmagoria of the aurora borealis.

the fecal phan­
tasmagoria of the aurora borealis.”
The syndrome is thus well characterized and includes flush­
ing, diarrhea, right-sided heart failure, and sometimes bronchial 
constriction and increased urinary levels of 5HIAA.60,61 This is the 
classic carcinoid syndrome, but some patients display only one or 
two of the features. Other symptoms related to the syndrome are 
weight loss, sweating, and pellagra-like skin lesions.
Development of the carcinoid syndrome is a function of tumor 
ma

 the carcinoid syndrome is a function of tumor 
mass, extent and localization of metastases, and localization of the 
primary tumor. The syndrome is most common in tumors origi­
nating in the small intestine and proximal colon; 40% to 60% of 
patients with these tumors experience the syndrome.27,57,60,61 The 
disorders are less common in patients with bronchial carcinoids and 
do not occur in patients with rectal carcinoids.55,62,63 The syndrome 
rarely occurs in patients with midgut carcinoids 

rarely occurs in patients with midgut carcinoids and a small tumor 
burden, such as only regional lymph node metastases.58 Patients 
with the full syndrome usually have multiple liver metastases. The 
association with hepatic metastases is due to efficient inactivation by 
the liver of amines and peptides released into the portal circulation. 
The venous drainage of liver metastases is directly into the systemic 
circulation and bypasses hepatic inactivation.64
Other carcinoid tumors likely to 

 inactivation.64
Other carcinoid tumors likely to be associated with the carci­
noid syndrome in the absence of liver metastases are ovarian car­
cinoids and bronchial carcinoids, which release mediators directly 
into the systemic rather than the portal circulation. Retroperito­
neal metastases from classic midgut carcinoids also release media­
tors directly into the circulation and might cause the carcinoid 
syndrome without any liver metastases.60,61
Flushing
Four types of flushing have been 

s.60,61
Flushing
Four types of flushing have been described in the literature: ery­
thematous, violaceous, prolonged, and bright red.60,61
The first and best known type is the sudden, diffuse, ery­
thematous flush, usually affecting the face, neck, and upper chest 
(i.e., the normal flushing area) (Fig. 45.11). This type of flush is 
S
A1-15
77 
114 
208 
248 
Chromostatin 
WE14 
Catestatin 
Chromacin I 
Chromacin II 
GE25
Vasostatins 
Pancreastatins
Parastatin
322 
338 
373 400409 437 
• Fig. 4

tins
322 
338 
• Fig. 45.9  The glycoprotein chromogranin A and related peptides, including GE25 and WE14.
Portal circulation
5HT into gut lumen
Golgi
5HT
5HT
5HT
5HIAA
5HT
5HTP
MAO VMAT
RER
Secretory granules
(CgA, pancreastatin
chromostatin,
tachykinins)
Prohormones
and CgA
Prosecretory
granules
(processing of
prohormones
and CgA)
Packaging of
prohormones and CgA
• Fig. 45.10  Schematic of an enterochromaffin cell. The initial step in 5-hydroxy­
tryptamine

in cell. The initial step in 5-hydroxy­
tryptamine (5HT ) synthesis is carrier transport of the amino acid tryptophan from 
blood into the cell across the cell membrane. Intracellular tryptophan is first con­
verted to 5-hydroxytryptophan (5HTP), which in turn is converted to 5HT and 
stored in secretory granules. The transport of 5HT into granules requires vesicu­
lar monoamine transporters (VMATs). Via the basal lateral membrane, 5HT can 
be released into the circulation. There is also a membr

leased into the circulation. There is also a membrane pump mechanism 
in the cell membrane responsible for amine reuptake. A minor part of 5HT can 
also be released into the gut lumen. Monoamine oxidase (MAO) degrades 5HT 
to 5-hydroxyindoleacetic acid (5HIAA). Peptide prohormones are synthesized 
in the rough endoplasmic reticulum (RER) together with chromogranin A (CgA) 
and other granule proteins. The products are transported to the Golgi apparatus 
for packaging into prosecretory granules. O

ratus 
for packaging into prosecretory granules. On stimulation, the secretory products 
are released from the granules by exocytosis.
1698
commonly of short duration, lasting from 1 to 5 minutes, and is 
related to early-stage midgut carcinoids. Patients usually experi­
ence a sensation of warmth during flushing and sometimes heart 
palpitations. This type of flushing is reported in 20% to 70% of 
patients with midgut carcinoid at onset of th

 of 
patients with midgut carcinoid at onset of the disease.26,60,62
The second type is the violaceous flush, which affects the same 
area of the body. It has roughly the same time course or some­
times lasts a little longer. Patients may also have facial telangiecta­
sia. This flush occurs during the later stages of midgut carcinoid 
(Fig. 45.12) and is normally not felt by patients because they have 
become accustomed to the flushing reaction.
The third type is prolonged flushing that usually 

The third type is prolonged flushing that usually lasts a cou­
ple of hours but can last up to several days. This flush sometimes 
involves the whole body and is associated with profuse lacrima­
tion, swelling of the salivary gland, hypotension, and facial edema 
(Fig. 45.13). These symptoms are usually associated with malig­
nant bronchial carcinoids.
The fourth type of flushing is a bright red, patchy flush seen in 
patients with chronic atrophic gastritis and ECL cell hyperplasia, or 
ECLoma 

ic gastritis and ECL cell hyperplasia, or 
ECLoma (derived from ECL cells). This type of flushing is related 
to an increased release of histamine and histamine metabolites.
Flushes may be spontaneous or may be precipitated by stress 
(physical and mental); infection; alcohol; certain foods (spicy); or 
drugs, such as by injections of catecholamines, calcium, or pen­
tagastrin (see later discussion). The pathophysiology of flushing 
in the carcinoid syndrome is not yet elucidated.65–67 It was pr

oid syndrome is not yet elucidated.65–67 It was previ­
ously believed to be totally related to excess production of sero­
tonin or serotonin metabolites.66 However, several patients with 
high levels of plasma serotonin did not have any flushing, nor 
did a serotonin antagonist (e.g., methysergide, cyproheptadine, or 
ketanserin) have any effect on the flushing.65,68
In a study from our own group in which we measured the 
release of tachykinins, neuropeptide K, and substance P during 
flushing p

neuropeptide K, and substance P during 
flushing provoked by pentagastrin or alcohol, a clear correlation 
was found between the onset and intensity of the flushing reaction 
and the release of tachykinins (see Fig. 45.6). Furthermore, when 
the release of tachykinins was blocked by prestimulatory adminis­
tration of octreotide, little or no flushing was observed in the same 
patient (Fig. 45.14).42–44 Other mediators of the flushing reaction 
may be kallikrein and bradykinins, which are release

y be kallikrein and bradykinins, which are released during pro­
voked flushing.33–35
Histamine may be a mediator of the flushes seen in lung car­
cinoids and in gastric carcinoids (ECLomas).37–39 Tachykinins, 
bradykinins, and histamines are well-known vasodilators, and soma­
tostatin analogues might alleviate flushing by reducing circulating 
levels of these agents (see later discussion).42–44,67–72 Furschgott and 
Zawadski have suggested that flushing is caused by an indirect vaso­
dilation me

lushing is caused by an indirect vaso­
dilation mediated by endothelium-derived relaxing factor (EDRF) 
or by nitric oxide released by 5HTP during platelet activation.73
B
A
• Fig. 45.11  Carcinoid syndrome before and after provocation. (A) Before flush provocation. (B) The same 
patient after pentagastrin-stimulated flush.
• Fig. 45.12  Long-lasting chronic flushing in a patient with long-standing 
carcinoid disease. Note the telangiectases.
1699
CHAPTER 45  Neuroendocrine Tumors and Related Di

9
The facial flushing associated with carcinoid tumors should be 
distinguished from idiopathic flushing and menopausal hot flushes. 
Patients with idiopathic flushes usually have a long history of flush­
ing starting early in life and sometimes with a family history without 
occurrence of a tumor. Menopausal hot flushes usually involve the 
whole body and are accompanied by intense sweating. Postmeno­
pausal women in whom a true carcinoid 

. Postmeno­
pausal women in whom a true carcinoid syndrome is developing 
can differentiate between the two types of flushes. 
Diarrhea
Diarrhea occurs in 30% to 80% of patients with the carcinoid 
syndrome.26,27,60,61 Its pathophysiology is poorly understood but 
is probably multifactorial. The diarrhea is often accompanied by 
abdominal cramping, and endocrine, paracrine, and mechanical 
factors contribute to this condition. A variety of tumor prod­
ucts, including serotonin, tachykinins, hist

prod­
ucts, including serotonin, tachykinins, histamines, kallikrein, and 
prostaglandins, can stimulate peristalsis, electromechanical activ­
ity, and tone in the intestine.67,74–76 Secretory diarrhea can occur 
with fluid and electrolyte imbalance. Malabsorption can result 
from intestinal resections, from lymphangiectasia, secondary to 
mesenteric fibrosis, from bacterial overgrowth, and secondary to 
a tumor partially obstructing the small bowel or rapid intestinal 
transit. Increased secret

wel or rapid intestinal 
transit. Increased secretion by the small bowel, malabsorption, or 
accelerated transit can overwhelm the normal storage and absorp­
tive capacity of proximal colon and result in diarrhea, which may 
be aggravated if the reabsorptive function of the colon is impaired.
In a study of patients with elevated serotonin levels and the 
carcinoid syndrome, transit time in the small bowel and colon 
was significantly decreased in comparison with that of normal 
subjects.77 The v

 comparison with that of normal 
subjects.77 The volume of the ascending colon was significantly 
smaller than that in normal subjects, and the postprandial colonic 
tone was markedly increased. This indicates that in patients in 
whom the carcinoid syndrome is associated with diarrhea, major 
alterations in gut motor function occur that affect both the small 
intestine and the colon. Many patients with carcinoid tumors 
have undergone wide resection of the small intestine, and they 
may be affe

tion of the small intestine, and they 
may be affected by the symptoms of short-bowel syndrome.
Serotonin is believed to be responsible for the diarrhea in the 
carcinoid syndrome by its effects on gut motility and intestinal 
electrolyte and fluid secretion.61,74–76 Serotonin receptor antago­
nists, such as ondansetron and ketanserin, relieve the diarrhea to a 
certain degree.74,78–80 Recently an inhibitor of the enzyme trypto­
phan-hydroxylase, telotristat ethyl, was shown to reduce serotonin 

 telotristat ethyl, was shown to reduce serotonin 
levels and concomitantly the number of bowel movements.81 
Carcinoid Heart Disease
A unique endocrine effect of carcinoid tumors is the development of 
plaque-like thickenings of the endocardium, valve leaflets, atria, and 
ventricles in 10% to 20% of the patients.82,83 This fibrotic involve­
ment causes stenosis and regurgitation of the blood flow. Findings of 
new collagen beneath the endothelium of the endocardium is almost 
pathognomonic for

um of the endocardium is almost 
pathognomonic for carcinoid heart disease.82–84 The incidence of 
these lesions depends on the diagnostic methodology. Echocardiog­
raphy can demonstrate early lesions in about 70% of patients with 
the carcinoid syndrome, whereas routine clinical examinations detect 
them in only 30% to 40%.82,83,85 These figures have significantly 
dropped to 5% to 10%, probably because of earlier diagnosis and the 
use of biologic antitumor treatments such as somatostatin anal

gic antitumor treatments such as somatostatin analogues 
and α-interferons. Both of these agents control the hormonal release 
and excess that are thought to be involved in the fibrotic process.
In a study performed in 1987,26 40% of patients with carcinoid 
tumors died of cardiac complications related to the carcinoid dis­
ease. Data from 2008 reveal that this complication is a rare event 
and that patients usually die of the effects of a progressive tumor.27
The precise mechanism behind the fi

ssive tumor.27
The precise mechanism behind the fibrosis in the right side 
of the heart has not been determined yet, but it occurs mainly 
• Fig. 45.13  This patient has lung carcinoid and carcinoid syndrome with 
severe, long-standing flushing, lacrimation, and a swollen face.
*** p < 0.001 
***
***
***
***
TKLI % change 
200
150
100
50
0
−50
−15
Placebo
SMS
0 5 
15 
45 
Minutes
P or 
SMS
Pentagastrin
• Fig. 45.14  Tachykinin levels (TKLI ) after stimulation with pentagastrin in 
patients with

er stimulation with pentagastrin in 
patients with classic midgut carcinoids. Pretreatment for 15 minutes with 
somatostatin (SMS) causes inhibition of tachykinin release and inhibition of 
the flush reaction (open circles). P, placebo.
1700
in patients with liver metastases who usually also have the carci­
noid syndrome.82,83 Substances inducing fibrosis are believed to 
be released directly into the right side of the heart and are then 
neut

nto the right side of the heart and are then 
neutralized or degraded as they pass through the pulmonary cir­
culation, because few patients present with similar lesions of the 
left side of the heart.82,83 However, patients with lung carcinoids 
occasionally display the same fibrotic changes in the left side of 
the heart. Histologically, the plaque-like thickenings in the endo­
cardium consist of myofibroblasts and fibroblasts embedded in a 
stroma that is rich in mucopolysaccharides and colla

roma that is rich in mucopolysaccharides and collagen.82
We have previously shown that the TGFβ family of growth 
factors is upregulated in carcinoid fibrous plaques on the right 
side of the heart.86 The TGFβ family of growth factors is known 
to stimulate matrix formation and collagen deposition. The sub­
stances that induce TGFβ locally in the heart are not known, but 
serotonin, tachykinins, and insulin-like growth factor-1 (IGF1) 
may be mediators.82,87
A correlation has been found between 

iators.82,87
A correlation has been found between circulating levels of sero­
tonin and tachykinins and the degree and frequency of carcinoid 
heart lesions. The weight-reducing drugs fenfluramine and dexfen­
fluramine appear to interfere with normal serotonin metabolism 
and have been associated with valvular lesions identical to those 
seen in carcinoid heart disease.88,89 However, treatment resulting in 
decreased urinary 5HIAA excretion does not result in regression of 
cardiac lesions.90 Tw

not result in regression of 
cardiac lesions.90 Two animal studies indicate that serotonin might 
play a significant role in the development of carcinoid heart dis­
ease. Serotonin is also known to induce TGFβ1 in in vitro experi­
ments.91–93 Connective tissue growth factor (CTGF) is produced by 
carcinoid tumor cells, and increased expression is found in patients 
with advanced fibrosis. CTGF is known to stimulate TGFβ.94 For 
early detection of carcinoid heart disease, measurement of NT-
proBN

 carcinoid heart disease, measurement of NT-
proBNP is the best diagnostic and prognostic marker.95 
Bronchial Constriction
A true asthma episode is rare in patients with the carcinoid syn­
drome.26,60,61 The causative agents of bronchial constriction are 
not known, but tachykinins and bradykinins have been suggested 
as mediators.96,97 These agents can constrict smooth muscle in the 
respiratory tract and can also cause local edema in the airways. 
Other Manifestations of the Carcinoid Syndrom

ys. 
Other Manifestations of the Carcinoid Syndrome
Fibrotic complications other than heart lesions may be found in 
patients with carcinoid tumors. They include intra-abdominal and 
retroperitoneal fibroses, occlusion of the mesenteric arteries and 
veins, Peyronie disease, and carcinoid arthropathy.60,61
Intra-abdominal fibrosis can lead to intestinal adhesions and bowel 
obstruction and is a more common cause of bowel obstruction than is 
the primary carcinoid tumor itself.58,98,99 Retroperit

primary carcinoid tumor itself.58,98,99 Retroperitoneal fibrosis can 
result in obstruction of the ureter that impairs kidney function, which 
sometimes requires treatment with ureteral stents.
Narrowing and occlusion of arteries and veins by fibrosis are 
potentially life threatening. Ischemic loops of the bowel might 
have to be removed, and this procedure ultimately causes short-
bowel syndrome.58,99
Other rare features of the syndrome are pellagra-like skin 
lesions with hyperkeratosis and p

lagra-like skin 
lesions with hyperkeratosis and pigmentation, myopathy, and 
sexual dysfunction.61 
Carcinoid Crisis
Carcinoid crisis has become rare since the introduction of treatment 
with somatostatin analogues.100 It might occur spontaneously or 
during induction of anesthesia, embolization procedures, chemo­
therapy, or infection. Carcinoid crisis is a clinical condition char­
acterized by severe flushing, diarrhea, hypotension, hyperthermia, 
and tachycardia. Without treatment, patients 

ia, 
and tachycardia. Without treatment, patients might die during 
the crisis.100–102
Intravenous (IV) or subcutaneous somatostatin analogues (or 
both) are given before, during, and after surgery to prevent the 
development of carcinoid crisis.100,102–104 Patients with metastatic 
lung carcinoids are particularly difficult to treat during crisis. IV 
infusions of octreotide at doses of 50 to 100 μg/hour, supple­
mented with histamine H1-receptor and H2-receptor blockers and 
IV sodium chloride

r and H2-receptor blockers and 
IV sodium chloride, are recommended.105 
Other Clinical Manifestations of Carcinoid 
Tumors
Ectopic secretion of CRH and ACTH from pulmonary carcinoid 
tumors and thymic carcinoids accounts for 1% of all cases of Cush­
ing syndrome.45,106 Acromegaly due to ectopic secretion of GHRH 
has also been reported in foregut carcinoids.46,107 Gastric carcinoid 
tumors make up less than 1% of gastric neoplasms. They can be 
separated into three distinct groups or types on t

separated into three distinct groups or types on the basis of clini­
cal and histologic characteristics and originate from gastric ECL 
cells.108 Type 1 is associated with chronic atrophic gastritis type 
A (80%). Type 2 is associated with Zollinger-Ellison syndrome as 
part of MEN1 syndrome (6%). Type 3 represents sporadic gastric 
carcinoids occurring without hypergastrinemia and pursue a more 
malignant course, with 50% to 60% developing metastases.108,109
About 80% of gastric carcinoids are 

tases.108,109
About 80% of gastric carcinoids are associated with chronic 
atrophic gastritis type A, and more than 50% of patients with 
these carcinoids also have pernicious anemia. These tumors are 
more common in women than in men and are usually identified 
endoscopically during diagnostic evaluation for anemia or abdom­
inal pain.108,110 They are often multifocal and localized in the gas­
tric fundus area, and they are derived from ECL cells. Patients 
have hypochlorhydria and hypergastrin

s. Patients 
have hypochlorhydria and hypergastrinemia. Gastrin hypersecre­
tion has been postulated to result in hyperplasia of the ECL cells, 
which might later develop into carcinoid tumors.111,112 Hyper­
plasia of ECL cells has been noticed in patients on long-standing 
proton-pump inhibitor therapy.113,114 
Diagnosis
The diagnosis of a suspected carcinoid tumor must take into con­
sideration molecular genetics, tumor biology, histopathology, bio­
chemistry, and localization. The diagnosis o

 bio­
chemistry, and localization. The diagnosis of a carcinoid tumor may 
be suspected from clinical symptoms suggesting the carcinoid syn­
drome or from the presence of other clinical symptoms, or it can be 
made in relatively asymptomatic patients from the histopathologic 
findings at surgery or after liver biopsy for unknown hepatic lesions.
In one study involving 154 consecutive patients with GI car­
cinoids found at surgery, 60% were asymptomatic.115 In patients 
with symptomatic tumors, t

omatic.115 In patients 
with symptomatic tumors, the time from onset of symptoms until 
diagnosis is often delayed 1 to 2 years.26 The current tumor biol­
ogy program includes growth factors (platelet-derived growth fac­
tor, epidermal growth factor, IGF1, TGF)116 and proliferation 
factors (measurements of the nuclear antigen Ki67) as a prolifera­
tion index. Such an index correlates with tumor aggressiveness and 
survival.117,118 Adhesion molecules such as CD44, particularly 
exon-V6 and exon-

ules such as CD44, particularly 
exon-V6 and exon-V9, have been related to improved survival.119 
Determination of the expression of angiogenic factors basic fibro­
blast growth factor (bFGF) and vascular endothelial growth factor 
1701
(VEGF) should also be included in a tumor biology program. 
Somatostatin analogues are cornerstones in the treatment of the 
carcinoid syndrome; therefore determination of the different sub­
types of somatos

termination of the different sub­
types of somatostatin receptors (SSTR1–SSTR5) with specific anti­
bodies is warranted.120,121 Rare cases with familial carcinoids should 
be analyzed with respect to loss of heterozygosity on chromosome 
11q13 and chromosome 18. Integrated genome-wide analysis, 
including exome and whole-genome sequencing, gene expression, 
DNA methylation, and copy number analysis, has identified three 
novel molecular subtypes of small intestinal NETs (carcinoids) with 
differ

of small intestinal NETs (carcinoids) with 
different clinical outcome. Epigenetic dysregulation is common in 
small intestinal NET.17 Recently we identified mutations in the 
MUTYH gene involved in protein of DNA from oxidative stress.18
Histopathologic Diagnosis
The histopathologic diagnosis of carcinoids is based on immuno­
histochemistry using antibodies against CgA, synaptophysin, and 
neuron-specific enolase. These immunohistochemical stains have 
replaced the old silver stains, the argyro

s have 
replaced the old silver stains, the argyrophil stains by Grimelius 
and Sevier-Munger. The argentaffin stain by Masson to demon­
strate content of serotonin has also been replaced by immuno­
cytochemistry with serotonin antibodies.10 These neuroendocrine 
markers can be supplemented by specific immunocytochemistry 
to different hormones such as substance P, gastrin, and ACTH. 
The WHO classification forms the basis for therapeutic deci­
sions (see earlier discussion) and therefore determ

ions (see earlier discussion) and therefore determination of Ki67 
(MIB1) for analysis of cell proliferation is mandatory. Antibodies 
to TTF1 and CDX2 give good information on the localization of 
the primary tumor in patients with unknown primary tumors.122 
Biochemical Diagnosis
In patients with flushing and other manifestations of the carci­
noid syndrome, the diagnosis can be established by measuring the 
urinary excretion of 5HIAA because levels are invariably elevated 
under these circums

evels are invariably elevated 
under these circumstances.123 Patients with carcinoid tumors usu­
ally have urinary 5HIAA levels of 100 to 3000 μmol/24 hours 
(15–60 mg/24 hours) (reference range: <50 μmol/24 hours [10 
mg/24 hours]). Assays for urinary 5HIAA include high-pressure 
liquid chromatography (HPLC) with electrochemical detection 
and colorimetric and fluorescence methods.124 Various foods and 
drugs can interfere with the measurement of urinary 5HIAA, and 
patients should avoid these 

f urinary 5HIAA, and 
patients should avoid these agents during the 24-hour sampling 
(Table 45.2).125 Normally two 24-hour urine collections are rec­
ommended. In a study of patients with malignant midgut car­
cinoid tumors, 60% to 73% presented with increased urinary 
5HIAA levels,27,60,61 with a specificity of almost 100%.
Today, measurement of urinary 5HIAA for diagnosis of carci­
noid tumor is the predominant biochemical analytic procedure. 
However, urinary and platelet measurements of ser

However, urinary and platelet measurements of serotonin itself can 
give additional information. In some studies, platelet serotonin lev­
els were more sensitive than urinary 5HIAA and urinary serotonin 
levels and were not affected by the patient’s diet, as are 5HIAA levels.
In a comparative study of 44 consecutive patients with carci­
noid tumor, the platelet serotonin, urinary 5HIAA, and urinary 
serotonin levels were measured. In foregut carcinoids the sensitivi­
ties were 50%, 29%, and 55%

cinoids the sensitivi­
ties were 50%, 29%, and 55%, respectively. For midgut carcinoids, 
the sensitivities were 100%, 92%, and 82%, respectively, and for 
hindgut carcinoids they were 20%, 0%, and 60%, respectively.126
A method to determine 5HIAA in serum has been developed 
and will replace the urine 5HIAA.127 Elevations of 5HIAA can 
occur in malabsorption states and a number of other conditions. 
Foregut carcinoids tend to produce an atypical carcinoid syn­
drome with increased plasma 5HTP, 

 carcinoid syn­
drome with increased plasma 5HTP, but not serotonin, because 
they lack the appropriate decarboxylase that results in normal uri­
nary 5HIAA.31,40 However, some of the 5HTP is decarboxylated 
in the intestine and other tissues, and many of these patients have 
slightly elevated urinary 5HT or 5HIAA levels.
Attempts have been made to identify more specific and sensitive 
serum markers for carcinoid tumors that might allow earlier diagno­
sis. One such marker is CgA. It has been sh

iagno­
sis. One such marker is CgA. It has been shown that CgA and CgB are 
more abundant than CgC in human neuroendocrine tissues.49,50,128 
In 44 patients with carcinoid tumors, CgA was increased in 99%, 
CgB in 88%, and CgC in only 6% (Fig. 45.15).129 It has been pro­
posed that CgA levels in plasma might reflect tumor size. In a study 
of 75 patients with midgut carcinoids and the carcinoid syndrome, 
	
Factors That Interfere With Urinary 
5HIAA Measurement
Foods
Drugs
Factors That Produce

Foods
Drugs
Factors That Produce False-Positive Results
Avocado
Acetaminophen
Banana
Acetanilide
Chocolate
Caffeine
Coffee
Fluorouracil
Eggplant
Guaifenesin
Pecan
l-Dopa
Pineapple
Melphalan
Plum
Mephenesin
Tea
Methamphetamine
Walnuts
Methocarbamol
Methysergide maleate
Phenmetrazine
Reserpine
Factors That Cause False-Negative Results
None
p-Chlorophenylalanine
Heparin
Imipramine
Isoniazid
Methenamine mandelate
Methyldopa
Monoamine oxidase

Monoamine oxidase inhibitors
Phenothiazine
5HIAA, 5-Hydroxyindoleacetic acid.
   
TABLE 45.2
1702
CgA was elevated in 87% of carcinoid patients. Furthermore, a cor­
relation between levels of plasma chromogranin and extent of disease 
was found (p < 0.0001).27 In the same study, urinary 5HIAA was 
elevated in 76% of midgut carcinoids, and there were no correlations 
with tumor size or extent of dis

 no correlations 
with tumor size or extent of disease.
CgA is a more sensitive marker than urinary 5HIAA in detect­
ing carcinoid tumors, but because CgA is released and secreted from 
various types of neuroendocrine tumors, the specificity is lower.129–132 
Therefore in a workup of patients with the carcinoid syndrome, 
one should combine the determination of plasma CgA with urinary 
5HIAA or serotonin. Importantly, plasma CgA might be elevated in 
other conditions such as chronic atrophic gas

 in 
other conditions such as chronic atrophic gastritis, treatment with pro­
ton-pump inhibitors, or impaired kidney function.133 Plasma neuron-
specific enolase (NSE) shows a lower sensitivity and specificity than 
does plasma CgA.129 Plasma NSE is particularly helpful in patients 
with lung NETs and other NETs with higher proliferation.134 Serum 
hCGα has been reported to be increased in 60% of patients with fore­
gut carcinoid tumors and in 50% of those with hindgut carcinoids but 
only 11% 

0% of those with hindgut carcinoids but 
only 11% in those patients with midgut carcinoids and the carcinoid 
syndrome. Plasma neuropeptide K levels have been reported to be ele­
vated in 46% of patients with midgut carcinoids, whereas only 9% of 
patients with foregut carcinoids displayed elevated levels.130,135 Plasma 
substance P has a sensitivity of 32% and a specificity of 85%.27,42–44 
Neurokinin A is reported to be a sensitive marker for carcinoids of 
the small intestine both for diagnos

arcinoids of 
the small intestine both for diagnosis and prognostication.136 Pancre­
atic polypeptide levels are also elevated in about one-third of patients 
with midgut carcinoids and in as many with foregut carcinoids.137 
Recently a gene transcript test (51 genes) analyzing circulating mRNA 
presented a sensitivity/specificity of 98/97% to detect neuroendocrine 
tumors and predicted outcome of therapy.137,138
During therapy with somatostatin analogues, neither plasma 
CgA nor urinary 5HIAA i

analogues, neither plasma 
CgA nor urinary 5HIAA is a reliable marker of tumor size because 
somatostatin inhibits the synthesis and release of the hormones 
without changes in tumor size. 
Localization Procedures
Numerous imaging techniques, including endoscopy, barium 
enema, chest radiography, ultrasonography, computed tomography 
(CT), magnetic resonance imaging (MRI), and angiography, have 
been used to determine the location of the primary tumor as 
well as the metastases in patients with 

tumor as 
well as the metastases in patients with carcinoid tumors. In more 
recent years, somatostatin-receptor scintigraphy (SRS) and iodin­
ated meta-iodobenzylguanidine (131I-MIBG) scanning have been 
used to localize and stage the disease.139–142 Bronchial carcinoids 
are usually detected by chest radiography, CT, or (occasionally) 
bronchoscopy.143 The primary midgut tumor is usually small and 
difficult to localize with traditional diagnostic methods such as 
barium enema, CT scan, or MRI

tic methods such as 
barium enema, CT scan, or MRI. Some of these tumors can be 
localized by angiography, capsule endoscopy, or SRS. Liver metas­
tases are usually detected by CT or MRI. At present, CT or MRI 
and SRS are the primary diagnostic modalities for tumor staging 
(Fig. 45.16). SRS is nowadays replaced by 68Ga-DOTATATE/
TOC positron emission tomography (PET), which shows signifi­
cantly higher sensitivity and specificity.144,145
Other sensitive methods are PET or 18F-DOPA-PET scan­
ni

sensitive methods are PET or 18F-DOPA-PET scan­
ning146 using 11C5HTP, the precursor of serotonin synthesis (Fig. 
45.17).147,148 These isotopes accumulate in carcinoid tumors 
(APUD mechanism); with the development of PET cameras, 
tumors as small as 0.5 cm in diameter can be detected.147 During 
treatment, a close relation has been found among changes in the 
PET scan, transport rate constant, and urinary 5HIAA, suggest­
ing that PET scanning may be useful in monitoring the results of 
therapy

ay be useful in monitoring the results of 
therapy. PET scanning using 18F-fluorodeoxyglucose (FDG-PET) 
has been considered not useful in detecting low-proliferating neu­
roendocrine tumors, but recent data indicate it to be beneficial in 
identifying poorly differentiated anaplastic tumors but also well-
differentiated tumors.149
Carcinoid tumors contain high-affinity receptors for soma­
tostatin in 80% to 100% of cases.120,121,150 The receptors are pres­
ent in both the primary tumor and meta

s are pres­
ent in both the primary tumor and metastases. Five subtypes of 
somatostatin receptors have been cloned (SSTR1–SSTR5), and 
somatostatin receptor type 2 is the predominant subtype expressed 
in carcinoid tumors.
The most commonly available somatostatin analogue, octreo­
tide, binds with high affinity to SSTR2 and with lower affinity 
to SSTR3 and SSTR5.151–153 SRS with 111In-DTPA-Phe-octreo­
tide (Octreoscan) has been reported to detect carcinoids with a 
p-CgA 
Log CgA (nmol/L) 
10,

ct carcinoids with a 
p-CgA 
10,000
1000
100
10
1
Carcinoid EPTs
MEN1 Normal
p-CgB
Log CgB (nmol/L) 
100
10
1
0.1
p-CgC
Log CgC (pmol/L) 
100
10
• Fig. 45.15  Plasma (p) levels of chromogranin A (CgA), CgB, and CgC in patients with various neuroendo­
crine tumors. EPTs, endocrine pancreatic tumors; MEN1, multiple endocrine neoplasia type 1.
1703
sensitivity of 80% to 90

ors and Related Disorders
sensitivity of 80% to 90% in patients.153,154 Many studies have 
demonstrated that SRS has greater sensitivity for localizing car­
cinoids compared with conventional imaging studies.154–157 
False-positive scans can be encountered in patients with granu­
lomas (e.g., sarcoidosis, tuberculosis), activated lymphocytes 
(lymphomas, chronic infection), thyroid diseases (goiter, thyroid­
itis), endocrine pancreatic tumors, and other endocrine tumors. 
Because of its high sen

d other endocrine tumors. 
Because of its high sensitivity and ability to image, whole-body 
SRS should be the initial imaging procedure to localize and estab­
lish the stage of the disease. Bone metastases, which are common 
with carcinoid tumors, are efficiently detected by SRS, which is as 
sensitive as traditional bone scanning with technetium.144,158–161 
68Ga-DOTATATE/TOC-PET scanning has now been devel­
oped, showing higher sensitivity than Octreoscan and replacing it 
worldwide (Fig. 45.

n Octreoscan and replacing it 
worldwide (Fig. 45.18).
Scintigraphy with 123I-MIBG has been applied in patients 
with midgut carcinoids with a sensitivity of about 50%, which is 
lower than that for SRS (80–90%). However, it can pick up carci­
noids in patients who are sensitive to therapy with 123I-MIBG.162 
The current imaging workup is summarized by Van Essen and 
coworkers.163
A diagnostic algorithm is outlined in Fig. 45.19. 
Treatment
Treatment of carcinoid tumors with the carcinoid syndro

ment of carcinoid tumors with the carcinoid syndrome 
requires a multimodal approach, including symptomatic control 
B
A
• Fig. 45.16  Bronchial carcinoid. (A) Somatostatin-receptor scintigraphy in a patient with a bronchial carci­
noid. (B) Computed tomography scan in the same patient.
• Fig. 45.17  Positron emission tomography (PET) scan with 11C-5-hy­
droxytryptophan. Note the metastasis in the liver.
NET–small intestine
• Fig. 45.18  68Ga-DOTA-octreotate positron emission tomography/com­
put

A-octreotate positron emission tomography/com­
puted tomography (PET/CT) scan of a patient with carcinoid tumors. 
Notice liver and lymph node metastases. NET, neuroendocrine tumor.
1704
as well as tumor reduction. Most patients with the carcinoid syn­
drome have metastatic disease. The therapeutic goals are to amelio­
rate and improve clinical symptoms, abrogate the tumor growth, 
improve quality of life, and (if possible) prolong overall sur

ity of life, and (if possible) prolong overall survival.
Symptomatic control of the carcinoid syndrome includes life­
style changes, diet supplementation, and specific medical treat­
ment that reduces the clinical symptoms related to the different 
components of the carcinoid syndrome. Avoiding stress, both psy­
chologic and physical, as well as substances such as alcohol, spicy 
foods, and medications that precipitate a flushing reaction might 
be sufficient in early cases.61
Production of sero

Production of serotonin by the tumor consumes tryptophan. 
Normally, about 1% of body tryptophan is used for production of 
serotonin; in carcinoid tumors, however, as much as 60% of the 
available tryptophan may be consumed for the synthesis of sero­
tonin, and this can result in tryptophan and niacin deficiencies. 
Therefore supplemental niacin to prevent the development of pel­
lagra has been recommended over the years. Many patients have 
undergone resection o

e years. Many patients have 
undergone resection of the terminal ileum, which can result in 
vitamin B12 and folic acid deficiencies. Supplementation is needed 
in these patients.
Heart failure due to carcinoid heart disease can require diuret­
ics or angiotensin-converting enzyme (ACE) inhibitors. A few 
patients need bronchodilators such as salbutamol, which interacts 
with α-adrenergic receptors and does not induce flushing. The 
diarrhea seen in the carcinoid syndrome might be controlled by 

 in the carcinoid syndrome might be controlled by 
loperamide or diphenoxylate.164 If patients still have the carcinoid 
syndrome, they receive somatostatin analogue treatment, which 
has replaced most of the earlier types of serotonin and serotonin 
receptor inhibitors. Serotonin inhibitors (e.g., parachlorophenyl­
alanine and α-methyldopa), which inhibit serotonin synthesis, 
and serotonin receptor antagonists (e.g., cyproheptadine, methy­
sergide, and ketanserin) are not used routinely clinic

ide, and ketanserin) are not used routinely clinically.164
These earlier treatments had limited efficacy in terms of inhib­
iting flushing and diarrhea and were accompanied by significant 
side effects. Telotristat ethyl, an inhibitor of the enzyme tryp­
tophan hydroxylase, reduces the serotonin levels. In one study 
the compound reduced bowel movements by 44% and urinary 
5HIAA levels by 75%. The patients experienced a significant 
improvement.165 A combination of histamine H1 and H2 receptor 

65 A combination of histamine H1 and H2 receptor 
antagonists is effective in the carcinoid syndrome that is caused by 
foregut carcinoids due to concomitant secretion of histamine and 
serotonin. Prednisolone in doses of 15 to 30 mg/day gives occa­
sional relief in some cases with severe flushing and diarrhea.164
Somatostatin Analogues
Although natural somatostatin-14 reduces symptoms in patients 
with the carcinoid syndrome,166 its use is limited by its short half-
life (∼2.5 minutes). During 

ed by its short half-
life (∼2.5 minutes). During the past 30 years, synthetic somatosta­
tin analogues (octapeptides) have been developed for clinical use. 
Octreotide is the most commonly available drug; other analogues 
are lanreotide and vapreotide.167–172
The somatostatin analogues used in clinical practice (octreo­
tide, lanreotide) (Fig. 45.20) bind to receptors SSTR1 and SSTR5 
and, with lower affinity, to SSTR3. They exert their cellular 
action through interaction with specific cell an

action through interaction with specific cell and transmembrane 
receptors belonging to the superfamily of G protein–coupled 
membrane receptors. They inhibit adenylate cyclase activity, 
activate phosphotyrosine phosphatases (PTPs), and modulate 
mitogen-activated protein kinases (MAPKs).153,155,173–175 Recep­
tor subtypes 2 and 5 modulate K+ and Ca2+ fluxes in the cell.173 
Activation of all these pathways results in inhibition of known 
growth factor production and release and has antiproli

th factor production and release and has antiproliferative 
effects.154,155,168–171,173,174
SSTR3 is known to mediate PTP-dependent apoptosis 
accompanied by activation of TP53 and BAX.175 Four of the five 
somatostatin receptor subtypes (SSTR2–SSTR5) undergo rapid 
internalization after ligand binding, which has been explored by 
tumor-targeted radioactive somatostatin analogue therapy.176–179
An antiproliferative effect has been reported, probably through 
a combination of SSTR2 and SSTR5 acti

bly through 
a combination of SSTR2 and SSTR5 activities, which inhibits 
MAPK and K+ and Ca2+ fluxes leading to cell cycle arrest173–175; 
the precise antitumor mechanism, however, is not known.
It is now known that different subtypes of somatostatin recep­
tors (SSTR1 and SSTR5) form heterodimers with dopamine 
receptor D2. This cross-talk modulates the intracellular signal and 
gives a fine tuning of the mediated effects.177
All five subtypes of somatostatin receptors are expressed in 
carcin

of somatostatin receptors are expressed in 
carcinoid tumors; they are expressed in various combinations, 
although some tumors express all five subtypes.177–179 The recep­
tors are expressed not only on tumor cells but also in peritumoral 
veins.180 Antiangiogenesis might be another antitumor mecha­
nism of somatostatin analogues.181
Subcutaneous administration of octreotide and lanreotide every 8 
to 12 hours can control the clinical symptoms in about 60% to 70% 
of patients with the carcinoid

n about 60% to 70% 
of patients with the carcinoid syndrome; these agents are considered 
the drugs of choice.182–187 Octreotide and lanreotide decrease sero­
tonin and urinary 5HIAA levels as well as plasma tachykinin and CgA 
Human somatostatin
Octreotide acetate
ala
gly
cys
lys
asn
phe
cys
ser
thr
phe
phe
thr
trp
lys
S
S
phe
thr
lys
S
S
D-
phe
thr
-ol
D-
trp
cys
cys
tyr
val
lys
S
S
D-
trp
cys
cys
Thr
-NH2
D-
βnal
• Fig. 45.19  Molecular structure of human somatostatin-14, octreotid

ular structure of human somatostatin-14, octreotide 
acetate, and lanreotide.
1705
levels. The recommended dose for octreotide is 100 to 150 μg two or 
three times a day, a standard treatment for controlling clinical symp­
toms.186 However, some patients require higher doses, up to a total 
of 3000 μg/day, to control the clinical symptoms and tumor growth, 
particularly during long-term therapy.188
Tachyphylaxis (reduced sensitivity) to som

apy.188
Tachyphylaxis (reduced sensitivity) to somatostatin analogues 
can develop during long-term therapy.189 Long-acting, slow-
release formulations of octreotide acetate and lanreotide have been 
developed, and doses of octreotide LAR of 20 to 30 mg given 
once a month or lanreotide Autogel 90 to 120 mg once a month 
control clinical symptoms and hormone levels in 50% to 60% 
of patients with the carcinoid syndrome.189,190 The long-acting 
formulations of somatostatin analogues have clearly 

rmulations of somatostatin analogues have clearly improved the 
quality of life of patients by reducing the number of injections 
and provide more stable control of clinical symptoms.189 A newer 
somatostatin analogue, pasireotide, binding to SSTR1, SSTR2, 
SSTR3, and SSTR5, has come into some clinical trials in carci­
noid tumors. In a phase II study of carcinoid patients refractory to 
octreotide, the number of bowel movements and flushing episodes 
was reduced in one-third of the patients.190

odes 
was reduced in one-third of the patients.190,191 However, the drug 
did not exhibit superiority to standard octreotide in a randomized 
trial in small intestinal NETs.189
High-dose therapy with lanreotide (12 mg/day) and octreotide 
(3 mg/day) has increased the percentage of patients demonstrating 
a significant reduction in tumor size (12% vs. 5% for the standard 
dose).192–195 Induction of apoptosis has been reported during high-
dose therapy,196 possibly mediated through activation of S

rapy,196 possibly mediated through activation of SSTR3. 
Ultrahigh-dose octreotide has generated significant antitumor 
responses in patients resistant to standard dose therapy.195,196 A 
prospective randomized study in midgut carcinoids (PROMID) 
demonstrated a significantly longer time to progression for octreo­
tide LAR compared to placebo.197 These data have changed the US 
National Comprehensive Cancer Network (NCCN) guidelines 
for treatment of carcinoid tumors. All types of carcinoid tumo

t of carcinoid tumors. All types of carcinoid tumors, 
irrespective of functionality, can now be treated with octreotide. 
The early results indicating an antitumor effect of somatostatin 
analogues is further supported by a 2014 study (CLARINET).198 
Lanreotide significantly prolongs progression-free survival time in 
nonfunctioning neuroendocrine tumors.198
For patients at risk for carcinoid crisis, somatostatin analogue 
therapy is the treatment of choice. Carcinoid crisis is a life-threat­
e

nt of choice. Carcinoid crisis is a life-threat­
ening complication of the carcinoid syndrome and can occur 
spontaneously or may be associated with stress and anesthesia, 
chemotherapy, and infections (see earlier discussion). Patients 
usually experience severe flushing, diarrhea, abdominal pain, and 
hypotension. Continuous infusion with somatostatin analogues, 
50 to 100 μg/hour, is recommended and usually alters the life-
threatening condition. It is also recommended that patients be 
given

on. It is also recommended that patients be 
given subcutaneous somatostatin analogues before surgery or 
other stressful situations.
Side effects of somatostatin analogue therapy have generally not 
been serious and occur in 20% to 40% of patients. They include 
pain at the injection site, gas formation, diarrhea, and abdomi­
nal cramping. Significant long-term side effects include gallstone 
formation, sludge in the gallbladder, steatorrhea, deterioration of 
glucose tolerance, and hypocalcemi

terioration of 
glucose tolerance, and hypocalcemia.103,184,185,188,189,195 The inci­
dence of gallstones in patients treated over the long term has var­
ied from 5% to 70%, and the incidence of symptomatic gallstones 
requiring surgical treatment is less than 10%.189 
Interferons
Interferon-α (IFNα)—alone or in combination with a soma­
tostatin analogue—is effective in the treatment of the carcinoid 
Carcinoid suspected 
Negative
Positive 
SRS 
Negative 
Positive 
Therapy 
Biochemical diagnosis

Negative 
Positive 
Therapy 
CgA, u5HIAA 
Consider other
alternatives
Follow-up
3-6 mo 
CT, MRI
US + biopsy 
CgA, Syn, NSE
Ki67 
PET
68Ga-DOTA octreotate
11C-5-HTP
• Fig. 45.20  Diagnostic algorithm for small intestinal neuroendocrine tumors (carcinoids). CgA, chromo­
granin A; 11C-5-HTP, 11C-5-hydroxytryptophan; CT, computed tomography; 5HIAA, 5-hydroxyindoleacetic 
acid; MRI, magnetic resonance imaging; NSE, neuron-specific enolase; PET, positron emission t

 neuron-specific enolase; PET, positron emission tomogra­
phy; SRS, somatostatin-receptor scintigraphy; Syn, synaptophysin; US, ultrasound.
1706
syndrome. Symptomatic and biochemical control may be obtained 
in 40% to 50% of patients with the recommended doses of 3 to 
5 million units of recombinant interferon alfa-2a or interferon 
alfa-2b three to five times per week subcutaneously.184,199,200–204 
Significant tumor reduction is reported in 

0–204 
Significant tumor reduction is reported in 10% to 20% of the 
patients.204–206
IFNα exerts a direct effect on the tumor cells by blocking 
cell division in the G1/S phase, by inhibiting protein and hor­
mone synthesis, and by reducing angiogenesis through inhi­
bition of angiogenic factors bFGF and VEGF. It has also an 
indirect effect through stimulation of the immune system, 
particularly T cells and natural killer cells.207,208 Response to 
IFNα can be predicted by analyzing induction 

 to 
IFNα can be predicted by analyzing induction of 2′,5′-oli­
goadenylate synthetase or protein kinase p68 (PKR), enzymes 
involved in cell cycle regulation and protein synthesis.208–212 
Long-acting formulations of IFNα are now available (pegylated 
interferons) that can be applied at doses of 80 to 150 μg/week 
subcutaneously.
Treatment with IFNα induces an intratumoral fibrosis that is 
not picked up by regular CT scanning or ultrasonography; there­
fore tumor size may remain unchanged.213 

; there­
fore tumor size may remain unchanged.213 The side effects of 
α-interferons are more pronounced than with somatostatin ana­
logues and include chronic fatigue syndrome, anemia, leukopenia, 
and thrombocytopenia as well as the development of autoimmune 
reactions in 10% to 15% of the patients.208,214 Most of the side 
effects are dose dependent and can be managed by individualizing 
the dose.
Patients with the carcinoid syndrome who have not responded 
to octreotide or IFNα alone may be 

not responded 
to octreotide or IFNα alone may be given a combination of both 
agents. Such combinations have generated symptomatic control in 
70% of patients and stabilization of tumor growth in 40% to 50% 
of patients.215–217 The combination also offers better tolerance of 
α-interferons when somatostatin analogues are added. Moreover, 
somatostatin analogue treatment is hampered by development 
of tachyphylaxis with time, which means less sensitivity to the 
somatostatin analogue, necessitat

sitivity to the 
somatostatin analogue, necessitating escalating doses and, finally, 
withdrawal of the compound for several months, when IFNα 
therapy can continue. Conversely, IFNα can be withdrawn and 
the somatostatin analogue can be continued if severe side effects 
to IFNα (mainly chronic fatigue syndrome or mental depression) 
develop.215–217 Recently, a prospective randomized phase III study 
between IFNα2b and bevacizumab showed similar antitumor 
activity.218 
Most agree t

antitumor 
Most agree that patients with classic midgut carcinoids and the 
carcinoid syndrome, in which tumors show low proliferation 
capacity, should not receive chemotherapy. The results in various 
studies have been disappointing: Response rates are not more than 
5% to 10%, are short lived, and are accompanied by considerable 
side effects.219,220 The combination of streptozotocin and 5-fluo­
rouracil, which has demonstrated antitumor effect in endocrine 
pancrea

emonstrated antitumor effect in endocrine 
pancreatic tumors, has not shown similar effects in classic mid­
gut carcinoids.220 In foregut carcinoids, which usually manifest 
a more malignant behavior, however, cytotoxic treatment may be 
attempted. Such combinations include streptozotocin plus 5-fluo­
rouracil, doxorubicin, cisplatin plus etoposide, and dacarbazine 
plus 5-fluorouracil.221–223 Temozolomide has significant efficacy 
in foregut carcinoids.224 All of these cytotoxic treatments can 

cinoids.224 All of these cytotoxic treatments can be 
combined with a somatostatin analogue. 
Other Agents
Tyrosine kinase receptors (platelet-derived growth factor recep­
tor [PDGFR] α/β, epidermal growth factor receptor [EGFR], 
VEGFR) are expressed in carcinoid tumor cells as well as in the 
stroma cells. Therefore treatment with tyrosine kinase receptor 
inhibitors has been attempted with objective response rates of 
about 10% to 15%.225 Mammalian target of rapamycin (mTOR) 
inhibitors are n

alian target of rapamycin (mTOR) 
inhibitors are new drugs that block the mTOR signaling path­
way, which is activated in many tumors. Everolimus alone or in 
combination with octreotide has generated 15% to 20% objec­
tive responses.226–228 Everolimus was evaluated in a randomized 
placebo-controlled study in nonfunctional lung or gastrointesti­
nal NETs (RADIANT4). Treatment with everolimus was associ­
ated with significant improvement in progression-free survival.225 
However, in another tria

ession-free survival.225 
However, in another trial in patients with carcinoid syndrome, 
no difference in overall survival was noted in patients receiving 
everolimus versus placebo (RADIANT2).228 
Surgery
Because most tumors in patients with the carcinoid syndrome 
are malignant at the time of clinical presentation, surgical cure 
is seldom obtained. Resection of local disease or regional nodular 
metastatic disease can cure some patients; however, even if radical 
surgery cannot be performed,

ver, even if radical 
surgery cannot be performed, debulking procedures and bypass 
should always be considered and can be performed at any time 
during the course of treatment.58,99,229
A more proactive attitude among surgeons has emerged, and 
currently wider resections and debulking procedures are being 
performed.229,230 Resection of primary tumor and locoregional 
metastases in patients has recently been debated. The final deci­
sion needs a prospective randomized trial.231 In contrast to o

 prospective randomized trial.231 In contrast to other 
metastatic tumors to the liver, in which liver transplantation has 
generally given poor results, an interest in liver transplantation is 
increasing for patients with metastatic carcinoids.232–234 In a review 
of 103 patients with malignant neuroendocrine tumors, includ­
ing both carcinoids and pancreatic endocrine tumors, 5-year and 
2-year survival rates were 16% and 47%, respectively; however, 
recurrence-free survival rate was less tha

wever, 
recurrence-free survival rate was less than 24%.234 Liver transplan­
tation might be considered in younger patients (<50 years of age) 
with a life-threatening uncontrolled carcinoid syndrome during 
medical therapy or tumor-targeted radioactive treatment with­
out known metastatic spread outside the liver (Milan criteria). 
However, a study from our group challenges the results of liver 
transplantation, with a 5-year survival time in patients younger 
than 55 years without surgery of 9

tients younger 
than 55 years without surgery of 92 ± 9 months compared with 
transplanted patients fulfilling the Milan criteria showing a 5-year 
survival time of 97 ± 6 months.232
Another means of tumor reduction is hepatic artery emboliza­
tion, which not only improves the carcinoid syndrome in about 
50% of the patients but also reduces the tumor size in as many. 
The therapeutic effect may last for 9 to 12 months, and the pro­
cedure can be repeated.235,236 Chemoembolization, simultane­
ou

 repeated.235,236 Chemoembolization, simultane­
ous embolization with surgical gel (Gelfoam), and chemotherapy 
(doxorubicin, mitomycin C, cisplatin, 5-fluorouracil), or IFNα 
has resulted in symptomatic improvement in a significant number 
of patients with the carcinoid syndrome.237,238 However, hepatic 
artery occlusion or embolization can result in serious side effects 
(nausea, vomiting, liver pain, fever) and major complications (hep­
atorenal syndrome, sepsis, gallbladder perforation, and 

al syndrome, sepsis, gallbladder perforation, and intestinal 
necrosis). Complications are seen in 5% to 7% of patients.236–238
1707
Other cytoreductive treatments include cryotherapy and radio­
frequency ablation.239 However, these procedures are limited to 
patients with smaller tumor burden, tumors less than 5 cm in 
diameter, and a limited number of metastases. 
External irradiation has demonstrated limited efficacy and is u

diation has demonstrated limited efficacy and is used 
mainly to palliate symptoms related to bone and brain metasta­
ses.240,241 MIBG is taken up by carcinoids and is concentrated. 
The possibility of radiolabeled MIBG therapy has been evalu­
ated in a limited number of patients. The response rate has been 
reported to be about 30% with 125I-MIBG or 131I-MIBG.242,243
Somatostatin analogue–based tumor-targeted radioactive treat­
ment has been applied using 111In-DTPA-octreotide. Symptom­
atic im

lied using 111In-DTPA-octreotide. Symptom­
atic improvement is reported in about 40% of the patients and 
tumor stabilization in about 30%.244 Indium-111 (111In) is a weak 
irradiator (Auger electrons) and has been replaced by yttrium-90 
(90Y) and lutetium-177 (177Lu) (β-emitters).245,247
Studies with 90Y-DOTA-octreotide have been reported with 
promising results.245 The relatively new isotope, 177Lu-DOTA-
octreotate, has come into clinical use with further improved 
results. Significant tumor 

with further improved 
results. Significant tumor reduction occurs in 30% to 40% of 
patients with advanced disease. However, it is more effective for 
small tumors. It is an attractive mode of treatment because the 
radioactive ligand, after binding to the receptor, is internalized 
and transported to the cell nucleus, causing DNA damage.245 
Because tumor cells usually have higher-density somatostatin 
receptors (SSTR2 and SSTR5) than do surrounding normal tis­
sues, the treatment might be bet

nding normal tis­
sues, the treatment might be better tolerated. A pivotal trial was 
recently published where small intestinal NETs were treated with 
either four cycles of 177Lu-DOTATATE or 60 mg octreotide LAR 
every 4 weeks (NETTER1). Patients undergoing radioactive treat­
ment showed a significant longer progression-free survival.248 This 
trial has now been the basis for registration by EMA and FDA for 
treatment of patients with nonsurgical metastatic gastrointestinal 
NETs. Most recently

l metastatic gastrointestinal 
NETs. Most recently, a genomic signature in blood was shown to 
predict response of PRRT with 177Lu octreotide.249
In liver-dominant disease, radioembolization might be a valid 
alternative using 90Y microspheres in the form of glass or resin 
beads injected into the hepatic artery.250 The current treatment of 
metastatic carcinoid tumor is summarized in an algorithm (Fig. 
45.21). 
Prognosis
Clinically, the carcinoid syndrome is generally a manifestation of 
advan

id syndrome is generally a manifestation of 
advanced disease. Carcinoids from various sites differ not only 
in the percentage developing the carcinoid syndrome but also in 
their aggressiveness. Survival rates for patients with various carci­
noids depend on the site, the extent of the tumor, and tumor biol­
ogy. In patients with only localized disease, the 5-year survival rate 
for midgut carcinoids is about 65%, not essentially higher than 
that for patients with regional metastases. In pati

hat for patients with regional metastases. In patients with distant 
metastases, the 5-year survival rate is reduced to 39%.26–28,251,252 
The relative 5-year, 10-year, and 15-year survival rates for midgut 
carcinoids were 67%, 54%, and 44%, respectively.251 The 5-year 
and 10-year survival rates for typical bronchial carcinoid are 95% 
and 80%. Atypical lung carcinoids have significantly shorter sur­
vival, only 50% 5-year survival rate.23
One of the main determinants of survival in carcinoid 

of the main determinants of survival in carcinoid patients 
is the presence of metastases. Female gender and a younger age 
are associated with a better prognosis. Other factors that corre­
late with lower survival rates are high CgA level at diagnosis and 
high proliferation index (Ki67).38,117 During the 1990s there was 
a reduced incidence of death from carcinoid heart disease, possibly 
a result of earlier diagnosis, active surgery, and the introduction 
of somatostatin analogues and α-inter

ntroduction 
of somatostatin analogues and α-interferons. In an earlier study 
performed by our group, 30% of the patients died of carcinoid 
heart complications.25 In a more recent 2008 study, this rate 
was reduced to less than 10%.27 Clinically significant carcinoid 
heart disease is now rare. Between 5% and 10% of patients with 
WHO Grade I 
Ki67 <3% 
WHO Grade II 
Ki67  3-20% 
WHO Grade III 
NEC-G3
Ki67 >20–50% 
Ki67 >50%
Streptozotocin + 5FU
Everolimus
PRRT  
Cispl/carbo
+ etoposide
Temozo

U
PRRT  
Temozolomide/
capecitabine
SMS with functioning tumors 
Alternative: Temozoloide/Capecitabine
SMS
PRRT
FN
Combinations 
Debulking
Surgery embolization
Radioembolization RF 
Temozolomide
± capecitabine
+ Avastin
PRRT
(Folfox)
(Folfiri) 
Metastatic NET 
• Fig. 45.21  Treatment algorithm for metastatic neuroendocrine tumor (NET). 5FU, 5-Fluorouracil; cispl/carbo, 
cisplatin/carboplatin; FN, fibronectin; IFN, interferon; NEC, neuroendocrine car

ronectin; IFN, interferon; NEC, neuroendocrine carcinoma; PNET, primitive neu­
roectodermal tumor; PRRT, peptide receptor radiotherapy; SMS, somatostatin; RF, radiofrequency; WHO, 
World Health Organization.
1708
carcinoids are at an increased risk for developing simultaneous 
adenocarcinoma of the large intestine. The occurrence of a sec­
ond malignancy is associated with a worse prognosis.23,26 In two 
studies (in 2011 and 2014) comparing al

26 In two 
studies (in 2011 and 2014) comparing almost 900 patients with 
midgut or hindgut tumors, survival correlated with the WHO 
grading system as well with the TNM staging. For grade 1 (G1) 
the 10-year survival rate was 80%, for G2 tumor 50% to 66%, 
and for G3 tumor 35%; for stage I and II tumors survival rate was 
100%, stage III 85%, and stage IV 35%.251
Significant negative prognostic factors for overall survival were 
old age at diagnosis, carcinoid heart disease, liver tumor load, h

osis, carcinoid heart disease, liver tumor load, high 
WHO grade, and peritoneal carcinomatosis.250,252 Locoregional 
surgery had a positive impact on survival.252 
Other Flushing Disorders
Medullary Thyroid Carcinoma and VIPoma
Other neuroendocrine tumors, such as medullary thyroid carci­
noma (MTC) and VIP-producing tumors (ganglioneuroma, endo­
crine pancreatic tumors), can manifest with flushing syndromes 
(Fig. 45.22).253,254 Patients might also have diarrhea, particularly 
those with VIP-p

also have diarrhea, particularly 
those with VIP-producing tumors, which are accompanied by 
a severe secretory diarrhea. In patients with MTC, flushing and 
diarrhea are infrequent symptoms and are seen mainly in patients 
with high circulating levels of calcitonin and CGRP.
The mechanism behind the flushing and diarrhea is unknown, 
but it has been postulated to be mediated through prostaglandins 
stimulated by calcitonin. The frequency of flushing and diar­
rhea is usually less than 5% in pat

hing and diar­
rhea is usually less than 5% in patients with advanced metastatic 
MTC.253,255 Treatment is directed against tumor growth and can 
consist of surgical resection, embolization of liver metastases, and 
cytotoxic treatment (doxorubicin-based combination therapies). 
Somatostatin analogue therapy can alleviate the diarrhea. Therapy 
with tyrosine kinase inhibitor vandetanib can reduce these symp­
toms and present an antitumor effect.255
VIPoma or WDHA (watery diarrhea, hypokalemia, a

55
VIPoma or WDHA (watery diarrhea, hypokalemia, and achlor­
hydria) syndrome (Verner-Morrison syndrome) is associated with 
severe secretory diarrhea (up to 15 L/day), and some patients 
also display a continuous whole-body violaceous flushing and 
hypotension.254,256,257 The syndrome also includes achlorhydria, 
hypokalemia, and metabolic acidosis and is related to overproduc­
tion of VIP and a related peptide-peptide histidine methionine. 
These patients have tumors in the pancreas, lung, or 

se patients have tumors in the pancreas, lung, or sympathetic 
ganglia.256,257
The diagnosis is confirmed by measuring plasma VIP, which 
usually exceeds 70 pmol/L.257
Treatment is directed against the tumor and hormone excess. 
Administration of somatostatin analogues by either subcutaneous 
or IV infusion in the worst cases can control clinical symptoms.258 
Cytotoxic treatment with streptozotocin-based combinations, 
5-fluorouracil, or doxorubicin is recommended for malig­
nant cases.256 Inhi

icin is recommended for malig­
nant cases.256 Inhibitors, such as mTOR (everolimus), demon­
strate an antitumor effect in VIPomas with reduction of clinical 
symptoms.225 
Mastocytosis and Related Disorders
Mastocytosis, as well as other systemic mast cell activation, is clini­
cally related to flushing disorders. Most patients with mastocyto­
sis have an indolent course, but some forms of mastocytosis are 
aggressive. Symptoms are attributed primarily to paroxysmal mast 
cell activation.259,260

marily to paroxysmal mast 
Most patients with mastocytosis have evidence of cutaneous 
involvement, the most common being multiple, small pigmented 
lesions that produce urticaria on stroking with a blunt object 
(Darier sign); this condition is called urticaria pigmentosa.261,262 
Another form of cutaneous mastocytosis is a more telangiectatic 
form called telangiectasia macularis eruptiva persistans. Hepato­
megaly and splenomegaly can be due to infiltration of mast cel

plenomegaly can be due to infiltration of mast cells, 
and hepatic fibrosis is also common.262,263
Bone involvement can be manifested by either osteoporosis or 
osteosclerosis.264 Systemic mastocytosis can also involve the GI tract 
with mucosal nodules in the ileum, stomach, and large bowel.265
Hematologic abnormalities are nonspecific, with marked mast 
cell infiltration of the bone, anemia, leukocytosis, sometimes 
lymphadenopathy, and eosinophilia.266 In a subgroup of patients, 
the mastocyt

hilia.266 In a subgroup of patients, 
the mastocytosis is secondary to primary hematologic disorders, 
usually myeloproliferative or myelodysplastic disease.266–268 Mast 
cell leukemia has been reported in rare cases.269 Some cases with 
FIP1-like 1/PDGFRα (FIP1L1-PDGFRA) gene arrangement 
overlap to chronic eosinophilic leukemia. They also manifest ele­
vated serum tryptase levels.270 A majority (>80%) of patients with 
systemic mastocytosis have an activating c-kit mutation in codon 
816 in th

e an activating c-kit mutation in codon 
816 in the mast cell.270
Clinical signs of systemic mastocytosis include flushing, 
tachycardia, hypotension, and sometimes nausea, vomiting, and 
diarrhea. This syndrome resembles the carcinoid syndrome. Hista­
mine is a potent vasodilator and is released from mast cells. Other 
mediators of the syndrome are the release of prostaglandin D2, 
tryptase, and heparin.270–273 Prostaglandin D2 is a more potent 
mediator than is histamine.
The diagnosis is made

The diagnosis is made by measurement of histamine and his­
tamine metabolites in the urine.272 Quantification of histamine 
metabolites (N-methylhistamine and methylimidazole acetic acid) 
appears to be more sensitive for overproduction of histamine in 
patients with mastocytosis.273 Measurement of endogenous pro­
duction of prostaglandin D2 can be assessed by quantifying the 
major urinary metabolite (9α-hydroxy-11,15-dioxo-2,3,18,19-
tet-ranorprost-5-ene-1,20-dioic

5-dioxo-2,3,18,19-
tet-ranorprost-5-ene-1,20-dioic acid).273 However, these measure­
ments can be done only in specialized laboratories. Measurement 
of the tryptase release is easier to perform, and increased quantities 
Accurate medical history
Exclude
Allergic reactions
Cardiovascular cancers
Congenital disorder
Dermatologic cancers
Food and alcohol
Menopause
Neuroendocrine tumors
Neurologic disorders
Physiologic cancers
Psychiatric disorders
Toxins
• Fig. 45.2

c cancers
Toxins
• Fig. 45.22  Evaluation of flushing disorders. (From Yale SH, Vasudeva S, 
Mazza JJ, et al. Disorders of flushing. Compr Ther. 2005;31:59–71.)
1709
of this granule-associated enzyme tryptase (>20 ng/mL) can be 
detected by immunoassay.274 Bone marrow analysis of CD25 cells 
might support the diagnosis of mast cell disease.275
Treatment depends on the severity of the disease. As in the 
treatment of al

everity of the disease. As in the 
treatment of allergic anaphylaxis, epinephrine is effective in revers­
ing the hypotension associated with mast cell mediator release275; 
thus these patients should have constant access to epinephrine in 
the form of subcutaneous injection or inhalation. Chronic therapy 
to prevent acute attacks includes antihistamine therapy combined 
with inhibition of prostaglandin biosynthesis. Blockade of both 
histamine H1 and H2 receptors is required to prevent the vaso

1 and H2 receptors is required to prevent the vasodila­
tor effect of histamine.246,275,276
Nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit 
the cyclooxygenase enzyme that catalyzes the formation of 
prostaglandins. Aspirin has been used, but some patients can­
not tolerate it because of side effects in the gut and allergic reac­
tions.277 In patients resistant to both antihistamines and NSAIDs, 
IFNα has been attempted with a reduction in mast cell numbers 
as well as excretion of mast ce

mast cell numbers 
as well as excretion of mast cell mediators. Treatment with IFNα is 
still considered experimental.278 A subset of patients who carry the 
FIP1L1-PDGFRA oncogene will achieve complete clinical, histo­
logic, and molecular remission with imatinib mesylate therapy, in 
contrast to those with c-kit D816V mutation.277
1709.e1
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on A, Biorck G, Bjorkman G, et al. Malignant carcinoid of 
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1710
46
Managing Reproductive Disorders 
in Cancer
MAHMOUD SALAMA, VICTORIA SANDLER, AND TERESA K. WOODRUFF
What Is Oncofertility, and Why Is It 
Important?
Scope of the Problem
Cancer affects more than 14 million individuals annually world­
wide, of whom 10% are of reproductive age.1 Among children and 
adolescents, cancer survival 

 Among children and 
adolescents, cancer survival rates range from 71% to 86%, mostly 
due to successful early detection and effective cancer therapies. 
However, up to 80% of children, adolescents, and adults with can­
cer receive treatment that may temporarily or permanently affect 
their reproductive health, including fertility, gonadal function, 
and psychosexual well-being.2 At the heart of the field of oncofer­
tility is the consideration of a patient’s reproductive health at the 
same tim

f a patient’s reproductive health at the 
same time that decisions are being made about life-preserving but 
fertility-threatening cancer therapy. Thus, a comprehensive discus­
sion of the endocrine consequences of cancer treatment, including 
fertility loss, dysfunction of steroid hormone–dependent tissues 
(e.g., bone and heart), and impaired psychosexual health, as well 
as a discussion of fertility preservation and contraception options, 
is critical for optimizing the future quality of life

critical for optimizing the future quality of life (QoL) of cancer 
survivors.3
Current oncology clinical practice guidelines recommend that 
health care providers engage in an active discussion about the 
potential effects of cancer therapy on future reproductive health 
prior to, during, and after treatment.4 Although there is grow­
ing awareness of the impact of cancer treatment on reproduc­
tive health among health care providers, up to 50% of patients 
with cancer are not offered counseling

f patients 
with cancer are not offered counseling on reproductive health 
prior to starting cancer treatment.2,5,6 Cancer patients are deeply 
What Is Oncofertility, and Why Is It Important?, 1710
Logistics of Fertility Preservation, 1713
Challenges of Fertility Preservation, 1720
Summary and Next Steps, 1724
KEY POINTS
	•	
With increasing cancer survival the adverse sequelae of the 
tumor, and treatment thereof, on multiple organ systems have 
increasingly been recognized. The

an systems have 
increasingly been recognized. The endocrine system is one of 
the most frequent organ systems to be affected, with greater 
than 40% of childhood cancer survivors showing abnormalities.
	•	
Oncofertility is a relatively new interdisciplinary field at the in­
tersection of oncology and reproductive medicine that expands 
fertility options for young cancer patients.
	•	
Aggressive gonadotoxic anticancer regimens including alkylat­
ing chemotherapy and total-body irradiation are 

­
ing chemotherapy and total-body irradiation are used often in 
treating young cancer patients. The risks of gonadotoxicity and 
subsequent iatrogenic fertility loss depend mainly on the type 
and stage of the disease, dose and dosage of anticancer therapy, 
and the age of the patient at the beginning of treatment.
	•	
If the risk of gonadotoxicity and subsequent iatrogenic fertility 
loss is greater than 50% and the patient desires to have children 
in the future, a fertility preservation and

ldren 
in the future, a fertility preservation and restoration strategy 
should be initiated before, during, and after chemotherapy and 
radiotherapy.
	•	
Fertility preservation and restoration strategy should be 
individualized and tailored to the patient’s circumstances after 
obtaining patient informed consent or that of the legal guard­
ians for children.
	•	
Fertility preservation and restoration options include gamete 
and gonadal tissue freezing and further autotransplantation, 
gonadal

freezing and further autotransplantation, 
gonadal protection, gonadal tissue bioengineering, stem cells 
reproductive technology, and neoadjuvant cytoprotective phar­
macotherapy.
	•	
A multidisciplinary oncofertility approach involving strong 
coordination among oncologists, gynecologists, endocrinolo­
gists, surgeons, reproductive biologists, research scientists, 
and patient navigators is essential to ensure a high standard of 
care.
1711
Chapter 46  Managing Reproductive Disorders  in Canc

apter 46  Managing Reproductive Disorders  in Cancer
interested in engaging in a discussion with their health care pro­
viders about reproductive health throughout the course of their 
cancer diagnosis, treatment, and follow-up. Among young women 
with cancer, 62% express having had plans for children before 
receiving their cancer diagnosis.4,7 In fact, concern about future 
fertility is a source of distress among women of reproductive age 
who are undergoing cancer therapy, with one-third repo

are undergoing cancer therapy, with one-third reporting 
symptoms of depression and/or anxiety that interfere with daily 
functioning and 15% requiring psychotropic medications.8 Even 
among those cancer patients who do receive counseling regard­
ing reproductive health prior to initiation of cancer therapy, many 
report dissatisfaction with the quantity and quality of the infor­
mation provided.4 For example, adult women with breast cancer 
report overall satisfaction with the fact that fertili

rt overall satisfaction with the fact that fertility preservation 
was mentioned prior to initiation of treatment, but many also 
note frustration with the focus on infertility as opposed to the 
variety of fertility preservation options and success rates, the cost 
of fertility preservation procedures, and experimental protocols. 
Importantly, adult women who felt they had received adequate 
and high-quality information about fertility preservation prior to 
initiation of cancer therapy were fi

ion prior to 
initiation of cancer therapy were five times more likely to pursue 
fertility preservation than women who did not receive counsel­
ing. These women also report less regret and better QoL even if 
they chose not to pursue fertility preservation prior to starting 
treatment.4,7 Among adult men with cancer, counseling regarding 
fertility preservation prior to cancer therapy is felt to be particu­
larly important, as fertility status is interwoven with their sense 
of masculinity and 

s interwoven with their sense 
of masculinity and their intimate relationships.9 Men pursuing 
fertility preservation report reassurance about the prospect of bio­
logic fatherhood as a reaffirmation of their masculine identity.9 
However, male cancer patients are less likely to be informed about 
potentially compromised fertility than their female counterparts.5
Furthermore, adolescent cancer patients face unique addi­
tional challenges in navigating issues related to personal body 
image, inti

ating issues related to personal body 
image, intimacy, relationships, and sexuality during and after 
cancer treatment. Both adolescents with cancer and their parents, 
who frequently serve as primary decision makers, express con­
cern about future fertility. However, parents of adolescents with 
cancer prioritize initiation of therapy over fertility preservation 
and underestimate their children’s concerns about future fertil­
ity.10 Parents of pediatric cancer patients sometimes regulate the 

 pediatric cancer patients sometimes regulate the 
information presented to younger cancer patients (12–15 years 
of age) to reduce distress during a particularly emotional time. 
Consequently, some adults with a history of cancer in childhood 
and adolescence convey dissatisfaction about having been fully 
or partially excluded from conversations regarding the impact 
of cancer treatment on their fertility by both their parents and 
their health care providers.11 Furthermore, many of these adul

 care providers.11 Furthermore, many of these adults 
report having received insufficient information about reassessing 
fertility potential after completion of cancer treatment, including 
counseling regarding safe sex practices and contraception, fam­
ily planning, and nonbiologic parenthood options.11 In fact, both 
adult survivors of pediatric cancer and their parents retrospectively 
express regret over inadequate allotment of time and attention to 
discussions about fertility prior to init

tion to 
discussions about fertility prior to initiation of cancer therapy 
and attribute responsibility to the health care providers for insuf­
ficient counseling.12 Parents of adult survivors of pediatric cancer 
also retrospectively report that their children should have been 
involved in discussions regarding future fertility, regardless of their 
age at cancer diagnosis. For many adult survivors and their par­
ents, discussion of future fertility prior to cancer therapy signifies 
hope for 

ility prior to cancer therapy signifies 
hope for the future.12
Reproductive health counseling should also include discussion 
of chronic comorbidities associated with cancer treatment and 
subsequent endocrine dysfunction, including osteoporosis, cardio­
vascular disease, cognitive dysfunction, and mental health issues.13 
Not only are certain cancer treatments (anthracyclines, trastu­
zumab, chest irradiation) associated with cardiac toxicity,14,15 but 
specific chemotherapeutic and immunologi

4,15 but 
specific chemotherapeutic and immunologic agents, cranial irradi­
ation, surgical tumor removal, and antihormone therapies are also 
linked with adverse metabolic parameters such as obesity, glucose 
intolerance, insulin resistance, hypertension, atherosclerosis, and 
dyslipidemia that increase the risk of cardiovascular disease.16,17 
In addition, many cancer treatments result in estrogen and testos­
terone deficiency by damaging the organs that produce these hor­
mones, thereby inhib

rgans that produce these hor­
mones, thereby inhibiting estrogen receptor–mediated signaling 
and aromatization of testosterone to estrogen and downregulating 
the hypothalamic-pituitary-gonadal axis. Considering that these 
steroid hormones are essential for bone health, cancer treatments 
that deplete estrogen and testosterone are associated with altered 
bone physiology, a rapid decline in bone mineral density, and a 
twofold increased risk of fracture in both men and women with 
cancer. In f

 fracture in both men and women with 
cancer. In fact, more than 80% of men with prostate cancer expe­
rience a decline in bone mineral density after medical or surgical 
castration, and women with breast cancer experience an acceler­
ated decline in bone density after onset of therapy-induced ovar­
ian insufficiency. Cancer therapy is also associated with chronic 
nutritional deficiencies that impede normal bone building and 
with chronic deconditioning and treatment-related neuropathies 
that 

itioning and treatment-related neuropathies 
that increase the risk of falls and subsequent fractures.18 Last, 
cancer survivors also experience greater mental health morbidity 
than the general population,19 with greater reported emotional 
and social distress among older, single, unemployed female cancer 
survivors of bone and central nervous system (CNS) tumors.20 
Sexuality, fertility, and parenthood are particularly important fac­
tors in psychosocial well-being among adult cancer survivors

ychosocial well-being among adult cancer survivors and 
are major contributors to the long-term psychological sequelae of 
cancer diagnosis and treatment.21 
Reproductive Health Concerns During Cancer 
Treatment
Fertility
Fertility is a critical component of reproductive health and plays 
a significant role in health-related QoL among cancer patients, 
especially women. In fact, 60% of women and more than 50% of 
men with cancer report a desire for parenthood.22 Temporary or 
permanent infertili

or parenthood.22 Temporary or 
permanent infertility after cancer treatment may affect up to 60% 
of cancer survivors. The impact of cancer treatment on fertility 
is often unpredictable and is influenced by the type of chemo­
therapy agent, dose of radiation, patient age, and gonadal reserve 
prior to therapy. For women with cancer, alkylating agents and 
pelvic irradiation are particularly toxic to primordial follicles, and 
these therapies deplete the ovarian reserve and shorten the repro­
du

lete the ovarian reserve and shorten the repro­
ductive window.23 For men with cancer, chemotherapy and pelvic 
irradiation are associated with azoospermia within 3 months of 
the start of treatment, and almost a quarter of male cancer survi­
vors experience persistent azoospermia or oligospermia. Chemo­
therapy and radiation may also result in unrepaired DNA damage 
in spermatogonia, which compromises fertility and/or increases 
the risk of congenital abnormalities in offspring.24
Biologic pare

enital abnormalities in offspring.24
Biologic parenthood is central to cancer patients’ sense of adult 
and gender identity, existential purpose, and perception of social 
acceptance. Female cancer survivors view gestation as a marker 
1712
of femininity, whereas male cancer survivors are more focused on 
biologic fatherhood as a marker of masculinity. Unknown fertility 
status further augments reproductive health-related anxiety among 
cancer

 reproductive health-related anxiety among 
cancer survivors, especially those who are young and single and 
may fear that potential infertility will adversely affect their ability 
to find a partner. Thus, reassessment of reproductive function after 
completion of cancer therapy is an important aspect of the com­
prehensive care of cancer patients. Notably, cancer survivors who 
are fertile may choose not to pursue conception due to concerns 
about cancer recurrence, transmission of cancer to o

out cancer recurrence, transmission of cancer to offspring, and 
the effect of their own morbidity and mortality on their future 
child.22 Thus, health care providers involved in the care of cancer 
patients must not only address the logistics of fertility preservation 
and initiation of cancer treatment but also provide compassionate 
support related to the emotional aspects of oncofertility that are 
important for patients’ overall and long-term well-being. 
Contraception During Cancer Therapy

m well-being. 
Despite the gonadotoxic effects of cancer therapy, many cancer 
patients remain fertile during treatment. Up to 10% of women 
undergoing cancer treatment do not consistently use a reliable 
contraceptive method,25 and 3% of women with cancer expe­
rience an unplanned pregnancy.26 Selection of an appropriate 
method of female contraception is guided by the type of cancer 
(e.g., a nonhormonal copper intrauterine device in women with 
breast cance

er intrauterine device in women with 
breast cancer vs. a contraceptive implant in women with cervical 
cancer), functional status and comorbidities (e.g., immunocom­
petence, thromboembolic risk, liver dysfunction), and personal 
preference.27 Barrier contraception provides protection against 
sexually transmitted infections22 but is not as effective as intrauter­
ine devices and contraceptive implants in preventing pregnancy 
due to high user failure rates. Emergency contraception, includ­
ing

ailure rates. Emergency contraception, includ­
ing a short course of oral agents, is considered safe for women 
with cancer. Options for male contraception are being expanded 
rapidly, and many have been shown to be acceptable among men. 
Methods that induce azoospermia include androgens alone or in 
combination with progestins, gonadotropin-releasing hormone 
(GnRH) agonists, GnRH antagonists, and vasectomy. Reversible 
inhibition of sperm under guidance using a polymer gel causes 
reversible s

 guidance using a polymer gel causes 
reversible sperm damage, whereas vaso-occlusive devices prevent 
transmission of sperm through the vas deferens.28 However, use 
of these male contraceptive methods is limited by the discomfort 
of administration, cost, stigma against male contraception, con­
cerns regarding effects on libido and sexual function, and ease of 
reversibility.29 Selective androgen receptor modulators and non­
hormonal agents are currently being investigated as contraceptive 
op

 currently being investigated as contraceptive 
options in men.29 Among women with cancer, contraceptive use is 
discordant with the desire for pregnancy. Many women with cancer 
indicate that they would opt for abortion if they became pregnant 
during cancer treatment, although abortion rates among pregnant 
women with cancer are not well reported. Cancer patients priori­
tize contraception as an important aspect of their care, but only a 
few studies have adequately documented the quantity and

tudies have adequately documented the quantity and quality 
of contraception counseling in cancer patients. Notably, 65% of 
women who participate in contraception counseling report using 
an effective method of contraception.25
Appropriate timing of conception after completion of cancer 
treatment is also an important aspect of reproductive health in 
cancer survivors. Conception should not be attempted sooner than 
6 months after the completion of chemotherapy and 12 months 
after completion o

 of chemotherapy and 12 months 
after completion of radiation therapy in women with cancer,23 
and no sooner than 12 to 24 months after completion of therapy 
in men given the risk of unrepaired DNA damage in ejaculated 
spermatozoa.24 Additionally, the risk of cancer recurrence must 
also be considered prior to attempting conception. Appropriately 
planned contraception allows cancer patients to time pregnancy 
to a stage of better maternal health and to optimize fertilization, 
embryonic devel

th and to optimize fertilization, 
embryonic development, and early pregnancy outcomes.24,30 
Gonadal Dysfunction
Women With Cancer
Women undergoing surgery, chemotherapy, or radiation therapy 
for the treatment of cancer are at an increased risk of premature 
ovarian insufficiency (POI). POI is characterized by amenorrhea 
and gonadal steroid hormone deficiency with concurrent eleva­
tion in serum follicle-stimulating hormone on at least two occa­
sions at least 1 month apart in women younger t

a­
sions at least 1 month apart in women younger than 40 years. 
The prevalence of POI among young female cancer survivors 
ranges widely at between 2% and 82%. Age at diagnosis, type of 
chemotherapy (e.g., alkylating agents and alkylating-like agents), 
dose and target area of radiation, and a diagnosis of Hodgkin 
lymphoma are risk factors for POI. Additionally, oophorectomy 
is associated with a rapid decline in estrogens and androgens that 
precipitates symptoms of POI, including hot flashe

precipitates symptoms of POI, including hot flashes, night sweats, 
vaginal dryness, and decreased sexual desire. Vulvectomy and radi­
cal hysterectomy add to scarring and pain of the perineum, further 
augmenting sexual dysfunction. Importantly, ovarian hormone 
deficiency is associated with increased morbidity (e.g., osteopo­
rosis and fracture risk, cardiovascular disease, mental illness, and 
cognitive decline) and mortality. Recovery of ovarian function 
is variable, somewhat unpredictable,

ian function 
is variable, somewhat unpredictable, and dependent on baseline 
ovarian reserve. Methods for better predicting ovarian reserve in 
female cancer survivors, including measurement of serum anti­
müllerian hormone, antral follicle count, and ovarian volume and 
surface area, are still being optimized. Standardized methods for 
assessing ovarian reserve (rather than or in addition to symptoms 
of POI) among female cancer survivors could be used to direct 
counseling on contraception an

 be used to direct 
counseling on contraception and guide screening for long-term 
complications of POI.31,32
Several strategies have been developed to address the symptoms 
of POI. Hot flashes may be managed with lifestyle changes such 
as weight management and avoidance of triggers; cognitive behav­
ioral therapy and hypnosis; and specific, nonhormonal medica­
tions.14 Incontinence, vaginal dryness, and dyspareunia associated 
with estrogen deficiency may be improved with pelvic floor physi­
c

iciency may be improved with pelvic floor physi­
cal therapy, clitoral therapy devices, avoidance of irritants, lubri­
cants/moisturizers, topical anesthetics, topical and intravaginal 
estrogen, testosterone, or dehydroepiandrosterone, and psycho­
therapy.14 Although hormone replacement therapy is sometimes 
unfavorable in women with hormone-sensitive cancers, such as 
breast, endometrial, cervical, and some ovarian cancers, a few 
studies have demonstrated that hormone replacement therapy in 

demonstrated that hormone replacement therapy in 
female survivors of early gynecologic cancer is not associated with 
an increased risk of cancer recurrence.14,33 However, alleviation 
of menopausal symptoms in female cancer survivors is not always 
associated with better QoL; thus, the psychosocial aspects of can­
cer survivorship must also be addressed.32 
Men With Cancer
Hypogonadism in men with cancer can result from direct damage 
to the testes, such as a consequence of surgical resection 

stes, such as a consequence of surgical resection of gonadal 
tumors or exposure to pelvic irradiation or systemic chemother­
apy. Testicular dysfunction can also occur secondary to pituitary 
or hypothalamic damage, as in the case of cranial radiotherapy 
1713
Chapter 46  Managing Reproductive Disorders  in Cancer
or resection of tumors of the CNS. Men with hypogonadism 
due to cancer therapy may present with ambiguous symptoms of 
decreased sexual desire or function, hot flashes, fatigue, slow

ual desire or function, hot flashes, fatigue, slowed 
mental processing, and dysthymia (persistent depressive disor­
der), which may be difficult to distinguish from the common side 
effects of cancer treatment. They may also present with objective 
changes in body habitus (e.g., decreased muscle tone, gynecomas­
tia, weight gain, redistribution of fat, loss of body hair), reduced 
testicular size, and normochromic normocytic anemia. Men 
undergoing bilateral orchidectomy, hypophysectomy, or pel

ing bilateral orchidectomy, hypophysectomy, or pelvic 
or cranial radiotherapy may develop symptoms of hypogonadism 
soon after treatment, whereas those undergoing systemic chemo­
therapy may experience a slower onset of hypogonadal symptoms. 
Many men treated with chemotherapy or radiation to the retro­
peritoneum will experience some manifestations of testosterone 
deficiency over the long term, with a higher risk among men older 
than 45 years.34 The effect of chemotherapy on testicular func­

.34 The effect of chemotherapy on testicular func­
tion is agent specific, even within the same chemotherapeutic cat­
egory.31 Baseline testicular volume is inversely associated with risk 
of hypogonadism after cancer treatment.34
Testosterone deficiency is linked to decreased sexual desire, 
erectile dysfunction, and orgasmic disorders that develop across 
the spectrum of subnormal and low testosterone concentrations. 
Symptoms of hypogonadism are associated with worse health-
related QoL. Test

re associated with worse health-
related QoL. Testosterone deficiency is also associated with osteo­
porosis, depression, metabolic dysfunction, cardiovascular disease, 
and mortality. Thus, it is important to assess for hypogonadism in 
men undergoing cancer treatment.34 Testosterone replacement is 
readily available but may not be an option for men with advanced 
prostate cancer or breast cancer.34 
Physical Changes Related to Cancer Therapy
Anatomic changes after mastectomy for the treatment 

atomic changes after mastectomy for the treatment of breast 
cancer or hysterectomy for the treatment of cervical or endome­
trial cancer adversely affect self-esteem, decrease sexual desire, and 
negatively affect a woman’s sense of sexuality and intimacy.22,35,36 
Nipple-sparing mastectomy has evolved as a surgical alternative 
that is associated with better cosmetic outcome, greater patient 
satisfaction, psychosocial and sexual well-being, and higher 
health-related QoL.37,38 Considering tha

d higher 
health-related QoL.37,38 Considering that few surgical alternatives 
are available for women with gynecologic cancer, the impact of 
gynecologic surgery on psychosocial well-being must be identified 
and addressed in this cohort of cancer patients. Men with prostate 
and testicular cancer also suffer from changes to body image, self-
perceived masculinity, and QoL after surgery.39 Among testicular 
cancer survivors who undergo surgical removal of testis (orchi­
ectomy) for cancer treat

removal of testis (orchi­
ectomy) for cancer treatment, 7% report problems with ejacula­
tion and erection, 17% report negative body image, 25% report 
decreased sexual interest, and 40% report reduced sexual activity 
3 years after completion of therapy. Sexual dysfunction is particu­
larly prevalent among men who undergo retroperitoneal lymph 
node dissection. Men who experience these unfavorable outcomes 
after cancer surgery also report lower marital satisfaction and com­
promised long-term 

 marital satisfaction and com­
promised long-term relationships.40 More than a quarter of men 
who undergo orchiectomy for unilateral testicular cancer opt for a 
testicular prosthesis, noting reasons related to regaining accepted 
male appearance, and more than 80% note long-term satisfaction 
with the prosthesis.41 It is clear that anatomic changes related to 
cancer therapy have significant implications for self-image, sexual 
health, and QoL among men and women with cancer and must 
be appro

among men and women with cancer and must 
be appropriately addressed in the comprehensive care of cancer 
survivors. 
Logistics of Fertility Preservation
Importance of Fertility Preservation and 
Restoration
When aggressive chemotherapy and radiotherapy are used, 
gonadotoxicity may occur as a side effect, leading to impair­
ment of reproductive function, impairment of gonadal steroid 
hormone–dependent tissues (e.g., bone and heart), and fertility 
loss. To avoid or at least mitigate these deva

ty 
loss. To avoid or at least mitigate these devastating complications, 
effective strategies for preserving and restoring fertility in young 
patients with cancer should be offered before, during, and after 
chemotherapy and radiotherapy.42–46
As the numbers of young cancer patients surviving into adult­
hood have increased due to advances in early diagnosis and treat­
ment, long-term health issues such as fertility have gained growing 
attention. To date, assisted reproduction technologies (A

on. To date, assisted reproduction technologies (ARTs), 
cryobiology, and experimental translational medicine protocols 
have been utilized to prevent or overcome chemotherapy- and 
radiotherapy-induced gonadotoxicity in patients with cancer to 
preserve and restore fertility. Each fertility preservation option 
has advantages and disadvantages and may not be feasible for all 
patients. A growing recognition of the importance of fertility pres­
ervation for patients with cancer is underscored by

rvation for patients with cancer is underscored by the numerous 
international fertility preservation and restoration guidelines that 
have been published in the past years by the American Society of 
Clinical Oncology (ASCO),47–49 the American Society for Repro­
ductive Medicine (ASRM),50,51 the European Society for Medical 
Oncology (ESMO),52,53 the American Oncofertility Consor­
tium,54,55 the International Society for Fertility Preservation,56–59 
the National Comprehensive Cancer Network,60

6–59 
the National Comprehensive Cancer Network,60 the American 
Academy of Pediatrics,61 the Association of Pediatric Hematol­
ogy/Oncology Nurses,62 and FertiPROTEKT (the German fertil­
ity preservation network).63–65 
Fertility Loss, Preservation, and Restoration in 
Female Cancer Patients
Epidemiology of Cancer in Young Female Patients
According to statistics from the United Kingdom (2013–2015), 
the most common cancers in prepubertal girls (age 0–14 years) are 
leukemias (29%), CNS malignan

age 0–14 years) are 
leukemias (29%), CNS malignancies (28%), lymphomas (7%), 
and renal (6%) cancers. The most common cancers in female ado­
lescents and young adults (AYA) (age 15–24 years) are lympho­
mas (20%), melanoma (16%), CNS (14%), and leukemias (7%). 
The most common cancers in adult females (age 25–49 years) are 
breast (44%), melanoma (9%), cervical (8%), CNS (6%), and 
ovarian (5%). The most common cancers that occur during the 
female reproductive years and may require aggressive 

ale reproductive years and may require aggressive gonadotoxic 
chemotherapy and radiotherapy are breast, cervical, leukemia, 
lymphoma, CNS, renal, and bone64 (Table 46.1). 
Risks of Gonadotoxicity and Fertility Loss in Young Female 
Patients
In young female patients undergoing cancer treatment, gonado­
toxicity, POI, premature ovarian failure (POF), and fertility loss 
are more likely to occur when the ovaries are exposed to alkylat­
ing chemotherapies such as cyclophosphamide, ifosfamide, and 

erapies such as cyclophosphamide, ifosfamide, and 
busulfan; after ionizing radiotherapy to the pelvis and abdo­
men; or following cranial or total-body irradiation. The degree of 
gonadotoxicity depends mainly on the type, dose, and duration of 
cancer therapy, as well as the age of the patient and the type and 
1714
stage of cancer. Some guidelines have attempted to classify cancer 
treatments according to their gonadotoxicity and associated

s according to their gonadotoxicity and associated risks 
of POF and fertility loss in women47–49,66,67 (Table 46.2). 
Fertility Preservation and Restoration Options for Young 
Female Patients
When the estimated risk of gonadotoxicity from cancer therapy is 
greater than 50%, fertility preservation strategies should be offered 
to young patients (<40 years of age) prior to starting treatment. 
Several established, debatable, and experimental options are avail­
able for fertility preservation for

ons are avail­
able for fertility preservation for young women and girls with can­
cer. Established options include embryo freezing and egg freezing. 
Debatable options include ovarian protection techniques such as 
treatment with GnRH analogues and hormonal suppression, sur­
gical ovarian transposition (oophoropexy), gonadal shielding, and 
the use of fractionated chemotherapy and radiotherapy. Experi­
mental options include ovarian tissue freezing for use in future 
autotransplantation, oocyte

ing for use in future 
autotransplantation, oocyte in vitro maturation (IVM), artificial 
ovary systems, stem cell transplantations, and neoadjuvant cyto­
protective pharmacotherapy, among others68–74 (Table 46.3 and 
Fig. 46.1). 
Established Options for Fertility Preservation in Women 
and Girls
Embryo Freezing
Embryo freezing was the first established method for preserving 
female fertility and is still considered the gold standard. It involves 
cryopreservation of in  vitro–fertilized mature 

cryopreservation of in  vitro–fertilized mature oocytes via slow 
freezing or vitrification; the latter is now preferred due to a better 
post-thaw survival rate.75–77 Embryo freezing requires prior ovar­
ian stimulation, mature oocyte retrieval, and sperm for in vitro 
fertilization (IVF).78–83 Therefore, it is not suitable for prepuber­
tal girls who have an inactive hypothalamic-pituitary-ovarian axis 
and for single women who do not wish to use donated sperm 
for personal, ethical, or reli

 use donated sperm 
for personal, ethical, or religious reasons. It is also not suitable 
for women with estrogen-sensitive cancers, such as breast and 
endometrial cancers, as conventional ovarian stimulation may 
lead to high serum estrogen levels. Alternative ovarian stimula­
tion protocols to minimize the effects of elevated estrogen use 
either tamoxifen (a selective estrogen receptor modulator)84,85 or 
letrozole (an aromatase inhibitor).86–91 In addition, conventional 
ovarian stimulation

–91 In addition, conventional 
ovarian stimulation may take up to several weeks and carries a risk 
of ovarian hyperstimulation syndrome); thus, it may not be a suit­
able option for women with highly aggressive malignancies (e.g., 
hematologic) that require immediate cancer treatment. For these 
women, random-start ovarian stimulation for emergency fertility 
preservation may be an option.92–99 The standard storage period 
for frozen embryos, eggs, and sperm is 10 years, although longer 
storag

gs, and sperm is 10 years, although longer 
storage periods may be possible. In healthy women, the live birth 
rate per frozen embryo transfer is nearly 30%.100–102 However, in 
women with cancer, the live birth rate per frozen embryo transfer 
is reduced to approximately 15%, without any increased risk for 
congenital abnormalities.103 
Egg Freezing
In 2012, the ASRM approved egg freezing as an established 
option for female fertility preservation.104 Egg freezing involves 
cryopreservation of 

on.104 Egg freezing involves 
cryopreservation of mature oocytes via slow freezing or vitrifica­
tion.105–108 Like embryo freezing, egg freezing requires prior ovar­
ian stimulation for mature oocyte retrieval but does not involve 
IVF. Thus, egg freezing is not suitable for prepubertal girls; how­
ever, it is an option for single women who do not want to use donor 
sperm. Egg freezing carries the same disadvantages related to ovar­
ian stimulation mentioned earlier for embryo freezing. In healt

on mentioned earlier for embryo freezing. In healthy 
women, the live birth rate per frozen oocyte is approximately 6% 
but continues to improve steadily due to advances in vitrification 
protocols and egg donation programs.109–113 However, in women 
with cancer, there is not enough data on egg freezing outcomes to 
estimate the live birth rate per frozen oocyte. To date, only a few 
live births have been reported after oocyte vitrification in women 
with cancer.113–115 Until more data become av

men 
with cancer.113–115 Until more data become available, extrapolat­
ing the outcomes of egg freezing to female cancer patients should 
be done with caution during oncofertility counseling. 
Debatable Options for Fertility Preservation in Women 
and Girls
GnRH Analogues and Hormonal Suppression
GnRH analogues are commonly prescribed in the fields of gyne­
cologic endocrinology and reproductive medicine; however, the 
role of GnRH analogue treatment before and during chemother­
apy to protect t

ment before and during chemother­
apy to protect the ovaries from damage is widely debated.116–120 
Some randomized trials, systematic reviews, and meta-analyses 
  Common Cancers Across the Life Span According to Reproductive Age in the United Kingdom (2013–
2015)
Female
Male
Prepubertal (0–14 years)
Leukemias (29%), central nervous system (28%), 
lymphoma (7%), and renal (6%)
Leukemias (31%), central nervous system (26%), 
and lymphoma (13%)
Adolescents and young adults (15–24 years)
Lymphoma

dolescents and young adults (15–24 years)
Lymphomas (20%), melanoma (16%), central 
nervous system (14%), and leukemias (7%)
Germ cell tumors (27%), lymphomas (22%), central 
nervous system (13%), and leukemias (10%)
Adults (25–49 years)
Breast (44%), melanoma (9%), cervix (8%), central 
nervous system (6%), and ovary (5%)
Testis (14%), melanoma (11%), central nervous 
system (10%), bowel (10%), and head and neck 
(8%)
Most common types of cancer that may require 
aggressive (gonadotoxic) antica

 that may require 
aggressive (gonadotoxic) anticancer chemo­
therapy and radiotherapy and necessitate 
prior fertility preservation measures
Breast, cervical, leukemia, lymphoma, central 
nervous system
Testicular, germ cell tumor, leukemia, lymphoma, 
central nervous system
Data from Cancer Research UK. Cancer incidence by age. Available at http://www.cancerresearchuk.org/health-professional/cancer-statistics/incidence/age. Accessed February 8, 2019.
   
TABLE 46.1
  Anticancer Therapy and Re

etoposide, cisplatin; CAF, cyclophosphamide, doxorubicin (Adria­
mycin), fluorouracil; CEF, cyclophosphamide, epirubicin, fluorouracil; ChlVPP, chlorambucil, vinblastine, procarbazine, prednisolone; CHOP, cyclophosphamide, hydroxydaunomycin, Oncovin, prednisone; 
CMF, cyclophosphamide, methotrexate, fluorouracil; COP, cyclophosphamide, Oncovin, prednisone; COPP, cyclophosphamide, Oncovin, procarbazine, prednisone; CVP, cyclophosphamide, vincristine, pred­
nisone; EVA, etoposide, vinblastine, Adr

ne, pred­
nisone; EVA, etoposide, vinblastine, Adriamycin (doxorubicin); MOPP, mechlorethamine, Oncovin (vincristine), procarbazine, prednisone; MVPP, mechlorethamine, vinblastine, procarbazine, prednisolone; 
NOVP, Novantrone (mitoxantrone), Oncovin, vinblastine, prednisone; OEPA, Oncovin, etoposide, prednisone, Adriamycin (doxorubicin).
   
TABLE 46.2
  Fertility Preservation Options for Patients Undergoing Gonadotoxic Anticancer Therapy
Option
Females
Males
Established
	•	 Embryo freezing
	
n female cancer survivors.119,121–126 The mechanism of action of 
GnRH analogues and their direct and indirect effects on ovaries 
are not fully understood. It is known that GnRH analogues sup­
press gonadotropin secretion from the pituitary gland and hence 
suppress ovarian function indirectly.127,128 Some theories sug­
gest that administration of GnRH analogues before and during 
chemotherapy suppresses the ovaries, decreasing the number of 
primordial follicles entering the growing pool; thes

imordial follicles entering the growing pool; these dormant fol­
licles are thought to be less sensitive to the gonadotoxic effects 
of chemotherapeutic agents. Other theories suggest a direct pro­
tective effect of GnRH analogues on the ovaries, including via 
upregulation of intraovarian antiapoptotic molecules and protec­
tion of ovarian germline stem cells.129,130 In most patients, GnRH 
analogues have not been shown to protect ovaries from radiother­
apy-induced gonadotoxicity. For this rea

adiother­
apy-induced gonadotoxicity. For this reason, GnRH analogues 
are not recommended for fertility preservation in female patients 
with cancer scheduled to undergo pelvic irradiation. According 
to ASCO, European Society for Medical Oncology, and ASRM 
guidelines,47–53 GnRH analogues and other hormonal suppres­
sion methods (e.g., oral contraceptives) should not be relied on as 
female fertility preservation methods. Nevertheless, several recent 
publications have encouraged the use of Gn

recent 
publications have encouraged the use of GnRH analogues to pre­
serve female fertility during chemotherapy.128,131–139 
Oophoropexy
Oophoropexy is the surgical transposition of ovaries away from 
the field of pelvic irradiation, which is often used to treat pelvic 
malignancies such as Hodgkin lymphoma, cervical carcinoma, 
vaginal carcinoma, and pelvic sarcoma. During oophoropexy, 
ovaries can be transposed either laterally toward the pelvic wall 
or medially behind the uterus. Although 

vic wall 
or medially behind the uterus. Although underutilized, oopho­
ropexy can be carried out via mini-laparotomy, laparoscopy, or 
even robotic surgery. The simplest and most successful technique 
is laparoscopic lateral oophoropexy. Oophoropexy has been 
Group
Method
Cryopreservation
Treatment
Recipient
Concerns
Hormone
Hormone
cycle
2-3 weeks
Hormone
Hormone
cycle
2-3 weeks
Laparoscopic
oophorectomy
Ejaculation
Mature sperm
extraction
Sperm
Testis
Testis
biopsy
Ova

re sperm
Sperm
Testis
Testis
biopsy
Ovarian
cortical strips
Cumulus-
oocyte
complexes
Zygote
or embryo
Mature
oocyte
Embryo
transfer
Patient or
gestational
surrogate
Patient or
surrogate
Partner
Delay in
cancer
treatment
Hormone
Availability
of appropriate
sperm donor
Ovarian
transplantation
In vitro follicle
maturation and
in vitro fertilization
or ICSI with
embryo transfer
ICSI with
Stem-cell
repopulation
Patient
Patient
Potential
reintroductio

Patient
Patient
Potential
reintroduction
of cancer cells
Experimental
Women
girls
Women
girls
Men
boys
girls
• Fig. 46.1  Options for fertility preservation in patients with cancer. ICSI, intracytoplasmic sperm injection. 
(Redrawn from Jeruss JS, Woodruff TK. Preservation of fertility in patients with cancer. N Engl J Med. 
2009;360[9]:902–911.)
1717
performed on one

Reproductive Disorders  in Cancer
performed on one ovary while the other ovary is excised for ovar­
ian tissue freezing.140–146 According to ASCO guidelines, the suc­
cess of oophoropexy in protecting ovaries and preserving fertility 
is debatable and varies according to the dose, site, and type of 
pelvic irradiation; the age of the patient; and whether radiation 
is combined with chemotherapy. There is also the probability of 
remigration of the transposed ovaries to their original positions 

e transposed ovaries to their original positions 
during the course of radiotherapy. Following successful oophoro­
pexy, spontaneous pregnancy may be possible without the need for 
ART. Oophoropexy does not protect ovaries from chemotherapy 
induced-gonadotoxicity. For this reason, it is not recommended 
for female patients with cancer scheduled to receive concomitant 
chemotherapeutic treatments.47,48 
Gonadal Shielding
Gonadal shielding should be routinely used during pelvic irra­
diation to p

be routinely used during pelvic irra­
diation to provide some ovarian protection, especially in young 
patients. Similar to oophoropexy, gonadal shielding does not 
protect ovaries from chemotherapy induced-gonadotoxicity and 
therefore has a limited role when chemotherapy is combined with 
radiation therapy.47–63 
Fractionated Chemotherapy and Radiotherapy
Fractionation of chemotherapy and radiotherapy doses may reduce 
gonadotoxicity and gonadal damage and should be attempted 
whenever possibl

l damage and should be attempted 
whenever possible.47–63,65 
Experimental Options for Fertility Preservation in Women 
and Girls
Ovarian Tissue Freezing and Autotransplantation
Ovarian tissue freezing is still considered an experimental method 
for female fertility preservation. It involves cryopreservation of 
surgically excised cortical ovarian tissues for later use in auto­
transplantation or in vitro oocyte maturation.147–151 Additionally, 
immature oocytes could be retrieved directly from 

immature oocytes could be retrieved directly from the extracted 
ovarian tissue for IVM and vitrification.152–157 At least half of one 
ovary is excised via laparoscopy or laparotomy before the initiation 
of cancer treatment. The extracted ovarian tissue is transported 
within 24 hours under special conditions to central cryobanks to 
be processed by experienced oncofertility teams.158–160 Ovarian 
tissue freezing is performed via slow freezing161–163; however, vitri­
fication protocols have be

61–163; however, vitri­
fication protocols have been attempted in numerous research trials 
with promising results.164–168
After recovery from cancer and when pregnancy is desired, the 
frozen ovarian tissue can be thawed and transplanted back into the 
same patient (autotransplantation), typically onto the remaining 
ovary or ovarian fossa. In the case of severe pelvic adhesions or poor 
pelvic vasculature due to previous irradiation, frozen-thawed ovar­
ian tissue can be autotransplanted heter

wed ovar­
ian tissue can be autotransplanted heterotopically to other sites, such 
as the subcutaneous space of abdominal wall or forearm. Following 
successful ovarian tissue freezing and autotransplantation, ovarian 
function may resume between 2 and 9 months postoperatively and 
may last for several years. After orthotopic autotransplantation, 
spontaneous pregnancy can be expected or patients with either 
orthotopic or heterotopic transplants can undergo ovarian stimula­
tion, oocyte retriev

 can undergo ovarian stimula­
tion, oocyte retrieval, and IVF. Although an international registry is 
still needed, at least 120 healthy babies have been born worldwide 
after ovarian tissue slow freezing and orthotopic autotransplanta­
tion, without any increased risk for miscarriage or congenital abnor­
malities. In such cases, the live birth rate per transplant was roughly 
estimated to be 25%. Ovarian tissue vitrification and heterotopic 
autotransplantation have resulted in few reported liv

otransplantation have resulted in few reported live births.169–173
Ovarian tissue freezing followed by autotransplantation may 
be the only suitable fertility preservation option for prepubertal 
girls, although few babies have been born in women whose ovar­
ian tissue was frozen before puberty.174,175 Compared with the 
established female fertility preservation options (embryo and egg 
freezing), ovarian tissue freezing and autotransplantation does not 
need prior ovarian stimulation and theref

oes not 
need prior ovarian stimulation and therefore does not delay cancer 
treatments. Moreover, this option can also restore both endocrine 
and reproductive ovarian function for several years.147–151
Transplantation of fresh and frozen-thawed ovarian tissue 
between monozygotic twin sisters has been successful, resulting in 
healthy live births.176–178 Whole ovary cryopreservation via slow 
freezing or vitrification has been studied in human and several 
animal species.179–182 However, the m

and several 
animal species.179–182 However, the major challenges associated 
with cryopreservation and autotransplantation of the whole ovary 
are the very high follicular loss due to related cryoinjury and post-
transplantation vascular complications and ischemia.183–187
Although promising, the use of autotransplantation of frozen-
thawed ovarian tissue for fertility preservation in patients with 
cancer presents a significant and serious concern of possible con­
tamination of ovarian tissue w

rn of possible con­
tamination of ovarian tissue with malignant cells, or the presence 
of minimal residual disease in the case of ovarian carcinomas or 
in malignancies that may metastasize in ovaries. According to 
several studies, systematic reviews, and meta-analyses assessing 
the risk of reintroducing malignant cells, autotransplantation 
of frozen-thawed ovarian tissue should be absolutely contrain­
dicated for women with any type of ovarian carcinoma or leu­
kemia (high risk). It may be 

an carcinoma or leu­
kemia (high risk). It may be an option in women with Hodgkin 
lymphoma, and breast, bone, and connective tissue malignan­
cies (mild risk), or with non-Hodgkin lymphoma and gastro­
intestinal cancers (moderate risk).150,188–191 To assess the risk 
of reintroducing malignant cells, several tests can be performed 
on the excised tissue prior to autotransplantation, including 
histologic examination, in  vitro culture, immunohistochemis­
try, polymerase chain reaction, multicol

tochemis­
try, polymerase chain reaction, multicolor flow cytometry, and 
long-term xenografting of ovarian tissue into immunodeficient 
mice.159,192–200 The first delivery in a leukemia survivor after 
autotransplantation of cryopreserved ovarian tissue evaluated 
for leukemia cells contamination prior to reimplantation has 
recently been reported.201 Although remarkable, it is not clear 
whether the risk of contamination is the same for all patients 
with leukemia; rather, it appears to be cas

ients 
with leukemia; rather, it appears to be case dependent, and the 
results should therefore be extrapolated to oncofertility care with 
caution. 
IVM of Oocytes
Considering that autotransplantation of frozen-thawed ovarian 
tissue should be absolutely contraindicated in some patients due 
to a high risk of ovarian tissue contamination with malignant cells 
or because of the presence of minimal residual disease, the safest 
way to restore fertility may be IVM of oocytes and artificial ovary 

tility may be IVM of oocytes and artificial ovary 
technology.
In  vitro, immature oocytes can be retrieved directly from 
extracted ovarian tissue and from unstimulated ovaries through a 
routine ultrasound-guided transvaginal approach. The retrieved 
immature oocytes are cultured in vitro for 24 to 48 hours to 
mature into metaphase II oocytes ready for either IVF or vit­
rification.202–205 Although IVM is technically challenging, it is 
feasible for all patients, including prepubertal girls. 

le for all patients, including prepubertal girls. Recently, 
the success rates of oocyte IVM have improved,206–209 and a 
few new reports have shown comparable results to traditional 
IVF.210,211 
1718
Artificial Ovary
The ovarian bioprosthetic, or artificial ovary, is a novel experi­
mental technology that aims to produce mature oocytes ready 
for IVF through an ex  vivo multistep strategy that includes 
sequential in  vitro culture of ovaria

t includes 
sequential in  vitro culture of ovarian tissue, follicles, and 
oocytes.212–214 Although successful only in mice,215,216 research 
is under way to improve artificial ovary methodology and out­
comes and help to establish this technology for translation to 
clinical care217 (Fig. 46.2).
Ovarian tissue culture (encapsulated in vitro follicle growth) 
involves in vitro culture of fresh or frozen-thawed human cortical 
ovarian tissue pieces from 6 to 10 days to activate the growth of pri

es from 6 to 10 days to activate the growth of pri­
mordial follicles within the tissue to reach preantral stages.218,219 
Preantral follicles are then isolated enzymatically and/or mechani­
cally from the cultured ovarian tissue and cultured individually for 
up to 4 weeks in a biodegradable three-dimensional (3D) micro­
environment made of alginate, matrigel, fibrin, or other biomate­
rials.220 The 3D in vitro follicle culture supports preantral follicle 
growth to the early antral stage.221–2

l follicle 
growth to the early antral stage.221–225 These in vitro–grown early 
antral follicles are then released enzymatically and/or mechani­
cally from the surrounding 3D microenvironment and punctured 
to release the enclosed oocytes.226 Finally, the released immature 
oocytes are cultured in vitro for 24 to 48 hours to mature into 
metaphase II oocytes ready for IVF or vitrification.152–157
Additional applications of the artificial ovary have included 
(1) IVF and/or vitrification of in  

ave included 
(1) IVF and/or vitrification of in  vitro–matured oocytes,152–157 
(2) retransplantation of in vitro–grown ovarian follicles within a 
3D biodegradable microenvironment with potential success in a 
xenotransplantation mouse model,227–231 and (3) retransplanta­
tion of in vitro–activated ovarian tissue with reported live births 
in patients with POF232–234 (see Fig. 46.2). Additional emerging 
technologies with potential applications in the field of female fer­
tility preservation a

 in the field of female fer­
tility preservation and restoration include tissue engineering235 
and 3D-printed organs.236 The 3D-printed bioprosthetic ovary 
is a novel experimental technology in which isolated ovarian fol­
licles are embedded in 3D-printed microporous hydrogel scaffolds 
to support follicle growth after transplantation in vivo. Full res­
toration of ovarian function after transplantation of a biopros­
thetic ovary in surgically sterilized mice has been accomplished. 
After natu

sterilized mice has been accomplished. 
After natural mating, mice with 3D-printed ovaries produced 
healthy pups.236 These findings open a new horizon for creating 
a 3D-printed artificial human ovary to restore fertility in cancer 
patients. 
Stem Cells
Due to recent advances in stem cell research over the past 
decades, many studies have examined potential applications 
in the field of female reproductive endocrinology and infertil­
ity. The recent discovery of ovarian (oogonial) stem cells a

ecent discovery of ovarian (oogonial) stem cells and 
the creation of induced pluripotent stem cells has revealed new 
opportunities to maintain ovarian follicle and oocyte produc­
tion, which normally halts just after birth. Stem cell–based 
therapies could potentially treat various conditions leading to 
infertility, including POF following aggressive gonadotoxic che­
motherapy and radiotherapy.237–245 Although promising, there 
is still a long way to go to translate stem cell science into rou

 way to go to translate stem cell science into routine 
clinical practice. 
Neoadjuvant Cytoprotective Pharmacotherapy
The development of mitigation strategies to reduce chemo­
therapy- and radiotherapy-induced gonadotoxicity is another 
approach being taken in fertility preservation research. Recently, 
several strategies have been attempted to reduce the gonadotoxic 
effects of specific chemotherapeutic agents in animal models, with 
promising results. Examples of such strategies include the u

results. Examples of such strategies include the use 
of nanoencapsulated arsenic trioxide,246 c-Abl-TAp63 pathway 
inactivation by imatinib,247,248 and use of the GnRH analogue 
triptorelin.123,249,250 S1P,251 AS101,252 and FTY720253 treatments 
have also been investigated as approaches to reduce radiotherapy-
induced gonadotoxicity. 
Male Patients With Cancer
Epidemiology of Cancer in Young Male Patients
According to statistics from the United 

 Patients
the most common cancers in prepubertal boys (age 0–14 years) are 
leukemias (31%), CNS cancers (26%), and lymphomas (13%). 
The most common cancers in AYA males (age 15–24 years) are 
germ cell tumors (27%), lymphomas (22%), CNS cancers (13%), 
and leukemias (10%). The most common cancers in adult males 
(age 25–49 years) are testicular (14%), melanoma (11%), CNS 
(10%), bowel (10%), and head and neck cancers (8%). The most 

l (10%), and head and neck cancers (8%). The most 
common cancers that arise during the male reproductive years that 
may require aggressive gonadotoxic chemotherapy and radiother­
apy are testicular cancer, germ cell tumors, leukemia, lymphoma, 
and CNS cancer64 (see Table 46.1). 
(1)
Ovarian Tissue Culture
(In vitro activation, IVA)
(C)
Retransplantation
of in vitro–activated.
ovarian tissue
(B)
of in vitro–grown.
ovarian follicles
(A)
In vitro fertilization and/or
freezing o

icles
(A)
freezing of in vitro–matured.
oocytes
(2)
Ovarian Follicle Culture
(In vitro growth, IVG)
(3)
Oocyte Culture
(In vitro maturation, IVM)
• Fig. 46.2  Artificial human ovary: the concept (blue boxes) and the applications (white boxes). (Redrawn 
from Salama M, Isachenko V, Isachenko E, Rahimi G, Mallmann P. Advances in fertility preservation of 
female patients with hematological malignancies. Expert Rev Hematol. 2017;10[11]:951–960.)
1719
Chapter 46  Managi

tol. 2017;10[11]:951–960.)
1719
Risks of Gonadotoxicity and Fertility Loss in Young Male 
Patients
In young male patients undergoing cancer treatment, gonado­
toxicity, azoospermia, and subsequent fertility loss usually occur 
when the testes are exposed to alkylating chemotherapies such 
as chlorambucil, nitrogen mustard, and cyclophosphamide, or 
to ionizing radiotherapy to the pelvis and abdomen or cranial 
or total-body irradiation. The 

bdomen or cranial 
or total-body irradiation. The degree of gonadotoxicity depends 
mainly on the type, dose, and dosage of cancer therapy, as well 
as the age of the patient and the type and stage of cancer. Cancer 
treatments have been classified according to their risk of gonado­
toxicity, azoospermia, and subsequent fertility loss in men24,254–259 
(see Table 46.2). 
Fertility Preservation and Restoration Options in Young 
Male Patients
As in female patients with cancer, when the estimated r

 female patients with cancer, when the estimated risk of 
gonadotoxicity is greater than 50%, fertility preservation strategies 
should be offered to young male patients (<40 years of age) prior 
to cancer treatment. To preserve the fertility of young men and 
boys with cancer, a few established, debatable, and experimental 
options can be offered. The established option is sperm freezing. 
Debatable options include the use of GnRH analogues and hor­
monal suppression, gonadal shielding, and fra

hor­
monal suppression, gonadal shielding, and fractionated chemo­
therapy and radiotherapy. Experimental options include testicular 
tissue freezing, stem cells, neoadjuvant cytoprotective pharmaco­
therapy, and others260–263 (see Table 46.3 and Fig. 46.1). 
Established Options for Fertility Preservation in Men and 
Boys
Sperm Freezing
Sperm freezing is the first and the only established method for 
male fertility cryopreservation; since the 1950s, sperm freezing 
has been considered the gold s

0s, sperm freezing 
has been considered the gold standard option.264,265 It involves 
sperm retrieval and cryopreservation of spermatozoa via slow 
freezing or vitrification; the latter approach is now preferred due 
to a better post-thaw survival rate.266,267 Recently, cryoprotectant-
free vitrification of human spermatozoa has been attempted and 
resulted in healthy live births.268–270 Under normal conditions, 
sperm samples are obtained from adults and adolescents males by 
masturbation. Howe

dults and adolescents males by 
masturbation. However, in case of ejaculation disorders, vibratory 
or electrical stimulation can be used to obtain sperm. In patients 
with retrograde ejaculation, spermatozoa can be isolated from 
urine. When obstructive or nonobstructive azoospermia is diag­
nosed, testicular sperm aspiration or extraction can be performed 
to obtain sperm.271–273 In clinical practice, frozen and fresh sperm 
have comparable live birth rates, and according to some reports, 
fro

e birth rates, and according to some reports, 
frozen sperm can be stored for more than 20 years without nega­
tive side effects. Healthy live births have been reported from IVF 
with semen stored for 21 years,274 from intrauterine insemina­
tion with semen stored for 28 years,275 and from intracytoplasmic 
sperm injection with semen stored for 40 years.276 
Debatable Options for Fertility Preservation in Men and 
Boys
The use of GnRH analogues before and 

 Suppression
The use of GnRH analogues before and during chemotherapy 
in males has been shown to have a gonadoprotective effect in 
mouse and rat models, but it does not appear to have the same 
effect in human and nonhuman primates.277,278 In most cases, 
GnRH analogues do not protect the testes from radiotherapy-
induced gonadotoxicity. Other hormonal methods to suppress 
testes with androgens and antiandrogens have not shown prom­
ising gonadoprotective effects in adult men. For these rea­
s

oprotective effects in adult men. For these rea­
sons, ASCO does not recommend the use of GnRH analogues 
and other hormonal suppression methods for male fertility 
preservation.47–49 
diation to provide some testicular protection, especially in 
young patients. However, gonadal shielding does not protect 
testes from chemotherapy-induced gonadotoxicity and there­
fore has a limited role when chemotherapy is combine

re has a limited role when chemotherapy is combined with 
radiation.47–63,65 
As in women and girls with cancer, fractionation of chemo­
therapy and radiotherapy doses may reduce gonadotoxic­
ity and gonadal damage and should be attempted whenever 
possible.47–63,65 
Experimental Options for Fertility Preservation in Men 
and Boys
Testicular Tissue Freezing
Testicular tissue freezing is an experimental option that may be 
offered when sperm production i

ption that may be 
offered when sperm production is not possible, such as in pre­
pubertal boys or in adults with azoospermia. In this technique, 
spermatogonial stem cells, either within the testicular tissue or 
as a cell suspension, are frozen via slow freezing or vitrification 
and stored for future autotransplantation, in  vitro culture, or 
in  vitro spermatogenesis.279 As with ovarian tissue, autotrans­
plantation of frozen-thawed testicular tissue may carry the risk 
of reintroducing mal

ar tissue may carry the risk 
of reintroducing malignant cells back into the patient, espe­
cially in patients with testicular cancer or malignancies that 
metastasize to the testes, such as leukemia. To date, testicular 
tissue freezing and spermatogonial stem cell autotransplantation, 
xenografting, in vitro culture, and in vitro spermatogenesis have 
proven more promising in mice than in other animal species and 
humans.271,279–285 
Stem Cells
The recent discovery of spermatogonial stem cells

The recent discovery of spermatogonial stem cells and creation 
of induced pluripotent stem cells have opened new horizons for 
the recovery and maintenance of spermatogenesis after aggressive 
gonadotoxic chemotherapy and radiotherapy.286–292 Although 
promising, much work still needs to be done to realize the 
promise of stem cell research for the clinical practice of fertility 
preservation. 
Although the development of neoadjuvant cytoprotective ph

h the development of neoadjuvant cytoprotective phar­
macotherapy strategies to reduce chemotherapy- and radiother­
apy-induced gonadotoxicity is an important topic of interest 
in fertility preservation research, few studies have examined 
the use of these strategies in men. Partial success has been 
observed with the immunomodulatory AS101 and carnitine in 
rodents.293,294 
Other Experimental Options
Recently, several research technologies have emerged with poten­
tial applications in the fiel

 emerged with poten­
tial applications in the field of male fertility preservation and res­
toration. Examples of such technologies include testicular tissue 
culture, in vitro spermatogenesis, and gene therapy.295–302 
1720
Decision-Making Strategies for Young Female 
and Male Patients With Cancer
To provide fertility preservation and restoration strategies to 
young patients with cancer, the treating center should be properly 
equipped, with

treating center should be properly 
equipped, with a highly skilled team of oncologists, gynecologists, 
andrologists, reproductive biologists, transplantation surgeons, 
and research scientists. Referring patients from oncology clinics, 
small medical centers, or general hospitals to highly specialized 
oncofertility centers is strongly encouraged to guarantee a high 
standard of care.1,303,304
For young patients with cancer, it is recommended that 
oncofertility counseling starts immediately a

hat 
oncofertility counseling starts immediately after diagnosis. If the 
patient is younger than 40 years and has a reasonable chance of 
survival, is in good health, and has satisfactory reproductive func­
tion, the cancer treatment plan should be reviewed by the oncofer­
tility team and the risk of gonadotoxicity and subsequent fertility 
loss should be assessed. If the risk is greater than 50% and the 
patient wishes to have children in the future, a fertility preserva­
tion strategy should 

uture, a fertility preserva­
tion strategy should be pursued before and during chemotherapy 
and radiotherapy. After the patient completes cancer therapy 
and crosses into the realm of survivorship, a new assessment of 
endocrine and reproductive function should be performed. If it is 
determined that the patient suffers from cancer therapy–induced 
fertility loss, the frozen gametes, gonadal tissue, or stem cells can 
be used to restore fertility. From a clinical perspective, fertility 
preserv

y. From a clinical perspective, fertility 
preservation and restoration strategies should be tailored to each 
patient’s circumstances and wishes, and may involve a combina­
tion of established, debatable, and experimental options (after 
obtaining informed consent from the patient or legal guardians 
in case of a child).1,305–309 As shown in Tables 46.4 and 46.5, 
plausible strategies according to patient’s gender and age can be 
attempted—when feasible and not contraindicated—before, dur­
ing,

feasible and not contraindicated—before, dur­
ing, and after cancer therapy.
If fertility preservation options are rejected, contraindicated, 
or unavailable, adoption and third-party reproduction (gamete 
donation and surrogacy) can be offered as alternatives when pos­
sible.310,311 If third-party reproduction services are not available 
or not legally allowed, patients may consider traveling abroad to 
pursue these options. This growing phenomenon has been called 
cross-border reproductive car

non has been called 
cross-border reproductive care or fertility tourism in the media.312–319 
Challenges of Fertility Preservation
Factors That Prohibit Oncofertility Support
Only about one third of AYA cancer survivors pursue fertil­
ity preservation.21 Barriers to oncofertility support include both 
patient- and provider-related factors.25 Female sex, age, ethnic­
ity, relationship status and current parenthood, religious beliefs, 
economic concerns, self-perceived lack of time, and emotional

ncerns, self-perceived lack of time, and emotional 
distress, as well as functional status, type of malignancy, and prog­
nosis at the time of diagnosis, are important contributors to the 
provision of and participation in adequate counseling on repro­
ductive health.25,26,320 Some cancer patients hesitate to initiate 
conversations about reproductive health with their health care 
team due to the assumption that the topic should first be intro­
duced by their providers.321 Furthermore, many pat

duced by their providers.321 Furthermore, many patients making 
fertility preservation decisions are in the early stages of coping with 
their cancer diagnosis and are overwhelmed by considerations of 
personal mortality, cancer recurrence, and comorbidities related to 
cancer therapy, in addition to evaluating the financial and ethical 
implications of fertility preservation.29
Although more than 80% of cancer health care providers 
believe that reproductive health is important for cancer patie

 reproductive health is important for cancer patients, 
more than 70% of providers feel unprepared to formally assess and 
address these concerns.321 Provider age, practice setting, perception 
  Plausible Fertility Preservation and Restoration Strategies for Female Patients With Cancer
Age
Before Anticancer Therapy 
(Fertility Preservation)
During Anticancer Therapy (Fertility 
Preservation)
After Anticancer Therapy 
(Fertility Restoration)
Prepubertal (0–14 
years)
	•	 Ovarian tissue freezin

easing hormone; ICSI, intracytoplasmic sperm injection; IVF, in vitro fertilization; IVM, in vitro maturation.
   
TABLE 46.4
1721
of inadequate training, time, and resources, and personal view 
of the patient’s desire and comfort with discussing reproductive 
health also influence whether oncofertility is included in compre­
hensive cancer care.25,321 Among providers caring for pediatric 
and adolescent cancer patients, discomfort with disc

d adolescent cancer patients, discomfort with discussing repro­
ductive health with sexually immature patients and their parents 
influences the extent to which reproductive health is addressed. In 
particular, providers express concerns regarding patients’ decision-
making capacity on issues pertaining to their future fertility.322
Both male and female cancer patients identify fertility as a 
high-priority element of their psychological and social well-being. 
Although an increasing number of c

al well-being. 
Although an increasing number of cancer patients are receiving 
information about fertility preservation prior to treatment, fertil­
ity preservation options are highly underutilized,21 partly due to 
insufficient referral to reproductive specialists after initial counsel­
ing.323 Patients who undergo evaluation by a reproductive spe­
cialist are more likely to report better understanding of fertility 
preservation options prior to starting treatment, regardless of 
whether they 

o starting treatment, regardless of 
whether they pursue these options in the future.323 Thus, effective 
execution of fertility preservation counseling requires an intricate 
level of communication and coordination and a shared sense of 
responsibility among those treating the cancer (oncologists, sur­
geons, radiologists) and fertility specialists and scientists.324 
Special Patient Populations
The relationship between reproductive health and cancer diag­
nosis and treatment is particularly mu

ancer diag­
nosis and treatment is particularly multifaceted for ethnic, cul­
tural, and sexual minorities. Provision of oncofertility counseling 
varies by ethnicity.320 Culturally specific beliefs about illness, 
spirituality, and mortality,325 trust in the medical system326 and 
medical decision making,327 sexuality,328,329 self-identity and 
autonomy,330 and QoL331 impact cancer patients’ perspectives 
on the role that reproductive health plays in their overall well-
being. Furthermore, pati

ys in their overall well-
being. Furthermore, patients’ religious and cultural beliefs, spe­
cifically regarding gamete collection and storage, fertilization, 
procreation, and surrogacy, guide their selection among differ­
ent fertility preservation options.332 Thus, culturally competent 
oncofertility care is important, as it can improve patient-provider 
communication, reduce decision regret, and improve health-
related QoL.326
Sexual minorities encounter distinct challenges relating to inti­

es encounter distinct challenges relating to inti­
macy, fertility, and parenthood prior to, during, and after cancer 
therapy. On the one hand, members of the lesbian, gay, bisexual, 
transgender, or queer (LGBTQ) community who have cancer 
experience similar concerns regarding relationships, fertility, and 
body image as their heterosexual counterparts. On the other hand, 
LGBTQ cancer patients also uniquely express beliefs in the “cho­
sen family,” which is a family founded on love and kinshi

ily,” which is a family founded on love and kinship and 
not necessarily blood or biologic relation; they are more open to 
alternative parenting options, such as adoption, gamete donation, 
and surrogacy, and report less impact of fertility on their relation­
ships compared with heterosexual cancer patients. Because many 
providers may not appropriately address these unique aspects of 
intimacy, relationships, and family building, almost one-third of 
LGBTQ cancer patients report unsatisfactory

rd of 
LGBTQ cancer patients report unsatisfactory counseling about 
reproductive health prior to cancer treatment.333
Decisions regarding fertility and family building are also chal­
lenging for pregnant women with cancer, who comprise about 
0.1% of the population of all pregnant women.334 Although rare, 
the common cancers diagnosed during pregnancy include breast 
(the most common) and cervical cancers, Hodgkin lymphoma, 
non-Hodgkin lymphoma, ovarian cancer, malignant melanoma, 
leukemia, a

, ovarian cancer, malignant melanoma, 
leukemia, and thyroid and colorectal cancers.335,336 Pregnant 
women with cancer may struggle with distinguishing between 
cancer- and pregnancy-related symptoms, are hesitant to undergo 
certain imaging or other diagnostic procedures, and are concerned 
about the possibility of missing clinically obscure cancers in the 
setting of pregnancy-related changes to their bodies. As such, 
they are at higher risk of pregnancy termination.337 Because of 
the compl

of pregnancy termination.337 Because of 
the complexity of cancer during pregnancy, guidelines for the pre­
natal care for women with cancer during pregnancy have been 
established.338 Treatment requiring surgery, radiotherapy, or che­
motherapy necessitates close monitoring by a multidisciplinary 
team of oncologists, obstetricians, neonatologists, cardiologists, 
pharmacologists, surgeons, radiation oncologists, ethicists, and 
mental health providers. Live birth rates as high as 86% have 
bee

oviders. Live birth rates as high as 86% have 
been reported among pregnant women with cancer, whereas the 
rates of preterm delivery, small for gestational age birth weight, 
and neonatal intensive care unit admission are 48%, 21%, and 
41%, respectively. Maternal age, type of cancer, extent of disease 
(systemic vs. localized), and type of chemotherapeutic agent (e.g., 
  Plausible Fertility Preservation and Restoration Strategies for Male Patients With Cancer
Age
(

platinum based, taxanes) are risk factors for certain adverse obstet­
ric and neonatal outcomes in pregnant women with cancer. Thus, 
changes to administration standards for chemotherapy (e.g., after 
the first trimester and no later than the 35th week of gestation) 
have been implemented to decrease the risk of congenital malfor­
mations and myelosuppression, respectively.339 Appropriate tim­
ing and perioperative care prior to surgery for cancer treatment 
have reduced the risk of preterm del

er treatment 
have reduced the risk of preterm delivery among women with 
cancer.340 Nonetheless, women who undergo radiation therapy 
as part of their cancer treatment experience a dose-dependent 
increase in the risk of miscarriage, preterm delivery, abnormal 
placentation, fetal malposition, and low fetal birth weight in 
subsequent pregnancies.31,341 The ultimate long-term impact of 
concurrent pregnancy and cancer treatment on cancer recurrence 
and overall maternal health is still unknown.

nce 
and overall maternal health is still unknown.342 Notably, growth, 
cardiac function, neuropsychological capacity, and overall health 
in kids of mothers who had cancer during pregnancy are compa­
rable to controls.343 
Financial and Ethical Considerations
About 50% of cancer survivors experience financial hardship 1 
year after diagnosis.344 In fact, cancer patients are more than 2.6 
times more likely to file for bankruptcy within 5 years of diagno­
sis, with patients 21 to 44 years of age

f diagno­
sis, with patients 21 to 44 years of age bearing the greatest risk 
compared with matched controls.345 “Financial toxicity” encom­
passes all of the financial considerations that patients with cancer 
experience throughout the course of their diagnosis, treatment, 
and follow-up. Financial toxicity is particularly burdensome for 
younger cancer patients and plays a significant role in personal 
choice about pursuing fertility preservation.345 Out-of-pocket 
costs of fertility preservat

on.345 Out-of-pocket 
costs of fertility preservation are shown in Fig. 46.3. Many health 
insurance plans do not cover costs associated with fertility preser­
vation, and only two states mandate insurance coverage for costs 
associated with iatrogenic infertility. Furthermore, some plans 
selectively cover the cost of ovarian stimulation but do not cover 
egg/embryo storage fees. Thus, fertility preservation may be an 
untenable option for many cancer patients of reproductive age.346 
In fact, 

cancer patients of reproductive age.346 
In fact, more than 20% of cancer patients cite cost as the pri­
mary reason for not pursuing fertility preservation.320 Disparities 
in financial hardship contribute to decreased health care–related 
QoL after cancer treatment, especially among certain ethnic 
minorities.344,347 As such, patient financial literacy and navigation 
programs are making strides toward attenuating the financial bur­
den of fertility preservation.348
The provision of adequate c

ility preservation.348
The provision of adequate counseling on reproductive health 
prior to cancer treatment and the timely implementation of can­
cer treatment are commonly at odds at the time of cancer diag­
nosis, calling forth complex ethical questions. Specific ethical 
concerns include purposeful disclosure of fertility preservation 
options despite the potential delay to initiation of cancer ther­
apy; ongoing awareness of patient confidentiality and privacy; 
procurement of assent and c

ntiality and privacy; 
procurement of assent and consent for fertility preservation in 
pediatric and adolescent patients; allocation of rights for future 
use, disposal, and donation of preserved tissue (including post­
humous reproduction); utilization of preimplantation genetic 
diagnosis in future reproduction; provision of the costs associated 
with fertility preservation between the patient and involved part­
ners; potential impact on QoL of subsequent offspring of cancer 
survivors; and a

f subsequent offspring of cancer 
survivors; and avoidance of false hope in discussions of future 
fertility in patients with poor prognosis.349 In prepubertal can­
cer patients, conversations about reproductive health, including 
Sperm banking
Testicular tissue
cryopreservation
Egg banking
Fertility preservation
Third-party reproduction
Embryo banking
Ovarian tissue
IVF with donor sperm
IVF with donor egg
Adoption
Surrogacy
Estimated cost ($)
0
10,000 20,000 30,000 40,000 50,00

mated cost ($)
0
10,000 20,000 30,000 40,000 50,000 60,000
40,000
25,000
16,000
12,000
4000
3000
3000
1000
70,000
Cancer treatment
• Fig. 46.3  Cost of fertility preservation versus third-party reproduction. Potential costs vary by state, insti­
tution, and insurance status. Fertility preservation costs do not include annual storage fees of reproductive 
tissues (approximately $250/year) or cost of in vitro fertilization (IVF; approximately $12,000). (Courtesy the 
Oncofertility Consortium and T

00). (Courtesy the 
Oncofertility Consortium and Teresa K. Woodruff.)
1723
current contraception, sexuality, and future fertility, as well as 
the logistics of acquiring semen from boys and ovarian tissue 
from girls, are largely shaped by parents’ and providers’ beliefs 
about the child’s sexual maturity.332 Adult survivors of cancer 
who underwent fertility preservation counseling at a young age 
may disagree with decisions made by their p

g age 
may disagree with decisions made by their parents at the time 
of cancer diagnosis.350 Many cancer survivors experience social 
and emotional pressure to use previously stored gonadal tissue or 
gametes for achievement of genetic parenthood, especially in the 
context of socially constructed ideals of femininity, masculinity, 
and parenthood.351 Furthermore, bioethical, legal, and regula­
tory concerns are particularly pertinent for young cancer patients 
considering experimental fertilit

cancer patients 
considering experimental fertility preservation options and for 
cancer survivors considering reproductive tissue allografting or 
isografting or gestational surrogacy as ways to achieve parent­
hood.352 Thus, many institutions have implemented task forces 
to help navigate the ethical issues that individual cancer patients 
face throughout the course of their cancer diagnosis, treatment, 
and follow-up. 
Methods for Addressing Challenges
Providing Personalized Risk Assessment
T

hallenges
The availability of information about the infertility risk posed by 
cancer or a particular cancer treatment is of the utmost impor­
tance for making an informed decision to undergo fertility pres­
ervation treatment. This is especially true for women, as the 
available fertility preservation techniques are costly and require 
several days or weeks to be completed. Several cohort studies have 
demonstrated the reduction of female and male fertilit

nstrated the reduction of female and male fertility after can­
cer treatment; the probability of having children was found to 
be reduced by half in the Scandinavian Cohort Study and the 
Childhood Cancer Survivor Study.353–355 However, prospective 
studies are still lacking or are of inadequate size, and random­
ized controlled trials are difficult to implement in this population 
of patients due to the amount of time required to assess fertility 
status after treatment. The sample size in part

y 
status after treatment. The sample size in particular is a difficult 
hurdle to overcome due to the heterogeneity of patient age, tumor 
stage, and treatment. Prospective or observational studies must 
last several years or decades to evaluate whether a particular can­
cer type or a cancer treatment prematurely reduces the ovarian 
reserve and fertility potential. Patient selection may also be biased, 
as the patients who enroll in these studies are highly interested 
in fertility. Despite th

es are highly interested 
in fertility. Despite these challenges, efforts have been made to 
identify individual predictors of infertility risk among patients 
with cancer, the most important of which are age in women and 
the type of cancer treatment in both men and women. Pretreat­
ment evaluation of the ovarian reserve or sperm production is an 
important first step in estimating infertility risk, as reproductive 
function may already be compromised in patients with cancer. 
After cancer trea

omised in patients with cancer. 
After cancer treatment, the recovery of ovarian or testicular func­
tion may occur months to years after radiation or chemotherapy. 
In men, semen analysis should be performed repeatedly to evalu­
ate the recovery of sperm production, which can be delayed years 
after treatment because stem cells of the testicle reinitiate sper­
matogenesis. Although most studies have used amenorrhea as a 
surrogate marker for fertility loss in women, it is not accurate; in 
fact

tility loss in women, it is not accurate; in 
fact, resumption of regular menses does not always signal fertility. 
New and more accurate markers of ovarian function are needed 
to counsel patients about infertility risk and fertility status before 
and after cancer treatment. 
Practice Management, Knowledge, and Access Barriers to 
Clinical Care
Oncofertility is a field that bridges reproductive science and oncol­
ogy in an effort to preserve reproductive function for patients 
diagnosed with c

productive function for patients 
diagnosed with cancer. Achieving this goal requires a close col­
laboration of specialists involved in the treatment of cancer and 
infertility, who often have competing priorities. Discussions with 
patients about the possible risks posed by cancer and its treatment 
on fertility and the options for fertility preservation are necessar­
ily complex—not only because multiple perspectives are in play 
but also because timing is crucial, particularly for patients w

use timing is crucial, particularly for patients with 
aggressive forms of cancer. This task can be particularly difficult 
in children with cancer, requiring practitioners to evaluate long-
term infertility risks and offer appropriate fertility preservation 
techniques as soon as the urgent need for gonadotoxic treatment 
is established. Fertility preservation options for the youngest 
patients (immature gamete retrieval and cryopreservation) remain 
largely experimental, and the availability o

main 
largely experimental, and the availability of this procedure is lim­
ited to only a few centers. Indeed, one of the major threats to this 
field is the need to have professional societies in many disciplines 
embrace fertility in their clinical setting—oncology, urology, allied 
health professionals, and reproductive endocrinologists (adult and 
pediatric) all need to be part of the equation.
Although the need for oncofertility and the collaboration 
between oncology and reproductive endoc

aboration 
between oncology and reproductive endocrinology is becoming 
globally recognized, with several scientific societies around the 
world establishing guidelines for fertility preservation in cancer 
patients,50,356 there is still a gap in access—to knowledge, to the 
procedures themselves, and to support. To more fully address the 
issue of iatrogenic infertility after cancer treatment, it is essential to 
share information about oncofertility with individual cancer treat­
ment centers, 

ility with individual cancer treat­
ment centers, reproductive endocrinology and infertility practices, 
and infertility clinics. Updates in the field should be sent regularly to 
each of these stakeholders. Social media and traditional media must 
be engaged. Cross-platform resources should be widely distributed 
and made available online. Mobile applications linking practitioners 
and patients to oncofertility care must be advertised extensively. 
Inclusion of Psychological Support Is Critical

y. 
Inclusion of Psychological Support Is Critical to Oncofertil­
ity Clinical Care Models
Psychological support during the oncofertility decision-making 
process is essential, especially for pediatric oncologic patients, 
who are presented with very specific challenges.357 They (or their 
parents) may be overwhelmed by the cancer diagnosis and focused 
on what is necessary to survive cancer rather than a discussion of 
possible future infertility. Even when a patient decides to undergo 
fertili

y. Even when a patient decides to undergo 
fertility preservation, they are more likely to select a quick “one-
stop” strategy, such as ovarian tissue cryopreservation, that does 
not delay the initiation of cancer treatment and does not require 
the patient to be actively engaged in the fertility preservation treat­
ment, as would be necessary for embryo or oocyte cryopreserva­
tion. This raises some concern, because in many cases patients may 
be choosing to undergo an experimental, but quicke

be choosing to undergo an experimental, but quicker, procedure 
when an established but slower one could be at least as effective 
and certainly less invasive. It is important that practices under­
stand the value of specialized oncofertility support personnel and 
models that train psychologists in oncofertility navigation and 
counseling who can provide psychological and decision-making 
support to patients at single centers or between centers. Building 
this capacity into oncofertility care i

 Building 
this capacity into oncofertility care is an important strategy that 
could be implemented on a macro (state, region, or country) scale. 
1724
Access and Affordability
The cost of fertility preservation is well beyond the reach of most 
people; in the United States, the average upfront cost of ART to 
cryopreserve oocytes is $9200, plus annual storage fees of approxi­
mately $300, and $4400 for thawing, fertilizing, and implanting 
t

 $4400 for thawing, fertilizing, and implanting 
the frozen eggs.358 Although insurance, reimbursement, and spe­
cific cost issues vary in Europe, Asia, and North America, the pri­
mary reason for the underuse of ART, which has been available for 
the past 35 years around the world, is the cost of the procedures 
and medications, both inside and outside the cancer setting. In 
many places in the world, including in countries in the global 
north (like the United States), infertility treatments a

(like the United States), infertility treatments are frequently 
not covered by health insurance.359 In the global south, infertility 
treatments are commonly seen as luxury items, given the lack of 
resources and the need to prioritize basic, lifesaving health care.360 
In Portugal, considerable effort has been made to improve finan­
cial support programs for ART, and today, public ART centers 
offer fertility preservation for men and women, with 69% of 
medication costs covered by social secur

h 69% of 
medication costs covered by social security.361 Country-by-coun­
try assessments of the costs of oncofertility are ongoing and will 
provide insight into future approaches to reduce cost as a bar­
rier to access for patients with cancer. The ability to have genetic 
children is important to many women and men throughout the 
world,360 and the World Health Organization considers infertility 
to be a global health issue.362 The significance of genetic parent­
hood and the public health p

ce of genetic parent­
hood and the public health perspective are important to factor 
into the arguments for pursuing fertility preservation, particularly 
in resource-limited environments. Ultimately, costs and priorities 
are intertwined and should be considered in equal measure. 
Public Awareness
Although public awareness about oncofertility has increased dramat­
ically in the past decade, there is still an overall lack of knowledge 
and understanding of the importance of fertility preservati

standing of the importance of fertility preservation in the 
cancer setting. The media is a powerful tool for disseminating health-
related news and has played an important role in educating the 
public about oncofertility. However, awareness remains low among 
individuals who are less likely to be reached via the news media, 
those who are less educated, those who have lower health literacy, 
and those who are from lower socioeconomic backgrounds. Public 
awareness is the first step in creating

s. Public 
awareness is the first step in creating public support for a given cause. 
Without widespread public support, it is difficult for a movement to 
gain momentum and engender real change. One of the barriers pre­
venting oncofertility from accumulating more public support is the 
perception that ART is an elective procedure that is not medically 
necessary. Yet several professional medical organizations categorize 
infertility as a disease, and there is substantial evidence demonstrat­
i

e, and there is substantial evidence demonstrat­
ing the physical, psychological, social, and economic impact of 
infertility and its treatment. Correcting this misperception and illus­
trating the health benefits of oncofertility, beyond fertility preserva­
tion, for patients with cancer is a major focus in the field. 
Distinguishing Between Oncofertility, Infertility, and 
Social Egg Freezing
Although oncofertility involves the same ART procedures used 
for infertility treatment, oncofertility

res used 
for infertility treatment, oncofertility is specifically focused on 
the needs young patients with cancer whose future fertility is 
threatened by the cancer or its treatment. It is important to rec­
ognize the differences between oncofertility patients and patients 
with traditional infertility. Unlike patients who seek treatment 
for infertility, oncofertility patients have anticipated iatrogenic 
infertility that is directly related to their lifesaving cancer treat­
ment. Unfortunat

to their lifesaving cancer treat­
ment. Unfortunately, these two categories are often conflated, 
leading to a similar exclusion of oncofertility procedures from 
insurance coverage as a form of infertility treatment.363 Clearly 
classifying oncofertility as part of the cancer treatment plan 
would help establish the difference between oncofertility and 
infertility treatments, as well as improve access to and insurance 
coverage for ART procedures specifically in the oncofertility 
setting.364

es specifically in the oncofertility 
Many in the public also have difficulty distinguishing between 
the use of ART for fertility preservation in cancer patients and the 
use of ART for fertility preservation to avoid age-related infertil­
ity—what has been called social egg freezing.365 In the past few 
years, social egg freezing by women who want to delay childbear­
ing has received a great deal of news media attention.366 Given 
low health and science literacy rates among the pub

ow health and science literacy rates among the public, people 
may not be able to immediately understand the different reasons 
for fertility preservation in cancer patients versus healthy women 
who are concerned about age-related infertility, especially since 
the same technologies are used in each setting. With the increas­
ing demand for social egg freezing, there is the concern that the 
cost of fertility preservation will increase for all patients, includ­
ing oncofertility patients. More 

atients, includ­
ing oncofertility patients. More work must be done to more 
clearly illustrate in plain language the differences in the use of 
ART in the oncofertility setting (e.g., for infertility) versus social 
egg freezing. 
Summary and Next Steps
Oncofertility sits at the fulcrum of disciplines and requires an inte­
gration of medical specialties—oncology, urology, and reproduc­
tive endocrinology—both with each other and with the emerging 
technologies that will be tomorrow’s breakthrou

g 
technologies that will be tomorrow’s breakthroughs. The loss of 
reproductive function due to cancer has a profound impact on 
endocrine health, fertility, and psychosocial well-being, and atten­
tion to this rapidly moving field is crucial for ensuring a high QoL 
among cancer survivors.
1724.e1
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of Medical
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of Medical
E L E V E N T H  
E D I T I O N
Arthur C. Guyton, M.D.†
Professor Emeritus
Department of Physiology and Biophysics
University of Mississippi Medical Center
Jackson, Mississippi
†Deceased
John E. Hall, Ph.D.
Professor and Chairman
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To
My Family
For their abundant support, for their patience and
understanding, and for their love
To
Arthur C. Guyton
For his imaginative and innovative research
For his dedication to education
For showing us the exc

his dedication to education
For showing us the excitement and joy of physiology
And for serving as an inspirational role model
Arthur C. Guyton, M.D.
1919–2003
I N  M E M O R I A M
The sudden loss of Dr. Arthur C. Guyton in an automobile accident on April 3,
2003, stunned and saddened all who were privileged to know him. Arthur
Guyton was a giant in the ﬁelds of physiology and medicine, a leader among
leaders, a master teacher, and an inspiring role model throughout the world.
Arthur Clifton Guy

ole model throughout the world.
Arthur Clifton Guyton was born in Oxford, Mississippi, to Dr. Billy S.
Guyton, a highly respected eye, ear, nose, and throat specialist, who later
became Dean of the University of Mississippi Medical School, and Kate Small-
wood Guyton, a mathematics and physics teacher who had been a missionary
in China before marriage. During his formative years,Arthur enjoyed watching
his father work at the Guyton Clinic, playing chess and swapping stories with
William Faulkner

g chess and swapping stories with
William Faulkner, and building sailboats (one of which he later sold to
Faulkner). He also built countless mechanical and electrical devices, which he
continued to do throughout his life. His brilliance shone early as he graduated
top in his class at the University of Mississippi. He later distinguished himself
at Harvard Medical School and began his postgraduate surgical training at
Massachusetts General Hospital.
His medical training was interrupted twice—once

l.
His medical training was interrupted twice—once to serve in the Navy during
World War II and again in 1946 when he was stricken with poliomyelitis during
his ﬁnal year of residency training. Suffering paralysis in his right leg, left arm,
and both shoulders, he spent nine months in Warm Springs, Georgia, recuper-
ating and applying his inventive mind to building the ﬁrst motorized wheelchair
controlled by a “joy stick,” a motorized hoist for lifting patients, special leg
braces, and other dev

ifting patients, special leg
braces, and other devices to aid the handicapped. For those inventions he
received a Presidential Citation.
He returned to Oxford where he devoted himself to teaching and research
at the University of Mississippi School of Medicine and was named Chair of the
Department of Physiology in 1948. In 1951 he was named one of the ten out-
standing men in the nation. When the University of Mississippi moved its
Medical School to Jackson in 1955, he rapidly developed one of t

 to Jackson in 1955, he rapidly developed one of the world’s
premier cardiovascular research programs. His remarkable life as a scientist,
author, and devoted father is detailed in a biography published on the occasion
of his “retirement” in 1989.1
A Great Physiologist. Arthur Guyton’s research contributions, which include
more than 600 papers and 40 books, are legendary and place him among the
greatest physiologists in history. His research covered virtually all areas of car-
diovascular regula

red virtually all areas of car-
diovascular regulation and led to many seminal concepts that are now an inte-
gral part of our understanding of cardiovascular disorders, such as hypertension,
heart failure, and edema. It is difﬁcult to discuss cardiovascular physiology
without including his concepts of cardiac output and venous return, negative
interstitial ﬂuid pressure and regulation of tissue ﬂuid volume and edema,
regulation of tissue blood ﬂow and whole body blood ﬂow autoregulation,
renal-

ow and whole body blood ﬂow autoregulation,
renal-pressure natriuresis, and long-term blood pressure regulation. Indeed, his
concepts of cardiovascular regulation are found in virtually every major text-
book of physiology.They have become so familiar that their origin is sometimes
forgotten.
One of Dr. Guyton’s most important scientiﬁc legacies was his application of
principles of engineering and systems analysis to cardiovascular regulation. He
used mathematical and graphical methods to quanti

used mathematical and graphical methods to quantify various aspects of circu-
latory function before computers were widely available. He built analog com-
puters and pioneered the application of large-scale systems analysis to modeling
the cardiovascular system before the advent of digital computers. As digital
computers became available, his cardiovascular models expanded dramatically
to include the kidneys and body ﬂuids, hormones, and the autonomic nervous
system, as well as cardiac and circ

onomic nervous
system, as well as cardiac and circulatory functions.2 He also provided the ﬁrst
comprehensive systems analysis of blood pressure regulation. This unique
approach to physiological research preceded the emergence of biomedical 
vii
viii
In Memoriam
engineering—a ﬁeld that he helped to establish and to
promote in physiology, leading the discipline into a
quantitative rather than a descriptive science.
It is a tribute to Arthur Guyton’s genius that his 
concepts of cardiovascular reg

’s genius that his 
concepts of cardiovascular regulation often seemed
heretical when they were ﬁrst presented, yet stimu-
lated investigators throughout the world to test them
experimentally.They are now widely accepted. In fact,
many of his concepts of cardiovascular regulation 
are integral components of what is now taught in 
most medical physiology courses. They continue to 
be the foundation for generations of cardiovascular
physiologists.
Dr. Guyton received more than 80 major honors
from

from diverse scientiﬁc and civic organizations and uni-
versities throughout the world.A few of these that are
especially relevant to cardiovascular research include
the Wiggers Award of the American Physiological
Society, the Ciba Award from the Council for High
Blood Pressure Research, The William Harvey Award
from the American Society of Hypertension, the
Research Achievement Award of the American Heart
Association, and the Merck Sharp & Dohme Awa

Heart
Association, and the Merck Sharp & Dohme Award 
of the International Society of Hypertension. It was
appropriate that in 1978 he was invited by the Royal
College of Physicians in London to deliver a special
lecture honoring the 400th anniversary of the birth of
William Harvey, who discovered the circulation of the
blood.
Dr. Guyton’s love of physiology was beautifully
articulated in his president’s address to the American
Physiological Society in 1975,3 appropriately entitled
Physiology, a

ety in 1975,3 appropriately entitled
Physiology, a Beauty and a Philosophy. Let me quote
just one sentence from his address: What other person,
whether he be a theologian, a jurist, a doctor of medi-
cine, a physicist, or whatever, knows more than you, a
physiologist, about life? For physiology is indeed an
explanation of life. What other subject matter is more
fascinating, more exciting, more beautiful than the
subject of life?
A Master Teacher. Although Dr. Guyton’s research
accomplishments ar

 Although Dr. Guyton’s research
accomplishments are legendary, his contributions as an
educator have probably had an even greater impact.
He and his wonderful wife Ruth raised ten children,
all of whom became outstanding physicians—a
remarkable educational achievement. Eight of the
Guyton children graduated from Harvard Medical
School, one from Duke Medical School, and one from
The University of Miami Medical School after receiv-
ing a Ph.D. from Harvard. An article published in
Reader’s Digest 

 Harvard. An article published in
Reader’s Digest in 1982 highlighted their extraordinary
family life.4
The success of the Guyton children did not occur by
chance. Dr. Guyton’s philosophy of education was to
“learn by doing.” The children participated in count-
less family projects that included the design and 
construction of their home and its heating system,
the swimming pool, tennis court, sailboats, go-carts 
and electrical cars, household gadgets, and electronic
instruments for their Oxfor

adgets, and electronic
instruments for their Oxford Instruments Company.
Television programs such as Good Morning America
and 20/20 described the remarkable home environ-
ment that Arthur and Ruth Guyton created to raise
their family. His devotion to family is beautifully
expressed in the dedication of his Textbook of Medical
Physiology5:
To
My father for his uncompromising principles that
guided my life
My mother for leading her children into intellectual
pursuits
My wife for her magniﬁcent dev

tellectual
pursuits
My wife for her magniﬁcent devotion to her family
My children for making everything worthwhile
Dr. Guyton was a master teacher at the University
of Mississippi for over 50 years. Even though he was
always busy with service responsibilities, research,
writing, and teaching, he was never too busy to talk
with a student who was having difﬁculty. He would
never accept an invitation to give a prestigious lecture
if it conﬂicted with his teaching schedule.
His contributions to educ

h his teaching schedule.
His contributions to education are also far reach-
ing through generations of physiology graduate 
students and postdoctoral fellows. He trained over 
150 scientists, at least 29 of whom became chairs of
their own departments and six of whom became pres-
idents of the American Physiological Society. He gave
students conﬁdence in their abilities and emphasized
his belief that “People who are really successful in the
research world are self-taught.” He insisted that his
tr

ch world are self-taught.” He insisted that his
trainees integrate their experimental ﬁndings into a
broad conceptual framework that included other
interacting systems. This approach usually led them 
to develop a quantitative analysis and a better 
understanding of the particular physiological systems
that they were studying. No one has been more pro-
liﬁc in training leaders of physiology than Arthur
Guyton.
Dr. Guyton’s Textbook of Medical Physiology, ﬁrst
published in 1956, quickly became th

siology, ﬁrst
published in 1956, quickly became the best-selling
medical physiology textbook in the world. He had a
gift for communicating complex ideas in a clear and
interesting manner that made studying physiology fun.
He wrote the book to teach his students, not to impress
his professional colleagues. Its popularity with stu-
dents has made it the most widely used physiology
textbook in history. This accomplishment alone was
enough to ensure his legacy.
The Textbook of Medical Physiology beg

his legacy.
The Textbook of Medical Physiology began as
lecture notes in the early 1950s when Dr. Guyton was
teaching the entire physiology course for medical stu-
dents at the University of Mississippi. He discovered
that the students were having difﬁculty with the text-
books that were available and began distributing
copies of his lecture notes. In describing his experi-
ence, Dr. Guyton stated that “Many textbooks of
medical physiology had become discursive, written pri-
marily by teachers o

come discursive, written pri-
marily by teachers of physiology for other teachers of
physiology, and written in language understood by
other teachers but not easily understood by the basic
student of medical physiology.”6
Through his Textbook of Medical Physiology, which
is translated into 13 languages, he has probably done
ix
more to teach physiology to the world than any other
individual in history. Unlike most major textbooks,
which often have 20 or more authors, the ﬁrst eight
ed

h often have 20 or more authors, the ﬁrst eight
editions were written entirely by Dr. Guyton—a feat
that is unprecedented for any major medical textbook.
For his many contributions to medical education, Dr.
Guyton received the 1996 Abraham Flexner Award
from the Association of American Medical Colleges
(AAMC). According to the AAMC, Arthur Guyton 
“. . . for the past 50 years has made an unparalleled
impact on medical education.” He is also honored each
year by The American Physiological Society

ed each
year by The American Physiological Society through
the Arthur C. Guyton Teaching Award.
An Inspiring Role Model. Dr. Guyton’s accomplish-
ments extended far beyond science,medicine,and edu-
cation. He was an inspiring role model for life as well
as for science. No one was more inspirational or inﬂu-
ential on my scientiﬁc career than Dr. Guyton. He
taught his students much more than physiology—
he taught us life, not so much by what he said but by
his unspoken courage and dedication to t

id but by
his unspoken courage and dedication to the highest
standards.
He had a special ability to motivate people through
his indomitable spirit. Although he was severely chal-
lenged by polio, those of us who worked with him
never thought of him as being handicapped. We were
too busy trying to keep up with him! His brilliant
mind, his indefatigable devotion to science, education,
and family, and his spirit captivated students and
trainees, professional colleagues, politicians, business
leader

fessional colleagues, politicians, business
leaders, and virtually everyone who knew him. He
would not succumb to the effects of polio. His courage
challenged and inspired us. He expected the best and
somehow brought out the very best in people.
We celebrate the magniﬁcent life of Arthur Guyton,
recognizing that we owe him an enormous debt. He
gave us an imaginative and innovative approach to
research and many new scientiﬁc concepts. He gave
countless students throughout the world a means of
und

tless students throughout the world a means of
understanding physiology and he gave many of us
exciting research careers. Most of all, he inspired us—
with his devotion to education, his unique ability to
bring out the best in those around him, his warm and
generous spirit, and his courage. We will miss him
tremendously, but he will remain in our memories as
a shining example of the very best in humanity.Arthur
Guyton was a real hero to the world, and his legacy is
everlasting.
1. Bri

, and his legacy is
1. Brinson C, Quinn J: Arthur C. Guyton—His Life, His
Family, His Achievements. Jackson, MS, Hederman
Brothers Press, 1989.
2. Guyton AC, Coleman TG, Granger HJ: Circulation:
overall regulation. Ann Rev Physiol 34:13–46, 1972.
3. Guyton AC: Past-President’s Address. Physiology, a
Beauty and a Philosophy. The Physiologist 8:495–501,
1975.
4. Bode R:A Doctor Who’s Dad to Seven Doctors—So Far!
Readers’ Digest, December, 1982, pp. 141–145.
5. Guyton AC: Te

est, December, 1982, pp. 141–145.
5. Guyton AC: Textbook of Medical Physiology. Philadel-
phia, Saunders, 1956.
6. Guyton AC: An author’s philosophy of physiology text-
book writing. Adv Physiol Ed 19: s1–s5, 1998.
John E. Hall
P
R
E
F
A
C
E
The ﬁrst edition of the Textbook of Medical Phys-
iology was written by Arthur C. Guyton almost 50
years ago. Unlike many major medical textbooks,
which often have 20 or more authors, the ﬁrst
eight editions of the Textbook of Medical Ph

 ﬁrst
eight editions of the Textbook of Medical Physi-
ology were written entirely by Dr. Guyton with
each new edition arriving on schedule for nearly
40 years. Over the years, Dr. Guyton’s textbook
became widely used throughout the world and was translated into 13 languages.
A major reason for the book’s unprecedented success was his uncanny ability
to explain complex physiologic principles in language easily understood by stu-
dents. His main goal with each edition was to instruct students in 

oal with each edition was to instruct students in physiology,
not to impress his professional colleagues. His writing style always maintained
the tone of a teacher talking to his students.
I had the privilege of working closely with Dr. Guyton for almost 30 years
and the honor of helping him with the 9th and 10th editions. For the 11th
edition, I have the same goal as in previous editions—to explain, in language
easily understood by students, how the different cells, tissues, and organs of the
h

 the different cells, tissues, and organs of the
human body work together to maintain life. This task has been challenging and
exciting because our rapidly increasing knowledge of physiology continues to
unravel new mysteries of body functions. Many new techniques for learning
about molecular and cellular physiology have been developed. We can present
more and more the physiology principles in the terminology of molecular and
physical sciences rather than in merely a series of separate and unexp

ther than in merely a series of separate and unexplained bio-
logical phenomena.This change is welcomed, but it also makes revision of each
chapter a necessity.
In this edition, I have attempted to maintain the same uniﬁed organization
of the text that has been useful to students in the past and to ensure that 
the book is comprehensive enough that students will wish to use it in later life
as a basis for their professional careers. I hope that this textbook conveys 
the majesty of the human bod

his textbook conveys 
the majesty of the human body and its many functions and that it stimulates
students to study physiology throughout their careers. Physiology is the link
between the basic sciences and medicine. The great beauty of physiology is 
that it integrates the individual functions of all the body’s different cells, tissues,
and organs into a functional whole, the human body. Indeed, the human 
body is much more than the sum of its parts, and life relies upon this total func-
tion, 

arts, and life relies upon this total func-
tion, not just on the function of individual body parts in isolation from the
others.
This brings us to an important question: How are the separate organs and
systems coordinated to maintain proper function of the entire body? Fortu-
nately, our bodies are endowed with a vast network of feedback controls that
achieve the necessary balances without which we would not be able to live.
Physiologists call this high level of internal bodily control homeosta

his high level of internal bodily control homeostasis. In
disease states, functional balances are often seriously disturbed and homeosta-
sis is impaired. And, when even a single disturbance reaches a limit, the whole
body can no longer live. One of the goals of this text, therefore, is to emphasize
the effectiveness and beauty of the body’s homeostasis mechanisms as well as
to present their abnormal function in disease.
Another objective is to be as accurate as possible. Suggestions and critiqu

e as accurate as possible. Suggestions and critiques
from many physiologists, students, and clinicians throughout the world have
been sought and then used to check factual accuracy as well as balance in the
text. Even so, because of the likelihood of error in sorting through many thou-
sands of bits of information, I wish to issue still a further request to all readers
to send along notations of error or inaccuracy. Physiologists understand the
importance of feedback for proper function of the h

mportance of feedback for proper function of the human body; so, too, is feed-
back important for progressive improvement of a textbook of physiology. To
the many persons who have already helped, I send sincere thanks.
xi
xii
Preface
A brief explanation is needed about several features
of the 11th edition. Although many of the chapters
have been revised to include new principles of physi-
ology, the text length has been closely monitored 
to limit the book size so that it can be used effec-
tive

t the book size so that it can be used effec-
tively in physiology courses for medical students and
health care professionals. Many of the ﬁgures have
also been redrawn and are now in full color. New 
references have been chosen primarily for their pres-
entation of physiologic principles, for the quality of
their own references, and for their easy accessibility.
Most of the selected references are from recently 
published scientiﬁc journals that can be freely
accessed from the PubMed internet s

 can be freely
accessed from the PubMed internet site at http://
www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed.
Use of these references, as well as cross-references
from them, can give the student almost complete cov-
erage of the entire ﬁeld of physiology.
Another feature is that the print is set in two sizes.
The material in small print is of several different kinds:
ﬁrst, anatomical, chemical, and other information that
is needed for immediate discussion but that most stu-
dents will learn

ate discussion but that most stu-
dents will learn in more detail in other courses; second,
physiologic information of special importance to
certain ﬁelds of clinical medicine; and, third, informa-
tion that will be of value to those students who may
wish to study particular physiologic mechanisms more
deeply.
The material in large print constitutes the funda-
mental physiologic information that students will
require in virtually all their medical activities and
studies.
I wish to express my tha

l activities and
studies.
I wish to express my thanks to many other persons
who have helped in preparing this book, including 
my colleagues in the Department of Physiology & 
Biophysics at the University of Mississippi Medical
Center who provided valuable suggestions. I am also
grateful to Ivadelle Osberg Heidke, Gerry McAlpin,
and Stephanie Lucas for their excellent secretarial
services, and to William Schmitt, Rebecca Gruliow,
Mary Anne Folcher, and the rest of the staff of 
Elsevier Saunders

r, and the rest of the staff of 
Elsevier Saunders for continued editorial and produc-
tion excellence.
Finally, I owe an enormous debt to Arthur Guyton
for an exciting career in physiology, for his friendship,
for the great privilege of contributing to the Textbook
of Medical Physiology, and for the inspiration that he
provided to all who knew him.
T A B L E  O F  C O N T E N T S
U N I T  I
Introduction to Physiology: The
Cell and General Physiology
C H A P T E

ology: The
C H A P T E R  1
Functional Organization of the 
Human Body and Control of the 
“Internal Environment”
3
Cells as the Living Units of the Body
3
Extracellular Fluid—The “Internal
Environment”
3
“Homeostatic” Mechanisms of the Major
Functional Systems
4
4
Extracellular Fluid Transport and Mixing 
System—The Blood Circulatory System
4
Origin of Nutrients in the Extracellular Fluid
5
Removal of Metabolic End Products
5
Regulation of Body Functions

bolic End Products
5
5
6
Control Systems of the Body
6
Examples of Control Mechanisms
6
Characteristics of Control Systems
7
Summary—Automaticity of the Body
9
C H A P T E R  2
The Cell and Its Functions
11
Organization of the Cell
11
Physical Structure of the Cell
12
Membranous Structures of the Cell
12
Cytoplasm and Its Organelles
14
Nucleus
17
Nuclear Membrane
17
Nucleoli and Formation of Ribosomes
18
Comparison of the Animal Cell with 
Precellular Fo

Precellular Forms of Life
18
Functional Systems of the Cell
19
Ingestion by the Cell—Endocytosis
19
Digestion of Pinocytotic and Phagocytic 
Foreign Substances Inside the Cell—
Function of the Lysosomes
20
Synthesis and Formation of Cellular 
Structures by Endoplasmic Reticulum 
and Golgi Apparatus
20
Extraction of Energy from Nutrients—
Function of the Mitochondria
22
Locomotion of Cells
24
Ameboid Movement
24
Cilia and Ciliary Movement
24
C H A P T E R  3
Ge

24
C H A P T E R  3
Genetic Control of Protein Synthesis,
Cell Function, and Cell Reproduction
27
Genes in the Cell Nucleus
27
Genetic Code
29
xiii
The DNA Code in the Cell Nucleus Is
Transferred to an RNA Code in the 
Cell Cytoplasm—The Process 
of Transcription
30
Synthesis of RNA
30
Assembly of the RNA Chain from Activated
Nucleotides Using the DNA Strand 
as a Template—The Process of 
“Transcription”
31
Messenger RNA—The Codons
31
Transfer RNA—The Anticodons
32
Ri

NA—The Codons
31
32
Ribosomal RNA
33
Formation of Proteins on the Ribosomes—
The Process of “Translation”
33
Synthesis of Other Substances in the 
Cell
35
Control of Gene Function and 
Biochemical Activity in Cells
35
Genetic Regulation
35
Control of Intracellular Function by 
Enzyme Regulation
36
The DNA-Genetic System Also Controls 
Cell Reproduction
37
Cell Reproduction Begins with Replication 
of DNA
37
Chromosomes and Their Replication
38
Cell Mitosis
38
Control 

 and Their Replication
38
38
Control of Cell Growth and Cell 
39
Cell Differentiation
40
Apoptosis—Programmed Cell Death
40
Cancer
40
U N I T  I I
Membrane Physiology, Nerve,
and Muscle
C H A P T E R  4
Transport of Substances Through 
the Cell Membrane
45
The Lipid Barrier of the Cell Membrane, 
and Cell Membrane Transport 
Proteins
45
Diffusion
46
Diffusion Through the Cell Membrane
46
Diffusion Through Protein Channels, and 
“Gating” of These Channels
47
Facilitated 

s, and 
47
Facilitated Diffusion
49
Factors That Affect Net Rate of Diffusion
50
Osmosis Across Selectively Permeable
Membranes—“Net Diffusion” of Water
51
“Active Transport” of Substances 
Through Membranes
52
Primary Active Transport
53
Secondary Active Transport—Co-Transport 
and Counter-Transport
54
Active Transport Through Cellular Sheets
55
xiv
Table of Contents
C H A P T E R  5
Membrane Potentials and Action 
Potentials
57
Basic Physics of Membrane 
5

entials
57
57
Membrane Potentials Caused by 
Diffusion
57
Measuring the Membrane Potential
58
Resting Membrane Potential of Nerves
59
Origin of the Normal Resting Membrane 
Potential
60
Nerve Action Potential
61
Voltage-Gated Sodium and Potassium 
Channels
62
Summary of the Events That Cause the 
Action Potential
64
Roles of Other Ions During the Action
Potential
64
Initiation of the Action Potential
65
Propagation of the Action Potential
65
Re-establishing 

gation of the Action Potential
65
Re-establishing Sodium and Potassium 
Ionic Gradients After Action Potentials 
Are Completed—Importance of Energy
66
Plateau in Some Action Potentials
66
Rhythmicity of Some Excitable Tissues—
Repetitive Discharge
67
Special Characteristics of Signal 
Transmission in Nerve Trunks
68
Excitation—The Process of Eliciting 
the Action Potential
69
“Refractory Period” After an Action 
Potential
70
Recording Membrane Potentials and 
Action Potentials
70
Inhi

Membrane Potentials and 
70
Inhibition of Excitability—“Stabilizers” 
and Local Anesthetics
70
C H A P T E R  6
Contraction of Skeletal Muscle
72
Physiologic Anatomy of Skeletal 
Muscle
72
Skeletal Muscle Fiber
72
General Mechanism of Muscle 
Contraction
74
Molecular Mechanism of Muscle 
74
Molecular Characteristics of the 
Contractile Filaments
75
Effect of Amount of Actin and Myosin 
Filament Overlap on Tension Developed 
by the Contracting Muscle
77
Relation of V

eloped 
77
Relation of Velocity of Contraction to 
Load
78
Energetics of Muscle Contraction
78
Work Output During Muscle Contraction
78
Sources of Energy for Muscle Contraction
79
Characteristics of Whole Muscle 
80
Mechanics of Skeletal Muscle Contraction
81
Remodeling of Muscle to Match Function
82
Rigor Mortis
83
C H A P T E R  7
Excitation of Skeletal Muscle:
Neuromuscular Transmission and
Excitation-Contraction Coupling
85
Transmission of Impulses from 

raction Coupling
85
Transmission of Impulses from Nerve 
Endings to Skeletal Muscle Fibers: 
The Neuromuscular Junction
85
Secretion of Acetylcholine by the Nerve 
Terminals
85
Molecular Biology of Acetyline 
Formation and Release
88
Drugs That Enhance or Block 
Transmission at the Neuromuscular 
Junction
88
Myasthenia Gravis
89
Muscle Action Potential
89
Spread of the Action Potential to the 
Interior of the Muscle Fiber by Way of
“Transverse Tubules”
89
89
Trans

bules”
89
89
Transverse Tubule–Sarcoplasmic Reticulum 
System
89
Release of Calcium Ions by the 
Sarcoplasmic Reticulum
90
C H A P T E R  8
Contraction and Excitation of 
Smooth Muscle
92
Contraction of Smooth Muscle
92
Types of Smooth Muscle
92
Contractile Mechanism in Smooth Muscle
93
Regulation of Contraction by Calcium Ions
95
Nervous and Hormonal Control of 
Smooth Muscle Contraction
95
Neuromuscular Junctions of Smooth 
Muscle
95
Membrane Potentials and Acti

 of Smooth 
Muscle
95
Membrane Potentials and Action Potentials 
in Smooth Muscle
96
Effect of Local Tissue Factors and 
Hormones to Cause Smooth Muscle 
Contraction Without Action Potentials
98
Source of Calcium Ions That Cause 
Contraction (1) Through the Cell 
Membrane and (2) from the Sarcoplasmic
Reticulum
99
U N I T  I I I
The Heart
C H A P T E R  9
Heart Muscle; The Heart as a Pump 
and Function of the Heart Valves
103
Physiology of Cardiac Muscle
103
Physiologic Anatomy of Cardiac Muscle

c Muscle
103
103
Action Potentials in Cardiac Muscle
104
The Cardiac Cycle
106
Diastole and Systole
106
Relationship of the Electrocardiogram to 
the Cardiac Cycle
107
Function of the Atria as Primer Pumps
107
Function of the Ventricles as Pumps
108
xv
Function of the Valves
109
Aortic Pressure Curve
109
Relationship of the Heart Sounds to 
Heart Pumping
109
Work Output of the Heart
110
Graphical Analysis of Ventricular Pumping
110
Chemical

hical Analysis of Ventricular Pumping
110
Chemical Energy Required for Cardiac
Contraction: Oxygen Utilization by 
the Heart
111
Regulation of Heart Pumping
111
Intrinsic Regulation of Heart Pumping—
The Frank-Starling Mechanism
111
Effect of Potassium and Calcium Ions on 
Heart Function
113
Effect of Temperature on Heart Function
114
Increasing the Arterial Pressure Load 
(up to a Limit) Does Not Decrease the 
Cardiac Output
114
C H A P T E R  1 0
Rhythmical Excitation of the Heart
116
Speciali

 0
116
Specialized Excitatory and Conductive 
System of the Heart
116
Sinus (Sinoatrial) Node
116
Internodal Pathways and Transmission of 
the Cardiac Impulse Through the Atria
118
Atrioventricular Node, and Delay of Impulse
Conduction from the Atria to the Ventricles
118
Rapid Transmission in the Ventricular 
Purkinje System
119
Transmission of the Cardiac Impulse in the
Ventricular Muscle
119
Summary of the Spread of the Cardiac 
Impulse Through the Heart
120

read of the Cardiac 
120
Control of Excitation and Conduction 
in the Heart
120
The Sinus Node as the Pacemaker of the 
Heart
120
Role of the Purkinje System in Causing
Synchronous Contraction of the 
121
Control of Heart Rhythmicity and Impulse
Conduction by the Cardiac Nerves: The
Sympathetic and Parasympathetic Nerves
121
C H A P T E R  1 1
The Normal Electrocardiogram
123
Characteristics of the Normal
Electrocardiogram
123
Depolarization Waves Ver

mal
123
Depolarization Waves Versus 
Repolarization Waves
123
Relationship of Atrial and Ventricular 
Contraction to the Waves of the
125
Voltage and Time Calibration of the
125
Methods for Recording 
Electrocardiograms
126
Pen Recorder
126
Flow of Current Around the Heart 
During the Cardiac Cycle
126
Recording Electrical Potentials from a 
Partially Depolarized Mass of Syncytial 
Cardiac Muscle
126
Flow of Electrical Currents in the Chest 

cle
126
Around the Heart
126
Electrocardiographic Leads
127
Three Bipolar Limb Leads
127
Chest Leads (Precordial Leads)
129
Augmented Unipolar Limb Leads
129
C H A P T E R  1 2
Electrocardiographic Interpretation 
of Cardiac Muscle and Coronary 
Blood Flow Abnormalities: Vectorial 
Analysis
131
Principles of Vectorial Analysis of
131
Use of Vectors to Represent Electrical 
131
Direction of a Vector Is Denoted in Terms 
of De

1
of Degrees
131
Axis for Each Standard Bipolar Lead and 
Each Unipolar Limb Lead
132
Vectorial Analysis of Potentials Recorded 
in Different Leads
133
Vectorial Analysis of the Normal
134
Vectors That Occur at Successive Intervals 
During Depolarization of the Ventricles—
The QRS Complex
134
Electrocardiogram During Repolarization—
The T Wave
134
Depolarization of the Atria—The P Wave
136
Vectorcardiogram
136
Mean Electrical Axis of t

136
136
Mean Electrical Axis of the Ventricular 
QRS—And Its Signiﬁcance
137
Determining the Electrical Axis from 
Standard Lead Electrocardiograms
137
Abnormal Ventricular Conditions That Cause 
Axis Deviation
138
Conditions That Cause Abnormal 
Voltages of the QRS Complex
140
Increased Voltage in the Standard Bipolar 
Limb Leads
140
Decreased Voltage of the Electrocardiogram
140
Prolonged and Bizarre Patterns of the 
QRS Complex
141
Prolonged QRS Complex as a Result of 
Cardia

x
141
Cardiac Hypertrophy or Dilatation
141
Prolonged QRS Complex Resulting from 
Purkinje System Blocks
141
Conditions That Cause Bizarre QRS 
Complexes
141
Current of Injury
141
Effect of Current of Injury on the QRS 
Complex
141
The J Point—The Zero Reference Potential 
for Analyzing Current of Injury
142
Coronary Ischemia as a Cause of Injury 
Potential
143
Abnormalities in the T Wave
145
Effect of Slow Conduction of the 
Depolarization Wave on the 
Char

onduction of the 
Characteristics of the T Wave
145
Shortened Depolarization in Portions of 
the Ventricular Muscle as a Cause of 
T Wave Abnormalities
145
xvi
C H A P T E R  1 3
Cardiac Arrhythmias and Their
Electrocardiographic Interpretation
147
Abnormal Sinus Rhythms
147
Tachycardia
147
Bradycardia
147
Sinus Arrhythmia
148
Abnormal Rhythms That Result from 
Block of Heart Signals Within the
Intracardiac Conduction Pathways
148
Sinoatrial Block
14

ardiac Conduction Pathways
148
148
Atrioventricular Block
148
Incomplete Atrioventricular Heart Block
149
Incomplete Intraventricular Block—
Electrical Alternans
150
Premature Contractions
150
Premature Atrial Contractions
150
A-V Nodal or A-V Bundle Premature 
Contractions
150
Premature Ventricular Contractions
151
Paroxysmal Tachycardia
151
Atrial Paroxysmal Tachycardia
152
Ventricular Paroxysmal Tachycardia
152
Ventricular Fibrillation
152
Phenomenon of Re-entry—“Circus 
Move

rillation
152
Movements” as the Basis for Ventricular
Fibrillation
153
Chain Reaction Mechanism of Fibrillation
153
Electrocardiogram in Ventricular Fibrillation
154
Electroshock Deﬁbrillation of the Ventricle
154
Hand Pumping of the Heart 
(Cardiopulmonary Resuscitation) as 
an Aid to Deﬁbrillation
155
Atrial Fibrillation
155
Atrial Flutter
156
Cardiac Arrest
156
U N I T  I V
The Circulation
C H A P T E R  1 4
Overview of the Circulation; Medical
Physics of Press

rview of the Circulation; Medical
Physics of Pressure, Flow, and 
161
Physical Characteristics of the 
161
Basic Theory of Circulatory Function
163
Interrelationships Among Pressure, 
Flow, and Resistance
164
Blood Flow
164
166
Resistance to Blood Flow
167
Effects of Pressure on Vascular Resistance 
and Tissue Blood Flow
170
C H A P T E R  1 5
Vascular Distensibility and Functions 
of the Arterial and Venous Systems
171
Vascular Distensibility
171
Vascular C

Systems
171
171
Vascular Compliance (or Vascular 
Capacitance)
171
Volume-Pressure Curves of the Arterial 
and Venous Circulations
172
Arterial Pressure Pulsations
173
Transmission of Pressure Pulses to the 
Peripheral Arteries
174
Clinical Methods for Measuring Systolic 
and Diastolic Pressures
175
Veins and Their Functions
176
Venous Pressures—Right Atrial Pressure 
(Central Venous Pressure) and 
Peripheral Venous Pressures
176
Blood Reservoir Function of the Veins
179

res
176
179
C H A P T E R  1 6
The Microcirculation and the 
Lymphatic System: Capillary Fluid
Exchange, Interstitial Fluid, and 
Lymph Flow
181
Structure of the Microcirculation and 
Capillary System
181
Flow of Blood in the Capillaries—
Vasomotion
182
Average Function of the Capillary System
183
Exchange of Water, Nutrients, and 
Other Substances Between the Blood 
and Interstitial Fluid
183
Diffusion Through the Capillary Membrane
183
The Interstitium and

gh the Capillary Membrane
183
The Interstitium and Interstitial Fluid
184
Fluid Filtration Across Capillaries Is
Determined by Hydrostatic and 
Colloid Osmotic Pressures, and 
Capillary Filtration Coefﬁcient
185
Capillary Hydrostatic Pressure
186
Interstitial Fluid Hydrostatic Pressure
187
Plasma Colloid Osmotic Pressure
188
Interstitial Fluid Colloid Osmotic Pressure
188
Exchange of Fluid Volume Through the 
Capillary Membrane
189
Starling Equilibrium for Capillary Exchange
189
Lymphatic System

ibrium for Capillary Exchange
189
190
Lymph Channels of the Body
190
Formation of Lymph
191
Rate of Lymph Flow
192
Role of the Lymphatic System in Controlling
Interstitial Fluid Protein Concentration, 
Interstitial Fluid Volume, and Interstitial 
Fluid Pressure
193
C H A P T E R  1 7
Local and Humoral Control of Blood 
Flow by the Tissues
195
Local Control of Blood Flow in Response 
to Tissue Needs
195
Mechanisms of Blood Flow Control
196
Acute Control of Local Blood Flow
196
Lo

ntrol
196
196
Long-Term Blood Flow Regulation
200
Development of Collateral Circulation—A
Phenomenon of Long-Term Local Blood 
Flow Regulation
201
Humoral Control of the Circulation
201
Vasoconstrictor Agents
201
Vasodilator Agents
202
Vascular Control by Ions and Other 
Chemical Factors
202
xvii
C H A P T E R  1 8
Nervous Regulation of the Circulation,
and Rapid Control of Arterial Pressure
204
Nervous Regulation of the Circulation
204
Autonom

204
Autonomic Nervous System
204
Role of the Nervous System in Rapid 
Control of Arterial Pressure
208
Increase in Arterial Pressure During Muscle
Exercise and Other Types of Stress
208
Reﬂex Mechanisms for Maintaining Normal 
Arterial Pressure
209
Central Nervous System Ischemic 
Response—Control of Arterial Pressure 
by the Brain’s Vasomotor Center in 
Response to Diminished Brain Blood 
Flow
212
Special Features of Nervous Control 
of Arterial Pressure
2

eatures of Nervous Control 
213
Role of the Skeletal Nerves and Skeletal 
Muscles in Increasing Cardiac Output 
and Arterial Pressure
213
Respiratory Waves in the Arterial Pressure
214
Arterial Pressure “Vasomotor” Waves—
Oscillation of Pressure Reﬂex Control 
Systems
214
C H A P T E R  1 9
Dominant Role of the Kidney in Long-
Term Regulation of Arterial Pressure 
and in Hypertension: The Integrated 
System for Pressure Control
216
Renal–Body Fluid System for Arterial 
Press

ol
216
Pressure Control
216
Quantitation of Pressure Diuresis as a Basis 
for Arterial Pressure Control
217
Chronic Hypertension (High Blood Pressure) 
Is Caused by Impaired Renal Fluid 
Excretion
220
The Renin-Angiotensin System: 
Its Role in Pressure Control and in 
223
Components of the Renin-Angiotensin 
System
223
Types of Hypertension in Which Angiotensin 
Is Involved: Hypertension Caused by a 
Renin-Secreting Tumor or by Infusion 
of Angi

y a 
of Angiotensin II
226
Other Types of Hypertension Caused by
Combinations of Volume Loading and
227
“Primary (Essential) Hypertension”
228
Summary of the Integrated, 
Multifaceted System for Arterial 
Pressure Regulation
230
C H A P T E R  2 0
Cardiac Output, Venous Return,
and Their Regulation
232
Normal Values for Cardiac Output at 
Rest and During Activity
232
Control of Cardiac Output by Venous 
Return—Role of the Frank-Starling
Mech

 by Venous 
Mechanism of the Heart
232
Cardiac Output Regulation Is the Sum of 
Blood Flow Regulation in All the Local 
Tissues of the Body—Tissue Metabolism
Regulates Most Local Blood Flow
233
The Heart Has Limits for the Cardiac Output 
That It Can Achieve
234
What Is the Role of the Nervous System in
Controlling Cardiac Output?
235
Pathologically High and Pathologically 
Low Cardiac Outputs
236
High Cardiac Output Caused by Reduced 
Total Peripheral Resistanc

tput Caused by Reduced 
Total Peripheral Resistance
236
Low Cardiac Output
237
A More Quantitative Analysis of Cardiac 
Output Regulation
237
Cardiac Output Curves Used in the 
Quantitative Analysis
237
Venous Return Curves
238
Analysis of Cardiac Output and Right Atrial
Pressure, Using Simultaneous Cardiac 
Output and Venous Return Curves
241
Methods for Measuring Cardiac 
Output
243
Pulsatile Output of the Heart as Measured 
by an Electromagnetic or Ultrasonic 
Flowmeter
243
Measurement of Car

ic or Ultrasonic 
Flowmeter
243
Measurement of Cardiac Output Using the 
Oxygen Fick Principle
244
Indicator Dilution Method for Measuring 
244
C H A P T E R  2 1
Muscle Blood Flow and Cardiac 
Output During Exercise; the 
Coronary Circulation and Ischemic 
Heart Disease
246
Blood Flow in Skeletal Muscle 
and Blood Flow Regulation 
During Exercise
246
Rate of Blood Flow Through the Muscles
246
Control of Blood Flow Through the Skeletal
Muscles
247
Total Body Circulatory Readjustme

etal
Muscles
247
Total Body Circulatory Readjustments 
247
Coronary Circulation
249
Physiologic Anatomy of the Coronary Blood 
Supply
249
Normal Coronary Blood Flow
249
Control of Coronary Blood Flow
250
Special Features of Cardiac Muscle 
251
Ischemic Heart Disease
252
Causes of Death After Acute Coronary 
Occlusion
253
Stages of Recovery from Acute 
Myocardial Infarction
254
Function of the Heart After Recovery 
from Myocardial Infarction
255
Pain in Coronary Heart D

255
Pain in Coronary Heart Disease
255
Surgical Treatment of Coronary Disease
256
C H A P T E R  2 2
Cardiac Failure
258
Dynamics of the Circulation in 
258
xviii
Acute Effects of Moderate Cardiac Failure
258
Chronic Stage of Failure—Fluid Retention 
Helps to Compensate Cardiac Output
259
Summary of the Changes That Occur After 
Acute Cardiac Failure—“Compensated 
Heart Failure”
260
Dynamics of Severe Cardiac Failure—
Decompensated Heart Fa

 of Severe Cardiac Failure—
Decompensated Heart Failure
260
Unilateral Left Heart Failure
262
Low-Output Cardiac Failure—
Cardiogenic Shock
262
Edema in Patients with Cardiac Failure
263
Cardiac Reserve
264
Quantitative Graphical Method for Analysis 
of Cardiac Failure
265
C H A P T E R  2 3
Heart Valves and Heart Sounds;
Dynamics of Valvular and Congenital 
Heart Defects
269
Heart Sounds
269
Normal Heart Sounds
269
Valvular Lesions
271
Abnormal Circulatory Dynamics in 
Valvular Heart Disease
27

Circulatory Dynamics in 
272
Dynamics of the Circulation in Aortic 
Stenosis and Aortic Regurgitation
272
Dynamics of Mitral Stenosis and Mitral
Regurgitation
273
Circulatory Dynamics During Exercise in 
Patients with Valvular Lesions
273
Congenital Heart Defects
274
Patent Ductus Arteriosus—A Left-to-Right 
Shunt
274
Tetralogy of Fallot—A Right-to-Left Shunt
274
Causes of Congenital Anomalies
276
Use of Extracorporeal Circulation 
During 

ies
276
During Cardiac Surgery
276
Hypertrophy of the Heart in Valvular 
and Congenital Heart Disease
276
C H A P T E R  2 4
Circulatory Shock and Physiology of 
Its Treatment
278
Physiologic Causes of Shock
278
Circulatory Shock Caused by Decreased 
278
Circulatory Shock That Occurs Without 
Diminished Cardiac Output
278
What Happens to the Arterial Pressure in
Circulatory Shock?
279
Tissue Deterioration Is the End Result of
Circulatory Shock, W

ioration Is the End Result of
Circulatory Shock, Whatever the Cause
279
Stages of Shock
279
Shock Caused by Hypovolemia—
Hemorrhagic Shock
279
Relationship of Bleeding Volume to 
Cardiac Output and Arterial Pressure
279
Progressive and Nonprogressive 
280
Irreversible Shock
284
Hypovolemic Shock Caused by Plasma 
Loss
284
Hypovolemic Shock Caused by Trauma
285
Neurogenic Shock—Increased Vascular
Capacity
285
Anaphylactic Shock and Histamine 
Shock
285
Septic Shock
286
Physiolog

nd Histamine 
Shock
285
286
Physiology of Treatment in Shock
286
Replacement Therapy
286
Treatment of Shock with Sympathomimetic 
Drugs—Sometimes Useful, Sometimes 
Not
287
Other Therapy
287
Circulatory Arrest
287
Effect of Circulatory Arrest on the Brain
287
U N I T  V
The Body Fluids and Kidneys
C H A P T E R  2 5
The Body Fluid Compartments:
Extracellular and Intracellular Fluids;
Interstitial Fluid and Edema
291
Fluid Intake and Output Are Balanced 
During Steady-State Condition

Output Are Balanced 
During Steady-State Conditions
291
Daily Intake of Water
291
Daily Loss of Body Water
291
Body Fluid Compartments
292
Intracellular Fluid Compartment
293
Extracellular Fluid Compartment
293
Blood Volume
293
Constituents of Extracellular and 
Intracellular Fluids
293
Ionic Composition of Plasma and 
Interstitial Fluid Is Similar
293
Important Constituents of the Intracellular 
Fluid
295
Measurement of Fluid Volumes in the 
Different Body Fluid Compartments—
The Indicator-Dilu

ferent Body Fluid Compartments—
The Indicator-Dilution Principle
295
Determination of Volumes of Speciﬁc 
295
Regulation of Fluid Exchange and 
Osmotic Equilibrium Between 
Intracellular and Extracellular Fluid
296
Basic Principles of Osmosis and 
Osmotic Pressure
296
Osmotic Equilibrium Is Maintained 
Between Intracellular and 
Extracellular Fluids
298
Volume and Osmolality of Extracellular 
and Intracellular Fluids in Abnormal 
States
299
Effect of Adding Saline Solutio

normal 
States
299
Effect of Adding Saline Solution to the 
Extracellular Fluid
299
Glucose and Other Solutions 
Administered for Nutritive Purposes
301
Clinical Abnormalities of Fluid Volume
Regulation: Hyponatremia and
Hypernatremia
301
Causes of Hyponatremia: Excess Water or 
Loss of Sodium
301
Causes of Hypernatremia: Water Loss or 
Excess Sodium
302
Edema: Excess Fluid in the Tissues
302
Intracellular Edema
302
Extracellular Edema
302
xix
Summary of Causes of Extracellular

of Contents
xix
Summary of Causes of Extracellular Edema
303
Safety Factors That Normally Prevent 
Edema
304
Fluids in the “Potential Spaces” of 
the Body
305
C H A P T E R  2 6
Urine Formation by the Kidneys:
I. Glomerular Filtration, Renal Blood 
Flow, and Their Control
307
Multiple Functions of the Kidneys in
307
Physiologic Anatomy of the Kidneys
308
General Organization of the Kidneys and 
Urinary Tract
308
Renal Blood Supply
309
The Nephron Is the Functional Unit of the 
Kidney

Kidney
310
Micturition
311
Physiologic Anatomy and Nervous
Connections of the Bladder
311
Transport of Urine from the Kidney 
Through the Ureters and into 
the Bladder
312
Innervation of the Bladder
312
Filling of the Bladder and Bladder Wall 
Tone; the Cystometrogram
312
Micturition Reﬂex
313
Facilitation or Inhibition of Micturition 
by the Brain
313
Abnormalities of Micturition
313
Urine Formation Results from 
Glomerular Filtration, Tubular 
Reabso

sults from 
Reabsorption, and Tubular Secretion
314
Filtration, Reabsorption, and Secretion of 
Different Substances
315
Glomerular Filtration—The First Step in 
Urine Formation
316
Composition of the Glomerular Filtrate
316
GFR Is About 20 Per Cent of the Renal 
Plasma Flow
316
Glomerular Capillary Membrane
316
Determinants of the GFR
317
Increased Glomerular Capillary Filtration
Coefﬁcient Increases GFR
318
Increased Bowman’s Capsule Hydrostatic 
Pressure Decrea

ased Bowman’s Capsule Hydrostatic 
Pressure Decreases GFR
318
Increased Glomerular Capillary Colloid 
Osmotic Pressure Decreases GFR
318
Increased Glomerular Capillary Hydrostatic
Pressure Increases GFR
319
Renal Blood Flow
320
Renal Blood Flow and Oxygen 
Consumption
320
Determinants of Renal Blood Flow
320
Blood Flow in the Vasa Recta of the Renal 
Medulla Is Very Low Compared with Flow 
in the Renal Cortex
321
Physiologic Control of Glomerular 
Filtration and Renal Blood Flow
321
Sympathetic 

321
Sympathetic Nervous System Activation 
Decreases GFR
321
Hormonal and Autacoid Control of Renal
322
Autoregulation of GFR and Renal 
Blood Flow
323
Importance of GFR Autoregulation in 
Preventing Extreme Changes in Renal 
Excretion
323
Role of Tubuloglomerular Feedback in
Autoregulation of GFR
323
Myogenic Autoregulation of Renal Blood 
Flow and GFR
325
Other Factors That Increase Renal Blood 
Flow and GFR: High Protein Intake and 
Increased Bloo

w and GFR: High Protein Intake and 
Increased Blood Glucose
325
C H A P T E R  2 7
II. Tubular Processing of the 
Glomerular Filtrate
327
Reabsorption and Secretion by the 
Renal Tubules
327
Tubular Reabsorption Is Selective and
Quantitatively Large
327
Tubular Reabsorption Includes 
Passive and Active Mechanisms
328
Active Transport
328
Passive Water Reabsorption by Osmosis 
Is Coupled Mainly to Sodium 
Reabsorption
332
Reabsorption of Chloride, Urea, and Other 

on
332
Solutes by Passive Diffusion
332
Reabsorption and Secretion Along 
Different Parts of the Nephron
333
Proximal Tubular Reabsorption
333
Solute and Water Transport in the Loop 
of Henle
334
Distal Tubule
336
Late Distal Tubule and Cortical Collecting 
Tubule
336
Medullary Collecting Duct
337
Summary of Concentrations of Different 
Solutes in the Different Tubular 
Segments
338
Regulation of Tubular Reabsorption
339
Glomerulotubular Balance—The Abi

 Reabsorption
339
Glomerulotubular Balance—The Ability 
of the Tubules to Increase Reabsorption 
Rate in Response to Increased Tubular 
Load
339
Peritubular Capillary and Renal Interstitial 
Fluid Physical Forces
339
Effect of Arterial Pressure on Urine 
Output—The Pressure-Natriuresis and 
Pressure-Diuresis Mechanisms
341
Hormonal Control of Tubular Reabsorption
342
Increases Sodium Reabsorption
343
Use of Clearance Methods to Quantify 
Kidney Function
343

Clearance Methods to Quantify 
343
Inulin Clearance Can Be Used to Estimate 
GFR
344
Creatine Clearance and Plasma Creatinine 
Clearance Can Be Used to Estimate 
GFR
344
PAH Clearance Can Be Used to Estimate 
Renal Plasma Flow
345
Filtration Fraction Is Calculated from GFR 
Divided by Renal Plasma Flow
346
Calculation of Tubular Reabsorption or 
Secretion from Renal Clearance
346
xx
C H A P T E R  2 8
Regulation of Extracellular Fluid
Osmolarity and Sodium 
Conc

of Extracellular Fluid
348
The Kidneys Excrete Excess Water 
by Forming a Dilute Urine
348
Antidiuretic Hormone Controls Urine 
348
Renal Mechanisms for Excreting a 
Dilute Urine
349
The Kidneys Conserve Water by 
Excreting a Concentrated Urine
350
Obligatory Urine Volume
350
Requirements for Excreting a Concentrated 
Urine—High ADH Levels and Hyperosmotic 
Renal Medulla
350
Countercurrent Mechanism Produces a
Hyperosmotic Renal Medullary Inters

ism Produces a
Hyperosmotic Renal Medullary Interstitium
351
Role of Distal Tubule and Collecting Ducts in
352
Urea Contributes to Hyperosmotic Renal 
Medullary Interstitium and to a 
Concentrated Urine
353
Countercurrent Exchange in the Vasa Recta
Preserves Hyperosmolarity of the 
354
Summary of Urine Concentrating Mechanism 
and Changes in Osmolarity in Different 
Segments of the Tubules
355
Quantifying Renal Urine Concentration 
and Dilution: “Free

ing Renal Urine Concentration 
and Dilution: “Free Water” and Osmolar
Clearances
357
Disorders of Urinary Concentrating 
Ability
357
Control of Extracellular Fluid Osmolarity 
and Sodium Concentration
358
Estimating Plasma Osmolarity from Plasma 
Sodium Concentration
358
Osmoreceptor-ADH Feedback System
358
ADH Synthesis in Supraoptic and 
Paraventricular Nuclei of the 
Hypothalamus and ADH Release from 
the Posterior Pituitary
359
Cardiovascular Reﬂex Stimulation of ADH 
Release by Decreased Ar

 Reﬂex Stimulation of ADH 
Release by Decreased Arterial Pressure 
and/or Decreased Blood Volume
360
Quantitative Importance of Cardiovascular 
Reﬂexes and Osmolarity in Stimulating 
ADH Secretion
360
Other Stimuli for ADH Secretion
360
Role of Thirst in Controlling Extracellular 
Fluid Osmolarity and Sodium 
361
Central Nervous System Centers for Thirst
361
Stimuli for Thirst
361
Threshold for Osmolar Stimulus of Drinking
362
Integrated Responses of Osmoreceptor-ADH 
and Thirst Me

rated Responses of Osmoreceptor-ADH 
and Thirst Mechanisms in Controlling
Extracellular Fluid Osmolarity and Sodium
362
Role of Angiotensin II and Aldosterone 
in Controlling Extracellular Fluid 
Osmolarity and Sodium Concentration
362
Salt-Appetite Mechanism for 
Controlling Extracellular Fluid 
Sodium Concentration and Volume
363
C H A P T E R  2 9
Renal Regulation of Potassium,
Calcium, Phosphate, and Magnesium;
Integration of Renal Mechanisms for
Control of Blood Volume and 
Ex

nal Mechanisms for
Extracellular Fluid Volume
365
Regulation of Potassium Excretion 
and Potassium Concentration in
365
Regulation of Internal Potassium 
Distribution
366
Overview of Renal Potassium Excretion
367
Potassium Secretion by Principal Cells of 
Late Distal and Cortical Collecting 
Tubules
367
Summary of Factors That Regulate 
Potassium Secretion: Plasma Potassium
Concentration, Aldosterone, Tubular Flow 
Rate, and Hydrogen Ion Concentra

ne, Tubular Flow 
Rate, and Hydrogen Ion Concentration
368
Control of Renal Calcium Excretion 
and Extracellular Calcium Ion 
371
Control of Calcium Excretion by the 
Kidneys
372
Regulation of Renal Phosphate Excretion
372
Control of Renal Magnesium Excretion 
and Extracellular Magnesium Ion
373
Integration of Renal Mechanisms for 
Control of Extracellular Fluid
373
Sodium Excretion Is Precisely Matched to 
Intake Under Steady-State Conditions
373
Sodium Excretion Is 

r Steady-State Conditions
373
Sodium Excretion Is Controlled by Altering
Glomerular Filtration or Tubular Sodium
Reabsorption Rates
374
Importance of Pressure Natriuresis and
Pressure Diuresis in Maintaining Body
Sodium and Fluid Balance
374
Pressure Natriuresis and Diuresis Are Key
Components of a Renal-Body Fluid 
Feedback for Regulating Body Fluid 
Volumes and Arterial Pressure
375
Precision of Blood Volume and Extracellular 
Fluid Volume Regulation
376
Distribution of Extracellular Fluid 
Be

lation
376
Between the Interstitial Spaces and
Vascular System
376
Nervous and Hormonal Factors Increase 
the Effectiveness of Renal-Body Fluid
377
Sympathetic Nervous System Control of 
Renal Excretion: Arterial Baroreceptor and 
Low-Pressure Stretch Receptor Reﬂexes
377
Role of Angiotensin II In Controlling Renal
Excretion
377
Role of Aldosterone in Controlling Renal 
Excretion
378
Role of ADH in Controlling Renal Water 
Excretion
379
Role 

DH in Controlling Renal Water 
Excretion
379
Role of Atrial Natriuretic Peptide in 
Controlling Renal Excretion
378
Integrated Responses to Changes in 
Sodium Intake
380
Conditions That Cause Large Increases 
in Blood Volume and Extracellular 
Fluid Volume
380
xxi
Increased Blood Volume and Extracellular 
Fluid Volume Caused by Heart Diseases
380
Increased Blood Volume Caused by 
Increased Capacity of Circulation
380
in Extracellular Fluid

That Cause Large Increases 
in Extracellular Fluid Volume but with
Normal Blood Volume
381
Nephrotic Syndrome—Loss of Plasma 
Proteins in Urine and Sodium Retention 
by the Kidneys
381
Liver Cirrhosis—Decreased Synthesis of 
Plasma Proteins by the Liver and 
Sodium Retention by the Kidneys
381
C H A P T E R  3 0
Regulation of Acid-Base Balance
383
Hydrogen Ion Concentration Is 
Precisely Regulated
383
Acids and Bases—Their Deﬁnitions 
and Meanings
383
Defenses Against Changes in Hydrogen 
Ion Co

s
383
Ion Concentration: Buffers, Lungs, 
and Kidneys
384
Buffering of Hydrogen Ions in the Body 
Fluids
385
Bicarbonate Buffer System
385
Quantitative Dynamics of the Bicarbonate 
Buffer System
385
Phosphate Buffer System
387
Proteins: Important Intracellular 
Buffers
387
Respiratory Regulation of Acid-Base 
Balance
388
Pulmonary Expiration of CO2 Balances 
Metabolic Formation of CO2
388
Increasing Alveolar Ventilation Decreases
Extracellular Fluid Hydrogen

Ventilation Decreases
Extracellular Fluid Hydrogen Ion 
Concentration and Raises pH
388
Increased Hydrogen Ion Concentration 
Stimulates Alveolar Ventilation
389
Renal Control of Acid-Base Balance
390
Secretion of Hydrogen Ions and 
Reabsorption of Bicarbonate Ions 
by the Renal Tubules
390
Hydrogen Ions Are Secreted by Secondary 
Active Transport in the Early Tubular 
Segments
391
Filtered Bicarbonate Ions Are Reabsorbed 
by Interaction with Hydrogen Ions in the 
Tubules
391
Primary Active Secr

rogen Ions in the 
Tubules
391
Primary Active Secretion of Hydrogen Ions in 
the Intercalated Cells of Late Distal and
Collecting Tubules
392
Combination of Excess Hydrogen Ions 
with Phosphate and Ammonia Buffers 
in the Tubule—A Mechanism for 
Generating “New” Bicarbonate Ions
392
Phosphate Buffer System Carries Excess 
Hydrogen Ions into the Urine and 
Generates New Bicarbonate
393
Excretion of Excess Hydrogen Ions and 
Generation of New Bicarbonate by the 
Ammonia Buffer System
393
Quantifyi

bonate by the 
393
Quantifying Renal Acid-Base Excretion
394
Regulation of Renal Tubular Hydrogen Ion
Secretion
395
Renal Correction of Acidosis—Increased
Excretion of Hydrogen Ions and 
Addition of Bicarbonate Ions to the
396
Acidosis Decreases the Ratio of HCO3
-/H+ in 
Renal Tubular Fluid
396
Renal Correction of Alkalosis—Decreased
Tubular Secretion of Hydrogen Ions 
and Increased Excretion of 
Bicarbonate Ions
396
Alkalosis Increases the Ratio of HCO

nate Ions
396
Alkalosis Increases the Ratio of HCO3
-/H+
in Renal Tubular Fluid
396
Clinical Causes of Acid-Base Disorders
397
Respiratory Acidosis Is Caused by 
Decreased Ventilation and Increased PCO2
397
Respiratory Alkalosis Results from Increased
Ventilation and Decreased PCO2
397
Metabolic Acidosis Results from Decreased
Extracellular Fluid Bicarbonate 
397
Treatment of Acidosis or Alkalosis
398
Clinical Measurements and Analysis of 
Acid-Base Disorders
398
Complex Acid-Base 

sis of 
398
Complex Acid-Base Disorders and Use of 
the Acid-Base Nomogram for Diagnosis
399
Use of Anion Gap to Diagnose Acid-Base 
Disorders
400
C H A P T E R  3 1
Kidney Diseases and Diuretics
402
Diuretics and Their Mechanisms of 
Action
402
Osmotic Diuretics Decrease Water 
Reabsorption by Increasing Osmotic 
Pressure of Tubular Fluid
402
“Loop” Diuretics Decrease Active 
Sodium-Chloride-Potassium Reabsorption 
in the Thick Ascending Loop of Henle
403
Thiazide Diuretics 

ck Ascending Loop of Henle
403
Thiazide Diuretics Inhibit Sodium-Chloride
Reabsorption in the Early Distal Tubule
404
Carbonic Anhydrase Inhibitors Block 
Sodium-Bicarbonate Reabsorption in the
Proximal Tubules
404
Competitive Inhibitors of Aldosterone 
Decrease Sodium Reabsorption from and
Potassium Secretion into the Cortical 
Collecting Tubule
404
Diuretics That Block Sodium Channels 
in the Collecting Tubules Decrease 
Sodium Reabsorption
404
Kidney Diseases
404
Acute Renal Failure
404
Prere

404
404
Prerenal Acute Renal Failure Caused by 
Decreased Blood Flow to the Kidney
405
Intrarenal Acute Renal Failure Caused by
Abnormalities within the Kidney
405
Postrenal Acute Renal Failure Caused by
Abnormalities of the Lower Urinary 
Tract
406
Physiologic Effects of Acute Renal Failure
406
Chronic Renal Failure: An Irreversible
Decrease in the Number of Functional
Nephrons
406
Vicious Circle of Chronic Renal Failure 
Leading to End-Stage Renal Disease
4

enal Failure 
407
Injury to the Renal Vasculature as a Cause 
of Chronic Renal Failure
408
xxii
Injury to the Glomeruli as a Cause of 
Chronic Renal Failure—
Glomerulonephritis
408
Injury to the Renal Interstitium as a 
Cause of Chronic Renal Failure—
Pyelonephritis
409
Nephrotic Syndrome—Excretion of Protein 
in the Urine Because of Increased 
Glomerular Permeability
409
Nephron Function in Chronic Renal Failure
409
Effects of Renal Failure o

ronic Renal Failure
409
Effects of Renal Failure on the Body 
Fluids—Uremia
411
Hypertension and Kidney Disease
412
Speciﬁc Tubular Disorders
413
Treatment of Renal Failure by Dialysis 
with an Artiﬁcial Kidney
414
U N I T  V I
Blood Cells, Immunity, and Blood
Clotting
C H A P T E R  3 2
Red Blood Cells, Anemia, and 
Polycythemia
419
Red Blood Cells (Erythrocytes)
419
Production of Red Blood Cells
420
Formation of Hemoglobin
424
Iron Metabolism
425
Life Span and Destruction of Red Blood 
Cells
4

25
Cells
426
Anemias
426
Effects of Anemia on Function of the 
Circulatory System
427
427
Effect of Polycythemia on Function of the
428
C H A P T E R  3 3
Resistance of the Body to Infection: I.
Leukocytes, Granulocytes, the 
Monocyte-Macrophage System, and
Inﬂammation
429
Leukocytes (White Blood Cells)
429
General Characteristics of Leukocytes
429
Genesis of the White Blood Cells
430
Life Span of the White Blood Cells
431
N

Cells
430
431
Neutrophils and Macrophages Defend 
Against Infections
431
Phagocytosis
431
Monocyte-Macrophage Cell System
(Reticuloendothelial System)
432
Inﬂammation: Role of Neutrophils and
Macrophages
434
434
Macrophage and Neutrophil Responses 
During Inﬂammation
434
Eosinophils
436
Basophils
436
Leukopenia
436
The Leukemias
437
Effects of Leukemia on the Body
437
C H A P T E R  3 4
Resistance of the Body to Infection: II.
Immunity and Allergy
4

 the Body to Infection: II.
439
439
Acquired (Adaptive) Immunity
439
Basic Types of Acquired Immunity
440
Both Types of Acquired Immunity Are 
Initiated by Antigens
440
Lymphocytes Are Responsible for 
Acquired Immunity
440
Preprocessing of the T and B Lymphocytes
440
T Lymphocytes and B-Lymphocyte 
Antibodies React Highly Speciﬁcally 
Against Speciﬁc Antigens—Role of 
Lymphocyte Clones
442
Origin of the Many Clones of Lymphocytes
442
Speciﬁc Attributes of th

Clones of Lymphocytes
442
Speciﬁc Attributes of the B-Lymphocyte 
System—Humoral Immunity and the 
Antibodies
443
Special Attributes of the T-Lymphocyte
System–Activated T Cells and Cell-
Mediated Immunity
446
Several Types of T Cells and Their Different
Functions
446
Tolerance of the Acquired Immunity 
System to One’s Own Tissues—Role 
of Preprocessing in the Thymus and 
448
Immunization by Injection of Antigens
448
Passive Immunity
449
Allergy and Hypersensitivity
449
Allergy Cause

449
449
Allergy Caused by Activated T Cells: 
Delayed-Reaction Allergy
449
Allergies in the “Allergic” Person, Who Has 
Excess IgE Antibodies
449
C H A P T E R  3 5
Blood Types; Transfusion; Tissue and 
Organ Transplantation
451
Antigenicity Causes Immune Reactions 
of Blood
451
O-A-B Blood Types
451
A and B Antigens—Agglutinogens
451
Agglutinins
452
Agglutination Process In Transfusion 
Reactions
452
Blood Typing
453
Rh Blood Types
453
Rh Immune Response
453
Transfu

453
453
Transfusion Reactions Resulting from 
Mismatched Blood Types
454
Transplantation of Tissues and Organs
455
Attempts to Overcome Immune Reactions 
in Transplanted Tissue
455
C H A P T E R  3 6
Hemostasis and Blood Coagulation
457
Events in Hemostasis
457
Vascular Constriction
457
Formation of the Platelet Plug
457
Blood Coagulation in the Ruptured 
Vessel
458
Fibrous Organization or Dissolution of the 
Blood Clot
458
xxiii
Mechanism of 

ood Clot
458
xxiii
Mechanism of Blood Coagulation
459
Conversion of Prothrombin to Thrombin
459
Conversion of Fibrinogen to Fibrin—
Formation of the Clot
460
Vicious Circle of Clot Formation
460
Initiation of Coagulation: Formation of 
Prothrombin Activator
461
Prevention of Blood Clotting in the 
Normal Vascular System—Intravascular 
Anticoagulants
463
Lysis of Blood Clots—Plasmin
464
Conditions That Cause Excessive 
Bleeding in Human Beings
464
Decreased Prothrombin, Factor V

n Human Beings
464
Decreased Prothrombin, Factor VII, 
Factor IX,and Factor X Caused by 
Vitamin K Deﬁciency
464
Hemophilia
465
465
Thromboembolic Conditions in the 
Human Being
465
Femoral Venous Thrombosis and Massive
466
Disseminated Intravascular Coagulation
466
Anticoagulants for Clinical Use
466
Heparin as an Intravenous Anticoagulant
466
Coumarins as Anticoagulants
466
Prevention of Blood Coagulation Outside 
the Body
466
Blood Coagulation Tests
467
Ble

side 
the Body
466
467
Bleeding Time
467
Clotting Time
467
Prothrombin Time
467
U N I T  V I I
Respiration
C H A P T E R  3 7
Pulmonary Ventilation
471
Mechanics of Pulmonary Ventilation
471
Muscles That Cause Lung Expansion and
471
Movement of Air In and Out of the Lungs 
and the Pressures That Cause the 
Movement
472
Effect of the Thoracic Cage on Lung 
Expansibility
474
Pulmonary Volumes and Capacities
475
Recording Changes in Pulmonary Volume—
Spirometry
4

ecording Changes in Pulmonary Volume—
475
Abbreviations and Symbols Used in 
Pulmonary Function Tests
476
Determination of Functional Residual 
Capacity, Residual Volume, and Total 
Lung Capacity—Helium Dilution Method
476
Minute Respiratory Volume Equals 
Respiratory Rate Times Tidal Volume
477
Alveolar Ventilation
477
“Dead Space” and Its Effect on Alveolar 
Ventilation
477
Rate of Alveolar Ventilation
478
Functions of the Respiratory 
Passageways
478
Trachea, Bronchi, and Bronchiol

y 
478
Trachea, Bronchi, and Bronchioles
478
Normal Respiratory Functions of the 
Nose
480
C H A P T E R  3 8
Pulmonary Circulation, Pulmonary 
Edema, Pleural Fluid
483
Physiologic Anatomy of the Pulmonary
483
Pressures in the Pulmonary System
483
Blood Volume of the Lungs
484
Blood Flow Through the Lungs and 
Its Distribution
485
Effect of Hydrostatic Pressure 
Gradients in the Lungs on Regional
Pulmonary Blood Flow
485
Zones 1, 2, and 3 of Pulmonary Blood Flow
48

w
485
485
Effect of Increased Cardiac Output on 
Pulmonary Blood Flow and Pulmonary 
Arterial Pressure During Heavy Exercise
486
Function of the Pulmonary Circulation 
When the Left Atrial Pressure Rises as a 
Result of Left-Sided Heart Failure
487
Pulmonary Capillary Dynamics
487
Capillary Exchange of Fluid in the Lungs, 
and Pulmonary Interstitial Fluid Dynamics
487
Pulmonary Edema
488
Fluid in the Pleural Cavity
489
C H A P T E R  3 9
Physical Princip

ral Cavity
489
C H A P T E R  3 9
Physical Principles of Gas Exchange;
Diffusion of Oxygen and Carbon 
Dioxide Through the Respiratory
Membrane
491
Physics of Gas Diffusion and Gas 
Partial Pressures
491
Molecular Basis of Gas Diffusion
491
Gas Pressures in a Mixture of Gases—
“Partial Pressures” of Individual Gases
491
Pressures of Gases Dissolved in Water 
and Tissues
492
Vapor Pressure of Water
492
Diffusion of Gases Through Fluids—
Pressure Difference Causes Net 
Diffusion
493
Diffusion of G

ifference Causes Net 
Diffusion
493
Diffusion of Gases Through Tissues
493
Composition of Alveolar Air—Its Relation 
to Atmospheric Air
493
Rate at Which Alveolar Air Is Renewed by
Atmospheric Air
494
Oxygen Concentration and Partial Pressure 
in the Alveoli
494
CO2 Concentration and Partial Pressure in 
the Alveoli
495
Expired Air
495
Diffusion of Gases Through the 
Respiratory Membrane
496
Factors That Affect the Rate of Gas 
Diffusion Through the Respiratory 
Membrane
498
Diffusing Capacity o

the Respiratory 
Membrane
498
Diffusing Capacity of the Respiratory 
Membrane
498
Effect of the Ventilation-Perfusion 
Ratio on Alveolar Gas Concentration
499
PO2-PCO2, V
.
A/Q
.
Diagram
500
Concept of the “Physiological Shunt” 
(When V
.
A/Q
.
Is Greater Than Normal)
500
Abnormalities of Ventilation-Perfusion Ratio
501
xxiv
C H A P T E R  4 0
Transport of Oxygen and Carbon 
Dioxide in Blood and Tissue Fluids
502
Transport of Oxygen from the Lungs to 
the Body Tissues
502
Diffu

ygen from the Lungs to 
502
Diffusion of Oxygen from the Alveoli to the
Pulmonary Capillary Blood
502
Transport of Oxygen in the Arterial Blood
503
Diffusion of Oxygen from the Peripheral 
Capillaries into the Tissue Fluid
503
Capillaries to the Tissue Cells
504
Diffusion of Carbon Dioxide from the 
Peripheral Tissue Cells into the 
Capillaries and from the Pulmonary 
Capillaries into the Alveoli
504
Role of Hemoglobin in Oxygen Transport

Alveoli
504
505
Reversible Combination of Oxygen with
Hemoglobin
505
Effect of Hemoglobin to “Buffer” the 
Tissue PO2
507
Factors That Shift the Oxygen-Hemoglobin
Dissociation Curve—Their Importance for 
Oxygen Transport
507
Metabolic Use of Oxygen by the Cells
508
Transport of Oxygen in the Dissolved State
509
Combination of Hemoglobin with Carbon
Monoxide—Displacement of Oxygen
509
Transport of Carbon Dioxide in the Blood
510
Chemical Forms in Which Carbo

ide in the Blood
510
Chemical Forms in Which Carbon Dioxide 
Is Transported
510
Carbon Dioxide Dissociation Curve
511
When Oxygen Binds with Hemoglobin, 
Carbon Dioxide Is Released (the Haldane 
Effect) to Increase CO2 Transport
511
Change in Blood Acidity During Carbon 
Dioxide Transport
512
Respiratory Exchange Ratio
512
C H A P T E R  4 1
Regulation of Respiration
514
Respiratory Center
514
Dorsal Respiratory Group of Neurons—Its 
Control of Inspiration and of Respiratory 
Rhythm
514
A Pneumo

nspiration and of Respiratory 
Rhythm
514
A Pneumotaxic Center Limits the Duration 
of Inspiration and Increases the 
Respiratory Rate
514
Ventral Respiratory Group of Neurons—
Functions in Both Inspiration and 
Expiration
515
Lung Inﬂation Signals Limit Inspiration—
The Hering-Breuer Inﬂation Reﬂex
515
Control of Overall Respiratory Center 
Activity
516
Chemical Control of Respiration
516
Direct Chemical Control of Respiratory 
Center Activity by Carbon Dioxide and 
Hydrogen Ions
516

y Carbon Dioxide and 
516
Peripheral Chemoreceptor System for 
Control of Respiratory Activity—Role 
of Oxygen in Respiratory Control
518
Effect of Low Arterial PO2 to Stimulate 
Alveolar Ventilation When Arterial Carbon 
Dioxide and Hydrogen Ion Concentrations
Remain Normal
519
Chronic Breathing of Low Oxygen Stimulates
Respiration Even More—The Phenomenon 
of “Acclimatization”
519
Composite Effects of PCO2, pH, and PO2 on 
519
Regulation of Respiration During

r Ventilation
519
Regulation of Respiration During 
Exercise
520
Other Factors That Affect Respiration
521
Sleep Apnea
522
C H A P T E R  4 2
Respiratory Insufﬁciency—
Pathophysiology, Diagnosis, Oxygen
Therapy
524
Useful Methods for Studying Respiratory
524
Study of Blood Gases and Blood pH
524
Measurement of Maximum Expiratory Flow
525
Forced Expiratory Vital Capacity and Forced
Expiratory Volume
526
Physiologic Peculiarities of Speciﬁc
Pulmonary Abnormalities
526
Chronic Pulmona

peciﬁc
526
Chronic Pulmonary Emphysema
526
Pneumonia
527
Atelectasis
528
Asthma
529
530
Hypoxia and Oxygen Therapy
530
Oxygen Therapy in Different Types of 
Hypoxia
530
Cyanosis
531
Hypercapnia
531
Dyspnea
532
Artiﬁcial Respiration
532
U N I T  V I I I
Aviation, Space, and Deep-Sea 
Diving Physiology
C H A P T E R  4 3
Aviation, High-Altitude, and Space
537
Effects of Low Oxygen Pressure on the 
Body
537
Alveolar PO2 at Different Elevations
537
Eff

y
537
537
Effect of Breathing Pure Oxygen on Alveolar 
PO2 at Different Altitudes
538
Acute Effects of Hypoxia
538
Acclimatization to Low PO2
539
Natural Acclimatization of Native Human 
Beings Living at High Altitudes
540
Reduced Work Capacity at High Altitudes 
and Positive Effect of Acclimatization
540
Acute Mountain Sickness and High-Altitude
540
Chronic Mountain Sickness
541
Effects of Acceleratory Forces on the 
Body in Aviation and Spac

eleratory Forces on the 
Body in Aviation and Space Physiology
541
Centrifugal Acceleratory Forces
541
Effects of Linear Acceleratory Forces on the 
Body
542
xxv
“Artiﬁcial Climate” in the Sealed 
Spacecraft
543
Weightlessness in Space
543
C H A P T E R  4 4
Physiology of Deep-Sea Diving and 
Other Hyperbaric Conditions
545
Effect of High Partial Pressures of 
Individual Gases on the Body
545
Nitrogen Narcosis at High Nitrogen 
Pressures
545
Oxygen Toxicity at High Pressures
54

Pressures
545
546
Carbon Dioxide Toxicity at Great Depths 
in the Sea
547
Decompression of the Diver After Excess 
Exposure to High Pressure
547
Scuba (Self-Contained Underwater 
Breathing Apparatus) Diving
549
Special Physiologic Problems in 
Submarines
550
Hyperbaric Oxygen Therapy
550
U N I T  I X
The Nervous System: A. General
Principles and Sensory Physiology
C H A P T E R  4 5
Organization of the Nervous System,
Basic Functions of Synapses,
“Transmitter Su

stem,
“Transmitter Substances”
555
General Design of the Nervous System
555
Central Nervous System Neuron: The Basic
Functional Unit
555
Sensory Part of the Nervous System—
Sensory Receptors
555
Motor Part of the Nervous System—
Effectors
556
Processing of Information—“Integrative” 
Function of the Nervous System
556
Storage of Information—Memory
557
Major Levels of Central Nervous System
Function
557
Spinal Cord Level
557
Lower Brain or Subcortical Level
558
Higher 

l
557
558
Higher Brain or Cortical Level
558
Comparison of the Nervous System 
with a Computer
558
Central Nervous System Synapses
559
Types of Synapses—Chemical and 
Electrical
559
Physiologic Anatomy of the Synapse
559
Chemical Substances That Function as 
Synaptic Transmitters
562
Electrical Events During Neuronal Excitation
564
Electrical Events During Neuronal 
566
Special Functions of Dendrites for Exciting
Neurons
568
Relation of State of Excita

r Exciting
Neurons
568
Relation of State of Excitation of the Neuron 
to Rate of Firing
569
Some Special Characteristics of 
Synaptic Transmission
570
C H A P T E R  4 6
Sensory Receptors, Neuronal Circuits 
for Processing Information
572
Types of Sensory Receptors and the 
Sensory Stimuli They Detect
572
Differential Sensitivity of Receptors
572
Transduction of Sensory Stimuli into 
Nerve Impulses
573
Local Electrical Currents at Nerve Endings—
Receptor Potentials
573
Adaptation of Receptors
57

573
575
Nerve Fibers That Transmit Different 
Types of Signals, and Their 
Physiologic Classiﬁcation
576
Transmission of Signals of Different 
Intensity in Nerve Tracts—Spatial and
Temporal Summation
577
Transmission and Processing of Signals 
in Neuronal Pools
578
Relaying of Signals Through Neuronal 
Pools
579
Prolongation of a Signal by a Neuronal 
Pool—“Afterdischarge”
581
Instability and Stability of Neuronal 
Circuits
583
Inhibitory Circuits as a

of Neuronal 
Circuits
583
Inhibitory Circuits as a Mechanism for 
Stabilizing Nervous System Function
583
Synaptic Fatigue as a Means for Stabilizing 
the Nervous System
583
C H A P T E R  4 7
Somatic Sensations: I. General
Organization, the Tactile and 
Position Senses
585
CLASSIFICATION OF SOMATIC SENSES
585
Detection and Transmission of Tactile
Sensations
585
Detection of Vibration
587
TICKLE AND ITCH
587
Sensory Pathways for Transmitting 
Somatic Signals into the Central 
587

atic Signals into the Central 
587
Dorsal Column–Medial Lemniscal System
588
Anterolateral System
588
Transmission in the Dorsal Column—
Medial Lemniscal System
588
Anatomy of the Dorsal Column—Medial 
Lemniscal System
588
Somatosensory Cortex
589
Somatosensory Association Areas
592
Overall Characteristics of Signal 
Transmission and Analysis in the Dorsal
Column–Medial Lemniscal System
592
594
Interpretation of Sensory Stimulus Intensity
593
Judgment of Stimulus I

sory Stimulus Intensity
593
Judgment of Stimulus Intensity
594
594
Transmission of Less Critical Sensory 
Signals in the Anterolateral Pathway
595
Anatomy of the Anterolateral Pathway
595
Some Special Aspects of 
Somatosensory Function
596
Function of the Thalamus in Somatic 
Sensation
596
xxvi
Cortical Control of Sensory Sensitivity—
“Corticofugal” Signals
597
Segmental Fields of Sensation—The 
Dermatomes
597
C H A P T E R  4 8
Somatic Sensations: II. Pain,
Hea

H A P T E R  4 8
Headache, and Thermal Sensations
598
Types of Pain and Their Qualities—
Fast Pain and Slow Pain
598
Pain Receptors and Their Stimulation
598
Rate of Tissue Damage as a Stimulus 
for Pain
599
Dual Pathways for Transmission of Pain
Signals into the Central Nervous 
System
600
Dual Pain Pathways in the Cord and Brain 
Stem—The Neospinothalamic Tract and 
the Paleospinothalamic Tract
600
Pain Suppression (“Analgesia”) System 
in the Brain and Spinal Cor

 (“Analgesia”) System 
in the Brain and Spinal Cord
602
Brain’s Opiate System—Endorphins and
602
Inhibition of Pain Transmission by 
Simultaneous Tactile Sensory Signals
603
Treatment of Pain by Electrical Stimulation
603
Referred Pain
603
Visceral Pain
603
Causes of True Visceral Pain
604
“Parietal Pain” Caused by Visceral Disease
604
Localization of Visceral Pain—“Visceral” 
and the “Parietal” Pain Transmission 
Pathways
604
Some Clinical Abnormalities of Pain 
and Other Somatic Se

inical Abnormalities of Pain 
and Other Somatic Sensations
605
Hyperalgesia
605
Herpes Zoster (Shingles)
605
Tic Douloureux
605
Brown-Séquard Syndrome
606
Headache
606
Headache of Intracranial Origin
606
Thermal Sensations
607
Thermal Receptors and Their Excitation
607
Transmission of Thermal Signals in the 
609
U N I T  X
The Nervous System: B.
The Special Senses
C H A P T E R  4 9
The Eye: I. Optics of Vision
613
Physical Principles of Optics
613
Refraction of Light
613
Applicat

les of Optics
613
613
Application of Refractive Principles to 
Lenses
613
Focal Length of a Lens
615
Formation of an Image by a Convex Lens
616
Measurement of the Refractive Power of a 
Lens—“Diopter”
616
Optics of the Eye
617
The Eye as a Camera
617
Mechanism of “Accommodation”
617
Pupillary Diameter
618
Errors of Refraction
619
Visual Acuity
621
Determination of Distance of an Object 
from the Eye—“Depth Perception”
621
Ophthalmoscope
622
Fluid System of the Eye—Intraocular

halmoscope
622
Fluid System of the Eye—Intraocular 
Fluid
623
Formation of Aqueous Humor by the Ciliary 
Body
623
Outﬂow of Aqueous Humor from the Eye
623
Intraocular Pressure
624
C H A P T E R  5 0
The Eye: II. Receptor and Neural 
Function of the Retina
626
Anatomy and Function of the 
Structural Elements of the Retina
626
Photochemistry of Vision
628
Rhodopsin-Retinal Visual Cycle, and 
Excitation of the Rods
629
Automatic Regulation of Retinal Sensitivity—
Light and Dark Adaptation
631
Color

l Sensitivity—
631
Color Vision
632
Tricolor Mechanism of Color Detection
632
Color Blindness
633
Neural Function of the Retina
633
Neural Circuitry of the Retina
633
Ganglion Cells and Optic Nerve Fibers
636
Excitation of the Ganglion Cells
637
C H A P T E R  5 1
The Eye: III. Central 
Neurophysiology of Vision
640
Visual Pathways
640
Function of the Dorsal Lateral Geniculate 
Nucleus of the Thalamus
640
Organization and Function of the Visual
Cortex
641
Layered Struct

d Function of the Visual
Cortex
641
Layered Structure of the Primary Visual 
Cortex
642
Two Major Pathways for Analysis of Visual
Information—(1) The Fast “Position” and
“Motion” Pathway; (2) The Accurate Color
Pathway
643
Neuronal Patterns of Stimulation During
Analysis of the Visual Image
643
Detection of Color
644
Effect of Removing the Primary Visual 
Cortex
644
Fields of Vision; Perimetry
644
Eye Movements and Their Control
645
Fixation Movements of the Eyes
645
“Fusion” of the Visual Image

ments of the Eyes
645
“Fusion” of the Visual Images from the 
Two Eyes
647
Autonomic Control of Accommodation 
and Pupillary Aperture
648
Control of Accommodation (Focusing the 
Eyes)
649
Control of Pupillary Diameter
649
C H A P T E R  5 2
The Sense of Hearing
651
Tympanic Membrane and the Ossicular 
System
651
xxvii
Conduction of Sound from the Tympanic 
Membrane to the Cochlea
651
Transmission of Sound Through Bone
652
Cochlea
652
Functional Anatomy of the Cochlea
652
Transm

a
652
652
Transmission of Sound Waves in the 
Cochlea—“Traveling Wave”
654
Function of the Organ of Corti
655
Determination of Sound Frequency—The 
“Place” Principle
656
Determination of Loudness
656
Central Auditory Mechanisms
657
Auditory Nervous Pathways
657
Function of the Cerebral Cortex in Hearing
658
Determination of the Direction from Which 
Sound Comes
660
Centrifugal Signals from the Central 
Nervous System to Lower Auditory 
Centers
660
Hearing Abnorm

stem to Lower Auditory 
Centers
660
Hearing Abnormalities
660
Types of Deafness
660
C H A P T E R  5 3
The Chemical Senses—Taste and 
Smell
663
Sense of Taste
663
Primary Sensations of Taste
663
Taste Bud and Its Function
664
Transmission of Taste Signals into the 
Central Nervous System
665
Taste Preference and Control of the Diet
666
Sense of Smell
667
Olfactory Membrane
667
Stimulation of the Olfactory Cells
667
Transmission of Smell Signals into the 
668
U N I T  X I
T

nto the 
668
U N I T  X I
The Nervous System: C. Motor and
Integrative Neurophysiology
C H A P T E R  5 4
Motor Functions of the Spinal Cord;
the Cord Reﬂexes
673
Organization of the Spinal Cord for Motor
Functions
673
Muscle Sensory Receptors—Muscle 
Spindles and Golgi Tendon Organs—
And Their Roles in Muscle Control
675
Receptor Function of the Muscle Spindle
675
Muscle Stretch Reﬂex
676
Role of the Muscle Spindle in Voluntary 
Motor Activity
678
Clinical Applications of

ntary 
678
Clinical Applications of the Stretch 
Reﬂex
678
Golgi Tendon Reﬂex
679
Function of the Muscle Spindles and Golgi 
Tendon Organs in Conjunction with Motor
Control from Higher Levels of the Brain
680
Flexor Reﬂex and the Withdrawal 
Reﬂexes
680
Crossed Extensor Reﬂex
681
Reciprocal Inhibition and Reciprocal
Innervation
681
Reﬂexes of Posture and Locomotion
682
Postural and Locomotive Reﬂexes of 
the Cord
682
Scratch Reﬂex
683
Spinal Cord Reﬂexes That Cause 
Muscle Spasm
6

683
683
Autonomic Reﬂexes in the Spinal 
Cord
683
Spinal Cord Transection and Spinal 
Shock
684
C H A P T E R  5 5
Cortical and Brain Stem Control of 
Motor Function
685
MOTOR CORTEX AND CORTICOSPINAL 
TRACT
685
Primary Motor Cortex
685
Premotor Area
686
Supplementary Motor Area
686
Some Specialized Areas of Motor Control 
Found in the Human Motor Cortex
686
Transmission of Signals from the Motor 
Cortex to the Muscles
687
Incoming Fiber Pathways to t

ex to the Muscles
687
Incoming Fiber Pathways to the Motor 
Cortex
688
Red Nucleus Serves as an Alternative 
Pathway for Transmitting Cortical Signals 
to the Spinal Cord
688
“Extrapyramidal” System
689
Excitation of the Spinal Cord Motor Control 
Areas by the Primary Motor Cortex and 
Red Nucleus
689
Role of the Brain Stem in Controlling 
691
Support of the Body Against Gravity—
Roles of the Reticular and Vestibular 
Nuclei
691
Vestibular Sensations and Maintenance 
of Equilibriu

stibular Sensations and Maintenance 
of Equilibrium
692
Vestibular Apparatus
692
Function of the Utricle and Saccule in the
Maintenance of Static Equilibrium
694
Detection of Head Rotation by the 
Semicircular Ducts
695
Vestibular Mechanisms for Stabilizing the 
Eyes
696
Other Factors Concerned with Equilibrium
696
Functions of Brain Stem Nuclei in 
Controlling Subconscious, Stereotyped
Movements
697
C H A P T E R  5 6
Contributions of the Cerebellum and 
Basal Ganglia to Overall Motor 
Control

llum and 
Control
698
Cerebellum and Its Motor Functions
698
Anatomical Functional Areas of the 
Cerebellum
699
Neuronal Circuit of the Cerebellum
700
Function of the Cerebellum in Overall 
Motor Control
703
Clinical Abnormalities of the Cerebellum
706
xxviii
Basal Ganglia—Their Motor Functions
707
Function of the Basal Ganglia in Executing 
Patterns of Motor Activity—The Putamen 
Circuit
708
Role of the Basal Ganglia for Cognitive 
Control of Se

 of the Basal Ganglia for Cognitive 
Control of Sequences of Motor Patterns—
The Caudate Circuit
709
Function of the Basal Ganglia to Change 
the Timing and to Scale the Intensity of
Movements
709
Functions of Speciﬁc Neurotransmitter 
Substances in the Basal Ganglial 
System
710
Integration of the Many Parts of the 
Total Motor Control System
712
Spinal Level
712
Hindbrain Level
712
Motor Cortex Level
712
What Drives Us to Action?
713
C H A P T E R  5 7
Cerebral Cortex, Intellectual Functions 

E R  5 7
of the Brain, Learning and Memory
714
Physiologic Anatomy of the Cerebral 
Cortex
714
Functions of Speciﬁc Cortical Areas
715
Association Areas
716
Comprehensive Interpretative Function of 
the Posterior Superior Temporal Lobe—
“Wernicke’s Area” (a General 
Interpretative Area)
718
Functions of the Parieto-occipitotemporal 
Cortex in the Nondominant Hemisphere
719
Higher Intellectual Functions of the 
Prefrontal Association Areas
719
Function of 

the 
719
Function of the Brain in 
Communication—Language Input 
and Language Output
720
Function of the Corpus Callosum and 
Anterior Commissure to Transfer 
Thoughts, Memories, Training, and 
Other Information Between the Two 
Cerebral Hemispheres
722
Thoughts, Consciousness, and Memory
723
Memory—Roles of Synaptic Facilitation and
Synaptic Inhibition
723
Short-Term Memory
724
Intermediate Long-Term Memory
724
Long-Term Memory
725
Consolidation of Memory
725
C H A 

Term Memory
725
725
C H A P T E R  5 8
Behavioral and Motivational 
Mechanisms of the Brain—The 
Limbic System and the 
728
Activating-Driving Systems of the Brain
728
Control of Cerebral Activity by Continuous
Excitatory Signals from the Brain Stem
728
Neurohormonal Control of Brain Activity
730
Limbic System
731
Functional Anatomy of the Limbic 
System; Key Position of the 
731
Hypothalamus, a Major Control 
Headquarters for the Limbic System
7

ajor Control 
732
Vegetative and Endocrine Control 
Functions of the Hypothalamus
733
Behavioral Functions of the Hypothalamus 
and Associated Limbic Structures
734
“Reward” and “Punishment” Function of 
the Limbic System
735
Importance of Reward or Punishment in 
Behavior
736
Speciﬁc Functions of Other Parts of 
736
Functions of the Hippocampus
736
Functions of the Amygdala
737
Function of the Limbic Cortex
738
C H A P T E R  5 9
States of Br

 Limbic Cortex
738
C H A P T E R  5 9
States of Brain Activity—Sleep, Brain
Waves, Epilepsy, Psychoses
739
Sleep
739
Slow-Wave Sleep
739
REM Sleep (Paradoxical Sleep, 
Desynchronized Sleep)
740
Basic Theories of Sleep
740
Physiologic Effects of Sleep
741
Brain Waves
741
Origin of Brain Waves
742
Effect of Varying Levels of Cerebral 
Activity on the Frequency of the EEG
743
Changes in the EEG at Different Stages of
Wakefulness and Sleep
743
Epilepsy
743
Grand Mal Epilepsy
743
Petit Mal Epilepsy
7

sy
743
743
744
Focal Epilepsy
744
Psychotic Behavior and Dementia—
Roles of Speciﬁc Neurotransmitter 
Systems
745
Depression and Manic-Depressive 
Psychoses—Decreased Activity of the
Norepinephrine and Serotonin 
Neurotransmitter Systems
745
Schizophrenia—Possible Exaggerated 
Function of Part of the Dopamine 
System
745
Alzheimer’s Disease—Amyloid Plaques 
and Depressed Memory
746
C H A P T E R  6 0
The Autonomic Nervous System and 
the Adrenal Medulla
748

nomic Nervous System and 
748
General Organization of the Autonomic
748
Physiologic Anatomy of the Sympathetic 
748
Preganglionic and Postganglionic 
Sympathetic Neurons
748
Physiologic Anatomy of the 
Parasympathetic Nervous System
750
Basic Characteristics of Sympathetic 
and Parasympathetic Function
750
Cholinergic and Adrenergic Fibers—
Secretion of Acetylcholine or 
750
Receptors on the Effector Organs
752
xx

rs on the Effector Organs
752
xxix
Excitatory and Inhibitory Actions of 
Sympathetic and Parasympathetic 
753
Effects of Sympathetic and Parasympathetic
Stimulation on Speciﬁc Organs
753
Function of the Adrenal Medullae
755
Relation of Stimulus Rate to Degree of 
Sympathetic and Parasympathetic Effect
756
Sympathetic and Parasympathetic “Tone”
756
Denervation Supersensitivity of Sympathetic 
and Parasympathetic Organs after 
Denervation
756
Autonomic Reﬂexes
757
Sti

s after 
756
757
Stimulation of Discrete Organs in Some
Instances and Mass Stimulation in 
Other Instances by the Sympathetic 
and Parasympathetic Systems
757
“Alarm” or “Stress” Response of the 
Sympathetic Nervous System
758
Medullary, Pontine, and Mesencephalic 
Control of the Autonomic Nervous 
System
758
Pharmacology of the Autonomic 
759
Drugs That Act on Adrenergic Effector 
Organs—Sympathomimetic Drugs
759
Drugs That Act on Cholinergic Effecto

ic Drugs
759
Drugs That Act on Cholinergic Effector 
Organs
759
Drugs That Stimulate or Block Sympathetic 
and Parasympathetic Postganglionic 
Neurons
759
C H A P T E R  6 1
Cerebral Blood Flow, Cerebrospinal 
Fluid, and Brain Metabolism
761
Cerebral Blood Flow
761
Normal Rate of Cerebral Blood Flow
761
Regulation of Cerebral Blood Flow
761
Cerebral Microcirculation
763
Cerebral Stroke Occurs When Cerebral 
Blood Vessels are Blocked
763
Cerebrospinal Fluid System
763
Cushioning Function of the C

inal Fluid System
763
Cushioning Function of the Cerebrospinal 
Fluid
763
Formation, Flow, and Absorption of 
Cerebrospinal Fluid
764
Cerebrospinal Fluid Pressure
765
Obstruction to Flow of Cerebrospinal Fluid 
Can Cause Hydrocephalus
766
Blood–Cerebrospinal Fluid and Blood-Brain
Barriers
766
Brain Edema
766
Brain Metabolism
767
U N I T  X I I
Gastrointestinal Physiology
C H A P T E R  6 2
General Principles of Gastrointestinal
Function—Motility, Nervous Control,
and Blood Circulation
771
Genera

 Nervous Control,
771
General Principles of Gastrointestinal 
Motility
771
Physiological Anatomy of the 
Gastrointestinal Wall
771
Neural Control of Gastrointestinal 
Function—Enteric Nervous System
773
Differences Between the Myenteric and
Submucosal Plexuses
774
Types of Neurotransmitters Secreted by 
Enteric Neurons
775
Hormonal Control of Gastrointestinal 
Motility
776
Functional Types of Movements in the
Gastrointestinal Tract
776
Propulsive Movements—Peristalsis
776
M

l Tract
776
776
Mixing Movements
777
Gastrointestinal Blood Flow—
“Splanchnic Circulation”
777
Anatomy of the Gastrointestinal Blood 
Supply
778
Effect of Gut Activity and Metabolic 
Factors on Gastrointestinal Blood 
Flow
778
Nervous Control of Gastrointestinal Blood 
Flow
779
C H A P T E R  6 3
Propulsion and Mixing of Food in the
Alimentary Tract
781
Ingestion of Food
781
Mastication (Chewing)
781
Swallowing (Deglutition)
782
Motor Functions of the Stomach
784

eglutition)
782
784
Storage Function of the Stomach
784
Mixing and Propulsion Of Food in the 
Stomach—The Basic Electrical Rhythm 
of the Stomach Wall
784
Stomach Emptying
785
Regulation of Stomach Emptying
785
Movements of the Small Intestine
786
Mixing Contractions (Segmentation 
Contractions)
786
Propulsive Movements
787
Function of the Ileocecal Valve
788
Movements of the Colon
788
Defecation
789
Other Autonomic Reﬂexes That Affect 
Bowel Activity
790
C H A P T

 Reﬂexes That Affect 
790
C H A P T E R  6 4
Secretory Functions of the Alimentary 
Tract
791
General Principles of Alimentary Tract
Secretion
791
Anatomical Types of Glands
791
Basic Mechanisms of Stimulation of the 
Alimentary Tract Glands
791
Basic Mechanism of Secretion by Glandular 
Cells
791
Lubricating and Protective Properties of 
Mucus, and Importance of Mucus in the
793
Secretion of Saliva
793
Nervous Regulation of Salivary Secretion
794
Esophageal

us Regulation of Salivary Secretion
794
Esophageal Secretion
795
xxx
Gastric Secretion
794
Characteristics of the Gastric Secretions
794
Pyloric Glands—Secretion of Mucus and 
Gastrin
797
Surface Mucous Cells
797
Stimulation of Gastric Acid Secretion
797
Regulation of Pepsinogen Secretion
798
Inhibition of Gastric Secretion by Other 
Post-Stomach Intestinal Factors
798
Chemical Composition of Gastrin And Other
Gastrointestinal Hormones
799
Pancreatic Secretion
799
Pancreatic Di

ormones
799
799
Pancreatic Digestive Enzymes
799
Secretion of Bicarbonate Ions
800
Regulation of Pancreatic Secretion
800
Secretion of Bile by the Liver; Functions 
of the Biliary Tree
802
Physiologic Anatomy of Biliary Secretion
802
Function of Bile Salts in Fat Digestion and
804
Liver Secretion of Cholesterol and 
Gallstone Formation
804
Secretions of the Small Intestine
805
Secretion of Mucus by Brunner’s Glands in 
the Duodenum
805
Secretion of Intestinal Dige

 in 
805
Secretion of Intestinal Digestive Juices by 
the Crypts of Lieberkühn
805
Regulation of Small Intestine Secretion—
Local Stimuli
806
Secretions of the Large Intestine
806
C H A P T E R  6 5
Digestion and Absorption in the
808
Digestion of the Various Foods by 
808
Digestion of Carbohydrates
809
Digestion of Proteins
810
Digestion of Fats
811
Basic Principles of Gastrointestinal 
812
Anatomical Basis of Absorption
812
Absorption i

12
812
Absorption in the Small Intestine
813
Absorption of Water
814
Absorption of Ions
814
Absorption of Nutrients
815
Absorption in the Large Intestine: 
Formation of Feces
817
C H A P T E R  6 6
Physiology of Gastrointestinal 
Disorders
819
Disorders of Swallowing and of the 
Esophagus
819
Disorders of the Stomach
819
Peptic Ulcer
820
Speciﬁc Causes of Peptic Ulcer in the 
821
Disorders of the Small Intestine
821
Abnormal Digestion of Food in the Sma

ntestine
821
Abnormal Digestion of Food in the Small
Intestine—Pancreatic Failure
821
Malabsorption by the Small Intestine 
Mucosa—Sprue
822
Disorders of the Large Intestine
822
822
Diarrhea
822
Paralysis of Defecation in Spinal Cord 
Injuries
823
General Disorders of the 
823
Vomiting
823
Nausea
824
Gastrointestinal Obstruction
824
U N I T  X I I I
Metabolism and Temperature
C H A P T E R  6 7
Metabolism of Carbohydrates,
and Formation of Adenosine

olism of Carbohydrates,
and Formation of Adenosine 
Triphosphate
829
Release of Energy from Foods, and the 
Concept of “Free Energy”
829
Role of Adenosine Triphosphate in 
829
Central Role of Glucose in 
Carbohydrate Metabolism
830
Transport of Glucose Through the 
Cell Membrane
831
Insulin Increases Facilitated Diffusion of 
Glucose
831
Phosphorylation of Glucose
831
Glycogen Is Stored in Liver and 
Muscle
831
Glycogenesis—The Process of Glycogen 
Formation
832
Removal of Stored Glyc

 of Glycogen 
Formation
832
Removal of Stored Glycogen—
832
Release of Energy from the Glucose 
Molecule by the Glycolytic Pathway
832
Summary of ATP Formation During the 
Breakdown of Glucose
836
Control of Energy Release from Stored 
Glycogen When the Body Needs Additional
Energy 
836
Anaerobic Release of Energy—“Anaerobic
Glycolysis”
836
Release of Energy from Glucose by the
Pentose Phosphate Pathway
837
Glucose Conversion to Glycogen or Fat
838
Formation of Carbohydrates from 

ycogen or Fat
838
Proteins and Fats—“Gluconeogenesis”
838
Blood Glucose
839
C H A P T E R  6 8
840
Transport of Lipids in the Body Fluids
840
Transport of Triglycerides and Other Lipids 
from the Gastrointestinal Tract by 
Lymph—The Chylomicrons
840
Removal of the Chylomicrons from the 
Blood
841
“Free Fatty Acids” Are Transported in the 
Blood in Combination with Albumin
841
xxxi
Lipoproteins—Their Special Function in
Transport

i
Transporting Cholesterol and 
841
Fat Deposits
842
842
Liver Lipids
842
Use of Triglycerides for Energy: 
Formation of Adenosine Triphosphate
842
Formation of Acetoacetic Acid in the 
Liver and Its Transport in the Blood
844
Synthesis of Triglycerides from 
844
Synthesis of Triglycerides from Proteins
845
Regulation of Energy Release from
846
Obesity
846
Phospholipids and Cholesterol
846
Phospholipi

846
846
846
847
Cellular Structural Functions of 
Phospholipids and Cholesterol—
Especially for Membranes
848
Atherosclerosis
848
Basic Causes of Atherosclerosis—The 
Roles of Cholesterol and Lipoproteins
850
Other Major Risk Factors for 
850
Prevention of Atherosclerosis
850
C H A P T E R  6 9
Protein Metabolism
852
Basic Properties
852
852
Transport and Storage of Amino Acids
854
Blood Amino Acids
854
Storage 

 of Amino Acids
854
854
Storage of Amino Acids as Proteins in the 
Cells
854
Functional Roles of the Plasma 
Proteins
855
Essential and Nonessential Amino Acids
855
Obligatory Degradation of Proteins
857
Hormonal Regulation of Protein 
857
C H A P T E R  7 0
The Liver as an Organ
859
Physiologic Anatomy of the Liver
859
Hepatic Vascular and Lymph 
Systems
859
Blood Flows Through the Liver from the 
Portal Vein and Hepatic Artery
860
The Liver Functions as a Blood Res

atic Artery
860
The Liver Functions as a Blood Reservoir
860
The Liver Has Very High Lymph Flow
860
Regulation of Liver Mass—Regeneration
860
Hepatic Macrophage System Serves a 
Blood-Cleansing Function
861
Metabolic Functions of the Liver
861
861
861
862
Other Metabolic Functions of the Liver
862
Measurement of Bilirubin in the Bile 
as a Clinical Diagnostic Tool
862
Jaundice—Excess Bilirubin in the 
863
C H A P T E R

ubin in the 
863
C H A P T E R  7 1
Dietary Balances; Regulation of 
Feeding; Obesity and Starvation;
Vitamins and Minerals
865
Energy Intake and Output Are Balanced 
Under Steady-State Conditions
865
Dietary Balances
865
Energy Available in Foods
865
Methods for Determining Metabolic 
Utilization of Proteins, Carbohydrates, 
and Fats
866
Regulation of Food Intake and Energy 
Storage
865
Neural Centers Regulate Food Intake
867
Factors That Regulate Quantity of Food 
Intake
87

Intake
870
Obesity
872
Decreased Physical Activity and 
Abnormal Feeding Regulation as 
Causes of Obesity
872
Treatment of Obesity
873
Inanition, Anorexia, and Cachexia
874
874
Vitamins
875
Vitamin A
875
Thiamine (Vitamin B1)
875
Niacin
876
Riboﬂavin (Vitamin B2)
876
Vitamin B12
876
Folic Acid (Pteroylglutamic Acid)
877
Pyridoxine (Vitamin B6)
877
Pantothenic Acid
877
Ascorbic Acid (Vitamin C)
877
Vitamin D
878
Vitamin E
878
Vitamin K
878
Miner

77
Vitamin D
878
Vitamin E
878
Vitamin K
878
Mineral Metabolism
878
C H A P T E R  7 2
Energetics and Metabolic Rate
881
Adenosine Triphosphate (ATP) 
Functions as an “Energy Currency” 
in Metabolism
881
Phosphocreatine Functions as an 
Accessory Storage Depot for Energy 
and as an “ATP Buffer”
882
Anaerobic Versus Aerobic Energy
882
Summary of Energy Utilization by the 
Cells
883
Control of Energy Release in the Cell
884
Metabolic Rate
884
Measurement of the Whole-Body Metabolic 
Rate
885
Energ

rement of the Whole-Body Metabolic 
Rate
885
Energy Metabolism—Factors That 
Inﬂuence Energy Output
885
Overall Energy Requirements for Daily 
Activities
885
Basal Metabolic Rate (BMR)—The 
Minimum Energy Expenditure for the 
Body to Exist
886
Energy Used for Physical Activities
887
Energy Used for Processing Food—
Thermogenic Effect of Food
887
Energy Used for Nonshivering 
Thermogenesis—Role of Sympathetic 
887
xxxii
C H A P T E R  7 3
Body Temperature, Temperatur

ts
C H A P T E R  7 3
Body Temperature, Temperature
Regulation, and Fever
889
Normal Body Temperatures
889
Body Temperature Is Controlled by 
Balancing Heat Production Against 
Heat Loss
889
Heat Production
889
Heat Loss
890
Regulation of Body Temperature—Role 
of the Hypothalamus
894
Neuronal Effector Mechanisms That 
Decrease or Increase Body Temperature
895
Concept of a “Set-Point” for Temperature 
Control
896
Behavioral Control of Body Temperature
897
Abnormalities of Body Temperature 
Regul

ature
897
898
Fever
898
Exposure of the Body to Extreme Cold
900
U N I T  X I V
Endocrinology and Reproduction
C H A P T E R  7 4
Introduction to Endocrinology
905
Coordination of Body Functions by 
Chemical Messengers
905
Chemical Structure and Synthesis of
Hormones
906
Hormone Secretion, Transport, and 
Clearance from the Blood
908
Feedback Control of Hormone Secretion
909
Transport of Hormones in the Blood
909
“Clearance” of Hormones from the Bloo

he Blood
909
“Clearance” of Hormones from the Blood
909
Mechanisms of Action of Hormones
910
Hormone Receptors and Their Activation
910
Intracellular Signaling After Hormone 
Receptor Activation
910
Second Messenger Mechanisms for 
Mediating Intracellular Hormonal 
Functions
912
Hormones That Act Mainly on the Genetic
Machinery of the Cell
915
Measurement of Hormone 
Concentrations in the Blood
915
Radioimmunoassay
915
Enzyme-Linked Immunosorbent Assay 
(ELISA)
916
C H A P T E R  7 5
Pituitary H

 Assay 
(ELISA)
916
C H A P T E R  7 5
Pituitary Hormones and Their Control 
by the Hypothalamus
918
Pituitary Gland and Its Relation to the
918
Hypothalamus Controls Pituitary 
Secretion
919
Hypothalamic-Hypophysial Portal Blood 
Vessels of the Anterior Pituitary Gland
920
Physiological Functions of Growth 
Hormone
921
Growth Hormone Promotes Growth of 
Many Body Tissues
922
Growth Hormone Has Several Metabolic 
Effects
922
Growth Hormone Stimulates Cartilage and 
Bone Growth
923
G

ormone Stimulates Cartilage and 
923
Growth Hormone Exerts Much of Its Effect 
Through Intermediate Substances Called
“Somatomedins” (Also Called “Insulin-Like
Growth Factors”)
923
Regulation of Growth Hormone Secretion
924
Abnormalities of Growth Hormone Secretion
926
Posterior Pituitary Gland and Its 
Relation to the Hypothalamus
927
Chemical Structures of ADH and Oxytocin
928
Physiological Functions of ADH
928
Oxytocic Hormone
929
C H A P T E R  7 6
Thyroid Metabolic Hormones
931

C H A P T E R  7 6
931
Synthesis and Secretion of the Thyroid
Metabolic Hormones
931
Iodine Is Required for Formation of 
Thyroxine
931
Iodide Pump (Iodide Trapping)
932
Thyroglobulin, and Chemistry of Thyroxine 
and Triiodothyronine Formation
932
Release of Thyroxine and Triiodothyronine 
from the Thyroid Gland
933
Transport of Thyroxine and Triiodothyronine 
to Tissues
934
Physiologic Functions of the Thyroid
Hormones
934
Thyroid Hormones Increase the 
Transcription 

934
Transcription of Large Numbers of Genes
934
Thyroid Hormones Increase Cellular 
Metabolic Activity
934
Effect of Thyroid Hormone on Growth
936
Effects of Thyroid Hormone on Speciﬁc 
Bodily Mechanisms
936
Regulation of Thyroid Hormone 
Secretion
938
Anterior Pituitary Secretion of TSH Is 
Regulated by Thyrotropin-Releasing 
Hormone from the Hypothalamus
938
Feedback Effect of Thyroid Hormone to 
Decrease Anterior Pituitary Secretion 
of TSH
939
Diseases of the 

r Pituitary Secretion 
of TSH
939
Diseases of the Thyroid
940
940
Symptoms of Hyperthyroidism
940
941
Cretinism
942
C H A P T E R  7 7
Adrenocortical Hormones
944
Synthesis and Secretion of 
944
Functions of the Mineralocorticoids-
947
Renal and Circulatory Effects of 
948
Aldosterone Stimulates Sodium and 
Potassium Transport in Sweat Glands, 
Salivary Glands, and Intestinal Epithelial 
Cells
949
xx

estinal Epithelial 
Cells
949
xxxiii
Calcium in the Plasma and Interstitial 
Fluid
978
Inorganic Phosphate in the Extracellular 
Fluids
979
Non-Bone Physiologic Effects of Altered 
Calcium and Phosphate Concentrations 
in the Body Fluids
979
Absorption and Excretion of Calcium and
Phosphate
980
Bone and Its Relation to Extracellular 
Calcium and Phosphate
980
Precipitation and Absorption of Calcium 
and Phosphate in Bone—Equilibrium 
with the Extracellular Fluids
981
Calcium Ex

rium 
981
Calcium Exchange Between Bone and 
982
Deposition and Absorption of Bone—
Remodeling of Bone
982
Vitamin D
983
985
985
Effect of Parathyroid Hormone on Calcium 
and Phosphate Concentrations in the 
986
Control of Parathyroid Secretion by 
Calcium Ion Concentration
988
988
Summary of Control of Calcium Ion
989
Pathophysiology of Parathyroid 
Hormone, Vi

on
989
Hormone, Vitamin D, and Bone 
Disease
990
990
Secondary Parathyroidism
991
Rickets—Vitamin D Deﬁciency
991
Osteoporosis—Decreased Bone Matrix
991
Physiology of the Teeth
992
Function of the Different Parts of the 
Teeth
992
Dentition
993
Mineral Exchange in Teeth
993
Dental Abnormalities
994
C H A P T E R  8 0
Reproductive and Hormonal 
Functions of the Male (and Function 
of the Pineal Gland)
996
Physiologic Anatomy of the Male 

Pineal Gland)
996
Sexual Organs
996
996
Steps of Spermatogenesis
996
Function of the Seminal Vesicles
999
Function of the Prostate Gland
999
Semen
999
Male Sexual Act
1001
Abnormal Spermatogenesis and Male 
Fertility
1001
Neuronal Stimulus for Performance of the 
1001
Stages of the Male Sexual Act
1002
Testosterone and Other Male Sex 
Hormones
1003
Secretion, Metabolism, and Chemistry of 
the Male Sex Hormone
1003
Functions of Test

ry of 
1003
Functions of Testosterone
1004
Basic Intracellular Mechanism of Action of
1006
Cellular Mechanism of Aldosterone Action
950
Possible Nongenomic Actions of 
Aldosterone and Other Steroid Hormones
950
Regulation of Aldosterone Secretion
950
Functions of the Glucocorticoids
950
Effects of Cortisol on Carbohydrate 
951
Effects of Cortisol on Protein Metabolism
952
Effects of Cortisol on Fat Metabolism
952
Cortisol Is Important in Resisting Str

abolism
952
Cortisol Is Important in Resisting Stress 
and Inﬂammation
952
Other Effects of Cortisol
954
Cellular Mechanism of Cortisol Action
954
Regulation of Cortisol Secretion by
Adrenocorticotropic Hormone from the 
955
957
Abnormalities of Adrenocortical 
Secretion
957
Hypoadrenalism-Addison’s Disease
957
Hyperadrenalism-Cushing’s Syndrome
958
Primary Aldosteronism (Conn’s Syndrome)
959
Adrenogenital Syndrome
959
C H A P T E R  7 8
Insulin, Glucagon, and D

me
959
C H A P T E R  7 8
Insulin, Glucagon, and Diabetes 
Mellitus
961
Insulin and Its Metabolic Effects
961
Effect of Insulin on Carbohydrate 
963
Effect of Insulin on Fat Metabolism
965
Effect of Insulin on Protein Metabolism 
and on Growth
966
Mechanisms of Insulin Secretion
967
Control of Insulin Secretion
968
Other Factors That Stimulate Insulin 
Secretion
969
Role of Insulin (and Other Hormones) in
“Switching” Between Carbohydrate and 
969
Glucagon and Its Func

te and 
969
Glucagon and Its Functions
970
Effects on Glucose Metabolism
970
Regulation of Glucagon Secretion
971
Somatostatin Inhibits Glucagon and 
971
Summary of Blood Glucose 
971
972
Type I Diabetes—Lack of Insulin Production 
by Beta Cells of the Pancreas
972
Type II Diabetes—Resistance to the 
Metabolic Effects of Insulin
974
Physiology of Diagnosis of Diabetes 
Mellitus
975
Treatment of Diabetes
976
Insulinoma—Hyperinsulinis

reatment of Diabetes
976
Insulinoma—Hyperinsulinism
976
C H A P T E R  7 9
Parathyroid Hormone, Calcitonin,
Calcium and Phosphate 
Metabolism, Vitamin D, Bone,
and Teeth
978
Overview of Calcium and Phosphate
Regulation in the Extracellular Fluid 
and Plasma
978
xxxiv
Control of Male Sexual Functions by 
Hormones from the Hypothalamus and 
Anterior Pituitary Gland
1006
Abnormalities of Male Sexual Function
1008
Prostate Gland and Its Abnormalities
1008
Hypogonadism in the Male
1

 Its Abnormalities
1008
1008
Testicular Tumors and Hypergonadism in 
the Male
1009
Pineal Gland—Its Function in Controlling
Seasonal Fertility in Some Animals
1009
C H A P T E R  8 1
Female Physiology Before Pregnancy 
and Female Hormones
1011
Physiologic Anatomy of the Female 
1011
Female Hormonal System
1011
Monthly Ovarian Cycle; Function of the
Gonadotropic Hormones
1012
Gonadotropic Hormones and Their Effects 
on the Ovaries
1012
Ovarian Follicle Growt

ffects 
1012
Ovarian Follicle Growth—The “Follicular” 
Phase of the Ovarian Cycle
1013
Corpus Luteum—“Luteal” Phase of the 
1014
Summary
1015
Functions of the Ovarian Hormones—
Estradiol and Progesterone
1016
Chemistry of the Sex Hormones
1016
Functions of the Estrogens—Their Effects 
on the Primary and Secondary Female Sex
1017
Functions of Progesterone
1018
Monthly Endometrial Cycle and Menstruation
1018
Regulation of the Female Monthly 
Rhythm—Inte

1018
Rhythm—Interplay Between the 
Ovarian and Hypothalamic-Pituitary
Hormones
1019
Feedback Oscillation of the Hypothalamic-
Pituitary-Ovarian System
1021
Puberty and Menarche
1021
Menopause
1022
Abnormalities of Secretion by the 
Ovaries
1023
Female Sexual Act
1023
Female Fertility
1024
C H A P T E R  8 2
Pregnancy and Lactation
1027
Maturation and Fertilization of the Ovum
1027
Transport of the Fertilized Ovum in the 
Fallopian Tube
1028
Implantation of the B

 in the 
1028
Implantation of the Blastocyst in the Uterus
1029
Early Nutrition of the Embryo
1029
Function of the Placenta
1029
Developmental and Physiologic Anatomy 
of the Placenta
1029
Hormonal Factors in Pregnancy
1031
Human Chorionic Gonadotropin and Its 
Effect to Cause Persistence of the 
Corpus Luteum and to Prevent 
Menstruation
1032
Secretion of Estrogens by the Placenta
1032
Secretion of Progesterone by the Placenta
1033
Human Chorionic Somatomammotropin
1033
Other Hor

3
1033
Other Hormonal Factors in Pregnancy
1034
Response of the Mother’s Body to 
Pregnancy
1034
Changes in the Maternal Circulatory System 
During Pregnancy
1035
1036
Increased Uterine Excitability Near Term
1036
Onset of Labor—A Positive Feedback 
Mechanism for Its Initiation
1037
Abdominal Muscle Contractions During 
Labor
1037
Mechanics of Parturition
1037
Separation and Delivery of the Placenta
1038
Labor Pains
1038
Involution of the Uterus Afte

038
1038
Involution of the Uterus After Parturition
1038
Lactation
1038
Development of the Breasts
1038
Initiation of Lactation—Function of 
Prolactin
1039
Ejection (or “Let-Down”) Process in Milk
Secretion—Function of Oxytocin
1040
Milk Composition and the Metabolic Drain 
on the Mother Caused by Lactation
1041
C H A P T E R  8 3
Fetal and Neonatal Physiology
1042
Growth and Functional Development 
of the Fetus
1042
Development of the Organ Systems
1042
Adjustments of the Infant to 

e Organ Systems
1042
Extrauterine Life
1044
Onset of Breathing
1044
Circulatory Readjustments at Birth
1045
Nutrition of the Neonate
1047
Special Functional Problems in the 
Neonate
1047
1047
1047
Fluid Balance, Acid-Base Balance, and 
Renal Function
1048
Liver Function
1048
Digestion, Absorption, and Metabolism 
of Energy Foods; and Nutrition
1048
Immunity
1049
Endocrine Problems
1049
Special Problems of Prematurity
1050
Immature Deve

1050
Immature Development of the Premature 
Infant
1050
Instability of the Homeostatic Control 
Systems in the Premature Infant
1050
Danger of Blindness Caused by Excess 
Oxygen Therapy in the Premature Infant
1051
Growth and Development of the Child
1051
Behavioral Growth
1052
U N I T  X V
Sports Physiology
C H A P T E R  8 4
1055
Muscles in Exercise
1055
Strength, Power, and Endurance of Muscles
1055
Muscle Metabolic Systems in Exercise
1056
Ph

1055
1056
Phosphocreatine-Creatine System
1057
xxxv
Body Fluids and Salt in Exercise
1065
Drugs and Athletes
1065
Body Fitness Prolongs Life
1066
Index
1067
Nutrients Used During Muscle Activity
1059
Effect of Athletic Training on Muscles and 
Muscle Performance
1060
Respiration in Exercise
1061
Cardiovascular System in Exercise
1062
Body Heat in Exercise
1065
U
N
I
T
I
Introduction to
Physiology:
The Cell and
General Physiology
1. Function

ology:
1. Functional Organization of the Human 
Body and Control of the “Internal Environment”
2. The Cell and Its Functions
3. Genetic Control of Protein Synthesis, 
C
H
A
P
T
E
R
1
3
Functional Organization of the
Human Body and Control of the
The goal of physiology is to explain the physical and
chemical factors that are responsible for the origin,
development, and progression of life. Each type of
lif

lopment, and progression of life. Each type of
life, from the simple virus to the largest tree or the
complicated human being, has its own functional
characteristics. Therefore, the vast ﬁeld of physiol-
ogy can be divided into viral physiology, bacterial
physiology, cellular physiology, plant physiology,
human physiology, and many more subdivisions.
Human Physiology. In human physiology, we attempt to explain the speciﬁc char-
acteristics and mechanisms of the human body that make it a living b

chanisms of the human body that make it a living being. The
very fact that we remain alive is almost beyond our control, for hunger makes
us seek food and fear makes us seek refuge. Sensations of cold make us look
for warmth. Other forces cause us to seek fellowship and to reproduce. Thus,
the human being is actually an automaton, and the fact that we are sensing,
feeling, and knowledgeable beings is part of this automatic sequence of life;
these special attributes allow us to exist under widely

 special attributes allow us to exist under widely varying conditions.
The basic living unit of the body is the cell. Each organ is an aggregate of many
different cells held together by intercellular supporting structures.
Each type of cell is specially adapted to perform one or a few particular 
functions. For instance, the red blood cells, numbering 25 trillion in each 
human being, transport oxygen from the lungs to the tissues. Although the red
cells are

e lungs to the tissues. Although the red
cells are the most abundant of any single type of cell in the body, there are 
about 75 trillion additional cells of other types that perform functions different 
from those of the red cell. The entire body, then, contains about 100 trillion 
cells.
Although the many cells of the body often differ markedly from one another,
all of them have certain basic characteristics that are alike. For instance, in all
cells, oxygen reacts with carbohydrate, fat, and 

cells, oxygen reacts with carbohydrate, fat, and protein to release the energy
required for cell function. Further, the general chemical mechanisms for chang-
ing nutrients into energy are basically the same in all cells, and all cells deliver
end products of their chemical reactions into the surrounding ﬂuids.
Almost all cells also have the ability to reproduce additional cells of their
own kind. Fortunately, when cells of a particular type are destroyed from one
cause or another, the remainin

 destroyed from one
cause or another, the remaining cells of this type usually generate new cells until
the supply is replenished.
Extracellular Fluid—The “Internal Environment”
About 60 per cent of the adult human body is ﬂuid, mainly a water solution of
ions and other substances. Although most of this ﬂuid is inside the cells and is
called intracellular ﬂuid, about one third is in the spaces outside the cells and
4
Unit I
Introduction to Physiology: The Cell and General Physiology
is called ex

logy: The Cell and General Physiology
is called extracellular ﬂuid. This extracellular ﬂuid is in
constant motion throughout the body. It is transported
rapidly in the circulating blood and then mixed
between the blood and the tissue ﬂuids by diffusion
through the capillary walls.
In the extracellular ﬂuid are the ions and nutrients
needed by the cells to maintain cell life. Thus, all cells
live in essentially the same environment—the extra-
cellular ﬂuid. For this reason, the extracellular ﬂuid

ular ﬂuid. For this reason, the extracellular ﬂuid is
also called the internal environment of the body, or the
milieu intérieur, a term introduced more than 100 years
ago by the great 19th-century French physiologist
Claude Bernard.
Cells are capable of living, growing, and performing
their special functions as long as the proper concen-
trations of oxygen, glucose, different ions, amino acids,
fatty substances, and other constituents are available
in this internal environment.
Differences Betwe

le
Differences Between Extracellular and Intracellular Fluids.
The extracellular ﬂuid contains large amounts of
sodium, chloride, and bicarbonate ions plus nutrients
for the cells, such as oxygen, glucose, fatty acids, and
amino acids. It also contains carbon dioxide that is
being transported from the cells to the lungs to be
excreted, plus other cellular waste products that are
being transported to the kidneys for excretion.
The intracellular ﬂuid differs signiﬁcan

xcretion.
The intracellular ﬂuid differs signiﬁcantly from 
the extracellular ﬂuid; speciﬁcally, it contains large
amounts of potassium, magnesium, and phosphate ions
instead of the sodium and chloride ions found in the
extracellular ﬂuid. Special mechanisms for transport-
ing ions through the cell membranes maintain the ion
concentration differences between the extracellular
and intracellular ﬂuids. These transport processes are
“Homeostatic” Mechanisms of
the Major Func

pter 4.
the Major Functional Systems
The term homeostasis is used by physiologists to mean
maintenance of nearly constant conditions in the inter-
nal environment. Essentially all organs and tissues of
the body perform functions that help maintain these
constant conditions. For instance, the lungs provide
oxygen to the extracellular ﬂuid to replenish the
oxygen used by the cells, the kidneys maintain con-
stant ion concentrations, and the gastrointestinal

tant ion concentrations, and the gastrointestinal
system provides nutrients.
A large segment of this text is concerned with the
manner in which each organ or tissue contributes to
homeostasis. To begin this discussion, the different
functional systems of the body and their contributions
to homeostasis are outlined in this chapter; then we
brieﬂy outline the basic theory of the body’s control
systems that allow the functional systems to operate in
support of one another.
Extracellular Fluid Trans

Extracellular Fluid Transport and
Mixing System—The Blood
Extracellular ﬂuid is transported through all parts of
the body in two stages. The ﬁrst stage is movement of
blood through the body in the blood vessels, and the
second is movement of ﬂuid between the blood capil-
laries and the intercellular spaces between the tissue
cells.
Figure 1–1 shows the overall circulation of blood.
All the blood in the circulation traverses the entire cir-
culatory cir

circulation traverses the entire cir-
culatory circuit an average of once each minute when
the body is at rest and as many as six times each minute
when a person is extremely active.
As blood passes through the blood capillaries,
continual exchange of extracellular ﬂuid also occurs
between the plasma portion of the blood and the 
Right
heart
pump
Left
heart
pump
Gut
Lungs
Kidneys
Excretion
of
electrolytes
Venous
end
Arterial
end
Capillaries
Nutrition and excretion
O2
CO2
Figure 1–1
Ge

aries
O2
CO2
Figure 1–1
General organization of the circulatory system.
Chapter 1
Functional Organization of the Human Body and Control of the “Internal Environment”
5
interstitial ﬂuid that ﬁlls the intercellular spaces. This
process is shown in Figure 1–2. The walls of the capil-
laries are permeable to most molecules in the plasma
of the blood, with the exception of the large plasma
protein molecules. Therefore, large amounts of ﬂuid
and its dissolved constituents diff

mounts of ﬂuid
and its dissolved constituents diffuse back and forth
between the blood and the tissue spaces, as shown by
the arrows. This process of diffusion is caused by
kinetic motion of the molecules in both the plasma and
the interstitial ﬂuid. That is, the ﬂuid and dissolved
molecules are continually moving and bouncing in all
directions within the plasma and the ﬂuid in the inter-
cellular spaces, and also through the capillary pores.
Few cells are located more than 50 micrometers from
a

 cells are located more than 50 micrometers from
a capillary, which ensures diffusion of almost any sub-
stance from the capillary to the cell within a few
seconds.Thus, the extracellular ﬂuid everywhere in the
body—both that of the plasma and that of the inter-
stitial ﬂuid—is continually being mixed, thereby 
maintaining almost complete homogeneity of the
extracellular ﬂuid throughout the body.
Origin of Nutrients in the 
Respiratory System. Figure 1–1 shows that each time 

spiratory System. Figure 1–1 shows that each time the
blood passes through the body, it also ﬂows through
the lungs. The blood picks up oxygen in the alveoli,
thus acquiring the oxygen needed by the cells. The
membrane between the alveoli and the lumen of the
pulmonary capillaries, the alveolar membrane, is only
0.4 to 2.0 micrometers thick, and oxygen diffuses by
molecular motion through the pores of this membrane
into the blood in the same manner that water and ions
diffuse through walls of th

er that water and ions
diffuse through walls of the tissue capillaries.
Gastrointestinal Tract. A large portion of the blood
pumped by the heart also passes through the walls of
the gastrointestinal tract. Here different dissolved
nutrients, including carbohydrates, fatty acids, and
amino acids, are absorbed from the ingested food into
the extracellular ﬂuid of the blood.
Liver and Other Organs That Perform Primarily Metabolic Func-
tions. Not all substances absorbed from the gastroin-
testinal 

l substances absorbed from the gastroin-
testinal tract can be used in their absorbed form by the
cells. The liver changes the chemical compositions of
many of these substances to more usable forms, and
other tissues of the body—fat cells, gastrointestinal
mucosa, kidneys, and endocrine glands—help modify
the absorbed substances or store them until they are
needed.
Musculoskeletal System. Sometimes the question is
asked, How does the musculoskeletal system ﬁt into
the homeostatic functions of th

tal system ﬁt into
the homeostatic functions of the body? The answer is
obvious and simple: Were it not for the muscles, the
body could not move to the appropriate place at the
appropriate time to obtain the foods required for
nutrition. The musculoskeletal system also provides
motility for protection against adverse surroundings,
without which the entire body, along with its homeo-
static mechanisms, could be destroyed instantaneously.
Removal of Carbon Dioxide

f Metabolic End Products
Removal of Carbon Dioxide by the Lungs. At the same time
that blood picks up oxygen in the lungs, carbon dioxide
is released from the blood into the lung alveoli;the res-
piratory movement of air into and out of the lungs
carries the carbon dioxide to the atmosphere. Carbon
dioxide is the most abundant of all the end products
of metabolism.
Kidneys. Passage of the blood through the kidneys
removes from the plasma most of the other substances
besides carbon dioxide that a

the other substances
besides carbon dioxide that are not needed by the
cells. These substances include different end products
of cellular metabolism, such as urea and uric acid; they
also include excesses of ions and water from the food
that might have accumulated in the extracellular ﬂuid.
The kidneys perform their function by ﬁrst ﬁltering
large quantities of plasma through the glomeruli into
the tubules and then reabsorbing into the blood those
substances needed by the body, such as glucose, 

e
substances needed by the body, such as glucose, amino
acids, appropriate amounts of water, and many of the
ions. Most of the other substances that are not needed
by the body, especially the metabolic end products
such as urea, are reabsorbed poorly and pass through
the renal tubules into the urine.
Nervous System. The nervous system is composed of
three major parts: the sensory input portion, the central
nervous system (or integrative portion), and the motor
output

tem (or integrative portion), and the motor
output portion. Sensory receptors detect the state of
the body or the state of the surroundings. For instance,
Venule
Arteriole
Figure 1–2
Diffusion of ﬂuid and dissolved constituents through the capillary
walls and through the interstitial spaces.
6
Unit I
receptors in the skin apprise one whenever an object
touches the skin at any point. The eyes are sensory
organs that give one a visual ima

eyes are sensory
organs that give one a visual image of the surrounding
area. The ears also are sensory organs. The central
nervous system is composed of the brain and spinal
cord. The brain can store information, generate
thoughts, create ambition, and determine reactions
that the body performs in response to the sensations.
Appropriate signals are then transmitted through the
motor output portion of the nervous system to carry
out one’s desires.
A large segment of the nervous system is called 

.
A large segment of the nervous system is called the
autonomic system. It operates at a subconscious level
and controls many functions of the internal organs,
including the level of pumping activity by the heart,
movements of the gastrointestinal tract, and secretion
by many of the body’s glands.
Hormonal System of Regulation. Located in the body are
eight major endocrine glands that secrete chemical
substances called hormones. Hormones are trans-
ported in the extracellular ﬂuid to all parts o

s-
ported in the extracellular ﬂuid to all parts of the body
to help regulate cellular function. For instance, thyroid
hormone increases the rates of most chemical reac-
tions in all cells, thus helping to set the tempo of bodily
activity. Insulin controls glucose metabolism; adreno-
cortical hormones control sodium ion, potassium ion,
and protein metabolism; and parathyroid hormone
controls bone calcium and phosphate. Thus, the hor-
mones are a system of regulation that complements
the nervous 

system of regulation that complements
the nervous system. The nervous system regulates
mainly muscular and secretory activities of the body,
whereas the hormonal system regulates many meta-
bolic functions.
Sometimes reproduction is not considered a homeo-
static function. It does, however, help maintain home-
ostasis by generating new beings to take the place of
those that are dying. This may sound like a permissive
usage of the term homeostasis, but it illustrates that, in
the ﬁna

m homeostasis, but it illustrates that, in
the ﬁnal analysis, essentially all body structures are
organized such that they help maintain the automatic-
ity and continuity of life.
The human body has thousands of control systems in
it. The most intricate of these are the genetic control
systems that operate in all cells to help control intra-
cellular function as well as extracellular function. This
subject is discussed in Chapter 3.
Many other control systems operate 

 in Chapter 3.
Many other control systems operate within the
organs to control functions of the individual parts 
of the organs; others operate throughout the entire
body to control the interrelations between the organs.
For instance, the respiratory system, operating in 
association with the nervous system, regulates the
concentration of carbon dioxide in the extracellular
ﬂuid. The liver and pancreas regulate the concentra-
tion of glucose in the extracellular ﬂuid, and the
kidneys regulate co

he extracellular ﬂuid, and the
kidneys regulate concentrations of hydrogen, sodium,
potassium, phosphate, and other ions in the extracel-
lular ﬂuid.
Regulation of Oxygen and Carbon Dioxide Concentrations in the
Extracellular Fluid. Because oxygen is one of the major
substances required for chemical reactions in the cells,
it is fortunate that the body has a special control 
mechanism to maintain an almost exact and constant
oxygen concentration in the extracellula

 constant
oxygen concentration in the extracellular ﬂuid. This
mechanism depends principally on the chemical char-
acteristics of hemoglobin, which is present in all red
blood cells. Hemoglobin combines with oxygen as the
blood passes through the lungs. Then, as the blood
passes through the tissue capillaries, hemoglobin,
because of its own strong chemical afﬁnity for oxygen,
does not release oxygen into the tissue ﬂuid if too
much oxygen is already there. But if the oxygen con-
centration in th

ady there. But if the oxygen con-
centration in the tissue ﬂuid is too low, sufﬁcient
oxygen is released to re-establish an adequate con-
centration. Thus, regulation of oxygen concentration 
in the tissues is vested principally in the chemical 
characteristics of hemoglobin itself. This regulation is
called the oxygen-buffering function of hemoglobin.
Carbon dioxide concentration in the extracellular
ﬂuid is regulated in a much different way. Carbon
dioxide is a major end product of the oxidati

rbon
dioxide is a major end product of the oxidative reac-
tions in cells. If all the carbon dioxide formed in the
cells continued to accumulate in the tissue ﬂuids, the
mass action of the carbon dioxide itself would soon
halt all energy-giving reactions of the cells. Fortu-
nately, a higher than normal carbon dioxide concen-
tration in the blood excites the respiratory center,
causing a person to breathe rapidly and deeply. This
increases expiration of carbon dioxide and, therefore,
removes exc

tion of carbon dioxide and, therefore,
removes excess carbon dioxide from the blood and
tissue ﬂuids. This process continues until the concen-
tration returns to normal.
Regulation of Arterial Blood Pressure. Several systems con-
tribute to the regulation of arterial blood pressure.
One of these, the baroreceptor system, is a simple and
excellent example of a rapidly acting control mecha-
nism. In the walls of the bifurcation region of the
carotid arteries in the neck, and also in the arch of th

d arteries in the neck, and also in the arch of the
aorta in the thorax, are many nerve receptors called
baroreceptors, which are stimulated by stretch of the
arterial wall.When the arterial pressure rises too high,
the baroreceptors send barrages of nerve impulses to
the medulla of the brain. Here these impulses inhibit
the vasomotor center, which in turn decreases the
number of impulses transmitted from the vasomotor
center through the sympathetic nervous system to the
heart and blood vessels.

tic nervous system to the
heart and blood vessels. Lack of these impulses causes
diminished pumping activity by the heart and also 
Chapter 1
7
dilation of the peripheral blood vessels, allowing
increased blood ﬂow through the vessels. Both of these
effects decrease the arterial pressure back toward
normal.
Conversely, a decrease in arterial pressure below
normal relaxes the stretch receptors, allowing the
vasomo

relaxes the stretch receptors, allowing the
vasomotor center to become more active than usual,
thereby causing vasoconstriction and increased heart
pumping, and raising arterial pressure back toward
normal.
Normal Ranges and Physical Characteristics
of Important Extracellular Fluid Constituents
Table 1–1 lists the more important constituents and
physical characteristics of extracellular ﬂuid, along
with their normal values, normal ranges, and maximum
limits without causing death. Note the narrow

imum
limits without causing death. Note the narrowness of
the normal range for each one. Values outside these
ranges are usually caused by illness.
Most important are the limits beyond which abnor-
malities can cause death. For example, an increase 
in the body temperature of only 11°F (7°C) above
normal can lead to a vicious cycle of increasing cellu-
lar metabolism that destroys the cells. Note also the
narrow range for acid-base balance in the body, with 
a normal pH value of 7.4 and lethal v

 body, with 
a normal pH value of 7.4 and lethal values only 
about 0.5 on either side of normal.Another important
factor is the potassium ion concentration, because
whenever it decreases to less than one third normal,
a person is likely to be paralyzed as a result of the
nerves’ inability to carry signals. Alternatively, if 
the potassium ion concentration increases to two or 
more times normal, the heart muscle is likely to be
severely depressed. Also, when the calcium ion con-
centration fall

d. Also, when the calcium ion con-
centration falls below about one half of normal, a
person is likely to experience tetanic contraction of
muscles throughout the body because of the sponta-
neous generation of excess nerve impulses in the
peripheral nerves. When the glucose concentration
falls below one half of normal, a person frequently
develops extreme mental irritability and sometimes
even convulsions.
These examples should give one an appreciation for
the extreme value and even the necessi

ciation for
the extreme value and even the necessity of the 
vast numbers of control systems that keep the body
operating in health; in the absence of any one of these
controls, serious body malfunction or death can result.
The aforementioned examples of homeostatic control
mechanisms are only a few of the many thousands in
the body, all of which have certain characteristics in
common. These characteristics are explained in this
section.
Negative Feedback Natur

explained in this
section.
Negative Feedback Nature of Most 
Control Systems
Most control systems of the body act by negative feed-
back, which can best be explained by reviewing some
of the homeostatic control systems mentioned pre-
viously. In the regulation of carbon dioxide concen-
tration, a high concentration of carbon dioxide in the
extracellular ﬂuid increases pulmonary ventilation.
This, in turn, decreases the extracellular ﬂuid carbon
dioxide concentration because the lungs expire grea

ioxide concentration because the lungs expire greater
amounts of carbon dioxide from the body. In other
words, the high concentration of carbon dioxide initi-
ates events that decrease the concentration toward
normal, which is negative to the initiating stimulus.
Conversely, if the carbon dioxide concentration falls
too low, this causes feedback to increase the concen-
tration. This response also is negative to the initiating
stimulus.
In the arterial pressure–regulating mechanisms, a
high press

erial pressure–regulating mechanisms, a
high pressure causes a series of reactions that promote
a lowered pressure, or a low pressure causes a series
of reactions that promote an elevated pressure.In both
instances, these effects are negative with respect to the
initiating stimulus.
Therefore, in general, if some factor becomes exces-
sive or deﬁcient, a control system initiates negative
feedback, which consists of a series of changes that
return the factor toward a certain mean value, thus
main

 the factor toward a certain mean value, thus
maintaining homeostasis.
“Gain” of a Control System. The degree of effectiveness
with which a control system maintains constant 
Table 1–1
Important Constituents and Physical Characteristics of Extracellular Fluid
Normal Range
Approximate Short-Term
Unit
Nonlethal Limit
Oxygen
40
35–45
10–1000
mm Hg
Carbon dioxide
40
35–45
5–80
mm Hg
Sodium ion
142
138–146
115–175
mmol/L
Potassium ion
4.2
3.8–5.0
1.5–9.0
mmol/L
Calcium ion
1.2
1.0–1.4
0.

3.8–5.0
1.5–9.0
mmol/L
1.2
1.0–1.4
0.5–2.0
mmol/L
Chloride ion
108
103–112
70–130
mmol/L
Bicarbonate ion
28
24–32
8–45
mmol/L
Glucose
85
75–95
20–1500
mg/dl
Body temperature
98.4 (37.0)
98–98.8 (37.0)
65–110 (18.3–43.3)
∞F (∞C)
Acid-base
7.4
7.3–7.5
6.9–8.0
pH
8
Unit I
conditions is determined by the gain of the negative
feedback. For instance, let us assume that a large
volume of blood is transfused into a person whose
bar

ume of blood is transfused into a person whose
baroreceptor pressure control system is not function-
ing, and the arterial pressure rises from the normal
level of 100 mm Hg up to 175 mm Hg. Then, let us
assume that the same volume of blood is injected into
the same person when the baroreceptor system is func-
tioning, and this time the pressure increases only
25 mm Hg. Thus, the feedback control system has
caused a “correction” of –50 mm Hg—that is, from
175 mm Hg to 125 mm Hg. There remains an 

is, from
175 mm Hg to 125 mm Hg. There remains an increase
in pressure of +25 mm Hg, called the “error,” which
means that the control system is not 100 per cent effec-
tive in preventing change. The gain of the system is
then calculated by the following formula:
Thus, in the baroreceptor system example, the correc-
tion is –50 mm Hg and the error persisting is +25 mm
Hg. Therefore, the gain of the person’s baroreceptor
system for control of arterial pressure is –50 divided
by +25, or –2. That is

ial pressure is –50 divided
by +25, or –2. That is, a disturbance that increases or
decreases the arterial pressure does so only one third
as much as would occur if this control system were not
present.
The gains of some other physiologic control systems
are much greater than that of the baroreceptor 
system. For instance, the gain of the system controlling
internal body temperature when a person is exposed
to moderately cold weather is about –33. Therefore,
one can see that the temperature cont

. Therefore,
one can see that the temperature control system is
much more effective than the baroreceptor pressure
control system.
Positive Feedback Can Sometimes Cause
Vicious Cycles and Death
One might ask the question, Why do essentially all
control systems of the body operate by negative feed-
back rather than positive feedback? If one considers
the nature of positive feedback, one immediately sees
that positive feedback does not lead to stability but to
instability and often death.
Figure 1

ility but to
Figure 1–3 shows an example in which death can
ensue from positive feedback. This ﬁgure depicts the
pumping effectiveness of the heart, showing that the
heart of a healthy human being pumps about 5 liters
of blood per minute. If the person is suddenly bled 2
liters, the amount of blood in the body is decreased to
such a low level that not enough blood is available for
the heart to pump effectively. As a result, the arterial
pressure falls,and the ﬂow of 

esult, the arterial
pressure falls,and the ﬂow of blood to the heart muscle
through the coronary vessels diminishes.This results in
weakening of the heart, further diminished pumping,
a further decrease in coronary blood ﬂow, and still
more weakness of the heart; the cycle repeats itself
again and again until death occurs. Note that each
cycle in the feedback results in further weakening of
the heart. In other words, the initiating stimulus causes
more of the same, which is positive feedback.
Ga

s
Gain
Correction
Error
=
Positive feedback is better known as a “vicious
cycle,” but a mild degree of positive feedback can be
overcome by the negative feedback control mecha-
nisms of the body, and the vicious cycle fails to
develop. For instance, if the person in the aforemen-
tioned example were bled only 1 liter instead of 2
liters, the normal negative feedback mechanisms for
controlling cardiac output and arterial pressure would
overbalance the

output and arterial pressure would
overbalance the positive feedback and the person
would recover, as shown by the dashed curve of 
Figure 1–3.
Positive Feedback Can Sometimes Be Useful. In some
instances, the body uses positive feedback to its advan-
tage. Blood clotting is an example of a valuable use of
positive feedback. When a blood vessel is ruptured 
and a clot begins to form, multiple enzymes called clot-
ting factors are activated within the clot itself. Some 
of these enzymes act on ot

 the clot itself. Some 
of these enzymes act on other unactivated enzymes 
of the immediately adjacent blood, thus causing 
more blood clotting. This process continues until the
hole in the vessel is plugged and bleeding no longer
occurs. On occasion, this mechanism can get out of
hand and cause the formation of unwanted clots.
In fact, this is what initiates most acute heart 
attacks, which are caused by a clot beginning on the
inside surface of an atherosclerotic plaque in a coro-
nary artery 

 an atherosclerotic plaque in a coro-
nary artery and then growing until the artery is
blocked.
Childbirth is another instance in which positive
feedback plays a valuable role. When uterine contrac-
tions become strong enough for the baby’s head to
begin pushing through the cervix, stretch of the cervix
sends signals through the uterine muscle back to the
5
4
3
2
1
0
1
Hours
Death
Bled 2 liters
Return to
normal
Bled 1 liter
Pumping effectiveness of heart
(Liters pumped per minute)
2
3
Figure 1–3

of heart
2
3
Figure 1–3
Recovery of heart pumping caused by negative feedback after 1
liter of blood is removed from the circulation. Death is caused by
positive feedback when 2 liters of blood are removed.
Chapter 1
9
body of the uterus, causing even more powerful 
contractions. Thus, the uterine contractions stretch 
the cervix, and the cervical stretch causes stronger 
contractions. 

e cervical stretch causes stronger 
contractions. When this process becomes powerful
enough, the baby is born. If it is not powerful enough,
the contractions usually die out, and a few days pass
before they begin again.
Another important use of positive feedback is 
for the generation of nerve signals. That is, when 
the membrane of a nerve ﬁber is stimulated, this 
causes slight leakage of sodium ions through sodium
channels in the nerve membrane to the ﬁber’s interior.
The sodium ions entering

e to the ﬁber’s interior.
The sodium ions entering the ﬁber then change 
the membrane potential, which in turn causes more
opening of channels, more change of potential,
still more opening of channels, and so forth. Thus, a
slight leak becomes an explosion of sodium entering
the interior of the nerve ﬁber, which creates the 
nerve action potential. This action potential in turn
causes electrical current to ﬂow along both the outside
and the inside of the ﬁber and initiates additional
action pote

e of the ﬁber and initiates additional
action potentials. This process continues again and
again until the nerve signal goes all the way to the end
of the ﬁber.
In each case in which positive feedback is useful, the
positive feedback itself is part of an overall negative
feedback process. For example, in the case of blood
clotting, the positive feedback clotting process is a 
negative feedback process for maintenance of normal
blood volume. Also, the positive feedback that causes
nerve signals a

 the positive feedback that causes
nerve signals allows the nerves to participate in 
thousands of negative feedback nervous control
systems.
More Complex Types of Control Systems—
Adaptive Control
Later in this text, when we study the nervous system,
we shall see that this system contains great numbers
of interconnected control mechanisms. Some are
simple feedback systems similar to those already 
discussed. Many are not. For instance, some move-
ments of the body occur so rapidly that there is

-
ments of the body occur so rapidly that there is not
enough time for nerve signals to travel from the
peripheral parts of the body all the way to the brain
and then back to the periphery again to control the
movement.Therefore, the brain uses a principle called
feed-forward control to cause required muscle con-
tractions. That is, sensory nerve signals from the
moving parts apprise the brain whether the movement
is performed correctly. If not, the brain corrects the
feed-forward signals that i

the brain corrects the
feed-forward signals that it sends to the muscles the
next time the movement is required. Then, if still
further correction is needed, this will be done again for
subsequent movements.This is called adaptive control.
Adaptive control, in a sense, is delayed negative 
feedback.
Thus, one can see how complex the feedback 
control systems of the body can be. A person’s life
depends on all of them. Therefore, a major share of 
this text is devoted to discussing these life-givi

this text is devoted to discussing these life-giving
mechanisms.
Summary—Automaticity 
of the Body
The purpose of this chapter has been to point out, ﬁrst,
the overall organization of the body and, second, the
means by which the different parts of the body operate
in harmony.To summarize, the body is actually a social
order of about 100 trillion cells organized into differ-
ent functional structures, some of which are called
organs. Each functional structure contributes its share
to the maintena

al structure contributes its share
to the maintenance of homeostatic conditions in the
extracellular ﬂuid, which is called the internal envi-
ronment. As long as normal conditions are maintained
in this internal environment, the cells of the body con-
tinue to live and function properly. Each cell beneﬁts
from homeostasis, and in turn, each cell contributes its
share toward the maintenance of homeostasis. This
reciprocal interplay provides continuous automaticity
of the body until one or more fu

uous automaticity
of the body until one or more functional systems lose
their ability to contribute their share of function.When
this happens, all the cells of the body suffer. Extreme
dysfunction leads to death; moderate dysfunction
leads to sickness.
Adolph EF: Physiological adaptations: hypertrophies and
superfunctions. Am Sci 60:608, 1972.
Bernard C: Lectures on the Phenomena of Life Common to
Animals and Plants. Springﬁeld, IL: Charles C Thomas,
1974.
Cabanac M: Regulation and th

arles C Thomas,
1974.
Cabanac M: Regulation and the ponderostat. Int J Obes
Relat Metab Disord 25(Suppl 5):S7, 2001.
Cannon WB: The Wisdom of the Body. New York: WW
Norton, 1932.
Conn PM, Goodman HM: Handbook of Physiology: Cellu-
lar Endocrinology. Bethesda: American Physiological
Society, 1997.
Csete ME, Doyle JC: Reverse engineering of biological com-
plexity. Science 295:1664, 2002.
Danzler WH (ed): Handbook of Physiology, Sec 13: Com-
parative Physiology. Bethesda: American Physiological
So

ve Physiology. Bethesda: American Physiological
Dickinson MH, Farley CT, Full RJ, et al: How animals move:
an integrative view. Science 288:100, 2000.
Garland T Jr, Carter PA: Evolutionary physiology.Annu Rev
Physiol 56:579, 1994.
Gelehrter TD, Collins FS: Principles of Medical Genetics.
Baltimore: Williams & Wilkins, 1995.
Guyton AC: Arterial Pressure and Hypertension. Philadel-
phia: WB Saunders, 1980.
Guyton AC, Jones CE, Coleman TG: Cardiac Output and Its
Regulation. Philadelp

n TG: Cardiac Output and Its
Regulation. Philadelphia: WB Saunders, 1973.
Guyton AC, Taylor AE, Granger HJ: Dynamics and 
Control of the Body Fluids. Philadelphia: WB Saunders,
1975.
Hoffman JF, Jamieson JD: Handbook of Physiology: Cell
Physiology. Bethesda: American Physiological Society,
1997.
Krahe R, Gabbiani F: Burst ﬁring in sensory systems. Nat
Rev Neurosci 5:13, 2004.
Lewin B: Genes VII. New York: Oxford University Press,
2000.
10
Unit I
Introduction to Physiology: The Cell and General P

Masoro EJ (ed): Handbook of Physiology, Sec 11: Aging.
Bethesda: American Physiological Society, 1995.
Milhorn HT: The Application of Control Theory to 
Physiological Systems.
Philadelphia:
WB Saunders,
1966.
Orgel LE: The origin of life on the earth. Sci Am 271:76,
1994.
Smith HW: From Fish to Philosopher. New York: Doubleday,
1961.
Thomson RC: Biomaterials Regulating Cell Function and
Tissue Development.Warrendale, PA: Materials Resea

Tissue Development.Warrendale, PA: Materials Research
Society, 1998.
Tjian R: Molecular machines that control genes. Sci Am
272:54, 1995.
C
H
A
P
T
E
R
 
2
11
Each of the 100 trillion cells in a human being is a
living structure that can survive for months or many
years, provided its surrounding ﬂuids contain appro-
priate nutrients. To understand the function of
organs and other structures of the body, it is essen-
tial that we ﬁrst understand the basic organization
o

l that we ﬁrst understand the basic organization
of the cell and the functions of its component 
parts.
A typical cell, as seen by the light microscope, is shown in Figure 2–1. Its two
major parts are the nucleus and the cytoplasm. The nucleus is separated from
the cytoplasm by a nuclear membrane, and the cytoplasm is separated from the
surrounding ﬂuids by a cell membrane, also called the plasma membrane.
The different substances that make up the cell are collectively c

ubstances that make up the cell are collectively called proto-
plasm. Protoplasm is composed mainly of ﬁve basic substances: water, elec-
trolytes, proteins, lipids, and carbohydrates.
Water. The principal ﬂuid medium of the cell is water, which is present in most
cells, except for fat cells, in a concentration of 70 to 85 per cent. Many cellular
chemicals are dissolved in the water. Others are suspended in the water as solid
particulates. Chemical reactions take place among the dissolved chemic

al reactions take place among the dissolved chemicals or at
the surfaces of the suspended particles or membranes.
Ions. The most important ions in the cell are potassium, magnesium, phosphate,
sulfate, bicarbonate, and smaller quantities of sodium, chloride, and calcium.
These are all discussed in more detail in Chapter 4, which considers the inter-
relations between the intracellular and extracellular ﬂuids.
The ions provide inorganic chemicals for cellular reactions. Also, they are
necessary f

for cellular reactions. Also, they are
necessary for operation of some of the cellular control mechanisms. For
instance, ions acting at the cell membrane are required for transmission of elec-
trochemical impulses in nerve and muscle ﬁbers.
Proteins. After water, the most abundant substances in most cells are proteins,
which normally constitute 10 to 20 per cent of the cell mass. These can be
divided into two types: structural proteins and functional proteins.
Structural proteins are present in 

onal proteins.
Structural proteins are present in the cell mainly in the form of long ﬁlaments
that themselves are polymers of many individual protein molecules. A promi-
nent use of such intracellular ﬁlaments is to form microtubules that provide the
“cytoskeletons” of such cellular organelles as cilia, nerve axons, the mitotic
spindles of mitosing cells, and a tangled mass of thin ﬁlamentous tubules that
hold the parts of the cytoplasm and nucleoplasm together in their respective

eoplasm together in their respective
compartments. Extracellularly, ﬁbrillar proteins are found especially in the col-
lagen and elastin ﬁbers of connective tissue and in blood vessel walls, tendons,
ligaments, and so forth.
The functional proteins are an entirely different type of protein, usually com-
posed of combinations of a few molecules in tubular-globular form. These 
12
Unit I
proteins are mainly the enzymes of the cell and, in

roteins are mainly the enzymes of the cell and, in con-
trast to the ﬁbrillar proteins, are often mobile in the
cell ﬂuid. Also, many of them are adherent to mem-
branous structures inside the cell. The enzymes come
into direct contact with other substances in the cell
ﬂuid and thereby catalyze speciﬁc intracellular chem-
ical reactions. For instance, the chemical reactions 
that split glucose into its component parts and then
combine these with oxygen to form carbon dioxide
and water while simu

oxygen to form carbon dioxide
and water while simultaneously providing energy for
cellular function are all catalyzed by a series of protein
enzymes.
Lipids. Lipids are several types of substances that are
grouped together because of their common property
of being soluble in fat solvents. Especially important
lipids are phospholipids
and cholesterol,
which
together constitute only about 2 per cent of the total
cell mass. The signiﬁcance of phospholipids and cho-
lesterol is that they are mainly 

olipids and cho-
lesterol is that they are mainly insoluble in water and,
therefore, are used to form the cell membrane and
intracellular membrane barriers that separate the dif-
ferent cell compartments.
In addition to phospholipids and cholesterol, some
cells contain large quantities of triglycerides, also
called neutral fat. In the fat cells, triglycerides often
account for as much as 95 per cent of the cell mass.The
fat stored in these cells represents the body’s main
storehouse of energy-gi

represents the body’s main
storehouse of energy-giving nutrients that can later be
dissoluted and used to provide energy wherever in the
body it is needed.
Carbohydrates. Carbohydrates have little structural
function in the cell except as parts of glycoprotein mol-
ecules, but they play a major role in nutrition of the
cell. Most human cells do not maintain large stores of
carbohydrates; the amount usually averages about 1
per cent of their total mass but increases to as much
as 3 per cent in mu

 mass but increases to as much
as 3 per cent in muscle cells and, occasionally, 6 per
cent in liver cells. However, carbohydrate in the 
form of dissolved glucose is always present in the 
surrounding extracellular ﬂuid so that it is readily
available to the cell. Also, a small amount of carbohy-
drate is virtually always stored in the cells in the form
of glycogen, which is an insoluble polymer of glucose
that can be depolymerized and used rapidly to supply
the cells’ energy needs.
Physical Str

ly to supply
The cell is not merely a bag of ﬂuid, enzymes, and
chemicals; it also contains highly organized physical
structures, called intracellular organelles. The physical
nature of each organelle is as important as the cell’s
chemical constituents for cell function. For instance,
without one of the organelles, the mitochondria, more
than 95 per cent of the cell’s energy release from nutri-
ents would cease immediately. The most importa

ri-
ents would cease immediately. The most important
organelles and other structures of the cell are shown
in Figure 2–2.
Most organelles of the cell are covered by membranes
composed primarily of lipids and proteins.These mem-
branes include the cell membrane, nuclear membrane,
membrane of the endoplasmic reticulum, and mem-
branes of the mitochondria, lysosomes, and Golgi
apparatus.
The lipids of the membranes provide a barrier that
impedes the movement of wat

provide a barrier that
impedes the movement of water and water-soluble
substances from one cell compartment to another
because water is not soluble in lipids.However,protein
molecules in the membrane often do penetrate all the
way through the membrane, thus providing specialized
pathways, often organized into actual pores, for
passage of speciﬁc substances through the membrane.
Also, many other membrane proteins are enzymes that
catalyze a multitude of different chemical reactions,
discussed her

ude of different chemical reactions,
discussed here and in subsequent chapters.
The cell membrane (also called the plasma mem-
brane), which envelops the cell, is a thin, pliable,
elastic structure only 7.5 to 10 nanometers thick. It 
is composed almost entirely of proteins and lipids.
The approximate composition is proteins, 55 per cent;
phospholipids, 25 per cent; cholesterol, 13 per cent;
other lipids, 4 per cent; and carbohydrates, 3 per cent.
Lipid Barrier of the Cell Membrane

es, 3 per cent.
Lipid Barrier of the Cell Membrane Impedes Water Penetration.
Figure 2–3 shows the structure of the cell membrane.
Its basic structure is a lipid bilayer, which is a thin,
double-layered ﬁlm of lipids—each layer only one
molecule thick—that is continuous over the entire cell
surface. Interspersed in this lipid ﬁlm are large globu-
lar protein molecules.
The basic lipid bilayer is composed of phospholipid
molecules. One end of each phospholipid molecule is
soluble in water; that i

 phospholipid molecule is
soluble in water; that is, it is hydrophilic. The other end
is soluble only in fats; that is, it is hydrophobic. The
Nucleoplasm
Cytoplasm
Nucleus
Nucleolus
Cell
membrane
Nuclear
membrane
Figure 2–1
Structure of the cell as seen with the light microscope.
Chapter 2
13
phosphate end of the phospholipid is hydrophilic, and
the fatty acid portion is hydrophobic.
Because the hydrophobic portions of the phospho-
lipid molecules are repelled by wate

 the phospho-
lipid molecules are repelled by water but are mutually
attracted to one another, they have a natural tendency
to attach to one another in the middle of the mem-
brane, as shown in Figure 2–3. The hydrophilic phos-
phate portions then constitute the two surfaces of the
complete cell membrane, in contact with intracellular
water on the inside of the membrane and extracellular
water on the outside surface.
The lipid layer in the middle of the membrane is
impermeable to the usual water

 of the membrane is
impermeable to the usual water-soluble substances,
such as ions, glucose, and urea. Conversely, fat-soluble
substances, such as oxygen, carbon dioxide, and
alcohol, can penetrate this portion of the membrane
with ease.
The cholesterol molecules in the membrane are also
lipid in nature because their steroid nucleus is highly
fat soluble. These molecules, in a sense, are dissolved
in the bilayer of the membrane. They mainly help
determine the degree of permeability (or imperme-

ability) of the bilayer to water-soluble constituents of
body ﬂuids. Cholesterol controls much of the ﬂuidity
of the membrane as well.
Cell Membrane Proteins. Figure 2–3 also shows globular
masses ﬂoating in the lipid bilayer. These are mem-
brane proteins, most of which are glycoproteins. Two
types of proteins occur: integral proteins that protrude
all the way through the membrane, and peripheral pro-
teins that are attached only to one surface

l pro-
teins that are attached only to one surface of the mem-
brane and do not penetrate all the way through.
Many of the integral proteins provide structural
channels (or pores) through which water molecules
and water-soluble substances, especially ions, can
diffuse between the extracellular and intracellular
ﬂuids.These protein channels also have selective prop-
erties that allow preferential diffusion of some sub-
stances over others.
Other integral proteins act as carrier proteins for
trans

ntegral proteins act as carrier proteins for
transporting substances that otherwise could not pen-
etrate the lipid bilayer. Sometimes these even trans-
port substances in the direction opposite to their
natural direction of diffusion, which is called “active
transport.” Still others act as enzymes.
Nucleolus
Cell
membrane
Lysosome
Secretory
granule
Centrioles
Microtubules
Nuclear
membrane
Granular
reticulum
Smooth
(agranular)
reticulum
Ribosomes
Glycogen
Go

nular)
reticulum
Ribosomes
Glycogen
Golgi
apparatus
Microfilaments
Chromosomes and DNA
Figure 2–2
Reconstruction of a typical
cell, showing the internal
organelles in the cytoplasm
and in the nucleus.
14
Unit I
Integral membrane proteins can also serve as recep-
tors for water-soluble chemicals, such as peptide hor-
mones, that do not easily penetrate the cell membrane.
Interaction of cell membrane receptors with speciﬁc
lig

action of cell membrane receptors with speciﬁc
ligands that bind to the receptor causes conforma-
tional changes in the receptor protein. This, in turn,
enzymatically activates the intracellular part of the
protein or induces interactions between the receptor
and proteins in the cytoplasm that act as second mes-
sengers, thereby relaying the signal from the extracel-
lular part of the receptor to the interior of the cell. In
this way, integral proteins spanning the cell membrane
provide a means 

oteins spanning the cell membrane
provide a means of conveying information about the
environment to the cell interior.
Peripheral protein molecules are often attached to
the integral proteins. These peripheral proteins func-
tion almost entirely as enzymes or as controllers of
transport of substances through the cell membrane
“pores.”
Membrane Carbohydrates—The Cell “Glycocalyx.” Mem-
brane carbohydrates occur almost invariably in 
combination with proteins or lipids in the form of gly-
coprotei

th proteins or lipids in the form of gly-
coproteins or glycolipids. In fact, most of the integral
proteins are glycoproteins, and about one tenth of the
membrane lipid molecules are glycolipids.The “glyco”
portions of these molecules almost invariably protrude
to the outside of the cell, dangling outward from the
cell surface. Many other carbohydrate compounds,
called proteoglycans—which are mainly carbohydrate
substances bound to small protein cores—are loosely
attached to the outer surface of

cores—are loosely
attached to the outer surface of the cell as well.
Thus, the entire outside surface of the cell often has a
loose carbohydrate coat called the glycocalyx.
The carbohydrate moieties attached to the outer
surface of the cell have several important functions:
(1) Many of them have a negative electrical charge,
which gives most cells an overall negative surface
charge that repels other negative objects. (2) The gly-
cocalyx of some cells attaches to the glycocalyx of
other cells, t

cells attaches to the glycocalyx of
other cells, thus attaching cells to one another. (3)
Many of the carbohydrates act as receptor substances
for binding hormones, such as insulin; when bound,
this combination activates attached internal proteins
that, in turn, activate a cascade of intracellular
enzymes. (4) Some carbohydrate moieties enter into
immune reactions, as discussed in Chapter 34.
The cytoplasm is ﬁlled with both minute and large dis-
persed particles and

th both minute and large dis-
persed particles and organelles.The clear ﬂuid portion
of the cytoplasm in which the particles are dispersed
is called cytosol; this contains mainly dissolved pro-
teins, electrolytes, and glucose.
Integral protein
fluid
fluid
Cytoplasm
Lipid
bilayer
Carbohydrate
protein
Figure 2–3
Structure of the cell membrane,
showing that it is composed
mainly of a lipid bilayer of phos-
pholipid molecules, but with large
n

ayer of phos-
numbers of protein molecules
protruding through the layer. 
Also, carbohydrate moieties are
attached to the protein molecules
on the outside of the membrane
and to additional protein mole-
cules on the inside. (Redrawn
from Lodish HF, Rothman JE: The
assembly of cell membranes. Sci
Am 
240:48, 
1979. 
Copyright
George V. Kevin.)
Chapter 2
15
Dispersed in the cytoplasm are neutral fat globules,
glycogen granules, ribosome

 neutral fat globules,
glycogen granules, ribosomes, secretory vesicles, and
ﬁve especially important organelles: the endoplasmic
reticulum, the Golgi apparatus, mitochondria, lyso-
somes, and peroxisomes.
Endoplasmic Reticulum
Figure 2–2 shows a network of tubular and ﬂat vesic-
ular structures in the cytoplasm; this is the endoplas-
mic reticulum. The tubules and vesicles interconnect
with one another. Also, their walls are constructed 
of lipid bilayer membranes that contain large amounts
of 

d bilayer membranes that contain large amounts
of proteins, similar to the cell membrane. The total
surface area of this structure in some cells—the liver
cells, for instance—can be as much as 30 to 40 times
the cell membrane area.
The detailed structure of a small portion of endo-
plasmic reticulum is shown in Figure 2–4. The space
inside the tubules and vesicles is ﬁlled with endoplas-
mic matrix, a watery medium that is different from the
ﬂuid in the cytosol outside the endoplasmic reticulum.

 in the cytosol outside the endoplasmic reticulum.
Electron micrographs show that the space inside the
endoplasmic reticulum is connected with the space
between the two membrane surfaces of the nuclear
membrane.
Substances formed in some parts of the cell enter
the space of the endoplasmic reticulum and are then
conducted to other parts of the cell. Also, the vast
surface area of this reticulum and the multiple enzyme
systems attached to its membranes provide machinery
for a major share of the m

ranes provide machinery
for a major share of the metabolic functions of the cell.
Ribosomes 
and 
the 
Granular 
Endoplasmic 
Reticulum.
Attached to the outer surfaces of many parts of the
endoplasmic reticulum are large numbers of minute
granular particles called ribosomes. Where these are
present, the reticulum is called the granular endoplas-
mic reticulum. The ribosomes are composed of a
mixture of RNA and proteins, and they function to
synthesize new protein molecules in the cell, as dis-
c

esize new protein molecules in the cell, as dis-
cussed later in this chapter and in Chapter 3.
Agranular Endoplasmic Reticulum. Part of the endoplasmic
reticulum has no attached ribosomes. This part is
called the agranular, or smooth, endoplasmic reticu-
lum. The agranular reticulum functions for the syn-
thesis of lipid substances and for other processes of the
cells promoted by intrareticular enzymes.
Golgi Apparatus
The Golgi apparatus, shown in Figure 2–5, is closely
related to the endoplas

 in Figure 2–5, is closely
related to the endoplasmic reticulum. It has mem-
branes similar to those of the agranular endoplasmic
reticulum. It usually is composed of four or more
stacked layers of thin, ﬂat, enclosed vesicles lying near
one side of the nucleus. This apparatus is prominent 
in secretory cells, where it is located on the side of 
the cell from which the secretory substances are 
extruded.
The Golgi apparatus functions in association with
the endoplasmic reticulum. As shown in Fig

on with
the endoplasmic reticulum. As shown in Figure 2–5,
small “transport vesicles” (also called endoplasmic
reticulum vesicles,or ER vesicles) continually pinch off
from the endoplasmic reticulum and shortly thereafter
fuse with the Golgi apparatus. In this way, substances
entrapped in the ER vesicles are transported from the
endoplasmic reticulum to the Golgi apparatus. The
transported substances are then processed in the Golgi
apparatus to form lysosomes, secretory vesicles, and
other cytop

orm lysosomes, secretory vesicles, and
other cytoplasmic components that are discussed later
in the chapter.
Matrix
Agranular
reticulum
Granular
reticulum
Figure 2–4
Structure of the endoplasmic reticulum. (Modiﬁed from DeRober-
tis EDP, Saez FA, DeRobertis EMF: Cell Biology, 6th ed. Philadel-
phia: WB Saunders, 1975.)
Golgi
apparatus
reticulum
ER vesicles
Golgi vesicles
Figure 2–5
A typical Golgi apparatus and its relationship to the endoplasmic
reticulum (ER

 its relationship to the endoplasmic
reticulum (ER) and the nucleus.
16
Unit I
Lysosomes
Lysosomes,
shown in Figure 2–2,
are vesicular
organelles that form by breaking off from the Golgi
apparatus and then dispersing throughout the cyto-
plasm.The lysosomes provide an intracellular digestive
system that allows the cell to digest (1) damaged cel-
lular structures, (2) food particles that have been
ingested by the cell, and (3) unwanted m

have been
ingested by the cell, and (3) unwanted matter such as
bacteria. The lysosome is quite different in different
types of cells, but it is usually 250 to 750 nanometers
in diameter. It is surrounded by a typical lipid bilayer
membrane and is ﬁlled with large numbers of small
granules 5 to 8 nanometers in diameter, which are
protein aggregates of as many as 40 different hydro-
lase (digestive) enzymes. A hydrolytic enzyme is
capable of splitting an organic compound into two or
more parts by

ting an organic compound into two or
more parts by combining hydrogen from a water mol-
ecule with one part of the compound and combining
the hydroxyl portion of the water molecule with the
other part of the compound. For instance, protein 
is hydrolyzed to form amino acids, glycogen is
hydrolyzed to form glucose, and lipids are hydrolyzed
to form fatty acids and glycerol.
Ordinarily,the membrane surrounding the lysosome
prevents the enclosed hydrolytic enzymes from
coming in contact with other 

rolytic enzymes from
coming in contact with other substances in the cell and,
therefore, prevents their digestive actions. However,
some conditions of the cell break the membranes of
some of the lysosomes, allowing release of the diges-
tive enzymes. These enzymes then split the organic
substances with which they come in contact into small,
highly diffusible substances such as amino acids and
glucose. Some of the more speciﬁc functions of lyso-
somes are discussed later in the chapter.
Peroxisom

omes are discussed later in the chapter.
Peroxisomes
Peroxisomes are similar physically to lysosomes, but
they are different in two important ways. First, they are
believed to be formed by self-replication (or perhaps
by budding off from the smooth endoplasmic reticu-
lum) rather than from the Golgi apparatus. Second,
they contain oxidases rather than hydrolases. Several
of the oxidases are capable of combining oxygen with
hydrogen ions derived from different intracellular
chemicals to form hydr

rom different intracellular
chemicals to form hydrogen peroxide (H2O2). Hydro-
gen peroxide is a highly oxidizing substance and is
used in association with catalase, another oxidase
enzyme present in large quantities in peroxisomes, to
oxidize many substances that might otherwise be poi-
sonous to the cell. For instance, about half the alcohol
a person drinks is detoxiﬁed by the peroxisomes of the
liver cells in this manner.
Secretory Vesicles
One of the important functions of many cells is secr

e of the important functions of many cells is secre-
tion of special chemical substances. Almost all such
secretory substances are formed by the endoplasmic
reticulum–Golgi apparatus system and are then
released from the Golgi apparatus into the cytoplasm
in the form of storage vesicles called secretory vesicles
or secretory granules. Figure 2–6 shows typical secre-
tory vesicles inside pancreatic acinar cells; these 
vesicles store protein proenzymes (enzymes that are
not yet activated). The pr

zymes (enzymes that are
not yet activated). The proenzymes are secreted later
through the outer cell membrane into the pancreatic
duct and thence into the duodenum, where they
become activated and perform digestive functions on
the food in the intestinal tract.
The mitochondria, shown in Figures 2–2 and 2–7, are
called the “powerhouses” of the cell. Without them,
cells would be unable to extract enough energy from
the nutrients, and essentially all cellular functions
would cease.
Mi

essentially all cellular functions
Mitochondria are present in all areas of each cell’s
cytoplasm, but the total number per cell varies from
less than a hundred up to several thousand, depending
on the amount of energy required by the cell. Further,
the mitochondria are concentrated in those portions
of the cell that are responsible for the major share of
its energy metabolism. They are also variable in size
and shape. Some are only a few hundred nanometers
Secretory
granules
Figure

a few hundred nanometers
Secretory
granules
Figure 2–6
Secretory granules (secretory vesicles) in acinar cells of the 
pancreas.
Inner membrane
Oxidative
enzymes
Outer chamber
Matrix
Crests
Figure 2–7
Structure of a mitochondrion. (Modiﬁed from DeRobertis EDP,
Saez FA, DeRobertis EMF: Cell Biology, 6th ed. Philadelphia: WB
Saunders, 1975.)
Chapter 2
17
in diameter and globular in shape, whereas others are
elongated—as large as 1 micromete

ereas others are
elongated—as large as 1 micrometer in diameter and
7 micrometers long; still others are branching and 
ﬁlamentous.
The basic structure of the mitochondrion, shown 
in Figure 2–7, is composed mainly of two lipid
bilayer–protein membranes: an outer membrane and
an inner membrane. Many infoldings of the inner
membrane form shelves onto which oxidative enzymes
are attached. In addition, the inner cavity of the mito-
chondrion is ﬁlled with a matrix that contains large
quantities of 

d with a matrix that contains large
quantities of dissolved enzymes that are necessary for
extracting energy from nutrients. These enzymes
operate in association with the oxidative enzymes on
the shelves to cause oxidation of the nutrients, thereby
forming carbon dioxide and water and at the same
time releasing energy. The liberated energy is used to
synthesize a “high-energy” substance called adenosine
triphosphate (ATP).ATP is then transported out of the
mitochondrion, and it diffuses througho

out of the
mitochondrion, and it diffuses throughout the cell to
release its own energy wherever it is needed for per-
forming cellular functions.The chemical details of ATP
formation by the mitochondrion are given in Chapter
67, but some of the basic functions of ATP in the cell
are introduced later in this chapter.
Mitochondria are self-replicative, which means that
one mitochondrion can form a second one, a third one,
and so on, whenever there is a need in the cell for
increased amounts of AT

 is a need in the cell for
increased amounts of ATP. Indeed, the mitochondria
contain DNA similar to that found in the cell nucleus.
In Chapter 3 we will see that DNA is the basic chem-
ical of the nucleus that controls replication of the cell.
The DNA of the mitochondrion plays a similar role,
controlling replication of the mitochondrion itself.
Filament and Tubular Structures of the Cell
The ﬁbrillar proteins of the cell are usually organized
into ﬁlaments or tubules. These originate as precur

nto ﬁlaments or tubules. These originate as precursor
protein molecules synthesized by ribosomes in the
cytoplasm. The precursor molecules then polymerize
to form ﬁlaments. As an example, large numbers of
actin ﬁlaments frequently occur in the outer zone of
the cytoplasm, called the ectoplasm, to form an elastic
support for the cell membrane. Also, in muscle cells,
actin and myosin ﬁlaments are organized into a special
contractile machine that is the basis for muscle con-
traction, as discussed 

 the basis for muscle con-
traction, as discussed in detail in Chapter 6.
A special type of stiff ﬁlament composed of poly-
merized tubulin molecules is used in all cells to con-
struct very strong tubular structures, the microtubules.
Figure 2–8 shows typical microtubules that were
teased from the ﬂagellum of a sperm.
Another example of microtubules is the tubular
skeletal structure in the center of each cilium that radi-
ates upward from the cell cytoplasm to the tip of the
cilium. This struct

ll cytoplasm to the tip of the
cilium. This structure is discussed later in the chapter
and is illustrated in Figure 2–17. Also, both the centri-
oles and the mitotic spindle of the mitosing cell are
composed of stiff microtubules.
Thus, a primary function of microtubules is to act as
a cytoskeleton, providing rigid physical structures for
certain parts of cells.
Nucleus
The nucleus is the control center of the cell. Brieﬂy, the
nucleus contains large quantities of DNA, which are
the genes. The 

large quantities of DNA, which are
the genes. The genes determine the characteristics of
the cell’s proteins, including the structural proteins, as
well as the intracellular enzymes that control cyto-
plasmic and nuclear activities.
The genes also control and promote reproduction of
the cell itself. The genes ﬁrst reproduce to give two
identical sets of genes; then the cell splits by a special
process called mitosis to form two daughter cells, each
of which receives one of the two sets of DNA ge

ch
of which receives one of the two sets of DNA genes.
All these activities of the nucleus are considered in
detail in the next chapter.
Unfortunately, the appearance of the nucleus under
the microscope does not provide many clues to the
mechanisms by which the nucleus performs its control
activities. Figure 2–9 shows the light microscopic
appearance of the interphase nucleus (during the
period between mitoses), revealing darkly staining
chromatin material
throughout the nucleoplasm.
During mito

n material
During mitosis, the chromatin material organizes in
the form of highly structured chromosomes, which can
then be easily identiﬁed using the light microscope, as
illustrated in the next chapter.
The nuclear membrane, also called the nuclear enve-
lope, is actually two separate bilayer membranes, one
inside the other. The outer membrane is continuous
with the endoplasmic reticulum of the cell cytoplasm,
and the space between the two nuclear m

cytoplasm,
and the space between the two nuclear membranes is
also continuous with the space inside the endoplasmic
reticulum, as shown in Figure 2–9.
Figure 2–8
Microtubules teased from the ﬂagellum of a sperm. (From 
Wolstenholme GEW, O’Connor M, and The publisher, JA Churchill,
1967. Figure 4, page 314. Copyright the Novartis Foundation 
formerly the Ciba Foundation.)
18
Unit I
The nuclear membrane is penetrated by several
thousand n

clear membrane is penetrated by several
thousand nuclear pores. Large complexes of protein
molecules are attached at the edges of the pores so
that the central area of each pore is only about 9
nanometers in diameter. Even this size is large enough
to allow molecules up to 44,000 molecular weight to
pass through with reasonable ease.
The nuclei of most cells contain one or more highly
staining structures called nucleoli. The nucleolus,
unlike most other organe

 nucleoli. The nucleolus,
unlike most other organelles discussed here, does not
have a limiting membrane.Instead,it is simply an accu-
mulation of large amounts of RNA and proteins of the
types found in ribosomes.The nucleolus becomes con-
siderably enlarged when the cell is actively synthesiz-
ing proteins.
Formation of the nucleoli (and of the ribosomes in
the cytoplasm outside the nucleus) begins in the
nucleus. First, speciﬁc DNA genes in the chromosomes
cause RNA to be synthesized. Some of 

 chromosomes
cause RNA to be synthesized. Some of this is stored 
in the nucleoli, but most of it is transported outward
through the nuclear pores into cytoplasm. Here, it is
used in conjunction with speciﬁc proteins to assemble
“mature” ribosomes that play an essential role in
forming cytoplasmic proteins, as discussed more fully
in Chapter 3.
Comparison of the Animal Cell
with Precellular Forms of Life
Many of us think of the cell as the lowest level of life.
However, the cell is a very compli

 level of life.
However, the cell is a very complicated organism that
required many hundreds of millions of years to
develop after the earliest form of life, an organism
similar to the present-day virus, ﬁrst appeared on
earth. Figure 2–10 shows the relative sizes of (1) the
smallest known virus, (2) a large virus, (3) a rickettsia,
(4) a bacterium, and (5) a nucleated cell, demonstrat-
ing that the cell has a diameter about 1000 times that
of the smallest virus and, therefore, a volume about 1

e smallest virus and, therefore, a volume about 1
billion times that of the smallest virus. Correspond-
ingly, the functions and anatomical organization of the
cell are also far more complex than those of the virus.
The essential life-giving constituent of the small
virus is a nucleic acid embedded in a coat of protein.
This nucleic acid is composed of the same basic nucleic
acid constituents (DNA or RNA) found in mammalian
cells, and it is capable of reproducing itself under
appropriate conditi

le of reproducing itself under
appropriate conditions. Thus, the virus propagates its
lineage from generation to generation and is therefore
a living structure in the same way that the cell and the
human being are living structures.
As life evolved, other chemicals besides nucleic acid
and simple proteins became integral parts of the
organism, and specialized functions began to develop
in different parts of the virus. A membrane formed
around the virus, and inside the membrane, a ﬂuid
matrix app

 virus, and inside the membrane, a ﬂuid
matrix appeared. Specialized chemicals then devel-
oped inside the ﬂuid to perform special functions;
many protein enzymes appeared that were capable of
catalyzing chemical reactions and, therefore, deter-
mining the organism’s activities.
In still later stages of life, particularly in the rick-
ettsial and bacterial stages, organelles developed inside
the organism, representing physical structures of
chemical aggregates that perform functions in a more
ef

cal aggregates that perform functions in a more
efﬁcient manner than can be achieved by dispersed
chemicals throughout the ﬂuid matrix.
Finally, in the nucleated cell, still more complex
organelles developed, the most important of which is
the nucleus itself. The nucleus distinguishes this type
of cell from all lower forms of life; the nucleus pro-
vides a control center for all cellular activities, and it
provides for exact reproduction of new cells genera-
tion after generation, each new cell 

ells genera-
tion after generation, each new cell having almost
exactly the same structure as its progenitor.
reticulum
Cytoplasm
Nuclear envelope–
outer and inner
membranes
Pores
Nucleolus
Chromatin material (DNA)
Figure 2–9
Structure of the nucleus.
15 nm — Small virus
150 nm — Large virus
350 nm — Rickettsia
1 mm   Bacterium
5 – 10 mm +
Cell
Figure 2–10
Comparison of sizes of precellular organisms with that of the
average cell in the human body.
Chapter 2
The Cell and 

ge cell in the human body.
Chapter 2
19
In the remainder of this chapter, we discuss several
representative functional systems of the cell that make
it a living organism.
If a cell is to live and grow and reproduce, it must
obtain nutrients and other substances from the sur-
rounding ﬂuids. Most substances pass through the cell
membrane by diffusion and active transport.
Diffusion involves simple movement

tive transport.
Diffusion involves simple movement through the
membrane caused by the random motion of the mol-
ecules of the substance; substances move either
through cell membrane pores or, in the case of lipid-
soluble substances, through the lipid matrix of the
membrane.
Active transport involves the actual carrying of a
substance through the membrane by a physical pro-
tein structure that penetrates all the way through the
membrane. These active transport mechanisms are so
important to cell

tive transport mechanisms are so
important to cell function that they are presented in
detail in Chapter 4.
Very large particles enter the cell by a specialized
function of the cell membrane called endocytosis. The
principal forms of endocytosis are pinocytosis and
phagocytosis. Pinocytosis means ingestion of minute
particles that form vesicles of extracellular ﬂuid and
particulate constituents inside the cell cytoplasm.
Phagocytosis means ingestion of large particles, such
as bacteria, whole ce

ion of large particles, such
as bacteria, whole cells, or portions of degenerating
tissue.
Pinocytosis. Pinocytosis occurs continually in the cell
membranes of most cells, but it is especially rapid 
in some cells. For instance, it occurs so rapidly in
macrophages that about 3 per cent of the total macro-
phage membrane is engulfed in the form of vesicles
each minute. Even so, the pinocytotic vesicles are so
small—usually only 100 to 200 nanometers in diame-
ter—that most of them can be seen onl

rs in diame-
ter—that most of them can be seen only with the elec-
tron microscope.
Pinocytosis is the only means by which most large
macromolecules, such as most protein molecules, can
enter cells. In fact, the rate at which pinocytotic vesi-
cles form is usually enhanced when such macro-
molecules attach to the cell membrane.
Figure 2–11 demonstrates the successive steps of
pinocytosis, showing three molecules of protein 
attaching to the membrane. These molecules usually
attach to specialized

ane. These molecules usually
attach to specialized protein receptors on the surface
of the membrane that are speciﬁc for the type of
protein that is to be absorbed.The receptors generally
are concentrated in small pits on the outer surface of
the cell membrane, called coated pits. On the inside of
the cell membrane beneath these pits is a latticework
of ﬁbrillar protein called clathrin, as well as other pro-
teins, perhaps including contractile ﬁlaments of actin
and myosin. Once the protein mole

laments of actin
and myosin. Once the protein molecules have bound
with the receptors, the surface properties of the local
membrane change in such a way that the entire pit
invaginates inward, and the ﬁbrillar proteins sur-
rounding the invaginating pit cause its borders to close
over the attached proteins as well as over a small
amount of extracellular ﬂuid. Immediately thereafter,
the invaginated portion of the membrane breaks away
from the surface of the cell, forming a pinocytotic
vesicle in

face of the cell, forming a pinocytotic
vesicle inside the cytoplasm of the cell.
What causes the cell membrane to go through the
necessary contortions to form pinocytotic vesicles
remains mainly a mystery.This process requires energy
from within the cell; this is supplied by ATP, a high-
energy substance discussed later in the chapter. Also,
it requires the presence of calcium ions in the extra-
cellular ﬂuid, which probably react with contractile
protein ﬁlaments beneath the coated pits to pro

le
protein ﬁlaments beneath the coated pits to provide
the force for pinching the vesicles away from the cell
membrane.
Phagocytosis. Phagocytosis occurs in much the same
way as pinocytosis, except that it involves large 
particles rather than molecules. Only certain cells 
have the capability of phagocytosis, most notably the
tissue macrophages and some of the white blood cells.
Phagocytosis is initiated when a particle such as a
bacterium, a dead cell, or tissue debris binds with
receptors on 

ad cell, or tissue debris binds with
receptors on the surface of the phagocyte. In the case
of bacteria, each bacterium usually is already attached
to a speciﬁc antibody, and it is the antibody that
attaches to the phagocyte receptors, dragging the bac-
terium along with it.This intermediation of antibodies
is called opsonization, which is discussed in Chapters
33 and 34.
Phagocytosis occurs in the following steps:
1. The cell membrane receptors attach to the surface
ligands of the particle.
2. 

attach to the surface
2. The edges of the membrane around the points of
attachment evaginate outward within a fraction of
a second to surround the entire particle; then,
progressively more and more membrane receptors
Receptors
Actin and myosin
Dissolving clathrin
Proteins
Coated pit
Clathrin
A
B
C
D
Figure 2–11
Mechanism of pinocytosis.
20
Unit I
attach to the particle ligands. All this occurs
suddenly in a zipp

rticle ligands. All this occurs
suddenly in a zipper-like manner to form a closed
phagocytic vesicle.
3. Actin and other contractile ﬁbrils in the cytoplasm
surround the phagocytic vesicle and contract
around its outer edge, pushing the vesicle to the
interior.
4. The contractile proteins then pinch the stem of
the vesicle so completely that the vesicle
separates from the cell membrane, leaving the
vesicle in the cell interior in the same way that
pinocytotic vesicles are formed.
Digestion of Pi

t
Digestion of Pinocytotic and
Phagocytic Foreign Substances Inside
the Cell—Function of the Lysosomes
Almost immediately after a pinocytotic or phagocytic
vesicle appears inside a cell, one or more lysosomes
become attached to the vesicle and empty their acid
hydrolases to the inside of the vesicle, as shown in
Figure 2–12. Thus, a digestive vesicle is formed inside
the cell cytoplasm in which the vesicular hydrolases
begin hydrolyzing the proteins, carbohydrate

lases
begin hydrolyzing the proteins, carbohydrates, lipids,
and other substances in the vesicle. The products of
digestion are small molecules of amino acids, glucose,
phosphates, and so forth that can diffuse through the
membrane of the vesicle into the cytoplasm. What is
left of the digestive vesicle, called the residual body,
represents indigestible substances. In most instances,
this is ﬁnally excreted through the cell membrane by
a process called exocytosis, which is essentially the
opposi

called exocytosis, which is essentially the
opposite of endocytosis.
Thus, the pinocytotic and phagocytic vesicles con-
taining lysosomes can be called the digestive organs of
the cells.
Regression of Tissues and Autolysis of Cells. Tissues of the
body often regress to a smaller size. For instance, this
occurs in the uterus after pregnancy, in muscles during
long periods of inactivity, and in mammary glands at
the end of lactation. Lysosomes are responsible for
much of this regression.The mechan

responsible for
much of this regression.The mechanism by which lack
of activity in a tissue causes the lysosomes to increase
their activity is unknown.
Another special role of the lysosomes is removal of
damaged cells or damaged portions of cells from
tissues. Damage to the cell—caused by heat, cold,
trauma, chemicals, or any other factor—induces lyso-
somes to rupture. The released hydrolases immedi-
ately begin to digest the surrounding organic
substances. If the damage is slight, only a porti

substances. If the damage is slight, only a portion of
the cell is removed, followed by repair of the cell. If
the damage is severe, the entire cell is digested, a
process called autolysis. In this way, the cell is com-
pletely removed, and a new cell of the same type ordi-
narily is formed by mitotic reproduction of an adjacent
cell to take the place of the old one.
The lysosomes also contain bactericidal agents that
can kill phagocytized bacteria before they can cause
cellular damage. These a

ria before they can cause
cellular damage. These agents include (1) lysozyme,
which dissolves the bacterial cell membrane; (2) lyso-
ferrin, which binds iron and other substances before
they can promote bacterial growth; and (3) acid at a
pH of about 5.0, which activates the hydrolases and
inactivates bacterial metabolic systems.
Synthesis and Formation of Cellular
Structures by Endoplasmic Reticulum
Speciﬁc Functions of the 
The extensiveness of the end

The extensiveness of the endoplasmic reticulum and
the Golgi apparatus in secretory cells has already been
emphasized. These structures are formed primarily of
lipid bilayer membranes similar to the cell membrane,
and their walls are loaded with protein enzymes that
catalyze the synthesis of many substances required by
the cell.
Most synthesis begins in the endoplasmic reticulum.
The products formed there are then passed on to the
Golgi apparatus, where they are further pro

to the
Golgi apparatus, where they are further processed
before being released into the cytoplasm. But ﬁrst, let
us note the speciﬁc products that are synthesized in
speciﬁc portions of the endoplasmic reticulum and the
Golgi apparatus.
Proteins Are Formed by the Granular Endoplasmic Reticulum.
The granular portion of the endoplasmic reticulum is
characterized by large numbers of ribosomes attached
to the outer surfaces of the endoplasmic reticulum
membrane. As we discuss in Chapter 3, protein m

um
membrane. As we discuss in Chapter 3, protein mole-
cules are synthesized within the structures of the ribo-
somes.The ribosomes extrude some of the synthesized
protein molecules directly into the cytosol, but they
also extrude many more through the wall of the endo-
plasmic reticulum to the interior of the endoplasmic
vesicles and tubules, that is, into the endoplasmic
matrix.
Pinocytotic or
phagocytic
vesicle
Lysosomes
Digestive vesicle
Residual body
Excretion
Figure 2–12
Digestion of subst

dual body
Excretion
Digestion of substances in pinocytotic or phagocytic vesicles by
enzymes derived from lysosomes.
Chapter 2
21
Synthesis of Lipids by the Smooth Endoplasmic Reticulum.
The endoplasmic reticulum also synthesizes lipids,
especially phospholipids and cholesterol. These are
rapidly incorporated into the lipid bilayer of the endo-
plasmic reticulum itself, thus causing the endoplasmic
reticulum to grow more extensive. This occurs mainly
in the

 to grow more extensive. This occurs mainly
in the smooth portion of the endoplasmic reticulum.
To keep the endoplasmic reticulum from growing
beyond the needs of the cell, small vesicles called ER
vesicles or transport vesicles continually break away
from the smooth reticulum; most of these vesicles then
migrate rapidly to the Golgi apparatus.
Other Functions of the Endoplasmic Reticulum. Other sig-
niﬁcant functions of the endoplasmic reticulum, espe-
cially the smooth reticulum, include the f

, espe-
cially the smooth reticulum, include the following:
1. It provides the enzymes that control glycogen
breakdown when glycogen is to be used for
energy.
2. It provides a vast number of enzymes that are
capable of detoxifying substances, such as drugs,
that might damage the cell. It achieves
detoxiﬁcation by coagulation, oxidation,
hydrolysis, conjugation with glycuronic acid, and
in other ways.
Speciﬁc Functions of the Golgi Apparatus
Synthetic Functions of the Golgi Apparatus. Although th

etic Functions of the Golgi Apparatus. Although the
major function of the Golgi apparatus is to provide
additional processing of substances already formed 
in the endoplasmic reticulum, it also has the capability
of synthesizing certain carbohydrates that cannot be
formed in the endoplasmic reticulum. This is especially
true for the formation of large saccharide polymers
bound with small amounts of protein; the most impor-
tant of these are hyaluronic acid and chondroitin sulfate.
A few of the m

ronic acid and chondroitin sulfate.
A few of the many functions of hyaluronic acid and
chondroitin sulfate in the body are as follows: (1) they
are the major components of proteoglycans secreted
in mucus and other glandular secretions; (2) they are
the major components of the ground substance outside
the cells in the interstitial spaces, acting as ﬁller
between collagen ﬁbers and cells; and (3) they are
principal components of the organic matrix in both
cartilage and bone.
Processing of Endoplas

in both
Processing of Endoplasmic Secretions by the Golgi Apparatus—
Formation of Vesicles. Figure 2–13 summarizes the
major functions of the endoplasmic reticulum and
Golgi apparatus. As substances are formed in the
endoplasmic reticulum, especially the proteins, they
are transported through the tubules toward portions
of the smooth endoplasmic reticulum that lie nearest
the Golgi apparatus.At this point, small transport vesi-
cles composed of small envelopes of smooth endo-

-
plasmic reticulum continually break away and diffuse
to the deepest layer of the Golgi apparatus.Inside these
vesicles are the synthesized proteins and other prod-
ucts from the endoplasmic reticulum.
The transport vesicles instantly fuse with the Golgi
apparatus and empty their contained substances into
the vesicular spaces of the Golgi apparatus. Here,
additional carbohydrate moieties are added to the
secretions. Also, an important function of

 to the
secretions. Also, an important function of the Golgi
apparatus is to compact the endoplasmic reticular
secretions into highly concentrated packets. As the
secretions pass toward the outermost layers of the
Golgi apparatus, the compaction and processing
proceed. Finally, both small and large vesicles contin-
ually break away from the Golgi apparatus, carrying
with them the compacted secretory substances, and in
turn, the vesicles diffuse throughout the cell.
To give an idea of the timing 

hroughout the cell.
To give an idea of the timing of these processes:
When a glandular cell is bathed in radioactive amino
acids, newly formed radioactive protein molecules can
be detected in the granular endoplasmic reticulum
within 3 to 5 minutes.Within 20 minutes, newly formed
proteins are already present in the Golgi apparatus,
and within 1 to 2 hours, radioactive proteins are
secreted from the surface of the cell.
Types of Vesicles Formed by the Golgi Apparatus—Secretory
Vesicles and Lysoso

 the Golgi Apparatus—Secretory
Vesicles and Lysosomes. In a highly secretory cell, the
vesicles formed by the Golgi apparatus are mainly
secretory vesicles containing protein substances that
are to be secreted through the surface of the cell mem-
brane. These secretory vesicles ﬁrst diffuse to the cell
membrane, then fuse with it and empty their sub-
stances to the exterior by the mechanism called exo-
cytosis. Exocytosis, in most cases, is stimulated by the
entry of calcium ions into the cell; 

lated by the
entry of calcium ions into the cell; calcium ions inter-
act with the vesicular membrane in some way that is
not understood and cause its fusion with the cell mem-
brane, followed by exocytosis—that is, opening of the
membrane’s outer surface and extrusion of its contents
outside the cell.
Some vesicles, however, are destined for intracellu-
lar use.
Ribosomes
Lysosomes
Secretory
vesicles
Protein
formation
Glycosylation
Transport
vesicles
Smooth
reticulum
Golgi
apparatus

icles
Smooth
reticulum
Golgi
apparatus
Granular
reticulum
Lipid
formation
Figure 2–13
Formation of proteins, lipids, and cellular vesicles by the endo-
plasmic reticulum and Golgi apparatus.
22
Unit I
Use of Intracellular Vesicles to Replenish Cellular Membranes.
Some of the intracellular vesicles formed by the Golgi
apparatus fuse with the cell membrane or with the
membranes of intracellular structures such as t

he
membranes of intracellular structures such as the
mitochondria and even the endoplasmic reticulum.
This increases the expanse of these membranes and
thereby replenishes the membranes as they are used
up. For instance, the cell membrane loses much of its
substance every time it forms a phagocytic or pinocy-
totic vesicle, and the vesicular membranes of the Golgi
apparatus continually replenish the cell membrane.
In summary, the membranous system of the endo-
plasmic reticulum and Golgi apparat

m of the endo-
plasmic reticulum and Golgi apparatus represents a
highly metabolic organ capable of forming new intra-
cellular structures as well as secretory substances to be
extruded from the cell.
The principal substances from which cells extract
energy are foodstuffs that react chemically with
oxygen—carbohydrates, fats, and proteins. In the
human body, essentially all carbohydrates are con-
verted into glucose by the digesti

drates are con-
verted into glucose by the digestive tract and liver
before they reach the other cells of the body. Similarly,
proteins are converted into amino acids and fats into
fatty acids. Figure 2–14 shows oxygen and the food-
stuffs—glucose,fatty acids,and amino acids—all enter-
ing the cell. Inside the cell, the foodstuffs react
chemically with oxygen, under the inﬂuence of
enzymes that control the reactions and channel the
energy released in the proper direction. The details of
ATP is a

d in the proper direction. The details of
ATP is a nucleotide composed of (1) the nitrogenous
base adenine, (2) the pentose sugar ribose, and (3)
three phosphate radicals. The last two phosphate rad-
icals are connected with the remainder of the molecule
by so-called high-energy phosphate bonds, which are
represented in the formula above by the symbol ~.
Under the physical and chemical conditions of the
body, each of these high-energy bonds contains about
12,000 calories of energy per mole of AT

ins about
12,000 calories of energy per mole of ATP, which is
many times greater than the energy stored in the
average chemical bond, thus giving rise to the term
high-energy bond. Further, the high-energy phosphate
bond is very labile, so that it can be split instantly on
demand whenever energy is required to promote
other intracellular reactions.
When ATP releases its energy, a phosphoric acid
radical is split away, and adenosine diphosphate (ADP)
is formed.This released energy is used to ener

DP)
is formed.This released energy is used to energize vir-
tually all of the cell’s other functions, such as synthe-
sis of substances and muscular contraction.
To reconstitute the cellular ATP as it is used up,
energy derived from the cellular nutrients causes ADP
and phosphoric acid to recombine to form new ATP,
and the entire process repeats over and over again. For
these reasons,ATP has been called the energy currency
of the cell because it can be spent and remade contin-
ually, having a tu

can be spent and remade contin-
ually, having a turnover time of only a few minutes.
Chemical Processes in the Formation of ATP—Role of the 
Mitochondria. On entry into the cells, glucose is sub-
jected to enzymes in the cytoplasm that convert it 
into pyruvic acid (a process called glycolysis). A small
amount of ADP is changed into ATP by the energy
released during this conversion, but this amount
O2
Cell
membrane
Glucose
AA
FA
Gl
Pyruvic acid
Acetoacetic
acid

AA
FA
Gl
acid
Nucleus
CO2
H2O
H2O
O2
CO2
ADP
ATP
2ADP
2ATP
36 ATP
36 ADP
O2
CO2+H2O
Figure 2–14
Formation of adenosine triphosphate (ATP) in the cell, showing
that most of the ATP is formed in the mitochondria. ADP, adeno-
sine diphosphate.
P
O
O
O-
O-
O-
OH
OH
H
NH2
H
N
N
N
C
C
C
N
C
C
C
O
C
H
H
O-
O
O
P
O
P
O
CH2
CH
HC
~
~
Phosphate
Adenosine triphosphate
Adenine
Ribose
all these digestive and metabolic functions are given
in Chapte

estive and metabolic functions are given
in Chapters 62 through 72.
Brieﬂy, almost all these oxidative reactions occur
inside the mitochondria, and the energy that is
released is used to form the high-energy compound
ATP. Then, ATP, not the original foodstuffs, is used
throughout the cell to energize almost all the subse-
quent intracellular metabolic reactions.
Functional Characteristics of ATP
Chapter 2
23
accounts for less than 5 per cent of the overall energy
metab

r less than 5 per cent of the overall energy
metabolism of the cell.
By far, the major portion of the ATP formed in the
cell, about 95 per cent, is formed in the mitochondria.
The pyruvic acid derived from carbohydrates, fatty
acids from lipids, and amino acids from proteins are
eventually converted into the compound acetyl-CoA
in the matrix of the mitochondrion. This substance, in
turn, is further dissoluted (for the purpose of extract-
ing its energy) by another series of enzymes in the
mitoch

energy) by another series of enzymes in the
mitochondrion matrix, undergoing dissolution in a
sequence of chemical reactions called the citric acid
cycle, or Krebs cycle. These chemical reactions are 
so important that they are explained in detail in
Chapter 67.
In this citric acid cycle, acetyl-CoA is split into its
component parts, hydrogen atoms and carbon dioxide.
The carbon dioxide diffuses out of the mitochondria
and eventually out of the cell; ﬁnally, it is excreted
from the body through 

ell; ﬁnally, it is excreted
from the body through the lungs.
The hydrogen atoms, conversely, are highly reactive,
and they combine instantly with oxygen that has also
diffused into the mitochondria.This releases a tremen-
dous amount of energy, which is used by the mito-
chondria to convert very large amounts of ADP to
ATP. The processes of these reactions are complex,
requiring the participation of large numbers of protein
enzymes that are integral parts of mitochondrial mem-
branous shelves th

ral parts of mitochondrial mem-
branous shelves that protrude into the mitochondrial
matrix.The initial event is removal of an electron from
the hydrogen atom, thus converting it to a hydrogen
ion. The terminal event is combination of hydrogen
ions with oxygen to form water plus the release of
tremendous amounts of energy to large globular pro-
teins, called ATP synthetase, that protrude like knobs
from the membranes of the mitochondrial shelves.
Finally, the enzyme ATP synthetase uses the energ

Finally, the enzyme ATP synthetase uses the energy
from the hydrogen ions to cause the conversion of
ADP to ATP. The newly formed ATP is transported
out of the mitochondria into all parts of the cell cyto-
plasm and nucleoplasm, where its energy is used to
energize multiple cell functions.
This overall process for formation of ATP is called
the chemiosmotic mechanism of ATP formation. The
chemical and physical details of this mechanism are
presented in Chapter 67, and many of the detailed
metab

nted in Chapter 67, and many of the detailed
metabolic functions of ATP in the body are presented
in Chapters 67 through 71.
Uses of ATP for Cellular Function. Energy from ATP is
used to promote three major categories of cellular
functions: (1) transport of substances through multiple
membranes in the cell, (2) synthesis of chemical 
compounds throughout the cell, and (3) mechanical
work. These uses of ATP are illustrated by examples
in Figure 2–15: (1) to supply energy for the transport
of sodi

15: (1) to supply energy for the transport
of sodium through the cell membrane, (2) to promote
protein synthesis by the ribosomes, and (3) to supply
the energy needed during muscle contraction.
In addition to membrane transport of sodium,
energy from ATP is required for membrane transport
of potassium ions, calcium ions, magnesium ions, phos-
phate ions, chloride ions, urate ions, hydrogen ions,
and many other ions and various organic substances.
Membrane transport is so important to cell functi

Membrane transport is so important to cell function
that some cells—the renal tubular cells, for instance—
use as much as 80 per cent of the ATP that they form
for this purpose alone.
In addition to synthesizing proteins, cells synthesize
phospholipids, cholesterol, purines, pyrimidines, and a
host of other substances. Synthesis of almost any
chemical compound requires energy. For instance, a
single protein molecule might be composed of as many
as several thousand amino acids attached to one
an

another by peptide linkages; the formation of each of
these linkages requires energy derived from the break-
down of four high-energy bonds; thus, many thousand
ATP molecules must release their energy as each
protein molecule is formed. Indeed, some cells use as
much as 75 per cent of all the ATP formed in the cell
simply to synthesize new chemical compounds, espe-
cially protein molecules; this is particularly true during
the growth phase of cells

particularly true during
the growth phase of cells.
The ﬁnal major use of ATP is to supply energy for
special cells to perform mechanical work. We see in
Chapter 6 that each contraction of a muscle ﬁber
requires expenditure of tremendous quantities of ATP
energy. Other cells perform mechanical work in other
ways, especially by ciliary and ameboid motion, which
are described later in this chapter. The source of
energy for all these types of mechanical work is ATP.
In summary, ATP is always availa

ical work is ATP.
In summary, ATP is always available to release its
energy rapidly and almost explosively wherever in the
cell it is needed. To replace the ATP used by the cell,
ADP
Na+
Na+
ATP
ATP
ADP
ATP
Muscle contraction
ADP
ATP
ADP
Ribosomes
Membrane
transport
reticulum
Figure 2–15
Use of adenosine triphosphate (ATP) (formed in the mitochon-
drion) to provide energy for three major cellular functions: mem-
brane transport, protein synthesis, and 

ons: mem-
brane transport, protein synthesis, and muscle contraction. ADP,
adenosine diphosphate.
24
Unit I
much slower chemical reactions break down carbohy-
drates, fats, and proteins and use the energy derived
from these to form new ATP. More than 95 per cent 
of this ATP is formed in the mitochondria, which
accounts for the mitochondria being called the “pow-
erhouses” of the cell.
By far the most important type 

ocomotion of Cells
By far the most important type of movement that occurs
in the body is that of the muscle cells in skeletal, cardiac,
and smooth muscle, which constitute almost 50 per cent
of the entire body mass. The specialized functions of
these cells are discussed in Chapters 6 through 9. Two
other types of movement—ameboid locomotion and
ciliary movement—occur in other cells.
Ameboid movement is movement of an entire cell in
relation to its surroundings, such as movement 

in
relation to its surroundings, such as movement of white
blood cells through tissues. It receives its name from the
fact that amebae move in this manner and have pro-
vided an excellent tool for studying the phenomenon.
Typically, ameboid locomotion begins with protrusion
of a pseudopodium from one end of the cell. The
pseudopodium projects far out, away from the cell 
body, and partially secures itself in a new tissue area.
Then the remainder of the cell is pulled toward the
pseudopodium. Fig

of the cell is pulled toward the
pseudopodium. Figure 2–16 demonstrates this process,
showing an elongated cell, the right-hand end of which
is a protruding pseudopodium. The membrane of this
end of the cell is continually moving forward, and the
membrane at the left-hand end of the cell is continually
following along as the cell moves.
Mechanism of Ameboid Locomotion. Figure 2–16 shows the
general principle of ameboid motion. Basically, it results
from continual formation of new cell membrane a

ts
from continual formation of new cell membrane at the
leading edge of the pseudopodium and continual
absorption of the membrane in mid and rear portions
of the cell. Also, two other effects are essential for
forward movement of the cell. The ﬁrst effect is attach-
ment of the pseudopodium to surrounding tissues so
that it becomes ﬁxed in its leading position, while the
remainder of the cell body is pulled forward toward the
point of attachment. This attachment is effected by
receptor proteins 

 This attachment is effected by
receptor proteins that line the insides of exocytotic vesi-
cles.When the vesicles become part of the pseudopodial
membrane, they open so that their insides evert to the
outside, and the receptors now protrude to the outside
and attach to ligands in the surrounding tissues.
At the opposite end of the cell, the receptors pull
away from their ligands and form new endocytotic vesi-
cles. Then, inside the cell, these vesicles stream toward
the pseudopodial end of the 

esicles stream toward
the pseudopodial end of the cell, where they are used
to form still new membrane for the pseudopodium.
The second essential effect for locomotion is to
provide the energy required to pull the cell body in the
direction of the pseudopodium. Experiments suggest
the following as an explanation: In the cytoplasm of all
cells is a moderate to large amount of the protein actin.
Much of the actin is in the form of single molecules that
do not provide any motive power; however, the

that
do not provide any motive power; however, these poly-
merize to form a ﬁlamentous network, and the network
contracts when it binds with an actin-binding protein
such as myosin. The whole process is energized by the
high-energy compound ATP. This is what happens in 
the pseudopodium of a moving cell, where such 
a network of actin ﬁlaments forms anew inside the
enlarging pseudopodium. Contraction also occurs in 
the ectoplasm of the cell body, where a preexisting 
actin network is already pr

, where a preexisting 
actin network is already present beneath the cell 
membrane.
Types of Cells That Exhibit Ameboid Locomotion. The most
common cells to exhibit ameboid locomotion in the
human body are the white blood cells when they move
out of the blood into the tissues in the form of tissue
macrophages. Other types of cells can also move by
ameboid locomotion under certain circumstances. For
instance, ﬁbroblasts move into a damaged area to help
repair the damage, and even the germinal cel

 help
repair the damage, and even the germinal cells of the
skin, though ordinarily completely sessile cells, move
toward a cut area to repair the rent. Finally, cell loco-
motion is especially important in development of the
embryo and fetus after fertilization of an ovum. For
instance, embryonic cells often must migrate long dis-
tances from their sites of origin to new areas during
development of special structures.
Control of Ameboid Locomotion—Chemotaxis. The most
important initiator of ame

on—Chemotaxis. The most
important initiator of ameboid locomotion is the
process called chemotaxis. This results from the appear-
ance of certain chemical substances in the tissues.
Any chemical substance that causes chemotaxis to
occur is called a chemotactic substance. Most cells 
that exhibit ameboid locomotion move toward the
source of a chemotactic substance—that is, from an area
of lower concentration toward an area of higher con-
centration—which is called positive chemotaxis. Some
cells 

n—which is called positive chemotaxis. Some
cells move away from the source, which is called nega-
tive chemotaxis.
But how does chemotaxis control the direction of
ameboid locomotion? Although the answer is not
certain, it is known that the side of the cell most exposed
to the chemotactic substance develops membrane
changes that cause pseudopodial protrusion.
Cilia and Ciliary Movements
A second type of cellular motion, ciliary movement, is a
whiplike movement of cilia on the surfaces of cells.

iplike movement of cilia on the surfaces of cells.This
Endocytosis
Surrounding tissue
Receptor binding
Pseudopodium
Exocytosis
Movement of cell
Figure 2–16
Ameboid motion by a cell.
Chapter 2
25
occurs in only two places in the human body: on the
sufaces of the respiratory airways and on the inside 
surfaces of the uterine tubes (fallopian tubes) of the
reproductive tract. In the nasal cavity and lower respi-
ratory airways, the whiplike motion of cilia causes a
layer 

ways, the whiplike motion of cilia causes a
layer of mucus to move at a rate of about 1 cm/min
toward the pharynx, in this way continually clearing
these passageways of mucus and particles that have
become trapped in the mucus. In the uterine tubes, the
cilia cause slow movement of ﬂuid from the ostium of
the uterine tube toward the uterus cavity; this move-
ment of ﬂuid transports the ovum from the ovary to the
uterus.
As shown in Figure 2–17, a cilium has the appearance
of a sharp-pointed stra

 cilium has the appearance
of a sharp-pointed straight or curved hair that projects
2 to 4 micrometers from the surface of the cell. Many
cilia often project from a single cell—for instance, as
many as 200 cilia on the surface of each epithelial cell
inside the respiratory passageways. The cilium is
covered by an outcropping of the cell membrane, and it
is supported by 11 microtubules—9 double tubules
located around the periphery of the cilium, and 2 single
tubules down the center, as demonstrat

nd 2 single
tubules down the center, as demonstrated in the cross
section shown in Figure 2–17. Each cilium is an out-
growth of a structure that lies immediately beneath the
cell membrane, called the basal body of the cilium.
The ﬂagellum of a sperm is similar to a cilium; in fact,
it has much the same type of structure and same type
of contractile mechanism. The ﬂagellum, however, is
much longer and moves in quasi-sinusoidal waves
instead of whiplike movements.
In the inset of Figure 2–17, mov

iplike movements.
In the inset of Figure 2–17, movement of the cilium is
shown. The cilium moves forward with a sudden, rapid
whiplike stroke 10 to 20 times per second, bending
sharply where it projects from the surface of the cell.
Then it moves backward slowly to its initial position.
The rapid forward-thrusting,
whiplike movement
pushes the ﬂuid lying adjacent to the cell in the direc-
tion that the cilium moves; the slow, dragging movement
in the backward direction has almost no effect on ﬂu

 the backward direction has almost no effect on ﬂuid
movement.As a result, the ﬂuid is continually propelled
in the direction of the fast-forward stroke. Because most
ciliated cells have large numbers of cilia on their sur-
faces and because all the cilia are oriented in the same
direction, this is an effective means for moving ﬂuids
from one part of the surface to another.
Mechanism of Ciliary Movement. Although not all aspects of
ciliary movement are clear, we do know the following:
First, the

nt are clear, we do know the following:
First, the nine double tubules and the two single tubules
are all linked to one another by a complex of protein
cross-linkages; this total complex of tubules and cross-
linkages is called the axoneme. Second, even after
removal of the membrane and destruction of other ele-
ments of the cilium besides the axoneme, the cilium can
still beat under appropriate conditions. Third, there are
two necessary conditions for continued beating of the
axoneme after remo

ns for continued beating of the
axoneme after removal of the other structures of the
cilium: (1) the availability of ATP and (2) appropriate
ionic conditions, especially appropriate concentrations
of magnesium and calcium. Fourth, during forward
motion of the cilium, the double tubules on the front
edge of the cilium slide outward toward the tip of the
cilium, while those on the back edge remain in place.
Fifth, multiple protein arms composed of the protein
dynein, which has ATPase enzymatic act

the protein
dynein, which has ATPase enzymatic activity, project
from each double tubule toward an adjacent double
tubule.
Given this basic information, it has been determined
that the release of energy from ATP in contact with the
ATPase dynein arms causes the heads of these arms to
“crawl” rapidly along the surface of the adjacent double
tubule. If the front tubules crawl outward while the back
tubules remain stationary, this will cause bending.
The way in which cilia contraction is controlled

.
The way in which cilia contraction is controlled is not
understood.The cilia of some genetically abnormal cells
do not have the two central single tubules, and these
cilia fail to beat. Therefore, it is presumed that some
signal, perhaps an electrochemical signal, is transmitted
along these two central tubules to activate the dynein
arms.
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Tip
Cross section
Forward stroke
Backward stroke
Membrane
Filament
Cell
membrane
Basal body
Rootlet
Basal plate
Ciliary stalk
Figure 2–17
Structure and function of the cilium. (Modiﬁed from Satir P: 

nd function of the cilium. (Modiﬁed from Satir P: Cilia.
Sci Am 204:108, 1961. Copyright Donald Garber: Executor of the
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26
Unit I
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H
A
P
T
E
R
 
tl Acad Sci U S A 100:8053,
2003.
C
H
A
P
T
E
R
 
3
27
Genetic Control of Protein
Synthesis, Cell Function,
and Cell Reproduction
Virtually everyone knows that the genes, located in
the nuclei of all cells of the body, control heredity
from parents to children, but most people do not
realize that these same genes also control day-to-
day function of all the body’s cells. The genes
control cell function by determining which sub-
stances are synthesized within the cell—which struc-
tures, which en

sized within the cell—which struc-
tures, which enzymes, which chemicals.
Figure 3–1 shows the general schema of genetic control. Each gene, which is
a nucleic acid called deoxyribonucleic acid (DNA), automatically controls the
formation of another nucleic acid, ribonucleic acid (RNA); this RNA then
spreads throughout the cell to control the formation of a speciﬁc protein.
Because there are more than 30,000 different genes in each cell, it is theoreti-
cally possible to form a very large number 

oreti-
cally possible to form a very large number of different cellular proteins.
Some of the cellular proteins are structural proteins, which, in association with
various lipids and carbohydrates, form the structures of the various intracellu-
lar organelles discussed in Chapter 2. However, by far the majority of the pro-
teins are enzymes that catalyze the different chemical reactions in the cells. For
instance, enzymes promote all the oxidative reactions that supply energy to the
cell, and th

e reactions that supply energy to the
cell, and they promote synthesis of all the cell chemicals, such as lipids, glyco-
gen, and adenosine triphosphate (ATP).
In the cell nucleus, large numbers of genes are attached end on end in extremely
long double-stranded helical molecules of DNA having molecular weights
measured in the billions. A very short segment of such a molecule is shown in
Figure 3–2. This molecule is composed of several simple chemical compounds
bound tog

sed of several simple chemical compounds
bound together in a regular pattern, details of which are explained in the next
few paragraphs.
Basic Building Blocks of DNA. Figure 3–3 shows the basic chemical compounds
involved in the formation of DNA. These include (1) phosphoric acid, (2) a
sugar called deoxyribose, and (3) four nitrogenous bases (two purines, adenine
and guanine, and two pyrimidines, thymine and cytosine). The phosphoric acid
and deoxyribose form the two helical strands that are th

oxyribose form the two helical strands that are the backbone of the DNA
molecule, and the nitrogenous bases lie between the two strands and connect
them, as illustrated in Figure 3–6.
Nucleotides. The ﬁrst stage in the formation of DNA is to combine one mole-
cule of phosphoric acid, one molecule of deoxyribose, and one of the four bases
to form an acidic nucleotide. Four separate nucleotides are thus formed, one for
each of the four bases: deoxyadenylic, deoxythymidylic, deoxyguanylic, and
deox

adenylic, deoxythymidylic, deoxyguanylic, and
deoxycytidylic acids. Figure 3–4 shows the chemical structure of deoxyadenylic
acid, and Figure 3–5 shows simple symbols for the four nucleotides that form
DNA.
Organization of the Nucleotides to Form Two Strands of DNA Loosely Bound to Each Other.
Figure 3–6 shows the manner in which multiple numbers of nucleotides are
28
Unit I
Gene (DNA)
RNA formation
Protein formation
Cell function
Cell 

NA formation
Cell structure
Cell enzymes
Figure 3–1
General schema by which the genes control cell function.
Figure 3–2
The helical, double-stranded structure of the gene. The outside
strands are composed of phosphoric acid and the sugar deoxyri-
bose. The internal molecules connecting the two strands of the
helix are purine and pyrimidine bases; these determine the “code”
of the gene.
Phosphoric acid
Deoxyribose
Bases
Purines
Pyrimidines
Guanine
Cytosine
Thymine

ases
Purines
Guanine
Cytosine
Thymine
C
C
O
C
C
C
H
H
H
H
N
N
N
N
N
H
Adenine
C
C
N
C
C
C
H
C
C
C
C
H
H
O
H
H
H
O
O
H
O
H
P
O
O
H
O
H
H
N
H
H
N
N
N
O
C
H
C
C
H
C
H
C
O
N
H
N
N
H
H
H
H
C
C
O
O
N
C
N
H
H
H
H
H
H
C
H
C
H
Figure 3–3
The basic building blocks of DNA.
Chapter 3
Genetic Control of Protein Synthesis, Cell Function, and Cell Reproduction
29
C
C
N
C
C
C
H
H
N
H
H
N
N
N
P
O
O
O
H
C
C
C
C
H
H
O
H
H
O
O
C
H
H
H
H
H
Adenine
Phosphate
Figure 3–4
Deoxyadenylic acid, one 

te
Figure 3–4
Deoxyadenylic acid, one of the nucleotides that make up DNA.
D
A
P
D
G
P
D
T
P
D
C
P
Deoxyadenylic acid
Deoxyguanylic acid
Deoxythymidylic acid
Deoxycytidylic acid
Figure 3–5
Symbols for the four nucleotides that combine to form DNA. 
Each nucleotide contains phosphoric acid (P), deoxyribose (D),
and one of the four nucleotide bases: A, adenine; T, thymine; 
G, guanine; or C, cytosine.
D
P
P
D
P
P
D
P
D
P
D
P
D
P
D
P
D
P
D
P
D
P
D
P
D
P
D
P
D
P
D
P
D
P
D
D
P
C
G
C
G
G
C

D
P
D
P
D
P
D
P
D
P
D
P
D
P
D
P
D
D
P
C
G
C
G
G
C
T
A
C
G
T
A
G
C
A
T
A
T
Figure 3–6
Arrangement of deoxyribose nucleotides
in a double strand of DNA.
bound together to form two strands of DNA. The two
strands are, in turn, loosely bonded with each other by
weak cross-linkages, illustrated in Figure 3–6 by the
central dashed lines. Note that the backbone of each
DNA strand is comprised of alternating phosphoric
acid and deoxyribose molecules. In turn, purine and
pyrimidine bases are attached to

 turn, purine and
pyrimidine bases are attached to the sides of the
deoxyribose molecules. Then, by means of loose
hydrogen bonds (dashed lines) between the purine
and pyrimidine bases, the two respective DNA strands
are held together. But note the following:
1. Each purine base adenine of one strand always
bonds with a pyrimidine base thymine of the other
strand, and
2. Each purine base guanine always bonds with a
pyrimidine base cytosine.
Thus, in Figure 3–6, the sequence of complementary
pair

 in Figure 3–6, the sequence of complementary
pairs of bases is CG, CG, GC,TA, CG,TA, GC,AT, and
AT. Because of the looseness of the hydrogen bonds,
the two strands can pull apart with ease, and they do
so many times during the course of their function in
the cell.
To put the DNA of Figure 3–6 into its proper phys-
ical perspective, one could merely pick up the two ends
and twist them into a helix. Ten pairs of nucleotides
are present in each full turn of the helix in the DNA
molecule, as shown 

l turn of the helix in the DNA
molecule, as shown in Figure 3–2.
The importance of DNA lies in its ability to control
the formation of proteins in the cell. It does this by
means of the so-called genetic code. That is, when the
two strands of a DNA molecule are split apart, this
exposes the purine and pyrimidine bases projecting to
the side of each DNA strand, as shown by the top
strand in Figure 3–7. It is these projecting bases that
form the genetic code.
30
Unit I

t
30
Unit I
The genetic code consists of successive “triplets” of
bases—that is, each three successive bases is a code
word. The successive triplets eventually control the
sequence of amino acids in a protein molecule that is
to be synthesized in the cell. Note in Figure 3–6 that
the top strand of DNA, reading from left to right, has
the genetic code GGC, AGA, CTT, the triplets being
separated from one another by 

 the triplets being
separated from one another by the arrows. As we
follow this genetic code through Figures 3–7 and 3–8,
we see that these three respective triplets are respon-
sible for successive placement of the three amino
acids, proline, serine, and glutamic acid, in a newly
formed molecule of protein.
The DNA Code in the Cell
Nucleus Is Transferred to 
an RNA Code in the Cell
Cytoplasm—The Process 
Because the DNA is located in the nucleus of the cell,
yet most of the fun

ed in the nucleus of the cell,
yet most of the functions of the cell are carried out in
the cytoplasm, there must be some means for the DNA
genes of the nucleus to control the chemical reactions
of the cytoplasm. This is achieved through the inter-
mediary of another type of nucleic acid, RNA, the for-
mation of which is controlled by the DNA of the
nucleus.Thus, as shown in Figure 3–7, the code is trans-
ferred to the RNA; this process is called transcription.
The RNA, in turn, diffuses from th

 transcription.
The RNA, in turn, diffuses from the nucleus through
nuclear pores into the cytoplasmic compartment,
where it controls protein synthesis.
During synthesis of RNA, the two strands of the DNA
molecule separate temporarily; one of these strands is
used as a template for synthesis of an RNA molecule.
The code triplets in the DNA cause formation of com-
plementary code triplets (called codons) in the RNA;
these codons, in turn, will control the sequence of
amino acids 

in turn, will control the sequence of
amino acids in a protein to be synthesized in the cell
cytoplasm.
Basic Building Blocks of RNA. The basic building blocks of
RNA are almost the same as those of DNA, except for
two differences.First,the sugar deoxyribose is not used
in the formation of RNA. In its place is another sugar
of slightly different composition, ribose, containing an
extra hydroxyl ion appended to the ribose ring struc-
ture. Second, thymine is replaced by another pyrimi-
dine, urac

 thymine is replaced by another pyrimi-
dine, uracil.
Formation of RNA Nucleotides. The basic building blocks
of RNA form RNA nucleotides, exactly as previously
described for DNA synthesis. Here again, four sepa-
rate nucleotides are used in the formation of RNA.
These nucleotides contain the bases adenine, guanine,
cytosine, and uracil. Note that these are the same bases
D
G
P
D
G
P
D
C
P
D
A
P
D
G
P
D
A
P
D
P
D
P
D
T
P
C
T
DNA strand
RNA molecule
RNA polymerase
R
C
P
P
R
C
P
R
G
R

le
R
C
P
P
R
C
P
R
G
R
U
P
P
R
C
P
R
U
P
P
R
G
P
P
P
R
A
P
Figure 3–7
Combination of ribose nucleotides
with a strand of DNA to form a 
molecule of RNA that carries the
genetic code from the gene to the 
cytoplasm. The RNA polymerase
enzyme moves along the DNA
strand and builds the RNA molecule.
Proline
Serine
Glutamic acid
R
C
P
P
R
C
P
R
G
P
R
U
P
R
C
P
R
U
P
R
G
P
R
A
P
R
A
Figure 3–8
Portion of an RNA molecule, showing three RNA
“codons”—CCG, UCU, and GAA—which co

wing three RNA
“codons”—CCG, UCU, and GAA—which control
attachment of the three amino acids proline,
serine, and glutamic acid, respectively, to the
growing RNA chain.
Chapter 3
31
as in DNA, except that uracil in RNA replaces
thymine in DNA.
“Activation” of the RNA Nucleotides. The next step in 
the synthesis of RNA is “activation” of the RNA
nucleotides by an enzyme, RNA polymerase. This
occurs by adding to each nucleot

 polymerase. This
occurs by adding to each nucleotide two extra phos-
phate radicals to form triphosphates (shown in Figure
3–7 by the two RNA nucleotides to the far right during
RNA chain formation). These last two phosphates are
combined with the nucleotide by high-energy phos-
phate bonds derived from ATP in the cell.
The result of this activation process is that 
large quantities of ATP energy are made available to
each of the nucleotides, and this energy is used to
promote the chemical reac

d this energy is used to
promote the chemical reactions that add each new
RNA nucleotide at the end of the developing RNA
chain.
Assembly of the RNA Chain from
Activated Nucleotides Using the DNA
Strand as a Template—The Process 
of “Transcription”
Assembly of the RNA molecule is accomplished in 
the manner shown in Figure 3–7 under the inﬂuence
of an enzyme, RNA polymerase. This is a large protein
enzyme that has many functional properties necessary
for formation of the RNA molecule. They are a

sary
for formation of the RNA molecule. They are as
follows:
1. In the DNA strand immediately ahead of the
initial gene is a sequence of nucleotides called 
the promoter. The RNA polymerase has an
appropriate complementary structure that
recognizes this promoter and becomes attached to
it. This is the essential step for initiating formation
of the RNA molecule.
2. After the RNA polymerase attaches to the
promoter, the polymerase causes unwinding of
about two turns of the DNA helix and separation

of
of the unwound portions of the two strands.
3. Then the polymerase moves along the DNA
strand, temporarily unwinding and separating the
two DNA strands at each stage of its movement.
As it moves along, it adds at each stage a new
activated RNA nucleotide to the end of the newly
forming RNA chain by the following steps:
a. First, it causes a hydrogen bond to form
between the end base of the DNA strand and
the base of an RNA nucleotide in the
nucl

rand and
nucleoplasm.
b. Then, one at a time, the RNA polymerase
breaks two of the three phosphate radicals
away from each of these RNA nucleotides,
liberating large amounts of energy from the
broken high-energy phosphate bonds; this
energy is used to cause covalent linkage of the
remaining phosphate on the nucleotide with
the ribose on the end of the growing RNA
chain.
c. When the RNA polymerase reaches the 
end of the DNA gene, it encounters a new
sequence 

sequence of DNA nucleotides called the 
chain-terminating sequence; this causes the
polymerase and the newly formed RNA chain
to break away from the DNA strand. Then the
polymerase can be used again and again to
form still more new RNA chains.
d. As the new RNA strand is formed, its weak
hydrogen bonds with the DNA template break
away, because the DNA has a high afﬁnity for
rebonding with its own complementary DNA
strand. Thus, the RNA chain is forced awa

tary DNA
strand. Thus, the RNA chain is forced away
from the DNA and is released into the
Thus, the code that is present in the DNA strand is
eventually transmitted in complementary form to the
RNA chain. The ribose nucleotide bases always
combine with the deoxyribose bases in the following
combinations:
DNA Base
RNA Base
guanine
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 
cytosine
cytosine   . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

. . . . . . . . . . . . . . . . . . . . . . . . . . 
guanine
adenine    . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 
uracil
thymine   . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 
adenine
Three Different Types of RNA. There are three different
types of RNA, each of which plays an independent and
entirely different role in protein formation:
1. Messenger RNA, which carries the genetic code to
the cytoplasm for controlling the type of protein
formed.

plasm for controlling the type of protein
formed.
2. Transfer RNA, which transports activated amino
acids to the ribosomes to be used in assembling
the protein molecule.
3. Ribosomal RNA, which, along with about 75
different proteins, forms ribosomes, the physical
and chemical structures on which protein
molecules are actually assembled.
Messenger RNA molecules are long, single RNA
strands that are suspended in the cytoplasm. These
molecules are composed of several hundr

asm. These
molecules are composed of several hundred to several
thousand RNA nucleotides in unpaired strands, and
they contain codons that are exactly complementary
to the code triplets of the DNA genes. Figure 3–8
shows a small segment of a molecule of messenger
RNA. Its codons are CCG, UCU, and GAA.These are
the codons for the amino acids proline, serine, and glu-
tamic acid. The transcription of these codons from the
DNA molecule to the RNA molecule is shown in
Figure 3–7.
RNA Codons for the 

lecule is shown in
RNA Codons for the Different Amino Acids. Table 3–1 gives
the RNA codons for the 20 common amino acids
32
Unit I
found in protein molecules. Note that most of the
amino acids are represented by more than one codon;
also, one codon represents the signal “start manufac-
turing the protein molecule,” and three codons repre-
sent “stop manufacturing the protein molecule.” In
Table 3–1, these two types of codon

 molecule.” In
Table 3–1, these two types of codons are designated CI
for “chain-initiating” and CT for “chain-terminating.”
Another type of RNA that plays an essential role in
protein synthesis is called transfer RNA, because it
transfers amino acid molecules to protein molecules as
the protein is being synthesized. Each type of transfer
RNA combines speciﬁcally with 1 of the 20 amino
acids that are to be incorporated into proteins. The
transfer RNA then acts as a ca

 into proteins. The
transfer RNA then acts as a carrier to transport its 
speciﬁc type of amino acid to the ribosomes, where
protein molecules are forming. In the ribosomes, each
speciﬁc type of transfer RNA recognizes a particular
codon on the messenger RNA (described later) and
thereby delivers the appropriate amino acid to the
appropriate place in the chain of the newly forming
protein molecule.
Transfer RNA, which contains only about 80
nucleotides, is a relatively small molecule in com-
par

otides, is a relatively small molecule in com-
parison with messenger RNA. It is a folded chain of
nucleotides with a cloverleaf appearance similar to
that shown in Figure 3–9. At one end of the molecule
is always an adenylic acid; it is to this that the trans-
ported amino acid attaches at a hydroxyl group of the
ribose in the adenylic acid.
Because the function of transfer RNA is to cause
attachment of a speciﬁc amino acid to a forming
protein chain, it is essential that each type of transfer

chain, it is essential that each type of transfer
RNA also have speciﬁcity for a particular codon in the
messenger RNA. The speciﬁc code in the transfer
RNA that allows it to recognize a speciﬁc codon is
again a triplet of nucleotide bases and is called an anti-
codon. This is located approximately in the middle 
of the transfer RNA molecule (at the bottom of the
cloverleaf conﬁguration shown in Figure 3–9). During
formation of the protein molecule, the anticodon bases
combine loosely by hydroge

le, the anticodon bases
combine loosely by hydrogen bonding with the codon
Forming protein
Alanine
Cysteine
Histidine
Alanine
Phenylalanine
Serine
Proline
Transfer RNA
Ribosome
Ribosome
Messenger
RNA movement
Start codon
AUG GCC UGU CAU GCC UUU UCC CCC AAA CAG GAC UAU
GGG
Figure 3–9
A messenger RNA strand is moving through two
ribosomes. As each “codon” passes through, 
an amino acid is added to the growing protein
chain, which is shown in the right-hand ribo-
some. The transfer RNA molecule tra

ght-hand ribo-
some. The transfer RNA molecule transports
each speciﬁc amino acid to the newly forming
protein.
Table 3–1
RNA Codons for Amino Acids and for Start and Stop
Amino Acid
RNA
Codons
Alanine
GCU
GCC
GCA
GCG
Arginine
CGU
CGC
CGA
CGG
AGA
AGG
Asparagine
AAU
AAC
Aspartic acid
GAU
GAC
Cysteine
UGU
UGC
GAA
GAG
Glutamine
CAA
CAG
Glycine
GGU
GGC
GGA
GGG
Histidine
CAU
CAC
Isoleucine
AUU
AUC
AUA
Leucine
CUU
CUC
CUA
CUG
UUA
UUG
Lysine
AAA
AAG
Methionine
AUG
UUU
UUC
Pr

ne
AAA
AAG
AUG
UUU
UUC
Proline
CCU
CCC
CCA
CCG
Serine
UCU
UCC
UCA
UCG
AGC
AGU
Threonine
ACU
ACC
ACA
ACG
UGG
Tyrosine
UAU
UAC
Valine
GUU
GUC
GUA
GUG
Start (CI)
AUG
Stop (CT)
UAA
UAG
UGA
CI, chain-initiating; CT, chain-terminating.
Chapter 3
33
bases of the messenger RNA. In this way, the respec-
tive amino acids are lined up one after another along
the messenger RNA chain, thus establish

ther along
the messenger RNA chain, thus establishing the
appropriate sequence of amino acids in the newly
forming protein molecule.
The third type of RNA in the cell is ribosomal RNA;
it constitutes about 60 per cent of the ribosome. The
remainder of the ribosome is protein, containing about
75 types of proteins that are both structural proteins
and enzymes needed in the manufacture of protein
molecules.
The ribosome is the physical structure in the cyto-
plasm on which protein mo

l structure in the cyto-
plasm on which protein molecules are actually syn-
thesized. However, it always functions in association
with the other two types of RNA as well: transfer RNA
transports amino acids to the ribosome for incorpora-
tion into the developing protein molecule, whereas
messenger RNA provides the information necessary
for sequencing the amino acids in proper order for
each speciﬁc type of protein to be manufactured.
Thus, the ribosome acts as a manufacturing plant in
which the 

bosome acts as a manufacturing plant in
which the protein molecules are formed.
Formation of Ribosomes in the Nucleolus. The DNA genes
for formation of ribosomal RNA are located in ﬁve
pairs of chromosomes in the nucleus, and each of these
chromosomes contains many duplicates of these par-
ticular genes because of the large amounts of riboso-
mal RNA required for cellular function.
As the ribosomal RNA forms, it collects in the
nucleolus, a specialized structure lying adjacent to the
chromosomes

alized structure lying adjacent to the
chromosomes.When large amounts of ribosomal RNA
are being synthesized, as occurs in cells that manufac-
ture large amounts of protein, the nucleolus is a large
structure, whereas in cells that synthesize little protein,
the nucleolus may not even be seen. Ribosomal RNA
is specially processed in the nucleolus, where it binds
with “ribosomal proteins” to form granular condensa-
tion products that are primordial subunits of ribo-
somes. These subunits are then

l subunits of ribo-
somes. These subunits are then released from the
nucleolus and transported through the large pores of
the nuclear envelope to almost all parts of the cyto-
plasm. After the subunits enter the cytoplasm, they 
are assembled to form mature, functional ribosomes.
Therefore, proteins are formed in the cytoplasm of the
cell, but not in the cell nucleus, because the nucleus
does not contain mature ribosomes.
Formation of Proteins on 
the Ribosomes—The Process 
of “Translation”
When

When a molecule of messenger RNA comes in contact
with a ribosome, it travels through the ribosome,
beginning at a predetermined end of the RNA mole-
cule speciﬁed by an appropriate sequence of RNA
bases called the “chain-initiating” codon. Then, as
shown in Figure 3–9, while the messenger RNA travels
through the ribosome, a protein molecule is formed—
a process called translation. Thus, the ribosome reads
the codons of the messenger RNA in much the 

reads
the codons of the messenger RNA in much the same
way that a tape is “read” as it passes through the play-
back head of a tape recorder. Then, when a “stop” (or
“chain-terminating”) codon slips past the ribosome,
the end of a protein molecule is signaled and the
protein molecule is freed into the cytoplasm.
Polyribosomes. A single messenger RNA molecule can
form protein molecules in several ribosomes at the
same time because the initial end of the RNA strand
can pass to a successive ribosom

of the RNA strand
can pass to a successive ribosome as it leaves the ﬁrst,
as shown at the bottom left in Figure 3–9 and in Figure
3–10. The protein molecules are in different stages of
development in each ribosome. As a result, clusters of
ribosomes frequently occur, 3 to 10 ribosomes being
attached to a single messenger RNA at the same time.
These clusters are called polyribosomes.
It is especially important to note that a messenger
RNA can cause the formation of a protein molecule
in any ribo

se the formation of a protein molecule
in any ribosome; that is, there is no speciﬁcity of ribo-
somes for given types of protein. The ribosome is
simply the physical manufacturing plant in which the
chemical reactions take place.
Many Ribosomes Attach to the Endoplasmic Reticulum. In
Chapter 2, it was noted that many ribosomes become
attached to the endoplasmic reticulum. This occurs
because the initial ends of many forming protein mol-
ecules have amino acid sequences that immediately
attach t

ave amino acid sequences that immediately
attach to speciﬁc receptor sites on the endoplasmic
reticulum; this causes these molecules to penetrate the
reticulum wall and enter the endoplasmic reticulum
matrix. This gives a granular appearance to those por-
tions of the reticulum where proteins are being formed
and entering the matrix of the reticulum.
Figure 3–10 shows the functional relation of mes-
senger RNA to the ribosomes and the manner in
which the ribosomes attach to the membrane of the
e

hich the ribosomes attach to the membrane of the
endoplasmic reticulum. Note the process of translation
occurring in several ribosomes at the same time in
response to the same strand of messenger RNA. Note
also the newly forming polypeptide (protein) chains
passing through the endoplasmic reticulum membrane
into the endoplasmic matrix.
Yet it should be noted that except in glandular cells
in which large amounts of protein-containing secre-
tory vesicles are formed, most proteins synthesized by
t

esicles are formed, most proteins synthesized by
the ribosomes are released directly into the cytosol
instead of into the endoplasmic reticulum. These pro-
teins are enzymes and internal structural proteins of
the cell.
Chemical Steps in Protein Synthesis. Some of the chemical
events that occur in synthesis of a protein molecule are
shown in Figure 3–11.This ﬁgure shows representative
reactions for three separate amino acids, AA1, AA2,
and AA20. The stages of the reactions are the follow-
ing: (

The stages of the reactions are the follow-
ing: (1) Each amino acid is activated by a chemical
34
Unit I
process in which ATP combines with the amino acid
to form an adenosine monophosphate complex with the
amino acid, giving up two high-energy phosphate
bonds in the process. (2) The activated amino acid,
having an excess of energy, then combines with its 
speciﬁc transfer RNA to form an amino acid–tRNA
complex and, at the same time, r

n amino acid–tRNA
complex and, at the same time, releases the adenosine
monophosphate. (3) The transfer RNA carrying the
amino acid complex then comes in contact with the
messenger RNA molecule in the ribosome, where 
the anticodon of the transfer RNA attaches tem-
porarily to its speciﬁc codon of the messenger RNA,
thus lining up the amino acid in appropriate sequence
to form a protein molecule. Then, under the inﬂuence
of the enzyme peptidyl transferase (one of the proteins
in the ribosome), p

ansferase (one of the proteins
in the ribosome), peptide bonds are formed between
the successive amino acids, thus adding progressively
to the protein chain. These chemical events require
energy from two additional high-energy phosphate
bonds, making a total of four high-energy bonds used
for each amino acid added to the protein chain. Thus,
the synthesis of proteins is one of the most energy-con-
suming processes of the cell.
Peptide Linkage. The successive amino acids in the
protein chain comb

e successive amino acids in the
protein chain combine with one another according to
the typical reaction:
Messenger
RNA
Ribosome
Amino acid
chain
Large
subunit
Small
subunit
Figure 3–10
Physical structure of the ribosomes,
as well as their functional relation to
messenger RNA, transfer RNA, and
the endoplasmic reticulum during
the formation of protein molecules.
(Courtesy of Dr. Don W. Fawcett,
Montana.)
Protein chain
+
ATP
tRNA1
tRNA1
+
GCC
Messenger RNA
GCC
GTP
+
ATP
A

TP
tRNA1
tRNA1
+
GCC
GCC
GTP
+
ATP
AMP
+
tRNA2
tRNA2
+
+
ATP
AMP
+
tRNA20
tRNA20
+
tRNA5
UGU
UGU
GTP
tRNA3
AAU
AAU
GTP
tRNA9
CAU
CAU
GTP
tRNA2
CGU
CGU
GTP
tRNA13
AUG
AUG
GTP
tRNA20
GUU
GUU
GTP
Activated amino acid
Amino acid
RNA-amino acyl complex
Complex between tRNA,
messenger RNA, and
amino acid
AMP
+
tRNA1
¸
Ô
Ô
˝
Ô
Ô
˛
AA1
AA1
AA1
AA1
AA1
AA2
AA2
AA2
AA20
AA20
AA20
AA5
AA5
AA3
AA3
AA9
AA9
AA2
AA2
AA13
AA13
AA20
AA20
Figure 3–11
Chemical events in the formation of
a protein mol

a protein molecule.
O
OH
C
C
R
NH2
R
COOH
C
N
H
H
O
C
C
R
NH2
H2O
R
COOH
C
N
H
+
+
Chapter 3
35
There are basically two methods by which the bio-
chemical activities in the cell are controlled. One of
these is genetic regulation, in which the degree of acti-
vation of the genes themselves is controlled, and the
other is enzyme regulation, in which the activity levels
of already formed 

n, in which the activity levels
of already formed enzymes in the cell are controlled.
The “Operon” of the Cell and Its Control of Biochemical Syn-
thesis—Function of the Promoter. Synthesis of a cellular
biochemical product usually requires a series of reac-
tions, and each of these reactions is catalyzed by a
special protein enzyme. Formation of all the enzymes
needed for the synthetic process often is controlled by
a sequence of genes located one after the other on 
the same

 of genes located one after the other on 
the same chromosomal DNA strand. This area of the
DNA strand is called an operon, and the genes respon-
sible for forming the respective enzymes are called
structural genes. In Figure 3–12, three respective 
structural genes are shown in an operon, and it is
demonstrated that they control the formation of three
respective enzymes that in turn cause synthesis of a
speciﬁc intracellular product.
Note in the ﬁgure the segment on the DNA strand
called the pr

 ﬁgure the segment on the DNA strand
called the promoter. This is a group of nucleotides that
has speciﬁc afﬁnity for RNA polymerase, as already
discussed. The polymerase must bind with this pro-
moter before it can begin traveling along the DNA
strand to synthesize RNA. Therefore, the promoter is
an essential element for activating the operon.
Control of the Operon by a “Repressor Protein”—The 
“Repressor Operator.” Also note in Figure 3–12 an addi-
tional band of nucleotides lying in the middl

ddi-
tional band of nucleotides lying in the middle of the
promoter. This area is called a repressor operator
because a “regulatory” protein can bind here and
prevent attachment of RNA polymerase to the pro-
moter, thereby blocking transcription of the genes of
In this chemical reaction, a hydroxyl radical (OH–)
is removed from the COOH portion of the ﬁrst amino
acid, and a hydrogen (H+) of the NH2 portion of the
other amino acid is removed. These combine to form
water, and the two reactive site

e combine to form
water, and the two reactive sites left on the two suc-
cessive amino acids bond with each other, resulting 
in a single molecule. This process is called peptide
linkage. As each additional amino acid is added, an
additional peptide linkage is formed.
Synthesis of Other
Substances in the Cell
Many thousand protein enzymes formed in the
manner just described control essentially all the other
chemical reactions that take place in cells. These
enzymes promote synthesis of lipids, g

ells. These
enzymes promote synthesis of lipids, glycogen, purines,
pyrimidines, and hundreds of other substances. We
discuss many of these synthetic processes in relation
to carbohydrate, lipid, and protein metabolism in
Chapters 67 through 69. It is by means of all these 
substances that the many functions of the cells are 
Control of Gene Function and
From our discussion thus far, it is clear that the genes
control both the physical and the chemical fu

enes
control both the physical and the chemical functions
of the cells. However, the degree of activation of
respective genes must be controlled as well; otherwise,
some parts of the cell might overgrow or some chem-
ical reactions might overact until they kill the cell.
Each cell has powerful internal feedback control
mechanisms that keep the various functional opera-
tions of the cell in step with one another. For each gene
(more than 30,000 genes in all), there is at least one
such feedback m

nes in all), there is at least one
such feedback mechanism.
¸
Ô
Ô
Ô
˝
Ô
Ô
Ô
˛
Gene A
Promoter
Inhibition of
the operator
Enzyme A
Enzyme B
Enzyme C
Synthesized
product
Substrates
(Negative feedback)
Repressor
operator
Activator
operator
Operon
Gene B
Gene C
Figure 3–12
Function of an operon to control synthesis of a non
protein intracellular product, such as an intra-
cellular metabolic chemical. Note that the synthe-
sized product exerts negative feedback to inh

the-
sized product exerts negative feedback to inhibit
the function of the operon, in this way automatically
controlling the concentration of the product itself.
36
Unit I
this operon. Such a negative regulatory protein is
called a repressor protein.
Control of the Operon by an “Activator Protein”—The “Activa-
tor Operator.” Note in Figure 3–12 another operator,
called the activator operator, that lies adjacent to but
ahead of the promo

ator, that lies adjacent to but
ahead of the promoter. When a regulatory protein
binds to this operator, it helps attract the RNA po-
lymerase to the promoter, in this way activating the
operon. Therefore, a regulatory protein of this type is
called an activator protein.
Negative Feedback Control of the Operon. Finally, note in
Figure 3–12 that the presence of a critical amount 
of a synthesized product in the cell can cause negative
feedback inhibition of the operon that is responsible
for its 

ibition of the operon that is responsible
for its synthesis. It can do this either by causing a 
regulatory repressor protein to bind at the repressor
operator or by causing a regulatory activator protein
to break its bond with the activator operator. In either
case, the operon becomes inhibited. Therefore, once
the required synthesized product has become abun-
dant enough for proper cell function, the operon
becomes dormant. Conversely, when the synthesized
product becomes degraded in the cell 

 synthesized
product becomes degraded in the cell and its concen-
tration decreases, the operon once again becomes
active. In this way, the desired concentration of the
product is controlled automatically.
Other Mechanisms for Control of Transcription by the Operon.
Variations in the basic mechanism for control of the
operon have been discovered with rapidity in the past
2 decades. Without giving details, let us list some of
them:
1. An operon frequently is controlled by a
regulatory gene locate

equently is controlled by a
regulatory gene located elsewhere in the genetic
complex of the nucleus. That is, the regulatory
gene causes the formation of a regulatory protein
that in turn acts either as an activator or as a
repressor substance to control the operon.
2. Occasionally, many different operons are
controlled at the same time by the same
regulatory protein. In some instances, the same
regulatory protein functions as an activator 
for one operon and as a repressor for another
operon. W

ne operon and as a repressor for another
operon. When multiple operons are controlled
simultaneously in this manner, all the operons
that function together are called a regulon.
3. Some operons are controlled not at the starting
point of transcription on the DNA strand but
farther along the strand. Sometimes the control is
not even at the DNA strand itself but during the
processing of the RNA molecules in the nucleus
before they are released into the cytoplasm;
rarely, control might occur at the

 the cytoplasm;
rarely, control might occur at the level of protein
formation in the cytoplasm during RNA
translation by the ribosomes.
4. In nucleated cells, the nuclear DNA is packaged
in speciﬁc structural units, the chromosomes.
Within each chromosome, the DNA is wound
around small proteins called histones, which in
turn are held tightly together in a compacted state
by still other proteins. As long as the DNA is in
this compacted state, it cannot function to form
RNA. However, multiple cont

annot function to form
RNA. However, multiple control mechanisms are
beginning to be discovered that can cause selected
areas of chromosomes to become decompacted
one part at a time so that partial RNA
transcription can occur. Even then, some speciﬁc
“transcriptor factor” controls the actual rate of
transcription by the separate operon in the
chromosome. Thus, still higher orders of control
are used for establishing proper cell function. In
addition, signals from outside the cell, such as
some o

ion, signals from outside the cell, such as
some of the body’s hormones, can activate speciﬁc
chromosomal areas and speciﬁc transcription
factors, thus controlling the chemical machinery
for function of the cell.
Because there are more than 30,000 different genes
in each human cell, the large number of ways in which
genetic activity can be controlled is not surprising.The
gene control systems are especially important for con-
trolling intracellular concentrations of amino acids,
amino acid deriv

ar concentrations of amino acids,
amino acid derivatives, and intermediate substrates
and products of carbohydrate, lipid, and protein
metabolism.
Control of Intracellular Function 
by Enzyme Regulation
In addition to control of cell function by genetic 
regulation, some cell activities are controlled by intra-
cellular inhibitors or activators that act directly on spe-
ciﬁc intracellular enzymes. Thus, enzyme regulation
represents a second category of mechanisms by which
cellular biochemical fu

ory of mechanisms by which
cellular biochemical functions can be controlled.
Enzyme Inhibition. Some chemical substances formed in
the cell have direct feedback effects in inhibiting the
speciﬁc enzyme systems that synthesize them. Almost
always the synthesized product acts on the ﬁrst
enzyme in a sequence, rather than on the subsequent
enzymes, usually binding directly with the enzyme and
causing an allosteric conformational change that inac-
tivates it. One can readily recognize the importance

vates it. One can readily recognize the importance of
inactivating the ﬁrst enzyme: this prevents buildup of
intermediary products that are not used.
Enzyme inhibition is another example of negative
feedback control; it is responsible for controlling 
intracellular concentrations of multiple amino acids,
purines, pyrimidines, vitamins, and other substances.
Enzyme Activation. Enzymes that are normally inactive
often can be activated when needed. An example of
this occurs when most of the ATP has

An example of
this occurs when most of the ATP has been depleted
in a cell. In this case, a considerable amount of cyclic
adenosine monophosphate (cAMP) begins to be
formed as a breakdown product of the ATP; the pres-
ence of this cAMP, in turn, immediately activates the
glycogen-splitting enzyme phosphorylase, liberating
glucose molecules that are rapidly metabolized and
their energy used for replenishment of the ATP stores.
Thus, cAMP acts as an enzyme activator for the
Chapter 3
Genetic Contr

n enzyme activator for the
Chapter 3
37
enzyme phosphorylase and thereby helps control
intracellular ATP concentration.
Another interesting instance of both enzyme inhi-
bition and enzyme activation occurs in the formation
of the purines and pyrimidines. These substances are
needed by the cell in approximately equal quantities
for formation of DNA and RNA. When purines are
formed, they inhibit the enzymes that are require

formed, they inhibit the enzymes that are required for
formation of additional purines. However, they acti-
vate the enzymes for formation of pyrimidines. Con-
versely, the pyrimidines inhibit their own enzymes but
activate the purine enzymes. In this way, there is con-
tinual cross-feed between the synthesizing systems 
for these two substances, resulting in almost exactly 
equal amounts of the two substances in the cells at all
times.
Summary. In summary, there are two principal methods
by wh

 In summary, there are two principal methods
by which cells control proper proportions and proper
quantities of different cellular constituents: (1) the
mechanism of genetic regulation and (2) the mecha-
nism of enzyme regulation. The genes can be either
activated or inhibited, and likewise, the enzyme
systems can be either activated or inhibited.These reg-
ulatory mechanisms most often function as feedback
control systems that continually monitor the cell’s 
biochemical composition and make cor

r the cell’s 
biochemical composition and make corrections as
needed. But on occasion, substances from without the
cell (especially some of the hormones discussed
throughout this text) also control the intracellular 
biochemical reactions by activating or inhibiting one
or more of the intracellular control systems.
The DNA-Genetic System Also
Controls Cell Reproduction
Cell reproduction is another example of the ubiqui-
tous role that the DNA-genetic system plays in all life
processes. The genes

etic system plays in all life
processes. The genes and their regulatory mechanisms
determine the growth characteristics of the cells and
also when or whether these cells will divide to form
new cells. In this way, the all-important genetic system
controls each stage in the development of the human
being, from the single-cell fertilized ovum to the whole
functioning body.Thus, if there is any central theme to
life, it is the DNA-genetic system.
Life Cycle of the Cell. The life cycle of a cell is 

fe Cycle of the Cell. The life cycle of a cell is the period
from cell reproduction to the next cell reproduction.
When mammalian cells are not inhibited and are repro-
ducing as rapidly as they can, this life cycle may be as
little as 10 to 30 hours. It is terminated by a series of
distinct physical events called mitosis that cause divi-
sion of the cell into two new daughter cells.The events
of mitosis are shown in Figure 3–13 and are described
later. The actual stage of mitosis, however, last

later. The actual stage of mitosis, however, lasts for
only about 30 minutes, so that more than 95 per cent
of the life cycle of even rapidly reproducing cells is
represented by the interval between mitosis, called
interphase.
Except in special conditions of rapid cellular repro-
duction, inhibitory factors almost always slow or stop
the uninhibited life cycle of the cell. Therefore, differ-
ent cells of the body actually have life cycle periods
that vary from as little as 10 hours for highly s

that vary from as little as 10 hours for highly stimu-
lated bone marrow cells to an entire lifetime of the
human body for most nerve cells.
Cell Reproduction Begins 
with Replication of DNA
As is true of almost all other important events in the
cell, reproduction begins in the nucleus itself. The ﬁrst
step is replication (duplication) of all DNA in the chro-
mosomes. Only after this has occurred can mitosis take
place.
The DNA begins to be duplicated some 5 to 10
hours before mitosis, and this

icated some 5 to 10
hours before mitosis, and this is completed in 4 to 8
hours. The net result is two exact replicas of all DNA.
These replicas become the DNA in the two new
A
B
C
D
E
F
G
H
Nucleolus
Centriole
Aster
Nuclear
membrane
Figure 3–13
Stages of cell reproduction. A, B,
and C,
Prophase. D,
Prometaphase. E, Metaphase. F, Anaphase. G and H, Telophase.
(From Margaret C. Gladbach, Estate of Mary E. and Dan Todd,
Kansas.)
38
Unit I
Introduction to Physiology: The Cell 

.)
38
Unit I
daughter cells that will be formed at mitosis. After
replication of the DNA, there is another period of 1 to
2 hours before mitosis begins abruptly. Even during
this period, preliminary changes are beginning to take
place that will lead to the mitotic process.
Chemical and Physical Events of DNA Replication. DNA is
replicated in much the same way that RNA is tran-
scribed in response to DNA, except for a few impor-
tant dif

 response to DNA, except for a few impor-
tant differences:
1. Both strands of the DNA in each chromosome
are replicated, not simply one of them.
2. Both entire strands of the DNA helix are
replicated from end to end, rather than small
portions of them, as occurs in the transcription of
RNA.
3. The principal enzymes for replicating DNA 
are a complex of multiple enzymes called DNA
polymerase, which is comparable to RNA
polymerase. It attaches to and moves along the
DNA template strand while anot

and moves along the
DNA template strand while another enzyme,
DNA ligase, causes bonding of successive DNA
nucleotides to one another, using high-energy
phosphate bonds to energize these attachments.
4. Formation of each new DNA strand occurs
simultaneously in hundreds of segments along
each of the two strands of the helix until the
entire strand is replicated. Then the ends of the
subunits are joined together by the DNA ligase
enzyme.
5. Each newly formed strand of DNA remains
attached by loose

wly formed strand of DNA remains
attached by loose hydrogen bonding to the
original DNA strand that was used as its template.
Therefore, two DNA helixes are coiled together.
6. Because the DNA helixes in each chromosome
are approximately 6 centimeters in length and
have millions of helix turns, it would be
impossible for the two newly formed DNA helixes
to uncoil from each other were it not for some
special mechanism. This is achieved by enzymes
that periodically cut each helix along its entire

length, rotate each segment enough to cause
separation, and then resplice the helix. Thus, the
two new helixes become uncoiled.
DNA Repair, DNA “Proofreading,” and “Mutation.” During
the hour or so between DNA replication and the
beginning of mitosis, there is a period of very active
repair and “proofreading” of the DNA strands.That is,
wherever inappropriate DNA nucleotides have been
matched up with the nucleotides of the original tem-
plate str

ith the nucleotides of the original tem-
plate strand, special enzymes cut out the defective
areas and replace these with appropriate complemen-
tary nucleotides. This is achieved by the same DNA
polymerases and DNA ligases that are used in repli-
cation. This repair process is referred to as DNA 
proofreading.
Because of repair and proofreading, the transcrip-
tion process rarely makes a mistake. But when a
mistake is made, this is called a mutation. The muta-
tion causes formation of some abno

tion. The muta-
tion causes formation of some abnormal protein in the
cell rather than a needed protein, often leading to
abnormal cellular function and sometimes even cell
death. Yet, given that there are 30,000 or more genes
in the human genome and that the period from one
human generation to another is about 30 years, one
would expect as many as 10 or many more mutations
in the passage of the genome from parent to child. As
a further protection, however, each human genome is
represented by tw

n, however, each human genome is
represented by two separate sets of chromosomes with
almost identical genes. Therefore, one functional gene
of each pair is almost always available to the child
despite mutations.
The DNA helixes of the nucleus are packaged in chro-
mosomes. The human cell contains 46 chromosomes
arranged in 23 pairs. Most of the genes in the two chro-
mosomes of each pair are identical or almost identical
to each other, so it is usually stated t

identical
to each other, so it is usually stated that the different
genes also exist in pairs, although occasionally this is
not the case.
In addition to DNA in the chromosome, there is a
large amount of protein in the chromosome, composed
mainly of many small molecules of electropositively
charged histones. The histones are organized into vast
numbers of small, bobbin-like cores. Small segments of
each DNA helix are coiled sequentially around one
core after another.
The histone cores play an im

e
The histone cores play an important role in the reg-
ulation of DNA activity because as long as the DNA
is packaged tightly, it cannot function as a template for
either the formation of RNA or the replication of new
DNA. Further, some of the regulatory proteins have
been shown to decondense the histone packaging of
the DNA and to allow small segments at a time to form
RNA.
Several nonhistone proteins are also major compo-
nents of chromosomes, functioning both as chromoso-

nts of chromosomes, functioning both as chromoso-
mal structural proteins and, in connection with the
genetic regulatory machinery, as activators, inhibitors,
and enzymes.
Replication of the chromosomes in their entirety
occurs during the next few minutes after replication of
the DNA helixes has been completed; the new DNA
helixes collect new protein molecules as needed. The
two newly formed chromosomes remain attached to
each other (until time for mitosis) at a point called the
centromere locat

or mitosis) at a point called the
centromere located near their center.These duplicated
but still attached chromosomes are called chromatids.
The actual process by which the cell splits into two new
cells is called mitosis. Once each chromosome has
been replicated to form the two chromatids, in many
cells, mitosis follows automatically within 1 or 2 hours.
Mitotic Apparatus: Function of the Centrioles. One of the
ﬁrst events of mitosis takes place in the cytoplasm,
occurring during 

is takes place in the cytoplasm,
occurring during the latter part of interphase in or
Chapter 3
39
around the small structures called centrioles. As shown
in Figure 3–13, two pairs of centrioles lie close to each
other near one pole of the nucleus. (These centrioles,
like the DNA and chromosomes, were also replicated
during interphase, usually shortly before replication of
the DNA.) Each centriole is a small cylindrical b

the DNA.) Each centriole is a small cylindrical body
about 0.4 micrometer long and about 0.15 micrometer
in diameter, consisting mainly of nine parallel tubular
structures arranged in the form of a cylinder. The two
centrioles of each pair lie at right angles to each other.
Each pair of centrioles, along with attached pericen-
triolar material, is called a centrosome.
Shortly before mitosis is to take place, the two pairs
of centrioles begin to move apart from each other.This
is caused by polym

move apart from each other.This
is caused by polymerization of protein microtubules
growing between the respective centriole pairs and
actually pushing them apart. At the same time, other
microtubules grow radially away from each of the cen-
triole pairs, forming a spiny star, called the aster, in
each end of the cell. Some of the spines of the aster
penetrate the nuclear membrane and help separate
the two sets of chromatids during mitosis.The complex
of microtubules extending between the two ne

mplex
of microtubules extending between the two new cen-
triole pairs is called the spindle, and the entire set of
microtubules plus the two pairs of centrioles is called
the mitotic apparatus.
Prophase. The ﬁrst stage of mitosis, called prophase, is
shown in Figure 3–13A, B, and C. While the spindle is
forming, the chromosomes of the nucleus (which in
interphase consist of loosely coiled strands) become
condensed into well-deﬁned chromosomes.
Prometaphase. During this stage (see Figure 3–13D),

ometaphase. During this stage (see Figure 3–13D),
the growing microtubular spines of the aster fragment
the nuclear envelope. At the same time, multiple
microtubules from the aster attach to the chromatids
at the centromeres, where the paired chromatids are
still bound to each other; the tubules then pull one
chromatid of each pair toward one cellular pole and
its partner toward the opposite pole.
Metaphase. During metaphase (see Figure 3–13E), the
two asters of the mitotic apparatus are pushed 

he
two asters of the mitotic apparatus are pushed farther
apart. This is believed to occur because the micro-
tubular spines from the two asters, where they inter-
digitate with each other to form the mitotic spindle,
actually push each other away. There is reason to
believe that minute contractile protein molecules
called “motor molecules,” perhaps composed of the
muscle protein actin, extend between the respective
spines and, using a stepping action as in muscle,
actively slide the spines in a

ction as in muscle,
actively slide the spines in a reverse direction along
each other. Simultaneously, the chromatids are pulled
tightly by their attached microtubules to the very
center of the cell, lining up to form the equatorial plate
of the mitotic spindle.
Anaphase. During this phase (see Figure 3–13F), the
two chromatids of each chromosome are pulled apart
at the centromere. All 46 pairs of chromatids are 
separated, forming two separate sets of 46 daughter
chromosomes. One of these sets 

ets of 46 daughter
chromosomes. One of these sets is pulled toward one
mitotic aster and the other toward the other aster as
the two respective poles of the dividing cell are pushed
still farther apart.
Telophase. In telophase (see Figure 3–13G and H), the
two sets of daughter chromosomes are pushed com-
pletely apart. Then the mitotic apparatus dissolutes,
and a new nuclear membrane develops around each
set of chromosomes. This membrane is formed from
portions of the endoplasmic reticulum that 

d from
portions of the endoplasmic reticulum that are already
present in the cytoplasm. Shortly thereafter, the cell
pinches in two, midway between the two nuclei.This is
caused by formation of a contractile ring of micro-
ﬁlaments composed of actin and probably myosin (the
two contractile proteins of muscle) at the juncture of
the newly developing cells that pinches them off from
each other.
Control of Cell Growth and 
We know that certain cells grow and reproduce all the
time

that certain cells grow and reproduce all the
time, such as the blood-forming cells of the bone
marrow,the germinal layers of the skin,and the epithe-
lium of the gut. Many other cells, however, such as
smooth muscle cells, may not reproduce for many
years. A few cells, such as the neurons and most stri-
ated muscle cells, do not reproduce during the entire
life of a person, except during the original period of
fetal life.
In certain tissues, an insufﬁciency of some types of
cells causes these t

insufﬁciency of some types of
cells causes these to grow and reproduce rapidly until
appropriate numbers of them are again available. For
instance, in some young animals, seven eighths of 
the liver can be removed surgically, and the cells of 
the remaining one eighth will grow and divide until the
liver mass returns almost to normal. The same occurs
for many glandular cells and most cells of the bone
marrow, subcutaneous tissue, intestinal epithelium,
and almost any other tissue except highly d

elium,
and almost any other tissue except highly differenti-
ated cells such as nerve and muscle cells.
We know little about the mechanisms that maintain
proper numbers of the different types of cells in 
the body. However, experiments have shown at least
three ways in which growth can be controlled. First,
growth often is controlled by growth factors that come
from other parts of the body. Some of these circulate
in the blood, but others originate in adjacent tissues.
For instance, the epitheli

te in adjacent tissues.
For instance, the epithelial cells of some glands, such
as the pancreas, fail to grow without a growth factor
from the sublying connective tissue of the gland.
Second, most normal cells stop growing when they
have run out of space for growth. This occurs when
cells are grown in tissue culture; the cells grow until
they contact a solid object, and then growth stops.
Third, cells grown in tissue culture often stop growing
when minute amounts of their own secretions are
allo

en minute amounts of their own secretions are
allowed to collect in the culture medium. This, too,
could provide a means for negative feedback control
of growth.
40
Unit I
Regulation of Cell Size. Cell size is determined almost
entirely by the amount of functioning DNA in the
nucleus. If replication of the DNA does not occur, the
cell grows to a certain size and thereafter remains at
that size. Conversely, it is possible, by use of the 

t size. Conversely, it is possible, by use of the chem-
ical colchicine, to prevent formation of the mitotic
spindle and therefore to prevent mitosis, even though
replication of the DNA continues. In this event, the
nucleus contains far greater quantities of DNA than 
it normally does, and the cell grows proportionately
larger. It is assumed that this results simply from
increased production of RNA and cell proteins, which
in turn cause the cell to grow larger.
A special cha

to grow larger.
A special characteristic of cell growth and cell division
is cell differentiation, which refers to changes in 
physical and functional properties of cells as they pro-
liferate in the embryo to form the different bodily
structures and organs. The description of an especially
interesting experiment that helps explain these pro-
cesses follows.
When the nucleus from an intestinal mucosal cell of
a frog is surgically implanted into a frog ovum from
which the ori

ally implanted into a frog ovum from
which the original ovum nucleus was removed, the
result is often the formation of a normal frog. This
demonstrates that even the intestinal mucosal cell,
which is a well-differentiated cell, carries all the 
necessary genetic information for development of all
structures required in the frog’s body.
Therefore, it has become clear that differentiation
results not from loss of genes but from selective
repression of different genetic operons. In fact, elec-
tron

 of different genetic operons. In fact, elec-
tron micrographs suggest that some segments of DNA
helixes wound around histone cores become so con-
densed that they no longer uncoil to form RNA mol-
ecules. One explanation for this is as follows: It has
been supposed that the cellular genome begins at a
certain stage of cell differentiation to produce a regu-
latory protein that forever after represses a select
group of genes. Therefore, the repressed genes never
function again. Regardless of the

ssed genes never
function again. Regardless of the mechanism, mature
human cells produce a maximum of about 8000 to
10,000 proteins rather than the potential 30,000 or
more if all genes were active.
Embryological experiments show that certain cells
in an embryo control differentiation of adjacent cells.
For instance, the primordial chorda-mesoderm is called
the primary organizer of the embryo because it forms
a focus around which the rest of the embryo develops.
It differentiates into a mesoderm

embryo develops.
It differentiates into a mesodermal axis that contains
segmentally arranged somites and, as a result of induc-
tions in the surrounding tissues, causes formation of
essentially all the organs of the body.
Another instance of induction occurs when the
developing eye vesicles come in contact with the ecto-
derm of the head and cause the ectoderm to thicken
into a lens plate that folds inward to form the lens of
the eye. Therefore, a large share of the embryo devel-
ops as a result

a large share of the embryo devel-
ops as a result of such inductions, one part of the body
affecting another part, and this part affecting still other
parts.
Thus, although our understanding of cell differenti-
ation is still hazy, we know many control mechanisms
by which differentiation could occur.
Apoptosis—Programmed 
Cell Death
The 100 trillion cells of the body are members of a
highly organized community in which the total number
of cells is regulated not only by controlling the rate of
c

is regulated not only by controlling the rate of
cell division but also by controlling the rate of cell
death. When cells are no longer needed or become a
threat to the organism, they undergo a suicidal pro-
grammed cell death, or apoptosis.This process involves
a speciﬁc proteolytic cascade that causes the cell to
shrink and condense, to disassemble its cytoskeleton,
and to alter its cell surface so that a neighboring
phagocytic cell, such as a macrophage, can attach to
the cell membrane and di

macrophage, can attach to
the cell membrane and digest the cell.
In contrast to programmed death, cells that die as a
result of an acute injury usually swell and burst due to
loss of cell membrane integrity, a process called cell
necrosis. Necrotic cells may spill their contents, causing
inﬂammation and injury to neighboring cells. Apopto-
sis, however, is an orderly cell death that results in 
disassembly and phagocytosis of the cell before any
leakage of its contents occurs, and neighboring ce

leakage of its contents occurs, and neighboring cells
usually remain healthy.
Apoptosis is initiated by activation of a family of
proteases called caspases. These are enzymes that are
synthesized and stored in the cell as inactive procas-
pases. The mechanisms of activation of caspases are
complex, but once activated, the enzymes cleave and
activate other procaspases, triggering a cascade that
rapidly breaks down proteins within the cell. The cell
thus dismantles itself, and its remains are rapi

l
thus dismantles itself, and its remains are rapidly
digested by neighboring phagocytic cells.
A tremendous amount of apoptosis occurs in tissues
that are being remodeled during development. Even
in adult humans, billions of cells die each hour in
tissues such as the intestine and bone marrow and 
are replaced by new cells. Programmed cell death,
however, is precisely balanced with the formation 
of new cells in healthy adults. Otherwise, the body’s
tissues would shrink or grow excessively. Rec

dy’s
tissues would shrink or grow excessively. Recent
studies suggest that abnormalities of apoptosis may
play a key role in neurodegenerative diseases such as
Alzheimer’s disease, as well as in cancer and auto-
immune disorders. Some drugs that have been used
successfully for chemotherapy appear to induce apop-
tosis in cancer cells.
Cancer
Cancer is caused in all or almost all instances by muta-
tion or by some other abnormal activation of cellular
genes that control cell growth and cell mitos

ular
genes that control cell growth and cell mitosis. The
Chapter 3
41
abnormal genes are called oncogenes. As many as 100
different oncogenes have been discovered.
Also present in all cells are antioncogenes, which
suppress the activation of speciﬁc oncogenes. There-
fore, loss of or inactivation of antioncogenes can allow
activation of oncogenes that lead to cancer.
Only a minute fraction of the cells that mutate in the

 a minute fraction of the cells that mutate in the
body ever lead to cancer.There are several reasons for
this. First, most mutated cells have less survival capa-
bility than normal cells and simply die. Second, only 
a few of the mutated cells that do survive become 
cancerous, because even most mutated cells still 
have normal feedback controls that prevent excessive
growth.
Third, those cells that are potentially cancerous are
often, if not usually, destroyed by the body’s immune
system befor

ually, destroyed by the body’s immune
system before they grow into a cancer. This occurs in
the following way: Most mutated cells form abnormal
proteins within their cell bodies because of their
altered genes, and these proteins activate the body’s
immune system, causing it to form antibodies or sen-
sitized lymphocytes that react against the cancerous
cells, destroying them. In support of this is the fact 
that in people whose immune systems have been sup-
pressed, such as in those taking immun

e been sup-
pressed, such as in those taking immunosuppressant
drugs after kidney or heart transplantation, the prob-
ability of a cancer’s developing is multiplied as much
as ﬁvefold.
Fourth, usually several different activated onco-
genes are required simultaneously to cause a cancer.
For instance, one such gene might promote rapid
reproduction of a cell line, but no cancer occurs
because there is not a simultaneous mutant gene to
form the needed blood vessels.
But what is it that causes the a

ed blood vessels.
But what is it that causes the altered genes? Con-
sidering that many trillions of new cells are formed
each year in humans, a better question might be, Why
is it that all of us do not develop millions or billions of
mutant cancerous cells? The answer is the incredible
precision with which DNA chromosomal strands are
replicated in each cell before mitosis can take place,
and also the proofreading process that cuts and repairs
any abnormal DNA strand before the mitotic process
i

y abnormal DNA strand before the mitotic process
is allowed to proceed. Yet, despite all these inherited
cellular precautions, probably one newly formed cell
in every few million still has signiﬁcant mutant 
characteristics.
Thus, chance alone is all that is required for muta-
tions to take place, so we can suppose that a large
number of cancers are merely the result of an unlucky
occurrence.
However, the probability of mutations can be
increased manyfold when a person is exposed to
certain chem

 manyfold when a person is exposed to
certain chemical, physical, or biological factors, includ-
ing the following:
1. It is well known that ionizing radiation, such as 
x-rays, gamma rays, and particle radiation from
radioactive substances, and even ultraviolet light
can predispose individuals to cancer. Ions formed
in tissue cells under the inﬂuence of such
radiation are highly reactive and can rupture
DNA strands, thus causing many mutations.
2. Chemical substances of certain types also have 

2. Chemical substances of certain types also have a
high propensity for causing mutations. It was
discovered long ago that various aniline dye
derivatives are likely to cause cancer, so that
workers in chemical plants producing such
substances, if unprotected, have a special
predisposition to cancer. Chemical substances that
can cause mutation are called carcinogens. The
carcinogens that currently cause the greatest
number of deaths are those in cigarette smoke.
They cause about one quarter of a

cigarette smoke.
They cause about one quarter of all cancer 
deaths.
3. Physical irritants also can lead to cancer, such as
continued abrasion of the linings of the intestinal
tract by some types of food. The damage to 
the tissues leads to rapid mitotic replacement of
the cells. The more rapid the mitosis, the greater
the chance for mutation.
4. In many families, there is a strong hereditary
tendency to cancer. This results from the fact that
most cancers require not one mutation but two or
mor

st cancers require not one mutation but two or
more mutations before cancer occurs. In those
families that are particularly predisposed to
cancer, it is presumed that one or more cancerous
genes are already mutated in the inherited
genome. Therefore, far fewer additional mutations
must take place in such family members before a
cancer begins to grow.
5. In laboratory animals, certain types of viruses can
cause some kinds of cancer, including leukemia.
This usually results in one of two ways. In 

emia.
This usually results in one of two ways. In the case
of DNA viruses, the DNA strand of the virus can
insert itself directly into one of the chromosomes
and thereby cause a mutation that leads to 
cancer. In the case of RNA viruses, some of these
carry with them an enzyme called reverse
transcriptase that causes DNA to be transcribed
from the RNA. The transcribed DNA then inserts
itself into the animal cell genome, leading to
cancer.
Invasive Characteristic of the Cancer Cell. The major dif

e Characteristic of the Cancer Cell. The major dif-
ferences between the cancer cell and the normal cell
are the following: (1) The cancer cell does not respect
usual cellular growth limits; the reason for this is that
these cells presumably do not require all the same
growth factors that are necessary to cause growth of
normal cells. (2) Cancer cells often are far less adhe-
sive to one another than are normal cells. Therefore,
they have a tendency to wander through the tissues, to
enter the bl

ncy to wander through the tissues, to
enter the blood stream, and to be transported all
through the body, where they form nidi for numerous
new cancerous growths. (3) Some cancers also produce
angiogenic factors that cause many new blood vessels
to grow into the cancer, thus supplying the nutrients
required for cancer growth.
Why Do Cancer Cells Kill? The answer to this question
usually is simple. Cancer tissue competes with normal
tissues for nutrients. Because cancer cells continue to
prolifer

trients. Because cancer cells continue to
proliferate indeﬁnitely, their number multiplying day
by day, cancer cells soon demand essentially all the
42
Unit I
nutrition available to the body or to an essential part
of the body.As a result, normal tissues gradually suffer
nutritive death.
Aranda A, Pascal A: Nuclear hormone re

ence, 2002.
Aranda A, Pascal A: Nuclear hormone receptors and gene
expression. Physiol Rev 81:1269, 2001.
Balmain A, Gray J, Ponder B: The genetics and genomics of
cancer. Nat Genet 33(Suppl):238, 2003.
Bowen ID, Bowen SM, Jones AH: Mitosis and Apoptosis:
Matters of Life and Death. London: Chapman & Hall,
1998.
Burke W: Genomics as a probe for disease biology. N Engl
J Med 349:969, 2003.
Caplen NJ, Mousses S: Short interfering RNA (siRNA)-
mediated RNA interference (RNAi) in human cells. Ann
N Y

ed RNA interference (RNAi) in human cells. Ann
N Y Acad Sci 1002:56, 2003.
Cooke MS, Evans MD, Dizdaroglu M, Lunec J: Oxidative
DNA damage:
mechanisms,
mutation,
and disease.
FASEB J 17:1195, 2003.
Cullen BR: Nuclear RNA export. J Cell Sci 116:587, 2003.
Fedier A, Fink D: Mutations in DNA mismatch repair genes:
implications for DNA damage signaling and drug sensi-
tivity. Int J Oncol 24:1039, 2004.
Hahn S: Structure and mechanism of the RNA polymerase
II transcription machinery. Nat Struct Mol B

erase
II transcription machinery. Nat Struct Mol Biol 11:394,
2004.
Hall JG: Genomic imprinting: nature and clinical relevance.
Annu Rev Med 48:35, 1997.
Jockusch BM,Hüttelmaier S,Illenberger S:From the nucleus
toward the cell periphery: a guided tour for mRNAs. News
Physiol Sci 18:7, 2003.
Kazazian HH Jr: Mobile elements: drivers of genome 
evolution. Science 303:1626, 2004.
Lewin B: Genes IV. Oxford: Oxford University Press, 2000.
Nabel GJ: Genetic, cellular and immune approaches to
disease th

etic, cellular and immune approaches to
disease therapy: past and future. Nat Med 10:135, 2004.
Pollard TD, Earnshaw WC: Cell Biology. Philadelphia:
Elsevier Science, 2002.
U
N
I
T
II
Membrane
Physiology, Nerve,
and Muscle
4. Transport of Substances Through the Cell Membrane
5. Membrane Potentials and Action Potentials
6. Contraction of Skeletal Muscle
7. Excitation of Skeletal Muscle: Neuromuscular
Transmission and Excitation-Contraction Coupling
8. Contraction and Excitation of Smooth Muscle
C

g
C
H
A
P
T
E
R
 
4
45
Transport of Substances Through
Figure 4–1 gives the approximate concentrations of
important electrolytes and other substances in the
extracellular ﬂuid and intracellular ﬂuid. Note that
the extracellular ﬂuid contains a large amount of
sodium but only a small amount of potassium.
Exactly the opposite is true of the intracellular ﬂuid.
Also, the extracellular ﬂuid contains a large amount
of chloride ions, whe

ﬂuid contains a large amount
of chloride ions, whereas the intracellular ﬂuid con-
tains very little. But the concentrations of phosphates and proteins in the intra-
cellular ﬂuid are considerably greater than those in the extracellular ﬂuid.These
differences are extremely important to the life of the cell. The purpose of this
chapter is to explain how the differences are brought about by the transport
mechanisms of the cell membranes.
and Cell Membrane T

Barrier of the Cell Membrane, 
and Cell Membrane Transport Proteins
The structure of the membrane covering the outside of every cell of the body
is discussed in Chapter 2 and illustrated in Figures 2–3 and 4–2.This membrane
consists almost entirely of a lipid bilayer, but it also contains large numbers of
protein molecules in the lipid, many of which penetrate all the way through the
membrane, as shown in Figure 4–2.
The lipid bilayer is not miscible with either the extracellular ﬂuid or the int

ible with either the extracellular ﬂuid or the intra-
cellular ﬂuid. Therefore, it constitutes a barrier against movement of water 
molecules and water-soluble substances between the extracellular and intracel-
lular ﬂuid compartments. However, as demonstrated in Figure 4–2 by the left-
most arrow, a few substances can penetrate this lipid bilayer, diffusing directly
through the lipid substance itself; this is true mainly of lipid-soluble substances,
as described later.
The protein molecules in 

ces,
The protein molecules in the membrane have entirely different properties for
transporting substances. Their molecular structures interrupt the continuity of
the lipid bilayer, constituting an alternative pathway through the cell mem-
brane. Most of these penetrating proteins, therefore, can function as transport
proteins. Different proteins function differently. Some have watery spaces all
the way through the molecule and allow free movement of water as well as
selected 

 allow free movement of water as well as
selected ions or molecules; these are called channel proteins. Others, called
carrier proteins, bind with molecules or ions that are to be transported; confor-
mational changes in the protein molecules then move the substances through
the interstices of the protein to the other side of the membrane. Both the
channel proteins and the carrier proteins are usually highly selective in the types
of molecules or ions that are allowed to cross the membrane.
“Dif

 ions that are allowed to cross the membrane.
“Diffusion” Versus “Active Transport.” Transport through the cell membrane, either
directly through the lipid bilayer or through the proteins, occurs by one of two
basic processes: diffusion or active transport.
Although there are many variations of these basic mechanisms, diffusion
means random molecular movement of substances molecule by molecule, either
through intermolecular spaces in the membrane or in combination with a carrier
46
Unit II
Membr

 or in combination with a carrier
46
Unit II
Membrane Physiology, Nerve, and Muscle
protein. The energy that causes diffusion is the energy
of the normal kinetic motion of matter.
By contrast, active transport means movement of
ions or other substances across the membrane in com-
bination with a carrier protein in such a way that the
carrier protein causes the substance to move against
an energy gradient, such as from a low-concentration
state to a high-concentration state. This movement
require

 a high-concentration state. This movement
requires an additional source of energy besides kinetic
energy. Following is a more detailed explanation of 
the basic physics and physical chemistry of these two
processes.
Diffusion
All molecules and ions in the body ﬂuids, including
water molecules and dissolved substances, are in con-
stant motion, each particle moving its own separate
way. Motion of these particles is what physicists call
“heat”—the greater the motion, the higher the tem-
perature—

 greater the motion, the higher the tem-
perature—and the motion never ceases under any
condition except at absolute zero temperature. When
a moving molecule, A, approaches a stationary mole-
cule, B, the electrostatic and other nuclear forces of
molecule A repel molecule B, transferring some of the
energy of motion of molecule A to molecule B. Con-
sequently, molecule B gains kinetic energy of motion,
while molecule A slows down, losing some of its
kinetic energy. Thus, as shown in Figure 4–3, 

its
kinetic energy. Thus, as shown in Figure 4–3, a single
molecule in a solution bounces among the other 
molecules ﬁrst in one direction, then another, then
another, and so forth, randomly bouncing thousands of
times each second. This continual movement of mole-
cules among one another in liquids or in gases is called
diffusion.
Ions diffuse in the same manner as whole molecules,
and even suspended colloid particles diffuse in a
similar manner, except that the colloids diffuse far 
less rapidl

 except that the colloids diffuse far 
less rapidly than molecular substances because of their
large size.
Diffusion through the cell membrane is divided into
two subtypes called simple diffusion and facilitated 
diffusion. Simple diffusion means that kinetic move-
ment of molecules or ions occurs through a membrane
opening or through intermolecular spaces without any
interaction with carrier proteins in the membrane.
The rate of diffusion is determined by the

mbrane.
The rate of diffusion is determined by the amount of
Na+ ---------------
K+ -----------------
Ca++ --------------
Mg++ --------------
Cl– ----------------
HCO3
– -----------
Phosphates-----
SO4– -------------
Glucose ---------
Amino acids ----
142 mEq/L ---------
4 mEq/L ------------
2.4 mEq/L ----------
1.2 mEq/L ----------
103 mEq/L ---------
28 mEq/L -----------
4 mEq/L -------------
1 mEq/L -------------
90 mg/dl ------------
30 mg/dl ------------
10 mEq/L 
140 mEq/L 
0.0001 mEq/L 
5

 ------------
10 mEq/L 
140 mEq/L 
58 mEq/L 
4 mEq/L 
10 mEq/L 
75 mEq/L 
2 mEq/L 
0 to 20 mg/dl 
200 mg/dl ? 
Neutral fat
0.5 g/dl-------------- 2 to 95 g/dl
PO2 ---------------
PCO2 -------------
pH -----------------
Proteins ----------
35 mm Hg --------- 
46 mm Hg ---------
7.4 -------------------
2 g/dl ----------------
(5 mEq/L)
20 mm Hg ? 
50 mm Hg ? 
7.0
16 g/dl
(40 mEq/L)
EXTRACELLULAR 
FLUID
INTRACELLULAR 
FLUID
Figure 4–1
Chemical compositions of

ELLULAR 
FLUID
Figure 4–1
Chemical compositions of extracellular and intracellular ﬂuids.
Channel
protein
Simple
diffusion
Facilitated
diffusion
Energy
Active transport
Diffusion
Carrier proteins
Figure 4–2
Transport pathways through the cell membrane, and the basic
mechanisms of transport.
Figure 4–3
Diffusion of a ﬂuid molecule during a thousandth of a second.
Chapter 4
Transport of Substances Through the Cell Membrane
47
substance available, the velocity of kinetic motion, and 
the number and

he velocity of kinetic motion, and 
the number and sizes of openings in the membrane
through which the molecules or ions can move.
Facilitated diffusion requires interaction of a carrier
protein. The carrier protein aids passage of the mole-
cules or ions through the membrane by binding 
chemically with them and shuttling them through the
membrane in this form.
Simple diffusion can occur through the cell mem-
brane by two pathways: (1) through the interstices of
the lipid bilayer if the diffusin

e interstices of
the lipid bilayer if the diffusing substance is lipid
soluble, and (2) through watery channels that pene-
trate all the way through some of the large transport
proteins, as shown to the left in Figure 4–2.
Diffusion of Lipid-Soluble Substances Through the Lipid Bilayer.
One of the most important factors that determines
how rapidly a substance diffuses through the lipid
bilayer is the lipid solubility of the substance. For
instance, the lipid solubilities of oxygen, nitrogen,
car

e, the lipid solubilities of oxygen, nitrogen,
carbon dioxide, and alcohols are high, so that all these
can dissolve directly in the lipid bilayer and diffuse
through the cell membrane in the same manner that
diffusion of water solutes occurs in a watery solution.
For obvious reasons, the rate of diffusion of each of
these substances through the membrane is directly
proportional to its lipid solubility. Especially large
amounts of oxygen can be transported in this way;
therefore, oxygen can be d

ransported in this way;
therefore, oxygen can be delivered to the interior of
the cell almost as though the cell membrane did not
exist.
Diffusion of Water and Other Lipid-Insoluble Molecules Through
Protein Channels. Even though water is highly insoluble
in the membrane lipids, it readily passes through chan-
nels in protein molecules that penetrate all the way
through the membrane. The rapidity with which water
molecules can move through most cell membranes is
astounding. As an example, the to

ell membranes is
astounding. As an example, the total amount of water
that diffuses in each direction through the red cell
membrane during each second is about 100 times as
great as the volume of the red cell itself.
Other lipid-insoluble molecules can pass through
the protein pore channels in the same way as water
molecules if they are water soluble and small enough.
However, as they become larger, their penetration falls
off rapidly. For instance, the diameter of the urea 
molecule is only 20 

ce, the diameter of the urea 
molecule is only 20 per cent greater than that of water,
yet its penetration through the cell membrane pores is
about 1000 times less than that of water. Even so, given
the astonishing rate of water penetration, this amount
of urea penetration still allows rapid transport of urea
through the membrane within minutes.
Diffusion Through Protein Channels,
and “Gating” of These Channels
Computerized three-dimensional reconstructions of
protein channels have demonstrated 

nstructions of
protein channels have demonstrated tubular pathways
all the way from the extracellular to the intracellu-
lar ﬂuid. Therefore, substances can move by simple 
diffusion directly along these channels from one side
of the membrane to the other. The protein channels
are distinguished by two important characteristics: (1)
they are often selectively permeable to certain sub-
stances, and (2) many of the channels can be opened
or closed by gates.
Selective Permeability of Protein Channel

y gates.
Selective Permeability of Protein Channels. Many of the
protein channels are highly selective for transport of
one or more speciﬁc ions or molecules. This results
from the characteristics of the channel itself, such as
its diameter, its shape, and the nature of the electrical
charges and chemical bonds along its inside surfaces.
To give an example, one of the most important of the
protein channels, the so-called sodium channel, is only
0.3 by 0.5 nanometer in diameter, but more importan

.3 by 0.5 nanometer in diameter, but more important,
the inner surfaces of this channel are strongly nega-
tively charged, as shown by the negative signs inside
the channel proteins in the top panel of Figure 4–4.
These strong negative charges can pull small dehy-
drated sodium ions into these channels,actually pulling
the sodium ions away from their hydrating water 
molecules. Once in the channel, the sodium ions
diffuse in either direction according to the usual laws
of diffusion. Thus, the so

rding to the usual laws
of diffusion. Thus, the sodium channel is speciﬁcally
selective for passage of sodium ions.
Conversely, another set of protein channels is selec-
tive for potassium transport, shown in the lower panel
of Figure 4–4. These channels are slightly smaller than
the sodium channels, only 0.3 by 0.3 nanometer, but
they are not negatively charged, and their chemical
bonds are different. Therefore, no strong attrac-
tive force is pulling ions into the channels, and the
potassium i

ulling ions into the channels, and the
potassium ions are not pulled away from the water
Gate
closed
Gate open
Outside
Inside
Na+
Na+
Gate
closed
Gate open
Outside
Inside
K+
K+
–
–
–
–
–
–
–
–
–
––
–
–
–
–
–
Figure 4–4
Transport of sodium and potassium ions through protein channels.
Also shown are conformational changes in the protein molecules
to open or close “gates” guarding the channels.
48
Unit II
molecules that hydrate them. The hydrated form of 
the 

ules that hydrate them. The hydrated form of 
the potassium ion is considerably smaller than the
hydrated form of sodium because the sodium ion
attracts far more water molecules than does potas-
sium. Therefore, the smaller hydrated potassium ions
can pass easily through this small channel, whereas the
larger hydrated sodium ions are rejected, thus provid-
ing selective permeability for a speciﬁc ion.
Gating of Protein Channels. Gating of protein channels
provides a means of controlling ion perm

 channels
provides a means of controlling ion permeability of the
channels. This is shown in both panels of Figure 4–4
for selective gating of sodium and potassium ions. It is
believed that some of the gates are actual gatelike
extensions of the transport protein molecule, which
can close the opening of the channel or can be lifted
away from the opening by a conformational change in
the shape of the protein molecule itself.
The opening and closing of gates are controlled in
two principal ways:
1

g of gates are controlled in
1. Voltage gating. In this instance, the molecular
conformation of the gate or of its chemical bonds
responds to the electrical potential across the cell
membrane. For instance, in the top panel of Figure
4–4, when there is a strong negative charge 
on the inside of the cell membrane, this presumably
could cause the outside sodium gates to remain
tightly closed; conversely, when the inside of the
membrane loses its negative charge, these gates
wou

embrane loses its negative charge, these gates
would open suddenly and allow tremendous
quantities of sodium to pass inward through the
sodium pores. This is the basic mechanism for
eliciting action potentials in nerves that are
responsible for nerve signals. In the bottom panel
of Figure 4–4, the potassium gates are on the
intracellular ends of the potassium channels, and
they open when the inside of the cell membrane
becomes positively charged. The opening of these
gates is partly responsible 

 The opening of these
gates is partly responsible for terminating the
action potential, as is discussed more fully in
Chapter 5.
2. Chemical (ligand) gating. Some protein channel
gates are opened by the binding of a chemical
substance (a ligand) with the protein; this causes a
conformational or chemical bonding change in the
protein molecule that opens or closes the gate. This
is called chemical gating or ligand gating. One of
the most important instances of chemical gating 
is the effect of ace

instances of chemical gating 
is the effect of acetylcholine on the so-called
acetylcholine channel. Acetylcholine opens the gate
of this channel, providing a negatively charged pore
about 0.65 nanometer in diameter that allows
uncharged molecules or positive ions smaller 
than this diameter to pass through. This gate is
exceedingly important for the transmission of nerve
signals from one nerve cell to another (see Chapter
45) and from nerve cells to muscle cells to cause
muscle contraction (see

s to muscle cells to cause
muscle contraction (see Chapter 7).
Open-State Versus Closed-State of Gated Channels.
Figure 4–5A shows an especially interesting charac-
teristic of most voltage-gated channels. This ﬁgure
shows two recordings of electrical current ﬂowing
through a single sodium channel when there was an
approximate 25-millivolt potential gradient across the
membrane. Note that the channel conducts current
either “all or none.” That is, the gate of the channel
snaps open and then snap

, the gate of the channel
snaps open and then snaps closed, each open state
lasting for only a fraction of a millisecond up to several
milliseconds. This demonstrates the rapidity with
2
4
Milliseconds
Open sodium channel
Picoamperes
6
8
10
Recorder
To recorder
0
A
B
3
3
0
0
Membrane
“patch”
Figure 4–5
A, Record of current ﬂow through a single voltage-gated sodium
channel, demonstrating the “all or none” principle for opening and
closing of the channel. B, The “patch-clamp” method for record-
in

hannel. B, The “patch-clamp” method for record-
ing current ﬂow through a single protein channel. To the left,
recording is performed from a “patch” of a living cell membrane.
To the right, recording is from a membrane patch that has been
torn away from the cell.
Chapter 4
49
pore large enough to transport a speciﬁc molecule
partway through. It also shows a binding “receptor” on
the inside of the protein carrier. The molecule to be
transported en

protein carrier. The molecule to be
transported enters the pore and becomes bound.Then,
in a fraction of a second, a conformational or chemi-
cal change occurs in the carrier protein, so that the
pore now opens to the opposite side of the membrane.
Because the binding force of the receptor is weak, the
thermal motion of the attached molecule causes it to
break away and to be released on the opposite side of
which changes can occur during the opening and
closing of the protein molecular gates. At

ing and
closing of the protein molecular gates. At one voltage
potential, the channel may remain closed all the time
or almost all the time, whereas at another voltage
level, it may remain open either all or most of the time.
At in-between voltages, as shown in the ﬁgure, the
gates tend to snap open and closed intermittently,
giving an average current ﬂow somewhere between
the minimum and the maximum.
Patch-Clamp Method for Recording Ion Current Flow Through
Single Channels. One might wonder how

Flow Through
Single Channels. One might wonder how it is technically
possible to record ion current ﬂow through single
protein channels as shown in Figure 4–5A. This has been
achieved by using the “patch-clamp” method illustrated
in Figure 4–5B. Very simply, a micropipette, having a tip
diameter of only 1 or 2 micrometers, is abutted against
the outside of a cell membrane. Then suction is applied
inside the pipette to pull the membrane against the tip
of the pipette.This creates a seal where the

e tip
of the pipette.This creates a seal where the edges of the
pipette touch the cell membrane. The result is a minute
membrane “patch” at the tip of the pipette through
which electrical current ﬂow can be recorded.
Alternatively, as shown to the right in Figure 4–5B,
the small cell membrane patch at the end of the pipette
can be torn away from the cell. The pipette with 
its sealed patch is then inserted into a free solution.
This allows the concentrations of ions both inside the
micropipette 

ncentrations of ions both inside the
micropipette and in the outside solution to be altered
as desired. Also, the voltage between the two sides of
the membrane can be set at will—that is, “clamped” to
a given voltage.
It has been possible to make such patches small
enough so that only a single channel protein is found 
in the membrane patch being studied. By varying the
concentrations of different ions, as well as the voltage
across the membrane, one can determine the transport 
characteristics 

 one can determine the transport 
characteristics of the single channel and also its gating 
properties.
Facilitated diffusion is also called carrier-mediated dif-
fusion because a substance transported in this manner
diffuses through the membrane using a speciﬁc carrier
protein to help. That is, the carrier facilitates diffusion
of the substance to the other side.
Facilitated diffusion differs from simple diffusion in
the following important way: Although the rate of
simpl

ollowing important way: Although the rate of
simple diffusion through an open channel increases
proportionately with the concentration of the diffus-
ing substance, in facilitated diffusion the rate of 
diffusion approaches a maximum, called Vmax, as the
concentration of the diffusing substance increases.This
difference between simple diffusion and facilitated dif-
fusion is demonstrated in Figure 4–6.The ﬁgure shows
that as the concentration of the diffusing substance
increases, the rate of sim

the diffusing substance
increases, the rate of simple diffusion continues to
increase proportionately, but in the case of facilitated
diffusion, the rate of diffusion cannot rise greater than
the Vmax level.
What is it that limits the rate of facilitated diffusion?
A probable answer is the mechanism illustrated in
Figure 4–7. This ﬁgure shows a carrier protein with a
Concentration of substance
Rate of diffusion
Simple diffusion
Vmax
Facilitated diffusion
Figure 4–6
Effect of concentration of a s

iffusion
Figure 4–6
Effect of concentration of a substance on rate of diffusion through
a membrane by simple diffusion and facilitated diffusion. This
shows that facilitated diffusion approaches a maximum rate called
the Vmax.
Transported
molecule
Binding point
Carrier protein
and
conformational
change
Release
of binding
Figure 4–7
Postulated mechanism for facilitated diffusion.
50
Unit II
the membrane. The rate at which molecules can be
transported by this

rate at which molecules can be
transported by this mechanism can never be greater
than the rate at which the carrier protein molecule 
can undergo change back and forth between its two
states. Note speciﬁcally, though, that this mechanism
allows the transported molecule to move—that is, to
“diffuse”—in either direction through the membrane.
Among the most important substances that cross
cell membranes by facilitated diffusion are glucose and
most of the amino acids. In the case of glucose, the
c

 of the amino acids. In the case of glucose, the
carrier molecule has been discovered, and it has a
molecular weight of about 45,000; it can also transport
several other monosaccharides that have structures
similar to that of glucose, including galactose. Also,
insulin can increase the rate of facilitated diffusion of
glucose as much as 10-fold to 20-fold. This is the prin-
cipal mechanism by which insulin controls glucose use
in the body, as discussed in Chapter 78.
Factors That Affect Net Rate

cussed in Chapter 78.
Factors That Affect Net Rate 
of Diffusion
By now it is evident that many substances can diffuse
through the cell membrane. What is usually important
is the net rate of diffusion of a substance in the desired
direction. This net rate is determined by several
factors.
Effect of Concentration Difference on Net Diffusion Through a
Membrane. Figure 4–8A shows a cell membrane with a
substance in high concentration on the outside and low
concentration on the inside.The rate at wh

and low
concentration on the inside.The rate at which the sub-
stance diffuses inward is proportional to the con-
centration of molecules on the outside, because this
concentration determines how many molecules strike
the outside of the membrane each second. Conversely,
the rate at which molecules diffuse outward is propor-
tional to their concentration inside the membrane.
Therefore, the rate of net diffusion into the cell is pro-
portional to the concentration on the outside minus
the concentr

he concentration on the outside minus
the concentration on the inside, or:
Net diffusion µ (Co - Ci)
in which Co is concentration outside and Ci is concen-
tration inside.
Effect of Membrane Electrical Potential on Diffusion of Ions—
The “Nernst Potential.” If an electrical potential is
applied across the membrane, as shown in Figure
4–8B, the electrical charges of the ions cause them to
move through the membrane even though no concen-
tration difference exists to cause movement. Thus, in
the le

fference exists to cause movement. Thus, in
the left panel of Figure 4–8B, the concentration of
negative ions is the same on both sides of the mem-
brane, but a positive charge has been applied to the
right side of the membrane and a negative charge to
the left, creating an electrical gradient across the mem-
brane. The positive charge attracts the negative ions,
whereas the negative charge repels them. Therefore,
net diffusion occurs from left to right.After much time,
large quantities of negat

o right.After much time,
large quantities of negative ions have moved to the
right, creating the condition shown in the right panel
of Figure 4–8B, in which a concentration difference of
the ions has developed in the direction opposite to the
electrical potential difference. The concentration dif-
ference now tends to move the ions to the left, while
the electrical difference tends to move them to the
right. When the concentration difference rises high
enough, the two effects balance each other.

s high
enough, the two effects balance each other. At normal
body temperature (37°C), the electrical difference 
that will balance a given concentration difference of
univalent ions—such as sodium (Na+) ions—can be
determined from the following formula, called the
Nernst equation:
in which EMF is the electromotive force (voltage)
between side 1 and side 2 of the membrane, C1 is the
concentration on side 1, and C2 is the concentration on
side 2. This equation is extremely important in under-
stan

his equation is extremely important in under-
standing the transmission of nerve impulses and is dis-
cussed in much greater detail in Chapter 5.
Effect of a Pressure Difference Across the Membrane. At
times,
considerable pressure difference develops
EMF in millivolts
61 log C
C
1
2
(
) = ±
-
- - –
+
+
-
-
-
-
-
-
-
-
-
-
-
- -
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
- - -
-
-
–
-
-
-
-
-
Outside
A
B
C
Inside
Membrane
Piston
P2
P1
Co
Ci
Figure 4–8
Effect of concentration difference (A), electrical

Effect of concentration difference (A), electrical potential differ-
ence affecting negative ions (B), and pressure difference (C) to
cause diffusion of molecules and ions through a cell membrane.
Chapter 4
51
between the two sides of a diffusible membrane.
This occurs, for instance, at the blood capillary mem-
brane in all tissues of the body. The pressure is about
20 mm Hg greater inside the capillary than outside.
Pressure actually means the s

illary than outside.
Pressure actually means the sum of all the forces of
the different molecules striking a unit surface area at
a given instant. Therefore, when the pressure is higher
on one side of a membrane than on the other, this
means that the sum of all the forces of the molecules
striking the channels on that side of the membrane is
greater than on the other side. In most instances, this
is caused by greater numbers of molecules striking the
membrane per second on one side than on the o

 the
membrane per second on one side than on the other
side.The result is that increased amounts of energy are
available to cause net movement of molecules from
the high-pressure side toward the low-pressure side.
This effect is demonstrated in Figure 4–8C, which
shows a piston developing high pressure on one side
of a “pore,” thereby causing more molecules to strike
the pore on this side and, therefore, more molecules to
“diffuse” to the other side.
Osmosis Across Selectively
Permeable Membrane

ide.
Permeable Membranes—
“Net Diffusion” of Water
By far the most abundant substance that diffuses
through the cell membrane is water. Enough water
ordinarily diffuses in each direction through the red
cell membrane per second to equal about 100 times the
volume of the cell itself. Yet, normally, the amount that
diffuses in the two directions is balanced so precisely
that zero net movement of water occurs.Therefore, the
volume of the cell remains constant. However, un

e
volume of the cell remains constant. However, under
certain conditions, a concentration difference for water
can develop across a membrane, just as concentration
differences for other substances can occur. When this
happens, net movement of water does occur across the
cell membrane, causing the cell either to swell or to
shrink, depending on the direction of the water move-
ment. This process of net movement of water caused
by a concentration difference of water is called
osmosis.
To give an e

ifference of water is called
osmosis.
To give an example of osmosis, let us assume the
conditions shown in Figure 4–9, with pure water on
one side of the cell membrane and a solution of
sodium chloride on the other side. Water molecules
pass through the cell membrane with ease, whereas
sodium and chloride ions pass through only with difﬁ-
culty. Therefore, sodium chloride solution is actually a
mixture of permeant water molecules and nonperme-
ant sodium and chloride ions, and the membrane is
sa

t sodium and chloride ions, and the membrane is
said to be selectively permeable to water but much less
so to sodium and chloride ions.Yet the presence of the
sodium and chloride has displaced some of the water
molecules on the side of the membrane where these
ions are present and, therefore, has reduced the con-
centration of water molecules to less than that of pure
water. As a result, in the example of Figure 4–9, more
water molecules strike the channels on the left side,
where there is pure 

he channels on the left side,
where there is pure water, than on the right side, where
the water concentration has been reduced. Thus, net
movement of water occurs from left to right—that is,
osmosis occurs from the pure water into the sodium
chloride solution.
If in Figure 4–9 pressure were applied to the sodium
chloride solution, osmosis of water into this solution
would be slowed, stopped, or even reversed.The exact
amount of pressure required to stop osmosis is called
the os

pressure required to stop osmosis is called
the osmotic pressure of the sodium chloride solution.
The principle of a pressure difference opposing
osmosis is demonstrated in Figure 4–10, which shows
a selectively permeable membrane separating two
columns of ﬂuid, one containing pure water and the
other containing a solution of water and any solute
that will not penetrate the membrane. Osmosis of
water from chamber B into chamber A causes the
levels of the ﬂuid columns to become farther and
farthe

s of the ﬂuid columns to become farther and
farther apart, until eventually a pressure difference
develops between the two sides of the membrane great
enough to oppose the osmotic effect.The pressure dif-
ference across the membrane at this point is equal to
the osmotic pressure of the solution that contains the
nondiffusible solute.
Importance of Number of Osmotic Particles (Molar Concentra-
tion) in Determining Osmotic Pressure. The osmotic pres-
sure exerted by particles in a solution, whethe

s-
sure exerted by particles in a solution, whether they
are molecules or ions, is determined by the number of
particles per unit volume of ﬂuid, not by the mass of
the particles. The reason for this is that each particle
in a solution, regardless of its mass, exerts, on average,
the same amount of pressure against the membrane.
That is, large particles, which have greater mass (m)
than small particles, move at slower velocities (v). The
small particles move at higher velocities in such a way
Os

rticles move at higher velocities in such a way
Osmosis
Water
NaCl solution
Figure 4–9
Osmosis at a cell membrane when a sodium chloride solution is
placed on one side of the membrane and water is placed on the
other side.
52
Unit II
that their average kinetic energies (k), determined by
the equation
are the same for each small particle as for each large
particle. Consequently, the factor that determines the
osmotic pressure of a solution is the concentrati

osmotic pressure of a solution is the concentration of
the solution in terms of number of particles (which is
the same as its molar concentration if it is a nondisso-
ciated molecule), not in terms of mass of the solute.
“Osmolality”—The Osmole. To express the concentration
of a solution in terms of numbers of particles, the unit
called the osmole is used in place of grams.
One osmole is 1 gram molecular weight of osmoti-
cally active solute. Thus, 180 grams of glucose, which 
is 1 gram molecul

us, 180 grams of glucose, which 
is 1 gram molecular weight of glucose, is equal to 1
osmole of glucose because glucose does not dissociate
into ions. Conversely, if a solute dissociates into two
ions, 1 gram molecular weight of the solute will
become 2 osmoles because the number of osmotically
active particles is now twice as great as is the case for
the nondissociated solute. Therefore, when fully disso-
ciated, 1 gram molecular weight of sodium chloride,
58.5 grams, is equal to 2 osmoles.
Thu

m chloride,
Thus, a solution that has 1 osmole of solute dissolved
in each kilogram of water is said to have an osmolality
of 1 osmole per kilogram, and a solution that has
1/1000 osmole dissolved per kilogram has an osmo-
lality of 1 milliosmole per kilogram. The normal 
k
mv
2
2
=
osmolality of the extracellular and intracellular ﬂuids
is about 300 milliosmoles per kilogram of water.
Relation of Osmolality to Osmotic Pressure. At normal body
temperature, 37°C

Osmotic Pressure. At normal body
temperature, 37°C, a concentration of 1 osmole per
liter will cause 19,300 mm Hg osmotic pressure in the
solution. Likewise, 1 milliosmole per liter concentra-
tion is equivalent to 19.3 mm Hg osmotic pressure.
Multiplying this value by the 300 milliosmolar con-
centration of the body ﬂuids gives a total calculated
osmotic pressure of the body ﬂuids of 5790 mm Hg.
The measured value for this, however, averages only
about 5500 mm Hg. The reason for this difference

y
about 5500 mm Hg. The reason for this difference is
that many of the ions in the body ﬂuids, such as sodium
and chloride ions, are highly attracted to one another;
consequently, they cannot move entirely unrestrained
in the ﬂuids and create their full osmotic pressure
potential. Therefore, on average, the actual osmotic
pressure of the body ﬂuids is about 0.93 times the cal-
culated value.
The Term “Osmolarity.” Because of the difﬁculty of meas-
uring kilograms of water in a solution, which is

-
uring kilograms of water in a solution, which is required
to determine osmolality, osmolarity, which is the
osmolar concentration expressed as osmoles per liter of
solution rather than osmoles per kilogram of water, is
used instead. Although, strictly speaking, it is osmoles
per kilogram of water (osmolality) that determines
osmotic pressure, for dilute solutions such as those in
the body, the quantitative differences between osmo-
larity and osmolality are less than 1 per cent. Because it
is 

smolality are less than 1 per cent. Because it
is far more practical to measure osmolarity than osmo-
lality, this is the usual practice in almost all physiologic
studies.
“Active Transport” of
Substances Through
Membranes
At times, a large concentration of a substance is
required in the intracellular ﬂuid even though the
extracellular ﬂuid contains only a small concentration.
This is true, for instance, for potassium ions. Con-
versely, it is important to keep the concentrations of
other ions v

portant to keep the concentrations of
other ions very low inside the cell even though their
concentrations in the extracellular ﬂuid are great.This
is especially true for sodium ions. Neither of these two
effects could occur by simple diffusion, because simple
diffusion eventually equilibrates concentrations on the
two sides of the membrane. Instead, some energy
source must cause excess movement of potassium ions
to the inside of cells and excess movement of sodium
ions to the outside of cells. 

 movement of sodium
ions to the outside of cells. When a cell membrane
moves molecules or ions “uphill” against a concentra-
tion gradient (or “uphill” against an electrical or pres-
sure gradient), the process is called active transport.
Different substances that are actively transported
through at least some cell membranes include sodium
ions, potassium ions, calcium ions, iron ions, hydrogen
ions, chloride ions, iodide ions, urate ions, several dif-
ferent sugars, and most of the amino acids.

l dif-
Semipermeable
membrane
Water
B
A
Non-
diffusible
solute
Figure 4–10
Demonstration of osmotic pressure caused by osmosis at a semi-
permeable membrane.
Chapter 4
53
Primary Active Transport and Secondary Active Transport.
Active transport is divided into two types according to
the source of the energy used to cause the transport:
primary active transport and secondary active trans-
port. In primar

nsport and secondary active trans-
port. In primary active transport, the energy is derived
directly from breakdown of adenosine triphosphate
(ATP) or of some other high-energy phosphate com-
pound. In secondary active transport, the energy is
derived secondarily from energy that has been stored
in the form of ionic concentration differences of 
secondary molecular or ionic substances between the
two sides of a cell membrane, created originally by
primary active transport. In both instances, tra

y
primary active transport. In both instances, transport
depends on carrier proteins that penetrate through the
cell membrane, as is true for facilitated diffusion.
However, in active transport, the carrier protein func-
tions differently from the carrier in facilitated diffu-
sion because it is capable of imparting energy to the
transported substance to move it against the electro-
chemical gradient. Following are some examples 
of primary active transport and secondary active 
transport, with 

e transport and secondary active 
transport, with more detailed explanations of their
principles of function.
Sodium-Potassium Pump
Among the substances that are transported by primary
active transport are sodium, potassium, calcium,
hydrogen, chloride, and a few other ions.
The active transport mechanism that has been
studied in greatest detail is the sodium-potassium
(Na+-K+) pump, a transport process that pumps
sodium ions outward through the cell membrane of all
cell

ions outward through the cell membrane of all
cells and at the same time pumps potassium ions from
the outside to the inside. This pump is responsible for
maintaining the sodium and potassium concentration
differences across the cell membrane, as well as for
establishing a negative electrical voltage inside the
cells. Indeed, Chapter 5 shows that this pump is also
the basis of nerve function, transmitting nerve signals
throughout the nervous system.
Figure 4–11 shows the basic physical component

em.
Figure 4–11 shows the basic physical components of
the Na+-K+ pump. The carrier protein is a complex of
two separate globular proteins: a larger one called the
a subunit, with a molecular weight of about 100,000,
and a smaller one called the b subunit, with a molec-
ular weight of about 55,000. Although the function of
the smaller protein is not known (except that it might
anchor the protein complex in the lipid membrane),
the larger protein has three speciﬁc features that are
important for 

has three speciﬁc features that are
important for the functioning of the pump:
1. It has three receptor sites for binding sodium ions
on the portion of the protein that protrudes to the
inside of the cell.
2. It has two receptor sites for potassium ions on the
outside.
3. The inside portion of this protein near the sodium
binding sites has ATPase activity.
To put the pump into perspective: When two potas-
sium ions bind on the outside of the carrier protein 
and three sodium ions bind on the ins

ier protein 
and three sodium ions bind on the inside, the ATPase
function of the protein becomes activated. This then
cleaves one molecule of ATP, splitting it to adenosine
diphosphate (ADP) and liberating a high-energy
phosphate bond of energy. This liberated energy is
then believed to cause a chemical and conformational
change in the protein carrier molecule, extruding the
three sodium ions to the outside and the two potas-
sium ions to the inside.
As with other enzymes, the Na+-K+ ATPase pum

side.
As with other enzymes, the Na+-K+ ATPase pump
can run in reverse. If the electrochemical gradients for
Na+ and K+ are experimentally increased enough so
that the energy stored in their gradients is greater than
the chemical energy of ATP hydrolysis, these ions will
move down their concentration gradients and the Na+-
K+ pump will synthesize ATP from ADP and phos-
phate. The phosphorylated form of the Na+-K+ pump,
therefore, can either donate its phosphate to ADP to
produce ATP or use the e

e its phosphate to ADP to
produce ATP or use the energy to change its confor-
mation and pump Na+ out of the cell and K+ into the
cell. The relative concentrations of ATP, ADP, and
phosphate, as well as the electrochemical gradients for
Na+ and K+, determine the direction of the enzyme
reaction. For some cells, such as electrically active
nerve cells, 60 to 70 per cent of the cells’ energy
requirement may be devoted to pumping Na+ out of
the cell and K+ into the cell.
Importance of the Na+-K+ Pu

 and K+ into the cell.
Importance of the Na+-K+ Pump for Controlling Cell Volume.
One of the most important functions of the Na+-K+
pump is to control the volume of each cell. Without
function of this pump, most cells of the body would
swell until they burst. The mechanism for controlling
the volume is as follows: Inside the cell are large
numbers of proteins and other organic molecules that
cannot escape from the cell. Most of these are nega-
tively charged and therefore attract large numbers o

vely charged and therefore attract large numbers of
potassium, sodium, and other positive ions as well. All
these molecules and ions then cause osmosis of water
to the interior of the cell. Unless this is checked, the
ATP
Inside
Outside
ATPase
ADP
+
Pi
3Na+
3Na+
2K+
2K+
Figure 4–11
Postulated mechanism of the sodium-potassium pump. ADP,
adenosine diphosphate; ATP, adenosine triphosphate; Pi, 
phosphate ion.
54
Unit II
cell will swell indeﬁnitely until it bu

and Muscle
cell will swell indeﬁnitely until it bursts. The normal
mechanism for preventing this is the Na+-K+ pump.
Note again that this device pumps three Na+ ions to
the outside of the cell for every two K+ ions pumped
to the interior. Also, the membrane is far less perme-
able to sodium ions than to potassium ions, so that
once the sodium ions are on the outside, they have a
strong tendency to stay there. Thus, this represents a
net loss of ions out of the cell, which initiates osmosis
of wa

ons out of the cell, which initiates osmosis
of water out of the cell as well.
If a cell begins to swell for any reason, this auto-
matically activates the Na+-K+ pump, moving still more
ions to the exterior and carrying water with them.
Therefore, the Na+-K+ pump performs a continual sur-
veillance role in maintaining normal cell volume.
Electrogenic Nature of the Na+-K+ Pump. The fact that the
Na+-K+ pump moves three Na+ ions to the exterior for
every two K+ ions to the interior means that a n

r
every two K+ ions to the interior means that a net of one
positive charge is moved from the interior of the cell to
the exterior for each cycle of the pump.This creates pos-
itivity outside the cell but leaves a deﬁcit of positive ions
inside the cell; that is, it causes negativity on the inside.
Therefore, the Na+-K+ pump is said to be electrogenic
because it creates an electrical potential across the cell
membrane. As discussed in Chapter 5, this electrical
potential is a basic requirement i

this electrical
potential is a basic requirement in nerve and muscle
ﬁbers for transmitting nerve and muscle signals.
Primary Active Transport of Calcium Ions
Another important primary active transport mecha-
nism is the calcium pump. Calcium ions are normally
maintained at extremely low concentration in the
intracellular cytosol of virtually all cells in the body, at
a concentration about 10,000 times less than that in the
extracellular ﬂuid. This is achieved mainly by two
primary active transp

is is achieved mainly by two
primary active transport calcium pumps. One is in the
cell membrane and pumps calcium to the outside of
the cell. The other pumps calcium ions into one or
more of the intracellular vesicular organelles of the
cell, such as the sarcoplasmic reticulum of muscle cells
and the mitochondria in all cells. In each of these
instances, the carrier protein penetrates the membrane
and functions as an enzyme ATPase, having the same
capability to cleave ATP as the ATPase of the s

me
capability to cleave ATP as the ATPase of the sodium
carrier protein. The difference is that this protein has
a highly speciﬁc binding site for calcium instead of for
sodium.
Primary Active Transport of Hydrogen Ions
At two places in the body, primary active transport of
hydrogen ions is very important: (1) in the gastric
glands of the stomach, and (2) in the late distal tubules
and cortical collecting ducts of the kidneys.
In the gastric glands, the deep-lying parietal cells
have the most po

ds, the deep-lying parietal cells
have the most potent primary active mechanism for
transporting hydrogen ions of any part of the body.
This is the basis for secreting hydrochloric acid in the
stomach digestive secretions. At the secretory ends of
the gastric gland parietal cells, the hydrogen ion con-
centration is increased as much as a millionfold and
then released into the stomach along with chloride
ions to form hydrochloric acid.
In the renal tubules are special intercalated cells in
the l

al tubules are special intercalated cells in
the late distal tubules and cortical collecting ducts that
also transport hydrogen ions by primary active trans-
port. In this case, large amounts of hydrogen ions are
secreted from the blood into the urine for the purpose
of eliminating excess hydrogen ions from the body
ﬂuids. The hydrogen ions can be secreted into the 
urine against a concentration gradient of about 
900-fold.
Energetics of Primary Active Transport
The amount of energy required to 

The amount of energy required to transport a sub-
stance actively through a membrane is determined by
how much the substance is concentrated during trans-
port. Compared with the energy required to concen-
trate a substance 10-fold, to concentrate it 100-fold
requires twice as much energy, and to concentrate 
it 1000-fold requires three times as much energy.
In other words, the energy required is proportional 
to the logarithm of the degree that the substance 
is concentrated,
a

 the degree that the substance 
as expressed by the following 
formula:
Thus, in terms of calories, the amount of energy
required to concentrate 1 osmole of substance 10-fold
is about 1400 calories; or to concentrate it 100-fold,
2800 calories. One can see that the energy expenditure
for concentrating substances in cells or for removing
substances from cells against a concentration gradient
can be tremendous. Some cells, such as those lining the
renal tubules and many glandular 

those lining the
renal tubules and many glandular cells, expend as
much as 90 per cent of their energy for this purpose
alone.
Secondary Active Transport—
Co-Transport and Counter-Transport
When sodium ions are transported out of cells by
primary active transport, a large concentration gradi-
ent of sodium ions across the cell membrane usually
develops—high concentration outside the cell and
very low concentration inside.This gradient represents
a storehouse of energy because the excess sodium
o

outside the cell membrane is always attempting to
diffuse to the interior. Under appropriate conditions,
this diffusion energy of sodium can pull other sub-
stances along with the sodium through the cell mem-
brane.This phenomenon is called co-transport; it is one
form of secondary active transport.
For sodium to pull another substance along with it,
a coupling mechanism is required. This is achieved by
means of still another carrier protein in th

ed by
means of still another carrier protein in the cell mem-
brane. The carrier in this instance serves as an attach-
ment point for both the sodium ion and the substance
to be co-transported. Once they both are attached, the
energy gradient of the sodium ion causes both the
sodium ion and the other substance to be transported
together to the interior of the cell.
Energy in calories per osmole
1400 log C
C
1
2
(
) =
Chapter 4
55
In counter-trans

nces Through the Cell Membrane
55
In counter-transport, sodium ions again attempt to
diffuse to the interior of the cell because of their large
concentration gradient. However, this time, the sub-
stance to be transported is on the inside of the cell and
must be transported to the outside. Therefore, the
sodium ion binds to the carrier protein where it pro-
jects to the exterior surface of the membrane, while 
the substance to be counter-transported binds to the
interior projection of the carrie

ted binds to the
interior projection of the carrier protein. Once both
have bound, a conformational change occurs, and
energy released by the sodium ion moving to the 
interior causes the other substance to move to the
exterior.
Co-Transport of Glucose and Amino Acids
Along with Sodium Ions
Glucose and many amino acids are transported into
most cells against large concentration gradients; the
mechanism of this is entirely by co-transport, as shown
in Figure 4–12. Note that the transport carrier 

n
in Figure 4–12. Note that the transport carrier protein
has two binding sites on its exterior side, one for
sodium and one for glucose.Also, the concentration of
sodium ions is very high on the outside and very low
inside, which provides energy for the transport. A
special property of the transport protein is that a con-
formational change to allow sodium movement to the
interior will not occur until a glucose molecule also
attaches. When they both become attached, the con-
formational change 

both become attached, the con-
formational change takes place automatically, and the
sodium and glucose are transported to the inside of the
cell at the same time. Hence, this is a sodium-glucose
co-transport mechanism.
Sodium co-transport of the amino acids occurs in the
same manner as for glucose, except that it uses a dif-
ferent set of transport proteins. Five amino acid trans-
port proteins have been identiﬁed, each of which is
responsible for transporting one subset of amino acids
with spe

or transporting one subset of amino acids
with speciﬁc molecular characteristics.
Sodium co-transport of glucose and amino acids
occurs especially through the epithelial cells of the
intestinal tract and the renal tubules of the kidneys to
promote absorption of these substances into the
blood, as is discussed in later chapters.
Other important co-transport mechanisms in at
least some cells include co-transport of chloride ions,
iodine ions, iron ions, and urate ions.
Sodium Counter-Transport of 

ions, and urate ions.
Sodium Counter-Transport of Calcium and
Two especially important counter-transport mecha-
nisms (transport in a direction opposite to the primary
ion) 
are 
sodium-calcium 
counter-transport
and
sodium-hydrogen counter-transport.
Sodium-calcium counter-transport occurs through
all or almost all cell membranes, with sodium ions
moving to the interior and calcium ions to the exterior,
both bound to the same transport protein in a counter-
transport mode. This is

port protein in a counter-
transport mode. This is in addition to primary active
transport of calcium that occurs in some cells.
Sodium-hydrogen 
counter-transport 
occurs 
in
several tissues. An especially important example is in
the proximal tubules of the kidneys, where sodium ions
move from the lumen of the tubule to the interior 
of the tubular cell, while hydrogen ions are counter-
transported into the tubule lumen.As a mechanism for
concentrating hydrogen ions, counter-transport is not
ne

trating hydrogen ions, counter-transport is not
nearly as powerful as the primary active transport of
hydrogen ions that occurs in the more distal renal
tubules, but it can transport extremely large numbers
of hydrogen ions, thus making it a key to hydrogen 
ion control in the body ﬂuids, as discussed in detail in
Chapter 30.
Active Transport Through 
Cellular Sheets
At many places in the body, substances must be trans-
ported all the way through a cellular sheet instead of
simply through the ce

 a cellular sheet instead of
simply through the cell membrane. Transport of this
type occurs through the (1) intestinal epithelium, (2)
epithelium of the renal tubules, (3) epithelium of all
exocrine glands, (4) epithelium of the gallbladder, and
(5) membrane of the choroid plexus of the brain and
other membranes.
The basic mechanism for transport of a substance
through a cellular sheet is (1) active transport through
the cell membrane on one side of the transporting cells
in the sheet, and then

e of the transporting cells
in the sheet, and then (2) either simple diffusion or
facilitated diffusion through the membrane on the
opposite side of the cell.
Figure 4–13 shows a mechanism for transport 
of sodium ions through the epithelial sheet of the
intestines, gallbladder, and renal tubules. This ﬁgure
shows that the epithelial cells are connected together
tightly at the luminal pole by means of junctions called
“kisses.” The brush border on the luminal surfaces of
the cells is permeable t

n the luminal surfaces of
the cells is permeable to both sodium ions and water.
Therefore, sodium and water diffuse readily from the
lumen into the interior of the cell. Then, at the basal
and lateral membranes of the cells, sodium ions are
actively transported into the extracellular ﬂuid of 
the surrounding connective tissue and blood vessels.
This creates a high sodium ion concentration gradient
across these membranes, which in turn causes osmosis
Na-binding
site
Na+
Na+
Glucose
Glucose
Glucos

sis
site
Na+
Na+
Glucose
Glucose
Glucose-binding
site
Figure 4–12
Postulated mechanism for sodium co-transport of glucose.
56
Unit II
of water as well. Thus, active transport of sodium ions
at the basolateral sides of the epithelial cells results in
transport not only of sodium ions but also of water.
These are the mechanisms by which almost all the
nutrients, ions, and other substances are absorbed into
the blood from the intestine; they are als

ed into
the blood from the intestine; they are also the way the
same substances are reabsorbed from the glomerular
ﬁltrate by the renal tubules.
Throughout this text are numerous examples of the
different types of transport discussed in this chapter.
Agre P, Kozono D: Aquaporin water channels: molecular
mechanisms for human diseases. FEBS Lett 555:72, 2003.
Benos DJ, Stanton BA: Functional domains within the
degenerin/epithelial sodium channel (Deg/ENaC) super-
family of ion channels.

 channel (Deg/ENaC) super-
family of ion channels. J Physiol 520:631, 1999.
Caplan MJ: Ion pump sorting in polarized renal epithelial
cells. Kidney Int 60:427, 2001.
Decoursey TE: Voltage-gated proton channels and other
proton transfer pathways. Physiol Rev 83:475, 2003.
De Weer P: A century of thinking about cell membranes.
Annu Rev Physiol 62:919, 2000.
Dolphin AC: G protein modulation of voltage-gated calcium
channels. Pharmacol Rev 55:607, 2003.
Jentsch TJ, Stein V, Weinreich F, Zdebik AA: M

03.
Jentsch TJ, Stein V, Weinreich F, Zdebik AA: Molecular
structure and physiological function of chloride channels.
Physiol Rev 82:503, 2002.
Kaupp UB, Seifert R: Cyclic nucleotide-gated ion channels.
Physiol Rev 82:769, 2002.
Kellenberger S, Schild L:
Epithelial sodium channel/
degenerin family of ion channels: a variety of functions for
a shared structure. Physiol Rev 82:735, 2002.
MacKinnon R: Potassium channels. FEBS Lett 555:62, 2003.
Peres A, Giovannardi S, Bossi E, Fesce R: Electrophysi

s A, Giovannardi S, Bossi E, Fesce R: Electrophysiolog-
ical insights into the mechanism of ion-coupled cotrans-
porters. News Physiol Sci 19:80, 2004.
Philipson KD, Nicoll DA, Ottolia M, et al: The Na+/Ca2+
exchange molecule: an overview.Ann N Y Acad Sci 976:1,
2002.
Rossier BC, Pradervand S, Schild L, Hummler E: Epithelial
sodium channel and the control of sodium balance: inter-
action between genetic and environmental factors. Annu
Rev Physiol 64:877, 2002.
Russell JM: Sodium-potassium-chlori

 64:877, 2002.
Russell JM: Sodium-potassium-chloride cotransport. Physiol
Rev 80:211, 2000.
Active
transport
Brush
border
Basement
membrane
Connective tissue
Osmosis
Osmosis
Lumen
Active
transport
Active
transport
Osmosis
Diffusion
Na+
Na+
and
H2O
Na+
Na+
Na+
Figure 4–13
Basic mechanism of active transport across a layer of cells.
C
H
A
P
T
E
R
 
5
57
Electrical potentials exist across the membranes of
virtually all cells of the body. In addition, some 

irtually all cells of the body. In addition, some cells,
such as nerve and muscle cells, are capable of gen-
erating rapidly changing electrochemical impulses
at their membranes, and these impulses are used to
transmit signals along the nerve or muscle mem-
branes. In still other types of cells, such as glandular
cells, macrophages, and ciliated cells, local changes
in membrane potentials also activate many of the cells’ functions. The present
discussion is concerned with membrane potentials gen

scussion is concerned with membrane potentials generated both at rest and
during action by nerve and muscle cells.
Basic Physics of Membrane Potentials
Membrane Potentials Caused by Diffusion
“Diffusion Potential” Caused by an Ion Concentration Difference on the Two Sides of the
Membrane. In Figure 5–1A, the potassium concentration is great inside a nerve
ﬁber membrane but very low outside the membrane. Let us assume that the
membrane in this instance is permeable to the potassium ions but not t

tance is permeable to the potassium ions but not to any
other ions. Because of the large potassium concentration gradient from inside
toward outside, there is a strong tendency for extra numbers of potassium ions
to diffuse outward through the membrane. As they do so, they carry positive
electrical charges to the outside, thus creating electropositivity outside the
membrane and electronegativity inside because of negative anions that remain
behind and do not diffuse outward with the potassium. W

d and do not diffuse outward with the potassium. Within a millisecond or
so, the potential difference between the inside and outside, called the diffusion
potential, becomes great enough to block further net potassium diffusion to the
exterior, despite the high potassium ion concentration gradient. In the normal
mammalian nerve ﬁber, the potential difference required is about 94 millivolts,
with negativity inside the ﬁber membrane.
Figure 5–1B shows the same phenomenon as in Figure 5–1A, but thi

ows the same phenomenon as in Figure 5–1A, but this time
with high concentration of sodium ions outside the membrane and low sodium
inside.These ions are also positively charged.This time, the membrane is highly
permeable to the sodium ions but impermeable to all other ions. Diffusion of
the positively charged sodium ions to the inside creates a membrane potential
of opposite polarity to that in Figure 5–1A, with negativity outside and positiv-
ity inside.Again, the membrane potential rises high

ty inside.Again, the membrane potential rises high enough within milliseconds
to block further net diffusion of sodium ions to the inside; however, this time,
in the mammalian nerve ﬁber, the potential is about 61 millivolts positive inside
the ﬁber.
Thus, in both parts of Figure 5–1, we see that a concentration difference of
ions across a selectively permeable membrane can, under appropriate condi-
tions, create a membrane potential. In later sections of this chapter, we show
that many of the r

ctions of this chapter, we show
that many of the rapid changes in membrane potentials observed during nerve
and muscle impulse transmission result from the occurrence of such rapidly
changing diffusion potentials.
58
Unit II
Relation of the Diffusion Potential to the Concentration Differ-
ence—The Nernst Potential. The diffusion potential level
across a membrane that exactly opposes the net diffu-
sion of a particular ion through the membrane is called
the 

particular ion through the membrane is called
the Nernst potential for that ion, a term that was intro-
duced in Chapter 4. The magnitude of this Nernst
potential is determined by the ratio of the concentra-
tions of that speciﬁc ion on the two sides of the mem-
brane. The greater this ratio, the greater the tendency
for the ion to diffuse in one direction, and therefore
the greater the Nernst potential required to prevent
additional net diffusion.The following equation, called
the Nernst equati

n.The following equation, called
the Nernst equation, can be used to calculate the
Nernst potential for any univalent ion at normal body
temperature of 98.6°F (37°C):
where EMF is electromotive force.
When using this formula, it is usually assumed that
the potential in the extracellular ﬂuid outside the
membrane remains at zero potential, and the Nernst
potential is the potential inside the membrane. Also,
the sign of the potential is positive (+) if the ion dif-
fusing from inside to outside is

) if the ion dif-
fusing from inside to outside is a negative ion, and it
is negative (–) if the ion is positive.Thus, when the con-
centration of positive potassium ions on the inside is
10 times that on the outside, the log of 10 is 1, so that
the Nernst potential calculates to be –61 millivolts
inside the membrane.
Calculation of the Diffusion Potential 
When the Membrane Is Permeable to 
Several Different Ions
When a membrane is permeable to several different
ions, the diffusion potential th

several different
ions, the diffusion potential that develops depends on
EMF millivolts
Concentration inside
Concentration outside
(
) = ± 61 log
three factors: (1) the polarity of the electrical charge
of each ion, (2) the permeability of the membrane (P)
to each ion, and (3) the concentrations (C) of the
respective ions on the inside (i) and outside (o) of 
the membrane. Thus, the following formula, called the
Goldman equation, or the Goldman-Hodgkin-Katz
equation, gives the calculated membran

odgkin-Katz
equation, gives the calculated membrane potential on
the inside of the membrane when two univalent posi-
tive ions, sodium (Na+) and potassium (K+), and one
univalent negative ion, chloride (Cl–), are involved.
Let us study the importance and the meaning of this
equation. First, sodium, potassium, and chloride ions
are the most important ions involved in the develop-
ment of membrane potentials in nerve and muscle
ﬁbers, as well as in the neuronal cells in the nervous
system. The con

 the neuronal cells in the nervous
system. The concentration gradient of each of these
ions across the membrane helps determine the voltage
of the membrane potential.
Second, the degree of importance of each of the ions
in determining the voltage is proportional to the mem-
brane permeability for that particular ion.That is,if the
membrane has zero permeability to both potassium
and chloride ions, the membrane potential becomes
entirely dominated by the concentration gradient of
sodium ions alon

 by the concentration gradient of
sodium ions alone, and the resulting potential will be
equal to the Nernst potential for sodium. The same
holds for each of the other two ions if the membrane
should become selectively permeable for either one of
them alone.
Third, a positive ion concentration gradient from
inside the membrane to the outside causes electroneg-
ativity inside the membrane.The reason for this is that
excess positive ions diffuse to the outside when their
concentration is higher in

 the outside when their
concentration is higher inside than outside.This carries
positive charges to the outside but leaves the nondif-
fusible negative anions on the inside, thus creating
electronegativity on the inside. The opposite effect
occurs when there is a gradient for a negative ion.That
is, a chloride ion gradient from the outside to the inside
causes negativity inside the cell because excess nega-
tively charged chloride ions diffuse to the inside, while
leaving the nondiffusible posi

o the inside, while
leaving the nondiffusible positive ions on the outside.
Fourth, as explained later, the permeability of the
sodium and potassium channels undergoes rapid
changes during transmission of a nerve impulse,
whereas the permeability of the chloride channels
does not change greatly during this process.Therefore,
rapid changes in sodium and potassium permeability
are primarily responsible for signal transmission in
nerves, which is the subject of most of the remainder
of this chapter

e subject of most of the remainder
of this chapter.
Measuring the Membrane
Potential
The method for measuring the membrane potential is
simple in theory but often difﬁcult in practice because
Na
Na
K
K
Cl
Cl
Na
Na
K
K
Cl
Cl
 (
)
= -
◊
+
+
+
+
+
+
+
+
-
-
+
+
+
-
-
61 log
C
P
C
P
C
P
C
P
C
P
C
P
i
i
o
o
o
i
-
+    –
+    –
+    –
+    –
+    –
+    –
+    –
+    –
+    –
–    +
–    +
–    +
–    +
–    +
–    +
–    +
–    +
–    +
–    +
Nerve fiber
(Anions)–
(Anions)–
(–94 mV)
K

–    +
(Anions)–
(Anions)–
(–94 mV)
K+
K+
–    +
–    +
–    +
–    +
–    +
–    +
–    +
–    +
–    +
+    –
+    –
+    –
+    –
+    –
+    –
+    –
+    –
+    –
+    –
(Anions)–
(Anions)–
(+61 mV)
Na+
Na+
DIFFUSION POTENTIALS 
A
B
Figure 5–1
A, Establishment of a “diffusion” potential across a nerve ﬁber
membrane, caused by diffusion of potassium ions from inside the
cell to outside through a membrane that is selectively permeable
only to potassium. B, Establishme

tively permeable
only to potassium. B, Establishment of a “diffusion potential” when
the nerve ﬁber membrane is permeable only to sodium ions. Note
that the internal membrane potential is negative when potassium
ions diffuse and positive when sodium ions diffuse because of
opposite concentration gradients of these two ions.
Chapter 5
Membrane Potentials and Action Potentials
59
of the small size of most of the ﬁbers. Figure 5–2 shows
a small pipette ﬁlled with an electrolyte solution. The
pipett

tte ﬁlled with an electrolyte solution. The
pipette is impaled through the cell membrane to 
the interior of the ﬁber. Then another electrode, called
the “indifferent electrode,” is placed in the extracellular
ﬂuid, and the potential difference between the inside
and outside of the ﬁber is measured using an appropri-
ate voltmeter. This voltmeter is a highly sophisticated
electronic apparatus that is capable of measuring very
small voltages despite extremely high resistance to elec-
trical ﬂow t

te extremely high resistance to elec-
trical ﬂow through the tip of the micropipette, which 
has a lumen diameter usually less than 1 micrometer
and a resistance more than a million ohms. For record-
ing rapid changes in the membrane potential during 
transmission of nerve impulses, the microelectrode is
connected to an oscilloscope, as explained later in the
chapter.
The lower part of Figure 5–3 shows the electrical
potential that is measured at each point in or near the
nerve ﬁber membrane, be

 each point in or near the
nerve ﬁber membrane, beginning at the left side of the
ﬁgure and passing to the right. As long as the electrode
is outside the nerve membrane, the recorded potential
is zero, which is the potential of the extracellular ﬂuid.
Then, as the recording electrode passes through the
voltage change area at the cell membrane (called the
electrical dipole layer), the potential decreases abruptly
to –90 millivolts. Moving across the center of the ﬁber,
the potential remains at a 

he center of the ﬁber,
the potential remains at a steady –90-millivolt level but
reverses back to zero the instant it passes through the
membrane on the opposite side of the ﬁber.
To create a negative potential inside the membrane,
only enough positive ions to develop the electrical
dipole layer at the membrane itself must be transported
outward. All the remaining ions inside the nerve ﬁber
can be both positive and negative, as shown in the upper
panel of Figure 5–3. Therefore, an incredibly sma

panel of Figure 5–3. Therefore, an incredibly small
number of ions needs to be transferred through the
membrane to establish the normal “resting potential” of
–90 millivolts inside the nerve ﬁber; this means that only
about 1/3,000,000 to 1/100,000,000 of the total positive
charges inside the ﬁber needs to be transferred.Also, an
equally small number of positive ions moving from
outside to inside the ﬁber can reverse the potential from
–90 millivolts to as much as +35 millivolts within as littl

volts to as much as +35 millivolts within as little
as 1/10,000 of a second. Rapid shifting of ions in this
manner causes the nerve signals discussed in subse-
quent sections of this chapter.
Resting Membrane Potential
of Nerves
The resting membrane potential of large nerve ﬁbers
when not transmitting nerve signals is about –90 
millivolts. That is, the potential inside the ﬁber is 90 
millivolts more negative than the potential in the extra-
cellular ﬂuid on the outside of the ﬁber. In the next

lular ﬂuid on the outside of the ﬁber. In the next few
paragraphs, we explain all the factors that determine
the level of this resting potential, but before doing so,
we must describe the transport properties of the resting
nerve membrane for sodium and potassium.
Active Transport of Sodium and Potassium Ions Through the
Membrane—The Sodium-Potassium (Na+-K+) Pump. First, let
us recall from Chapter 4 that all cell membranes of the
body have a powerful Na+-K+ that continually pumps
sodium ions to

erful Na+-K+ that continually pumps
sodium ions to the outside of the cell and potassium
ions to the inside, as illustrated on the left-hand side in
Figure 5–4. Further, note that this is an electrogenic
pump because more positive charges are pumped to
the outside than to the inside (three Na+ ions to the
outside for each two K+ ions to the inside), leaving a
net deﬁcit of positive ions on the inside; this causes a
negative potential inside the cell membrane.
—
+
0
+ + + + + + + + + + + 
+ + + +

ell membrane.
—
+
0
+ + + + +     
+ + + + + + + + +  
+ + + + + + + +   
Silver–silver
chloride 
electrode
KCI
– – – – – – – – –        – – – – – – –            
(–90 
mV)
 – – – – – – – – – –      
– – – – –           
Figure 5–2
Measurement of the membrane potential of the nerve ﬁber using
a microelectrode.
–90
Electrical potential
(millivolts)
0
– + – + – + – + – + – + – + –
+ – + + – – + – + – – + + – +
– + –

 – + – + – + –
Figure 5–3
Distribution of positively and negatively charged ions in the extra-
cellular ﬂuid surrounding a nerve ﬁber and in the ﬂuid inside the
ﬁber; note the alignment of negative charges along the inside
surface of the membrane and positive charges along the 

ce of the membrane and positive charges along the outside
surface. The lower panel displays the abrupt changes in mem-
brane potential that occur at the membranes on the two sides of
the ﬁber.
60
Unit II
The Na+-K+ also causes large concentration gradi-
ents for sodium and potassium across the resting nerve
membrane. These gradients are the following:
Na+ (outside):
142 mEq/L
Na+ (inside):
14 mEq/L
K+ (outside):
4 mEq/L
K+ (inside):
140 mEq/L
The ratios of 

de):
4 mEq/L
140 mEq/L
The ratios of these two respective ions from the inside
to the outside are
Na+
inside/Na+
outside = 0.1
K+
inside/K+
outside = 35.0
Leakage of Potassium and Sodium Through the Nerve Mem-
brane. The right side of Figure 5–4 shows a channel
protein in the nerve membrane through which potas-
sium and sodium ions can leak, called a potassium-
sodium (K+-Na+) “leak” channel. The emphasis is on
potassium leakage because, on average, the channels
are far more permeab

use, on average, the channels
are far more permeable to potassium than to sodium,
normally about 100 times as permeable. As discussed
later, this differential in permeability is exceedingly
important in determining the level of the normal
resting membrane potential.
Origin of the Normal Resting
Membrane Potential
Figure 5–5 shows the important factors in the estab-
lishment of the normal resting membrane potential of
–90 millivolts. They are as follows.
Contribution of the Potassium Diffusion Po

ollows.
Contribution of the Potassium Diffusion Potential. In Figure
5–5A, we make the assumption that the only move-
ment of ions through the membrane is diffusion of
potassium ions, as demonstrated by the open channels
between the potassium symbols (K+) inside and
outside the membrane. Because of the high ratio of
potassium ions inside to outside, 35:1, the Nernst
potential corresponding to this ratio is –94 millivolts
because the logarithm of 35 is 1.54, and this times –61
millivolts is –94 m

35 is 1.54, and this times –61
millivolts is –94 millivolts. Therefore, if potassium ions
were the only factor causing the resting potential, the
resting potential inside the ﬁber would be equal to –94
millivolts, as shown in the ﬁgure.
Contribution of Sodium Diffusion Through the Nerve Membrane.
Figure 5–5B shows the addition of slight permeability
of the nerve membrane to sodium ions, caused by the
minute diffusion of sodium ions through the K+-Na+
3Na+
2K+
Na+
K+
Na+
K+
Na+
Na+-K+ pump
K+ -Na

-Na+
3Na+
2K+
Na+
K+
Na+
K+
Na+
K+ -Na+
"leak" channels
ATP
ADP
Outside
K+
Figure 5–4
Functional characteristics of the Na+-K+ pump and of the K+-Na+
“leak” channels. ADP, adenosine diphosphate; ATP, adenosine
triphosphate.
+
+
 
+
+
+
+
+
+
-
-
 
-
-
-
-
-
-
+
+
+
+
+
+
-
-
-
-
-
-
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
142 mEq/L
14 mEq/L
142 mEq/L
4 mEq/L
4 mEq/L
140 mEq/L
(Anions)-
(Anions)-
(–90 mV)
Na+
Na+
K+
4 mEq/L
K+
Na+
pump
A
B
C
Diffusi

0 mV)
Na+
Na+
K+
4 mEq/L
K+
Na+
pump
A
B
C
Diffusion
K+
K+
pump
Diffusion
140 mEq/L
(-94 mV)
(–94 mV)
K+
14 mEq/L
(+61 mV)
(–86 mV)
Na+
140 mEq/L
(–94 mV)
K+
Figure 5–5
Establishment of resting membrane potentials in nerve ﬁbers
under three conditions: A, when the membrane potential is caused
entirely by potassium diffusion alone; B, when the membrane
potential is caused by diffusion of both sodium and potassium
ions; and C, when the membrane potential is caused by diffusion
of both sodium and p

ential is caused by diffusion
of both sodium and potassium ions plus pumping of both these
ions by the Na+-K+ pump.
Chapter 5
61
leak channels. The ratio of sodium ions from inside to
outside the membrane is 0.1,and this gives a calculated
Nernst potential for the inside of the membrane of 
+61 millivolts. But also shown in Figure 5–5B is the
Nernst potential for potassium diffusion of –94 milli-
volts. How do these interact with each other, and what
wil

ow do these interact with each other, and what
will be the summated potential? This can be answered
by using the Goldman equation described previously.
Intuitively, one can see that if the membrane is highly
permeable to potassium but only slightly permeable to
sodium,it is logical that the diffusion of potassium con-
tributes far more to the membrane potential than does
the diffusion of sodium. In the normal nerve ﬁber, the
permeability of the membrane to potassium is about
100 times as great a

embrane to potassium is about
100 times as great as its permeability to sodium. Using
this value in the Goldman equation gives a potential
inside the membrane of –86 millivolts, which is near
the potassium potential shown in the ﬁgure.
Contribution of the Na+-K+ Pump. In Figure 5–5C, the 
Na+-K+ pump is shown to provide an additional con-
tribution to the resting potential. In this ﬁgure, there
is continuous pumping of three sodium ions to the
outside for each two potassium ions pumped to the
in

tside for each two potassium ions pumped to the
inside of the membrane. The fact that more sodium
ions are being pumped to the outside than potassium
to the inside causes continual loss of positive charges
from inside the membrane; this creates an additional
degree of negativity (about –4 millivolts additional) on
the inside beyond that which can be accounted for by
diffusion alone. Therefore, as shown in Figure 5–5C,
the net membrane potential with all these factors
operative at the same time i

ith all these factors
operative at the same time is about –90 millivolts.
In summary, the diffusion potentials alone caused by
potassium and sodium diffusion would give a mem-
brane potential of about –86 millivolts, almost all of
this being determined by potassium diffusion.Then, an
additional –4 millivolts is contributed to the mem-
brane potential by the continuously acting electro-
genic Na+-K+ pump, giving a net membrane potential
of –90 millivolts.
Nerve signals are 

livolts.
Nerve signals are transmitted by action potentials,
which are rapid changes in the membrane potential
that spread rapidly along the nerve ﬁber membrane.
Each action potential begins with a sudden change
from the normal resting negative membrane potential
to a positive potential and then ends with an almost
equally rapid change back to the negative potential.
To conduct a nerve signal, the action potential moves
along the nerve ﬁber until it comes to the ﬁber’s 
en

ng the nerve ﬁber until it comes to the ﬁber’s 
end.
The upper panel of Figure 5–6 shows the changes
that occur at the membrane during the action poten-
tial, with transfer of positive charges to the interior of
the ﬁber at its onset and return of positive charges to
the exterior at its end. The lower panel shows graphi-
cally the successive changes in membrane potential
over a few 10,000ths of a second, illustrating the 
explosive onset of the action potential and the 
almost equally rapid reco

ction potential and the 
almost equally rapid recovery.
The successive stages of the action potential are as
follows.
Resting Stage. This is the resting membrane potential
before the action potential begins. The membrane is
said to be “polarized” during this stage because of the
–90 millivolts negative membrane potential that is
present.
Depolarization Stage. At this time, the membrane sud-
denly becomes very permeable to sodium ions, allow-
ing tremendous numbers of positively charged sodium
io

tremendous numbers of positively charged sodium
ions to diffuse to the interior of the axon. The normal
“polarized” state of –90 millivolts is immediately neu-
tralized by the inﬂowing positively charged sodium
ions, with the potential rising rapidly in the positive
direction. This is called depolarization. In large nerve
ﬁbers, the great excess of positive sodium ions moving
to the inside causes the membrane potential to actu-
ally “overshoot” beyond the zero level and to become
somewhat positi

eyond the zero level and to become
somewhat positive. In some smaller ﬁbers, as well as in
KCI
0.7
0.5 0.6
0.3 0.4
0.1 0.2
0
+35
0
–90
Millivolts
Overshoot
De
po
lar
iza
tion
Repol
ariz
ati
on
Resting
—
+
0
+ + + + +     
+ + + +  
+ + + + + +    
chloride 
electrode
+ + + +         – – – – – –           
  – – – –           
– – – –       
   + + + +      
+ + + +         
– – – – –         
 – – – –         
– – – – 
Figure 5–6
Typical action potential recorded by th

Figure 5–6
Typical action potential recorded by the method shown in the
upper panel of the ﬁgure.
62
Unit II
many central nervous system neurons, the potential
merely approaches the zero level and does not over-
shoot to the positive state.
Repolarization Stage. Within a few 10,000ths of a second
after the membrane becomes highly permeable to
sodium ions, the sodium channels begin to close and
the potassium channels open more than normal.
Then, rapid diffus

channels open more than normal.
Then, rapid diffusion of potassium ions to the exterior
re-establishes the normal negative resting mem-
brane potential. This is called repolarization of the 
membrane.
To explain more fully the factors that cause both
depolarization and repolarization, we need to describe
the special characteristics of two other types of trans-
port channels through the nerve membrane: the
voltage-gated sodium and potassium channels.
Voltage-Gated Sodium and 
T

ls.
The necessary actor in causing both depolarization
and repolarization of the nerve membrane during the
action potential is the voltage-gated sodium channel. A
voltage-gated potassium channel also plays an impor-
tant role in increasing the rapidity of repolarization of
the membrane. These two voltage-gated channels are 
in addition to the Na+-K+ pump and the K+-Na+ leak 
channels.
Voltage-Gated Sodium Channel—Activation
and Inactivation of the Cha

ium Channel—Activation
and Inactivation of the Channel
The upper panel of Figure 5–7 shows the voltage-gated
sodium channel in three separate states. This channel
has two gates—one near the outside of the channel
called the activation gate, and another near the inside
called the inactivation gate. The upper left of the ﬁgure
depicts the state of these two gates in the normal
resting membrane when the membrane potential is
–90 millivolts. In this state, the activation gate is closed,
which preven

state, the activation gate is closed,
which prevents any entry of sodium ions to the inte-
rior of the ﬁber through these sodium channels.
Activation of the Sodium Channel. When the membrane
potential becomes less negative than during the
resting state, rising from –90 millivolts toward zero, it
ﬁnally reaches a voltage—usually somewhere between
–70 and –50 millivolts—that causes a sudden confor-
mational change in the activation gate, ﬂipping it all
the way to the open position. This is called 

 all
the way to the open position. This is called the acti-
vated state; during this state, sodium ions can pour
inward through the channel, increasing the sodium
permeability of the membrane as much as 500- to
5000-fold.
Inactivation of the Sodium Channel. The upper right panel
of Figure 5–7 shows a third state of the sodium
channel. The same increase in voltage that opens the
activation gate also closes the inactivation gate. The
inactivation gate, however, closes a few 10,000ths of 
a second 

ate, however, closes a few 10,000ths of 
a second after the activation gate opens. That is, the
conformational change that ﬂips the inactivation gate
to the closed state is a slower process than the con-
formational change that opens the activation gate.
Therefore, after the sodium channel has remained
open for a few 10,000ths of a second, the inactivation
gate closes, and sodium ions no longer can pour to the
inside of the membrane. At this point, the membrane
potential begins to recover back t

t, the membrane
potential begins to recover back toward the resting
membrane state, which is the repolarization process.
Another important characteristic of the sodium
channel inactivation process is that the inactivation
gate will not reopen until the membrane potential
returns to or near the original resting membrane
potential level. Therefore, it usually is not possible for
the sodium channels to open again without the nerve
ﬁber’s ﬁrst repolarizing.
Voltage-Gated Potassium Channel and 
Its A

rizing.
Its Activation
The lower panel of Figure 5–7 shows the voltage-gated
potassium channel in two states: during the resting
state (left) and toward the end of the action potential
(right). During the resting state, the gate of the potas-
sium channel is closed, and potassium ions are pre-
vented from passing through this channel to the
exterior. When the membrane potential rises from 
–90 millivolts toward zero, this voltage change causes
a conformationa

d zero, this voltage change causes
a conformational opening of the gate and allows
increased potassium diffusion outward through the
channel. However, because of the slight delay in
opening of the potassium channels, for the most part,
gate
Inactivation
gate
Inside
Resting
(-90 mV)
Activated
(-90 to +35 mV)
Inactivated
(+35 to -90 mV,
delayed)
Resting
(-90 mV)
Slow activation
(+35 to -90 mV)
Na+
Na+
K+
K+
Na+
Figure 5–7
Characteristics of the voltage-gated sodium (top) and potassium
(

 of the voltage-gated sodium (top) and potassium
(bottom) channels, showing successive activation and inactiva-
tion of the sodium channels and delayed activation of the potas-
sium channels when the membrane potential is changed from the
normal resting negative value to a positive value.
Chapter 5
63
they open just at the same time that the sodium chan-
nels are beginning to close because of inactivation.
Thus, the decrease in sodium entry to the cell a

.
Thus, the decrease in sodium entry to the cell and the
simultaneous increase in potassium exit from the cell
combine to speed the repolarization process, leading
to full recovery of the resting membrane potential
within another few 10,000ths of a second.
Research Method for Measuring the Effect of Voltage on Opening
and Closing of the Voltage-Gated Channels—The “Voltage
Clamp.” The original research that led to quantitative
understanding of the sodium and potassium channels
was so ingenious th

 sodium and potassium channels
was so ingenious that it led to Nobel Prizes for the sci-
entists responsible, Hodgkin and Huxley.The essence of
these studies is shown in Figures 5–8 and 5–9.
Figure 5–8 shows an experimental apparatus called a
voltage clamp, which is used to measure ﬂow of ions
through the different channels. In using this apparatus,
two electrodes are inserted into the nerve ﬁber. One of
these is to measure the voltage of the membrane poten-
tial, and the other is to conduct ele

brane poten-
tial, and the other is to conduct electrical current into
or out of the nerve ﬁber. This apparatus is used in the
following way: The investigator decides which voltage
he or she wants to establish inside the nerve ﬁber. The
electronic portion of the apparatus is then adjusted to
the desired voltage, and this automatically injects either
positive or negative electricity through the current elec-
trode at whatever rate is required to hold the voltage,
as measured by the voltage electr

old the voltage,
as measured by the voltage electrode, at the level set by
the operator.When the membrane potential is suddenly
increased by this voltage clamp from –90 millivolts to
zero, the voltage-gated sodium and potassium channels
open, and sodium and potassium ions begin to pour
through the channels. To counterbalance the effect of
these ion movements on the desired setting of the intra-
cellular voltage, electrical current is injected automati-
cally through the current electrode of the 

omati-
cally through the current electrode of the voltage clamp
to maintain the intracellular voltage at the required
steady zero level. To achieve this, the current injected
must be equal to but of opposite polarity to the net
current ﬂow through the membrane channels. To
measure how much current ﬂow is occurring at each
instant, the current electrode is connected to an oscillo-
scope that records the current ﬂow, as demonstrated on
the screen of the oscilloscope in Figure 5–8. Finally, the
inv

f the oscilloscope in Figure 5–8. Finally, the
investigator adjusts the concentrations of the ions to
other than normal levels both inside and outside the
nerve ﬁber and repeats the study. This can be done
easily when using large nerve ﬁbers removed from some
crustaceans, especially the giant squid axon, which in
some cases is as large as 1 millimeter in diameter.When
sodium is the only permeant ion in the solutions inside
and outside the squid axon, the voltage clamp measures
current ﬂow only t

xon, the voltage clamp measures
current ﬂow only through the sodium channels. When
potassium is the only permeant ion, current ﬂow only
through the potassium channels is measured.
Another means for studying the ﬂow of ions through
an individual type of channel is to block one type of
channel at a time. For instance, the sodium channels can
be blocked by a toxin called tetrodotoxin by applying it
to the outside of the cell membrane where the sodium
activation gates are located. Conversely, tetrae

m
activation gates are located. Conversely, tetraethylam-
monium ion blocks the potassium channels when it is
applied to the interior of the nerve ﬁber.
Figure 5–9 shows typical changes in conductance of
the voltage-gated sodium and potassium channels when
the membrane potential is suddenly changed by use of
the voltage clamp from –90 millivolts to +10 millivolts
and then, 2 milliseconds later, back to –90 millivolts.
Note the sudden opening of the sodium channels (the
activation stage) within a

he sodium channels (the
activation stage) within a small fraction of a millisecond
after the membrane potential is increased to the posi-
tive value. However, during the next millisecond or so,
the sodium channels automatically close (the inactiva-
tion stage).
Note the opening (activation) of the potassium chan-
nels. These open slowly and reach their full open state
only after the sodium channels have almost completely
closed. Further, once the potassium channels open, they
remain open for the

 potassium channels open, they
remain open for the entire duration of the positive
membrane potential and do not close again until after
the membrane potential is decreased back to a negative
value.
Current
electrode
Voltage
electrode
Amplifier
Electrode
in fluid
Figure 5–8
“Voltage clamp” method for studying ﬂow of ions through speciﬁc
channels.
Conductance
 (mmho/cm2)
 Time (milliseconds)
0
1
2
3
20
10
0
30
–90 mV
+10 mV
–90 mV
Membrane potential
I
n
a
c
ti
v
a
t
Na+ channel
K+ chan

al
I
n
a
c
ti
v
a
t
K+ channel
Figure 5–9
Typical changes in conductance of sodium and potassium ion
channels when the membrane potential is suddenly increased
from the normal resting value of –90 millivolts to a positive value
of +10 millivolts for 2 milliseconds. This ﬁgure shows that the
sodium channels open (activate) and then close (inactivate)
before the end of the 2 milliseconds, whereas the potassium chan-
nels only open (activate), and the rate of opening is much 

 open (activate), and the rate of opening is much slower
than that of the sodium channels.
64
Unit II
Summary of the Events That Cause
Figure 5–10 shows in summary form the sequential
events that occur during and shortly after the action
potential. The bottom of the ﬁgure shows the changes
in membrane conductance for sodium and potassium
ions. During the resting state, before the action 
potential begins, the conductance for potassium i

potential begins, the conductance for potassium ions is
50 to 100 times as great as the conductance for sodium
ions. This is caused by much greater leakage of potas-
sium ions than sodium ions through the leak channels.
However, at the onset of the action potential, the
sodium channels instantaneously become activated
and allow up to a 5000-fold increase in sodium con-
ductance. Then the inactivation process closes the
sodium channels within another fraction of a millisec-
ond. The onset of the

ther fraction of a millisec-
ond. The onset of the action potential also causes
voltage gating of the potassium channels, causing them
to begin opening more slowly a fraction of a millisec-
ond after the sodium channels open.At the end of the
action potential, the return of the membrane potential
to the negative state causes the potassium channels to
close back to their original status, but again, only after
an additional millisecond or more delay.
The middle portion of Figure 5–10 shows the rat

y.
The middle portion of Figure 5–10 shows the ratio
of sodium conductance to potassium conductance at
each instant during the action potential, and above this
is the action potential itself. During the early portion
of the action potential, the ratio of sodium to potas-
sium conductance increases more than 1000-fold.
Therefore, far more sodium ions ﬂow to the interior of
the ﬁber than do potassium ions to the exterior. This
is what causes the membrane potential to become 
positive at the action

mbrane potential to become 
positive at the action potential onset.Then the sodium
channels begin to close and the potassium channels to
open, so that the ratio of conductance shifts far in
favor of high potassium conductance but low sodium
conductance. This allows very rapid loss of potassium
ions to the exterior but virtually zero ﬂow of 
sodium ions to the interior. Consequently, the action
potential quickly returns to its baseline level.
Roles of Other Ions During
Thus f

s of Other Ions During
Thus far, we have considered only the roles of sodium
and potassium ions in the generation of the action
potential. At least two other types of ions must be con-
sidered: negative anions and calcium ions.
Impermeant Negatively Charged Ions (Anions) Inside the Nerve
Axon. Inside the axon are many negatively charged ions
that cannot go through the membrane channels. They
include the anions of protein molecules and of many
organic phosphate compounds, sul

cules and of many
organic phosphate compounds, sulfate compounds, and
so forth. Because these ions cannot leave the interior of
the axon, any deﬁcit of positive ions inside the mem-
brane leaves an excess of these impermeant negative
anions. Therefore, these impermeant negative ions are
responsible for the negative charge inside the ﬁber
when there is a net deﬁcit of positively charged potas-
sium ions and other positive ions.
Calcium Ions. The membranes of almost all cells of the
body have a ca

embranes of almost all cells of the
body have a calcium pump similar to the sodium pump,
and calcium serves along with (or instead of) sodium in
some cells to cause most of the action potential. Like
the sodium pump, the calcium pump pumps calcium
ions from the interior to the exterior of the cell mem-
brane (or into the endoplasmic reticulum of the cell),
creating a calcium ion gradient of about 10,000-fold.
This leaves an internal cell concentration of calcium
ions of about 10–7 molar, in cont

ation of calcium
ions of about 10–7 molar, in contrast to an external con-
centration of about 10–3 molar.
In addition, there are voltage-gated calcium channels.
These channels are slightly permeable to sodium ions as
well as to calcium ions; when they open, both calcium
and sodium ions ﬂow to the interior of the ﬁber. There-
fore, these channels are also called Ca++-Na+ channels.
The calcium channels are slow to become activated,
requiring 10 to 20 times as long for activation as the
sodium cha

 20 times as long for activation as the
sodium channels. Therefore, they are called slow chan-
nels, in contrast to the sodium channels, which are called
fast channels.
Calcium channels are numerous in both cardiac
muscle and smooth muscle. In fact, in some types of
smooth muscle, the fast sodium channels are hardly
(mmho/cm2)
Na+ conductance
K+ conductance
 Milliseconds
0.5
0
1.5
1.0
0.1
0.01
0.005
1
10
100
0.001
0.01
0.1
1
10
100
Overshoot
Positive
afterpotential
+60
+40
+20
0
–20

ershoot
Positive
+60
+40
+20
0
–20
–40
–60
–80
–100
Membrane potential (mV)
Action potential
Ratio of conductances
Na+
K+ 
Figure 5–10
Changes in sodium and potassium conductance during the
course of the action potential. Sodium conductance increases
several thousand-fold during the early stages of the action poten-
tial, whereas potassium conductance increases only about 30-fold
during the latter stages of the action potential and for a short
period thereafter. (These curves were

 for a short
period thereafter. (These curves were constructed from theory pre-
sented in papers by Hodgkin and Huxley but transposed from
squid axon to apply to the membrane potentials of large mam-
malian nerve ﬁbers.)
Chapter 5
65
present, so that the action potentials are caused almost
entirely by activation of slow calcium channels.
Increased Permeability of the Sodium Channels When
There Is a Deﬁcit of Calcium Ions. The concentration of
calcium ion

 of Calcium Ions. The concentration of
calcium ions in the extracellular ﬂuid also has a pro-
found effect on the voltage level at which the sodium
channels become activated. When there is a deﬁcit of
calcium ions, the sodium channels become activated
(opened) by very little increase of the membrane poten-
tial from its normal, very negative level. Therefore, the
nerve ﬁber becomes highly excitable, sometimes dis-
charging repetitively without provocation rather than
remaining in the resting sta

ovocation rather than
remaining in the resting state. In fact, the calcium ion
concentration needs to fall only 50 per cent below
normal before spontaneous discharge occurs in some
peripheral nerves, often causing muscle “tetany.” This is
sometimes lethal because of tetanic contraction of the
respiratory muscles.
The probable way in which calcium ions affect the
sodium channels is as follows:These ions appear to bind
to the exterior surfaces of the sodium channel protein
molecule. The positive c

he sodium channel protein
molecule. The positive charges of these calcium ions 
in turn alter the electrical state of the channel protein
itself, in this way altering the voltage level required to
open the sodium gate.
Up to this point, we have explained the changing
sodium and potassium permeability of the membrane,
as well as the development of the action potential
itself, but we have not explained what initiates the
action potential. The answer is quite simp

tes the
action potential. The answer is quite simple.
A Positive-Feedback Vicious Cycle Opens the Sodium Channels.
First, as long as the membrane of the nerve ﬁber
remains undisturbed, no action potential occurs in the
normal nerve. However, if any event causes enough
initial rise in the membrane potential from –90 milli-
volts toward the zero level, the rising voltage itself
causes many voltage-gated sodium channels to begin
opening.This allows rapid inﬂow of sodium ions, which
causes a further

rapid inﬂow of sodium ions, which
causes a further rise in the membrane potential, thus
opening still more voltage-gated sodium channels and
allowing more streaming of sodium ions to the interior
of the ﬁber. This process is a positive-feedback vicious
cycle that, once the feedback is strong enough, contin-
ues until all the voltage-gated sodium channels have
become activated (opened).Then, within another frac-
tion of a millisecond, the rising membrane potential
causes closure of the sodium cha

embrane potential
causes closure of the sodium channels as well as
opening of potassium channels, and the action poten-
tial soon terminates.
Threshold for Initiation of the Action Potential. An action
potential will not occur until the initial rise in mem-
brane potential is great enough to create the vicious
cycle described in the preceding paragraph. This
occurs when the number of Na+ ions entering the ﬁber
becomes greater than the number of K+ ions leaving
the ﬁber. A sudden rise in membrane

+ ions leaving
the ﬁber. A sudden rise in membrane potential of 15
to 30 millivolts usually is required.Therefore, a sudden
increase in the membrane potential in a large nerve
ﬁber from –90 millivolts up to about –65 millivolts
usually causes the explosive development of an action
potential. This level of –65 millivolts is said to be the
threshold for stimulation.
Propagation of the 
In the preceding paragraphs, we discussed the action
potential as it occurs at one spot on the m

action
potential as it occurs at one spot on the membrane.
However, an action potential elicited at any one point
on an excitable membrane usually excites adjacent
portions of the membrane, resulting in propagation of
the action potential along the membrane. This mecha-
nism is demonstrated in Figure 5–11. Figure 5–11A
shows a normal resting nerve ﬁber, and Figure 5–11B
shows a nerve ﬁber that has been excited in its mid-
portion—that is, the midportion suddenly develops
increased permeability t

portion suddenly develops
increased permeability to sodium. The arrows show a
“local circuit” of current ﬂow from the depolarized
areas of the membrane to the adjacent resting mem-
brane areas. That is, positive electrical charges are
carried by the inward-diffusing sodium ions through
the depolarized membrane and then for several mil-
limeters in both directions along the core of the axon.
These positive charges increase the voltage for a dis-
tance of 1 to 3 millimeters inside the large myelin

ance of 1 to 3 millimeters inside the large myelinated
+ + + + + + + + + + + + – – + + + + + + + + +
+ + + + + + + + + + – – – – + + + + + + + +
 – – + + + + + + + + + + + + + + + + + + – – 
 + + – – – – – – – – – – – – – – – – – – + +
 + + – – – – – – – – – – – – – – – – – – + + 
– – – – – – – – – – – – + + – – – – – – – – – 
+ + + + + + + + + + + + + + + + + +

– – – – – – – 
+ + + + + + + + + + + + + + + + + + + + + + +
– – – – – – – – – – – – – – – – – – – – – – – 
– – – – – – – – – – + + + + – – – – – – – – 
A
B
C
D
Figure 5–11
Propagation of action potentials in both directions along a con-
ductive ﬁber.
66
Unit II
ﬁber to above

mbrane Physiology, Nerve, and Muscle
ﬁber to above the threshold voltage value for initiat-
ing an action potential.Therefore, the sodium channels
in these new areas immediately open, as shown in
Figure 5–11C and D, and the explosive action poten-
tial spreads. These newly depolarized areas produce
still more local circuits of current ﬂow farther along
the membrane, causing progressively more and more
depolarization.
Thus,
the depolarization process
travels along the entire length of the ﬁber. T

ess
travels along the entire length of the ﬁber. This trans-
mission of the depolarization process along a nerve or
muscle ﬁber is called a nerve or muscle impulse.
Direction of Propagation. As demonstrated in Figure
5–11, an excitable membrane has no single direction
of propagation, but the action potential travels in all
directions away from the stimulus—even along all
branches of a nerve ﬁber—until the entire membrane
has become depolarized.
All-or-Nothing Principle. Once an action potential 

ll-or-Nothing Principle. Once an action potential has
been elicited at any point on the membrane of a
normal ﬁber, the depolarization process travels over
the entire membrane if conditions are right, or it does
not travel at all if conditions are not right.This is called
the all-or-nothing principle, and it applies to all normal
excitable tissues. Occasionally, the action potential
reaches a point on the membrane at which it does not
generate sufﬁcient voltage to stimulate the next area
of the m

ﬁcient voltage to stimulate the next area
of the membrane. When this occurs, the spread of
depolarization stops. Therefore, for continued propa-
gation of an impulse to occur, the ratio of action poten-
tial to threshold for excitation must at all times be
greater than 1. This “greater than 1” requirement is
called the safety factor for propagation.
Re-establishing Sodium and
Potassium Ionic Gradients
After Action Potentials Are
Completed—Importance 
of Energy Metabolism
The transmission of each

nce 
The transmission of each action potential along a
nerve ﬁber reduces very slightly the concentration 
differences of sodium and potassium inside and
outside the membrane, because sodium ions diffuse to
the inside during depolarization and potassium ions
diffuse to the outside during repolarization. For a
single action potential, this effect is so minute that 
it cannot be measured. Indeed, 100,000 to 50 million
impulses can be transmitted by large nerve ﬁbers
before the

can be transmitted by large nerve ﬁbers
before the concentration differences reach the point
that action potential conduction ceases. Even so, with
time, it becomes necessary to re-establish the sodium
and potassium membrane concentration differences.
This is achieved by action of the Na+-K+ pump in the
same way as described previously in the chapter for
the original establishment of the resting potential.That
is, sodium ions that have diffused to the interior of the
cell during the action poten

o the interior of the
cell during the action potentials and potassium ions
that have diffused to the exterior must be returned to
their original state by the Na+-K+ pump. Because this
pump requires energy for operation, this “recharging”
of the nerve ﬁber is an active metabolic process, using
energy derived from the adenosine triphosphate
(ATP) energy system of the cell. Figure 5–12 shows
that the nerve ﬁber produces excess heat during
recharging, which is a measure of energy expenditure
when th

, which is a measure of energy expenditure
when the nerve impulse frequency increases.
A special feature of the Na+-K+ ATPase pump is that
its degree of activity is strongly stimulated when excess
sodium ions accumulate inside the cell membrane. In
fact, the pumping activity increases approximately in
proportion to the third power of this intracellular
sodium concentration. That is, as the internal sodium
concentration rises from 10 to 20 mEq/L, the activity
of the pump does not merely double bu

the activity
of the pump does not merely double but increases
about eightfold. Therefore, it is easy to understand
how the “recharging” process of the nerve ﬁber can be
set rapidly into motion whenever the concentration
differences of sodium and potassium ions across the
membrane begin to “run down.”
Plateau in Some Action
In some instances, the excited membrane does not
repolarize immediately after depolarization; instead,
the potential remains on a plateau near the peak of the
spike

al remains on a plateau near the peak of the
spike potential for many milliseconds, and only then
does repolarization begin. Such a plateau is shown 
in Figure 5–13; one can readily see that the plateau
greatly prolongs the period of depolarization. This
type of action potential occurs in heart muscle ﬁbers,
where the plateau lasts for as long as 0.2 to 0.3 second
and causes contraction of heart muscle to last for this
same long period.
The cause of the plateau is a combination of several
factor

 of the plateau is a combination of several
factors. First, in heart muscle, two types of channels
Heat production
Impulses per second
0
100
200
300
At rest
Figure 5–12
Heat production in a nerve ﬁber at rest and at progressively
increasing rates of stimulation.
Chapter 5
67
enter into the depolarization process: (1) the usual
voltage-activated sodium channels, called fast chan-
nels, and (2) voltage-activated calcium-sodium chan-
nels, which are slow to

vated calcium-sodium chan-
nels, which are slow to open and therefore are called
slow channels. Opening of fast channels causes the
spike portion of the action potential, whereas the slow,
prolonged opening of the slow calcium-sodium chan-
nels mainly allows calcium ions to enter the ﬁber,
which is largely responsible for the plateau portion of
the action potential as well.
A second factor that may be partly responsible for
the plateau is that the voltage-gated potassium chan-
nels are slower th

e voltage-gated potassium chan-
nels are slower than usual to open, often not opening
very much until the end of the plateau.This delays the
return of the membrane potential toward its normal
negative value of –80 to –90 millivolts.
RHYTHMICITY OF SOME
EXCITABLE TISSUES—
REPETITIVE DISCHARGE
Repetitive self-induced discharges occur normally in
the heart, in most smooth muscle, and in many of the
neurons of the central nervous system. These rhyth-
mical discharges cause (1) the rhythmical beat of

mical discharges cause (1) the rhythmical beat of the
heart, (2) rhythmical peristalsis of the intestines, and
(3) such neuronal events as the rhythmical control of
breathing.
Also, almost all other excitable tissues can discharge
repetitively if the threshold for stimulation of the
tissue cells is reduced low enough. For instance, even
large nerve ﬁbers and skeletal muscle ﬁbers, which
normally are highly stable, discharge repetitively when
they are placed in a solution that contains the drug 

y are placed in a solution that contains the drug 
veratrine or when the calcium ion concentration falls
below a critical value, both of which increase sodium
permeability of the membrane.
Re-excitation Process Necessary for Spontaneous Rhythmicity.
For spontaneous rhythmicity to occur, the membrane
even in its natural state must be permeable enough to
sodium ions (or to calcium and sodium ions through
the slow calcium-sodium channels) to allow automatic
membrane depolarization. Thus, Figure 5–1

utomatic
membrane depolarization. Thus, Figure 5–14 shows
that the “resting” membrane potential in the rhythmi-
cal control center of the heart is only –60 to –70 milli-
volts. This is not enough negative voltage to keep the
sodium and calcium channels totally closed.Therefore,
the following sequence occurs: (1) some sodium and
calcium ions ﬂow inward; (2) this increases the mem-
brane voltage in the positive direction, which further
increases membrane permeability; (3) still more ions
ﬂow inwar

mbrane permeability; (3) still more ions
ﬂow inward; and (4) the permeability increases more,
and so on, until an action potential is generated.Then,
at the end of the action potential, the membrane re-
polarizes. After another delay of milliseconds or
seconds, spontaneous excitability causes depolariza-
tion again, and a new action potential occurs sponta-
neously.This cycle continues over and over and causes
self-induced rhythmical excitation of the excitable
tissue.
Why does the membrane of t

f the excitable
tissue.
Why does the membrane of the heart control center
not depolarize immediately after it has become re-
polarized, rather than delaying for nearly a second
before the onset of the next action potential? The
answer can be found by observing the curve labeled
“potassium conductance” in Figure 5–14. This shows
that toward the end of each action potential, and con-
tinuing for a short period thereafter, the membrane
becomes excessively permeable to potassium ions.The
excessive o

sively permeable to potassium ions.The
excessive outﬂow of potassium ions carries tremen-
dous numbers of positive charges to the outside of the
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
+60
+40
+20
0
–20
–40
–60
–80
–100
Plateau
Seconds
Figure 5–13
Action potential (in millivolts) from a Purkinje ﬁber of the heart,
showing a “plateau.”
1
0
Seconds
2
3
+60
+40
+20
0
–20
–40
–60
Potassium
conductance
Hyperpolarization
Rhythmical
action
potentials
Threshold
Figure 5–14

action
Threshold
Rhythmical action potentials (in millivolts) similar to those recorded
in the rhythmical control center of the heart. Note their relationship
to potassium conductance and to the state of hyperpolarization.
68
Unit II
membrane, leaving inside the ﬁber considerably more
negativity than would otherwise occur. This continues
for nearly a second after the preceding action poten-
tial is over, thus drawing the membrane pote

oten-
tial is over, thus drawing the membrane potential
nearer to the potassium Nernst potential.This is a state
called hyperpolarization, also shown in Figure 5–14.As
long as this state exists, self–re-excitation will not
occur. But the excess potassium conductance (and the
state of hyperpolarization) gradually disappears, as
shown after each action potential is completed in the
ﬁgure, thereby allowing the membrane potential again
to increase up to the threshold for excitation. Then,
suddenly, 

 to the threshold for excitation. Then,
suddenly, a new action potential results, and the
process occurs again and again.
Special Characteristics 
of Signal Transmission 
in Nerve Trunks
Myelinated and Unmyelinated Nerve Fibers. Figure 5–15
shows a cross section of a typical small nerve, revealing
many large nerve ﬁbers that constitute most of the
cross-sectional area. However, a more careful look
reveals many more very small ﬁbers lying between 
the large ones. The large ﬁbers are myelinated, a

the large ones. The large ﬁbers are myelinated, and the
small ones are unmyelinated. The average nerve trunk
contains about twice as many unmyelinated ﬁbers as
myelinated ﬁbers.
Figure 5–16 shows a typical myelinated ﬁber. The
central core of the ﬁber is the axon, and the membrane
of the axon is the membrane that actually conducts the
action potential.The axon is ﬁlled in its center with axo-
plasm, which is a viscid intracellular ﬂuid. Surrounding
the axon is a myelin sheath that is often much

ing
the axon is a myelin sheath that is often much thicker
than the axon itself. About once every 1 to 3 milli-
meters along the length of the myelin sheath is a node
of Ranvier.
The myelin sheath is deposited around the axon by
Schwann cells in the following manner: The membrane
of a Schwann cell ﬁrst envelops the axon. Then the
Schwann cell rotates around the axon many times,
laying down multiple layers of Schwann cell membrane
containing the lipid substance sphingomyelin. This sub-
stance is 

ipid substance sphingomyelin. This sub-
stance is an excellent electrical insulator that decreases
ion ﬂow through the membrane about 5000-fold.At the
juncture between each two successive Schwann cells
along the axon, a small uninsulated area only 2 to 3
micrometers in length remains where ions still can ﬂow
with ease through the axon membrane between the
extracellular ﬂuid and the intracellular ﬂuid inside the
axon. This area is called the node of Ranvier.
“Saltatory” Conduction in Myelinated F

“Saltatory” Conduction in Myelinated Fibers from Node to Node.
Even though almost no ions can ﬂow through the thick
myelin sheaths of myelinated nerves, they can ﬂow with
ease through the nodes of Ranvier. Therefore, action
potentials occur only at the nodes. Yet the action poten-
tials are conducted from node to node, as shown in
Figure 5–17; this is called saltatory conduction. That is,
electrical current ﬂows through the surrounding extra-
cellular ﬂuid outside the myelin sheath a

g extra-
cellular ﬂuid outside the myelin sheath as well as
through the axoplasm inside the axon from node to
node, exciting successive nodes one after another. Thus,
Figure 5–15
Cross section of a small nerve trunk containing both myelinated
and unmyelinated ﬁbers.
Axon
A
B
Node of Ranvier
Unmyelinated axons
Schwann cell nucleus
Schwann cell cytoplasm
Schwann cell
nucleus
cytoplasm
Myelin
sheath
Figure 5–16
Function of the Schwann cell to insulate nerve ﬁbers. A, Wrapping
of a Schw

ell to insulate nerve ﬁbers. A, Wrapping
of a Schwann cell membrane around a large axon to form the
myelin sheath of the myelinated nerve ﬁber. B, Partial wrapping of
the membrane and cytoplasm of a Schwann cell around multiple
unmyelinated nerve ﬁbers (shown in cross section). (A, Modiﬁed
from Leeson TS, Leeson R: Histology. Philadelphia: WB Saunders,
1979.)
Chapter 5
69
the nerve impulse jumps down the ﬁber, which is the
origin of the term “saltatory.”

ﬁber, which is the
Saltatory conduction is of value for two reasons. First,
by causing the depolarization process to jump long
intervals along the axis of the nerve ﬁber, this mecha-
nism increases the velocity of nerve transmission in
myelinated ﬁbers as much as 5- to 50-fold. Second,
saltatory conduction conserves energy for the axon
because only the nodes depolarize, allowing perhaps
100 times less loss of ions than would otherwise be nec-
essary, and therefore

than would otherwise be nec-
essary, and therefore requiring little metabolism for re-
establishing the sodium and potassium concentration
differences across the membrane after a series of nerve
impulses.
Still another feature of saltatory conduction in 
large myelinated ﬁbers is the following: The excellent
insulation afforded by the myelin membrane and the 50-
fold decrease in membrane capacitance allow repolar-
ization to occur with very little transfer of ions.
Velocity of Conduction in Nerv

e transfer of ions.
Velocity of Conduction in Nerve Fibers. The velocity of con-
duction in nerve ﬁbers varies from as little as 0.25 m/sec
in very small unmyelinated ﬁbers to as great as 100 m/
sec (the length of a football ﬁeld in 1 second) in very
large myelinated ﬁbers.
Excitation—The Process 
of Eliciting the Action
Potential
Basically, any factor that causes sodium ions to begin to
diffuse inward through the membrane in sufﬁcient
numbers can set off automatic regenerative opening of
the so

n set off automatic regenerative opening of
the sodium channels. This can result from mechanical
disturbance of the membrane, chemical effects on the
membrane, or passage of electricity through the mem-
brane. All these are used at different points in the body
to elicit nerve or muscle action potentials: mechanical
pressure to excite sensory nerve endings in the skin,
chemical neurotransmitters to transmit signals from one
neuron to the next in the brain, and electrical current to
transmit signa

he brain, and electrical current to
transmit signals between successive muscle cells in the
heart and intestine. For the purpose of understanding
the excitation process, let us begin by discussing the
principles of electrical stimulation.
Excitation of a Nerve Fiber by a Negatively Charged Metal 
Electrode. The usual means for exciting a nerve or
muscle in the experimental laboratory is to apply elec-
tricity to the nerve or muscle surface through two small
electrodes, one of which is negatively

h two small
electrodes, one of which is negatively charged and 
the other positively charged. When this is done, the
excitable membrane becomes stimulated at the negative 
electrode.
The cause of this effect is the following: Remember
that the action potential is initiated by the opening of
voltage-gated sodium channels. Further, these channels
are opened by a decrease in the normal resting electri-
cal voltage across the membrane. That is, negative
current from the electrode decreases the volta

ive
current from the electrode decreases the voltage on the
outside of the membrane to a negative value nearer to
the voltage of the negative potential inside the ﬁber.
This decreases the electrical voltage across the mem-
brane and allows the sodium channels to open, result-
ing in an action potential. Conversely, at the positive
electrode, the injection of positive charges on the
outside of the nerve membrane heightens the voltage
difference across the membrane rather than lessening
it. This c

ross the membrane rather than lessening
it. This causes a state of hyperpolarization, which actu-
ally decreases the excitability of the ﬁber rather than
causing an action potential.
Threshold for Excitation, and “Acute Local Potentials.” A weak
negative electrical stimulus may not be able to excite a
ﬁber. However, when the voltage of the stimulus is
increased, there comes a point at which excitation does
take place. Figure 5–18 shows the effects of successively
applied stimuli of progressing s

s of successively
applied stimuli of progressing strength. A very weak
stimulus at point A causes the membrane potential to
change from –90 to –85 millivolts, but this is not a sufﬁ-
cient change for the automatic regenerative processes
of the action potential to develop. At point B, the stim-
ulus is greater,but again,the intensity is still not enough.
The stimulus does, however, disturb the membrane
potential locally for as long as 1 millisecond or more
after both of these weak stimuli. These 

d or more
after both of these weak stimuli. These local potential
changes are called acute local potentials, and when they
fail to elicit an action potential, they are called acute
subthreshold potentials.
Myelin sheath
1
2
3
Axoplasm
Figure 5–17
Saltatory conduction along a myelinated axon. Flow of electrical
current from node to node is illustrated by the arrows.
1
2
Threshold
Acute
subthreshold
Action potentials
3
C
B
A
D
4
0
+60
+40
+20
-20

s
3
C
B
A
D
4
0
+60
+40
+20
-20
-40
-60
0
Figure 5–18
Effect of stimuli of increasing voltages to elicit an action potential.
Note development of “acute subthreshold potentials” when the
stimuli are below the threshold value required for eliciting an
action potential.
70
Unit II
At point C in Figure 5–18, the stimulus is even
stronger. Now the local potential has barely reached the
level required to elicit an action potential, called the
t

quired to elicit an action potential, called the
threshold level, but this occurs only after a short “latent
period.” At point D, the stimulus is still stronger, the
acute local potential is also stronger, and the action
potential occurs after less of a latent period.
Thus, this ﬁgure shows that even a very weak stimu-
lus causes a local potential change at the membrane, but
the intensity of the local potential must rise to a thresh-
old level before the action potential is set off.
“Refractory 

fore the action potential is set off.
“Refractory Period” After an Action
Potential, During Which a New
Stimulus Cannot Be Elicited
A new action potential cannot occur in an excitable ﬁber
as long as the membrane is still depolarized from the pre-
ceding action potential.The reason for this is that shortly
after the action potential is initiated,the sodium channels
(or calcium channels, or both) become inactivated, and
no amount of excitatory signal applied to these channels
at this point will o

nal applied to these channels
at this point will open the inactivation gates. The only
condition that will allow them to reopen is for the mem-
brane potential to return to or near the original resting
membrane potential level. Then, within another small
fraction of a second, the inactivation gates of the chan-
nels open, and a new action potential can be initiated.
The period during which a second action potential
cannot be elicited, even with a strong stimulus, is 
called the absolute refracto

 strong stimulus, is 
called the absolute refractory period. This period for
large myelinated nerve ﬁbers is about 1/2500 second.
Therefore, one can readily calculate that such a ﬁber 
can transmit a maximum of about 2500 impulses per
second.
Inhibition of Excitability—
“Stabilizers” and Local Anesthetics
In contrast to the factors that increase nerve excitabil-
ity, still others, called membrane-stabilizing factors, can
decrease excitability. For instance, a high extracellular
ﬂuid calcium ion 

r instance, a high extracellular
ﬂuid calcium ion concentration decreases membrane
permeability to sodium ions and simultaneously reduces
excitability. Therefore, calcium ions are said to be a 
“stabilizer.”
Local Anesthetics. Among the most important stabilizers
are the many substances used clinically as local anes-
thetics, including procaine and tetracaine. Most of these
act directly on the activation gates of the sodium chan-
nels, making it much more difﬁcult for these gates to
open, thereb

much more difﬁcult for these gates to
open, thereby reducing membrane excitability. When
excitability has been reduced so low that the ratio of
action potential strength to excitability threshold (called
the “safety factor”) is reduced below 1.0, nerve impulses
fail to pass along the anesthetized nerves.
Recording Membrane
Potentials and Action
Cathode Ray Oscilloscope. Earlier in this chapter, we noted
that the membrane potential changes extremely rapidly
during the course of an acti

ges extremely rapidly
during the course of an action potential. Indeed, most
of the action potential complex of large nerve ﬁbers
takes place in less than 1/1000 second. In some ﬁgures
of this chapter, an electrical meter has been shown
recording these potential changes. However, it must be
understood that any meter capable of recording most
action potentials must be capable of responding
extremely rapidly. For practical purposes, the only
common type of meter that is capable of responding
accur

 type of meter that is capable of responding
accurately to the rapid membrane potential changes is
the cathode ray oscilloscope.
Figure 5–19 shows the basic components of a cathode
ray oscilloscope.The cathode ray tube itself is composed
basically of an electron gun and a ﬂuorescent screen
against which electrons are ﬁred. Where the electrons
hit the screen surface, the ﬂuorescent material glows. If
the electron beam is moved across the screen, the spot
stimulator
Electronic
sweep cir

he spot
sweep circuit
Horizontal
plates
Vertical
plates
Electron gun
Plugs
Nerve
amplifier
Recorded
action potential
Electron
beam
Stimulus
artifact
Figure 5–19
Cathode ray oscilloscope for recording transient
action potentials.
Chapter 5
71
of glowing light also moves and draws a ﬂuorescent line
on the screen.
In addition to the electron gun and ﬂuorescent
surface, the cathode ray tube is provided with two set

ace, the cathode ray tube is provided with two sets
of electrically charged plates—one set positioned on the
two sides of the electron beam, and the other set posi-
tioned above and below.Appropriate electronic control
circuits change the voltage on these plates so that the
electron beam can be bent up or down in response to
electrical signals coming from recording electrodes on
nerves.The beam of electrons also is swept horizontally
across the screen at a constant time rate by an internal
elect

creen at a constant time rate by an internal
electronic circuit of the oscilloscope. This gives the
record shown on the face of the cathode ray tube in 
the ﬁgure, giving a time base horizontally and voltage
changes from the nerve electrodes shown vertically.
Note at the left end of the record a small stimulus arti-
fact caused by the electrical stimulus used to elicit the
nerve action potential. Then further to the right is 
the recorded action potential itself.
Alberts B, Johnson A,

potential itself.
Grillner S: The motor infrastructure: from ion channels to
neuronal networks. Nat Rev Neurosci 4:573, 2003.
Hodgkin AL: The Conduction of the Nervous Impulse.
Springﬁeld, IL: Charles C Thomas, 1963.
Hodgkin AL, Huxley AF: Quantitative description of 
membrane current and its application to conduction and
excitation in nerve. J Physiol (Lond) 117:500, 1952.
Kleber AG,

 nerve. J Physiol (Lond) 117:500, 1952.
Kleber AG, Rudy Y: Basic mechanisms of cardiac impulse
propagation and associated arrhythmias. Physiol Rev
84:431, 2004.
Lu Z: Mechanism of rectiﬁcation in inward-rectiﬁer K+
channels. Annu Rev Physiol 66:103, 2004.
Matthews GG: Cellular Physiology of Nerve and Muscle.
Malden, MA: Blackwell Science, 1998.
Perez-Reyes E:
Molecular physiology of low-voltage-
activated T-type calcium channels. Physiol Rev 83:117,
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Poliak S, Peles E: The local differentia

17,
2003.
Poliak S, Peles E: The local differentiation of myelinated
axons at nodes of Ranvier. Nat Rev Neurosci 12:968, 2003.
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C
H
A
P
T
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6
72
Contraction of 
Skeletal Muscle
About 40 per cent of the body is sk

keletal Muscle
About 40 per cent of the body is skeletal muscle,
and perhaps another 10 per cent is smooth and
cardiac muscle. Some of the same basic principles of
contraction apply to all these different types of
muscle. In this chapter, function of skeletal muscle
is considered mainly; the specialized functions of
smooth muscle are discussed in Chapter 8, and
cardiac muscle is discussed in Chapter 9.
Physiologic Anatomy of Skeletal Muscle
Figure 6–1 shows the organization

tal Muscle Fiber
Figure 6–1 shows the organization of skeletal muscle, demonstrating that all
skeletal muscles are composed of numerous ﬁbers ranging from 10 to 80
micrometers in diameter. Each of these ﬁbers is made up of successively smaller
subunits, also shown in Figure 6–1 and described in subsequent paragraphs.
In most skeletal muscles, each ﬁber extends the entire length of the muscle.
Except for about 2 per cent of the ﬁbers, each ﬁber is usually innervated by only
one nerve ending, loca

 usually innervated by only
one nerve ending, located near the middle of the ﬁber.
Sarcolemma. The sarcolemma is the cell membrane of the muscle ﬁber. The sar-
colemma consists of a true cell membrane, called the plasma membrane, and an
outer coat made up of a thin layer of polysaccharide material that contains
numerous thin collagen ﬁbrils.At each end of the muscle ﬁber, this surface layer
of the sarcolemma fuses with a tendon ﬁber, and the tendon ﬁbers in turn collect
into bundles to form the 

on ﬁbers in turn collect
into bundles to form the muscle tendons that then insert into the bones.
Myoﬁbrils; Actin and Myosin Filaments. Each muscle ﬁber contains several hundred
to several thousand myoﬁbrils, which are demonstrated by the many small open
dots in the cross-sectional view of Figure 6–1C. Each myoﬁbril (Figure 6–1D
and E) is composed of about 1500 adjacent myosin ﬁlaments and 3000 actin ﬁl-
aments, which are large polymerized protein molecules that are responsible for
the actual m

in molecules that are responsible for
the actual muscle contraction.These can be seen in longitudinal view in the elec-
tron micrograph of Figure 6–2 and are represented diagrammatically in Figure
6–1, parts E through L. The thick ﬁlaments in the diagrams are myosin, and the
thin ﬁlaments are actin.
Note in Figure 6–1E that the myosin and actin ﬁlaments partially interdigi-
tate and thus cause the myoﬁbrils to have alternate light and dark bands, as
illustrated in Figure 6–2. The light bands con

 as
illustrated in Figure 6–2. The light bands contain only actin ﬁlaments and are
called I bands because they are isotropic to polarized light. The dark bands
contain myosin ﬁlaments, as well as the ends of the actin ﬁlaments where they
overlap the myosin, and are called A bands because they are anisotropic to
polarized light. Note also the small projections from the sides of the myosin 
ﬁlaments in Figure 6–1E and L. These are cross-bridges. It is the interaction
between these cross-bridges an

 is the interaction
between these cross-bridges and the actin ﬁlaments that causes contraction.
Figure 6–1E also shows that the ends of the actin ﬁlaments are attached to a
so-called Z disc. From this disc, these ﬁlaments extend in both directions to
Chapter 6
73
interdigitate with the myosin ﬁlaments. The Z disc,
which itself is composed of ﬁlamentous proteins dif-
ferent from the actin and myosin ﬁlaments, passes
crosswise across the myoﬁbril and also crosswise f

crosswise across the myoﬁbril and also crosswise from
myoﬁbril to myoﬁbril, attaching the myoﬁbrils to one
another all the way across the muscle ﬁber. Therefore,
the entire muscle ﬁber has light and dark bands, as do
the individual myoﬁbrils.These bands give skeletal and
cardiac muscle their striated appearance.
The portion of the myoﬁbril (or of the whole muscle
ﬁber) that lies between two successive Z discs is called
a sarcomere. When the muscle ﬁber is contracted, as
shown at the bottom of Fi

e ﬁber is contracted, as
shown at the bottom of Figure 6–4, the length of 
the sarcomere is about 2 micrometers. At this length,
the actin ﬁlaments completely overlap the myosin 
ﬁlaments, and the tips of the actin ﬁlaments are just
beginning to overlap one another. We will see later
that, at this length, the muscle is capable of generating
its greatest force of contraction.
What Keeps the Myosin and Actin Filaments in Place?
Titin Filamentous Molecules. The side-by-side rela-
tionship between t

lecules. The side-by-side rela-
tionship between the myosin and actin ﬁlaments is dif-
ﬁcult to maintain. This is achieved by a large number
of ﬁlamentous molecules of a protein called titin. Each
titin molecule has a molecular weight of about 3
million, which makes it one of the largest protein mol-
ecules in the body. Also, because it is ﬁlamentous, it is
very springy. These springy titin molecules act as a
framework that holds the myosin and actin ﬁlaments
in place so that the contractile mac

ctin ﬁlaments
in place so that the contractile machinery of the sar-
comere will work. There is reason to believe that the
G-Actin molecules
F-Actin filament
Myofilaments
Myosin filament
Myosin molecule
Light
meromyosin
Heavy
Muscle fasciculus
SKELETAL MUSCLE
Muscle
Muscle fiber
Myofibril
H
band
Z
disc
A
band
Sarcomere Z
H
D
E
F
G
H
I
N
M
L
K
J
B
C
Z
I
band
A
Figure 6–1
Organization of skeletal muscle,
from the gross to the molecular
level. F, G, H, and I are cross
sections at the lev

evel. F, G, H, and I are cross
sections at the levels indicated.
(Drawing 
by 
Sylvia 
Colard
Keene. Modiﬁed from Fawcett
DW: Bloom and Fawcett: A Text-
book of Histology. Philadelphia:
WB Saunders, 1986.)
74
Unit II
titin molecule itself acts as template for initial forma-
tion of portions of the contractile ﬁlaments of the 
sarcomere, especially the myosin ﬁlaments.
Sarcoplasm. The many myoﬁbrils of each muscle ﬁber
are suspended side by side in the muscl

uscle ﬁber
are suspended side by side in the muscle ﬁber. The
spaces between the myoﬁbrils are ﬁlled with intracel-
lular ﬂuid called sarcoplasm, containing large quanti-
ties of potassium, magnesium, and phosphate, plus
multiple protein enzymes. Also present are tremen-
dous numbers of mitochondria that lie parallel to the
myoﬁbrils. These supply the contracting myoﬁbrils
with large amounts of energy in the form of adenosine
triphosphate (ATP) formed by the mitochondria.
Sarcoplasmic Reticulum.

ormed by the mitochondria.
Sarcoplasmic Reticulum. Also in the sarcoplasm sur-
rounding the myoﬁbrils of each muscle ﬁber is an
extensive reticulum (Figure 6–3), called the sarcoplas-
mic reticulum. This reticulum has a special organiza-
tion that is extremely important in controlling muscle
contraction, as discussed in Chapter 7.The very rapidly
contracting types of muscle ﬁbers have especially
extensive sarcoplasmic reticula.
General Mechanism of Muscle
The initiation and execution

of Muscle
The initiation and execution of muscle contraction
occur in the following sequential steps.
1. An action potential travels along a motor nerve to
its endings on muscle ﬁbers.
2. At each ending, the nerve secretes a small amount
of the neurotransmitter substance acetylcholine.
3. The acetylcholine acts on a local area of the muscle
ﬁber membrane to open multiple “acetylcholine-
gated” channels through protein molecules ﬂoating
in the membrane.
4. Opening of the acetylcholine

g
4. Opening of the acetylcholine-gated channels allows
large quantities of sodium ions to diffuse to the
interior of the muscle ﬁber membrane. This initiates
an action potential at the membrane.
5. The action potential travels along the muscle ﬁber
membrane in the same way that action potentials
travel along nerve ﬁber membranes.
6. The action potential depolarizes the muscle
membrane, and much of the action potential
electricity ﬂows through the center of the muscle
ﬁber. Here

y ﬂows through the center of the muscle
ﬁber. Here it causes the sarcoplasmic reticulum to
release large quantities of calcium ions that have
been stored within this reticulum.
7. The calcium ions initiate attractive forces between
the actin and myosin ﬁlaments, causing them to
slide alongside each other, which is the contractile
process.
8. After a fraction of a second, the calcium ions are
pumped back into the sarcoplasmic reticulum by a
Ca++ membrane pump, and they remain stored in
the reticu

embrane pump, and they remain stored in
the reticulum until a new muscle action potential
comes along; this removal of calcium ions from the
myoﬁbrils causes the muscle contraction to cease.
We now describe the molecular machinery of the
muscle contractile process.
Molecular Mechanism 
of Muscle Contraction
Sliding Filament Mechanism of Muscle Contraction. Figure
6–4 demonstrates the basic mechanism of muscle con-
traction. It shows the relaxed state of a sarcomere
(top) and the contracted state

tate of a sarcomere
(top) and the contracted state (bottom). In the relaxed
state, the ends of the actin ﬁlaments extending from
two successive Z discs barely begin to overlap one
another.Conversely,in the contracted state,these actin
ﬁlaments have been pulled inward among the myosin
Figure 6–2
Electron micrograph of muscle myoﬁbrils showing the detailed
organization of actin and myosin ﬁlaments. Note the mitochondria
lying between the myoﬁbrils. (From Fawcett DW: The Cell.
Philadelphia: WB Saun

 (From Fawcett DW: The Cell.
Philadelphia: WB Saunders, 1981.)
Figure 6–3
Sarcoplasmic reticulum in the extracellular spaces between the
myoﬁbrils, showing a longitudinal system paralleling the myoﬁ-
brils. Also shown in cross section are T tubules (arrows) that lead
to the exterior of the ﬁber membrane and are important for con-
ducting the electrical signal into the center of the muscle ﬁber.
(From Fawcett DW: The Cell. Philadelphia: WB Saunders, 1981.)
Chapter 6

s, 1981.)
Chapter 6
75
ﬁlaments, so that their ends overlap one another to
their maximum extent. Also, the Z discs have been
pulled by the actin ﬁlaments up to the ends of the
myosin ﬁlaments.Thus, muscle contraction occurs by a
sliding ﬁlament mechanism.
But what causes the actin ﬁlaments to slide inward
among the myosin ﬁlaments? This is caused by forces
generated by interaction of the cross-bridges from 
the myosin ﬁlaments with the actin ﬁlaments. Under
resting

in ﬁlaments with the actin ﬁlaments. Under
resting conditions, these forces are inactive, but when
an action potential travels along the muscle ﬁber,
this causes the sarcoplasmic reticulum to release large
quantities of calcium ions that rapidly surround the
myoﬁbrils. The calcium ions in turn activate the forces
between the myosin and actin ﬁlaments, and contrac-
tion begins. But energy is needed for the contractile
process to proceed. This energy comes from high-
energy bonds in the ATP molecu

gy comes from high-
energy bonds in the ATP molecule, which is degraded
to adenosine diphosphate (ADP) to liberate the
energy. In the next few sections, we describe what is
known about the details of these molecular processes
of contraction.
Molecular Characteristics of the
Myosin Filament. The myosin ﬁlament is composed of
multiple myosin molecules, each having a molecular
weight of about 480,000. Figure 6–5A shows an indi-
vidual molecule; Figure 6–5B shows the organizati

vidual molecule; Figure 6–5B shows the organization
of many molecules to form a myosin ﬁlament, as well
as interaction of this ﬁlament on one side with the ends
of two actin ﬁlaments.
The myosin molecule (see Figure 6–5A) is composed
of six polypeptide chains—two heavy chains, each with
a molecular weight of about 200,000, and four light
chains with molecular weights of about 20,000 each.
The two heavy chains wrap spirally around each other
to form a double helix, which is called the tail of th

orm a double helix, which is called the tail of the
myosin molecule. One end of each of these chains is
folded bilaterally into a globular polypeptide structure
called a myosin head. Thus, there are two free heads
at one end of the double-helix myosin molecule. The
four light chains are also part of the myosin head, two
to each head. These light chains help control the func-
tion of the head during muscle contraction.
The myosin ﬁlament is made up of 200 or more indi-
vidual myosin molecules.The

p of 200 or more indi-
vidual myosin molecules.The central portion of one of
these ﬁlaments is shown in Figure 6–5B, displaying the
tails of the myosin molecules bundled together to form
the body of the ﬁlament, while many heads of the 
molecules hang outward to the sides of the body.Also,
part of the body of each myosin molecule hangs to the
side along with the head, thus providing an arm that
extends the head outward from the body, as shown in
the ﬁgure.The protruding arms and heads together a

the ﬁgure.The protruding arms and heads together are
called cross-bridges. Each cross-bridge is ﬂexible at
two points called hinges—one where the arm leaves
the body of the myosin ﬁlament, and the other where
the head attaches to the arm. The hinged arms allow
the heads either to be extended far outward from the
body of the myosin ﬁlament or to be brought close to
the body. The hinged heads in turn participate in the
actual contraction process, as discussed in the follow-
ing sections.
The total

s discussed in the follow-
The total length of each myosin ﬁlament is uniform,
almost exactly 1.6 micrometers. Note, however, that
there are no cross-bridge heads in the very center of
the myosin ﬁlament for a distance of about 0.2
micrometer because the hinged arms extend away
from the center.
I
A
I
Z
Z
I
A
Relaxed
Contracted
I
Z
Z
Figure 6–4
Relaxed and contracted states of a myoﬁbril showing (top) sliding
of the actin ﬁlaments (pink) into the spaces between the myosin
ﬁlaments (

ink) into the spaces between the myosin
ﬁlaments (red), and (bottom) pulling of the Z membranes toward
Tail
Head
A
B
Two heavy chains
Light chains
Actin filaments
Hinges
Body
Cross-bridges
Figure 6–5
A, Myosin molecule. B, Combination of many myosin molecules
to form a myosin ﬁlament. Also shown are thousands of myosin
cross-bridges and interaction between the heads of the cross-
bridges with adjacent actin ﬁlaments.
76
Unit II
N

6
Unit II
Now, to complete the picture, the myosin ﬁlament
itself is twisted so that each successive pair of cross-
bridges is axially displaced from the previous pair by
120 degrees.This ensures that the cross-bridges extend
in all directions around the ﬁlament.
ATPase Activity of the Myosin Head. Another feature
of the myosin head that is essential for muscle con-
traction is that it functions as an ATPase enzyme. As
explained later, this property allows 

 enzyme. As
explained later, this property allows the head to cleave
ATP and to use the energy derived from the ATP’s
high-energy phosphate bond to energize the contrac-
tion process.
Actin Filament. The actin ﬁlament is also complex.
It is composed of three protein components: actin,
tropomyosin, and troponin.
The backbone of the actin ﬁlament is a double-
stranded F-actin protein molecule, represented by the
two lighter-colored strands in Figure 6–6. The two
strands are wound in a helix in the

e 6–6. The two
strands are wound in a helix in the same manner as the
myosin molecule.
Each strand of the double F-actin helix is composed
of polymerized G-actin molecules, each having a
molecular weight of about 42,000. Attached to each
one of the G-actin molecules is one molecule of 
ADP. It is believed that these ADP molecules are the
active sites on the actin ﬁlaments with which the cross-
bridges of the myosin ﬁlaments interact to cause
muscle contraction.The active sites on the two F-actin

le contraction.The active sites on the two F-actin
strands of the double helix are staggered, giving one
active site on the overall actin ﬁlament about every 2.7
nanometers.
Each actin ﬁlament is about 1 micrometer long. The
bases of the actin ﬁlaments are inserted strongly into
the Z discs; the ends of the ﬁlaments protrude in both
directions to lie in the spaces between the myosin 
molecules, as shown in Figure 6–4.
Tropomyosin Molecules. The actin ﬁlament also con-
tains another protein, trop

ctin ﬁlament also con-
tains another protein, tropomyosin. Each molecule of
tropomyosin has a molecular weight of 70,000 and a
length of 40 nanometers.These molecules are wrapped
spirally around the sides of the F-actin helix. In the
resting state, the tropomyosin molecules lie on top of
the active sites of the actin strands, so that attraction
cannot occur between the actin and myosin ﬁlaments
to cause contraction.
Troponin and Its Role in Muscle Contraction. Attached
intermittently along the s

e Contraction. Attached
intermittently along the sides of the tropomyosin mol-
ecules are still other protein molecules called troponin.
These are actually complexes of three loosely bound
protein subunits, each of which plays a speciﬁc role in
controlling muscle contraction. One of the subunits
(troponin I) has a strong afﬁnity for actin, another
(troponin T) for tropomyosin, and a third (troponin C)
for calcium ions. This complex is believed to attach 
the tropomyosin to the actin.The strong a

 attach 
the tropomyosin to the actin.The strong afﬁnity of the
troponin for calcium ions is believed to initiate the
contraction process, as explained in the next section.
Interaction of One Myosin Filament, 
Two Actin Filaments, and Calcium Ions 
to Cause Contraction
Inhibition of the Actin Filament by the Troponin-Tropomyosin
Complex; Activation by Calcium Ions. A pure actin ﬁlament
without the presence of the troponin-tropomyosin
complex (but in the presence of magnesium ions and
ATP) binds 

 in the presence of magnesium ions and
ATP) binds instantly and strongly with the heads 
of the myosin molecules. Then, if the troponin-
tropomyosin complex is added to the actin ﬁlament,
the binding between myosin and actin does not take
place. Therefore, it is believed that the active sites on
the normal actin ﬁlament of the relaxed muscle are
inhibited or physically covered by the troponin-
tropomyosin complex. Consequently, the sites cannot
attach to the heads of the myosin ﬁlaments to cause

ttach to the heads of the myosin ﬁlaments to cause
contraction. Before contraction can take place, the
inhibitory effect of the troponin-tropomyosin complex
must itself be inhibited.
This brings us to the role of the calcium ions. In 
the presence of large amounts of calcium ions, the
inhibitory effect of the troponin-tropomyosin on 
the actin ﬁlaments is itself inhibited. The mechanism
of this is not known, but one suggestion is the follow-
ing:When calcium ions combine with troponin C, each
mo

When calcium ions combine with troponin C, each
molecule of which can bind strongly with up to four
calcium ions, the troponin complex supposedly under-
goes a conformational change that in some way tugs
on the tropomyosin molecule and moves it deeper into
the groove between the two actin strands.This “uncov-
ers” the active sites of the actin, thus allowing these to
attract the myosin cross-bridge heads and cause con-
traction to proceed. Although this is a hypothetical
mechanism, it does empha

gh this is a hypothetical
mechanism, it does emphasize that the normal relation
between the troponin-tropomyosin complex and actin
is altered by calcium ions, producing a new condition
that leads to contraction.
Interaction Between the “Activated” Actin Filament and the
Myosin Cross-Bridges—The “Walk-Along” Theory of Contrac-
tion. As soon as the actin ﬁlament becomes activated
by the calcium ions, the heads of the cross-bridges
from the myosin ﬁlaments become attracted to the
active sites of th

laments become attracted to the
active sites of the actin ﬁlament, and this, in some way,
causes contraction to occur. Although the precise
Active sites
F-actin
Tropomyosin
Troponin complex
Figure 6–6
Actin ﬁlament, composed of two helical strands of F-actin mole-
cules and two strands of tropomyosin molecules that ﬁt in the
grooves between the actin strands. Attached to one end of each
tropomyosin molecule is a troponin
complex that initiates 
contraction.
Chapter 6
Contraction of Skeletal Musc

ontraction.
Chapter 6
77
manner by which this interaction between the cross-
bridges and the actin causes contraction is still partly
theoretical, one hypothesis for which considerable evi-
dence exists is the “walk-along” theory (or “ratchet”
theory) of contraction.
Figure 6–7 demonstrates this postulated walk-along
mechanism for contraction.The ﬁgure shows the heads
of two cross-bridges attaching to and disengaging from
active sites of an actin ﬁlament. It is pos

g from
active sites of an actin ﬁlament. It is postulated that
when a head attaches to an active site, this attach-
ment simultaneously causes profound changes in the
intramolecular forces between the head and arm of its
cross-bridge. The new alignment of forces causes the
head to tilt toward the arm and to drag the actin ﬁla-
ment along with it. This tilt of the head is called the
power stroke. Then, immediately after tilting, the head
automatically breaks away from the active site. Next,
the h

ally breaks away from the active site. Next,
the head returns to its extended direction. In this 
position, it combines with a new active site farther
down along the actin ﬁlament; then the head tilts again
to cause a new power stroke, and the actin ﬁlament
moves another step. Thus, the heads of the cross-
bridges bend back and forth and step by step walk
along the actin ﬁlament, pulling the ends of two suc-
cessive actin ﬁlaments toward the center of the myosin
ﬁlament.
Each one of the cross-br

er of the myosin
ﬁlament.
Each one of the cross-bridges is believed to operate
independently of all others, each attaching and pulling
in a continuous repeated cycle. Therefore, the greater
the number of cross-bridges in contact with the actin
ﬁlament at any given time, the greater, theoretically,
the force of contraction.
ATP as the Source of Energy for Contraction—Chemical Events
in the Motion of the Myosin Heads. When a muscle con-
tracts, work is performed and energy is required. Large
amoun

k is performed and energy is required. Large
amounts of ATP are cleaved to form ADP during the
contraction process; the greater the amount of work
performed by the muscle, the greater the amount of
ATP that is cleaved, which is called the Fenn effect. The
following sequence of events is believed to be the
means by which this occurs:
1. Before contraction begins, the heads of the cross-
bridges bind with ATP. The ATPase activity of the
myosin head immediately cleaves the ATP but
leaves the cleava

 immediately cleaves the ATP but
leaves the cleavage products, ADP plus phosphate
ion, bound to the head. In this state, the
conformation of the head is such that it extends
perpendicularly toward the actin ﬁlament but is
not yet attached to the actin.
2. When the troponin-tropomyosin complex binds
with calcium ions, active sites on the actin ﬁlament
are uncovered, and the myosin heads then bind
with these, as shown in Figure 6–7.
3. The bond between the head of the cross-bridge
and the active s

ween the head of the cross-bridge
and the active site of the actin ﬁlament causes a
conformational change in the head, prompting the
head to tilt toward the arm of the cross-bridge.
This provides the power stroke for pulling the
actin ﬁlament. The energy that activates the 
power stroke is the energy already stored, like a
“cocked” spring, by the conformational change
that occurred in the head when the ATP molecule
was cleaved earlier.
4. Once the head of the cross-bridge tilts, this allows
rele

e head of the cross-bridge tilts, this allows
release of the ADP and phosphate ion that were
previously attached to the head. At the site of
release of the ADP, a new molecule of ATP binds.
This binding of new ATP causes detachment of
the head from the actin.
5. After the head has detached from the actin, the
new molecule of ATP is cleaved to begin the next
cycle, leading to a new power stroke. That is,
the energy again “cocks” the head back to its
perpendicular condition, ready to begin the new

ts
power stroke cycle.
6. When the cocked head (with its stored energy
derived from the cleaved ATP) binds with a new
active site on the actin ﬁlament, it becomes
uncocked and once again provides a new power
stroke.
Thus, the process proceeds again and again until the
actin ﬁlaments pull the Z membrane up against 
the ends of the myosin ﬁlaments or until the load on
the muscle becomes too great for further pulling to
occur.
Effect of Amount of Acti

further pulling to
occur.
Effect of Amount of Actin and Myosin
Filament Overlap on Tension
Developed by the Contracting Muscle
Figure 6–8 shows the effect of sarcomere length and
amount of myosin-actin ﬁlament overlap on the active
tension developed by a contracting muscle ﬁber.To the
right, shown in black, are different degrees of overlap
of the myosin and actin ﬁlaments at different sarco-
mere lengths. At point D on the diagram, the actin 
ﬁlament has pulled all the way out to the end of the

ment has pulled all the way out to the end of the
myosin ﬁlament, with no actin-myosin overlap. At this
point, the tension developed by the activated muscle
is zero. Then, as the sarcomere shortens and the actin
ﬁlament begins to overlap the myosin ﬁlament, the
tension increases progressively until the sarcomere
length decreases to about 2.2 micrometers. At this
point, the actin ﬁlament has already overlapped all 
the cross-bridges of the myosin ﬁlament but has not
yet reached the center of the 

ﬁlament but has not
yet reached the center of the myosin ﬁlament. With
Actin filament
Hinges
Power
stroke
Movement
Figure 6–7
“Walk-along” mechanism for contraction of the muscle.
78
Unit II
further shortening, the sarcomere maintains full
tension until point B is reached, at a sarcomere length
of about 2 micrometers. At this point, the ends of the
two actin ﬁlaments begin to overlap each other in
addition to overlapping the myo

rlap each other in
addition to overlapping the myosin ﬁlaments. As the
sarcomere length falls from 2 micrometers down to
about 1.65 micrometers, at point A, the strength of
contraction decreases rapidly. At this point, the two Z
discs of the sarcomere abut the ends of the myosin ﬁl-
aments. Then, as contraction proceeds to still shorter
sarcomere lengths, the ends of the myosin ﬁlaments
are crumpled and, as shown in the ﬁgure, the strength
of contraction approaches zero, but the entire muscle
ha

traction approaches zero, but the entire muscle
has now contracted to its shortest length.
Effect of Muscle Length on Force of Contraction in the Whole
Intact Muscle. The top curve of Figure 6–9 is similar to
that in Figure 6–8, but the curve in Figure 6–9 depicts
tension of the intact, whole muscle rather than of a
single muscle ﬁber. The whole muscle has a large
amount of connective tissue in it; also, the sarcomeres
in different parts of the muscle do not always contract
the same amount. Ther

uscle do not always contract
the same amount. Therefore, the curve has somewhat
different dimensions from those shown for the 
individual muscle ﬁber,but it exhibits the same general
form for the slope in the normal range of contraction,
as noted in Figure 6–9.
Note in Figure 6–9 that when the muscle is at its
normal resting length, which is at a sarcomere length
of about 2 micrometers, it contracts upon activation
with the approximate maximum force of contraction.
However, the increase in tensi

rce of contraction.
However, the increase in tension that occurs during
contraction, called active tension, decreases as the
muscle is stretched beyond its normal length—that is,
to a sarcomere length greater than about 2.2 microm-
eters. This is demonstrated by the decreased length of
the arrow in the ﬁgure at greater than normal muscle
length.
Relation of Velocity of Contraction
to Load
A skeletal muscle contracts extremely rapidly when it
contracts against no load—to a state of full contracti

racts against no load—to a state of full contraction
in about 0.1 second for the average muscle. When loads
are applied, the velocity of contraction becomes pro-
gressively less as the load increases, as shown in Figure
6–10.That is, when the load has been increased to equal
the maximum force that the muscle can exert, the veloc-
ity of contraction becomes zero and no contraction
results, despite activation of the muscle ﬁber.
This decreasing velocity of contraction with load is
caused by the fa

ocity of contraction with load is
caused by the fact that a load on a contracting muscle
is a reverse force that opposes the contractile force
caused by muscle contraction. Therefore, the net force
that is available to cause velocity of shortening is 
correspondingly reduced.
Energetics of Muscle
Work Output During Muscle
When a muscle contracts against a load, it performs
work. This means that energy is transferred from the
muscle to the external load to lift an object t

he
muscle to the external load to lift an object to a greater
height or to overcome resistance to movement.
In mathematical terms, work is deﬁned by the 
following equation:
3
4
2
1
0
100
50
0
Length of sarcomere (micrometers)
A
B C
D
C
B
A
D
Tension developed
(per cent)
Figure 6–8
Length-tension diagram for a single fully contracted sarcomere,
showing maximum strength of contraction when the sarcomere is
2.0 to 2.2 micrometers in length. At the upper right are the rela-
tive positions of the ac

upper right are the rela-
tive positions of the actin and myosin ﬁlaments at different sar-
comere lengths from point A to point D. (Modiﬁed from Gordon
AM, Huxley AF, Julian FJ: The length-tension diagram of single
vertebrate striated muscle ﬁbers. J Physiol 171:28P, 1964.)
0
Length 
Tension of muscle
1/2
normal
2 x
normal
Normal
Increase in tension
during contraction
Tension during
Normal range of contraction
Tension
before contraction
D
Figure 6–9
Relation of muscle length to tens

ion
D
Figure 6–9
Relation of muscle length to tension in the muscle both before and
during muscle contraction.
Chapter 6
79
W = L ¥ D
in which W is the work output, L is the load, and D is
the distance of movement against the load.The energy
required to perform the work is derived from the
chemical reactions in the muscle cells during contrac-
tion, as described in the following sections.
Sources of Energy for Muscle
We have already seen that muscle con

e
We have already seen that muscle contraction depends
on energy supplied by ATP. Most of this energy is
required to actuate the walk-along mechanism by
which the cross-bridges pull the actin ﬁlaments, but
small amounts are required for (1) pumping calcium
ions from the sarcoplasm into the sarcoplasmic retic-
ulum after the contraction is over, and (2) pumping
sodium and potassium ions through the muscle ﬁber
membrane to maintain appropriate ionic environment
for propagation of muscl

opriate ionic environment
for propagation of muscle ﬁber action potentials.
The concentration of ATP in the muscle ﬁber, about
4 millimolar, is sufﬁcient to maintain full contraction
for only 1 to 2 seconds at most.The ATP is split to form
ADP, which transfers energy from the ATP molecule
to the contracting machinery of the muscle ﬁber.Then,
as described in Chapter 2, the ADP is rephosphory-
lated to form new ATP within another fraction of a
second, which allows the muscle to continue its con-
t

nd, which allows the muscle to continue its con-
traction. There are several sources of the energy for
this rephosphorylation.
The ﬁrst source of energy that is used to reconsti-
tute the ATP is the substance phosphocreatine, which
carries a high-energy phosphate bond similar to the
bonds of ATP. The high-energy phosphate bond of
phosphocreatine has a slightly higher amount of free
energy than that of each ATP bond, as is discussed
more fully in Chapters 67 and 72. Therefore, phospho-
creatine i

Chapters 67 and 72. Therefore, phospho-
creatine is instantly cleaved, and its released energy
causes bonding of a new phosphate ion to ADP to
reconstitute the ATP. However, the total amount of
phosphocreatine in the muscle ﬁber is also very little—
only about ﬁve times as great as the ATP. Therefore,
the combined energy of both the stored ATP and 
the phosphocreatine in the muscle is capable of
causing maximal muscle contraction for only 5 to 8
seconds.
The second important source of energy, wh

seconds.
The second important source of energy, which is
used to reconstitute both ATP and phosphocreatine,
is “glycolysis” of glycogen previously stored in the
muscle cells. Rapid enzymatic breakdown of the glyco-
gen to pyruvic acid and lactic acid liberates energy 
that is used to convert ADP to ATP; the ATP can 
then be used directly to energize additional muscle 
contraction and also to re-form the stores of 
phosphocreatine.
The importance of this glycolysis mechanism is
twofold. First, th

of this glycolysis mechanism is
twofold. First, the glycolytic reactions can occur even
in the absence of oxygen, so that muscle contraction
can be sustained for many seconds and sometimes up
to more than a minute, even when oxygen delivery
from the blood is not available. Second, the rate of for-
mation of ATP by the glycolytic process is about 2.5
times as rapid as ATP formation in response to cellu-
lar foodstuffs reacting with oxygen. However, so 
many end products of glycolysis accumulate i

, so 
many end products of glycolysis accumulate in the
muscle cells that glycolysis also loses its capability to
sustain maximum muscle contraction after about 1
minute.
The third and ﬁnal source of energy is oxidative
metabolism. This means combining oxygen with the
end products of glycolysis and with various other cel-
lular foodstuffs to liberate ATP. More than 95 per cent
of all energy used by the muscles for sustained, long-
term contraction is derived from this source. The
foodstuffs that

n is derived from this source. The
foodstuffs that are consumed are carbohydrates, fats,
and protein. For extremely long-term maximal muscle
activity—over a period of many hours—by far the
greatest proportion of energy comes from fats, but for
periods of 2 to 4 hours, as much as one half of the
energy can come from stored carbohydrates.
The detailed mechanisms of these energetic
processes are discussed in Chapters 67 through 72.
In addition, the importance of the different mecha-
nisms of energy

importance of the different mecha-
nisms of energy release during performance of 
different sports is discussed in Chapter 84 on sports
physiology.
Efﬁciency of Muscle Contraction. The efﬁciency of an
engine or a motor is calculated as the percentage of
energy input that is converted into work instead of heat.
The percentage of the input energy to muscle (the
chemical energy in nutrients) that can be converted into
work, even under the best conditions, is less than 25 per
cent, with the remainde

tions, is less than 25 per
cent, with the remainder becoming heat. The reason for
this low efﬁciency is that about one half of the energy
in foodstuffs is lost during the formation of ATP, and
even then, only 40 to 45 per cent of the energy in the
ATP itself can later be converted into work.
2
3
4
0
1
30
20
10
0
Load-opposing contraction (kg)
Velocity of contraction (cm/sec)
Figure 6–10
Relation of load to velocity of contraction in a skeletal muscle 
with a cross section of 1 square centimeter 

scle 
with a cross section of 1 square centimeter and a length of 8 
centimeters.
80
Unit II
Force of contraction
0
40
80
120
160
200
Duration of
depolarization
Ocular
muscle
Gastrocnemius
Soleus
Figure 6–12
Duration of isometric contractions for different types of mammalian
skeletal muscles, showing a latent period between the action
potential (depolarization) and muscle contraction.
Maximum efﬁciency can be realized only when the
muscle contr

ﬁciency can be realized only when the
muscle contracts at a moderate velocity. If the muscle
contracts slowly or without any movement, small
amounts of maintenance heat are released during con-
traction, even though little or no work is performed,
thereby decreasing the conversion efﬁciency to as little
as zero. Conversely, if contraction is too rapid, large pro-
portions of the energy are used to overcome viscous
friction within the muscle itself,and this,too,reduces the
efﬁciency of contractio

f,and this,too,reduces the
efﬁciency of contraction. Ordinarily, maximum efﬁ-
ciency is developed when the velocity of contraction is
about 30 per cent of maximum.
Characteristics of Whole
Muscle Contraction
Many features of muscle contraction can be demon-
strated by eliciting single muscle twitches. This can be
accomplished by instantaneous electrical excitation of
the nerve to a muscle or by passing a short electrical
stimulus through the muscle itself, giving rise to a single,
sudden contrac

le itself, giving rise to a single,
sudden contraction lasting for a fraction of a second.
Isometric Versus Isotonic Contraction. Muscle contraction is
said to be isometric when the muscle does not shorten
during contraction and isotonic when it does shorten but
the tension on the muscle remains constant throughout
the contraction. Systems for recording the two types of
muscle contraction are shown in Figure 6–11.
In the isometric system, the muscle contracts against
a force transducer without d

cle contracts against
a force transducer without decreasing the muscle
length, as shown on the right in Figure 6–11. In the iso-
tonic system, the muscle shortens against a ﬁxed load;
this is illustrated on the left in the ﬁgure, showing a
muscle lifting a pan of weights. The characteristics of
isotonic contraction depend on the load against which
the muscle contracts, as well as the inertia of the load.
However, the isometric system records strictly changes
in force of muscle contraction itself

ctly changes
in force of muscle contraction itself. Therefore, the iso-
metric system is most often used when comparing the
functional characteristics of different muscle types.
Characteristics of Isometric Twitches Recorded from Different
Muscles. The human body has many sizes of skeletal
muscles—from the very small stapedius muscle in the
middle ear, measuring only a few millimeters long and
a millimeter or so in diameter, up to the very large
quadriceps muscle, a half million times as large a

quadriceps muscle, a half million times as large as the
stapedius. Further, the ﬁbers may be as small as 10
micrometers in diameter or as large as 80 micrometers.
Finally, the energetics of muscle contraction vary con-
siderably from one muscle to another.Therefore, it is no
wonder that the mechanical characteristics of muscle
contraction differ among muscles.
Figure 6–12 shows records of isometric contractions
of three types of skeletal muscle: an ocular muscle,
which has a duration of isometri

an ocular muscle,
which has a duration of isometric contraction of less
than 1/40 second; the gastrocnemius muscle, which has
a duration of contraction of about 1/15 second; and the
soleus muscle, which has a duration of contrac-
tion of about 1/3 second. It is interesting that these dura-
tions of contraction are adapted to the functions of the
respective muscles.
Ocular movements must be
extremely rapid to maintain ﬁxation of the eyes on 
speciﬁc objects to provide accuracy of vision. The gas-

ﬁc objects to provide accuracy of vision. The gas-
trocnemius muscle must contract moderately rapidly 
to provide sufﬁcient velocity of limb movement for
running and jumping, and the soleus muscle is con-
cerned principally with slow contraction for continual,
long-term support of the body against gravity.
Fast Versus Slow Muscle Fibers. As we discuss more fully in
Chapter 84 on sports physiology, every muscle of the
body is composed of a mixture of so-called fast and slow
muscle ﬁbers, with sti

 of so-called fast and slow
muscle ﬁbers, with still other ﬁbers gradated between
these two extremes. The muscles that react rapidly are
composed mainly of “fast” ﬁbers with only small
numbers of the slow variety. Conversely, the muscles
that respond slowly but with prolonged contraction are
composed mainly of “slow” ﬁbers. The differences
between these two types of ﬁbers are as follows.
Fast Fibers. (1) Large ﬁbers for great strength of con-
traction. (2) Extensive sarcoplasmic reticulum for ra

ction. (2) Extensive sarcoplasmic reticulum for rapid
release of calcium ions to initiate contraction. (3) Large
amounts of glycolytic enzymes for rapid release of
energy by the glycolytic process. (4) Less extensive
blood supply because oxidative metabolism is of 
Kymograph
Muscle
electrodes
Weights
ISOTONIC SYSTEM
ISOMETRIC SYSTEM
Electronic force
transducer
To electronic
recorder
Figure 6–11
Isotonic and isometric systems for recording muscle contractions.
C

etric systems for recording muscle contractions.
Chapter 6
81
secondary importance. (5) Fewer mitochondria, also
because oxidative metabolism is secondary.
Slow Fibers. (1) Smaller ﬁbers. (2) Also innervated by
smaller nerve ﬁbers. (3) More extensive blood vessel
system and capillaries to supply extra amounts of
oxygen.(4) Greatly increased numbers of mitochondria,
also to support high levels of oxidative metabolism. (5)
Fibers contain large amounts of myoglobin, a

. (5)
Fibers contain large amounts of myoglobin, an iron-
containing protein similar to hemoglobin in red blood
cells. Myoglobin combines with oxygen and stores it
until needed; this also greatly speeds oxygen transport
to the mitochondria. The myoglobin gives the slow
muscle a reddish appearance and the name red muscle,
whereas a deﬁcit of red myoglobin in fast muscle gives
it the name white muscle.
Mechanics of Skeletal Muscle
Motor Unit. Each motoneuron that leaves the spinal cord

 Unit. Each motoneuron that leaves the spinal cord
innervates multiple muscle ﬁbers, the number depend-
ing on the type of muscle. All the muscle ﬁbers inner-
vated by a single nerve ﬁber are called a motor unit. In
general, small muscles that react rapidly and whose
control must be exact have more nerve ﬁbers for fewer
muscle ﬁbers (for instance, as few as two or three muscle
ﬁbers per motor unit in some of the laryngeal muscles).
Conversely, large muscles that do not require ﬁne
control, such 

rge muscles that do not require ﬁne
control, such as the soleus muscle, may have several
hundred muscle ﬁbers in a motor unit. An average
ﬁgure for all the muscles of the body is questionable, but
a good guess would be about 80 to 100 muscle ﬁbers to
a motor unit.
The muscle ﬁbers in each motor unit are not all
bunched together in the muscle but overlap other motor
units in microbundles of 3 to 15 ﬁbers. This interdigita-
tion allows the separate motor units to contract in
support of one another

 motor units to contract in
support of one another rather than entirely as individ-
ual segments.
Muscle Contractions of Different Force—Force Summation.
Summation means the adding together of individual
twitch contractions to increase the intensity of overall
muscle contraction. Summation occurs in two ways: (1)
by increasing the number of motor units contracting
simultaneously, which is called multiple ﬁber summa-
tion, and (2) by increasing the frequency of contraction,
which is called freque

e frequency of contraction,
which is called frequency summation and can lead to
tetanization.
Multiple Fiber Summation. When the central nervous
system sends a weak signal to contract a muscle, the
smaller motor units of the muscle may be stimulated in
preference to the larger motor units. Then, as the
strength of the signal increases, larger and larger motor
units begin to be excited as well, with the largest motor
units often having as much as 50 times the contractile
force of the smallest uni

50 times the contractile
force of the smallest units. This is called the size princi-
ple. It is important, because it allows the gradations of
muscle force during weak contraction to occur in small
steps, whereas the steps become progressively greater
when large amounts of force are required. The cause of
this size principle is that the smaller motor units are
driven by small motor nerve ﬁbers, and the small
motoneurons in the spinal cord are more excitable than
the larger ones, so they natural

re excitable than
the larger ones, so they naturally are excited ﬁrst.
Another important feature of multiple ﬁber summa-
tion is that the different motor units are driven asyn-
chronously by the spinal cord, so that contraction
alternates among motor units one after the other, thus
providing smooth contraction even at low frequencies
of nerve signals.
Frequency Summation and Tetanization. Figure 6–13
shows the principles of frequency summation and
tetanization. To the left are displayed individu

d
tetanization. To the left are displayed individual twitch
contractions occurring one after another at low fre-
quency of stimulation. Then, as the frequency increases,
there comes a point where each new contraction occurs
before the preceding one is over.As a result, the second
contraction is added partially to the ﬁrst, so that the
total strength of contraction rises progressively with
increasing frequency. When the frequency reaches a
critical level, the successive contractions eventually
be

l level, the successive contractions eventually
become so rapid that they fuse together, and the whole
muscle contraction appears to be completely smooth
and continuous, as shown in the ﬁgure. This is called
tetanization. At a slightly higher frequency, the strength
of contraction reaches its maximum, so that any addi-
tional increase in frequency beyond that point has no
further effect in increasing contractile force.This occurs
because enough calcium ions are maintained in the
muscle sarcoplas

alcium ions are maintained in the
muscle sarcoplasm, even between action potentials, so
that full contractile state is sustained without allowing
any relaxation between the action potentials.
Maximum Strength of Contraction. The maximum strength
of tetanic contraction of a muscle operating at a normal
muscle length averages between 3 and 4 kilograms per
square centimeter of muscle, or 50 pounds per square
inch. Because a quadriceps muscle can have as much as
16 square inches of muscle belly, as 

e as much as
16 square inches of muscle belly, as much as 800 pounds
of tension may be applied to the patellar tendon. Thus,
one can readily understand how it is possible for
muscles to pull their tendons out of their insertions in
bone.
Changes in Muscle Strength at the Onset of Contraction—The
Staircase Effect (Treppe). When a muscle begins to con-
tract after a long period of rest, its initial strength of
Rate of stimulation (times per second)
5
10
15
20
25
30
35
40
45 50
55
Strength of muscl

5
10
15
20
25
30
35
40
45 50
55
Strength of muscle contraction
Tetanization
Figure 6–13
Frequency summation and tetanization.
82
Unit II
contraction may be as little as one half its strength 10
to 50 muscle twitches later. That is, the strength of con-
traction increases to a plateau, a phenomenon called the
staircase effect, or treppe.
Although all the possible causes of the staircase effect
are not known, it is believed to be caused primarily by
increasi

 it is believed to be caused primarily by
increasing calcium ions in the cytosol because of the
release of more and more ions from the sarcoplasmic
reticulum with each successive muscle action potential
and failure of the sarcoplasm to recapture the ions
immediately.
Skeletal Muscle Tone. Even when muscles are at rest, a
certain amount of tautness usually remains. This is
called muscle tone. Because normal skeletal muscle
ﬁbers do not contract without an action potential to
stimulate the ﬁbers, 

thout an action potential to
stimulate the ﬁbers, skeletal muscle tone results entirely
from a low rate of nerve impulses coming from the
spinal cord. These, in turn, are controlled partly by
signals transmitted from the brain to the appropriate
spinal cord anterior motoneurons and partly by signals
that originate in muscle spindles located in the muscle
itself. Both of these are discussed in relation to muscle
spindle and spinal cord function in Chapter 54.
Muscle Fatigue. Prolonged and strong 

 Chapter 54.
Muscle Fatigue. Prolonged and strong contraction of a
muscle leads to the well-known state of muscle fatigue.
Studies in athletes have shown that muscle fatigue
increases in almost direct proportion to the rate of
depletion of muscle glycogen. Therefore, fatigue results
mainly from inability of the contractile and metabolic
processes of the muscle ﬁbers to continue supplying the
same work output. However, experiments have also
shown that transmission of the nerve signal through the

that transmission of the nerve signal through the
neuromuscular junction, which is discussed in Chapter
7, can diminish at least a small amount after intense pro-
longed muscle activity, thus further diminishing muscle
contraction. Interruption of blood ﬂow through a con-
tracting muscle leads to almost complete muscle fatigue
within 1 or 2 minutes because of the loss of nutrient
supply, especially loss of oxygen.
Lever Systems of the Body. Muscles operate by applying
tension to their points of 

es operate by applying
tension to their points of insertion into bones, and the
bones in turn form various types of lever systems. Figure
6–14 shows the lever system activated by the biceps
muscle to lift the forearm. If we assume that a large
biceps muscle has a cross-sectional area of 6 square
inches, the maximum force of contraction would be
about 300 pounds. When the forearm is at right angles
with the upper arm, the tendon attachment of the biceps
is about 2 inches anterior to the fulcrum a

biceps
is about 2 inches anterior to the fulcrum at the elbow,
and the total length of the forearm lever is about 14
inches. Therefore, the amount of lifting power of the
biceps at the hand would be only one seventh of the 300
pounds of muscle force, or about 43 pounds. When the
arm is fully extended, the attachment of the biceps is
much less than 2 inches anterior to the fulcrum, and the
force with which the hand can be brought forward is
also much less than 43 pounds.
In short, an analysis of 

uch less than 43 pounds.
In short, an analysis of the lever systems of the body
depends on knowledge of (1) the point of muscle inser-
tion, (2) its distance from the fulcrum of the lever, (3)
the length of the lever arm, and (4) the position of the
lever. Many types of movement are required in the
body, some of which need great strength and others 
of which need large distances of movement. For this
reason, there are many different types of muscle; some
are long and contract a long distance, an

le; some
are long and contract a long distance, and some are
short but have large cross-sectional areas and can
provide extreme strength of contraction over short dis-
tances. The study of different types of muscles, lever
systems, and their movements is called kinesiology
and is an important scientiﬁc component of human
physioanatomy.
“Positioning” of a Body Part by Contraction of Agonist and Antag-
onist Muscles on Opposite Sides of a Joint—“Coactivation” 
of Antagonist Muscles. Virtually all 

activation” 
of Antagonist Muscles. Virtually all body movements are
caused by simultaneous contraction of agonist and
antagonist muscles on opposite sides of joints. This 
is called coactivation of the agonist and antagonist
muscles,and it is controlled by the motor control centers
of the brain and spinal cord.
The position of each separate part of the body, such
as an arm or a leg, is determined by the relative degrees
of contraction of the agonist and antagonist sets of
muscles. For instance,

nist and antagonist sets of
muscles. For instance, let us assume that an arm or a leg
is to be placed in a midrange position. To achieve 
this, agonist and antagonist muscles are excited about
equally. Remember that an elongated muscle contracts
with more force than a shortened muscle, which was
demonstrated in Figure 6–9, showing maximum strength
of contraction at full functional muscle length and
almost no strength of contraction at half normal length.
Therefore, the elongated muscle on one si

 length.
Therefore, the elongated muscle on one side of a joint
can contract with far greater force than the shorter
muscle on the opposite side. As an arm or leg moves
toward its midposition, the strength of the longer
muscle decreases, whereas the strength of the shorter
muscle increases until the two strengths equal each
other. At this point, movement of the arm or leg stops.
Thus, by varying the ratios of the degree of activation
of the agonist and antagonist muscles, the nervous
system dire

st and antagonist muscles, the nervous
system directs the positioning of the arm or leg.
We learn in Chapter 54 that the motor nervous
system has additional important mechanisms to com-
pensate for different muscle loads when directing this
positioning process.
Remodeling of Muscle 
to Match Function
All the muscles of the body are continually being
remodeled to match the functions that are required of
Figure 6–14
Lever system activated by the biceps muscle.
Chapter 6
Contraction of Skeletal Mus

ceps muscle.
Chapter 6
83
them. Their diameters are altered, their lengths are
altered, their strengths are altered, their vascular sup-
plies are altered, and even the types of muscle ﬁbers are
altered at least slightly.This remodeling process is often
quite rapid, within a few weeks. Indeed, experiments in
animals have shown that muscle contractile proteins in
some smaller, more active muscles can be replaced in as
little as 2 weeks.
Muscle Hypertrophy and Muscle

s
Muscle Hypertrophy and Muscle Atrophy. When the total
mass of a muscle increases, this is called muscle hyper-
trophy. When it decreases, the process is called muscle
atrophy.
Virtually all muscle hypertrophy results from an
increase in the number of actin and myosin ﬁlaments in
each muscle ﬁber, causing enlargement of the individ-
ual muscle ﬁbers; this is called simply ﬁber hypertrophy.
Hypertrophy occurs to a much greater extent when the
muscle is loaded during the contra

extent when the
muscle is loaded during the contractile process. Only a
few strong contractions each day are required to cause
signiﬁcant hypertrophy within 6 to 10 weeks.
The manner in which forceful contraction leads to
hypertrophy is not known.It is known,however,that the
rate of synthesis of muscle contractile proteins is far
greater when hypertrophy is developing, leading also to
progressively greater numbers of both actin and myosin
ﬁlaments in the myoﬁbrils, often increasing as much as
50

s in the myoﬁbrils, often increasing as much as
50 per cent. In turn, some of the myoﬁbrils themselves
have been observed to split within hypertrophying
muscle to form new myoﬁbrils, but how important this
is in usual muscle hypertrophy is still unknown.
Along with the increasing size of myoﬁbrils, the
enzyme systems that provide energy also increase. This
is especially true of the enzymes for glycolysis, allowing
rapid supply of energy during short-term forceful
muscle contraction.
When a muscl

ort-term forceful
When a muscle remains unused for many weeks, the
rate of decay of the contractile proteins is more rapid
than the rate of replacement.Therefore, muscle atrophy
occurs.
Adjustment of Muscle Length. Another type of hyper-
trophy occurs when muscles are stretched to greater
than normal length. This causes new sarcomeres to be
added at the ends of the muscle ﬁbers, where they attach
to the tendons. In fact, new sarcomeres can be added 
as rapidly as several per 

arcomeres can be added 
as rapidly as several per minute in newly developing
muscle,
illustrating the rapidity of this type of 
hypertrophy.
Conversely, when a muscle continually remains short-
ened to less than its normal length, sarcomeres at the
ends of the muscle ﬁbers can actually disappear. It is by
these processes that muscles are continually remodeled
to have the appropriate length for proper muscle 
Hyperplasia of Muscle Fibers. Under rare conditions of
extreme muscle force

ers. Under rare conditions of
extreme muscle force generation, the actual number of
muscle ﬁbers has been observed to increase (but only
by a few percentage points), in addition to the ﬁber
hypertrophy process. This increase in ﬁber number is
called ﬁber hyperplasia. When it does occur, the mech-
anism is linear splitting of previously enlarged ﬁbers.
Effects of Muscle Denervation. When a muscle loses its
nerve supply, it no longer receives the contractile signals
that are required to maintain n

ontractile signals
that are required to maintain normal muscle size.
Therefore, atrophy begins almost immediately. After
about 2 months, degenerative changes also begin to
appear in the muscle ﬁbers themselves. If the nerve
supply to the muscle grows back rapidly, full return 
of function can occur in as little as 3 months, but from
that time onward, the capability of functional return
becomes less and less, with no further return of function
after 1 to 2 years.
In the ﬁnal stage of denervation 

er 1 to 2 years.
In the ﬁnal stage of denervation atrophy, most of the
muscle ﬁbers are destroyed and replaced by ﬁbrous and
fatty tissue. The ﬁbers that do remain are composed of
a long cell membrane with a lineup of muscle cell nuclei
but with few or no contractile properties and little or no
capability of regenerating myoﬁbrils if a nerve does
regrow.
The ﬁbrous tissue that replaces the muscle ﬁbers
during denervation atrophy also has a tendency to con-
tinue shortening for many months, which

cy to con-
tinue shortening for many months, which is called con-
tracture. Therefore, one of the most important problems
in the practice of physical therapy is to keep atrophying
muscles from developing debilitating and disﬁguring
contractures. This is achieved by daily stretching of the
muscles or use of appliances that keep the muscles
stretched during the atrophying process.
Recovery of Muscle Contraction in Poliomyelitis: Devel-
opment of Macromotor Units. When some but not all
nerve ﬁbers 

cromotor Units. When some but not all
nerve ﬁbers to a muscle are destroyed, as commonly
occurs in poliomyelitis, the remaining nerve ﬁbers
branch off to form new axons that then innervate many
of the paralyzed muscle ﬁbers. This causes large motor
units called macromotor units, which can contain as
many as ﬁve times the normal number of muscle ﬁbers
for each motoneuron coming from the spinal cord. This
decreases the ﬁneness of control one has over the
muscles but does allow the muscles to regai

er the
muscles but does allow the muscles to regain varying
degrees of strength.
Several hours after death, all the muscles of the body go
into a state of contracture called “rigor mortis”; that is,
the muscles contract and become rigid, even without
action potentials.This rigidity results from loss of all the
ATP, which is required to cause separation of the cross-
bridges from the actin ﬁlaments during the relaxation
process. The muscles remain in rigor until the muscle
proteins d

uscles remain in rigor until the muscle
proteins deteriorate about 15 to 25 hours later, which
presumably results from autolysis caused by enzymes
released from lysosomes. All these events occur more
rapidly at higher temperatures.
Berchtold MW, Brinkmeier H, Muntener M: Calcium ion 
in skeletal muscle: its crucial role for muscle function,
plasticity, and disease. Physiol Rev 80:1215, 2000.
Brooks SV: Current topics for teaching skeletal muscle phys-
iology. Adv Physiol Educ 27:171, 

tal muscle phys-
iology. Adv Physiol Educ 27:171, 2003.
Clausen T: Na+-K+ pump regulation and skeletal muscle con-
tractility. Physiol Rev 83:1269, 2003.
Glass DJ: Molecular mechanisms modulating muscle mass.
Trends Mol Med 8:344, 2003.
Glass DJ: Signalling pathways that mediate skeletal muscle
hypertrophy and atrophy. Nat Cell Biol 5:87, 2003.
Gordon AM, Homsher E, Regnier M: Regulation of con-
traction in striated muscle. Physiol Rev 80:853, 2000.
84
Unit II
Membrane Physiology, Nerve, and Mus

00.
84
Unit II
Gordon AM, Regnier M, Homsher E: Skeletal and cardiac
muscle contractile activation: tropomyosin “rocks and
rolls.” News Physiol Sci 16:49, 2001.
Huxley AF, Gordon AM: Striation patterns in active and
passive shortening of muscle. Nature (Lond) 193:280,
1962.
Huxley HE: A personal view of muscle and motility mecha-
nisms. Annu Rev Physiol 58:1, 1996.
Jurkat-Rott K, Lerche H, Lehmann-Horn F: Skeletal muscle
channelopathies. J Neurol 249:1493, 

eletal muscle
channelopathies. J Neurol 249:1493, 2002.
Kjær M: Role of extracellular matrix in adaptation of tendon
and skeletal muscle to mechanical loading. Physiol Rev
84:649, 2004.
MacIntosh BR: Role of calcium sensitivity modulation in
skeletal muscle performance. News Physiol Sci 18:222,
2003.
Sieck GC, Regnier M: Plasticity and energetic demands of
contraction in skeletal and cardiac muscle. J Appl

contraction in skeletal and cardiac muscle. J Appl Physiol
90:1158, 2001.
Stamler JS, Meissner G: Physiology of nitric oxide in skele-
tal muscle. Physiol Rev 81:209, 2001.
Szent-Gyorgyi AG: Regulation of contraction by calcium
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7
85
Transmission of Impulses
from Nerve Endings to
Skeletal Muscle Fibers: 
The Neuromuscular
Junction
The skeletal mus

bers: 
Junction
The skeletal muscle ﬁbers are innervated by large,
myelinated nerve ﬁbers that originate from large motoneurons in the anterior
horns of the spinal cord. As pointed out in Chapter 6, each nerve ﬁber, after
entering the muscle belly, normally branches and stimulates from three to
several hundred skeletal muscle ﬁbers. Each nerve ending makes a junction,
called the neuromuscular junction, with the muscle ﬁber near its midpoint. The
action potential initiated in th

its midpoint. The
action potential initiated in the muscle ﬁber by the nerve signal travels in both
directions toward the muscle ﬁber ends. With the exception of about 2 per cent
of the muscle ﬁbers, there is only one such junction per muscle ﬁber.
Physiologic Anatomy of the Neuromuscular Junction—The Motor End Plate. Figure 7–1A
and B shows the neuromuscular junction from a large, myelinated nerve ﬁber
to a skeletal muscle ﬁber. The nerve ﬁber forms a complex of branching nerve
terminals that i

orms a complex of branching nerve
terminals that invaginate into the surface of the muscle ﬁber but lie outside the
muscle ﬁber plasma membrane. The entire structure is called the motor end
plate. It is covered by one or more Schwann cells that insulate it from the 
surrounding ﬂuids.
Figure 7–1C shows an electron micrographic sketch of the junction between
a single axon terminal and the muscle ﬁber membrane. The invaginated mem-
brane is called the synaptic gutter or synaptic trough, and the sp

the synaptic gutter or synaptic trough, and the space between the
terminal and the ﬁber membrane is called the synaptic space or synaptic cleft.
This space is 20 to 30 nanometers wide. At the bottom of the gutter are numer-
ous smaller folds of the muscle membrane called subneural clefts, which greatly
increase the surface area at which the synaptic transmitter can act.
In the axon terminal are many mitochondria that supply adenosine triphos-
phate (ATP), the energy source that is used for synth

te (ATP), the energy source that is used for synthesis of an excitatory 
transmitter acetylcholine. The acetylcholine in turn excites the muscle 
ﬁber membrane. Acetylcholine is synthesized in the cytoplasm of the terminal,
but it is absorbed rapidly into many small synaptic vesicles, about 300,000 
of which are normally in the terminals of a single end plate. In the synaptic
space are large quantities of the enzyme acetylcholinesterase, which destroys
acetylcholine a few milliseconds after it h

stroys
acetylcholine a few milliseconds after it has been released from the synaptic
vesicles.
Secretion of Acetylcholine by the Nerve Terminals
When a nerve impulse reaches the neuromuscular junction, about 125 vesicles
of acetylcholine are released from the terminals into the synaptic space. Some
of the details of this mechanism can be seen in Figure 7–2, which shows an
expanded view of a synaptic space with the neural membrane above and the
muscle membrane and its subneural clefts below.
86
U

scle membrane and its subneural clefts below.
86
Unit II
On the inside surface of the neural membrane are
linear dense bars, shown in cross section in Figure 7–2.
To each side of each dense bar are protein particles
that penetrate the neural membrane;these are voltage-
gated calcium channels. When an action potential
spreads over the terminal, these channels open and
allow calcium ions to diffuse from the synaptic space
to the interior of the nerve terminal

naptic space
to the interior of the nerve terminal.The calcium ions,
in turn, are believed to exert an attractive inﬂuence on
the acetylcholine vesicles, drawing them to the neural
membrane adjacent to the dense bars. The vesicles
then fuse with the neural membrane and empty their
acetylcholine into the synaptic space by the process of
exocytosis.
Although some of the aforementioned details are
speculative, it is known that the effective stimulus for
causing acetylcholine release from the vesicl

 for
causing acetylcholine release from the vesicles is entry
of calcium ions and that acetylcholine from the vesi-
cles is then emptied through the neural membrane
adjacent to the dense bars.
Effect of Acetylcholine on the Postsynaptic Muscle Fiber Mem-
brane to Open Ion Channels. Figure 7–2 also shows many
very small acetylcholine receptors in the muscle ﬁber
membrane; these are acetylcholine-gated ion channels,
and they are located almost entirely near the mouths
of the subneural clefts lying

rely near the mouths
of the subneural clefts lying immediately below the
dense bar areas, where the acetylcholine is emptied
into the synaptic space.
Each receptor is a protein complex that has a total
molecular weight of 275,000.The complex is composed
Axon
Myofibrils
A
C
B
Teloglial cell
Terminal nerve
branches
Muscle
nuclei
Myelin
sheath
Synaptic vesicles
Axon terminal in
synaptic trough
Subneural clefts
Figure 7–1
Different views of the motor end
plate. A,
Longitudinal section
through the en

 end
plate. A,
through the end plate. B, Surface
view of the end plate. C, Electron
micrographic appearance of the
contact point between a single
axon terminal and the muscle
ﬁber membrane. (Redrawn from
Fawcett DW, as modiﬁed from
Couteaux R, in Bloom W, Fawcett
DW: A Textbook of Histology.
Philadelphia: 
WB 
Saunders,
1986.)
Calcium
channels
Neural
membrane
Muscle
membrane
Release
sites
Vesicles
Dense bar
and
acetylcholinesterase
receptors
Subneu

cetylcholinesterase
receptors
Subneural cleft
Figure 7–2
Release of acetylcholine from synaptic vesicles at the neural
membrane of the neuromuscular junction. Note the proximity of
the release sites in the neural membrane to the acetylcholine
receptors in the muscle membrane, at the mouths of the subneural
clefts.
Chapter 7
Excitation of Skeletal Muscle: Neuromuscular Transmission and Excitation-Contraction Coupling
87
of ﬁve subunit proteins, two alpha proteins and one
each of bet

t proteins, two alpha proteins and one
each of beta, delta, and gamma proteins.These protein
molecules penetrate all the way through the mem-
brane, lying side by side in a circle to form a tubular
channel, illustrated in Figure 7–3.The channel remains
constricted, as shown in section A of the ﬁgure, until
two acetylcholine molecules attach respectively to the
two alpha subunit proteins. This causes a conforma-
tional change that opens the channel, as shown in
section B of the ﬁgure.
The opened 

l, as shown in
The opened acetylcholine channel has a diameter of
about 0.65 nanometer, which is large enough to allow
the important positive ions—sodium (Na+), potassium
(K+), and calcium (Ca++)—to move easily through the
opening. Conversely, negative ions, such as chloride
ions, do not pass through because of strong negative
charges in the mouth of the channel that repel these
negative ions.
In practice, far more sodium ions ﬂow through the
acetylcholine channels than a

ions ﬂow through the
acetylcholine channels than any other ions, for two
reasons. First, there are only two positive ions in large
concentration: sodium ions in the extracellular ﬂuid,
and potassium ions in the intracellular ﬂuid. Second,
the very negative potential on the inside of the muscle
membrane, –80 to –90 millivolts, pulls the positively
charged sodium ions to the inside of the ﬁber, while
simultaneously preventing efﬂux of the positively
charged potassium ions when they attempt to pass

y
outward.
As shown in Figure 7–3B, the principal effect of
opening the acetylcholine-gated channels is to allow
large numbers of sodium ions to pour to the inside of
the ﬁber, carrying with them large numbers of positive
charges. This creates a local positive potential change
inside the muscle ﬁber membrane, called the end plate
potential. In turn, this end plate potential initiates an
action potential that spreads along the muscle mem-
brane and

ntial that spreads along the muscle mem-
brane and thus causes muscle contraction.
Destruction of the Released Acetylcholine by Acetyl-
cholinesterase. The acetylcholine, once released into
the synaptic space, continues to activate the acetyl-
choline receptors as long as the acetylcholine persists
in the space. However, it is removed rapidly by two
means: (1) Most of the acetylcholine is destroyed 
by the enzyme acetylcholinesterase, which is attached
mainly to the spongy layer of ﬁne connectiv

tached
mainly to the spongy layer of ﬁne connective tissue
that ﬁlls the synaptic space between the presynaptic
nerve terminal and the postsynaptic muscle mem-
brane. (2) A small amount of acetylcholine diffuses out
of the synaptic space and is then no longer available
to act on the muscle ﬁber membrane.
The short time that the acetylcholine remains in the
synaptic space—a few milliseconds at most—normally
is sufﬁcient to excite the muscle ﬁber. Then the rapid
removal of the acetylcholine preven

Then the rapid
removal of the acetylcholine prevents continued
muscle re-excitation after the muscle ﬁber has re-
covered from its initial action potential.
End Plate Potential and Excitation of the Skeletal Muscle Fiber.
The sudden insurgence of sodium ions into the muscle
ﬁber when the acetylcholine channels open causes the
electrical potential inside the ﬁber at the local area of
the end plate to increase in the positive direction as
much as 50 to 75 millivolts, creating a local potential
cal

0 to 75 millivolts, creating a local potential
called the end plate potential. Recall from Chapter 5
that a sudden increase in nerve membrane potential 
of more than 20 to 30 millivolts is normally sufﬁcient
to initiate more and more sodium channel opening,
thus initiating an action potential at the muscle ﬁber
membrane.
Figure 7–4 shows the principle of an end plate
potential initiating the action potential. This ﬁgure
shows three separate end plate potentials. End plate
potentials A and C are 

late potentials. End plate
potentials A and C are too weak to elicit an action
potential, but they do produce weak local end plate
voltage changes, as recorded in the ﬁgure. By contrast,
end plate potential B is much stronger and causes
enough sodium channels to open so that the self-
regenerative effect of more and more sodium ions
Ach
Na+
A
–
–
–
–
–
–
–
–
–
–
–
–
B
Figure 7–3
Acetylcholine channel. A, Closed state. B, After acetylcholine
(Ach) has become attached and a conformational change h

 has become attached and a conformational change has
opened the channel, allowing sodium ions to enter the muscle
ﬁber and excite contraction. Note the negative charges at the
channel mouth that prevent passage of negative ions such as
chloride ions.
88
Unit II
ﬂowing to the interior of the ﬁber initiates an action
potential. The weakness of the end plate potential at
point A was caused by poisoning of the muscle ﬁber
with curare, a drug that blocks the gat

uscle ﬁber
with curare, a drug that blocks the gating action of
acetylcholine on the acetylcholine channels by com-
peting for the acetylcholine receptor sites. The weak-
ness of the end plate potential at point C resulted from
the effect of botulinum toxin, a bacterial poison that
decreases the quantity of acetylcholine release by the
nerve terminals.
Safety Factor for Transmission at the Neuromuscular Junction;
Fatigue of the Junction. Ordinarily, each impulse that
arrives at the neuromuscular

ly, each impulse that
arrives at the neuromuscular junction causes about
three times as much end plate potential as that
required to stimulate the muscle ﬁber. Therefore, the
normal neuromuscular junction is said to have a high
safety factor. However, stimulation of the nerve ﬁber
at rates greater than 100 times per second for several
minutes often diminishes the number of acetylcholine
vesicles so much that impulses fail to pass into the
muscle ﬁber. This is called fatigue of the neuromuscu-
la

ﬁber. This is called fatigue of the neuromuscu-
lar junction,and it is the same effect that causes fatigue
of synapses in the central nervous system when the
synapses are overexcited. Under normal functioning
conditions, measurable fatigue of the neuromuscular
junction occurs rarely, and even then only at the most
exhausting levels of muscle activity.
Molecular Biology of
Acetylcholine Formation 
and Release
Because the neuromuscular junction is large enough to
be studied easily, it is one of th

large enough to
be studied easily, it is one of the few synapses of the
nervous system for which most of the details of chemi-
cal transmission have been worked out. The formation
and release of acetylcholine at this junction occur in the
following stages:
1. Small vesicles, about 40 nanometers in size, are
formed by the Golgi apparatus in the cell body of
the motoneuron in the spinal cord. These vesicles
are then transported by axoplasm that “streams”
through the core of the axon from the centr

reams”
through the core of the axon from the central 
cell body in the spinal cord all the way to the
neuromuscular junction at the tips of the peripheral
nerve ﬁbers. About 300,000 of these small vesicles
collect in the nerve terminals of a single skeletal
muscle end plate.
2. Acetylcholine is synthesized in the cytosol of the
nerve ﬁber terminal but is immediately transported
through the membranes of the vesicles to their
interior, where it is stored in highly concentrated
form, about 10,000 m

stored in highly concentrated
form, about 10,000 molecules of acetylcholine in
each vesicle.
3. When an action potential arrives at the nerve
terminal, it opens many calcium channels in the
membrane of the nerve terminal because this
terminal has an abundance of voltage-gated
calcium channels. As a result, the calcium ion
concentration inside the terminal membrane
increases about 100-fold, which in turn increases
the rate of fusion of the acetylcholine vesicles with
the terminal membrane about 1

holine vesicles with
the terminal membrane about 10,000-fold. This
fusion makes many of the vesicles rupture, allowing
exocytosis of acetylcholine into the synaptic space.
About 125 vesicles usually rupture with each 
action potential. Then, after a few milliseconds, the
acetylcholine is split by acetylcholinesterase 
into acetate ion and choline, and the choline is
reabsorbed actively into the neural terminal to be
reused to form new acetylcholine. This sequence of
events occurs within a period

ne. This sequence of
events occurs within a period of 5 to 10
milliseconds.
4. The number of vesicles available in the nerve
ending is sufﬁcient to allow transmission of only a
few thousand nerve-to-muscle impulses. Therefore,
for continued function of the neuromuscular
junction, new vesicles need to be re-formed rapidly.
Within a few seconds after each action potential is
over, “coated pits” appear in the terminal nerve
membrane, caused by contractile proteins in the
nerve ending, especially th

actile proteins in the
nerve ending, especially the protein clathrin, which
is attached to the membrane in the areas of the
original vesicles. Within about 20 seconds, the
proteins contract and cause the pits to break away
to the interior of the membrane, thus forming new
vesicles. Within another few seconds, acetylcholine
is transported to the interior of these vesicles, and
they are then ready for a new cycle of acetylcholine
release.
Drugs That Enhance or
Block Transmission at the
Neuromuscul

t Enhance or
Neuromuscular Junction
Drugs That Stimulate the Muscle Fiber by Acetylcholine-Like
Action. Many compounds, including methacholine, car-
bachol, and nicotine, have the same effect on the muscle
ﬁber as does acetylcholine. The difference between
0
15
30
45
60
75
C
B
A
+60
+40
+20
0
–20
–40
–60
–80
–100
Figure 7–4
End plate potentials (in millivolts). A, Weakened end plate poten-
tial recorded in a curarized muscle, too weak to elicit a

corded in a curarized muscle, too weak to elicit an action
potential. B, Normal end plate potential eliciting a muscle action
potential. C, Weakened end plate potential caused by botulinum
toxin that decreases end plate release of acetylcholine, again too
weak to elicit a muscle action potential.
Chapter 7
89
these drugs and acetylcholine is that the drugs are not
destroyed by cholinesterase or are dest

gs are not
destroyed by cholinesterase or are destroyed so slowly
that their action often persists for many minutes to
several hours.The drugs work by causing localized areas
of depolarization of the muscle ﬁber membrane at the
motor end plate where the acetylcholine receptors 
are located. Then, every time the muscle ﬁber recovers
from a previous contraction, these depolarized areas, by
virtue of leaking ions, initiate a new action potential,
thereby causing a state of muscle spasm.
Drugs 
That

ereby causing a state of muscle spasm.
Drugs 
That 
Stimulate 
the 
Neuromuscular 
Junction 
by 
Inactivating Acetylcholinesterase. Three particularly well-
known 
drugs,
neostigmine,
physostigmine,
and 
diisopropyl ﬂuorophosphate,
inactivate the acetyl-
cholinesterase in the synapses so that it no longer
hydrolyzes acetylcholine. Therefore, with each succes-
sive nerve impulse, additional acetylcholine accumu-
lates and stimulates the muscle ﬁber repetitively. This
causes muscle spasm when even

r repetitively. This
causes muscle spasm when even a few nerve impulses
reach the muscle. Unfortunately, it also can cause death
due to laryngeal spasm, which smothers the person.
Neostigmine and physostigmine combine with acetyl-
cholinesterase to inactivate the acetylcholinesterase 
for up to several hours, after which these drugs are 
displaced from the acetylcholinesterase so that the
esterase once again becomes active. Conversely, diiso-
propyl ﬂuorophosphate, which has military potential 

pyl ﬂuorophosphate, which has military potential 
as a powerful “nerve” gas poison, inactivates acetyl-
cholinesterase for weeks, which makes this a particu-
larly lethal poison.
Drugs That Block Transmission at the Neuromuscular Junction. A
group of drugs known as curariform drugs can prevent
passage of impulses from the nerve ending into the
muscle. For instance, D-tubocurarine blocks the action
of acetylcholine on the muscle ﬁber acetylcholine recep-
tors, thus preventing sufﬁcient increase i

 recep-
tors, thus preventing sufﬁcient increase in permeability
of the muscle membrane channels to initiate an action
potential.
Myasthenia gravis, which occurs in about 1 in every
20,000 persons, causes muscle paralysis because of
inability of the neuromuscular junctions to transmit
enough signals from the nerve ﬁbers to the muscle
ﬁbers. Pathologically, antibodies that attack the acetyl-
choline-gated sodium ion transport proteins have been
demonstrated in the blood of most 

teins have been
demonstrated in the blood of most patients with 
myasthenia gravis. Therefore, it is believed that myas-
thenia gravis is an autoimmune disease in which the
patients have developed immunity against their own
acetylcholine-activated ion channels.
Regardless of the cause, the end plate potentials that
occur in the muscle ﬁbers are mostly too weak to stimu-
late the muscle ﬁbers. If the disease is intense enough,
the patient dies of paralysis—in particular, paralysis of
the respirat

paralysis—in particular, paralysis of
the respiratory muscles.The disease usually can be ame-
liorated for several hours by administering neostigmine
or some other anticholinesterase drug, which allows
larger than normal amounts of acetylcholine to accu-
mulate in the synaptic space. Within minutes, some of
these paralyzed people can begin to function almost
normally, until a new dose of neostigmine is required a
few hours later.
Almost everything discussed in Chapter 5 r

tential
Almost everything discussed in Chapter 5 regarding
initiation and conduction of action potentials in nerve
ﬁbers applies equally to skeletal muscle ﬁbers, except
for quantitative differences. Some of the quantitative
aspects of muscle potentials are the following:
1. Resting membrane potential: about –80 to –90
millivolts in skeletal ﬁbers—the same as in large
myelinated nerve ﬁbers.
2. Duration of action potential: 1 to 5 milliseconds in
skeletal muscle—about ﬁve times as long as in
lar

large myelinated nerves.
3. Velocity of conduction: 3 to 5 m/sec—about 1/13
the velocity of conduction in the large myelinated
nerve ﬁbers that excite skeletal muscle.
Spread of the Action Potential to the
The skeletal muscle ﬁber is so large that action poten-
tials spreading along its surface membrane cause
almost no current ﬂow deep within the ﬁber. Yet, to
cause maximum muscle contracti

n the ﬁber. Yet, to
cause maximum muscle contraction, current must pen-
etrate deeply into the muscle ﬁber to the vicinity of the
separate myoﬁbrils.This is achieved by transmission of
action potentials along transverse tubules (T tubules)
that penetrate all the way through the muscle ﬁber
from one side of the ﬁber to the other, as illustrated in
Figure 7–5. The T tubule action potentials cause
release of calcium ions inside the muscle ﬁber in the
immediate vicinity of the myoﬁbrils, and these c

e
immediate vicinity of the myoﬁbrils, and these calcium
ions then cause contraction. This overall process is
called excitation-contraction coupling.
Excitation-Contraction
Coupling
Transverse Tubule–Sarcoplasmic
Reticulum System
Figure 7–5 shows myoﬁbrils surrounded by the T
tubule–sarcoplasmic reticulum system. The T tubules
are very small and run transverse to the myoﬁbrils.
They begin at the cell membrane and penetrate all the
way from one side of the muscle ﬁber to the opposite
side. Not sh

de of the muscle ﬁber to the opposite
side. Not shown in the ﬁgure is the fact that these
tubules branch among themselves so that they form
entire planes of T tubules interlacing among all the
separate myoﬁbrils. Also, where the T tubules originate
from the cell membrane, they are open to the exterior
of the muscle ﬁber. Therefore, they communicate with
the extracellular ﬂuid surrounding the muscle ﬁber,
and they themselves contain extracellular ﬂuid in their
lumens. In other words, the T tubule

ﬂuid in their
lumens. In other words, the T tubules are actually
internal extensions of the cell membrane. Therefore,
when an action potential spreads over a muscle ﬁber
90
Unit II
membrane, a potential change also spreads along the
T tubules to the deep interior of the muscle ﬁber. The
electrical currents surrounding these T tubules then
elicit the muscle contraction.
Figure 7–5 also shows a sarcoplasmic reticulum, in
yellow. This is composed of two major 

ticulum, in
yellow. This is composed of two major parts: (1) large
chambers called terminal cisternae that abut the T
tubules, and (2) long longitudinal tubules that sur-
round all surfaces of the actual contracting myoﬁbrils.
Release of Calcium Ions by the
One of the special features of the sarcoplasmic reticu-
lum is that within its vesicular tubules is an excess of
calcium ions in high concentration, and many of these
ions are released from each vesicle when an action
p

ns are released from each vesicle when an action
potential occurs in the adjacent T tubule.
Figure 7–6 shows that the action potential of the T
tubule causes current ﬂow into the sarcoplasmic re-
ticular cisternae where they abut the T tubule. This in
turn causes rapid opening of large numbers of calcium
channels through the membranes of the cisternae as
well as their attached longitudinal tubules. These 
channels remain open for a few milliseconds; during
this time, enough calcium ions are rele

ds; during
this time, enough calcium ions are released into the
sarcoplasm surrounding the myoﬁbrils to cause con-
traction, as discussed in Chapter 6.
Calcium Pump for Removing Calcium Ions from the Myoﬁbrillar
Fluid After Contraction Occurs. Once the calcium ions
have been released from the sarcoplasmic tubules and
have diffused among the myoﬁbrils, muscle contrac-
tion continues as long as the calcium ions remain in
high concentration. However, a continually active
calcium pump located in the

, a continually active
calcium pump located in the walls of the sarcoplasmic
reticulum pumps calcium ions away from the myoﬁ-
brils back into the sarcoplasmic tubules.This pump can
concentrate the calcium ions about 10,000-fold inside
the tubules. In addition, inside the reticulum is a
protein called calsequestrin that can bind up to 40
times more calcium.
Excitatory “Pulse” of Calcium Ions. The normal resting
state concentration (less than 10-7 molar) of calcium
ions in the cytosol that bathes 

molar) of calcium
ions in the cytosol that bathes the myoﬁbrils is too
little to elicit contraction. Therefore, the troponin-
tropomyosin complex keeps the actin ﬁlaments inhib-
ited and maintains a relaxed state of the muscle.
Conversely, full excitation of the T tubule and 
sarcoplasmic reticulum system causes enough re-
lease of calcium ions to increase the concentration 
in the myoﬁbrillar ﬂuid to as high as 2 ¥ 10-4 molar
A band
I band
Sarcotubules
Z line
Triad of the
reticulum
S

tubules
Z line
reticulum
Sarcolemma
Transverse
tubule
tubule
Terminal
cisternae
Sarcoplasmic
reticulum
Figure 7–5
Transverse (T) tubule–sarcoplas-
mic reticulum system. Note that 
the T tubules communicate with the
outside of the cell membrane, and
deep in the muscle ﬁber, each T
tubule lies adjacent to the ends of
longitudinal sarcoplasmic reticulum
tubules that surround all sides of
the actual myoﬁbrils that contract.
This illustration was drawn

oﬁbrils that contract.
This illustration was drawn from frog
muscle, which has one T tubule per
sarcomere, located at the Z line. A
similar arrangement is found in
mammalian 
heart 
muscle, 
but
mammalian skeletal muscle has
two T tubules per sarcomere,
located at the A-I band junctions.
(Redrawn from Bloom W, Fawcett
Philadelphia: WB Saunders, 1986.
Modiﬁed after Peachey LD: J Cell
Biol 25:209, 1965. Drawn by Sylvia
Colard Keene.)
Chapter 7
Excitation of Skeletal Mu

Chapter 7
91
concentration, a 500-fold increase, which is about 10
times the level required to cause maximum muscle
contraction. Immediately thereafter, the calcium pump
depletes the calcium ions again. The total duration of
this calcium “pulse” in the usual skeletal muscle ﬁber
lasts about 1/20 of a second, although it may last
several times as long in some ﬁbers and several times
less i

mes as long in some ﬁbers and several times
less in others. (In heart muscle, the calcium pulse lasts
about 1/3 of a second because of the long duration of
the cardiac action potential.)
During this calcium pulse, muscle contraction
occurs. If the contraction is to continue without inter-
ruption for long intervals, a series of calcium pulses
must be initiated by a continuous series of repetitive
action potentials, as discussed in Chapter 6.
Also see references for Chapters 5 and 6.
A

rences
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Amonof MJ: Electromyography in Clinical Practice. New
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Brown RH Jr: Dystrophin-associated proteins and the mus-
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Chaudhuri A, Behan PO: Fatigue in neurological disorders.
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Engel AG, Ohno K, Shen XM, Sine SM: Congenital myas-
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 Shen XM, Sine SM: Congenital myas-
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Calcium pump
ATP
required
Myosin filaments
Ca
Ca
Ca++
Ca++
Figure 7–6
Excitation-contraction coupling in
the muscle, showing (1) an action
potential that causes release of
calcium ions from the sarcoplas-
mic reticulum and then (2) re-
uptake of the calcium ions by a
calcium pump.
C
H
A
P
T
E
R
8
92
Contraction and Excitation 
of Smooth Muscle
Cont

Contraction of Smooth
Muscle
In Chapters 6 and 7, the discussion was concerned
with skeletal muscle. We now turn to smooth
muscle, which is composed of far smaller ﬁbers—
usually 1 to 5 micrometers in diameter and only 20
to 500 micrometers in length. In contrast, skeletal
muscle ﬁbers are as much as 30 times greater in diameter and hundreds of times
as long. Many of the same principles of contraction apply to smooth muscle as
to skeletal muscle. Mos

 apply to smooth muscle as
to skeletal muscle. Most important, essentially the same attractive forces
between myosin and actin ﬁlaments cause contraction in smooth muscle as in
skeletal muscle, but the internal physical arrangement of smooth muscle ﬁbers
is very different.
The smooth muscle of each organ is distinctive from that of most other organs
in several ways: (1) physical dimensions, (2) organization into bundles or sheets,
(3) response to different types of stimuli

sheets,
(3) response to different types of stimuli, (4) characteristics of innervation, and
(5) function. Yet, for the sake of simplicity, smooth muscle can generally be
divided into two major types, which are shown in Figure 8–1: multi-unit smooth
muscle and unitary (or single-unit) smooth muscle.
Multi-Unit Smooth Muscle. This type of smooth muscle is composed of discrete,
separate smooth muscle ﬁbers. Each ﬁber operates independently of the others
and often is innervated by a single nerve end

hers
and often is innervated by a single nerve ending, as occurs for skeletal muscle
ﬁbers. Further, the outer surfaces of these ﬁbers, like those of skeletal muscle
ﬁbers, are covered by a thin layer of basement membrane–like substance, a
mixture of ﬁne collagen and glycoprotein that helps insulate the separate ﬁbers
from one another.
The most important characteristic of multi-unit smooth muscle ﬁbers is that
each ﬁber can contract independently of the others, and their control is exerted
mainl

 of the others, and their control is exerted
mainly by nerve signals. In contrast, a major share of control of unitary smooth
muscle is exerted by non-nervous stimuli. Some examples of multi-unit smooth
muscle are the ciliary muscle of the eye, the iris muscle of the eye, and the pilo-
erector muscles that cause erection of the hairs when stimulated by the sym-
pathetic nervous system.
Unitary Smooth Muscle. The term “unitary” is confusing because it does not mean
single muscle ﬁbers. Instead, i

e it does not mean
single muscle ﬁbers. Instead, it means a mass of hundreds to thousands of
smooth muscle ﬁbers that contract together as a single unit. The ﬁbers usually
are arranged in sheets or bundles, and their cell membranes are adherent to one
another at multiple points so that force generated in one muscle ﬁber can be
transmitted to the next. In addition, the cell membranes are joined by many gap
junctions through which ions can ﬂow freely from one muscle cell to the next
so that action

ly from one muscle cell to the next
so that action potentials or simple ion ﬂow without action potentials can travel
Chapter 8
Contraction and Excitation of Smooth Muscle
93
from one ﬁber to the next and cause the muscle ﬁbers
to contract together.This type of smooth muscle is also
known as syncytial smooth muscle because of its syn-
cytial interconnections among ﬁbers. It is also called
visceral smooth muscle because it is found in the walls
of most viscera of the body, including the gut, bile

most viscera of the body, including the gut, bile
ducts, ureters, uterus, and many blood vessels.
Contractile Mechanism in 
Chemical Basis for Smooth 
Smooth muscle contains both actin and myosin ﬁla-
ments, having chemical characteristics similar to those
of the actin and myosin ﬁlaments in skeletal muscle.
It does not contain the normal troponin complex 
that is required in the control of skeletal muscle con-
traction, so the mechanism for control of contractio

action, so the mechanism for control of contraction
is different. This is discussed in detail later in this
chapter.
Chemical studies have shown that actin and myosin
ﬁlaments derived from smooth muscle interact with
each other in much the same way that they do in skele-
tal muscle. Further, the contractile process is activated
by calcium ions, and adenosine triphosphate (ATP) is
degraded to adenosine diphosphate (ADP) to provide
the energy for contraction.
There are, however, major differences 

There are, however, major differences between the
physical organization of smooth muscle and that of
skeletal muscle, as well as differences in excitation-
contraction coupling, control of the contractile process
by calcium ions, duration of contraction, and amount
of energy required for contraction.
Physical Basis for Smooth Muscle Contraction
Smooth muscle does not have the same striated
arrangement of actin and myosin ﬁlaments as is found
in skeletal muscle.Instead,electron microg

s found
in skeletal muscle.Instead,electron micrographic tech-
niques suggest the physical organization exhibited in
Figure 8–2. This ﬁgure shows large numbers of actin
ﬁlaments attached to so-called dense bodies. Some of
these bodies are attached to the cell membrane.
Others are dispersed inside the cell. Some of the mem-
brane dense bodies of adjacent cells are bonded
together by intercellular protein bridges. It is mainly
through these bonds that the force of contraction is
transmitted from o

hat the force of contraction is
transmitted from one cell to the next.
Multi-unit smooth muscle
A
Unitary smooth muscle
Small artery
Medial
muscle fibers
B
Adventitia
Figure 8–1
Multi-unit (A) and unitary (B) smooth muscle.
Actin
filaments
Dense bodies
Cell membrane
Figure 8–2
Physical structure of smooth muscle. The upper left-hand ﬁber
shows actin ﬁlaments radiating from dense bodies. The lower left-
hand ﬁber and the right-hand diagram demonstrate the relation of

he right-hand diagram demonstrate the relation of
myosin ﬁlaments to actin ﬁlaments.
94
Unit II
Interspersed among the actin ﬁlaments in the
muscle ﬁber are myosin ﬁlaments. These have a diam-
eter more than twice that of the actin ﬁlaments. In
electron micrographs, one usually ﬁnds 5 to 10 times
as many actin ﬁlaments as myosin ﬁlaments.
To the right in Figure 8–2 is a postulated structure
of an individual contractile unit within a smooth
muscle cell, show

contractile unit within a smooth
muscle cell, showing large numbers of actin ﬁlaments
radiating from two dense bodies; the ends of these 
ﬁlaments overlap a myosin ﬁlament located midway
between the dense bodies. This contractile unit is
similar to the contractile unit of skeletal muscle, but
without the regularity of the skeletal muscle structure;
in fact, the dense bodies of smooth muscle serve the
same role as the Z discs in skeletal muscle.
There is another difference: Most of the myosin ﬁla

here is another difference: Most of the myosin ﬁla-
ments have what are called “sidepolar” cross-bridges
arranged so that the bridges on one side hinge in one
direction and those on the other side hinge in the
opposite direction. This allows the myosin to pull an
actin ﬁlament in one direction on one side while simul-
taneously pulling another actin ﬁlament in the oppo-
site direction on the other side. The value of this
organization is that it allows smooth muscle cells to
contract as much as 8

llows smooth muscle cells to
contract as much as 80 per cent of their length instead
of being limited to less than 30 per cent, as occurs in
skeletal muscle.
Comparison of Smooth Muscle Contraction
and Skeletal Muscle Contraction
Although most skeletal muscles contract and relax
rapidly, most smooth muscle contraction is prolonged
tonic contraction, sometimes lasting hours or even
days.Therefore,it is to be expected that both the physi-
cal and the chemical characteristics of smooth muscle
versu

he chemical characteristics of smooth muscle
versus skeletal muscle contraction would differ.
Following are some of the differences.
Slow Cycling of the Myosin Cross-Bridges. The rapidity 
of cycling of the myosin cross-bridges in smooth
muscle—that is, their attachment to actin, then release
from the actin, and reattachment for the next cycle—
is much, much slower in smooth muscle than in skele-
tal muscle; in fact, the frequency is as little as 1/10 to
1/300 that in skeletal muscle. Yet the fr

 1/10 to
1/300 that in skeletal muscle. Yet the fraction of time
that the cross-bridges remain attached to the actin ﬁla-
ments, which is a major factor that determines the
force of contraction, is believed to be greatly increased
in smooth muscle. A possible reason for the slow
cycling is that the cross-bridge heads have far less
ATPase activity than in skeletal muscle, so that degra-
dation of the ATP that energizes the movements 
of the cross-bridge heads is greatly reduced, with
correspondin

bridge heads is greatly reduced, with
corresponding slowing of the rate of cycling.
Energy Required to Sustain Smooth Muscle Contraction. Only
1/10 to 1/300 as much energy is required to sustain the
same tension of contraction in smooth muscle as in
skeletal muscle.This, too, is believed to result from the
slow attachment and detachment cycling of the cross-
bridges and because only one molecule of ATP is
required for each cycle, regardless of its duration.
This sparsity of energy utilization by

s duration.
This sparsity of energy utilization by smooth muscle
is exceedingly important to the overall energy
economy of the body, because organs such as the intes-
tines, urinary bladder, gallbladder, and other viscera
often maintain tonic muscle contraction almost 
indeﬁnitely.
Slowness of Onset of Contraction and Relaxation of the Total
Smooth Muscle Tissue. A typical smooth muscle tissue
begins to contract 50 to 100 milliseconds after it is
excited, reaches full contraction about 0.5 secon

excited, reaches full contraction about 0.5 second later,
and then declines in contractile force in another 1 to
2 seconds, giving a total contraction time of 1 to 3
seconds. This is about 30 times as long as a single 
contraction of an average skeletal muscle ﬁber. But
because there are so many types of smooth muscle,
contraction of some types can be as short as 0.2 second
or as long as 30 seconds.
The slow onset of contraction of smooth muscle,
as well as its prolonged contraction, is caused 

,
as well as its prolonged contraction, is caused by 
the slowness of attachment and detachment of the 
cross-bridges with the actin ﬁlaments. In addition, the
initiation of contraction in response to calcium ions is
much slower than in skeletal muscle, as discussed later.
Force of Muscle Contraction. Despite the relatively few
myosin ﬁlaments in smooth muscle, and despite the
slow cycling time of the cross-bridges, the maximum
force of contraction of smooth muscle is often greater
than that of 

on of smooth muscle is often greater
than that of skeletal muscle—as great as 4 to 6 kg/cm2
cross-sectional area for smooth muscle, in comparison
with 3 to 4 kilograms for skeletal muscle. This great
force of smooth muscle contraction results from the
prolonged period of attachment of the myosin cross-
bridges to the actin ﬁlaments.
“Latch” Mechanism for Prolonged Holding of Contractions of
Smooth Muscle. Once smooth muscle has developed full
contraction, the amount of continuing excitation
usua

traction, the amount of continuing excitation
usually can be reduced to far less than the initial 
level, yet the muscle maintains its full force of con-
traction. Further, the energy consumed to maintain
contraction is often minuscule, sometimes as little as
1/300 the energy required for comparable sustained 
skeletal muscle contraction. This is called the “latch”
mechanism.
The importance of the latch mechanism is that it 
can maintain prolonged tonic contraction in smooth
muscle for hours wit

d tonic contraction in smooth
muscle for hours with little use of energy. Little con-
tinued excitatory signal is required from nerve ﬁbers
or hormonal sources.
Stress-Relaxation of Smooth Muscle. Another impor-
tant characteristic of smooth muscle, especially the vis-
ceral unitary type of smooth muscle of many hollow
organs, is its ability to return to nearly its original 
force of contraction seconds or minutes after it has
been elongated or shortened. For example, a sudden
increase in ﬂuid v

hortened. For example, a sudden
increase in ﬂuid volume in the urinary bladder, thus
stretching the smooth muscle in the bladder wall,
causes an immediate large increase in pressure in the
bladder. However, during the next 15 seconds to a
Chapter 8
95
minute or so, despite continued stretch of the bladder
wall, the pressure returns almost exactly back to the
original level. Then, when the volume is increased by
another step, the same effect occurs agai

eased by
another step, the same effect occurs again.
Conversely, when the volume is suddenly decreased,
the pressure falls very low at ﬁrst but then rises back
in another few seconds or minutes to or near to the
original level. These phenomena are called stress-
relaxation and reverse stress-relaxation. Their impor-
tance is that, except for short periods of time, they
allow a hollow organ to maintain about the same
amount of pressure inside its lumen despite long-term,
large changes in volume.

lumen despite long-term,
Regulation of Contraction 
by Calcium Ions
As is true for skeletal muscle, the initiating stimulus
for most smooth muscle contraction is an increase in
intracellular calcium ions. This increase can be caused
in different types of smooth muscle by nerve stimula-
tion of the smooth muscle ﬁber, hormonal stimulation,
stretch of the ﬁber, or even change in the chemical
environment of the ﬁber.
Yet smooth muscle does not contain troponin, the
regulato

oth muscle does not contain troponin, the
regulatory protein that is activated by calcium ions 
to cause skeletal muscle contraction. Instead, smooth
muscle contraction is activated by an entirely different
mechanism, as follows.
Combination of Calcium Ions with Calmodulin—Activation of
Myosin Kinase and Phosphorylation of the Myosin Head. In
place of troponin, smooth muscle cells contain a large
amount of another regulatory protein called calmod-
ulin. Although this protein is similar to tropon

-
ulin. Although this protein is similar to troponin, it is
different in the manner in which it initiates contrac-
tion. Calmodulin does this by activating the myosin
cross-bridges. This activation and subsequent contrac-
tion occur in the following sequence:
1. The calcium ions bind with calmodulin.
2. The calmodulin-calcium combination joins with
and activates myosin kinase, a phosphorylating
enzyme.
3. One of the light chains of each myosin 
head, called the regulatory chain, becomes
phosphor

ead, called the regulatory chain, becomes
phosphorylated in response to this myosin kinase.
When this chain is not phosphorylated, the
attachment-detachment cycling of the myosin
head with the actin ﬁlament does not occur. But
when the regulatory chain is phosphorylated, the
head has the capability of binding repetitively
with the actin ﬁlament and proceeding through
the entire cycling process of intermittent “pulls,”
the same as occurs for skeletal muscle, thus
causing muscle contraction.
Cessa

tal muscle, thus
Cessation of Contraction—Role of Myosin Phosphatase.
When the calcium ion concentration falls below a crit-
ical level, the aforementioned processes automatically
reverse, except for the phosphorylation of the myosin
head. Reversal of this requires another enzyme,
myosin phosphatase, located in the ﬂuids of the
smooth muscle cell, which splits the phosphate from
the regulatory light chain. Then the cycling stops and
contraction ceases.The time require

ling stops and
contraction ceases.The time required for relaxation of
muscle contraction, therefore, is determined to a great
extent by the amount of active myosin phosphatase in
the cell.
Possible Mechanism for Regulation of the
Latch Phenomenon
Because of the importance of the latch phenomenon
in smooth muscle, and because this phenomenon
allows long-term maintenance of tone in many smooth
muscle organs without much expenditure of energy,
many attempts have been made to explain it. Among
the m

attempts have been made to explain it. Among
the many mechanisms that have been postulated, one
of the simplest is the following.
When the myosin kinase and myosin phosphatase
enzymes are both strongly activated, the cycling fre-
quency of the myosin heads and the velocity of 
contraction are great. Then, as the activation of the
enzymes decreases, the cycling frequency decreases,
but at the same time,the deactivation of these enzymes
allows the myosin heads to remain attached to the
actin ﬁlame

myosin heads to remain attached to the
actin ﬁlament for a longer and longer proportion of
the cycling period. Therefore, the number of heads
attached to the actin ﬁlament at any given time
remains large. Because the number of heads attached
to the actin determines the static force of contraction,
tension is maintained, or “latched”; yet little energy 
is used by the muscle, because ATP is not degraded 
to ADP except on the rare occasion when a head
detaches.
Nervous and Hormonal Control
of Smoo

ead
detaches.
of Smooth Muscle Contraction
Although skeletal muscle ﬁbers are stimulated exclu-
sively by the nervous system, smooth muscle can be
stimulated to contract by multiple types of signals: by
nervous signals, by hormonal stimulation, by stretch of
the muscle, and in several other ways. The principal
reason for the difference is that the smooth muscle
membrane contains many types of receptor proteins
that can initiate the contractile process. Still other
re

n initiate the contractile process. Still other
receptor proteins inhibit smooth muscle contraction,
which is another difference from skeletal muscle.
Therefore, in this section, we discuss nervous control
of smooth muscle contraction, followed by hormonal
control and other means of control.
Neuromuscular Junctions 
Physiologic Anatomy of Smooth Muscle Neuromuscular Junc-
tions. Neuromuscular junctions of the highly struc-
tured type found on skeletal muscle ﬁbers do not occur
i

type found on skeletal muscle ﬁbers do not occur
in smooth muscle. Instead, the autonomic nerve ﬁbers
that innervate smooth muscle generally branch 
96
Unit II
diffusely on top of a sheet of muscle ﬁbers, as shown
in Figure 8–3. In most instances, these ﬁbers do not
make direct contact with the smooth muscle ﬁber cell
membranes but instead form so-called diffuse junc-
tions that secrete their transmitter substance into the
matrix coating of the smooth muscl

stance into the
matrix coating of the smooth muscle often a few
nanometers to a few micrometers away from the
muscle cells; the transmitter substance then diffuses to
the cells. Furthermore, where there are many layers of
muscle cells, the nerve ﬁbers often innervate only the
outer layer, and muscle excitation travels from this
outer layer to the inner layers by action potential con-
duction in the muscle mass or by additional diffusion
of the transmitter substance.
The axons that innervate smoo

ansmitter substance.
The axons that innervate smooth muscle ﬁbers do
not have typical branching end feet of the type in the
motor end plate on skeletal muscle ﬁbers. Instead,
most of the ﬁne terminal axons have multiple vari-
cosities distributed along their axes.At these points the
Schwann cells that envelop the axons are interrupted
so that transmitter substance can be secreted through
the walls of the varicosities. In the varicosities are vesi-
cles similar to those in the skeletal muscle end

-
cles similar to those in the skeletal muscle end plate
that contain transmitter substance. But, in contrast to
the vesicles of skeletal muscle junctions, which always
contain acetylcholine, the vesicles of the autonomic
nerve ﬁber endings contain acetylcholine in some
ﬁbers and norepinephrine in others—and occasionally
other substances as well.
In a few instances, particularly in the multi-unit type
of smooth muscle, the varicosities are separated from
the muscle cell membrane by as little as 

ted from
the muscle cell membrane by as little as 20 to 30
nanometers—the same width as the synaptic cleft that
occurs in the skeletal muscle junction.These are called
contact junctions, and they function in much the same
way as the skeletal muscle neuromuscular junction;the
rapidity of contraction of these smooth muscle ﬁbers
is considerably faster than that of ﬁbers stimulated by
the diffuse junctions.
Excitatory and Inhibitory Transmitter Substances Secreted at
the Smooth Muscle Neuromuscular

tances Secreted at
the Smooth Muscle Neuromuscular Junction. The most
important transmitter substances secreted by the 
autonomic nerves innervating smooth muscle are
acetylcholine and norepinephrine, but they are never
secreted by the same nerve ﬁbers. Acetylcholine is an
excitatory transmitter substance for smooth muscle
ﬁbers in some organs but an inhibitory transmitter for
smooth muscle in other organs. When acetylcholine
excites a muscle ﬁber, norepinephrine ordinarily
inhibits it. Converse

r, norepinephrine ordinarily
inhibits it. Conversely, when acetylcholine inhibits a
ﬁber, norepinephrine usually excites it.
But why these different responses? The answer is
that both acetylcholine and norepinephrine excite or
inhibit smooth muscle by ﬁrst binding with a receptor
protein on the surface of the muscle cell membrane.
Some of the receptor proteins are excitatory receptors,
whereas others are inhibitory receptors. Thus, the type
of receptor determines whether the smooth muscle is
inh

ceptor determines whether the smooth muscle is
inhibited or excited and also determines which of the
two transmitters, acetylcholine or norepinephrine, is
effective in causing the excitation or inhibition. These
receptors are discussed in more detail in Chapter 60 in
relation to function of the autonomic nervous system.
Membrane Potentials and Action
Potentials in Smooth Muscle
Membrane Potentials in Smooth Muscle. The quantitative
voltage of the membrane potential of smooth muscle
depends on th

 membrane potential of smooth muscle
depends on the momentary condition of the muscle. In
the normal resting state, the intracellular potential is
usually about -50 to -60 millivolts, which is about 30
millivolts less negative than in skeletal muscle.
Action Potentials in Unitary Smooth Muscle. Action poten-
tials occur in unitary smooth muscle (such as visceral
muscle) in the same way that they occur in skeletal
muscle. They do not normally occur in many, if not
most, multi-unit types of smooth

r in many, if not
most, multi-unit types of smooth muscle, as discussed
in a subsequent section.
The action potentials of visceral smooth muscle
occur in one of two forms: (1) spike potentials or 
(2) action potentials with plateaus.
Spike Potentials. Typical spike action potentials, such
as those seen in skeletal muscle, occur in most types of
unitary smooth muscle. The duration of this type of
action potential is 10 to 50 milliseconds, as shown in
Figure 8–4A. Such action potentials can be eli

 in
Figure 8–4A. Such action potentials can be elicited in
many ways, for example, by electrical stimulation, by
the action of hormones on the smooth muscle, by the
action of transmitter substances from nerve ﬁbers, by
stretch, or as a result of spontaneous generation in the
muscle ﬁber itself, as discussed subsequently.
Action Potentials with Plateaus. Figure 8–4C shows a
smooth muscle action potential with a plateau. The
onset of this action potential is similar to that of the
typical spike po

tential is similar to that of the
typical spike potential. However, instead of rapid
repolarization of the muscle ﬁber membrane, the repo-
larization is delayed for several hundred to as much 
as 1000 milliseconds (1 second). The importance of 
the plateau is that it can account for the prolonged
Varicosities
Visceral
Multi-unit
Figure 8–3
Innervation of smooth muscle.
Chapter 8
97
contraction that occurs in some types of smooth
muscle, such as the ure

rs in some types of smooth
muscle, such as the ureter, the uterus under some con-
ditions, and certain types of vascular smooth muscle.
(Also, this is the type of action potential seen in
cardiac muscle ﬁbers that have a prolonged period of
contraction, as discussed in Chapters 9 and 10.)
Importance of Calcium Channels in Generating the
Smooth Muscle Action Potential. The smooth muscle
cell membrane has far more voltage-gated calcium
channels than does skeletal muscle but few voltage-
gated sodi

n does skeletal muscle but few voltage-
gated sodium channels.Therefore, sodium participates
little in the generation of the action potential in most
smooth muscle. Instead, ﬂow of calcium ions to the
interior of the ﬁber is mainly responsible for the action
potential. This occurs in the same self-regenerative
way as occurs for the sodium channels in nerve ﬁbers
and in skeletal muscle ﬁbers. However, the calcium
channels open many times more slowly than do
sodium channels, and they also remain o

ly than do
sodium channels, and they also remain open much
longer. This accounts in large measure for the pro-
longed plateau action potentials of some smooth
muscle ﬁbers.
Another important feature of calcium ion entry into
the cells during the action potential is that the calcium
ions act directly on the smooth muscle contractile
mechanism to cause contraction. Thus, the calcium
performs two tasks at once.
Slow Wave Potentials in Unitary Smooth Muscle, and Sponta-
neous Generation of Action Po

 Muscle, and Sponta-
neous Generation of Action Potentials. Some smooth
muscle is self-excitatory.That is, action potentials arise
within the smooth muscle cells themselves without 
an extrinsic stimulus. This often is associated with a
basic slow wave rhythm of the membrane potential.
A typical slow wave in a visceral smooth muscle of 
the gut is shown in Figure 8–4B. The slow wave itself
is not the action potential. That is, it is not a self-
regenerative process that spreads progressively ove

egenerative process that spreads progressively over
the membranes of the muscle ﬁbers. Instead, it is a
local property of the smooth muscle ﬁbers that make
up the muscle mass.
The cause of the slow wave rhythm is unknown. One
suggestion is that the slow waves are caused by waxing
and waning of the pumping of positive ions (presum-
ably sodium ions) outward through the muscle ﬁber
membrane; that is, the membrane potential becomes
more negative when sodium is pumped rapidly and
less negative when 

n sodium is pumped rapidly and
less negative when the sodium pump becomes less
active. Another suggestion is that the conductances of
the ion channels increase and decrease rhythmically.
The importance of the slow waves is that, when they
are strong enough, they can initiate action potentials.
The slow waves themselves cannot cause muscle con-
traction, but when the peak of the negative slow wave
potential inside the cell membrane rises in the positive
direction from -60 to about -35 millivolts 

sitive
direction from -60 to about -35 millivolts (the
approximate threshold for eliciting action potentials in
most visceral smooth muscle), an action potential
develops and spreads over the muscle mass.Then con-
traction does occur. Figure 8–4B demonstrates this
effect, showing that at each peak of the slow wave, one
or more action potentials occur. These repetitive
sequences of action potentials elicit rhythmical con-
traction of the smooth muscle mass. Therefore, the
slow waves are called pa

scle mass. Therefore, the
slow waves are called pacemaker waves. In Chapter 62,
we see that this type of pacemaker activity controls the
rhythmical contractions of the gut.
Excitation of Visceral Smooth Muscle by Muscle Stretch.
When visceral (unitary) smooth muscle is stretched
sufﬁciently, spontaneous action potentials usually 
are generated. They result from a combination of (1)
the normal slow wave potentials and (2) decrease in
overall negativity of the membrane potential caused
by the stre

ivity of the membrane potential caused
by the stretch itself.This response to stretch allows the
gut wall, when excessively stretched, to contract auto-
matically and rhythmically. For instance, when the gut
is overﬁlled by intestinal contents, local automatic con-
tractions often set up peristaltic waves that move the
contents away from the overﬁlled intestine, usually in
the direction of the anus.
Depolarization of Multi-Unit Smooth Muscle
Without Action Potentials
The smooth muscle ﬁbers of m

out Action Potentials
The smooth muscle ﬁbers of multi-unit smooth muscle
(such as the muscle of the iris of the eye or the 
0
A
C
B
50
100
+20
0
–40
–60
0
–25
–50
Seconds
0
10
20
30
Slow waves
Seconds
0
0.1
0.2
0.3
0.4
Figure 8–4
A, Typical smooth muscle action potential (spike potential) elicited
by an external stimulus. B, Repetitive spike potentials, elicited by
slow rhythmical electrical waves that occur spontaneously in the
smooth muscle of the intestinal

ontaneously in the
smooth muscle of the intestinal wall. C, Action potential with a
plateau, recorded from a smooth muscle ﬁber of the uterus.
98
Unit II
piloerector muscle of each hair) normally contract
mainly in response to nerve stimuli.The nerve endings
secrete acetylcholine in the case of some multi-unit
smooth muscles and norepinephrine in the case of
others. In both instances, the transmitter substances
cause depolarization of the smooth muscle memb

ces
cause depolarization of the smooth muscle membrane,
and this in turn elicits contraction. Action potentials
usually do not develop; the reason is that the ﬁbers are
too small to generate an action potential. (When
action potentials are elicited in visceral unitary smooth
muscle, 30 to 40 smooth muscle ﬁbers must depolarize
simultaneously before a self-propagating action poten-
tial ensues.) Yet, in small smooth muscle cells, even
without an action potential, the local depolarization
(called 

ction potential, the local depolarization
(called the junctional potential) caused by the nerve
transmitter substance itself spreads “electrotonically”
over the entire ﬁber and is all that is needed to cause
Effect of Local Tissue Factors and
Hormones to Cause Smooth Muscle
Probably half of all smooth muscle contraction is 
initiated by stimulatory factors acting directly on 
the smooth muscle contractile machinery and without

scle contractile machinery and without
action potentials. Two types of non-nervous and
non–action 
potential 
factors 
often
involved are (1) local tissue chemical factors and (2)
various hormones.
Smooth Muscle Contraction in Response to Local Tissue 
Chemical Factors. In Chapter 17, we discuss control of 
contraction of the arterioles, meta-arterioles, and pre-
capillary sphincters. The smallest of these vessels have
little or no nervous supply. Yet the smooth muscle is
highly con

ervous supply. Yet the smooth muscle is
highly contractile, responding rapidly to changes in
local chemical conditions in the surrounding intersti-
tial ﬂuid.
In the normal resting state, many of these small
blood vessels remain contracted.But when extra blood
ﬂow to the tissue is needed, multiple factors can relax
the vessel wall, thus allowing for increased ﬂow. In this
way, a powerful local feedback control system controls
the blood ﬂow to the local tissue area. Some of the 
speciﬁc control f

 local tissue area. Some of the 
speciﬁc control factors are as follows:
1. Lack of oxygen in the local tissues causes smooth
muscle relaxation and, therefore, vasodilatation.
2. Excess carbon dioxide causes vasodilatation.
3. Increased hydrogen ion concentration causes
vasodilatation.
Adenosine, lactic acid, increased potassium ions,
diminished calcium ion concentration, and increased
body temperature can all cause local vasodilatation.
Effects of Hormones on Smooth Muscle Contraction. Most
cir

of Hormones on Smooth Muscle Contraction. Most
circulating hormones in the blood affect smooth
muscle contraction to some degree, and some have
profound effects. Among the more important of 
these are norepinephrine, epinephrine, acetylcholine,
angiotensin, endothelin, vasopressin, oxytocin, sero-
tonin, and histamine.
A hormone causes contraction of a smooth muscle
when the muscle cell membrane contains hormone-
gated excitatory receptors for the respective hormone.
Conversely, the hormone caus

e respective hormone.
Conversely, the hormone causes inhibition if the mem-
brane contains inhibitory receptors for the hormone
rather than excitatory receptors.
Mechanisms of Smooth Muscle Excitation or Inhibition by Hor-
mones or Local Tissue Factors. Some hormone receptors
in the smooth muscle membrane open sodium or
calcium ion channels and depolarize the membrane,
the same as after nerve stimulation. Sometimes action
potentials result, or action potentials that are already
occurring may be 

tion potentials that are already
occurring may be enhanced. In other cases, depolari-
zation occurs without action potentials, and this depo-
larization allows calcium ion entry into the cell, which
promotes the contraction.
Inhibition, in contrast, occurs when the hormone (or
other tissue factor) closes the sodium and calcium
channels to prevent entry of these positive ions; inhi-
bition also occurs if the normally closed potassium
channels are opened, allowing positive potassium ions
to diffus

opened, allowing positive potassium ions
to diffuse out of the cell. Both of these actions increase
the degree of negativity inside the muscle cell, a 
state called hyperpolarization, which strongly inhibits
Sometimes smooth muscle contraction or inhibition
is initiated by hormones without directly causing any
change in the membrane potential. In these instances,
the hormone may activate a membrane receptor that
does not open any ion channels but instead causes an
internal ch

any ion channels but instead causes an
internal change in the muscle ﬁber, such as release of
calcium ions from the intracellular sarcoplasmic re-
ticulum; the calcium then induces contraction. To
inhibit contraction, other receptor mechanisms are
known to activate the enzyme adenylate cyclase or
guanylate cyclase in the cell membrane; the portions of
the receptors that protrude to the interior of the cells 
are coupled to these enzymes, causing the formation
of cyclic adenosine monophosphate (c

the formation
of cyclic adenosine monophosphate (cAMP) or cyclic
guanosine monophosphate (cGMP), so-called second
messengers. The cAMP or cGMP has many effects, one
of which is to change the degree of phosphorylation of
several enzymes that indirectly inhibit contraction.
The  pump that moves calcium ions from the sar-
coplasm into the sarcoplasmic reticulum is activated,
as well as the cell membrane pump that moves calcium
ions out of the cell itself; these effects reduce the
calcium ion concen

tself; these effects reduce the
calcium ion concentration in the sarcoplasm, thereby
inhibiting contraction.
Smooth muscles have considerable diversity in how
they initiate contraction or relaxation in response to
different hormones, neurotransmitters, and other 
substances. In some instances, the same substance 
may cause either relaxation or contraction of smooth
muscles in different locations. For example, norepi-
nephrine inhibits contraction of smooth muscle in the
intestine but stimulates 

 of smooth muscle in the
intestine but stimulates contraction of smooth muscle
in blood vessels.
Chapter 8
99
Source of Calcium Ions That Cause
Contraction (1) Through the Cell
Membrane and (2) from the
Although the contractile process in smooth muscle,
as in skeletal muscle, is activated by calcium ions,
the source of the calcium ions differs; the difference 
is that the sarcoplasmic reticulum, which provides 
virtually all the 

smic reticulum, which provides 
virtually all the calcium ions for skeletal muscle con-
traction, is only slightly developed in most smooth
muscle. Instead, almost all the calcium ions that 
cause contraction enter the muscle cell from the extra-
cellular ﬂuid at the time of the action potential or
other stimulus. That is, the concentration of calcium
ions in the extracellular ﬂuid is greater than 10-3 molar,
in comparison with less than 10-7 molar inside the
smooth muscle cell; this causes rapi

ar inside the
smooth muscle cell; this causes rapid diffusion of 
the calcium ions into the cell from the extracellular
ﬂuid when the calcium pores open. The time required
for this diffusion to occur averages 200 to 300 mil-
liseconds and is called the latent period before con-
traction begins. This latent period is about 50 times 
as great for smooth muscle as for skeletal muscle 
Role of the Smooth Muscle Sarcoplasmic Reticulum. Figure
8–5 shows a few slightly developed sarcoplasm

gure
8–5 shows a few slightly developed sarcoplasmic
tubules that lie near the cell membrane in some larger
smooth muscle cells. Small invaginations of the 
cell membrane, called caveolae, abut the surfaces of
these tubules. The caveolae suggest a rudimentary
analog of the transverse tubule system of skeletal
muscle. When an action potential is transmitted into
the caveolae, this is believed to excite calcium ion
release from the abutting sarcoplasmic tubules in the
same way that action potentia

asmic tubules in the
same way that action potentials in skeletal muscle
transverse tubules cause release of calcium ions from
the skeletal muscle longitudinal sarcoplasmic tubules.
In general, the more extensive the sarcoplasmic re-
ticulum in the smooth muscle ﬁber, the more rapidly
it contracts.
Effect on Smooth Muscle Contraction Caused by Changing of
Extracellular Calcium Ion Concentration. Although chang-
ing the extracellular ﬂuid calcium ion concentration
from normal has little effect on 

on concentration
from normal has little effect on the force of contrac-
tion of skeletal muscle, this is not true for most smooth
muscle. When the extracellular ﬂuid calcium ion con-
centration falls to about 1/3 to 1/10 normal, smooth
muscle contraction usually ceases. Therefore, the 
force of contraction of smooth muscle usually is 
highly dependent on extracellular ﬂuid calcium ion
A Calcium Pump Is Required to Cause Smooth Muscle 
Relaxation. To cause relaxation of smooth musc

le 
Relaxation. To cause relaxation of smooth muscle after
it has contracted, the calcium ions must be removed
from the intracellular ﬂuids. This removal is achieved
by a calcium pump that pumps calcium ions out 
of the smooth muscle ﬁber back into the extra-
cellular ﬂuid, or into a sarcoplasmic reticulum, if it 
is present. This pump is slow-acting in comparison 
with the fast-acting sarcoplasmic reticulum pump in 
skeletal muscle. Therefore, a single smooth muscle
contraction often lasts for 

 single smooth muscle
contraction often lasts for seconds rather than hun-
dredths to tenths of a second, as occurs for skeletal
muscle.
Blaustein MP, Lederer WJ: Sodium/calcium exchange:
its physiological implications. Physiol Rev 79:763, 1999.
Davis MJ, Hill MA: Signaling mechanisms underlying 
the vascular myogenic response. Physiol Rev 79:387,
1999.
Harnett KM, Biancani P: Calcium-dependent and calcium-
independent contractions in smooth m

 and calcium-
independent contractions in smooth muscles. Am J Med
115(Suppl 3A):24S, 2003.
Horowitz A, Menice CB, Laporte R, Morgan KG: Mecha-
nisms of smooth muscle contraction. Physiol Rev 76:967,
1996.
Kamm KE, Stull JT: Regulation of smooth muscle contrac-
tile elements by second messengers. Annu Rev Physiol
51:299, 1989.
Kuriyama H, Kitamura K, Itoh T, Inoue R: Physiological fea-
tures of visceral smooth muscle cells, with special refer-
ence to receptors and ion channels. Physiol Rev 78:8

ce to receptors and ion channels. Physiol Rev 78:811,
1998.
Lee CH, Poburko D, Kuo KH, et al: Ca2+ oscillations, gradi-
ents, and homeostasis in vascular smooth muscle. Am J
Physiol Heart Circ Physiol 282:H1571, 2002.
Rybalkin SD, Yan C, Bornfeldt KE, Beavo JA: Cyclic GMP
phosphodiesterases and regulation of smooth muscle
function. Circ Res 93:280, 2003.
Caveolae
reticulum
Figure 8–5
Sarcoplasmic tubules in a large smooth muscle ﬁber showing their
relation to invaginations in the ce

 showing their
relation to invaginations in the cell membrane called caveolae.
100
Unit II
Somlyo AP, Somlyo AV: Ca2+ sensitivity of smooth muscle
and nonmuscle myosin II: modulated by G proteins,
kinases, and myosin phosphatase. Physiol Rev 83:1325,
2003.
Stephens NL: Airway smooth muscle. Lung 179:333, 2001.
Walker JS, Wingard CJ, Murphy RA: Energetics of cross-
bridge phosphorylation and contraction in vascular
smooth muscle. Hypertension 23:1106, 1994.

scular
Webb RC: Smooth muscle contraction and relaxation. Adv
Physiol Educ 27:201, 2003.
U
N
I
T
III
The Heart
9. Heart Muscle; The Heart as a Pump and Function 
of the Heart Valves
10. Rhythmical Excitation of the Heart
11. The Normal Electrocardiogram
12. Electrocardiographic Interpretation of Cardiac 
Muscle and Coronary Blood Flow Abnormalities: 
Vectorial Analysis
13. Cardiac Arrhythmias and Their 
C
H
A
P
T
E
R

C
H
A
P
T
E
R
9
103
Heart Muscle; The Heart as 
a Pump and Function of the 
Heart Valves
With this chapter we begin discussion of the heart
and circulatory system. The heart, shown in Figure
9–1, is actually two separate pumps: a right heart
that pumps blood through the lungs, and a left heart
that pumps blood through the peripheral organs. In
turn, each of these hearts is a pulsatile two-chamber
pump composed of an atrium and a ventricle. Each
atrium is a wea

of an atrium and a ventricle. Each
atrium is a weak primer pump for the ventricle,
helping to move blood into the ventricle. The ventricles then supply the main
pumping force that propels the blood either (1) through the pulmonary circu-
lation by the right ventricle or (2) through the peripheral circulation by the left
ventricle.
Special mechanisms in the heart cause a continuing succession of heart con-
tractions called cardiac rhythmicity, transmitting action potentials throughout
the heart m

nsmitting action potentials throughout
the heart muscle to cause the heart’s rhythmical beat. This rhythmical control
system is explained in Chapter 10. In this chapter, we explain how the heart
operates as a pump, beginning with the special features of heart muscle itself.
The heart is composed of three major types of cardiac muscle: atrial muscle, ven-
tricular muscle, and specialized excitatory and conductive muscle ﬁbers. The
atrial and ventricular types of muscl

e ﬁbers. The
atrial and ventricular types of muscle contract in much the same way as skele-
tal muscle, except that the duration of contraction is much longer. Conversely,
the specialized excitatory and conductive ﬁbers contract only feebly because
they contain few contractile ﬁbrils; instead, they exhibit either automatic rhyth-
mical electrical discharge in the form of action potentials or conduction of the
action potentials through the heart, providing an excitatory system that controls
the r

providing an excitatory system that controls
the rhythmical beating of the heart.
Figure 9–2 shows a typical histological picture of cardiac muscle, demonstrat-
ing cardiac muscle ﬁbers arranged in a latticework, with the ﬁbers dividing,
recombining, and then spreading again. One also notes immediately from this
ﬁgure that cardiac muscle is striated in the same manner as in typical skeletal
muscle. Further, cardiac muscle has typical myoﬁbrils that contain a

ardiac muscle has typical myoﬁbrils that contain actin and
myosin ﬁlaments almost identical to those found in skeletal muscle; these ﬁla-
ments lie side by side and slide along one another during contraction in the
same manner as occurs in skeletal muscle (see Chapter 6). But in other ways,
cardiac muscle is quite different from skeletal muscle, as we shall see.
Cardiac Muscle as a Syncytium. The dark areas crossing the cardiac muscle ﬁbers
in Figure 9–2 are called intercalated discs; they are a

gure 9–2 are called intercalated discs; they are actually cell membranes that
separate individual cardiac muscle cells from one another. That is, cardiac
muscle ﬁbers are made up of many individual cells connected in series and in
parallel with one another.
104
Unit III
The Heart
At each intercalated disc the cell membranes fuse
with one another in such a way that they form perme-
able “communicating” junctions (gap junctions) that
allow almost totally free diffusion of ions. Therefore,
from a f

otally free diffusion of ions. Therefore,
from a functional point of view, ions move with ease
in the intracellular ﬂuid along the longitudinal axes of
the cardiac muscle ﬁbers, so that action potentials
travel easily from one cardiac muscle cell to the next,
past the intercalated discs. Thus, cardiac muscle is a
syncytium of many heart muscle cells in which the
cardiac cells are so interconnected that when one 
of these cells becomes excited, the action potential
spreads to all of them, spreadi

e action potential
spreads to all of them, spreading from cell to cell
throughout the latticework interconnections.
The heart actually is composed of two syncytiums:
the atrial syncytium that constitutes the walls of the
two atria, and the ventricular syncytium that consti-
tutes the walls of the two ventricles.The atria are sepa-
rated from the ventricles by ﬁbrous tissue that
surrounds the atrioventricular (A-V) valvular open-
ings between the atria and ventricles. Normally, poten-
tials are n

atria and ventricles. Normally, poten-
tials are not conducted from the atrial syncytium into
the ventricular syncytium directly through this ﬁbrous
tissue. Instead, they are conducted only by way of a
specialized conductive system called the A-V bundle,
a bundle of conductive ﬁbers several millimeters in
diameter that is discussed in detail in Chapter 10.
This division of the muscle of the heart into two
functional syncytiums allows the atria to contract a
short time ahead of ventricular contra

 contract a
short time ahead of ventricular contraction, which is
important for effectiveness of heart pumping.
The action potential recorded in a ventricular muscle
ﬁber, shown in Figure 9–3, averages about 105 milli-
volts, which means that the intracellular potential rises
from a very negative value, about -85 millivolts,
between beats to a slightly positive value, about +20
millivolts, during each beat. After the initial spike, the
membrane remains depolar

er the initial spike, the
membrane remains depolarized for about 0.2 second,
exhibiting a plateau as shown in the ﬁgure, followed at
the end of the plateau by abrupt repolarization. The
presence of this plateau in the action potential causes
ventricular contraction to last as much as 15 times as
long in cardiac muscle as in skeletal muscle.
Aorta
Pulmonary artery
Inferior
vena cava
Superior
vena cava
Right ventricle
Tricuspid
valve
Pulmonary
valve
Right atrium
Pulmonary
vein
Left atrium
Mitral v

e
Pulmonary
vein
Mitral valve
Aortic valve
Left
ventricle
Lungs
HEAD AND UPPER EXTREMITY
TRUNK AND LOWER EXTREMITY
Figure 9–1
Structure of the heart, and course of blood ﬂow through the heart
chambers and heart valves.
Figure 9–2
“Syncytial,” interconnecting nature of cardiac muscle ﬁbers.
0
+20
–100
–80
–60
–40
–20
0
+20
–60
–40
–20
–100
–80
Seconds
1
2
3
4
0
Purkinje fiber
Ventricular muscle
Plateau
Plateau
Figure 9–3
Rhythmical action potentials (in millivo

igure 9–3
Rhythmical action potentials (in millivolts) from a Purkinje ﬁber 
and from a ventricular muscle ﬁber, recorded by means of 
microelectrodes.
Chapter 9
Heart Muscle; The Heart as a Pump and Function of the Heart Valves
105
What Causes the Long Action Potential and the Plateau? At
this point, we must ask the questions:Why is the action
potential of cardiac muscle so long, and why does it
have a plateau, whereas that of skeletal muscle does
not? The basic biophysical answers to these que

es
not? The basic biophysical answers to these questions
were presented in Chapter 5, but they merit summa-
rizing here as well.
At least two major differences between the mem-
brane properties of cardiac and skeletal muscle
account for the prolonged action potential and the
plateau in cardiac muscle. First, the action potential of
skeletal muscle is caused almost entirely by sudden
opening of large numbers of so-called fast sodium
channels that allow tremendous numbers of sodium
ions to enter t

allow tremendous numbers of sodium
ions to enter the skeletal muscle ﬁber from the extra-
cellular ﬂuid.These channels are called “fast” channels
because they remain open for only a few thousandths
of a second and then abruptly close. At the end of this
closure, repolarization occurs, and the action potential
is over within another thousandth of a second or so.
In cardiac muscle, the action potential is caused by
opening of two types of channels: (1) the same fast
sodium channels as those in ske

 (1) the same fast
sodium channels as those in skeletal muscle and (2)
another entirely different population of slow calcium
channels,
which are also called calcium-sodium 
channels. This second population of channels differs
from the fast sodium channels in that they are slower
to open and, even more important, remain open 
for several tenths of a second. During this time, a large
quantity of both calcium and sodium ions ﬂows
through these channels to the interior of the cardiac
muscle ﬁber, an

els to the interior of the cardiac
muscle ﬁber, and this maintains a prolonged period of
depolarization, causing the plateau in the action poten-
tial. Further, the calcium ions that enter during this
plateau phase activate the muscle contractile process,
while the calcium ions that cause skeletal muscle con-
traction are derived from the intracellular sarcoplas-
mic reticulum.
The second major functional difference between
cardiac muscle and skeletal muscle that helps account
for both the prolo

letal muscle that helps account
for both the prolonged action potential and its plateau
is this: Immediately after the onset of the action poten-
tial, the permeability of the cardiac muscle membrane
for potassium ions decreases about ﬁvefold, an effect
that does not occur in skeletal muscle. This decreased
potassium permeability may result from the excess
calcium inﬂux through the calcium channels just
noted. Regardless of the cause, the decreased potas-
sium permeability greatly decreases the 

ed potas-
sium permeability greatly decreases the outﬂux of 
positively charged potassium ions during the action
potential plateau and thereby prevents early return of
the action potential voltage to its resting level. When
the slow calcium-sodium channels do close at the end
of 0.2 to 0.3 second and the inﬂux of calcium and
sodium ions ceases, the membrane permeability for
potassium ions also increases rapidly; this rapid loss of
potassium from the ﬁber immediately returns the
membrane potentia

the ﬁber immediately returns the
membrane potential to its resting level, thus ending
the action potential.
Velocity of Signal Conduction in Cardiac Muscle. The veloci-
ty of conduction of the excitatory action potential
signal along both atrial and ventricular muscle ﬁbers is
about 0.3 to 0.5 m/sec, or about 1/250 the velocity in very
large nerve ﬁbers and about 1/10 the velocity in skele-
tal muscle ﬁbers. The velocity of conduction in the 
specialized heart conductive system—in the Purkinje
ﬁ

cialized heart conductive system—in the Purkinje
ﬁbers—is as great as 4 m/sec in most parts of the
system, which allows reasonably rapid conduction of
the excitatory signal to the different parts of the heart,
as explained in Chapter 10.
Refractory Period of Cardiac Muscle. Cardiac muscle, like
all excitable tissue, is refractory to restimulation
during the action potential. Therefore, the refractory
period of the heart is the interval of time, as shown to
the left in Figure 9–4, during which a 

s shown to
the left in Figure 9–4, during which a normal cardiac
impulse cannot re-excite an already excited area of
cardiac muscle. The normal refractory period of the
ventricle is 0.25 to 0.30 second, which is about the
duration of the prolonged plateau action potential.
There is an additional relative refractory period of
Seconds
1
2
3
0
Relative refractory 
period
Refractory period
Early premature
Later premature
Figure 9–4
Force of ventricular he

 of contraction
Figure 9–4
Force of ventricular heart muscle
contraction, showing also dura-
tion of the refractory period and
relative refractory period, plus the
effect of premature contraction.
Note that premature contractions
do not cause wave summation, as
occurs in skeletal muscle.
106
Unit III
The Heart
about 0.05 second during which the muscle is more dif-
ﬁcult than normal to excite but nevertheless can be
excited by a very strong excitatory signal, as demon-
strated by the early “prema

tory signal, as demon-
strated by the early “premature” contraction in the
second example of Figure 9–4. The refractory period
of atrial muscle is much shorter than that for the ven-
tricles (about 0.15 second for the atria compared with
0.25 to 0.30 second for the ventricles).
Excitation-Contraction Coupling—Function 
of Calcium Ions and the Transverse Tubules
The term “excitation-contraction coupling” refers to
the mechanism by which the action potential causes
the myoﬁbrils of muscle to contr

 potential causes
the myoﬁbrils of muscle to contract.This was discussed
for skeletal muscle in Chapter 7. Once again, there are
differences in this mechanism in cardiac muscle that
have important effects on the characteristics of cardiac
As is true for skeletal muscle, when an action poten-
tial passes over the cardiac muscle membrane, the
action potential spreads to the interior of the cardiac
muscle ﬁber along the membranes of the transverse
(T) tubules. The T tubule actio

 of the transverse
(T) tubules. The T tubule action potentials in turn act
on the membranes of the longitudinal sarcoplasmic
tubules to cause release of calcium ions into the
muscle sarcoplasm from the sarcoplasmic reticulum.In
another few thousandths of a second, these calcium
ions diffuse into the myoﬁbrils and catalyze the chemi-
cal reactions that promote sliding of the actin and
myosin ﬁlaments along one another; this produces the
Thus far, this mechanism of excitation-c

traction.
Thus far, this mechanism of excitation-contraction
coupling is the same as that for skeletal muscle, but
there is a second effect that is quite different. In addi-
tion to the calcium ions that are released into the 
sarcoplasm from the cisternae of the sarcoplasmic
reticulum, a large quantity of extra calcium ions also
diffuses into the sarcoplasm from the T tubules them-
selves at the time of the action potential. Indeed,
without this extra calcium from the T tubules, the
strength of

 extra calcium from the T tubules, the
strength of cardiac muscle contraction would be
reduced considerably because the sarcoplasmic re-
ticulum of cardiac muscle is less well developed than
that of skeletal muscle and does not store enough
calcium to provide full contraction. Conversely, the T
tubules of cardiac muscle have a diameter 5 times as
great as that of the skeletal muscle tubules, which
means a volume 25 times as great. Also, inside the T
tubules is a large quantity of mucopolysacchar

e T
tubules is a large quantity of mucopolysaccharides that
are electronegatively charged and bind an abundant
store of calcium ions, keeping these always available
for diffusion to the interior of the cardiac muscle ﬁber
when a T tubule action potential appears.
The strength of contraction of cardiac muscle
depends to a great extent on the concentration of
calcium ions in the extracellular ﬂuids. The reason for
this is that the openings of the T tubules pass directly
through the cardiac muscle 

 tubules pass directly
through the cardiac muscle cell membrane into the
extracellular spaces surrounding the cells, allowing the
same extracellular ﬂuid that is in the cardiac muscle
interstitium to percolate through the T tubules as well.
Consequently, the quantity of calcium ions in the T
tubule system—that is, the availability of calcium ions
to cause cardiac muscle contraction—depends to a
great extent on the extracellular ﬂuid calcium ion 
(By way of contrast, the strength o

(By way of contrast, the strength of skeletal muscle
contraction is hardly affected by moderate changes in
extracellular ﬂuid calcium concentration because
skeletal muscle contraction is caused almost entirely
by calcium ions released from the sarcoplasmic retic-
ulum inside the skeletal muscle ﬁber itself.)
At the end of the plateau of the cardiac action
potential, the inﬂux of calcium ions to the interior of
the muscle ﬁber is suddenly cut off, and the calcium
ions in the sarcop

ddenly cut off, and the calcium
ions in the sarcoplasm are rapidly pumped back out of
the muscle ﬁbers into both the sarcoplasmic reticulum
and the T tubule–extracellular ﬂuid space. As a result,
the contraction ceases until a new action potential
comes along.
Duration of Contraction. Cardiac muscle begins to contract
a few milliseconds after the action potential begins and
continues to contract until a few milliseconds after the
action potential ends. Therefore, the duration of con-
traction of

 ends. Therefore, the duration of con-
traction of cardiac muscle is mainly a function of the
duration of the action potential, including the plateau—
about 0.2 second in atrial muscle and 0.3 second in ven-
tricular muscle.
The cardiac events that occur from the beginning of
one heartbeat to the beginning of the next are called
the cardiac cycle. Each cycle is initiated by sponta-
neous generation of an action potential in the sinus
node, as explained in Chapter 10. This node 

sinus
node, as explained in Chapter 10. This node is located
in the superior lateral wall of the right atrium near the
opening of the superior vena cava, and the action
potential travels from here rapidly through both atria
and then through the A-V bundle into the ventricles.
Because of this special arrangement of the conducting
system from the atria into the ventricles, there is a
delay of more than 0.1 second during passage of the
cardiac impulse from the atria into the ventricles. This
allows

se from the atria into the ventricles. This
allows the atria to contract ahead of ventricular con-
traction, thereby pumping blood into the ventricles
before the strong ventricular contraction begins. Thus,
the atria act as primer pumps for the ventricles, and
the ventricles in turn provide the major source of
power for moving blood through the body’s vascular
system.
The cardiac cycle consists of a period of relaxation
called diastole, during which the heart ﬁlls with blood

d diastole, during which the heart ﬁlls with blood,
followed by a period of contraction called systole.
Figure 9–5 shows the different events during the
cardiac cycle for the left side of the heart.The top three
curves show the pressure changes in the aorta, left ven-
tricle, and left atrium, respectively. The fourth curve
depicts the changes in left ventricular volume, the ﬁfth
Chapter 9
107
the electrocardiogram, and the sixth 

t Valves
107
the electrocardiogram, and the sixth a phonocardio-
gram, which is a recording of the sounds produced by
the heart—mainly by the heart valves—as it pumps. It
is especially important that the reader study in detail
this ﬁgure and understand the causes of all the events
shown.
Relationship of the Electrocardiogram
to the Cardiac Cycle
The electrocardiogram in Figure 9–5 shows the P, Q,
R, S, and T waves, which are discussed in Chapters 11,
12, and 13. They are electrical voltages gene

 11,
12, and 13. They are electrical voltages generated by
the heart and recorded by the electrocardiograph from
the surface of the body.
The P wave is caused by spread of depolarization
through the atria, and this is followed by atrial con-
traction, which causes a slight rise in the atrial pres-
sure curve immediately after the electrocardiographic
P wave.
About 0.16 second after the onset of the P wave, the
QRS waves appear as a result of electrical depolariza-
tion of the ventricles, which i

trical depolariza-
tion of the ventricles, which initiates contraction of the
ventricles and causes the ventricular pressure to begin
rising, as also shown in the ﬁgure. Therefore, the QRS
complex begins slightly before the onset of ventricu-
lar systole.
Finally, one observes the ventricular T wave in the
electrocardiogram. This represents the stage of repo-
larization of the ventricles when the ventricular muscle
ﬁbers begin to relax. Therefore, the T wave occurs
slightly before the end of ven

, the T wave occurs
slightly before the end of ventricular contraction.
Blood normally ﬂows continually from the great veins
into the atria; about 80 per cent of the blood ﬂows
directly through the atria into the ventricles even
before the atria contract. Then, atrial contraction
usually causes an additional 20 per cent ﬁlling of the
ventricles. Therefore, the atria simply function as
120
100
Pressure (mm Hg)
80
60
40
20
0
130
90
Volume (ml)
50
Systole
1st
2

80
60
40
20
0
130
90
50
Systole
1st
2nd
3rd
Systole
Diastole
Q
S
T
R
P
a
c
v
Phonocardiogram
Ventricular volume
Ventricular pressure
Atrial pressure
Aortic pressure
A-V valve
opens
A-V valve
closes
closes
Aortic 
valve
opens
Isovolumic
Ejection
Isovolumic 
Rapid inflow
Diastasis
Atrial systole
Figure 9–5
Events of the cardiac cycle for left ventricular function, showing changes in left atrial pressure, left ventricular pressure, 

 left atrial pressure, left ventricular pressure, aortic pressure,
ventricular volume, the electrocardiogram, and the phonocardiogram.
108
Unit III
The Heart
primer pumps that increase the ventricular pumping
effectiveness as much as 20 per cent. However, the
heart can continue to operate under most conditions
even without this extra 20 per cent effectiveness
because it normally has the capability of pumping 300
to 400 per cent more blood than is required by the
resting body.Therefore, when the 

 required by the
resting body.Therefore, when the atria fail to function,
the difference is unlikely to be noticed unless a person
exercises; then acute signs of heart failure occasionally
develop, especially shortness of breath.
Pressure Changes in the Atria—The a, c, and v Waves. In the
atrial pressure curve of Figure 9–5, three minor pressure
elevations, called the a, c, and v atrial pressure waves, are
noted.
The a wave is caused by atrial contraction. Ordinarily,
the right atrial pressure i

ntraction. Ordinarily,
the right atrial pressure increases 4 to 6 mm Hg during
atrial contraction, and the left atrial pressure increases
about 7 to 8 mm Hg.
The c wave occurs when the ventricles begin to 
contract; it is caused partly by slight backﬂow of blood
into the atria at the onset of ventricular contraction 
but mainly by bulging of the A-V valves backward
toward the atria because of increasing pressure in the
ventricles.
The v wave occurs toward the end of ventricular 
contraction; it 

rs toward the end of ventricular 
contraction; it results from slow ﬂow of blood into the
atria from the veins while the A-V valves are closed
during ventricular contraction. Then, when ventricular
contraction is over, the A-V valves open, allowing this
stored atrial blood to ﬂow rapidly into the ventricles and
causing the v wave to disappear.
Filling of the Ventricles. During ventricular systole, large
amounts of blood accumulate in the right and left atria
b

 of blood accumulate in the right and left atria
because of the closed A-V valves. Therefore, as soon
as systole is over and the ventricular pressures fall
again to their low diastolic values, the moderately
increased pressures that have developed in the atria
during ventricular systole immediately push the A-V
valves open and allow blood to ﬂow rapidly into the
ventricles, as shown by the rise of the left ventricular
volume curve in Figure 9–5.This is called the period of
rapid ﬁlling of the ve

his is called the period of
rapid ﬁlling of the ventricles.
The period of rapid ﬁlling lasts for about the ﬁrst
third of diastole. During the middle third of diastole,
only a small amount of blood normally ﬂows into the
ventricles; this is blood that continues to empty into
the atria from the veins and passes through the atria
directly into the ventricles.
During the last third of diastole, the atria contract
and give an additional thrust to the inﬂow of blood
into the ventricles; this accounts 

inﬂow of blood
into the ventricles; this accounts for about 20 per cent
of the ﬁlling of the ventricles during each heart cycle.
Emptying of the Ventricles During Systole
Period of Isovolumic (Isometric) Contraction. Immedi-
ately after ventricular contraction begins, the ventric-
ular pressure rises abruptly, as shown in Figure 9–5,
causing the A-V valves to close. Then an additional
0.02 to 0.03 second is required for the ventricle to build
up sufﬁcient pressure to push the semilunar (aortic
a

sufﬁcient pressure to push the semilunar (aortic
and pulmonary) valves open against the pressures in
the aorta and pulmonary artery. Therefore, during this
period, contraction is occurring in the ventricles, but
there is no emptying. This is called the period of iso-
volumic or isometric contraction, meaning that tension
is increasing in the muscle but little or no shortening
of the muscle ﬁbers is occurring.
Period of Ejection. When the left ventricular pressure
rises slightly above 80 mm Hg (a

tricular pressure
rises slightly above 80 mm Hg (and the right ventricu-
lar pressure slightly above 8 mm Hg), the ventricular
pressures push the semilunar valves open. Immedi-
ately, blood begins to pour out of the ventricles, with
about 70 per cent of the blood emptying occurring
during the ﬁrst third of the period of ejection and the
remaining 30 per cent emptying during the next two
thirds. Therefore, the ﬁrst third is called the period of
rapid ejection, and the last two thirds, the period 

pid ejection, and the last two thirds, the period of
slow ejection.
Period of Isovolumic (Isometric) Relaxation. At the end of
systole, ventricular relaxation begins suddenly, allow-
ing both the right and left intraventricular pressures
to decrease rapidly. The elevated pressures in the dis-
tended large arteries that have just been ﬁlled with
blood from the contracted ventricles immediately
push blood back toward the ventricles, which snaps the
aortic and pulmonary valves closed. For another 0

aortic and pulmonary valves closed. For another 0.03
to 0.06 second, the ventricular muscle continues to
relax, even though the ventricular volume does not
change, giving rise to the period of isovolumic or iso-
metric relaxation. During this period, the intraven-
tricular pressures decrease rapidly back to their low
diastolic levels. Then the A-V valves open to begin a
new cycle of ventricular pumping.
End-Diastolic Volume, End-Systolic Volume, and Stroke Volume
Output. During diastole, normal

 and Stroke Volume
Output. During diastole, normal ﬁlling of the ventricles
increases the volume of each ventricle to about 110 to
120 milliliters. This volume is called the end-diastolic
volume. Then, as the ventricles empty during systole,
the volume decreases about 70 milliliters, which is
called the stroke volume output.The remaining volume
in each ventricle, about 40 to 50 milliliters, is called the
end-systolic volume. The fraction of the end-diastolic
volume that is ejected is called the 

nd-diastolic
volume that is ejected is called the ejection fraction—
usually equal to about 60 per cent.
When the heart contracts strongly, the end-systolic
volume can be decreased to as little as 10 to 20 milli-
liters. Conversely, when large amounts of blood ﬂow
into the ventricles during diastole, the ventricular end-
diastolic volumes can become as great as 150 to 180
milliliters in the healthy heart. By both increasing the
end-diastolic volume and decreasing the end-systolic
volume, the str

me and decreasing the end-systolic
volume, the stroke volume output can be increased to
more than double normal.
Chapter 9
109
Atrioventricular Valves. The A-V valves (the tricuspid
and mitral valves) prevent backﬂow of blood from the
ventricles to the atria during systole, and the semilu-
nar valves (the aortic and pulmonary artery valves)
prevent backﬂow from the aorta and pulmonary ar-
teries into the ve

rom the aorta and pulmonary ar-
teries into the ventricles during diastole. These valves,
shown in Figure 9–6 for the left ventricle, close and
open passively. That is, they close when a backward
pressure gradient pushes blood backward, and they
open when a forward pressure gradient forces blood
in the forward direction. For anatomical reasons, the
thin, ﬁlmy A-V valves require almost no backﬂow to
cause closure, whereas the much heavier semilunar
valves require rather rapid backﬂow for a few 
m

r
Function of the Papillary Muscles. Figure 9–6 also
shows papillary muscles that attach to the vanes of the
A-V valves by the chordae tendineae. The papillary
muscles contract when the ventricular walls contract,
but contrary to what might be expected, they do not
help the valves to close. Instead, they pull the vanes of
the valves inward toward the ventricles to prevent
their bulging too far backward toward the atria during
ventricul

too far backward toward the atria during
ventricular contraction. If a chorda tendinea becomes
ruptured or if one of the papillary muscles becomes
paralyzed, the valve bulges far backward during ven-
tricular contraction, sometimes so far that it leaks
severely and results in severe or even lethal cardiac
incapacity.
Aortic and Pulmonary Artery Valves. The aortic and pul-
monary artery semilunar valves function quite differ-
ently from the A-V valves. First, the high pressures in
the arteries at

lves. First, the high pressures in
the arteries at the end of systole cause the semilunar
valves to snap to the closed position, in contrast to the
much softer closure of the A-V valves.Second,because
of smaller openings, the velocity of blood ejection
through the aortic and pulmonary valves is far greater
than that through the much larger A-V valves. Also,
because of the rapid closure and rapid ejection, the
edges of the aortic and pulmonary valves are subjected
to much greater mechanical abras

ves are subjected
to much greater mechanical abrasion than are the 
A-V valves. Finally, the A-V valves are supported by
the chordae tendineae, which is not true for the semi-
lunar valves. It is obvious from the anatomy of the
aortic and pulmonary valves (as shown for the aortic
valve at the bottom of Figure 9–6) that they must be
constructed with an especially strong yet very pliable
ﬁbrous tissue base to withstand the extra physical
stresses.
When the left ventricle cont

When the left ventricle contracts, the ventricular pres-
sure increases rapidly until the aortic valve opens.
Then, after the valve opens, the pressure in the ventri-
cle rises much less rapidly, as shown in Figure 9–5,
because blood immediately ﬂows out of the ventricle
into the aorta and then into the systemic distribution
arteries.
The entry of blood into the arteries causes the walls
of these arteries to stretch and the pressure to increase
to about 120 mm Hg.
Next, at 

pressure to increase
Next, at the end of systole, after the left ventricle
stops ejecting blood and the aortic valve closes, the
elastic walls of the arteries maintain a high pressure in
the arteries, even during diastole.
A so-called incisura occurs in the aortic pressure
curve when the aortic valve closes. This is caused by a
short period of backward ﬂow of blood immediately
before closure of the valve, followed by sudden cessa-
tion of the backﬂow.
After the aortic valve h

ssa-
After the aortic valve has closed, the pressure in the
aorta decreases slowly throughout diastole because
the blood stored in the distended elastic arteries ﬂows
continually through the peripheral vessels back to the
veins. Before the ventricle contracts again, the aortic
pressure usually has fallen to about 80 mm Hg (dias-
tolic pressure), which is two thirds the maximal pres-
sure of 120 mm Hg (systolic pressure) that occurs in
the aorta during ventricular contraction

occurs in
the aorta during ventricular contraction.
The pressure curves in the right ventricle and pul-
monary artery are similar to those in the aorta, except
that the pressures are only about one sixth as great, as
discussed in Chapter 14.
Relationship of the Heart
Sounds to Heart Pumping
When listening to the heart with a stethoscope, one does
not hear the opening of the valves because this is a 
Cusp
Cusp
MITRAL VALVE
AORTIC VALVE
Papillary muscles
Chordae tendineae
Figure 9–6
Mitral and aor

uscles
Figure 9–6
Mitral and aortic valves (the left ventricular valves).
110
Unit III
The Heart
relatively slow process that normally makes no noise.
However, when the valves close, the vanes of the valves
and the surrounding ﬂuids vibrate under the inﬂuence
of sudden pressure changes, giving off sound that travels
in all directions through the chest.
When the ventricles contract, one ﬁrst hears a sound
caused by closure of the A-V valves.The vibration is low
in pitch and rela

 A-V valves.The vibration is low
in pitch and relatively long-lasting and is known as the
ﬁrst heart sound. When the aortic and pulmonary valves
close at the end of systole, one hears a rapid snap
because these valves close rapidly, and the surroundings
vibrate for a short period. This sound is called the
second heart sound. The precise causes of the heart
sounds are discussed more fully in Chapter 23, in rela-
tion to listening to the sounds with the stethoscope.
The st

h the stethoscope.
The stroke work output of the heart is the amount of
energy that the heart converts to work during each
heartbeat while pumping blood into the arteries. Minute
work output is the total amount of energy converted to
work in 1 minute; this is equal to the stroke work output
times the heart rate per minute.
Work output of the heart is in two forms. First, by far
the major proportion is used to move the blood from
the low-pressure veins to the high-pressur

od from
the low-pressure veins to the high-pressure arteries.
This is called volume-pressure work or external work.
Second, a minor proportion of the energy is used to
accelerate the blood to its velocity of ejection through
the aortic and pulmonary valves. This is the kinetic
energy of blood ﬂow component of the work output.
Right ventricular external work output is normally
about one sixth the work output of the left ventricle
because of the sixfold difference in systolic pressures
that the tw

xfold difference in systolic pressures
that the two ventricles pump. The additional work
output of each ventricle required to create kinetic
energy of blood ﬂow is proportional to the mass of
blood ejected times the square of velocity of ejection.
Ordinarily, the work output of the left ventricle
required to create kinetic energy of blood ﬂow is only
about 1 per cent of the total work output of the ventri-
cle and therefore is ignored in the calculation of the
total stroke work output. But in ce

ulation of the
total stroke work output. But in certain abnormal con-
ditions, such as aortic stenosis, in which blood ﬂows with
great velocity through the stenosed valve, more than 
50 per cent of the total work output may be required to
create kinetic energy of blood ﬂow.
Graphical Analysis of Ventricular
Pumping
Figure 9–7 shows a diagram that is especially useful in
explaining the pumping mechanics of the left ventricle.
The most important components of the diagram are 
the two curves labele

mponents of the diagram are 
the two curves labeled “diastolic pressure”
and 
“systolic pressure.” These curves are volume-pressure
curves.
The diastolic pressure curve is determined by ﬁlling
the heart with progressively greater volumes of blood
and then measuring the diastolic pressure immediately
before ventricular contraction occurs, which is the end-
diastolic pressure of the ventricle.
The systolic pressure curve is determined by record-
ing the systolic pressure achieved during ventricula

g the systolic pressure achieved during ventricular
contraction at each volume of ﬁlling.
Until the volume of the noncontracting ventricle rises
above about 150 milliliters, the “diastolic” pressure 
does not increase greatly. Therefore, up to this volume,
blood can ﬂow easily into the ventricle from the atrium.
Above 150 milliliters, the ventricular diastolic pressure
increases rapidly, partly because of ﬁbrous tissue in the
heart that will stretch no more and partly because the
pericardium tha

tch no more and partly because the
pericardium that surrounds the heart becomes ﬁlled
nearly to its limit.
During ventricular contraction, the “systolic” pres-
sure increases even at low ventricular volumes and
reaches a maximum at a ventricular volume of 150 to
170 milliliters. Then, as the volume increases still
further, the systolic pressure actually decreases under
some conditions, as demonstrated by the falling systolic
pressure curve in Figure 9–7, because at these great
volumes, the actin

ure 9–7, because at these great
volumes, the actin and myosin ﬁlaments of the cardiac
muscle ﬁbers are pulled apart far enough that the
strength of each cardiac ﬁber contraction becomes less
than optimal.
Note especially in the ﬁgure that the maximum sys-
tolic pressure for the normal left ventricle is between
250 and 300 mm Hg, but this varies widely with each
person’s heart strength and degree of heart stimulation
by cardiac nerves. For the normal right ventricle,
the maximum systolic pressure

mal right ventricle,
the maximum systolic pressure is between 60 and 
80 mm Hg.
“Volume-Pressure Diagram” During the Cardiac Cycle; Cardiac
Work Output. The red lines in Figure 9–7 form a loop
called the volume-pressure diagram of the cardiac cycle
for normal function of the left ventricle. It is divided into
four phases.
Phase I: Period of ﬁlling. This phase in the volume-
pressure diagram begins at a ventricular volume of
about 45 milliliters and a diastolic pressure near 
0 mm Hg. Forty-ﬁve m

nd a diastolic pressure near 
0 mm Hg. Forty-ﬁve milliliters is the amount of
250
0
0
Left ventricular volume (ml)
Intraventricular pressure (mm Hg)
250
200
150
100
50
50
100
150
200
300
Systolic pressure
EW
III
IV
I
II
Diastolic 
pressure
Period of ejection
Period of filling
Figure 9–7
Relationship between left ventricular volume and intraventricular
pressure during diastole and systole. Also shown by the heavy
red lines is the “volume-pressure diagr

 the heavy
red lines is the “volume-pressure diagram,” demonstrating
changes in intraventricular volume and pressure during the
normal cardiac cycle. EW, net external work.
Chapter 9
111
blood that remains in the ventricle after the
previous heartbeat and is called the end-systolic
volume. As venous blood ﬂows into the ventricle
from the left atrium, the ventricular volume
normally increases to about 115 milliliters, called
the e

y increases to about 115 milliliters, called
the end-diastolic volume, an increase of 70
milliliters. Therefore, the volume-pressure diagram
during phase I extends along the line labeled “I,”
with the volume increasing to 115 milliliters and the
diastolic pressure rising to about 5 mm Hg.
Phase II: Period of isovolumic contraction. During
isovolumic contraction, the volume of the ventricle
does not change because all valves are closed.
However, the pressure inside the ventricle increases
to equa

he pressure inside the ventricle increases
to equal the pressure in the aorta, at a pressure
value of about 80 mm Hg, as depicted by the arrow
end of the line labeled “II.”
Phase III: Period of ejection. During ejection, the
systolic pressure rises even higher because of still
more contraction of the ventricle. At the same time,
the volume of the ventricle decreases because the
aortic valve has now opened and blood ﬂows out of
the ventricle into the aorta. Therefore, the curve
labeled “III” trac

the aorta. Therefore, the curve
labeled “III” traces the changes in volume and
systolic pressure during this period of ejection.
Phase IV: Period of isovolumic relaxation. At the end
of the period of ejection, the aortic valve closes, and
the ventricular pressure falls back to the diastolic
pressure level. The line labeled “IV” traces this
decrease in intraventricular pressure without any
change in volume. Thus, the ventricle returns to its
starting point, with about 45 milliliters of blood 
lef

ing point, with about 45 milliliters of blood 
left in the ventricle and at an atrial pressure near 
0 mm Hg.
Readers well trained in the basic principles of physics
should recognize that the area subtended by this func-
tional volume-pressure diagram (the tan shaded area,
labeled EW) represents the net external work output of
the ventricle during its contraction cycle. In experimen-
tal studies of cardiac contraction, this diagram is used
for calculating cardiac work output.
When the heart pump

lculating cardiac work output.
When the heart pumps large quantities of blood, the
area of the work diagram becomes much larger.That is,
it extends far to the right because the ventricle ﬁlls with
more blood during diastole, it rises much higher because
the ventricle contracts with greater pressure, and it
usually extends farther to the left because the ventricle
contracts to a smaller volume—especially if the ventri-
cle is stimulated to increased activity by the sympathetic
nervous system.
Con

ed activity by the sympathetic
Concepts of Preload and Afterload. In assessing the contrac-
tile properties of muscle, it is important to specify the
degree of tension on the muscle when it begins to con-
tract, which is called the preload, and to specify the load
against which the muscle exerts its contractile force,
which is called the afterload.
For cardiac contraction, the preload is usually con-
sidered to be the end-diastolic pressure when the ven-
tricle has become ﬁlled.

c pressure when the ven-
The afterload of the ventricle is the pressure in the
artery leading from the ventricle. In Figure 9–7, this cor-
responds to the systolic pressure described by the phase
III curve of the volume-pressure diagram. (Sometimes
the afterload is loosely considered to be the resistance
in the circulation rather than the pressure.)
The importance of the concepts of preload and after-
load is that in many abnormal functional states of the
heart or circul

 abnormal functional states of the
heart or circulation, the pressure during ﬁlling of the
ventricle (the preload), the arterial pressure against
which the ventricle must contract (the afterload), or
both are severely altered from normal.
Chemical Energy Required
for Cardiac Contraction:
Oxygen Utilization by 
the Heart
Heart muscle, like skeletal muscle, uses chemical energy
to provide the work of contraction. This energy is
derived mainly from oxidative metabolism of fatty acids
and, to a less

oxidative metabolism of fatty acids
and, to a lesser extent, of other nutrients, especially
lactate and glucose. Therefore, the rate of oxygen con-
sumption by the heart is an excellent measure of the
chemical energy liberated while the heart performs its
work.The different chemical reactions that liberate this
energy are discussed in Chapters 67 and 68.
Efﬁciency of Cardiac Contraction. During heart muscle con-
traction, most of the expended chemical energy is con-
verted into heat and a much s

mical energy is con-
verted into heat and a much smaller portion into work
output. The ratio of work output to total chemical
energy expenditure is called the efﬁciency of cardiac
contraction, or simply efﬁciency of the heart. Maximum
efﬁciency of the normal heart is between 20 and 25 per
cent. In heart failure, this can decrease to as low as 5 to
10 per cent.
When a person is at rest, the heart pumps only 4 to 6
liters of blood each minute. During severe exercise,
th

s of blood each minute. During severe exercise,
the heart may be required to pump four to seven times
this amount. The basic means by which the volume
pumped by the heart is regulated are (1) intrinsic
cardiac regulation of pumping in response to changes
in volume of blood ﬂowing into the heart and (2)
control of heart rate and strength of heart pumping by
the autonomic nervous system.
Intrinsic Regulation of Heart
Pumping—The Frank-Starling
Mechanism
In Chapter 20, we will learn that under most

anism
In Chapter 20, we will learn that under most condi-
tions, the amount of blood pumped by the heart each
minute is determined almost entirely by the rate of
blood ﬂow into the heart from the veins,which is called
venous return. That is, each peripheral tissue of the
body controls its own local blood ﬂow, and all the local
tissue ﬂows combine and return by way of the veins to
the right atrium. The heart, in turn, automatically
pumps this incoming blood into the arteries, so that it
can ﬂow a

ming blood into the arteries, so that it
can ﬂow around the circuit again.
This intrinsic ability of the heart to adapt to increas-
ing volumes of inﬂowing blood is called the Frank-
Starling mechanism of the heart, in honor of Frank and
112
Unit III
The Heart
Starling, two great physiologists of a century ago. Basi-
cally, the Frank-Starling mechanism means that the
greater the heart muscle is stretched during ﬁlling,
the greater is the force of contraction and the greater
the quantity of blood

 contraction and the greater
the quantity of blood pumped into the aorta. Or, stated
another way: Within physiologic limits, the heart pumps
all the blood that returns to it by the way of the veins.
What Is the Explanation of the Frank-Starling Mechanism?
When an extra amount of blood ﬂows into the ventri-
cles, the cardiac muscle itself is stretched to greater
length. This in turn causes the muscle to contract with
increased force because the actin and myosin ﬁlaments
are brought to a more near

tin and myosin ﬁlaments
are brought to a more nearly optimal degree of
overlap for force generation. Therefore, the ventricle,
because of its increased pumping, automatically
pumps the extra blood into the arteries.
This ability of stretched muscle, up to an optimal
length, to contract with increased work output is char-
acteristic of all striated muscle,as explained in Chapter
6, and is not simply a characteristic of cardiac muscle.
In addition to the important effect of lengthening
the heart m

to the important effect of lengthening
the heart muscle, still another factor increases heart
pumping when its volume is increased. Stretch of the
right atrial wall directly increases the heart rate by 
10 to 20 per cent; this, too, helps increase the amount
of blood pumped each minute, although its contribu-
tion is much less than that of the Frank-Starling 
Ventricular Function Curves
One of the best ways to express the functional ability
of the ventricles to pump blood is by ventri

ility
of the ventricles to pump blood is by ventricular func-
tion curves, as shown in Figures 9–8 and 9–9. Figure
9–8 shows a type of ventricular function curve called
the stroke work output curve. Note that as the atrial
pressure for each side of the heart increases, the stroke
work output for that side increases until it reaches the
limit of the ventricle’s pumping ability.
Figure 9–9 shows another type of ventricular func-
tion curve called the ventricular volume output curve.
The two curves

he ventricular volume output curve.
The two curves of this ﬁgure represent function of the
two ventricles of the human heart based on data
extrapolated from lower animals. As the right and left
atrial pressures increase, the respective ventricular
volume outputs per minute also increase.
Thus, ventricular function curves are another way of
expressing the Frank-Starling mechanism of the heart.
That is, as the ventricles ﬁll in response to higher atrial
pressures, each ventricular volume and stren

trial
pressures, each ventricular volume and strength of
cardiac muscle contraction increase, causing the 
heart to pump increased quantities of blood into the
arteries.
Control of the Heart by the Sympathetic and
Parasympathetic Nerves
The pumping effectiveness of the heart also is con-
trolled by the sympathetic
and parasympathetic
(vagus) nerves, which abundantly supply the heart, as
shown in Figure 9–10. For given levels of input atrial
pressure, the amount of blood pumped each minute
(cardi

ure, the amount of blood pumped each minute
(cardiac output) often can be increased more than 100
per cent by sympathetic stimulation. By contrast, the
output can be decreased to as low as zero or almost
zero by vagal (parasympathetic) stimulation.
Mechanisms of Excitation of the Heart by the Sympathetic
Nerves. Strong sympathetic stimulation can increase
the heart rate in young adult humans from the normal
rate of 70 beats per minute up to 180 to 200 and, rarely,
even 250 beats per minute. Also

o 200 and, rarely,
even 250 beats per minute. Also, sympathetic stimula-
tion increases the force of heart contraction to as much
as double normal, thereby increasing the volume of
blood pumped and increasing the ejection pressure.
40
20
30
0
10
Left mean atrial
pressure
(mm Hg)
10
20
0
Left ventricular
stroke work
(gram meters)
4
2
3
0
1
Right mean atrial
pressure
(mm Hg)
10
20
0
Right ventricular
Figure 9–8
Left and right ventricular function curves recorded from dogs

ght ventricular function curves recorded from dogs,
depicting ventricular stroke work output as a function of left and
right mean atrial pressures. (Curves reconstructed from data in
Sarnoff SJ: Myocardial contractility as described by ventricular
function curves. Physiol Rev 35:107, 1955.)
Ventricular output (L/min)
Atrial pressure (mm Hg)
0
–4
+8
+4
+12
+16
10
5
0
15
Left ventricle
Figure 9–9
Approximate normal right and left ventricular volume output
curves for the normal rest

ntricular volume output
curves for the normal resting human heart as extrapolated from
data obtained in dogs and data from human beings.
Chapter 9
113
Thus, sympathetic stimulation often can increase the
maximum cardiac output as much as twofold to three-
fold, in addition to the increased output caused by the
Frank-Starling mechanism already discussed.
Conversely, inhibition of the sympathetic nerves to
the heart can decrease ca

he sympathetic nerves to
the heart can decrease cardiac pumping to a moderate
extent in the following way: Under normal conditions,
the sympathetic nerve ﬁbers to the heart discharge
continuously at a slow rate that maintains pumping at
about 30 per cent above that with no sympathetic stimu-
lation. Therefore, when the activity of the sympathetic
nervous system is depressed below normal, this
decreases both heart rate and strength of ventricular
muscle contraction, thereby decreasing the level o

muscle contraction, thereby decreasing the level of
cardiac pumping as much as 30 per cent below normal.
Parasympathetic (Vagal) Stimulation of the Heart. Strong
stimulation of the parasympathetic nerve ﬁbers in the
vagus nerves to the heart can stop the heartbeat for a
few seconds, but then the heart usually “escapes” and
beats at a rate of 20 to 40 beats per minute as long 
as the parasympathetic stimulation continues. In 
addition, strong vagal stimulation can decrease the
strength of heart m

l stimulation can decrease the
strength of heart muscle contraction by 20 to 30 
per cent.
The vagal ﬁbers are distributed mainly to the atria
and not much to the ventricles, where the power con-
traction of the heart occurs. This explains the effect of
vagal stimulation mainly to decrease heart rate rather
than to decrease greatly the strength of heart contrac-
tion. Nevertheless, the great decrease in heart rate
combined with a slight decrease in heart contraction
strength can decrease ventric

in heart contraction
strength can decrease ventricular pumping 50 per cent
or more.
Effect of Sympathetic or Parasympathetic Stimulation on the
Cardiac Function Curve. Figure 9–11 shows four cardiac
function curves. They are similar to the ventricular
function curves of Figure 9–9. However, they represent
function of the entire heart rather than of a single ven-
tricle; they show the relation between right atrial pres-
sure at the input of the right heart and cardiac output
from the left ventric

ght heart and cardiac output
from the left ventricle into the aorta.
The curves of Figure 9–11 demonstrate that at any
given right atrial pressure, the cardiac output increases
during 
increased 
sympathetic 
stimulation 
and
decreases during increased parasympathetic stimula-
tion. These changes in output caused by nerve stimu-
lation result both from changes in heart rate and from
changes in contractile strength of the heart because
both change in response to the nerve stimulation.
Effect of P

 in response to the nerve stimulation.
Effect of Potassium and Calcium Ions
on Heart Function
In the discussion of membrane potentials in Chapter
5, it was pointed out that potassium ions have a
marked effect on membrane potentials,and in Chapter
6 it was noted that calcium ions play an especially
important role in activating the muscle contractile
process. Therefore, it is to be expected that the con-
centration of each of these two ions in the extracellu-
lar ﬂuids should also have important e

extracellu-
lar ﬂuids should also have important effects on cardiac
pumping.
Effect of Potassium Ions. Excess potassium in the extra-
cellular ﬂuids causes the heart to become dilated and
nerves
nerves
Vagi
S-A
node
A-V
node
Sympathetic chains
Figure 9–10
Cardiac sympathetic and parasympathetic nerves. (The vagus
nerves to the heart are parasympathetic nerves.)
Cardiac output (L/min)
Right atrial pressure (mm Hg)
0
–4
+8
+4
10
5
0
20
15
25
Maximum sympathetic 

4
10
5
0
20
15
25
Normal sympathetic
Zero sympathetic 
(Parasympathetic 
stimulation)
Figure 9–11
Effect on the cardiac output curve of different degrees of sympa-
thetic or parasympathetic stimulation.
114
Unit III
The Heart
ﬂaccid and also slows the heart rate. Large quantities
also can block conduction of the cardiac impulse from
the atria to the ventricles through the A-V bundle.
Elevation of potassium concentration to only 8 to 
12 mE

ion of potassium concentration to only 8 to 
12 mEq/L—two to three times the normal value—can
cause such weakness of the heart and abnormal
rhythm that this can cause death.
These effects result partially from the fact that a
high potassium concentration in the extracellular
ﬂuids decreases the resting membrane potential in 
the cardiac muscle ﬁbers, as explained in Chapter 5.
As the membrane potential decreases, the intensity of
the action potential also decreases, which makes con-
traction of 

tial also decreases, which makes con-
traction of the heart progressively weaker.
Effect of Calcium Ions. An excess of calcium ions causes
effects almost exactly opposite to those of potassium
ions, causing the heart to go toward spastic contrac-
tion.This is caused by a direct effect of calcium ions to
initiate the cardiac contractile process, as explained
earlier in the chapter.
Conversely,deﬁciency of calcium ions causes cardiac
ﬂaccidity, similar to the effect of high potassium.
Fortunately,

ilar to the effect of high potassium.
Fortunately, however, calcium ion levels in the blood
normally are regulated within a very narrow range.
Therefore, cardiac effects of abnormal calcium con-
centrations are seldom of clinical concern.
Effect of Temperature on 
Increased body temperature, as occurs when one has
fever, causes a greatly increased heart rate, sometimes
to as fast as double normal. Decreased temperature
causes a greatly decreased heart rate, falling to as low
as a 

atly decreased heart rate, falling to as low
as a few beats per minute when a person is near death
from hypothermia in the body temperature range of
60° to 70°F. These effects presumably result from 
the fact that heat increases the permeability of the
cardiac muscle membrane to ions that control heart
rate, resulting in acceleration of the self-excitation
process.
Contractile strength of the heart often is enhanced
temporarily by a moderate increase in temperature, as
occurs during body exercis

ease in temperature, as
occurs during body exercise, but prolonged elevation
of temperature exhausts the metabolic systems of the
heart and eventually causes weakness. Therefore,
optimal function of the heart depends greatly on
proper control of body temperature by the tempera-
ture control mechanisms explained in Chapter 73.
Increasing the Arterial Pressure Load
(up to a Limit) Does Not Decrease the
Note in Figure 9–12 that increasing the arterial pres-
sure in the aorta does not

sing the arterial pres-
sure in the aorta does not decrease the cardiac output
until the mean arterial pressure rises above about 
160 mm Hg. In other words, during normal function of
the heart at normal systolic arterial pressures (80 to
140 mm Hg), the cardiac output is determined almost
entirely by the ease of blood ﬂow through the body’s
tissues, which in turn controls venous return of 
blood to the heart. This is the principal subject of
Chapter 20.
Bers DM: Cardiac excitation-co

pter 20.
Bers DM: Cardiac excitation-contraction coupling. Nature
415:198, 2002.
Brette F, Orchard C:T-tubule function in mammalian cardiac
myocytes. Circ Res 92:1182, 2003.
Brutsaert DL: Cardiac endothelial-myocardial signaling: its
role in cardiac growth, contractile performance, and rhyth-
micity. Physiol Rev 83:59, 2003.
Clancy CE, Kass RS: Defective cardiac ion channels: from
mutations to clinical syndromes. J Clin Invest 110:1075,
2002.
Fozzard HA: Cardiac sodium and calcium cha

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2002.
Fozzard HA: Cardiac sodium and calcium channels: a history
of excitatory currents. Cardiovasc Res 55:1, 2002.
Fuchs F, Smith SH: Calcium, cross-bridges, and the Frank-
Starling relationship. News Physiol Sci 16:5, 2001.
Guyton AC: Determination of cardiac output by equating
venous return curves with cardiac response curves. Physiol
Rev 35:123, 1955.
Guyton AC, Jones CE, Coleman TG: Circulatory Physiology:
Cardiac Output and Its Regulation, 2nd ed. Philadelphia:
WB Saunders, 1973.
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Herring N, Danson EJ, Paterson DJ: Cholinergic control of
heart rate by nitric oxide is site speciﬁc. News Physiol Sci
17:202, 2002.
Korzick DH: Regulation of cardiac excitation-contraction
coupling: a cellular update.Adv Physiol Educ 27:192, 2003.
Olson EN: A decade of discoveries in cardiac biology. Nat
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ystem, vol 1: The Heart. New
York: Oxford University Press, 2002.
 Arterial pressure (mm Hg)
50
100
0
200
250
150
3
2
1
0
4
5
Figure 9–12
Constancy of cardiac output up to a pressure level of 160 mm Hg.
Only when the arterial pressure rises above this normal limit does
the increasing pressure load cause the cardiac output to fall 
signiﬁcantly.
Chapter 9
115
Rudy Y: From genome to physiome: int

rt Valves
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Rudy Y: From genome to physiome: integrative models of
cardiac excitation. Ann Biomed Eng 28:945, 2000.
Sarnoff SJ: Myocardial contractility as described by ven-
tricular function curves. Physiol Rev 35:107, 1955.
Starling EH: The Linacre Lecture on the Law of the Heart.
London: Longmans Green, 1918.
Sussman MA, Anversa P: Myocardial aging and senescence:
where have the stem cells gone? Annu Rev Physiol 66:29,
2004.
Zucker IH, Schultz HD, Li YF, et al: The origin of sympa-
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C
H
A
P
T
E
R
1
0
116
Rhythmical Excitation 
of the Heart
The heart is endowed with a special system for (1)
generating rhythmical electrical impulses to cause
rhythmical contraction of the heart muscle and (2)
conducting these impulses rapidly through the
heart.When this system functions normally, the atria
contract about one sixth of a seco

ally, the atria
contract about one sixth of a second ahead of ven-
tricular contraction, which allows ﬁlling of the ven-
tricles before they pump the blood through the
lungs and peripheral circulation. Another special importance of the system is
that it allows all portions of the ventricles to contract almost simultaneously,
which is essential for most effective pressure generation in the ventricular
chambers.
This rhythmical and conductive system of the heart is susceptible to damage
by heart d

m of the heart is susceptible to damage
by heart disease, especially by ischemia of the heart tissues resulting from poor
coronary blood ﬂow. The result is often a bizarre heart rhythm or abnormal
sequence of contraction of the heart chambers, and the pumping effectiveness
of the heart often is affected severely, even to the extent of causing death.
Specialized Excitatory and Conductive System
Figure 10–1 shows the specialized excitatory and conductive system of the heart
that contr

tory and conductive system of the heart
that controls cardiac contractions. The ﬁgure shows the sinus node (also called
sinoatrial or S-A node), in which the normal rhythmical impulse is generated;
the internodal pathways that conduct the impulse from the sinus node to the
atrioventricular (A-V) node; the A-V node, in which the impulse from the atria
is delayed before passing into the ventricles; the A-V bundle, which conducts
the impulse from the atria into the ventricles; and the left and righ

e atria into the ventricles; and the left and right bundle
branches of Purkinje ﬁbers, which conduct the cardiac impulse to all parts of
the ventricles.
The sinus node (also called sinoatrial node) is a small, ﬂattened, ellipsoid strip
of specialized cardiac muscle about 3 millimeters wide, 15 millimeters long, and
1 millimeter thick. It is located in the superior posterolateral wall of the right
atrium immediately below and slightly lateral to the opening of the superior

nd slightly lateral to the opening of the superior
vena cava. The ﬁbers of this node have almost no contractile muscle ﬁlaments
and are each only 3 to 5 micrometers in diameter, in contrast to a diameter of
10 to 15 micrometers for the surrounding atrial muscle ﬁbers. However, the
sinus nodal ﬁbers connect directly with the atrial muscle ﬁbers, so that any
action potential that begins in the sinus node spreads immediately into the atrial
muscle wall.
Automatic Electrical Rhythmicity of the Sinus

all.
Automatic Electrical Rhythmicity of the Sinus Fibers
Some cardiac ﬁbers have the capability of self-excitation, a process that can
cause automatic rhythmical discharge and contraction. This is especially true of
Chapter 10
117
the ﬁbers of the heart’s specialized conducting system,
including the ﬁbers of the sinus node. For this reason,
the sinus node ordinarily controls the rate of beat 
of the entire heart, as discussed in detail later in 
this chapter. 

t, as discussed in detail later in 
this chapter. First, let us describe this automatic 
rhythmicity.
Mechanism of Sinus Nodal Rhythmicity. Figure 10–2 shows
action potentials recorded from inside a sinus nodal
ﬁber for three heartbeats and, by comparison, a single
ventricular muscle ﬁber action potential. Note that the
“resting membrane potential” of the sinus nodal ﬁber
between discharges has a negativity of about -55 to 
-60 millivolts, in comparison with -85 to -90 millivolts
for the ventric

parison with -85 to -90 millivolts
for the ventricular muscle ﬁber.The cause of this lesser
negativity is that the cell membranes of the sinus ﬁbers
are naturally leaky to sodium and calcium ions, and
positive charges of the entering sodium and calcium
ions neutralize much of the intracellular negativity.
Before attempting to explain the rhythmicity of the
sinus nodal ﬁbers, ﬁrst recall from the discussions of
Chapters 5 and 9 that cardiac muscle has three types
of membrane ion channels that pla

 has three types
of membrane ion channels that play important roles in
causing the voltage changes of the action potential.
They are (1) fast sodium channels, (2) slow sodium-
calcium channels, and (3) potassium channels. Opening
of the fast sodium channels for a few 10,000ths of a
second is responsible for the rapid upstroke spike of
the action potential observed in ventricular muscle,
because of rapid inﬂux of positive sodium ions to the
interior of the ﬁber. Then the “plateau” of the ven-
tri

or of the ﬁber. Then the “plateau” of the ven-
tricular action potential is caused primarily by slower
opening of the slow sodium-calcium channels, which
lasts for about 0.3 second. Finally, opening of potas-
sium channels allows diffusion of large amounts of
positive potassium ions in the outward direction
through the ﬁber membrane and returns the mem-
brane potential to its resting level.
But there is a difference in the function of these
channels in the sinus nodal ﬁber because the “resting”

els in the sinus nodal ﬁber because the “resting”
potential is much less negative—only -55 millivolts in
the nodal ﬁber instead of the -90 millivolts in the ven-
tricular muscle ﬁber. At this level of -55 millivolts,
the fast sodium channels mainly have already become
“inactivated,” which means that they have become
blocked. The cause of this is that any time the mem-
brane potential remains less negative than about -55
millivolts for more than a few milliseconds, the inacti-
vation gates on the

 few milliseconds, the inacti-
vation gates on the inside of the cell membrane that
close the fast sodium channels become closed and
remain so. Therefore, only the slow sodium-calcium
channels can open (i.e., can become “activated”) and
thereby cause the action potential. As a result, the
atrial nodal action potential is slower to develop than
the action potential of the ventricular muscle. Also,
after the action potential does occur, return of the
potential to its negative state occurs slowly a

he
potential to its negative state occurs slowly as well,
rather than the abrupt return that occurs for the 
ventricular ﬁber.
Self-Excitation of Sinus Nodal Fibers. Because of the
high sodium ion concentration in the extracellular
ﬂuid outside the nodal ﬁber, as well as a moderate
number of already open sodium channels, positive
sodium ions from outside the ﬁbers normally tend to
leak to the inside. Therefore, between heartbeats,
inﬂux of positively charged sodium ions causes a slow
rise in the

vely charged sodium ions causes a slow
rise in the resting membrane potential in the positive
direction. Thus, as shown in Figure 10–2, the “resting”
potential gradually rises between each two heartbeats.
When the potential reaches a threshold voltage of
A-V node
A-V bundle
Right 
bundle
branch
Left 
bundle
branch
Sinus
node
Internodal
pathways
Figure 10–1
Sinus node, and the Purkinje system of the heart, showing also
the A-V node, atrial internodal pathways, and ventricular bundle
branches.
Sin

dal pathways, and ventricular bundle
branches.
Sinus
nodal fiber
Ventricular
muscle fiber
Threshold for
discharge
“Resting 
potential”
Seconds
0
1
2
3
0
+20
–40
–80
Figure 10–2
Rhythmical discharge of a sinus nodal ﬁber. Also, the sinus nodal
action potential is compared with that of a ventricular muscle ﬁber.
118
Unit III
The Heart
about -40 millivolts, the sodium-calcium channels
become “activated,” thus causing the action potential.
Therefore, basically, the inherent leakiness of t

Therefore, basically, the inherent leakiness of the sinus
nodal ﬁbers to sodium and calcium ions causes their
self-excitation.
Why does this leakiness to sodium and calcium ions
not cause the sinus nodal ﬁbers to remain depolarized
all the time? The answer is that two events occur
during the course of the action potential to prevent
this. First, the sodium-calcium channels become inac-
tivated (i.e., they close) within about 100 to 150 mil-
liseconds after opening, and second, at about the same

conds after opening, and second, at about the same
time, greatly increased numbers of potassium channels
open.Therefore, inﬂux of positive calcium and sodium
ions through the sodium-calcium channels ceases,
while at the same time large quantities of positive
potassium ions diffuse out of the ﬁber. Both of these
effects reduce the intracellular potential back to its
negative resting level and therefore terminate the
action potential. Furthermore, the potassium channels
remain open for another few

the potassium channels
remain open for another few tenths of a second, tem-
porarily continuing movement of positive charges out
of the cell, with resultant excess negativity inside the
ﬁber; this is called hyperpolarization. The hyperpolar-
ization state initially carries the “resting” membrane
potential down to about -55 to -60 millivolts at the
termination of the action potential.
Last, we must explain why this new state of hyper-
polarization is not maintained forever. The reason is
that dur

 is not maintained forever. The reason is
that during the next few tenths of a second after the
action potential is over, progressively more and more
potassium channels close. The inward-leaking sodium
and calcium ions once again overbalance the outward
ﬂux of potassium ions, and this causes the “resting”
potential to drift upward once more, ﬁnally reaching
the threshold level for discharge at a potential of about
-40 millivolts. Then the entire process begins again:
self-excitation to cause the

process begins again:
self-excitation to cause the action potential, recovery
from the action potential, hyperpolarization after the
action potential is over, drift of the “resting” potential
to threshold, and ﬁnally re-excitation to elicit another
cycle. This process continues indeﬁnitely throughout a
person’s life.
Internodal Pathways and
Transmission of the Cardiac Impulse
Through the Atria
The ends of the sinus nodal ﬁbers connect directly with
surrounding atrial muscle ﬁbers. Therefore, act

th
surrounding atrial muscle ﬁbers. Therefore, action
potentials originating in the sinus node travel outward
into these atrial muscle ﬁbers. In this way, the action
potential spreads through the entire atrial muscle mass
and, eventually, to the A-V node. The velocity of con-
duction in most atrial muscle is about 0.3 m/sec, but
conduction is more rapid, about 1 m/sec, in several
small bands of atrial ﬁbers. One of these, called the
anterior interatrial band, passes through the anterior
walls of

ratrial band, passes through the anterior
walls of the atria to the left atrium. In addition, three
other small bands curve through the anterior, lateral,
and posterior atrial walls and terminate in the A-V
node; shown in Figures 10–1 and 10–3, these are called,
respectively, the anterior, middle, and posterior inter-
nodal pathways. The cause of more rapid velocity of
conduction in these bands is the presence of special-
ized conduction ﬁbers.These ﬁbers are similar to even
more rapidly conduct

ese ﬁbers are similar to even
more rapidly conducting “Purkinje ﬁbers” of the ven-
tricles, which will be discussed.
Atrioventricular Node, and Delay 
of Impulse Conduction from the Atria
to the Ventricles
The atrial conductive system is organized so that the
cardiac impulse does not travel from the atria into 
the ventricles too rapidly; this delay allows time for the
atria to empty their blood into the ventricles before
ventricular contraction begins. It is primarily the A-V
node and its adjac

begins. It is primarily the A-V
node and its adjacent conductive ﬁbers that delay this
transmission into the ventricles.
The A-V node is located in the posterior wall of the
right atrium immediately behind the tricuspid valve,
as shown in Figure 10–1. And Figure 10–3 shows dia-
grammatically the different parts of this node, plus 
its connections with the entering atrial internodal
pathway ﬁbers and the exiting A-V bundle. The ﬁgure
also shows the approximate intervals of time in 
Atrioventricul

e approximate intervals of time in 
Atrioventricular
fibrous tissue
Transitional fibers
A-V node
(0.12)
(0.03)
pathways
septum
(0.16)
Penetrating portion
of A-V bundle
Distal portion of
A-V bundle
Left bundle branch
Right bundle branch
Figure 10–3
Organization of the A-V node. The numbers represent the interval
of time from the origin of the impulse in the sinus node. The values
have been extrapolated to human beings.
Chapter 10
119
fract

er 10
119
fractions of a second between initial onset of the
cardiac impulse in the sinus node and its subsequent
appearance in the A-V nodal system. Note that the
impulse, after traveling through the internodal path-
ways, reaches the A-V node about 0.03 second after its
origin in the sinus node. Then there is a delay of
another 0.09 second in the A-V node itself before the
impulse enters the penetrating portion of the A-V
bundle, where it passes into the vent

n of the A-V
bundle, where it passes into the ventricles.A ﬁnal delay
of another 0.04 second occurs mainly in this penetrat-
ing A-V bundle, which is composed of multiple small
fascicles passing through the ﬁbrous tissue separating
the atria from the ventricles.
Thus, the total delay in the A-V nodal and A-V
bundle system is about 0.13 second.This, in addition to
the initial conduction delay of 0.03 second from the
sinus node to the A-V node, makes a total delay of 0.16
second before the excitat

es a total delay of 0.16
second before the excitatory signal ﬁnally reaches the
contracting muscle of the ventricles.
Cause of the Slow Conduction. The slow conduction in the
transitional, nodal, and penetrating A-V bundle ﬁbers
is caused mainly by diminished numbers of gap junc-
tions between successive cells in the conducting path-
ways, so that there is great resistance to conduction of
excitatory ions from one conducting ﬁber to the next.
Therefore, it is easy to see why each succeeding cell

refore, it is easy to see why each succeeding cell is
slow to be excited.
Rapid Transmission in the Ventricular
Special Purkinje ﬁbers lead from the A-V node
through the A-V bundle into the ventricles. Except for
the initial portion of these ﬁbers where they penetrate
the A-V ﬁbrous barrier, they have functional charac-
teristics that are quite the opposite of those of the 
A-V nodal ﬁbers. They are very large ﬁbers, even
larger than the normal ventricular muscle ﬁbers, and
they 

han the normal ventricular muscle ﬁbers, and
they transmit action potentials at a velocity of 1.5 to
4.0 m/sec, a velocity about 6 times that in the usual
ventricular muscle and 150 times that in some of the
A-V nodal ﬁbers. This allows almost instantaneous
transmission of the cardiac impulse throughout the
entire remainder of the ventricular muscle.
The rapid transmission of action potentials by Purk-
inje ﬁbers is believed to be caused by a very high level
of permeability of the gap junctions 

y high level
of permeability of the gap junctions at the intercalated
discs between the successive cells that make up the
Purkinje ﬁbers. Therefore, ions are transmitted easily
from one cell to the next, thus enhancing the velocity
of transmission.The Purkinje ﬁbers also have very few
myoﬁbrils, which means that they contract little or not
at all during the course of impulse transmission.
One-Way Conduction Through the A-V Bundle. A special
characteristic of the A-V bundle is the inability, exce

cteristic of the A-V bundle is the inability, except
in abnormal states, of action potentials to travel back-
ward from the ventricles to the atria. This prevents 
re-entry of cardiac impulses by this route from the
ventricles to the atria, allowing only forward conduc-
tion from the atria to the ventricles.
Furthermore, it should be recalled that everywhere,
except at the A-V bundle, the atrial muscle is sepa-
rated from the ventricular muscle by a continuous
ﬁbrous barrier, a portion of which 

y a continuous
ﬁbrous barrier, a portion of which is shown in Figure
10–3. This barrier normally acts as an insulator to
prevent passage of the cardiac impulse between atrial
and ventricular muscle through any other route
besides forward conduction through the A-V bundle
itself. (In rare instances, an abnormal muscle bridge
does penetrate the ﬁbrous barrier elsewhere besides 
at the A-V bundle. Under such conditions, the cardiac
impulse can re-enter the atria from the ventricles and
cause a seri

ter the atria from the ventricles and
cause a serious cardiac arrhythmia.)
Distribution of the Purkinje Fibers in the Ventricles—The Left
and Right Bundle Branches. After penetrating the ﬁbrous
tissue between the atrial and ventricular muscle, the
distal portion of the A-V bundle passes downward in
the ventricular septum for 5 to 15 millimeters toward
the apex of the heart, as shown in Figures 10–1 and
10–3.Then the bundle divides into left and right bundle
branches that lie beneath the endocard

ight bundle
branches that lie beneath the endocardium on the two
respective sides of the ventricular septum. Each
branch spreads downward toward the apex of the ven-
tricle, progressively dividing into smaller branches.
These branches in turn course sidewise around each
ventricular chamber and back toward the base of the
heart. The ends of the Purkinje ﬁbers penetrate about
one third the way into the muscle mass and ﬁnally
become continuous with the cardiac muscle ﬁbers.
From the time the cardia

the cardiac muscle ﬁbers.
From the time the cardiac impulse enters the bundle
branches in the ventricular septum until it reaches the
terminations of the Purkinje ﬁbers, the total elapsed
time averages only 0.03 second. Therefore, once the
cardiac impulse enters the ventricular Purkinje con-
ductive system, it spreads almost immediately to the
entire ventricular muscle mass.
in the Ventricular Muscle
Once the impulse reaches the ends of the Purkinje
ﬁbers, it 

mpulse reaches the ends of the Purkinje
ﬁbers, it is transmitted through the ventricular muscle
mass by the ventricular muscle ﬁbers themselves. The
velocity of transmission is now only 0.3 to 0.5 m/sec,
one sixth that in the Purkinje ﬁbers.
The cardiac muscle wraps around the heart in a
double spiral, with ﬁbrous septa between the spiraling
layers; therefore, the cardiac impulse does not neces-
sarily travel directly outward toward the surface of the
heart but instead angulates toward the surfa

f the
heart but instead angulates toward the surface along
the directions of the spirals. Because of this, transmis-
sion from the endocardial surface to the epicardial
surface of the ventricle requires as much as another
0.03 second, approximately equal to the time required
for transmission through the entire ventricular portion
120
Unit III
The Heart
of the Purkinje system. Thus, the total time for trans-
mission of the cardiac impulse from the initial bundle
branches to the last of the ventri

 initial bundle
branches to the last of the ventricular muscle ﬁbers in
the normal heart is about 0.06 second.
Summary of the Spread of the Cardiac
Figure 10–4 shows in summary form the transmission
of the cardiac impulse through the human heart. The
numbers on the ﬁgure represent the intervals of time,
in fractions of a second, that lapse between the origin
of the cardiac impulse in the sinus node and its appear-
ance at each respective point in the heart. Note that
th

t each respective point in the heart. Note that
the impulse spreads at moderate velocity through the
atria but is delayed more than 0.1 second in the A-V
nodal region before appearing in the ventricular septal
A-V bundle. Once it has entered this bundle, it spreads
very rapidly through the Purkinje ﬁbers to the entire
endocardial surfaces of the ventricles.Then the impulse
once again spreads slightly less rapidly through the
ventricular muscle to the epicardial surfaces.
It is extremely importan

 the epicardial surfaces.
It is extremely important that the student learn in
detail the course of the cardiac impulse through the
heart and the precise times of its appearance in each
separate part of the heart, because a thorough quanti-
tative knowledge of this process is essential to the
understanding of electrocardiography, to be discussed
in Chapters 11 through 13.
Control of Excitation and
Conduction in the Heart
The Sinus Node as the Pacemaker 
In the discussion thus far of 

maker 
In the discussion thus far of the genesis and transmis-
sion of the cardiac impulse through the heart, we have
noted that the impulse normally arises in the sinus
node. In some abnormal conditions, this is not the case.
A few other parts of the heart can exhibit intrinsic
rhythmical excitation in the same way that the sinus
nodal ﬁbers do; this is particularly true of the A-V
nodal and Purkinje ﬁbers.
The A-V nodal ﬁbers, when not stimulated from
some outside source, discharg

 not stimulated from
some outside source, discharge at an intrinsic rhyth-
mical rate of 40 to 60 times per minute, and the Purk-
inje ﬁbers discharge at a rate somewhere between 
15 and 40 times per minute.These rates are in contrast
to the normal rate of the sinus node of 70 to 80 times
per minute.
The  question we must ask is: Why does the sinus
node rather than the A-V node or the Purkinje ﬁbers
control the heart’s rhythmicity? The answer derives
from the fact that the discharge rate of the 

ives
from the fact that the discharge rate of the sinus node
is considerably faster than the natural self-excitatory
discharge rate of either the A-V node or the Purkinje
ﬁbers. Each time the sinus node discharges, its impulse
is conducted into both the A-V node and the Purkinje
ﬁbers, also discharging their excitable membranes. But
the sinus node discharges again before either the A-V
node or the Purkinje ﬁbers can reach their own thresh-
olds for self-excitation. Therefore, the new impulse
fro

or self-excitation. Therefore, the new impulse
from the sinus node discharges both the A-V node and
the Purkinje ﬁbers before self-excitation can occur in
either of these.
Thus, the sinus node controls the beat of the heart
because its rate of rhythmical discharge is faster than
that of any other part of the heart.Therefore, the sinus
node is virtually always the pacemaker of the normal
heart.
Abnormal Pacemakers—“Ectopic” Pacemaker. Occasionally
some other part of the heart develops a rhythmica

some other part of the heart develops a rhythmical dis-
charge rate that is more rapid than that of the sinus
node. For instance, this sometimes occurs in the A-V
node or in the Purkinje ﬁbers when one of these
becomes abnormal. In either case, the pacemaker of
the heart shifts from the sinus node to the A-V node
or to the excited Purkinje ﬁbers. Under rarer condi-
tions, a place in the atrial or ventricular muscle devel-
ops excessive excitability and becomes the pacemaker.
A pacemaker elsewhe

ity and becomes the pacemaker.
A pacemaker elsewhere than the sinus node is
called an “ectopic” pacemaker. An ectopic pacemaker
causes an abnormal sequence of contraction of the 
different parts of the heart and can cause signiﬁcant
debility of heart pumping.
Another cause of shift of the pacemaker is blockage
of transmission of the cardiac impulse from the sinus
node to the other parts of the heart. The new pace-
maker then occurs most frequently at the A-V node or
.04
.03
.07
.07
.07
.05
.03
.

y at the A-V node or
.04
.03
.07
.07
.07
.05
.03
.00
A-V
S-A
.09
.06
.16
.19
.19
.22
.21
.21
.20
.18
.18
.17
.17
Figure 10–4
Transmission of the cardiac impulse through the heart, showing
the time of appearance (in fractions of a second after initial
appearance at the sinoatrial node) in different parts of the heart.
Chapter 10
121
in the penetrating portion of the A-V bundle on the
way to the ventricles.
When A-V block occurs—that is, when the cardiac
impulse 

-V block occurs—that is, when the cardiac
impulse fails to pass from the atria into the ventricles
through the A-V nodal and bundle system—the atria
continue to beat at the normal rate of rhythm of the
sinus node, while a new pacemaker usually develops
in the Purkinje system of the ventricles and drives the
ventricular muscle at a new rate somewhere between
15 and 40 beats per minute. After sudden A-V bundle
block, the Purkinje system does not begin to emit its
intrinsic rhythmical impulses unti

gin to emit its
intrinsic rhythmical impulses until 5 to 20 seconds later
because, before the blockage, the Purkinje ﬁbers had
been “overdriven” by the rapid sinus impulses and,
consequently, are in a suppressed state. During these
5 to 20 seconds, the ventricles fail to pump blood, and
the person faints after the ﬁrst 4 to 5 seconds because
of lack of blood ﬂow to the brain. This delayed pickup
of the heartbeat is called Stokes-Adams syndrome. If
the delay period is too long, it can lead to dea

f
the delay period is too long, it can lead to death.
Role of the Purkinje System 
in Causing Synchronous Contraction
of the Ventricular Muscle
It is clear from our description of the Purkinje system
that normally the cardiac impulse arrives at almost all
portions of the ventricles within a narrow span of time,
exciting the ﬁrst ventricular muscle ﬁber only 0.03 to
0.06 second ahead of excitation of the last ventricular
muscle ﬁber.This causes all portions of the ventricular
muscle in both ventr

l portions of the ventricular
muscle in both ventricles to begin contracting at
almost the same time and then to continue contract-
ing for about another 0.3 second.
Effective pumping by the two ventricular chambers
requires this synchronous type of contraction. If the
cardiac impulse should travel through the ventricles
slowly, much of the ventricular mass would contract
before contraction of the remainder, in which case the
overall pumping effect would be greatly depressed.
Indeed, in some typ

ct would be greatly depressed.
Indeed, in some types of cardiac debilities, several of
which are discussed in Chapters 12 and 13, slow trans-
mission does occur, and the pumping effectiveness of
the ventricles is decreased as much as 20 to 30 per cent.
Control of Heart Rhythmicity and
Impulse Conduction by the Cardiac
Nerves: The Sympathetic and
The heart is supplied with both sympathetic and
parasympathetic nerves, as shown in Figure 9-10 of
Chapter 9. The parasympathetic

n in Figure 9-10 of
Chapter 9. The parasympathetic nerves (the vagi) are
distributed mainly to the S-A and A-V nodes, to a
lesser extent to the muscle of the two atria, and very
little directly to the ventricular muscle. The sympa-
thetic nerves, conversely, are distributed to all parts of
the heart, with strong representation to the ventricu-
lar muscle as well as to all the other areas.
Parasympathetic (Vagal) Stimulation Can Slow or Even Block
Cardiac Rhythm and Conduction—“Ventricular Escape

Cardiac Rhythm and Conduction—“Ventricular Escape.”
Stimulation of the parasympathetic nerves to the heart
(the vagi) causes the hormone acetylcholine to be
released at the vagal endings. This hormone has two
major effects on the heart. First, it decreases the rate
of rhythm of the sinus node, and second, it decreases
the excitability of the A-V junctional ﬁbers between
the atrial musculature and the A-V node, thereby
slowing transmission of the cardiac impulse into the
Weak to mode

 cardiac impulse into the
Weak to moderate vagal stimulation slows the rate
of heart pumping, often to as little as one half normal.
And strong stimulation of the vagi can stop completely
the rhythmical excitation by the sinus node or block
completely transmission of the cardiac impulse from
the atria into the ventricles through the A-V mode. In
either case, rhythmical excitatory signals are no longer
transmitted into the ventricles. The ventricles stop
beating for 5 to 20 seconds, b

The ventricles stop
beating for 5 to 20 seconds, but then some point in the
Purkinje ﬁbers, usually in the ventricular septal
portion of the A-V bundle, develops a rhythm of its
own and causes ventricular contraction at a rate of 
15 to 40 beats per minute. This phenomenon is called
ventricular escape.
Mechanism of the Vagal Effects. The acetylcholine
released at the vagal nerve endings greatly increases
the permeability of the ﬁber membranes to potassium
ions, which allows rapid leakage of pota

potassium
ions, which allows rapid leakage of potassium out of
the conductive ﬁbers. This causes increased negativity
inside the ﬁbers, an effect called hyperpolarization,
which makes this excitable tissue much less excitable,
as explained in Chapter 5.
In the sinus node, the state of hyperpolarization
decreases the “resting” membrane potential of the
sinus nodal ﬁbers to a level considerably more nega-
tive than usual, to -65 to -75 millivolts rather than 
the normal level of -55 to -60 millivo

ather than 
the normal level of -55 to -60 millivolts. Therefore,
the initial rise of the sinus nodal membrane potential
caused by inward sodium and calcium leakage
requires much longer to reach the threshold potential
for excitation. This greatly slows the rate of rhythmic-
ity of these nodal ﬁbers. If the vagal stimulation is
strong enough, it is possible to stop entirely the rhyth-
mical self-excitation of this node.
In the A-V node, a state of hyperpolarization caused
by vagal stimulation ma

f hyperpolarization caused
by vagal stimulation makes it difﬁcult for the small
atrial ﬁbers entering the node to generate enough elec-
tricity to excite the nodal ﬁbers. Therefore, the safety
factor for transmission of the cardiac impulse through
the transitional ﬁbers into the A-V nodal ﬁbers
decreases. A moderate decrease simply delays con-
duction of the impulse, but a large decrease blocks
conduction entirely.
Effect of Sympathetic Stimulation on Cardiac Rhythm and Con-
duction. Sympathetic

on on Cardiac Rhythm and Con-
duction. Sympathetic stimulation causes essentially the
opposite effects on the heart to those caused by vagal
stimulation, as follows: First, it increases the rate of
sinus nodal discharge. Second, it increases the rate 
of conduction as well as the level of excitability in all
122
Unit III
The Heart
portions of the heart. Third, it increases greatly the
force of contraction of all the cardiac musculature,
both atrial and ventricular, as discussed in Chapter 9.
In 

al and ventricular, as discussed in Chapter 9.
In short, sympathetic stimulation increases the
overall activity of the heart. Maximal stimulation 
can almost triple the frequency of heartbeat and can
increase the strength of heart contraction as much as
twofold.
Mechanism of the Sympathetic Effect. Stimulation of
the sympathetic nerves releases the hormone norepi-
nephrine at the sympathetic nerve endings.The precise
mechanism by which this hormone acts on cardiac
muscle ﬁbers is somewhat unclea

ne acts on cardiac
muscle ﬁbers is somewhat unclear, but the belief is 
that it increases the permeability of the ﬁber mem-
brane to sodium and calcium ions. In the sinus node,
an increase of sodium-calcium permeability causes 
a more positive resting potential and also causes
increased rate of upward drift of the diastolic mem-
brane potential toward the threshold level for self-
excitation,
thus accelerating self-excitation and,
therefore, increasing the heart rate.
In the A-V node and A-V bun

easing the heart rate.
In the A-V node and A-V bundles, increased
sodium-calcium permeability makes it easier for the
action potential to excite each succeeding portion of
the conducting ﬁber bundles, thereby decreasing the
conduction time from the atria to the ventricles.
The increase in permeability to calcium ions is at
least partially responsible for the increase in contrac-
tile strength of the cardiac muscle under the inﬂuence
of sympathetic stimulation, because calcium ions play
a powerfu

c stimulation, because calcium ions play
a powerful role in exciting the contractile process of
the myoﬁbrils.
Blatter LA, Kockskamper J, Sheehan KA, et al: Local
calcium gradients during excitation-contraction coupling
and alternans in atrial myocytes. J Physiol 546:19, 2003.
Ferrier GR, Howlett SE: Cardiac excitation-contraction cou-
pling: role of membrane potential in regulation of con-
traction.Am J Physiol Heart Circ Physiol 280:H1928,2001.
Gentlesk PJ, Markwood TT, Atwood JE: C

H1928,2001.
Gentlesk PJ, Markwood TT, Atwood JE: Chronotropic
incompetence in a young adult: case report and literature
review. Chest 125:297, 2004.
Huikuri HV, Castellanos A, Myerburg RJ: Sudden death due
to cardiac arrhythmias. N Engl J Med 345:1473, 2001.
Hume JR, Duan D, Collier ML, et al: Anion transport in
heart. Physiol Rev 80:31, 2000.
James TN: Structure and function of the sinus node,AV node
and His bundle of the human heart: part I—structure. Prog
Cardiovasc Dis 45:235, 2002.
James TN

ucture. Prog
and His bundle of the human heart: part II—function.
Prog Cardiovasc Dis 45:327, 2003.
Kaupp UB, Seifert R: Molecular diversity of pacemaker ion
channels. Annu Rev Physiol 63:235, 2001.
Kléber AG, Rudy Y: Basic mechanisms of cardiac impulse
Leclercq C, Hare JM:Ventricular resynchronization: current
state of the art. Circulation 109

ization: current
state of the art. Circulation 109:296, 2004.
Mazgalev 
TN,
Ho 
SY,
Anderson 
RH:
Anatomic-
electrophysiological correlations concerning the pathways
for atrioventricular conduction. Circulation 103:2660,
2001.
Petrashevskaya NN, Koch SE, Bodi I, Schwartz A: Calcium
cycling, historic overview and perspectives: role for auto-
nomic ner

verview and perspectives: role for auto-
nomic nervous system regulation. J Mol Cell Cardiol
34:885, 2002.
Priori SG: Inherited arrhythmogenic diseases: the com-
plexity beyond monogenic disorders. Circ Res 94:140,
2004.
Roden DM, Balser JR, George AL Jr, Anderson ME:
Cardiac ion channels. Annu Rev Physiol 64:431, 2002.
Schram G, Pourrier M, Melnyk P, Nattel S: Differential dis-
tribution of cardiac ion channel expression as a basis for
regional specialization in electrical function. Circ Res
90

specialization in electrical function. Circ Res
90:939, 2002.
Surawicz B: Electrophysiologic Basis of ECG and Cardiac
Arrhythmias. Baltimore: Williams & Wilkins, 1995.
Waldo AL: Mechanisms of atrial ﬁbrillation. J Cardiovasc
Electrophysiol 14(12 Suppl):S267, 2003.
Yasuma F,Hayano J:Respiratory sinus arrhythmia:why does
the heartbeat synchronize with respiratory rhythm? Chest
125:683, 2004.
C
H
A
P
T
E
R
1
1
123
When the cardiac impulse passes through the heart,
elect

he cardiac impulse passes through the heart,
electrical current also spreads from the heart into
the adjacent tissues surrounding the heart. A small
portion of the current spreads all the way to the
surface of the body. If electrodes are placed on the
skin on opposite sides of the heart, electrical 
potentials generated by the current can be recorded;
the recording is known as an electrocardiogram.
A normal electrocardiogram for two beats of the heart is shown in Figure 11–1.
Characteristics of 

heart is shown in Figure 11–1.
The normal electrocardiogram (see Figure 11–1) is composed of a P wave, a
QRS complex, and a T wave. The QRS complex is often, but not always, three
separate waves: the Q wave, the R wave, and the S wave.
The P wave is caused by electrical potentials generated when the atria depo-
larize before atrial contraction begins. The QRS complex is caused by poten-
tials generated when the ventricles depolarize before contract

ted when the ventricles depolarize before contraction, that is, as the
depolarization wave spreads through the ventricles. Therefore, both the P wave
and the components of the QRS complex are depolarization waves.
The T wave is caused by potentials generated as the ventricles recover from
the state of depolarization. This process normally occurs in ventricular muscle
0.25 to 0.35 second after depolarization, and the T wave is known as a repolar-
ization wave.
Thus, the electrocardiogram is compo

Thus, the electrocardiogram is composed of both depolarization and repo-
larization waves. The principles of depolarization and repolarization are 
discussed in Chapter 5.The distinction between depolarization waves and repo-
larization waves is so important in electrocardiography that further clariﬁcation
is needed.
Depolarization Waves Versus Repolarization Waves
Figure 11–2 shows a single cardiac muscle ﬁber in four stages of depolarization
and repolarization, the color red desi

olarization
and repolarization, the color red designating depolarization. During depolar-
ization, the normal negative potential inside the ﬁber reverses and becomes
slightly positive inside and negative outside.
In Figure 11–2A, depolarization, demonstrated by red positive charges inside
and red negative charges outside, is traveling from left to right. The ﬁrst half of
the ﬁber has already depolarized, while the remaining half is still polarized.
Therefore, the left electrode on the outside of

d.
Therefore, the left electrode on the outside of the ﬁber is in an area of nega-
tivity, and the right electrode is in an area of positivity; this causes the meter to
record positively. To the right of the muscle ﬁber is shown a record of changes
in potential between the two electrodes, as recorded by a high-speed recording
meter. Note that when depolarization has reached the halfway mark in Figure
11–2A, the record has risen to a maximum positive value.
124
Unit III
The Heart
In Figure 11–2B,

ive value.
124
Unit III
The Heart
In Figure 11–2B, depolarization has extended over
the entire muscle ﬁber, and the recording to the right
has returned to the zero baseline because both elec-
trodes are now in areas of equal negativity. The com-
pleted wave is a depolarization wave because it results
from spread of depolarization along the muscle ﬁber
membrane.
Figure 11–2C shows halfway repolarization of the
same muscle ﬁber, with positivity returning to the
outside of the ﬁber. At this point, 

urning to the
outside of the ﬁber. At this point, the left electrode is
in an area of positivity, and the right electrode is in an
area of negativity. This is opposite to the polarity in
Figure 11–2A. Consequently, the recording, as shown
to the right, becomes negative.
In Figure 11–2D, the muscle ﬁber has completely
repolarized, and both electrodes are now in areas of
positivity, so that no potential difference is recorded
between them. Thus, in the recording to the right, the
potential returns

 the recording to the right, the
potential returns once more to zero. This completed
negative wave is a repolarization wave because it
results from spread of repolarization along the muscle
ﬁber membrane.
Relation of the Monophasic Action Potential of Ventricular
Muscle to the QRS and T Waves in the Standard Electrocardio-
gram. The monophasic action potential of ventricular
muscle,discussed in Chapter 10,normally lasts between
0.25 and 0.35 second.The top part of Figure 11–3 shows
a monophasic 

nd.The top part of Figure 11–3 shows
a monophasic action potential recorded from a micro-
electrode inserted to the inside of a single ventricular
muscle ﬁber. The upsweep of this action potential is
caused by depolarization, and the return of the poten-
tial to the baseline is caused by repolarization.
Note in the lower half of the ﬁgure a simultaneous
recording of the electrocardiogram from this same
ventricle, which shows the QRS waves appearing at
+2
Atria Ventricles
RR interval
P
R
T
S
Q
P-

ng at
+2
P
R
T
S
Q
P-R interval
= 0.16 sec
Q-T interval
S-T
segment
+1
0
–1
0
0.2
0.4
0.6
0.8
1.0
1.2
1.6
1.4
Time (sec)
Figure 11–1
Normal electrocardiogram.
+
+
–
–
+
–
+
–
Depolarization
wave
Repolarization
wave
+ + + + + + + + +
+ + + + + + + + +
+ + + + + + + 
+ + + + + + + + + + + + + + + + 
– – – – – – – – – + + + + + + + 
– – – – – – – – – – – – – – – – 
– – – – – – – – – – – – – – –

– – – – – – – – – – 
+ + + + + + + + + – – – – – – – 
+ + + + + + + 
– – – – – – –
– – – – – – – 
– – – – – – – 
– – – – – – – 
– – – – – – – – – 
– – – – – – – – – 
– – – – – – – – – 
– – – – – – – – – 
A
B
C
D
0.30 second
–
–
+
+
0
Figure 11–2
Recording the depolarization wave (A and B) and
the repolarization wave (C and D) from a cardiac
muscle ﬁber.
Chapter 11
T

(C and D) from a cardiac
Chapter 11
125
the beginning of the monophasic action potential and
the T wave appearing at the end. Note especially that
no potential is recorded in the electrocardiogram when
the ventricular muscle is either completely polarized 
or completely depolarized. Only when the muscle is
partly polarized and partly depolarized does current
ﬂow from one part of the ventricles to another part,
and therefore current also ﬂows to the surfa

part,
and therefore current also ﬂows to the surface of the
body to produce the electrocardiogram.
Relationship of Atrial and Ventricular
Before contraction of muscle can occur,depolarization
must spread through the muscle to initiate the chemi-
cal processes of contraction. Refer again to Figure
11–1; the P wave occurs at the beginning of contrac-
tion of the atria, and the QRS complex of waves occurs
at the beginning of contraction of the ventr

ccurs
at the beginning of contraction of the ventricles. The
ventricles remain contracted until after repolarization
has occurred, that is, until after the end of the T wave.
The atria repolarize about 0.15 to 0.20 second after
termination of the P wave. This is also approxima-
tely when the QRS complex is being recorded in 
the electrocardiogram. Therefore, the atrial repo-
larization wave, known as the atrial T wave, is usually
obscured by the much larger QRS complex. For 
this reason, an atri

much larger QRS complex. For 
this reason, an atrial T wave seldom is observed in the
electrocardiogram.
The ventricular repolarization wave is the T wave of
the normal electrocardiogram. Ordinarily, ventricular
muscle begins to repolarize in some ﬁbers about 0.20
second after the beginning of the depolarization wave
(the QRS complex), but in many other ﬁbers, it takes
as long as 0.35 second. Thus, the process of ventricular
repolarization extends over a long period, about 0.15
second. For this 

s over a long period, about 0.15
second. For this reason, the T wave in the normal elec-
trocardiogram is a prolonged wave, but the voltage of
the T wave is considerably less than the voltage of the
QRS complex, partly because of its prolonged length.
All recordings of electrocardiograms are made with
appropriate calibration lines on the recording paper.
Either these calibration lines are already ruled on the
paper, as is the case when a pen 

ady ruled on the
paper, as is the case when a pen recorder is used, or
they are recorded on the paper at the same time that
the electrocardiogram is recorded, which is the case
with the photographic types of electrocardiographs.
As shown in Figure 11–1, the horizontal calibration
lines are arranged so that 10 of the small line divisions
upward or downward in the standard electrocardio-
gram represent 1 millivolt,with positivity in the upward
direction and negativity in the downward direction.
Th

ction and negativity in the downward direction.
The vertical lines on the electrocardiogram are time
calibration lines. Each inch in the horizontal direction
is 1 second, and each inch is usually broken into ﬁve
segments by dark vertical lines; the intervals between
these dark lines represent 0.20 second.The 0.20 second
intervals are then broken into ﬁve smaller intervals by
thin lines, each of which represents 0.04 second.
Normal Voltages in the Electrocardiogram. The recorded
voltages of the w

 Electrocardiogram. The recorded
voltages of the waves in the normal electrocardiogram
depend on the manner in which the electrodes are
applied to the surface of the body and how close the
electrodes are to the heart. When one electrode is
placed directly over the ventricles and a second elec-
trode is placed elsewhere on the body remote from the
heart, the voltage of the QRS complex may be as great
as 3 to 4 millivolts. Even this voltage is small in com-
parison with the monophasic action poten

l in com-
parison with the monophasic action potential of 110
millivolts recorded directly at the heart muscle mem-
brane. When electrocardiograms are recorded from
electrodes on the two arms or on one arm and one leg,
the voltage of the QRS complex usually is 1.0 to 1.5
millivolt from the top of the R wave to the bottom of
the S wave; the voltage of the P wave is between 0.1
and 0.3 millivolt; and that of the T wave is between 0.2
and 0.3 millivolt.
P-Q or P-R Interval. The time between the beg

olt.
P-Q or P-R Interval. The time between the beginning of
the P wave and the beginning of the QRS complex is
the interval between the beginning of electrical exci-
tation of the atria and the beginning of excitation of
the ventricles. This period is called the P-Q interval.
The normal P-Q interval is about 0.16 second. (Often
this interval is called the P-R interval because the Q
wave is likely to be absent.)
Q-T Interval. Contraction of the ventricle lasts almost
from the beginning of the Q w

ntricle lasts almost
from the beginning of the Q wave (or R wave, if 
the Q wave is absent) to the end of the T wave. This
T
R
S
Q
Figure 11–3
Above, Monophasic action potential from a ventricular muscle
ﬁber during normal cardiac function, showing rapid depolarization
and then repolarization occurring slowly during the plateau stage
but rapidly toward the end. Below, Electrocardiogram recorded
simultaneously.
126
Unit III
The Heart
interval is called the Q-T interv

it III
The Heart
interval is called the Q-T interval and ordinarily is
about 0.35 second.
Rate of Heartbeat as Determined from the Electrocardiogram.
The rate of heartbeat can be determined easily from
an electrocardiogram because the heart rate is the
reciprocal of the time interval between two successive
heartbeats. If the interval between two beats as deter-
mined from the time calibration lines is 1 second, the
heart rate is 60 beats per minute. The normal interval
between two successive QRS

te. The normal interval
between two successive QRS complexes in the adult
person is about 0.83 second. This is a heart rate of
60/0.83 times per minute, or 72 beats per minute.
Methods for Recording
Sometimes the electrical currents generated by the
cardiac muscle during each beat of the heart change
electrical potentials and polarities on the respective
sides of the heart in less than 0.01 second. Therefore,
it is essential that any apparatus for recording electro-
cardiogram

at any apparatus for recording electro-
cardiograms be capable of responding rapidly to these
changes in potentials.
Many modern clinical electrocardiographs use com-
puter-based systems and electronic display, while
others use a direct pen recorder that writes the elec-
trocardiogram with a pen directly on a moving sheet
of paper. Sometimes the pen is a thin tube connected
at one end to an inkwell, and its recording end is con-
nected to a powerful electromagnet system that is
capa

ed to a powerful electromagnet system that is
capable of moving the pen back and forth at high
speed. As the paper moves forward, the pen records
the electrocardiogram.The movement of the pen is con-
trolled by appropriate electronic ampliﬁers connected 
to electrocardiographic electrodes on the patient.
Other pen recording systems use special paper that
does not require ink in the recording stylus. One such
paper turns black when it is exposed to heat; the stylus
itself is made very hot by elec

o heat; the stylus
itself is made very hot by electrical current ﬂowing
through its tip. Another type turns black when electri-
cal current ﬂows from the tip of the stylus through the
paper to an electrode at its back. This leaves a black
line on the paper where the stylus touches.
Flow of Current Around 
the Heart During the 
Cardiac Cycle
Recording Electrical Potentials from a
Partially Depolarized Mass of
Syncytial Cardiac Muscle
Figure 11–4 shows a syncytial mass of cardiac muscle
that has b

hows a syncytial mass of cardiac muscle
that has been stimulated at its centralmost point.
Before stimulation, all the exteriors of the muscle cells
had been positive and the interiors negative. For
reasons presented in Chapter 5 in the discussion of
membrane potentials, as soon as an area of cardiac
syncytium becomes depolarized, negative charges leak
to the outsides of the depolarized muscle ﬁbers,
making this part of the surface electronegative, as rep-
resented by the negative signs in Figur

e, as rep-
resented by the negative signs in Figure 11–4. The
remaining surface of the heart, which is still polarized,
is represented by the positive signs.Therefore, a meter
connected with its negative terminal on the area of
depolarization and its positive terminal on one of the
still-polarized areas, as shown to the right in the ﬁgure,
records positively.
Two other electrode placements and meter readings
are also demonstrated in Figure 11–4. These should be
studied carefully, and the reader 

These should be
studied carefully, and the reader should be able to
explain the causes of the respective meter readings.
Because the depolarization spreads in all directions
through the heart, the potential differences shown 
in the ﬁgure persist for only a few thousandths of 
a second, and the actual voltage measurements can 
be accomplished only with a high-speed recording
Flow of Electrical Currents in the
Chest Around the Heart
Figure 11–5 shows the ventricular muscle lying within

ure 11–5 shows the ventricular muscle lying within
the chest. Even the lungs, although mostly ﬁlled 
with air, conduct electricity to a surprising extent, and
ﬂuids in other tissues surrounding the heart conduct
electricity even more easily. Therefore, the heart 
is actually suspended in a conductive medium.
When one portion of the ventricles depolarizes and
therefore becomes electronegative with respect to the
+++++
+++++
++++++
 – – – – – – – – –
 – – – – – – – – –
 – – – – – – – –  
 – – – – 

 –
 – – – – – – – – –
 – – – – – – – –  
 – – – – – 
–
–
+
+
–
+
–
+
0
–
+
0
–
+
0
+++++
+++++
+++++
+++++
++++++++
+++++
+
+
+
+
+
+
+
+
+
+
+
+
+++++++++++
++++++++++
++++++
++++++
+++++
+++++
++++++
+++++
++++++
+
– – – – – – –
+
+
+
+
 – – – – – – – – – 
+++++
+++++
+++
+
+ +
+
 – – – – – – – – – 
 – – – – – – – – – 
 – – – – – – – – – 
 – – – – – – – – –  
 – – – – – – –   
   – – – –   
  – –   
+
+
Figure 11–4
Instantaneous potentials develop on the surface of a cardiac
muscle mass that h

lop on the surface of a cardiac
muscle mass that has been depolarized in its center.
Chapter 11
127
remainder, electrical current ﬂows from the depolar-
ized area to the polarized area in large circuitous
routes, as noted in the ﬁgure.
It should be recalled from the discussion of the
Purkinje system in Chapter 10 that the cardiac impulse
ﬁrst arrives in the ventricles in the septum and shortly
thereafter spreads to the inside surfaces of the remain-
der of the ventri

e inside surfaces of the remain-
der of the ventricles, as shown by the red areas and the
negative signs in Figure 11–5. This provides elec-
tronegativity on the insides of the ventricles and elec-
tropositivity on the outer walls of the ventricles, with
electrical current ﬂowing through the ﬂuids surround-
ing the ventricles along elliptical paths, as demon-
strated by the curving arrows in the ﬁgure. If one
algebraically averages all the lines of current ﬂow (the
elliptical lines), one ﬁnds th

of current ﬂow (the
elliptical lines), one ﬁnds that the average current ﬂow
occurs with negativity toward the base of the heart and
with positivity toward the apex.
During most of the remainder of the depolarization
process, current also continues to ﬂow in this same
direction, while depolarization spreads from the endo-
cardial surface outward through the ventricular
muscle mass.Then, immediately before depolarization
has completed its course through the ventricles, the
average direction of cu

hrough the ventricles, the
average direction of current ﬂow reverses for about
0.01 second, ﬂowing from the ventricular apex toward
the base, because the last part of the heart to become
depolarized is the outer walls of the ventricles near the
base of the heart.
Thus, in normal heart ventricles, current ﬂows from
negative to positive primarily in the direction from 
the base of the heart toward the apex during almost
the entire cycle of depolarization, except at the very
end. And if a meter is 

zation, except at the very
end. And if a meter is connected to electrodes on 
the surface of the body as shown in Figure 11–5, the
electrode nearer the base will be negative, whereas 
the electrode nearer the apex will be positive, and 
the recording meter will show positive recording in the
Figure 11–6 shows electrical connections between the
patient’s limbs and the electrocardiograph for record-
ing electrocardiograms from 

rdiograph for record-
ing electrocardiograms from the so-called standard
bipolar limb leads. The term “bipolar” means that the
electrocardiogram is recorded from two electrodes
+
-
-
-
-
--------
++++
++
+
+
+
+
+
+
+
+
+
+
+
+++
0
A
B
-
-
+
+
Figure 11–5
Flow of current in the chest around partially depolarized 
-
+
-
+
-
+
+0.3 mV
+0.7 mV
+0.5 mV
+1.0 mV
Lead III
+1.2 mV
Lead II
Lead I
-0.2 mV
0
-
+
-
+
-
-
+
+
0
-
+
0
-
+
Figure 11–6
Conventional arrangement of electrod

+
Conventional arrangement of electrodes for recording the stan-
dard electrocardiographic leads. Einthoven’s triangle is superim-
posed on the chest.
128
Unit III
The Heart
located on different sides of the heart, in this case, on
the limbs. Thus, a “lead” is not a single wire connect-
ing from the body but a combination of two wires and
their electrodes to make a complete circuit between
the body and the electrocardiograph. The electrocar-
diograph in each instance is represented b

trocar-
diograph in each instance is represented by an elec-
trical meter in the diagram, although the actual
electrocardiograph is a high-speed recording meter
with a moving paper.
Lead I. In recording limb lead I, the negative terminal
of the electrocardiograph is connected to the right arm
and the positive terminal to the left arm. Therefore,
when the point where the right arm connects to the
chest is electronegative with respect to the point where
the left arm connects, the electrocardiograp

where
the left arm connects, the electrocardiograph records
positively, that is, above the zero voltage line in the
electrocardiogram.When the opposite is true, the elec-
trocardiograph records below the line.
Lead II. To record limb lead II, the negative terminal of
the electrocardiograph is connected to the right arm and
the positive terminal to the left leg. Therefore, when the
right arm is negative with respect to the left leg, the
electrocardiograph records positively.
Lead III. To record l

diograph records positively.
Lead III. To record limb lead III, the negative terminal
of the electrocardiograph is connected to the left arm
and the positive terminal to the left leg. This means that
the electrocardiograph records positively when the left
arm is negative with respect to the left leg.
Einthoven’s Triangle. In Figure 11–6, the triangle, called
Einthoven’s triangle, is drawn around the area of the
heart.This illustrates that the two arms and the left leg
form apices of a triangle s

 arms and the left leg
form apices of a triangle surrounding the heart. The
two apices at the upper part of the triangle represent
the points at which the two arms connect electrically
with the ﬂuids around the heart, and the lower apex is
the point at which the left leg connects with the ﬂuids.
Einthoven’s Law. Einthoven’s law states that if the
electrical potentials of any two of the three bipolar
limb electrocardiographic leads are known at any
given instant, the third one can be determined 

y
mathematically by simply summing the ﬁrst two (but
note that the positive and negative signs of the 
different leads must be observed when making this
summation).
For instance, let us assume that momentarily, as
noted in Figure 11–6, the right arm is -0.2 millivolt
(negative) with respect to the average potential in 
the body, the left arm is + 0.3 millivolt (positive), and
the left leg is +1.0 millivolt (positive). Observing the
meters in the ﬁg

llivolt (positive). Observing the
meters in the ﬁgure, it can be seen that lead I records
a positive potential of +0.5 millivolt, because this is the
difference between the -0.2 millivolt on the right arm
and the +0.3 millivolt on the left arm. Similarly, lead
III records a positive potential of +0.7 millivolt, and
lead II records a positive potential of +1.2 millivolts
because these are the instantaneous potential differ-
ences between the respective pairs of limbs.
Now, note that the sum of th

ctive pairs of limbs.
Now, note that the sum of the voltages in leads I and
III equals the voltage in lead II; that is, 0.5 plus 
0.7 equals 1.2. Mathematically, this principle, called
Einthoven’s law, holds true at any given instant while
the three “standard” bipolar electrocardiograms are
being recorded.
Normal Electrocardiograms Recorded from the Three Standard
Bipolar Limb Leads. Figure 11–7 shows recordings of the
electrocardiograms in leads I, II, and III. It is obvious
that the electrocar

 I, II, and III. It is obvious
that the electrocardiograms in these three leads are
similar to one another because they all record positive
P waves and positive T waves, and the major portion
of the QRS complex is also positive in each 
On analysis of the three electrocardiograms, it can
be shown, with careful measurements and proper
observance of polarities, that at any given instant the
sum of the potentials in leads I and III equals the
potential in lead II, thus illustrati

I equals the
potential in lead II, thus illustrating the validity of
Einthoven’s law.
Because the recordings from all the bipolar limb
leads are similar to one another, it does not matter
greatly which lead is recorded when one wants to 
diagnose different cardiac arrhythmias, because diag-
nosis of arrhythmias depends mainly on the time 
relations between the different waves of the cardiac
cycle. But when one wants to diagnose damage in 
the ventricular or atrial muscle or in the Purkinje 
cond

tricular or atrial muscle or in the Purkinje 
conducting system, it does matter greatly which leads
are recorded,
because abnormalities of cardiac 
muscle contraction or cardiac impulse conduction do
I
II
III
Figure 11–7
Normal electrocardiograms recorded from the three standard
electrocardiographic leads.
Chapter 11
129
change the patterns of the electrocardiograms
markedly in some leads yet may not affect other leads.
Electrocardiographic interpretation of these tw

s.
Electrocardiographic interpretation of these two 
types of conditions—cardiac myopathies and cardiac
arrhythmias—is discussed separately in Chapters 12
and 13.
Often electrocardiograms are recorded with one elec-
trode placed on the anterior surface of the chest
directly over the heart at one of the points shown in
Figure 11–8.This electrode is connected to the positive
terminal of the electrocardiograph, and the negative
electrode, called the indifferent electr

 negative
electrode, called the indifferent electrode, is connected
through equal electrical resistances to the right arm,
left arm, and left leg all at the same time, as also shown
in the ﬁgure. Usually six standard chest leads are
recorded, one at a time, from the anterior chest wall,
the chest electrode being placed sequentially at the six
points shown in the diagram. The different recordings
are known as leads V1, V2, V3, V4, V5, and V6.
Figure 11–9 illustrates the electrocardiograms of the

re 11–9 illustrates the electrocardiograms of the
healthy heart as recorded from these six standard
chest leads. Because the heart surfaces are close 
to the chest wall, each chest lead records mainly the
electrical potential of the cardiac musculature imme-
diately beneath the electrode. Therefore, relatively
minute abnormalities in the ventricles, particularly in
the anterior ventricular wall, can cause marked
changes in the electrocardiograms recorded from indi-
vidual chest leads.
In leads V

recorded from indi-
In leads V1 and V2, the QRS recordings of the
normal heart are mainly negative because, as shown in
Figure 11–8, the chest electrode in these leads is nearer
to the base of the heart than to the apex, and the base
of the heart is the direction of electronegativity during
most of the ventricular depolarization process. Con-
versely, the QRS complexes in leads V4, V5, and V6 are
mainly positive because the chest electrode in these
leads is nearer the heart a

est electrode in these
leads is nearer the heart apex, which is the direction
of electropositivity during most of depolarization.
Another system of leads in wide use is the augmented
unipolar limb lead. In this type of recording, two of 
the limbs are connected through electrical resistances
to the negative terminal of the electrocardiograph,
-
+
0
-
+
1
2
3 456
LA
RA
5000
ohms
5000
ohms
5000
ohms
Figure 11–8
Connections of the body with the electrocardiograph for r

ions of the body with the electrocardiograph for record-
ing chest leads. LA, left arm; RA, right arm.
V1
V2
V3
V4
V5
V6
Figure 11–9
Normal electrocardiograms recorded from the six standard chest
leads.
aVR
aVL
aVF
Figure 11–10
Normal electrocardiograms recorded from the three augmented
unipolar limb leads.
130
Unit III
The Heart
and the third limb is connected to the positive termi-
nal. When the positive terminal is on the right arm,
the lead is known as the aVR lead; when on the left
arm, the

s known as the aVR lead; when on the left
arm, the aVL lead; and when on the left leg, the aVF
lead.
Normal recordings of the augmented unipolar limb
leads are shown in Figure 11–10. They are all similar
to the standard limb lead recordings, except that the
recording from the aVR lead is inverted. (Why does
this inversion occur? Study the polarity connections to
the electrocardiograph to determine this.)
See references for Chapter 13.
C
H
A
P
T
E
R
 
1
2
131
Electrocardiographic 
Inte

C
H
A
P
T
E
R
 
1
2
131
Interpretation of Cardiac Muscle
and Coronary Blood Flow
Abnormalities: Vectorial Analysis
From the discussion in Chapter 10 of impulse trans-
mission through the heart, it is obvious that any
change in the pattern of this transmission can cause
abnormal electrical potentials around the heart and,
consequently, alter the shapes of the waves in the
electrocardiogram. For this reason, almost all
serious abnormalities of the heart muscle can be
diagnose

 abnormalities of the heart muscle can be
diagnosed by analyzing the contours of the different
waves in the different electrocardiographic leads.
Principles of Vectorial Analysis 
of Electrocardiograms
Use of Vectors to Represent Electrical Potentials
Before it is possible to understand how cardiac abnormalities affect the con-
tours of the electrocardiogram, one must ﬁrst become thoroughly familiar with
the concept of vectors and vectorial analysis as applied to electrical potentials
in and aro

sis as applied to electrical potentials
in and around the heart.
Several times in Chapter 11 it was pointed out that heart current ﬂows in a
particular direction in the heart at a given instant during the cardiac cycle. A
vector is an arrow that points in the direction of the electrical potential gener-
ated by the current ﬂow, with the arrowhead in the positive direction. Also, by
convention, the length of the arrow is drawn proportional to the voltage of the
“Resultant” Vector in th

voltage of the
“Resultant” Vector in the Heart at Any Given Instant. Figure 12–1 shows, by the shaded
area and the negative signs, depolarization of the ventricular septum and parts
of the apical endocardial walls of the two ventricles.At this instant of heart exci-
tation, electrical current ﬂows between the depolarized areas inside the heart
and the nondepolarized areas on the outside of the heart, as indicated by the
long elliptical arrows. Some current also ﬂows inside the heart c

 arrows. Some current also ﬂows inside the heart chambers directly
from the depolarized areas toward the still polarized areas. Overall, consider-
ably more current ﬂows downward from the base of the ventricles toward the
apex than in the upward direction. Therefore, the summated vector of the gen-
erated potential at this particular instant, called the instantaneous mean vector,
is represented by the long black arrow drawn through the center of the ventri-
cles in a direction from base toward a

the ventri-
cles in a direction from base toward apex. Furthermore, because the summated
current is considerable in quantity, the potential is large, and the vector is long.
Direction of a Vector Is Denoted in Terms of Degrees
When a vector is exactly horizontal and directed toward the person’s left side,
the vector is said to extend in the direction of 0 degrees, as shown in Figure 
132
Unit III
The Heart
12–2. From this zero reference point, the scale of
vectors rotates clockwise: when the vec

e scale of
vectors rotates clockwise: when the vector extends
from above and straight downward, it has a direction
of +90 degrees; when it extends from the person’s left
to right, it has a direction of +180 degrees; and when
it extends straight upward, it has a direction of -90 (or
+270) degrees.
In a normal heart, the average direction of the
vector during spread of the depolarization wave
through the ventricles, called the mean QRS vector, is
about +59 degrees, which is shown by vector A drawn

bout +59 degrees, which is shown by vector A drawn
through the center of Figure 12–2 in the +59-degree
direction. This means that during most of the depolar-
ization wave, the apex of the heart remains positive
with respect to the base of the heart, as discussed later
Axis for Each Standard Bipolar Lead
and Each Unipolar Limb Lead
In Chapter 11, the three standard bipolar and the three
unipolar limb leads are described. Each lead is actu-
ally a pair of electrodes connected to th

is actu-
ally a pair of electrodes connected to the body on
opposite sides of the heart, and the direction from neg-
ative electrode to positive electrode is called the “axis”
of the lead. Lead I is recorded from two electrodes
placed respectively on the two arms. Because the elec-
trodes lie exactly in the horizontal direction, with 
the positive electrode to the left, the axis of lead I is 
0 degrees.
In recording lead II, electrodes are placed on the
right arm and left leg. The right arm conn

 on the
right arm and left leg. The right arm connects to the
torso in the upper right-hand corner and the left leg
connects in the lower left-hand corner. Therefore, the
direction of this lead is about +60 degrees.
By similar analysis, it can be seen that lead III has
an axis of about +120 degrees; lead aVR, +210 degrees;
aVF, +90 degrees; and aVL -30 degrees.The directions
of the axes of all these leads are shown in Figure 12–3,
which is known as the hexagonal reference system. The
polarities 

as the hexagonal reference system. The
polarities of the electrodes are shown by the plus and
minus signs in the ﬁgure. The reader must learn these
axes and their polarities, particularly for the bipolar
limb leads I, II, and III, to understand the remainder of
this chapter.
- -
-
-
-
-
-
-
-
--- - -------------
-
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
++++++ + + + + + + + + ++
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
++
+
+
+
Figure 12–1
Mean vector through the partially depolarized ventricles.
-90∞
+270∞
-90∞

 partially depolarized ventricles.
-90∞
+270∞
-90∞
180∞
-100∞
0∞
59∞
120∞
A
Figure 12–2
Vectors drawn to represent potentials for several different hearts,
and the “axis” of the potential (expressed in degrees) for each
heart.
90∞
60∞
0∞
-30∞
210∞
aVR
aVR
aVF
aVL
aVL
I
II
III
III
I
120∞
-
+
+
+
-
+
+
-
-
-
+
-
Figure 12–3
Axes of the three bipolar and three unipolar leads.
Chapter 12
Electrocardiographic Interpretation of Cardiac Muscle and Coronary Blood Flow Abnormalities
133
Vectorial Analysi

ary Blood Flow Abnormalities
133
Vectorial Analysis of Potentials
Recorded in Different Leads
Now that we have discussed, ﬁrst, the conventions for
representing potentials across the heart by means of
vectors and, second, the axes of the leads, it is possible
to use these together to determine the instantaneous
potential that will be recorded in the electrocardio-
gram of each lead for a given vector in the heart, as
follows.
Figure 12–4 shows a partially depolarized heart;
vector A represents t

partially depolarized heart;
vector A represents the instantaneous mean direction
of current ﬂow in the ventricles. In this instance, the
direction of the vector is +55 degrees, and the voltage
of the potential, represented by the length of vector A,
is 2 millivolts. In the diagram below the heart, vector
A is shown again, and a line is drawn to represent the
axis of lead I in the 0-degree direction. To determine
how much of the voltage in vector A will be recorded
in lead I, a line perpendicula

 A will be recorded
in lead I, a line perpendicular to the axis of lead I is
drawn from the tip of vector A to the lead I axis, and
a so-called projected vector (B) is drawn along the lead
I axis.The arrow of this projected vector points toward
the positive end of the lead I axis, which means that
the record momentarily being recorded in the electro-
cardiogram of lead I is positive.And the instantaneous
recorded voltage will be equal to the length of B
divided by the length of A times 2 millivo

th of B
divided by the length of A times 2 millivolts, or about
1 millivolt.
Figure 12–5 shows another example of vectorial
analysis. In this example, vector A represents the elec-
trical potential and its axis at a given instant during
ventricular depolarization in a heart in which the left
side of the heart depolarizes more rapidly than the
right. In this instance, the instantaneous vector has a
direction of 100 degrees, and its voltage is again 2 mil-
livolts.To determine the potential actual

n 2 mil-
livolts.To determine the potential actually recorded in
lead I, we draw a perpendicular line from the tip of
vector A to the lead I axis and ﬁnd projected vector B.
Vector B is very short and this time in the negative
direction, indicating that at this particular instant, the
recording in lead I will be negative (below the zero line
in the electrocardiogram), and the voltage recorded
will be slight, about -0.3 millivolts. This ﬁgure demon-
strates that when the vector in the heart is in

n-
strates that when the vector in the heart is in a direc-
tion almost perpendicular to the axis of the lead, the
voltage recorded in the electrocardiogram of this lead
is very low. Conversely, when the heart vector has
almost exactly the same axis as the lead axis, essentially
the entire voltage of the vector will be recorded.
Vectorial Analysis of Potentials in the Three Standard 
Bipolar Limb Leads. In Figure 12–6, vector A depicts the
-
A
B
I
I
A
+
Figure 12–4
Determination of a projected v

I
I
A
+
Determination of a projected vector B along the axis of lead I when
vector A represents the instantaneous potential in the ventricles.
-
B
I
I
A
+
Figure 12–5
Determination of the projected vector B along the axis of lead I
when vector A represents the instantaneous potential in the 
A
B
C
D
I
II
III
III
II
I
+
+
+
-
-
-
Figure 12–6
Determination of projected vectors in leads I, II, and III when
13

 the instantaneous potential in the ventricles.
134
Unit III
The Heart
instantaneous electrical potential of a partially depo-
larized heart. To determine the potential recorded at
this instant in the electrocardiogram for each one of
the three standard bipolar limb leads, perpendicular
lines (the dashed lines) are drawn from the tip of
vector A to the three lines representing the axes of the
three different standard leads, as shown in the ﬁgure.
The projected vector B depicts the potential reco

The projected vector B depicts the potential recorded
at that instant in lead I, projected vector C depicts the
potential in lead II, and projected vector D depicts the
potential in lead III. In each of these, the record in 
the electrocardiogram is positive—that is, above the
zero line—because the projected vectors point in the
positive directions along the axes of all the leads.
The potential in lead I (vector B) is about one half that
of the actual potential in the heart (vector A); in lead

actual potential in the heart (vector A); in lead
II (vector C), it is almost equal to that in the heart; and
in lead III (vector D), it is about one third that in the
heart.
An identical analysis can be used to determine
potentials recorded in augmented limb leads, except
that the respective axes of the augmented leads (see
Figure 12–3) are used in place of the standard bipolar
limb lead axes used for Figure 12–6.
Vectorial Analysis of the
Normal Electrocardiogram
Vectors That Occur at Successi

l Electrocardiogram
Vectors That Occur at Successive
Intervals During Depolarization of the
Ventricles—The QRS Complex
When the cardiac impulse enters the ventricles
through the atrioventricular bundle, the ﬁrst part of
the ventricles to become depolarized is the left endo-
cardial surface of the septum. Then depolarization
spreads rapidly to involve both endocardial surfaces of
the septum, as demonstrated by the shaded portion 
of the ventricle in Figure 12–7A. Next, depolarization
spreads alon

in Figure 12–7A. Next, depolarization
spreads along the endocardial surfaces of the remain-
der of the two ventricles, as shown in Figure 12–7B and
C. Finally, it spreads through the ventricular muscle to
the outside of the heart, as shown progressively in
Figure 12–7C, D, and E.
At each stage in Figure 12–7, parts A to E, the
instantaneous mean electrical potential of the ventri-
cles is represented by a red vector superimposed on
the ventricle in each ﬁgure. Each of these vectors is
then analy

in each ﬁgure. Each of these vectors is
then analyzed by the method described in the preced-
ing section to determine the voltages that will be
recorded at each instant in each of the three standard
electrocardiographic leads. To the right in each ﬁgure
is shown progressive development of the electrocar-
diographic QRS complex. Keep in mind that a positive
vector in a lead will cause recording in the electrocar-
diogram above the zero line, whereas a negative vector
will cause recording below th

as a negative vector
will cause recording below the zero line.
Before proceeding with further consideration of
vectorial analysis, it is essential that this analysis of the
successive normal vectors presented in Figure 12–7 be
understood. Each of these analyses should be studied
in detail by the procedure given here. A short
summary of this sequence follows.
In Figure 12–7A, the ventricular muscle has just
begun to be depolarized, representing an instant about
0.01 second after the onset of depo

 instant about
0.01 second after the onset of depolarization. At this
time, the vector is short because only a small portion
of the ventricles—the septum—is depolarized. There-
fore, all electrocardiographic voltages are low, as
recorded to the right of the ventricular muscle for each
of the leads. The voltage in lead II is greater than the
voltages in leads I and III because the heart vector
extends mainly in the same direction as the axis of lead
II.
In Figure 12–7B, which represents about 0.0

ad
II.
In Figure 12–7B, which represents about 0.02 second
after onset of depolarization, the heart vector is long
because much of the ventricular muscle mass has
become depolarized. Therefore, the voltages in all
electrocardiographic leads have increased.
In Figure 12–7C, about 0.035 second after onset of
depolarization, the heart vector is becoming shorter
and the recorded electrocardiographic voltages are
lower because the outside of the heart apex is now
electronegative, neutralizing much of

 apex is now
electronegative, neutralizing much of the positivity on
the other epicardial surfaces of the heart.Also, the axis
of the vector is beginning to shift toward the left side
of the chest because the left ventricle is slightly slower
to depolarize than the right.Therefore, the ratio of the
voltage in lead I to that in lead III is increasing.
In Figure 12–7D, about 0.05 second after onset of
depolarization, the heart vector points toward the base
of the left ventricle, and it is short be

the base
of the left ventricle, and it is short because only a
minute portion of the ventricular muscle is still polar-
ized positive. Because of the direction of the vector at
this time, the voltages recorded in leads II and III are
both negative—that is, below the line—whereas the
voltage of lead I is still positive.
In Figure 12–7E, about 0.06 second after onset of
depolarization, the entire ventricular muscle mass is
depolarized, so that no current ﬂows around the heart
and no electrical pot

urrent ﬂows around the heart
and no electrical potential is generated. The vector 
becomes zero, and the voltages in all leads become
zero.
Thus, the QRS complexes are completed in the
three standard bipolar limb leads.
Sometimes the QRS complex has a slight negative
depression at its beginning in one or more of the leads,
which is not shown in Figure 12–7; this depression is
the Q wave.When it occurs, it is caused by initial depo-
larization of the left side of the septum before the right
side,

the left side of the septum before the right
side, which creates a weak vector from left to right for
a fraction of a second before the usual base-to-apex
vector occurs. The major positive deﬂection shown in
Figure 12–7 is the R wave, and the ﬁnal negative
deﬂection is the S wave.
Electrocardiogram During
Repolarization—The T Wave
After the ventricular muscle has become depolarized,
about 0.15 second later, repolarization begins and 
Chapter 12
Electrocardiographic Interpretation of Cardiac Musc

lectrocardiographic Interpretation of Cardiac Muscle and Coronary Blood Flow Abnormalities
135
proceeds until complete at about 0.35 second.This repo-
larization causes the T wave in the electrocardiogram.
Because the septum and endocardial areas of the
ventricular muscle depolarize ﬁrst, it seems logical that
these areas should repolarize ﬁrst as well. However,
this is not the usual case because the septum and other
endocardial areas have a longer period of contraction
than most of the external

er period of contraction
than most of the external surfaces of the heart. There-
fore, the greatest portion of ventricular muscle mass to
repolarize ﬁrst is the entire outer surface of the ventri-
cles, especially near the apex of the heart. The endocar-
dial areas, conversely, normally repolarize last. This
sequence of repolarization is postulated to be caused
by the high blood pressure inside the ventricles during
contraction, which greatly reduces coronary blood
ﬂow to the endocardium, thereb

uces coronary blood
ﬂow to the endocardium, thereby slowing repolariza-
tion in the endocardial areas.
Because the outer apical surfaces of the ventricles
repolarize before the inner surfaces, the positive end
of the overall ventricular vector during repolarization
is toward the apex of the heart. As a result, the normal
T wave in all three bipolar limb leads is positive, which
is also the polarity of most of the normal QRS complex.
In Figure 12–8, ﬁve stages of repolarization of the
ventricles 

8, ﬁve stages of repolarization of the
ventricles are denoted by progressive increase of the
white areas—the repolarized areas. At each stage,
the vector extends from the base of the heart toward
the apex until it disappears in the last stage. At 
ﬁrst, the vector is relatively small because the area of
repolarization is small. Later, the vector becomes
−
−
−
I
I
I
+
+
+
II
II
II
III
III
A
III
−
−
−
I
I
I
+
+
+
II
II
II
III
III
B
III
−
I
I
I
+
+
+
II
II
II
III
III
C
III
−
−
−
−
−
I
I
I
+
+
+
II

+
II
II
II
III
III
C
III
−
−
−
−
−
I
I
I
+
+
+
II
II
II
III
III
D
III
−
−
−
I
I
I
+
+
+
II
II
II
III
III
E
III
Figure 12–7
Shaded areas of the ventricles are depolarized (–); nonshaded areas are still polarized (+). The ventricular vectors and QRS complexes 
0.01 second after onset of ventricular depolarization (A); 0.02 second after onset of depolarization (B); 0.035 second after onset of 
depolarization (C); 0.05 second after onset of depolarization (D); and after depolarization of the ventric

ation (D); and after depolarization of the ventricles is complete, 0.06 second after
onset (E).
136
Unit III
The Heart
stronger because of greater degrees of repolarization.
Finally, the vector becomes weaker again because the
areas of depolarization still persisting become so slight
that the total quantity of current ﬂow decreases.These
changes also demonstrate that the vector is greatest
when about half the heart is in the polarized state and
about half is depolarized.
The changes in the elect

bout half is depolarized.
The changes in the electrocardiograms of the three
standard limb leads during repolarization are noted
under each of the ventricles, depicting the progressive
stages of repolarization. Thus, over about 0.15 second,
the period of time required for the whole process to
take place, the T wave of the electrocardiogram is 
generated.
Depolarization of the Atria—
The P Wave
Depolarization of the atria begins in the sinus node
and spreads in all directions over the atria. Ther

and spreads in all directions over the atria. Therefore,
the point of original electronegativity in the atria is
about at the point of entry of the superior vena cava
where the sinus node lies, and the direction of initial
depolarization is denoted by the black vector in Figure
12–9. Furthermore, the vector remains generally in this
direction throughout the process of normal atrial
depolarization. Because this direction is generally 
in the positive directions of the axes of the three 
standard 

ive directions of the axes of the three 
standard bipolar limb leads I, II, and III, the electro-
cardiograms recorded from the atria during depolar-
ization are also usually positive in all three of these
leads, as shown in Figure 12–9. This record of atrial
depolarization is known as the atrial P wave.
Repolarization of the Atria—The Atrial T Wave. Spread of
depolarization through the atrial muscle is much
slower than in the ventricles because the atria have no
Purkinje system for fast conduct

the atria have no
Purkinje system for fast conduction of the depolariza-
tion signal. Therefore, the musculature around the
sinus node becomes depolarized a long time before the
musculature in distal parts of the atria. Because of this,
the area in the atria that also becomes repolarized ﬁrst
is the sinus nodal region, the area that had originally
become depolarized ﬁrst. Thus, when repolarization
begins, the region around the sinus node becomes pos-
itive with respect to the rest of the atria.T

pos-
itive with respect to the rest of the atria.Therefore, the
atrial repolarization vector is backward to the vector of
depolarization. (Note that this is opposite to the effect
that occurs in the ventricles.) Therefore, as shown to
the right in Figure 12–9, the so-called atrial T wave
follows about 0.15 second after the atrial P wave, but
this T wave is on the opposite side of the zero refer-
ence line from the P wave; that is, it is normally nega-
tive rather than positive in the three stand

nega-
tive rather than positive in the three standard bipolar
limb leads.
In the normal electrocardiogram, the atrial T wave
appears at about the same time that the QRS complex
of the ventricles appears.Therefore, it is almost always
totally obscured by the large ventricular
QRS
complex, although in some very abnormal states, it
does appear in the recorded electrocardiogram.
It has been noted in the discussion up to this point that
the vector of current ﬂow through the heart cha

at
the vector of current ﬂow through the heart changes
rapidly 
as 
the 
impulse 
spreads 
through 
the
myocardium. It changes in two aspects: First, the
vector increases and decreases in length because of
increasing and decreasing voltage of the vector.
Second, the vector changes direction because of
I
-
-
-
I
I
+
+
+
II
II
III
III
II
III
Figure 12–8
Generation of the T wave during repolarization of the ventricles,
showing also vectorial analysis of the ﬁrst stage of repolarization.
The total t

s of the ﬁrst stage of repolarization.
The total time from the beginning of the T wave to its end is
approximately 0.15 second.
I
P
T
-
-
-
-
-
-
-
--
- --
-
I
I
+
+
+
+ +
+
+
+
+
+
+
+
+ +++ + +
+
+
+
+
+
+
+
+
+
II
II
III
III
SA
II
III
Figure 12–9
Depolarization of the atria and generation of the P wave, showing
the maximum vector through the atria and the resultant vectors in
the three standard leads. At the right are the atrial P and T waves.
SA, sinoatrial node.
Chapter 12
Electrocardiograp

Chapter 12
137
changes in the average direction of the electrical
potential from the heart. The so-called vectorcardio-
gram depicts these changes at different times during
the cardiac cycle, as shown in Figure 12–10.
In the large vectorcardiogram of Figure 12–10, point
5 is the zero reference point, and this point is the neg-
ative end of all the successive vectors. While the heart

nd of all the successive vectors. While the heart
muscle is polarized between heartbeats, the positive
end of the vector remains at the zero point because
there is no vectorial electrical potential. However,
as soon as current begins to ﬂow through the ventri-
cles at the beginning of ventricular depolarization, the
positive end of the vector leaves the zero reference
point.
When the septum ﬁrst becomes depolarized, the
vector extends downward toward the apex of the ven-
tricles, but it is relat

ward the apex of the ven-
tricles, but it is relatively weak, thus generating the ﬁrst
portion of the ventricular vectorcardiogram, as shown
by the positive end of vector 1. As more of the 
ventricular muscle becomes depolarized, the vector
becomes stronger and stronger, usually swinging
slightly to one side.Thus, vector 2 of Figure 12–10 rep-
resents the state of depolarization of the ventricles
about 0.02 second after vector 1. After another 0.02
second, vector 3 represents the potential, and 

02
second, vector 3 represents the potential, and vector 4
occurs in another 0.01 second. Finally, the ventricles
become totally depolarized, and the vector becomes
zero once again, as shown at point 5.
The elliptical ﬁgure generated by the positive ends
of the vectors is called the QRS vectorcardiogram.
Vectorcardiograms can be recorded on an oscilloscope
by connecting body surface electrodes from the neck
and lower abdomen to the vertical plates of the oscil-
loscope and connecting chest surfa

s of the oscil-
loscope and connecting chest surface electrodes from
each side of the heart to the horizontal plates. When
the vector changes, the spot of light on the oscilloscope
follows the course of the positive end of the changing
vector, thus inscribing the vectorcardiogram on the
oscilloscopic screen.
Mean Electrical Axis 
of the Ventricular QRS—
And Its Signiﬁcance
The vectorcardiogram during ventricular depolariza-
tion (the QRS vectorcardiogram) shown in Figure 
12–10 is that of a norm

rdiogram) shown in Figure 
12–10 is that of a normal heart. Note from this vec-
torcardiogram that the preponderant direction of the
vectors of the ventricles during depolarization is
mainly toward the apex of the heart. That is, during
most of the cycle of ventricular depolarization, the
direction of the electrical potential (negative to posi-
tive) is from the base of the ventricles toward the apex.
This preponderant direction of the potential during
depolarization is called the mean electrica

during
depolarization is called the mean electrical axis of the
ventricles. The mean electrical axis of the normal 
ventricles is 59 degrees. In many pathological condi-
tions of the heart, this direction changes markedly—
sometimes even to opposite poles of the heart.
Determining the Electrical Axis from
Clinically, the electrical axis of the heart usually is esti-
mated from the standard bipolar limb lead electrocar-
diograms rather than from the vectorcardiogr

ocar-
diograms rather than from the vectorcardiogram.
Figure 12–11 shows a method for doing this. After
recording the standard leads, one determines the net
potential and polarity of the recordings in leads I and
III. In lead I of Figure 12–11, the recording is positive,
and in lead III, the recording is mainly positive but
negative during part of the cycle. If any part of a
recording is negative, this negative potential is sub-
tracted from the positive part of the potential to deter-
mine the 

positive part of the potential to deter-
mine the net potential for that lead, as shown by the
arrow to the right of the QRS complex for lead III.
5
4
1
2
3
4
5
3
2
QRS
T
1
Figure 12–10
QRS and T vectorcardiograms.
I
III
III
III
I
+
+
-
-
II
I
59∞
-60∞
0∞
180∞
120∞
Figure 12–11
Plotting the mean electrical axis of the ventricles from two elec-
trocardiographic leads (leads I and III).
138
Unit III
The Heart
Then each net potential for leads I and III is plotted
on t

 net potential for leads I and III is plotted
on the axes of the respective leads, with the base of the
potential at the point of intersection of the axes, as
shown in Figure 12–11.
If the net potential of lead I is positive, it is plotted
in a positive direction along the line depicting lead I.
Conversely, if this potential is negative, it is plotted in
a negative direction. Also, for lead III, the net poten-
tial is placed with its base at the point of intersection,
and, if positive, it is plo

point of intersection,
and, if positive, it is plotted in the positive direction
along the line depicting lead III. If it is negative, it is
plotted in the negative direction.
To determine the vector of the total QRS ventricu-
lar mean electrical potential, one draws perpendicular
lines (the dashed lines in the ﬁgure) from the apices of
leads I and III, respectively. The point of intersection
of these two perpendicular lines represents, by vecto-
rial analysis, the apex of the mean QRS vector in

rial analysis, the apex of the mean QRS vector in the
ventricles, and the point of intersection of the lead I
and lead III axes represents the negative end of the
mean vector.Therefore, the mean QRS vector is drawn
between these two points. The approximate average
potential generated by the ventricles during depolar-
ization is represented by the length of this mean QRS
vector, and the mean electrical axis is represented by
the direction of the mean vector. Thus, the orientation
of the mean ele

mean vector. Thus, the orientation
of the mean electrical axis of the normal ventricles,
as determined in Figure 12–11, is 59 degrees positive
(+59 degrees).
Abnormal Ventricular Conditions That
Cause Axis Deviation
Although the mean electrical axis of the ventricles
averages about 59 degrees, this axis can swing even 
in the normal heart from about 20 degrees to about
100 degrees. The causes of the normal variations are
mainly anatomical differences in the Purkinje distri-
bution system or in t

nces in the Purkinje distri-
bution system or in the musculature itself of different
hearts. However, a number of abnormal conditions of
the heart can cause axis deviation beyond the normal
limits, as follows.
Change in the Position of the Heart in the Chest. If the heart
itself is angulated to the left, the mean electrical axis
of the heart also shifts to the left. Such shift occurs (1)
at the end of deep expiration, (2) when a person lies
down, because the abdominal contents press upward
again

 because the abdominal contents press upward
against the diaphragm, and (3) quite frequently in
stocky, fat people whose diaphragms normally press
upward against the heart all the time.
Likewise, angulation of the heart to the right causes
the mean electrical axis of the ventricles to shift to the
right. This occurs (1) at the end of deep inspiration,
(2) when a person stands up, and (3) normally in tall,
lanky people whose hearts hang downward.
Hypertrophy of One Ventricle. When one ventricle g

Hypertrophy of One Ventricle. When one ventricle greatly
hypertrophies, the axis of the heart shifts toward the
hypertrophied ventricle for two reasons. First, a far
greater quantity of muscle exists on the hypertrophied
side of the heart than on the other side, and this allows
excess generation of electrical potential on that side.
Second, more time is required for the depolarization
wave to travel through the hypertrophied ventricle
than through the normal ventricle. Consequently, the
normal v

h the normal ventricle. Consequently, the
normal ventricle becomes depolarized considerably in
advance of the hypertrophied ventricle, and this causes
a strong vector from the normal side of the heart
toward the hypertrophied side, which remains strongly
positively charged. Thus, the axis deviates toward the
hypertrophied ventricle.
Vectorial Analysis of Left Axis Deviation Resulting from
Hypertrophy of the Left Ventricle. Figure 12–12 shows
the three standard bipolar limb lead electrocardio-
gr

three standard bipolar limb lead electrocardio-
grams. Vectorial analysis demonstrates left axis devia-
tion with mean electrical axis pointing in the
-15-degree direction. This is a typical electrocardio-
gram caused by increased muscle mass of the left ven-
tricle. In this instance, the axis deviation was caused by
hypertension (high arterial blood pressure), which
caused the left ventricle to hypertrophy so that it could
pump blood against elevated systemic arterial pres-
sure. A similar pict

vated systemic arterial pres-
sure. A similar picture of left axis deviation occurs
when the left ventricle hypertrophies as a result of
aortic valvular stenosis, aortic valvular regurgitation, or
any number of congenital heart conditions in which the
left ventricle enlarges while the right ventricle remains
relatively normal in size.
Vectorial Analysis of Right Axis Deviation Resulting
from Hypertrophy of the Right Ventricle. The electro-
cardiogram of Figure 12–13 shows intense right axis
devi

gram of Figure 12–13 shows intense right axis
deviation, to an electrical axis of 170 degrees, which is
I
I
I
II
III
III
III
+
+
-
-
Figure 12–12
Left axis deviation in a hypertensive heart (hypertrophic left ven-
tricle). Note the slightly prolonged QRS complex as well.
Chapter 12
139
111 degrees to the right of the normal 59-degree mean
ventricular QRS axis. The right axis deviation demon-
strated in th

xis. The right axis deviation demon-
strated in this ﬁgure was caused by hypertrophy of the
right ventricle as a result of congenital pulmonary
valve stenosis. Right axis deviation also can occur in
other congenital heart conditions that cause hypertro-
phy of the right ventricle,such as tetralogy of Fallot and
interventricular septal defect.
Bundle Branch Block Causes Axis Deviation. Ordinarily, the
lateral walls of the two ventricles depolarize at almost
the same instant because both the left 

 at almost
the same instant because both the left and the right
bundle branches of the Purkinje system transmit the
cardiac impulse to the two ventricular walls at almost
the same instant. As a result, the potentials generated
by the two ventricles (on the two opposite sides of the
heart) almost neutralize each other. But if only one of
the major bundle branches is blocked, the cardiac
impulse spreads through the normal ventricle long
before it spreads through the other. Therefore, depo-
larizat

preads through the other. Therefore, depo-
larization of the two ventricles does not occur even
nearly simultaneously, and the depolarization poten-
tials do not neutralize each other.As a result, axis devi-
ation occurs as follows.
Vectorial Analysis of Left Axis Deviation in Left Bundle
Branch Block. When the left bundle branch is
blocked, cardiac depolarization spreads through the
right ventricle two to three times as rapidly as through
the left ventricle. Consequently, much of the left ven-

ft ventricle. Consequently, much of the left ven-
tricle remains polarized for as long as 0.1 second after
the right ventricle has become totally depolarized.
Thus, the right ventricle becomes electronegative,
whereas the left ventricle remains electropositive
during most of the depolarization process, and a strong
vector projects from the right ventricle toward the left
ventricle. In other words, there is intense left axis devi-
ation of about -50 degrees because the positive end of
the vector 

50 degrees because the positive end of
the vector points toward the left ventricle. This is
demonstrated in Figure 12–14, which shows typical left
axis deviation resulting from left bundle branch block.
Because of slowness of impulse conduction when
the Purkinje system is blocked, in addition to axis devi-
ation, the duration of the QRS complex is greatly pro-
longed because of extreme slowness of depolarization
in the affected side of the heart. One can see this by
observing the excessive width

 One can see this by
observing the excessive widths of the QRS waves in
Figure 12–14.This is discussed in greater detail later in
the chapter. This extremely prolonged QRS complex
differentiates bundle branch block from axis deviation
caused by hypertrophy.
Vectorial Analysis of Right Axis Deviation in Right
Bundle Branch Block. When the right bundle branch
is blocked, the left ventricle depolarizes far more
rapidly than the right ventricle, so that the left side of
the ventricles becomes electr

hat the left side of
the ventricles becomes electronegative as long as 
I
I
I
III
+
+
-
-
II
III
III
Figure 12–13
High-voltage electrocardiogram in congenital pulmonary valve
stenosis with right ventricular hypertrophy. Intense right axis devi-
ation and a slightly prolonged QRS complex also are seen.
I
I
I
III
+
+
-
-
II
III
III
Figure 12–14
Left axis deviation caused by left bundle branch block. Note also
the greatly prolonged QRS complex.
140
Unit III
The Heart
0.1 second before the right. Th

Unit III
The Heart
0.1 second before the right. Therefore, a strong vector
develops, with its negative end toward the left ventri-
cle and its positive end toward the right ventricle. In
other words, intense right axis deviation occurs. Right
axis deviation caused by right bundle branch block is
demonstrated, and its vector is analyzed, in Figure
12–15, which shows an axis of about 105 degrees
instead of the normal 59 degrees and a prolonged QRS
complex because of slow conduction.
Conditions Tha

Conditions That Cause
Abnormal Voltages of the QRS
Complex
Increased Voltage in the Standard
Bipolar Limb Leads
Normally, the voltages in the three standard bipolar
limb leads, as measured from the peak of the R wave
to the bottom of the S wave, vary between 0.5 and 2.0
millivolts, with lead III usually recording the lowest
voltage and lead II the highest. However, these rela-
tions are not invariable, even for the normal heart. In
general, when the sum of the

 the normal heart. In
general, when the sum of the voltages of all the QRS
complexes of the three standard leads is greater than
4 millivolts, the patient is considered to have a high-
voltage electrocardiogram.
The cause of high-voltage QRS complexes most
often is increased muscle mass of the heart, which 
ordinarily results from hypertrophy of the muscle in
response to excessive load on one part of the heart or
the other. For example, the right ventricle hypertro-
phies when it must pump blood

 ventricle hypertro-
phies when it must pump blood through a stenotic pul-
monary valve, and the left ventricle hypertrophies
when a person has high blood pressure. The increased
quantity of muscle causes generation of increased
quantities of electricity around the heart. As a result,
the electrical potentials recorded in the electrocardio-
graphic leads are considerably greater than normal, as
shown in Figures 12–12 and 12–13.
Decreased Voltage of the
Decreased Voltage Caused 

of the
Decreased Voltage Caused by Cardiac Myopathies. One of
the most common causes of decreased voltage of the
QRS complex is a series of old myocardial artery
infarctions with resultant diminished muscle mass. This
also causes the depolarization wave to move through
the ventricles slowly and prevents major portions of
the heart from becoming massively depolarized all at
once. Consequently, this condition causes some pro-
longation of the QRS complex along with the
decreased 

ation of the QRS complex along with the
decreased voltage. Figure 12–16 shows a typical low-
voltage electrocardiogram with prolongation of the
QRS complex, which is common after multiple small
infarctions of the heart have caused local delays of
impulse conduction and reduced voltages due to loss
of muscle mass throughout the ventricles.
Decreased Voltage Caused by Conditions Surrounding the
Heart. One of the most important causes of decreased
voltage in electrocardiographic leads is ﬂuid in th

oltage in electrocardiographic leads is ﬂuid in the
pericardium. Because extracellular ﬂuid conducts elec-
trical currents with great ease, a large portion of the
electricity ﬂowing out of the heart is conducted from
one part of the heart to another through the pericar-
dial ﬂuid.Thus, this effusion effectively “short-circuits”
the electrical potentials generated by the heart,
decreasing the electrocardiographic voltages that
reach the outside surfaces of the body. Pleural effu-
sion, to a lesse

rfaces of the body. Pleural effu-
sion, to a lesser extent, also can “short-circuit” the
electricity around the heart, so that the voltages at the
I
II
III
III
+
I
I
+
-
-III
Figure 12–15
Right axis deviation caused by right bundle branch block. Note
also the greatly prolonged QRS complex.
I
II
III
Figure 12–16
Low-voltage electrocardiogram following local damage through-
out the ventricles caused by previous myocardial infarction.
Chapter 12
Electrocardiographic Interpretation of Cardiac Muscle

ctrocardiographic Interpretation of Cardiac Muscle and Coronary Blood Flow Abnormalities
141
surface of the body and in the electrocardiograms are
decreased.
Pulmonary emphysema can decrease the electrocar-
diographic potentials, but by a different method from
that of pericardial effusion. In pulmonary emphysema,
conduction of electrical current through the lungs is
depressed considerably because of excessive quantity
of air in the lungs. Also, the chest cavity enlarges, and
the lungs tend to en

he chest cavity enlarges, and
the lungs tend to envelop the heart to a greater extent
than normally. Therefore, the lungs act as an insulator
to prevent spread of electrical voltage from the heart
to the surface of the body, and this results in decreased
electrocardiographic potentials in the various leads.
Prolonged and Bizarre
Patterns of the QRS Complex
Prolonged QRS Complex as a Result
of Cardiac Hypertrophy or Dilatation
The QRS complex lasts as long as depolarization con-
tinues to spread 

s as long as depolarization con-
tinues to spread through the ventricles—that is, as long
as part of the ventricles is depolarized and part is 
still polarized. Therefore, prolonged conduction of the
impulse through the ventricles always causes a pro-
longed QRS complex. Such prolongation often occurs
when one or both ventricles are hypertrophied or
dilated, owing to the longer pathway that the impulse
must then travel. The normal QRS complex lasts 0.06
to 0.08 second, whereas in hypertrophy or 

ts 0.06
to 0.08 second, whereas in hypertrophy or dilatation of
the left or right ventricle, the QRS complex may be
prolonged to 0.09 to 0.12 second.
Prolonged QRS Complex Resulting
from Purkinje System Blocks
When the Purkinje ﬁbers are blocked, the cardiac
impulse must then be conducted by the ventricular
muscle instead of by way of the Purkinje system. This
decreases the velocity of impulse conduction to about
one third of normal. Therefore, if complete block of
one of the bundle branches occ

f complete block of
one of the bundle branches occurs, the duration of the
QRS complex usually is increased to 0.14 second or
greater.
In general, a QRS complex is considered to be
abnormally long when it lasts more than 0.09 second;
when it lasts more than 0.12 second, the prolongation
is almost certainly caused by pathological block some-
where in the ventricular conduction system, as shown
by the electrocardiograms for bundle branch block in
Figures 12–14 and 12–15.
Conditions That Cause Biza

igures 12–14 and 12–15.
Conditions That Cause Bizarre QRS
Complexes
Bizarre patterns of the QRS complex most frequently
are caused by two conditions: (1) destruction of
cardiac muscle in various areas throughout the 
ventricular system, with replacement of this muscle by
scar tissue, and (2) multiple small local blocks in the
conduction of impulses at many points in the Purkinje
system. As a result, cardiac impulse conduction
becomes irregular, causing rapid shifts in voltages and
axis deviation

ausing rapid shifts in voltages and
axis deviations.This often causes double or even triple
peaks in some of the electrocardiographic leads, such
as those shown in Figure 12–14.
Many different cardiac abnormalities, especially those
that damage the heart muscle itself, often cause part
of the heart to remain partially or totally depolarized
all the time. When this occurs, current ﬂows between
the pathologically depolarized and the normally polar-
ized areas even between heartbe

he normally polar-
ized areas even between heartbeats. This is called a
current of injury. Note especially that the injured part
of the heart is negative, because this is the part that is
depolarized and emits negative charges into the sur-
rounding ﬂuids, whereas the remainder of the heart is
neutral or positive polarity.
Some abnormalities that can cause current of injury
are (1) mechanical trauma, which sometimes makes
the membranes remain so permeable that full repo-
larization cannot take p

permeable that full repo-
larization cannot take place; (2) infectious processes
that damage the muscle membranes; and (3) ischemia
of local areas of heart muscle caused by local coronary
occlusions, which is by far the most common cause of
current of injury in the heart. During ischemia, not
enough nutrients from the coronary blood supply are
available to the heart muscle to maintain normal mem-
brane polarization.
Effect of Current of Injury on the 
In Figure 12–17, a small area in

 the 
In Figure 12–17, a small area in the base of the left ven-
tricle is newly infarcted (loss of coronary blood ﬂow).
Therefore, during the T-P interval—that is, when the
normal ventricular muscle is totally polarized—abnor-
mal negative current still ﬂows from the infarcted area
at the base of the left ventricle and spreads toward the
rest of the ventricles. The vector of this “current of
injury,” as shown in the ﬁrst heart in the ﬁgure, is in a
direction of about 125 degrees, wi

 ﬁgure, is in a
direction of about 125 degrees, with the base of the
vector, the negative end, toward the injured muscle.As
shown in the lower portions of the ﬁgure, even before
the QRS complex begins, this vector causes an initial
record in lead I below the zero potential line, because
the projected vector of the current of injury in lead I
points toward the negative end of the lead I axis. In
lead II, the record is above the line because the pro-
jected vector points more toward the positive t

o-
jected vector points more toward the positive terminal
of the lead. In lead III, the projected vector points in
the same direction as the positive terminal of lead III,
so that the record is positive. Furthermore, because the
vector lies almost exactly in the direction of the axis
of lead III, the voltage of the current of injury in lead
III is much greater than in either lead I or lead II.
142
Unit III
The Heart
As the heart then proceeds through its normal
process of depolarization, the sep

ough its normal
process of depolarization, the septum ﬁrst becomes
depolarized; then the depolarization spreads down to
the apex and back toward the bases of the ventricles.
The last portion of the ventricles to become totally
depolarized is the base of the right ventricle, because
the base of the left ventricle is already totally and per-
manently depolarized. By vectorial analysis, the suc-
cessive stages of electrocardiogram generation by the
depolarization wave traveling through the ventricl

depolarization wave traveling through the ventricles
can be constructed graphically, as demonstrated in the
lower part of Figure 12–17.
When the heart becomes totally depolarized, at the
end of the depolarization process (as noted by the
next-to-last stage in Figure 12–17), all the ventricular
muscle is in a negative state. Therefore, at this instant
in the electrocardiogram, no current ﬂows from the
ventricles to the electrocardiographic electrodes
because now both the injured heart muscle and 

des
because now both the injured heart muscle and the
contracting muscle are depolarized.
Next, as repolarization takes place, all of the heart
ﬁnally repolarizes, except the area of permanent depo-
larization in the injured base of the left ventricle.Thus,
repolarization causes a return of the current of injury
in each lead, as noted at the far right in Figure 12–17.
The J Point—The Zero Reference
Potential for Analyzing Current 
of Injury
One would think that the electrocardiograph
machines fo

ould think that the electrocardiograph
machines for recording electrocardiograms could
determine when no current is ﬂowing around the
heart. However, many stray currents exist in the body,
such as currents resulting from “skin potentials” and
from differences in ionic concentrations in different
ﬂuids of the body. Therefore, when two electrodes are
connected between the arms or between an arm and a
leg, these stray currents make it impossible for one to
predetermine the exact zero reference leve

 one to
predetermine the exact zero reference level in the
electrocardiogram. For these reasons, the following
procedure must be used to determine the zero poten-
tial level: First, one notes the exact point at which the
wave of depolarization just completes its passage
through the heart, which occurs at the end of the QRS
complex. At exactly this point, all parts of the ventri-
cles have become depolarized, including both the
damaged parts and the normal parts, so that no current
is ﬂowing arou

he normal parts, so that no current
is ﬂowing around the heart. Even the current of injury
disappears at this point.Therefore, the potential of the
electrocardiogram at this instant is at zero voltage.This
point is known as the “J point” in the electrocardio-
gram, as shown in Figure 12–18.
Then, for analysis of the electrical axis of the injury
potential caused by a current of injury, a horizontal
line is drawn in the electrocardiogram for each lead at
the level of the J point. This horizontal 

lead at
the level of the J point. This horizontal line is then the
zero potential level in the electrocardiogram from
which all potentials caused by currents of injury must
be measured.
Use of the J Point in Plotting Axis of Injury Potential. Figure
12–18 shows electrocardiograms (leads I and III) from
an injured heart. Both records show injury potentials.
In other words, the J point of each of these two elec-
trocardiograms is not on the same line as the T-P
segment. In the ﬁgure, a horizontal 

ne as the T-P
segment. In the ﬁgure, a horizontal line has been
drawn through the J point to represent the zero
-
-
-
I
I
+
+
+
II
II
III
III
Current
of injury
Injured area
Current
of injury
I
J
J
J
II
III
Figure 12–17
Effect of a current of injury on
the electrocardiogram.
Chapter 12
143
voltage level in each of the two recordings. The injury
potential in each lead is the difference between the
voltage o

 each lead is the difference between the
voltage of the electrocardiogram immediately before
onset of the P wave and the zero voltage level deter-
mined from the J point. In lead I, the recorded voltage
of the injury potential is above the zero potential level
and is, therefore, positive. Conversely, in lead III, the
injury potential is below the zero voltage level and,
therefore, is negative.
At the bottom in Figure 12–18, the respective injury
potentials in leads I and III are plotted on the c

potentials in leads I and III are plotted on the coordi-
nates of these leads, and the resultant vector of the
injury potential for the whole ventricular muscle mass
is determined by vectorial analysis as described. In this
instance, the resultant vector extends from the right
side of the ventricles toward the left and slightly
upward, with an axis of about -30 degrees. If one
places this vector for the injury potential directly over
the ventricles, the negative end of the vector points
toward t

es, the negative end of the vector points
toward the permanently depolarized, “injured” area of
the ventricles. In the example shown in Figure 12–18,
the injured area would be in the lateral wall of the
right ventricle.
This analysis is obviously complex. However, it is
essential that the student go over it again and 
again until he or she understands it thoroughly. No
other aspect of electrocardiographic analysis is more
Coronary Ischemia as a Cause 
of Injury Potential
Insufﬁcient b

emia as a Cause 
Insufﬁcient blood ﬂow to the cardiac muscle depresses
the metabolism of the muscle for three reasons: (1)
lack of oxygen, (2) excess accumulation of carbon
dioxide, and (3) lack of sufﬁcient food nutrients.
Consequently, repolarization of the muscle membrane
cannot occur in areas of severe myocardial ischemia.
Often the heart muscle does not die because the blood
ﬂow is sufﬁcient to maintain life of the muscle even
though it is not sufﬁcient to cause repolari

 even
though it is not sufﬁcient to cause repolarization of the
membranes. As long as this state exists, an injury
potential continues to ﬂow during the diastolic portion
(the T-P portion) of each heart cycle.
Extreme ischemia of the cardiac muscle occurs after
coronary occlusion, and a strong current of injury
ﬂows from the infarcted area of the ventricles during
the T-P interval between heartbeats, as shown in
Figures 12–19 and 12–20. Therefore, one of the most
important diagnostic features of

, one of the most
important diagnostic features of electrocardiograms
recorded after acute coronary thrombosis is the
current of injury.
Acute Anterior Wall Infarction. Figure 12–19 shows the
electrocardiogram in the three standard bipolar limb
I
I
0
I
0
“J” point
“J” point
+-
+
+
-
-III
III
0
III
0
+-
Figure 12–18
J point as the zero reference potential of the electrocardiograms
for leads I and II. Also, the method for plotting the axis of the injury
potential is shown by the lowermost panel.
I

njury
I
I
I
II
V2
+
+
-
-III
III
III
Figure 12–19
Current of injury in acute anterior wall infarction. Note the intense
injury potential in lead V2.
144
Unit III
The Heart
leads and in one chest lead (lead V2 ) recorded from a
patient with acute anterior wall cardiac infarction.The
most important diagnostic feature of this electrocar-
diogram is the intense injury potential in chest lead V2.
If one draws a zero horizontal potential line through
the J po

 a zero horizontal potential line through
the J point of this electrocardiogram, a strong negative
injury potential during the T-P interval is found, which
means that the chest electrode over the front of the
heart is in an area of strongly negative potential. In
other words, the negative end of the injury potential
vector in this heart is against the anterior chest wall.
This means that the current of injury is emanating
from the anterior wall of the ventricles, which diag-
noses this condition

f the ventricles, which diag-
noses this condition as anterior wall infarction.
Analyzing the injury potentials in leads I and III,
one ﬁnds a negative potential in lead I and a positive
potential in lead III. This means that the resultant
vector of the injury potential in the heart is about +150
degrees, with the negative end pointing toward the left
ventricle and the positive end pointing toward the
right ventricle. Thus, in this particular electrocardio-
gram, the current of injury is coming 

ctrocardio-
gram, the current of injury is coming mainly from the
left ventricle as well as from the anterior wall of the
heart.Therefore, one would conclude that this anterior
wall infarction almost certainly is caused by thrombo-
sis of the anterior descending branch of the left coro-
nary artery.
Posterior Wall Infarction. Figure 12–20 shows the three
standard bipolar limb leads and one chest lead (lead
V2) from a patient with posterior wall infarction. The
major diagnostic feature of this el

nfarction. The
major diagnostic feature of this electrocardiogram is
also in the chest lead. If a zero potential reference line
is drawn through the J point of this lead, it is readily
apparent that during the T-P interval, the potential of
the current of injury is positive. This means that the
positive end of the vector is in the direction of the
anterior chest wall, and the negative end (injured end
of the vector) points away from the chest wall. In other
words, the current of injury is coming

l. In other
words, the current of injury is coming from the back
of the heart opposite to the anterior chest wall, which
is the reason this type of electrocardiogram is the basis
for diagnosing posterior wall infarction.
If one analyzes the injury potentials from leads II
and III of Figure 12–20, it is readily apparent that the
injury potential is negative in both leads. By vectorial
analysis, as shown in the ﬁgure, one ﬁnds that the
resultant vector of the injury potential is about -95
degrees,

ctor of the injury potential is about -95
degrees, with the negative end pointing downward and
the positive end pointing upward. Thus, because the
infarct, as indicated by the chest lead, is on the poste-
rior wall of the heart and, as indicated by the injury
potentials in leads II and III, is in the apical portion of
the heart, one would suspect that this infarct is near
the apex on the posterior wall of the left ventricle.
Infarction in Other Parts of the Heart. By the same proce-
dures demons

arts of the Heart. By the same proce-
dures demonstrated in the preceding discussions of
anterior and posterior wall infarctions, it is possible to
determine the locus of any infarcted area emitting a
current of injury, regardless of which part of the heart
is involved. In making such vectorial analyses, it must
be remembered that the positive end of the injury
potential vector points toward the normal cardiac
muscle, and the negative end points toward the injured
portion of the heart that is em

toward the injured
portion of the heart that is emitting the current of injury.
Recovery from Acute Coronary Thrombosis. Figure 12–21
shows a V3 chest lead from a patient with acute pos-
terior wall infarction, demonstrating changes in the
electrocardiogram from the day of the attack to 1
II
I
II
II
+
+
-
-III
III
III
V2
Figure 12–20
Injury potential in acute posterior wall, apical infarction.
Same day
1 week
2 weeks
1 year
Figure 12–21
Recovery of the myocardium after moderate posterior wall in

of the myocardium after moderate posterior wall infarc-
tion, demonstrating disappearance of the injury potential that is
present on the ﬁrst day after the infarction and still slightly present
at 1 week.
Chapter 12
145
week later, 3 weeks later, and ﬁnally 1 year later.
From this electrocardiogram, one can see that the
injury potential is strong immediately after the 
acute attack (T-P segment displaced 

ly after the 
acute attack (T-P segment displaced positively from
the S-T segment). However, after about 1 week, the
injury potential has diminished considerably, and after
3 weeks, it is gone. After that, the electrocardiogram
does not change greatly during the next year. This 
is the usual recovery pattern after acute cardiac 
infarction of moderate degree, showing that the new
collateral coronary blood ﬂow
develops enough 
to re-establish appropriate nutrition to most of the
infarcted area.
C

opriate nutrition to most of the
Conversely, in some patients with coronary infarc-
tion, the infarcted area never redevelops adequate
coronary blood supply. Often, some of the heart
muscle dies, but if the muscle does not die, it will con-
tinue to show an injury potential as long as the
ischemia exists, particularly during bouts of exercise
when the heart is overloaded.
Old Recovered Myocardial Infarction. Figure 12–22 shows
leads I and III after anterior infarction and leads I

ds I and III after anterior infarction and leads I and
III after posterior infarction about 1 year after the
acute heart attack. The records show what might be
called the “ideal” conﬁgurations of the QRS complex
in these types of recovered myocardial infarction.
Usually a Q wave has developed at the beginning of
the QRS complex in lead I in anterior infarction
because of loss of muscle mass in the anterior wall of
the left ventricle, but in posterior infarction, a Q wave
has developed at the beg

rior infarction, a Q wave
has developed at the beginning of the QRS complex
in lead III because of loss of muscle in the posterior
apical part of the ventricle.
These conﬁgurations are certainly not found in all
cases of old cardiac infarction.Local loss of muscle and
local points of cardiac signal conduction block can
cause very bizarre QRS patterns (especially promi-
nent Q waves, for instance), decreased voltage, and
QRS prolongation.
Current of Injury in Angina Pectoris. “Angina pectoris”
me

of Injury in Angina Pectoris. “Angina pectoris”
means pain from the heart felt in the pectoral regions
of the upper chest. This pain usually also radiates into
the left neck area and down the left arm.The pain typ-
ically is caused by moderate ischemia of the heart.
Usually, no pain is felt as long as the person is quiet,
but as soon as he or she overworks the heart, the pain
appears.
An injury potential sometimes appears in the elec-
trocardiogram during an attack of severe angina pec-
toris,be

am during an attack of severe angina pec-
toris,because the coronary insufﬁciency becomes great
enough to prevent adequate repolarization of some
areas of the heart during diastole.
Earlier in the chapter, it was pointed out that the T
wave is normally positive in all the standard bipolar
limb leads and that this is caused by repolarization of
the apex and outer surfaces of the ventricles ahead 
of the intraventricular surfaces. That is, the T wave
becomes abnormal wh

 surfaces. That is, the T wave
becomes abnormal when the normal sequence of
repolarization does not occur. Several factors can
change this sequence of repolarization.
Effect of Slow Conduction of the
Depolarization Wave on the
Referring back to Figure 12–14, note that the QRS
complex is considerably prolonged.The reason for this
prolongation is delayed conduction in the left ventricle
resulting from left bundle branch block. This causes
the left ventricle to become 

h block. This causes
the left ventricle to become depolarized about 0.08
second after depolarization of the right ventricle,
which gives a strong mean QRS vector to the left.
However, the refractory periods of the right and left
ventricular muscle masses are not greatly different
from each other. Therefore, the right ventricle begins
to repolarize long before the left ventricle; this causes
strong positivity in the right ventricle and negativity in
the left ventricle at the time that the T wave 

in
the left ventricle at the time that the T wave is devel-
oping. In other words, the mean axis of the T wave is
now deviated to the right, which is opposite the mean
electrical axis of the QRS complex in the same elec-
trocardiogram. Thus, when conduction of the depolar-
ization impulse through the ventricles is greatly
delayed, the T wave is almost always of opposite polar-
ity to that of the QRS complex.
Shortened Depolarization in Portions
of the Ventricular Muscle as a Cause
of T Wave Abno

f the Ventricular Muscle as a Cause
of T Wave Abnormalities
If the base of the ventricles should exhibit an abnor-
mally short period of depolarization, that is, a short-
ened action potential, repolarization of the ventricles
would not begin at the apex as it normally does.
Anterior
I
Q
III
Posterior
I
Q
III
Figure 12–22
Electrocardiograms of anterior and posterior wall infarctions that
occurred about 1 year previously, showing a Q wave in lead I in
anterior wall infarction and a Q wave in lead

I in
anterior wall infarction and a Q wave in lead III in posterior wall
infarction.
146
Unit III
The Heart
Instead, the base of the ventricles would repolarize
ahead of the apex, and the vector of repolarization
would point from the apex toward the base of the
heart, opposite to the standard vector of repolariza-
tion. Consequently, the T wave in all three standard
leads would be negative rather than the usual positive.
Thus, the simple fact that the base of the ventricles has
a shortened perio

t the base of the ventricles has
a shortened period of depolarization is sufﬁcient to
cause marked changes in the T wave, even to the
extent of changing the entire T wave polarity, as shown
in Figure 12–23.
Mild ischemia is by far the most common cause of
shortening of depolarization of cardiac muscle,
because this increases current ﬂow through the potas-
sium channels. When the ischemia occurs in only one
area of the heart, the depolarization period of this area
decreases out of proportion to t

riod of this area
decreases out of proportion to that in other portions.
As a result, deﬁnite changes in the T wave can take
place. The ischemia might result from chronic, pro-
gressive coronary occlusion; acute coronary occlusion;
or relative coronary insufﬁciency that occurs during
exercise.
One means for detecting mild coronary insufﬁciency
is to have the patient exercise and to record the elec-
trocardiogram, noting whether changes occur in the T
waves. The changes in the T waves need not be

he T
waves. The changes in the T waves need not be spe-
ciﬁc, because any change in the T wave in any lead—
inversion, for instance, or a biphasic wave—is often
evidence enough that some portion of the ventricular
muscle has a period of depolarization out of propor-
tion to the rest of the heart, caused by mild to moder-
ate coronary insufﬁciency.
Effect of Digitalis on the T Wave. As discussed in Chapter
22, digitalis is a drug that can be used during coronary
insufﬁciency to increase the stren

during coronary
insufﬁciency to increase the strength of cardiac muscle
contraction. But when overdosages of digitalis are
given, depolarization duration in one part of the ven-
tricles may be increased out of proportion to that of
other parts. As a result, nonspeciﬁc changes, such as T
wave inversion or biphasic T waves, may occur in one
or more of the electrocardiographic leads. A biphasic
T wave caused by excessive administration of digitalis
is shown in Figure 12–24. Therefore, changes in th

is shown in Figure 12–24. Therefore, changes in the T
wave during digitalis administration are often the ear-
liest signs of digitalis toxicity.
T
T
T
T
Figure 12–23
Inverted T wave resulting from mild ischemia at the apex of the
T
T
Figure 12–24
Biphasic T wave caused by digitalis toxicity.
C
H
A
P
T
E
R
 
1
3
147
Cardiac Arrhythmias and 
Their Electrocardiographic
Some of the most distressing types of heart malfunction
occur 

most distressing types of heart malfunction
occur not as a result of abnormal heart muscle but
because of abnormal rhythm of the heart. For instance,
sometimes the beat of the atria is not coordinated with
the beat of the ventricles, so that the atria no longer
function as primer pumps for the ventricles.
The purpose of this chapter is to discuss the 
physiology of common cardiac arrhythmias and their
effects on heart pumping, as well as their diagnosis by
electrocardiography.The causes of the c

agnosis by
electrocardiography.The causes of the cardiac arrhythmias are usually one or a com-
bination of the following abnormalities in the rhythmicity-conduction system of the
heart:
1. Abnormal rhythmicity of the pacemaker
2. Shift of the pacemaker from the sinus node to another place in the heart
3. Blocks at different points in the spread of the impulse through the heart
4. Abnormal pathways of impulse transmission through the heart
5. Spontaneous generation of spurious impulses in almost 

taneous generation of spurious impulses in almost any part of the heart
The term “tachycardia” means fast heart rate, usually deﬁned in an adult person as
faster than 100 beats per minute. An electrocardiogram recorded from a patient
with tachycardia is shown in Figure 13–1. This electrocardiogram is normal except
that the heart rate, as determined from the time intervals between QRS complexes,
is about 150 per minute instead of the normal 72 per minute.
The ge

minute instead of the normal 72 per minute.
The general causes of tachycardia include increased body temperature, stimulation
of the heart by the sympathetic nerves, or toxic conditions of the heart.
The heart rate increases about 10 beats per minute for each degree Fahrenheit
(18 beats per degree Celsius) increase in body temperature, up to a body tempera-
ture of about 105°F (40.5°C); beyond this, the heart rate may decrease because of
progressive debility of the heart muscle as a result of th

ive debility of the heart muscle as a result of the fever. Fever causes tachy-
cardia because increased temperature increases the rate of metabolism of the sinus
node, which in turn directly increases its excitability and rate of rhythm.
Many factors can cause the sympathetic nervous system to excite the heart, as we
discuss at multiple points in this text. For instance, when a patient loses blood and
passes into a state of shock or semishock, sympathetic reﬂex stimulation of the heart
often inc

mpathetic reﬂex stimulation of the heart
often increases the heart rate to 150 to 180 beats per minute.
Simple weakening of the myocardium usually increases the heart rate because
the weakened heart does not pump blood into the arterial tree to a normal extent,
and this elicits sympathetic reﬂexes to increase the heart rate.
The term “bradycardia” means a slow heart rate, usually deﬁned as fewer than 60
beats per minute. Bradycardia is shown by the electrocardiogram in Figure 13–2.
B

s shown by the electrocardiogram in Figure 13–2.
Bradycardia in Athletes. The athlete’s heart is larger and considerably stronger than
that of a normal person, which allows the athlete’s heart to pump a large stroke
volume output per beat even during periods of rest. When the athlete is at rest,
excessive quantities of blood pumped into the arterial tree with each beat initiate
feedback circulatory reﬂexes or other effects to cause bradycardia.
148
Unit III
The Heart
Vagal Stimulation as a Cause

48
Unit III
The Heart
Vagal Stimulation as a Cause of Bradycardia. Any circulatory
reﬂex that stimulates the vagus nerves causes release of
acetylcholine at the vagal endings in the heart, thus
giving a parasympathetic effect. Perhaps the most strik-
ing example of this occurs in patients with carotid sinus
syndrome. In these patients, the pressure receptors
(baroreceptors) in the carotid sinus region of the carotid
artery walls are excessively sensitive. Therefore, even
mild external pressure o

ensitive. Therefore, even
mild external pressure on the neck elicits a strong
baroreceptor reﬂex, causing intense vagal-acetylcholine
effects on the heart, including extreme bradycardia.
Indeed, sometimes this reﬂex is so powerful that it actu-
ally stops the heart for 5 to 10 seconds.
Figure 13–3 shows a cardiotachometer recording of the
heart rate, at ﬁrst during normal and then (in the second
half of the record) during deep respiration. A cardiota-
chometer is an instrument t

spiration. A cardiota-
chometer is an instrument that records by the height of
successive spikes the duration of the interval between
the successive QRS complexes in the electrocar-
diogram. Note from this record that the heart rate
increased and decreased no more than 5 per cent during
quiet respiration (left half of the record). Then, during
deep respiration, the heart rate increased and decreased
with each respiratory cycle by as much as 30 per cent.
Sinus arrhythmia can result from any one o

r cent.
Sinus arrhythmia can result from any one of many cir-
culatory conditions that alter the strengths of the sym-
pathetic and parasympathetic nerve signals to the heart
sinus node. In the “respiratory” type of sinus arrhyth-
mia, as shown in Figure 13–3, this results mainly from
“spillover” of signals from the medullary respiratory
center into the adjacent vasomotor center during inspi-
ratory and expiratory cycles of respiration.The spillover
signals cause alternate increase and decrease 

ver
signals cause alternate increase and decrease in the
number of impulses transmitted through the sympa-
thetic and vagus nerves to the heart.
Abnormal Rhythms That
Result from Block of Heart
Signals Within the
Intracardiac Conduction
Pathways
In rare instances, the impulse from the sinus node is
blocked before it enters the atrial muscle. This phe-
nomenon is demonstrated in Figure 13–4, which shows
sudden cessation of P waves, with resultant standstill of
the atria. However,

, with resultant standstill of
the atria. However, the ventricles pick up a new rhythm,
the impulse usually originating spontaneously in the
atrioventricular (A-V) node, so that the rate of the ven-
tricular QRS-T complex is slowed but not otherwise
altered.
The only means by which impulses ordinarily can pass
from the atria into the ventricles is through the A-V
bundle, also known as the bundle of His. Conditions that
can either decrease the rate of impulse conduction in

either decrease the rate of impulse conduction in
this bundle or block the impulse entirely are as follows:
1. Ischemia of the A-V node or A-V bundle ﬁbers
often delays or blocks conduction from the atria to
the ventricles. Coronary insufﬁciency can cause
ischemia of the A-V node and bundle in the same
way that it can cause ischemia of the myocardium.
2. Compression of the A-V bundle by scar tissue or by
calciﬁed portions of the heart can depress or block
conduction from the atria to the ventric

 or block
conduction from the atria to the ventricles.
3. Inﬂammation of the A-V node or A-V bundle
can depress conductivity from the atria to the
ventricles. Inﬂammation results frequently from
Figure 13–1
Sinus tachycardia (lead I).
Figure 13–2
Sinus bradycardia (lead III).
60
70
80
100
120
Heart rate
Figure 13–3
Sinus arrhythmia as recorded by a cardiotachometer. To the left is
the record when the subject was breathing normally; to the right,
when breathing deeply.
SA block
Figure 13–4
Sinoat

SA block
Sinoatrial nodal block, with A-V nodal rhythm during the block
period (lead III).
Chapter 13
Cardiac Arrhythmias and Their Electrocardiographic Interpretation
149
different types of myocarditis, caused, for example,
by diphtheria or rheumatic fever.
4. Extreme stimulation of the heart by the vagus nerves
in rare instances blocks impulse conduction
through the A-V node. Such vagal excitation
occasionally results from strong stimulation of the
barorecept

 results from strong stimulation of the
baroreceptors in people with carotid sinus
syndrome, discussed earlier in relation to
bradycardia.
Incomplete Atrioventricular 
Heart Block
Prolonged P-R (or P-Q) Interval—First Degree Block. The usual
lapse of time between beginning of the P wave and
beginning of the QRS complex is about 0.16 second
when the heart is beating at a normal rate. This so-
called P-R interval usually decreases in length with
faster heartbeat and increases with slower heartbeat

ster heartbeat and increases with slower heartbeat. In
general, when the P-R interval increases to greater than
0.20 second,the P-R interval is said to be prolonged,and
the patient is said to have ﬁrst degree incomplete heart
block.
Figure 13–5 shows an electrocardiogram with pro-
longed P-R interval; the interval in this instance is about
0.30 second instead of the normal 0.20 or less.Thus, ﬁrst
degree block is deﬁned as a delay of conduction from
the atria to the ventricles but not actual bloc

om
the atria to the ventricles but not actual blockage of con-
duction. The P-R interval seldom increases above 0.35
to 0.45 second because,by that time,conduction through
the A-V bundle is depressed so much that conduction
stops entirely. One means for determining the severity
of some heart diseases—acute rheumatic heart disease,
for instance—is to measure the P-R interval.
Second Degree Block. When conduction through the A-V
bundle is slowed enough to increase the P-R interval to
0.25 to 0.45 

ough to increase the P-R interval to
0.25 to 0.45 second, the action potential sometimes is
strong enough to pass through the bundle into the ven-
tricles and sometimes is not strong enough. In this
instance, there will be an atrial P wave but no QRS-T
wave, and it is said that there are “dropped beats” of the
ventricles. This condition is called second degree heart
block.
Figure 13–6 shows P-R intervals of 0.30 second, as
well as one dropped ventricular beat as a result of
failure of conduction

tricular beat as a result of
failure of conduction from the atria to the ventricles.
At times, every other beat of the ventricles is
dropped, so that a “2:1 rhythm” develops, with the atria
beating twice for every single beat of the ventricles. At
other times, rhythms of 3:2 or 3:1 also develop.
Complete A-V Block (Third Degree Block). When the condi-
tion causing poor conduction in the A-V node or A-V
bundle becomes severe, complete block of the impulse
from the atria into the ventricles occurs

 impulse
from the atria into the ventricles occurs. In this instance,
the ventricles spontaneously establish their own signal,
usually originating in the A-V node or A-V bundle.
Therefore, the P waves become dissociated from the
QRS-T complexes, as shown in Figure 13–7. Note that
the rate of rhythm of the atria in this electrocardiogram
is about 100 beats per minute, whereas the rate of ven-
tricular beat is less than 40 per minute. Furthermore,
there is no relation between the rhythm of the P w

there is no relation between the rhythm of the P waves
and that of the QRS-T complexes because the ventri-
cles have “escaped” from control by the atria, and they
are beating at their own natural rate, controlled most
often by rhythmical signals generated in the A-V node
or A-V bundle.
Stokes-Adams 
Syndrome—Ventricular 
Escape. In 
some
patients with A-V block, the total block comes and goes;
that is, impulses are conducted from the atria into the
ventricles for a period of time and then sudden

he
ventricles for a period of time and then suddenly
impulses are not conducted. The duration of block may
be a few seconds, a few minutes, a few hours, or even
weeks or longer before conduction returns. This condi-
tion occurs in hearts with borderline ischemia of the
conductive system.
Each time A-V conduction ceases,the ventricles often
do not start their own beating until after a delay of 5 to
30 seconds. This results from the phenomenon called
overdrive suppression. This means that ventricu

ed
overdrive suppression. This means that ventricular
excitability is at ﬁrst in a suppressed state because the
ventricles have been driven by the atria at a rate greater
than their natural rate of rhythm. However, after a few
seconds, some part of the Purkinje system beyond the
block, usually in the distal part of the A-V node beyond
the blocked point in the node, or in the A-V bundle,
begins discharging rhythmically at a rate of 15 to 40
times per minute and acting as the pacemaker of the
vent

per minute and acting as the pacemaker of the
ventricles. This is called ventricular escape.
P
P
P
P
P
Figure 13–5
Prolonged P-R interval caused by ﬁrst degree A-V heart block
(lead II).
P
P
P
P
P
P
Dropped beat
Figure 13–6
Second degree A-V block, showing occasional failure of the ven-
tricles to receive the excitatory signals (lead V3).
P
P
P
P
P
P
P
P
P
P
Figure 13–7
Complete A-V block (lead II).
150
Unit III
The Heart
Because the brain cannot remain active for more than
4 to 7 seconds withou

 remain active for more than
4 to 7 seconds without blood supply, most patients faint
a few seconds after complete block occurs because the
heart does not pump any blood for 5 to 30 seconds,
until the ventricles “escape.” After escape, however, the
slowly beating ventricles usually pump enough blood to
allow rapid recovery from the faint and then to sustain
the person. These periodic fainting spells are known as
the Stokes-Adams syndrome.
Occasionally the interval of ventricular standstill at
th

nally the interval of ventricular standstill at
the onset of complete block is so long that it becomes
detrimental to the patient’s health or even causes death.
Consequently, most of these patients are provided 
with an artiﬁcial pacemaker, a small battery-operated
electrical stimulator planted beneath the skin, with 
electrodes usually connected to the right ventricle. The
pacemaker provides continued rhythmical impulses
that take control of the ventricles.
El

 ventricles.
Most of the same factors that can cause A-V block can
also block impulse conduction in the peripheral ven-
tricular Purkinje system. Figure 13–8 shows the condi-
tion known as electrical alternans, which results from
partial intraventricular block every other heartbeat.
This electrocardiogram also shows tachycardia (rapid
heart rate), which is probably the reason the block has
occurred, because when the rate of the heart is rap

ccurred, because when the rate of the heart is rapid, it
may be impossible for some portions of the Purkinje
system to recover from the previous refractory period
quickly enough to respond during every succeeding
heartbeat.Also, many conditions that depress the heart,
such as ischemia, myocarditis, or digitalis toxicity, can
cause incomplete intraventricular block, resulting in
electrical alternans.
A premature contraction is a contraction of the heart
before the time that

is a contraction of the heart
before the time that normal contraction would have
been expected.This condition is also called extrasystole,
premature beat, or ectopic beat.
Causes of Premature Contractions. Most premature con-
tractions result from ectopic foci in the heart, which emit
abnormal impulses at odd times during the cardiac
rhythm. Possible causes of ectopic foci are (1) local
areas of ischemia; (2) small calciﬁed plaques at differ-
ent points in the heart, which press against the adja

 points in the heart, which press against the adjacent
cardiac muscle so that some of the ﬁbers are irritated;
and (3) toxic irritation of the A-V node, Purkinje
system, or myocardium caused by drugs, nicotine, or caf-
feine. Mechanical initiation of premature contractions 
is also frequent during cardiac catheterization; large
numbers of premature contractions often occur when
the catheter enters the right ventricle and presses
against the endocardium.
Figure 13–9 

ardium.
Figure 13–9 shows a single premature atrial contraction.
The P wave of this beat occurred too soon in the heart
cycle; the P-R interval is shortened, indicating that the
ectopic origin of the beat is in the atria near the A-V
node. Also, the interval between the premature con-
traction and the next succeeding contraction is slightly
prolonged, which is called a compensatory pause. One
of the reasons for this is that the premature contraction
originated in th

is that the premature contraction
originated in the atrium some distance from the sinus
node, and the impulse had to travel through a consid-
erable amount of atrial muscle before it discharged the
sinus node.Consequently,the sinus node discharged late
in the premature cycle, and this made the succeeding
sinus node discharge also late in appearing.
Premature atrial contractions occur frequently in 
otherwise healthy people. Indeed, they often occur in
athletes whose hearts are in very healthy co

ur in
athletes whose hearts are in very healthy condition.
Mild toxic conditions resulting from such factors as
smoking, lack of sleep, ingestion of too much coffee,
alcoholism, and use of various drugs can also initiate
such contractions.
Pulse Deﬁcit. When the heart contracts ahead of sched-
ule, the ventricles will not have ﬁlled with blood nor-
mally, and the stroke volume output during that
contraction is depressed or almost absent. Therefore,
the pulse wave passing to the peripheral arteri

e,
the pulse wave passing to the peripheral arteries after a
premature contraction may be so weak that it cannot be
felt in the radial artery. Thus, a deﬁcit in the number of
radial pulses occurs when compared with the actual
number of contractions of the heart.
A-V Nodal or A-V Bundle Premature
Figure 13–10 shows a premature contraction that 
originated in the A-V node or in the A-V bundle. The
P wave is missing from the electrocardiographic record
Figure 13–8
Partial intraventricu

iographic record
Partial intraventricular block—“electrical alternans” (lead III).
Premature beat
Figure 13–9
Atrial premature beat (lead I).
Chapter 13
151
of the premature contraction. Instead, the P wave is
superimposed onto the QRS-T complex because the
cardiac impulse traveled backward into the atria at 
the same time that it traveled forward into the ventri-
cles; this P wave slightly distorts the QRS-T complex,

 this P wave slightly distorts the QRS-T complex,
but the P wave itself cannot be discerned as such. In
general, A-V nodal premature contractions have 
the same signiﬁcance and causes as atrial premature
contractions.
The electrocardiogram of Figure 13–11 shows a series
of premature ventricular contractions (PVCs) alternat-
ing with normal contractions. PVCs cause speciﬁc
effects in the electrocardiogram, as follows:
1. The QRS complex is usually considerably
p

lows:
prolonged. The reason is that the impulse is
conducted mainly through slowly conducting
muscle of the ventricles rather than through the
Purkinje system.
2. The QRS complex has a high voltage for the
following reasons: when the normal impulse passes
through the heart, it passes through both ventricles
nearly simultaneously; consequently, in the normal
heart, the depolarization waves of the two sides 
of the heart—mainly of opposite polarity to eac

s 
of the heart—mainly of opposite polarity to each
other—partially neutralize each other in the
electrocardiogram. When a PVC occurs, the
impulse almost always travels in only one direction,
so that there is no such neutralization effect,
and one entire side or end of the ventricles is
depolarized ahead of the other; this causes large
electrical potentials, as shown for the PVCs in
Figure 13–11.
3. After almost all PVCs, the T wave has an electrical
potential polarity exactly opposite to that o

ical
potential polarity exactly opposite to that of the
QRS complex, because the slow conduction of the
impulse through the cardiac muscle causes the
muscle ﬁbers that depolarize ﬁrst also to repolarize
ﬁrst.
Some PVCs are relatively benign in their effects on
overall pumping by the heart; they can result from such
factors as cigarettes, coffee, lack of sleep, various mild
toxic states, and even emotional irritability. Conversely,
many other PVCs result from stray impulses or re-
entrant signals

 result from stray impulses or re-
entrant signals that originate around the borders of
infarcted or ischemic areas of the heart. The presence
of such PVCs is not to be taken lightly. Statistics show
that people with signiﬁcant numbers of PVCs have a
much higher than normal chance of developing sponta-
neous lethal ventricular ﬁbrillation, presumably initi-
ated by one of the PVCs. This is especially true when
the PVCs occur during the vulnerable period for
causing ﬁbrillation, just at the end o

 period for
causing ﬁbrillation, just at the end of the T wave when
the ventricles are coming out of refractoriness, as
explained later in the chapter.
Vector Analysis of the Origin of an Ectopic Premature Ventricular
Contraction. In Chapter 12, the principles of vectorial
analysis are explained. Applying these principles, one
can determine from the electrocardiogram in Figure
13–11 the point of origin of the PVC as follows: Note
that the potentials of the premature contractions in
leads II and 

als of the premature contractions in
leads II and III are both strongly positive. Plotting these
potentials on the axes of leads II and III and solving by
vectorial analysis for the mean QRS vector in the heart,
one ﬁnds that the vector of this premature contraction
has its negative end (origin) at the base of the heart and
its positive end toward the apex. Thus, the ﬁrst portion
of the heart to become depolarized during this prema-
ture contraction is near the base of the ventricles, which
ther

ion is near the base of the ventricles, which
therefore is the locus of the ectopic focus.
Some abnormalities in different portions of the heart,
including the atria, the Purkinje system, or the ventri-
cles, can occasionally cause rapid rhythmical discharge
of impulses that spread in all directions throughout the
heart. This is believed to be caused most frequently by
P
P
P
P
P
T
T
T
T
T
Figure 13–10
A-V nodal premature contraction (lead III).
–
+
+
–
II
II

al premature contraction (lead III).
–
+
+
–
II
II
III
III
III
II
Figure 13–11
Premature ventricular contractions (PVCs) demonstrated by the
large abnormal QRS-T complexes (leads II and III). Axis of the
premature contractions is plotted in accordance with the princi-
ples of vectorial analysis explained in Chapter 12; this shows the
origin of the PVC to be near the base of the ventricles.
152
Unit III
The Heart
re-entrant circus movement feedback pathways that set
up local repeated self–re-exci

k pathways that set
up local repeated self–re-excitation. Because of the
rapid rhythm in the irritable focus, this focus becomes
the pacemaker of the heart.
The term “paroxysmal” means that the heart rate
becomes rapid in paroxysms, with the paroxysm 
beginning suddenly and lasting for a few seconds, a 
few minutes, a few hours, or much longer. Then the
paroxysm usually ends as suddenly as it began, with 
the pacemaker of the heart instantly shifting back to 
the sinus node.
Paroxysmal tachycard

ting back to 
Paroxysmal tachycardia often can be stopped by elic-
iting a vagal reﬂex. A type of vagal reﬂex sometimes
elicited for this purpose is to press on the neck in the
regions of the carotid sinuses, which may cause enough
of a vagal reﬂex to stop the paroxysm. Various drugs
may also be used. Two drugs frequently used are quini-
dine and lidocaine, either of which depresses the normal
increase in sodium permeability of the cardiac muscle
membrane during generation of the

e cardiac muscle
membrane during generation of the action potential,
thereby often blocking the rhythmical discharge of the
focal point that is causing the paroxysmal attack.
Figure 13–12 demonstrates in the middle of the record
a sudden increase in the heart rate from about 95 to
about 150 beats per minute. On close study of the elec-
trocardiogram during the rapid heartbeat, an inverted P
wave is seen before each QRS-T complex, and this P
wave is partially superim

RS-T complex, and this P
wave is partially superimposed onto the normal T wave
of the preceding beat. This indicates that the origin 
of this paroxysmal tachycardia is in the atrium, but
because the P wave is abnormal in shape, the origin is
not near the sinus node.
A-V Nodal Paroxysmal Tachycardia. Paroxysmal tachycardia
often results from an aberrant rhythm that involves the
A-V node. This usually causes almost normal QRS-T
complexes but totally missing or obscured P waves.
Atrial or A-V nodal

y missing or obscured P waves.
Atrial or A-V nodal paroxysmal tachycardia, both of
which are called supraventricular tachycardias, usually
occurs in young,otherwise healthy people,and they gen-
erally grow out of the predisposition to tachycardia
after adolescence. In general, supraventricular tachy-
cardia frightens a person tremendously and may cause
weakness during the paroxysm, but only seldom does
permanent harm come from the attack.
Figure 13–13 shows a t

ular Paroxysmal Tachycardia
Figure 13–13 shows a typical short paroxysm of ven-
tricular tachycardia.The electrocardiogram of ventricu-
lar paroxysmal tachycardia has the appearance of a
series of ventricular premature beats occurring one
after another without any normal beats interspersed.
Ventricular paroxysmal tachycardia is usually a
serious condition for two reasons. First, this type of
tachycardia usually does not occur unless considerable
ischemic damage is present in the ventricles. Seco

ischemic damage is present in the ventricles. Second,
ventricular tachycardia frequently initiates the lethal
condition of ventricular ﬁbrillation because of rapid
repeated stimulation of the ventricular muscle, as we
discuss in the next section.
Sometimes intoxication from the heart treatment
drug digitalis causes irritable foci that lead to ventricu-
lar tachycardia. Conversely, quinidine, which increases
the refractory period and threshold for excitation of
cardiac muscle, may be used to bloc

 excitation of
cardiac muscle, may be used to block irritable foci
causing ventricular tachycardia.
The most serious of all cardiac arrhythmias is ventricu-
lar ﬁbrillation, which, if not stopped within 1 to 3
minutes, is almost invariably fatal. Ventricular ﬁbrilla-
tion results from cardiac impulses that have gone
berserk within the ventricular muscle mass, stimulating
ﬁrst one portion of the ventricular muscle, then another
portion, then another, and eventually feedin

other
portion, then another, and eventually feeding back onto
itself to re-excite the same ventricular muscle over and
over—never stopping. When this happens, many small
portions of the ventricular muscle will be contracting at
the same time, while equally as many other portions will
be relaxing. Thus, there is never a coordinate contrac-
tion of all the ventricular muscle at once, which is
required for a pumping cycle of the heart. Despite
massive movement of stimulatory signals throughout
the 

ve movement of stimulatory signals throughout
the ventricles, the ventricular chambers neither enlarge
nor contract but remain in an indeterminate stage of
partial contraction, pumping either no blood or negligi-
ble amounts. Therefore, after ﬁbrillation begins, uncon-
sciousness occurs within 4 to 5 seconds for lack of blood
ﬂow to the brain, and irretrievable death of tissues
begins to occur throughout the body within a few
minutes.
Multiple factors can spark the beginning of ventricu-
lar ﬁbr

ctors can spark the beginning of ventricu-
lar ﬁbrillation—a person may have a normal heartbeat
one moment, but 1 second later, the ventricles are in ﬁb-
rillation. Especially likely to initiate ﬁbrillation are (1)
sudden electrical shock of the heart, or (2) ischemia of
the heart muscle, of its specialized conducting system,
or both.
Figure 13–12
Atrial paroxysmal tachycardia—onset in middle of record (lead I).
Figure 13–13
Ventricular paroxysmal tachycardia (lead III).
Chapter 13
Cardiac Arrhy

l tachycardia (lead III).
Chapter 13
153
Phenomenon of Re-entry—“Circus
Movements” as the Basis for
When the normal cardiac impulse in the normal heart
has traveled through the extent of the ventricles, it has
no place to go because all the ventricular muscle is
refractory and cannot conduct the impulse farther.
Therefore, that impulse dies, and the heart awaits a new
action potential to begin in the atria

waits a new
action potential to begin in the atrial sinus node.
Under some circumstances, however, this normal
sequence of events does not occur. Therefore, let us
explain more fully the background conditions that can
initiate re-entry and lead to “circus movements,” which
in turn cause ventricular ﬁbrillation.
Figure 13–14 shows several small cardiac muscle
strips cut in the form of circles. If such a strip is stimu-
lated at the 12 o’clock position so that the impulse travels
in only one direc

tion so that the impulse travels
in only one direction, the impulse spreads progressively
around the circle until it returns to the 12 o’clock posi-
tion. If the originally stimulated muscle ﬁbers are still in
a refractory state, the impulse then dies out because
refractory muscle cannot transmit a second impulse. But
there are three different conditions that can cause this
impulse to continue to travel around the circle, that is,
to cause “re-entry” of the impulse into muscle that 
has already 

try” of the impulse into muscle that 
has already been excited. This is called a “circus 
movement.”
First, if the pathway around the circle is too long, by
the time the impulse returns to the 12 o’clock position,
the originally stimulated muscle will no longer be
refractory and the impulse will continue around the
circle again and again.
Second, if the length of the pathway remains constant
but the velocity of conduction becomes decreased
enough, an increased interval of time will elapse before

, an increased interval of time will elapse before
the impulse returns to the 12 o’clock position. By this
time, the originally stimulated muscle might be out of
the refractory state, and the impulse can continue
around the circle again and again.
Third, the refractory period of the muscle might
become greatly shortened. In this case, the impulse could
also continue around and around the circle.
All these conditions occur in different pathological
states of the human heart, as follows: (1) A lon

l
states of the human heart, as follows: (1) A long pathway
typically occurs in dilated hearts. (2) Decreased rate of
conduction frequently results from (a) blockage of the
Purkinje system, (b) ischemia of the muscle, (c) high
blood potassium levels, or (d) many other factors. (3) 
A shortened refractory period commonly occurs in
response to various drugs, such as epinephrine, or after
repetitive electrical stimulation. Thus, in many cardiac
disturbances, re-entry can cause abnormal patterns of

urbances, re-entry can cause abnormal patterns of
cardiac contraction or abnormal cardiac rhythms that
ignore the pace-setting effects of the sinus node.
Chain Reaction Mechanism 
of Fibrillation
In ventricular ﬁbrillation, one sees many separate and
small contractile waves spreading at the same time in
different directions over the cardiac muscle. The re-
entrant impulses in ﬁbrillation are not simply a single
impulse moving in a circle, as shown in Figure 13–14.
Instead, they have degenerated 

n in Figure 13–14.
Instead, they have degenerated into a series of multiple
wave fronts that have the appearance of a “chain reac-
tion.” One of the best ways to explain this process in 
ﬁbrillation is to describe the initiation of ﬁbrillation by
electric shock caused by 60-cycle alternating electric
current.
Fibrillation Caused by 60-Cycle Alternating Current. At a
central point in the ventricles of heart A in Figure 13–15,
a 60-cycle electrical stimulus is applied through a 
stimulating electr

 stimulus is applied through a 
stimulating electrode. The ﬁrst cycle of the electrical
stimulus causes a depolarization wave to spread in all
directions, leaving all the muscle beneath the electrode
in a refractory state.After about 0.25 second, part of this
muscle begins to come out of the refractory state. Some
portions come out of refractoriness before other 
Absolutely
refractory
Relatively
LONG PATHWAY
NORMAL PATHWAY
Figure 13–14
Circus movement, showing an

L PATHWAY
Circus movement, showing annihilation of the impulse in the short
pathway and continued propagation of the impulse in the long
pathway.
Stimulus
point
Dividing
impulses
Blocked
impulse
A
B
Figure 13–15
A, Initiation of ﬁbrillation in a heart when patches of refractory mus-
culature are present. B, Continued propagation of ﬁbrillatory
impulses in the ﬁbrillating ventricle.
154
Unit III
The Heart
portions. This state of events is depicted in heart A by
many lighter patches, 

s is depicted in heart A by
many lighter patches, which represent excitable cardiac
muscle, and dark patches, which represent still refrac-
tory muscle. Now, continuing 60-cycle stimuli from the
electrode can cause impulses to travel only in certain
directions through the heart but not in all directions.
Thus, in heart A, certain impulses travel for short dis-
tances, until they reach refractory areas of the heart, and
then are blocked. But other impulses pass between the
refractory areas and co

 impulses pass between the
refractory areas and continue to travel in the excitable
areas.Then, several events transpire in rapid succession,
all occurring simultaneously and eventuating in a state
of ﬁbrillation.
First,block of the impulses in some directions but suc-
cessful transmission in other directions creates one of
the necessary conditions for a re-entrant signal to
develop—that is, transmission of some of the depolar-
ization waves around the heart in only some directions
but not other

nd the heart in only some directions
but not other directions.
Second, the rapid stimulation of the heart causes two
changes in the cardiac muscle itself, both of which pre-
dispose to circus movement: (1) The velocity of conduc-
tion through the heart muscle decreases, which allows a
longer time interval for the impulses to travel around
the heart. (2) The refractory period of the muscle is
shortened, allowing re-entry of the impulse into previ-
ously excited heart muscle within a much shorter 

ously excited heart muscle within a much shorter time
than normally.
Third, one of the most important features of ﬁbrilla-
tion is the division of impulses, as demonstrated in heart
A. When a depolarization wave reaches a refractory
area in the heart, it travels to both sides around the
refractory area. Thus, a single impulse becomes two
impulses. Then, when each of these reaches another
refractory area, it, too, divides to form two more
impulses. In this way, many new wave fronts are contin-
u

s. In this way, many new wave fronts are contin-
ually being formed in the heart by progressive chain
reactions until, ﬁnally, there are many small depolariza-
tion waves traveling in many directions at the same
time. Furthermore, this irregular pattern of impulse
travel causes many circuitous routes for the impulses to
travel, greatly lengthening the conductive pathway, which
is one of the conditions that sustains the ﬁbrillation. It
also results in a continual irregular pattern of patchy
refra

s in a continual irregular pattern of patchy
refractory areas in the heart.
One can readily see when a vicious circle has been
initiated: More and more impulses are formed; these
cause more and more patches of refractory muscle, and
the refractory patches cause more and more division of
the impulses.Therefore, any time a single area of cardiac
muscle comes out of refractoriness, an impulse is close
at hand to re-enter the area.
Heart B in Figure 13–15 demonstrates the ﬁnal state
that develops in

13–15 demonstrates the ﬁnal state
that develops in ﬁbrillation. Here one can see many
impulses traveling in all directions, some dividing and
increasing the number of impulses, whereas others are
blocked by refractory areas. In fact, a single electric
shock during this vulnerable period frequently can lead
to an odd pattern of impulses spreading multidirection-
ally around refractory areas of muscle, which will lead
to ﬁbrillation.
Electrocardiogram in Ventricular
In ventricular ﬁbr

ram in Ventricular
In ventricular ﬁbrillation, the electrocardiogram is
bizarre (Figure 13–16) and ordinarily shows no ten-
dency toward a regular rhythm of any type. During the
ﬁrst few seconds of ventricular ﬁbrillation, relatively
large masses of muscle contract simultaneously, and this
causes coarse, irregular waves in the electrocardiogram.
After another few seconds, the coarse contractions of
the ventricles disappear, and the electrocardiogram
changes into a new pattern of low

lectrocardiogram
changes into a new pattern of low-voltage, very irregu-
lar waves. Thus, no repetitive electrocardiographic pat-
tern can be ascribed to ventricular ﬁbrillation. Instead,
the ventricular muscle contracts at as many as 30 to 50
small patches of muscle at a time, and electrocardio-
graphic potentials change constantly and spasmodically
because the electrical currents in the heart ﬂow ﬁrst in
one direction and then in another and seldom repeat
any speciﬁc cycle.
The voltages of the

ldom repeat
The voltages of the waves in the electrocardiogram 
in ventricular ﬁbrillation are usually about 0.5 millivolt
when ventricular ﬁbrillation ﬁrst begins, but they 
decay rapidly so that after 20 to 30 seconds, they 
are usually only 0.2 to 0.3 millivolt. Minute voltages of
0.1 millivolt or less may be recorded for 10 minutes or
longer after ventricular ﬁbrillation begins. As already
pointed out, because no pumping of blood occurs 
during ventricular ﬁbrillation, thi

 blood occurs 
during ventricular ﬁbrillation, this state is lethal unless
stopped by some heroic therapy, such as immediate
electroshock through the heart, as explained in the next
section.
Electroshock Deﬁbrillation 
of the Ventricles
Although a moderate alternating-current voltage
applied directly to the ventricles almost invariably
throws the ventricles into ﬁbrillation, a strong high-
voltage alternating electrical current passed through the
ventricles for a fraction of a second can stop ﬁb

ventricles for a fraction of a second can stop ﬁbrillation
by throwing all the ventricular muscle into refractori-
ness simultaneously. This is accomplished by passing
intense current through large electrodes placed on two
sides of the heart. The current penetrates most of the
ﬁbers of the ventricles at the same time, thus stimulat-
ing essentially all parts of the ventricles simultaneously
and causing them all to become refractory. All action
potentials stop, and the heart remains quiescent fo

otentials stop, and the heart remains quiescent for 3 to
5 seconds, after which it begins to beat again, usually
with the sinus node or some other part of the heart
becoming the pacemaker. However, the same re-entrant
focus that had originally thrown the ventricles into ﬁb-
rillation often is still present, in which case ﬁbrillation
may begin again immediately.
When electrodes are applied directly to the two sides
of the heart, ﬁbrillation can usually be stopped using 110
volts of 60-cycle alter

ually be stopped using 110
volts of 60-cycle alternating current applied for 0.1
Figure 13–16
Ventricular ﬁbrillation (lead II).
Chapter 13
155
second or 1000 volts of direct current applied for a few
thousandths of a second. When applied through two
electrodes on the chest wall, as shown in Figure 13–17,
the usual procedure is to charge a large electrical 
capacitor up to several thousand volts and then to cause
the capacitor to 

thousand volts and then to cause
the capacitor to discharge for a few thousandths of a
second through the electrodes and through the heart.
In our laboratory, the heart of a single anesthetized 
dog was deﬁbrillated 130 times through the chest wall,
and the animal lived thereafter in perfectly normal 
Hand Pumping of the Heart
(Cardiopulmonary Resuscitation) as
Unless deﬁbrillated within 1 minute after ﬁbrillation
begins, the heart is usually too weak to be rev

on
begins, the heart is usually too weak to be revived by
deﬁbrillation because of the lack of nutrition from coro-
nary blood ﬂow. However, it is still possible to revive the
heart by preliminarily pumping the heart by hand
(intermittent hand squeezing) and then deﬁbrillating
the heart later. In this way, small quantities of blood are
delivered into the aorta and a renewed coronary blood
supply develops. Then, after a few minutes of hand
pumping, electrical deﬁbrillation often becomes possi-
bl

, electrical deﬁbrillation often becomes possi-
ble. Indeed, ﬁbrillating hearts have been pumped by
hand for as long as 90 minutes followed by successful
deﬁbrillation.
A technique for pumping the heart without opening
the chest consists of intermittent thrusts of pressure on
the chest wall along with artiﬁcial respiration. This, plus
deﬁbrillation, is called cardiopulmonary resuscitation,
or CPR.
Lack of blood ﬂow to the brain for more than 5 to 
8 minutes usually causes permanent mental impair

8 minutes usually causes permanent mental impair-
ment or even destruction of brain tissue. Even if the
heart is revived, the person may die from the effects of
brain damage or may live with permanent mental
impairment.
Remember that except for the conducting pathway
through the A-V bundle, the atrial muscle mass is 
separated from the ventricular muscle mass by ﬁbrous
tissue. Therefore, ventricular ﬁbrillation often occurs
without atrial ﬁbrillation. Likewise, ﬁbrillation 

without atrial ﬁbrillation. Likewise, ﬁbrillation often
occurs in the atria without ventricular ﬁbrillation
(shown to the right in Figure 13–19).
The mechanism of atrial ﬁbrillation is identical to that
of ventricular ﬁbrillation, except that the process occurs
only in the atrial muscle mass instead of the ventricular
mass. A frequent cause of atrial ﬁbrillation is atrial
enlargement resulting from heart valve lesions that
prevent the atria from emptying adequately into the
ventricles, or from v

emptying adequately into the
ventricles, or from ventricular failure with excess
damming of blood in the atria. The dilated atrial walls
provide ideal conditions of a long conductive pathway
as well as slow conduction, both of which predispose to
atrial ﬁbrillation.
Pumping Characteristics of the Atria During Atrial Fibrillation.
For the same reasons that the ventricles will not pump
blood during ventricular ﬁbrillation, neither do the atria
pump blood in atrial ﬁbrillation. Therefore, the atria

 blood in atrial ﬁbrillation. Therefore, the atria
become useless as primer pumps for the ventricles.Even
so, blood ﬂows passively through the atria into the 
ventricles, and the efﬁciency of ventricular pumping is
decreased only 20 to 30 per cent. Therefore, in contrast
to the lethality of ventricular ﬁbrillation, a person can
live for months or even years with atrial ﬁbrillation,
although at reduced efﬁciency of overall heart pumping.
Electrocardiogram in Atrial Fibrillation. Figure 13–18 show

rdiogram in Atrial Fibrillation. Figure 13–18 shows
the electrocardiogram during atrial ﬁbrillation. Numer-
ous small depolarization waves spread in all directions
through the atria during atrial ﬁbrillation. Because the
waves are weak and many of them are of opposite
polarity at any given time, they usually almost com-
pletely electrically neutralize one another.Therefore, in
the electrocardiogram, one can see either no P waves
from the atria or only a ﬁne, high-frequency, very low
voltage wavy

 only a ﬁne, high-frequency, very low
voltage wavy record. Conversely, the QRS-T complexes
are normal unless there is some pathology of the ven-
tricles, but their timing is irregular, as explained next.
Irregularity of Ventricular Rhythm During Atrial Fibrillation.
When the atria are ﬁbrillating, impulses arrive from the
atrial muscle at the A-V node rapidly but also irregu-
larly. Because the A-V node will not pass a second
impulse for about 0.35 second after a previous one, at
least 0.35 seco

35 second after a previous one, at
least 0.35 second must elapse between one ventricular
contraction and the next. Then an additional but 
Several thousand volts
for a few milliseconds
Handle for
application
of pressure
Electrode
Figure 13–17
Application of electrical current to the chest to stop ventricular 
ﬁbrillation.
Figure 13–18
Atrial ﬁbrillation (lead I). The waves that can be seen are ventric-
ular QRS and T waves.
156
Unit III
The Heart
variable interval of 0 to 0.6 second occurs befor

variable interval of 0 to 0.6 second occurs before one of
the irregular atrial ﬁbrillatory impulses happens to
arrive at the A-V node. Thus, the interval between suc-
cessive ventricular contractions varies from a minimum
of about 0.35 second to a maximum of about 0.95
second, causing a very irregular heartbeat. In fact, this
irregularity, demonstrated by the variable spacing of the
heartbeats in the electrocardiogram of Figure 13–18, is
one of the clinical ﬁndings used to diagnose the condi-
t

the clinical ﬁndings used to diagnose the condi-
tion. Also, because of the rapid rate of the ﬁbrillatory
impulses in the atria, the ventricle is driven at a fast
heart rate, usually between 125 and 150 beats per
minute.
Electroshock Treatment of Atrial Fibrillation. In the same
manner that ventricular ﬁbrillation can be converted
back to a normal rhythm by electroshock, so too can
atrial ﬁbrillation be converted by electroshock.The pro-
cedure is essentially the same as for ventricular ﬁbrilla-

s essentially the same as for ventricular ﬁbrilla-
tion conversion—passage of a single strong electric
shock through the heart, which throws the entire heart
into refractoriness for a few seconds; a normal rhythm
often follows if the heart is capable of this.
Atrial ﬂutter is another condition caused by a circus
movement in the atria. It is different from atrial ﬁbril-
lation, in that the electrical signal travels as a single
large wave always in one direction around and around
th

 wave always in one direction around and around
the atrial muscle mass, as shown to the left in Figure
13–19.Atrial ﬂutter causes a rapid rate of contraction of
the atria, usually between 200 and 350 beats per minute.
However, because one side of the atria is contracting
while the other side is relaxing, the amount of blood
pumped by the atria is slight. Furthermore, the signals
reach the A-V node too rapidly for all of them to be
passed into the ventricles, because the refractory
periods of the

 ventricles, because the refractory
periods of the A-V node and A-V bundle are too long
to pass more than a fraction of the atrial signals. There-
fore, there are usually two to three beats of the atria for
every single beat of the ventricles.
Figure 13–20 shows a typical electrocardiogram in
atrial ﬂutter. The P waves are strong because of con-
traction of semicoordinate masses of muscle. However,
note in the record that a QRS-T complex follows an
atrial P wave only once for every two to three 

an
atrial P wave only once for every two to three beats of
the atria, giving a 2:1 or 3:1 rhythm.
A ﬁnal serious abnormality of the cardiac rhythmicity-
conduction system is cardiac arrest. This results from
cessation of all electrical control signals in the heart.
That is, no spontaneous rhythm remains.
Cardiac arrest is especially likely to occur during deep
anesthesia, when many patients develop severe hypoxia
because of inadequate respiration. The hypoxia pre-
vents the muscle

ate respiration. The hypoxia pre-
vents the muscle ﬁbers and conductive ﬁbers from main-
taining normal electrolyte concentration differentials
across their membranes, and their excitability may be so
affected that the automatic rhythmicity disappears.
In most instances of cardiac arrest from anesthesia,
prolonged 
cardiopulmonary 
resuscitation 
(many
minutes or even hours) is quite successful in re-
establishing a normal heart rhythm. In some patients,
severe myocardial disease can cause perma

atients,
severe myocardial disease can cause permanent or
semipermanent cardiac arrest, which can cause death.
To treat the condition, rhythmical electrical impulses
from an implanted electronic cardiac pacemaker have
been used successfully to keep patients alive for months
to years.
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omyopathy. Heart
Greenland P, Gaziano JM: Clinical practice: selecting asymp-
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Atrial flutter
Figure 13–19
Pathways of impulses in atrial ﬂutter and atrial ﬁbrillation.
Figure 13–20
Atrial ﬂutter—2:1 and 3:1 atrial to ventricle rhythm (lead I).
Chapter 13
157
Hurst JW: Current sta

iographic Interpretation
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Jalife J:Ventricular ﬁbrillation: mechanisms of initiation and
maintenance. Annu Rev Physiol 62:25, 2000.
Lee TH, Boucher CA: Clinical practice: noninvasive tests in
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344:1840, 2001.
Lehmann MH, Morady F: QT interval: metric for cardiac
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rval: metric for cardiac
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Levy S: Pharmacologic management of atrial ﬁbrillation: cur-
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2001.
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channels in vasospastic angina. J Clin Invest 110:153,
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Maron BJ: Sudden death in young athletes. N Engl J Med
349:1064, 2003.
Nattel S: New ideas about atrial ﬁbrillation 50 years on.
Nature 415:219, 2002.
Roden DM: Drug-induced prolongation of the QT interva

en DM: Drug-induced prolongation of the QT interval.
N Engl J Med 350:1013, 2004.
Swynghedauw B, Baillard C, Milliez P: The long QT interval
is not only inherited but is also linked to cardiac hyper-
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Topol EJ:A guide to therapeutic decision-making in patients
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Wang K, Asinger RW, Marriott HJ: ST-segment elevation in
conditions other than acute myocardial infarction

conditions other than acute myocardial infarction. N Engl
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Yan GX, Lankipalli RS, Burke JF, et al: Ventricular repolar-
ization components on the electrocardiogram: cellular
basis and clinical signiﬁcance. J Am Coll Cardiol 42:401,
2003.
Zimetbaum PJ, Josephson ME: Use of the electrocardiogram
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Zipes DP, Jalife J: Cardiac Electrophysiology, 3rd ed.
Philadelphia: WB Saunders, 1999.
U
N
I
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IV
14. Over

aunders, 1999.
U
N
I
T
IV
14. Overview of the Circulation; Medical Physics of
Pressure, Flow, and Resistance
15. Vascular Distensibility and Functions of the 
Arterial and Venous Systems
16. The Microcirculation and the Lymphatic System:
Capillary Fluid Exchange, Interstitial Fluid, 
and Lymph Flow
17. Local and Humoral Control of Blood Flow 
by the Tissues
18. Nervous Regulation of the Circulation, and 
Rapid Control of Arterial Pressure
19. Dominant Role of the Kidney in Long-T

Pressure
19. Dominant Role of the Kidney in Long-Term
Regulation of Arterial Pressure and in Hypertension: 
The Integrated System for Pressure Control
20. Cardiac Output, Venous Return, and 
Their Regulation
21. Muscle Blood Flow and Cardiac Output During
Exercise; the Coronary Circulation and Ischemic 
22. Cardiac Failure
23. Heart Valves and Heart Sounds; Dynamics of 
Valvular and Congenital Heart Defects
24. Circulatory Shock and Physiology of Its Treatment
C
H
A
P
T
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R
1
4
161

 Physiology of Its Treatment
C
H
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P
T
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1
4
161
Overview of the Circulation;
Medical Physics of Pressure,
The function of the circulation is to service the
needs of the body tissues—to transport nutrients to
the body tissues, to transport waste products away,
to conduct hormones from one part of the body to
another, and, in general, to maintain an appropriate
environment in all the tissue ﬂuids of the body for
optimal survival and function of the cells.
The rate of blood 

ival and function of the cells.
The rate of blood ﬂow through most tissues is
controlled in response to tissue need for nutrients. The heart and circulation in
turn are controlled to provide the necessary cardiac output and arterial pres-
sure to cause the needed tissue blood ﬂow. What are the mechanisms for con-
trolling blood volume and blood ﬂow, and how does this relate to all the other
functions of the circulation? These are some of the topics and questions that we
discuss in this section o

cs and questions that we
discuss in this section on the circulation.
Physical Characteristics of the Circulation
The circulation, shown in Figure 14–1, is divided into the systemic circulation
and the pulmonary circulation. Because the systemic circulation supplies blood
ﬂow to all the tissues of the body except the lungs, it is also called the greater
circulation or peripheral circulation.
Functional Parts of the Circulation. Before discussing the details of circulatory func-
tion, it is import

he details of circulatory func-
tion, it is important to understand the role of each part of the circulation.
The function of the arteries is to transport blood under high pressure to the
tissues. For this reason, the arteries have strong vascular walls, and blood ﬂows
at a high velocity in the arteries.
The arterioles are the last small branches of the arterial system; they act as
control conduits through which blood is released into the capillaries. The arte-
riole has a strong muscular wall t

ries. The arte-
riole has a strong muscular wall that can close the arteriole completely or can,
by relaxing, dilate it severalfold, thus having the capability of vastly altering
blood ﬂow in each tissue bed in response to the need of the tissue.
The function of the capillaries is to exchange ﬂuid, nutrients, electrolytes,
hormones, and other substances between the blood and the interstitial ﬂuid.
To serve this role, the capillary walls are very thin and have numerous minute
capillary pores perm

thin and have numerous minute
capillary pores permeable to water and other small molecular substances.
The venules collect blood from the capillaries, and they gradually coalesce
into progressively larger veins.
The veins function as conduits for transport of blood from the venules back
to the heart; equally important, they serve as a major reservoir of extra blood.
Because the pressure in the venous system is very low, the venous walls are thin.
Even so, they are muscular enough to contract or 

Even so, they are muscular enough to contract or expand and thereby act as a
controllable reservoir for the extra blood, either a small or a large amount,
depending on the needs of the circulation.
Volumes of Blood in the Different Parts of the Circulation. Figure 14–1 gives an
overview of the circulation and lists the percentage of the total blood volume
162
Unit IV
in major segments of the circulation. For instance,
about 84 per cent of the entire blood volume of the
body is i

r cent of the entire blood volume of the
body is in the systemic circulation, and 16 per cent in
heart and lungs. Of the 84 per cent in the systemic cir-
culation, 64 per cent is in the veins, 13 per cent in the
arteries, and 7 per cent in the systemic arterioles and
capillaries. The heart contains 7 per cent of the blood,
and the pulmonary vessels, 9 per cent.
Most surprising is the low blood volume in the cap-
illaries. It is here, however, that the most important
function of the circulation o

t the most important
function of the circulation occurs, diffusion of sub-
stances back and forth between the blood and the
tissues. This function is discussed in detail in Chapter
16.
Cross-Sectional Areas and Velocities of Blood Flow. If all the
systemic vessels of each type were put side by side,
their approximate total cross-sectional areas for the
average human being would be as follows:
Note particularly the much larger cross-sectional
areas of the veins than of the arteries, averaging abo

s of the veins than of the arteries, averaging about
four times those of the corresponding arteries. This
explains the large storage of blood in the venous
system in comparison with the arterial system.
Because the same volume of blood must ﬂow
through each segment of the circulation each minute,
the velocity of blood ﬂow is inversely proportional to
vascular cross-sectional area. Thus, under resting con-
ditions, the velocity averages about 33 cm/sec in the
aorta but only 1/1000 as rapidly in t

m/sec in the
aorta but only 1/1000 as rapidly in the capillaries,about
0.3 mm/sec. However, because the capillaries have a
typical length of only 0.3 to 1 millimeter, the blood
remains in the capillaries for only 1 to 3 seconds. This
short time is surprising because all diffusion of 
nutrient food substances and electrolytes that occurs
through the capillary walls must do so in this exceed-
ingly short time.
Pressures in the Various Portions of the Circulation. Because
the heart pumps blood cont

he Circulation. Because
the heart pumps blood continually into the aorta,
the mean pressure in the aorta is high, averaging about
100 mm Hg. Also, because heart pumping is pulsatile,
the arterial pressure alternates between a systolic pres-
sure level of 120 mm Hg and a diastolic pressure level
of 80 mm Hg, as shown on the left side of Figure 14–2.
As the blood ﬂows through the systemic circulation,
its mean pressure falls progressively to about 0 mm Hg
by the time it reaches the termination of 

0 mm Hg
by the time it reaches the termination of the venae
cavae where they empty into the right atrium of the
heart.
The pressure in the systemic capillaries varies from
as high as 35 mm Hg near the arteriolar ends to as low
as 10 mm Hg near the venous ends, but their average
“functional” pressure in most vascular beds is about 
17 mm Hg, a pressure low enough that little of the
plasma leaks through the minute pores of the capillary
walls, even though nutrients can diffuse easily through
these

 though nutrients can diffuse easily through
these same pores to the outlying tissue cells.
Note at the far right side of Figure 14–2 the respec-
tive pressures in the different parts of the pulmonary
circulation. In the pulmonary arteries, the pressure is 
pulsatile, just as in the aorta, but the pressure level is
far less: pulmonary artery systolic pressure averages
about 25 mm Hg and diastolic pressure 8 mm Hg, with
a mean pulmonary arterial pressure of only 16 mm Hg.
The mean pulmonary capil

ressure of only 16 mm Hg.
The mean pulmonary capillary pressure averages only
7 mm Hg. Yet the total blood ﬂow through the lungs
each minute is the same as through the systemic cir-
culation. The low pressures of the pulmonary system
are in accord with the needs of the lungs, because 
all that is required is to expose the blood in the 
Systemic
vessels
Arteries–13%
Arterioles
and
capillaries–7%
Heart–7%
Aorta
Pulmonary circulation–9%
Veins, venules,
and venous
sinuses–64%
Inferior
vena cava
Supe

es,
and venous
Inferior
vena cava
Superior
vena cava
Figure 14–1
Distribution of blood (in percentage of total blood) in the different
parts of the circulatory system.
Vessel
Cross-Sectional Area (cm2)
Aorta
2.5
Small arteries
20
40
2500
Venules
250
Small veins
80
Venae cavae
8
Chapter 14
Overview of the Circulation; Medical Physics of Pressure, Flow, and Resistance 
163
pulmonary capillaries to oxygen and other gases in the
pulmonary alveoli.
Basic Theory of C

 gases in the
Basic Theory of Circulatory
Function
Although the details of circulatory function are
complex, there are three basic principles that underlie
all functions of the system.
1. The rate of blood ﬂow to each tissue of the body
is almost always precisely controlled in relation to
the tissue need. When tissues are active, they need
greatly increased supply of nutrients and
therefore much more blood ﬂow than when at
rest—occasionally as much as 20 to 30 times 
the resti

—occasionally as much as 20 to 30 times 
the resting level. Yet the heart normally cannot
increase its cardiac output more than four to
seven times greater than resting levels. Therefore,
it is not possible simply to increase blood ﬂow
everywhere in the body when a particular tissue
demands increased ﬂow. Instead, the microvessels
of each tissue continuously monitor tissue needs,
such as the availability of oxygen and other
nutrients and the accumulation of carbon dioxide
and other tissue waste 

mulation of carbon dioxide
and other tissue waste products, and these in turn
act directly on the local blood vessels, dilating or
constricting them, to control local blood ﬂow
precisely to that level required for the tissue
activity. Also, nervous control of the circulation
from the central nervous system provides
additional help in controlling tissue blood 
ﬂow.
2. The cardiac output is controlled mainly by the
sum of all the local tissue ﬂows. When blood ﬂows
through a tissue, it immediately 

 When blood ﬂows
through a tissue, it immediately returns by way of
the veins to the heart. The heart responds
automatically to this increased inﬂow of blood by
pumping it immediately into the arteries from
whence it had originally come. Thus, the heart acts
as an automaton, responding to the demands of
the tissues. The heart, however, often needs help
in the form of special nerve signals to make it
pump the required amounts of blood ﬂow.
3. In general the arterial pressure is controlled
indepen

eneral the arterial pressure is controlled
independently of either local blood ﬂow control
or cardiac output control. The circulatory 
system is provided with an extensive system for
controlling the arterial blood pressure.
For instance, if at any time the pressure falls
signiﬁcantly below the normal level of about 
100 mm Hg, within seconds a barrage of nervous
reﬂexes elicits a series of circulatory changes to
raise the pressure back toward normal. The
nervous signals especially (a) increase t

mal. The
nervous signals especially (a) increase the force of
heart pumping, (b) cause contraction of the large
venous reservoirs to provide more blood to the
heart, and (c) cause generalized constriction of
most of the arterioles throughout the body so that
more blood accumulates in the large arteries to
increase the arterial pressure. Then, over more
prolonged periods, hours and days, the kidneys
play an additional major role in pressure control
both by secreting pressure-controlling hormones

l
and by regulating the blood volume.
Thus, in summary, the needs of the individual tissues
are served speciﬁcally by the circulation. In the
remainder of this chapter, we begin to discuss the basic
details of the management of tissue blood ﬂow and
control of cardiac output and arterial pressure.
0
Systemic
Pulmonary
60
80
100
120
40
20
0
Aorta
Large arteries
Venules
Large veins
Ven

es
Venules
Pulmonary arteries
Venules
Pulmonary veins
Figure 14–2
Normal blood pressures in the different portions of the circulatory system when a person is lying in the horizontal position.
164
Unit IV
Interrelationships Among
Pressure, Flow, and
Blood ﬂow through a blood vessel is determined by
two factors: (1) pressure difference of the blood be-
tween the two ends of the vessel, also sometimes 

tween the two ends of the vessel, also sometimes called
“pressure gradient” along the vessel, which is the force
that pushes the blood through the vessel, and (2) the
impediment to blood ﬂow through the vessel, which is
called vascular resistance. Figure 14–3 demonstrates
these relationships, showing a blood vessel segment
located anywhere in the circulatory system.
P1 represents the pressure at the origin of the vessel;
at the other end, the pressure is P2. Resistance occurs
as a result of fri

essure is P2. Resistance occurs
as a result of friction between the ﬂowing blood and
the intravascular endothelium all along the inside of
the vessel. The ﬂow through the vessel can be cal-
culated by the following formula, which is called
Ohm’s law:
in which F is blood ﬂow, DP is the pressure difference
(P1 - P2) between the two ends of the vessel, and R is
the resistance. This formula states, in effect, that the
blood ﬂow is directly proportional to the pressure dif-
ference but inversely prop

al to the pressure dif-
ference but inversely proportional to the resistance.
Note that it is the difference in pressure between the
two ends of the vessel, not the absolute pressure in the
vessel, that determines rate of ﬂow. For example, if 
the pressure at both ends of a vessel is 100 mm Hg and
yet no difference exists between the two ends, there
will be no ﬂow despite the presence of 100 mm Hg 
pressure.
Ohm’s law, illustrated in Equation 1, expresses the
most important of all the relations 

expresses the
most important of all the relations that the reader
needs to understand to comprehend the hemodynam-
ics of the circulation. Because of the extreme impor-
tance of this formula, the reader should also become
familiar with its other algebraic forms:
Blood Flow
Blood ﬂow means simply the quantity of blood that
passes a given point in the circulation in a given period
R
P
F
= D
DP
F
R
=
¥
F
P
R
= D
of time.Ordinarily,blood ﬂow is expressed in milliliters
per minute or liters per minut

ssed in milliliters
per minute or liters per minute, but it can be expressed
in milliliters per second or in any other unit of ﬂow.
The overall blood ﬂow in the total circulation of an
adult person at rest is about 5000 ml/min.This is called
the cardiac output because it is the amount of blood
pumped into the aorta by the heart each minute.
Methods for Measuring Blood Flow. Many mechanical and
mechanoelectrical devices can be inserted in series with
a blood vessel or, in some instances, applied 

ith
a blood vessel or, in some instances, applied to the
outside of the vessel to measure ﬂow. They are called
ﬂowmeters.
Electromagnetic Flowmeter. One of the most important
devices for measuring blood ﬂow without opening the
vessel is the electromagnetic ﬂowmeter, the principles
of which are illustrated in  Figure 14–4. Figure 14–4A
shows the generation of electromotive force (electrical
voltage) in a wire that is moved rapidly in a cross-wise
direction through a magnetic ﬁeld. This is the wel

direction through a magnetic ﬁeld. This is the well-
known principle for production of electricity by the
electric generator. Figure 14–4B shows that the same
principle applies for generation of electromotive force
in blood that is moving through a magnetic ﬁeld. In 
this case, a blood vessel is placed between the poles 
of a strong magnet, and electrodes are placed on the 
two sides of the vessel perpendicular to the magnetic
lines of force. When blood ﬂows through the vessel,
an electrical vol

n blood ﬂows through the vessel,
an electrical voltage proportional to the rate of blood 
ﬂow is generated between the two electrodes, and this
is recorded using an appropriate voltmeter or electronic
recording apparatus. Figure 14–4C shows an actual
“probe” that is placed on a large blood vessel to record
its blood ﬂow. The probe contains both the strong
magnet and the electrodes.
A special advantage of the electromagnetic ﬂowme-
ter is that it can record changes in ﬂow in less than 1/100
of a 

can record changes in ﬂow in less than 1/100
of a second, allowing accurate recording of pulsatile
changes in ﬂow as well as steady ﬂow.
Ultrasonic Doppler Flowmeter. Another type of ﬂowmeter
that can be applied to the outside of the vessel and that
has many of the same advantages as the electromagnetic
ﬂowmeter is the ultrasonic Doppler ﬂowmeter, shown in
Figure 14–5. A minute piezoelectric crystal is mounted
at one end in the wall of the device. This crystal, when
energized with an appropriate

. This crystal, when
energized with an appropriate electronic apparatus,
transmits ultrasound at a frequency of several hun-
dred thousand cycles per second downstream along the
ﬂowing blood.A portion of the sound is reﬂected by the
red blood cells in the ﬂowing blood.The reﬂected ultra-
sound waves then travel backward from the blood cells
toward the crystal. These reﬂected waves have a lower
frequency than the transmitted wave because the red
cells are moving away from the transmitter crystal.

ells are moving away from the transmitter crystal. This
is called the Doppler effect. (It is the same effect that
one experiences when a train approaches and passes by
while blowing its whistle. Once the whistle has passed
by the person, the pitch of the sound from the whistle
suddenly becomes much lower than when the train is
approaching.)
For the ﬂowmeter shown in Figure 14–5, the high-
frequency ultrasound wave is intermittently cut off, and
the reﬂected wave is received back onto the crystal

he reﬂected wave is received back onto the crystal and
ampliﬁed greatly by the electronic apparatus. Another
portion of the electronic apparatus determines the 
Pressure gradient
P1
P2
Blood flow
Figure 14–3
Interrelationships among pressure, resistance, and blood ﬂow.
Chapter 14
165
frequency difference between the transmitted wave and
the reﬂected wave, thus determining the velocity of
blood ﬂow.
Like th

hus determining the velocity of
blood ﬂow.
Like the electromagnetic ﬂowmeter, the ultrasonic
Doppler ﬂowmeter is capable of recording rapid, pul-
satile changes in ﬂow as well as steady ﬂow.
Laminar Flow of Blood in Vessels. When blood ﬂows at a
steady rate through a long, smooth blood vessel, it ﬂows
in streamlines, with each layer of blood remaining the
same distance from the vessel wall. Also, the central
most portion of the blood stays in the center of the
vessel.This type of ﬂow is called l

 center of the
vessel.This type of ﬂow is called laminar ﬂow or stream-
line ﬂow, and it is the opposite of turbulent ﬂow, which
is blood ﬂowing in all directions in the vessel and 
continually mixing within the vessel, as discussed 
subsequently.
Parabolic Velocity Proﬁle During Laminar Flow. When laminar
ﬂow occurs, the velocity of ﬂow in the center of the
vessel is far greater than that toward the outer edges.
This is demonstrated in Figure 14–6. In Figure 14–6A, a
vessel contains two ﬂuids, 

–6. In Figure 14–6A, a
vessel contains two ﬂuids, the one at the left colored by
a dye and the one at the right a clear ﬂuid, but there is
no ﬂow in the vessel. When the ﬂuids are made to ﬂow,
a parabolic interface develops between them, as shown
1 second later in Figure 14–6B; the portion of ﬂuid adja-
cent to the vessel wall has hardly moved, the portion
slightly away from the wall has moved a small distance,
and the portion in the center of the vessel has moved a
long distance. This effect is

e vessel has moved a
long distance. This effect is called the “parabolic proﬁle
for velocity of blood ﬂow.”
The cause of the parabolic proﬁle is the following:The
ﬂuid molecules touching the wall barely move because
–
+
0
–
+
0
N
S
+
+
–
–
A
B
N
S
C
Figure 14–4
Flowmeter of the electromagnetic
type, showing generation of an
electrical voltage in a wire as it
passes through an electromag-
netic ﬁeld (A); generation of an
electrical voltage in electrodes on
a blood vessel when the vessel 
is place

trodes on
is placed in a strong magnetic
ﬁeld and blood ﬂows through 
the vessel (B); and a modern
electromagnetic ﬂowmeter probe
for chronic implantation around
blood vessels (C).
Crystal
Reflected
wave
Transmitted
wave
Figure 14–5
Ultrasonic Doppler ﬂowmeter.
A
B
C
Figure 14–6
A, Two ﬂuids (one dyed red, and the other clear) before ﬂow
begins; B, the same ﬂuids 1 second after ﬂow begins; C, turbu-
lent ﬂow, with elements of the ﬂuid moving in a disorderly patter

elements of the ﬂuid moving in a disorderly pattern.
166
Unit IV
of adherence to the vessel wall. The next layer of mol-
ecules slips over these, the third layer over the second,
the fourth layer over the third, and so forth. Therefore,
the ﬂuid in the middle of the vessel can move rapidly
because many layers of slipping molecules exist
between the middle of the vessel and the vessel wall;
thus, each layer toward the center ﬂows progressively
more rapidly than the outer layers.
T

rogressively
Turbulent Flow of Blood Under Some Conditions. When the rate
of blood ﬂow becomes too great, when it passes by an
obstruction in a vessel, when it makes a sharp turn, or
when it passes over a rough surface, the ﬂow may then
become turbulent, or disorderly, rather than streamline
(see Figure 14–6C).Turbulent ﬂow means that the blood
ﬂows crosswise in the vessel as well as along the vessel,
usually forming whorls in the blood called eddy cur-
rents.

orming whorls in the blood called eddy cur-
rents. These are similar to the whirlpools that one 
frequently sees in a rapidly ﬂowing river at a point of
obstruction.
When eddy currents are present, the blood ﬂows with
much greater resistance than when the ﬂow is stream-
line because eddies add tremendously to the overall
friction of ﬂow in the vessel.
The tendency for turbulent ﬂow increases in direct
proportion to the velocity of blood ﬂow, the diameter of
the blood vessel, and the density of t

diameter of
the blood vessel, and the density of the blood, and is
inversely proportional to the viscosity of the blood, in
accordance with the following equation:
where Re is Reynolds’ number and is the measure of the
tendency for turbulence to occur, n is the mean velocity
of blood ﬂow (in centimeters/second), d is the vessel
diameter (in centimeters), r is density, and h is the vis-
cosity (in poise). The viscosity of blood is normally
about 1/30 poise, and the density is only slightly greate

/30 poise, and the density is only slightly greater
than 1. When Reynolds’ number rises above 200 to 400,
turbulent ﬂow will occur at some branches of vessels but
will die out along the smooth portions of the vessels.
However, when Reynolds’ number rises above approx-
imately 2000, turbulence will usually occur even in a
straight, smooth vessel.
Reynolds’ number for ﬂow in the vascular system
even normally rises to 200 to 400 in large arteries; as a
result there is almost always some turbulence 

s a
result there is almost always some turbulence of ﬂow at
the branches of these vessels. In the proximal portions
of the aorta and pulmonary artery, Reynolds’ number
can rise to several thousand during the rapid phase of
ejection by the ventricles; this causes considerable tur-
bulence in the proximal aorta and pulmonary artery
where many conditions are appropriate for turbulence:
(1) high velocity of blood ﬂow, (2) pulsatile nature of
the ﬂow, (3) sudden change in vessel diameter, and 
(4) la

 (3) sudden change in vessel diameter, and 
(4) large vessel diameter. However, in small vessels,
Reynolds’ number is almost never high enough to cause
turbulence.
Standard Units of Pressure. Blood pressure almost always
is measured in millimeters of mercury (mm Hg)
because the mercury manometer (shown in Figure
14–7) has been used since antiquity as the standard
Re =
◊
◊
n
r
h
d
reference for measuring pressure.Actually, blood pres-
sure means the force exerted by the blood again

s-
sure means the force exerted by the blood against any
unit area of the vessel wall. When one says that the
pressure in a vessel is 50 mm Hg, one means that the
force exerted is sufﬁcient to push a column of mercury
against gravity up to a level 50 mm high. If the pres-
sure is 100 mm Hg, it will push the column of mercury
up to 100 millimeters.
Occasionally, pressure is measured in centimeters of
water (cm H2O). A pressure of 10 cm H2O means a
pressure sufﬁcient to raise a column of water aga

pressure sufﬁcient to raise a column of water against
gravity to a height of 10 centimeters. One millimeter of
mercury pressure equals 1.36 cm water pressure
because the speciﬁc gravity of mercury is 13.6 times
that of water, and 1 centimeter is 10 times as great as
1 millimeter.
High-Fidelity Methods for Measuring Blood Pressure. The
mercury in the mercury manometer has so much inertia
that it cannot rise and fall rapidly. For this reason, the
mercury manometer, although excellent for recordin

mercury manometer, although excellent for recording
steady pressures, cannot respond to pressure changes
that occur more rapidly than about one cycle every 2 to
3 seconds. Whenever it is desired to record rapidly
changing pressures, some other type of pressure
recorder is needed. Figure 14–8 demonstrates the basic
principles of three electronic pressure transducers com-
monly used for converting blood pressure and/or rapid
changes in pressure into electrical signals and then
recording the electr

o electrical signals and then
recording the electrical signals on a high-speed electri-
cal recorder. Each of these transducers uses a very thin,
highly stretched metal membrane that forms one wall
of the ﬂuid chamber. The ﬂuid chamber in turn is con-
nected through a needle or catheter to the blood vessel
in which the pressure is to be measured.When the pres-
sure is high, the membrane bulges slightly, and when it
is low, it returns toward its resting position.
Anticoagulant
solution
Float
0 pr

esting position.
solution
Float
0 pressure
Mercury
Mercury
manometer
Moving sooted
paper
100 mm Hg pressure
Figure 14–7
Recording arterial pressure with a mercury manometer, a method
that has been used in the manner shown for recording pressure
throughout the history of physiology.
Chapter 14
167
In Figure 14–8A, a simple metal plate is placed a few
hundredths of a centimeter above the membrane.When
the

redths of a centimeter above the membrane.When
the membrane bulges, the membrane comes closer to
the plate, which increases the electrical capacitance
between these two, and this change in capacitance can
be recorded using an appropriate electronic system.
In Figure 14–8B, a small iron slug rests on the mem-
brane, and this can be displaced upward into a center
space inside an electrical wire coil. Movement of the
iron into the coil increases the inductance of the coil,
and this, too, can be rec

 inductance of the coil,
and this, too, can be recorded electronically.
Finally, in Figure 14–8C, a very thin, stretched resist-
ance wire is connected to the membrane.When this wire
is stretched greatly, its resistance increases; when it is
stretched less, its resistance decreases. These changes,
too, can be recorded by an electronic system.
With some of these high-ﬁdelity types of recording
systems, pressure cycles up to 500 cycles per second
have been recorded accurately. In common use are
re

ave been recorded accurately. In common use are
recorders capable of registering pressure changes that
occur as rapidly as 20 to 100 cycles per second, in the
manner shown on the recording paper in Figure 14–8C.
Units of Resistance. Resistance is the impediment to
blood ﬂow in a vessel, but it cannot be measured by
any direct means. Instead, resistance must be calcu-
lated from measurements of blood ﬂow and pressure
difference between two points in the vessel. If the
pre

rence between two points in the vessel. If the
pressure difference between two points is 1 mm Hg
and the ﬂow is 1 ml/sec, the resistance is said to be 1
peripheral resistance unit, usually abbreviated PRU.
Expression of Resistance in CGS Units. Occasionally, a
basic physical unit called the CGS (centimeters, grams,
seconds) unit is used to express resistance. This unit is
dyne seconds/centimeters5. Resistance in these units can
be calculated by the following formula:
Total Peripheral Vascular Re

he following formula:
Total Peripheral Vascular Resistance and Total Pulmonary Vas-
cular Resistance. The rate of blood ﬂow through the
entire circulatory system is equal to the rate of blood
pumping by the heart—that is, it is equal to the cardiac
output. In the adult human being, this is approximately
100 ml/sec. The pressure difference from the systemic
arteries to the systemic veins is about 100 mm Hg.
Therefore, the resistance of the entire systemic circu-
lation, called the total periphera

systemic circu-
lation, called the total peripheral resistance, is about
100/100, or 1 PRU.
In conditions in which all the blood vessels through-
out the body become strongly constricted, the total
peripheral resistance occasionally rises to as high as 4
PRU. Conversely, when the vessels become greatly
dilated, the resistance can fall to as little as 0.2 PRU.
In the pulmonary system, the mean pulmonary arte-
rial pressure averages 16 mm Hg and the mean left
atrial pressure averages 2 mm Hg, givi

e mean left
atrial pressure averages 2 mm Hg, giving a net pres-
sure difference of 14 mm. Therefore, when the cardiac
output is normal at about 100 ml/sec, the total 
pulmonary vascular resistance calculates to be about
0.14 PRU (about one seventh that in the systemic 
circulation).
“Conductance” of Blood in a Vessel and Its Relation to Resis-
tance. Conductance is a measure of the blood ﬂow
through a vessel for a given pressure difference. This
is generally expressed in terms of milliliters pe

is generally expressed in terms of milliliters per second
per millimeter of mercury pressure, but it can also be
expressed in terms of liters per second per millimeter
of mercury or in any other units of blood ﬂow and
pressure.
It is evident that conductance is the exact reciprocal
of resistance in accord with the following equation:
Very Slight Changes in Diameter of a Vessel Can Change Its
Conductance Tremendously! Slight changes in the diame-
ter of a vessel cause tremendous changes in the v

ter of a vessel cause tremendous changes in the vessel’s
ability to conduct blood when the blood ﬂow is stream-
lined. This is demonstrated by the experiment illus-
trated in Figure 14–9A, which shows three vessels with
relative diameters of 1, 2, and 4 but with the same pres-
sure difference of 100 mm Hg between the two ends
of the vessels. Although the diameters of these vessels
=
1
R in dyne
cm
mm Hg
ml sec
sec
5
1333
Ê
Ë
ˆ
¯ =
¥
A
B
C
Figure 14–8
Principles of three t

Ë
ˆ
¯ =
¥
A
B
C
Principles of three types of electronic transducers for recording
rapidly changing blood pressures (explained in the text).
168
Unit IV
increase only fourfold, the respective ﬂows are 1, 16,
and 256 ml/mm, which is a 256-fold increase in ﬂow.
Thus, the conductance of the vessel increases in pro-
portion to the fourth power of the diameter, in accor-
dance with the following formula:
Poiseuille’s Law. The cause of this great increase in con-
ductance w

he cause of this great increase in con-
ductance when the diameter increases can be explained
by referring to Figure 14–9B, which shows cross sections
of a large and a small vessel.The concentric rings inside
the vessels indicate that the velocity of ﬂow in each ring
is different from that in the adjacent rings because of
laminar ﬂow, which was discussed earlier in the chapter.
That is, the blood in the ring touching the wall of the
vessel is barely ﬂowing because of its adherence to 
the vascul

ely ﬂowing because of its adherence to 
the vascular endothelium.The next ring of blood toward
the center of the vessel slips past the ﬁrst ring and,
therefore, ﬂows more rapidly.The third, fourth, ﬁfth, and
sixth rings likewise ﬂow at progressively increasing
velocities. Thus, the blood that is near the wall of the
vessel ﬂows extremely slowly, whereas that in the
middle of the vessel ﬂows extremely rapidly.
In the small vessel, essentially all the blood is near the
wall, so that the extremely 

the blood is near the
wall, so that the extremely rapidly ﬂowing central
stream of blood simply does not exist. By integrating the
velocities of all the concentric rings of ﬂowing blood and
multiplying them by the areas of the rings, one can
derive the following formula, known as Poiseuille’s law:
in which F is the rate of blood ﬂow, DP is the pressure
difference between the ends of the vessel, r is the radius
of the vessel, l is length of the vessel, and h is viscosity
of the blood.
F
l
= p
h
D

ssel, and h is viscosity
F
l
= p
h
DPr4
8
Diameter
µ
4
Note particularly in this equation that the rate of
blood ﬂow is directly proportional to the fourth power
of the radius of the vessel, which demonstrates once
again that the diameter of a blood vessel (which is equal
to twice the radius) plays by far the greatest role of all
factors in determining the rate of blood ﬂow through a
vessel.
Importance of the Vessel Diameter “Fourth Power Law” in
Determining Arteriolar 

eter “Fourth Power Law” in
Determining Arteriolar Resistance. In the systemic circula-
tion, about two thirds of the total systemic resistance
to blood ﬂow is arteriolar resistance in the small 
arterioles. The internal diameters of the arterioles
range from as little as 4 micrometers to as great as 
25 micrometers. However, their strong vascular walls
allow the internal diameters to change tremendously,
often as much as fourfold. From the fourth power law
discussed above that relates blood ﬂow 

 power law
discussed above that relates blood ﬂow to diameter of
the vessel,one can see that a fourfold increase in vessel
diameter can increase the ﬂow as much as 256-fold.
Thus, this fourth power law makes it possible for 
the arterioles, responding with only small changes in
diameter to nervous signals or local tissue chemical
signals, either to turn off almost completely the blood
ﬂow to the tissue or at the other extreme to cause a
vast increase in ﬂow. Indeed, ranges of blood ﬂow of
more t

ease in ﬂow. Indeed, ranges of blood ﬂow of
more than 100-fold in separate tissue areas have been
recorded between the limits of maximum arteriolar
constriction and maximum arteriolar dilatation.
Resistance to Blood Flow in Series and Parallel Vascular Cir-
cuits. Blood pumped by the heart ﬂows from the high
pressure part of the systemic circulation (i.e., aorta) to
the low pressure side (i.e., vena cava) through many
miles of blood vessels arranged in series and in paral-
lel. The arteries, art

ged in series and in paral-
lel. The arteries, arterioles, capillaries, venules, and
veins are collectively arranged in series. When blood
vessels are arranged in series, ﬂow through each blood
vessel is the same and the total resistance to blood ﬂow
(Rtotal) is equal to the sum of the resistances of each
vessel:
The total peripheral vascular resistance is therefore
equal to the sum of resistances of the arteries, arteri-
oles, capillaries, venules, and veins. In the example
shown in Figure 14–1

es, and veins. In the example
shown in Figure 14–10A, the total vascular resistance
is equal to the sum of R1 and R2.
Blood vessels branch extensively to form parallel
circuits that supply blood to the many organs and
tissues of the body. This parallel arrangement permits
each tissue to regulate its own blood ﬂow, to a great
extent, independently of ﬂow to other tissues.
For blood vessels arranged in parallel (Figure
14–10B), the total resistance to blood ﬂow is expressed
as:
It is obvious that 

 to blood ﬂow is expressed
as:
It is obvious that for a given pressure gradient,
far greater amounts of blood will ﬂow through this
1
1
1
1
1
1
2
3
4
R
R
R
R
R
total
=
+
+
+
. . .
R
R
R
R
R . . .
total
1
2
3
4
=
+
+
+
P =
100 mm
Hg
d = 1
d = 2
d = 4
1 ml/min
16 ml/min
256 ml/min
Large vessel
Small vessel
A
B
Figure 14–9
A, Demonstration of the effect of vessel diameter on blood ﬂow.
B, Concentric rings of blood ﬂowing at different velocities; the
farther away from the vessel wall, the faster the

farther away from the vessel wall, the faster the ﬂow.
Chapter 14
169
parallel system than through any of the individual
blood vessels. Therefore, the total resistance is far less
than the resistance of any single blood vessel. Flow
through each of the parallel vessels in Figure 14–10B
is determined by the pressure gradient and its own
resistance, not the resistance of the other parallel
blood vessels. However, incr

of the other parallel
blood vessels. However, increasing the resistance of
any of the blood vessels increases the total vascular
resistance.
It may seem paradoxical that adding more blood
vessels to a circuit reduces the total vascular resist-
ance. Many parallel blood vessels, however, make it
easier for blood to ﬂow through the circuit because
each parallel vessel provides another pathway, or con-
ductance, for blood ﬂow. The total conductance (Ctotal)
for blood ﬂow is the sum of the conductan

(Ctotal)
for blood ﬂow is the sum of the conductance of each
parallel pathway:
For example, brain, kidney, muscle, gastrointestinal,
skin, and coronary circulations are arranged in paral-
lel, and each tissue contributes to the overall conduc-
tance of the systemic circulation. Blood ﬂow through
each tissue is a fraction of the total blood ﬂow (cardiac
output) and is determined by the resistance (the recip-
rocal of conductance) for blood ﬂow in the tissue, as
well as the pressure gradient.There

the tissue, as
well as the pressure gradient.Therefore, amputation of
a limb or surgical removal of a kidney also removes a
parallel circuit and reduces the total vascular conduc-
tance and total blood ﬂow (i.e., cardiac output) while
increasing total peripheral vascular resistance.
Effect of Blood Hematocrit and Blood
Viscosity on Vascular Resistance and 
Blood Flow
Note especially that another of the important factors
in Poiseuille’s equation is the viscosity of the blood.
The greater the visc

s the viscosity of the blood.
The greater the viscosity, the less the ﬂow in a vessel
if all other factors are constant. Furthermore, the vis-
cosity of normal blood is about three times as great as
the viscosity of water.
But what makes the blood so viscous? It is mainly
the large numbers of suspended red cells in the blood,
C
C
C
C
C . . .
total
1
2
3
4
=
+
+
+
each of which exerts frictional drag against adjacent
cells and against the wall of the blood vessel.
Hematocrit. The percentage of th

the blood vessel.
Hematocrit. The percentage of the blood that is cells is
called the hematocrit. Thus, if a person has a hemat-
ocrit of 40, this means that 40 per cent of the blood
volume is cells and the remainder is plasma.The hema-
tocrit of men averages about 42, while that of women
averages about 38. These values vary tremendously,
depending on whether the person has anemia, on the
degree of bodily activity, and on the altitude at which
the person resides. These changes in hematocrit are

e person resides. These changes in hematocrit are
discussed in relation to the red blood cells and their
oxygen transport function in Chapter 32.
Hematocrit is determined by centrifuging blood in
a calibrated tube, as shown in Figure 14–11. The cali-
bration allows direct reading of the percentage of cells.
Effect of Hematocrit on Blood Viscosity. The viscosity of
blood increases drastically as the hematocrit increases,
as shown in Figure 14–12.The viscosity of whole blood
at normal hematocrit i

he viscosity of whole blood
at normal hematocrit is about 3; this means that three
times as much pressure is required to force whole
blood as to force water through the same blood vessel.
When the hematocrit rises to 60 or 70, which it often
does in polycythemia, the blood viscosity can become
as great as 10 times that of water, and its ﬂow through
blood vessels is greatly retarded.
Other factors that affect blood viscosity are the
plasma protein concentration and types of proteins in
the plasma

 concentration and types of proteins in
the plasma, but these effects are so much less than 
the effect of hematocrit that they are not signiﬁcant
A
B
R1
R1
R2
R2
R3 R4
Figure 14–10
Vascular resistances: A, in series and B, in parallel.
Normal
Anemia
0
10
20
30
40
50
60
70
80
90
100
0
10
20
30
40
50
60
70
80
90
100
0
10
20
30
40
50
60
70
80
90
100
Figure 14–11
Hematocrits in a healthy (normal) person and in patients with
anemia and polycythemia.
170
Unit IV
considera

olycythemia.
170
Unit IV
considerations in most hemodynamic studies. The vis-
cosity of blood plasma is about 1.5 times that of water.
Effects of Pressure on Vascular
Resistance and Tissue Blood Flow
From the discussion thus far, one might expect an
increase in arterial pressure to cause a proportionate
increase in blood ﬂow through the various tissues of
the body. However, the effect of pressure on blood
ﬂow is greater than one would expect, as shown by the
upward curving lines 

ould expect, as shown by the
upward curving lines in Figure 14–13. The reason for
this is that an increase in arterial pressure not only
increases the force that pushes blood through the
vessels but also distends the vessels at the same time,
which decreases vascular resistance. Thus, greater
pressure increases the ﬂow in both of these ways.
Therefore, for most tissues, blood ﬂow at 100 mm Hg
arterial pressure is usually four to six times as great as
blood ﬂow at 50 mm Hg instead of two times as

t as
blood ﬂow at 50 mm Hg instead of two times as would
be true if there were no effect of increasing pressure
to increase vascular diameter.
Note also in Figure 14–13 the large changes in 
blood ﬂow that can be caused by either increased 
or decreased sympathetic nerve stimulation of the
peripheral blood vessels. Thus, as shown in the ﬁgure,
inhibition of sympathetic activity greatly dilates the
vessels and can increase the blood ﬂow twofold or
more. Conversely, very strong sympathetic stimula

more. Conversely, very strong sympathetic stimulation
can constrict the vessels so much that blood ﬂow occa-
sionally decreases to as low as zero for a few seconds
despite high arterial pressure.
See references for Chapter 15.
Viscosity (water = 1)
0
10
20
30
40
50
60
70
0
9
10
8
7
6
5
4
3
2
1
Viscosity of whole blood
Viscosity of plasma
Viscosity of water
Normal blood
Figure 14–12
Effect of hematocrit on blood viscosity. (Water viscosity = 1.)
Blood flow (ml/min)
0
0
1
2

Water viscosity = 1.)
0
0
1
2
3
4
5
6
7
20
40
60
80 100 120 140 160 180 200
Arterial pressure (mm Hg)
Normal
Critical
closing
pressure
Figure 14–13
Effect of arterial pressure on blood ﬂow through a blood vessel at
different degrees of vascular tone caused by increased or
decreased sympathetic stimulation of the vessel.
C
H
A
P
T
E
R
1
5
171
Vascular Distensibility and
Functions of the Arterial and
Venous Systems
Vascular Disten

of the Arterial and
A valuable characteristic of the vascular system is
that all blood vessels are distensible. We have seen
one example of this in Chapter 14: When the pres-
sure in blood vessels is increased, this dilates the
blood vessels and therefore decreases their resist-
ance. The result is increased blood ﬂow not only
because of increased pressure but also because of decreased resistance, usually
giving at least twice as much ﬂow increase for each 

ving at least twice as much ﬂow increase for each increase in pressure as one
might expect.
Vascular distensibility also plays other important roles in circulatory function.
For example, the distensible nature of the arteries allows them to accommodate
the pulsatile output of the heart and to average out the pressure pulsations.This
provides smooth, continuous ﬂow of blood through the very small blood vessels
of the tissues.
The most distensible by far of all the vessels are the veins. Even slig

by far of all the vessels are the veins. Even slight
increases in venous pressure cause the veins to store 0.5 to 1.0 liter of extra
blood. Therefore, the veins provide a reservoir function for storing large quan-
tities of extra blood that can be called into use whenever required elsewhere in
the circulation.
Units of Vascular Distensibility. Vascular distensibility normally is expressed as the
fractional increase in volume for each millimeter of mercury rise in pressure, in
accordance with the

f mercury rise in pressure, in
accordance with the following formula:
That is, if 1 mm Hg causes a vessel that originally contained 10 millimeters of
blood to increase its volume by 1 milliliter, the distensibility would be 0.1 per
mm Hg, or 10 per cent per mm Hg.
Difference in Distensibility of the Arteries and the Veins. Anatomically, the walls
of the arteries are far stronger than those of the veins. Consequently, the arter-
ies, on average, are about eight times less distensible than the vei

re about eight times less distensible than the veins. That is, a
given increase in pressure causes about eight times as much increase in blood
in a vein as in an artery of comparable size.
In the pulmonary circulation, the pulmonary vein distensibilities are similar
to those of the systemic circulation. But, the pulmonary arteries normally
operate under pressures about one sixth of those in the systemic arterial system,
and their distensibilities are correspondingly greater, about six times the 

 are correspondingly greater, about six times the dis-
tensibility of systemic arteries.
Vascular Compliance (or Vascular Capacitance)
In hemodynamic studies, it usually is much more important to know the total
quantity of blood that can be stored in a given portion of the circulation for
each millimeter of mercury pressure rise than to know the distensibilities of the
Vascular distensibility
Increase in volume
Increase in pressure
Original volume
=
¥
172
Unit IV
individual vesse

e
=
¥
172
Unit IV
individual vessels. This value is called the compliance
or capacitance of the respective vascular bed; that is,
Compliance and distensibility are quite different. A
highly distensible vessel that has a slight volume may
have far less compliance than a much less distensible
vessel that has a large volume because compliance is
equal to distensibility times volume.
The compliance of a systemic vein is about 24 times
that of its corresponding artery because it is ab

that of its corresponding artery because it is about 8
times as distensible and it has a volume about 3 times
as great (8 ¥ 3 = 24).
Volume-Pressure Curves of the
Arterial and Venous Circulations
A convenient method for expressing the relation of
pressure to volume in a vessel or in any portion of the
circulation is to use the so-called volume-pressure
curve. The red and blue solid curves in Figure 15–1 
represent, respectively, the volume-pressure curves 
of the normal systemic arterial system

ure curves 
of the normal systemic arterial system and venous
system, showing that when the arterial system of the
average adult person (including all the large arteries,
small arteries, and arterioles) is ﬁlled with about 700
milliliters of blood, the mean arterial pressure is 
100 mm Hg, but when it is ﬁlled with only 400 milli-
liters of blood, the pressure falls to zero.
Vascular compliance
=
In the entire systemic venous system, the volume
normally ra

ire systemic venous system, the volume
normally ranges from 2000 to 3500 milliliters, and a
change of several hundred millimeters in this volume
is required to change the venous pressure only 3 to
5 mm Hg. This mainly explains why as much as one
half liter of blood can be transfused into a healthy
person in only a few minutes without greatly altering
function of the circulation.
Effect of Sympathetic Stimulation or Sympathetic Inhibition on
the Volume-Pressure Relations of the Arterial and Venou

olume-Pressure Relations of the Arterial and Venous
Systems. Also shown in Figure 15–1 are the effects that
exciting or inhibiting the vascular sympathetic nerves
has on the volume-pressure curves. It is evident that
increase in vascular smooth muscle tone caused by
sympathetic stimulation increases the pressure at each
volume of the arteries or veins, whereas sympathetic
inhibition decreases the pressure at each volume.
Control of the vessels in this manner by the sympa-
thetics is a valuable m

 this manner by the sympa-
thetics is a valuable means for diminishing the 
dimensions of one segment of the circulation, thus
transferring blood to other segments. For instance, an
increase in vascular tone throughout the systemic cir-
culation often causes large volumes of blood to shift
into the heart, which is one of the principal methods
that the body uses to increase heart pumping.
Sympathetic control of vascular capacitance is also
highly important during hemorrhage. Enhancement of
sympat

important during hemorrhage. Enhancement of
sympathetic tone, especially to the veins, reduces the
vessel sizes enough that the circulation continues to
operate almost normally even when as much as 25 per
cent of the total blood volume has been lost.
Delayed Compliance 
(Stress-Relaxation) of Vessels
The term “delayed compliance” means that a vessel
exposed to increased volume at ﬁrst exhibits a large
increase in pressure, but progressive delayed stretching
of smooth muscle in the vessel wall al

 stretching
of smooth muscle in the vessel wall allows the pressure
to return back toward normal over a period of minutes
to hours. This effect is shown in Figure 15–2. In this
ﬁgure, the pressure is recorded in a small segment of a
vein that is occluded at both ends. An extra volume of
blood is suddenly injected until the pressure rises from
5 to 12 mm Hg. Even though none of the blood is
removed after it is injected, the pressure begins to
decrease immediately and approaches about 9 mm Hg
afte

ease immediately and approaches about 9 mm Hg
after several minutes. In other words, the volume of
blood injected causes immediate elastic distention of the
vein, but then the smooth muscle ﬁbers of the vein begin
to “creep” to longer lengths, and their tensions corre-
spondingly decrease. This effect is a characteristic of all
smooth muscle tissue and is called stress-relaxation,
which was explained in Chapter 8.
Delayed compliance is a valuable mechanism by
which the circulation can accommodat

 mechanism by
which the circulation can accommodate much extra
blood when necessary, such as after too large a transfu-
sion. Delayed compliance in the reverse direction is one
of the ways in which the circulation automatically
adjusts itself over a period of minutes or hours to dimin-
ished blood volume after serious hemorrhage.
0
0
20
40
60
80
100
120
140
500
1000 1500
2000
2500 3000
3500
Sympathetic stimulation
Sympathetic inhibition
Arterial system
Venous system

pathetic inhibition
Normal volume
Figure 15–1
“Volume-pressure curves” of the systemic arterial and venous
systems, showing the effects of stimulation or inhibition of the sym-
pathetic nerves to the circulatory system.
Chapter 15
Vascular Distensibility and Functions of the Arterial and Venous Systems
173
With each beat of the heart a new surge of blood ﬁlls
the arteries. Were it not for distensibility of the arte-
rial system, all of t

 distensibility of the arte-
rial system, all of this new blood would have to ﬂow
through the peripheral blood vessels almost instanta-
neously, only during cardiac systole, and no ﬂow would
occur during diastole. However, normally the compli-
ance of the arterial tree reduces the pressure pulsa-
tions to almost no pulsations by the time the blood
reaches the capillaries; therefore, tissue blood ﬂow is
mainly continuous with very little pulsation.
A typical record of the pressure pulsations at t

.
A typical record of the pressure pulsations at the
root of the aorta is shown in Figure 15–3. In the
healthy young adult, the pressure at the top of each
pulse, called the systolic pressure, is about 120 mm Hg.
At the lowest point of each pulse, called the diastolic
pressure, it is about 80 mm Hg.The difference between
these two pressures, about 40 mm Hg, is called the
pulse pressure.
Two major factors affect the pulse pressure: (1) the
stroke volume output of the heart and (2) the compli-
anc

volume output of the heart and (2) the compli-
ance (total distensibility) of the arterial tree. A third,
less important factor is the character of ejection from
the heart during systole.
In general, the greater the stroke volume output, the
greater the amount of blood that must be accommo-
dated in the arterial tree with each heartbeat, and,
therefore, the greater the pressure rise and fall during
systole and diastole, thus causing a greater pulse pres-
sure. Conversely, the less the compliance

se pres-
sure. Conversely, the less the compliance of the arte-
rial system, the greater the rise in pressure for a given
stroke volume of blood pumped into the arteries. For
instance, as demonstrated by the middle top curves in
Figure 15–4, the pulse pressure in old age sometimes
rises to as much as twice normal, because the arteries
have become hardened with arteriosclerosis and there-
fore are relatively noncompliant.
In effect, then, pulse pressure is determined ap-
proximately by the ratio 

essure is determined ap-
proximately by the ratio of stroke volume output to
compliance of the arterial tree.Any condition of the cir-
culation that affects either of these two factors also
affects the pulse pressure.
0
0
12
10
8
6
4
2
14
60
40
20
80
Minutes
Increased
 volume
Decreased 
volume
Delayed 
compliance
Delayed
 compliance
Figure 15–2
Effect on the intravascular pressure of injecting a volume of blood
into a venous segment and later removing the excess blood,
demonstra

ent and later removing the excess blood,
demonstrating the principle of delayed compliance.
Seconds
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
+20
–80
–80
–80
Slow rise
to peak
Sharp
incisura
Exponential diastolic decline
(may be distorted by
reflected wave)
Sharp
upstroke
Figure 15–3
Pressure pulse contour recorded from the ascending aorta.
(Redrawn from Opdyke DF: Fed Proc 11:734, 1952.)
160
120
80
160
120
80
40
0
Normal
Normal
Patent ductus
arteriosus
Aortic
regurgitation
Arte

Aortic
Arteriosclerosis Aortic stenosis
Figure 15–4
Aortic pressure pulse contours in arteriosclerosis, aortic stenosis,
patent ductus arteriosus, and aortic regurgitation.
174
Unit IV
Abnormal Pressure Pulse Contours
Some conditions of the circulation also cause abnor-
mal contours of the pressure pulse wave in addition 
to altering the pulse pressure. Especially distinctive
among these are aortic stenosis, patent ductus arterio-
sus, and a

aortic stenosis, patent ductus arterio-
sus, and aortic regurgitation, each of which is shown in
Figure 15–4.
In aortic stenosis, the diameter of the aortic valve
opening is reduced signiﬁcantly, and the aortic pres-
sure pulse is decreased signiﬁcantly because of dimin-
ished blood ﬂow outward through the stenotic valve.
In patent ductus arteriosus, one half or more of the
blood pumped into the aorta by the left ventricle ﬂows
immediately backward through the wide-open ductus
into the pulmonary

rd through the wide-open ductus
into the pulmonary artery and lung blood vessels, thus
allowing the diastolic pressure to fall very low before
the next heartbeat.
In aortic regurgitation, the aortic valve is absent or
will not close completely. Therefore, after each heart-
beat, the blood that has just been pumped into the
aorta ﬂows immediately backward into the left ventri-
cle. As a result, the aortic pressure can fall all the way
to zero between heartbeats. Also, there is no incisura
in the 

een heartbeats. Also, there is no incisura
in the aortic pulse contour because there is no aortic
valve to close.
Transmission of Pressure Pulses 
to the Peripheral Arteries
When the heart ejects blood into the aorta during
systole, at ﬁrst only the proximal portion of the aorta
becomes distended because the inertia of the blood
prevents sudden blood movement all the way to the
periphery. However, the rising pressure in the proxi-
mal aorta rapidly overcomes this inertia, and the wave
front of d

ly overcomes this inertia, and the wave
front of distention spreads farther and farther along
the aorta, as shown in Figure 15–5.This is called trans-
mission of the pressure pulse in the arteries.
The velocity of pressure pulse transmission in the
normal aorta is 3 to 5 m/sec; in the large arterial
branches, 7 to 10 m/sec; and in the small arteries, 15 to
35 m/sec. In general, the greater the compliance of
each vascular segment, the slower the velocity, which
explains the slow transmission in t

elocity, which
explains the slow transmission in the aorta and the
much faster transmission in the much less compliant
small distal arteries. In the aorta, the velocity of trans-
mission of the pressure pulse is 15 or more times the
velocity of blood ﬂow because the pressure pulse is
simply a moving wave of pressure that involves little
forward total movement of blood volume.
Damping of the Pressure Pulses in the Smaller Arteries, Arteri-
oles, and Capillaries. Figure 15–6 shows typical changes

nd Capillaries. Figure 15–6 shows typical changes
in the contours of the pressure pulse as the pulse
travels into the peripheral vessels. Note especially in
the three lower curves that the intensity of pulsation
becomes progressively less in the smaller arteries,
the arterioles, and, especially, the capillaries. In fact,
only when the aortic pulsations are extremely large or
the arterioles are greatly dilated can pulsations be
observed in the capillaries.
Wave fronts
Figure 15–5
Progressive stag

illaries.
Progressive stages in transmission of the pressure pulse along the
aorta.
0
1
2
Time (seconds)
Incisura
Radial artery
Arteriole
Capillary
Diastole
Proximal aorta
Femoral artery
Systole
Figure 15–6
Changes in the pulse pressure contour as the pulse wave travels
toward the smaller vessels.
Chapter 15
175
This progressive diminution of the pulsations in the
periphery is called damping of the p

ations in the
periphery is called damping of the pressure pulses.The
cause of this is twofold: (1) resistance to blood move-
ment in the vessels and (2) compliance of the vessels.
The resistance damps the pulsations because a small
amount of blood must ﬂow forward at the pulse wave
front to distend the next segment of the vessel; the
greater the resistance, the more difﬁcult it is for this to
occur. The compliance damps the pulsations because
the more compliant a vessel, the greater the quantity

 more compliant a vessel, the greater the quantity
of blood required at the pulse wave front to cause an
increase in pressure. Therefore, in effect, the degree of
damping is almost directly proportional to the product
of resistance times compliance.
Clinical Methods for Measuring
Systolic and Diastolic Pressures
It is not reasonable to use pressure recorders that
require needle insertion into an artery for making
routine pressure measurements in human patients,
although these are used on occasio

human patients,
although these are used on occasion when special
studies are necessary. Instead, the clinician determines
systolic and diastolic pressures by indirect means,
usually by the auscultatory method.
Auscultatory Method. Figure 15–7 shows the auscultatory
method for determining systolic and diastolic arterial
pressures.A stethoscope is placed over the antecubital
artery and a blood pressure cuff is inﬂated around the
upper arm. As long as the cuff continues to compress
the arm with too

as the cuff continues to compress
the arm with too little pressure to close the brachial
artery, no sounds are heard from the antecubital artery
with the stethoscope. However, when the cuff pressure
is great enough to close the artery during part of the
arterial pressure cycle, a sound then is heard with each
pulsation. These sounds are called Korotkoff sounds.
The exact cause of Korotkoff sounds is still 
debated, but they are believed to be caused mainly 
by blood jetting through the partly oc

sed mainly 
by blood jetting through the partly occluded vessel.
The jet causes turbulence in the vessel beyond the cuff,
and this sets up the vibrations heard through the 
stethoscope.
In determining blood pressure by the auscultatory
method, the pressure in the cuff is ﬁrst elevated well
above arterial systolic pressure. As long as this cuff
pressure is higher than systolic pressure, the brachial
artery remains collapsed so that no blood jets into the
lower artery during any part of the pressu

nto the
lower artery during any part of the pressure cycle.
Therefore, no Korotkoff sounds are heard in the lower
artery. But then the cuff pressure gradually is reduced.
Just as soon as the pressure in the cuff falls below sys-
tolic pressure, blood begins to slip through the artery
beneath the cuff during the peak of systolic pressure,
and one begins to hear tapping sounds from the ante-
cubital artery in synchrony with the heartbeat.As soon
as these sounds begin to be heard, the pressure leve

 these sounds begin to be heard, the pressure level
indicated by the manometer connected to the cuff is
about equal to the systolic pressure.
As the pressure in the cuff is lowered still more, the
Korotkoff sounds change in quality, having less of the
tapping quality and more of a rhythmical and harsher
quality.Then, ﬁnally, when the pressure in the cuff falls
to equal diastolic pressure, the artery no longer closes
during diastole, which means that the basic factor
causing the sounds (the jetti

hat the basic factor
causing the sounds (the jetting of blood through a
squeezed artery) is no longer present. Therefore, the
sounds suddenly change to a mufﬂed quality, then dis-
appear entirely after another 5- to 10-millimeter drop
in cuff pressure. One notes the manometer pressure
when the Korotkoff sounds change to the mufﬂed
quality; this pressure is about equal to the diastolic
pressure. The auscultatory method for determining
systolic and diastolic pressures is not entirely accurate,
but

 diastolic pressures is not entirely accurate,
but it usually gives values within 10 per cent of those
determined by direct catheter measurement from
inside the arteries.
Normal Arterial Pressures as Measured by the Auscultatory
Method. Figure 15–8 shows the approximate normal
systolic and diastolic arterial pressures at different
ages. The progressive increase in pressure with age
results from the effects of aging on the blood pressure
control mechanisms. We shall see in Chapter 19 that
the kid

echanisms. We shall see in Chapter 19 that
the kidneys are primarily responsible for this long-
term regulation of arterial pressure; and it is well
known that the kidneys exhibit deﬁnitive changes with
age, especially after the age of 50 years.
A slight extra increase in systolic pressure usually
occurs beyond the age of 60 years. This results from
hardening of the arteries, which itself is an end-stage
result of atherosclerosis. The ﬁnal effect is a bounding
systolic pressure with considerable

 is a bounding
systolic pressure with considerable increase in pulse
pressure, as previously explained.
Mean Arterial Pressure. The mean arterial pressure is the
average of the arterial pressures measured millisecond
120
100
Sounds
150
mm Hg
100
50
0
80
Figure 15–7
Auscultatory method for measuring systolic and diastolic arterial
pressures.
176
Unit IV
by millisecond over a period of time. It is not equal to
the average of systolic and diastolic pressure because
the arterial pres

c and diastolic pressure because
the arterial pressure remains nearer to diastolic pres-
sure than to systolic pressure during the greater part
of the cardiac cycle. Therefore, the mean arterial pres-
sure is determined about 60 per cent by the diastolic
pressure and 40 per cent by the systolic pressure. Note
in Figure 15–8 that the mean pressure (solid green
line) at all ages is nearer to the diastolic pressure than
to the systolic pressure.
For years, the veins were c

ns and Their Functions
For years, the veins were considered to be nothing
more than passageways for ﬂow of blood to the heart,
but it has become apparent that they perform other
special functions that are necessary for operation of
the circulation. Especially important, they are capable
of constricting and enlarging and thereby storing
either small or large quantities of blood and making
this blood available when it is required by the remain-
der of the circulation. The peripheral veins can also

 of the circulation. The peripheral veins can also
propel blood forward by means of a so-called venous
pump, and they even help to regulate cardiac output,
an exceedingly important function that is described in
detail in Chapter 20.
Venous Pressures—Right Atrial
Pressure (Central Venous Pressure)
and Peripheral Venous Pressures
To understand the various functions of the veins, it is
ﬁrst necessary to know something about pressure in
the veins and what determines the pressure.
Blood from all the 

 what determines the pressure.
Blood from all the systemic veins ﬂows into the right
atrium of the heart; therefore, the pressure in the right
atrium is called the central venous pressure.
Right atrial pressure is regulated by a balance
between (1) the ability of the heart to pump blood out
of the right atrium and ventricle into the lungs and (2)
the tendency for blood to ﬂow from the peripheral veins
into the right atrium. If the right heart is pumping
strongly, the right atrial pressure decrea

pumping
strongly, the right atrial pressure decreases. Con-
versely, weakness of the heart elevates the right atrial
pressure. Also, any effect that causes rapid inﬂow of
blood into the right atrium from the peripheral veins
elevates the right atrial pressure. Some of the factors
that can increase this venous return (and thereby
increase the right atrial pressure) are (1) increased
blood volume, (2) increased large vessel tone through-
out the body with resultant increased peripheral
venous pres

dy with resultant increased peripheral
venous pressures, and (3) dilatation of the arterioles,
which decreases the peripheral resistance and allows
rapid ﬂow of blood from the arteries into the veins.
The same factors that regulate right atrial pressure
also enter into the regulation of cardiac output
because the amount of blood pumped by the heart
depends on both the ability of the heart to pump and
the tendency for blood to ﬂow into the heart from the
peripheral vessels. Therefore, we will dis

rom the
peripheral vessels. Therefore, we will discuss regula-
tion of right atrial pressure in much more depth in
Chapter 20 in connection with regulation of cardiac
output.
The normal right atrial pressure is about 0 mm Hg,
which is equal to the atmospheric pressure around the
body. It can increase to 20 to 30 mm Hg under very
abnormal conditions, such as (1) serious heart failure
or (2) after massive transfusion of blood, which greatly
increases the total blood volume and causes excessive
qua

es the total blood volume and causes excessive
quantities of blood to attempt to ﬂow into the heart
from the peripheral vessels.
The lower limit to the right atrial pressure is usually
about -3 to -5 mm Hg below atmospheric pressure.
This is also the pressure in the chest cavity that sur-
rounds the heart. The right atrial pressure approaches
these low values when the heart pumps with excep-
tional vigor or when blood ﬂow into the heart from the
peripheral vessels is greatly depressed, such as a

peripheral vessels is greatly depressed, such as after
severe hemorrhage.
Venous Resistance and Peripheral 
Venous Pressure
Large veins have so little resistance to blood ﬂow when
they are distended that the resistance then is almost
zero and is of almost no importance. However, as
shown in Figure 15–9,most of the large veins that enter
the thorax are compressed at many points by the sur-
rounding tissues, so that blood ﬂow is impeded at these
points. For instance, the veins from the arms are co

ints. For instance, the veins from the arms are com-
pressed by their sharp angulations over the ﬁrst rib.
Also, the pressure in the neck veins often falls so low
that the atmospheric pressure on the outside of the
neck causes these veins to collapse. Finally, veins
coursing through the abdomen are often compressed
by different organs and by the intra-abdominal pres-
sure, so that they usually are at least partially collapsed
to an ovoid or slitlike state. For these reasons, the 
large veins do 

ike state. For these reasons, the 
large veins do usually offer some resistance to blood
ﬂow, and because of this, the pressure in the more
0
0
150
100
200
60
40
20
80
50
Systolic
Mean
Diastolic
Figure 15–8
Changes in systolic, diastolic, and mean arterial pressures with
age. The shaded areas show the approximate normal ranges.
Chapter 15
177
peripheral small veins in a person lying down is usua

ipheral small veins in a person lying down is usually
+4 to +6 mm Hg greater than the right atrial pressure.
Effect of High Right Atrial Pressure on Peripheral Venous Pres-
sure. When the right atrial pressure rises above its
normal value of 0 mm Hg, blood begins to back up in
the large veins. This enlarges the veins, and even the
collapse points in the veins open up when the right
atrial pressure rises above +4 to +6 mm Hg. Then, as
the right atrial pressure rises still further, the addi-
tiona

rial pressure rises still further, the addi-
tional increase causes a corresponding rise in periph-
eral venous pressure in the limbs and elsewhere.
Because the heart must be weakened greatly to cause
a rise in right atrial pressure as high as +4 to +6 mm
Hg, one often ﬁnds that the peripheral venous pres-
sure is not noticeably elevated even in the early stages
of heart failure.
Effect of Intra-abdominal Pressure on Venous Pressures of the
Leg. The pressure in the abdominal cavity of a recum-
b

The pressure in the abdominal cavity of a recum-
bent person normally averages about +6 mm Hg,
but it can rise to +15 to +30 mm Hg as a result of 
pregnancy, large tumors, or excessive ﬂuid (called
“ascites”) in the abdominal cavity. When the intra-
abdominal pressure does rise, the pressure in the veins
of the legs must rise above the abdominal pressure
before the abdominal veins will open and allow 
the blood to ﬂow from the legs to the heart. Thus,
if the intra-abdominal pressure is +20 mm Hg

Thus,
if the intra-abdominal pressure is +20 mm Hg, the
lowest possible pressure in the femoral veins is also
+20 mm Hg.
Effect of Gravitational Pressure on Venous
Pressure
In any body of water that is exposed to air, the pres-
sure at the surface of the water is equal to atmospheric
pressure, but the pressure rises 1 mm Hg for each 13.6
millimeters of distance below the surface. This pres-
sure results from the weight of the water and there-
fore is called gravitational pressure or hydrostatic

e is called gravitational pressure or hydrostatic
pressure.
Gravitational pressure also occurs in the vascular
system of the human being because of weight of the
blood in the vessels, as shown in Figure 15–10. When
a person is standing, the pressure in the right atrium
remains about 0 mm Hg because the heart pumps into
the arteries any excess blood that attempts to accu-
mulate at this point. However, in an adult who is stand-
ing absolutely still, the pressure in the veins of the feet
is about 

l, the pressure in the veins of the feet
is about +90 mm Hg simply because of the gravita-
tional weight of the blood in the veins between 
the heart and the feet. The venous pressures at other
levels of the body are proportionately between 0 and
90 mm Hg.
In the arm veins, the pressure at the level of the top
rib is usually about +6 mm Hg because of compression
of the subclavian vein as it passes over this rib. The
gravitational pressure down the length of the arm then
is determined by the dist

e length of the arm then
is determined by the distance below the level of this
rib. Thus, if the gravitational difference between the
Atmospheric
pressure
collapse in neck
Axillary collapse
Abdominal
pressure
collapse
Intrathoracic
pressure = -4 mm Hg
Rib collapse
Figure 15–9
Compression points that tend to collapse the veins entering the
thorax.
Sagittal sinus
-10 mm
0 mm
0 mm
+ 6 mm
+ 8 mm
+ 22 mm
+ 35 mm
+ 40 mm
+ 90 mm
Figure 15–10
Effect of gravitational pressure on the venous pressures thr

gravitational pressure on the venous pressures through-
out the body in the standing person.
178
Unit IV
level of the rib and the hand is +29 mm Hg, this grav-
itational pressure is added to the +6 mm Hg pressure
caused by compression of the vein as it crosses the rib,
making a total of +35 mm Hg pressure in the veins of
the hand.
The neck veins of a person standing upright collapse
almost completely all the way to the skull because of
atmospheric pressure on the outside of the n

se of
atmospheric pressure on the outside of the neck. This
collapse causes the pressure in these veins to remain
at zero along their entire extent. The reason for this is
that any tendency for the pressure to rise above this
level opens the veins and allows the pressure to fall
back to zero because of ﬂow of the blood. Conversely,
any tendency for the neck vein pressure to fall below
zero collapses the veins still more, which further
increases their resistance and again returns the pres-
sure b

heir resistance and again returns the pres-
sure back to zero.
The veins inside the skull, on the other hand, are in
a noncollapsible chamber (the skull cavity) so that
they cannot collapse. Consequently, negative pressure
can exist in the dural sinuses of the head; in the stand-
ing position, the venous pressure in the sagittal sinus
at the top of the brain is about -10 mm Hg because of
the hydrostatic “suction” between the top of the skull
and the base of the skull.Therefore, if the sagittal s

the base of the skull.Therefore, if the sagittal sinus
is opened during surgery, air can be sucked immedi-
ately into the venous system; the air may even pass
downward to cause air embolism in the heart, and
death can ensue.
Effect of the Gravitational Factor on Arterial and Other Pres-
sures. The gravitational factor also affects pressures in
the peripheral arteries and capillaries, in addition to its
effects in the veins.For instance,a standing person who
has a mean arterial pressure of 100 mm

 person who
has a mean arterial pressure of 100 mm Hg at the level
of the heart has an arterial pressure in the feet of about
190 mm Hg. Therefore, when one states that the arte-
rial pressure is 100 mm Hg, this generally means that
this is the pressure at the gravitational level of the
heart but not necessarily elsewhere in the arterial
vessels.
Venous Valves and the “Venous Pump”: 
Their Effects on Venous Pressure
Were it not for valves in the veins, the gravitational
pressure effect would cau

veins, the gravitational
pressure effect would cause the venous pressure in the
feet always to be about +90 mm Hg in a standing adult.
However, every time one moves the legs, one tightens
the muscles and compresses the veins in or adjacent to
the muscles, and this squeezes the blood out of the
veins. But the valves in the veins, shown in Figure
15–11, are arranged so that the direction of venous
blood ﬂow can be only toward the heart. Conse-
quently, every time a person moves the legs or even
te

tly, every time a person moves the legs or even
tenses the leg muscles, a certain amount of venous
blood is propelled toward the heart. This pumping
system is known as the “venous pump” or “muscle
pump,” and it is efﬁcient enough that under ordinary
circumstances, the venous pressure in the feet of a
walking adult remains less than +20 mm Hg.
If a person stands perfectly still, the venous pump
does not work, and the venous pressures in the lower
legs increase to the full gravitational value of 9

legs increase to the full gravitational value of 90 mm
Hg in about 30 seconds. The pressures in the capillar-
ies also increase greatly, causing ﬂuid to leak from the
circulatory system into the tissue spaces. As a result,
the legs swell, and the blood volume diminishes.
Indeed, 10 to 20 per cent of the blood volume can be
lost from the circulatory system within the 15 to 30
minutes of standing absolutely still, as often occurs
when a soldier is made to stand at rigid attention.
Venous Valve Inc

made to stand at rigid attention.
Venous Valve Incompetence Causes “Varicose” Veins. The
valves of the venous system frequently become
“incompetent” or sometimes even are destroyed. This
is especially true when the veins have been over-
stretched by excess venous pressure lasting weeks or
months, as occurs in pregnancy or when one stands
most of the time. Stretching the veins increases their
cross-sectional areas, but the leaﬂets of the valves do
not increase in size.Therefore, the leaﬂets of th

not increase in size.Therefore, the leaﬂets of the valves
no longer close completely. When this develops, the
pressure in the veins of the legs increases greatly
because of failure of the venous pump; this further
increases the sizes of the veins and ﬁnally destroys the
function of the valves entirely. Thus, the person devel-
ops “varicose veins,” which are characterized by large,
bulbous protrusions of the veins beneath the skin of
the entire leg, particularly the lower leg.
Whenever people wi

eg, particularly the lower leg.
Whenever people with varicose veins stand for more
than a few minutes, the venous and capillary pressures
become very high, and leakage of ﬂuid from the cap-
illaries causes constant edema in the legs. The edema
in turn prevents adequate diffusion of nutritional
materials from the capillaries to the muscle and skin
cells, so that the muscles become painful and weak,
and the skin frequently becomes gangrenous and
ulcerates. The best treatment for such a condition i

cerates. The best treatment for such a condition is
Deep vein
Perforating
vein
Superficial
vein
Valve
Figure 15–11
Venous valves of the leg.
Chapter 15
179
continual elevation of the legs to a level at least as high
as the heart. Tight binders on the legs also can be of
considerable assistance in preventing the edema and
its sequelae.
Clinical Estimation of Venous Pressure. The venous pressure
often can be estimated by simp

The venous pressure
often can be estimated by simply observing the degree
of distention of the peripheral veins—especially of the
neck veins. For instance, in the sitting position, the neck
veins are never distended in the normal quietly resting
person. However, when the right atrial pressure be-
comes increased to as much as +10 mm Hg, the lower
veins of the neck begin to protrude; and at +15 mm Hg
atrial pressure essentially all the veins in the neck
become distended.
Direct Measurement of Ven

e neck
Direct Measurement of Venous Pressure and 
Right Atrial Pressure
Venous pressure can also be measured with ease by
inserting a needle directly into a vein and connecting it
to a pressure recorder. The only means by which right
atrial pressure can be measured accurately is by insert-
ing a catheter through the peripheral veins and into the
right atrium. Pressures measured through such central
venous catheters are used almost routinely in some types
of hospitalized cardiac

st routinely in some types
of hospitalized cardiac patients to provide constant
assessment of heart pumping ability.
Pressure Reference Level for Measuring Venous and Other
Circulatory Pressures
In discussions up to this point, we often have spoken of
right atrial pressure as being 0 mm Hg and arterial pres-
sure as being 100 mm Hg, but we have not stated the
gravitational level in the circulatory system to which this
pressure is referred. There is one point in the circula-
tory system at which 

is one point in the circula-
tory system at which gravitational pressure factors
caused by changes in body position of a healthy person
usually do not affect the pressure measurement by more
than 1 to 2 mm Hg.This is at or near the level of the tri-
cuspid valve, as shown by the crossed axes in Figure
15–12.Therefore, all circulatory pressure measurements
discussed in this text are referred to this level, which is
called the reference level for pressure measurement.
The reason for lack of gravit

ressure measurement.
The reason for lack of gravitational effects at the 
tricuspid valve is that the heart automatically prevents
signiﬁcant gravitational changes in pressure at this point
in the following way:
If the pressure at the tricuspid valve rises slightly
above normal, the right ventricle ﬁlls to a greater extent
than usual, causing the heart to pump blood more
rapidly and therefore to decrease the pressure at the tri-
cuspid valve back toward the normal mean value. Con-
versely, if th

 toward the normal mean value. Con-
versely, if the pressure falls, the right ventricle fails to ﬁll
adequately, its pumping decreases, and blood dams up
in the venous system until the pressure at the tricuspid
level again rises to the normal value. In other words,
the heart acts as a feedback regulator of pressure at the
tricuspid valve.
When a person is lying on his or her back, the tricus-
pid valve is located at almost exactly 60 per cent of the
chest thickness in front of the back.This is t

the
chest thickness in front of the back.This is the zero pres-
sure reference level for a person lying down.
As pointed out in Chapter 14, more than 60 per cent
of all the blood in the circulatory system is usually in
the veins. For this reason and also because the veins
are so compliant, it is said that the venous system
serves as a blood reservoir for the circulation.
When blood is lost from the body and the arterial
pressure begins to fall, nervous signa

he arterial
pressure begins to fall, nervous signals are elicited
from the carotid sinuses and other pressure-sensitive
areas of the circulation, as discussed in Chapter 18.
These in turn elicit nerve signals from the brain and
spinal cord mainly through sympathetic nerves to the
veins, causing them to constrict.This takes up much of
the slack in the circulatory system caused by the lost
blood. Indeed, even after as much as 20 per cent of the
total blood volume has been lost, the circulatory
sys

al blood volume has been lost, the circulatory
system often functions almost normally because of this
variable reservoir function of the veins.
Speciﬁc Blood Reservoirs. Certain portions of the circu-
latory system are so extensive and/or so compliant that
they are called “speciﬁc blood reservoirs.” These
include (1) the spleen, which sometimes can decrease
in size sufﬁciently to release as much as 100 milliliters
of blood into other areas of the circulation; (2) the
liver, the sinuses of which 

 circulation; (2) the
liver, the sinuses of which can release several hundred
milliliters of blood into the remainder of the circula-
tion; (3) the large abdominal veins, which can con-
tribute as much as 300 milliliters; and (4) the venous
plexus beneath the skin, which also can contribute
several hundred milliliters. The heart and the lungs,
although not parts of the systemic venous reservoir
system, must also be considered blood reservoirs. The
heart, for instance, shrinks during sympathetic 

e
heart, for instance, shrinks during sympathetic stimu-
lation and in this way can contribute some 50 to 100
milliliters of blood; the lungs can contribute another
100 to 200 milliliters when the pulmonary pressures
decrease to low values.
The Spleen as a Reservoir for Storing Red Blood Cells.
Figure 15–13 shows that the spleen has two separate
areas for storing blood: the venous sinuses and the
pulp. The sinuses can swell the same as any other part
of the venous system and store whole blood.
R

part
Natural reference
point
Figure 15–12
Reference point for circulatory pressure measurement (located
near the tricuspid valve).
180
Unit IV
In the splenic pulp, the capillaries are so permeable
that whole blood, including the red blood cells, oozes
through the capillary walls into a trabecular mesh,
forming the red pulp. The red cells are trapped by the
trabeculae, while the plasma ﬂows on into the venous

abeculae, while the plasma ﬂows on into the venous
sinuses and then into the general circulation.As a con-
sequence, the red pulp of the spleen is a special reser-
voir that contains large quantities of concentrated red
blood cells. These can then be expelled into the general
circulation whenever the sympathetic nervous system
becomes excited and causes the spleen and its vessels
to contract. As much as 50 milliliters of concentrated
red blood cells can be released into the circulation,
raising 

lls can be released into the circulation,
raising the hematocrit 1 to 2 per cent.
In other areas of the splenic pulp are islands of white
blood cells, which collectively are called the white pulp.
Here lymphoid cells are manufactured similar to those
manufactured in the lymph nodes.They are part of the
body’s immune system, described in Chapter 34.
Blood-Cleansing Function of the Spleen—Removal of Old Cells
Blood cells passing through the splenic pulp before
entering the sinuses undergo thorough

 pulp before
entering the sinuses undergo thorough squeezing.
Therefore,it is to be expected that fragile red blood cells
would not withstand the trauma. For this reason, many
of the red blood cells destroyed in the body have their
ﬁnal demise in the spleen. After the cells rupture, the
released hemoglobin and the cell stroma are digested
by the reticuloendothelial cells of the spleen, and the
products of digestion are mainly reused by the body as
nutrients, often for making new blood cells.
Ret

s
Reticuloendothelial Cells of the Spleen
The pulp of the spleen contains many large phagocytic
reticuloendothelial cells, and the venous sinuses are
lined with similar cells. These cells function as part of a
cleansing system for the blood, acting in concert with a
similar system of reticuloendothelial cells in the venous
sinuses of the liver.When the blood is invaded by infec-
tious agents, the reticuloendothelial cells of the spleen
rapidly remove 

uloendothelial cells of the spleen
rapidly remove debris, bacteria, parasites, and so forth.
Also, in many chronic infectious processes, the spleen
enlarges in the same manner that lymph nodes enlarge
and then performs its cleansing function even more
avidly.
Badeer HS: Hemodynamics for medical students. Am J
Physiol (Adv Physiol Educ) 25:44, 2001.
Benetos A, Waeber B, Izzo J, Mitchell G, et al: Inﬂuence of
age, risk factors, and cardiovascular and renal disease on
arterial stiffness:

ovascular and renal disease on
arterial stiffness: clinical applications. Am J Hypertens
15:1101, 2002.
Gimbrone MA Jr,Topper JN, Nagel T, et al: Endothelial dys-
function, hemodynamic forces, and atherogenesis. Ann 
N Y Acad Sci 902:230, 2000.
Guyton 
AC:
Arterial 
Pressure 
and 
Hypertension.
Philadelphia: WB Saunders Co, 1980.
Guyton AC, Jones CE: Central venous pressure: physiologi-
cal signiﬁcance and clinical implications. Am Heart J 86:
431, 1973.
Guyton AC, Jones CE, Coleman TG: Circulat

1, 1973.
Cardiac Output and Its Regulation. Philadelphia: WB
Saunders Co, 1973.
Hall J: Integration and regulation of cardiovascular function.
Am J Physiol (Adv Physiol Educ) 22:s174, 1999.
Hicks JW, Badeer HS: Gravity and the circulation: “open”
vs. “closed” systems. Am J Physiol 262:R725-R732, 1992.
Jones DW, Appel LJ, Sheps SG, et al: Measuring blood pres-
sure accurately: New and persistent challenges. JAMA
289:1027, 2003.
Lakatta EG, 

tent challenges. JAMA
Lakatta EG, Levy D: Arterial and cardiac aging: major 
shareholders in cardiovascular disease enterprises. Part I:
Aging arteries: a “set up” for vascular disease. Circulation
107:139, 2003.
McAlister FA, Straus SE: Evidence based treatment of
hypertension. Measurement of blood pressure: an evi-
dence based review. BMJ 322:908, 2001.
O’Brien E, Asmar R, Beilin L, et al: European Society 
of Hypertension recommendations for conventional,
ambulatory and home b

mendations for conventional,
ambulatory and home blood pressure measurement. J
Hypertens 21:821, 2003.
Perloff D, Grim C, Flack J, et al: Human blood pressure
determination 
by 
sphygmomanometry.
88:2460, 1993.
Rothe CF, Gersting JM: Cardiovascular interactions: an
interactive tutorial and mathematical model.Am J Physiol
(Adv Physiol Educ) 26:98, 2002.
Safar ME, Levy BI, Struijker-Boudier H: Current perspec-
tives on arterial stiffness and pulse pressure in hyperten-
sion and cardiov

s and pulse pressure in hyperten-
sion and cardiovascular diseases. Circulation 107:2864,
2003.
Verdecchia P, Angeli F, Gattobigio R: Clinical usefulness of
ambulatory blood pressure monitoring. J Am Soc Nephrol
15(Suppl 1):S30, 2004.
Pulp
Vein
Artery
Venous sinuses
Figure 15–13
Functional structures of the spleen. (Courtesy of Dr. Don W.
Fawcett, Montana.)
C
H
A
P
T
E
R
1
6
181
The Microcirculation and the
Lymphatic System: Capillary 
Fluid Exchange, Interstitial Fluid,
and Lymph Fl

The most purposeful function of the circulation
occurs in the microcirculation: this is transport of
nutrients to the tissues and removal of cell excreta.
The small arterioles control blood ﬂow to each
tissue area, and local conditions in the tissues in turn
control the diameters of the arterioles. Thus, each
tissue, in most instances, controls its own blood ﬂow
in relation to its individual needs, a subject that is
discussed in detail in Chap

eds, a subject that is
discussed in detail in Chapter 17.
The walls of the capillaries are extremely thin, constructed of single-layer,
highly permeable endothelial cells. Therefore, water, cell nutrients, and cell
excreta can all interchange quickly and easily between the tissues and the cir-
culating blood.
The peripheral circulation of the whole body has about 10 billion capillaries
with a total surface area estimated to be 500 to 700 square meters (about one-
eighth the surface area of a foo

eters (about one-
eighth the surface area of a football ﬁeld). Indeed, it is rare that any single func-
tional cell of the body is more than 20 to 30 micrometers away from a capillary.
Structure of the Microcirculation and
The microcirculation of each organ is organized speciﬁcally to serve that organ’s
needs. In general, each nutrient artery entering an organ branches six to eight times
before the arteries become small enough to be called arterioles, which generally
have intern

 be called arterioles, which generally
have internal diameters of only 10 to 15 micrometers.Then the arterioles themselves
branch two to ﬁve times, reaching diameters of 5 to 9 micrometers at their ends
where they supply blood to the capillaries.
The arterioles are highly muscular, and their diameters can change manyfold.The
metarterioles (the terminal arterioles) do not have a continuous muscular coat, but
smooth muscle ﬁbers encircle the vessel at intermittent points, as shown in Figure
16–1 b

 at intermittent points, as shown in Figure
16–1 by the black dots on the sides of the metarteriole.
At the point where each true capillary originates from a metarteriole, a smooth
muscle ﬁber usually encircles the capillary. This is called the precapillary sphincter.
This sphincter can open and close the entrance to the capillary.
The venules are larger than the arterioles and have a much weaker muscular coat.
Yet it must be remembered that the pressure in the venules is much less than that
in 

pressure in the venules is much less than that
in the arterioles, so that the venules still can contract considerably despite the weak
muscle.
This typical arrangement of the capillary bed is not found in all parts of the body;
however, some similar arrangement serves the same purposes. Most important, the
metarterioles and the precapillary sphincters are in close contact with the tissues
they serve. Therefore, the local conditions of the tissues—the concentrations of
nutrients, end products of 

—the concentrations of
nutrients, end products of metabolism, hydrogen ions, and so forth—can cause direct
effects on the vessels in controlling local blood ﬂow in each small tissue area.
Structure of the Capillary Wall. Figure 16–2 shows the ultramicroscopic structure of
typical endothelial cells in the capillary wall as found in most organs of the body,
especially in muscles and connective tissue. Note that the wall is composed of
182
Unit IV
a unicellular layer of endothelial 

he Circulation
a unicellular layer of endothelial cells and is sur-
rounded by a very thin basement membrane on the
outside of the capillary. The total thickness of the cap-
illary wall is only about 0.5 micrometer. The internal
diameter of the capillary is 4 to 9 micrometers, barely
large enough for red blood cells and other blood cells
to squeeze through.
“Pores” in the Capillary Membrane. Studying Figure 16–2,
one sees two very small passageways connecting the
interior of the capillary with t

ys connecting the
interior of the capillary with the exterior. One of these
is an intercellular cleft, which is the thin-slit, curving
channel that lies at the bottom of the ﬁgure between
adjacent endothelial cells. Each cleft is interrupted
periodically by short ridges of protein attachments
that hold the endothelial cells together, but between
these ridges ﬂuid can percolate freely through the
cleft. The cleft normally has a uniform spacing with a
width of about 6 to 7 nanometers (60 to 70 ang

h a
width of about 6 to 7 nanometers (60 to 70 angstroms),
slightly smaller than the diameter of an albumin
Because the intercellular clefts are located only at
the edges of the endothelial cells, they usually repre-
sent no more than 1/1000 of the total surface area of
the capillary wall. Nevertheless, the rate of thermal
motion of water molecules as well as most water-
soluble ions and small solutes is so rapid that all of
these diffuse with ease between the interior and exte

se diffuse with ease between the interior and exte-
rior of the capillaries through these “slit-pores,” the
intercellular clefts.
Also present in the endothelial cells are many
minute plasmalemmal vesicles. These form at one
surface of the cell by imbibing small packets of plasma
or extracellular ﬂuid. They can then move slowly
through the endothelial cell. It also has been postu-
lated that some of these vesicles coalesce to form
vesicular channels all the way through the endothelial
cell, whic

els all the way through the endothelial
cell, which is demonstrated to the right in Figure 16–2.
However, careful measurements in laboratory animals
probably have proved that these vesicular forms of
transport are quantitatively of little importance.
Special Types of “Pores” Occur in the Capillaries of Certain
Organs. The “pores” in the capillaries of some organs
have special characteristics to meet the peculiar needs
of the organs. Some of these characteristics are as
follows:
1. In the brain, 

 characteristics are as
follows:
1. In the brain, the junctions between the capillary
endothelial cells are mainly “tight” junctions that
allow only extremely small molecules such as
water, oxygen, and carbon dioxide to pass into or
out of the brain tissues.
2. In the liver, the opposite is true. The clefts between
the capillary endothelial cells are wide open, so
that almost all dissolved substances of the plasma,
including the plasma proteins, can pass from the
blood into the liver tissues.
3.

can pass from the
3. The pores of the gastrointestinal capillary
membranes are midway between those of the
muscles and those of the liver.
4. In the glomerular tufts of the kidney, numerous
small oval windows called fenestrae penetrate all
the way through the middle of the endothelial 
cells, so that tremendous amounts of very small
molecular and ionic substances (but not the large
molecules of the plasma proteins) can ﬁlter through
the glomeruli without having to p

an ﬁlter through
the glomeruli without having to pass through the
clefts between the endothelial cells.
Flow of Blood in the
Capillaries—Vasomotion
Blood usually does not ﬂow continuously through the
capillaries. Instead, it ﬂows intermittently, turning on
and off every few seconds or minutes.The cause of this
intermittency is the phenomenon called vasomotion,
which means intermittent contraction of the metarte-
rioles and precapillary sphincters (and sometimes
even the very small arterioles as 

 (and sometimes
even the very small arterioles as well).
Regulation of Vasomotion. The most important factor
found thus far to affect the degree of opening and
closing of the metarterioles and precapillary sphinc-
ters is the concentration of oxygen in the tissues.When
Preferential channel
Metarteriole
Arteriole
Venule
Precapillary
sphincter
True capillaries
Figure 16–1
Structure of the mesenteric capillary bed. (Redrawn from Zweifach
BW: Factors Regulating Blood Pressure. New York: Josiah Macy,

 Regulating Blood Pressure. New York: Josiah Macy,
Jr., Foundation, 1950.)
Endothelial
cell
Plasmalemmal
vesicles
Vesicular
channel??
Basement
membrane
Intercellular
cleft
Figure 16–2
Structure of the capillary wall. Note especially the intercellular cleft
at the junction between adjacent endothelial cells; it is believed
that most water-soluble substances diffuse through the capillary
membrane along the clefts.
Chapter 16
The Microcirculation and the Lymphatic System
183
the rate of oxygen usag

d the Lymphatic System
183
the rate of oxygen usage by the tissue is great so that
tissue oxygen concentration decreases below normal,
the intermittent periods of capillary blood ﬂow occur
more often, and the duration of each period of ﬂow
lasts longer, thereby allowing the capillary blood to
carry increased quantities of oxygen (as well as other
nutrients) to the tissues. This effect, along with multi-
ple other factors that control local tissue blood ﬂow, is
discussed in Chapter 17.
Average Fu

 blood ﬂow, is
Average Function of the 
Despite the fact that blood ﬂow through each capillary
is intermittent, so many capillaries are present in the
tissues that their overall function becomes averaged.
That is, there is an average rate of blood ﬂow through
each tissue capillary bed, an average capillary pressure
within the capillaries, and an average rate of transfer of
substances between the blood of the capillaries and the
surrounding interstitial ﬂ

the capillaries and the
surrounding interstitial ﬂuid. In the remainder of this
chapter, we will be concerned with these averages,
although one must remember that the average func-
tions are, in reality, the functions of literally billions of
individual capillaries, each operating intermittently in
response to local conditions in the tissues.
Exchange of Water, Nutrients,
and Other Substances
Between the Blood and
Interstitial Fluid
Diffusion Through the 
By far the most impor

ough the 
By far the most important means by which substances
are transferred between the plasma and the interstitial
ﬂuid is diffusion. Figure 16–3 demonstrates this
process, showing that as the blood ﬂows along the
lumen of the capillary, tremendous numbers of water
molecules and dissolved particles diffuse back and
forth through the capillary wall, providing continual
mixing between the interstitial ﬂuid and the plasma.
Diffusion results from thermal motion of the water 
mo

usion results from thermal motion of the water 
molecules and dissolved substances in the ﬂuid, the 
different molecules and ions moving ﬁrst in one direc-
tion and then another, bouncing randomly in every
direction.
Lipid-Soluble Substances Can Diffuse Directly Through the Cell
Membranes of the Capillary Endothelium. If a substance is
lipid soluble, it can diffuse directly through the cell
membranes of the capillary without having to go
through the pores. Such substances include oxygen and
carb

the pores. Such substances include oxygen and
carbon dioxide. Because these substances can per-
meate all areas of the capillary membrane, their rates
of transport through the capillary membrane are 
many times faster than the rates for lipid-insoluble
substances, such as sodium ions and glucose that can
go only through the pores.
Water-Soluble, Non-Lipid-Soluble Substances Diffuse Only
Through Intercellular “Pores” in the Capillary Membrane.
Many substances needed by the tissues are soluble in

y substances needed by the tissues are soluble in
water but cannot pass through the lipid membranes of
the endothelial cells; such substances include water
molecules themselves, sodium ions, chloride ions, and
glucose. Despite the fact that not more than 1/1000 of
the surface area of the capillaries is represented by the
intercellular clefts between the endothelial cells, the
velocity of thermal molecular motion in the clefts is so
great that even this small area is sufﬁcient to allow
tremendous

n this small area is sufﬁcient to allow
tremendous diffusion of water and water-soluble sub-
stances through these cleft-pores. To give one an idea
of the rapidity with which these substances diffuse, the
rate at which water molecules diffuse through the cap-
illary membrane is about 80 times as great as the rate at
which plasma itself ﬂows linearly along the capillary.
That is, the water of the plasma is exchanged with 
the water of the interstitial ﬂuid 80 times before the
plasma can ﬂow the e

tial ﬂuid 80 times before the
plasma can ﬂow the entire distance through the 
capillary.
Effect of Molecular Size on Passage Through the
Pores. The width of the capillary intercellular cleft-
pores, 6 to 7 nanometers, is about 20 times the diame-
ter of the water molecule, which is the smallest
molecule that normally passes through the capillary
pores. Conversely, the diameters of plasma protein
molecules are slightly greater than the width of the
pores. Other substances, such as sodium ions, ch

e
pores. Other substances, such as sodium ions, chloride
ions, glucose, and urea, have intermediate diameters.
Therefore, the permeability of the capillary pores for
Arterial end
Venous end
Blood capillary
Lymphatic
capillary
Figure 16–3
Diffusion of ﬂuid molecules and dissolved substances between
the capillary and interstitial ﬂuid spaces.
184
Unit IV
different substances varies according to their molecu-
lar diameters.
Table 16–1 gives the relative permeabilities of the
capilla

1 gives the relative permeabilities of the
capillary pores in skeletal muscle for substances com-
monly encountered, demonstrating, for instance, that
the permeability for glucose molecules is 0.6 times that
for water molecules, whereas the permeability for
albumin molecules is very, very slight, only 1/1000 that
for water molecules.
A word of caution must be issued at this point.
The capillaries in different tissues have extreme dif-
ferences in their permeabilities. For instance, the mem-
bran

 their permeabilities. For instance, the mem-
brane of the liver capillary sinusoids is so permeable
that even plasma proteins pass freely through these
walls, almost as easily as water and other substances.
Also, the permeability of the renal glomerular mem-
brane for water and electrolytes is about 500 times the
permeability of the muscle capillaries, but this is not
true for the plasma proteins; for these, the capillary
permeabilities are very slight, as in other tissues and
organs. When we s

 slight, as in other tissues and
organs. When we study these different organs later in
this text, it should become clear why some tissues—the
liver,for instance—require greater degrees of capillary
permeability than others to transfer tremendous
amounts of nutrients between the blood and liver
parenchymal cells, and the kidneys to allow ﬁltration
of large quantities of ﬂuid for formation of urine.
Effect of Concentration Difference on Net Rate of Diffusion
Through the Capillary Membrane. The “ne

 Diffusion
Through the Capillary Membrane. The “net” rate of diffu-
sion of a substance through any membrane is propor-
tional to the concentration difference of the substance
between the two sides of the membrane. That is, the
greater the difference between the concentrations of
any given substance on the two sides of the capillary
membrane, the greater the net movement of the sub-
stance in one direction through the membrane. For
instance, the concentration of oxygen in capillary
blood is norm

concentration of oxygen in capillary
blood is normally greater than in the interstitial ﬂuid.
Therefore, large quantities of oxygen normally move
from the blood toward the tissues. Conversely, the con-
centration of carbon dioxide is greater in the tissues
than in the blood, which causes excess carbon dioxide
to move into the blood and to be carried away from
the tissues.
The rates of diffusion through the capillary mem-
branes of most nutritionally important substances are
so great that only sl

lly important substances are
so great that only slight concentration differences
sufﬁce to cause more than adequate transport between
the plasma and interstitial ﬂuid. For instance, the con-
centration of oxygen in the interstitial ﬂuid immedi-
ately outside the capillary is no more than a few per
cent less than its concentration in the plasma of the
blood, yet this slight difference causes enough oxygen
to move from the blood into the interstitial spaces to
provide all the oxygen required for t

al spaces to
provide all the oxygen required for tissue metabolism,
often as much as several liters of oxygen per minute
during very active states of the body.
The Interstitium and 
About one sixth of the total volume of the body con-
sists of spaces between cells, which collectively are
called the interstitium. The ﬂuid in these spaces is the
interstitial ﬂuid.
The structure of the interstitium is shown in Figure
16–4. It contains two major types of solid structures:
(1) coll

ains two major types of solid structures:
(1) collagen ﬁber bundles and (2) proteoglycan ﬁla-
ments. The collagen ﬁber bundles extend long dis-
tances in the interstitium. They are extremely strong
and therefore provide most of the tensional strength
of the tissues.The proteoglycan ﬁlaments, however, are
extremely thin coiled or twisted molecules composed
of about 98 per cent hyaluronic acid and 2 per cent
protein. These molecules are so thin that they can
never be seen with a light microscope a

t they can
never be seen with a light microscope and are difﬁcult
Table 16–1
Relative Permeability of Skeletal Muscle Capillary Pores to
Different-Sized Molecules
Substance
Molecular Weight
Permeability
Water
18
1.00
NaCl
58.5
0.96
Urea
60
0.8
Glucose
180
0.6
Sucrose
342
0.4
Inulin
5,000
0.2
Myoglobin
17,600
0.03
68,000
0.01
Albumin
69,000
0.001
Data from Pappenheimer JR: Passage of molecules through capillary walls.
Physiol Rev 33:387, 1953.
Free fluid
vesicles
Rivulets
of free
fluid

, 1953.
Free fluid
vesicles
Rivulets
of free
fluid
Proteoglycan
filaments
Collagen fiber
bundles
Capillary
Figure 16–4
Structure of the interstitium. Proteoglycan ﬁlaments are every-
where in the spaces between the collagen ﬁber bundles. Free ﬂuid
vesicles and small amounts of free ﬂuid in the form of rivulets
occasionally also occur.
Chapter 16
185
to demonstrate even with the electron microscope.
Nevertheless, they form a mat of very ﬁne reticular 

vertheless, they form a mat of very ﬁne reticular ﬁl-
aments aptly described as a “brush pile.”
“Gel” in the Interstitium. The ﬂuid in the interstitium is
derived by ﬁltration and diffusion from the capillaries.
It contains almost the same constituents as plasma
except for much lower concentrations of proteins
because proteins do not pass outward through the
pores of the capillaries with ease. The interstitial ﬂuid
is entrapped mainly in the minute spaces among the
proteoglycan ﬁlaments. This co

te spaces among the
proteoglycan ﬁlaments. This combination of proteo-
glycan ﬁlaments and ﬂuid entrapped within them has
the characteristics of a gel and therefore is called tissue
gel.
Because of the large number of proteoglycan ﬁla-
ments, it is difﬁcult for ﬂuid to ﬂow easily through the
tissue gel. Instead, it mainly diffuses through the gel;
that is, it moves molecule by molecule from one place
to another by kinetic, thermal motion rather than by
large numbers of molecules moving together.

han by
Diffusion through the gel occurs about 95 to 99 per
cent as rapidly as it does through free ﬂuid. For the
short distances between the capillaries and the tissue
cells, this diffusion allows rapid transport through the
interstitium not only of water molecules but also of
electrolytes, small molecular weight nutrients, cellular
excreta, oxygen, carbon dioxide, and so forth.
“Free” Fluid in the Interstitium. Although almost all the
ﬂuid in the inte

rstitium. Although almost all the
ﬂuid in the interstitium normally is entrapped within
the tissue gel, occasionally small rivulets of “free” ﬂuid
and small free ﬂuid vesicles are also present, which
means ﬂuid that is free of the proteoglycan molecules
and therefore can ﬂow freely. When a dye is injected
into the circulating blood, it often can be seen to ﬂow
through the interstitium in the small rivulets, usually
coursing along the surfaces of collagen ﬁbers or sur-
faces of cells.
The amount 

collagen ﬁbers or sur-
The amount of “free” ﬂuid present in normal tissues
is slight, usually much less than 1 per cent. Conversely,
when the tissues develop edema, these small pockets
and rivulets of free ﬂuid expand tremendously until one
half or more of the edema ﬂuid becomes freely ﬂowing
ﬂuid independent of the proteoglycan ﬁlaments.
Fluid Filtration Across
Capillaries Is Determined 
by Hydrostatic and Colloid
Osmotic Pressures, and
The hydros

es, and
The hydrostatic pressure in the capillaries tends to
force ﬂuid and its dissolved substances through the
capillary pores into the interstitial spaces. Conversely,
osmotic pressure caused by the plasma proteins
(called colloid osmotic pressure) tends to cause ﬂuid
movement by osmosis from the interstitial spaces 
into the blood. This osmotic pressure exerted by 
the plasma proteins normally prevents signiﬁcant loss
of ﬂuid volume from the blood into the int

nt loss
of ﬂuid volume from the blood into the interstitial
spaces.
Also important is the lymphatic system, which
returns to the circulation the small amounts of excess
protein and ﬂuid that leak from the blood into the
interstitial spaces. In the remainder of this chapter, we
discuss the mechanisms that control capillary ﬁltration
and lymph ﬂow function together to regulate the
respective volumes of the plasma and the interstitial
ﬂuid.
Four Primary Hydrostatic and Colloid Osmotic Forces Determ

mary Hydrostatic and Colloid Osmotic Forces Determine
Fluid Movement Through the Capillary Membrane. Figure
16–5 shows the four primary forces that determine
whether ﬂuid will move out of the blood into the inter-
stitial ﬂuid or in the opposite direction. These forces,
called “Starling forces” in honor of the physiologist
who ﬁrst demonstrated their importance, are:
1. The capillary pressure (Pc), which tends to force
ﬂuid outward through the capillary membrane.
2. The interstitial ﬂuid pressur

pillary membrane.
2. The interstitial ﬂuid pressure (Pif), which tends 
to force ﬂuid inward through the capillary
membrane when Pif is positive but outward when
Pif is negative.
3. The capillary plasma colloid osmotic pressure
(Pp), which tends to cause osmosis of ﬂuid inward
through the capillary membrane.
4. The interstitial ﬂuid colloid osmotic pressure (Pif),
which tends to cause osmosis of ﬂuid outward
If the sum of these forces, the net ﬁltration pressure,

 sum of these forces, the net ﬁltration pressure,
is positive, there will be a net ﬂuid ﬁltration across the
capillaries. If the sum of the Starling forces is negative,
there will be a net ﬂuid absorption from the intersti-
tial spaces into the capillaries. The net ﬁltration pres-
sure (NFP) is calculated as:
As discussed later, the NFP is slightly positive under
normal conditions, resulting in a net ﬁltration of ﬂuid
across the capillaries into the interstitial space in most
organs. The rate of

the interstitial space in most
organs. The rate of ﬂuid ﬁltration in a tissue is also
determined by the number and size of the pores in
each capillary as well as the number of capillaries in
NFP
Pc
Pif
p
if
=
-
-
+
P
P
Capillary
pressure
(Pc)
Plasma colloid
osmotic pressure
(Pp)
fluid pressure
(Pif)
Interstitial fluid
colloid osmotic pressure
(Pif)
Figure 16–5
Fluid pressure and colloid osmotic pressure forces operate at the
capillary membrane, tending to move ﬂuid either outward or

y membrane, tending to move ﬂuid either outward or
inward through the membrane pores.
186
Unit IV
which blood is ﬂowing. These factors are usually
expressed together as the capillary ﬁltration coefﬁcient
(Kf). The Kf is therefore a measure of the capacity of
the capillary membranes to ﬁlter water for a given
NFP and is usually expressed as ml/min per mm Hg
net ﬁltration pressure.
The rate of capillary ﬂuid ﬁltration is therefore
determined as:
In the following sections we discuss

etermined as:
In the following sections we discuss in detail each 
of the forces that determine the rate of capillary ﬂuid
ﬁltration.
Two experimental methods have been used to esti-
mate the capillary hydrostatic pressure: (1) direct
micropipette cannulation of the capillaries, which has
given an average mean capillary pressure of about 25
mm Hg, and (2) indirect functional measurement of the
capillary pressure, which has given a capillary pressure
averaging about

ich has given a capillary pressure
averaging about 17 mm Hg.
Micropipette Method for Measuring Capillary Pressure. To
measure pressure in a capillary by cannulation, a
microscopic glass pipette is thrust directly into the cap-
illary, and the pressure is measured by an appropriate
micromanometer system. Using this method, capillary
pressures have been measured in capillaries of
exposed tissues of animals and in large capillary loops
of the eponychium at the base of the ﬁngernail in
humans. These

hium at the base of the ﬁngernail in
humans. These measurements have given pressures of
30 to 40 mm Hg in the arterial ends of the capillaries,
10 to 15 mm Hg in the venous ends, and about 25 mm
Hg in the middle.
Isogravimetric Method for Indirectly Measuring “Functional”
Capillary Pressure. Figure 16–6 demonstrates an iso-
gravimetric method for indirectly estimating capillary
pressure. This ﬁgure shows a section of gut held up by
one arm of a gravimetric balance. Blood is perfused
through the 

ravimetric balance. Blood is perfused
through the blood vessels of the gut wall. When the
arterial pressure is decreased, the resulting decrease in
capillary pressure allows the osmotic pressure of the
plasma proteins to cause absorption of ﬂuid out of the
gut wall and makes the weight of the gut decrease.This
immediately causes displacement of the balance arm.
To prevent this weight decrease, the venous pressure
is increased an amount sufﬁcient to overcome the
effect of decreasing the arterial 

to overcome the
effect of decreasing the arterial pressure. In other
words, the capillary pressure is kept constant while
simultaneously (1) decreasing the arterial pressure
and (2) increasing the venous pressure.
In the graph in the lower part of the ﬁgure, the
changes in arterial and venous pressures that exactly
nullify all weight changes are shown. The arterial and
venous lines meet each other at a value of 17 mm Hg.
Therefore, the capillary pressure must have remained
at this same level of 

pressure must have remained
at this same level of 17 mm Hg throughout these
maneuvers; otherwise, either ﬁltration or absorption of
Filtration
K
NFP
f
=
¥
ﬂuid through the capillary walls would have occurred.
Thus, in a roundabout way, the “functional” capillary
pressure is measured to be about 17 mm Hg.
Why Is the Functional Capillary Pressure Lower than Capillary
Pressure Measured by the Micropipette Method? It is clear
that the aforementioned two methods do not give the
same capillary pressur

two methods do not give the
same capillary pressure. However, the isogravimetric
method determines the capillary pressure that exactly
balances all the forces tending to move ﬂuid into or
out of the capillaries. Because such a balance of forces
is the normal state, the average functional capillary
pressure must be close to the pressure measured by
the isogravimetric method. Therefore, one is justiﬁed
in believing that the true functional capillary pressure
averages about 17 mm Hg.
It is easy to 

y pressure
It is easy to explain why the cannulation methods
give higher pressure values. The most important
reason is that these measurements usually are made 
in capillaries whose arterial ends are open and when
Arterial pressure
Gut
100
80
60
Pressure
40
20
0100
50
Capillary pressure
= 17 mm Hg
Arterial
Venous
Arterial pressure    –    venous pressure
0
Venous pressure
Figure 16–6
Isogravimetric method for measuring capillary pressure.
Chapter 16
The Microcirculation 

pillary pressure.
Chapter 16
187
blood is actively ﬂowing into the capillary. However, it
should be recalled from the earlier discussion of cap-
illary vasomotion that the metarterioles and precapil-
lary sphincters normally are closed during a large part
of the vasomotion cycle. When closed, the pressure in
the capillaries beyond the closures should be almost
equal to the pressure at the venous ends of the capil-
laries, about 10 mm Hg. Therefore, w

of the capil-
laries, about 10 mm Hg. Therefore, when averaged
over a period of time, one would expect the functional
mean capillary pressure to be much nearer to the pres-
sure in the venous ends of the capillaries than to the
pressure in the arterial ends.
There are two other reasons why the functional cap-
illary pressure is less than the values measured by can-
nulation. First, there are far more capillaries nearer to
the venules than to the arterioles. Second, the venous
capillaries are sev

arterioles. Second, the venous
capillaries are several times as permeable as the arte-
rial capillaries. Both of these effects further decrease
the functional capillary pressure to a lower value.
As is true for the measurement of capillary pressure,
there are several methods for measuring interstitial
ﬂuid pressure, and each of these gives slightly differ-
ent values but usually values that are a few millime-
ters of mercury less than atmospheric pressure,

e-
ters of mercury less than atmospheric pressure, that is,
values called negative interstitial ﬂuid pressure. The
methods most widely used have been (1) direct can-
nulation of the tissues with a micropipette, (2) meas-
urement of the pressure from implanted perforated
capsules, and (3) measurement of the pressure from a
cotton wick inserted into the tissue.
Measurement 
of 
Interstitial 
Fluid 
Pressure 
Using 
the
Micropipette. The same type of micropipette used for
measuring capillary pressu

f micropipette used for
measuring capillary pressure can also be used in some
tissues for measuring interstitial ﬂuid pressure. The tip
of the micropipette is about 1 micrometer in diameter,
but even this is 20 or more times larger than the sizes
of the spaces between the proteoglycan ﬁlaments of the
interstitium. Therefore, the pressure that is measured is
probably the pressure in a free ﬂuid pocket.
The ﬁrst pressures measured using the micropipette
method ranged from -1 to +2 mm Hg but were u

pette
method ranged from -1 to +2 mm Hg but were usually
slightly positive. With experience and improved equip-
ment for making such measurements, more recent pres-
sures have averaged about -2 mm Hg, giving average
pressure values in loose tissues, such as skin, that are
slightly less than atmospheric pressure.
Measurement of Interstitial Free Fluid Pressure in Implanted Per-
forated Hollow Capsules. Interstitial free ﬂuid pressure
measured by this method when using 2-centimeter
diameter capsul

his method when using 2-centimeter
diameter capsules in normal loose subcutaneous tissue
averages about -6 mm Hg, but with smaller capsules,
the values are not greatly different from the -2 mm Hg
measured by the micropipette in Figure 16-7.
Measurement of Interstitial Free Fluid Pressure by Means of a
Cotton Wick.
Still another method is to insert into a
tissue a small Teﬂon tube with about eight cotton ﬁbers
protruding from its end.The cotton ﬁbers form a “wick”
that makes excellent contact wit

ers form a “wick”
that makes excellent contact with the tissue ﬂuids and
transmits interstitial ﬂuid pressure into the Teﬂon tube:
the pressure can then be measured from the tube by
usual manometric means. Pressures measured by this
technique in loose subcutaneous tissue also have been
negative, usually measuring -1 to -3 mm Hg.
Interstitial Fluid Pressures in Tightly 
Encased Tissues
Some tissues of the body are surrounded by tight
encasements, such as the cranial vault around the
brain, the st

such as the cranial vault around the
brain, the strong ﬁbrous capsule around the kidney, the
ﬁbrous sheaths around the muscles, and the sclera
around the eye. In most of these, regardless of the
method used for measurement, the interstitial ﬂuid
pressures are usually positive. However, these intersti-
tial ﬂuid pressures almost invariably are still less than
the pressures exerted on the outsides of the tissues by
their encasements. For instance, the cerebrospinal ﬂuid
pressure surrounding the br

the cerebrospinal ﬂuid
pressure surrounding the brain of an animal lying on
its side averages about +10 mm Hg, whereas the brain
interstitial ﬂuid pressure averages about +4 to +6 mm
Hg. In the kidneys, the capsular pressure surrounding
the kidney averages about +13 mm Hg, whereas the
reported renal interstitial ﬂuid pressures have averaged
about +6 mm Hg.
Thus, if one remembers that the
pressure exerted on the skin is atmospheric pressure,
which is considered to be zero pressure, one might for-

 is considered to be zero pressure, one might for-
mulate a general rule that the normal interstitial ﬂuid
pressure is usually several millimeters of mercury neg-
ative with respect to the pressure that surrounds each
tissue.
Is the True Interstitial Fluid Pressure in Loose
Subcutaneous Tissue Subatmospheric?
The concept that the interstitial ﬂuid pressure is sub-
atmospheric in many if not most tissues of the body
began with clinical observations that could not be
explained by the previously he

s that could not be
explained by the previously held concept that intersti-
tial ﬂuid pressure was always positive. Some of the per-
tinent observations are the following:
1. When a skin graft is placed on a concave surface
of the body, such as in an eye socket after
removal of the eye, before the skin becomes
attached to the sublying socket, ﬂuid tends to
collect underneath the graft. Also, the skin
attempts to shorten, with the result that it tends to
pull it away from the concavity. Neverthel

ends to
pull it away from the concavity. Nevertheless,
some negative force underneath the skin causes
absorption of the ﬂuid and usually literally pulls
the skin back into the concavity.
2. Less than 1 mm Hg of positive pressure is
required to inject tremendous volumes of ﬂuid
into loose subcutaneous tissues, such as beneath
the lower eyelid, in the axillary space, and in the
scrotum. Amounts of ﬂuid calculated to be more
than 100 times the amount of ﬂuid normally in the
interstitial space, when

t of ﬂuid normally in the
interstitial space, when injected into these areas,
cause no more than about 2 mm Hg of positive
pressure. The importance of these observations is
that they show that such tissues do not have
188
Unit IV
strong ﬁbers that can prevent the accumulation of
ﬂuid. Therefore, some other mechanism, such as a
negative ﬂuid pressure system, must be available
to prevent such ﬂuid accumulation.
3. In most natural cavities of the body where there 
is free ﬂuid in dy

vities of the body where there 
is free ﬂuid in dynamic equilibrium with the
surrounding interstitial ﬂuids, the pressures that
have been measured have been negative. Some of
these are the following:
Intrapleural space: -8 mm Hg
Joint synovial spaces: -4 to -6 mm Hg
Epidural space: -4 to -6 mm Hg
4. The implanted capsule for measuring the
interstitial ﬂuid pressure can be used to record
dynamic changes in this pressure. The changes are
approximately those that one would calculate to
occur (1) wh

ely those that one would calculate to
occur (1) when the arterial pressure is increased
or decreased, (2) when ﬂuid is injected into the
surrounding tissue space, or (3) when a highly
concentrated colloid osmotic agent is injected into
the blood to absorb ﬂuid from the tissue spaces. It
is not likely that these dynamic changes could be
recorded this accurately unless the capsule
pressure closely approximated the true interstitial
pressure.
Summary—An Average Value for Negative Interstitial Fluid

y—An Average Value for Negative Interstitial Fluid
Pressure in Loose Subcutaneous Tissue. Although the
aforementioned different methods give slightly differ-
ent values for interstitial ﬂuid pressure, there currently
is a general belief among most physiologists that the
true interstitial ﬂuid pressure in loose subcutaneous
tissue is slightly less subatmospheric, averaging about
-3 mm Hg.
Pumping by the Lymphatic System Is the
Basic Cause of the Negative Interstitial 
The lymphatic

egative Interstitial 
The lymphatic system is discussed later in the chapter,
but we need to understand here the basic role that this
system plays in determining interstitial ﬂuid pressure.
The lymphatic system is a “scavenger” system that
removes excess ﬂuid, excess protein molecules, debris,
and other matter from the tissue spaces. Normally,
when ﬂuid enters the terminal lymphatic capillaries,
the lymph vessel walls automatically contract for a few
seconds and pump the ﬂuid into

 contract for a few
seconds and pump the ﬂuid into the blood circulation.
This overall process creates the slight negative pres-
sure that has been measured for ﬂuid in the interstitial
spaces.
Proteins in the Plasma Cause Colloid Osmotic Pressure. In the
basic discussion of osmotic pressure in Chapter 4, it
was pointed out that only those molecules or ions that
fail to pass through the pores of a semiperme-
able membrane exert osmotic pressure. Because the
protei

embrane exert osmotic pressure. Because the
proteins are the only dissolved constituents in the
plasma and interstitial ﬂuids that do not readily pass
through the capillary pores, it is the proteins of the
plasma and interstitial ﬂuids that are responsible for
the osmotic pressures on the two sides of the capillary
membrane. To distinguish this osmotic pressure from
that which occurs at the cell membrane, it is called
either colloid osmotic pressure or oncotic pressure. The
term “colloid” osmoti

ure or oncotic pressure. The
term “colloid” osmotic pressure is derived from the
fact that a protein solution resembles a colloidal solu-
tion despite the fact that it is actually a true molecu-
lar solution.
Normal Values for Plasma Colloid Osmotic Pressure. The
colloid osmotic pressure of normal human plasma
averages about 28 mm Hg; 19 mm of this is caused by
molecular effects of the dissolved protein and 9 mm by
the Donnan effect—that is, extra osmotic pressure
caused by sodium, potassium, an

a osmotic pressure
caused by sodium, potassium, and the other cations
held in the plasma by the proteins.
Effect of the Different Plasma Proteins on Colloid Osmotic 
Pressure. The plasma proteins are a mixture that con-
tains albumin, with an average molecular weight of
69,000; globulins, 140,000; and ﬁbrinogen, 400,000.Thus,
1 gram of globulin contains only half as many molecules
as 1 gram of albumin, and 1 gram of ﬁbrinogen contains
only one sixth as many molecules as 1 gram of albumin.
It sho

xth as many molecules as 1 gram of albumin.
It should be recalled from the discussion of osmotic
pressure in Chapter 4 that osmotic pressure is deter-
mined by the number of molecules dissolved in a ﬂuid
rather than by the mass of these molecules. Therefore,
when corrected for number of molecules rather than
mass, the following chart gives both the relative mass
concentrations (g/dl) of the different types of proteins
in normal plasma and their respective contributions to
the total plasma colloi

espective contributions to
the total plasma colloid osmotic pressure (Pp).
g/dl
Pp (mm Hg)
Albumin
4.5
21.8
Globulins
2.5
6.0
Fibrinogen
0.3
0.2
Total
7.3
28.0
Thus, about 80 per cent of the total colloid osmotic
pressure of the plasma results from the albumin frac-
tion, 20 per cent from the globulins, and almost none
from the ﬁbrinogen. Therefore, from the point of view
of capillary and tissue ﬂuid dynamics, it is mainly
albumin that is important.
Interstitial Fluid Colloid 
A

nt.
Although the size of the usual capillary pore is smaller
than the molecular sizes of the plasma proteins, this is
not true of all the pores. Therefore, small amounts of
plasma proteins do leak through the pores into the
interstitial spaces.
The total quantity of protein in the entire 12 liters
of interstitial ﬂuid of the body is slightly greater than
the total quantity of protein in the plasma itself, but
Chapter 16
The Microcirculation and the Ly

lf, but
Chapter 16
189
because this volume is four times the volume of
plasma, the average protein concentration of the inter-
stitial ﬂuid is usually only 40 per cent of that in plasma,
or about 3 g/dl. Quantitatively, one ﬁnds that the
average interstitial ﬂuid colloid osmotic pressure for
this concentration of proteins is about 8 mm Hg.
Exchange of Fluid Volume Through
the Capillary Membrane
Now that the different factors affecting ﬂuid move-
ment

at the different factors affecting ﬂuid move-
ment through the capillary membrane have been 
discussed, it is possible to put all these together to 
see how the capillary system maintains normal ﬂuid
volume distribution between the plasma and the inter-
stitial ﬂuid.
The average capillary pressure at the arterial ends
of the capillaries is 15 to 25 mm Hg greater than at the
venous ends. Because of this difference, ﬂuid “ﬁlters”
out of the capillaries at their arterial ends, but at their
venous e

ries at their arterial ends, but at their
venous ends ﬂuid is reabsorbed back into the capillar-
ies. Thus, a small amount of ﬂuid actually “ﬂows”
through the tissues from the arterial ends of the cap-
illaries to the venous ends. The dynamics of this ﬂow
are as follows.
Analysis of the Forces Causing Filtration at the Arterial End of
the Capillary. The approximate average forces operative
at the arterial end of the capillary that cause movement
through the capillary membrane are shown as follow

through the capillary membrane are shown as follows:
Thus, the summation of forces at the arterial end of
the capillary shows a net ﬁltration pressure of 13 mm
Hg, tending to move ﬂuid outward through the capil-
lary pores.
This 13 mm Hg ﬁltration pressure causes, on the
average, about 1/200 of the plasma in the ﬂowing blood
to ﬁlter out of the arterial ends of the capillaries into
the interstitial spaces each time the blood passes
through the capillaries.
Analysis of Reabsorption at the Venous 

pillaries.
Analysis of Reabsorption at the Venous End of the Capillary.
The low blood pressure at the venous end of the cap-
illary changes the balance of forces in favor of absorp-
tion as follows:
Thus, the force that causes ﬂuid to move into the
capillary, 28 mm Hg, is greater than that opposing
reabsorption, 21 mm Hg. The difference, 7 mm Hg, is
the net reabsorption pressure at the venous ends of the
capillaries. This reabsorption pressure is considerably
less than the ﬁltration pressure at 

 considerably
less than the ﬁltration pressure at the capillary arterial
ends, but remember that the venous capillaries are
more numerous and more permeable than the arterial
capillaries, so that less reabsorption pressure is
required to cause inward movement of ﬂuid.
The reabsorption pressure causes about nine tenths
of the ﬂuid that has ﬁltered out of the arterial ends of
the capillaries to be reabsorbed at the venous ends.
The remaining one tenth ﬂows into the lymph vessels
and returns to the

nth ﬂows into the lymph vessels
and returns to the circulating blood.
Starling Equilibrium for 
Capillary Exchange
E. H. Starling pointed out over a century ago that
under normal conditions, a state of near-equilibrium
exists at the capillary membrane. That is, the amount
of ﬂuid ﬁltering outward from the arterial ends of cap-
illaries equals almost exactly the ﬂuid returned to the
circulation by absorption. The slight disequilibrium
that does occur accounts for the small amount of ﬂuid
that is 

cur accounts for the small amount of ﬂuid
that is eventually returned by way of the lymphatics.
The following chart shows the principles of the Star-
ling equilibrium.For this chart,the pressures in the arte-
rial and venous capillaries are averaged to calculate
mean functional capillary pressure for the entire length
of the capillary. This calculates to be 17.3 mm Hg.
mm Hg
Forces tending to move ﬂuid outward:
Capillary pressure (arterial end of capillary)
30
Negative interstitial free ﬂuid pre

 capillary)
30
Negative interstitial free ﬂuid pressure
3
Interstitial ﬂuid colloid osmotic pressure
8
total outward force
41
Forces tending to move ﬂuid inward:
Plasma colloid osmotic pressure
28
total inward force
28
Summation of forces:
Outward
41
Inward
28
net outward force (at arterial end)
13
mm Hg
28
28
Capillary pressure (venous end of capillary)
10
Negative interst

sure (venous end of capillary)
10
3
8
21
Inward
28
Outward
21
net inward force
7
mm Hg
Mean forces tending to move ﬂuid outward:
Mean capillary pressure
17.3
3.0
8.0
28.3
Mean force tending to move ﬂuid inward:
28.0
28.0

loid osmotic pressure
28.0
28.0
Summation of mean forces:
Outward
28.3
Inward
28.0
net outward force
0.3
190
Unit IV
Thus, for the total capillary circulation, we ﬁnd a
near-equilibrium between the total outward forces,
28.3 mm Hg, and the total inward force, 28.0 mm Hg.
This slight imbalance of forces, 0.3 mm Hg, causes
slightly more ﬁltration of ﬂuid into the interstitial
spaces than reabsorption.This slight excess of ﬁltration
is called net ﬁltration, and it

xcess of ﬁltration
is called net ﬁltration, and it is the ﬂuid that must be
returned to the circulation through the lymphatics.The
normal rate of net ﬁltration in the entire body is only
about 2 milliliters per minute.
Filtration Coefﬁcient. In the above example, an average
net imbalance of forces at the capillary membranes 
of 0.3 mm Hg causes net ﬂuid ﬁltration in the entire
body of 2 ml/min. Expressing this for each millimeter
of mercury imbalance, one ﬁnds a net ﬁltration rate 
of 6.67 milli

ance, one ﬁnds a net ﬁltration rate 
of 6.67 milliliters of ﬂuid per minute per millimeter of
mercury for the entire body. This is called the whole
body capillary ﬁltration coefﬁcient.
The ﬁltration coefﬁcient can also be expressed for
separate parts of the body in terms of rate of ﬁltration
per minute per millimeter of mercury per 100 grams
of tissue. On this basis, the ﬁltration coefﬁcient of the
average tissue is about 0.01 ml/min/mm Hg/100 g of
tissue. But, because of extreme differences in 

of
tissue. But, because of extreme differences in perme-
abilities of the capillary systems in different tissues,
this coefﬁcient varies more than 100-fold among the
different tissues. It is very small in both brain and
muscle, moderately large in subcutaneous tissue, large
in the intestine, and extreme in the liver and glomeru-
lus of the kidney where the pores are either numerous
or wide open. By the same token, the permeation 
of proteins through the capillary membranes varies
greatly as well

ugh the capillary membranes varies
greatly as well. The concentration of protein in the
interstitial ﬂuid of muscles is about 1.5 g/dl; in subcu-
taneous tissue, 2 g/dl; in intestine, 4 g/dl; and in liver,
6 g/dl.
Effect of Abnormal Imbalance of Forces 
at the Capillary Membrane
If the mean capillary pressure rises above 17 mm Hg,
the net force tending to cause ﬁltration of ﬂuid into the
tissue spaces rises.Thus, a 20 mm Hg rise in mean cap-
illary pressure causes an increase in net ﬁltration pr

ry pressure causes an increase in net ﬁltration pres-
sure from 0.3 mm Hg to 20.3 mm Hg, which results in
68 times as much net ﬁltration of ﬂuid into the inter-
stitial spaces as normally occurs. To prevent accumu-
lation of excess ﬂuid in these spaces would require 68
times the normal ﬂow of ﬂuid into the lymphatic
system, an amount that is 2 to 5 times too much for the
lymphatics to carry away. As a result, ﬂuid will begin
to accumulate in the interstitial spaces, and edema will
result.
Conver

interstitial spaces, and edema will
result.
Conversely, if the capillary pressure falls very low,
net reabsorption of ﬂuid into the capillaries will occur
instead of net ﬁltration, and the blood volume will
increase at the expense of the interstitial ﬂuid volume.
These effects of imbalance at the capillary membrane
in relation to the development of different kinds of
edema are discussed in Chapter 25.
The lymphatic system represents an accessory route
through which ﬂuid can ﬂow 

nts an accessory route
through which ﬂuid can ﬂow from the interstitial
spaces into the blood. Most important, the lymphatics
can carry proteins and large particulate matter away
from the tissue spaces, neither of which can be
removed by absorption directly into the blood capil-
laries. This return of proteins to the blood from the
interstitial spaces is an essential function without
which we would die within about 24 hours.
Almost all tissues of the body have special 

 Body
Almost all tissues of the body have special lymph chan-
nels that drain excess ﬂuid directly from the interstitial
spaces. The exceptions include the superﬁcial portions
of the skin, the central nervous system, the endomysium
of muscles, and the bones. But, even these tissues have
minute interstitial channels called prelymphatics
through which interstitial ﬂuid can ﬂow; this ﬂuid even-
tually empties either into lymphatic vessels or, in the
case of the brain, into the cerebrospinal ﬂuid an

case of the brain, into the cerebrospinal ﬂuid and then
directly back into the blood.
Essentially all the lymph vessels from the lower part
of the body eventually empty into the thoracic duct,
which in turn empties into the blood venous system at
the juncture of the left internal jugular vein and left sub-
clavian vein, as shown in Figure 16–8.
Lymph from the left side of the head, the left arm, and
parts of the chest region also enters the thoracic duct
before it empties into the veins.
Lymph 

acic duct
Lymph from the right side of the neck and head, the
right arm, and parts of the right thorax enters the right
lymph duct (much smaller than the thoracic duct), which
empties into the blood venous system at the juncture of
the right subclavian vein and internal jugular vein.
Terminal Lymphatic Capillaries and Their Permeability. Most
of the ﬂuid ﬁltering from the arterial ends of blood 
Fluid filled cavity
To
measure
pressure
Skin
Implanted
capsule
Bloo

ty
To
measure
pressure
Skin
Implanted
capsule
Blood
vessels
FIGURE 16–7
Perforated capsule method for measuring interstitial ﬂuid 
pressure.
Chapter 16
191
capillaries ﬂows among the cells and ﬁnally is reab-
sorbed back into the venous ends of the blood capil-
laries; but on the average, about 1/10 of the ﬂuid
instead enters the lymphatic capillaries and returns to
the blood through the lymphatic system rather than
through the venous capillaries.The

tem rather than
through the venous capillaries.The total quantity of all
this lymph is normally only 2 to 3 liters each day.
The ﬂuid that returns to the circulation by way of
the lymphatics is extremely important because sub-
stances of high molecular weight, such as proteins,
cannot be absorbed from the tissues in any other way,
although they can enter the lymphatic capillaries
almost unimpeded. The reason for this is a special
structure of the lymphatic capillaries, demonstrated in
Figure 16–

 lymphatic capillaries, demonstrated in
Figure 16–9. This ﬁgure shows the endothelial cells of
the lymphatic capillary attached by anchoring ﬁla-
ments to the surrounding connective tissue. At the
junctions of adjacent endothelial cells, the edge of one
endothelial cell overlaps the edge of the adjacent cell
in such a way that the overlapping edge is free to ﬂap
inward, thus forming a minute valve that opens to the
interior of the lymphatic capillary. Interstitial ﬂuid,
along with its suspended 

lary. Interstitial ﬂuid,
along with its suspended particles, can push the valve
open and ﬂow directly into the lymphatic capillary. But
this ﬂuid has difﬁculty leaving the capillary once it has
entered because any backﬂow closes the ﬂap valve.
Thus, the lymphatics have valves at the very tips of the
terminal lymphatic capillaries as well as valves along
their larger vessels up to the point where they empty
into the blood circulation.
Lymph is derived from interstitial ﬂuid tha

 Lymph
Lymph is derived from interstitial ﬂuid that ﬂows into
the lymphatics. Therefore, lymph as it ﬁrst enters the
terminal lymphatics has almost the same composition
as the interstitial ﬂuid.
Cervical nodes
Sentinel node
Subclavian vein
R. lymphatic duct
Thoracic duct
Axillary nodes
Cisterna chyli
Abdominal nodes
Inguinal nodes
Peripheral lymphatics
FIGURE 16–8
Lymphatic system.
192
Unit IV
The protein concentration in the interstitial ﬂuid of
most tissues averages about 2 g/d

rstitial ﬂuid of
most tissues averages about 2 g/dl,and the protein con-
centration of lymph ﬂowing from these tissues is near
this value. Conversely, lymph formed in the liver has a
protein concentration as high as 6 g/dl, and lymph
formed in the intestines has a protein concentration as
high as 3 to 4 g/dl. Because about two thirds of all
lymph normally is derived from the liver and intes-
tines, the thoracic duct lymph, which is a mixture of
lymph from all areas of the body, usually has a pro

ymph from all areas of the body, usually has a protein
concentration of 3 to 5 g/dl.
The lymphatic system is also one of the major routes
for absorption of nutrients from the gastrointestinal
tract, especially for absorption of virtually all fats in
food, as discussed in Chapter 65. Indeed, after a fatty
meal, thoracic duct lymph sometimes contains as much
as 1 to 2 per cent fat.
Finally, even large particles, such as bacteria, can
push their way between the endothelial cells of the
lymphatic ca

 between the endothelial cells of the
lymphatic capillaries and in this way enter the lymph.
As the lymph passes through the lymph nodes, these
particles are almost entirely removed and destroyed,
as discussed in Chapter 33.
About 100 milliliters per hour of lymph ﬂows through
the thoracic duct of a resting human, and approxi-
mately another 20 milliliters ﬂows into the circulation
each hour through other channels, making a total esti-
mated lymph ﬂow of about 120 ml/hr or 2 t

al esti-
mated lymph ﬂow of about 120 ml/hr or 2 to 3 liters
per day.
Effect of Interstitial Fluid Pressure on Lymph Flow. Figure
16–10 shows the effect of different levels of interstitial
ﬂuid pressure on lymph ﬂow as measured in dog legs.
Note that normal lymph ﬂow is very little at intersti-
tial ﬂuid pressures more negative than the normal
value of -6 mm Hg.Then, as the pressure rises to 0 mm
Hg (atmospheric pressure), ﬂow increases more than
20-fold.Therefore, any factor that increases inte

20-fold.Therefore, any factor that increases interstitial
ﬂuid pressure also increases lymph ﬂow if the lymph
vessels are functioning normally. Such factors include
∑Elevated capillary pressure
∑Decreased plasma colloid osmotic pressure
∑Increased interstitial ﬂuid colloid osmotic pressure
∑Increased permeability of the capillaries
All of these cause a balance of ﬂuid exchange at the
blood capillary membrane to favor ﬂuid movement
into the interstitium, thus increasing interstiti

into the interstitium, thus increasing interstitial ﬂuid
volume, interstitial ﬂuid pressure, and lymph ﬂow all
at the same time.
However, note in Figure 16–10 that when the inter-
stitial ﬂuid pressure becomes 1 or 2 millimeters greater
than atmospheric pressure (greater than 0 mm Hg),
lymph ﬂow fails to rise any further at still higher pres-
sures. This results from the fact that the increasing
tissue pressure not only increases entry of ﬂuid into
the lymphatic capillaries but also compresses 

nto
the lymphatic capillaries but also compresses the
outside surfaces of the larger lymphatics, thus imped-
ing lymph ﬂow. At the higher pressures, these two
factors balance each other almost exactly, so that
lymph ﬂow reaches what is called the “maximum
lymph ﬂow rate.” This is illustrated by the upper level
plateau in Figure 16–10.
Lymphatic Pump Increases Lymph Flow. Valves exist in all
lymph channels; typical valves are shown in Figure
Anchoring filaments
Valves
FIGURE 16–

helial cells
Valves
FIGURE 16–9
Special structure of the lymphatic capillaries that permits passage
of substances of high molecular weight into the lymph.
Relative lymph flow
0
2
4
2
0
–2
–4
–6
4
PT (mm Hg)
2 times/
mm Hg
7 times/
mm Hg
FIGURE 16–10
Relation between interstitial ﬂuid pressure and lymph ﬂow in the
leg of a dog. Note that lymph ﬂow reaches a maximum when the
interstitial pressure, PT, rises slightly above atmospheric pressure
(0 mm Hg). (Courtesy Drs. Harry Gib

heric pressure
(0 mm Hg). (Courtesy Drs. Harry Gibson and Aubrey Taylor.)
Chapter 16
193
16–11 in collecting lymphatics into which the lym-
phatic capillaries empty.
Motion pictures of exposed lymph vessels, both 
in animals and in human beings, show that when a 
collecting lymphatic or larger lymph vessel becomes
stretched with ﬂuid, the smooth muscle in the wall of
the vessel automatically contracts. Furthermore, each
segment of the lymph vessel be

. Furthermore, each
segment of the lymph vessel between successive valves
functions as a separate automatic pump. That is, even
slight ﬁlling of a segment causes it to contract, and the
ﬂuid is pumped through the next valve into the next
lymphatic segment. This ﬁlls the subsequent segment,
and a few seconds later it, too, contracts, the process
continuing all along the lymph vessel until the ﬂuid is
ﬁnally emptied into the blood circulation. In a very
large lymph vessel such as the thoracic duct

 very
large lymph vessel such as the thoracic duct, this lym-
phatic pump can generate pressures as great as 50 to
100 mm Hg.
Pumping Caused by External Intermittent Compression
of the Lymphatics. In addition to the pumping caused
by intrinsic intermittent contraction of the lymph
vessel walls, any external factor that intermittently
compresses the lymph vessel also can cause pumping.
In order of their importance, such factors are:
∑Contraction of surrounding skeletal muscles
∑Movement of the pa

f surrounding skeletal muscles
∑Movement of the parts of the body
∑Pulsations of arteries adjacent to the lymphatics
∑Compression of the tissues by objects outside the
body
The lymphatic pump becomes very active during exer-
cise, often increasing lymph ﬂow 10- to 30-fold. Con-
versely, during periods of rest, lymph ﬂow is sluggish,
almost zero.
Lymphatic Capillary Pump. The terminal lymphatic capil-
lary is also capable of pumping lymph, in addition to
the lymph pumping by the larger lymph vess

tion to
the lymph pumping by the larger lymph vessels.
As explained earlier in the chapter, the walls of the
lymphatic capillaries are tightly adherent to the sur-
rounding tissue cells by means of their anchoring 
ﬁlaments. Therefore, each time excess ﬂuid enters 
the tissue and causes the tissue to swell, the anchoring
ﬁlaments pull on the wall of the lymphatic capillary,
and ﬂuid ﬂows into the terminal lymphatic capillary
through the junctions between the endothelial cells.
Then, when the tis

 between the endothelial cells.
Then, when the tissue is compressed, the pressure
inside the capillary increases and causes the overlap-
ping edges of the endothelial cells to close like valves.
Therefore, the pressure pushes the lymph forward into
the collecting lymphatic instead of backward through
the cell junctions.
The lymphatic capillary endothelial cells also
contain a few contractile actomyosin ﬁlaments. In
some animal tissues (e.g., the bat’s wing) these ﬁla-
ments have been observed to

bat’s wing) these ﬁla-
ments have been observed to cause rhythmical con-
traction of the lymphatic capillaries in the same way
that many of the small blood and larger lymphatic
vessels also contract rhythmically. Therefore, it is 
probable that at least part of lymph pumping results
from lymph capillary endothelial cell contraction 
in addition to contraction of the larger muscular 
lymphatics.
Summary of Factors That Determine Lymph Flow. From the
above discussion, one can see that the two prim

he
above discussion, one can see that the two primary
factors that determine lymph ﬂow are (1) the intersti-
tial ﬂuid pressure and (2) the activity of the lymphatic
pump. Therefore, one can state that, roughly, the rate
of lymph ﬂow is determined by the product of intersti-
tial ﬂuid pressure times the activity of the lymphatic
pump.
Role of the Lymphatic System in
Controlling Interstitial Fluid Protein
Concentration, Interstitial Fluid
Volume, and Interstitial Fluid Pressure
It is already clea

and Interstitial Fluid Pressure
It is already clear that the lymphatic system functions
as an “overﬂow mechanism” to return to the circula-
tion excess proteins and excess ﬂuid volume from the
tissue spaces. Therefore, the lymphatic system also
plays a central role in controlling (1) the concentration
of proteins in the interstitial ﬂuids, (2) the volume of
interstitial ﬂuid, and (3) the interstitial ﬂuid pressure.
Let us explain how these factors interact.
First, remember that small amounts of 

s interact.
First, remember that small amounts of proteins 
leak continuously out of the blood capillaries into the
Pores
Lymphatic
capillaries
Collecting
lymphatic
Valves
FIGURE 16–11
Structure of lymphatic capillaries
and 
a 
collecting 
lymphatic,
showing 
also 
the 
lymphatic
valves.
194
Unit IV
interstitium.Only minute amounts,if any,of the leaked
proteins return to the circulation by way of the venous
ends of the blood capillaries.Therefore, these proteins
tend to accumulat

laries.Therefore, these proteins
tend to accumulate in the interstitial ﬂuid, and this in
turn increases the colloid osmotic pressure of the
interstitial ﬂuids.
Second, the increasing colloid osmotic pressure in
the interstitial ﬂuid shifts the balance of forces at the
blood capillary membranes in favor of ﬂuid ﬁltration
into the interstitium. Therefore, in effect, ﬂuid is
translocated osmotically outward through the capil-
lary wall by the proteins and into the interstitium, thus
increasing bot

ins and into the interstitium, thus
increasing both interstitial ﬂuid volume and interstitial
ﬂuid pressure.
Third, the increasing interstitial ﬂuid pressure
greatly increases the rate of lymph ﬂow, as explained
previously. This in turn carries away the excess inter-
stitial ﬂuid volume and excess protein that has accu-
mulated in the spaces.
Thus, once the interstitial ﬂuid protein concentra-
tion reaches a certain level and causes a comparable
increase in interstitial ﬂuid volume and interstit

increase in interstitial ﬂuid volume and interstitial
ﬂuid pressure, the return of protein and ﬂuid by way
of the lymphatic system becomes great enough to
balance exactly the rate of leakage of these into the
interstitium from the blood capillaries. Therefore, the
quantitative values of all these factors reach a steady
state; they will remain balanced at these steady state
levels until something changes the rate of leakage of
proteins and ﬂuid from the blood capillaries.
Signiﬁcance of Negative 

om the blood capillaries.
Signiﬁcance of Negative Interstitial Fluid
Pressure as a Means for Holding the 
Body Tissues Together
Traditionally, it has been assumed that the different
tissues of the body are held together entirely by 
connective tissue ﬁbers. However, at many places in
the body, connective tissue ﬁbers are very weak or
even absent. This occurs particularly at points where
tissues slide over one another, such as the skin sliding
over the back of the hand or over the face. Yet even

r the back of the hand or over the face. Yet even
at these places, the tissues are held together by the
negative interstitial ﬂuid pressure, which is actually a
partial vacuum. When the tissues lose their negative
pressure, ﬂuid accumulates in the spaces and the 
condition known as edema occurs, which is discussed
in Chapter 25.
Aukland K, Reed RK: Interstitial-lymphatic mechanisms in
the control of extracellular ﬂuid volume. Physiol Rev 73:1,
1993.
D’Amico G, Bazzi C: Pathophysiology

ev 73:1,
1993.
D’Amico G, Bazzi C: Pathophysiology of proteinuria.
Kidney Int 63:809, 2003.
Dejana E: Endothelial cell-cell junctions: happy together.
Nat Rev Mol Cell Biol 5:261, 2004.
Frank PG, Woodman SE, Park DS, Lisanti MP: Caveolin,
caveolae, and endothelial cell function. Arterioscler
Thromb Vasc Biol 23:1161, 2003.
Gashev AA: Physiologic aspects of lymphatic contractile
function: current perspectives. Ann N Y Acad Sci 979:178,
2002.
Guyton AC: Concept of negative interstitial pressure ba

n AC: Concept of negative interstitial pressure based
on pressures in implanted perforated capsules. Circ Res
12:399, 1963.
Guyton AC: Interstitial ﬂuid pressure: II. Pressure-volume
curves of interstitial space. Circ Res 16:452, 1965.
Guyton AC, Granger HJ, Taylor AE: Interstitial ﬂuid pres-
sure. Physiol Rev 51:527, 1971.
Guyton AC, Prather J, Scheel K, McGehee J: Interstitial ﬂuid
pressure: IV. Its effect on ﬂuid movement through the 
capillary wall. Circ Res 19:1022, 1966.
Guyton AC, Scheel 

y wall. Circ Res 19:1022, 1966.
Guyton AC, Scheel K, Murphree D: Interstitial ﬂuid pres-
sure: III. Its effect on resistance to tissue ﬂuid mobility.
Circ Res 19:412, 1966.
Guyton AC,Taylor AE, Granger HJ: Circulatory Physiology
II. Dynamics and Control of the Body Fluids. Philadel-
phia: WB Saunders Co, 1975.
Michel CC, Curry FE: Microvascular permeability. Physiol
Rev 79:703, 1999.
Miyasaka M, Tanaka T: Lymphocyte trafﬁcking across high
endothelial venules: dogmas and enigmas. Nat Rev
Immunol 

lial venules: dogmas and enigmas. Nat Rev
Immunol 4:360, 2004.
Oliver G: Lymphatic vasculature development. Nat Rev
Immunol 4:35, 2004.
Rippe B,
Rosengren BI,
Carlsson O,
Venturoli D:
Transendothelial transport: the vesicle controversy. J Vasc
Res 39:375, 2002.
Taylor AE, Granger DN: Exchange of macromolecules
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(eds): Handbook of Physiology. Sec. 2, Vol. IV. Bethesda:
American Physiological Society, 1984, p 467.
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E
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1
7
195
Local a

iety, 1984, p 467.
C
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1
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195
Local and Humoral Control of
Blood Flow by the Tissues
Local Control of Blood 
Flow in Response to 
Tissue Needs
One of the most fundamental principles of circula-
tory function is the ability of each tissue to control
its own local blood ﬂow in proportion to its meta-
bolic needs.
What are some of the speciﬁc needs of the tissues for blood ﬂow? The answer
to this is manyfold, including the following:
1. Delivery of oxygen to the tissues
2. Delivery of 

 Delivery of oxygen to the tissues
2. Delivery of other nutrients, such as glucose, amino acids, and fatty acids
3. Removal of carbon dioxide from the tissues
4. Removal of hydrogen ions from the tissues
5. Maintenance of proper concentrations of other ions in the tissues
6. Transport of various hormones and other substances to the different tissues
Certain organs have special requirements. For instance, blood ﬂow to the skin
determines heat loss from the body and in this way helps to control bo

 from the body and in this way helps to control body tem-
perature. Also, delivery of adequate quantities of blood plasma to the kidneys
allows the kidneys to excrete the waste products of the body.
We shall see that most of these factors exert extreme degrees of local blood
ﬂow control.
Variations in Blood Flow in Different Tissues and Organs. Note in Table 17–1 the very
large blood ﬂows in some organs—for example, several hundred milliliters per
minute per 100 grams of thyroid or adrenal gland

r
minute per 100 grams of thyroid or adrenal gland tissue and a total blood ﬂow
of 1350 ml/min in the liver, which is 95 ml/min/100 g of liver tissue.
Also note the extremely large blood ﬂow through the kidneys—1100 ml/min.
This extreme amount of ﬂow is required for the kidneys to perform their func-
tion of cleansing the blood of waste products.
Conversely, most surprising is the low blood ﬂow to all the inactive muscles
of the body, only a total of 750 ml/min, even though the muscles constitut

l of 750 ml/min, even though the muscles constitute
between 30 and 40 per cent of the total body mass. In the resting state, the meta-
bolic activity of the muscles is very low, and so also is the blood ﬂow, only 
4 ml/min/100 g. Yet, during heavy exercise, muscle metabolic activity can
increase more than 60-fold and the blood ﬂow as much as 20-fold, increasing to
as high as 16,000 ml/min in the body’s total muscle vascular bed (or 80 ml/
min/100 g of muscle).
Importance of Blood Flow Control by

 g of muscle).
Importance of Blood Flow Control by the Local Tissues. One might ask the simple ques-
tion: Why not simply allow a very large blood ﬂow all the time through every
tissue of the body, always enough to supply the tissue’s needs whether the activ-
ity of the tissue is little or great? The answer is equally simple:To do this would
require many times more blood ﬂow than the heart can pump.
Experiments have shown that the blood ﬂow to each tissue usually is regu-
lated at the minimal le

ch tissue usually is regu-
lated at the minimal level that will supply the tissue’s requirements— no more,
no less. For instance, in tissues for which the most important requirement is
delivery of oxygen, the blood ﬂow is always controlled at a level only slightly
more than required to maintain full tissue oxygenation but no more than this.
196
Unit IV
By controlling local blood ﬂow in such an exact way,
the tissues almost never suffer from oxygen nutritional
deﬁciency, and yet t

uffer from oxygen nutritional
deﬁciency, and yet the workload on the heart is kept
at a minimum.
Mechanisms of Blood 
Flow Control
Local blood ﬂow control can be divided into two
phases: (1) acute control and (2) long-term control.
Acute control is achieved by rapid changes in local
vasodilation or vasoconstriction of the arterioles,
metarterioles, and precapillary sphincters, occurring
within seconds to minutes to provide very rapid main-
tenance of appropriate local tissue blood ﬂow.
Long-term

e of appropriate local tissue blood ﬂow.
Long-term control, however, means slow, controlled
changes in ﬂow over a period of days, weeks, or even
months. In general, these long-term changes provide
even better control of the ﬂow in proportion to the
needs of the tissues. These changes come about as a
result of an increase or decrease in the physical sizes
and numbers of actual blood vessels supplying the
tissues.
Effect of Tissue Metabolism on Local Blood Flow. F

Effect of Tissue Metabolism on Local Blood Flow. Figure
17–1 shows the approximate quantitative acute effect
on blood ﬂow of increasing the rate of metabolism in
a local tissue, such as in a skeletal muscle. Note that an
increase in metabolism up to eight times normal
increases the blood ﬂow acutely about fourfold.
Acute Local Blood Flow Regulation When Oxygen Availability
Changes. One of the most necessary of the metabolic
nutrients is oxygen. Whenever the availability of
oxygen to the tissues 

henever the availability of
oxygen to the tissues decreases, such as (1) at high alti-
tude at the top of a high mountain, (2) in pneumonia,
(3) in carbon monoxide poisoning (which poisons the
ability of hemoglobin to transport oxygen), or (4) in
cyanide poisoning (which poisons the ability of the
tissues to use oxygen), the blood ﬂow through the
tissues increases markedly. Figure 17–2 shows that as
the arterial oxygen saturation decreases to about 25
per cent of normal, the blood ﬂow through an

ut 25
per cent of normal, the blood ﬂow through an isolated
leg increases about threefold; that is, the blood ﬂow
increases almost enough, but not quite enough, to
make up for the decreased amount of oxygen in the
blood, thus almost maintaining an exact constant
supply of oxygen to the tissues.
Total cyanide poisoning of oxygen usage by a local
tissue area can cause local blood ﬂow to increase as
much as sevenfold, thus demonstrating the extreme
effect of oxygen deﬁciency to increase blood ﬂow.

There are two basic theories for the regulation of
local blood ﬂow when either the rate of tissue metab-
olism changes or the availability of oxygen changes.
They are (1) the vasodilator theory and (2) the oxygen
lack theory.
Vasodilator Theory for Acute Local Blood Flow Regu-
lation—Possible Special Role of Adenosine. Accord-
ing to this theory, the greater the rate of metabolism
or the less the availability of oxygen or some other
nutrients to 

availability of oxygen or some other
nutrients to a tissue, the greater the rate of formation
of vasodilator substances in the tissue cells. The
vasodilator substances then are believed to diffuse
through the tissues to the precapillary sphincters,
metarterioles, and arterioles to cause dilation. Some of
the different vasodilator substances that have been
suggested are adenosine, carbon dioxide, adenosine
phosphate compounds, histamine, potassium ions, and
hydrogen tons.
Most of the vasodilator 

 ions, and
Most of the vasodilator theories assume that the
vasodilator substance is released from the tissue
mainly in response to oxygen deﬁciency. For instance,
Table 17–1
Blood Flow to Different Organs and Tissues Under Basal
Conditions
Per cent
ml/min
ml/min/100 g
Brain
14
700
50
Heart
4
200
70
Bronchi
2
100
25
Kidneys
22
1100
360
Liver
27
1350
95
Portal
(21)
1050
Arterial
(6)
300
Muscle (inactive state)
15
750
4
Bone
5
250
3
Skin (cool weather)
6
300
3
Thyroid gland
1
50
160

6
300
3
1
50
160
Adrenal glands
0.5
25
300
Other tissues
3.5
175
1.3
Total
100.0
5000
Based mainly on data compiled by Dr. L. A. Sapirstein.
Blood flow (x normal)
0
0
2
3
4
7
6
5
4
3
2
1
8
Rate of metabolism (x normal)
1
Normal level
Figure 17–1
Effect of increasing rate of metabolism on tissue blood ﬂow.
Chapter 17
Local and Humoral Control of Blood Flow by the Tissues
197
experiments have shown that decreased availability of
oxygen can cause both adenosine and

vailability of
oxygen can cause both adenosine and lactic acid (con-
taining hydrogen ions) to be released into the spaces
between the tissue cells; these substances then cause
intense acute vasodilation and therefore are respon-
sible, or partially responsible, for the local blood ﬂow
regulation.
Many physiologists have suggested that the sub-
stance adenosine is the most important of the local
vasodilators for controlling local blood ﬂow. For
example, minute quantities of adenosine are release

xample, minute quantities of adenosine are released
from heart muscle cells when coronary blood ﬂow
becomes too little, and this causes enough local vasodi-
lation in the heart to return coronary blood ﬂow 
back to normal. Also, whenever the heart becomes
more active than normal and the heart’s metabolism
increases an extra amount, this, too, causes increased
utilization of oxygen, followed by (1) decreased
oxygen concentration in the heart muscle cells with (2)
consequent degradation of adenosi

e cells with (2)
consequent degradation of adenosine triphosphate
(ATP), which (3) increases the release of adenosine. It
is believed that much of this adenosine leaks out of the
heart muscle cells to cause coronary vasodilation, pro-
viding increased coronary blood ﬂow to supply the
increased nutrient demands of the active heart.
Although research evidence is less clear, many phys-
iologists also have suggested that the same adenosine
mechanism is the most important controller of blood
ﬂow in s

is the most important controller of blood
ﬂow in skeletal muscle and many other tissues as 
well as in the heart. The problem with the different
vasodilator theories of local blood ﬂow regulation 
has been the following: It has been difﬁcult to prove
that sufﬁcient quantities of any single vasodilator sub-
stance (including adenosine) are indeed formed in the
tissues to cause all the measured increase in blood
ﬂow. But a combination of several different vasodila-
tors could increase the blood ﬂo

fferent vasodila-
tors could increase the blood ﬂow sufﬁciently.
Oxygen Lack Theory for Local Blood Flow Control.
Although the vasodilator theory is widely accepted,
several critical facts have made other physiologists
favor still another theory, which can be called either
the oxygen lack theory or, more accurately, the 
nutrient lack theory (because other nutrients besides
oxygen are involved). Oxygen (and other nutrients 
as well) is required as one of the metabolic nutrients
to cause vascular

s one of the metabolic nutrients
to cause vascular muscle contraction.Therefore, in the
absence of adequate oxygen, it is reasonable to 
believe that the blood vessels simply would relax 
and therefore naturally dilate. Also, increased utiliza-
tion of oxygen in the tissues as a result of increased
metabolism theoretically could decrease the availabil-
ity of oxygen to the smooth muscle ﬁbers in the 
local blood vessels, and this, too, would cause local
vasodilation.
A mechanism by which the oxy

e local
A mechanism by which the oxygen lack theory
could operate is shown in Figure 17–3. This ﬁgure
shows a tissue unit, consisting of a metarteriole with a
single sidearm capillary and its surrounding tissue. At
the origin of the capillary is a precapillary sphincter,
and around the metarteriole are several other smooth
muscle ﬁbers. Observing such a tissue under a micro-
scope—for example, in a bat’s wing—one sees that the
precapillary sphincters are normally either completely

pillary sphincters are normally either completely
open or completely closed.The number of precapillary
sphincters that are open at any given time is roughly
proportional to the requirements of the tissue for
nutrition. The precapillary sphincters and metarteri-
oles open and close cyclically several times per minute,
with the duration of the open phases being propor-
tional to the metabolic needs of the tissues for oxygen.
The cyclical opening and closing is called vasomotion.
Let us explain how

d closing is called vasomotion.
Let us explain how oxygen concentration in the
local tissue could regulate blood ﬂow through the area.
Because smooth muscle requires oxygen to remain
75
100
0
2
3
50
25
Arterial oxygen saturation (per cent)
1
Figure 17–2
Effect of decreasing arterial oxygen saturation on blood ﬂow
through an isolated dog leg.
Precapillary sphincter
Sidearm capillary
Figure 17–3
Diagram of a tissue unit area for explanation of acute local feed-
b

e unit area for explanation of acute local feed-
back control of blood ﬂow, showing a metarteriole passing through
the tissue and a sidearm capillary with its precapillary sphincter
for controlling capillary blood ﬂow.
198
Unit IV
contracted, one might assume that the strength of con-
traction of the sphincters would increase with an
increase in oxygen concentration. Consequently, when
the oxygen concentration in the tissue rises above 
a certain level, the precapillary and metar

above 
a certain level, the precapillary and metarteriole
sphincters presumably would close until the tissue
cells consume the excess oxygen. But when the excess
oxygen is gone and the oxygen concentration falls low
enough, the sphincters would open once more to begin
the cycle again.
Thus, on the basis of available data, either a
vasodilator substance theory or an oxygen lack theory
could explain acute local blood ﬂow regulation in
response to the metabolic needs of the tissues.
Probably the tr

he metabolic needs of the tissues.
Probably the truth lies in a combination of the two
Possible Role of Other Nutrients Besides Oxygen in Control of
Local Blood Flow. Under special conditions, it has been
shown that lack of glucose in the perfusing blood can
cause local tissue vasodilation. Also, it is possible that
this same effect occurs when other nutrients, such as
amino acids or fatty acids, are deﬁcient, although this
has not been studied adequately. In addition, vasodi-
lation

en studied adequately. In addition, vasodi-
lation occurs in the vitamin deﬁciency disease beriberi,
in which the patient has deﬁciencies of the vitamin B
substances thiamine, niacin, and riboﬂavin. In this
disease, the peripheral vascular blood ﬂow everywhere
in the body often increases twofold to threefold.
Because these vitamins all are needed for oxygen-
induced phosphorylation that is required to produce
ATP in the tissue cells, one can well understand how
deﬁciency of these vitamins might 

 understand how
deﬁciency of these vitamins might lead to diminished
smooth muscle contractile ability and therefore also
local vasodilation.
Special Examples of Acute “Metabolic”
Control of Local Blood Flow
The mechanisms that we have described thus far for
local blood ﬂow control are called “metabolic mecha-
nisms” because all of them function in response to the
metabolic needs of the tissues. Two additional special
examples of metabolic control of local blood ﬂow are
reactive hyperemia and ac

l of local blood ﬂow are
reactive hyperemia and active hyperemia.
Reactive Hyperemia. When the blood supply to a tissue
is blocked for a few seconds to as long an hour or more
and then is unblocked, blood ﬂow through the tissue
usually increases immediately to four to seven times
normal; this increased ﬂow will continue for a few
seconds if the block has lasted only a few seconds but
sometimes continues for as long as many hours if the
blood ﬂow has been stopped for an hour or more.This
phenomen

has been stopped for an hour or more.This
phenomenon is called reactive hyperemia.
Reactive hyperemia is another manifestation of the
local “metabolic” blood ﬂow regulation mechanism;
that is, lack of ﬂow sets into motion all of those factors
that cause vasodilation. After short periods of vascu-
lar occlusion, the extra blood ﬂow during the reactive
hyperemia phase lasts long enough to repay almost
exactly the tissue oxygen deﬁcit that has accrued
during the period of occlusion. This mechanism

ed
emphasizes the close connection between local blood
ﬂow regulation and delivery of oxygen and other nutri-
ents to the tissues.
Active Hyperemia. When any tissue becomes highly
active, such as an exercising muscle, a gastrointestinal
gland during a hypersecretory period, or even the
brain during rapid mental activity, the rate of blood
ﬂow through the tissue increases. Here again, by
simply applying the basic principles of local blood ﬂow
control

ng the basic principles of local blood ﬂow
control, one can easily understand this active hyper-
emia. The increase in local metabolism causes the cells
to devour tissue ﬂuid nutrients extremely rapidly and
also to release large quantities of vasodilator sub-
stances. The result is to dilate the local blood vessels
and, therefore, to increase local blood ﬂow. In this way,
the active tissue receives the additional nutrients
required to sustain its new level of function. As
pointed out earlier, ac

 new level of function. As
pointed out earlier, active hyperemia in skeletal
muscle can increase local muscle blood ﬂow as much
as 20-fold during intense exercise.
“Autoregulation” of Blood Flow When the
Arterial Pressure Changes from Normal—
“Metabolic” and “Myogenic” Mechanisms
In any tissue of the body, an acute increase in arterial
pressure causes immediate rise in blood ﬂow. But,
within less than a minute, the blood ﬂow in most
tissues returns almost to the normal level, even though
the art

ns almost to the normal level, even though
the arterial pressure is kept elevated. This return of
ﬂow toward normal is called “autoregulation of blood
ﬂow.” After autoregulation has occurred, the local
blood ﬂow in most body tissues will be related to arte-
rial pressure approximately in accord with the solid
“acute” curve in Figure 17–4. Note that between an
100
50
0
0.5
0
1.0
1.5
2.0
2.5
150
200
250
Acute
Long-term
Figure 17–4
Effect of different

g)
Acute
Long-term
Effect of different levels of arterial pressure on blood ﬂow through
a muscle. The solid red curve shows the effect if the arterial pres-
sure is raised over a period of a few minutes. The dashed green
curve shows the effect if the arterial pressure is raised extremely
slowly over a period of many weeks.
Chapter 17
199
arterial pressure of about 70 mm Hg and 175 mm Hg,
the blood ﬂow increases only 30 per cent e

 mm Hg,
the blood ﬂow increases only 30 per cent even though
the arterial pressure increases 150 per cent.
For almost a century, two views have been proposed
to explain this acute autoregulation mechanism. They
have been called (1) the metabolic theory and (2) the
myogenic theory.
The metabolic theory can be understood easily by
applying the basic principles of local blood ﬂow regu-
lation discussed in previous sections. Thus, when the
arterial pressure becomes too great, the excess ﬂow
provides

ressure becomes too great, the excess ﬂow
provides too much oxygen and too many other nutri-
ents to the tissues. These nutrients (especially oxygen)
then cause the blood vessels to constrict and the ﬂow
to return nearly to normal despite the increased 
pressure.
The myogenic theory, however, suggests that still
another mechanism not related to tissue metabolism
explains the phenomenon of autoregulation. This
theory is based on the observation that sudden stretch
of small blood vessels causes th

at sudden stretch
of small blood vessels causes the smooth muscle of the
vessel wall to contract for a few seconds. Therefore, it
has been proposed that when high arterial pressure
stretches the vessel, this in turn causes reactive vascu-
lar constriction that reduces blood ﬂow nearly back to
normal. Conversely, at low pressures, the degree of
stretch of the vessel is less, so that the smooth muscle
relaxes and allows increased ﬂow.
The myogenic response is inherent to vascular
smooth muscle and

response is inherent to vascular
smooth muscle and can occur in the absence of neural
or hormonal inﬂuences. It is most pronounced in arte-
rioles but can also be observed in arteries, venules,
veins, and even lymphatic vessels. Myogenic contrac-
tion is initiated by stretch-induced vascular depolar-
ization, which then rapidly increases calcium ion entry
from the extracellular ﬂuid into the cells, causing them
to contract. Changes in vascular pressure may also
open or close other ion channels t

essure may also
open or close other ion channels that inﬂuence vascu-
lar contraction. The precise mechanisms by which
changes in pressure cause opening or closing of vas-
cular ion channels are still uncertain, but likely involve
mechanical effects of pressure on extracellular pro-
teins that are tethered to cytoskeleton elements of the
vascular wall or to the ion channels themselves.
The myogenic mechanism may be important in pre-
venting excessive stretch of blood vessel when blood
pressure i

sive stretch of blood vessel when blood
pressure is increased. However, the importance of 
the myogenic mechanism in blood ﬂow regulation 
is unclear because this pressure sensing mechanism
cannot directly detect changes in blood ﬂow in the
tissue. Indeed metabolic factors appear to override 
the myogenic mechanism in circumstances where the
metabolic demands of the tissues are signiﬁcantly
increased, such as during vigorous muscle exercise,
which can cause dramatic increases in skeletal muscle

h can cause dramatic increases in skeletal muscle
blood ﬂow.
Special Mechanisms for Acute Blood Flow
Control in Speciﬁc Tissues
Although the general mechanisms for local blood ﬂow
control discussed thus far are present in almost all
tissues of the body, distinctly different mechanisms
operate in a few special areas. They all are discussed
throughout this text in relation to speciﬁc organs, but
two notable ones are as follows:
1. In the kidneys, blood ﬂow control is vested mainly
in a mechanism c

lood ﬂow control is vested mainly
in a mechanism called tubuloglomerular feedback,
in which the composition of the ﬂuid in the early
distal tubule is detected by an epithelial structure
of the distal tubule itself called the macula densa.
This is located where the distal tubule lies adjacent
to the afferent and efferent arterioles at the
nephron juxtaglomerular apparatus. When too
much ﬂuid ﬁlters from the blood through the
glomerulus into the tubular system, appropriate
feedback signals from th

bular system, appropriate
feedback signals from the macula densa cause
constriction of the afferent arterioles, in this way
reducing both renal blood ﬂow and glomerular
ﬁltration rate back to or near to normal. The 
details of this mechanism are discussed in 
Chapter 26.
2. In the brain, in addition to control of blood ﬂow by
tissue oxygen concentration, the concentrations of
carbon dioxide and hydrogen ions play very
prominent roles. An increase of either or both of
these dilates the cerebral v

 of either or both of
these dilates the cerebral vessels and allows rapid
washout of the excess carbon dioxide or hydrogen
ions from the brain tissues. This is important
because the level of excitability of the brain itself is
highly dependent on exact control of both carbon
dioxide concentration and hydrogen ion
concentration. This special mechanism for cerebral
blood ﬂow control is presented in Chapter 61.
Mechanism for Dilating Upstream Arteries
When Microvascular Blood Flow Increases—
The En

es
The Endothelium-Derived Relaxing Factor
(Nitric Oxide)
The local mechanisms for controlling tissue blood ﬂow
can dilate only the very small arteries and arterioles in
each tissue because tissue cell vasodilator substances
or tissue cell oxygen deﬁciency can reach only these
vessels, not the intermediate and larger arteries back
upstream. Yet, when blood ﬂow through a micro-
vascular portion of the circulation increases, this sec-
ondarily entrains anot

lation increases, this sec-
ondarily entrains another mechanism that does dilate
the larger arteries as well. This mechanism is the 
The endothelial cells lining the arterioles and small
arteries synthesize several substances that, when
released, can affect the degree of relaxation or con-
traction of the arterial wall. The most important of
these is a vasodilator substance called endothelium-
derived relaxing factor (EDRF), which is composed
principally of nitric oxide, which has a h

omposed
principally of nitric oxide, which has a half-life in the
blood of only 6 seconds. Rapid ﬂow of blood through
the arteries and arterioles causes shear stress on the
endothelial cells because of viscous drag of the blood
against the vascular walls. This stress contorts the
endothelial cells in the direction of ﬂow and causes sig-
niﬁcant increase in the release of nitric oxide. The
nitric oxide then relaxes the blood vessels. This is for-
tunate because it increases the diameters of the
u

upstream arterial blood vessels whenever microvascu-
lar blood ﬂow increases downstream. Without such a
response, the effectiveness of local blood ﬂow control
200
Unit IV
would be signiﬁcantly decreased because a signiﬁcant
part of the resistance to blood ﬂow is in the upstream
small arteries.
Thus far, most of the mechanisms for local blood ﬂow
regulation that we have discussed act within a few
seco

ation that we have discussed act within a few
seconds to a few minutes after the local tissue condi-
tions have changed. Yet, even after full activation 
of these acute mechanisms, the blood ﬂow usually 
is adjusted only about three quarters of the way to 
the exact additional requirements of the tissues.
For instance, when the arterial pressure suddenly is
increased from 100 to 150 mm Hg, the blood ﬂow
increases almost instantaneously about 100 per cent.
Then, within 30 seconds to 2 minutes, th

per cent.
Then, within 30 seconds to 2 minutes, the ﬂow
decreases back to about 15 per cent above the origi-
nal control value. This illustrates the rapidity of the
acute mechanisms for local blood ﬂow regulation, but
at the same time, it demonstrates that the regulation is
still incomplete because there remains an excess 15 per
cent increase in blood ﬂow.
However, over a period of hours, days, and weeks, a
long-term type of local blood ﬂow regulation develops
in addition to the acute regulation

ation develops
in addition to the acute regulation. This long-term 
regulation gives far more complete regulation. For
instance, in the aforementioned example, if the arte-
rial pressure remains at 150 mm Hg indeﬁnitely, within
a few weeks the blood ﬂow through the tissues gradu-
ally reapproaches almost exactly the normal ﬂow level.
Figure 17–4 shows by the dashed green curve the
extreme effectiveness of this long-term local blood
ﬂow regulation. Note that once the long-term regula-
tion has ha

. Note that once the long-term regula-
tion has had time to occur, long-term changes in arte-
rial pressure between 50 and 250 mm Hg have little
effect on the rate of local blood ﬂow.
Long-term regulation of blood ﬂow is especially
important when the long-term metabolic demands 
of a tissue change. Thus, if a tissue becomes chroni-
cally overactive and therefore requires chronically
increased quantities of oxygen and other nutrients, the
arterioles and capillary vessels usually increase both in

es and capillary vessels usually increase both in
number and size within a few weeks to match the
needs of the tissue—unless the circulatory system has
become pathological or too old to respond.
Mechanism of Long-Term Regulation—
Change in “Tissue Vascularity”
The mechanism of long-term local blood ﬂow regula-
tion is principally to change the amount of vascularity
of the tissues. For instance, if the metabolism in a 
given tissue is increased for a prolonged period,
vascularity increases; if th

r a prolonged period,
vascularity increases; if the metabolism is decreased,
vascularity decreases.
Thus, there is actual physical reconstruction of 
the tissue vasculature to meet the needs of the tissues.
This reconstruction occurs rapidly (within days) in
extremely young animals. It also occurs rapidly in new
growth tissue, such as in scar tissue and cancerous
tissue; however, it occurs much more slowly in old,
well-established tissues. Therefore, the time required
for long-term regulation to

ore, the time required
for long-term regulation to take place may be only 
a few days in the neonate or as long as months in 
the elderly person. Furthermore, the ﬁnal degree of
response is much better in younger tissues than in
older, so that in the neonate, the vascularity will adjust
to match almost exactly the needs of the tissue for
blood ﬂow, whereas in older tissues, vascularity fre-
quently lags far behind the needs of the tissues.
Role of Oxygen in Long-Term Regulation. Oxygen is impor-

f Oxygen in Long-Term Regulation. Oxygen is impor-
tant not only for acute control of local blood ﬂow but
also for long-term control. One example of this is
increased vascularity in tissues of animals that live at
high altitudes, where the atmospheric oxygen is low.A
second example is that fetal chicks hatched in low
oxygen have up to twice as much tissue blood vessel
conductivity as is normally true.This same effect is also
dramatically demonstrated in premature human
babies put into oxygen ten

ated in premature human
babies put into oxygen tents for therapeutic purposes.
The excess oxygen causes almost immediate cessation
of new vascular growth in the retina of the premature
baby’s eyes and even causes degeneration of some of
the small vessels that already have formed.Then when
the infant is taken out of the oxygen tent, there is
explosive overgrowth of new vessels to make up for
the sudden decrease in available oxygen; indeed, there
is often so much overgrowth that the retinal vessel

s often so much overgrowth that the retinal vessels
grow out from the retina into the eye’s vitreous humor;
and this eventually causes blindness. (This condition is
called retrolental ﬁbroplasia.)
Importance of Vascular Endothelial Growth
Factor in Formation of New Blood Vessels
A dozen or more factors that increase growth of new
blood vessels have been found, almost all of which are
small peptides. Three of those that have been best
characterized are vascular endothelial growth factor
(VEGF), ﬁ

d are vascular endothelial growth factor
(VEGF), ﬁbroblast growth factor, and angiogenin, each
of which has been isolated from tissues that have inad-
equate blood supply. Presumably, it is deﬁciency of
tissue oxygen or other nutrients, or both, that leads to
formation of the vascular growth factors (also called
“angiogenic factors”).
Essentially all the angiogenic factors promote new
vessel growth in the same way.They cause new vessels
to sprout from other small vessels. The ﬁrst step is dis-
s

 from other small vessels. The ﬁrst step is dis-
solution of the basement membrane of the endothelial
cells at the point of sprouting.This is followed by rapid
reproduction of new endothelial cells that stream
outward through the vessel wall in extended cords
directed toward the source of the angiogenic factor.
The cells in each cord continue to divide and rapidly
fold over into a tube. Next, the tube connects with
another tube budding from another donor vessel
(another arteriole or venule) and 

er donor vessel
(another arteriole or venule) and forms a capillary
loop through which blood begins to ﬂow. If the ﬂow is
great enough, smooth muscle cells eventually invade
the wall, so that some of the new vessels eventually
grow to be new arterioles or venules or perhaps even
larger vessels. Thus, angiogenesis explains the manner
in which metabolic factors in local tissues can cause
growth of new vessels.
Chapter 17
201
Certain other subs

f Blood Flow by the Tissues
201
Certain other substances, such as some steroid hor-
mones, have exactly the opposite effect on small blood
vessels, occasionally even causing dissolution of vas-
cular cells and disappearance of vessels. Therefore,
blood vessels can also be made to disappear when not
needed.
Vascularity Is Determined by Maximum Blood Flow Need, Not
by Average Need. An especially valuable characteristic
of long-term vascular control is that vascularity is
determined mainly by the m

 is that vascularity is
determined mainly by the maximum level of blood
ﬂow need rather than by average need. For instance,
during heavy exercise the need for whole body blood
ﬂow often increases to six to eight times the resting
blood ﬂow. This great excess of ﬂow may not be
required for more than a few minutes each day. Nev-
ertheless, even this short need can cause enough
VEGF to be formed by the muscles to increase their
vascularity as required. Were it not for this capability,
every time th

ed. Were it not for this capability,
every time that a person attempted heavy exercise, the
muscles would fail to receive the required nutrients,
especially the required oxygen, so that the muscles
simply would fail to contract.
However, after extra vascularity does develop, the
extra blood vessels normally remain mainly vasocon-
stricted, opening to allow extra ﬂow only when ap-
propriate local stimuli such as oxygen lack, nerve
vasodilatory stimuli, or other stimuli call forth the
required ext

muli, or other stimuli call forth the
required extra ﬂow.
Development of Collateral
Circulation—A Phenomenon of Long-
Term Local Blood Flow Regulation
When an artery or a vein is blocked in virtually any
tissue of the body, a new vascular channel usually
develops around the blockage and allows at least
partial resupply of blood to the affected tissue. The
ﬁrst stage in this process is dilation of small vascular
loops that already connect the vessel above the block-
age to the vessel below.This d

el above the block-
age to the vessel below.This dilation occurs within the
ﬁrst minute or two, indicating that the dilation is
simply a neurogenic or metabolic relaxation of the
muscle ﬁbers of the small vessels involved. After this
initial opening of collateral vessels, the blood ﬂow
often is still less than one quarter that needed to
supply all the tissue needs. However, further opening
occurs within the ensuing hours, so that within 1 day
as much as half the tissue needs may be met, and
with

much as half the tissue needs may be met, and
within a few days often all the tissue needs.
The collateral vessels continue to grow for many
months thereafter, almost always forming multiple
small collateral channels rather than one single large
vessel. Under resting conditions, the blood ﬂow usually
returns very near to normal, but the new channels
seldom become large enough to supply the blood ﬂow
needed during strenuous tissue activity. Thus, the
development of collateral vessels follows the 

the
development of collateral vessels follows the usual
principles of both acute and long-term local blood ﬂow
control, the acute control being rapid neurogenic and
metabolic dilation, followed chronically by manifold
growth and enlargement of new vessels over a period
of weeks and months.
The most important example of the development of
collateral blood vessels occurs after thrombosis of one
of the coronary arteries. Almost all people by the age
of 60 years have had at least one of the smaller 

of 60 years have had at least one of the smaller branch
coronary vessels close. Yet most people do not know
that this has happened because collaterals have devel-
oped rapidly enough to prevent myocardial damage. It
is in those other instances in which coronary insufﬁ-
ciency occurs too rapidly or too severely for collater-
als to develop that serious heart attacks occur.
Humoral Control of the
Humoral control of the circulation means control by
substances secreted or absorbed into 

s control by
substances secreted or absorbed into the body ﬂuids—
such as hormones and ions. Some of these substances
are formed by special glands and transported in 
the blood throughout the entire body. Others are
formed in local tissue areas and cause only local cir-
culatory effects. Among the most important of the
humoral factors that affect circulatory function are the
following.
Norepinephrine and Epinephrine. Norepinephrine is an
especially powerful vasoconstrictor

nephrine is an
especially powerful vasoconstrictor hormone; epi-
nephrine is less so and in some tissues even causes mild
vasodilation. (A special example of vasodilation
caused by epinephrine occurs to dilate the coronary
arteries during increased heart activity.)
When the sympathetic nervous system is stimulated
in most or all parts of the body during stress or exer-
cise, the sympathetic nerve endings in the individual
tissues release norepinephrine, which excites the heart
and contracts the 

phrine, which excites the heart
and contracts the veins and arterioles. In addition,
the sympathetic nerves to the adrenal medullae cause
these glands to secrete both norepinephrine and epi-
nephrine into the blood. These hormones then circu-
late to all areas of the body and cause almost the same
effects on the circulation as direct sympathetic stimu-
lation, thus providing a dual system of control: (1)
direct nerve stimulation and (2) indirect effects of 
norepinephrine and/or epinephrine in t

effects of 
norepinephrine and/or epinephrine in the circulating
blood.
Angiotensin II. Angiotensin II is another powerful vaso-
constrictor substance. As little as one millionth of a
gram can increase the arterial pressure of a human
being 50 mm Hg or more.
The effect of angiotensin II is to constrict powerfully
the small arterioles. If this occurs in an isolated 
tissue area, the blood ﬂow to that area can be 
severely depressed. However, the real importance of
angiotensin II is that it normal

eal importance of
angiotensin II is that it normally acts on many of the
arterioles of the body at the same time to increase the
202
Unit IV
total peripheral resistance, thereby increasing the arte-
rial pressure. Thus, this hormone plays an integral role
in the regulation of arterial pressure, as is discussed in
detail in Chapter 19.
Vasopressin. Vasopressin,
also 
called 
antidiuretic
hormone, is even more powerful than angiotensin II as
a vasoconstrictor, thus making it one of

sin II as
a vasoconstrictor, thus making it one of the body’s
most potent vascular constrictor substances. It is
formed in nerve cells in the hypothalamus of the brain
(see Chapter 75) but is then transported downward by
nerve axons to the posterior pituitary gland, where it
is ﬁnally secreted into the blood.
It is clear that vasopressin could have enormous
effects on circulatory function. Yet, normally, only
minute amounts of vasopressin are secreted, so that
most physiologists have thought tha

reted, so that
most physiologists have thought that vasopressin plays
little role in vascular control. However, experiments
have shown that the concentration of circulating blood
vasopressin after severe hemorrhage can rise high
enough to increase the arterial pressure as much as 
60 mm Hg. In many instances, this can, by itself, bring
the arterial pressure almost back up to normal.
Vasopressin has a major function to increase greatly
water reabsorption from the renal tubules back into
the blood

orption from the renal tubules back into
the blood (discussed in Chapter 28), and therefore 
to help control body ﬂuid volume. That is why this
hormone is also called antidiuretic hormone.
Endothelin—A Powerful Vasoconstrictor in Damaged Blood
Vessels. Still another vasoconstrictor substance that
ranks along with angiotensin and vasopressin in its
vasoconstrictor capability is a large 21 amino acid
peptide called endothelin,
which requires only
nanogram quantities to cause powerful vasoconstric-

anogram quantities to cause powerful vasoconstric-
tion. This substance is present in the endothelial cells
of all or most blood vessels. The usual stimulus for
release is damage to the endothelium, such as that
caused by crushing the tissues or injecting a trauma-
tizing chemical into the blood vessel. After severe
blood vessel damage, release of local endothelin and
subsequent vasoconstriction helps to prevent exten-
sive bleeding from arteries as large as 5 millimeters in
diameter that might 

 as large as 5 millimeters in
diameter that might have been torn open by crushing
injury.
Bradykinin. Several substances called kinins cause pow-
erful vasodilation when formed in the blood and tissue
ﬂuids of some organs.
The kinins are small polypeptides that are split away
by proteolytic enzymes from alpha2-globulins in the
plasma or tissue ﬂuids. A proteolytic enzyme of par-
ticular importance for this purpose is kallikrein, which
is present in the blood and tissue ﬂuids i

, which
is present in the blood and tissue ﬂuids in an inactive
form. This inactive kallikrein is activated by macera-
tion of the blood, by tissue inﬂammation, or by other
similar chemical or physical effects on the blood or
tissues. As kallikrein becomes activated, it acts imme-
diately on alpha2-globulin to release a kinin called
kallidin that then is converted by tissue enzymes into
bradykinin. Once formed, bradykinin persists for only
a few minutes because it is inactivated by the enzyme
ca

minutes because it is inactivated by the enzyme
carboxypeptidase or by converting enzyme, the same
enzyme that also plays an essential role in activating
angiotensin, as discussed in Chapter 19. The activated
kallikrein enzyme is destroyed by a kallikrein inhibitor
also present in the body ﬂuids.
Bradykinin 
causes 
both 
powerful 
arteriolar 
dilation and increased capillary permeability. For
instance, injection of 1 microgram of bradykinin into
the brachial artery of a person increases blood ﬂ

the brachial artery of a person increases blood ﬂow
through the arm as much as sixfold, and even smaller
amounts injected locally into tissues can cause marked
local edema resulting from increase in capillary pore
size.
There is reason to believe that kinins play special
roles in regulating blood ﬂow and capillary leakage of
ﬂuids in inﬂamed tissues. It also is believed that
bradykinin plays a normal role to help regulate blood
ﬂow in the skin as well as in the salivary and gastroin-
testinal g

s well as in the salivary and gastroin-
testinal glands.
Histamine. Histamine is released in essentially every
tissue of the body if the tissue becomes damaged or
inﬂamed or is the subject of an allergic reaction. Most
of the histamine is derived from mast cells in the
damaged tissues and from basophils in the blood.
Histamine has a powerful vasodilator effect on 
the arterioles and, like bradykinin, has the ability to
increase greatly capillary porosity, allowing leakage 
of both ﬂuid and plasm

porosity, allowing leakage 
of both ﬂuid and plasma protein into the tissues. In
many pathological conditions, the intense arteriolar
dilation and increased capillary porosity produced 
by histamine cause tremendous quantities of ﬂuid 
to leak out of the circulation into the tissues, inducing
edema. The local vasodilatory and edema-producing
effects 
of 
histamine 
are 
especially 
prominent 
during allergic reactions and are discussed in 
Chapter 34.
Vascular Control by Ions and Other
Chemical 

r 34.
Many different ions and other chemical factors can
either dilate or constrict local blood vessels. Most of
them have little function in overall regulation of the
circulation, but some speciﬁc effects are:
1. An increase in calcium ion concentration causes
vasoconstriction. This results from the general
effect of calcium to stimulate smooth muscle
contraction, as discussed in Chapter 8.
2. An increase in potassium ion concentration causes

An increase in potassium ion concentration causes
vasodilation. This results from the ability of
potassium ions to inhibit smooth muscle
3. An increase in magnesium ion concentration
causes powerful vasodilation because magnesium
ions inhibit smooth muscle contraction.
4. An increase in hydrogen ion concentration
(decrease in pH) causes dilation of the arterioles.
Chapter 17
203
Conversely, slight decrease in hydrogen ion
concen

concentration causes arteriolar constriction.
5. Anions that have signiﬁcant effects on blood
vessels are acetate and citrate, both of which cause
mild degrees of vasodilation.
6. An increase in carbon dioxide concentration
causes moderate vasodilation in most tissues, but
marked vasodilation in the brain. Also, carbon
dioxide in the blood, acting on the brain
vasomotor center, has an extremely powerful
indirect effect, transmitted through the
sympathe

sympathetic nervous vasoconstrictor system, to
cause widespread vasoconstriction throughout the
body.
Adair TH, Gay WJ, Montani JP: Growth regulation of the
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Campbell WB, Gauthier KM: What is new in endothelium-
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Chang L, Kaipainen A, Folkman J: Lymphangiogenesis new
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n J: Lymphangiogenesis new
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Cowley AW Jr, Mori T, Mattson D, Zou AP: Role of renal
NO production in the regulation of medullary blood ﬂow.
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Davis MJ, Hill MA: Signaling mechanisms underlying the
vascular myogenic response. Physiol Rev 79:387, 1999.
Erdös EG, Marcic BM: Kinins, receptors, kininases and
inhibitors—where did they lead us? Biol Chem 382:43,
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Ferrara N, Gerber HP, LeCouter J:The biol

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Granger HJ, Guyton AC: Autoregulation of the total sys-
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Guyton AC, Coleman TG, Granger HJ: Circulation: overall
regulation. Annu Rev Physiol 34:13, 1972.
Hall JE, Brands MW, Henegar JR: Angiotensin II and long-
term arterial pressure regulation: the overriding domi-
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Harder DR, Zhang C, Gebremedhin D: Astrocytes function
in matching blood ﬂow to metabolic activity. News Physiol
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Hester RL, Hammer LW: Venular-arteriolar communication
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Comp Physiol 282:R1280, 2002.
Iglarz M, Schiffrin EL: Role of endothelin-1 in hypertension.
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C
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A
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T
E
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1
8
204
Nervous Regulation of the
Circulation, and Rapid Control 

al Pressure
As discussed in Chapter 17, adjustment of blood
ﬂow tissue by tissue is mainly the function of local
tissue blood ﬂow control mechanisms. We shall see
in this chapter that nervous control of the circula-
tion has more global functions, such as redistribut-
ing blood ﬂow to different areas of the body, increasing or decreasing pumping
activity by the heart, and, especially, providing very rapid control of systemic
arterial pressure.
The nervous sy

trol of systemic
The nervous system controls the circulation almost entirely through the auto-
nomic nervous system. The total function of this system is presented in Chapter
60, and this subject was also introduced in Chapter 17. For our present discus-
sion, we need to present still other speciﬁc anatomical and functional charac-
teristics, as follows.
By far the most important part of the autonomic nervous system for regulating
the circulation is th

ervous system for regulating
the circulation is the sympathetic nervous system. The parasympathetic nervous
system also contributes speciﬁcally to regulation of heart function, as we shall
see later in the chapter.
Sympathetic Nervous System. Figure 18–1 shows the anatomy of sympathetic
nervous control of the circulation. Sympathetic vasomotor nerve ﬁbers leave
the spinal cord through all the thoracic spinal nerves and through the ﬁrst one
or two lumbar spinal nerves. They then pass immediately 

 lumbar spinal nerves. They then pass immediately into a sympathetic
chain, one of which lies on each side of the vertebral column. Next, they pass
by two routes to the circulation: (1) through speciﬁc sympathetic nerves that
innervate mainly the vasculature of the internal viscera and the heart, as shown
on the right side of Figure 18–1, and (2) almost immediately into peripheral por-
tions of the spinal nerves distributed to the vasculature of the peripheral areas.
The precise pathways of thes

the peripheral areas.
The precise pathways of these ﬁbers in the spinal cord and in the sympathetic
chains are discussed more fully in Chapter 60.
Sympathetic Innervation of the Blood Vessels. Figure 18–2 shows distribution
of sympathetic nerve ﬁbers to the blood vessels, demonstrating that in most
tissues all the vessels except the capillaries, precapillary sphincters, and metar-
terioles are innervated.
The innervation of the small arteries and arterioles allows sympathetic stim-
ulation to in

 arterioles allows sympathetic stim-
ulation to increase resistance to blood ﬂow and thereby to decrease rate of blood
ﬂow through the tissues.
The innervation of the large vessels, particularly of the veins, makes it possi-
ble for sympathetic stimulation to decrease the volume of these vessels.This can
push blood into the heart and thereby play a major role in regulation of heart
pumping, as we shall see later in this and subsequent chapters.
Chapter 18
Nervous Regulation of the Circulation, a

hapter 18
Nervous Regulation of the Circulation, and Rapid Control of Arterial Pressure
205
Sympathetic Nerve Fibers to the Heart. In addition to
sympathetic nerve ﬁbers supplying the blood vessels,
sympathetic ﬁbers also go directly to the heart, as
shown in Figure 18–1 and also discussed in Chapter 9.
It should be recalled that sympathetic stimulation
markedly increases the activity of the heart, both
increasing the heart rate and enhancing its strength
and volume of pumping.
Parasympathetic C

 strength
Parasympathetic Control of Heart Function, Especially Heart
Rate. Although the parasympathetic nervous system is
exceedingly important for many other autonomic
functions of the body, such as control of multiple gas-
trointestinal actions, it plays only a minor role in 
regulation of the circulation. Its most important circu-
latory effect is to control heart rate by way of parasym-
pathetic nerve ﬁbers to the heart in the vagus 
nerves, shown in Figure 18–1 by th

t in the vagus 
nerves, shown in Figure 18–1 by the dashed red line
from the brain medulla directly to the heart.
The effects of parasympathetic stimulation on 
heart function were discussed in detail in Chapter 9.
Vasoconstrictor
Sympathetic chain
Cardioinhibitor
Vasodilator
Blood
vessels
Vasomotor center
Blood
vessels
Heart
Vagus
Figure 18–1
Anatomy of sympathetic nervous
control of the circulation. Also
shown by the red dashed line is a
vagus nerve that carries parasym-
pathetic signals to th

nerve that carries parasym-
pathetic signals to the heart.
Arteries
vasoconstriction
Venules
Veins
Figure 18–2
Sympathetic innervation of the systemic circulation.
206
Unit IV
Principally, parasympathetic stimulation causes a
marked decrease in heart rate and a slight decrease in
heart muscle contractility.
Sympathetic Vasoconstrictor System and 
Its Control by the Central Nervous System
The sympathetic nerves carry tremendous numbers of
vasocon

athetic nerves carry tremendous numbers of
vasoconstrictor nerve ﬁbers and only a few vasodilator
ﬁbers. The vasoconstrictor ﬁbers are distributed to
essentially all segments of the circulation, but more to
some tissues than others. This sympathetic vasocon-
strictor effect is especially powerful in the kidneys,
intestines, spleen, and skin but much less potent in
skeletal muscle and the brain.
Vasomotor Center in the Brain and Its Control of the Vasocon-
strictor System. Located bilaterally mai

 Vasocon-
strictor System. Located bilaterally mainly in the retic-
ular substance of the medulla and of the lower third
of the pons, shown in Figures 18–1 and 18–3, is an area
called the vasomotor center. This center transmits
parasympathetic impulses through the vagus nerves to
the heart and transmits sympathetic impulses through
the spinal cord and peripheral sympathetic nerves 
to virtually all arteries, arterioles, and veins of the
body.
Although the total organization of the vasomotor
cent

hough the total organization of the vasomotor
center is still unclear, experiments have made it 
possible to identify certain important areas in this
center, as follows:
1. A vasoconstrictor area located bilaterally in the
anterolateral portions of the upper medulla. The
neurons originating in this area distribute their
ﬁbers to all levels of the spinal cord, where they
excite preganglionic vasoconstrictor neurons of the
sympathetic nervous system.
2. A vasodilator area located bilaterally in th

m.
2. A vasodilator area located bilaterally in the
anterolateral portions of the lower half of the
medulla. The ﬁbers from these neurons project
upward to the vasoconstrictor area just described;
they inhibit the vasoconstrictor activity of this area,
thus causing vasodilation.
3. A sensory area located bilaterally in the tractus
solitarius in the posterolateral portions of the
medulla and lower pons. The neurons of this area
receive sensory nerve signals from the circulatory
system mainly thro

ve signals from the circulatory
system mainly through the vagus and
glossopharyngeal nerves, and output signals from
this sensory area then help to control activities of
both the vasoconstrictor and vasodilator areas of
the vasomotor center, thus providing “reﬂex”
control of many circulatory functions. An example
is the baroreceptor reﬂex for controlling arterial
pressure, which we describe later in this chapter.
Continuous Partial Constriction of the Blood Vessels Is Nor-
mally Caused by Sympat

f the Blood Vessels Is Nor-
mally Caused by Sympathetic Vasoconstrictor Tone. Under
normal conditions, the vasoconstrictor area of the
vasomotor center transmits signals continuously to 
the sympathetic vasoconstrictor nerve ﬁbers over the
entire body, causing continuous slow ﬁring of these
ﬁbers at a rate of about one half to two impulses per
second.This continual ﬁring is called sympathetic vaso-
constrictor tone. These impulses normally maintain a
partial state of contraction in the blood ves

in a
partial state of contraction in the blood vessels, called
vasomotor tone.
Figure 18–4 demonstrates the signiﬁcance of vaso-
constrictor tone. In the experiment of this ﬁgure,
total spinal anesthesia was administered to an animal.
This blocked all transmission of sympathetic nerve
impulses from the spinal cord to the periphery. As a
result, the arterial pressure fell from 100 to 50 mm Hg,
demonstrating the effect of losing vasoconstrictor tone
throughout the body. A few minutes later, a smal

e
throughout the body. A few minutes later, a small
amount of the hormone norepinephrine was injected
into the blood (norepinephrine is the principal 
vasoconstrictor hormonal substance secreted at the
endings of the sympathetic vasoconstrictor nerve
ﬁbers throughout the body). As this injected hormone
was transported in the blood to all blood vessels, the
vessels once again became constricted, and the arterial
pressure rose to a level even greater than normal for
1 to 3 minutes, until the norep

er than normal for
1 to 3 minutes, until the norepinephrine was destroyed.
Control of Heart Activity by the Vasomotor Center. At the
same time that the vasomotor center is controlling the
amount of vascular constriction, it also controls heart
activity. The lateral portions of the vasomotor center
transmit excitatory impulses through the sympathetic
nerve ﬁbers to the heart when there is need to increase
heart rate and contractility. Conversely, when there is
need to decrease heart pumping, the 

when there is
need to decrease heart pumping, the medial portion of
Reticular
substance
Motor
Cingulate
Orbital
Temporal
Pons
Medulla
VASOMOTOR
CENTER
VASOCONSTRICTOR 
VASODILATOR
Mesencephalon
Figure 18–3
Areas of the brain that play important roles in the nervous regula-
tion of the circulation. The dashed lines represent inhibitory 
pathways.
Chapter 18
207
the vasomotor center sends signals to the adjacent
dorsal m

otor center sends signals to the adjacent
dorsal motor nuclei of the vagus nerves, which then
transmit parasympathetic impulses through the vagus
nerves to the heart to decrease heart rate and heart
contractility. Therefore, the vasomotor center can
either increase or decrease heart activity. Heart rate
and strength of heart contraction ordinarily increase
when vasoconstriction occurs and ordinarily decrease
when vasoconstriction is inhibited.
Control of the Vasomotor Center by Higher Nervous Ce

ntrol of the Vasomotor Center by Higher Nervous Centers.
Large numbers of small neurons located throughout
the reticular substance of the pons, mesencephalon,
and diencephalon can either excite or inhibit the 
vasomotor center. This reticular substance is shown 
in Figure 18–3 by the rose-colored area. In general, the
neurons in the more lateral and superior portions 
of the reticular substance cause excitation, whereas 
the more medial and inferior portions cause inhibition.
The hypothalamus pl

ior portions cause inhibition.
The hypothalamus plays a special role in controlling
the vasoconstrictor system because it can exert either
powerful excitatory or inhibitory effects on the vaso-
motor center. The posterolateral portions of the hypo-
thalamus cause mainly excitation, whereas the anterior
portion can cause either mild excitation or inhibition,
depending on the precise part of the anterior hypo-
thalamus stimulated.
Many parts of the cerebral cortex can also excite or
inhibit the va

 cerebral cortex can also excite or
inhibit the vasomotor center. Stimulation of the motor
cortex, for instance, excites the vasomotor center
because of impulses transmitted downward into the
hypothalamus and thence to the vasomotor center.
Also, stimulation of the anterior temporal lobe, the
orbital areas of the frontal cortex, the anterior part of
the cingulate gyrus, the amygdala, the septum, and the
hippocampus can all either excite or inhibit the vaso-
motor center, depending on the precise

t the vaso-
motor center, depending on the precise portions of
these areas that are stimulated and on the intensity of
stimulus.Thus, widespread basal areas of the brain can
have profound effects on cardiovascular function.
Norepinephrine—The Sympathetic Vasoconstrictor Transmitter
Substance. The substance secreted at the endings of the
vasoconstrictor nerves is almost entirely norepineph-
rine. Norepinephrine acts directly on the alpha adren-
ergic receptors of the vascular smooth muscle to cau

gic receptors of the vascular smooth muscle to cause
vasoconstriction, as discussed in Chapter 60.
Adrenal Medullae and Their Relation to the Sympathetic Vaso-
constrictor System. Sympathetic impulses are transmit-
ted to the adrenal medullae at the same time that they
are transmitted to the blood vessels. They cause the
medullae to secrete both epinephrine and norepineph-
rine into the circulating blood.These two hormones are
carried in the blood stream to all parts of the body,
where they act 

d stream to all parts of the body,
where they act directly on all blood vessels, usually 
to cause vasoconstriction, but in an occasional tissue
epinephrine causes vasodilation because it also has 
a “beta” adrenergic receptor stimulatory effect, which
dilates rather than constricts certain vessels, as dis-
cussed in Chapter 60.
Sympathetic Vasodilator System and its Control by the Central
Nervous System. The sympathetic nerves to skeletal
muscles carry sympathetic vasodilator ﬁbers as well as
c

s carry sympathetic vasodilator ﬁbers as well as
constrictor ﬁbers. In lower animals such as the cat, these
dilator ﬁbers release acetylcholine, not norepinephrine,
at their endings, although in primates, the vasodilator
Seconds
0
5
10
15
20
25
150
125
100
75
50
25
0
Total spinal 
anesthesia
Injection of norepinephrine
Figure 18–4
Effect of total spinal anesthesia on
the arterial pressure, showing
marked decrease in pressure
resulting from loss of “vasomotor
tone.”
208

sure
tone.”
208
Unit IV
effect is believed to be caused by epinephrine exciting
speciﬁc beta adrenergic receptors in the muscle 
vasculature.
The pathway for central nervous system control of
the vasodilator system is shown by the dashed lines in
Figure 18–3. The principal area of the brain controlling
this system is the anterior hypothalamus.
Possible Unimportance of the Sympathetic Vasodilator System.
It is doubtful that the sympathetic vasodil

ystem.
It is doubtful that the sympathetic vasodilator system
plays an important role in the control of the circulation
in the human being because complete block of the sym-
pathetic nerves to the muscles hardly affects the ability
of these muscles to control their own blood ﬂow in
response to their needs. Yet some experiments suggest
that at the onset of exercise, the sympathetic vasodila-
tor system might cause initial vasodilation in skeletal
muscles to allow anticipatory increase in blood ﬂo

muscles to allow anticipatory increase in blood ﬂow even
before the muscles require increased nutrients.
Emotional 
Fainting—Vasovagal 
Syncope. A 
particularly
interesting vasodilatory reaction occurs in people who
experience intense emotional disturbances that cause
fainting. In this case, the muscle vasodilator system
becomes activated, and at the same time, the vagal car-
dioinhibitory center transmits strong signals to the heart
to slow the heart rate markedly. The arterial pressure
falls r

heart rate markedly. The arterial pressure
falls rapidly, which reduces blood ﬂow to the brain and
causes the person to lose consciousness. This overall
effect is called vasovagal syncope. Emotional fainting
begins with disturbing thoughts in the cerebral cortex.
The pathway probably then goes to the vasodilatory
center of the anterior hypothalamus next to the vagal
centers of the medulla, to the heart through the vagus
nerves, and also through the spinal cord to the sympa-
thetic vasodilator ne

he spinal cord to the sympa-
thetic vasodilator nerves of the muscles.
Role of the Nervous System 
in Rapid Control of 
One of the most important functions of nervous
control of the circulation is its capability to cause rapid
increases in arterial pressure. For this purpose, the
entire vasoconstrictor and cardioaccelerator functions
of the sympathetic nervous system are stimulated
together. At the same time, there is reciprocal inhibi-
tion of parasympathetic vagal inhibitory 

 inhibi-
tion of parasympathetic vagal inhibitory signals to the
heart.Thus, three major changes occur simultaneously,
each of which helps to increase arterial pressure. They
are as follows:
1. Almost all arterioles of the systemic circulation 
are constricted. This greatly increases the total
peripheral resistance, thereby increasing the arterial
pressure.
2. The veins especially (but the other large vessels of
the circulation as well) are strongly constricted.
This displaces blood out of the l

gly constricted.
This displaces blood out of the large peripheral
blood vessels toward the heart, thus increasing the
volume of blood in the heart chambers. The stretch
of the heart then causes the heart to beat with far
greater force and therefore to pump increased
quantities of blood. This, too, increases the arterial
pressure.
3. Finally, the heart itself is directly stimulated by the
autonomic nervous system, further enhancing
cardiac pumping. Much of this is caused by an
increase in the hea

. Much of this is caused by an
increase in the heart rate, the rate sometimes
increasing to as great as three times normal. In
addition, sympathetic nervous signals have a
signiﬁcant direct effect to increase contractile 
force of the heart muscle, this, too, increasing the
capability of the heart to pump larger volumes of
blood. During strong sympathetic stimulation, the
heart can pump about two times as much blood as
under normal conditions. This contributes still more
to the acute rise in art

is contributes still more
to the acute rise in arterial pressure.
Rapidity of Nervous Control of Arterial Pressure. An espe-
cially important characteristic of nervous control of
arterial pressure is its rapidity of response, beginning
within seconds and often increasing the pressure to
two times normal within 5 to 10 seconds. Conversely,
sudden inhibition of nervous cardiovascular stimula-
tion can decrease the arterial pressure to as little as
one half normal within 10 to 40 seconds. Therefore

one half normal within 10 to 40 seconds. Therefore,
nervous control of arterial pressure is by far the most
rapid of all our mechanisms for pressure control.
Increase in Arterial Pressure 
During Muscle Exercise and Other
Types of Stress
An important example of the ability of the nervous
system to increase the arterial pressure is the increase
in pressure that occurs during muscle exercise. During
heavy exercise, the muscles require greatly increased
blood ﬂow. Part of this increase results from

ased
blood ﬂow. Part of this increase results from local
vasodilation of the muscle vasculature caused by
increased metabolism of the muscle cells, as explained
in Chapter 17. Additional increase results from simul-
taneous elevation of arterial pressure caused by sym-
pathetic stimulation of the overall circulation during
exercise. In most heavy exercise, the arterial pressure
rises about 30 to 40 per cent, which increases blood
ﬂow almost an additional twofold.
The increase in arterial pressur

ditional twofold.
The increase in arterial pressure during exercise
results mainly from the following effect: At the same
time that the motor areas of the brain become acti-
vated to cause exercise, most of the reticular activat-
ing system of the brain stem is also activated, which
includes greatly increased stimulation of the vasocon-
strictor and cardioacceleratory areas of the vasomotor
center. These increase the arterial pressure instanta-
neously to keep pace with the increase in muscle 
a

eously to keep pace with the increase in muscle 
activity.
In many other types of stress besides muscle exer-
cise, a similar rise in pressure can also occur. For
instance, during extreme fright, the arterial pressure
sometimes rises to as high as double normal within a
few seconds. This is called the alarm reaction, and it
provides an excess of arterial pressure that can imme-
diately supply blood to any or all muscles of the body
that might need to respond instantly to cause ﬂight
from danger.

d to respond instantly to cause ﬂight
Chapter 18
209
Reﬂex Mechanisms for Maintaining
Normal Arterial Pressure
Aside from the exercise and stress functions of the
autonomic nervous system to increase arterial pres-
sure, there are multiple subconscious special nervous
control mechanisms that operate all the time to main-
tain the arterial pressure at or near normal. Almost 
all of these are negative feedba

r normal. Almost 
all of these are negative feedback reﬂex mechanisms,
which we explain in the following sections.
The Baroreceptor Arterial Pressure Control
System—Baroreceptor Reﬂexes
By far the best known of the nervous mechanisms for
arterial pressure control is the baroreceptor reﬂex.
Basically, this reﬂex is initiated by stretch receptors,
called either baroreceptors or pressoreceptors, located
at speciﬁc points in the walls of several large systemic
arteries. A rise in arterial pressure s

e systemic
arteries. A rise in arterial pressure stretches the
baroreceptors and causes them to transmit signals into
the central nervous system. “Feedback” signals are
then sent back through the autonomic nervous system
to the circulation to reduce arterial pressure down-
ward toward the normal level.
Physiologic Anatomy of the Baroreceptors and Their Innerva-
tion. Baroreceptors are spray-type nerve endings that
lie in the walls of the arteries;they are stimulated when
stretched. A few barorec

;they are stimulated when
stretched. A few baroreceptors are located in the wall
of almost every large artery of the thoracic and neck
regions; but, as shown in Figure 18–5, baroreceptors
are extremely abundant in (1) the wall of each inter-
nal carotid artery slightly above the carotid bifurca-
tion, an area known as the carotid sinus, and (2) the
wall of the aortic arch.
Figure 18–5 shows that signals from the “carotid
baroreceptors” are transmitted through very small
Hering’s nerves to the gl

itted through very small
Hering’s nerves to the glossopharyngeal nerves in 
the high neck, and then to the tractus solitarius in the
medullary area of the brain stem. Signals from the
“aortic baroreceptors” in the arch of the aorta are
transmitted through the vagus nerves also to the same
tractus solitarius of the medulla.
Response of the Baroreceptors to Pressure. Figure 18–6
shows the effect of different arterial pressure levels on
the rate of impulse transmission in a Hering’s carotid
sinus n

impulse transmission in a Hering’s carotid
sinus nerve. Note that the carotid sinus baroreceptors
are not stimulated at all by pressures between 0 and
50 to 60 mm Hg, but above these levels, they respond
progressively more rapidly and reach a maximum at
about 180 mm Hg. The responses of the aortic barore-
ceptors are similar to those of the carotid receptors
except that they operate, in general, at pressure levels
about 30 mm Hg higher.
Note especially that in the normal operating range
of arter

cially that in the normal operating range
of arterial pressure, around 100 mm Hg, even a slight
change in pressure causes a strong change in the
baroreﬂex signal to readjust arterial pressure back
toward normal. Thus, the baroreceptor feedback
mechanism functions most effectively in the pressure
range where it is most needed.
Glossopharyngeal nerve
Hering’s nerve
Vagus nerve
Aortic baroreceptors
Carotid body
Carotid sinus
Figure 18–5
The baroreceptor system for controlling arterial pressure.
Num

ptor system for controlling arterial pressure.
Number of impulses from carotid
sinus nerves per second
80
0
160
244
DI
DP = maximum
Arterial blood pressure (mm Hg)
Figure 18–6
Activation of the baroreceptors at different levels of arterial 
pressure. DI, change in carotid sinus nerve impulses per second;
DP, change in arterial blood pressure in mm Hg.
210
Unit IV
The baroreceptors respond extremely rapidly to
changes in arterial pressure; in fact, the rate of impulse
ﬁring increa

ressure; in fact, the rate of impulse
ﬁring increases in the fraction of a second during each
systole and decreases again during diastole. Further-
more, the baroreceptors respond much more to a
rapidly changing pressure than to a stationary pres-
sure. That is, if the mean arterial pressure is 150 mm
Hg but at that moment is rising rapidly, the rate of
impulse transmission may be as much as twice that
when the pressure is stationary at 150 mm Hg.
Circulatory Reﬂex Initiated by the Baroreceptors

.
Circulatory Reﬂex Initiated by the Baroreceptors. After the
baroreceptor signals have entered the tractus solitar-
ius of the medulla, secondary signals inhibit the vaso-
constrictor center of the medulla and excite the vagal
parasympathetic center. The net effects are (1) vasodi-
lation of the veins and arterioles throughout the
peripheral circulatory system and (2) decreased heart
rate and strength of heart contraction. Therefore, exci-
tation of the baroreceptors by high pressure in the
art

n of the baroreceptors by high pressure in the
arteries reﬂexly causes the arterial pressure to decrease
because of both a decrease in peripheral resistance
and a decrease in cardiac output. Conversely, low pres-
sure has opposite effects, reﬂexly causing the pressure
to rise back toward normal.
Figure 18–7 shows a typical reﬂex change in arterial
pressure caused by occluding the two common carotid
arteries. This reduces the carotid sinus pressure; as a
result, the baroreceptors become inactive 

e; as a
result, the baroreceptors become inactive and lose
their inhibitory effect on the vasomotor center. The
vasomotor center then becomes much more active
than usual, causing the aortic arterial pressure to rise
and remain elevated during the 10 minutes that the
carotids are occluded. Removal of the occlusion allows
the pressure in the carotid sinuses to rise, and the
carotid sinus reﬂex now causes the aortic pressure to
fall immediately to slightly below normal as a momen-
tary overcompensa

lightly below normal as a momen-
tary overcompensation and then return to normal in
another minute.
Function of the Baroreceptors During Changes in Body
Posture. The ability of the baroreceptors to maintain
relatively constant arterial pressure in the upper body
is important when a person stands up after having
been lying down. Immediately on standing, the arterial
pressure in the head and upper part of the body tends
to fall, and marked reduction of this pressure could
cause loss of consciousne

n of this pressure could
cause loss of consciousness. However, the falling pres-
sure at the baroreceptors elicits an immediate reﬂex,
resulting in strong sympathetic discharge throughout
the body. This minimizes the decrease in pressure in
the head and upper body.
Pressure “Buffer” Function of the Baroreceptor
Control System. Because the baroreceptor system
opposes either increases or decreases in arterial pres-
sure,it is called a pressure buffer system,and the nerves
from the baroreceptors ar

er system,and the nerves
from the baroreceptors are called buffer nerves.
Figure 18–8 shows the importance of this buffer
function of the baroreceptors.The upper record in this
ﬁgure shows an arterial pressure recording for 2 hours
from a normal dog, and the lower record shows an
arterial pressure recording from a dog whose barore-
ceptor nerves from both the carotid sinuses and the
aorta had been removed. Note the extreme variability
of pressure in the denervated dog caused by simple
events of 

 in the denervated dog caused by simple
events of the day, such as lying down, standing, excite-
ment, eating, defecation, and noises.
Figure 18–9 shows the frequency distributions of the
mean arterial pressures recorded for a 24-hour day in
both the normal dog and the denervated dog. Note
that when the baroreceptors were functioning nor-
mally the mean arterial pressure remained throughout
the day within a narrow range between 85 and 
115 mm Hg—indeed, during most of the day at almost
exactly 1

indeed, during most of the day at almost
exactly 100 mm Hg. Conversely, after denervation of
the baroreceptors, the frequency distribution curve
became the broad, low curve of the ﬁgure, showing
that the pressure range increased 2.5-fold, frequently
falling to as low as 50 mm Hg or rising to over 160 mm
Hg. Thus, one can see the extreme variability of pres-
sure in the absence of the arterial baroreceptor
system.
In summary, a primary purpose of the arterial
baroreceptor system is to reduce the 

the arterial
baroreceptor system is to reduce the minute by minute
variation in arterial pressure to about one third that
which would occur were the baroreceptor system not
present.
Are the Baroreceptors Important in Long-Term Regulation of
Arterial Pressure? Although the arterial baroreceptors
provide powerful moment-to-moment control of 
arterial pressure,
their importance in long-term 
blood pressure regulation has been controversial. One
reason that the baroreceptors have been considered
150

on that the baroreceptors have been considered
150
Both common
carotids clamped
Carotids released
100
50
0
0
2
4
6
8
Minutes
10
12
14
Figure 18–7
Typical carotid sinus reﬂex effect on aortic arterial pressure
caused by clamping both common carotids (after the two vagus
nerves have been cut).
Chapter 18
211
by some physiologists to be relatively unimportant in
chronic regulation of arterial pre

 unimportant in
chronic regulation of arterial pressure chronically is
that they tend to reset in 1 to 2 days to the pressure
level to which they are exposed. That is, if the arterial
pressure rises from the normal value of 100 mm Hg to
160 mm Hg, a very high rate of baroreceptor impulses
are at ﬁrst transmitted. During the next few minutes,
the rate of ﬁring diminishes considerably; then it
diminishes much more slowly during the next 1 to 2
days, at the end of which time the rate of ﬁring will

, at the end of which time the rate of ﬁring will
have returned to nearly normal despite the fact that
the mean arterial pressure still remains at 160 mm Hg.
Conversely, when the arterial pressure falls to a very
low level, the baroreceptors at ﬁrst transmit no
impulses, but gradually, over 1 to 2 days, the rate of
baroreceptor ﬁring returns toward the control level.
This “resetting” of the baroreceptors may attenuate
their potency as a control system for correcting dis-
turbances that tend to c

ystem for correcting dis-
turbances that tend to change arterial pressure for
longer than a few days at a time. Experimental studies,
however, have suggested that the baroreceptors do 
not completely reset and may therefore contribute to
long-term blood pressure regulation, especially by
inﬂuencing sympathetic nerve activity of the kidneys.
For example, with prolonged increases in arterial pres-
sure, the baroreceptor reﬂexes may mediate decreases
in renal sympathetic nerve activity that promote

s
increased excretion of sodium and water by the
kidneys. This, in turn, causes a gradual decrease in
blood volume, which helps to restore arterial pressure
toward normal. Thus, long-term regulation of mean
arterial pressure by the baroreceptors requires 
interaction with additional systems, principally the
renal–body ﬂuid–pressure control system (along with
its associated nervous and hormonal mechanisms), dis-
cussed in Chapters 19 and 29.
Contro

anisms), dis-
Control of Arterial Pressure by the Carotid and Aortic 
Chemoreceptors—Effect of Oxygen Lack on Arterial Pressure.
Closely associated with the baroreceptor pressure
control system is a chemoreceptor reﬂex that operates in
much the same way as the baroreceptor reﬂex except
that chemoreceptors, instead of stretch receptors, initi-
ate the response.
The chemoreceptors are chemosensitive cells sensi-
tive to oxygen lack, carbon dioxide excess, and hydro-
g

o oxygen lack, carbon dioxide excess, and hydro-
gen ion excess. They are located in several small
chemoreceptor organs about 2 millimeters in size (two
carotid bodies, one of which lies in the bifurcation of
each common carotid artery, and usually one to three
aortic bodies adjacent to the aorta). The chemo-
receptors excite nerve ﬁbers that, along with the 
NORMAL
24
DENERVATED
Time (min)
200
100
0
200
100
0
24
Figure 18–8
Two-hour records of arterial pressure in a no

18–8
Two-hour records of arterial pressure in a normal dog (above) and
in the same dog (below) several weeks after the baroreceptors
had been denervated. (Redrawn from Cowley AW Jr, Liard JF,
Guyton AC: Role of baroreceptor reﬂex in daily control of arterial
blood pressure and other variables in dogs. Circ Res 32:564,
1973. By permission of the American Heart Association, Inc.)
Percentage of occurrence
0
50
100
150
200
250
0
1
2
3
4
5
6
Mean arterial pressure (mm Hg)
Normal
Denervated
Figure 18–

rial pressure (mm Hg)
Normal
Figure 18–9
Frequency distribution curves of the arterial pressure for a 24-hour
period in a normal dog and in the same dog several weeks after
the baroreceptors had been denervated. (Redrawn from Cowley
AW Jr, Liard JP, Guyton AC: Role of baroreceptor reﬂex in daily
control of arterial blood pressure and other variables in dogs. Circ
Res 32:564, 1973. By permission of the American Heart Associa-
tion, Inc.)
212
Unit IV
baroreceptor ﬁbers, 

)
212
Unit IV
baroreceptor ﬁbers, pass through Hering’s nerves and
the vagus nerves into the vasomotor center of the brain
stem.
Each carotid or aortic body is supplied with an 
abundant blood ﬂow through a small nutrient artery, so
that the chemoreceptors are always in close contact 
with arterial blood. Whenever the arterial pressure falls
below a critical level, the chemoreceptors become stim-
ulated because diminished blood ﬂow causes decreased
oxygen as well as excess buildu

w causes decreased
oxygen as well as excess buildup of carbon dioxide and
hydrogen ions that are not removed by the slowly
ﬂowing blood.
The signals transmitted from the chemoreceptors
excite the vasomotor center, and this elevates the arterial
pressure back toward normal. However, this chemo-
receptor reﬂex is not a powerful arterial pressure con-
troller until the arterial pressure falls below 80 mm Hg.
Therefore, it is at the lower pressures that this reﬂex
becomes important to help prevent s

hat this reﬂex
becomes important to help prevent still further fall in
pressure.
The chemoreceptors are discussed in much more
detail in Chapter 41 in relation to respiratory control,
in which they play a far more important role than in
pressure control.
Atrial and Pulmonary Artery Reﬂexes That Help Regulate Arterial
Pressure and Other Circulatory Factors. Both the atria and
the pulmonary arteries have in their walls stretch recep-
tors called low-pressure receptors. They are similar to
the baro

w-pressure receptors. They are similar to
the baroreceptor stretch receptors of the large systemic
arteries.These low-pressure receptors play an important
role, especially in minimizing arterial pressure changes
in response to changes in blood volume. To give an
example, if 300 milliliters of blood suddenly are infused
into a dog with allreceptors intact, the arterial pressure
rises only about 15 mm Hg. With the arterial barorecep-
tors denervated, the pressure rises about 40 mm Hg. If
the low-p

d, the pressure rises about 40 mm Hg. If
the low-pressure receptors also are denervated, the pres-
sure rises about 100 mm Hg.
Thus, one can see that even though the low-pressure
receptors in the pulmonary artery and in the atria
cannot detect the systemic arterial pressure, they do
detect simultaneous increases in pressure in the low-
pressure areas of the circulation caused by increase in
volume, and they elicit reﬂexes parallel to the barore-
ceptor reﬂexes to make the total reﬂex system more

potent for control of arterial pressure.
Atrial Reﬂexes That Activate the Kidneys—The “Volume Reﬂex.”
Stretch of the atria also causes signiﬁcant reﬂex dilation
of the afferent arterioles in the kidneys. And still 
other signals are transmitted simultaneously from the
atria to the hypothalamus to decrease secretion of
antidiuretic hormone. The decreased afferent arteriolar
resistance in the kidneys causes the glomerular capillary
pressure to ris

ys causes the glomerular capillary
pressure to rise, with resultant increase in ﬁltration of
ﬂuid into the kidney tubules. The diminution of antidi-
uretic hormone diminishes the reabsorption of water
from the tubules. Combination of these two effects—
increase in glomerular ﬁltration and decrease in reab-
sorption of the ﬂuid—increases ﬂuid loss by the kidneys
and reduces an increased blood volume back toward
normal. (We will also see in Chapter 19 that atrial
stretch caused by increased blood 

 19 that atrial
stretch caused by increased blood volume also elicits a
hormonal effect on the kidneys—release of atrial natri-
uretic peptide that adds still further to the excretion of
ﬂuid in the urine and return of blood volume toward
normal.)
All these mechanisms that tend to return the blood
volume back toward normal after a volume overload act
indirectly as pressure controllers as well as blood
volume controllers because excess volume drives the
heart to greater cardiac output and leads, 

es the
heart to greater cardiac output and leads, therefore, to
greater arterial pressure.This volume reﬂex mechanism
is discussed again in Chapter 29, along with other mech-
anisms of blood volume control.
Atrial Reﬂex Control of Heart Rate (the Bainbridge Reﬂex). An
increase in atrial pressure also causes an increase in
heart rate, sometimes increasing the heart rate as much
as 75 per cent. A small part of this increase is caused by
a direct effect of the increased atrial volume to stretch
the

fect of the increased atrial volume to stretch
the sinus node: it was pointed out in Chapter 10 that
such direct stretch can increase the heart rate as much
as 15 per cent. An additional 40 to 60 per cent increase
in rate is caused by a nervous reﬂex called the Bain-
bridge reﬂex. The stretch receptors of the atria that elicit
the Bainbridge reﬂex transmit their afferent signals
through the vagus nerves to the medulla of the brain.
Then efferent signals are transmitted back through
vagal and sym

signals are transmitted back through
vagal and sympathetic nerves to increase heart rate and
strength of heart contraction. Thus, this reﬂex helps
prevent damming of blood in the veins, atria, and pul-
monary circulation.
Central Nervous System Ischemic
Response—Control of Arterial
Pressure by the Brain’s Vasomotor
Center in Response to Diminished
Brain Blood Flow
Most nervous control of blood pressure is achieved 
by reﬂexes that originate in the baroreceptors, the
chemoreceptors, and the low-p

e baroreceptors, the
chemoreceptors, and the low-pressure receptors, all of
which are located in the peripheral circulation outside
the brain. However, when blood ﬂow to the vasomotor
center in the lower brain stem becomes decreased
severely enough to cause nutritional deﬁciency—that is,
to cause cerebral ischemia—the vasoconstrictor and
cardioaccelerator neurons in the vasomotor center
respond directly to the ischemia and become strongly
excited.When this occurs, the systemic arterial pressure

.When this occurs, the systemic arterial pressure
often rises to a level as high as the heart can possibly
pump. This effect is believed to be caused by failure of
the slowly ﬂowing blood to carry carbon dioxide away
from the brain stem vasomotor center: at low levels of
blood ﬂow to the vasomotor center, the local con-
centration of carbon dioxide increases greatly and 
has an extremely potent effect in stimulating the sym-
pathetic vasomotor nervous control areas in the brain’s
medulla.
It is 

rvous control areas in the brain’s
medulla.
It is possible that other factors, such as buildup of
lactic acid and other acidic substances in the vasomotor
center, also contribute to the marked stimulation and
elevation in arterial pressure. This arterial pressure ele-
vation in response to cerebral ischemia is known as the
central nervous system ischemic response, or simply CNS
ischemic response.
The magnitude of the ischemic effect on vasomotor
activity is tremendous: it can elevate the mean ar

activity is tremendous: it can elevate the mean arterial
pressure for as long as 10 minutes sometimes to as high
as 250 mm Hg. The degree of sympathetic vasoconstric-
tion caused by intense cerebral ischemia is often so great
that some of the peripheral vessels become totally or
almost totally occluded. The kidneys, for instance, often
Chapter 18
213
entirely cease their production of urine because of renal

r production of urine because of renal
arteriolar constriction in response to the sympathetic
discharge. Therefore, the CNS ischemic response is one
of the most powerful of all the activators of the sympa-
thetic vasoconstrictor system.
Importance of the CNS Ischemic Response as a Regulator of Arte-
rial Pressure. Despite the powerful nature of the CNS
ischemic response, it does not become signiﬁcant until
the arterial pressure falls far below normal, down to 
60 mm Hg and below, reaching its gr

rmal, down to 
60 mm Hg and below, reaching its greatest degree of
stimulation at a pressure of 15 to 20 mm Hg. Therefore,
it is not one of the normal mechanisms for regulating
arterial pressure. Instead, it operates principally as an
emergency pressure control system that acts rapidly and
very powerfully to prevent further decrease in arterial
pressure whenever blood ﬂow to the brain decreases dan-
gerously close to the lethal level. It is sometimes called
the “last ditch stand” pressure contro

imes called
the “last ditch stand” pressure control mechanism.
Cushing Reaction. The so-called Cushing reaction is a
special type of CNS ischemic response that results from
increased pressure of the cerebrospinal ﬂuid around 
the brain in the cranial vault. For instance, when the
cerebrospinal ﬂuid pressure rises to equal the arterial
pressure, it compresses the whole brain as well as the
arteries in the brain and cuts off the blood supply to the
brain.This initiates a CNS ischemic response that

brain.This initiates a CNS ischemic response that causes
the arterial pressure to rise. When the arterial pressure
has risen to a level higher than the cerebrospinal ﬂuid
pressure, blood will ﬂow once again into the vessels of
the brain to relieve the brain ischemia. Ordinarily, the
blood pressure comes to a new equilibrium level slightly
higher than the cerebrospinal ﬂuid pressure, thus allow-
ing blood to begin again to ﬂow through the brain. A
typical Cushing reaction is shown in Figure 18–1

A
typical Cushing reaction is shown in Figure 18–10,
caused in this instance by pumping ﬂuid under pressure
into the cranial vault around the brain. The Cushing
reaction helps protect the vital centers of the brain from
loss of nutrition if ever the cerebrospinal ﬂuid pressure
rises high enough to compress the cerebral arteries.
Special Features of Nervous
Role of the Skeletal Nerves and
Skeletal Muscles in Increasing
Cardiac Output and 
Although mo

Although most rapidly acting nervous control of the cir-
culation is effected through the autonomic nervous
system, at least two conditions in which the skeletal
nerves and muscles also play major roles in circulatory
responses are the following.
Abdominal Compression Reﬂex. When a baroreceptor or
chemoreceptor reﬂex is elicited, nerve signals are trans-
mitted simultaneously through skeletal nerves to skele-
tal muscles of the body, particularly to the abd

-
tal muscles of the body, particularly to the abdominal
muscles. This compresses all the venous reservoirs of 
the abdomen, helping to translocate blood out of the
abdominal vascular reservoirs toward the heart. As a
result, increased quantities of blood are made available
for the heart to pump.This overall response is called the
abdominal compression reﬂex. The resulting effect on
the circulation is the same as that caused by sympathetic
vasoconstrictor impulses when they constrict the veins:

nstrictor impulses when they constrict the veins:
an increase in both cardiac output and arterial pressure.
The abdominal compression reﬂex is probably much
more important than has been realized in the past
because it is well known that people whose skeletal
muscles have been paralyzed are considerably more
prone to hypotensive episodes than are people with
normal skeletal muscles.
Increased Cardiac Output and Arterial Pressure Caused by 
Skeletal Muscle Contraction During Exercise. When the 
sk

l Muscle Contraction During Exercise. When the 
skeletal muscles contract during exercise, they compress
blood vessels throughout the body. Even anticipation of
exercise tightens the muscles, thereby compressing 
the vessels in the muscles and in the abdomen. The
Connector to
subarachnoid
space
Pressure
bottle
Arterial
pressure
Pen
recorder
CSF pressure
reduced
raised
Zero
pressure
Moving paper
Arterial
pressure
Figure 18–10
“Cushing reaction,” showing a
rapid rise in art

10
rapid rise in arterial pressure
resulting from increased cere-
brospinal ﬂuid (CSF) pressure.
214
Unit IV
resulting effect is to translocate blood from the periph-
eral vessels into the heart and lungs and, therefore, to
increase the cardiac output. This is an essential effect in
helping to cause the ﬁvefold to sevenfold increase in
cardiac output that sometimes occurs in heavy exercise.
The increase in cardiac output in turn is an essential
ingre

se in cardiac output in turn is an essential
ingredient in increasing the arterial pressure during
exercise, an increase usually from a normal mean of 
100 mm Hg up to 130 to 160 mm Hg.
Respiratory Waves in the 
With each cycle of respiration, the arterial pressure
usually rises and falls 4 to 6 mm Hg in a wavelike
manner, causing respiratory waves in the arterial pres-
sure. The waves result from several different effects,
some of which are reﬂex in nature, as follows:
1. Many

 of which are reﬂex in nature, as follows:
1. Many of the “breathing signals” that arise in the
respiratory center of the medulla “spill over” into
the vasomotor center with each respiratory cycle.
2. Every time a person inspires, the pressure in the
thoracic cavity becomes more negative than usual,
causing the blood vessels in the chest to expand.
This reduces the quantity of blood returning to the
left side of the heart and thereby momentarily
decreases the cardiac output and arterial pressure

decreases the cardiac output and arterial pressure.
3. The pressure changes caused in the thoracic vessels
by respiration can excite vascular and atrial stretch
Although it is difﬁcult to analyze the exact relations
of all these factors in causing the respiratory pressure
waves,the net result during normal respiration is usually
an increase in arterial pressure during the early part 
of expiration and a decrease in pressure during the
remainder of the respiratory cycle. During deep re

remainder of the respiratory cycle. During deep respi-
ration, the blood pressure can rise and fall as much as
20 mm Hg with each respiratory cycle.
Arterial Pressure “Vasomotor”
Waves—Oscillation of Pressure
Reﬂex Control Systems
Often while recording arterial pressure from an animal,
in addition to the small pressure waves caused by res-
piration, some much larger waves are also noted—as
great as 10 to 40 mm Hg at times—that rise and fall
more slowly than the respiratory waves.The duration of

slowly than the respiratory waves.The duration of
each cycle varies from 26 seconds in the anesthetized
dog to 7 to 10 seconds in the unanesthetized human.
These waves are called vasomotor waves or “Mayer
waves.” Such records are demonstrated in Figure 18–11,
showing the cyclical rise and fall in arterial pressure.
The cause of vasomotor waves is “reﬂex oscillation”
of one or more nervous pressure control mechanisms,
some of which are the following.
Oscillation of the Baroreceptor and Chemorecep

ng.
Oscillation of the Baroreceptor and Chemoreceptor Reﬂexes.
The vasomotor waves of Figure 18–11B are often seen
in experimental pressure recordings, although usually
much less intense than shown in the ﬁgure. They are
caused mainly by oscillation of the baroreceptor reﬂex.
That is, a high pressure excites the baroreceptors;
this then inhibits the sympathetic nervous system and
lowers the pressure a few seconds later. The decreased
pressure in turn reduces the baroreceptor stimulation
and allo

turn reduces the baroreceptor stimulation
and allows the vasomotor center to become active once
again, elevating the pressure to a high value. The
response is not instantaneous, and it is delayed until a
few seconds later. This high pressure then initiates
another cycle, and the oscillation continues on and on.
The chemoreceptor reﬂex can also oscillate to give the
same type of waves. This reﬂex usually oscillates simul-
taneously with the baroreceptor reﬂex.It probably plays
the major role in c

ceptor reﬂex.It probably plays
the major role in causing vasomotor waves when the
arterial pressure is in the range of 40 to 80 mm Hg
because in this low range, chemoreceptor control of the
circulation becomes powerful, whereas baroreceptor
control becomes weaker.
Oscillation of the CNS Ischemic Response. The record in
Figure 18–11A resulted from oscillation of the CNS
ischemic pressure control mechanism. In this experi-
ment, the cerebrospinal ﬂuid pressure was raised to 
160 mm Hg, which compr

uid pressure was raised to 
160 mm Hg, which compressed the cerebral vessels and
initiated a CNS ischemic pressure response up to 
200 mm Hg. When the arterial pressure rose to such a
high value, the brain ischemia was relieved and the sym-
pathetic nervous system became inactive. As a result,
the arterial pressure fell rapidly back to a much lower
value, causing brain ischemia once again. The ischemia
then initiated another rise in pressure. Again the
ischemia was relieved and again the pressur

in the
ischemia was relieved and again the pressure fell. This
repeated itself cyclically as long as the cerebrospinal
ﬂuid pressure remained elevated.
Thus, any reﬂex pressure control mechanism can
oscillate if the intensity of “feedback” is strong enough
and if there is a delay between excitation of the pres-
sure receptor and the subsequent pressure response.
The vasomotor waves are of considerable theoretical
importance because they show that the nervous reﬂexes
that control arterial pressur

 the nervous reﬂexes
that control arterial pressure obey the same principles
as those applicable to mechanical and electrical control
systems. For instance, if the feedback “gain” is too great
in the guiding mechanism of an automatic pilot for an
airplane and there is also delay in response time of the
guiding mechanism, the plane will oscillate from side to
side instead of following a straight course.
200 
160 
120 
80 
40 
0
A
B
100
60
Figure 18–11
A, Vasomotor waves caused by

B
100
60
A, Vasomotor waves caused by oscillation of the CNS ischemic
response. B, Vasomotor waves caused by baroreceptor reﬂex
oscillation.
Chapter 18
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ren S, et al: Sustained acti-
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C
H
A
P
T
E
R
 
1
9
216
Dominant Role of the Kidney 
in Long-Term Regulation of
Arterial Pressure and in
Hypertension: The Integrated
Short-term control of arterial pressure by the sym-
pathetic nervous system, as discussed in Chapter 18,
occurs primarily through the effects of the nerv

,
occurs primarily through the effects of the nervous
system on total peripheral vascular resistance and
capacitance, and on cardiac pumping ability.
The body, however, also has powerful mecha-
nisms for regulating arterial pressure week 
after week and month after month. This long-term
control of arterial pressure is closely intertwined with homeostasis of body ﬂuid
volume, which is determined by the balance between the ﬂuid intake and
output. For long-term survival, ﬂuid intake and output must

or long-term survival, ﬂuid intake and output must be precisely bal-
anced, a task that is performed by multiple nervous and hormonal controls, and
by local control systems within the kidneys that regulate their excretion of salt
and water. In this chapter we discuss these renal–body ﬂuid systems that play
a dominant role in long-term blood pressure regulation.
The renal–body ﬂuid system for arterial pressure control is a simple one: When
th

arterial pressure control is a simple one: When
the body contains too much extracellular ﬂuid, the blood volume and arterial
pressure rise.The rising pressure in turn has a direct effect to cause the kidneys
to excrete the excess extracellular ﬂuid, thus returning the pressure back toward
normal.
In the phylogenetic history of animal development, this renal–body ﬂuid
system for pressure control is a primitive one. It is fully operative in one of the
lowest of vertebrates, the hagﬁsh. This animal

the
lowest of vertebrates, the hagﬁsh. This animal has a low arterial pressure, only
8 to 14 mm Hg, and this pressure increases almost directly in proportion to its
blood volume. The hagﬁsh continually drinks sea water, which is absorbed into
its blood, increasing the blood volume as well as the pressure. However, when
the pressure rises too high, the kidney simply excretes the excess volume into
the urine and relieves the pressure.At low pressure, the kidney excretes far less
ﬂuid than is inges

e, the kidney excretes far less
ﬂuid than is ingested. Therefore, because the hagﬁsh continues to drink, extra-
cellular ﬂuid volume, blood volume, and pressure all build up again to the higher
levels.
Throughout the ages, this primitive mechanism of pressure control has sur-
vived almost exactly as it functions in the hagﬁsh; in the human being, kidney
output of water and salt is just as sensitive to pressure changes as in the hagﬁsh,
if not more so. Indeed, an increase in arterial pressure in 

e so. Indeed, an increase in arterial pressure in the human of only a
few millimeters of mercury can double renal output of water, which is called
pressure diuresis, as well as double the output of salt, which is called pressure
natriuresis.
Chapter 19
217
In the human being, the renal–body ﬂuid system for
arterial pressure control, just as in the hagﬁsh, is the
fundamental basis for long-term arterial pressure
control. However, through the stages of ev

ressure
control. However, through the stages of evolution,
multiple reﬁnements have been added to make this
system much more exact in its control in the human
being. An especially important reﬁnement, as we shall
see, has been addition of the renin-angiotensin 
Quantitation of Pressure Diuresis as 
a Basis for Arterial Pressure Control
Figure 19–1 shows the approximate average effect of
different arterial pressure levels on urinary volume
output by an isolated kidney, demonstrating ma

ume
output by an isolated kidney, demonstrating markedly
increased output of volume as the pressure rises. This
increased urinary output is the phenomenon of pres-
sure diuresis. The curve in this ﬁgure is called a renal
urinary  output curve or a renal function curve. In the
human being, at an arterial pressure of 50 mm Hg, the
urine output is essentially zero. At 100 mm Hg it is
normal, and at 200 mm Hg it is about six to eight times
normal. Furthermore, not only does increasing the
arterial p

rthermore, not only does increasing the
arterial pressure increase urine volume output, but it
causes approximately equal increase in sodium output,
which is the phenomenon of pressure natriuresis.
An Experiment Demonstrating the Renal–Body Fluid System for
Arterial Pressure Control. Figure 19–2 shows the results
of a research experiment in dogs in which all the
nervous reﬂex mechanisms for blood pressure control
were ﬁrst blocked. Then the arterial pressure was sud-
denly elevated by infusing a

ial pressure was sud-
denly elevated by infusing about 400 milliliters of
blood intravenously. Note the instantaneous increase
in cardiac output to about double normal and increase 
in mean arterial pressure to 205 mm Hg, 115 mm Hg
above its resting level. Shown by the middle curve is
the effect of this increased arterial pressure on urine
output, which increased 12-fold. Along with this
tremendous loss of ﬂuid in the urine, both the cardiac
output and the arterial pressure returned to normal
du

ut and the arterial pressure returned to normal
during the subsequent hour.Thus, one sees an extreme
capability of the kidneys to eliminate ﬂuid volume
from the body in response to high arterial pressure and
in so doing to return the arterial pressure back to
normal.
Graphical Analysis of Pressure Control by the Renal–Body Fluid
Mechanism, Demonstrating an “Inﬁnite Feedback Gain”
Feature. Figure 19–3 shows a graphical method that
can be used for analyzing arterial pressure control by
the renal–b

analyzing arterial pressure control by
the renal–body ﬂuid system. This analysis is based on
two separate curves that intersect each other: (1) the
renal output curve for water and salt in response to
rising arterial pressure, which is the same renal output
curve as that shown in Figure 19–1, and (2) the curve
(or line) that represents the net water and salt intake.
Over a long period, the water and salt output must
equal the intake. Furthermore, the only place on the
graph in Figure 19–3 at whi

 the only place on the
graph in Figure 19–3 at which output equals intake is
Urinary volume output (x normal)
0
20
40
60
0
8
7
6
5
4
3
2
1
Figure 19–1
Typical renal urinary output curve measured in a perfused isolated
kidney, showing pressure diuresis when the arterial pressure rises
above normal.
0 10 20 30 40 50 60
120
Infusion period
50
75
100
125
150
175
200
225
0
1
2
3
4
1000
2000
3000
4000
(ml/min)
Urinary o

0
2000
3000
4000
(ml/min)
Urinary output
(ml/min)
(mm Hg)
Figure 19–2
Increases in cardiac output, urinary output, and arterial pressure
caused by increased blood volume in dogs whose nervous pres-
sure control mechanisms had been blocked. This ﬁgure shows
return of arterial pressure to normal after about an hour of ﬂuid
loss into the urine. (Courtesy Dr. William Dobbs.)
218
Unit IV
where the two curves intersect, which is called the
equilibrium p

urves intersect, which is called the
equilibrium point. Now, let us see what happens if the
arterial pressure becomes some value that is different
from that at the equilibrium point.
First, assume that the arterial pressure rises to 
150 mm Hg. At this level, the graph shows that renal
output of water and salt is about three times as great
as the intake.Therefore, the body loses ﬂuid, the blood
volume decreases, and the arterial pressure decreases.
Furthermore, this “negative balance” of ﬂuid wi

s.
Furthermore, this “negative balance” of ﬂuid will not
cease until the pressure falls all the way back exactly
to the equilibrium level. Indeed, even when the arte-
rial pressure is only 1 mm Hg greater than the equi-
librium level, there still is slightly more loss of water
and salt than intake, so that the pressure continues to
fall that last 1 mm Hg until the pressure eventually
returns exactly to the equilibrium point.
If the arterial pressure falls below the equilibrium
point, the intake 

ure falls below the equilibrium
point, the intake of water and salt is greater than the
output. Therefore, body ﬂuid volume increases, blood
volume increases, and the arterial pressure rises until
once again it returns exactly to the equilibrium point.
This return of the arterial pressure always exactly 
back to the equilibrium point is the inﬁnite feedback
gain principle for control of arterial pressure by the
renal–body ﬂuid mechanism.
Two Determinants of the Long-Term Arterial Pressure Level.

rminants of the Long-Term Arterial Pressure Level. In
Figure 19–3, one can also see that two basic long-term
factors determine the long-term arterial pressure level.
This can be explained as follows.
As long as the two curves representing (1) renal
output of salt and water and (2) intake of salt and
water remain exactly as they are shown in Figure 19–3,
the long-term mean arterial pressure level will always
readjust exactly to 100 mm Hg, which is the pressure
level depicted by the equilibrium po

 the pressure
level depicted by the equilibrium point of this ﬁgure.
Furthermore, there are only two ways in which the
pressure of this equilibrium point can be changed from
the 100 mm Hg level. One of these is by shifting the
pressure level of the renal output curve for salt and
water; and the other is by changing the level of the
water and salt intake line.Therefore, expressed simply,
the two primary determinants of the long-term arte-
rial pressure level are as follows:
1. The degree of press

ssure level are as follows:
1. The degree of pressure shift of the renal output
curve for water and salt
2. The level of the water and salt intake line
Operation of these two determinants in the control
of arterial pressure is demonstrated in Figure 19–4. In
Figure 19–4A, some abnormality of the kidneys has
caused the renal output curve to shift 50 mm Hg in the
high-pressure direction (to the right). Note that the
equilibrium point has also shifted to 50 mm Hg higher
than normal. Therefore, one 

ed to 50 mm Hg higher
than normal. Therefore, one can state that if the renal
output curve shifts to a new pressure level, so will the
Intake or output (x normal)
0
50
100
150
250
0
2
4
6
8
Equilibrium point
Renal output of water and salt
Water and salt intake
Figure 19–3
Analysis of arterial pressure regulation by equating the “renal
output curve” with the “salt and water intake curve.” The equilib-
rium point describes the level to which the arterial pressure will
be 

 the level to which the arterial pressure will
be regulated. (That small portion of the salt and water intake that
is lost from the body through nonrenal routes is ignored in this and
similar ﬁgures in this chapter.)
0
50
100
150
200
250
0
2
4
6
8
Normal
Elevated
pressure
0
50
100
150
200
250
0
2
4
6
8
Normal
Elevated
pressure
A
B
Figure 19–4
Two ways in which the arterial pressure can be increased: A, by
shifting the renal output curve in th

eased: A, by
shifting the renal output curve in the right-hand direction toward
a higher pressure level or B, by increasing the intake level of salt
and water.
Chapter 19
219
arterial pressure follow to this new pressure level
within a few days.
Figure 19–4B shows how a change in the level of salt
and water intake also can change the arterial pressure.
In this case, the intake level has increased fourfold and
the equilibrium point has shifted to a press

d and
the equilibrium point has shifted to a pressure level of
160 mm Hg, 60 mm Hg above the normal level. Con-
versely, a decrease in the intake level would reduce the
Thus, it is impossible to change the long-term mean
arterial pressure level to a new value without changing
one or both of the two basic determinants of long-term
arterial pressure—either (1) the level of salt and water
intake or (2) the degree of shift of the renal function
curve along the pressure axis. Howev

enal function
curve along the pressure axis. However, if either of
these is changed, one ﬁnds the arterial pressure there-
after to be regulated at a new pressure level, at the
pressure level at which the two new curves intersect.
Failure of Increased Total Peripheral
Resistance to Elevate the Long-Term Level 
of Arterial Pressure if Fluid Intake and Renal
Function Do Not Change
Now is the chance for the reader to see whether he or
she really understands the renal–body ﬂuid mecha-
nism for arter

erstands the renal–body ﬂuid mecha-
nism for arterial pressure control. Recalling the basic
equation for arterial pressure—arterial pressure equals
cardiac output times total peripheral resistance—it is
clear that an increase in total peripheral resistance
should elevate the arterial pressure. Indeed, when the
total peripheral resistance is acutely increased, the arte-
rial pressure does rise immediately. Yet if the kidneys
continue to function normally, the acute rise in arterial
pressure usual

ormally, the acute rise in arterial
pressure usually is not maintained. Instead, the arte-
rial pressure returns all the way to normal within a day
or so. Why?
The answer to this is the following: Increasing resist-
ance in the blood vessels everywhere else in the body
besides in the kidneys does not change the equilibrium
point for blood pressure control as dictated by the
kidneys (see again Figures 19–3 and 19–4). Instead, the
kidneys immediately begin to respond to the high arte-
rial pressur

ly begin to respond to the high arte-
rial pressure, causing pressure diuresis and pressure
natriuresis. Within hours, large amounts of salt and
water are lost from the body, and this continues until
the arterial pressure returns exactly to the pressure
level of the equilibrium point.
As proof of this principle that changes in total
peripheral resistance do not affect the long-term level
of arterial pressure if function of the kidneys is still
normal, carefully study Figure 19–5. This ﬁgure show

rmal, carefully study Figure 19–5. This ﬁgure shows
the approximate cardiac outputs and the arterial pres-
sures in different clinical conditions in which the long-
term total peripheral resistance is either much less than
or much greater than normal, but kidney excretion of
salt and water is normal. Note in all these different
clinical conditions that the arterial pressure is also
exactly normal.
(A word of caution! Many times when the total
peripheral resistance increases, this increases the
i

ipheral resistance increases, this increases the
intrarenal vascular resistance at the same time, which
alters the function of the kidney and can cause 
hypertension by shifting the renal function curve to a
higher pressure level, in the manner shown in Figure
19–4A. We see an example of this later in this chapter
when we discuss hypertension caused by vasoconstric-
tor mechanisms. But it is the increase in renal resistance
that is the culprit, not the increased total peripheral
resistance—an im

ot the increased total peripheral
resistance—an important distinction!)
Increased Fluid Volume Can Elevate Arterial
Pressure by Increasing Cardiac Output or 
The overall mechanism by which increased extracellu-
lar ﬂuid volume elevates arterial pressure is given in
the schema of Figure 19–6. The sequential events are
(1) increased extracellular ﬂuid volume (2) increases
the blood volume, which (3) increases the mean cir-
culatory ﬁlling pressure, which (4) increases v

r-
culatory ﬁlling pressure, which (4) increases venous
return of blood to the heart, which (5) increases
cardiac output, which (6) increases arterial pressure.
Note especially in this schema the two ways in which
an increase in cardiac output can increase the arterial
pressure. One of these is the direct effect of increased
cardiac output to increase the pressure, and the other
is an indirect effect to raise total peripheral vascular
resistance through autoregulation of blood ﬂow. The
second ef

through autoregulation of blood ﬂow. The
second effect can be explained as follows.
Referring back to Chapter 17, let us recall that
whenever an excess amount of blood ﬂows through a
Arterial pressure  and cardiac output
(per cent of normal)
Total peripheral resistance
160
40
60
80
100
120
140
200
0
50
100
150
Beriberi
Anemia
Paget's disease
Pulmonary disease
Normal
Removal of four limbs
AV shunts
output
Cardiac
Figure 19–5
Re

sm
output
Cardiac
Relations of total peripheral resistance to the long-term levels of
arterial pressure and cardiac output in different clinical abnor-
malities. In these conditions, the kidneys were functioning nor-
mally. Note that changing the whole-body total peripheral
resistance caused equal and opposite changes in cardiac output
but in all cases had no effect on arterial pressure. (Redrawn from
Guyton AC: Arterial Pressure and Hypertension. Philadelphia: WB
S

rial Pressure and Hypertension. Philadelphia: WB
Saunders Co, 1980.)
220
Unit IV
tissue, the local tissue vasculature constricts and
decreases the blood ﬂow back toward normal. This
phenomenon is called “autoregulation,” which means
simply regulation of blood ﬂow by the tissue itself.
When increased blood volume increases the cardiac
output, the blood ﬂow increases in all tissues of the
body, so that this autoregulation mechanism constricts
blood vessels all over the body. This i

constricts
blood vessels all over the body. This in turn increases
the total peripheral resistance.
Finally, because arterial pressure is equal to cardiac
output times total peripheral resistance, the secondary
increase in total peripheral resistance that results from
the autoregulation mechanism helps greatly in increas-
ing the arterial pressure. For instance, only a 5 to 10
per cent increase in cardiac output can increase the
arterial pressure from the normal mean arterial pres-
sure of 100 m

 from the normal mean arterial pres-
sure of 100 mm Hg up to 150 mm Hg. In fact, the slight
increase in cardiac output is often unmeasurable.
Importance of Salt (NaCl) in the Renal–Body
Fluid Schema for Arterial Pressure Regulation
Although the discussions thus far have emphasized the
importance of volume in regulation of arterial pres-
sure, experimental studies have shown that an increase
in salt intake is far more likely to elevate the arterial
pressure than is an increase in water intake.The

l
pressure than is an increase in water intake.The reason
for this is that pure water is normally excreted by the
kidneys almost as rapidly as it is ingested, but salt 
is not excreted so easily. As salt accumulates in the
body, it also indirectly increases the extracellular ﬂuid
volume for two basic reasons:
1. When there is excess salt in the extracellular ﬂuid,
the osmolality of the ﬂuid increases, and this in
turn stimulates the thirst center in the brain,
making the person drink extra amoun

 in the brain,
making the person drink extra amounts of 
water to return the extracellular salt
concentration to normal. This increases the
extracellular ﬂuid volume.
2. The increase in osmolality caused by the excess
salt in the extracellular ﬂuid also stimulates the
hypothalamic-posterior pituitary gland secretory
mechanism to secrete increased quantities of
antidiuretic hormone. (This is discussed in Chapter
28.) The antidiuretic hormone then causes the
kidneys to reabsorb greatly increased q

causes the
kidneys to reabsorb greatly increased quantities 
of water from the renal tubular ﬂuid, thereby
diminishing the excreted volume of urine but
increasing the extracellular ﬂuid volume.
Thus, for these important reasons, the amount of salt
that accumulates in the body is the main determinant
of the extracellular ﬂuid volume. Because only small
increases in extracellular ﬂuid and blood volume can
often increase the arterial pressure greatly, accumula-
tion of even a small amount of extra 

y, accumula-
tion of even a small amount of extra salt in the body
can lead to considerable elevation of arterial pressure.
Chronic Hypertension (High Blood
Pressure) Is Caused by Impaired
Renal Fluid Excretion
When a person is said to have chronic hypertension (or
“high blood pressure”), it is meant that his or her mean
arterial pressure is greater than the upper range of the
accepted normal measure. A mean arterial pressure
greater than 110 mm Hg (normal is about 90 mm Hg)
is considered to be 

Hg (normal is about 90 mm Hg)
is considered to be hypertensive. (This level of mean
pressure occurs when the diastolic blood pressure is
greater than about 90 mm Hg and the systolic pressure
is greater than about 135 mm Hg.) In severe hyper-
tension, the mean arterial pressure can rise to 150 
to 170 mm Hg, with diastolic pressure as high as 
130 mm Hg and systolic pressure occasionally as high
as 250 mm Hg.
Even moderate elevation of arterial pressure leads
to shortened life expectancy. At seve

essure leads
to shortened life expectancy. At severely high pres-
sures—mean arterial pressures 50 per cent or more
above normal—a person can expect to live no more
than a few more years unless appropriately treated.
The lethal effects of hypertension are caused mainly
in three ways:
1. Excess workload on the heart leads to early heart
failure and coronary heart disease, often causing
death as a result of a heart attack.
2. The high pressure frequently damages a major
blood vessel in the brain, 

uently damages a major
blood vessel in the brain, followed by death of
Increased extracellular fluid volume
Increased blood volume
Increased mean circulatory filling pressure
Increased venous return of blood to the heart
Increased cardiac output
Autoregulation
Increased total
peripheral resistance
Increased arterial pressure
Figure 19–6
Sequential steps by which increased extracellular ﬂuid volume
increases the arterial pressure. Note especially that increased
cardiac output has both a direct ef

that increased
cardiac output has both a direct effect to increase arterial pres-
sure and an indirect effect by ﬁrst increasing the total peripheral
Chapter 19
221
major portions of the brain; this is a cerebral
infarct. Clinically it is called a “stroke.” Depending
on which part of the brain is involved, a stroke
can cause paralysis, dementia, blindness, or
multiple other serious brain disorders.
3. High pressure almost always causes injur

rders.
3. High pressure almost always causes injury in 
the kidneys, producing many areas of renal
destruction and, eventually, kidney failure, uremia,
and death.
Lessons learned from the type of hypertension
called “volume-loading hypertension” have been
crucial in understanding the role of the renal–body
ﬂuid volume mechanism for arterial pressure regula-
tion. Volume-loading hypertension means hyperten-
sion caused by excess accumulation of extracellular
ﬂuid in the body, some examples of whi

tracellular
ﬂuid in the body, some examples of which follow.
Experimental Volume-Loading Hypertension Caused by Reduced
Renal Mass Along with Simultaneous Increase in Salt Intake.
Figure 19–7 shows a typical experiment demonstrating
volume-loading hypertension in a group of dogs with
70 per cent of their kidney mass removed. At the ﬁrst
circled point on the curve, the two poles of one of the
kidneys were removed, and at the second circled point,
the entire opposite kidney was removed, leaving th

the entire opposite kidney was removed, leaving the
animals with only 30 per cent of normal renal mass.
Note that removal of this amount of kidney mass
increased the arterial pressure an average of only 6
mm Hg. Then, the dogs were given salt solution to
drink instead of water. Because salt solution fails to
quench the thirst, the dogs drank two to four times the
normal amounts of volume, and within a few days,
their average arterial pressure rose to about 40 mm Hg
above normal. After 2 weeks, t

e to about 40 mm Hg
above normal. After 2 weeks, the dogs were given tap
water again instead of salt solution; the pressure
returned to normal within 2 days. Finally, at the end of
the experiment, the dogs were given salt solution
again, and this time the pressure rose much more
rapidly to an even higher level because the dogs had
already learned to tolerate the salt solution and there-
fore drank much more. Thus, this experiment demon-
strates volume-loading hypertension.
If the reader consider

olume-loading hypertension.
If the reader considers again the basic determinants
of long-term arterial pressure regulation, he or she can
immediately understand why hypertension occurred in
the volume-loading experiment of Figure 19–7. First,
reduction of the kidney mass to 30 per cent of normal
greatly reduced the ability of the kidneys to excrete
salt and water. Therefore, salt and water accumulated
in the body and in a few days raised the arterial pres-
sure high enough to excrete the excess 

rial pres-
sure high enough to excrete the excess salt and water
intake.
Sequential Changes in Circulatory Function During the Devel-
opment of Volume-Loading Hypertension. It is especially
instructive to study the sequential changes in circula-
tory function during progressive development of
volume-loading hypertension. Figure 19–8 shows these
sequential changes. A week or so before the point
labeled “0” days, the kidney mass had already been
decreased to only 30 per cent of normal. Then, at th

creased to only 30 per cent of normal. Then, at this
Mean arterial pressure
(per cent of control)
Days
0
20
40
60
80
100
0
100
110
120
130
140
150
35–45% of  left
kidney removed
0.9% NaCl
0.9% NaCl
Tap water
Entire right
Figure 19–7
Average effect on arterial pressure of drinking 0.9 per cent saline solution instead of water in four dogs with 70 per cent of their 
renal tissue removed. (Redrawn from Langston JB, Guyton AC, Douglas BH, Dorsett PE: Circ Res 12:508, 1963. By permissi

BH, Dorsett PE: Circ Res 12:508, 1963. By permission of the
American Heart Association, Inc.)
222
Unit IV
point, the intake of salt and water was increased to
about six times normal and kept at this high intake
thereafter. The acute effect was to increase extracellu-
lar ﬂuid volume, blood volume, and cardiac output to
20 to 40 per cent above normal. Simultaneously, the
arterial pressure began to rise but not nearly so much
at ﬁrst as did the ﬂuid volumes and cardiac output.The
r

t as did the ﬂuid volumes and cardiac output.The
reason for this slower rise in pressure can be discerned
by studying the total peripheral resistance curve,which
shows an initial decrease in total peripheral resistance.
This decrease was caused by the baroreceptor mecha-
nism discussed in Chapter 18, which tried to prevent
the rise in pressure. However, after 2 to 4 days, the
baroreceptors adapted (reset) and were no longer able
to prevent the rise in pressure. At this time, the arte-
rial press

se in pressure. At this time, the arte-
rial pressure had risen almost to its full height because
of the increase in cardiac output, even though the total
peripheral resistance was still almost at the normal
level.
After these early acute changes in the circulatory
variables had occurred, more prolonged secondary
changes occurred during the next few weeks. Espe-
cially important was a progressive increase in total
peripheral resistance,while at the same time the cardiac
output decreased almost a

he same time the cardiac
output decreased almost all the way back to normal,
mainly as a result of the long-term blood ﬂow autoreg-
ulation mechanism that is discussed in detail in
Chapter 17 and earlier in this chapter.That is, after the
cardiac output had risen to a high level and had initi-
ated the hypertension, the excess blood ﬂow through
the tissues then caused progressive constriction of the
local arterioles, thus returning the local blood ﬂows 
in all the body tissues and also the cardi

d ﬂows 
in all the body tissues and also the cardiac output
almost all the way back to normal, while simultane-
ously causing a secondary increase in total peripheral
Note, too, that the extracellular ﬂuid volume and
blood volume returned almost all the way back to
normal along with the decrease in cardiac output.This
resulted from two factors: First, the increase in arteri-
olar resistance decreased the capillary pressure, which
allowed the ﬂuid in the tissue spaces to be absorbed
b

wed the ﬂuid in the tissue spaces to be absorbed
back into the blood. Second, the elevated arterial 
pressure now caused the kidneys to excrete the excess
volume of ﬂuid that had initially accumulated in the
body.
Last, let us take stock of the ﬁnal state of the circu-
lation several weeks after the initial onset of volume
loading. We ﬁnd the following effects:
1. Hypertension
2. Marked increase in total peripheral resistance
3. Almost complete return of the extracellular ﬂuid
volume, blood volu

eturn of the extracellular ﬂuid
volume, blood volume, and cardiac output back to
normal
Days
0
2
4
6
8
10
12
14
0
110
120
130
140
150
18
20
22
24
26
28
5.0
5.5
6.0
6.5
7.0
5.0
5.5
6.0
15
16
17
18
19
20
33%
20%
40%
–13% 
30%
40%
33%
5%
4%
5%
fluid volume
(liters)
Blood 
volume
(liters)
(L/min)
Total
peripheral
(mm Hg/L/min)
Arterial 
pressure
(mm Hg)
Figure 19–8
Progressive changes in important circulatory
system variables during the ﬁrst few weeks of
volum

ystem variables during the ﬁrst few weeks of
volume-loading hypertension. Note especially the
initial increase in cardiac output as the basic
cause of the hypertension. Subsequently, the auto-
regulation mechanism returns the cardiac output
almost to normal while simultaneously causing a
secondary increase in total peripheral resistance.
(Modiﬁed from Guyton AC: Arterial Pressure and
Hypertension. Philadelphia: WB Saunders Co,
1980.)
Chapter 19
223
Ther

he Integrated System for Pressure Control
223
Therefore, we can divide volume-loading hyperten-
sion into two separate sequential stages:The ﬁrst stage
results from increased ﬂuid volume causing increased
cardiac output. This increase in cardiac output causes
the hypertension. The second stage in volume-loading
hypertension is characterized by high blood pressure
and high total peripheral resistance but return of the
cardiac output so near to normal that the usual meas-
uring techniques frequent

mal that the usual meas-
uring techniques frequently cannot detect an abnor-
mally elevated cardiac output.
Thus, the increased total peripheral resistance in
volume-loading hypertension occurs after the hyper-
tension has developed and, therefore, is secondary to
the hypertension rather than being the cause of the
hypertension.
Volume-Loading Hypertension in Patients 
Who Have No Kidneys but Are Being
Maintained on an Artiﬁcial Kidney
When a patient is maintained on an artiﬁcial kidney, it
is e

ient is maintained on an artiﬁcial kidney, it
is especially important to keep the patient’s body ﬂuid
volume at a normal level—that is, it is important 
to remove an appropriate amount of water and salt
each time the patient is dialyzed. If this is not done and
extracellular ﬂuid volume is allowed to increase,
hypertension almost invariably develops in exactly the
same way as shown in Figure 19–8. That is, the cardiac
output increases at ﬁrst and causes hypertension.Then
the autoregulation mecha

 causes hypertension.Then
the autoregulation mechanism returns the cardiac
output back toward normal while causing a secondary
increase in total peripheral resistance. Therefore, in
the end, the hypertension is a high peripheral resist-
ance type of hypertension.
Hypertension Caused by 
Another type of volume-loading hypertension is
caused by excess aldosterone in the body or, occa-
sionally, by excesses of other types of steroids.A small
tumor in one of the adrenal glands 

eroids.A small
tumor in one of the adrenal glands occasionally
secretes large quantities of aldosterone, which is 
the condition called “primary aldosteronism.” As dis-
cussed in Chapter 29, aldosterone increases the rate of
reabsorption of salt and water by the tubules of the
kidneys, thereby reducing the loss of these in the 
urine while at the same time causing an increase in
blood volume and extracellular ﬂuid volume. Conse-
quently, hypertension occurs. And, if salt intake is
increased at t

sion occurs. And, if salt intake is
increased at the same time, the hypertension becomes
even greater. Furthermore, if the condition persists for
months or years, the excess arterial pressure often
causes pathological changes in the kidneys that make
the kidneys retain even more salt and water in addi-
tion to that caused directly by the aldosterone. There-
fore, the hypertension often ﬁnally becomes lethally
severe.
Here again, in the early stages of this type of hyper-
tension, the cardiac out

es of this type of hyper-
tension, the cardiac output is increased, but in later
stages, the cardiac output generally returns almost to
normal while the total peripheral resistance becomes
secondarily elevated, as explained earlier in the
chapter for primary volume-loading hypertension.
The Renin-Angiotensin
System: Its Role in Pressure
Control and in Hypertension
Aside from the capability of the kidneys to control
arterial pressure through changes in extracellular 
ﬂuid volume, the kidneys also

es in extracellular 
ﬂuid volume, the kidneys also have another powerful
mechanism for controlling pressure. It is the renin-
angiotensin system.
Renin is a protein enzyme released by the kidneys
when the arterial pressure falls too low. In turn, it
raises the arterial pressure in several ways, thus
helping to correct the initial fall in pressure.
Components of the Renin-Angiotensin
System
Figure 19–9 shows the functional steps by which the
renin-angiotensin system helps to regulate arterial
pre

-angiotensin system helps to regulate arterial
pressure.
Renin is synthesized and stored in an inactive form
called prorenin in the juxtaglomerular cells (JG cells)
of the kidneys. The JG cells are modiﬁed smooth
muscle cells located in the walls of the afferent 
Decreased
arterial pressure
Renin (kidney)
Renin substrate
(angiotensinogen)
Angiotensinase
(Inactivated)
Renal retention
of salt and water
Converting
enzyme
(lun

retention
enzyme
(lung)
Figure 19–9
Renin-angiotensin vasoconstrictor mechanism for arterial pres-
sure control.
224
Unit IV
arterioles immediately proximal to the glomeruli.When
the arterial pressure falls, intrinsic reactions in the
kidneys themselves cause many of the prorenin mole-
cules in the JG cells to split and release renin. Most of
the renin enters the renal blood and then passes out
of the kidneys to circulate throughout the entire body.
H

kidneys to circulate throughout the entire body.
However, small amounts of the renin do remain in the
local ﬂuids of the kidney and initiate several intrarenal
Renin itself is an enzyme, not a vasoactive sub-
stance. As shown in the schema of Figure 19–9,
renin acts enzymatically on another plasma protein,
a globulin called renin substrate (or angiotensinogen),
to release a 10-amino acid peptide, angiotensin I.
Angiotensin I has mild vasoconstrictor properties but
not enough to cause 

asoconstrictor properties but
not enough to cause signiﬁcant changes in circulatory
function.The renin persists in the blood for 30 minutes
to 1 hour and continues to cause formation of still
more angiotensin I during this entire time.
Within a few seconds to minutes after formation of
angiotensin I, two additional amino acids are split
from the angiotensin I to form the 8-amino acid
peptide angiotensin II. This conversion occurs almost
entirely in the lungs while the blood ﬂows through the
smal

in the lungs while the blood ﬂows through the
small vessels of the lungs, catalyzed by an enzyme
called converting enzyme that is present in the
endothelium of the lung vessels.
Angiotensin II is an extremely powerful vasocon-
strictor, and it also affects circulatory function in other
ways as well. However, it persists in the blood only 
for 1 or 2 minutes because it is rapidly inactivated by
multiple blood and tissue enzymes collectively called
angiotensinases.
During its persistence in the bl

During its persistence in the blood, angiotensin 
II has two principal effects that can elevate arterial
pressure. The ﬁrst of these, vasoconstriction in many
areas of the body, occurs rapidly. Vasoconstriction
occurs intensely in the arterioles and much less so in
the veins. Constriction of the arterioles increases the
total peripheral resistance, thereby raising the arterial
pressure, as demonstrated at the bottom of the schema
in Figure 19–9. Also, the mild constriction of t

a
in Figure 19–9. Also, the mild constriction of the veins
promotes increased venous return of blood to the
heart, thereby helping the heart pump against the
increasing pressure.
The second principal means by which angiotensin
increases the arterial pressure is to decrease excretion
of both salt and water by the kidneys. This slowly
increases the extracellular ﬂuid volume, which then
increases the arterial pressure during subsequent
hours and days. This long-term effect, acting through
the extra

s. This long-term effect, acting through
the extracellular ﬂuid volume mechanism, is even
more powerful than the acute vasoconstrictor mecha-
nism in eventually raising the arterial pressure.
Rapidity and Intensity of the 
Vasoconstrictor Pressure Response to 
the Renin-Angiotensin System
Figure 19–10 shows a typical experiment demon-
strating the effect of hemorrhage on the arterial 
pressure under two separate conditions: (1) with the
renin-angiotensin system functioning and (2) without
the sy

iotensin system functioning and (2) without
the system functioning (the system was interrupted 
by a renin-blocking antibody). Note that after hemor-
rhage—enough to cause acute decrease of the arterial
pressure to 50 mm Hg—the arterial pressure rose 
back to 83 mm Hg when the renin-angiotensin 
system was functional. Conversely, it rose to only 
60 mm Hg when the renin-angiotensin system was
blocked. This shows that the renin-angiotensin system
is powerful enough to return the arterial pressure

is powerful enough to return the arterial pressure at
least halfway back to normal within a few minutes
after severe hemorrhage. Therefore, sometimes it 
can be of lifesaving service to the body, especially in
circulatory shock.
Note also that the renin-angiotensin vasocon-
strictor system requires about 20 minutes to become
fully active. Therefore, it is somewhat slower to act 
for pressure control than are the nervous reﬂexes 
and the sympathetic norepinephrine-epinephrine
system.
Effect of An

ic norepinephrine-epinephrine
system.
Effect of Angiotensin in the Kidneys to 
Cause Renal Retention of Salt and Water—
An Especially Important Means for 
Long-Term Control of Arterial Pressure
Angiotensin causes the kidneys to retain both salt and
water in two major ways:
1. Angiotensin acts directly on the kidneys to cause
salt and water retention.
2. Angiotensin causes the adrenal glands to secrete
aldosterone, and the aldosterone in turn increases
salt and water reabsorption by the kidney tu

eases
salt and water reabsorption by the kidney tubules.
Thus, whenever excess amounts of angiotensin cir-
culate in the blood, the entire long-term renal–body
ﬂuid mechanism for arterial pressure control auto-
matically becomes set to a higher arterial pressure
level than normal.
Hemorrhage
0
10
20
30
40
0
25
50
75
100
Minutes
With
renin-angiotensin system
Without
Figure 19–10
Pressure-compensating effect of the renin-angiotensin vasocon-
stric

ing effect of the renin-angiotensin vasocon-
strictor system after severe hemorrhage. (Drawn from experiments
by Dr. Royce Brough.)
Chapter 19
225
Mechanisms of the Direct Renal Effects of Angiotensin to Cause
Renal Retention of Salt and Water. Angiotensin has several
direct renal effects that make the kidneys retain salt
and water. One major effect is to constrict the renal
arterioles, thereby diminishing blood ﬂow through 
the kidneys. As a result, le

ng blood ﬂow through 
the kidneys. As a result, less ﬂuid ﬁlters through the
glomeruli into the tubules.Also, the slow ﬂow of blood
reduces the pressure in the peritubular capillaries,
which causes rapid reabsorption of ﬂuid from the
tubules. And still a third effect is that angiotensin has
important direct actions on the tubular cells them-
selves to increase tubular reabsorption of sodium and
water. The total result of all these effects is signiﬁcant,
sometimes decreasing urine output less tha

iﬁcant,
sometimes decreasing urine output less than one ﬁfth
of normal.
Stimulation of Aldosterone Secretion by Angiotensin, and the
Effect of Aldosterone in Increasing Salt and Water Retention 
by the Kidneys. Angiotensin is also one of the most 
powerful stimulators of aldosterone secretion by the
adrenal glands, as we shall discuss in relation to body
ﬂuid regulation in Chapter 29 and in relation to
adrenal gland function in Chapter 77.Therefore, when
the renin-angiotensin system becomes acti

re, when
the renin-angiotensin system becomes activated, the
rate of aldosterone secretion usually also increases;
and an important subsequent function of aldosterone
is to cause marked increase in sodium reabsorption by
the kidney tubules, thus increasing the total body
extracellular ﬂuid sodium. This increased sodium then
causes water retention, as already explained, increas-
ing the extracellular ﬂuid volume and leading second-
arily to still more long-term elevation of the arterial
pressure.

more long-term elevation of the arterial
pressure.
Thus both the direct effect of angiotensin on the
kidney and its effect acting through aldosterone are
important in long-term arterial pressure control.
However, research in our own laboratory has sug-
gested that the direct effect of angiotensin on the
kidneys is perhaps three or more times as potent as the
indirect effect acting through aldosterone—even
though the indirect effect is the one most widely
known.
Quantitative Analysis of Arterial 

t widely
known.
Quantitative Analysis of Arterial Pressure Changes Caused by
Angiotensin. Figure 19–11 shows a quantitative analysis
of the effect of angiotensin in arterial pressure control.
This ﬁgure shows two renal output curves as well as a
line depicting normal level of sodium intake. The left-
hand renal output curve is that measured in dogs whose
renin-angiotensin system had been blocked by the drug
captopril (which blocks the conversion of angiotensin I
to angiotensin II, the active for

of angiotensin I
to angiotensin II, the active form of angiotensin). The
right-hand curve was measured in dogs infused contin-
uously with angiotensin II at a level about 2.5 times the
normal rate of angiotensin formation in the blood. Note
the shift of the renal output curve toward higher pres-
sure levels under the inﬂuence of angiotensin II. This
shift is caused by both the direct effects of angiotensin
on the kidney and the indirect effect acting through
aldosterone secretion, as explained a

ting through
aldosterone secretion, as explained above.
Finally, note the two equilibrium points, one for zero
angiotensin showing an arterial pressure level of 75 mm
Hg, and one for elevated angiotensin showing a 
pressure level of 115 mm Hg. Therefore, the effect of
angiotensin to cause renal retention of salt and water
can have a powerful effect in promoting chronic eleva-
tion of the arterial pressure.
Role of the Renin-Angiotensin System in
Maintaining a Normal Arterial Pressure
Despite Wid

Despite Wide Variations in Salt Intake
One of the most important functions of the renin-
angiotensin system is to allow a person to eat either
very small or very large amounts of salt without
causing great changes in either extracellular ﬂuid
volume or arterial pressure. This function is explained
by the schema in Figure 19–12, which shows that the
initial effect of increased salt intake is to elevate the
extracellular ﬂuid volume and this in turn to elevat

tracellular ﬂuid volume and this in turn to elevate
the arterial pressure.Then, the increased arterial pres-
sure causes increased blood ﬂow through the kidneys,
which reduces the rate of secretion of renin to a much
lower level and leads sequentially to decreased renal
retention of salt and water, return of the extracellular
ﬂuid volume almost to normal, and, ﬁnally, return of
the arterial pressure also almost to normal. Thus, the
renin-angiotensin system is an automatic feedback
mechanism that

sin system is an automatic feedback
mechanism that helps maintain the arterial pressure at
or near the normal level even when salt intake is
increased. Or, when salt intake is decreased below
normal, exactly opposite effects take place.
To emphasize the efﬁcacy of the renin-angiotensin
system in controlling arterial pressure, when the
system functions normally, the pressure rises no more
Sodium intake and output (times normal)
0
60
80
100
120
140
160
0
2
4
6
8
10
Angiot

160
0
2
4
6
8
10
Angiotensin levels in the blood
(times normal)
0
2.5
Equilibrium
points
Normal
Intake
Figure 19–11
Effect of two angiotensin II levels in the blood on the renal output
curve, showing regulation of the arterial pressure at an equilibrium
point of 75 mm Hg when the angiotensin II level is low and at 
115 mm Hg when the angiotensin II level is high.
226
Unit IV
than 4 to 6 mm Hg in response to as much as a 50-fold
increase in salt intake. 

 to as much as a 50-fold
increase in salt intake. Conversely, when the renin-
angiotensin system is blocked, the same increase in salt
intake sometimes causes the pressure to rise 10 times
the normal increase, often as much as 50 to 60 mm Hg.
Types of Hypertension in Which
Angiotensin Is Involved: Hypertension
Caused by a Renin-Secreting Tumor
or by Infusion of Angiotensin II
Occasionally a tumor of the renin-secreting juxta-
glomerular cells (the JG cells) occurs and secretes
tremendous quantit

e JG cells) occurs and secretes
tremendous quantities of renin; in turn, equally large
quantities of angiotensin II are formed. In all patients
in whom this has occurred, severe hypertension has
developed. Also, when large amounts of angiotensin
are infused continuously for days or weeks into
animals,
similar 
severe 
long-term 
hypertension 
develops.
We have already noted that angiotensin can increase
the arterial pressure in two ways:
1. By constricting the arterioles throughout the entire
bo

nstricting the arterioles throughout the entire
body, thereby increasing the total peripheral
resistance and arterial pressure; this effect occurs
within seconds after one begins to infuse
angiotensin.
2. By causing the kidneys to retain salt and water;
over a period of days, this, too, causes hypertension
and is the principal cause of the long-term
continuation of the elevated pressure.
“One-Kidney” Goldblatt Hypertension. When one kidney is
removed and a constrictor is placed on the renal arte

oved and a constrictor is placed on the renal artery
of the remaining kidney, as shown in Figure 19–13, the
immediate effect is greatly reduced pressure in the
renal artery beyond the constrictor, as demonstrated
by the dashed curve in the ﬁgure.Then, within seconds
or minutes, the systemic arterial pressure begins to rise
and continues to rise for several days. The pressure
usually rises rapidly for the ﬁrst hour or so, and this is
Increased salt intake
Increased extracellular volume
Increased 

t intake
Decreased renin and angiotensin
Decreased renal retention of salt and water
Return of extracellular volume almost to normal
Return of arterial pressure almost to normal
Figure 19–12
Sequential events by which increased salt intake increases the
arterial pressure, but feedback decrease in activity of the renin
angiotensin system returns the arterial pressure almost to the
normal level.
Times normal
Days
0
4
8
12

evel.
Days
0
4
8
12
0
1
7
50
100
150
200
Renin secretion
Systemic arterial
pressure
Distal renal arterial
pressure
Renal artery constricted
Constriction released
Figure 19–13
Effect of placing a constricting clamp on the renal artery of one
kidney after the other kidney has been removed. Note the
changes in systemic arterial pressure, renal artery pressure distal
to the clamp, and rate of renin secretion. The resulting hyperten-
sion is called “one-kidney” Goldblatt

ng hyperten-
sion is called “one-kidney” Goldblatt hypertension.
Chapter 19
227
followed by a slower additional rise during the next
several days. When the systemic arterial pressure
reaches its new stable pressure level, the renal arterial
pressure (the dashed curve in the ﬁgure) will have
returned almost all the way back to normal.The hyper-
tension produced in this way is called “one-kidney”
Goldblatt hypertension in honor of Dr. Goldblatt, who
ﬁrst 

 hypertension in honor of Dr. Goldblatt, who
ﬁrst studied the important quantitative features of
hypertension caused by renal artery constriction.
The early rise in arterial pressure in Goldblatt
hypertension is caused by the renin-angiotensin vaso-
constrictor mechanism. That is, because of poor blood
ﬂow through the kidney after acute constriction of the
renal artery, large quantities of renin are secreted by
the kidney, as demonstrated by the lowermost curve in
Figure 19–13, and this causes i

lowermost curve in
Figure 19–13, and this causes increased angiotensin 
II and aldosterone in the blood. The angiotensin in
turn raises the arterial pressure acutely. The secretion
of renin rises to a peak in an hour or so but returns 
nearly to normal in 5 to 7 days because the renal arte-
rial pressure by that time has also risen back to
normal, so that the kidney is no longer ischemic.
The second rise in arterial pressure is caused by
retention of salt and water by the constricted kidney
(tha

n of salt and water by the constricted kidney
(that is also stimulated by angiotensin II and aldos-
terone). In 5 to 7 days, the body ﬂuid volume will have
increased enough to raise the arterial pressure to its
new sustained level.The quantitative value of this sus-
tained pressure level is determined by the degree of
constriction of the renal artery.That is, the aortic pres-
sure must rise high enough so that renal arterial pres-
sure distal to the constrictor is enough to cause normal
urine ou

the constrictor is enough to cause normal
urine output.
“Two-Kidney” Goldblatt Hypertension. Hypertension also
can result when the artery to only one kidney is con-
stricted while the artery to the other kidney is normal.
This hypertension results from the following mecha-
nism: The constricted kidney secretes renin and also
retains salt and water because of decreased renal arte-
rial pressure in this kidney. Then the “normal” oppo-
site kidney retains salt and water because of the renin
produce

etains salt and water because of the renin
produced by the ischemic kidney. This renin causes
formation of angiotension II and aldosterone both of
which circulate to the opposite kidney and cause it also
to retain salt and water.Thus, both kidneys, but for dif-
ferent reasons, become salt and water retainers. Con-
sequently, hypertension develops.
Hypertension Caused by Diseased Kidneys That Secrete Renin
Chronically. Often, patchy areas of one or both kidneys
are diseased and become ischemic be

r both kidneys
are diseased and become ischemic because of local
vascular constrictions, whereas other areas of the
kidneys are normal.When this occurs, almost identical
effects occur as in the two-kidney type of Goldblatt
hypertension. That is, the patchy ischemic kidney
tissue secretes renin, and this in turn, acting through
the formation of angiotensin II, causes the remaining
kidney mass also to retain salt and water. Indeed, one
of the most common causes of renal hypertension,
especially in

common causes of renal hypertension,
especially in older persons, is such patchy ischemic
Other Types of Hypertension
Caused by Combinations of Volume
Loading and Vasoconstriction
Hypertension in the Upper Part of the Body Caused by Coarctation
of the Aorta. One out of every few thousand babies is
born with pathological constriction or blockage of the
aorta at a point beyond the aortic arterial branches to
the head and arms but proximal to the renal arteries,
a condition called c

oximal to the renal arteries,
a condition called coarctation of the aorta. When this
occurs, blood ﬂow to the lower body is carried by mul-
tiple, small collateral arteries in the body wall, with
much vascular resistance between the upper aorta and
the lower aorta. As a consequence, the arterial pressure
in the upper part of the may be 40-50 per cent higher
than that in the lower body.
The mechanism of this upper-body hypertension is
almost identical to that of one-kidney Goldblatt hyper-
tensio

ical to that of one-kidney Goldblatt hyper-
tension.That is, when a constrictor is placed on the aorta
above the renal arteries, the blood pressure in both
kidneys at ﬁrst falls, renin is secreted, angiotensin and
aldosterone are formed, and hypertension occurs in the
upper body. The arterial pressure in the lower body at
the level of the kidneys rises approximately to normal,
but high pressure persists in the upper body. The
kidneys are no longer ischemic, so that secretion of
renin and formati

r ischemic, so that secretion of
renin and formation of angiotensin and aldosterone
return to normal. Likewise, in coarctation of the aorta,
the arterial pressure in the lower body is usually almost
normal, whereas the pressure in the upper body is far
higher than normal.
Role of Autoregulation in the Hypertension Caused by Aortic
Coarctation. A signiﬁcant feature of hypertension caused
by aortic coarctation is that blood ﬂow in the arms,
where the pressure may be 40 to 60 per cent above
normal,

he pressure may be 40 to 60 per cent above
normal, is almost exactly normal.Also, blood ﬂow in the
legs, where the pressure is not elevated, is almost exactly
normal. How could this be, with the pressure in the
upper body 40 to 60 per cent greater than in the lower
body? The answer is not that there are differences in
vasoconstrictor substances in the blood of the upper and
lower body, because the same blood ﬂows to both areas.
Likewise, the nervous system innervates both areas of
the circulatio

ous system innervates both areas of
the circulation similarly, so that there is no reason to
believe that there is a difference in nervous control of
the blood vessels. The only reasonable answer is that
long-term autoregulation develops so nearly completely
that the local blood ﬂow control mechanisms have com-
pensated almost 100 per cent for the differences in pres-
sure. The result is that, in both the high-pressure area
and the low-pressure area, the local blood ﬂow is con-
trolled almost ex

rea, the local blood ﬂow is con-
trolled almost exactly in accord with the needs of the
tissue and not in accord with the level of the pressure.
One of the reasons these observations are so important
is that they demonstrate how nearly complete the long-
term autoregulation process can be.
Hypertension in Preeclampsia (Toxemia of Pregnancy). Approx-
imately 5 to 10 per cent  of expectant mothers develop
a syndrome called preeclampsia (also called toxemia of
pregnancy). One of the manifestations 

 toxemia of
pregnancy). One of the manifestations of preeclampsia
is hypertension that usually subsides after delivery of
the baby. Although the precise causes of preeclampsia
are not completely understood, ischemia of the placenta
and subsequent release by the placenta of toxic factors
are believed to play a role in causing many of the man-
228
Unit IV
ifestations of this disorder,including hypertension in the
mother. Substances released by the ischemic placenta,
in turn, cause 

released by the ischemic placenta,
in turn, cause dysfunction of vascular endothelial cells
throughout the body, including the blood vessels of the
kidneys.This endothelial dysfunction decreases release of
nitric oxide and other vasodilator substances, causing
vasoconstriction, decreased rate of ﬂuid ﬁltration from
the glomeruli into the renal tubules, impaired renal-
pressure natriuresis, and development of hypertension.
Another pathological abnormality that may con-
tribute to hypertension in 

ormality that may con-
tribute to hypertension in preeclampsia is thickening of
the kidney glomerular membranes (perhaps caused by
an autoimmune process), which also reduces the rate of
glomerular ﬂuid ﬁltration. For obvious reasons, the arte-
rial pressure level required to cause normal formation
of urine becomes elevated, and the long-term level of
arterial pressure becomes correspondingly elevated.
These patients are especially prone to extra degrees of
hypertension when they have excess salt

degrees of
hypertension when they have excess salt intake.
Neurogenic Hypertension. Acute neurogenic hypertension
can be caused by strong stimulation of the sympathetic
nervous system. For instance, when a person becomes
excited for any reason or at times during states of
anxiety, the sympathetic system becomes excessively
stimulated, peripheral vasoconstriction occurs every-
where in the body, and acute hypertension ensues.
Acute Neurogenic Hypertension Caused by Sectioning the Barore-
ceptor N

rtension Caused by Sectioning the Barore-
ceptor Nerves. Another type of acute neurogenic hyper-
tension occurs when the nerves leading from the
baroreceptors are cut or when the tractus solitarius 
is destroyed in each side of the medulla oblongata
(these are the areas where the nerves from the carotid 
and aortic baroreceptors connect in the brain stem).
The sudden cessation of normal nerve signals from 
the baroreceptors has the same effect on the nervous
pressure control mechanisms as a sudd

 the nervous
pressure control mechanisms as a sudden reduction 
of the arterial pressure in the aorta and carotid arteries.
That is, loss of the normal inhibitory effect on the 
vasomotor center caused by normal baroreceptor
nervous signals allows the vasomotor center suddenly
to become extremely active and the mean arterial pres-
sure to increase from 100 mm Hg to as high as 160 mm
Hg. The pressure returns to nearly normal within about
2 days because the response of the vasomotor center to
the 

cause the response of the vasomotor center to
the absent baroreceptor signal fades away, which is
called central “resetting” of the baroreceptor pressure
control mechanism. Therefore, the neurogenic hyper-
tension caused by sectioning the baroreceptor nerves is
mainly an acute type of hypertension, not a chronic
type.
Spontaneous Hereditary Hypertension in Lower Animals. Spon-
taneous hereditary hypertension has been observed in
a number of strains of lower animals, including several
different s

ns of lower animals, including several
different strains of rats, at least one strain of rabbits, and
at least one strain of dogs. In the strain of rats that has
been studied to the greatest extent, the Okamoto strain,
there is evidence that in early development of the
hypertension, the sympathetic nervous system is con-
siderably more active than in normal rats. However, in
the late stages of this type of hypertension, two struc-
tural changes have been observed in the nephrons of the
kidneys: 

ave been observed in the nephrons of the
kidneys: (1) increased preglomerular renal arterial
resistance and (2) decreased permeability of the
glomerular membranes. These structural changes could
easily be the basis for the long-term continuance of the
hypertension. In other strains of hypertensive rats,
impaired renal function also has been observed.
About 90 to 95 per cent of all people who have hyper-
tension are said to have “primary hypertension,” also
wide

are said to have “primary hypertension,” also
widely known as “essential hypertension” by many cli-
nicians.These terms mean simply that the hypertension
is of unknown origin, in contrast to those forms of
hypertension that are secondary to known causes, such
as renal artery stenosis. In some patients with primary
hypertension, there is a strong hereditary tendency, the
same as occurs in animal strains of genetic hyperten-
sion discussed above.
In most patients, excess weight gain and sedentary

n most patients, excess weight gain and sedentary
lifestyle appear to play a major role in causing hyper-
tension. The majority of patients with hypertension 
are overweight, and studies of different populations
suggest that excess weight gain and obesity may
account for as much as 65 to 70 percent of the risk 
for developing primary hypertension. Clinical studies
have clearly shown the value of weight loss for reduc-
ing blood pressure in most patients with hypertension.
In fact, new clinical g

atients with hypertension.
In fact, new clinical guidelines for treating hyper-
tension recommend increased physical activity and
weight loss as a ﬁrst step in treating most patients with
Some of the characteristics of primary hypertension
caused by excess weight gain and obesity include:
1. Cardiac output is increased due, in part, to the
additional blood ﬂow required for the extra
adipose tissue. However, blood ﬂow in the heart,
kidneys, gastrointestinal tract, and skeletal muscl

idneys, gastrointestinal tract, and skeletal muscle
also increases with weight gain due to increased
metabolic rate and growth of the organs and tissues
in response to their increased metabolic demands.
As the hypertension is sustained for many months
and years, total peripheral vascular resistance may
be increased.
2. Sympathetic nerve activity, especially in the kidneys,
is increased in overweight patients. The causes of
increased sympathetic activity in obesity are not
fully understood, but r

ctivity in obesity are not
fully understood, but recent studies suggest that
hormones, such as leptin, released from fat cells
may directly stimulate multiple regions of the
hypothalamus, which, in turn, have an excitatory
inﬂuence on the vasomotor centers of the brain
medulla.
3. Angiotensin II and aldosterone levels are increased
two- to threefold in many obese patients. This may
be caused partly by increased sympathetic nerve
stimulation, which increases renin release by the
kidneys and there

h increases renin release by the
kidneys and therefore formation of angiotensin II,
which, in turn, stimulates the adrenal gland to
secrete aldosterone.
4. The renal-pressure natriuresis mechanism is
impaired, and the kidneys will not excrete adequate
amounts of salt and water unless the arterial
pressure is high or unless kidney function is
somehow improved. In other words, if the mean
arterial pressure in the essential hypertensive
person is 150 mm Hg, acute reduction of the mean
arterial pres

0 mm Hg, acute reduction of the mean
arterial pressure artiﬁcially to the normal value of
Chapter 19
229
100 mm Hg (but without otherwise altering renal
function except for the decreased pressure) will
cause almost total anuria, and the person will retain
salt and water until the pressure rises back to the
elevated value of 150 mm Hg. Chronic reductions
in arterial pressure with effective antihypertensive
therapies, however, usually do not cause marked

e
salt and water retention by the kidneys because
these therapies also improve renal-pressure
natriuresis, as discussed below.
Experimental studies in obese animals and obese
patients suggest that impaired renal-pressure natri-
uresis in obesity hypertension is caused mainly by
increased renal tubular reabsorption of salt and 
water due to increased sympathetic nerve activity and
increased levels of angiotensin II and aldosterone.
However, if hyper

 angiotensin II and aldosterone.
However, if hypertension is not effectively treated,
there may also be vascular damage in the kidneys that
can reduce the glomerular ﬁltration rate and increase
the severity of the hypertension. Eventually uncon-
trolled hypertension associated with obesity can lead
to severe vascular injury and complete loss of kidney
function.
Graphical Analysis of Arterial Pressure Control in Essential
Hypertension. Figure 19–14 is a graphical analysis of
essential hypertensio

4 is a graphical analysis of
essential hypertension. The curves of this ﬁgure are
called sodium-loading renal function curves because
the arterial pressure in each instance is increased 
very slowly, over many days or weeks, by gradually
increasing the level of sodium intake. The sodium-
loading type of curve can be determined by increasing
the level of sodium intake to a new level every few
days, then waiting for the renal output of sodium to
come into balance with the intake, and at the same
t

me into balance with the intake, and at the same
time recording the changes in arterial pressure.
When this procedure is used in essential hyperten-
sive patients, two types of curves, shown to the right in
Figure 19–14, can be recorded in essential hyperten-
sive patients, one called (1) nonsalt-sensitive hyperten-
sion and the other (2) salt-sensitive hypertension. Note
in both instances that the curves are shifted to the
right, to a much higher pressure level than for normal
people. Now, let 

r pressure level than for normal
people. Now, let us plot on this same graph (1) a
normal level of salt intake and (2) a high level of salt
intake representing 3.5 times the normal intake. In 
the case of the person with nonsalt-sensitive essential
hypertension, the arterial pressure does not increase
signiﬁcantly when changing from normal salt intake 
to high salt intake. Conversely, in those patients who
have salt-sensitive essential hypertension, the high salt
intake signiﬁcantly exacerbates 

on, the high salt
intake signiﬁcantly exacerbates the hypertension.
Two additional points should be emphasized: (1)
Salt-sensitivity of blood pressure is not an all-or-none
characteristic—it is a quantitative characteristic, with
some individuals being more salt-sensitive than others.
(2) Salt-sensitivity of blood pressure is not a ﬁxed
characteristic; instead, blood pressure usually becomes
more salt-sensitive as a person ages, especially after 50
or 60 years of age.
The reason for the differen

50
The reason for the difference between nonsalt-
sensitive essential hypertension and salt-sensitive
hypertension is presumably related to structural or
functional differences in the kidneys of these two types
of hypertensive patients. For example, salt-sensitive
hypertension may occur with different types of chronic
renal disease due to gradual loss of the functional units
of the kidneys (the nephrons) or to normal aging as
discussed in Chapter 31. Abnormal function of the

renin-angiotensin system can also cause blood pres-
sure to become salt-sensitive, as discussed previously
Treatment of Essential Hypertension. Current guidelines
for treating hypertension recommend, as a ﬁrst step,
lifestyle modiﬁcations that are aimed at increasing
physical activity and weight loss in most patients.
Unfortunately, many patients are unable to lose
weight, and pharmacological treatment with antihy-
pertensive dru

rmacological treatment with antihy-
pertensive drugs must be initiated.
Two general classes of drugs are used to treat hyper-
tension: (1) vasodilator drugs that increase renal blood
ﬂow and (2) natriuretic or diuretic drugs that decrease
tubular reabsorption of salt and water.
Vasodilator drugs usually cause vasodilation in
many other tissues of the body as well as in the
kidneys. Different ones act in one of the following
ways: (1) by inhibiting sympathetic nervous signals to
the kidneys or by

g sympathetic nervous signals to
the kidneys or by blocking the action of the sympa-
thetic transmitter substance on the renal vasculature,
(2) by directly relaxing the smooth muscle of the renal
vasculature, or (3) by blocking the action of the 
Salt intake and output
0
50
150
0
1
2
3
4
5
6
Nonsalt-sensitive
Normal
Normal
Essential
hypertension
Salt-sensitive
High intake
E
B
A
D
B1
Normal intake
100
C
Figure 19–14
Analysis of arterial pressure regulation

ure 19–14
Analysis of arterial pressure regulation in (1) nonsalt-sensitive
essential hypertension and (2) salt-sensitive essential hyperten-
sion. (Redrawn from Guyton AC, Coleman TG, Young DB, et al:
Salt balance and long-term blood pressure control. Annu Rev Med
31:15, 1980. With permission, from the Annual Review of Medi-
cine, ” 1980, by Annual Reviews http://www.AnnualReviews.org.)
230
Unit IV
renin-angiotensin system on the renal vasculature or
renal tubules.
Those drugs t

 renal vasculature or
Those drugs that reduce reabsorption of salt and
water by the renal tubules include especially drugs that
block active transport of sodium through the tubular
wall; this blockage in turn also prevents the reabsorp-
tion of water, as explained earlier in the chapter.These
natriuretic or diuretic drugs are discussed in greater
detail in Chapter 31.
Summary of the Integrated,
Multifaceted System for
Arterial Pressure Regulation
By now, it is clear that arterial 

sure Regulation
By now, it is clear that arterial pressure is regulated
not by a single pressure controlling system but instead
by several interrelated systems, each of which per-
forms a speciﬁc function. For instance, when a person
bleeds severely so that the pressure falls suddenly, two
problems confront the pressure control system. The
ﬁrst is survival, that is, to return the arterial pressure
immediately to a high enough level that the person can
live through the acute episode.The second is

n can
live through the acute episode.The second is to return
the blood volume eventually to its normal level so that
the circulatory system can re-establish full normality,
including return of the arterial pressure all the way
back to its normal value, not merely back to a pressure
level required for survival.
In Chapter 18, we saw that the ﬁrst line of de-
fense against acute changes in arterial pressure is 
the nervous control system. In this chapter, we have
emphasized a second line of defens

hapter, we have
emphasized a second line of defense achieved mainly
by kidney mechanisms for long-term control of arte-
rial pressure. However, there are other pieces to the
puzzle. Figure 19–15 helps to put these together.
Figure 19–15 shows the approximate immediate
(seconds and minutes) and long-term (hours and days)
control responses, expressed as feedback gain, of eight
arterial pressure control mechanism. These mecha-
nisms can be divided into three groups: (1) those that
react rapidly, wi

nto three groups: (1) those that
react rapidly, within seconds or minutes; (2) those that
respond over an intermediate time period, minutes or
hours; and (3) those that provide long-term arterial
pressure regulation, days, months, and years. Let us see
how they ﬁt together as a total, integrated system for
Rapidly Acting Pressure Control Mechanisms, Acting Within
Seconds or Minutes. The rapidly acting pressure control
mechanisms are almost entirely acute nervous reﬂexes
or othe

 are almost entirely acute nervous reﬂexes
or other nervous responses. Note in Figure 19–15 the
three mechanisms that show responses within seconds.
They are (1) the baroreceptor feedback mechanism,
(2) the central nervous system ischemic mechanism,
and (3) the chemoreceptor mechanism. Not only do
these mechanisms begin to react within seconds, but
they are also powerful.After any acute fall in pressure,
as might be caused by severe hemorrhage, the nervous
mechanisms combine (1) to cause constri

he nervous
mechanisms combine (1) to cause constriction of the
veins and provide transfer of blood into the heart,
(2) to cause increased heart rate and contractility of
the heart to provide greater pumping capacity by the
heart, and (3) to cause constriction of most peripheral
arterioles to impede ﬂow of blood out of the arteries;
all these effects occur almost instantly to raise the
arterial pressure back into a survival range.
When the pressure suddenly rises too high, as might
occur in respo

e suddenly rises too high, as might
occur in response to rapid overadministration of a
blood transfusion, the same control mechanisms
operate in the reverse direction, again returning the
pressure back toward normal.
Pressure Control Mechanisms That Act After Many Minutes.
Several pressure control mechanisms exhibit signiﬁ-
cant responses only after a few minutes following
acute arterial pressure change. Three of these, shown
in Figure 19–15, are (1) the renin-angiotensin vaso-
constrictor mecha

 (1) the renin-angiotensin vaso-
constrictor mechanism, (2) stress-relaxation of the 
vasculature, and (3) shift of ﬂuid through the tissue
capillary walls in and out of the circulation to readjust
the blood volume as needed.
We have already described at length the role of the
renin-angiotensin vasoconstrictor system to provide a
semi-acute means for increasing the arterial pressure
when this is needed. The stress-relaxation mechanism
is demonstrated by the following example: When the
pressure i

ated by the following example: When the
pressure in the blood vessels becomes too high, they
become stretched and keep on stretching more and
more for minutes or hours; as a result, the pressure in
Maximum feedback gain at optimal pressure
Time after sudden change in pressure
•
0 1530 1 2 4 8 1632 1
1 2 4 8 16
2 4 816
0
1
2
3
4
5
6
7
8
9
10
11
Acute change in pressure at this time
Seconds
Minutes
Renin-angiotensin-vasoconstriction
Hours
Days
Baroreceptors
Capillary
Fluid
          shift
•
• !!
C

receptors
Capillary
Fluid
          shift
•
• !!
C
N
S
ischemic respons
e
C
hem
ore
ce
ptors
Stress relax
ati
on
A
ld
os
te
ro
n
e
Rena
l–b
lo
od
vol
um
e pr
essure
control
Figure 19–15
Approximate potency of various arterial pressure control mecha-
nisms at different time intervals after onset of a disturbance to the
arterial pressure. Note especially the inﬁnite gain (∞) of the renal
body ﬂuid pressure control mechanism that occurs after a few
weeks’ time. (Redrawn from Guyton AC: Arterial Pre

weeks’ time. (Redrawn from Guyton AC: Arterial Pressure and
Hypertension. Philadelphia: WB Saunders Co, 1980.)
Chapter 19
231
the vessels falls toward normal.This continuing stretch
of the vessels, called stress-relaxation, can serve as an
intermediate-term pressure “buffer.”
The capillary ﬂuid shift mechanism means simply
that any time capillary pressure falls too low, ﬂuid is
absorbed through the capillary membranes from the
tissues into the circulati

lary membranes from the
tissues into the circulation, thus building up the blood
volume and increasing the pressure in the circulation.
Conversely, when the capillary pressure rises too high,
ﬂuid is lost out of the circulation into the tissues, thus
reducing the blood volume as well as virtually all the
pressures throughout the circulation.
These three intermediate mechanisms become
mostly activated within 30 minutes to several hours.
During this time, the nervous mechanisms usually
become less

s time, the nervous mechanisms usually
become less and less effective, which explains the
importance of these non-nervous, intermediate time
pressure control measures.
Long-Term Mechanisms for Arterial Pressure Regulation. The
goal of this chapter has been to explain the role of the
kidneys in long-term control of arterial pressure. To
the far right in Figure 19–15 is shown the renal-blood
volume pressure control mechanism (which is the
same as the renal–body ﬂuid pressure control mecha-
nism), 

he renal–body ﬂuid pressure control mecha-
nism), demonstrating that it takes a few hours to begin
showing signiﬁcant response. Yet it eventually devel-
ops a feedback gain for control of arterial pressure
equal to inﬁnity. This means that this mechanism can
eventually return the arterial pressure all the way back,
not merely partway back, to that pressure level that
provides normal output of salt and water by the
kidneys. By now, the reader should be familiar with
this concept, which has been t

ld be familiar with
this concept, which has been the major point of this
chapter.
It must also be remembered that many factors can
affect the pressure-regulating level of the renal–body
ﬂuid mechanism. One of these, shown in Figure 19–15,
is aldosterone. A decrease in arterial pressure leads
within minutes to an increase in aldosterone secretion,
and over the next hour or days, this plays an important
role in modifying the pressure control characteristics
of the renal–body ﬂuid mechanism. Especi

teristics
of the renal–body ﬂuid mechanism. Especially impor-
tant is interaction of the renin-angiotensin system with
the aldosterone and renal ﬂuid mechanisms. For
instance, a person’s salt intake varies tremendously
from one day to another. We have seen in this chapter
that the salt intake can decrease to as little as 1/10
normal or can increase to 10 to 15 times normal and
yet the regulated level of the mean arterial pressure
will change only a few millimeters of mercury if the
renin-angiote

 a few millimeters of mercury if the
renin-angiotensin-aldosterone system is fully opera-
tive. But, without a functional renin-angiotensin-aldos-
terone system, blood pressure becomes very sensitive
to changes in salt intake.Thus, arterial pressure control
begins with the lifesaving measures of the nervous
pressure controls, then continues with the sustaining
characteristics of the intermediate pressure controls,
and, ﬁnally, is stabilized at the long-term pressure level
by the renal–body ﬂuid 

e long-term pressure level
by the renal–body ﬂuid mechanism. This long-term
mechanism in turn has multiple interactions with the
renin-angiotensin-aldosterone system, the nervous
system, and several other factors that provide special
blood pressure control capabilities for special pur-
poses.
Chobanian AV, Bakris GL, Black HR, et al: Joint National
Committee on Prevention, Detection, Evaluation, and
Treatment of High Blood Pressure. National High Blood
Pressure Education Program Coord

tional High Blood
Pressure Education Program Coordinating Committee.
Seventh Report of the Joint National Committee on 
prevention, detection, evaluation, and treatment of high
blood pressure. Hypertension 42:1206, 2003.
Cowley AW J: Long-term control of arterial blood pressure.
Physiol Rev 72:231, 1992.
Granger JP, Alexander BT, Bennett WA, Khalil RA: Patho-
physiology of pregnancy-induced hypertension. Am J
Hypertens 14:178S, 2001.
Granger JP,Alexander BT:Abnormal pressure-natriuresis in
hyper

lexander BT:Abnormal pressure-natriuresis in
hypertension: role of nitric oxide. Acta Physiol Scand
168:161, 2000.
Guyton AC: Blood pressure control—special role of the
kidneys and body ﬂuids. Science 252:1813, 1991.
Guyton AC, Coleman TG, Cowley AW Jr, et al:Arterial pres-
sure regulation: overriding dominance of the kidneys in
long-term regulation and in hypertension. Am J Med
52:584, 1972.
Hall JE:The kidney,

ension. Am J Med
Hall JE:The kidney, hypertension, and obesity. Hypertension
41:625, 2003.
1999.
Hall JE, Guyton AC, Brands MW: Pressure-volume regula-
tion in hypertension. Kidney Int Suppl 55:S35, 1996.
Hall JE, Jones DW, Kuo JJ, et al: Impact of the obesity epi-
demic on hypertension and renal disease. Curr Hype

demic on hypertension and renal disease. Curr Hypertens
Rep 5:386, 2003.
Lifton RP, Gharavi AG, Geller DS: Molecular mechanisms
of human hypertension. Cell 104:545, 2001.
Manning RD Jr, Hu L, Tan DY, Meng S: Role of abnormal
nitric oxide systems in salt-sensitive hypertension. Am J
Hypertens 14:68S, 2001.
Oparil S, Zaman MA, Calhoun DA: Pathogenesis of hyper-
tension. Ann Intern Med 139:761, 2003.
O’Shaughnessy KM, Karet FE: Salt handling and hyperten-
sion. J Clin Invest 113:1075, 2004.
Reckelh

erten-
Reckelhoff JF: Gender differences in the regulation of blood
pressure. Hypertension 37:1199, 2001.
Rossier BC: Negative regulators of sodium transport in the
kidney: key factors in understanding salt-sensitive hyper-
tension? J Clin Invest 111:947, 2003.
C
H
A
P
T
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2
0
232
Cardiac output is the quantity of blood pumped into
the aorta each minute by the heart. This is 
also the quantity of blood th

 the heart. This is 
also the quantity of blood that ﬂows through the 
circulation. Cardiac output is perhaps the most
important factor that we have to consider in rela-
tion to the circulation.
Venous return is the quantity of blood ﬂowing
from the veins into the right atrium each minute.
The venous return and the cardiac output must equal each other except for a
few heartbeats at a time when blood is temporarily stored in or removed from
the heart and lungs.
Normal Values for Cardiac Output at

art and lungs.
Normal Values for Cardiac Output at Rest and
During Activity
Cardiac output varies widely with the level of activity of the body. The follow-
ing factors, among others, directly affect cardiac output: (1) the basic level of
body metabolism, (2) whether the person is exercising, (3) the person’s age, and
(4) size of the body.
For young, healthy men, resting cardiac output averages about 5.6 L/min.
For women, this value is about 4.9 L/min. When one considers the factor of age
as wel

L/min. When one considers the factor of age
as well—because with increasing age, body activity diminishes—the average
cardiac output for the resting adult, in round numbers, is often stated to be
almost exactly 5 L/min.
Cardiac Index
Experiments have shown that the cardiac output increases approximately in pro-
portion to the surface area of the body.Therefore, cardiac output is frequently stated
in terms of the cardiac index, which is the cardiac output per square meter of body
surface area. Th

c output per square meter of body
surface area. The normal human being weighing 70 kilograms has a body surface
area of about 1.7 square meters, which means that the normal average cardiac index
for adults is about 3 L/min/m2 of body surface area.
Effect of Age on Cardiac Output. Figure 20–1 shows the cardiac output, expressed
as cardiac index, at different ages. Rising rapidly to a level greater than 
4 L/min/m2 at age 10 years, the cardiac index declines to about 2.4 L/min/m2 at
age 80 years. 

x declines to about 2.4 L/min/m2 at
age 80 years. We will see later in the chapter that the cardiac output is regu-
lated throughout life almost directly in proportion to the overall bodily meta-
bolic activity. Therefore, the declining cardiac index is indicative of declining
activity with age.
Return—Role of the Frank-Starling 
When one states that cardiac output is controlled by venous return, this means
that it is not the heart itse

s return, this means
that it is not the heart itself that is the primary controller of cardiac output.
Chapter 20
Cardiac Output, Venous Return, and Their Regulation
233
Instead, it is the various factors of the peripheral cir-
culation that affect ﬂow of blood into the heart from
the veins, called venous return, that are the primary
controllers.
The main reason peripheral factors are usually more
important than the heart itself in controlling cardiac
output is that the heart has a built-in mech

rdiac
output is that the heart has a built-in mechanism that
normally allows it to pump automatically whatever
amount of blood that ﬂows into the right atrium from
the veins. This mechanism, called the Frank-Starling
law of the heart, was discussed in Chapter 9. Basically,
this law states that when increased quantities of blood
ﬂow into the heart, the increased blood stretches the
walls of the heart chambers. As a result of the stretch,
the cardiac muscle contracts with increased force, and
this

ac muscle contracts with increased force, and
this empties the extra blood that has entered from the
systemic circulation. Therefore, the blood that ﬂows
into the heart is automatically pumped without delay
into the aorta and ﬂows again through the circulation.
Another important factor, discussed in Chapter 10,
is that stretching the heart causes the heart to pump
faster—at an increased heart rate.That is,stretch of the
sinus node in the wall of the right atrium has a direct
effect on the rhythm

the right atrium has a direct
effect on the rhythmicity of the node itself to increase
heart rate as much as 10 to 15 per cent. In addition,
the stretched right atrium initiates a nervous reﬂex
called the Bainbridge reﬂex, passing ﬁrst to the vaso-
motor center of the brain and then back to the heart
by way of the sympathetic nerves and vagi, also to
increase the heart rate.
Under most normal unstressful conditions, the
cardiac output is controlled almost entirely by periph-
eral factors that de

ed almost entirely by periph-
eral factors that determine venous return. However,
we shall see later in the chapter that if the returning
blood does become more than the heart can pump,
then the heart becomes the limiting factor that deter-
mines cardiac output.
Cardiac Output Regulation Is the Sum
of Blood Flow Regulation in All the
Local Tissues of the Body—Tissue
Metabolism Regulates Most Local
Blood Flow
The venous return to the heart is the sum of all the
local blood ﬂows through all the in

sum of all the
local blood ﬂows through all the individual tissue 
segments of the peripheral circulation. Therefore, it
follows that cardiac output regulation is the sum of all
the local blood ﬂow regulations.
The mechanisms of local blood ﬂow regulation were
discussed in Chapter 17. In most tissues, blood ﬂow
increases mainly in proportion to each tissue’s metab-
olism. For instance, local blood ﬂow almost always
increases when tissue oxygen consumption increases;
this effect is demonstrated i

nsumption increases;
this effect is demonstrated in Figure 20–2 for different
levels of exercise. Note that at each increasing level of
work output during exercise, the oxygen consumption
and the cardiac output increase in parallel to each
other.
To summarize, cardiac output is determined by the
sum of all the various factors throughout the body that
control local blood ﬂow. All the local blood ﬂows
summate to form the venous return, and the heart
automatically pumps this returning blood back in

t
automatically pumps this returning blood back into
the arteries to ﬂow around the system again.
Cardiac index (L/min/m2)
0
10
20
30
40
50
60
70
80
0
1
2
3
4
1
2
3
4
0
Figure 20–1
Cardiac index for the human being (cardiac output per square
meter of surface area) at different ages. (Redrawn from Guyton
AC, Jones CE, Coleman TB: Circulatory Physiology: Cardiac
Output and Its Regulation. 2nd ed. Philadelphia: WB Saunders Co,
1973.)
Cardiac output (L/min/m2)
O

rdiac index (L/min/m2)
Oxygen consumption (L/min)
0
200 400 600 800 1000120014001600
0
5
10
15
20
25
15
10
5
0
30
35
0
1
2
3
4
Work output during exercise (kg-m/min)
and cardiac index
Oxygen
consumption
Dexter
Douglas
Christensen
Donald
1951
1922
1931
1055
Figure 20–2
Effect of increasing levels of exercise to increase cardiac output
(red solid line) and oxygen consumption (blue dashed line).
(Redrawn from Guyton AC, Jones CE, Coleman TB: Circulatory
Phys

 Guyton AC, Jones CE, Coleman TB: Circulatory
Physiology: Cardiac Output and Its Regulation. 2nd ed. Philadel-
phia: WB Saunders Co, 1973.)
234
Unit IV
Effect of Total Peripheral Resistance on the Long-Term Cardiac
Output Level. Figure 20–3 is the same as Figure 19–5.
It is repeated here to illustrate an extremely important
principle in cardiac output control: Under most
normal conditions, the long-term cardiac output level
varies reciprocally with changes in total peripheral
res

 reciprocally with changes in total peripheral
resistance. Note in Figure 20–3 that when the total
peripheral resistance is exactly normal (at the 100 per
cent mark in the ﬁgure), the cardiac output is also
normal. Then, when the total peripheral resistance
increases above normal, the cardiac output falls; con-
versely, when the total peripheral resistance decreases,
the cardiac output increases. One can easily under-
stand this by reconsidering one of the forms of Ohm’s
law, as expressed in Cha

one of the forms of Ohm’s
law, as expressed in Chapter 14:
The meaning of this formula, and of Figure 20–3, is
simply the following: Any time the long-term level of
total peripheral resistance changes (but no other func-
tions of the circulation change), the cardiac output
changes quantitatively in exactly the opposite direction.
The Heart Has Limits for the Cardiac
Output That It Can Achieve
There are deﬁnite limits to the amount of blood that
the heart can pump, which can be expressed quantita

he heart can pump, which can be expressed quantita-
tively in the form of cardiac output curves.
=
Figure 20–4 demonstrates the normal cardiac output
curve, showing the cardiac output per minute at each
level of right atrial pressure.This is one type of cardiac
function curve, which was discussed in Chapter 9. Note
that the plateau level of this normal cardiac output
curve is about 13 L/min, 2.5 times the normal cardiac
output of about

/min, 2.5 times the normal cardiac
output of about 5 L/min. This means that the normal
human heart, functioning without any special stimula-
tion, can pump an amount of venous return up to
about 2.5 times the normal venous return before the
heart becomes a limiting factor in the control of
cardiac output.
Shown in Figure 20–4 are several other cardiac
output curves for hearts that are not pumping nor-
mally. The uppermost curves are for hypereffective
hearts that are pumping better than normal. 

ctive
hearts that are pumping better than normal. The low-
ermost curves are for hypoeffective hearts that are
pumping at levels below normal.
Factors That Can Cause Hypereffective Heart
Only two types of factors usually can make the heart
a better pump than normal.They are (1) nervous stim-
ulation and (2) hypertrophy of the heart muscle.
Effect of Nervous Excitation to Increase Heart Pumping. In
Chapter 9, we saw that a combination of (1) sympa-
thetic stimulation and (2) parasympathetic inhib

-
thetic stimulation and (2) parasympathetic inhibition
does two things to increase the pumping effectiveness
of the heart: (1) it greatly increases the heart rate—
sometimes, in young people, from the normal level of
Arterial pressure or cardiac output
(percentage of normal)
40
60
80
100
120
140
160
0
50
100
150
200
output
Beriberi
AV shunts
Anemia
Normal
Paget’s disease
Removal of both arms and legs

s disease
C
ar
di
a
c
Figure 20–3
Chronic effect of different levels of total peripheral resistance on
cardiac output, showing a reciprocal relationship between total
peripheral resistance and cardiac output. (Redrawn from Guyton
AC: Arterial Pressure and Hypertension. Philadelphia: WB 
–4
0
+4
+8
0
5
10
15
20
25
Normal
Hypereffective
Hypoeffective
Figure 20–4
Cardiac output curv

tive
Cardiac output curves for the normal heart and for hypoeffective
and hypereffective hearts. (Redrawn from Guyton AC, Jones CE,
Coleman TB: Circulatory Physiology: Cardiac Output and Its 
Regulation. 2nd ed. Philadelphia: WB Saunders Co, 1973.)
Chapter 20
235
72 beats/min up to 180 to 200 beats/min—and (2) it
increases the strength of heart contraction (which is
called increased “contractility”) to twice its normal

led increased “contractility”) to twice its normal
strength. Combining these two effects, maximal
nervous excitation of the heart can raise the plateau
level of the cardiac output curve to almost twice the
plateau of the normal curve, as shown by the 25-liter
level of the uppermost curve in Figure 20–4.
Increased Pumping Effectiveness Caused by Heart Hypertrophy.
A long-term increased workload, but not so much
excess load that it damages the heart, causes the heart
muscle to increase in mass and

t, causes the heart
muscle to increase in mass and contractile strength in
the same way that heavy exercise causes skeletal
muscles to hypertrophy. For instance, it is common for
the hearts of marathon runners to be increased in mass
by 50 to 75 per cent.This increases the plateau level of
the cardiac output curve, sometimes 60 to 100 per cent,
and therefore allows the heart to pump much greater
than usual amounts of cardiac output.
When one combines nervous excitation of the heart
and hypertrop

ines nervous excitation of the heart
and hypertrophy, as occurs in marathon runners, the
total effect can allow the heart to pump as much 30 to
40 L/min, about 21/2 times normal; this increased level
of pumping is one of the most important factors in
determining the runner’s running time.
Factors That Cause a Hypoeffective Heart
Any factor that decreases the heart’s ability to pump
blood causes hypoeffectivity. Some of the factors that
can do this are the following:
Coronary artery blockage, cau

s are the following:
Coronary artery blockage, causing a “heart
attack”
Inhibition of nervous excitation of the heart
Pathological factors that cause abnormal heart
rhythm or rate of heartbeat
Valvular heart disease
Increased arterial pressure against which the
heart must pump, such as in hypertension
Congenital heart disease
Cardiac hypoxia
What Is the Role of the 
Nervous System in Controlling
Cardiac Output?
Importance of the Nervous System in
Maintaining the Arterial Pressure Whe

us System in
Maintaining the Arterial Pressure When the
Venous Return and Cardiac Output Increase
Figure 20–5 shows an important difference in cardiac
output control with and without a functioning auto-
nomic nervous system. The solid curves demonstrate
the effect in the normal dog of intense dilation of 
the peripheral blood vessels caused by administering
the drug dinitrophenol, which increased the metabo-
lism of virtually all tissues of the body about fourfold.
Note that with nervous control

ody about fourfold.
Note that with nervous control to keep the arterial
pressure from falling, dilating all the peripheral blood
vessels caused almost no change in arterial pressure
but increased the cardiac output almost fourfold.
However, after autonomic control of the nervous
system had been blocked, none of the normal circula-
tory reﬂexes for maintaining the arterial pressure
could function, and vasodilation of the vessels with
dinitrophenol (dashed curves) then caused a profound
fall in ar

 (dashed curves) then caused a profound
fall in arterial pressure to about one half normal, and
the cardiac output rose only 1.6-fold instead of 4-fold.
Thus, maintenance of a normal arterial pressure by
the nervous reﬂexes, by mechanisms explained in
Chapter 18, is essential to achieve high cardiac outputs
when the peripheral tissues dilate their vessels to
increase the venous return.
Effect of the Nervous System to Increase the Arterial Pressure
During Exercise. During exercise, intense increa

e
During Exercise. During exercise, intense increase in
metabolism in active skeletal muscles acts directly on
the muscle arterioles to relax them and to allow ade-
quate oxygen and other nutrients needed to sustain
muscle contraction. Obviously, this greatly decreases
the total peripheral resistance, which normally would
decrease the arterial pressure also. However, the
nervous system immediately compensates. The same
brain activity that sends motor signals to the muscles
sends simultaneous sig

otor signals to the muscles
sends simultaneous signals into the autonomic nervous
centers of the brain to excite circulatory activity,
causing large vein constriction, increased heart rate,
and increased contractility of the heart. All these
changes acting together increase the arterial pressure
above normal, which in turn forces still more blood
ﬂow through the active muscles.
In summary, when local tissue vessels dilate and
thereby increase venous return and cardiac output
 (L/m

ous return and cardiac output
 (L/min)
0
2
3
4
5
6
 (mm Hg)
0
50
75
100
With pressure control
Dinitrophenol
Without pressure control
0
10
20
30
Minutes
Figure 20–5
Experiment in a dog to demonstrate the importance of nervous
maintenance of the arterial pressure as a prerequisite for cardiac
output control. Note that with pressure control, the metabolic stim-
ulant dinitrophenol increases cardiac output greatly; without pres-
sure control, the arterial pressure fa

thout pres-
sure control, the arterial pressure falls and the cardiac output rises
very little. (Drawn from experiments by Dr. M. Banet.)
236
Unit IV
above normal, the nervous system plays an exceed-
ingly important role in preventing the arterial pressure
from falling to disastrously low levels. In fact, during
exercise, the nervous system goes even further, pro-
viding additional signals to raise the arterial pressure
even above normal, which serves to increase the
cardiac outp

 normal, which serves to increase the
cardiac output an extra 30 to 100 per cent.
Pathologically High and
Pathologically Low 
Cardiac Outputs
In healthy human beings, the cardiac outputs are 
surprisingly constant from one person to another.
However, multiple clinical abnormalities can cause
either high or low cardiac outputs. Some of the more
important of these are shown in Figure 20–6.
High Cardiac Output Caused 
by Reduced Total Peripheral
The left side of Figure 20–6 identiﬁes con

istance
The left side of Figure 20–6 identiﬁes conditions that
commonly cause cardiac outputs higher than normal.
One of the distinguishing features of these conditions is
that they all result from chronically reduced total periph-
eral resistance. None of them result from excessive 
excitation of the heart itself, which we will explain sub-
sequently. For the present, let us look at some of the
conditions that can decrease the peripheral resistance
and at the same time increase the cardiac outp

nce
and at the same time increase the cardiac output to
1. Beriberi. This disease is caused by insufﬁcient
quantity of the vitamin thiamine (vitamin B1) in the
diet. Lack of this vitamin causes diminished ability
of the tissues to use some cellular nutrients, and the
local tissue blood ﬂow mechanisms in turn cause
marked compensatory peripheral vasodilation.
Sometimes the total peripheral resistance decreases
to as little as one-half normal. Consequently, the
long-term levels of ve

f normal. Consequently, the
long-term levels of venous return and cardiac
output also often increase to twice normal.
2. Arteriovenous ﬁstula (shunt). Earlier, we pointed
out that whenever a ﬁstula (also called an AV
shunt) occurs between a major artery and a major
vein, tremendous amounts of blood ﬂow directly
from the artery into the vein. This, too, greatly
decreases the total peripheral resistance and,
likewise, increases the venous return and cardiac
output.
3. Hyperthyroidism. In hyperthyr

d cardiac
output.
3. Hyperthyroidism. In hyperthyroidism, the
metabolism of most tissues of the body becomes
greatly increased. Oxygen usage increases, and
vasodilator products are released from the tissues.
Therefore, the total peripheral resistance decreases
markedly because of the local tissue blood 
ﬂow control reactions throughout the body;
consequently, the venous return and cardiac output
often increase to 40 to 80 per cent above normal.
4. Anemia. In anemia, two peripheral effects greatl

. Anemia. In anemia, two peripheral effects greatly
decrease the total peripheral resistance. One of
these is reduced viscosity of the blood, resulting
from the decreased concentration of red blood
Beriberi (5)
AV shunts (33)
Hyperthyroidism (29)
Anemia (75)
Anxiety (21)
Pulmonary disease (29)
Pregnancy (46)
Paget’s disease (9)
Control (young adults) (308)
Hypertension (47)
Mild valve disease (31)
Myocardial infarction (22)
Mild shock (4)
Severe valve disease (29)
Traumatic shock (4)
Cardiac sho

valve disease (29)
Cardiac shock (7)
Control (young adults)
Average 45-year-old adult
200
175
150
125
100
75
50
25
0
7
6
5
4
3
2
1
0
Cardiac index
(L/min/m2)
Figure 20–6
Cardiac output in different pathological conditions. The numbers in parentheses indicate number of patients studied in each condition.
(Redrawn from Guyton AC, Jones CE, Coleman TB: Circulatory Physiology: Cardiac Output and Its Regulation. 2nd ed. Philadelphia: 
WB Saunde

d Its Regulation. 2nd ed. Philadelphia: 
WB Saunders Co, 1973.)
Chapter 20
237
cells. The other is diminished delivery of oxygen to
the tissues, which causes local vasodilation. As a
consequence, the cardiac output increases greatly.
Any other factor that decreases the total peripheral
resistance chronically also increases the cardiac output.
Figure 20–6 shows at the far right several conditions that
cause abnormally low card

 several conditions that
cause abnormally low cardiac output. These conditions
fall into two categories: (1) those abnormalities that
cause the pumping effectiveness of the heart to fall too
low and (2) those that cause venous return to fall too
low.
Decreased Cardiac Output Caused by Cardiac Factors. When-
ever the heart becomes severely damaged, regardless 
of the cause, its limited level of pumping may fall below
that needed for adequate blood ﬂow to the tissues.Some
examples of this include 

 ﬂow to the tissues.Some
examples of this include (1) severe coronary blood vessel
blockage and consequent myocardial infarction, (2)
severe valvular heart disease, (3) myocarditis, (4) cardiac
tamponade, and (5) cardiac metabolic derangements.
The effects of several of these are shown on the right in
Figure 20–6, demonstrating the low cardiac outputs that
result.
When the cardiac output falls so low that the tissues
throughout the body begin to suffer nutritional deﬁ-
ciency, the condition is c

suffer nutritional deﬁ-
ciency, the condition is called cardiac shock. This is dis-
cussed fully in Chapter 22 in relation to cardiac failure.
Decrease in Cardiac Output Caused by Non-cardiac Peripheral
Factors—Decreased Venous Return. Anything that inter-
feres with venous return also can lead to decreased
cardiac output. Some of these factors are the following:
1. Decreased blood volume. By far, the most 
common non-cardiac peripheral factor that leads 
to decreased cardiac output is decreased

at leads 
to decreased cardiac output is decreased blood
volume, resulting most often from hemorrhage.
It is clear why this condition decreases the cardiac
output: Loss of blood decreases the ﬁlling of the
vascular system to such a low level that there is not
enough blood in the peripheral vessels to create
peripheral vascular pressures high enough to push
the blood back to the heart.
2. Acute venous dilation. On some occasions, the
peripheral veins become acutely vasodilated. This
results most 

ins become acutely vasodilated. This
results most often when the sympathetic nervous
system suddenly becomes inactive. For instance,
fainting often results from sudden loss of
sympathetic nervous system activity, which causes
the peripheral capacitative vessels, especially the
veins, to dilate markedly. This decreases the ﬁlling
pressure of the vascular system because the blood
volume can no longer create adequate pressure 
in the now ﬂaccid peripheral blood vessels. As a
result, the blood “pool

pheral blood vessels. As a
result, the blood “pools” in the vessels and does not
return to the heart.
3. Obstruction of the large veins. On rare occasions,
the large veins leading into the heart become
obstructed, so that the blood in the peripheral
vessels cannot ﬂow back into the heart.
Consequently, the cardiac output falls markedly.
4. Decreased tissue mass, especially decreased skeletal
muscle mass. With normal aging or with prolonged
periods of physical inactivity, there is usually a
reduc

s of physical inactivity, there is usually a
reduction in the size of the skeletal muscles. This, in
turn, decreases the total oxygen consumption and
blood ﬂow needs of the muscles, resulting in
decreases in skeletal muscle blood ﬂow and cardiac
output.
Regardless of the cause of low cardiac output,
whether it be a peripheral factor or a cardiac factor, if
ever the cardiac output falls below that level required
for adequate nutrition of the tissues, the person is said
to suffer circulatory shock

es, the person is said
to suffer circulatory shock. This condition can be lethal
within a few minutes to a few hours. Circulatory shock
is such an important clinical problem that it is discussed
in detail in Chapter 24.
A More Quantitative Analysis 
of Cardiac Output Regulation
Our discussion of cardiac output regulation thus far is
adequate for understanding the factors that control
cardiac output in most simple conditions. However, to
understand cardiac output regulation in especially
stressfu

d cardiac output regulation in especially
stressful situations, such as the extremes of exercise,
cardiac failure, and circulatory shock, a more complex
quantitative analysis is presented in the following 
sections.
To perform the more quantitative analysis, it is nec-
essary to distinguish separately the two primary factors
concerned with cardiac output regulation: (1) the
pumping ability of the heart, as represented by cardiac
output curves, and (2) the peripheral factors that affect
ﬂow of bl

d (2) the peripheral factors that affect
ﬂow of blood from the veins into the heart, as repre-
sented by venous return curves. Then one can put these
curves together in a quantitative way to show how they
interact with each other to determine cardiac output,
venous return,and right atrial pressure at the same time.
Cardiac Output Curves Used in the
Some of the cardiac output curves used to depict quan-
titative heart pumping effectiveness have already been
shown in Figure 2

 effectiveness have already been
shown in Figure 20–4. However, an additional set of
curves is required to show the effect on cardiac output
caused by changing external pressures on the outside of
the heart, as explained in the next section.
Effect of External Pressure Outside the Heart on Cardiac Output
Curves. Figure 20–7 shows the effect of changes in exter-
nal cardiac pressure on the cardiac output curve. The
normal external pressure is equal to the normal
intrapleural pressure (the pressur

l to the normal
intrapleural pressure (the pressure in the chest cavity),
which is -4 mm Hg. Note in the ﬁgure that a rise in
intrapleural pressure, to -2 mm Hg, shifts the entire
cardiac output curve to the right by the same amount.
This shift occurs because to ﬁll the cardiac chambers
with blood requires an extra 2 mm Hg right atrial pres-
sure to overcome the increased pressure on the outside
of the heart. Likewise, an increase in intrapleural pres-
sure to +2 mm Hg requires a 6 mm Hg increas

 pres-
sure to +2 mm Hg requires a 6 mm Hg increase in right
atrial pressure from the normal -4 mm Hg, which shifts
the entire cardiac output curve 6 mm Hg to the right.
Some of the factors that can alter the intrapleural
pressure and thereby shift the cardiac output curve are
1. Cyclical changes of intrapleural pressure during
respiration, which are about ±2 mm Hg during
normal breathing but can be as much as 
±50 mm Hg during strenuous breathing.
238
Unit IV
2. B

enuous breathing.
238
Unit IV
2. Breathing against a negative pressure, which shifts
the curve to a more negative right atrial pressure
(to the left).
3. Positive pressure breathing, which shifts the curve to
the right.
4. Opening the thoracic cage, which increases the
intrapleural pressure to 0 mm Hg and shifts the
cardiac output curve to the right 4 mm Hg.
5. Cardiac tamponade, which means accumulation of 
a large quantity of ﬂuid in the pericardial cavity
around the heart with

id in the pericardial cavity
around the heart with resultant increase in external
cardiac pressure and shifting of the curve to the
right. Note in Figure 20–7 that cardiac tamponade
shifts the upper parts of the curves farther to the
right than the lower parts because the external
“tamponade” pressure rises to higher values as the
chambers of the heart ﬁll to increased volumes
during high cardiac output.
Combinations of Different Patterns of Cardiac Output Curves.
Figure 20–8 shows that the ﬁnal

iac Output Curves.
Figure 20–8 shows that the ﬁnal cardiac output curve can
change as a result of simultaneous changes in (a) exter-
nal cardiac pressure and (b) effectiveness of the heart as
a pump. Thus, by knowing what is happening to the
external pressure as well as to the capability of the heart
as a pump, one can express the momentary ability of the
heart to pump blood by a single cardiac output curve.
There remains the entire systemic circulation that must
be consider

 entire systemic circulation that must
be considered before total analysis of cardiac regulation
can be achieved.To analyze the function of the systemic
circulation, we ﬁrst remove the heart and lungs from the
circulation of an animal and replace them with a pump
and artiﬁcial oxygenator system.Then, different factors,
such blood volume, vascular resistances, and central
venous pressure in the right atrium, are altered to
determine how the systemic circulation operates in
different circulatory s

ic circulation operates in
different circulatory states. In these studies, one ﬁnds
three principal factors that affect venous return to the
heart from the systemic circulation.They are as follows:
1. Right atrial pressure, which exerts a backward force
on the veins to impede ﬂow of blood from the
veins into the right atrium.
2. Degree of ﬁlling of the systemic circulation
(measured by the mean systemic ﬁlling pressure),
which forces the systemic blood toward the heart
(this is the pressure meas

 blood toward the heart
(this is the pressure measured everywhere in the
systemic circulation when all ﬂow of blood is
stopped—we discuss this in detail later).
3. Resistance to blood ﬂow between the peripheral
vessels and the right atrium.
These factors can all be expressed quantitatively 
by the venous return curve, as we explain in the next 
sections.
Normal Venous Return Curve
In the same way that the cardiac output curve relates
pumping of blood by the heart to right atrial pressure,
the ve

lood by the heart to right atrial pressure,
the venous return curve relates venous return also to
right atrial pressure—that is, the venous ﬂow of blood
into the heart from the systemic circulation at different
levels of right atrial pressure.
The curve in Figure 20–9 is the normal venous return
curve. This curve shows that when heart pumping capa-
bility becomes diminished and causes the right atrial
pressure to rise, the backward force of the rising atrial
pressure on the veins of the systemic

ising atrial
pressure on the veins of the systemic circulation
decreases venous return of blood to the heart. If all
nervous circulatory reﬂexes are prevented from acting,
venous return decreases to zero when the right atrial
pressure rises to about +7 mm Hg. Such a slight rise in
right atrial pressure causes a drastic decrease in venous
return because the systemic circulation is a distensible
bag, so that any increase in back pressure causes blood
to dam up in this bag instead of returning to t

od
to dam up in this bag instead of returning to the heart.
At the same time that the right atrial pressure is rising
and causing venous stasis, pumping by the heart also
0
–4
5
10
15
0
+4
+8
+12
C
ardiac tam
ponad
e
N
o
r
m
al
(in
trapleural p
ressur
e
=
–
4
m
m
H
g
)
In
tra
pleu
ral pres
su
re
=
–
5
.
5
m
m
I
nt
ra
pleu
ral pressure =
–
2
m
m
H
g
I
nt
ra
pleu
ral pressure = +
2
m
m
H
g
Figure 20–7
Cardiac output curves at different levels of

 20–7
Cardiac output curves at different levels of intrapleural pressure
and at different degrees of cardiac tamponade. (Redrawn from
Guyton AC, Jones CE, Coleman TB: Circulatory Physiology:
Cardiac Output and Its Regulation. 2nd ed. Philadelphia: WB
Saunders Co, 1973.)
0
–4
5
10
15
0
+4
+8
+12
Normal
Hypereffective–increased
intrapleural pressure
Hypoeffective–reduced
Figure 20–8
Combinations of two major patterns of car

ure 20–8
Combinations of two major patterns of cardiac output curves
showing the effect of alterations in both extracardiac pressure and
effectiveness of the heart as a pump. (Redrawn from Guyton AC,
Jones CE, Coleman TB: Circulatory Physiology: Cardiac Output
and Its Regulation. 2nd ed. Philadelphia: WB Saunders Co, 1973.)
Chapter 20
239
approaches zero because of decreasing venous return.
Both the arterial and the venous pressures come to
equ

 the arterial and the venous pressures come to
equilibrium when all ﬂow in the systemic circulation
ceases at a pressure of 7 mm Hg, which, by deﬁnition, is
the mean systemic ﬁlling pressure (Psf).
Plateau in the Venous Return Curve at Negative Atrial 
Pressures—Caused by Collapse of the Large Veins. When the
right atrial pressure falls below zero—that is, below
atmospheric pressure—further increase in venous
return almost ceases. And by the time the right atrial
pressure has fallen to about -2 

 the right atrial
pressure has fallen to about -2 mm Hg, the venous
return will have reached a plateau. It remains at this
plateau level even though the right atrial pressure falls
to -20 mm Hg,-50 mm Hg,or even further.This plateau
is caused by collapse of the veins entering the chest.Neg-
ative pressure in the right atrium sucks the walls of the
veins together where they enter the chest, which pre-
vents any additional ﬂow of blood from the peripheral
veins. Consequently, even very negative pr

ipheral
veins. Consequently, even very negative pressures in the
right atrium cannot increase venous return signiﬁcantly
above that which exists at a normal atrial pressure of 
0 mm Hg.
Mean Circulatory Filling Pressure and Mean Systemic Filling
Pressure, and Their Effect on Venous Return
When heart pumping is stopped by shocking the heart
with electricity to cause ventricular ﬁbrillation or is
stopped in any other way, ﬂow of blood everywhere in
the circulation ceases a few seconds later.Withou

the circulation ceases a few seconds later.Without blood
ﬂow, the pressures everywhere in the circulation
become equal. This equilibrated pressure level is called
the mean circulatory ﬁlling pressure.
Effect of Blood Volume on Mean Circulatory Filling Pressure.
The greater the volume of blood in the circulation, the
greater is the mean circulatory ﬁlling pressure because
extra blood volume stretches the walls of the vascula-
ture. The red curve in Figure 20–10 shows the approxi-
mate normal eff

in Figure 20–10 shows the approxi-
mate normal effect of different levels of blood volume
on the mean circulatory ﬁlling pressure. Note that at a
blood volume of about 4000 milliliters, the mean circu-
latory ﬁlling pressure is close to zero because this is the
“unstressed volume” of the circulation, but at a volume
of 5000 milliliters, the ﬁlling pressure is the normal value
of 7 mm Hg. Similarly, at still higher volumes, the mean
circulatory ﬁlling pressure increases almost linearly.
Effect of

ling pressure increases almost linearly.
Effect of Sympathetic Nervous Stimulation of the Circulation on
Mean Circulatory Filling Pressure. The green curve and blue
curve in Figure 20–10 show the effects, respectively,
of high and low levels of sympathetic nervous activity
on the mean circulatory ﬁlling pressure. Strong sympa-
thetic stimulation constricts all the systemic blood
vessels as well as the larger pulmonary blood vessels
and even the chambers of the heart. Therefore, the
capacity of t

hambers of the heart. Therefore, the
capacity of the system decreases, so that at each level of
blood volume, the mean circulatory ﬁlling pressure is
increased. At normal blood volume, maximal sympa-
thetic stimulation increases the mean circulatory ﬁlling
pressure from 7 mm Hg to about 2.5 times that value, or
about 17 mm Hg.
Venous return (L/min)
0
5
–8
–4
0
+4
+8
Transitional zone
Plateau
Down slope
Mean
systemic
filling
pressure
Figure 20–9
Normal venous return 

pressure (mm Hg)
Normal venous return curve. The
plateau is caused by collapse of
the large veins entering the chest
when the right atrial pressure falls
below 
atmospheric 
pressure.
Note also that venous return
becomes zero when the right
atrial pressure rises to equal the
mean systemic ﬁlling pressure.
0
2
4
6
8
10
12
14
–0
1000 2000 3000 4000 5000 6000 7000
Mean circulatory filling pressure (mm Hg)
Strong sympathetic
Normal circulatory
system
Complete symp

timulation
system
Complete sympathetic
Figure 20–10
Effect of changes in total blood volume on the mean circulatory
ﬁlling pressure (i.e., “volume-pressure curves” for the entire cir-
culatory system). These curves also show the effects of strong
sympathetic stimulation and complete sympathetic inhibition.
240
Unit IV
Conversely, complete inhibition of the sympathetic
nervous system relaxes both the blood vessels and the
heart, decreasi

xes both the blood vessels and the
heart, decreasing the mean circulatory ﬁlling pressure
from the normal value of 7 mm Hg down to about 4 mm
Hg. Before leaving Figure 20–10, note speciﬁcally how
steep the curves are.This means that even slight changes
in blood volume or slight changes in the capacity of the
system caused by various levels of sympathetic activity
can have large effects on the mean circulatory ﬁlling
pressure.
Mean Systemic Filling Pressure and Its Relation to Mean Circula-
tory 

g Pressure and Its Relation to Mean Circula-
tory Filling Pressure. The mean systemic ﬁlling pressure,
Psf, is slightly different from the mean circulatory ﬁlling
pressure. It is the pressure measured everywhere in the
systemic circulation after blood ﬂow has been stopped
by clamping the large blood vessels at the heart, so that
the pressures in the systemic circulation can be meas-
ured independently from those in the pulmonary circu-
lation. The mean systemic pressure, although almost
impossib

e mean systemic pressure, although almost
impossible to measure in the living animal, is the impor-
tant pressure for determining venous return. The mean
systemic ﬁlling pressure,however,is almost always nearly
equal to the mean circulatory ﬁlling pressure because the
pulmonary circulation has less than one eighth as much
capacitance as the systemic circulation and only about
one tenth as much blood volume.
Effect on the Venous Return Curve of Changes in Mean Systemic
Filling Pressure. Figure 20

anges in Mean Systemic
Filling Pressure. Figure 20–11 shows the effects on the
venous return curve caused by increasing or decreasing
the mean systemic ﬁlling pressure (Psf). Note in Figure
20–11 that the normal mean systemic ﬁlling pressure is
7 mm Hg. Then, for the uppermost curve in the ﬁgure,
the mean systemic ﬁlling pressure has been increased to
14 mm Hg, and for the lowermost curve, has been
decreased to 3.5 mm Hg.These curves demonstrate that
the greater the mean systemic ﬁlling pressure

that
the greater the mean systemic ﬁlling pressure (which
also means the greater the “tightness” with which the
circulatory system is ﬁlled with blood) the more the
venous return curve shifts upward and to the right.
Conversely,the lower the mean systemic ﬁlling pressure,
the more the curve shifts downward and to the left.
To express this another way, the greater the system is
ﬁlled, the easier it is for blood to ﬂow into the heart.
The less the ﬁlling, the more difﬁcult it is for blood to
ﬂow i

ﬁlling, the more difﬁcult it is for blood to
ﬂow into the heart.
“Pressure Gradient for Venous Return”—When This Is Zero, There
Is No Venous Return. When the right atrial pressure rises
to equal the mean systemic ﬁlling pressure, there is no
longer any pressure difference between the peripheral
vessels and the right atrium. Consequently, there can no
longer be any blood ﬂow from any peripheral vessels
back to the right atrium. However, when the right atrial
pressure falls progressively lower tha

ight atrial
pressure falls progressively lower than the mean sys-
temic ﬁlling pressure, the ﬂow to the heart increases
proportionately, as one can see by studying any of the
venous return curves in Figure 20–11.That is, the greater
the difference between the mean systemic ﬁlling pressure
and the right atrial pressure, the greater becomes the
venous return. Therefore, the difference between these
two pressures is called the pressure gradient for venous
return.
Resistance to Venous Return
In the 

venous
return.
In the same way that mean systemic ﬁlling pressure rep-
resents a pressure pushing venous blood from the
periphery toward the heart, there is also resistance to
this venous ﬂow of blood. It is called the resistance to
venous return. Most of the resistance to venous return
occurs in the veins, although some occurs in the arteri-
oles and small arteries as well.
Why is venous resistance so important in determin-
ing the resistance to venous return? The an

ermin-
ing the resistance to venous return? The answer is that
when the resistance in the veins increases, blood begins
to be dammed up, mainly in the veins themselves. But
the venous pressure rises very little because the veins
are highly distensible. Therefore, this rise in venous
pressure is not very effective in overcoming the resist-
ance, and blood ﬂow into the right atrium decreases
drastically. Conversely, when arteriolar and small artery
resistances increase, blood accumulates in the ar

resistances increase, blood accumulates in the arteries,
which have a capacitance only 1/30 as great as that of
the veins. Therefore, even slight accumulation of blood
in the arteries raises the pressure greatly—30 times as
much as in the veins—and this high pressure does over-
come much of the increased resistance. Mathematically,
it turns out that about two thirds of the so-called “resist-
ance to venous return” is determined by venous resist-
ance, and about one third by the arteriolar and s

ance, and about one third by the arteriolar and small
artery resistance.
Venous return can be calculated by the following
formula:
in which VR is venous return, Psf is mean systemic
ﬁlling pressure, PRA is right atrial pressure, and RVR
is resistance to venous return. In the healthy human
adult, the values for these are as follows: venous return
equals 5 L/min, mean systemic ﬁlling pressure equals 
7 mm Hg, right atrial pressure equals 0 mm Hg, and
resistance to venous return equals 1.4 mm Hg p

and
resistance to venous return equals 1.4 mm Hg per liter
of blood ﬂow.
VR
Psf
PRA
RVR
=
-
–4
0
+4
+8
+12
0
5
10
Normal
Psf = 3.5
Psf = 7
Psf = 14
Figure 20–11
Venous return curves showing the normal curve when the mean
systemic ﬁlling pressure (Psf) is 7 mm Hg, and showing the effect
of altering the Psf to either 3.5 or 14 mm Hg. (Redrawn from
Cardiac Output and Its Regulation. 2nd ed. 

ology:
Chapter 20
241
Effect of Resistance to Venous Return on the Venous Return
Curve. Figure 20–12 demonstrates the effect of different
levels of resistance to venous return on the venous
return curve, showing that a decrease in this resistance
to one-half normal allows twice as much ﬂow of blood
and, therefore, rotates the curve upward to twice as great
a slope. 

tates the curve upward to twice as great
a slope. Conversely, an increase in resistance to twice
normal rotates the curve downward to one-half as great
a slope.
Note also that when the right atrial pressure rises to
equal the mean systemic ﬁlling pressure, venous return
becomes zero at all levels of resistance to venous return
because when there is no pressure gradient to cause
ﬂow of blood, it makes no difference what the resistance
is in the circulation; the ﬂow is still zero. Therefore, the
h

rculation; the ﬂow is still zero. Therefore, the
highest level to which the right atrial pressure can rise,
regardless of how much the heart might fail, is equal to
the mean systemic ﬁlling pressure.
Combinations of Venous Return Curve Patterns. Figure 20–13
shows effects on the venous return curve caused by
simultaneous changes in mean systemic pressure (Psf)
and resistance to venous return, demonstrating that
both these factors can operate simultaneously.
Analysis of Cardiac Output and
Right A

ltaneously.
Right Atrial Pressure, Using
Simultaneous Cardiac Output and
In the complete circulation, the heart and the systemic
circulation must operate together. This means that (1)
the venous return from the systemic circulation must
equal the cardiac output from the heart and (2) the right
atrial pressure is the same for both the heart and the
systemic circulation.
Therefore, one can predict the cardiac output and
right atrial pressure in t

 the cardiac output and
right atrial pressure in the following way: (1) Determine
the momentary pumping ability of the heart and depict
this in the form of a cardiac output curve; (2) determine
the momentary state of ﬂow from the systemic circula-
tion into the heart and depict this in the form of a
venous return curve; and (3) “equate” these curves
against each other, as shown in Figure 20–14.
Two curves in the ﬁgure depict the normal cardiac
output curve (red line) and the normal venous return

tput curve (red line) and the normal venous return
curve (blue line). There is only one point on the graph,
point A, at which the venous return equals the cardiac
output and at which the right atrial pressure is the same
for both the heart and the systemic circulation. There-
fore, in the normal circulation, the right atrial pressure,
cardiac output, and venous return are all depicted by
point A, called the equilibrium point, giving a normal
value for cardiac output of 5 liters per minute and a

e for cardiac output of 5 liters per minute and a
right atrial pressure of 0 mm Hg .
Effect of Increased Blood Volume on Cardiac Output. A sudden
increase in blood volume of about 20 per cent increases
the cardiac output to about 2.5 to 3 times normal. An
analysis of this effect is shown in Figure 20–14. Imme-
diately on infusing the large quantity of extra blood, the
increased ﬁlling of the system causes the mean systemic
ﬁlling pressure (Psf) to increase to 16 mm Hg, which
shifts the venous re

o increase to 16 mm Hg, which
shifts the venous return curve to the right. At the same
time, the increased blood volume distends the blood
vessels, thus reducing their resistance and thereby
reducing the resistance to venous return, which rotates
the curve upward. As a result of these two effects, the
venous return curve of Figure 20–14 is shifted to the
–4
0
+4
+8
0
5
10
15
20
1/2 resistance
N
o
r
m
al
2 ¥
re
si
stanc
e
Psf = 7
Figu

o
r
m
al
2 ¥
re
si
stanc
e
Psf = 7
Figure 20–12
Venous return curves depicting the effect of altering the “resist-
ance to venous return.” Psf, mean systemic ﬁlling pressure.
–4
0
+4
+8
+12
0
5
10
15
Normal resistance
2 ¥ resistance
1/3 resistance
Psf = 10
Psf = 7
Psf 

/2 resistance
Psf = 10
Psf = 7
Psf = 2.3
Psf = 10.5
Figure 20–13
Combinations of the major patterns of venous return curves,
showing the effects of simultaneous changes in mean systemic
ﬁlling pressure (Psf) and in “resistance to venous return.”
242
Unit IV
right. This new curve equates with the cardiac output
curv

his new curve equates with the cardiac output
curve at point B, showing that the cardiac output and
venous return increase 2.5 to 3 times, and that the right
atrial pressure rises to about +8 mm Hg.
Further Compensatory Effects Initiated in Response to Increased
Blood Volume. The greatly increased cardiac output
caused by increased blood volume lasts for only a few
minutes because several compensatory effects immedi-
ately begin to occur: (1) The increased cardiac output
increases the capillary 

 increased cardiac output
increases the capillary pressure so that ﬂuid begins to
transude out of the capillaries into the tissues, thereby
returning the blood volume toward normal. (2) The
increased pressure in the veins causes the veins to 
continue distending gradually by the mechanism called
stress-relaxation, especially causing the venous blood
reservoirs, such as the liver and spleen, to distend, thus
reducing the mean systemic pressure. (3) The excess
blood ﬂow through the peripheral tiss

) The excess
blood ﬂow through the peripheral tissues causes
autoregulatory increase in the peripheral resistance,
thus increasing the resistance to venous return. These
factors cause the mean systemic ﬁlling pressure to
return back toward normal and the resistance vessels of
the systemic circulation to constrict. Therefore, gradu-
ally, over a period of 10 to 40 minutes, the cardiac output
returns almost to normal.
Effect of Sympathetic Stimulation on Cardiac Output. Sympa-
thetic stimulation a

ion on Cardiac Output. Sympa-
thetic stimulation affects both the heart and the sys-
temic circulation: (1) It makes the heart a stronger pump.
(2) In the systemic circulation, it increases the mean 
systemic ﬁlling pressure because of contraction of 
the peripheral vessels—especially the veins—and it
increases the resistance to venous return.
In Figure 20–15, the normal cardiac output and
venous return curves are depicted; these equate with
each other at point A, which represents a normal venou

 other at point A, which represents a normal venous
return and cardiac output of 5 L/min and a right atrial
pressure of 0 min Hg. Note in the ﬁgure that maximal
sympathetic stimulation (green curves) increases the
mean systemic ﬁlling pressure to 17 mm Hg (depicted
by the point at which the venous return curve reaches
the zero venous return level).And the sympathetic stim-
ulation also increases pumping effectiveness of the
heart by nearly 100 per cent. As a result, the cardiac
output rises from

r cent. As a result, the cardiac
output rises from the normal value at equilibrium point
A to about double normal at equilibrium point D—and
yet the right atrial pressure hardly changes. Thus, differ-
ent degrees of sympathetic stimulation can increase the
cardiac output progressively to about twice normal for
short periods of time, until other compensatory effects
occur within seconds or minutes.
Effect of Sympathetic Inhibition on Cardiac Output. The sym-
pathetic nervous system can be blocked

t. The sym-
pathetic nervous system can be blocked by inducing
total spinal anesthesia or by using some drug, such as
hexamethonium, that blocks transmission of nerve
signals through the autonomic ganglia. The lowermost
curves in Figure 20–15 show the effect of sympathetic
inhibition caused by total spinal anesthesia, demon-
strating that (1) the mean systemic ﬁlling pressure falls
to about 4 mm Hg and (2) the effectiveness of the heart
as a pump decreases to about 80 per cent of normal. The
car

 decreases to about 80 per cent of normal. The
cardiac output falls from point A to point B, which is a
decrease to about 60 per cent of normal.
Effect of Opening a Large Arteriovenous Fistula. Figure 20–16
shows various stages of circulatory changes that occur
after opening a large arteriovenous ﬁstula, that is, after
making an opening directly between a large artery and
a large vein.
1. The two red curves crossing at point A show the
normal condition.
Cardiac output and venous return (L/min)
–

dition.
–4
0
+4
+8
+12
+16
0
5
10
15
20
B
A
Psf = 7
Psf = 16
Figure 20–14
The two solid curves demonstrate an analysis of cardiac output
and right atrial pressure when the cardiac output (red line) and
venous return (blue line) curves are normal. Transfusion of blood
equal to 20 per cent of the blood volume causes the venous return
curve to become the dashed curve; as a result, the cardiac output
and right atrial pressure shi

, the cardiac output
and right atrial pressure shift from point A to point B. Psf, mean
systemic ﬁlling pressure.
–4
0
+4
+8
+12
+16
0
5
10
15
20
25
B
A
D
C
Maximal sympathetic
Moderate sympathetic
Spinal anesthesia
Normal
Figure 20–15
Analysis of the effect on cardiac output of (1) moderate sympa-
thetic stimulation (from point A to point C), (2) maximal sympa-
thetic stimulation (point D), and (3) sy

al sympa-
thetic stimulation (point D), and (3) sympathetic inhibition caused
by total spinal anesthesia (point B). (Redrawn from Guyton AC,
Chapter 20
243
2. The curves crossing at point B show the circulatory
condition immediately after opening the large 
ﬁstula. The principal effects are (1) a sudden and
precipitous r

incipal effects are (1) a sudden and
precipitous rotation of the venous return curve
upward caused by the large decrease in resistance to
venous return when blood is allowed to ﬂow with
almost no impediment directly from the large
arteries into the venous system, bypassing most 
of the resistance elements of the peripheral
circulation, and (2) a slight increase in the level of
the cardiac output curve because opening the ﬁstula
decreases the peripheral resistance and allows 
an acute fall in art

pheral resistance and allows 
an acute fall in arterial pressure against which 
the heart can pump more easily. The net result,
depicted by point B, is an increase in cardiac output
from 5 L/min up to 13 L/min and an increase in
right atrial pressure to about +3 mm Hg.
3. Point C represents the effects about 1 minute later,
after the sympathetic nerve reﬂexes have restored
the arterial pressure almost to normal and caused
two other effects: (1) an increase in the mean
systemic ﬁlling pressure (b

n increase in the mean
systemic ﬁlling pressure (because of constriction of
all veins and arteries) from 7 to 9 mm Hg, thus
shifting the venous return curve 2 mm Hg to the
right, and (2) further elevation of the cardiac
output curve because of sympathetic nervous
excitation of the heart. The cardiac output now
rises to almost 16 L/min, and the right atrial
pressure to about 4 mm Hg.
4. Point D shows the effect after several more weeks.
By this time, the blood volume has increased
because the sli

me, the blood volume has increased
because the slight reduction in arterial pressure and
the sympathetic stimulation have both reduced
kidney output of urine. The mean systemic ﬁlling
pressure has now risen to +12 mm Hg, shifting the
venous return curve another 3 mm Hg to the right.
Also, the prolonged increased workload on the
heart has caused the heart muscle to hypertrophy
slightly, raising the level of the cardiac output curve
still further. Therefore, point D shows a cardiac
output now of a

Therefore, point D shows a cardiac
output now of almost 20 L/min and a right atrial
pressure of about 6 mm Hg.
Other Analyses of Cardiac Output Regulation. In Chapter 21,
analysis of cardiac output regulation during exercise is
presented, and in Chapter 22, analyses of cardiac output
regulation at various stages of congestive heart failure
are shown.
Methods for Measuring
In animal experiments, one can cannulate the aorta, pul-
monary artery, or great veins entering the heart and

ary artery, or great veins entering the heart and
measure the cardiac output using any type of ﬂowme-
ter.An electromagnetic or ultrasonic ﬂowmeter can also
be placed on the aorta or pulmonary artery to measure
In the human, except in rare instances, cardiac output
is measured by indirect methods that do not require
surgery. Two of the methods commonly used are the
oxygen Fick method and the indicator dilution method.
Pulsatile Output of the Heart as
Measured by an Electromagneti

tput of the Heart as
Measured by an Electromagnetic or
Ultrasonic Flowmeter
Figure 20–17 shows a recording in a dog of blood ﬂow
in the root of the aorta made using an electromagnetic
ﬂowmeter. It demonstrates that the blood ﬂow rises
rapidly to a peak during systole, and then at the end of
systole reverses for a fraction of a second. This reverse
ﬂow causes the aortic valve to close and the ﬂow to
return to zero.
–4
0
+4
+8
+12
0
5
10
15
20
Right atrial 

 (L/min)
–4
0
+4
+8
+12
0
5
10
15
20
A
B
C
D
Figure 20–16
Analysis of successive changes in cardiac output and right atrial
pressure in a human being after a large arteriovenous (AV) ﬁstula
is suddenly opened. The stages of the analysis, as shown by the
equilibrium points, are A, normal conditions; B, immediately after
opening the AV ﬁstula; C, 1 minute or so after the sympathetic
reﬂexes have become active; and D, several weeks after the blood
volume has increased 

everal weeks after the blood
volume has increased and the heart has begun to hypertrophy.
0
1
2
Seconds
Flow (L/min)
20
15
10
5
0
Figure 20–17
Pulsatile blood ﬂow in the root of the aorta recorded using an elec-
tromagnetic ﬂowmeter.
244
Unit IV
Measurement of Cardiac Output Using
the Oxygen Fick Principle
The Fick principle is e

The Fick principle is explained by Figure 20–18. This
ﬁgure shows that 200 milliliters of oxygen are being
absorbed from the lungs into the pulmonary blood
each minute. It also shows that the blood entering the
right heart has an oxygen concentration of 160 milli-
liters per liter of blood, whereas that leaving the left
heart has an oxygen concentration of 200 milliliters
per liter of blood. From these data, one can calculate
that each liter of blood passing through th

culate
that each liter of blood passing through the lungs
absorbs 40 milliliters of oxygen.
Because the total quantity of oxygen absorbed into
the blood from the lungs each minute is 200 milliliters,
dividing 200 by 40 calculates a total of ﬁve 1-liter por-
tions of blood that must pass through the pulmonary
circulation each minute to absorb this amount of
oxygen. Therefore, the quantity of blood ﬂowing
through the lungs each minute is 5 liters, which is also
a measure of the cardiac output. Thu

which is also
a measure of the cardiac output. Thus, the cardiac
output can be calculated by the following formula:
In applying this Fick procedure for measuring
cardiac output in the human being, mixed venous
blood is usually obtained through a catheter inserted
up the brachial vein of the forearm, through the sub-
clavian vein, down to the right atrium, and, ﬁnally, into
the right ventricle or pulmonary artery. And systemic
arterial blood can then be obtained from any systemic
artery in the bo

hen be obtained from any systemic
artery in the body. The rate of oxygen absorption by
the lungs is measured by the rate of disappearance of
oxygen from the respired air, using any type of oxygen
meter.
Cardiac output L min
O absorbed per minute by the lungs ml min
Arteriovenous O  difference ml L of blood
2
2
(
)
=
(
)
(
)
Indicator Dilution Method for
Measuring Cardiac Output
To measure cardiac output by the so-called “indicator
dilution method,” a small amount of indicator, such as
a dye, is 

,” a small amount of indicator, such as
a dye, is injected into a large systemic vein or, prefer-
ably, into the right atrium. This passes rapidly through
the right side of the heart, then through the blood
vessels of the lungs, through the left side of the heart
and, ﬁnally, into the systemic arterial system. The con-
centration of the dye is recorded as the dye passes
through one of the peripheral arteries, giving a curve as
shown in Figure 20–19. In each of these instances, 5 mil-
ligrams of 

19. In each of these instances, 5 mil-
ligrams of Cardio-Green dye was injected at zero time.
In the top recording, none of the dye passed into the
arterial tree until about 3 seconds after the injection, but
then the arterial concentration of the dye rose rapidly
to a maximum in about 6 to 7 seconds. After that, the
concentration fell rapidly, but before the concentration
reached zero, some of the dye had already circulated all
the way through some of the peripheral systemic vessels
and returne

ome of the peripheral systemic vessels
and returned through the heart for a second time. Con-
sequently, the dye concentration in the artery began to
rise again. For the purpose of calculation, it is necessary
to extrapolate the early down-slope of the curve to the
zero point, as shown by the dashed portion of each
curve. In this way, the extrapolated time-concentration
curve of the dye in the systemic artery without recircu-
lation can be measured in its ﬁrst portion and estimated
reasonably ac

ed in its ﬁrst portion and estimated
reasonably accurately in its latter portion.
Once the extrapolated time-concentration curve has
been determined, one then calculates the mean con-
centration of dye in the arterial blood for the duration
of the curve. For instance, in the top example of Figure
20–19, this was done by measuring the area under the
O2 = 
160 ml/L
right heart
O2 = 
200 ml/L
left heart
Oxygen used = 200 ml/min
LUNGS
Cardiac output =
5000 ml/min
Figure 20–18
Fick principle for dete

=
Fick principle for determining cardiac output.
Dye concentration in artery (mg/dl)
10
20
30
0
0.1
0.2
0.3
0.4
0.5
Seconds
0
10
20
30
0
0.1
0.2
0.3
0.4
0.5
5 mg
injected
5 mg
injected
0
Figure 20–19
Extrapolated dye concentration curves used to calculate two sep-
arate cardiac outputs by the dilution method. (The rectangular
areas are the calculated average concentrations of dye in the arte-
rial blood for the durations of the respective extrapolated curves.)
Chapter 20

of the respective extrapolated curves.)
Chapter 20
245
entire initial and extrapolated curve and then averaging
the concentration of dye for the duration of the curve;
one can see from the shaded rectangle straddling the
curve in the upper ﬁgure that the average concentration
of dye was 0.25 mg/dl of blood and that the duration 
of this average value was 12 seconds. A total of 5 mil-
ligrams of dye had been injected at the beginning of the
expe

dye had been injected at the beginning of the
experiment. For blood carrying only 0.25 milligram of
dye in each deciliter to carry the entire 5 milligrams of
dye through the heart and lungs in 12 seconds, a total 
of 20 1-deciliter portions of blood would have passed
through the heart during the 12 seconds, which would
be the same as a cardiac output of 2 L/12 sec, or 
10 L/min. We leave it to the reader to calculate the
cardiac output from the bottom extrapolated curve of
Figure 20–19. To summa

ottom extrapolated curve of
Figure 20–19. To summarize, the cardiac output can be
determined using the following formula:
Brengelmann GL: A critical analysis of the view that right
atrial pressure determines venous return. J Appl Physiol
94:849, 2003.
Gaasch WH, Zile MR: Left ventricular diastolic dysfunction
and diastolic heart failure. Annu Rev Med. 55:373, 2004.
Cardiac output ml min
Milligrams of dye injected  60
Average concentration of dye
in each milliliter of blood
for the dur

ion of dye
for the duration of the curve
the curve
in seconds
(
) =
¥
Ê
Ë
Á
ˆ
¯
˜ ¥
Ê
Ë
Á
ˆ
¯
˜
Guyton AC: Venous return. In: Hamilton WF (ed): Hand-
book of Physiology. Sec 2, Vol. 2. Baltimore, Williams &
Wilkins, 1963, p 1099.

overall
Guyton AC, Lindsey AW, Kaufmann BN: Effect of mean cir-
culatory ﬁlling pressure and other peripheral circulatory
factors on cardiac output.Am J Physiol 180:463-468, 1955.
Koch WJ, Lefkowitz RJ, Rockman HA: Functional conse-
quences of altering myocardial adrenergic receptor sig-
naling. Annu Rev Physiol 62:237, 2000.
Ro

tor sig-
Rockman HA, Koch WJ, Lefkowitz RJ: Seven-transmem-
brane-spanning receptors and heart function. Nature
415:206, 2002.
Rothe CF: Mean circulatory ﬁlling pressure: its meaning and
measurement. J Appl Physiol 74:499, 1993.
Rothe CF: Reﬂex control of veins and vascular capacitance.
Physiol Rev 63:1281, 1983.
Sarnoff SJ, Berglund E:Ventricular function. 1. Starling’s law
of the heart, studied by means of simultaneous right and
left ventricular function 

 simultaneous right and
left ventricular function curves in the dog. Circulation
9:706-718, 1953.
Uemura K, Sugimachi M, Kawada T, et al: A novel frame-
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C
H
A
P
T
E
R
 
2
1
246
Muscle Blood Flow and Cardiac
Output During Exercise; the
Coronary Circulation and Ischemic
In this chapter we cons

and Ischemic
In this chapter we consider (1) blood ﬂow to the
skeletal muscles and (2) coronary blood ﬂow 
to the heart. Regulation of each of these is 
achieved mainly by local control of vascular 
resistance in response to muscle tissue metabolic
needs.
In addition, related subjects are discussed, such as
(1) cardiac output control during exercise, (2) char-
acteristics of heart attacks, and (3) the pain of angina pectoris.
Blood Flow in Skeletal Muscle and Blood Flow
Regulation 

Flow in Skeletal Muscle and Blood Flow
Regulation During Exercise
Very strenuous exercise is one of the most stressful conditions that the normal
circulatory system faces.This is true because there is such a large mass of skele-
tal muscle in the body, all of it requiring large amounts of blood ﬂow. Also, the
cardiac output often must increase in the non-athlete to four to ﬁve times
normal, or in the well-trained athlete to six to seven times normal.
During

mal.
During rest, blood ﬂow through skeletal muscle averages 3 to 4 ml/min/100 g of
muscle.During extreme exercise in the well-conditioned athlete,this can increase
15- to 25-fold, rising to 50 to 80 ml/min/100 g of muscle.
Blood Flow During Muscle Contractions. Figure 21–1 shows a record of blood ﬂow
changes in a calf muscle of a human leg during strong rhythmical muscular exer-
cise. Note that the ﬂow increases and decreases with each muscle contraction.

eases and decreases with each muscle contraction.
At the end of the contractions, the blood ﬂow remains very high for a few
seconds but then fades toward normal during the next few minutes.
The cause of the lower ﬂow during the muscle contraction phase of exercise
is compression of the blood vessels by the contracted muscle. During strong
tetanic contraction, which causes sustained compression of the blood vessels, the
blood ﬂow can be almost stopped, but this also causes rapid weakening of the

pped, but this also causes rapid weakening of the
Increased Blood Flow in Muscle Capillaries During Exercise. During rest, some muscle
capillaries have little or no ﬂowing blood. But during strenuous exercise, all 
the capillaries open. This opening of dormant capillaries diminishes the dis-
tance that oxygen and other nutrients must diffuse from the capillaries to the
contracting muscle ﬁbers and sometimes contributes a twofold to threefold
increased capillary surface area through 

hreefold
increased capillary surface area through which oxygen and nutrients can diffuse
from the blood.
Chapter 21
Muscle Blood Flow and Cardiac Output During Exercise
247
Control of Blood Flow Through the
Skeletal Muscles
Local Regulation—Decreased Oxygen in Muscle Greatly
Enhances Flow. The tremendous increase in muscle
blood ﬂow that occurs during skeletal muscle activity
is caused primarily by chemical effects acting directly
on the muscle arterioles to cause dilation. One of the
most impor

rterioles to cause dilation. One of the
most important chemical effects is reduction of oxygen
in the muscle tissues. That is, during muscle activity,
the muscle uses oxygen rapidly, thereby decreasing the
oxygen concentration in the tissue ﬂuids. This in turn
causes local arteriolar vasodilation both because the
arteriolar walls cannot maintain contraction in the
absence of oxygen and because oxygen deﬁciency
causes release of vasodilator substances. The most
important vasodilator substance is 

nces. The most
important vasodilator substance is probably adeno-
sine, but experiments have shown that even large
amounts of adenosine infused directly into a muscle
artery cannot sustain vasodilation in skeletal muscle
for more than about 2 hours.
Fortunately, even after the muscle blood vessels
have become insensitive to the vasodilator effects of
adenosine, still other vasodilator factors continue to
maintain increased capillary blood ﬂow as long as the
exercise continues.These factors inclu

long as the
exercise continues.These factors include (1) potassium
ions, (2) adenosine triphosphate (ATP), (3) lactic acid,
and (4) carbon dioxide. We still do not know quanti-
tatively how great a role each of these plays in in-
creasing muscle blood ﬂow during muscle activity;
this subject was discussed in additional detail in
Chapter 17.
Nervous Control of Muscle Blood Flow. In addition to local
tissue vasodilator mechanisms, skeletal muscles are
provided with sympathetic vasoconstrictor nerv

are
provided with sympathetic vasoconstrictor nerves and
(in some species of animals) sympathetic vasodilator
nerves as well.
Sympathetic Vasoconstrictor Nerves. The sympa-
thetic vasoconstrictor nerve ﬁbers secrete norepineph-
rine at their nerve endings.When maximally activated,
this can decrease blood ﬂow through resting muscles
to as little as one half to one third normal. This vaso-
constriction is of physiologic importance in circulatory
shock and during other periods of stress when it is 

ock and during other periods of stress when it is 
necessary to maintain a normal or even high arterial
pressure.
In addition to the norepinephrine secreted at the
sympathetic vasoconstrictor nerve endings, the medul-
lae of the two adrenal glands also secrete large
amounts of norepinephrine plus even more epineph-
rine into the circulating blood during strenuous exer-
cise.The circulating norepinephrine acts on the muscle
vessels to cause a vasoconstrictor effect similar to that
caused by direc

constrictor effect similar to that
caused by direct sympathetic nerve stimulation. The
epinephrine, however, often has a slight vasodilator
effect because epinephrine excites more of the beta
adrenergic receptors of the vessels, which are vasodila-
tor receptors, in contrast to the alpha vasoconstrictor
receptors excited especially by norepinephrine. These
receptors are discussed in Chapter 60.
Total Body Circulatory Readjustments
Three major effects occur during exercise that ar

Three major effects occur during exercise that are
essential for the circulatory system to supply the
tremendous blood ﬂow required by the muscles. They
are (1) mass discharge of the sympathetic nervous
system throughout the body with consequent stimula-
tory effects on the entire circulation, (2) increase in
arterial pressure, and (3) increase in cardiac output.
Effects of Mass Sympathetic Discharge
At the onset of exercise, signals are transmitted not
only from the brain to the muscles to cau

tted not
only from the brain to the muscles to cause muscle
contraction but also into the vasomotor center to ini-
tiate mass sympathetic discharge throughout the body.
Simultaneously, the parasympathetic signals to the
heart are attenuated. Therefore, three major circula-
tory effects result.
First, the heart is stimulated to greatly increased
heart rate and increased pumping strength as a result
of the sympathetic drive to the heart plus release of
the heart from normal parasympathetic inhibit

e of
the heart from normal parasympathetic inhibition.
Second, most of the arterioles of the peripheral 
circulation are strongly contracted, except for the 
arterioles in the active muscles, which are strongly
vasodilated by the local vasodilator effects in the
muscles as noted above. Thus, the heart is stimulated
to supply the increased blood ﬂow required by the
muscles, while at the same time blood ﬂow through
most nonmuscular areas of the body is temporarily
reduced, thereby temporarily “len

y is temporarily
reduced, thereby temporarily “lending” their blood
supply to the muscles. This accounts for as much as 
2 L/min of extra blood ﬂow to the muscles, which is
Blood flow (100 ml/min)
0
10
16
18
0
20
40
Minutes
Rhythmic exercise
Calf
flow
Figure 21–1
Effects of muscle exercise on blood ﬂow in the calf of a leg during
strong rhythmical contraction. The blood ﬂow was much less
during contractions than between contractions. (Adapted from
Barcroft and Dornhorst: J Physiol 109:402, 1949.

m
Barcroft and Dornhorst: J Physiol 109:402, 1949.)
248
Unit IV
exceedingly important when one thinks of a person
running for his life—even a fractional increase in
running speed may make the difference between life
and death. Two of the peripheral circulatory systems,
the coronary and cerebral systems, are spared this vaso-
constrictor effect because both these circulatory areas
have poor vasoconstrictor innervation—fortunately so
because both the heart and the brain are as esse

o
because both the heart and the brain are as essential
to exercise as are the skeletal muscles.
Third, the muscle walls of the veins and other capac-
itative areas of the circulation are contracted power-
fully, which greatly increases the mean systemic ﬁlling
pressure. As we learned in Chapter 20, this is one of
the most important factors in promoting increase in
venous return of blood to the heart and, therefore, in
increasing the cardiac output.
Increase in Arterial Pressure During Exercise—

ut.
An Important Result of Increased 
Sympathetic Stimulation
One of the most important effects of increased sym-
pathetic stimulation in exercise is to increase the arte-
rial pressure. This results from multiple stimulatory
effects, including (1) vasoconstriction of the arterioles
and small arteries in most tissues of the body except
the active muscles, (2) increased pumping activity 
by the heart, and (3) a great increase in mean systemic
ﬁlling 

 and (3) a great increase in mean systemic
ﬁlling pressure caused mainly by venous contraction.
These effects, working together, virtually always
increase the arterial pressure during exercise. This
increase can be as little as 20 mm Hg or as great as 
80 mm Hg, depending on the conditions under which
the exercise is performed. When a person performs
exercise under tense conditions but uses only a few
muscles, the sympathetic nervous response still occurs
everywhere in the body. In the few activ

ll occurs
everywhere in the body. In the few active muscles,
vasodilation occurs, but everywhere else in the body
the effect is mainly vasoconstriction, often increasing
the mean arterial pressure to as high as 170 mm Hg.
Such a condition might occur in a person standing 
on a ladder and nailing with a hammer on the ceiling
above. The tenseness of the situation is obvious.
Conversely,
when a person performs massive 
whole-body exercise, such as running or swimming,
the increase in arterial press

unning or swimming,
the increase in arterial pressure is often only 20 to 
40 mm Hg. This lack of a large increase in pressure
results from the extreme vasodilation that occurs
simultaneously in large masses of active muscle.
Why Is the Arterial Pressure Increase During Exercise Impor-
tant? When muscles are stimulated maximally in a lab-
oratory experiment but without allowing the arterial
pressure to rise, muscle blood ﬂow seldom rises more
than about eightfold. Yet, we know from studies of
ma

n about eightfold. Yet, we know from studies of
marathon runners that muscle blood ﬂow can increase
from as little as 1 L/min for the whole body during rest
to at least 20 L/min during maximal activity.Therefore,
it is clear that muscle blood ﬂow can increase much
more than occurs in the aforementioned simple labo-
ratory experiment. What is the difference? Mainly, the
arterial pressure rises during normal exercise. Let us
assume, for instance, that the arterial pressure rises 
30 per cent, a co

hat the arterial pressure rises 
30 per cent, a common increase during heavy exercise.
This 30 per cent increase causes 30 per cent more force
to push blood through the muscle tissue vessels. But
this is not the only important effect; the extra pressure
also stretches the walls of the vessels so much that
muscle total ﬂow often rises to more than 20 times
normal.
Importance of the Increase in Cardiac Output
Many different physiologic effects occur at the same
time during exercise

gic effects occur at the same
time during exercise to increase cardiac output
approximately in proportion to the degree of exercise.
In fact, the ability of the circulatory system to provide
increased cardiac output for delivery of oxygen and
other nutrients to the muscles during exercise is
equally as important as the strength of the muscles
themselves in setting the limit for continued muscle
work. For instance, marathon runners who can
increase their cardiac outputs the most are generally
the

e their cardiac outputs the most are generally
the same persons who have record-breaking running
times.
Graphical Analysis of the Changes in Cardiac Output During
Heavy Exercise. Figure 21–2 shows a graphical analysis
of the large increase in cardiac output that occurs
during heavy exercise. The cardiac output and venous
return curves crossing at point A give the analysis for
the normal circulation; and the curves crossing at point
B analyze heavy exercise. Note that the great increase
in cardia

y exercise. Note that the great increase
in cardiac output requires signiﬁcant changes in both
the cardiac output curve and the venous return curve,
as follow.
The increased level of the cardiac output curve is
easy to understand. It results almost entirely from
sympathetic stimulation of the heart that causes (1)
0
Cardiac output and 
venous return (L/min)
–4
0
+4
+8
+12
+16
+20
+24
5
10
15
20
25
B
A
Figure 21–2
Graphical analysis of change in cardiac output and ri

phical analysis of change in cardiac output and right atrial
pressure with onset of strenuous exercise. Black curves, normal
circulation. Red curves, heavy exercise.
Chapter 21
249
increased heart rate, often up to rates as high as 170 to
190 beats/min, and (2) increased strength of contrac-
tion of the heart, often to as much as twice normal.
Without this increased level of the output curve, the
increase in cardiac output would be limited to 

he
increase in cardiac output would be limited to the
plateau level of the normal heart, which would be a
maximum increase of cardiac output of only about 
2.5-fold rather than the 4-fold that can commonly be
achieved by the untrained runner and the 7-fold that
can be achieved in some marathon runners.
Now study the venous return curves. If no change
occurred from the normal venous return curve, the
cardiac output could hardly rise at all in exercise
because the upper plateau level of the normal

cise
because the upper plateau level of the normal venous
return curve is only 6 L/min. Yet two important
changes do occur:
1. The mean systemic ﬁlling pressure rises
tremendously at the onset of heavy exercise. This
results partly from the sympathetic stimulation 
that contracts the veins and other capacitative parts
of the circulation. In addition, tensing of the
abdominal and other skeletal muscles of the body
compresses many of the internal vessels, thus
providing more compression of the ent

essels, thus
providing more compression of the entire
capacitative vascular system, causing a still greater
increase in mean systemic ﬁlling pressure.
During maximal exercise, these two effects
together can increase the mean systemic ﬁlling
pressure from a normal level of 7 mm Hg to as
high as 30 mm Hg.
2. The slope of the venous return curve rotates
upward. This is caused by decreased resistance in
virtually all the blood vessels in active muscle
tissue, which also causes resistance to venous
r

e
return to decrease, thus increasing the upward
slope of the venous return curve.
Therefore, the combination of increased mean sys-
temic ﬁlling pressure and decreased resistance to
venous return raises the entire level of the venous
return curve.
In response to the changes in both the venous return
curve and the cardiac output curve, the new equilib-
rium point in Figure 21–2 for cardiac output and 
right atrial pressure is now point B, in contras

right atrial pressure is now point B, in contrast to the
normal level at point A. Note especially that the right
atrial pressure has hardly changed, having risen only
1.5 mm Hg. In fact, in a person with a strong heart,
the right atrial pressure often falls below normal in
very heavy exercise because of the greatly increased
sympathetic stimulation of the heart during exercise.
About one third of all deaths in the afﬂuent society of
the Western world result from coronary a

ociety of
the Western world result from coronary artery disease,
and almost all elderly people have at least some
impairment of the coronary artery circulation. For this
reason, understanding normal and pathological physi-
ology of the coronary circulation is one of the most
important subjects in medicine.
Physiologic Anatomy of the Coronary
Blood Supply
Figure 21–3 shows the heart and its coronary blood
supply. Note that the main coronary arteries lie on the
surface of the heart and smaller art

es lie on the
surface of the heart and smaller arteries then pene-
trate from the surface into the cardiac muscle mass. It
is almost entirely through these arteries that the heart
receives its nutritive blood supply. Only the inner 1/10
millimeter of the endocardial surface can obtain sig-
niﬁcant nutrition directly from the blood inside the
cardiac chambers, so that this source of muscle nutri-
tion is minuscule.
The left coronary artery supplies mainly the anterior
and left lateral portions of

s mainly the anterior
and left lateral portions of the left ventricle, whereas
the right coronary artery supplies most of the right
ventricle as well as the posterior part of the left ven-
tricle in 80 to 90 per cent of people.
Most of the coronary venous blood ﬂow from the
left ventricular muscle returns to the right atrium of
the heart by way of the coronary sinus—which is about
75 per cent of the total coronary blood ﬂow.And most
of the coronary venous blood from the right ventricu-
lar muscl

ry venous blood from the right ventricu-
lar muscle returns through small anterior cardiac veins
that ﬂow directly into the right atrium, not by way of
the coronary sinus. A very small amount of coronary
venous blood also ﬂows back into the heart through
very minute thebesian veins, which empty directly into
all chambers of the heart.
The resting coronary blood ﬂow in the human being
averages about 225 ml/min, which is about 4 to 5 per
cent of the total cardiac output.

about 4 to 5 per
During strenuous exercise, the heart in the young
adult increases its cardiac output fourfold to seven-
fold, and it pumps this blood against a higher than
normal arterial pressure. Consequently, the work
Left coronary
artery
Right coronary
artery
Figure 21–3
The coronary arteries.
250
Unit IV
output of the heart under severe conditions may
increase sixfold to ninefold. At the same time, the
coronary blood ﬂow increa

d. At the same time, the
coronary blood ﬂow increases threefold to fourfold to
supply the extra nutrients needed by the heart. This
increase is not as much as the increase in workload,
which means that the ratio of energy expenditure by
the heart to coronary blood ﬂow increases. Thus, the
“efﬁciency” of cardiac utilization of energy increases
to make up for the relative deﬁciency of coronary
blood supply.
Phasic Changes in Coronary Blood Flow During Systole and
Diastole—Effect of Cardiac Muscle 

ing Systole and
Diastole—Effect of Cardiac Muscle Compression. Figure
21–4 shows the changes in blood ﬂow through the
nutrient capillaries of the left ventricular coronary
system in milliliters per minute in the human heart
during systole and diastole, as extrapolated from
experiments in lower animals. Note from this diagram
that the coronary capillary blood ﬂow in the left ven-
tricle muscle falls to a low value during systole, which
is opposite to ﬂow in vascular beds elsewhere in the
body. Th

 to ﬂow in vascular beds elsewhere in the
body. The reason for this is strong compression of the
left ventricular muscle around the intramuscular
vessels during systolic contraction.
During diastole, the cardiac muscle relaxes and no
longer obstructs blood ﬂow through the left ventricu-
lar muscle capillaries, so that blood ﬂows rapidly
during all of diastole.
Blood ﬂow through the coronary capillaries of the
right ventricle also undergoes phasic changes during
the cardiac cycle, but because the

 changes during
the cardiac cycle, but because the force of contraction
of the right ventricular muscle is far less than that of
the left ventricular muscle, the inverse phasic changes
are only partial in contrast to those in the left ventric-
ular muscle.
Epicardial Versus Subendocardial Coronary Blood Flow—Effect
of Intramyocardial Pressure. Figure 21–5 demonstrates
the special arrangement of the coronary vessels at dif-
ferent depths in the heart muscle, showing on the outer
surface epicardia

art muscle, showing on the outer
surface epicardial coronary arteries that supply most of
the muscle. Smaller, intramuscular arteries derived
from the epicardial arteries penetrate the muscle,
supplying the needed nutrients. Lying immediately
beneath the endocardium is a plexus of subendocar-
dial arteries. During systole, blood ﬂow through the
subendocardial plexus of the left ventricle, where the
intramuscular coronary vessels are compressed greatly
by ventricular muscle contraction, tends to 

eatly
by ventricular muscle contraction, tends to be reduced.
But the extra vessels of the subendocardial plexus nor-
mally compensate for this. Later in the chapter, we will
see that this peculiar difference between blood ﬂow in
the epicardial and subendocardial arteries plays an
important role in certain types of coronary ischemia.
Local Muscle Metabolism Is the Primary
Controller of Coronary Flow
Blood ﬂow through the coronary system is regulated
mostly by local

h the coronary system is regulated
mostly by local arteriolar vasodilation in response to
cardiac muscle need for nutrition. That is, whenever
the vigor of cardiac contraction is increased, regard-
less of cause, the rate of coronary blood ﬂow also
increases. Conversely, decreased heart activity is
accompanied by decreased coronary ﬂow. This local
regulation of coronary blood ﬂow is almost identical
to that occurring in many other tissues of the body,
especially in the skeletal muscles all over 

body,
especially in the skeletal muscles all over the body.
Oxygen Demand as a Major Factor in Local Coronary Blood Flow
Regulation. Blood ﬂow in the coronaries usually is reg-
ulated almost exactly in proportion to the need of the
cardiac musculature for oxygen. Normally, about 70
per cent of the oxygen in the coronary arterial blood
is removed as the blood ﬂows through the heart
muscle. Because not much oxygen is left, very little
additional oxygen can be supplied to the heart mus-
culature un

ygen can be supplied to the heart mus-
culature unless the coronary blood ﬂow increases. For-
tunately, the coronary blood ﬂow does increase almost
in direct proportion to any additional metabolic con-
sumption of oxygen by the heart.
However, the exact means by which increased
oxygen consumption causes coronary dilation has 
not been determined. It is speculated by many
research workers that a decrease in the oxygen 
0
100
200
300
Coronary blood flow (ml/min)
Systole
Diastole
Figure 21–4
Phasic

 flow (ml/min)
Systole
Diastole
Phasic ﬂow of blood through the coronary capillaries of the human
left ventricle during cardiac systole and diastole (as extrapolated
from measured ﬂows in dogs).
Cardiac
muscle
Epicardial coronary arteries
Subendocardial arterial plexus
Figure 21–5
Diagram of the epicardial, intramuscular, and subendocardial
coronary vasculature.
Chapter 21
251
concentration in the heart causes vasodilator sub-
stan

ntration in the heart causes vasodilator sub-
stances to be released from the muscle cells and that
these dilate the arterioles. A substance with great
vasodilator propensity is adenosine. In the presence of
very low concentrations of oxygen in the muscle cells,
a large proportion of the cell’s ATP degrades to adeno-
sine monophosphate; then small portions of this are
further degraded and release adenosine into the tissue
ﬂuids of the heart muscle, with resultant increase in
local coronary blood

e, with resultant increase in
local coronary blood ﬂow. After the adenosine causes
vasodilation, much of it is reabsorbed into the cardiac
cells to be reused.
Adenosine is not the only vasodilator product 
that has been identiﬁed. Others include adenosine
phosphate compounds, potassium ions, hydrogen 
ions,
carbon dioxide,
bradykinin,
and,
possibly,
prostaglandins and nitric oxide.
Yet,difﬁculties with the vasodilator hypothesis exist.
First, pharmacologic agents that block or partially
block th

rmacologic agents that block or partially
block the vasodilator effect of adenosine do not
prevent coronary vasodilation caused by increased
heart muscle activity. Second, studies in skeletal
muscle have shown that continued infusion of adeno-
sine maintains vascular dilation for only 1 to 3 hours,
and yet muscle activity still dilates the local blood
vessels even when the adenosine can no longer dilate
them. Therefore, the other vasodilator mechanisms
listed above must be remembered.
Nervous Co

anisms
Nervous Control of Coronary Blood Flow
Stimulation of the autonomic nerves to the heart can
affect coronary blood ﬂow both directly and indirectly.
The direct effects result from action of the nervous
transmitter substances acetylcholine from the vagus
nerves and norepinephrine and epinephrine from the
sympathetic nerves on the coronary vessels them-
selves. The indirect effects result from secondary
changes in coronary blood ﬂow caused by increased or
dec

s in coronary blood ﬂow caused by increased or
decreased activity of the heart.
The indirect effects, which are mostly opposite to
the direct effects, play a far more important role in
normal control of coronary blood ﬂow. Thus, sympa-
thetic stimulation, which releases norepinephrine and
epinephrine, increases both heart rate and heart con-
tractility as well as increases the rate of metabolism of
the heart. In turn, the increased metabolism of the
heart sets off local blood ﬂow regulatory mech

the
heart sets off local blood ﬂow regulatory mechanisms
for dilating the coronary vessels, and the blood ﬂow
increases approximately in proportion to the meta-
bolic needs of the heart muscle. In contrast, vagal stim-
ulation, with its release of acetylcholine, slows the
heart and has a slight depressive effect on heart con-
tractility.These effects in turn decrease cardiac oxygen
consumption and, therefore, indirectly constrict the
coronary arteries.
Direct Effects of Nervous Stimuli on the Co

eries.
Direct Effects of Nervous Stimuli on the Coronary Vasculature.
The distribution of parasympathetic (vagal) nerve
ﬁbers to the ventricular coronary system is not very
great. However, the acetylcholine released by parasym-
pathetic stimulation has a direct effect to dilate the coro-
nary arteries.
There is much more extensive sympathetic innerva-
tion of the coronary vessels. In Chapter 60, we see that
the sympathetic transmitter substances norepineph-
rine and epinephrine can have either v

norepineph-
rine and epinephrine can have either vascular con-
strictor or vascular dilator effects, depending on the
presence or absence of constrictor or dilator receptors
in the blood vessel walls.The constrictor receptors are
called alpha receptors and the dilator receptors are
called beta receptors. Both alpha and beta receptors
exist in the coronary vessels. In general, the epicardial
coronary vessels have a preponderance of alpha recep-
tors, whereas the intramuscular arteries may have a

rs, whereas the intramuscular arteries may have a
preponderance of beta receptors. Therefore, sympa-
thetic stimulation can, at least theoretically, cause
slight overall coronary constriction or dilation, but
usually constriction. In some people, the alpha vaso-
constrictor effects seem to be disproportionately
severe, and these people can have vasospastic myocar-
dial ischemia during periods of excess sympathetic
drive, often with resultant anginal pain.
Metabolic factors—especially myocardial 

nal pain.
Metabolic factors—especially myocardial oxygen
consumption—are the major controllers of myocardial
blood ﬂow. Whenever the direct effects of nervous
stimulation alter the coronary blood ﬂow in the wrong
direction, the metabolic control of coronary ﬂow
usually overrides the direct coronary nervous effects
within seconds.
Special Features of Cardiac 
Muscle Metabolism
The basic principles of cellular metabolism, discussed
in Chapters 67 through 72, apply to cardiac muscle the
same as for

hrough 72, apply to cardiac muscle the
same as for other tissues, but there are some quantita-
tive differences. Most important, under resting condi-
tions, cardiac muscle normally consumes fatty acids to
supply most of its energy instead of carbohydrates
(about 70 per cent of the energy is derived from fatty
acids). However, as is also true of other tissues, under
anaerobic or ischemic conditions, cardiac metabolism
must call on anaerobic glycolysis mechanisms for
energy. Unfortunately, glycoly

ysis mechanisms for
energy. Unfortunately, glycolysis consumes tremen-
dous quantities of the blood glucose and at the same
time forms large amounts of lactic acid in the cardiac
tissue, which is probably one of the causes of cardiac
pain in cardiac ischemic conditions, as discussed later
As is true in other tissues, more than 95 per cent of
the metabolic energy liberated from foods is used to
form ATP in the mitochondria. This ATP in turn acts
as the conveyer of energy for card

TP in turn acts
as the conveyer of energy for cardiac muscular con-
traction and other cellular functions. In severe coro-
nary ischemia, the ATP degrades ﬁrst to adenosine
diphosphate, then to adenosine monophosphate and
adenosine. Because the cardiac muscle cell membrane
is slightly permeable to adenosine, much of this 
can diffuse from the muscle cells into the circulating
blood.
The released adenosine is believed to be one of the
substances that causes dilation of the coronary arteri-
oles d

hat causes dilation of the coronary arteri-
oles during coronary hypoxia, as discussed earlier.
252
Unit IV
However, loss of adenosine also has a serious cellular
consequence. Within as little as 30 minutes of severe
coronary ischemia,as occurs after a myocardial infarct,
about one half of the adenine base can be lost from
the affected cardiac muscle cells. Furthermore, this loss
can be replaced by new synthesis of adenine at a rate
of only 2 per cent per hour. Therefore, once a 

te
of only 2 per cent per hour. Therefore, once a serious
bout of coronary ischemia has persisted for 30 or more
minutes, relief of the ischemia may be too late to save
the lives of the cardiac cells. This almost certainly is
one of the major causes of cardiac cellular death
during myocardial ischemia.
The most common cause of death in Western culture
is ischemic heart disease, which results from insufﬁ-
cient coronary blood ﬂow.About 35 per cent of people
in the United St

d ﬂow.About 35 per cent of people
in the United States die of this cause. Some deaths
occur suddenly as a result of acute coronary occlusion
or ﬁbrillation of the heart, whereas other deaths occur
slowly over a period of weeks to years as a result of
progressive weakening of the heart pumping process.
In this chapter, we discuss acute coronary ischemia
caused by acute coronary occlusion and myocardial
infarction. In Chapter 22, we discuss congestive heart
failure, the most frequent cause of whic

ive heart
failure, the most frequent cause of which is slowly
increasing coronary ischemia and weakening of the
cardiac muscle.
Atherosclerosis as a Cause of Ischemic Heart Disease. The
most frequent cause of diminished coronary blood
ﬂow is atherosclerosis. The atherosclerotic process is
discussed in connection with lipid metabolism in
Chapter 68. Brieﬂy, this process is the following.
In people who have genetic predisposition to ath-
erosclerosis, or in people who eat excessive quantities 
of 

is, or in people who eat excessive quantities 
of cholesterol and have a sedentary lifestyle, large
quantities of cholesterol gradually become deposited
beneath the endothelium at many points in arteries
throughout the body. Gradually, these areas of deposit
are invaded by ﬁbrous tissue and frequently become
calciﬁed. The net result is the development of athero-
sclerotic plaques that actually protrude into the vessel
lumens and either block or partially block blood ﬂow.
A common site for develo

artially block blood ﬂow.
A common site for development of atherosclerotic
plaques is the ﬁrst few centimeters of the major coro-
Acute Coronary Occlusion
Acute occlusion of a coronary artery most frequently
occurs in a person who already has underlying ath-
erosclerotic coronary heart disease but almost never
in a person with a normal coronary circulation. Acute
occlusion can result from any one of several effects,
two of which are the following:
1. The atherosclerotic plaque can

e the following:
1. The atherosclerotic plaque can cause a local blood
clot called a thrombus, which in turn occludes 
the artery. The thrombus usually occurs where the
arteriosclerotic plaque has broken through the
endothelium, thus coming in direct contact with
the ﬂowing blood. Because the plaque presents an
unsmooth surface, blood platelets adhere to it,
ﬁbrin is deposited, and red blood cells become
entrapped to form a blood clot that grows until 
it occludes the vessel. Or, occasionally, t

until 
it occludes the vessel. Or, occasionally, the 
clot breaks away from its attachment on the
atherosclerotic plaque and ﬂows to a more
peripheral branch of the coronary arterial tree,
where it blocks the artery at that point. A
thrombus that ﬂows along the artery in this way
and occludes the vessel more distally is called a
coronary embolus.
2. Many clinicians believe that local muscular spasm
of a coronary artery also can occur. The spasm
might result from direct irritation of the smooth
m

ight result from direct irritation of the smooth
muscle of the arterial wall by the edges of an
arteriosclerotic plaque, or it might result from
local nervous reﬂexes that cause excess coronary
vascular wall contraction. The spasm may then
lead to secondary thrombosis of the vessel.
Lifesaving Value of Collateral Circulation in the Heart. The
degree of damage to the heart muscle caused either by
slowly developing atherosclerotic constriction of the
coronary arteries or by sudden coronary occlusi

he
coronary arteries or by sudden coronary occlusion is
determined to a great extent by the degree of collat-
eral circulation that has already developed or that can
open within minutes after the occlusion.
In a normal heart, almost no large communications
exist among the larger coronary arteries. But many
anastomoses do exist among the smaller arteries 
sized 20 to 250 micrometers in diameter, as shown in
Figure 21–6.
When a sudden occlusion occurs in one of the larger
coronary arteries, the sm

urs in one of the larger
coronary arteries, the small anastomoses begin to
Artery
Vein
Artery
Vein
Figure 21–6
Minute anastomoses in the normal coronary arterial system.
Chapter 21
253
dilate within seconds. But the blood ﬂow through these
minute collaterals is usually less than one half that
needed to keep alive most of the cardiac muscle that
they now supply; the diameters of the collateral vessels
do not enlarge much more for the next 8 to 

essels
do not enlarge much more for the next 8 to 24 hours.
But then collateral ﬂow does begin to increase, dou-
bling by the second or third day and often reaching
normal or almost normal coronary ﬂow within about
1 month. Because of these developing collateral chan-
nels, many patients recover almost completely from
various degrees of coronary occlusion when the area
of muscle involved is not too great.
When atherosclerosis constricts the coronary arter-
ies slowly over a period of many years 

ary arter-
ies slowly over a period of many years rather than 
suddenly, collateral vessels can develop at the same
time while the atherosclerosis becomes more and
more severe. Therefore, the person may never experi-
ence an acute episode of cardiac dysfunction. But,
eventually, the sclerotic process develops beyond the
limits of even the collateral blood supply to provide
the needed blood ﬂow, and sometimes the collateral
blood vessels themselves develop atherosclerosis.
When this occurs, the h

s develop atherosclerosis.
When this occurs, the heart muscle becomes severely
limited in its work output, often so much so that the
heart cannot pump even normally required amounts
of blood ﬂow. This is one of the most common causes
of the cardiac failure that occurs in vast numbers of
older people.
Immediately after an acute coronary occlusion, blood
ﬂow ceases in the coronary vessels beyond the occlu-
sion except for small amounts of collateral ﬂow from
surrounding vesse

l amounts of collateral ﬂow from
surrounding vessels.The area of muscle that has either
zero ﬂow or so little ﬂow that it cannot sustain cardiac
muscle function is said to be infarcted. The overall
process is called a myocardial infarction.
Soon after the onset of the infarction, small amounts
of collateral blood begin to seep into the infarcted area,
and this, combined with progressive dilation of local
blood vessels, causes the area to become overﬁlled with
stagnant blood. Simultaneously the m

verﬁlled with
stagnant blood. Simultaneously the muscle ﬁbers use
the last vestiges of the oxygen in the blood, causing 
the hemoglobin to become totally de-oxygenated.
Therefore, the infarcted area takes on a bluish-brown
hue, and the blood vessels of the area appear to be
engorged despite lack of blood ﬂow. In later stages, the
vessel walls become highly permeable and leak ﬂuid;
the local muscle tissue becomes edematous, and the
cardiac muscle cells begin to swell because of dimin-
ished cellu

cells begin to swell because of dimin-
ished cellular metabolism.Within a few hours of almost
no blood supply, the cardiac muscle cells die.
Cardiac muscle requires about 1.3 milliliters of
oxygen per 100 grams of muscle tissue per minute just
to remain alive.This is in comparison with about 8 mil-
liliters of oxygen per 100 grams delivered to the
normal resting left ventricle each minute. Therefore, if
there is even 15 to 30 per cent of normal resting 
coronary blood ﬂow, the muscle will not di

esting 
coronary blood ﬂow, the muscle will not die. In the
central portion of a large infarct, however, where 
there is almost no collateral blood ﬂow, the muscle
does die.
Subendocardial Infarction. The subendocardial muscle
frequently becomes infarcted even when there is no
evidence of infarction in the outer surface portions of
the heart. The reason for this is that the subendocar-
dial muscle has extra difﬁculty obtaining adequate
blood ﬂow because the blood vessels in the subendo-
cardium 

because the blood vessels in the subendo-
cardium are intensely compressed by systolic contrac-
tion of the heart, as explained earlier. Therefore, any
condition that compromises blood ﬂow to any area of
the heart usually causes damage ﬁrst in the subendo-
cardial regions, and the damage then spreads outward
toward the epicardium.
Causes of Death After Acute 
Coronary Occlusion
The most common causes of death after acute
myocardial infarction are (1) decreased cardiac output;
(2) damming of bloo

 (1) decreased cardiac output;
(2) damming of blood in the pulmonary blood vessels
and then death resulting from pulmonary edema; (3)
ﬁbrillation of the heart; and, occasionally, (4) rupture of
the heart.
Decreased Cardiac Output—Systolic Stretch and Cardiac
Shock. When some of the cardiac muscle ﬁbers are not
functioning and others are too weak to contract with
great force, the overall pumping ability of the affected
ventricle is proportionately depressed. Indeed, the
overall pumping strength o

 depressed. Indeed, the
overall pumping strength of the infarcted heart is often
decreased more than one might expect because of a
phenomenon called systolic stretch, shown in Figure
21–7. That is, when the normal portions of the ventric-
ular muscle contract, the ischemic portion of the
muscle, whether this be dead or simply nonfunctional,
instead of contracting is forced outward by the pres-
sure that develops inside the ventricle. Therefore,
Normal contraction
Non-functional
muscle
Systolic s

ormal contraction
muscle
Systolic stretch
Figure 21–7
Systolic stretch in an area of ischemic cardiac muscle.
254
Unit IV
much of the pumping force of the ventricle is dissi-
pated by bulging of the area of nonfunctional cardiac
muscle.
When the heart becomes incapable of contracting
with sufﬁcient force to pump enough blood into the
peripheral arterial tree, cardiac failure and death of
peripheral tissues ensue as a result of peripheral
ischemia. This condition is

a result of peripheral
ischemia. This condition is called coronary shock, car-
diogenic shock, cardiac shock, or low cardiac output
failure. It is discussed more fully in the next chapter.
Cardiac shock almost always occurs when more than
40 per cent of the left ventricle is infarcted.And death
occurs in about 85 per cent of patients once they
develop cardiac shock.
Damming of Blood in the Body’s Venous System. When the
heart is not pumping blood forward, it must be
damming blood in the atria an

 forward, it must be
damming blood in the atria and in the blood vessels 
of the lungs or in the systemic circulation. This leads
to increased capillary pressures, particularly in the
lungs.
This damming of blood in the veins often causes
little difﬁculty during the ﬁrst few hours after myocar-
dial infarction. Instead, symptoms develop a few days
later for the following reason: The acutely diminished
cardiac output leads to diminished blood ﬂow to the
kidneys. Then, for reasons that are discuss

to the
kidneys. Then, for reasons that are discussed in
Chapter 22. the kidneys fail to excrete enough urine.
This adds progressively to the total blood volume and,
therefore,leads to congestive symptoms.Consequently,
many patients who seemingly are getting along well
during the ﬁrst few days after onset of heart failure will
suddenly develop acute pulmonary edema and often
will die within a few hours after appearance of the
initial pulmonary symptoms.
Fibrillation of the Ventricles After Myocar

ptoms.
Fibrillation of the Ventricles After Myocardial Infarction.
Many people who die of coronary occlusion die
because of sudden ventricular ﬁbrillation. The ten-
dency to develop ﬁbrillation is especially great after a
large infarction, but ﬁbrillation can sometimes occur
after small occlusions as well. Indeed, some patients
with chronic coronary insufﬁciency die suddenly from
ﬁbrillation without any acute infarction.
There are two especially dangerous periods after
coronary infarction during

dangerous periods after
coronary infarction during which ﬁbrillation is most
likely to occur. The ﬁrst is during the ﬁrst 10 minutes
after the infarction occurs.Then there is a short period
of relative safety, followed by a secondary period of
cardiac irritability beginning 1 hour or so later and
lasting for another few hours. Fibrillation can also
occur many days after the infarct but less likely so.
At least four factors enter into the tendency for the
heart to ﬁbrillate:
1. Acute loss of bloo

 for the
1. Acute loss of blood supply to the cardiac muscle
causes rapid depletion of potassium from the
ischemic musculature. This also increases the
potassium concentration in the extracellular 
ﬂuids surrounding the cardiac muscle ﬁbers.
Experiments in which potassium has been injected
into the coronary system have demonstrated that
an elevated extracellular potassium concentration
increases the irritability of the cardiac
musculature and, therefore, its likelihood of
ﬁbr

ﬁbrillating.
2. Ischemia of the muscle causes an “injury current,”
which is described in Chapter 12 in relation to
electrocardiograms in patients with acute
myocardial infarction. That is, the ischemic
musculature often cannot completely repolarize
its membranes after a heart beat, so that the
external surface of this muscle remains negative
with respect to normal cardiac muscle membrane
potential elsewhere in the heart. Therefore,
electric current 

sewhere in the heart. Therefore,
electric current ﬂows from this ischemic area of
the heart to the normal area and can elicit
abnormal impulses that can cause ﬁbrillation.
3. Powerful sympathetic reﬂexes often develop after
massive infarction, principally because the heart
does not pump an adequate volume of blood into
the arterial tree. The sympathetic stimulation also
increases irritability of the cardiac muscle and
thereby predisposes to ﬁbrillation.
4. Cardiac muscle weakness caused by the
m

lation.
myocardial infarction often causes the ventricle to
dilate excessively. This increases the pathway length
for impulse conduction in the heart and frequently
causes abnormal conduction pathways all the way
around the infarcted area of the cardiac muscle.
Both of these effects predispose to development
of circus movements because, as discussed in
Chapter 13, excess prolongation of conduction
pathways in the ventricles allows impulses to re-
enter mu

 in the ventricles allows impulses to re-
enter muscle that is already recovering from
refractoriness, thereby initiating a “circus
movement” cycle of new excitation and causing
the process to continue on and on.
Rupture of the Infarcted Area. During the ﬁrst day or so
after an acute infarct, there is little danger of rupture
of the ischemic portion of the heart, but a few days
later, the dead muscle ﬁbers begin to degenerate, and
the heart wall becomes stretched very thin. When this
happens, th

becomes stretched very thin. When this
happens, the dead muscle bulges outward severely
with each heart contraction, and this systolic stretch
becomes greater and greater until ﬁnally the heart rup-
tures. In fact, one of the means used in assessing
progress of severe myocardial infarction is to record
by cardiac imaging (i.e., x-rays) whether the degree of
systolic stretch is worsening.
When a ventricle does rupture, loss of blood into the
pericardial space causes rapid development of cardiac
t

ardial space causes rapid development of cardiac
tamponade—that is, compression of the heart from the
outside by blood collecting in the pericardial cavity.
Because of this compression of the heart, blood cannot
ﬂow into the right atrium, and the patient dies of sud-
denly decreased cardiac output.
Stages of Recovery from Acute
The upper left part of Figure 21–8 shows the effects of
acute coronary occlusion in a patient with a small area
Chapter 21
Muscle Blood Flow and Car

 a small area
Chapter 21
255
of muscle ischemia; to the right is shown a heart with
a large area of ischemia. When the area of ischemia is
small, little or no death of the muscle cells may occur,
but part of the muscle often does become temporarily
nonfunctional because of inadequate nutrition to
support muscle contraction.
When the area of ischemia is large, some of the
muscle ﬁbers in the center of the area die rapidly,
within 1 to 3 hours w

ter of the area die rapidly,
within 1 to 3 hours where there is total cessation of
coronary blood supply. Immediately around the dead
area is a nonfunctional area, with failure of contrac-
tion and usually failure of impulse conduction. Then,
extending circumferentially around the nonfunctional
area is an area that is still contracting but weakly so
because of mild ischemia.
Replacement of Dead Muscle by Scar Tissue. In the lower
part of Figure 21–8, the various stages of recovery after
a large 

1–8, the various stages of recovery after
a large myocardial infarction are shown. Shortly after
the occlusion, the muscle ﬁbers in the center of the
ischemic area die. Then, during the ensuing days, this
area of dead ﬁbers becomes bigger because many of
the marginal ﬁbers ﬁnally succumb to the prolonged
ischemia.At the same time, because of enlargement of
collateral arterial channels supplying the outer rim of
the infarcted area, much of the nonfunctional muscle
recovers. After a few days to th

functional muscle
recovers. After a few days to three weeks, most of 
the nonfunctional muscle becomes functional again or
dies—one or the other. In the meantime, ﬁbrous tissue
begins developing among the dead ﬁbers because
ischemia can stimulate growth of ﬁbroblasts and
promote development of greater than normal quanti-
ties of ﬁbrous tissue.Therefore, the dead muscle tissue
is gradually replaced by ﬁbrous tissue. Then, because
it is a general property of ﬁbrous tissue to undergo
progressive co

roperty of ﬁbrous tissue to undergo
progressive contraction and dissolution, the ﬁbrous
scar may grow smaller over a period of several months
to a year.
Finally, the normal areas of the heart gradually
hypertrophy to compensate at least partially for the
lost dead cardiac musculature. By these means, the
heart recovers either partially or almost completely
Value of Rest in Treating Myocardial Infarction. The degree
of cardiac cellular death is determined by the degree
of isc

 cellular death is determined by the degree
of ischemia and the workload on the heart muscle.
When the workload is greatly increased, such as during
exercise, in severe emotional strain, or as a result of
fatigue, the heart needs increased oxygen and other
nutrients for sustaining its life. Furthermore, anasto-
motic blood vessels that supply blood to ischemic
areas of the heart must also still supply the areas of
the heart that they normally supply. When the heart
becomes excessively active, th

ply. When the heart
becomes excessively active, the vessels of the normal
musculature become greatly dilated. This allows most
of the blood ﬂowing into the coronary vessels to ﬂow
through the normal muscle tissue, thus leaving little
blood to ﬂow through the small anastomotic channels
into the ischemic area, so that the ischemic condition
worsens. This condition is called the “coronary steal”
syndrome. Consequently, one of the most important
factors in the treatment of a patient with myocardial

ors in the treatment of a patient with myocardial
infarction is observance of absolute body rest during
the recovery process.
Function of the Heart After Recovery
Occasionally, a heart that has recovered from a large
myocardial infarction returns almost to full functional
capability, but more frequently its pumping capability
is permanently decreased below that of a healthy
heart. This does not mean that the person is necessar-
ily a cardiac invalid or that the resting

ecessar-
ily a cardiac invalid or that the resting cardiac output
is depressed below normal, because the normal heart
is capable of pumping 300 to 400 per cent more blood
per minute than the body requires during rest—that is,
a normal person has a “cardiac reserve” of 300 to 400
per cent. Even when the cardiac reserve is reduced to
as little as 100 per cent, the person can still perform
normal activity of a quiet, restful type but not strenu-
ous exercise that would overload the heart.
Pain in C

 exercise that would overload the heart.
Normally, a person cannot “feel” his or her heart, but
ischemic cardiac muscle often does cause pain sensa-
tion—sometimes severe pain. Exactly what causes this
pain is not known, but it is believed that ischemia
causes the muscle to release acidic substances, such as
lactic acid, or other pain-promoting products, such as
histamine, kinins, or cellular proteolytic enzymes, that
are not removed rapidly enough by the slowly mo

at
are not removed rapidly enough by the slowly moving
coronary blood ﬂow. The high concentrations of these
abnormal products then stimulate pain nerve endings
in the cardiac muscle, sending pain impulses through
sensory afferent nerve ﬁbers into the central nervous
system.
Angina Pectoris
In most people who develop progressive constriction
of their coronary arteries, cardiac pain, called angina
Mild
ischemia
Non-
functional
Mild
ischemia
Non-
Dead fibers
Nonfunctional
Fib

nctional
Fibrous tissue
Figure 21–8
Top, Small and large areas of coronary ischemia. Bottom, Stages
of recovery from myocardial infarction.
256
Unit IV
pectoris, begins to appear whenever the load on the
heart becomes too great in relation to the available
coronary blood ﬂow. This pain is usually felt beneath
the upper sternum over the heart, and in addition it is
often referred to distant surface areas of the body,
most commonly to the left 

face areas of the body,
most commonly to the left arm and left shoulder but
also frequently to the neck and even to the side of the
face.The reason for this distribution of pain is that the
heart originates during embryonic life in the neck, as
do the arms. Therefore, both the heart and these
surface areas of the body receive pain nerve ﬁbers
from the same spinal cord segments.
Most people who have chronic angina pectoris feel
pain when they exercise or when they experience 
emotions that increa

cise or when they experience 
emotions that increase metabolism of the heart or
temporarily constrict the coronary vessels because of
sympathetic vasoconstrictor nerve signals. The pain
usually lasts for only a few minutes. However, some
patients have such severe and lasting ischemia that the
pain is present all the time. The pain is frequently
described as hot, pressing, and constricting; it is of such
quality that it usually makes the patient stop all unnec-
essary body activity and come to a 

top all unnec-
essary body activity and come to a complete state of
rest.
Treatment with Drugs. Several vasodilator drugs, when
administered during an acute anginal attack, can 
often give immediate relief from the pain. Commonly 
used vasodilators are nitroglycerin and other nitrate
drugs.
A second class of drugs that are used for prolonged
treatment of angina pectoris is the beta blockers,
such as propranolol. These drugs block sympathetic
beta adrenergic receptors, which prevents sympathetic

 adrenergic receptors, which prevents sympathetic
enhancement of heart rate and cardiac metabolism
during exercise or emotional episodes. Therefore,
therapy with a beta blocker decreases the need 
of the heart for extra metabolic oxygen during 
stressful conditions. For obvious reasons, this can also
reduce the number of anginal attacks as well as their
severity.
Surgical Treatment of 
Coronary Disease
Aortic-Coronary Bypass Surgery. In many patients with
coronary ischemia, the constricted areas

ents with
coronary ischemia, the constricted areas of the coro-
nary arteries are located at only a few discrete points
blocked by atherosclerotic disease, and the coronary
vessels elsewhere are normal or almost normal.A sur-
gical procedure was developed in the 1960s, called
aortic-coronary bypass, for removing a section of a
subcutaneous vein from an arm or leg and then graft-
ing this vein from the root of the aorta to the side of
a peripheral coronary artery beyond the atheroscle-
rotic bloc

 coronary artery beyond the atheroscle-
rotic blockage point. One to ﬁve such grafts are usually
performed, each of which supplies a peripheral coro-
nary artery beyond a block.
Anginal pain is relieved in most patients. Also, in
patients whose hearts have not become too severely
damaged before the operation, the coronary bypass
procedure may provide the patient with normal sur-
vival expectation. Conversely, if the heart has already
been severely damaged, the bypass procedure is likely
to be of

y damaged, the bypass procedure is likely
to be of little value.
Coronary Angioplasty. Since the 1980s, a procedure has
been used to open partially blocked coronary vessels
before they become totally occluded. This procedure,
called coronary artery angioplasty, is the following:
A small balloon-tipped catheter, about 1 millimeter in
diameter, is passed under radiographic guidance into
the coronary system and pushed through the partially
occluded artery until the balloon portion of the
catheter s

artery until the balloon portion of the
catheter straddles the partially occluded point. Then
the balloon is inﬂated with high pressure, which
markedly stretches the diseased artery. After this pro-
cedure is performed, the blood ﬂow through the vessel
often increases threefold to fourfold, and more than
three quarters of the patients who undergo the proce-
dure are relieved of the coronary ischemic symptoms
for at least several years, although many of the patients
still eventually require coron

any of the patients
still eventually require coronary bypass surgery.
Still newer procedures for opening atherosclerotic
coronary arteries are constantly in experimental
development. One of these employs a laser beam from
the tip of a coronary artery catheter aimed at the 
atherosclerotic lesion. The laser literally dissolves the
lesion without substantially damaging the rest of 
the arterial wall.
Another development has been a minute metal
“sleeve” placed inside a coronary artery dilated by
an

eve” placed inside a coronary artery dilated by
angioplasty to hold the artery open, thus preventing its
restenosis.
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rotic plaque: a multifocal disease. Circulation 107:2072,
2003.
Dalal H, Evans PH, Campbell JL: Recent developments in
secondary prevention and cardiac rehabilitation after
acute myocardial infarction. BMJ 328:693, 2004.
Freedman SB

cardial infarction. BMJ 328:693, 2004.
Freedman SB, Isner JM: Therapeutic angiogenesis for coro-
nary artery disease. Ann Intern Med 136:54, 2002.
Gehlbach BK, Geppert E: The pulmonary manifestations of
left heart failure. Chest 125:669, 2004.
Guyton AC, Jones CE, Coleman TG: Circulatory Pathology:
Hao H, Gabbiani J, Bochaton-Piallat M: Arterial smooth
muscle cell heterogeneity: implications for atherosclerosis
and restenosis

y: implications for atherosclerosis
and restenosis development. Arterioscler Thromb Vasc
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2002.
Hochman JS: Cardiogenic shock complicating acute myocar-
dial infarction: expanding the paradigm. Circulation 107:
2998, 2003.
Joyner MJ, Dietz NM: Sympathetic vasodilation in human
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Chapter 21
Muscle Blood Flow and Cardiac Output Du

Chapter 21
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Coronary collaterals: an important and underexposed
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2003.
Libby P: Inﬂammation in atherosclerosis. Nature 420:868,
2002.
Richardson RS: Oxygen transport and utilization: an inte-
gration of the muscle systems. Adv Physiol Educ 27:183,
2003.
Ridker PM: Clinical application of C-reactive protein for
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tion 107:363, 2003.
Tsai AG, Johnson PC, Intaglietta M: Oxygen gradients in the
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83:1113, 2003.
C
H
A
P
T
E
R
 
2
2
258
One of the most important ailments that must be
treated by the physician is cardiac failure (“heart
failure”). This 

sician is cardiac failure (“heart
failure”). This can result from any heart condition
that reduces the ability of the heart to pump blood.
The cause usually is decreased contractility of the
myocardium resulting from diminished coronary
blood ﬂow. However, failure can also be caused by
damaged heart valves, external pressure around the
heart, vitamin B deﬁciency, primary cardiac muscle disease, or any other abnor-
mality that makes the heart a hypoeffective pump. In this chapter, we discuss
main

poeffective pump. In this chapter, we discuss
mainly cardiac failure caused by ischemic heart disease resulting from partial
blockage of the coronary blood vessels. In Chapter 23, we discuss valvular and
congenital heart disease.
Deﬁnition of Cardiac Failure. The term “cardiac failure” means simply failure of
the heart to pump enough blood to satisfy the needs of the body.
Dynamics of the Circulation in Cardiac Failure
If a heart suddenly becomes severel

ardiac Failure
If a heart suddenly becomes severely damaged, such as by myocardial infarc-
tion, the pumping ability of the heart is immediately depressed.As a result, two
main effects occur: (1) reduced cardiac output and (2) damming of blood in the
veins, resulting in increased venous pressure.
The progressive changes in heart pumping effectiveness at different times
after an acute myocardial infarction are shown graphically in Figure 22–1. The
top curve of this ﬁgure shows a normal cardiac ou

top curve of this ﬁgure shows a normal cardiac output curve. Point A on this
curve is the normal operating point, showing a normal cardiac output under
resting conditions of 5 L/min and a right atrial pressure of 0 mm Hg.
Immediately after the heart becomes damaged, the cardiac output curve
becomes greatly lowered, falling to the lowest curve at the bottom of the graph.
Within a few seconds, a new circulatory state is established at point B rather
than point A, illustrating that the cardiac out

er
than point A, illustrating that the cardiac output has fallen to 2 L/min, about
two-ﬁfths normal, whereas the right atrial pressure has risen to +4 mm Hg
because venous blood returning to the heart from the body is dammed up in
the right atrium. This low cardiac output is still sufﬁcient to sustain life for
perhaps a few hours, but it is likely to be associated with fainting. Fortunately,
this acute stage usually lasts for only a few seconds because sympathetic nerve
reﬂexes occur immediately

ecause sympathetic nerve
reﬂexes occur immediately and compensate, to a great extent, for the damaged
heart, as follows.
Compensation for Acute Cardiac Failure by Sympathetic Nervous Reﬂexes. When the
cardiac output falls precariously low, many of the circulatory reﬂexes discussed
in Chapter 18 are immediately activated.The best known of these is the barore-
ceptor reﬂex, which is activated by diminished arterial pressure. It is probable
that the chemoreceptor reﬂex, the central nervous system i

 chemoreceptor reﬂex, the central nervous system ischemic response,
and even reﬂexes that originate in the damaged heart also contribute to activat-
ing the sympathetic nervous system. But whatever the reﬂexes might be, the
sympathetics become strongly stimulated within a few seconds, and the
Chapter 22
259
parasympathetic nervous signals to the heart become
reciprocally inhibited at the same time.
Strong sympathetic stimulation has two major
effects on the circulation: ﬁrst on t

as two major
effects on the circulation: ﬁrst on the heart itself,
and second on the peripheral vasculature. If all the
ventricular musculature is diffusely damaged but is
still functional, sympathetic stimulation strengthens
this damaged musculature. If part of the muscle is non-
functional and part of it is still normal, the normal
muscle is strongly stimulated by sympathetic stimula-
tion, in this way partially compensating for the non-
functional muscle. Thus, the heart, one way or another,

onal muscle. Thus, the heart, one way or another,
becomes a stronger pump. This effect is also demon-
strated in Figure 22–1, showing after sympathetic com-
pensation about twofold elevation of the very low
cardiac output curve.
Sympathetic stimulation also increases venous
return because it increases the tone of most of the
blood vessels of the circulation, especially the veins,
raising the mean systemic ﬁlling pressure to 12 to 14
mm Hg, almost 100 per cent above normal. As dis-
cussed in Chap

 100 per cent above normal. As dis-
cussed in Chapter 20, this increased ﬁlling pressure
greatly increases the tendency for blood to ﬂow from
the veins back into the heart. Therefore, the damaged
heart becomes primed with more inﬂowing blood than
usual, and the right atrial pressure rises still further,
which helps the heart to pump still larger quantities of
blood. Thus, in Figure 22–1, the new circulatory state
is depicted by point C, showing a cardiac output of 
4.2 L/min and a right atrial p

 cardiac output of 
4.2 L/min and a right atrial pressure of 5 mm Hg.
The sympathetic reﬂexes become maximally devel-
oped in about 30 seconds.Therefore, a person who has
a sudden, moderate heart attack might experience
nothing more than cardiac pain and a few seconds of
fainting. Shortly thereafter, with the aid of the sympa-
thetic reﬂex compensations, the cardiac output may
return to a level adequate to sustain the person if he
or she remains quiet, although the pain might persist.
Chronic St

quiet, although the pain might persist.
Chronic Stage of Failure—Fluid
Retention Helps to Compensate
After the ﬁrst few minutes of an acute heart attack, a
prolonged semi-chronic state begins, characterized
mainly by two events: (1) retention of ﬂuid by the
kidneys and (2) varying degrees of recovery of the
heart itself over a period of weeks to months, as illus-
trated by the light green curve in Figure 22–1; this was
also discussed in Chapter 21.
Renal Retention of Fluid and Inc

ed in Chapter 21.
Renal Retention of Fluid and Increase in Blood
Volume Occur for Hours to Days
A low cardiac output has a profound effect on renal
function, sometimes causing anuria when the cardiac
output falls to one-half to two-thirds normal. In
general, the urine output remains reduced below
normal as long as the cardiac output and arterial pres-
sure remain signiﬁcantly less than normal, and urine
output usually does not return all the way to normal
after an acute heart attack until the ca

to normal
after an acute heart attack until the cardiac output and
arterial pressure rise either all the way back to normal
or almost to normal.
Moderate Fluid Retention in Cardiac Failure Can Be Beneﬁcial.
Many cardiologists formerly considered ﬂuid retention
always to have a detrimental effect in cardiac failure.
But it is now known that a moderate increase in body
ﬂuid and blood volume is an important factor in
helping to compensate for the diminished pumping
ability of the heart by increasin

minished pumping
ability of the heart by increasing the venous return.
The increased blood volume increases venous return
in two ways: First, it increases the mean systemic ﬁlling
pressure, which increases the pressure gradient for
causing venous ﬂow of blood toward the heart. Second,
it distends the veins, which reduces the venous resist-
ance and allows even more ease of ﬂow of blood to the
heart.
If the heart is not too greatly damaged, this
increased venous return can often fully compensate

ncreased venous return can often fully compensate
for the heart’s diminished pumping ability—enough in
fact that even when the heart’s pumping ability is
reduced to as low as 40 to 50 per cent of normal, the
increased venous return can often cause an entirely
normal cardiac output as long as the person remains
in a quiet resting state.
When the heart’s pumping capability is reduced 
still more, blood ﬂow to the kidneys ﬁnally becomes
too low for the kidneys to excrete enough salt and
water to eq

the kidneys to excrete enough salt and
water to equal salt and water intake. Therefore, ﬂuid
0
5
10
15
–4
–2
0
+2
+4
+6
+8
+10 +12 +14
Normal heart
Partially recovered heart
Damaged heart + sympathetic stimulation
Acutely damaged heart
A
B
C
D
Figure 22–1
Progressive changes in the cardiac output curve after acute
myocardial infarction. Both the cardiac output and right atrial pres-
sure change progressively from point A to point D (illustrate

 progressively from point A to point D (illustrated by
the black line) over a period of seconds, minutes, days, and
weeks.
260
Unit IV
retention begins and continues indeﬁnitely, unless
major therapeutic procedures are used to prevent this.
Furthermore, because the heart is already pumping at
its maximum pumping capacity, this excess ﬂuid no
longer has a beneﬁcial effect on the circulation.Instead,
severe edema develops throughout the body, which
can be very detrimental in itself

 the body, which
can be very detrimental in itself and can lead to death.
Detrimental Effects of Excess Fluid Retention in Severe Cardiac
Failure. In contrast to the beneﬁcial effects of moder-
ate ﬂuid retention in cardiac failure, in severe failure
extreme excesses of ﬂuid can have serious physiologi-
cal consequences. They include (1) overstretching of
the heart, thus weakening the heart still more; (2) 
ﬁltration of ﬂuid into the lungs, causing pulmonary
edema and consequent deoxygenation of

ng pulmonary
edema and consequent deoxygenation of the blood;
and (3) development of extensive edema in most parts
of the body.These detrimental effects of excessive ﬂuid
are discussed in later sections of this chapter.
Recovery of the Myocardium After 
After a heart becomes suddenly damaged as a result
of myocardial infarction,
the natural reparative
processes of the body begin immediately to help
restore normal cardiac function. For instance, a new
collateral blood supply

ction. For instance, a new
collateral blood supply begins to penetrate the periph-
eral portions of the infarcted area of the heart, often
causing much of the heart muscle in the fringe areas
to become functional again. Also, the undamaged
portion of the heart musculature hypertrophies, in this
way offsetting much of the cardiac damage.
The degree of recovery depends on the type of
cardiac damage, and it varies from no recovery to
almost complete recovery. After acute myocardial
infarction, the 

 recovery. After acute myocardial
infarction, the heart ordinarily recovers rapidly during
the ﬁrst few days and weeks and achieves most of its
ﬁnal state of recovery within 5 to 7 weeks, although
mild degrees of additional recovery can continue for
months.
The Cardiac Output Curve After Partial Recovery. Figure 22–1
shows function of the partially recovered heart a week
or so after acute myocardial infarction. By this time,
considerable ﬂuid has been retained in the body and
the tendency for ve

 been retained in the body and
the tendency for venous return has increased
markedly as well; therefore, the right atrial pressure
has risen even more. As a result, the state of the cir-
culation is now changed from point C to point D,which
shows a normal cardiac output of 5 L/min but right
atrial pressure increased to 6 mm Hg.
Because the cardiac output has returned to normal,
renal output of ﬂuid also returns to normal, and no
further ﬂuid retention occurs, except that the retention
of ﬂuid th

ntion occurs, except that the retention
of ﬂuid that has already occurred continues to maintain
moderate excesses of ﬂuid. Therefore, except for the
high right atrial pressure represented by point D in
this ﬁgure, the person now has essentially normal car-
diovascular dynamics as long as he or she remains at
rest.
If the heart recovers to a signiﬁcant extent and if
adequate ﬂuid volume has been retained, the sympa-
thetic stimulation gradually abates toward normal for
the following reasons: The 

ates toward normal for
the following reasons: The partial recovery of the 
heart can elevate the cardiac output curve the same as
sympathetic stimulation can. Therefore, as the heart
recovers even slightly, the fast pulse rate, cold skin,
and pallor resulting from sympathetic stimulation in
the acute stage of cardiac failure gradually disappear.
Summary of the Changes That Occur
After Acute Cardiac Failure—
“Compensated Heart Failure”
To summarize the events discussed in the past few sec-
tions 

e the events discussed in the past few sec-
tions describing the dynamics of circulatory changes
after an acute, moderate heart attack, we can divide
the stages into (1) the instantaneous effect of the
cardiac damage; (2) compensation by the sympathetic
nervous system, which occurs mainly within the ﬁrst 30
seconds to 1 minute; and (3) chronic compensations
resulting from partial heart recovery and renal reten-
tion of ﬂuid. All these changes are shown graphically
by the black curve in Figure 22

 shown graphically
by the black curve in Figure 22–1. The progression of
this curve shows the normal state of the circulation
(point A), the state a few seconds after the heart attack
but before sympathetic reﬂexes have occurred (point
B), the rise in cardiac output toward normal caused 
by sympathetic stimulation (point C), and ﬁnal return
of the cardiac output almost exactly to normal after
several days to several weeks of partial cardiac recov-
ery and ﬂuid retention (point D). This ﬁnal stat

-
ery and ﬂuid retention (point D). This ﬁnal state is
called compensated heart failure.
Compensated Heart Failure. Note especially in Figure
22–1 that the maximum pumping ability of the partly
recovered heart, as depicted by the plateau level of the
light green curve, is still depressed to less than one-half
normal. This demonstrates that an increase in right
atrial pressure can maintain the cardiac output at a
normal level despite continued weakness of the heart.
Thus many people, especially o

kness of the heart.
Thus many people, especially older people, have
normal resting cardiac outputs but mildly to moder-
ately elevated right atrial pressures because of various
degrees of “compensated heart failure.”These persons
may not know that they have cardiac damage because
the damage often has occurred a little at a time, and
the compensation has occurred concurrently with the
progressive stages of damage.
When a person is in compensated heart failure, any
attempt to perform heavy exercis

eart failure, any
attempt to perform heavy exercise usually causes
immediate return of the symptoms of acute failure
because the heart is not able to increase its pumping
capacity to the levels required for the exercise. There-
fore, it is said that the cardiac reserve is reduced in
compensated heart failure. This concept of cardiac
reserve is discussed more fully later in the chapter.
If the heart becomes severely damaged, no amoun

re
If the heart becomes severely damaged, no amount of
compensation, either by sympathetic nervous reﬂexes
Chapter 22
261
or by ﬂuid retention, can make the excessively weak-
ened heart pump a normal cardiac output. As a con-
sequence, the cardiac output cannot rise high enough
to make the kidneys excrete normal quantities of ﬂuid.
Therefore, ﬂuid continues to be retained, the person
develops more and more edema, and this state of
events eventually leads to death. This is called 

events eventually leads to death. This is called decom-
pensated heart failure. Thus, the main cause of decom-
pensated heart failure is failure of the heart to pump
sufﬁcient blood to make the kidneys excrete daily the
necessary amounts of ﬂuid.
Graphical Analysis of Decompensated Heart Failure. Figure
22–2 shows greatly depressed cardiac output at differ-
ent times (points A to F) after the heart has become
severely weakened. Point A on this curve represents
the approximate state of the circu

urve represents
the approximate state of the circulation before any
compensation has occurred, and point B, the state a
few minutes later after sympathetic stimulation has
compensated as much as it can but before ﬂuid reten-
tion has begun. At this time, the cardiac output has
risen to 4 L/min and the right atrial pressure has risen
to 5 mm Hg. The person appears to be in reasonably
good condition, but this state will not remain stable
because the cardiac output has not risen high enough
to caus

e cardiac output has not risen high enough
to cause adequate kidney excretion of ﬂuid; therefore,
ﬂuid retention continues and can eventually be the
cause of death. These events can be explained quanti-
tatively in the following way.
Note the straight line in Figure 22–2, at a cardiac
output level of 5 L/min. This is approximately the crit-
ical cardiac output level that is required in the normal
adult person to make the kidneys re-establish normal
ﬂuid balance—that is, for the output of salt an

al
ﬂuid balance—that is, for the output of salt and water
to be as great as the intake of these. At any cardiac
output below this level, all the ﬂuid-retaining mecha-
nisms discussed in the earlier section remain in play
and the body ﬂuid volume increases progressively.
And because of this progressive increase in ﬂuid
volume, the mean systemic ﬁlling pressure of the 
circulation continues to rise; this forces progressively
increasing quantities of blood from the person’s
peripheral veins into th

f blood from the person’s
peripheral veins into the right atrium, thus increasing
the right atrial pressure. After 1 day or so, the state of
the circulation changes in Figure 22–2 from point B to
point C—the right atrial pressure rising to 7 mm Hg
and the cardiac output to 4.2 L/min. Note again that
the cardiac output is still not high enough to cause
normal renal output of ﬂuid; therefore, ﬂuid continues
to be retained. After another day or so, the right atrial
pressure rises to 9 mm Hg, and th

the right atrial
pressure rises to 9 mm Hg, and the circulatory state
becomes that depicted by point D. Still, the cardiac
output is not enough to establish normal ﬂuid 
balance.
After another few days of ﬂuid retention, the right
atrial pressure has risen still further, but by now,
cardiac function is beginning to decline toward a lower
level.This decline is caused by overstretch of the heart,
edema of the heart muscle, and other factors that
diminish the heart’s pumping performance. It is 
now

minish the heart’s pumping performance. It is 
now clear that further retention of ﬂuid will be more
detrimental than beneﬁcial to the circulation. Yet the
cardiac output still is not high enough to bring about
normal renal function, so that ﬂuid retention not 
only continues but accelerates because of the falling
cardiac output (and falling arterial pressure that also
occurs). Consequently, within a few days, the state 
of the circulation has reached point F on the curve,
with the cardiac outpu

ached point F on the curve,
with the cardiac output now less than 2.5 L/min and
the right atrial pressure 16 mm Hg. This state has
approached or reached incompatibility with life, and
the patient dies.This state of heart failure in which the
failure continues to worsen is called decompensated
heart failure.
Thus, one can see from this analysis that failure of
the cardiac output (and arterial pressure) to rise to the
critical level required for normal renal function results
in (1) progressive ret

rmal renal function results
in (1) progressive retention of more and more ﬂuid,
which causes (2) progressive elevation of the mean
systemic ﬁlling pressure, and (3) progressive elevation
of the right atrial pressure until ﬁnally the heart is so
overstretched or so edematous that it cannot pump
even moderate quantities of blood and, therefore, fails
completely. Clinically, one detects this serious condi-
tion of decompensation principally by the progressing
edema, especially edema of the lungs, w

rogressing
edema, especially edema of the lungs, which leads 
to bubbling rales in the lungs and to dyspnea (air
hunger). All clinicians know that lack of appropriate
therapy when this state of events occurs leads to rapid
death.
Treatment 
of 
Decompensation. The 
decompensation
process can often be stopped by (1) strengthening the
heart in any one of several ways, especially by admin-
istration of a cardiotonic drug, such as digitalis, so that
the heart becomes strong enough to pump adequate
q

quantities of blood required to make the kidneys func-
tion normally again, or (2) administering diuretic drugs
to increase kidney excretion while at the same time
reducing water and salt intake, which brings about a
balance between ﬂuid intake and output despite low
0
2.5
5.0
Cardiac output 
(L/min)
–4
0
+4
+8
+12
+16
A
F
B
C
D
E
Critical cardiac output level
for normal fluid balance
Figure 22–2
Great

t level
Greatly depressed cardiac output that indicates decompensated
heart disease. Progressive ﬂuid retention raises the right atrial
pressure over a period of days, and the cardiac output progresses
from point A to point F, until death occurs.
262
Unit IV
Both methods stop the decompensation process by
re-establishing normal ﬂuid balance, so that at least as
much ﬂuid leaves the body as enters it.
Mechanism of Action of the Cardiotonic Drug

rs it.
Mechanism of Action of the Cardiotonic Drugs Such as
Digitalis. Cardiotonic drugs, such as digitalis, when
administered to a person with a healthy heart, have
little effect on increasing the contractile strength of 
the cardiac muscle. However, when administered to a
person with a chronically failing heart, the same drugs
can sometimes increase the strength of the failing
myocardium as much as 50 to 100 per cent. Therefore,
they are one of the mainstays of therapy in chronic

 the mainstays of therapy in chronic
Digitalis and other cardiotonic glycosides are
believed to strengthen heart contraction by increasing
the quantity of calcium ions in muscle ﬁbers. In the
failing heart muscle, the sarcoplasmic reticulum fails 
to accumulate normal quantities of calcium and,
therefore, cannot release enough calcium ions into 
the free-ﬂuid compartment of the muscle ﬁbers to
cause full contraction of the muscle. One effect of dig-
italis is to depress the calciu

One effect of dig-
italis is to depress the calcium pump of the cell mem-
brane of the cardiac muscle ﬁbers.This pump normally
pumps calcium ions out of the muscle. However, in the
case of a failing heart, extra calcium is needed to
increase the muscle contractile force. Therefore, it is
usually beneﬁcial to depress the calcium pumping
mechanism a moderate amount using digitalis, allow-
ing the muscle ﬁber intracellular calcium level to rise
slightly.
In the discuss

htly.
In the discussions thus far in this chapter, we have con-
sidered failure of the heart as a whole. Yet, in a large
number of patients, especially those with early acute
failure, left-sided failure predominates over right-
sided failure, and in rare instances, the right side fails
without signiﬁcant failure of the left side. Therefore,
we need especially to discuss the special features of
unilateral heart failure.
When the left side of the heart fails without c

e.
When the left side of the heart fails without con-
comitant failure of the right side, blood continues to
be pumped into the lungs with usual right heart vigor,
whereas it is not pumped adequately out of the lungs
by the left heart into the systemic circulation. As 
a result, the mean pulmonary ﬁlling pressure rises
because of shift of large volumes of blood from the sys-
temic circulation into the pulmonary circulation.
As the volume of blood in the lungs increases, the
pulmonary capillary p

 in the lungs increases, the
pulmonary capillary pressure increases, and if this rises
above a value approximately equal to the colloid
osmotic pressure of the plasma, about 28 mm Hg, ﬂuid
begins to ﬁlter out of the capillaries into the lung inter-
stitial spaces and alveoli, resulting in pulmonary
edema.
Thus, among the most important problems of left
heart failure are pulmonary vascular congestion and
pulmonary edema. In severe, acute left heart failure,
pulmonary edema occasionally occurs so 

t failure,
pulmonary edema occasionally occurs so rapidly that
it can cause death by suffocation in 20 to 30 minutes,
which we discuss more fully later in the chapter.
In many instances after acute heart attacks and 
often after prolonged periods of slow progressive
cardiac deterioration, the heart becomes incapable of
pumping even the minimal amount of blood ﬂow
required to keep the body alive. Consequently, all the
body tissues begin to suffer and 

quently, all the
body tissues begin to suffer and even to deteriorate,
often leading to death within a few hours to a few days.
The picture then is one of circulatory shock, as
explained in Chapter 24. Even the cardiovascular
system suffers from lack of nutrition, and it, too (along
with the remainder of the body),deteriorates,thus has-
tening death. This circulatory shock syndrome caused
by inadequate cardiac pumping is called cardiogenic
shock or simply cardiac shock. Once a person devel-
ops 

or simply cardiac shock. Once a person devel-
ops cardiogenic shock, the survival rate is often less
than 15 per cent.
Vicious Circle of Cardiac Deterioration in Cardiogenic Shock.
The discussion of circulatory shock in Chapter 24
emphasizes the tendency for the heart to become pro-
gressively more damaged when its coronary blood
supply is reduced during the course of the shock. That
is, the low arterial pressure that occurs during shock
reduces the coronary blood supply even more. This
makes th

the coronary blood supply even more. This
makes the heart still weaker, which makes the arterial
pressure fall still more, which makes the shock still
worse, the process eventually becoming a vicious circle
of cardiac deterioration. In cardiogenic shock caused
by myocardial infarction, this problem is greatly com-
pounded by already existing coronary vessel blockage.
For instance, in a healthy heart, the arterial pres-
sure usually must be reduced below about 45 mm Hg
before cardiac deterioratio

d below about 45 mm Hg
before cardiac deterioration sets in. However, in a
heart that already has a blocked major coronary
vessel, deterioration sets in when the coronary arterial
pressure falls below 80 to 90 mm Hg. In other words,
even a small decrease in arterial pressure can now 
set off a vicious circle of cardiac deterioration. For 
this reason, in treating myocardial infarction, it is
extremely important to prevent even short periods of
hypotension.
Physiology of Treatment. Often a patien

potension.
Physiology of Treatment. Often a patient dies of cardio-
genic shock before the various compensatory
processes can return the cardiac output (and arterial
pressure) to a life-sustaining level. Therefore, treat-
ment of this condition is one of the most important
problems in the management of acute heart attacks.
Immediate administration of digitalis is often used
for strengthening the heart if the ventricular muscle
shows signs of deterioration. Also, infusion of whole
blood, plasma, 

rioration. Also, infusion of whole
blood, plasma, or a blood pressure–raising drug is used
to sustain the arterial pressure. If the arterial pressure
can be elevated high enough, the coronary blood ﬂow
Chapter 22
263
often will increase enough to prevent the vicious circle
of deterioration. And this allows enough time for
appropriate compensatory mechanisms in circulatory
system to correct the shock.
Some success has also been achieved in saving the
lives of patients in cardiogen

ieved in saving the
lives of patients in cardiogenic shock by using one of
the following procedures: (1) surgically removing the
clot in the coronary artery, often in combination with
coronary bypass graft, or (2) catheterizing the blocked
coronary artery and infusing either streptokinase or
tissue-type plasminogen activator enzymes that cause
dissolution of the clot. The results occasionally are
astounding when one of these procedures is instituted
within the ﬁrst hour of cardiogenic shock but 

ted
within the ﬁrst hour of cardiogenic shock but of little,
if any, beneﬁt after 3 hours.
Edema in Patients with
Inability of Acute Cardiac Failure to Cause Peripheral Edema.
Acute left heart failure can cause terriﬁc and rapid
congestion of the lungs, with development of pul-
monary edema and even death within minutes to
hours.
However, either left or right heart failure is very
slow to cause peripheral edema. This can best be
explained by referring to Figure 22–3. When a previ

xplained by referring to Figure 22–3. When a previ-
ously healthy heart acutely fails as a pump, the aortic
pressure falls and the right atrial pressure rises.As the
cardiac output approaches zero, these two pressures
approach each other at an equilibrium value of about
13 mm Hg.Capillary pressure also falls from its normal
value of 17 mm Hg to the new equilibrium pressure 
of 13 mm Hg. Thus, severe acute cardiac failure often
causes a fall in peripheral capillary pressure rather than
a rise. Th

ripheral capillary pressure rather than
a rise. Therefore, animal experiments, as well as expe-
rience in humans, show that acute cardiac failure
almost never causes immediate development of peri-
pheral edema.
Long-Term Fluid Retention by the Kidneys—
The Cause of Peripheral Edema in Persisting
Heart Failure
After the ﬁrst day or so of overall heart failure or of
right-ventricular heart failure, peripheral edema does
begin to occur principally because of ﬂuid retention by
the kidneys. The reten

ecause of ﬂuid retention by
the kidneys. The retention of ﬂuid increases the mean
systemic ﬁlling pressure, resulting in increased ten-
dency for blood to return to the heart. This elevates
the right atrial pressure to a still higher value and
returns the arterial pressure back toward normal.
Therefore, the capillary pressure now also rises
markedly, thus causing loss of ﬂuid into the tissues and
development of severe edema.
There are three known causes of the reduced renal
output of urine durin

 causes of the reduced renal
output of urine during cardiac failure, all of which are
equally important but in different ways.
1. Decreased glomerular ﬁltration. A decrease in
cardiac output has a tendency to reduce the
glomerular pressure in the kidneys because 
of (1) reduced arterial pressure and (2) intense
sympathetic constriction of the afferent arterioles 
of the kidney. As a consequence, except in the
mildest degrees of heart failure, the glomerular
ﬁltration rate becomes less than norma

 glomerular
ﬁltration rate becomes less than normal. It is 
clear from the discussion of kidney function in
Chapters 26 through 29 that even a slight decrease
in glomerular ﬁltration often markedly decreases
urine output. When the cardiac output falls to
about one-half normal, this can result in almost
complete anuria.
2. Activation of the renin-angiotensin system and
increased reabsorption of water and salt by the
renal tubules. The reduced blood ﬂow to the
kidneys causes marked increase in ren

d ﬂow to the
kidneys causes marked increase in renin secretion
by the kidneys, and this in turn causes the
formation of angiotensin, as described in Chapter
19. The angiotensin in turn has a direct effect on
the arterioles of the kidneys to decrease further
the blood ﬂow through the kidneys, which
especially reduces the pressure in the capillaries
surrounding the renal tubules, promoting greatly
increased reabsorption of both water and salt
from the tubules. Therefore, loss of water and salt
int

the tubules. Therefore, loss of water and salt
into the urine decreases greatly, and large
quantities of salt and water accumulate in the
blood and interstitial ﬂuids everywhere in the
body.
3. Increased aldosterone secretion. In the chronic
stage of heart failure, large quantities of
aldosterone are secreted by the adrenal cortex.
This results mainly from the effect of angiotensin
to stimulate aldosterone secretion by the adrenal
cortex. But some of the increase in aldosterone
secretion often r

e of the increase in aldosterone
secretion often results from increased plasma
0
20
40
60
80
100
Normal
1/2 Normal
Zero
Mean aortic pressure
Right atrial pressure
13 mm Hg
Figure 22–3
Progressive changes in mean aortic pressure, peripheral tissue
capillary pressure, and right atrial pressure as the cardiac output
falls from normal to zero.
264
Unit IV
potassium. Excess potassium is one of the most
powerful stimuli known for aldos

s one of the most
powerful stimuli known for aldosterone secretion,
and the potassium concentration rises in response
to reduced renal function in cardiac failure.
The elevated aldosterone level further increases
the reabsorption of sodium from the renal
tubules. This in turn leads to a secondary increase
in water reabsorption for two reasons: First, as the
sodium is reabsorbed, it reduces the osmotic
pressure in the tubules but increases the osmotic
pressure in the renal interstitial ﬂuids; the

otic
pressure in the renal interstitial ﬂuids; these
changes promote osmosis of water into the blood.
Second, the absorbed sodium and anions that go
with the sodium, mainly chloride ions, increase 
the osmotic concentration of the extracellular 
ﬂuid everywhere in the body. This elicits
antidiuretic hormone secretion by the
hypothalamic–posterior pituitary gland system
(discussed in Chapter 29). The antidiuretic
hormone in turn promotes still greater increase in
tubular reabsorption of water.
Ro

ater increase in
Role of Atrial Natriuretic Factor to Delay Onset of Cardiac
Decompensation. Atrial natriuretic factor (ANF) is a
hormone released by the atrial walls of the heart when
they become stretched. Because heart failure almost
always causes excessive increase in both the right and
left atrial pressures that stretch the atrial walls, the cir-
culating levels of ANF in the blood increase ﬁvefold
to tenfold in severe heart failure. The ANF in turn has
a dire

n severe heart failure. The ANF in turn has
a direct effect on the kidneys to increase greatly their
excretion of salt and water. Therefore, ANF plays a
natural role to help prevent extreme congestive symp-
toms during cardiac failure. The renal effects of ANF
are discussed in Chapter 29.
Acute Pulmonary Edema in Late-Stage Heart
Failure—Another Lethal Vicious Circle
A frequent cause of death in heart failure is acute pul-
monary edema occurring in patients who have already
had chronic heart fai

in patients who have already
had chronic heart failure for a long time. When this
occurs in a person without new cardiac damage, it
usually is set off by some temporary overload of the
heart, such as might result from a bout of heavy exer-
cise, some emotional experience, or even a severe cold.
The acute pulmonary edema is believed to result from
the following vicious circle:
1. A temporarily increased load on the already weak
left ventricle initiates the vicious circle. Because of
limited pumpi

iates the vicious circle. Because of
limited pumping capacity of the left heart, blood
begins to dam up in the lungs.
2. The increased blood in the lungs elevates the
pulmonary capillary pressure, and a small amount
of ﬂuid begins to transude into the lung tissues
and alveoli.
3. The increased ﬂuid in the lungs diminishes the
degree of oxygenation of the blood.
4. The decreased oxygen in the blood further
weakens the heart and also weakens the arterioles
everywhere in the body, thus causing peri

terioles
everywhere in the body, thus causing peripheral
5. The peripheral vasodilation increases venous
return of blood from the peripheral circulation
still more.
6. The increased venous return further increases the
damming of the blood in the lungs, leading to still
more transudation of ﬂuid, more arterial oxygen
desaturation, more venous return, and so forth.
Thus, a vicious circle has been established.
Once this vicious circle has proceeded beyond a
certain critical point, it 

has proceeded beyond a
certain critical point, it will continue until death of the
patient unless heroic therapeutic measures are used
within minutes. The types of heroic therapeutic meas-
ures that can reverse the process and save the patient’s
life include the following:
1. Putting tourniquets on both arms and legs to
sequester much of the blood in the veins and,
therefore, decrease the workload on the left side
of the heart
2. Bleeding the patient
3. Giving a rapidly acting diuretic, such as

ient
furosemide, to cause rapid loss of ﬂuid from the
body
4. Giving the patient pure oxygen to breathe to
reverse the blood oxygen desaturation, the heart
deterioration, and the peripheral vasodilation
5. Giving the patient a rapidly acting cardiotonic
drug, such as digitalis, to strengthen the heart
This vicious circle of acute pulmonary edema can
proceed so rapidly that death can occur in 20 minutes
to 1 hour. Therefore, any procedure that is to be

to 1 hour. Therefore, any procedure that is to be suc-
cessful must be instituted immediately.
The maximum percentage that the cardiac output can
increase above normal is called the cardiac reserve.
Thus, in the healthy young adult, the cardiac reserve is
300 to 400 per cent. In athletically trained persons, it
is occasionally 500 to 600 per cent. But in heart failure,
there is no cardiac reserve. As an example of normal
reserve, during severe exercise the cardiac output of a
he

 during severe exercise the cardiac output of a
healthy young adult can rise to about ﬁve times
normal; this is an increase above normal of 400 per
cent—that is, a cardiac reserve of 400 per cent.
Any factor that prevents the heart from pumping
blood satisfactorily will decrease the cardiac reserve.
This can result from ischemic heart disease, primary
myocardial disease, vitamin deﬁciency that affects
cardiac muscle, physical damage to the myocardium,
valvular heart disease, and many other facto

dium,
valvular heart disease, and many other factors, some of
which are shown in Figure 22–4.
Diagnosis of Low Cardiac Reserve—Exercise Test. As long as
persons with low cardiac reserve remain in a state of
rest, they usually will not know that they have heart
disease. However, a diagnosis of low cardiac reserve
usually can be easily made by requiring the person to
exercise either on a treadmill or by walking up and
down steps, either of which requires greatly increased
cardiac output.The increa

quires greatly increased
cardiac output.The increased load on the heart rapidly
uses up the small amount of reserve that is available,
Chapter 22
265
and the cardiac output soon fails to rise high enough
to sustain the body’s new level of activity. The acute
effects are as follows:
1. Immediate and sometimes extreme shortness of
breath (dyspnea) resulting from failure of the
heart to pump sufﬁcient blood to the tissues,
thereby causing tissue ischemia and creating a
sensation of 

using tissue ischemia and creating a
sensation of air hunger
2. Extreme muscle fatigue resulting from muscle
ischemia, thus limiting the person’s ability to
continue with the exercise
3. Excessive increase in heart rate because the
nervous reﬂexes to the heart overreact in an
attempt to overcome the inadequate cardiac
output
Exercise tests are part of the armamentarium of the
cardiologist. These tests take the place of cardiac
output measurements that cannot be made with ease
in most clinical se

 that cannot be made with ease
in most clinical settings.
Quantitative Graphical Method for
Analysis of Cardiac Failure
Although it is possible to understand most general
principles of cardiac failure using mainly qualitative
logic, as we have done thus far in this chapter, one can
grasp the importance of the different factors in cardiac
failure with far greater depth by using more quantita-
tive approaches. One such approach is the graphical
method for analysis of cardiac output regulation intr

hod for analysis of cardiac output regulation intro-
duced in Chapter 20. In the remaining sections of this
chapter, we analyze several aspects of cardiac failure,
using this graphical technique.
Graphical Analysis of Acute Heart Failure and
Chronic Compensation
Figure 22–5 shows cardiac output and venous return
curves for different states of the heart and peripheral
circulation. The two curves passing through Point A
are (1) the normal cardiac output curve and (2) the
normal venous return curve

utput curve and (2) the
normal venous return curve.As pointed out in Chapter
20, there is only one point on each of these two curves
at which the circulatory system can operate.This point
is where the two curves cross at point A.Therefore, the
normal state of the circulation is a cardiac output and
venous return of 5 L/min and a right atrial pressure of
0 mm Hg.
Effect of Acute Heart Attack. During the ﬁrst few seconds
after a moderately severe heart attack, the cardiac
output curve falls to the

eart attack, the cardiac
output curve falls to the lowermost curve. During these
few seconds, the venous return curve still has not
changed because the peripheral circulatory system is
still operating normally.Therefore, the new state of the
circulation is depicted by point B, where the new
cardiac output curve crosses the normal venous return
curve. Thus, the right atrial pressure rises immedi-
ately to 4 mm Hg, whereas the cardiac output falls to
2 L/min.
Effect of Sympathetic Reﬂexes. Within 

to
2 L/min.
Effect of Sympathetic Reﬂexes. Within the next 30
seconds, the sympathetic reﬂexes become very active.
They affect both the cardiac output and the venous
return curves, raising both of them. Sympathetic stim-
ulation can increase the plateau level of the cardiac
output curve as much as 30 to 100 per cent. It can also
increase the mean systemic ﬁlling pressure (depicted
by the point where the venous return curve crosses the
zero venous return axis) by several millimeters of
mercury—in

 return axis) by several millimeters of
mercury—in this ﬁgure, from a normal value of 7 mm
Hg up to 10 mm Hg. This increase in mean systemic
ﬁlling pressure shifts the entire venous return curve to
the right and upward. The new cardiac output and
venous return curves now equilibrate at point C, that
is, at a right atrial pressure of +5 mm Hg and a cardiac
output of 4 L/min.
0
100
200
300
400
500
600
Cardiac reserve (%)
Moderate
coronary
disease
Diphtheria
Severe
coronary
thrombosis
Mild
valvular

iphtheria
Severe
coronary
Mild
valvular
disease
Severe
valvular
disease
Athlete
Normal
Normal
operation
Figure 22–4
Cardiac reserve in different conditions, showing less than zero
reserve for two of the conditions.
0
5
10
15
–4
–2
0
2
4
6
8
10
12
14
Cardiac output and
A
B
C
D
Normal
Figure 22–5
Progressive changes in cardiac output and right atrial pressure
during different stages of cardiac failure.
266
Unit IV
Compe

cardiac failure.
266
Unit IV
Compensation During the Next Few Days. During the
ensuing week, the cardiac output and venous return
curves rise further because of (1) some recovery of the
heart and (2) renal retention of salt and water, which
raises the mean systemic ﬁlling pressure still further—
this time up to +12 mm Hg. The two new curves now
equilibrate at point D. Thus, the cardiac output has
now returned to normal. The right atrial pressure,
however, has risen still further 

atrial pressure,
however, has risen still further to +6 mm Hg. Because
the cardiac output is now normal, renal output is also
normal, so that a new state of equilibrated ﬂuid
balance has been achieved.The circulatory system will
continue to function at point D and remain stable, with
a normal cardiac output and an elevated right atrial
pressure, until some additional extrinsic factor changes
either the cardiac output curve or the venous return
curve.
Using this technique for analysis, one can se

rve.
Using this technique for analysis, one can see espe-
cially the importance of moderate ﬂuid retention and
how it eventually leads to a new stable state of the cir-
culation in mild to moderate heart failure. And one
can also see the interrelation between mean systemic
ﬁlling pressure and cardiac pumping at various
degrees of heart failure.
Note that the events described in Figure 22–5 are
the same as those presented in Figure 22–1, but in
Figure 22–5, they are presented in a more quantitati

gure 22–5, they are presented in a more quantitative
manner.
Graphical Analysis of “Decompensated”
The black cardiac output curve in Figure 22–6 is the
same as the curve shown in Figure 22–2, a very low
curve that has already reached a degree of recovery as
great as this heart can achieve. In this ﬁgure, we have
added venous return curves that occur during succes-
sive days after the acute fall of the cardiac output
curve to this low level. At point A, the curve at time
zero equa

low level. At point A, the curve at time
zero equates with the normal venous return curve to
give a cardiac output of about 3 L/min. However, stim-
ulation of the sympathetic nervous system, caused by
this low cardiac output, increases the mean systemic
ﬁlling pressure within 30 seconds from 7 to 10.5 mm
Hg. This shifts the venous return curve upward and to
the right to produce the curve labeled “autonomic
compensation.” Thus, the new venous return curve
equates with the cardiac output curve at 

rn curve
equates with the cardiac output curve at point B. The
cardiac output has been improved to a level of 4 L/min
but at the expense of an additional rise in right atrial
pressure to 5 mm Hg.
The cardiac output of 4 L/min is still too low to cause
the kidneys to function normally. Therefore, ﬂuid con-
tinues to be retained, and the mean systemic ﬁlling
pressure rises from 10.5 to almost 13 mm Hg. Now the
venous return curve becomes that labeled “2nd day”
and equilibrates with the cardiac out

ed “2nd day”
and equilibrates with the cardiac output curve at point
C. The cardiac output rises to 4.2 L/min and the right
atrial pressure to 7 mm Hg.
During the succeeding days, the cardiac output
never rises quite high enough to re-establish normal
renal function. Fluid continues to be retained, the
mean systemic ﬁlling pressure continues to rise, the
venous return curve continues to shift to the right, and
the equilibrium point between the venous return curve
and the cardiac output curve als

nous return curve
and the cardiac output curve also shifts progressively
to point D, to point E, and, ﬁnally, to point F.The equi-
libration process is now on the down slope of the
cardiac output curve, so that further retention of ﬂuid
causes only more severe cardiac edema and a detri-
mental effect on cardiac output. The condition accel-
erates downhill until death occurs.
Thus, “decompensation” results from the fact that
the cardiac output curve never rises to the critical level
of 5 L/min ne

ve never rises to the critical level
of 5 L/min needed to re-establish normal kidney excre-
tion of ﬂuid that would be required to cause balance
between ﬂuid input and output.
Treatment of Decompensated Heart Disease with Digitalis. Let
us assume that the stage of decompensation has
already reached point E in Figure 22–6, and let us
proceed to the same point E in Figure 22–7. At this
time, digitalis is given to strengthen the heart. This
raises the cardiac output curve to the level shown in
Figu

he cardiac output curve to the level shown in
Figure 22–7, but there is not an immediate change in
the venous return curve. Therefore, the new cardiac
output curve equates with the venous return curve at
point G. The cardiac output is now 5.7 L/min, a value
greater than the critical level of 5 liters required to
make the kidneys excrete normal amounts of urine.
Therefore, the kidneys eliminate much more ﬂuid than
normally, causing diuresis, a well-known therapeutic
effect of digitalis.
The progr

l-known therapeutic
The progressive loss of ﬂuid over a period of several
days reduces the mean systemic ﬁlling pressure back
down to 11.5 mm Hg, and the new venous return curve
becomes the curve labeled “Several days later.” This
curve equates with the cardiac output curve of the 
digitalized heart at point H, at an output of 5 L/min
and a right atrial pressure of 4.6 mm Hg. This cardiac
0
5
10
15
–4
–2
0
2
4
6
8
10
12
14
16
Right at

8th day
6th day
4th day
2nd day
Critical cardiac
output level
for normal
fluid balance
Au
to
n
o
mic co
mpensation
A
B
C
D
F
No
rm
al
ve
nous r
eturn
E
Figure 22–6
Graphical analysis of decompensated heart disease showing
progressive shift of the venous return curve to the right as a result
of continued ﬂuid retention.
Chapter 22
267
output is precisely that required for normal ﬂuid
balance. Therefore, no add

equired for normal ﬂuid
balance. Therefore, no additional ﬂuid will be lost and
none will be gained. Consequently, the circulatory
system has now stabilized, or in other words, the
decompensation of the heart failure has been “com-
pensated.” And to state this another way, the ﬁnal
steady-state condition of the circulation is deﬁned by
the crossing point of three curves: the cardiac output
curve, the venous return curve, and the critical level
for normal ﬂuid balance. The compensatory mecha-
nis

r normal ﬂuid balance. The compensatory mecha-
nisms automatically stabilize the circulation when all
three curves cross at the same point.
Graphical Analysis of High-Output 
Figure 22–8 gives an analysis of two types of high-
output cardiac failure. One of these is caused by an
arteriovenous ﬁstula that overloads the heart because
of excessive venous return, even though the pumping
capability of the heart is not depressed. The other is
caused by beriberi, in which the venous ret

her is
caused by beriberi, in which the venous return is
greatly increased because of diminished systemic vas-
cular resistance, but at the same time, the pumping
capability of the heart is depressed.
Arteriovenous Fistula. The “normal” curves of Figure
22–8 depict the normal cardiac output and normal
venous return curves. These equate with each other at
point A, which depicts a normal cardiac output of 5 L/
min and a normal right atrial pressure of 0 mm Hg.
Now let us assume that the systemic r

 of 0 mm Hg.
Now let us assume that the systemic resistance (the
total peripheral resistance) becomes greatly decreased
because of opening a large arteriovenous ﬁstula (a
direct opening between a large artery and a large
vein). The venous return curve rotates upward to give
the curve labeled “AV ﬁstula.” This venous return
curve equates with the normal cardiac output curve at
point B, with a cardiac output of 12.5 L/min and a right
atrial pressure of 3 mm Hg. Thus, the cardiac output
has become 

e of 3 mm Hg. Thus, the cardiac output
has become greatly elevated, the right atrial pressure
is slightly elevated, and there are mild signs of periph-
eral congestion. If the person attempts to exercise, he
or she will have little cardiac reserve because the heart
is already being used almost to maximum capacity to
pump the extra blood through the arteriovenous
ﬁstula. This condition resembles a failure condition
and is called “high-output failure,” but in reality, the
heart is overloaded by ex

re,” but in reality, the
heart is overloaded by excess venous return.
Beriberi. Figure 22–8 shows the approximate changes
in the cardiac output and venous return curves caused
by beriberi. The decreased level of the cardiac output
curve is caused by weakening of the heart because of
the avitaminosis (mainly lack of thiamine) that causes
the beriberi syndrome.The weakening of the heart has
decreased the blood ﬂow to the kidneys.Therefore, the
kidneys have retained a large amount of extra body
ﬂui

eys have retained a large amount of extra body
ﬂuid, which in turn has increased the mean systemic
ﬁlling pressure (represented by the point where the
venous return curve now intersects the zero cardiac
output level) from the normal value of 7 mm Hg up to
11 mm Hg.This has shifted the venous return curve to
the right. Finally, the venous return curve has rotated
upward from the normal curve because the avita-
minosis has dilated the peripheral blood vessels, as
explained in Chapter 17.
The two b

ood vessels, as
The two blue curves (cardiac output curve and
venous return curve) intersect with each other at 
point C, which describes the circulatory condition in
beriberi, with a right atrial pressure in this instance of
9 mm Hg and a cardiac output about 65 per cent above
normal; this high cardiac output occurs despite the
weak heart, as demonstrated by the depressed plateau
level of the cardiac output curve.
0
5
10
15
–4
–2
0
2
4
6
8
10
12
14
16

15
–4
–2
0
2
4
6
8
10
12
14
16
Critical cardiac output
level for normal
G
E
H
S
ev
er
al
days later
S
ev
er
el
y f
ail
in
g
he
art
Di
git
ali
ze
d
he
ar
t
Fir
st
da
y
Figure 22–7
Treatment of decompensated heart disease showing the effect of
digitalis in elevating the cardiac output curve, this in turn causing
increased urine output and progressive shift of the venous return
curve to the left.
0
5
10
15
20
25
–

venous return
0
5
10
15
20
25
–4 –2
0
2
4
6
8
10 12 14 16
B
A
C
AV fistula
Normal cardiac 
output curve
Beriberi
heart 
disease
Normal 
venous 
return 
curve
Figure 22–8
Graphical analysis of two types of conditions that can cause high-
output cardiac failure: (1) arteriovenous (AV) ﬁstula and (2)
beriberi heart disease.
268
Unit IV
Andrew P: Diastolic heart failure demystiﬁed. Ch

s
Andrew P: Diastolic heart failure demystiﬁed. Chest 124:744,
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Braunwald E, Zipes DP, Liby P: A Textbook for Cardiovas-
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Dorn GW 2nd, Molkentin JD: Manipulating cardiac con-
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lation 109:150, 2004.
Ducas J, Grech ED: ABC of interventional cardiology. Per-
cutaneous coronary intervention: cardiogenic shock. BMJ
326:1450, 2003.
Eikelboom J, White H, Yusuf S: The evolving role 

Eikelboom J, White H, Yusuf S: The evolving role of direct
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Floras JS: Sympathetic activation in human heart failure:
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therapeutic opportunities. Acta
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BK, Geppert E: The pulmonary manifestations of
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C
H
A
P
T
E
R
 
2
3
269
Dynamics of Valvular and
Congenital Heart Def

nds;
Function of the heart valves was discussed in
Chapter 9, where it was pointed out that closing of
the valves causes audible sounds. Ordinarily, no
audible sounds occur when the valves open. In this
chapter, we ﬁrst discuss the factors that cause the
sounds in the heart under normal and abnormal
conditions. Then we discuss what happens in the
overall circulatory system when valvular or congen-
ital heart defects are present.
Norm

Listening with a stethoscope to a normal heart, one hears a sound usually
described as “lub,dub,lub,dub.”The “lub”is associated with closure of the atrio-
ventricular (A-V) valves at the beginning of systole, and the “dub” is associated
with closure of the semilunar (aortic and pulmonary) valves at the end of
systole. The “lub” sound is called the ﬁrst heart sound, and the “dub” is called
the second heart sound, because the normal

 called
the second heart sound, because the normal pumping cycle of the heart is con-
sidered to start when the A-V valves close at the onset of ventricular systole.
Causes of the First and Second Heart Sounds. The earliest explanation for the cause
of the heart sounds was that the “slapping” together of the valve leaﬂets sets
up vibrations. However, this has been shown to cause little, if any, of the sound,
because the blood between the leaﬂets cushions the slapping effect and pre-
vents signiﬁ

cushions the slapping effect and pre-
vents signiﬁcant sound. Instead, the cause is vibration of the taut valves imme-
diately after closure, along with vibration of the adjacent walls of the heart and
major vessels around the heart. That is, in generating the ﬁrst heart sound, con-
traction of the ventricles ﬁrst causes sudden backﬂow of blood against the 
A-V valves (the tricuspid and mitral valves), causing them to close and bulge
toward the atria until the chordae tendineae abruptly stop the

tria until the chordae tendineae abruptly stop the back bulging.
The elastic tautness of the chordae tendineae and of the valves then causes the
back surging blood to bounce forward again into each respective ventricle.This
causes the blood and the ventricular walls, as well as the taut valves, to vibrate
and causes vibrating turbulence in the blood. The vibrations travel through the
adjacent tissues to the chest wall, where they can be heard as sound by using
the stethoscope.
The second heart s

sound by using
The second heart sound results from sudden closure of the semilunar valves
at the end of systole. When the semilunar valves close, they bulge backward
toward the ventricles, and their elastic stretch recoils the blood back into the
arteries, which causes a short period of reverberation of blood back and forth
between the walls of the arteries and the semilunar valves, as well as between
these valves and the ventricular walls. The vibrations occurring in the arteri

ular walls. The vibrations occurring in the arterial
walls are then transmitted mainly along the arteries.When the vibrations of the
vessels or ventricles come into contact with a “sounding board,” such as the
chest wall, they create sound that can be heard.
270
Unit IV
Duration and Pitch of the First and Second Heart Sounds. The
duration of each of the heart sounds is slightly more
than 0.10 second—the ﬁrst sound about 0.14 second,
and the second about 0.11 second. The reason fo

d,
and the second about 0.11 second. The reason for the
shorter second sound is that the semilunar valves are
more taut than the A-V valves, so that they vibrate for
a shorter time than do the A-V valves.
The audible range of frequency (pitch) in the ﬁrst
and second heart sounds, as shown in Figure 23–1,
begins at the lowest frequency the ear can detect,
about 40 cycles/sec, and goes up above 500 cycles/sec.
When special electronic apparatus is used to record
these sounds, by far a larger propor

sed to record
these sounds, by far a larger proportion of the re-
corded sound is at frequencies and sound levels below
the audible range, going down to 3 to 4 cycles/sec and
peaking at about 20 cycles/sec, as illustrated by the
lower shaded area in Figure 23–1. For this reason,
major portions of the heart sounds can be recorded
electronically in phonocardiograms even though they
cannot be heard with a stethoscope.
The second heart sound normally has a higher fre-
quency than the ﬁrst heart soun

 has a higher fre-
quency than the ﬁrst heart sound for two reasons: (1)
the tautness of the semilunar valves in comparison
with the much less taut A-V valves, and (2) the greater
elastic coefﬁcient of the taut arterial walls that provide
the principal vibrating chambers for the second sound,
in comparison with the much looser, less elastic ven-
tricular chambers that provide the vibrating system for
the ﬁrst heart sound. The clinician uses these differ-
ences to distinguish special characterist

 differ-
ences to distinguish special characteristics of the two
respective sounds.
Third Heart Sound. Occasionally a weak, rumbling third
heart sound is heard at the beginning of the middle
third of diastole. A logical but unproved explanation
of this sound is oscillation of blood back and forth
between the walls of the ventricles initiated by inrush-
ing blood from the atria. This is analogous to running
water from a faucet into a paper sack, the inrushing
water reverberating back and forth be

he inrushing
water reverberating back and forth between the walls
of the sack to cause vibrations in its walls. The reason
the third heart sound does not occur until the middle
third of diastole is believed to be that in the early part
of diastole, the ventricles are not ﬁlled sufﬁciently to
create even the small amount of elastic tension neces-
sary for reverberation. The frequency of this sound is
usually so low that the ear cannot hear it, yet it can
often be recorded in the phonocardiogram.

 it can
Atrial Heart Sound (Fourth Heart Sound). An atrial heart
sound can sometimes be recorded in the phonocar-
diogram, but it can almost never be heard with a
stethoscope because of its weakness and very low 
frequency—usually 20 cycles/sec or less. This sound
occurs when the atria contract, and presumably, it is
caused by the inrush of blood into the ventricles, which
initiates vibrations similar to those of the third heart
sound.
Chest Surface Area

those of the third heart
sound.
Chest Surface Areas for Auscultation 
of Normal Heart Sounds
Listening to the sounds of the body, usually with the
aid of a stethoscope, is called auscultation. Figure 23–2
shows the areas of the chest wall from which the dif-
ferent heart valvular sounds can best be distinguished.
Although the sounds from all the valves can be heard
from all these areas, the cardiologist distinguishes 
the sounds from the different valves by a process of
Inaudible
Heart sounds
an

t valves by a process of
Inaudible
and murmurs
Speech
area
100
10
1
0.1
0.01
0.001
0.0001
0
8
32
64 128 256 512
2048 4096
1024
Dynes/cm2
Frequency in cycles per second
Threshold of audibility
Figure 23–1
Amplitude of different-frequency vibrations in the heart sounds
and heart murmurs in relation to the threshold of audibility,
showing that the range of sounds that can be heard is between
40 and 520 cycles/sec. (Modiﬁed from Butterworth JS, Chassin
JL, McGra

c. (Modiﬁed from Butterworth JS, Chassin
JL, McGrath JJ: Cardiac Auscultation, 2nd ed. New York: Grune
& Stratton, 1960.)
Mitral area
Tricuspid area
Aortic area
Pulmonic area
Figure 23–2
Chest areas from which sound from each valve is best heard.
Chapter 23
Heart Valves and Heart Sounds; Dynamics of Valvular and Congenital Heart Defects
271
elimination. That is, he or she moves the stethoscope
from one area to another, noting the loudness of the
sounds in different areas and gradually picking ou

sounds in different areas and gradually picking out the
sound components from each valve.
The areas for listening to the different heart sounds
are not directly over the valves themselves. The aortic
area is upward along the aorta because of sound trans-
mission up the aorta, and the pulmonic area is upward
along the pulmonary artery. The tricuspid area is over
the right ventricle, and the mitral area is over the apex
of the left ventricle, which is the portion of the heart
nearest the surface o

 is the portion of the heart
nearest the surface of the chest; the heart is rotated 
so that the remainder of the left ventricle lies more
posteriorly.
If a microphone specially designed to detect low-fre-
quency sound is placed on the chest, the heart sounds
can be ampliﬁed and recorded by a high-speed record-
ing apparatus. The recording is called a phonocardio-
gram, and the heart sounds appear as waves, as shown
schematically in Figure 23–3. Recording A is an
example of norma

in Figure 23–3. Recording A is an
example of normal heart sounds, showing the vibra-
tions of the ﬁrst, second, and third heart sounds and
even the very weak atrial sound. Note speciﬁcally that
the third and atrial heart sounds are each a very low
rumble.The third heart sound can be recorded in only
one third to one half of all people, and the atrial heart
sound can be recorded in perhaps one fourth of all
people.
Rheumatic Valvular Lesions
By far the greatest number of valvular

lar Lesions
By far the greatest number of valvular lesions results
from rheumatic fever. Rheumatic fever is an autoim-
mune disease in which the heart valves are likely to be
damaged or destroyed. It is usually initiated by strep-
tococcal toxin in the following manner.
The sequence of events almost always begins with a
preliminary streptococcal infection caused speciﬁcally
by group A hemolytic streptococci. These bacteria ini-
tially cause a sore throat, scarlet fever, or middle ear
infection. 

e throat, scarlet fever, or middle ear
infection. But the streptococci also release several dif-
ferent proteins against which the person’s reticuloen-
dothelial system produces antibodies. The antibodies
react not only with the streptococcal protein but also
with other protein tissues of the body, often causing
severe immunologic damage.These reactions continue
to take place as long as the antibodies persist in the
blood—1 year or more.
Rheumatic fever causes damage especially in
certain suscep

c fever causes damage especially in
certain susceptible areas, such as the heart valves. The
degree of heart valve damage is directly correlated
with the concentration and persistence of the anti-
bodies. The principles of immunity that relate to this
type of reaction are discussed in Chapter 34, and it is
noted in Chapter 31 that acute glomerular nephritis of
the kidneys has a similar immunologic basis.
In rheumatic fever, large hemorrhagic, ﬁbrinous,
bulbous lesions grow along the inﬂamed edge

inous,
bulbous lesions grow along the inﬂamed edges of 
the heart valves. Because the mitral valve receives
more trauma during valvular action than any of the
other valves, it is the one most often seriously
damaged, and the aortic valve is second most fre-
quently damaged. The right heart valves, the tricuspid
and pulmonary valves, are usually affected much less
severely, probably because the low-pressure stresses
that act on these valves are slight compared with 
the high-pressure stresses tha

ight compared with 
the high-pressure stresses that act on the left heart
valves.
Scarring of the Valves. The lesions of acute rheumatic
fever frequently occur on adjacent valve leaﬂets simul-
taneously, so that the edges of the leaﬂets become
stuck together.Then, weeks, months, or years later, the
lesions become scar tissue, permanently fusing por-
tions of adjacent valve leaﬂets. Also, the free edges of
the leaﬂets,which are normally ﬁlmy and free-ﬂapping,
often become solid, scarred masses.
A

ree-ﬂapping,
A valve in which the leaﬂets adhere to one another
so extensively that blood cannot ﬂow through it nor-
mally is said to be stenosed. Conversely,when the valve
edges are so destroyed by scar tissue that they cannot
close as the ventricles contract, regurgitation (back-
ﬂow) of blood occurs when the valve should be closed.
Stenosis usually does not occur without the coexis-
tence of at least some degree of regurgitation, and vice
versa.
Other Cause

gree of regurgitation, and vice
versa.
Other Causes of Valvular Lesions. Stenosis or lack of one
or more leaﬂets of a valve also occurs occasionally as
a congenital defect. Complete lack of leaﬂets is rare;
Aortic regurgitation
Mitral regurgitation
Aortic stenosis
Normal
Patent ductus 
Mitral stenosis
Diastole
Systole
Diastole
Systole
A
B
C
D
E
F
1st
2nd
3rd
Atrial
Figure 23–3
Phonocardiograms from normal and abnormal hearts.
272
Unit IV
congenital stenosis is more com

IV
congenital stenosis is more common, as is discussed
later in this chapter.
Heart Murmurs Caused by Valvular Lesions
As shown by the phonocardiograms in Figure 23–3,
many abnormal heart sounds, known as “heart
murmurs,” occur when there are abnormalities of the
valves, as follows.
Systolic Murmur of Aortic Stenosis. In aortic stenosis,
blood is ejected from the left ventricle through only a
small ﬁbrous opening of the aortic valve. Because of
the resistance to ejection, sometim

ve. Because of
the resistance to ejection, sometimes the blood pres-
sure in the left ventricle rises as high as 300 mm Hg,
while the pressure in the aorta is still normal. Thus,
a nozzle effect is created during systole, with blood
jetting at tremendous velocity through the small
opening of the valve. This causes severe turbulence of
the blood in the root of the aorta.The turbulent blood
impinging against the aortic walls causes intense vibra-
tion, and a loud murmur (see recording B, Figure 23

ion, and a loud murmur (see recording B, Figure 23–3)
occurs during systole and is transmitted throughout
the superior thoracic aorta and even into the large
arteries of the neck. This sound is harsh and in severe
stenosis may be so loud that it can be heard several
feet away from the patient. Also, the sound vibrations
can often be felt with the hand on the upper chest and
lower neck, a phenomenon known as a “thrill.”
Diastolic Murmur of Aortic Regurgitation. In aortic regurgi-
tation, no abnor

Regurgitation. In aortic regurgi-
tation, no abnormal sound is heard during systole,
but during diastole, blood ﬂows backward from the
high-pressure aorta into the left ventricle, causing 
a “blowing” murmur of relatively high pitch with a
swishing quality heard maximally over the left ventri-
cle (see recording D, Figure 23–3).This murmur results
from turbulence of blood jetting backward into the
blood already in the low-pressure diastolic left 
Systolic Murmur of Mitral Regurgitatio

Systolic Murmur of Mitral Regurgitation. In mitral regurgi-
tation, blood ﬂows backward through the mitral valve
into the left atrium during systole. This also causes a
high-frequency “blowing,” swishing sound (see record-
ing C, Figure 23–3) similar to that of aortic regurgita-
tion but occurring during systole rather than diastole.
It is transmitted most strongly into the left atrium.
However, the left atrium is so deep within the chest
that it is difﬁcult to hear this sound direct

hest
that it is difﬁcult to hear this sound directly over the
atrium. As a result, the sound of mitral regurgitation
is transmitted to the chest wall mainly through the left
ventricle to the apex of the heart.
Diastolic Murmur of Mitral Stenosis. In mitral stenosis,
blood passes with difﬁculty through the stenosed
mitral valve from the left atrium into the left ventri-
cle, and because the pressure in the left atrium seldom
rises above 30 mm Hg, a large pressure differential
forcing blood from t

a large pressure differential
forcing blood from the left atrium into the left ventri-
cle does not develop. Consequently, the abnormal
sounds heard in mitral stenosis (see recording E,
Figure 23–3) are usually weak and of very low 
frequency, so that most of the sound spectrum is below
the low-frequency end of human hearing.
During the early part of diastole, a left ventricle with
a stenotic mitral valve has so little blood in it and its
walls are so ﬂabby that blood does not reverberate
back a

re so ﬂabby that blood does not reverberate
back and forth between the walls of the ventricle. For
this reason, even in severe mitral stenosis, no murmur
may be heard during the ﬁrst third of diastole. Then,
after partial ﬁlling, the ventricle has stretched enough
for blood to reverberate, and a low rumbling murmur
begins.
Phonocardiograms of Valvular Murmurs. Phonocardio-
grams B, C, D, and E of Figure 23–3 show, respectively,
idealized records obtained from patients with aortic
stenosis, mitra

obtained from patients with aortic
stenosis, mitral regurgitation, aortic regurgitation, and
mitral stenosis. It is obvious from these phonocardio-
grams that the aortic stenotic lesion causes the loudest
murmur, and the mitral stenotic lesion causes the
weakest. The phonocardiograms show how the inten-
sity of the murmurs varies during different portions of
systole and diastole, and the relative timing of each
murmur is also evident. Note especially that the
murmurs of aortic stenosis and mitra

ally that the
murmurs of aortic stenosis and mitral regurgitation
occur only during systole, whereas the murmurs of
aortic regurgitation and mitral stenosis occur only
during diastole. If the reader does not understand this
timing, extra review should be undertaken until it is
Abnormal Circulatory
Dynamics in Valvular 
Dynamics of the Circulation in Aortic
In aortic stenosis, the contracting left ventricle fails to
empty adequately, whe

ting left ventricle fails to
empty adequately, whereas in aortic regurgitation,
blood ﬂows backward into the ventricle from the aorta
after the ventricle has just pumped the blood into the
aorta. Therefore, in either case, the net stroke volume
output of the heart is reduced.
Several important compensations take place that
can ameliorate the severity of the circulatory defects.
Some of these compensations are the following.
Hypertrophy of the Left Ventricle. In both aortic stenosis
and aortic re

t Ventricle. In both aortic stenosis
and aortic regurgitation, the left ventricular muscula-
ture hypertrophies because of the increased ventricu-
lar workload.
In regurgitation, the left ventricular chamber also
enlarges to hold all the regurgitant blood from the
aorta. Sometimes the left ventricular muscle mass
increases fourfold to ﬁvefold, creating a tremendously
large left side of the heart.
When the aortic valve is seriously stenosed, the
hypertrophied muscle allows the left ventricle to
d

ypertrophied muscle allows the left ventricle to
develop as much as 400 mm Hg intraventricular pres-
sure at systolic peak.
Chapter 23
273
In severe aortic regurgitation, sometimes the hyper-
trophied muscle allows the left ventricle to pump a
stroke volume output as great as 250 milliliters,
although as much as three fourths of this blood returns
to the ventricle during diastole, and only one fourth
ﬂows through th

ring diastole, and only one fourth
ﬂows through the aorta to the body.
Increase in Blood Volume. Another effect that helps
compensate for the diminished net pumping by the left
ventricle is increased blood volume. This results from
(1) an initial slight decrease in arterial pressure, plus
(2) peripheral circulatory reﬂexes that the decrease in
pressure induces.These together diminish renal output
of urine, causing the blood volume to increase and 
the mean arterial pressure to return to normal. 

the mean arterial pressure to return to normal. Also,
red cell mass eventually increases because of a slight
degree of tissue hypoxia.
The increase in blood volume tends to increase
venous return to the heart.This, in turn, causes the left
ventricle to pump with the extra power required to
overcome the abnormal pumping dynamics.
Eventual Failure of the Left Ventricle, and
Development of Pulmonary Edema
In the early stages of aortic stenosis or aortic regurgi-
tation, the intrinsic ability of t

aortic regurgi-
tation, the intrinsic ability of the left ventricle to adapt
to increasing loads prevents signiﬁcant abnormalities
in circulatory function in the person during rest, other
than increased work output required of the left ven-
tricle. Therefore, considerable degrees of aortic steno-
sis or aortic regurgitation often occur before the
person knows that he or she has serious heart disease
(such as a resting left ventricular systolic pressure as
high as 200 mm Hg in aortic stenosis or 

essure as
high as 200 mm Hg in aortic stenosis or a left ventric-
ular stroke volume output as high as double normal in
aortic regurgitation).
Beyond a critical stage in these aortic valve lesions,
the left ventricle ﬁnally cannot keep up with the work
demand. As a consequence, the left ventricle dilates
and cardiac output begins to fall; blood simultaneously
dams up in the left atrium and in the lungs behind the
failing left ventricle. The left atrial pressure rises pro-
gressively, and at mean

atrial pressure rises pro-
gressively, and at mean left atrial pressures above 25
to 40 mm Hg, serious edema appears in the lungs, as
discussed in detail in Chapter 38.
Dynamics of Mitral Stenosis and
Mitral Regurgitation
In mitral stenosis, blood ﬂow from the left atrium into
the left ventricle is impeded, and in mitral regurgita-
tion, much of the blood that has ﬂowed into the left
ventricle during diastole leaks back into the left atrium
during systole rather than being pumped into the
aorta.

g systole rather than being pumped into the
aorta.Therefore, either of these conditions reduces net
movement of blood from the left atrium into the left
Pulmonary Edema in Mitral Valvular Disease. The buildup 
of blood in the left atrium causes progressive increase
in left atrial pressure, and this eventually results in
development of serious pulmonary edema. Ordinarily,
lethal edema does not occur until the mean left atrial
pressure rises above 25 mm Hg and sometimes as high
as 40 mm

ises above 25 mm Hg and sometimes as high
as 40 mm Hg, because the lung lymphatic vasculature
enlarges manyfold and can carry ﬂuid away from the
lung tissues extremely rapidly.
Enlarged Left Atrium and Atrial Fibrillation. The high left
atrial pressure in mitral valvular disease also causes
progressive enlargement of the left atrium, which
increases the distance that the cardiac electrical exci-
tatory impulse must travel in the atrial wall. This
pathway may eventually become so long that it pre

pathway may eventually become so long that it pre-
disposes to development of excitatory signal circus
movements, as discussed in Chapter 13. Therefore,
in late stages of mitral valvular disease, especially in
mitral stenosis, atrial ﬁbrillation usually occurs. This
further reduces the pumping effectiveness of the heart
and causes further cardiac debility.
Compensation in Early Mitral Valvular Disease. As also
occurs in aortic valvular disease and in many types of
congenital heart disease, the 

nd in many types of
congenital heart disease, the blood volume increases
in mitral valvular disease principally because of dimin-
ished excretion of water and salt by the kidneys. This
increased blood volume increases venous return to the
heart, thereby helping to overcome the effect of the
cardiac debility.Therefore, after compensation, cardiac
output may fall only minimally until the late stages of
mitral valvular disease,even though the left atrial pres-
sure is rising.
As the left atrial pre

trial pres-
As the left atrial pressure rises, blood begins to dam
up in the lungs, eventually all the way back to the pul-
monary artery. In addition, incipient edema of the
lungs causes pulmonary arteriolar constriction. These
two effects together increase systolic pulmonary arte-
rial pressure and also right ventricular pressure, some-
times to as high as 60 mm Hg, which is more than
double normal.This, in turn, causes hypertrophy of the
right side of the heart, which partiall

phy of the
right side of the heart, which partially compensates for
its increased workload.
Circulatory Dynamics During Exercise
in Patients with Valvular Lesions
During exercise, large quantities of venous blood are
returned to the heart from the peripheral circulation.
Therefore, all the dynamic abnormalities that occur in
the different types of valvular heart disease become
tremendously exacerbated. Even in mild valvular
heart disease, in which the symptoms may be unrec-
ognizable at rest, se

h the symptoms may be unrec-
ognizable at rest, severe symptoms often develop
during heavy exercise. For instance, in patients with
aortic valvular lesions, exercise can cause acute left
ventricular failure followed by acute pulmonary
edema. Also, in patients with mitral disease, exercise
can cause so much damming of blood in the lungs that
serious or even lethal pulmonary edema may ensue in
as little as 10 minutes.
Even in mild to moderate cases of valvular disease,
the patient’s cardiac reserv

 of valvular disease,
the patient’s cardiac reserve diminishes in proportion
274
Unit IV
to the severity of the valvular dysfunction. That is, the
cardiac output does not increase as much as it should
during exercise. Therefore, the muscles of the body
fatigue rapidly because of too little increase in muscle
blood ﬂow.
Dynamics in Congenital 
Occasionally, the heart or its associated blood vessels
are malformed during fetal life; the defect is c

s
are malformed during fetal life; the defect is called a
congenital anomaly. There are three major types of
congenital anomalies of the heart and its associated
vessels: (1) stenosis of the channel of blood ﬂow at
some point in the heart or in a closely allied major
blood vessel; (2) an anomaly that allows blood to ﬂow
backward from the left side of the heart or aorta to the
right side of the heart or pulmonary artery, thus failing
to ﬂow through the systemic circulation—called a left-
to-right

h the systemic circulation—called a left-
to-right shunt; and (3) an anomaly that allows blood to
ﬂow directly from the right side of the heart into the
left side of the heart, thus failing to ﬂow through the
lungs—called a right-to-left shunt.
The effects of the different stenotic lesions are easily
understood. For instance, congenital aortic valve steno-
sis results in the same dynamic effects as aortic valve
stenosis caused by other valvular lesions, namely, a
tendency to develop serious pulm

esions, namely, a
tendency to develop serious pulmonary edema and a
reduced cardiac output.
Another type of congenital stenosis is coarctation 
of the aorta, often occurring near the level of the
diaphragm. This causes the arterial pressure in the
upper part of the body (above the level of the coarc-
tation) to be much greater than the pressure in the
lower body because of the great resistance to blood
ﬂow through the coarctation to the lower body; part of
the blood must go around the coarctatio

y; part of
the blood must go around the coarctation through
small collateral arteries, as discussed in Chapter 19.
Patent Ductus Arteriosus—
A Left-to-Right Shunt
During fetal life, the lungs are collapsed, and the
elastic compression of the lungs that keeps the alveoli
collapsed keeps most of the lung blood vessels col-
lapsed as well. Therefore, resistance to blood ﬂow
through the lungs is so great that the pulmonary arte-
rial pressure is high in the fetus. Also, because of low
resistance to 

 in the fetus. Also, because of low
resistance to blood ﬂow from the aorta through the
large vessels of the placenta, the pressure in the aorta
of the fetus is lower than normal—in fact, lower than
in the pulmonary artery.This causes almost all the pul-
monary arterial blood to ﬂow through a special artery
present in the fetus that connects the pulmonary artery
with the aorta (Figure 23–4), called the ductus arterio-
sus, thus bypassing the lungs. This allows immediate
recirculation of the blood

. This allows immediate
recirculation of the blood through the systemic arter-
ies of the fetus without the blood going through the
lungs. This lack of blood ﬂow through the lungs is not
detrimental to the fetus because the blood is oxy-
genated by the placenta.
Closure of the Ductus Arteriosus After Birth. As soon as a
baby is born and begins to breathe, the lungs inﬂate;
not only do the alveoli ﬁll with air, but also the resist-
ance to blood ﬂow through the pulmonary vascular
tree decreases t

ow through the pulmonary vascular
tree decreases tremendously, allowing the pulmonary
arterial pressure to fall. Simultaneously, the aortic
pressure rises because of sudden cessation of blood
ﬂow from the aorta through the placenta. Thus, the
pressure in the pulmonary artery falls, while that in 
the aorta rises.As a result, forward blood ﬂow through
the ductus arteriosus ceases suddenly at birth, and in
fact, blood begins to ﬂow backward through the ductus
from the aorta into the pulmonary arte

 the ductus
from the aorta into the pulmonary artery. This new
state of backward blood ﬂow causes the ductus arte-
riosus to become occluded within a few hours to a few
days in most babies, so that blood ﬂow through the
ductus does not persist.The ductus is believed to close
because the oxygen concentration of the aortic blood
now ﬂowing through it is about twice as high as that
of the blood ﬂowing from the pulmonary artery into
the ductus during fetal life. The oxygen presumably
constricts the 

 fetal life. The oxygen presumably
constricts the muscle in the ductus wall. This is dis-
cussed further in Chapter 83.
Unfortunately, in about 1 of every 5500 babies, the
ductus does not close, causing the condition known as
patent ductus arteriosus, which is shown in Figure 23–4.
Dynamics of the Circulation with a Persistent Patent Ductus.
During the early months of an infant’s life, a patent
ductus usually does not cause severely abnormal func-
tion. But as the child grows older, the differen

-
tion. But as the child grows older, the differential
between the high pressure in the aorta and the lower
Head and upper
extremities
Trunk and lower
Right
lung
Left lung
Pulmonary
artery
Aorta
Ductus
Left
pulmonary
artery
Figure 23–4
Patent ductus arteriosus, showing by the intensity of the pink color
that dark venous blood changes into oxygenated blood at differ-
ent points in the circulation. The right-hand diagram shows back-
ﬂow of blood from the aorta into the pulmo

s back-
ﬂow of blood from the aorta into the pulmonary artery and then
through the lungs for a second time.
Chapter 23
275
pressure in the pulmonary artery progressively in-
creases, with corresponding increase in backward ﬂow
of blood from the aorta into the pulmonary artery.
Also, the high aortic blood pressure usually causes the
diameter of the partially open ductus to increase with
time, making the condition eve

us to increase with
time, making the condition even worse.
Recirculation Through the Lungs. In an older child
with a patent ductus,one half to two thirds of the aortic
blood ﬂows backward through the ductus into the pul-
monary artery, then through the lungs, and ﬁnally back
into the left ventricle and aorta, passing through the
lungs and left side of the heart two or more times for
every one time that it passes through the systemic cir-
culation. These people do not show cyanosis until later
in

. These people do not show cyanosis until later
in life, when the heart fails or the lungs become con-
gested. Indeed, early in life, the arterial blood is often
better oxygenated than normal because of the extra
times it passes through the lungs.
Diminished Cardiac and Respiratory Reserve. The
major effects of patent ductus arteriosus on the patient
are decreased cardiac and respiratory reserve.The left
ventricle is pumping about two or more times the
normal cardiac output, and the maximum that

es the
normal cardiac output, and the maximum that it can
pump after hypertrophy of the heart has occurred is
about four to seven times normal. Therefore, during
exercise, the net blood ﬂow through the remainder of
the body can never increase to the levels required for
strenuous activity. With even moderately strenuous
exercise, the person is likely to become weak and may
even faint from momentary heart failure.
The high pressures in the pulmonary vessels caused
by excess ﬂow through the lungs o

y vessels caused
by excess ﬂow through the lungs often lead to pul-
monary congestion and pulmonary edema. As a result
of the excessive load on the heart, and especially
because the pulmonary congestion becomes progres-
sively more severe with age, most patients with uncor-
rected patent ductus die from heart disease between
ages 20 and 40 years.
Heart Sounds: Machinery Murmur. In a newborn infant
with patent ductus arteriosus, occasionally no abnor-
mal heart sounds are heard because the quanti

nor-
mal heart sounds are heard because the quantity of
reverse blood ﬂow through the ductus may be insufﬁ-
cient to cause a heart murmur. But as the baby grows
older, reaching age 1 to 3 years, a harsh, blowing
murmur begins to be heard in the pulmonary artery
area of the chest, as shown in recording F, Figure 23–3.
This sound is much more intense during systole when
the aortic pressure is high and much less intense
during diastole when the aortic pressure falls low, so
that the murmur waxes an

ic pressure falls low, so
that the murmur waxes and wanes with each beat of
the heart, creating the so-called machinery murmur.
Surgical Treatment. Surgical treatment of patent ductus
arteriosus is extremely simple; one need only ligate the
patent ductus or divide it and then close the two ends.
In fact, this was one of the ﬁrst successful heart sur-
geries ever performed.
Tetralogy of Fallot—
A Right-to-Left Shunt
Tetralogy of Fallot is shown in Figure 23–5; it is the
most common cause of “blue

 Figure 23–5; it is the
most common cause of “blue baby.” Most of the blood
bypasses the lungs, so the aortic blood is mainly unoxy-
genated venous blood. In this condition, four abnor-
malities of the heart occur simultaneously:
1. The aorta originates from the right ventricle
rather than the left, or it overrides a hole in the
septum, as shown in Figure 23–5, receiving blood
from both ventricles.
2. The pulmonary artery is stenosed, so that much
lower than normal amounts of blood pass from
the

h
the right ventricle into the lungs; instead, most of
the blood passes directly into the aorta, thus
bypassing the lungs.
3. Blood from the left ventricle ﬂows either through
a ventricular septal hole into the right ventricle
and then into the aorta or directly into the aorta
that overrides this hole.
4. Because the right side of the heart must pump
large quantities of blood against the high pressure
in the aorta, its musculature is highly developed,

causing an enlarged right ventricle.
Abnormal Circulatory Dynamics. It is readily apparent that
the major physiological difﬁculty caused by tetralogy
of Fallot is the shunting of blood past the lungs without
its becoming oxygenated. As much as 75 per cent of
the venous blood returning to the heart passes directly
Right
lung
Left lung
Figure 23–5
Tetralogy of Fallot, showing by the intensity 

3–5
Tetralogy of Fallot, showing by the intensity of the pink color that
most of the dark venous blood is shunted from the right ventricle
into the aorta without passing through the lungs.
276
Unit IV
from the right ventricle into the aorta without becom-
ing oxygenated.
A diagnosis of tetralogy of Fallot is usually based on
(1) the fact that the baby’s skin is cyanotic (blue); (2)
measurement of high systolic pressure in the right 
ventricle, recorded through a catheter; (3) cha

t 
ventricle, recorded through a catheter; (3) charac-
teristic changes in the radiological silhouette of the 
heart, showing an enlarged right ventricle; and (4)
angiograms (x-ray pictures) showing abnormal blood
ﬂow through the interventricular septal hole and into
the overriding aorta, but much less ﬂow through the
stenosed pulmonary artery.
Surgical Treatment. Tetralogy of Fallot can usually be
treated successfully by surgery. The usual operation is
to open the pulmonary stenosis, close the 

tion is
to open the pulmonary stenosis, close the septal defect,
and reconstruct the ﬂow pathway into the aorta.When
surgery is successful, the average life expectancy
increases from only 3 to 4 years to 50 or more years.
One of the most common causes of congenital heart
defects is a viral infection in the mother during the ﬁrst
trimester of pregnancy when the fetal heart is being
formed. Defects are particularly prone to develop
when the expectant mother contracts

one to develop
when the expectant mother contracts German
measles; thus, obstetricians often advise termination of
pregnancy if German measles occurs in the ﬁrst
trimester.
Some congenital defects of the heart are hereditary,
because the same defect has been known to occur in
identical twins as well as in succeeding generations.
Children of patients surgically treated for congenital
heart disease have about a 10 times greater chance of
having congenital heart disease than other children do.
Cong

genital heart disease than other children do.
Congenital defects of the heart are also frequently
associated with other congenital defects of the baby’s
body.
Use of Extracorporeal
Circulation During 
Cardiac Surgery
It is almost impossible to repair intracardiac defects
surgically while the heart is still pumping. Therefore,
many types of artiﬁcial heart-lung machines have been
developed to take the place of the heart and lungs
during the course of operation. Such a system is called
extracorpor

 of operation. Such a system is called
extracorporeal circulation. The system consists princi-
pally of a pump and an oxygenating device. Almost
any type of pump that does not cause hemolysis of the
blood seems to be suitable.
Methods used for oxygenating blood include (1)
bubbling oxygen through the blood and removing the
bubbles from the blood before passing it back into the
patient, (2) dripping the blood downward over the sur-
faces of plastic sheets in the presence of oxygen, (3)
passing th

c sheets in the presence of oxygen, (3)
passing the blood over surfaces of rotating discs, or (4)
passing the blood between thin membranes or through
thin tubes that are permeable to oxygen and carbon
dioxide.
The different systems have all been fraught with
many difﬁculties, including hemolysis of the blood,
development of small clots in the blood, likelihood of
small bubbles of oxygen or small emboli of antifoam
agent passing into the arteries of the patient, necessity
for large quantities of 

of the patient, necessity
for large quantities of blood to prime the entire
system, failure to exchange adequate quantities of
oxygen, and necessity to use heparin to prevent blood
coagulation in the extracorporeal system. Heparin also
interferes with adequate hemostasis during the surgi-
cal procedure. Yet despite these difﬁculties, in the
hands of experts, patients can be kept alive on artiﬁ-
cial heart-lung machines for many hours while opera-
tions are performed on the inside of the heart.
H

Hypertrophy of the Heart 
in Valvular and Congenital
Hypertrophy of cardiac muscle is one of the most
important mechanisms by which the heart adapts to
increased workloads, whether these loads are caused
by increased pressure against which the heart muscle
must contract or by increased cardiac output that 
must be pumped. Some physicians believe that the
increased strength of contraction of the heart muscle
causes the hypertrophy; ot

ion of the heart muscle
causes the hypertrophy; others believe that the
increased metabolic rate of the muscle is the primary
stimulus. Regardless of which of these is correct, one
can calculate approximately how much hypertrophy
will occur in each chamber of the heart by multiplying
ventricular output by the pressure against which the
ventricle must work, with emphasis on pressure. Thus,
hypertrophy occurs in most types of valvular and con-
genital disease, sometimes causing heart weights as
gr

tal disease, sometimes causing heart weights as
great as 800 grams instead of the normal 300 grams.
Arad M, Seidman JG, Seidman CE: Phenotypic diversity in
hypertrophic cardiomyopathy. Hum Mol Genet 11:2499,
2002.
Braunwald E, Seidman CE, Sigwart U: Contemporary eval-
uation and management of hypertrophic cardiomyopathy.
Circulation 106:1312, 2002.
Brickner ME, Hillis LD, Lange RA: Congenital heart disease
in adults: second of two parts. N Engl J Med 342:334, 2000.
Gottdiener JS: Over

s. N Engl J Med 342:334, 2000.
Gottdiener JS: Overview of stress echocardiography: uses,
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4
278
Circulatory Shock and 
Physiology of Its Treatment
Circulatory sho

k and 
Circulatory shock means generalized inadequate
blood ﬂow through the body, to the extent that the
body tissues are damaged because of too little ﬂow,
especially because of too little oxygen and other
nutrients delivered to the tissue cells. Even the car-
diovascular system itself—the heart musculature,
walls of the blood vessels, vasomotor system, and
other circulatory parts—begins to deteriorate, so
that the shock, once begun, is prone to become progressively 

ock, once begun, is prone to become progressively worse.
Circulatory Shock Caused by Decreased Cardiac Output
Shock usually results from inadequate cardiac output. Therefore, any condition
that reduces the cardiac output far below normal will likely lead to circulatory
shock. Two types of factors can severely reduce cardiac output:
1. Cardiac abnormalities that decrease the ability of the heart to pump blood.
These include especially myocardial infarction but also tox

lude especially myocardial infarction but also toxic states of 
the heart, severe heart valve dysfunction, heart arrhythmias, and other
conditions. The circulatory shock that results from diminished cardiac
pumping ability is called cardiogenic shock. This is discussed in detail in
Chapter 22, where it is pointed out that as many as 85 per cent of people
who develop cardiogenic shock do not survive.
2. Factors that decrease venous return also decrease cardiac output because
the heart cannot pump

rease cardiac output because
the heart cannot pump blood that does not ﬂow into it. The most common
cause of decreased venous return is diminished blood volume, but venous
return can also be reduced as a result of decreased vascular tone, especially
of the venous blood reservoirs, or obstruction to blood ﬂow at some point
in the circulation, especially in the venous return pathway to the heart.
Circulatory Shock That Occurs Without Diminished
Occasionally, the cardiac output is no

diac Output
Occasionally, the cardiac output is normal or even greater than normal, yet the
person is in circulatory shock. This can result from (1) excessive metabolism of
the body, so that even a normal cardiac output is inadequate, or (2) abnormal
tissue perfusion patterns, so that most of the cardiac output is passing through
blood vessels besides those that supply the local tissues with nutrition.
The speciﬁc causes of shock are discussed later in the chapter. For the present,
it is importa

ter in the chapter. For the present,
it is important to note that all of them lead to inadequate delivery of nutrients
to critical tissues and critical organs and also cause inadequate removal of cellu-
lar waste products from the tissues.
Chapter 24
Circulatory Shock and Physiology of Its Treatment
279
What Happens to the Arterial Pressure
in Circulatory Shock?
In the minds of many physicians, the arterial pressure
level is the principal measure of adequacy of circula-
tory function. However, t

 of adequacy of circula-
tory function. However, the arterial pressure can often
be seriously misleading. At times, a person may be in
severe shock and still have an almost normal arterial
pressure because of powerful nervous reﬂexes that
keep the pressure from falling. At other times, the 
arterial pressure can fall to half of normal, but the
person still has normal tissue perfusion and is not in
shock.
In most types of shock, especially shock caused by
severe blood loss, the arterial blood pre

aused by
severe blood loss, the arterial blood pressure decreases
at the same time as the cardiac output decreases,
although usually not as much.
Tissue Deterioration Is the End Result
of Circulatory Shock, Whatever 
the Cause
Once circulatory shock reaches a critical state of sever-
ity, regardless of its initiating cause, the shock itself
breeds more shock. That is, the inadequate blood ﬂow
causes the body tissues to begin deteriorating, includ-
ing the heart and circulatory system itself. Thi

-
ing the heart and circulatory system itself. This causes
an even greater decrease in cardiac output, and a
vicious circle ensues, with progressively increasing cir-
culatory shock, less adequate tissue perfusion, more
shock, and so forth until death. It is with this late stage
of circulatory shock that we are especially concerned,
because appropriate physiologic treatment can often
reverse the rapid slide to death.
Because the characteristics of circulatory shock
change with di

haracteristics of circulatory shock
change with different degrees of severity, shock is
divided into the following three major stages:
1. A nonprogressive stage (sometimes called 
the compensated stage), in which the normal
circulatory compensatory mechanisms eventually
cause full recovery without help from outside
therapy.
2. A progressive stage, in which, without therapy,
the shock becomes steadily worse until death.
3. An irreversible stage, in which the shock has
progressed to such an extent

n which the shock has
progressed to such an extent that all forms of
known therapy are inadequate to save the
person’s life, even though, for the moment, the
person is still alive.
Now, let us discuss the stages of circulatory shock
caused by decreased blood volume,which illustrate the
basic principles. Then we can consider special charac-
teristics of shock initiated by other causes.
Shock Caused by
Hypovolemia—Hemorrhagic
Shock
Hypovolemia means diminished blood volume. Hem-
orrhage is the mos

s diminished blood volume. Hem-
orrhage is the most common cause of hypovolemic
shock. Hemorrhage decreases the ﬁlling pressure of the
circulation and, as a consequence, decreases venous
return. As a result, the cardiac output falls below
normal, and shock may ensue.
Relationship of Bleeding Volume to
Figure 24–1 shows the approximate effects on both
cardiac output and arterial pressure of removing blood
from the circulatory system over a period of about 30
m

the circulatory system over a period of about 30
minutes. About 10 per cent of the total blood volume
can be removed with almost no effect on either arte-
rial pressure or cardiac output, but greater blood loss
usually diminishes the cardiac output ﬁrst and later the
arterial pressure, both of which fall to zero when about
35 to 45 per cent of the total blood volume has been
removed.
Sympathetic Reﬂex Compensations in Shock—Their Special
Value to Maintain Arterial Pressure. The decrease in arte-

 Maintain Arterial Pressure. The decrease in arte-
rial pressure after hemorrhage—as well as decreases
in pressures in the pulmonary arteries and veins in the
thorax—causes powerful sympathetic reﬂexes (initi-
ated mainly by the arterial baroreceptors and other
vascular stretch receptors, as explained in Chapter 18).
These reﬂexes stimulate the sympathetic vasoconstric-
tor system throughout the body, resulting in three
important effects: (1) The arterioles constrict in most
parts of the systemi

 arterioles constrict in most
parts of the systemic circulation, thereby increasing
the total peripheral resistance. (2) The veins and
venous reservoirs constrict, thereby helping to main-
tain adequate venous return despite diminished blood
 (percentage of normal)
0
10
20
30
40
50
0
50
100
Percentage of total blood removed
Arterial 
pressure
Cardiac
output
Figure 24–1
Effect of hemorrhage on cardiac output and arterial pressure.
280
Unit IV
v

d arterial pressure.
280
Unit IV
volume. (3) Heart activity increases markedly, some-
times increasing the heart rate from the normal value
of 72 beats/min to as high as 160 to 180 beats/min.
Value of the Sympathetic Nervous Reﬂexes. In the
absence of the sympathetic reﬂexes, only 15 to 20 per
cent of the blood volume can be removed over a
period of 30 minutes before a person dies; this is in
contrast to a 30 to 40 per cent loss of blood volume
that a person can sustain when the 

f blood volume
that a person can sustain when the reﬂexes are intact.
Therefore, the reﬂexes extend the amount of blood
loss that can occur without causing death to about
twice that which is possible in their absence.
Greater Effect of the Sympathetic Nervous Reﬂexes in
Maintaining Arterial Pressure than in Maintaining
Cardiac Output. Referring again to Figure 24–1, note
that the arterial pressure is maintained at or near
normal levels in the hemorrhaging person longer than
is the cardiac output

orrhaging person longer than
is the cardiac output. The reason for this is that the
sympathetic reﬂexes are geared more for maintaining
arterial pressure than for maintaining output. They
increase the arterial pressure mainly by increasing the
total peripheral resistance, which has no beneﬁcial
effect on cardiac output;however,the sympathetic con-
striction of the veins is important to keep venous return
and cardiac output from falling too much, in addition
to their role in maintaining arterial 

in addition
to their role in maintaining arterial pressure.
Especially interesting is the second plateau occur-
ring at about 50 mm Hg in the arterial pressure curve
of Figure 24–1. This results from activation of the
central nervous system ischemic response, which
causes extreme stimulation of the sympathetic nervous
system when the brain begins to suffer from lack of
oxygen or from excess buildup of carbon dioxide, as
discussed in Chapter 18. This effect of the central
nervous system ischemic 

his effect of the central
nervous system ischemic response can be called the
“last-ditch stand” of the sympathetic reﬂexes in their
attempt to keep the arterial pressure from falling too
low.
Protection of Coronary and Cerebral Blood Flow by 
the Reﬂexes. A special value of the maintenance 
of normal arterial pressure even in the presence of
decreasing cardiac output is protection of blood ﬂow
through the coronary and cerebral circulatory systems.
The sympathetic stimulation does not cause signi

.
The sympathetic stimulation does not cause signiﬁcant
constriction of either the cerebral or the cardiac
vessels. In addition, in both these vascular beds, local
blood ﬂow autoregulation is excellent, which prevents
moderate decreases in arterial pressure from signiﬁ-
cantly decreasing their blood ﬂows. Therefore, blood
ﬂow through the heart and brain is maintained essen-
tially at normal levels as long as the arterial pressure
does not fall below about 70 mm Hg, despite the fact
that blood ﬂo

low about 70 mm Hg, despite the fact
that blood ﬂow in some other areas of the body might
be decreased to as little as one third to one quarter
normal by this time because of vasoconstriction.
Progressive and Nonprogressive
Figure 24–2 shows an experiment that we performed
in dogs to demonstrate the effects of different degrees
of sudden acute hemorrhage on the subsequent course
of arterial pressure. The dogs were bled rapidly until
their arterial pressures fell to different le

ntil
their arterial pressures fell to different levels. Those
dogs whose pressures fell immediately to no lower
than 45 mm Hg (groups I, II, and III) all eventually
recovered; the recovery occurred rapidly if the pres-
sure fell only slightly (group I) but occurred slowly if
it fell almost to the 45 mm Hg level (group III). When
the arterial pressure fell below 45 mm Hg (groups IV,
V, and VI), all the dogs died, although many of them
hovered between life and death for hours before the
circulator

een life and death for hours before the
circulatory system deteriorated to the stage of death.
This experiment demonstrates that the circulatory
system can recover as long as the degree of hemor-
rhage is no greater than a certain critical amount.
Crossing this critical threshold by even a few milliliters
of blood loss makes the eventual difference between
life and death.Thus, hemorrhage beyond a certain crit-
ical level causes shock to become progressive. That is,
the shock itself causes still 

ogressive. That is,
the shock itself causes still more shock, and the condi-
tion becomes a vicious circle that eventually leads to
deterioration of the circulation and to death.
Nonprogressive Shock—Compensated Shock
If shock is not severe enough to cause its own pro-
gression, the person eventually recovers. Therefore,
shock of this lesser degree is called nonprogressive
(percentage of control value)
0
60
120
180
240
300
360
0
10
20
30
40
50
60
70
80
90
100
Time in minutes
I

10
20
30
40
50
60
70
80
90
100
I
II
III
IV
V
VI
Figure 24–2
Time course of arterial pressure in dogs after dif-
ferent degrees of acute hemorrhage. Each curve
represents average results from six dogs.
Chapter 24
281
shock. It is also called compensated shock, meaning
that the sympathetic reﬂexes and other factors com-
pensate enough to prevent further deterioration of the
circulation.
The factors that cause a person to recover fr

ion.
The factors that cause a person to recover from
moderate degrees of shock are all the negative feed-
back control mechanisms of the circulation that
attempt to return cardiac output and arterial pressure
back to normal levels. They include the following:
1. Baroreceptor reﬂexes, which elicit powerful
sympathetic stimulation of the circulation.
2. Central nervous system ischemic response,
which elicits even more powerful sympathetic
stimulation throughout the body but is not
activated signiﬁ

on throughout the body but is not
activated signiﬁcantly until the arterial pressure
falls below 50 mm Hg.
3. Reverse stress-relaxation of the circulatory system,
which causes the blood vessels to contract around
the diminished blood volume, so that the blood
volume that is available more adequately ﬁlls the
4. Formation of angiotensin by the kidneys, which
constricts the peripheral arteries and also causes
decreased output of water and salt by the 
kidneys, both of which help preve

and salt by the 
kidneys, both of which help prevent progression
of shock.
5. Formation of vasopressin (antidiuretic hormone)
by the posterior pituitary gland, which constricts
the peripheral arteries and veins and greatly
increases water retention by the kidneys.
6. Compensatory mechanisms that return the blood
volume back toward normal, including absorption
of large quantities of ﬂuid from the intestinal
tract, absorption of ﬂuid into the blood capillaries
from the interstitial spaces of the b

 capillaries
from the interstitial spaces of the body,
conservation of water and salt by the kidneys, and
increased thirst and increased appetite for salt,
which make the person drink water and eat salty
foods if able.
The sympathetic reﬂexes provide immediate help
toward bringing about recovery because they become
maximally activated within 30 seconds to a minute
after hemorrhage.
The angiotensin and vasopressin mechanisms, as
well as the reverse stress-relaxation that causes 
contraction of th

e stress-relaxation that causes 
contraction of the blood vessels and venous reservoirs,
all require 10 minutes to 1 hour to respond completely,
but they aid greatly in increasing the arterial pressure
or increasing the circulatory ﬁlling pressure and
thereby increasing the return of blood to the heart.
Finally, readjustment of blood volume by absorption
of ﬂuid from the interstitial spaces and intestinal tract,
as well as oral ingestion and absorption of additional
quantities of water and salt,

rption of additional
quantities of water and salt, may require from 1 to 48
hours, but recovery eventually takes place, provided
the shock does not become severe enough to enter the
progressive stage.
“Progressive Shock” Is Caused by a Vicious
Circle of Cardiovascular Deterioration
Figure 24–3 shows some of the positive feedbacks that
further depress cardiac output in shock, thus causing
the shock to become progressive. Some of the more
important feedbacks are the following.
Cardiac Depression. 

 feedbacks are the following.
Cardiac Depression. When the arterial pressure falls low
enough, coronary blood ﬂow decreases below that
required for adequate nutrition of the myocardium.
This weakens the heart muscle and thereby decreases
the cardiac output more. Thus, a positive feedback
cycle has developed, whereby the shock becomes more
and more severe.
Figure 24–4 shows cardiac output curves extrapo-
lated to the human heart from experiments in dogs,
demonstrating progressive deterioration of

n dogs,
demonstrating progressive deterioration of the heart
at different times after the onset of shock. A dog was
bled until the arterial pressure fell to 30 mm Hg, and
the pressure was held at this level by further bleeding
or retransfusion of blood as required. Note from the
second curve in the ﬁgure that there was little deteri-
oration of the heart during the ﬁrst 2 hours, but by 4
hours, the heart had deteriorated about 40 per cent;
then, rapidly, during the last hour of the experiment
(a

rapidly, during the last hour of the experiment
(after 4 hours of low coronary blood pressure), the
heart deteriorated completely.
Thus, one of the important features of progressive
shock, whether it is hemorrhagic in origin or caused in
another way, is eventual progressive deterioration of
the heart. In the early stages of shock, this plays very
little role in the condition of the person, partly because
deterioration of the heart is not severe during the ﬁrst
hour or so of shock, but mainly bec

uring the ﬁrst
hour or so of shock, but mainly because the heart has
tremendous reserve capability that normally allows 
it to pump 300 to 400 per cent more blood than is
required by the body for adequate bodywide tissue
nutrition. In the latest stages of shock, however, dete-
rioration of the heart is probably the most important
factor in the ﬁnal lethal progression of the shock.
Vasomotor Failure. In the early stages of shock, various
circulatory reﬂexes cause intense activity of the 
sympathe

ry reﬂexes cause intense activity of the 
sympathetic nervous system. This, as discussed earlier,
helps delay depression of the cardiac output and 
especially helps prevent decreased arterial pressure.
However, there comes a point when diminished blood
ﬂow to the brain’s vasomotor center depresses the
center so much that it, too, becomes progressively less
active and ﬁnally totally inactive. For instance, com-
plete circulatory arrest to the brain causes, during the
ﬁrst 4 to 8 minutes, the most

n causes, during the
ﬁrst 4 to 8 minutes, the most intense of all sympathetic
discharges,but by the end of 10 to 15 minutes,the vaso-
motor center becomes so depressed that no further
evidence of sympathetic discharge can be demon-
strated. Fortunately, the vasomotor center usually does
not fail in the early stages of shock if the arterial 
pressure remains above 30 mm Hg.
Blockage of Very Small Vessels—“Sludged Blood.” In time,
blockage occurs in many of the very small blood
vessels in the circ

n many of the very small blood
vessels in the circulatory system, and this also causes
the shock to progress. The initiating cause of this
blockage is sluggish blood ﬂow in the microvessels.
Because tissue metabolism continues despite the low
ﬂow, large amounts of acid, both carbonic acid and
282
Unit IV
lactic acid, continue to empty into the local blood
vessels and greatly increase the local acidity of the
blood. This acid, plus other deterioration products
from the ischemic ti

 other deterioration products
from the ischemic tissues, causes local blood aggluti-
nation, resulting in minute blood clots, leading to very
small plugs in the small vessels. Even if the vessels do
not become plugged, an increased tendency for the
blood cells to stick to one another makes it more dif-
ﬁcult for blood to ﬂow through the microvasculature,
giving rise to the term sludged blood.
Increased Capillary Permeability. After many hours of
capillary hypoxia and lack of other nutrients, the

capillary hypoxia and lack of other nutrients, the 
permeability of the capillaries gradually increases,
and large quantities of ﬂuid begin to transude into the
tissues. This decreases the blood volume even more,
with a resultant further decrease in cardiac output,
making the shock still more severe. Capillary hypoxia
does not cause increased capillary permeability until
the late stages of prolonged shock.
Release of Toxins by Ischemic Tissue. Throughout the
history of research in the ﬁeld of sh

roughout the
history of research in the ﬁeld of shock, it has been
suggested that shock causes tissues to release toxic
substances, such as histamine, serotonin, and tissue
enzymes, that cause further deterioration of the circu-
latory system. Quantitative studies have proved the
signiﬁcance of at least one toxin, endotoxin, in some
types of shock.
Cardiac Depression Caused by Endotoxin. Endotoxin
is released from the bodies of dead gram-negative 
bacteria in the intestines. Diminished blood ﬂow

bacteria in the intestines. Diminished blood ﬂow to 
the intestines often causes enhanced formation and
absorption of this toxic substance. The circulating
toxin then causes increased cellular metabolism
despite inadequate nutrition of the cells; this has a spe-
ciﬁc effect on the heart muscle, causing cardiac depres-
sion. Endotoxin can play a major role in some types
of shock, especially “septic shock,” discussed later in
Decreased systemic blood flow
Increased
capillary
permeab

ed systemic blood flow
Increased
capillary
Decreased nutrition of tissues
Decreased cardiac nutrition
Decreased nutrition of brain
Decreased venous return
Decreased
Tissue ischemia
Decreased cardiac output 
Decreased arterial pressure
Decreased vasomotor
activity
Cardiac depression
Vascular dilation
Venous pooling
of blood
Decreased nutrition
of vascular system
Release of
toxins
Intravascular clotting
Figure 24–3
Different types of “positive feedback” that can lead to p

nt types of “positive feedback” that can lead to progression of shock.
Chapter 24
283
Generalized Cellular Deterioration. As shock becomes
severe, many signs of generalized cellular deteriora-
tion occur throughout the body. One organ especially
affected is the liver, as illustrated in Figure 24–5. This
occurs mainly because of lack of enough nutrients to
support the normally high rate of metabolism in liver
cells, but also partly because of the 

sm in liver
cells, but also partly because of the extreme exposure
of the liver cells to any vascular toxin or other abnor-
mal metabolic factor occurring in shock.
Among the damaging cellular effects that are
known to occur in most body tissues are the following:
1. Active transport of sodium and potassium
through the cell membrane is greatly diminished.
As a result, sodium and chloride accumulate in
the cells, and potassium is lost from the cells. In
addition, the cells begin to swell.
2. Mito

ls. In
2. Mitochondrial activity in the liver cells, as well as
in many other tissues of the body, becomes
severely depressed.
3. Lysosomes in the cells in widespread tissue areas
begin to break open, with intracellular release of
hydrolases that cause further intracellular
deterioration.
4. Cellular metabolism of nutrients, such as glucose,
eventually becomes greatly depressed in the last
stages of shock. The actions of some hormones
are depressed as well, in

actions of some hormones
are depressed as well, including almost 100 per
cent depression of the action of insulin.
All these effects contribute to further deterioration
of many organs of the body, including especially (1)
the liver, with depression of its many metabolic and
detoxiﬁcation functions; (2) the lungs, with eventual
development of pulmonary edema and poor ability to
oxygenate the blood; and (3) the heart, thereby further
depressing its contractility.
Tissue Necrosis in Severe Shock—Pa

 contractility.
Tissue Necrosis in Severe Shock—Patchy Areas of
Necrosis Occur Because of Patchy Blood Flows in 
Different Organs. Not all cells of the body are equally
damaged by shock, because some tissues have better
blood supplies than others. For instance, the cells adja-
cent to the arterial ends of capillaries receive better
nutrition than cells adjacent to the venous ends of the
same capillaries. Therefore, one would expect more
nutritive deﬁciency around the venous ends of capil-
laries

 deﬁciency around the venous ends of capil-
laries than elsewhere. This is precisely the effect that
Crowell discovered in studying tissue areas in many
parts of the body. For instance, Figure 24–5 shows
necrosis in the center of a liver lobule, the portion of
the lobule that is last to be exposed to the blood as it
passes through the liver sinusoids.
Similar punctate lesions occur in heart muscle,
although here a deﬁnite repetitive pattern, such as
occurs in the liver, cannot be demonstrated. N

 as
occurs in the liver, cannot be demonstrated. Never-
theless, the cardiac lesions play an important role in
leading to the ﬁnal irreversible stage of shock. Deteri-
orative lesions also occur in the kidneys, especially in
the epithelium of the kidney tubules, leading to kidney
failure and occasionally uremic death several days
later. Deterioration of the lungs also often leads to res-
piratory distress and death several days later—called
the shock lung syndrome.
Acidosis in Shock. Most metabo

hock lung syndrome.
Acidosis in Shock. Most metabolic derangements that
occur in shocked tissue can lead to blood acidosis all
through the body. This results from poor delivery of
oxygen to the tissues, which greatly diminishes oxida-
tive metabolism of the foodstuffs. When this occurs,
the cells obtain most of their energy by the anaerobic
–4
0
4
8
12
0
5
10
15
0 time
2 hours
4 hours
43/4 hours
41/2 hours
5 hours
Figure 24–4
Cardiac output cu

s
41/2 hours
5 hours
Cardiac output curves of the heart at different times after hemor-
rhagic shock begins. (These curves are extrapolated to the human
heart from data obtained in dog experiments by Dr. J. W. Crowell.)
Figure 24–5
Necrosis of the central portion of a liver lobule in severe circula-
tory shock. (Courtesy Dr. J. W. Crowell.)
284
Unit IV
process of glycolysis, which leads to tremendous quan-
tities of excess lactic acid in the blood. In addition, poor
b

cess lactic acid in the blood. In addition, poor
blood ﬂow through tissues prevents normal removal 
of carbon dioxide. The carbon dioxide reacts locally 
in the cells with water to form high concentrations of
intracellular carbonic acid; this, in turn, reacts with
various tissue chemicals to form still other intracellu-
lar acidic substances.Thus, another deteriorative effect
of shock is both generalized and local tissue acidosis,
leading to further progression of the shock itself.
Positive Feed

her progression of the shock itself.
Positive Feedback Deterioration of Tissues 
in Shock and the Vicious Circle of 
Progressive Shock
All the factors just discussed that can lead to further
progression of shock are types of positive feedback.
That is, each increase in the degree of shock causes a
further increase in the shock.
However, positive feedback does not necessarily
lead to a vicious circle. Whether a vicious circle devel-
ops depends on the intensity of the positive feedback.
In mild d

 the intensity of the positive feedback.
In mild degrees of shock, the negative feedback mech-
anisms of the circulation—sympathetic reﬂexes,
reverse stress-relaxation mechanism of the blood
reservoirs, absorption of ﬂuid into the blood from the
interstitial spaces, and others—can easily overcome
the positive feedback inﬂuences and, therefore, cause
recovery. But in severe degrees of shock, the deterio-
rative feedback mechanisms become more and more
powerful, leading to such rapid deterioration

more
powerful, leading to such rapid deterioration of the
circulation that all the normal negative feedback
systems of circulatory control acting together cannot
return the cardiac output to normal.
Considering once again the principles of positive
feedback and vicious circle discussed in Chapter 1, one
can readily understand why there is a critical cardiac
output level above which a person in shock recovers
and below which a person enters a vicious circle of cir-
culatory deterioration that pro

ous circle of cir-
culatory deterioration that proceeds until death.
After shock has progressed to a certain stage, transfu-
sion or any other type of therapy becomes incapable
of saving the person’s life. The person is then said to
be in the irreversible stage of shock. Ironically, even in
this irreversible stage, therapy can, on rare occasions,
return the arterial pressure and even the cardiac
output to normal or near normal for short periods, but
the circulatory system neve

for short periods, but
the circulatory system nevertheless continues to dete-
riorate, and death ensues in another few minutes to
few hours.
Figure 24–6 demonstrates this effect, showing that
transfusion during the irreversible stage can some-
times cause the cardiac output (as well as the arterial
pressure) to return to normal. However, the cardiac
output soon begins to fall again, and subsequent trans-
fusions have less and less effect. By this time, multiple
deteriorative changes have occurre

 time, multiple
deteriorative changes have occurred in the muscle
cells of the heart that may not necessarily affect 
the heart’s immediate ability to pump blood but, over
a long period, depress heart pumping enough to cause
death. Beyond a certain point, so much tissue damage
has occurred, so many destructive enzymes have been
released into the body ﬂuids, so much acidosis has
developed, and so many other destructive factors are
now in progress that even a normal cardiac output for
a few minute

that even a normal cardiac output for
a few minutes cannot reverse the continuing deterio-
ration.Therefore, in severe shock, a stage is eventually
reached at which the person will die even though vig-
orous therapy might still return the cardiac output to
normal for short periods.
Depletion of Cellular High-Energy Phosphate Reserves in 
Irreversible Shock. The high-energy phosphate reserves
in the tissues of the body, especially in the liver and
the heart, are greatly diminished in severe degre

the heart, are greatly diminished in severe degrees of
shock. Essentially all the creatine phosphate has been
degraded, and almost all the adenosine triphosphate
has downgraded to adenosine diphosphate, adenosine
monophosphate, and, eventually, adenosine. Then
much of this adenosine diffuses out of the cells into the
circulating blood and is converted into uric acid, a 
substance that cannot re-enter the cells to reconstitute
the adenosine phosphate system. New adenosine can
be synthesized at a

hate system. New adenosine can
be synthesized at a rate of only about 2 per cent of 
the normal cellular amount an hour, meaning that
once the high-energy phosphate stores of the cells are
depleted, they are difﬁcult to replenish.
Thus, one of the most devastating end results of
deterioration in shock, and the one that is perhaps
most signiﬁcant for development of the ﬁnal state of
irreversibility, is this cellular depletion of these high-
energy compounds.
Hypovolemic Shock Caused 
by Plasma Lo

 compounds.
by Plasma Loss
Loss of plasma from the circulatory system, even
without loss of red blood cells, can sometimes be
severe enough to reduce the total blood volume
0
30
60
90
120
150
0
25
50
75
100
Minutes
stage
Irreversible shock
Figure 24–6
Failure of transfusion to prevent death in irreversible shock.
Chapter 24
285
markedly, causing typ

siology of Its Treatment
285
markedly, causing typical hypovolemic shock similar in
almost all details to that caused by hemorrhage.Severe
plasma loss occurs in the following conditions:
1. Intestinal obstruction is often a cause of severely
reduced plasma volume. Distention of the
intestine in intestinal obstruction partly blocks
venous blood ﬂow in the intestinal walls, which
increases intestinal capillary pressure. This in turn
causes ﬂuid to leak from the capillaries into the
intestinal wall

leak from the capillaries into the
intestinal walls and also into the intestinal lumen.
Because the lost ﬂuid has a high protein content,
the result is reduced total blood plasma protein as
well as reduced plasma volume.
2. In almost all patients who have severe burns or
other denuding conditions of the skin, so much
plasma is lost through the denuded skin areas 
that the plasma volume becomes markedly
reduced.
The hypovolemic shock that results from plasma
loss has almost the same characteristi

from plasma
loss has almost the same characteristics as the shock
caused by hemorrhage, except for one additional com-
plicating factor: the blood viscosity increases greatly as
a result of increased red blood cell concentration in
the remaining blood, and this exacerbates the slug-
gishness of blood ﬂow.
Loss of ﬂuid from all ﬂuid compartments of the
body is called dehydration; this, too, can reduce the
blood volume and cause hypovolemic shock similar to
that resulting from hemorrhage. Some of 

imilar to
that resulting from hemorrhage. Some of the causes of
this type of shock are (1) excessive sweating, (2) ﬂuid
loss in severe diarrhea or vomiting, (3) excess loss of
ﬂuid by nephrotic kidneys, (4) inadequate intake of
ﬂuid and electrolytes, or (5) destruction of the adrenal
cortices, with loss of aldosterone secretion and conse-
quent failure of the kidneys to reabsorb sodium,
chloride, and water, which occurs in the absence of 
the adrenocortical hormone aldosterone.
Hypovolemic Shock

enocortical hormone aldosterone.
by Trauma
One of the most common causes of circulatory shock
is trauma to the body. Often the shock results simply
from hemorrhage caused by the trauma, but it can also
occur even without hemorrhage, because extensive
contusion of the body can damage the capillaries 
sufﬁciently to allow excessive loss of plasma into the
tissues. This results in greatly reduced plasma volume,
with resultant hypovolemic shock.
Various attempts have been m

nt hypovolemic shock.
Various attempts have been made to implicate toxic
factors released by the traumatized tissues as one of
the causes of shock after trauma. However, cross-
transfusion experiments into normal animals have
failed to show signiﬁcant toxic elements.
In summary, traumatic shock seems to result 
mainly from hypovolemia, although there might also
be a moderate degree of concomitant neurogenic 
shock caused by loss of vasomotor tone, as discussed
next.
Neurogenic Shock—Increased
Va

, as discussed
next.
Vascular Capacity
Shock occasionally results without any loss of blood
volume. Instead, the vascular capacity increases so
much that even the normal amount of blood becomes
incapable of ﬁlling the circulatory system adequately.
One of the major causes of this is sudden loss of vaso-
motor tone throughout the body, resulting especially in
massive dilation of the veins. The resulting condition
is known as neurogenic shock.
The role of vascular capaci

n as neurogenic shock.
The role of vascular capacity in helping to regulate
circulatory function was discussed in Chapter 15,
where it was pointed out that either an increase in vas-
cular capacity or a decrease in blood volume reduces
the mean systemic ﬁlling pressure, which reduces
venous return to the heart. Diminished venous return
caused by vascular dilation is called venous pooling of
blood.
Causes of Neurogenic Shock. Some neurogenic factors
that can cause loss of vasomotor tone include t

rs
that can cause loss of vasomotor tone include the 
1. Deep general anesthesia often depresses the
vasomotor center enough to cause vasomotor
paralysis, with resulting neurogenic shock.
2. Spinal anesthesia, especially when this extends all
the way up the spinal cord, blocks the sympathetic
nervous outﬂow from the nervous system and can
be a potent cause of neurogenic shock.
3. Brain damage is often a cause of vasomotor
paralysis. Many patients who have had brain
concussion or contu

ny patients who have had brain
concussion or contusion of the basal regions of
the brain develop profound neurogenic shock.
Also, even though brain ischemia for a few
minutes almost always causes extreme vasomotor
stimulation, prolonged ischemia (lasting longer
than 5 to 10 minutes) can cause the opposite
effect—total inactivation of the vasomotor
neurons in the brain stem, with consequent
development of severe neurogenic shock.
Anaphylactic Shock and
Histamine Shock
Anaphylaxis is an allergic c

k and
Anaphylaxis is an allergic condition in which the
cardiac output and arterial pressure often decrease
drastically. This is discussed in Chapter 34. It results
primarily from an antigen-antibody reaction that takes
place immediately after an antigen to which the person
is sensitive enters the circulation. One of the principal
effects is to cause the basophils in the blood and mast
cells in the pericapillary tissues to release histamine or
a histamine-like substance. The hist

 histamine or
a histamine-like substance. The histamine causes (1) an
increase in vascular capacity because of venous dila-
tion, thus causing a marked decrease in venous return;
(2) dilation of the arterioles, resulting in greatly
reduced arterial pressure; and (3) greatly increased
capillary permeability, with rapid loss of ﬂuid and
286
Unit IV
protein into the tissue spaces. The net effect is a great
reduction in venous return and sometimes such
serious shock that the person d

and sometimes such
serious shock that the person dies within minutes.
Intravenous injection of large amounts of histamine
causes “histamine shock,” which has characteristics
almost identical to those of anaphylactic shock.
A condition that was formerly known by the popular
name “blood poisoning” is now called septic shock
by most clinicians. This refers to a bacterial infection
widely disseminated to many areas of the body, with
the infection being borne through the blood from one
t

infection being borne through the blood from one
tissue to another and causing extensive damage.There
are many varieties of septic shock because of the many
types of bacterial infections that can cause it and
because infection in different parts of the body 
produces different effects.
Septic shock is extremely important to the clinician,
because other than cardiogenic shock, septic shock is
the most frequent cause of shock-related death in the
modern hospital.
Some of the typical causes of sept

odern hospital.
Some of the typical causes of septic shock include
1. Peritonitis caused by spread of infection from the
uterus and fallopian tubes, sometimes resulting
from instrumental abortion performed under
unsterile conditions.
2. Peritonitis resulting from rupture of the
gastrointestinal system, sometimes caused by
intestinal disease and sometimes by wounds.
3. Generalized bodily infection resulting from
spread of a skin infection such as streptococcal 
or staphylococcal in

ection such as streptococcal 
or staphylococcal infection.
4. Generalized gangrenous infection resulting
speciﬁcally from gas gangrene bacilli, spreading
ﬁrst through peripheral tissues and ﬁnally by way
of the blood to the internal organs, especially the
liver.
5. Infection spreading into the blood from the
kidney or urinary tract, often caused by colon
bacilli.
Special Features of Septic Shock. Because of the multiple
types of septic shock, it is difﬁcult to categorize this
condition. Some fea

is difﬁcult to categorize this
condition. Some features often observed are:
1. High fever.
2. Often marked vasodilation throughout the body,
especially in the infected tissues.
3. High cardiac output in perhaps half of patients,
caused by arteriolar dilation in the infected tissues
and by high metabolic rate and vasodilation
elsewhere in the body, resulting from bacterial
toxin stimulation of cellular metabolism and from
high body temperature.
4. Sludging of the blood, caused by red cell
aggluti

 Sludging of the blood, caused by red cell
agglutination in response to degenerating tissues.
5. Development of micro–blood clots in widespread
areas of the body, a condition called disseminated
intravascular coagulation. Also, this causes the
blood clotting factors to be used up, so that
hemorrhaging occurs in many tissues, especially in
the gut wall of the intestinal tract.
In early stages of septic shock, the patient usually
does not have signs of circulatory collapse but only
signs of the ba

s of circulatory collapse but only
signs of the bacterial infection. As the infection
becomes more severe, the circulatory system usually
becomes involved either because of direct extension
of the infection or secondarily as a result of toxins
from the bacteria,with resultant loss of plasma into the
infected tissues through deteriorating blood capillary
walls. There ﬁnally comes a point at which deteriora-
tion of the circulation becomes progressive in the same
way that progression occurs in all

ive in the same
way that progression occurs in all other types of shock.
The end stages of septic shock are not greatly differ-
ent from the end stages of hemorrhagic shock, even
though the initiating factors are markedly different in
the two conditions.
Physiology of Treatment 
in Shock
Blood and Plasma Transfusion. If a person is in shock
caused by hemorrhage, the best possible therapy is
usually transfusion of whole blood. If the shock is
caused by plasma loss, the best th

If the shock is
caused by plasma loss, the best therapy is administra-
tion of plasma; when dehydration is the cause, admin-
istration of an appropriate electrolyte solution can
correct the shock.
Whole blood is not always available, such as under
battleﬁeld conditions. Plasma can usually substitute
adequately for whole blood because it increases the
blood volume and restores normal hemodynamics.
Plasma cannot restore a normal hematocrit, but the
human body can usually stand a decrease in hemat-

ocrit to about half of normal before serious con-
sequences result, if cardiac output is adequate.
Therefore, in emergency conditions, it is reasonable to
use plasma in place of whole blood for treatment of
hemorrhagic or most other types of hypovolemic
shock.
Sometimes plasma is unavailable. In these instances,
various plasma substitutes have been developed that
perform almost exactly the same hemodynamic func-
tions as plasma. One of these is 

emodynamic func-
tions as plasma. One of these is dextran solution.
Dextran Solution as a Plasma Substitute. The principal
requirement of a truly effective plasma substitute is
that it remain in the circulatory system—that is, not
ﬁlter through the capillary pores into the tissue spaces.
In addition, the solution must be nontoxic and must
contain appropriate electrolytes to prevent derange-
ment of the body’s extracellular ﬂuid electrolytes on
To remain in the circulation, the pl

ministration.
To remain in the circulation, the plasma substitute
must contain some substance that has a large enough
molecular size to exert colloid osmotic pressure. One
of the most satisfactory substances developed for this
Chapter 24
287
purpose is dextran, a large polysaccharide polymer of
glucose. Certain bacteria secrete dextran as a by-
product of their growth, and commercial dextran can
be manufactured using a bacterial culture procedure

e manufactured using a bacterial culture procedure.
By varying the growth conditions of the bacteria, the
molecular weight of the dextran can be controlled to
the desired value. Dextrans of appropriate molecular
size do not pass through the capillary pores and, there-
fore, can replace plasma proteins as colloid osmotic
agents.
Few toxic reactions have been observed when using
puriﬁed dextran to provide colloid osmotic pressure;
therefore, solutions containing this substance have
proved to be a 

ons containing this substance have
proved to be a satisfactory substitute for plasma in
most ﬂuid replacement therapy.
Treatment of Shock with
Sympathomimetic Drugs—Sometimes
Useful, Sometimes Not
A sympathomimetic drug is a drug that mimics sym-
pathetic stimulation. These drugs include norepineph-
rine, epinephrine, and a large number of long-acting
drugs that have the same effect as epinephrine and
norepinephrine.
In two types of shock, sympathomimetic drugs have
proved to be especially beneﬁ

homimetic drugs have
proved to be especially beneﬁcial. The ﬁrst of these is
neurogenic shock, in which the sympathetic nervous
system is severely depressed. Administering a sympa-
thomimetic drug takes the place of the diminished
sympathetic actions and can often restore full circula-
tory function.
The second type of shock in which sympatho-
mimetic drugs are valuable is anaphylactic shock, in
which excess histamine plays a prominent role. The
sympathomimetic drugs have a vasoconstrictor effec

sympathomimetic drugs have a vasoconstrictor effect
that opposes the vasodilating effect of histamine.
Therefore, either norepinephrine or another sympa-
thomimetic drug is often lifesaving.
Sympathomimetic drugs have not proved to be very
valuable in hemorrhagic shock. The reason is that in
this type of shock, the sympathetic nervous system is
almost always maximally activated by the circulatory
reﬂexes already; so much norepinephrine and epi-
nephrine are already circulating in the blood that 

ephrine are already circulating in the blood that sym-
pathomimetic drugs have essentially no additional
beneﬁcial effect.
Treatment by the Head-Down Position. When the pressure
falls too low in most types of shock, especially in 
hemorrhagic and neurogenic shock,placing the patient
with the head at least 12 inches lower than the feet
helps tremendously in promoting venous return,
thereby also increasing cardiac output. This head-
down position is the ﬁrst essential step in the tre

own position is the ﬁrst essential step in the treat-
ment of many types of shock.
Oxygen Therapy. Because the major deleterious effect of
most types of shock is too little delivery of oxygen to
the tissues, giving the patient oxygen to breathe can be
of beneﬁt in many instances. However, this frequently
is far less beneﬁcial than one might expect, because the
problem in most types of shock is not inadequate oxy-
genation of the blood by the lungs but inadequate
transport of the blood after it i

s but inadequate
transport of the blood after it is oxygenated.
Treatment with Glucocorticoids (Adrenal Cortex Hormones That
Control Glucose Metabolism). Glucocorticoids are fre-
quently given to patients in severe shock for several
reasons: (1) experiments have shown empirically that
glucocorticoids frequently increase the strength of the
heart in the late stages of shock; (2) glucocorticoids
stabilize lysosomes in tissue cells and thereby prevent
release of lysosomal enzymes into the cytoplasm

nt
release of lysosomal enzymes into the cytoplasm of the
cells, thus preventing deterioration from this source;
and (3) glucocorticoids might aid in the metabolism of
glucose by the severely damaged cells.
A condition closely allied to circulatory shock is cir-
culatory arrest, in which all blood ﬂow stops. This
occurs frequently on the surgical operating table as a
result of cardiac arrest or ventricular ﬁbrillation.
Ventricular ﬁbrillation can usually be stopped by
strong e

lar ﬁbrillation can usually be stopped by
strong electroshock of the heart, the basic principles
of which are described in Chapter 13.
Cardiac arrest often results from too little oxygen in
the anesthetic gaseous mixture or from a depressant
effect of the anesthesia itself.A normal cardiac rhythm
can usually be restored by removing the anesthetic and
immediately applying cardiopulmonary resuscitation
procedures, while at the same time supplying the
patient’s lungs with adequate quantities of ven

he
patient’s lungs with adequate quantities of ventilatory
oxygen.
Effect of Circulatory Arrest 
on the Brain
A special problem in circulatory arrest is to prevent
detrimental effects in the brain as a result of the arrest.
In general, more than 5 to 8 minutes of total circula-
tory arrest can cause at least some degree of perma-
nent brain damage in more than half of patients.
Circulatory arrest for as long as 10 to 15 minutes
almost always permanently destroys signiﬁcant
amounts of mental powe

anently destroys signiﬁcant
amounts of mental power.
For many years, it was taught that this detrimental
effect on the brain was caused by the acute cerebral
hypoxia that occurs during circulatory arrest.
However, experiments have shown that if blood clots
are prevented from occurring in the blood vessels of
the brain, this will also prevent most of the early dete-
rioration of the brain during circulatory arrest. For
instance,in animal experiments performed by Crowell,
all the blood was removed

ts performed by Crowell,
all the blood was removed from the animal’s blood
vessels at the beginning of circulatory arrest and 
then replaced at the end of circulatory arrest so that
no intravascular blood clotting could occur. In this
288
Unit IV
experiment, the brain was usually able to withstand up
to 30 minutes of circulatory arrest without permanent
brain damage.Also,administration of heparin or strep-
tokinase (to prevent blood coagulation) before cardiac
arrest was shown to

od coagulation) before cardiac
arrest was shown to increase the survivability of the
brain up to two to four times longer than usual.
It is likely that the severe brain damage that occurs
from circulatory arrest is caused mainly by permanent
blockage of many small blood vessels by blood clots,
thus leading to prolonged ischemia and eventual death
of the neurons.
Annane D, Sebille V, Charpentier C, et al: Effect of treat-
ment with low doses of hydrocortisone and ﬂudrocorti-
sone on mo

doses of hydrocortisone and ﬂudrocorti-
sone on mortality in patients with septic shock. JAMA
288:862, 2002.
Burry LD, Wax RS: Role of corticosteroids in septic shock.
Ann Pharmacother 38:464, 2004.
Creteur J, Vincent JL: Hemoglobin solutions. Crit Care Med
31(12 Suppl):S698, 2003.
Crowell JW, Guyton AC: Evidence favoring a cardiac mech-
anism in irreversible hemorrhagic shock. Am J Physiol
201:893, 1961.
Crowell JW, Smith EE: Oxygen deﬁcit and irreversible 
hemorrhagic shock. Am J Physiol 206:3

rreversible 
hemorrhagic shock. Am J Physiol 206:313, 1964.
de Jonge E, Levi M: Effects of different plasma substitutes
on blood coagulation: a comparative review. Crit Care
Med 29:1261, 2001.
Goodnough LT, Shander A: Evolution in alternatives to
blood transfusion. Hematol J 4:87, 2003.
Granger DN, Stokes KY, Shigematsu T, et al: Splanchnic
ischaemia-reperfusion injury: mechanistic insights pro-
vided by mutant mice. Acta Physiol Scand 173:83, 2001.
Guyton AC, Jones CE, Coleman TG: Circulatory P

01.
Saunders, 1973.
Ledgerwood AM, Lucas CE: A review of studies on the
effects of hemorrhagic shock and resuscitation on the
coagulation proﬁle. J Trauma 54(5 Suppl):S68, 2003.
Martin GS, Mannino DM, Eaton S, Moss M: The epidemiol-
ogy of sepsis in the United States from 1979 through 2000.
N Engl J Med 348:1546, 2003.
McLean-Tooke AP, Bethune CA, Fay AC, Spickett GP:
Adrenaline in the tr

hune CA, Fay AC, Spickett GP:
Adrenaline in the treatment of anaphylaxis: what is the
evidence? BMJ 327:1332, 2003.
Proctor KG: Blood substitutes and experimental models of
trauma. J Trauma 54(5 Suppl):S106, 2003.
Rivers E, Nguyen B, Havstad S, et al: The Early Goal-
Directed Therapy Collaborative Group. Early goal-
directed therapy in the treatment of severe sepsis and
septic shock. N Engl J Med 345:1368, 2001.
Toh CH, Dennis M: Disseminated intravascular coagulation:
old disease, new hope. BMJ

ravascular coagulation:
old disease, new hope. BMJ 327:974, 2003.
Wernly JA: Ischemia, reperfusion, and the role of surgery in
the treatment of cardiogenic shock secondary to acute
myocardial infarction: an interpretative review. J Surg Res
117:6, 2004.
Wilson M, Davis DP, Coimbra R: Diagnosis and monitoring
of hemorrhagic shock during the initial resuscitation of
multiple trauma patients: a review. J Emerg Med 24:413,
2003.
U
N
I
T
V
The Body Fluids 
25. The Body Fluid Compartments:

uids 
25. The Body Fluid Compartments: Extracellular and
Intracellular Fluids; Interstitial Fluid and Edema
26. Urine Formation by the Kidneys: I. Glomerular
Filtration, Renal Blood Flow, and Their Control
27. Urine Formation by the Kidneys: II. Tubular
Processing of the Glomerular Filtrate
28. Regulation of Extracellular Fluid Osmolarity 
29. Renal Regulation of Potassium, Calcium, 
Phosphate, and Magnesium; Integration of Renal
Mechanisms for Control of Blo

Integration of Renal
Mechanisms for Control of Blood Volume and
Extracellular Fluid Volume 
30. Regulation of Acid-Base Balance
31. Kidney Diseases and Diuretics
C
H
A
P
T
E
R
 
2
5
291
Extracellular and 
Intracellular Fluids;
The maintenance of a relatively constant volume
and a stable composition of the body ﬂuids is essen-
tial for homeostasis,as discussed in Chapter 1.Some
of the most common and important problems in
clinical medicine

common and important problems in
clinical medicine arise because of abnormalities in
the control systems that maintain this constancy of
the body ﬂuids. In this chapter and in the following
chapters on the kidneys, we discuss the overall reg-
ulation of body ﬂuid volume, constituents of the extracellular ﬂuid, acid-base
balance, and control of ﬂuid exchange between extracellular and intracellular
Fluid Intake and Output Are Balanced During
Steady-State Conditions
The relative const

 During
The relative constancy of the body ﬂuids is remarkable because there is con-
tinuous exchange of ﬂuid and solutes with the external environment as well as
within the different compartments of the body. For example, there is a highly
variable ﬂuid intake that must be carefully matched by equal output from the
body to prevent body ﬂuid volumes from increasing or decreasing.
Water is added to the body by two major sources: (1) it is ingested in 

 body by two major sources: (1) it is ingested in the form
of liquids or water in the food, which together normally add about 2100 ml/day
to the body ﬂuids, and (2) it is synthesized in the body as a result of oxidation
of carbohydrates, adding about 200 ml/day. This provides a total water intake
of about 2300 ml/day (Table 25–1). Intake of water, however, is highly variable
among different people and even within the same person on different days,
depending on climate, habits, and level of physi

,
depending on climate, habits, and level of physical activity.
Insensible Water Loss. Some of the water losses cannot be precisely regulated. For
example, there is a continuous loss of water by evaporation from the respira-
tory tract and diffusion through the skin, which together account for about 
700 ml/day of water loss under normal conditions. This is termed insensible 
water loss because we are not consciously aware of it, even though it occurs con-
tinually in al

e of it, even though it occurs con-
tinually in all living humans.
The insensible water loss through the skin occurs independently of sweating
and is present even in people who are born without sweat glands; the average
water loss by diffusion through the skin is about 300 to 400 ml/day. This loss is
minimized by the cholesterol-ﬁlled corniﬁed layer of the skin, which provides a
barrier against excessive loss by diffusion. When the corniﬁed layer becomes
292
Unit V
de

 becomes
292
Unit V
denuded, as occurs with extensive burns, the rate of
evaporation can increase as much as 10-fold, to 3 to 
5 L/day. For this reason, burn victims must be given
large amounts of ﬂuid, usually intravenously, to
balance ﬂuid loss.
Insensible water loss through the respiratory tract
averages about 300 to 400 ml/day. As air enters the 
respiratory tract, it becomes saturated with moisture,
to a vapor pressure of about 47 mm Hg, before it is
expelled. Be

ssure of about 47 mm Hg, before it is
expelled. Because the vapor pressure of the inspired
air is usually less than 47 mm Hg, water is continuously
lost through the lungs with respiration. In cold
weather, the atmospheric vapor pressure decreases to
nearly 0, causing an even greater loss of water from
the lungs as the temperature decreases. This explains
the dry feeling in the respiratory passages in cold
weather.
Fluid Loss in Sweat. The amount of water lost by sweat-
ing is highly variable, de

of water lost by sweat-
ing is highly variable, depending on physical activity
and environmental temperature. The volume of sweat
normally is about 100 ml/day, but in very hot weather
or during heavy exercise, water loss in sweat occa-
sionally increases to 1 to 2 L/hour. This would rapidly
deplete the body ﬂuids if intake were not also
increased by activating the thirst mechanism discussed
in Chapter 29.
Water Loss in Feces. Only a small amount of water 
(100 ml/day) normally is lost in the fec

of water 
(100 ml/day) normally is lost in the feces. This can
increase to several liters a day in people with severe
diarrhea. For this reason, severe diarrhea can be life
threatening if not corrected within a few days.
Water Loss by the Kidneys. The remaining water loss from
the body occurs in the urine excreted by the kidneys.
There are multiple mechanisms that control the rate
of urine excretion. In fact, the most important means
by which the body maintains a balance between water
intake and

 body maintains a balance between water
intake and output, as well as a balance between intake
and output of most electrolytes in the body, is by con-
trolling the rates at which the kidneys excrete these
substances. For example, urine volume can be as low
as 0.5 L/day in a dehydrated person or as high as 
20 L/day in a person who has been drinking tremen-
dous amounts of water.
This variability of intake is also true for most of the
electrolytes of the body, such as sodium, chloride, and
potass

 of the body, such as sodium, chloride, and
potassium. In some people, sodium intake may be as
low as 20 mEq/day, whereas in others, sodium intake
may be as high as 300 to 500 mEq/day.The kidneys are
faced with the task of adjusting the excretion rate of
water and electrolytes to match precisely the intake of
these substances, as well as compensating for excessive
losses of ﬂuids and electrolytes that occur in certain
disease states. In Chapters 26 through 30, we discuss
the mechanisms that allo

26 through 30, we discuss
the mechanisms that allow the kidneys to perform
these remarkable tasks.
The total body ﬂuid is distributed mainly between two
compartments: the extracellular ﬂuid and the intracel-
lular ﬂuid (Figure 25–1). The extracellular ﬂuid is
divided into the interstitial ﬂuid and the blood plasma.
There is another small compartment of ﬂuid that is
referred to as transcellular ﬂuid. This compartment
includes ﬂuid in the synovial, peritoneal, pericardial,

es ﬂuid in the synovial, peritoneal, pericardial,
and intraocular spaces, as well as the cerebrospinal
ﬂuid; it is usually considered to be a specialized type
of extracellular ﬂuid, although in some cases, its com-
position may differ markedly from that of the plasma
or interstitial ﬂuid.All the transcellular ﬂuids together
constitute about 1 to 2 liters.
Table 25–1
Daily Intake and Output of Water (ml/day)
Prolonged,
Normal
Heavy Exercise
Intake
Fluids ingested
2100
?
From metabolism
200
200
To

2100
?
200
200
Total intake
2300
?
Output
Insensible—skin
350
350
Insensible—lungs
350
650
Sweat
100
5000
Feces
100
100
Urine
1400
500
Total output
2300
6600
Plasma
3.0 L
fluid
11.0 L
INTAKE
fluid
28.0 L
Capillary membrane
Lymphatics
fluid (14.0 L)
OUTPUT
•Kidneys
•Lungs
•Feces
•Sweat
•Skin
Figure 25–1
Summary of body ﬂuid regulation, including the major body ﬂuid
compartments and the membranes that separate 

ﬂuid
compartments and the membranes that separate these compart-
ments. The values shown are for an average 70-kilogram person.
Chapter 25
The Body Fluid Compartments: Extracellular and Intracellular Fluids; Interstitial Fluid and Edema
293
In the average 70-kilogram adult human, the total
body water is about 60 per cent of the body weight, or
about 42 liters.This percentage can change, depending
on age, gender, and degree of obesity. As a person
grows older, the percentage of total body weight 

grows older, the percentage of total body weight that
is ﬂuid gradually decreases. This is due in part to the
fact that aging is usually associated with an increased
percentage of the body weight being fat, which
decreases the percentage of water in the body.Because
women normally have more body fat than men, they
contain slightly less water than men in proportion to
their body weight. Therefore, when discussing the
“average” body ﬂuid compartments, we should realize
that variations exist, depe

nts, we should realize
that variations exist, depending on age, gender, and
percentage of body fat.
About 28 of the 42 liters of ﬂuid in the body are inside
the 75 trillion cells and are collectively called the intra-
cellular ﬂuid. Thus, the intracellular ﬂuid constitutes
about 40 per cent of the total body weight in an
“average” person.
The ﬂuid of each cell contains its individual mixture
of different constituents, but the concentrations of
these substances are

ts, but the concentrations of
these substances are similar from one cell to another.
In fact, the composition of cell ﬂuids is remarkably
similar even in different animals, ranging from the
most primitive microorganisms to humans. For this
reason, the intracellular ﬂuid of all the different 
cells together is considered to be one large ﬂuid 
compartment.
Compartment
All the ﬂuids outside the cells are collectively called
the extracellular ﬂuid.Together these ﬂuids account for

xtracellular ﬂuid.Together these ﬂuids account for
about 20 per cent of the body weight, or about 14 liters
in a normal 70-kilogram adult. The two largest com-
partments of the extracellular ﬂuid are the interstitial
ﬂuid, which makes up more than three fourths of the
extracellular ﬂuid, and the plasma, which makes up
almost one fourth of the extracellular ﬂuid, or about 3
liters. The plasma is the noncellular part of the blood;
it exchanges substances continuously with the intersti-
tial ﬂuid t

tances continuously with the intersti-
tial ﬂuid through the pores of the capillary mem-
branes.These pores are highly permeable to almost all
solutes in the extracellular ﬂuid except the proteins.
Therefore, the extracellular ﬂuids are constantly
mixing, so that the plasma and interstitial ﬂuids have
about the same composition except for proteins, which
have a higher concentration in the plasma.
Blood contains both extracellular ﬂuid (the ﬂuid in
plasma) and intracellular ﬂuid (the

d (the ﬂuid in
plasma) and intracellular ﬂuid (the ﬂuid in the red
blood cells). However, blood is considered to be a sep-
arate ﬂuid compartment because it is contained in a
chamber of its own, the circulatory system. The blood
volume is especially important in the control of car-
diovascular dynamics.
The average blood volume of adults is about 7 per
cent of body weight, or about 5 liters. About 60 per
cent of the blood is plasma and 40 per cent is red blood
cells, but these percentages can va

t is red blood
cells, but these percentages can vary considerably in
different people, depending on gender, weight, and
other factors.
Hematocrit (Packed Red Cell Volume). The hematocrit is
the fraction of the blood composed of red blood cells,
as determined by centrifuging blood in a “hematocrit
tube” until the cells become tightly packed in the
bottom of the tube. It is impossible to completely pack
the red cells together; therefore, about 3 to 4 per cent
of the plasma remains entrapped among 

 4 per cent
of the plasma remains entrapped among the cells, and
the true hematocrit is only about 96 per cent of the
measured hematocrit.
In men, the measured hematocrit is normally about
0.40, and in women, it is about 0.36. In severe anemia,
the hematocrit may fall as low as 0.10, a value that is
barely sufﬁcient to sustain life. Conversely, there are
some conditions in which there is excessive production
of red blood cells, resulting in polycythemia. In these
conditions, the hematocrit can r

ythemia. In these
conditions, the hematocrit can rise to 0.65.
Constituents of Extracellular
and Intracellular Fluids
Comparisons of the composition of the extracellular
ﬂuid,including the plasma and interstitial ﬂuid,and the
intracellular ﬂuid are shown in Figures 25–2 and 25–3
and in Table 25–2.
Ionic Composition of Plasma and
Because the plasma and interstitial ﬂuid are separated
only by highly permeable capillary membranes, their
ionic composition is similar. Th

 membranes, their
ionic composition is similar. The most important dif-
ference between these two compartments is the higher
concentration of protein in the plasma; because the
capillaries have a low permeability to the plasma pro-
teins, only small amounts of proteins are leaked into
the interstitial spaces in most tissues.
Because of the Donnan effect, the concentration of
positively charged ions (cations) is slightly greater
(about 2 per cent) in the plasma than in the intersti-
tial ﬂuid. Th

 in the plasma than in the intersti-
tial ﬂuid. The plasma proteins have a net negative
charge and, therefore, tend to bind cations, such as
sodium and potassium ions, thus holding extra
amounts of these cations in the plasma along with the
plasma proteins. Conversely, negatively charged ions
(anions) tend to have a slightly higher concentration
in the interstitial ﬂuid compared with the plasma,
because the negative charges of the plasma proteins
repel the negatively charged anions. For practica

repel the negatively charged anions. For practical 
purposes, however, the concentration of ions in the
294
Unit V
–––
EXTRACELLULAR
Cations
Anions
mEq/L
INTRACELLULAR
150
150
100
100
50
50
0
Ca++
Na+
HCO3
-
PO  4 and organic anions
Protein
Mg++
K+
Cl-
Figure 25–2
Major cations and anions of the intracellular and extracellular
ﬂuids. The concentrations of Ca++ and Mg++ represent the sum of
these two ions. The concentrations shown represent the total of
free ions and 

ations shown represent the total of
free ions and complexed ions.
Phospholipids – 280 mg/dl
Cholesterol – 150 mg/dl
Glucose – 100 mg/dl
Urea – 15 mg/dl
Lactic acid – 10 mg/dl
Uric acid – 3 mg/dl
Creatinine – 1.5 mg/dl
Bilirubin – 0.5 mg/dl
Bile salts – trace
Neutral fat – 125 mg/dl
Figure 25–3
Nonelectrolytes of the plasma.
Table 25–2
Osmolar Substances in Extracellular and Intracellular Fluids
Plasma (mOsm/L H2O)
Interstitial (mOsm/L H2O)
Intracellular (mOsm/L H2O)
Na+
142
139
14
K+
4.2
4.0
140

ellular (mOsm/L H2O)
Na+
142
139
14
K+
4.2
4.0
140
Ca++
1.3
1.2
0
Mg+
0.8
0.7
20
Cl–
108
108
4
HCO3
–
24
28.3
10
HPO4
–, H2PO4
–
2
2
11
SO4
–
0.5
0.5
1
Phosphocreatine
45
Carnosine
14
2
2
8
Creatine
0.2
0.2
9
Lactate
1.2
1.2
1.5
5
Hexose monophosphate
3.7
Glucose
5.6
5.6
Protein
1.2
0.2
4
Urea
4
4
4
Others
4.8
3.9
10
Total mOsm/L
301.8
300.8
301.2
Corrected osmolar activity (mOsm/L)
282.0
281.0
281.0
Total osmotic pressure at 37∞C (mm Hg)
5443
5423
5423
Chapter

ic pressure at 37∞C (mm Hg)
5443
5423
5423
Chapter 25
295
interstitial ﬂuid and in the plasma is considered to be
about equal.
Referring again to Figure 25–2, one can see that 
the extracellular ﬂuid, including the plasma and the
interstitial ﬂuid, contains large amounts of sodium 
and chloride ions, reasonably large amounts of bicar-
bonate ions, but only small quantities of potassium,
calcium, mag

t only small quantities of potassium,
calcium, magnesium, phosphate, and organic acid ions.
The composition of extracellular ﬂuid is carefully
regulated by various mechanisms, but especially by the
kidneys, as discussed later. This allows the cells to
remain continually bathed in a ﬂuid that contains the
proper concentration of electrolytes and nutrients for
optimal cell function.
Important Constituents of the
Intracellular Fluid
The intracellular ﬂuid is separated from the extracel-
lular ﬂuid 

r ﬂuid is separated from the extracel-
lular ﬂuid by a cell membrane that is highly permeable
to water but not to most of the electrolytes in the body.
In contrast to the extracellular ﬂuid, the intracellu-
lar ﬂuid contains only small quantities of sodium and
chloride ions and almost no calcium ions. Instead, it
contains large amounts of potassium and phosphate
ions plus moderate quantities of magnesium and
sulfate ions, all of which have low concentrations in the
extracellular ﬂuid.Also, cells

ncentrations in the
extracellular ﬂuid.Also, cells contain large amounts of
protein, almost four times as much as in the plasma.
Measurement of Fluid
Volumes in the Different Body
Fluid Compartments—The
Indicator-Dilution Principle
The volume of a ﬂuid compartment in the body can be
measured by placing an indicator substance in the
compartment, allowing it to disperse evenly through-
out the compartment’s ﬂuid, and then analyzing the
extent to which the substance becomes diluted. Figure
25–4 sho

ich the substance becomes diluted. Figure
25–4 shows this “indicator-dilution” method of meas-
uring the volume of a ﬂuid compartment, which is
based on the principle of conservation of mass. This
means that the total mass of a substance after disper-
sion in the ﬂuid compartment will be the same as the
total mass injected into the compartment.
In the example shown in Figure 25–4, a small
amount of dye or other substance contained in the
syringe is injected into a chamber, and the substance
is a

is injected into a chamber, and the substance
is allowed to disperse throughout the chamber until it
becomes mixed in equal concentrations in all areas.
Then a sample of ﬂuid containing the dispersed sub-
stance is removed and the concentration is analyzed
chemically, photoelectrically, or by other means. If
none of the substance leaks out of the compartment,
the total mass of substance in the compartment
(Volume B ¥ Concentration B) will equal the total
mass of the substance injected (Volume A 

he total
mass of the substance injected (Volume A ¥ Con-
centration A). By simple rearrangement of the equa-
tion, one can calculate the unknown volume of
chamber B as
Note that all one needs to know for this calculation 
is (1) the total amount of substance injected into 
the chamber (the numerator of the equation) and (2)
the concentration of the ﬂuid in the chamber after the
substance has been dispersed (the denominator).
For example, if 1 milliliter of a solution containing 
10 mg/ml of dye 

liliter of a solution containing 
10 mg/ml of dye is dispersed into chamber B and 
the ﬁnal concentration in the chamber is 0.01 mil-
ligram for each milliliter of ﬂuid, the unknown volume
of the chamber can be calculated as follows:
This method can be used to measure the volume of
virtually any compartment in the body as long as (1)
the indicator disperses evenly throughout the com-
partment, (2) the indicator disperses only in the com-
partment that is being measured, and (3) the indicator
is 

 that is being measured, and (3) the indicator
is not metabolized or excreted. Several substances can
be used to measure the volume of each of the differ-
ent body ﬂuids.
Determination of Volumes 
of Speciﬁc Body Fluid
Compartments
Measurement of Total Body Water. Radioactive water
(tritium, 3H2O) or heavy water (deuterium, 2H2O) can
be used to measure total body water. These forms of
water mix with the total body water within a few hours
after being injected into the blood, and the dilution
Vol

eing injected into the blood, and the dilution
Volume B
ml
10 mg ml
0.01 mg ml
ml
=
¥
=
1
1000
Volume B
Volume A
Concentration A
Concentration B
=
¥
Indicator Mass A = Volume A x Concentration A
Indicator Mass B = Volume B x Concentration B
Volume B = Indicator Mass B / Concentration B
Indicator Mass A = Indicator Mass B
Figure 25–4
Indicator-dilution method for measuring ﬂuid volumes.
296
Unit V
principle can be used to calculate total body water
(Table 25–3). Anothe

to calculate total body water
(Table 25–3). Another substance that has been used to
measure total body water is antipyrine, which is very
lipid soluble and can rapidly penetrate cell membranes
and distribute itself uniformly throughout the intracel-
lular and extracellular compartments.
Measurement of Extracellular Fluid Volume. The volume of
extracellular ﬂuid can be estimated using any of several
substances that disperse in the plasma and interstitial
ﬂuid but do not readily permeate the cell 

stitial
ﬂuid but do not readily permeate the cell membrane.
They include radioactive sodium, radioactive chloride,
radioactive iothalamate, thiosulfate ion, and inulin.
When any one of these substances is injected into the
blood, it usually disperses almost completely through-
out the extracellular ﬂuid within 30 to 60 minutes. Some
of these substances, however, such as radioactive
sodium, may diffuse into the cells in small amounts.
Therefore, one frequently speaks of the sodium space or
the in

ne frequently speaks of the sodium space or
the inulin space, instead of calling the measurement the
true extracellular ﬂuid volume.
Calculation 
of 
Intracellular 
Volume. The 
intracellular
volume cannot be measured directly. However, it can be
calculated as
Intracellular volume = Total body water 
– Extracellular volume
Measurement of Plasma Volume. To measure plasma
volume, a substance must be used that does not readily
penetrate capillary membranes but remains in the vas-
cular system after

mbranes but remains in the vas-
cular system after injection. One of the most commonly
used substances for measuring plasma volume is serum
albumin labeled with radioactive iodine (125I-albumin).
Also, dyes that avidly bind to the plasma proteins, such
as Evans blue dye (also called T-1824), can be used to
measure plasma volume.
Calculation of Interstitial Fluid Volume. Interstitial ﬂuid
volume cannot be measured directly, but it can be cal-
culated as
Interstitial ﬂuid volume = Extracellular ﬂu

ted as
Interstitial ﬂuid volume = Extracellular ﬂuid volume 
– Plasma volume
Measurement of Blood Volume. If one measures plasma
volume using the methods described earlier, blood
volume can also be calculated if one knows the 
hematocrit (the fraction of the total blood volume com-
posed of cells), using the following equation:
For example, if plasma volume is 3 liters and hemat-
ocrit is 0.40, total blood volume would be calculated as
Another way to measure blood volume is to inject
into the ci

y to measure blood volume is to inject
into the circulation red blood cells that have been
labeled with radioactive material.After these mix in the
circulation, the radioactivity of a mixed blood sample
can be measured, and the total blood volume can be cal-
culated using the dilution principle. A substance fre-
quently used to label the red blood cells is radioactive
chromium (51Cr), which binds tightly with the red blood
cells.
Regulation of Fluid Exchange
and Osmotic Equilibrium
Between Intra

uid Exchange
Between Intracellular and
A frequent problem in treating seriously ill patients is
maintaining adequate ﬂuids in one or both of the intra-
cellular and extracellular compartments. As discussed
in Chapter 16 and later in this chapter, the relative
amounts of extracellular ﬂuid distributed between the
plasma and interstitial spaces are determined mainly
by the balance of hydrostatic and colloid osmotic
forces across the capillary membranes.

id osmotic
The distribution of ﬂuid between intracellular and
extracellular compartments, in contrast, is determined
mainly by the osmotic effect of the smaller solutes—
especially sodium, chloride, and other electrolytes—
acting across the cell membrane. The reason for this is
that the cell membranes are highly permeable to water
but relatively impermeable to even small ions such as
sodium and chloride. Therefore, water moves across
the cell membrane rapid

refore, water moves across
the cell membrane rapidly, so that the intracellular
ﬂuid remains isotonic with the extracellular ﬂuid.
In the next section, we discuss the interrelations
between intracellular and extracellular ﬂuid volumes
and the osmotic factors that can cause shifts of ﬂuid
between these two compartments.
Basic Principles of Osmosis
and Osmotic Pressure
The basic principles of osmosis and osmotic pressure
were presented in Chapter 4. Therefore, we review
here only the most importan

. Therefore, we review
here only the most important aspects of these princi-
ples as they apply to volume regulation.
Osmosis is the net diffusion of water across a selec-
tively permeable membrane from a region of high water
concentration to one that has a lower water concentra-
tion. When a solute is added to pure water, this reduces
3 liters
1
liters
-
=
0 4
5
.
Total blood volume
Plasma volume
1
=
-
Table 25–3
Measurement of Body Fluid Volumes
Volume
Indicators
Total body water
3H

luid Volumes
Volume
3H2O, 2H2O, antipyrine
Extracellular ﬂuid
22Na, 125I-iothalamate, thiosulfate, inulin
Intracellular ﬂuid
(Calculated as Total body water – 
Extracellular ﬂuid volume)
125I-albumin, Evans blue dye (T-1824)
Blood volume
51Cr-labeled red blood cells, or calculated 
as Blood volume = Plasma volume/
(1 - Hematocrit)
Interstitial ﬂuid
(Calculated as Extracellular ﬂuid 
volume - Plasma volume)
From Guyton AC, Hall JE: Human Physiology and Me

)
From Guyton AC, Hall JE: Human Physiology and Mechanisms of Disease,
6th ed. Philadelphia: WB Saunders, 1997.
Chapter 25
297
the concentration of water in the mixture. Thus, the
higher the solute concentration in a solution, the lower
the water concentration. Further, water diffuses from
a region of low solute concentration (high water con-
centration) to one with a high solute concentration
(low 

ion) to one with a high solute concentration
(low water concentration).
Because cell membranes are relatively impermeable
to most solutes but highly permeable to water (i.e.,
selectively permeable), whenever there is a higher
concentration of solute on one side of the cell mem-
brane, water diffuses across the membrane toward the
region of higher solute concentration. Thus, if a solute
such as sodium chloride is added to the extracellular
ﬂuid, water rapidly diffuses from the cells through the
c

ater rapidly diffuses from the cells through the
cell membranes into the extracellular ﬂuid until the
water concentration on both sides of the membrane
becomes equal. Conversely, if a solute such as sodium
chloride is removed from the extracellular ﬂuid, water
diffuses from the extracellular ﬂuid through the cell
membranes and into the cells. The rate of diffusion of
water is called the rate of osmosis.
Relation Between Moles and Osmoles. Because the water
concentration of a solution depends on 

 the water
concentration of a solution depends on the number of
solute particles in the solution, a concentration term is
needed to describe the total concentration of solute
particles, regardless of their exact composition. The
total number of particles in a solution is measured in
osmoles. One osmole (osm) is equal to 1 mole (mol)
(6.02 ¥ 1023) of solute particles. Therefore, a solution
containing 1 mole of glucose in each liter has a con-
centration of 1 osm/L. If a molecule dissociates into

ration of 1 osm/L. If a molecule dissociates into
two ions (giving two particles), such as sodium chlo-
ride ionizing to give chloride and sodium ions, then a
solution containing 1 mol/L will have an osmolar con-
centration of 2 osm/L. Likewise, a solution that con-
tains 1 mole of a molecule that dissociates into three
ions, such as sodium sulfate (Na2SO4), will contain 
3 osm/L. Thus, the term osmole refers to the number
of osmotically active particles in a solution rather than
to the molar co

articles in a solution rather than
to the molar concentration.
In general, the osmole is too large a unit for express-
ing osmotic activity of solutes in the body ﬂuids.
The term milliosmole (mOsm), which equals 1/1000
osmole, is commonly used.
Osmolality and Osmolarity. The osmolal concentration of
a solution is called osmolality when the concentration
is expressed as osmoles per kilogram of water; it is
called osmolarity when it is expressed as osmoles per
liter of solution. In dilute solution

 osmoles per
liter of solution. In dilute solutions such as the body
ﬂuids, these two terms can be used almost synony-
mously because the differences are small. In most
cases, it is easier to express body ﬂuid quantities in
liters of ﬂuid rather than in kilograms of water.There-
fore, most of the calculations used clinically and the
calculations expressed in the next several chapters are
based on osmolarities rather than osmolalities.
Osmotic Pressure. Osmosis of water molecules across a
selecti

ssure. Osmosis of water molecules across a
selectively permeable membrane can be opposed by
applying a pressure in the direction opposite that of
the osmosis. The precise amount of pressure required
to prevent the osmosis is called the osmotic pressure.
Osmotic pressure, therefore, is an indirect measure-
ment of the water and solute concentrations of a solu-
tion. The higher the osmotic pressure of a solution,
the lower the water concentration and the higher the
solute concentration of the solu

nd the higher the
solute concentration of the solution.
Relation Between Osmotic Pressure and Osmolarity. The
osmotic pressure of a solution is directly proportional
to the concentration of osmotically active particles in
that solution. This is true regardless of whether the
solute is a large molecule or a small molecule. For
example, one molecule of albumin with a molecular
weight of 70,000 has the same osmotic effect as one
molecule of glucose with a molecular weight of 180.
One molecule of so

with a molecular weight of 180.
One molecule of sodium chloride, however, has two
osmotically active particles, Na+ and Cl–, and therefore
has twice the osmotic effect of either an albumin mol-
ecule or a glucose molecule.Thus, the osmotic pressure
of a solution is proportional to its osmolarity, a
measure of the concentration of solute particles.
Expressed mathematically, according to van’t Hoff’s
law, osmotic pressure (p) can be calculated as
p = CRT
where C is the concentration of solutes in 

 = CRT
where C is the concentration of solutes in osmoles per
liter, R is the ideal gas constant, and T is the absolute
temperature in degrees kelvin (273° + centigrade°). If
p is expressed in millimeters of mercury (mm Hg), the
unit of pressure commonly used for biological ﬂuids,
and T is normal body temperature (273° + 37° =
310° kelvin), the value of p calculates to be about
19,300 mm Hg for a solution having a concentration 
of 1 osm/L. This means that for a concentration of 
1 mOsm/L, p is 

means that for a concentration of 
1 mOsm/L, p is equal to 19.3 mm Hg. Thus, for each
milliosmole concentration gradient across the cell
membrane, 19.3 mm Hg osmotic pressure is exerted.
Calculation of the Osmolarity and Osmotic Pressure of a 
Solution. Using van’t Hoff’s law, one can calculate the
potential osmotic pressure of a solution, assuming that
the cell membrane is impermeable to the solute.
For example, the osmotic pressure of a 0.9 per cent
sodium chloride solution is calculated as fo

 cent
sodium chloride solution is calculated as follows: A 
0.9 per cent solution means that there is 0.9 gram of
sodium chloride per 100 milliliters of solution, or 
9 g/L. Because the molecular weight of sodium chlo-
ride is 58.5 g/mol, the molarity of the solution is 9 g/L
divided by 58.5 g/mol, or about 0.154 mol/L. Because
each molecule of sodium chloride is equal to 
2 osmoles, the osmolarity of the solution is 0.154 ¥ 2,
or 0.308 osm/L. Therefore, the osmolarity of this solu-
tion is 308 

erefore, the osmolarity of this solu-
tion is 308 mOsm/L. The potential osmotic pressure 
of this solution would therefore be 308 mOsm/L 
¥ 19.3 mm Hg/mOsm/L, or 5944 mm Hg.
This calculation is only an approximation, because
sodium and chloride ions do not behave entirely inde-
pendently in solution because of interionic attraction
between them. One can correct for these deviations
from the predictions of van’t Hoff’s law by using a 
correction factor called the osmotic coefﬁcient. For
298
Unit 

actor called the osmotic coefﬁcient. For
298
Unit V
sodium chloride, the osmotic coefﬁcient is about 0.93.
Therefore, the actual osmolarity of a 0.9 per cent
sodium chloride solution is 308 ¥ 0.93, or about 
286 mOsm/L. For practical reasons, the osmotic coef-
ﬁcients of different solutes are sometimes neglected in
determining the osmolarity and osmotic pressures of
physiologic solutions.
Osmolarity of the Body Fluids. Turning back to Table 25–2,
note the approximate 

 Turning back to Table 25–2,
note the approximate osmolarity of the various osmot-
ically active substances in plasma, interstitial ﬂuid, and
intracellular ﬂuid. Note that about 80 per cent of the
total osmolarity of the interstitial ﬂuid and plasma is
due to sodium and chloride ions, whereas for intracel-
lular ﬂuid, almost half the osmolarity is due to potas-
sium ions, and the remainder is divided among many
other intracellular substances.
As shown in Table 25–2, the total osmolarity of each

shown in Table 25–2, the total osmolarity of each
of the three compartments is about 300 mOsm/L, with
the plasma being about 1 mOsm/L greater than that
of the interstitial and intracellular ﬂuids.The slight dif-
ference between plasma and interstitial ﬂuid is caused
by the osmotic effects of the plasma proteins, which
maintain about 20 mm Hg greater pressure in the cap-
illaries than in the surrounding interstitial spaces, as
discussed in Chapter 16.
Corrected Osmolar Activity of the Body Fluids

 16.
Corrected Osmolar Activity of the Body Fluids. At the
bottom of Table 25–2 are shown corrected osmolar
activities of plasma, interstitial ﬂuid, and intracellular
ﬂuid.The reason for these corrections is that molecules
and ions in solution exert interionic and intermolecu-
lar attraction or repulsion from one solute molecule to
the next, and these two effects can cause, respectively,
a slight decrease or an increase in the osmotic “activ-
ity” of the dissolved substance.
Total Osmotic Pressu

” of the dissolved substance.
Total Osmotic Pressure Exerted by the Body Fluids. Table
25–2 also shows the total osmotic pressure in millime-
ters of mercury that would be exerted by each of the
different ﬂuids if it were placed on one side of the cell
membrane with pure water on the other side. Note
that this total pressure averages about 5443 mm Hg for
plasma, which is 19.3 times the corrected osmolarity of
282 mOsm/L for plasma.
Osmotic Equilibrium Is
Maintained Between
Intracellular and 
Ext

brium Is
Large osmotic pressures can develop across the cell
membrane with relatively small changes in the con-
centrations of solutes in the extracellular ﬂuid. As dis-
cussed earlier, for each milliosmole concentration
gradient of an impermeant solute (one that will not
permeate the cell membrane), about 19.3 mm Hg
osmotic pressure is exerted across the cell membrane.
If the cell membrane is exposed to pure water and the
osmolarity of 

ne is exposed to pure water and the
osmolarity of intracellular ﬂuid is 282 mOsm/L, the
potential osmotic pressure that can develop across the
cell membrane is more than 5400 mm Hg.This demon-
strates the large force that can move water across the
cell membrane when the intracellular and extracellu-
lar ﬂuids are not in osmotic equilibrium.As a result of
these forces, relatively small changes in the concen-
tration of impermeant solutes in the extracellular ﬂuid
can cause large changes in cell v

tracellular ﬂuid
can cause large changes in cell volume.
Isotonic, Hypotonic, and Hypertonic Fluids. The effects of
different concentrations of impermeant solutes in the
extracellular ﬂuid on cell volume are shown in Figure
25–5. If a cell is placed in a solution of impermeant
solutes having an osmolarity of 282 mOsm/L, the cells
will not shrink or swell because the water concentra-
tion in the intracellular and extracellular ﬂuids is equal
and the solutes cannot enter or leave the cell. Such a

he solutes cannot enter or leave the cell. Such a
solution is said to be isotonic because it neither shrinks
nor swells the cells. Examples of isotonic solutions
include a 0.9 per cent solution of sodium chloride or a
5 per cent glucose solution.These solutions are impor-
tant in clinical medicine because they can be infused
into the blood without the danger of upsetting osmotic
equilibrium between the intracellular and extracellu-
lar ﬂuids.
If a cell is placed into a hypotonic solution that ha

a cell is placed into a hypotonic solution that has
a lower concentration of impermeant solutes (less than
282 mOsm/L), water will diffuse into the cell, causing
it to swell; water will continue to diffuse into the cell,
diluting the intracellular ﬂuid while also concentrating
the extracellular ﬂuid until both solutions have about
the same osmolarity. Solutions of sodium chloride with
a concentration of less than 0.9 per cent are hypotonic
and cause cells to swell.
HYPOTONIC
Cell swells
200 mOsm

use cells to swell.
HYPOTONIC
200 mOsm/L
HYPERTONIC
Cell shrinks
360 mOsm/L
ISOTONIC
No change
280 mOsm/L
B
C
A
Figure 25–5
Effects of isotonic (A), hypertonic (B), and hypotonic (C) solutions
on cell volume.
Chapter 25
299
If a cell is placed in a hypertonic solution having a
higher concentration of impermeant solutes, water will
ﬂow out of the cell into the extracellular ﬂuid, con-
cen

 of the cell into the extracellular ﬂuid, con-
centrating the intracellular ﬂuid and diluting the extra-
cellular ﬂuid. In this case, the cell will shrink until the
two concentrations become equal. Sodium chloride
solutions of greater than 0.9 per cent are hypertonic.
Isosmotic, Hyperosmotic, and Hypo-osmotic Fluids. The
terms isotonic, hypotonic, and hypertonic refer to
whether solutions will cause a change in cell volume.
The tonicity of solutions depends on the concentration
of impermeant sol

ons depends on the concentration
of impermeant solutes. Some solutes, however, can
permeate the cell membrane. Solutions with an osmo-
larity the same as the cell are called isosmotic, regard-
less of whether the solute can penetrate the cell
membrane.
The terms hyperosmotic and hypo-osmotic refer to
solutions that have a higher or lower osmolarity,
respectively, compared with the normal extracellular
ﬂuid, without regard for whether the solute permeates
the cell membrane. Highly permeating subs

ermeates
the cell membrane. Highly permeating substances,
such as urea, can cause transient shifts in ﬂuid volume
between the intracellular and extracellular ﬂuids, but
given enough time, the concentrations of these sub-
stances eventually become equal in the two compart-
ments and have little effect on intracellular volume
under steady-state conditions.
Osmotic Equilibrium Between Intracellular and Extracellular
Fluids Is Rapidly Attained. The transfer of ﬂuid across the
cell membrane occurs so

ransfer of ﬂuid across the
cell membrane occurs so rapidly that any differences
in osmolarities between these two compartments are
usually corrected within seconds or, at the most,
minutes. This rapid movement of water across the cell
membrane does not mean that complete equilibrium
occurs between the intracellular and extracellular
compartments throughout the whole body within the
same short period. The reason for this is that ﬂuid
usually enters the body through the gut and must be
transported

s the body through the gut and must be
transported by the blood to all tissues before complete
osmotic equilibrium can occur. It usually takes about
30 minutes to achieve osmotic equilibrium everywhere
in the body after drinking water.
Volume and Osmolality of
Extracellular and Intracellular
Fluids in Abnormal States
Some of the different factors that can cause extracel-
lular and intracellular volumes to change markedly are
ingestion of water, dehydration, intravenous infusion
of different type

ehydration, intravenous infusion
of different types of solutions, loss of large amounts of
ﬂuid from the gastrointestinal tract, and loss of abnor-
mal amounts of ﬂuid by sweating or through the
kidneys.
One can calculate both the changes in intracellular
and extracellular ﬂuid volumes and the types of
therapy that should be instituted if the following basic
principles are kept in mind:
1. Water moves rapidly across cell membranes;
therefore, the osmolarities of intracellular and
extracellular ﬂ

 osmolarities of intracellular and
extracellular ﬂuids remain almost exactly equal 
to each other except for a few minutes after a
change in one of the compartments.
2. Cell membranes are almost completely
impermeable to many solutes; therefore, the
number of osmoles in the extracellular or
intracellular ﬂuid generally remains constant
unless solutes are added to or lost from the
extracellular compartment.
With these basic principles in mind, we can analyze
the effects of different abnormal ﬂuid

can analyze
the effects of different abnormal ﬂuid conditions on
extracellular and intracellular ﬂuid volumes and 
osmolarities.
Effect of Adding Saline Solution 
to the Extracellular Fluid
If an isotonic saline solution is added to the extracel-
lular ﬂuid compartment, the osmolarity of the extra-
cellular ﬂuid does not change; therefore, no osmosis
occurs through the cell membranes. The only effect is
an increase in extracellular ﬂuid volume (Figure
25–6A).The sodium and chloride largely remai

igure
25–6A).The sodium and chloride largely remain in the
extracellular ﬂuid because the cell membrane behaves
as though it were virtually impermeable to the sodium
chloride.
If a hypertonic solution is added to the extracellular
ﬂuid, the extracellular osmolarity increases and causes
osmosis of water out of the cells into the extracellular
compartment (see Figure 25–6B).Again, almost all the
added sodium chloride remains in the extracellular
compartment, and ﬂuid diffuses from the cells into t

mpartment, and ﬂuid diffuses from the cells into the
extracellular space to achieve osmotic equilibrium.
The net effect is an increase in extracellular volume
(greater than the volume of ﬂuid added), a decrease in
intracellular volume, and a rise in osmolarity in both
If a hypotonic solution is added to the extracellular
ﬂuid,the osmolarity of the extracellular ﬂuid decreases
and some of the extracellular water diffuses into the
cells until the intracellular and extracellular compa

ls until the intracellular and extracellular compart-
ments have the same osmolarity (see Figure 25–6C).
Both the intracellular and the extracellular volumes
are increased by the addition of hypotonic ﬂuid,
although the intracellular volume increases to a
greater extent.
Calculation of Fluid Shifts and Osmolarities After Infusion 
of Hypertonic Saline. We can calculate the sequential
effects of infusing different solutions on extracellular
and intracellular ﬂuid volumes and osmolarities. For
exa

tracellular ﬂuid volumes and osmolarities. For
example, if 2 liters of a hypertonic 3.0 per cent sodium
chloride solution are infused into the extracellular
ﬂuid compartment of a 70-kilogram patient whose
initial plasma osmolarity is 280 mOsm/L, what would
be the intracellular and extracellular ﬂuid volumes and
osmolarities after osmotic equilibrium?
The ﬁrst step is to calculate the initial conditions,
including the volume,
concentration,
and total 
300
Unit V
millio

tal 
300
Unit V
milliosmoles in each compartment. Assuming that
extracellular ﬂuid volume is 20 per cent of body weight
and intracellular ﬂuid volume is 40 per cent of body
weight, the following volumes and concentrations 
can be calculated.
Step 1. Initial Conditions
Volume
Total
(Liters)
(mOsm/L)
(mOsm)
14
280
3,920
28
280
7,840
Total body ﬂuid
42
280
11,760
Next, we calculate the total milliosmoles added to
the ex

e calculate the total milliosmoles added to
the extracellular ﬂuid in 2 liters of 3.0 per cent sodium
chloride. A 3.0 per cent solution means that there are
3.0 g/100 ml, or 30 grams of sodium chloride per liter.
Because the molecular weight of sodium chloride is
about 58.5 g/mol, this means that there is about 0.513
mole of sodium chloride per liter of solution. For 2
liters of solution, this would be 1.026 mole of sodium
chloride. Because 1 mole of sodium chloride is about
equal to 2 osmoles (

e of sodium chloride is about
equal to 2 osmoles (sodium chloride has two osmoti-
cally active particles per mole), the net effect of adding
2 liters of this solution is to add 2051 milliosmoles of
sodium chloride to the extracellular ﬂuid.
In Step 2, we calculate the instantaneous effect of
adding 2051 milliosmoles of sodium chloride to the
extracellular ﬂuid along with 2 liters of volume. There
would be no change in the intracellular ﬂuid concen-
tration or volume, and there would be no osmoti

n-
tration or volume, and there would be no osmotic
equilibrium. In the extracellular ﬂuid, however, there
would be an additional 2051 milliosmoles of total
solute, yielding a total of 5791 milliosmoles. Because
the extracellular compartment now has 16 liters of
volume, the concentration can be calculated by divid-
ing 5791 milliosmoles by 16 liters to yield a con-
centration of 373 mOsm/L. Thus, the following values
would occur instantly after adding the solution.
Step 2. Instantaneous Effect o

dding the solution.
Step 2. Instantaneous Effect of Adding 2 Liters of 
3.0 Per Cent Sodium Chloride
Volume 
Total 
(Liters)
(mOsm/L)
(mOsm)
16
373
5,971
28
280
7,840
44
No equilibrium
13,811
In the third step, we calculate the volumes and con-
centrations that would occur within a few minutes
after osmotic equilibrium develops. In this case, the
concentrations in the intracellular and extracellular
ﬂuid compartments would be e

lar and extracellular
ﬂuid compartments would be equal and can be calcu-
lated by dividing the total milliosmoles in the body,
13,811, by the total volume, which is now 44 liters.This
yields a concentration of 313.9 mOsm/L.Therefore, all
the body ﬂuid compartments will have this same con-
centration after osmotic equilibrium. Assuming that
no solute or water has been lost from the body and
that there is no movement of sodium chloride into or
out of the cells, we then calculate the volumes of 
th

of the cells, we then calculate the volumes of 
the intracellular and extracellular compartments. The
intracellular ﬂuid volume is calculated by dividing the
total milliosmoles in the intracellular ﬂuid (7840) by
Osmolarity
20
Normal State
A. Add Isotonic NaCl
C. Add Hypotonic NaCl
B. Add Hypertonic NaCl
30
40
10
Intracellular fluid
300
200
100
0
Volume (liters)
Extracellular fluid
Figure 25–6
Effect of adding isotonic, hyper-
tonic, and hypotonic solutions 
to the extracellular 

ic, and hypotonic solutions 
to the extracellular ﬂuid after
osmotic equilibrium. The normal
state is indicated by the solid
lines, and the shifts from normal
are shown by the shaded areas.
The volumes of intracellular and
extracellular ﬂuid compartments
are shown in the abscissa of each
diagram, and the osmolarities of
these compartments are shown
on the ordinates.
Chapter 25
301
the concentration 

nterstitial Fluid and Edema
301
the concentration (313.9 mOsm/L), to yield a volume
of 24.98 liters. Extracellular ﬂuid volume is calculated
by dividing the total milliosmoles in extracellular ﬂuid
(5971) by the concentration (313.9 mOsm/L), to yield
a volume of 19.02 liters. Again, these calculations are
based on the assumption that the sodium chloride
added to the extracellular ﬂuid remains there and does
not move into the cells.
Step 3. Effect of Adding 2 Liters of 3.0 Per Cent Sodium
Chlorid

 of Adding 2 Liters of 3.0 Per Cent Sodium
Chloride After Osmotic Equilibrium
Volume
Total
(Liters)
(mOsm/L)
(mOsm)
19.02
313.9
5,971
24.98
313.9
7,840
44.0
313.9
13,811
Thus, one can see from this example that adding 2
liters of a hypertonic sodium chloride solution causes
more than a 5-liter increase in extracellular ﬂuid
volume while decreasing intracellular ﬂuid volume by
almost 3 liters.
This method of calculating changes i

ost 3 liters.
This method of calculating changes in intracellular
and extracellular ﬂuid volumes and osmolarities can
be applied to virtually any clinical problem of ﬂuid
volume regulation. The reader should be familiar with
such calculations because an understanding of the
mathematical aspects of osmotic equilibrium between
intracellular and extracellular ﬂuid compartments is
essential for understanding almost all ﬂuid abnormal-
ities of the body and their treatment.
Glucose and Other 
Solution

y and their treatment.
Solutions Administered 
for Nutritive Purposes
Many types of solutions are administered intravenously
to provide nutrition to people who cannot otherwise
take adequate amounts of nutrition. Glucose solutions
are widely used, and amino acid and homogenized fat
solutions are used to a lesser extent. When these solu-
tions are administered, their concentrations of osmoti-
cally active substances are usually adjusted nearly to
isotonicity, or they are given 

adjusted nearly to
isotonicity, or they are given slowly enough that they do
not upset the osmotic equilibrium of the body ﬂuids.
After the glucose or other nutrients are metabolized, an
excess of water often remains, especially if additional
ﬂuid is ingested. Ordinarily, the kidneys excrete this in
the form of a very dilute urine.The net result, therefore,
is the addition of only nutrients to the body.
Clinical Abnormalities of
Fluid Volume Regulation:
Hyponatremia and
The primary

lation:
The primary measurement that is readily available to
the clinician for evaluating a patient’s ﬂuid status is the
plasma sodium concentration. Plasma osmolarity is not
routinely measured, but because sodium and its associ-
ated anions (mainly chloride) account for more than 
90 per cent of the solute in the extracellular ﬂuid,
plasma sodium concentration is a reasonable indicator
of plasma osmolarity under many conditions. When
plasma sodium concentration is

ny conditions. When
plasma sodium concentration is reduced more than a
few milliequivalents below normal (about 142 mEq/L),
a person is said to have hyponatremia. When plasma
sodium concentration is elevated above normal, a
person is said to have hypernatremia.
Causes of Hyponatremia: Excess
Water or Loss of Sodium
Decreased plasma sodium concentration can result
from loss of sodium chloride from the extracellular ﬂuid
or addition of excess water to the extracellular ﬂuid
(Table 25–4). A primary

 to the extracellular ﬂuid
(Table 25–4). A primary loss of sodium chloride usually
results in hypo-osmotic dehydration and is associated
with decreased extracellular ﬂuid volume. Conditions
that can cause hyponatremia owing to loss of sodium
chloride include diarrhea and vomiting. Overuse of
diuretics that inhibit the ability of the kidneys to con-
serve sodium and certain types of sodium-wasting
kidney diseases can also cause modest degrees of
hyponatremia. Finally, Addison’s disease, which res

yponatremia. Finally, Addison’s disease, which results
from decreased secretion of the hormone aldosterone,
impairs the ability of the kidneys to reabsorb sodium
and can cause a modest degree of hyponatremia.
Hyponatremia can also be associated with excess
water retention, which dilutes the sodium in the extra-
cellular ﬂuid, a condition that is referred to as hypo-
osmotic overhydration. For example, excessive secretion
of antidiuretic hormone, which causes the kidney
Table 25–4
Abnormalities o

which causes the kidney
Table 25–4
Abnormalities of Body Fluid Volume Regulation: Hyponatremia and Hypernatremia
Plasma Na+
Cause
Hypo-osmotic dehydration
Adrenal insufﬁciency; overuse of diuretics
Ø
Ø
≠
Hypo-osmotic overhydration
Excess ADH; bronchogenic tumor
Ø
≠
≠
Hyper-osmotic dehydration
Diabetes insipidus; excessive sweating
≠
Ø
Ø
Hyper-osmotic overhydration
Cushing’s disease; primary aldosteronism
≠
≠
Ø
ADH,

ushing’s disease; primary aldosteronism
≠
≠
Ø
ADH, antidiuretic hormone.
302
Unit V
tubules to reabsorb more water, can lead to hypona-
tremia and overhydration.
Causes of Hypernatremia: Water
Loss or Excess Sodium
Increased plasma sodium concentration, which also
causes increased osmolarity, can be due to either loss of
water from the extracellular ﬂuid, which concentrates
the sodium ions, or excess sodium in the extracellular
ﬂuid. When there is primary loss of wate

acellular
ﬂuid. When there is primary loss of water from the
extracellular ﬂuid, this results in hyperosmotic dehydra-
tion. This condition can occur from an inability to
secrete antidiuretic hormone, which is needed for the
kidneys to conserve water. As a result of lack of anti-
diuretic hormone, the kidneys excrete large amounts 
of dilute urine (a disorder referred to as diabetes
insipidus), causing dehydration and increased concen-
tration of sodium chloride in the extracellular ﬂuid. In
cer

 sodium chloride in the extracellular ﬂuid. In
certain types of renal diseases, the kidneys cannot
respond to antidiuretic hormone, also causing a type of
nephrogenic diabetes insipidus. A more common cause
of hypernatremia associated with decreased extracellu-
lar ﬂuid volume is dehydration caused by water intake
that is less than water loss, as can occur with sweating
during prolonged, heavy exercise.
Hypernatremia can also occur as a result of excessive
sodium chloride added to the extracellu

 excessive
sodium chloride added to the extracellular ﬂuid. This
often results in hyperosmotic overhydration because
excess extracellular sodium chloride is usually associ-
ated with at least some degree of water retention by the
kidneys as well. For example, excessive secretion of the
sodium-retaining hormone aldosterone can cause a mild
degree of hypernatremia and overhydration.The reason
that the hypernatremia is not more severe is that
increased aldosterone secretion causes the kidneys to
re

sed aldosterone secretion causes the kidneys to
reabsorb greater amounts of water as well as sodium.
Thus, in analyzing abnormalities of plasma sodium
concentration and deciding on proper therapy, one
should ﬁrst determine whether the abnormality is
caused by a primary loss or gain of sodium or a primary
loss or gain of water.
Edema: Excess Fluid 
in the Tissues
Edema refers to the presence of excess ﬂuid in the
body tissues. In most instances, edema occurs mainly
in the extracellular ﬂuid compa

dema occurs mainly
in the extracellular ﬂuid compartment, but it can
involve intracellular ﬂuid as well.
Two conditions are especially prone to cause intracel-
lular swelling: (1) depression of the metabolic systems
of the tissues, and (2) lack of adequate nutrition to 
the cells. For example, when blood ﬂow to a tissue 
is decreased, the delivery of oxygen and nutrients is
reduced. If the blood ﬂow becomes too low to main-
tain normal tissue metabolism, the cell membrane
ion

in normal tissue metabolism, the cell membrane
ionic pumps become depressed. When this occurs,
sodium ions that normally leak into the interior of the
cell can no longer be pumped out of the cells, and the
excess sodium ions inside the cells cause osmosis of
water into the cells. Sometimes this can increase intra-
cellular volume of a tissue area—even of an entire
ischemic leg, for example—to two to three times
normal. When this occurs, it is usually a prelude to
death of the tissue.
Intracellul

ally a prelude to
Intracellular edema can also occur in inﬂamed
tissues. Inﬂammation usually has a direct effect on the
cell membranes to increase their permeability, allow-
ing sodium and other ions to diffuse into the interior
of the cell, with subsequent osmosis of water into the
cells.
Extracellular ﬂuid edema occurs when there is excess
ﬂuid accumulation in the extracellular spaces. There
are two general causes of extracellular edema: (1)
abnormal le

ral causes of extracellular edema: (1)
abnormal leakage of ﬂuid from the plasma to the inter-
stitial spaces across the capillaries, and (2) failure of
the lymphatics to return ﬂuid from the interstitium
back into the blood. The most common clinical cause
of interstitial ﬂuid accumulation is excessive capillary
ﬂuid ﬁltration.
Factors That Can Increase Capillary Filtration
To understand the causes of excessive capillary ﬁltra-
tion, it is useful to review the determinants of capillary
ﬁltration 

to review the determinants of capillary
ﬁltration discussed in Chapter 16. Mathematically, cap-
illary ﬁltration rate can be expressed as
Filtration = Kf ¥ (Pc – Pif – pc + pif),
where Kf is the capillary ﬁltration coefﬁcient (the
product of the permeability and surface area of the
capillaries), Pc is the capillary hydrostatic pressure, Pif
is the interstitial ﬂuid hydrostatic pressure, pc is the
capillary plasma colloid osmotic pressure, and pif is the
interstitial ﬂuid colloid osmotic pressure

 is the
interstitial ﬂuid colloid osmotic pressure. From this
equation, one can see that any one of the following
changes can increase the capillary ﬁltration rate:
∑Increased capillary ﬁltration coefﬁcient.
∑Increased capillary hydrostatic pressure.
∑Decreased plasma colloid osmotic pressure.
Lymphatic Blockage Causes Edema
When lymphatic blockage occurs, edema can become
especially severe because plasma proteins that leak
into the interstitium have no other way to be removed.
The rise in prote

have no other way to be removed.
The rise in protein concentration raises the colloid
osmotic pressure of the interstitial ﬂuid, which draws
even more ﬂuid out of the capillaries.
Blockage of lymph ﬂow can be especially severe
with infections of the lymph nodes, such as occurs with
infection by ﬁlaria nematodes. Blockage of the lymph
vessels can occur in certain types of cancer or after
surgery in which lymph vessels are removed or
obstructed. For example, large numbers of lymph
vessels are remo

r example, large numbers of lymph
vessels are removed during radical mastectomy,
impairing removal of ﬂuid from the breast and arm
areas and causing edema and swelling of the tissue
Chapter 25
303
spaces. A few lymph vessels eventually regrow after
this type of surgery, so that the interstitial edema is
usually temporary.
Summary of Causes of 
A large number of conditions can cau

ellular Edema
A large number of conditions can cause ﬂuid accumu-
lation in the interstitial spaces by the abnormal leaking
of ﬂuid from the capillaries or by preventing the lym-
phatics from returning ﬂuid from the interstitium back
to the circulation. The following is a partial list of con-
ditions that can cause extracellular edema by these two
types of abnormalities:
I. Increased capillary pressure
A. Excessive kidney retention of salt and water
1. Acute or chronic kidney failure
2. Mineralo

ter
2. Mineralocorticoid excess
B. High venous pressure and venous constriction
1. Heart failure
2. Venous obstruction
3. Failure of venous pumps
(a) Paralysis of muscles
(b) Immobilization of parts of the body
(c) Failure of venous valves
C. Decreased arteriolar resistance
1. Excessive body heat
2. Insufﬁciency of sympathetic nervous system
3. Vasodilator drugs
II. Decreased plasma proteins
A. Loss of proteins in urine (nephrotic
syndrome)
B. Loss of protein f

in urine (nephrotic
syndrome)
B. Loss of protein from denuded skin areas
1. Burns
2. Wounds
C. Failure to produce proteins
1. Liver disease (e.g., cirrhosis)
2. Serious protein or caloric malnutrition
III. Increased capillary permeability
A. Immune reactions that cause release of
histamine and other immune products
B. Toxins
C. Bacterial infections
D. Vitamin deﬁciency, especially vitamin C
E. Prolonged ischemia
F.
Burns
IV. Blockage of lymph return
A. Cancer
B. Infections (e.g., ﬁlaria nematode

urn
A. Cancer
B. Infections (e.g., ﬁlaria nematodes)
C. Surgery
D. Congenital absence or abnormality of
lymphatic vessels
Edema Caused by Heart Failure. One of the most serious and
most common causes of edema is heart failure. In heart
failure, the heart fails to pump blood normally from the
veins into the arteries; this raises venous pressure and
capillary pressure, causing increased capillary ﬁltration.
In addition, the arterial pressure tends to fall, causing
decreased excretion of salt and w

to fall, causing
decreased excretion of salt and water by the kidneys,
which increases blood volume and further raises capil-
lary hydrostatic pressure to cause still more edema.
Also, diminished blood ﬂow to the kidneys stimulates
secretion of renin, causing increased formation of
angiotensin II and increased secretion of aldosterone,
both of which cause additional salt and water retention
by the kidneys.Thus, in untreated heart failure, all these
factors acting together cause serious generaliz

se
factors acting together cause serious generalized extra-
cellular edema.
In patients with left-sided heart failure but without
signiﬁcant failure of the right side of the heart, blood is
pumped into the lungs normally by the right side of the
heart but cannot escape easily from the pulmonary
veins to the left side of the heart because this part of the
heart has been greatly weakened. Consequently, all 
the pulmonary vascular pressures, including pulmon-
ary capillary pressure, rise far above 

ng pulmon-
ary capillary pressure, rise far above normal, causing
serious and life-threatening pulmonary edema. When
untreated, ﬂuid accumulation in the lungs can rapidly
progress, causing death within a few hours.
Edema Caused by Decreased Kidney Excretion of Salt and Water.
As discussed earlier, most sodium chloride added to the
blood remains in the extracellular compartment, and
only small amounts enter the cells. Therefore, in kidney
diseases that compromise urinary excretion of salt and
wat

 that compromise urinary excretion of salt and
water, large amounts of sodium chloride and water are
added to the extracellular ﬂuid. Most of this salt and
water leaks from the blood into the interstitial spaces,
but some remains in the blood. The main effects of this
are to cause (1) widespread increases in interstitial ﬂuid
volume (extracellular edema) and (2) hypertension
because of the increase in blood volume, as explained
in Chapter 19. As an example, children who develop
acute glomerulone

an example, children who develop
acute glomerulonephritis, in which the renal glomeruli
are injured by inﬂammation and therefore fail to ﬁlter
adequate amounts of ﬂuid, also develop serious 
extracellular ﬂuid edema in the entire body; along with
the edema, these children usually develop severe 
Edema Caused by Decreased Plasma Proteins. A reduction in
plasma concentration of proteins because of either
failure to produce normal amounts of proteins or
leakage of proteins from the pl

nts of proteins or
leakage of proteins from the plasma causes the plasma
colloid osmotic pressure to fall. This leads to increased
capillary ﬁltration throughout the body as well as extra-
One of the most important causes of decreased
plasma protein concentration is loss of proteins in the
urine in certain kidney diseases, a condition referred to
as nephrotic syndrome. Multiple types of renal diseases
can damage the membranes of the renal glomeruli,
causing the membranes to becom

he renal glomeruli,
causing the membranes to become leaky to the plasma
proteins and often allowing large quantities of these
proteins to pass into the urine. When this loss exceeds
the ability of the body to synthesize proteins, a reduc-
tion in plasma protein concentration occurs. Serious
generalized edema occurs when the plasma protein con-
centration falls below 2.5 g/100 ml.
Cirrhosis of the liver is another condition that causes
a reduction in plasma protein concentration. Cirrhosis
means 

 in plasma protein concentration. Cirrhosis
means development of large amounts of ﬁbrous tissue
among the liver parenchymal cells. One result is failure
of these cells to produce sufﬁcient plasma proteins,
leading to decreased plasma colloid osmotic pressure
and the generalized edema that goes with this condition.
Another way that liver cirrhosis causes edema is that
the liver ﬁbrosis sometimes compresses the abdominal
portal venous drainage vessels as they pass through the
liver before emptying

els as they pass through the
liver before emptying back into the general circulation.
Blockage of this portal venous outﬂow raises capillary
hydrostatic pressure throughout the gastrointestinal
area and further increases ﬁltration of ﬂuid out of 
the plasma into the intra-abdominal areas. When this
304
Unit V
occurs, the combined effects of decreased plasma
protein concentration and high portal capillary pres-
sures cause transudation of large amounts of ﬂuid and
prot

use transudation of large amounts of ﬂuid and
protein into the abdominal cavity, a condition referred
to as ascites.
Safety Factors That Normally 
Prevent Edema
Even though many disturbances can cause edema,
usually the abnormality must be severe before serious
edema develops.The reason for this is that three major
safety factors prevent excessive ﬂuid accumulation in
the interstitial spaces: (1) low compliance of the inter-
stitium when interstitial ﬂuid pressure is in the nega-
tive pressure r

tial ﬂuid pressure is in the nega-
tive pressure range, (2) the ability of lymph ﬂow 
to increase 10- to 50-fold, and (3) washdown of inter-
stitial ﬂuid protein concentration, which reduces 
interstitial ﬂuid colloid osmotic pressure as capillary
ﬁltration increases.
Safety Factor Caused by Low Compliance 
of the Interstitium in the Negative 
Pressure Range
In Chapter 16, we noted that interstitial ﬂuid hydro-
static pressure in most loose subcutaneous tissues of
the body is slightly less than 

taneous tissues of
the body is slightly less than atmospheric pressure,
averaging about –3 mm Hg. This slight suction in the
tissues helps hold the tissues together. Figure 25–7
shows the approximate relations between different
levels of interstitial ﬂuid pressure and interstitial ﬂuid
volume, as extrapolated to the human being from
animal studies. Note in Figure 25–7 that as long as the
interstitial ﬂuid pressure is in the negative range, small
changes in interstitial ﬂuid volume are associated

changes in interstitial ﬂuid volume are associated with
relatively large changes in interstitial ﬂuid hydrostatic
pressure.Therefore, in the negative pressure range, the
compliance of the tissues, deﬁned as the change in
volume per millimeter of mercury pressure change, is
low.
How does the low compliance of the tissues in the
negative pressure range act as a safety factor against
edema? To answer this question, recall the determi-
nants of capillary ﬁltration discussed previously.When
interstit

lary ﬁltration discussed previously.When
interstitial ﬂuid hydrostatic pressure increases, this
increased pressure tends to oppose further capillary
ﬁltration. Therefore, as long as the interstitial ﬂuid
hydrostatic pressure is in the negative pressure range,
small increases in interstitial ﬂuid volume cause rela-
tively large increases in interstitial ﬂuid hydrostatic
pressure, opposing further ﬁltration of ﬂuid into the
tissues.
Because the normal interstitial ﬂuid hydrostatic
pressure is –3 m

mal interstitial ﬂuid hydrostatic
pressure is –3 mm Hg, the interstitial ﬂuid hydrostatic
pressure must increase by about 3 mm Hg before large
amounts of ﬂuid will begin to accumulate in the tissues.
Therefore, the safety factor against edema is a change
of interstitial ﬂuid pressure of about 3 mm Hg.
Once interstitial ﬂuid pressure rises above 0 mm Hg,
the compliance of the tissues increases markedly,
allowing large amounts of ﬂuid to accumulate in the
tissues with relatively small additional i

 in the
tissues with relatively small additional increases in
interstitial ﬂuid hydrostatic pressure. Thus, in the pos-
itive tissue pressure range, this safety factor against
edema is lost because of the large increase in compli-
ance of the tissues.
Importance of Interstitial Gel in Preventing Fluid Accumulation in
the Interstitium. Note in Figure 25–7 that in normal
tissues with negative interstitial ﬂuid pressure, virtually
all the ﬂuid in the interstitium is in gel form.That is, the
ﬂuid is

e interstitium is in gel form.That is, the
ﬂuid is bound in a proteoglycan meshwork so that there
are virtually no “free” ﬂuid spaces larger than a few
hundredths of a micrometer in diameter. The impor-
tance of the gel is that it prevents ﬂuid from ﬂowing
easily through the tissues because of impediment from
the “brush pile” of trillions of proteoglycan ﬁlaments.
Also, when the interstitial ﬂuid pressure falls to very
negative values, the gel does not contract greatly
because the meshwork of pr

es not contract greatly
because the meshwork of proteoglycan ﬁlaments offers
an elastic resistance to compression. In the negative
ﬂuid pressure range, the interstitial ﬂuid volume does
not change greatly, regardless of whether the degree of
suction is only a few millimeters of mercury negative
pressure or 10 to 20 mm Hg negative pressure. In other
words, the compliance of the tissues is very low in the
negative pressure range.
Interstitial fluid volume (liters)
-10
-8
-6
-4
-2
0
(Low compliance

 volume (liters)
-10
-8
-6
-4
-2
0
(Low compliance)
Total interstitial fluid
(High
compliance)
Normal
2
4
6
60
56
52
48
44
40
36
32
28
24
20
16
12
8
4
0
Interstitial free fluid pressure
(mm Hg)
(mm Hg)
Free fluid
Gel fluid
Figure 25–7
Relation between interstitial ﬂuid hydrostatic pressure and inter-
stitial ﬂuid volumes, including total volume, free ﬂuid volume, and
gel ﬂuid volume, for loose tissues such as skin. Note that signiﬁ-
cant amounts of free ﬂuid occu

. Note that signiﬁ-
cant amounts of free ﬂuid occur only when the interstitial ﬂuid pres-
sure becomes positive. (Modiﬁed from Guyton AC, Granger HJ,
Taylor AE: Interstitial ﬂuid pressure. Physiol Rev 51:527, 1971.)
Chapter 25
305
By contrast, when interstitial ﬂuid pressure rises to
the positive pressure range, there is a tremendous accu-
mulation of free ﬂuid in the tissues. In this pressure
range

f free ﬂuid in the tissues. In this pressure
range, the tissues are compliant, allowing large amounts
of ﬂuid to accumulate with relatively small additional
increases in interstitial ﬂuid hydrostatic pressure. Most
of the extra ﬂuid that accumulates is “free ﬂuid”because
it pushes the brush pile of proteoglycan ﬁlaments apart.
Therefore, the ﬂuid can ﬂow freely through the tissue
spaces because it is not in gel form. When this occurs,
the edema is said to be pitting edema because one can
press t

s said to be pitting edema because one can
press the thumb against the tissue area and push the
ﬂuid out of the area. When the thumb is removed, a pit
is left in the skin for a few seconds until the ﬂuid ﬂows
back from the surrounding tissues. This type of edema
is distinguished from nonpitting edema, which occurs
when the tissue cells swell instead of the interstitium or
when the ﬂuid in the interstitium becomes clotted with
ﬁbrinogen so that it cannot move freely within the tissue
spaces.
Impo

 cannot move freely within the tissue
spaces.
Importance of the Proteoglycan Filaments as a “Spacer” for the
Cells and in Preventing Rapid Flow of Fluid in the Tissues. The
proteoglycan ﬁlaments, along with much larger collagen
ﬁbrils in the interstitial spaces, act as a “spacer” between
the cells. Nutrients and ions do not diffuse readily
through cell membranes; therefore, without adequate
spacing between the cells, these nutrients, electrolytes,
and cell waste products could not be rapidly exc

,
and cell waste products could not be rapidly exchanged
between the blood capillaries and cells located at a dis-
tance from one another.
The proteoglycan ﬁlaments also prevent ﬂuid from
ﬂowing too easily through the tissue spaces. If it were
not for the proteoglycan ﬁlaments, the simple act of a
person standing up would cause large amounts of inter-
stitial ﬂuid to ﬂow from the upper body to the lower
body. When too much ﬂuid accumulates in the intersti-
tium, as occurs in edema, this extra ﬂu

 intersti-
tium, as occurs in edema, this extra ﬂuid creates large
channels that allow the ﬂuid to ﬂow readily through the
interstitium. Therefore, when severe edema occurs in
the legs, the edema ﬂuid often can be decreased by
simply elevating the legs.
Even though ﬂuid does not ﬂow easily through the
tissues in the presence of the compacted proteoglycan
ﬁlaments, different substances within the ﬂuid can
diffuse through the tissues at least 95 per cent as easily
as they normally diffuse. Therefo

r cent as easily
as they normally diffuse. Therefore, the usual diffusion
of nutrients to the cells and the removal of waste prod-
ucts from the cells are not compromised by the proteo-
glycan ﬁlaments of the interstitium.
Increased Lymph Flow as a Safety Factor
Against Edema
A major function of the lymphatic system is to return
to the circulation the ﬂuid and proteins ﬁltered from
the capillaries into the interstitium. Without this con-
tinuous return of the ﬁltered proteins and ﬂuid to the
blo

return of the ﬁltered proteins and ﬂuid to the
blood, the plasma volume would be rapidly depleted,
and interstitial edema would occur.
The lymphatics act as a safety factor against edema
because lymph ﬂow can increase 10- to 50-fold when
ﬂuid begins to accumulate in the tissues. This allows
the lymphatics to carry away large amounts of ﬂuid
and proteins in response to increased capillary ﬁltra-
tion, preventing the interstitial pressure from rising
into the positive pressure range. The safety fa

ng
into the positive pressure range. The safety factor
caused by increased lymph ﬂow has been calculated to
be about 7 mm Hg.
“Washdown” of the Interstitial Fluid Protein as
a Safety Factor Against Edema
As increased amounts of ﬂuid are ﬁltered into the
interstitium, the interstitial ﬂuid pressure increases,
causing increased lymph ﬂow. In most tissues, the
protein concentration of the interstitium decreases as
lymph ﬂow is increased, because larger amounts of
protein are carried away than can b

ger amounts of
protein are carried away than can be ﬁltered out of the
capillaries; the reason for this is that the capillaries 
are relatively impermeable to proteins, compared 
with the lymph vessels. Therefore, the proteins are
“washed out” of the interstitial ﬂuid as lymph ﬂow
increases.
Because the interstitial ﬂuid colloid osmotic pres-
sure caused by the proteins tends to draw ﬂuid out of
the capillaries, decreasing the interstitial ﬂuid proteins
lowers the net ﬁltration force across the 

roteins
lowers the net ﬁltration force across the capillaries and
tends to prevent further accumulation of ﬂuid. The
safety factor from this effect has been calculated to be
about 7 mm Hg.
Summary of Safety Factors That
Putting together all the safety factors against edema, we
ﬁnd the following:
1. The safety factor caused by low tissue compliance
in the negative pressure range is about 3 mm Hg.
2. The safety factor caused by increased lymph ﬂow is
3. The safety fact

sed lymph ﬂow is
3. The safety factor caused by washdown of proteins
from the interstitial spaces is about 7 mm Hg.
Therefore, the total safety factor against edema is
about 17 mm Hg. This means that the capillary pres-
sure in a peripheral tissue could theoretically rise by
17 mm Hg, or approximately double the normal value,
before marked edema would occur.
Fluids in the “Potential
Spaces” of the Body
Perhaps the best way to describe a “potential space”
is to list some examples: 

ibe a “potential space”
is to list some examples: pleural cavity, pericardial
cavity, peritoneal cavity, and synovial cavities, includ-
ing both the joint cavities and the bursae. Virtually all
these potential spaces have surfaces that almost touch
each other, with only a thin layer of ﬂuid in between,
and the surfaces slide over each other.To facilitate the
sliding, a viscous proteinaceous ﬂuid lubricates the 
surfaces.
Fluid Is Exchanged Between the Capillaries and the Potential
Spaces. The su

n the Capillaries and the Potential
Spaces. The surface membrane of a potential space
usually does not offer signiﬁcant resistance to the
passage of ﬂuids, electrolytes, or even proteins, which
all move back and forth between the space and the
interstitial ﬂuid in the surrounding tissue with relative
ease. Therefore, each potential space is in reality a
306
Unit V
large tissue space. Consequently, ﬂuid in the capillar-
ies adjacent to the potential space diffuses not 

ies adjacent to the potential space diffuses not only
into the interstitial ﬂuid but also into the potential
space.
Lymphatic Vessels Drain Protein from the Potential Spaces.
Proteins collect in the potential spaces because of
leakage out of the capillaries, similar to the collection
of protein in the interstitial spaces throughout the
body. The protein must be removed through lymphat-
ics or other channels and returned to the circulation.
Each potential space is either directly or indirectly
c

potential space is either directly or indirectly
connected with lymph vessels. In some cases, such as
the pleural cavity and peritoneal cavity, large lymph
vessels arise directly from the cavity itself.
Edema Fluid in the Potential Spaces Is Called “Effusion.”
When edema occurs in the subcutaneous tissues adja-
cent to the potential space, edema ﬂuid usually collects
in the potential space as well, and this ﬂuid is called
effusion. Thus, lymph blockage or any of the multiple
abnormalities that c

ockage or any of the multiple
abnormalities that can cause excessive capillary ﬁltra-
tion can cause effusion in the same way that intersti-
tial edema is caused.The abdominal cavity is especially
prone to collect effusion ﬂuid, and in this instance, the
effusion is called ascites. In serious cases, 20 liters or
more of ascitic ﬂuid can accumulate.
The other potential spaces, such as the pleural
cavity, pericardial cavity, and joint spaces, can become
seriously swollen when there is generalized 

ecome
seriously swollen when there is generalized edema.
Also, injury or local infection in any one of the cavi-
ties often blocks the lymph drainage, causing isolated
swelling in the cavity.
The dynamics of ﬂuid exchange in the pleural cavity
are discussed in detail in Chapter 38. These dynamics
are mainly representative of all the other potential
spaces as well. It is especially interesting that the
normal ﬂuid pressure in most or all of the potential
spaces in the nonedematous state is negati

tential
spaces in the nonedematous state is negative in the
same way that this pressure is negative (subatmos-
pheric) in loose subcutaneous tissue. For instance, the
interstitial ﬂuid hydrostatic pressure is normally about
–7 to –8 mm Hg in the pleural cavity, –3 to –5 mm Hg
in the joint spaces, and –5 to –6 mm Hg in the peri-
cardial cavity.
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Saaristo A, Karkkainen MJ, Alitalo K: Insights into the
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C
H
A
P
T
E
R
 
2
6
307
I. Glomerular Filtration, Renal
Blood Flow, and Their Control
Multiple Functions of the
Kidneys in Homeostasis
Most people are familiar with one important func-
tion of the kidneys—to rid the body of waste mate-
rials that are either ingested or produced by
metabolism. A second function that is especially
critical is to control the volume and composition of
the body ﬂuids. For wat

 volume and composition of
the body ﬂuids. For water and virtually all electrolytes in the body, the balance
between intake (due to ingestion or metabolic production) and output (due to
excretion or metabolic consumption) is maintained in large part by the kidneys.
This regulatory function of the kidneys maintains the stable environment of the
cells necessary for them to perform their various activities.
The kidneys perform their most important functions by ﬁltering the plasma
and removing subst

unctions by ﬁltering the plasma
and removing substances from the ﬁltrate at variable rates, depending on the
needs of the body. Ultimately, the kidneys “clear” unwanted substances from
the ﬁltrate (and therefore from the blood) by excreting them in the urine while
returning substances that are needed back to the blood.
Although this chapter and the next few chapters focus mainly on the control
of renal excretion, it is important to recognize that the kidneys serve multiple
functions, including t

 the kidneys serve multiple
functions, including the following:
∑Excretion of metabolic waste products and foreign chemicals
∑Regulation of water and electrolyte balances
∑Regulation of body ﬂuid osmolality and electrolyte concentrations
∑Regulation of arterial pressure
∑Regulation of acid-base balance
∑Secretion, metabolism, and excretion of hormones
∑Gluconeogenesis
Excretion of Metabolic Waste Products, Foreign Chemicals, Drugs, and Hormone Metabo-
lites. The kidneys are the primary means for

tabo-
lites. The kidneys are the primary means for eliminating waste products of
metabolism that are no longer needed by the body.These products include urea
(from the metabolism of amino acids), creatinine (from muscle creatine), uric
acid (from nucleic acids), end products of hemoglobin breakdown (such as
bilirubin), and metabolites of various hormones. These waste products must be
eliminated from the body as rapidly as they are produced. The kidneys also
eliminate most toxins and other foreig

idneys also
eliminate most toxins and other foreign substances that are either produced by
the body or ingested, such as pesticides, drugs, and food additives.
Regulation of Water and Electrolyte Balances. For maintenance of homeostasis, excre-
tion of water and electrolytes must precisely match intake. If intake exceeds
excretion, the amount of that substance in the body will increase. If intake is
less than excretion, the amount of that substance in the body will decrease.
Intake of water and 

ce in the body will decrease.
Intake of water and many electrolytes is governed mainly by a person’s eating
and drinking habits, requiring the kidneys to adjust their excretion rates to
match the intake of various substances. Figure 26–1 shows the response of the
kidneys to a sudden 10-fold increase in sodium intake from a low level of 
30 mEq/day to a high level of 300 mEq/day. Within 2 to 3 days after raising the
308
Unit V
sodium intake, renal excretion also increa

Kidneys
sodium intake, renal excretion also increases to about
300 mEq/day, so that a balance between intake and
output is re-established. However, during the 2 to 3
days of renal adaptation to the high sodium intake,
there is a modest accumulation of sodium that raises
extracellular ﬂuid volume slightly and triggers hor-
monal changes and other compensatory responses 
that signal the kidneys to increase their sodium 
excretion.
The capacity of the kidneys to alter sodium excre-
tion in response

he kidneys to alter sodium excre-
tion in response to changes in sodium intake is enor-
mous. Experimental studies have shown that in many
people, sodium intake can be increased to 1500 mEq/
day (more than 10 times normal) or decreased to 
10 mEq/day (less than one tenth normal) with rela-
tively small changes in extracellular ﬂuid volume or
plasma sodium concentration. This is also true for
water and for most other electrolytes, such as chloride,
potassium, calcium, hydrogen, magnesium, and pho

,
potassium, calcium, hydrogen, magnesium, and phos-
phate ions. In the next few chapters, we discuss the spe-
ciﬁc mechanisms that permit the kidneys to perform
these amazing feats of homeostasis.
Regulation of Arterial Pressure. As discussed in Chapter
19, the kidneys play a dominant role in long-term reg-
ulation of arterial pressure by excreting variable
amounts of sodium and water. The kidneys also con-
tribute to short-term arterial pressure regulation by
secreting vasoactive factors or su

e regulation by
secreting vasoactive factors or substances, such as
renin, that lead to the formation of vasoactive prod-
ucts (e.g., angiotensin II).
Regulation of Acid-Base Balance. The kidneys contribute
to acid-base regulation, along with the lungs and body
ﬂuid buffers, by excreting acids and by regulating the
body ﬂuid buffer stores. The kidneys are the only
means of eliminating from the body certain types of
acids, such as sulfuric acid and phosphoric acid, gen-
erated by the metabolism o

d phosphoric acid, gen-
erated by the metabolism of proteins.
Regulation of Erythrocyte Production. The kidneys secrete
erythropoietin, which stimulates the production of red
blood cells, as discussed in Chapter 32. One important
stimulus for erythropoietin secretion by the kidneys is
hypoxia. The kidneys normally account for almost all
the erythropoietin secreted into the circulation. In
people with severe kidney disease or who have had
their kidneys removed and have been placed on
hemodialysis

dneys removed and have been placed on
hemodialysis, severe anemia develops as a result of
decreased erythropoietin production.
Regulation of 1,25–Dihydroxyvitamin D3
Production. The
kidneys produce the active form of vitamin D, 1,25-
dihydroxyvitamin D3 (calcitriol), by hydroxylating this
vitamin at the “number 1” position. Calcitriol is essen-
tial for normal calcium deposition in bone and calcium
reabsorption by the gastrointestinal tract.As discussed
in Chapter 79, calcitriol plays an importa

scussed
in Chapter 79, calcitriol plays an important role in
calcium and phosphate regulation.
Glucose Synthesis. The kidneys synthesize glucose from
amino acids and other precursors during prolonged
fasting, a process referred to as gluconeogenesis. The
kidneys’ capacity to add glucose to the blood during
prolonged periods of fasting rivals that of the liver.
With chronic kidney disease or acute failure of the
kidneys, these homeostatic functions are disrupted,
and severe abnormalities of body 

s are disrupted,
and severe abnormalities of body ﬂuid volumes and
composition rapidly occur. With complete renal
failure, enough potassium, acids, ﬂuid, and other sub-
stances accumulate in the body to cause death within
a few days, unless clinical interventions such as
hemodialysis are initiated to restore, at least partially,
the body ﬂuid and electrolyte balances.
Physiologic Anatomy 
of the Kidneys
General Organization of the Kidneys
and Urinary Tract
The two kidneys lie on the posterior wa

nary Tract
The two kidneys lie on the posterior wall of the
abdomen, outside the peritoneal cavity (Figure 26–2).
Each kidney of the adult human weighs about 150
grams and is about the size of a clenched ﬁst. The
medial side of each kidney contains an indented region
called the hilum through which pass the renal artery
and vein, lymphatics, nerve supply, and ureter, which
carries the ﬁnal urine from the kidney to the bladder,
where it is stored until emptied. The kidney is sur-
rounded by a toug

ntil emptied. The kidney is sur-
rounded by a tough, ﬁbrous capsule that protects its
delicate inner structures.
Sodium intake and
excretion
(mEq/day)
(Liters)
-4
-2
0
2
4
6
8
10
12
14
300
Sodium
retention
Intake
Excretion
Sodium
loss
200
100
0
Time (days)
15
10
5
Figure 26–1
Effect of increasing sodium intake 10-fold (from 30 to 300 mEq/
day) on urinary sodium excretion and extracellular ﬂuid volume.
The shaded areas represent the net sodium retention or t

aded areas represent the net sodium retention or the net
sodium loss, determined from the difference between sodium
intake and sodium excretion.
Chapter 26
Urine Formation by the Kidneys: I. Glomerular Filtration, Renal Blood Flow, and Their Control
309
If the kidney is bisected from top to bottom, the two
major regions that can be visualized are the outer
cortex and the inner region referred to as the medulla.
The medulla is divided into multiple cone-shaped
masses of tissue called renal pyrami

e cone-shaped
masses of tissue called renal pyramids. The base of
each pyramid originates at the border between the
cortex and medulla and terminates in the papilla,
which projects into the space of the renal pelvis, a
funnel-shaped continuation of the upper end of the
ureter. The outer border of the pelvis is divided into
open-ended pouches called major calyces that extend
downward and divide into minor calyces, which collect
urine from the tubules of each papilla. The walls of the
calyces, pel

les of each papilla. The walls of the
calyces, pelvis, and ureter contain contractile elements
that propel the urine toward the bladder, where urine
is stored until it is emptied by micturition, discussed in
Blood ﬂow to the two kidneys is normally about 22 per
cent of the cardiac output, or 1100 ml/min. The renal
artery enters the kidney through the hilum and then
branches progressively to form the interlobar arteries,
arcuate arteries, interlobular art

lobar arteries,
arcuate arteries, interlobular arteries (also called radial
arteries) and afferent arterioles, which lead to the
glomerular capillaries, where large amounts of ﬂuid
and solutes (except the plasma proteins) are ﬁltered to
begin urine formation (Figure 26–3).The distal ends of
the capillaries of each glomerulus coalesce to form the
efferent arteriole, which leads to a second capillary
network, the peritubular capillaries, that surrounds the
The renal circulation is u

unds the
The renal circulation is unique in that it has two cap-
illary beds, the glomerular and peritubular capillaries,
which are arranged in series and separated by the
efferent arterioles, which help regulate the hydrostatic
pressure in both sets of capillaries. High hydro-
static pressure in the glomerular capillaries (about
60 mm Hg) causes rapid ﬂuid ﬁltration, whereas a
much lower hydrostatic pressure in the peritubular
capillaries (about 13 mm Hg) permits rapid ﬂuid reab-

illaries (about 13 mm Hg) permits rapid ﬂuid reab-
sorption. By adjusting the resistance of the afferent
and efferent arterioles, the kidneys can regulate the
hydrostatic pressure in both the glomerular and the
peritubular capillaries, thereby changing the rate of
Nephron (enlarged)
Minor calyx
Major calyx
Papilla
Renal cortex
Renal medulla
Renal pelvis
Renal pyramid
Capsule
Kidney
Bladder
Urethra
Ureter
Kidney
Ureter
Renal
artery
Figure 26–2
General 
organization 
of 
the
kidneys and the urinar

eral 
of 
the
kidneys and the urinary system.
Renal artery
apparatus
Efferent
arteriole
Bowman's
capsule
Glomerulus
Interlobar
arteries
Arcuate arteries
Cortical
collecting tubule
Collecting duct
Loop of
Henle
Proximal tubule
Interlobular
arterioles
Segmental
arteries
Afferent
arteriole
Arcuate
artery
Distal tubule
Arcuate
vein
Peritubular
Figure 26–3
Section of the human kidney showing the major vessels that
supply the blood ﬂow to the kidney and schema

that
supply the blood ﬂow to the kidney and schematic of the micro-
circulation of each nephron.
310
Unit V
glomerular ﬁltration, tubular reabsorption, or both in
response to body homeostatic demands.
The peritubular capillaries empty into the vessels of
the venous system, which run parallel to the arteriolar
vessels and progressively form the interlobular vein,
arcuate vein, interlobar vein, and renal vein, which
leaves the kidney beside the renal artery and ureter.

es the kidney beside the renal artery and ureter.
The Nephron Is the Functional Unit 
of the Kidney
Each kidney in the human contains about 1 million
nephrons, each capable of forming urine. The kidney
cannot regenerate new nephrons.Therefore, with renal
injury, disease, or normal aging, there is a gradual
decrease in nephron number.After age 40, the number
of functioning nephrons usually decreases about 10
per cent every 10 years; thus, at age 80, many people
have 40 per cent fewer functioning 

0, many people
have 40 per cent fewer functioning nephrons than they
did at age 40. This loss is not life threatening because
adaptive changes in the remaining nephrons allow
them to excrete the proper amounts of water,
electrolytes, and waste products, as discussed in
Chapter 31.
Each nephron contains (1) a tuft of glomerular cap-
illaries called the glomerulus, through which large
amounts of ﬂuid are ﬁltered from the blood, and (2) a
long tubule in which the ﬁltered ﬂuid is converted into
urin

e in which the ﬁltered ﬂuid is converted into
urine on its way to the pelvis of the kidney (see Figure
26–3).
The glomerulus contains a network of branching
and anastomosing glomerular capillaries that, com-
pared with other capillaries, have high hydrostatic
pressure (about 60 mm Hg). The glomerular capillar-
ies are covered by epithelial cells, and the total
glomerulus is encased in Bowman’s capsule. Fluid 
ﬁltered from the glomerular capillaries ﬂows into
Bowman’s capsule and then into the pr

es ﬂows into
Bowman’s capsule and then into the proximal tubule,
which lies in the cortex of the kidney (Figure 26–4).
From the proximal tubule, ﬂuid ﬂows into the loop
of Henle, which dips into the renal medulla. Each loop
consists of a descending and an ascending limb. The
walls of the descending limb and the lower end of the
ascending limb are very thin and therefore are called
the thin segment of the loop of Henle. After the ascend-
ing limb of the loop has returned partway back to the
corte

of the loop has returned partway back to the
cortex, its wall becomes much thicker, and it is referred
to as the thick segment of the ascending limb.
At the end of the thick ascending limb is a short
segment, which is actually a plaque in its wall, known
as the macula densa. As we discuss later, the macula
densa plays an important role in controlling nephron
function. Beyond the macula densa, ﬂuid enters the
distal tubule, which, like the proximal tubule, lies in the
renal cortex. This is follow

l tubule, lies in the
renal cortex. This is followed by the connecting tubule
and the cortical collecting tubule, which lead to the cor-
tical collecting duct. The initial parts of 8 to 10 cortical
collecting ducts join to form a single larger collecting
duct that runs downward into the medulla and
becomes the medullary collecting duct. The collecting
ducts merge to form progressively larger ducts that
eventually empty into the renal pelvis through the tips
of the renal papillae. In each kidney,

gh the tips
of the renal papillae. In each kidney, there are about
250 of the very large collecting ducts, each of which
collects urine from about 4000 nephrons.
Regional Differences in Nephron Structure: Cortical and 
Juxtamedullary Nephrons. Although each nephron has all
the components described earlier, there are some dif-
ferences, depending on how deep the nephron lies
within the kidney mass. Those nephrons that have
glomeruli located in the outer cortex are called corti-
cal nephrons; they

 outer cortex are called corti-
cal nephrons; they have short loops of Henle that 
penetrate only a short distance into the medulla
(Figure 26–5).
About 20 to 30 per cent of the nephrons have
glomeruli that lie deep in the renal cortex near the
medulla and are called juxtamedullary nephrons.
These nephrons have long loops of Henle that dip
deeply into the medulla, in some cases all the way to
the tips of the renal papillae.
The 
vascular 
structures 
supplying 
the 
jux-
tamedullary nephrons als

res 
supplying 
the 
jux-
tamedullary nephrons also differ from those supplying
the cortical nephrons. For the cortical nephrons, the
entire tubular system is surrounded by an extensive
network of peritubular capillaries. For the jux-
tamedullary nephrons, long efferent arterioles extend
from the glomeruli down into the outer medulla and
then divide into specialized peritubular capillaries
called vasa recta that extend downward into the
medulla, lying side by side with the loops of Henle.
Like t

lying side by side with the loops of Henle.
Like the loops of Henle, the vasa recta return toward
the cortex and empty into the cortical veins. This 
specialized network of capillaries in the medulla plays
an essential role in the formation of a concentrated
urine.
Cortical
Loop of Henle:
   Thick segment of
   ascending limb
   Thin segment of
   Descending limb
Medullary
Medulla
Cor

cting duct
Medullary
Medulla
Cortex
Cortical
Bowman's capsule
Connecting tubule
Figure 26–4
Basic tubular segments of the nephron. The relative lengths of the
different tubular segments are not drawn to scale.
Chapter 26
311
Micturition is the process by which the urinary bladder
empties when it becomes ﬁlled.This involves two main
steps: First, the bladde

ed.This involves two main
steps: First, the bladder ﬁlls progressively until the
tension in its walls rises above a threshold level; this
elicits the second step, which is a nervous reﬂex called
the micturition reﬂex that empties the bladder or,if this
fails, at least causes a conscious desire to urinate.
Although the micturition reﬂex is an autonomic spinal
cord reﬂex, it can also be inhibited or facilitated by
centers in the cerebral cortex or brain stem.
Physiologic Anatomy and
Nervous Connec

brain stem.
Nervous Connections 
of the Bladder
The urinary bladder, shown in Figure 26–6, is a smooth
muscle chamber composed of two main parts: (1) the
body, which is the major part of the bladder in which
urine collects, and (2) the neck, which is a funnel-shaped
extension of the body, passing inferiorly and anteriorly
into the urogenital triangle and connecting with the
urethra.The lower part of the bladder neck is also called
the posterior urethra because of its rela

o called
the posterior urethra because of its relation to the
urethra.
The smooth muscle of the bladder is called the detru-
sor muscle. Its muscle ﬁbers extend in all directions and,
when contracted, can increase the pressure in the
bladder to 40 to 60 mm Hg. Thus, contraction of the
detrusor muscle is a major step in emptying the bladder.
Smooth muscle cells of the detrusor muscle fuse with
one another so that low-resistance electrical pathways
exist from one muscle cell to the other. Therefor

exist from one muscle cell to the other. Therefore, an
action potential can spread throughout the detrusor
muscle, from one muscle cell to the next, to cause con-
traction of the entire bladder at once.
On the posterior wall of the bladder, lying immedi-
ately above the bladder neck, is a small triangular area
called the trigone. At the lowermost apex of the trigone,
the bladder neck opens into the posterior urethra, and
the two ureters enter the bladder at the uppermost
angles of the trigone. 

e bladder at the uppermost
angles of the trigone. The trigone can be identiﬁed by
the fact that its mucosa, the inner lining of the bladder,
is smooth, in contrast to the remaining bladder mucosa,
which is folded to form rugae. Each ureter, as it enters
the bladder, courses obliquely through the detrusor
muscle and then passes another 1 to 2 centimeters
beneath the bladder mucosa before emptying into the
bladder.
Cortex
Medulla
Outer zone
Inner zone
Efferent
arteriole
Afferent
arteriole
Collecti

one
Efferent
arteriole
Afferent
arteriole
duct
   artery   
   vein   
   artery   
   vein   
Thick loop
of Henle
Juxtamedullary
nephron
Cortical
nephron
Duct of
Bellini
Vasa
recta
Thin loop
of Henle
Figure 26–5
Schematic of relations between blood vessels and tubular struc-
tures and differences between cortical and juxtamedullary
nephrons.
L1
L2
L3
L4
L5
S1
S2
S3
S4
Ureter
Body
Pudendal
Sympathetics
Parasympathetics
Trigone
Bladder neck
(posterior urethra)
Ext

etics
Trigone
External sphincter
Figure 26–6
Urinary bladder and its innervation.
312
Unit V
The bladder neck (posterior urethra) is 2 to 3 cen-
timeters long, and its wall is composed of detrusor
muscle interlaced with a large amount of elastic tissue.
The muscle in this area is called the internal sphincter.
Its natural tone normally keeps the bladder neck and
posterior urethra empty of urine and, therefore, pre-
vents emptying of th

of urine and, therefore, pre-
vents emptying of the bladder until the pressure in the
main part of the bladder rises above a critical threshold.
Beyond the posterior urethra, the urethra passes
through the urogenital diaphragm, which contains a
layer of muscle called the external sphincter of the
bladder. This muscle is a voluntary skeletal muscle, in
contrast to the muscle of the bladder body and bladder
neck, which is entirely smooth muscle. The external
sphincter muscle is under voluntary con

e external
sphincter muscle is under voluntary control of the
nervous system and can be used to consciously prevent
urination even when involuntary controls are attempt-
ing to empty the bladder.
The principal nerve supply of the bladder is by way of
the pelvic nerves, which connect with the spinal cord
through the sacral plexus, mainly connecting with cord
segments S-2 and S-3. Coursing through the pelvic
nerves are both sensory nerve ﬁbers and motor nerve
ﬁbers. The 

th sensory nerve ﬁbers and motor nerve
ﬁbers. The sensory ﬁbers detect the degree of stretch in
the bladder wall. Stretch signals from the posterior
urethra are especially strong and are mainly re-
sponsible for initiating the reﬂexes that cause bladder
emptying.
The motor nerves transmitted in the pelvic nerves are
parasympathetic ﬁbers. These terminate on ganglion
cells located in the wall of the bladder. Short postgan-
glionic nerves then innervate the detrusor muscle.
In addition to the pelv

rvate the detrusor muscle.
In addition to the pelvic nerves, two other types of
innervation are important in bladder function. Most
important are the skeletal motor ﬁbers transmitted
through the pudendal nerve to the external bladder
sphincter. These are somatic nerve ﬁbers that innervate
and control the voluntary skeletal muscle of the sphinc-
ter. Also, the bladder receives sympathetic innervation
from the sympathetic chain through the hypogastric
nerves, connecting mainly with the L-2 segment

ric
nerves, connecting mainly with the L-2 segment of the
spinal cord. These sympathetic ﬁbers stimulate mainly
the blood vessels and have little to do with bladder con-
traction. Some sensory nerve ﬁbers also pass by way of
the sympathetic nerves and may be important in the
sensation of fullness and, in some instances, pain.
Transport of Urine from the
Kidney Through the Ureters
and into the Bladder
Urine that is expelled from the bladder has essentially
the same composition as ﬂuid ﬂowing out 

sentially
the same composition as ﬂuid ﬂowing out of the col-
lecting ducts; there are no signiﬁcant changes in the
composition of urine as it ﬂows through the renal
calyces and ureters to the bladder.
Urine ﬂowing from the collecting ducts into the
renal calyces stretches the calyces and increases their
inherent pacemaker activity, which in turn initiates
peristaltic contractions that spread to the renal pelvis
and then downward along the length of the ureter,
thereby forcing urine from the ren

 of the ureter,
thereby forcing urine from the renal pelvis toward the
bladder. The walls of the ureters contain smooth
muscle and are innervated by both sympathetic and
parasympathetic nerves as well as by an intramural
plexus of neurons and nerve ﬁbers that extends along
the entire length of the ureters. As with other visceral
smooth muscle, peristaltic contractions in the ureter are
enhanced by parasympathetic stimulation and inhibited
by sympathetic stimulation.
The ureters enter the bladder

athetic stimulation.
The ureters enter the bladder through the detrusor
muscle in the trigone region of the bladder, as shown
in Figure 26–6. Normally, the ureters course obliquely
for several centimeters through the bladder wall. The
normal tone of the detrusor muscle in the bladder wall
tends to compress the ureter, thereby preventing back-
ﬂow of urine from the bladder when pressure builds
up in the bladder during micturition or bladder 
compression. Each peristaltic wave along the ureter
inc

ession. Each peristaltic wave along the ureter
increases the pressure within the ureter so that the
region passing through the bladder wall opens and
allows urine to ﬂow into the bladder.
In some people, the distance that the ureter courses
through the bladder wall is less than normal, so that
contraction of the bladder during micturition does not
always lead to complete occlusion of the ureter. As a
result, some of the urine in the bladder is propelled
backward into the ureter, a condition call

opelled
backward into the ureter, a condition called vesi-
coureteral reﬂux. Such reﬂux can lead to enlargement
of the ureters and, if severe, can increase the pressure
in the renal calyces and structures of the renal
medulla, causing damage to these regions.
Pain Sensation in the Ureters, and the Ureterorenal Reﬂex.
The ureters are well supplied with pain nerve ﬁbers.
When a ureter becomes blocked (e.g., by a ureteral
stone), intense reﬂex constriction occurs, associated
with severe pain. Also,

riction occurs, associated
with severe pain. Also, the pain impulses cause a sym-
pathetic reﬂex back to the kidney to constrict the renal
arterioles, thereby decreasing urine output from the
kidney. This effect is called the ureterorenal reﬂex and
is important for preventing excessive ﬂow of ﬂuid into
the pelvis of a kidney with a blocked ureter.
Filling of the Bladder 
and Bladder Wall Tone; 
the Cystometrogram
Figure 26–7 shows the approximate changes in intra-
vesicular pressure as the bladd

 changes in intra-
vesicular pressure as the bladder ﬁlls with urine. When
there is no urine in the bladder, the intravesicular pres-
sure is about 0, but by the time 30 to 50 milliliters of
urine has collected, the pressure rises to 5 to 10 cen-
timeters of water. Additional urine—200 to 300 milli-
liters—can collect with only a small additional rise in
pressure; this constant level of pressure is caused by
intrinsic tone of the bladder wall itself. Beyond 300 to
400 milliliters, collection of 

elf. Beyond 300 to
400 milliliters, collection of more urine in the bladder
causes the pressure to rise rapidly.
Superimposed on the tonic pressure changes during
ﬁlling of the bladder are periodic acute increases in
pressure that last from a few seconds to more than a
minute. The pressure peaks may rise only a few cen-
timeters of water or may rise to more than 100 
Chapter 26
313
centimeters of water.

 Flow, and Their Control
313
centimeters of water. These pressure peaks are called
micturition waves in the cystometrogram and are caused
by the micturition reﬂex.
Referring again to Figure 26–7, one can see that as the
bladder ﬁlls, many superimposed micturition contrac-
tions begin to appear, as shown by the dashed spikes.
They are the result of a stretch reﬂex initiated by
sensory stretch receptors in the bladder wall, especially
by the receptors in the posterior urethra whe

ally
by the receptors in the posterior urethra when this
area begins to ﬁll with urine at the higher bladder pres-
sures. Sensory signals from the bladder stretch recep-
tors are conducted to the sacral segments of the cord
through the pelvic nerves and then reﬂexively back
again to the bladder through the parasympathetic
nerve ﬁbers by way of these same nerves.
When the bladder is only partially ﬁlled, these mic-
turition contractions usually relax spontaneously after
a fraction of a minute, th

lax spontaneously after
a fraction of a minute, the detrusor muscles stop con-
tracting, and pressure falls back to the baseline.As the
bladder continues to ﬁll, the micturition reﬂexes
become more frequent and cause greater contractions
of the detrusor muscle.
Once a micturition reﬂex begins, it is “self-regener-
ative.” That is, initial contraction of the bladder acti-
vates the stretch receptors to cause a greater increase
in sensory impulses to the bladder and posterior
urethra, which causes

to the bladder and posterior
urethra, which causes a further increase in reﬂex con-
traction of the bladder; thus, the cycle is repeated again
and again until the bladder has reached a strong
degree of contraction. Then, after a few seconds to
more than a minute, the self-regenerative reﬂex begins
to fatigue and the regenerative cycle of the micturition
reﬂex ceases, permitting the bladder to relax.
Thus, the micturition reﬂex is a single complete cycle
of (1) progressive and rapid increase of p

e cycle
of (1) progressive and rapid increase of pressure, (2) a
period of sustained pressure, and (3) return of the
pressure to the basal tone of the bladder. Once a mic-
turition reﬂex has occurred but has not succeeded in
emptying the bladder, the nervous elements of this
reﬂex usually remain in an inhibited state for a few
minutes to 1 hour or more before another micturition
reﬂex occurs. As the bladder becomes more and more
ﬁlled, micturition reﬂexes occur more and more often
and more and m

n reﬂexes occur more and more often
and more and more powerfully.
Once the micturition reﬂex becomes powerful
enough, it causes another reﬂex, which passes through
the pudendal nerves to the external sphincter to inhibit
it. If this inhibition is more potent in the brain than the
voluntary constrictor signals to the external sphincter,
urination will occur. If not, urination will not occur
until the bladder ﬁlls still further and the micturition
reﬂex becomes more powerful.
Facilitation or Inhib

Facilitation or Inhibition of Micturition
The micturition reﬂex is a completely autonomic
spinal cord reﬂex, but it can be inhibited or facilitated
by centers in the brain. These centers include (1)
strong facilitative and inhibitory centers in the brain
stem, located mainly in the pons, and (2) several centers
located in the cerebral cortex that are mainly inhibitory
but can become excitatory.
The micturition reﬂex is the basic cause of micturi-
tion, b

ition reﬂex is the basic cause of micturi-
tion, but the higher centers normally exert ﬁnal control
of micturition as follows:
1. The higher centers keep the micturition reﬂex
partially inhibited, except when micturition is
desired.
2. The higher centers can prevent micturition, even
if the micturition reﬂex occurs, by continual tonic
contraction of the external bladder sphincter until
a convenient time presents itself.
3. When it is time to urinate, the cortical centers can
facilitate the sacra

ate, the cortical centers can
facilitate the sacral micturition centers to help
initiate a micturition reﬂex and at the same time
inhibit the external urinary sphincter so that
urination can occur.
Voluntary urination is usually initiated in the fol-
lowing way: First, a person voluntarily contracts his or
her abdominal muscles, which increases the pressure
in the bladder and allows extra urine to enter the
bladder neck and posterior urethra under pressure,
thus stretching their walls. This stim

r pressure,
thus stretching their walls. This stimulates the stretch
receptors, which excites the micturition reﬂex and
simultaneously inhibits the external urethral sphincter.
Ordinarily, all the urine will be emptied, with rarely
more than 5 to 10 milliliters left in the bladder.
Atonic Bladder Caused by Destruction of Sensory Nerve Fibers.
Micturition reﬂex contraction cannot occur if the
sensory nerve ﬁbers from the bladder to the spinal cord
contract

he bladder to the spinal cord
contractions
Basal cystometrogram
Intravesical pressure
(centimeters of water)
Volume (milliliters)
0
100
200
300
400
40
30
20
10
0
Figure 26–7
Normal cystometrogram, showing also acute pressure waves
(dashed spikes) caused by micturition reﬂexes.
314
Unit V
are destroyed, thereby preventing transmission of
stretch signals from the bladder. When this happens, a
person loses bladder control, despite intact efferent
ﬁbers from t

dder control, despite intact efferent
ﬁbers from the cord to the bladder and despite intact
neurogenic connections within the brain. Instead of
emptying periodically, the bladder ﬁlls to capacity and
overﬂows a few drops at a time through the urethra.This
is called overﬂow incontinence.
A common cause of atonic bladder is crush injury to
the sacral region of the spinal cord. Certain diseases can
also cause damage to the dorsal root nerve ﬁbers that
enter the spinal cord. For example, syphilis ca

at
enter the spinal cord. For example, syphilis can cause
constrictive ﬁbrosis around the dorsal root nerve ﬁbers,
destroying them. This condition is called tabes dorsalis,
and the resulting bladder condition is called tabetic
bladder.
Automatic Bladder Caused by Spinal Cord Damage Above the
Sacral Region. If the spinal cord is damaged above the
sacral region but the sacral cord segments are still intact,
typical micturition reﬂexes can still occur. However,
they are no longer controlled by the 

ur. However,
they are no longer controlled by the brain. During the
ﬁrst few days to several weeks after the damage to the
cord has occurred, the micturition reﬂexes are sup-
pressed because of the state of “spinal shock” caused by
the sudden loss of facilitative impulses from the brain
stem and cerebrum. However, if the bladder is emptied
periodically by catheterization to prevent bladder injury
caused by overstretching of the bladder, the excitability
of the micturition reﬂex gradually increas

ability
of the micturition reﬂex gradually increases until typical
micturition reﬂexes return; then, periodic (but unan-
nounced) bladder emptying occurs.
Some patients can still control urination in this con-
dition by stimulating the skin (scratching or tickling) in
the genital region, which sometimes elicits a micturition
reﬂex.
Uninhibited Neurogenic Bladder Caused by Lack of Inhibitory
Signals from the Brain. Another abnormality of micturi-
tion is the so-called uninhibited neurogenic bladd

tion is the so-called uninhibited neurogenic bladder,
which results in frequent and relatively uncontrolled
micturition. This condition derives from partial damage
in the spinal cord or the brain stem that interrupts most
of the inhibitory signals. Therefore, facilitative impulses
passing continually down the cord keep the sacral
centers so excitable that even a small quantity of urine
elicits an uncontrollable micturition reﬂex, thereby pro-
moting frequent urination.
Urine Formation Results 
f

ing frequent urination.
from Glomerular Filtration,
Tubular Reabsorption, and
Tubular Secretion
The rates at which different substances are excreted 
in the urine represent the sum of three renal pro-
cesses, shown in Figure 26–8: (1) glomerular ﬁltration,
(2) reabsorption of substances from the renal tubules
into the blood, and (3) secretion of substances 
from the blood into the renal tubules. Expressed 
mathematically,
Urinary excretion rate = Filtration rate 
- Reabs

- Reabsorption rate + Secretion rate
Urine formation begins when a large amount of
ﬂuid that is virtually free of protein is ﬁltered from the
glomerular capillaries into Bowman’s capsule. Most
substances in the plasma,except for proteins,are freely
ﬁltered, so that their concentration in the glomerular
ﬁltrate in Bowman’s capsule is almost the same as in
the plasma. As ﬁltered ﬂuid leaves Bowman’s capsule
and passes through the tubules, it is modiﬁed by

e
and passes through the tubules, it is modiﬁed by reab-
sorption of water and speciﬁc solutes back into the
blood or by secretion of other substances from the 
peritubular capillaries into the tubules.
Figure 26–9 shows the renal handling of four hypo-
thetical substances. The substance shown in panel A is
freely ﬁltered by the glomerular capillaries but is
neither reabsorbed nor secreted. Therefore, its excre-
tion rate is equal to the rate at which it was ﬁltered.
Certain waste products in th

which it was ﬁltered.
Certain waste products in the body, such as creatinine,
are handled by the kidneys in this manner, allowing
excretion of essentially all that is ﬁltered.
In panel B, the substance is freely ﬁltered but is also
partly reabsorbed from the tubules back into the
blood. Therefore, the rate of urinary excretion is less
than the rate of ﬁltration at the glomerular capillaries.
In this case, the excretion rate is calculated as the ﬁl-
tration rate minus the reabsorption rate.This i

l-
tration rate minus the reabsorption rate.This is typical
for many of the electrolytes of the body.
In panel C, the substance is freely ﬁltered at the
glomerular capillaries but is not excreted into the
1. Filtration
2. Reabsorption
3. Secretion
4. Excretion
1
2
3
4
Renal
vein
Urinary excretion
Excretion = Filtration – Reabsorption + Secretion
Bowman's
capsule
Afferent
arteriole
Efferent
arteriole
Figure 26–8
Basic kidney processes that determine 

Basic kidney processes that determine the composition of the
urine. Urinary excretion rate of a substance is equal to the rate at
which the substance is ﬁltered minus its reabsorption rate plus the
rate at which it is secreted from the peritubular capillary blood into
the tubules.
Chapter 26
315
urine because all the ﬁltered substance is reabsorbed
from the tubules back into the blood. This 

sorbed
from the tubules back into the blood. This pattern
occurs for some of the nutritional substances in the
blood, such as amino acids and glucose, allowing them
to be conserved in the body ﬂuids.
The substance in panel D is freely ﬁltered at the
glomerular capillaries and is not reabsorbed, but addi-
tional quantities of this substance are secreted from
the peritubular capillary blood into the renal tubules.
This pattern often occurs for organic acids and bases,
permitting them to be rapidly

nic acids and bases,
permitting them to be rapidly cleared from the blood
and excreted in large amounts in the urine. The excre-
tion rate in this case is calculated as ﬁltration rate plus
tubular secretion rate.
For each substance in the plasma, a particular com-
bination of ﬁltration, reabsorption, and secretion
occurs. The rate at which the substance is excreted in
the urine depends on the relative rates of these three
basic renal processes.
Filtration, Reabsorption, and
Secretion of Differen

iltration, Reabsorption, and
Secretion of Different Substances
In general, tubular reabsorption is quantitatively more
important than tubular secretion in the formation of
urine, but secretion plays an important role in deter-
mining the amounts of potassium and hydrogen ions
and a few other substances that are excreted in the
urine. Most substances that must be cleared from the
blood, especially the end products of metabolism such
as urea, creatinine, uric acid, and urates, are poorly
reabsorbe

inine, uric acid, and urates, are poorly
reabsorbed and are therefore excreted in large
amounts in the urine. Certain foreign substances and
drugs are also poorly reabsorbed but, in addition, are
secreted from the blood into the tubules, so that their
excretion rates are high. Conversely, electrolytes, such
as sodium ions, chloride ions, and bicarbonate ions, are
highly reabsorbed, so that only small amounts appear
in the urine. Certain nutritional substances, such as
amino acids and glucose, ar

al substances, such as
amino acids and glucose, are completely reabsorbed
from the tubules and do not appear in the urine even
though large amounts are ﬁltered by the glomerular
capillaries.
Each of the processes—glomerular ﬁltration,
tubular reabsorption, and tubular secretion—is regu-
lated according to the needs of the body. For example,
when there is excess sodium in the body, the rate at
which sodium is ﬁltered increases and a smaller frac-
tion of the ﬁltered sodium is reabsorbed, resultin

tion of the ﬁltered sodium is reabsorbed, resulting in
increased urinary excretion of sodium.
For most substances, the rates of ﬁltration and reab-
sorption are extremely large relative to the rates of
excretion.Therefore, subtle adjustments of ﬁltration or
reabsorption can lead to relatively large changes in
renal excretion. For example, an increase in glomeru-
lar ﬁltration rate (GFR) of only 10 per cent (from 180
to 198 L/day) would raise urine volume 13-fold (from
1.5 to 19.5 L/day) if tubul

e volume 13-fold (from
1.5 to 19.5 L/day) if tubular reabsorption remained
constant. In reality, changes in glomerular ﬁltration
and tubular reabsorption usually act in a coordinated
manner to produce the necessary changes in renal
Why Are Large Amounts of Solutes Filtered and Then Reab-
sorbed by the Kidneys? One might question the wisdom
of ﬁltering such large amounts of water and solutes
and then reabsorbing most of these substances. One
advantage of a high GFR is that it allows th

. One
advantage of a high GFR is that it allows the kidneys
to rapidly remove waste products from the body that
depend primarily on glomerular ﬁltration for their
excretion. Most waste products are poorly reabsorbed
by the tubules and, therefore, depend on a high GFR
for effective removal from the body.
A second advantage of a high GFR is that it allows
all the body ﬂuids to be ﬁltered and processed by the
kidney many times each day. Because the entire
plasma volume is only about 3 liters, where

entire
plasma volume is only about 3 liters, whereas the GFR
is about 180 L/day, the entire plasma can be ﬁltered
and processed about 60 times each day.This high GFR
allows the kidneys to precisely and rapidly control the
volume and composition of the body ﬂuids.
Substance A
Urine
A. Filtration only
Substance B
Urine
B. Filtration, partial
reabsorption
Substance C
Urine
C. Filtration, complete
Substance D
Urine
D. Filtration, secretion
Figure 26–9
Renal handling of four hypothetical

on
Renal handling of four hypothetical substances. A, The substance
is freely ﬁltered but not reabsorbed. B, The substance is freely ﬁl-
tered, but part of the ﬁltered load is reabsorbed back in the blood.
C, The substance is freely ﬁltered but is not excreted in the urine
because all the ﬁltered substance is reabsorbed from the tubules
into the blood. D, The substance is freely ﬁltered and is not reab-
sorbed but is secreted from the peritubular capillary blood into the
renal tubule

 peritubular capillary blood into the
316
Unit V
Glomerular Filtration—The
First Step in Urine Formation
Composition of the Glomerular
Filtrate
Urine formation begins with ﬁltration of large
amounts of ﬂuid through the glomerular capillaries
into Bowman’s capsule. Like most capillaries, the
glomerular capillaries are relatively impermeable to
proteins, so that the ﬁltered ﬂuid (called the glomeru-
lar ﬁltrate) is essentially protein-free and devoid of
c

trate) is essentially protein-free and devoid of
cellular elements, including red blood cells.
The concentrations of other constituents of the
glomerular ﬁltrate, including most salts and organic
molecules, are similar to the concentrations in the
plasma. Exceptions to this generalization include a few
low-molecular-weight substances, such as calcium and
fatty acids, that are not freely ﬁltered because they are
partially bound to the plasma proteins. Almost one
half of the plasma calcium and mos

ins. Almost one
half of the plasma calcium and most of the plasma fatty
acids are bound to proteins, and these bound portions
are not ﬁltered through the glomerular capillaries.
GFR Is About 20 Per Cent of the
As in other capillaries, the GFR is determined by (1)
the balance of hydrostatic and colloid osmotic forces
acting across the capillary membrane and (2) the cap-
illary ﬁltration coefﬁcient (Kf), the product of the per-
meability and ﬁltering surface area of the capillari

ability and ﬁltering surface area of the capillaries.
The glomerular capillaries have a much higher rate of
ﬁltration than most other capillaries because of a high
glomerular hydrostatic pressure and a large Kf. In the
average adult human, the GFR is about 125 ml/min, or
180 L/day.The fraction of the renal plasma ﬂow that is
ﬁltered (the ﬁltration fraction) averages about 0.2; this
means that about 20 per cent of the plasma ﬂowing
through the kidney is ﬁltered through the glomerular

ney is ﬁltered through the glomerular
capillaries. The ﬁltration fraction is calculated as
follows:
Filtration fraction = GFR/Renal plasma ﬂow
The glomerular capillary membrane is similar to that
of other capillaries, except that it has three (instead 
of the usual two) major layers: (1) the endothelium of
the capillary, (2) a basement membrane, and (3) a layer
of epithelial cells (podocytes) surrounding the outer
surface of the capillary basement membrane (Figure
2

rface of the capillary basement membrane (Figure
26–10). Together, these layers make up the ﬁltration
barrier, which, despite the three layers, ﬁlters several
hundred times as much water and solutes as the usual
capillary membrane. Even with this high rate of ﬁltra-
tion, the glomerular capillary membrane normally pre-
vents ﬁltration of plasma proteins.
The high ﬁltration rate across the glomerular capil-
lary membrane is due partly to its special charac-
teristics. The capillary endothelium is

al charac-
teristics. The capillary endothelium is perforated by
thousands of small holes called fenestrae, similar to the
fenestrated capillaries found in the liver.Although the
fenestrations are relatively large, endothelial cells are
richly endowed with ﬁxed negative charges that hinder
the passage of plasma proteins.
Surrounding the endothelium is the basement mem-
brane, which consists of a meshwork of collagen and
proteoglycan ﬁbrillae that have large spaces through
which large amounts of 

 have large spaces through
which large amounts of water and small solutes can
ﬁlter. The basement membrane effectively prevents 
ﬁltration of plasma proteins, in part because of 
strong negative electrical charges associated with the
proteoglycans.
The ﬁnal part of the glomerular membrane is a layer
of epithelial cells that line the outer surface of the
glomerulus. These cells are not continuous but have
long footlike processes (podocytes) that encircle the
outer surface of the capillaries (see 

ncircle the
outer surface of the capillaries (see Figure 26–10).
The foot processes are separated by gaps called 
slit pores through which the glomerular ﬁltrate moves.
The epithelial cells, which also have negative charges,
provide additional restriction to ﬁltration of plasma
proteins. Thus, all layers of the glomerular capillary
wall provide a barrier to ﬁltration of plasma 
proteins.
Efferent arteriole
Bowman's space
Capillary loops
Afferent arteriole
Slit pores
B

y loops
Slit pores
Basement
membrane
Fenestrations
Podocytes
A
B
Figure 26–10
A, Basic ultrastructure of the glomerular capillaries. B, Cross
section of the glomerular capillary membrane and its major com-
ponents: capillary endothelium, basement membrane, and epithe-
lium (podocytes).
Chapter 26
317
Filterability of Solutes Is Inversely Related 

317
Filterability of Solutes Is Inversely Related to Their
Size. The glomerular capillary membrane is thicker
than most other capillaries, but it is also much more
porous and therefore ﬁlters ﬂuid at a high rate. Despite
the high ﬁltration rate, the glomerular ﬁltration barrier
is selective in determining which molecules will ﬁlter,
based on their size and electrical charge.
Table 26–1 lists the effect of molecular size on ﬁl-
terability of different molecules. A ﬁlterability of 1.0
means that t

rent molecules. A ﬁlterability of 1.0
means that the substance is ﬁltered as freely as water;
a ﬁlterability of 0.75 means that the substance is ﬁl-
tered only 75 per cent as rapidly as water. Note that
electrolytes such as sodium and small organic com-
pounds such as glucose are freely ﬁltered. As the
molecular weight of the molecule approaches that of
albumin, the ﬁlterability rapidly decreases, approach-
ing zero.
Negatively Charged Large Molecules Are Filtered Less Easily
Than Positively Cha

cules Are Filtered Less Easily
Than Positively Charged Molecules of Equal Molecular Size.
The molecular diameter of the plasma protein albumin
is only about 6 nanometers, whereas the pores of 
the glomerular membrane are thought to be about 
8 nanometers (80 angstroms). Albumin is restricted
from ﬁltration, however, because of its negative charge
and the electrostatic repulsion exerted by negative
charges of the glomerular capillary wall proteoglycans.
Figure 26–11 shows how electrical charge af

ycans.
Figure 26–11 shows how electrical charge affects the
ﬁltration of different molecular weight dextrans by the
glomerulus. Dextrans are polysaccharides that can be
manufactured as neutral molecules or with negative or
positive charges. Note that for any given molecular
radius, positively charged molecules are ﬁltered much
more readily than negatively charged molecules.
Neutral dextrans are also ﬁltered more readily than
negatively charged dextrans of equal molecular
weight. The reason for t

xtrans of equal molecular
weight. The reason for these differences in ﬁlterability
is that the negative charges of the basement mem-
brane and the podocytes provide an important means
for restricting large negatively charged molecules,
including the plasma proteins.
In certain kidney diseases, the negative charges on
the basement membrane are lost even before there are
noticeable changes in kidney histology, a condition
referred to as minimal change nephropathy. As a result
of this loss of negat

nge nephropathy. As a result
of this loss of negative charges on the basement 
membranes,
some of the lower-molecular-weight 
proteins, especially albumin, are ﬁltered and appear 
in the urine, a condition known as proteinuria or 
The GFR is determined by (1) the sum of the hydro-
static and colloid osmotic forces across the glomerular
membrane, which gives the net ﬁltration pressure, and
(2) the glomerular capillary ﬁltration coefﬁcient,
Kf. Expressed mathem

pillary ﬁltration coefﬁcient,
Kf. Expressed mathematically, the GFR equals the
product of Kf and the net ﬁltration pressure:
GFR = Kf ¥ Net ﬁltration pressure
The net ﬁltration pressure represents the sum of the
hydrostatic and colloid osmotic forces that either favor
or oppose ﬁltration across the glomerular capillaries
(Figure 26–12). These forces include (1) hydrostatic
pressure inside the glomerular capillaries (glomerular
hydrostatic pressure, PG), which promotes ﬁltration;
(2) the hydrosta

e, PG), which promotes ﬁltration;
(2) the hydrostatic pressure in Bowman’s capsule (PB)
outside the capillaries, which opposes ﬁltration; (3) the
colloid osmotic pressure of the glomerular capillary
plasma proteins (pG), which opposes ﬁltration; and 
(4) the colloid osmotic pressure of the proteins in
Bowman’s capsule (pB), which promotes ﬁltration.
(Under normal conditions, the concentration of
protein in the glomerular ﬁltrate is so low that the
colloid osmotic pressure of the Bowman’s capsule

e
colloid osmotic pressure of the Bowman’s capsule ﬂuid
is considered to be zero.)
The GFR can therefore be expressed as
GFR = Kf ¥ (PG – PB – pG + pB)
Table 26–1
Filterability of Substances by Glomerular Capillaries Based
on Molecular Weight
Substance
Filterability
Water
18
1.0
Sodium
23
1.0
Glucose
180
1.0
Inulin
5,500
1.0
Myoglobin
17,000
0.75
Albumin
69,000
0.005
Effective molecular radius (A)
Relative filterability
1.0
0.8
0.6
0.4
0.2
0
18
22

elative filterability
1.0
0.8
0.6
0.4
0.2
0
18
22
26
30
Polycationic dextran
Polyanionic dextran
Neutral dextran
34
38
42
Figure 26–11
Effect of size and electrical charge of dextran on its ﬁlterability by
the glomerular capillaries. A value of 1.0 indicates that the sub-
stance is ﬁltered as freely as water, whereas a value of 0 indicates
that it is not ﬁltered. Dextrans are polysaccharides that can be
manufactured as neutral molecules or with negative or positive
charges and with varying molec

egative or positive
charges and with varying molecular weights.
318
Unit V
Although the normal values for the determinants of
GFR have not been measured directly in humans, they
have been estimated in animals such as dogs and rats.
Based on the results in animals, the approximate
normal forces favoring and opposing glomerular ﬁl-
tration in humans are believed to be as follows (see
Figure 26–12):
Forces Favoring Filtration (mm Hg)
Glomerular hydrostatic pressure
60
Bo

tion (mm Hg)
60
Bowman’s capsule colloid osmotic pressure
0
Forces Opposing Filtration (mm Hg)
Bowman’s capsule hydrostatic pressure
18
Glomerular capillary colloid osmotic pressure
32
Net ﬁltration pressure = 60 – 18 – 32 = +10 mm Hg
Some of these values can change markedly under
different physiologic conditions, whereas others are
altered mainly in disease states, as discussed later.
Increased Glomerular Capillary
Filtration Coefﬁcient Increases GFR
The Kf is a 

y
The Kf is a measure of the product of the hydraulic
conductivity and surface area of the glomerular capil-
laries. The Kf cannot be measured directly, but it is 
estimated experimentally by dividing the rate of
glomerular ﬁltration by net ﬁltration pressure:
Kf = GFR/Net ﬁltration pressure
Because total GFR for both kidneys is about 125 ml/
min and the net ﬁltration pressure is 10 mm Hg, the
normal Kf is calculated to be about 12.5 ml/min/mm
Hg of ﬁltration 

ulated to be about 12.5 ml/min/mm
Hg of ﬁltration pressure. When Kf is expressed per 
100 grams of kidney weight, it averages about 
4.2 ml/min/mm Hg, a value about 400 times as high as
the Kf of most other capillary systems of the body; the
average Kf of many other tissues in the body is only
about 0.01 ml/min/mm Hg per 100 grams. This high Kf
for the glomerular capillaries contributes tremen-
dously to their rapid rate of ﬂuid ﬁltration.
Although increased Kf raises GFR and decreased Kf
reduce

gh increased Kf raises GFR and decreased Kf
reduces GFR, changes in Kf probably do not provide
a primary mechanism for the normal day-to-day regu-
lation of GFR. Some diseases, however, lower Kf by
reducing the number of functional glomerular capil-
laries (thereby reducing the surface area for ﬁltration)
or by increasing the thickness of the glomerular 
capillary membrane and reducing its hydraulic con-
ductivity. For example, chronic, uncontrolled hyper-
tension and diabetes mellitus gradually

led hyper-
tension and diabetes mellitus gradually reduce Kf by 
increasing the thickness of the glomerular capillary
basement membrane and, eventually, by damaging the
capillaries so severely that there is loss of capillary
function.
Increased Bowman’s Capsule
Hydrostatic Pressure Decreases GFR
Direct measurements, using micropipettes, of hydro-
static pressure in Bowman’s capsule and at different
points in the proximal tubule suggest that a reasonable
estimate for Bowman’s capsule pressure in 

sonable
estimate for Bowman’s capsule pressure in humans is
about 18 mm Hg under normal conditions. Increasing
the hydrostatic pressure in Bowman’s capsule reduces
GFR, whereas decreasing this pressure raises GFR.
However, changes in Bowman’s capsule pressure nor-
mally do not serve as a primary means for regulating
GFR.
In certain pathological states associated with
obstruction of the urinary tract, Bowman’s capsule
pressure can increase markedly, causing serious reduc-
tion of GFR. For example

y, causing serious reduc-
tion of GFR. For example, precipitation of calcium or
of uric acid may lead to “stones” that lodge in the
urinary tract, often in the ureter, thereby obstructing
outﬂow of the urinary tract and raising Bowman’s
capsule pressure. This reduces GFR and eventually
can damage or even destroy the kidney unless the
obstruction is relieved.
Colloid Osmotic Pressure 
As blood passes from the afferent arteriole through
the glomerular c

om the afferent arteriole through
the glomerular capillaries to the efferent arterioles,
the plasma protein concentration increases about 
20 per cent (Figure 26–13). The reason for this is that
about one ﬁfth of the ﬂuid in the capillaries ﬁlters into
Bowman’s 
capsule,
thereby 
concentrating 
the
glomerular plasma proteins that are not ﬁltered.
Assuming that the normal colloid osmotic pressure 
of plasma entering the glomerular capillaries is 
28 mm Hg, this value usually rises to about 36 mm 

28 mm Hg, this value usually rises to about 36 mm Hg 
hydrostatic
pressure
(60 mm Hg)
Net filtration
pressure
(10 mm Hg)
=
–
–
oncotic
pressure
(32 mm Hg)
Bowman's
capsule
pressure
(18 mm Hg)
Afferent
arteriole
Efferent
arteriole
Bowman's
capsule
pressure
(18 mm Hg)
Bowman's
capsule
pressure
(18 mm Hg)
pressure
(60 mm Hg)
pressure
(60 mm Hg)
colloid osmotic
pressure
(32 mm Hg)
pressure
(32 m

2 mm Hg)
pressure
(32 mm Hg)
Figure 26–12
Summary of forces causing ﬁltration by the glomerular capillaries.
The values shown are estimates for healthy humans.
Chapter 26
319
by the time the blood reaches the efferent end of the
capillaries. Therefore, the average colloid osmotic
pressure of the glomerular capillary plasma proteins 
is midway between 28 and 36 mm Hg, or about 

eins 
32 mm Hg.
Thus, two factors that inﬂuence the glomerular cap-
illary colloid osmotic pressure are (1) the arterial
plasma colloid osmotic pressure and (2) the fraction 
of plasma ﬁltered by the glomerular capillaries (ﬁltra-
tion fraction). Increasing the arterial plasma colloid
osmotic pressure raises the glomerular capillary
colloid osmotic pressure, which in turn decreases
GFR.
Increasing the ﬁltration fraction also concentrates the
plasma pr

ﬁltration fraction also concentrates the
plasma proteins and raises the glomerular colloid
osmotic pressure (see Figure 26–13). Because the ﬁl-
tration fraction is deﬁned as GFR/renal plasma ﬂow,
the ﬁltration fraction can be increased either by raising
GFR or by reducing renal plasma ﬂow. For example, a
reduction in renal plasma ﬂow with no initial change
in GFR would tend to increase the ﬁltration fraction,
which would raise the glomerular capillary colloid
osmotic pressure and tend to reduce 

llary colloid
osmotic pressure and tend to reduce GFR. For this
reason, changes in renal blood ﬂow can inﬂuence GFR
independently of changes in glomerular hydrostatic
pressure.
With increasing renal blood ﬂow, a lower fraction of
the plasma is initially ﬁltered out of the glomerular
capillaries, causing a slower rise in the glomerular 
capillary colloid osmotic pressure and less inhibitory
effect on GFR. Consequently, even with a constant
glomerular hydrostatic pressure, a greater rate of blood

lar hydrostatic pressure, a greater rate of blood
ﬂow into the glomerulus tends to increase GFR, and a
lower rate of blood ﬂow into the glomerulus tends to
decrease GFR.
Hydrostatic Pressure Increases GFR
The glomerular capillary hydrostatic pressure has
been estimated to be about 60 mm Hg under normal
conditions. Changes in glomerular hydrostatic pres-
sure serve as the primary means for physiologic regu-
lation of GFR. Increases in glomerular hydrostatic
pressure

 GFR. Increases in glomerular hydrostatic
pressure raise GFR, whereas decreases in glomerular
hydrostatic pressure reduce GFR.
Glomerular hydrostatic pressure is determined by
three variables, each of which is under physiologic
control: (1) arterial pressure, (2) afferent arteriolar
resistance, and (3) efferent arteriolar resistance.
Increased arterial pressure tends to raise glomeru-
lar hydrostatic pressure and, therefore, to increase
GFR. (However, as discussed later, this effect is
buffered 

ever, as discussed later, this effect is
buffered by autoregulatory mechanisms that maintain
a relatively constant glomerular pressure as blood
pressure ﬂuctuates.)
Increased resistance of afferent arterioles reduces
glomerular hydrostatic pressure and decreases GFR.
Conversely, dilation of the afferent arterioles increases
both glomerular hydrostatic pressure and GFR
(Figure 26–14).
Distance along
glomerular capillary
Glomerular colloid
(mm H

apillary
(mm Hg)
40
38
36
34
32
30
28
Afferent
end
Efferent
end
fraction
fraction
Normal
Figure 26–13
Increase in colloid osmotic pressure in plasma ﬂowing through the
glomerular capillary. Normally, about one ﬁfth of the ﬂuid in the
glomerular capillaries ﬁlters into Bowman’s capsule, thereby con-
centrating the plasma proteins that are not ﬁltered. Increases in
the ﬁltration fraction (glomerular ﬁltration rate/renal plasma ﬂow)
increase

omerular ﬁltration rate/renal plasma ﬂow)
increase the rate at which the plasma colloid osmotic pressure
rises along the glomerular capillary; decreases in the ﬁltration
fraction have the opposite effect.
Efferent arteriolar resistance
(X normal)
(X normal)
Glomerular filtration
rate (ml/min)
150
100
60
0
Renal blood flow
(ml/min)
2000
1400
800
200
0
1
Normal
Renal blood
flow
filtration
rate
2
3
4
Afferent arteriolar resistance
(X normal)
Afferent arteri

t arteriolar resistance
(X normal)
(X normal)
250
100
150
100
50
0
(ml/min)
2000
1400
800
200
0
1
Normal
flow
rate
2
3
4
Figure 26–14
Effect of change in afferent arteriolar resistance or efferent arteri-
olar resistance on glomerular ﬁltration rate and renal blood ﬂow.
320
Unit V
Constriction of the efferent arterioles increases the
resistance to outﬂ

ferent arterioles increases the
resistance to outﬂow from the glomerular capillaries.
This raises the glomerular hydrostatic pressure, and as
long as the increase in efferent resistance does not
reduce renal blood ﬂow too much, GFR increases
slightly (see Figure 26–14). However, because efferent
arteriolar constriction also reduces renal blood ﬂow,
the ﬁltration fraction and glomerular colloid osmotic
pressure increase as efferent arteriolar resistance
increases. Therefore, if the constriction o

stance
increases. Therefore, if the constriction of efferent
arterioles is severe (more than about a threefold
increase in efferent arteriolar resistance), the rise in
colloid osmotic pressure exceeds the increase in
glomerular capillary hydrostatic pressure caused by
efferent arteriolar constriction. When this occurs, the
net force for ﬁltration actually decreases, causing a
reduction in GFR.
Thus, efferent arteriolar constriction has a biphasic
effect on GFR. At moderate levels of constriction

effect on GFR. At moderate levels of constriction,
there is a slight increase in GFR, but with severe con-
striction, there is a decrease in GFR. The primary
cause of the eventual decrease in GFR is as follows:
As efferent constriction becomes severe and as plasma
protein concentration increases, there is a rapid, non-
linear increase in colloid osmotic pressure caused by
the Donnan effect; the higher the protein concentra-
tion, the more rapidly the colloid osmotic pressure
rises because of th

y the colloid osmotic pressure
rises because of the interaction of ions bound to the
plasma proteins, which also exert an osmotic effect, as
To summarize, constriction of afferent arterioles
reduces GFR. However, the effect of efferent arterio-
lar constriction depends on the severity of the con-
striction; modest efferent constriction raises GFR, but
severe efferent constriction (more than a threefold
increase in resistance) tends to reduce GFR.
Table 26–2 summarizes th

nce) tends to reduce GFR.
Table 26–2 summarizes the factors that can decrease
GFR.
In an average 70-kilogram man, the combined blood
ﬂow through both kidneys is about 1100 ml/min, or
about 22 per cent of the cardiac output. Considering
the fact that the two kidneys constitute only about 0.4
per cent of the total body weight, one can readily see
that they receive an extremely high blood ﬂow com-
pared with other organs.
As with other tissues, blood ﬂow supplies the
kidneys with n

her tissues, blood ﬂow supplies the
kidneys with nutrients and removes waste products.
However, the high ﬂow to the kidneys greatly exceeds
this need. The purpose of this additional ﬂow is to
supply enough plasma for the high rates of glomeru-
lar ﬁltration that are necessary for precise regulation
of body ﬂuid volumes and solute concentrations. As
might be expected, the mechanisms that regulate renal
blood ﬂow are closely linked to the control of GFR
and the excretory functions of the kidneys.

f GFR
Renal Blood Flow and Oxygen
On a per gram weight basis, the kidneys normally
consume oxygen at twice the rate of the brain but have
almost seven times the blood ﬂow of the brain. Thus,
the oxygen delivered to the kidneys far exceeds their
metabolic needs, and the arterial-venous extraction of
oxygen is relatively low compared with that of most
other tissues.
A large fraction of the oxygen consumed by the
kidneys is related to the high

gen consumed by the
kidneys is related to the high rate of active sodium
reabsorption by the renal tubules. If renal blood ﬂow
and GFR are reduced and less sodium is ﬁltered, less
sodium is reabsorbed and less oxygen is consumed.
Therefore, renal oxygen consumption varies in pro-
portion to renal tubular sodium reabsorption, which in
turn is closely related to GFR and the rate of sodium
ﬁltered (Figure 26–15). If glomerular ﬁltration com-
pletely ceases, renal sodium reabsorption also ceases,
an

 ceases, renal sodium reabsorption also ceases,
and oxygen consumption decreases to about one
fourth normal. This residual oxygen consumption
reﬂects the basic metabolic needs of the renal cells.
Renal blood ﬂow is determined by the pressure gradi-
ent across the renal vasculature (the difference
between renal artery and renal vein hydrostatic pres-
sures), divided by the total renal vascular resistance:
Renal artery pressure is about equal to systemic
arterial p

ery pressure is about equal to systemic
arterial pressure, and renal vein pressure averages
about 3 to 4 mm Hg under most conditions.As in other
vascular beds, the total vascular resistance through the
kidneys is determined by the sum of the resistances in
the individual vasculature segments, including the
arteries, arterioles, capillaries, and veins (Table 26–3).
Renal artery pressure
Renal vein pressure)
Total renal vascular resistance
-
(
Table 26–2
Factors That Can Decrease the Glomerular Fi

e 26–2
Factors That Can Decrease the Glomerular Filtration Rate
(GFR)
Physical Determinants*
Physiologic/Pathophysiologic Causes
Ø Kf Æ Ø GFR
Renal disease, diabetes mellitus,
≠PB Æ Ø GFR
Urinary tract obstruction (e.g., kidney 
stones)
≠pG Æ Ø GFR
Ø Renal blood ﬂow, increased plasma 
proteins
Ø PG Æ Ø GFR
Ø AP Æ Ø PG
Ø Arterial pressure (has only small effect 
due to autoregulation)
Ø RE Æ Ø PG
Ø Angiotensin II (drugs that block 
angiotensin II formation)
≠RA Æ Ø PG
≠Sympathetic ac

giotensin II formation)
≠RA Æ Ø PG
≠Sympathetic activity, vasoconstrictor 
hormones (e.g., norepinephrine,
endothelin)
* Opposite changes in the determinants usually increase GFR.
Kf, glomerular ﬁltration coefﬁcient; PB, Bowman’s capsule hydrostatic pres-
sure; pG, glomerular capillary colloid osmotic pressure; PG, glomerular capil-
lary hydrostatic pressure;AP, systemic arterial pressure; RE, efferent arteriolar
resistance; RA, afferent arteriolar resistance.
Chapter 26
Urine Formation by the K

ar resistance.
Chapter 26
321
Most of the renal vascular resistance resides in three
major segments: interlobular arteries, afferent arteri-
oles, and efferent arterioles. Resistance of these vessels
is controlled by the sympathetic nervous system,
various hormones, and local internal renal control
mechanisms, as discussed later. An increase in the
resistance of any of the vascular segments of the
kidne

tance of any of the vascular segments of the
kidneys tends to reduce the renal blood ﬂow, whereas
a decrease in vascular resistance increases renal blood
ﬂow if renal artery and renal vein pressures remain
constant.
Although changes in arterial pressure have some
inﬂuence on renal blood ﬂow, the kidneys have effec-
tive mechanisms for maintaining renal blood ﬂow and
GFR relatively constant over an arterial pressure
range between 80 and 170 mm Hg, a process called
autoregulation. This capacity fo

 a process called
autoregulation. This capacity for autoregulation occurs
through mechanisms that are completely intrinsic to
the kidneys, as discussed later in this chapter.
Blood Flow in the Vasa Recta of the
Renal Medulla Is Very Low Compared
with Flow in the Renal Cortex
The outer part of the kidney, the renal cortex, receives
most of the kidney’s blood ﬂow.Blood ﬂow in the renal
medulla accounts for only 1 to 2 per cent of the total
renal blood ﬂow. Flow to the renal medulla is supplied
by 

ood ﬂow. Flow to the renal medulla is supplied
by a specialized portion of the peritubular capillary
system called the vasa recta.These vessels descend into
the medulla in parallel with the loops of Henle and
then loop back along with the loops of Henle and
return to the cortex before emptying into the venous
system.As discussed in Chapter 28, the vasa recta play
an important role in allowing the kidneys to form a
concentrated urine.
Physiologic Control of
Glomerular Filtration and
Renal Blood F

Control of
The determinants of GFR that are most variable and
subject to physiologic control include the glomerular
hydrostatic pressure and the glomerular capillary
colloid osmotic pressure. These variables, in turn, are
inﬂuenced by the sympathetic nervous system, hor-
mones and autacoids (vasoactive substances that are
released in the kidneys and act locally), and other
feedback controls that are intrinsic to the kidneys.
A

insic to the kidneys.
Activation Decreases GFR
Essentially all the blood vessels of the kidneys, includ-
ing the afferent and the efferent arterioles, are richly
innervated by sympathetic nerve ﬁbers. Strong activa-
tion of the renal sympathetic nerves can constrict the
renal arterioles and decrease renal blood ﬂow and
Oxygen consumption
(ml/min/100 gm kidney weight)
3.0
2.5
2.0
1.5
0.5
1.0
0
Sodium reabsorption
(mEq/min per 100 g kidney weight)
(mE

n per 100 g kidney weight)
0
5
10
Basal oxygen consumption
15
20
Figure 26–15
Relationship between oxygen consumption and sodium reab-
sorption in dog kidneys. (Kramer K, Deetjen P: Relation of renal
oxygen consumption to blood supply and glomerular ﬁltration
during variations of blood pressure. Pﬂugers Arch Physiol
271:782, 1960.)
Table 26–3
Approximate Pressures and Vascular Resistances in the Circulation of a Normal Kidney
Pressure in Vess

he Circulation of a Normal Kidney
Pressure in Vessel (mm Hg)
Vessel
Beginning
End
Per Cent of Total Renal Vascular Resistance
100
100
~0
Interlobar, arcuate, and interlobular arteries
~100
85
~16
85
60
~26
Glomerular capillaries
60
59
~1
59
18
~43
Peritubular capillaries
18
8
~10
Interlobar, interlobular, and arcuate veins
8
4
~4
Renal vein
4
~4
~0
322
Unit V
GFR. Moderate or mild sympathetic stimulation has
little in

rate or mild sympathetic stimulation has
little inﬂuence on renal blood ﬂow and GFR. For
example, reﬂex activation of the sympathetic nervous
system resulting from moderate decreases in pressure
at the carotid sinus baroreceptors or cardiopulmonary
receptors has little inﬂuence on renal blood ﬂow or
GFR.
The renal sympathetic nerves seem to be most
important in reducing GFR during severe, acute dis-
turbances lasting for a few minutes to a few hours,
such as those elicited by the defense reactio

urs,
such as those elicited by the defense reaction, brain
ischemia, or severe hemorrhage. In the healthy resting
person, sympathetic tone appears to have little inﬂu-
ence on renal blood ﬂow.
Hormonal and Autacoid Control 
of Renal Circulation
There are several hormones and autacoids that can
inﬂuence GFR and renal blood ﬂow, as summarized in
Table 26–4.
Norepinephrine, Epinephrine, and Endothelin Constrict Renal
Blood Vessels and Decrease GFR. Hormones that constrict
afferent and efferent arte

Hormones that constrict
afferent and efferent arterioles, causing reductions in
GFR and renal blood ﬂow, include norepinephrine and
epinephrine released from the adrenal medulla. In
general, blood levels of these hormones parallel the
activity of the sympathetic nervous system; thus, nor-
epinephrine and epinephrine have little inﬂuence on
renal hemodynamics except under extreme conditions,
such as severe hemorrhage.
Another vasoconstrictor, endothelin, is a peptide
that can be released by damag

thelin, is a peptide
that can be released by damaged vascular endothelial
cells of the kidneys as well as by other tissues. The
physiologic role of this autacoid is not completely
understood. However, endothelin may contribute to
hemostasis (minimizing blood loss) when a blood
vessel is severed, which damages the endothelium 
and releases this powerful vasoconstrictor. Plasma
endothelin levels also are increased in certain disease
states associated with vascular injury, such as toxemia
of pregna

ed with vascular injury, such as toxemia
of pregnancy, acute renal failure, and chronic uremia,
and may contribute to renal vasoconstriction and
decreased GFR in some of these pathophysiologic
Angiotensin II Constricts Efferent Arterioles. A powerful
renal vasoconstrictor, angiotensin II, can be consid-
ered a circulating hormone as well as a locally pro-
duced autacoid because it is formed in the kidneys 
as well as in the systemic circulation. Because
angiotensin II preferentially 

irculation. Because
angiotensin II preferentially constricts efferent arteri-
oles, increased angiotensin II levels raise glomerular
hydrostatic pressure while reducing renal blood ﬂow.
It should be kept in mind that increased angiotensin
II formation usually occurs in circumstances associ-
ated with decreased arterial pressure or volume 
depletion, which tend to decrease GFR. In these cir-
cumstances, the increased level of angiotensin II, by
constricting efferent arterioles, helps prevent decr

nstricting efferent arterioles, helps prevent decreases
in glomerular hydrostatic pressure and GFR; at the
same time, though, the reduction in renal blood ﬂow
caused by efferent arteriolar constriction contributes
to decreased ﬂow through the peritubular capillaries,
which in turn increases reabsorption of sodium and
water, as discussed in Chapter 27.
Thus, increased angiotensin II levels that occur with
a low-sodium diet or volume depletion help preserve
GFR and maintain normal excretion of met

 preserve
GFR and maintain normal excretion of metabolic
waste products such as urea and creatinine that
depend on glomerular ﬁltration for their excretion; at
the same time, the angiotensin II–induced constriction
of efferent arterioles increases tubular reabsorption of
sodium and water, which helps restore blood volume
and blood pressure. This effect of angiotensin II in
helping to “autoregulate” GFR is discussed in more
detail later in this chapter.
Endothelial-Derived Nitric Oxide Decreases 

apter.
Endothelial-Derived Nitric Oxide Decreases Renal Vascular
Resistance and Increases GFR. An autacoid that de-
creases renal vascular resistance and is released by the
vascular endothelium throughout the body is endothe-
lial-derived nitric oxide. A basal level of nitric oxide
production appears to be important for maintaining
vasodilation of the kidneys. This allows the kidneys to
excrete normal amounts of sodium and water. There-
fore, administration of drugs that inhibit this normal
form

inistration of drugs that inhibit this normal
formation of nitric oxide increases renal vascular
resistance and decreases GFR and urinary sodium
excretion, eventually causing high blood pressure. In
some hypertensive patients, impaired nitric oxide pro-
duction could be the cause of increased renal vaso-
constriction and increased blood pressure.
Prostaglandins and Bradykinin Tend to Increase GFR. Hor-
mones and autacoids that cause vasodilation and
increased renal blood ﬂow and GFR include the

and
prostaglandins (PGE2 and PGI2) and bradykinin.
These substances are discussed in Chapter 17.
Although these vasodilators do not appear to be of
major importance in regulating renal blood ﬂow or
GFR in normal conditions, they may dampen the renal
vasoconstrictor effects of the sympathetic nerves or
angiotensin II, especially their effects to constrict the
afferent arterioles.
By opposing vasoconstriction of afferent arterioles,
the prostaglandins 

iction of afferent arterioles,
the prostaglandins may help prevent excessive reduc-
tions in GFR and renal blood ﬂow. Under stressful
Table 26–4
Hormones and Autacoids That Inﬂuence Glomerular
Filtration Rate (GFR)
Hormone or Autacoid
Effect on GFR
Ø
Ø
Ø
¨Æ (prevents Ø) 
Endothelial-derived nitric oxide
≠
≠
Chapter 26
323
conditions, su

l Blood Flow, and Their Control
323
conditions, such as volume depletion or after surgery,
the administration of nonsteroidal anti-inﬂammatory
agents, such as aspirin, that inhibit prostaglandin syn-
thesis may cause signiﬁcant reductions in GFR.
Autoregulation of GFR and
Feedback mechanisms intrinsic to the kidneys nor-
mally keep the renal blood ﬂow and GFR relatively
constant, despite marked changes in arterial blood
pressure. These mechanisms still function in blood-
perfuse

 These mechanisms still function in blood-
perfused kidneys that have been removed from the
body, independent of systemic inﬂuences. This relative
constancy of GFR and renal blood ﬂow is referred to
as autoregulation (Figure 26–16).
The primary function of blood ﬂow autoregulation
in most tissues other than the kidneys is to maintain
the delivery of oxygen and nutrients at a normal level
and to remove the waste products of metabolism,
despite changes in the arterial pressure. In the kidneys,
the

nges in the arterial pressure. In the kidneys,
the normal blood ﬂow is much higher than that
required for these functions. The major function of
autoregulation in the kidneys is to maintain a rela-
tively constant GFR and to allow precise control of
renal excretion of water and solutes.
The GFR normally remains autoregulated (that is,
remains relatively constant), despite considerable
arterial pressure ﬂuctuations that occur during a
person’s usual activities. For instance, a decrease in
arteria

al activities. For instance, a decrease in
arterial pressure to as low as 75 mm Hg or an increase
to as high as 160 mm Hg changes GFR only a few 
percentage points. In general, renal blood ﬂow is auto-
regulated in parallel with GFR, but GFR is more efﬁ-
ciently autoregulated under certain conditions.
Importance of GFR Autoregulation 
in Preventing Extreme Changes in
Renal Excretion
The autoregulatory mechanisms of the kidney are not
100 per cent perfect, but they do prevent potentially
large ch

 perfect, but they do prevent potentially
large changes in GFR and renal excretion of water and
solutes that would otherwise occur with changes in
blood pressure. One can understand the quantitative
importance of autoregulation by considering the rela-
tive magnitudes of glomerular ﬁltration, tubular reab-
sorption, and renal excretion and the changes in renal
excretion that would occur without autoregulatory
Normally, GFR is about 180 L/day and tubular reab-
sorption is 178.5 L/day,

0 L/day and tubular reab-
sorption is 178.5 L/day, leaving 1.5 L/day of ﬂuid to be
excreted in the urine. In the absence of autoregulation,
a relatively small increase in blood pressure (from 100
to 125 mm Hg) would cause a similar 25 per cent
increase in GFR (from about 180 to 225 L/day). If
tubular reabsorption remained constant at 178.5 L/
day, this would increase the urine ﬂow to 46.5 L/day
(the difference between GFR and tubular reabsorp-
tion)—a total increase in urine of more than 30-fold

on)—a total increase in urine of more than 30-fold.
Because the total plasma volume is only about 3 liters,
such a change would quickly deplete the blood
volume.
But in reality, such a change in arterial pressure
exerts much less of an effect on urine volume for 
two reasons: (1) renal autoregulation prevents large
changes in GFR that would otherwise occur, and (2)
there are additional adaptive mechanisms in the renal
tubules that allow them to increase their reabsorption
rate when GFR rises, a 

ncrease their reabsorption
rate when GFR rises, a phenomenon referred to as
glomerulotubular balance (discussed in Chapter 27).
Even with these special control mechanisms, changes
in arterial pressure still have signiﬁcant effects on
renal excretion of water and sodium; this is referred to
as pressure diuresis or pressure natriuresis, and it is
crucial in the regulation of body ﬂuid volumes and
arterial pressure, as discussed in Chapters 19 and 29.
Role of Tubuloglomerular Feedback 
in Autoregul

9.
in Autoregulation of GFR
To perform the function of autoregulation, the kidneys
have a feedback mechanism that links changes in
sodium chloride concentration at the macula densa
with the control of renal arteriolar resistance. This
feedback helps ensure a relatively constant delivery of
sodium chloride to the distal tubule and helps prevent
spurious ﬂuctuations in renal excretion that would
otherwise occur. In many circumstances, this feedback
Renal blood fl

n many circumstances, this feedback
(ml/min)
1600
1200
800
400
   rate
0
160
120
80
40
0
(mm Hg)
(mm Hg)
50
100
150
200
Urine output
(ml/min)
8
6
4
2
0
Figure 26–16
Autoregulation of renal blood ﬂow and glomerular ﬁltration rate but
lack of autoregulation of urine ﬂow during changes in renal arte-
rial pressure.
324
Unit V
autoregulates renal

 V
autoregulates renal blood ﬂow and GFR in parallel.
However, because this mechanism is speciﬁcally
directed toward stabilizing sodium chloride delivery to
the distal tubule, there are instances when GFR is
autoregulated at the expense of changes in renal blood
ﬂow, as discussed later.
The tubuloglomerular feedback mechanism has 
two components that act together to control GFR:
(1) an afferent arteriolar feedback mechanism and (2)
an efferent arteriolar feedback mech

anism and (2)
an efferent arteriolar feedback mechanism. These
feedback mechanisms depend on special anatomical
arrangements of the juxtaglomerular complex (Figure
26–17).
The juxtaglomerular complex consists of macula
densa cells in the initial portion of the distal tubule and
juxtaglomerular cells in the walls of the afferent and
efferent arterioles. The macula densa is a specialized
group of epithelial cells in the distal tubules that comes
in close contact with the afferent and efferent arte

 close contact with the afferent and efferent arteri-
oles. The macula densa cells contain Golgi apparatus,
which are intracellular secretory organelles directed
toward the arterioles, suggesting that these cells may
be secreting a substance toward the arterioles.
Decreased Macula Densa Sodium Chloride Causes Dilation of
Afferent Arterioles and Increased Renin Release. The
macula densa cells sense changes in volume delivery
to the distal tubule by way of signals that are not com-
pletely underst

y way of signals that are not com-
pletely understood. Experimental studies suggest that
decreased GFR slows the ﬂow rate in the loop of
Henle, causing increased reabsorption of sodium and
chloride ions in the ascending loop of Henle, thereby
reducing the concentration of sodium chloride at the
macula densa cells. This decrease in sodium chloride
concentration initiates a signal from the macula densa
that has two effects (Figure 26–18): (1) it decreases
resistance to blood ﬂow in the afferent ar

creases
resistance to blood ﬂow in the afferent arterioles,
which raises glomerular hydrostatic pressure and helps
return GFR toward normal, and (2) it increases renin
release from the juxtaglomerular cells of the afferent
and efferent arterioles, which are the major storage
sites for renin. Renin released from these cells then
functions as an enzyme to increase the formation of
angiotensin I, which is converted to angiotensin II.
Finally, the angiotensin II constricts the efferent arte-
rioles,

giotensin II constricts the efferent arte-
rioles, thereby increasing glomerular hydrostatic pres-
sure and returning GFR toward normal.
These two components of the tubuloglomerular
feedback mechanism, operating together by way of the
special anatomical structure of the juxtaglomerular
apparatus, provide feedback signals to both the affer-
ent and the efferent arterioles for efﬁcient autoregu-
lation of GFR during changes in arterial pressure.
When both of these mechanisms are functioning
togeth

en both of these mechanisms are functioning
together, the GFR changes only a few percentage
points, even with large ﬂuctuations in arterial pressure
between the limits of 75 and 160 mm Hg.
cells
Afferent
arteriole
Internal
elastic
lamina
Efferent
arteriole
Smooth
muscle
fiber
Distal
tubule
Basement
membrane
Figure 26–17
Structure of the juxtaglomerular apparatus, demonstrating its pos-
sible feedback role in the control of nephron function.
Glom

back role in the control of nephron function.
Glomerular hydrostatic
pressure
-
-
GFR
Renin
Proximal
NaCl
NaCl
Efferent
arteriolar
Afferent
Figure 26–18
Macula densa feedback mechanism for autoregulation of
glomerular hydrostatic pressure and glomerular ﬁltration rate
(GFR) during decreased renal arterial pressure.
Chapter 26
Urine Formation by the Kidneys: I. Glomerular Filtration, Renal Blood Flow, and 

: I. Glomerular Filtration, Renal Blood Flow, and Their Control
325
Blockade of Angiotensin II Formation Further Reduces GFR During
Renal Hypoperfusion. As discussed earlier, a preferential
constrictor action of angiotensin II on efferent arteri-
oles helps prevent serious reductions in glomerular
hydrostatic pressure and GFR when renal perfusion
pressure falls below normal. The administration of
drugs that block the formation of angiotensin II
(angiotensin-converting enzyme inhibitors) or that

angiotensin-converting enzyme inhibitors) or that
block the action of angiotensin II (angiotensin II antag-
onists) causes greater reductions in GFR than usual
when the renal arterial pressure falls below normal.
Therefore, an important complication of using these
drugs to treat patients who have hypertension because
of renal artery stenosis (partial blockage of the renal
artery) is a severe decrease in GFR that can, in 
some cases, cause acute renal failure. Nevertheless,
angiotensin II–blockin

enal failure. Nevertheless,
angiotensin II–blocking drugs can be useful therapeutic
agents in many patients with hypertension, congestive
heart failure, and other conditions, as long as they are
monitored to ensure that severe decreases in GFR do
not occur.
Myogenic Autoregulation of Renal
Blood Flow and GFR
Another mechanism that contributes to the main-
tenance of a relatively constant renal blood ﬂow 
and GFR is the ability of individual blood vessels to
resist stretching during increased art

 vessels to
resist stretching during increased arterial pressure, a
phenomenon referred to as the myogenic mechanism.
Studies of individual blood vessels (especially small
arterioles) throughout the body have shown that they
respond to increased wall tension or wall stretch by
contraction of the vascular smooth muscle. Stretch 
of the vascular wall allows increased movement of
calcium ions from the extracellular ﬂuid into the cells,
causing them to contract through the mechanisms dis-
cussed in 

to contract through the mechanisms dis-
cussed in Chapter 8.This contraction prevents overdis-
tention of the vessel and at the same time, by raising
vascular resistance, helps prevent excessive increases
in renal blood ﬂow and GFR when arterial pressure
Although the myogenic mechanism probably oper-
ates in most arterioles throughout the body, its impor-
tance in renal blood ﬂow and GFR autoregulation has
been questioned by some physiologists because this
pressure-sensitive mechanism

ologists because this
pressure-sensitive mechanism has no means of directly
detecting changes in renal blood ﬂow or GFR per se.
Other Factors That Increase Renal
Blood Flow and GFR: High Protein
Intake and Increased Blood Glucose
Although renal blood ﬂow and GFR are relatively
stable under most conditions, there are circumstances in
which these variables change signiﬁcantly. For example,
a high protein intake is known to increase both renal
blood ﬂow and GFR. With a chronic high-protein diet,
su

 ﬂow and GFR. With a chronic high-protein diet,
such as one that contains large amounts of meat, the
increases in GFR and renal blood ﬂow are due partly to
growth of the kidneys. However, GFR and renal blood
ﬂow increase 20 to 30 per cent within 1 or 2 hours after
a person eats a high-protein meal.
The exact mechanisms by which this occurs are still
not completely understood, but one possible explana-
tion is the following: A high-protein meal increases the
release of amino acids into the blood,

creases the
release of amino acids into the blood, which are reab-
sorbed in the proximal tubule. Because amino acids 
and sodium are reabsorbed together by the proximal
tubules, increased amino acid reabsorption also stimu-
lates sodium reabsorption in the proximal tubules. This
decreases sodium delivery to the macula densa, which
elicits a tubuloglomerular feedback–mediated decrease
in resistance of the afferent arterioles, as discussed
earlier.The decreased afferent arteriolar resistance then

.The decreased afferent arteriolar resistance then
raises renal blood ﬂow and GFR. This increased GFR
allows sodium excretion to be maintained at a nearly
normal level while increasing the excretion of the waste
products of protein metabolism, such as urea.
A similar mechanism may also explain the marked
increases in renal blood ﬂow and GFR that occur with
large increases in blood glucose levels in uncontrolled
diabetes mellitus. Because glucose, like some of the
amino acids, is also reabsorbed 

 like some of the
amino acids, is also reabsorbed along with sodium in 
the proximal tubule, increased glucose delivery to the
tubules causes them to reabsorb excess sodium along
with glucose. This, in turn, decreases delivery of sodium
chloride to the macula densa, activating a tubu-
loglomerular feedback–mediated dilation of the affer-
ent arterioles and subsequent increases in renal blood
ﬂow and GFR.
These examples demonstrate that renal blood ﬂow
and GFR per se are not the primary variables

d ﬂow
and GFR per se are not the primary variables controlled
by the tubuloglomerular feedback mechanism. The
main purpose of this feedback is to ensure a constant
delivery of sodium chloride to the distal tubule, where
ﬁnal processing of the urine takes place. Thus, distur-
bances that tend to increase reabsorption of sodium
chloride at tubular sites before the macula densa tend
to elicit increased renal blood ﬂow and GFR, which
helps return distal sodium chloride delivery toward
normal so that

tal sodium chloride delivery toward
normal so that normal rates of sodium and water excre-
tion can be maintained (see Figure 26–18).
An opposite sequence of events occurs when proxi-
mal tubular reabsorption is reduced. For example, when
the proximal tubules are damaged (which can occur as
a result of poisoning by heavy metals, such as mercury,
or large doses of drugs, such as tetracyclines), their
ability to reabsorb sodium chloride is decreased. As a
consequence, large amounts of sodium chlor

d. As a
consequence, large amounts of sodium chloride are
delivered to the distal tubule and, without appropriate
compensations, would quickly cause excessive volume
depletion.
One of the important compensatory
responses appears to be a tubuloglomerular feed-
back–mediated renal vasoconstriction that occurs in
response to the increased sodium chloride delivery to
the macula densa in these circumstances. These exam-
ples again demonstrate the importance of this feedback
mechanism in ensuring that

rtance of this feedback
mechanism in ensuring that the distal tubule receives
the proper rate of delivery of sodium chloride, other
tubular ﬂuid solutes, and tubular ﬂuid volume so that
appropriate amounts of these substances are excreted
in the urine.
Beeuwkes R III: The vascular organization of the kidney.
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Bell PD, Lapointe JY, Peti-Peterdi J: Macula densa cell sig-
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Unit V
Blantz RC

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Deen WM, Lazzara MJ, Myers BD: Structura

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Deen WM, Lazzara MJ, Myers BD: Structural determinants
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future. Hypertension 41:621, 2003.
Hall JE: Angiotensin II and long-term arterial pressure reg-
ulation: the overriding dominance of the kidney. J Am Soc
Nephrol 10 (Suppl 12):s258, 1999.
Hall JE, Brands MW: The renin-angiotensin-aldosterone
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l mechanisms and circulatory homeostasis. In
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Haraldsson B, Sörensson J: Why do we not all have protein-
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, Levine DZ: Paracrine factors in tubu-
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C
H
A
P
T
E
R
 
2
7
327
II. Tubular Processing of the
Reabsorption and Secretion
As the glomerular ﬁltrate enters the renal tu

ules
As the glomerular ﬁltrate enters the renal tubules,
it ﬂows sequentially through the successive parts of
the tubule—the proximal tubule, the loop of Henle,
the distal tubule, the collecting tubule, and, ﬁnally,
the collecting duct—before it is excreted as urine.
Along this course, some substances are selectively reabsorbed from the tubules
back into the blood, whereas others are secreted from the blood into the tubular
lumen. Eventually, the urine that is formed and all the substances in th

 urine that is formed and all the substances in the urine
represent the sum of three basic renal processes—glomerular ﬁltration, tubular
reabsorption, and tubular secretion—as follows:
Urinary excretion = Glomerular ﬁltration - Tubular reabsorption 
+ Tubular secretion
For many substances, reabsorption plays a much more important role than
does secretion in determining the ﬁnal urinary excretion rate. However, secre-
tion accounts for signiﬁcant amounts of potassium ions, hydrogen ions, and a
fe

amounts of potassium ions, hydrogen ions, and a
few other substances that appear in the urine.
Tubular Reabsorption Is Selective and Quantitatively Large
Table 27–1 shows the renal handling of several substances that are all freely ﬁl-
tered in the kidneys and reabsorbed at variable rates.
The rate at which each of these substances is ﬁltered is calculated as
Filtration = Glomerular ﬁltration rate ¥ Plasma concentration
This calculation assumes that the substance is freely ﬁltered and not bound

hat the substance is freely ﬁltered and not bound
to plasma proteins. For example, if plasma glucose concentration is 1 g/L, the
amount of glucose ﬁltered each day is about 180 L/day ¥ 1 g/L, or 180 g/day.
Because virtually none of the ﬁltered glucose is normally excreted, the rate of
glucose reabsorption is also 180 g/day.
From Table 27–1, two things are immediately apparent. First, the processes of
glomerular ﬁltration and tubular reabsorption are quantitatively very large rel-
ative to urinar

are quantitatively very large rel-
ative to urinary excretion for many substances. This means that a small change
in glomerular ﬁltration or tubular reabsorption can potentially cause a relatively
large change in urinary excretion. For example, a 10 per cent decrease in tubular
reabsorption, from 178.5 to 160.7 L/day, would increase urine volume from 1.5
to 19.3 L/day (almost a 13-fold increase) if the glomerular ﬁltration rate (GFR)
remained constant. In reality, however, changes in tubular rea

stant. In reality, however, changes in tubular reabsorption and
glomerular ﬁltration are closely coordinated,so that large ﬂuctuations in urinary
excretion are avoided.
Second, unlike glomerular ﬁltration, which is relatively nonselective (that is,
essentially all solutes in the plasma are ﬁltered except the plasma proteins or
substances bound to them), tubular reabsorption is highly selective. Some sub-
stances, such as glucose and amino acids, are almost completely reabsorbed
328
Unit V
The Bo

are almost completely reabsorbed
328
Unit V
from the tubules, so that the urinary excretion rate is
essentially zero. Many of the ions in the plasma, such
as sodium, chloride, and bicarbonate, are also highly
reabsorbed, but their rates of reabsorption and urinary
excretion are variable, depending on the needs of the
body. Certain waste products, such as urea and creati-
nine, conversely, are poorly reabsorbed from the
tubules and excreted in relatively large amounts.

Therefore, by controlling the rate at which they
reabsorb different substances, the kidneys regulate the
excretion of solutes independently of one another, a
capability that is essential for precise control of the
composition of body ﬂuids. In this chapter, we discuss
the mechanisms that allow the kidneys to selectively
reabsorb or secrete different substances at variable
rates.
Tubular Reabsorption 
Includes Passive and 
Active Mechanisms
For a

ion 
For a substance to be reabsorbed, it must ﬁrst be trans-
ported (1) across the tubular epithelial membranes
into the renal interstitial ﬂuid and then (2) through the
peritubular capillary membrane back into the blood
(Figure 27–1). Thus, reabsorption of water and solutes
includes a series of transport steps. Reabsorption
across the tubular epithelium into the interstitial ﬂuid
includes active or passive transport by way of the same
basic mechanisms di

e transport by way of the same
basic mechanisms discussed in Chapter 4 for transport
across other membranes of the body. For instance,
water and solutes can be transported either through
the cell membranes themselves (transcellular route) or
through the junctional spaces between the cells (para-
cellular route). Then, after absorption across the
tubular epithelial cells into the interstitial ﬂuid, water
and solutes are transported the rest of the way through
the peritubular capillary walls into 

 way through
the peritubular capillary walls into the blood by ultra-
ﬁltration (bulk ﬂow) that is mediated by hydrostatic
and colloid osmotic forces. The peritubular capillaries
behave very much like the venous ends of most other
capillaries because there is a net reabsorptive force
that moves the ﬂuid and solutes from the interstitium
into the blood.
Active transport can move a solute against an elec-
trochemical gradient and requires energy derived
from metabolism. Transport 

equires energy derived
from metabolism. Transport that is coupled directly to
an energy source, such as the hydrolysis of adenosine
triphosphate (ATP), is termed primary active trans-
port. A good example of this is the sodium-potassium
ATPase pump that functions throughout most parts of
the renal tubule.Transport that is coupled indirectly to
an energy source, such as that due to an ion gradient,
is referred to as secondary active transport. Reabsorp-
tion of glucose by the renal tubule is an e

bsorp-
tion of glucose by the renal tubule is an example of
secondary active transport. Although solutes can be
reabsorbed by active and/or passive mechanisms by
the tubule, water is always reabsorbed by a passive
Table 27–1
Filtration, Reabsorption, and Excretion Rates of Different Substances by the Kidneys
Amount Filtered
Amount Reabsorbed
Amount Excreted
% of Filtered Load Reabsorbed
Glucose (g/day)
180
180
0
100
Bicarbonate (mEq/day)
4,320
4,318
2
>99.9
Sodium (mEq/day)
25,560
25,410
150
99.

318
2
>99.9
25,560
25,410
150
99.4
Chloride (mEq/day)
19,440
19,260
180
99.1
Potassium (mEq/day)
756
664
92
87.8
Urea (g/day)
46.8
23.4
23.4
50
Creatinine (g/day)
1.8
0
1.8
0
capillary
Tubular
cells
Lumen
Paracellular
path
Transcellular
path
Solutes
H2O
FILTRATION
EXCRETION
REABSORPTION
Bulk
flow
Blood
Active
Passive
(diffusion)
Osmosis
ATP
Figure 27–1
Reabsorption of ﬁltered water and solutes from the tubular lumen
across the tubular epithelial cells, through the re

cross the tubular epithelial cells, through the renal interstitium, and
back into the blood. Solutes are transported through the cells (trans-
cellular route) by passive diffusion or active transport, or between
the cells (paracellular route) by diffusion. Water is transported
through the cells and between the tubular cells by osmosis. Trans-
port of water and solutes from the interstitial ﬂuid into the peri-
tubular capillaries occurs by ultraﬁltration (bulk ﬂow).
Chapter 27
Urine Formation by 

tration (bulk ﬂow).
Chapter 27
Urine Formation by the Kidneys: II. Tubular Processing of the Glomerular Filtrate
329
(nonactive) physical mechanism called osmosis, which
means water diffusion from a region of low solute con-
centration (high water concentration) to one of high
solute concentration (low water concentration).
Solutes Can Be Transported Through Epithelial Cells or Between
Cells. Renal tubular cells, like other epithelial cells, are
held together by tight junctions. Lateral intercel

held together by tight junctions. Lateral intercellular
spaces lie behind the tight junctions and separate the
epithelial cells of the tubule.Solutes can be reabsorbed
or secreted across the cells by way of the transcellular
pathway or between the cells by moving across the
tight junctions and intercellular spaces by way of 
the paracellular pathway. Sodium is a substance that
moves through both routes, although most of the
sodium is transported through the transcellular
pathway. In some nephron

through the transcellular
pathway. In some nephron segments, especially the
proximal tubule, water is also reabsorbed across the
paracellular pathway, and substances dissolved in 
the water, especially potassium, magnesium, and chlo-
ride ions, are carried with the reabsorbed ﬂuid
between the cells.
Primary Active Transport Through the Tubular Membrane Is
Linked to Hydrolysis of ATP. The special importance of
primary active transport is that it can move solutes
against an electrochemical gradien

an move solutes
against an electrochemical gradient. The energy for this
active transport comes from the hydrolysis of ATP by
way of membrane-bound ATPase; the ATPase is also
a component of the carrier mechanism that binds and
moves solutes across the cell membranes.The primary
active transporters that are known include sodium-
potassium ATPase,
hydrogen ATPase,
hydrogen-
potassium ATPase, and calcium ATPase.
A good example of a primary active transport
system is the reabsorption of sodium ions 

ansport
system is the reabsorption of sodium ions across the
proximal tubular membrane, as shown in Figure 27–2.
On the basolateral sides of the tubular epithelial cell,
the cell membrane has an extensive sodium-potassium
ATPase system that hydrolyzes ATP and uses the
released energy to transport sodium ions out of the
cell into the interstitium. At the same time, potassium
is transported from the interstitium to the inside of the
cell. The operation of this ion pump maintains low
intracellular 

tion of this ion pump maintains low
intracellular sodium and high intracellular potassium
concentrations and creates a net negative charge of
about -70 millivolts within the cell. This pumping of
sodium out of the cell across the basolateral membrane
of the cell favors passive diffusion of sodium across the
luminal membrane of the cell, from the tubular lumen
into the cell, for two reasons: (1) There is a concen-
tration gradient favoring sodium diffusion into the cell
because intracellular sodi

diffusion into the cell
because intracellular sodium concentration is low 
(12 mEq/L) and tubular ﬂuid sodium concentration is
high (140 mEq/L). (2) The negative, -70-millivolt,
intracellular potential attracts the positive sodium ions
from the tubular lumen into the cell.
Active 
reabsorption 
of 
sodium 
by 
sodium-
potassium ATPase occurs in most parts of the tubule.
In certain parts of the nephron, there are additional
provisions for moving large amounts of sodium into
the cell. In the proxi

arge amounts of sodium into
the cell. In the proximal tubule, there is an extensive
brush border on the luminal side of the membrane
(the side that faces the tubular lumen) that multiplies
the surface area about 20-fold. There are also sodium
carrier proteins that bind sodium ions on the luminal
surface of the membrane and release them inside the
cell, providing facilitated diffusion of sodium through
the membrane into the cell. These sodium carrier pro-
teins are also important for secondary ac

ier pro-
teins are also important for secondary active transport
of other substances, such as glucose and amino acids,
as discussed later.
Thus, the net reabsorption of sodium ions from the
tubular lumen back into the blood involves at least
three steps:
1. Sodium diffuses across the luminal membrane
(also called the apical membrane) into the cell
down an electrochemical gradient established 
by the sodium-potassium ATPase pump on the
basolateral side of the membrane.
2. Sodium is transported ac

 side of the membrane.
2. Sodium is transported across the basolateral
membrane against an electrochemical gradient 
by the sodium-potassium ATPase pump.
3. Sodium, water, and other substances are
reabsorbed from the interstitial ﬂuid into the
peritubular capillaries by ultraﬁltration, a passive
process driven by the hydrostatic and colloid
osmotic pressure gradients.
Secondary Active Reabsorption Through the Tubular Membrane.
In secondary active transport, two or more substances
interact with a

 transport, two or more substances
interact with a speciﬁc membrane protein (a carrier
molecule) and are transported together across the
membrane. As one of the substances (for instance,
sodium) diffuses down its electrochemical gradient,
the energy released is used to drive another substance
capillary
Tubular
epithelial cells
Basement
membrane
Intercellular space
fluid
Tubular
lumen
(-3 mv)
(-70 mV)
Brush border
(luminal
membrane)
Basal
channels
ATP
ATP
N

border
(luminal
membrane)
Basal
channels
ATP
ATP
Na+
Na+
Na+
K+
K+
Figure 27–2
Basic mechanism for active transport of sodium through the tubular
epithelial cell.The sodium-potassium pump transports sodium from
the interior of the cell across the basolateral membrane, creating a
low intracellular sodium concentration and a negative intracellular
electrical potential. The low intracellular sodium concentration and
the negative electrical potential cause sodium ions to diffuse from
the tubular lum

 cause sodium ions to diffuse from
the tubular lumen into the cell through the brush border.
330
Unit V
(for instance, glucose) against its electrochemical 
gradient. Thus, secondary active transport does not
require energy directly from ATP or from other high-
energy phosphate sources. Rather, the direct source of
the energy is that liberated by the simultaneous facil-
itated diffusion of another transported substance
down its own electrochemical gradient.
Figure 27–

Figure 27–3 shows secondary active transport of
glucose and amino acids in the proximal tubule. In
both instances, a speciﬁc carrier protein in the brush
border combines with a sodium ion and an amino acid
or a glucose molecule at the same time. These trans-
port mechanisms are so efﬁcient that they remove vir-
tually all the glucose and amino acids from the tubular
lumen. After entry into the cell, glucose and amino
acids exit across the basolateral membr

 and amino
acids exit across the basolateral membranes by facili-
tated diffusion, driven by the high glucose and amino
acid concentrations in the cell.
Although transport of glucose against a chemical
gradient does not directly use ATP, the reabsorption
of glucose depends on energy expended by the
primary active sodium-potassium ATPase pump in the
basolateral membrane. Because of the activity of 
this pump, an electrochemical gradient for facilitated
diffusion of sodium across the luminal membr

tated
diffusion of sodium across the luminal membrane is
maintained, and it is this downhill diffusion of sodium
to the interior of the cell that provides the energy for
the simultaneous uphill transport of glucose across the
luminal membrane. Thus, this reabsorption of glucose
is referred to as “secondary active transport” because
glucose itself is reabsorbed uphill against a chemical
gradient, but it is “secondary” to primary active trans-
port of sodium.
Another important point is that a subs

 of sodium.
Another important point is that a substance is said
to undergo “active” transport when at least one of the
steps in the reabsorption involves primary or second-
ary active transport, even though other steps in 
the reabsorption process may be passive. For glucose
reabsorption, secondary active transport occurs at the
luminal membrane, but passive facilitated diffusion
occurs at the basolateral membrane, and passive
uptake by bulk ﬂow occurs at the peritubular 
Secondary 

occurs at the peritubular 
Secondary Active Secretion into the Tubules. Some sub-
stances are secreted into the tubules by secondary
active transport. This often involves counter-transport
of the substance with sodium ions. In counter-
transport, the energy liberated from the downhill
movement of one of the substances (for example,
sodium ions) enables uphill movement of a second
substance in the opposite direction.
One example of counter-transport, shown in Figure
27–3, is the acti

unter-transport, shown in Figure
27–3, is the active secretion of hydrogen ions coupled
to sodium reabsorption in the luminal membrane of
the proximal tubule. In this case, sodium entry into the
cell is coupled with hydrogen extrusion from the cell
by sodium-hydrogen counter-transport. This transport
is mediated by a speciﬁc protein in the brush border
of the luminal membrane. As sodium is carried to the
interior of the cell, hydrogen ions are forced outward
in the opposite direction into the tu

rced outward
in the opposite direction into the tubular lumen. The
basic principles of primary and secondary active trans-
port are discussed in additional detail in Chapter 4.
Pinocytosis—An Active Transport Mechanism for Reabsorption
of Proteins. Some parts of the tubule, especially the
proximal tubule, reabsorb large molecules such as 
proteins by pinocytosis. In this process, the protein
attaches to the brush border of the luminal membrane,
and this portion of the membrane then invaginates t

nd this portion of the membrane then invaginates to
the interior of the cell until it is completely pinched off
and a vesicle is formed containing the protein. Once
inside the cell, the protein is digested into its con-
stituent amino acids,which are reabsorbed through the
basolateral membrane into the interstitial ﬂuid.
Because pinocytosis requires energy, it is considered a
form of active transport.
Transport Maximum for Substances That Are Actively Reab-
sorbed. For most substances that are a

ively Reab-
sorbed. For most substances that are actively reab-
sorbed or secreted, there is a limit to the rate at which
the solute can be transported, often referred to as the
transport maximum. This limit is due to saturation 
of the speciﬁc transport systems involved when the
amount of solute delivered to the tubule (referred to
as tubular load) exceeds the capacity of the carrier
proteins and speciﬁc enzymes involved in the trans-
port process.
Tubular
cells
-70 mV
Co-transport

ss.
Tubular
cells
-70 mV
fluid
Tubular
lumen
Na+
Na+
Na+
Na+
ATP
Na+
Na+
K+
K+
-70 mV
Counter-transport
ATP
Na+
Na+
K+
K+
Na+
Na+
H+
H+
Glucose
Glucose
Figure 27–3
Mechanisms of secondary active transport.The upper cell shows the
co-transport of glucose and amino acids along with sodium ions
through the apical side of the tubular epithelial cells, followed by
facilitated diffusion through the basolateral membranes. The lower
cell shows the counte

lateral membranes. The lower
cell shows the counter-transport of hydrogen ions from the interior
of the cell across the apical membrane and into the tubular lumen;
movement of sodium ions into the cell,down an electrochemical gra-
dient established by the sodium-potassium pump on the basolateral
membrane, provides the energy for transport of the hydrogen ions
from inside the cell into the tubular lumen.
Chapter 27

II. Tubular Processing of the Glomerular Filtrate
331
The glucose transport system in the proximal tubule
is a good example. Normally, measurable glucose does
not appear in the urine because essentially all the ﬁl-
tered glucose is reabsorbed in the proximal tubule.
However, when the ﬁltered load exceeds the capabil-
ity of the tubules to reabsorb glucose, urinary excre-
tion of glucose does occur.
In the adult human, the transport maximum for
glucose averages about 375 mg/min, whereas the ﬁl-
t

ucose averages about 375 mg/min, whereas the ﬁl-
tered load of glucose is only about 125 mg/min (GFR
¥ plasma glucose = 125 ml/min ¥ 1 mg/ml). With large
increases in GFR and/or plasma glucose concentration
that increase the ﬁltered load of glucose above 375 mg/
min, the excess glucose ﬁltered is not reabsorbed and
passes into the urine.
Figure 27–4 shows the relation between plasma con-
centration of glucose, ﬁltered load of glucose, tubular
transport maximum for glucose, and rate of glucose
lo

nsport maximum for glucose, and rate of glucose
loss in the urine. Note that when the plasma glucose
concentration is 100 mg/100 mL and the ﬁltered load
is at its normal level, 125 mg/min, there is no loss of
glucose in the urine. However, when the plasma con-
centration of glucose rises above about 200 mg/100 ml,
increasing the ﬁltered load to about 250 mg/min, a
small amount of glucose begins to appear in the urine.
This point is termed the threshold for glucose. Note
that this appearance of g

eshold for glucose. Note
that this appearance of glucose in the urine (at the
threshold) occurs before the transport maximum is
reached. One reason for the difference between
threshold and transport maximum is that not all
nephrons have the same transport maximum for
glucose, and some of the nephrons excrete glucose
before others have reached their transport maximum.
The overall transport maximum for the kidneys, which
is normally about 375 mg/min, is reached when all
nephrons have reached their

n, is reached when all
nephrons have reached their maximal capacity to reab-
sorb glucose.
The plasma glucose of a healthy person almost
never becomes high enough to cause excretion of
glucose in the urine, even after eating a meal.
However, in uncontrolled diabetes mellitus, plasma
glucose may rise to high levels,causing the ﬁltered load
of glucose to exceed the transport maximum and
resulting in urinary glucose excretion. Some of the
important transport maximums for substances actively
reabsor

transport maximums for substances actively
reabsorbed by the tubules are as follows:
Substance
Transport Maximum
Glucose
375 mg/min
Phosphate
0.10 mM/min
Sulfate
0.06 mM/min
1.5 mM/min
Urate
15 mg/min
Lactate
75 mg/min
Plasma protein
30 mg/min
Transport Maximums for Substances That Are Actively
Secreted. Substances that are actively secreted also
exhibit transport maximums as follows:
Substance
16 mg/min
Para-aminohippuric acid
80 mg/min
Substances That A

ara-aminohippuric acid
80 mg/min
Substances That Are Actively Transported but Do Not Exhibit a
Transport Maximum. The reason that actively trans-
ported solutes often exhibit a transport maximum is
that the transport carrier system becomes saturated as
the tubular load increases.Substances that are passively
reabsorbed do not demonstrate a transport maximum
because their rate of transport is determined by other
factors, such as (1) the electrochemical gradient for
diffusion of the substance acro

mical gradient for
diffusion of the substance across the membrane, (2)
the permeability of the membrane for the substance,
and (3) the time that the ﬂuid containing the substance
remains within the tubule. Transport of this type is
referred to as gradient-time transport because the rate
of transport depends on the electrochemical gradient
and the time that the substance is in the tubule, which
in turn depends on the tubular ﬂow rate.
Some actively transported substances also have char-
acteristi

y transported substances also have char-
acteristics of gradient-time transport. An example is
sodium reabsorption in the proximal tubule. The 
main reason that sodium transport in the proximal
tubule does not exhibit a transport maximum is that
other factors limit the reabsorption rate besides the
maximum rate of active transport. For example, in the
proximal tubules, the maximum transport capacity of
the basolateral sodium-potassium ATPase pump is
usually far greater than the actual rate of ne

 is
usually far greater than the actual rate of net sodium
Glucose filtered load, reabsorption
or excretion (mg/min)
Plasma glucose concentration
(mg/100 ml)
0
800
700
600
500
400
300
200
100
Filtered
load
Filtered
load
900
800
700
600
500
400
300
200
100
Normal
Threshold
Transport
maximum
Excretion
0
Figure 27–4
Relations among the ﬁltered load of glucose, the rate of glucose
reabsorption by the renal tubules, and the rate of glucose excretion
in the urine. The transport maximum is

e excretion
in the urine. The transport maximum is the maximum rate at which
glucose can be reabsorbed from the tubules. The threshold for
glucose refers to the ﬁltered load of glucose at which glucose ﬁrst
begins to be excreted in the urine.
332
Unit V
reabsorption. One of the reasons for this is that a sig-
niﬁcant amount of sodium transported out of the 
cell leaks back into the tubular lumen through the
epithelial tight junctions. The rate at which this back-
leak

tight junctions. The rate at which this back-
leak occurs depends on several factors, including (1)
the permeability of the tight junctions and (2) the
interstitial physical forces, which determine the rate of
bulk ﬂow reabsorption from the interstitial ﬂuid into
the peritubular capillaries. Therefore, sodium trans-
port in the proximal tubules obeys mainly gradient-
time 
transport 
principles 
rather 
than 
tubular
maximum transport characteristics. This means that
the greater the concentratio

tics. This means that
the greater the concentration of sodium in the proxi-
mal tubules, the greater its reabsorption rate.Also, the
slower the ﬂow rate of tubular ﬂuid, the greater the
percentage of sodium that can be reabsorbed from 
the proximal tubules.
In the more distal parts of the nephron, the epithe-
lial cells have much tighter junctions and transport
much smaller amounts of sodium. In these segments,
sodium reabsorption exhibits a transport maximum
similar to that for other actively t

sport maximum
similar to that for other actively transported substances.
Furthermore, this transport maximum can be increased
in response to certain hormones, such as aldosterone.
Passive Water Reabsorption 
by Osmosis Is Coupled Mainly 
to Sodium Reabsorption
When solutes are transported out of the tubule by
either primary or secondary active transport, their
concentrations tend to decrease inside the tubule
while increasing in the renal interstitium. This creates
a concentration difference tha

itium. This creates
a concentration difference that causes osmosis of
water in the same direction that the solutes are trans-
ported, from the tubular lumen to the renal intersti-
tium. Some parts of the renal tubule, especially the
proximal tubule, are highly permeable to water, and
water reabsorption occurs so rapidly that there is only
a small concentration gradient for solutes across the
tubular membrane.
A large part of the osmotic ﬂow of water occurs
through the so-called tight junctions b

ter occurs
through the so-called tight junctions between the
epithelial cells as well as through the cells themselves.
The reason for this, as already discussed, is that the
junctions between the cells are not as tight as their
name would imply, and they allow signiﬁcant diffusion
of water and small ions. This is especially true in the
proximal tubules, which have a high permeability for
water and a smaller but signiﬁcant permeability to
most ions, such as sodium, chloride, potassium,
calcium, a

s, such as sodium, chloride, potassium,
calcium, and magnesium.
As water moves across the tight junctions by
osmosis, it can also carry with it some of the solutes, a
process referred to as solvent drag. And because the
reabsorption of water, organic solutes, and ions is
coupled to sodium reabsorption, changes in sodium
reabsorption signiﬁcantly inﬂuence the reabsorption
of water and many other solutes.
In the more distal parts of the nephron, beginning
in the loop of Henle and extending through

ginning
in the loop of Henle and extending through the col-
lecting tubule, the tight junctions become far less per-
meable to water and solutes, and the epithelial cells
also have a greatly decreased membrane surface area.
Therefore, water cannot move easily across the tubular
membrane by osmosis. However, antidiuretic hormone
(ADH) greatly increases the water permeability in the
distal and collecting tubules, as discussed later.
Thus, water movement across the tubular epithe-
lium can occur on

ement across the tubular epithe-
lium can occur only if the membrane is permeable to
water, no matter how large the osmotic gradient. In the
proximal tubule, the water permeability is always high,
and water is reabsorbed as rapidly as the solutes. In
the ascending loop of Henle, water permeability is
always low, so that almost no water is reabsorbed,
despite a large osmotic gradient.Water permeability in
the last parts of the tubules—the distal tubules, col-
lecting tubules, and collecting ducts

ubules, col-
lecting tubules, and collecting ducts—can be high or
low, depending on the presence or absence of ADH.
Reabsorption of Chloride, Urea, and
Other Solutes by Passive Diffusion
When sodium is reabsorbed through the tubular
epithelial cell, negative ions such as chloride are trans-
ported along with sodium because of electrical poten-
tials. That is, transport of positively charged sodium
ions out of the lumen leaves the inside of the lumen
negatively charged, compared with the intersti

men
negatively charged, compared with the interstitial
ﬂuid. This causes chloride ions to diffuse passively
through the paracellular pathway. Additional reab-
sorption of chloride ions occurs because of a chloride
concentration gradient that develops when water is
reabsorbed from the tubule by osmosis, thereby con-
centrating the chloride ions in the tubular lumen
(Figure 27–5). Thus, the active reabsorption of sodium
is closely coupled to the passive reabsorption of chlo-
ride by way of an elec

ssive reabsorption of chlo-
ride by way of an electrical potential and a chloride
concentration gradient.
Passive Cl-
Passive urea
Na+ reabsorption
H2O reabsorption
Lumen
negative
potential
Lumen
negative
potential
Luminal Cl-
Luminal
urea
Luminal
urea
Figure 27–5
Mechanisms by which water, chloride, and urea reabsorption are
coupled with sodium reabsorption.
Chapter 27
Urine Formation by the Kidneys: I

tion.
Chapter 27
333
Chloride ions can also be reabsorbed by secondary
active transport. The most important of the secondary
active transport processes for chloride reabsorption
involves co-transport of chloride with sodium across
the luminal membrane.
Urea is also passively reabsorbed from the tubule,
but to a much lesser extent than chloride ions. As
water is reabsorbed from the tubules (by osmosis
coupled to sod

sorbed from the tubules (by osmosis
coupled to sodium reabsorption), urea concentration
in the tubular lumen increases (see Figure 27–5). This
creates a concentration gradient favoring the reab-
sorption of urea. However, urea does not permeate 
the tubule as readily as water. In some parts of the
nephron, especially the inner medullary collecting
duct, passive urea reabsorption is facilitated by speciﬁc
urea transporters. Yet only about one half of the urea
that is ﬁltered by the glomerular cap

 of the urea
that is ﬁltered by the glomerular capillaries is reab-
sorbed from the tubules. The remainder of the urea
passes into the urine, allowing the kidneys to excrete
large amounts of this waste product of metabolism.
Another waste product of metabolism, creatinine, is
an even larger molecule than urea and is essentially
impermeant to the tubular membrane. Therefore,
almost none of the creatinine that is ﬁltered is reab-
sorbed, so that virtually all the creatinine ﬁltered by
the glomerul

rtually all the creatinine ﬁltered by
the glomerulus is excreted in the urine.
Along Different Parts of 
the Nephron
In the previous sections, we discussed the basic princi-
ples by which water and solutes are transported across
the tubular membrane. With these generalizations in
mind, we can now discuss the different characteristics
of the individual tubular segments that enable them to
perform their speciﬁc excretory functions. Only the
tubular transport functions th

functions. Only the
tubular transport functions that are quantitatively
most important are discussed, especially as they relate
to the reabsorption of sodium, chloride, and water. In
subsequent chapters, we discuss the reabsorption and
secretion of other speciﬁc substances in different parts
of the tubular system.
Normally, about 65 per cent of the ﬁltered load of
sodium and water and a slightly lower percentage of
ﬁltered chloride are reabsorbed by the proximal
tub

ltered chloride are reabsorbed by the proximal
tubule before the ﬁltrate reaches the loops of Henle.
These percentages can be increased or decreased in
different physiologic conditions, as discussed later.
Proximal Tubules Have a High Capacity for Active and Passive
Reabsorption. The high capacity of the proximal tubule
for reabsorption results from its special cellular char-
acteristics, as shown in Figure 27–6. The proximal
tubule epithelial cells are highly metabolic and have
large numbers of

lls are highly metabolic and have
large numbers of mitochondria to support potent
active transport processes. In addition, the proximal
tubular cells have an extensive brush border on the
luminal (apical) side of the membrane as well as an
extensive labyrinth of intercellular and basal channels,
all of which together provide an extensive membrane
surface area on the luminal and basolateral sides of the
epithelium for rapid transport of sodium ions and
other substances.
The extensive membrane sur

s and
The extensive membrane surface of the epithelial
brush border is also loaded with protein carrier mole-
cules that transport a large fraction of the sodium ions
across the luminal membrane linked by way of the co-
transport mechanism with multiple organic nutrients
such as amino acids and glucose.The remainder of the
sodium is transported from the tubular lumen into the
cell by counter-transport mechanisms, which reabsorb
sodium while secreting other substances into the
t

tubular lumen, especially hydrogen ions. As discussed
in Chapter 30, the secretion of hydrogen ions into the
tubular lumen is an important step in the removal of
bicarbonate ions from the tubule (by combining H+
with the HCO3
_ to form H2CO3, which then dissociates
into H2O and CO2).
Although the sodium-potassium ATPase pump pro-
vides the major force for reabsorption of sodium,
chloride, and water throughout the proximal tubule,
there are some di

 throughout the proximal tubule,
there are some differences in the mechanisms by
which sodium and chloride are transported through
the luminal side of the early and late portions of the
proximal tubular membrane.
In the ﬁrst half of the proximal tubule, sodium is
reabsorbed by co-transport along with glucose, amino
acids, and other solutes. But in the second half of the
proximal tubule, little glucose and amino acids remain
to be reabsorbed. Instead, sodium is now reabsorbed
mainly with chloride

ead, sodium is now reabsorbed
mainly with chloride ions.The second half of the prox-
imal tubule has a relatively high concentration of 
chloride (around 140 mEq/L) compared with the early
65%
Isosmotic
H+, organic acids, bases
Na+, Cl-, HCO3
-, K+,
H2O, glucose, amino acids
Figure 27–6
Cellular ultrastructure and primary transport characteristics of the
proximal tubule. The proximal tubules reabsorb about 65 per cent
of the ﬁltered sodium, chloride, bicarbonate, and potassium an

ed sodium, chloride, bicarbonate, and potassium and
essentially all the ﬁltered glucose and amino acids. The proximal
tubules also secrete organic acids, bases, and hydrogen ions into the
tubular lumen.
334
Unit V
proximal tubule (about 105 mEq/L) because when
sodium is reabsorbed, it preferentially carries with it
glucose, bicarbonate, and organic ions in the early
proximal tubule, leaving behind a solution that has a
higher concentration of chloride. In the second h

higher concentration of chloride. In the second half of
the proximal tubule, the higher chloride concentration
favors the diffusion of this ion from the tubule lumen
through the intercellular junctions into the renal inter-
Concentrations of Solutes Along the Proximal Tubule. Figure
27–7 summarizes the changes in concentrations of
various solutes along the proximal tubule. Although
the amount of sodium in the tubular ﬂuid decreases
markedly along the proximal tubule, the concentra

markedly along the proximal tubule, the concentration
of sodium (and the total osmolarity) remains relatively
constant because water permeability of the proximal
tubules is so great that water reabsorption keeps pace
with sodium reabsorption. Certain organic solutes,
such as glucose, amino acids, and bicarbonate, are
much more avidly reabsorbed than water, so that their
concentrations decrease markedly along the length of
the proximal tubule. Other organic solutes that are less
permeant and not

her organic solutes that are less
permeant and not actively reabsorbed, such as creati-
nine, increase their concentration along the proximal
tubule. The total solute concentration, as reﬂected by
osmolarity, remains essentially the same all along the
proximal tubule because of the extremely high per-
meability of this part of the nephron to water.
Secretion of Organic Acids and Bases by the Proximal Tubule.
The proximal tubule is also an important site for secre-
tion of organic acids and bases

nt site for secre-
tion of organic acids and bases such as bile salts,
oxalate, urate, and catecholamines. Many of these sub-
stances are the end products of metabolism and must
be rapidly removed from the body. The secretion of
these substances into the proximal tubule plus ﬁltra-
tion into the proximal tubule by the glomerular capil-
laries and the almost total lack of reabsorption by the
tubules, all combined, contribute to rapid excretion in
the urine.
In addition to the waste products of me

the urine.
In addition to the waste products of metabolism, the
kidneys secrete many potentially harmful drugs or
toxins directly through the tubular cells into the
tubules and rapidly clear these substances from the
blood. In the case of certain drugs, such as penicillin
and salicylates, the rapid clearance by the kidneys
creates a problem in maintaining a therapeutically
effective drug concentration.
Another compound that is rapidly secreted by the
proximal tubule is para-aminohippuric acid (P

 the
proximal tubule is para-aminohippuric acid (PAH).
PAH is secreted so rapidly that the average person 
can clear about 90 per cent of the PAH from the
plasma ﬂowing through the kidneys and excrete it in
the urine. For this reason, the rate of PAH clearance
can be used to estimate the renal plasma ﬂow, as dis-
cussed later.
Solute and Water Transport in the
Loop of Henle
The loop of Henle consists of three functionally dis-
tinct segments: the thin descending segment, the thin
ascending segme

 thin descending segment, the thin
ascending segment, and the thick ascending segment.
The thin descending and thin ascending segments, as
their names imply, have thin epithelial membranes
with no brush borders, few mitochondria, and minimal
levels of metabolic activity (Figure 27–8).
The descending part of the thin segment is highly
permeable to water and moderately permeable to
most solutes, including urea and sodium. The function
of this nephron segment is mainly to allow simple dif-
fusion o

on segment is mainly to allow simple dif-
fusion of substances through its walls. About 20 per
cent of the ﬁltered water is reabsorbed in the loop of
Henle,and almost all of this occurs in the thin descend-
ing limb. The ascending limb, including both the thin
and the thick portions, is virtually impermeable to
water, a characteristic that is important for concen-
trating the urine.
The thick segment of the loop of Henle, which
begins about halfway up the ascending limb, has thick
epithelial cel

ay up the ascending limb, has thick
epithelial cells that have high metabolic activity and
are capable of active reabsorption of sodium, chloride,
and potassium (see Figure 27–8).About 25 per cent of
the ﬁltered loads of sodium, chloride, and potassium
are reabsorbed in the loop of Henle, mostly in the
thick ascending limb. Considerable amounts of other
ions, such as calcium, bicarbonate, and magnesium, are
also reabsorbed in the thick ascending loop of Henle.
The thin segment of the ascending l

loop of Henle.
The thin segment of the ascending limb has a much
lower reabsorptive capacity than the thick segment,
% Total proximal tubule length
Tubular fluid / plasma concentration
20
0
Glucose
Urea
HCO3
-
Cl-
Na+
40
60
80
100
5.0
2.0
1.0
0.5
0.05
0.2
0.1
0.01
Figure 27–7
Changes in concentrations of different substances in tubular ﬂuid
along the proximal convoluted tubule relative to the concentrations
of these substances in the plasma and in the glomerular

ese substances in the plasma and in the glomerular ﬁltrate. A
value of 1.0 indicates that the concentration of the substance in the
tubular ﬂuid is the same as the concentration in the plasma. Values
below 1.0 indicate that the substance is reabsorbed more avidly than
water, whereas values above 1.0 indicate that the substance is reab-
sorbed to a lesser extent than water or is secreted into the tubules.
Chapter 27

 II. Tubular Processing of the Glomerular Filtrate
335
and the thin descending limb does not reabsorb sig-
niﬁcant amounts of any of these solutes.
An important component of solute reabsorption in
the thick ascending limb is the sodium-potassium
ATPase pump in the epithelial cell basolateral mem-
branes. As in the proximal tubule, the reabsorption of
other solutes in the thick segment of the ascending
loop of Henle is closely linked to the reabsorptive
capability of the sodium-potassium ATPase p

rptive
capability of the sodium-potassium ATPase pump,
which maintains a low intracellular sodium concentra-
tion. The low intracellular sodium concentration in
turn provides a favorable gradient for movement 
of sodium from the tubular ﬂuid into the cell. In the
thick ascending loop, movement of sodium across 
the luminal membrane is mediated primarily by a 
1-sodium,
2-chloride,
1-potassium 
co-transporter
(Figure 27–9). This co-transport protein carrier in the
luminal membrane uses the potent

in carrier in the
luminal membrane uses the potential energy released
by downhill diffusion of sodium into the cell to drive
the reabsorption of potassium into the cell against a
The thick ascending limb of the loop of Henle is 
the site of action of the powerful “loop” diuretics
furosemide, ethacrynic acid, and bumetanide, all of
which inhibit the action of the sodium 2-chloride,
potassium co-transporter. These diuretics are dis-
cussed in Chapter 31.
There is also signi

are dis-
There is also signiﬁcant paracellular reabsorption of
cations, such as Mg++, Ca++, Na+, and K+, in the thick
ascending limb owing to the slight positive charge of
the tubular lumen relative to the interstitial ﬂuid.
Although the 1-sodium, 2-chloride, 1-potassium co-
transporter moves equal amounts of cations and
anions into the cell, there is a slight backleak of potas-
sium ions into the lumen, creating a positive charge of
about +8 millivolts in the tubular lumen

charge of
about +8 millivolts in the tubular lumen. This positive
charge forces cations such as Mg++ and Ca++ to diffuse
from the tubular lumen through the paracellular space
and into the interstitial ﬂuid.
The thick ascending limb also has a sodium-
hydrogen counter-transport mechanism in its luminal
cell membrane that mediates sodium reabsorption and
hydrogen secretion in this segment.
The thick segment of the ascending loop of Henle is
virtually impermeable to water. Therefore, most of the
wa

ly impermeable to water. Therefore, most of the
water delivered to this segment remains in the tubule,
Thick ascending
loop of Henle
25%
Hypo-
osmotic
H+
Na+, Cl-, K+,
Ca++, HCO3
-, Mg++
Thin descending
H2O
Figure 27–8
Cellular ultrastructure and transport characteristics of the thin
descending loop of Henle (top) and the thick ascending segment of
the loop of Henle (bottom).The descending part of the thin segment
of the loop of Henle is highly permeable to water and moderately
per

le is highly permeable to water and moderately
permeable to most solutes but has few mitochondria and little or no
active reabsorption. The thick ascending limb of the loop of Henle
reabsorbs about 25 per cent of the ﬁltered loads of sodium, chloride,
and potassium, as well as large amounts of calcium, bicarbonate,
and magnesium. This segment also secretes hydrogen ions into the
Renal
interstitial
fluid
Tubular
lumen
(+8 mV)
Tubular
cells
diffusion
Mg++, Ca++
Mg++, Ca

+8 mV)
Tubular
cells
diffusion
Mg++, Ca++
Mg++, Ca++
Na+, K+
Na+, K+
ATP
Na+
Na+
Cl-
Cl-
K+
K+
K+
K+
Na+
Na+
H+
H+
• Furosemide
• Ethacrynic acid
• Bumetanide
-
Na+
Na+
2Cl-
2Cl-
K+
K+
Figure 27–9
Mechanisms of sodium, chloride, and potassium transport in the
thick ascending loop of Henle.The sodium-potassium ATPase pump
in the basolateral cell membrane maintains a low intracellular
sodium concentration and a negative electrical potential in the cell.
The 1-sodium, 2-chloride, 1-p

tential in the cell.
The 1-sodium, 2-chloride, 1-potassium co-transporter in the luminal
membrane transports these three ions from the tubular lumen into
the cells, using the potential energy released by diffusion of sodium
down an electrochemical gradient into the cells. Sodium is also
transported into the tubular cell by sodium-hydrogen counter-trans-
port. The positive charge (+8 mV) of the tubular lumen relative to
the interstitial ﬂuid forces cations such as Mg++ and Ca++ to diffuse
from th

s cations such as Mg++ and Ca++ to diffuse
from the lumen to the interstitial ﬂuid via the paracellular pathway.
336
Unit V
despite reabsorption of large amounts of solute. The
tubular ﬂuid in the ascending limb becomes very dilute
as it ﬂows toward the distal tubule, a feature that is
important in allowing the kidneys to dilute or concen-
trate the urine under different conditions, as we
discuss much more fully in Chapter 28.
The thick segment of the as

pter 28.
The thick segment of the ascending limb of the loop
of Henle empties into the distal tubule. The very ﬁrst
portion of the distal tubule forms part of the juxta-
glomerular complex that provides feedback control of
GFR and blood ﬂow in this same nephron. The next
part of the distal tubule is highly convoluted and has
many of the same reabsorptive characteristics of the
thick segment of the ascending limb of the loop of
Henle. That is, it avidly reabsorbs most of the ions,
i

. That is, it avidly reabsorbs most of the ions,
including sodium, potassium, and chloride, but is vir-
tually impermeable to water and urea. For this reason,
it is referred to as the diluting segment because it also
dilutes the tubular ﬂuid.
Approximately 5 percent of the ﬁltered load of
sodium chloride is reabsorbed in the early distal
tubule. The sodium-chloride co-transporter moves
sodium chloride from the tubular lumen into the cell,
and the sodium-potassium ATPase pump transports
sodium ou

 sodium-potassium ATPase pump transports
sodium out of the cell across the basolateral mem-
brane (Figure 27–10). Chloride diffuses out of the cell
into the renal interstitial ﬂuid through chloride chan-
nels in the basolateral membrane. The thiazide diuret-
ics, which are widely used to treat disorders such as
hypertension and heart failure, inhibit the sodium-
chloride co-transporter.
Late Distal Tubule and Cortical
The second half of the distal tubule and the subse-
quent co

 half of the distal tubule and the subse-
quent cortical collecting tubule have similar functional
characteristics. Anatomically, they are composed of
two distinct cell types, the principal cells and the inter-
calated cells (Figure 27–11). The principal cells reab-
sorb sodium and water from the lumen and secrete
potassium ions into the lumen. The intercalated cells
reabsorb potassium ions and secrete hydrogen ions
into the tubular lumen.
Principal Cells Reabsorb Sodium and Secrete Potassium.
S

pal Cells Reabsorb Sodium and Secrete Potassium.
Sodium reabsorption and potassium secretion by the
principal cells depend on the activity of a sodium-
potassium ATPase pump in each cell’s basolateral
membrane (Figure 27–12). This pump maintains a low
sodium concentration inside the cell and, therefore,
favors sodium diffusion into the cell through special
Renal
fluid
Tubular
lumen
(-10mV)
Tubular
cells
ATP
Na+
Na+
Cl-
Cl-
K+
K+
Thiazide diuretics:
-
Na+
Na+
Cl-

zide diuretics:
-
Na+
Na+
Cl-
Cl-
Figure 27–10
Mechanism of sodium chloride transport in the early distal tubule.
Sodium and chloride are transported from the tubular lumen into
the cell by a co-transporter that is inhibited by thiazide diuretics.
Sodium is pumped out of the cell by sodium-potassium ATPase and
chloride diffuses into the interstitial ﬂuid via chloride channels.
Intercalated
cells
Early distal tubule
Late distal tubule
and collecting tubule
Principal
cells
Na+,

 tubule
Principal
cells
Na+, Cl-, Ca++, Mg++
Na+, Cl-
(+ADH) H2O
HCO3
-
H+
K+
K+
Figure 27–11
Cellular ultrastructure and transport characteristics of the early
distal tubule and the late distal tubule and collecting tubule. The
early distal tubule has many of the same characteristics as the thick
ascending loop of Henle and reabsorbs sodium, chloride, calcium,
and magnesium but is virtually impermeable to water and urea.The
late distal tubules and cortical collecting tubul

late distal tubules and cortical collecting tubules are composed of
two distinct cell types, the principal cells and the intercalated cells.
The principal cells reabsorb sodium from the lumen and secrete
potassium ions into the lumen.The intercalated cells reabsorb potas-
sium and bicarbonate ions from the lumen and secrete hydrogen
ions into the lumen. The reabsorption of water from this tubular
segment is controlled by the concentration of antidiuretic hormone.
Chapter 27
Urine Formation by t

idiuretic hormone.
Chapter 27
337
channels. The secretion of potassium by these cells
from the blood into the tubular lumen involves two
steps: (1) Potassium enters the cell because of the
sodium-potassium ATPase pump, which maintains a
high intracellular potassium concentration, and then
(2) once in the cell, potassium diffuses down its con-
centration gradient across the luminal membrane into
the tubular ﬂuid.
Th

oss the luminal membrane into
The principal cells are the primary sites of action 
of the potassium-sparing diuretics, including spirono-
lactone,
eplerenone,
amiloride,
and triamterene.
Aldosterone antagonists compete with aldosterone for
receptor sites in the principal cells and therefore
inhibit the stimulatory effects of aldosterone on
sodium reabsorption and potassium secretion. Sodium
channel blockers directly inhibit the entry of sodium
into the sodium channels of the lu

entry of sodium
into the sodium channels of the luminal membranes
and therefore reduce the amount of sodium that can
be transported across the basolateral membranes by
the sodium-potassium ATPase pump. This, in turn,
decreases transport of potassium into the cells and
ultimately reduces potassium secretion into the
tubular ﬂuid. For this reason the sodium channel
blockers as well as the aldosterone antagonists
decrease urinary excretion of potassium and act as
potassium-sparing diuretics.
Interc

ium and act as
Intercalated Cells Avidly Secrete Hydrogen and Reabsorb Bicar-
bonate and Potassium Ions. Hydrogen ion secretion by
the intercalated cells is mediated by a hydrogen-
ATPase transport mechanism. Hydrogen is generated
in this cell by the action of carbonic anhydrase on
water and carbon dioxide to form carbonic acid, which
then dissociates into hydrogen ions and bicarbonate
ions. The hydrogen ions are then secreted into the
tubular lumen, and for each hyd

 secreted into the
tubular lumen, and for each hydrogen ion secreted, a
bicarbonate ion becomes available for reabsorption
across the basolateral membrane.A more detailed dis-
cussion of this mechanism is presented in Chapter 30.
The intercalated cells can also reabsorb potassium
ions.
The functional characteristics of the late distal tubule
and cortical collecting tubule can be summarized as
follows:
1. The tubular membranes of both segments are
almost completely impermeable to urea, similar 
t

to the diluting segment of the early distal tubule;
thus, almost all the urea that enters these
segments passes on through and into the
collecting duct to be excreted in the urine,
although some reabsorption of urea occurs 
in the medullary collecting ducts.
2. Both the late distal tubule and the cortical
collecting tubule segments reabsorb sodium ions,
and the rate of reabsorption is controlled by
hormones, especially aldosterone. At the same
tim

time, these segments secrete potassium ions 
from the peritubular capillary blood into the
tubular lumen, a process that is also controlled 
by aldosterone and by other factors such as 
the concentration of potassium ions in the body
ﬂuids.
3. The intercalated cells of these nephron segments
avidly secrete hydrogen ions by an active
hydrogen-ATPase mechanism. This process is
different from the secondary active secretion of
hydrogen ions by the proxi

ary active secretion of
hydrogen ions by the proximal tubule because it is
capable of secreting hydrogen ions against a large
concentration gradient, as much as 1000 to 1. This
is in contrast to the relatively small gradient (4- to
10-fold) for hydrogen ions that can be achieved
by secondary active secretion in the proximal
tubule. Thus, the intercalated cells play a key role
in acid-base regulation of the body ﬂuids.
4. The permeability of the late distal tubule and
cortical collecting duct to 

ate distal tubule and
cortical collecting duct to water is controlled by
the concentration of ADH, which is also called
vasopressin. With high levels of ADH, these
tubular segments are permeable to water, but 
in the absence of ADH, they are virtually
impermeable to water. This special characteristic
provides an important mechanism for controlling
the degree of dilution or concentration of the
urine.
Although the medullary collecting ducts reabsorb less
than 10 per cent

ry collecting ducts reabsorb less
than 10 per cent of the ﬁltered water and sodium, they
are the ﬁnal site for processing the urine and, there-
fore, play an extremely important role in determining
the ﬁnal urine output of water and solutes.
The epithelial cells of the collecting ducts are nearly
cuboidal in shape with smooth surfaces and relatively
Renal
fluid
Tubular
lumen
(-50 mV)
Tubular
cells
ATP
Na+
Na+
K+
K+
Na+
Na+
Cl-
Cl-
K+
K+
Na+ channel blockers
• Amiloride
• Triamterene

K+
-
Aldosterone antagonists
• Spironolactone
• Eplerenone
-
Figure 27–12
Mechanism of sodium chloride reabsorption and potassium secre-
tion in the late distal tubules and cortical collecting tubules. Sodium
enters the cell through special channels and is transported out of the
cell by the sodium-potassium ATPase pump. Aldosterone antago-
nists compete with aldosterone for binding sites in the cell and
therefore inhibit the effects of aldosterone 

 and
therefore inhibit the effects of aldosterone to stimulate sodium reab-
sorption and potassium secretion. Sodium channel blockers directly
inhibit the entry of sodium into the sodium channels.
338
Unit V
few mitochondria (Figure 27–13). Special characteris-
tics of this tubular segment are as follows:
1. The permeability of the medullary collecting duct
to water is controlled by the level of ADH. With
high levels of ADH, water is avidly reabsorbed
into the medulla

f ADH, water is avidly reabsorbed
into the medullary interstitium, thereby reducing
the urine volume and concentrating most of the
solutes in the urine.
2. Unlike the cortical collecting tubule, the
medullary collecting duct is permeable to urea.
Therefore, some of the tubular urea is reabsorbed
into the medullary interstitium, helping to raise
the osmolality in this region of the kidneys and
contributing to the kidneys’ overall ability to form
a concentrated urine.
3. The medullary collecting d

 concentrated urine.
3. The medullary collecting duct is capable of
secreting hydrogen ions against a large
concentration gradient, as also occurs in the
cortical collecting tubule. Thus, the medullary
collecting duct also plays a key role in regulating
acid-base balance.
Summary of Concentrations of
Different Solutes in the Different
Tubular Segments
Whether a solute will become concentrated in the
tubular ﬂuid is determined by the relative degree of
reabsorption of that solute versus the reabs

ee of
reabsorption of that solute versus the reabsorption of
water. If a greater percentage of water is reabsorbed,
the substance becomes more concentrated. If a greater
percentage of the solute is reabsorbed, the substance
becomes more diluted.
Figure 27–14 shows the degree of concentration of
several substances in the different tubular segments.
All the values in this ﬁgure represent the tubular ﬂuid
concentration divided by the plasma concentration of
a substance. If plasma concentration of t

ation of
a substance. If plasma concentration of the substance
is assumed to be constant, any change in the ratio 
of tubular ﬂuid/plasma concentration rate reﬂects
changes in tubular ﬂuid concentration.
As the ﬁltrate moves along the tubular system, the
concentration rises to progressively greater than 1.0 if
more water is reabsorbed than solute, or if there has
been a net secretion of the solute into the tubular ﬂuid.
If the concentration ratio becomes progressively less
than 1.0, this means t

 becomes progressively less
than 1.0, this means that relatively more solute has
been reabsorbed than water.
The substances represented at the top of Figure
27–14, such as creatinine, become highly concentrated
in the urine. In general, these substances are not
needed by the body, and the kidneys have become
adapted to reabsorb them only slightly or not at all, or
even to secrete them into the tubules, thereby excret-
ing especially great quantities into the urine. Con-
versely, the substances r

ies into the urine. Con-
versely, the substances represented toward the bottom
of the ﬁgure, such as glucose and amino acids, are all
strongly reabsorbed; these are all substances that the
body needs to conserve, and almost none of them are
lost in the urine.
Medullary
collecting duct
Na+, Cl-
Urea
HCO3
-
(+ADH) H2O
H+
Figure 27–13
Cellular ultrastructure and transport characteristics of the
medullary collecting duct. The medullary collecting ducts actively
reabsorb sodium and secrete hydrogen i

ts actively
reabsorb sodium and secrete hydrogen ions and are permeable to
urea, which is reabsorbed in these tubular segments. The reabsorp-
tion of water in medullary collecting ducts is controlled by the con-
centration of antidiuretic hormone.
Tubular fluid/plasma concentration
Proximal
tubule
Loop of
Henle
Distal
tubule
tubule
PAH
PAH
Cl
Cl
Cl
Cl
K
Na
Na
to 585
to 585
to 140
to 140
to 125
to 125
HCO3
HCO3
K
and Na
K
and Na
Glucose
Glucose
Protein
Protein
Ami

ine
Glucose
Glucose
Protein
Protein
Inulin
Inulin
Urea
Urea
100.0
50.0
20.0
10.0
5.0
2.0
1.0
0.50
0.20
0.10
0.05
0.02
Figure 27–14
Changes in average concentrations of different substances at differ-
ent points in the tubular system relative to the concentration of that
substance in the plasma and in the glomerular ﬁltrate.A value of 1.0
indicates that the concentration of the substance in the tubular ﬂuid
is the same as the concentration of that substance in t

e same as the concentration of that substance in the plasma.
Values below 1.0 indicate that the substance is reabsorbed more
avidly than water, whereas values above 1.0 indicate that the sub-
stance is reabsorbed to a lesser extent than water or is secreted into
Chapter 27
339
Tubular Fluid /Plasma Inulin Concentration Ratio Can Be Used
to Measure Water Reabsorption by the Renal Tubules. Inulin,
a poly

 Reabsorption by the Renal Tubules. Inulin,
a polysaccharide used to measure GFR, is not reab-
sorbed or secreted by the renal tubules. Changes in
inulin concentration at different points along the renal
tubule, therefore, reﬂect changes in the amount of
water present in the tubular ﬂuid. For example, the
tubular ﬂuid/plasma concentration ratio for inulin
rises to about 3.0 at the end of the proximal tubules,
indicating that inulin concentration in the tubular ﬂuid
is 3 times greater than in the

in the tubular ﬂuid
is 3 times greater than in the plasma and in the
glomerular ﬁltrate. Since inulin is not secreted or reab-
sorbed from the tubules, a tubular ﬂuid/plasma con-
centration ratio of 3.0 means that only one third of the
water that was ﬁltered remains in the renal tubule and
that two thirds of the ﬁltered water has been reab-
sorbed as the ﬂuid passes through the proximal tubule.
At the end of the collecting ducts, the tubular
ﬂuid/plasma inulin concentration ratio rises to about

/plasma inulin concentration ratio rises to about
125 (see Figure 27–14), indicating that only 1/125 of
the ﬁltered water remains in the tubule and that more
than 99% has been reabsorbed.
Regulation of Tubular
Because it is essential to maintain a precise balance
between tubular reabsorption and glomerular ﬁltra-
tion, there are multiple nervous, hormonal, and local
control mechanisms that regulate tubular reabsorp-
tion, just as there are for control of glomerular ﬁltra-
tion. An i

re are for control of glomerular ﬁltra-
tion. An important feature of tubular reabsorption is
that reabsorption of some solutes can be regulated
independently of others, especially through hormonal
control mechanisms.
Glomerulotubular Balance—The
Ability of the Tubules to Increase
Reabsorption Rate in Response 
to Increased Tubular Load
One of the most basic mechanisms for controlling
tubular reabsorption is the intrinsic ability of the
tubules to increase their reabsorption rate in response
to 

o increase their reabsorption rate in response
to increased tubular load (increased tubular inﬂow).
This phenomenon is referred to as glomerulotubular
balance. For example, if GFR is increased from 125 ml/
min to 150 ml/min, the absolute rate of proximal
tubular reabsorption also increases from about 81 ml/
min (65 per cent of GFR) to about 97.5 ml/min (65 per
cent of GFR). Thus, glomerulotubular balance refers
to the fact that the total rate of reabsorption increases
as the ﬁltered load increas

reabsorption increases
as the ﬁltered load increases, even though the 
percentage of GFR reabsorbed in the proximal 
tubule remains relatively constant at about 65 per
cent.
Some degree of glomerulotubular balance also
occurs in other tubular segments, especially the loop
of Henle. The precise mechanisms responsible for this
are not fully understood but may be due partly to
changes in physical forces in the tubule and surround-
ing renal interstitium, as discussed later. It is clear that
the mec

tium, as discussed later. It is clear that
the mechanisms for glomerulotubular balance can
occur independently of hormones and can be demon-
strated in completely isolated kidneys or even in com-
pletely isolated proximal tubular segments.
The importance of glomerulotubular balance is that
it helps to prevent overloading of the distal tubular
segments when GFR increases. Glomerulotubular
balance acts as a second line of defense to buffer the
effects of spontaneous changes in GFR on urine
output.

cts of spontaneous changes in GFR on urine
output. (The ﬁrst line of defense, discussed earlier,
includes the renal autoregulatory mechanisms, espe-
cially tubuloglomerular feedback, which help prevent
changes in GFR.) Working together, the autoregula-
tory and glomerulotubular balance mechanisms
prevent large changes in ﬂuid ﬂow in the distal tubules
when the arterial pressure changes or when there 
are other disturbances that would otherwise wreak
havoc with the maintenance of sodium and volum

eak
havoc with the maintenance of sodium and volume
homeostasis.
Peritubular Capillary and Renal
Interstitial Fluid Physical Forces
Hydrostatic and colloid osmotic forces govern the rate
of reabsorption across the peritubular capillaries,
just as these physical forces control ﬁltration in the
glomerular capillaries. Changes in peritubular capil-
lary reabsorption can in turn inﬂuence the hydrostatic
and colloid osmotic pressures of the renal interstitium
and, ultimately, reabsorption of water an

rstitium
and, ultimately, reabsorption of water and solutes from
the renal tubules.
Normal Values for Physical Forces and Reabsorption Rate. As
the glomerular ﬁltrate passes through the renal
tubules, more than 99 per cent of the water and most
of the solutes are normally reabsorbed. Fluid and elec-
trolytes are reabsorbed from the tubules into the renal
interstitium and from there into the peritubular capil-
laries. The normal rate of peritubular capillary reab-
sorption is about 124 ml/min.
Re

r capillary reab-
Reabsorption across the peritubular capillaries can
be calculated as
Reabsorption = Kf ¥ Net reabsorptive force
The net reabsorptive force represents the sum of the
or oppose reabsorption across the peritubular capil-
laries. These forces include (1) hydrostatic pressure
inside the peritubular capillaries (peritubular hydro-
static pressure [Pc]), which opposes reabsorption;
(2) hydrostatic p

c]), which opposes reabsorption;
(2) hydrostatic pressure in the renal interstitium (Pif)
outside the capillaries, which favors reabsorption; (3)
colloid osmotic pressure of the peritubular capillary
plasma proteins (pc), which favors reabsorption; and
(4) colloid osmotic pressure of the proteins in the renal
interstitium (pif), which opposes reabsorption.
Figure 27–15 shows the approximate normal forces
that favor and oppose peritubular reabsorption.
340
Unit V
Becau

tion.
340
Unit V
Because the normal peritubular capillary pressure
averages about 13 mm Hg and renal interstitial ﬂuid
hydrostatic pressure averages 6 mm Hg, there is a pos-
itive hydrostatic pressure gradient from the peritubu-
lar capillary to the interstitial ﬂuid of about 7 mm Hg,
which opposes ﬂuid reabsorption. This is more than
counterbalanced by the colloid osmotic pressures that
favor reabsorption. The plasma colloid osmotic pres-
sure, which favors reabsorpt

colloid osmotic pres-
sure, which favors reabsorption, is about 32 mm Hg,
and the colloid osmotic pressure of the interstitium,
which opposes reabsorption, is 15 mm Hg, causing a
net colloid osmotic force of about 17 mm Hg, favoring
reabsorption. Therefore, subtracting the net hydro-
static forces that oppose reabsorption (7 mm Hg) from
the net colloid osmotic forces that favor reabsorption
(17 mm Hg) gives a net reabsorptive force of about 
10 mm Hg. This is a high value, similar to that found

m Hg. This is a high value, similar to that found
in the glomerular capillaries, but in the opposite 
The other factor that contributes to the high rate of
ﬂuid reabsorption in the peritubular capillaries is a
large ﬁltration coefﬁcient (Kf) because of the high
hydraulic conductivity and large surface area of the
capillaries. Because the reabsorption rate is normally
about 124 ml/min and net reabsorption pressure is 
10 mm Hg, Kf normally is about 12.4 ml/min/mm Hg.
Regulation of Peri

lly is about 12.4 ml/min/mm Hg.
Regulation of Peritubular Capillary Physical Forces. The two
determinants of peritubular capillary reabsorption
that are directly inﬂuenced by renal hemodynamic
changes are the hydrostatic and colloid osmotic pres-
sures of the peritubular capillaries. The peritubular
capillary hydrostatic pressure is inﬂuenced by the arte-
rial pressure and resistances of the afferent and efferent
arterioles. (1) Increases in arterial pressure tend to
raise peritubular capillary 

rial pressure tend to
raise peritubular capillary hydrostatic pressure and
decrease reabsorption rate. This effect is buffered to
some extent by autoregulatory mechanisms that main-
tain relatively constant renal blood ﬂow as well as 
relatively constant hydrostatic pressures in the renal
blood vessels. (2) Increase in resistance of either the
afferent or the efferent arterioles reduces peritubular
capillary hydrostatic pressure and tends to increase
reabsorption rate. Although constriction of t

ease
reabsorption rate. Although constriction of the 
efferent arterioles increases glomerular capillary
hydrostatic pressure, it lowers peritubular capillary
hydrostatic pressure.
The second major determinant of peritubular capil-
lary reabsorption is the colloid osmotic pressure of the
plasma in these capillaries; raising the colloid osmotic
pressure increases peritubular capillary reabsorption.
The colloid osmotic pressure of peritubular capillaries
is determined by (1) the systemic plasma co

laries
is determined by (1) the systemic plasma colloid
osmotic pressure; increasing the plasma protein con-
centration of systemic blood tends to raise peritubular
capillary colloid osmotic pressure, thereby increasing
reabsorption; and (2) the ﬁltration fraction; the higher
the ﬁltration fraction, the greater the fraction of
plasma ﬁltered through the glomerulus and, conse-
quently, the more concentrated the protein becomes in
the plasma that remains behind. Thus, increasing the
ﬁltration frac

emains behind. Thus, increasing the
ﬁltration fraction also tends to increase the peritubu-
lar capillary reabsorption rate. Because ﬁltration frac-
tion is deﬁned as the ratio of GFR/renal plasma ﬂow,
increased ﬁltration fraction can occur as a result of
increased GFR or decreased renal plasma ﬂow. Some
renal vasoconstrictors, such as angiotensin II, increase
peritubular capillary reabsorption by decreasing renal
plasma ﬂow and increasing ﬁltration fraction, as dis-
Changes in the

ion fraction, as dis-
Changes in the peritubular capillary Kf can also
inﬂuence the reabsorption rate because Kf is a
measure of the permeability and surface area of the
capillaries. Increases in Kf raise reabsorption, whereas
decreases in Kf lower peritubular capillary reabsorp-
tion. Kf remains relatively constant in most physiologic
conditions. Table 27–2 summarizes the factors that 
can inﬂuence the peritubular capillary reabsorption
rate.
Renal Interstitial Hydrostatic and Col

ption
rate.
Renal Interstitial Hydrostatic and Colloid Osmotic Pressures.
Ultimately, changes in peritubular capillary physical
forces inﬂuence tubular reabsorption by changing the
physical forces in the renal interstitium surrounding
the tubules. For example, a decrease in the reabsorp-
tive force across the peritubular capillary membranes,
caused by either increased peritubular capillary hydro-
static pressure or decreased peritubular capillary
colloid osmotic pressure, reduces the uptake of ﬂ

colloid osmotic pressure, reduces the uptake of ﬂuid
and solutes from the interstitium into the peritubular
capillaries. This in turn raises renal interstitial ﬂuid
hydrostatic pressure and decreases interstitial ﬂuid
colloid osmotic pressure because of dilution of the
proteins in the renal interstitium. These changes then
decrease the net reabsorption of ﬂuid from the renal
tubules into the interstitium, especially in the proximal
tubules.
ATP
Na+
Na+
Tubular
cells
Tubular
cells
Tubular
lumen

Na+
Na+
Tubular
cells
Tubular
cells
Tubular
lumen
Tubular
lumen
fluid
fluid
capillary
capillary
H2O
Na+
Na+
10 mm Hg
Net reabsorption
pressure
10 mm Hg
pressure
Pc
13 mm Hg
Pc
13 mm Hg
πc
32 mm Hg
πc
32 mm Hg
πif
15 mm Hg
πif
15 mm Hg
Pif
6 mm Hg
Pif
6 mm Hg
H2O
Bulk
flow
Bulk
flow
Figure 27–15
Summary of the hydrostatic and colloid osmotic forces that deter-
mine ﬂuid reabsorption by the peritubular capillaries. The numeri-
cal 

n by the peritubular capillaries. The numeri-
cal values shown are estimates of the normal values for humans.The
net reabsorptive pressure is normally about 10 mm Hg, causing ﬂuid
and solutes to be reabsorbed into the peritubular capillaries as they
are transported across the renal tubular cells. ATP, adenosine
triphosphate; Pc, peritubular capillary hydrostatic pressure; Pif, inter-
stitial ﬂuid hydrostatic pressure; pc, peritubular capillary colloid
osmotic pressure; pif, interstitial ﬂuid col

lloid
osmotic pressure; pif, interstitial ﬂuid colloid osmotic pressure.
Chapter 27
341
The mechanisms by which changes in interstitial
ﬂuid hydrostatic and colloid osmotic pressures inﬂu-
ence tubular reabsorption can be understood by exam-
ining the pathways through which solute and water are
reabsorbed (Figure 27–16). Once the solutes enter the
intercellular channels or renal interstitium by active
transport or 

nels or renal interstitium by active
transport or passive diffusion, water is drawn from the
tubular lumen into the interstitium by osmosis. And
once the water and solutes are in the interstitial spaces,
they can either be swept up into the peritubular cap-
illaries or diffuse back through the epithelial junctions
into the tubular lumen. The so-called tight junctions
between the epithelial cells of the proximal tubule are
actually leaky, so that considerable amounts of sodium
can diffuse in both

considerable amounts of sodium
can diffuse in both directions through these junctions.
With the normal high rate of peritubular capillary
reabsorption, the net movement of water and solutes
is into the peritubular capillaries with little backleak
into the lumen of the tubule. However, when peri-
tubular capillary reabsorption is reduced, there is
increased interstitial ﬂuid hydrostatic pressure and a
tendency for greater amounts of solute and water to
backleak into the tubular lumen, thereby red

er to
backleak into the tubular lumen, thereby reducing the
rate of net reabsorption (refer again to Figure 27–16).
The opposite is true when there is increased peri-
tubular capillary reabsorption above the normal level.
An initial increase in reabsorption by the peritubular
capillaries tends to reduce interstitial ﬂuid hydrostatic
pressure and raise interstitial ﬂuid colloid osmotic
pressure. Both of these forces favor movement of ﬂuid
and solutes out of the tubular lumen and into the inter-
s

tes out of the tubular lumen and into the inter-
stitium; therefore, backleak of water and solutes into
the tubular lumen is reduced, and net tubular reab-
sorption increases.
Thus, through changes in the hydrostatic and colloid
osmotic pressures of the renal interstitium, the uptake
of water and solutes by the peritubular capillaries is
closely matched to the net reabsorption of water and
solutes from the tubular lumen into the interstitium.
Therefore, in general, forces that increase peritubul

refore, in general, forces that increase peritubular
capillary reabsorption also increase reabsorption from
the renal tubules. Conversely, hemodynamic changes
that inhibit peritubular capillary reabsorption also
inhibit tubular reabsorption of water and solutes.
Effect of Arterial Pressure on Urine
Output—The Pressure-Natriuresis and
Even small increases in arterial pressure often cause
marked increases in urinary excretion of sodium and
water, phenomena that are ref

retion of sodium and
water, phenomena that are referred to as pressure
natriuresis and pressure diuresis. Because of the
autoregulatory mechanisms described in Chapter 26,
increasing the arterial pressure between the limits of
75 and 160 mm Hg usually has only a small effect on
renal blood ﬂow and GFR.The slight increase in GFR
that does occur contributes in part to the effect of
increased arterial pressure on urine output. When
GFR autoregulation is impaired, as often occurs in
kidney disease, 

n is impaired, as often occurs in
kidney disease, increases in arterial pressure cause
much larger increases in GFR.
Table 27–2
Factors That Can Inﬂuence Peritubular Capillary
≠Pc Æ Ø Reabsorption
• Ø RA Æ ≠Pc
• Ø RE Æ ≠Pc
• ≠Arterial Pressure Æ ≠Pc
≠pc Æ ≠Reabsorption
• ≠pA Æ ≠pc
• ≠FF Æ ≠pc
≠Kf Æ ≠Reabsorption
Pc, peritubular capillary hydrostatic pressure; RA and RE, afferent and effer-
ent arteriolar resistances, respectively; pc, peritubular capillary colloid osmotic
pressure; 

, peritubular capillary colloid osmotic
pressure; pA, arterial plasma colloid osmotic pressure; FF, ﬁltration fraction;
Kf, peritubular capillary ﬁltration coefﬁcient.
Decreased reabsorption
Normal
Normal
ATP
ATP
Tubular
cells
Tubular
cells
Lumen
Lumen
Backleak
Backleak
fluid
fluid
capillary
capillary
Net
Net
ATP
ATP
Increased
backleak
Increased
backleak
 Decreased net
r

kleak
Pc
Pc
πc
πc
Pc
Pc
πc
πc
Figure 27–16
Proximal tubular and peritubular capillary reabsorption under
normal conditions (top) and during decreased peritubular capillary
reabsorption (bottom) caused by either increasing peritubular cap-
illary hydrostatic pressure (Pc) or decreasing peritubular capillary
colloid osmotic pressure (pc). Reduced peritubular capillary reab-
sorption, in turn, decreases the net reabsorption of solutes and wate

decreases the net reabsorption of solutes and water
by increasing the amounts of solutes and water that leak back into
the tubular lumen through the tight junctions of the tubular epithe-
lial cells, especially in the proximal tubule.
342
Unit V
A second effect of increased renal arterial pressure
that raises urine output is that it decreases the per-
centage of the ﬁltered load of sodium and water that
is reabsorbed by the tubules. The mechanisms respon-
sible for th

y the tubules. The mechanisms respon-
sible for this effect include a slight increase in peri-
tubular capillary hydrostatic pressure, especially in the
vasa recta of the renal medulla, and a subsequent
increase in the renal interstitial ﬂuid hydrostatic pres-
sure. As discussed earlier, an increase in the renal
interstitial ﬂuid hydrostatic pressure enhances back-
leak of sodium into the tubular lumen, thereby reduc-
ing the net reabsorption of sodium and water and
further increasing the rate o

sodium and water and
further increasing the rate of urine output when renal
arterial pressure rises.
A third factor that contributes to the pressure-
natriuresis and pressure-diuresis mechanisms is
reduced angiotensin II formation.Angiotensin II itself
increases sodium reabsorption by the tubules; it also
stimulates aldosterone secretion,
which further
increases sodium reabsorption. Therefore, decreased
angiotensin II formation contributes to the decreased
tubular sodium reabsorption that occurs

 decreased
tubular sodium reabsorption that occurs when arterial
pressure is increased.
Hormonal Control of Tubular
Precise regulation of body ﬂuid volumes and solute
concentrations requires the kidneys to excrete differ-
ent solutes and water at variable rates, sometimes
independently of one another. For example, when
potassium intake is increased, the kidneys must
excrete more potassium while maintaining normal
excretion of sodium and other electrolytes. Likewise,
when sodium inta

and other electrolytes. Likewise,
when sodium intake is changed, the kidneys must
appropriately adjust urinary sodium excretion without
major changes in excretion of other electrolytes.
Several hormones in the body provide this speciﬁcity
of tubular reabsorption for different electrolytes and
water.Table 27–3 summarizes some of the most impor-
tant hormones for regulating tubular reabsorption,
their principal sites of action on the renal tubule, and
their effects on solute and water excretion. S

and
their effects on solute and water excretion. Some 
of these hormones are discussed in more detail in
Chapters 28 and 29, but we brieﬂy review their renal
tubular actions in the next few paragraphs.
Aldosterone Increases Sodium Reabsorption and Increases
Potassium Secretion. Aldosterone, secreted by the zona
glomerulosa cells of the adrenal cortex, is an impor-
tant regulator of sodium reabsorption and potassium
secretion by the renal tubules. The primary site 
of aldosterone action is on the

 The primary site 
of aldosterone action is on the principal cells of the 
cortical collecting tubule. The mechanism by which
aldosterone increases sodium reabsorption while at
the same time increasing potassium secretion is by
stimulating the sodium-potassium ATPase pump on
the basolateral side of the cortical collecting tubule
membrane. Aldosterone also increases the sodium
permeability of the luminal side of the membrane.
The cellular mechanisms of aldosterone action are 
discussed in Chapter

ms of aldosterone action are 
discussed in Chapter 77.
In the absence of aldosterone,as occurs with adrenal
destruction or malfunction (Addison’s disease), there
is marked loss of sodium from the body and accumu-
lation of potassium. Conversely, excess aldosterone
secretion, as occurs in patients with adrenal tumors
(Conn’s syndrome) is associated with sodium retention
and potassium depletion.Although day-to-day regula-
tion of sodium balance can be maintained as long as
minimal levels of aldost

 be maintained as long as
minimal levels of aldosterone are present, the inabil-
ity to appropriately adjust aldosterone secretion
greatly impairs the regulation of renal potassium
excretion and potassium concentration of the body
ﬂuids. Thus, aldosterone is even more important as a
regulator of potassium concentration than it is for
sodium concentration.
Angiotensin II Increases Sodium and Water Reabsorption.
Angiotensin II is perhaps the body’s most powerful
sodium-retaining hormone. As discus

 most powerful
sodium-retaining hormone. As discussed in Chapter
19, angiotensin II formation increases in circumstances
associated with low blood pressure and/or low extra-
cellular ﬂuid volume, such as during hemorrhage or
loss of salt and water from the body ﬂuids. The
increased formation of angiotensin II helps to return
blood pressure and extracellular volume toward
normal by increasing sodium and water reabsorption
from the renal tubules through three main effects:
1. Angiotensin II stimul

rough three main effects:
1. Angiotensin II stimulates aldosterone secretion,
which in turn increases sodium reabsorption.
2. Angiotensin II constricts the efferent arterioles,
which has two effects on peritubular capillary
dynamics that raise sodium and water
Table 27–3
Hormones That Regulate Tubular Reabsorption
Hormone
Effects
Collecting tubule and duct
≠NaCl, H2O reabsorption, ≠K+ secretion
Proximal tubule, thick ascending loop of Henle/distal
≠NaCl,

ubule, thick ascending loop of Henle/distal
≠NaCl, H2O reabsorption, ≠H+ secretion
tubule, collecting tubule
Antidiuretic hormone
Distal tubule/collecting tubule and duct
≠H2O reabsorption
Atrial natriuretic peptide
Ø NaCl reabsorption
Proximal tubule, thick ascending loop of 
Ø PO4
- - - reabsorption, ≠Ca++ reabsorption
Henle/distal tubule
Chapter 27
343

 Tubular Processing of the Glomerular Filtrate
343
reabsorption. First, efferent arteriolar constriction
reduces peritubular capillary hydrostatic pressure,
which increases net tubular reabsorption,
especially from the proximal tubules. Second,
efferent arteriolar constriction, by reducing renal
blood ﬂow, raises ﬁltration fraction in the
glomerulus and increases the concentration of
proteins and the colloid osmotic pressure in 
the peritubular capillaries; this increases the
reabsorptive force 

apillaries; this increases the
reabsorptive force at the peritubular capillaries
and raises tubular reabsorption of sodium and
water.
3. Angiotensin II directly stimulates sodium
reabsorption in the proximal tubules, the loops of
Henle, the distal tubules, and the collecting tubules.
One of the direct effects of angiotensin II is to
stimulate the sodium-potassium ATPase pump on
the tubular epithelial cell basolateral membrane.
A second effect is to stimulate sodium-hydrogen
exchange in the lumin

to stimulate sodium-hydrogen
exchange in the luminal membrane, especially 
in the proximal tubule. Thus, angiotensin II
stimulates sodium transport across both the
luminal and the basolateral surfaces of the
epithelial cell membrane in the tubules.
These multiple actions of angiotensin II cause
marked sodium retention by the kidneys when
angiotensin II levels are increased.
ADH Increases Water Reabsorption. The most important
renal action of ADH is to increase the water perme-
ability of the dis

is to increase the water perme-
ability of the distal tubule, collecting tubule, and col-
lecting duct epithelia. This effect helps the body to
conserve water in circumstances such as dehydration.
In the absence of ADH, the permeability of the distal
tubules and collecting ducts to water is low, causing the
kidneys to excrete large amounts of dilute urine.Thus,
the actions of ADH play a key role in controlling the
degree of dilution or concentration of the urine, as dis-
cussed further in Chapte

ion of the urine, as dis-
cussed further in Chapters 28 and 75.
ADH binds to speciﬁc V2 receptors in the late distal
tubules,
collecting tubules,
and collecting ducts,
increasing the formation of cyclic AMP and activating
protein kinases.This, in turn, stimulates the movement
of an intracellular protein, called aquaporin-2 (AQP-
2), to the luminal side of the cell membranes.The mol-
ecules of AQP-2 cluster together and fuse with the cell
membrane by exocytosis to form water channels that
permit 

 by exocytosis to form water channels that
permit rapid diffusion of water through the cells.There
are other aquaporins, AQP-3 and AQP-4, in the baso-
lateral side of the cell membrane that provide a path
for water to rapidly exit the cells, although these 
are not believed to be regulated by ADH. Chronic
increases in ADH levels also increase the formation of
AQP-2 protein in the renal tubular cells by stimulat-
ing AQP-2 gene transcription.When the concentration
of ADH decreases, the molecules 

the concentration
of ADH decreases, the molecules of AQP-2 are shut-
tled back to the cell cytoplasm, thereby removing the
water channels from the luminal membrane and
reducing water permeability.
Atrial Natriuretic Peptide Decreases Sodium and Water 
Reabsorption. Speciﬁc cells of the cardiac atria, when
distended because of plasma volume expansion,
secrete a peptide called atrial natriuretic peptide.
Increased levels of this peptide in turn inhibit the reab-
sorption of sodium and water by the

ibit the reab-
sorption of sodium and water by the renal tubules,
especially in the collecting ducts. This decreased
sodium and water reabsorption increases urinary
excretion, which helps to return blood volume back
toward normal.
Parathyroid 
Hormone 
Increases 
Calcium 
Reabsorption.
Parathyroid hormone is one of the most important
calcium-regulating hormones in the body. Its principal
action in the kidneys is to increase tubular reabsorp-
tion of calcium, especially in the distal tubules and

 of calcium, especially in the distal tubules and
perhaps also in the loops of Henle. Parathyroid
hormone also has other actions, including inhibition of
phosphate reabsorption by the proximal tubule and
stimulation of magnesium reabsorption by the loop of
Henle, as discussed in Chapter 29.
Activation Increases Sodium
Activation of the sympathetic nervous system can
decrease sodium and water excretion by constricting
the renal arterioles, thereby reducing 

nstricting
the renal arterioles, thereby reducing GFR. Sympa-
thetic activation also increases sodium reabsorption in
the proximal tubule, the thick ascending limb of the
loop of Henle, and perhaps in more distal parts of the
renal tubule. And ﬁnally, sympathetic nervous system
stimulation increases renin release and angiotensin II
formation, which adds to the overall effect to increase
tubular reabsorption and decrease renal excretion of
sodium.
Use of Clearance Methods 
to Quantify Kidney Func

to Quantify Kidney Function
The rates at which different substances are “cleared”
from the plasma provide a useful way of quantitating
the effectiveness with which the kidneys excrete
various substances (Table 27–4). By deﬁnition, the
renal clearance of a substance is the volume of plasma
that is completely cleared of the substance by the
kidneys per unit time. This concept is somewhat
abstract because there is no single volume of plasma
that is completely cleared of a

e volume of plasma
that is completely cleared of a substance. However,
renal clearance provides a useful way of quantifying
the excretory function of the kidneys and, as discussed
later, can be used to quantify the rate at which blood
ﬂows through the kidneys as well as the basic functions
of the kidneys: glomerular ﬁltration, tubular reabsorp-
tion, and tubular secretion.
To illustrate the clearance principle, consider the fol-
lowing example: If the plasma passing through the
kidneys contains 

f the plasma passing through the
kidneys contains 1 milligram of a substance in each mil-
liliter and if 1 milligram of this substance is also
excreted into the urine each minute, then 1 ml/min 
of the plasma is “cleared” of the substance. Thus,
344
Unit V
clearance refers to the volume of plasma that would be
necessary to supply the amount of substance excreted
in the urine per unit time. Stated mathematically,
Cs ¥ Ps = Us ¥ V,
where Cs is the clearance rate of a su

s = Us ¥ V,
where Cs is the clearance rate of a substance s, Ps is the
plasma concentration of the substance, Us is the urine
concentration of that substance, and V is the urine ﬂow
rate. Rearranging this equation, clearance can be
expressed as
Thus, renal clearance of a substance is calculated from
the urinary excretion rate (Us ¥ V) of that substance
divided by its plasma concentration.
Inulin Clearance Can Be Used 
to Estimate GFR
If a substance is freely ﬁltered (ﬁltered as freely as
water) 

ce is freely ﬁltered (ﬁltered as freely as
water) and is not reabsorbed or secreted by the renal
tubules, then the rate at which that substance is excreted
in the urine (Us ¥ V) is equal to the ﬁltration rate of the
substance by the kidneys (GFR ¥ Ps). Thus,
GFR ¥ Ps = Us ¥ V
The GFR, therefore, can be calculated as the clearance
of the substance as follows:
A substance that ﬁts these criteria is inulin, a polysac-
charide molecule with a molecular weight of about
GFR
U
V
P
C
s
s
s
=
¥
=
C
U
V
P

ar weight of about
GFR
U
V
P
C
s
s
s
=
¥
=
C
U
V
P
s
s
s
=
¥
5200. Inulin, which is not produced in the body, is found
in the roots of certain plants and must be administered
intravenously to a patient to measure GFR.
Figure 27–17 shows the renal handling of inulin. In
this example, the plasma concentration is 1 mg/ml, urine
concentration is 125 mg/ml, and urine ﬂow rate is 1 ml/
min. Therefore, 125 mg/min of inulin passes into the
urine. Then, inulin clearance is calculated as the urine
excreti

nulin clearance is calculated as the urine
excretion rate of inulin divided by the plasma concen-
tration, which yields a value of 125 ml/min. Thus, 125
milliliters of plasma ﬂowing through the kidneys must
be ﬁltered to deliver the inulin that appears in the urine.
Inulin is not the only substance that can be used for
determining GFR. Other substances that have been
used clinically to estimate GFR include radioactive
iothalamate and creatinine.
Creatinine Clearance and Plasma
Creatinine Concent

Creatinine Concentration Can Be
Used to Estimate GFR
Creatinine is a by-product of muscle metabolism and is
cleared from the body ﬂuids almost entirely by glomeru-
lar ﬁltration. Therefore, the clearance of creatinine can
also be used to assess GFR. Because measurement of
creatinine clearance does not require intravenous infu-
sion into the patient, this method is much more widely
used than inulin clearance for estimating GFR clinically.
However, creatinine cleara

imating GFR clinically.
However, creatinine clearance is not a perfect marker
of GFR because a small amount of it is secreted by the
tubules, so that the amount of creatinine excreted
slightly exceeds the amount ﬁltered. There is normally
a slight error in measuring plasma creatinine that 
leads to an overestimate of the plasma creatinine 
Table 27–4
Use of Clearance to Quantify Kidney Function
Term
Equation
Units
Clearance rate (Cs)
ml/min
Glomerular ﬁltration rate (GFR)
Clearance ratio
None
Ef

rular ﬁltration rate (GFR)
None
Effective renal plasma ﬂow (ERPF)
ml/min
Renal plasma ﬂow (RPF)
ml/min
Renal blood ﬂow (RBF)
ml/min
Excretion rate
Excretion rate = Us ¥ V
mg/min, mmol/min, or mEq/min
Reabsorption rate
Reabsorption rate = Filtered load - Excretion rate
= (GFR ¥ Ps) - (Us ¥ V)
Secretion rate
Secretion rate = Excretion rate - Filtered load
S, a substance; U, urine concentration;V, urine ﬂow rate; P, plasma co

rine concentration;V, urine ﬂow rate; P, plasma concentration; PAH, para-aminohippuric acid; PPAH, renal arterial PAH concentration; EPAH, PAH
extraction ratio; VPAH, renal venous PAH concentration.
RBF
RPF
1
=
-
RPF
C
E
U
V P
P
V
P
U
V
P
V
PAH
PAH
PAH
PAH
PAH
PAH
PAH
PAH
PAH
PAH
PAH
=
=
¥
(
)
-
(
)
=
¥
-
ERPF
C
U
V
P
PAH
PAH
PAH
=
=
¥
C
C
s
inulin
=
GFR
U
V
P
inulin
inulin
=
¥
C
U
V
P
s
s
s
=
¥
Chapter 27
Urine Formation by the Kidneys: II. Tubular Processing of the G

on by the Kidneys: II. Tubular Processing of the Glomerular Filtrate
345
concentration, and fortuitously, these two errors tend to
cancel each other. Therefore, creatinine clearance pro-
vides a reasonable estimate of GFR.
In some cases, it may not be practical to collect urine
in a patient for measuring creatinine clearance (CCr).An
approximation of changes in GFR, however, can be
obtained by simply measuring plasma creatinine con-
centration (PCr), which is inversely proportional to GFR:
If GF

Cr), which is inversely proportional to GFR:
If GFR suddenly decreases by 50%, the kidneys will
transiently ﬁlter and excrete only half as much creati-
nine, causing accumulation of creatinine in the body
ﬂuids and raising plasma concentration. Plasma con-
centration of creatinine will continue to rise until the ﬁl-
tered load of creatinine (PCr ¥ GFR) and creatinine
excretion (UCr ¥ V) return to normal and a balance
between creatinine production and creatinine excretion
is reestablished. This w

 and creatinine excretion
is reestablished. This will occur when plasma creatinine
increases to approximately twice normal, as shown in
Figure 27–18. If GFR falls to one-fourth normal, plasma
creatinine would increase to about 4 times normal, and
a decrease of GFR to one-eighth normal would raise
plasma creatinine to 8 times normal.Thus, under steady-
state conditions, the creatinine excretion rate equals the
rate of creatinine production, despite reductions in
GFR. However, this normal rate of 

e reductions in
GFR. However, this normal rate of creatinine excretion
occurs at the expense of elevated plasma creatinine con-
centration, as shown in Figure 27–19.
PAH Clearance Can Be Used to
Estimate Renal Plasma Flow
Theoretically, if a substance is completely cleared from
the plasma, the clearance rate of that substance is equal
GFR
C
U
V
P
Cr
Cr
Cr
ª
=
¥
to the total renal plasma ﬂow. In other words, the
amount of the substance delivered to the kidneys in 
the blood (renal plasma ﬂow ¥ Ps

o the kidneys in 
the blood (renal plasma ﬂow ¥ Ps) would be equal to 
the amount excreted in the urine (Us ¥ V). Thus, renal
plasma ﬂow (RPF) could be calculated as
Because the GFR is only about 20 per cent of the
total plasma ﬂow, a substance that is completely cleared
from the plasma must be excreted by tubular secretion
as well as glomerular ﬁltration (Figure 27–20). There is
no known substance that is completely cleared by the
kidneys. One substance, however, PAH, is about 90 per
cent clear

ubstance, however, PAH, is about 90 per
cent cleared from the plasma. Therefore, the clearance
of PAH can be used as an approximation of renal
plasma ﬂow. To be more accurate, one can correct for
the percentage of PAH that is still in the blood when it
leaves the kidneys. The percentage of PAH removed
from the blood is known as the extraction ratio of PAH
and averages about 90 per cent in normal kidneys. In
diseased kidneys, this extraction ratio may be reduced
RPF
U
V
P
C
s
s
s
=
¥
=
Uinulin = 

 may be reduced
RPF
U
V
P
C
s
s
s
=
¥
=
Uinulin = 125 mg/ml
Pinulin = 1 mg/ml
V = 1 ml/min
Amount filtered = Amount excreted
GFR x Pinulin = Uinulin x V
Uinulin x V
Pinulin
GFR = 
GFR = 125 ml/min 
Figure 27–17
Measurement of glomerular ﬁltration rate (GFR) from the renal
clearance of inulin. Inulin is freely ﬁltered by the glomerular cap-
illaries but is not reabsorbed by the renal tubules. Pinulin, plasma
inulin concentration; Uinulin, urine inulin concentration; V, urine ﬂow
rate.
Creatinine 

ulin concentration; V, urine ﬂow
rate.
Creatinine production and
renal excretion (g/day)
Days 
2
1
0
0
1
2
3
4
Positive balance
Excretion    GFR x PCreatinine
Serum creatinine
concentration (mg/dl)
2
1
0
GFR (ml/min)
100
50
0
Figure 27–18
Effect of reducing glomerular ﬁltration rate (GRF) by 50 per cent
on serum creatinine concentration and on creatinine excretion 
rate when the production rate of creatinine remains constant. 
PCreatinine, plasma creatinine concentration.
346
Unit V
T

ine, plasma creatinine concentration.
346
Unit V
because of inability of damaged tubules to secrete PAH
into the tubular ﬂuid.
The calculation of RPF can be demonstrated by the
following example: Assume that the plasma concentra-
tion of PAH is 0.01 mg/ml, urine concentration is 5.85
mg/ml, and urine ﬂow rate is 1 ml/min. PAH clearance
can be calculated from the rate of urinary PAH excre-
tion (5.85 mg/ml ¥ 1 ml/min) divided by the plasma PAH
concentration (0.01 mg/ml

ivided by the plasma PAH
concentration (0.01 mg/ml).Thus, clearance of PAH cal-
culates to be 585 ml/min.
If the extraction ratio for PAH is 90 per cent, the
actual renal plasma ﬂow can be calculated by dividing
585 ml/min by 0.9, yielding a value of 650 ml/min. Thus,
total renal plasma ﬂow can be calculated as
Total renal plasma ﬂow =
Clearance of PAH/Extraction ratio of PAH
The extraction ratio (EPAH) is calculated as the dif-
ference between the renal arterial PAH (PPAH) and renal
venous PAH 

he renal arterial PAH (PPAH) and renal
venous PAH (VPAH) concentrations, divided by the renal
arterial PAH concentration:
One can calculate the total blood ﬂow through the
kidneys from the total renal plasma ﬂow and hematocrit
(the percentage of red blood cells in the blood). If the
hematocrit is 0.45 and the total renal plasma ﬂow is 
650 ml/min, the total blood ﬂow through both kidneys
is 650/(1 - 0.45), or 1182 ml/min.
E
P
V
P
PAH
PAH
PAH
PAH
=
-
Filtration Fraction Is Calculated
from GFR Div

=
-
from GFR Divided by Renal 
To calculate the ﬁltration fraction, which is the fraction
of plasma that ﬁlters through the glomerular mem-
brane, one must ﬁrst know the renal plasma ﬂow (PAH
clearance) and the GFR (inulin clearance). If renal
plasma ﬂow is 650 ml/min and GFR is 125 ml/min, the
ﬁltration fraction (FF) is calculated as
FF = GFR/RPF = 125/650 = 0.19
Calculation of Tubular Reabsorption
or Secretion from Renal Clearances
If the rates of 

r Secretion from Renal Clearances
If the rates of glomerular ﬁltration and renal excretion
of a substance are known, one can calculate whether
there is a net reabsorption or a net secretion of that sub-
stance by the renal tubules. For example, if the rate of
excretion of the substance (Us ¥ V) is less than the ﬁl-
tered load of the substance (GFR ¥ Ps), then some of
the substance must have been reabsorbed from the
renal tubules. Conversely, if the excretion rate of the
substance is greater than

he excretion rate of the
substance is greater than its ﬁltered load, then the rate
at which it appears in the urine represents the sum of
the rate of glomerular ﬁltration plus tubular secretion.
The following example demonstrates the calculation
of tubular reabsorption. Assume the following labora-
tory values for a patient were obtained:
Plasma creatinine concentration
(mg/100 ml) 
Glomerular filtration rate
(ml/min) 
14
12
10
8
6
4
2
25
50
Normal
75
100
125
150
Figure 27–19
Approximate relatio

al
75
100
125
150
Approximate relationship between glomerular ﬁltration rate (GFR)
and plasma creatinine concentration under steady-state conditions.
Decreasing GFR by 50 per cent will increase plasma creatinine to
twice normal if creatinine production by the body remains constant.
UPAH = 5.85 mg/ml
PPAH = 0.01 mg/ml
V = 1 ml/min
UPAH x V
PPAH
Renal plasma flow
= 
Renal venous
PAH =
0.001 mg/ml
Figure 27–20
Measurement of renal plasma ﬂow from the renal clearance of para-
aminohippu

a ﬂow from the renal clearance of para-
aminohippuric acid (PAH). PAH is freely ﬁltered by the glomerular
capillaries and is also secreted from the peritubular capillary blood
into the tubular lumen. The amount of PAH in the plasma of the
renal artery is about equal to the amount of PAH excreted in 
the urine. Therefore, the renal plasma ﬂow can be calculated from
the clearance of PAH (CPAH). To be more accurate, one can correct
for the percentage of PAH that is still in the blood when it leaves

e of PAH that is still in the blood when it leaves
the kidneys. PPAH, arterial plasma PAH concentration; UPAH, urine
PAH concentration; V, urine ﬂow rate.
Chapter 27
347
Urine ﬂow rate = 1 ml/min
Urine concentration of sodium (UNa) = 70 mEq/L 
= 70 mEq/ml
Plasma sodium concentration = 140 mEq/L 
= 140 mEq/ml
GFR (inulin clearance) = 100 ml/min
In this example, the ﬁltered sodium load is GFR ¥ PNa,
or 100 ml/min ¥ 1

ltered sodium load is GFR ¥ PNa,
or 100 ml/min ¥ 140 mEq/ml = 14,000 mEq/min. Urinary
sodium excretion (UNa ¥ urine ﬂow rate) is 70 mEq/min.
Therefore, tubular reabsorption of sodium is the differ-
ence between the ﬁltered load and urinary excretion, or
14,000 mEq/min - 70 mEq/min = 13,930 mEq/min.
Comparisons of Inulin Clearance with Clearances of Different
Solutes. The following generalizations can be made by
comparing the clearance of a substance with the clear-
ance of inulin, a measure of G

nce with the clear-
ance of inulin, a measure of GFR: (1) if the clearance
rate of the substance equals that of inulin, the substance
is only ﬁltered and not reabsorbed or secreted; (2) if the
clearance rate of a substance is less than inulin clear-
ance, the substance must have been reabsorbed by the
nephron tubules; and (3) if the clearance rate of a sub-
stance is greater than that of inulin, the substance must
be secreted by the nephron tubules. Listed below are
the approximate clearance rat

es. Listed below are
the approximate clearance rates for some of the sub-
stances normally handled by the kidneys:
Substance
Clearance Rate (ml/min)
Glucose
0
Sodium
0.9
Chloride
1.3
Potassium
12.0
Phosphate
25.0
Inulin
125.0
140.0
Aronson PS:Ion exchangers mediating NaCl transport in the
renal proximal tubule. Cell Biochem Biophys 36:147, 2002.
Benos DJ, Fuller CM, Shlyonsky VG, et al: Amiloride-
sensitive Na+ channels: insights and outlooks. News
Physiol Sci 12:55, 1997.

ights and outlooks. News
Féraille 
E,
Doucet 
A:
Sodium-potassium-adenosine-
triphosphatase–dependent sodium transport in the
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Granger JP, Alexander BT, Llinas M: Mechanisms of pres-
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he Kidney—Physiology
2000.
Humphreys MH, Valentin J-P: Natriuretic hormonal agents.
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Kellenberger S, Schild L: Epithelial sodium channel/
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Nielsen S, Frøkiær J, Marples D, et al: Aquaporins in the
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uaporins in the
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Palmer LG, Frindt G: Aldosterone and potassium secretion
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Rahn KH, Heidenreich S, Bruckner D: How to assess
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 cotransport. Physiol
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C
H
A
P
T
E
R
 
2
8
348
Osmolarit

8
348
Osmolarity and Sodium
For the cells of the body to function properly, they
must be bathed in extracellular ﬂuid with a rela-
tively constant concentration of electrolytes and
other solutes. The total concentration of solutes in
the extracellular ﬂuid—and therefore the osmolar-
ity—is determined by the amount of solute divided
by the volume of the extracellular ﬂuid. Thus, to a
large extent, extracellular ﬂuid sodium concentra-
tion and osmol

xtracellular ﬂuid sodium concentra-
tion and osmolarity are regulated by the amount of extracellular water. The
body water in turn is controlled by (1) ﬂuid intake, which is regulated by factors
that determine thirst, and (2) renal excretion of water, which is controlled by
multiple factors that inﬂuence glomerular ﬁltration and tubular reabsorption.
In this chapter, we discuss speciﬁcally (1) the mechanisms that cause the
kidneys to eliminate excess water by excreting a dilute urine; (2) the me

cess water by excreting a dilute urine; (2) the mecha-
nisms that cause the kidneys to conserve water by excreting a concentrated
urine; (3) the renal feedback mechanisms that control the extracellular ﬂuid
sodium concentration and osmolarity; and (4) the thirst and salt appetite mech-
anisms that determine the intakes of water and salt, which also help to control
extracellular ﬂuid volume, osmolarity, and sodium concentration.
The Kidneys Excrete Excess Water by Forming
a Dilute Urine
The norma

e Excess Water by Forming
The normal kidney has tremendous capability to vary the relative proportions
of solutes and water in the urine in response to various challenges. When there
is excess water in the body and body ﬂuid osmolarity is reduced, the kidney can
excrete urine with an osmolarity as low as 50 mOsm/L, a concentration that is
only about one sixth the osmolarity of normal extracellular ﬂuid. Conversely,
when there is a deﬁcit of water and extracellular ﬂuid osmolarity 

deﬁcit of water and extracellular ﬂuid osmolarity is high, the
kidney can excrete urine with a concentration of 1200 to 1400 mOsm/L. Equally
important, the kidney can excrete a large volume of dilute urine or a small
volume of concentrated urine without major changes in rates of excretion of
solutes such as sodium and potassium. This ability to regulate water excretion
independently of solute excretion is necessary for survival, especially when ﬂuid
intake is limited.
Antidiuretic Hormone Contro

uid
Antidiuretic Hormone Controls Urine Concentration
There is a powerful feedback system for regulating plasma osmolarity and
sodium concentration that operates by altering renal excretion of water inde-
pendently of the rate of solute excretion. A primary effector of this feedback is
antidiuretic hormone (ADH), also called vasopressin.
When osmolarity of the body ﬂuids increases above normal (that is, the
solutes in the body ﬂuids become too concentrated), the posterior pitu

ﬂuids become too concentrated), the posterior pituitary
gland secretes more ADH, which increases the permeability of the distal tubules
and collecting ducts to water, as discussed in Chapter 27. This allows large
amounts of water to be reabsorbed and decreases urine volume but does not
Chapter 28
Regulation of Extracellular Fluid Osmolarity and Sodium Concentration
349
markedly alter the rate of renal excretion of the
solutes.
When there is excess water in the body and extra-
cellular ﬂuid osmol

s water in the body and extra-
cellular ﬂuid osmolarity is reduced, the secretion of
ADH by the posterior pituitary decreases, thereby
reducing the permeability of the distal tubule and col-
lecting ducts to water, which causes large amounts of
dilute urine to be excreted. Thus, the rate of ADH
secretion determines, to a large extent, whether the
kidney excretes a dilute or a concentrated urine.
Renal Mechanisms for Excreting a
When there is a large excess of water in the body, the

there is a large excess of water in the body, the
kidney can excrete as much as 20 L/day of dilute urine,
with a concentration as low as 50 mOsm/L.The kidney
performs this impressive feat by continuing to reab-
sorb solutes while failing to reabsorb large amounts of
water in the distal parts of the nephron, including the
late distal tubule and the collecting ducts.
Figure 28–1 shows the approximate renal responses
in a human after ingestion of 1 liter of water. Note that
urine volume increases t

liter of water. Note that
urine volume increases to about six times normal
within 45 minutes after the water has been drunk.
However, the total amount of solute excreted remains
relatively constant because the urine formed becomes
very dilute and urine osmolarity decreases from 600 to
about 100 mOsm/L. Thus, after ingestion of excess
water, the kidney rids the body of the excess water but
does not excrete excess amounts of solutes.
When the glomerular ﬁltrate is initially formed,
its osmolarity 

rular ﬁltrate is initially formed,
its osmolarity is about the same as that of plasma 
(300 mOsm/L).To excrete excess water, it is necessary
to dilute the ﬁltrate as it passes along the tubule. This
is achieved by reabsorbing solutes to a greater extent
than water, as shown in Figure 28–2, but this occurs
only in certain segments of the tubular system as
follows.
Tubular Fluid Remains Isosmotic in the Proximal Tubule. As
ﬂuid ﬂows through the proximal tubule, solutes and
water are reabsorbed in 

ximal tubule, solutes and
water are reabsorbed in equal proportions, so that
little change in osmolarity occurs; that is, the proximal
tubule ﬂuid remains isosmotic to the plasma, with an
osmolarity of about 300 mOsm/L.As ﬂuid passes down
the descending loop of Henle, water is reabsorbed by
osmosis and the tubular ﬂuid reaches equilibrium with
the surrounding interstitial ﬂuid of the renal medulla,
which is very hypertonic—about two to four times the
osmolarity of the original glomerular ﬁltrate

 the
osmolarity of the original glomerular ﬁltrate. There-
fore, the tubular ﬂuid becomes more concentrated as
it ﬂows into the inner medulla.
Tubular Fluid Becomes Dilute in the Ascending Loop of Henle.
In the ascending limb of the loop of Henle, especially
in the thick segment, sodium, potassium, and chloride
are avidly reabsorbed. However, this portion of the
tubular segment is impermeable to water, even in 
Urinary solute
excretion
(mOsm/min)
1.2
0.6
0
0
180
120
60
Urine flow 

1.2
0.6
0
0
180
120
60
Urine flow rate
(ml/min)
6
4
2
0
(mOsm/L)
800
Urine
osmolarity
Plasma
400
0
Drink 1.0 L H2O
Figure 28–1
Water diuresis in a human after ingestion of 1 liter of water. Note
that after water ingestion, urine volume increases and urine osmo-
larity decreases, causing the excretion of a large volume of dilute
urine; however, the total amount of solute excreted by the kidneys
remains relatively constant. These responses of the kidneys
preve

ely constant. These responses of the kidneys
prevent plasma osmolarity from decreasing markedly during
excess water ingestion.
NaCl
NaCl
NaCl
NaCl
NaCl
300
300
400
400
Medulla
Cortex
600
400
600
H2O
H2O
600
300
100
100
70
50
Figure 28–2
Formation of a dilute urine when antidiuretic hormone (ADH) levels
are very low. Note that in the ascending loop of Henle, the tubular
ﬂuid becomes very dilute. In the distal tubules and collecting
tubules, the tubular ﬂuid is further diluted by the reabsorption 

bular ﬂuid is further diluted by the reabsorption of
sodium chloride and the failure to reabsorb water when ADH
levels are very low. The failure to reabsorb water and continued
reabsorption of solutes lead to a large volume of dilute urine.
(Numerical values are in milliosmoles per liter.)
350
Unit V
the presence of large amounts of ADH.Therefore, the
tubular ﬂuid becomes more dilute as it ﬂows up the
ascending loop of Henle into the early distal tubule,
with the osmo

 Henle into the early distal tubule,
with the osmolarity decreasing progressively to about
100 mOsm/L by the time the ﬂuid enters the early
distal tubular segment. Thus, regardless of whether
ADH is present or absent, ﬂuid leaving the early distal
tubular segment is hypo-osmotic, with an osmolarity of
only about one third the osmolarity of plasma.
Tubular Fluid in Distal and Collecting Tubules Is Further Diluted
in the Absence of ADH. As the dilute ﬂuid in the early
distal tubule passes into the

te ﬂuid in the early
distal tubule passes into the late distal convoluted
tubule, cortical collecting duct, and collecting duct,
there is additional reabsorption of sodium chloride. In
the absence of ADH, this portion of the tubule is also
impermeable to water,and the additional reabsorption
of solutes causes the tubular ﬂuid to become even
more dilute, decreasing its osmolarity to as low as 
50 mOsm/L.The failure to reabsorb water and the con-
tinued reabsorption of solutes lead to a large volu

inued reabsorption of solutes lead to a large volume
of dilute urine.
To summarize, the mechanism for forming a dilute
urine is to continue reabsorbing solutes from the distal
segments of the tubular system while failing to reab-
sorb water. In healthy kidneys, ﬂuid leaving the
ascending loop of Henle and early distal tubule is
always dilute, regardless of the level of ADH. In the
absence of ADH, the urine is further diluted in the late
distal tubule and collecting ducts, and a large volume
of d

bule and collecting ducts, and a large volume
of dilute urine is excreted.
The Kidneys Conserve Water
by Excreting a Concentrated
Urine
The ability of the kidney to form a urine that is more
concentrated than plasma is essential for survival of
mammals that live on land, including humans.Water is
continuously lost from the body through various
routes, including the lungs by evaporation into the
expired air, the gastrointestinal tract by way of the
feces, the skin through evaporation and perspira

e
feces, the skin through evaporation and perspiration,
and the kidneys through the excretion of urine. Fluid
intake is required to match this loss, but the ability of
the kidney to form a small volume of concentrated
urine minimizes the intake of ﬂuid required to main-
tain homeostasis, a function that is especially impor-
tant when water is in short supply.
When there is a water deﬁcit in the body, the kidney
forms a concentrated urine by continuing to excrete
solutes while increasing water re

nuing to excrete
solutes while increasing water reabsorption and
decreasing the volume of urine formed. The human
kidney can produce a maximal urine concentration of
1200 to 1400 mOsm/L, four to ﬁve times the osmolar-
ity of plasma. Some desert animals, such as the Aus-
tralian hopping mouse, can concentrate urine to as
high as 10,000 mOsm/L. This allows the mouse to
survive in the desert without drinking water; sufﬁcient
water can be obtained through the food ingested and
water produced in the 

rough the food ingested and
water produced in the body by metabolism of the food.
Animals adapted to aquatic environments, such as the
beaver, have minimal urine concentrating ability; they
can concentrate the urine to only about 500 mOsm/L.
The maximal concentrating ability of the kidney dic-
tates how much urine volume must be excreted each
day to rid the body of waste products of metabolism and
ions that are ingested. A normal 70-kilogram human
must excrete about 600 m

 normal 70-kilogram human
must excrete about 600 milliosmoles of solute each day.
If maximal urine concentrating ability is 1200 mOsm/L,
the minimal volume of urine that must be excreted,
called the obligatory urine volume, can be calculated as
This minimal loss of volume in the urine contributes to
dehydration, along with water loss from the skin, respi-
ratory tract, and gastrointestinal tract, when water is not
available to drink.
The limited ability of the human kidney to con-
centrate the u

ability of the human kidney to con-
centrate the urine to a maximal concentration of 
1200 mOsm/L explains why severe dehydration occurs
if one attempts to drink seawater. Sodium chloride 
concentration in the oceans averages about 3.0 to 3.5
per cent, with an osmolarity between about 1000 and
1200 mOsm/L. Drinking 1 liter of seawater with a con-
centration of 1200 mOsm/L would provide a total
sodium chloride intake of 1200 milliosmoles. If maximal
urine concentrating ability is 1200 mOsm/L, the

al
urine concentrating ability is 1200 mOsm/L, the amount
of urine volume needed to excrete 1200 milliosmoles
would be 1200 milliosmoles divided by 1200 mOsm/L,
or 1.0 liter. Why then does drinking seawater cause
dehydration? The answer is that the kidney must also
excrete other solutes, especially urea, which contribute
about 600 mOsm/L when the urine is maximally con-
centrated. Therefore, the maximum concentration of
sodium chloride that can be excreted by the kidneys is
about 600 mOsm/L. Thu

e excreted by the kidneys is
about 600 mOsm/L. Thus, for every liter of seawater
drunk, 2 liters of urine volume would be required to rid
the body of 1200 milliosmoles of sodium chloride
ingested in addition to other solutes such as urea. This
would result in a net ﬂuid loss of 1 liter for every liter
of seawater drunk, explaining the rapid dehydration
that occurs in shipwreck victims who drink seawater.
However, a shipwreck victim’s pet Australian hopping
mouse could drink with impunity all the

an hopping
mouse could drink with impunity all the seawater it
wanted.
Requirements for Excreting a
Concentrated Urine—High ADH Levels
and Hyperosmotic Renal Medulla
The basic requirements for forming a concentrated
urine are (1) a high level of ADH, which increases the
permeability of the distal tubules and collecting ducts
to water, thereby allowing these tubular segments to
avidly reabsorb water, and (2) a high osmolarity of the
renal medullary interstitial ﬂuid, which provides the
osmotic gr

y interstitial ﬂuid, which provides the
osmotic gradient necessary for water reabsorption to
occur in the presence of high levels of ADH.
600 mOsm day
1200 mOsm L
L day
= 0 5.
Chapter 28
351
The renal medullary interstitium surrounding the
collecting ducts normally is very hyperosmotic, so that
when ADH levels are high, water moves through the
tubular membrane by osmosis into the renal intersti-
tium; from there it is carried 

he renal intersti-
tium; from there it is carried away by the vasa recta
back into the blood. Thus, the urine concentrating
ability is limited by the level of ADH and by the
degree of hyperosmolarity of the renal medulla. We
discuss the factors that control ADH secretion later,
but for now, what is the process by which renal
medullary interstitial ﬂuid becomes hyperosmotic?
This process involves the operation of the countercur-
rent mechanism.
The countercurrent mechanism depends on the
special 

e countercurrent mechanism depends on the
special anatomical arrangement of the loops of Henle
and the vasa recta, the specialized peritubular capillar-
ies of the renal medulla. In the human, about 25 per
cent of the nephrons are juxtamedullary nephrons,
with loops of Henle and vasa recta that go deeply into
the medulla before returning to the cortex. Some of
the loops of Henle dip all the way to the tips of the
renal papillae that project from the medulla into the
renal pelvis. Paralleling the

the medulla into the
renal pelvis. Paralleling the long loops of Henle are 
the vasa recta, which also loop down into the medulla
before returning to the renal cortex. And ﬁnally, the
collecting ducts, which carry urine through the hyper-
osmotic renal medulla before it is excreted, also play
a critical role in the countercurrent mechanism.
Countercurrent Mechanism 
Produces a Hyperosmotic Renal
Medullary Interstitium
The osmolarity of interstitial ﬂuid in almost all parts
of the body is about 3

al ﬂuid in almost all parts
of the body is about 300 mOsm/L, which is similar to
the plasma osmolarity. (As discussed in Chapter 25,
the corrected osmolar activity, which accounts for
intermolecular attraction and repulsion, is about 
282 mOsm/L.) The osmolarity of the interstitial ﬂuid
in the medulla of the kidney is much higher, increas-
ing progressively to about 1200 to 1400 mOsm/L in the
pelvic tip of the medulla. This means that the renal
medullary interstitium has accumulated solutes in
g

edullary interstitium has accumulated solutes in
great excess of water. Once the high solute concentra-
tion in the medulla is achieved, it is maintained by a
balanced inﬂow and outﬂow of solutes and water in
the medulla.
The major factors that contribute to the buildup of
solute concentration into the renal medulla are as
follows:
1. Active transport of sodium ions and co-transport
of potassium, chloride, and other ions out of the
thick portion of the ascending limb of the loop 
of Henle into t

of the ascending limb of the loop 
of Henle into the medullary interstitium
2. Active transport of ions from the collecting ducts
into the medullary interstitium
3. Facilitated diffusion of large amounts of urea
from the inner medullary collecting ducts into the
medullary interstitium
4. Diffusion of only small amounts of water from the
medullary tubules into the medullary interstitium,
far less than the reabsorption of solutes into the
Special Characteristics of Loop of H

 interstitium
Special Characteristics of Loop of Henle That Cause Solutes to
Be Trapped in the Renal Medulla. The transport character-
istics of the loops of Henle are summarized in Table
28–1, along with the characteristics of the proximal
tubules, distal tubules, cortical collecting tubules, and
inner medullary collecting ducts.
The most important cause of the high medullary
osmolarity is active transport of sodium and co-
transport of potassium, chloride, and other ions from
the thick ascendi

m, chloride, and other ions from
the thick ascending loop of Henle into the interstitium.
This pump is capable of establishing about a 200-
milliosmole concentration gradient between the
tubular lumen and the interstitial ﬂuid. Because the
thick ascending limb is virtually impermeable to water,
the solutes pumped out are not followed by osmotic
ﬂow of water into the interstitium. Thus, the active
transport of sodium and other ions out of the thick
ascending loop adds solutes in excess of water t

k
ascending loop adds solutes in excess of water to the
renal medullary interstitium. There is some passive
reabsorption of sodium chloride from the thin ascend-
ing limb of Henle’s loop, which is also impermeable to
water, adding further to the high solute concentration
of the renal medullary interstitium.
The descending limb of Henle’s loop, in contrast to
the ascending limb, is very permeable to water, and the
tubular ﬂuid osmolarity quickly becomes equal to the
renal medullary osmolarity. Th

ecomes equal to the
renal medullary osmolarity. Therefore, water diffuses
out of the descending limb of Henle’s loop into the
interstitium, and the tubular ﬂuid osmolarity gradually
rises as it ﬂows toward the tip of the loop of Henle.
Steps Involved in Causing Hyperosmotic Renal Medullary Inter-
stitium. With these characteristics of the loop of Henle
in mind, let us now discuss how the renal medulla
becomes hyperosmotic. First, assume that the loop of
Henle is ﬁlled with ﬂuid with a concentrat

loop of
Henle is ﬁlled with ﬂuid with a concentration of 
300 mOsm/L, the same as that leaving the proximal
tubule (Figure 28–3, step 1). Next, the active pump of
the thick ascending limb on the loop of Henle is turned
on, reducing the concentration inside the tubule and
Table 28–1
Summary of Tubule Characteristics—Urine Concentration
Active NaCl
Transport
H2O
NaCl
Urea
++
++
+
+
Thin descending limb
0
++
+
+
Thin ascending limb
0
0
+
+
Thick ascending limb
++
0
0
0

ending limb
0
0
+
+
++
0
0
0
+
+ADH
0
0
Cortical collecting
+
+ADH
0
0
tubule
Inner medullary
+
+ADH
0
++ADH
0, minimal level of active transport or permeability; +, moderate level of active
transport or permeability; ++, high level of active transport or permeability;
+ADH, permeability to water or urea is increased by ADH.
352
Unit V
raising the interstitial concentration; this pump estab-
lishes a 200-mOsm/L concen

ation; this pump estab-
lishes a 200-mOsm/L concentration gradient between
the tubular ﬂuid and the interstitial ﬂuid (step 2). The
limit to the gradient is about 200 mOsm/L because
paracellular diffusion of ions back into the tubule
eventually counterbalances transport of ions out of the
lumen when the 200-mOsm/L concentration gradient
is achieved.
Step 3 is that the tubular ﬂuid in the descending limb
of the loop of Henle and the interstitial ﬂuid quickly
reach osmotic equilibrium because of o

uid quickly
reach osmotic equilibrium because of osmosis of water
out of the descending limb. The interstitial osmolarity
is maintained at 400 mOsm/L because of continued
transport of ions out of the thick ascending loop of
Henle. Thus, by itself, the active transport of sodium
chloride out of the thick ascending limb is capable of
establishing only a 200-mOsm/L concentration gradi-
ent, much less than that achieved by the countercur-
rent system.
Step 4 is additional ﬂow of ﬂuid into the loop o

.
Step 4 is additional ﬂow of ﬂuid into the loop of
Henle from the proximal tubule, which causes the
hyperosmotic ﬂuid previously formed in the descend-
ing limb to ﬂow into the ascending limb. Once this ﬂuid
is in the ascending limb, additional ions are pumped
into the interstitium, with water remaining behind,
until a 200-mOsm/L osmotic gradient is established,
with the interstitial ﬂuid osmolarity rising to 
500 mOsm/L (step 5).Then, once again, the ﬂuid in the
descending limb reaches equilib

n, the ﬂuid in the
descending limb reaches equilibrium with the hyper-
osmotic medullary interstitial ﬂuid (step 6), and as the
hyperosmotic tubular ﬂuid from the descending limb
of the loop of Henle ﬂows into the ascending limb, still
more solute is continuously pumped out of the tubules
and deposited into the medullary interstitium.
These steps are repeated over and over, with the net
effect of adding more and more solute to the medulla
in excess of water; with sufﬁcient time, this process
gra

ss of water; with sufﬁcient time, this process
gradually traps solutes in the medulla and multiplies 
the concentration gradient established by the active
pumping of ions out of the thick ascending loop of
Henle, eventually raising the interstitial ﬂuid osmolar-
ity to 1200 to 1400 mOsm/L as shown in step 7.
Thus, the repetitive reabsorption of sodium chloride
by the thick ascending loop of Henle and continued
inﬂow of new sodium chloride from the proximal
tubule into the loop of Henle is called

e proximal
tubule into the loop of Henle is called the countercur-
rent multiplier. The sodium chloride reabsorbed from
the ascending loop of Henle keeps adding to the newly
arrived sodium chloride, thus “multiplying” its con-
centration in the medullary interstitium.
Role of Distal Tubule and 
Collecting Ducts in Excreting a
When the tubular ﬂuid leaves the loop of Henle and
ﬂows into the distal convoluted tubule in the renal
cortex, the ﬂuid is dilute, with an osmolarity of 

cortex, the ﬂuid is dilute, with an osmolarity of only
about 100 mOsm/L (Figure 28–4). The early distal
300
300
300
300
300
300
300
300
300
300
300
300
400
400
400
300
300
300
300
200
200
200
200
2
400
400
400
300
400
400
400
200
200
200
200
400
300
400
400
300
300
400
400
200
200
400
400
3
4
1
350
500
500
300
350
500
500
150
150
300
300
6
700
300
1000
1200
300
700
1000
1200
100
500
800
1000
7
Repeat Steps 4-6
5
350
500
500
300
300
400
400
150
150
300
300
Figure 28–3
Countercurrent multiplier sy

0
300
300
Countercurrent multiplier system in the loop of Henle for producing a hyperosmotic renal medulla. (Numerical values are in milliosmoles
per liter.)
Chapter 28
353
tubule further dilutes the tubular ﬂuid because this
segment, like the ascending loop of Henle, actively
transports sodium chloride out of the tubule but is rel-
atively impermeable to water.
As ﬂuid ﬂows into the cortical collecting tubule, the

ﬂuid ﬂows into the cortical collecting tubule, the
amount of water reabsorbed is critically dependent on
the plasma concentration of ADH. In the absence of
ADH, this segment is almost impermeable to water
and fails to reabsorb water but continues to reabsorb
solutes and further dilutes the urine. When there is a
high concentration of ADH, the cortical collecting
tubule becomes highly permeable to water, so that
large amounts of water are now reabsorbed from the
tubule into the cortex interstitiu

sorbed from the
tubule into the cortex interstitium, where it is swept
away by the rapidly ﬂowing peritubular capillaries.
The fact that these large amounts of water are reab-
sorbed into the cortex, rather than into the renal
medulla, helps to preserve the high medullary intersti-
tial ﬂuid osmolarity.
As the tubular ﬂuid ﬂows along the medullary col-
lecting ducts, there is further water reabsorption from
the tubular ﬂuid into the interstitium, but the total
amount of water is relatively small

 but the total
amount of water is relatively small compared with that
added to the cortex interstitium.The reabsorbed water
is quickly carried away by the vasa recta into the
venous blood. When high levels of ADH are present,
the collecting ducts become permeable to water, so
that the ﬂuid at the end of the collecting ducts has
essentially the same osmolarity as the interstitial ﬂuid
of the renal medulla—about 1200 mOsm/L (see Figure
28–3).Thus, by reabsorbing as much water as possible,
the kidn

by reabsorbing as much water as possible,
the kidneys form a highly concentrated urine, excret-
ing normal amounts of solutes in the urine while
adding water back to the extracellular ﬂuid and com-
pensating for deﬁcits of body water.
Urea Contributes to Hyperosmotic
Renal Medullary Interstitium and to a
Thus far, we have considered only the contribution of
sodium chloride to the hyperosmotic renal medullary
interstitium. However, urea contributes about 40 
to 50 per cent of t

er, urea contributes about 40 
to 50 per cent of the osmolarity (500-600 mOsm/L) of
the renal medullary interstitium when the kidney 
is forming a maximally concentrated urine. Unlike
sodium chloride, urea is passively reabsorbed from the
tubule. When there is water deﬁcit and blood concen-
trations of ADH are high, large amounts of urea are
passively reabsorbed from the inner medullary col-
lecting ducts into the interstitium.
The mechanism for reabsorption of urea into the
renal medulla is as 

reabsorption of urea into the
renal medulla is as follows: As water ﬂows up the
ascending loop of Henle and into the distal and corti-
cal collecting tubules, little urea is reabsorbed because
these segments are impermeable to urea (see Table
28–1). In the presence of high concentrations of ADH,
water is reabsorbed rapidly from the cortical col-
lecting tubule and the urea concentration increases
rapidly because urea is not very permeant in this part
of the tubule. Then, as the tubular ﬂuid ﬂows

part
of the tubule. Then, as the tubular ﬂuid ﬂows into the
inner medullary collecting ducts, still more water reab-
sorption takes place, causing an even higher concen-
tration of urea in the ﬂuid. This high concentration of
urea in the tubular ﬂuid of the inner medullary col-
lecting duct causes urea to diffuse out of the tubule
into the renal interstitium. This diffusion is greatly
facilitated by speciﬁc urea transporters. One of these
urea transporters, UT-AI, is activated by ADH,
increasing

ansporters, UT-AI, is activated by ADH,
increasing transport of urea out of the inner medullary
collecting duct even more when ADH levels are ele-
vated. The simultaneous movement of water and urea
out of the inner medullary collecting ducts maintains
a high concentration of urea in the tubular ﬂuid and,
eventually, in the urine, even though urea is being
reabsorbed.
The fundamental role of urea in contributing to
urine concentrating ability is evidenced by the fact that
people who ingest a high

videnced by the fact that
people who ingest a high-protein diet, yielding large
amounts of urea as a nitrogenous “waste” product,
can concentrate their urine much better than people
whose protein intake and urea production are low.
Malnutrition is associated with a low urea concentra-
tion in the medullary interstitium and considerable
impairment of urine concentrating ability.
Recirculation of Urea from Collecting Duct to Loop of Henle
Contributes to Hyperosmotic Renal Medulla. A person
usually

es to Hyperosmotic Renal Medulla. A person
usually excretes about 20 to 50 per cent of the ﬁltered
load of urea. In general, the rate of urea excretion is
determined mainly by two factors: (1) the concentra-
tion of urea in the plasma and (2) the glomerular ﬁl-
tration rate (GFR). In patients with renal disease who
have large reductions of GFR, the plasma urea con-
centration increases markedly, returning the ﬁltered
urea load and urea excretion rate to the normal level
(equal to the rate of ure

rate to the normal level
(equal to the rate of urea production), despite the
reduced GFR.
NaCl
Medulla
Cortex
1200
1200
300
100
300
600
1200
H2O
NaCl
NaCl
600
1200
Urea
H2O
NaCl
600
600
H2O
NaCl
Urea
300
H2O
Figure 28–4
Formation of a concentrated urine when antidiuretic hormone
(ADH) levels are high. Note that the ﬂuid leaving the loop of Henle
is dilute but becomes concentrated as water is absorbed from the
distal tubules and collecting tubules. With high ADH levels, the
osmolarity of the urin

. With high ADH levels, the
osmolarity of the urine is about the same as the osmolarity of 
the renal medullary interstitial ﬂuid in the papilla, which is about
1200 mOsm/L. (Numerical values are in milliosmoles per liter.)
354
Unit V
In the proximal tubule, 40 to 50 per cent of the ﬁl-
tered urea is reabsorbed, but even so, the tubular ﬂuid
urea concentration increases because urea is not
nearly as permeant as water.The concentration of urea
continues to rise as the 

he concentration of urea
continues to rise as the tubular ﬂuid ﬂows into the thin
segments of the loop of Henle, partly because of water
reabsorption out of the descending loop of Henle but
also because of some secretion of urea into the thin
loop of Henle from the medullary interstitium (Figure
28–5).
The thick limb of the loop of Henle, the distal
tubule, and the cortical collecting tubule are all rela-
tively impermeable to urea, and very little urea reab-
sorption occurs in these tubular seg

le urea reab-
sorption occurs in these tubular segments. When the
kidney is forming a concentrated urine and high levels
of ADH are present, the reabsorption of water from
the distal tubule and cortical collecting tubule further
raises the tubular ﬂuid concentration of urea. And as
this urea ﬂows into the inner medullary collecting
duct, the high tubular ﬂuid concentration of urea 
and speciﬁc urea transporters cause urea to diffuse
into the medullary interstitium. A moderate share of
the urea t

llary interstitium. A moderate share of
the urea that moves into the medullary interstitium
eventually diffuses into the thin loop of Henle, so that
it passes upward through the ascending loop of Henle,
the distal tubule, the cortical collecting tubule, and
back down into the medullary collecting duct again. In
this way, urea can recirculate through these terminal
parts of the tubular system several times before it is
excreted. Each time around the circuit contributes to
a higher concentration o

he circuit contributes to
a higher concentration of urea.
This urea recirculation provides an additional mech-
anism for forming a hyperosmotic renal medulla.
Because urea is one of the most abundant waste prod-
ucts that must be excreted by the kidneys, this mech-
anism for concentrating urea before it is excreted is
essential to the economy of the body ﬂuid when water
is in short supply.
When there is excess water in the body and low
levels of ADH, the inner medullary collecting ducts
have a m

ADH, the inner medullary collecting ducts
have a much lower permeability to both water and
urea, and more urea is excreted in the urine.
Countercurrent Exchange in the Vasa
Recta Preserves Hyperosmolarity 
of the Renal Medulla
Blood ﬂow must be provided to the renal medulla to
supply the metabolic needs of the cells in this part of
the kidney. Without a special medullary blood ﬂow
system, the solutes pumped into the renal medulla by
the countercurrent multiplier system would be rapidly
dissipate

rrent multiplier system would be rapidly
dissipated.
There are two special features of the renal
medullary blood ﬂow that contribute to the preserva-
tion of the high solute concentrations:
1. The medullary blood ﬂow is low, accounting for
less than 5 per cent of the total renal blood ﬂow.
This sluggish blood ﬂow is sufﬁcient to supply 
the metabolic needs of the tissues but helps to
minimize solute loss from the medullary
interstitium.
2. The vasa recta serve as countercurrent exchangers,
minim

sa recta serve as countercurrent exchangers,
minimizing washout of solutes from the medullary
The countercurrent exchange mechanism operates as
follows (Figure 28–6): Blood enters and leaves the
medulla by way of the vasa recta at the boundary 
of the cortex and renal medulla. The vasa recta, like
other capillaries, are highly permeable to solutes in 
the blood, except for the plasma proteins. As blood
descends into the medulla toward the papillae, it
becomes progressively more con

rd the papillae, it
becomes progressively more concentrated, partly 
by solute entry from the interstitium and partly by 
loss of water into the interstitium. By the time the
blood reaches the tips of the vasa recta, it has a con-
centration of about 1200 mOsm/L, the same as that of
the medullary interstitium. As blood ascends back
toward the cortex, it becomes progressively less 
concentrated as solutes diffuse back out into the
medullary interstitium and as water moves into the
vasa recta.
Thu

titium and as water moves into the
Thus, although there is a large amount of ﬂuid and
solute exchange across the vasa recta, there is little net
dilution of the concentration of the interstitial ﬂuid at
each level of the renal medulla because of the U shape
Urea
Urea
Urea
Urea
20% remaining
100% remaining
50% remaining
100%
remaining
4.5
30
H2O
Cortex
Outer
medulla
Inner
medulla
15
7
30
300
300
500
550
4.5
Figure 28–5
Recirculation of urea absorbed from the medullary collecting duct

 urea absorbed from the medullary collecting duct
into the interstitial ﬂuid. This urea diffuses into the thin loop of
Henle, and then passes through the distal tubules, and ﬁnally
passes back into the collecting duct. The recirculation of urea
helps to trap urea in the renal medulla and contributes to the
hyperosmolarity of the renal medulla. The heavy dark lines, from
the thick ascending loop of Henle to the medullary collecting
ducts, indicate that these segments are not very permeable to
ure

 that these segments are not very permeable to
urea. (Numerical values are in milliosmoles per liter of urea during
antidiuresis, when large amounts of antidiuretic hormone are
present. Percentages of the ﬁltered load of urea that remain in the
tubules are indicated in the boxes.)
Chapter 28
355
of the vasa recta capillaries, which act as countercur-
rent exchangers. Thus, the vasa recta do not create the
medullary hyperosmola

vasa recta do not create the
medullary hyperosmolarity, but they do prevent it from
being dissipated.
The U-shaped structure of the vessels minimizes
loss of solute from the interstitium but does not
prevent the bulk ﬂow of ﬂuid and solutes into the
blood through the usual colloid osmotic and hydro-
static pressures that favor reabsorption in these capil-
laries. Thus, under steady-state conditions, the vasa
recta carry away only as much solute and water as is
absorbed from the medullary tubules

nd water as is
absorbed from the medullary tubules, and the high
concentration of solutes established by the counter-
current mechanism is maintained.
Increased Medullary Blood Flow Can Reduce Urine Concentrat-
ing Ability. Certain vasodilators can markedly increase
renal medullary blood ﬂow, thereby “washing out”
some of the solutes from the renal medulla and reduc-
ing maximum urine concentrating ability. Large
increases in arterial pressure can also increase the
blood ﬂow of the renal medulla

n also increase the
blood ﬂow of the renal medulla to a greater extent
than in other regions of the kidney and tend to wash
out the hyperosmotic interstitium, thereby reducing
urine concentrating ability. As discussed earlier,
maximum concentrating ability of the kidney is deter-
mined not only by the level of ADH but also by the
osmolarity of the renal medulla interstitial ﬂuid. Even
with maximal levels of ADH, urine concentrating
ability will be reduced if medullary blood ﬂow
increases enough 

e reduced if medullary blood ﬂow
increases enough to reduce the hyperosmolarity in the
renal medulla.
Summary of Urine Concentrating
Mechanism and Changes in
Osmolarity in Different Segments 
of the Tubules
The changes in osmolarity and volume of the tubular
ﬂuid as it passes through the different parts of the
nephron are shown in Figure 28–7.
Proximal Tubule. About 65 per cent of the ﬁltered 
electrolytes are reabsorbed in the proximal tubule.
However, the tubular membranes are highly perme-
ab

owever, the tubular membranes are highly perme-
able to water,so that whenever solutes are reabsorbed,
water also diffuses through the tubular membrane by
osmosis.Therefore, the osmolarity of the ﬂuid remains
about the same as the glomerular ﬁltrate,300 mOsm/L.
Descending Loop of Henle. As ﬂuid ﬂows down the
descending loop of Henle, water is absorbed into the
medulla. The descending limb is highly permeable to
water but much less permeable to sodium chloride and
urea. Therefore, the osmolarity 

dium chloride and
urea. Therefore, the osmolarity of the ﬂuid ﬂowing
through the descending loop gradually increases until
it is equal to that of the surrounding interstitial ﬂuid,
which is about 1200 mOsm/L when the blood concen-
tration of ADH is high. When a dilute urine is being
formed, owing to low ADH concentrations, the med-
ullary interstitial osmolarity is less than 1200 mOsm/L;
consequently, the descending loop tubular ﬂuid osmo-
larity also becomes less concentrated. This is due
partl

 also becomes less concentrated. This is due
partly to the fact that less urea is absorbed into the
medullary interstitium from the collecting ducts when
ADH levels are low and the kidney is forming a large
volume of dilute urine.
Thin Ascending Loop of Henle. The thin ascending limb is
essentially impermeable to water but reabsorbs some
sodium chloride. Because of the high concentration of
sodium chloride in the tubular ﬂuid, owing to water
removal from the descending loop of Henle, there is
so

val from the descending loop of Henle, there is
some passive diffusion of sodium chloride from the
thin ascending limb into the medullary interstitium.
Thus, the tubular ﬂuid becomes more dilute as the
sodium chloride diffuses out of the tubule and water
remains in the tubule. Some of the urea absorbed into
the medullary interstitium from the collecting ducts
also diffuses into the ascending limb, thereby return-
ing the urea to the tubular system and helping to
prevent its washout from the rena

m and helping to
prevent its washout from the renal medulla. This urea
recycling is an additional mechanism that contributes
to the hyperosmotic renal medulla.
Thick Ascending Loop of Henle. The thick part of the
ascending loop of Henle is also virtually impermeable
to water, but large amounts of sodium, chloride, potas-
sium, and other ions are actively transported from 
the tubule into the medullary interstitium. Therefore,
ﬂuid in the thick ascending limb of the loop of Henle
becomes very dil

cending limb of the loop of Henle
becomes very dilute, falling to a concentration of
about 100 mOsm/L.
Vasa recta
mOsm/L
mOsm/L
300
600
900
1200
H2O
600
H2O
1000
Solute
Solute
Solute
Solute
Solute
Solute
H2O
800
350
300
600
800
1000
600
800
1000
1200
Figure 28–6
Countercurrent exchange in the vasa recta. Plasma ﬂowing down
the descending limb of the vasa recta becomes more hyperos-
motic because of diffusion of water out of the blood and diffusion
of solutes from the renal interstit

 and diffusion
of solutes from the renal interstitial ﬂuid into the blood. In the
ascending limb of the vasa recta, solutes diffuse back into the
interstitial ﬂuid and water diffuses back into the vasa recta. Large
amounts of solutes would be lost from the renal medulla without
the U shape of the vasa recta capillaries. (Numerical values are
in milliosmoles per liter.)
356
Unit V
Early Distal Tubule. The early distal tubule has proper-
ties similar to those of the thi

ubule has proper-
ties similar to those of the thick ascending loop of
Henle, so that further dilution of the tubular ﬂuid
occurs as solutes are reabsorbed while water remains
in the tubule.
Late Distal Tubule and Cortical Collecting Tubules. In the late
distal tubule and cortical collecting tubules, the osmo-
larity of the ﬂuid depends on the level of ADH. With
high levels of ADH, these tubules are highly perme-
able to water, and signiﬁcant amounts of water are
reabsorbed. Urea, however, is no

unts of water are
reabsorbed. Urea, however, is not very permeant in
this part of the nephron, resulting in increased urea
concentration as water is reabsorbed.This allows most
of the urea delivered to the distal tubule and collect-
ing tubule to pass into the inner medullary collecting
ducts, from which it is eventually reabsorbed or
excreted in the urine. In the absence of ADH, little
water is reabsorbed in the late distal tubule and corti-
cal collecting tubule; therefore, osmolarity decrease

 collecting tubule; therefore, osmolarity decreases
even further because of continued active reabsorption
of ions from these segments.
Inner Medullary Collecting Ducts. The concentration of
ﬂuid in the inner medullary collecting ducts also
depends on (1) ADH and (2) the osmolarity of the
medullary interstitium established by the countercur-
rent mechanism. In the presence of large amounts of
ADH, these ducts are highly permeable to water, and
water diffuses from the tubule into the interstitium

er diffuses from the tubule into the interstitium
until osmotic equilibrium is reached, with the tubular
ﬂuid having about the same concentration as the renal
medullary interstitium (1200 to 1400 mOsm/L). Thus,
a very concentrated but small volume of urine is pro-
duced when ADH levels are high. Because water reab-
sorption increases urea concentration in the tubular
ﬂuid, and because the inner medullary collecting ducts
have speciﬁc urea transporters that greatly facilitate
diffusion, much of t

rters that greatly facilitate
diffusion, much of the highly concentrated urea in 
the ducts diffuses out of the tubular lumen into the
medullary interstitium. This absorption of the urea
into the renal medulla contributes to the high osmo-
larity of the medullary interstitium and the high con-
centrating ability of the kidney.
There are several important points to consider that
may not be obvious from this discussion. First,
although sodium chloride is one of the principal
solutes that contribut

ide is one of the principal
solutes that contributes to the hyperosmolarity of the
medullary interstitium, the kidney can, when needed,
excrete a highly concentrated urine that contains little
sodium chloride. The hyperosmolarity of the urine in
these circumstances is due to high concentrations of
other solutes, especially of waste products such as urea
and creatinine. One condition in which this occurs is
dehydration accompanied by low sodium intake. As
discussed in Chapter 29, low sodium intak

take. As
discussed in Chapter 29, low sodium intake stimulates
formation of the hormones angiotensin II and aldos-
terone, which together cause avid sodium reabsorption
from the tubules while leaving the urea and other
solutes to maintain the highly concentrated urine.
Second, large quantities of dilute urine can be
excreted without increasing the excretion of sodium.
This is accomplished by decreasing ADH secretion,
which reduces water reabsorption in the more distal
tubular segments without si

ion in the more distal
tubular segments without signiﬁcantly altering sodium
reabsorption.
And ﬁnally, we should keep in mind that there is 
an obligatory urine volume, which is dictated by the
maximum concentrating ability of the kidney and the
amount of solute that must be excreted. Therefore, if
large amounts of solute must be excreted, they must
be accompanied by the minimal amount of water 
Osmolarity  (mOsm/L)
Diluting segment
Late distal
Cortical
Medullary
Effect of ADH
1200
900
600
300
2

ortical
Medullary
1200
900
600
300
200
100
0
Proximal
tubule
125 ml
44 ml
25 ml
Distal
tubule
tubule
and duct
Urine
20 ml
8 ml
0.2 ml
25 ml
Figure 28–7
Changes in osmolarity of the
tubular ﬂuid as it passes through
the different tubular segments in
the presence of high levels of
antidiuretic hormone (ADH) and
in the absence of ADH. (Numeri-
cal values indicate the approxi-
mate volumes in milliliters per
minute or in osmolarities in mil-
liosmoles per liter

ute or in osmolarities in mil-
liosmoles per liter of ﬂuid ﬂow-
ing along the different tubular
segments.)
Chapter 28
357
necessary to excrete them. For example, if 1200 mil-
liosmoles of solute must be excreted each day, this
requires at least 1 liter of urine if maximal urine con-
centrating ability is 1200 mOsm/L.
Quantifying Renal Urine
Concentration and Dilution:
“Free Water” and 
Osmolar Clearances
The process of concent

er” and 
The process of concentrating or diluting the urine
requires the kidneys to excrete water and solutes some-
what independently. When the urine is dilute, water is
excreted in excess of solutes. Conversely, when the urine
is concentrated, solutes are excreted in excess of water.
The total clearance of solutes from the blood can be
expressed as the osmolar clearance (Cosm); this is the
volume of plasma cleared of solutes each minute, in 
the same way that clearance of a 

each minute, in 
the same way that clearance of a single substance is cal-
culated:
where Uosm is the urine osmolarity, V is the urine ﬂow
rate, and Posm is the plasma osmolarity. For example, if
plasma osmolarity is 300 mOsm/L, urine osmolarity is
600 mOsm/L, and urine ﬂow rate is 1 ml/min (0.001 L/
min), the rate of osmolar excretion is 0.6 mOsm/min
(600 mOsm/L ¥ 0.001 L/min) and osmolar clearance is
0.6 mOsm/min divided by 300 mOsm/L, or 0.002 L/min
(2.0 ml/min). This means that 2 milliliters

 L/min
(2.0 ml/min). This means that 2 milliliters of plasma are
being cleared of solute each minute.
Relative Rates at Which Solutes and Water Are Excreted Can Be
Assessed Using the Concept of “Free-Water Clearance.” Free-
water clearance (CH2O) is calculated as the difference
between water excretion (urine ﬂow rate) and osmolar
clearance:
Thus, the rate of free-water clearance represents the
rate at which solute-free water is excreted by the
kidneys. When free-water clearance is positive, exce

dneys. When free-water clearance is positive, excess
water is being excreted by the kidneys; when free-water
clearance is negative, excess solutes are being removed
from the blood by the kidneys and water is being 
conserved.
Using the example discussed earlier, if urine ﬂow 
rate is 1 ml/min and osmolar clearance is 2 ml/min,
free-water clearance would be -1 ml/min. This means
that instead of water being cleared from the kidneys in
excess of solutes, the kidneys are actually returning
water bac

utes, the kidneys are actually returning
water back to the systemic circulation, as occurs during
water deﬁcits. Thus, whenever urine osmolarity is greater
than plasma osmolarity, free-water clearance will be neg-
ative, indicating water conservation.
When the kidneys are forming a dilute urine (that is,
urine osmolarity is less than plasma osmolarity), free-
water clearance will be a positive value, denoting that
water is being removed from the plasma by the kidneys
in excess of solutes. Thus, 

plasma by the kidneys
in excess of solutes. Thus, water free of solutes, called
“free water,” is being lost from the body and the plasma
is being concentrated when free-water clearance is 
positive.
C
V
C
V
U
V
P
H O
osm
osm
osm
2
=
-
=
-
¥
(
)
(
)
C
U
V
P
osm
osm
osm
=
¥
Disorders of Urinary
Concentrating Ability
An impairment in the ability of the kidneys to concen-
trate or dilute the urine appropriately can occur with
one or more of the following abnormalities:
1. Inappropriate secretion of 

wing abnormalities:
1. Inappropriate secretion of ADH. Either too much
or too little ADH secretion results in abnormal
ﬂuid handling by the kidneys.
2. Impairment of the countercurrent mechanism. A
hyperosmotic medullary interstitium is required for
maximal urine concentrating ability. No matter how
much ADH is present, maximal urine concentration
is limited by the degree of hyperosmolarity of the
medullary interstitium.
3. Inability of the distal tubule, collecting tubule, and
collecting ducts 

l tubule, collecting tubule, and
collecting ducts to respond to ADH.
Failure to Produce ADH: “Central” Diabetes Insipidus. An
inability to produce or release ADH from the posterior
pituitary can be caused by head injuries or infections, or
it can be congenital. Because the distal tubular segments
cannot reabsorb water in the absence of ADH, this con-
dition, called “central” diabetes insipidus, results in the
formation of a large volume of dilute urine, with urine
volumes that can exceed 15 L/da

 urine, with urine
volumes that can exceed 15 L/day. The thirst mecha-
nisms, discussed later in this chapter, are activated when
excessive water is lost from the body; therefore, as long
as the person drinks enough water, large decreases in
body ﬂuid water do not occur. The primary abnormal-
ity observed clinically in people with this condition is
the large volume of dilute urine. However, if water
intake is restricted, as can occur in a hospital setting
when ﬂuid intake is restricted or the pa

l setting
when ﬂuid intake is restricted or the patient is uncon-
scious (for example, because of a head injury), severe
dehydration can rapidly occur.
The treatment for central diabetes insipidus is admin-
istration of a synthetic analog of ADH, desmopressin,
which acts selectively on V2 receptors to increase water
permeability in the late distal and collecting tubules.
Desmopressin can be given by injection,as a nasal spray,
or orally, and rapidly restores urine output toward
normal.
Inability

dly restores urine output toward
normal.
Inability of the Kidneys to Respond to ADH: “Nephrogenic”
Diabetes Insipidus. There are circumstances in which
normal or elevated levels of ADH are present but the
renal tubular segments cannot respond appropriately.
This condition is referred to as “nephrogenic” diabetes
insipidus
because the abnormality resides in the
kidneys.This abnormality can be due to either failure of
the countercurrent mechanism to form a hyperosmotic
renal medullary interstitium

o form a hyperosmotic
renal medullary interstitium or failure of the distal and
collecting tubules and collecting ducts to respond to
ADH. In either case, large volumes of dilute urine are
formed, which tends to cause dehydration unless ﬂuid
intake is increased by the same amount as urine volume
is increased.
Many types of renal diseases can impair the concen-
trating mechanism, especially those that damage the
renal medulla. Also, impairment of the function of the
loop of Henle, as occurs with 

the function of the
loop of Henle, as occurs with diuretics that inhibit elec-
trolyte reabsorption by this segment, can compromise
urine concentrating ability. And certain drugs, such as
lithium (used to treat manic-depressive disorders) 
and tetracyclines (used as antibiotics), can impair the
ability of the distal nephron segments to respond to
ADH.
358
Unit V
Nephrogenic diabetes insipidus can be distinguished
from central diabetes insipidus by administration of
de

central diabetes insipidus by administration of
desmopressin, the synthetic analog of ADH. Lack of a
prompt decrease in urine volume and an increase in
urine osmolarity within 2 hours after injection of
desmopressin is strongly suggestive of nephrogenic dia-
betes insipidus. The treatment for nephrogenic diabetes
insipidus is to correct, if possible, the underlying renal
disorder. The hypernatremia can also be attenuated by
a low-sodium diet and administration of a diuretic that
enhances renal s

administration of a diuretic that
enhances renal sodium excretion, such as a thiazide
diuretic.
Regulation of extracellular ﬂuid osmolarity and
sodium concentration are closely linked because
sodium is the most abundant ion in the extracellular
compartment. Plasma sodium concentration is nor-
mally regulated within close limits of 140 to 145 mEq/
L, with an average concentration of about 142 mEq/L.
Osmolarity averages about 300 m

f about 142 mEq/L.
Osmolarity averages about 300 mOsm/L (about 
282 mOsm/L when corrected for interionic attraction)
and seldom changes more than ±2 to 3 per cent. As 
discussed in Chapter 25, these variables must be pre-
cisely controlled because they determine the distribu-
tion of ﬂuid between the intracellular and extracellular
Estimating Plasma Osmolarity from
Plasma Sodium Concentration
In most clinical laboratories, plasma osmolarity is not
routinely measured. However, becau

molarity is not
routinely measured. However, because sodium and its
associated anions account for about 94 per cent of the
solute in the extracellular compartment, plasma osmo-
larity (Posm) can be roughly approximated as
Posm = 2.1 ¥ Plasma sodium concentration
For instance, with a plasma sodium concentration of
142 mEq/L, the plasma osmolarity would be estimated
from the formula above to be about 298 mOsm/L. To
be more exact, especially in conditions associated with
renal disease, the contribu

itions associated with
renal disease, the contribution of two other solutes,
glucose and urea, should be included. Such estimates
of plasma osmolarity are usually accurate within a few
percentage points of those measured directly.
Normally, sodium ions and associated anions (pri-
marily bicarbonate and chloride) represent about 94
per cent of the extracellular osmoles, with glucose and
urea contributing about 3 to 5 per cent of the total
osmoles. However, because urea easily permeates
most cell 

 However, because urea easily permeates
most cell membranes, it exerts little effective osmotic
pressure under steady-state conditions. Therefore, the
sodium ions in the extracellular ﬂuid and associated
anions are the principal determinants of ﬂuid move-
ment across the cell membrane. Consequently, we can
discuss the control of osmolarity and control of sodium
ion concentration at the same time.
Although multiple mechanisms control the amount
of sodium and water excretion by the kidneys, two
pr

 sodium and water excretion by the kidneys, two
primary systems are especially involved in regulating
the concentration of sodium and osmolarity of extra-
cellular ﬂuid: (1) the osmoreceptor-ADH system and
(2) the thirst mechanism.
Osmoreceptor-ADH 
Feedback System
Figure 28–8 shows the basic components of the
osmoreceptor-ADH feedback system for control of
extracellular ﬂuid sodium concentration and osmolar-
ity. When osmolarity (plasma sodium concentration)
increases above normal because of wa

oncentration)
increases above normal because of water deﬁcit, for
example, this feedback system operates as follows:
1. An increase in extracellular ﬂuid osmolarity
(which in practical terms means an increase in
plasma sodium concentration) causes the special
nerve cells called osmoreceptor cells, located in
the anterior hypothalamus near the supraoptic
nuclei, to shrink.
2. Shrinkage of the osmoreceptor cells causes them
to ﬁre, sending nerve signals to additional nerve
cells in the supraoptic 

gnals to additional nerve
cells in the supraoptic nuclei, which then relay
Water deficit
-
Extracellular osmolarity
Osmoreceptors
Plasma ADH
ADH secretion
(posterior pituitary)
H2O permeability in
distal tubules,
collecting ducts
H2O excreted
Figure 28–8
Osmoreceptor-antidiuretic hormone (ADH) feedback mechanism
for regulating extracellular ﬂuid osmolarity in response to a water
deﬁcit.
Chapter 28
359
these si

d Osmolarity and Sodium Concentration
359
these signals down the stalk of the pituitary gland
to the posterior pituitary.
3. These action potentials conducted to the posterior
pituitary stimulate the release of ADH, which is
stored in secretory granules (or vesicles) in the
nerve endings.
4. ADH enters the blood stream and is transported
to the kidneys, where it increases the water
permeability of the late distal tubules, cortical
collecting tubules, and medullary collecting ducts.
5. The increa

les, and medullary collecting ducts.
5. The increased water permeability in the distal
nephron segments causes increased water
reabsorption and excretion of a small volume of
Thus, water is conserved in the body while sodium and
other solutes continue to be excreted in the urine.This
causes dilution of the solutes in the extracellular ﬂuid,
thereby correcting the initial excessively concentrated
extracellular ﬂuid.
The opposite sequence of events occurs when the
extracellular

e sequence of events occurs when the
extracellular ﬂuid becomes too dilute (hypo-osmotic).
For example, with excess water ingestion and a
decrease in extracellular ﬂuid osmolarity, less ADH is
formed, the renal tubules decrease their permeability
for water, less water is reabsorbed, and a large volume
of dilute urine is formed. This in turn concentrates the
body ﬂuids and returns plasma osmolarity toward
normal.
ADH Synthesis in Supraoptic and
Paraventricular Nuclei of the
Hypothalamus and ADH R

raventricular Nuclei of the
Hypothalamus and ADH Release 
from the Posterior Pituitary
Figure 28–9 shows the neuroanatomy of the hypothal-
amus and the pituitary gland, where ADH is synthe-
sized and released. The hypothalamus contains two
types of magnocellular (large) neurons that synthesize
ADH in the supraoptic and paraventricular nuclei of
the hypothalamus, about ﬁve sixths in the supraoptic
nuclei and about one sixth in the paraventricular
nuclei. Both of these nuclei have axonal extension

nuclei. Both of these nuclei have axonal extensions to
the posterior pituitary. Once ADH is synthesized, it is
transported down the axons of the neurons to their
tips, terminating in the posterior pituitary gland.When
the supraoptic and paraventricular nuclei are stimu-
lated by increased osmolarity or other factors, nerve
impulses pass down these nerve endings, changing
their membrane permeability and increasing calcium
entry. ADH stored in the secretory granules (also
called vesicles) of the n

secretory granules (also
called vesicles) of the nerve endings is released in
response to increased calcium entry. The released
ADH is then carried away in the capillary blood of the
posterior pituitary into the systemic circulation.
Secretion of ADH in response to an osmotic stimu-
lus is rapid, so that plasma ADH levels can increase
severalfold within minutes, thereby providing a rapid
means for altering renal excretion of water.
A second neuronal area important in controlling
osmolarity and A

nal area important in controlling
osmolarity and ADH secretion is located along the
anteroventral region of the third ventricle, called the
AV3V region. At the upper part of this region is a
structure called the subfornical organ, and at the infe-
rior part is another structure called the organum vas-
culosum of the lamina terminalis. Between these two
organs is the median preoptic nucleus, which has mul-
tiple nerve connections with the two organs as well as
with the supraoptic nuclei and the b

ns as well as
with the supraoptic nuclei and the blood pressure
control centers in the medulla of the brain. Lesions of
the AV3V region cause multiple deﬁcits in the control
of ADH secretion, thirst, sodium appetite, and blood
pressure. Electrical stimulation of this region or stim-
ulation by angiotensin II can alter ADH secretion,
thirst, and sodium appetite.
In the vicinity of the AV3V region and the supraop-
tic nuclei are neuronal cells that are excited by small
increases in extracellular ﬂ

 are excited by small
increases in extracellular ﬂuid osmolarity; hence, the
term osmoreceptors has been used to describe these
neurons.These cells send nerve signals to the supraop-
tic nuclei to control their ﬁring and secretion of ADH.
It is also likely that they induce thirst in response to
increased extracellular ﬂuid osmolarity.
ADH
Urine:
   decreased flow
   and concentrated
Cardiopulmonary
receptors
neuron
Pituitary
Posterior
lobe
Anterior
lob

smoreceptors
Pituitary
Posterior
lobe
Anterior
lobe
Supraoptic
neuron
Figure 28–9
Neuroanatomy of the hypothalamus, where antidiuretic hormone
(ADH) is synthesized, and the posterior pituitary gland, where
ADH is released.
360
Unit V
Both the subfornical organ and the organum vascu-
losum of the lamina terminalis have vascular supplies
that lack the typical blood-brain barrier that impedes
the diffusion of most ions from the blood into the brain
tissue.This makes it p

om the blood into the brain
tissue.This makes it possible for ions and other solutes
to cross between the blood and the local interstitial
ﬂuid in this region. As a result, the osmoreceptors
rapidly respond to changes in osmolarity of the extra-
cellular ﬂuid, exerting powerful control over the secre-
tion of ADH and over thirst, as discussed later.
Cardiovascular Reﬂex Stimulation 
of ADH Release by Decreased 
Arterial Pressure and/or Decreased
ADH release is also controlled by car

ADH release is also controlled by cardiovascular
reﬂexes that respond to decreases in blood pressure
and/or blood volume, including (1) the arterial barore-
ceptor reﬂexes and (2) the cardiopulmonary reﬂexes,
both of which are discussed in Chapter 18.These reﬂex
pathways originate in high-pressure regions of the cir-
culation, such as the aortic arch and carotid sinus, and
in the low-pressure regions, especially in the cardiac
atria. Afferent stimuli are carried by the vagus and
glo

 Afferent stimuli are carried by the vagus and
glossopharyngeal nerves with synapses in the nuclei of
the tractus solitarius. Projections from these nuclei
relay signals to the hypothalamic nuclei that control
ADH synthesis and secretion.
Thus, in addition to increased osmolarity, two other
stimuli increase ADH secretion: (1) decreased arterial
pressure and (2) decreased blood volume. Whenever
blood pressure and blood volume are reduced, such as
occurs during hemorrhage, increased ADH secretion

causes increased ﬂuid reabsorption by the kidneys,
helping to restore blood pressure and blood volume
Quantitative Importance of
Cardiovascular Reﬂexes and
Osmolarity in Stimulating 
As shown in Figure 28–10, either a decrease in effec-
tive blood volume or an increase in extracellular ﬂuid
osmolarity stimulates ADH secretion. However,ADH
is considerably more sensitive to small changes in
osmolarity than to similar ch

 to small changes in
osmolarity than to similar changes in blood volume.
For example, a change in plasma osmolarity of only 
1 per cent is sufﬁcient to increase ADH levels. By 
contrast, after blood loss, plasma ADH levels do not
change appreciably until blood volume is reduced by
about 10 per cent. With further decreases in blood
volume,ADH levels rapidly increase.Thus, with severe
decreases in blood volume, the cardiovascular reﬂexes
play a major role in stimulating ADH secretion. The
usual da

or role in stimulating ADH secretion. The
usual day-to-day regulation of ADH secretion during
simple dehydration is effected mainly by changes in
plasma osmolarity. Decreased blood volume, however,
greatly enhances the ADH response to increased
osmolarity.
ADH secretion can also be increased or decreased by
other stimuli to the central nervous system as well as
by various drugs and hormones, as shown in Table
28–2. For example, nausea is a potent stimulus for
Plas

 For example, nausea is a potent stimulus for
Plasma ADH (pg/ml)
Per cent change
PAVP = 2.5 D Osm + 2.0
PAVP = 1.3 e–0.17 vol.
0
5
10
15
20
25
30
35
40
50
45
0
Isotonic volume depletion
Isovolemic osmotic increase
5
10
15
20
Figure 28–10
The effect of increased plasma osmolarity or decreased blood
volume on the level of plasma (P) antidiuretic hormone (ADH), also
called arginine vasopressin (AVP). (Redrawn from Dunn FL,
Brennan TJ, Nelson AE, Robertson GL: The role of blood osmo-
lality and volu

ertson GL: The role of blood osmo-
lality and volume in regulating vasopressin secretion in the rat. J
Clin Invest 52(12):3212, 1973. By copyright permission of the
American Society of Clinical Investigation.)
Table 28–2
Regulation of ADH Secretion
Increase ADH
Decrease ADH
≠Plasma osmolarity
Ø Plasma osmolarity
Ø Blood volume
≠Blood volume
Ø Blood pressure
≠Blood pressure
Nausea
Hypoxia
Drugs:
Drugs:
Morphine
Alcohol
Nicotine
Clonidine (antihypertensive drug)
Haloperidol (dopam

rtensive drug)
Haloperidol (dopamine blocker)
Chapter 28
361
ADH release, which may increase to as much as 100
times normal after vomiting. Also, drugs such as nico-
tine and morphine stimulate ADH release, whereas
some drugs, such as alcohol, inhibit ADH release. The
marked diuresis that occurs after ingestion of alcohol
is due in part to inhibition of ADH release.
Role of Thirst in Controlling

ease.
Extracellular Fluid Osmolarity
The kidneys minimize ﬂuid loss during water deﬁcits
through the osmoreceptor-ADH feedback system.
Adequate ﬂuid intake, however, is necessary to coun-
terbalance whatever ﬂuid loss does occur through
sweating and breathing and through the gastrointesti-
nal tract. Fluid intake is regulated by the thirst mech-
anism, which, together with the osmoreceptor-ADH
mechanism, maintains precise control of extracel

H
mechanism, maintains precise control of extracellular
ﬂuid osmolarity and sodium concentration.
Many of the same factors that stimulate ADH secre-
tion also increase thirst, which is deﬁned as the con-
scious desire for water.
Central Nervous System Centers 
for Thirst
Referring again to Figure 28–9, the same area along
the anteroventral wall of the third ventricle that pro-
motes ADH release also stimulates thirst. Located
anterolaterally in the preoptic nucleus is another small
area that, wh

he preoptic nucleus is another small
area that, when stimulated electrically, causes immedi-
ate drinking that continues as long as the stimulation
lasts. All these areas together are called the thirst
center.
The neurons of the thirst center respond to injec-
tions of hypertonic salt solutions by stimulating drink-
ing behavior. These cells almost certainly function as
osmoreceptors to activate the thirst mechanism, in 
the same way that the osmoreceptors stimulate ADH
release.
Increased osmola

oreceptors stimulate ADH
release.
Increased osmolarity of the cerebrospinal ﬂuid in
the third ventricle has essentially the same effect to
promote drinking. It is likely that the organum vascu-
losum of the lamina terminalis, which lies immediately
beneath the ventricular surface at the inferior end of
the AV3V region, is intimately involved in mediating
this response.
Table 28–3 summarizes some of the known stimuli for
thirst. One of the most important is increased extra-
cel

 One of the most important is increased extra-
cellular ﬂuid osmolarity, which causes intracellular
dehydration in the thirst centers, thereby stimulating
the sensation of thirst. The value of this response is
obvious: it helps to dilute extracellular ﬂuids and
returns osmolarity toward normal.
Decreases in extracellular ﬂuid volume and arterial
pressure also stimulate thirst by a pathway that is inde-
pendent of the one stimulated by increased plasma
osmolarity. Thus, blood volume loss by hemor

lasma
osmolarity. Thus, blood volume loss by hemorrhage
stimulates thirst even though there might be no change
in plasma osmolarity. This probably occurs because of
neutral input from cardiopulmonary and systemic
arterial baroreceptors in the circulation.
A third important stimulus for thirst is angiotensin II.
Studies in animals have shown that angiotensin II acts
on the subfornical organ and on the organum vascu-
losum of the lamina terminalis. These regions are
outside the blood-brain barrier

 These regions are
outside the blood-brain barrier, and peptides such as
angiotensin II diffuse into the tissues. Because
angiotensin II is also stimulated by factors associated
with hypovolemia and low blood pressure, its effect on
thirst helps to restore blood volume and blood pres-
sure toward normal, along with the other actions 
of angiotensin II on the kidneys to decrease ﬂuid
Dryness of the mouth and mucous membranes of the
esophagus can elicit the sensation of thirst.As a resu

hagus can elicit the sensation of thirst.As a result,
a thirsty person may receive relief from thirst almost
immediately after drinking water, even though the
water has not been absorbed from the gastrointestinal
tract and has not yet had an effect on extracellular
ﬂuid osmolarity.
Gastrointestinal and pharyngeal stimuli inﬂuence
thirst. For example, in animals that have an esophageal
opening to the exterior so that water is never absorbed
into the blood, partial relief of thirst occurs after 
d

he blood, partial relief of thirst occurs after 
drinking, although the relief is only temporary. Also,
gastrointestinal distention may partially alleviate
thirst; for instance, simple inﬂation of a balloon in the
stomach can relieve thirst. However, relief of thirst
sensations through gastrointestinal or pharyngeal
mechanisms is short-lived; the desire to drink is com-
pletely satisﬁed only when plasma osmolarity and/or
blood volume returns to normal.
The ability of animals and humans to “meter

ormal.
The ability of animals and humans to “meter” ﬂuid
intake is important because it prevents overhydration.
After a person drinks water, 30 to 60 minutes may be
required for the water to be reabsorbed and dis-
tributed throughout the body. If the thirst sensation
were not temporarily relieved after drinking water, the
person would continue to drink more and more, even-
tually leading to overhydration and excess dilution of
Table 28–3
Control of Thirst
Increase Thirst
Decrease Thirst
≠Osmolar

of Thirst
≠Osmolarity
Ø Osmolarity
≠Angiotensin
Ø Angiotensin II
Dryness of mouth
Gastric distention
362
Unit V
the body ﬂuids. Experimental studies have repeatedly
shown that animals drink almost exactly the amount
necessary to return plasma osmolarity and volume to
normal.
Threshold for Osmolar Stimulus 
of Drinking
The kidneys must continually excrete at least some
ﬂuid, e

eys must continually excrete at least some
ﬂuid, even in a dehydrated person, to rid the body of
excess solutes that are ingested or produced by metab-
olism.Water is also lost by evaporation from the lungs
and the gastrointestinal tract and by evaporation and
sweating from the skin. Therefore, there is always a
tendency for dehydration, with resultant increased
ity. When the sodium concentration increases only
about 2 mEq/L above normal, the 

on increases only
about 2 mEq/L above normal, the thirst mechanism is
activated, causing a desire to drink water.This is called
the threshold for drinking. Thus, even small increases
in plasma osmolarity are normally followed by water
intake, which restores extracellular ﬂuid osmolarity
and volume toward normal. In this way, the extracel-
lular ﬂuid osmolarity and sodium concentration are
precisely controlled.
Integrated Responses of
Osmoreceptor-ADH and Thirst
Mechanisms in Controlling
Extracel

-ADH and Thirst
Extracellular Fluid Osmolarity and
In a healthy person, the osmoreceptor-ADH and thirst
mechanisms work in parallel to precisely regulate
extracellular ﬂuid osmolarity and sodium concentra-
tion, despite the constant challenges of dehydration.
Even with additional challenges, such as high salt
intake, these feedback systems are able to keep plasma
osmolarity reasonably constant. Figure 28–11 shows
that an increase in sodium intake to

e 28–11 shows
that an increase in sodium intake to as high as six times
normal has only a small effect on plasma sodium con-
centration as long as the ADH and thirst mechanisms
are both functioning normally.
When either the ADH or the thirst mechanism fails,
the other ordinarily can still control extracellular
osmolarity and sodium concentration with reasonable
effectiveness, as long as there is enough ﬂuid intake 
to balance the daily obligatory urine volume and 
water losses caused by respirat

 urine volume and 
water losses caused by respiration, sweating, or 
gastrointestinal losses. However, if both the ADH 
and thirst mechanisms fail simultaneously, plasma
sodium concentration and osmolarity are very poorly
controlled; thus, when sodium intake is increased 
after blocking the total ADH-thirst system, relatively
large changes in plasma sodium concentration do
occur. In the absence of the ADH-thirst mechanisms,
no other feedback mechanism is capable of ad-
equately regulating plasma

anism is capable of ad-
equately regulating plasma sodium concentration and
Role of Angiotensin II and
Aldosterone in Controlling
As discussed in Chapter 27, both angiotensin II and
aldosterone play an important role in regulating sodium
reabsorption by the renal tubules. When sodium intake
is low, increased levels of these hormones stimulate
sodium reabsorption by the kidneys and, therefore,
prevent large sodium losses, even th

d, therefore,
prevent large sodium losses, even though sodium intake
may be reduced to as low as 10 per cent of normal. Con-
versely, with high sodium intake, decreased formation of
these hormones permits the kidneys to excrete large
amounts of sodium.
Because of the importance of angiotensin II and
aldosterone in regulating sodium excretion by the
kidneys, one might mistakenly infer that they also 
play an important role in regulating extracellular ﬂuid
Although these horm

lar ﬂuid
Although these hormones
increase the amount of sodium in the extracellular ﬂuid,
they also increase the extracellular ﬂuid volume by
increasing reabsorption of water along with the sodium.
Therefore, angiotensin II and aldosterone have little
effect on sodium concentration, except under extreme
This relative unimportance of aldosterone in regulat-
ing extracellular ﬂuid sodium concentration is shown by
the experiment of Figure 28–12. This ﬁgure shows th

he experiment of Figure 28–12. This ﬁgure shows the
effect on plasma sodium concentration of changing
Normal
ADH and
thirst
systems
blocked
Plasma sodium concentration (mEq/L)
Sodium intake (mEq/day)
136
152
148
144
140
0
30
60
90
120
150
180
Figure 28–11
Effect of large changes in sodium intake on extracellular ﬂuid
sodium concentration in dogs under normal conditions (red line)
and after the antidiuretic hormone (ADH) and thirst feedback
systems had been blocked (blue line). Note that control 

s had been blocked (blue line). Note that control of extra-
cellular ﬂuid sodium concentration is poor in the absence of these
feedback systems. (Courtesy Dr. David B. Young.)
Chapter 28
363
sodium intake more than sixfold under two conditions:
(1) under normal conditions and (2) after the aldos-
terone feedback system has been blocked by removing
the adrenal glands and infusing the animals with aldos-
terone at a constant rat

g the animals with aldos-
terone at a constant rate so that plasma levels could not
change upward or downward. Note that when sodium
intake was increased sixfold, plasma concentration
changed only about 1 to 2 per cent in either case. This
indicates that even without a functional aldosterone
feedback system, plasma sodium concentration can be
well regulated. The same type of experiment has been
conducted after blocking angiotensin II formation, with
the same result.
There are two primary reasons

ith
There are two primary reasons why changes in
angiotensin II and aldosterone do not have a major
effect on plasma sodium concentration. First, as dis-
cussed earlier, angiotensin II and aldosterone increase
both sodium and water reabsorption by the renal
tubules, leading to increases in extracellular ﬂuid
volume and sodium quantity but little change in sodium
concentrations. Second, as long as the ADH-thirst mech-
anism is functional, any tendency toward increased
plasma sodi

ctional, any tendency toward increased
plasma sodium concentration is compensated for by
increased water intake or increased plasma ADH secre-
tion, which tends to dilute the extracellular ﬂuid back
toward normal. The ADH-thirst system far overshad-
ows the angiotensin II and aldosterone systems for reg-
ulating sodium concentration under normal conditions.
Even in patients with primary aldosteronism, who have
extremely high levels of aldosterone, the plasma sodium
concentration usually increase

, the plasma sodium
concentration usually increases only about 3 to 5 mEq/L
Under extreme conditions, caused by complete loss of
aldosterone secretion because of adrenalectomy or in
patients with Addison’s disease (severely impaired
secretion or total lack of aldosterone), there is tremen-
dous loss of sodium by the kidneys, which can lead to
reductions in plasma sodium concentration. One of the
reasons for this is that large losses of sodium eventually
cause severe volume depletio

 of sodium eventually
cause severe volume depletion and decreased blood
pressure, which can activate the thirst mechanism
through the cardiovascular reﬂexes. This leads to a
further dilution of the plasma sodium concentration,
even though the increased water intake helps to mini-
mize the decrease in body ﬂuid volumes under these
Thus, there are extreme situations in which plasma
sodium concentration may change signiﬁcantly, even
with a functional ADH-thirst mechanism. Even so, the
A

 a functional ADH-thirst mechanism. Even so, the
ADH-thirst mechanism is by far the most powerful
feedback system in the body for controlling extracellu-
lar ﬂuid osmolarity and sodium concentration.
Salt-Appetite Mechanism 
for Controlling Extracellular
Fluid Sodium Concentration
and Volume
Maintenance of normal extracellular ﬂuid volume and
sodium concentration requires a balance between
sodium excretion and sodium intake. In modern civi-
lizations, sodium intake is almost always greater than

ions, sodium intake is almost always greater than
necessary for homeostasis. In fact, the average sodium
intake for individuals in industrialized cultures eating
processed foods usually ranges between 100 and 
200 mEq/day, even though humans can survive and
function normally on 10 to 20 mEq/day. Thus, most
people eat far more sodium than is necessary for home-
ostasis, and there is evidence that our usual high sodium
intake may contribute to cardiovascular disorders such
as hypertension.
Salt ap

iovascular disorders such
Salt appetite is due in part to the fact that animals and
humans like salt and eat it regardless of whether they
are salt-deﬁcient. There is also a regulatory component
to salt appetite in which there is a behavioral drive to
obtain salt when there is sodium deﬁciency in the body.
This is particularly important in herbivores, which nat-
urally eat a low-sodium diet, but salt craving may also
be important in humans who have extreme deﬁciency
of sodium, s

 in humans who have extreme deﬁciency
of sodium, such as occurs in Addison’s disease. In this
instance, there is deﬁciency of aldosterone secretion,
which causes excessive loss of sodium in the urine and
leads to decreased extracellular ﬂuid volume and
decreased sodium concentration; both of these changes
elicit the desire for salt.
In general, the two primary stimuli that are believed to
increase salt appetite are (1) decreased extracellular ﬂuid
sodium concentration and (2) decreased blood vol

d
sodium concentration and (2) decreased blood volume
or blood pressure, associated with circulatory insufﬁ-
ciency. These are the same major stimuli that elicit thirst.
The neuronal mechanism for salt appetite is analo-
gous to that of the thirst mechanism. Some of the same
neuronal centers in the AV3V region of the brain seem
to be involved, because lesions in this region frequently
affect both thirst and salt appetite simultaneously in
animals. Also, circulatory reﬂexes elicited by low blood

. Also, circulatory reﬂexes elicited by low blood
pressure or decreased blood volume affect both thirst
and salt appetite at the same time.
Plasma sodium concentration
(mEq/L)
Sodium intake (mEq/L)
100
110
120
130
140
150
0
30
60
Normal
Aldosterone system blocked
90
120
150
180
210
Figure 28–12

ne system blocked
90
120
150
180
210
and after the aldosterone feedback system had been blocked
(blue line). Note that sodium concentration is maintained relatively
constant over this wide range of sodium intakes, with or without
aldosterone feedback control. (Courtesy Dr. David B. Young.)
364
Unit V
Dwyer TM, Schmidt-Nielsen B: T

 Fluids and Kidneys
Dwyer TM, Schmidt-Nielsen B: The renal pelvis: machinery
that concentrates urine in the papilla. News Physiol Sci
18:1, 2003.
Edwards A, Delong MJ, Pallone TL: Interstitial water and
solute recovery by inner medullary vasa recta. Am J
Physiol Renal Physiol 278:F257, 2000.
Fitzsimons JT: Angiotensin, thirst, and sodium appetite.
Physiol Rev 78:583, 1998.
Knepper MA, Saidel GM, Hascall VC, Dwyer T: Concentra-
tion of solutes in the renal inner medulla: interstitial
hyaluronan a

the renal inner medulla: interstitial
hyaluronan as a mechano-osmotic transducer. Am J
Physiol Renal Physiol 284:F433, 2003.
Kozono D,Yasui M, King LS,Agre P:Aquaporin water chan-
nels: atomic structure molecular dynamics meet clinical
medicine. J Clin Invest 109:1395, 2002.
McKinley MJ, Johnson AK: The physiological regulation of
thirst and ﬂuid intake. News Physiol Sci 19:1, 2004.
Morello JP, Bichet DG: Nephrogenic diabetes insipidus.
Annu Rev Physiol 63:607, 2001.
Nielsen S, Frokiaer J, Marpl

Physiol 63:607, 2001.
Nielsen S, Frokiaer J, Marples D, et al: Aquaporins in the
2002.
Pallone TL, Turner MR, Edwards A, Jamison RL: Counter-
current exchange in the renal medulla. Am J Physiol
Regul Integr Comp Physiol 284:R1153, 2003.
Robertson GL:Vasopressin. In Seldin DW, Giebisch G (eds):
The Kidney—Physiology and Pathophysiology, 3rd ed.
New York: Raven Press, 2000.
Sands JM, Layton HE: Urine concentrating mechanism and
its regulation

: Urine concentrating mechanism and
its regulation. In Seldin DW, Giebisch G (eds): The
York: Raven Press, 2000.
Sands JM: Molecular mechanisms of urea transport. J Mem-
brane Biol 191:149, 2003.
Stricker EM, Sved AF: Controls of vasopressin secretion and
thirst: similarities and dissimilarities in signals. Physiol
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Verbalis JG: Diabetes insipidus. Rev Endocr Metab Disord
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365
Renal Regulation o

, 2003.
C
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E
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2
9
365
Calcium, Phosphate, and
Magnesium; Integration 
of Renal Mechanisms for 
Regulation of Potassium
Excretion and Potassium
Concentration in
Extracellular ﬂuid potassium concentration nor-
mally is regulated precisely at about 4.2 mEq/L,
seldom rising or falling more than ± 0.3 mEq/L. This precise control is neces-
sary because many cell functions are very sensi

s-
sary because many cell functions are very sensitive to changes in extracellular
ﬂuid potassium concentration. For instance, an increase in plasma potassium
concentration of only 3 to 4 mEq/L can cause cardiac arrhythmias, and higher
concentrations can lead to cardiac arrest or ﬁbrillation.
A special difﬁculty in regulating extracellular potassium concentration is the
fact that over 98 per cent of the total body potassium is contained in the cells
and only 2 per cent in the extracellular ﬂuid 

lls
and only 2 per cent in the extracellular ﬂuid (Figure 29–1). For a 70-kilogram
adult, who has about 28 liters of intracellular ﬂuid (40 per cent of body weight)
and 14 liters of extracellular ﬂuid (20 per cent of body weight), about 3920 mil-
liequivalents of potassium are inside the cells and only about 59 milliequiva-
lents are in the extracellular ﬂuid.Also, the potassium contained in a single meal
is often as high as 50 milliequivalents, and the daily intake usually ranges
between 50 and

and the daily intake usually ranges
between 50 and 200 mEq/day; therefore, failure to rapidly rid the extracellular
ﬂuid of the ingested potassium could cause life-threatening hyperkalemia
(increased plasma potassium concentration). Likewise, a small loss of potassium
from the extracellular ﬂuid could cause severe hypokalemia (low plasma potas-
sium concentration) in the absence of rapid and appropriate compensatory
responses.
Maintenance of potassium balance depends primarily on excretion by th

ssium balance depends primarily on excretion by the
kidneys because the amount excreted in the feces is only about 5 to 10 per cent
of the potassium intake. Thus, the maintenance of normal potassium balance
requires the kidneys to adjust their potassium excretion rapidly and precisely
to wide variations in intake, as is also true for most other electrolytes.
Control of potassium distribution between the extracellular and intracellular
compartments also plays an important role in potassium homeos

s also plays an important role in potassium homeostasis. Because
over 98 per cent of the total body potassium is contained in the cells, they can
serve as an overﬂow site for excess extracellular ﬂuid potassium during hyper-
kalemia or as a source of potassium during hypokalemia. Thus, redistribution
of potassium between the intra- and extracellular ﬂuid compartments pro-
vides a ﬁrst line of defense against changes in extracellular ﬂuid potassium 
366
Unit V
The Body Fluids and K

m 
366
Unit V
Regulation of Internal Potassium
After ingestion of a normal meal, extracellular ﬂuid
potassium concentration would rise to a lethal level if
the ingested potassium did not rapidly move into the
cells. For example, absorption of 40 milliequivalents of
potassium (the amount contained in a meal rich in veg-
etables and fruit) into an extracellular ﬂuid volume of
14 liters would raise plasma potassium concentration
by about 2.9 m

aise plasma potassium concentration
by about 2.9 mEq/L if all the potassium remained in
the extracellular compartment. Fortunately, most of
the ingested potassium rapidly moves into the cells
until the kidneys can eliminate the excess. Table 29–1
summarizes some of the factors that can inﬂuence the
distribution of potassium between the intra- and extra-
cellular compartments.
Insulin Stimulates Potassium Uptake into Cells. One of the
most important factors that increase cell potassium
uptake aft

nt factors that increase cell potassium
uptake after a meal is insulin. In people who have
insulin deﬁciency owing to diabetes mellitus, the rise in
plasma potassium concentration after eating a meal is
much greater than normal. Injections of insulin,
however, can help to correct the hyperkalemia.
Aldosterone Increases Potassium Uptake into Cells. Increased
potassium intake also stimulates secretion of aldos-
terone, which increases cell potassium uptake. Excess
aldosterone secretion (Conn’s syn

m uptake. Excess
aldosterone secretion (Conn’s syndrome) is almost
invariably associated with hypokalemia, due in part to
movement of extracellular potassium into the cells.
Conversely, patients with deﬁcient aldosterone produc-
tion (Addison’s disease) often have clinically signiﬁcant
hyperkalemia due to accumulation of potassium in 
the extracellular space as well as to renal retention of
potassium.
b-Adrenergic Stimulation Increases Cellular Uptake of Potassium.
Increased secretion of catecho

ptake of Potassium.
Increased secretion of catecholamines, especially epi-
nephrine, can cause movement of potassium from the
extracellular to the intracellular ﬂuid, mainly by activa-
tion of b2-adrenergic receptors. Treatment of hyperten-
sion with b-adrenergic receptor blockers, such as
propranolol, causes potassium to move out of the cells
and creates a tendency toward hyperkalemia.
Acid-Base Abnormalities Can Cause Changes in Potassium Distri-
bution. Metabolic acidosis increases extracellu

i-
bution. Metabolic acidosis increases extracellular potas-
sium concentration, in part by causing loss of potassium
from the cells, whereas metabolic alkalosis decreases
extracellular ﬂuid potassium concentration. Although
the mechanisms responsible for the effect of hydrogen
ion concentration on potassium internal distribution are
not completely understood, one effect of increased
hydrogen ion concentration is to reduce the activity 
of the sodium-potassium adenosine triphosphatase
(ATPase) p

dium-potassium adenosine triphosphatase
(ATPase) pump. This in turn decreases cellular uptake
of potassium and raises extracellular potassium 
Cell Lysis Causes Increased Extracellular Potassium Concentra-
tion. As cells are destroyed, the large amounts of potas-
sium contained in the cells are released into the
extracellular compartment. This can cause signiﬁcant
hyperkalemia if large amounts of tissue are destroyed,
as occurs with severe muscle injury or with red blood
cell lysi

h severe muscle injury or with red blood
cell lysis.
Strenuous Exercise Can Cause Hyperkalemia by Releasing Potas-
sium from Skeletal Muscle. During prolonged exercise,
potassium is released from skeletal muscle into the
extracellular ﬂuid. Usually the hyperkalemia is mild, but
it may be clinically signiﬁcant after heavy exercise in
patients treated with b-adrenergic blockers or in indi-
viduals with insulin deﬁciency. In rare instances, hyper-
kalemia after exercise may be severe enough to caus

alemia after exercise may be severe enough to cause
cardiac arrhythmias and sudden death.
Increased Extracellular Fluid Osmolarity Causes Redistribution of
Potassium from the Cells to Extracellular Fluid. Increased
extracellular ﬂuid osmolarity causes osmotic ﬂow of
water out of the cells.The cellular dehydration increases
intracellular potassium concentration, thereby pro-
moting diffusion of potassium out of the cells and 
increasing extracellular ﬂuid potassium concentration.
Decreased extrac

lar ﬂuid potassium concentration.
Decreased extracellular ﬂuid osmolarity has the oppo-
site effect. In diabetes mellitus, large increases in plasma
glucose raise extracellular osmolarity, causing cell 
K+ intake
100 mEq/day
K+ output
    Urine 92 mEq/day
    Feces  8 mEq/day
           100 mEq/day
fluid K+
4.2 mEq/L
x 14 L
fluid K+
140 mEq/L
x 28 L
59 m Eq
3920 mEq
Figure 29–1
Normal potassium intake, distribution of potassium in the body
ﬂuids, and potassium output 

potassium in the body
ﬂuids, and potassium output from the body.
Table 29–1
Factors That Can Alter Potassium Distribution Between the
Intra- and Extracellular Fluid
Factors That Shift K+ into Cells
Factors That Shift K+ Out of Cells
(Decrease Extracellular [K+])
(Increase Extracellular [K+])
• Insulin
• Insulin deﬁciency (diabetes
• Aldosterone
mellitus)
• b-adrenergic stimulation
• Aldosterone deﬁciency 
• Alkalosis
(Addison’s disease)
• b-adrenergic blockade
• Acidosis
• Cell lysis
• Strenuous

ergic blockade
• Acidosis
• Strenuous exercise
• Increased extracellular ﬂuid 
Chapter 29
Renal Regulation; Integration of Renal Mechanisms
367
dehydration and movement of potassium from the cells
into the extracellular ﬂuid.
Overview of Renal Potassium
Excretion
Potassium excretion is determined by the sum of three
renal processes: (1) the rate of potassium ﬁltration
(GFR multiplied by the plasma potassium concentra-
tion), (2) the rate of potassium reabsorption by the
t

), (2) the rate of potassium reabsorption by the
tubules, and (3) the rate of potassium secretion by 
the tubules. The normal rate of potassium ﬁltration is
about 756 mEq/day (GFR, 180 L/day multiplied by
plasma potassium, 4.2 mEq/L); this rate of ﬁltration is
usually relatively constant because of the autoregula-
tory mechanisms for GFR discussed previously and
the precision with which plasma potassium concentra-
tion is regulated. Severe decreases in GFR in certain
renal diseases, however, can

ses in GFR in certain
renal diseases, however, can cause serious potassium
accumulation and hyperkalemia.
Figure 29–2 summarizes the tubular handling of
potassium under normal conditions.About 65 per cent
of the ﬁltered potassium is reabsorbed in the proximal
tubule.Another 25 to 30 per cent of the ﬁltered potas-
sium is reabsorbed in the loop of Henle, especially in
the thick ascending part where potassium is actively
co-transported along with sodium and chloride. In
both the proximal tubule an

odium and chloride. In
both the proximal tubule and the loop of Henle, a rel-
atively constant fraction of the ﬁltered potassium load
is reabsorbed. Changes in potassium reabsorption in
these segments can inﬂuence potassium excretion, but
most of the day-to-day variation of potassium excre-
tion is not due to changes in reabsorption in the prox-
imal tubule or loop of Henle.
Most Daily Variation in Potassium Excretion Is Caused by
Changes in Potassium Secretion in Distal and Collecting
Tubules. 

ssium Secretion in Distal and Collecting
Tubules. The most important sites for regulating potas-
sium excretion are the principal cells of the late distal
tubules and cortical collecting tubules. In these tubular
segments, potassium can at times be reabsorbed or at
other times be secreted, depending on the needs of the
body.With a normal potassium intake of 100 mEq/day,
the kidneys must excrete about 92 mEq/day (the
remaining 8 milliequivalents are lost in the feces).
About one third (31 mEq/day

re lost in the feces).
About one third (31 mEq/day) of this amount of
potassium is secreted into the distal and collecting
tubules.
With high potassium intakes, the required extra
excretion of potassium is achieved almost entirely by
increasing the secretion of potassium into the distal
and collecting tubules. In fact, with extremely high
potassium diets, the rate of potassium excretion can
exceed the amount of potassium in the glomerular ﬁl-
trate, indicating a powerful mechanism for secreting

te, indicating a powerful mechanism for secreting
When potassium intake is reduced below normal,
the secretion rate of potassium in the distal and 
collecting tubules decreases, causing a reduction in
urinary potassium secretion. With extreme reductions
in potassium intake, there is net reabsorption of potas-
sium in the distal segments of the nephron, and potas-
sium excretion can fall to 1 per cent of the potassium
in the glomerular ﬁltrate (to less than 10 mEq/day).
With potassium 

ﬁltrate (to less than 10 mEq/day).
With potassium intakes below this level, severe
hypokalemia can develop.
Thus, most of the day-to-day regulation of potas-
sium excretion occurs in the late distal and cortical 
collecting tubules, where potassium can be either 
reabsorbed or secreted, depending on the needs of the
body. In the next section, we consider the basic mech-
anisms of potassium secretion and the factors that reg-
ulate this process.
Potassium Secretion by Principal
Cells of Late Dist

otassium Secretion by Principal
Cells of Late Distal and Cortical
The cells in the late distal and cortical collecting
tubules that secrete potassium are called principal cells
65%
(491 mEq/day)
4%
(31 mEq/day)
12%
(92 mEq/day)
756 mEq/day
(180 L/day x 4.2 mEq/L)
27%
(204 mEq/day)
Figure 29–2
Renal tubular sites of potassium reabsorp-
tion and secretion. Potassium is reab-
sorbed in the proximal tubule and in the
ascending loop of Henle, so that only about
8 per cent of the ﬁl

 of Henle, so that only about
8 per cent of the ﬁltered load is delivered to
the distal tubule. Secretion of potassium
into the late distal tubules and collecting
ducts adds to the amount delivered, so that
the daily excretion is about 12 per cent of
the potassium ﬁltered at the glomerular
capillaries. The percentages indicate how
much of the ﬁltered load is reabsorbed or
secreted into the different tubular seg-
ments.
368
Unit V
and make up about 90 per cent of the e

and make up about 90 per cent of the epithelial cells
in these regions. Figure 29–3 shows the basic cellular
mechanisms of potassium secretion by the principal
cells.
Secretion of potassium from the blood into the
tubular lumen is a two-step process, beginning with
uptake from the interstitium into the cell by the
sodium-potassium ATPase pump in the basolateral
membrane of the cell; this pump moves sodium out of
the cell into the interstitium and at the same time
moves potassium to t

rstitium and at the same time
moves potassium to the interior of the cell.The second
step of the process is passive diffusion of potassium
from the interior of the cell into the tubular ﬂuid. The
sodium-potassium ATPase pump creates a high intra-
cellular potassium concentration, which provides the
driving force for passive diffusion of potassium from
the cell into the tubular lumen.The luminal membrane
of the principal cells is highly permeable to potassium.
One reason for this high permeabilit

to potassium.
One reason for this high permeability is that there are
special channels that are speciﬁcally permeable to
potassium ions, thus allowing these ions to diffuse
across the membrane.
Control of Potassium Secretion by Principal Cells. The
primary factors that control potassium secretion by the
principal cells of the late distal and cortical collecting
tubules are (1) the activity of the sodium-potassium
ATPase pump, (2) the electrochemical gradient for
potassium secretion from the bloo

cal gradient for
potassium secretion from the blood to the tubular
lumen, and (3) the permeability of the luminal mem-
brane for potassium. These three determinants of
potassium secretion are in turn regulated by the
factors discussed later.
Intercalated Cells Can Reabsorb Potassium During Potassium
Depletion. In circumstances associated with severe
potassium depletion, there is a cessation of potassium
secretion and actually a net reabsorption of potas-
sium in the late distal and collecting tu

f potas-
sium in the late distal and collecting tubules. This 
reabsorption occurs through the intercalated cells;
although this reabsorptive process is not completely
understood, one mechanism believed to contribute 
is a hydrogen-potassium ATPase transport mechanism
located in the luminal membrane.This transporter reab-
sorbs potassium in exchange for hydrogen ions secreted
into the tubular lumen, and the potassium then diffuses
through the basolateral membrane of the cell into the
blood. This

olateral membrane of the cell into the
blood. This transporter is necessary to allow potassium
reabsorption during extracellular ﬂuid potassium deple-
tion, but under normal conditions it plays a small role
in controlling the excretion of potassium.
Summary of Factors That Regulate
Potassium Secretion: Plasma
Potassium Concentration,
Aldosterone, Tubular Flow Rate, and
Hydrogen Ion Concentration
Because normal regulation of potassium excretion
occurs mainly as a result of changes in potassium
se

curs mainly as a result of changes in potassium
secretion by the principal cells of the late distal and
collecting tubules, we will discuss the primary factors
that inﬂuence secretion by these cells.The most impor-
tant factors that stimulate potassium secretion by the
principal cells include (1) increased extracellular ﬂuid
potassium concentration, (2) increased aldosterone,
and (3) increased tubular ﬂow rate.
One factor that decreases potassium secretion is
increased hydrogen ion concentration

 secretion is
increased hydrogen ion concentration (acidosis).
Increased Extracellular Fluid Potassium Concentration Stimu-
lates Potassium Secretion. The rate of potassium secre-
tion in the late distal and cortical collecting tubules is
directly stimulated by increased extracellular ﬂuid
potassium concentration, leading to increases in potas-
sium excretion, as shown in Figure 29–4. This effect 
is especially pronounced when extracellular ﬂuid
potassium concentration rises above about 4.1 mEq/

potassium concentration rises above about 4.1 mEq/L,
slightly less than the normal concentration. Increased
plasma potassium concentration, therefore, serves as
one of the most important mechanisms for increasing
potassium secretion and regulating extracellular ﬂuid
potassium ion concentration.
There are three mechanisms by which increased
extracellular ﬂuid potassium concentration raises
potassium secretion: (1) Increased extracellular ﬂuid
potassium concentration stimulates the sodium-
potassi

ssium concentration stimulates the sodium-
potassium ATPase pump, thereby increasing potas-
sium uptake across the basolateral membrane. This in
turn increases intracellular potassium ion concentra-
tion, causing potassium to diffuse across the luminal
membrane into the tubule. (2) Increased extracellular
potassium concentration increases the potassium gra-
dient from the renal interstitial ﬂuid to the interior of
the epithelial cell; this reduces backleakage of potas-
sium ions from inside the 

s backleakage of potas-
sium ions from inside the cells through the basolateral
membrane. (3) Increased potassium concentration
stimulates aldosterone secretion by the adrenal cortex,
which further stimulates potassium secretion, as dis-
cussed next.
ATP
Na+
Na+
K+
K+
Na+
Na+
Na+
Na+
K+
K+
Renal
fluid
Renal
fluid
Tubular
lumen
Tubular
lumen
Principal
cells
Principal
cells
0 mV
0 mV
-50 mV
-50 mV
-70 mV
-70 mV
K+
K+
Figure 29–3
Mechanisms of potassium secretion and sodiu

e 29–3
Mechanisms of potassium secretion and sodium reabsorption by
the principal cells of the late distal and collecting tubules.
Chapter 29
369
Aldosterone Stimulates Potassium Secretion. In Chapter 27,
we discuss the fact that aldosterone stimulates active
reabsorption of sodium ions by the principal cells of
the late distal tubules and collecting ducts. This effect
is mediated through a sodium-potassium ATPase
pump that transports sodium outw

-potassium ATPase
pump that transports sodium outward through the
basolateral membrane of the cell and into the blood at
the same time that it pumps potassium into the cell.
Thus, aldosterone also has a powerful effect to control
the rate at which the principal cells secrete potassium.
A second effect of aldosterone is to increase the per-
meability of the luminal membrane for potassium,
further adding to the effectiveness of aldosterone in
stimulating potassium secretion. Therefore, aldos-
tero

lating potassium secretion. Therefore, aldos-
terone has a powerful effect to increase potassium
excretion, as shown in Figure 29–4.
Increased Extracellular Potassium Ion Concentration Stimu-
lates Aldosterone Secretion. In negative feedback control
systems, the factor that is controlled usually has a 
feedback effect on the controller. In the case of the
aldosterone-potassium control system, the rate of
aldosterone secretion by the adrenal gland is con-
trolled strongly by extracellular ﬂuid po

 is con-
trolled strongly by extracellular ﬂuid potassium ion
concentration. Figure 29–5 shows that an increase in
plasma potassium concentration of about 3 mEq/L can
increase plasma aldosterone concentration from
nearly 0 to as high as 60 ng/100 ml, a concentration
almost 10 times normal.
The effect of potassium ion concentration to stimu-
late aldosterone secretion is part of a powerful feed-
back system for regulating potassium excretion, as
shown in Figure 29–6. In this feedback system, an
i

hown in Figure 29–6. In this feedback system, an
increase in plasma potassium concentration stimulates
aldosterone secretion and, therefore, increases the
blood level of aldosterone (block 1). The increase in
blood aldosterone then causes a marked increase in
potassium excretion by the kidneys (block 2). The
increased potassium excretion then reduces the extra-
cellular ﬂuid potassium concentration back toward
normal (blocks 3 and 4). Thus, this feedback mecha-
nism acts synergistically with the

feedback mecha-
nism acts synergistically with the direct effect of
Urinary potassium excretion
4
3
2
1
0
0
1
Effect of aldosterone
Effect of extracellular
K+ concentration
2
3
5
4
Extracellular potassium concentration
(mEq/L)
(mEq/L)
1
2
3
5
4
Plasma aldosterone (times normal)
Figure 29–4
Effect of plasma aldosterone concentration (red line) and extra-
cellular potassium ion conce

(red line) and extra-
cellular potassium ion concentration (black line) on the rate of
urinary potassium excretion. These factors stimulate potassium
secretion by the principal cells of the cortical collecting tubules.
(Drawn from data in Young DB, Paulsen AW: Interrelated effects
of aldosterone and plasma potassium on potassium excretion. Am
J Physiol 244:F28, 1983.)
Approximate plasma aldosterone
concentration (ng/100 ml plasma)
0
70
60
50
40
30
20
10
3.0
6.5
5.5
4.5
6.0
3.5
4.0
5.0
Serum pota

0
20
10
3.0
6.5
5.5
4.5
6.0
3.5
4.0
5.0
Serum potassium concentration (mEq/L)
Figure 29–5
Effect of extracellular ﬂuid potassium ion concentration on plasma
aldosterone concentration. Note that small changes in potassium
concentration cause large changes in aldosterone concentration.
Ald.
Ald.
1
K+
K+
4
3
Ald.
Ald.
2
K+
excretion
K+
excretion
K+
K+
+
-
K

rone
+
-
K+ intake
K+ intake
Figure 29–6
Basic feedback mechanism for control of extracellular ﬂuid potas-
sium concentration by aldosterone (Ald.).
370
Unit V
increased extracellular potassium concentration to
elevate potassium excretion when potassium intake is
raised (Figure 29–7).
Blockade of Aldosterone Feedback System Greatly Impairs
Control of Potassium Concentration. In the absence of
aldosterone secretion, as occurs in 

he absence of
aldosterone secretion, as occurs in patients with
Addison’s disease, renal secretion of potassium is
impaired, thus causing extracellular ﬂuid potassium
concentration to rise to dangerously high levels. Con-
versely, with excess aldosterone secretion (primary
aldosteronism), potassium secretion becomes greatly
increased, causing potassium loss by the kidneys and
thus leading to hypokalemia.
The special quantitative importance of the aldos-
terone feedback system in controlling pota

 aldos-
terone feedback system in controlling potassium con-
centration is shown in Figure 29–8. In this experiment,
potassium intake was increased almost sevenfold in
dogs under two conditions: (1) under normal condi-
tions and (2) after the aldosterone feedback system
had been blocked by removing the adrenal glands and
placing the animals on a ﬁxed rate of aldosterone infu-
sion so that plasma aldosterone concentration could
neither increase nor decrease.
Note that in the normal animals, a sev

r decrease.
Note that in the normal animals, a sevenfold
increase in potassium intake caused only a slight in-
crease in potassium concentration, from 4.2 to 4.3 mEq/
L. Thus, when the aldosterone feedback system is 
functioning normally, potassium concentration is pre-
cisely controlled, despite large changes in potassium
intake.
When the aldosterone feedback system was
blocked, the same increases in potassium intake
caused a much larger increase in potassium concen-
tration, from 3.8 to almost

e in potassium concen-
tration, from 3.8 to almost 4.7 mEq/L. Thus, control of
potassium concentration is greatly impaired when the
aldosterone feedback system is blocked. A similar
impairment of potassium regulation is observed in
humans with poorly functioning aldosterone feedback
systems, such as occurs in patients with either primary
aldosteronism (too much aldosterone) or Addison’s
disease (too little aldosterone).
Increased Distal Tubular Flow Rate Stimulates Potassium
Secretion. A rise in

low Rate Stimulates Potassium
Secretion. A rise in distal tubular ﬂow rate, as occurs
with volume expansion, high sodium intake, or diuretic
drug treatment, stimulates potassium secretion. Con-
versely, a decrease in distal tubular ﬂow rate, as caused
by sodium depletion, reduces potassium secretion.
The mechanism for the effect of high-volume ﬂow
rate is as follows: When potassium is secreted into the
tubular ﬂuid, the luminal concentration of potassium
increases, thereby reducing the driving f

otassium
increases, thereby reducing the driving force for potas-
sium diffusion across the luminal membrane. With
increased tubular ﬂow rate, the secreted potassium is
continuously ﬂushed down the tubule, so that the rise
in tubular potassium concentration becomes mini-
mized. Therefore, net potassium secretion is stimu-
lated by increased tubular ﬂow rate.
The effect of increased tubular ﬂow rate is especially
important in helping to preserve normal potassium
excretion during changes in sodium

ormal potassium
excretion during changes in sodium intake. For
example, with a high sodium intake, there is decreased
aldosterone secretion, which by itself would tend to
decrease the rate of potassium secretion and, there-
fore, reduce urinary excretion of potassium. However,
the high distal tubular ﬂow rate that occurs with a high
sodium intake tends to increase potassium secretion
K+ intake
Plasma K+
K+
 secretion
tubules
Figure 29–7

secretion
tubules
Primary mechanisms by which high potassium intake raises
potassium excretion. Note that increased plasma potassium con-
centration directly raises potassium secretion by the cortical col-
lecting tubules and indirectly increases potassium secretion by
raising plasma aldosterone concentration.
Plasma potassium concentration
(mEq/day)
3.8
4.8
4.6
4.4
4.2
4.0
0
30
60
90
120
150
180
210
Potassium intake (mEq/day)
Aldosteron

ke (mEq/day)
system
blocked
system
blocked
Normal
Normal
Figure 29–8
Effect of large changes in potassium intake on extracellular ﬂuid
potassium concentration under normal conditions (red line) and
after the aldosterone feedback had been blocked (blue line). Note
that after blockade of the aldosterone system, regulation of potas-
sium concentration was greatly impaired. (Courtesy Dr. David B.
Young.)
Chapter 29
Renal Regulation; Integration of R

ng.)
Chapter 29
371
(Figure 29–9), as discussed in the previous paragraph.
Therefore, the two effects of high sodium intake,
decreased aldosterone secretion and the high tubular
ﬂow rate, counterbalance each other, so that there is
little change in potassium excretion. Likewise, with a
low sodium intake, there is little change in potassium
excretion because of the counterbalancing effects of
increased aldosterone secretion and decreased tubular
ﬂ

ased aldosterone secretion and decreased tubular
ﬂow rate on potassium secretion.
Acute Acidosis Decreases Potassium Secretion. Acute
increases in hydrogen ion concentration of the extra-
cellular ﬂuid (acidosis) reduce potassium secretion,
whereas decreased hydrogen ion concentration (alka-
losis) increases potassium secretion. The primary
mechanism by which increased hydrogen ion concen-
tration inhibits potassium secretion is by reducing the
activity of the sodium-potassium ATPase pump. This

ctivity of the sodium-potassium ATPase pump. This
in turn decreases intracellular potassium concentra-
tion and subsequent passive diffusion of potassium
across the luminal membrane into the tubule.
With more prolonged acidosis, lasting over a period of
several days, there is an increase in urinary potassium
excretion. The mechanism for this effect is due in part
to an effect of chronic acidosis to inhibit proximal
tubular sodium chloride and water reabsorption, which
increases distal volume del

er reabsorption, which
increases distal volume delivery, thereby stimulating 
the secretion of potassium. This effect overrides 
the inhibitory effect of hydrogen ions on the sodium-
potassium ATPase pump. Thus, chronic acidosis leads 
to a loss of potassium, whereas acute acidosis leads to
decreased potassium excretion.
Control of Renal Calcium
Excretion and Extracellular
The mechanisms for regulating calcium ion concen-
tration are discussed in detail in Chapter 79, a

-
tration are discussed in detail in Chapter 79, along with
the endocrinology of the calcium-regulating hormones
parathyroid hormone (PTH) and calcitonin. There-
fore, calcium ion regulation is discussed only brieﬂy in
Extracellular ﬂuid calcium ion concentration 
normally remains tightly controlled within a few 
percentage points of its normal level, 2.4 mEq/L.
When calcium ion concentration falls to low levels
(hypocalcemia), the excitability of nerve and muscle
cells increases m

excitability of nerve and muscle
cells increases markedly and can in extreme cases
result in hypocalcemic tetany. This is characterized by
spastic skeletal muscle contractions. Hypercalcemia
(increased calcium concentration) depresses neuromus-
cular excitability and can lead to cardiac arrhythmias.
About 50 per cent of the total calcium in the plasma
(5 mEq/L) exists in the ionized form, which is the form
that has biological activity at cell membranes. The
remainder is either bound to the plasm

branes. The
remainder is either bound to the plasma proteins
(about 40 per cent) or complexed in the non-ionized
form with anions such as phosphate and citrate (about
10 per cent).
Changes in plasma hydrogen ion concentration can
inﬂuence the degree of calcium binding to plasma pro-
teins.With acidosis, less calcium is bound to the plasma
proteins. Conversely, in alkalosis, a greater amount of
calcium is bound to the plasma proteins. Therefore,
patients with alkalosis are more susceptible to hyp

atients with alkalosis are more susceptible to hypocal-
cemic tetany.
As with other substances in the body, the intake of
calcium must be balanced with the net loss of calcium
over the long term. Unlike ions such as sodium and
chloride, however, a large share of calcium excretion
occurs in the feces. The usual rate of dietary calcium
intake is about 1000 mg/day, with about 900 mg/day of
calcium excreted in the feces. Under certain condi-
tions, fecal calcium excretion can exceed calcium
ingestio

ecal calcium excretion can exceed calcium
ingestion because calcium can also be secreted into 
the intestinal lumen. Therefore, the gastrointestinal
tract and the regulatory mechanisms that inﬂuence
intestinal calcium absorption and secretion play a
major role in calcium homeostasis, as discussed in
Chapter 79.
Almost all the calcium in the body (99 per cent) is
stored in the bone, with only about 1 per cent in the
extracellular ﬂuid and 0.1 per cent in the intracellular
ﬂuid. The bone, therefor

cent in the intracellular
ﬂuid. The bone, therefore, acts as a large reservoir 
for storing calcium and as a source of calcium when
extracellular ﬂuid calcium concentration tends to
decrease.
One of the most important regulators of bone uptake
and release of calcium is PTH.When extracellular ﬂuid
Na+ intake
-
+
GFR
Unchanged K+
excretion
Distal tubular
flow rate
K+
 secretion
ducts
Proximal
Figure 29–9
Effect of high sodium intake on renal

Effect of high sodium intake on renal excretion of potassium. Note
that a high-sodium diet decreases plasma aldosterone, which
tends to decrease potassium secretion by the cortical collecting
tubules. However, the high-sodium diet simultaneously increases
ﬂuid delivery to the cortical collecting duct, which tends to
increase potassium secretion. The opposing effects of a high-
sodium diet counterbalance each other, so that there is little
change in potassium excretion.
372
Unit V
Th

ittle
372
Unit V
calcium concentration falls below normal, the parathy-
roid glands are directly stimulated by the low calcium
levels to promote increased secretion of PTH. This
hormone then acts directly on the bones to increase
the resorption of bone salts (release of salts from the
bones) and, therefore, to release large amounts of
calcium into the extracellular ﬂuid, thereby returning
calcium levels back toward normal. When calcium i

calcium levels back toward normal. When calcium ion
concentration is elevated, PTH secretion decreases, so
that almost no bone resorption now occurs; instead,
excess calcium is deposited in the bones because of
new bone formation. Thus, the day-to-day regulation
of calcium ion concentration is mediated in large part
by the effect of PTH on bone resorption.
The bones, however, do not have an inexhaustible
supply of calcium. Therefore, over the long term, the
intake of calcium must be balanced wi

ng term, the
intake of calcium must be balanced with calcium
excretion by the gastrointestinal tract and the kidneys.
The most important regulator of calcium reabsorption
at both of these sites is PTH. Thus, PTH regulates
plasma calcium concentration through three main
effects: (1) by stimulating bone resorption; (2) by stim-
ulating activation of vitamin D, which then increases
intestinal reabsorption of calcium; and (3) by directly
increasing renal tubular calcium reabsorption (Figure
29–10). 

enal tubular calcium reabsorption (Figure
29–10). The control of gastrointestinal calcium reab-
sorption and calcium exchange in the bones is dis-
cussed elsewhere, and the remainder of this section
focuses on the mechanisms that control renal calcium
Control of Calcium Excretion 
Because calcium is both ﬁltered and reabsorbed in the
kidneys but not secreted, the rate of renal calcium
excretion is calculated as
Renal calcium excretion = Calcium ﬁltered 
- Calcium reabso

cium excretion = Calcium ﬁltered 
- Calcium reabsorbed
Only about 50 per cent of the plasma calcium is
ionized, with the remainder being bound to the plasma
proteins or complexed with anions such as phosphate.
Therefore, only about 50 per cent of the plasma
calcium can be ﬁltered at the glomerulus. Normally,
about 99 per cent of the ﬁltered calcium is reabsorbed
by the tubules, with only about 1 per cent of the ﬁl-
tered calcium being excreted.About 65 per cent of the
ﬁltered calcium is reabsorb

out 65 per cent of the
ﬁltered calcium is reabsorbed in the proximal tubule,
25 to 30 per cent is reabsorbed in the loop of Henle,
and 4 to 9 per cent is reabsorbed in the distal and col-
lecting tubules. This pattern of reabsorption is similar
to that for sodium.
As is true with the other ions, calcium excretion is
adjusted to meet the body’s needs.With an increase in
calcium intake, there is also increased renal calcium
excretion, although much of the increase of calcium
intake is eliminated i

 of the increase of calcium
intake is eliminated in the feces. With calcium deple-
tion, calcium excretion by the kidneys decreases as a
result of enhanced tubular reabsorption.
One of the primary controllers of renal tubular
calcium reabsorption is PTH. With increased levels of
PTH, there is increased calcium reabsorption in the
thick ascending loops of Henle and distal tubules,
which reduces urinary excretion of calcium. Con-
versely, reduction of PTH promotes calcium excretion
by decreasing r

 of PTH promotes calcium excretion
by decreasing reabsorption in the loops of Henle and
distal tubules.
In the proximal tubule, calcium reabsorption usually
parallels sodium and water reabsorption. Therefore,
in instances of extracellular volume expansion or
increased arterial pressure—both of which decrease
proximal sodium and water reabsorption—there is
also reduction in calcium reabsorption and, con-
sequently, increased urinary excretion of calcium.
Conversely, with extracellular volume cont

alcium.
Conversely, with extracellular volume contraction or
decreased blood pressure, calcium excretion decreases
primarily because of increased proximal tubular 
Another factor that inﬂuences calcium reabsorption
is the plasma concentration of phosphate. An increase
in plasma phosphate stimulates PTH, which increases
calcium reabsorption by the renal tubules, thereby
reducing calcium excretion. The opposite occurs with
reduction in plasma phosphate concentration.
Calcium reabsorp

n plasma phosphate concentration.
Calcium reabsorption is also stimulated by meta-
bolic acidosis and inhibited by metabolic alkalosis.
Most of the effect of hydrogen ion concentration on
calcium excretion results from changes in calcium
reabsorption in the distal tubule.
A summary of the factors that are known to inﬂu-
ence calcium excretion by the renal tubules is shown
in Table 29–2.
Regulation of Renal Phosphate
Excretion
Phosphate excretion by the kidneys is controlled 
primarily by an over

by the kidneys is controlled 
primarily by an overﬂow mechanism that can be
explained as follows: The renal tubules have a normal
transport maximum for reabsorbing phosphate of
about 0.1 mM/min. When less than this amount 
of phosphate is present in the glomerular ﬁltrate,
Vitamin D3
PTH
[Ca++]
Intestinal Ca++
Renal Ca++
Ca++ release
from bones
Figure 29–10
Compensatory responses to decreased plasma ionized calcium
concentration mediated by parathyroid hormon

lcium
concentration mediated by parathyroid hormone (PTH) and
vitamin D.
Chapter 29
373
essentially all the ﬁltered phosphate is reabsorbed.
When more than this is present, the excess is excreted.
Therefore, phosphate normally begins to spill into the
urine when its concentration in the extracellular ﬂuid
rises above a threshold of about 0.8 mM/L, which gives
a tubular load of phosphate of about 0.1 mM/min,
assuming a GFR of 125 ml/min. Because m

.1 mM/min,
assuming a GFR of 125 ml/min. Because most people
ingest large quantities of phosphate in milk products
and meat, the concentration of phosphate is usually
maintained above 1 mM/L, a level at which there is
continual excretion of phosphate into the urine.
Changes in tubular phosphate reabsorption can also
inﬂuence phosphate excretion. For instance, a diet low
in phosphate can, over time, increase the reabsorptive
transport maximum for phosphate, thereby reducing
the tendency for phosp

phosphate, thereby reducing
the tendency for phosphate to spill over into the urine.
PTH can play a signiﬁcant role in regulating phos-
phate concentration through two effects: (1) PTH 
promotes bone resorption, thereby dumping large
amounts of phosphate ions into the extracellular ﬂuid
from the bone salts, and (2) PTH decreases the trans-
port maximum for phosphate by the renal tubules,
so that a greater proportion of the tubular phosphate
is lost in the urine. Thus, whenever plasma PTH is
incr

st in the urine. Thus, whenever plasma PTH is
increased, tubular phosphate reabsorption is decreased
and more phosphate is excreted. These interrelations
among phosphate, PTH, and calcium are discussed in
more detail in Chapter 79.
Control of Renal Magnesium
Magnesium Ion Concentration
More than one half of the body’s magnesium is stored
in the bones. Most of the rest resides within the cells,
with less than 1 per cent located in the extracellular
ﬂuid. Although the t

 located in the extracellular
ﬂuid. Although the total plasma magnesium concen-
tration is about 1.8 mEq/L, more than one half of this
is bound to plasma proteins. Therefore, the free
ionized concentration of magnesium is only about 
0.8 mEq/L.
The normal daily intake of magnesium is about 250
to 300 mg/day, but only about one half of this intake is
absorbed by the gastrointestinal tract. To maintain
magnesium balance, the kidneys must excrete this
absorbed magnesium, about one half the daily in

is
absorbed magnesium, about one half the daily intake
of magnesium, or 125 to 150 mg/day. The kidneys 
normally excrete about 10 to 15 per cent of the mag-
nesium in the glomerular ﬁltrate.
Renal excretion of magnesium can increase
markedly during magnesium excess or can decrease to
almost nil during magnesium depletion. Because mag-
nesium is involved in many biochemical processes in
the body, including activation of many enzymes, its
concentration must be closely regulated.
Regulation of magn

tion must be closely regulated.
Regulation of magnesium excretion is achieved
mainly by changing tubular reabsorption. The proxi-
mal tubule usually reabsorbs only about 25 per cent of
the ﬁltered magnesium. The primary site of reabsorp-
tion is the loop of Henle, where about 65 per cent of
the ﬁltered load of magnesium is reabsorbed. Only a
small amount (usually less than 5 per cent) of the 
ﬁltered magnesium is reabsorbed in the distal and 
collecting tubules.
The mechanisms that regulate magn

lecting tubules.
The mechanisms that regulate magnesium excretion
are not well understood, but the following distur-
bances lead to increased magnesium excretion: (1)
increased extracellular ﬂuid magnesium concentra-
tion, (2) extracellular volume expansion, and (3)
increased extracellular ﬂuid calcium concentration.
Mechanisms for Control 
Extracellular ﬂuid volume is determined mainly by the
balance between intake and output of water and salt.
In mos

etween intake and output of water and salt.
In most cases, salt and ﬂuid intakes are dictated by a
person’s habits rather than by physiologic control
mechanisms. Therefore, the burden of extracellular
volume regulation is usually placed on the kidneys,
which must adapt their excretion of salt and water to
match intake of salt and water under steady-state 
In discussing the regulation of extracellular ﬂuid
volume, we also consider the factors that regulate the
amount of sodium chlorid

factors that regulate the
amount of sodium chloride in the extracellular ﬂuid,
because changes in extracellular ﬂuid sodium chloride
content usually cause parallel changes in extracellular
ﬂuid volume, provided the antidiuretic hormone
(ADH)-thirst mechanisms are also operative. When
the ADH-thirst mechanisms are functioning normally,
a change in the amount of sodium chloride in the
extracellular ﬂuid is matched by a similar change in 
the amount of extracellular water, so that osmolality
and so

 of extracellular water, so that osmolality
and sodium concentration are maintained relatively
constant.
Sodium Excretion Is Precisely
Matched to Intake Under Steady-State
An important consideration in overall control of
sodium excretion—or excretion of any electrolyte, for
that matter—is that under steady-state conditions,
excretion by the kidneys is determined by intake. To
Table 29–2
Factors That Alter Renal Calcium Excretion
Ø Calcium Excretion
≠Calcium Excretion
≠Parathyroid horm

ium Excretion
≠Parathyroid hormone (PTH)
Ø PTH
Ø Extracellular ﬂuid volume
≠Extracellular ﬂuid volume
≠Plasma phosphate
Ø Plasma phosphate
Metabolic alkalosis
Vitamin D3
374
Unit V
maintain life, a person must, over the long term,
excrete almost precisely the amount of sodium
ingested. Therefore, even with disturbances that cause
major changes in kidney function, balance between
intake and output o

dney function, balance between
intake and output of sodium usually is restored within
a few days.
If disturbances of kidney function are not too
severe, sodium balance may be achieved mainly by
intrarenal adjustments with minimal changes in extra-
cellular ﬂuid volume or other systemic adjustments.
But when perturbations to the kidneys are severe and
intrarenal compensations are exhausted, systemic
adjustments must be invoked, such as changes in blood
pressure, changes in circulating hormones, a

blood
pressure, changes in circulating hormones, and alter-
ations of sympathetic nervous system activity. These
adjustments are costly in terms of overall homeostasis
because they cause other changes throughout the 
body that may, in the long run, be damaging. These
compensations, however, are necessary because a 
sustained imbalance between ﬂuid and electrolyte
intake and excretion would quickly lead to accumula-
tion or loss of electrolytes and ﬂuid, causing cardio-
vascular collapse within a

d ﬂuid, causing cardio-
vascular collapse within a few days.Thus, one can view
the systemic adjustments that occur in response to
abnormalities of kidney function as a necessary trade-
off that brings electrolyte and ﬂuid excretion back in
balance with intake.
Sodium Excretion Is Controlled by
Altering Glomerular Filtration or
Tubular Sodium Reabsorption Rates
The two variables that inﬂuence sodium and water
excretion are the rates of ﬁltration and the rates of
reabsorption:
Excretion = Glomerul

nd the rates of
Excretion = Glomerular ﬁltration - Tubular 
GFR normally is about 180 L/day, tubular reab-
sorption is 178.5 L/day, and urine excretion is 1.5
L/day. Thus, small changes in GFR or tubular reab-
sorption potentially can cause large changes in renal
excretion. For example, a 5 per cent increase in GFR
(to 189 L/day) would cause a 9 L/day increase in urine
volume, if tubular compensations did not occur; this
would quickly cause catastrophic changes in body

s
would quickly cause catastrophic changes in body ﬂuid
volumes. Similarly, small changes in tubular reabsorp-
tion, in the absence of compensatory adjustments of
GFR, would also lead to dramatic changes in urine
volume and sodium excretion. Tubular reabsorption
and GFR usually are regulated precisely, so that excre-
tion by the kidneys can be exactly matched to intake
of water and electrolytes.
Even with disturbances that alter GFR or tubular
reabsorption, changes in urinary excretion are mini-

absorption, changes in urinary excretion are mini-
mized by various buffering mechanisms. For example,
if the kidneys become greatly vasodilated and GFR
increases (as can occur with certain drugs or high
fever), this raises sodium chloride delivery to the
tubules, which in turn leads to at least two intrarenal
compensations: (1) increased tubular reabsorption of
much of the extra sodium chloride ﬁltered, called
glomerulotubular balance, and (2) macula densa feed-
back, in which increased sodium 

acula densa feed-
back, in which increased sodium chloride delivery to
the distal tubule causes afferent arteriolar constriction
and return of GFR toward normal. Likewise, abnor-
malities of tubular reabsorption in the proximal tubule
or loop of Henle are partially compensated for by
these same intrarenal feedbacks.
Because neither of these two mechanisms operates
perfectly to restore distal sodium chloride delivery all
the way back to normal, changes in either GFR or
tubular reabsorption can le

anges in either GFR or
tubular reabsorption can lead to signiﬁcant changes in
urine sodium and water excretion.When this happens,
other feedback mechanisms come into play, such as
changes in blood pressure and changes in various 
hormones, that eventually return sodium excretion to
equal sodium intake. In the next few sections, we
review how these mechanisms operate together to
control sodium and water balance and in so doing act
also to control extracellular ﬂuid volume. We should
keep in mind,

extracellular ﬂuid volume. We should
keep in mind, however, that all these feedback mech-
anisms control renal excretion of sodium and water by
altering either GFR or tubular reabsorption.
Importance of Pressure
Natriuresis and Pressure
Diuresis in Maintaining Body
One of the most basic and powerful mechanisms for
control of blood volume and extracellular ﬂuid
volume, as well as for the maintenance of sodium and
ﬂuid balance, is the effect of blood pressure on sodium
and

nce, is the effect of blood pressure on sodium
and water excretion—called the pressure natriuresis
and pressure diuresis mechanisms, respectively. As 
discussed in Chapter 19, this feedback between the
kidneys and the circulatory system also plays a domi-
nant role in long-term blood pressure regulation.
Pressure diuresis refers to the effect of increased
blood pressure to raise urinary volume excretion,
whereas pressure natriuresis refers to the rise in
sodium excretion that occurs with elevate

 rise in
sodium excretion that occurs with elevated blood pres-
sure. Because pressure diuresis and natriuresis usually
occur in parallel, we refer to these mechanisms simply
as “pressure natriuresis” in the following discussion.
Figure 29–11 shows the effect of arterial pressure on
urinary sodium output. Note that acute increases in
blood pressure of 30 to 50 mm Hg cause a twofold to
threefold increase in urinary sodium output. This
effect is independent of changes in activity of the sym-
pathe

dependent of changes in activity of the sym-
pathetic nervous system or of various hormones, such
as angiotensin II, ADH, or aldosterone, because pres-
sure natriuresis can be demonstrated in an isolated
kidney that has been removed from the inﬂuence of
these factors.With chronic increases in blood pressure,
the effectiveness of pressure natriuresis is greatly
enhanced because the increased blood pressure also,
after a short time delay, suppresses renin release and,
therefore, decreases formatio

s renin release and,
therefore, decreases formation of angiotensin II and
Chapter 29
375
aldosterone. As discussed previously, decreased levels
of angiotensin II and aldosterone inhibit renal tubular
reabsorption of sodium, thereby amplifying the direct
effects of increased blood pressure to raise sodium and
water excretion.
Pressure Natriuresis and Diuresis Are
Key Components of a Renal-Body
Fluid Feedback for Regulating Body
Fluid Volumes and A

d Feedback for Regulating Body
Fluid Volumes and Arterial Pressure
The effect of increased blood pressure to raise urine
output is part of a powerful feedback system that oper-
ates to maintain balance between ﬂuid intake and
output, as shown in Figure 29–12. This is the same
mechanism that is discussed in Chapter 19 for arterial
pressure control.The extracellular ﬂuid volume, blood
volume, cardiac output, arterial pressure, and urine
output are all controlled at the same time as separate
parts 

all controlled at the same time as separate
parts of this basic feedback mechanism.
During changes in sodium and ﬂuid intake,this feed-
back mechanism helps to maintain ﬂuid balance and
to minimize changes in blood volume, extracellular
ﬂuid volume, and arterial pressure as follows:
1. An increase in ﬂuid intake (assuming that sodium
accompanies the ﬂuid intake) above the level of
urine output causes a temporary accumulation of
ﬂuid in the body.
2. As long as ﬂuid intake exceeds urine output, ﬂu

2. As long as ﬂuid intake exceeds urine output, ﬂuid
accumulates in the blood and interstitial spaces,
causing parallel increases in blood volume and
extracellular ﬂuid volume. As discussed later, the
actual increases in these variables are usually
small because of the effectiveness of this
feedback.
3. An increase in blood volume raises mean
circulatory ﬁlling pressure.
4. An increase in mean circulatory ﬁlling pressure
raises the pressure gradient for venous return.
5. An increased pressure gr

ent for venous return.
5. An increased pressure gradient for venous return
elevates cardiac output.
6. An increased cardiac output raises arterial
pressure.
7. An increased arterial pressure increases urine
output by way of pressure diuresis. The steepness
of the normal pressure natriuresis relation
indicates that only a slight increase in blood
pressure is required to raise urinary excretion
severalfold.
8. The increased ﬂuid excretion balances the
increased intake, and further accumulation of 

the
increased intake, and further accumulation of ﬂuid
is prevented.
Urinary sodium or volume
output (times normal)
0
8
6
4
2
0
20 40
60
Chronic
Acute
Figure 29–11
Acute and chronic effects of arterial pressure on sodium output
by the kidneys (pressure natriuresis). Note that chronic increases
in arterial pressure cause much greater increases in sodium
output than those measured during acute increases in arterial
pres

e measured during acute increases in arterial
pressure.
Arterial
pressure
Blood
volume
Mean circulatory
filling pressure
Venous
return
Vascular
capacity
Rate of change of
Renal fluid
excretion
Nonrenal
fluid loss
Fluid
intake
Cardiac
output
Heart strength
Total peripheral
Figure 29–12
Basic renal–body ﬂuid feedback
mechanism 
for 
control 
of 
blood volume, extracellular ﬂuid
volume, and arterial pressure.
Solid 

llular ﬂuid
Solid 
lines 
indicate 
positive
effects, and dashed lines indicate
negative effects.
376
Unit V
Thus, the renal-body ﬂuid feedback mechanism
operates to prevent continuous accumulation of salt
and water in the body during increased salt and water
intake. As long as kidney function is normal and the
pressure diuresis mechanism is operating effectively,
large changes in salt and water intake can be accom-
modated with only sli

d water intake can be accom-
modated with only slight changes in blood volume,
extracellular ﬂuid volume, cardiac output, and arterial
pressure.
The opposite sequence of events occurs when ﬂuid
intake falls below normal. In this case, there is a ten-
dency toward decreased blood volume and extracellu-
lar ﬂuid volume, as well as reduced arterial pressure.
Even a small decrease in blood pressure causes a 
large decrease in urine output, thereby allowing ﬂuid
balance to be maintained with minimal 

lowing ﬂuid
balance to be maintained with minimal changes in
blood pressure, blood volume, or extracellular ﬂuid
volume. The effectiveness of this mechanism in pre-
venting large changes in blood volume is demon-
strated in Figure 29–13, which shows that changes in
blood volume are almost imperceptible despite large
variations in daily intake of water and electrolytes,
except when intake becomes so low that it is not suf-
ﬁcient to make up for ﬂuid losses caused by evapora-
tion or other inescap

id losses caused by evapora-
tion or other inescapable losses.
Precision of Blood Volume and
Extracellular Fluid Volume Regulation
By studying Figure 29–12, one can see why the blood
volume remains almost exactly constant despite
extreme changes in daily ﬂuid intake. The reason for
this is the following: (1) a slight change in blood
volume causes a marked change in cardiac output, (2)
a slight change in cardiac output causes a large change
in blood pressure, and (3) a slight change in blood
pres

od pressure, and (3) a slight change in blood
pressure causes a large change in urine output. These
factors work together to provide effective feedback
control of blood volume.
The same control mechanisms operate whenever
there is a loss of whole blood because of hemorrhage.
In this case, ﬂuid is retained by the kidneys, and other
parallel processes occur to reconstitute the red blood
cells and plasma proteins in the blood. If abnormali-
ties of red blood cell volume remain, such as occurs
when 

red blood cell volume remain, such as occurs
when there is deﬁciency of erythropoietin or other
factors needed to stimulate red blood cell production,
the plasma volume will simply make up the difference,
and the overall blood volume will return essentially to
normal despite the low red blood cell mass.
Distribution of Extracellular
Fluid Between the Interstitial
Spaces and Vascular System
From Figure 29–12 it is apparent that blood volume
and extracellular ﬂuid volume are usually controlled
in 

tracellular ﬂuid volume are usually controlled
in parallel with each other. Ingested ﬂuid initially goes
into the blood, but it rapidly becomes distributed
between the interstitial spaces and the plasma. There-
fore, blood volume and extracellular ﬂuid volume
usually are controlled simultaneously.
There are circumstances, however, in which the dis-
tribution of extracellular ﬂuid between the interstitial
spaces and blood can vary greatly. As discussed in
Chapter 25, the principal factors that ca

ussed in
Chapter 25, the principal factors that can cause accu-
mulation of ﬂuid in the interstitial spaces include (1)
increased capillary hydrostatic pressure, (2) decreased
plasma colloid osmotic pressure, (3) increased perme-
ability of the capillaries, and (4) obstruction of lym-
phatic vessels. In all these conditions, an unusually high
proportion of the extracellular ﬂuid becomes distrib-
uted to the interstitial spaces.
Figure 29–14 shows the normal distribution of ﬂuid
between the inter

 the normal distribution of ﬂuid
between the interstitial spaces and the vascular system
and the distribution that occurs in edema states.When
small amounts of ﬂuid accumulate in the blood as a
result of either too much ﬂuid intake or a decrease in
renal output of ﬂuid, about 20 to 30 per cent of it stays
in the blood and increases the blood volume. The
remainder is distributed to the interstitial spaces.
When the extracellular ﬂuid volume rises more than
30 to 50 per cent above normal, almost a

more than
30 to 50 per cent above normal, almost all the addi-
tional ﬂuid goes into the interstitial spaces and little
remains in the blood. This occurs because once the
interstitial ﬂuid pressure rises from its normally nega-
tive value to become positive, the tissue interstitial
spaces become compliant, and large amounts of ﬂuid
then pour into the tissues without interstitial ﬂuid
pressure rising much more. In other words, the safety
factor against edema, owing to a rising interstitial 
ﬂuid 

ainst edema, owing to a rising interstitial 
ﬂuid pressure that counteracts ﬂuid accumulation in
the tissues, is lost once the tissues become highly 
compliant.
Thus, under normal conditions, the interstitial 
spaces act as an “overﬂow” reservoir for excess ﬂuid,
Blood volume (liters)
0
6
5
4
3
2
1
0
1
Death
2
3
4
5
6
7
8
Daily fluid intake
(water and electrolytes) (L/day)
Figure 29–13
Approximate effect of changes in 

ay)
Approximate effect of changes in daily ﬂuid intake on blood
volume. Note that blood volume remains relatively constant in the
normal range of daily ﬂuid intakes.
Chapter 29
377
sometimes increasing in volume 10 to 30 liters. This
causes edema, as explained in Chapter 25, but it also
acts as an important overﬂow release valve for the cir-
culation, protecting the cardiovascular system against
dangerous overload that could lead to 

tem against
dangerous overload that could lead to pulmonary
edema and cardiac failure.
To summarize, extracellular ﬂuid volume and blood
volume are controlled simultaneously, but the quanti-
tative amounts of ﬂuid distribution between the 
interstitium and the blood depend on the physical
properties of the circulation and the interstitial spaces
as well as on the dynamics of ﬂuid exchange through
the capillary membranes.
Nervous and Hormonal
Factors Increase the
Effectiveness of Renal-Body
Fluid

ors Increase the
Fluid Feedback Control
In Chapter 27, we discuss the nervous and hormonal
factors that inﬂuence GFR and tubular reabsorption
and, therefore, renal excretion of salt and water.These
nervous and hormonal mechanisms usually act in
concert with the pressure natriuresis and pressure
diuresis mechanisms, making them more effective in
minimizing the changes in blood volume, extracellular
ﬂuid volume, and arterial pressure that occur in
response to day-to-day

rial pressure that occur in
response to day-to-day challenges. However, abnor-
malities of kidney function or of the various nervous
and hormonal factors that inﬂuence the kidneys can
lead to serious changes in blood pressure and body
ﬂuid volumes, as discussed later.
Sympathetic Nervous System 
Control of Renal Excretion: Arterial
Baroreceptor and Low-Pressure
Stretch Receptor Reﬂexes
Because the kidneys receive extensive sympathetic
innervation, changes in sympathetic activity can alter
renal 

, changes in sympathetic activity can alter
renal sodium and water excretion as well as regulation
of extracellular ﬂuid volume under some conditions.
For example, when blood volume is reduced by hem-
orrhage, the pressures in the pulmonary blood vessels
and other low-pressure regions of the thorax decrease,
causing reﬂex activation of the sympathetic nervous
system. This in turn increases renal sympathetic nerve
activity, which has several effects to decrease sodium
and water excretion: (1) con

ts to decrease sodium
and water excretion: (1) constriction of the renal arte-
rioles, with resultant decreased GFR; (2) increased
tubular reabsorption of salt and water; and (3) stimu-
lation of renin release and increased angiotensin II
and aldosterone formation, both of which further
increase tubular reabsorption. And if the reduction in
blood volume is great enough to lower systemic arte-
rial pressure, further activation of the sympathetic
nervous system occurs because of decreased stretch 

ervous system occurs because of decreased stretch of
the arterial baroreceptors located in the carotid sinus
and aortic arch. All these reﬂexes together play an
important role in the rapid restitution of blood volume
that occurs in acute conditions such as hemorrhage.
Also, reﬂex inhibition of renal sympathetic activity
may contribute to the rapid elimination of excess ﬂuid
in the circulation that occurs after eating a meal that
contains large amounts of salt and water.
Role of Angiotensin II In

ounts of salt and water.
Role of Angiotensin II In Controlling
One of the body’s most powerful controllers of sodium
excretion is angiotensin II. Changes in sodium and
ﬂuid intake are associated with reciprocal changes in
angiotensin II formation, and this in turn contributes
greatly to the maintenance of body sodium and ﬂuid
balances. That is, when sodium intake is elevated 
above normal, renin secretion is decreased, causing
decreased 
angiotensin 
II 
Because
angiot

reased 
II 
Because
angiotensin II has several important effects in increas-
ing tubular reabsorption of sodium, as explained in
Chapter 27, a reduced level of angiotensin II decreases
tubular reabsorption of sodium and water, thus
increasing the kidneys’ excretion of sodium and water.
The net result is to minimize the rise in extracellular
ﬂuid volume and arterial pressure that would other-
wise occur when sodium intake increases.
Conversely, when sodium intake is reduce

ncreases.
Conversely, when sodium intake is reduced below
normal, increased levels of angiotensin II cause
sodium and water retention and oppose reductions in
arterial blood pressure that would otherwise occur.
Thus, changes in activity of the renin-angiotensin
system act as a powerful ampliﬁer of the pressure
natriuresis mechanism for maintaining stable blood
pressures and body ﬂuid volumes.
0
8
7
6
5
4
3
2
1
0
5
10
15
20
25
30
35
Edema
Normal value
Death
40
E

5
30
35
Edema
Death
40
Extracellular fluid volume (liters)
Figure 29–14
Approximate relation between extracellular ﬂuid volume and blood
volume, showing a nearly linear relation in the normal range but
also showing the failure of blood volume to continue rising when
the extracellular ﬂuid volume becomes excessive. When this
occurs, the additional extracellular ﬂuid volume resides in the
interstitial spaces, and edema results.
378
Unit

e
378
Unit V
Importance of Angiotensin II in Increasing Effectiveness of
Pressure Natriuresis. The importance of angiotensin II
in making the pressure natriuresis mechanism more
effective is shown in Figure 29–15. Note that when the
angiotensin control of natriuresis is fully functional,
the pressure natriuresis curve is steep (normal curve),
indicating that only minor changes in blood pressure
are needed to increase sodium excr

 blood pressure
are needed to increase sodium excretion when sodium
intake is raised.
In contrast, when angiotensin levels cannot be
decreased in response to increased sodium intake
(high angiotensin II curve), as occurs in some hyper-
tensive patients who have impaired ability to decrease
renin secretion, the pressure natriuresis curve is not
nearly as steep. Therefore, when sodium intake is
raised, much greater increases in arterial pressure are
needed to increase sodium excretion and maintain

e
sodium balance. For example, in most people, a 10-fold
increase in sodium intake causes an increase of only a
few millimeters of mercury in arterial pressure,
whereas in subjects who cannot suppress angiotensin
II formation appropriately in response to excess
sodium, the same rise in sodium intake causes blood
pressure to rise as much as 50 mm Hg.Thus, the inabil-
ity to suppress angiotensin II formation when there is
excess sodium reduces the s

ormation when there is
excess sodium reduces the slope of pressure natriure-
sis and makes arterial pressure very salt sensitive, as
discussed in Chapter 19.
The use of drugs to block the effects of angiotensin
II has proved to be important clinically for improv-
ing the kidneys’ ability to excrete salt and water.
When angiotensin II formation is blocked with an
angiotensin-converting enzyme inhibitor (see Figure
29–15) or an angiotensin II receptor antagonist, the
renal–pressure natriuresis cur

tor antagonist, the
renal–pressure natriuresis curve is shifted to lower
pressures; this indicates an enhanced ability of the
kidneys to excrete sodium because normal levels of
sodium excretion can now be maintained at reduced
arterial pressures. This shift of pressure natriuresis
provides the basis for the chronic blood pressure–
lowering effects in hypertensive patients of the
angiotensin-converting 
enzyme 
inhibitors 
and
angiotensin II receptor antagonists.
Excessive Angiotensin II Does Not

tor antagonists.
Excessive Angiotensin II Does Not Cause Large Increases in
Extracellular Fluid Volume Because Increased Arterial Pressure
Counterbalances Angiotensin-Mediated Sodium Retention.
Although angiotensin II is one of the most powerful
sodium- and water-retaining hormones in the body,
neither a decrease nor an increase in circulating
angiotensin II has a large effect on extracellular ﬂuid
volume or blood volume. The reason for this is 
that with large increases in angiotensin II levels

that with large increases in angiotensin II levels, as
occurs with a renin-secreting tumor of the kidney,
the high angiotensin II levels initially cause sodium
and water retention by the kidneys and a small
increase in extracellular ﬂuid volume. This also initi-
ates a rise in arterial pressure that quickly increases
kidney output of sodium and water, thereby overcom-
ing the sodium- and water-retaining effects of the
angiotensin II and re-establishing a balance between
intake and output of sodi

ishing a balance between
intake and output of sodium at a higher blood pres-
sure. Conversely, after blockade of angiotensin II for-
mation, as occurs when an angiotensin-converting
enzyme inhibitor is administered, there is initial loss of
sodium and water, but the fall in blood pressure offsets
this effect, and sodium excretion is once again restored
to normal.
Role of Aldosterone in Controlling
Aldosterone increases sodium reabsorption, especially
in the cortical collecting tu

sorption, especially
in the cortical collecting tubules.The increased sodium
reabsorption is also associated with increased water
reabsorption and potassium secretion. Therefore, the
net effect of aldosterone is to make the kidneys retain
sodium and water but to increase potassium excretion
Sodium intake and output
12
10
8
6
4
2
0
60
Angiotensin blockade
Normal
High angiotensin II
80
100
120
140
160
Figure 29–15
Effect of excessive angiotens

(mm Hg)
Effect of excessive angiotensin II 
formation and effect of blocking
II 
formation 
on 
the
renal–pressure natriuresis curve. Note
that high levels of angiotensin II for-
mation decrease the slope of pressure
natriuresis, making blood pressure
very sensitive to changes in sodium
intake. Blockade of angiotensin II for-
mation shifts pressure natriuresis to
lower blood pressures.
Chapter 29
379
The function of aldo

ation of Renal Mechanisms
379
The function of aldosterone in regulating sodium
balance is closely related to that described for
angiotensin II.That is, with reduction in sodium intake,
the increased angiotensin II levels that occur stimulate
aldosterone secretion, which in turn contributes to the
reduction in urinary sodium excretion and, therefore,
to the maintenance of sodium balance. Conversely,
with high sodium intake, suppression of aldosterone
formation decreases tubular reabsorption, allo

one
formation decreases tubular reabsorption, allowing
the kidneys to excrete larger amounts of sodium.Thus,
changes in aldosterone formation also aid the pressure
natriuresis mechanism in maintaining sodium balance
during variations in salt intake.
During Chronic Oversecretion of Aldosterone, Kidneys
“Escape” from Sodium Retention as Arterial Pressure Rises.
Although aldosterone has powerful effects on sodium
reabsorption, when there is excessive infusion of
aldosterone or excessive formation o

e infusion of
aldosterone or excessive formation of aldosterone, as
occurs in patients with tumors of the adrenal gland
(Conn’s syndrome), the increased sodium reabsorp-
tion and decreased sodium excretion by the kidneys
are transient. After 1 to 3 days of sodium and water
retention, the extracellular ﬂuid volume rises by about
10 to 15 per cent and there is a simultaneous increase
in arterial blood pressure. When the arterial pressure
rises sufﬁciently, the kidneys “escape” from the sodium
and 

ciently, the kidneys “escape” from the sodium
and water retention and thereafter excrete amounts of
sodium equal to the daily intake, despite continued
presence of high levels of aldosterone. The primary
reason for the escape is the pressure natriuresis and
diuresis that occur when the arterial pressure rises.
In patients with adrenal insufﬁciency who do not
secrete enough aldosterone (Addison’s disease), there
is increased excretion of sodium and water, reduction
in extracellular ﬂuid volume, a

d water, reduction
in extracellular ﬂuid volume, and a tendency toward
low blood pressure. In the complete absence of aldos-
terone, the volume depletion may be severe unless the
person is allowed to eat large amounts of salt and
drink large amounts of water to balance the increased
urine output of salt and water.
Role of ADH in Controlling Renal
Water Excretion
As discussed in Chapter 28, ADH plays an important
role in allowing the kidneys to form a small volume of
concentrated urine while excr

rm a small volume of
concentrated urine while excreting normal amounts of
salt. This effect is especially important during water
deprivation, which strongly elevates plasma levels of
ADH that in turn increase water reabsorption by the
kidneys and help to minimize the decreases in extra-
cellular ﬂuid volume and arterial pressure that would
otherwise occur. Water deprivation for 24 to 48 hours
normally causes only a small decrease in extracellular
ﬂuid volume and arterial pressure. However, if th

ﬂuid volume and arterial pressure. However, if the
effects of ADH are blocked with a drug that antago-
nizes the action of ADH to promote water reabsorp-
tion in the distal and collecting tubules, the same
period of water deprivation causes a substantial fall in
both extracellular ﬂuid volume and arterial pressure.
Conversely, when there is excess extracellular volume,
decreased ADH levels reduce reabsorption of water by
the kidneys, thus helping to rid the body of the excess
volume.
Excess ADH

g to rid the body of the excess
volume.
Excess ADH Secretion Usually Causes Only Small Increases in
Extracellular Fluid Volume but Large Decreases in Sodium Con-
centration. Although ADH is important in regulating
extracellular ﬂuid volume, excessive levels of ADH
seldom cause large increases in arterial pressure or
extracellular ﬂuid volume. Infusion of large amounts
of ADH into animals initially causes renal retention of
water and a 10 to 15 per cent increase in extracellular
ﬂuid volume.As th

r cent increase in extracellular
ﬂuid volume.As the arterial pressure rises in response
to this increased volume, much of the excess volume
is excreted because of the pressure diuresis mecha-
nism. After several days of ADH infusion, the blood
volume and extracellular ﬂuid volume are elevated no
more than 5 to 10 per cent, and the arterial pressure is
also elevated by less than 10 mm Hg. The same is true
for patients with inappropriate ADH syndrome, in
which ADH levels may be elevated severalfol

me, in
which ADH levels may be elevated severalfold.
Thus, high levels of ADH do not cause major
increases of either body ﬂuid volume or arterial pres-
sure, although high ADH levels can cause severe reduc-
tions in extracellular sodium ion concentration. The
reason for this is that increased water reabsorption by
the kidneys dilutes the extracellular sodium, and at the
same time, the small increase in blood pressure that
does occur causes loss of sodium from the extracellu-
lar ﬂuid in the urin

f sodium from the extracellu-
lar ﬂuid in the urine through pressure natriuresis.
In patients who have lost their ability to secrete
ADH because of destruction of the supraoptic nuclei,
the urine volume may become 5 to 10 times normal.
This is almost always compensated for by ingestion of
enough water to maintain ﬂuid balance. If free access
to water is prevented, the inability to secrete ADH
may lead to marked reductions in blood volume and
Role of Atrial Natriuretic Peptide 

in Controlling Renal Excretion
Thus far, we have discussed mainly the role of sodium-
and water-retaining hormones in controlling extracel-
lular ﬂuid volume. However, several different natri-
uretic hormones may also contribute to volume
regulation. One of the most important of the natri-
uretic hormones is a peptide referred to as atrial natri-
uretic peptide (ANP), released by the cardiac atrial
muscle ﬁbers. The stimulus for release of this 

al
muscle ﬁbers. The stimulus for release of this peptide
appears to be overstretch of the atria, which can result
from excess blood volume. Once released by the
cardiac atria, ANP enters the circulation and acts on
the kidneys to cause small increases in GFR and
decreases in sodium reabsorption by the collecting
ducts. These combined actions of ANP lead to
increased excretion of salt and water, which helps to
compensate for the excess blood volume.
Changes in ANP levels probably help to minimiz

me.
Changes in ANP levels probably help to minimize
changes in blood volume during various disturbances,
380
Unit V
such as increased salt and water intake. However,
excessive production of ANP or even complete lack of
ANP does not cause major changes in blood volume
because these effects can easily be overcome by small
changes in blood pressure, acting through pressure
natriuresis. For example, infusions of large amounts of
ANP initially raise urine output of salt an

nts of
ANP initially raise urine output of salt and water and
cause slight decreases in blood volume. In less than 24
hours, this effect is overcome by a slight decrease in
blood pressure that returns urine output toward
normal, despite continued excess of ANP.
Integrated Responses to
Changes in Sodium Intake
The integration of the different control systems that
regulate sodium and ﬂuid excretion under normal 
conditions can be summarized by examining the
homeostatic responses to progressive inc

ining the
homeostatic responses to progressive increases in
dietary sodium intake. As discussed previously, the
kidneys have an amazing capability to match their
excretion of salt and water to intakes that can range
from as low as one tenth of normal to as high as 10
times normal.
High Sodium Intake Suppresses Antinatriuretic Systems and
Activates Natriuretic Systems. As sodium intake is
increased, sodium output initially lags slightly behind
intake.The time delay results in a small increase in 

ake.The time delay results in a small increase in the
cumulative sodium balance, which causes a slight
increase in extracellular ﬂuid volume. It is mainly this
small increase in extracellular ﬂuid volume that trig-
gers various mechanisms in the body to increase
sodium excretion. These mechanisms include the 
1. Activation of low pressure receptor reﬂexes that
originate from the stretch receptors of the right
atrium and the pulmonary blood vessels. Signals
from the stretch receptors g

lood vessels. Signals
from the stretch receptors go to the brain stem
and there inhibit sympathetic nerve activity to the
kidneys to decrease tubular sodium reabsorption.
This mechanism is most important in the ﬁrst few
hours—or perhaps the ﬁrst day—after a large
increase in salt and water intake.
2. Small increases in arterial pressure, caused by
volume expansion, raise sodium excretion through
pressure natriuresis.
3. Suppression of angiotensin II formation, caused by
increased arterial pressu

 II formation, caused by
increased arterial pressure and extracellular ﬂuid
volume expansion, decreases tubular sodium
reabsorption by eliminating the normal effect of
angiotensin II to increase sodium reabsorption.
Also, reduced angiotensin II decreases
aldosterone secretion, thus further reducing
tubular sodium reabsorption.
4. Stimulation of natriuretic systems, especially ANP,
contributes further to increased sodium excretion.
Thus, the combined activation of natriuretic systems
and suppress

ned activation of natriuretic systems
and suppression of sodium- and water-retaining
systems leads to an increase in sodium excretion when
sodium intake is increased. The opposite changes take
place when sodium intake is reduced below normal
levels.
Large Increases in Blood
Volume and Extracellular
Despite the powerful regulatory mechanisms that main-
tain blood volume and extracellular ﬂuid volume rea-
sonably constant, there are abnormal conditions that
can c

constant, there are abnormal conditions that
can cause large increases in both of these variables.
Almost all of these conditions result from circulatory
abnormalities.
Increased Blood Volume and
Extracellular Fluid Volume Caused
by Heart Diseases
In congestive heart failure, blood volume may increase
15 to 20 per cent, and extracellular ﬂuid volume some-
times increases by 200 per cent or more. The reason for
this can be understood by re-examination of Figure
29–12. Initially, heart failure red

tion of Figure
29–12. Initially, heart failure reduces cardiac output and,
consequently, decreases arterial pressure. This in turn
activates multiple sodium-retaining systems, especially
the renin-angiotensin, aldosterone, and sympathetic
nervous systems. In addition, the low blood pressure
itself causes the kidneys to retain salt and water. There-
fore, the kidneys retain volume in an attempt to return
the arterial pressure and cardiac output toward normal.
Indeed, if the heart failure is not t

ward normal.
Indeed, if the heart failure is not too severe, the rise in
blood volume can often return cardiac output and arte-
rial pressure virtually all the way to normal, and sodium
excretion will eventually increase back to normal,
although there will remain excess extracellular ﬂuid
volume and blood volume to keep the weakened heart
pumping adequately. However, if the heart is greatly
weakened, arterial pressure will not be able to increase
enough to restore urine output to normal. When th

enough to restore urine output to normal. When this
occurs, the kidneys continue to retain volume until the
person develops severe circulatory congestion and
eventually dies of pulmonary edema.
In myocardial failure, heart valvular disease, and 
congenital abnormalities of the heart, an important 
circulatory compensation is an increase in blood
volume, which helps to return cardiac output and blood
pressure to normal. This allows even the weakened
heart to maintain a life-sustaining level of c

ned
heart to maintain a life-sustaining level of cardiac
output.
Increased Blood Volume Caused by
Any condition that increases vascular capacity will also
cause the blood volume to increase to ﬁll this extra
capacity. An increase in vascular capacity initially
reduces mean circulatory ﬁlling pressure (see Figure
29–12), which leads to decreased cardiac output and
decreased arterial pressure. The fall in pressure causes
salt and water retention by the kidneys unt

causes
salt and water retention by the kidneys until the blood
Chapter 29
381
volume increases sufﬁciently to ﬁll the extra capacity.
For example, in pregnancy the increased vascular capac-
ity of the uterus, placenta, and other enlarged organs 
of the woman’s body regularly increases the blood
volume 15 to 25 per cent. Similarly, in patients who 
have large varicose veins of the legs, which in rare
instances may hold up to an extra liter of bloo

re
instances may hold up to an extra liter of blood,
the blood volume simply increases to ﬁll the extra 
vascular capacity. In these cases, salt and water are
retained by the kidneys until the total vascular bed is
ﬁlled enough to raise blood pressure to the level
required to balance renal output of ﬂuid with daily
intake of ﬂuid.
Large Increases in
Extracellular Fluid 
Volume but with Normal
There are several conditions in which extracellular ﬂuid
volume becom

onditions in which extracellular ﬂuid
volume becomes markedly increased but blood volume
remains normal or even slightly reduced. These condi-
tions are usually initiated by leakage of ﬂuid and protein
into the interstitium, which tends to decrease the blood
volume. The kidneys’ response to these conditions is
similar to the response after hemorrhage. That is, the
kidneys retain salt and water in an attempt to restore
blood volume toward normal. Much of the extra ﬂuid,
however, leaks into the in

Much of the extra ﬂuid,
however, leaks into the interstitium, causing further
edema.
Nephrotic Syndrome—Loss of
Plasma Proteins in Urine and
The general mechanisms that lead to extracellular
edema are reviewed in Chapter 25. One of the most
important clinical causes of edema is the so-called
nephrotic syndrome.
In nephrotic syndrome,
the
glomerular capillaries leak large amounts of protein
into the ﬁltrate and the urine because of an increased
permeability of the 

urine because of an increased
permeability of the glomerulus. Thirty to 50 grams of
plasma protein can be lost in the urine each day, some-
times causing the plasma protein concentration to fall
to less than one-third normal. As a consequence of the
decreased plasma protein concentration, the plasma
colloid osmotic pressure falls to low levels. This causes
the capillaries all over the body to ﬁlter large amounts
of ﬂuid into the various tissues, which in turn causes
edema and decreases the plasm

which in turn causes
edema and decreases the plasma volume.
Renal sodium retention in nephrotic syndrome
occurs through multiple mechanisms activated by
leakage of protein and ﬂuid from the plasma into the
interstitial ﬂuid, including activation of various sodium-
retaining systems such as the renin-angiotensin system,
aldosterone, and possibly the sympathetic nervous
system. The kidneys continue to retain sodium and
water until plasma volume is restored nearly to normal.
However, because of the

restored nearly to normal.
However, because of the large amount of sodium and
water retention, the plasma protein concentration
becomes further diluted, causing still more ﬂuid to leak
into the tissues of the body. The net result is massive
ﬂuid retention by the kidneys until tremendous extra-
cellular edema occurs unless treatment is instituted to
restore the plasma proteins.
Liver Cirrhosis—Decreased
Synthesis of Plasma Proteins 
by the Liver and Sodium Retention
A similar seque

nd Sodium Retention
A similar sequence of events occurs in cirrhosis of the
liver as in nephrotic syndrome, except that in liver cir-
rhosis, the reduction in plasma protein concentration
results from destruction of the liver cells, thus reducing
the ability of the liver to synthesize enough plasma 
proteins. Cirrhosis is also associated with large amounts
of ﬁbrous tissue in the liver structure, which greatly
impedes the ﬂow of portal blood through the liver.
This in turn raises 

rtal blood through the liver.
This in turn raises capillary pressure throughout the
portal vascular bed, which also contributes to the
leakage of ﬂuid and proteins into the peritoneal cavity,
a condition called ascites. Once ﬂuid and protein are lost
from the circulation, the renal responses are similar to
those observed in other conditions associated with
decreased plasma volume. That is, the kidneys continue
to retain salt and water until plasma volume and arte-
rial pressure are restored to n

a volume and arte-
rial pressure are restored to normal. In some cases,
plasma volume may actually increase above normal
because of increased vascular capacity in cirrhosis;
the high pressures in the portal circulation can 
greatly distend veins and therefore increase vascular
capacity.
Antunes-Rodrigues J, de Castro M, Elias LL, et al: Neu-
84:169, 2004.
Cowley AW Jr: Long-term control of arterial pressure.
Physiol Rev 72:231, 1

ontrol of arterial pressure.
Fitzsimmons JT: Physiology and pathophysiology of thirst
and salt appetite. In Seldin DW, Giebisch G (eds): The
York: Raven Press, 2000, pp 1153-1174.
Giebisch GH: A trail of research on potassium. Kidney Int
62:1498, 2002.
Giebisch G, Hebert SC, Wang WH: New aspects of renal
potassium transport. Pﬂugers Arch 446:289, 2003.
sure natriur

der BT, Llinas M: Mechanisms of pres-
sure natriuresis. Curr Hypertens Rep 4:15, 2002.
Guise TA, Mundy GR: Disorders of calcium metabolism. In
2000, pp 1811-1840.
Seldin DW, Giebisch G

 circulatory homeostasis. In
Nephrol 10(Suppl 12):s258, 1999.
382
Unit V
Murer H, Hernando N, Forster I, Biber J: Regulation of
Na/Pi transporter in the proximal tubule. Annu Rev
Physiol 65:531, 2003.
Rosa R, Epstein FH: Extrarena

hysiol 65:531, 2003.
Rosa R, Epstein FH: Extrarenal potassium metabolism. In
2000, pp 1551-1574.
Suki WN, Lederer ED, Rouse D: Renal transport of calcium
magnesium and phosphate. In: Brenner BM (ed): The
Kidney, 6th ed. Philadelphia: WB Saunders, 2000, pp 520-
574.
Warnock DG. Renal genetic disorders related to K+ and
Mg2+. Annu Rev Physiol 64:845, 2002.
Young DB: Quantitative analysis of aldos

45, 2002.
Young DB: Quantitative analysis of aldosterone’s role in
potassium regulation. Am J Physiol 255: F811, 1988.
Young DB: Analysis of long-term potassium regulation.
Endocr Rev 6:24, 1985.
C
H
A
P
T
E
R
 
3
0
383
Regulation of hydrogen ion (H+) balance is similar
in some ways to the regulation of other ions in the
body. For instance, to achieve homeostasis, there
must be a balance between the intake or production
of H+ and the net removal of H+ from the bod

ction
of H+ and the net removal of H+ from the body.And,
as is true for other ions, the kidneys play a key role
in regulating H+ removal. However, precise control
of extracellular ﬂuid H+ concentration involves
much more than simple elimination of H+ by the kidneys. There are also multi-
ple acid-base buffering mechanisms involving the blood, cells, and lungs that are
essential in maintaining normal H+ concentrations in both the extracellular and
the intracellular ﬂuid.
In this chapter, the vari

In this chapter, the various mechanisms that contribute to the regulation of
H+ concentration are discussed, with special emphasis on the control of renal
H+ secretion and renal reabsorption, production, and excretion of bicarbonate
ions (HCO3
–), one of the key components of acid-base control systems in the
body ﬂuids.
Precise H+ regulation is essential because the activities of almost all enzyme
systems in the body are

ities of almost all enzyme
systems in the body are inﬂuenced by H+ concentration. Therefore, changes in
hydrogen concentration alter virtually all cell and body functions.
Compared with other ions, the H+ concentration of the body ﬂuids normally
is kept at a low level. For example, the concentration of sodium in extracellu-
lar ﬂuid (142 mEq/L) is about 3.5 million times as great as the normal concen-
tration of H+, which averages only 0.00004 mEq/L. Equally important, the
normal variation in H+

q/L. Equally important, the
normal variation in H+ concentration in extracellular ﬂuid is only about one mil-
lionth as great as the normal variation in sodium ion (Na+) concentration.Thus,
the precision with which H+ is regulated emphasizes its importance to the
various cell functions.
A hydrogen ion is a single free proton released from a hydrogen atom. Molecules
containing hydrogen atoms that can release hydrogen ions in solutions are referred
to

release hydrogen ions in solutions are referred
to as acids. An example is hydrochloric acid (HCl), which ionizes in water to form
hydrogen ions (H+) and chloride ions (Cl–). Likewise, carbonic acid (H2CO3) ionizes
in water to form H+ and bicarbonate ions (HCO3
–).
A base is an ion or a molecule that can accept an H+. For example, HCO3
– is a
base because it can combine with H+ to form H2CO3. Likewise, HPO4
= is a base
because it can accept an H+ to form H2PO4
–. The proteins in the body also fu

 to form H2PO4
–. The proteins in the body also function
as bases, because some of the amino acids that make up proteins have net negative
charges that readily accept H+. The protein hemoglobin in the red blood cells and
proteins in the other cells of the body are among the most important of the body’s
bases.
The terms base and alkali are often used synonymously. An alkali is a molecule
formed by the combination of one or more of the alkaline metals—sodium, potas-
sium, lithium, and so forth—wit

als—sodium, potas-
sium, lithium, and so forth—with a highly basic ion such as a hydroxyl ion (OH–).
384
Unit V
The base portion of these molecules reacts quickly with
H+ to remove it from solution; they are, therefore,
typical bases. For similar reasons, the term alkalosis
refers to excess removal of H+ from the body ﬂuids, in
contrast to the excess addition of H+, which is referred
to as acidosis.
Strong and Weak Acids and Bases. A strong acid is one that
rapidly di

ds and Bases. A strong acid is one that
rapidly dissociates and releases especially large
amounts of H+ in solution. An example is HCl. Weak
acids have less tendency to dissociate their ions and,
therefore, release H+ with less vigor. An example is
H2CO3. A strong base is one that reacts rapidly and
strongly with H+ and, therefore, quickly removes these
from a solution. A typical example is OH–, which reacts
with H+ to form water (H2O). A typical weak base is
HCO3
– because it binds with H+ much

 weak base is
HCO3
– because it binds with H+ much more weakly than
does OH–. Most of the acids and bases in the extracel-
lular ﬂuid that are involved in normal acid-base regula-
tion are weak acids and bases.The most important ones
that we discuss in detail are H2CO3 and bicarbonate
base.
Normal Hydrogen Ion Concentration and pH of Body Fluids 
and Changes That Occur in Acidosis and Alkalosis. As dis-
cussed earlier, the blood H+ concentration is normally 
maintained within tight limits around

is normally 
maintained within tight limits around a normal value 
of about 0.00004 mEq/L (40 nEq/L). Normal variations
are only about 3 to 5 nEq/L, but under extreme 
conditions, the H+ concentration can vary from as low
as 10 nEq/L to as high as 160 nEq/L without causing
death.
Because H+ concentration normally is low, and
because these small numbers are cumbersome, it is cus-
tomary to express H+ concentration on a logarithm
scale, using pH units. pH is related to the actual H+ con-
centratio

its. pH is related to the actual H+ con-
centration by the following formula (H+ concentration
[H+] is expressed in equivalents per liter):
For 
example,
the 
normal 
[H+] 
is 
40 nEq/L
(0.00000004 Eq/L). Therefore, the normal pH is
pH = –log [0.00000004]
pH = 7.4
From this formula, one can see that pH is inversely
related to the H+ concentration; therefore, a low pH 
corresponds to a high H+ concentration, and a high pH
corresponds to a low H+ concentration.
The normal pH of arterial blood is 7

oncentration.
The normal pH of arterial blood is 7.4, whereas the
pH of venous blood and interstitial ﬂuids is about 7.35
because of the extra amounts of carbon dioxide (CO2)
released from the tissues to form H2CO3 in these ﬂuids
(Table 30–1). Because the normal pH of arterial blood
is 7.4, a person is considered to have acidosis when the
pH falls below this value and to have alkalosis when the
pH rises above 7.4. The lower limit of pH at which a
person can live more than a few hours is about 6.

person can live more than a few hours is about 6.8, and
the upper limit is about 8.0.
Intracellular pH usually is slightly lower than plasma
pH because the metabolism of the cells produces acid,
especially H2CO3. Depending on the type of cells, the
pH of intracellular ﬂuid has been estimated to range
between 6.0 and 7.4. Hypoxia of the tissues and poor
blood ﬂow to the tissues can cause acid accumulation
and decreased intracellular pH.
pH
log
1
H+
=
[
] = -log H+
[
]
The pH of urine can range f

H+
=
[
] = -log H+
[
]
The pH of urine can range from 4.5 to 8.0, depending
on the acid-base status of the extracellular ﬂuid. As dis-
cussed later, the kidneys play a major role in correcting
abnormalities of extracellular ﬂuid H+ concentration by
excreting acids or bases at variable rates.
An extreme example of an acidic body ﬂuid is the
HCl secreted into the stomach by the oxyntic (parietal)
cells of the stomach mucosa, as discussed in Chapter 64.
The H+ concentration in these cells is about 

 64.
The H+ concentration in these cells is about 4 million
times greater than the hydrogen concentration in blood,
with a pH of 0.8.
In the remainder of this chapter, we discuss the reg-
ulation of extracellular ﬂuid H+ concentration.
Defenses Against Changes in
Hydrogen Ion Concentration:
Buffers, Lungs, and Kidneys
There are three primary systems that regulate the H+
concentration in the body ﬂuids to prevent acidosis or
alkalosis: (1) the chemical acid-base buffer systems of
the body ﬂuids, 

mical acid-base buffer systems of
the body ﬂuids, which immediately combine with acid
or base to prevent excessive changes in H+ concentra-
tion; (2) the respiratory center, which regulates the
removal of CO2 (and, therefore, H2CO3) from the
extracellular ﬂuid; and (3) the kidneys, which can
excrete either acid or alkaline urine, thereby readjust-
ing the extracellular ﬂuid H+ concentration toward
normal during acidosis or alkalosis.
When there is a change in H+ concentration, the
buffer systems

s a change in H+ concentration, the
buffer systems of the body ﬂuids react within a fraction
of a second to minimize these changes. Buffer systems
do not eliminate H+ from or add them to the body 
but only keep them tied up until balance can be re-
established.
The second line of defense, the respiratory system,
also acts within a few minutes to eliminate CO2 and,
therefore, H2CO3 from the body.
These ﬁrst two lines of defense keep the H+ con-
centration from changing too much until the more
slo

ntration from changing too much until the more
slowly responding third line of defense, the kidneys,
can eliminate the excess acid or base from the body.
Although the kidneys are relatively slow to respond
compared with the other defenses, over a period of
hours to several days, they are by far the most power-
ful of the acid-base regulatory systems.
Table 30–1
pH and H+ Concentration of Body Fluids
H+ Concentration (mEq/L)
pH
Arterial blood
4.0 ¥ 10–5
7.40
Venous blood
4.5 ¥ 

4.0 ¥ 10–5
7.40
4.5 ¥ 10–5
7.35
4.5 ¥ 10–5
7.35
1 ¥ 10–3 to 4 ¥ 10–5
6.0 to 7.4
Urine
3 ¥ 10–2 to 1 ¥ 10–5
4.5 to 8.0
Gastric HCl
160
0.8
Chapter 30
385
Buffering of Hydrogen Ions 
A buffer is any substance that can reversibly bind H+.
The general form of the buffering reaction is
In this example, a free H+ combines with the buffer to
form a weak acid (H buffer) that can either rem

to
form a weak acid (H buffer) that can either remain as
an unassociated molecule or dissociate back to buffer
and H+. When the H+ concentration increases, the 
reaction is forced to the right, and more H+ binds to
the buffer, as long as buffer is available. Conversely,
when the H+ concentration decreases, the reaction
shifts toward the left, and H+ is released from the
buffer. In this way, changes in H+ concentration are
minimized.
The importance of the body ﬂuid buffers can be
quickly realized

e of the body ﬂuid buffers can be
quickly realized if one considers the low concentration
of H+ in the body ﬂuids and the relatively large
amounts of acids produced by the body each day. For
example, about 80 milliequivalents of hydrogen is
either ingested or produced each day by metabolism,
whereas the H+ concentration of the body ﬂuids nor-
mally is only about 0.00004 mEq/L.Without buffering,
the daily production and ingestion of acids would
cause huge changes in body ﬂuid H+ concentration.
Th

use huge changes in body ﬂuid H+ concentration.
The action of acid-base buffers can perhaps best be
explained by considering the buffer system that is
quantitatively the most important in the extracellular
ﬂuid—the bicarbonate buffer system.
The bicarbonate buffer system consists of a water solu-
tion that contains two ingredients: (1) a weak acid,
H2CO3, and (2) a bicarbonate salt, such as NaHCO3.
H2CO3 is formed in the body by the reaction of CO2
with H2O.
This reacti

 body by the reaction of CO2
with H2O.
This reaction is slow, and exceedingly small amounts
of H2CO3 are formed unless the enzyme carbonic
anhydrase is present. This enzyme is especially abun-
dant in the walls of the lung alveoli, where CO2 is
released; carbonic anhydrase is also present in the
epithelial cells of the renal tubules, where CO2 reacts
with H2O to form H2CO3.
H2CO3 ionizes weakly to form small amounts of H+
and HCO3
–.
The second component of the system, bicarbonate
salt, occurs p

omponent of the system, bicarbonate
salt, occurs predominantly as sodium bicarbonate
(NaHCO3) in the extracellular ﬂuid.
NaHCO3
ionizes almost completely to form HCO3
– and Na+, as
follows:
NaHCO
Na
HCO
3
+
3
+
-
¨
æ Æ
æ
æ
H C
H
HCO
2
+
3
3
+
-
¨
O
æ Æ
æ
æ
Æ
CO
H O
H CO
2
2
carbonic
anhydrase
2
3
+
¨ææ
ææ
æ Æ
ææ
æ
Buffer
H
H Buffer
+
+
¨
æ Æ
æ
æ
æ Æ
æ
æ
Now, putting the entire system together, we have the
Because of the weak dissociation of H2CO3, the H+
concentration is extremely sma

on of H2CO3, the H+
concentration is extremely small.
When a strong acid such as HCl is added to the
bicarbonate buffer solution, the increased H+ released
from the acid (HCl Æ H+ + Cl–) is buffered by HCO3
–.
≠H+ + HCO3
– Æ H2CO3 Æ CO2 + H2O
As a result, more H2CO3 is formed, causing increased
CO2 and H2O production. From these reactions, one
can see that H+ from the strong acid HCl reacts with
HCO3
– to form the very weak acid H2CO3, which in
turn forms CO2 and H2O.The excess CO2 greatly stim-

urn forms CO2 and H2O.The excess CO2 greatly stim-
ulates respiration, which eliminates the CO2 from the
The opposite reactions take place when a strong
base, such as sodium hydroxide (NaOH), is added to
the bicarbonate buffer solution.
NaOH + H2CO3 Æ NaHCO3 + H2O
In this case, the OH– from the NaOH combines with
H2CO3 to form additional HCO3
–.Thus, the weak base
NaHCO3 replaces the strong base NaOH.At the same
time, the concentration of H2CO3 decreases (because
it reacts wi

entration of H2CO3 decreases (because
it reacts with NaOH), causing more CO2 to combine
with H2O to replace the H2CO3.
The net result, therefore, is a tendency for the CO2
levels in the blood to decrease, but the decreased CO2
in the blood inhibits respiration and decreases the rate
of CO2 expiration.The rise in blood HCO3
– that occurs
is compensated for by increased renal excretion of
HCO3
–.
Quantitative Dynamics of the
All acids, including H2CO3, are ionized to some

em
All acids, including H2CO3, are ionized to some extent.
From mass balance considerations, the concentrations
of H+ and HCO3
– are proportional to the concentration
of H2CO3.
For any acid, the concentration of the acid relative 
to its dissociated ions is deﬁned by the dissociation 
constant K¢.
(1)
This equation indicates that in an H2CO3 solution, the
amount of free H+ is equal to
(2)
H
K
H CO
HCO
+
2
3
3
=
¢ ¥
-
¢ =
¥
-
K
H
HCO
H CO
+
3
2
3
H CO
HCO
H
2
3
+
3
+
-
¨
æ Æ
æ
æ
Æ
CO
H O
H CO
HCO

H CO
HCO
H
2
3
+
3
+
-
¨
æ Æ
æ
æ
Æ
CO
H O
H CO
HCO
H
NaOH
Na
2
2
2
3
3
+
+
≠
+
+
+
-
æ Æ
æ
æ
æ Æ
æ
æ
CO
H O
H CO
H
HCO
+
Na
2
2
2
3
+
3
–
+
+
+
Ï
Ì
Ó
¨
æ Æ
æ
æ
æ Æ
æ
æ æ
¨
æ Æ
æ
æ
Æ
386
Unit V
The concentration of undissociated H2CO3 cannot be
measured in solution because it rapidly dissociates into
CO2 and H2O or to H+ and HCO3
–. However, the CO2
dissolved in the blood is directly proportional to the
amount of undissociated H2CO3. Therefore, equation 2
can be rewrit

ociated H2CO3. Therefore, equation 2
can be rewritten as
(3)
The dissociation constant (K) for equation 3 is only
about 1/400 of the dissociation constant (K¢) of equation
2 because the proportionality ratio between H2CO3 and
CO2 is 1:400.
Equation 3 is written in terms of the total amount of
CO2 dissolved in solution. However, most clinical labo-
ratories measure the blood CO2 tension (PCO2) rather
than the actual amount of CO2. Fortunately, the amount
of CO2 in the blood is a linear function o

 amount
of CO2 in the blood is a linear function of PCO2 times
the solubility coefﬁcient for CO2; under physiologic 
conditions,
the solubility coefﬁcient for CO2
is 
0.03 mmol/mm Hg at body temperature.This means that
0.03 millimole of H2CO3 is present in the blood for each
millimeter of mercury PCO2 measured.Therefore, equa-
tion 3 can be rewritten as
(4)
Henderson-Hasselbalch Equation. As discussed earlier, it is
customary to express H+ concentration in pH units
rather than in actual concentr

tration in pH units
rather than in actual concentrations. Recall that pH is
deﬁned as pH = –log H+.
The dissociation constant can be expressed in a
similar manner.
pK = –log K
Therefore, we can express the H+ concentration in
equation 4 in pH units by taking the negative logarithm
of that equation, which yields
(5)
Therefore,
(6)
Rather than work with a negative logarithm, we can
change the sign of the logarithm and invert the numer-
ator and denominator in the last term, using the law of
logari

ominator in the last term, using the law of
logarithms to yield
(7)
For the bicarbonate buffer system, the pK is 6.1, and
equation 7 can be written as
(8)
Equation 8 is the Henderson-Hasselbalch equation,
and with it, one can calculate the pH of a solution if 
the molar concentration of HCO3
– and the PCO2 are
known.
From the Henderson-Hasselbalch equation, it is
apparent that an increase in HCO3
– concentration
causes the pH to rise, shifting the acid-base balance
pH
6
HCO
P
3
2
=
+
¥
-
.
log .

e acid-base balance
pH
6
HCO
P
3
2
=
+
¥
-
.
log .
1
0 03
co
pH
pK
HCO
P
3
2
=
+
¥
(
)
-
log
.0 03
co
pH
pK
P
HCO
2
3
=
-
¥
(
)
-
log
.0 03
co
-
= -
-
¥
(
)
-
log H
pK
P
HCO
+
2
3
log
log
.0 03
co
H
K
0.03
P
HCO
+
2
3
=
¥
¥
(
)
-
co
H
K
CO
HCO
+
2
3
=
¥
-
toward alkalosis. An increase in PCO2 causes the pH to
decrease, shifting the acid-base balance toward acidosis.
The Henderson-Hasselbalch equation, in addition to
deﬁning the determinants of normal pH regulation and
acid-base balance in the ex

rmal pH regulation and
acid-base balance in the extracellular ﬂuid, provides
insight into the physiologic control of acid and base
composition of the extracellular ﬂuid. As discussed
later, the bicarbonate concentration is regulated mainly
by the kidneys, whereas the PCO2 in extracellular ﬂuid is
controlled by the rate of respiration. By increasing the
rate of respiration, the lungs remove CO2 from the
plasma, and by decreasing respiration, the lungs elevate
PCO2. Normal physiologic acid-base ho

ungs elevate
PCO2. Normal physiologic acid-base homeostasis results
from the coordinated efforts of both of these organs, the
lungs and the kidneys, and acid-base disorders occur
when one or both of these control mechanisms are
impaired, thus altering either the bicarbonate concen-
tration or the PCO2 of extracellular ﬂuid.
When disturbances of acid-base balance result from
a primary change in extracellular ﬂuid bicarbonate con-
centration, they are referred to as metabolic acid-base
disorders. 

are referred to as metabolic acid-base
disorders. Therefore, acidosis caused by a primary
decrease in bicarbonate concentration is termed meta-
bolic acidosis, whereas alkalosis caused by a primary
increase in bicarbonate concentration is called meta-
bolic alkalosis. Acidosis caused by an increase in PCO2
is called respiratory acidosis, whereas alkalosis caused
by a decrease in PCO2 is termed respiratory alkalosis.
Bicarbonate Buffer System Titration Curve. Figure 30–1 shows
the changes in pH o

ation Curve. Figure 30–1 shows
the changes in pH of the extracellular ﬂuid when the
ratio of HCO3
– to CO2 in extracellular ﬂuid is altered.
When the concentrations of these two components are
equal, the right-hand portion of equation 8 becomes the
log of 1, which is equal to 0. Therefore, when the two
components of the buffer system are equal, the pH of
the solution is the same as the pK (6.1) of the bicar-
bonate buffer system.When base is added to the system,
part of the dissolved CO2 is conv

d to the system,
part of the dissolved CO2 is converted into HCO3
–,
causing an increase in the ratio of HCO3
– to CO2 and
increasing the pH, as is evident from the Henderson-
Hasselbalch equation.When acid is added, it is buffered
Acid added
Per cent of buffer in form of
H2CO3 and CO2
0
25
50
75
100
HCO3
-
Base added
0
25
50
75
100
8
7
6
5
4
pH
pK
Normal 
operating
point in body
Figure 30–1
Titration curve for bicarbonate buffer system showing the pH of
extracellul

bonate buffer system showing the pH of
extracellular ﬂuid when the percentages of buffer in the form of
HCO3
– and CO2 (or H2CO3) are altered.
Chapter 30
387
by HCO3
–, which is then converted into dissolved CO2,
decreasing the ratio of HCO3
– to CO2 and decreasing
the pH of the extracellular ﬂuid.
“Buffer Power” Is Determined by the Amount and Relative Con-
centrations of the Buffer Components. From the titration
curve in Figure 30–1, several points are apparent.

curve in Figure 30–1, several points are apparent. First,
the pH of the system is the same as the pK when each
of the components (HCO3
– and CO2) constitutes 50 per
cent of the total concentration of the buffer system.
Second, the buffer system is most effective in the central
part of the curve, where the pH is near the pK of the
system. This means that the change in pH for any given
amount of acid or base added to the system is least when
the pH is near the pK of the system. The buffer system
i

 is near the pK of the system. The buffer system
is still reasonably effective for 1.0 pH unit on either side
of the pK, which for the bicarbonate buffer system
extends from a pH of about 5.1 to 7.1 units. Beyond
these limits, the buffering power rapidly diminishes.
And when all the CO2 has been converted into HCO3
–
or when all the HCO3
– has been converted into CO2, the
system has no more buffering power.
The absolute concentration of the buffers is also an
important factor in determining the 

rs is also an
important factor in determining the buffer power of a
system. With low concentrations of the buffers, only a
small amount of acid or base added to the solution
changes the pH considerably.
Bicarbonate Buffer System Is the Most Important Extracellular
Buffer. From the titration curve shown in Figure 30–1,
one would not expect the bicarbonate buffer system to
be powerful, for two reasons: First, the pH of the extra-
cellular ﬂuid is about 7.4, whereas the pK of the bicar-
bonate buff

bout 7.4, whereas the pK of the bicar-
bonate buffer system is 6.1. This means that there is
about 20 times as much of the bicarbonate buffer
system in the form of HCO3
– as in the form of dis-
solved CO2. For this reason, this system operates on
the portion of the buffering curve where the slope is
low and the buffering power is poor. Second, the con-
centrations of the two elements of the bicarbonate
system, CO2 and HCO3
–, are not great.
Despite these characteristics, the bicarbonate buffer
s

te these characteristics, the bicarbonate buffer
system is the most powerful extracellular buffer in the
body. This apparent paradox is due mainly to the fact
that the two elements of the buffer system, HCO3
– and
CO2,are regulated,respectively,by the kidneys and the
lungs, as discussed later. As a result of this regulation,
the pH of the extracellular ﬂuid can be precisely con-
trolled by the relative rate of removal and addition of
HCO3
– by the kidneys and the rate of removal of CO2
by the lu

e kidneys and the rate of removal of CO2
by the lungs.
Although the phosphate buffer system is not impor-
tant as an extracellular ﬂuid buffer, it plays a major
role in buffering renal tubular ﬂuid and intracellular
ﬂuids.
The main elements of the phosphate buffer system
are H2PO4
– and HPO4
=. When a strong acid such as
HCl is added to a mixture of these two substances, the
hydrogen is accepted by the base HPO4
= and con-
verted to H2PO4
–.
HCl + Na2HPO4 Æ NaH2PO4 + NaCl

–.
The result of this reaction is that the strong acid, HCl,
is replaced by an additional amount of a weak acid,
NaH2PO4, and the decrease in pH is minimized.
When a strong base, such as NaOH, is added to the
buffer system, the OH– is buffered by the H2PO4
– to
form additional amounts of HPO4
= + H2O.
NaOH + NaH2PO4 Æ Na2HPO4 + H2O
In this case, a strong base, NaOH, is traded for a weak
base, NaH2PO4, causing only a slight increase in 
pH.
The phos

4, causing only a slight increase in 
pH.
The phosphate buffer system has a pK of 6.8, which
is not far from the normal pH of 7.4 in the body ﬂuids;
this allows the system to operate near its maximum
buffering power. However, its concentration in the
extracellular ﬂuid is low, only about 8 per cent of the
concentration of the bicarbonate buffer. Therefore,
the total buffering power of the phosphate system in
the extracellular ﬂuid is much less than that of the
bicarbonate buffering system.
In co

an that of the
In contrast to its rather insigniﬁcant role as an extra-
cellular buffer, the phosphate buffer is especially
important in the tubular ﬂuids of the kidneys, for two
reasons: (1) phosphate usually becomes greatly con-
centrated in the tubules, thereby increasing the buffer-
ing power of the phosphate system, and (2) the tubular
ﬂuid usually has a considerably lower pH than 
the extracellular ﬂuid does, bringing the operating
range of the buffer closer t

ringing the operating
range of the buffer closer to the pK (6.8) of the 
system.
The phosphate buffer system is also important in
buffering intracellular ﬂuid because the concentration
of phosphate in this ﬂuid is many times that in the
extracellular ﬂuid. Also, the pH of intracellular ﬂuid is
lower than that of extracellular ﬂuid and therefore is
usually closer to the pK of the phosphate buffer
system compared with the extracellular ﬂuid.
Proteins: Important
Intracellular Buffers
Proteins are a

ns: Important
Proteins are among the most plentiful buffers in the
body because of their high concentrations, especially
within the cells.
The pH of the cells, although slightly lower than in
the extracellular ﬂuid, nevertheless changes approxi-
mately in proportion to extracellular ﬂuid pH changes.
There is a slight amount of diffusion of H+ and HCO3
–
through the cell membrane, although these ions
require several hours to come to equilibrium with the
extracellular ﬂuid, e

come to equilibrium with the
extracellular ﬂuid, except for rapid equilibrium that
occurs in the red blood cells. CO2, however, can rapidly
diffuse through all the cell membranes. This diffusion
of the elements of the bicarbonate buffer system causes
the pH in intracellular ﬂuid to change when there are
changes in extracellular pH. For this reason, the buffer
systems within the cells help prevent changes in the
pH of extracellular ﬂuid but may take several hours to
become maximally effective.
38

ke several hours to
388
Unit V
In the red blood cell, hemoglobin (Hb) is an impor-
tant buffer, as follows:
Approximately 60 to 70 per cent of the total chemi-
cal buffering of the body ﬂuids is inside the cells, and
most of this results from the intracellular proteins.
However, except for the red blood cells, the slowness
with which H+ and HCO3
– move through the cell mem-
branes often delays for several hours the maximum
ability of the in

ys for several hours the maximum
ability of the intracellular proteins to buffer extracel-
lular acid-base abnormalities.
In addition to the high concentration of proteins in
the cells, another factor that contributes to their
buffering power is the fact that the pKs of many of
these protein systems are fairly close to 7.4.
Isohydric Principle: All Buffers 
in a Common Solution Are 
in Equilibrium with the Same
We have been discussing buffer systems as though they
oper

been discussing buffer systems as though they
operated individually in the body ﬂuids. However, they
all work together, because H+ is common to the reac-
tions of all the systems. Therefore, whenever there is a
change in H+ concentration in the extracellular ﬂuid, the
balance of all the buffer systems changes at the same
time. This phenomenon is called the isohydric principle
and is illustrated by the following formula:
K1, K2, K3 are the dissociation constants of three
respective acids, HA1, HA

ation constants of three
respective acids, HA1, HA2, HA3, and A1, A2, A3 are the
concentrations of the free negative ions that constitute
the bases of the three buffer systems.
The implication of this principle is that any condition
that changes the balance of one of the buffer systems
also changes the balance of all the others because the
buffer systems actually buffer one another by shifting
H+ back and forth between them.
Respiratory Regulation 
of Acid-Base Balance
The second line of defense

n 
The second line of defense against acid-base distur-
bances is control of extracellular ﬂuid CO2 concentra-
tion by the lungs.An increase in ventilation eliminates
CO2 from extracellular ﬂuid, which, by mass action,
reduces the H+ concentration. Conversely, decreased
ventilation increases CO2, thus also increasing H+ con-
centration in the extracellular ﬂuid.
Pulmonary Expiration of CO2
Balances Metabolic Formation of CO2
CO2 is formed continually in the body by intracell

CO2 is formed continually in the body by intracellular
metabolic processes.After it is formed, it diffuses from
the cells into the interstitial ﬂuids and blood, and the
H
K
HA
A
K
HA
A
K
HA
A
+
1
1
1
2
2
2
3
3
3
=
¥
=
¥
=
¥
H+ + Hb
HHb
¨
æ Æ
æ
æ
¨
æ Æ
æ
æ
ﬂowing blood transports it to the lungs, where it dif-
fuses into the alveoli and then is transferred to the
atmosphere by pulmonary ventilation. About 1.2 mol/
L of dissolved CO2 normally is in the extracellular
ﬂuid, corresponding to a Pco2 o

 the extracellular
ﬂuid, corresponding to a Pco2 of 40 mm Hg.
If the rate of metabolic formation of CO2 increases,
the Pco2 of the extracellular ﬂuid is likewise increased.
Conversely, a decreased metabolic rate lowers the
Pco2. If the rate of pulmonary ventilation is increased,
CO2 is blown off from the lungs, and the Pco2 in the
extracellular ﬂuid decreases. Therefore, changes in
either pulmonary ventilation or the rate of CO2 for-
mation by the tissues can change the extracellular ﬂuid
Pco2.

e tissues can change the extracellular ﬂuid
Pco2.
Increasing Alveolar Ventilation
Decreases Extracellular Fluid
Hydrogen Ion Concentration and
Raises pH
If the metabolic formation of CO2 remains constant,
the only other factor that affects Pco2 in extracellular
ﬂuid is the rate of alveolar ventilation. The higher the
alveolar ventilation, the lower the Pco2; conversely, the
lower the alveolar ventilation rate, the higher the Pco2.
As discussed previously, when CO2 concentration
increases, the H2

eviously, when CO2 concentration
increases, the H2CO3 concentration and H+ concentra-
tion also increase, thereby lowering extracellular 
ﬂuid pH.
Figure 30–2 shows the approximate changes in
blood pH that are caused by increasing or decreasing
the rate of alveolar ventilation. Note that increasing
alveolar ventilation to about twice normal raises the
pH of the extracellular ﬂuid by about 0.23. If the pH
of the body ﬂuids is 7.40 with normal alveolar ventila-
tion, doubling the ventilation rate 

olar ventila-
tion, doubling the ventilation rate raises the pH to
Normal
pH change in body fluids
+0.3
-0.3
-0.4
-0.5
-0.1
-0.2
+0.2
+0.1
0
0.5
1.0
1.5
2.0
2.5
Rate of alveolar ventilation
(normal = 1)
Figure 30–2
Change in extracellular ﬂuid pH caused by increased or
decreased rate of alveolar ventilation, expressed as times normal.
Chapter 30
389
about 7.63. Conversely, a decrease in alveolar ventila-
tion to one fourth normal reduces the pH by 0.45.That
is, if

e fourth normal reduces the pH by 0.45.That
is, if the pH is 7.4 at a normal alveolar ventilation,
reducing the ventilation to one fourth normal reduces
the pH to 6.95. Because the alveolar ventilation rate
can change markedly, from as low as 0 to as high as 15
times normal, one can easily understand how much the
pH of the body ﬂuids can be changed by the respira-
tory system.
Increased Hydrogen Ion
Concentration Stimulates 
Not only does the alveolar ventilation rate inﬂuen

Not only does the alveolar ventilation rate inﬂuence
H+ concentration by changing the Pco2 of the body
ﬂuids, but the H+ concentration affects the rate of
alveolar ventilation. Thus, Figure 30–3 shows that the
alveolar ventilation rate increases four to ﬁve times
normal as the pH decreases from the normal value of
7.4 to the strongly acidic value of 7.0. Conversely, when
plasma pH rises above 7.4, this causes a decrease in the
ventilation rate. As one can see from the graph, the
change in ventil

s one can see from the graph, the
change in ventilation rate per unit pH change is much
greater at reduced levels of pH (corresponding to ele-
vated H+ concentration) compared with increased
levels of pH. The reason for this is that as the alveolar
ventilation rate decreases, owing to an increase in pH
(decreased H+ concentration), the amount of oxygen
added to the blood decreases and the partial pressure
of oxygen (PO2) in the blood also decreases, which
stimulates the ventilation rate. Therefo

es, which
stimulates the ventilation rate. Therefore, the respira-
tory compensation for an increase in pH is not nearly
as effective as the response to a marked reduction in
pH.
Feedback Control of Hydrogen Ion Concentration by the Respi-
ratory System. Because increased H+ concentration
stimulates respiration, and because increased alveolar
ventilation decreases the H+ concentration, the respi-
ratory system acts as a typical negative feedback con-
troller of H+ concentration.
That is, wheneve

con-
That is, whenever the H+ concentration increases
above normal, the respiratory system is stimulated,
and alveolar ventilation increases. This decreases the
PCO2 in extracellular ﬂuid and reduces H+ concentra-
tion back toward normal. Conversely, if H+ concen-
tration falls below normal, the respiratory center
becomes depressed,alveolar ventilation decreases,and
H+ concentration increases back toward normal.
Efﬁciency of Respiratory Control of Hydrogen Ion Concen

ency of Respiratory Control of Hydrogen Ion Concen-
tration. Respiratory control cannot return the H+
concentration all the way back to normal when a dis-
turbance outside the respiratory system has altered
pH. Ordinarily, the respiratory mechanism for con-
trolling H+ concentration has an effectiveness between
50 and 75 per cent, corresponding to a feedback gain
of 1 to 3. That is, if the H+ concentration is suddenly
increased by adding acid to the extracellular ﬂuid and
pH falls from 7.4 to 7.

the extracellular ﬂuid and
pH falls from 7.4 to 7.0, the respiratory system can
return the pH to a value of about 7.2 to 7.3. This
response occurs within 3 to 12 minutes.
Buffering Power of the Respiratory System. Respiratory reg-
ulation of acid-base balance is a physiologic type of
buffer system because it acts rapidly and keeps the H+
concentration from changing too much until the slowly
responding kidneys can eliminate the imbalance. In
general, the overall buffering power of the respiratory

al, the overall buffering power of the respiratory
system is one to two times as great as the buffering
power of all other chemical buffers in the extracellu-
lar ﬂuid combined. That is, one to two times as much
acid or base can normally be buffered by this mecha-
nism as by the chemical buffers.
Impairment of Lung Function Can Cause Respiratory Acidosis.
We have discussed thus far the role of the normal res-
piratory mechanism as a means of buffering changes
in H+ concentration. However, abnorm

ering changes
in H+ concentration. However, abnormalities of respi-
ration can also cause changes in H+ concentration. For
example, an impairment of lung function, such as
severe emphysema, decreases the ability of the lungs
to eliminate CO2; this causes a buildup of CO2 in the
extracellular ﬂuid and a tendency toward respiratory
acidosis. Also, the ability to respond to metabolic aci-
dosis is impaired because the compensatory reduc-
tions in PCO2 that would normally occur by means of
increased

O2 that would normally occur by means of
increased ventilation are blunted. In these circum-
stances, the kidneys represent the sole remaining phys-
iologic mechanism for returning pH toward normal
after the initial chemical buffering in the extracellular
ﬂuid has occurred.
≠[H+] Æ  ≠Alveolar ventilation
Ø
ØPCO2
–
Alveolar ventilation (normal = 1)
4
3
2
1
0
7.0
7.1
7.2
7.3
7.4
7.5
7.6
pH of arterial blood
Figure 30–3
Effect of blood pH on the rate of alveolar ventilation.
390
Unit V
The Body Flu

e of alveolar ventilation.
390
Unit V
Renal Control of 
Acid-Base Balance
The kidneys control acid-base balance by excreting
either an acidic or a basic urine. Excreting an acidic
urine reduces the amount of acid in extracellular ﬂuid,
whereas excreting a basic urine removes base from the
The overall mechanism by which the kidneys
excrete acidic or basic urine is as follows: Large
numbers of HCO3
– are ﬁltered continuously into the
tubules, and if 

are ﬁltered continuously into the
tubules, and if they are excreted into the urine, this
removes base from the blood.Large numbers of H+ are
also secreted into the tubular lumen by the tubular
epithelial cells, thus removing acid from the blood. If
more H+ is secreted than HCO3
– is ﬁltered, there will
be a net loss of acid from the extracellular ﬂuid. Con-
versely, if more HCO3
– is ﬁltered than H+ is secreted,
there will be a net loss of base.
As discussed previously, each day the body produce

As discussed previously, each day the body produces
about 80 milliequivalents of nonvolatile acids, mainly
from the metabolism of proteins.These acids are called
nonvolatile because they are not H2CO3 and, there-
fore, cannot be excreted by the lungs. The primary
mechanism for removal of these acids from the body
is renal excretion. The kidneys must also prevent the
loss of bicarbonate in the urine, a task that is quanti-
tatively more important than the excretion of non-
volatile acids. Each da

than the excretion of non-
volatile acids. Each day the kidneys ﬁlter about 4320
milliequivalents of bicarbonate (180 L/day ¥ 24 mEq/
L); under normal conditions, almost all this is reab-
sorbed from the tubules, thereby conserving the
primary buffer system of the extracellular ﬂuid.
As discussed later, both the reabsorption of bicar-
bonate and the excretion of H+ are accomplished
through the process of H+ secretion by the tubules.
Because the HCO3
– must react with a secreted H+ to
form H2CO3 

CO3
form H2CO3 before it can be reabsorbed, 4320 mil-
liequivalents of H+ must be secreted each day just to
reabsorb the ﬁltered bicarbonate. Then an additional
80 milliequivalents of H+ must be secreted to rid the
body of the nonvolatile acids produced each day, for a
total of 4400 milliequivalents of H+ secreted into the
tubular ﬂuid each day.
When there is a reduction in the extracellular ﬂuid
H+ concentration (alkalosis), the kidneys fail to reab-
sorb all 

n (alkalosis), the kidneys fail to reab-
sorb all the ﬁltered bicarbonate, thereby increasing the
excretion of bicarbonate. Because HCO3
– normally
buffers hydrogen in the extracellular ﬂuid, this loss of
bicarbonate is the same as adding an H+ to the extra-
cellular ﬂuid. Therefore, in alkalosis, the removal of
HCO3
– raises the extracellular ﬂuid H+ concentration
back toward normal.
In acidosis, the kidneys do not excrete bicarbonate
into the urine but reabsorb all the ﬁltered bicarbonate
and 

rine but reabsorb all the ﬁltered bicarbonate
and produce new bicarbonate, which is added back to
the extracellular ﬂuid. This reduces the extracellular
ﬂuid H+ concentration back toward normal.
Thus, the kidneys regulate extracellular ﬂuid H + con-
centration through three fundamental mechanisms: (1)
secretion of H +, (2) reabsorption of ﬁltered HCO3
-, and
(3) production of new HCO3
-. All these processes are
accomplished through the same basic mechanism, as
discussed in the next few sections.

 mechanism, as
Secretion of Hydrogen Ions
and Reabsorption of
Bicarbonate Ions by the 
Hydrogen ion secretion and bicarbonate reabsorption
occur in virtually all parts of the tubules except the
descending and ascending thin limbs of the loop of
Henle. Figure 30–4 summarizes bicarbonate reabsorp-
tion along the tubule. Keep in mind that for each bicar-
bonate reabsorbed, an H+ must be secreted.
About 80 to 90 per cent of the bicarbonate reab-
sorp

ut 80 to 90 per cent of the bicarbonate reab-
sorption (and H+ secretion) occurs in the proximal
tubule, so that only a small amount of bicarbonate
ﬂows into the distal tubules and collecting ducts. In the
85%
(3672 mEq/day)
>4.9%
(215 mEq/day)
(1 mEq/day)
4320 mEq/day
10%
(432 mEq/day)
Figure 30–4
Reabsorption of bicarbonate in differ-
ent segments of the renal tubule. The
percentages of the ﬁltered load of
bicarbonate absorbed by the various
tubular segments are shown, as well
as the number of

bular segments are shown, as well
as the number of milliequivalents 
reabsorbed per day under normal 
Chapter 30
391
thick ascending loop of Henle, another 10 per cent of
the ﬁltered bicarbonate is reabsorbed, and the remain-
der of the reabsorption takes place in the distal tubule
and collecting duct.As discussed previously, the mech-
anism by which bicarbonate is reabsorbed also
involves tubular secretion of H+, but different tubular
segments accompl

tion of H+, but different tubular
segments accomplish this task differently.
Hydrogen Ions Are Secreted 
by Secondary Active Transport 
in the Early Tubular Segments
The epithelial cells of the proximal tubule, the thick
segment of the ascending loop of Henle, and the early
distal tubule all secrete H+ into the tubular ﬂuid 
by sodium-hydrogen counter-transport, as shown in
Figure 30–5. This secondary active secretion of H+
is coupled with the transport of Na+ into the cell at 
the luminal membr

ansport of Na+ into the cell at 
the luminal membrane by the sodium-hydrogen
exchanger protein, and the energy for H+ secretion
against a concentration gradient is derived from the
sodium gradient favoring Na+ movement into the cell.
This gradient is established by the sodium-potassium
adenosine triphosphatase (ATPase) pump in the baso-
lateral membrane. More than 90 per cent of the bicar-
bonate is reabsorbed in this manner, requiring about
3900 milliequivalents of H+ to be secreted each day by

 milliequivalents of H+ to be secreted each day by
the tubules. This mechanism, however, does not estab-
lish a very high H+ concentration in the tubular ﬂuid;
the tubular ﬂuid becomes very acidic only in the col-
lecting tubules and collecting ducts.
Figure 30–5 shows how the process of H+ secretion
achieves bicarbonate reabsorption. The secretory
process begins when CO2 either diffuses into the
tubular cells or is formed by metabolism in the tubular
epithelial cells. CO2, under the inﬂuence of

bular
epithelial cells. CO2, under the inﬂuence of the enzyme
carbonic anhydrase, combines with H2O to form
H2CO3, which dissociates into HCO3
– and H+. The H+
is secreted from the cell into the tubular lumen by
sodium-hydrogen counter-transport. That is, when an
Na+ moves from the lumen of the tubule to the inte-
rior of the cell, it ﬁrst combines with a carrier protein
in the luminal border of the cell membrane; at the
same time, an H+ in the interior of the cells combines
with the carrier pro

nterior of the cells combines
with the carrier protein. The Na+ moves into the cell
down a concentration gradient that has been estab-
lished by the sodium-potassium ATPase pump in the
basolateral membrane. The gradient for Na+ move-
ment into the cell then provides the energy for moving
H+ in the opposite direction from the interior of the
cell to the tubular lumen.
The HCO3
– generated in the cell (when H+ dissoci-
ates from H2CO3) then moves downhill across the
basolateral membrane into the r

ownhill across the
basolateral membrane into the renal interstitial ﬂuid
and the peritubular capillary blood. The net result is
that for every H+ secreted into the tubular lumen, an
HCO3
– enters the blood.
Filtered Bicarbonate Ions Are
Reabsorbed by Interaction with
Hydrogen Ions in the Tubules
Bicarbonate ions do not readily permeate the luminal
membranes of the renal tubular cells; therefore,
HCO3
– that is ﬁltered by the glomerulus cannot be
directly reabsorbed. Instead, HCO3
– is reabsorbed

– is reabsorbed by
a special process in which it ﬁrst combines with H+ to
form H2CO3, which eventually becomes CO2 and H2O,
as shown in Figure 30–5.
This reabsorption of HCO3
– is initiated by a reaction
in the tubules between HCO3
– ﬁltered at the glomeru-
lus and H+ secreted by the tubular cells. The H2CO3
formed then dissociates into CO2 and H2O. The CO2
can move easily across the tubular membrane; there-
fore, it instantly diffuses into the tubular cell, wh

e, it instantly diffuses into the tubular cell, where it
recombines with H2O, under the inﬂuence of carbonic
anhydrase, to generate a new H2CO3 molecule. This
H2CO3 in turn dissociates to form HCO3
– and H+; the
HCO3
– then diffuses through the basolateral mem-
brane into the interstitial ﬂuid and is taken up into the
peritubular capillary blood. The transport of HCO3
across the basolateral membrane is facilitated by 
two mechanisms: (1) Na+-HCO3
– co-transport and (2)
Cl–-HCO3
– exchange.
Thus,

Cl–-HCO3
Thus, each time an H + is formed in the tubular epithe-
lial cells, an HCO3
- is also formed and released back
into the blood. The net effect of these reactions is
“reabsorption” of HCO3
– from the tubules, although
the HCO3
– that actually enters the extracellular ﬂuid
is not the same as that ﬁltered into the tubules. The
reabsorption of ﬁltered HCO3
– does not result in net
secretion of H+ because the secreted H+ combines with
the ﬁltered HCO3
– and

e secreted H+ combines with
– and is therefore not excreted.
HCO3
- + H+
HCO3
- + H+
ATP
Na+
Na+
K+
K+
Na+
Na+
Na+ + HCO3
-
Na+ + HCO3
-
H+
H+
+
CO2 + H2O
CO2 + H2O
H2CO3
H2CO3
H2O
H2O
Renal
fluid
Renal
fluid
Tubular
lumen
Tubular
lumen
Carbonic
anhydrase
Carbonic
anhydrase
Tubular cells
CO2
CO2
CO2
CO2
H2CO3
H2CO3
Figure 30–5
Cellular mechanisms for (1) active secretion of hydrogen ions into
the renal tubule; (2) tubular reabsorption of b

to
the renal tubule; (2) tubular reabsorption of bicarbonate ions by
combination with hydrogen ions to form carbonic acid, which dis-
sociates to form carbon dioxide and water; and (3) sodium ion
reabsorption in exchange for hydrogen ions secreted. This pattern
of hydrogen ion secretion occurs in the proximal tubule, the thick
ascending segment of the loop of Henle, and the early distal
tubule.
392
Unit V
Bicarbonate Ions Are “Titrated” Against Hydrogen Ions in the
Tu

ons Are “Titrated” Against Hydrogen Ions in the
Tubules. Under normal conditions, the rate of tubular
H+ secretion is about 4400 mEq/day, and the rate of ﬁl-
tration by HCO3
– is about 4320 mEq/day. Thus, the
quantities of these two ions entering the tubules are
almost equal, and they combine with each other to
form CO2 and H2O. Therefore, it is said that HCO3
–
and H+ normally “titrate” each other in the tubules.
The titration process is not quite exact because there
is usually a slight excess 

te exact because there
is usually a slight excess of H+ in the tubules to be
excreted in the urine. This excess H+ (about 80 mEq/
day) rids the body of nonvolatile acids produced by
metabolism. As discussed later, most of this H+ is 
not excreted as free H+ but rather in combination with
other urinary buffers,
especially phosphate and
ammonia.
When there is an excess of HCO3
– over H+ in the
urine, as occurs in metabolic alkalosis, the excess
HCO3
– cannot be reabsorbed; therefore, the excess
HC

3
HCO3
– is left in the tubules and eventually excreted
into the urine, which helps correct the metabolic 
alkalosis.
In acidosis, there is excess H+ relative to HCO3
–,
causing complete reabsorption of the bicarbonate;
the excess H+ passes into the urine. The excess H+ is
buffered in the tubules by phosphate and ammonia
and eventually excreted as salts. Thus, the basic mech-
anism by which the kidneys correct either acidosis or
alkalosis is incomple

s correct either acidosis or
alkalosis is incomplete titration of H+ against HCO3
–,
leaving one or the other to pass into the urine and be
removed from the extracellular ﬂuid.
Primary Active Secretion of Hydrogen
Ions in the Intercalated Cells of Late
Distal and Collecting Tubules
Beginning in the late distal tubules and continuing
through the remainder of the tubular system, the
tubular epithelium secretes H+ by primary active
transport. The characteristics of this transport are dif-
ferent fr

aracteristics of this transport are dif-
ferent from those discussed for the proximal tubule,
loop of Henle, and early distal tubule.
The mechanism for primary active H+ secretion is
shown in Figure 30–6. It occurs at the luminal mem-
brane of the tubular cell, where H+ is transported
directly by a speciﬁc protein, a hydrogen-transporting
ATPase. The energy required for pumping the H+ is
derived from the breakdown of ATP to adenosine
diphosphate.
Primary active secretion of H+ occurs in a specia

Primary active secretion of H+ occurs in a special
type of cell called the intercalated cells of the late distal
tubule and in the collecting tubules. Hydrogen ion
secretion in these cells is accomplished in two steps:
(1) the dissolved CO2 in this cell combines with H2O
to form H2CO3, and (2) the H2CO3 then dissociates
into HCO3
–, which is reabsorbed into the blood, plus
H+, which is secreted into the tubule by means of the
hydrogen-ATPase mechanism. For each H+ secreted,
an HCO3
– is reabsor

hanism. For each H+ secreted,
an HCO3
– is reabsorbed, similar to the process in the
proximal tubules.The main difference is that H+ moves
across the luminal membrane by an active H+ pump
instead of by counter-transport, as occurs in the early
parts of the nephron.
Although the secretion of H+ in the late distal tubule
and collecting tubules accounts for only about 5 per
cent of the total H+ secreted, this mechanism is impor-
tant in forming a maximally acidic urine. In the prox-
imal tubules, H

ximally acidic urine. In the prox-
imal tubules, H+ concentration can be increased only
about threefold to fourfold, and the tubular ﬂuid pH
can be reduced to only about 6.7, although large
amounts of H+ are secreted by this nephron segment.
However, H+ concentration can be increased as much
as 900-fold in the collecting tubules.This decreases the
pH of the tubular ﬂuid to about 4.5, which is the lower
limit of pH that can be achieved in normal kidneys.
Combination of Excess
Hydrogen Ions with P

idneys.
Hydrogen Ions with Phosphate
and Ammonia Buffers in 
the Tubule—A Mechanism 
for Generating “New”
When H+ is secreted in excess of the bicarbonate ﬁl-
tered into the tubular ﬂuid, only a small part of the
excess H+ can be excreted in the ionic form (H+) in the
urine. The reason for this is that the minimal urine pH
is about 4.5, corresponding to an H+ concentration 
of 10–4.5 mEq/L, or 0.03 mEq/L. Thus, for each liter of
urine formed, a maximum of o

us, for each liter of
urine formed, a maximum of only about 0.03 mil-
liequivalent of free H+ can be excreted. To excrete the
80 milliequivalents of nonvolatile acid formed by
metabolism each day, about 2667 liters of urine would
have to be excreted if the H+ remained free in 
solution.
ATP
Cl-
Cl-
Cl-
Cl-
Cl-
Cl-
Carbonic
anhydrase
Carbonic
anhydrase
HCO3
- + H+
HCO3
- + H+
H+
H+
+
H2CO3
H2CO3
H2O
H2O
CO2
CO2
CO2
CO2
Renal
fluid
Renal
fluid

Renal
fluid
Renal
fluid
Tubular
lumen
Tubular
lumen
Figure 30–6
Primary active secretion of hydrogen ions through the luminal
membrane of the intercalated epithelial cells of the late distal and
collecting tubules. Note that one bicarbonate ion is absorbed for
each hydrogen ion secreted, and a chloride ion is passively
secreted along with the hydrogen ion.
Chapter 30
393
The excretion of large amounts of H+ (on occasion
as much as 500 mEq

arge amounts of H+ (on occasion
as much as 500 mEq/day) in the urine is accomplished
primarily by combining the H+ with buffers in the
tubular ﬂuid. The most important buffers are phos-
phate buffer and ammonia buffer. There are other
weak buffer systems, such as urate and citrate, that are
much less important.
When H+ is titrated in the tubular ﬂuid with HCO3
–,
this results in the reabsorption of one HCO3
– for each
H+ secreted, as discussed earlier. But when there are
excess H+ in the urine, 

rlier. But when there are
excess H+ in the urine, they combine with buffers other
than HCO3
–, and this results in the generation of new
HCO3
– that can also enter the blood.Thus, when there
is excess H+ in the extracellular ﬂuid, the kidneys not
only reabsorb all the ﬁltered HCO3
– but also generate
new HCO3
–, thereby helping to replenish the HCO3
–
lost from the extracellular ﬂuid in acidosis. In the next
two sections, we discuss the mechanisms by which
phosphate and ammonia buffers contribut

s by which
phosphate and ammonia buffers contribute to the gen-
eration of new HCO3
–.
Phosphate Buffer System Carries
Excess Hydrogen Ions into the Urine
and Generates New Bicarbonate
The phosphate buffer system is composed of HPO4
=
and H2PO4
–. Both become concentrated in the tubular
ﬂuid because of their relatively poor reabsorption and
because of the reabsorption of water from the tubular
ﬂuid. Therefore, although phosphate is not an impor-
tant extracellular ﬂuid buffer, it is much more ef

tant extracellular ﬂuid buffer, it is much more effective
as a buffer in the tubular ﬂuid.
Another factor that makes phosphate important as
a tubular buffer is the fact that the pK of this system
is about 6.8. Under normal conditions, the urine is
slightly acidic, and the urine pH is near the pK of the
phosphate buffer system.Therefore, in the tubules, the
phosphate buffer system normally functions near its
most effective range of pH.
Figure 30–7 shows the sequence of events by which
H+ is excre

 shows the sequence of events by which
H+ is excreted in combination with phosphate buffer
and the mechanism by which new bicarbonate is added
to the blood. The process of H+ secretion into the
tubules is the same as described earlier. As long as
there is excess HCO3
– in the tubular ﬂuid, most of the
secreted H+ combines with HCO3
–. However, once all
the HCO3
– has been reabsorbed and is no longer avail-
able to combine with H+, any excess H+ can combine
with HPO4
= and other tubular buffers. 

an combine
with HPO4
= and other tubular buffers. After the H+
combines with HPO4
= to form H2PO4
–, it can be
excreted as a sodium salt (NaH2PO4), carrying with it
the excess hydrogen.
There is one important difference in this sequence
of H+ excretion from that discussed previously. In this
case, the HCO3
– that is generated in the tubular cell
and enters the peritubular blood represents a net gain
of HCO3
– by the blood, rather than merely a replace-
ment of ﬁltered HCO3
–. Therefore, whenever

place-
–. Therefore, whenever an H+
secreted into the tubular lumen combines with a buffer
other than HCO3
-, the net effect is addition of a new
HCO3
- to the blood. This demonstrates one of the
mechanisms by which the kidneys are able to replen-
ish the extracellular ﬂuid stores of HCO3
–.
Under normal conditions, much of the ﬁltered phos-
phate is reabsorbed, and only about 30 to 40 mEq/day
is available for buffering H+. Therefore, much of the
buffering of excess H+ in th

herefore, much of the
buffering of excess H+ in the tubular ﬂuid in acidosis
occurs through the ammonia buffer system.
Excretion of Excess Hydrogen Ions
and Generation of New Bicarbonate
by the Ammonia Buffer System
A second buffer system in the tubular ﬂuid that is even
more important quantitatively than the phosphate
buffer system is composed of ammonia (NH3) and the
ammonium ion (NH4
+). Ammonium ion is synthesized
from glutamine, which comes mainly from the metab-
olism of amino acids in the

mainly from the metab-
olism of amino acids in the liver. The glutamine deliv-
ered to the kidneys is transported into the epithelial
cells of the proximal tubules, thick ascending limb of
the loop of Henle, and distal tubules (Figure 30–8).
Once inside the cell, each molecule of glutamine is
metabolized in a series of reactions to ultimately form
two NH4
+ and two HCO3
–. The NH4
+ is secreted into
the tubular lumen by a counter-transport mechanism
in exchange for sodium, which is reabsorbed. T

ism
in exchange for sodium, which is reabsorbed. The
HCO3
– is transported across the basolateral mem-
brane, along with the reabsorbed Na+, into the inter-
stitial ﬂuid and is taken up by the peritubular
capillaries. Thus, for each molecule of glutamine
metabolized in the proximal tubules, two NH4
+ are
secreted into the urine and two HCO3
– are reabsorbed
into the blood. The HCO3
– generated by this process
constitutes new bicarbonate.
In the collecting tubules, the addition of NH4
+ to the
tu

ollecting tubules, the addition of NH4
+ to the
tubular ﬂuids occurs through a different mechanism
(Figure 30–9). Here, H+ is secreted by the tubular
ATP
Na+
Na+
K+
K+
Na+
Na+
Na+
Na+
Na+ + NaHPO4
- 
- 
H+ + NaHPO4
-
H+ + NaHPO4
-
HCO3
-
HCO3
-
HCO3
- + H+
HCO3
- + H+
+
NaH2PO4
NaH2PO4
H2CO3
H2CO3
H2O
H2O
Carbonic
anhydrase
Carbonic
anhydrase
CO2
CO2
CO2
CO2
Renal
fluid
Renal
fluid
Tubular
lumen
Tubular
lumen
Figure 30–7
Bufferin

d
Tubular
lumen
Tubular
lumen
Buffering of secreted hydrogen ions by ﬁltered phosphate
(NaHPO4
–). Note that a new bicarbonate ion is returned to the
blood for each NaHPO4
– that reacts with a secreted hydrogen ion.
394
Unit V
membrane into the lumen, where it combines with
NH3 to form NH4
+, which is then excreted. The col-
lecting ducts are permeable to NH3, which can easily
diffuse into the tubular lumen. However, the luminal
membrane of this part of th

. However, the luminal
membrane of this part of the tubules is much less per-
meable to NH4
+; therefore, once the H+ has reacted
with NH3 to form NH4
+, the NH4
+ is trapped in the
tubular lumen and eliminated in the urine. For each
NH4
+ excreted, a new HCO3
- is generated and added to
the blood.
Chronic Acidosis Increases NH4
+ Excretion. One of the most
important features of the renal ammonium-ammonia
buffer system is that it is subject to physiologic control.
An increase in extracellular ﬂu

ysiologic control.
An increase in extracellular ﬂuid H+ concentration
stimulates renal glutamine metabolism and, therefore,
increases the formation of NH4
+ and new HCO3
– to be
used in H+ buffering; a decrease in H+ concentration
has the opposite effect.
Under normal conditions, the amount of H+ elimi-
nated by the ammonia buffer system accounts for
about 50 per cent of the acid excreted and 50 per cent
of the new HCO3
– generated by the kidneys. However,
with chronic acidosis, the rate of NH4

. However,
+ excretion can
increase to as much as 500 mEq/day. Therefore, with
chronic acidosis, the dominant mechanism by which
acid is eliminated is excretion of NH4
+. This also pro-
vides the most important mechanism for generating
new bicarbonate during chronic acidosis.
Quantifying Renal 
Acid-Base Excretion
Based on the principles discussed earlier, we can quan-
titate the kidneys’ net excretion of acid or net addition
or elimination of bicarbonate f

id or net addition
or elimination of bicarbonate from the blood as
follows.
Bicarbonate excretion is calculated as the urine ﬂow
rate multiplied by urinary bicarbonate concentration.
This number indicates how rapidly the kidneys are
removing HCO3
– from the blood (which is the same as
adding an H+ to the blood). In alkalosis, the loss of
HCO3
– helps return the plasma pH toward normal.
The amount of new bicarbonate contributed to 
the blood at any given time is equal to the amount of
H+ secreted

y given time is equal to the amount of
H+ secreted that ends up in the tubular lumen with
nonbicarbonate urinary buffers. As discussed previ-
ously, the primary sources of nonbicarbonate urinary
buffers are NH4
+ and phosphate. Therefore, the
amount of HCO3
– added to the blood (and H+ excreted
by 
NH4
+) 
is 
calculated 
by 
measuring 
NH4
+
excretion (urine ﬂow rate multiplied by urinary NH4
+
concentration).
The rest of the nonbicarbonate, non-NH4
+ buffer
excreted in the urine is measured by

-NH4
+ buffer
excreted in the urine is measured by determining a
value known as titratable acid. The amount of titrat-
able acid in the urine is measured by titrating the urine
with a strong base, such as NaOH, to a pH of 7.4, the
pH of normal plasma, and the pH of the glomerular
ﬁltrate.This titration reverses the events that occurred
in the tubular lumen when the tubular ﬂuid was
titrated by excreted H+. Therefore, the number of mil-
liequivalents of NaOH required to return the urinary
pH to 7

nts of NaOH required to return the urinary
pH to 7.4 equals the number of milliequivalents of H+
added to the tubular ﬂuid that combined with phos-
phate and other organic buffers. The titratable acid
measurement does not include H+ in association with
NH4
+, because the pK of the ammonia-ammonium
reaction is 9.2, and titration with NaOH to a pH of 7.4
does not remove the H+ from NH4
+.
Thus, the net acid excretion by the kidneys can be
assessed as
Net acid excretion = NH4
+ excretion + Urinary 

as
titratable acid – Bicarbonate excretion
Cl-
Cl-
+ Cl-
+ Cl-
Na+
Na+
Na+
Na+
Renal
fluid
Renal
fluid
Tubular
lumen
Tubular
lumen
Proximal
tubular cells
Proximal
Glutamine
Glutamine
Glutamine
Glutamine
Glutamine
Glutamine
2HCO3
-
2HCO3
-
2NH4
+
2NH4
+
NH4
+
NH4
+
NH4
+
NH4
+
Figure 30–8
Production and secretion of ammonium ion (NH4
+) by proximal
tubular cells. Glutamine is metabolized in the cell, yielding 

s. Glutamine is metabolized in the cell, yielding NH4
+
and bicarbonate. The NH4
+ is secreted into the lumen by a
sodium-NH4
+ pump. For each glutamine molecule metabolized,
two NH4
+ are produced and secreted and two HCO3
– are returned
to the blood.
K+
K+
Carbonic
anhydrase
Carbonic
anhydrase
ATP
ATP
Na+
Na+
NH3
NH3
Cl-
Cl-
Renal
fluid
Renal
fluid
Tubular
lumen
Tubular
lumen
HCO3
- + H+
HCO3
- + H+
NH3
NH3
NH4
+ + Cl-

 cells
HCO3
- + H+
HCO3
- + H+
NH3
NH3
NH4
+ + Cl-
NH4
+ + Cl-
H+
H+
+
H2CO3
H2CO3
H2O
H2O
CO2
CO2
CO2
CO2
Figure 30–9
Buffering of hydrogen ion secretion by ammonia (NH3) in the col-
lecting tubules. Ammonia diffuses into the tubular lumen, where it
reacts with secreted hydrogen ions to form NH4
+, which is then
excreted. For each NH4
– is formed in the
tubular cells and returned to the blood.
Chapter 30
395
The reason we subtract bicarbona

-Base Balance
395
The reason we subtract bicarbonate excretion is that
the loss of HCO3
– is the same as the addition of H+ to
the blood. To maintain acid-base balance, the net acid
excretion must equal the nonvolatile acid production
in the body. In acidosis, the net acid excretion increases
markedly, especially because of increased NH4
+ excre-
tion, thereby removing acid from the blood. The net
acid excretion also equals the rate of net HCO3
– addi-
tion to the blood. Therefore, in acidosis, 

 addi-
tion to the blood. Therefore, in acidosis, there is a net
addition of HCO3
– back to the blood as more NH4
+ and
urinary titratable acid are excreted.
In alkalosis, titratable acid and NH4
+ excretion drop
to 0, whereas HCO3
– excretion increases. Therefore, in
alkalosis, there is a negative net acid secretion. This
means that there is a net loss of HCO3
– from the blood
(which is the same as adding H+ to the blood) and that
no new HCO3
– is generated by the kidneys.
Regulation of Renal T

is generated by the kidneys.
Regulation of Renal Tubular Hydrogen
Ion Secretion
As discussed earlier, H+ secretion by the tubular
epithelium is necessary for both HCO3
– reabsorption
and generation of new HCO3
– associated with titrat-
able acid formation. Therefore, the rate of H+ secre-
tion must be carefully regulated if the kidneys are to
effectively perform their functions in acid-base home-
ostasis. Under normal conditions, the kidney tubules
must secrete at least enough H+ to reabsorb alm

es
must secrete at least enough H+ to reabsorb almost all
the HCO3
– that is ﬁltered, and there must be enough
H+ left over to be excreted as titratable acid or NH4
+
to rid the body of the nonvolatile acids produced each
day from metabolism.
In alkalosis, tubular secretion of H+ must be reduced
to a level that is too low to achieve complete HCO3
–
reabsorption, enabling the kidneys to increase HCO3
–
excretion. In this condition, titratable acid and
ammonia are not excreted because there is no 

 and
ammonia are not excreted because there is no excess
H+ available to combine with nonbicarbonate buffers;
therefore, there is no new HCO3
– added to the 
urine in alkalosis. During acidosis, the tubular H+
secretion must be increased sufﬁciently to reabsorb 
all the ﬁltered HCO3
– and still have enough H+ left
over to excrete large amounts of NH4
+ and titratable
acid, thereby contributing large amounts of new
HCO3
– to the total body extracellular ﬂuid. The 
most important stimuli for incre

llular ﬂuid. The 
most important stimuli for increasing H+ secretion by
the tubules in acidosis are (1) an increase in PCO2
of the extracellular ﬂuid and (2) an increase in H+
concentration of the extracellular ﬂuid (decreased 
pH).
The tubular cells respond directly to an increase in
PCO2 of the blood, as occurs in respiratory acidosis,
with an increase in the rate of H+ secretion as follows:
The increased PCO2 raises the PCO2 of the tubular cells,
causing increased formation of H+ in the tubul

ls,
causing increased formation of H+ in the tubular 
cells, which in turn stimulates the secretion of H+. The
second factor that stimulates H+ secretion is an
increase in extracellular ﬂuid H+
(decreased pH).
A special factor that can increase H+ secretion under
some pathophysiologic conditions is excessive aldos-
terone secretion. Aldosterone stimulates the secretion
of H+ by the intercalated cells of the collecting duct.
Therefore, oversecretion of aldosterone, as occurs in
Conn

e, oversecretion of aldosterone, as occurs in
Conn’s syndrome, can cause excessive secretion of H+
into the tubular ﬂuid and, consequently, increased
amounts of bicarbonate added back to the blood. This
usually causes alkalosis in patients with excessive
aldosterone secretion.
The tubular cells usually respond to a decrease in H+
concentration (alkalosis) by reducing H+ secretion.
The decreased H+ secretion results from decreased
extracellular PCO2, as occurs in respiratory alkalosis, or
from a 

O2, as occurs in respiratory alkalosis, or
from a decrease in H+ concentration per se, as occurs
in both respiratory and metabolic alkalosis.
Table 30–2 summarizes the major factors that inﬂu-
ence H+ secretion and HCO3
– reabsorption. Some of
these are not directly related to the regulation of acid-
base balance. For example, H+ secretion is coupled 
to Na+ reabsorption by the Na+-H+ exchanger in the
proximal tubule and thick ascending loop of Henle.
Therefore, factors that stimulate Na+ reabso

enle.
Therefore, factors that stimulate Na+ reabsorption,
such as decreased extracellular ﬂuid volume, may also
secondarily increase H+ secretion.
Extracellular ﬂuid volume depletion stimulates
sodium reabsorption by the renal tubules and
increases H+
secretion and HCO3
–
through multiple mechanisms, including (1) increased
angiotensin II levels,which directly stimulate the activ-
ity of the Na+-H+ exchanger in the renal tubules, and
(2) increased aldosterone levels, which stimulate

(2) increased aldosterone levels, which stimulate H+
secretion by the intercalated cells of the cortical col-
lecting tubules. Therefore, extracellular ﬂuid volume
depletion tends to cause alkalosis due to excess H+
– reabsorption.
Changes in plasma potassium concentration can
also inﬂuence H+ secretion, with hypokalemia stimu-
lating and hyperkalemia inhibiting H+ secretion in the
proximal tubule. A decreased plasma potassium con-
centration tends to increase the H+ concentr

 con-
centration tends to increase the H+ concentration in
the renal tubular cells. This, in turn, stimulates H+
– reabsorption and leads to alka-
losis. Hyperkalemia decreases H+ secretion and HCO3
–
reabsorption and tends to cause acidosis.
Table 30–2
Factors That Increase or Decrease H+ Secretion and HCO3
-
Reabsorption by the Renal Tubules
Increase H+ Secretion and 
Decrease H+ Secretion and 
HCO3
– Reabsorption 
HCO3
– Reabsorption
≠PCO2
Ø PCO2
≠H+, Ø HCO3
–
Ø H+, ≠HCO3
–

bsorption
≠PCO2
Ø PCO2
≠H+, Ø HCO3
–
Ø H+, ≠HCO3
–
≠Angiotensin II
≠Aldosterone
Ø Aldosterone
396
Unit V
Renal Correction of Acidosis—
Increased Excretion of
Hydrogen Ions and Addition 
of Bicarbonate Ions to the
Now that we have described the mechanisms by which
the kidneys secrete H+ and reabsorb HCO3
–, we can
explain how the kidneys readjust the pH 

–, we can
explain how the kidneys readjust the pH of the extra-
cellular ﬂuid when it becomes abnormal.
Referring 
to 
equation 
8,
the 
Henderson-
Hasselbalch equation, we can see that acidosis occurs
when the ratio of HCO3
– to CO2 in the extracellular
ﬂuid decreases, thereby decreasing pH. If this ratio
decreases because of a fall in HCO3
–, the acidosis is
referred to as metabolic acidosis. If the pH falls
because of an increase in PCO2, the acidosis is referred
to as respiratory acidosis.
A

acidosis is referred
Acidosis Decreases the Ratio of
HCO3
-/H+ in Renal Tubular Fluid
Both respiratory and metabolic acidosis cause a
decrease in the ratio of HCO3
– to H+ in the renal
tubular ﬂuid.As a result, there is an excess of H+ in the
renal tubules,causing complete reabsorption of HCO3
–
and still leaving additional H+ available to combine
with the urinary buffers NH4
+ and HPO4
=. Thus, in aci-
dosis, the kidneys reabsorb all the ﬁltered HCO3
– and
contribute

eys reabsorb all the ﬁltered HCO3
– and
contribute new HCO3
– through the formation of NH4
+
and titratable acid.
In metabolic acidosis, an excess of H + over HCO3
–
occurs in the tubular ﬂuid primarily because of
decreased ﬁltration of HCO3
-. This decreased ﬁltration
of HCO3
– is caused mainly by a decrease in the extra-
cellular ﬂuid concentration of HCO3
–.
In respiratory acidosis, the excess H+ in the tubular
ﬂuid is due mainly to the rise in extracellular ﬂuid
PCO2, which stimulates H+ sec

n extracellular ﬂuid
PCO2, which stimulates H+ secretion.
As discussed previously, with chronic acidosis,
regardless of whether it is respiratory or metabolic,
there is an increase in the production of NH4
+, which
further contributes to the excretion of H+ and the addi-
tion of new HCO3
– to the extracellular ﬂuid. With
severe chronic acidosis, as much as 500 mEq/day of H+
can be excreted in the urine, mainly in the form of
NH4
+; this, in turn, contributes up to 500 mEq/day of
new HCO3
– that 

 contributes up to 500 mEq/day of
new HCO3
– that is added to the blood.
Thus, with chronic acidosis, the increased secretion
of H+ by the tubules helps eliminate excess H+ from
the body and increases the quantity of HCO3
– in the
extracellular ﬂuid. This increases the bicarbonate part
of the bicarbonate buffer system, which, in accordance
with the Henderson-Hasselbalch equation, helps raise
the extracellular pH and corrects the acidosis. If the
acidosis is metabolically mediated, additional com

acidosis is metabolically mediated, additional com-
pensation by the lungs causes a reduction in PCO2, also
helping to correct the acidosis.
Table 30–3 summarizes the characteristics associ-
ated with respiratory and metabolic acidosis as well as
respiratory and metabolic alkalosis, which are dis-
cussed in the next section. Note that in respiratory 
acidosis, there is a reduction in pH, an increase in
extracellular ﬂuid H+ concentration, and an increase in
PCO2, which is the initial cause of th

increase in
PCO2, which is the initial cause of the acidosis. The
compensatory response is an increase in plasma 
HCO3
-, caused by the addition of new bicarbonate to
the extracellular ﬂuid by the kidneys. The rise in HCO3
–
helps offset the increase in PCO2, thereby returning the
plasma pH toward normal.
In metabolic acidosis, there is also a decrease in pH
and a rise in extracellular ﬂuid H+ concentration.
However, in this case, the primary abnormality is a
decrease in plasma HCO3
–. The prima

ormality is a
–. The primary compensa-
tions include increased ventilation rate, which reduces
PCO2, and renal compensation, which, by adding new
bicarbonate to the extracellular ﬂuid, helps minimize
the initial fall in extracellular HCO3
- concentration.
Renal Correction of
Alkalosis—Decreased Tubular
and Increased Excretion 
of Bicarbonate Ions
The compensatory responses to alkalosis are basically
opposite to those that occur in acidosis. In a

lly
opposite to those that occur in acidosis. In alkalosis,
the ratio of HCO3
– to CO2 in the extracellular ﬂuid
increases, causing a rise in pH (a decrease in H+
concentration), as is evident from the Henderson-
Hasselbalch equation.
Alkalosis Increases the Ratio of 
HCO3
Regardless of whether the alkalosis is caused by meta-
bolic or respiratory abnormalities, there is still an
increase in the ratio of HCO3
tubular ﬂuid. The net effect of this i

n the renal
tubular ﬂuid. The net effect of this is an excess of
HCO3
– that cannot be reabsorbed from the tubules
and is, therefore, excreted in the urine. Thus, in alka-
losis, HCO3
– is removed from the extracellular ﬂuid by
Table 30–3
Characteristics of Primary Acid-Base Disturbances
pH
H+
PCO2
HCO3
–
Normal
7.4
40 mEq/L
40 mm Hg
24 mEq/L
Ø
≠
≠≠
≠
≠
Ø
ØØ
Ø
Ø
≠
Ø
ØØ
≠
Ø
≠
≠≠
The primary event is indicated by the

sis
≠
Ø
≠
≠≠
The primary event is indicated by the double arrows (≠≠or ØØ). Note that
respiratory acid-base disorders are initiated by an increase or decrease in
PCO2, whereas metabolic disorders are initiated by an increase or decrease in
HCO3
–.
Chapter 30
397
renal excretion, which has the same effect as adding an
H+ to the extracellular ﬂuid. This helps return the H+
concentration and pH back toward normal.
Table 30–3 shows the overall characteristics of res-

le 30–3 shows the overall characteristics of res-
piratory and metabolic alkalosis. In respiratory alkalo-
sis, there is an increase in extracellular ﬂuid pH and a
decrease in H+ concentration. The cause of the alkalo-
sis is a decrease in plasma PCO2, caused by hyperventi-
lation. The reduction in PCO2 then leads to a decrease
in the rate of H+ secretion by the renal tubules. The
decrease in H+ secretion reduces the amount of H+ in
the renal tubular ﬂuid. Consequently, there is not
enough H+ to

ular ﬂuid. Consequently, there is not
enough H+ to react with all the HCO3
– that is ﬁltered.
Therefore, the HCO3
– that cannot react with H+ is not
reabsorbed and is excreted in the urine. This results in
a decrease in plasma HCO3
– concentration and cor-
rection of the alkalosis. Therefore, the compensatory
response to a primary reduction in PCO2 in respiratory
alkalosis is a reduction in plasma HCO3
- concentration,
caused by increased renal excretion of HCO3
-.
In metabolic alkalosis, there 

xcretion of HCO3
-.
In metabolic alkalosis, there is also an increase in
plasma pH and a decrease in H+ concentration.
The cause of metabolic alkalosis, however, is a rise in
the extracellular ﬂuid HCO3
– concentration. This is
partly compensated for by a reduction in the respira-
tion rate, which increases PCO2 and helps return the
extracellular ﬂuid pH toward normal. In addition, the
increase in HCO3
– concentration in the extracellular
ﬂuid leads to an increase in the ﬁltered load of HCO3
–,

ads to an increase in the ﬁltered load of HCO3
–,
which in turn causes an excess of HCO3
– over H+
secreted in the renal tubular ﬂuid. The excess HCO3
–
in the tubular ﬂuid fails to be reabsorbed because
there is no H+ to react with, and it is excreted in the
urine. In metabolic alkalosis, the primary compensa-
tions are decreased ventilation, which raises PCO2, and
increased renal HCO3
- excretion, which helps compen-
sate for the initial rise in extracellular ﬂuid HCO3
-
Clinica

n extracellular ﬂuid HCO3
-
Clinical Causes of 
Respiratory Acidosis Is Caused 
by Decreased Ventilation and
Increased PCO2
From the previous discussion, it is obvious that any
factor that decreases the rate of pulmonary ventilation
also increases the PCO2 of extracellular ﬂuid.This causes
an increase in H2CO3 and H+ concentration, thus re-
sulting in acidosis. Because the acidosis is caused by 
an abnormality in respiration, it is called respiratory 
acidosis.

n respiration, it is called respiratory 
acidosis.
Respiratory acidosis can occur from pathological
conditions that damage the respiratory centers or 
that decrease the ability of the lungs to eliminate CO2.
For example, damage to the respiratory center in the
medulla oblongata can lead to respiratory acidosis.Also,
obstruction of the passageways of the respiratory tract,
pneumonia, emphysema, or decreased pulmonary mem-
brane surface area, as well as any factor that interferes
with the exchange

ll as any factor that interferes
with the exchange of gases between the blood and the
alveolar air, can cause respiratory acidosis.
In respiratory acidosis, the compensatory responses
available are (1) the buffers of the body ﬂuids and (2)
the kidneys, which require several days to compensate
for the disorder.
Respiratory Alkalosis Results 
from Increased Ventilation and
Decreased PCO2
Respiratory alkalosis is caused by overventilation by
the lungs. Rarely does this occur because of physical
pat

gs. Rarely does this occur because of physical
pathological conditions. However, a psychoneurosis can
occasionally cause overbreathing to the extent that a
person becomes alkalotic.
A physiologic type of respiratory alkalosis occurs
when a person ascends to high altitude.The low oxygen
content of the air stimulates respiration, which causes
excess loss of CO2 and development of mild respiratory
alkalosis. Again, the major means for compensation are
the chemical buffers of the body ﬂuids and the 

re
the chemical buffers of the body ﬂuids and the ability
of the kidneys to increase HCO3
– excretion.
Metabolic Acidosis Results from
Decreased Extracellular Fluid
Bicarbonate Concentration
The term metabolic acidosis refers to all other types of
acidosis besides those caused by excess CO2 in the body
ﬂuids.Metabolic acidosis can result from several general
causes: (1) failure of the kidneys to excrete metabolic
acids normally formed in the body, (2) formation of
excess quantities of metabolic 

, (2) formation of
excess quantities of metabolic acids in the body, (3)
addition of metabolic acids to the body by ingestion or
infusion of acids, and (4) loss of base from the body
ﬂuids, which has the same effect as adding an acid to the
body ﬂuids. Some speciﬁc conditions that cause meta-
bolic acidosis are the following.
Renal Tubular Acidosis. This type of acidosis results from
a defect in renal secretion of H+ or in reabsorption of
HCO3
–, or both. These disorders are generally of two
typ

 or both. These disorders are generally of two
types: (1) impairment of renal tubular HCO3
– reab-
sorption, causing loss of HCO3
– in the urine, or (2)
inability of the renal tubular H+ secretory mechanism to
establish a normal acidic urine, causing the excretion of
an alkaline urine. In these cases, inadequate amounts of
titratable acid and NH4
+ are excreted, so that there is
net accumulation of acid in the body ﬂuids. Some causes
of renal tubular acidosis include chronic renal failure,
insuf

ular acidosis include chronic renal failure,
insufﬁcient aldosterone secretion (Addison’s disease),
and several hereditary and acquired disorders that
impair tubular function, such as Fanconi’s syndrome.
Diarrhea. Severe diarrhea is probably the most frequent
cause of metabolic acidosis. The cause of this acidosis is
the loss of large amounts of sodium bicarbonate into the
feces. The gastrointestinal secretions normally contain
large amounts of bicarbonate, and diarrhea results in
the loss of HC

icarbonate, and diarrhea results in
– from the body, which has the same
effect as losing large amounts of bicarbonate in the
urine. This form of metabolic acidosis is particularly
serious and can cause death, especially in young 
children.
Vomiting of Intestinal Contents. Vomiting of gastric con-
tents alone would cause loss of acid and a tendency
398
Unit V
toward alkalosis because the stomach secretions are
highly acidic. However, vomiting large amo

ons are
highly acidic. However, vomiting large amounts from
deeper in the gastrointestinal tract, which sometimes
occurs, causes loss of bicarbonate and results in meta-
bolic acidosis in the same way that diarrhea causes 
acidosis.
Diabetes Mellitus. Diabetes mellitus is caused by lack of
insulin secretion by the pancreas (type I diabetes) or 
by insufﬁcient insulin secretion to compensate for
decreased sensitivity to the effects of insulin (type II
diabetes).In the absence of sufﬁcient insulin

e II
diabetes).In the absence of sufﬁcient insulin,the normal
use of glucose for metabolism is prevented. Instead,
some of the fats are split into acetoacetic acid, and this
is metabolized by the tissues for energy in place of
glucose.With severe diabetes mellitus, blood acetoacetic
acid levels can rise very high, causing severe metabolic
acidosis. In an attempt to compensate for this acidosis,
large amounts of acid are excreted in the urine, some-
times as much as 500 mmol/day.
Ingestion of Aci

e-
Ingestion of Acids. Rarely are large amounts of acids
ingested in normal foods. However, severe metabolic
acidosis occasionally results from the ingestion of
certain acidic poisons. Some of these include acetylsali-
cylics (aspirin) and methyl alcohol (which forms formic
acid when it is metabolized).
Chronic Renal Failure. When kidney function declines
markedly, there is a buildup of the anions of weak acids
in the body ﬂuids that are not being excreted by the
k

he body ﬂuids that are not being excreted by the
kidneys. In addition, the decreased glomerular ﬁltration
rate reduces the excretion of phosphates and NH4
+,
which reduces the amount of bicarbonate added back
to the body ﬂuids. Thus, chronic renal failure can be
associated with severe metabolic acidosis.
Metabolic Alkalosis Is Caused 
by Increased Extracellular Fluid
When there is excess retention of HCO3
– or loss of H+
from the body, this results in metabolic alkalosi

from the body, this results in metabolic alkalosis. Meta-
bolic alkalosis is not nearly as common as metabolic aci-
dosis, but some of the causes of metabolic alkalosis are
as follows.
Administration of Diuretics (Except the Carbonic Anhydrase
Inhibitors). All diuretics cause increased ﬂow of ﬂuid
along the tubules, usually causing increased ﬂow in the
distal and collecting tubules. This leads to increased
reabsorption of Na+ from these parts of the nephrons.
Because the sodium reabsorption her

 the nephrons.
Because the sodium reabsorption here is coupled with
H+ secretion, the enhanced sodium reabsorption also
leads to an increase in H+ secretion and an increase in
bicarbonate reabsorption. These changes lead to the
development of alkalosis, characterized by increased
extracellular ﬂuid bicarbonate concentration.
Excess Aldosterone. When large amounts of aldosterone
are secreted by the adrenal glands, a mild metabolic
alkalosis develops. As discussed previously, aldosterone
promotes 

ps. As discussed previously, aldosterone
promotes extensive reabsorption of Na+ from the distal
and collecting tubules and at the same time stimulates
the secretion of H+ by the intercalated cells of the col-
lecting tubules. This increased secretion of H+ leads to
its increased excretion by the kidneys and, therefore,
metabolic alkalosis.
Vomiting of Gastric Contents. Vomiting of the gastric con-
tents alone, without vomiting of the lower gastrointesti-
nal contents, causes loss of the HCl secr

intesti-
nal contents, causes loss of the HCl secreted by the
stomach mucosa.The net result is a loss of acid from the
extracellular ﬂuid and development of metabolic alka-
losis.This type of alkalosis occurs especially in neonates
who have pyloric obstruction caused by hypertrophied
pyloric sphincter muscles.
Ingestion of Alkaline Drugs. A common cause of metabolic
alkalosis is ingestion of alkaline drugs, such as sodium
bicarbonate, for the treatment of gastritis or peptic
ulcer.
Treatment of 

atment of gastritis or peptic
ulcer.
Treatment of Acidosis 
or Alkalosis
The best treatment for acidosis or alkalosis is to correct
the condition that caused the abnormality. This is often
difﬁcult, especially in chronic diseases that cause
impaired lung function or kidney failure. In these cir-
cumstances, various agents can be used to neutralize the
excess acid or base in the extracellular ﬂuid.
To neutralize excess acid, large amounts of sodium
bicarbonate can be ingested by mouth. The sodium

m
bicarbonate is absorbed from the gastrointestinal tract
into the blood and increases the bicarbonate portion of
the bicarbonate buffer system, thereby increasing pH
toward normal. Sodium bicarbonate can also be infused
intravenously, but because of the potentially dangerous
physiologic effects of such treatment, other substances
are often used instead, such as sodium lactate and
sodium gluconate. The lactate and gluconate portions of
the molecul

 The lactate and gluconate portions of
the molecules are metabolized in the body, leaving the
sodium in the extracellular ﬂuid in the form of sodium
bicarbonate and thereby increasing the pH of the ﬂuid
For the treatment of alkalosis, ammonium chloride
can be administered by mouth. When the ammonium
chloride is absorbed into the blood, the ammonia
portion is converted by the liver into urea. This 
reaction liberates HCl, which immediately reacts with
the buffers of the body ﬂuids 

diately reacts with
the buffers of the body ﬂuids to shift the H+ concentra-
tion in the acidic direction. Ammonium chloride 
occasionally is infused intravenously, but NH4
+ is 
highly toxic,
and this procedure can be dan-
gerous. Another substance used occasionally is lysine
monohydrochloride.
Clinical Measurements and
Analysis of Acid-Base
Disorders
Appropriate therapy of acid-base disorders requires
proper diagnosis. The simple acid-base disorders
described previously can be diagnosed by ana

rders
described previously can be diagnosed by analyzing
three measurements from an arterial blood sample: pH,
plasma bicarbonate concentration, and PCO2.
The diagnosis of simple acid-base disorders involves
several steps, as shown in Figure 30–10. By exam-
ining the pH, one can determine whether the disorder 
is acidosis or alkalosis. A pH less than 7.4 indicates 
acidosis, whereas a pH greater than 7.4 indicates 
Chapter 30
399
The second step is to e

n of Acid-Base Balance
399
The second step is to examine the plasma PCO2 and
HCO3
– concentration. The normal value for PCO2 is
about 40 mm Hg, and for HCO3
–, it is 24 mEq/L. If the
disorder has been characterized as acidosis and the
plasma PCO2 is increased, there must be a respiratory
component to the acidosis. After renal compensation,
the plasma HCO3
– concentration in respiratory acidosis
would tend to increase above normal. Therefore, the
expected values for a simple respiratory acidosis 

expected values for a simple respiratory acidosis would
be reduced plasma pH, increased PCO2, and increased
plasma HCO3
–
concentration after partial renal 
compensation.
For metabolic acidosis, there would also be a decrease
in plasma pH. However, with metabolic acidosis, the
primary abnormality is a decrease in plasma HCO3
–
concentration. Therefore, if a low pH is associated with
a low HCO3
– concentration, there must be a metabolic
component to the acidosis. In simple metabolic acidosis,
the

to the acidosis. In simple metabolic acidosis,
the PCO2 is reduced because of partial respiratory com-
pensation, in contrast to respiratory acidosis, in which
PCO2 is increased. Therefore, in simple metabolic acido-
sis, one would expect to ﬁnd a low pH, a low plasma
HCO3
- concentration, and a reduction in PCO2 after
partial respiratory compensation.
The procedures for categorizing the types of alkalo-
sis involve the same basic steps. First, alkalosis implies
that there is an increase in plas

lkalosis implies
that there is an increase in plasma pH. If the increase in
pH is associated with decreased PCO2, there must be a
respiratory component to the alkalosis. If the rise in pH
is associated with increased HCO3
–, there must be a
metabolic component to the alkalosis. Therefore, in
simple respiratory alkalosis, one would expect to ﬁnd
increased pH, decreased PCO2, and decreased HCO3
-
concentration in the plasma. In simple metabolic alkalo-
sis, one would expect to ﬁnd increased pH, in

alo-
sis, one would expect to ﬁnd increased pH, increased
-, and increased PCO2.
Complex Acid-Base Disorders and
Use of the Acid-Base Nomogram 
for Diagnosis
In some instances, acid-base disorders are not accom-
panied by appropriate compensatory responses. When
this occurs, the abnormality is referred to as a mixed
acid-base disorder. This means that there are two or
more underlying causes for the acid-base disturbance.
For example, a patient with low pH would be catego-
rized as ac

a patient with low pH would be catego-
rized as acidotic. If the disorder was metabolically medi-
ated, this would also be accompanied by a low plasma
HCO3
– concentration and, after appropriate respiratory
compensation, a low PCO2. However, if the low plasma
pH and low HCO3
– concentration are associated with
elevated PCO2, one would suspect a respiratory compo-
nent to the acidosis as well as a metabolic component.
Therefore, this disorder would be categorized as a
mixed acidosis. This could o

d be categorized as a
mixed acidosis. This could occur, for example, in a
patient with acute HCO3
– loss from the gastrointestinal
tract because of diarrhea (metabolic acidosis) who also
has emphysema (respiratory acidosis).
A convenient way to diagnose acid-base disorders is
to use an acid-base nomogram, as shown in Figure
30–11. This diagram can be used to determine the type
of acidosis or alkalosis, as well as its severity. In this 
acid-base diagram, pH, HCO3
– concentration, and PCO2
values

diagram, pH, HCO3
values intersect according to the Henderson-Hassel-
balch equation. The central open circle shows normal
values and the deviations that can still be considered
within the normal range. The shaded areas of the
diagram show the 95 per cent conﬁdence limits for the
normal compensations to simple metabolic and respira-
tory disorders.
When using this diagram, one must assume that suf-
ﬁcient time has elapsed for a full compensatory
response, which is 6 to 

d for a full compensatory
response, which is 6 to 12 hours for the ventilatory com-
pensations in primary metabolic disorders and 3 to 5
days for the metabolic compensations in primary respi-
ratory disorders. If a value is within the shaded area, this
suggests that there is a simple acid-base disturbance.
Conversely, if the values for pH, bicarbonate, or PCO2
lie outside the shaded area, this suggests that there may
be a mixed acid-base disorder.
It is important to recognize that an acid-base v

.
It is important to recognize that an acid-base value
within the shaded area does not always mean that there
is a simple acid-base disorder. With this reservation in
mind, the acid-base diagrams can be used as a quick
means of determining the speciﬁc type and severity of
an acid-base disorder.
For example, assume that the arterial plasma from a
patient yields the following values: pH 7.30, plasma
HCO3
– concentration 12.0 mEq/L, and plasma PCO2
25 mm Hg. With these values, one can look at the
d

diagram and ﬁnd that this represents a simple metabolic
acidosis, with appropriate respiratory compensation
that reduces the PCO2 from its normal value of 40 mm
Hg to 25 mm Hg.
A second example would be a patient with the fol-
lowing values: pH 7.15, plasma HCO3
– concentration 
7 mEq/L, and plasma PCO2 50 mm Hg. In this example,
the patient is acidotic, and there appears to be a meta-
bolic component because the plasma HCO3
– concentra-
tion is l

ent because the plasma HCO3
tion is lower than the normal value of 24 mEq/L.
However, the respiratory compensation that would
normally reduce PCO2 is absent, and PCO2 is slightly
increased above the normal value of 40 mm Hg. This is
Arterial blood sample
pH?
Alkalosis
Acidosis
Metabolic
Respiratory
>7.4
<7.4
HCO3
-
>24 mEq/L
Pco2
<40 mm Hg
Pco2
>40 mm Hg
Pco2
<40 mm Hg
HCO3
-
<24 mEq/L
HCO3
-
>24 mEq/L
HCO3
-
<24 mEq/L
Pco2
>40 mm Hg
compensation

Metabolic
Renal
Renal
Figure 30–10
Analysis of simple acid-base disorders. If the compensatory
responses are markedly different from those shown at the bottom
of the ﬁgure, one should suspect a mixed acid-base disorder.
400
Unit V
consistent with a mixed acid-base disturbance con-
sisting of metabolic acidosis as well as a respiratory
component.
The acid-base diagram serves as a q

ory
The acid-base diagram serves as a quick way to assess
the type and severity of disorders that may be con-
tributing to abnormal pH, PCO2, and plasma bicarbon-
ate concentrations. In a clinical setting, the patient’s
history and other physical ﬁndings also provide impor-
tant clues concerning causes and treatment of the acid-
base disorders.
Use of Anion Gap to Diagnose 
The concentrations of anions and cations in plasma
must be equal to maintain electrical neut

n plasma
must be equal to maintain electrical neutrality. There-
fore, there is no real “anion gap” in the plasma.
However, only certain cations and anions are routinely
measured in the clinical laboratory.The cation normally
measured is Na+, and the anions are usually Cl– and
HCO3
–. The “anion gap” (which is only a diagnostic
concept) is the difference between unmeasured anions
and unmeasured cations, and is estimated as
Plasma anion gap = [Na+] – [HCO3
–] – [Cl–]
= 144 – 24 – 108 = 10 mEq/L
T

] – [HCO3
–] – [Cl–]
The anion gap will increase if unmeasured anions rise
or if unmeasured cations fall. The most important
unmeasured cations include calcium, magnesium, and
potassium, and the major unmeasured anions are
albumin, phosphate, sulfate, and other organic anions.
Usually the unmeasured anions exceed the unmeasured
cations, and the anion gap ranges between 8 and 
16 mEq/L.
The plasma anion gap is used mainly in diagnosing
different causes of metabolic aci

ly in diagnosing
different causes of metabolic acidosis. In metabolic aci-
dosis, the plasma HCO3
– is reduced. If the plasma
sodium concentration is unchanged, the concentration
of anions (either Cl– or an unmeasured anion) must
increase to maintain electroneutrality. If plasma Cl–
increases in proportion to the fall in plasma HCO3
–, the
anion gap will remain normal, and this is often referred
to as hyperchloremic metabolic acidosis.
If the decrease in plasma HCO3
– is not accompanied
by incre

rease in plasma HCO3
by increased Cl–, there must be increased levels of
unmeasured anions and therefore an increase in the cal-
culated anion gap. Metabolic acidosis caused by excess
nonvolatile acids (besides HCl), such as lactic acid or
ketoacids, is associated with an increased plasma anion
gap because the fall in HCO3
– is not matched by an
equal increase in Cl–. Some examples of metabolic aci-
dosis associated with a normal or increased anion gap
are shown in Table 30–

rmal or increased anion gap
are shown in Table 30–4. By calculating the anion gap,
one can narrow some of the potential causes of meta-
bolic acidosis.
Table 30–4
Metabolic Acidosis Associated with Normal or Increased
Plasma Anion Gap
Increased Anion Gap 
Normal Anion Gap 
(Normochloremia)
(Hyperchloremia)
Diabetes mellitus (ketoacidosis)
Diarrhea
Lactic acidosis
Renal tubular acidosis
Carbonic anhydrase inhibitors
Aspirin (acetylsalicylic acid) 
Addison’s disease
poisoning

acetylsalicylic acid) 
poisoning
Methanol poisoning
Ethylene glycol poisoning
Arterial plasma [HCO3
-] (mEq/L) 
20
24
28
32
36
40
44
48
52
56
60
16
12
8
4
0
7.40
7.50
7.60
7.70
7.80
7.30
7.20
7.10
120
100 90
80
70
60
50
40
35
30
25
20
15
10
110
7.00
Arterial blood pH
Chronic
respiratory
acidosis
Metabolic
alkalosis
Acute
alkalosis
Metabolic
acidosis
Acute
acidosis
Chronic
alkalosis
Normal
Pco2 (mm Hg)
Figure 30–11
Acid

Normal
Acid-base nomogram showing
arterial blood pH, arterial plasma
HCO3
–, and PCO2 values. The
central open circle shows the
approximate limits for acid-base
status in normal people. The
shaded areas in the nomogram
show 
the 
approximate 
limits 
for the normal compensations
caused by simple metabolic and
respiratory disorders. For values
lying outside the shaded areas,
one should suspect a mixed acid-
base disorder. (Adapted from
Cogan MG, Rector FC Jr

ase disorder. (Adapted from
Cogan MG, Rector FC Jr: Acid-
Base Disorders in the Kidney, 3rd
ed. Philadelphia: WB Saunders,
1986.)
Chapter 30
401
Alpern RJ: Renal acidiﬁcation mechanisms. In Brenner BM
(ed):The Kidney, 6th ed. Philadelphia:WB Saunders, 2000,
pp 455-519.
Capasso G, Unwin R, Rizzo M, et al: Bicarbonate transport
along the loop of Henle: molecular mechanisms and reg-
ulation. J Nephrol 15(Suppl 5):S88, 2002.
Decoursey TE: Voltage-gated prot

ppl 5):S88, 2002.
Gennari FJ, Maddox DA: Renal regulation of acid-base
homeostasis. In Seldin DW, Giebisch G (eds): The
York: Raven Press, 2000, pp 2015-2054.
Good DW: Ammonium transport by the thick ascending
limb of Henle’s loop. Ann Rev Physiol 56:623, 1994.
Igarashi I, Sekine T, Inatomi J, Seki G: Unraveling the molec-
ular pathogenesis 

, Seki G: Unraveling the molec-
ular pathogenesis of isolated proximal renal tubular aci-
dosis. J Am Soc Nephrol 13:2171, 2002.
Karet FE: Inherited distal renal tubular acidosis. J Am Soc
Nephrol 13:2178, 2002.
Laffey JG, Kavanagh BP: Hypocapnia. N Engl J Med 347:43,
2002.
Lemann J Jr, Bushinsky DA, Hamm LL: Bone buffering of
acid and base in humans. Am J Physiol Renal Physiol
285:F811, 2003.
Madias NE, Adrogue HJ: Cross-talk between two organs:
how the kidney responds to disruption of acid-bas

how the kidney responds to disruption of acid-base
balance by the lung. Nephron Physiol 93:61, 2003.
Wagner CA, Geibel JP:Acid-base transport in the collecting
duct. J Nephrol 15(Suppl 5):S112, 2002.
Wesson DE, Alpern RJ, Seldin DW: Clinical syndromes of
metabolic alkalosis. In Seldin DW, Giebisch G (eds): The
York: Raven Press, 2000, pp 2055-2072.
White NH: Management of diabetic ketoacidosis. Rev
Endocr Metab Disord 4:343, 2003.
C
H
A
P
T
E
R

Rev
C
H
A
P
T
E
R
 
3
1
402
Diuretics and Their
Mechanisms of Action
A diuretic is a substance that increases the rate of urine
volume output, as the name implies. Most diuretics also
increase urinary excretion of solutes, especially sodium
and chloride. In fact, most diuretics that are used clin-
ically act by decreasing the rate of sodium reabsorption
from the tubules, which causes natriuresis (increased
sodium output), which in tu

natriuresis (increased
sodium output), which in turn causes diuresis (increased water output). That is, in
most cases, increased water output occurs secondary to inhibition of tubular 
sodium reabsorption, because sodium remaining in the tubules acts osmotically to
decrease water reabsorption. Because the renal tubular reabsorption of many
solutes, such as potassium, chloride, magnesium, and calcium, is also inﬂuenced sec-
ondarily by sodium reabsorption, many diuretics raise renal output of the

sorption, many diuretics raise renal output of these solutes
as well.
The most common clinical use of diuretics is to reduce extracellular ﬂuid volume,
especially in diseases associated with edema and hypertension. As discussed in
Chapter 25, loss of sodium from the body mainly decreases extracellular ﬂuid
volume; therefore, diuretics are most often administered in clinical conditions in
which extracellular ﬂuid volume is expanded.
Some diuretics can increase urine output more than 20-fold withi

 can increase urine output more than 20-fold within a few minutes
after they are administered. However, the effect of most diuretics on renal output
of salt and water subsides within a few days (Figure 31–1). This is due to activation
of other compensatory mechanisms initiated by decreased extracellular ﬂuid
volume. For example, a decrease in extracellular ﬂuid volume often reduces arterial
pressure and glomerular ﬁltration rate (GFR) and increases renin secretion and
angiotensin II formation; a

es renin secretion and
angiotensin II formation; all these responses, together, eventually override the
chronic effects of the diuretic on urine output.Thus, in the steady state, urine output
becomes equal to intake, but only after reductions in arterial pressure and extra-
cellular ﬂuid volume have occurred, relieving the hypertension or edema that
prompted the use of diuretics in the ﬁrst place.
The many diuretics available for clinical use have different mechanisms of action
and, therefore, i

e different mechanisms of action
and, therefore, inhibit tubular reabsorption at different sites along the renal
nephron. The general classes of diuretics and their mechanisms of action are shown
in Table 31–1.
Osmotic Diuretics Decrease Water Reabsorption 
by Increasing Osmotic Pressure of Tubular Fluid
Injection into the blood stream of substances that are not easily reabsorbed by the
renal tubules, such as urea, mannitol, and sucrose, causes a marked increase in the
concentration of osmotical

 marked increase in the
concentration of osmotically active molecules in the tubules. The osmotic pressure
of these solutes then greatly reduces water reabsorption, ﬂushing large amounts of
tubular ﬂuid into the urine.
Large volumes of urine are also formed in certain diseases associated with excess
solutes that fail to be reabsorbed from the tubular ﬂuid. For example, when the
blood glucose concentration rises to high levels in diabetes mellitus, the increased
ﬁltered load of glucose into the t

, the increased
ﬁltered load of glucose into the tubules exceeds their capacity to reabsorb glucose
(i.e., exceeds their transport maximum for glucose). Above a plasma glucose con-
centration of about 250 mg/dl, little of the extra glucose is reabsorbed by the tubules;
instead, the excess glucose remains in the tubules, acts as an osmotic diuretic, and
causes rapid loss of ﬂuid into the urine. In patients with diabetes mellitus, the high
Chapter 31
403
urine output 

31
403
urine output is balanced by a high level of ﬂuid intake
owing to activation of the thirst mechanism.
“Loop” Diuretics Decrease Active
Sodium-Chloride-Potassium
Reabsorption in the Thick
Ascending Loop of Henle
Furosemide, ethacrynic acid, and bumetanide are power-
ful diuretics that decrease active reabsorption in the
thick ascending limb of the loop of Henle by blocking
the 1-sodium, 2-chloride, 1-potassium co-transporter
located in the luminal membrane of t

o-transporter
located in the luminal membrane of the epithelial cells.
These diuretics are among the most powerful of the clin-
ically used diuretics.
By blocking active sodium-chloride-potassium co-
transport in the luminal membrane of the loop of Henle,
the loop diuretics raise urine output of sodium, chloride,
potassium, and other electrolytes, as well as water, for
two reasons: (1) they greatly increase the quantities of
solutes delivered to the distal parts of the nephrons, and
these act as

the distal parts of the nephrons, and
these act as osmotic agents to prevent water reabsorp-
tion as well; and (2) they disrupt the countercurrent
multiplier system by decreasing absorption of ions from
the loop of Henle into the medullary interstitium,
thereby decreasing the osmolarity of the medullary
interstitial ﬂuid. Because of this effect, loop diuretics
impair the ability of the kidneys to either concentrate
or dilute the urine. Urinary dilution is impaired because
the inhibition of sodiu

lution is impaired because
the inhibition of sodium and chloride reabsorption in
the loop of Henle causes more of these ions to be
excreted along with increased water excretion. Urinary
concentration is impaired because the renal medullary
interstitial ﬂuid concentration of these ions, and there-
fore renal medullary osmolarity, is reduced. Conse-
quently, reabsorption of ﬂuid from the collecting ducts
is decreased, so that the maximal concentrating ability
of the kidneys is also greatly reduced

ing ability
of the kidneys is also greatly reduced. In addition,
decreased renal medullary interstitial ﬂuid osmolarity
reduces absorption of water from the descending loop
of Henle. Because of these multiple effects, 20 to 30 per
cent of the glomerular ﬁltrate may be delivered into 
the urine, causing, under acute conditions, urine output
to be as great as 25 times normal for at least a few
minutes.
volume (liters)
Sodium excretion or
sodium intake (mEq/day)
200
100
15.0
14.

retion or
200
100
15.0
14.0
13.0
8
6
4
2
0
-4
-2
Excretion
Intake
Diuretic therapy
Figure 31–1
Sodium excretion and extracellular ﬂuid volume during diuretic
administration. The immediate increase in sodium excretion is
accompanied by a decrease in extracellular ﬂuid volume. If
sodium intake is held constant, compensatory mechanisms will
eventually return sodium excretion to equal sodium intake, thus
re-establishing sodium balance.
Table 31–1
Classes of Diuret

shing sodium balance.
Table 31–1
Classes of Diuretics, Their Mechanisms of Action, and Tubular Sites of Action
Class of Diuretic
Tubular Site of Action
Osmotic diuretics (mannitol)
Inhibit water and solute reabsorption by increasing
Mainly proximal tubules
osmolarity of tubular ﬂuid
Loop diuretics (furosemide, bumetanide)
Inhibit Na+-K+-Cl- co-transport in luminal membrane
Thick ascending loop of Henle
Thiazide diuretics (hydrochlorothiazide,
Inhibit Na+-Cl- co-transport in l

ochlorothiazide,
Inhibit Na+-Cl- co-transport in luminal membrane
Early distal tubules
chlorthalidone)
Carbonic anhydrase inhibitors 
Inhibit H+ secretion and HCO3
– reabsorption, which 
Proximal tubules
(acetazolamide)
reduces Na+ reabsorption
Aldosterone antagonists 
Inhibit action of aldosterone on tubular receptor,
Collecting tubules
(spironolactone, eplerenone)
decrease Na+ reabsorption, and decrease
K+ secretion
Sodium channel blockers 
Block entry of Na+ into Na+ channels of luminal
Colle

ck entry of Na+ into Na+ channels of luminal
(triamterene, amiloride)
membrane, decrease Na+ reabsorption, and
decrease K+ secretion
404
Unit V
Thiazide Diuretics Inhibit Sodium-
Chloride Reabsorption in the Early
The thiazide derivatives, such as chlorothiazide, act
mainly on the early distal tubules to block the sodium-
chloride co-transporter in the luminal membrane of the
tubular cells. Under favorable conditions, these agents
caus

lls. Under favorable conditions, these agents
cause 5 to 10 per cent of the glomerular ﬁltrate to pass
into the urine.This is about the same amount of sodium
normally reabsorbed by the distal tubules.
Carbonic Anhydrase Inhibitors
Block Sodium-Bicarbonate
Reabsorption in the Proximal
Tubules
Acetazolamide inhibits the enzyme carbonic anhydrase,
which is critical for the reabsorption of bicarbonate in
the proximal tubule, as discussed in Chapter 30. Car-
bonic anhydrase is abundant in the proxima

0. Car-
bonic anhydrase is abundant in the proximal tubule, the
primary site of action of carbonic anhydrase inhibitors.
Some carbonic anhydrase is also present in other
tubular cells, such as in the intercalated cells of the col-
lecting tubule.
Because hydrogen ion secretion and bicarbonate
reabsorption in the proximal tubules are coupled to
sodium reabsorption through the sodium-hydrogen ion
counter-transport mechanism in the luminal membrane,
decreasing bicarbonate reabsorption also reduces

sodium reabsorption. The blockage of sodium and
bicarbonate reabsorption from the tubular ﬂuid causes
these ions to remain in the tubules and act as an osmotic
diuretic. Predictably, a disadvantage of the carbonic
anhydrase inhibitors is that they cause some degree of
acidosis because of the excessive loss of bicarbonate
ions in the urine.
Competitive Inhibitors of
Aldosterone Decrease Sodium
Reabsorption from and Potassium
Secretion into the Cor

sorption from and Potassium
Secretion into the Cortical
Spironolactone and eplerenone are aldosterone antago-
nists that compete with aldosterone for receptor sites 
in the cortical collecting tubule epithelial cells and,
therefore, can decrease the reabsorption of sodium and
secretion of potassium in this tubular segment.As a con-
sequence, sodium remains in the tubules and acts as an
osmotic diuretic, causing increased excretion of water as
well as sodium. Because these drugs

on of water as
well as sodium. Because these drugs also block the
effect of aldosterone to promote potassium secretion in
the tubules, they decrease the excretion of potassium.
Aldosterone antagonists also cause movement of potas-
sium from the cells to the extracellular ﬂuid. In some
instances, this causes extracellular ﬂuid potassium con-
centration to increase excessively. For this reason,
spironolactone and other aldosterone inhibitors are
referred to as potassium-sparing diuretics. Many of 

ferred to as potassium-sparing diuretics. Many of the
other diuretics cause loss of potassium in the urine, in
contrast to the aldosterone antagonists, which “spare”
the loss of potassium.
Diuretics That Block Sodium
Channels in the Collecting Tubules
Decrease Sodium Reabsorption
Amiloride and triamterene also inhibit sodium reab-
sorption and potassium secretion in the collecting
tubules, similar to the effects of spironolactone.
However, at the cellular level, these drugs act directly
to block

 cellular level, these drugs act directly
to block the entry of sodium into the sodium channels
of the luminal membrane of the collecting tubule
epithelial cells. Because of this decreased sodium entry
into the epithelial cells, there is also decreased sodium
transport across the cells’ basolateral membranes and,
therefore, decreased activity of the sodium-potassium-
adenosine triphosphatase pump.This decreased activity
reduces the transport of potassium into the cells and
ultimately decreases t

otassium into the cells and
ultimately decreases the secretion of potassium into the
tubular ﬂuid. For this reason, the sodium channel block-
ers are also potassium-sparing diuretics and decrease
the urinary excretion rate of potassium.
Diseases of the kidneys are among the most important
causes of death and disability in many countries
throughout the world. For example, in 2004, more than
20 million adults in the United States were estimated
to have chronic kidney disease.
Sever

re estimated
Severe kidney diseases can be divided into two main
categories: (1) acute renal failure, in which the kidneys
abruptly stop working entirely or almost entirely but
may eventually recover nearly normal function, and
(2) chronic renal failure, in which there is progressive
loss of function of more and more nephrons that grad-
ually decreases overall kidney function. Within these
two general categories, there are many speciﬁc kidney
diseases that can aff

here are many speciﬁc kidney
diseases that can affect the kidney blood vessels,
glomeruli, tubules, renal interstitium, and parts of the
urinary tract outside the kidney, including the ureters
and bladder. In this chapter, we discuss speciﬁc physi-
ologic abnormalities that occur in a few of the more
important types of kidney diseases.
The causes of acute renal failure can be divided into
three main categories:
1. Acute renal failure resulting from decreased
blood supply to t

failure resulting from decreased
blood supply to the kidneys; this condition is
often referred to as prerenal acute renal failure
to reﬂect the fact that the abnormality occurs 
in a system before the kidneys. This can be a
consequence of heart failure with reduced cardiac
output and low blood pressure or conditions
associated with diminished blood volume and low
blood pressure, such as severe hemorrhage.
2. Intrarenal acute renal failure resulting from
abnormalities within the kidney itself, in

ng from
abnormalities within the kidney itself, including
those that affect the blood vessels, glomeruli, or
tubules.
3. Postrenal acute renal failure, resulting from
obstruction of the urinary collecting system
Chapter 31
405
anywhere from the calyces to the outﬂow from
the bladder. The most common causes of
obstruction of the urinary tract outside the kidney
are kidney stones, caused by precipitation of
calcium, urate, or cystine.
Prerenal Acute Renal Failure
Caus

ate, or cystine.
Caused by Decreased Blood Flow
to the Kidney
The kidneys normally receive an abundant blood supply
of about 1100 ml/min, or about 20 to 25 per cent of the
cardiac output.The main purpose of this high blood ﬂow
to the kidneys is to provide enough plasma for the high
rates of glomerular ﬁltration needed for effective regu-
lation of body ﬂuid volumes and solute concentrations.
Therefore, decreased renal blood ﬂow is usually accom-
panied by decreased G

 blood ﬂow is usually accom-
panied by decreased GFR and decreased urine output
of water and solutes. Consequently, conditions that
acutely diminish blood ﬂow to the kidneys usually cause
oliguria, which refers to diminished urine output below
the level of intake of water and solutes.This causes accu-
mulation of water and solutes in the body ﬂuids. If renal
blood ﬂow is markedly reduced, total cessation of urine
output can occur, a condition referred to as anuria.
As long as renal blood ﬂow doe

erred to as anuria.
As long as renal blood ﬂow does not fall below about
20 to 25 per cent of normal, acute renal failure can
usually be reversed if the cause of the ischemia is cor-
rected before damage to the renal cells has occurred.
Unlike some tissues, the kidney can endure a relatively
large reduction in blood ﬂow before actual damage to
the renal cells occurs.The reason for this is that as renal
blood ﬂow is reduced, the GFR and the amount of
sodium chloride ﬁltered by the glomeruli (as w

 of
sodium chloride ﬁltered by the glomeruli (as well as 
the ﬁltration rate of water and other electrolytes) are
reduced. This decreases the amount of sodium chloride
that must be reabsorbed by the tubules, which uses most
of the energy and oxygen consumed by the normal
kidney. Therefore, as renal blood ﬂow and GFR fall,
the requirement for renal oxygen consumption is also
reduced. As the GFR approaches zero, oxygen con-
sumption of the kidney approaches the rate that is
required to keep the re

pproaches the rate that is
required to keep the renal tubular cells alive even when
they are not reabsorbing sodium. When blood ﬂow is
reduced below this basal requirement, which is usually
less than 20 to 25 per cent of the normal renal blood
ﬂow, the renal cells start to become hypoxic, and further
decreases in renal blood ﬂow, if prolonged, will cause
damage or even death of the renal cells, especially the
tubular epithelial cells. If the cause of prerenal acute
renal failure is not corrected

e of prerenal acute
renal failure is not corrected and ischemia of the kidney
persists longer than a few hours,this type of renal failure
can evolve into intrarenal acute renal failure, as dis-
cussed later. Acute reduction of renal blood ﬂow is a
common cause of acute renal failure in hospitalized
patients. Table 31–2 shows some of the common causes
of decreased renal blood ﬂow and prerenal acute renal
failure.
Intrarenal Acute Renal Failure
Caused by Abnormalities 
Within the Kidney
Abnormalit

sed by Abnormalities 
Abnormalities that originate within the kidney and that
abruptly diminish urine output fall into the general cat-
egory of intrarenal acute renal failure. This category of
acute renal failure can be further divided into (1) 
conditions that injure the glomerular capillaries or
other small renal vessels, (2) conditions that damage the
renal tubular epithelium, and (3) conditions that cause
damage to the renal interstitium. This type of classiﬁca-
tion refer

l interstitium. This type of classiﬁca-
tion refers to the primary site of injury, but because the
renal vasculature and tubular system are functionally
interdependent, damage to the renal blood vessels can
lead to tubular damage, and primary tubular damage
can lead to damage of the renal blood vessels. Causes
of intrarenal acute renal failure are listed in Table 31–3.
Acute Renal Failure Caused by Glomerulonephritis. Acute
glomerulonephritis is a type of intrarenal acute renal
failure usually c

a type of intrarenal acute renal
failure usually caused by an abnormal immune reaction
that damages the glomeruli. In about 95 per cent of the
patients with this disease, damage to the glomeruli
occurs 1 to 3 weeks after an infection elsewhere in the
body, usually caused by certain types of group A beta
streptococci.The infection may have been a streptococ-
cal sore throat, streptococcal tonsillitis, or even strepto-
coccal infection of the skin. It is not the infection itself
that damages the k

 It is not the infection itself
that damages the kidneys. Instead, over a few weeks, as
antibodies develop against the streptococcal antigen,
the antibodies and antigen react with each other to form
an insoluble immune complex that becomes entrapped
in the glomeruli, especially in the basement membrane
portion of the glomeruli.
Once the immune complex has deposited in the
glomeruli, many of the cells of the glomeruli begin to
proliferate, but mainly the mesangial cells that lie
Table 31–2
Some C

nly the mesangial cells that lie
Table 31–2
Some Causes of Prerenal Acute Renal Failure
Intravascular volume depletion
Hemorrhage (trauma, surgery, postpartum, gastrointestinal)
Diarrhea or vomiting
Burns
Cardiac failure
Valvular damage
Peripheral vasodilation and resultant hypotension
Anaphylactic shock
Anesthesia
Sepsis, severe infections
Primary renal hemodynamic abnormalities
Renal artery stenosis, embolism, or thrombosis of renal artery 
or vein
Table 31–3
Some Causes 

s of renal artery 
or vein
Table 31–3
Some Causes of Intrarenal Acute Renal Failure
Small vessel and/or glomerular injury
Vasculitis (polyarteritis nodosa)
Cholesterol emboli
Malignant hypertension
Acute glomerulonephritis
Tubular epithelial injury (tubular necrosis)
Acute tubular necrosis due to ischemia
Acute tubular necrosis due to toxins (heavy metals, ethylene 
glycol, insecticides, poison mushrooms, carbon tetrachloride)
Renal interstitial injury
Acute pyelonephritis
Acute allergic interst

injury
Acute allergic interstitial nephritis
406
Unit V
between the endothelium and the epithelium. In addi-
tion, large numbers of white blood cells become
entrapped in the glomeruli. Many of the glomeruli
become blocked by this inﬂammatory reaction, and
those that are not blocked usually become excessively
permeable, allowing both protein and red blood cells to
leak from the blood of the glomerular capillaries into
the glomerular ﬁltrate. In sev

ar capillaries into
the glomerular ﬁltrate. In severe cases, either total or
almost complete renal shutdown occurs.
The acute inﬂammation of the glomeruli usually sub-
sides in about 2 weeks, and in most patients, the kidneys
return to almost normal function within the next few
weeks to few months. Sometimes, however, many of the
glomeruli are destroyed beyond repair, and in a small
percentage of patients, progressive renal deterioration
continues indeﬁnitely, leading to chronic renal failure, a

s indeﬁnitely, leading to chronic renal failure, as
described in a subsequent section of this chapter.
Tubular Necrosis as a Cause of Acute Renal Failure. Another
cause of intrarenal acute renal failure is tubular necro-
sis, which means destruction of epithelial cells in the
tubules. Some common causes of tubular necrosis are
(1) severe ischemia and inadequate supply of oxygen
and nutrients to the tubular epithelial cells and (2)
poisons, toxins, or medications that destroy the tubular
epitheli

, or medications that destroy the tubular
epithelial cells.
Acute Tubular Necrosis Caused by Severe Renal
Ischemia. Severe ischemia of the kidney can result from
circulatory shock or any other disturbance that severely
impairs the blood supply to the kidney. If the ischemia
is severe enough to seriously impair the delivery of
nutrients and oxygen to the renal tubular epithelial
cells, and if the insult is prolonged, damage or eventual
destruction of the epithelial cells can occur. When this
happ

 of the epithelial cells can occur. When this
happens, tubular cells “slough off” and plug many of the
nephrons, so that there is no urine output from the
blocked nephrons; the affected nephrons often fail to
excrete urine even when renal blood ﬂow is restored to
normal, as long as the tubules remain plugged.The most
common causes of ischemic damage to the tubular
epithelium are the prerenal causes of acute renal failure
associated with circulatory shock, as discussed earlier in
Ac

ry shock, as discussed earlier in
Acute Tubular Necrosis Caused by Toxins or Medications.
There is a long list of renal poisons and medications that
can damage the tubular epithelium and cause acute renal
failure. Some of these are carbon tetrachloride, heavy
metals (such as mercury and lead),ethylene glycol (which
is a major component in antifreeze), various insecticides,
various medications (such as tetracyclines) used as
antibiotics, and cis-platinum, which is used in treating
c

ics, and cis-platinum, which is used in treating
certain cancers. Each of these substances has a speciﬁc
toxic action on the renal tubular epithelial cells, causing
death of many of them. As a result, the epithelial cells
slough away from the basement membrane and plug the
tubules. In some instances, the basement membrane also
is destroyed. If the basement membrane remains intact,
new tubular epithelial cells can grow along the surface
of the membrane, so that the tubule repairs itself within
10

brane, so that the tubule repairs itself within
10 to 20 days.
Postrenal Acute Renal Failure
Caused by Abnormalities of the
Lower Urinary Tract
Multiple abnormalities in the lower urinary tract can
block or partially block urine ﬂow and therefore lead to
acute renal failure even when the kidneys’ blood supply
and other functions are initially normal. If the urine
output of only one kidney is diminished, no major
change in body ﬂuid composition will occur because the
contralateral kidney can incr

ll occur because the
contralateral kidney can increase its urine output sufﬁ-
ciently to maintain relatively normal levels of extracel-
lular electrolytes and solutes as well as normal
extracellular ﬂuid volume. With this type of renal
failure, normal kidney function can be restored if the
basic cause of the problem is corrected within a few
hours. But chronic obstruction of the urinary tract,
lasting for several days or weeks, can lead to irreversible
kidney damage. Some of the causes of postre

rsible
kidney damage. Some of the causes of postrenal acute
failure include (1) bilateral obstruction of the ureters or
renal pelvises caused by large stones or blood clots, (2)
bladder obstruction, and (3) obstruction of the urethra.
Physiologic Effects of Acute 
A major physiologic effect of acute renal failure is
retention in the blood and extracellular ﬂuid of water,
waste products of metabolism, and electrolytes. This
can lead to water and salt overload, which in turn can
lead

to water and salt overload, which in turn can
lead to edema and hypertension. Excessive retention
of potassium, however, is often a more serious threat
to patients with acute renal failure, because increases
in plasma potassium concentration (hyperkalemia) to
more than about 8 mEq/L (only twice normal) can be
fatal. Because the kidneys are also unable to excrete
sufﬁcient hydrogen ions, patients with acute renal
failure develop metabolic acidosis, which in itself can
be lethal or can aggravate t

, which in itself can
be lethal or can aggravate the hyperkalemia.
In the most severe cases of acute renal failure, com-
plete anuria occurs.The patient will die in 8 to 14 days
unless kidney function is restored or unless an artiﬁ-
cial kidney is used to rid the body of the excessive
retained water, electrolytes, and waste products of
metabolism. Other effects of diminished urine output,
as well as treatment with an artiﬁcial kidney, are dis-
cussed in the next section in relation to chronic re

ssed in the next section in relation to chronic renal
failure.
Chronic Renal Failure: 
An Irreversible Decrease 
in the Number of Functional
Nephrons
Chronic renal failure results from progressive and irre-
versible loss of large numbers of functioning nephrons.
Serious clinical symptoms often do not occur until the
number of functional nephrons falls to at least 70 to
75 per cent below normal. In fact, relatively normal
blood concentrations of most electrolytes and normal
body ﬂuid volumes can 

ost electrolytes and normal
body ﬂuid volumes can still be maintained until the
number of functioning nephrons decreases below 20
to 25 per cent of normal.
Table 31–4 gives some of the most important causes
of chronic renal failure. In general, chronic renal
Chapter 31
407
failure, like acute renal failure, can occur because of
disorders of the blood vessels, glomeruli, tubules, renal
interstitium, and lower urinary tract. Despite the wide
variety of diseases that c

tract. Despite the wide
variety of diseases that can lead to chronic renal
failure, the end result is essentially the same—a
decrease in the number of functional nephrons.
Vicious Circle of Chronic Renal
Failure Leading to End-Stage 
In many cases, an initial insult to the kidney leads to
progressive deterioration of kidney function and
further loss of nephrons to the point where the person
must be placed on dialysis treatment or transplanted
with a functional kidney to survive. Th

ansplanted
with a functional kidney to survive. This condition is
referred to as end-stage renal disease.
Studies in laboratory animals have shown that sur-
gical removal of large portions of the kidney initially
causes adaptive changes in the remaining nephrons
that lead to increased blood ﬂow, increased GFR, and
increased urine output in the surviving nephrons. The
exact mechanisms responsible for these changes are
not well understood but involve hypertrophy (growth
of the various structures o

ve hypertrophy (growth
of the various structures of the surviving nephrons) as
well as functional changes that decrease vascular
resistance and tubular reabsorption in the surviving
nephrons. These adaptive changes permit a person to
excrete normal amounts of water and solutes even
when kidney mass is reduced to 20 to 25 per cent of
normal. Over a period of several years, however, the
renal functional changes may lead to further injury of
the remaining nephrons, particularly to the glomeruli
of 

aining nephrons, particularly to the glomeruli
of these nephrons.
The cause of this additional injury is not known, but
some investigators believe that it may be related in
part to increased pressure or stretch of the remaining
glomeruli, which occurs as a result of functional
vasodilation or increased blood pressure; the chronic
increase in pressure and stretch of the small arterioles
and glomeruli are believed to cause sclerosis of these
vessels (replacement of normal tissue with connective
ti

s (replacement of normal tissue with connective
tissue). These sclerotic lesions can eventually obliter-
ate the glomerulus, leading to further reduction in
kidney function, further adaptive changes in the
remaining nephrons, and a slowly progressing vicious
circle that eventually terminates in end-stage renal
disease (Figure 31–2). The only proven method 
of slowing down this progressive loss of kidney func-
tion is to lower arterial pressure and glomerular
hydrostatic pressure, especially by u

d glomerular
hydrostatic pressure, especially by using drugs such 
as angiotensin-converting enzyme inhibitors or
angiotensin II antagonists.
Table 31–5 gives the most common causes of end-
stage renal disease. In the early 1980s, glomeru-
lonephritis in all its various forms was believed to be
the most common initiating cause of end-stage renal
disease. In recent years, diabetes mellitus and hyper-
tension have become recognized as the leading causes
of end-stage renal disease, together account

auses
of end-stage renal disease, together accounting for
approximately 70 per cent of all chronic renal failure.
Excessive weight gain (obesity) appears to be the
most important risk factor for the two main causes of
end-stage renal disease—diabetes and hypertension.
As discussed in Chapter 78, type II diabetes, which is
Table 31–4
Some Causes of Chronic Renal Failure
Metabolic disorders
Obesity
Renal vascular disorders
Nephrosclerosis-

ascular disorders
Nephrosclerosis-hypertension
Immunologic disorders
Polyarteritis nodosa
Primary tubular disorders
Nephrotoxins (analgesics, heavy metals)
Urinary tract obstruction
Renal calculi
Hypertrophy of prostate
Urethral constriction
Congenital disorders
Polycystic disease
Congenital absence of kidney tissue (renal hypoplasia)
sclerosis
+
pressure
and/or
Arterial

s
+
pressure
and/or
Arterial
pressure
Nephron
number
Primary
kidney disease
and vasodilation
of surviving
nephrons
Figure 31–2
Vicious circle that can occur with primary kidney disease. Loss of
nephrons because of disease may increase pressure and ﬂow in
the surviving glomerular capillaries, which in turn may eventually
injure these “normal” capillaries as well, thus causing progressive
sclerosis and eventual loss of these glomeruli.
408
Unit V
The Body Fluids a

s of these glomeruli.
408
Unit V
closely linked to obesity, accounts for approximately
90 per cent of all diabetes mellitus. Excess weight gain
is also a major cause of essential hypertension,
accounting for as much as 65 to 75 per cent of the risk
for developing hypertension in adults. In addition to
causing renal injury through diabetes and hyperten-
sion, obesity may have additive or synergistic effects
to worsen renal function in patients with pre-existing
kidney 

nal function in patients with pre-existing
Injury to the Renal Vasculature as a
Cause of Chronic Renal Failure
Many types of vascular lesions can lead to renal
ischemia and death of kidney tissue. The most common
of these are (1) atherosclerosis of the larger renal arter-
ies, with progressive sclerotic constriction of the vessels;
(2) ﬁbromuscular hyperplasia of one or more of the
large arteries, which also causes occlusion of the vessels;
and (3) nephrosclerosis, caused by scle

e vessels;
and (3) nephrosclerosis, caused by sclerotic lesions of
the smaller arteries, arterioles, and glomeruli.
Atherosclerotic or hyperplastic lesions of the large
arteries frequently affect one kidney more than the
other and, therefore, cause unilaterally diminished
kidney function. As discussed in Chapter 19, hyperten-
sion often occurs when the artery of one kidney is 
constricted while the artery of the other kidney is 
still normal, a condition analogous to “two-kidney”
Goldblatt hyper

ondition analogous to “two-kidney”
Goldblatt hypertension.
Benign nephrosclerosis, the most common form of
kidney disease, is seen to at least some extent in about
70 per cent of postmortem examinations in people who
die after the age of 60. This type of vascular lesion
occurs in the smaller interlobular arteries and in the
afferent arterioles of the kidney. It is believed to begin
with leakage of plasma through the intimal membrane
of these vessels. This causes ﬁbrinoid deposits to
develop in t

els. This causes ﬁbrinoid deposits to
develop in the medial layers of these vessels, followed
by progressive thickening of the vessel wall that even-
tually constricts the vessels and, in some cases, occludes
them. Because there is essentially no collateral circula-
tion among the smaller renal arteries, occlusion of one
or more of them causes destruction of a comparable
number of nephrons. Therefore, much of the kidney
tissue becomes replaced by small amounts of ﬁbrous
tissue.When sclerosis occ

 small amounts of ﬁbrous
tissue.When sclerosis occurs in the glomeruli, the injury
is referred to as glomerulosclerosis.
Nephrosclerosis and glomerulosclerosis occur to
some extent in most people after the fourth decade of
life, causing about a 10 per cent decrease in the number
of functional nephrons each 10 years after age 40
(Figure 31–3). This loss of glomeruli and overall
nephron function is reﬂected by a progressive decrease
in both renal blood ﬂow and GFR. Even in “normal”
people, kidney 

lood ﬂow and GFR. Even in “normal”
people, kidney plasma ﬂow and GFR decrease by 40 to
50 per cent by age 80.
The frequency and severity of nephrosclerosis and
glomerulosclerosis are greatly increased by concurrent
hypertension or diabetes mellitus. In fact, diabetes mel-
litus and hypertension are the two most important
causes of end-stage renal disease, as discussed previ-
ously. Thus, benign nephrosclerosis in association with
severe hypertension can lead to a rapidly progressing
malignant ne

ion can lead to a rapidly progressing
malignant nephrosclerosis. The characteristic histologi-
cal features of malignant nephrosclerosis include large
amounts of ﬁbrinoid deposits in the arterioles and pro-
gressive thickening of the vessels, with severe ischemia
occurring in the affected nephrons. For unknown
reasons, the incidence of malignant nephrosclerosis and
severe glomerulosclerosis is signiﬁcantly higher in
blacks than in whites of similar ages who have similar
degrees of severity of hy

ar ages who have similar
degrees of severity of hypertension or diabetes.
Injury to the Glomeruli as a Cause
of Chronic Renal Failure—
Chronic glomerulonephritis can be caused by several
diseases that cause inﬂammation and damage to the
capillary loops in the glomeruli of the kidneys. In con-
trast to the acute form of this disease, chronic glomeru-
lonephritis is a slowly progressive disease that often
leads to irreversible renal failure. It may be a primary
kidney disease, f

nal failure. It may be a primary
kidney disease, following acute glomerulonephritis, or it
may be secondary to systemic diseases, such as lupus
erythematosus.
In most cases, chronic glomerulonephritis begins with
accumulation of precipitated antigen-antibody com-
plexes in the glomerular membrane. In contrast to acute
glomerulonephritis, streptococcal infections account for
only a small percentage of patients with the chronic
form of glomerulonephritis. The results of the accumu-
lation of antig

hritis. The results of the accumu-
lation of antigen-antibody complex in the glomerular
membranes are inﬂammation, progressive thickening of
the membranes, and eventual invasion of the glomeruli
Table 31–5
Most Common Causes of End-Stage Renal Disease (ESRD)
Cause
Percentage of Total ESRD Patients
44
26
8
Polycystic kidney disease
2
Other/unknown
20
Glomeruli (x 106)
0.5
0.0
1.0
2.0
2.5
1.5
0
20
40
60
80
Figure 31–3
Effect of aging on

0
60
80
Effect of aging on the number of functional glomeruli.
Chapter 31
409
by ﬁbrous tissue. In the later stages of the disease, the
glomerular capillary ﬁltration coefﬁcient becomes
greatly reduced because of decreased numbers of ﬁlter-
ing capillaries in the glomerular tufts and because of
thickened glomerular membranes. In the ﬁnal stages of
the disease, many glomeruli are replaced by ﬁbrous
tissue and are, therefore, unable to ﬁlter ﬂu

rous
tissue and are, therefore, unable to ﬁlter ﬂuid.
Injury to the Renal Interstitium as a
Primary or secondary disease of the renal interstitium
is referred to as interstitial nephritis. In general, this can
result from vascular, glomerular, or tubular damage that
destroys individual nephrons, or it can involve primary
damage to the renal interstitium by poisons, drugs, and
bacterial infections.
Renal interstitial injury caused by bacterial infec-

nal interstitial injury caused by bacterial infec-
tion is called pyelonephritis. The infection can result
from different types of bacteria but especially from
Escherichia coli that originate from fecal contamination
of the urinary tract. These bacteria reach the kidneys
either by way of the blood stream or, more commonly,
by ascension from the lower urinary tract by way of the
ureters to the kidneys.
Although the normal bladder is able to clear bacteria
readily, there are two general clinical c

bacteria
readily, there are two general clinical conditions that
may interfere with the normal ﬂushing of bacteria from
the bladder: (1) the inability of the bladder to empty
completely, leaving residual urine in the bladder, and 
(2) the existence of obstruction of urine outﬂow. With
impaired ability to ﬂush bacteria from the bladder, the
bacteria multiply and the bladder becomes inﬂamed, a
condition termed cystitis. Once cystitis has occurred, it
may remain localized without ascending to the k

it
may remain localized without ascending to the kidney,
or in some people, bacteria may reach the renal pelvis
because of a pathological condition in which urine is
propelled up one or both of the ureters during micturi-
tion. This condition is called vesicoureteral reﬂux and is
due to the failure of the bladder wall to occlude the
ureter during micturition; as a result, some of the urine
is propelled upward toward the kidney, carrying with it
bacteria that can reach the renal pelvis and renal

acteria that can reach the renal pelvis and renal
medulla, where they can initiate the infection and
inﬂammation associated with pyelonephritis.
Pyelonephritis begins in the renal medulla and there-
fore usually affects the function of the medulla more
than it affects the cortex, at least in the initial stages.
Because one of the primary functions of the medulla is
to provide the countercurrent mechanism for concen-
trating urine, patients with pyelonephritis frequently
have markedly impaired ab

yelonephritis frequently
have markedly impaired ability to concentrate the urine.
With long-standing pyelonephritis, invasion of the
kidneys by bacteria not only causes damage to the renal
medulla interstitium but also results in progressive
damage of renal tubules, glomeruli, and other structures
throughout the kidney. Consequently, large parts of
functional renal tissue are lost, and chronic renal failure
can develop.
Nephrotic Syndrome—Excretion 
of Protein in the Urine Because of
Increased G

on 
Increased Glomerular Permeability
Many patients with kidney disease develop the
nephrotic syndrome, which is characterized by loss of
large quantities of plasma proteins into the urine. In
some instances, this occurs without evidence of other
major abnormalities of kidney function, but more often
it is associated with some degree of renal failure.
The cause of the protein loss in the urine is increased
permeability of the glomerular membrane. Therefore,
any

ability of the glomerular membrane. Therefore,
any disease that increases the permeability of this mem-
brane can cause the nephrotic syndrome. Such diseases
include (1) chronic glomerulonephritis, which affects
primarily the glomeruli and often causes greatly
increased permeability of the glomerular membrane;
(2) amyloidosis, which results from deposition of an
abnormal proteinoid substance in the walls of the blood
vessels and seriously damages the basement membrane
of the glomeruli; and (3) m

 the basement membrane
of the glomeruli; and (3) minimal change nephrotic syn-
drome, which is associated with no major abnormality
in the glomerular capillary membrane that can be
detected with light microscopy.As discussed in Chapter
26, minimal change nephropathy has been found to 
be associated with loss of the negative charges that are
normally present in the glomerular capillary basement
membrane. Immunologic studies have also shown
abnormal immune reactions in some cases, suggesting
that 

l immune reactions in some cases, suggesting
that the loss of the negative charges may have resulted
from antibody attack on the membrane. Loss of normal
negative charges in the basement membrane of the
glomerular capillaries allows proteins,
especially
albumin, to pass through the glomerular membrane
with ease because the negative charges in the basement
membrane normally repel the negatively charged
plasma proteins.
Minimal change nephropathy can occur in adults, but
more frequently it occurs 

an occur in adults, but
more frequently it occurs in children between the ages
of 2 and 6 years. Increased permeability of the glomeru-
lar capillary membrane occasionally allows as much as
40 grams of plasma protein loss into the urine each day,
which is an extreme amount for a young child. There-
fore, the child’s plasma protein concentration often falls
below 2 g/dl, and the colloid osmotic pressure falls from
a normal value of 28 to less than 10 mm Hg. As a con-
sequence of this low colloid 

 10 mm Hg. As a con-
sequence of this low colloid osmotic pressure in the
plasma, large amounts of ﬂuid leak from the capillaries
all over the body into most of the tissues, causing severe
edema, as discussed in Chapter 25.
Nephron Function in Chronic 
Loss of Functional Nephrons Requires the Surviving Nephrons
to Excrete More Water and Solutes. It would be reasonable
to suspect that decreasing the number of functional
nephrons, which reduces the GFR, would also cause
major decreas

ch reduces the GFR, would also cause
major decreases in renal excretion of water and
solutes. Yet patients who have lost as much as 75 per
cent of their nephrons are able to excrete normal
amounts of water and electrolytes without serious
accumulation of any of these in the body ﬂuids.
Further reduction in the number of nephrons,
however, leads to electrolyte and ﬂuid retention, and
death usually ensues when the number of nephrons
falls below 5 to 10 per cent of normal.
In contrast to the electr

o 10 per cent of normal.
In contrast to the electrolytes, many of the waste
products of metabolism, such as urea and creatinine,
accumulate almost in proportion to the number of
nephrons that have been destroyed.The reason for this
410
Unit V
is that substances such as creatinine and urea depend
largely on glomerular ﬁltration for their excretion, and
they are not reabsorbed as avidly as the electrolytes.
Creatinine, for example, is not reabsorbed at all, and
the excr

or example, is not reabsorbed at all, and
the excretion rate is equal to the rate at which it is 
ﬁltered.
Creatinine ﬁltration rate 
= GFR ¥ Plasma creatinine concentration 
= Creatinine excretion rate
Therefore, if GFR decreases, the creatinine excre-
tion rate also transiently decreases, causing accumula-
tion of creatinine in the body ﬂuids and raising plasma
concentration until the excretion rate of creatinine
returns to normal—the same rate at which creatinine
is produced in the body (Figu

 at which creatinine
is produced in the body (Figure 31–4). Thus, under
steady-state conditions, the creatinine excretion rate
equals the rate of creatinine production,despite reduc-
tions in GFR; however, this normal rate of creatinine
excretion occurs at the expense of elevated plasma
creatinine concentration, as shown in curve A of
Figure 31–5.
Some solutes, such as phosphate, urate, and hydro-
gen ions, are often maintained near the normal range
until GFR falls below 20 to 30 per cent of nor

nge
until GFR falls below 20 to 30 per cent of normal.
Thereafter, the plasma concentrations of these sub-
stances rise, but not in proportion to the fall in GFR,
as shown in curve B of Figure 31–5. Maintenance of
relatively constant plasma concentrations of these
solutes as GFR declines is accomplished by excreting
progressively larger fractions of the amounts of these
solutes that are ﬁltered at the glomerular capillaries;
this occurs by decreasing the rate of tubular reab-
sorption or, in som

sing the rate of tubular reab-
sorption or, in some instances, by increasing tubular
secretion rates.
In the case of sodium and chloride ions, their plasma
concentrations are maintained virtually constant 
even with severe decreases in GFR (see curve C of
Figure 31–5). This is accomplished by greatly de-
creasing tubular reabsorption of these electrolytes.
For example, with a 75 per cent loss of functional
nephrons, each surviving nephron must excrete four
times as much sodium and four times as 

crete four
times as much sodium and four times as much volume
as under normal conditions (Table 31–6).
Part of this adaptation occurs because of increased
blood ﬂow and increased GFR in each of the surviv-
ing nephrons, owing to hypertrophy of the blood
vessels and glomeruli, as well as functional changes
that cause the blood vessels to vasodilate. Even with
large decreases in the total GFR, normal rates of renal
Days 
2
1
0
0
1
2
3
4
Positive ba

xcretion (g/day)
Days 
2
1
0
0
1
2
3
4
2
1
0
100
50
0
Figure 31–4
Effect of reducing glomerular ﬁltration rate (GFR) by 50 per cent
on serum creatinine concentration and on creatinine excretion rate
when the production rate of creatinine remains constant.
Plasma concentration
0
25
50
75
100
A
B
C
Figure 31–5
Representative patterns of ad

l)
A
B
C
Representative patterns of adaptation for different types of solutes
in chronic renal failure. Curve A shows the approximate changes
in the plasma concentrations of solutes such as creatinine and
urea that are ﬁltered and poorly reabsorbed. Curve B shows the
approximate concentrations for solutes such as phosphate and
urate. Curve C shows the approximate concentrations for solutes
such as sodium and chloride.
Chapter 31
411
excretion can still b

y Diseases and Diuretics
411
excretion can still be maintained by decreasing the
rate at which the tubules reabsorb water and solutes.
Isosthenuria—Inability of the Kidney to Concentrate or Dilute
the Urine. One important effect of the rapid rate of
tubular ﬂow that occurs in the remaining nephrons of
diseased kidneys is that the renal tubules lose their
ability to concentrate or dilute the urine. The concen-
trating ability of the kidney is impaired mainly because
(1) the rapid ﬂow of tubular ﬂ

ired mainly because
(1) the rapid ﬂow of tubular ﬂuid through the collect-
ing ducts prevents adequate water reabsorption, and
(2) the rapid ﬂow through both the loop of Henle and
the collecting ducts prevents the countercurrent mech-
anism from operating effectively to concentrate the
medullary interstitial ﬂuid solutes. Therefore, as pro-
gressively more nephrons are destroyed, the maximum
concentrating ability of the kidney declines, and urine
osmolarity and speciﬁc gravity (a measure of the 

osmolarity and speciﬁc gravity (a measure of the total
solute concentration) approach the osmolarity and
speciﬁc gravity of the glomerular ﬁltrate, as shown in
Figure 31–6.
The diluting mechanism in the kidney is also
impaired when the number of nephrons decreases
because the rapid ﬂushing of ﬂuid through the loops
of Henle and the high load of solutes such as urea
cause a relatively high solute concentration in the
tubular ﬂuid of this part of the nephron. As a conse-
quence, the diluting capa

the nephron. As a conse-
quence, the diluting capacity of the kidney is impaired,
and the minimal urine osmolality and speciﬁc gravity
approach those of the glomerular ﬁltrate. Because the
concentrating mechanism becomes impaired to a
greater extent than does the diluting mechanism in
chronic renal failure, an important clinical test of renal
function is to determine how well the kidneys can 
concentrate urine when a person’s water intake is
restricted for 12 or more hours.
Effects of Renal Fail

ricted for 12 or more hours.
Effects of Renal Failure on the Body
The effect of complete renal failure on the body ﬂuids
depends on (1) water and food intake and (2) the
degree of impairment of renal function. Assuming that
a person with complete renal failure continues to ingest
the same amounts of water and food, the concentrations
of different substances in the extracellular ﬂuid are
approximately those shown in Figure 31–7. Important
effects include (1) generalized edema result

rtant
effects include (1) generalized edema resulting from
water and salt retention, (2) acidosis resulting from
failure of the kidneys to rid the body of normal acidic
products, (3) high concentration of the nonprotein nitro-
gens—especially urea, creatinine, and uric acid—result-
ing from failure of the body to excrete the metabolic
end products of proteins, and (4) high concentrations of
other substances excreted by the kidney, including
phenols, sulfates, phosphates, potassium, and guanidine

ls, sulfates, phosphates, potassium, and guanidine
bases. This total condition is called uremia because of
the high concentration of urea in the body ﬂuids.
Table 31–6
Total Kidney Excretion and Excretion per Nephron in 
75% Loss of 
Normal
Nephrons
Number of nephrons
2,000,000
500,000
Total GFR (ml/min)
125
40
Single nephron GFR (nl/min)
62.5
80
Volume excreted for all 
1.5
1.5
nephrons (ml/min)
Volume excreted per nephron
0.75
3.0
(nl/min)
GFR, glomerular ﬁltration rate.
Specific

(nl/min)
Specific gravity of urine
2,000,000 1,500,000 1,000,000
Isosthenuria
Glomerular filtrate specific gravity
500,000
0
1.000
1.010
1.020
1.030
1.040
1.050
Number of nephrons in both kidneys
Figure 31–6
Development of isosthenuria in a patient with decreased numbers
of functional nephrons.
Decrease
Increase
3
0
6
Kidney shutdown
Normal
NPN
Water
Phenols
HPO4
=
HCO3
-
SO4
=
K+
H+
Na+
9
12
Days
Figure 31–7
Effect of kidney failure on extracellular ﬂuid constit

ct of kidney failure on extracellular ﬂuid constituents. NPN,
nonprotein nitrogens.
412
Unit V
Water Retention and Development of Edema in Renal Failure. If
water intake is restricted immediately after acute renal
failure begins, the total body ﬂuid content may become
only slightly increased. If ﬂuid intake is not limited and
the patient drinks in response to the normal thirst mech-
anisms, the body ﬂuids begin to increase immediately
and rapidly.
With chronic partial

ease immediately
With chronic partial kidney failure, accumulation of
ﬂuid may not be severe, as long as salt and ﬂuid intake
are not excessive, until kidney function falls to 25 per
cent of normal or lower.The reason for this, as discussed
previously, is that the surviving nephrons excrete larger
amounts of salt and water. Even the small ﬂuid reten-
tion that does occur, along with increased secretion of
renin and angiotensin II that usually occurs in ischemic
kidney disease, often

t usually occurs in ischemic
kidney disease, often causes severe hypertension in
chronic renal failure. Almost all patients with kidney
function so reduced as to require dialysis to preserve life
develop hypertension. In most of these patients, severe
reduction of salt intake or removal of extracellular ﬂuid
by dialysis can control the hypertension. The remaining
patients continue to have hypertension even after
excess sodium has been removed by dialysis. In this
group, removal of the ischemic k

dialysis. In this
group, removal of the ischemic kidneys usually corrects
the hypertension (as long as ﬂuid retention is prevented
by dialysis) because it removes the source of excessive
renin secretion and subsequent increased angiotensin II
Uremia—Increase in Urea and Other Nonprotein Nitrogens
(Azotemia). The nonprotein nitrogens include urea, uric
acid, creatinine, and a few less important compounds.
These, in general, are the end products of protein
metabolism and must be removed

products of protein
metabolism and must be removed from the body to
ensure continued normal protein metabolism in the
cells. The concentrations of these, particularly of urea,
can rise to as high as 10 times normal during 1 to 2
weeks of total renal failure. With chronic renal failure,
the concentrations rise approximately in proportion to
the degree of reduction in functional nephrons. For this
reason, measuring the concentrations of these sub-
stances, especially of urea and creatinine, provid

stances, especially of urea and creatinine, provides an
important means for assessing the degree of renal
failure.
Acidosis in Renal Failure. Each day the body normally pro-
duces about 50 to 80 millimoles more metabolic acid
than metabolic alkali. Therefore, when the kidneys fail
to function, acid accumulates in the body ﬂuids. The
buffers of the body ﬂuids normally can buffer 500 to
1000 millimoles of acid without lethal increases in extra-
cellular ﬂuid hydrogen ion concentration, and the pho

lular ﬂuid hydrogen ion concentration, and the phos-
phate compounds in the bones can buffer an additional
few thousand millimoles of hydrogen ion. However,
when this buffering power is used up, the blood pH falls
drastically, and the patient will become comatose and
die if the pH falls below about 6.8.
Anemia in Chronic Renal Failure Caused by Decreased Erythro-
poietin Secretion. Patients with severe chronic renal
failure almost always develop anemia. The most im-
portant cause of this is decr

anemia. The most im-
portant cause of this is decreased renal secretion of 
erythropoietin, which stimulates the bone marrow to
produce red blood cells. If the kidneys are seriously
damaged, they are unable to form adequate quantities
of erythropoietin, which leads to diminished red blood
cell production and consequent anemia.
Osteomalacia in Chronic Renal Failure Caused by Decreased 
Production of Active Vitamin D and by Phosphate Retention by 
the Kidneys. Prolonged renal failure also causes o

the Kidneys. Prolonged renal failure also causes osteo-
malacia, a condition in which the bones are partially
absorbed and, therefore, become greatly weakened. An
important cause of this condition is the following:
Vitamin D must be converted by a two-stage process,
ﬁrst in the liver and then in the kidneys, into 1,25–
dihydroxycholecalciferol before it is able to promote
calcium absorption from the intestine. Therefore,
serious damage to the kidney greatly reduces the blood
concentration of act

ney greatly reduces the blood
concentration of active vitamin D, which in turn
decreases intestinal absorption of calcium and the avail-
ability of calcium to the bones.
Another important cause of demineralization of the
skeleton in chronic renal failure is the rise in serum
phosphate concentration that occurs as a result of
decreased GFR. This rise in serum phosphate causes
increased binding of phosphate with calcium in the
plasma, thus decreasing the plasma serum ionized
calcium concentration,

ng the plasma serum ionized
calcium concentration, which in turn stimulates parathy-
roid hormone secretion. This secondary hyperparathy-
roidism then stimulates the release of calcium from
bones, causing further demineralization of the bones.
As discussed earlier in this chapter, hypertension can
exacerbate injury to the glomeruli and blood vessels of
the kidneys and is a major cause of end-stage renal
disease. Conversely, abnormalities of kidney function
can cau

nversely, abnormalities of kidney function
can cause hypertension,as discussed in detail in Chapter
19. Thus, the relation between hypertension and kidney
disease can, in some instances, propagate a vicious
circle: primary kidney damage leads to increased blood
pressure, which in turn causes further damage to the
kidneys, further increases in blood pressure, and so
forth, until end-stage renal disease develops.
Not all types of kidney disease cause hypertension,
because damage to certain portion

se hypertension,
because damage to certain portions of the kidney cause
uremia without hypertension. Nevertheless, some types
of renal damage are particularly prone to cause hyper-
tension. A classiﬁcation of kidney disease relative 
to hypertensive or nonhypertensive effects is the 
Renal Lesions That Reduce the Ability of the Kidneys to Excrete
Sodium and Water Promote Hypertension. Renal lesions that
decrease the ability of the kidneys to excrete sodium
and water almost invariably 

eys to excrete sodium
and water almost invariably cause hypertension. There-
fore, lesions that either decrease GFR or increase
tubular reabsorption usually lead to hypertension of
varying degrees. Some speciﬁc types of renal abnormal-
ities that can cause hypertension are as follows:
1. Increased renal vascular resistance, which reduces
renal blood ﬂow and GFR. An example is
hypertension caused by renal artery stenosis.
2. Decreased glomerular capillary ﬁltration coefﬁcient,
which reduces GFR. 

apillary ﬁltration coefﬁcient,
which reduces GFR. An example of this is chronic
glomerulonephritis, which causes inﬂammation and
thickening of the glomerular capillary membranes,
thereby reducing the glomerular capillary ﬁltration
coefﬁcient.
3. Excessive tubular sodium reabsorption. An example
is hypertension caused by excessive aldosterone
secretion, which increases sodium reabsorption
mainly in the cortical collecting tubules.
Chapter 31
413
Once hypertension has

413
Once hypertension has developed, renal excretion of
sodium and water returns to normal because the high
arterial pressure causes pressure natriuresis and pres-
sure diuresis, so that intake and output of sodium 
and water become balanced once again. Even when
there are large increases in renal vascular resistance or
decreases in the glomerular capillary coefﬁcient, the
GFR may still return to nearly normal levels after the
arterial blood pressure rises. Likewise, whe

r the
arterial blood pressure rises. Likewise, when tubular
reabsorption is increased, as occurs with excessive
aldosterone secretion, the urinary excretion rate is ini-
tially reduced but then returns to normal as arterial
pressure rises. Thus, after hypertension develops, there
may be no sign of impaired excretion of sodium and
water other than the hypertension. As explained in
Chapter 19, normal excretion of sodium and water at an
elevated arterial pressure means that pressure natriure-
sis a

erial pressure means that pressure natriure-
sis and pressure diuresis have been reset to a higher
Hypertension Caused by Patchy Renal Damage and Increased
Renal Secretion of Renin. If one part of the kidney is
ischemic and the remainder is not ischemic, such as
occurs when one renal artery is severely constricted, the
ischemic renal tissue secretes large quantities of renin.
This secretion leads to the formation of angiotensin II,
which can cause hypertension.The most likely 

 II,
which can cause hypertension.The most likely sequence
of events in causing this hypertension, as discussed in
Chapter 19, is (1) the ischemic kidney tissue itself
excretes less than normal amounts of water and salt; (2)
the renin secreted by the ischemic kidney, and subse-
quent increased angiotensin II formation, affects the
nonischemic kidney tissue, causing it also to retain salt
and water; and (3) excess salt and water cause hyper-
tension in the usual manner.
A similar type of hyperten

on in the usual manner.
A similar type of hypertension can result when patchy
areas of one or both kidneys become ischemic as a
result of arteriosclerosis or vascular injury in speciﬁc
portions of the kidneys. When this occurs, the ischemic
nephrons excrete less salt and water but secrete greater
amounts of renin, which causes increased angiotensin 
II formation. The high levels of angiotensin II then
impair the ability of the surrounding otherwise normal
nephrons to excrete sodium and water. As

se normal
nephrons to excrete sodium and water. As a result,
hypertension develops, which restores the overall 
excretion of sodium and water by the kidney, so that
balance between intake and output of salt and water 
is maintained, but at the expense of high blood 
pressure.
Kidney Diseases That Cause Loss of Entire Nephrons Lead to Renal
Failure But May Not Cause Hypertension. Loss of large
numbers of whole nephrons, such as occurs with the loss
of one kidney and part of another kidney, almost

s
of one kidney and part of another kidney, almost always
leads to renal failure if the amount of kidney tissue lost
is great enough. If the remaining nephrons are normal
and the salt intake is not excessive, this condition might
not cause clinically signiﬁcant hypertension, because
even a slight rise in blood pressure will raise the GFR
and decrease tubular sodium reabsorption sufﬁciently
to promote enough water and salt excretion in the urine,
even with the few nephrons that remain intact.
How

However, a patient with this type of abnormality may
become severely hypertensive if additional stresses are
imposed, such as eating a large amount of salt. In this
case, the kidneys simply cannot clear adequate quanti-
ties of salt with the small number of functioning
nephrons that remain.
In Chapter 27, we point out that several mechanisms are
responsible for transporting different individual sub-
stances across the tubul

different individual sub-
stances across the tubular epithelial membranes. In
Chapter 3, we also point out that each cellular enzyme
and each carrier protein is formed in response to a
respective gene in the nucleus. If any required gene
happens to be absent or abnormal, the tubules may be
deﬁcient in one of the appropriate carrier proteins or
one of the enzymes needed for solute transport by the
renal tubular epithelial cells. For this reason, many
hereditary tubular disorders occur because of 

any
hereditary tubular disorders occur because of the trans-
port of individual substances or groups of substances
through the tubular membrane. In addition, damage to
the tubular epithelial membrane by toxins or ischemia
can cause important renal tubular disorders.
Renal Glycosuria—Failure of the Kidneys to Reabsorb Glucose.
In this condition, the blood glucose concentration may
be normal, but the transport mechanism for tubular
reabsorption of glucose is greatly limited or absent.
Consequently

glucose is greatly limited or absent.
Consequently, despite a normal blood glucose level,
large amounts of glucose pass into the urine each day.
Because diabetes mellitus is also associated with the
presence of glucose in the urine, renal glycosuria, which
is a relatively benign condition,must be ruled out before
making a diagnosis of diabetes mellitus.
Aminoaciduria—Failure of the Kidneys to Reabsorb Amino Acids.
Some amino acids share mutual transport systems for
reabsorption, whereas other am

ansport systems for
reabsorption, whereas other amino acids have their own
distinct transport systems. Rarely, a condition called
generalized aminoaciduria results from deﬁcient reab-
sorption of all amino acids; more frequently, deﬁciencies
of speciﬁc carrier systems may result in (1) essential
cystinuria, in which large amounts of cystine fail to be
reabsorbed and often crystallize in the urine to form
renal stones; (2) simple glycinuria, in which glycine fails
to be reabsorbed; or (3) beta-am

ich glycine fails
to be reabsorbed; or (3) beta-aminoisobutyricaciduria,
which occurs in about 5 per cent of all people but appar-
ently has no major clinical signiﬁcance.
Renal Hypophosphatemia—Failure of the Kidneys to Reabsorb
Phosphate. In renal hypophosphatemia,
the renal
tubules fail to reabsorb large enough quantities of phos-
phate ions when the phosphate concentration of the
body ﬂuids falls very low. This condition usually does
not cause serious immediate abnormalities, because 
the ph

e serious immediate abnormalities, because 
the phosphate concentration of the extracellular ﬂuid
can vary widely without causing major cellular dys-
function. Over a long period, a low phosphate level
causes diminished calciﬁcation of the bones, causing the
person to develop rickets. This type of rickets is refrac-
tory to vitamin D therapy, in contrast to the rapid
response of the usual type of rickets, as discussed in
Renal Tubular Acidosis—Failure of the Tubules to Secrete Hydro-

 Acidosis—Failure of the Tubules to Secrete Hydro-
gen Ions. In this condition, the renal tubules are unable
to secrete adequate amounts of hydrogen ions. As a
result, large amounts of sodium bicarbonate are contin-
ually lost in the urine. This causes a continued state of
metabolic acidosis, as discussed in Chapter 30.This type
of renal abnormality can be caused by hereditary dis-
orders, or it can occur as a result of widespread injury
to the renal tubules.
414
Unit V
The Body Fluids and Kidne

enal tubules.
414
Unit V
Nephrogenic Diabetes Insipidus—Failure of the Kidneys to
Respond to Antidiuretic Hormone. Occasionally, the renal
tubules do not respond to antidiuretic hormone, causing
large quantities of dilute urine to be excreted. As long
as the person is supplied with plenty of water, this con-
dition seldom causes severe difﬁculty. However, when
adequate quantities of water are not available, the
person rapidly becomes dehydrated.
Fanconi’s Syndrome—A G

rapidly becomes dehydrated.
Fanconi’s Syndrome—A Generalized Reabsorptive Defect of the
Renal Tubules. Fanconi’s syndrome is usually associated
with increased urinary excretion of virtually all amino
acids, glucose, and phosphate. In severe cases, other
manifestations are also observed, such as (1) failure 
to reabsorb sodium bicarbonate, which results in 
metabolic acidosis; (2) increased excretion of potassium
and sometimes calcium; and (3) nephrogenic diabetes
insipidus.
There are multiple ca

hrogenic diabetes
There are multiple causes of Fanconi’s syndrome,
which results from a generalized inability of the renal
tubular cells to transport various substances. Some of
these causes include (1) hereditary defects in cell trans-
port mechanisms, (2) toxins or drugs that injure the
renal tubular epithelial cells, and (3) injury to the renal
tubular cells as a result of ischemia. The proximal
tubular cells are especially affected in Fanconi’s syn-
drome caused by tubular injury,

 in Fanconi’s syn-
drome caused by tubular injury, because these cells
reabsorb and secrete many of the drugs and toxins that
can cause damage.
Treatment of Renal Failure
by Dialysis with an 
Artiﬁcial Kidney
Severe loss of kidney function, either acutely or chron-
ically, is a threat to life and requires removal of toxic
waste products and restoration of body ﬂuid volume
and composition toward normal. This can be accom-
plished by dialysis with an artiﬁcial kidney. In certain
types of acute ren

an artiﬁcial kidney. In certain
types of acute renal failure, an artiﬁcial kidney may be
used to tide the patient over until the kidneys resume
their function. If the loss of kidney function is irre-
versible, it is necessary to perform dialysis chronically
to maintain life. In the United States alone, nearly
300,000 people with irreversible renal failure or even
total kidney removal are being maintained by dialysis
with artiﬁcial kidneys. Because dialysis cannot maintain
completely normal body 

e dialysis cannot maintain
completely normal body ﬂuid composition and cannot
replace all the multiple functions performed by the
kidneys, the health of patients maintained on artiﬁcial
kidneys usually remains signiﬁcantly impaired.A better
treatment for permanent loss of kidney function is to
restore functional kidney tissue by means of a kidney
transplant.
Basic Principles of Dialysis. The basic principle of the arti-
ﬁcial kidney is to pass blood through minute blood
channels bounded by a thi

ood through minute blood
channels bounded by a thin membrane. On the other
side of the membrane is a dialyzing ﬂuid into which
unwanted substances in the blood pass by diffusion.
Figure 31–8 shows the components of one type of arti-
ﬁcial kidney in which blood ﬂows continually between
two thin membranes of cellophane; outside the mem-
brane is a dialyzing ﬂuid. The cellophane is porous
enough to allow the constituents of the plasma, except
the plasma proteins, to diffuse in both directions—from

asma proteins, to diffuse in both directions—from
plasma into the dialyzing ﬂuid or from the dialyzing
ﬂuid back into the plasma. If the concentration of a sub-
stance is greater in the plasma than in the dialyzing
ﬂuid, there will be a net transfer of the substance from
the plasma into the dialyzing ﬂuid.
The rate of movement of solute across the dialyzing
membrane depends on (1) the concentration gradient
of the solute between the two solutions, (2) the per-
meability of the membrane to the so

, (2) the per-
meability of the membrane to the solute, (3) the surface
area of the membrane, and (4) the length of time 
that the blood and ﬂuid remain in contact with the 
membrane.
Thus, the maximum rate of solute transfer occurs ini-
tially when the concentration gradient is greatest (when
dialysis is begun) and slows down as the concentration
gradient is dissipated. In a ﬂowing system, as is the case
with “hemodialysis,” in which blood and dialysate ﬂuid
ﬂow through the artiﬁcial kidney, th

ialysate ﬂuid
ﬂow through the artiﬁcial kidney, the dissipation of the
concentration gradient can be reduced and diffusion of
solute across the membrane can be optimized by
increasing the ﬂow rate of the blood, the dialyzing ﬂuid,
or both.
In normal operation of the artiﬁcial kidney, blood
ﬂows continually or intermittently back into the vein.
The total amount of blood in the artiﬁcial kidney at any
one time is usually less than 500 milliliters, the rate of
ﬂow may be several hundred milliliters

the rate of
ﬂow may be several hundred milliliters per minute, and
Bubble
trap
Fresh dialyzing
solution
Used dialyzing
solution
Constant
temperature
bath
membrane
Flowing
dialysate
Flowing
blood
Dialyzer
Blood in
Blood out
Dialysate
in
Dialysate
out
Waste
products
Water
Figure 31–8
Principles of dialysis with an artiﬁcial kidney.
Chapter 31
415
the total diffusion surface area is between 0.6 and 2.5
square meters. To prevent coagulation of the blood in

are meters. To prevent coagulation of the blood in
the artiﬁcial kidney,a small amount of heparin is infused
into the blood as it enters the artiﬁcial kidney. In addi-
tion to diffusion of solutes, mass transfer of solutes and
water can be produced by applying a hydrostatic pres-
sure to force the ﬂuid and solutes across the membranes
of the dialyzer; such ﬁltration is called bulk ﬂow.
Dialyzing Fluid. Table 31–7 compares the constituents in
a typical dialyzing ﬂuid with those in normal plasma a

pical dialyzing ﬂuid with those in normal plasma and
uremic plasma. Note that the concentrations of ions and
other substances in dialyzing ﬂuid are not the same as
the concentrations in normal plasma or in uremic
plasma. Instead, they are adjusted to levels that are
needed to cause appropriate movement of water and
solutes through the membrane during dialysis.
Note that there is no phosphate, urea, urate, sulfate,
or creatinine in the dialyzing ﬂuid; however, these are
present in high concentrat

uid; however, these are
present in high concentrations in the uremic blood.
Therefore, when a uremic patient is dialyzed, these sub-
stances are lost in large quantities into the dialyzing
ﬂuid.
The effectiveness of the artiﬁcial kidney can be
expressed in terms of the amount of plasma that is
cleared of different substances each minute, which, as
discussed in Chapter 27, is the primary means for
expressing the functional effectiveness of the kidneys
themselves to rid the body of unwanted substa

neys
themselves to rid the body of unwanted substances.
Most artiﬁcial kidneys can clear urea from the plasma
at a rate of 100 to 225 ml/min, which shows that at least
for the excretion of urea, the artiﬁcial kidney can func-
tion about twice as rapidly as two normal kidneys
together, whose urea clearance is only 70 ml/min. Yet
the artiﬁcial kidney is used for only 4 to 6 hours per day,
three times a week.Therefore, the overall plasma clear-
ance is still considerably limited when the artiﬁcial

 is still considerably limited when the artiﬁcial
kidney replaces the normal kidneys.Also, it is important
to keep in mind that the artiﬁcial kidney cannot replace
some of the other functions of the kidneys, such as
secretion of erythropoietin, which is necessary for red
blood cell production.
Andreoli TE (ed): Cecil’s Essentials of Medicine, 6th ed.
Philadelphia: WB Saunders, 2004.
Fishbane SA, Scribner BH: Blood pressure control in dialy-
sis patients. Semin Dial 15:144, 2002.
Hall 

ialy-
Hall JE, Henegar JR, Dwyer TM, et al: Is obesity a major
cause of chronic renal disease? Adv Ren Replace Ther
11:41, 2004.
Hostetter TH: Prevention of the development and progres-
sion of renal disease. J Am Soc Nephrol 14(Suppl 2):S144,
2003.
Levey AS, Beto JA, Coronado BE, et al: Controlling the epi-
demic of cardiovascular disease in chronic renal disease.
What do we know? Wh

ease in chronic renal disease.
What do we know? What do we need to learn? Where do
we go from here? National Kidney Foundation Task Force
on Cardiovascular Disease.Am J Kidney Dis 32:853, 1998.
Luke RG: Chronic renal failure. In: Goldman F, Bennett JC
(eds): Cecil Textbook of Medicine, 21st ed. Philadelphia:
WB Saunders, 2000, pp 571-578.
Mitch WE: Acute renal failure. In: Goldman F, Bennett JC
WB Saunders, 2000, pp 567-570.
Molitoris BA:

phia:
Molitoris BA: Transitioning to therapy in ischemic acute
renal failure. J Am Soc Nephrol 14:265, 2003.
Sarnak MJ, Levey AS, Schoolwerth AC, et al: Kidney disease
as a risk factor for development of cardiovascular disease.
Hypertension 42:1050, 2003.
Schrier RW: Atlas of Diseases of the Kidney. http://
www.kidneyatlas.org/.
Shankar SS, Brater DC: Loop diuretics: from the Na-K-2Cl
transporter to clinical use. Am J Physiol Renal Physiol
284:F11, 2003.
Singri N, 

 J Physiol Renal Physiol
Singri N, Ahya SN, Levin ML: Acute renal failure. JAMA
289:747, 2003.
United States Renal Data System. http://www.usrds.org/.
Wilcox CS: New insights into diuretic use in patients with
chronic renal disease. J Am Soc Nephrol 13:798, 2002.
Table 31–7
Comparison of Dialyzing Fluid with Normal and Uremic
Plasma
Normal 
Dialyzing
Uremic
Constituent
Plasma
Fluid
Plasma
Electrolytes (mEq/L)
Na+
142
133
142
K+
5
1.0
7
Ca++
3
3.0
2
Mg++
1.5
1.5
1.5
Cl–
107
105
107

.0
7
Ca++
3
3.0
2
Mg++
1.5
1.5
1.5
Cl–
107
105
107
HCO3
–
24
35.7
14
Lactate–
1.2
1.2
1.2
HPO4
=
3
0
9
Urate–
0.3
0
2
Sulfate=
0.5
0
3
Nonelectrolytes
Glucose
100
125
100
Urea
26
0
200
1
0
6
U
N
I
T
VI
Blood Cells,
Immunity, and 
Blood Clotting
32. Red Blood Cells, Anemia, and Polycythemia
33. Resistance of the Body to Infection: I. Leukocytes,
Granulocytes, the Monocyte-Macrophage System, 
34. Resistance of the Body to Infection: II. Immunity 
and Allergy
35. Blood Ty

 Infection: II. Immunity 
35. Blood Types; Transfusion; Tissue and Organ
36. Hemostasis and Blood Coagulation
C
H
A
P
T
E
R
 
3
2
419
Red Blood Cells, Anemia,
and Polycythemia
With this chapter we begin discussing the blood cells
and cells of the macrophage system and lymphatic
system. We ﬁrst present the functions of red blood
cells, which are the most abundant cells of the blood
and are necessary for the delivery of oxygen to the
tissues.
Red Blood Cells (Erythrocyt

oxygen to the
tissues.
The major function of red blood cells, also known as erythrocytes, is to trans-
port hemoglobin, which in turn carries oxygen from the lungs to the tissues. In
some lower animals, hemoglobin circulates as free protein in the plasma, not
enclosed in red blood cells. When it is free in the plasma of the human being,
about 3 per cent of it leaks through the capillary membrane into the tissue
spaces or through the glomerular membrane of the kidne

es or through the glomerular membrane of the kidney into the glomerular
ﬁltrate each time the blood passes through the capillaries. Therefore, for hemo-
globin to remain in the human blood stream, it must exist inside red blood cells.
The red blood cells have other functions besides transport of hemoglobin.
For instance, they contain a large quantity of carbonic anhydrase, an enzyme
that catalyzes the reversible reaction between carbon dioxide (CO2) and water
to form carbonic acid (H2CO3), incre

O2) and water
to form carbonic acid (H2CO3), increasing the rate of this reaction several thou-
sandfold. The rapidity of this reaction makes it possible for the water of the
blood to transport enormous quantities of CO2 in the form of bicarbonate ion
(HCO3
–) from the tissues to the lungs, where it is reconverted to CO2 and
expelled into the atmosphere as a body waste product. The hemoglobin in the
cells is an excellent acid-base buffer (as is true of most proteins), so that the red
blood cells

rue of most proteins), so that the red
blood cells are responsible for most of the acid-base buffering power of whole
blood.
Shape and Size of Red Blood Cells. Normal red blood cells, shown in Figure 32–3,
are biconcave discs having a mean diameter of about 7.8 micrometers and 
a thickness of 2.5 micrometers at the thickest point and 1 micrometer or less 
in the center. The average volume of the red blood cell is 90 to 95 cubic 
micrometers.
The shapes of red blood cells can change remarkably as

shapes of red blood cells can change remarkably as the cells squeeze
through capillaries. Actually, the red blood cell is a “bag” that can be deformed
into almost any shape. Furthermore, because the normal cell has a great excess
of cell membrane for the quantity of material inside, deformation does not
stretch the membrane greatly and, consequently, does not rupture the cell, as
would be the case with many other cells.
Concentration of Red Blood Cells in the Blood. In normal men, the average nu

 Cells in the Blood. In normal men, the average number
of red blood cells per cubic millimeter is 5,200,000 (±300,000);in normal women,
it is 4,700,000 (±300,000). Persons living at high altitudes have greater numbers
of red blood cells. This is discussed later.
Quantity of Hemoglobin in the Cells. Red blood cells have the ability to concentrate
hemoglobin in the cell ﬂuid up to about 34 grams in each 100 milliliters of cells.
420
Unit VI
Blood Cells, Immunity, and Blood Clotting
The concentrati

ells, Immunity, and Blood Clotting
The concentration does not rise above this value,
because this is the metabolic limit of the cell’s hemo-
globin-forming mechanism. Furthermore, in normal
people, the percentage of hemoglobin is almost always
near the maximum in each cell. However, when hemo-
globin formation is deﬁcient, the percentage of hemo-
globin in the cells may fall considerably below this
value, and the volume of the red cell may also decrease
because of diminished hemoglobin to ﬁll th

ecrease
because of diminished hemoglobin to ﬁll the cell.
When the hematocrit (the percentage of blood that
is cells—normally, 40 to 45 per cent) and the quantity
of hemoglobin in each respective cell are normal, the
whole blood of men contains an average of 15 grams
of hemoglobin per 100 milliliters of cells; for women,
it contains an average of 14 grams per 100 milliliters.
As discussed in connection with blood transport of
oxygen in Chapter 40, each gram of pure hemoglobin
is capable of combi

, each gram of pure hemoglobin
is capable of combining with 1.34 milliliters of oxygen.
Therefore, in a normal man, a maximum of about 20
milliliters of oxygen can be carried in combination
with hemoglobin in each 100 milliliters of blood, and
in a normal woman, 19 milliliters of oxygen can be
carried.
Areas of the Body That Produce Red Blood Cells. In the early
weeks of embryonic life,primitive,nucleated red blood
cells are produced in the yolk sac. During the midd

ells are produced in the yolk sac. During the middle
trimester of gestation, the liver is the main organ for
production of red blood cells, but reasonable numbers
are also produced in the spleen and lymph nodes.Then,
during the last month or so of gestation and after birth,
red blood cells are produced exclusively in the bone
marrow.
As demonstrated in Figure 32–1, the bone marrow
of essentially all bones produces red blood cells until
a person is 5 years old. The marrow of the long bones,
excep

s 5 years old. The marrow of the long bones,
except for the proximal portions of the humeri and
tibiae, becomes quite fatty and produces no more red
blood cells after about age 20 years. Beyond this age,
most red cells continue to be produced in the marrow
of the membranous bones, such as the vertebrae,
sternum, ribs, and ilia. Even in these bones, the marrow
becomes less productive as age increases.
Genesis of Blood Cells
Pluripotential Hematopoietic Stem Cells, Growth Inducers, and
Differentia

ietic Stem Cells, Growth Inducers, and
Differentiation Inducers. The blood cells begin their lives
in the bone marrow from a single type of cell called
the pluripotential hematopoietic stem cell, from which
all the cells of the circulating blood are eventually
derived. Figure 32–2 shows the successive divisions of
the pluripotential cells to form the different circulat-
ing blood cells. As these cells reproduce, a small
portion of them remains exactly like the original
pluripotential cells and i

actly like the original
pluripotential cells and is retained in the bone marrow
to maintain a supply of these, although their numbers
diminish with age. Most of the reproduced cells,
however, differentiate to form the other cell types
shown to the right in Figure 32–2. The intermediate-
stage cells are very much like the pluripotential 
stem cells, even though they have already become
committed to a particular line of cells and are called
committed stem cells.
The different committed stem cells,

ed stem cells.
The different committed stem cells, when grown in
culture, will produce colonies of speciﬁc types of blood
cells. A committed stem cell that produces erythro-
cytes is called a colony-forming unit–erythrocyte, and
the abbreviation CFU-E is used to designate this type
of stem cell. Likewise, colony-forming units that form
granulocytes and monocytes have the designation
CFU-GM, and so forth.
Growth and reproduction of the different stem 
cells are controlled by multiple proteins cal

tem 
cells are controlled by multiple proteins called growth
inducers. Four major growth inducers have been
described, each having different characteristics. One of
these, interleukin-3, promotes growth and reproduc-
tion of virtually all the different types of committed
stem cells, whereas the others induce growth of only
speciﬁc types of cells.
The growth inducers promote growth but not dif-
ferentiation of the cells.This is the function of another
set of proteins called differentiation induce

ther
set of proteins called differentiation inducers. Each of
these causes one type of committed stem cell to dif-
ferentiate one or more steps toward a ﬁnal adult blood
cell.
Formation of the growth inducers and differentia-
tion inducers is itself controlled by factors outside the
bone marrow. For instance, in the case of erythrocytes
(red blood cells), exposure of the blood to low oxygen
for a long time results in growth induction, differenti-
ation, and production of greatly increased number

ation, and production of greatly increased numbers of
erythrocytes, as discussed later in the chapter. In the
case of some of the white blood cells, infectious 
diseases cause growth, differentiation, and eventual
formation of speciﬁc types of white blood cells that are
needed to combat each infection.
Stages of Differentiation of Red Blood Cells
The ﬁrst cell that can be identiﬁed as belonging to the
red blood cell series is the proerythroblast, shown at
the starting point in Figure 32–3. Unde

, shown at
the starting point in Figure 32–3. Under appropriate
0
25
50
75
100
Cellularity (per cent)
0
5 10 15 20
30
40
50
60
70
Rib
Tib
ia
(sh
a
ft
)
Femu
r
(
s
h
a
ft
)
Sternum
Vertebra
Figure 32–1
Relative rates of red blood cell production in the bone marrow of
different bones at different ages.
Chapter 32
Red Blood Cells, Anemia, and Polycythemia
421
PHSC
(Pluripotent
stem cell)
PHSC 
CFU-S
(Colony-forming
unit–spleen)
CFU-B
unit–blast)
CFU-E
(Colo

een)
CFU-B
CFU-E
unit–erythrocytes)
CFU-GM
(Colony-forming unit–
granulocytes, monocytes)
CFU-M
megakaryocytes)
LSC
(Lymphoid stem cell)
B lymphocytes
T lymphocytes
Megakaryocytes
Macrocytes
Granulocytes
Erythrocytes
Monocytes
(Neutrophils)
(Eosinophils)
(Basophils)
Platelets
Figure 32–2
Formation of the multiple different
blood cells from the original pluripo-
tent hematopoietic stem cell (PHSC)
in the bone marrow.
Proerythroblas

tem cell (PHSC)
Proerythroblast
Basophil
erythroblast
Microcytic,
hypochromic anemia
Megaloblastic anemia
Erythroblastosis fetalis
Sickle cell anemia
Polychromatophil
Orthochromatic
Reticulocyte
GENESIS OF RBC
Figure 32–3
Genesis of normal red blood cells (RBCs) and characteristics of RBCs in different types of anemias.
422
Unit VI
stimulation, large numbers of these cells are formed
from the CFU

rge numbers of these cells are formed
from the CFU-E stem cells.
Once the proerythroblast has been formed, it
divides multiple times, eventually forming many
mature red blood cells. The ﬁrst-generation cells are
called basophil erythroblasts because they stain with
basic dyes; the cell at this time has accumulated very
little hemoglobin. In the succeeding generations, as
shown in Figure 32–3, the cells become ﬁlled with
hemoglobin to a concentration of about 34 per cent,
the nucleus condenses to

ion of about 34 per cent,
the nucleus condenses to a small size, and its ﬁnal
remnant is absorbed or extruded from the cell. At the
same time, the endoplasmic reticulum is also reab-
sorbed. The cell at this stage is called a reticulocyte
because it still contains a small amount of basophilic
material, consisting of remnants of the Golgi appara-
tus, mitochondria, and a few other cytoplasmic
organelles. During this reticulocyte stage, the cells pass
from the bone marrow into the blood capillarie

ass
from the bone marrow into the blood capillaries by
diapedesis (squeezing through the pores of the capil-
lary membrane).
The remaining basophilic material in the reticulo-
cyte normally disappears within 1 to 2 days, and the
cell is then a mature erythrocyte. Because of the short
life of the reticulocytes, their concentration among all
the red cells of the blood is normally slightly less than
1 per cent.
Regulation of Red Blood Cell Production—Role
of Erythropoietin
The total mass of red blo

n—Role
The total mass of red blood cells in the circulatory
system is regulated within narrow limits, so that (1) an
adequate number of red cells is always available to
provide sufﬁcient transport of oxygen from the lungs
to the tissues,yet (2) the cells do not become so numer-
ous that they impede blood ﬂow.What we know about
this control mechanism is diagrammed in Figure 32–4
and is as follows.
Tissue Oxygenation Is the Most Essential Regulator of Red
Blood Cell Production. A

ssential Regulator of Red
Blood Cell Production. Any condition that causes the
quantity of oxygen transported to the tissues to
decrease ordinarily increases the rate of red blood cell
production. Thus, when a person becomes extremely
anemic as a result of hemorrhage or any other condi-
tion, the bone marrow immediately begins to produce
large quantities of red blood cells. Also, destruction of
major portions of the bone marrow by any means,
especially by x-ray therapy, causes hyperplasia of the

cially by x-ray therapy, causes hyperplasia of the
remaining bone marrow, thereby attempting to supply
the demand for red blood cells in the body.
At very high altitudes, where the quantity of oxygen
in the air is greatly decreased, insufﬁcient oxygen is
transported to the tissues, and red cell production is
greatly increased. In this case, it is not the concentra-
tion of red blood cells in the blood that controls red
cell production but the amount of oxygen transported
to the tissues in relati

unt of oxygen transported
to the tissues in relation to tissue demand for oxygen.
Various diseases of the circulation that cause
decreased blood ﬂow through the peripheral vessels,
and particularly those that cause failure of oxygen
absorption by the blood as it passes through the lungs,
can also increase the rate of red cell production. This
is especially apparent in prolonged cardiac failure and
in many lung diseases, because the tissue hypoxia
resulting from these conditions increases red cel

resulting from these conditions increases red cell pro-
duction, with a resultant increase in hematocrit and
usually total blood volume as well.
Erythropoietin Stimulates Red Cell Production, and Its Forma-
tion Increases in Response to Hypoxia. The principal stim-
ulus for red blood cell production in low oxygen states
is a circulating hormone called erythropoietin, a gly-
coprotein with a molecular weight of about 34,000. In
the absence of erythropoietin, hypoxia has little or no
effect in st

thropoietin, hypoxia has little or no
effect in stimulating red blood cell production. But
when the erythropoietin system is functional, hypoxia
causes a marked increase in erythropoietin produc-
tion, and the erythropoietin in turn enhances red
blood cell production until the hypoxia is relieved.
Role of the Kidneys in Formation of Erythropoietin. In
the normal person, about 90 per cent of all erythro-
poietin is formed in the kidneys; the remainder is
formed mainly in the liver. It is not know

nder is
formed mainly in the liver. It is not known exactly
where in the kidneys the erythropoietin is formed.
One likely possibility is that the renal tubular epithe-
lial cells secrete the erythropoietin, because anemic
blood is unable to deliver enough oxygen from the
peritubular capillaries to the highly oxygen-consuming
tubular cells, thus stimulating erythropoietin pro-
duction.
Tissue Oxygenation
Red Blood Cells
Proerythroblasts
Hematopoietic Stem Cells
Decreases
Kidney
Decreases
Factors 

tic Stem Cells
Decreases
Kidney
Decreases
Factors that decrease
oxygenation
1. Low blood volume
2. Anemia
3. Low hemoglobin
4. Poor blood flow
5. Pulmonary disease
Figure 32–4
Function of the erythropoietin mechanism to increase production
of red blood cells when tissue oxygenation decreases.
Chapter 32
423
At times, hypoxia in other parts of the body, but 
not in the kidneys, stimulates kidney erythropoietin
secretion, which suggests that

dney erythropoietin
secretion, which suggests that there might be some
nonrenal sensor that sends an additional signal to the
kidneys to produce this hormone. In particular, both
norepinephrine and epinephrine and several of the
prostaglandins stimulate erythropoietin production.
When both kidneys are removed from a person or
when the kidneys are destroyed by renal disease, the
person invariably becomes very anemic because the 10
per cent of the normal erythropoietin formed in other
tissues (mai

normal erythropoietin formed in other
tissues (mainly in the liver) is sufﬁcient to cause only
one third to one half the red blood cell formation
needed by the body.
Effect of Erythropoietin in Erythrogenesis. When an
animal or a person is placed in an atmosphere of low
oxygen, erythropoietin begins to be formed within
minutes to hours, and it reaches maximum production
within 24 hours. Yet almost no new red blood cells
appear in the circulating blood until about 5 days later.
From this fact, as

blood until about 5 days later.
From this fact, as well as other studies, it has been
determined that the important effect of erythropoietin
is to stimulate the production of proerythroblasts from
hematopoietic stem cells in the bone marrow. In ad-
dition, once the proerythroblasts are formed, the 
erythropoietin causes these cells to pass more rapidly
through the different erythroblastic stages than they
normally do, further speeding up the production of
new red blood cells.The rapid production

uction of
new red blood cells.The rapid production of cells con-
tinues as long as the person remains in a low oxygen
state or until enough red blood cells have been pro-
duced to carry adequate amounts of oxygen to the
tissues despite the low oxygen; at this time, the rate of
erythropoietin production decreases to a level that will
maintain the required number of red cells but not an
excess.
In the absence of erythropoietin, few red blood 
cells are formed by the bone marrow. At the other
extre

 are formed by the bone marrow. At the other
extreme, when large quantities of erythropoietin 
are formed available, and if there is plenty of iron and
other required nutrients available, the rate of red
blood cell production can rise to perhaps 10 or more
times normal. Therefore, the erythropoietin mecha-
nism for controlling red blood cell production is a
powerful one.
Maturation of Red Blood Cells—Requirement
for Vitamin B12 (Cyanocobalamin) and 
Folic Acid
Because of the continuing need to r

nd 
Folic Acid
Because of the continuing need to replenish red blood
cells, the erythropoietic cells of the bone marrow are
among the most rapidly growing and reproducing cells
in the entire body. Therefore, as would be expected,
their maturation and rate of production are affected
greatly by a person’s nutritional status.
Especially important for ﬁnal maturation of the red
blood cells are two vitamins, vitamin B12 and folic acid.
Both of these are essential for the synthesis of DNA,
because eac

re essential for the synthesis of DNA,
because each in a different way is required for the for-
mation of thymidine triphosphate, one of the essential
building blocks of DNA. Therefore, lack of either
vitamin B12 or folic acid causes abnormal and dimin-
ished DNA and, consequently, failure of nuclear 
maturation and cell division. Furthermore, the ery-
throblastic cells of the bone marrow, in addition to
failing to proliferate rapidly, produce mainly larger
than normal red cells called macrocyte

inly larger
than normal red cells called macrocytes, and the cell
itself has a ﬂimsy membrane and is often irregular,
large, and oval instead of the usual biconcave disc.
These poorly formed cells, after entering the circulat-
ing blood, are capable of carrying oxygen normally, but
their fragility causes them to have a short life, one half
to one third normal.Therefore, it is said that deﬁciency
of either vitamin B12 or folic acid causes maturation
failure in the process of erythropoiesis.
Matur

on
Maturation Failure Caused by Poor Absorption of Vitamin 
B12 from the Gastrointestinal Tract—Pernicious Anemia. A
common cause of red blood cell maturation failure is
failure to absorb vitamin B12 from the gastrointestinal
tract. This often occurs in the disease pernicious
anemia, in which the basic abnormality is an atrophic
gastric mucosa that fails to produce normal gastric
secretions. The parietal cells of the gastric glands
secrete a glycoprotein

cells of the gastric glands
secrete a glycoprotein called intrinsic factor, which
combines with vitamin B12 in food and makes the B12
available for absorption by the gut. It does this in the
following way: (1) Intrinsic factor binds tightly with
the vitamin B12. In this bound state, the B12 is protected
from digestion by the gastrointestinal secretions. (2)
Still in the bound state, intrinsic factor binds to speciﬁc
receptor sites on the brush border membranes of the
mucosal cells in the ileum. 

rder membranes of the
mucosal cells in the ileum. (3) Then, vitamin B12 is
transported into the blood during the next few hours
by the process of pinocytosis, carrying intrinsic factor
and the vitamin together through the membrane. Lack
of intrinsic factor, therefore, causes diminished avail-
ability of vitamin B12 because of faulty absorption of
the vitamin.
Once vitamin B12 has been absorbed from the gas-
trointestinal tract, it is ﬁrst stored in large quantities in
the liver, then released sl

in large quantities in
the liver, then released slowly as needed by the bone
marrow.The minimum amount of vitamin B12 required
each day to maintain normal red cell maturation is
only 1 to 3 micrograms, and the normal storage in the
liver and other body tissues is about 1000 times this
amount.Therefore,3 to 4 years of defective B12 absorp-
tion are usually required to cause maturation failure
anemia.
Failure of Maturation Caused by Deﬁciency of Folic Acid
(Pteroylglutamic Acid). Folic acid is a n

lic Acid
(Pteroylglutamic Acid). Folic acid is a normal constituent
of green vegetables, some fruits, and meats (especially
liver). However, it is easily destroyed during cooking.
Also, people with gastrointestinal absorption abnor-
malities, such as the frequently occurring small intes-
tinal disease called sprue, often have serious difﬁculty
absorbing both folic acid and vitamin B12. Therefore,
in many instances of maturation failure, the cause is
deﬁciency of intestinal absorption of both fol

 is
deﬁciency of intestinal absorption of both folic acid
and vitamin B12.
424
Unit VI
Synthesis of hemoglobin begins in the proerythro-
blasts and continues even into the reticulocyte stage of
the red blood cells.Therefore, when reticulocytes leave
the bone marrow and pass into the blood stream, they
continue to form minute quantities of hemoglobin for
another day or so until they become mature erythro-
cytes.
Figure 32–5 shows t

 become mature erythro-
cytes.
Figure 32–5 shows the basic chemical steps in the
formation of hemoglobin. First, succinyl-CoA, formed
in the Krebs metabolic cycle (as explained in Chapter
67), binds with glycine to form a pyrrole molecule. In
turn, four pyrroles combine to form protoporphyrin
IX, which then combines with iron to form the heme
molecule. Finally, each heme molecule combines with
a long polypeptide chain, a globin synthesized by 
ribosomes, forming a subunit of hemoglobin called a

bosomes, forming a subunit of hemoglobin called a
hemoglobin chain (Figure 32–6). Each chain has a
molecular weight of about 16,000; four of these in turn
bind together loosely to form the whole hemoglobin
molecule.
There are several slight variations in the different
subunit hemoglobin chains, depending on the amino
acid composition of the polypeptide portion. The dif-
ferent types of chains are designated alpha chains, beta
chains, gamma chains, and delta chains. The most
common form of hemogl

, and delta chains. The most
common form of hemoglobin in the adult human
being, hemoglobin A, is a combination of two alpha
chains and two beta chains. Hemoglobin A has a
molecular weight of 64,458.
Because each hemoglobin chain has a heme pros-
thetic group containing an atom of iron, and because
there are four hemoglobin chains in each hemoglobin
molecule, one ﬁnds four iron atoms in each hemoglo-
bin molecule; each of these can bind loosely with one
molecule of oxygen, making a total of four

ith one
molecule of oxygen, making a total of four molecules
of oxygen (or eight oxygen atoms) that can be trans-
ported by each hemoglobin molecule.
The types of hemoglobin chains in the hemoglobin
molecule determine the binding afﬁnity of the hemo-
globin for oxygen. Abnormalities of the chains can
alter the physical characteristics of the hemoglobin
molecule as well. For instance, in sickle cell anemia, the
amino acid valine is substituted for glutamic acid at
one point in each of the two bet

 glutamic acid at
one point in each of the two beta chains. When this
type of hemoglobin is exposed to low oxygen, it forms
elongated crystals inside the red blood cells that are
sometimes 15 micrometers in length. These make it
almost impossible for the cells to pass through many
small capillaries, and the spiked ends of the crystals are
likely to rupture the cell membranes, leading to sickle
cell anemia.
Combination of Hemoglobin with Oxygen. The most impor-
tant feature of the hemoglobin mole

he most impor-
tant feature of the hemoglobin molecule is its ability
to combine loosely and reversibly with oxygen. This
ability is discussed in detail in Chapter 40 in relation
A
C
HC
P
C
N
H
(pyrrole)
CH
2 succinyl-CoA + 2 glycine
4 pyrrole
protoporphyrin IX
protoporphyrin IX + Fe++
heme
heme + polypeptide
hemoglobin chain (a or b)
2 a chains + 2 b chains
hemoglobin A
I.
II.
III.
IV.
V.
Figure 32–5
Formation of hemoglobin.
CH2
CH
CH
CH2
H3C
CH3
H
C
A
B
N
(–)N
O2
Fe
CH
CH3
H3C
C
CH2
CH2
CH2
CH

3
H
C
A
B
N
(–)N
O2
Fe
CH
CH3
H3C
C
CH2
CH2
CH2
CH2
COOH
COOH
C
H
D
HC
N(–)
N
Polypepitide
(hemoglobin chain–a or b)
Figure 32–6
Basic structure of the hemoglobin molecule, showing one of the
four heme chains that bind together to form the hemoglobin 
molecule.
Chapter 32
425
to respiration, because the primary function of hemo-
globin in the body is to combine with oxygen in the
lungs and then to release this oxygen readily in the
peripheral tissue capi

 this oxygen readily in the
peripheral tissue capillaries, where the gaseous tension
of oxygen is much lower than in the lungs.
Oxygen does not combine with the two positive
bonds of the iron in the hemoglobin molecule. Instead,
it binds loosely with one of the so-called coordination
bonds of the iron atom. This is an extremely loose
bond, so that the combination is easily reversible. Fur-
thermore, the oxygen does not become ionic oxygen
but is carried as molecular oxygen (composed of two
oxyge

carried as molecular oxygen (composed of two
oxygen atoms) to the tissues, where, because of the
loose,readily reversible combination,it is released into
the tissue ﬂuids still in the form of molecular oxygen
rather than ionic oxygen.
Because iron is important for the formation not only
of hemoglobin but also of other essential elements in
the body (e.g., myoglobin, cytochromes, cytochrome
oxidase, peroxidase, catalase), it is important to under-
stand the means by which iron is 

ortant to under-
stand the means by which iron is utilized in the body.
The total quantity of iron in the body averages 4 to 5
grams, about 65 per cent of which is in the form of
hemoglobin. About 4 per cent is in the form of myo-
globin, 1 per cent is in the form of the various heme
compounds that promote intracellular oxidation, 0.1
per cent is combined with the protein transferrin in the
blood plasma, and 15 to 30 per cent is stored for later
use, mainly in the reticuloendothelial system and 

use, mainly in the reticuloendothelial system and liver
parenchymal cells, principally in the form of ferritin.
Transport and Storage of Iron. Transport, storage, and
metabolism of iron in the body are diagrammed in
Figure 32–7 and can be explained as follows: When
iron is absorbed from the small intestine, it immedi-
ately combines in the blood plasma with a beta 
globulin, apotransferrin, to form transferrin, which is
then transported in the plasma. The iron is loosely
bound in the transferrin

asma. The iron is loosely
bound in the transferrin and, consequently, can be
released to any tissue cell at any point in the body.
Excess iron in the blood is deposited especially in the
liver hepatocytes and less in the reticuloendothelial
cells of the bone marrow.
In the cell cytoplasm, iron combines mainly with 
a protein, apoferritin, to form ferritin. Apoferritin 
has a molecular weight of about 460,000, and 
varying quantities of iron can combine in clusters of
iron radicals with this larg

ombine in clusters of
iron radicals with this large molecule; therefore,
ferritin may contain only a small amount of iron or a
large amount. This iron stored as ferritin is called
storage iron.
Smaller quantities of the iron in the storage pool are
in an extremely insoluble form called hemosiderin.
This is especially true when the total quantity of iron
in the body is more than the apoferritin storage pool
can accommodate. Hemosiderin collects in cells in the
form of large clusters that can be o

 cells in the
form of large clusters that can be observed micro-
scopically as large particles. In contrast, ferritin parti-
cles are so small and dispersed that they usually can
be seen in the cell cytoplasm only with the electron
microscope.
When the quantity of iron in the plasma falls low,
some of the iron in the ferritin storage pool is removed
easily and transported in the form of transferrin in the
plasma to the areas of the body where it is needed. A
unique characteristic of the transfer

is needed. A
unique characteristic of the transferrin molecule is that
it binds strongly with receptors in the cell membranes
of erythroblasts in the bone marrow. Then, along with
its bound iron, it is ingested into the erythroblasts by
endocytosis. There the transferrin delivers the iron
directly to the mitochondria, where heme is synthe-
sized. In people who do not have adequate quantities
of transferrin in their blood, failure to transport iron
to the erythroblasts in this manner can cause se

n
to the erythroblasts in this manner can cause severe
hypochromic anemia—that is, red cells that contain
much less hemoglobin than normal.
Degrading hemoglobin
Free
iron
Tissues
Ferritin
Hemosiderin
Bilirubin (excreted)
Transferrin–Fe
Red Cells
Blood loss–0.7 mg Fe
daily in menses
Fe++ absorbed
(small intestine)
Fe excreted–0.6 mg daily
Plasma
Free iron
Heme
Enzymes
Figure 32–7
Iron transport and metabolism.
426
Unit VI
When red b

ood Cells, Immunity, and Blood Clotting
When red blood cells have lived their life span and
are destroyed, the hemoglobin released from the cells
is ingested by monocyte-macrophage cells. There,
iron is liberated and is stored mainly in the ferritin
pool to be used as needed for the formation of new
hemoglobin.
Daily Loss of Iron. A man excretes about 0.6 milligram
of iron each day, mainly into the feces. Additional
quantities of iron are lost when bleeding occurs. For a
woman,additional menstru

en bleeding occurs. For a
woman,additional menstrual loss of blood brings long-
term iron loss to an average of about 1.3 mg/day.
Absorption of Iron from the Intestinal Tract
Iron is absorbed from all parts of the small intestine,
mostly by the following mechanism. The liver secretes
moderate amounts of apotransferrin into the bile,
which ﬂows through the bile duct into the duodenum.
Here, the apotransferrin binds with free iron and also
with certain iron compounds, such as hemoglobin and
myoglo

tain iron compounds, such as hemoglobin and
myoglobin from meat, two of the most important
sources of iron in the diet. This combination is called
transferrin. It, in turn, is attracted to and binds with
receptors in the membranes of the intestinal epithelial
cells. Then, by pinocytosis, the transferrin molecule,
carrying its iron store, is absorbed into the epithelial
cells and later released into the blood capillaries
beneath these cells in the form of plasma transferrin.
Iron absorption from 

 form of plasma transferrin.
Iron absorption from the intestines is extremely
slow, at a maximum rate of only a few milligrams per
day.This means that even when tremendous quantities
of iron are present in the food, only small proportions
can be absorbed.
Regulation of Total Body Iron by Controlling Rate of Absorption.
When the body has become saturated with iron so that
essentially all apoferritin in the iron storage areas is
already combined with iron, the rate of additional iron
absorption fr

th iron, the rate of additional iron
absorption from the intestinal tract becomes greatly
decreased. Conversely, when the iron stores have
become depleted, the rate of absorption can acceler-
ate probably ﬁve or more times normal. Thus, total
body iron is regulated mainly by altering the rate of
Life Span and Destruction of Red
Blood Cells
When red blood cells are delivered from the bone
marrow into the circulatory system, they normally cir-
culate an average of 120 days before being

ly cir-
culate an average of 120 days before being destroyed.
Even though mature red cells do not have a nucleus,
mitochondria, or endoplasmic reticulum, they do have
cytoplasmic enzymes that are capable of metabolizing
glucose and forming small amounts of adenosine
triphosphate. These enzymes also (1) maintain pliabil-
ity of the cell membrane, (2) maintain membrane
transport of ions, (3) keep the iron of the cells’ hemo-
globin in the ferrous form rather than ferric form, and
(4) prevent oxida

orm rather than ferric form, and
(4) prevent oxidation of the proteins in the red cells.
Even so, the metabolic systems of old red cells become
progressively less active, and the cells become more
and more fragile, presumably because their life
processes wear out.
Once the red cell membrane becomes fragile, the
cell ruptures during passage through some tight spot
of the circulation. Many of the red cells self-destruct
in the spleen, where they squeeze through the red pulp
of the spleen. There, t

ueeze through the red pulp
of the spleen. There, the spaces between the structural
trabeculae of the red pulp, through which most of the
cells must pass, are only 3 micrometers wide, in com-
parison with the 8-micrometer diameter of the red 
cell. When the spleen is removed, the number of old
abnormal red cells circulating in the blood increases
considerably.
Destruction of Hemoglobin. When red blood cells burst
and release their hemoglobin, the hemoglobin is
phagocytized almost immediately by m

hemoglobin is
phagocytized almost immediately by macrophages in
many parts of the body, but especially by the Kupffer
cells of the liver and macrophages of the spleen and
bone marrow. During the next few hours to days, the
macrophages release iron from the hemoglobin and
pass it back into the blood, to be carried by transfer-
rin either to the bone marrow for the production of
new red blood cells or to the liver and other tissues for
storage in the form of ferritin. The porphyrin portion
of the 

he form of ferritin. The porphyrin portion
of the hemoglobin molecule is converted by the
macrophages, through a series of stages, into the bile
pigment bilirubin, which is released into the blood and
later removed from the body by secretion through the
liver into the bile; this is discussed in relation to liver
function in Chapter 70.
Anemias
Anemia means deﬁciency of hemoglobin in the blood,
which can be caused by either too few red blood cells
or too little hemoglobin in the cells. Some types

or too little hemoglobin in the cells. Some types of
anemia and their physiologic causes are the following.
Blood Loss Anemia. After rapid hemorrhage, the body
replaces the ﬂuid portion of the plasma in 1 to 3 days,
but this leaves a low concentration of red blood cells.
If a second hemorrhage does not occur, the red blood
cell concentration usually returns to normal within 3
to 6 weeks.
In chronic blood loss, a person frequently cannot
absorb enough iron from the intestines to form hemo-
globi

nough iron from the intestines to form hemo-
globin as rapidly as it is lost. Red cells are then pro-
duced that are much smaller than normal and have too
little hemoglobin inside them, giving rise to microcytic,
hypochromic anemia, which is shown in Figure 32–3.
Aplastic Anemia. Bone marrow aplasia means lack of
functioning bone marrow. For instance, a person
exposed to gamma ray radiation from a nuclear bomb
blast can sustain complete destruction of bone
marrow, followed in a few weeks by leth

on of bone
marrow, followed in a few weeks by lethal anemia.
Likewise, excessive x-ray treatment, certain industrial
chemicals, and even drugs to which the person might
be sensitive can cause the same effect.
Chapter 32
427
Megaloblastic Anemia. Based on the earlier discussions
of vitamin B12, folic acid, and intrinsic factor from the
stomach mucosa, one can readily understand that loss
of any one of these can lead to slow reproduction of
erythroblasts i

e can lead to slow reproduction of
erythroblasts in the bone marrow. As a result, the red
cells grow too large, with odd shapes, and are called
megaloblasts. Thus, atrophy of the stomach mucosa,
as occurs in pernicious anemia, or loss of the entire
stomach after surgical total gastrectomy can lead to
megaloblastic anemia. Also, patients who have intes-
tinal sprue, in which folic acid, vitamin B12, and other
vitamin B compounds are poorly absorbed, often
develop megaloblastic anemia. Because in 

d, often
develop megaloblastic anemia. Because in these states
the erythroblasts cannot proliferate rapidly enough to
form normal numbers of red blood cells, those red cells
that are formed are mostly oversized, have bizarre
shapes, and have fragile membranes. These cells
rupture easily, leaving the person in dire need of an
adequate number of red cells.
Hemolytic Anemia. Different abnormalities of the red
blood cells, many of which are hereditarily acquired,
make the cells fragile, so that they

ily acquired,
make the cells fragile, so that they rupture easily as
they go through the capillaries, especially through the
spleen. Even though the number of red blood cells
formed may be normal, or even much greater than
normal in some hemolytic diseases, the life span of the
fragile red cell is so short that the cells are destroyed
faster than they can be formed, and serious anemia
results. Some of these types of anemia are the 
In hereditary spherocytosis, the red cells are very
s

hereditary spherocytosis, the red cells are very
small and spherical rather than being biconcave discs.
These cells cannot withstand compression forces
because they do not have the normal loose, baglike cell
membrane structure of the biconcave discs. On passing
through the splenic pulp and some other tight vas-
cular beds, they are easily ruptured by even slight 
compression.
In sickle cell anemia, which is present in 0.3 to 1.0
per cent of West African and American blacks,the cells
have an abno

African and American blacks,the cells
have an abnormal type of hemoglobin called hemo-
globin S, containing faulty beta chains in the hemo-
globin molecule, as explained earlier in the chapter.
When this hemoglobin is exposed to low concentra-
tions of oxygen, it precipitates into long crystals inside
the red blood cell. These crystals elongate the cell and
give it the appearance of a sickle rather than a bicon-
cave disc. The precipitated hemoglobin also damages
the cell membrane, so that the c

obin also damages
the cell membrane, so that the cells become highly
fragile, leading to serious anemia. Such patients fre-
quently experience a vicious circle of events called a
sickle cell disease “crisis,” in which low oxygen tension
in the tissues causes sickling, which leads to ruptured
red cells, which causes a further decrease in oxygen
tension and still more sickling and red cell destruction.
Once the process starts, it progresses rapidly, eventu-
ating in a serious decrease in red blood

, eventu-
ating in a serious decrease in red blood cells within a
few hours and, often, death.
In erythroblastosis fetalis, Rh-positive red blood
cells in the fetus are attacked by antibodies from 
an Rh-negative mother. These antibodies make the
Rh-positive cells fragile, leading to rapid rupture and
causing the child to be born with serious anemia. This
is discussed in Chapter 35 in relation to the Rh factor
of blood. The extremely rapid formation of new red
cells to make up for the destroyed 

ion of new red
cells to make up for the destroyed cells in erythro-
blastosis fetalis causes a large number of early blast
forms of red cells to be released from the bone marrow
Effects of Anemia on Function of the
The viscosity of the blood, which was discussed in
Chapter 14, depends almost entirely on the blood con-
centration of red blood cells. In severe anemia, the
blood viscosity may fall to as low as 1.5 times that of
water rather than the normal value o

times that of
water rather than the normal value of about 3. This
decreases the resistance to blood ﬂow in the periph-
eral blood vessels, so that far greater than normal
quantities of blood ﬂow through the tissues and return
to the heart, thereby greatly increasing cardiac output.
Moreover, hypoxia resulting from diminished trans-
port of oxygen by the blood causes the peripheral
tissue blood vessels to dilate, allowing a further
increase in the return of blood to the heart and
increasing the c

 return of blood to the heart and
increasing the cardiac output to a still higher level—
sometimes three to four times normal.Thus, one of the
major effects of anemia is greatly increased cardiac
output, as well as increased pumping workload on the
heart.
The increased cardiac output in anemia partially
offsets the reduced oxygen-carrying effect of the
anemia, because even though each unit quantity of
blood carries only small quantities of oxygen, the rate
of blood ﬂow may be increased enough so

, the rate
of blood ﬂow may be increased enough so that almost
normal quantities of oxygen are actually delivered to
the tissues. However, when a person with anemia
begins to exercise, the heart is not capable of pumping
much greater quantities of blood than it is already
pumping.
Consequently,
during 
exercise,
which 
greatly increases tissue demand for oxygen, extreme
tissue hypoxia results, and acute cardiac failure
ensues.
Secondary Polycythemia. Whenever the tissues become
hypo

ary Polycythemia. Whenever the tissues become
hypoxic because of too little oxygen in the breathed
air, such as at high altitudes, or because of failure of
oxygen delivery to the tissues, such as in cardiac
failure,
the blood-forming organs automatically
produce large quantities of extra red blood cells. This
condition is called secondary polycythemia, and the
red cell count commonly rises to 6 to 7 million/mm3,
about 30 per cent above normal.
A common type of secondary polycythemia, called
phys

common type of secondary polycythemia, called
physiologic polycythemia, occurs in natives who live at
altitudes of 14,000 to 17,000 feet, where the atmos-
pheric oxygen is very low.The blood count is generally
6 to 7 million/mm3; this allows these people to perform
428
Unit VI
reasonably high levels of continuous work even in a
rareﬁed atmosphere.
Polycythemia Vera (Erythremia). In addition to those
people who have physiologic polycythemia, others
have a

e who have physiologic polycythemia, others
have a pathological condition known as polycythemia
vera, in which the red blood cell count may be 7 to 8
million/mm3 and the hematocrit may be 60 to 70 per
cent instead of the normal 40 to 45 per cent. Poly-
cythemia vera is caused by a genetic aberration in the
hemocytoblastic cells that produce the blood cells.The
blast cells no longer stop producing red cells when too
many cells are already present.This causes excess pro-
duction of red blood cells

This causes excess pro-
duction of red blood cells in the same manner that a
breast tumor causes excess production of a speciﬁc
type of breast cell. It usually causes excess production
of white blood cells and platelets as well.
In polycythemia vera, not only does the hematocrit
increase, but the total blood volume also increases, on
some occasions to almost twice normal.As a result, the
entire vascular system becomes intensely engorged. In
addition, many blood capillaries become plugged by
the 

ion, many blood capillaries become plugged by
the viscous blood; the viscosity of the blood in poly-
cythemia vera sometimes increases from the normal of
3 times the viscosity of water to 10 times that of water.
Effect of Polycythemia on Function of
the Circulatory System
Because of the greatly increased viscosity of the blood
in polycythemia, blood ﬂow through the peripheral
blood vessels is often very sluggish. In accordance with
the factors that regulate return of blood to the heart,
as discu

at regulate return of blood to the heart,
as discussed in Chapter 20, increasing blood viscosity
decreases the rate of venous return to the heart. Con-
versely, the blood volume is greatly increased in poly-
cythemia, which tends to increase venous return.
Actually, the cardiac output in polycythemia is not far
from normal, because these two factors more or less
neutralize each other.
The arterial pressure is also normal in most people
with polycythemia, although in about one third of
them, the 

ythemia, although in about one third of
them, the arterial pressure is elevated.This means that
the blood pressure–regulating mechanisms can usually
offset the tendency for increased blood viscosity to
increase peripheral resistance and, thereby, increase
arterial pressure. Beyond certain limits, however, these
regulations fail, and hypertension develops.
The color of the skin depends to a great extent on
the quantity of blood in the skin subpapillary venous
plexus. In polycythemia vera, the qua

llary venous
plexus. In polycythemia vera, the quantity of blood in
this plexus is greatly increased. Further, because the
blood passes sluggishly through the skin capillaries
before entering the venous plexus, a larger than
normal quantity of hemoglobin is deoxygenated. The
blue color of all this deoxygenated hemoglobin masks
the red color of the oxygenated hemoglobin. There-
fore, a person with polycythemia vera ordinarily has a
ruddy complexion with a bluish (cyanotic) tint to the
skin.
Refer

n with a bluish (cyanotic) tint to the
skin.
Alayash AI: Oxygen therapeutics: can we tame haemoglo-
bin? Nat Rev Drug Discov 3:152, 2004.
Brissot P, Troadec MB, Loreal O: The clinical relevance of
new insights in iron transport and metabolism. Curr
Hematol Rep 3:107, 2004.
Claster S, Vichinsky EP: Managing sickle cell disease. BMJ
327:1151, 2003.
Fandrey J: Oxygen-dependent and tissue-speciﬁc regulation
of erythropoietin gene expression. Am J Physiol Regul
Integr Comp Physiol 286:R977

n. Am J Physiol Regul
Integr Comp Physiol 286:R977, 2004.
Hallberg L: Perspectives on nutritional iron deﬁciency.Annu
Rev Nutr 21:1, 2001.
Hentze MW, Muckenthaler MU, Andrews NC: Balancing
acts: molecular control of mammalian iron metabolism.
Cell 117:285, 2004.
Lappin T: The cellular biology of erythropoietin receptors.
Oncologist 8(Suppl 1):15, 2003.
Maxwell P: HIF-1: an oxygen response system with special
relevance to the kidney. J Am Soc Nephrol 14:2712, 2003.
Persons DA: Update on gene ther

rol 14:2712, 2003.
Persons DA: Update on gene therapy for hemoglobin disor-
ders. Curr Opin Mol Ther 5:508, 2003.
Pietrangelo A: Hereditary hemochromatosis—a new look at
an old disease. N Engl J Med 350:2383, 2004.
Shah S, Vega R: Hereditary spherocytosis. Pediatr Rev
25:168, 2004.
Tefferi A: A contemporary approach to the diagnosis and
management of polycythemia vera. Curr Hematol Rep
2:237, 2003.
Trigg ME: Hematopoietic stem cells. Pediatrics 113(4
Suppl):1051, 2004.
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Resistance of the Body 
to Infection: I. Leukocytes,
Granulocytes, the 
Monocyte-Macrophage System,
Our bodies are exposed continually to bacteria,
viruses, fungi, and parasites, all of which occur 
normally and to varying degrees in the skin, the
mouth, the respiratory passageways, the intestinal
tract,the lining membranes of the eyes,and even the
urinary tract. Many of these infectious agents are
capable of causing serious abnorma

ious agents are
capable of causing serious abnormal physiologic
function or even death if they invade the deeper
tissues. In addition, we are exposed intermittently to other highly infectious bac-
teria and viruses besides those that are normally present, and these can cause
acute lethal diseases such as pneumonia, streptococcal infection, and typhoid
fever.
Our bodies have a special system for combating the different infectious and
toxic agents.This is comprised of blood leukocytes (white blood

This is comprised of blood leukocytes (white blood cells) and tissue
cells derived from leukocytes.These cells work together in two ways to prevent
disease: (1) by actually destroying invading bacteria or viruses by phagocytosis,
and (2) by forming antibodies and sensitized lymphocytes, one or both of which
may destroy or inactivate the invader. This chapter is concerned with the ﬁrst
of these methods, and Chapter 34 with the second.
The leukocytes, also called whi

White Blood Cells)
The leukocytes, also called white blood cells, are the mobile units of the body’s
protective system. They are formed partially in the bone marrow (granulocytes
and monocytes and a few lymphocytes) and partially in the lymph tissue (lym-
phocytes and plasma cells). After formation, they are transported in the blood
to different parts of the body where they are needed.
The real value of the white blood cells is that most of them are speciﬁcally
transported to areas of serious in

are speciﬁcally
transported to areas of serious infection and inﬂammation, thereby providing a
rapid and potent defense against infectious agents. As we see later, the granu-
locytes and monocytes have a special ability to “seek out and destroy” a foreign
invader.
Types of White Blood Cells. Six types of white blood cells are normally present in
the blood. They are polymorphonuclear neutrophils, polymorphonuclear
eosinophils, polymorphonuclear basophils, mon

lear
eosinophils, polymorphonuclear basophils, monocytes, lymphocytes, and, occa-
sionally, plasma cells. In addition, there are large numbers of platelets, which are
fragments of another type of cell similar to the white blood cells found in the
bone marrow, the megakaryocyte. The ﬁrst three types of cells, the polymor-
phonuclear cells, all have a granular appearance, as shown in cell numbers 7,
430
Unit VI
10, and 12 in Figure 33–1, for which reason t

ting
10, and 12 in Figure 33–1, for which reason they are
called granulocytes, or, in clinical terminology,“polys,”
because of the multiple nuclei.
The granulocytes and monocytes protect the body
against invading organisms mainly by ingesting
them—that is, by phagocytosis. The lymphocytes and
plasma cells function mainly in connection with the
immune system; this is discussed in Chapter 34. Finally,
the function of platelets is speciﬁcally to activate the
blood clotting mechanism, which is discu

ivate the
blood clotting mechanism, which is discussed in
Chapter 36.
Concentrations of the Different White Blood Cells in the Blood.
The adult human being has about 7000 white blood
cells per microliter of blood (in comparison with 5
million red blood cells). Of the total white blood cells,
the normal percentages of the different types are
approximately the following:
Polymorphonuclear neutrophils
62.0%
Polymorphonuclear eosinophils
2.3%
Polymorphonuclear basophils
0.4%
Monocytes 
5.3%
Lymphocy

phonuclear basophils
0.4%
Monocytes 
5.3%
Lymphocytes
30.0%
The number of platelets, which are only cell frag-
ments, in each microliter of blood is normally about
300,000.
Early differentiation of the pluripotential hematopoi-
etic stem cell into the different types of committed
stem cells is shown in Figure 32–2 in the previous
chapter.Aside from those cells committed to form red
blood cells, two major lineages of white blood cells are
formed, the myelocytic an

of white blood cells are
formed, the myelocytic and the lymphocytic lineages.
The left side of Figure 33–1 shows the myelocytic
lineage, beginning with the myeloblast; the right 
shows the lymphocytic lineage, beginning with the 
lymphoblast.
The granulocytes and monocytes are formed only 
in the bone marrow. Lymphocytes and plasma cells 
are produced mainly in the various lymphogenous
tissues—especially the lymph glands, spleen, thymus,
tonsils, and various pockets of lymphoid tissue 
1
2
3
4
5

 and various pockets of lymphoid tissue 
1
2
3
4
5
6
7
10
12
16
9
8
11
13
14
15
Genesis of Myelocytes
Genesis of Lymphocytes
Figure 33–1
Genesis of white blood cells. The
different cells of the myelocyte
series 
are 
1, 
myeloblast; 
2,
promyelocyte; 3, megakaryocyte;
4, neutrophil myelocyte; 5, young
neutrophil 
metamyelocyte; 
6,
“band” neutrophil metamyelocyte;
7, polymorphonuclear neutrophil;
8, 
eosinophil 
myelocyte; 
9,
10,
polymorphonuclear 
eosinophil;
11, ba

locyte; 
10,
11, basophil myelocyte; 12, poly-
morphonuclear basophil; 13–16,
stages of monocyte formation.
Diapedesis
Chemotaxis
source
Chemotactic
substance
Increased
Margination
Figure 33–2
Movement of neutrophils by diapedesis through capillary pores
and by chemotaxis toward an area of tissue damage.
Chapter 33
Resistance of the Body to Infection: I. Leukocytes, Granulocytes, the Monocyte-Macrophage System
431
elsewhere in the body, such as in the 

 System
431
elsewhere in the body, such as in the bone marrow and
in so-called Peyer’s patches underneath the epithelium
in the gut wall.
The white blood cells formed in the bone marrow
are stored within the marrow until they are needed in
the circulatory system. Then, when the need arises,
various factors cause them to be released (these
factors are discussed later). Normally, about three
times as many white blood cells are stored in the
marrow as circulate in the entire blood.This represents
a

as circulate in the entire blood.This represents
about a 6-day supply of these cells.
The lymphocytes are mostly stored in the various
lymphoid tissues, except for a small number that are
temporarily being transported in the blood.
As shown in Figure 33–1, megakaryocytes (cell 3)
are also formed in the bone marrow. These megakary-
ocytes fragment in the bone marrow; the small frag-
ments, known as platelets (or thrombocytes), then pass
into the blood. They are very important in the initia-
tion 

lood. They are very important in the initia-
tion of blood clotting.
The life of the granulocytes after being released from
the bone marrow is normally 4 to 8 hours circulating
in the blood and another 4 to 5 days in tissues where
they are needed. In times of serious tissue infection,
this total life span is often shortened to only a few
hours because the granulocytes proceed even more
rapidly to the infected area, perform their functions,
and, in the process, 

ea, perform their functions,
and, in the process, are themselves destroyed.
The monocytes also have a short transit time, 10 to
20 hours in the blood, before wandering through the
capillary membranes into the tissues. Once in the
tissues, they swell to much larger sizes to become tissue
macrophages, and, in this form, can live for months
unless destroyed while performing phagocytic func-
tions. These tissue macrophages are the basis of the
tissue macrophage system, discussed in greater detail
la

 macrophage system, discussed in greater detail
later, which provides continuing defense against 
infection.
Lymphocytes enter the circulatory system continu-
ally, along with drainage of lymph from the lymph
nodes and other lymphoid tissue. After a few hours,
they pass out of the blood back into the tissues by dia-
pedesis. Then, still later, they re-enter the lymph and
return to the blood again and again; thus, there is con-
tinual circulation of lymphocytes through the body.
The lymphocytes h

of lymphocytes through the body.
The lymphocytes have life spans of weeks or months;
this life span depends on the body’s need for these
cells.
The platelets in the blood are replaced about once
every 10 days; in other words, about 30,000 platelets
are formed each day for each microliter of blood.
Neutrophils and Macrophages
Defend Against Infections
It is mainly the neutrophils and tissue macrophages
that attack and destroy invading bacteria, viruses, and
other injurious agents. The neutrophils

ruses, and
other injurious agents. The neutrophils are mature
cells that can attack and destroy bacteria even in the
circulating blood. Conversely, the tissue macrophages
begin life as blood monocytes, which are immature
cells while still in the blood and have little ability to
ﬁght infectious agents at that time. However, once they
enter the tissues, they begin to swell—sometimes
increasing their diameters as much as ﬁvefold—to as
great as 60 to 80 micrometers, a size that can barely be
seen wi

 80 micrometers, a size that can barely be
seen with the naked eye. These cells are now called
macrophages, and they are extremely capable of com-
bating intratissue disease agents.
White Blood Cells Enter the Tissue Spaces by Diapedesis.
Neutrophils and monocytes can squeeze through the
pores of the blood capillaries by diapedesis. That is,
even though a pore is much smaller than a cell, a small
portion of the cell slides through the pore at a time;
the portion sliding through is momentarily co

ime;
the portion sliding through is momentarily constricted
to the size of the pore, as shown in Figure 33–2.
White Blood Cells Move Through Tissue Spaces by Ameboid
Motion. Both neutrophils and macrophages can move
through the tissues by ameboid motion, described in
Chapter 2. Some cells move at velocities as great as 
40 mm/min, a distance as great as their own length 
each minute.
White Blood Cells Are Attracted to Inﬂamed Tissue Areas by
Chemotaxis. Many different chemical substances in the

otaxis. Many different chemical substances in the
tissues cause both neutrophils and macrophages to
move toward the source of the chemical. This phe-
nomenon, shown in Figure 33–2, is known as chemo-
taxis. When a tissue becomes inﬂamed, at least a dozen
different products are formed that can cause chemo-
taxis toward the inﬂamed area. They include (1) some
of the bacterial or viral toxins, (2) degenerative prod-
ucts of the inﬂamed tissues themselves, (3) several
reaction products of the “compl

elves, (3) several
reaction products of the “complement complex”
(discussed in Chapter 34) activated in inﬂamed tissues,
and (4) several reaction products caused by plasma 
clotting in the inﬂamed area, as well as other 
substances.
As shown in Figure 33–2, chemotaxis depends on
the concentration gradient of the chemotactic sub-
stance. The concentration is greatest near the source,
which directs the unidirectional movement of the
white cells. Chemotaxis is effective up to 100 microm-
eters away

emotaxis is effective up to 100 microm-
eters away from an inﬂamed tissue.Therefore, because
almost no tissue area is more than 50 micrometers
away from a capillary, the chemotactic signal can easily
move hordes of white cells from the capillaries into the
inﬂamed area.
The most important function of the neutrophils and
macrophages is phagocytosis, which means cellular
ingestion of the offending agent. Phagocytes must be
selective of the material that is phagocytized; other-
wise, n

 the material that is phagocytized; other-
wise, normal cells and structures of the body might be
432
Unit VI
ingested. Whether phagocytosis will occur depends
especially on three selective procedures.
First, most natural structures in the tissues have
smooth surfaces, which resist phagocytosis. But if the
surface is rough, the likelihood of phagocytosis is
Second, most natural substances of the body have
protective protein coats that repel th

e body have
protective protein coats that repel the phagocytes.
Conversely, most dead tissues and foreign particles
have no protective coats, which makes them subject to
phagocytosis.
Third, the immune system of the body (described in
detail in Chapter 34) develops antibodies against infec-
tious agents such as bacteria. The antibodies then
adhere to the bacterial membranes and thereby make
the bacteria especially susceptible to phagocytosis. To
do this, the antibody molecule also combines with 

do this, the antibody molecule also combines with the
C3 product of the complement cascade, which is an
additional part of the immune system discussed in the
next chapter. The C3 molecules, in turn, attach to
receptors on the phagocyte membrane, thus initiating
phagocytosis. This selection and phagocytosis process
is called opsonization.
Phagocytosis by Neutrophils. The neutrophils entering
the tissues are already mature cells that can immedi-
ately begin phagocytosis. On approaching a particle 

ely begin phagocytosis. On approaching a particle to
be phagocytized, the neutrophil ﬁrst attaches itself to
the particle and then projects pseudopodia in all direc-
tions around the particle. The pseudopodia meet one
another on the opposite side and fuse. This creates an
enclosed chamber that contains the phagocytized par-
ticle.Then the chamber invaginates to the inside of the
cytoplasmic cavity and breaks away from the outer cell
membrane to form a free-ﬂoating phagocytic vesicle
(also called

orm a free-ﬂoating phagocytic vesicle
(also called a phagosome) inside the cytoplasm. A
single neutrophil can usually phagocytize 3 to 20 bac-
teria before the neutrophil itself becomes inactivated
and dies.
Phagocytosis by Macrophages. Macrophages are the end-
stage product of monocytes that enter the tissues from
the blood. When activated by the immune system as
described in Chapter 34, they are much more power-
ful phagocytes than neutrophils, often capable of
phagocytizing as many as 100 bac

 often capable of
phagocytizing as many as 100 bacteria. They also have
the ability to engulf much larger particles, even whole
red blood cells or, occasionally, malarial parasites,
whereas neutrophils are not capable of phagocytizing
particles much larger than bacteria. Also, after digest-
ing particles, macrophages can extrude the residual
products and often survive and function for many
more months.
Once Phagocytized, Most Particles Are Digested by Intracellu-
lar Enzymes. Once a foreign part

ed by Intracellu-
lar Enzymes. Once a foreign particle has been phagocy-
tized, lysosomes and other cytoplasmic granules in 
the neutrophil or macrophage immediately come in
contact with the phagocytic vesicle, and their mem-
branes fuse, thereby dumping many digestive enzymes
and bactericidal agents into the vesicle. Thus, the
phagocytic vesicle now becomes a digestive vesicle,
and digestion of the phagocytized particle begins
Both neutrophils and macrophages contain an
abundance o

neutrophils and macrophages contain an
abundance of lysosomes ﬁlled with proteolytic
enzymes especially geared for digesting bacteria 
and other foreign protein matter. The lysosomes of
macrophages (but not of neutrophils) also contain
large amounts of lipases, which digest the thick lipid
membranes possessed by some bacteria such as the
tuberculosis bacillus.
Both Neutrophils and Macrophages Can Kill Bacteria. In 
addition to the digestion of ingested bacteria in 
phagosomes, neutrophils and ma

gested bacteria in 
phagosomes, neutrophils and macrophages contain
bactericidal agents that kill most bacteria even when
the lysosomal enzymes fail to digest them.This is espe-
cially important, because some bacteria have protec-
tive coats or other factors that prevent their
destruction by digestive enzymes. Much of the killing
effect results from several powerful oxidizing agents
formed by enzymes in the membrane of the phago-
some or by a special organelle called the peroxisome.
These oxidiz

cial organelle called the peroxisome.
These oxidizing agents include large quantities of
superoxide (O2
–), hydrogen peroxide (H2O2), and
hydroxyl ions (–OH–), all of which are lethal to most
bacteria, even in small quantities. Also, one of the 
lysosomal enzymes, myeloperoxidase, catalyzes the
reaction between H2O2 and chloride ions to form
hypochlorite, which is exceedingly bactericidal.
Some bacteria, however, notably the tuberculosis
bacillus, have coats that are resistant to lysosomal
diges

, have coats that are resistant to lysosomal
digestion and also secrete substances that partially
resist the killing effects of the neutrophils and
macrophages. These bacteria are responsible for 
many of the chronic diseases, an example of which is
tuberculosis.
Monocyte-Macrophage Cell
System (Reticuloendothelial
System)
In the preceding paragraphs, we described the
macrophages mainly as mobile cells that are capable
of wandering through the tissues. However, after
entering the tissues and bec

ssues. However, after
entering the tissues and becoming macrophages,
another large portion of monocytes becomes attached
to the tissues and remains attached for months or even
years until they are called on to perform speciﬁc local
protective functions. They have the same capabilities
as the mobile macrophages to phagocytize large 
quantities of bacteria, viruses, necrotic tissue, or other
foreign particles in the tissue.And, when appropriately
stimulated, they can break away from their attach-

timulated, they can break away from their attach-
ments and once again become mobile macrophages
that respond to chemotaxis and all the other stimuli
related to the inﬂammatory process.Thus, the body has
a widespread “monocyte-macrophage system” in vir-
tually all tissue areas.
The total combination of monocytes,
mobile
macrophages, ﬁxed tissue macrophages, and a few 
Chapter 33
433
specialized endot

e Monocyte-Macrophage System
433
specialized endothelial cells in the bone marrow,
spleen, and lymph nodes is called the reticuloendothe-
lial system. However, all or almost all these cells orig-
inate from monocytic stem cells; therefore, the
reticuloendothelial system is almost synonymous with
the monocyte-macrophage system. Because the term
reticuloendothelial system
is much better known 
in medical literature than the term monocyte-
macrophage system, it should be remembered as a 
generalize

e system, it should be remembered as a 
generalized phagocytic system located in all tissues,
especially in those tissue areas where large quantities
of particles, toxins, and other unwanted substances
must be destroyed.
Tissue Macrophages in the Skin and Subcutaneous Tissues (His-
tiocytes). Although the skin is mainly impregnable to
infectious agents, this is no longer true when the skin
is broken. When infection begins in a subcutaneous
tissue and local inﬂammation ensues, local tissue
macrop

 and local inﬂammation ensues, local tissue
macrophages can divide in situ and form still more
macrophages. Then they perform the usual functions
of attacking and destroying the infectious agents, as
described earlier.
Macrophages in the Lymph Nodes. Essentially no particu-
late matter that enters the tissues, such as bacteria, can
be absorbed directly through the capillary membranes
into the blood. Instead, if the particles are not
destroyed locally in the tissues, they enter the lymph
and ﬂow 

ally in the tissues, they enter the lymph
and ﬂow to the lymph nodes located intermittently
along the course of the lymph ﬂow. The foreign parti-
cles are then trapped in these nodes in a meshwork of
sinuses lined by tissue macrophages.
Figure 33–3 illustrates the general organization of
the lymph node, showing lymph entering through the
lymph node capsule by way of afferent lymphatics,
then ﬂowing through the nodal medullary sinuses, and
ﬁnally passing out the hilus into efferent lymphatics
tha

passing out the hilus into efferent lymphatics
that eventually empty into the venous blood.
Large numbers of macrophages line the lymph
sinuses, and if any particles enter the sinuses by way
of the lymph, the macrophages phagocytize them and
prevent general dissemination throughout the body.
Alveolar Macrophages in the Lungs. Another route by
which invading organisms frequently enter the body 
is through the lungs. Large numbers of tissue
macrophages are present as integral components of
the alv

ages are present as integral components of
the alveolar walls. They can phagocytize particles that
become entrapped in the alveoli. If the particles are
digestible, the macrophages can also digest them and
release the digestive products into the lymph. If the
particle is not digestible, the macrophages often form
a “giant cell” capsule around the particle until such
time—if ever—that it can be slowly dissolved. Such
capsules are frequently formed around tuberculosis
bacilli, silica dust particle

 around tuberculosis
bacilli, silica dust particles, and even carbon particles.
Macrophages (Kupffer Cells) in the Liver Sinusoids. Still
another favorite route by which bacteria invade the
body is through the gastrointestinal tract. Large
numbers of bacteria from ingested food constantly
pass through the gastrointestinal mucosa into the
portal blood. Before this blood enters the general cir-
culation, it passes through the sinusoids of the liver;
these sinusoids are lined with tissue macrophage

;
these sinusoids are lined with tissue macrophages
called Kupffer cells, shown in Figure 33–4. These cells
form such an effective particulate ﬁltration system that
almost none of the bacteria from the gastrointestinal
tract succeeds in passing from the portal blood into the
general systemic circulation. Indeed, motion pictures
of phagocytosis by Kupffer cells have demonstrated
phagocytosis of a single bacterium in less than 1/100 of a
second.
Afferent lymphatics
Capsule
Subcapsular
sinus
Germin

ferent lymphatics
Capsule
sinus
Germinal
center
Medullary cord
Hilus
Lymph in
medullary
sinuses
Valve
Primary
nodule
Efferent lymphatics
Figure 33–3
Functional diagram of a lymph node. (Redrawn from Ham AW: 
Histology, 6th ed. Philadelphia: JB Lippincott, 1969.) (Modiﬁed
from Gartner LP, Hiatt JL: Color Textbook of Histology, 2nd ed.
Philadelphia, WB Saunders, 2001.)
Kupffer cells
Figure 33–4
Kupffer cells lining the liver sinusoids, showing phagocytosis of
India ink particles into t

showing phagocytosis of
India ink particles into the cytoplasm of the Kupffer cells.
(Redrawn from Copenhaver WM, et al: Bailey’s Textbook of His-
tology, 10th ed. Baltimore: Williams & Wilkins, 1971.)
434
Unit VI
Macrophages of the Spleen and Bone Marrow. If an invad-
ing organism succeeds in entering the general circula-
tion, there are other lines of defense by the tissue
macrophage system, especially by macrophages of the
spleen and bone marrow. In b

by macrophages of the
spleen and bone marrow. In both these tissues,
macrophages have become entrapped by the reticular
meshwork of the two organs, and when foreign parti-
cles come in contact with these macrophages, they are
phagocytized.
The spleen is similar to the lymph nodes, except that
blood, instead of lymph, ﬂows through the tissue
spaces of the spleen. Figure 33–5 shows a small periph-
eral segment of spleen tissue. Note that a small artery
penetrates from the splenic capsule into the 

tery
penetrates from the splenic capsule into the splenic
pulp and terminates in small capillaries. The capillar-
ies are highly porous, allowing whole blood to pass out
of the capillaries into cords of red pulp. The blood then
gradually squeezes through the trabecular meshwork
of these cords and eventually returns to the circulation
through the endothelial walls of the venous sinuses.
The trabeculae of the red pulp are lined with vast
numbers of macrophages, and the venous sinuses are
also line

 macrophages, and the venous sinuses are
also lined with macrophages. This peculiar passage of
blood through the cords of the red pulp provides an
exceptional means of phagocytizing unwanted debris
in the blood, including especially old and abnormal red
blood cells.
Inﬂammation: Role of
When tissue injury occurs, whether caused by bacteria,
trauma, chemicals, heat, or any other phenomenon,
multiple substances are released by the injured tissues
and cause d

es are released by the injured tissues
and cause dramatic secondary changes in the sur-
rounding uninjured tissues. This entire complex of
tissue changes is called inﬂammation.
Inﬂammation is characterized by (1) vasodilation of
the local blood vessels, with consequent excess local
blood ﬂow; (2) increased permeability of the capillar-
ies, allowing leakage of large quantities of ﬂuid into 
the interstitial spaces; (3) often clotting of the ﬂuid in
the interstitial spaces because of excessive am

in
the interstitial spaces because of excessive amounts of 
ﬁbrinogen and other proteins leaking from the capil-
laries; (4) migration of large numbers of granulocytes
and monocytes into the tissue; and (5) swelling of the
tissue cells. Some of the many tissue products that
cause these reactions are histamine, bradykinin, sero-
tonin, prostaglandins, several different reaction prod-
ucts of the complement system (described in Chapter
34), reaction products of the blood clotting system, and
multi

n products of the blood clotting system, and
multiple substances called lymphokines
that are
released by sensitized T cells (part of the immune
system; also discussed in Chapter 34). Several of these
substances strongly activate the macrophage system,
and within a few hours, the macrophages begin to
devour the destroyed tissues. But at times, the
macrophages also further injure the still-living tissue
cells.
“Walling-Off” Effect of Inﬂammation. One of the ﬁrst
results of inﬂammation is to “wall 

ne of the ﬁrst
results of inﬂammation is to “wall off” the area of
injury from the remaining tissues. The tissue spaces
and the lymphatics in the inﬂamed area are blocked
by ﬁbrinogen clots so that after a while, ﬂuid barely
ﬂows through the spaces. This walling-off process
delays the spread of bacteria or toxic products.
The intensity of the inﬂammatory process is usually
proportional to the degree of tissue injury. For
instance, when staphylococci invade tissues, they
release extremely lethal 

cci invade tissues, they
release extremely lethal cellular toxins. As a result,
inﬂammation develops rapidly—indeed, much more
rapidly than the staphylococci themselves can multi-
ply and spread. Therefore, local staphylococcal in-
fection is characteristically walled off rapidly and
prevented from spreading through the body. Strepto-
cocci, in contrast, do not cause such intense local tissue
destruction. Therefore, the walling-off process devel-
ops slowly over many hours, while many streptococ

ops slowly over many hours, while many streptococci
reproduce and migrate. As a result, streptococci often
have a far greater tendency to spread through the 
body and cause death than do staphylococci, even
though staphylococci are far more destructive to the
tissues.
Macrophage and Neutrophil
Responses During Inﬂammation
Tissue Macrophage Is a First Line of Defense Against Infection.
Within minutes after inﬂammation begins,
the
macrophages already present in the tissues, whether
histiocytes in

ady present in the tissues, whether
histiocytes in the subcutaneous tissues,
alveolar
macrophages in the lungs, microglia in the brain, or
others, immediately begin their phagocytic actions.
Pulp
Vein
Artery
Figure 33–5
Functional structures of the spleen. (Modiﬁed from Bloom W,
Fawcett DW: A Textbook of Histology, 10th ed. Philadelphia: WB
Chapter 33
435
W

Granulocytes, the Monocyte-Macrophage System
435
When activated by the products of infection and
inﬂammation, the ﬁrst effect is rapid enlargement of
each of these cells. Next, many of the previously sessile
macrophages break loose from their attachments 
and become mobile, forming the ﬁrst line of defense
against infection during the ﬁrst hour or so. The
numbers of these early mobilized macrophages often
are not great, but they are lifesaving.
Neutrophil Invasion of the Inﬂamed Area Is a Second

eutrophil Invasion of the Inﬂamed Area Is a Second Line of
Defense. Within the ﬁrst hour or so after inﬂammation
begins, large numbers of neutrophils begin to invade
the inﬂamed area from the blood. This is caused by
products from the inﬂamed tissues that initiate the fol-
lowing reactions: (1) They alter the inside surface of
the capillary endothelium, causing neutrophils to stick
to the capillary walls in the inﬂamed area. This effect
is called margination and is shown in Figure 33–2. (2)
They

 margination and is shown in Figure 33–2. (2)
They cause the intercellular attachments between the
endothelial cells of the capillaries and small venules 
to loosen, allowing openings large enough for neu-
trophils to pass by diapedesis directly from the blood
into the tissue spaces. (3) Other products of inﬂam-
mation then cause chemotaxis of the neutrophils
toward the injured tissues, as explained earlier.
Thus, within several hours after tissue damage
begins, the area becomes well supplied wi

e damage
begins, the area becomes well supplied with neu-
trophils. Because the blood neutrophils are already
mature cells, they are ready to immediately begin their
scavenger functions for killing bacteria and removing
foreign matter.
Acute Increase in Number of Neutrophils in the Blood—“Neu-
trophilia.” Also within a few hours after the onset of
acute, severe inﬂammation, the number of neutrophils
in the blood sometimes increases fourfold to ﬁve-
fold—from a normal of 4000 to 5000 to 15,000 to

e-
25,000 neutrophils per microliter. This is called neu-
trophilia, which means an increase in the number of
neutrophils in the blood. Neutrophilia is caused by
products of inﬂammation that enter the blood stream,
are transported to the bone marrow, and there act on
the stored neutrophils of the marrow to mobilize these
into the circulating blood. This makes even more neu-
trophils available to the inﬂamed tissue area.
Second Macrophage Invasion i

 inﬂamed tissue area.
Second Macrophage Invasion into the Inﬂamed Tissue Is a Third
Line of Defense. Along with the invasion of neutrophils,
monocytes from the blood enter the inﬂamed tissue
and enlarge to become macrophages. However, the
number of monocytes in the circulating blood is low:
also, the storage pool of monocytes in the bone
marrow is much less than that of neutrophils. There-
fore, the buildup of macrophages in the inﬂamed tissue
area is much slower than that of neutrophils, requir

ea is much slower than that of neutrophils, requiring
several days to become effective. Furthermore, even
after invading the inﬂamed tissue, monocytes are still
immature cells, requiring 8 hours or more to swell to
much larger sizes and develop tremendous quantities
of lysosomes; only then do they acquire the full capac-
ity of tissue macrophages for phagocytosis. Yet, after
several days to several weeks, the macrophages ﬁnally
come to dominate the phagocytic cells of the inﬂamed
area because of

he phagocytic cells of the inﬂamed
area because of greatly increased bone marrow pro-
duction of new monocytes, as explained later.
As already pointed out, macrophages can phagocy-
tize far more bacteria (about ﬁve times as many) and
far larger particles, including even neutrophils them-
selves and large quantities of necrotic tissue, than can
neutrophils. Also, the macrophages play an important
role in initiating the development of antibodies, as we
discuss in Chapter 34.
Increased Production o

s we
Increased Production of Granulocytes and Monocytes by the
Bone Marrow Is a Fourth Line of Defense. The fourth line of
defense is greatly increased production of both gran-
ulocytes and monocytes by the bone marrow. This
results from stimulation of the granulocytic and mono-
cytic progenitor cells of the marrow. However, it 
takes 3 to 4 days before newly formed granulocytes
and monocytes reach the stage of leaving the bone
marrow. If the stimulus from the inﬂamed tiss

bone
marrow. If the stimulus from the inﬂamed tissue con-
tinues, the bone marrow can continue to produce these
cells in tremendous quantities for months and even
years, sometimes at a rate 20 to 50 times normal.
Feedback Control of the Macrophage and
Neutrophil Responses
Although more than two dozen factors have been
implicated in control of the macrophage response to
inﬂammation, ﬁve of these are believed to play domi-
nant roles. They are shown in Figure 33–6 and consist
of (1) tumor necrosis

n in Figure 33–6 and consist
of (1) tumor necrosis factor (TNF), (2) interleukin-1
(IL-1), (3) granulocyte-monocyte colony-stimulating
factor (GM-CSF), (4) granulocyte colony-stimulating
factor (G-CSF), and (5) monocyte colony-stimulating
factor (M-CSF).These factors are formed by activated
macrophage cells in the inﬂamed tissues and in smaller
quantities by other inﬂamed tissue cells.
The cause of the increased production of granulo-
cytes and monocytes by the bone marrow is mainly 
the three c

onocytes by the bone marrow is mainly 
the three colony-stimulating factors, one of which,
GM-CSF, stimulates both granulocyte and monocyte
production; the other two, G-CSF and M-CSF, stimu-
late granulocyte and monocyte production, respec-
tively.This combination of TNF, IL-1, and colony-stim-
ulating factors provides a powerful feedback mecha-
nism that begins with tissue inﬂammation and
proceeds to formation of large numbers of defensive
white blood cells that help remove the cause of the
inﬂ

 blood cells that help remove the cause of the
inﬂammation.
Formation of Pus
When neutrophils and macrophages engulf large
numbers of bacteria and necrotic tissue, essentially 
all the neutrophils and many, if not most, of the
macrophages eventually die. After several days, a
cavity is often excavated in the inﬂamed tissues that
contains varying portions of necrotic tissue, dead neu-
trophils, dead macrophages, and tissue ﬂuid. This
mixture is commonly known as pus. After the infec-
tion has bee

mmonly known as pus. After the infec-
tion has been suppressed, the dead cells and necrotic
tissue in the pus gradually autolyze over a period of
days, and the end products are eventually absorbed
436
Unit VI
into the surrounding tissues and lymph until most of
the evidence of tissue damage is gone.
The eosinophils normally constitute about 2 per cent
of all the blood leukocytes. Eosinophils are weak
phagocytes, and they exhibit chemotaxis, b

re weak
phagocytes, and they exhibit chemotaxis, but in com-
parison with the neutrophils, it is doubtful that the
eosinophils are signiﬁcant in protecting against the
usual types of infection.
Eosinophils, however, are often produced in large
numbers in people with parasitic infections, and they
migrate in large numbers into tissues diseased by par-
asites. Although most parasites are too large to be
phagocytized by eosinophils or any other phagocytic
cells, eosinophils attach themselves to the

ocytic
cells, eosinophils attach themselves to the parasites by
way of special surface molecules and release sub-
stances that kill many of the parasites. For instance,
one of the most widespread infections is schistosomi-
asis, a parasitic infection found in as many as one third
of the population of some Third World countries; the
parasite can invade any part of the body. Eosinophils
attach themselves to the juvenile forms of the parasite
and kill many of them. They do so in several ways: (1)
b

ll many of them. They do so in several ways: (1)
by releasing hydrolytic enzymes from their granules,
which are modiﬁed lysosomes; (2) probably by also
releasing highly reactive forms of oxygen that are
especially lethal to parasites; and (3) by releasing from
the granules a highly larvacidal polypeptide called
major basic protein.
In a few areas of the world, another parasitic disease
that causes eosinophilia is trichinosis. This results from
invasion of the body’s muscles by the Trichinella pa

vasion of the body’s muscles by the Trichinella par-
asite (“pork worm”) after a person eats undercooked
infested pork.
Eosinophils also have a special propensity to collect
in tissues in which allergic reactions occur, such as in
the peribronchial tissues of the lungs in people with
asthma and in the skin after allergic skin reactions.
This is caused at least partly by the fact that many mast
cells and basophils participate in allergic reactions, as
we discuss in the next paragraph. The mast ce

, as
we discuss in the next paragraph. The mast cells and
basophils release an eosinophil chemotactic factor that
causes eosinophils to migrate toward the inﬂamed
allergic tissue. The eosinophils are believed to detox-
ify some of the inﬂammation-inducing substances
released by the mast cells and basophils and probably
also to phagocytize and destroy allergen-antibody
complexes, thus preventing excess spread of the local
inﬂammatory process.
Basophils
The basophils in the circulating blood are s

phils
The basophils in the circulating blood are similar to
the large tissue mast cells located immediately outside
many of the capillaries in the body. Both mast cells and
basophils liberate heparin into the blood, a substance
that can prevent blood coagulation.
The mast cells and basophils also release histamine,
as well as smaller quantities of bradykinin and 
serotonin. Indeed, it is mainly the mast cells in
inﬂamed tissues that release these substances during
The mast cells and

 substances during
The mast cells and basophils play an exceedingly
important role in some types of allergic reactions
because the type of antibody that causes allergic reac-
tions, the immunoglobulin E (IgE) type (see Chapter
34), has a special propensity to become attached to
mast cells and basophils. Then, when the speciﬁc
antigen for the speciﬁc IgE antibody subsequently
reacts with the antibody, the resulting attachment of
antigen to antibody causes the mast cell or basophil 
t

en to antibody causes the mast cell or basophil 
to rupture and release exceedingly large quantities of
histamine, bradykinin, serotonin, heparin, slow-reacting
substance of anaphylaxis, and a number of lysosomal
enzymes. These cause local vascular and tissue reac-
tions that cause many, if not most, of the allergic man-
ifestations. These reactions are discussed in greater
detail in Chapter 34.
A clinical condition known as leukopenia occasionally
occurs in which the bone marrow prod

 occasionally
occurs in which the bone marrow produces very few
Activated
INFLAMMATION
TNF
IL-1
GM-CSF
G-CSF
M-CSF
GM-CSF
G-CSF
M-CSF
Monocytes/macrophages
Bone marrow
TNF
IL-1
Endothelial cells,
fibroblasts,
lymphocytes
Figure 33–6
Control of bone marrow production of granulocytes and mono-
cyte-macrophages in response to multiple growth factors released
from activated macrophages in an inﬂamed tissue. G-CSF, gran-
ulocyte colony-stimulating factor; GM-CSF, granulocyte-m

e colony-stimulating factor; GM-CSF, granulocyte-monocyte
colony-stimulating factor; IL-1, interleukin-1; M-CSF, monocyte
colony-stimulating factor; TNF, tumor necrosis factor.
Chapter 33
437
white blood cells, leaving the body unprotected against
many bacteria and other agents that might invade the
tissues.
Normally, the human body lives in symbiosis with
many bacteria, because all the mucous membra

 with
many bacteria, because all the mucous membranes of
the body are constantly exposed to large numbers of
bacteria. The mouth almost always contains various
spirochetal, pneumococcal, and streptococcal bacteria,
and these same bacteria are present to a lesser extent
in the entire respiratory tract. The distal gastrointesti-
nal tract is especially loaded with colon bacilli. Fur-
thermore, one can always ﬁnd bacteria on the surfaces
of the eyes, urethra, and vagina. Any decrease in the
number 

, urethra, and vagina. Any decrease in the
number of white blood cells immediately allows inva-
sion of adjacent tissues by bacteria that are already
present.
Within 2 days after the bone marrow stops produc-
ing white blood cells, ulcers may appear in the mouth
and colon, or the person might develop some form of
severe respiratory infection. Bacteria from the ulcers
rapidly invade surrounding tissues and the blood.
Without treatment, death often ensues in less than a
week after acute total leuk

 ensues in less than a
week after acute total leukopenia begins.
Irradiation of the body by x-rays or gamma rays, or
exposure to drugs and chemicals that contain benzene
or anthracene nuclei, is likely to cause aplasia of the
bone marrow. Indeed, some common drugs, such as
chloramphenicol (an antibiotic), thiouracil (used to
treat thyrotoxicosis), and even various barbiturate
hypnotics, on very rare occasions cause leukopenia,
thus setting off the entire infectious sequence of this
malady.
After

e entire infectious sequence of this
malady.
After moderate irradiation injury to the bone
marrow, some stem cells, myeloblasts, and hemocyto-
blasts may remain undestroyed in the marrow and are
capable of regenerating the bone marrow, provided
sufﬁcient time is available. A patient properly treated
with transfusions, plus antibiotics and other drugs to
ward off infection, usually develops enough new bone
marrow within weeks to months for blood cell con-
centrations to return to normal.
The Leuk

ell con-
Uncontrolled production of white blood cells can be
caused by cancerous mutation of a myelogenous or
lymphogenous cell. This causes leukemia, which is
usually characterized by greatly increased numbers 
of abnormal white blood cells in the circulating 
blood.
Types of Leukemia. Leukemias are divided into two
general types: lymphocytic leukemias and myeloge-
nous leukemias. The lymphocytic leukemias are caused
by cancerous production of lymp

ukemias are caused
by cancerous production of lymphoid cells, usually
beginning in a lymph node or other lymphocytic tissue
and spreading to other areas of the body. The second
type of leukemia, myelogenous leukemia, begins by
cancerous production of young myelogenous cells in
the bone marrow and then spreads throughout the
body so that white blood cells are produced in many
extramedullary tissues—especially in the lymph nodes,
spleen, and liver.
In myelogenous leukemia, the cancerous process
oc

occasionally produces partially differentiated cells,
resulting in what might be called neutrophilic
leukemia, eosinophilic leukemia, basophilic leukemia,
or monocytic leukemia. More frequently, however, the
leukemia cells are bizarre and undifferentiated and
not identical to any of the normal white blood cells.
Usually, the more undifferentiated the cell, the more
acute is the leukemia, often leading to death within a
few months if untreated. With

ng to death within a
few months if untreated. With some of the more dif-
ferentiated cells, the process can be chronic, some-
times developing slowly over 10 to 20 years. Leukemic
cells, especially the very undifferentiated cells, are
usually nonfunctional for providing the normal pro-
tection against infection.
The ﬁrst effect of leukemia is metastatic growth of
leukemic cells in abnormal areas of the body.
Leukemic cells from the bone marrow may reproduce
so gre

ic cells from the bone marrow may reproduce
so greatly that they invade the surrounding bone,
causing pain and, eventually, a tendency for bones to
fracture easily.
Almost all leukemias eventually spread to the
spleen, lymph nodes, liver, and other vascular regions,
regardless of whether the origin of the leukemia is in
the bone marrow or the lymph nodes. Common effects
in leukemia are the development of infection, severe
anemia, and a bleeding tendency caused by thrombo-
cytopenia (lack of plat

endency caused by thrombo-
cytopenia (lack of platelets). These effects result
mainly from displacement of the normal bone marrow
and lymphoid cells by the nonfunctional leukemic
cells.
Finally, perhaps the most important effect of
leukemia on the body is excessive use of metabolic
substrates by the growing cancerous cells. The
leukemic tissues reproduce new cells so rapidly that
tremendous demands are made on the body reserves
for foodstuffs, speciﬁc amino acids, and vitamins. Con-
sequently, t

eciﬁc amino acids, and vitamins. Con-
sequently, the energy of the patient is greatly depleted,
and excessive utilization of amino acids by the
leukemic cells causes especially rapid deterioration of
the normal protein tissues of the body. Thus, while the
leukemic tissues grow, other tissues become debili-
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439
Resistance of the Body to
Infection: II. Immunity and Allergy
The human body has the ability to resist almost all
types of organisms or toxins that tend to d

st all
types of organisms or toxins that tend to damage the
tissues and organs. This capability is called immu-
nity. Much of immunity is acquired immunity that
does not develop until after the body is ﬁrst
attacked by a bacterium, virus, or toxin, often
requiring weeks or months to develop the immunity. An additional portion of
immunity results from general processes, rather than from processes directed
at speciﬁc disease organisms. This is called innate immunity. It includes the 
1.

ed innate immunity. It includes the 
1. Phagocytosis of bacteria and other invaders by white blood cells and cells
of the tissue macrophage system, as described in Chapter 33.
2. Destruction of swallowed organisms by the acid secretions of the stomach
and the digestive enzymes.
3. Resistance of the skin to invasion by organisms.
4. Presence in the blood of certain chemical compounds that attach to
foreign organisms or toxins and destroy them. Some of these compounds
are (1) lysozyme, 

y them. Some of these compounds
are (1) lysozyme, a mucolytic polysaccharide that attacks bacteria and
causes them to dissolute; (2) basic polypeptides, which react with and
inactivate certain types of gram-positive bacteria; (3) the complement
complex that is described later, a system of about 20 proteins that can be
activated in various ways to destroy bacteria; and (4) natural killer
lymphocytes that can recognize and destroy foreign cells, tumor cells, and
even some infected cells.
This inna

mor cells, and
This innate immunity makes the human body resistant to such diseases as
some paralytic viral infections of animals, hog cholera, cattle plague, and dis-
temper—a viral disease that kills a large percentage of dogs that become
afﬂicted with it. Conversely, many lower animals are resistant or even immune
to many human diseases, such as poliomyelitis, mumps, human cholera, measles,
and syphilis, which are very damaging or even lethal to human beings.
Acquire

y damaging or even lethal to human beings.
In addition to its generalized innate immunity, the human body has the ability
to develop extremely powerful speciﬁc immunity against individual invading
agents such as lethal bacteria, viruses, toxins, and even foreign tissues from other
animals. This is called acquired or adaptive immunity. Acquired immunity is
caused by a special immune system that forms antibodies and/or activated lym-
phocytes that attack and destroy th

activated lym-
phocytes that attack and destroy the speciﬁc invading organism or toxin. It is
with this acquired immunity mechanism and some of its associated reactions—
especially the allergies—that this chapter is concerned.
Acquired immunity can often bestow extreme protection. For instance,
certain toxins, such as the paralytic botulinum toxin or the tetanizing toxin of
tetanus, can be protected against in doses as high as 100,000 times the amount
that would be lethal without immunity.This i

mount
that would be lethal without immunity.This is the reason the treatment process
known as immunization is so important in protecting human beings against
440
Unit VI
disease and against toxins, as explained in the course
Two basic but closely allied types of acquired immu-
nity occur in the body. In one of these the body 
develops circulating antibodies, which are globulin
molecules in the blood plasm

s, which are globulin
molecules in the blood plasma that are capable of
attacking the invading agent. This type of immunity is
called humoral immunity or B-cell immunity (because
B lymphocytes produce the antibodies). The second
type of acquired immunity is achieved through the
formation of large numbers of activated T lymphocytes
that are speciﬁcally crafted in the lymph nodes to
destroy the foreign agent. This type of immunity is
called cell-mediated immunity or T-cell immunity
(because the ac

diated immunity or T-cell immunity
(because the activated lymphocytes are T lympho-
cytes). We shall see shortly that both the antibodies
and the activated lymphocytes are formed in the lym-
phoid tissues of the body. Let us discuss the initiation
of the immune process by antigens.
Both Types of Acquired Immunity Are
Because acquired immunity does not develop until
after invasion by a foreign organism or toxin, it is clear
that the body must have some mechanism for recog-
n

hat the body must have some mechanism for recog-
nizing this invasion. Each toxin or each type of organ-
ism almost always contains one or more speciﬁc
chemical compounds in its makeup that are different
from all other compounds. In general, these are pro-
teins or large polysaccharides, and it is they that initi-
ate the acquired immunity.These substances are called
antigens (antibody generations).
For a substance to be antigenic, it usually must have
a high molecular weight, 8000 or greater. F

t have
a high molecular weight, 8000 or greater. Furthermore,
the process of antigenicity usually depends on regu-
larly recurring molecular groups, called epitopes, on
the surface of the large molecule. This also explains
why proteins and large polysaccharides are almost
always antigenic, because both of these have this ster-
eochemical characteristic.
Lymphocytes Are Responsible for
Acquired immunity is the product of the body’s lym-
phocytes. In people who have a genetic lac

’s lym-
phocytes. In people who have a genetic lack of lym-
phocytes or whose lymphocytes have been destroyed
by radiation or chemicals, no acquired immunity can
develop. And within days after birth, such a person
dies of fulminating bacterial infection unless treated
by heroic measures. Therefore, it is clear that the lym-
phocytes are essential to survival of the human being.
The lymphocytes are located most extensively in the
lymph nodes, but they are also found in special lym-
phoid tissues 

they are also found in special lym-
phoid tissues such as the spleen, submucosal areas of
the gastrointestinal tract, thymus, and bone marrow.
The lymphoid tissue is distributed advantageously in
the body to intercept invading organisms or toxins
before they can spread too widely.
In most instances, the invading agent ﬁrst enters the
tissue ﬂuids and then is carried by way of lymph vessels
to the lymph node or other lymphoid tissue. For
instance, the lymphoid tissue of the gastrointestinal
walls

 the lymphoid tissue of the gastrointestinal
walls is exposed immediately to antigens invading
from the gut. The lymphoid tissue of the throat and
pharynx (the tonsils and adenoids) is well located to
intercept antigens that enter by way of the upper res-
piratory tract.The lymphoid tissue in the lymph nodes
is exposed to antigens that invade the peripheral
tissues of the body.And, ﬁnally, the lymphoid tissue of
the spleen, thymus, and bone marrow plays the speciﬁc
role of intercepting antigenic

w plays the speciﬁc
role of intercepting antigenic agents that have suc-
ceeded in reaching the circulating blood.
Two Types of Lymphocytes Promote “Cell-Mediated” Immunity
or “Humoral” Immunity—the T and the B Lymphocytes.
Although most lymphocytes in normal lymphoid
tissue look alike when studied under the microscope,
these cells are distinctly divided into two major 
populations. One of the populations, the T lympho-
cytes, is responsible for forming the activated lympho-
cytes that provide “

forming the activated lympho-
cytes that provide “cell-mediated” immunity, and the
other population, the B lymphocytes, is responsible 
for forming antibodies that provide “humoral”
immunity.
Both types of lymphocytes are derived originally in
the embryo from pluripotent hematopoietic stem cells
that form lymphocytes as one of their most important
offspring as they differentiate. Almost all of the lym-
phocytes that are formed eventually end up in the 
lymphoid tissue, but before doing so, they 

n the 
lymphoid tissue, but before doing so, they are further
differentiated or “preprocessed” in the following ways.
The lymphocytes that are destined to eventually
form activated T lymphocytes ﬁrst migrate to and are
preprocessed in the thymus gland, and thus they are
called “T” lymphocytes to designate the role of 
the thymus. They are responsible for cell-mediated
immunity.
The other population of lymphocytes—the B lym-
phocytes that are destined to form antibodies—are
preprocessed in the li

ined to form antibodies—are
preprocessed in the liver during midfetal life and in the
bone marrow in late fetal life and after birth.This pop-
ulation of cells was ﬁrst discovered in birds, which have
a special preprocessing organ called the bursa of Fabri-
cius. For this reason, these lymphocytes are called “B”
lymphocytes to designate the role of the bursa, and
they are responsible for humoral immunity. Figure
34–1 shows the two lymphocyte systems for the for-
mation, respectively, of (1) the 

ems for the for-
mation, respectively, of (1) the activated T lympho-
cytes and (2) the antibodies.
Preprocessing of the T and B
Although all lymphocytes in the body originate from
lymphocyte-committed stem cells of the embryo, these
Chapter 34
Resistance of the Body to Infection: II. Immunity and Allergy
441
stem cells themselves are incapable of forming directly
either activated T lymphocytes or antibodies. Before
they can do so, they must be further differentiated in
appropriate p

ey must be further differentiated in
appropriate processing areas as follows.
Thymus Gland Preprocesses the T Lymphocytes. The T lym-
phocytes, after origination in the bone marrow, ﬁrst
migrate to the thymus gland. Here they divide rapidly
and at the same time develop extreme diversity for
reacting against different speciﬁc antigens.That is, one
thymic lymphocyte develops speciﬁc reactivity against
one antigen.Then the next lymphocyte develops speci-
ﬁcity against another antigen. This continue

peci-
ﬁcity against another antigen. This continues until
there are thousands of different types of thymic lym-
phocytes with speciﬁc reactivities against many thou-
sands of different antigens. These different types of
preprocessed T lymphocytes now leave the thymus
and spread by way of the blood throughout the body
to lodge in lymphoid tissue everywhere.
The thymus also makes certain that any T lympho-
cytes leaving the thymus will not react against proteins
or other antigens that are present 

ainst proteins
or other antigens that are present in the body’s 
own tissues; otherwise, the T lymphocytes would be
lethal to the person’s own body in only a few days.
The thymus selects which T lymphocytes will be
released by ﬁrst mixing them with virtually all the spe-
ciﬁc “self-antigens” from the body’s own tissues. If a T
lymphocyte reacts, it is destroyed and phagocytized
instead of being released. This happens to as many as
90 per cent of the cells. Thus, the only cells that are
ﬁnally re

the cells. Thus, the only cells that are
ﬁnally released are those that are nonreactive against
the body’s own antigens—they react only against anti-
gens from an outside source, such as from a bacterium,
a toxin, or even transplanted tissue from another
person.
Most of the preprocessing of T lymphocytes in the
thymus occurs shortly before birth of a baby and for a
few months after birth. Beyond this period, removal of
the thymus gland diminishes (but does not eliminate)
the T-lymphocytic immune

 (but does not eliminate)
the T-lymphocytic immune system. However, removal
of the thymus several months before birth can prevent
development of all cell-mediated immunity. Because
this cellular type of immunity is mainly responsible for
rejection of transplanted organs, such as hearts and
kidneys, one can transplant organs with much less like-
lihood of rejection if the thymus is removed from an
animal a reasonable time before its birth.
Liver and Bone Marrow Preprocess the B Lymphocytes. Much

nd Bone Marrow Preprocess the B Lymphocytes. Much
less is known about the details for preprocessing B
lymphocytes than for preprocessing T lymphocytes. In
the human being, B lymphocytes are known to be pre-
processed in the liver during midfetal life and in the
bone marrow during late fetal life and after birth.
B lymphocytes are different from T lymphocytes in
two ways: First, instead of the whole cell developing
reactivity against the antigen, as occurs for the T lym-
phocytes, the B lymphocyt

as occurs for the T lym-
phocytes, the B lymphocytes actively secrete antibod-
ies that are the reactive agents. These agents are large
protein molecules that are capable of combining with
and destroying the antigenic substance, which is
explained elsewhere in this chapter and in Chapter 
33. Second, the B lymphocytes have even greater
diversity than the T lymphocytes, thus forming many
millions of types of B-lymphocyte antibodies with dif-
ferent speciﬁc reactivities. After preprocessing, the B

t speciﬁc reactivities. After preprocessing, the B
lymphocytes, like the T lymphocytes, migrate to lym-
phoid tissue throughout the body, where they lodge
near but slightly removed from the T-lymphocyte
areas.
Thymus
Cell-Mediated Immunity
Humoral Immunity
Antigen
Activated T
Antigen
Plasma
cells
 Fetal liver,
bone marrow
Stem cell
Lymph node
Figure 34–1
Formation of antibodies and sen-
sitized lymphocytes by a lymph
node in response to antigens

ymphocytes by a lymph
node in response to antigens.
This ﬁgure also shows the origin
of thymic (T) and bursal (B) lym-
phocytes that respectively are
responsible for the cell-mediated
and humoral immune processes.
442
Unit VI
T Lymphocytes and B-Lymphocyte
Antibodies React Highly Speciﬁcally
Against Speciﬁc Antigens—Role of
When speciﬁc antigens come in contact with T and B
lymphocytes in the lymphoid tissue, certain of the T
lymphocyte

n the lymphoid tissue, certain of the T
lymphocytes become activated to form activated T
cells, and certain of the B lymphocytes become acti-
vated to form antibodies. The activated T cells and
antibodies in turn react highly speciﬁcally against the
particular types of antigens that initiated their devel-
opment. The mechanism of this speciﬁcity is the 
Millions of Speciﬁc Types of Lymphocytes Are Stored in the
Lymphoid Tissue. Millions of different types of pre-
formed B lymphocytes 

s of different types of pre-
formed B lymphocytes and preformed T lymphocytes
that are capable of forming highly speciﬁc types of
antibodies or T cells have been stored in the lymph
tissue, as explained earlier. Each of these preformed
lymphocytes is capable of forming only one type of
antibody or one type of T cell with a single type of
speciﬁcity. And only the speciﬁc type of antigen with
which it can react can activate it.Once the speciﬁc lym-
phocyte is activated by its antigen, it reproduce

phocyte is activated by its antigen, it reproduces wildly,
forming tremendous numbers of duplicate lympho-
cytes. If it is a B lymphocyte, its progeny will eventu-
ally secrete the speciﬁc type of antibody that then
circulates throughout the body. If it is a T lymphocyte,
its progeny are speciﬁc sensitized T cells that are
released into the lymph and then carried to the blood
and circulated through all the tissue ﬂuids and back
into the lymph, sometimes circulating around and
around in this cir

ometimes circulating around and
around in this circuit for months or years.
All the different lymphocytes that are capable of
forming one speciﬁcity of antibody or T cell are called
a clone of lymphocytes. That is, the lymphocytes in
each clone are alike and are derived originally from
one or a few early lymphocytes of its speciﬁc type.
Origin of the Many Clones 
of Lymphocytes
Only several hundred to a few thousand genes code
for the millions of different types of antibodies and T
lymphocytes. 

 different types of antibodies and T
lymphocytes. At ﬁrst, it was a mystery how it was pos-
sible for so few genes to code for the millions of dif-
ferent speciﬁcities of antibody molecules or T cells that
can be produced by the lymphoid tissue, especially
when one considers that a single gene is usually nec-
essary for the formation of each different type of
protein. This mystery has now been solved.
The whole gene for forming each type of T cell or B
cell is never present in the original stem 

l or B
cell is never present in the original stem cells from
which the functional immune cells are formed.Instead,
there are only “gene segments”—actually, hundreds of
such segments—but not whole genes. During prepro-
cessing of the respective T- and B-cell lymphocytes,
these gene segments become mixed with one another
in random combinations, in this way ﬁnally forming
whole genes.
Because there are several hundred types of gene
segments, as well as millions of different combinations
in which th

 as millions of different combinations
in which the segments can be arranged in single cells,
one can understand the millions of different cell gene
types that can occur. For each functional T or B lym-
phocyte that is ﬁnally formed, the gene structure codes
for only a single antigen speciﬁcity. These mature cells
then become the highly speciﬁc T and B cells that
spread to and populate the lymphoid tissue.
Mechanism for Activating a Clone 
Each clone of lymphocytes is responsive t

phocytes
Each clone of lymphocytes is responsive to only a
single type of antigen (or to several similar antigens
that have almost exactly the same stereochemical
characteristics).The reason for this is the following: In
the case of the B lymphocytes, each of these has on the
surface of its cell membrane about 100,000 antibody
molecules that will react highly speciﬁcally with 
only one speciﬁc type of antigen. Therefore, when 
the appropriate antigen comes along, it immediately
attaches to the a

igen comes along, it immediately
attaches to the antibody in the cell membrane;
this leads to the activation process, which we describe
in more detail subsequently. In the case of the T 
lymphocytes, molecules similar to antibodies, called
surface receptor proteins (or T-cell markers), are on 
the surface of the T-cell membrane, and these, too,
are highly speciﬁc for one speciﬁed activating 
antigen.
Role of Macrophages in the Activation Process. Aside from
the lymphocytes in lymphoid tissue, li

 Aside from
the lymphocytes in lymphoid tissue, literally millions
of macrophages are also present in the same tissue.
These line the sinusoids of the lymph nodes, spleen,
and other lymphoid tissue, and they lie in apposition
to many of the lymph node lymphocytes. Most invad-
ing organisms are ﬁrst phagocytized and partially
digested by the macrophages, and the antigenic prod-
ucts are liberated into the macrophage cytosol. The
macrophages then pass these antigens by cell-to-cell
contact directl

ass these antigens by cell-to-cell
contact directly to the lymphocytes, thus leading to
activation of the speciﬁed lymphocytic clones. The
macrophages, in addition, secrete a special activating
substance that promotes still further growth and
reproduction of the speciﬁc lymphocytes. This sub-
stance is called interleukin-1.
Role of the T Cells in Activation of the B Lymphocytes. Most
antigens activate both T lymphocytes and B lympho-
cytes at the same time. Some of the T cells that are
formed, c

 same time. Some of the T cells that are
formed, called helper cells, secrete speciﬁc substances
(collectively called lymphokines) that activate the 
speciﬁc B lymphocytes. Indeed, without the aid of
these helper T cells, the quantity of antibodies formed
by the B lymphocytes is usually slight. We will discuss
this cooperative relationship between helper T cells
and B cells again after we have an opportunity 
to describe the mechanisms of the T-cell system of
immunity.
Chapter 34
Resistance of t

ell system of
immunity.
Chapter 34
443
Speciﬁc Attributes of the 
B-Lymphocyte System—Humoral
Immunity and the Antibodies
Formation of Antibodies by Plasma Cells. Before exposure
to a speciﬁc antigen, the clones of B lymphocytes
remain dormant in the lymphoid tissue. On entry of a
foreign antigen, macrophages in the lymphoid tissue
phagocytize the antigen and then present it to adjacent
B lymphocytes. In addition, the antigen is prese

t
B lymphocytes. In addition, the antigen is presented to
T cells at the same time, and activated helper T cells
are formed. These helper cells also contribute to
extreme activation of the B lymphocytes, as we discuss
more fully later.
Those B lymphocytes speciﬁc for the antigen imme-
diately enlarge and take on the appearance of 
lymphoblasts. Some of the lymphoblasts further dif-
ferentiate to form plasmablasts, which are precursors
of plasma cells. In the plasmablasts, the cytoplasm
expands a

ells. In the plasmablasts, the cytoplasm
expands and the rough endoplasmic reticulum vastly
proliferates. The plasmablasts then begin to divide at
a rate of about once every 10 hours for about nine divi-
sions, giving in 4 days a total population of about 500
cells for each original plasmablast.The mature plasma
cell then produces gamma globulin antibodies at an
extremely rapid rate—about 2000 molecules per
second for each plasma cell. In turn, the antibodies are
secreted into the lymph and carr

he antibodies are
secreted into the lymph and carried to the circulating
blood.This process continues for several days or weeks
until ﬁnally exhaustion and death of the plasma cells
occur.
Formation of “Memory” Cells—Difference Between Primary
Response and Secondary Response. A few of the lym-
phoblasts formed by activation of a clone of B lym-
phocytes do not go on to form plasma cells but instead
form moderate numbers of new B lymphocytes similar
to those of the original clone. In other words,

ar
to those of the original clone. In other words, the B-
cell population of the speciﬁcally activated clone
becomes greatly enhanced, and the new B lympho-
cytes are added to the original lymphocytes of the
same clone.They also circulate throughout the body to
populate all the lymphoid tissue; immunologically,
however, they remain dormant until activated once
again by a new quantity of the same antigen. These
lymphocytes are called memory cells. Subsequent
exposure to the same antigen will caus

 Subsequent
exposure to the same antigen will cause a much more
rapid and much more potent antibody response this
second time around, because there are many more
memory cells than there were original B lymphocytes
of the speciﬁc clone.
Figure 34–2 shows the differences between the
primary response for forming antibodies that occurs on
ﬁrst exposure to a speciﬁc antigen and the secondary
response that occurs after second exposure to the same
antigen. Note the 1-week delay in the appearance of
the

en. Note the 1-week delay in the appearance of
the primary response, its weak potency, and its short
life. The secondary response, by contrast, begins
rapidly after exposure to the antigen (often within
hours), is far more potent, and forms antibodies for
many months rather than for only a few weeks. The
increased potency and duration of the secondary
response explain why immunization is usually accom-
plished by injecting antigen in multiple doses with
periods of several weeks or several months

es with
periods of several weeks or several months between
injections.
Nature of the Antibodies
The antibodies are gamma globulins called immuno-
globulins (abbreviated as Ig), and they have molecu-
lar weights between 160,000 and 970,000.They usually
constitute about 20 per cent of all the plasma proteins.
All the immunoglobulins are composed of combi-
nations of light and heavy polypeptide chains. Most are
a combination of two light and two heavy chains,
as shown in Figure 34–3. However, some 

vy chains,
as shown in Figure 34–3. However, some of the
immunoglobulins have combinations of as many as 
10 heavy and 10 light chains, which gives rise to 
high-molecular-weight immunoglobulins. Yet, in all
immunoglobulins, each heavy chain is paralleled by a
light chain at one of its ends,thus forming a heavy-light
pair, and there are always at least 2 and as many as 10
such pairs in each immunoglobulin molecule.
Figure 34–3 shows by the circled area a designated
end of each light and heavy ch

d area a designated
end of each light and heavy chain, called the variable
portion; the remainder of each chain is called the con-
stant portion.The variable portion is different for each
speciﬁcity of antibody, and it is this portion that
attaches speciﬁcally to a particular type of antigen.The
constant portion of the antibody determines other
properties of the antibody, establishing such factors as
diffusivity of the antibody in the tissues, adherence of
the antibody to speciﬁc structures with

herence of
the antibody to speciﬁc structures within the tissues,
attachment to the complement complex, the ease with
which the antibodies pass through membranes, and
other biological properties of the antibody.
0
2
4
8
6
Weeks
Concentration of antibody
0
64
32
16
8
128
Secondary
Primary
Figure 34–2
Time course of the antibody response in the circulating blood to
a primary injection of antigen and to a secondary injection several
months later.
444
Unit VI
Blood Cells, Immunity, and Blood Clottin

4
Unit VI
Speciﬁcity of Antibodies. Each antibody is speciﬁc for a
particular antigen; this is caused by its unique struc-
tural organization of amino acids in the variable por-
tions of both the light and heavy chains. The amino
acid organization has a different steric shape for each
antigen speciﬁcity, so that when an antigen comes in
contact with it, multiple prosthetic groups of the
antigen ﬁt as a mirror image with those of the anti-
body, thus allo

rror image with those of the anti-
body, thus allowing rapid and tight bonding between
the antibody and the antigen. When the antibody is
highly speciﬁc,there are so many bonding sites that the
antibody-antigen coupling is exceedingly strong, held
together by (1) hydrophobic bonding, (2) hydrogen
bonding, (3) ionic attractions, and (4) van der Waals
forces. It also obeys the thermodynamic mass action
law.
Ka is called the afﬁnity constant and is a measure of
how tightly the antibody binds with t

a measure of
how tightly the antibody binds with the antigen.
Note, especially, in Figure 34–3 that there are two
variable sites on the illustrated antibody for attach-
ment of antigens, making this type of antibody biva-
lent. A small proportion of the antibodies, which
consist of combinations of up to 10 light and 10 heavy
chains, have as many as 10 binding sites.
Classes of Antibodies. There are ﬁve general classes of
antibodies, respectively named IgM, IgG, IgA, IgD,
and IgE. Ig stands for i

named IgM, IgG, IgA, IgD,
and IgE. Ig stands for immunoglobulin, and the other
ﬁve respective letters designate the respective classes.
For the purpose of our present limited discussion,
two of these classes of antibodies are of particular
importance: IgG, which is a bivalent antibody and con-
stitutes about 75 per cent of the antibodies of the
normal person, and IgE, which constitutes only a small
percentage of the antibodies but is especially involved
K
Concentration of bound antibody-antigen

nvolved
K
Concentration of antigen
a =
¥
in allergy. The IgM class is also interesting because 
a large share of the antibodies formed during the
primary response are of this type. These antibodies
have 10 binding sites that make them exceedingly
effective in protecting the body against invaders, even
though there are not many IgM antibodies.
Mechanisms of Action of Antibodies
Antibodies act mainly in two ways to protect the body

bodies act mainly in two ways to protect the body
against invading agents: (1) by direct attack on the
invader and (2) by activation of the “complement
system” that then has multiple means of its own for
destroying the invader.
Direct Action of Antibodies on Invading Agents. Figure 34–4
shows antibodies (designated by the red Y-shaped
bars) reacting with antigens (designated by the shaded
objects). Because of the bivalent nature of the anti-
bodies and the multiple antigen sites on most invad-
i

es and the multiple antigen sites on most invad-
ing agents, the antibodies can inactivate the invading
agent in one of several ways, as follows:
1. Agglutination, in which multiple large particles
with antigens on their surfaces, such as bacteria or
red cells, are bound together into a clump
2. Precipitation, in which the molecular complex of
soluble antigen (such as tetanus toxin) and
antibody becomes so large that it is rendered
insoluble and precipitates
3. Neutralization, in which the antib

precipitates
3. Neutralization, in which the antibodies cover the
toxic sites of the antigenic agent
4. Lysis, in which some potent antibodies are
occasionally capable of directly attacking
membranes of cellular agents and thereby cause
rupture of the agent
These direct actions of antibodies attacking the anti-
genic invaders often are not strong enough to play a
major role in protecting the body against the invader.
Most of the protection comes through the amplifying
effects of the complement s

through the amplifying
effects of the complement system described next.
Antigen-binding sites
Antigen
Light
chain
Heavy
chain
Constant portion
Variable portion
S • S
S • S
S • S
S • S
Figure 34–3
Structure of the typical IgG antibody, showing it to be composed
of two heavy polypeptide chains and two light polypeptide chains.
The antigen binds at two different sites on the variable portions of
the chains.
Antigen
Figure 34–4
Binding of antigen molecules to one another by bivalent 
anti

antigen molecules to one another by bivalent 
antibodies.
Chapter 34
445
Complement System for Antibody Action
“Complement” is a collective term that describes a
system of about 20 proteins,many of which are enzyme
precursors. The principal actors in this system are 11
proteins designated C1 through C9, B, and D, shown
in Figure 34–5. All these are present normally among
the plasma proteins in the blood as well as among the
proteins t

teins in the blood as well as among the
proteins that leak out of the capillaries into the tissue
spaces. The enzyme precursors are normally inactive,
but they can be activated mainly by the so-called
classic pathway.
Classic Pathway. The classic pathway is initiated by an
antigen-antibody reaction. That is, when an antibody
binds with an antigen, a speciﬁc reactive site on the
“constant” portion of the antibody becomes uncov-
ered, or “activated,” and this in turn binds directly with
the C1 mol

,” and this in turn binds directly with
the C1 molecule of the complement system, setting
into motion a “cascade” of sequential reactions, shown
in Figure 34–5, beginning with activation of the proen-
zyme C1 itself. The C1 enzymes that are formed then
activate successively increasing quantities of enzymes in
the later stages of the system, so that from a small
beginning, an extremely large “ampliﬁed” reaction
occurs. Multiple end products are formed, as shown to
the right in the ﬁgure, and seve

rmed, as shown to
the right in the ﬁgure, and several of these cause
important effects that help to prevent damage to the
body’s tissues caused by the invading organism or
toxin. Among the more important effects are the 
1. Opsonization and phagocytosis. One of the
products of the complement cascade, C3b,
strongly activates phagocytosis by both
neutrophils and macrophages, causing these cells
to engulf the bacteria to which the antigen-
antibody complexes are attached. This process is

-
called opsonization. It often enhances the number
of bacteria that can be destroyed by many
hundredfold.
2. Lysis. One of the most important of all the
products of the complement cascade is the lytic
complex, which is a combination of multiple
complement factors and designated C5b6789.
This has a direct effect of rupturing the cell
membranes of bacteria or other invading
organisms.
3. Agglutination. The complement products also
change the surfac

on. The complement products also
change the surfaces of the invading organisms,
causing them to adhere to one another, thus
promoting agglutination.
4. Neutralization of viruses. The complement
enzymes and other complement products can
attack the structures of some viruses and thereby
render them nonvirulent.
5. Chemotaxis. Fragment C5a initiates chemotaxis of
neutrophils and macrophages, thus causing large
numbers of these phagocytes to migrate into the
tissue area adjacent to the antigenic age

into the
tissue area adjacent to the antigenic agent.
6. Activation of mast cells and basophils. Fragments
C3a, C4a, and C5a activate mast cells and
basophils, causing them to release histamine,
heparin, and several other substances into the
local ﬂuids. These substances in turn cause
increased local blood ﬂow, increased leakage of
ﬂuid and plasma protein into the tissue, and other
local tissue reactions that help inactivate or
immobilize the antigenic agent. The same factors
play a major role i

igenic agent. The same factors
play a major role in inﬂammation (which was
Antigen–antibody complex
Micro-organism +
B and D
Opsonization of bacteria
Activate mast
cells and basophils
Chemotaxis of
white blood cells
C1
C3
C5
C4 + C2
C42 + C4a
C1
C3b + C3a
C8 + C9
C5b + C5a
C6 + C7
C5b67
C5b6789
Lysis of cells
Figure 34–5
Cascade of reactions during 
activation of the classic pathway
of complement. (Modiﬁed from
Alexander JW, Good RA: Funda-
mentals of Clinical Immunology.
WB 
Saun

ls of Clinical Immunology.
WB 
Saunders,
1977.)
446
Unit VI
discussed in Chapter 33) and in allergy, as we
discuss later.
7. Inﬂammatory effects. In addition to inﬂammatory
effects caused by activation of the mast cells and
basophils, several other complement products
contribute to local inﬂammation. These products
cause (1) the already increased blood ﬂow to
increase still further, (2) the capillary leakage of
proteins to be increased, an

 capillary leakage of
proteins to be increased, and (3) the interstitial
ﬂuid proteins to coagulate in the tissue spaces,
thus preventing movement of the invading
organism through the tissues.
Special Attributes of the 
T-Lymphocyte System–Activated 
T Cells and Cell-Mediated Immunity
Release of Activated T Cells from Lymphoid Tissue and Forma-
tion of Memory Cells. On exposure to the proper antigen,
as presented by adjacent macrophages, the T lympho-
cytes of a speciﬁc lymphocyte clone prolifer

mpho-
cytes of a speciﬁc lymphocyte clone proliferate and
release large numbers of activated, speciﬁcally react-
ing T cells in ways that parallel antibody release by
activated B cells. The principal difference is that
instead of releasing antibodies, whole activated T cells
are formed and released into the lymph. These then
pass into the circulation and are distributed through-
out the body, passing through the capillary walls into
the tissue spaces, back into the lymph and blood once
again, an

aces, back into the lymph and blood once
again, and circulating again and again throughout the
body, sometimes lasting for months or even years.
Also, T-lymphocyte memory cells are formed in the
same way that B memory cells are formed in the anti-
body system.That is, when a clone of T lymphocytes is
activated by an antigen, many of the newly formed
lymphocytes are preserved in the lymphoid tissue to
become additional T lymphocytes of that speciﬁc
clone; in fact, these memory cells even spread t

c
clone; in fact, these memory cells even spread through-
out the lymphoid tissue of the entire body. Therefore,
on subsequent exposure to the same antigen anywhere
in the body, release of activated T cells occurs far more
rapidly and much more powerfully than had occurred
during ﬁrst exposure.
Antigen-Presenting Cells, MHC Proteins, and Antigen Recep-
tors on the T Lymphocytes. T-cell responses are extremely
antigen speciﬁc, like the antibody responses of B cells,
and are at least as important 

sponses of B cells,
and are at least as important as antibodies in de-
fending against infection. In fact, acquired immune
responses usually require assistance from T cells to
begin the process, and T cells play a major role in actu-
ally helping to eliminate invading pathogens.
Although B lymphocytes recognize intact antigens,
T lymphocytes respond to antigens only when they are
bound to speciﬁc molecules called MHC proteins on
the surface of antigen-presenting cells in the lymphoid
tissues (Fi

tigen-presenting cells in the lymphoid
tissues (Figure 34–6). The three major types of
antigen-presenting cells are macrophages, B lympho-
cytes, and dendritic cells. The dendritic cells, the most
potent of the antigen-presenting cells, are located
throughout the body, and their only known function is
to present antigens to T cells. Interaction of cell adhe-
sion proteins is critical in permitting the T cells to bind
to antigen-presenting cells long enough to become
activated.
The MHC proteins a

ong enough to become
The MHC proteins are encoded by a large group of
genes called the major histocompatibility complex
(MHC). The MHC proteins bind peptide fragments 
of antigen proteins that are degraded inside antigen-
presenting cells and then transport them to the cell
surface. There are two types of MHC proteins: (1)
MHC I proteins, which present antigens to cytotoxic T
cells, and (2) MHC II proteins, which present antigens
to T helper cells. The speciﬁc functions of cytotoxic a

helper cells. The speciﬁc functions of cytotoxic and
helper T cells are discussed later.
The antigens on the surface of antigen-presenting
cells bind with receptor molecules on the surfaces of T
cells in the same way that they bind with plasma
protein antibodies.These receptor molecules are com-
posed of a variable unit similar to the variable portion
of the humoral antibody, but its stem section is ﬁrmly
bound to the cell membrane of the T lymphocyte.
There are as many as 100,000 receptor sites

ocyte.
There are as many as 100,000 receptor sites on a single
T cell.
Several Types of T Cells and Their
Different Functions
It has become clear that there are multiple types of T
cells. They are classiﬁed into three major groups: (1)
helper T cells, (2) cytotoxic T cells, and (3) suppressor
T cells. The functions of each of these are distinct.
Helper T Cells—Their Role in Overall
Regulation of Immunity
The helper T cells are by far the most numerous of the
T cells, usually constituting more th

erous of the
T cells, usually constituting more than three quarters
Cell-cell
adhesion
proteins
T-cell receptor
Foreign protein
MHC protein
T cell
Antigen-
presenting
cell
Figure 34–6
Activation of T cells requires interaction of T-cell receptors with an
antigen (foreign protein) that is transported to the surface of the
antigen-presenting cell by a major histocompatibility complex
(MHC) protein. Cell-to-cell adhesion proteins enable the T cell 
to bind to the antigen-presenting cell long enough

to bind to the antigen-presenting cell long enough to become 
Chapter 34
447
of all of them. As their name implies, they help in the
functions of the immune system, and they do so in
many ways. In fact, they serve as the major regulator
of virtually all immune functions, as shown in Figure
34–7.They do this by forming a series of protein medi-
ators, called lymphokines, that act on other cells of the
immune system as well a

 act on other cells of the
immune system as well as on bone marrow cells.
Among the important lymphokines secreted by the
helper T cells are the following:
Interleukin-2
Interleukin-3
Interleukin-4
Interleukin-5
Interleukin-6
Granulocyte-monocyte colony-stimulating factor
Interferon-g
Speciﬁc Regulatory Functions of the Lymphokines. In the
absence of the lymphokines from the helper T cells,
the remainder of the immune system is almost para-
lyzed. In fact, it is the helper T cells that are inact

. In fact, it is the helper T cells that are inactivated
or destroyed by the acquired immunodeﬁciency syn-
drome (AIDS) virus, which leaves the body almost
totally unprotected against infectious disease, there-
fore leading to the now well-known debilitating and
lethal effects of AIDS. Some of the speciﬁc regulatory
functions are the following.
Stimulation of Growth and Proliferation of Cytotoxic T
Cells and Suppressor T Cells. In the absence of helper
T cells, the clones for producing cytotoxic

helper
T cells, the clones for producing cytotoxic T cells and
suppressor T cells are activated only slightly by most
antigens. The lymphokine interleukin-2 has an espe-
cially strong stimulatory effect in causing growth and
proliferation of both cytotoxic and suppressor T cells.
In addition, several of the other lymphokines have less
potent effects.
Stimulation of B-Cell Growth and Differentiation to
Form Plasma Cells and Antibodies. The direct actions
of antigen to cause B-cell growth, prolife

actions
of antigen to cause B-cell growth, proliferation, for-
mation of plasma cells, and secretion of antibodies are
also slight without the “help” of the helper T cells.
Almost all the interleukins participate in the B-cell
response, but especially interleukins 4, 5, and 6. In fact,
these three interleukins have such potent effects on
the B cells that they have been called B-cell stimulat-
ing factors or B-cell growth factors.
Activation of the Macrophage System. The lym-
phokines also affect

e Macrophage System. The lym-
phokines also affect the macrophages. First, they slow
or stop the migration of the macrophages after they
have been chemotactically attracted into the inﬂamed
tissue area, thus causing great accumulation of
macrophages. Second, they activate the macrophages
to cause far more efﬁcient phagocytosis, allowing them
to attack and destroy increasing numbers of invading
bacteria or other tissue-destroying agents.
Feedback Stimulatory Effect on the Helper Cells Them-
selve

Stimulatory Effect on the Helper Cells Them-
selves. Some of the lymphokines, especially inter-
leukin-2, have a direct positive feedback effect in
stimulating activation of the helper T cells themselves.
This acts as an ampliﬁer by further enhancing the
helper cell response as well as the entire immune
response to an invading antigen.
Cytotoxic T Cells
The cytotoxic T cell is a direct-attack cell that is
capable of killing micro-organisms and, at times, even
some of the body’s own cells. For th

t times, even
some of the body’s own cells. For this reason, these
cells are called killer cells.The receptor proteins on the
surfaces of the cytotoxic cells cause them to bind
tightly to those organisms or cells that contain the
appropriate binding-speciﬁc antigen. Then, they kill
the attacked cell in the manner shown in Figure 34–8.
After binding, the cytotoxic T cell secretes hole-
forming proteins, called perforins, that literally punch
round holes in the membrane of the attacked cell.
Then 

 holes in the membrane of the attacked cell.
Then ﬂuid ﬂows rapidly into the cell from the intersti-
tial space. In addition, the cytotoxic T cell releases
cytotoxic substances directly into the attacked cell.
Almost immediately, the attacked cell becomes greatly
swollen, and it usually dissolves shortly thereafter.
Especially important, these cytotoxic killer cells can
pull away from the victim cells after they have
punched holes and delivered cytotoxic substances and
Preprocessor
areas
Lymphok

ytotoxic substances and
areas
Lymphokines!!
Antigen
Antigen
Processed
antigen
MHC
B cell
Suppressor
T cells
Plasma
cells
Cytotoxic
T cells
Helper
T cells
Antigen-specific receptor
IgM
IgG
IgA
IgE
areas
Interleukin-1
Figure 34–7
Regulation of the immune system, emphasizing a pivotal role of
the helper T cells. MHC, major histocompatibility complex.
448
Unit VI
then move on to kill more cells. Indeed,

 Clotting
then move on to kill more cells. Indeed, some of these
cells persist for months in the tissues.
Some of the cytotoxic T cells are especially lethal to
tissue cells that have been invaded by viruses because
many virus particles become entrapped in the mem-
branes of the tissue cells and attract T cells in response
to the viral antigenicity. The cytotoxic cells also play
an important role in destroying cancer cells, heart
transplant cells, or other types of cells that are foreign
to the 

, or other types of cells that are foreign
to the person’s own body.
Suppressor T Cells
Much less is known about the suppressor T cells than
about the others, but they are capable of suppressing
the functions of both cytotoxic and helper T cells. It is
believed that these suppressor functions serve the
purpose of preventing the cytotoxic cells from causing
excessive immune reactions that might be damaging to
the body’s own tissues. For this reason, the suppressor
cells are classiﬁed, along with 

n, the suppressor
cells are classiﬁed, along with the helper T cells, as reg-
ulatory T cells. It is probable that the suppressor T-cell
system plays an important role in limiting the ability
of the immune system to attack a person’s own body
tissues, called immune tolerance, as we discuss in the
next section.
Tolerance of the Acquired Immunity
System to One’s Own Tissues—Role
of Preprocessing in the Thymus and
If a person should become immune to his or her own
tissues, the process o

me immune to his or her own
tissues, the process of acquired immunity would
destroy the individual’s own body. The immune mech-
anism normally “recognizes” a person’s own tissues 
as being distinctive from bacteria or viruses, and the
person’s immunity system forms few antibodies or
activated T cells against his or her own antigens.
Most Tolerance Results from Clone Selection During Prepro-
cessing. It is believed that most tolerance develops
during preprocessing of T lymphocytes in the thymus
a

ing preprocessing of T lymphocytes in the thymus
and of B lymphocytes in the bone marrow.The reason
for this belief is that injecting a strong antigen into a
fetus while the lymphocytes are being preprocessed in
these two areas prevents development of clones of
lymphocytes in the lymphoid tissue that are speciﬁc for
the injected antigen. Experiments have shown that
speciﬁc immature lymphocytes in the thymus, when
exposed to a strong antigen, become lymphoblastic,
proliferate considerably, and th

me lymphoblastic,
proliferate considerably, and then combine with the
stimulating antigen—an effect that is believed to cause
the cells themselves to be destroyed by the thymic
epithelial cells before they can migrate to and colonize
the total body lymphoid tissue.
It is believed that during the preprocessing of lym-
phocytes in the thymus and bone marrow, all or most
of those clones of lymphocytes that are speciﬁc to
damage the body’s own tissues are self-destroyed
because of their continual ex

s are self-destroyed
because of their continual exposure to the body’s 
antigens.
Failure of the Tolerance Mechanism Causes Autoimmune Dis-
eases. Sometimes people lose their immune tolerance
of their own tissues.This occurs to a greater extent the
older a person becomes. It usually occurs after destruc-
tion of some of the body’s own tissues, which releases
considerable quantities of “self-antigens” that cir-
culate in the body and presumably cause acquired
immunity in the form of either activa

use acquired
immunity in the form of either activated T cells or 
Several speciﬁc diseases that result from autoim-
munity include (1) rheumatic fever, in which the body
becomes immunized against tissues in the joints and
heart, especially the heart valves, after exposure to a
speciﬁc type of streptococcal toxin that has an epitope
in its molecular structure similar to the structure of
some of the body’s own self-antigens; (2) one type of
glomerulonephritis, in which the person becom

e of
glomerulonephritis, in which the person becomes
immunized against the basement membranes of
glomeruli; (3) myasthenia gravis, in which immunity
develops against the acetylcholine receptor proteins of
the neuromuscular junction, causing paralysis; and (4)
lupus erythematosus, in which the person becomes
immunized against many different body tissues at the
same time, a disease that causes extensive damage and
often rapid death.
Immunization has been used 

 Injection of Antigens
Immunization has been used for many years to
produce acquired immunity against speciﬁc diseases.A
person can be immunized by injecting dead organisms
that are no longer capable of causing disease but that
still have some of their chemical antigens. This type of
immunization is used to protect against typhoid fever,
Cytotoxic
and
digestive
enzymes
Cytotoxic
T cells
(killer cells)
Antigen
receptors
Antigen
Attacked
cell
Specific
binding
Figure 34–8
Direct destruction of an i

fic
binding
Direct destruction of an invading cell by sensitized lymphocytes
(cytotoxic T cells).
Chapter 34
449
whooping cough, diphtheria, and many other types of
bacterial diseases.
Immunity can be achieved against toxins that have
been treated with chemicals so that their toxic nature
has been destroyed even though their antigens for
causing immunity are still intact. This procedure is
used in immunizing against tetanu

his procedure is
used in immunizing against tetanus, botulism, and
other similar toxic diseases.
And, ﬁnally, a person can be immunized by being
infected with live organisms that have been “attenu-
ated.”That is, these organisms either have been grown
in special culture media or have been passed through
a series of animals until they have mutated enough
that they will not cause disease but do still carry spe-
ciﬁc antigens required for immunization. This proce-
dure is used to protect against po

on. This proce-
dure is used to protect against poliomyelitis, yellow
fever,measles,smallpox,and many other viral diseases.
Thus far, all the acquired immunity we have discussed
has been active immunity. That is, the person’s own
body develops either antibodies or activated T cells in
response to invasion of the body by a foreign antigen.
However, temporary immunity can be achieved in a
person without injecting any antigen. This is done by
infusing antibodies, activated T cells,

is done by
infusing antibodies, activated T cells, or both obtained
from the blood of someone else or from some other
animal that has been actively immunized against the
antigen.
Antibodies last in the body of the recipient for 2 to
3 weeks, and during that time, the person is protected
against the invading disease. Activated T cells last for
a few weeks if transfused from another person but
only for a few hours to a few days if transfused 
from an animal. Such transfusion of antibodies or T
lym

an animal. Such transfusion of antibodies or T
lymphocytes to confer immunity is called passive
immunity.
ALLERGY AND
HYPERSENSITIVITY
An important undesirable side effect of immunity is
the development, under some conditions, of allergy or
other types of immune hypersensitivity. There are
several types of allergy and other hypersensitivities,
some of which occur only in people who have a spe-
ciﬁc allergic tendency.
Allergy Caused by Activated T Cells:
Delayed-reaction 

 Cells:
Delayed-reaction allergy is caused by activated T cells
and not by antibodies. In the case of poison ivy, the
toxin of poison ivy in itself does not cause much harm
to the tissues. However, on repeated exposure, it does
cause the formation of activated helper and cytotoxic
T cells. Then, after subsequent exposure to the poison
ivy toxin, within a day or so, the activated T cells
diffuse from the circulating blood in large numbers
into the skin to respond to the p

in large numbers
into the skin to respond to the poison ivy toxin. And,
at the same time, these T cells elicit a cell-mediated
type of immune reaction. Remembering that this type
of immunity can cause release of many toxic sub-
stances from the activated T cells as well as extensive
invasion of the tissues by macrophages along with
their subsequent effects, one can well understand that
the eventual result of some delayed-reaction allergies
can be serious tissue damage. The damage normally
occurs

 serious tissue damage. The damage normally
occurs in the tissue area where the instigating antigen
is present, such as in the skin in the case of poison ivy,
or in the lungs to cause lung edema or asthmatic
attacks in the case of some airborne antigens.
Allergies in the “Allergic” Person,
Who Has Excess IgE Antibodies
Some people have an “allergic” tendency. Their aller-
gies are called atopic allergies because they are caused
by a nonordinary response of the immune system.The
allergic tendency

esponse of the immune system.The
allergic tendency is genetically passed from parent to
child and is characterized by the presence of large
quantities of IgE antibodies in the blood. These anti-
bodies are called reagins or sensitizing antibodies to
distinguish them from the more common IgG anti-
bodies. When an allergen (deﬁned as an antigen 
that reacts speciﬁcally with a speciﬁc type of IgE
reagin antibody) enters the body, an allergen-reagin
reaction lakes place, and a subsequent allergic re

reaction lakes place, and a subsequent allergic reaction
occurs.
A special characteristic of the IgE antibodies (the
reagins) is a strong propensity to attach to mast cells
and basophils. Indeed, a single mast cell or basophil
can bind as many as half a million molecules of IgE
antibodies. Then, when an antigen (an allergen) that
has multiple binding sites binds with several IgE anti-
bodies that are already attached to a mast cell or
basophil, this causes immediate change in the mem-
brane of t

his causes immediate change in the mem-
brane of the mast cell or basophil, perhaps resulting
from a physical effect of the antibody molecules to
contort the cell membrane. At any rate, many of the
mast cells and basophils rupture; others release special
agents immediately or shortly thereafter,including his-
tamine, protease, slow-reacting substance of anaphy-
laxis (which is a mixture of toxic leukotrienes),
eosinophil chemotactic substance, neutrophil chemo-
tactic substance, heparin, and pla

utrophil chemo-
tactic substance, heparin, and platelet activating factors.
These substances cause such effects as dilation of the
local blood vessels; attraction of eosinophils and neu-
trophils to the reactive site; increased permeability of
the capillaries with loss of ﬂuid into the tissues; and
contraction of local smooth muscle cells. Therefore,
several different tissue responses can occur, depend-
ing on the type of tissue in which the allergen-reagin
reaction occurs. Among the different t

rgen-reagin
reaction occurs. Among the different types of allergic
reactions caused in this manner are the following.
Anaphylaxis. When a speciﬁc allergen is injected
directly into the circulation, the allergen can react with
basophils of the blood and mast cells in the tissues
450
Unit VI
located immediately outside the small blood vessels 
if the basophils and mast cells have been sensitized 
by attachment of IgE reagins.Therefore, a widespread
allergi

ent of IgE reagins.Therefore, a widespread
allergic reaction occurs throughout the vascular
system and closely associated tissues. This is called
anaphylaxis. Histamine is released into the circulation
and causes body-wide vasodilation as well as increased
permeability of the capillaries with resultant marked
loss of plasma from the circulation. An occasional
person who experiences this reaction dies of circula-
tory shock within a few minutes unless treated with
epinephrine to oppose the effect

less treated with
epinephrine to oppose the effects of the histamine.
Also released from the activated basophils and mast
cells is a mixture of leukotrienes called slow-reacting
substance of anaphylaxis.These leukotrienes can cause
spasm of the smooth muscle of the bronchioles, elicit-
ing an asthma-like attack, sometimes causing death by
suffocation.
Urticaria. Urticaria results from antigen entering spe-
ciﬁc skin areas and causing localized anaphylactoid
reactions. Histamine released locally 

aphylactoid
reactions. Histamine released locally causes (1) vasodi-
lation that induces an immediate red ﬂare and (2)
increased local permeability of the capillaries that
leads to local circumscribed areas of swelling of the
skin within another few minutes. The swellings are
commonly called hives. Administration of antihista-
mine drugs to a person before exposure will prevent
the hives.
Hay Fever. In hay fever, the allergen-reagin reaction
occurs in the nose. Histamine released in response to

rs in the nose. Histamine released in response to
the reaction causes local intranasal vascular dilation,
with resultant increased capillary pressure as well as
increased capillary permeability. Both these effects
cause rapid ﬂuid leakage into the nasal cavities and
into associated deeper tissues of the nose; and the
nasal linings become swollen and secretory. Here
again, use of antihistamine drugs can prevent this
swelling reaction. But other products of the allergen-
reagin reaction can still 

oducts of the allergen-
reagin reaction can still cause irritation of the nose,
eliciting the typical sneezing syndrome.
Asthma. Asthma often occurs in the “allergic” type of
person. In such a person, the allergen-reagin reaction
occurs in the bronchioles of the lungs. Here, an impor-
tant product released from the mast cells is believed
to be the slow-reacting substance of anaphylaxis, which
causes spasm of the bronchiolar smooth muscle. Con-
sequently, the person has difﬁculty breathing until 

quently, the person has difﬁculty breathing until the
reactive products of the allergic reaction have been
removed. Administration of antihistaminics has less
effect on the course of asthma because histamine does
not appear to be the major factor eliciting the asth-
matic reaction.
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A
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3
5
451
Blood Types; Transfusion; Tissue
and Organ Transplantation
Antigenicity Causes

ssue
Antigenicity Causes Immune
Reactions of Blood
When blood transfusions from one person to
another were ﬁrst attempted, immediate or delayed
agglutination and hemolysis of the red blood cells
often occurred, resulting in typical transfusion reac-
tions that frequently led to death. Soon it was dis-
covered that the bloods of different people have different antigenic and immune
properties, so that antibodies in the plasma of one blood will react with anti-
gens on the

sma of one blood will react with anti-
gens on the surfaces of the red cells of another blood type. If proper precau-
tions are taken, one can determine ahead of time whether the antibodies 
and antigens present in the donor and recipient bloods will cause a transfusion
reaction.
Multiplicity of Antigens in the Blood Cells. At least 30 commonly occurring antigens
and hundreds of other rare antigens, each of which can at times cause antigen-
antibody reactions, have been found in human blood cell

ody reactions, have been found in human blood cells, especially on the sur-
faces of the cell membranes. Most of the antigens are weak and therefore are
of importance principally for studying the inheritance of genes to establish
parentage.
Two particular types of antigens are much more likely than the others to
cause blood transfusion reactions. They are the O-A-B system of antigens and
the Rh system.
Two antigens—type A and type B—occur on the s

gens
Two antigens—type A and type B—occur on the surfaces of the red blood cells
in a large proportion of human beings. It is these antigens (also called agglu-
tinogens because they often cause blood cell agglutination) that cause most
blood transfusion reactions. Because of the way these agglutinogens are inher-
ited, people may have neither of them on their cells, they may have one, or they
may have both simultaneously.
Major O-A-B Blood Types. In transfusing blood from one person to another,

. In transfusing blood from one person to another, the
bloods of donors and recipients are normally classiﬁed into four major O-A-B
blood types, as shown in Table 35–1, depending on the presence or absence of
the two agglutinogens, the A and B agglutinogens.When neither A nor B agglu-
tinogen is present, the blood is type O. When only type A agglutinogen is
present, the blood is type A.When only type B agglutinogen is present, the blood
is type B. When both A and B agglutinogens are present, the

. When both A and B agglutinogens are present, the blood is type AB.
Genetic Determination of the Agglutinogens. Two genes, one on each of two paired
chromosomes, determine the O-A-B blood type.These genes can be any one of
three types but only one type on each of the two chromosomes: type O, type A,
452
Unit VI
or type B. The type O gene is either functionless or
almost functionless, so that it causes no signiﬁcant type
O agglutinogen on the cells. Conv

 signiﬁcant type
O agglutinogen on the cells. Conversely, the type A
and type B genes do cause strong agglutinogens on the
cells.
The six possible combinations of genes, as shown in
Table 35–1, are OO, OA, OB, AA, BB, and AB. These
combinations of genes are known as the genotypes,
and each person is one of the six genotypes.
One can also observe from Table 35–1 that a person
with genotype OO produces no agglutinogens, and
therefore the blood type is O.A person with genotype
OA or AA produces typ

 is O.A person with genotype
OA or AA produces type A agglutinogens and there-
fore has blood type A. Genotypes OB and BB give
type B blood, and genotype AB gives type AB blood.
Relative Frequencies of the Different Blood Types.
The prevalence of the different blood types among one
group of persons studied was approximately:
when type A agglutinogens are not present in the cells,
and anti-B agglutinins when type B agglutinogens are
not in the cells. Figure 35–1 shows the changing titers
of the a

ls. Figure 35–1 shows the changing titers
of the anti-A and anti-B agglutinins at different ages.
A maximum titer is usually reached at 8 to 10 years 
of age, and this gradually declines throughout the
remaining years of life.
Origin of Agglutinins in the Plasma. The agglutinins are
gamma globulins, as are almost all antibodies, and they
are produced by the same bone marrow and lymph
gland cells that produce antibodies to any other anti-
gens. Most of them are IgM and IgG immunoglobulin
molecule

st of them are IgM and IgG immunoglobulin
But why are these agglutinins produced in people
who do not have the respective agglutinogens in their
red blood cells? The answer to this is that small
amounts of type A and B antigens enter the body in
food, in bacteria, and in other ways, and these sub-
stances initiate the development of the anti-A and
anti-B agglutinins.
For instance, infusion of group A antigen into a
recipient having a non-A blood type causes a typical
immune response w

on-A blood type causes a typical
immune response with formation of greater quantities
of anti-A agglutinins than ever. Also, the neonate has
few, if any, agglutinins, showing that agglutinin forma-
tion occurs almost entirely after birth.
Agglutination Process In Transfusion
Reactions
When bloods are mismatched so that anti-A or anti-B
plasma agglutinins are mixed with red blood cells that
contain A or B agglutinogens, respectively, the red
Table 35–1
Blood Types with Their Genotypes and Their C

 35–1
Blood Types with Their Genotypes and Their Constituent
Agglutinogens and Agglutinins
Genotypes
Blood Types
Agglutinogens
OO
O
—
Anti-A and
Anti-B
OA or AA
A
A
Anti-B
OB or BB
B
B
Anti-A
AB
AB
A and B
—
0
100
200
300
400
0
10 20 30 40 50 60 70 80 90 100
Average titer of agglutinins
Age of person (years)
Anti-A agglutinins in
groups B and O blood
Anti-B agglutinins in
groups A and O blood
Figure 35–1
Average titers of anti-A and anti-B agglutinins in the plasmas of
people with di

nti-B agglutinins in the plasmas of
people with different blood types.
O
47%
A
41%
B
9%
AB
3%
It is obvious from these percentages that the O and 
A genes occur frequently, whereas the B gene is 
infrequent.
When type A agglutinogen is not present in a person’s
red blood cells, antibodies known as anti-A agglutinins
develop in the plasma. Also, when type B agglutino-
gen is not present in the red blood cells, antibodies
known as anti-B agglutinins develop in the plasma.
Thus, referri

B agglutinins develop in the plasma.
Thus, referring once again to Table 35–1, note that
type O blood, although containing no agglutinogens,
does contain both anti-A and anti-B agglutinins; type
A blood contains type A agglutinogens and anti-B
agglutinins; type B blood contains type B agglutino-
gens and anti-A agglutinins. Finally, type AB blood
contains both A and B agglutinogens but no 
agglutinins.
Titer of the Agglutinins at Different Ages. Immediately after
birth, the quantity of agglutini

Immediately after
birth, the quantity of agglutinins in the plasma is
almost zero. Two to 8 months after birth, an infant
begins to produce agglutinins—anti-A agglutinins
Chapter 35
Blood Types; Transfusion; Tissue and Organ Transplantation
453
cells agglutinate as a result of the agglutinins’ attach-
ing themselves to the red blood cells. Because the
agglutinins have two binding sites (IgG type) or 10
binding sites (IgM type), a single agglutinin can attach
to two or more red blood cells at the

n can attach
to two or more red blood cells at the same time,
thereby causing the cells to be bound together by the
agglutinin. This causes the cells to clump, which is the
process of “agglutination.” Then these clumps plug
small blood vessels throughout the circulatory system.
During ensuing hours to days, either physical distor-
tion of the cells or attack by phagocytic white blood
cells destroys the membranes of the agglutinated cells,
releasing hemoglobin into the plasma, which is called
“he

ng hemoglobin into the plasma, which is called
“hemolysis” of the red blood cells.
Acute Hemolysis Occurs in Some Transfusion Reactions.
Sometimes, when recipient and donor bloods are mis-
matched, immediate hemolysis of red cells occurs in
the circulating blood. In this case, the antibodies cause
lysis of the red blood cells by activating the comple-
ment system, which releases proteolytic enzymes (the
lytic complex) that rupture the cell membranes, as
described in Chapter 34. Immediate intrava

nes, as
described in Chapter 34. Immediate intravascular
hemolysis is far less common than agglutination fol-
lowed by delayed hemolysis, because not only does
there have to be a high titer of antibodies for lysis to
occur, but also a different type of antibody seems to
be required, mainly the IgM antibodies; these anti-
bodies are called hemolysins.
Before giving a transfusion to a person, it is necessary
to determine the blood type of the recipient’s blood
and the blood type of th

 of the recipient’s blood
and the blood type of the donor blood so that the
bloods can be appropriately matched. This is called
blood typing and blood matching, and these are per-
formed in the following way: The red blood cells are
ﬁrst separated from the plasma and diluted with saline.
One portion is then mixed with anti-A agglutinin and
another portion with anti-B agglutinin. After several
minutes, the mixtures are observed under a micro-
scope. If the red blood cells have become clumped—
tha

e. If the red blood cells have become clumped—
that is, “agglutinated”—one knows that an antibody-
antigen reaction has resulted.
Table 35–2 lists the presence (+) or absence (-) of
agglutination of the four types of red blood cells. Type
O red blood cells have no agglutinogens and therefore
do not react with either the anti-A or the anti-B agglu-
tinins. Type A blood has A agglutinogens and there-
fore agglutinates with anti-A agglutinins. Type B 
blood has B agglutinogens and agglutinates with

B 
blood has B agglutinogens and agglutinates with 
anti-B agglutinins. Type AB blood has both A and B
agglutinogens and agglutinates with both types of
Along with the O-A-B blood type system, the Rh
blood type system is also important when transfusing
blood. The major difference between the O-A-B
system and the Rh system is the following: In the 
O-A-B system, the plasma agglutinins responsible for
causing transfusion reactions develop spontaneously,
whereas in the R

 reactions develop spontaneously,
whereas in the Rh system, spontaneous agglutinins
almost never occur. Instead, the person must ﬁrst be
massively exposed to an Rh antigen, such as by trans-
fusion of blood containing the Rh antigen, before
enough agglutinins to cause a signiﬁcant transfusion
reaction will develop.
Rh Antigens—“Rh-Positive” and “Rh-Negative” People.
There are six common types of Rh antigens, each of
which is called an Rh factor. These types are desig-
nated C, D, E, c, d, and e.

These types are desig-
nated C, D, E, c, d, and e.A person who has a C antigen
does not have the c antigen, but the person missing the
C antigen always has the c antigen. The same is true
for the D-d and E-e antigens. Also, because of the
manner of inheritance of these factors, each person has
one of each of the three pairs of antigens.
The type D antigen is widely prevalent in the 
population and considerably more antigenic than 
the other Rh antigens. Anyone who has this type of
antigen is sai

tigens. Anyone who has this type of
antigen is said to be Rh positive, whereas a person who
does not have type D antigen is said to be Rh negative.
However, it must be noted that even in Rh-negative
people, some of the other Rh antigens can still cause
transfusion reactions, although the reactions are
usually much milder.
About 85 per cent of all white people are Rh posi-
tive and 15 per cent, Rh negative. In American blacks,
the percentage of Rh-positives is about 95, whereas in
African blacks,

-positives is about 95, whereas in
African blacks, it is virtually 100 per cent.
Formation of Anti-Rh Agglutinins. When red blood cells
containing Rh factor are injected into a person whose
blood does not contain the Rh factor—that is, into 
an Rh-negative person—anti-Rh agglutinins develop
slowly, reaching maximum concentration of agglu-
tinins about 2 to 4 months later.This immune response
occurs to a much greater extent in some people than
in others. With multiple exposures

ome people than
in others. With multiple exposures to the Rh factor,
an Rh-negative person eventually becomes strongly
“sensitized” to Rh factor.
Table 35–2
Blood Typing, Showing Agglutination of Cells of the
Different Blood Types with Anti-A or Anti-B Agglutinins 
in the Sera
Red Blood Cell Types
Sera
Anti-A
Anti-B
O
-
-
A
+
-
B
-
+
AB
+
+
454
Unit VI
Characteristics of Rh Transfusion Reactions. If an Rh-
negative person has never before been exposed to

-
negative person has never before been exposed to Rh-
positive blood, transfusion of Rh-positive blood into
that person will likely cause no immediate reaction.
However, anti-Rh antibodies can develop in sufﬁcient
quantities during the next 2 to 4 weeks to cause agglu-
tination of those transfused cells that are still circulat-
ing in the blood.These cells are then hemolyzed by the
tissue macrophage system. Thus, a delayed transfusion
reaction occurs, although it is usually mild. On subse-
quen

ccurs, although it is usually mild. On subse-
quent transfusion of Rh-positive blood into the same
person, who is now already immunized against the Rh
factor, the transfusion reaction is greatly enhanced and
can be immediate and as severe as a transfusion reac-
tion caused by mismatched type A or B blood.
Erythroblastosis Fetalis (“Hemolytic Disease
of the Newborn”)
Erythroblastosis fetalis is a disease of the fetus and
newborn child characterized by agglutination and
phagocytosis of the fetus’s

d by agglutination and
phagocytosis of the fetus’s red blood cells. In most
instances of erythroblastosis fetalis, the mother is Rh
negative and the father Rh positive. The baby has
inherited the Rh-positive antigen from the father, and
the mother develops anti-Rh agglutinins from expo-
sure to the fetus’s Rh antigen. In turn, the mother’s
agglutinins diffuse through the placenta into the fetus
and cause red blood cell agglutination.
Incidence of the Disease. An Rh-negative mother having
her ﬁrs

 the Disease. An Rh-negative mother having
her ﬁrst Rh-positive child usually does not develop
sufﬁcient anti-Rh agglutinins to cause any harm.
However, about 3 per cent of second Rh-positive
babies exhibit some signs of erythroblastosis fetalis;
about 10 per cent of third babies exhibit the disease;
and the incidence rises progressively with subsequent
pregnancies.
Effect of the Mother’s Antibodies on the Fetus. After anti-
Rh antibodies have formed in the mother, they diffuse
slowly through th

rmed in the mother, they diffuse
slowly through the placental membrane into the
fetus’s blood. There they cause agglutination of the
fetus’s blood. The agglutinated red blood cells subse-
quently hemolyze, releasing hemoglobin into the
blood. The fetus’s macrophages then convert the
hemoglobin into bilirubin, which causes the baby’s
skin to become yellow (jaundiced).The antibodies can
also attack and damage other cells of the body.
Clinical Picture of Erythroblastosis. The jaundiced, ery-
throbl

re of Erythroblastosis. The jaundiced, ery-
throblastotic newborn baby is usually anemic at birth,
and the anti-Rh agglutinins from the mother usually
circulate in the infant’s blood for another 1 to 2
months after birth, destroying more and more red
The hematopoietic tissues of the infant attempt to
replace the hemolyzed red blood cells. The liver and
spleen become greatly enlarged and produce red
blood cells in the same manner that they normally do
during the middle of gestation. 

 they normally do
during the middle of gestation. Because of the rapid
production of red cells, many early forms of red blood
cells, including many nucleated blastic forms, are
passed from the baby’s bone marrow into the circula-
tory system, and it is because of the presence of these
nucleated blastic red blood cells that the disease is
called erythroblastosis fetalis.
Although the severe anemia of erythroblastosis
fetalis is usually the cause of death, many children who
barely survive the anem

f death, many children who
barely survive the anemia exhibit permanent mental
impairment or damage to motor areas of the brain
because of precipitation of bilirubin in the neuronal
cells, causing destruction of many, a condition called
kernicterus.
Treatment of the Erythroblastotic Neonate. One treatment
for erythroblastosis fetalis is to replace the neonate’s
blood with Rh-negative blood.About 400 milliliters of
Rh-negative blood is infused over a period of 1.5 or
more hours while the neonate’s

 a period of 1.5 or
more hours while the neonate’s own Rh-positive blood
is being removed. This procedure may be repeated
several times during the ﬁrst few weeks of life, mainly
to keep the bilirubin level low and thereby prevent
kernicterus. By the time these transfused Rh-negative
cells are replaced with the infant’s own Rh-positive
cells, a process that requires 6 or more weeks, the anti-
Rh agglutinins that had come from the mother will
have been destroyed.
Prevention of Erythroblastosis Fet

been destroyed.
Prevention of Erythroblastosis Fetalis. The D antigen of
the Rh blood group system is the primary culprit in
causing immunization of an Rh-negative mother to 
an Rh-positive fetus. In the 1970’s, a dramatic reduc-
tion in the incidence of erythroblastosis fetalis was
achieved with the development of Rh immunoglobu-
lin globin, an anti-D antibody that is administered to
the expectant mother starting at 28 to 30 weeks of 
gestation. The anti-D antibody is also administered to
Rh-ne

 The anti-D antibody is also administered to
Rh-negative women who deliver Rh-positive babies to
prevent sensitization of the mothers to the D antigen.
This greatly reduces the risk of developing large
amounts of D antibodies during the second pregnancy.
The mechanism by which Rh immunoglobulin
globin prevents sensitization of the D antigen is not
completely understood, but one effect of the anti-D
antibody is to inhibit antigen-induced B lymphocyte
antibody production in the expectant mother. T

yte
antibody production in the expectant mother. The
administered anti-D antibody also attaches to D-
antigen sites on Rh-positive fetal red blood cells that
may cross the placenta and enter the circulation of 
the expectant mother, thereby interfering with the
immune response to the D antigen.
Transfusion Reactions Resulting 
from Mismatched Blood Types
If donor blood of one blood type is transfused into a
recipient who has another blood type, a transfusion
reaction is likely to occur in which 

 transfusion
reaction is likely to occur in which the red blood cells
of the donor blood are agglutinated. It is rare that the
transfused blood causes agglutination of the recipient’s
cells, for the following reason: The plasma portion of
the donor blood immediately becomes diluted by all
the plasma of the recipient, thereby decreasing the
Chapter 35
455
titer of the infused agglutinins to a level usually too
low to cause agglutination. 

o a level usually too
low to cause agglutination. Conversely, the small
amount of infused blood does not signiﬁcantly dilute
the agglutinins in the recipient’s plasma.Therefore, the
recipient’s agglutinins can still agglutinate the mis-
matched donor cells.
As explained earlier, all transfusion reactions even-
tually cause either immediate hemolysis resulting from
hemolysins or later hemolysis resulting from phagocy-
tosis of agglutinated cells. The hemoglobin released
from the red cells is then

The hemoglobin released
from the red cells is then converted by the phagocytes
into bilirubin and later excreted in the bile by the liver,
as discussed in Chapter 70.The concentration of biliru-
bin in the body ﬂuids often rises high enough to cause
jaundice—that is, the person’s internal tissues and skin
become colored with yellow bile pigment. But if liver
function is normal, the bile pigment will be excreted
into the intestines by way of the liver bile, so that jaun-
dice usually does not app

iver bile, so that jaun-
dice usually does not appear in an adult person unless
more than 400 milliliters of blood is hemolyzed in less
than a day.
Acute Kidney Shutdown After Transfusion Reactions. One of
the most lethal effects of transfusion reactions is
kidney failure, which can begin within a few minutes
to few hours and continue until the person dies of
renal failure.
The kidney shutdown seems to result from three
causes: First, the antigen-antibody reaction of the
transfusion reaction rel

-antibody reaction of the
transfusion reaction releases toxic substances from the
hemolyzing blood that cause powerful renal vasocon-
striction. Second, loss of circulating red cells in the
recipient, along with production of toxic substances
from the hemolyzed cells and from the immune reac-
tion, often causes circulatory shock.The arterial blood
pressure falls very low, and renal blood ﬂow and urine
output decrease. Third, if the total amount of free
hemoglobin released into the circulating bl

f free
hemoglobin released into the circulating blood is
greater than the quantity that can bind with “hapto-
globin” (a plasma protein that binds small amounts of
hemoglobin), much of the excess leaks through the
glomerular membranes into the kidney tubules. If this
amount is still slight, it can be reabsorbed through the
tubular epithelium into the blood and will cause no
harm; if it is great, then only a small percentage is reab-
sorbed. Yet water continues to be reabsorbed, causing
the tubul

ater continues to be reabsorbed, causing
the tubular hemoglobin concentration to rise so high
that the hemoglobin precipitates and blocks many of
the kidney tubules. Thus, renal vasoconstriction, circu-
latory shock, and renal tubular blockage together
cause acute renal shutdown. If the shutdown is com-
plete and fails to resolve,the patient dies within a week
to 12 days, as explained in Chapter 31, unless treated
with an artiﬁcial kidney.
Transplantation of Tissues
and Organs
Most of the differ

lantation of Tissues
and Organs
Most of the different antigens of red blood cells that
cause transfusion reactions are also widely present 
in other cells of the body, and each bodily tissue has
its own additional complement of antigens. Conse-
quently, foreign cells transplanted anywhere into the
body of a recipient can produce immune reactions. In
other words, most recipients are just as able to resist
invasion by foreign tissue cells as to resist invasion by
foreign bacteria or red cells.
Aut

ist invasion by
Autografts, Isografts, Allografts, and Xenografts. A trans-
plant of a tissue or whole organ from one part of the
same animal to another part is called an autograft;
from one identical twin to another, an isograft; from
one human being to another or from any animal to
another animal of the same species, an allograft; and
from a lower animal to a human being or from an
animal of one species to one of another species, a
xenograft.
Transplantation of C

another species, a
Transplantation of Cellular Tissues. In the case of auto-
grafts and isografts, cells in the transplant contain vir-
tually the same types of antigens as in the tissues of
the recipient and will almost always continue to live
normally and indeﬁnitely if an adequate blood supply
is provided.
At the other extreme, in the case of xenografts,
immune reactions almost always occur, causing death
of the cells in the graft within 1 day to 5 weeks after
transplantation unles

ithin 1 day to 5 weeks after
transplantation unless some speciﬁc therapy is used to
prevent the immune reactions.
Some of the different cellular tissues and organs that
have been transplanted as allografts, either experi-
mentally or for therapeutic purposes, from one person
to another are skin, kidney, heart, liver, glandular
tissue, bone marrow, and lung.With proper “matching”
of tissues between persons, many kidney allografts
have been successful for at least 5 to 15 years, and allo-
graft li

ful for at least 5 to 15 years, and allo-
graft liver and heart transplants for 1 to 15 years.
Attempts to Overcome Immune
Reactions in Transplanted Tissue
Because of the extreme potential importance of trans-
planting certain tissues and organs, serious attempts
have been made to prevent antigen-antibody reactions
associated with transplantation. The following speciﬁc
procedures have met with some degrees of clinical or
experimental success.
Tissue Typing—The HLA Complex of Antigens
The most im

sue Typing—The HLA Complex of Antigens
The most important antigens for causing graft rejec-
tion are a complex called the HLA antigens. Six of
these antigens are present on the tissue cell mem-
branes of each person, but there are about 150 dif-
ferent HLA antigens to choose from. Therefore, this
represents more than a trillion possible combinations.
Consequently, it is virtually impossible for two
persons, except in the case of identical twins, to have
the same six HLA antigens. Development of 

to have
the same six HLA antigens. Development of signiﬁ-
cant immunity against any one of these antigens can
cause graft rejection.
The HLA antigens occur on the white blood cells as
well as on the tissue cells. Therefore, tissue typing for
456
Unit VI
these antigens is done on the membranes of lympho-
cytes that have been separated from the person’s
blood. The lymphocytes are mixed with appropriate
antisera and complement; after incubation, the cells
a

sera and complement; after incubation, the cells
are tested for membrane damage, usually by testing
the rate of trans-membrane uptake by the lymphocytic
cells of a special dye.
Some of the HLA antigens are not severely anti-
genic, for which reason a precise match of some 
antigens between donor and recipient is not always
essential to allow allograft acceptance. Therefore, by
obtaining the best possible match between donor and
recipient, the grafting procedure has become far less
hazardous. The

fting procedure has become far less
hazardous. The best success has been with tissue-type
matches between siblings and between parent and
child. The match in identical twins is exact, so that
transplants between identical twins are almost never
rejected because of immune reactions.
Prevention of Graft Rejection by Suppressing
the Immune System
If the immune system were completely suppressed,
graft rejection would not occur. In fact, in an occa-
sional person who has serious depression of the
imm

ional person who has serious depression of the
immune system, grafts can be successful without the
use of signiﬁcant therapy to prevent rejection. But in
the normal person, even with the best possible tissue
typing, allografts seldom resist rejection for more than
a few days or weeks without use of speciﬁc therapy to
suppress the immune system. Furthermore, because
the T cells are mainly the portion of the immune
system important for killing grafted cells, their sup-
pression is much more import

ted cells, their sup-
pression is much more important than suppression of
plasma antibodies. Some of the therapeutic agents that
have been used for this purpose include the following:
1. Glucocorticoid hormones isolated from adrenal
cortex glands (or drugs with glucocorticoid-like
activity), which suppress the growth of all
lymphoid tissue and, therefore, decrease
formation of antibodies and T cells.
2. Various drugs that have a toxic effect on the
lymphoid system and, therefore, block formation

he
of antibodies and T cells, especially the drug
azathioprine.
3. Cyclosporine, which has a speciﬁc inhibitory effect
on the formation of helper T cells and, therefore,
is especially efﬁcacious in blocking the T-cell
rejection reaction. This has proved to be one of
the most valuable of all the drugs because it does
not depress some other portions of the immune
system.
Use of these agents often leaves the person unpro-
tected from infectious diseas

es the person unpro-
tected from infectious disease; therefore, sometimes
bacterial and viral infections become rampant.In addi-
tion, the incidence of cancer is several times as great
in an immunosuppressed person, presumably because
the immune system is important in destroying many
early cancer cells before they can begin to proliferate.
To summarize, transplantation of living tissues in
human beings has had very limited but important
success. When someone does ﬁnally succeed in block-
ing the

When someone does ﬁnally succeed in block-
ing the immune response of the recipient without at
the same time destroying the recipient’s speciﬁc immu-
nity for disease, the story will change overnight.
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C
H
A
P
T
E
R
 
3
6
457
The term hemostasis means prevention of blood
loss. Whenever a vessel is severed or ruptured,
hemostasis is achieved by several mechanisms: (1)
vascular constriction, (2) formation of a platelet
plug, (3) formation of a blood clot as a result of
blood coagulation, and (4) eventual growth of
ﬁbrous tissue int

tion, and (4) eventual growth of
ﬁbrous tissue into the blood clot to close the hole in the vessel permanently.
Immediately after a blood vessel has been cut or ruptured, the trauma to the
vessel wall itself causes the smooth muscle in the wall to contract; this instan-
taneously reduces the ﬂow of blood from the ruptured vessel. The contraction
results from (1) local myogenic spasm, (2) local autacoid factors from the trau-
matized tissues and blood platelets, and (3) nerv

matized tissues and blood platelets, and (3) nervous reﬂexes. The nervous
reﬂexes are initiated by pain nerve impulses or other sensory impulses that orig-
inate from the traumatized vessel or nearby tissues. However, even more vaso-
constriction probably results from local myogenic contraction of the blood
vessels initiated by direct damage to the vascular wall. And, for the smaller
vessels, the platelets are responsible for much of the vasoconstriction by releas-
ing a vasoconstrictor substan

striction by releas-
ing a vasoconstrictor substance, thromboxane A2.
The more severely a vessel is traumatized, the greater the degree of vascular
spasm. The spasm can last for many minutes or even hours, during which time
the processes of platelet plugging and blood coagulation can take place.
If the cut in the blood vessel is very small—indeed, many very small vascular
holes do develop throughout the body each day—the cut is often sealed by a
platelet plug, rath

y—the cut is often sealed by a
platelet plug, rather than by a blood clot.To understand this, it is important that
we ﬁrst discuss the nature of platelets themselves.
Physical and Chemical Characteristics of Platelets
Platelets (also called thrombocytes) are minute discs 1 to 4 micrometers in diam-
eter. They are formed in the bone marrow from megakaryocytes, which are
extremely large cells of the hematopoietic series in the marrow; the megakary-
ocytes fragment into the minute platelets either 

ocytes fragment into the minute platelets either in the bone marrow or soon
after entering the blood, especially as they squeeze through capillaries. The
normal concentration of platelets in the blood is between 150,000 and 300,000
per microliter.
Platelets have many functional characteristics of whole cells, even though
they do not have nuclei and cannot reproduce. In their cytoplasm are such active
factors as (1) actin and myosin molecules, which are contractile proteins similar
to those foun

ich are contractile proteins similar
to those found in muscle cells, and still another contractile protein, throm-
bosthenin, that can cause the platelets to contract; (2) residuals of both the
endoplasmic reticulum and the Golgi apparatus that synthesize various enzymes
and especially store large quantities of calcium ions; (3) mitochondria and
458
Unit VI
enzyme systems that are capable of forming adenosine
triphosphate
(ATP) and adenosine diphosphate

sine
(ADP);
(4) 
enzyme 
systems 
that 
synthesize
prostaglandins, which are local hormones that cause
many vascular and other local tissue reactions; (5) an
important protein called ﬁbrin-stabilizing factor, which
we discuss later in relation to blood coagulation; and
(6) a growth factor that causes vascular endothelial
cells, vascular smooth muscle cells, and ﬁbroblasts to
multiply and grow, thus causing cellular growth that
eventually helps repair 

sing cellular growth that
eventually helps repair damaged vascular walls.
The cell membrane of the platelets is also important.
On its surface is a coat of glycoproteins that repulses
adherence to normal endothelium and yet causes
adherence to injured areas of the vessel wall, especially
to injured endothelial cells and even more so to any
exposed collagen from deep within the vessel wall. In
addition,
the platelet membrane contains large
amounts of phospholipids that activate multiple stages
in

 of phospholipids that activate multiple stages
in the blood-clotting process, as we discuss later.
Thus, the platelet is an active structure. It has a half-
life in the blood of 8 to 12 days, so that over several
weeks its functional processes run out. Then it is 
eliminated from the circulation mainly by the tissue
macrophage system. More than one half of the
platelets are removed by macrophages in the spleen,
where the blood passes through a latticework of tight
trabeculae.
Mechanism of the P

atticework of tight
Mechanism of the Platelet Plug
Platelet repair of vascular openings is based on several
important functions of the platelet itself. When
platelets come in contact with a damaged vascular
surface, especially with collagen ﬁbers in the vascular
wall, the platelets themselves immediately change
their own characteristics drastically. They begin to
swell; they assume irregular forms with numerous irra-
diating pseudopods protruding from their surfaces;
their contractil

s protruding from their surfaces;
their contractile proteins contract forcefully and cause
the release of granules that contain multiple active
factors; they become sticky so that they adhere to col-
lagen in the tissues and to a protein called von Wille-
brand factor that leaks into the traumatized tissue
from the plasma; they secrete large quantities of ADP;
and their enzymes form thromboxane A2. The ADP
and thromboxane in turn act on nearby platelets to
activate them as well, and the stickine

atelets to
activate them as well, and the stickiness of these addi-
tional platelets causes them to adhere to the original
activated platelets.
Therefore, at the site of any opening in a blood
vessel wall, the damaged vascular wall activates suc-
cessively increasing numbers of platelets that them-
selves attract more and more additional platelets, thus
forming a platelet plug. This is at ﬁrst a loose plug, but
it is usually successful in blocking blood loss if the vas-
cular opening is small. T

g blood loss if the vas-
cular opening is small. Then, during the subsequent
process of blood coagulation, ﬁbrin threads form.
These attach tightly to the platelets, thus constructing
an unyielding plug.
Importance of the Platelet Mechanism for Closing Vascular
Holes. The platelet-plugging mechanism is extremely
important for closing minute ruptures in very small
blood vessels that occur many thousands of times
daily. Indeed, multiple small holes through the
endothelial cells themselves are ofte

 through the
endothelial cells themselves are often closed by
platelets actually fusing with the endothelial cells to
form additional endothelial cell membrane. A person
who has few blood platelets develops each day liter-
ally thousands of small hemorrhagic areas under the
skin and throughout the internal tissues, but this does
not occur in the normal person.
Blood Coagulation in the 
Ruptured Vessel
The third mechanism for hemostasis is formation of
the blood clot. The clot begins to develop i

on of
the blood clot. The clot begins to develop in 15 to 20
seconds if the trauma to the vascular wall has been
severe, and in 1 to 2 minutes if the trauma has been
minor.Activator substances from the traumatized vas-
cular wall, from platelets, and from blood proteins
adhering to the traumatized vascular wall initiate the
clotting process.The physical events of this process are
shown in Figure 36–1, and Table 36–1 lists the most
important of the clotting factors.
Within 3 to 6 minutes after ru

e clotting factors.
Within 3 to 6 minutes after rupture of a vessel, if the
vessel opening is not too large, the entire opening or
broken end of the vessel is ﬁlled with clot. After 20
minutes to an hour, the clot retracts; this closes the
vessel still further. Platelets also play an important role
in this clot retraction, as is discussed later.
Fibrous Organization or Dissolution 
of the Blood Clot
Once a blood clot has formed, it can follow one of two
courses: (1) It can become invaded by ﬁbro

 of two
courses: (1) It can become invaded by ﬁbroblasts,
1. Severed vessel
2. Platelets agglutinate
3. Fibrin appears
5. Clot retraction occurs
4. Fibrin clot forms
Figure 36–1
Clotting process in a traumatized blood vessel. (Modiﬁed from
Seegers WH: Hemostatic Agents, 1948. Courtesy of Charles C
Thomas, Publisher, Ltd., Springﬁeld, IL.)
Chapter 36
459
which subsequently form connective tissue all through
the clot, or (2) it can dissolve. The usual course for a

t, or (2) it can dissolve. The usual course for a
clot that forms in a small hole of a vessel wall is inva-
sion by ﬁbroblasts, beginning within a few hours after
the clot is formed (which is promoted at least partially
by growth factor secreted by platelets). This continues
to complete organization of the clot into ﬁbrous tissue
within about 1 to 2 weeks.
Conversely, when excess blood has leaked into the
tissues and tissue clots have occurred where they are
not needed, special substances within

ere they are
not needed, special substances within the clot itself
usually become activated. These function as enzymes
to dissolve the clot, as discussed later in the chapter.
Mechanism of Blood
Coagulation
Basic Theory. More than 50 important substances that
cause or affect blood coagulation have been found in
the blood and in the tissues—some that promote coag-
ulation, called procoagulants, and others that inhibit
coagulation, called anticoagulants. Whether blood will
coagulate depends on the

lants. Whether blood will
coagulate depends on the balance between these two
groups of substances. In the blood stream, the anti-
coagulants normally predominate, so that the blood
does not coagulate while it is circulating in the blood
vessels. But when a vessel is ruptured, procoagulants
from the area of tissue damage become “activated”
and override the anticoagulants, and then a clot does
develop.
General Mechanism. All research workers in the ﬁeld of
blood coagulation agree that clotting tak

 ﬁeld of
blood coagulation agree that clotting takes place in
three essential steps: (1) In response to rupture of the
vessel or damage to the blood itself, a complex cascade
of chemical reactions occurs in the blood involving
more than a dozen blood coagulation factors. The net
result is formation of a complex of activated sub-
stances collectively called prothrombin activator. (2)
The prothrombin activator catalyzes conversion of
prothrombin into thrombin. (3) The thrombin acts as
an enzyme to

to thrombin. (3) The thrombin acts as
an enzyme to convert ﬁbrinogen into ﬁbrin ﬁbers that
enmesh platelets, blood cells, and plasma to form the
clot.
Let us discuss ﬁrst the mechanism by which the
blood clot itself is formed, beginning with conversion
of prothrombin to thrombin; then we will come back
to the initiating stages in the clotting process by which
prothrombin activator is formed.
Conversion of Prothrombin 
to Thrombin
First, prothrombin activator is formed as a result of
rupture of a

in activator is formed as a result of
rupture of a blood vessel or as a result of damage 
to special substances in the blood. Second, the pro-
thrombin activator, in the presence of sufﬁcient
amounts of ionic Ca++, causes conversion of pro-
thrombin to thrombin (Figure 36–2).Third, the throm-
bin causes polymerization of ﬁbrinogen molecules into
ﬁbrin ﬁbers within another 10 to 15 seconds. Thus, the
rate-limiting factor in causing blood coagulation is
usually the formation of prothrombin activat

on is
usually the formation of prothrombin activator and
not the subsequent reactions beyond that point,
because these terminal steps normally occur rapidly to
form the clot itself.
Platelets also play an important role in the con-
version of prothrombin to thrombin because much 
of the prothrombin ﬁrst attaches to prothrombin 
Table 36–1
Clotting Factors in Blood and Their Synonyms
Clotting Factor
Synonyms
Factor I
Prothrombin
Factor II
Tissue factor
Factor III; tissue thromboplastin

II
Calcium
Factor IV
Factor V
Proaccelerin; labile factor; Ac-globulin
(Ac-G)
Factor VII
Serum prothrombin conversion 
accelerator (SPCA); proconvertin;
stable factor
Factor VIII
Antihemophilic factor (AHF);
antihemophilic globulin (AHG);
antihemophilic factor A
Factor IX
Plasma thromboplastin component 
(PTC); Christmas factor;
antihemophilic factor B
Factor X
Stuart factor; Stuart-Prower factor
Factor XI
Plasma thromboplastin antecedent 
(PTA); a

ctor XI
(PTA); antihemophilic factor C
Factor XII
Hageman factor
Factor XIII
Fibrin-stabilizing factor
Prekallikrein
Fletcher factor
High-molecular-weight
Fitzgerald factor; HMWK
kininogen
(high-molecular-weight) kininogen
Platelets
Thrombin
Fibrin fibers
Cross-linked fibrin fibers
Thrombin
activated
Fibrinogen monomer
Fibrinogen 
activator
Ca++
Ca++
fibrin-stabilizing
factor
Figure 36–2
Schema for conversion of prothrombin to thrombin an

chema for conversion of prothrombin to thrombin and polymer-
ization of ﬁbrinogen to form ﬁbrin ﬁbers.
460
Unit VI
receptors on the platelets already bound to the
damaged tissue.
Prothrombin and Thrombin. Prothrombin is a plasma
protein, an alpha2-globulin, having a molecular weight
of 68,700. It is present in normal plasma in a concen-
tration of about 15 mg/dl. It is an unstable protein that
can split easily into smaller compounds, one of which
is thro

asily into smaller compounds, one of which
is thrombin, which has a molecular weight of 33,700,
almost exactly one half that of prothrombin.
Prothrombin is formed continually by the liver, and
it is continually being used throughout the body for
blood clotting. If the liver fails to produce prothrom-
bin, in a day or so prothrombin concentration in 
the plasma falls too low to provide normal blood 
coagulation.
Vitamin K is required by the liver for normal for-
mation of prothrombin as well as f

for normal for-
mation of prothrombin as well as for formation of a
few other clotting factors. Therefore, either lack of
vitamin K or the presence of liver disease that pre-
vents normal prothrombin formation can decrease the
prothrombin level so low that a bleeding tendency
results.
Fibrinogen. Fibrinogen is a high-molecular-weight
protein (MW = 340,000) that occurs in the plasma in
quantities of 100 to 700 mg/dl. Fibrinogen is formed i

tities of 100 to 700 mg/dl. Fibrinogen is formed in
the liver, and liver disease can decrease the concen-
tration of circulating ﬁbrinogen, as it does the concen-
tration of prothrombin, pointed out above.
Because of its large molecular size, little ﬁbrinogen
normally leaks from the blood vessels into the inter-
stitial ﬂuids,and because ﬁbrinogen is one of the essen-
tial factors in the coagulation process, interstitial ﬂuids
ordinarily do not coagulate. Yet, when the permeabil-
ity of the capi

oagulate. Yet, when the permeabil-
ity of the capillaries becomes pathologically increased,
ﬁbrinogen does then leak into the tissue ﬂuids in suf-
ﬁcient quantities to allow clotting of these ﬂuids in
much the same way that plasma and whole blood can
clot.
Action of Thrombin on Fibrinogen to Form Fibrin. Thrombin
is a protein enzyme with weak proteolytic capabilities.
It acts on ﬁbrinogen to remove four low-molecular-
weight peptides from each molecule of ﬁbrinogen,
forming one molecule of ﬁbrin

lecule of ﬁbrinogen,
forming one molecule of ﬁbrin monomer that has the
automatic capability to polymerize with other ﬁbrin
monomer molecules to form ﬁbrin ﬁbers. Therefore,
many ﬁbrin monomer molecules polymerize within
seconds into long ﬁbrin ﬁbers that constitute the retic-
ulum of the blood clot.
In the early stages of polymerization, the ﬁbrin
monomer molecules are held together by weak non-
covalent hydrogen bonding, and the newly forming
ﬁbers are not cross-linked with one another; theref

bers are not cross-linked with one another; therefore,
the resultant clot is weak and can be broken apart with
ease. But another process occurs during the next few
minutes that greatly strengthens the ﬁbrin reticulum.
This involves a substance called ﬁbrin-stabilizing
factor that is present in small amounts in normal
plasma globulins but is also released from platelets
entrapped in the clot. Before ﬁbrin-stabilizing factor
can have an effect on the ﬁbrin ﬁbers, it must itself be
activated. The s

he ﬁbrin ﬁbers, it must itself be
activated. The same thrombin that causes ﬁbrin for-
mation also activates the ﬁbrin-stabilizing factor.
Then this activated substance operates as an enzyme
to cause covalent bonds between more and more of the
ﬁbrin monomer molecules, as well as multiple cross-
linkages between adjacent ﬁbrin ﬁbers, thus adding
tremendously to the three-dimensional strength of the
ﬁbrin meshwork.
Blood Clot. The clot is composed of a meshwork of
ﬁbrin ﬁbers running in all directi

f a meshwork of
ﬁbrin ﬁbers running in all directions and entrapping
blood cells, platelets, and plasma. The ﬁbrin ﬁbers also
adhere to damaged surfaces of blood vessels; there-
fore, the blood clot becomes adherent to any vascular
opening and thereby prevents further blood loss.
Clot Retraction—Serum. Within a few minutes after a
clot is formed, it begins to contract and usually
expresses most of the ﬂuid from the clot within 20 to
60 minutes.The ﬂuid expressed is called serum because
all its ﬁ

e ﬂuid expressed is called serum because
all its ﬁbrinogen and most of the other clotting factors
have been removed; in this way, serum differs from
plasma. Serum cannot clot because it lacks these
factors.
Platelets are necessary for clot retraction to occur.
Therefore, failure of clot retraction is an indication
that the number of platelets in the circulating blood
might be low. Electron micrographs of platelets in
blood clots show that they become attached to the
ﬁbrin ﬁbers in such a way tha

come attached to the
ﬁbrin ﬁbers in such a way that they actually bond 
different ﬁbers together.
Furthermore,
platelets
entrapped in the clot continue to release procoagulant
substances, one of the most important of which is
ﬁbrin-stabilizing factor, which causes more and more
cross-linking bonds between adjacent ﬁbrin ﬁbers. In
addition, the platelets themselves contribute directly
to clot contraction by activating platelet throm-
bosthenin, actin, and myosin molecules, which are all
contracti

tin, and myosin molecules, which are all
contractile proteins in the platelets and cause strong
contraction of the platelet spicules attached to the
ﬁbrin. This also helps compress the ﬁbrin meshwork
into a smaller mass. The contraction is activated and
accelerated by thrombin as well as by calcium ions
released from calcium stores in the mitochondria,
endoplasmic reticulum, and Golgi apparatus of the
platelets.
As the clot retracts, the edges of the broken blood
vessel are pulled together, thus

 the broken blood
vessel are pulled together, thus contributing still
further to the ultimate state of hemostasis.
Once a blood clot has started to develop, it normally
extends within minutes into the surrounding blood.
That is, the clot itself initiates a vicious circle (positive
feedback) to promote more clotting. One of the most
Chapter 36
461
important causes of this is the fact that the proteolytic
action of thrombin allows i

t that the proteolytic
action of thrombin allows it to act on many of the
other blood-clotting factors in addition to ﬁbrinogen.
For instance, thrombin has a direct proteolytic effect
on prothrombin itself, tending to convert this into still
more thrombin, and it acts on some of the blood-clot-
ting factors responsible for formation of prothrombin
activator. (These effects, discussed in subsequent para-
graphs, include acceleration of the actions of Factors
VIII, IX, X, XI, and XII and aggregati

 of Factors
VIII, IX, X, XI, and XII and aggregation of platelets.)
Once a critical amount of thrombin is formed,a vicious
circle develops that causes still more blood clotting
and more and more thrombin to be formed; thus,
the blood clot continues to grow until blood leakage
ceases.
Initiation of Coagulation: Formation 
of Prothrombin Activator
Now that we have discussed the clotting process itself,
we must turn to the more complex mechanisms that
initiate clotting in the ﬁrst place. These mech

at
initiate clotting in the ﬁrst place. These mechanisms
are set into play by (1) trauma to the vascular wall and
adjacent tissues, (2) trauma to the blood, or (3) contact
of the blood with damaged endothelial cells or with
collagen and other tissue elements outside the blood
vessel. In each instance, this leads to the formation of
prothrombin activator, which then causes prothrombin
conversion to thrombin and all the subsequent clotting
steps.
Prothrombin activator is generally considered to be

rothrombin activator is generally considered to be
formed in two ways, although, in reality, the two ways
interact constantly with each other: (1) by the extrin-
sic pathway that begins with trauma to the vascular
wall and surrounding tissues and (2) by the intrinsic
pathway that begins in the blood itself.
In both the extrinsic and the intrinsic pathways, a
series of different plasma proteins called blood-
clotting factors play major roles.Most of these are inac-
tive forms of proteolytic enzym

of these are inac-
tive forms of proteolytic enzymes. When converted to
the active forms, their enzymatic actions cause the suc-
cessive, cascading reactions of the clotting process.
Most of the clotting factors, which are listed in Table
36–1, are designated by Roman numerals. To indicate
the activated form of the factor, a small letter “a” is
added after the Roman numeral, such as Factor VIIIa
to indicate the activated state of Factor VIII.
Extrinsic Pathway for Initiating Clotting
The extrins

rinsic Pathway for Initiating Clotting
The extrinsic pathway for initiating the formation of
prothrombin activator begins with a traumatized vas-
cular wall or traumatized extravascular tissues that
come in contact with the blood. This leads to the fol-
lowing steps, as shown in Figure 36–3:
1. Release of tissue factor. Traumatized tissue
releases a complex of several factors called tissue
factor or tissue thromboplastin. This factor is
composed especially of phospholipids from the
membranes of 

especially of phospholipids from the
membranes of the tissue plus a lipoprotein
complex that functions mainly as a proteolytic
enzyme.
VIIa
Vll
Thrombin
X
Activated X (Xa)
Ca++
Ca++
Ca++
Activator
Platelet
phospholipids
V
Tissue trauma
(1)
(2)
(3)
Figure 36–3
Extrinsic pathway for initiating blood clotting.
462
Unit VI
2. Activation of Factor X—role of Factor VII and
tissue factor. The lipoprotein complex of tissue
f

factor further complexes with blood coagulation
Factor VII and, in the presence of calcium ions,
acts enzymatically on Factor X to form activated
Factor X (Xa).
3. Effect of activated Factor X (Xa) to form
prothrombin activator—role of Factor V. The
activated Factor X combines immediately with
tissue phospholipids that are part of tissue factor
or with additional phospholipids released from
platelets as well as with Factor V to form the
complex ca

ts as well as with Factor V to form the
complex called prothrombin activator. Within a
few seconds, in the presence of calcium ions
(Ca++), this splits prothrombin to form thrombin,
and the clotting process proceeds as already
explained. At ﬁrst, the Factor V in the
prothrombin activator complex is inactive, but
once clotting begins and thrombin begins to form,
the proteolytic action of thrombin activates Factor
V. This then becomes an additional strong
accelerator of prothrombin activation. Thu

 strong
accelerator of prothrombin activation. Thus, in the
ﬁnal prothrombin activator complex, activated
Factor X is the actual protease that causes
splitting of prothrombin to form thrombin;
activated Factor V greatly accelerates this
protease activity, and platelet phospholipids act as
a vehicle that further accelerates the process. Note
especially the positive feedback effect of thrombin,
acting through Factor V, to accelerate the entire
process once it begins.
Intrinsic Pathway for Initiati

ess once it begins.
Intrinsic Pathway for Initiating Clotting
The second mechanism for initiating formation of pro-
thrombin activator, and therefore for initiating clot-
ting, begins with trauma to the blood itself or exposure
of the blood to collagen from a traumatized blood
vessel wall. Then the process continues through the
series of cascading reactions shown in Figure 36–4.
1. Blood trauma causes (1) activation of Factor XII
and (2) release of platelet phospholipids. Trauma
to the blood or 

of platelet phospholipids. Trauma
to the blood or exposure of the blood to vascular
XII
Activated XII (XIIa)
XI
Activated XI (XIa)
X
IX
(1)
(2)
(3)
(4)
(5)
Activated IX (IXa)
(HMW kininogen, prekallikrein)
Ca++
Ca++
Ca++
Ca++
VIII
Thrombin
VIIIa
Thrombin
Thrombin
V
Activator
Platelet
Blood trauma or
contact with collagen
Platelet
Figure 36–4
Intrinsic pathway for initiating
blood clotting.
Chapter 36
Hemostasis and Blood Coagul

d clotting.
Chapter 36
463
wall collagen alters two important clotting factors
in the blood: Factor XII and the platelets. When
Factor XII is disturbed, such as by coming into
contact with collagen or with a wettable surface
such as glass, it takes on a new molecular
conﬁguration that converts it into a proteolytic
enzyme called “activated Factor XII.”
Simultaneously, the blood trauma also damages
the platelets because of adherence to either
collagen or a wettabl

cause of adherence to either
collagen or a wettable surface (or by damage 
in other ways), and this releases platelet
phospholipids that contain the lipoprotein called
platelet factor 3, which also plays a role in
subsequent clotting reactions.
2. Activation of Factor XI. The activated Factor XII
acts enzymatically on Factor XI to activate this
factor as well, which is the second step in the
intrinsic pathway. This reaction also requires
HMW (high-molecular-weight) kininogen and is
accelerated b

h-molecular-weight) kininogen and is
accelerated by prekallikrein.
3. Activation of Factor IX by activated Factor XI.
The activated Factor XI then acts enzymatically
on Factor IX to activate this factor also.
4. Activation of Factor X—role of Factor VIII.
The activated Factor IX, acting in concert with
activated Factor VIII and with the platelet
phospholipids and factor 3 from the traumatized
platelets, activates Factor X. It is clear that when
either Factor VIII or platelets are in short supply

ither Factor VIII or platelets are in short supply,
this step is deﬁcient. Factor VIII is the factor that
is missing in a person who has classic hemophilia,
for which reason it is called antihemophilic factor.
Platelets are the clotting factor that is lacking in
the bleeding disease called thrombocytopenia.
5. Action of activated Factor X to form prothrombin
activator—role of Factor V. This step in the
intrinsic pathway is the same as the last step in
the extrinsic pathway. That is, activated Fa

ep in
the extrinsic pathway. That is, activated Factor X
combines with Factor V and platelet or tissue
phospholipids to form the complex called
prothrombin activator. The prothrombin activator
in turn initiates within seconds the cleavage of
prothrombin to form thrombin, thereby setting
into motion the ﬁnal clotting process, as described
earlier.
Role of Calcium Ions in the Intrinsic and
Extrinsic Pathways
Except for the ﬁrst two steps in the intrinsic pathway,
calcium ions are required for prom

rinsic pathway,
calcium ions are required for promotion or accelera-
tion of all the blood-clotting reactions. Therefore, in
the absence of calcium ions, blood clotting by either
pathway does not occur.
In the living body, the calcium ion concentration
seldom falls low enough to signiﬁcantly affect the
kinetics of blood clotting. But, when blood is removed
from a person, it can be prevented from clotting by
reducing the calcium ion concentration below the
threshold level for clotting, either by 

below the
threshold level for clotting, either by deionizing the
calcium by causing it to react with substances such as
citrate ion or by precipitating the calcium with sub-
stances such as oxalate ion.
Interaction Between the Extrinsic 
and Intrinsic Pathways—Summary 
of Blood-Clotting Initiation
It is clear from the schemas of the intrinsic and extrin-
sic systems that after blood vessels rupture, clotting
occurs by both pathways simultaneously. Tissue factor
initiates the extrinsic pathway, w

. Tissue factor
initiates the extrinsic pathway, whereas contact of
Factor XII and platelets with collagen in the vascular
wall initiates the intrinsic pathway.
An especially important difference between the
extrinsic and intrinsic pathways is that the extrinsic
pathway can be explosive; once initiated, its speed of
completion to the ﬁnal clot is limited only by the
amount of tissue factor released from the traumatized
tissues and by the quantities of Factors X, VII, and V
in the blood. With sev

es of Factors X, VII, and V
in the blood. With severe tissue trauma, clotting can
occur in as little as 15 seconds. The intrinsic pathway
is much slower to proceed, usually requiring 1 to 6
minutes to cause clotting.
Prevention of Blood Clotting 
in the Normal Vascular System—
Intravascular Anticoagulants
Endothelial Surface Factors. Probably the most important
factors for preventing clotting in the normal vascular
system are (1) the smoothness of the endothelial cell
surface,which prevents cont

f the endothelial cell
surface,which prevents contact activation of the intrin-
sic clotting system; (2) a layer of glycocalyx on the
endothelium (glycocalyx is a mucopolysaccharide
adsorbed to the surfaces of the endothelial cells),
which repels clotting factors and platelets, thereby pre-
venting activation of clotting; and (3) a protein bound
with the endothelial membrane, thrombomodulin,
which binds thrombin. Not only does the binding of
thrombin with thrombomodulin slow the clotting
process

mbin with thrombomodulin slow the clotting
process by removing thrombin, but the thrombomod-
ulin-thrombin complex also activates a plasma protein,
protein C, that acts as an anticoagulant by inactivating
activated Factors V and VIII.
When the endothelial wall is damaged, its smooth-
ness and its glycocalyx-thrombomodulin layer are lost,
which activates both Factor XII and the platelets,
thus setting off the intrinsic pathway of clotting. If
Factor XII and platelets come in contact with the
sube

or XII and platelets come in contact with the
subendothelial collagen, the activation is even more
powerful.
Antithrombin Action of Fibrin and Antithrombin III. Among
the most important anticoagulants in the blood itself
are those that remove thrombin from the blood. The
most powerful of these are (1) the ﬁbrin ﬁbers that
themselves are formed during the process of clotting
and (2) an alpha-globulin called antithrombin III or
antithrombin-heparin cofactor.
While a clot is forming, about 85 to 90

 cofactor.
While a clot is forming, about 85 to 90 per cent of
the thrombin formed from the prothrombin becomes
adsorbed to the ﬁbrin ﬁbers as they develop.This helps
prevent the spread of thrombin into the remaining
blood and, therefore, prevents excessive spread of the
clot.
464
Unit VI
The thrombin that does not adsorb to the ﬁbrin
ﬁbers soon combines with antithrombin III, which
further blocks the effect of the thrombin on the ﬁ-
brinogen and then al

ect of the thrombin on the ﬁ-
brinogen and then also inactivates the thrombin itself
during the next 12 to 20 minutes.
Heparin. Heparin is another powerful anticoagulant,
but its concentration in the blood is normally low, so
that only under special physiologic conditions does 
it have signiﬁcant anticoagulant effects. However,
heparin is used widely as a pharmacological agent in
medical practice in much higher concentrations to
prevent intravascular clotting.
The heparin molecule is a highly ne

ular clotting.
The heparin molecule is a highly negatively charged
conjugated polysaccharide. By itself, it has little or no
anticoagulant properties, but when it combines with
antithrombin III, the effectiveness of antithrombin III
for removing thrombin increases by a hundredfold to
a thousandfold, and thus it acts as an anticoagulant.
Therefore, in the presence of excess heparin, removal
of free thrombin from the circulating blood by
antithrombin III is almost instantaneous.
The complex of hep

in III is almost instantaneous.
The complex of heparin and antithrombin III
removes several other activated coagulation factors in
addition to thrombin, further enhancing the effective-
ness of anticoagulation. The others include activated
Factors XII, XI, X, and IX.
Heparin is produced by many different cells of the
body, but especially large quantities are formed by the
basophilic mast cells located in the pericapillary con-
nective tissue throughout the body. These cells contin-
ually secrete

ughout the body. These cells contin-
ually secrete small quantities of heparin that diffuse
into the circulatory system. The basophil cells of the
blood, which are functionally almost identical to the
mast cells, release small quantities of heparin into 
the plasma.
Mast cells are abundant in tissue surrounding the
capillaries of the lungs and to a lesser extent capillar-
ies of the liver. It is easy to understand why large quan-
tities of heparin might be needed in these areas
because the capil

n might be needed in these areas
because the capillaries of the lungs and liver receive
many embolic clots formed in slowly ﬂowing venous
blood; sufﬁcient formation of heparin prevents further
growth of the clots.
The plasma proteins contain a euglobulin called 
plasminogen (or proﬁbrinolysin) that, when activated,
becomes a substance called plasmin (or ﬁbrinolysin).
Plasmin is a proteolytic enzyme that resembles trypsin,
the most important proteolytic digestive enzy

sin,
the most important proteolytic digestive enzyme of
pancreatic secretion. Plasmin digests ﬁbrin ﬁbers and
some other protein coagulants such as ﬁbrinogen,
Factor V, Factor VIII, prothrombin, and Factor XII.
Therefore, whenever plasmin is formed, it can cause
lysis of a clot by destroying many of the clotting
factors, thereby sometimes even causing hypocoagula-
bility of the blood.
Activation of Plasminogen to Form Plasmin: Then Lysis of Clots.
When a clot is formed, a large amount of plasmin

.
When a clot is formed, a large amount of plasminogen
is trapped in the clot along with other plasma proteins.
This will not become plasmin or cause lysis of the clot
until it is activated. The injured tissues and vascular
endothelium very slowly release a powerful activator
called tissue plasminogen activator (t-PA) that a few
days later, after the clot has stopped the bleeding,
eventually converts plasminogen to plasmin, which in
turn removes the remaining unnecessary blood clot. In
fact, man

the remaining unnecessary blood clot. In
fact, many small blood vessels in which blood ﬂow has
been blocked by clots are reopened by this mecha-
nism. Thus, an especially important function of the
plasmin system is to remove minute clots from millions
of tiny peripheral vessels that eventually would
become occluded were there no way to clear them.
Excessive Bleeding in 
Human Beings
Excessive bleeding can result from deﬁciency of any
one of the many blood-clotting factors. 

cy of any
one of the many blood-clotting factors. Three particu-
lar types of bleeding tendencies that have been studied
to the greatest extent are discussed here: bleeding
caused by (1) vitamin K deﬁciency, (2) hemophilia, and
(3) thrombocytopenia (platelet deﬁciency).
Decreased Prothrombin, Factor VII,
Factor IX, and Factor X Caused 
by Vitamin K Deﬁciency
With few exceptions, almost all the blood-clotting
factors are formed by the liver. Therefore, diseases of
the liver such as hepatitis, cir

fore, diseases of
the liver such as hepatitis, cirrhosis, and acute yellow
atrophy can sometimes depress the clotting system so
greatly that the patient develops a severe tendency to
bleed.
Another cause of depressed formation of clotting
factors by the liver is vitamin K deﬁciency. Vitamin K
is necessary for liver formation of ﬁve of the impor-
tant clotting factors: prothrombin, Factor VII, Factor
IX, Factor X, and protein C. In the absence of vitamin
K, subsequent insufﬁciency of these coagul

vitamin
K, subsequent insufﬁciency of these coagulation
factors in the blood can lead to serious bleeding ten-
dencies.
Vitamin K is continually synthesized in the intes-
tinal tract by bacteria, so that vitamin K deﬁciency
seldom occurs in the normal person as a result of
vitamin K absence from the diet (except in neonates
before they establish their intestinal bacterial ﬂora).
However, in gastrointestinal disease, vitamin K deﬁ-
ciency often occurs as a result of poor absorption of
fats from t

curs as a result of poor absorption of
fats from the gastrointestinal tract. The reason is that
vitamin K is fat-soluble and ordinarily is absorbed into
the blood along with the fats.
One of the most prevalent causes of vitamin K deﬁ-
ciency is failure of the liver to secrete bile into the gas-
trointestinal tract (which occurs either as a result of
obstruction of the bile ducts or as a result of liver
disease). Lack of bile prevents adequate fat digestion
Chapter 36
Hemostasis and Blood Coagula

 digestion
Chapter 36
465
and absorption and, therefore, depresses vitamin K
absorption as well. Thus, liver disease often causes
decreased production of prothrombin and some other
clotting factors both because of poor vitamin K
absorption and because of the diseased liver cells.
Because of this, vitamin K is injected into all surgical
patients with liver disease or with obstructed bile ducts
before performing the surgical procedure. Ordinarily,
if vitamin K is g

 surgical procedure. Ordinarily,
if vitamin K is given to a deﬁcient patient 4 to 8 hours
before the operation and the liver parenchymal cells
are at least one-half normal in function, sufﬁcient 
clotting factors will be produced to prevent excessive
bleeding during the operation.
Hemophilia is a bleeding disease that occurs almost
exclusively in males. In 85 per cent of cases, it is caused
by an abnormality or deﬁciency of Factor VIII; this
type of hemophilia is called hemophilia A o

; this
type of hemophilia is called hemophilia A or classic
hemophilia. About 1 of every 10,000 males in the
United States has classic hemophilia. In the other 15
per cent of hemophilia patients, the bleeding tendency
is caused by deﬁciency of Factor IX. Both of these
factors are transmitted genetically by way of the
female chromosome. Therefore, almost never will a
woman have hemophilia because at least one of her
two X chromosomes will have the appropriate genes.
If one of her X chromosomes is

 appropriate genes.
If one of her X chromosomes is deﬁcient, she will be
a hemophilia carrier, transmitting the disease to half of
her male offspring and transmitting the carrier state to
half of her female offspring.
The bleeding trait in hemophilia can have various
degrees of severity, depending on the character of the
genetic deﬁciency. Bleeding usually does not occur
except after trauma, but in some patients, the degree
of trauma required to cause severe and prolonged
bleeding may be so mild

cause severe and prolonged
bleeding may be so mild that it is hardly noticeable.
For instance, bleeding can often last for days after
extraction of a tooth.
Factor VIII has two active components, a large com-
ponent with a molecular weight in the millions and a
smaller component with a molecular weight of about
230,000. The smaller component is most important in
the intrinsic pathway for clotting, and it is deﬁciency
of this part of Factor VIII that causes classic hemo-
philia. Another bleeding 

hat causes classic hemo-
philia. Another bleeding disease with somewhat dif-
ferent characteristics, called von Willebrand’s disease,
results from loss of the large component.
When a person with classic hemophilia experiences
severe prolonged bleeding, almost the only therapy
that is truly effective is injection of puriﬁed Factor
VIII. The cost of Factor VIII is high, and its availabil-
ity is limited because it can be gathered only from
human blood and only in extremely small quantities.
Thromb

ood and only in extremely small quantities.
Thrombocytopenia means the presence of very low
numbers of platelets in the circulating blood. People
with thrombocytopenia have a tendency to bleed, as
do hemophiliacs, except that the bleeding is usually
from many small venules or capillaries, rather than
from larger vessels as in hemophilia. As a result, small
punctate hemorrhages occur throughout all the body
tissues. The skin of such a person displays many small,
purplish blotches

ch a person displays many small,
purplish blotches, giving the disease the name throm-
bocytopenic purpura. As stated earlier, platelets are
especially important for repair of minute breaks in
capillaries and other small vessels.
Ordinarily, bleeding will not occur until the number
of platelets in the blood falls below 50,000/ml, rather
than the normal 150,000 to 300,000. Levels as low as
10,000/ml are frequently lethal.
Even without making speciﬁc platelet counts in the
blood, sometimes one can

ﬁc platelet counts in the
blood, sometimes one can suspect the existence of
thrombocytopenia if the person’s blood fails to retract,
because, as pointed out earlier, clot retraction is nor-
mally dependent on release of multiple coagulation
factors from the large numbers of platelets entrapped
in the ﬁbrin mesh of the clot.
Most people with thrombocytopenia have the
disease known as idiopathic thrombocytopenia, which
means thrombocytopenia of unknown cause. In most
of these people, it has been d

nown cause. In most
of these people, it has been discovered that for
unknown reasons, speciﬁc antibodies have formed and
react against the platelets themselves to destroy them.
Relief from bleeding for 1 to 4 days can often be
effected in a patient with thrombocytopenia by 
giving fresh whole blood transfusions that contain
large numbers of platelets. Also, splenectomy is often
helpful, sometimes effecting almost complete cure
because the spleen normally removes large numbers
of platelets from t

normally removes large numbers
of platelets from the blood.
Thromboembolic Conditions 
in the Human Being
Thrombi and Emboli. An abnormal clot that develops in
a blood vessel is called a thrombus. Once a clot has
developed, continued ﬂow of blood past the clot is
likely to break it away from its attachment and cause
the clot to ﬂow with the blood; such freely ﬂowing clots
are known as emboli. Also, emboli that originate in
large arteries or in the left side of the heart can ﬂow
peripherally and 

e left side of the heart can ﬂow
peripherally and plug arteries or arterioles in the
brain, kidneys, or elsewhere. Emboli that originate in
the venous system or in the right side of the heart gen-
erally ﬂow into the lungs to cause pulmonary arterial
embolism.
Cause of Thromboembolic Conditions. The causes of throm-
boembolic conditions in the human being are usually
twofold: (1) Any roughened endothelial surface of a
vessel—as may be caused by arteriosclerosis, infection,
or trauma—is likely to

rteriosclerosis, infection,
or trauma—is likely to initiate the clotting process. (2)
Blood often clots when it ﬂows very slowly through
blood vessels, where small quantities of thrombin and
other procoagulants are always being formed.
466
Unit VI
Use of t-PA in Treating Intravascular Clots. Genetically
engineered t-PA (tissue plasminogen activator) is
available. When delivered directly to a thrombosed
area through a catheter, it is effective in activati

ea through a catheter, it is effective in activating
plasminogen to plasmin, which in turn can dissolve
some intravascular clots. For instance, if used within
the ﬁrst hour or so after thrombotic occlusion of a
coronary artery, the heart is often spared serious
damage.
Femoral Venous Thrombosis and
Massive Pulmonary Embolism
Because clotting almost always occurs when blood
ﬂow is blocked for many hours in any vessel of the
body, the immobility of patients conﬁned to bed plus
the practice of prop

 patients conﬁned to bed plus
the practice of propping the knees with pillows often
causes intravascular clotting because of blood stasis in
one or more of the leg veins for hours at a time. Then
the clot grows, mainly in the direction of the slowly
moving venous blood, sometimes growing the entire
length of the leg veins and occasionally even up into
the common iliac vein and inferior vena cava. Then,
about 1 time out of every 10, a large part of the clot
disengages from its attachments to the 

f the clot
disengages from its attachments to the vessel wall and
ﬂows freely with the venous blood through the right
side of the heart and into the pulmonary arteries to
cause massive blockage of the pulmonary arteries,
called massive pulmonary embolism. If the clot is large
enough to occlude both of the pulmonary arteries at
the same time, immediate death ensues. If only one
pulmonary artery is blocked, death may not occur, or
the embolism may lead to death a few hours to several
days later be

lead to death a few hours to several
days later because of further growth of the clot within
the pulmonary vessels. But, again, t-PA therapy can be
a lifesaver.
Disseminated Intravascular
Occasionally the clotting mechanism becomes acti-
vated in widespread areas of the circulation, giving rise
to the condition called disseminated intravascular
coagulation. This often results from the presence of
large amounts of traumatized or dying tissue in the
body that releases great quantities 

tissue in the
body that releases great quantities of tissue factor into
the blood. Frequently, the clots are small but numer-
ous, and they plug a large share of the small periph-
eral blood vessels. This occurs especially in patients
with widespread septicemia, in which either circulat-
ing bacteria or bacterial toxins—especially endotox-
ins—activate the clotting mechanisms. Plugging of
small peripheral vessels greatly diminishes delivery of
oxygen and other nutrients to the tissues—a situatio

ygen and other nutrients to the tissues—a situation
that leads to or exacerbates circulatory shock. It is
partly for this reason that septicemic shock is lethal in
85 per cent or more of patients.
A peculiar effect of disseminated intravascular
coagulation is that the patient on occasion begins 
to bleed. The reason for this is that so many of the 
clotting factors are removed by the widespread clot-
ting that too few procoagulants remain to allow
normal hemostasis of the remaining blood.
Antico

w
Anticoagulants for 
Clinical Use
In some thromboembolic conditions, it is desirable to
delay the coagulation process. Various anticoagulants
have been developed for this purpose. The ones most
useful clinically are heparin and the coumarins.
Heparin as an Intravenous
Commercial heparin is extracted from several differ-
ent animal tissues and prepared in almost pure form.
Injection of relatively small quantities, about 0.5 to 
1 mg/kg of b

ively small quantities, about 0.5 to 
1 mg/kg of body weight, causes the blood-clotting time
to increase from a normal of about 6 minutes to 30 or
more minutes. Furthermore, this change in clotting
time occurs instantaneously, thereby immediately pre-
venting or slowing further development of a throm-
boembolic condition.
The action of heparin lasts about 1.5 to 4 hours.The
injected heparin is destroyed by an enzyme in the
blood known as heparinase.
When a coumarin, s

se.
When a coumarin, such as warfarin, is given to a
patient, the plasma levels of prothrombin and Factors
VII, IX, and X, all formed by the liver, begin to fall,
indicating that warfarin has a potent depressant effect
on liver formation of these compounds. Warfarin
causes this effect by competing with vitamin K for
reactive sites in the enzymatic processes for formation
of prothrombin and the other three clotting factors,
thereby blocking the action of vitamin K.
Aft

ors,
After administration of an effective dose of war-
farin, the coagulant activity of the blood decreases to
about 50 per cent of normal by the end of 12 hours and
to about 20 per cent of normal by the end of 24 hours.
In other words, the coagulation process is not blocked
immediately but must await the natural consumption
of the prothrombin and the other affected coagulation
factors already present in the plasma. Normal coagu-
lation usually returns 1

the plasma. Normal coagu-
lation usually returns 1 to 3 days after discontinuing
coumarin therapy.
Prevention of Blood Coagulation
Outside the Body
Although blood removed from the body and held in a
glass test tube normally clots in about 6 minutes, blood
collected in siliconized containers often does not clot
for 1 hour or more. The reason for this delay is that
preparing the surfaces of the containers with silicone
Chapter 36
467
prevents contact activation of 

od Coagulation
467
prevents contact activation of platelets and Factor XII,
the two principal factors that initiate the intrinsic clot-
ting mechanism. Conversely, untreated glass contain-
ers allow contact activation of the platelets and Factor
XII, with rapid development of clots.
Heparin can be used for preventing coagulation of
blood outside the body as well as in the body. Heparin
is especially used in surgical procedures in which the
blood must be passed through a heart-lung machine 
or ar

must be passed through a heart-lung machine 
or artiﬁcial kidney machine and then back into the
person.
Various substances that decrease the concentration
of calcium ions in the blood can also be used for pre-
venting blood coagulation outside the body. For
instance, a soluble oxalate compound mixed in a very
small quantity with a sample of blood causes precipi-
tation of calcium oxalate from the plasma and thereby
decreases the ionic calcium level so much that blood
coagulation is blocked.
Any 

el so much that blood
Any substance that deionizes the blood calcium will
prevent coagulation.The negatively charged citrate ion
is especially valuable for this purpose, mixed with
blood usually in the form of sodium, ammonium, or
potassium citrate. The citrate ion combines with
calcium in the blood to cause an un-ionized calcium
compound, and the lack of ionic calcium prevents
coagulation. Citrate anticoagulants have an important
advantage over the oxalate anticoagulants

mportant
advantage over the oxalate anticoagulants because
oxalate is toxic to the body, whereas moderate quan-
tities of citrate can be injected intravenously. After
injection, the citrate ion is removed from the blood
within a few minutes by the liver and is polymerized
into glucose or metabolized directly for energy.
Consequently, 500 milliliters of blood that has been
rendered incoagulable by citrate can ordinarily be
transfused into a recipient within a few minutes
without dire consequences

ent within a few minutes
without dire consequences. But if the liver is damaged
or if large quantities of citrated blood or plasma are
given too rapidly (within fractions of a minute), the
citrate ion may not be removed quickly enough, and
the citrate can, under these conditions, greatly depress
the level of calcium ion in the blood, which can result
in tetany and convulsive death.
When a sharp-pointed knife is used to pierce the tip
of the ﬁnger or lobe of 

is used to pierce the tip
of the ﬁnger or lobe of the ear, bleeding ordinarily lasts
for 1 to 6 minutes. The time depends largely on the
depth of the wound and the degree of hyperemia in
the ﬁnger or ear lobe at the time of the test. Lack of
any one of several of the clotting factors can prolong
the bleeding time, but it is especially prolonged by lack
of platelets.
Many methods have been devised for determining
blood clotting times. The one most widely used is to
collect blood in 

. The one most widely used is to
collect blood in a chemically clean glass test tube and
then to tip the tube back and forth about every 30
seconds until the blood has clotted. By this method,
the normal clotting time is 6 to 10 minutes. Procedures
using multiple test tubes have also been devised for
determining clotting time more accurately.
Unfortunately, the clotting time varies widely,
depending on the method used for measuring it, so it
is no longer used in many clinics. Instead, measure-
m

o longer used in many clinics. Instead, measure-
ments of the clotting factors themselves are made,
using sophisticated chemical procedures.
Prothrombin time gives an indication of the concen-
tration of prothrombin in the blood. Figure 36–5 shows
the relation of prothrombin concentration to pro-
thrombin time. The method for determining pro-
thrombin time is the following.
Blood removed from the patient is immediately
oxalated so that none of the prothrombin can change
into thr

o that none of the prothrombin can change
into thrombin. Then, a large excess of calcium ion and
tissue factor is quickly mixed with the oxalated blood.
The excess calcium nulliﬁes the effect of the oxalate,
and the tissue factor activates the prothrombin-to-
thrombin reaction by means of the extrinsic clotting
pathway. The time required for coagulation to take
place is known as the prothrombin time. The shortness
of the time is determined mainly by prothrombin con-
centration. The normal prothr

 by prothrombin con-
centration. The normal prothrombin time is about 12
seconds. In each laboratory, a curve relating pro-
thrombin concentration to prothrombin time, such as
0
6.25
12.5
25.0
50.0
Concentration (per cent of normal)
0
10
20
30
40
50
60
100
Prothrombin time
(seconds)
Figure 36–5
Relation of prothrombin concentration in the blood to “prothrom-
bin time.”
468
Unit VI
that shown in Figure 36–5, is drawn for the method
used so that the prothr

5, is drawn for the method
used so that the prothrombin in the blood can be quan-
tiﬁed.
Tests similar to that for prothrombin time have been
devised to determine the quantities of other blood
clotting factors. In each of these tests, excesses of
calcium ions and all the other factors besides the one
being tested are added to oxalated blood all at once.
Then the time required for coagulation is determined
in the same manner as for prothrombin time. If the
factor being tested is deﬁcient, the coa

e. If the
factor being tested is deﬁcient, the coagulation time is
prolonged. The time itself can then be used to quanti-
tate the concentration of the factor.
Brass LF: Thrombin and platelet activation. Chest 124(3
Suppl):18S, 2003.
Caprini JA, Glase CJ, Anderson CB, Hathaway K: Labora-
tory markers in the diagnosis of venous thromboem-
bolism. Circulation 109(12 Suppl 1):I4, 2004.
Dorsam RT, Kunapuli SP: Central role of the P2Y12 recep-
tor in platelet activation. J Clin Invest 113:

ep-
tor in platelet activation. J Clin Invest 113:340, 2004.
Fisher M, Brott TG: Emerging therapies for acute ischemic
stroke: new therapies on trial. Stroke 34:359, 2003.
Geddis AE, Kaushansky K: Inherited thrombocytopenias:
toward a molecular understanding of disorders of platelet
production. Curr Opin Pediatr 16:15, 2004.
Kahn SR, Ginsberg JS: Relationship between deep venous
thrombosis and the postthrombotic syndrome. Arch
Intern Med 164:17, 2004.
Koreth R,Weinert C,Weisdorf DJ, Key NS: Meas

2004.
Koreth R,Weinert C,Weisdorf DJ, Key NS: Measurement of
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lar coagulation. Br J Haematol 124:567, 2004.
Lindsberg PJ, Kaste M: Thrombolysis for acute stroke. Curr
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Roberts HR, Monroe DM, Escobar MA: Current concepts
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plications for therapy. Anesthesiology
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Saenko EL, Ananyeva NM, Shima M, et al: The future of
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Tsai HM:Advances in the pathogenesis, diagnosis, and treat-
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sis, and treat-
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thrombocytopenic purpura. Arterioscler Thromb Vasc
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VandenDriessche T, Collen D, Chuah MK: Gene therapy for
the hemophilias. J Thromb Haemost 1:1550, 2003.
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U
N
I
T
VII
37. Pulmonary Ventilation
38. Pulmonary Circulation, Pulmonary Edema, 
Pleural Fluid
39. Physical Principles of Gas Exchange; Diffusion 
of Oxygen and Carbon Dioxide Through the 
40. Transport of Oxygen and Carbon Dioxide in 
Bloo

0. Transport of Oxygen and Carbon Dioxide in 
Blood and Tissue Fluids
41. Regulation of Respiration
42. Respiratory Insufﬁciency—Pathophysiology,
Diagnosis, Oxygen Therapy
C
H
A
P
T
E
R
 
3
7
471
The goals of respiration are to provide oxygen to
the tissues and to remove carbon dioxide. To
achieve these goals, respiration can be divided into
four major functions: (1) pulmonary ventilation,
which means the inﬂow and outﬂow of air between
the atmosphere and the lung alveoli; 

 air between
the atmosphere and the lung alveoli; (2) diffusion 
of oxygen and carbon dioxide between the alveoli
and the blood; (3) transport of oxygen and carbon
dioxide in the blood and body ﬂuids to and from the body’s tissue cells; and 
(4) regulation of ventilation and other facets of respiration. This chapter is a 
discussion of pulmonary ventilation, and the subsequent ﬁve chapters 
cover other respiratory functions plus the physiology of special respiratory
Mechanics of P

 special respiratory
Muscles That Cause Lung Expansion and Contraction
The lungs can be expanded and contracted in two ways: (1) by downward and
upward movement of the diaphragm to lengthen or shorten the chest cavity,
and (2) by elevation and depression of the ribs to increase and decrease the
anteroposterior diameter of the chest cavity. Figure 37–1 shows these two
methods.
Normal quiet breathing is accomplished almost entirely by the ﬁrst meth

g is accomplished almost entirely by the ﬁrst method,
that is, by movement of the diaphragm. During inspiration, contraction of the
diaphragm pulls the lower surfaces of the lungs downward. Then, during expi-
ration, the diaphragm simply relaxes, and the elastic recoil of the lungs, chest
wall, and abdominal structures compresses the lungs and expels the air. During
heavy breathing, however, the elastic forces are not powerful enough to cause
the necessary rapid expiration, so that extra force i

 necessary rapid expiration, so that extra force is achieved mainly by con-
traction of the abdominal muscles, which pushes the abdominal contents
upward against the bottom of the diaphragm, thereby compressing the lungs.
The second method for expanding the lungs is to raise the rib cage. This
expands the lungs because, in the natural resting position, the ribs slant down-
ward, as shown on the left side of Figure 37–1, thus allowing the sternum to fall
backward toward the vertebral column. But 

to fall
backward toward the vertebral column. But when the rib cage is elevated, the
ribs project almost directly forward, so that the sternum also moves forward,
away from the spine, making the anteroposterior thickness of the chest about
20 per cent greater during maximum inspiration than during expiration. There-
fore, all the muscles that elevate the chest cage are classiﬁed as muscles of inspi-
ration, and those muscles that depress the chest cage are classiﬁed as muscles
of expiration. The

t cage are classiﬁed as muscles
of expiration. The most important muscles that raise the rib cage are the exter-
nal intercostals, but others that help are the (1) sternocleidomastoid muscles,
which lift upward on the sternum; (2) anterior serrati, which lift many of the
ribs; and (3) scaleni, which lift the ﬁrst two ribs.
The muscles that pull the rib cage downward during expiration are mainly
the (1) abdominal recti, which have the powerful effect of pulling downward on
the lower ribs at the s

ect of pulling downward on
the lower ribs at the same time that they and other abdominal muscles also com-
press the abdominal contents upward against the diaphragm, and (2) internal
intercostals.
472
Unit VII
Figure 37–1 also shows the mechanism by which the
external and internal intercostals act to cause inspira-
tion and expiration. To the left, the ribs during 
expiration are angled downward, and the external
intercostals are elongated forward and downward. As
they contract, they

gated forward and downward. As
they contract, they pull the upper ribs forward in rela-
tion to the lower ribs, and this causes leverage on the
ribs to raise them upward, thereby causing inspiration.
The internal intercostals function exactly in the oppo-
site manner,functioning as expiratory muscles because
they angle between the ribs in the opposite direction
and cause opposite leverage.
Movement of Air In and Out of the
Lungs and the Pressures 
That Cause the Movement
The lung is an elastic s

The lung is an elastic structure that collapses like a
balloon and expels all its air through the trachea when-
ever there is no force to keep it inﬂated. Also, there
are no attachments between the lung and the walls of
the chest cage,except where it is suspended at its hilum
from the mediastinum. Instead, the lung “ﬂoats” in the
thoracic cavity, surrounded by a thin layer of pleural
ﬂuid that lubricates movement of the lungs within the
cavity. Further, continual suctio

lungs within the
cavity. Further, continual suction of excess ﬂuid into
lymphatic channels maintains a slight suction between
the visceral surface of the lung pleura and the parietal
pleural surface of the thoracic cavity. Therefore, the
lungs are held to the thoracic wall as if glued there,
except that they are well lubricated and can slide
freely as the chest expands and contracts.
Pleural Pressure and Its Changes 
During Respiration
Pleural pressure is the pressure of the ﬂuid in the thin
spa

essure is the pressure of the ﬂuid in the thin
space between the lung pleura and the chest wall
pleura. As noted earlier, this is normally a slight
suction, which means a slightly negative pressure. The
normal pleural pressure at the beginning of inspiration
is about –5 centimeters of water, which is the amount
of suction required to hold the lungs open to their
resting level. Then, during normal inspiration, expan-
sion of the chest cage pulls outward on the lungs with
greater force and creates

utward on the lungs with
greater force and creates more negative pressure, to an
average of about –7.5 centimeters of water.
These relationships between pleural pressure and
changing lung volume are demonstrated in Figure
37–2, showing in the lower panel the increasing nega-
tivity of the pleural pressure from –5 to –7.5 during
inspiration and in the upper panel an increase in lung
volume of 0.5 liter.Then, during expiration, the events
are essentially reversed.
Alveolar Pressure
Alveolar pressu

tially reversed.
Alveolar pressure is the pressure of the air inside the
lung alveoli. When the glottis is open and no air is
ﬂowing into or out of the lungs, the pressures in all
parts of the respiratory tree, all the way to the alveoli,
are equal to atmospheric pressure, which is considered
to be zero reference pressure in the airways—that is,
0 centimeters water pressure. To cause inward ﬂow of
air into the alveoli during inspiration, the pressure in
the alveoli must fall to

piration, the pressure in
the alveoli must fall to a value slightly below atmos-
pheric pressure (below 0). The second curve (labeled
“alveolar pressure”) of Figure 37–2 demonstrates that
during normal inspiration, alveolar pressure decreases
to about –1 centimeter of water. This slight negative
Abdominals
contracted
Elevated
rib cage
EXPIRATION
INSPIRATION
Diaphragmatic
Increased
vertical diameter
Increased
A–P diameter
External
intercostals
Internal
relaxed

rcostals
Internal
relaxed
Figure 37–1
Contraction and expansion of the thoracic cage during expiration
and inspiration, demonstrating diaphragmatic contraction, func-
tion of the intercostal muscles, and elevation and depression of
the rib cage.
Pressure (cm H2O)
Volume change (liters)
0.25
0
+2
0
–2
–4
–6
–8
0.50
Inspiration
Transpulmonary pressure
Lung volume
Alveolar pressure
Pleural pressure
Figure 37–2
Changes in lung volume, alveolar pressure, pleural pre

ges in lung volume, alveolar pressure, pleural pressure, and
transpulmonary pressure during normal breathing.
Chapter 37
473
pressure is enough to pull 0.5 liter of air into the lungs
in the 2 seconds required for normal quiet inspiration.
During expiration, opposite pressures occur: The
alveolar pressure rises to about +1 centimeter of water,
and this forces the 0.5 liter of inspired air out of the
lungs during the 2 to 3 seconds of expiration.
Transpulmonary Pressure. Fin

econds of expiration.
Transpulmonary Pressure. Finally, note in Figure 37–2 the
difference between the alveolar pressure and the
pleural pressure. This is called the transpulmonary
pressure. It is the pressure difference between that in
the alveoli and that on the outer surfaces of the lungs,
and it is a measure of the elastic forces in the lungs
that tend to collapse the lungs at each instant of res-
piration, called the recoil pressure.
Compliance of the Lungs
The extent to which the lungs wil

nce of the Lungs
The extent to which the lungs will expand for each unit
increase in transpulmonary pressure (if enough time is
allowed to reach equilibrium) is called the lung com-
pliance. The total compliance of both lungs together in
the normal adult human being averages about 200 mil-
liliters of air per centimeter of water transpulmonary
pressure. That is, every time the transpulmonary pres-
sure increases 1 centimeter of water, the lung volume,
after 10 to 20 seconds, will expand 200 mill

lume,
after 10 to 20 seconds, will expand 200 milliliters.
Compliance Diagram of the Lungs. Figure 37–3 is a diagram
relating lung volume changes to changes in transpul-
monary pressure. Note that the relation is different for
inspiration and expiration. Each curve is recorded by
changing the transpulmonary pressure in small steps
and allowing the lung volume to come to a steady level
between successive steps. The two curves are called,
respectively, the inspiratory compliance curve and the
expi

ely, the inspiratory compliance curve and the
expiratory compliance curve, and the entire diagram is
called the compliance diagram of the lungs.
The characteristics of the compliance diagram are
determined by the elastic forces of the lungs.These can
be divided into two parts: (1) elastic forces of the lung
tissue itself and (2) elastic forces caused by surface
tension of the ﬂuid that lines the inside walls of the
alveoli and other lung air spaces.
The elastic forces of the lung tissue are dete

es.
The elastic forces of the lung tissue are determined
mainly by elastin and collagen ﬁbers interwoven
among the lung parenchyma. In deﬂated lungs, these
ﬁbers are in an elastically contracted and kinked state;
then, when the lungs expand, the ﬁbers become
stretched and unkinked, thereby elongating and exert-
ing even more elastic force.
The elastic forces caused by surface tension are
much more complex. The signiﬁcance of surface
tension is shown in Figure 37–4, which compares the
compliance 

own in Figure 37–4, which compares the
compliance diagram of the lungs when ﬁlled with
saline solution and when ﬁlled with air.When the lungs
are ﬁlled with air, there is an interface between the
alveolar ﬂuid and the air in the alveoli. In the case of
the saline solution–ﬁlled lungs, there is no air-ﬂuid
interface; therefore, the surface tension effect is not
present—only tissue elastic forces are operative in the
saline solution–ﬁlled lung.
Note that transpleural pressures required to expand
a

e that transpleural pressures required to expand
air-ﬁlled lungs are about three times as great as those
required to expand saline solution–ﬁlled lungs. Thus,
one can conclude that the tissue elastic forces tending
to cause collapse of the air-ﬁlled lung represent only
about one third of the total lung elasticity, whereas the
Lung volume change (liters)
0.50
0.25
0
–4
–5
–6
Pleural pressure (cm H2O)
Figure 37–3
Compliance diagram in a healthy person. This diagram shows
com

iagram in a healthy person. This diagram shows
compliance of the lungs alone.
0.50
0.25
0
0
–2
–4
–6
–8
Saline-filled
Air-filled
Figure 37–4
Comparison of the compliance diagrams of saline-ﬁlled and air-
ﬁlled lungs when the alveolar pressure is maintained at atmos-
pheric pressure (0 cm H2O) and pleural pressure is changed.
474
Unit VII
ﬂuid-air surface tension forces in the alveoli represent
about two thir

ion forces in the alveoli represent
about two thirds.
The ﬂuid-air surface tension elastic forces of the
lungs also increase tremendously when the substance
called surfactant is not present in the alveolar ﬂuid. Let
us now discuss surfactant and its relation to the surface
tension forces.
Surfactant, Surface Tension, and Collapse 
of the Alveoli
Principle of Surface Tension. When water forms a surface
with air, the water molecules on the surface of the
water have an especially strong attraction 

of the
water have an especially strong attraction for one
another. As a result, the water surface is always
attempting to contract. This is what holds raindrops
together: that is, there is a tight contractile membrane
of water molecules around the entire surface of the
raindrop. Now let us reverse these principles and see
what happens on the inner surfaces of the alveoli.
Here, the water surface is also attempting to contract.
This results in an attempt to force the air out of the
alveoli throug

attempt to force the air out of the
alveoli through the bronchi and, in doing so, causes the
alveoli to try to collapse. The net effect is to cause an
elastic contractile force of the entire lungs, which is
called the surface tension elastic force.
Surfactant and Its Effect on Surface Tension. Surfactant is a
surface active agent in water, which means that it
greatly reduces the surface tension of water. It is
secreted by special surfactant-secreting epithelial cells
called type II alveolar epit

ting epithelial cells
called type II alveolar epithelial cells, which constitute
about 10 per cent of the surface area of the alveoli.
These cells are granular, containing lipid inclusions
that are secreted in the surfactant into the alveoli.
Surfactant is a complex mixture of several 
phospholipids, proteins, and ions. The most important
components are the phospholipid dipalmitoylphos-
phatidylcholine, surfactant apoproteins, and calcium
ions. The dipalmitoylphosphatidylcholine, along with
seve

he dipalmitoylphosphatidylcholine, along with
several less important phospholipids, is responsible 
for reducing the surface tension. It does this by not 
dissolving uniformly in the ﬂuid lining the alveolar
surface. Instead, part of the molecule dissolves,
while the remainder spreads over the surface of the
water in the alveoli. This surface has from one 
twelfth to one half the surface tension of a pure water
surface.
In quantitative terms, the surface tension of differ-
ent water ﬂuids is app

 surface tension of differ-
ent water ﬂuids is approximately the following: pure
water, 72 dynes/cm; normal ﬂuids lining the alveoli but
without surfactant, 50 dynes/cm; normal ﬂuids lining
the alveoli and with normal amounts of surfactant
included, between 5 and 30 dynes/cm.
Pressure in Occluded Alveoli Caused by Surface Tension. If the
air passages leading from the alveoli of the lungs are
blocked, the surface tension in the alveoli tends to col-
lapse the alveoli. This creates positive pressu

l-
lapse the alveoli. This creates positive pressure in the
alveoli, attempting to push the air out. The amount of
pressure generated in this way in an alveolus can be cal-
culated from the following formula:
For the average-sized alveolus with a radius of about
100 micrometers and lined with normal surfactant, this
calculates to be about 4 centimeters of water pressure
(3 mm Hg). If the alveoli were lined with pure water
without any surfactant, the pressure would calculate to
be about 18 centim

the pressure would calculate to
be about 18 centimeters of water pressure, 4.5 times as
great. Thus, one sees how important surfactant is in
reducing alveolar surface tension and therefore also
reducing the effort required by the respiratory muscles
to expand the lungs.
Effect of Alveolar Radius on the Pressure Caused by Surface
Tension. Note from the preceding formula that the pres-
sure generated as a result of surface tension in the
alveoli is inversely affected by the radius of the alveo-
lu

 inversely affected by the radius of the alveo-
lus, which means that the smaller the alveolus, the
greater the alveolar pressure caused by the surface
tension. Thus, when the alveoli have half the normal
radius (50 instead of 100 micrometers), the pressures
noted earlier are doubled. This is especially signiﬁcant
in small premature babies, many of whom have alveoli
with radii less than one quarter that of an adult person.
Further, surfactant does not normally begin to be
secreted into the alveo

s not normally begin to be
secreted into the alveoli until between the sixth and
seventh months of gestation, and in some cases, even
later than that. Therefore, many premature babies have
little or no surfactant in the alveoli when they are born,
and their lungs have an extreme tendency to collapse,
sometimes as great as six to eight times that in a normal
adult person. This causes the condition called respira-
tory distress syndrome of the newborn. It is fatal if 
not treated with strong measu

orn. It is fatal if 
not treated with strong measures, especially properly
applied continuous positive pressure breathing.
Effect of the Thoracic Cage 
on Lung Expansibility
Thus far, we have discussed the expansibility of the
lungs alone, without considering the thoracic cage.The
thoracic cage has its own elastic and viscous charac-
teristics, similar to those of the lungs; even if the lungs
were not present in the thorax, muscular effort would
still be required to expand the thoracic cage.
Com

Compliance of the Thorax and the 
Lungs Together
The compliance of the entire pulmonary system (the
lungs and thoracic cage together) is measured while
expanding the lungs of a totally relaxed or paralyzed
person.To do this, air is forced into the lungs a little at
a time while recording lung pressures and volumes.To
inﬂate this total pulmonary system, almost twice as
much pressure is needed as to inﬂate the same lungs
after removal from the chest c

ﬂate the same lungs
after removal from the chest cage.Therefore, the com-
pliance of the combined lung-thorax system is almost
exactly one half that of the lungs alone—110 milliliters
of volume per centimeter of water pressure for 
the combined system, compared with 200 ml/cm for
the lungs alone. Furthermore, when the lungs are
expanded to high volumes or compressed to low
volumes, the limitations of the chest become extreme;
Pressure
Surface tension
Radius of alveolus
=
¥
2
Chapter 37
Pulmonary

sion
=
¥
2
Chapter 37
475
when near these limits, the compliance of the com-
bined lung-thorax system can be less than one ﬁfth
that of the lungs alone.
“Work” of Breathing
We have already pointed out that during normal quiet
breathing, all respiratory muscle contraction occurs
during inspiration; expiration is almost entirely a
passive process caused by elastic recoil of the lungs and
chest cage. Thus, under resting conditions, the respira-
tory muscles 

der resting conditions, the respira-
tory muscles normally perform “work” to cause inspi-
ration but not to cause expiration.
The work of inspiration can be divided into three
fractions: (1) that required to expand the lungs against
the lung and chest elastic forces, called compliance work
or elastic work; (2) that required to overcome the vis-
cosity of the lung and chest wall structures, called tissue
resistance work; and (3) that required to overcome
airway resistance to movement of air into 

vercome
airway resistance to movement of air into the lungs,
called airway resistance work.
Energy Required for Respiration. During normal quiet respi-
ration, only 3 to 5 per cent of the total energy expended
by the body is required for pulmonary ventilation. But
during heavy exercise, the amount of energy required
can increase as much as 50-fold, especially if the person
has any degree of increased airway resistance or de-
creased pulmonary compliance. Therefore, one of the
major limitations o

pliance. Therefore, one of the
major limitations on the intensity of exercise that can
be performed is the person’s ability to provide enough
muscle energy for the respiratory process alone.
Pulmonary Volumes 
and Capacities
Recording Changes in Pulmonary
Volume—Spirometry
A simple method for studying pulmonary ventilation
is to record the volume movement of air into and out
of the lungs, a process called spirometry. A typical
basic spirometer is shown in Figure 37–5. It consists of
a drum inver

 shown in Figure 37–5. It consists of
a drum inverted over a chamber of water, with the
drum counterbalanced by a weight. In the drum is a
breathing gas, usually air or oxygen; a tube connects
the mouth with the gas chamber. When one breathes
into and out of the chamber, the drum rises and falls,
and an appropriate recording is made on a moving
sheet of paper.
Figure 37–6 shows a spirogram indicating changes in
lung volume under different conditions of breathing.
For ease in describing the event

ons of breathing.
For ease in describing the events of pulmonary venti-
lation, the air in the lungs has been subdivided in this
diagram into four volumes and four capacities, which
are the average for a young adult man.
Pulmonary Volumes
To the left in Figure 37–6 are listed four pulmonary
lung volumes that, when added together, equal the
maximum volume to which the lungs can be expanded.
The signiﬁcance of each of these volumes is the 
1. The tidal volume is the volume of air inspir

g:
1. The tidal volume is the volume of air inspired or
expired with each normal breath; it amounts to
about 500 milliliters in the adult male.
2. The inspiratory reserve volume is the extra volume
of air that can be inspired over and above the
normal tidal volume when the person inspires
with full force; it is usually equal to about 
3000 milliliters.
3. The expiratory reserve volume is the maximum
extra volume of air that can be expired by
forceful expiration after the end of a normal 
tidal e

eful expiration after the end of a normal 
tidal expiration; this normally amounts to about
1100 milliliters.
4. The residual volume is the volume of air
remaining in the lungs after the most forceful
expiration; this volume averages about 
1200 milliliters.
Pulmonary Capacities
In describing events in the pulmonary cycle, it is 
sometimes desirable to consider two or more of the
volumes together. Such combinations are called pul-
monary capacities. To the right in Figure 37–6 are listed
the imp

es. To the right in Figure 37–6 are listed
the important pulmonary capacities, which can be
described as follows:
1. The inspiratory capacity equals the tidal volume
plus the inspiratory reserve volume. This is the
amount of air (about 3500 milliliters) a person can
breathe in, beginning at the normal expiratory
level and distending the lungs to the maximum
amount.
2. The functional residual capacity equals the
expiratory reserve volume plus the residual
volume. This is the amount of air that re

residual
volume. This is the amount of air that remains in
the lungs at the end of normal expiration (about
2300 milliliters).
3. The vital capacity equals the inspiratory reserve
volume plus the tidal volume plus the expiratory
reserve volume. This is the maximum amount of
air a person can expel from the lungs after ﬁrst
ﬁlling the lungs to their maximum extent and then
Recording
drum
Mouthpiece
Counterbalancing
weight
Floating
drum
Oxygen
chamber
Water
Figure 37–5
Spirometer.
476
Unit VII
Resp

er
Water
476
Unit VII
expiring to the maximum extent (about 
4600 milliliters).
4. The total lung capacity is the maximum volume to
which the lungs can be expanded with the greatest
possible effort (about 5800 milliliters); it is equal
to the vital capacity plus the residual volume.
All pulmonary volumes and capacities are about 20
to 25 per cent less in women than in men, and they are
greater in large and athletic people than in small and
asthenic people.
Abb

etic people than in small and
Abbreviations and Symbols Used 
in Pulmonary Function Studies
Spirometry is only one of many measurement proce-
dures that the pulmonary physician uses daily. Many of
these measurement procedures depend heavily on
mathematical computations. To simplify these calcula-
tions as well as the presentation of pulmonary function
data, a number of abbreviations and symbols have
become standardized. The more important of these are
given in Table 37–1. Using 

important of these are
given in Table 37–1. Using these symbols, we present
here a few simple algebraic exercises showing some of
the interrelations among the pulmonary volumes and
capacities; the student should think through and verify
these interrelations.
VC = IRV + VT + ERV
VC = IC + ERV
TLC = VC + RV
TLC = IC + FRC
FRC = ERV + RV
Determination of Functional
Residual Capacity, Residual
Volume, and Total Lung Capacity—
Helium Dilution Method
The functional residual capacity (FRC), which is th

he functional residual capacity (FRC), which is the
volume of air that remains in the lungs at the end of
each normal expiration, is important to lung function.
Because its value changes markedly in some types of
pulmonary disease, it is often desirable to measure this
capacity. The spirometer cannot be used in a direct way
to measure the functional residual capacity, because the
air in the residual volume of the lungs cannot be expired
into the spirometer, and this volume constitutes about
one 

spirometer, and this volume constitutes about
one half of the functional residual capacity. To measure
functional residual capacity, the spirometer must be
used in an indirect manner, usually by means of a
helium dilution method, as follows.
A spirometer of known volume is ﬁlled with air mixed
with helium at a known concentration. Before breath-
ing from the spirometer, the person expires normally.At
the end of this expiration, the remaining volume in the
lungs is equal to the functional residua

me in the
lungs is equal to the functional residual capacity.At this
point, the subject immediately begins to breathe from
the spirometer, and the gases of the spirometer mix with
the gases of the lungs. As a result, the helium becomes
diluted by the functional residual capacity gases, and the
volume of the functional residual capacity can be cal-
culated from the degree of dilution of the helium, using
the following formula:
FRC
Ci
Cf
Vi
He
He
Spir
=
-
Ê
Ë
ˆ
¯
1
Lung volume (ml)
5000
6000
1000

pir
=
-
Ê
Ë
ˆ
¯
1
5000
6000
1000
2000
3000
4000
Time
Inspiratory
capacity
reserve
volume
Expiratory
reserve volume
Vital
capacity
Total lung
capacity
Tidal
volume
residual
capacity
Residual
volume
Figure 37–6
Diagram showing respiratory excursions during
normal breathing and during maximal inspiration
and maximal expiration.
Chapter 37
477
where FRC is functional residual capacity, CiHe is initial

 residual capacity, CiHe is initial
concentration of helium in the spirometer, CfHe is ﬁnal
concentration of helium in the spirometer, and ViSpir is
initial volume of the spirometer.
Once the FRC has been determined, the residual
volume (RV) can be determined by subtracting expira-
tory reserve volume (ERV), as measured by normal
spirometry, from the FRC. Also, the total lung capacity
(TLC) can be determined by adding the inspiratory
capacity (IC) to the FRC. That is,
RV = FRC – ERV
and
TLC = FR

) to the FRC. That is,
and
TLC = FRC + IC
Minute Respiratory Volume
Equals Respiratory Rate
Times Tidal Volume
The minute respiratory volume is the total amount of
new air moved into the respiratory passages each
minute; this is equal to the tidal volume times the res-
piratory rate per minute. The normal tidal volume is
about 500 milliliters, and the normal respiratory rate
is about 12 breaths per minute. Therefore, the minute
respiratory volume averages about 6 L/min. A person
c

piratory volume averages about 6 L/min. A person
can live for a short period with a minute respiratory
volume as low as 1.5 L/min and a respiratory rate of
only 2 to 4 breaths per minute.
The respiratory rate occasionally rises to 40 to 50
per minute, and the tidal volume can become as great
as the vital capacity, about 4600 milliliters in a young
adult man. This can give a minute respiratory volume
greater than 200 L/min, or more than 30 times normal.
Most people cannot sustain more than one ha

ormal.
Most people cannot sustain more than one half to two
thirds these values for longer than 1 minute.
The ultimate importance of pulmonary ventilation is
to continually renew the air in the gas exchange areas
of the lungs, where air is in proximity to the pulmonary
blood. These areas include the alveoli, alveolar sacs,
alveolar ducts, and respiratory bronchioles.The rate at
which new air reaches these areas is called alveolar
ventilation.
“Dead Space” and Its Effect 
on 

olar
on Alveolar Ventilation
Some of the air a person breathes never reaches the
gas exchange areas but simply ﬁlls respiratory pas-
sages where gas exchange does not occur, such as the
nose, pharynx, and trachea.This air is called dead space
air because it is not useful for gas exchange.
On expiration, the air in the dead space is expired
ﬁrst, before any of the air from the alveoli reaches the
atmosphere. Therefore, the dead space is very disad-
vantag

e. Therefore, the dead space is very disad-
vantageous for removing the expiratory gases from the
lungs.
Measurement of the Dead Space Volume. A simple method
for measuring dead space volume is demonstrated by
the graph in Figure 37–7. In making this measurement,
the subject suddenly takes a deep breath of oxygen.This
Table 37–1
Abbreviations and Symbols for Pulmonary Function
VT
tidal volume
PB
atmospheric pressure
FRC
functional residual capacity
Palv
alveolar pressure
ERV
expiratory reserve v

ty
Palv
ERV
expiratory reserve volume
Ppl
pleural pressure
RV
residual volume
PO2
partial pressure of oxygen
IC
inspiratory capacity
PCO2
partial pressure of carbon dioxide
IRV
inspiratory reserve volume
PN2
partial pressure of nitrogen
TLC
total lung capacity
PaO2
partial pressure of oxygen in arterial blood
VC
vital capacity
PaCO2
partial pressure of carbon dioxide in arterial blood
Raw
resistance of the airways to ﬂow of air into 
PAO2
partial pressure of oxygen in alveolar 

into 
PAO2
partial pressure of oxygen in alveolar gas
the lung
C
PACO2
partial pressure of carbon dioxide in alveolar gas
VD
volume of dead space gas
PAH2O
partial pressure of water in alveolar gas
VA
volume of alveolar gas
R
respiratory exchange ratio
V
.
I
inspired volume of ventilation per minute
cardiac output
V
.
E
expired volume of ventilation per minute
V
.
s
shunt ﬂow
V
.
A
alveolar ventilation per minute
CaO2
concentration of oxygen in arterial blood
V
.
o2
rate of oxygen upt

oxygen in arterial blood
V
.
o2
rate of oxygen uptake per minute
Cv¯o2
concentration of oxygen in mixed venous blood
V
.
co2
amount of carbon dioxide eliminated per 
SO2
percentage saturation of hemoglobin with oxygen
minute
V
.
co
rate of carbon monoxide uptake per minute
SaO2
percentage saturation of hemoglobin with oxygen 
in arterial blood
Dlo2
diffusing capacity of the lungs for oxygen
DlCO
diffusing capacity of the lungs for carbon 
monoxide
˙Q
478
Unit VII
ﬁlls the entire dead

e
˙Q
478
Unit VII
ﬁlls the entire dead space with pure oxygen. Some
oxygen also mixes with the alveolar air but does not
completely replace this air. Then the person expires
through a rapidly recording nitrogen meter, which
makes the record shown in the ﬁgure. The ﬁrst portion
of the expired air comes from the dead space regions of
the respiratory passageways, where the air has been
completely replaced by oxygen. Therefore, in the early
part of the record, only oxygen appears, and th

ly
part of the record, only oxygen appears, and the nitro-
gen concentration is zero. Then, when alveolar air
begins to reach the nitrogen meter, the nitrogen con-
centration rises rapidly, because alveolar air containing
large amounts of nitrogen begins to mix with the dead
space air. After still more air has been expired, all the
dead space air has been washed from the passages, and
only alveolar air remains.Therefore, the recorded nitro-
gen concentration reaches a plateau level equal to its

oncentration reaches a plateau level equal to its
concentration in the alveoli, as shown to the right in the
ﬁgure.With a little thought, the student can see that the
gray area represents the air that has no nitrogen in it;
this area is a measure of the volume of dead space air.
For exact quantiﬁcation, the following equation is used:
where VD is dead space air and VE is the total volume
of expired air.
Let us assume, for instance, that the gray area on the
graph is 30 square centimeters, the pi

area on the
graph is 30 square centimeters, the pink area is 70
square centimeters, and the total volume expired is 500
milliliters. The dead space would be
Normal Dead Space Volume. The normal dead space air in
a young adult man is about 150 milliliters.This increases
slightly with age.
30
30
70
500
150
+
¥
, or 
ml
V
Gray area  V
Pink area
Gray area
D
E
=
¥
+
Anatomic Versus Physiologic Dead Space. The method just
described for measuring the dead space measures the
volume of all the space of t

ad space measures the
volume of all the space of the respiratory system other
than the alveoli and their other closely related gas
exchange areas; this space is called the anatomic dead
space. On occasion, some of the alveoli themselves are
nonfunctional or only partially functional because of
absent or poor blood ﬂow through the adjacent pul-
monary capillaries.Therefore, from a functional point of
view, these alveoli must also be considered dead space.
When the alveolar dead space is included 

d space.
When the alveolar dead space is included in the total
measurement of dead space, this is called the physio-
logic dead space, in contradistinction to the anatomic
dead space. In a normal person, the anatomic and phys-
iologic dead spaces are nearly equal because all alveoli
are functional in the normal lung, but in a person with
partially functional or nonfunctional alveoli in some
parts of the lungs, the physiologic dead space may be as
much as 10 times the volume of the anatomic dead

space, or 1 to 2 liters. These problems are discussed
further in Chapter 39 in relation to pulmonary gaseous
exchange and in Chapter 42 in relation to certain pul-
monary diseases.
Alveolar ventilation per minute is the total volume of
new air entering the alveoli and adjacent gas exchange
areas each minute. It is equal to the respiratory rate
times the amount of new air that enters these areas
with each breath.
A = F

ir that enters these areas
A = Freq • (VT – VD)
where 
A is the volume of alveolar ventilation per
minute, Freq is the frequency of respiration per
minute, VT is the tidal volume, and VD is the physio-
logic dead space volume.
Thus, with a normal tidal volume of 500 milliliters, a
normal dead space of 150 milliliters, and a respiratory
rate of 12 breaths per minute, alveolar ventilation
equals 12 ¥ (500 – 150), or 4200 ml/min.
Alveolar ventilation is one of the major factors
de

lveolar ventilation is one of the major factors
determining the concentrations of oxygen and carbon
dioxide in the alveoli.Therefore, almost all discussions
of gaseous exchange in the following chapters on the
respiratory system emphasize alveolar ventilation.
Functions of the Respiratory
Figure 37–8 shows the respiratory system, demonstrat-
ing especially the respiratory passageways.The air is dis-
tributed to the lungs by way of the trachea, bronch

ributed to the lungs by way of the trachea, bronchi, and
bronchioles.
One of the most important problems in all the respi-
ratory passageways is to keep them open and allow easy
passage of air to and from the alveoli. To keep the
trachea from collapsing, multiple cartilage rings extend
about ﬁve sixths of the way around the trachea. In the
walls of the bronchi, less extensive curved cartilage
plates also maintain a reasonable amount of rigidity yet
allow sufﬁcient motion for the lungs to expand 

et
allow sufﬁcient motion for the lungs to expand and 
V.
V.
Per cent nitrogen
0
20
40
0
100
Inspiration of pure oxygen
200
300
400
500
60
80
Air expired (ml)
Recorded n
i
t
r
o
g
e
n
Figure 37–7
Record of the changes in nitrogen concentration in the expired air
after a single previous inspiration of pure oxygen. This record can
be used to calculate dead space, as discussed in the text.
Chapter 37
479
contract. These plates become progressively less exten-
siv

 These plates become progressively less exten-
sive in the later generations of bronchi and are gone in
the bronchioles, which usually have diameters less than
1.5 millimeters.The bronchioles are not prevented from
collapsing by the rigidity of their walls. Instead, they are
kept expanded mainly by the same transpulmonary
pressures that expand the alveoli. That is, as the alveoli
enlarge, the bronchioles also enlarge, but not as much.
Muscular Wall of the Bronchi and Bronchioles and Its Control.

ll of the Bronchi and Bronchioles and Its Control. In
all areas of the trachea and bronchi not occupied by car-
tilage plates, the walls are composed mainly of smooth
muscle. Also, the walls of the bronchioles are almost
entirely smooth muscle, with the exception of the most
terminal bronchiole, called the respiratory bronchiole,
which is mainly pulmonary epithelium and underlying
ﬁbrous tissue plus a few smooth muscle ﬁbers. Many
obstructive diseases of the lung result from narrowing
of the sma

eases of the lung result from narrowing
of the smaller bronchi and larger bronchioles, often
because of excessive contraction of the smooth muscle
itself.
Resistance to Airﬂow in the Bronchial Tree. Under normal
respiratory conditions, air ﬂows through the respiratory
passageways so easily that less than 1 centimeter of
water pressure gradient from the alveoli to the atmos-
phere is sufﬁcient to cause enough airﬂow for quiet
breathing. The greatest amount of resistance to airﬂow
occurs not in th

st amount of resistance to airﬂow
occurs not in the minute air passages of the terminal
bronchioles but in some of the larger bronchioles and
bronchi near the trachea.The reason for this high resist-
ance is that there are relatively few of these larger
bronchi in comparison with the approximately 65,000
parallel terminal bronchioles, through each of which
only a minute amount of air must pass.
Yet in disease conditions, the smaller bronchioles
often play a far greater role in determining airﬂow

ften play a far greater role in determining airﬂow
resistance because of their small size and because they
are easily occluded by (1) muscle contraction in their
walls, (2) edema occurring in the walls, or (3) mucus col-
lecting in the lumens of the bronchioles.
Nervous and Local Control of the Bronchiolar Musculature—
“Sympathetic” Dilation of the Bronchioles. Direct control of
the bronchioles by sympathetic nerve ﬁbers is relatively
weak because few of these ﬁbers penetrate to the
central port

e few of these ﬁbers penetrate to the
central portions of the lung. However, the bronchial tree
is very much exposed to norepinephrine and epineph-
rine released into the blood by sympathetic stimulation
of the adrenal gland medullae. Both these hormones—
especially epinephrine, because of its greater stimula-
tion of beta-adrenergic receptors—cause dilation of the
bronchial tree.
Parasympathetic Constriction of the Bronchioles. A few
parasympathetic nerve ﬁbers derived from the vagus
nerves pen

etic nerve ﬁbers derived from the vagus
nerves penetrate the lung parenchyma. These nerves
secrete acetylcholine and, when activated, cause mild 
to moderate constriction of the bronchioles. When a
disease process such as asthma has already caused some
bronchiolar constriction, superimposed parasympa-
thetic nervous stimulation often worsens the condition.
When this occurs, administration of drugs that block the
effects of acetylcholine, such as atropine, can sometimes
relax the respiratory pass

atropine, can sometimes
relax the respiratory passages enough to relieve the
Sometimes the parasympathetic nerves are also acti-
vated by reﬂexes that originate in the lungs. Most of
these begin with irritation of the epithelial membrane
of the respiratory passageways themselves, initiated by
noxious gases, dust, cigarette smoke, or bronchial infec-
tion. Also, a bronchiolar constrictor reﬂex often occurs
when microemboli occlude small pulmonary arteries.
Local Secretory Factors Oft

ll pulmonary arteries.
Local Secretory Factors Often Cause Bronchiolar Constriction.
Several substances formed in the lungs themselves are
Epiglottis
Pharynx
Esophagus
Pulmonary
capillary
Alveolus
O2
O2
O2
CO2
CO2
CO2
Conchae
Alveoli
Glottis
Larynx, vocal
cords
Trachea
Figure 37–8
Respiratory passages.
480
Unit VII
often quite active in causing bronchiolar constriction.
Two of the most important of these are histamine and
slow reactive substance of 

hese are histamine and
slow reactive substance of anaphylaxis. Both of these are
released in the lung tissues by mast cells during allergic
reactions, especially those caused by pollen in the air.
Therefore, they play key roles in causing the airway
obstruction that occurs in allergic asthma; this is espe-
cially true of the slow reactive substance of anaphylaxis.
The same irritants that cause parasympathetic con-
strictor reﬂexes of the airways—smoke, dust, sulfur
dioxide, and some of the acidi

smoke, dust, sulfur
dioxide, and some of the acidic elements in smog—often
act directly on the lung tissues to initiate local, non-
nervous reactions that cause obstructive constriction of
the airways.
Mucus Lining the Respiratory Passageways, and Action of Cilia
to Clear the Passageways
All the respiratory passages, from the nose to the ter-
minal bronchioles, are kept moist by a layer of mucus
that coats the entire surface. The mucus is secreted
partly by individual mucous goblet cells in the 

d
partly by individual mucous goblet cells in the epithe-
lial lining of the passages and partly by small submu-
cosal glands. In addition to keeping the surfaces moist,
the mucus traps small particles out of the inspired air
and keeps most of these from ever reaching the alveoli.
The mucus itself is removed from the passages in the
following manner.
The entire surface of the respiratory passages, both in
the nose and in the lower passages down as far as the
terminal bronchioles, is lined with c

s far as the
terminal bronchioles, is lined with ciliated epithelium,
with about 200 cilia on each epithelial cell. These cilia
beat continually at a rate of 10 to 20 times per second
by the mechanism explained in Chapter 2, and the direc-
tion of their “power stroke” is always toward the
pharynx. That is, the cilia in the lungs beat upward,
whereas those in the nose beat downward. This contin-
ual beating causes the coat of mucus to ﬂow slowly, at a
velocity of a few millimeters per minute, tow

at a
velocity of a few millimeters per minute, toward the
pharynx.Then the mucus and its entrapped particles are
either swallowed or coughed to the exterior.
Cough Reﬂex
The bronchi and trachea are so sensitive to light touch
that very slight amounts of foreign matter or other
causes of irritation initiate the cough reﬂex. The larynx
and carina (the point where the trachea divides into the
bronchi) are especially sensitive, and the terminal bron-
chioles and even the alveoli are sensitive to cor

chioles and even the alveoli are sensitive to corrosive
chemical stimuli such as sulfur dioxide gas or chlorine
gas. Afferent nerve impulses pass from the respiratory
passages mainly through the vagus nerves to the
medulla of the brain. There, an automatic sequence of
events is triggered by the neuronal circuits of the
medulla, causing the following effect.
First, up to 2.5 liters of air are rapidly inspired.
Second, the epiglottis closes, and the vocal cords shut
tightly to entrap the air with

he vocal cords shut
tightly to entrap the air within the lungs. Third, the
abdominal muscles contract forcefully, pushing against
the diaphragm while other expiratory muscles, such as
the internal intercostals, also contract forcefully. Conse-
quently, the pressure in the lungs rises rapidly to as
much as 100 mm Hg or more. Fourth, the vocal cords
and the epiglottis suddenly open widely, so that air
under this high pressure in the lungs explodes outward.
Indeed, sometimes this air is expelled at

outward.
Indeed, sometimes this air is expelled at velocities
ranging from 75 to 100 miles per hour. Importantly, the
strong compression of the lungs collapses the bronchi
and trachea by causing their noncartilaginous parts to
invaginate inward, so that the exploding air actually
passes through bronchial and tracheal slits. The rapidly
moving air usually carries with it any foreign matter that
is present in the bronchi or trachea.
Sneeze Reﬂex
The sneeze reﬂex is very much like the cough reﬂex,

e sneeze reﬂex is very much like the cough reﬂex,
except that it applies to the nasal passageways instead
of the lower respiratory passages. The initiating stimu-
lus of the sneeze reﬂex is irritation in the nasal pas-
sageways; the afferent impulses pass in the ﬁfth cranial
nerve to the medulla, where the reﬂex is triggered. A
series of reactions similar to those for the cough reﬂex
takes place; however, the uvula is depressed, so that
large amounts of air pass rapidly through the nose, thus
he

unts of air pass rapidly through the nose, thus
helping to clear the nasal passages of foreign matter.
Normal Respiratory Functions 
of the Nose
As air passes through the nose, three distinct normal
respiratory functions are performed by the nasal cavi-
ties: (1) the air is warmed by the extensive surfaces of
the conchae and septum, a total area of about 160
square centimeters (see Figure 37–8); (2) the air is
almost completely humidiﬁed even before it passes
beyond the nose; and (3) the air is 

ore it passes
beyond the nose; and (3) the air is partially ﬁltered.
These functions together are called the air conditioning
function of the upper respiratory passageways. Ordi-
narily, the temperature of the inspired air rises to within
1°F of body temperature and to within 2 to 3 per cent
of full saturation with water vapor before it reaches the
trachea. When a person breathes air through a tube
directly into the trachea (as through a tracheostomy),
the cooling and especially the drying effec

tomy),
the cooling and especially the drying effect in the lower
lung can lead to serious lung crusting and infection.
Filtration Function of the Nose. The hairs at the entrance to
the nostrils are important for ﬁltering out large par-
ticles. Much more important, though, is the removal 
of particles by turbulent precipitation. That is, the air
passing through the nasal passageways hits many
obstructing vanes: the conchae (also called turbinates,
because they cause turbulence of the air), the se

because they cause turbulence of the air), the septum,
and the pharyngeal wall. Each time air hits one of these
obstructions, it must change its direction of movement.
The particles suspended in the air, having far more mass
and momentum than air, cannot change their direction
of travel as rapidly as the air can. Therefore, they con-
tinue forward, striking the surfaces of the obstructions,
and are entrapped in the mucous coating and trans-
ported by the cilia to the pharynx to be swallowed.
Si

ed by the cilia to the pharynx to be swallowed.
Size of Particles Entrapped in the Respiratory Passages.
The nasal turbulence mechanism for removing particles
from air is so effective that almost no particles larger
than 6 micrometers in diameter enter the lungs through
the nose. This size is smaller than the size of red blood
cells.
Of the remaining particles, many that are between 1
and 5 micrometers settle in the smaller bronchioles as a
result of gravitational precipitation. For instance, te

t of gravitational precipitation. For instance, termi-
nal bronchiolar disease is common in coal miners
because of settled dust particles. Some of the still
smaller particles (smaller than 1 micrometer in diame-
ter) diffuse against the walls of the alveoli and adhere
to the alveolar ﬂuid. But many particles smaller than 0.5
Chapter 37
481
micrometer in diameter remain suspended in the alve-
olar air and are expelled by expiration. For instance, the
particles of cigarette s

ration. For instance, the
particles of cigarette smoke are about 0.3 micrometer.
Almost none of these particles are precipitated in the
respiratory passageways before they reach the alveoli.
Unfortunately, up to one third of them do precipitate in
the alveoli by the diffusion process, with the balance
remaining suspended and expelled in the expired air.
Many of the particles that become entrapped in 
the alveoli are removed by alveolar macrophages, as
explained in Chapter 33, and others are carr

s, as
explained in Chapter 33, and others are carried away by
the lung lymphatics. An excess of particles can cause
growth of ﬁbrous tissue in the alveolar septa, leading to
permanent debility.
Vocalization
Speech involves not only the respiratory system but 
also (1) speciﬁc speech nervous control centers in the
cerebral cortex, which are discussed in Chapter 57; (2)
respiratory control centers of the brain; and (3) the
articulation and resonance structures of the mouth and
nasal cavities. Spee

e structures of the mouth and
nasal cavities. Speech is composed of two mechanical
functions: (1) phonation, which is achieved by the
larynx, and (2) articulation, which is achieved by the
structures of the mouth.
Phonation. The larynx, shown in Figure 37–9A, is espe-
cially adapted to act as a vibrator.The vibrating element
is the vocal folds, commonly called the vocal cords. The
vocal cords protrude from the lateral walls of the larynx
toward the center of the glottis; they are stretched and
p

he center of the glottis; they are stretched and
positioned by several speciﬁc muscles of the larynx
itself.
Figure 37–9B shows the vocal cords as they are seen
when looking into the glottis with a laryngoscope.
During normal breathing, the cords are wide open to
allow easy passage of air. During phonation, the cords
move together so that passage of air between them will
cause vibration.The pitch of the vibration is determined
mainly by the degree of stretch of the cords, but also by
how tightly

e of stretch of the cords, but also by
how tightly the cords are approximated to one another
and by the mass of their edges.
Figure 37–9A shows a dissected view of the vocal
folds after removal of the mucous epithelial lining.
Immediately inside each cord is a strong elastic ligament
called the vocal ligament. This is attached anteriorly to
the large thyroid cartilage, which is the cartilage that
projects forward from the anterior surface of the neck
and is called the “Adam’s apple.” Posteriorly

neck
and is called the “Adam’s apple.” Posteriorly, the vocal
ligament is attached to the vocal processes of two ary-
tenoid cartilages. The thyroid cartilage and the arytenoid
cartilages articulate from below with another cartilage
not shown in Figure 37–9, the cricoid cartilage.
The vocal cords can be stretched by either forward
rotation of the thyroid cartilage or posterior rotation of
the arytenoid cartilages, activated by muscles stretching
from the thyroid cartilage and arytenoid cartilage

from the thyroid cartilage and arytenoid cartilages to
the cricoid cartilage. Muscles located within the vocal
cords lateral to the vocal ligaments, the thyroarytenoid
muscles, can pull the arytenoid cartilages toward the
thyroid cartilage and, therefore, loosen the vocal cords.
Also, slips of these muscles within the vocal cords can
change the shapes and masses of the vocal cord edges,
sharpening them to emit high-pitched sounds and blunt-
ing them for the more bass sounds.
Finally, several oth

hem for the more bass sounds.
Finally, several other sets of small laryngeal muscles
lie between the arytenoid cartilages and the cricoid car-
tilage and can rotate these cartilages inward or outward
or pull their bases together or apart to give the various
conﬁgurations of the vocal cords shown in Figure 37–9B.
Articulation and Resonance. The three major organs of
articulation are the lips, tongue, and soft palate. They
need not be discussed in detail because we are all famil-
iar with their mo

detail because we are all famil-
iar with their movements during speech and other
vocalizations.
The resonators include the mouth, the nose and asso-
ciated nasal sinuses, the pharynx, and even the chest
cavity. Again, we are all familiar with the resonating
qualities of these structures. For instance, the function
of the nasal resonators is demonstrated by the change
in voice quality when a person has a severe cold that
blocks the air passages to these resonators.
Carr MJ, Undem BJ: 

o these resonators.
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Thyroarytenoid
muscle
Full
abduction
Gentle
abduction
Stage
whisper
Phonation
Intermediate position–
loud whisper
Lateral
cricoarytenoid
muscle
Posterior
muscle
arytenoid
muscle
Thyroid
cartilage
Arytenoid
cartilage
Vocal
ligament
A
B
Figure 37–9
A,
Anatomy 

rtilage
Vocal
ligament
A
B
A,
Anatomy 
of 
the 
larynx. 
B, Laryngeal function in phona-
tion, showing the positions of the
vocal cords during different
types of phonation. (Modiﬁed
from Greene MC: The Voice and
Its Disorders, 4th ed. Philadel-
phia: JB Lippincott, 1980.)
482
Unit VII
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C
H
A
P
T

honosurgery.
C
H
A
P
T
E
R
 
3
8
483
Pulmonary Circulation,
Pulmonary Edema, Pleural Fluid
Some aspects of blood ﬂow distribution and other
hemodynamics are particular to the pulmonary cir-
culation and are especially important for gas
exchange in the lungs.The present discussion is con-
cerned with these special features of the pulmonary
Pulmonary Vessels. The pulmonary artery extends only 5 centime

ssels. The pulmonary artery extends only 5 centimeters beyond the apex
of the right ventricle and then divides into right and left main branches that supply
blood to the two respective lungs.
The pulmonary artery is thin, with a wall thickness one third that of the aorta.
The pulmonary arterial branches are very short, and all the pulmonary arteries, even
the smaller arteries and arterioles, have larger diameters than their counterpart sys-
temic arteries. This, combined with the fact that the v

 arteries. This, combined with the fact that the vessels are thin and distensible,
gives the pulmonary arterial tree a large compliance, averaging almost 7 ml/mm Hg,
which is similar to that of the entire systemic arterial tree. This large compliance
allows the pulmonary arteries to accommodate the stroke volume output of the
The pulmonary veins, like the pulmonary arteries, are also short. They immedi-
ately empty their efﬂuent blood into the left atrium, to be pumped by the le

blood into the left atrium, to be pumped by the left heart
through the systemic circulation.
Bronchial Vessels. Blood also ﬂows to the lungs through small bronchial arteries that
originate from the systemic circulation, amounting to about 1 to 2 per cent of the
total cardiac output. This bronchial arterial blood is oxygenated blood, in contrast
to the partially deoxygenated blood in the pulmonary arteries. It supplies the sup-
porting tissues of the lungs, including the connective tissue, septa,

the lungs, including the connective tissue, septa, and large and
small bronchi. After this bronchial and arterial blood has passed through the sup-
porting tissues, it empties into the pulmonary veins and enters the left atrium, rather
than passing back to the right atrium. Therefore, the ﬂow into the left atrium and
the left ventricular output are about 1 to 2 per cent greater than the right ventric-
ular output.
Lymphatics. Lymph vessels are present in all the supportive tissues of the lung, b

esent in all the supportive tissues of the lung, begin-
ning in the connective tissue spaces that surround the terminal bronchioles, cours-
ing to the hilum of the lung, and thence mainly into the right thoracic lymph duct.
Particulate matter entering the alveoli is partly removed by way of these channels,
and plasma protein leaking from the lung capillaries is also removed from the lung
tissues, thereby helping to prevent pulmonary edema.
Pressure Pulse Curve i

res in the Pulmonary System
Pressure Pulse Curve in the Right Ventricle. The pressure pulse curves of the right
ventricle and pulmonary artery are shown in the lower portion of Figure 38–1.
These curves are contrasted with the much higher aortic pressure curve shown
in the upper portion of the ﬁgure. The systolic pressure in the right ventricle of
the normal human being averages about 25 mm Hg, and the diastolic pressure
484
Unit VII
averages about 0 to 1 mm Hg, values that are only 

averages about 0 to 1 mm Hg, values that are only one
ﬁfth those for the left ventricle.
Pressures in the Pulmonary Artery. During systole, the
pressure in the pulmonary artery is essentially equal
to the pressure in the right ventricle, as also shown 
in Figure 38–1. However, after the pulmonary valve
closes at the end of systole, the ventricular pressure
falls precipitously, whereas the pulmonary arterial
pressure falls more slowly as blood ﬂows through the
capillaries of the lungs.
As shown i

s through the
As shown in Figure 38–2, the systolic pulmonary
arterial pressure averages about 25 mm Hg in the
normal human being, the diastolic pulmonary arterial
pressure is about 8 mm Hg, and the mean pulmonary
arterial pressure is 15 mm Hg.
Pulmonary Capillary Pressure. The mean pulmonary cap-
illary pressure, as diagrammed in Figure 38–2, is about
7 mm Hg. The importance of this low capillary pres-
sure is discussed in detail later in the chapter in rela-
tion to ﬂ

 in detail later in the chapter in rela-
tion to ﬂuid exchange functions of the pulmonary
Left Atrial and Pulmonary Venous Pressures. The mean
pressure in the left atrium and the major pulmonary
veins averages about 2 mm Hg in the recumbent
human being, varying from as low as 1 mm Hg to as
high as 5 mm Hg. It usually is not feasible to measure
a human being’s left atrial pressure using a direct
measuring device because it is difﬁcult to pass a
catheter through the heart chambers int

 to pass a
catheter through the heart chambers into the left
atrium. However, the left atrial pressure can often be
estimated with moderate accuracy by measuring the
so-called pulmonary wedge pressure. This is achieved
by inserting a catheter ﬁrst through a peripheral vein
to the right atrium, then through the right side of the
heart and through the pulmonary artery into one of
the small branches of the pulmonary artery, ﬁnally
pushing the catheter until it wedges tightly in the small
branch.
Th

er until it wedges tightly in the small
branch.
The pressure measured through the catheter, called
the “wedge pressure,” is about 5 mm Hg. Because all
blood ﬂow has been stopped in the small wedged
artery, and because the blood vessels extending
beyond this artery make a direct connection with the
pulmonary capillaries, this wedge pressure is usually
only 2 to 3 mm Hg greater than the left atrial pressure.
When the left atrial pressure rises to high values,
the pulmonary wedge pressure also rise

igh values,
the pulmonary wedge pressure also rises. Therefore,
wedge pressure measurements can be used to clinically
study changes in pulmonary capillary pressure and 
left atrial pressure in patients with congestive heart
failure.
The blood volume of the lungs is about 450 milliliters,
about 9 per cent of the total blood volume of the entire
circulatory system.Approximately 70 milliliters of this
pulmonary blood volume is in the pulmonary capil-
laries, and the remain

 is in the pulmonary capil-
laries, and the remainder is divided about equally
between the pulmonary arteries and the veins.
Lungs as a Blood Reservoir. Under various physiological
and pathological conditions, the quantity of blood in
the lungs can vary from as little as one half normal up
to twice normal. For instance, when a person blows out
air so hard that high pressure is built up in the lungs—
such as when blowing a trumpet—as much as 250 mil-
liliters of blood can be expelled from the pul

il-
liliters of blood can be expelled from the pulmonary
circulatory system into the systemic circulation.
Also, loss of blood from the systemic circulation by
hemorrhage can be partly compensated for by the
0
0
8
25
75
120
2
1
Aortic pressure curve
Pulmonary artery curve
Right ventricular curve
Seconds
Figure 38–1
Pressure pulse contours in the right ventricle, pulmonary artery,
and aorta.
Pulmonary
artery
Left
atrium
Pulmonary
0
2
8
7
15
M
D
S
25
Left
atrium
Pulmon

0
2
8
7
15
M
D
S
25
Left
atrium
Pulmonary
mm Hg
Figure 38–2
Pressures in the different vessels of the lungs. D, diastolic; M,
mean; S, systolic; red curve, arterial pulsations.
Chapter 38
Pulmonary Circulation, Pulmonary Edema, Pleural Fluid
485
automatic shift of blood from the lungs into the sys-
temic vessels.
Shift of Blood Between the Pulmonary and Systemic
Circulatory Systems as a Result of Cardiac Pathology.
Failure of the left side of the heart or increased resist

 of the left side of the heart or increased resist-
ance to blood ﬂow through the mitral valve as a result
of mitral stenosis or mitral regurgitation causes blood
to dam up in the pulmonary circulation, sometimes
increasing the pulmonary blood volume as much as
100 per cent and causing large increases in the pul-
monary vascular pressures. Because the volume of the
systemic circulation is about nine times that of the pul-
monary system, a shift of blood from one system to the
other affects the p

f blood from one system to the
other affects the pulmonary system greatly but usually
has only mild systemic circulatory effects.
Blood Flow Through the Lungs
and Its Distribution
The blood ﬂow through the lungs is essentially equal
to the cardiac output. Therefore, the factors that
control cardiac output—mainly peripheral factors,
as discussed in Chapter 20—also control pulmonary
blood ﬂow. Under most conditions, the pulmonary
vessels act as passive, distensible tubes that enlarge
with increasi

sive, distensible tubes that enlarge
with increasing pressure and narrow with decreasing
pressure. For adequate aeration of the blood to 
occur, it is important for the blood to be distributed to
those segments of the lungs where the alveoli are 
best oxygenated. This is achieved by the following
Effect of Diminished Alveolar Oxygen on Local Alveolar Blood
Flow—Automatic Control of Pulmonary Blood Flow Distribution.
When the concentration of oxygen in the air of the
alveoli decreases 

ion of oxygen in the air of the
alveoli decreases below normal—especially when it
falls below 70 per cent of normal (below 73 mm Hg
Po2)—the adjacent blood vessels constrict, with the
vascular resistance increasing more than ﬁvefold at
extremely low oxygen levels. This is opposite to the
effect observed in systemic vessels, which dilate rather
than constrict in response to low oxygen. It is believed
that the low oxygen concentration causes some yet
undiscovered vasoconstrictor substance to be re

et
undiscovered vasoconstrictor substance to be released
from the lung tissue; this substance promotes con-
striction of the small arteries and arterioles. It has been
suggested that this vasoconstrictor might be secreted
by the alveolar epithelial cells when they become
hypoxic.
This effect of low oxygen on pulmonary vascular
resistance has an important function: to distribute
blood ﬂow where it is most effective. That is, if some
alveoli are poorly ventilated so that their oxygen con-
centrati

orly ventilated so that their oxygen con-
centration becomes low,the local vessels constrict.This
causes the blood to ﬂow through other areas of the
lungs that are better aerated, thus providing an auto-
matic control system for distributing blood ﬂow to the
pulmonary areas in proportion to their alveolar
oxygen pressures.
Effect of Hydrostatic
Pressure Gradients in the
Lungs on Regional
In Chapter 15, it was pointed out that the blood pres-
sure in the foot of a standing pe

 the blood pres-
sure in the foot of a standing person can be as much as
90 mm Hg greater than the pressure at the level of the
heart. This is caused by hydrostatic pressure—that is, by
the weight of the blood itself in the blood vessels. The
same effect, but to a lesser degree, occurs in the lungs.
In the normal, upright adult, the lowest point in the
lungs is about 30 centimeters below the highest point.
This represents a 23 mm Hg pressure difference, about
15 mm Hg of which is above the heart

erence, about
15 mm Hg of which is above the heart and 8 below.That
is, the pulmonary arterial pressure in the uppermost
portion of the lung of a standing person is about 15 mm
Hg less than the pulmonary arterial pressure at the level
of the heart, and the pressure in the lowest portion of
the lungs is about 8 mm Hg greater. Such pressure dif-
ferences have profound effects on blood ﬂow through
the different areas of the lungs. This is demonstrated by
the lower curve in Figure 38–3, which depict

ed by
the lower curve in Figure 38–3, which depicts blood ﬂow
per unit of lung tissue at different levels of the lung in
the upright person. Note that in the standing position at
rest, there is little ﬂow in the top of the lung but about
ﬁve times as much ﬂow in the bottom. To help explain
these differences, one often describes the lung as being
divided into three zones, as shown in Figure 38–4.
In each zone, the patterns of blood ﬂow are quite 
different.
Zones 1, 2, and 3 of Pulmonary
Blood Fl

Blood Flow
The capillaries in the alveolar walls are distended by the
blood pressure inside them, but simultaneously, they are
Middle
Lung level
Top
Bottom
Standing at rest
Exercise
Blood flow
(per unit of tissue)
Figure 38–3
Blood ﬂow at different levels in the lung of an upright person at
rest and during exercise. Note that when the person is at rest, the
blood ﬂow is very low at the top of the lungs; most of the ﬂow is
through the bottom of the lung.

ost of the ﬂow is
486
Unit VII
compressed by the alveolar air pressure on their out-
sides. Therefore, any time the lung alveolar air pressure
becomes greater than the capillary blood pressure,
the capillaries close and there is no blood ﬂow. Under
different normal and pathological lung conditions, one
may ﬁnd any one of three possible zones of pulmonary
blood ﬂow, as follows:
Zone 1: No blood ﬂow during all portions of the
cardiac cycle because the lo

g all portions of the
cardiac cycle because the local alveolar capillary
pressure in that area of the lung never rises higher
than the alveolar air pressure during any part of the
cardiac cycle
Zone 2: Intermittent blood ﬂow only during the pul-
monary arterial pressure peaks because the systolic
pressure is then greater than the alveolar air pres-
sure, but the diastolic pressure is less than the alve-
olar air pressure
Zone 3: Continuous blood ﬂow because the alveolar
capillary pressure remain

ﬂow because the alveolar
capillary pressure remains greater than alveolar air
pressure during the entire cardiac cycle
Normally, the lungs have only zones 2 and 3 blood
ﬂow—zone 2 (intermittent ﬂow) in the apices, and zone
3 (continuous ﬂow) in all the lower areas. For example,
when a person is in the upright position, the pulmonary
arterial pressure at the lung apex is about 15 mm Hg
less than the pressure at the level of the heart. There-
fore, the apical systolic pressure is only 10 mm Hg 
(2

 the apical systolic pressure is only 10 mm Hg 
(25 mm Hg at heart level minus 15 mm Hg hydrostatic
pressure difference). This 10 mm Hg apical blood pres-
sure is greater than the zero alveolar air pressure,so that
blood ﬂows through the pulmonary apical capillaries
during cardiac systole. Conversely, during diastole, the 
8 mm Hg diastolic pressure at the level of the heart is
not sufﬁcient to push the blood up the 15 mm Hg hydro-
static pressure gradient required to cause diastolic cap-
illary

e gradient required to cause diastolic cap-
illary ﬂow.Therefore, blood ﬂow through the apical part
of the lung is intermittent, with ﬂow during systole but
cessation of ﬂow during diastole; this is called zone 2
blood ﬂow. Zone 2 blood ﬂow begins in the normal
lungs about 10 centimeters above the midlevel of the
heart and extends from there to the top of the lungs.
In the lower regions of the lungs, from about 10 cen-
timeters above the level of the heart all the way to the
bottom of the lungs,

 the heart all the way to the
bottom of the lungs, the pulmonary arterial pressure
during both systole and diastole remains greater than
the zero alveolar air pressure. Therefore, there is con-
tinuous ﬂow through the alveolar capillaries, or zone 3
blood ﬂow. Also, when a person is lying down, no part
of the lung is more than a few centimeters above the
level of the heart. In this case, blood ﬂow in a normal
person is entirely zone 3 blood ﬂow, including the lung
apices.
Zone 1 Blood Flow Occur

including the lung
apices.
Zone 1 Blood Flow Occurs Only Under Abnormal Conditions.
Zone 1 blood ﬂow, which is blood ﬂow at no time during
the cardiac cycle, occurs when either the pulmonary sys-
tolic arterial pressure is too low or the alveolar pressure
is too high to allow ﬂow. For instance, if an upright
person is breathing against a positive air pressure so
that the intra-alveolar air pressure is at least 10 mm Hg
greater than normal but the pulmonary systolic blood
pressure is normal, one 

 pulmonary systolic blood
pressure is normal, one would expect zone 1 blood
ﬂow—no blood ﬂow—in the lung apices. Another
instance in which zone 1 blood ﬂow occurs is in an
upright person whose pulmonary systolic arterial pres-
sure is exceedingly low, as might occur after severe
blood loss.
Effect of Exercise on Blood Flow Through the Different Parts of
the Lungs. Referring again to Figure 38–3, one sees that
the blood ﬂow in all parts of the lung increases during
exercise.The increase in ﬂow in

g increases during
exercise.The increase in ﬂow in the top of the lung may
be 700 to 800 per cent, whereas the increase in the lower
part of the lung may be no more than 200 to 300 per
cent. The reason for these differences is that the pul-
monary vascular pressures rise enough during exercise
to convert the lung apices from a zone 2 pattern into a
zone 3 pattern of ﬂow.
Effect of Increased Cardiac Output 
on Pulmonary Blood Flow and
Pulmonary Arterial Pressure During
During heavy

terial Pressure During
During heavy exercise, blood ﬂow through the lungs
increases fourfold to sevenfold. This extra ﬂow is
accommodated in the lungs in three ways: (1) by
increasing the number of open capillaries, sometimes
as much as threefold; (2) by distending all the capil-
laries and increasing the rate of ﬂow through each 
capillary more than twofold; and (3) by increasing the
pulmonary arterial pressure. In the normal person, the
ﬁrst two changes decrease pulmonary vascul

on, the
ﬁrst two changes decrease pulmonary vascular resist-
ance so much that the pulmonary arterial pressure
rises very little, even during maximum exercise; this
effect is shown in Figure 38–5.
Artery
Vein
PALV
ZONE 1
Ppc
Artery
Vein
PALV
ZONE 2
Ppc
Artery
Vein
PALV
ZONE 3
Ppc
Figure 38–4
Mechanics of blood ﬂow in the three blood ﬂow zones of the lung:
zone 1, no ﬂow—alveolar air pressure (PALV) is greater than arte-
rial pressure; zone 2, intermittent ﬂow—systolic arterial pressure
rises hig

ermittent ﬂow—systolic arterial pressure
rises higher than alveolar air pressure, but diastolic arterial pres-
sure falls below alveolar air pressure; and zone 3, continuous
ﬂow—arterial pressure and pulmonary capillary pressure (Ppc)
remain greater than alveolar air pressure at all times.
Chapter 38
487
The ability of the lungs to accommodate greatly
increased blood ﬂow during exercise without increas-
ing the pulmonary arterial pressure con

t increas-
ing the pulmonary arterial pressure conserves the
energy of the right side of the heart. This ability also
prevents a signiﬁcant rise in pulmonary capillary pres-
sure, thus also preventing the development of pul-
monary edema.
Function of the Pulmonary Circulation
When the Left Atrial Pressure Rises
as a Result of Left-Sided Heart Failure
The left atrial pressure in a healthy person almost
never rises above +6 mm Hg, even during the most
strenuous exercise. These small changes in lef

ost
strenuous exercise. These small changes in left atrial
pressure have virtually no effect on pulmonary circu-
latory function because this merely expands the pul-
monary venules and opens up more capillaries so that
blood continues to ﬂow with almost equal ease from
the pulmonary arteries.
When the left side of the heart fails, however, blood
begins to dam up in the left atrium. As a result, the 
left atrial pressure can rise on occasion from its normal
value of 1 to 5 mm Hg all the way up to

its normal
value of 1 to 5 mm Hg all the way up to 40 to 
50 mm Hg. The initial rise in atrial pressure, up to
about 7 mm Hg, has very little effect on pulmonary cir-
culatory function. But when the left atrial pressure
rises to greater than 7 or 8 mm Hg, further increases
in left atrial pressure above these levels cause almost
equally great increases in pulmonary arterial pressure,
thus causing a concomitant increased load on the right
heart. Any increase in left atrial pressure above 7 or 
8 m

y increase in left atrial pressure above 7 or 
8 mm Hg increases the capillary pressure almost
equally as much.When the left atrial pressure has risen
above 30 mm Hg, causing similar increases in capillary
pressure, pulmonary edema is likely to develop, as we
discuss later in the chapter.
Exchange of gases between the alveolar air and the
pulmonary capillary blood is discussed in the next
chapter. However, it is important for us to note here
that the alveolar walls a

tant for us to note here
that the alveolar walls are lined with so many capil-
laries that, in most places, the capillaries almost touch
one another side by side.Therefore, it is often said that
the capillary blood ﬂows in the alveolar walls as a
“sheet of ﬂow,” rather than in individual capillaries.
Pulmonary Capillary Pressure. No direct measurements 
of pulmonary capillary pressure have ever been 
made. However,“isogravimetric” measurement of pul-
monary capillary pressure, using a technique 

pul-
monary capillary pressure, using a technique described
in Chapter 16, has given a value of 7 mm Hg. This is
probably nearly correct, because the mean left atrial
pressure is about 2 mm Hg and the mean pulmonary
arterial pressure is only 15 mm Hg, so the mean
pulmonary capillary pressure must lie somewhere
between these two values.
Length of Time Blood Stays in the Pulmonary Capillaries.
From histological study of the total cross-sectional
area of all the pulmonary capillaries, it can be cal

ea of all the pulmonary capillaries, it can be calcu-
lated that when the cardiac output is normal, blood
passes through the pulmonary capillaries in about 0.8
second. When the cardiac output increases, this can
shorten to as little as 0.3 second.The shortening would
be much greater were it not for the fact that additional
capillaries, which normally are collapsed, open up to
accommodate the increased blood ﬂow. Thus, in only
a fraction of a second, blood passing through the 
alveolar capillarie

nd, blood passing through the 
alveolar capillaries becomes oxygenated and loses its
excess carbon dioxide.
Capillary Exchange of Fluid in the
Lungs, and Pulmonary Interstitial
Fluid Dynamics
The dynamics of ﬂuid exchange across the lung capil-
lary membranes are qualitatively the same as for
peripheral tissues. However, quantitatively, there are
important differences, as follows:
1. The pulmonary capillary pressure is low, about 
7 mm Hg, in comparison with a considerably
higher functional capi

parison with a considerably
higher functional capillary pressure in the
peripheral tissues of about 17 mm Hg.
2. The interstitial ﬂuid pressure in the lung is 
slightly more negative than that in the peripheral
subcutaneous tissue. (This has been measured in
two ways: by a micropipette inserted into the
pulmonary interstitium, giving a value of about 
–5 mm Hg, and by measuring the absorption
pressure of ﬂuid from the alveoli, giving a value of
about –8 mm Hg.)
0
30 
20 
10 
0
4
8
1

bout –8 mm Hg.)
0
30 
20 
10 
0
4
8
12
16
20
24
Pulmonary arterial
Figure 38–5
Effect on mean pulmonary arterial pressure caused by increasing
the cardiac output during exercise.
488
Unit VII
3. The pulmonary capillaries are relatively leaky to
protein molecules, so that the colloid osmotic
pressure of the pulmonary interstitial ﬂuid is
about 14 mm Hg, in comparison with less than
half this value in the peripheral tissues.
4. The a

alf this value in the peripheral tissues.
4. The alveolar walls are extremely thin, and the
alveolar epithelium covering the alveolar surfaces
is so weak that it can be ruptured by any positive
pressure in the interstitial spaces greater than
alveolar air pressure (greater than 0 mm Hg),
which allows dumping of ﬂuid from the interstitial
spaces into the alveoli.
Now let us see how these quantitative differences
affect pulmonary ﬂuid dynamics.
Interrelations Between Interstitial Fluid Pressure an

errelations Between Interstitial Fluid Pressure and Other
Pressures in the Lung. Figure 38–6 shows a pulmonary
capillary, a pulmonary alveolus, and a lymphatic capil-
lary draining the interstitial space between the blood
capillary and the alveolus. Note the balance of forces
at the blood capillary membrane, as follows:
mm Hg
Forces tending to cause movement of ﬂuid outward from the
capillaries and into the pulmonary interstitium:
7
1

ure
7
14
Negative interstitial ﬂuid pressure
8
TOTAL OUTWARD FORCE
29
Forces tending to cause absorption of ﬂuid into the capillaries:
28
TOTAL INWARD FORCE
28
Thus, the normal outward forces are slightly greater
than the inward forces, providing a mean ﬁltration
pressure at the pulmonary capillary membrane; this
can be calculated as follows:
mm Hg
Total outward force
+29
Total inward force
–28
MEAN FILTRATION PRESSURE
+1

–28
+1
This ﬁltration pressure causes a slight continual ﬂow
of ﬂuid from the pulmonary capillaries into the inter-
stitial spaces, and except for a small amount that evap-
orates in the alveoli, this ﬂuid is pumped back to the
circulation through the pulmonary lymphatic system.
Negative Pulmonary Interstitial Pressure and the Mechanism
for Keeping the Alveoli “Dry.” One of the most important
problems in lung function is to understand why the
alveoli d

n lung function is to understand why the
alveoli do not normally ﬁll with ﬂuid. One’s ﬁrst incli-
nation is to think that the alveolar epithelium is strong
enough and continuous enough to keep ﬂuid from
leaking out of the interstitial spaces into the alveoli.
This is not true, because experiments have shown 
that there are always openings between the alveolar
epithelial cells through which even large protein mol-
ecules, as well as water and electrolytes, can pass.
However, if one remembers that

trolytes, can pass.
However, if one remembers that the pulmonary cap-
illaries and the pulmonary lymphatic system normally
maintain a slight negative pressure in the interstitial
spaces, it is clear that whenever extra ﬂuid appears 
in the alveoli, it will simply be sucked mechanically
into the lung interstitium through the small openings
between the alveolar epithelial cells. Then the excess
ﬂuid is either carried away through the pulmonary
lymphatics or absorbed into the pulmonary capillaries.

hatics or absorbed into the pulmonary capillaries.
Thus, under normal conditions, the alveoli are kept
“dry,” except for a small amount of ﬂuid that seeps
from the epithelium onto the lining surfaces of the
alveoli to keep them moist.
Pulmonary edema occurs in the same way that edema
occurs elsewhere in the body.Any factor that causes the
pulmonary interstitial ﬂuid pressure to rise from the
negative range into the positive range will cause rapid
ﬁlling of the pulmonary interstit

will cause rapid
ﬁlling of the pulmonary interstitial spaces and alveoli
with large amounts of free ﬂuid.
The most common causes of pulmonary edema are as
follows:
1. Left-sided heart failure or mitral valve disease, with
consequent great increases in pulmonary venous
pressure and pulmonary capillary pressure and
ﬂooding of the interstitial spaces and alveoli.
2. Damage to the pulmonary blood capillary
membranes caused by infections such as
pneumonia or by breathing noxious substances
such as ch

onia or by breathing noxious substances
such as chlorine gas or sulfur dioxide gas. Each of
these causes rapid leakage of both plasma proteins
and ﬂuid out of the capillaries and into both the
lung interstitial spaces and the alveoli.
“Pulmonary Edema Safety Factor.” Experiments in animals
have shown that the pulmonary capillary pressure nor-
Hydrostatic
pressure
-8
+7
-14
-8
Osmotic
pressure
Net
pressure
CAPILLARY
Pressures Causing Fluid Movement
ALVEOLUS
Lymphatic pump
(+1)
(0)
(Evaporation)
(

t
ALVEOLUS
(+1)
(0)
(Surface
tension
at pore)
-28
-5
-4
-8
Figure 38–6
Hydrostatic and osmotic forces at the capillary (left) and alveolar
membrane (right) of the lungs. Also shown is the tip end of a lym-
phatic vessel (center) that pumps ﬂuid from the pulmonary inter-
stitial spaces. (Modiﬁed from Guyton AC, Taylor AE, Granger HJ:
Circulatory Physiology II: Dynamics and Control of the Body
Fluids. Philadelphia: WB Saunders, 1975.)
Chapter 38
Pulmonary Circulation, 

aunders, 1975.)
Chapter 38
489
mally must rise to a value at least equal to the colloid
osmotic pressure of the plasma inside the capillaries
before signiﬁcant pulmonary edema will occur. To give
an example, Figure 38–7 shows how different levels of
left atrial pressure increase the rate of pulmonary
edema formation in dogs. Remember that every time
the left atrial pressure rises to high values, the pul-
monary capillary pressure rises to a l

s, the pul-
monary capillary pressure rises to a level 1 to 2 mm Hg
greater than the left atrial pressure. In these experi-
ments, as soon as the left atrial pressure rose above 
23 mm Hg (causing the pulmonary capillary pressure to
rise above 25 mm Hg), ﬂuid began to accumulate in the
lungs. This ﬂuid accumulation increased even more
rapidly with further increases in capillary pressure. The
plasma colloid osmotic pressure during these experi-
ments was equal to this 25 mm Hg critical pressure
l

nts was equal to this 25 mm Hg critical pressure
level. Therefore, in the human being, whose normal
plasma colloid osmotic pressure is 28 mm Hg, one can
predict that the pulmonary capillary pressure must 
rise from the normal level of 7 mm Hg to more than
28 mm Hg to cause pulmonary edema, giving an acute
safety factor against pulmonary edema of 21 mm Hg.
Safety Factor in Chronic Conditions. When the pul-
monary capillary pressure remains elevated chronically
(for at least 2 weeks), the lungs be

d chronically
(for at least 2 weeks), the lungs become even more
resistant to pulmonary edema because the lymph
vessels expand greatly, increasing their capability of car-
rying ﬂuid away from the interstitial spaces perhaps as
much as 10-fold. Therefore, in patients with chronic
mitral stenosis, pulmonary capillary pressures of 40 to
45 mm Hg have been measured without the develop-
ment of lethal pulmonary edema.
Rapidity of Death in Acute Pulmonary Edema. When the pul-
monary capillary pressur

nary Edema. When the pul-
monary capillary pressure rises even slightly above the
safety factor level, lethal pulmonary edema can occur
within hours, or even within 20 to 30 minutes if the cap-
illary pressure rises 25 to 30 mm Hg above the safety
factor level. Thus, in acute left-sided heart failure, in
which the pulmonary capillary pressure occasionally
does rise to 50 mm Hg, death frequently ensues in less
than 30 minutes from acute pulmonary edema.
When the lungs 

edema.
When the lungs expand and contract during normal
breathing, they slide back and forth within the pleural
cavity. To facilitate this, a thin layer of mucoid ﬂuid lies
between the parietal and visceral pleurae.
Figure 38–8 shows the dynamics of ﬂuid exchange in
the pleural space. The pleural membrane is a porous,
mesenchymal, serous membrane through which small
amounts of interstitial ﬂuid transude continually into
the pleural space. These ﬂuids carry with them t

o
the pleural space. These ﬂuids carry with them tissue
proteins, giving the pleural ﬂuid a mucoid characteris-
tic, which is what allows extremely easy slippage of the
moving lungs.
The total amount of ﬂuid in each pleural cavity is nor-
mally slight, only a few milliliters. Whenever the quan-
tity becomes more than barely enough to begin ﬂowing
in the pleural cavity, the excess ﬂuid is pumped away by
lymphatic vessels opening directly from the pleural
cavity into (1) the mediastinum, (2) the s

pleural
cavity into (1) the mediastinum, (2) the superior surface
of the diaphragm, and (3) the lateral surfaces of the
parietal pleura. Therefore, the pleural space—the space
between the parietal and visceral pleurae—is called a
potential space because it normally is so narrow that it
is not obviously a physical space.
“Negative Pressure” in Pleural Fluid. A negative force is
always required on the outside of the lungs to keep the
lungs expanded. This is provided by negative pressure
in the nor

. This is provided by negative pressure
in the normal pleural space.The basic cause of this neg-
ative pressure is pumping of ﬂuid from the space by the
lymphatics (which is also the basis of the negative pres-
sure found in most tissue spaces of the body). Because
the normal collapse tendency of the lungs is about 
–4 mm Hg, the pleural ﬂuid pressure must always be at
x
x
x
x
x x
x
x
x
x
x
x
x
x
x
x
xxx
xxx
xxx
10
9
8
7
6
5
4
3
2
1
0
0
5
10
15
20
25
Left atrial pressure (mm Hg)
Rate of edema fo

0
25
Rate of edema formation = edema fluid per hour
30
35
40
45
50
dry weight of lung
Figure 38–7
Rate of ﬂuid loss into the lung tissues when the left
atrial pressure (and pulmonary capillary pressure)
is increased. (From Guyton AC, Lindsey AW: Effect
of elevated left atrial pressure and decreased
plasma protein concentration on the development
of pulmonary edema. Circ Res 7:649, 1959.)
490
Unit VII
least as negative as –4 mm Hg to keep the lungs
expande

 as negative as –4 mm Hg to keep the lungs
expanded. Actual measurements have shown that the
pressure is usually about –7 mm Hg, which is a few mil-
limeters of mercury more negative than the collapse
pressure of the lungs. Thus, the negativity of the pleural
ﬂuid keeps the normal lungs pulled against the parietal
pleura of the chest cavity, except for an extremely thin
layer of mucoid ﬂuid that acts as a lubricant.
Pleural Effusion. Pleural effusion means the collection of
large amounts of free

sion means the collection of
large amounts of free ﬂuid in the pleural space.The effu-
sion is analogous to edema ﬂuid in the tissues and can
be called “edema of the pleural cavity.” The causes of
the effusion are the same as the causes of edema in
other tissues (discussed in Chapter 25), including (1)
blockage of lymphatic drainage from the pleural cavity;
(2) cardiac failure, which causes excessively high periph-
eral and pulmonary capillary pressures, leading to
excessive transudation of ﬂuid

essures, leading to
excessive transudation of ﬂuid into the pleural cavity;
(3) greatly reduced plasma colloid osmotic pressure,
thus allowing excessive transudation of ﬂuid; and (4)
infection or any other cause of inﬂammation of the sur-
faces of the pleural cavity, which breaks down the cap-
illary membranes and allows rapid dumping of both
plasma proteins and ﬂuid into the cavity.
Guazz

 of
Guazzi M: Alveolar-capillary membrane dysfunction in
heart failure: evidence of a pathophysiologic role. Chest
124:1090, 2003.
Guyton AC, Lindsey AW: Effect of elevated left atrial pres-
sure and decreased plasma protein concentration on the
development of pulmonary edema. Circ Res 7:649, 1959.
Guyton AC, Parker JC, Taylor AE, et al: Forces governing
water movement in the lung. In: Fishman AP, Renkin EM
(eds): Pulmonary Edema. Baltimore: Waverly Press

M
(eds): Pulmonary Edema. Baltimore: Waverly Press, 1979,
p 65.
Guyton AC,Taylor AE, Granger HJ: Circulatory Physiology.
Hoschele S, Mairbaurl H: Alveolar ﬂooding at high altitude:
failure of reabsorption? News Physiol Sci 18:55, 2003.
Matalon S, Hardiman KM, Jain L, et al: Regulation of ion
channel structure and function by reactive oxygen-

hannel structure and function by reactive oxygen-
nitrogen species. Am J Physiol Lung Cell Mol Physiol
285:L1184, 2003.
Miserocchi G, Negrini D, Passi A, De Luca G: Development
of lung edema: interstitial ﬂuid dynamics and molecular
structure. News Physiol Sci 16:66, 2001.
Schoene RB: Limits of human lung function at high altitude.
J Exp Biol 204:3121, 2001.
Taylor AE, Guyton AC, Bishop VS: Permeability of the alve-
olar membrane to solutes. Circ Res 16:353, 1965.
Wallace J: Update in pulmonary 

 Res 16:353, 1965.
Wallace J: Update in pulmonary diseases. Ann Intern Med
139:499, 2003.
West JB: Respiratory Physiology—The Essentials, 5th ed.
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West JB: Invited review: pulmonary capillary stress failure.
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Artery
Vein
Figure 38–8
Dynamics of ﬂuid exchange in the intrapleural space.
C
H
A
P
T
E
R
 
3
9
491
Physical Principles of Gas
Exchange; Diffusion of Oxygen 
and Carbon Dioxide Through 
the Respirator

Oxygen 
the Respiratory Membrane
After the alveoli are ventilated with fresh air, the
next step in the respiratory process is diffusion of
oxygen from the alveoli into the pulmonary blood
and diffusion of carbon dioxide in the opposite
direction, out of the blood. The process of diffusion
is simply the random motion of molecules inter-
twining their way in all directions through 
the respiratory membrane and adjacent ﬂuids.
However, in respiratory physiology, one is c

uids.
However, in respiratory physiology, one is concerned not only with the basic
mechanism by which diffusion occurs but also with the rate at which it occurs;
this is a much more complex problem, requiring a deeper understanding of the
physics of diffusion and gas exchange.
All the gases of concern in respiratory physiology are simple molecules that are free
to move among one another, which is the process cal

o move among one another, which is the process called “diffusion.”This is also true
of gases dissolved in the ﬂuids and tissues of the body.
For diffusion to occur, there must be a source of energy. This is provided by the
kinetic motion of the molecules themselves. Except at absolute zero temperature,
all molecules of all matter are continually undergoing motion. For free molecules
that are not physically attached to others, this means linear movement at high veloc-
ity until they strike other 

vement at high veloc-
ity until they strike other molecules. Then they bounce away in new directions and
continue until striking other molecules again. In this way, the molecules move
rapidly and randomly among one another.
Net Diffusion of a Gas in One Direction—Effect of a Concentration Gradient. If a gas chamber
or a solution has a high concentration of a particular gas at one end of the chamber
and a low concentration at the other end, as shown in Figure 39–1, net diffusion of
the gas will o

wn in Figure 39–1, net diffusion of
the gas will occur from the high-concentration area toward the low-concentration
area. The reason is obvious: There are far more molecules at end A of the chamber
to diffuse toward end B than there are molecules to diffuse in the opposite direc-
tion. Therefore, the rates of diffusion in each of the two directions are proportion-
ately different, as demonstrated by the lengths of the arrows in the ﬁgure.
Gas Pressures in a Mixture of Gases—“Partial Pressures”

essures in a Mixture of Gases—“Partial Pressures”
of Individual Gases
Pressure is caused by multiple impacts of moving molecules against a surface.There-
fore, the pressure of a gas acting on the surfaces of the respiratory passages and
alveoli is proportional to the summated force of impact of all the molecules of that
gas striking the surface at any given instant. This means that the pressure is directly
proportional to the concentration of the gas molecules.
492
Unit VII
In respir

 gas molecules.
492
Unit VII
In respiratory physiology, one deals with mixtures of
gases, mainly of oxygen, nitrogen, and carbon dioxide.
The rate of diffusion of each of these gases is directly
proportional to the pressure caused by that gas alone,
which is called the partial pressure of that gas. The
concept of partial pressure can be explained as follows.
Consider air, which has an approximate composition
of 79 per cent nitrogen and 21 per cent oxygen. The 
total pressure of this 

d 21 per cent oxygen. The 
total pressure of this mixture at sea level averages 
760 mm Hg. It is clear from the preceding description of
the molecular basis of pressure that each gas contributes
to the total pressure in direct proportion to its concen-
tration. Therefore, 79 per cent of the 760 mm Hg is
caused by nitrogen (600 mm Hg) and 21 per cent by
oxygen (160 mm Hg). Thus, the “partial pressure” of
nitrogen in the mixture is 600 mm Hg, and the “partial
pressure” of oxygen is 160 mm Hg; the

the “partial
pressure” of oxygen is 160 mm Hg; the total pressure is
760 mm Hg, the sum of the individual partial pressures.
The partial pressures of individual gases in a mixture are
designated by the symbols Po2, Pco2, Pn2, Ph2o, Phe, and
so forth.
Pressures of Gases Dissolved in
Water and Tissues
Gases dissolved in water or in body tissues also exert
pressure, because the dissolved gas molecules are
moving randomly and have kinetic energy. Further,
when the gas dissolved in ﬂuid encounters a 

ther,
when the gas dissolved in ﬂuid encounters a surface,
such as the membrane of a cell, it exerts its own partial
pressure in the same way that a gas in the gas phase
does. The partial pressures of the separate dissolved
gases are designated the same as the partial pressures
in the gas state, that is, Po2, Pco2, Pn2, Phe, and so forth.
Factors That Determine the Partial Pressure of a Gas Dissolved in
a Fluid. The partial pressure of a gas in a solution is
determined not only by its concentrat

 solution is
determined not only by its concentration but also by the
solubility coefﬁcient of the gas. That is, some types of
molecules, especially carbon dioxide, are physically or
chemically attracted to water molecules, whereas others
are repelled. When molecules are attracted, far more of
them can be dissolved without building up excess partial
pressure within the solution. Conversely, in the case of
those that are repelled, high partial pressure will
develop with fewer dissolved molecules.

ssure will
develop with fewer dissolved molecules.These relations
are expressed by the following formula, which is Henry’s
law:
Partial pressure
Concentration of dissolved gas
Solubility coefﬁcient
=
When partial pressure is expressed in atmospheres 
(1 atmosphere pressure equals 760 mm Hg) and con-
centration is expressed in volume of gas dissolved in
each volume of water, the solubility coefﬁcients for
important respiratory gases at body temperature are the
Oxygen
0.024
Carbon dioxi

ature are the
Oxygen
0.024
0.57
Carbon monoxide
0.018
Nitrogen
0.012
Helium
0.008
From this table, one can see that carbon dioxide is
more than 20 times as soluble as oxygen. Therefore, the
partial pressure of carbon dioxide (for a given concen-
tration) is less than one twentieth that exerted by
oxygen.
Diffusion of Gases Between the Gas Phase in the Alveoli and the
Dissolved Phase in the Pulmonary Blood. The partial pressure
of each gas in the alveolar respiratory gas

essure
of each gas in the alveolar respiratory gas mixture tends
to force molecules of that gas into solution in the blood
of the alveolar capillaries. Conversely, the molecules of
the same gas that are already dissolved in the blood 
are bouncing randomly in the ﬂuid of the blood, and
some of these bouncing molecules escape back into the
alveoli. The rate at which they escape is directly pro-
portional to their partial pressure in the blood.
But in which direction will net diffusion of the gas

 in which direction will net diffusion of the gas
occur? The answer is that net diffusion is determined by
the difference between the two partial pressures. If the
partial pressure is greater in the gas phase in the alveoli,
as is normally true for oxygen, then more molecules will
diffuse into the blood than in the other direction.Alter-
natively, if the partial pressure of the gas is greater in
the dissolved state in the blood, which is normally true
for carbon dioxide, then net diffusion will 

 true
for carbon dioxide, then net diffusion will occur toward
the gas phase in the alveoli.
When nonhumidiﬁed air is breathed into the respiratory
passageways, water immediately evaporates from the
surfaces of these passages and humidiﬁes the air. This
results from the fact that water molecules, like the dif-
ferent dissolved gas molecules, are continually escaping
from the water surface into the gas phase. The partial
pressure that the water molecules exert to escape
th

essure that the water molecules exert to escape
through the surface is called the vapor pressure of the
water. At normal body temperature, 37°C, this vapor
pressure is 47 mm Hg. Therefore, once the gas mixture
has become fully humidiﬁed—that is, once it is in “equi-
librium” with the water—the partial pressure of the
water vapor in the gas mixture is 47 mm Hg.This partial
pressure, like the other partial pressures, is designated
Ph2o.
The vapor pressure of water depends entirely on the
temperatu

ressure of water depends entirely on the
temperature of the water. The greater the temperature,
the greater the kinetic activity of the molecules and,
therefore, the greater the likelihood that the water mol-
ecules will escape from the surface of the water into the
gas phase. For instance, the water vapor pressure at 
0°C is 5 mm Hg, and at 100°C it is 760 mm Hg. But the
most important value to remember is the vapor pres-
sure at body temperature, 47 mm Hg; this value appears
in many of our sub

e, 47 mm Hg; this value appears
in many of our subsequent discussions.
Dissolved gas molecules
A
B
Figure 39–1
Diffusion of oxygen from one end of a chamber (A) to the other
(B). The difference between the lengths of the arrows represents
net diffusion.
Chapter 39
Physical Principles of Gas Exchange; Diffusion of Oxygen and Carbon Dioxide
493
Diffusion of Gases Through
Fluids—Pressure Difference Causes
Net Diffusion
Now, let us return to the problem of diffusion. From the
preceding discussion, i

lem of diffusion. From the
preceding discussion, it is clear that when the partial
pressure of a gas is greater in one area than in another
area, there will be net diffusion from the high-pressure
area toward the low-pressure area. For instance, re-
turning to Figure 39–1, one can readily see that the mol-
ecules in the area of high pressure, because of their
greater number, have a greater statistical chance of
moving randomly into the area of low pressure than 
do molecules attempting to go in 

w pressure than 
do molecules attempting to go in the other direction.
However, some molecules do bounce randomly from
the area of low pressure toward the area of high pres-
sure.Therefore, the net diffusion of gas from the area of
high pressure to the area of low pressure is equal to the
number of molecules bouncing in this forward direction
minus the number bouncing in the opposite direction;
this is proportional to the gas partial pressure difference
between the two areas, called simply the p

ference
between the two areas, called simply the pressure dif-
ference for causing diffusion.
Quantifying the Net Rate of Diffusion in Fluids. In addition to
the pressure difference, several other factors affect the
rate of gas diffusion in a ﬂuid. They are (1) the solubil-
ity of the gas in the ﬂuid, (2) the cross-sectional area of
the ﬂuid, (3) the distance through which the gas must
diffuse, (4) the molecular weight of the gas, and (5) the
temperature of the ﬂuid. In the body, the last of the

perature of the ﬂuid. In the body, the last of these
factors, the temperature, remains reasonably constant
and usually need not be considered.
The greater the solubility of the gas, the greater the
number of molecules available to diffuse for any given
partial pressure difference. The greater the cross-
sectional area of the diffusion pathway, the greater the
total number of molecules that diffuse. Conversely,
the greater the distance the molecules must diffuse, the
longer it will take the molec

es must diffuse, the
longer it will take the molecules to diffuse the entire dis-
tance. Finally, the greater the velocity of kinetic move-
ment of the molecules, which is inversely proportional
to the square root of the molecular weight, the greater
the rate of diffusion of the gas. All these factors can be
expressed in a single formula, as follows:
in which D is the diffusion rate, DP is the partial pres-
sure difference between the two ends of the diffusion
D
P
A
S
d
MW
µ
¥
¥
¥
D
,
pathway, A

 the diffusion
D
P
A
S
d
MW
µ
¥
¥
¥
D
,
pathway, A is the cross-sectional area of the pathway, S
is the solubility of the gas, d is the distance of diffusion,
and MW is the molecular weight of the gas.
It is obvious from this formula that the characteristics
of the gas itself determine two factors of the formula:
solubility and molecular weight. Together, these two
factors determine the diffusion coefﬁcient of the gas,
which is proportional to S/
. That is, the relative
rates at which different 

rates at which different gases at the same partial pres-
sure levels will diffuse are proportional to their diffu-
sion coefﬁcients.Assuming that the diffusion coefﬁcient
for oxygen is 1, the relative diffusion coefﬁcients for dif-
ferent gases of respiratory importance in the body ﬂuids
Oxygen
1.0
20.3
0.81
Nitrogen
0.53
Helium
0.95
The gases that are of respiratory importance are all
high

es that are of respiratory importance are all
highly soluble in lipids and, consequently, are highly
soluble in cell membranes. Because of this, the major
limitation to the movement of gases in tissues is the rate
at which the gases can diffuse through the tissue water
instead of through the cell membranes. Therefore, dif-
fusion of gases through the tissues, including through
the respiratory membrane, is almost equal to the diffu-
sion of gases in water, as given in the preceding list.
Composit

in water, as given in the preceding list.
Composition of Alveolar 
Air—Its Relation to
Alveolar air does not have the same concentrations of
gases as atmospheric air by any means, which can
readily be seen by comparing the alveolar air compo-
sition in Table 39–1 with that of atmospheric air.There
are several reasons for the differences. First, the alve-
olar air is only partially replaced by atmospheric air
with each breath. Second, oxygen is constantly being
absorbed into the p

nd, oxygen is constantly being
absorbed into the pulmonary blood from the alveolar
air. Third, carbon dioxide is constantly diffusing from
the pulmonary blood into the alveoli. And fourth, dry
MW
Table 39–1
Partial Pressures of Respiratory Gases as They Enter and Leave the Lungs (at Sea Level)
Atmospheric Air*
Humidiﬁed Air
Alveolar Air
(mm Hg)
(mm Hg)
(mm Hg)
(mm Hg)
N2
597.0
(78.62%)
563.4
(74.09%)
569.0
(74.9%)
566.0
(74.5%)
O2
159.0
(20.84%)
149.3
(19.67%)
104.0
(13.6%)
120.0
(15

.0
(20.84%)
149.3
(19.67%)
104.0
(13.6%)
120.0
(15.7%)
CO2
0.3
(0.04%)
0.3
(0.04%)
40.0
(5.3%)
27.0
(3.6%)
H2O
3.7
(0.50%)
47.0
(6.20%)
47.0
(6.2%)
47.0
(6.2%)
TOTAL
760.0
(100.0%)
760.0
(100.0%)
760.0
(100.0%)
760.0
(100.0%)
* On an average cool, clear day.
494
Unit VII
atmospheric air that enters the respiratory passages is
humidiﬁed even before it reaches the alveoli.
Humidiﬁcation of the Air in the Respiratory Passages. Table
39–1 shows that atmospheric air is composed almost
ent

 shows that atmospheric air is composed almost
entirely of nitrogen and oxygen; it normally contains
almost no carbon dioxide and little water vapor.
However,as soon as the atmospheric air enters the res-
piratory passages, it is exposed to the ﬂuids that cover
the respiratory surfaces. Even before the air enters the
alveoli, it becomes (for all practical purposes) totally
humidiﬁed.
The partial pressure of water vapor at a normal
body temperature of 37°C is 47 mm Hg, which is there-
fore the pa

e of 37°C is 47 mm Hg, which is there-
fore the partial pressure of water vapor in the alveo-
lar air. Because the total pressure in the alveoli 
cannot rise to more than the atmospheric pressure
(760 mm Hg at sea level), this water vapor simply
dilutes all the other gases in the inspired air.Table 39–1
also shows that humidiﬁcation of the air dilutes the
oxygen partial pressure at sea level from an average
of 159 mm Hg in atmospheric air to 149 mm Hg in the
humidiﬁed air, and it dilutes the nit

mm Hg in the
humidiﬁed air, and it dilutes the nitrogen partial pres-
sure from 597 to 563 mm Hg.
Rate at Which Alveolar Air Is Renewed
by Atmospheric Air
In Chapter 37, it was pointed out that the average male
functional residual capacity of the lungs (the volume
of air remaining in the lungs at the end of normal expi-
ration) measures about 2300 milliliters. Yet only 350
milliliters of new air is brought into the alveoli with
each normal inspiration, and this same amount of old
alveolar air is

ation, and this same amount of old
alveolar air is expired. Therefore, the volume of alve-
olar air replaced by new atmospheric air with each
breath is only one seventh of the total, so that multi-
ple breaths are required to exchange most of the alve-
olar air. Figure 39–2 shows this slow rate of renewal of
the alveolar air. In the ﬁrst alveolus of the ﬁgure, an
excess amount of a gas is present in the alveoli, but
note that even at the end of 16 breaths, the excess 
gas still has not been comp

6 breaths, the excess 
gas still has not been completely removed from the
alveoli.
Figure 39–3 demonstrates graphically the rate at
which excess gas in the alveoli is normally removed,
showing that with normal alveolar ventilation, about
one half the gas is removed in 17 seconds. When a
person’s rate of alveolar ventilation is only one half
normal, one half the gas is removed in 34 seconds, and
when the rate of ventilation is twice normal, one half
is removed in about 8 seconds.
Importance of th

lf
Importance of the Slow Replacement of Alveolar Air. The slow
replacement of alveolar air is of particular importance
in preventing sudden changes in gas concentrations in
the blood. This makes the respiratory control mecha-
nism much more stable than it would be otherwise, and
it helps prevent excessive increases and decreases in
tissue oxygenation, tissue carbon dioxide concentra-
tion, and tissue pH when respiration is temporarily
interrupted.
Oxygen Concentra

ation is temporarily
Oxygen Concentration and Partial
Pressure in the Alveoli
Oxygen is continually being absorbed from the alveoli
into the blood of the lungs, and new oxygen is contin-
ually being breathed into the alveoli from the atmos-
phere.The more rapidly oxygen is absorbed, the lower
its concentration in the alveoli becomes; conversely,
the more rapidly new oxygen is breathed into the
alveoli from the atmosphere, the higher its concentra-
tion becomes. Therefore, oxygen con

its concentra-
tion becomes. Therefore, oxygen concentration in the
alveoli, and its partial pressure as well, is controlled by
(1) the rate of absorption of oxygen into the blood and
(2) the rate of entry of new oxygen into the lungs by
the ventilatory process.
1st breath
2nd breath
3rd breath
8th breath
4th breath
12th breath
16th breath
Figure 39–2
Expiration of a gas from an alveolus with successive breaths.
0
100 
80 
60 
40 
20 
0
10
40
50
30
20
60
Concentration of gas
(per cent of origina

30
20
60
(per cent of original concentration)
1/2 no
rmal alv
eolar
ven
tila
tio
n
2 ¥
nor
m
a
l
a
l
v
e
ol
ar
ven
tilation
Nor
mal
alv
eol
ar
ve
nti
lat
io
n
Figure 39–3
Rate of removal of excess gas from alveoli.
Chapter 39
495
Figure 39–4 shows the effect of both alveolar venti-
lation and rate of oxygen absorption into the blood on
the alveolar partial pressure of oxygen (Po2). One

curve represents oxygen absorption at a rate of 
250 ml/min, and the other curve represents a rate of
1000 ml/min.At a normal ventilatory rate of 4.2 L/min
and an oxygen consumption of 250 ml/min, the normal
operating point in Figure 39–4 is point A. The ﬁgure
also shows that when 1000 milliliters of oxygen is being
absorbed each minute, as occurs during moderate
exercise, the rate of alveolar ventilation must increase
fourfold to maintain the a

ntilation must increase
fourfold to maintain the alveolar Po2 at the normal
value of 104 mm Hg.
Another effect shown in Figure 39–4 is that an
extremely marked increase in alveolar ventilation can
never increase the alveolar Po2 above 149 mm Hg as
long as the person is breathing normal atmospheric 
air at sea level pressure, because this is the maximum
Po2 in humidiﬁed air at this pressure. If the person
breathes gases that contain partial pressures of oxygen
higher than 149 mm Hg, the alveolar 

res of oxygen
higher than 149 mm Hg, the alveolar Po2 can approach
these higher pressures at high rates of ventilation.
CO2 Concentration and Partial
Carbon dioxide is continually being formed in the
body and then carried in the blood to the alveoli; it is
continually being removed from the alveoli by venti-
lation. Figure 39–5 shows the effects on the alveolar
partial pressure of carbon dioxide (Pco2) of both alve-
olar ventilation and two rates of carbon dioxide excre-

entilation and two rates of carbon dioxide excre-
tion, 200 and 800 ml/min. One curve represents a
normal rate of carbon dioxide excretion of 200 ml/min.
At the normal rate of alveolar ventilation of 4.2 L/min,
the operating point for alveolar Pco2 is at point A in
Figure 39–5—that is, 40 mm Hg.
Two other facts are also evident from Figure 39–5:
First, the alveolar PCO2 increases directly in proportion
to the rate of carbon dioxide excretion, as represented
by the fourfold elevation of the curve

represented
by the fourfold elevation of the curve (when 800 
milliliters of CO2 are excreted per minute). Second,
the alveolar PCO2 decreases in inverse proportion to
alveolar ventilation. Therefore, the concentrations and
partial pressures of both oxygen and carbon dioxide in
the alveoli are determined by the rates of absorption
or excretion of the two gases and by the amount of
alveolar ventilation.
Expired air is a combination of dead space air and alve-
olar air; its overall com

dead space air and alve-
olar air; its overall composition is therefore determined
0
150 
125 
100 
75 
50 
25 
0
10
15
25
5
20
40
35
30
Upper limit at maximum ventilation
250 ml O2/min
1000 ml O2/min
Normal alveolar PO2
Alveolar partial pressure
of oxygen (mm Hg)
Alveolar ventilation (L/min)
A
Figure 39–4
Effect of alveolar ventilation on the alveolar PO2 at
two rates of oxygen absorption from the alveoli—
250 ml/min and 1000 ml/min. Point A is the normal
operating point.
Normal alveolar PCO2
0

the normal
0
175 
150 
125 
100 
75 
50 
25 
0
10
15
25
5
20
40
35
30
A
200 ml CO2/min
800 ml CO2/min
of CO2 (mm Hg)
Figure 39–5
Effect of alveolar ventilation on the alveolar PCO2 at two rates of
carbon dioxide excretion from the blood—800 ml/min and 200 ml/
min. Point A is the normal operating point.
496
Unit VII
by (1) the amount of the expired air that is dead space
air and (2) the amount

ired air that is dead space
air and (2) the amount that is alveolar air. Figure 39–6
shows the progressive changes in oxygen and carbon
dioxide partial pressures in the expired air during the
course of expiration. The ﬁrst portion of this air, the
dead space air from the respiratory passageways, is
typical humidiﬁed air, as shown in Table 39–1.Then, pro-
gressively more and more alveolar air becomes mixed
with the dead space air until all the dead space air has
ﬁnally been washed out and nothing

d space air has
ﬁnally been washed out and nothing but alveolar air is
expired at the end of expiration. Therefore, the method
of collecting alveolar air for study is simply to collect 
a sample of the last portion of the expired air after
forceful expiration has removed all the dead space 
air.
Normal expired air, containing both dead space air
and alveolar air, has gas concentrations and partial 
pressures approximately as shown in Table 39–1—that
is, concentrations between those of alveolar a

hat
is, concentrations between those of alveolar air and
humidiﬁed atmospheric air.
Respiratory Unit. Figure 39–7 shows the respiratory unit
(also called “respiratory lobule”), which is composed
of a respiratory bronchiole, alveolar ducts, atria, and
alveoli. There are about 300 million alveoli in the two
lungs, and each alveolus has an average diameter of
about 0.2 millimeter. The alveolar walls are extremely
thin, and between the alveoli is a

s are extremely
thin, and between the alveoli is an almost solid
network of interconnecting capillaries, shown in
Figure 39–8. Indeed, because of the extensiveness of
the capillary plexus, the ﬂow of blood in the alveolar
wall has been described as a “sheet” of ﬂowing blood.
Thus, it is obvious that the alveolar gases are in very
close proximity to the blood of the pulmonary capil-
laries. Further, gas exchange between the alveolar air
and the pulmonary blood occurs through the mem-
branes of al

lmonary blood occurs through the mem-
branes of all the terminal portions of the lungs, not
merely in the alveoli themselves.All these membranes
are collectively known as the respiratory membrane,
also called the pulmonary membrane.
Carbon dioxide (PCO2)
Oxygen  (PO2)
0
160 
140 
120 
100 
80 
60 
40 
20 
0
100
200
Milliliters of air expired
Pressures of O2 and CO2
(mm Hg)
500
400
300
Alveolar air
and dead
space air
Dead
space
air
Figure 39–6
Oxygen and carbon dioxide partial pressu

gure 39–6
Oxygen and carbon dioxide partial pressures in the
various portions of normal expired air.
Atrium
Alveoli
Terminal bronchiole
Alveolar
duct
Alveolar sacs
Respiratory bronchiole
Figure 39–7
Respiratory unit. (Redrawn from Miller WS: The Lung. Springﬁeld,
Ill: Charles C Thomas, 1947.)
Chapter 39
497
Respiratory Membrane. Figure 39–9 shows the ultrastruc-
ture of the respiratory membrane drawn in cross
section on 

he respiratory membrane drawn in cross
section on the left and a red blood cell on the right. It
also shows the diffusion of oxygen from the alveolus
into the red blood cell and diffusion of carbon dioxide
in the opposite direction. Note the following different
layers of the respiratory membrane:
1. A layer of ﬂuid lining the alveolus and containing
surfactant that reduces the surface tension of the
alveolar ﬂuid
2. The alveolar epithelium composed of thin
3. An epithelial basem

ed of thin
3. An epithelial basement membrane
4. A thin interstitial space between the alveolar
epithelium and the capillary membrane
5. A capillary basement membrane that in many
places fuses with the alveolar epithelial basement
membrane
6. The capillary endothelial membrane
Despite the large number of layers, the overall
thickness of the respiratory membrane in some areas
is as little as 0.2 micrometer, and it averages about 0.6
micrometer, except where there are cell nuclei.

.6
micrometer, except where there are cell nuclei. From
histological studies, it has been estimated that the total
surface area of the respiratory membrane is about 70
square meters in the normal adult human male.This is
equivalent to the ﬂoor area of a 25–by-30–foot room.
The total quantity of blood in the capillaries of the
lungs at any given instant is 60 to 140 milliliters. Now
imagine this small amount of blood spread over the
entire surface of a 25–by-30–foot ﬂoor, and it is easy
to unders

 of a 25–by-30–foot ﬂoor, and it is easy
to understand the rapidity of the respiratory exchange
of oxygen and carbon dioxide.
The average diameter of the pulmonary capillaries
is only about 5 micrometers, which means that red
blood cells must squeeze through them.The red blood
cell membrane usually touches the capillary wall, so
that oxygen and carbon dioxide need not pass through
signiﬁcant amounts of plasma as they diffuse between
the alveolus and the red cell. This, too, increases the
rapidit

and the red cell. This, too, increases the
rapidity of diffusion.
Vein
Artery
Perivascular
interstitial space
Interstitial space
Lymphatic
vessel
Alveolus
Alveolus
Alveolus
B
A
Alveolus
Figure 39–8
A, Surface view of capillaries in an alveolar wall. B, Cross-sec-
tional view of alveolar walls and their vascular supply. (A, From
Maloney JE, Castle BL: Pressure-diameter relations of capillaries
and small blood vessels in frog lung. Respir Physiol 7:150, 1969.
Reproduced by permission o

ir Physiol 7:150, 1969.
Reproduced by permission of ASP Biological and Medical Press,
North-Holland Division.)
Capillary basement membrane
Capillary endothelium
Epithelial
basement
membrane
Alveolar
Fluid and
surfactant
layer
Alveolus
Capillary
Diffusion
Diffusion
Oxygen
Red blood
cell
Figure 39–9
Ultrastructure of the alveolar respiratory membrane, shown in
cross section.
498
Unit VII
Factors That Affect the Rate 
of Gas Diffusion Through

ors That Affect the Rate 
of Gas Diffusion Through the
Referring to the earlier discussion of diffusion of gases
in water, one can apply the same principles and math-
ematical formulas to diffusion of gases through the
respiratory membrane. Thus, the factors that deter-
mine how rapidly a gas will pass through the mem-
brane are (1) the thickness of the membrane, (2) the
surface area of the membrane, (3) the diffusion coefﬁ-
cient of the gas in the substance of the membrane,

cient of the gas in the substance of the membrane, and
(4) the partial pressure difference of the gas between
the two sides of the membrane.
The thickness of the respiratory membrane occa-
sionally increases—for instance, as a result of edema
ﬂuid in the interstitial space of the membrane and in
the alveoli—so that the respiratory gases must then
diffuse not only through the membrane but also
through this ﬂuid. Also, some pulmonary diseases
cause ﬁbrosis of the lungs, which can increase the
thic

 ﬁbrosis of the lungs, which can increase the
thickness of some portions of the respiratory mem-
brane. Because the rate of diffusion through the mem-
brane is inversely proportional to the thickness of the
membrane, any factor that increases the thickness to
more than two to three times normal can interfere sig-
niﬁcantly with normal respiratory exchange of gases.
The surface area of the respiratory membrane can be
greatly decreased by many conditions. For instance,
removal of an entire lung de

itions. For instance,
removal of an entire lung decreases the total surface
area to one half normal. Also, in emphysema, many of
the alveoli coalesce, with dissolution of many alveolar
walls. Therefore, the new alveolar chambers are much
larger than the original alveoli, but the total surface
area of the respiratory membrane is often decreased
as much as ﬁvefold because of loss of the alveolar
walls. When the total surface area is decreased to
about one third to one fourth normal, exchange of
ga

out one third to one fourth normal, exchange of
gases through the membrane is impeded to a signiﬁ-
cant degree, even under resting conditions, and during
competitive sports and other strenuous exercise, even
the slightest decrease in surface area of the lungs can
be a serious detriment to respiratory exchange of
gases.
The diffusion coefﬁcient for transfer of each gas
through the respiratory membrane depends on the
gas’s solubility in the membrane and, inversely, on the
square root of the gas’s 

e and, inversely, on the
square root of the gas’s molecular weight. The rate 
of diffusion in the respiratory membrane is almost
exactly the same as that in water, for reasons explained
earlier. Therefore, for a given pressure difference,
carbon dioxide diffuses about 20 times as rapidly as
oxygen. Oxygen diffuses about twice as rapidly as
nitrogen.
The pressure difference across the respiratory mem-
brane is the difference between the partial pressure of
the gas in the alveoli and the partial p

essure of
the gas in the alveoli and the partial pressure of the
gas in the pulmonary capillary blood.The partial pres-
sure represents a measure of the total number of mol-
ecules of a particular gas striking a unit area of the
alveolar surface of the membrane in unit time, and the
pressure of the gas in the blood represents the number
of molecules that attempt to escape from the blood 
in the opposite direction. Therefore, the difference
between these two pressures is a measure of the net
tend

n these two pressures is a measure of the net
tendency for the gas molecules to move through the
membrane. When the partial pressure of a gas in the
alveoli is greater than the pressure of the gas in 
the blood, as is true for oxygen, net diffusion from the
alveoli into the blood occurs; when the pressure of the
gas in the blood is greater than the partial pressure in
the alveoli, as is true for carbon dioxide, net diffusion
from the blood into the alveoli occurs.
Diffusing Capacity of the 
Resp

he alveoli occurs.
The ability of the respiratory membrane to exchange
a gas between the alveoli and the pulmonary blood 
is expressed in quantitative terms by the respiratory
membrane’s diffusing capacity, which is deﬁned as 
the volume of a gas that will diffuse through the mem-
brane each minute for a partial pressure difference of
1 mm Hg. All the factors discussed earlier that affect
diffusion through the respiratory membrane can affect
this d

through the respiratory membrane can affect
this diffusing capacity.
Diffusing Capacity for Oxygen. In the average young man,
the diffusing capacity for oxygen under resting condi-
tions averages 21 ml/min/mm Hg. In functional terms,
what does this mean? The mean oxygen pressure 
difference across the respiratory membrane during
normal, quiet breathing is about 11 mm Hg. Multipli-
cation of this pressure by the diffusing capacity (11 ¥
21) gives a total of about 230 milliliters of oxygen dif-
fu

a total of about 230 milliliters of oxygen dif-
fusing through the respiratory membrane each minute;
this is equal to the rate at which the resting body uses
oxygen.
Change in Oxygen Diffusing Capacity During Exercise.
During strenuous exercise or other conditions that
greatly increase pulmonary blood ﬂow and alveolar
ventilation, the diffusing capacity for oxygen increases
in young men to a maximum of about 65 ml/min/
mm Hg, which is three times the diffusing capacity
under resting conditions. 

 the diffusing capacity
under resting conditions. This increase is caused by
several factors, among which are (1) opening up of
many previously dormant pulmonary capillaries or
extra dilation of already open capillaries, thereby
increasing the surface area of the blood into which the
oxygen can diffuse; and (2) a better match between 
the ventilation of the alveoli and the perfusion of the
alveolar capillaries with blood, called the ventilation-
perfusion ratio, which is explained in detail late

perfusion ratio, which is explained in detail later in 
this chapter.Therefore, during exercise, oxygenation of
the blood is increased not only by increased alveolar
ventilation but also by greater diffusing capacity of 
the respiratory membrane for transporting oxygen into
the blood.
Diffusing Capacity for Carbon Dioxide. The diffusing capac-
ity for carbon dioxide has never been measured
Chapter 39
499
because of the f

 of Oxygen and Carbon Dioxide
499
because of the following technical difﬁculty: Carbon
dioxide diffuses through the respiratory membrane so
rapidly that the average Pco2 in the pulmonary blood
is not far different from the Pco2 in the alveoli—the
average difference is less than 1 mm Hg—and with the
available techniques, this difference is too small to be
measured.
Nevertheless, measurements of diffusion of other
gases have shown that the diffusing capacity varies
directly with the diffusion coef

g capacity varies
directly with the diffusion coefﬁcient of the particular
gas.Because the diffusion coefﬁcient of carbon dioxide
is slightly more than 20 times that of oxygen, one
would expect a diffusing capacity for carbon dioxide
under resting conditions of about 400 to 450 ml/min/
mm Hg and during exercise of about 1200 to 1300 ml/
min/mm Hg. Figure 39–10 compares the measured or
calculated diffusing capacities of carbon monoxide,
oxygen, and carbon dioxide at rest and during exercise,
show

d carbon dioxide at rest and during exercise,
showing the extreme diffusing capacity of carbon
dioxide and the effect of exercise on the diffusing
capacity of each of these gases.
Measurement of Diffusing Capacity—The Carbon Monoxide
Method. The oxygen diffusing capacity can be calculated
from measurements of (1) alveolar Po2, (2) Po2 in the
pulmonary capillary blood, and (3) the rate of oxygen
uptake by the blood. However, measuring the Po2 in the
pulmonary capillary blood is so difﬁcult and so

he
pulmonary capillary blood is so difﬁcult and so impre-
cise that it is not practical to measure oxygen diffusing
capacity by such a direct procedure, except on an exper-
imental basis.
To obviate the difﬁculties encountered in measuring
oxygen diffusing capacity directly, physiologists usually
measure carbon monoxide diffusing capacity instead
and then calculate the oxygen diffusing capacity from
this. The principle of the carbon monoxide method is 
the following: A small amount of carbon mon

od is 
the following: A small amount of carbon monoxide is
breathed into the alveoli, and the partial pressure of the
carbon monoxide in the alveoli is measured from appro-
priate alveolar air samples. The carbon monoxide pres-
sure in the blood is essentially zero,because hemoglobin
combines with this gas so rapidly that its pressure never
has time to build up. Therefore, the pressure difference
of carbon monoxide across the respiratory membrane is
equal to its partial pressure in the alveolar 

 is
equal to its partial pressure in the alveolar air sample.
Then, by measuring the volume of carbon monoxide
absorbed in a short period and dividing this by the 
alveolar carbon monoxide partial pressure, one can
determine accurately the carbon monoxide diffusing
capacity.
To convert carbon monoxide diffusing capacity to
oxygen diffusing capacity, the value is multiplied by a
factor of 1.23 because the diffusion coefﬁcient for
oxygen is 1.23 times that for carbon monoxide. Thus,
the average di

mes that for carbon monoxide. Thus,
the average diffusing capacity for carbon monoxide 
in young men at rest is 17 ml/min/mm Hg, and the 
diffusing capacity for oxygen is 1.23 times this, or 
21 ml/min/mm Hg.
Effect of the Ventilation-
Perfusion Ratio on Alveolar
Gas Concentration
In the early part of this chapter, we learned that two
factors determine the Po2 and the Pco2 in the alveoli: (1)
the rate of alveolar ventilation and (2) the rate of trans-
fer of oxygen and carbon dioxide through the

rans-
fer of oxygen and carbon dioxide through the respira-
tory membrane. These earlier discussions made the
assumption that all the alveoli are ventilated equally
and that blood ﬂow through the alveolar capillaries is
the same for each alveolus. However, even normally to
some extent, and especially in many lung diseases, some
areas of the lungs are well ventilated but have almost
no blood ﬂow, whereas other areas may have excellent
blood ﬂow but little or no ventilation. In either of these
con

t little or no ventilation. In either of these
conditions, gas exchange through the respiratory mem-
brane is seriously impaired, and the person may suffer
severe respiratory distress despite both normal total
ventilation and normal total pulmonary blood ﬂow, but
with the ventilation and blood ﬂow going to different
parts of the lungs. Therefore, a highly quantitative
concept has been developed to help us understand res-
piratory exchange when there is imbalance between
alveolar ventilation and 

ere is imbalance between
alveolar ventilation and alveolar blood ﬂow. This
concept is called the ventilation-perfusion ratio.
In quantitative terms, the ventilation-perfusion ratio
is expressed as V
.
a/Q
.
. When V
.
a (alveolar ventilation) is
normal for a given alveolus and Q
.
(blood ﬂow) is also
normal for the same alveolus, the ventilation-perfusion
ratio (V
.
a/Q
.
) is also said to be normal. When the venti-
lation (V
.
a) is zero, yet there is still perfusion (Q
.
) of the
alveolus, the

ere is still perfusion (Q
.
) of the
alveolus, the V
.
a/Q
.
is zero. Or, at the other extreme,
when there is adequate ventilation (V
.
a) but zero perfu-
sion (Q
.
), the ratio V
.
a/Q
.
is inﬁnity. At a ratio of either
zero or inﬁnity, there is no exchange of gases through
the respiratory membrane of the affected alveoli, which
1300 
1200 
1100 
1000 
900 
800 
700 
600 
500 
400 
300 
200 
100 
0
CO2
O2
CO
Resting
Exercise
Diffusing capacity (ml/min/mm Hg)
Figure 39–10
Diffusing capacities fo

ml/min/mm Hg)
Diffusing capacities for carbon monoxide, oxygen, and carbon
dioxide in the normal lungs under resting conditions and during
exercise.
500
Unit VII
explains the importance of this concept. Therefore, let
us explain the respiratory consequences of these two
extremes.
Alveolar Oxygen and Carbon Dioxide Partial Pressures When V
.
A/Q
.
Equals Zero. When V
.
a/Q
.
is equal to zero—that is, without
any alveolar ventilation—the air in the alveolus 
comes to equil

entilation—the air in the alveolus 
comes to equilibrium with the blood oxygen and carbon
dioxide because these gases diffuse between the blood
and the alveolar air. Because the blood that perfuses 
the capillaries is venous blood returning to the lungs
from the systemic circulation, it is the gases in this 
blood with which the alveolar gases equilibrate. In
Chapter 40, we will learn that the normal venous blood
(v¯) has a Po2 of 40 mm Hg and a Pco2 of 45 mm Hg.
Therefore, these are also the no

Pco2 of 45 mm Hg.
Therefore, these are also the normal partial pressures of
these two gases in alveoli that have blood ﬂow but no
.
A/Q
.
Equals Inﬁnity. The effect on the alveolar gas partial pres-
sures when V
.
a/Q
.
equals inﬁnity is entirely different
from the effect when V
.
a/Q
.
equals zero because now
there is no capillary blood ﬂow to carry oxygen away or
to bring carbon dioxide to the alveoli.Therefore, instead
o

carbon dioxide to the alveoli.Therefore, instead
of the alveolar gases coming to equilibrium with the
venous blood, the alveolar air becomes equal to the
humidiﬁed inspired air. That is, the air that is inspired
loses no oxygen to the blood and gains no carbon
dioxide from the blood. And because normal inspired
and humidiﬁed air has a Po2 of 149 mm Hg and a Pco2
of 0 mm Hg, these will be the partial pressures of these
two gases in the alveoli.
Gas Exchange and Alveolar Partial Pressures When V
.

s Exchange and Alveolar Partial Pressures When V
.
A/Q
.
Is
Normal. When there is both normal alveolar ventilation
and normal alveolar capillary blood ﬂow (normal alve-
olar perfusion), exchange of oxygen and carbon dioxide
through the respiratory membrane is nearly optimal,
and alveolar Po2 is normally at a level of 104 mm Hg,
which lies between that of the inspired air (149 mm Hg)
and that of venous blood (40 mm Hg). Likewise,
alveolar Pco2 lies between two extremes; it is normally
40 mm Hg,in

s between two extremes; it is normally
40 mm Hg,in contrast to 45 mm Hg in venous blood and
0 mm Hg in inspired air.Thus, under normal conditions,
the alveolar air Po2 averages 104 mm Hg and the Pco2
averages 40 mm Hg.
.
A/Q
.
Diagram
The concepts presented in the preceding sections can be
shown in graphical form, as demonstrated in Figure
39–11, called the Po2-Pco2, V
.
a/Q
.
diagram. The curve in
the diagram represents all possible Po2 and Pco2 com-
binations between the limits of 

Po2 and Pco2 com-
binations between the limits of V
.
a/Q
.
equals zero and 
V
.
a/Q
.
equals inﬁnity when the gas pressures in the
venous blood are normal and the person is breathing 
air at sea-level pressure. Thus, point v¯ is the plot of Po2
and Pco2 when V
.
a/Q
.
equals zero. At this point, the Po2
is 40 mm Hg and the Pco2 is 45 mm Hg, which are the
values in normal venous blood.
At the other end of the curve,when V
.
a/Q
.
equals inﬁn-
ity, point I represents inspired air, showing Po2 to 

, point I represents inspired air, showing Po2 to be 149
mm Hg while Pco2 is zero. Also plotted on the curve 
is the point that represents normal alveolar air when
V
.
a/Q
.
is normal. At this point, Po2 is 104 mm Hg and
Pco2 is 40 mm Hg.
Concept of “Physiologic Shunt”
(When V
.
A/Q
.
Is Below Normal)
Whenever V
.
a/Q
.
is below normal, there is inadequate
ventilation to provide the oxygen needed to fully oxy-
genate the blood ﬂowing through the alveolar capil-
laries. Therefore, a certain fract

alveolar capil-
laries. Therefore, a certain fraction of the venous 
blood passing through the pulmonary capillaries does
not become oxygenated. This fraction is called shunted
blood. Also, some additional blood ﬂows through
bronchial vessels rather than through alveolar capillar-
ies, normally about 2 per cent of the cardiac output; this,
too, is unoxygenated, shunted blood.
The total quantitative amount of shunted blood per
minute is called the physiologic shunt. This physiologic
shunt is meas

 physiologic shunt. This physiologic
shunt is measured in clinical pulmonary function labo-
ratories by analyzing the concentration of oxygen in
both mixed venous blood and arterial blood, along with
simultaneous measurement of cardiac output. From
these values, the physiologic shunt can be calculated by
the following equation:
in which Q
.
ps is the physiologic shunt blood ﬂow per
minute, Q
.
t is cardiac output per minute, CiO2 is the con-
centration of oxygen in the arterial blood if there is

ration of oxygen in the arterial blood if there is an
“ideal” ventilation-perfusion ratio, CaO2 is the measured
concentration of oxygen in the arterial blood, and Cv¯O2
is the measured concentration of oxygen in the mixed
venous blood.
The greater the physiologic shunt, the greater the
amount of blood that fails to be oxygenated as it passes
through the lungs.
Concept of the “Physiologic Dead
Space” (When V
.
A/Q
.
Is Greater 
Than Normal)
When ventilation of some of the alveoli is great but
alv

entilation of some of the alveoli is great but
alveolar blood ﬂow is low, there is far more available
oxygen in the alveoli than can be transported away from
Q
.
Q
.
Ci
Ca
Ci
C
O
O
O
O
2
2
2
2
ps
t
v
=
-
-
.
50 
40 
30 
20 
10
0
20
40
60
80
100 120 140 160
(PO2 = 40)
(PCO2 = 45)
Normal alveolar
air
(PO2 = 104)
(PCO2 = 40)
(PO2 = 149)
(PCO2 = 0)
I
VA /Q = 0
VA /Q = Normal
v
VA /Q = ∞
PCO2 (mm Hg)
PO2 (mm Hg)
Figure 39–11
Normal PO2-PCO2, V
.
A/Q
.
diagram.
Chapter 39
Physical Principles of Gas Ex

diagram.
Chapter 39
501
the alveoli by the ﬂowing blood.Thus, the ventilation of
these alveoli is said to be wasted. The ventilation of the
anatomical dead space areas of the respiratory pas-
sageways is also wasted. The sum of these two types of
wasted ventilation is called the physiologic dead space.
This is measured in the clinical pulmonary function lab-
oratory by making appropriate blood and expiratory
gas measure

aking appropriate blood and expiratory
gas measurements and using the following equation,
called the Bohr equation:
in which Vdphys is the physiologic dead space, Vt is the
tidal volume, PaCO2 is the partial pressure of carbon
dioxide in the arterial blood, and 
is the average
partial pressure of carbon dioxide in the entire expired
air.
When the physiologic dead space is great, much of the
work of ventilation is wasted effort because so much of
the ventilating air never reaches the blood.
Abnor

Abnormalities of Ventilation-
Perfusion Ratio
Abnormal V
.
A/Q
.
in the Upper and Lower Normal Lung. In a
normal person in the upright position, both pulmonary
capillary blood ﬂow and alveolar ventilation are con-
siderably less in the upper part of the lung than in the
lower part; however, blood ﬂow is decreased consider-
ably more than ventilation is. Therefore, at the top of
the lung, V
.
a/Q
.
is as much as 2.5 times as great as the
ideal value, w

s much as 2.5 times as great as the
ideal value, which causes a moderate degree of physio-
logic dead space in this area of the lung.
At the other extreme, in the bottom of the lung, there
is slightly too little ventilation in relation to blood ﬂow,
with V
.
a/Q
.
as low as 0.6 times the ideal value. In this
area, a small fraction of the blood fails to become 
normally oxygenated, and this represents a physiologic
shunt.
In both extremes, inequalities of ventilation and 
perfusion decrease sligh

ities of ventilation and 
perfusion decrease slightly the lung’s effectiveness 
for exchanging oxygen and carbon dioxide. However,
during exercise, blood ﬂow to the upper part of the lung
increases markedly, so that far less physiologic dead
space occurs, and the effectiveness of gas exchange now
approaches optimum.
Abnormal V
.
A/Q
.
in Chronic Obstructive Lung Disease. Most
people who smoke for many years develop various
degrees of bronchial obstruction; in a large share of
these persons, this

struction; in a large share of
these persons, this condition eventually becomes so
severe that they develop serious alveolar air trapping
and resultant emphysema. The emphysema in turn
causes many of the alveolar walls to be destroyed.Thus,
P
CO2
e
V
V
Pa
P
Pa
phys
CO
CO
CO
2
2
2
d
t
e
=
-
,
two abnormalities occur in smokers to cause abnormal
V
.
a/Q
.
. First, because many of the small bronchioles are
obstructed, the alveoli beyond the obstructions are
unventilated, causing a V
.
a/Q
.
that ap

ions are
.
a/Q
.
that approaches zero.
Second, in those areas of the lung where the alveolar
walls have been mainly destroyed but there is still 
alveolar ventilation, most of the ventilation is wasted
because of inadequate blood ﬂow to transport the blood
gases.
Thus, in chronic obstructive lung disease, some areas
of the lung exhibit serious physiologic shunt, and other
areas exhibit serious physiologic dead space. Both these
conditions tremendously decrease the effec

h these
conditions tremendously decrease the effectiveness of
the lungs as gas exchange organs, sometimes reducing
their effectiveness to as little as one tenth normal. In
fact, this is the most prevalent cause of pulmonary dis-
ability today.
Albert R, Spiro S, Jett J: Comprehensive Respiratory Medi-
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Cole RP: CO2 and lung mechanical or gas exchange func-
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eolar-capillary membrane dysfunction in
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 Sec 3,Vol 1. Baltimore:Williams
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502
Transport of Oxygen and Carbon
Once oxygen has diffused from the alveoli into 
the pulmonary blood, it is transported to the
peripheral tissue capillaries almost entirely in com-
bination with hemoglobin. The presence of hemo-
globin in the red blood cells allows the blood to
transport 30 to 100 times as much oxygen as could
be transported in the form of dissolved oxygen in
the water of the blood.
In the body’s

ed oxygen in
In the body’s tissue cells, oxygen reacts with various foodstuffs to form large
quantities of carbon dioxide. This carbon dioxide enters the tissue capillaries
and is transported back to the lungs. Carbon dioxide, like oxygen, also combines
with chemical substances in the blood that increase carbon dioxide transport
15- to 20-fold.
The purpose of this chapter is to present both qualitatively and quantitatively
the physical and chemical principles of oxygen an

the physical and chemical principles of oxygen and carbon dioxide transport in
the blood and tissue ﬂuids.
Transport of Oxygen from the Lungs to the
Body Tissues
In Chapter 39, we pointed out that gases can move from one point to another
by diffusion and that the cause of this movement is always a partial pressure
difference from the ﬁrst point to the next.Thus, oxygen diffuses from the alveoli
into the pulmonary capillary blood because the oxygen partial pressure (Po2)
in the alveoli is greate

en partial pressure (Po2)
in the alveoli is greater than the Po2 in the pulmonary capillary blood. In the
other tissues of the body, a higher Po2 in the capillary blood than in the tissues
causes oxygen to diffuse into the surrounding cells.
Conversely, when oxygen is metabolized in the cells to form carbon dioxide,
the intracellular carbon dioxide pressure (Pco2) rises to a high value, which
causes carbon dioxide to diffuse into the tissue capillaries. After blood ﬂows to
the lungs, the carbon 

laries. After blood ﬂows to
the lungs, the carbon dioxide diffuses out of the blood into the alveoli, because
the Pco2 in the pulmonary capillary blood is greater than that in the alveoli.
Thus, the transport of oxygen and carbon dioxide by the blood depends on both
diffusion and the ﬂow of blood. We now consider quantitatively the factors
responsible for these effects.
Diffusion of Oxygen from the Alveoli to the Pulmonary
Capillary Blood
The top part of Figure 40–1 shows a pulmonary alveolus ad

 part of Figure 40–1 shows a pulmonary alveolus adjacent to a pul-
monary capillary, demonstrating diffusion of oxygen molecules between the
alveolar air and the pulmonary blood. The Po2 of the gaseous oxygen in the
alveolus averages 104 mm Hg, whereas the Po2 of the venous blood entering the
pulmonary capillary at its arterial end averages only 40 mm Hg because a large
amount of oxygen was removed from this blood as it passed through the periph-
eral tissues. Therefore, the initial pressure dif

eral tissues. Therefore, the initial pressure difference that causes oxygen to
diffuse into the pulmonary capillary is 104 – 40, or 64 mm Hg. In the graph at
Chapter 40
Transport of Oxygen and Carbon Dioxide in Blood and Tissue Fluids
503
the bottom of the ﬁgure, the curve shows the rapid rise
in blood Po2 as the blood passes through the capillary;
the blood Po2 rises almost to that of the alveolar air by
the time the blood has moved a third of the distance
through the capillary, becoming almos

the distance
through the capillary, becoming almost 104 mm Hg.
Uptake of Oxygen by the Pulmonary Blood During Exercise.
During strenuous exercise, a person’s body may
require as much as 20 times the normal amount of
oxygen. Also, because of increased cardiac output
during exercise, the time that the blood remains in the
pulmonary capillary may be reduced to less than one
half normal. Yet, because of the great safety factor for
diffusion of oxygen through the pulmonary mem-
brane, the blood still

 through the pulmonary mem-
brane, the blood still becomes almost saturated with
oxygen by the time it leaves the pulmonary capillaries.
First, it was pointed out in Chapter 39 that the dif-
fusing capacity for oxygen increases almost threefold
during exercise; this results mainly from increased
surface area of capillaries participating in the diffusion
and also from a more nearly ideal ventilation-perfu-
sion ratio in the upper part of the lungs.
Second, note i

tio in the upper part of the lungs.
Second, note in the curve of Figure 40–1 that under
nonexercising conditions, the blood becomes almost
saturated with oxygen by the time it has passed
through one third of the pulmonary capillary, and little
additional oxygen normally enters the blood during
the latter two thirds of its transit. That is, the blood
normally stays in the lung capillaries about three 
times as long as necessary to cause full oxygenation.
Therefore, during exercise, even with a sh

nation.
Therefore, during exercise, even with a shortened time
of exposure in the capillaries, the blood can still
become fully oxygenated, or nearly so.
Transport of Oxygen in the 
Arterial Blood
About 98 per cent of the blood that enters the left
atrium from the lungs has just passed through the alve-
olar capillaries and has become oxygenated up to a Po2
of about 104 mm Hg. Another 2 per cent of the blood
has passed from the aorta through the bronchial cir-
culation, which supplies mainly the

bronchial cir-
culation, which supplies mainly the deep tissues of the
lungs and is not exposed to lung air. This blood ﬂow is
called “shunt ﬂow,” meaning that blood is shunted past
the gas exchange areas. On leaving the lungs, the Po2
of the shunt blood is about that of normal systemic
venous blood, about 40 mm Hg. When this blood 
combines in the pulmonary veins with the oxygenated
blood from the alveolar capillaries, this so-called
venous admixture of blood causes the Po2 of the blood
enterin

xture of blood causes the Po2 of the blood
entering the left heart and pumped into the aorta to
fall to about 95 mm Hg. These changes in blood Po2 at
different points in the circulatory system are shown in
Figure 40–2.
Diffusion of Oxygen from the
Peripheral Capillaries into the 
Tissue Fluid
When the arterial blood reaches the peripheral tissues,
its Po2 in the capillaries is still 95 mm Hg.Yet, as shown
in Figure 40–3, the Po2 in the interstitial ﬂuid that sur-
rounds the tissue cells averages

al ﬂuid that sur-
rounds the tissue cells averages only 40 mm Hg. Thus,
there is a tremendous initial pressure difference that
causes oxygen to diffuse rapidly from the capillary
Alveolus PO2 = 104 mm Hg
PO2 = 40 mm Hg
Blood PO2 (mm Hg)
Blood PO2
PO2 = 104 mm Hg
Pulmonary Capillary
Alveolar oxygen partial pressure
110
100
90
80
70
60
50
40
Arterial End
Venous End
Figure 40–1
Uptake of oxygen by the pulmonary capillary blood. (The curve in
this ﬁgure was constructed from data in Milhorn HT Jr, Pu

ure was constructed from data in Milhorn HT Jr, Pulley PE
Jr: A theoretical study of pulmonary capillary gas exchange and
venous admixture. Biophys J 8:337, 1968.)
100 
80 
60 
40 
20 
0
Systemic
venous
blood
Systemic
arterial
blood
Systemic
Systemic
venous
blood
Mixed with
pulmonary
shunt blood
PO2
Pulmonary
Figure 40–2
Changes in PO2 in the pulmonary capillary blood, systemic arte-
rial blood, and systemic capillary blood, demonstrating the effect
of “venous admixture.”

d, demonstrating the effect
504
Unit VII
blood into the tissues—so rapidly that the capillary Po2
falls almost to equal the 40 mm Hg pressure in the
interstitium. Therefore, the Po2 of the blood leaving
the tissue capillaries and entering the systemic veins is
also about 40 mm Hg.
Effect of Rate of Blood Flow on Interstitial Fluid PO2. If the
blood ﬂow through a particular tissue is increased,
greater quantities of oxygen are transported into the
tissue, and th

 of oxygen are transported into the
tissue, and the tissue Po2 becomes correspondingly
higher. This is shown in Figure 40–4. Note that an
increase in ﬂow to 400 per cent of normal increases 
the Po2 from 40 mm Hg (at point A in the ﬁgure) to 
66 mm Hg (at point B). However, the upper limit to 
which the Po2 can rise, even with maximal blood ﬂow, is
95 mm Hg, because this is the oxygen pressure in the
arterial blood. Conversely, if blood ﬂow through the
tissue decreases, the tissue Po2 also decre

gh the
tissue decreases, the tissue Po2 also decreases, as shown
at point C.
Effect of Rate of Tissue Metabolism on Interstitial Fluid PO2. If
the cells use more oxygen for metabolism than nor-
mally, this reduces the interstitial ﬂuid Po2. Figure 40–4
also demonstrates this effect, showing reduced intersti-
tial ﬂuid Po2 when the cellular oxygen consumption is
increased, and increased Po2 when consumption is
In summary, tissue Po2 is determined by a balance
between (1) the rate of ox

determined by a balance
between (1) the rate of oxygen transport to the tissues
in the blood and (2) the rate at which the oxygen is used
by the tissues.
Peripheral Capillaries to the 
Tissue Cells
Oxygen is always being used by the cells. Therefore,
the intracellular Po2 in the peripheral tissue cells
remains lower than the Po2 in the peripheral capillar-
ies.Also, in many instances, there is considerable phys-
ical distance between the capillaries and the cells.
Th

distance between the capillaries and the cells.
Therefore, the normal intracellular Po2 ranges from 
as low as 5 mm Hg to as high as 40 mm Hg, averaging
(by direct measurement in lower animals) 23 mm Hg.
Because only 1 to 3 mm Hg of oxygen pressure is 
normally required for full support of the chemical
processes that use oxygen in the cell, one can see that
even this low intracellular Po2 of 23 mm Hg is more
than adequate and provides a large safety factor.
Diffusion of Carbon Dioxide from 
the 

ety factor.
the Peripheral Tissue Cells into the
Capillaries and from the Pulmonary
When oxygen is used by the cells, virtually all of it
becomes carbon dioxide, and this increases the intra-
cellular Pco2; because of this high tissue cell Pco2,
carbon dioxide diffuses from the cells into the tissue
capillaries and is then carried by the blood to the lungs.
In the lungs, it diffuses from the pulmonary capillaries
into the alveoli and

rom the pulmonary capillaries
into the alveoli and is expired.
Thus, at each point in the gas transport chain, carbon
dioxide diffuses in the direction exactly opposite to the
diffusion of oxygen. Yet there is one major difference
between diffusion of carbon dioxide and of oxygen:
carbon dioxide can diffuse about 20 times as rapidly as
oxygen.Therefore,the pressure differences required to
cause carbon dioxide diffusion are,in each instance,far
less than the pressure differences required to cause

ss than the pressure differences required to cause
oxygen diffusion. The CO2 pressures are approxi-
mately the following:
1. Intracellular Pco2, 46 mm Hg; interstitial Pco2,
45 mm Hg. Thus, there is only a 1 mm Hg pressure
differential, as shown in Figure 40–5.
2. Pco2 of the arterial blood entering the tissues,
40 mm Hg; Pco2 of the venous blood leaving the
tissues, 45 mm Hg. Thus, as shown in Figure 40–5,
the tissue capillary blood comes almost exactly 
to equilibrium with the interstitial Pco

 exactly 
to equilibrium with the interstitial Pco2 of 
45 mm Hg.
3. Pco2 of the blood entering the pulmonary
capillaries at the arterial end, 45 mm Hg; Pco2 of
the alveolar air, 40 mm Hg. Thus, only a 5 mm Hg
pressure difference causes all the required carbon
dioxide diffusion out of the pulmonary capillaries
of capillary
Venous end
40 mm Hg
23 mm Hg
PO2 = 95 mm Hg
Figure 40–3
Diffusion of oxygen from a tissue capillary to the cells. (PO2 in

issue capillary to the cells. (PO2 in
interstitial ﬂuid = 40 mm Hg, and in tissue cells = 23 mm Hg.)
Upper limit of infinite blood flow
0
100 
80 
60 
40 
20 
0
200
300
Blood flow (per cent of normal)
500
100
400
700
600
A
B
C
Interstitial fluid PO2 (mm Hg)
1/4
n
or
m
a
l
O
2
c
o
ns
um
ptio
n
N
or
m
al
O
2
co
nsu
mpti
on
4 ¥
nor
mal
O2 c
ons
ump
tion
Figure 40–4
Effect of blood ﬂow and rate of oxygen consumption on tissue PO2.
Chapter 40
Transport of Oxygen and Carbon Dioxide in Blood and Tissue

t of Oxygen and Carbon Dioxide in Blood and Tissue Fluids
505
into the alveoli. Furthermore, as shown in Figure
40–6, the Pco2 of the pulmonary capillary blood
falls to almost exactly equal the alveolar Pco2 of
40 mm Hg before it has passed more than about
one third the distance through the capillaries. This
is the same effect that was observed earlier for
oxygen diffusion, except that it is in the opposite
Effect of Rate of Tissue Metabolism and Tissue Blood Flow on
Interstitial PCO2

abolism and Tissue Blood Flow on
Interstitial PCO2. Tissue capillary blood ﬂow and tissue
metabolism affect the Pco2 in ways exactly opposite to
their effect on tissue Po2. Figure 40–7 shows these
effects, as follows:
1. A decrease in blood ﬂow from normal (point A)
to one quarter normal (point B) increases
peripheral tissue Pco2 from the normal value of 
45 mm Hg to an elevated level of 60 mm Hg.
Conversely, increasing the blood ﬂow to six 
times normal (point C) decreases the interstitial
Pco2

s normal (point C) decreases the interstitial
Pco2 from the normal value of 45 mm Hg to 
41 mm Hg, down to a level almost equal to the
Pco2 in the arterial blood (40 mm Hg) entering
the tissue capillaries.
2. Note also that a 10-fold increase in tissue
metabolic rate greatly elevates the interstitial ﬂuid
Pco2 at all rates of blood ﬂow, whereas decreasing
the metabolism to one quarter normal causes the
interstitial ﬂuid Pco2 to fall to about 41 mm Hg,
closely approaching that of the arterial blo

m Hg,
closely approaching that of the arterial blood,
40 mm Hg.
Role of Hemoglobin in Oxygen
Transport
Normally, about 97 per cent of the oxygen transported
from the lungs to the tissues is carried in chemical
combination with hemoglobin in the red blood cells.
The remaining 3 per cent is transported in the dis-
solved state in the water of the plasma and blood cells.
Thus, under normal conditions, oxygen is carried to the
tissues almost entirely by hemoglobin.
Reversible Combination of Oxygen
w

 by hemoglobin.
with Hemoglobin
The chemistry of hemoglobin is presented in Chapter
32, where it was pointed out that the oxygen molecule
combines loosely and reversibly with the heme portion
of hemoglobin. When Po2 is high, as in the pulmonary
capillaries, oxygen binds with the hemoglobin, but
when Po2 is low, as in the tissue capillaries, oxygen is
released from the hemoglobin. This is the basis for
Venous end
PCO2 = 40 mm

Venous end
PCO2 = 40 mm Hg
45 mm Hg
46 mm Hg
PCO2 = 45 mm Hg
Figure 40–5
Uptake of carbon dioxide by the blood in the tissue capillaries.
(PCO2 in tissue cells = 46 mm Hg, and in interstitial ﬂuid =
45 mm Hg.)
Alveolus PCO2 = 40 mm Hg
Blood PCO2 (mm Hg)
Alveolar carbon dioxide partial pressure
45
44
43
42
41
40
Venous End
Pulmonary capillary blood
Figure 40–6
Diffusion of carbon dioxide from the pulmonary

0–6
Diffusion of carbon dioxide from the pulmonary blood into the alve-
olus. (This curve was constructed from data in Milhorn HT Jr,
Pulley PE Jr: A theoretical study of pulmonary capillary gas
exchange and venous admixture. Biophys J 8:337, 1968.)
Normal metabolism
Lower limit of infinite blood flow
0
120 
100 
80 
60 
40 
20 
0
200
300
500
100
400
600
A
B
Interstitial fluid PCO2 (mm Hg)
C
1/4 normal metabolism
10 normal metabolism
Figure 40–7
Effect of blood ﬂ

0 normal metabolism
Effect of blood ﬂow and metabolic rate on peripheral tissue PCO2.
506
Unit VII
almost all oxygen transport from the lungs to the
tissues.
Oxygen-Hemoglobin Dissociation Curve. Figure 40–8 shows
the oxygen-hemoglobin dissociation curve, which
demonstrates a progressive increase in the percentage
of hemoglobin bound with oxygen as blood Po2
increases, which is called the per cent saturation of
hemoglobin. Because the blood leaving the lungs and
entering

. Because the blood leaving the lungs and
entering the systemic arteries usually has a Po2 of
about 95 mm Hg, one can see from the dissociation
curve that the usual oxygen saturation of systemic arte-
rial blood averages 97 per cent. Conversely, in normal
venous blood returning from the peripheral tissues,the
Po2 is about 40 mm Hg, and the saturation of hemo-
globin averages 75 per cent.
Maximum Amount of Oxygen That Can Combine with the 
Hemoglobin of the Blood. The blood of a normal person
con

bin of the Blood. The blood of a normal person
contains about 15 grams of hemoglobin in each 100
milliliters of blood, and each gram of hemoglobin can
bind with a maximum of 1.34 milliliters of oxygen (1.39
milliliters when the hemoglobin is chemically pure, but
impurities such as methemoglobin reduce this).There-
fore, 15 times 1.34 equals 20.1, which means that, on
average, the 15 grams of hemoglobin in 100 milliliters
of blood can combine with a total of almost exactly 20
milliliters of oxyge

 a total of almost exactly 20
milliliters of oxygen if the hemoglobin is 100 per cent
saturated. This is usually expressed as 20 volumes per
cent. The oxygen-hemoglobin dissociation curve for
the normal person can also be expressed in terms of
volume per cent of oxygen, as shown by the far right
scale in Figure 40–8, instead of per cent saturation of
Amount of Oxygen Released from the Hemoglobin When Sys-
temic Arterial Blood Flows Through the Tissues. The total
quantity of oxygen bo

rough the Tissues. The total
quantity of oxygen bound with hemoglobin in normal
systemic arterial blood, which is 97 per cent saturated,
is about 19.4 milliliters per 100 milliliters of blood.This
is shown in Figure 40–9. On passing through the tissue
capillaries, this amount is reduced, on average, to 
14.4 milliliters (Po2 of 40 mm Hg, 75 per cent saturated
hemoglobin). Thus, under normal conditions, about 5
milliliters of oxygen are transported from the lungs to
the tissues by each 100 millil

d from the lungs to
the tissues by each 100 milliliters of blood ﬂow.
Transport of Oxygen During Strenuous Exercise. During
heavy exercise, the muscle cells use oxygen at a rapid
rate, which, in extreme cases, can cause the muscle
interstitial ﬂuid Po2 to fall from the normal 40 mm Hg
100
120
0
100 
90 
80 
70 
60 
50 
40 
30 
20 
10
0
20
30
50
10
40
60
70
Pressure of oxygen in blood (PO2) (mm Hg)
80
90
110
Volumes (%)
130 140
Oxygenated blood
leaving the lungs
20 
18 
16 
14 
12 
10 
8 
6 
4 
2

aving the lungs
20 
18 
16 
14 
12 
10 
8 
6 
4 
2 
0
Reduced blood returning
from tissues
Hemoglobin saturation (%)
Figure 40–8
Oxygen-hemoglobin dissociation curve.
0
Oxygen in blood (volumes %)
Venous blood in exercise
Normal venous blood
Normal arterial blood
20 
18 
16 
14 
12 
10 
8 
6 
4 
2 
0
60
80
120
40
20
100
140
O
2
b
o
u
nd
wit
h h
em
oglo
bin
Figure 40–9
Effect of blood PO2 on the quantity of oxygen bound with hemo-
globin in each 100 milli

f oxygen bound with hemo-
globin in each 100 milliliters of blood.
Chapter 40
507
to as low as 15 mm Hg. At this low pressure, only 4.4
milliliters of oxygen remain bound with the hemoglo-
bin in each 100 milliliters of blood, as shown in Figure
40–9. Thus, 19.4 – 4.4, or 15 milliliters, is the quantity
of oxygen actually delivered to the tissues by each 100
milliliters of blood ﬂow. Thus, three times as much
oxygen as normal is d

w. Thus, three times as much
oxygen as normal is delivered in each volume of blood
that passes through the tissues. And keep in mind that
the cardiac output can increase to six to seven times
normal in well-trained marathon runners. Thus, multi-
plying the increase in cardiac output (six- to sevenfold)
by the increase in oxygen transport in each volume of
blood (threefold) gives a 20-fold increase in oxygen
transport to the tissues. We see later in the chapter 
that several other factors facilit

in the chapter 
that several other factors facilitate delivery of oxygen
into muscles during exercise, so that muscle tissue Po2
often falls very little below normal even during very
strenuous exercise.
Utilization Coefﬁcient. The percentage of the blood that
gives up its oxygen as it passes through the tissue cap-
illaries is called the utilization coefﬁcient. The normal
value for this is about 25 per cent, as is evident from
the preceding discussion—that is, 25 per cent of the
oxygenated hemog

ssion—that is, 25 per cent of the
oxygenated hemoglobin gives its oxygen to the tissues.
During strenuous exercise, the utilization coefﬁcient in
the entire body can increase to 75 to 85 per cent. And
in local tissue areas where blood ﬂow is extremely 
slow or the metabolic rate is very high, utilization 
coefﬁcients approaching 100 per cent have been
recorded—that is, essentially all the oxygen is given to
Effect of Hemoglobin to “Buffer” the
Tissue PO2
Although hemoglobin is neces

uffer” the
Tissue PO2
Although hemoglobin is necessary for the transport of
oxygen to the tissues, it performs another function
essential to life. This is its function as a “tissue oxygen
buffer” system. That is, the hemoglobin in the blood is
mainly responsible for stabilizing the oxygen pressure
in the tissues. This can be explained as follows.
Role of Hemoglobin in Maintaining Nearly Constant PO2 in the
Tissues. Under basal conditions, the tissues require
about 5 milliliters of oxygen from ea

sues require
about 5 milliliters of oxygen from each 100 milliliters
of blood passing through the tissue capillaries. Refer-
ring back to the oxygen-hemoglobin dissociation curve
in Figure 40–9, one can see that for the normal 5 mil-
liliters of oxygen to be released per 100 milliliters of
blood ﬂow, the Po2 must fall to about 40 mm Hg.
Therefore, the tissue Po2 normally cannot rise above
this 40 mm Hg level, because if it did, the amount of
oxygen needed by the tissues would not be released
fro

en needed by the tissues would not be released
from the hemoglobin. In this way, the hemoglobin nor-
mally sets an upper limit on the oxygen pressure in the
tissues at about 40 mm Hg.
Conversely, during heavy exercise, extra amounts of
oxygen (as much as 20 times normal) must be deliv-
ered from the hemoglobin to the tissues. But this can
be achieved with little further decrease in tissue Po2
because of (1) the steep slope of the dissociation curve
and (2) the increase in tissue blood ﬂow caused

ve
and (2) the increase in tissue blood ﬂow caused by the
decreased Po2; that is, a very small fall in Po2 causes
large amounts of extra oxygen to be released from the
hemoglobin. It can be seen, then, that the hemoglobin
in the blood automatically delivers oxygen to the
tissues at a pressure that is held rather tightly between
about 15 and 40 mm Hg.
When Atmospheric Oxygen Concentration Changes Markedly,
the Buffer Effect of Hemoglobin Still Maintains Almost Constant
Tissue PO2. The normal Po2 

ntains Almost Constant
Tissue PO2. The normal Po2 in the alveoli is about 
104 mm Hg, but as one ascends a mountain or ascends
in an airplane, the Po2 can easily fall to less than half
this amount. Alternatively, when one enters areas of
compressed air, such as deep in the sea or in pressur-
ized chambers, the Po2 may rise to 10 times this level.
Even so, the tissue Po2 changes little.
It can be seen from the oxygen-hemoglobin dissoci-
ation curve in Figure 40–8 that when the alveolar Po2
is dec

e in Figure 40–8 that when the alveolar Po2
is decreased to as low as 60 mm Hg, the arterial hemo-
globin is still 89 per cent saturated with oxygen—only
8 per cent below the normal saturation of 97 per cent.
Further, the tissues still remove about 5 milliliters of
oxygen from each 100 milliliters of blood passing
through the tissues; to remove this oxygen, the Po2
of the venous blood falls to 35 mm Hg—only 5 mm Hg
below the normal value of 40 mm Hg. Thus, the tissue
Po2 hardly changes, despite 

 Hg. Thus, the tissue
Po2 hardly changes, despite the marked fall in alveolar
Po2 from 104 to 60 mm Hg.
Conversely, when the alveolar Po2 rises as high as
500 mm Hg, the maximum oxygen saturation of
hemoglobin can never rise above 100 per cent, which
is only 3 per cent above the normal level of 97 per cent.
Only a small amount of additional oxygen dissolves in
the ﬂuid of the blood, as will be discussed subse-
quently. Then, when the blood passes through the
tissue capillaries and loses several 

tissue capillaries and loses several milliliters of oxygen
to the tissues, this reduces the Po2 of the capillary
blood to a value only a few millimeters greater than
the normal 40 mm Hg. Consequently, the level of alve-
olar oxygen may vary greatly—from 60 to more than
500 mm Hg Po2—and still the Po2 in the peripheral
tissues does not vary more than a few millimeters from
normal, demonstrating beautifully the tissue “oxygen
buffer” function of the blood hemoglobin system.
Factors Th

unction of the blood hemoglobin system.
Factors That Shift the Oxygen-
Hemoglobin Dissociation Curve—
Their Importance for Oxygen
Transport
The oxygen-hemoglobin dissociation curves of Figures
40–8 and 40–9 are for normal, average blood.
However, a number of factors can displace the disso-
ciation curve in one direction or the other in the
manner shown in Figure 40–10. This ﬁgure shows 
that when the blood becomes slightly acidic, with the
pH decreasing from the normal value of 7.4 to 7.2, the
o

reasing from the normal value of 7.4 to 7.2, the
oxygen-hemoglobin dissociation curve shifts,
on
508
Unit VII
average, about 15 per cent to the right. Conversely, an
increase in pH from the normal 7.4 to 7.6 shifts the
curve a similar amount to the left.
In addition to pH changes, several other factors are
known to shift the curve. Three of these, all of which
shift the curve to the right, are (1) increased carbon
dioxide concentration, (2) increased blood tempera-
ture, and (3) incr

n, (2) increased blood tempera-
ture, and (3) increased 2,3-biphosphoglycerate (BPG),
a metabolically important phosphate compound
present in the blood in different concentrations under
different metabolic conditions.
Increased Delivery of Oxygen to the Tissues When Carbon
Dioxide and Hydrogen Ions Shift the Oxygen-Hemoglobin Disso-
ciation Curve—The Bohr Effect. A shift of the oxygen-
hemoglobin dissociation curve to the right in response
to increases in blood carbon dioxide and hydrogen
ions h

reases in blood carbon dioxide and hydrogen
ions has a signiﬁcant effect by enhancing the release
of oxygen from the blood in the tissues and enhancing
oxygenation of the blood in the lungs.This is called the
Bohr effect, which can be explained as follows: As the
blood passes through the tissues, carbon dioxide dif-
fuses from the tissue cells into the blood.This increases
the blood Po2, which in turn raises the blood H2CO3
(carbonic acid) and the hydrogen ion concentration.
These effects shift 

e hydrogen ion concentration.
These effects shift the oxygen-hemoglobin dissocia-
tion curve to the right and downward, as shown in
Figure 40–10, forcing oxygen away from the hemoglo-
bin and therefore delivering increased amounts of
oxygen to the tissues.
Exactly the opposite effects occur in the lungs,
where carbon dioxide diffuses from the blood into 
the alveoli. This reduces the blood Pco2 and decreases
the 
hydrogen 
ion 
shifting 
the 
oxygen-hemoglobin dissociation curve t

fting 
the 
oxygen-hemoglobin dissociation curve to the left and
upward. Therefore, the quantity of oxygen that binds
with the hemoglobin at any given alveolar Po2
becomes considerably increased, thus allowing greater
oxygen transport to the tissues.
Effect of BPG to Shift the Oxygen-Hemoglobin Dissociation
Curve. The normal BPG in the blood keeps the
oxygen-hemoglobin dissociation curve shifted slightly
to the right all the time. In hypoxic conditions that 
last longer than a few hours, the qua

itions that 
last longer than a few hours, the quantity of BPG in
the blood increases considerably, thus shifting the
oxygen-hemoglobin dissociation curve even farther to
the right. This causes oxygen to be released to the
tissues at as much as 10 mm Hg higher tissue oxygen
pressure than would be the case without this increased 
BPG. Therefore, under some conditions, the BPG
mechanism can be important for adaptation to
hypoxia, especially to hypoxia caused by poor tissue
blood ﬂow.
Shift of the 

xia caused by poor tissue
blood ﬂow.
Shift of the Dissociation Curve During Exercise. During exer-
cise, several factors shift the dissociation curve con-
siderably to the right, thus delivering extra amounts 
of oxygen to the active, exercising muscle ﬁbers. The
exercising muscles, in turn, release large quantities of
carbon dioxide; this and several other acids released
by the muscles increase the hydrogen ion concentra-
tion in the muscle capillary blood. In addition, the tem-
perature of the

llary blood. In addition, the tem-
perature of the muscle often rises 2° to 3°C, which can
increase oxygen delivery to the muscle ﬁbers even
more.All these factors act together to shift the oxygen-
hemoglobin dissociation curve of the muscle capillary
blood considerably to the right. This right-hand shift
of the curve forces oxygen to be released from the
blood hemoglobin to the muscle at Po2 levels as great
as 40 mm Hg, even when 70 per cent of the oxygen has
already been removed from the hemog

the oxygen has
already been removed from the hemoglobin. Then, in
the lungs, the shift occurs in the opposite direction,
allowing the pickup of extra amounts of oxygen from
the alveoli.
Effect of Intracellular PO2 on Rate of Oxygen Usage. Only 
a minute level of oxygen pressure is required in the
cells for normal intracellular chemical reactions to
take place. The reason for this is that the respiratory
enzyme systems of the cell, which are discussed in
Chapt

 systems of the cell, which are discussed in
Chapter 67, are geared so that when the cellular Po2 is
more than 1 mm Hg, oxygen availability is no longer a
limiting factor in the rates of the chemical reactions.
Instead, the main limiting factor is the concentration
of adenosine diphosphate (ADP) in the cells. This
effect is demonstrated in Figure 40–11, which shows
the relation between intracellular Po2 and the rate of
oxygen usage at different concentrations of ADP. Note
that whenever the intra

oncentrations of ADP. Note
that whenever the intracellular Po2 is above 1 mm Hg,
the rate of oxygen usage becomes constant for any
given concentration of ADP in the cell. Conversely,
when the ADP concentration is altered, the rate of
0
100 
90 
80 
70 
60 
50 
40 
30 
20 
10 
0
7.6
pH
7.4
7.2
60
50
80
70
120130
40
30
20
10
100110
90
140
Shift to right:
(1) Increased hydrogen ions
(2) Increased CO2
(3) Increased temperature
(4) Increased BPG
Pressure of oxygen in blood (

re
Figure 40–10
Shift of the oxygen-hemoglobin dissociation curve to the right
caused by an increase in hydrogen ion concentration (decrease
in pH). BPG, 2,3-biphosphoglycerate.
Chapter 40
509
oxygen usage changes in proportion to the change in
ADP concentration.
As explained in Chapter 3, when adenosine triphos-
phate (ATP) is used in the cells to provide energy, it is
c

P) is used in the cells to provide energy, it is
converted into ADP. The increasing concentration 
of ADP increases the metabolic usage of oxygen as 
it combines with the various cell nutrients, releasing
energy that reconverts the ADP back to ATP. Under
normal operating conditions, the rate of oxygen usage
by the cells is controlled ultimately by the rate of energy
expenditure within the cells—that is, by the rate at which
ADP is formed from ATP.
Effect of Diffusion Distance from the Capillary 

.
Effect of Diffusion Distance from the Capillary to the Cell on
Oxygen Usage. Tissue cells are seldom more than 50
micrometers away from a capillary, and oxygen nor-
mally can diffuse readily enough from the capillary to
the cell to supply all the required amounts of oxygen
for metabolism. However, occasionally, cells are
located farther from the capillaries, and the rate of
oxygen diffusion to these cells can become so low that
intracellular Po2 falls below the critical level required
to maint

2 falls below the critical level required
to maintain maximal intracellular metabolism. Thus,
under these conditions, oxygen usage by the cells is
said to be diffusion limited and is no longer deter-
mined by the amount of ADP formed in the cells.
But this almost never occurs, except in pathological
states.
Effect of Blood Flow on Metabolic Use of Oxygen. The total
amount of oxygen available each minute for use in 
any given tissue is determined by (1) the quantity of
oxygen that can be transpor

by (1) the quantity of
oxygen that can be transported to the tissue in each
100 milliliters of blood and (2) the rate of blood ﬂow.
If the rate of blood ﬂow falls to zero, the amount of
available oxygen also falls to zero. Thus, there are
times when the rate of blood ﬂow through a tissue 
can be so low that tissue Po2 falls below the critical 
1 mm Hg required for intracellular metabolism. Under
these conditions, the rate of tissue usage of oxygen 
is blood ﬂow limited. Neither diffusion-limited

n 
is blood ﬂow limited. Neither diffusion-limited nor
blood ﬂow–limited oxygen states can continue for
long, because the cells receive less oxygen than is
required to continue the life of the cells.
Transport of Oxygen in the
Dissolved State
At the normal arterial Po2 of 95 mm Hg, about 0.29 mil-
liliter of oxygen is dissolved in every 100 milliliters of
water in the blood, and when the Po2 of the blood falls
to the normal 40 mm Hg in the tissue capillaries, only
0.12 milliliter of oxygen remai

 capillaries, only
0.12 milliliter of oxygen remains dissolved. In other
words, 0.17 milliliter of oxygen is normally transported
in the dissolved state to the tissues by each 100 milli-
liters of arterial blood ﬂow. This compares with almost
5 milliliters of oxygen transported by the red cell hemo-
globin. Therefore, the amount of oxygen transported to
the tissues in the dissolved state is normally slight, only
about 3 per cent of the total, as compared with 97 per
cent transported by the hemog

compared with 97 per
cent transported by the hemoglobin.
During strenuous exercise, when hemoglobin release
of oxygen to the tissues increases another threefold, the
relative quantity of oxygen transported in the dissolved
state falls to as little as 1.5 per cent. But if a person
breathes oxygen at very high alveolar Po2 levels, the
amount transported in the dissolved state can become
much greater, sometimes so much so that a serious
excess of oxygen occurs in the tissues, and “oxygen poi-
sonin

ygen occurs in the tissues, and “oxygen poi-
soning” ensues. This often leads to brain convulsions
and even death, as discussed in detail in Chapter 44 
in relation to the high-pressure breathing of oxygen
among deep-sea divers.
Combination of Hemoglobin with
Carbon Monoxide—Displacement 
of Oxygen
Carbon monoxide combines with hemoglobin at the
same point on the hemoglobin molecule as does oxygen;
it can therefore displace oxygen from the hemoglobin,
thereby decreasing the oxygen carrying capac

obin,
thereby decreasing the oxygen carrying capacity of
blood. Further, it binds with about 250 times as much
tenacity as oxygen, which is demonstrated by the carbon
monoxide–hemoglobin dissociation curve in Figure
40–12. This curve is almost identical to the oxygen-
hemoglobin dissociation curve, except that the carbon
monoxide partial pressures, shown on the abscissa, are
at a level 1/250 of those for the oxygen-hemoglobin dis-
sociation curve of Figure 40–8. Therefore, a carbon
monoxide part

 of Figure 40–8. Therefore, a carbon
monoxide partial pressure of only 0.4 mm Hg in the
alveoli, 1/250 that of normal alveolar oxygen (100 mm Hg
Po2), allows the carbon monoxide to compete equally
with the oxygen for combination with the hemoglobin
and causes half the hemoglobin in the blood to become
bound with carbon monoxide instead of with oxygen.
a carbon monoxide pressure of only 
0
1.5 
1.0 
0.5
0
ADP = 11/2 normal
ADP = Normal resting level
ADP = 
3
2
Intracellular PO2 (mm Hg)

resting level
ADP = 
3
2
1
4
1/2 normal
Rate of oxygen usage
(times normal resting level)
Figure 40–11
Effect of intracellular adenosine diphosphate (ADP) and PO2 on
rate of oxygen usage by the cells. Note that as long as the intra-
cellular PO2 remains above 1 mm Hg, the controlling factor for the
rate of oxygen usage is the intracellular concentration of ADP.
510
Unit VII
0.6 mm Hg (a volume concentration of less than one
part per thousand in air) can be l

f less than one
part per thousand in air) can be lethal.
Even though the oxygen content of blood is greatly
reduced in carbon monoxide poisoning, the Po2 of the
blood may be normal. This makes exposure to carbon
monoxide especially dangerous, because the blood is
bright red and there are no obvious signs of hypoxemia,
such as a bluish color of the ﬁngertips or lips (cyanosis).
Also, Po2 is not reduced, and the feedback mechanism
that usually stimulates increased respiration rate in
response to l

ulates increased respiration rate in
response to lack of oxygen (usually reﬂected by a low
Po2) is absent. Because the brain is one of the ﬁrst
organs affected by lack of oxygen, the person may
become disoriented and unconscious before becoming
aware of the danger.
A patient severely poisoned with carbon monoxide
can be treated by administering pure oxygen, because
oxygen at high alveolar pressure can displace carbon
monoxide rapidly from its combination with hemoglo-
bin. The patient can also b

bination with hemoglo-
bin. The patient can also beneﬁt from simultaneous
administration of 5 per cent carbon dioxide, because 
this strongly stimulates the respiratory center, which
increases alveolar ventilation and reduces the alveolar
carbon monoxide. With intensive oxygen and carbon
dioxide therapy, carbon monoxide can be removed from
the blood as much as 10 times as rapidly as without
therapy.
Transport of Carbon Dioxide 
in the Blood
Transport of carbon dioxide by the blood is not nearly

port of carbon dioxide by the blood is not nearly
as problematical as transport of oxygen is, because
even in the most abnormal conditions, carbon dioxide
can usually be transported in far greater quantities
than oxygen can be. However, the amount of carbon
dioxide in the blood has a lot to do with the acid-base
balance of the body ﬂuids, which is discussed in
Chapter 30. Under normal resting conditions, an
average of 4 milliliters of carbon dioxide is transported
from the tissues to the lungs i

ide is transported
from the tissues to the lungs in each 100 milliliters of
blood.
Chemical Forms in Which Carbon
Dioxide Is Transported
To begin the process of carbon dioxide transport,
carbon dioxide diffuses out of the tissue cells in the
dissolved molecular carbon dioxide form. On entering
the tissue capillaries, the carbon dioxide initiates a host
of almost instantaneous physical and chemical reac-
tions, shown in Figure 40–13, which are essential for
carbon dioxide transport.
Transport of 

ential for
Transport of Carbon Dioxide in the Dissolved State. A small
portion of the carbon dioxide is transported in the dis-
solved state to the lungs. Recall that the Pco2 of venous
blood is 45 mm Hg and that of arterial blood is 
40 mm Hg.The amount of carbon dioxide dissolved in
the ﬂuid of the blood at 45 mm Hg is about 2.7 ml/dl
(2.7 volumes per cent). The amount dissolved at 
40 mm Hg is about 2.4 milliliters, or a difference of 0.3
milliliter.Therefore, only a

r a difference of 0.3
milliliter.Therefore, only about 0.3 milliliter of carbon
dioxide is transported in the dissolved form by each
100 milliliters of blood ﬂow.This is about 7 per cent of
all the carbon dioxide normally transported.
Transport of Carbon Dioxide in the Form of Bicarbonate Ion
Reaction of Carbon Dioxide with Water in the Red
Blood Cells—Effect of Carbonic Anhydrase. The dis-
solved carbon dioxide in the blood reacts with water
to form carbonic acid. This reaction would occur 
muc

form carbonic acid. This reaction would occur 
much too slowly to be of importance were it not for
the fact that inside the red blood cells is a protein
enzyme called carbonic anhydrase, which catalyzes the
0.2
Gas pressure of carbon monoxide (mm Hg)
0
100 
90 
80 
70 
60 
50 
40 
30 
20 
10
0
0.1
0.4
0.3
Figure 40–12
Carbon monoxide–hemoglobin dissociation curve. Note the
extremely low carbon monoxide pressures at which carbon
monoxide combines with hemoglobin.
Interst

 carbon
fluid
Capillary
Red blood cell
Plasma
Carbonic
anhydrase
1. CO2
CO2 transported as:
=    7%
=  23%
=  70%
2. Hgb • CO2
3.
Hgb
Hgb
Cl
HHgb
Hgb • CO2
H2CO3
H2O
H2O
CO2
CO2
CO2
Cell
CO2
H2O +
+
H+
+
+
HCO3
–
Cl
HCO3
–
HCO3

Figure 40–13
Transport of carbon dioxide in the blood.
Chapter 40
511
reaction between carbon dioxide and water and accel-
erates its reaction rate about 50

water and accel-
erates its reaction rate about 5000-fold. Therefore,
instead of requiring many seconds or minutes to occur,
as is true in the plasma, the reaction occurs so rapidly
in the red blood cells that it reaches almost complete
equilibrium within a very small fraction of a second.
This allows tremendous amounts of carbon dioxide to
react with the red blood cell water even before the
blood leaves the tissue capillaries.
Dissociation of Carbonic Acid into Bicarbonate and
Hydrogen Ions. In

rbonic Acid into Bicarbonate and
Hydrogen Ions. In another fraction of a second, the
carbonic acid formed in the red cells (H2CO3) disso-
ciates into hydrogen and bicarbonate ions (H+ and
HCO3
–). Most of the hydrogen ions then combine with
the hemoglobin in the red blood cells, because the
hemoglobin protein is a powerful acid-base buffer. In
turn, many of the bicarbonate ions diffuse from the red
cells into the plasma, while chloride ions diffuse into
the red cells to take their place. This is

se into
the red cells to take their place. This is made possible
by the presence of a special bicarbonate-chloride
carrier protein in the red cell membrane that shuttles
these two ions in opposite directions at rapid veloci-
ties. Thus, the chloride content of venous red blood
cells is greater than that of arterial red cells, a phe-
nomenon called the chloride shift.
The reversible combination of carbon dioxide with
water in the red blood cells under the inﬂuence of car-
bonic anhydrase accounts

nder the inﬂuence of car-
bonic anhydrase accounts for about 70 per cent of the
carbon dioxide transported from the tissues to the
lungs. Thus, this means of transporting carbon dioxide
is by far the most important. Indeed, when a carbonic
anhydrase inhibitor (acetazolamide) is administered
to an animal to block the action of carbonic anhydrase
in the red blood cells, carbon dioxide transport from
the tissues becomes so poor that the tissue Pco2 can be
made to rise to 80 mm Hg instead of the nor

can be
made to rise to 80 mm Hg instead of the normal 
45 mm Hg.
Transport of Carbon Dioxide in Combination with Hemoglobin
and Plasma Proteins—Carbaminohemoglobin. In addition to
reacting with water, carbon dioxide reacts directly with
amine radicals of the hemoglobin molecule to form 
the compound carbaminohemoglobin (CO2Hgb). This
combination of carbon dioxide and hemoglobin is a
reversible reaction that occurs with a loose bond, so
that the carbon dioxide is easily released into the
alveoli,

arbon dioxide is easily released into the
alveoli, where the Pco2 is lower than in the pulmonary
A small amount of carbon dioxide also reacts in the
same way with the plasma proteins in the tissue capil-
laries. This is much less signiﬁcant for the transport of
carbon dioxide because the quantity of these proteins
in the blood is only one fourth as great as the quantity
of hemoglobin.
The quantity of carbon dioxide that can be carried
from the peripheral tissues to the lungs by carb

d
from the peripheral tissues to the lungs by carbamino
combination with hemoglobin and plasma proteins is
about 30 per cent of the total quantity transported—
that is, normally about 1.5 milliliters of carbon dioxide
in each 100 milliliters of blood. However, because this
reaction is much slower than the reaction of carbon
dioxide with water inside the red blood cells, it is
doubtful that under normal conditions this carbamino
mechanism transports more than 20 per cent of the
total carbon dioxi

ts more than 20 per cent of the
total carbon dioxide.
The curve shown in Figure 40–14—called the carbon
dioxide dissociation curve—depicts the dependence 
of total blood carbon dioxide in all its forms on Pco2.
Note that the normal blood Pco2 ranges between the
limits of 40 mm Hg in arterial blood and 45 mm Hg in
venous blood, which is a very narrow range. Note also
that the normal concentration of carbon dioxide in the
blood in all its different forms is about 

 in the
blood in all its different forms is about 50 volumes per
cent, but only 4 volumes per cent of this is exchanged
during normal transport of carbon dioxide from the
tissues to the lungs. That is, the concentration rises to
about 52 volumes per cent as the blood passes through
the tissues and falls to about 48 volumes per cent as it
passes through the lungs.
When Oxygen Binds with
Hemoglobin, Carbon Dioxide Is
Released (the Haldane Effect) 
to Increase CO2 Transport
Earlier in the chapter, 

Earlier in the chapter, it was pointed out that an
increase in carbon dioxide in the blood causes oxygen
to be displaced from the hemoglobin (the Bohr effect),
which is an important factor in increasing oxygen
transport. The reverse is also true: binding of oxygen
with hemoglobin tends to displace carbon dioxide
from the blood. Indeed, this effect, called the Haldane
effect,
is quantitatively far more important in 
40
0
80 
70 
60 
50 
40 
30 
20 
10
0
20
10
30
50
70
90

80 
70 
60 
50 
40 
30 
20 
10
0
20
10
30
50
70
90
110
60
80
100
120
CO2 in blood (volumes per cent)
Gas pressure of carbon dioxide (mm Hg)
Normal operating range
Figure 40–14
Carbon dioxide dissociation curve.
512
Unit VII
promoting carbon dioxide transport than is the Bohr
effect in promoting oxygen transport.
The Haldane effect results from the simple fact that
the combination of oxygen with hemoglobin in the
lungs causes the hemoglobin to become a stronger
acid.This displaces car

lobin to become a stronger
acid.This displaces carbon dioxide from the blood and
into the alveoli in two ways: (1) The more highly acidic
hemoglobin has less tendency to combine with carbon
dioxide to form carbaminohemoglobin, thus displacing
much of the carbon dioxide that is present in the car-
bamino form from the blood. (2) The increased acidity
of the hemoglobin also causes it to release an excess
of hydrogen ions, and these bind with bicarbonate 
ions to form carbonic acid; this then disso

onate 
ions to form carbonic acid; this then dissociates into 
water and carbon dioxide, and the carbon dioxide is
released from the blood into the alveoli and, ﬁnally,
into the air.
Figure 40–15 demonstrates quantitatively the signif-
icance of the Haldane effect on the transport of carbon
dioxide from the tissues to the lungs.This ﬁgure shows
small portions of two carbon dioxide dissociation
curves: (1) when the Po2 is 100 mm Hg, which is the
case in the blood capillaries of the lungs, and (2)

ase in the blood capillaries of the lungs, and (2) when
the Po2 is 40 mm Hg, which is the case in the tissue cap-
illaries. Point A shows that the normal Pco2 of 
45 mm Hg in the tissues causes 52 volumes per cent of
carbon dioxide to combine with the blood. On enter-
ing the lungs, the Pco2 falls to 40 mm Hg and the Po2
rises to 100 mm Hg. If the carbon dioxide dissociation
curve did not shift because of the Haldane effect, the
carbon dioxide content of the blood would fall only to
50 volumes p

ntent of the blood would fall only to
50 volumes per cent, which would be a loss of only 2
volumes per cent of carbon dioxide. However, the
increase in Po2 in the lungs lowers the carbon dioxide
dissociation curve from the top curve to the lower
curve of the ﬁgure, so that the carbon dioxide content
falls to 48 volumes per cent (point B). This represents
an additional 2 volumes per cent loss of carbon
dioxide. Thus, the Haldane effect approximately
doubles the amount of carbon dioxide released f

ly
doubles the amount of carbon dioxide released from
the blood in the lungs and approximately doubles the
pickup of carbon dioxide in the tissues.
Change in Blood Acidity During
Carbon Dioxide Transport
The carbonic acid formed when carbon dioxide enters
the blood in the peripheral tissues decreases the blood
pH. However, reaction of this acid with the acid-base
buffers of the blood prevents the hydrogen ion concen-
tration from rising greatly (and the pH from falling
greatly). Ordinarily, arte

and the pH from falling
greatly). Ordinarily, arterial blood has a pH of about
7.41, and as the blood acquires carbon dioxide in the
tissue capillaries, the pH falls to a venous value of about
7.37. In other words, a pH change of 0.04 unit takes
place. The reverse occurs when carbon dioxide is
released from the blood in the lungs, with the pH rising
to the arterial value of 7.41 once again. In heavy exer-
cise or other conditions of high metabolic activity, or
when blood ﬂow through the tissues 

c activity, or
when blood ﬂow through the tissues is sluggish, the
decrease in pH in the tissue blood (and in the tissues
themselves) can be as much as 0.50, about 12 times
normal, thus causing signiﬁcant tissue acidosis.
The discerning student will have noted that normal
transport of oxygen from the lungs to the tissues by each
100 milliliters of blood is about 5 milliliters, whereas
normal transport of carbon dioxide from the tissues 
to the lungs is about 4 millilit

from the tissues 
to the lungs is about 4 milliliters. Thus, under normal
resting conditions, only about 82 per cent as much
carbon dioxide is expired from the lungs as oxygen is
taken up by the lungs. The ratio of carbon dioxide
output to oxygen uptake is called the respiratory
exchange ratio (R). That is,
The value for R changes under different metabolic
conditions. When a person is using exclusively carbohy-
drates for body metabolism, R rises to 1.00. Conversely,
when a person is using exclu

 to 1.00. Conversely,
when a person is using exclusively fats for metabolic
energy, the R level falls to as low as 0.7. The reason 
for this difference is that when oxygen is metabolized
with carbohydrates, one molecule of carbon dioxide is
formed for each molecule of oxygen consumed; when
oxygen reacts with fats, a large share of the oxygen
combines with hydrogen atoms from the fats to form
water instead of carbon dioxide. In other words, when
fats are metabolized, the respiratory quotient of t

ats are metabolized, the respiratory quotient of the
chemical reactions in the tissues is about 0.70 instead of
1.00. (The tissue respiratory quotient is discussed in
Chapter 71.) For a person on a normal diet consuming
average amounts of carbohydrates, fats, and proteins,
the average value for R is considered to be 0.825.
R
Rate of carbon dioxide output
Rate of oxygen uptake
=
35
55 
50
45
45

de output
=
35
55 
50
45
45
50 
40
A
B
PO2 = 100 mm Hg
PCO2
Figure 40–15
Portions of the carbon dioxide dissociation curve when the PO2 is
100 mm Hg or 40 mm Hg. The arrow represents the Haldane
effect on the transport of carbon dioxide, as discussed in the text.
Chapter 40
513
Dempsey JA, Wagner PD: Exercise-induced arterial hypox-
emia. J Appl Physiol 87:1997, 

ced arterial hypox-
emia. J Appl Physiol 87:1997, 1999.
Geers C, Gros G: Carbon dioxide transport and carbonic
anhydrase in blood and muscle. Physiol Rev 80:681,
2000.
Henry RP, Swenson ER: The distribution and physiological
signiﬁcance of carbonic anhydrase in vertebrate gas
exchange organs. Respir Physiol 121:1, 2000.
Jones AM, Koppo K, Burnley M: Effects of prior exercise on
metabolic and gas exchange responses to exercise. Sports
Med 33:949, 2003.
Nikinmaa M:
Membrane transport and control o

2003.
Membrane transport and control of 
hemoglobin-oxygen afﬁnity in nucleated erythrocytes.
Physiol Rev 72:301, 1992.
Piiper J: Perfusion, diffusion and their heterogeneities limit-
ing blood-tissue O2 transfer in muscle.Acta Physiol Scand
168:603, 2000.
2003.
Roy TK, Popel AS: Theoretical predictions of end-capillary
Po2 in muscles of athletic and nonathletic animals a

2 in muscles of athletic and nonathletic animals at
Vo2max. Am J Physiol 271:H721, 1996.
Wagner PD: Diffusive resistance to O2 transport in muscle.
Acta Physiol Scand 168:609, 2000.
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Case-Based Approach. Philadelphia: Lippin-
C
H
A
P
T
E
R
 
4
1
514
The nervous system normally adjusts the rate of
alveolar ventilation almost exactly to the demands
of the body so that the oxygen pressure (Po2) and
carbon dioxide pressure (Pco2) in the arterial blood
are hardly altered even during heavy exercise and
most other types of respirator

 heavy exercise and
most other types of respiratory stress. This chapter
describes the function of this neurogenic system for
regulation of respiration.
The respiratory center is composed of several groups of neurons located bilat-
erally in the medulla oblongata and pons of the brain stem, as shown in Figure
41–1. It is divided into three major collections of neurons: (1) a dorsal respira-
tory group, located in the dorsal portion of the medulla, which mainly causes
inspirati

tion of the medulla, which mainly causes
inspiration; (2) a ventral respiratory group, located in the ventrolateral part of
the medulla, which mainly causes expiration; and (3) the pneumotaxic center,
located dorsally in the superior portion of the pons, which mainly controls rate
and depth of breathing.The dorsal respiratory group of neurons plays the most
fundamental role in the control of respiration.Therefore, let us discuss its func-
tion ﬁrst.
Dorsal Respiratory Group of Neurons—Its Contro

st.
Dorsal Respiratory Group of Neurons—Its Control 
of Inspiration and of Respiratory Rhythm
The dorsal respiratory group of neurons extends most of the length of the
medulla. Most of its neurons are located within the nucleus of the tractus soli-
tarius, although additional neurons in the adjacent reticular substance of the
medulla also play important roles in respiratory control. The nucleus of the
tractus solitarius is the sensory termination of both the vagal and the glos-
sopharyngeal nerv

 of both the vagal and the glos-
sopharyngeal nerves, which transmit sensory signals into the respiratory center
from (1) peripheral chemoreceptors, (2) baroreceptors, and (3) several types of
receptors in the lungs.
Rhythmical Inspiratory Discharges from the Dorsal Respiratory Group. The basic rhythm
of respiration is generated mainly in the dorsal respiratory group of neurons.
Even when all the peripheral nerves entering the medulla have been sectioned
and the brain stem transected both above 

ectioned
and the brain stem transected both above and below the medulla, this group of
neurons still emits repetitive bursts of inspiratory neuronal action potentials. The
basic cause of these repetitive discharges is unknown. In primitive animals,
neural networks have been found in which activity of one set of neurons excites
a second set, which in turn inhibits the ﬁrst. Then, after a period of time, the
mechanism repeats itself, continuing throughout the life of the animal. There-
fore, most 

oughout the life of the animal. There-
fore, most respiratory physiologists believe that some similar network of
neurons is present in the human being, located entirely within the medulla; it
probably involves not only the dorsal respiratory group but adjacent areas of
the medulla as well, and is responsible for the basic rhythm of respiration.
Inspiratory “Ramp” Signal. The nervous signal that is transmitted to the inspira-
tory muscles, mainly the diaphragm, is not an instantaneous burst of ac

the diaphragm, is not an instantaneous burst of action
Chapter 41
515
potentials. Instead, in normal respiration, it begins
weakly and increases steadily in a ramp manner for
about 2 seconds. Then it ceases abruptly for approxi-
mately the next 3 seconds, which turns off the excita-
tion of the diaphragm and allows elastic recoil of the
lungs and the chest wall to cause expiration. Next, the
inspiratory signal begins again for another cycle; this
cycle repeats again and

in for another cycle; this
cycle repeats again and again, with expiration occur-
ring in between. Thus, the inspiratory signal is a ramp
signal. The obvious advantage of the ramp is that it
causes a steady increase in the volume of the lungs
during inspiration, rather than inspiratory gasps.
There are two qualities of the inspiratory ramp that
are controlled, as follows:
1. Control of the rate of increase of the ramp signal,
so that during heavy respiration, the ramp
increases rapidly and theref

respiration, the ramp
increases rapidly and therefore ﬁlls the lungs
rapidly.
2. Control of the limiting point at which the ramp
suddenly ceases. This is the usual method for
controlling the rate of respiration; that is, the
earlier the ramp ceases, the shorter the duration
of inspiration. This also shortens the duration of
expiration. Thus, the frequency of respiration is
A Pneumotaxic Center Limits the
Duration of Inspiration and Increases
the Respiratory Rate
A pneumotaxic center, 

reases
A pneumotaxic center, located dorsally in the nucleus
parabrachialis of the upper pons, transmits signals to
the inspiratory area. The primary effect of this center
is to control the “switch-off” point of the inspiratory
ramp, thus controlling the duration of the ﬁlling phase
of the lung cycle. When the pneumotaxic signal is
strong, inspiration might last for as little as 0.5 second,
thus ﬁlling the lungs only slightly; when the pneumo-
taxic signal is weak, inspirati

; when the pneumo-
taxic signal is weak, inspiration might continue for 5
or more seconds, thus ﬁlling the lungs with a great
excess of air.
The function of the pneumotaxic center is primarily
to limit inspiration. This has a secondary effect of
increasing the rate of breathing, because limitation 
of inspiration also shortens expiration and the entire
period of each respiration. A strong pneumotaxic
signal can increase the rate of breathing to 30 to 40
breaths per minute, whereas a weak pneumot

0 to 40
breaths per minute, whereas a weak pneumotaxic
signal may reduce the rate to only 3 to 5 breaths per
minute.
Ventral Respiratory Group of
Neurons—Functions in Both
Inspiration and Expiration
Located in each side of the medulla, about 5 millime-
ters anterior and lateral to the dorsal respiratory group
of neurons, is the ventral respiratory group of neurons,
found in the nucleus ambiguus rostrally and the
nucleus retroambiguus caudally. The function of this
neuronal group differs from tha

e function of this
neuronal group differs from that of the dorsal respi-
ratory group in several important ways:
1. The neurons of the ventral respiratory group
remain almost totally inactive during normal quiet
respiration. Therefore, normal quiet breathing is
caused only by repetitive inspiratory signals from
the dorsal respiratory group transmitted mainly to
the diaphragm, and expiration results from elastic
recoil of the lungs and thoracic cage.
2. There is no evidence that the ventral respi

ge.
2. There is no evidence that the ventral respiratory
neurons participate in the basic rhythmical
oscillation that controls respiration.
3. When the respiratory drive for increased
pulmonary ventilation becomes greater than
normal, respiratory signals spill over into the
ventral respiratory neurons from the basic
oscillating mechanism of the dorsal respiratory
area. As a consequence, the ventral respiratory
area contributes extra respiratory drive as well.
4. Electrical stimulation of a few o

rive as well.
4. Electrical stimulation of a few of the neurons in
the ventral group causes inspiration, whereas
stimulation of others causes expiration. Therefore,
these neurons contribute to both inspiration and
expiration. They are especially important in
providing the powerful expiratory signals to the
abdominal muscles during very heavy expiration.
Thus, this area operates more or less as an
overdrive mechanism when high levels of
pulmonary ventilation are required, especially
during heavy 

ventilation are required, especially
during heavy exercise.
Lung Inﬂation Signals Limit
Inspiration—The Hering-Breuer
Inﬂation Reﬂex
In addition to the central nervous system respiratory
control mechanisms operating entirely within the
brain stem, sensory nerve signals from the lungs also
Pneumotaxic center
Fourth ventricle
Dorsal respiratory
group (inspiration)
Vagus and
glossopharyngeal
? Apneustic center
Inhibits
Ventral respiratory
group (expiration
and inspiration)
Respiratory motor
pathway

iration
pathways
Figure 41–1
Organization of the respiratory center.
516
Unit VII
help control respiration. Most important, located in
the muscular portions of the walls of the bronchi and
bronchioles throughout the lungs are stretch receptors
that transmit signals through the vagi into the dorsal
respiratory group of neurons when the lungs become
overstretched.These signals affect inspiration in much
the same way as signals from the pneumotaxic cen

h
the same way as signals from the pneumotaxic center;
that is, when the lungs become overly inﬂated, the
stretch receptors activate an appropriate feedback
response that “switches off” the inspiratory ramp and
thus stops further inspiration.This is called the Hering-
Breuer inﬂation reﬂex. This reﬂex also increases the
rate of respiration, as is true for signals from the pneu-
motaxic center.
In human beings, the Hering-Breuer reﬂex probably
is not activated until the tidal volume increases to

not activated until the tidal volume increases to
more than three times normal (greater than about 1.5
liters per breath). Therefore, this reﬂex appears to be
mainly a protective mechanism for preventing excess
lung inﬂation rather than an important ingredient in
normal control of ventilation.
Control of Overall Respiratory 
Center Activity
Up to this point, we have discussed the basic mecha-
nisms for causing inspiration and expiration, but it is
also important to know how the intensity of the 

s
also important to know how the intensity of the respi-
ratory control signals is increased or decreased to
match the ventilatory needs of the body. For example,
during heavy exercise, the rates of oxygen usage and
carbon dioxide formation are often increased to as
much as 20 times normal, requiring commensurate
increases in pulmonary ventilation.The major purpose
of the remainder of this chapter is to discuss this
control of ventilation in accord with the respiratory
needs of the body.
Chemica

rd with the respiratory
Chemical Control 
of Respiration
The ultimate goal of respiration is to maintain proper
concentrations of oxygen, carbon dioxide, and hydro-
gen ions in the tissues. It is fortunate, therefore, that
respiratory activity is highly responsive to changes in
each of these.
Excess carbon dioxide or excess hydrogen ions in
the blood mainly act directly on the respiratory center
itself, causing greatly increased strength of both the
inspiratory and the expirat

d strength of both the
inspiratory and the expiratory motor signals to the
Oxygen, in contrast, does not have a signiﬁcant
direct effect on the respiratory center of the brain in
controlling respiration. Instead, it acts almost entirely
on peripheral chemoreceptors located in the carotid
and aortic bodies, and these in turn transmit appropri-
ate nervous signals to the respiratory center for
control of respiration.
Let us discuss ﬁrst the stimulation of the respiratory
cente

cuss ﬁrst the stimulation of the respiratory
center itself by carbon dioxide and hydrogen ions.
Direct Chemical Control of
Respiratory Center Activity by Carbon
Dioxide and Hydrogen Ions
Chemosensitive Area of the Respiratory Center. We have dis-
cussed mainly three areas of the respiratory center:
the dorsal respiratory group of neurons, the ventral
respiratory group, and the pneumotaxic center. It 
is believed that none of these is affected directly by
changes in blood carbon dioxide concentra

ectly by
changes in blood carbon dioxide concentration or
hydrogen ion concentration. Instead, an additional
neuronal area, a chemosensitive area, shown in Figure
41–2, is located bilaterally, lying only 0.2 millimeter
beneath the ventral surface of the medulla. This area
is highly sensitive to changes in either blood Pco2 or
hydrogen ion concentration, and it in turn excites the
other portions of the respiratory center.
Excitation of the Chemosensitive Neurons by
Hydrogen Ions Is Likely the Pri

nsitive Neurons by
Hydrogen Ions Is Likely the Primary Stimulus
The sensor neurons in the chemosensitive area are
especially excited by hydrogen ions; in fact, it is
believed that hydrogen ions may be the only impor-
tant direct stimulus for these neurons. However,
hydrogen ions do not easily cross the blood-brain
barrier. For this reason, changes in hydrogen ion con-
centration in the blood have considerably less effect 
in stimulating the chemosensitive neurons than do
changes in blood carbon 

sensitive neurons than do
changes in blood carbon dioxide, even though carbon
dioxide is believed to stimulate these neurons second-
arily by changing the hydrogen ion concentration, as
explained in the following section.
Carbon Dioxide Stimulates the 
Chemosensitive Area
Although carbon dioxide has little direct effect in
stimulating the neurons in the chemosensitive area, it
Chemosensitive
area
Inspiratory area
H+ + HCO3
-
H2CO3
CO2 + H2O
Figure 41–2
Stimulation of the brain stem inspiratory a

e 41–2
Stimulation of the brain stem inspiratory area by signals from the
chemosensitive area located bilaterally in the medulla, lying only
a fraction of a millimeter beneath the ventral medullary surface.
Note also that hydrogen ions stimulate the chemosensitive area,
but carbon dioxide in the ﬂuid gives rise to most of the hydrogen
ions.
Chapter 41
517
does have a potent indirect effect. It does this by react-
ing with the water of the tissues to form carbonic acid,

h the water of the tissues to form carbonic acid,
which dissociates into hydrogen and bicarbonate ions;
the hydrogen ions then have a potent direct stimula-
tory effect on respiration. These reactions are shown
in Figure 41–2.
Why does blood carbon dioxide have a more potent
effect in stimulating the chemosensitive neurons than
do blood hydrogen ions? The answer is that the blood-
brain barrier is not very permeable to hydrogen ions,
but carbon dioxide passes through this barrier almost
as if th

ioxide passes through this barrier almost
as if the barrier did not exist. Consequently, whenever
the blood Pco2 increases, so does the Pco2 of both the
interstitial ﬂuid of the medulla and the cerebrospinal
ﬂuid. In both these ﬂuids, the carbon dioxide immedi-
ately reacts with the water to form new hydrogen ions.
Thus, paradoxically, more hydrogen ions are released
into the respiratory chemosensitive sensory area of the
medulla when the blood carbon dioxide concentration
increases than when th

arbon dioxide concentration
increases than when the blood hydrogen ion concen-
tration increases. For this reason, respiratory center
activity is increased very strongly by changes in blood
carbon dioxide, a fact that we subsequently discuss
quantitatively.
Decreased Stimulatory Effect of Carbon Dioxide After the First
1 to 2 Days. Excitation of the respiratory center by
carbon dioxide is great the ﬁrst few hours after the
blood carbon dioxide ﬁrst increases, but then it grad-
ually declines ove

st increases, but then it grad-
ually declines over the next 1 to 2 days, decreasing to
about one ﬁfth the initial effect. Part of this decline
results from renal readjustment of the hydrogen ion
concentration in the circulating blood back toward
normal after the carbon dioxide ﬁrst increases the
hydrogen concentration. The kidneys achieve this by
increasing the blood bicarbonate, which binds with the
hydrogen ions in the blood and cerebrospinal ﬂuid to
reduce their concentrations. But even more

ﬂuid to
reduce their concentrations. But even more important,
over a period of hours, the bicarbonate ions also 
slowly diffuse through the blood-brain and blood–
cerebrospinal ﬂuid barriers and combine directly with
the hydrogen ions adjacent to the respiratory neurons
as well, thus reducing the hydrogen ions back to near
normal. A change in blood carbon dioxide concentra-
tion therefore has a potent acute effect on controlling
respiratory drive but only a weak chronic effect after
a few days’ 

 but only a weak chronic effect after
a few days’ adaptation.
Quantitative Effects of Blood PCO2
and Hydrogen Ion Concentration on 
Figure 41–3 shows quantitatively the approximate
effects of blood Pco2 and blood pH (which is an
inverse logarithmic measure of hydrogen ion concen-
tration) on alveolar ventilation. Note especially the
very marked increase in ventilation caused by an
increase in Pco2 in the normal range between 35 and
75 mm Hg. This demonstrates the tremendous 

35 and
75 mm Hg. This demonstrates the tremendous effect
that carbon dioxide changes have in controlling respi-
ration. By contrast, the change in respiration in the
normal blood pH range between 7.3 and 7.5 is less than
one tenth as great.
Unimportance of Oxygen for Control of the
Changes in oxygen concentration have virtually no
direct effect on the respiratory center itself to alter 
respiratory drive (although oxygen changes do have
an indirect effect, acting through the p

s do have
an indirect effect, acting through the peripheral
chemoreceptors, as explained in the next section).
We learned in Chapter 40 that the hemoglobin-
oxygen buffer system delivers almost exactly normal
amounts of oxygen to the tissues even when the pul-
monary Po2 changes from a value as low as 60 mm Hg
up to a value as high as 1000 mm Hg.Therefore, except
under special conditions, adequate delivery of oxygen
can occur despite changes in lung ventilation ranging
from slightly below one ha

ung ventilation ranging
from slightly below one half normal to as high as 20 or
more times normal.This is not true for carbon dioxide,
because both the blood and tissue Pco2 changes
inversely with the rate of pulmonary ventilation; thus,
the processes of animal evolution have made carbon
dioxide the major controller of respiration, not
oxygen.
Yet, for those special conditions in which the tissues
get into trouble for lack of oxygen, the body has a
special mechanism for respiratory control locat

 a
special mechanism for respiratory control located in
the peripheral chemoreceptors, outside the brain res-
piratory center; this mechanism responds when the
blood oxygen falls too low, mainly below a Po2 of 
70 mm Hg, as explained in the next section.
20
30
40
50
60
70
PCO2
pH
80
90
100
Alveolar ventilation (basal rate = 1)
Normal
11
10
9
8
7
6
5
4
3
2
1
0
7.6
7.5
7.4
7.3
7.2
7.1
7.0
6.9
pH
Figure 41–3
Effects of increased arterial blood PCO2 and decreased arterial pH
(increased 

l blood PCO2 and decreased arterial pH
(increased hydrogen ion concentration) on the rate of alveolar
518
Unit VII
Peripheral Chemoreceptor
System for Control of
Respiratory Activity—Role of
Oxygen in Respiratory Control
In addition to control of respiratory activity by the res-
piratory center itself, still another mechanism is avail-
able for controlling respiration. This is the peripheral
chemoreceptor system, shown in Figure 41–4. Special
nervous chemical receptors, 

 Figure 41–4. Special
nervous chemical receptors, called chemoreceptors, are
located in several areas outside the brain. They are
especially important for detecting changes in oxygen
in the blood, although they also respond to a lesser
extent to changes in carbon dioxide and hydrogen ion
concentrations. The chemoreceptors transmit nervous
signals to the respiratory center in the brain to help
regulate respiratory activity.
Most of the chemoreceptors are in the carotid
bodies. However, a few are 

ors are in the carotid
bodies. However, a few are also in the aortic bodies,
shown in the lower part of Figure 41–4, and a very few
are located elsewhere in association with other arter-
ies of the thoracic and abdominal regions.
The carotid bodies are located bilaterally in the
bifurcations of the common carotid arteries. Their
afferent nerve ﬁbers pass through Hering’s nerves to
the glossopharyngeal nerves and then to the dorsal 
respiratory area of the medulla. The aortic bodies are
located a

ea of the medulla. The aortic bodies are
located along the arch of the aorta; their afferent 
nerve ﬁbers pass through the vagi, also to the dorsal
medullary respiratory area.
Each of the chemoreceptor bodies receives its own
special blood supply through a minute artery directly
from the adjacent arterial trunk. Further, blood ﬂow
through these bodies is extreme, 20 times the weight
of the bodies themselves each minute. Therefore, the
percentage of oxygen removed from the ﬂowing blood
is virtual

of oxygen removed from the ﬂowing blood
is virtually zero. This means that the chemoreceptors
are exposed at all times to arterial blood, not venous
blood, and their Po2s are arterial Po2s.
Stimulation of the Chemoreceptors by Decreased Arterial
Oxygen. When the oxygen concentration in the arterial
blood falls below normal, the chemoreceptors become
strongly stimulated. This is demonstrated in Figure
41–5, which shows the effect of different levels of arte-
rial Po2 on the rate of nerve impulse 

ls of arte-
rial Po2 on the rate of nerve impulse transmission from
a carotid body. Note that the impulse rate is particu-
larly sensitive to changes in arterial Po2 in the range of
60 down to 30 mm Hg, a range in which hemoglobin
saturation with oxygen decreases rapidly.
Effect of Carbon Dioxide and Hydrogen Ion Concentration on
Chemoreceptor Activity. An increase in either carbon
dioxide concentration or hydrogen ion concentration
also excites the chemoreceptors and, in this way,
indirectly in

the chemoreceptors and, in this way,
indirectly increases respiratory activity. However, the
direct effects of both these factors in the respiratory
center itself are so much more powerful than their
effects mediated through the chemoreceptors (about
seven times as powerful) that, for practical purposes,
the indirect effects of carbon dioxide and hydrogen
ions through the chemoreceptors do not need to be
considered. Yet there is one difference between the
peripheral and central effects of carbon

tween the
peripheral and central effects of carbon dioxide: the
stimulation by way of the peripheral chemoreceptors
occurs as much as ﬁve times as rapidly as central stim-
ulation, so that the peripheral chemoreceptors might
be especially important in increasing the rapidity of
response to carbon dioxide at the onset of exercise.
Basic Mechanism of Stimulation of the Chemoreceptors by
Oxygen Deﬁciency. The exact means by which low Po2
excites the nerve endings in the carotid and aortic
bodies is

 nerve endings in the carotid and aortic
bodies is still unknown. However, these bodies have
Aortic bodies
Medulla
Figure 41–4
Respiratory control by peripheral chemoreceptors in the carotid
and aortic bodies.
0
100
200
300
400
500
Carotid body nerve
impulses per second
0
200
400
600
800
Arterial PO2 (mm Hg)
Figure 41–5
Effect of arterial PO2 on impulse rate from the carotid body of a
cat. 
Chapter 41
519
multiple highly c

41
519
multiple highly characteristic glandular-like cells,
called glomus cells, that synapse directly or indirectly
with the nerve endings. Some investigators have sug-
gested that these glomus cells might function as the
chemoreceptors and then stimulate the nerve endings.
But other studies suggest that the nerve endings them-
selves are directly sensitive to the low Po2.
Effect of Low Arterial PO2 to Stimulate
Alveolar Ventilation When Arterial Carbon
Dioxide and Hyd

r Ventilation When Arterial Carbon
Figure 41–6 shows the effect of low arterial Po2 on
alveolar ventilation when the Pco2 and the hydrogen
ion concentration are kept constant at their normal
levels. In other words, in this ﬁgure, only the ventila-
tory drive due to the effect of low oxygen on the
chemoreceptors is active. The ﬁgure shows almost no
effect on ventilation as long as the arterial Po2 remains
greater than 100 mm Hg. But at pressur

Po2 remains
greater than 100 mm Hg. But at pressures lower than
100 mm Hg, ventilation approximately doubles when
the arterial Po2 falls to 60 mm Hg and can increase as
much as ﬁvefold at very low Po2s. Under these condi-
tions, low arterial Po2 obviously drives the ventilatory
process quite strongly.
Mountain climbers have found that when they ascend
a mountain slowly, over a period of days rat

scend
a mountain slowly, over a period of days rather than a
period of hours, they breathe much more deeply and
therefore can withstand far lower atmospheric oxygen
concentrations than when they ascend rapidly. This is
called acclimatization.
The reason for acclimatization is that, within 2 to 3
days, the respiratory center in the brain stem loses
about four ﬁfths of its sensitivity to changes in Pco2
and hydrogen ions. Therefore, the excess ventilatory
blow-off of carbon dioxide that normally w

ilatory
blow-off of carbon dioxide that normally would inhibit
an increase in respiration fails to occur, and low
oxygen can drive the respiratory system to a much
higher level of alveolar ventilation than under acute
conditions. Instead of the 70 per cent increase in ven-
tilation that might occur after acute exposure to low
oxygen, the alveolar ventilation often increases 400 
to 500 per cent after 2 to 3 days of low oxygen; this
helps immensely in supplying additional oxygen to the
mountain c

y in supplying additional oxygen to the
mountain climber.
Composite Effects of PCO2, pH, and
PO2 on Alveolar Ventilation
Figure 41–7 gives a quick overview of the manner in
which the chemical factors Po2, Pco2, and pH—
together—affect alveolar ventilation. To understand
this diagram, ﬁrst observe the four red curves.
These curves were recorded at different levels of 
arterial Po2—40 mm Hg, 50 mm Hg, 60 mm Hg, and
100 mm Hg. For each of these curves, the Pco2 was
changed from lower to higher leve

es, the Pco2 was
changed from lower to higher levels. Thus, this
160 140 120 100
80
60
40
20
0
0
1
2
3
4
5
6
7
0
20
30
40
Arterial PCO2 (mm Hg)
PCO2
Figure 41–6
The lower curve demonstrates the effect of different levels of arte-
rial PO2 on alveolar ventilation, showing a sixfold increase in ven-
tilation as the PO2 decreases from the normal level of 100 mm Hg
to 20 mm Hg. The upper line shows that the arterial PCO2 was kept
at 

per line shows that the arterial PCO2 was kept
at a constant level during the measurements of this study; pH also
was kept constant.
0
10
40
pH = 7.4
pH = 7.3
5040
50 60
100
60
100
PO2 (mm Hg)
20
30
40
50
60
0
10
20
30
40
50
60
Alveolar PCO2 (mm Hg)
Figure 41–7
Composite diagram showing the interrelated effects of PCO2, PO2,
and pH on alveolar ventilation. (Drawn from data in Cunningham
DJC, Lloyd BB: The Regulation of Human Respiration. Oxford:
Blackwell Scientiﬁc P

f Human Respiration. Oxford:
Blackwell Scientiﬁc Publications, 1963.)
520
Unit VII
“family” of red curves represents the combined effects
of alveolar Pco2 and Po2 on ventilation.
Now observe the green curves. The red curves were
measured at a blood pH of 7.4; the green curves were
measured at a pH of 7.3. We now have two families of
curves representing the combined effects of Pco2 and
Po2 on ventilation at two different pH values. Still
other families of curves would be displaced to 

ll
other families of curves would be displaced to the right
at higher pHs and displaced to the left at lower pHs.
Thus, using this diagram, one can predict the level of
alveolar ventilation for most combinations of alveolar
Pco2, alveolar Po2, and arterial pH.
In strenuous exercise, oxygen consumption and carbon
dioxide formation can increase as much as 20-fold.Yet,
as illustrated in Figure 41–8, in the healthy athlete,
alveolar ventilation ordinarily in

ealthy athlete,
alveolar ventilation ordinarily increases almost exactly
in step with the increased level of oxygen metabolism.
The arterial Po2, Pco2, and pH remain almost exactly
normal.
In trying to analyze what causes the increased ven-
tilation during exercise, one is tempted to ascribe this
ions, plus a decrease in blood oxygen. However, this is
questionable, because measurements of arterial Pco2,
pH, and Po2 show that none of these values 

 Pco2,
pH, and Po2 show that none of these values changes
signiﬁcantly during exercise, so that none of them
becomes abnormal enough to stimulate respiration.
Therefore, the question must be asked: What causes
intense ventilation during exercise? At least one effect
seems to be predominant. The brain, on transmitting
motor impulses to the exercising muscles, is believed
to transmit at the same time collateral impulses into
the brain stem to excite the respiratory center. This is
analogous to the

e the respiratory center. This is
analogous to the stimulation of the vasomotor center
of the brain stem during exercise that causes a simul-
taneous increase in arterial pressure.
Actually, when a person begins to exercise, a large
share of the total increase in ventilation begins imme-
diately on initiation of the exercise, before any blood
chemicals have had time to change. It is likely that
most of the increase in respiration results from neuro-
genic signals transmitted directly into the br

ro-
genic signals transmitted directly into the brain stem
respiratory center at the same time that signals go to
the body muscles to cause muscle contraction.
Interrelation Between Chemical Factors and Nervous: Factors
in the Control of Respiration During Exercise. When a
person exercises, direct nervous signals presumably
stimulate the respiratory center almost the proper
amount to supply the extra oxygen required for exer-
cise and to blow off extra carbon dioxide. Occasion-
ally, however, th

 extra carbon dioxide. Occasion-
ally, however, the nervous respiratory control signals
are either too strong or too weak. Then chemical
factors play a signiﬁcant role in bringing about the
ﬁnal adjustment of respiration required to keep the
oxygen, carbon dioxide, and hydrogen ion concentra-
tions of the body ﬂuids as nearly normal as possible.
This is demonstrated in Figure 41–9, which shows in
the lower curve changes in alveolar ventilation during
a 1-minute period of exercise and in the uppe

ring
a 1-minute period of exercise and in the upper curve
changes in arterial Pco2. Note that at the onset of exer-
cise, the alveolar ventilation increases instantaneously,
without an initial increase in arterial Pco2. In fact, this
0
1.0
Moderate
exercise
Severe
exercise
2.0
3.0
4.0
Total ventilation (L/min)
0
20
40
60
80
100
110
120
O2 consumption (L/min)
Figure 41–8
Effect of exercise on oxygen consumption and ventilatory rate.
(From Gray JS: Pulmonary Ventilation and Its Physiological Regu-

 Pulmonary Ventilation and Its Physiological Regu-
lation. Springﬁeld, Ill: Charles C Thomas, 1950.)
0
1
2
Arterial PCO2
 (mm Hg)
36
Exercise
38
40
42
44
Minutes
Alveolar ventilation
(L/min)
2
6
10
14
18
Figure 41–9
Changes in alveolar ventilation (bottom curve) and arterial PCO2
(top curve) during a 1-minute period of exercise and also after ter-
mination of exercise. (Extrapolated to the human being from data
in dogs in Bainton CR: Effect of speed vs grade and shivering on
ventilation in dogs 

eed vs grade and shivering on
ventilation in dogs during active exercise. J Appl Physiol 33:778,
1972.)
Chapter 41
521
increase in ventilation is usually great enough so that
at ﬁrst it actually decreases arterial Pco2 below normal,
as shown in the ﬁgure. The presumed reason that the
ventilation forges ahead of the buildup of blood
carbon dioxide is that the brain provides an “anticipa-
tory” stimulation of respiration at the onset of exer-
cise, causing extra alveolar 

t the onset of exer-
cise, causing extra alveolar ventilation even before it
is needed. However, after about 30 to 40 seconds,
the amount of carbon dioxide released into the blood
from the active muscles approximately matches the
increased rate of ventilation, and the arterial Pco2
returns essentially to normal even as the exercise con-
tinues, as shown toward the end of the 1-minute period
of exercise in the ﬁgure.
Figure 41–10 summarizes the control of respiration
during exercise in still anot

ntrol of respiration
during exercise in still another way, this time more
quantitatively.The lower curve of this ﬁgure shows the
effect of different levels of arterial Pco2 on alveolar
ventilation when the body is at rest—that is, not exer-
cising.The upper curve shows the approximate shift of
this ventilatory curve caused by neurogenic drive from
the respiratory center that occurs during heavy exer-
cise. The points indicated on the two curves show the
arterial Pco2 ﬁrst in the resting state an

how the
arterial Pco2 ﬁrst in the resting state and then in the
exercising state. Note in both instances that the Pco2
is at the normal level of 40 mm Hg. In other words, the
neurogenic factor shifts the curve about 20-fold in the
upward direction, so that ventilation almost matches
the rate of carbon dioxide release, thus keeping arte-
rial Pco2 near its normal value. The upper curve of
Figure 41–10 also shows that if, during exercise, the
arterial Pco2 does change from its normal value of 
40 

ial Pco2 does change from its normal value of 
40 mm Hg, it has an extra stimulatory effect on venti-
lation at a Pco2 greater than 40 mm Hg and a depres-
sant effect at a Pco2 less than 40 mm Hg.
Possibility That the Neurogenic Factor for Control of Ventila-
tion During Exercise Is a Learned Response. Many experi-
ments suggest that the brain’s ability to shift the
ventilatory response curve during exercise, as shown
in Figure 41–10, is at least partly a learned response.
That is, with repeated

 partly a learned response.
That is, with repeated periods of exercise, the brain
becomes progressively more able to provide the
proper signals required to keep the blood Pco2 at its
normal level.Also, there is reason to believe that even
the cerebral cortex is involved in this learning, because
experiments that block only the cortex also block the
learned response.
Other Factors That 
Affect Respiration
Voluntary Control of Respiration. Thus far, we have dis-
cussed the involuntary system for t

, we have dis-
cussed the involuntary system for the control of respi-
ration. However, we all know that for short periods of
time, respiration can be controlled voluntarily and that
one can hyperventilate or hypoventilate to such an
extent that serious derangements in Pco2, pH, and Po2
can occur in the blood.
Effect of Irritant Receptors in the Airways. The epithelium of
the trachea, bronchi, and bronchioles is supplied with
sensory nerve endings called pulmonary irritant recep-
tors that are s

s called pulmonary irritant recep-
tors that are stimulated by many incidents. These cause
coughing and sneezing, as discussed in Chapter 39.They
may also cause bronchial constriction in such diseases
as asthma and emphysema.
Function of Lung “J Receptors.” A few sensory nerve
endings have been described in the alveolar walls in 
juxtaposition to the pulmonary capillaries—hence the
name “J receptors.”They are stimulated especially when
the pulmonary capillaries become engorged with blood
or when

ary capillaries become engorged with blood
or when pulmonary edema occurs in such conditions as
congestive heart failure.Although the functional role of
the J receptors is not clear, their excitation may give the
person a feeling of dyspnea.
Effect of Brain Edema. The activity of the respiratory
center may be depressed or even inactivated by acute
brain edema resulting from brain concussion. For
instance, the head might be struck against some solid
object, after which the damaged brain tissues s

id
object, after which the damaged brain tissues swell,
compressing the cerebral arteries against the cranial
vault and thus partially blocking cerebral blood supply.
Occasionally, respiratory depression resulting from
brain edema can be relieved temporarily by intravenous
injection of hypertonic solutions such as highly concen-
trated mannitol solution. These solutions osmotically
remove some of the ﬂuids of the brain, thus relieving
intracranial pressure and sometimes re-establishing res-
pira

l pressure and sometimes re-establishing res-
piration within a few minutes.
20
30
40
50
Exercise
60
80
100
0
140
120
100
80
60
40
20
Resting
Normal
Figure 41–10
Approximate effect of maximum exercise in an athlete to shift the
alveolar PCO2-ventilation response curve to a level much higher
than normal. The shift, believed to be caused by neurogenic
factors, is almost exactly the right amount to maintain arterial PCO2
at the normal level of 40 m

maintain arterial PCO2
at the normal level of 40 mm Hg both in the resting state and
522
Unit VII
Anesthesia. Perhaps the most prevalent cause of respi-
ratory depression and respiratory arrest is overdosage
with anesthetics or narcotics. For instance, sodium pen-
tobarbital depresses the respiratory center considerably
more than many other anesthetics, such as halothane.At
one time, morphine was used as an anesthetic, but this
drug is now used only as an adjun

thetic, but this
drug is now used only as an adjunct to anesthetics
because it greatly depresses the respiratory center while
having less ability to anesthetize the cerebral cortex.
Periodic Breathing
An abnormality of respiration called periodic breathing
occurs in a number of disease conditions. The person
breathes deeply for a short interval and then breathes
slightly or not at all for an additional interval, with the
cycle repeating itself over and over. One type of peri-
odic breathing, Che

er and over. One type of peri-
odic breathing, Cheyne-Stokes breathing, is character-
ized by slowly waxing and waning respiration occur-
ring about every 40 to 60 seconds, as illustrated in Figure
41–11.
Basic Mechanism of Cheyne-Stokes Breathing. The basic
cause of Cheyne-Stokes breathing is the following:
When a person overbreathes, thus blowing off too much
carbon dioxide from the pulmonary blood while at the
same time increasing blood oxygen, it takes several
seconds before the changed pulm

, it takes several
seconds before the changed pulmonary blood can be
transported to the brain and inhibit the excess ventila-
tion. By this time, the person has already overventilated
for an extra few seconds. Therefore, when the overven-
tilated blood ﬁnally reaches the brain respiratory
center, the center becomes depressed an excessive
amount. Then the opposite cycle begins. That is, carbon
dioxide increases and oxygen decreases in the alveoli.
Again, it takes a few seconds before the brain ca

Again, it takes a few seconds before the brain can
respond to these new changes. When the brain does
respond, the person breathes hard once again, and the
cycle repeats.
The basic cause of Cheyne-Stokes breathing occurs 
in everyone. However, under normal conditions, this
mechanism is highly “damped.” That is, the ﬂuids of the
blood and the respiratory center control areas have
large amounts of dissolved and chemically bound
carbon dioxide and oxygen. Therefore, normally, the
lungs cannot build

xygen. Therefore, normally, the
lungs cannot build up enough extra carbon dioxide or
depress the oxygen sufﬁciently in a few seconds to cause
the next cycle of the periodic breathing. But under two
separate conditions, the damping factors can be over-
ridden, and Cheyne-Stokes breathing does occur:
1. When a long delay occurs for transport of blood
from the lungs to the brain, changes in carbon
dioxide and oxygen in the alveoli can continue 
for many more seconds than usual. Under these
conditio

many more seconds than usual. Under these
conditions, the storage capacities of the alveoli and
pulmonary blood for these gases are exceeded;
then, after a few more seconds, the periodic
respiratory drive becomes extreme, and Cheyne-
Stokes breathing begins. This type of Cheyne-
Stokes breathing often occurs in patients with
severe cardiac failure because blood ﬂow is slow,
thus delaying the transport of blood gases from the
lungs to the brain. In fact, in patients with chronic
heart failure, Ch

n fact, in patients with chronic
heart failure, Cheyne-Stokes breathing can
sometimes occur on and off for months.
2. A second cause of Cheyne-Stokes breathing is
increased negative feedback gain in the respiratory
control areas. This means that a change in blood
carbon dioxide or oxygen causes a far greater
change in ventilation than normally. For instance,
instead of the normal 2- to 3-fold increase in
ventilation that occurs when the Pco2 rises 
3 mm Hg, the same 3 mm Hg rise might increase
v

s 
ventilation 10- to 20-fold. The brain feedback
tendency for periodic breathing is now strong
enough to cause Cheyne-Stokes breathing without
extra blood ﬂow delay between the lungs and
brain. This type of Cheyne-Stokes breathing occurs
mainly in patients with brain damage. The brain
damage often turns off the respiratory drive
entirely for a few seconds; then an extra intense
increase in blood carbon dioxide turns it back on
with great force. Chey

on dioxide turns it back on
with great force. Cheyne-Stokes breathing of this
type is frequently a prelude to death from brain
malfunction.
Typical records of changes in pulmonary and respira-
tory center Pco2 during Cheyne-Stokes breathing are
shown in Figure 41–11. Note that the Pco2 of the 
pulmonary blood changes in advance of the Pco2 of 
the respiratory neurons. But the depth of respiration
corresponds with the Pco2 in the brain, not with the 
Pco2 in the pulmonary blood where the ventilat

he 
Pco2 in the pulmonary blood where the ventilation is
occurring.
The term apnea means absence of spontaneous breath-
ing. Occasional apneas occur during normal sleep, but
in persons with sleep apnea, the frequency and duration
are greatly increased, with episodes of apnea lasting for
10 seconds or longer and occurring 300 to 500 times
each night. Sleep apneas can be caused by obstruction
of the upper airways, especially the pharynx, or by
impaired central nervous system respirator

, or by
impaired central nervous system respiratory drive.
Obstructive Sleep Apnea Is Caused by Blockage of the Upper
Airway. The muscles of the pharynx normally keep this
passage open to allow air to ﬂow into the lungs during
inspiration. During sleep, these muscles usually relax,
but the airway passage remains open enough to permit
adequate airﬂow. Some individuals have an especially
Depth of
respiration
PCO2 of
neurons
PCO2 of
lung blood
center excited
Figure 41–11
Che

 blood
Cheyne-Stokes breathing, showing changing PCO2 in the pul-
monary blood (red line) and delayed changes in the PCO2 of the
ﬂuids of the respiratory center (blue line).
Chapter 41
523
narrow passage, and relaxation of these muscles during
sleep causes the pharynx to completely close so that air
cannot ﬂow into the lungs.
In persons with sleep apnea, loud snoring and labored
breathing occur soon after falling asleep. The snori

eathing occur soon after falling asleep. The snoring
proceeds, often becoming louder, and is then inter-
rupted by a long silent period during which no breath-
ing (apnea) occurs. These periods of apnea result 
in signiﬁcant decreases in Po2 and increases in Pco2,
which greatly stimulate respiration. This, in turn, causes
sudden attempts to breathe, which result in loud snorts
and gasps followed by snoring and repeated episodes of
apnea. The periods of apnea and labored breathing are
repeated se

ods of apnea and labored breathing are
repeated several hundred times during the night, result-
ing in fragmented, restless sleep. Therefore, patients
with sleep apnea usually have excessive daytime drowsi-
ness as well as other disorders, including increased sym-
pathetic activity, high heart rates, pulmonary and
systemic hypertension, and a greatly elevated risk for
cardiovascular disease.
Obstructive sleep apnea most commonly occurs in
older, obese persons in whom there is increased fat dep-

obese persons in whom there is increased fat dep-
osition in the soft tissues of the pharynx or compression
of the pharynx due to excessive fat masses in the neck.
In a few individuals, sleep apnea may be associated with
nasal obstruction, a very large tongue, enlarged tonsils,
or certain shapes of the palate that greatly increase
resistance to the ﬂow of air to the lungs during inspira-
tion. The most common treatments of obstructive sleep
apnea include (1) surgery to remove excess fat tissue 

 include (1) surgery to remove excess fat tissue 
at the back of the throat (a procedure called uvu-
lopalatopharyngoplasty), to remove enlarged tonsils or
adenoids, or to create an opening in the trachea (tra-
cheostomy) to bypass the obstructed airway during
sleep, and (2) nasal ventilation with continuous positive
airway pressure (CPAP).
“Central” Sleep Apnea Occurs When the Neural Drive to Respira-
tory Muscles Is Transiently Abolished. In a few persons 
with sleep apnea, the central nervous

few persons 
with sleep apnea, the central nervous system drive to
the ventilatory muscles transiently ceases. Disorders
that can cause cessation of the ventilatory drive during
sleep include damage to the central respiratory centers
or abnormalities of the respiratory neuromuscular appa-
ratus. Patients affected by central sleep apnea may have
decreased ventilation when they are awake, although
they are fully capable of normal voluntary breathing.
During sleep, their breathing disorders usually

g.
During sleep, their breathing disorders usually worsen,
resulting in more frequent episodes of apnea that
decrease Po2 and increase Pco2 until a critical level is
reached that eventually stimulates respiration. These
transient instabilities of respiration cause restless sleep
and clinical features similar to those observed in
obstructive sleep apnea.
In most patients, the cause of central sleep apnea is
unknown, although instability of the respiratory drive
can result from strokes or other di

piratory drive
can result from strokes or other disorders that make the
respiratory centers of the brain less responsive to the
stimulatory effects of carbon dioxide and hydrogen ions.
Patients with this disease are extremely sensitive to
even small doses of sedatives or narcotics, which further
reduce the responsiveness of the respiratory centers to
the stimulatory effects of carbon dioxide. Medications
that stimulate the respiratory centers can sometimes be
helpful, but ventilation with CPAP a

 sometimes be
helpful, but ventilation with CPAP at night is usually
necessary.
Dean JB, Ballantyne D, Cardone DL, et al: Role of gap junc-
tions in CO2 chemoreception and respiratory control. Am
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Dutschmann M, Paton JF: Inhibitory synaptic mechanisms
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Feldman JL, Mitchell GS, Nattie EE: Breathing: rhythmicity,
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city, chemosensitivity. Annu Rev Neurosci 26:239,
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Forster HV: Plasticity in the control of breathing following
sensory denervation. J Appl Physiol 94:784, 2003.
Hilaire G, Pasaro R: Genesis and control of the respiratory
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Howard RS, Rudd AG, Wolfe CD, Williams AJ: Pathophysi-
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 pathways and control of the
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Kara T, Narkiewicz K, Somers VK: Chemoreﬂexes—physi-
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Morris KF, Baekey DM, Nuding SC, et al: Neural network
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2003.
Mortola JP, Frappell PB:Ventilatory responses to changes in
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Sharp FR, Bernaudin M: HIF1 and oxygen sensing in the
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diovascular and respiratory systems and their interactions
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nteractions
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T
E
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4
2
524

131:121, 2002.
C
H
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P
T
E
R
 
4
2
524
Pathophysiology, Diagnosis,
Oxygen Therapy
Diagnosis and treatment of most respiratory disor-
ders depend heavily on understanding the basic
physiologic principles of respiration and gas
exchange. Some respiratory diseases result from
inadequate ventilation. Others result from abnor-
malities of diffusion through the pulmonary mem-
brane or abnormal blood transport of gases
between the lungs and tissues. Therapy is often
entirely d

the lungs and tissues. Therapy is often
entirely different for these diseases, so it is no longer satisfactory simply to make
a diagnosis of “respiratory insufﬁciency.”
Useful Methods for Studying Respiratory Abnormalities
In the previous few chapters, we have discussed a number of methods for study-
ing respiratory abnormalities, including measuring vital capacity, tidal air, func-
tional residual capacity, dead space, physiologic shunt, and physiologic dead
space. This array of measurements is

siologic dead
space. This array of measurements is only part of the armamentarium of the
clinical pulmonary physiologist. Some other interesting tools are described here.
Among the most fundamental of all tests of pulmonary performance are deter-
minations of the blood Po2, CO2, and pH. It is often important to make these
measurements rapidly as an aid in determining appropriate therapy for acute
respiratory distress or acute abnormalities of acid-base balance. 

ress or acute abnormalities of acid-base balance. Several simple
and rapid methods have been developed to make these measurements within
minutes, using no more than a few drops of blood. They are the following.
Determination of Blood pH. Blood pH is measured using a glass pH electrode of
the type used in all chemical laboratories. However, the electrodes used for this
purpose are miniaturized. The voltage generated by the glass electrode is a
direct measure of pH, and this is generally read dire

ect measure of pH, and this is generally read directly from a voltmeter scale,
or it is recorded on a chart.
Determination of Blood CO2. A glass electrode pH meter can also be used to deter-
mine blood CO2 in the following way: When a weak solution of sodium bicar-
bonate is exposed to carbon dioxide gas, the carbon dioxide dissolves in the
solution until an equilibrium state is established. In this equilibrium state, the
pH of the solution is a function of the carbon dioxide and bicarbonate ion

function of the carbon dioxide and bicarbonate ion con-
centrations in accordance with the Henderson-Hasselbalch equation that is
explained in Chapter 30; that is,
When the glass electrode is used to measure CO2 in blood, a miniature glass
electrode is surrounded by a thin plastic membrane. In the space between the
electrode and plastic membrane is a solution of sodium bicarbonate of known
pH
HCO
CO
3
2
=
-
lo
+
g
6.1
Chapter 42
Respiratory Insufﬁciency—Pathophysiology, Diagnosis, Oxygen Therapy

ﬁciency—Pathophysiology, Diagnosis, Oxygen Therapy
525
concentration. Blood is then superfused onto the outer
surface of the plastic membrane, allowing carbon
dioxide to diffuse from the blood into the bicarbonate
solution. Only a drop or so of blood is required. Next,
the pH is measured by the glass electrode, and the CO2
is calculated by use of the above formula.
Determination of Blood PO2. The concentration of oxygen
in a ﬂuid can be measured by a technique called
polarography. Electric curre

by a technique called
polarography. Electric current is made to ﬂow
between a small negative electrode and the solution.
If the voltage of the electrode is more than -0.6 volt
different from the voltage of the solution, oxygen will
deposit on the electrode. Furthermore, the rate of
current ﬂow through the electrode will be directly pro-
portional to the concentration of oxygen (and there-
fore to PO2 as well). In practice, a negative platinum
electrode with a surface area of about 1 square mil-

ctrode with a surface area of about 1 square mil-
limeter is used, and this is separated from the blood 
by a thin plastic membrane that allows diffusion of
oxygen but not diffusion of proteins or other sub-
stances that will “poison” the electrode.
Often all three of the measuring devices for pH,
CO2, and Po2 are built into the same apparatus, and all
these measurements can be made within a minute or
so using a single, droplet-size sample of blood. Thus,
changes in the blood gases and pH can be

od. Thus,
changes in the blood gases and pH can be followed
almost moment by moment at the bedside.
Measurement of Maximum 
Expiratory Flow
In many respiratory diseases, particularly in asthma,
the resistance to airﬂow becomes especially great
during expiration, sometimes causing tremendous dif-
ﬁculty in breathing. This has led to the concept called
maximum expiratory ﬂow, which can be deﬁned as
follows: When a person expires with great force, the
expiratory airﬂow reaches a maximum ﬂow beyond

he
which the ﬂow cannot be increased any more even
with greatly increased additional force. This is the
maximum expiratory ﬂow. The maximum expiratory
ﬂow is much greater when the lungs are ﬁlled with a
large volume of air than when they are almost empty.
These principles can be understood by referring to
Figure 42–1.
Figure 42–1A shows the effect of increased pressure
applied to the outsides of the alveoli and air passage-
ways caused by compressin

alveoli and air passage-
ways caused by compressing the chest cage.The arrows
indicate that the same pressure compresses the 
outsides of both the alveoli and the bronchioles.There-
fore, not only does this pressure force air from 
the alveoli toward the bronchioles, but it also tends 
to collapse the bronchioles at the same time, which 
will oppose movement of air to the exterior. Once 
the bronchioles have almost completely collapsed,
further expiratory force can still greatly increase the
alv

xpiratory force can still greatly increase the
alveolar pressure, but it also increases the degree of
bronchiolar collapse and airway resistance by an equal
amount, thus preventing further increase in ﬂow.
Therefore, beyond a critical degree of expiratory force,
a maximum expiratory ﬂow has been reached.
Figure 42–1B shows the effect of different degrees
of lung collapse (and therefore of bronchiolar collapse
as well) on the maximum expiratory ﬂow. The curve
recorded in this section shows the ma

w. The curve
recorded in this section shows the maximum expira-
tory ﬂow at all levels of lung volume after a healthy
person ﬁrst inhales as much air as possible and then
expires with maximum expiratory effort until he or she
can expire at no greater rate. Note that the person
quickly reaches a maximum expiratory airﬂow of more
than 400 L/min. But regardless of how much addi-
tional expiratory effort the person exerts, this is still
the maximum ﬂow rate that he or she can achieve.
Note also that

ow rate that he or she can achieve.
Note also that as the lung volume becomes smaller,
the maximum expiratory ﬂow rate also becomes less.
The main reason for this is that in the enlarged lung
the bronchi and bronchioles are held open partially by
way of elastic pull on their outsides by lung structural
elements; however, as the lung becomes smaller, these
structures are relaxed, so that the bronchi and bron-
chioles are collapsed more easily by external chest
pressure, thus progressively reducin

xternal chest
pressure, thus progressively reducing the maximum
expiratory ﬂow rate as well.
Abnormalities of the Maximum Expiratory Flow-Volume Curve.
Figure 42–2 shows the normal maximum expiratory
ﬂow-volume curve, along with two additional ﬂow-
volume curves recorded in two types of lung diseases:
constricted lungs and partial airway obstruction. Note
that the constricted lungs have both reduced total lung
capacity (TLC) and reduced residual volume (RV).
6
0
1
2
3
A
B
Residual
volume
Total l

volume (RV).
6
0
1
2
3
A
B
Residual
volume
Total lung
capacity
Maximum expiratory flow
4
5
Expiratory air flow (L/min)
Lung volume (liters)
0
500
400
300
200
100
Figure 42–1
A, Collapse of the respiratory passageway during maximum expi-
ratory effort, an effect that limits expiratory ﬂow rate. B, Effect of
lung volume on the maximum expiratory air ﬂow, showing
decreasing maximum expiratory air ﬂow as the lung volume
becomes smaller.
526
Unit VII
Furthermore, because the lung cannot e

Furthermore, because the lung cannot expand to a
normal maximum volume, even with the greatest pos-
sible expiratory effort, the maximal expiratory ﬂow
cannot rise to equal that of the normal curve. Con-
stricted lung diseases include ﬁbrotic diseases of the
lung itself, such as tuberculosis and silicosis, and dis-
eases that constrict the chest cage, such as kyphosis,
scoliosis, and ﬁbrotic pleurisy.
In diseases with airway obstruction, it is usually
much more difﬁcult to expire tha

on, it is usually
much more difﬁcult to expire than to inspire because
the closing tendency of the airways is greatly increased
by the extra positive pressure required in the chest 
to cause expiration. By contrast, the extra negative
pleural pressure that occurs during inspiration actually
“pulls” the airways open at the same time that it
expands the alveoli. Therefore, air tends to enter the
lung easily but then becomes trapped in the lungs.
Over a period of months or years, this effect increa

er a period of months or years, this effect increases
both the TLC and the RV, as shown by the green curve
in Figure 42–2. Also, because of the obstruction of the
airways and because they collapse more easily than
normal airways, the maximum expiratory ﬂow rate is
greatly reduced.
The classic disease that causes severe airway
obstruction is asthma. Serious airway obstruction also
occurs in some stages of emphysema.
Forced Expiratory Vital Capacity and
Forced Expiratory Volume
Another exceedingly

y and
Another exceedingly useful clinical pulmonary test,
and one that is also simple, is to make a record on a
spirometer of the forced expiratory vital capacity
(FVC). Such a record is shown in Figure 42–3A for a
person with normal lungs and in Figure 42–3B for a
person with partial airway obstruction. In performing
the FVC maneuver, the person ﬁrst inspires maximally
to the total lung capacity, then exhales into the
spirometer with maximum expiratory effort as rapidly

irometer with maximum expiratory effort as rapidly
and as completely as possible.The total distance of the
downslope of the lung volume record represents the
FVC, as shown in the ﬁgure.
Now, study the difference between the two records
(1) for normal lungs and (2) for partial airway obstruc-
tion. The total volume changes of the FVCs are not
greatly different,indicating only a moderate difference
in basic lung volumes in the two persons. There is,
however, a major difference in the amounts of ai

,
however, a major difference in the amounts of air that
these persons can expire each second, especially during
the ﬁrst second. Therefore, it is customary to compare
the recorded forced expiratory volume during the ﬁrst
second (FEV1) with the normal. In the normal person
(see Figure 42–3A), the percentage of the FVC that is
expired in the ﬁrst second divided by the total FVC
(FEV1/FVC%) is 80 per cent. However, note in Figure
42–3B
that, with airway obstruction, this value
decreased to only 47

irway obstruction, this value
decreased to only 47 per cent. In serious airway
obstruction, as often occurs in acute asthma, this can
decrease to less than 20 per cent.
Physiologic Peculiarities 
of Speciﬁc Pulmonary
The term pulmonary emphysema literally means
excess air in the lungs. However, this term is usually
7
TLC
RV
6
5
4
3
2
1
0
0
500
Normal
Airway obstruction
Constricted lungs
400
300
200
100
Fig

 obstruction
400
300
200
100
Figure 42–2
Effect of two respiratory abnormalities—constricted lungs and
airway obstruction—-on the maximum expiratory ﬂow-volume
curve. TLC, total lung capacity; RV, residual volume.
0
1
2
3
4
5
6
7
Maximum
inspiration
FEV1
FVC
FEV1/FVC%
= 80%
AIRWAY OBSTRUCTION
Seconds
NORMAL
4
3
2
1
0
A
0
1
2
3
4
5
6
7
FEV1
FVC
FEV1/FVC%
= 47%
4
3
2
1
0
B
Figure 42–3
Recordings during the forced vital capacity maneuver: A, in a
health

the forced vital capacity maneuver: A, in a
healthy person and B, in a person with partial airway obstruction.
(The “zero” on the volume scale is residual volume.)
Chapter 42
527
used to describe complex obstructive and destructive
process of the lungs caused by many years of smoking.
It results from the following major pathophysiologic
changes in the lungs:
1. Chronic infection, caused by inhaling smoke or
other substances that

 caused by inhaling smoke or
other substances that irritate the bronchi and
bronchioles. The chronic infection seriously
deranges the normal protective mechanisms of the
airways, including partial paralysis of the cilia of
the respiratory epithelium, an effect caused by
nicotine. As a result, mucus cannot be moved
easily out of the passageways. Also, stimulation 
of excess mucus secretion occurs, which further
exacerbates the condition. Too, inhibition of the
alveolar macrophages occurs, so that

bition of the
alveolar macrophages occurs, so that they become
less effective in combating infection.
2. The infection, excess mucus, and inﬂammatory
edema of the bronchiolar epithelium together
cause chronic obstruction of many of the smaller
airways.
3. The obstruction of the airways makes it especially
difﬁcult to expire, thus causing entrapment of 
air in the alveoli and overstretching them. This,
combined with the lung infection, causes marked
destruction of as much as 50 to 80 per cent of 

ed
destruction of as much as 50 to 80 per cent of the
alveolar walls. Therefore, the ﬁnal picture of the
emphysematous lung is that shown in Figures
42–4 (top) and 42–5.
The physiologic effects of chronic emphysema are
extremely varied, depending on the severity of the
disease and the relative degrees of bronchiolar ob-
struction versus lung parenchymal destruction.Among
the different abnormalities are the following:
1. The bronchiolar obstruction increases airway
resistance and results in great

n increases airway
resistance and results in greatly increased work of
breathing. It is especially difﬁcult for the person
to move air through the bronchioles during
expiration because the compressive force on the
outside of the lung not only compresses the
alveoli but also compresses the bronchioles,
which further increases their resistance during
expiration.
2. The marked loss of alveolar walls greatly
decreases the diffusing capacity of the lung, which
reduces the ability of the lungs to oxyg

ng, which
reduces the ability of the lungs to oxygenate the
blood and remove carbon dioxide from the blood.
3. The obstructive process is frequently much worse
in some parts of the lungs than in other parts,
so that some portions of the lungs are well
ventilated, while other portions are poorly
ventilated. This often causes extremely abnormal
ventilation-perfusion ratios, with a very low 
V
.
a/Q
.
in some parts (physiologic shunt), resulting
in poor aeration of the blood, and very high V
.
a/Q

poor aeration of the blood, and very high V
.
a/Q
.
in other parts (physiologic dead space), resulting
in wasted ventilation, both effects occurring in the
same lungs.
4. Loss of large portions of the alveolar walls also
decreases the number of pulmonary capillaries
through which blood can pass. As a result, the
pulmonary vascular resistance often increases
markedly, causing pulmonary hypertension. This in
turn overloads the right side of the heart and
frequently causes right-sided heart failure

rt and
frequently causes right-sided heart failure.
Chronic emphysema usually progresses slowly over
many years. The person develops both hypoxia and
hypercapnia because of hypoventilation of many
alveoli plus loss of alveolar walls. The net result of 
all these effects is severe, prolonged, devastating air
hunger that can last for years until the hypoxia and
hypercapnia cause death—a high penalty to pay for
smoking.
Pneumonia
The term pneumonia includes any inﬂammatory con-
dition of the lung i

includes any inﬂammatory con-
dition of the lung in which some or all of the alveoli
are ﬁlled with ﬂuid and blood cells, as shown in Figure
42–5. A common type of pneumonia is bacterial pneu-
monia, caused most frequently by pneumococci.
This disease begins with infection in the alveoli; the
pulmonary membrane becomes inﬂamed and highly
porous so that ﬂuid and even red and white blood cells
Figure 42–4
Contrast of the emphysematous lung (top ﬁgure) with the normal
lung (bottom ﬁgure), showing e

re) with the normal
lung (bottom ﬁgure), showing extensive alveolar destruction in
emphysema. (Reproduced with permission of Patricia Delaney
and the Department of Anatomy, The Medical College of 
Wisconsin.)
528
Unit VII
leak out of the blood into the alveoli.Thus,the infected
alveoli become progressively ﬁlled with ﬂuid and cells,
and the infection spreads by extension of bacteria or
virus from alveolus to alveolus. Eventually, large areas
of the lungs, sometimes whole lobes or eve

e areas
of the lungs, sometimes whole lobes or even a whole
lung, become “consolidated,” which means that they
are ﬁlled with ﬂuid and cellular debris.
In pneumonia, the gas exchange functions of the
lungs change in different stages of the disease. In early
stages, the pneumonia process might well be localized
to only one lung, with alveolar ventilation reduced
while blood ﬂow through the lung continues normally.
This results in two major pulmonary abnormalities: (1)
reduction in the total avail

ry abnormalities: (1)
reduction in the total available surface area of the res-
piratory membrane and (2) decreased ventilation-
perfusion ratio. Both these effects cause hypoxemia
(low blood oxygen) and hypercapnia (high blood
carbon dioxide).
Figure 42–6 shows the effect of the decreased 
ventilation-perfusion ratio in pneumonia, showing that
the blood passing through the aerated lung becomes
97 per cent saturated with oxygen, whereas that
passing through the unaerated lung is about 60 per
cen

ing through the unaerated lung is about 60 per
cent saturated.Therefore,the average saturation of the
blood pumped by the left heart into the aorta is only
about 78 per cent, which is far below normal.
Atelectasis means collapse of the alveoli. It can occur
in localized areas of a lung or in an entire lung. Its most
common causes are (1) total obstruction of the airway
or (2) lack of surfactant in the ﬂuids lining the alveoli.
Airway Obstruction. The airway obstruction type of
atelec

Obstruction. The airway obstruction type of
atelectasis usually results from (1) blockage of many
small bronchi with mucus or (2) obstruction of a major
bronchus by either a large mucus plug or some solid
object such as a tumor. The air entrapped beyond the
block is absorbed within minutes to hours by the blood
ﬂowing in the pulmonary capillaries. If the lung tissue
is pliable enough, this will lead simply to collapse of
the alveoli. However, if the lung is rigid because of
ﬁbrotic tissue and ca

the lung is rigid because of
ﬁbrotic tissue and cannot collapse, absorption of air
from the alveoli creates very negative pressures within
the alveoli, which pull ﬂuid out of the pulmonary cap-
illaries into the alveoli, thus causing the alveoli to ﬁll
completely with edema ﬂuid.This almost always is the
effect that occurs when an entire lung becomes
atelectatic, a condition called massive collapse of the
lung.
The effects on overall pulmonary function caused by
massive collapse (atelectasis) of

nction caused by
massive collapse (atelectasis) of an entire lung are
shown in Figure 42–7. Collapse of the lung tissue not
only occludes the alveoli but also almost always
increases the resistance to blood ﬂow through the pul-
monary vessels of the collapsed lung. This resistance
Fluid and blood cells
Confluent alveoli
Edema
Normal
Pneumonia
Emphysema
Figure 42–5
Lung alveolar changes in pneumonia and emphysema.
Pneumonia
Pulmonary arterial blood
60% saturated with O2
Left
pulmonary
vein 60%
sa

d
Left
pulmonary
vein 60%
saturated
Right
pulmonary
vein 97%
saturated
Aorta:
Blood 1/2 = 97%
          1/2 = 60%
Mean      = 78%
Figure 42–6
Effect of pneumonia on percentage saturation of oxygen in the
pulmonary artery, the right and left pulmonary veins, and the aorta.
Chapter 42
529
increase occurs partially because of the lung collapse
itself, which compresses and folds the vessels as the
volume of the

presses and folds the vessels as the
volume of the lung decreases. In addition, hypoxia in
the collapsed alveoli causes additional vasoconstric-
tion, as explained in Chapter 38.
Because of the vascular constriction, blood ﬂow
through the atelectatic lung becomes slight. Fortu-
nately, most of the blood is routed through the venti-
lated lung and therefore becomes well aerated. In the
situation shown in Figure 42–7, ﬁve sixths of the blood
passes through the aerated lung and only one sixth
throu

 through the aerated lung and only one sixth
through the unaerated lung. As a result, the overall
ventilation-perfusion ratio is only moderately com-
promised, so that the aortic blood has only mild
oxygen desaturation despite total loss of ventilation in
an entire lung.
Lack of “Surfactant” as a Cause of Lung Collapse. The secre-
tion and function of surfactant in the alveoli were 
discussed in Chapter 37. It was pointed out that the
surfactant is secreted by special alveolar epithelial
cells i

is secreted by special alveolar epithelial
cells into the ﬂuids that coat the inside surface of the
alveoli. The surfactant in turn decreases the surface
tension in the alveoli 2- to 10-fold, which normally
plays a major role in preventing alveolar collapse.
However, in a number of conditions, such as in hyaline
membrane disease (also called respiratory distress syn-
drome), which often occurs in newborn premature
babies, the quantity of surfactant secreted by the
alveoli is so greatly depressed

nt secreted by the
alveoli is so greatly depressed that the surface tension
of the alveolar ﬂuid becomes several times normal.
This causes a serious tendency for the lungs of these
babies to collapse or to become ﬁlled with ﬂuid. As
explained in Chapter 37, many of these infants die of
suffocation when large portions of the lungs become
atelectatic.
Asthma
Asthma is characterized by spastic contraction of the
smooth muscle in the bronchioles, which partially
obstructs the bronchioles and causes 

ch partially
obstructs the bronchioles and causes extremely difﬁ-
cult breathing. It occurs in 3 to 5 per cent of all people
at some time in life.
The usual cause of asthma is contractile hypersensi-
tivity of the bronchioles in response to foreign sub-
stances in the air. In about 70 per cent of patients
younger than age 30 years, the asthma is caused by
allergic hypersensitivity, especially sensitivity to plant
pollens. In older people, the cause is almost always
hypersensitivity to nonallerge

se is almost always
hypersensitivity to nonallergenic types of irritants in
the air, such as irritants in smog.
The allergic reaction that occurs in the allergic type
of asthma is believed to occur in the following way:The
typical allergic person has a tendency to form abnor-
mally large amounts of IgE antibodies, and these anti-
bodies cause allergic reactions when they react with the
speciﬁc antigens that have caused them to develop in
the ﬁrst place, as explained in Chapter 34. In asthma,
the

 place, as explained in Chapter 34. In asthma,
these antibodies are mainly attached to mast cells that
are present in the lung interstitium in close association
with the bronchioles and small bronchi.When the asth-
matic person breathes in pollen to which he or she is
sensitive (that is, to which the person has developed
IgE antibodies), the pollen reacts with the mast cell–
attached antibodies and causes the mast cells to release
several different substances. Among them are (a) 
histamine, (b) 

nt substances. Among them are (a) 
histamine, (b) slow-reacting substance of anaphylaxis
(which is a mixture of leukotrienes), (c) eosinophilic
chemotactic factor, and (d) bradykinin. The combined
effects of all these factors, especially the slow-reacting
substance of anaphylaxis, are to produce (1) localized
edema in the walls of the small bronchioles, as well as
secretion of thick mucus into the bronchiolar lumens,
and (2) spasm of the bronchiolar smooth muscle.
Therefore, the airway resistanc

lar smooth muscle.
Therefore, the airway resistance increases greatly.
As discussed earlier in this chapter, the bronchiolar
diameter becomes more reduced during expiration
than during inspiration in asthma, caused by bronchi-
olar collapse during expiratory effort that compresses
the outsides of the bronchioles. Because the bronchi-
oles of the asthmatic lungs are already partially
occluded, further occlusion resulting from the external
pressure creates especially severe obstruction during
expi

 creates especially severe obstruction during
expiration. That is, the asthmatic person often can
inspire quite adequately but has great difﬁculty expir-
ing. Clinical measurements show (1) greatly reduced
maximum expiratory rate and (2) reduced timed expi-
ratory volume. Also, all of this together results in
dyspnea, or “air hunger,” which is discussed later in
The functional residual capacity and residual volume
of the lung become especially increased during the
acute asthmatic a

 especially increased during the
acute asthmatic attack because of the difﬁculty in
expiring air from the lungs. Also, over a period of
years, the chest cage becomes permanently enlarged,
causing a “barrel chest,” and both the functional resid-
ual capacity and lung residual volume become perma-
nently increased.
Left
pulmonary
vein 60%
saturated-
flow 1/5 normal
Right
pulmonary
vein 97%
saturated
Aorta:
Blood 5/6 = 97%
          1/6 = 6

saturated
Aorta:
          1/6 = 60%
Mean saturation
               = 91%
Figure 42–7
Effect of atelectasis on aortic blood oxygen saturation.
530
Unit VII
In tuberculosis, the tubercle bacilli cause a peculiar
tissue reaction in the lungs, including (1) invasion of
the infected tissue by macrophages and (2) “walling
off” of the lesion by ﬁbrous tissue to form the so-called
tubercle. This walling-off process helps to limit further
transmission of the tube

ss helps to limit further
transmission of the tubercle bacilli in the lungs and
therefore is part of the protective process against
extension of the infection. However, in about 3 per
cent of all people who develop tuberculosis, if
untreated, the walling-off process fails and tubercle
bacilli spread throughout the lungs, often causing
extreme destruction of lung tissue with formation of
large abscess cavities.
Thus, tuberculosis in its late stages is characterized
by many areas of ﬁbrosis throug

s is characterized
by many areas of ﬁbrosis throughout the lungs, as well
as reduced total amount of functional lung tissue.
These effects cause (1) increased “work” on the part
of the respiratory muscles to cause pulmonary venti-
lation and reduced vital capacity and breathing capac-
ity; (2) reduced total respiratory membrane surface area
and increased thickness of the respiratory membrane,
causing progressively diminished pulmonary diffusing
capacity; and (3) abnormal ventilation-perfusion ra

apacity; and (3) abnormal ventilation-perfusion ratio
in the lungs, further reducing overall pulmonary diffu-
sion of oxygen and carbon dioxide.
Almost any of the conditions discussed in the past 
few sections of this chapter can cause serious degrees
of bodywide cellular hypoxia. Sometimes, oxygen
therapy is of great value; other times, it is of moderate
value; and, at still other times, it is of almost no value.
Therefore, it is important to understand the different

fore, it is important to understand the different
types of hypoxia; then we can discuss the physiologic
principles of oxygen therapy. The following is a
descriptive classiﬁcation of the causes of hypoxia:
1. Inadequate oxygenation of the blood in the lungs
because of extrinsic reasons
a. Deﬁciency of oxygen in the atmosphere
b. Hypoventilation (neuromuscular disorders)
2. Pulmonary disease
a. Hypoventilation caused by increased airway
resistance or decreased pulmonary compliance
b. Abnormal alve

or decreased pulmonary compliance
b. Abnormal alveolar ventilation-perfusion ratio
(including either increased physiologic dead
space or increased physiologic shunt)
c. Diminished respiratory membrane diffusion
3. Venous-to-arterial shunts (“right-to-left” cardiac
shunts)
4. Inadequate oxygen transport to the tissues by the
blood
a. Anemia or abnormal hemoglobin
b. General circulatory deﬁciency
c. Localized circulatory deﬁciency (peripheral,
cerebral, coronary vessels)
d. Tissue edema
5. Inadequ

bral, coronary vessels)
5. Inadequate tissue capability of using oxygen
a. Poisoning of cellular oxidation enzymes
b. Diminished cellular metabolic capacity for using
oxygen, because of toxicity, vitamin deﬁciency, or
other factors
This classiﬁcation of the types of hypoxia is mainly
self-evident from the discussions earlier in the chapter.
Only one of the types of hypoxia in the classiﬁcation
needs further elaboration: this is the hypoxia caused
by inadequate capability of the b

e hypoxia caused
by inadequate capability of the body’s tissue cells to
use oxygen.
Inadequate Tissue Capability to Use Oxygen. The classic
cause of inability of the tissues to use oxygen is cyanide
poisoning, in which the action of the enzyme
cytochrome oxidase is completely blocked by the
cyanide—to such an extent that the tissues simply
cannot use oxygen even when plenty is available.Also,
deﬁciencies of some of the tissue cellular oxidative
enzymes or of other elements in the tissue oxidativ

nzymes or of other elements in the tissue oxidative
system can lead to this type of hypoxia. A special
example occurs in the disease beriberi, in which several
important steps in tissue utilization of oxygen and
formation of carbon dioxide are compromised because
of vitamin B deﬁciency.
Effects of Hypoxia on the Body. Hypoxia, if severe enough,
can cause death of cells throughout the body, but in
less severe degrees it causes principally (1) depressed
mental activity, sometimes culminating in co

essed
mental activity, sometimes culminating in coma, and
(2) reduced work capacity of the muscles.These effects
are speciﬁcally discussed in Chapter 43 in relation to
high-altitude physiology.
Oxygen Therapy in Different Types 
of Hypoxia
Oxygen can be administered by (1) placing the
patient’s head in a “tent” that contains air fortiﬁed
with oxygen, (2) allowing the patient to breathe either
pure oxygen or high concentrations of oxygen from 
a mask, or (3) administering oxygen through an
intran

ask, or (3) administering oxygen through an
intranasal tube.
Recalling the basic physiologic principles of the 
different types of hypoxia, one can readily decide
when oxygen therapy will be of value and, if so, how
valuable.
In atmospheric hypoxia, oxygen therapy can com-
pletely correct the depressed oxygen level in the
inspired gases and, therefore, provide 100 per cent
effective therapy.
In hypoventilation hypoxia, a person breathing 100
per cent oxygen can move ﬁve times as much oxygen
into

cent oxygen can move ﬁve times as much oxygen
into the alveoli with each breath as when breathing
normal air. Therefore, here again oxygen therapy can
be extremely beneﬁcial. (However, this provides no
beneﬁt for the excess blood carbon dioxide also caused
by the hypoventilation.)
In hypoxia caused by impaired alveolar membrane
diffusion, essentially the same result occurs as in
hypoventilation hypoxia, because oxygen therapy can
increase the Po2 in the lung alveoli from the normal
value of abou

 in the lung alveoli from the normal
value of about 100 mm Hg to as high as 600 mm Hg.
Chapter 42
531
This raises the oxygen pressure gradient for diffusion
of oxygen from the alveoli to the blood from the
normal value of 60 mm Hg to as high as 560 mm Hg,
an increase of more than 800 per cent.This highly ben-
eﬁcial effect of oxygen therapy in diffusion hypoxia is
demonstrated in Figure 42–8, which shows that the
pulmonary blood

 Figure 42–8, which shows that the
pulmonary blood in this patient with pulmonary
edema picks up oxygen three to four times as rapidly
as would occur with no therapy.
In hypoxia caused by anemia, abnormal hemoglobin
transport of oxygen, circulatory deﬁciency, or physio-
logic shunt, oxygen therapy is of much less value
because normal oxygen is already available in the
alveoli.The problem instead is that one or more of the
mechanisms for transporting oxygen from the lungs to
the tissues is deﬁcie

ing oxygen from the lungs to
the tissues is deﬁcient. Even so, a small amount of
extra oxygen, between 7 and 30 per cent, can be trans-
ported in the dissolved state in the blood when alveo-
lar oxygen is increased to maximum even though the
amount transported by the hemoglobin is hardly
altered.This small amount of extra oxygen may be the
difference between life and death.
In the different types of hypoxia caused by inade-
quate tissue use of oxygen, there is abnormality neither
of oxygen picku

ygen, there is abnormality neither
of oxygen pickup by the lungs nor of transport to the
tissues. Instead, the tissue metabolic enzyme system is
simply incapable of using the oxygen that is delivered.
Therefore, oxygen therapy is of hardly any measura-
ble beneﬁt.
Cyanosis
The term cyanosis means blueness of the skin, and its
cause is excessive amounts of deoxygenated hemoglo-
bin in the skin blood vessels, especially in the capil-
laries. This deoxygenated hemoglobin has an intense 
dark blue-p

eoxygenated hemoglobin has an intense 
dark blue-purple color that is transmitted through the
skin.
In general, deﬁnite cyanosis appears whenever the
arterial blood contains more than 5 grams of deoxy-
genated hemoglobin in each 100 milliliters of blood.A
person with anemia almost never becomes cyanotic
because there is not enough hemoglobin for 5 grams
to be deoxygenated in 100 milliliters of arterial blood.
Conversely, in a person with excess red blood cells, as
occurs in polycythemia vera, th

ed blood cells, as
occurs in polycythemia vera, the great excess of avail-
able hemoglobin that can become deoxygenated leads
frequently to cyanosis, even under otherwise normal
Hypercapnia means excess carbon dioxide in the body
ﬂuids.
One might suspect, on ﬁrst thought, that any respi-
ratory condition that causes hypoxia would also cause
hypercapnia. However, hypercapnia usually occurs 
in association with hypoxia only when the hypoxia 
is caused by hypoventilation or 

when the hypoxia 
is caused by hypoventilation or circulatory deﬁciency.
The reasons for this are the following.
Hypoxia caused by too little oxygen in the air, too
little hemoglobin, or poisoning of the oxidative
enzymes has to do only with the availability of oxygen
or use of oxygen by the tissues. Therefore, it is readily
understandable that hypercapnia is not a concomitant
of these types of hypoxia.
In hypoxia resulting from poor diffusion through
the pulmonary membrane or through the tissue

rough
the pulmonary membrane or through the tissues,
serious hypercapnia usually does not occur at the same
time because carbon dioxide diffuses 20 times as
rapidly as oxygen. If hypercapnia does begin to occur,
this immediately stimulates pulmonary ventilation,
which corrects the hypercapnia but not necessarily the
hypoxia.
Conversely, in hypoxia caused by hypoventilation,
carbon dioxide transfer between the alveoli and the
atmosphere is affected as much as is oxygen transfer.
Hypercapnia then 

d as much as is oxygen transfer.
Hypercapnia then occurs along with the hypoxia. And
in circulatory deﬁciency, diminished ﬂow of blood
decreases carbon dioxide removal from the tissues,
resulting in tissue hypercapnia in addition to tissue
hypoxia. However, the transport capacity of the blood
for carbon dioxide is more than three times that for
oxygen, so that the resulting tissue hypercapnia is
much less than the tissue hypoxia.
When the alveolar Pco2 rises above about 60 to 
75 mm Hg, an other

r Pco2 rises above about 60 to 
75 mm Hg, an otherwise normal person by then is
breathing about as rapidly and deeply as he or she 
can, and “air hunger,” also called dyspnea, becomes
severe.
If the Pco2 rises to 80 to 100 mm Hg, the person
becomes lethargic and sometimes even semicomatose.
Anesthesia and death can result when the Pco2 rises
to 120 to 150 mm Hg. At these higher levels of Pco2,
the excess carbon dioxide now begins to depress res-
piration rather than stimulate it, thus causing a 

piration rather than stimulate it, thus causing a vicious
circle: (1) more carbon dioxide, (2) further decrease in
Venous end
PO2 in alveoli and blood (mm Hg)
Blood in pulmonary capillary
0
100
Capillary blood
Alveolar PO2 with tent therapy
Pulmonary edema +  O2 therapy
Pulmonary edema with no therapy
200
300
Figure 42–8
Absorption of oxygen into the pulmonary capillary blood in pul-
monary edema with and without oxygen tent therapy.
532
Unit VII
resp

oxygen tent therapy.
532
Unit VII
respiration, (3) then more carbon dioxide, and so
forth—culminating rapidly in a respiratory death.
Dyspnea
Dyspnea means mental anguish associated with inabil-
ity to ventilate enough to satisfy the demand for air.A
common synonym is air hunger.
At least three factors often enter into the develop-
ment of the sensation of dyspnea. They are (1) abnor-
mality of respiratory gases in the body ﬂuids, especially
hypercapnia and, to a much less extent, hy

ecially
hypercapnia and, to a much less extent, hypoxia; (2)
the amount of work that must be performed by the res-
piratory muscles to provide adequate ventilation; and
(3) state of mind.
A person becomes very dyspneic especially from
excess buildup of carbon dioxide in the body ﬂuids.At
times, however, the levels of both carbon dioxide and
oxygen in the body ﬂuids are normal, but to attain this
normality of the respiratory gases, the person has to
breathe forcefully. In these instances, the for

to
breathe forcefully. In these instances, the forceful
activity of the respiratory muscles frequently gives the
person a sensation of dyspnea.
Finally, the person’s respiratory functions may be
normal and still dyspnea may be experienced because
of an abnormal state of mind.This is called neurogenic
dyspnea or emotional dyspnea. For instance, almost
anyone momentarily thinking about the act of breath-
ing may suddenly start taking breaths a little more
deeply than ordinarily because of a feelin

le more
deeply than ordinarily because of a feeling of mild
dyspnea. This feeling is greatly enhanced in people
who have a psychological fear of not being able to
receive a sufﬁcient quantity of air, such as on entering
small or crowded rooms.
Resuscitator. Many types of respiratory resuscitators
are available, and each has its own characteristic prin-
ciples of operation. The resuscitator shown in Figure
42–9A consists of a tank supply of oxygen or air; a
mechanism for app

 tank supply of oxygen or air; a
mechanism for applying intermittent positive pressure
and, with some machines, negative pressure as well;
and a mask that ﬁts over the face of the patient or a
connector for joining the equipment to an endotra-
cheal tube.This apparatus forces air through the mask
or endotracheal tube into the lungs of the patient
during the positive-pressure cycle of the resuscitator
and then usually allows the air to ﬂow passively out of
the lungs during the remainder of the cy

ly out of
the lungs during the remainder of the cycle.
Earlier resuscitators often caused damage to the
lungs because of excessive positive pressure. Their
usage was at one time greatly decried. However, resus-
citators now have adjustable positive-pressure limits
that are commonly set at 12 to 15 cm H2O pressure for
normal lungs (but sometimes much higher for non-
compliant lungs).
Tank Respirator (the “Iron-Lung”). Figure 42–9B shows the
tank respirator with a patient’s body inside the tank
an

espirator with a patient’s body inside the tank
and the head protruding through a ﬂexible but airtight
collar. At the end of the tank opposite the patient’s
head a motor-driven leather diaphragm moves back
and forth with sufﬁcient excursion to raise and lower
the pressure inside the tank.As the leather diaphragm
moves inward, positive pressure develops around the
body and causes expiration; as the diaphragm moves
outward, negative pressure causes inspiration. Check
valves on the respirator contr

 inspiration. Check
valves on the respirator control the positive and 
negative pressures. Ordinarily these pressures are
adjusted so that the negative pressure that causes
inspiration falls to -10 to -20 cm H2O and the positive
pressure rises to 0 to +5 cm H2O.
Effect of the Resuscitator and the Tank Respirator on Venous
Return. When air is forced into the lungs under posi-
tive pressure by a resuscitator, or when the pressure
around the patient’s body is reduced by the tank 
respirator, the pr

’s body is reduced by the tank 
respirator, the pressure inside the lungs becomes
greater than pressure everywhere else in the body.
Flow of blood into the chest and heart from the
peripheral veins becomes impeded. As a result, use 
of excessive pressures with either the resuscitator or
the tank respirator can reduce the cardiac output—
sometimes to lethal levels. For instance, continuous
exposure for more than a few minutes to greater than
30 mm Hg positive pressure in the lungs can cause
death

 Hg positive pressure in the lungs can cause
death because of inadequate venous return to the
heart.
Positive
pressure
valve
Negative
pressure
valve
Mechanism
for applying
positive and
negative
pressure
A
B
Leather diaphragm
Figure 42–9
A, Resuscitator. B, Tank respirator.
Chapter 42
533
Barnes PJ, Adcock IM: How do corticoster

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Cardoso WV: Molecular regulation of lung development.
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Carter EP, Garat C, Imamura M: Continual emerging roles
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Dwyer TM: Cigarette smoke-induce

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Dwyer TM: Cigarette smoke-induced airway inﬂammation
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Naureckas ET, Solway J: Clinical practice. Mild asthma.
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Page S, Ammit AJ, Black JL, Armour CL: Human mast cell
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U
N
I
T
VIII
Aviation, Space, and
Deep-Sea Diving
43. Aviation, High-Altitude, and Space Physiology
44. Physiology of Deep-Sea Diving and Other 
Hyperbaric Conditions
C
H
A
P
T
E
R
 
4
3
537
Aviation, High-Altitude,
and Space Physiology
As we 

iation, High-Altitude,
As we have ascended to higher and higher altitudes
in aviation, mountain climbing, and space vehicles,
it has become progressively more important to
understand the effects of altitude and low gas pres-
sures (as well as several other factors—acceleratory
forces, weightlessness, and so forth) on the human
body. This chapter deals with these problems.
Effects of Low Oxygen Pressure on the Body
Barometric Pressures at Different Altitudes. Table 43–1 gives

Pressures at Different Altitudes. Table 43–1 gives the approximate baro-
metric and oxygen pressures at different altitudes, showing that at sea level, the
barometric pressure is 760 mm Hg; at 10,000 feet, only 523 mm Hg; and at
50,000 feet, 87 mm Hg. This decrease in barometric pressure is the basic cause
of all the hypoxia problems in high-altitude physiology because, as the baro-
metric pressure decreases, the atmospheric oxygen partial pressure decreases
proportionately, remaining at all tim

re decreases
proportionately, remaining at all times slightly less than 21 per cent of the total
barometric pressure—Po2 at sea level about 159 mm Hg, but at 50,000 feet only
18 mm Hg.
Carbon Dioxide and Water Vapor Decrease the Alveolar Oxygen. Even at high altitudes,
carbon dioxide is continually excreted from the pulmonary blood into the
alveoli.Also, water vaporizes into the inspired air from the respiratory surfaces.
These two gases dilute the oxygen in 

ry surfaces.
These two gases dilute the oxygen in the alveoli, thus reducing the oxygen con-
centration. Water vapor pressure in the alveoli remains 47 mm Hg as long as
the body temperature is normal, regardless of altitude.
In the case of carbon dioxide, during exposure to very high altitudes, the alve-
olar Pco2 falls from the sea-level value of 40 mm Hg to lower values. In the
acclimatized person, who increases his or her ventilation about ﬁvefold, the Pco2
falls to about 7 mm Hg because of i

fold, the Pco2
falls to about 7 mm Hg because of increased respiration.
Now let us see how the pressures of these two gases affect the alveolar
oxygen. For instance, assume that the barometric pressure falls from the normal
sea-level value of 760 mm Hg to 253 mm Hg, which is the usual measured value
at the top of 29,028–foot Mount Everest. Forty-seven millimeters of mercury of
this must be water vapor, leaving only 206 mm Hg for all the other gases. In the
acclimatized person, 7 mm of the 206 mm

es. In the
acclimatized person, 7 mm of the 206 mm Hg must be carbon dioxide, leaving
only 199 mm Hg. If there were no use of oxygen by the body, one ﬁfth of this
199 mm Hg would be oxygen and four ﬁfths would be nitrogen; that is, the Po2
in the alveoli would be 40 mm Hg. However, some of this remaining alveolar
oxygen is continually being absorbed into the blood, leaving about 35 mm Hg
oxygen pressure in the alveoli. At the summit of Mount Everest, only the best
of acclimatized people can bare

est, only the best
of acclimatized people can barely survive when breathing air. But the effect is
very different when the person is breathing pure oxygen, as we see in the fol-
lowing discussions.
538
Unit VIII
Aviation, Space, and Deep-Sea Diving Physiology
Alveolar PO2 at Different Altitudes. The ﬁfth column of
Table 43–1 shows the approximate Po2s in the alveoli
at different altitudes when one is breathing air for both
the unacclimatized and the acclimatized person.At sea
level, the alveolar

the acclimatized person.At sea
level, the alveolar Po2 is 104 mm Hg; at 20,000 feet alti-
tude, it falls to about 40 mm Hg in the unacclimatized
person but only to 53 mm Hg in the acclimatized. The
difference between these two is that alveolar ventila-
tion increases much more in the acclimatized person
than in the unacclimatized person, as we discuss later.
Saturation of Hemoglobin with Oxygen at Different Altitudes.
Figure 43–1 shows arterial blood oxygen saturation at
different altitudes whil

lood oxygen saturation at
different altitudes while a person is breathing air and
while breathing oxygen. Up to an altitude of about
10,000 feet, even when air is breathed, the arterial
oxygen saturation remains at least as high as 90 per
cent.Above 10,000 feet, the arterial oxygen saturation
falls rapidly, as shown by the blue curve of the ﬁgure,
until it is slightly less than 70 per cent at 20,000 feet
and much less at still higher altitudes.
Effect of Breathing Pure Oxygen on
Alveolar PO2 at 

ffect of Breathing Pure Oxygen on
Alveolar PO2 at Different Altitudes
When a person breathes pure oxygen instead of air,
most of the space in the alveoli formerly occupied by
nitrogen becomes occupied by oxygen. At 30,000 feet,
an aviator could have an alveolar Po2 as high as 
139 mm Hg instead of the 18 mm Hg when breathing
air (see Table 43–1).
The red curve of Figure 43–1 shows arterial blood
hemoglobin oxygen saturation at different altitudes
when one is breathing pure oxygen. Note that the 

when one is breathing pure oxygen. Note that the sat-
uration remains above 90 per cent until the aviator
ascends to about 39,000 feet; then it falls rapidly to
about 50 per cent at about 47,000 feet.
The “Ceiling” When Breathing Air 
and When Breathing Oxygen in an
Unpressurized Airplane
Comparing the two arterial blood oxygen satura-
tion curves in Figure 43–1, one notes that an aviator
breathing pure oxygen in an unpressurized airplane
can ascend to far higher altitudes than one breathing
ai

cend to far higher altitudes than one breathing
air. For instance, the arterial saturation at 47,000 feet
when one is breathing oxygen is about 50 per cent and
is equivalent to the arterial oxygen saturation at 23,000
feet when one is breathing air. In addition, because an
unacclimatized person usually can remain conscious
until the arterial oxygen saturation falls to 50 per cent,
for short exposure times the ceiling for an aviator in
an unpressurized airplane when breathing air is about
23,000 

rized airplane when breathing air is about
23,000 feet and when breathing pure oxygen is about
47,000 feet, provided the oxygen-supplying equipment
operates perfectly.
Some of the important acute effects of hypoxia in the
unacclimatized person breathing air, beginning at an
Table 43–1
Effects of Acute Exposure to Low Atmospheric Pressures on Alveolar Gas Concentrations and Arterial Oxygen Saturation*
Breathing Air
Breathing Pure Oxygen
Arterial
Arterial
Barometric
PO2 in

ng Pure Oxygen
Arterial
Arterial
PO2 in
PCO2 in
PO2 in
Oxygen
PCO2 in
PO2 in
Oxygen
Pressure
Air
Alveoli
Alveoli
Saturation
Alveoli
Alveoli
Altitude (ft)
(mm Hg)
(mm Hg)
(mm Hg)
(mm Hg)
(%)
(mm Hg)
(mm Hg)
(%)
0
760
159
40 (40)
104 (104)
97 (97)
40
673
100
10,000
523
110
36 (23)
67 (77)
90 (92)
40
436
100
20,000
349
73
24 (10)
40 (53)
73 (85)
40
262
100
30,000
226
47
24 (7)
18 (30)
24 (38)
40
139
99
40,000
141
29
36
58
84
50,000
87
18
24
16
15
* Numbers in parentheses are a

,000
87
18
24
16
15
* Numbers in parentheses are acclimatized values.
0
10
20
30
40
50
Altitude (thousands of feet)
50
60
70
80
90
100
Breathing pure oxygen
Breathing air
Figure 43–1
Effect of high altitude on arterial oxygen saturation when breath-
ing air and when breathing pure oxygen.
Chapter 43
Aviation, High-Altitude, and Space Physiology
539
altitude of about 12,000 feet, are drowsiness, lassitude,
mental and muscle fatigue, sometimes headache, occa-

tal and muscle fatigue, sometimes headache, occa-
sionally nausea,and sometimes euphoria.These effects
progress to a stage of twitchings or seizures above
18,000 feet and end, above 23,000 feet in the unaccli-
matized person, in coma, followed shortly thereafter
by death.
One of the most important effects of hypoxia is
decreased mental proﬁciency, which decreases judg-
ment, memory, and performance of discrete motor
movements. For instance, if an unacclimatized aviator
stays at 15,000 feet for 1

 unacclimatized aviator
stays at 15,000 feet for 1 hour, mental proﬁciency ordi-
narily falls to about 50 per cent of normal, and after
18 hours at this level it falls to about 20 per cent of
normal.
A person remaining at high altitudes for days, weeks,
or years becomes more and more acclimatized to the
low Po2, so that it causes fewer deleterious effects on
the body. And it becomes possible for the person to
work harder without hypoxic effects or to ascend to
still hi

r without hypoxic effects or to ascend to
still higher altitudes.
The principal means by which acclimatization comes
about are (1) a great increase in pulmonary ventila-
tion, (2) increased numbers of red blood cells, (3)
increased diffusing capacity of the lungs, (4) increased
vascularity of the peripheral tissues, and (5) increased
ability of the tissue cells to use oxygen despite low Po2.
Increased Pulmonary Ventilation—Role of Arterial Chemore-
ceptors. Immediate exposure to low Po2 stimulat

e-
ceptors. Immediate exposure to low Po2 stimulates the
arterial chemoreceptors, and this increases alveolar
ventilation to a maximum of about 1.65 times normal.
Therefore, compensation occurs within seconds for the
high altitude, and it alone allows the person to rise
several thousand feet higher than would be possible
without the increased ventilation. Then, if the person
remains at very high altitude for several days, the
chemoreceptors increase ventilation still more, up to
about ﬁve times 

ase ventilation still more, up to
about ﬁve times normal.
The immediate increase in pulmonary ventilation on
rising to a high altitude blows off large quantities of
carbon dioxide, reducing the Pco2 and increasing the
pH of the body ﬂuids. These changes inhibit the brain
stem respiratory center and thereby oppose the effect
of low PO2 to stimulate respiration by way of the
peripheral arterial chemoreceptors in the carotid and
aortic bodies. But during the ensuing 2 to 5 days, this
inhibition fad

uring the ensuing 2 to 5 days, this
inhibition fades away, allowing the respiratory center
to respond with full force to the peripheral chemore-
ceptor stimulus from hypoxia, and ventilation in-
creases to about ﬁve times normal.
The cause of this fading inhibition is believed to be
mainly a reduction of bicarbonate ion concentration in
the cerebrospinal ﬂuid as well as in the brain tissues.
This in turn decreases the pH in the ﬂuids surround-
ing the chemosensitive neurons of the respiratory
ce

g the chemosensitive neurons of the respiratory
center, thus increasing the respiratory stimulatory
activity of the center.
An important mechanism for the gradual decrease
in bicarbonate concentration is compensation by the
kidneys for the respiratory alkalosis, as discussed in
Chapter 30. The kidneys respond to decreased Pco2
by reducing hydrogen ion secretion and increasing
bicarbonate excretion. This metabolic compensation
for the respiratory alkalosis gradually reduces plasma
and cerebrospin

alkalosis gradually reduces plasma
and cerebrospinal ﬂuid bicarbonate concentration and
pH toward normal and removes part of the inhibitory
effect on respiration of low hydrogen ion concentra-
tion. Thus, the respiratory centers are much more
responsive to the peripheral chemoreceptor stimulus
caused by the hypoxia after the kidneys compensate
for the alkalosis.
Increase in Red Blood Cells and Hemoglobin Concentration
During Acclimatization. As discussed in Chapter 32,
hypoxia is the principal s

iscussed in Chapter 32,
hypoxia is the principal stimulus for causing an
increase in red blood cell production. Ordinarily, when
a person remains exposed to low oxygen for weeks at
a time, the hematocrit rises slowly from a normal value
of 40 to 45 to an average of about 60, with an average
increase in whole blood hemoglobin concentration
from normal of 15 g/dl to about 20 g/dl.
In addition, the blood volume also increases, often
by 20 to 30 per cent, and this increase times the
increased blood 

cent, and this increase times the
increased blood hemoglobin concentration gives an
increase in total body hemoglobin of 50 or more per
cent.
Increased Diffusing Capacity After Acclimatization. It will 
be recalled that the normal diffusing capacity for
oxygen through the pulmonary membrane is about 
21 ml/mm Hg/min, and this diffusing capacity can
increase as much as threefold during exercise. A
similar increase in diffusing capacity occurs at high
altitude.
Part of the increase results from in

igh
altitude.
Part of the increase results from increased pul-
monary capillary blood volume, which expands the
capillaries and increases the surface area through
which oxygen can diffuse into the blood.Another part
results from an increase in lung air volume, which
expands the surface area of the alveolar-capillary
interface still more. A ﬁnal part results from an
increase in pulmonary arterial blood pressure; this
forces blood into greater numbers of alveolar capil-
laries than normally—especi

ers of alveolar capil-
laries than normally—especially in the upper parts 
of the lungs, which are poorly perfused under usual
Peripheral Circulatory System Changes During Acclimatiza-
tion—Increased Tissue Capillarity. The cardiac output
often increases as much as 30 per cent immediately
after a person ascends to high altitude but then
decreases back toward normal over a period of weeks
as the blood hematocrit increases, so that the amount
of oxygen transported to the peripheral bod

amount
of oxygen transported to the peripheral body tissues
remains about normal.
Another circulatory adaptation is growth of
increased numbers of systemic circulatory capillaries
in the nonpulmonary tissues, which is called in-
creased tissue capillarity (or angiogenesis). This occurs
540
Unit VIII
especially in animals born and bred at high altitudes
but less so in animals that later in life become exposed
to high altitude.
In active tissues expo

e exposed
In active tissues exposed to chronic hypoxia, the
increase in capillarity is especially marked. For
instance, capillary density in right ventricular muscle
increases markedly because of the combined effects 
of hypoxia and excess workload on the right ventricle
caused by pulmonary hypertension at high altitude.
Cellular Acclimatization. In animals native to altitudes of
13,000 to 17,000 feet, cell mitochondria and cellular
oxidative enzyme systems are slightly more pl

ular
oxidative enzyme systems are slightly more plentiful
than in sea-level inhabitants. Therefore, it is presumed
that the tissue cells of high altitude–acclimatized
human beings also can use oxygen more effectively
than can their sea-level counterparts.
Natural Acclimatization 
of Native Human Beings Living 
at High Altitudes
Many native human beings in the Andes and in the
Himalayas live at altitudes above 13,000 feet—one
group in the Peruvian Andes lives at an altitude of
17,500 feet and wor

 Andes lives at an altitude of
17,500 feet and works a mine at an altitude of 19,000
feet. Many of these natives are born at these altitudes
and live there all their lives. In all aspects of acclima-
tization, the natives are superior to even the best-
acclimatized lowlanders, even though the lowlanders
might also have lived at high altitudes for 10 or more
years. Acclimatization of the natives begins in infancy.
The chest size, especially, is greatly increased, whereas
the body size is somewhat

eatly increased, whereas
the body size is somewhat decreased, giving a high
ratio of ventilatory capacity to body mass. In addition,
their hearts, which from birth onward pump extra
amounts of cardiac output, are considerably larger
than the hearts of lowlanders.
Delivery of oxygen by the blood to the tissues is also
highly facilitated in these natives. For instance, Figure
43–2 shows oxygen-hemoglobin dissociation curves for
natives who live at sea level and for their counterparts
who live at 1

sea level and for their counterparts
who live at 15,000 feet. Note that the arterial oxygen
Po2 in the natives at high altitude is only 40 mm Hg,
but because of the greater quantity of hemoglobin,
the quantity of oxygen in their arterial blood is greater
than that in the blood of the natives at the lower 
altitude. Note also that the venous Po2 in the high-
altitude natives is only 15 mm Hg less than the venous
Po2 for the lowlanders, despite the very low arterial
Po2, indicating that oxygen tra

 very low arterial
Po2, indicating that oxygen transport to the tissues is
exceedingly effective in the naturally acclimatized
high-altitude natives.
Reduced Work Capacity at 
High Altitudes and Positive Effect 
of Acclimatization
In addition to the mental depression caused by
hypoxia, as discussed earlier, the work capacity of all
muscles is greatly decreased in hypoxia. This includes
not only skeletal muscles but also cardiac muscles.
In general, work capacity is reduced in direct pro-
portion

l, work capacity is reduced in direct pro-
portion to the decrease in maximum rate of oxygen
uptake that the body can achieve.
To give an idea of the importance of acclimatization
in increasing work capacity, consider this: The work
capacities as per cent of normal for unacclimatized and
acclimatized people at an altitude of 17,000 feet are as
follows:
Work capacity
Unacclimatized 
50
Acclimatized for 2 months 
68
Native living at 13,200 feet but working at 
17,000 feet 
87

ng at 13,200 feet but working at 
87
Thus, naturally acclimatized native persons can
achieve a daily work output even at high altitude
almost equal to that of a lowlander at sea level, but
even well-acclimatized lowlanders can almost never
achieve this result.
Acute Mountain Sickness and 
High-Altitude Pulmonary Edema
A small percentage of people who ascend rapidly to
high altitudes become acutely sick and can die if not
0
20
40
60
80
100 120
140
Pressure of oxygen in blood (PO2) (m

0
100 120
140
Quantity of oxygen in blood (vol %)
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
Sea-level dwellers
(Venous values)
Mountain dwellers
(15,000 ft)
(Arterial values)
X
X
X
X
Figure 43–2
Oxygen-hemoglobin dissociation curves for blood of high-altitude
residents (red curve) and sea-level residents (blue curve),
showing the respective arterial and venous PO2 levels and oxygen
contents as recorded in their native surroundings. (Data from
Oxygen-dissoc

heir native surroundings. (Data from
Oxygen-dissociation curves for bloods of high-altitude and sea-
level residents. PAHO Scientiﬁc Publication No. 140, Life at High
Altitudes, 1966.)
Chapter 43
541
given oxygen or removed to a low altitude. The sick-
ness begins from a few hours up to about 2 days after
ascent. Two events frequently occur:
1. Acute cerebral edema. This is believed to result
from local vasodilation of the cerebral blood
vessels, cau

al vasodilation of the cerebral blood
vessels, caused by the hypoxia. Dilation of the
arterioles increases blood ﬂow into the capillaries,
thus increasing capillary pressure, which in turn
causes ﬂuid to leak into the cerebral tissues.
The cerebral edema can then lead to severe
disorientation and other effects related to cerebral
2. Acute pulmonary edema. The cause of this is still
unknown, but a suggested answer is the following:
The severe hypoxia causes the pulmonary arterioles
t

e severe hypoxia causes the pulmonary arterioles
to constrict potently, but the constriction is much
greater in some parts of the lungs than in other
parts, so that more and more of the pulmonary
blood ﬂow is forced through fewer and fewer still
unconstricted pulmonary vessels. The postulated
result is that the capillary pressure in these areas of
the lungs becomes especially high and local edema
occurs. Extension of the process to progressively
more areas of the lungs leads to spreading
pulmona

pulmonary edema and severe pulmonary
dysfunction that can be lethal. Allowing the person
to breathe oxygen usually reverses the process
within hours.
Occasionally, a person who remains at high altitude
too long develops chronic mountain sickness, in which
the following effects occur: (1) the red cell mass and
hematocrit become exceptionally high, (2) the pul-
monary arterial pressure becomes elevated even more
than the normal e

ssure becomes elevated even more
than the normal elevation that occurs during acclima-
tization, (3) the right side of the heart becomes greatly
enlarged, (4) the peripheral arterial pressure begins to
fall, (5) congestive heart failure ensues, and (6) death
often follows unless the person is removed to a lower
altitude.
The causes of this sequence of events are probably
threefold: First, the red cell mass becomes so great that
the blood viscosity increases severalfold; this increased
viscosity 

y increases severalfold; this increased
viscosity tends to decrease tissue blood ﬂow so that
oxygen delivery also begins to decrease. Second, the
pulmonary arterioles become vasoconstricted because
of the lung hypoxia. This results from the hypoxic 
vascular constrictor effect that normally operates to
divert blood ﬂow from low-oxygen to high-oxygen
alveoli, as explained in Chapter 38. But because all the
alveoli are now in the low-oxygen state, all the arteri-
oles become constricted, the pulmo

all the arteri-
oles become constricted, the pulmonary arterial pres-
sure rises excessively, and the right side of the heart
fails. Third, the alveolar arteriolar spasm diverts much
of the blood ﬂow through nonalveolar pulmonary
vessels, thus causing an excess of pulmonary shunt
blood ﬂow where the blood is poorly oxygenated; this
further compounds the problem. Most of these people
recover within days or weeks when they are moved to
a lower altitude.
Effects of Acceleratory Forces
on the Body i

tude.
on the Body in Aviation and
Space Physiology
Because of rapid changes in velocity and direction of
motion in airplanes or spacecraft, several types of
acceleratory forces affect the body during ﬂight.At the
beginning of ﬂight, simple linear acceleration occurs;
at the end of ﬂight, deceleration; and every time the
vehicle turns, centrifugal acceleration.
When an airplane makes a turn, the force of cen-
trifugal acceleration is 

 turn, the force of cen-
trifugal acceleration is determined by the following 
relation:
in which f is centrifugal acceleratory force, m is the
mass of the object, v is velocity of travel, and r is radius
of curvature of the turn. From this formula, it is
obvious that as the velocity increases, the force of 
centrifugal acceleration increases in proportion to the
square of the velocity. It is also obvious that the force
of acceleration is directly proportional to the sharpness
of the turn (the l

y proportional to the sharpness
of the turn (the less the radius).
of 
Acceleratory 
Force—“G.” When 
an
aviator is simply sitting in his seat, the force with which
he is pressing against the seat results from the pull of
gravity and is equal to his weight. The intensity of this
force is said to be +1 G because it is equal to the pull
of gravity. If the force with which he presses against
the seat becomes ﬁve times his normal weight during
pull-out from a dive, the force acting on t

during
pull-out from a dive, the force acting on the seat is 
+5 G.
If the airplane goes through an outside loop so that
the person is held down by his seat belt, negative G is
applied to his body; if the force with which he is held
down by his belt is equal to the weight of his body, the
negative force is -1 G.
Effects of Centrifugal Acceleratory Force on the Body—
(Positive G)
Effects on the Circulatory System. The most impor-
tant effect of centrifugal acceleration is on the circu-
latory sys

ntrifugal acceleration is on the circu-
latory system, because blood is mobile and can be
translocated by centrifugal forces.
When an aviator is subjected to positive G, blood is
centrifuged toward the lowermost part of the body.
Thus, if the centrifugal acceleratory force is +5 G and
the person is in an immobilized standing position,
the pressure in the veins of the feet becomes greatly
increased (to about 450 mm Hg). In the sitting posi-
tion, the pressure becomes nearly 300 mm Hg.And, as
pres

the pressure becomes nearly 300 mm Hg.And, as
pressure in the vessels of the lower body increases,
these vessels passively dilate so that a major portion
of the blood from the upper body is translocated into
the lower vessels. Because the heart cannot pump
f
mv
r
=
2
542
Unit VIII
unless blood returns to it, the greater the quantity of
blood “pooled” in this way in the lower body, the less
that is available for the cardiac output.
Figure 43–3 shows

vailable for the cardiac output.
Figure 43–3 shows the changes in systolic and 
diastolic arterial pressures (top and bottom curves,
respectively) in the upper body when a centrifugal
acceleratory force of +3.3 G is suddenly applied to a
sitting person. Note that both these pressures fall
below 22 mm Hg for the ﬁrst few seconds after the
acceleration begins but then return to a systolic pres-
sure of about 55 mm Hg and a diastolic pressure of 
20 mm Hg within another 10 to 15 seconds. This sec-

 mm Hg within another 10 to 15 seconds. This sec-
ondary recovery is caused mainly by activation of the
baroreceptor reﬂexes.
Acceleration greater than 4 to 6 G causes “black-
out” of vision within a few seconds and unconscious-
ness shortly thereafter. If this great degree of
acceleration is continued, the person will die.
Effects on the Vertebrae. Extremely high acceleratory
forces for even a fraction of a second can fracture the
vertebrae. The degree of positive acceleration that the
average 

 degree of positive acceleration that the
average person can withstand in the sitting position
before vertebral fracture occurs is about 20 G.
Negative G. The effects of negative G on the body are
less dramatic acutely but possibly more damaging per-
manently than the effects of positive G.An aviator can
usually go through outside loops up to negative accel-
eratory forces of -4 to -5 G without causing perma-
nent harm, although causing intense momentary
hyperemia of the head. Occasionally, psyc

omentary
hyperemia of the head. Occasionally, psychotic distur-
bances lasting for 15 to 20 minutes occur as a result of
brain edema.
Occasionally, negative G forces can be so great 
(-20 G, for instance) and centrifugation of the blood
into the head is so great that the cerebral blood pres-
sure reaches 300 to 400 mm Hg, sometimes causing
small vessels on the surface of the head and in the
brain to rupture. However, the vessels inside the
cranium show less tendency for rupture than would be
exp

m show less tendency for rupture than would be
expected for the following reason: The cerebrospinal
ﬂuid is centrifuged toward the head at the same time
that blood is centrifuged toward the cranial vessels,
and the greatly increased pressure of the cerebrospinal
ﬂuid acts as a cushioning buffer on the outside of the
brain to prevent intracerebral vascular rupture.
Because the eyes are not protected by the cranium,
intense hyperemia occurs in them during strong nega-
tive G. As a result, the eyes

 during strong nega-
tive G. As a result, the eyes often become temporarily
blinded with “red-out.”
Protection of the Body Against Centrifugal Acceleratory
Forces. Speciﬁc procedures and apparatus have been
developed to protect aviators against the circulatory
collapse that might occur during positive G. First, if 
the aviator tightens his or her abdominal muscles to
an extreme degree and leans forward to compress the
abdomen, some of the pooling of blood in the large
vessels of the abdomen can 

 of blood in the large
vessels of the abdomen can be prevented, there-
by delaying the onset of blackout. Also, special “anti-
G”suits have been devised to prevent pooling of blood
in the lower abdomen and legs. The simplest of these
applies positive pressure to the legs and abdomen by
inﬂating compression bags as the G increases. Theo-
retically, a pilot submerged in a tank or suit of water
might experience little effect of G forces on the circu-
lation because the pressures developed in the wa

-
lation because the pressures developed in the water
pressing on the outside of the body during centrifugal
acceleration would almost exactly balance the forces
acting in the body. However, the presence of air in 
the lungs still allows displacement of the heart, lung
tissues, and diaphragm into seriously abnormal posi-
tions despite submersion in water. Therefore, even if
this procedure were used, the limit of safety almost
certainly would still be less than 10 G.
Effects of Linear Accelerator

l be less than 10 G.
Effects of Linear Acceleratory Forces
on the Body
Acceleratory Forces in Space Travel. Unlike an airplane, a
spacecraft cannot make rapid turns; therefore, cen-
trifugal acceleration is of little importance except
when the spacecraft goes into abnormal gyrations.
However, blast-off acceleration and landing decelera-
tion can be tremendous; both of these are types 
of linear acceleration, one positive and the other 
negative.
Figure 43–4 shows an approximate proﬁle of accel-

eration during blast-off in a three-stage spacecraft,
demonstrating that the ﬁrst-stage booster causes accel-
eration as high as 9 G, and the second-stage booster as
high as 8 G. In the standing position, the human body
could not withstand this much acceleration, but in a
semireclining position transverse to the axis of acceler-
ation, this amount of acceleration can be withstood
with ease despite the fact that the acceleratory forces
continue fo

 the fact that the acceleratory forces
continue for as long as several minutes at a time.
Therefore, we see the reason for the reclining seats
used by astronauts.
0
5
10
15
20
25
30
Time from start of G to symptoms
(sec)
(mm Hg)
0
50
100
Figure 43–3
Changes in systolic (top of curve) and diastolic (bottom of curve)
arterial pressures after abrupt and continuing exposure of a sitting
person to an acceleratory force from top to bottom of 3.3. G. (Data
from Martin EE, Henry JP: Ef

ttom of 3.3. G. (Data
from Martin EE, Henry JP: Effects of time and temperature upon
tolerance to positive acceleration. J Aviation Med 22:382, 1951.)
Chapter 43
543
Problems also occur during deceleration when the
spacecraft re-enters the atmosphere. A person travel-
ing at Mach 1 (the speed of sound and of fast air-
planes) can be safely decelerated in a distance of about
0.12 mile, whereas a person traveling at a speed of
Mach 100 (a speed possibl

 traveling at a speed of
Mach 100 (a speed possible in interplanetary space
travel) would require a distance of about 10,000 miles
for safe deceleration. The principal reason for this dif-
ference is that the total amount of energy that must 
be dispelled during deceleration is proportional to 
the square of the velocity, which alone increases the
required distance for decelerations between Mach 1
versus Mach 100 about 10,000-fold. But in addition to
this, a human being can withstand far less de

n to
this, a human being can withstand far less deceleration
if the period of deceleration lasts for a long time than
for a short time. Therefore, deceleration must be
accomplished much more slowly from high velocities
than is necessary at lower velocities.
Deceleratory Forces Associated with Parachute Jumps.
When the parachuting aviator leaves the airplane, his
velocity of fall is at ﬁrst exactly 0 feet per second.
However, because of the acceleratory force of gravity,
within 1 second his veloc

ratory force of gravity,
within 1 second his velocity of fall is 32 feet per second
(if there is no air resistance); in 2 seconds it is 64 feet
per second; and so on.As the velocity of fall increases,
the air resistance tending to slow the fall also increases.
Finally, the deceleratory force of the air resistance
exactly balances the acceleratory force of gravity, so
that after falling for about 12 seconds, the person will
be falling at a “terminal velocity” of 109 to 119 miles
per hour (175 fee

al velocity” of 109 to 119 miles
per hour (175 feet per second). If the parachutist has
already reached terminal velocity before opening his
parachute, an “opening shock load” of up to 1200
pounds can occur on the parachute shrouds.
The usual-sized parachute slows the fall of the para-
chutist to about one ninth the terminal velocity. In
other words, the speed of landing is about 20 feet per
second, and the force of impact against the earth is
1/81 the impact force without a parachute. Even so,

81 the impact force without a parachute. Even so,
the force of impact is still great enough to cause con-
siderable damage to the body unless the parachutist 
is properly trained in landing. Actually, the force of
impact with the earth is about the same as that which
would be experienced by jumping without a parachute
from a height of about 6 feet. Unless forewarned, the
parachutist will be tricked by his senses into striking
the earth with extended legs, and this will result in
tremendous decel

ded legs, and this will result in
tremendous deceleratory forces along the skeletal axis
of the body, resulting in fracture of his pelvis, verte-
brae, or leg. Consequently, the trained parachutist
strikes the earth with knees bent but muscles taut to
cushion the shock of landing.
“Artiﬁcial Climate” in the
Sealed Spacecraft
Because there is no atmosphere in outer space, an arti-
ﬁcial atmosphere and climate must be produced in a
spacecraft. Most important, the oxygen concentration
must remain h

 important, the oxygen concentration
must remain high enough and the carbon dioxide con-
centration low enough to prevent suffocation. In some
earlier space missions, a capsule atmosphere contain-
ing pure oxygen at about 260 mm Hg pressure was
used, but in the modern space shuttle, gases about
equal to those in normal air are used, with four times
as much nitrogen as oxygen and a total pressure of 
760 mm Hg. The presence of nitrogen in the mixture
greatly diminishes the likelihood of ﬁre and e

ure
greatly diminishes the likelihood of ﬁre and explosion.
It also protects against development of local patches
of lung atelectasis that often occur when breathing
pure oxygen because oxygen is absorbed rapidly when
small bronchi are temporarily blocked by mucous
plugs.
For space travel lasting more than several months,
it is impractical to carry along an adequate oxygen
supply. For this reason, recycling techniques have been
proposed for use of the same oxygen over and over
again. Some recycl

f the same oxygen over and over
again. Some recycling processes depend on purely
physical procedures, such as electrolysis of water to
release oxygen. Others depend on biological methods,
such as use of algae with their large store of chloro-
phyll to release oxygen from carbon dioxide by the
process of photosynthesis. A completely satisfactory
system for recycling has yet to be achieved.
A person in an orbiting satellite or a nonpropelled
spacecraft experiences weightles

or a nonpropelled
spacecraft experiences weightlessness, or a state of
near-zero G force, which is sometimes called micro-
gravity. That is, the person is not drawn toward the
bottom, sides, or top of the spacecraft but simply ﬂoats
inside its chambers. The cause of this is not failure of
gravity to pull on the body, because gravity from any
nearby heavenly body is still active. However, the
gravity acts on both the spacecraft and the person at
0
1
2
3
4
5
Minutes
Acceleration (G)
0
10
8
6
4
2
F

1
2
3
4
5
Minutes
0
10
8
6
4
2
First
booster
Second
booster
Space
ship
Figure 43–4
Acceleratory forces during takeoff of a spacecraft.
544
Unit VIII
the same time, so that both are pulled with exactly the
same acceleratory forces and in the same direction.
For this reason, the person simply is not attracted
toward any speciﬁc wall of the spacecraft.
Physiologic Problems of Weightlessness (Microgravity). The
physiologic problems of

ssness (Microgravity). The
physiologic problems of weightlessness have not
proved to be of much signiﬁcance, as long as the period
of weightlessness is not too long. Most of the problems
that do occur are related to three effects of the weight-
lessness: (1) motion sickness during the ﬁrst few days
of travel, (2) translocation of ﬂuids within the body
because of failure of gravity to cause normal hydro-
static pressures, and (3) diminished physical activity
because no strength of muscle contract

al activity
because no strength of muscle contraction is required
to oppose the force of gravity.
Almost 50 per cent of astronauts experience motion
sickness, with nausea and sometimes vomiting, during
the ﬁrst 2 to 5 days of space travel. This probably
results from an unfamiliar pattern of motion signals
arriving in the equilibrium centers of the brain, and at
the same time lack of gravitational signals.
The observed effects of prolonged stay in space 
are the following: (1) decrease in blood v

 space 
are the following: (1) decrease in blood volume, (2)
decrease in red blood cell mass, (3) decrease in muscle
strength and work capacity, (4) decrease in maximum
cardiac output, and (5) loss of calcium and phosphate
from the bones, as well as loss of bone mass. Most of
these same effects also occur in people who lie in bed
for an extended period of time. For this reason, exer-
cise programs are carried out by astronauts during
prolonged space missions.
In previous space laboratory expedit

ace missions.
In previous space laboratory expeditions in which
the exercise program had been less vigorous, the astro-
nauts had severely decreased work capacities for the
ﬁrst few days after returning to earth. They also had a
tendency to faint (and still do, to some extent) when
they stood up during the ﬁrst day or so after return to
gravity because of diminished blood volume and
diminished responses of the arterial pressure control
Cardiovascular, Muscle, and Bone “Deconditioning

.
Cardiovascular, Muscle, and Bone “Deconditioning” During
Prolonged Exposure to Weightlessness. During very long
space ﬂights and prolonged exposure to microgravity,
gradual “deconditioning” effects occur on the cardio-
vascular system, skeletal muscles, and bone despite rig-
orous exercise during the ﬂight. Studies of astronauts
on space ﬂights lasting several months have shown that
they may lose as much 1.0 percent of their bone mass
each month even though they continue to exercise.
Substanti

h even though they continue to exercise.
Substantial atrophy of cardiac and skeletal muscles
also occurs during prolonged exposure to a micro-
gravity environment.
One of the most serious effects is cardiovascular
“deconditioning”, which includes decreased work
capacity, reduced blood volume, impaired barorecep-
tor reﬂexes, and reduced orthostatic tolerance. These
changes greatly limit the astronauts’ ability to stand
upright or perform normal daily activities after return-
ing to the full grav

aily activities after return-
ing to the full gravity of Earth. Astronauts returning
from space ﬂights lasting 4 to 6 months are also sus-
ceptible to bone fractures and may require several
weeks before they return to pre-ﬂight cardiovascular,
bone, and muscle ﬁtness. As space ﬂights become
longer in preparation for possible human exploration
of other planets, such as Mars, the effects of prolonged
microgravity could pose a very serious threat to astro-
nauts after they land, especially in the e

 astro-
nauts after they land, especially in the event of an
emergency landing. Therefore, considerable research
effort has been directed toward developing counter-
measures, in addition to exercise, that can prevent or
more effectively attenuate these changes. One such
countermeasure that is being tested is the application
of intermittent “artiﬁcial gravity” caused by short
periods (e.g., 1 hour each day) of centrifugal accelera-
tion of the astronauts while they sit in specially
designed short

ronauts while they sit in specially
designed short-arm centrifuges that create forces of up
to 2 to 3 G.
Adams GR, Caiozzo VJ, Baldwin KM: Skeletal muscle
unweighting: spaceﬂight and ground-based models. J Appl
Physiol 95:2185, 2003.
Alfrey CP, Udden MM, Leach-Huntoon C, et al: Control of
red blood cell mass in spaceﬂight. J Appl Physiol 81:98,
1996.
Convertino VA: Mechanisms of microgravity induced ortho-
static int

chanisms of microgravity induced ortho-
static intolerance: implications for effective countermea-
sures. J Gravit Physiol 9:1, 2002.
Eckberg DL: Bursting into space: alterations of sympathetic
control by space travel. Acta Physiol Scand 177:299, 2003.
Hackett PH, Roach RC: High-altitude illness. N Engl J Med
345:107, 2001.
Harm DL, Jennings RT, Meck JV, et al: Gender issues related
to spaceﬂight: a NASA perspective. J Appl Physiol
91:2374, 2001.
Hochachka PW, Beatty CL, Burelle Y, et al: The la

Hochachka PW, Beatty CL, Burelle Y, et al: The lactate
paradox in human high-altitude physiological perform-
ance. News Physiol Sci 17:122, 2002.
Hultgren HN: High-altitude pulmonary edema: current con-
cepts. Annu Rev Med 47:267, 1996.
Rupert JL, Hochachka PW: Genetic approaches to under-
standing human adaptation to altitude in the Andes. J Exp
Biol 204(Pt 18):3151, 2001.
Smith S

e Andes. J Exp
Smith SM, Heer M: Calcium and bone metabolism during
space ﬂight. Nutrition 18:849, 2002.
West JB: Climbing Mount Everest without oxygen. News
Physiol Sci 1:25, 1986.
West JB: Man in space. News Physiol Sci 1:198, 1986.
West JB: High Life—History of High-Altitude Physiology
and Medicine. Bethesda, MD: American Physiological
Zhang LF: Vascular adaptation to microgravity: what have
we learned? J Appl Physiol 91:2415, 2001.
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H
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P
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E
R
 
4

d? J Appl Physiol 91:2415, 2001.
C
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P
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E
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4
4
545
Physiology of Deep-Sea Diving 
and Other Hyperbaric Conditions
When human beings descend beneath the sea, the
pressure around them increases tremendously. To
keep the lungs from collapsing, air must be supplied
at very high pressure to keep them inﬂated. This
exposes the blood in the lungs also to extremely
high alveolar gas pressure, a condition called hyper-
barism. Beyond certain limits, these high pressures
can cause tremendous alterat

 these high pressures
can cause tremendous alterations in body physiol-
ogy and can be lethal.
Relationship of Pressure to Sea Depth. A column of seawater 33 feet deep exerts the
same pressure at its bottom as the pressure of the atmosphere above the sea.
Therefore, a person 33 feet beneath the ocean surface is exposed to 2 atmos-
pheres pressure, 1 atmosphere of pressure caused by the weight of the air above
the water and the second atmosphere by the weight of the water itself. At 66
feet the p

y the weight of the water itself. At 66
feet the pressure is 3 atmospheres, and so forth, in accord with the table in
Figure 44–1.
Effect of Sea Depth on the Volume of Gases-Boyle’s Law. Another important effect of
depth is compression of gases to smaller and smaller volumes. The lower part
of Figure 44–1 shows a bell jar at sea level containing 1 liter of air. At 33 feet
beneath the sea, where the pressure is 2 atmospheres, the volume has been com-
pressed to only one-half liter, and at 8 atmos

om-
pressed to only one-half liter, and at 8 atmospheres (233 feet) to one-eighth
liter.Thus, the volume to which a given quantity of gas is compressed is inversely
proportional to the pressure. This is a principle of physics called Boyle’s law,
which is extremely important in diving physiology because increased pressure
can collapse the air chambers of the diver’s body, especially the lungs, and often
causes serious damage.
Many times in this chapter it is necessary to refer to actual volume ve

apter it is necessary to refer to actual volume versus 
sea-level volume. For instance, we might speak of an actual volume of 1 liter at
a depth of 300 feet; this is the same quantity of air as a sea-level volume of 
10 liters.
Effect of High Partial Pressures of Individual
Gases on the Body
The individual gases to which a diver is exposed when breathing air are nitro-
gen, oxygen, and carbon dioxide; each of these at times can cause signiﬁcant
physiologic effects at high pressures.
Nitrogen Nar

hysiologic effects at high pressures.
Nitrogen Narcosis at High Nitrogen Pressures
About four ﬁfths of the air is nitrogen. At sea-level pressure, the nitrogen has
no signiﬁcant effect on bodily function, but at high pressures it can cause varying
degrees of narcosis. When the diver remains beneath the sea for an hour or
more and is breathing compressed air, the depth at which the ﬁrst symptoms of
mild narcosis appear is about 120 feet. At this level the diver begins to exhibit
546
Unit VIII
Avi

evel the diver begins to exhibit
546
Unit VIII
joviality and to lose many of his or her cares. At 150
to 200 feet, the diver becomes drowsy. At 200 to 250
feet, his or her strength wanes considerably, and the
diver often becomes too clumsy to perform the work
required. Beyond 250 feet (8.5 atmospheres pressure),
the diver usually becomes almost useless as a result of
nitrogen narcosis if he or she remains at these depths
too long.
Nitrogen narcosis

emains at these depths
too long.
Nitrogen narcosis has characteristics similar to those
of alcohol intoxication, and for this reason it has 
frequently been called “raptures of the depths.” The
mechanism of the narcotic effect is believed to be the
same as that of most other gas anesthetics. That is, it
dissolves in the fatty substances in neuronal mem-
branes and, because of its physical effect on altering
ionic conductance through the membranes, reduces
neuronal excitability.
Oxygen Toxicity a

, reduces
Effect of Very High PO2 on Blood Oxygen Transport. When the
Po2 in the blood rises above 100 mm Hg, the amount
of oxygen dissolved in the water of the blood increases
markedly. This is shown in Figure 44–2, which depicts
the same oxygen-hemoglobin dissociation curve as
that shown in Chapter 40 but with the alveolar Po2
extended to more than 3000 mm Hg. Also depicted by
the lowest curve in the ﬁgure is the volume of oxygen
dissolv

curve in the ﬁgure is the volume of oxygen
dissolved in the ﬂuid of the blood at each Po2 level.
Note that in the normal range of alveolar Po2 (below
120 mm Hg), almost none of the total oxygen in the
blood is accounted for by dissolved oxygen, but as the
oxygen pressure rises into the thousands of millime-
ters of mercury, a large portion of the total oxygen is
then dissolved in the water of the blood, in addition to
that bound with hemoglobin.
Effect of High Alveolar PO2 on Tissue PO2. Let us 

Effect of High Alveolar PO2 on Tissue PO2. Let us assume that
the Po2 in the lungs is about 3000 mm Hg (4 atmos-
pheres pressure). Referring to Figure 44–2, one ﬁnds
that this represents a total oxygen content in each 100
milliliters of blood of about 29 volumes per cent, as
demonstrated by point A in the ﬁgure—this means 
20 volumes per cent bound with hemoglobin and 9
volumes per cent dissolved in the blood water. As this
blood passes through the tissue capillaries and the
tissues use their no

he tissue capillaries and the
tissues use their normal amount of oxygen, about 
5 milliliters from each 100 milliliters of blood, the
Depth (feet)
Sea level
33
66
100
133
166
200
300
400
500
Atmosphere(s)
1 liter
Sea level
33 ft
1/2 liter
1/4 liter
1/8 liter
1
2
3
4
5
6
7
10
13
16
100 ft
233 ft
Figure 44–1
Effect of sea depth on pressure (top table) and on gas volume
(bottom).
0
760
Total O2 in blood
Oxygen
poisoning
Combined with
hemoglobin
Oxygen-hemoglobin dissociation curve
B
A
Dissolved in

en-hemoglobin dissociation curve
B
A
water of blood
oxygen pressure
1560
2280
3040
Oxygen partial pressure in lungs (mm Hg)
Oxygen in blood (volumes per cent)
0
5
10
15
20
25
30
Figure 44–2
Quantity of oxygen dissolved in the ﬂuid of the blood and in com-
bination with hemoglobin at very high PO2s.
Chapter 44
Physiology of Deep-Sea Diving and Other Hyperbaric Conditions
547
oxygen content on leaving the tissue capillaries is still
24 volumes per cent (point B in the 

ries is still
24 volumes per cent (point B in the ﬁgure). At this
point, the Po2 is approximately 1200 mm Hg, which
means that oxygen is delivered to the tissues at this
extremely high pressure instead of at the normal value
of 40 mm Hg. Thus, once the alveolar Po2 rises above
a critical level, the hemoglobin-oxygen buffer mecha-
nism (discussed in Chapter 40) is no longer capable 
of keeping the tissue Po2 in the normal, safe range
between 20 and 60 mm Hg.
Acute Oxygen Poisoning. The extremely 

d 60 mm Hg.
Acute Oxygen Poisoning. The extremely high tissue Po2
that occurs when oxygen is breathed at very high alve-
olar oxygen pressure can be detrimental to many of
the body’s tissues. For instance, breathing oxygen at 
4 atmospheres pressure of oxygen (Po2 = 3040 mm Hg)
will cause brain seizures followed by coma in most
people within 30 to 60 minutes. The seizures often
occur without warning and, for obvious reasons, are
likely to be lethal to divers submerged beneath the 
sea.
Other sym

al to divers submerged beneath the 
sea.
Other symptoms encountered in acute oxygen poi-
soning include nausea, muscle twitchings, dizziness,
disturbances of vision, irritability, and disorientation.
Exercise greatly increases the diver’s susceptibility to
oxygen toxicity, causing symptoms to appear much
earlier and with far greater severity than in the resting
person.
Excessive Intracellular Oxidation as a Cause of
Nervous System Oxygen Toxicity—“Oxidizing Free
Radicals.” Molecular oxygen (O2) 

—“Oxidizing Free
Radicals.” Molecular oxygen (O2) has little capability
of oxidizing other chemical compounds. Instead, it
must ﬁrst be converted into an “active” form of
oxygen. There are several forms of active oxygen
called oxygen free radicals. One of the most important
of these is the superoxide free radical O2
_, and another
is the peroxide radical in the form of hydrogen perox-
ide. Even when the tissue Po2 is normal at the level of
40 mm Hg, small amounts of free radicals are contin-
ual

Hg, small amounts of free radicals are contin-
ually being formed from the dissolved molecular
oxygen. Fortunately, the tissues also contain multiple
enzymes that rapidly remove these free radicals,
including peroxidases, catalases, and superoxide dis-
mutases. Therefore, so long as the hemoglobin-oxygen
buffering mechanism maintains a normal tissue Po2,
the oxidizing free radicals are removed rapidly enough
that they have little or no effect in the tissues.
Above a critical alveolar Po2 (above 

Above a critical alveolar Po2 (above about 2 
atmospheres Po2), the hemoglobin-oxygen buffering
mechanism fails, and the tissue Po2 can then rise to
hundreds or thousands of millimeters of mercury. At
these high levels, the amounts of oxidizing free radi-
cals literally swamp the enzyme systems designed to
remove them, and now they can have serious destruc-
tive and even lethal effects on the cells. One of the
principal effects is to oxidize the polyunsaturated fatty
acids that are 

 oxidize the polyunsaturated fatty
acids that are essential components of many of the cell
membranes. Another effect is to oxidize some of the
cellular enzymes, thus damaging severely the cellular
metabolic systems. The nervous tissues are especially
susceptible because of their high lipid content.
Therefore, most of the acute lethal effects of acute
oxygen toxicity are caused by brain dysfunction.
Chronic Oxygen Poisoning Causes Pulmonary Disabil-
ity. A person can be exposed to only 1 atmosphe

l-
ity. A person can be exposed to only 1 atmosphere
pressure of oxygen almost indeﬁnitely without devel-
oping the acute oxygen toxicity of the nervous system
just described. However, after only about 12 hours of
1 atmosphere oxygen exposure, lung passageway con-
gestion, pulmonary edema, and atelectasis caused by
damage to the linings of the bronchi and alveoli begin
to develop. The reason for this effect in the lungs but
not in other tissues is that the air spaces of the lungs
are directly ex

s that the air spaces of the lungs
are directly exposed to the high oxygen pressure, but
oxygen is delivered to the other body tissues at almost
normal Po2 because of the hemoglobin-oxygen buffer
system.
Carbon Dioxide Toxicity at Great
Depths in the Sea
If the diving gear is properly designed and functions
properly, the diver has no problem due to carbon
dioxide toxicity because depth alone does not increase
the carbon dioxide partial pressure in the alveoli. This
is true because depth does not

n the alveoli. This
is true because depth does not increase the rate of
carbon dioxide production in the body, and as long as
the diver continues to breathe a normal tidal volume
and expires the carbon dioxide as it is formed, alveo-
lar carbon dioxide pressure will be maintained at a
normal value.
In certain types of diving gear, however, such as the
diving helmet and some types of rebreathing appara-
tuses, carbon dioxide can build up in the dead space
air of the apparatus and be rebreathed by

ad space
air of the apparatus and be rebreathed by the diver.
Up to an alveolar carbon dioxide pressure (Pco2) of
about 80 mm Hg, twice that in normal alveoli, the
diver usually tolerates this buildup by increasing the
minute respiratory volume a maximum of 8- to 11-fold
to compensate for the increased carbon dioxide.
Beyond 80-mm Hg alveolar Pco2, the situation
becomes intolerable, and eventually the respiratory
center begins to be depressed, rather than excited,
because of the negative tissue 

ther than excited,
because of the negative tissue metabolic effects of high
Pco2. The diver’s respiration then begins to fail rather
than to compensate. In addition, the diver develops
severe respiratory acidosis, and varying degrees of
lethargy, narcosis, and ﬁnally even anesthesia, as dis-
cussed in Chapter 42.
Decompression of the Diver After
Excess Exposure to High Pressure
When a person breathes air under high pressure for a
long time, the amount of nitrogen dissolved in the
body ﬂuids incr

mount of nitrogen dissolved in the
body ﬂuids increases. The reason for this is the fol-
lowing: Blood ﬂowing through the pulmonary capil-
laries becomes saturated with nitrogen to the same
high pressure as that in the alveolar breathing mixture.
And over several more hours, enough nitrogen is
548
Unit VIII
carried to all the tissues of the body to raise their tissue
Pn2 also to equal the Pn2 in the breathing air.
Because nitrogen is not metabolize

 breathing air.
Because nitrogen is not metabolized by the body, it
remains dissolved in all the body tissues until the nitro-
gen pressure in the lungs is decreased back to some
lower level, at which time the nitrogen can be removed
by the reverse respiratory process; however, this
removal often takes hours to occur and is the source
of multiple problems collectively called decompression
sickness.
Volume of Nitrogen Dissolved in the Body Fluids at Different
Depths. At sea level, almost exactly 

at Different
Depths. At sea level, almost exactly 1 liter of nitrogen
is dissolved in the entire body. Slightly less than one
half of this is dissolved in the water of the body and a
little more than one half in the fat of the body. This is
true because nitrogen is ﬁve times as soluble in fat as
in water.
After the diver has become saturated with nitrogen,
the sea-level volume of nitrogen dissolved in the body
at different depths is as follows:
nitrogen pressure (Pn2 = 3918 mm Hg), about 6.5 
ti

itrogen pressure (Pn2 = 3918 mm Hg), about 6.5 
times the normal amount of nitrogen in the tissues.
As long as the diver remains deep beneath the sea, the
pressure against the outside of his or her body 
(5000 mm Hg) compresses all the body tissues sufﬁ-
ciently to keep the excess nitrogen gas dissolved. But
when the diver suddenly rises to sea level (Figure
44–3B), the pressure on the outside of the body
becomes only 1 atmosphere (760 mm Hg), while the
gas pressure inside the body ﬂuids is the 

ile the
gas pressure inside the body ﬂuids is the sum of the
pressures of water vapor, carbon dioxide, oxygen, and
nitrogen, or a total of 4065 mm Hg, 97 per cent of
which is caused by the nitrogen. Obviously, this total
value of 4065 mm Hg is far greater than the 760 mm
Hg pressure on the outside of the body. Therefore, the
gases can escape from the dissolved state and form
actual bubbles, composed almost entirely of nitrogen,
both in the tissues and in the blood where they plug
many small bloo

s and in the blood where they plug
many small blood vessels.The bubbles may not appear
for many minutes to hours, because sometimes the
gases can remain dissolved in the “supersaturated”
state for hours before bubbling.
Symptoms of Decompression Sickness (“Bends”).
The symptoms of decompression sickness are caused
by gas bubbles blocking many blood vessels in 
different tissues. At ﬁrst, only the smallest vessels 
are blocked by minute bubbles, but as the bubbles 
coalesce, progressively larger 

ut as the bubbles 
coalesce, progressively larger vessels are affected.
Feet
Liters
0
1
33
2
100
4
200
7
300
10
Several hours are required for the gas pressures of
nitrogen in all the body tissues to come nearly to equi-
librium with the gas pressure of nitrogen in the alveoli.
The reason for this is that the blood does not ﬂow
rapidly enough and the nitrogen does not diffuse
rapidly enough to cause instantaneous equilibrium.
The nitrogen dissolved in the water of the body comes
to almost comple

ed in the water of the body comes
to almost complete equilibrium in less than 1 hour, but
the fat tissue, requiring ﬁve times as much transport of
nitrogen and having a relatively poor blood supply,
reaches equilibrium only after several hours. For this
reason, if a person remains at deep levels for only a
few minutes, not much nitrogen dissolves in the body
ﬂuids and tissues, whereas if the person remains at a
deep level for several hours, both the body water and
body fat become saturated with 

the body water and
body fat become saturated with nitrogen.
Decompression Sickness (Synonyms: Bends, Compressed Air
Sickness, Caisson Disease, Diver’s Paralysis, Dysbarism). If a
diver has been beneath the sea long enough that large
amounts of nitrogen have dissolved in his or her body
and the diver then suddenly comes back to the surface
of the sea, signiﬁcant quantities of nitrogen bubbles
can develop in the body ﬂuids either intracellularly or
extracellularly and can cause minor or serious da

extracellularly and can cause minor or serious damage
in almost any area of the body, depending on the
number and sizes of bubbles formed; this is called
decompression sickness.
The principles underlying bubble formation are
shown in Figure 44–3. In Figure 44–3A, the diver’s
tissues have become equilibrated to a high dissolved
Before
Pressure Outside Body
A
B
O2 = 1044 mm Hg
N2 = 3956
Total = 5000 mm Hg
After sudden
 decompression
O2 = 159 mm Hg
N2 = 601
Total = 760 mm Hg
Total = 

N2 = 601
Total = 4065
Body
Gaseous pressure
in the body fluids
H2O = 47 mm Hg
CO2 = 40
O2   = 60
N2   = 3918
Body
CO2 = 40
O2   = 60
N2   = 3918
Figure 44–3
Gaseous pressures both inside and outside the body, showing (A)
saturation of the body to high gas pressures when breathing air
at a total pressure of 5000 mm Hg, and (B) the great excesses of
intra-body pressures that are responsible for bubble 

ra-body pressures that are responsible for bubble formation in
the tissues when the lung intra-alveolar pressure body is 
suddenly returned from 5000 mm Hg to normal pressure of 
760 mm Hg.
Chapter 44
549
Tissue ischemia and sometimes tissue death are the
result.
In most people with decompression sickness, the
symptoms are pain in the joints and muscles of the legs
and arms, affecting 85 to 90 per cent of those persons
who develop dec

85 to 90 per cent of those persons
who develop decompression sickness. The joint pain
accounts for the term “bends” that is often applied to
this condition.
In 5 to 10 per cent of people with decompression
sickness, nervous system symptoms occur, ranging
from dizziness in about 5 per cent to paralysis or col-
lapse and unconsciousness in as many as 3 per cent.
The paralysis may be temporary, but in some instances,
damage is permanent.
Finally, about 2 per cent of people with decompres-
sion sick

out 2 per cent of people with decompres-
sion sickness develop “the chokes,” caused by massive
numbers of microbubbles plugging the capillaries of
the lungs; this is characterized by serious shortness of
breath, often followed by severe pulmonary edema
and, occasionally, death.
Nitrogen Elimination from the Body; Decompression Tables. If
a diver is brought to the surface slowly, enough of the
dissolved nitrogen can usually be eliminated by expi-
ration through the lungs to prevent decompression

sickness. About two thirds of the total nitrogen is lib-
erated in 1 hour and about 90 per cent in 6 hours.
Decompression tables have been prepared by the
U.S. Navy that detail procedures for safe decompres-
sion.To give the student an idea of the decompression
process, a diver who has been breathing air and has
been on the sea bottom for 60 minutes at a depth of
190 feet is decompressed according to the following
schedule:
10 minutes at 50 feet 

 to the following
schedule:
10 minutes at 50 feet depth
17 minutes at 40 feet depth
19 minutes at 30 feet depth
50 minutes at 20 feet depth
84 minutes at 10 feet depth
Thus, for a work period on the bottom of only 
1 hour, the total time for decompression is about 
3 hours.
Tank Decompression and Treatment of Decompression Sick-
ness. Another procedure widely used for decompres-
sion of professional divers is to put the diver into a
pressurized tank and then to lower the pressure grad-
ually bac

ank and then to lower the pressure grad-
ually back to normal atmospheric pressure, using
essentially the same time schedule as noted above.
Tank decompression is even more important for
treating people in whom symptoms of decompression
sickness develop minutes or even hours after they
have returned to the surface. In this case, the diver 
is recompressed immediately to a deep level. Then
decompression is carried out over a period several
times as long as the usual decompression period.
“Saturat

s long as the usual decompression period.
“Saturation Diving” and Use of Helium-Oxygen Mixtures in Deep
Dives. When divers must work at very deep levels—
between 250 feet and nearly 1000 feet—they fre-
quently live in a large compression tank for days or
weeks at a time, remaining compressed at a pressure
level near that at which they will be working. This
keeps the tissues and ﬂuids of the body saturated with
the gases to which they will be exposed while diving.
Then, when they return to the sa

sed while diving.
Then, when they return to the same tank after
working, there are no signiﬁcant changes in pressure,
so that decompression bubbles do not occur.
In very deep dives, especially during saturation
diving, helium is usually used in the gas mixture
instead of nitrogen for three reasons: (1) it has only
about one ﬁfth the narcotic effect of nitrogen; (2) only
about one half as much volume of helium dissolves in
the body tissues as nitrogen, and the volume that does
dissolve diffuses o

ogen, and the volume that does
dissolve diffuses out of the tissues during decompres-
sion several times as rapidly as does nitrogen, thus
reducing the problem of decompression sickness; and
(3) the low density of helium (one seventh the density
of nitrogen) keeps the airway resistance for breathing
at a minimum, which is very important because highly
compressed nitrogen is so dense that airway resistance
can become extreme, sometimes making the work of
breathing beyond endurance.
Finally, in ve

work of
Finally, in very deep dives it is important to reduce
the oxygen concentration in the gaseous mixture
because otherwise oxygen toxicity would result. For
instance, at a depth of 700 feet (22 atmospheres of
pressure), a 1 per cent oxygen mixture will provide all
the oxygen required by the diver, whereas a 21 per
cent mixture of oxygen (the percentage in air) deliv-
ers a Po2 to the lungs of more than 4 atmospheres,
a level very likely to cause seizures in as li

es,
a level very likely to cause seizures in as little as 
30 minutes.
Scuba (Self-Contained
Underwater Breathing
Apparatus) Diving
Before the 1940s, almost all diving was done using a
diving helmet connected to a hose through which air
was pumped to the diver from the surface. Then, in
1943, Jacques Cousteau popularized a self-contained
underwater breathing apparatus, known as the SCUBA
apparatus. The type of SCUBA apparatus used in
more than 99 per cent of all sports and commercial
diving is t

 per cent of all sports and commercial
diving is the open-circuit demand system shown in
Figure 44–4.This system consists of the following com-
ponents: (1) one or more tanks of compressed air or
some other breathing mixture, (2) a ﬁrst-stage “reduc-
ing” valve for reducing the very high pressure from the
tanks to a low pressure level, (3) a combination inhala-
tion “demand” valve and exhalation valve that allows
air to be pulled into the lungs with slight negative pres-
sure of breathing and th

ith slight negative pres-
sure of breathing and then to be exhaled into the sea
at a pressure level slightly positive to the surrounding
water pressure, and (4) a mask and tube system with
small “dead space.”
The demand system operates as follows: The ﬁrst-
stage reducing valve reduces the pressure from the
tanks so that the air delivered to the mask has a pres-
sure only a few mm Hg greater than the surrounding
water pressure. The breathing mixture does not 
ﬂow continually into the mask. Inste

ure does not 
ﬂow continually into the mask. Instead, with each
550
Unit VIII
inspiration, slight extra negative pressure in the
demand valve of the mask pulls the diaphragm of the
valve open, and this automatically releases air from
the tank into the mask and lungs. In this way, only the
amount of air needed for inhalation enters the mask.
Then, on expiration, the air cannot go back into the
tank but instead is expired into the sea.
The most impor

ut instead is expired into the sea.
The most important problem in use of the self-
contained underwater breathing apparatus is the
limited amount of time one can remain beneath the
sea surface; for instance, only a few minutes are possi-
ble at a 200-foot depth. The reason for this is that
tremendous airﬂow from the tanks is required to wash
carbon dioxide out of the lungs—the greater the
depth, the greater the airﬂow in terms of quantity of
air per minute that is required, because the volumes
h

per minute that is required, because the volumes
have been compressed to small sizes.
Special Physiologic Problems
in Submarines
Escape from Submarines. Essentially the same problems
encountered in deep-sea diving are often met in rela-
tion to submarines, especially when it is necessary 
to escape from a submerged submarine. Escape is 
possible from as deep as 300 feet without using any
However,
proper use of rebreathing
devices, especially when using helium, theoretically
can allow 

cially when using helium, theoretically
can allow escape from as deep as 600 feet or perhaps
more.
One of the major problems of escape is prevention
of air embolism. As the person ascends, the gases in
the lungs expand and sometimes rupture a pulmonary
blood vessel, forcing the gases to enter the vessel and
cause air embolism of the circulation.Therefore, as the
person ascends, he or she must make a special effort
to exhale continually.
Health Problems in the Submarine Internal Environment.
Exce

oblems in the Submarine Internal Environment.
Except for escape, submarine medicine generally
centers around several engineering problems to keep
hazards out of the internal environment. First, in
atomic submarines, there exists the problem of ra-
diation hazards, but with appropriate shielding, the
amount of radiation received by the crew submerged
beneath the sea has been less than normal radiation
received above the surface of the sea from cosmic rays.
Second, poisonous gases on occasion esca

mic rays.
Second, poisonous gases on occasion escape into the
atmosphere of the submarine and must be controlled
rapidly. For instance, during several weeks’ submer-
gence, cigarette smoking by the crew can liberate
enough carbon monoxide, if not removed rapidly,
to cause carbon monoxide poisoning. And, on occa-
sion, even freon gas has been found to diffuse out of
refrigeration systems in sufﬁcient quantity to cause
toxicity.
The intense oxidizing properties of high-pr

herapy
The intense oxidizing properties of high-pressure
oxygen (hyperbaric oxygen) can have valuable 
therapeutic effects in several important clinical 
conditions. Therefore, large pressure tanks are now
available in many medical centers into which patients
can be placed and treated with hyperbaric oxygen.
The oxygen is usually administered at Po2s of 2 to 
3 atmospheres of pressure through a mask or intra-
tracheal tube, whereas the gas around the body is
normal air compressed to the same hig

 the body is
normal air compressed to the same high-pressure
level.
It is believed that the same oxidizing free radicals
responsible for oxygen toxicity are also responsible for
at least some of the therapeutic beneﬁts. Some of the
conditions in which hyperbaric oxygen therapy has
been especially beneﬁcial follow.
Probably the most successful use of hyperbaric
oxygen has been for treatment of gas gangrene. The
bacteria that cause this condition, clostridial organ-
isms, grow best under anaerobic

clostridial organ-
isms, grow best under anaerobic conditions and 
stop growing at oxygen pressures greater than about
70 mm Hg. Therefore, hyperbaric oxygenation of the
tissues can frequently stop the infectious process
entirely and thus convert a condition that formerly was
almost 100 per cent fatal into one that is cured in most
instances by early treatment with hyperbaric therapy.
Other conditions in which hyperbaric oxygen
therapy has been either valuable or possibly valuable
include decomp

ither valuable or possibly valuable
include decompression sickness, arterial gas embolism,
Mask
Demand valve
First stage
valve
Hose
Air cylinders
Figure 44–4
Open-circuit demand type of SCUBA apparatus.
Chapter 44
551
carbon monoxide poisoning,
osteomyelitis,
and
myocardial infarction.
Butler PJ: Diving beyond the limits. News Physiol Sci 16:222,
2001.
Kooyman GL, Ponganis PJ:The physiological basis of diving
to depth: bird

J:The physiological basis of diving
to depth: birds and mammals. Annu Rev Physiol 60:19,
1998.
Leach RM, Rees PJ, Wilmshurst P: Hyperbaric oxygen
therapy. BMJ 317:1140, 1998.
Neuman TS:Arterial gas embolism and decompression sick-
ness. News Physiol Sci 17:77, 2002.
Nilsson GE: Surviving anoxia with the brain turned on. News
Physiol Sci 16:217, 2001.
Russi EW: Diving and the risk of barotrauma. Thorax
53(Suppl 2):S20, 1998.
Wang C, Schwaitzberg S, Berliner E, et al: Hyperbaric
oxygen for treatin

, Berliner E, et al: Hyperbaric
oxygen for treating wounds: a systematic review of the 
literature. Arch Surg 138:272, 2003.
Wang J, Li F, Calhoun JH, Mader JT: The role and effective-
ness of adjunctive hyperbaric oxygen therapy in the man-
agement of musculoskeletal disorders. J Postgrad Med
48:226, 2002
West JB, Fu Z, Gaeth AP, Short RV: Fetal lung development
in the elephant reﬂects the adaptations required for snor-
keling in adult life. Respir Physiol Neurobiol 138:325,
2003.
U
N
I
T
IX
Th

pir Physiol Neurobiol 138:325,
2003.
U
N
I
T
IX
The Nervous 
System: A. General
Principles and
Sensory Physiology
45. Organization of the Nervous System, Basic 
Functions of Synapses, “Transmitter Substances”
46. Sensory Receptors, Neuronal Circuits for 
Processing Information
47. Somatic Sensations: I. General Organization, 
the Tactile and Position Senses
48. Somatic Sensations: II. Pain, Headache, 
and Thermal Sensations
C
H
A
P
T
E
R
 
4
5
555
Organization of the Nervous
System, Basic Functi

5
System, Basic Functions of
Synapses, “Transmitter Substances”
The nervous system is unique in the vast complex-
ity of thought processes and control actions it can
perform. It receives each minute literally millions of
bits of information from the different sensory
nerves and sensory organs and then integrates all
these to determine responses to be made by the
body.
However, before beginning this discussion of the
nervous system, the reader should review Chapters 5 

rvous system, the reader should review Chapters 5 and 7, which present the
principles of membrane potentials and transmission of signals in nerves and
through neuromuscular junctions.
Central Nervous System Neuron: The Basic 
The central nervous system contains more than 100 billion neurons. Figure 45–1
shows a typical neuron of a type found in the brain motor cortex. Incoming
signals enter this neuron through synapses located mostly on the ne

s neuron through synapses located mostly on the neuronal den-
drites, but also on the cell body. For different types of neurons, there may be
only a few hundred or as many as 200,000 such synaptic connections from input
ﬁbers. Conversely, the output signal travels by way of a single axon leaving the
neuron. Then, this axon has many separate branches to other parts of the
nervous system or peripheral body.
A special feature of most synapses is that the signal normally passes only in
the forward d

t the signal normally passes only in
the forward direction (from the axon of a preceding neuron to dendrites on cell
membranes of subsequent neurons). This forces the signal to travel in required
directions for performing speciﬁc nervous functions.
Sensory Part of the Nervous System—Sensory Receptors
Most activities of the nervous system are initiated by sensory experience 
exciting sensory receptors, whether visual receptors in the eyes, auditory 
receptors in the ears, tactile receptors on the

y 
receptors in the ears, tactile receptors on the surface of the body, or other 
kinds of receptors.This sensory experience can either cause immediate reaction
from the brain, or memory of the experience can be stored in the brain 
for minutes, weeks, or years and determine bodily reactions at some future 
date.
Figure 45–2 shows the somatic portion of the sensory system, which transmits
sensory information from the receptors of the entire body surface and from
some deep structures. This inform

surface and from
some deep structures. This information enters the central nervous system
through peripheral nerves and is conducted immediately to multiple sensory
areas in (1) the spinal cord at all levels; (2) the reticular substance of the
medulla, pons, and mesencephalon of the brain; (3) the cerebellum; (4) the thal-
amus; and (5) areas of the cerebral cortex.
556
Unit IX
The Nervous System: A. General Principles and Sensory Physiology
Effectors
The most i

r Part of the Nervous System—
Effectors
The most important eventual role of the nervous
system is to control the various bodily activities. This
is achieved by controlling (1) contraction of appropri-
ate skeletal muscles throughout the body, (2) contrac-
tion of smooth muscle in the internal organs, and 
(3) secretion of active chemical substances by both
exocrine and endocrine glands in many parts of the
body. These activities are collectively called motor
functions of the nervous system, and 

called motor
functions of the nervous system, and the muscles and
glands are called effectors because they are the actual
anatomical structures that perform the functions dic-
tated by the nerve signals.
Figure 45–3 shows the “skeletal” motor nerve axis of
the nervous system for controlling skeletal muscle con-
traction. Operating parallel to this axis is another
system, called the autonomic nervous system, for con-
trolling smooth muscles, glands, and other internal
bodily systems; this is disc

s, and other internal
bodily systems; this is discussed in Chapter 60.
Note in Figure 45–3 that the skeletal muscles can be
controlled from many levels of the central nervous
system, including (1) the spinal cord; (2) the reticular
substance of the medulla, pons, and mesencephalon;
(3) the basal ganglia; (4) the cerebellum; and (5) the
motor cortex. Each of these areas plays its own spe-
ciﬁc role, the lower regions concerned primarily with
automatic, instantaneous muscle responses to sensory
st

atic, instantaneous muscle responses to sensory
stimuli, and the higher regions with deliberate
complex muscle movements controlled by the thought
processes of the brain.
Processing of Information—
“Integrative” Function of the 
One of the most important functions of the nervous
system is to process incoming information in such a
way that appropriate mental and motor responses will
occur. More than 99 per cent of all sensory informa-
tion is discarded by the brain as irrelevant an

a-
tion is discarded by the brain as irrelevant and unim-
portant. For instance, one is ordinarily unaware of the
Brain
Spinal cord
Second-order
neurons
Axon
Synapses
Cell body
Dendrites
Figure 45–1
Structure of a large neuron in the brain, showing its important func-
tional parts. (Redrawn from Guyton AC: Basic Neuroscience:
Anatomy and Physiology. Philadelphia: WB Saunders Co, 1987.)
Golgi tendon
apparatus
Motor cortex
Thalamus
Bulboreticular
formation
Pons
Somesthetic areas
Medulla

reticular
formation
Pons
Medulla
Skin
Pain, cold,
warmth (Free
nerve ending)
Pressure
(Pacinian corpuscle)
(Expanded tip
receptor)
Touch
(Meissner's corpuscle)
Muscle spindle
Kinesthetic receptor
Joint
Muscle
Figure 45–2
Somatosensory axis of the nervous system.
Chapter 45
Organization of the Nervous System, Basic Functions of Synapses, “Transmitter Substances”
557
parts of the body that are in contact with clothing, as
well as of the seat pressure when sitting. Lik

 as
well as of the seat pressure when sitting. Likewise,
attention is drawn only to an occasional object in one’s
ﬁeld of vision, and even the perpetual noise of our sur-
roundings is usually relegated to the subconscious.
But, when important sensory information excites
the mind, it is immediately channeled into proper inte-
grative and motor regions of the brain to cause desired
responses. This channeling and processing of informa-
tion is called the integrative function of the nervous
system. 

d the integrative function of the nervous
system. Thus, if a person places a hand on a hot stove,
the desired instantaneous response is to lift the hand.
And other associated responses follow, such as moving
the entire body away from the stove, and perhaps even
shouting with pain.
Role of Synapses in Processing Information. The synapse 
is the junction point from one neuron to the next.
Later in this chapter, we will discuss the details of
synaptic function. However, it is important to point out

ic function. However, it is important to point out
here that synapses determine the directions that the
nervous signals will spread through the nervous
system. Some synapses transmit signals from one
neuron to the next with ease, whereas others transmit
signals only with difﬁculty. Also, facilitatory and
inhibitory signals from other areas in the nervous
system can control synaptic transmission, sometimes
opening the synapses for transmission and at other
times closing them. In addition, some po

 at other
times closing them. In addition, some postsynaptic
neurons respond with large numbers of output
impulses, and others respond with only a few.Thus, the
synapses perform a selective action, often blocking
weak signals while allowing strong signals to pass, but
at other times selecting and amplifying certain weak
signals, and often channeling these signals in many
directions rather than only one direction.
Only a small fraction of even the most important
sens

y a small fraction of even the most important
sensory information usually causes immediate motor
response. But much of the information is stored for
future control of motor activities and for use in 
the thinking processes. Most storage occurs in the 
cerebral cortex, but even the basal regions of the 
brain and the spinal cord can store small amounts of 
information.
The storage of information is the process we call
memory, and this, too, is a function of the synapses.
That is, each time certai

unction of the synapses.
That is, each time certain types of sensory signals pass
through sequences of synapses, these synapses become
more capable of transmitting the same type of signal
the next time, a process called facilitation. After the
sensory signals have passed through the synapses a
large number of times, the synapses become so facili-
tated that signals generated within the brain itself can
also cause transmission of impulses through the same
sequences of synapses, even when the sens

the same
sequences of synapses, even when the sensory input is
not excited.This gives the person a perception of expe-
riencing the original sensations, although the percep-
tions are only memories of the sensations.
We know little about the precise mechanisms by
which long-term facilitation of synapses occurs in the
memory process, but what is known about this and
other details of the sensory memory process are dis-
cussed in Chapter 57.
Once memories have been stored in the nervous
system, the

mories have been stored in the nervous
system, they become part of the brain processing
mechanism for future “thinking.” That is, the thinking
processes of the brain compare new sensory experi-
ences with stored memories; the memories then help
to select the important new sensory information and
to channel this into appropriate memory storage areas
for future use or into motor areas to cause immediate
bodily responses.
Major Levels of Central
Nervous System Function
The human nervous system has 

vous System Function
The human nervous system has inherited special func-
tional capabilities from each stage of human evolu-
tionary development. From this heritage, three major
levels of the central nervous system have speciﬁc func-
tional characteristics: (1) the spinal cord level, (2) the
lower brain or subcortical level, and (3) the higher
brain or cortical level.
Alpha motor fiber
Motor
area
Motor nerve
to muscles
Thalamus
Putamen
Globus pallidus
Subthalamic nucleus
Bulboreticul

n
Bulboreticular formation
Gamma motor fiber
Caudate
nucleus
Figure 45–3
Skeletal motor nerve axis of the nervous system.
558
Unit IX
We often think of the spinal cord as being only a
conduit for signals from the periphery of the body to
the brain, or in the opposite direction from the brain
back to the body. This is far from the truth. Even after
the spinal cord 

is far from the truth. Even after
the spinal cord has been cut in the high neck region,
many highly organized spinal cord functions still
occur. For instance, neuronal circuits in the cord can
cause (1) walking movements, (2) reﬂexes that with-
draw portions of the body from painful objects, (3)
reﬂexes that stiffen the legs to support the body
against gravity,and (4) reﬂexes that control local blood
vessels, gastrointestinal movements, or urinary excre-
tion. In fact, the upper levels of the ne

y excre-
tion. In fact, the upper levels of the nervous system
often operate not by sending signals directly to the
periphery of the body but by sending signals to 
the control centers of the cord, simply “commanding”
the cord centers to perform their functions.
Many, if not most, of what we call subconscious activ-
ities of the body are controlled in the lower areas 
of the brain—in the medulla, pons, mesencephalon,
hypothalamus,
thalamus,
cerebellum,
and 
basal

on,
thalamus,
and 
basal
ganglia. For instance, subconscious control of arterial
pressure and respiration is achieved mainly in the
medulla and pons. Control of equilibrium is a com-
bined function of the older portions of the cerebellum
and the reticular substance of the medulla, pons, and
mesencephalon. Feeding reﬂexes, such as salivation
and licking of the lips in response to the taste of food,
are controlled by areas in the medulla, pons, mesen-
cephalon, amygdala, 

 in the medulla, pons, mesen-
cephalon, amygdala, and hypothalamus. And many
emotional patterns, such as anger, excitement, sexual
response, reaction to pain, and reaction to pleasure,
can still occur after destruction of much of the cere-
bral cortex.
After the preceding account of the many nervous
system functions that occur at the cord and lower brain
levels, one may ask, what is left for the cerebral cortex
to do? The answer to this is complex, but it begins wi

o? The answer to this is complex, but it begins with
the fact that the cerebral cortex is an extremely large
memory storehouse. The cortex never functions alone
but always in association with lower centers of the
Without the cerebral cortex, the functions of the
lower brain centers are often imprecise.The vast store-
house of cortical information usually converts these
functions to determinative and precise operations.
Finally, the cerebral cortex is essential for most of
our tho

e cerebral cortex is essential for most of
our thought processes, but it cannot function by itself.
In fact, it is the lower brain centers, not the cortex,
that initiate wakefulness in the cerebral cortex, thus
opening its bank of memories to the thinking machin-
ery of the brain. Thus, each portion of the nervous
system performs speciﬁc functions. But it is the cortex
that opens a world of stored information for use by the
mind.
Comparison of the 
Nervous System with 
a Computer
When computers 

e 
a Computer
When computers were ﬁrst developed, it soon became
apparent that these machines have many features in
common with the nervous system. First, all computers
have input circuits that are comparable to the sensory
portion of the nervous system, and output circuits that
are comparable to the motor portion of the nervous
system.
In simple computers, the output signals are controlled
directly by the input signals, operating in a manner
similar to that of simple reﬂexe

ating in a manner
similar to that of simple reﬂexes of the spinal cord. In
more complex computers,the output is determined both
by input signals and by information that has already
been stored in memory in the computer, which is anal-
ogous to the more complex reﬂex and processing mech-
anisms of our higher nervous system. Furthermore,
as computers become even more complex, it is neces-
sary to add still another unit, called the central process-
ing unit, that determines the sequence of all oper

ing unit, that determines the sequence of all operations.
This unit is analogous to the control mechanisms in our
brain that direct our attention ﬁrst to one thought or
sensation or motor activity, then to another, and so
forth, until complex sequences of thought or action take
place.
Figure 45–4 is a simple block diagram of a computer.
Even a rapid study of this diagram demonstrates its sim-
ilarity to the nervous system. The fact that the basic
components of the general-purpose computer are an

components of the general-purpose computer are anal-
ogous to those of the human nervous system demon-
strates that the brain is basically a computer that
continuously collects sensory information and uses this
along with stored information to compute the daily
course of bodily activity.
Problem
Answer
Output
Result of
operations
Initial
data
Procedure
for solution
Central
processing unit
Computational
unit
Information
storage
Input
Figure 45–4
Block diagram of a general-purpose computer, showi

Block diagram of a general-purpose computer, showing the basic
components and their interrelations.
Chapter 45
559
Synapses
Every medical student is aware that information is
transmitted in the central nervous system mainly in the
form of nerve action potentials, called simply “nerve
impulses,” through a succession of neurons, one after
another. However, in addition, each impulse (1) 

r
another. However, in addition, each impulse (1) may
be blocked in its transmission from one neuron to the
next, (2) may be changed from a single impulse into
repetitive impulses, or (3) may be integrated with
impulses from other neurons to cause highly intricate
patterns of impulses in successive neurons. All these
functions can be classiﬁed as synaptic functions of
neurons.
Types of Synapses—Chemical 
and Electrical
There are two major types of synapses: (1) the chemi-
cal synapse and (2) the

f synapses: (1) the chemi-
cal synapse and (2) the electrical synapse.
Almost all the synapses used for signal transmission
in the central nervous system of the human being are
chemical synapses. In these, the ﬁrst neuron secretes at
its nerve ending synapse a chemical substance called a
neurotransmitter (or often called simply transmitter
substance), and this transmitter in turn acts on recep-
tor proteins in the membrane of the next neuron to
excite the neuron, inhibit it, or modify its sensit

xcite the neuron, inhibit it, or modify its sensitivity in
some other way. More than 40 important transmitter
substances have been discovered thus far. Some of 
the best known are acetylcholine, norepinephrine,
epinephrine,
histamine,
gamma-aminobutyric acid
(GABA), glycine, serotonin, and glutamate.
Electrical synapses, in contrast, are characterized by
direct open ﬂuid channels that conduct electricity from
one cell to the next. Most of these consist of small
protein tubular structures called 

onsist of small
protein tubular structures called gap junctions that
allow free movement of ions from the interior of one
cell to the interior of the next. Such junctions were dis-
cussed in Chapter 4. Only a few examples of gap junc-
tions have been found in the central nervous system.
However, it is by way of gap junctions and other
similar junctions that action potentials are transmitted
from one smooth muscle ﬁber to the next in visceral
smooth muscle (Chapter 8) and from one cardiac
muscle 

th muscle (Chapter 8) and from one cardiac
muscle cell to the next in cardiac muscle (Chapter 10).
“One-Way” Conduction at Chemical Synapses. Chemical
synapses have one exceedingly important characteris-
tic that makes them highly desirable for transmitting
most nervous system signals: they always transmit 
the signals in one direction: that is, from the neuron 
that secretes the transmitter substance, called the
presynaptic neuron, to the neuron on which the trans-
mitter acts, called the posts

 on which the trans-
mitter acts, called the postsynaptic neuron. This is the
principle of one-way conduction at chemical synapses,
and it is quite different from conduction through elec-
trical synapses, which often transmit signals in either
Think for a moment about the extreme importance
of the one-way conduction mechanism. It allows
signals to be directed toward speciﬁc goals. Indeed, it
is this speciﬁc transmission of signals to discrete and
highly focused areas both within the n

iscrete and
highly focused areas both within the nervous system
and at the terminals of the peripheral nerves that
allows the nervous system to perform its myriad func-
tions of sensation, motor control, memory, and many
others.
Figure 45–5 shows a typical anterior motor neuron in
the anterior horn of the spinal cord. It is composed of
three major parts: the soma, which is the main body of
the neuron; a single axon, which extends from the
soma into a peripheral

xon, which extends from the
soma into a peripheral nerve that leaves the spinal
cord; and the dendrites, which are great numbers of
branching projections of the soma that extend as 
much as 1 millimeter into the surrounding areas of the
cord.
As many as 10,000 to 200,000 minute synaptic knobs
called presynaptic terminals lie on the surfaces of the
dendrites and soma of the motor neuron, about 80 to
95 per cent of them on the dendrites and only 5 to 20
per cent on the soma. These presynaptic term

to 20
per cent on the soma. These presynaptic terminals are
the ends of nerve ﬁbrils that originate from many
other neurons. Later, it will become evident that many
Dendrites
Axon
Soma
Figure 45–5
Typical anterior motor neuron, showing presynaptic terminals on
the neuronal soma and dendrites. Note also the single axon.
560
Unit IX
of these presynaptic terminals are excitatory—that is,
they secrete a transmitter substance that excit

s,
they secrete a transmitter substance that excites the
postsynaptic neuron. But other presynaptic terminals
are inhibitory—they secrete a transmitter substance
that inhibits the postsynaptic neuron.
Neurons in other parts of the cord and brain differ
from the anterior motor neuron in (1) the size of the
cell body; (2) the length, size, and number of dendrites,
ranging in length from almost zero to many centime-
ters; (3) the length and size of the axon; and (4) the
number of presynaptic termin

the axon; and (4) the
number of presynaptic terminals, which may range
from only a few to as many as 200,000. These differ-
ences make neurons in different parts of the nervous
system react differently to incoming synaptic signals
and, therefore, perform many different functions.
Presynaptic Terminals. Electron microscopic studies of
the presynaptic terminals show that they have varied
anatomical forms, but most resemble small round or
oval knobs and, therefore, are sometimes called termi-
nal k

 and, therefore, are sometimes called termi-
nal knobs, boutons, end-feet, or synaptic knobs.
Figure 45–6 illustrates the basic structure of a
synapse, showing a single presynaptic terminal on the
membrane surface of a postsomatic neuron.The presy-
naptic terminal is separated from the postsynaptic
neuronal soma by a synaptic cleft having a width
usually of 200 to 300 angstroms. The terminal has 
two internal structures important to the excitatory or
inhibitory function of the synapse: the trans

y or
inhibitory function of the synapse: the transmitter vesi-
cles and the mitochondria. The transmitter vesicles
contain the transmitter substance that, when released
into the synaptic cleft, either excites or inhibits the
postsynaptic neuron—excites if the neuronal mem-
brane contains excitatory receptors, inhibits if the
membrane contains inhibitory receptors. The mito-
chondria provide adenosine triphosphate (ATP),
which in turn supplies the energy for synethesizing
new transmitter substanc

 energy for synethesizing
new transmitter substance.
When an action potential spreads over a presynap-
tic terminal, depolarization of its membrane causes 
a small number of vesicles to empty into the cleft.
The released transmitter in turn causes an immediate
change in permeability characteristics of the postsy-
naptic neuronal membrane, and this leads to excitation
or inhibition of the postsynaptic neuron, depending on
the neuronal receptor characteristics.
Mechanism by Which an Action Potenti

racteristics.
Mechanism by Which an Action Potential
Causes Transmitter Release from the
Presynaptic Terminals—Role of Calcium Ions
The membrane of the presynaptic terminal is called
the presynaptic membrane. It contains large numbers
of voltage-gated calcium channels. When an action
potential depolarizes the presynaptic membrane,
these calcium channels open and allow large numbers
of calcium ions to ﬂow into the terminal. The quantity
of transmitter substance that is then released from 
the ter

tter substance that is then released from 
the terminal into the synaptic cleft is directly related
to the number of calcium ions that enter. The precise
mechanism by which the calcium ions cause this
release is not known, but it is believed to be the 
When the calcium ions enter the presynaptic termi-
nal, it is believed that they bind with special protein
molecules on the inside surface of the presynaptic
membrane, called release sites. This binding in turn
causes the release sites 

es. This binding in turn
causes the release sites to open through the mem-
brane, allowing a few transmitter vesicles to release
their transmitter into the cleft after each single action
potential. For those vesicles that store the neurotrans-
mitter acetylcholine, between 2000 and 10,000 mole-
cules of acetylcholine are present in each vesicle, and
there are enough vesicles in the presynaptic terminal
to transmit from a few hundred to more than 10,000
Action of the Transmitte

10,000
Action of the Transmitter Substance 
on the Postsynaptic Neuron—Function of
“Receptor Proteins”
The membrane of the postsynaptic neuron contains
large numbers of receptor proteins, also shown in
Figure 45–6. The molecules of these receptors have
two important components: (1) a binding component
that protrudes outward from the membrane into the
synaptic cleft—here it binds the neurotransmitter
coming from the presynaptic terminal—and (2) an
ionophore component that passe

terminal—and (2) an
ionophore component that passes all the way through
the postsynaptic membrane to the interior of the post-
synaptic neuron. The ionophore in turn is one of two
types: (1) an ion channel that allows passage of speci-
ﬁed types of ions through the membrane or (2) a
“second messenger” activator that is not an ion channel
but instead is a molecule that protrudes into the cell
cytoplasm and activates one or more substances inside
the postsynaptic neuron. These substances in turn
s

he postsynaptic neuron. These substances in turn
serve as “second messengers” to increase or decrease
speciﬁc cellular functions.
Transmitter vesicles
Synaptic cleft
(200-300
angstroms)
Presynaptic
terminal
Soma of neuron
Receptor
proteins
Figure 45–6
Physiologic anatomy of the synapse.
Chapter 45
561
Ion Channels. The ion channels in the postsynaptic neu-
ronal membrane are usually of two type

naptic neu-
ronal membrane are usually of two types: (1) cation
channels that most often allow sodium ions to pass
when opened, but sometimes allow potassium and/or
calcium ions as well, and (2) anion channels that allow
mainly chloride ions to pass but also minute quantities
of other anions.
The cation channels that conduct sodium ions are
lined with negative charges. These charges attract the
positively charged sodium ions into the channel when
the channel diameter increases to a size larger t

the channel diameter increases to a size larger than
that of the hydrated sodium ion. But those same neg-
ative charges repel chloride ions and other anions and
prevent their passage.
For the anion channels, when the channel diameters
become large enough, chloride ions pass into the
channels and on through to the opposite side, whereas
sodium, potassium, and calcium cations are blocked,
mainly because their hydrated ions are too large to
pass.
We will learn later that when cation channels open

e will learn later that when cation channels open
and allow positively charged sodium ions to enter, the
positive electrical charges of the sodium ions will in
turn excite this neuron. Therefore, a transmitter sub-
stance that opens cation channels is called an excita-
tory transmitter. Conversely, opening anion channels
allows negative electrical charges to enter, which
inhibits the neuron. Therefore, transmitter substances
that open these channels are called inhibitory 
transmitters.
When a tr

els are called inhibitory 
When a transmitter substance activates an ion
channel, the channel usually opens within a fraction of
a millisecond; when the transmitter substance is no
longer present, the channel closes equally rapidly. The
opening and closing of ion channels provide a means
for very rapid control of postsynaptic neurons.
“Second Messenger” System in the Postsynaptic Neuron.
Many functions of the nervous system—for instance,
the process of memory—require prolonged chan

ance,
the process of memory—require prolonged changes in
neurons for seconds to months after the initial trans-
mitter substance is gone.The ion channels are not suit-
able for causing prolonged postsynaptic neuronal
changes because these channels close within millisec-
onds after the transmitter substance is no longer
present. However, in many instances, prolonged post-
synaptic neuronal excitation or inhibition is achieved
by activating a “second messenger” chemical system
inside the postsynap

nd messenger” chemical system
inside the postsynaptic neuronal cell itself, and then it
is the second messenger that causes the prolonged
effect.
There are several types of second messenger
systems. One of the most common types uses a group
of proteins called G-proteins. Figure 45–7 shows in the
upper left corner a membrane receptor protein. A 
G-protein is attached to the portion of the receptor
that protrudes into the interior of the cell. The 
G-protein in turn consists of three components: a

G-protein in turn consists of three components: an
alpha (a) component that is the activator portion of
the G-protein, and beta (b) and gamma (g) compo-
nents that are attached to the alpha component and
also to the inside of the cell membrane adjacent to the
receptor protein. On activation by a nerve impulse, the
alpha portion of the G-protein separates from the beta
and gamma portions and then is free to move within
the cytoplasm of the cell.
Inside the cytoplasm, the separated alpha compo-
n

nent performs one or more of multiple functions,
depending on the speciﬁc characteristic of each type
of neuron. Shown in Figure 45–7 are four changes that
can occur. They are as follows:
1. Opening speciﬁc ion channels through the
postsynaptic cell membrane. Shown in the upper
right of the ﬁgure is a potassium channel that is
opened in response to the G-protein; this channel
often stays open for a prolonged time, in contrast
to rapid closure of d

 prolonged time, in contrast
to rapid closure of directly activated ion channels
that do not use the second messenger system.
Transmitter substance
G-protein
Opens
channel
K+
Membrane
enzyme
Potassium
channel
Receptor
protein
1
2
3
4
Activates one or
more intracellular
enzymes
Activates gene
Specific cellular
chemical activators
Proteins and
structural changes
Activates
enzymes
ATP
cAMP
or
g
b
a
a
GTP
cGMP
Figure 45–7
“Second messenger” system by
which a transmitter substance from

ger” system by
an initial neuron can activate a
second neuron by ﬁrst releasing 
a “G-protein” into the second
neuron’s cytoplasm. Four subse-
quent possible effects of the 
G-protein are shown, including 1,
opening an ion channel in the
membrane of the second neuron;
2, activating an enzyme system in
the neuron’s membrane; 3, activat-
ing an intracellular enzyme system;
and/or 4, causing gene transcrip-
tion in the second neuron.
562
Unit IX
The Nervous System

 the second neuron.
562
Unit IX
2. Activation of cyclic adenosine monophosphate
(cAMP) or cyclic guanosine monophosphate
(cGMP) in the neuronal cell. Recall that either
cyclic AMP or cyclic GMP can activate highly
speciﬁc metabolic machinery in the neuron and,
therefore, can initiate any one of many chemical
results, including long-term changes in cell
structure itself, which in turn alters long-term
excitability of the neuron.
3. 

n alters long-term
3. Activation of one or more intracellular enzymes.
The G-protein can directly activate one or more
intracellular enzymes. In turn the enzymes can
cause any one of many speciﬁc chemical functions
in the cell.
4. Activation of gene transcription. This is one of the
most important effects of activation of the second
messenger systems because gene transcription 
can cause formation of new proteins within the
neuron, thereby changing its metabolic machi

n the
neuron, thereby changing its metabolic machinery
or its structure. Indeed, it is well known that
structural changes of appropriately activated
neurons do occur, especially in long-term memory
It is clear that activation of second messenger
systems within the neuron, whether they be of the 
G-protein type or of other types, is extremely impor-
tant for changing the long-term response characteris-
tics of different neuronal pathways. We will return to
this subject in more detail i

s. We will return to
this subject in more detail in Chapter 57 when we
discuss memory functions of the nervous system.
Excitatory or Inhibitory Receptors in the
Postsynaptic Membrane
Some postsynaptic receptors, when activated, cause
excitation of the postsynaptic neuron,and others cause
inhibition.The importance of having inhibitory as well
as excitatory types of receptors is that this gives an
additional dimension to nervous function, allowing
restraint of nervous action as well as excitation.

The different molecular and membrane mechanisms
used by the different receptors to cause excitation or
inhibition include the following.
Excitation
1. Opening of sodium channels to allow large
numbers of positive electrical charges to ﬂow to the
interior of the postsynaptic cell. This raises the
intracellular membrane potential in the positive
direction up toward the threshold level for
excitation. It is by far the most widely used means
for cau

n. It is by far the most widely used means
for causing excitation.
2. Depressed conduction through chloride or
potassium channels, or both. This decreases the
diffusion of negatively charged chloride ions to
the inside of the postsynaptic neuron or decreases
the diffusion of positively charged potassium ions
to the outside. In either instance, the effect is to
make the internal membrane potential more
positive than normal, which is excitatory.
3. Various changes in the internal metabolism of the

 Various changes in the internal metabolism of the
postsynaptic neuron to excite cell activity or, in
some instances, to increase the number of
excitatory membrane receptors or decrease the
number of inhibitory membrane receptors.
1. Opening of chloride ion channels through the
postsynaptic neuronal membrane. This allows
rapid diffusion of negatively charged chloride ions
from outside the postsynaptic neuron to the
inside, thereby carrying negative charges inward
and increasing the ne

ying negative charges inward
and increasing the negativity inside, which is
inhibitory.
2. Increase in conductance of potassium ions out of
the neuron. This allows positive ions to diffuse to
the exterior, which causes increased negativity
inside the neuron; this is inhibitory.
3. Activation of receptor enzymes that inhibit cellular
metabolic functions that increase the number of
inhibitory synaptic receptors or decrease the
number of excitatory receptors.
Chemical Substances That Function
as Sy

as Synaptic Transmitters
More than 50 chemical substances have been proved
or postulated to function as synaptic transmitters.
Many of them are listed in Tables 45–1 and 45–2, which
give two groups of synaptic transmitters. One group
comprises small-molecule, rapidly acting transmitters.
The other is made up of a large number of neuropep-
tides of much larger molecular size that are usually
much more slowly acting.
The small-molecule, rapidly acting t

lowly acting.
The small-molecule, rapidly acting transmitters are
the ones that cause most acute responses of the
nervous system, such as transmission of sensory signals
to the brain and of motor signals back to the muscles.
The neuropeptides, in contrast, usually cause more
prolonged actions, such as long-term changes in
numbers of neuronal receptors, long-term opening or
closure of certain ion channels, and possibly even long-
term changes in numbers of synapses or sizes of
synapses.
Table 45–

umbers of synapses or sizes of
synapses.
Table 45–1
Small-Molecule, Rapidly Acting Transmitters
Class I
Class II: The Amines
Dopamine
Serotonin
Histamine
Class III: Amino Acids
Gamma-aminobutyric acid (GABA)
Glycine
Glutamate
Aspartate
Class IV
Chapter 45
563
In most cases, the small-molecule types of tran

rs
In most cases, the small-molecule types of transmitters
are synthesized in the cytosol of the presynaptic ter-
minal and are absorbed by means of active transport
into the many transmitter vesicles in the terminal.
Then, each time an action potential reaches the presy-
naptic terminal, a few vesicles at a time release their
transmitter into the synaptic cleft. This usually occurs
within a millisecond or less by the mechanism
described earlier. The subsequent action of the small-
molecule type

 The subsequent action of the small-
molecule type of transmitter on the membrane recep-
tors of the postsynaptic neuron usually also occurs
within another millisecond or less. Most often the
effect is to increase or decrease conductance through
ion channels; an example is to increase sodium con-
ductance, which causes excitation, or to increase potas-
sium or chloride conductance, which causes inhibition.
Recycling of the Small-Molecule Types of Vesicles. The vesi-
cles that store and release s

 Vesicles. The vesi-
cles that store and release small-molecule transmitters
are continually recycled and used over and over again.
After they fuse with the synaptic membrane and open
to release their transmitter substance,the vesicle mem-
brane at ﬁrst simply becomes part of the synaptic
membrane. However, within seconds to minutes, the
vesicle portion of the membrane invaginates back to
the inside of the presynaptic terminal and pinches 
off to form a new vesicle. And the new vesicular 
membra

 form a new vesicle. And the new vesicular 
membrane still contains appropriate enzyme proteins
or transport proteins required for synthesizing and/or
concentrating new transmitter substance inside the
vesicle.
Acetylcholine is a typical small-molecule transmit-
ter that obeys the principles of synthesis and release
stated earlier.This transmitter substance is synthesized
in the presynaptic terminal from acetyl coenzyme A
and choline in the presence of the enzyme choline
acetyltransferase.Then i

nce of the enzyme choline
acetyltransferase.Then it is transported into its speciﬁc
vesicles. When the vesicles later release the acetyl-
choline into the synaptic cleft during synaptic neu-
ronal signal transmission, the acetylcholine is rapidly
split again to acetate and choline by the enzyme
cholinesterase, which is present in the proteoglycan
reticulum that ﬁlls the space of the synaptic cleft. And
then again, inside the presynaptic terminal, the vesi-
cles are recycled; choline is actively 

 the vesi-
cles are recycled; choline is actively transported back
into the terminal to be used again for synthesis of new
acetylcholine.
Characteristics of Some of the More Important Small-Molecule
Transmitters. The most important of the small-molecule
transmitters are the following.
Acetylcholine is secreted by neurons in many areas
of the nervous system but speciﬁcally by (1) the ter-
minals of the large pyramidal cells from the motor
cortex, (2) several different types of neurons in the
basa

(2) several different types of neurons in the
basal ganglia, (3) the motor neurons that innervate the
skeletal muscles, (4) the preganglionic neurons of 
the autonomic nervous system, (5) the postganglionic
neurons of the parasympathetic nervous system, and
(6) some of the postganglionic neurons of the sympa-
thetic nervous system. In most instances, acetylcholine
has an excitatory effect; however, it is known to have
inhibitory effects at some peripheral parasympathetic
nerve endings, such as i

eripheral parasympathetic
nerve endings, such as inhibition of the heart by the
vagus nerves.
Norepinephrine is secreted by the terminals of many
neurons whose cell bodies are located in the brain
stem and hypothalamus. Speciﬁcally, norepinephrine-
secreting neurons located in the locus ceruleus in the
pons send nerve ﬁbers to widespread areas of the brain
to help control overall activity and mood of the mind,
such as increasing the level of wakefulness. In most of
these areas, norepinephrine pr

fulness. In most of
these areas, norepinephrine probably activates excita-
tory receptors, but in a few areas, it activates inhibitory
receptors instead. Norepinephrine is also secreted by
most postganglionic neurons of the sympathetic
nervous system, where it excites some organs but
inhibits others.
Dopamine is secreted by neurons that originate in
the substantia nigra.The termination of these neurons
is mainly in the striatal region of the basal ganglia.The
effect of dopamine is usually inhibi

l ganglia.The
effect of dopamine is usually inhibition.
Glycine is secreted mainly at synapses in the 
spinal cord. It is believed to always act as an inhibitory
transmitter.
GABA (gamma-aminobutyric acid) is secreted by
nerve terminals in the spinal cord, cerebellum, basal
ganglia, and many areas of the cortex. It is believed
always to cause inhibition.
Table 45–2
Neuropeptide, Slowly Acting Transmitters or Growth
Factors
Hypothalamic-releasing hormones
Luteinizing

hormones
Luteinizing hormone–releasing hormone
Somatostatin (growth hormone inhibitory factor)
Pituitary peptides
b-Endorphin
a-Melanocyte-stimulating hormone
Prolactin
Oxytocin
Peptides that act on gut and brain
Leucine enkephalin
Methionine enkephalin
Gastrin
Vasoactive intestinal polypeptide (VIP)
Brain-derived neurotropic 

IP)
Brain-derived neurotropic factor
Insulin
Glucagon
From other tissues
Bradykinin
Carnosine
Sleep peptides
564
Unit IX
Glutamate is secreted by the presynaptic terminals
in many of the sensory pathways entering the central
nervous system, as well as in many areas of the cere-
bral cortex. It probably always causes excitation.
Serotonin is secreted by nuclei that originate 

n.
Serotonin is secreted by nuclei that originate in the
median raphe of the brain stem and project to many
brain and spinal cord areas, especially to the dorsal
horns of the spinal cord and to the hypothalamus.
Serotonin acts as an inhibitor of pain pathways in the
cord, and an inhibitor action in the higher regions of
the nervous system is believed to help control the
mood of the person, perhaps even to cause sleep.
Nitric oxide is especially secreted by nerve terminals
in areas of the brain r

ecreted by nerve terminals
in areas of the brain responsible for long-term behav-
ior and for memory.Therefore, this transmitter system
might in the future explain some behavior and
memory functions that thus far have deﬁed under-
standing. Nitric oxide is different from other small-
molecule transmitters in its mechanism of formation
in the presynaptic terminal and in its actions on the
postsynaptic neuron. It is not preformed and stored in
vesicles in the presynaptic terminal as are other tran

cles in the presynaptic terminal as are other trans-
mitters. Instead, it is synthesized almost instantly as
needed, and it then diffuses out of the presynaptic 
terminals over a period of seconds rather than being
released in vesicular packets. Next, it diffuses into
postsynaptic neurons nearby. In the postsynaptic
neuron, it usually does not greatly alter the membrane
potential but instead changes intracellular metabolic
functions that modify neuronal excitability for
seconds, minutes, or perh

euronal excitability for
seconds, minutes, or perhaps even longer.
The neuropeptides are an entirely different class of
transmitters that are synthesized differently and
whose actions are usually slow and in other ways quite
different from those of the small-molecule transmit-
ters. The neuropeptides are not synthesized in the
cytosol of the presynaptic terminals. Instead, they are
synthesized as integral parts of large-protein mole-
cules by ribosomes in the neuronal cell body.
Th

-
The protein molecules then enter the spaces inside
the endoplasmic reticulum of the cell body and subse-
quently inside the Golgi apparatus, where two changes
occur: First, the neuropeptide-forming protein is enzy-
matically split into smaller fragments, some of which
are either the neuropeptide itself or a precursor of it.
Second, the Golgi apparatus packages the neuropep-
tide into minute transmitter vesicles that are released
into the cytoplasm.

ter vesicles that are released
into the cytoplasm. Then the transmitter vesicles are
transported all the way to the tips of the nerve ﬁbers
by axonal streaming of the axon cytoplasm, traveling
at the slow rate of only a few centimeters per day.
Finally, these vesicles release their transmitter at the
neuronal terminals in response to action potentials in
the same manner as for small-molecule transmitters.
However, the vesicle is autolyzed and is not reused.
Because of this laborious method of fo

not reused.
Because of this laborious method of forming the
neuropeptides, much smaller quantities of them are
usually released than of the small-molecule transmit-
ters.This is partly compensated for by the fact that the
neuropeptides are generally a thousand or more times
as potent as the small-molecule transmitters. Another
important characteristic of the neuropeptides is that
they often cause much more prolonged actions. Some
of these actions include prolonged closure of calcium
channels, pr

 include prolonged closure of calcium
channels, prolonged changes in the metabolic machin-
ery of cells, prolonged changes in activation or deacti-
vation of speciﬁc genes in the cell nucleus, and/or 
prolonged alterations in numbers of excitatory or
inhibitory receptors. Some of these effects last for
days, but others perhaps for months or years. Our
knowledge of the functions of the neuropeptides is
only beginning to develop.
Electrical Events During 
Neuronal Excitation
The electrical events 

During 
The electrical events in neuronal excitation have been
studied especially in the large motor neurons of the
anterior horns of the spinal cord.Therefore, the events
described in the next few sections pertain essentially
to these neurons. Except for quantitative differences,
they apply to most other neurons of the nervous
system as well.
Resting Membrane Potential of the Neuronal Soma. Figure
45–8 shows the soma of a spinal motor neuron, indi-
cating a resting membrane 

nal motor neuron, indi-
cating a resting membrane potential of about -65 mil-
livolts. This is somewhat less negative than the -90
millivolts found in large peripheral nerve ﬁbers and in
skeletal muscle ﬁbers; the lower voltage is important
because it allows both positive and negative control 
of the degree of excitability of the neuron. That is,
decreasing the voltage to a less negative value makes
the membrane of the neuron more excitable, whereas
increasing this voltage to a more negative val

eas
increasing this voltage to a more negative value makes
the neuron less excitable. This is the basis for the two
Dendrite
Axon
Axon hillock
14 mEq/L
(Pumps)
?
Pump
120 mEq/L
8 mEq/L
-65
mV
Na+: 142 mEq/L
K+: 4.5 mEq/L
Cl-: 107 mEq/L
Figure 45–8
Distribution of sodium, potassium, and chloride ions across the
neuronal somal membrane; origin of the intrasomal membrane
Chapter 45
565
modes of func

napses, “Transmitter Substances”
565
modes of function of the neuron—either excitation or
inhibition—as explained in detail in the next sections.
Concentration Differences of Ions Across the Neuronal
Somal Membrane. Figure 45–8 also shows the concen-
tration differences across the neuronal somal mem-
brane of the three ions that are most important for
neuronal function: sodium ions, potassium ions, and
chloride ions. At the top, the sodium ion concentra-
tion is shown to be high in the extracell

ncentra-
tion is shown to be high in the extracellular ﬂuid
(142 mEq/L) but low inside the neuron (14 mEq/L).
This sodium concentration gradient is caused by a
strong somal membrane sodium pump that continually
pumps sodium out of the neuron.
The ﬁgure also shows that potassium ion concentra-
tion is high inside the neuronal soma (120 mEq/L) but
low in the extracellular ﬂuid (4.5 mEq/L). It shows that
there is a potassium pump (the other half of the Na+-
K+ pump) that pumps potassium to the inte

the Na+-
K+ pump) that pumps potassium to the interior.
Figure 45–8 shows the chloride ion to be of high con-
centration in the extracellular ﬂuid but low concentra-
tion inside the neuron. It also shows that the membrane
is quite permeable to chloride ions and that there may
be a weak chloride pump. Yet most of the reason 
for the low concentration of chloride ions inside the
neuron is the -65 millivolts in the neuron. That is, this
negative voltage repels the negatively charged chlo-
ride ions

tage repels the negatively charged chlo-
ride ions, forcing them outward through the pores until
the concentration is much less inside the membrane
than outside.
Let us recall at this point what we learned in Chap-
ters 4 and 5 about the relation of ionic concentration
differences to membrane potentials. It will be recalled
that an electrical potential across the cell membrane
can oppose movement of ions through a membrane if
the potential is of proper polarity and magnitude. A
potential that ex

proper polarity and magnitude. A
potential that exactly opposes movement of an ion is
called the Nernst potential for that ion; the equation
for this is the following:
where EMF is the Nernst potential in millivolts on the
inside of the membrane. The potential will be negative
(-) for positive ions and positive (+) for negative ions.
Now, let us calculate the Nernst potential that will
exactly oppose the movement of each of the three sep-
arate ions: sodium, potassium, and chloride.
For the sodi

ons: sodium, potassium, and chloride.
For the sodium concentration difference shown in
Figure 45–8, 142 mEq/L on the exterior and 14 mEq/L
on the interior, the membrane potential that will
exactly oppose sodium ion movement through the
sodium channels calculates to be +61 millivolts.
However, the actual membrane potential is -65 milli-
volts, not +61 millivolts. Therefore, those sodium ions
that leak to the interior are immediately pumped back
to the exterior by the sodium pump, thus maintaining

 the exterior by the sodium pump, thus maintaining
the -65 millivolt negative potential inside the neuron.
For potassium ions, the concentration gradient is 
120 mEq/L inside the neuron and 4.5 mEq/L outside.
This calculates to a Nernst potential of -86 millivolts
inside the neuron, which is more negative than the 
EMF mV
(
) = ±
¥
Ê
Ë
ˆ
¯
61
log
-65 that actually exists. Therefore, because of the high
intracellular potassium ion concentration, there is

ntracellular potassium ion concentration, there is a
net tendency for potassium ions to diffuse to the
outside of the neuron, but this is opposed by continual
pumping of these potassium ions back to the 
interior.
Finally, the chloride ion gradient, 107 mEq/L outside
and 8 mEq/L inside, yields a Nernst potential of -70
millivolts inside the neuron, which is only slightly more
negative than the actual measured value of -65 milli-
volts.Therefore, chloride ions tend to leak very slightly
to the in

chloride ions tend to leak very slightly
to the interior of the neuron, but those few that do leak
are moved back to the exterior, perhaps by an active
chloride pump.
Keep these three Nernst potentials in mind and
remember the direction in which the different ions
tend to diffuse because this information is important
in understanding both excitation and inhibition of the
neuron by synapse activation or inactivation of ion
channels.
Uniform Distribution of Electrical Potential Inside the
Soma. Th

bution of Electrical Potential Inside the
Soma. The interior of the neuronal soma contains a
highly conductive electrolytic solution, the intracellu-
lar ﬂuid of the neuron. Furthermore, the diameter of
the neuronal soma is large (from 10 to 80 microme-
ters), causing almost no resistance to conduction of
electric current from one part of the somal interior to
another part.Therefore, any change in potential in any
part of the intrasomal ﬂuid causes an almost exactly
equal change in potential at 

es an almost exactly
equal change in potential at all other points inside the
soma (that is, as long as the neuron is not transmitting
an action potential). This is an important principle,
because it plays a major role in “summation” of signals
entering the neuron from multiple sources, as we shall
see in subsequent sections of this chapter.
Effect of Synaptic Excitation on the Postsynaptic Membrane—
Excitatory Postsynaptic Potential. Figure 45–9A shows the
resting neuron with an unexcited presy

A shows the
resting neuron with an unexcited presynaptic terminal
resting on its surface. The resting membrane potential
everywhere in the soma is -65 millivolts.
Figure 45–9B shows a presynaptic terminal that has
secreted an excitatory transmitter into the cleft
between the terminal and the neuronal somal mem-
brane. This transmitter acts on the membrane excita-
tory receptor to increase the membrane’s permeability 
to Na+. Because of the large sodium concentration 
gradient and large electrica

sodium concentration 
gradient and large electrical negativity inside the
neuron, sodium ions diffuse rapidly to the inside of the
membrane.
The rapid inﬂux of positively charged sodium ions
to the interior neutralizes part of the negativity of the
resting membrane potential.Thus, in Figure 45–9B, the
resting membrane potential has increased in the posi-
tive direction from -65 to -45 millivolts. This positive
increase in voltage above the normal resting neuronal
potential—that is, to a less neg

 resting neuronal
potential—that is, to a less negative value—is called
the excitatory postsynaptic potential
(or EPSP),
because if this potential rises high enough in the pos-
itive direction, it will elicit an action potential in the
postsynaptic neuron, thus exciting it. (In this case, the
566
Unit IX
EPSP is +20 millivolts—that is, 20 millivolts more pos-
itive than the resting value.)
However, we must issue a word of warning. 

value.)
However, we must issue a word of warning. Dis-
charge of a single presynaptic terminal can never
increase the neuronal potential from -65 millivolts 
all the way up to -45 millivolts. An increase of this
magnitude requires simultaneous discharge of many
terminals—about 40 to 80 for the usual anterior motor
neuron—at the same time or in rapid succession. This
occurs by a process called summation, which is dis-
cussed in detail in the next sections.
Generation of Action Potentials in the I

sections.
Generation of Action Potentials in the Initial Segment of the
Axon Leaving the Neuron—Threshold for Excitation. When
the EPSP rises high enough in the positive direction,
there comes a point at which this initiates an action
potential in the neuron. However, the action potential
does not begin adjacent to the excitatory synapses.
Instead, it begins in the initial segment of the axon
where the axon leaves the neuronal soma. The main
reason for this point of origin of the action potentia

on for this point of origin of the action potential
is that the soma has relatively few voltage-gated
sodium channels in its membrane, which makes it dif-
ﬁcult for the EPSP to open the required number of
sodium channels to elicit an action potential. Con-
versely, the membrane of the initial segment has seven
times as great a concentration of voltage-gated sodium
channels as does the soma and, therefore, can gener-
ate an action potential with much greater ease than
can the soma. The EPSP that 

uch greater ease than
can the soma. The EPSP that will elicit an action
potential in the axon initial segment is between +10
and +20 millivolts. This is in contrast to the +30 or +40
millivolts or more required on the soma.
Once the action potential begins, it travels periph-
erally along the axon and usually also backward over
the soma. In some instances, it travels backward into
the dendrites, too, but not into all of them, because
they, like the neuronal soma, have very few voltage-
gated sod

he neuronal soma, have very few voltage-
gated sodium channels and therefore frequently
cannot generate action potentials at all.Thus, in Figure
45–9B, the threshold for excitation of the neuron is
shown to be about -45 millivolts, which represents an
EPSP of +20 millivolts—that is, 20 millivolts more pos-
itive than the normal resting neuronal potential of 
-65 millivolts.
Electrical Events During Neuronal
Effect of Inhibitory Synapses on the Postsynaptic Membrane—
Inhibitory Postsyn

s on the Postsynaptic Membrane—
Inhibitory Postsynaptic Potential. The inhibitory synapses
open mainly chloride channels, allowing easy passage
of chloride ions. Now, to understand how the
inhibitory synapses inhibit the postsynaptic neuron,
we must recall what we learned about the Nernst
potential for chloride ions. We calculated the Nernst
potential for chloride ions to be about -70 millivolts.
This potential is more negative than the -65 millivolts
normally present inside the resting neuronal

volts
normally present inside the resting neuronal mem-
brane. Therefore, opening the chloride channels 
will allow negatively charged chloride ions to move
from the extracellular ﬂuid to the interior, which 
will make the interior membrane potential more 
negative than normal, approaching the -70 millivolt
level.
Opening potassium channels will allow positively
charged potassium ions to move to the exterior, and
this will also make the interior membrane potential
more negative than usual. Thus,

membrane potential
more negative than usual. Thus, both chloride inﬂux
and potassium efﬂux increase the degree of intracel-
lular negativity, which is called hyperpolarization. This
inhibits the neuron because the membrane potential is
even more negative than the normal intracellular
potential. Therefore, an increase in negativity beyond
the normal resting membrane potential level is called
an inhibitory postsynaptic potential (IPSP).
Figure 45–9C shows the effect on the membrane
potential cause

C shows the effect on the membrane
potential caused by activation of inhibitory synapses,
allowing chloride inﬂux into the cell and/or potassium
efﬂux out of the cell, with the membrane potential
decreasing from its normal value of -65 millivolts to
the more negative value of -70 millivolts. This mem-
brane potential is 5 millivolts more negative than
normal and is therefore an IPSP of -5 millivolts, which
inhibits transmission of the nerve signal through the
synapse.
Resting Neuron
Spread of
ac

l through the
synapse.
Spread of
Excited Neuron
A
B
C
Initial segment
of axon
Excitatory
Na+
influx
Cl- influx
-65 mV
-45 mV
Inhibited Neuron
K+
efflux
-70 mV
Figure 45–9
Three states of a neuron. A, Resting neuron, with a normal intra-
neuronal potential of -65 millivolts. B, Neuron in an excited state,
with a less negative intraneuronal potential (-45 millivolts) caused
by sodium inﬂux. C, Neuron in an inhibited state, with a more neg-
ative intraneur

 inhibited state, with a more neg-
ative intraneuronal membrane potential (-70 millivolts) caused by
potassium ion efﬂux, chloride ion inﬂux, or both.
Chapter 45
567
Presynaptic Inhibition
In addition to inhibition caused by inhibitory synapses
operating at the neuronal membrane, which is called
postsynaptic inhibition, another type of inhibition
often occurs at the presynaptic terminals before the
signal e

s at the presynaptic terminals before the
signal ever reaches the synapse.This type of inhibition,
called presynaptic inhibition, occurs in the following
way.
Presynaptic inhibition is caused by release of an
inhibitory substance onto the outsides of the presy-
naptic nerve ﬁbrils before their own endings terminate
on the postsynaptic neuron. In most instances, the
inhibitory transmitter substance is GABA (gamma-
aminobutyric acid). This has a speciﬁc effect of
opening anion channels, allowing l

eciﬁc effect of
opening anion channels, allowing large numbers of
chloride ions to diffuse into the terminal ﬁbril. The
negative charges of these ions inhibit synaptic trans-
mission because they cancel much of the excitatory
effect of the positively charged sodium ions that also
enter the terminal ﬁbrils when an action potential
arrives.
Presynaptic inhibition occurs in many of the sensory
pathways in the nervous system. In fact, adjacent
sensory nerve ﬁbers often mutually inhibit one
another, 

y nerve ﬁbers often mutually inhibit one
another, which minimizes sideways spread and mixing
of signals in sensory tracts. We discuss the importance
of this phenomenon more fully in subsequent chapters.
Time Course of Postsynaptic Potentials
When an excitatory synapse excites the anterior motor
neuron, the neuronal membrane becomes highly per-
meable to sodium ions for 1 to 2 milliseconds. During
this very short time, enough sodium ions diffuse
rapidly to the interior of the postsynaptic motor
n

apidly to the interior of the postsynaptic motor
neuron to increase its intraneuronal potential by a few
millivolts, thus creating the excitatory postsynaptic
potential (EPSP) shown by the blue and green curves
of Figure 45–10. This potential then slowly declines
over the next 15 milliseconds because this is the time
required for the excess positive charges to leak out of
the excited neuron and to re-establish the normal
Precisely the opposite effect occurs for an IPS

l.
Precisely the opposite effect occurs for an IPSP; that
is, the inhibitory synapse increases the permeability of
the membrane to potassium or chloride ions, or both,
for 1 to 2 milliseconds, and this decreases the intra-
neuronal potential to a more negative value than
normal, thereby creating the IPSP. This potential also
dies away in about 15 milliseconds.
Other types of transmitter substances can excite or
inhibit the postsynaptic neuron for much longer
periods—for hundreds of milliseconds 

 much longer
periods—for hundreds of milliseconds or even for
seconds, minutes, or hours. This is especially true for
some of the neuropeptide types of transmitter 
“Spatial Summation” in Neurons—
Threshold for Firing
Excitation of a single presynaptic terminal on the
surface of a neuron almost never excites the neuron.
The reason for this is that sufﬁcient transmitter sub-
stance is released by a single terminal to cause an
EPSP usually no greater than 0.5 to 1 millivolt, instead
of

lly no greater than 0.5 to 1 millivolt, instead
of the 10 to 20 millivolts normally required to reach
threshold for excitation. However, many presynaptic
terminals are usually stimulated at the same time.
Even though these terminals are spread over wide
areas of the neuron, their effects can still summate;
that is,they can add to one another until neuronal exci-
tation does occur. The reason for this is the following:
It was pointed out earlier that a change in potential at
any single point with

hat a change in potential at
any single point within the soma will cause the poten-
tial to change everywhere inside the soma almost
exactly equally. This is true because of the very high
electrical conductivity inside the large neuronal cell
body. Therefore, for each excitatory synapse that dis-
charges simultaneously, the total intrasomal potential
becomes more positive by 0.5 to 1.0 millivolt. When
12
14
16
Excitatory postsynaptic
potential
Resting membrane potential
16 synap

ptic
potential
16 synapses firing
16
8
4
8 synapses firing
4 synapses firing
0
4
6
8
2
10
–80
–60
–40
–20
+20
0
16
8
4
Figure 45–10
Excitatory postsynaptic potentials, showing that
simultaneous ﬁring of only a few synapses will not
cause sufﬁcient summated potential to elicit an
action potential, but that simultaneous ﬁring of
many synapses will raise the summated potential
to threshold for excitation and cause a superim-
posed action potential.

ation and cause a superim-
568
Unit IX
the EPSP becomes great enough, the threshold for
ﬁring will be reached and an action potential will
develop spontaneously in the initial segment of the
axon. This is demonstrated in Figure 45–10. The
bottom postsynaptic potential in the ﬁgure was caused
by simultaneous stimulation of 4 synapses; the next
higher potential was caused by stimulation of 8
synapses; ﬁnally, 

 was caused by stimulation of 8
synapses; ﬁnally, a still higher EPSP was caused by
stimulation of 16 synapses. In this last instance, the
ﬁring threshold had been reached, and an action
potential was generated in the axon.
This effect of summing simultaneous postsynaptic
potentials by activating multiple terminals on widely
spaced areas of the neuronal membrane is called
spatial summation.
“Temporal Summation”
Each time a presynaptic terminal ﬁres, the released
transmitter substance opens the m

es, the released
transmitter substance opens the membrane channels
for at most a millisecond or so. But the changed post-
synaptic potential lasts up to 15 milliseconds after the
synaptic membrane channels have already closed.
Therefore, a second opening of the same channels can
increase the postsynaptic potential to a still greater
level, and the more rapid the rate of stimulation, the
greater the postsynaptic potential becomes. Thus, suc-
cessive discharges from a single presynaptic terminal,

ve discharges from a single presynaptic terminal,
if they occur rapidly enough, can add to one another;
that is, they can “summate.” This type of summation is
called temporal summation.
Simultaneous Summation of Inhibitory and Excitatory Postsy-
naptic Potentials. If an IPSP is tending to decrease the
membrane potential to a more negative value while an
EPSP is tending to increase the potential at the same
time, these two effects can either completely or par-
tially nullify each other. Thus, if 

etely or par-
tially nullify each other. Thus, if a neuron is being
excited by an EPSP, an inhibitory signal from another
source can often reduce the postsynaptic potential to
less than threshold value for excitation, thus turning
off the activity of the neuron.
“Facilitation” of Neurons
Often the summated postsynaptic potential is excita-
tory but has not risen high enough to reach the thresh-
old for ﬁring by the postsynaptic neuron. When this
happens, the neuron is said to be facilitated. Tha

happens, the neuron is said to be facilitated. That is, its
membrane potential is nearer the threshold for ﬁring
than normal, but not yet at the ﬁring level. Conse-
quently, another excitatory signal entering the neuron
from some other source can then excite the neuron
very easily. Diffuse signals in the nervous system often
do facilitate large groups of neurons so that they can
respond quickly and easily to signals arriving from
other sources.
Special Functions of Dendrites 
for Exciting Neuron

pecial Functions of Dendrites 
for Exciting Neurons
Large Spatial Field of Excitation of the Dendrites. The den-
drites of the anterior motor neurons often extend 500
to 1000 micrometers in all directions from the neu-
ronal soma. And these dendrites can receive signals
from a large spatial area around the motor neuron.
This provides a vast opportunity for summation of
signals from many separate presynaptic nerve ﬁbers.
It is also important that between 80 and 95 per cent
of all the presynaptic 

between 80 and 95 per cent
of all the presynaptic terminals of the anterior motor
neuron terminate on dendrites, in contrast to only 5 to
20 per cent terminating on the neuronal soma. There-
fore, the preponderant share of the excitation is pro-
vided by signals transmitted by way of the dendrites.
Most Dendrites Cannot Transmit Action Potentials—But They
Can Transmit Signals Within the Same Neuron by Electrotonic
Conduction. Most dendrites fail to transmit action
potentials because their membra

to transmit action
potentials because their membranes have relatively
few voltage-gated sodium channels, and their thresh-
olds for excitation are too high for action potentials to
occur. Yet they do transmit electrotonic current down
the dendrites to the soma.Transmission of electrotonic
current means direct spread of electrical current by ion
conduction in the ﬂuids of the dendrites but without
generation of action potentials. Stimulation (or inhibi-
tion) of the neuron by this current has spe

nhibi-
tion) of the neuron by this current has special charac-
Decrement of Electrotonic Conduction in the Den-
drites—Greater Excitatory (or Inhibitory) Effect by
Synapses Located Near the Soma. In Figure 45–11,
multiple excitatory and inhibitory synapses are shown
stimulating the dendrites of a neuron. On the two den-
drites to the left, there are excitatory effects near the
tip ends; note the high levels of excitatory postsynap-
tic potentials at these ends—that is, not

stsynap-
tic potentials at these ends—that is, note the less neg-
ative membrane potentials at these points. However, a
-40
-40
-30
-30
-10
-20
-20
-20
-35 -50 -60
-50 -60
-70 -75
-60 mV
I
I
I
I
I
I
-40
-50
-75
-70
-60
E
E
E
E
E
E
E
E
E
E
Figure 45–11
Stimulation of a neuron by presynaptic terminals located on den-
drites, showing, especially, decremental conduction of excitatory
(E) electrotonic potentials in the two dendrites to the left and inhi-
bition (I) of dendritic excitation in the dend

hi-
bition (I) of dendritic excitation in the dendrite that is uppermost.
A powerful effect of inhibitory synapses at the initial segment of
the axon is also shown.
Chapter 45
569
large share of the excitatory postsynaptic potential is
lost before it reaches the soma. The reason is that the
dendrites are long, and their membranes are thin and
at least partially permeable to potassium and chloride
ions, maki

lly permeable to potassium and chloride
ions, making them “leaky” to electric current. There-
fore, before the excitatory potentials can reach the
soma, a large share of the potential is lost by leakage
through the membrane. This decrease in membrane
potential as it spreads electrotonically along dendrites
toward the soma is called decremental conduction.
The farther the excitatory synapse is from the soma
of the neuron, the greater will be the decrement, and
the less will be excitatory signal r

ecrement, and
the less will be excitatory signal reaching the soma.
Therefore, those synapses that lie near the soma have
far more effect in causing neuron excitation or inhibi-
tion than those that lie far away from the soma.
Summation of Excitation and Inhibition in Dendrites. The
uppermost dendrite of Figure 45–11 is shown to be
stimulated by both excitatory and inhibitory synapses.
At the tip of the dendrite is a strong excitatory post-
synaptic potential, but nearer the soma are two
inhibit

tic potential, but nearer the soma are two
inhibitory synapses acting on the same dendrite.These
inhibitory synapses provide a hyperpolarizing voltage
that completely nulliﬁes the excitatory effect and
indeed transmits a small amount of inhibition by elec-
trotonic conduction toward the soma. Thus, dendrites
can summate excitatory and inhibitory postsynaptic
potentials in the same way that the soma can. Also
shown in the ﬁgure are several inhibitory synapses
located directly on the axon hillock 

ory synapses
located directly on the axon hillock and initial axon
segment. This location provides especially powerful
inhibition because it has the direct effect of increasing
the threshold for excitation at the very point where the
action potential is normally generated.
Relation of State of Excitation of the
Neuron to Rate of Firing
“Excitatory State.” The “excitatory state” of a neuron is
deﬁned as the summated degree of excitatory drive to
the neuron. If there is a higher degree of excitati

he neuron. If there is a higher degree of excitation
than inhibition of the neuron at any given instant, then
it is said that there is an excitatory state. Conversely, if
there is more inhibition than excitation, then it is said
that there is an inhibitory state.
When the excitatory state of a neuron rises above
the threshold for excitation, the neuron will ﬁre repet-
itively as long as the excitatory state remains at that
level. Figure 45–12 shows responses of three types of
neurons to varying 

ws responses of three types of
neurons to varying levels of excitatory state. Note that
neuron 1 has a low threshold for excitation, whereas
neuron 3 has a high threshold. But note also that
neuron 2 has the lowest maximum frequency of dis-
charge, whereas neuron 3 has the highest maximum
frequency.
Some neurons in the central nervous system ﬁre
continuously because even the normal excitatory 
state is above the threshold level. Their frequency of
ﬁring can usually be increased still more by fur

f
ﬁring can usually be increased still more by further
increasing their excitatory state. The frequency can be
decreased, or ﬁring can even be stopped, by superim-
posing an inhibitory state on the neuron. Thus, dif-
ferent neurons respond differently, have different
thresholds for excitation, and have widely differing
maximum frequencies of discharge. With a little imag-
ination, one can readily understand the importance of
having different neurons with these many types of
response characterist

ons with these many types of
response characteristics to perform the widely varying
functions of the nervous system.
Frequency of discharge per second
Excitatory state (arbitrary units)
0
5
Threshold
Neuron 1
Neuron 2
Neuron 3
10
15
20
25
30
35
0
600
500
400
300
200
100
Figure 45–12
Response characteristics of different types of
neurons to different levels of excitatory state.
570
Unit IX
Some Special Characteristics
of Synaptic Tr

iology
of Synaptic Transmission
Fatigue 
of 
Synaptic 
Transmission. When 
excitatory
synapses are repetitively stimulated at a rapid rate, the
number of discharges by the postsynaptic neuron is at
ﬁrst very great, but the ﬁring rate becomes progres-
sively less in succeeding milliseconds or seconds. This
is called fatigue of synaptic transmission.
Fatigue is an exceedingly important characteristic of
synaptic function because when areas of the nervous
system become 

n because when areas of the nervous
system become overexcited, fatigue causes them to
lose this excess excitability after awhile. For example,
fatigue is probably the most important means by which
the excess excitability of the brain during an epileptic
seizure is ﬁnally subdued so that the seizure ceases.
Thus, the development of fatigue is a protective mech-
anism against excess neuronal activity. This is dis-
cussed further in the description of reverberating
neuronal circuits in Chapter 46.

of reverberating
The mechanism of fatigue is mainly exhaustion or
partial exhaustion of the stores of transmitter sub-
stance in the presynaptic terminals.The excitatory ter-
minals on many neurons can store enough excitatory
transmitter to cause only about 10,000 action poten-
tials, and the transmitter can be exhausted in only a
few seconds to a few minutes of rapid stimulation. Part
of the fatigue process probably results from two other
factors as well: (1) pr

bly results from two other
factors as well: (1) progressive inactivation of many of
the postsynaptic membrane receptors and (2) slow
development of abnormal concentrations of ions
inside the postsynaptic neuronal cell.
Effect of Acidosis or Alkalosis on Synaptic Transmission. Most
neurons are highly responsive to changes in pH of 
the surrounding interstitial ﬂuids. Normally, alkalosis
greatly increases neuronal excitability. For instance, a
rise in arterial blood pH from the 7.4 norm to 7.8 to 

 in arterial blood pH from the 7.4 norm to 7.8 to 
8.0 often causes cerebral epileptic seizures because of
increased excitability of some or all of the cerebral
neurons. This can be demonstrated especially well by
asking a person who is predisposed to epileptic seizures
to overbreathe. The overbreathing blows off carbon
dioxide and therefore elevates the pH of the blood
momentarily, but even this short time can often precip-
itate an epileptic attack.
Conversely, acidosis greatly depresses neuro

tack.
Conversely, acidosis greatly depresses neuronal activ-
ity; a fall in pH from 7.4 to below 7.0 usually causes a
comatose state. For instance, in very severe diabetic or
uremic acidosis, coma virtually always develops.
Effect of Hypoxia on Synaptic Transmission. Neuronal
excitability is also highly dependent on an adequate
supply of oxygen. Cessation of oxygen for only a few
seconds can cause complete inexcitability of some
neurons.This is observed when the brain’s blood ﬂow is
temporarily 

bserved when the brain’s blood ﬂow is
temporarily interrupted, because within 3 to 7 seconds,
the person becomes unconscious.
Effect of Drugs on Synaptic Transmission. Many drugs are
known to increase the excitability of neurons, and
others are known to decrease excitability. For instance,
caffeine, theophylline, and theobromine, which are
found in coffee, tea, and cocoa, respectively, all increase
neuronal excitability, presumably by reducing the
threshold for excitation of neurons.
Strychnine 

e
Strychnine is one of the best known of all agents that
increase excitability of neurons. However, it does not 
do this by reducing the threshold for excitation of 
the neurons; instead, it inhibits the action of some nor-
mally inhibitory transmitter substances, especially the
inhibitory effect of glycine in the spinal cord.Therefore,
the effects of the excitatory transmitters become over-
whelming, and the neurons become so excited that they
go into rapidl

neurons become so excited that they
go into rapidly repetitive discharge, resulting in severe
tonic muscle spasms.
Most anesthetics increase the neuronal membrane
threshold for excitation and thereby decrease synaptic
transmission at many points in the nervous system.
Because many of the anesthetics are especially lipid-
soluble, it has been reasoned that some of them might
change the physical characteristics of the neuronal
membranes, making them less responsive to excitatory
agents.
Synaptic D

m less responsive to excitatory
agents.
Synaptic Delay. During transmission of a neuronal signal
from a presynaptic neuron to a postsynaptic neuron, a
certain amount of time is consumed in the process of
(1) discharge of the transmitter substance by the presy-
naptic terminal, (2) diffusion of the transmitter to the
postsynaptic neuronal membrane, (3) action of the
transmitter on the membrane receptor, (4) action of 
the receptor to increase the membrane permeability,
and (5) inward diffusion of

membrane permeability,
and (5) inward diffusion of sodium to raise the excita-
tory postsynaptic potential to a high enough level to
elicit an action potential. The minimal period of time
required for all these events to take place, even when
large numbers of excitatory synapses are stimulated
simultaneously, is about 0.5 millisecond. This is called
the synaptic delay. Neurophysiologists can measure the
minimal delay time between an input volley of impulses
into a pool of neurons and the consequ

of impulses
into a pool of neurons and the consequent output volley.
From the measure of delay time, one can then estimate
the number of series neurons in the circuit.
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572
Sensory Receptors, Neuronal
Circuits for Processing Information
Input to the nervous system is provided by sensory
receptors that detect such sensory stimuli as 

ory
receptors that detect such sensory stimuli as touch,
sound, light, pain, cold, and warmth. The purpose of
this chapter is to discuss the basic mechanisms by
which these receptors change sensory stimuli into
nerve signals that are then conveyed to and
processed in the central nervous system.
Types of Sensory Receptors and the Sensory
Stimuli They Detect
Table 46–1 lists and classiﬁes most of the body’s sensory receptors. This table
shows that there are ﬁve basic types of sensory receptors: (1

there are ﬁve basic types of sensory receptors: (1) mechanorecep-
tors, which detect mechanical compression or stretching of the receptor or of
tissues adjacent to the receptor; (2) thermoreceptors, which detect changes in
temperature, some receptors detecting cold and others warmth; (3) nociceptors
(pain receptors), which detect damage occurring in the tissues, whether physi-
cal damage or chemical damage;(4) electromagnetic receptors,which detect light
on the retina of the eye; and (5) chemore

ct light
on the retina of the eye; and (5) chemoreceptors, which detect taste in the
mouth, smell in the nose, oxygen level in the arterial blood, osmolality of the
body ﬂuids, carbon dioxide concentration, and perhaps other factors that make
up the chemistry of the body.
In this chapter, we discuss the function of a few speciﬁc types of receptors,
primarily peripheral mechanoreceptors, to illustrate some of the principles by
which receptors operate. Other receptors are discussed in other chapte

ate. Other receptors are discussed in other chapters in rela-
tion to the sensory systems that they subserve. Figure 46–1 shows some of the
types of mechanoreceptors found in the skin or in deep tissues of the body.
The ﬁrst question that must be answered is, how do two types of sensory recep-
tors detect different types of sensory stimuli? The answer is, by “differential sen-
sitivities.”That is, each type of receptor is highly sensitive to one type of stim

f receptor is highly sensitive to one type of stimulus
for which it is designed and yet is almost nonresponsive to other types of sensory
stimuli. Thus, the rods and cones of the eyes are highly responsive to light but
are almost completely nonresponsive to normal ranges of heat, cold, pressure
on the eyeballs, or chemical changes in the blood. The osmoreceptors of the
supraoptic nuclei in the hypothalamus detect minute changes in the osmolality
of the body ﬂuids but have never been known to res

of the body ﬂuids but have never been known to respond to sound. Finally, pain
receptors in the skin are almost never stimulated by usual touch or pressure
stimuli but do become highly active the moment tactile stimuli become severe
enough to damage the tissues.
Modality of Sensation—The “Labeled Line” Principle
Each of the principal types of sensation that we can experience—pain, touch,
sight, sound, and so forth—is called a modality of sensation. Yet despite the 
fact that we experience these 

n. Yet despite the 
fact that we experience these different modalities of sensation, nerve ﬁbers
Chapter 46
Sensory Receptors, Neuronal Circuits for Processing Information
573
transmit only impulses. Therefore, how is it that 
different nerve ﬁbers transmit different modalities of
sensation?
The answer is that each nerve tract terminates at a
speciﬁc point in the central nervous system, and 
the type of sensation felt when a nerve ﬁber is stimu-
lated is determined by the point in the nervous sy

lated is determined by the point in the nervous system
to which the ﬁber leads. For instance, if a pain ﬁber 
is stimulated, the person perceives pain regardless 
of what type of stimulus excites the ﬁber.The stimulus
can be electricity, overheating of the ﬁber, crushing 
of the ﬁber, or stimulation of the pain nerve ending 
by damage to the tissue cells. In all these instances,
the person perceives pain. Likewise, if a touch ﬁber is
stimulated by electrical excitation of a touch receptor
or in 

y electrical excitation of a touch receptor
or in any other way, the person perceives touch
because touch ﬁbers lead to speciﬁc touch areas in 
the brain. Similarly, ﬁbers from the retina of the eye
terminate in the vision areas of the brain, ﬁbers from
the ear terminate in the auditory areas of the brain,
and temperature ﬁbers terminate in the temperature
areas.
This speciﬁcity of nerve ﬁbers for transmitting only
one modality of sensation is called the labeled line
principle.
Transduction of S

lled the labeled line
Transduction of Sensory
Stimuli into Nerve Impulses
Local Electrical Currents at Nerve
Endings—Receptor Potentials
All sensory receptors have one feature in common.
Whatever the type of stimulus that excites the recep-
tor, its immediate effect is to change the membrane
Free nerve
endings
Expanded tip
receptor
Tactile hair
Pacinian
corpuscle
Meissner’s
corpuscle
Krause’s
corpuscle
Rufﬁni’s
end-organ
apparatus
Muscle
spindle
Figure 46–1
Several types 

pparatus
Muscle
spindle
Several types of somatic sensory nerve endings.
Table 46–1
Classiﬁcation of Sensory Receptors
I. Mechanoreceptors
Skin tactile sensibilities (epidermis and dermis)
Free nerve endings
Expanded tip endings
Merkel’s discs
Plus several other variants
Spray endings
Rufﬁni’s endings
Encapsulated endings
Meissner’s corpuscles
Krause’s corpuscles
Hair end-organs
Deep tissue sensibilities
Encapsulat

 endings
Pacinian corpuscles
Plus a few other variants
Muscle endings
Muscle spindles
Golgi tendon receptors
Hearing
Sound receptors of cochlea
Vestibular receptors
Baroreceptors of carotid sinuses and aorta
II. Thermoreceptors
Cold
Cold receptors
Warmth
Warm receptors
III. Nociceptors
Pain
IV. Electromagnetic receptors
Vision
Rods
Cones
V. Chemoreceptors
Taste
Receptors of taste buds
Smell
Recep

eceptors
Taste
Smell
Receptors of olfactory epithelium
Arterial oxygen
Receptors of aortic and carotid bodies
Neurons in or near supraoptic nuclei
Blood CO2
Receptors in or on surface of medulla and in aortic and 
carotid bodies
Blood glucose, amino acids, fatty acids
Receptors in hypothalamus
574
Unit IX
electrical potential of the receptor. This change in
potential is called a receptor potential

change in
potential is called a receptor potential.
Mechanisms of Receptor Potentials. Different receptors
can be excited in one of several ways to cause recep-
tor potentials: (1) by mechanical deformation of the
receptor, which stretches the receptor membrane and
opens ion channels; (2) by application of a chemical to
the membrane, which also opens ion channels; (3) by
change of the temperature of the membrane, which
alters the permeability of the membrane; or (4) by the
effects of electromagn

the membrane; or (4) by the
effects of electromagnetic radiation, such as light on a
retinal visual receptor, which either directly or indi-
rectly changes the receptor membrane characteristics
and allows ions to ﬂow through membrane channels.
It will be recognized that these four means of exciting
receptors correspond in general with the different
types of known sensory receptors. In all instances, the
basic cause of the change in membrane potential is a
change in membrane permeability of the r

tial is a
change in membrane permeability of the receptor,
which allows ions to diffuse more or less readily
through the membrane and thereby to change the
transmembrane potential.
Maximum Receptor Potential Amplitude. The maximum
amplitude of most sensory receptor potentials is about
100 millivolts,but this level occurs only at an extremely
high intensity of sensory stimulus. This is about the
same maximum voltage recorded in action potentials
and is also the change in voltage when the membrane

nd is also the change in voltage when the membrane
becomes maximally permeable to sodium ions.
Relation of the Receptor Potential to Action Potentials. When
the receptor potential rises above the threshold for
eliciting action potentials in the nerve ﬁber attached
to the receptor, then action potentials occur, as illus-
trated in Figure 46–2. Note also that the more the
receptor potential rises above the threshold level, the
greater becomes the action potential frequency.
Receptor Potential of t

ction potential frequency.
Receptor Potential of the Pacinian Corpuscle—
An Example of Receptor Function
The student should at this point restudy the anatomi-
cal structure of the pacinian corpuscle shown in Figure
46–1. Note that the corpuscle has a central nerve ﬁber
extending through its core. Surrounding this are mul-
tiple concentric capsule layers, so that compression
anywhere on the outside of the corpuscle will elongate,
indent, or otherwise deform the central ﬁber.
Now study Figure 46–3

ise deform the central ﬁber.
Now study Figure 46–3, which shows only the
central ﬁber of the pacinian corpuscle after all capsule
layers but one have been removed. The tip of the
central ﬁber inside the capsule is unmyelinated, but
the ﬁber does become myelinated (the blue sheath
shown in the ﬁgure) shortly before leaving the cor-
puscle to enter a peripheral sensory nerve.
The ﬁgure also shows the mechanism by which a
receptor potential is produced in the pacinian corpus-
cle. Observe the small

ced in the pacinian corpus-
cle. Observe the small area of the terminal ﬁber that
has been deformed by compression of the corpuscle,
and note that ion channels have opened in the mem-
brane, allowing positively charged sodium ions to
diffuse to the interior of the ﬁber. This creates
increased positivity inside the ﬁber, which is the
“receptor potential.” The receptor potential in turn
induces a local circuit of current ﬂow, shown by the
arrows, that spreads along the nerve ﬁber. At the ﬁrst
node

hat spreads along the nerve ﬁber. At the ﬁrst
node of Ranvier, which itself lies inside the capsule of
the pacinian corpuscle, the local current ﬂow depolar-
izes the ﬁber membrane at this node, which then sets
off typical action potentials that are transmitted along
the nerve ﬁber toward the central nervous system.
Relation Between Stimulus Intensity and the Receptor Poten-
tial. Figure 46–4 shows the changing amplitude of the
receptor potential caused by progressively stronger
mechanical 
comp

 caused by progressively stronger
compression 
(increasing 
“stimulus
strength”) applied experimentally to the central core
of a pacinian corpuscle. Note that the amplitude
Membrane potential (millivolts)
0
10
20
30
40
Receptor potential
Threshold
60
80
100 120 140
+30
-30
-60
-90
0
Figure 46–2
Typical relation between receptor potential and action potentials
when the receptor potential rises above threshold level.
+
+
+
+
+
+

potential rises above threshold level.
+
+
+
+
+
+
+
+
+
+ + + +
++ + + + + + + + + + + + + + +
+ + + + + + + + + + + + + + +
+ + + +
+
+
+
+
+
+
+
+
+
+
+
+
+
- - - -
- - - -
+ + + +
+ + + +
Deformed
area
Action
potential
Node of
Ranvier
Figure 46–3
Excitation of a sensory nerve ﬁber by a receptor potential pro-
duced in a pacinian corpuscle. (Modiﬁed from Loëwenstein WR:
Excitation and inactivation in a receptor membrane. Ann N Y Acad
Sci 94:510, 1961.)
Chapter 46
Sensory Re

 N Y Acad
Chapter 46
575
increases rapidly at ﬁrst but then progressively less
rapidly at high stimulus strength.
In turn, the frequency of repetitive action potentials
transmitted from sensory receptors increases approx-
imately in proportion to the increase in receptor
potential. Putting this principle together with the data
in Figure 46–4, one can see that very intense stimula-
tion of the receptor causes progr

intense stimula-
tion of the receptor causes progressively less and less
additional increase in numbers of action potentials.
This is an exceedingly important principle that is appli-
cable to almost all sensory receptors. It allows the
receptor to be sensitive to very weak sensory experi-
ence and yet not reach a maximum ﬁring rate until the
sensory experience is extreme. This allows the recep-
tor to have an extreme range of response, from very
weak to very intense.
Ano

Another characteristic of all sensory receptors is that
they adapt either partially or completely to any con-
stant stimulus after a period of time. That is, when a
continuous sensory stimulus is applied, the receptor
responds at a high impulse rate at ﬁrst and then at a
progressively slower rate until ﬁnally the rate of action
potentials decreases to very few or often to none at all.
Figure 46–5 shows typical adaptation of certain
types of receptor

ws typical adaptation of certain
types of receptors. Note that the pacinian corpuscle
adapts extremely rapidly and hair receptors adapt
within a second or so, whereas some joint capsule and
muscle spindle receptors adapt slowly.
Furthermore, some sensory receptors adapt to a far
greater extent than others. For example, the pacinian
corpuscles adapt to “extinction” within a few hun-
dredths of a second, and the receptors at the bases of
the hairs adapt to extinction within a second or more.
It is

adapt to extinction within a second or more.
It is probable that all other mechanoreceptors eventu-
ally adapt almost completely, but some require hours
or days to do so, for which reason they are called “non-
adapting” receptors. The longest measured time for
complete adaptation of a mechanoreceptor is about 
2 days, which is the adaptation time for many caro-
tid and aortic baroreceptors. Conversely, some of 
the nonmechanoreceptors—the chemoreceptors and 
pain receptors, for instance—probably

ceptors and 
pain receptors, for instance—probably never adapt
completely.
Mechanisms by Which Receptors Adapt. The mechanism of
receptor adaptation is different for each type of recep-
tor, in much the same way that development of a
receptor potential is an individual property. For
instance, in the eye, the rods and cones adapt by chang-
ing the concentrations of their light-sensitive chemi-
cals (which is discussed in Chapter 50).
In the case of the mechanoreceptors, the receptor
that has been

f the mechanoreceptors, the receptor
that has been studied in greatest detail is the pacinian
corpuscle. Adaptation occurs in this receptor in two
ways. First, the pacinian corpuscle is a viscoelastic
structure so that when a distorting force is suddenly
applied to one side of the corpuscle, this force is
instantly transmitted by the viscous component of the
corpuscle directly to the same side of the central nerve
ﬁber, thus eliciting a receptor potential. However,
within a few hundredths of a s

potential. However,
within a few hundredths of a second, the ﬂuid within
the corpuscle redistributes, so that the receptor poten-
tial is no longer elicited. Thus, the receptor potential
Amplitude of observed
receptor potential (per cent)
Stimulus strength
(per cent)
0
20
40
60
80
100
0
10
20
30
40
50
60
70
80
90
100
Figure 46–4
Relation of amplitude of receptor potential to strength of a
mechanical stimulus applied to a pacinian corpuscle. (Data from
Loëwenstein WR: Excitation and inactivation 

 from
Loëwenstein WR: Excitation and inactivation in a receptor mem-
brane. Ann N Y Acad Sci 94:510, 1961.)
Seconds
0
1
2
3
4
5
6
7
8
0
250
Joint capsule receptors
200
150
100
50
Hair receptor
Pacinian corpuscle
Figure 46–5
Adaptation of different types of receptors, showing rapid adapta-
tion of some receptors and slow adaptation of others.
576
Unit IX
appears at the onset of compression but disa

ology
appears at the onset of compression but disappears
within a small fraction of a second even though the
compression continues.
The second mechanism of adaptation of the pacin-
ian corpuscle, but a much slower one, results from a
process called accommodation, which occurs in the
nerve ﬁber itself. That is, even if by chance the central
core ﬁber should continue to be distorted, the tip of
the nerve ﬁber itself gradually becomes “accommo-
dated” to the stimulus. This probably results from 
pr

d” to the stimulus. This probably results from 
progressive “inactivation” of the sodium channels in
the nerve ﬁber membrane, which means that sodium
current ﬂow through the channels causes them gradu-
ally to close, an effect that seems to occur for all 
or most cell membrane sodium channels, as was
explained in Chapter 5.
Presumably, these same two general mechanisms of
adaptation apply also to the other types of mechano-
receptors. That is, part of the adaptation results from
readjustments in

rt of the adaptation results from
readjustments in the structure of the receptor itself,
and part from an electrical type of accommodation in
the terminal nerve ﬁbril.
Slowly Adapting Receptors Detect Continuous Stimulus
Strength—The “Tonic” Receptors. Slowly adapting recep-
tors continue to transmit impulses to the brain as long
as the stimulus is present (or at least for many minutes
or hours). Therefore, they keep the brain constantly
apprised of the status of the body and its relation to
its

 of the status of the body and its relation to
its surroundings. For instance, impulses from the
muscle spindles and Golgi tendon apparatuses allow
the nervous system to know the status of muscle con-
traction and load on the muscle tendon at each instant.
Other slowly adapting receptors include (1) recep-
tors of the macula in the vestibular apparatus, (2) pain
receptors, (3) baroreceptors of the arterial tree, and (4)
chemoreceptors of the carotid and aortic bodies.
Because the slowly adapting

tid and aortic bodies.
Because the slowly adapting receptors can continue
to transmit information for many hours, they are called
tonic receptors.
Rapidly Adapting Receptors Detect Change in Stimulus
Strength—The “Rate Receptors,” “Movement Receptors,” 
or “Phasic Receptors.” Receptors that adapt rapidly
cannot be used to transmit a continuous signal because
these receptors are stimulated only when the stimulus
strength changes. Yet they react strongly while a
change is actually taking place. Th

rongly while a
change is actually taking place. Therefore, these recep-
tors are called rate receptors, movement receptors, or
phasic receptors. Thus, in the case of the pacinian cor-
puscle, sudden pressure applied to the tissue excites
this receptor for a few milliseconds, and then its exci-
tation is over even though the pressure continues. But
later, it transmits a signal again when the pressure is
released. In other words, the pacinian corpuscle is
exceedingly important in apprising the ner

scle is
exceedingly important in apprising the nervous system
of rapid tissue deformations, but it is useless for trans-
mitting information about constant conditions in the
body.
Importance of the Rate Receptors—Their Predictive Function.
If one knows the rate at which some change in bodily
status is taking place, one can predict in one’s mind the
state of the body a few seconds or even a few minutes
later. For instance, the receptors of the semicircular
canals in the vestibular apparatus of th

icircular
canals in the vestibular apparatus of the ear detect the
rate at which the head begins to turn when one runs
around a curve. Using this information, a person can
predict how much he or she will turn within the next
2 seconds and can adjust the motion of the legs ahead
of time to keep from losing balance. Likewise, recep-
tors located in or near the joints help detect the rates
of movement of the different parts of the body. For
instance, when one is running, information from the
joint 

, when one is running, information from the
joint rate receptors allows the nervous system to
predict where the feet will be during any precise frac-
tion of the next second. Therefore, appropriate motor
signals can be transmitted to the muscles of the legs to
make any necessary anticipatory corrections in posi-
tion so that the person will not fall. Loss of this pre-
dictive function makes it impossible for the person to
run.
Nerve Fibers That Transmit
Different Types of Signals,
and Their Phys

ransmit
and Their Physiologic
Classiﬁcation
Some signals need to be transmitted to or from the
central nervous system extremely rapidly; otherwise,
the information would be useless. An example of this is
the sensory signals that apprise the brain of the momen-
tary positions of the legs at each fraction of a second
during running. At the other extreme, some types of
sensory information, such as that depicting prolonged,
aching pain, do not need to be transmitted rapid

,
aching pain, do not need to be transmitted rapidly, so
that slowly conducting ﬁbers will sufﬁce. As shown in
Figure 46–6, nerve ﬁbers come in all sizes between 0.5
and 20 micrometers in diameter—the larger the diame-
ter, the greater the conducting velocity. The range of
conducting velocities is between 0.5 and 120 m/sec.
General Classiﬁcation of Nerve Fibers. Shown in Figure 46–6
is a “general classiﬁcation” and a “sensory nerve classi-
ﬁcation” of the different types of nerve ﬁbers. In the
g

n” of the different types of nerve ﬁbers. In the
general classiﬁcation, the ﬁbers are divided into types A
and C, and the type A ﬁbers are further subdivided into
a, b, g, and d ﬁbers.
Type A ﬁbers are the typical large and medium-sized
myelinated ﬁbers of spinal nerves. Type C ﬁbers are the
small unmyelinated nerve ﬁbers that conduct impulses
at low velocities. The C ﬁbers constitute more than one
half of the sensory ﬁbers in most peripheral nerves as
well as all the postganglionic autonomic ﬁb

ves as
well as all the postganglionic autonomic ﬁbers.
The sizes, velocities of conduction, and functions of
the different nerve ﬁber types are also given in Figure
46–6. Note that a few large myelinated ﬁbers can trans-
mit impulses at velocities as great as 120 m/sec, a 
distance in 1 second that is longer than a football ﬁeld.
Conversely, the smallest ﬁbers transmit impulses as
slowly as 0.5 m/sec, requiring about 2 seconds to go from
the big toe to the spinal cord.
Alternative Classiﬁcation 

toe to the spinal cord.
Alternative Classiﬁcation Used by Sensory Physiologists.
Certain recording techniques have made it possible to
separate the type Aa ﬁbers into two subgroups; yet
Chapter 46
577
these same recording techniques cannot distinguish
easily between Ab and Ag ﬁbers. Therefore, the fol-
lowing classiﬁcation is frequently used by sensory 
physiologists:
Group Ia
Fibers from the annulospiral endings of muscle spindles

 from the annulospiral endings of muscle spindles
(average about 17 microns in diameter; these are a-type
A ﬁbers in the general classiﬁcation).
Group Ib
Fibers from the Golgi tendon organs (average about 16
micrometers in diameter; these also are a-type A
ﬁbers).
Group II
Fibers from most discrete cutaneous tactile receptors
and from the ﬂower-spray endings of the muscle spin-
dles (average about 8 micrometers in diameter; these
are b- and g-type A ﬁbers in the general classiﬁcation).
Group III

e A ﬁbers in the general classiﬁcation).
Group III
Fibers carrying temperature, crude touch, and pricking
pain sensations (average about 3 micrometers in diame-
ter; they are d-type A ﬁbers in the general classiﬁcation).
Group IV
Unmyelinated ﬁbers carrying pain, itch, temperature,
and crude touch sensations (0.5 to 2 micrometers in
diameter; they are type C ﬁbers in the general classiﬁ-
cation).
Transmission of Signals 
of Different Intensity in 
Nerve Tracts—Spatial and
One 

One of the characteristics of each signal that always
must be conveyed is signal intensity—for instance, the
intensity of pain. The different gradations of intensity
Nerve fiber diameter (micrometers)
20
15
Myelinated
Unmyelinated
10
5
1 2.0
0.5
Conduction velocity (m/sec.)
30
60
80
IB
IA
I
II
III
IV
C
d
g
b
A
a
(primary ending)
(secondary ending)
6 2.0
0.5
10
5
2.0
0.5
Muscle tendon
(Golgi tendon organ)
Hair receptors
S

uscle tendon
Sensory functions
Sensory nerve classification
General classification
Diameter (micrometers)
(type Aa)
(type Ag)
(type C)
Motor function
Deep pressure
and touch
Pricking pain
Cold
Warmth
Aching pain
Tickle
Crude touch
and pressure
Vibration
(pacinian corpuscle)
High discrimination touch
(Meissner's expanded tips)
15
1
120
20
Figure 46–6
Physiologic classiﬁcations and functions of nerve
ﬁbers.
578
Unit IX

cations and functions of nerve
ﬁbers.
578
Unit IX
can be transmitted either by using increasing numbers
of parallel ﬁbers or by sending more action potentials
along a single ﬁber. These two mechanisms are called,
respectively,
spatial 
summation 
and 
temporal 
summation.
Spatial Summation. Figure 46–7 shows the phenomenon
of spatial summation, whereby increasing signal
strength is transmitted by using progressively greater
numbers

transmitted by using progressively greater
numbers of ﬁbers. This ﬁgure shows a section of skin
innervated by a large number of parallel pain ﬁbers.
Each of these arborizes into hundreds of minute free
nerve endings that serve as pain receptors. The entire
cluster of ﬁbers from one pain ﬁber frequently covers
an area of skin as large as 5 centimeters in diameter.
This area is called the receptor ﬁeld of that ﬁber. The
number of endings is large in the center of the ﬁeld but
diminishes toward the

n the center of the ﬁeld but
diminishes toward the periphery. One can also see
from the ﬁgure that the arborizing ﬁbrils overlap those
from other pain ﬁbers.Therefore, a pinprick of the skin
usually stimulates endings from many different pain
ﬁbers simultaneously. When the pinprick is in the
center of the receptive ﬁeld of a particular pain ﬁber,
the degree of stimulation of that ﬁber is far greater
than when it is in the periphery of the ﬁeld, because
the number of free nerve endings in the mid

ecause
the number of free nerve endings in the middle of the
ﬁeld is much greater than at the periphery.
Thus, the lower part of Figure 46–7 shows three
views of the cross section of the nerve bundle leading
from the skin area. To the left is the effect of a weak
stimulus, with only a single nerve ﬁber in the middle
of the bundle stimulated strongly (represented by the
red-colored ﬁber), whereas several adjacent ﬁbers are
stimulated weakly (half-red ﬁbers). The other two
views of the nerve cross

red ﬁbers). The other two
views of the nerve cross section show the effect of a
moderate stimulus and a strong stimulus, with pro-
gressively more ﬁbers being stimulated. Thus, the
stronger signals spread to more and more ﬁbers. This
is the phenomenon of spatial summation.
Temporal Summation. A second means for transmitting
signals of increasing strength is by increasing the fre-
quency of nerve impulses in each ﬁber, which is called
temporal summation. Figure 46–8 demonstrates this,
showing in 

mation. Figure 46–8 demonstrates this,
showing in the upper part a changing strength of signal
and in the lower part the actual impulses transmitted
by the nerve ﬁber.
Transmission and Processing
of Signals in Neuronal Pools
The central nervous system is composed of thousands
to millions of neuronal pools; some of these contain
few neurons, while others have vast numbers. For
instance, the entire cerebral cortex could be consid-
ered to be a single large neuronal pool.Other neuronal
pools includ

le large neuronal pool.Other neuronal
pools include the different basal ganglia and the 
speciﬁc nuclei in the thalamus, cerebellum, mesen-
cephalon, pons, and medulla. Also, the entire dorsal
gray matter of the spinal cord could be considered one
long pool of neurons.
Each neuronal pool has its own special organization
that causes it to process signals in its own unique way,
Weak
stimulus
Moderate
stimulus
Skin
Nerve
Pin
Strong
stimulus
Figure 46–7
Pattern of stimulation of pain ﬁbers in a nerv

6–7
Pattern of stimulation of pain ﬁbers in a nerve leading from an area
of skin pricked by a pin. This is an example of spatial summation.
Strength of signal
(impulses per second)
Impulses
Time
0
80
60
40
20
Figure 46–8
Translation of signal strength into a frequency-modulated series
of nerve impulses, showing the strength of signal (above) and the
separate nerve impulses (below). This is an example of temporal
Chapter 46
Sensory Receptors, Neuronal Circuits for Processing Informatio

ptors, Neuronal Circuits for Processing Information
579
thus allowing the total consortium of pools to achieve
the multitude of functions of the nervous system. Yet
despite their differences in function, the pools also
have many similar principles of function, described in
the following pages.
Relaying of Signals Through 
Neuronal Pools
Organization of Neurons for Relaying Signals. Figure 46–9 is
a schematic diagram of several neurons in a neuronal
pool, showing “input” ﬁbers to the left and “ou

al
pool, showing “input” ﬁbers to the left and “output”
ﬁbers to the right. Each input ﬁber divides hundreds
to thousands of times, providing a thousand or more
terminal ﬁbrils that spread into a large area in the pool
to synapse with dendrites or cell bodies of the neurons
in the pool. The dendrites usually also arborize and
spread hundreds to thousands of micrometers in the
pool.
The neuronal area stimulated by each incoming
nerve ﬁber is called its stimulatory ﬁeld. Note in Figure
46–9 that l

d its stimulatory ﬁeld. Note in Figure
46–9 that large numbers of the terminals from each
input ﬁber lie on the nearest neuron in its “ﬁeld,” but
progressively fewer terminals lie on the neurons
farther away.
Threshold and Subthreshold Stimuli—Excitation or Facilitation.
From the discussion of synaptic function in Chapter
45, it will be recalled that discharge of a single 
excitatory presynaptic terminal almost never causes an
action potential in a postsynaptic neuron. Instead,
large numbers of 

 a postsynaptic neuron. Instead,
large numbers of input terminals must discharge on
the same neuron either simultaneously or in rapid suc-
cession to cause excitation. For instance, in Figure
46–9, let us assume that six terminals must discharge
almost simultaneously to excite any one of the
neurons. If the student counts the number of terminals
on each one of the neurons from each input ﬁber, he
or she will see that input ﬁber 1 has more than enough
terminals to cause neuron a to discharge. The

ough
terminals to cause neuron a to discharge. The stimulus
from input ﬁber 1 to this neuron is said to be an 
excitatory stimulus; it is also called a suprathreshold
stimulus because it is above the threshold required for
excitation.
Input ﬁber 1 also contributes terminals to neurons
b and c, but not enough to cause excitation. Never-
theless, discharge of these terminals makes both these
neurons more likely to be excited by signals arriving
through other incoming nerve ﬁbers. Therefore, the
st

ough other incoming nerve ﬁbers. Therefore, the
stimuli to these neurons are said to be subthreshold,
and the neurons are said to be facilitated.
Similarly, for input ﬁber 2, the stimulus to neuron d
is a suprathreshold stimulus, and the stimuli to neurons
b and c are subthreshold, but facilitating, stimuli.
Figure 46–9 represents a highly condensed version
of a neuronal pool because each input nerve ﬁber
usually provides massive numbers of branching termi-
nals to hundreds or thousands of neuro

hing termi-
nals to hundreds or thousands of neurons in its distri-
bution “ﬁeld,” as shown in Figure 46–10. In the central
portion of the ﬁeld in this ﬁgure, designated by the
circled area, all the neurons are stimulated by the
incoming ﬁber. Therefore, this is said to be the dis-
charge zone of the incoming ﬁber, also called the
excited zone or liminal zone. To each side, the neurons
are facilitated but not excited, and these areas are
called the facilitated zone, also called the subthreshold

he facilitated zone, also called the subthreshold
zone or subliminal zone.
Inhibition of a Neuronal Pool. We must also remember
that some incoming ﬁbers inhibit neurons, rather than
exciting them. This is the opposite of facilitation, and
the entire ﬁeld of the inhibitory branches is called the
inhibitory zone. The degree of inhibition in the center
of this zone is great because of large numbers of
endings in the center; it becomes progressively less
toward its edges.
1
2
d
c
b
a
Figure 46–9
Bas

less
1
2
d
c
b
a
Basic organization of a neuronal pool.
Facilitated zone
Input nerve
fiber
Discharge zone
Figure 46–10
“Discharge” and “facilitated” zones of a neuronal pool.
580
Unit IX
Divergence of Signals Passing Through
Often it is important for weak signals entering a neu-
ronal pool to excite far greater numbers of nerve ﬁbers
leaving the pool. This phenomenon is 

 nerve ﬁbers
leaving the pool. This phenomenon is called diver-
gence. Two major types of divergence occur and have
entirely different purposes.
An amplifying type of divergence is shown in Figure
46–11A.This means simply that an input signal spreads
to an increasing number of neurons as it passes
through successive orders of neurons in its path. This
type of divergence is characteristic of the corticospinal
pathway in its control of skeletal muscles, with a single
large pyramidal cell in the mo

cles, with a single
large pyramidal cell in the motor cortex capable, under
highly facilitated conditions, of exciting as many as
10,000 muscle ﬁbers.
The second type of divergence, shown in Figure
46–11B, is divergence into multiple tracts. In this case,
the signal is transmitted in two directions from the
pool. For instance, information transmitted up the
dorsal columns of the spinal cord takes two courses in
the lower part of the brain: (1) into the cerebellum and
(2) on through the lower reg

to the cerebellum and
(2) on through the lower regions of the brain to the
thalamus and cerebral cortex. Likewise, in the thala-
mus, almost all sensory information is relayed both
into still deeper structures of the thalamus and at the
same time to discrete regions of the cerebral cortex.
Convergence of Signals
Convergence means signals from multiple inputs
uniting to excite a single neuron. Figure 46–12A shows
convergence from a single source. That is, multiple ter-
minals from a single incomi

That is, multiple ter-
minals from a single incoming ﬁber tract terminate on
the same neuron. The importance of this is that
neurons are almost never excited by an action poten-
tial from a single input terminal. But action potentials
converging on the neuron from multiple terminals
provide enough spatial summation to bring the neuron
to the threshold required for discharge.
Convergence can also result from input signals (exci-
tatory or inhibitory) from multiple sources, as shown
in Figure 46–1

ry) from multiple sources, as shown
in Figure 46–12B. For instance, the interneurons of the
spinal cord receive converging signals from (1) periph-
eral nerve ﬁbers entering the cord, (2) propriospinal
ﬁbers passing from one segment of the cord to
another, (3) corticospinal ﬁbers from the cerebral
cortex, and (4) several other long pathways descend-
ing from the brain into the spinal cord.Then the signals
from the interneurons converge on the anterior motor
neurons to control muscle function.
Su

erior motor
Such convergence allows summation of information
from different sources, and the resulting response is a
summated effect of all the different types of informa-
tion. Convergence is one of the important means by
which the central nervous system correlates, sum-
mates, and sorts different types of information.
Neuronal Circuit with Both Excitatory and
Inhibitory Output Signals
Sometimes an incoming signal to a neuronal pool
causes an output excitator

gnal to a neuronal pool
causes an output excitatory signal going in one direc-
tion and at the same time an inhibitory signal going
elsewhere. For instance, at the same time that an exci-
tatory signal is transmitted by one set of neurons in the
spinal cord to cause forward movement of a leg, an
inhibitory signal is transmitted through a separate set
of neurons to inhibit the muscles on the back of the
leg so that they will not oppose the forward move-
ment. This type of circuit is characteristi

 move-
ment. This type of circuit is characteristic for control-
ling all antagonistic pairs of muscles, and it is called the
reciprocal inhibition circuit.
Figure 46–13 shows the means by which the inhibi-
tion is achieved. The input ﬁber directly excites the
excitatory output pathway, but it stimulates an inter-
mediate inhibitory neuron (neuron 2), which secretes
a different type of transmitter substance to inhibit the
second output pathway from the pool. This type of
circuit is also importan

om the pool. This type of
circuit is also important in preventing overactivity in
many parts of the brain.
 
Divergence in same tract
A
Divergence in multiple tracts
B
Figure 46–11
“Divergence” in neuronal pathways. A, Divergence within a
pathway to cause “ampliﬁcation” of the signal. B, Divergence into
multiple tracts to transmit the signal to separate areas.
Convergence from
single source
A
multiple sources
Source
#3
Source
#2
Source
#1
Source
B
Figure 46–12
“Convergence” of m

Source
#1
Source
B
“Convergence” of multiple input ﬁbers onto a single neuron. 
A, Multiple input ﬁbers from a single source. B, Input ﬁbers from
multiple separate sources.
Chapter 46
581
Prolongation of a Signal by a
Neuronal Pool—“Afterdischarge”
Thus far, we have considered signals that are merely
relayed through neuronal pools. However, in many
instances, a signal entering a pool causes a prolonged
output discharge,

tering a pool causes a prolonged
output discharge, called afterdischarge, lasting a 
few milliseconds to as long as many minutes after 
the incoming signal is over. The most important 
mechanisms by which afterdischarge occurs are the
Synaptic Afterdischarge. When excitatory synapses dis-
charge on the surfaces of dendrites or soma of a
neuron, a postsynaptic electrical potential develops in
the neuron and lasts for many milliseconds, especially
when some of the long-acting synaptic t

when some of the long-acting synaptic transmitter sub-
stances are involved. As long as this potential lasts, it
can continue to excite the neuron, causing it to trans-
mit a continuous train of output impulses, as was
explained in Chapter 45.Thus, as a result of this synap-
tic “afterdischarge” mechanism alone, it is possible for
a single instantaneous input signal to cause a sustained
signal output (a series of repetitive discharges) lasting
for many milliseconds.
Reverberatory (Osc

 lasting
Reverberatory (Oscillatory) Circuit as a Cause of Signal Prolon-
gation. One of the most important of all circuits in the
entire nervous system is the reverberatory, or oscilla-
tory, circuit. Such circuits are caused by positive feed-
back within the neuronal circuit that feeds back to
re-excite the input of the same circuit. Consequently,
once stimulated, the circuit may discharge repetitively
for a long time.
Several possible varieties of reverberatory circuits

veral possible varieties of reverberatory circuits
are shown in Figure 46–14. The simplest, shown in
Figure 46–14A, involves only a single neuron. In this
case, the output neuron simply sends a collateral nerve
ﬁber back to its own dendrites or soma to restimulate
itself. Although this type of circuit probably is not an
important one, theoretically, once the neuron dis-
charges, the feedback stimuli could keep the neuron
discharging for a protracted time thereafter.
Figure 46–14B shows a few add

ted time thereafter.
Figure 46–14B shows a few additional neurons in the
feedback circuit, which causes a longer delay between
initial discharge and the feedback signal. Figure
46–14C shows a still more complex system in which
both facilitatory and inhibitory ﬁbers impinge on the
reverberating circuit.A facilitatory signal enhances the
intensity and frequency of reverberation, whereas an
inhibitory signal depresses or stops the reverberation.
Figure 46–14D shows that most reverberating path-
way

ure 46–14D shows that most reverberating path-
ways are constituted of many parallel ﬁbers. At each
cell station, the terminal ﬁbrils spread widely. In such
a system, the total reverberating signal can be either
weak or strong, depending on how many parallel nerve
ﬁbers are momentarily involved in the reverberation.
Characteristics of Signal Prolongation from a Rever-
beratory Circuit. Figure 46–15 shows output signals
from a typical reverberatory circuit. The input stimu-
lus may last only 1 mi

y circuit. The input stimu-
lus may last only 1 millisecond or so,and yet the output
can last for many milliseconds or even minutes. The
ﬁgure demonstrates that the intensity of the output
signal usually increases to a high value early in rever-
beration and then decreases to a critical point,at which
it suddenly ceases entirely. The cause of this sudden
cessation of reverberation is fatigue of synaptic junc-
tions in the circuit. Fatigue beyond a certain critical
level lowers the stimulation of

a certain critical
level lowers the stimulation of the next neuron in the
circuit below threshold level so that the circuit feed-
back is suddenly broken.
Excitatory synapse
#1
#3
#2
Inhibitory synapse
Input fiber
Figure 46–13
Inhibitory circuit. Neuron 2 is an inhibitory neuron.
Input
Input
C
D
B
A
Output
Input
Output
Input
Output
Input
Output
Facilitation
Figure 46–14
Reverberatory circuits of increasing complexity.
582
Unit IX
The Nervous System: A. General Pr

ity.
582
Unit IX
The duration of the total signal before cessation can
also be controlled by signals from other parts of the
brain that inhibit or facilitate the circuit. Almost these
exact patterns of output signals are recorded from the
motor nerves exciting a muscle involved in a ﬂexor
reﬂex after pain stimulation of the foot (as shown later
in Figure 46–18).
Continuous Signal Output from Some 
Neuronal Circuits
Some neuronal ci

tput from Some 
Some neuronal circuits emit output signals continu-
ously, even without excitatory input signals. At least
two mechanisms can cause this effect: (1) continuous
intrinsic neuronal discharge and (2) continuous rever-
beratory signals.
Continuous Discharge Caused by Intrinsic Neuronal Excitabil-
ity. Neurons, like other excitable tissues, discharge
repetitively if their level of excitatory membrane
potential rises above a certain threshold level. The
membrane poten

bove a certain threshold level. The
membrane potentials of many neurons even normally
are high enough to cause them to emit impulses con-
tinually. This occurs especially in many of the neurons
of the cerebellum, as well as in most of the interneu-
rons of the spinal cord. The rates at which these cells
emit impulses can be increased by excitatory signals or
decreased by inhibitory signals;inhibitory signals often
can decrease the rate of ﬁring to zero.
Continuous Signals Emitted from Reverberat

o zero.
Continuous Signals Emitted from Reverberating Circuits as 
a Means for Transmitting Information. A reverberating
circuit that does not fatigue enough to stop reverber-
ation is a source of continuous impulses. And excita-
tory impulses entering the reverberating pool can
increase the output signal, whereas inhibition can
decrease or even extinguish the signal.
Figure 46–16 shows a continuous output signal from
a pool of neurons. The pool may be emitting impulses
because of intrinsic neur

may be emitting impulses
because of intrinsic neuronal excitability or as a result
of reverberation. Note that an excitatory input signal
greatly increases the output signal, whereas an
inhibitory input signal greatly decreases the output.
Those students who are familiar with radio trans-
mitters will recognize this to be a carrier wave type of
information transmission. That is, the excitatory and
inhibitory control signals are not the cause of the
output signal, but they do control its changing

he
output signal, but they do control its changing level of
intensity. Note that this carrier wave system allows a
decrease in signal intensity as well as an increase,
whereas up to this point, the types of information
transmission we have discussed have been mainly pos-
itive information rather than negative information.
This type of information transmission is used by the
autonomic nervous system to control such functions as
vascular tone, gut tone, degree of constriction of the
iris in the ey

tone, degree of constriction of the
iris in the eye, and heart rate. That is, the nerve exci-
tatory signal to each of these can be either increased
or decreased by accessory input signals into the rever-
berating neuronal pathway.
Rhythmical Signal Output
Many neuronal circuits emit rhythmical output
signals—for instance, a rhythmical respiratory signal
originates in the respiratory centers of the medulla and
pons. This respiratory rhythmical signal continues
throughout life. Other rhythmical s

gnal continues
throughout life. Other rhythmical signals, such as those
that cause scratching movements by the hind leg of a
dog or the walking movements of any animal, require
input stimuli into the respective circuits to initiate the
rhythmical signals.
All or almost all rhythmical signals that have been
studied experimentally have been found to result from
reverberating circuits or a succession of sequential
reverberating circuits that feed excitatory or inhibitory
signals in a circular pathw

citatory or inhibitory
signals in a circular pathway from one neuronal pool
to the next.
Output pulse rate
Time
Inhibited
Input stimulus
Normal
Figure 46–15
Typical pattern of the output signal from a reverberatory circuit
after a single input stimulus, showing the effects of facilitation and
inhibition.
Time
Output
Figure 46–16
Continuous output from either a reverberating circuit or a pool of
intrinsically discharging neurons. This ﬁgure al

f
intrinsically discharging neurons. This ﬁgure also shows the effect
of excitatory or inhibitory input signals.
Chapter 46
583
Excitatory or inhibitory signals can also increase 
or decrease the amplitude of the rhythmical signal
output. Figure 46–17, for instance, shows changes in
the respiratory signal output in the phrenic nerve.
When the carotid body is stimulated by arterial oxygen
deﬁciency, both the frequency and the amplitu

ygen
deﬁciency, both the frequency and the amplitude of 
the respiratory rhythmical output signal increase 
progressively.
Instability and Stability 
of Neuronal Circuits
Almost every part of the brain connects either directly
or indirectly with every other part, and this creates a
serious problem. If the ﬁrst part excites the second, the
second the third, the third the fourth, and so on until
ﬁnally the signal re-excites the ﬁrst part, it is clear that
an excitatory signal entering any part of 

ar that
an excitatory signal entering any part of the brain
would set off a continuous cycle of re-excitation of all
parts. If this should occur, the brain would be inun-
dated by a mass of uncontrolled reverberating
signals—signals that would be transmitting no infor-
mation but, nevertheless, would be consuming the cir-
cuits of the brain so that none of the informational
signals could be transmitted. Such an effect occurs in
widespread areas of the brain during epileptic seizures.
How does th

f the brain during epileptic seizures.
How does the central nervous system prevent this
from happening all the time? The answer lies mainly
in two basic mechanisms that function throughout the
central nervous system: (1) inhibitory circuits and (2)
fatigue of synapses.
Inhibitory Circuits as a Mechanism for
Two types of inhibitory circuits in widespread areas 
of the brain help prevent excessive spread of signals:
(1) inhibitory feedback circuits that return f

ls:
(1) inhibitory feedback circuits that return from the
termini of pathways back to the initial excitatory
neurons of the same pathways—these circuits occur in
virtually all sensory nervous pathways and inhibit
either the input neurons or the intermediate neurons
in the sensory pathway when the termini become
overly excited; and (2) some neuronal pools that exert
gross inhibitory control over widespread areas of the
brain—for instance, many of the basal ganglia exert
inhibitory inﬂuences throu

the basal ganglia exert
inhibitory inﬂuences throughout the muscle control
system.
Synaptic Fatigue as a Means for
Stabilizing the Nervous System
Synaptic fatigue means simply that synaptic transmis-
sion becomes progressively weaker the more pro-
longed and more intense the period of excitation.
Figure 46–18 shows three successive records of a ﬂexor
reﬂex elicited in an animal caused by inﬂicting pain in
the footpad of the paw. Note in each record that the
strength of contraction progressively 

rd that the
strength of contraction progressively “decrements”—
that is, its strength diminishes; much of this effect is
caused by fatigue of synapses in the ﬂexor reﬂex
circuit. Furthermore, the shorter the interval between
successive ﬂexor reﬂexes, the less the intensity of the
subsequent reﬂex response.
Automatic Short-Term Adjustment of Pathway Sensitivity by the
Fatigue Mechanism. Now let us apply this phenomenon
of fatigue to other pathways in the brain. Those that
Phrenic nerve output
Inc

 in the brain. Those that
Increasing carotid
body stimulation
Figure 46–17
The rhythmical output of summated nerve impulses from the 
respiratory center, showing that progressively increasing stimula-
tion of the carotid body increases both the intensity and the fre-
quency of the phrenic nerve signal to the diaphragm to increase
respiration.
Flexor muscle contraction force (g)
Seconds
0
15
30
45
60
0
10
20
30
40
50
Flexor reflexes–decremental responses
Stimulus
Figure 46–18

flexes–decremental responses
Stimulus
Successive ﬂexor reﬂexes showing fatigue of conduction through
the reﬂex pathway.
584
Unit IX
are overused usually become fatigued, so that their
sensitivities decrease. Conversely, those that are
underused become rested, and their sensitivities
increase. Thus, fatigue and recovery from fatigue con-
stitute an important short-term means of moderating
the sensitivities of the differ

eans of moderating
the sensitivities of the different nervous system cir-
cuits. These help to keep the circuits operating in a
range of sensitivity that allows effective function.
Long-Term Changes in Synaptic Sensitivity Caused by Automatic
Downregulation or Upregulation of Synaptic Receptors. The
long-term sensitivities of synapses can be changed
tremendously by upregulating the number of receptor
proteins at the synaptic sites when there is underac-
tivity, and downregulating the receptors w

nderac-
tivity, and downregulating the receptors when there is
overactivity. The mechanism for this is the following:
Receptor proteins are being formed constantly by the
endoplasmic reticular–Golgi apparatus system and are
constantly being inserted into the receptor neuron
synaptic membrane. However, when the synapses are
overused so that excesses of transmitter substance
combine with the receptor proteins, many of these
receptors are inactivated and removed from the synap-
tic membrane.
It is 

d and removed from the synap-
It is indeed fortunate that upregulation and down-
regulation of receptors, as well as other control mech-
anisms for adjusting synaptic sensitivity, continually
adjust the sensitivity in each circuit to almost the exact
level required for proper function.Think for a moment
how serious it would be if the sensitivities of only a few
of these circuits were abnormally high; one might then
expect almost continual muscle cramps, seizures, psy-
chotic distur

tinual muscle cramps, seizures, psy-
chotic disturbances, hallucinations, mental tension,
or other nervous disorders. But fortunately, the auto-
matic controls normally readjust the sensitivities of the
circuits back to controllable ranges of reactivity any
time the circuits begin to be too active or too
depressed.
Buzsaki G: Large-scale recording of neuronal ensembles.
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Baev KV: Biological Neural Networks. Boston: Birkhauser,
1998.
Basar E: Brain Function and

ton: Birkhauser,
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Basar E: Brain Function and Oscillations. Berlin: Springer,
1998.
Fain GL, Matthews HR, Cornwall MC, Koutalos Y: Adapta-
tion in vertebrate photoreceptors. Physiol Rev 81:117,
2001.
Gandevia SC: Spinal and supraspinal factors in human
muscle fatigue. Physiol Rev 81:1725, 2001.
Gebhart GF: Descending modulation of pain. Neurosci
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Ivry RB, Robe

ing cells. Physiol Rev 81:685, 2001.
Ivry RB, Robertson LC: The Two Sides of Perception. Cam-
bridge, MA: MIT Press, 1998.
Krupa B, Liu G: Does the fusion pore contribute to synap-
tic plasticity? Trends Neurosci 27:62, 2004.
McLachlan EM: Transmission of signals through sympa-
thetic ganglia—modulation, integration or simply distri-
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Mombaerts P: Genes 

a Physiol Scand 177:227, 2003.
Mombaerts P: Genes and ligands for odorant, vomeronasal
and taste receptors. Nat Rev Neurosci 5:263, 2004.
Palmer MJ, von Gersdorff H: Phasic transmitter release from
tonic neurons. Neuron 35:600, 2002.
Pearson KG: Neural adaptation in the generation of rhyth-
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Renteria RC, Johnson J, Copenhagen DR: Need rods? Get
glycine receptors and taurine. Neuron 41:839, 2004.
Richerson GB, Wu Y: Dynamic equilibrium of neurotrans-
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on GB, Wu Y: Dynamic equilibrium of neurotrans-
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4
7
585
Somatic Sensations: I. Gener

H
A
P
T
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4
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585
Organization, the Tactile and
The somatic senses are the nervous mechanisms that
collect sensory information from all over the body.
These senses are in contradistinction to the special
senses, which mean speciﬁcally vision, hearing,
smell, taste, and equilibrium.
The somatic senses can be classiﬁed into three physiologic types: (1) the
mechanoreceptive somatic senses, which include both tactile

ceptive somatic senses, which include both tactile and position sensa-
tions that are stimulated by mechanical displacement of some tissue of the body;
(2) the thermoreceptive senses, which detect heat and cold; and (3) the pain
sense, which is activated by any factor that damages the tissues.
This chapter deals with the mechanoreceptive tactile and position senses.
Chapter 48 discusses the thermoreceptive and pain senses. The tactile senses
include touch, pressure, vibration, and tickle senses,

ude touch, pressure, vibration, and tickle senses, and the position senses
include static position and rate of movement senses.
Other Classiﬁcations of Somatic Sensations. Somatic sensations are also often
grouped together in other classes, as follows.
Exteroreceptive sensations are those from the surface of the body. Proprio-
ceptive sensations are those having to do with the physical state of the body,
including position sensations, tendon and muscle sensations, pressure sensations
from the bo

muscle sensations, pressure sensations
from the bottom of the feet, and even the sensation of equilibrium (which is
often considered a “special” sensation rather than a somatic sensation).
Visceral sensations are those from the viscera of the body; in using this term,
one usually refers speciﬁcally to sensations from the internal organs.
Deep sensations are those that come from deep tissues, such as from fasciae,
muscles, and bone. These include mainly “deep” pressure, pain, and vibration.
Detec

mainly “deep” pressure, pain, and vibration.
Detection and Transmission of 
Tactile Sensations
Interrelations Among the Tactile Sensations of Touch, Pressure, and Vibration. Although
touch, pressure, and vibration are frequently classiﬁed as separate sensations,
they are all detected by the same types of receptors. There are three 
principal differences among them: (1) touch sensation generally results from
stimulation of tactile receptors in the skin or in tissues immediately beneath the
skin; 

 skin or in tissues immediately beneath the
skin; (2) pressure sensation generally results from deformation of deeper
tissues; and (3) vibration sensation results from rapidly repetitive sensory
signals, but some of the same types of receptors as those for touch and pressure
are used.
Tactile Receptors. There are at least six entirely different types of tactile recep-
tors, but many more similar to these also exist. Some were shown in Figure 46–1
of the previous chapter; their special characteri

of the previous chapter; their special characteristics are the following.
586
Unit IX
First, some free nerve endings, which are found
everywhere in the skin and in many other tissues, can
detect touch and pressure. For instance, even light
contact with the cornea of the eye, which contains no
other type of nerve ending besides free nerve endings,
can nevertheless elicit touch and pressure sensations.
Second, a touch receptor with 

ressure sensations.
Second, a touch receptor with great sensitivity is the
Meissner’s corpuscle (illustrated in Figure 46–1), an
elongated encapsulated nerve ending of a large (type
Ab) myelinated sensory nerve ﬁber. Inside the capsu-
lation are many branching terminal nerve ﬁlaments.
These corpuscles are present in the nonhairy parts of
the skin and are particularly abundant in the ﬁnger-
tips, lips, and other areas of the skin where one’s ability
to discern spatial locations of touch sensation

ty
to discern spatial locations of touch sensations is
highly developed. Meissner’s corpuscles adapt in a
fraction of a second after they are stimulated, which
means that they are particularly sensitive to movement
of objects over the surface of the skin as well as to low-
frequency vibration.
Third, the ﬁngertips and other areas that contain
large numbers of Meissner’s corpuscles usually also
contain large numbers of expanded tip tactile receptors,
one type of which is Merkel’s discs, shown in 

rs,
one type of which is Merkel’s discs, shown in Figure
47–1. The hairy parts of the skin also contain moder-
ate numbers of expanded tip receptors, even though
they have almost no Meissner’s corpuscles. These
receptors differ from Meissner’s corpuscles in that
they transmit an initially strong but partially adapting
signal and then a continuing weaker signal that adapts
only slowly. Therefore, they are responsible for giving
steady-state signals that allow one to determine con-
tinuous touch o

s that allow one to determine con-
tinuous touch of objects against the skin.
Merkel’s discs are often grouped together in a
receptor organ called the Iggo dome receptor, which
projects upward against the underside of the epithe-
lium of the skin, as also shown in Figure 47–1. This
causes the epithelium at this point to protrude
outward, thus creating a dome and constituting an
extremely sensitive receptor. Also note that the entire
group of Merkel’s discs is innervated by a single large
myelina

el’s discs is innervated by a single large
myelinated nerve ﬁber (type Ab). These receptors,
along with the Meissner’s corpuscles discussed earlier,
play extremely important roles in localizing touch 
sensations to speciﬁc surface areas of the body and in
determining the texture of what is felt.
Fourth, slight movement of any hair on the body
stimulates a nerve ﬁber entwining its base. Thus, each
hair and its basal nerve ﬁber, called the hair end-organ,
are also a touch receptor. This receptor a

-organ,
are also a touch receptor. This receptor adapts readily
and, like Meissner’s corpuscles, detects mainly (a)
movement of objects on the surface of the body or (b)
initial contact with the body.
Fifth, located in the deeper layers of the skin and
also in still deeper internal tissues are many Rufﬁni’s
end-organs, which are multibranched, encapsulated
endings, as shown in Figure 46–1. These endings adapt
very slowly and, therefore, are important for signaling
continuous states of deformatio

tant for signaling
continuous states of deformation of the tissues, such as
heavy prolonged touch and pressure signals. They are
also found in joint capsules and help to signal the
degree of joint rotation.
Sixth, pacinian corpuscles, which were discussed in
detail in Chapter 46, lie both immediately beneath the
skin and deep in the fascial tissues of the body. They
are stimulated only by rapid local compression of the
tissues because they adapt in a few hundredths of a
second. Therefore, they a

in a few hundredths of a
second. Therefore, they are particularly important for
detecting tissue vibration or other rapid changes in the
mechanical state of the tissues.
Transmission of Tactile Signals in Peripheral Nerve Fibers.
Almost all specialized sensory receptors, such as
Meissner’s corpuscles, Iggo dome receptors, hair
10 mm
FF
CF
A
AA
C
E
Figure 47–1
Iggo dome receptor. Note the
multiple numbers of Merkel’s
discs connecting to a single large
myelinated ﬁber and abutting
tightly the unde

arge
tightly the undersurface of the
epithelium. (From Iggo A, Muir
AR: The structure and function of
a slowly adapting touch corpus-
cle in hairy skin. J Physiol 200:
763, 1969.)
Chapter 47
Somatic Sensations: I. General Organization, the Tactile and Position Senses
587
receptors, pacinian corpuscles, and Rufﬁni’s endings,
transmit their signals in type Ab nerve ﬁbers that have
transmission velocities ranging from 30 to 70 m/sec.
Conversely, free nerve ending tactil

 to 70 m/sec.
Conversely, free nerve ending tactile receptors trans-
mit signals mainly by way of the small type Ad myeli-
nated ﬁbers that conduct at velocities of only 5 to 
30 m/sec.
Some tactile free nerve endings transmit by way of
type C unmyelinated ﬁbers at velocities from a frac-
tion of a meter up to 2 m/sec; these send signals into
the spinal cord and lower brain stem, probably sub-
serving mainly the sensation of tickle.
Thus, the more critical types of sensory signals—
those that he

e critical types of sensory signals—
those that help to determine precise localization on
the skin, minute gradations of intensity, or rapid
changes in sensory signal intensity—are all transmit-
ted in more rapidly conducting types of sensory nerve
ﬁbers. Conversely, the cruder types of signals, such as
crude pressure, poorly localized touch, and especially
tickle, are transmitted by way of much slower, very
small nerve ﬁbers that require much less space in the
nerve bundle than the fast ﬁbers.

ss space in the
All tactile receptors are involved in detection of vibra-
tion, although different receptors detect different fre-
quencies of vibration. Pacinian corpuscles can detect
signal vibrations from 30 to 800 cycles per second
because they respond extremely rapidly to minute and
rapid deformations of the tissues, and they also trans-
mit their signals over type Ab nerve ﬁbers, which can
transmit as many as 1000 impulses per second

h can
transmit as many as 1000 impulses per second. Low-
frequency vibrations from 2 up to 80 cycles per second,
in contrast, stimulate other tactile receptors, especially
Meissner’s corpuscles, which are less rapidly adapting
than pacinian corpuscles.
Neurophysiologic studies have demonstrated the exis-
tence of very sensitive, rapidly adapting mechano-
receptive free nerve endings that elicit only the tickle
and itch sensations. Furthermore, these endings are
found almost exclu

 Furthermore, these endings are
found almost exclusively in superﬁcial layers of the
skin, which is also the only tissue from which the tickle
and itch sensations usually can be elicited. These sen-
sations are transmitted by very small type C, unmyeli-
nated ﬁbers similar to those that transmit the aching,
slow type of pain.
The purpose of the itch sensation is presumably to
call attention to mild surface stimuli such as a ﬂea
crawling on the skin or a ﬂy about to bite, and the
elicited signals

in or a ﬂy about to bite, and the
elicited signals then activate the scratch reﬂex or other
maneuvers that rid the host of the irritant. Itch can 
be relieved by scratching if this removes the irritant 
or if the scratch is strong enough to elicit pain. The
pain signals are believed to suppress the itch signals 
in the cord by lateral inhibition, as described in
Chapter 48.
Sensory Pathways for
Transmitting Somatic 
Signals into the Central
Almost all sensory information from the 

us System
Almost all sensory information from the somatic seg-
ments of the body enters the spinal cord through the
dorsal roots of the spinal nerves. However, from 
the entry point into the cord and then to the brain, the
sensory signals are carried through one of two alter-
native sensory pathways:(1) the dorsal column–medial
lemniscal system or (2) the anterolateral system. These
two systems come back together partially at the level
of the thalamus.
The dorsal column–medial lemniscal system, 

lamus.
The dorsal column–medial lemniscal system, as its
name implies, carries signals upward to the medulla of
the brain mainly in the dorsal columns of the cord.
Then, after the signals synapse and cross to the oppo-
site side in the medulla, they continue upward through
the brain stem to the thalamus by way of the medial
lemniscus.
Conversely, signals in the anterolateral system,
immediately after entering the spinal cord from the
dorsal spinal nerve roots, synapse in the dorsal 
horns of the

l nerve roots, synapse in the dorsal 
horns of the spinal gray matter, then cross to the oppo-
site side of the cord and ascend through the anterior
and lateral white columns of the cord. They terminate
at all levels of the lower brain stem and in the 
thalamus.
The dorsal column–medial lemniscal system is com-
posed of large, myelinated nerve ﬁbers that transmit
signals to the brain at velocities of 30 to 110 m/sec,
whereas the anterolateral system is composed of
smaller myelinated ﬁbers that t

tem is composed of
smaller myelinated ﬁbers that transmit signals at
velocities ranging from a few meters per second up to
40 m/sec.
Another difference between the two systems is 
that the dorsal column–medial lemniscal system has a
high degree of spatial orientation of the nerve ﬁbers
with respect to their origin, while the anterolateral
system has much less spatial orientation. These 
differences immediately characterize the types of
sensory information that can be transmitted by the 
two syst

formation that can be transmitted by the 
two systems. That is, sensory information that must 
be transmitted rapidly and with temporal and 
spatial ﬁdelity is transmitted mainly in the dorsal
column–medial lemniscal system; that which does not
need to be transmitted rapidly or with great spatial
ﬁdelity is transmitted mainly in the anterolateral
system.
The anterolateral system has a special capability
that the dorsal system does not have: the ability to
transmit a broad spectrum of sensory mod

bility to
transmit a broad spectrum of sensory modalities—
pain, warmth, cold, and crude tactile sensations;
most of these are discussed in detail in Chapter 48.
The dorsal system is limited to discrete types of
mechanoreceptive sensations.
With this differentiation in mind, we can now 
list the types of sensations transmitted in the two 
systems.
588
Unit IX
Dorsal Column–Medial 
1. Touch sensations requiring a hi

emniscal System
1. Touch sensations requiring a high degree of
localization of the stimulus
2. Touch sensations requiring transmission of ﬁne
gradations of intensity
3. Phasic sensations, such as vibratory sensations
4. Sensations that signal movement against the skin
5. Position sensations from the joints
6. Pressure sensations having to do with ﬁne degrees
of judgment of pressure intensity
1. Pain
2. Thermal sensations, including both warmth and
cold sensations
3. Crude to

luding both warmth and
3. Crude touch and pressure sensations capable only
of crude localizing ability on the surface of the
body
4. Tickle and itch sensations
5. Sexual sensations
Transmission in the 
Dorsal Column–Medial
Anatomy of the Dorsal
On entering the spinal cord through the spinal nerve
dorsal roots, the large myelinated ﬁbers from the spe-
cialized mechanoreceptors divide almost immediately
to form a medial branch and a l

almost immediately
to form a medial branch and a lateral branch, shown by
the right-hand ﬁber entering through the spinal root in
Figure 47–2. The medial branch turns medially ﬁrst and
then upward in the dorsal column, proceeding by way
of the dorsal column pathway all the way to the brain.
The lateral branch enters the dorsal horn of the cord
gray matter, then divides many times to provide termi-
nals that synapse with local neurons in the intermediate
and anterior portions of the cord gray mat

mediate
and anterior portions of the cord gray matter. These
local neurons in turn serve three functions: (1) A major
share of them give off ﬁbers that enter the dorsal
columns of the cord and then travel upward to the brain.
(2) Many of the ﬁbers are very short and terminate
locally in the spinal cord gray matter to elicit local spinal
cord reﬂexes, which are discussed in Chapter 54. (3)
Others give rise to the spinocerebellar tracts, which we
will discuss in Chapter 56 in relation to the funct

ill discuss in Chapter 56 in relation to the function of
the cerebellum.
The Dorsal Column–Medial Lemniscal Pathway. Note in Figure
47–3 that nerve ﬁbers entering the dorsal columns 
pass uninterrupted up to the dorsal medulla, where 
they synapse in the dorsal column nuclei (the cuneate
and gracile nuclei). From there, second-order neurons
VII
VI
V
IV
III
II
I
VIII
IX
Dorsal
column
Anterolateral
spinothalamic
pathway
Lamina marginalis
Substantia gelatinosa
Spinal nerve
Tract of
Lissauer
Spinoce

 gelatinosa
Tract of
Lissauer
Spinocervical
tract
Dorsal
spinocerebellar
tract
Ventral
tract
Figure 47–2
Cross section of the spinal cord, showing the anatomy of the cord
gray matter and of ascending sensory tracts in the white columns
of the spinal cord.
Medulla oblongata
Cortex
Dorsal root and spinal
ganglion
Medial lemniscus
Dorsal column nuclei
Ascending branches of
dorsal root fibers
Spinocervical tract
Ventrobasal
complex
of thalamus
Internal
capsul

ct
complex
Internal
capsule
Figure 47–3
The dorsal column–medial lemniscal pathway for transmitting crit-
ical types of tactile signals. (Modiﬁed from Ranson SW, Clark SL:
Anatomy of the Nervous System. Philadelphia: WB Saunders Co,
1959.)
Chapter 47
589
decussate immediately to the opposite side of the brain
stem and continue upward through the medial lemnisci
to the thalamus. In this pathway th

edial lemnisci
to the thalamus. In this pathway through the brain stem,
each medial lemniscus is joined by additional ﬁbers
from the sensory nuclei of the trigeminal nerve; these
ﬁbers subserve the same sensory functions for the head
that the dorsal column ﬁbers subserve for the body.
In the thalamus, the medial lemniscal ﬁbers terminate
in the thalamic sensory relay area,called the ventrobasal
complex. From the ventrobasal complex, third-order
nerve ﬁbers project, as shown in Figure 47–4, mainl

erve ﬁbers project, as shown in Figure 47–4, mainly to
the postcentral gyrus of the cerebral cortex, which is
called somatic sensory area I (as shown in Figure 47–6,
these ﬁbers also project to a smaller area in the lateral
parietal cortex called somatic sensory area II).
Spatial Orientation of the Nerve Fibers in the
Dorsal Column-Medial Lemniscal System
One of the distinguishing features of the dorsal
column–medial lemniscal system is a distinct spatial
orientation of nerve ﬁbers from the indi

t spatial
orientation of nerve ﬁbers from the individual parts of
the body that is maintained throughout. For instance,
in the dorsal columns of the spinal cord,the ﬁbers from
the lower parts of the body lie toward the center of the
cord, whereas those that enter the cord at progres-
sively higher segmental levels form successive layers
laterally.
In the thalamus, distinct spatial orientation is still
maintained, with the tail end of the body represented
by the most lateral portions of the ventr

resented
by the most lateral portions of the ventrobasal
complex and the head and face represented by the
medial areas of the complex. Because of the crossing
of the medial lemnisci in the medulla, the left side of
the body is represented in the right side of the thala-
mus, and the right side of the body in the left side of
the thalamus.
Before discussing the role of the cerebral cortex in
somatic sensation, we need to give an orientation to
the various areas of the cortex.

an orientation to
the various areas of the cortex. Figure 47–5 is a map
of the human cerebral cortex, showing that it is divided
into about 50 distinct areas called Brodmann’s areas
based on histological structural differences. This map
is important because virtually all neurophysiologists
and neurologists use it to refer by number to many of
the different functional areas of the human cortex.
Note in the ﬁgure the large central ﬁssure (also
called central sulcus) that extends horizontally acros

ed central sulcus) that extends horizontally across
the brain. In general, sensory signals from all modali-
ties of sensation terminate in the cerebral cortex
immediately posterior to the central ﬁssure. And,
generally, the anterior half of the parietal lobe is 
concerned almost entirely with reception and inter-
pretation of somatosensory signals. But the posterior
half of the parietal lobe provides still higher levels of
interpretation.
Visual signals terminate in the occipital lobe, and
audit

signals terminate in the occipital lobe, and
auditory signals in the temporal lobe.
Conversely, that portion of the cerebral cortex ante-
rior to the central ﬁssure and constituting the poste-
rior half of the frontal lobe is called the motor cortex
and is devoted almost entirely to control of muscle
contractions and body movements. A major share of
this motor control is in response to somatosensory
Lower extremity
Upper
extremity
Ventrobasal complex of thalamus
POSTCENTRAL GYRUS
MESENCEPHALON
S

plex of thalamus
Spinothalamic tract
Trunk
Face
Figure 47–4
Projection of the dorsal column–medial lemniscal system through
the thalamus to the somatosensory cortex. (Modiﬁed from Brodal
A: Neurological Anatomy in Relation to Clinical Medicine. New
York: Oxford University Press, 1969, by permission of Oxford 
Central fissure
Lateral fissure
4
5
3 2
7a
7a
39
19
18
17
37
21
20
38
45
47
11
10
46
9
8
6
1
40
41
42
22
43
44
Figure 47–

5
47
11
10
46
9
8
6
1
40
41
42
22
43
44
Figure 47–5
Structurally distinct areas, called Brodmann’s areas, of the human
cerebral cortex. Note speciﬁcally areas 1, 2, and 3, which consti-
tute primary somatosensory area I, and areas 5 and 7, which con-
stitute the somatosensory association area.
590
Unit IX
signals received from the sensory portions of the
cortex, which keep the motor cortex informed at each
instant about the positio

 cortex informed at each
instant about the positions and motions of the differ-
ent body parts.
Somatosensory Areas I and II. Figure 47–6 shows two sep-
arate sensory areas in the anterior parietal lobe called
somatosensory area I and somatosensory area II. The
reason for this division into two areas is that a distinct
and separate spatial orientation of the different 
parts of the body is found in each of these two 
areas. However, somatosensory area I is so much 
more extensive and so much mor

 area I is so much 
more extensive and so much more important than
somatosensory area II that in popular usage, the term
“somatosensory cortex” almost always means area I.
Somatosensory area I has a high degree of localiza-
tion of the different parts of the body, as shown by the
names of virtually all parts of the body in Figure 47–6.
By contrast, localization is poor in somatosensory area
II, although roughly, the face is represented anteriorly,
the arms centrally, and the legs posteriorly.
Li

,
Little is known about the function of somatosensory
area II. It is known that signals enter this area from
the brain stem, transmitted upward from both sides of
the body. In addition, many signals come secondarily
from somatosensory area I as well as from other
sensory areas of the brain, even from the visual and
auditory areas. Projections from somatosensory area I
are required for function of somatosensory area II.
However, removal of parts of so

tosensory area II.
However, removal of parts of somatosensory area II
has no apparent effect on the response of neurons in
somatosensory area I. Thus, much of what we know
about somatic sensation appears to be explained by
the functions of somatosensory area I.
Spatial Orientation of Signals from Different Parts of the Body
in Somatosensory Area I. Somatosensory area I lies
immediately behind the central ﬁssure, located in the
postcentral gyrus of the human cerebral cortex (in
Brodmann’s areas 3

f the human cerebral cortex (in
Brodmann’s areas 3, 1, and 2).
Figure 47–7 shows a cross section through the brain
at the level of the postcentral gyrus, demonstrating
representations of the different parts of the body in
separate regions of somatosensory area I. Note,
however, that each lateral side of the cortex receives
sensory information almost exclusively from the oppo-
site side of the body.
Some areas of the body are represented by large
areas in the somatic cortex—the lips the greatest 

areas in the somatic cortex—the lips the greatest of all,
followed by the face and thumb—whereas the trunk
and lower part of the body are represented by rela-
tively small areas. The sizes of these areas are directly
proportional to the number of specialized sensory
receptors in each respective peripheral area of the
body. For instance, a great number of specialized nerve
endings are found in the lips and thumb, whereas only
a few are present in the skin of the body trunk.
Note also that the hea

the skin of the body trunk.
Note also that the head is represented in the most
lateral portion of somatosensory area I, and the lower
part of the body is represented medially.
Layers of the Somatosensory Cortex and 
Their Function
The cerebral cortex contains six layers of neurons,
beginning with layer I next to the brain surface and
extending progressively deeper to layer VI, shown in
Figure 47–8. As would be expected, the neurons in
each layer perform functions different from those 
in other l

perform functions different from those 
in other layers. Some of these functions are:
area I
area II
Thigh
Thorax
Neck
Shoulder
Hand
Fingers
Tongue
Leg
Arm
Face
Abdomen
Figure 47–6
Two somatosensory cortical areas, somatosensory areas I and II.
Intra-abdominal
Pharynx
Tongue
Teeth, gums, and jaw
Lower lip
Lips
Upper lip
Face
Nose
Eye
Thumb
Index finger
Middle finger
Ring finger
Little finger
Hand
Wrist
Forearm
Elbow
Arm
Shoulder
Head
Neck
Trunk
Hip
Leg
Foot
Toes
Genit

m
Shoulder
Head
Neck
Trunk
Hip
Leg
Foot
Toes
Genitals
Figure 47–7
Representation of the different areas of the body in somatosen-
sory area I of the cortex. (From Penﬁeld W, Rasmussen T: Cere-
bral Cortex of Man: A Clinical Study of Localization of Function.
New York: Hafner, 1968.)
Chapter 47
591
1. The incoming sensory signal excites neuronal
layer IV ﬁrst; then the signal spreads toward the
surface of the cortex and 

gnal spreads toward the
surface of the cortex and also toward deeper
layers.
2. Layers I and II receive diffuse, nonspeciﬁc input
signals from lower brain centers that facilitate
speciﬁc regions of the cortex; this system is
described in Chapter 57. This input mainly
controls the overall level of excitability of the
respective regions stimulated.
3. The neurons in layers II and III send axons to
related portions of the cerebral cortex on the
opposite side of the brain through the corpus
callosum

site side of the brain through the corpus
callosum.
4. The neurons in layers V and VI send axons to the
deeper parts of the nervous system. Those in layer
V are generally larger and project to more distant
areas, such as to the basal ganglia, brain stem,
and spinal cord where they control signal
transmission. From layer VI, especially large
numbers of axons extend to the thalamus,
providing signals from the cerebral cortex that
interact with and help to control the excitatory
levels of incoming 

help to control the excitatory
levels of incoming sensory signals entering the
thalamus.
The Sensory Cortex Is Organized in Vertical
Columns of Neurons; Each Column Detects a
Different Sensory Spot on the Body with a
Speciﬁc Sensory Modality
Functionally, the neurons of the somatosensory cortex
are arranged in vertical columns extending all the way
through the six layers of the cortex, each column
having a diameter of 0.3 to 0.5 millimeter and con-
taining perhaps 10,000 neuronal cell bodies. Ea

n-
taining perhaps 10,000 neuronal cell bodies. Each of
these columns serves a single speciﬁc sensory modal-
ity, some columns responding to stretch receptors
around joints, some to stimulation of tactile hairs,
others to discrete localized pressure points on the skin,
and so forth. At layer IV, where the input sensory
signals ﬁrst enter the cortex, the columns of neurons
function almost entirely separately from one another.
At other levels of the columns, interactions occur that
initiate analys

e columns, interactions occur that
initiate analysis of the meanings of the sensory signals.
In the most anterior 5 to 10 millimeters of the post-
central gyrus, located deep in the central ﬁssure in
Brodmann’s area 3a, an especially large share of the
vertical columns respond to muscle, tendon, and joint
stretch receptors. Many of the signals from these
sensory columns then spread anteriorly, directly to 
the motor cortex located immediately forward of the
central ﬁssure. These signals play a m

ward of the
central ﬁssure. These signals play a major role in 
controlling the efﬂuent motor signals that activate
sequences of muscle contraction.
As one moves posteriorly in somatosensory area I,
more and more of the vertical columns respond to
slowly adapting cutaneous receptors, and then still
farther posteriorly,greater numbers of the columns are
sensitive to deep pressure.
In the most posterior portion of somatosensory area
I,about 6 per cent of the vertical columns respond only
when a st

ent of the vertical columns respond only
when a stimulus moves across the skin in a particular
direction. Thus, this is a still higher order of interpre-
tation of sensory signals; the process becomes even
more complex as the signals spread farther backward
from somatosensory area I into the parietal cortex, an
area called the somatosensory association area, as we
discuss subsequently.
Functions of Somatosensory Area I
Widespread bilateral excision of somatosensory area I
causes loss of the foll

on of somatosensory area I
causes loss of the following types of sensory judgment:
1. The person is unable to localize discretely the
different sensations in the different parts of the
body. However, he or she can localize these
sensations crudely, such as to a particular hand, to
a major level of the body trunk, or to one of the
legs. Thus, it is clear that the brain stem, thalamus,
or parts of the cerebral cortex not normally
considered to be concerned with somatic
sensations can perform some 

oncerned with somatic
sensations can perform some degree of
localization.
2. The person is unable to judge critical degrees of
pressure against the body.
3. The person is unable to judge the weights of
objects.
4. The person is unable to judge shapes or forms of
objects. This is called astereognosis.
I
VIb
VIa
V
IV
III
II
Figure 47–8
Structure of the cerebral cortex, showing I, molecular layer; II,
external granular layer; III, layer of small pyramidal cells; IV, inter-
nal granular layer; V, la

amidal cells; IV, inter-
nal granular layer; V, large pyramidal cell layer; and VI, layer of
fusiform or polymorphic cells. (From Ranson SW, Clark SL [after
Brodmann]: Anatomy of the Nervous System. Philadelphia: WB
Saunders, 1959.)
592
Unit IX
5. The person is unable to judge texture of materials
because this type of judgment depends on highly
critical sensations caused by movement of the
ﬁngers over the surface to be judged.
Note

 of the
Note that in the list nothing has been said about loss
of pain and temperature sense. In speciﬁc absence 
of only somatosensory area I, appreciation of these
sensory modalities is still preserved both in quality and
intensity. But the sensations are poorly localized, indi-
cating that pain and temperature localization depend
greatly on the topographical map of the body in
somatosensory area I to localize the source.
Br

ize the source.
Brodmann’s areas 5 and 7 of the cerebral cortex,
located in the parietal cortex behind somatosensory
area I (see Figure 47–5), play important roles in deci-
phering deeper meanings of the sensory information
in the somatosensory areas. Therefore, these areas are
called somatosensory association areas.
Electrical stimulation in a somatosensory associa-
tion area can occasionally cause an awake person to
experience a complex body sensation, sometimes

 to
experience a complex body sensation, sometimes even
the “feeling” of an object such as a knife or a ball.
Therefore, it seems clear that the somatosensory asso-
ciation area combines information arriving from mul-
tiple points in the primary somatosensory area to
decipher its meaning. This also ﬁts with the anatomi-
cal arrangement of the neuronal tracts that enter the
somatosensory association area because it receives
signals from (1) somatosensory area I, (2) the ven-
trobasal nuclei of th

sensory area I, (2) the ven-
trobasal nuclei of the thalamus, (3) other areas of the
thalamus, (4) the visual cortex, and (5) the auditory
cortex.
Effect of Removing the Somatosensory Association Area—
Amorphosynthesis. When the somatosensory association
area is removed on one side of the brain, the person
loses ability to recognize complex objects and complex
forms felt on the opposite side of the body. In addi-
tion, he or she loses most of the sense of form of his
or her own body or body part

 sense of form of his
or her own body or body parts on the opposite side.
In fact, the person is mainly oblivious to the opposite
side of the body—that is, forgets that it is there.
Therefore, he or she also often forgets to use the other
side for motor functions as well. Likewise, when
feeling objects, the person tends to recognize only 
one side of the object and forgets that the other side
even exists. This complex sensory deﬁcit is called
amorphosynthesis.
Overall Characteristics of Signal
T

rphosynthesis.
Transmission and Analysis 
in the Dorsal Column–Medial
Basic Neuronal Circuit in the Dorsal Column–Medial Lemniscal
System. The lower part of Figure 47–9 shows the basic
organization of the neuronal circuit of the spinal cord
dorsal column pathway, demonstrating that at each
synaptic stage, divergence occurs. The upper curves of
the ﬁgure show that the cortical neurons that discharge
to the greatest extent are those in a central p

ge
to the greatest extent are those in a central part of the
cortical “ﬁeld” for each respective receptor. Thus, a
weak stimulus causes only the centralmost neurons to
ﬁre. A stronger stimulus causes still more neurons to
ﬁre, but those in the center discharge at a considerably
more rapid rate than do those farther away from the
center.
Two-Point Discrimination. A method frequently used to
test tactile discrimination is to determine a person’s
so-called “two-point” discriminatory ability. In thi

-called “two-point” discriminatory ability. In this
test, two needles are pressed lightly against the skin at
the same time, and the person determines whether two
points of stimulus are felt or one point. On the tips of
the ﬁngers, a person can distinguish two separate
points even when the needles are as close together as
1 to 2 millimeters. However, on the person’s back,
the needles must usually be as far apart as 30 to 70 
Discharges per second
Cortex
Thalamus
Single-point

Cortex
Thalamus
Single-point stimulus on skin
Strong stimulus
Moderate
stimulus
Weak
stimulus
Figure 47–9
Transmission of a pinpoint stimulus signal to the cerebral cortex.
Chapter 47
593
millimeters before two separate points can be
detected.The reason for this difference is the different
numbers of specialized tactile receptors in the two
areas.
Figure 47–10 shows the mechanism by which the
dorsa

igure 47–10 shows the mechanism by which the
dorsal column pathway (as well as all other sensory
pathways) transmits two-point discriminatory infor-
mation. This ﬁgure shows two adjacent points on the
skin that are strongly stimulated as well as the areas
of the somatosensory cortex (greatly enlarged) that
are excited by signals from the two stimulated points.
The blue curve shows the spatial pattern of cortical
excitation when both skin points are stimulated simul-
taneously. Note that the resu

re stimulated simul-
taneously. Note that the resultant zone of excitation
has two separate peaks.These two peaks, separated by
a valley, allow the sensory cortex to detect the pres-
ence of two stimulatory points, rather than a single
point. The capability of the sensorium to distinguish
this presence of two points of stimulation is strongly
inﬂuenced by another mechanism, lateral inhibition, as
explained in the next section.
Effect of Lateral Inhibition (Also Called Surround Inhibition) to
Inc

nhibition (Also Called Surround Inhibition) to
Increase the Degree of Contrast in the Perceived Spatial
Pattern. As pointed out in Chapter 46, virtually every
sensory pathway, when excited, gives rise simultane-
ously to lateral inhibitory signals; these spread to the
sides of the excitatory signal and inhibit adjacent
neurons. For instance, consider an excited neuron 
in a dorsal column nucleus. Aside from the central
excitatory signal, short lateral pathways transmit
inhibitory signals to the 

teral pathways transmit
inhibitory signals to the surrounding neurons. That is,
these signals pass through additional interneurons that
secrete an inhibitory transmitter.
The importance of lateral inhibition is that it blocks
lateral spread of the excitatory signals and, therefore,
increases the degree of contrast in the sensory pattern
perceived in the cerebral cortex.
In the case of the dorsal column system, lateral
inhibitory signals occur at each synaptic level—for
instance, in (1) the dorsa

each synaptic level—for
instance, in (1) the dorsal column nuclei of the
medulla, (2) the ventrobasal nuclei of the thalamus,
and (3) the cortex itself. At each of these levels, the
lateral inhibition helps to block lateral spread of the
excitatory signal. As a result, the peaks of excitation
stand out, and much of the surrounding diffuse stimu-
lation is blocked. This effect is demonstrated by the
two red curves in Figure 47–10, showing complete 
separation of the peaks when the intensity of la

separation of the peaks when the intensity of lateral
inhibition is great.
Transmission of Rapidly Changing and Repetitive Sensations.
The dorsal column system also is of particular impor-
tance in apprising the sensorium of rapidly changing
peripheral conditions. Based on recorded action
potentials, this system can recognize changing stimuli
that occur in as little as 1/400 of a second.
Vibratory Sensation. Vibratory signals are rapidly
repetitive and can be detected as vibration up to 700
cy

tive and can be detected as vibration up to 700
cycles per second. The higher-frequency vibratory
signals originate from the pacinian corpuscles in the
skin and deeper tissues, but lower-frequency signals
(below about 200 per second) can originate from
Meissner’s corpuscles as well. These signals are trans-
mitted only in the dorsal column pathway. For this
reason, application of vibration (e.g., from a “tuning
fork”) to different peripheral parts of the body is an
important tool used by neurolo

s of the body is an
important tool used by neurologists for testing func-
tional integrity of the dorsal columns.
Interpretation of Sensory 
Stimulus Intensity
The ultimate goal of most sensory stimulation is to
apprise the psyche of the state of the body and its sur-
roundings. Therefore, it is important that we discuss
brieﬂy some of the principles related to transmission of
sensory stimulus intensity to the higher levels of the
The ﬁrst question that comes to mind is, how is i

The ﬁrst question that comes to mind is, how is it pos-
sible for the sensory system to transmit sensory experi-
ences of tremendously varying intensities? For instance,
the auditory system can detect the weakest possible
whisper but can also discern the meanings of an explo-
sive sound, even though the sound intensities of these
two experiences can vary more than 10 billion times; the
eyes can see visual images with light intensities that vary
as much as a half million times; and the skin can 

as much as a half million times; and the skin can detect
pressure differences of 10,000 to 100,000 times.
As a partial explanation of these effects, Figure 46–4
in the previous chapter shows the relation of the recep-
tor potential produced by the pacinian corpuscle to the
Cortex
Two adjacent points
strongly stimulated
Figure 47–10
Transmission of signals to the cortex from two adjacent pinpoint
stimuli. The blue curve represents the pattern of cortical stimula-
tion withou

sents the pattern of cortical stimula-
tion without “surround” inhibition, and the two red curves repre-
sent the pattern when “surround” inhibition does occur.
594
Unit IX
intensity of the sensory stimulus. At low stimulus inten-
sity, slight changes in intensity increase the potential
markedly, whereas at high levels of stimulus intensity,
further increases in receptor potential are slight. Thus,
the pacinian corpuscle is capable

re slight. Thus,
the pacinian corpuscle is capable of accurately measur-
ing extremely minute changes in stimulus at low-
intensity levels, but at high-intensity levels, the change
in stimulus must be much greater to cause the same
amount of change in receptor potential.
The transduction mechanism for detecting sound by
the cochlea of the ear demonstrates still another
method for separating gradations of stimulus intensity.
When sound stimulates a speciﬁc point on the basilar
membrane, weak soun

a speciﬁc point on the basilar
membrane, weak sound stimulates only those hair cells
at the point of maximum sound vibration. But as the
sound intensity increases, many more hair cells in each
direction farther away from the maximum vibratory
point also become stimulated. Thus, signals are trans-
mitted over progressively increasing numbers of nerve
ﬁbers, which is another mechanism by which stimulus
intensity is transmitted to the central nervous system.
This mechanism, plus the direct effect o

s system.
This mechanism, plus the direct effect of stimulus inten-
sity on impulse rate in each nerve ﬁber,as well as several
other mechanisms, makes it possible for some sensory
systems to operate reasonably faithfully at stimulus
intensity levels changing as much as millions of times.
Importance of the Tremendous Intensity Range of Sensory 
Reception. Were it not for the tremendous intensity
range of sensory reception that we can experience, the
various sensory systems would more often than n

he
various sensory systems would more often than not be
operating in the wrong range. This is demonstrated by
the attempts of most people, when taking photographs
with a camera, to adjust the light exposure without using
a light meter. Left to intuitive judgment of light inten-
sity, a person almost always overexposes the ﬁlm on
bright days and greatly underexposes the ﬁlm at twi-
light. Yet, that person’s own eyes are capable of dis-
criminating with great detail visual objects in bright
sunlig

 with great detail visual objects in bright
sunlight or at twilight;the camera cannot do this without
very special manipulation because of the narrow criti-
cal range of light intensity required for proper exposure
of ﬁlm.
Weber-Fechner Principle—Detection of “Ratio” of Stimulus
Strength. In the mid-1800s,Weber ﬁrst and Fechner later
proposed the principle that gradations of stimulus
strength are discriminated approximately in proportion
to the logarithm of stimulu

ximately in proportion
to the logarithm of stimulus strength. That is, a person
already holding 30 grams weight in his or her hand can
barely detect an additional 1-gram increase in weight.
And, when already holding 300 grams, he or she can
barely detect a 10-gram increase in weight. Thus, in this
instance, the ratio of the change in stimulus strength
required for detection remains essentially constant,
about 1 to 30, which is what the logarithmic principle
means. To express this mathematically.

c principle
Interpreted signal strength = Log (Stimulus)
+ Constant
More recently, it has become evident that the Weber-
Fechner principle is quantitatively accurate only for
higher intensities of visual, auditory, and cutaneous
sensory experience and applies only poorly to most
other types of sensory experience. Yet the Weber-
Fechner principle is still a good one to remember,
because it emphasizes that the greater the background
sensory intensity, the gre

 greater the background
sensory intensity, the greater an additional change must
be for the psyche to detect the change.
Power Law. Another attempt by physiopsychologists to
ﬁnd a good mathematical relation is the following
formula, known as the power law.
Interpreted signal strength = K • (Stimulus - k)y
In this formula, the exponent y and the constants K and
k are different for each type of sensation.
When this power law relation is plotted on a graph
using double logarithmic coordinates, as s

a graph
using double logarithmic coordinates, as shown in
Figure 47–11, and when appropriate quantitative values
for the constants y, K, and k are found, a linear relation
can be attained between interpreted stimulus strength
and actual stimulus strength over a large range for
almost any type of sensory perception.
The position senses are frequently also called proprio-
ceptive senses. They can be divided into two subtypes:
(1) static position sense, which means conscious per-
ce

atic position sense, which means conscious per-
ception of the orientation of the different parts of 
the body with respect to one another, and (2) rate of
movement sense, also called kinesthesia or dynamic
proprioception.
Position Sensory Receptors. Knowledge of position, both
static and dynamic, depends on knowing the degrees
of angulation of all joints in all planes and their rates
of change.Therefore, multiple different types of recep-
tors help to determine joint angulation and are used
0
1

elp to determine joint angulation and are used
0
10
100
1000
10,000
Stimulus strength (arbitrary units)
Interpreted stimulus strength
(arbitrary units)
0
10
20
50
100
200
500
Figure 47–11
Graphical demonstration of the “power law” relation between
actual stimulus strength and strength that the psyche interprets it
to be. Note that the power law does not hold at either very weak
or very strong stimulus strengths.
Chapter 47
Somatic Sensations: I. General Organization, the Tactile and Position Sen

General Organization, the Tactile and Position Senses
595
together for position sense. Both skin tactile receptors
and deep receptors near the joints are used. In the 
case of the ﬁngers, where skin receptors are in great
abundance, as much as half of position recognition is
believed to be detected through the skin receptors.
Conversely, for most of the larger joints of the body,
deep receptors are more important.
For determining joint angulation in mid ranges of
motion, among the most important

 in mid ranges of
motion, among the most important receptors are the
muscle spindles. They are also exceedingly important
in helping to control muscle movement, as we shall see
in Chapter 54. When the angle of a joint is changing,
some muscles are being stretched while others are
loosened, and the net stretch information from the
spindles is transmitted into the computational system
of the spinal cord and higher regions of the dorsal
column system for deciphering joint angulations.
At the extrem

m for deciphering joint angulations.
At the extremes of joint angulation, stretch of the
ligaments and deep tissues around the joints is an addi-
tional important factor in determining position. Types
of sensory endings used for this are the pacinian cor-
puscles, Rufﬁni’s endings, and receptors similar to the
Golgi tendon receptors found in muscle tendons.
The pacinian corpuscles and muscle spindles are
especially adapted for detecting rapid rates of change.
It is likely that these are the rece

es of change.
It is likely that these are the receptors most responsi-
ble for detecting rate of movement.
Processing of Position Sense Information in the Dorsal
Column–Medial Lemniscal Pathway. Referring to Figure
47–12, one sees that thalamic neurons responding 
to joint rotation are of two categories: (1) those 
maximally stimulated when the joint is at full rotation
and (2) those maximally stimulated when the joint is
at minimal rotation. Thus, the signals from the indi-
vidual joint recepto

s, the signals from the indi-
vidual joint receptors are used to tell the psyche how
much each joint is rotated.
Transmission of Less Critical
Sensory Signals in the
Anterolateral Pathway
The anterolateral pathway for transmitting sensory
signals up the spinal cord and into the brain, in con-
trast to the dorsal column pathway, transmits sensory
signals that do not require highly discrete localization
of the signal source and do not require discrimination
of ﬁne gradations of intensity. These ty

imination
of ﬁne gradations of intensity. These types of signals
include pain, heat, cold, crude tactile, tickle, itch, and
sexual sensations. In Chapter 48, pain and temperature
sensations will be discussed speciﬁcally.
Anatomy of the 
The spinal cord anterolateral ﬁbers originate mainly in
dorsal horn laminae I, IV, V, and VI (see Figure 47–2).
These laminae are where many of the dorsal root
sensory nerve ﬁbers terminate after entering the cord.
As shown in Figure 47–13, 

fter entering the cord.
As shown in Figure 47–13, the anterolateral ﬁbers
cross immediately in the anterior commissure of the
cord to the opposite anterior and lateral white columns,
where they turn upward toward the brain by way of the
anterior spinothalamic and lateral spinothalamic tracts.
The upper terminus of the two spinothalamic tracts is
mainly twofold: (1) throughout the reticular nuclei of the
brain stem and (2) in two different nuclear complexes
of the thalamus, the ventrobasal comple

 complexes
of the thalamus, the ventrobasal complex and the
intralaminar nuclei. In general, the tactile signals are
transmitted mainly into the ventrobasal complex, ter-
minating in some of the same thalamic nuclei where the
dorsal column tactile signals terminate. From here, the
signals are transmitted to the somatosensory cortex
along with the signals from the dorsal columns.
Conversely, only a small fraction of the pain signals
project directly to the ventrobasal complex of the thal-
amus. I

ly to the ventrobasal complex of the thal-
amus. Instead, most pain signals terminate in the retic-
ular nuclei of the brain stem and from there are relayed
to the intralaminar nuclei of the thalamus where the
pain signals are further processed, as discussed in
greater detail in Chapter 48.
Characteristics of Transmission in the Anterolateral Pathway.
In general, the same principles apply to transmission
in the anterolateral pathway as in the dorsal
column–medial lemniscal system, except for the

sal
column–medial lemniscal system, except for the fol-
lowing differences: (1) the velocities of transmission
are only one third to one half those in the dorsal
column–medial lemniscal system, ranging between 8
and 40 m/sec; (2) the degree of spatial localization of
signals is poor; (3) the gradations of intensities are also
far less accurate, most of the sensations being recog-
nized in 10 to 20 gradations of strength, rather than as
many as 100 gradations for the dorsal column system;
0
60
80

0 gradations for the dorsal column system;
0
60
80
100
120
140
160
180
Degrees
0
20
40
60
80
100
#1
#2
#3
#4
#5
Figure 47–12
Typical responses of ﬁve different thalamic neurons in the thala-
mic ventrobasal complex when the knee joint is moved through
its range of motion. (Data from Mountcastle VB, Poggie GF, Werner
G: The relation of thalamic cell response to peripheral stimuli
varied over an intensive continuum. J Neurophysiol 26:807, 1963.)
596
Unit IX
The Nervous System: 

ol 26:807, 1963.)
596
Unit IX
and (4) the ability to transmit rapidly changing or
rapidly repetitive signals is poor.
Thus, it is evident that the anterolateral system is a
cruder type of transmission system than the dorsal
column–medial lemniscal system. Even so, certain
modalities of sensation are transmitted only in this
system and not at all in the dorsal column–medial lem-
niscal system. They are pain, temperature, tickle, itc

al system. They are pain, temperature, tickle, itch,
and sexual sensations, in addition to crude touch and
pressure.
Some Special Aspects of
Function of the Thalamus 
in Somatic Sensation
When the somatosensory cortex of a human being is
destroyed, that person loses most critical tactile sensi-
bilities, but a slight degree of crude tactile sensibility
does return.Therefore, it must be assumed that the thal-
amus (as well as other lower centers) has a slight ability
to dis

s other lower centers) has a slight ability
to discriminate tactile sensation, even though the thala-
mus normally functions mainly to relay this type of
information to the cortex.
Conversely, loss of the somatosensory cortex has little
effect on one’s perception of pain sensation and only 
a moderate effect on the perception of temperature.
Therefore, there is much reason to believe that the
lower brain stem, the thalamus, and other associated
basal regions of the brain play dominant roles in d

asal regions of the brain play dominant roles in dis-
crimination of these sensibilities. It is interesting that
these sensibilities appeared very early in the phyloge-
netic development of animals, whereas the critical
tactile sensibilities and the somatosensory cortex were
late developments.
Internal
capsule
Cortex
ganglion
and intralaminar
nuclei of the
thalamus
Lateral
division of the
anterolatera

 the
thalamus
Lateral
anterolateral
pathway
Anterior
pathway
Figure 47–13
Anterior and lateral divisions of the anterolateral sensory pathway.
C2
T2
T2
T3
T5
T7
T9
T10
T11
T12
L1
S3
S2
S4&5
S2
L5
L3
L1
L2
L3
L4
L5
S1
C2
C3
C4
C5
T4
T5
T4
T2
T5
T6
T7
T8
T9
T10
T12
T11
L2
L3
L4
L5
L5
L4
S1
C6
C7 C8
T4
T6
T8
C3
C4
C5
Figure 47–14
Dermatomes. (Modiﬁed from Grinker RR, Sahs AL: Neurology, 6th
ed. Springﬁeld, IL: Charles C Thomas, 1966. Courtesy of Charles

d, IL: Charles C Thomas, 1966. Courtesy of Charles
C Thomas, Publisher, Ltd., Springﬁeld, Illinois.)
Chapter 47
597
Cortical Control of Sensory
Sensitivity—“Corticofugal” Signals
In addition to somatosensory signals transmitted from
the periphery to the brain, corticofugal signals are trans-
mitted in the backward direction from the cerebral
cortex to the lower sensory relay stations of the thala-
mus, medulla, and spin

elay stations of the thala-
mus, medulla, and spinal cord; they control the intensity
of sensitivity of the sensory input.
Corticofugal signals are almost entirely inhibitory, so
that when sensory input intensity becomes too great,the
corticofugal signals automatically decrease transmission
in the relay nuclei. This does two things: First, it
decreases lateral spread of the sensory signals into adja-
cent neurons and, therefore, increases the degree of
sharpness in the signal pattern. Second, it

ree of
sharpness in the signal pattern. Second, it keeps the
sensory system operating in a range of sensitivity that is
not so low that the signals are ineffectual nor so high
that the system is swamped beyond its capacity to dif-
ferentiate sensory patterns. This principle of corticofu-
gal sensory control is used by all sensory systems, not
only the somatic system, as explained in subsequent
chapters.
Segmental Fields of Sensation—
The Dermatomes
Each spinal nerve innervates a “segmental ﬁeld”

es
Each spinal nerve innervates a “segmental ﬁeld” of the
skin called a dermatome. The different dermatomes are
shown in Figure 47–14. They are shown in the ﬁgure as
if there were distinct borders between the adjacent der-
matomes, which is far from true because much overlap
exists from segment to segment.
The ﬁgure shows that the anal region of the body lies
in the dermatome of the most distal cord segment, der-
matome S5. In the embryo, this is the tail region and the
most distal portion of th

 the tail region and the
most distal portion of the body. The legs originate
embryologically from the lumbar and upper sacral seg-
ments (L2 to S3), rather than from the distal sacral seg-
ments, which is evident from the dermatomal map. One
can use a dermatomal map as shown in Figure 47–14 to
determine the level in the spinal cord at which a cord
injury has occurred when the peripheral sensations are
disturbed by the injury.
Bosco G, Poppele RE: Proprioception from a spinocerebel-
la

Poppele RE: Proprioception from a spinocerebel-
lar perspective. Physiol Rev 81:539, 2001.
Chen R, Cohen LG, Hallett M: Nervous system reorganiza-
tion following injury. Neuroscience 111(4):761, 2002.
Cohen YE, Andersen RA: A common reference frame for
movement plans in the posterior parietal cortex. Nat Rev
Neurosci 3:553, 2002.
Craig AD: Pain mechanisms: labeled lines versus conver-
gence in central processing.Annu Rev Neurosci 26:1,2003.
Foeller E, Feldman DE: Synaptic basis for developmental

er E, Feldman DE: Synaptic basis for developmental
plasticity in somatosensory cortex. Curr Opin Neurobiol
14:89, 2004.
Haines DE:
New York:
Ivry RB, Spencer RM: The neural representation of time.
Curr Opin Neurobiol 14:225, 2004.
Janig W, Baron R: Complex regional pain syndrome: mystery
explained? Lancet Neurol 2:687, 2003.
Jeannerod M:The mechanism of self

rol 2:687, 2003.
Jeannerod M:The mechanism of self-recognition in humans.
Behav Brain Res 142:1, 2003.
Johnson KO: The roles and functions of cutaneous
mechanoreceptors. Curr Opin Neurobiol 11:455, 2001.
Lumb BM: Hypothalamic and midbrain circuitry that dis-
tinguishes between escapable and inescapable pain. News
Physiol Sci 19:22, 2004.
Maravita A, Spence C, Driver J: Multisensory integration

a A, Spence C, Driver J: Multisensory integration
and the body schema: close to hand and within reach. Curr
Biol 13:R531, 2003.
Pears S, Jackson SR: Cognitive neuroscience: vision and
touch are constant companions. Curr Biol 14:R349, 2004.
Petersen CC: The barrel cortex—integrating molecular, cel-
lular and systems physiology. Pﬂugers Arch 447:126, 2003.
Pouget A, Deneve S, Duhamel JR: A computational per-
spective on the neural basis of multisensory spatial repre-
sentations. Nat Rev Neurosci 3

sory spatial repre-
sentations. Nat Rev Neurosci 3:741, 2002.
Sommer MA: The role of the thalamus in motor control.
Curr Opin Neurobiol 13:663, 2003.
Suga N, Ma X: Multiparametric corticofugal modulation and
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4
8
598
Somatic Sensations:
II. Pain, Headache, and 
Many, if not most, ailments of the b

al Sensations
Many, if not most, ailments of the body cause pain.
Furthermore, the ability to diagnose different dis-
eases depends to a great extent on a physician’s
knowledge of the different qualities of pain. For
these reasons, the ﬁrst part of this chapter is devoted
mainly to pain and to the physiologic bases of some
associated clinical phenomena.
Pain Is a Protective Mechanism. Pain occurs whenever any tissues are being
damaged, and it causes the individual to react to remove the pain sti

ses the individual to react to remove the pain stimulus. Even
such simple activities as sitting for a long time on the ischia can cause tissue
destruction because of lack of blood ﬂow to the skin where it is compressed by
the weight of the body. When the skin becomes painful as a result of the
ischemia, the person normally shifts weight subconsciously. But a person who
has lost the pain sense, as after spinal cord injury, fails to feel the pain and, there-
fore, fails to shift.This soon results 

nd, there-
fore, fails to shift.This soon results in total breakdown and desquamation of the
skin at the areas of pressure.
Types of Pain and Their Qualities—Fast Pain
and Slow Pain
Pain has been classiﬁed into two major types: fast pain and slow pain. Fast pain
is felt within about 0.1 second after a pain stimulus is applied, whereas slow pain
begins only after 1 second or more and then increases slowly over many seconds
and sometimes even minutes. During the course of this chapter, we shall se

es. During the course of this chapter, we shall see
that the conduction pathways for these two types of pain are different and that
each of them has speciﬁc qualities.
Fast pain is also described by many alternative names, such as sharp pain,
pricking pain, acute pain, and electric pain. This type of pain is felt when a needle
is stuck into the skin,when the skin is cut with a knife,or when the skin is acutely
burned. It is also felt when the skin is subjected to electric shock. Fast-sharp
pain 

n is subjected to electric shock. Fast-sharp
pain is not felt in most deeper tissues of the body.
Slow pain also goes by many names, such as slow burning pain, aching pain,
throbbing pain, nauseous pain, and chronic pain. This type of pain is usually
associated with tissue destruction. It can lead to prolonged, unbearable suffer-
ing. It can occur both in the skin and in almost any deep tissue or organ.
Pain Receptors Are Free Nerve Endings. The pain receptor

eceptors Are Free Nerve Endings. The pain receptors in the skin and other
tissues are all free nerve endings. They are widespread in the superﬁcial layers
of the skin as well as in certain internal tissues, such as the periosteum, the arte-
rial walls, the joint surfaces, and the falx and tentorium in the cranial vault. Most
other deep tissues are only sparsely supplied with pain endings; nevertheless,
any widespread tissue damage can summate to cause the slow-chronic-aching
type of pain in most

cause the slow-chronic-aching
type of pain in most of these areas.
Chapter 48
Somatic Sensations: II. Pain, Headache, and Thermal Sensations
599
Three Types of Stimuli Excite Pain Receptors—Mechanical,
Thermal, and Chemical. Pain can be elicited by multiple
types of stimuli. They are classiﬁed as mechanical,
thermal, and chemical pain stimuli. In general, fast pain
is elicited by the mechanical and thermal types of
stimuli, whereas slow pain can be elicited by all three
types.
Some of the chemic

be elicited by all three
types.
Some of the chemicals that excite the chemical type
of pain are bradykinin, serotonin, histamine, potassium
ions, acids, acetylcholine, and proteolytic enzymes. In
addition, prostaglandins and substance P enhance the
sensitivity of pain endings but do not directly excite
them. The chemical substances are especially impor-
tant in stimulating the slow, suffering type of pain that
occurs after tissue injury.
Nonadapting Nature of Pain Receptors. In contrast to most

ing Nature of Pain Receptors. In contrast to most
other sensory receptors of the body, pain receptors
adapt very little and sometimes not at all. In fact,
under some conditions, excitation of pain ﬁbers
becomes progressively greater, especially so for 
slow-aching-nauseous pain, as the pain stimulus 
continues.This increase in sensitivity of the pain recep-
tors is called hyperalgesia. One can readily understand
the importance of this failure of pain receptors to
adapt, because it allows the pai

pain receptors to
adapt, because it allows the pain to keep the person
apprised of a tissue-damaging stimulus as long as it
persists.
Rate of Tissue Damage as a Stimulus
for Pain
The average person begins to perceive pain when the
skin is heated above 45°C, as shown in Figure 48–1.
This is also the temperature at which the tissues begin
to be damaged by heat; indeed, the tissues are even-
tually destroyed if the temperature remains above this
level indeﬁnitely.Therefore, it is immediately appare

el indeﬁnitely.Therefore, it is immediately apparent
that pain resulting from heat is closely correlated 
with the rate at which damage to the tissues is occur-
ring and not with the total damage that has already
occurred.
The intensity of pain is also closely correlated with
the rate of tissue damage from causes other than heat,
such as bacterial infection, tissue ischemia, tissue con-
tusion, and so forth.
Special Importance of Chemical Pain Stimuli During Tissue
Damage. Extracts from damaged 

imuli During Tissue
Damage. Extracts from damaged tissue cause intense
pain when injected beneath the normal skin. Most of
the chemicals listed earlier that excite the chemical
pain receptors can be found in these extracts. One
chemical that seems to be more painful than others is
bradykinin. Many researchers have suggested that
bradykinin might be the agent most responsible for
causing pain following tissue damage. Also, the inten-
sity of the pain felt correlates with the local increase
in pot

ain felt correlates with the local increase
in potassium ion concentration or the increase in pro-
teolytic enzymes that directly attack the nerve endings
and excite pain by making the nerve membranes more
permeable to ions.
Tissue Ischemia as a Cause of Pain. When blood ﬂow to a
tissue is blocked, the tissue often becomes very painful
within a few minutes. The greater the rate of metabo-
lism of the tissue, the more rapidly the pain appears.
For instance, if a blood pressure cuff is placed arou

 instance, if a blood pressure cuff is placed around
the upper arm and inﬂated until the arterial blood ﬂow
ceases, exercise of the forearm muscles sometimes 
can cause muscle pain within 15 to 20 seconds. In the
absence of muscle exercise, the pain may not appear
for 3 to 4 minutes even though the muscle blood ﬂow
remains zero.
One of the suggested causes of pain during ischemia
is accumulation of large amounts of lactic acid in the
tissues, formed as a consequence of anaerobic metab-
olism (me

med as a consequence of anaerobic metab-
olism (metabolism without oxygen). It is also proba-
ble that other chemical agents, such as bradykinin and
proteolytic enzymes, are formed in the tissues because
of cell damage and that these, in addition to lactic acid,
stimulate the pain nerve endings.
Muscle Spasm as a Cause of Pain. Muscle spasm is also 
a common cause of pain, and it is the basis of many
clinical pain syndromes. This pain probably results 
partially from the direct effect of muscle 

sults 
partially from the direct effect of muscle spasm in
stimulating mechanosensitive pain receptors, but it
might also result from the indirect effect of muscle
spasm to compress the blood vessels and cause
ischemia. Also, the spasm increases the rate of metab-
olism in the muscle tissue, thus making the relative
ischemia even greater, creating ideal conditions for the
release of chemical pain-inducing substances.
43
44
45
46
47
Temperature (∞C)
Number of subjects
Figure 48–1
Distribution cur

C)
Distribution curve obtained from a large number of persons
showing the minimal skin temperature that will cause pain. (Mod-
iﬁed from Hardy DJ: Nature of pain. J Clin Epidemiol 4:22, 1956.)
600
Unit IX
Dual Pathways for
Transmission of Pain 
Even though all pain receptors are free nerve endings,
these endings use two separate pathways for transmit-
ting pain 

use two separate pathways for transmit-
ting pain signals into the central nervous system. The
two pathways mainly correspond to the two types of
pain—a fast-sharp pain pathway and a slow-chronic
pain pathway.
Peripheral Pain Fibers—“Fast” and “Slow” Fibers. The fast-
sharp pain signals are elicited by either mechanical 
or thermal pain stimuli; they are transmitted in the
peripheral nerves to the spinal cord by small type Ad
ﬁbers at velocities between 6 and 30 m/sec.Conversely,
the slow-chroni

between 6 and 30 m/sec.Conversely,
the slow-chronic type of pain is elicited mostly by
chemical types of pain stimuli but sometimes by 
persisting mechanical or thermal stimuli. This slow-
chronic pain is transmitted to the spinal cord by type
C ﬁbers at velocities between 0.5 and 2 m/sec.
Because of this double system of pain innervation,
a sudden painful stimulus often gives a “double” pain
sensation: a fast-sharp pain that is transmitted to the
brain by the Ad ﬁber pathway, followed a second 

e
brain by the Ad ﬁber pathway, followed a second or so
later by a slow pain that is transmitted by the C ﬁber
pathway.The sharp pain apprises the person rapidly of
a damaging inﬂuence and, therefore, plays an impor-
tant role in making the person react immediately to
remove himself or herself from the stimulus. The slow
pain tends to become greater over time.This sensation
eventually produces the intolerable suffering of long-
continued pain and makes the person keep trying to
relieve the cause

 makes the person keep trying to
relieve the cause of the pain.
On entering the spinal cord from the dorsal spinal
roots, the pain ﬁbers terminate on relay neurons in the
dorsal horns. Here again, there are two systems for
processing the pain signals on their way to the brain,
as shown in Figures 48–2 and 48–3.
Dual Pain Pathways in the 
Cord and Brain Stem—The
Neospinothalamic Tract and the
Paleospinothalamic Tract
On entering the spinal cord, the pain signals take two
pathways to the brain, th

he pain signals take two
pathways to the brain, through (1) the neospinothala-
mic tract and (2) the paleospinothalamic tract.
Neospinothalamic Tract for Fast Pain. The fast type Ad
pain ﬁbers transmit mainly mechanical and acute
thermal pain. They terminate mainly in lamina I
(lamina marginalis) of the dorsal horns, as shown in
Figure 48–2, and there excite second-order neurons of
the neospinothalamic tract. These give rise to long
ﬁbers that cross immediately to the opposite side of
the cord t

oss immediately to the opposite side of
the cord through the anterior commissure and then
turn upward, passing to the brain in the anterolateral
columns.
Termination of the Neospinothalamic Tract in the Brain
Stem and Thalamus. A few ﬁbers of the neospinothal-
amic tract terminate in the reticular areas of the brain
stem,but most pass all the way to the thalamus without
interruption, terminating in the ventrobasal complex
along with the dorsal column–medial lemniscal tract
for tactile sensations

lumn–medial lemniscal tract
for tactile sensations, as was discussed in Chapter 47.
A few ﬁbers also terminate in the posterior nuclear
Fast-sharp
pain fibers
Spinal
nerve
C Ad
Slow-chronic
pathway
IX VIII
VII
VI
V
IV
III
II
I
Substantia
gelatinosa
Lamina
marginalis
Tract of
Lissauer
Figure 48–2
Transmission of both “fast-sharp” and “slow-chronic” pain signals
into and through the spinal cord on their way to the brain.
To: Somatosensory areas
complex and
pos

o: Somatosensory areas
posterior
nuclear
group
"Fast" Pain
Fibers
"Slow" Pain
Fibers
Thalamus
Reticular
formation
Intralaminar
nuclei
Pain tracts
Figure 48–3
Transmission of pain signals into the brain stem, thalamus, and
cerebral cortex by way of the fast pricking pain pathway and the
slow burning pain pathway.
Chapter 48
601
group of the thalamus. From these thalamic areas, the
signals are transmitted to oth

thalamic areas, the
signals are transmitted to other basal areas of the brain
as well as to the somatosensory cortex.
Capability of the Nervous System to Localize Fast Pain
in the Body. The fast-sharp type of pain can be local-
ized much more exactly in the different parts of the
body than can slow-chronic pain. However, when only
pain receptors are stimulated, without the simultane-
ous stimulation of tactile receptors, even fast pain may
be poorly localized, often only within 10 centimeters
or

rly localized, often only within 10 centimeters
or so of the stimulated area.Yet when tactile receptors
that excite the dorsal column–medial lemniscal system
are simultaneously stimulated, the localization can be
nearly exact.
Glutamate, the Probable Neurotransmitter of the Type
Ad Fast Pain Fibers. It is believed that glutamate is the
neurotransmitter substance secreted in the spinal cord
at the type Ad pain nerve ﬁber endings. This is one of
the most widely used excitatory transmitters in the

e most widely used excitatory transmitters in the
central nervous system, usually having a duration of
action lasting for only a few milliseconds.
Paleospinothalamic Pathway for Transmitting Slow-Chronic
Pain. The paleospinothalamic pathway is a much older
system and transmits pain mainly from the peripheral
slow-chronic type C pain ﬁbers, although it does trans-
mit some signals from type Ad ﬁbers as well. In this
pathway, the peripheral ﬁbers terminate in the spinal
cord almost entirely in lam

erminate in the spinal
cord almost entirely in laminae II and III of the dorsal
horns, which together are called the substantia gelati-
nosa, as shown by the lateral most dorsal root type C
ﬁber in Figure 48–2. Most of the signals then pass
through one or more additional short ﬁber neurons
within the dorsal horns themselves before entering
mainly lamina V, also in the dorsal horn. Here the last
neurons in the series give rise to long axons that mostly
join the ﬁbers from the fast pain pathway, p

ostly
join the ﬁbers from the fast pain pathway, passing ﬁrst
through the anterior commissure to the opposite side
of the cord, then upward to the brain in the anterolat-
eral pathway.
Substance P, the Probable Slow-Chronic Neurotrans-
mitter of Type C Nerve Endings. Research experi-
ments suggest that type C pain ﬁber terminals entering
the spinal cord secrete both glutamate transmitter and
substance P transmitter. The glutamate transmitter
acts instantaneously and lasts for only a few millisec

 instantaneously and lasts for only a few millisec-
onds. Substance P is released much more slowly, build-
ing up in concentration over a period of seconds or
even minutes. In fact, it has been suggested that the
“double” pain sensation one feels after a pinprick
might result partly from the fact that the glutamate
transmitter gives a faster pain sensation, whereas the
substance P transmitter gives a more lagging sensa-
tion. Regardless of the yet unknown details, it seems
clear that glutamate i

t unknown details, it seems
clear that glutamate is the neurotransmitter most
involved in transmitting fast pain into the central
nervous system, and substance P is concerned with
slow-chronic pain.
Projection of the Paleospinothalamic Pathway (Slow-
Chronic Pain Signals) into the Brain Stem and Thala-
mus. The slow-chronic paleospinothalamic pathway
terminates widely in the brain stem, in the large
shaded area shown in Figure 48–3. Only one tenth to
one fourth of the ﬁbers pass all the way to t

h to
one fourth of the ﬁbers pass all the way to the thala-
mus. Instead, most terminate in one of three areas: (1)
the reticular nuclei of the medulla, pons, and mesen-
cephalon; (2) the tectal area of the mesencephalon
deep to the superior and inferior colliculi; or (3) the
periaqueductal gray region surrounding the aqueduct
of Sylvius. These lower regions of the brain appear to
be important for feeling the suffering types of pain,
because animals whose brains have been sectioned
above the mes

als whose brains have been sectioned
above the mesencephalon to block pain signals from
reaching the cerebrum still evince undeniable evi-
dence of suffering when any part of the body is trau-
matized. From the brain stem pain areas, multiple
short-ﬁber neurons relay the pain signals upward into
the intralaminar and ventrolateral nuclei of the thala-
mus and into certain portions of the hypothalamus and
other basal regions of the brain.
Very Poor Capability of the Nervous System to Local-
ize Pr

 Capability of the Nervous System to Local-
ize Precisely the Source of Pain Transmitted in the
Slow-Chronic Pathway. Localization of pain transmit-
ted by way of the paleospinothalamic pathway is poor.
For instance, slow-chronic pain can usually be local-
ized only to a major part of the body, such as to one
arm or leg but not to a speciﬁc point on the arm or leg.
This is in keeping with the multisynaptic, diffuse con-
nectivity of this pathway. It explains why patients often
have serious difﬁc

 It explains why patients often
have serious difﬁculty in localizing the source of some
chronic types of pain.
Function of the Reticular Formation, Thalamus, and Cerebral
Cortex in the Appreciation of Pain. Complete removal of
the somatic sensory areas of the cerebral cortex does
not destroy an animal’s ability to perceive pain.There-
fore, it is likely that pain impulses entering the brain
stem reticular formation, the thalamus, and other
lower brain centers cause conscious perception of
pain. 

brain centers cause conscious perception of
pain. This does not mean that the cerebral cortex has
nothing to do with normal pain appreciation; electri-
cal stimulation of cortical somatosensory areas does
cause a human being to perceive mild pain from about
3 per cent of the points stimulated. However, it is
believed that the cortex plays an especially important
role in interpreting pain quality, even though pain per-
ception might be principally the function of lower
centers.
Special Capability

 the function of lower
centers.
Special Capability of Pain Signals to Arouse Overall Brain
Excitability. Electrical stimulation in the reticular areas
of the brain stem and in the intralaminar nuclei of the
thalamus, the areas where the slow-suffering type of
pain terminates, has a strong arousal effect on nervous
activity throughout the entire brain. In fact, these two
areas constitute part of the brain’s principal “arousal
system,”which is discussed in Chapter 59.This explains
why it is almost

ussed in Chapter 59.This explains
why it is almost impossible for a person to sleep when
he or she is in severe pain.
602
Unit IX
Surgical Interruption of Pain Pathways. When a person has
severe and intractable pain (sometimes resulting from
rapidly spreading cancer), it is necessary to relieve the
pain. To do this, the pain nervous pathways can be cut
at any one of several points. If the pain is in the lower
part of the body, a co

If the pain is in the lower
part of the body, a cordotomy in the thoracic region of
the spinal cord often relieves the pain for a few weeks
to a few months. To do this, the spinal cord on the side
opposite to the pain is partially cut in its anterolateral
quadrant to interrupt the anterolateral sensory pathway.
A cordotomy, however, is not always successful in
relieving pain, for two reasons. First, many pain ﬁbers
from the upper part of the body do not cross to the
opposite side of the spinal c

 do not cross to the
opposite side of the spinal cord until they have reached
the brain, so that the cordotomy does not transect these
ﬁbers. Second, pain frequently returns several months
later, partly as a result of sensitization of other pathways
that normally are too weak to be effectual (e.g., sparse
pathways in the dorsolateral cord). Another experi-
mental operative procedure to relieve pain has been to
cauterize speciﬁc pain areas in the intralaminar nuclei
in the thalamus, which often r

intralaminar nuclei
in the thalamus, which often relieves suffering types of
pain while leaving intact one’s appreciation of “acute”
pain, an important protective mechanism.
Pain Suppression
(“Analgesia”) System in the
Brain and Spinal Cord
The degree to which a person reacts to pain varies
tremendously. This results partly from a capability of
the brain itself to suppress input of pain signals to the
nervous system by activating a pain control system,
called an analgesia system.
The analgesia s

ystem,
The analgesia system is shown in Figure 48–4. It 
consists of three major components: (1) The periaque-
ductal gray and periventricular areas of the mesen-
cephalon and upper pons surround the aqueduct of
Sylvius and portions of the third and fourth ventricles.
Neurons from these areas send signals to (2) the raphe
magnus nucleus, a thin midline nucleus located in 
the lower pons and upper medulla, and the nucleus
reticularis paragigantocellularis, located late

us
reticularis paragigantocellularis, located laterally in 
the medulla. From these nuclei, second-order signals
are transmitted down the dorsolateral columns in the
spinal cord to (3) a pain inhibitory complex located in
the dorsal horns of the spinal cord. At this point, the
analgesia signals can block the pain before it is relayed
to the brain.
Electrical stimulation either in the periaqueductal
gray area or in the raphe magnus nucleus can suppress
many strong pain signals entering by way of 

press
many strong pain signals entering by way of the dorsal
spinal roots. Also, stimulation of areas at still higher
levels of the brain that excite the periaqueductal gray
area can also suppress pain. Some of these areas are
(1) the periventricular nuclei in the hypothalamus, lying
adjacent to the third ventricle, and (2) to a lesser
extent, the medial forebrain bundle, also in the hypo-
thalamus.
Several transmitter substances are involved in the
analgesia system; especially involved are enke

the
analgesia system; especially involved are enkephalin
and serotonin. Many nerve ﬁbers derived from the
periventricular nuclei and from the periaqueductal
gray area secrete enkephalin at their endings. Thus, as
shown in Figure 48–4, the endings of many ﬁbers in 
the raphe magnus nucleus release enkephalin when
stimulated.
Fibers originating in this area send signals to the
dorsal horns of the spinal cord to secrete serotonin at
their endings. The serotonin causes local cord neurons
to secrete 

he serotonin causes local cord neurons
to secrete enkephalin as well. The enkephalin is
believed to cause both presynaptic and postsynaptic
inhibition of incoming type C and type Ad pain ﬁbers
where they synapse in the dorsal horns.
Thus, the analgesia system can block pain signals at
the initial entry point to the spinal cord. In fact, it can
also block many local cord reﬂexes that result from
pain signals, especially withdrawal reﬂexes described
in Chapter 54.
Brain’s Opiate System—Endorphins

d
and Enkephalins
More than 35 years ago it was discovered that injec-
tion of minute quantities of morphine either into the
periventricular nucleus around the third ventricle or
nuclei
Periaqueductal gray
Enkephalin neurons
Pons
Raphe magnus
nucleus
Medulla
Serotonergic
neurons
neurons
somatosensory tract
Aqueduct
Fourth
ventricle
Third
ventricle
Pain fibers
pain inhibition
Figure 

le
Figure 48–4
Analgesia system of the brain and spinal cord, showing (1) inhi-
bition of incoming pain signals at the cord level and (2) presence
of enkephalin-secreting neurons that suppress pain signals in
both the cord and the brain stem.
Chapter 48
603
into the periaqueductal gray area of the brain stem
causes an extreme degree of analgesia. In subsequent
studies, it has been found that mo

. In subsequent
studies, it has been found that morphine-like agents,
mainly the opiates, also act at many other points in the
analgesia system, including the dorsal horns of the
spinal cord. Because most drugs that alter excitability
of neurons do so by acting on synaptic receptors, it was
assumed that the “morphine receptors” of the analge-
sia system must be receptors for some morphine-like
neurotransmitter that is naturally secreted in the
brain. Therefore, an extensive search was undertaken

ain. Therefore, an extensive search was undertaken
for the natural opiate of the brain.About a dozen such
opiate-like substances have now been found at differ-
ent points of the nervous system; all are breakdown
products of three large protein molecules: pro-
opiomelanocortin, proenkephalin, and prodynorphin.
Among the more important of these opiate-like 
substances are b-endorphin,
met-enkephalin,
leu-
enkephalin, and dynorphin.
The two enkephalins are found in the brain stem
and spinal cord, i

ins are found in the brain stem
and spinal cord, in the portions of the analgesia system
described earlier, and b-endorphin is present in both
the hypothalamus and the pituitary gland. Dynorphin
is found mainly in the same areas as the enkephalins,
but in much lower quantities.
Thus, although the ﬁne details of the brain’s opiate
system are not understood, activation of the analgesia
system by nervous signals entering the periaqueductal
gray and periventricular areas, or inactivation of pain
pat

periventricular areas, or inactivation of pain
pathways by morphine-like drugs, can almost totally
suppress many pain signals entering through the
peripheral nerves.
Inhibition of Pain Transmission 
by Simultaneous Tactile 
Sensory Signals
Another important event in the saga of pain control was
the discovery that stimulation of large type Ab sensory
ﬁbers from peripheral tactile receptors can depress
transmission of pain signals from the same body area.
This presumably results from local lateral

y area.
This presumably results from local lateral inhibition in
the spinal cord. It explains why such simple maneuvers
as rubbing the skin near painful areas is often effective
in relieving pain. And it probably also explains why lin-
iments are often useful for pain relief.
This mechanism and the simultaneous psychogenic
excitation of the central analgesia system are probably
also the basis of pain relief by acupuncture.
Treatment of Pain by Electrical
Several clinical procedures h

ectrical
Several clinical procedures have been developed for
suppressing pain by electrical stimulation. Stimulating
electrodes are placed on selected areas of the skin or,
on occasion, implanted over the spinal cord, supposedly
to stimulate the dorsal sensory columns.
In some patients, electrodes have been placed stereo-
taxically in appropriate intralaminar nuclei of the thal-
amus or in the periventricular or periaqueductal area of
the diencephalon. The patient can then personally

control the degree of stimulation. Dramatic relief has
been reported in some instances. Also, pain relief has
been reported to last for as long as 24 hours after only
a few minutes of stimulation.
Often a person feels pain in a part of the body that is
fairly remote from the tissue causing the pain. This is
called referred pain. For instance, pain in one of the
visceral organs often is referred to an area on the body
surface. Knowl

 is referred to an area on the body
surface. Knowledge of the different types of referred
pain is important in clinical diagnosis because in many
visceral ailments the only clinical sign is referred pain.
Mechanism of Referred Pain. Figure 48–5 shows the prob-
able mechanism by which most pain is referred. In the
ﬁgure, branches of visceral pain ﬁbers are shown to
synapse in the spinal cord on the same second-order
neurons (1 and 2) that receive pain signals from the
skin.When the visceral pain 

pain signals from the
skin.When the visceral pain ﬁbers are stimulated, pain
signals from the viscera are conducted through at least
some of the same neurons that conduct pain signals
from the skin, and the person has the feeling that the
sensations originate in the skin itself.
In clinical diagnosis, pain from the different viscera of
the abdomen and chest is one of the few criteria that
can be used for diagnosing visceral inﬂammation, vis-
ceral infectious disease, and other visc

ion, vis-
ceral infectious disease, and other visceral ailments.
Often, the viscera have sensory receptors for no other
modalities of sensation besides pain. Also, visceral pain
differs from surface pain in several important aspects.
One of the most important differences between
surface pain and visceral pain is that highly localized
types of damage to the viscera seldom cause severe
pain. For instance, a surgeon can cut the gut entirely in
two in a patient who is awake without causing signiﬁ-
c

n a patient who is awake without causing signiﬁ-
cant pain. Conversely, any stimulus that causes diffuse
stimulation of pain nerve endings throughout a viscus
causes pain that can be severe. For instance, ischemia
Skin nerve
fibers
Visceral
nerve fibers
1
2
Figure 48–5
Mechanism of referred pain and referred hyperalgesia.
604
Unit IX
caused by occluding the blood supply to a large area of
gut stimulates many diffuse pain ﬁbers at t

rea of
gut stimulates many diffuse pain ﬁbers at the same time
and can result in extreme pain.
Any stimulus that excites pain nerve endings in diffuse
areas of the viscera can cause visceral pain. Such stimuli
include ischemia of visceral tissue, chemical damage to
the surfaces of the viscera, spasm of the smooth muscle
of a hollow viscus, excess distention of a hollow viscus,
and stretching of the connective tissue surrounding or
within the viscus. Essentially all v

urrounding or
within the viscus. Essentially all visceral pain that orig-
inates in the thoracic and abdominal cavities is trans-
mitted through small type C pain ﬁbers and, therefore,
can transmit only the chronic-aching-suffering type of
pain.
Ischemia. Ischemia causes visceral pain in the same way
that it does in other tissues, presumably because of the
formation of acidic metabolic end products or tissue-
degenerative products such as bradykinin, proteolytic
enzymes, or others that stimulate

nin, proteolytic
enzymes, or others that stimulate pain nerve endings.
Chemical Stimuli. On occasion, damaging substances leak
from the gastrointestinal tract into the peritoneal cavity.
For instance, proteolytic acidic gastric juice often leaks
through a ruptured gastric or duodenal ulcer. This juice
causes widespread digestion of the visceral peritoneum,
thus stimulating broad areas of pain ﬁbers. The pain is
usually excruciatingly severe.
Spasm of a Hollow Viscus. Spasm of a portion of the gu

m of a Hollow Viscus. Spasm of a portion of the gut,
the gallbladder, a bile duct, a ureter, or any other hollow
viscus can cause pain, possibly by mechanical stimula-
tion of the pain nerve endings.Or the spasm might cause
diminished blood ﬂow to the muscle, combined with the
muscle’s increased metabolic need for nutrients, thus
causing severe pain.
Often pain from a spastic viscus occurs in the form of
cramps, with the pain increasing to a high degree of
severity and then subsiding. This proce

 degree of
severity and then subsiding. This process continues
intermittently, once every few minutes.The intermittent
cycles result from periods of contraction of smooth
muscle. For instance, each time a peristaltic wave travels
along an overly excitable spastic gut, a cramp occurs.
The cramping type of pain frequently occurs in appen-
dicitis, gastroenteritis, constipation, menstruation, par-
turition, gallbladder disease, or ureteral obstruction.
Overdistention of a Hollow Viscus. Extreme ove

on.
Overdistention of a Hollow Viscus. Extreme overﬁlling of a
hollow viscus also can result in pain, presumably
because of overstretch of the tissues themselves.
Overdistention can also collapse the blood vessels that
encircle the viscus or that pass into its wall, thus perhaps
promoting ischemic pain.
Insensitive Viscera. A few visceral areas are almost com-
pletely insensitive to pain of any type.These include the
parenchyma of the liver and the alveoli of the lungs.Yet
the liver capsule is e

he alveoli of the lungs.Yet
the liver capsule is extremely sensitive to both direct
trauma and stretch, and the bile ducts are also sensitive
to pain. In the lungs, even though the alveoli are insen-
sitive, both the bronchi and the parietal pleura are very
sensitive to pain.
“Parietal Pain” Caused by 
Visceral Disease
When a disease affects a viscus, the disease process
often spreads to the parietal peritoneum, pleura, or 
pericardium. These parietal surfaces, like the skin, are
supplied with e

ietal surfaces, like the skin, are
supplied with extensive pain innervation from the
peripheral spinal nerves. Therefore, pain from the pari-
etal wall overlying a viscus is frequently sharp. An
example can emphasize the difference between this
pain and true visceral pain: a knife incision through the
parietal peritoneum is very painful, whereas a similar
cut through the visceral peritoneum or through a gut
wall is not very painful, if painful at all.
Localization of Visceral Pain—
“Visceral” an

 all.
“Visceral” and the “Parietal” Pain
Transmission Pathways
Pain from the different viscera is frequently difﬁcult to
localize, for a number of reasons. First, the patient’s
brain does not know from ﬁrsthand experience that the
different internal organs exist; therefore, any pain that
originates internally can be localized only generally.
Second, sensations from the abdomen and thorax are
transmitted through two pathways to the central
nervous system—the true vi

pathways to the central
nervous system—the true visceral pathway and the pari-
etal pathway. True visceral pain is transmitted via pain
sensory ﬁbers within the autonomic nerve bundles, and
the sensations are referred to surface areas of the body
often far from the painful organ. Conversely, parietal
sensations are conducted directly into local spinal
nerves from the parietal peritoneum, pleura, or peri-
cardium, and these sensations are usually localized
directly over the painful area.
Localiza

localized
Localization of Referred Pain Transmitted via Visceral Pathways.
When visceral pain is referred to the surface of the body,
the person generally localizes it in the dermatomal
segment from which the visceral organ originated in the
embryo, not necessarily where the visceral organ now
lies. For instance, the heart originated in the neck and
upper thorax, so that the heart’s visceral pain ﬁbers pass
upward along the sympathetic sensory nerves and enter
the

along the sympathetic sensory nerves and enter
the spinal cord between segments C-3 and T-5. There-
fore, as shown in Figure 48–6, pain from the heart is
referred to the side of the neck, over the shoulder, over
the pectoral muscles, down the arm, and into the sub-
sternal area of the upper chest. These are the areas of
the body surface that send their own somatosensory
nerve ﬁbers into the C-3 to T-5 cord segments. Most fre-
quently, the pain is on the left side rather than on the
right because

 on the left side rather than on the
right because the left side of the heart is much more fre-
quently involved in coronary disease than the right.
The stomach originated approximately from the
seventh to ninth thoracic segments of the embryo.
Therefore, stomach pain is referred to the anterior epi-
gastrium above the umbilicus, which is the surface area
of the body subserved by the seventh through ninth tho-
racic segments. Figure 48–6 shows several other surface
areas to which visceral pain i

veral other surface
areas to which visceral pain is referred from other
organs, representing in general the areas in the embryo
from which the respective organs originated.
Parietal Pathway for Transmission of Abdominal and Thoracic
Pain. Pain from the viscera is frequently localized to two
surface areas of the body at the same time because of
Chapter 48
605
the dual transmission of pain through the referred vis-
ceral pathway and th

ain through the referred vis-
ceral pathway and the direct parietal pathway. Thus,
Figure 48–7 shows dual transmission from an inﬂamed
appendix. Pain impulses pass ﬁrst from the appendix
through visceral pain ﬁbers located within sympathetic
nerve bundles, and then into the spinal cord at about 
T-10 or T-11; this pain is referred to an area around the
umbilicus and is of the aching, cramping type. Pain
impulses also often originate in the parietal peritoneum
where the inﬂamed appendix touches o

al peritoneum
where the inﬂamed appendix touches or is adherent to
the abdominal wall. These cause pain of the sharp type
directly over the irritated peritoneum in the right lower
quadrant of the abdomen.
Some Clinical Abnormalities
of Pain and Other 
Somatic Sensations
A pain nervous pathway sometimes becomes exces-
sively excitable; this gives rise to hyperalgesia, which
means hypersensitivity to pain.Possible causes of hyper-
algesia are (1) excessive sensitivity of the pain rece

sia are (1) excessive sensitivity of the pain receptors
themselves, which is called primary hyperalgesia, and
(2) facilitation of sensory transmission, which is called
secondary hyperalgesia.
An example of primary hyperalgesia is the extreme
sensitivity of sunburned skin, which results from 
sensitization of the skin pain endings by local tissue
products from the burn—perhaps histamine, perhaps
prostaglandins, perhaps others. Secondary hyperalgesia
frequently results from lesions in the spinal c

ia
frequently results from lesions in the spinal cord or the
thalamus. Several of these lesions are discussed in sub-
sequent sections.
Occasionally herpesvirus infects a dorsal root ganglion.
This causes severe pain in the dermatomal segment sub-
served by the ganglion, thus eliciting a segmental type
of pain that circles halfway around the body.The disease
is called herpes zoster, or “shingles,” because of a skin
eruption that often ensues.
The cause of the pain is pre

on that often ensues.
The cause of the pain is presumably infection of the
pain neuronal cells in the dorsal root ganglion by the
virus. In addition to causing pain, the virus is carried by
neuronal cytoplasmic ﬂow outward through the neu-
ronal peripheral axons to their cutaneous origins. Here
the virus causes a rash that vesiculates within a few days
and then crusts over within another few days, all of this
occurring within the dermatomal area served by the
infected dorsal root.

served by the
Lancinating pain occasionally occurs in some people
over one side of the face in the sensory distribution area
(or part of the area) of the ﬁfth or ninth nerves; this phe-
nomenon is called tic douloureux (or trigeminal neural-
gia or glossopharyngeal neuralgia). The pain feels like
sudden electrical shocks, and it may appear for only a
few seconds at a time or may be almost continuous.
Often it is set off by exceedingly sensitive trigger areas

is set off by exceedingly sensitive trigger areas
on the surface of the face, in the mouth, or inside the
throat—almost always by a mechanoreceptive stimulus
rather than a pain stimulus. For instance, when the
patient swallows a bolus of food, as the food touches a
tonsil, it might set off a severe lancinating pain in the
mandibular portion of the ﬁfth nerve.
The pain of tic douloureux can usually be blocked by
surgically cutting the peripheral nerve from the hyper-
sensitive area. The sensory p

erve from the hyper-
sensitive area. The sensory portion of the ﬁfth nerve is
Heart
Esophagus
Liver and
gallbladder
Stomach
Pylorus
Umbilicus
Appendix and
small intestine
Right kidney
Left kidney
Colon
Ureter
Figure 48–6
Surface areas of referred pain from different visceral organs.
Visceral pain
T-10
L-1
Parietal pain
Figure 48–7
Visceral and parietal transmission of pain signals from the 
appendix.
606
Unit IX
often sectioned imm

nciples and Sensory Physiology
often sectioned immediately inside the cranium, where
the motor and sensory roots of the ﬁfth nerve separate
from each other, so that the motor portions, which are
needed for many jaw movements, can be spared while
the sensory elements are destroyed. This operation
leaves the side of the face anesthetic, which in itself may
be annoying. Furthermore, sometimes the operation is
unsuccessful, indicating that the lesion that causes the
pain might be in the sensory nucl

 that causes the
pain might be in the sensory nucleus in the brain stem
and not in the peripheral nerves.
If the spinal cord is transected entirely, all sensations
and motor functions distal to the segment of transection
are blocked, but if the spinal cord is transected on only
one side, the Brown-Séquard syndrome occurs. The
effects of such transection can be predicted from a
knowledge of the cord ﬁber tracts shown in Figure 48–8.
All motor functions are blocked on the si

re 48–8.
All motor functions are blocked on the side of the tran-
section in all segments below the level of the transec-
tion. Yet only some of the modalities of sensation are
lost on the transected side, and others are lost on the
opposite side. The sensations of pain, heat, and cold—
sensations served by the spinothalamic pathway—are
lost on the opposite side of the body in all dermatomes
two to six segments below the level of the transection.
By contrast, the sensations that are transmitted 

By contrast, the sensations that are transmitted only in
the dorsal and dorsolateral columns—kinesthetic and
position sensations, vibration sensation, discrete local-
ization, and two-point discrimination—are lost on the
side of the transection in all dermatomes below the level
of the transection. Discrete “light touch” is impaired 
on the side of the transection because the principal
pathway for the transmission of light touch, the dorsal
column, is transected. That is, the ﬁbers in this colum

n, is transected. That is, the ﬁbers in this column
do not cross to the opposite side until they reach the
medulla of the brain. “Crude touch,” which is poorly
localized, still persists because of partial transmission in
the opposite spinothalamic tract.
Headache
Headaches are a type of pain referred to the surface of
the head from deep head structures. Some headaches
result from pain stimuli arising inside the cranium, but
others result from pain arising outside the cranium, such
as from the na

n arising outside the cranium, such
as from the nasal sinuses.
Pain-Sensitive Areas in Cranial Vault. The brain tissues
themselves are almost totally insensitive to pain. Even
cutting or electrically stimulating the sensory areas 
of the cerebral cortex only occasionally causes pain;
instead, it causes prickly types of paresthesias on the
area of the body represented by the portion of the
sensory cortex stimulated. Therefore, it is likely that
much or most of the 

 Therefore, it is likely that
much or most of the pain of headache is not caused by
damage within the brain itself.
Conversely, tugging on the venous sinuses around the
brain, damaging the tentorium, or stretching the dura at
the base of the brain can cause intense pain that is rec-
ognized as headache. Also, almost any type of trauma-
tizing, crushing, or stretching stimulus to the blood
vessels of the meninges can cause headache. An espe-
cially sensitive structure is the middle meningeal arte

y sensitive structure is the middle meningeal artery,
and neurosurgeons are careful to anesthetize this artery
speciﬁcally when performing brain operations under
local anesthesia.
Areas of the Head to Which Intracranial Headache Is Referred.
Stimulation of pain receptors in the cerebral vault
above the tentorium, including the upper surface of the
tentorium itself, initiates pain impulses in the cerebral
portion of the ﬁfth nerve and, therefore, causes referred
headache to the front half of the 

causes referred
headache to the front half of the head in the surface
areas supplied by this somatosensory portion of the ﬁfth
cranial nerve, as shown in Figure 48–9.
pain 
impulses 
from 
beneath 
the 
tentorium enter the central nervous system mainly
through the glossopharyngeal, vagal, and second cervi-
cal nerves, which also supply the scalp above, behind,
and slightly below the ear. Subtentorial pain stimuli
cause “occipital headache” referred to the posterior
part of the head.

ache” referred to the posterior
Lateral
corticospinal
Rubrospinal
Olivospinal
Tectospinal
Vestibulospinal
Descending
Tracts
Ventral
Fasciculus cuneatus
Fasciculus gracilis
Dorsal
Ventral
Ventral spinothalamic
Spinotectal
Ascending
Tracts
Lateral
Figure 48–8
Cross section of the spinal cord, showing principal ascending
tracts on the right and principal descending tracts on the left.
Brain stem and
cerebellar vault
heada

on the left.
headaches
Cerebral vault
headaches
Nasal sinus
and eye
headaches
Figure 48–9
Areas of headache resulting from different causes.
Chapter 48
607
Types of Intracranial Headache
Headache of Meningitis. One of the most severe
headaches of all is that resulting from meningitis, which
causes inﬂammation of all the meninges, including the
sensitive areas of the dura and the sensitive areas
around 

 areas of the dura and the sensitive areas
around the venous sinuses. Such intense damage can
cause extreme headache pain referred over the entire
head.
Headache Caused by Low Cerebrospinal Fluid Pressure.
Removing as little as 20 milliliters of ﬂuid from the
spinal canal, particularly if the person remains in an
upright position, often causes intense intracranial
headache. Removing this quantity of ﬂuid removes part
of the ﬂotation for the brain that is normally provided
by the cerebrospinal ﬂu

 that is normally provided
by the cerebrospinal ﬂuid. The weight of the brain
stretches and otherwise distorts the various dural 
surfaces and thereby elicits the pain that causes the
headache.
Migraine Headache. Migraine headache is a special type
of headache that is thought to result from abnormal vas-
cular phenomena, although the exact mechanism is
unknown. Migraine headaches often begin with various
prodromal sensations, such as nausea, loss of vision in
part of the ﬁeld of vision, visual a

 of vision in
part of the ﬁeld of vision, visual aura, and other types
of sensory hallucinations. Ordinarily, the prodromal
symptoms begin 30 minutes to 1 hour before the 
beginning of the headache. Any theory that explains
migraine headache must also explain the prodromal
symptoms.
One of the theories of the cause of migraine
headaches is that prolonged emotion or tension causes
reﬂex vasospasm of some of the arteries of the head,
including arteries that supply the brain. The vasospasm
theoreti

ries that supply the brain. The vasospasm
theoretically produces ischemia of portions of the brain,
and this is responsible for the prodromal symptoms.
Then, as a result of the intense ischemia, something
happens to the vascular walls, perhaps exhaustion of
smooth muscle contraction, to allow the blood vessels
to become ﬂaccid and incapable of maintaining vascu-
lar tone for 24 to 48 hours. The blood pressure in the
vessels causes them to dilate and pulsate intensely, and
it is postulated that t

and pulsate intensely, and
it is postulated that the excessive stretching of the walls
of the arteries—including some extracranial arteries,
such as the temporal artery—causes the actual pain 
of migraine headaches. Other theories of the cause 
of migraine headaches include spreading cortical de-
pression, psychological abnormalities, and vasospasm
caused by excess local potassium in the cerebral extra-
cellular ﬂuid.
There may be a genetic predisposition to migraine
headaches, because a positiv

sposition to migraine
headaches, because a positive family history for
migraine has been reported in 65 to 90 per cent of cases.
Migraine headaches also occur about twice as fre-
quently in women as in men.
Alcoholic Headache. As many people have experi-
enced, a headache usually follows an alcoholic binge. It
is most likely that alcohol, because it is toxic to tissues,
directly irritates the meninges and causes the intracra-
nial pain.
Headache Caused by Constipation. Constipation causes
headac

Caused by Constipation. Constipation causes
headache in many people. Because it has been shown
that constipation headache can occur in people whose
pain sensory tracts in the spinal cord have been cut, we
know that this headache is not caused by nervous
impulses from the colon. Therefore, it may result from
absorbed toxic products or from changes in the circula-
tory system resulting from loss of ﬂuid into the gut.
Extracranial Types of Headache
Headache Resulting from Muscle Spasm. Emotional te

Headache Resulting from Muscle Spasm. Emotional tension
often causes many of the muscles of the head, especially
those muscles attached to the scalp and the neck
muscles attached to the occiput, to become spastic, and
it is postulated that this is one of the common causes of
headache. The pain of the spastic head muscles suppos-
edly is referred to the overlying areas of the head and
gives one the same type of headache as intracranial
lesions do.
Headache Caused by Irritation of Nasal and Access

Headache Caused by Irritation of Nasal and Accessory Nasal
Structures. The mucous membranes of the nose and
nasal sinuses are sensitive to pain, but not intensely so.
Nevertheless, infection or other irritative processes in
widespread areas of the nasal structures often summate
and cause headache that is referred behind the eyes or,
in the case of frontal sinus infection, to the frontal sur-
faces of the forehead and scalp, as shown in Figure 48–9.
Also, pain from the lower sinuses, such as fro

–9.
Also, pain from the lower sinuses, such as from the max-
illary sinuses, can be felt in the face.
Headache Caused by Eye Disorders. Difﬁculty in focusing
one’s eyes clearly may cause excessive contraction of
the eye ciliary muscles in an attempt to gain clear vision.
Even though these muscles are extremely small, it is
believed that tonic contraction of them can cause retro-
orbital headache. Also, excessive attempts to focus 
the eyes can result in reﬂex spasm in various facial 
and extraoc

sult in reﬂex spasm in various facial 
and extraocular muscles, which is a possible cause of
headache.
A second type of headache that originates in the eyes
occurs when the eyes are exposed to excessive irradia-
tion by light rays, especially ultraviolet light. Looking at
the sun or the arc of an arc-welder for even a few
seconds may result in headache that lasts from 24 to 48
hours. The headache sometimes results from “actinic”
irritation of the conjunctivae, and the pain is referred to
the sur

 conjunctivae, and the pain is referred to
the surface of the head or retro-orbitally. However,
focusing intense light from an arc or the sun on the
retina can also burn the retina, and this could be the
cause of the headache.
Thermal Receptors and 
Their Excitation
The human being can perceive different gradations of
cold and heat, from freezing cold to cold to cool to
indifferent to warm to hot to burning hot.
Thermal gradations are discriminated by at least
three types of s

ons are discriminated by at least
three types of sensory receptors: cold receptors,
warmth receptors, and pain receptors. The pain recep-
tors are stimulated only by extreme degrees of heat or
cold and, therefore, are responsible, along with the
cold and warmth receptors, for “freezing cold” and
“burning hot” sensations.
The cold and warmth receptors are located imme-
diately under the skin at discrete separated spots. In
608
Unit IX
The Nervous System: A. General Principles and Sensory Physiolo

System: A. General Principles and Sensory Physiology
most areas of the body, there are 3 to 10 times as many
cold spots as warmth spots, and the number in differ-
ent areas of the body varies from 15 to 25 cold spots
per square centimeter in the lips to 3 to 5 cold spots
per square centimeter in the ﬁnger to less than 1 cold
spot per square centimeter in some broad surface
areas of the trunk.
Although the existence of distinctive warmth nerve
endings is quite certain, based on psychological test

ings is quite certain, based on psychological tests,
they have not been identiﬁed histologically. They are
presumed to be free nerve endings, because warmth
signals are transmitted mainly over type C nerve ﬁbers
at transmission velocities of only 0.4 to 2 m/sec.
Conversely, a deﬁnitive cold receptor has been iden-
tiﬁed. It is a special, small type Ad myelinated nerve
ending that branches a number of times, the tips of
which protrude into the bottom surfaces of basal epi-
dermal cells. Signals a

tom surfaces of basal epi-
dermal cells. Signals are transmitted from these recep-
tors via type Ad nerve ﬁbers at velocities of about 
20 m/sec. Some cold sensations are believed to be
transmitted in type C nerve ﬁbers as well, which sug-
gests that some free nerve endings also might function
as cold receptors.
Stimulation of Thermal Receptors—Sensations of Cold, Cool,
Indifferent, Warm, and Hot. Figure 48–10 shows the
effects of different temperatures on the responses of
four types of nerve ﬁb

ratures on the responses of
four types of nerve ﬁbers: (1) a pain ﬁber stimulated
by cold, (2) a cold ﬁber, (3) a warmth ﬁber, and (4) a
pain ﬁber stimulated by heat. Note especially that
these ﬁbers respond differently at different levels of
temperature. For instance, in the very cold region, only
the cold-pain ﬁbers are stimulated (if the skin becomes
even colder, so that it nearly freezes or actually does
freeze, these ﬁbers cannot be stimulated). As the tem-
perature rises to +10° to 15°C, t

ed). As the tem-
perature rises to +10° to 15°C, the cold-pain impulses
cease, but the cold receptors begin to be stimulated,
reaching peak stimulation at about 24°C and fading
out slightly above 40°C. Above about 30°C, the
warmth receptors begin to be stimulated, but these
also fade out at about 49°C. Finally, at around 45°C,
the heat-pain ﬁbers begin to be stimulated by heat and,
paradoxically, some of the cold ﬁbers begin to be stim-
ulated again, possibly because of damage to the cold
ending

ain, possibly because of damage to the cold
endings caused by the excessive heat.
One can understand from Figure 48–10 that a
person determines the different gradations of thermal
sensations by the relative degrees of stimulation of the
different types of endings. One can also understand
why extreme degrees of both cold and heat can be
painful and why both these sensations, when intense
enough, may give almost the same quality of sensa-
tion—that is, freezing cold and burning hot sensations
feel

 is, freezing cold and burning hot sensations
feel almost alike.
Stimulatory Effects of Rising and Falling Temperature—Adap-
tation of Thermal Receptors. When a cold receptor is sud-
denly subjected to an abrupt fall in temperature, it
becomes strongly stimulated at ﬁrst, but this stimula-
tion fades rapidly during the ﬁrst few seconds and pro-
gressively more slowly during the next 30 minutes or
more. In other words, the receptor “adapts” to a great
extent, but never 100 per cent.
Thus, it is e

reat
Thus, it is evident that the thermal senses respond
markedly to changes in temperature, in addition to
being able to respond to steady states of temperature.
This means that when the temperature of the skin is
actively falling, a person feels much colder than when
the temperature remains cold at the same level. Con-
versely, if the temperature is actively rising, the person
feels much warmer than he or she would at the same
temperature if it were constant. Th

ld at the same
temperature if it were constant. The response to
changes in temperature explains the extreme degree
of heat one feels on ﬁrst entering a tub of hot water
and the extreme degree of cold felt on going from a
heated room to the out-of-doors on a cold day.
5
10
Freezing
cold
Indiffer-
ent
Burning
hot
Cold
Cool
Warm
Hot
20
30
40
50
60
15
25
35
45
55
10
Cold-pain
Heat-pain
Cold-receptors
Warmth receptors
8
6
4
2
Figure 48–10
Discharge frequencies at 

 per second
Discharge frequencies at different skin tempera-
tures of a cold-pain ﬁber, a cold ﬁber, a warmth
ﬁber, and a heat-pain ﬁber.
Chapter 48
609
Mechanism of Stimulation of 
Thermal Receptors
It is believed that the cold and warmth receptors are
stimulated by changes in their metabolic rates, and
that these changes result from the fact that tempera-
ture alters the rate of intracellular chemical reactions
more th

e rate of intracellular chemical reactions
more than twofold for each 10°C change. In other
words, thermal detection probably results not from
direct physical effects of heat or cold on the nerve
endings but from chemical stimulation of the endings
as modiﬁed by temperature.
Spatial Summation of Thermal Sensations. Because the
number of cold or warm endings in any one surface
area of the body is slight, it is difﬁcult to judge grada-
tions of temperature when small skin areas are stimu-
lated. H

perature when small skin areas are stimu-
lated. However, when a large skin area is stimulated
all at once, the thermal signals from the entire area
summate. For instance, rapid changes in temperature
as little as 0.01°C can be detected if this change affects
the entire surface of the body simultaneously. Con-
versely, temperature changes 100 times as great often
will not be detected when the affected skin area is only
1 square centimeter in size.
Transmission of Thermal Signals 
in the Nervous 

.
in the Nervous System
In general, thermal signals are transmitted in pathways
parallel to those for pain signals. On entering the
spinal cord, the signals travel for a few segments
upward or downward in the tract of Lissauer and then
terminate mainly in laminae I, II, and III of the dorsal
horns—the same as for pain. After a small amount of
processing by one or more cord neurons, the signals
enter long, ascending thermal ﬁbers that cross to the
opposite anterol

g thermal ﬁbers that cross to the
opposite anterolateral sensory tract and terminate in
both (1) the reticular areas of the brain stem and (2)
the ventrobasal complex of the thalamus.
A few thermal signals are also relayed to the cere-
bral somatic sensory cortex from the ventrobasal
complex. Occasionally a neuron in cortical somatic
sensory area I has been found by microelectrode
studies to be directly responsive to either cold or warm
stimuli on a speciﬁc area of the skin.However,removal
of th

n a speciﬁc area of the skin.However,removal
of the entire cortical postcentral gyrus in the human
being reduces but does not abolish the ability to dis-
tinguish gradations of temperature.
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U
N
I
T
X
System: B. The
Special Senses
49. The 

he Nervous 
49. The Eye: I. Optics of Vision
50. The Eye: II. Receptor and Neural 
51. The Eye: III. Central Neurophysiology of Vision
52. The Sense of Hearing
53. The Chemical Senses—Taste and Smell
C
H
A
P
T
E
R
 
4
9
613
Physical Principles 
of Optics
Before it is possible to understand the optical
system of the eye, the student must ﬁrst be thor-
oughly familiar with the basic principles of optics,
including th

 with the basic principles of optics,
including the physics of light refraction, focusing,
depth of focus, and so forth. A brief review of these
physical principles is presented; then the optics of the eye is discussed.
Refractive Index of a Transparent Substance. Light rays travel through air at a velocity of
about 300,000 km/sec, but they travel much slower through transparent solids 
and liquids. The refractive index of a transparent substance is the ratio of the 
velocity

ransparent substance is the ratio of the 
velocity of light in air to the velocity in the substance. The refractive index of air
itself is 1.00. Thus, if light travels through a particular type of glass at a velocity of
200,000 km/sec, the refractive index of this glass is 300,000 divided by 200,000, or
1.50.
Refraction of Light Rays at an Interface Between Two Media with Different Refractive Indices.
When light rays traveling forward in a beam (as shown in Figure 49–1A) strike an
interface that

as shown in Figure 49–1A) strike an
interface that is perpendicular to the beam, the rays enter the second medium
without deviating from their course. The only effect that occurs is decreased veloc-
ity of transmission and shorter wavelength, as shown in the ﬁgure by the shorter
distances between wave fronts.
If the light rays pass through an angulated interface as shown in Figure 49–1B,
the rays bend if the refractive indices of the two media are different from each other.
In this particular ﬁg

e different from each other.
In this particular ﬁgure, the light rays are leaving air, which has a refractive index
of 1.00, and are entering a block of glass having a refractive index of 1.50.When the
beam ﬁrst strikes the angulated interface, the lower edge of the beam enters the
glass ahead of the upper edge.The wave front in the upper portion of the beam con-
tinues to travel at a velocity of 300,000 km/sec, while that which entered the glass
travels at a velocity of 200,000 km/sec. This cau

travels at a velocity of 200,000 km/sec. This causes the upper portion of the wave
front to move ahead of the lower portion, so that the wave front is no longer ver-
tical but angulated to the right. Because the direction in which light travels is always
perpendicular to the plane of the wave front, the direction of travel of the light beam
bends downward.
This bending of light rays at an angulated interface is known as refraction. Note
particularly that the degree of refraction increases as a 

arly that the degree of refraction increases as a function of (1) the ratio of
the two refractive indices of the two transparent media and (2) the degree of angu-
lation between the interface and the entering wave front.
Application of Refractive Principles to Lenses
Convex Lens Focuses Light Rays. Figure 49–2 shows parallel light rays entering a convex
lens.The light rays passing through the center of the lens strike the lens exactly per-
pendicular to the lens surface and, therefore, pass thro

ular to the lens surface and, therefore, pass through the lens without being
refracted. Toward either edge of the lens, however, the light rays strike a progres-
sively more angulated interface. The outer rays bend more and more toward the
center, which is called convergence of the rays. Half the bending occurs when 
the rays enter the lens, and half as they exit from the opposite side. (At this time,
the student should pause and analyze why the rays bend toward the center on
leaving the lens.) 

rays bend toward the center on
leaving the lens.) Finally, if the lens has exactly the proper curvature, parallel light
614
Unit X
The Nervous System: B. The Special Senses
rays passing through each part of the lens will be bent
exactly enough so that all the rays will pass through a
single point, which is called the focal point.
Concave Lens Diverges Light Rays. Figure 49–3 shows the
effect of a concave lens on parallel light rays. The rays
that enter the center of the lens strike an interface 

 enter the center of the lens strike an interface that
is perpendicular to the beam and, therefore, do not
refract. The rays at the edge of the lens enter the lens
ahead of the rays in the center. This is opposite to the
effect in the convex lens, and it causes the peripheral
light rays to diverge from the light rays that pass through
the center of the lens. Thus, the concave lens diverges
light rays, but the convex lens converges light rays.
Cylindrical Lens Bends Light Rays in Only One Plane—C

indrical Lens Bends Light Rays in Only One Plane—Comparison
with Spherical Lenses. Figure 49–4 shows both a convex
spherical lens and a convex cylindrical lens. Note that
the cylindrical lens bends light rays from the two sides
of the lens but not from the top or the bottom. That is,
bending occurs in one plane but not the other.Thus, par-
allel light rays are bent to a focal line. Conversely, light
rays that pass through the spherical lens are refracted
A
B
Glass
Figure 49–1
Light r

efracted
A
B
Glass
Light rays entering a glass surface perpendicular to the light rays
(A) and a glass surface angulated to the light rays (B). This ﬁgure
demonstrates that the distance between waves after they enter
the glass is shortened to about two thirds that in air. It also shows
that light rays striking an angulated glass surface are bent.
Light from
distant source
Focal length
Figure 49–2
Bending of light rays at each surface of a convex spherical lens,
showing th

ach surface of a convex spherical lens,
showing that parallel light rays are focused to a focal point.
Light from
Figure 49–3
Bending of light rays at each surface of a concave spherical lens,
showing that parallel light rays are diverged.
A
B
Figure 49–4
A, Point focus of parallel light rays by a spherical convex lens. 
B, Line focus of parallel light rays by a cylindrical convex lens.
Chapter 49
615
at all edges of the lens (in both planes) toward th

t all edges of the lens (in both planes) toward the
central ray, and all the rays come to a focal point.
The cylindrical lens is well demonstrated by a test
tube full of water. If the test tube is placed in a beam of
sunlight and a piece of paper is brought progressively
closer to the opposite side of the tube, a certain distance
will be found at which the light rays come to a focal line.
The spherical lens is demonstrated by an ordinary mag-
nifying glass. If such a lens is placed in a beam of 

ying glass. If such a lens is placed in a beam of sun-
light and a piece of paper is brought progressively closer
to the lens, the light rays will impinge on a common
focal point at an appropriate distance.
Concave cylindrical lenses diverge light rays in only
one plane in the same manner that convex cylindrical
lenses converge light rays in one plane.
Combination of Two Cylindrical Lenses at Right Angles
Equals a Spherical Lens. Figure 49–5B shows two
convex cylindrical lenses at right angles t

ws two
convex cylindrical lenses at right angles to each other.
The vertical cylindrical lens converges the light rays that
pass through the two sides of the lens, and the horizon-
tal lens converges the top and bottom rays. Thus, all the
light rays come to a single-point focus. In other words,
two cylindrical lenses crossed at right angles to each other
perform the same function as one spherical lens of the
same refractive power.
The distance beyond a convex lens at which

 a Lens
The distance beyond a convex lens at which parallel rays
converge to a common focal point is called the focal
length of the lens.The diagram at the top of Figure 49–6
demonstrates this focusing of parallel light rays.
In the middle diagram, the light rays that enter the
convex lens are not parallel but are diverging because
the origin of the light is a point source not far away from
the lens itself. Because these rays are diverging outward
from the point source, it can be seen from the d

d
from the point source, it can be seen from the diagram
that they do not focus at the same distance away from
the lens as do parallel rays. In other words, when rays
of light that are already diverging enter a convex lens,
the distance of focus on the other side of the lens is
farther from the lens than is the focal length of the lens
for parallel rays.
The bottom diagram of Figure 49–6 shows light rays
that are diverging toward a convex lens that has far
greater curvature than that of the othe

at has far
greater curvature than that of the other two lenses in
the ﬁgure. In this diagram, the distance from the lens at
which the light rays come to focus is exactly the same
as that from the lens in the ﬁrst diagram, in which the
lens is less convex but the rays entering it are parallel.
This demonstrates that both parallel rays and diverging
rays can be focused at the same distance beyond a lens,
provided the lens changes its convexity.
The relation of focal length of the lens, distance of

 relation of focal length of the lens, distance of the
point source of light, and distance of focus is expressed
by the following formula:
in which f is the focal length of the lens for parallel rays,
a is the distance of the point source of light from the
lens, and b is the distance of focus on the other side of
the lens.
1
f
a
b
=
+
1
1
A
B
Point source of light
Point focus
Line focus
Figure 49–5
A, Focusing of light from a point source to a line focus by a cylin-
drical 

a point source to a line focus by a cylin-
drical lens. B, Two cylindrical convex lenses at right angles to
each other, demonstrating that one lens converges light rays in
one plane and the other lens converges light rays in the plane at
a right angle. The two lenses combined give the same point focus
as that obtained with a single spherical convex lens.
Focal
points
Light from distant source
Point source
Figure 49–6
The two upper lenses of this ﬁgure have the same focal length,
but the light ra

ﬁgure have the same focal length,
but the light rays entering the top lens are parallel, whereas those
entering the middle lens are diverging; the effect of parallel versus
diverging rays on the focal distance is shown. The bottom lens
has far more refractive power than either of the other two lenses
(i.e., has a much shorter focal length), demonstrating that the
stronger the lens is, the nearer to the lens the point focus is.
616
Unit X
Formation of an 

ous System: B. The Special Senses
Formation of an Image by a 
Convex Lens
Figure 49–7A shows a convex lens with two point
sources of light to the left. Because light rays pass
through the center of a convex lens without being
refracted in either direction, the light rays from each
point source of light are shown to come to a point focus
on the opposite side of the lens directly in line with the
point source and the center of the lens.
Any object in front of the lens is, in reality, a mosaic
of p

in front of the lens is, in reality, a mosaic
of point sources of light. Some of these points are very
bright, some are very weak, and they vary in color. Each
point source of light on the object comes to a separate
point focus on the opposite side of the lens in line with
the lens center. If a white sheet of paper is placed at the
focus distance from the lens, one can see an image of
the object, as demonstrated in Figure 49–7B. However,
this image is upside down with respect to the original
obj

ge is upside down with respect to the original
object, and the two lateral sides of the image are
reversed. This is the method by which the lens of a
camera focuses images on ﬁlm.
Measurement of the Refractive
Power of a Lens—“Diopter”
The more a lens bends light rays, the greater is its
“refractive power.”This refractive power is measured in
terms of diopters. The refractive power in diopters of a
convex lens is equal to 1 meter divided by its focal
length. Thus, a spherical lens that converges

ocal
length. Thus, a spherical lens that converges parallel
light rays to a focal point 1 meter beyond the lens has
a refractive power of +1 diopter, as shown in Figure
49–8. If the lens is capable of bending parallel light rays
twice as much as a lens with a power of +1 diopter, it is
said to have a strength of +2 diopters, and the light rays
come to a focal point 0.5 meter beyond the lens. A lens
capable of converging parallel light rays to a focal point
only 10 centimeters (0.10 meter) beyond

ocal point
only 10 centimeters (0.10 meter) beyond the lens has a
refractive power of +10 diopters.
Point sources
Focal points
A
B
Figure 49–7
A, Two point sources of light
focused at two separate points on
opposite sides of the lens. B, For-
mation of an image by a convex
spherical lens.
1
diopter
2
diopters
10
diopters
1 meter
Figure 49–8
Effect of lens strength on the focal distance.
The refractive power of concave lenses cannot be
stated in terms of the focal distance beyond the lens
because

erms of the focal distance beyond the lens
because the light rays diverge, rather than focus to a
point. However, if a concave lens diverges light rays at
the same rate that a 1-diopter convex lens converges
them, the concave lens is said to have a dioptric strength
of –1. Likewise, if the concave lens diverges light rays as
much as a +10-diopter lens converges them, this lens is
said to have a strength of –10 diopters.
Concave lenses “neutralize” the refractive power of
convex lenses. Thus, pla

” the refractive power of
convex lenses. Thus, placing a 1-diopter concave lens
immediately in front of a 1-diopter convex lens results
in a lens system with zero refractive power.
The strengths of cylindrical lenses are computed in
the same manner as the strengths of spherical lenses,
except that the axis of the cylindrical lens must be stated
in addition to its strength. If a cylindrical lens focuses
Chapter 49
617
parallel light rays to a line focus 1 meter beyond

parallel light rays to a line focus 1 meter beyond the
lens,it has a strength of +1 diopter.Conversely,if a cylin-
drical lens of a concave type diverges light rays as much
as a +1-diopter cylindrical lens converges them, it has a
strength of –1 diopter. If the focused line is horizontal,
its axis is said to be 0 degrees. If it is vertical, its axis is
90 degrees.
The eye, shown in Figure 49–9, is optically equivalent
to the usual photographic camera. It has

quivalent
to the usual photographic camera. It has a lens system,
a variable aperture system (the pupil), and a retina
that corresponds to the ﬁlm.The lens system of the eye
is composed of four refractive interfaces: (1) the inter-
face between air and the anterior surface of the
cornea, (2) the interface between the posterior surface
of the cornea and the aqueous humor, (3) the interface
between the aqueous humor and the anterior surface
of the lens of the eye, and (4) the interface between
the

lens of the eye, and (4) the interface between
the posterior surface of the lens and the vitreous
humor. The internal index of air is 1; the cornea, 1.38;
the aqueous humor, 1.33; the crystalline lens (on
average), 1.40; and the vitreous humor, 1.34.
Reduced Eye. If all the refractive surfaces of the eye are
algebraically added together and then considered to
be one single lens, the optics of the normal eye may be
simpliﬁed and represented schematically as a “reduced
eye.” This is useful in simp

tically as a “reduced
eye.” This is useful in simple calculations. In the
reduced eye, a single refractive surface is considered
to exist, with its central point 17 millimeters in front of
the retina and a total refractive power of 59 diopters
when the lens is accommodated for distant vision.
About two thirds of the 59 diopters of refractive
power of the eye is provided by the anterior surface of
the cornea (not by the eye lens). The principal reason
for this is that the refractive index of the 

ason
for this is that the refractive index of the cornea is
markedly different from that of air, while the refrac-
tive index of the eye lens is not greatly different 
from the indices of the aqueous humor and vitreous
humor.
The total refractive power of the internal lens of the
eye, as it normally lies in the eye surrounded by ﬂuid
on each side, is only 20 diopters, about one third the
total refractive power of the eye. But the importance
of the internal lens is that, in response to nervous
si

e internal lens is that, in response to nervous
signals from the brain, its curvature can be increased
markedly to provide “accommodation,” which is dis-
cussed later in the chapter.
Formation of an Image on the Retina. In the same manner
that a glass lens can focus an image on a sheet of paper,
the lens system of the eye can focus an image on the
retina.The image is inverted and reversed with respect
to the object. However, the mind perceives objects in
the upright position despite the upside-d

jects in
the upright position despite the upside-down orienta-
tion on the retina because the brain is trained to con-
sider an inverted image as the normal.
In children, the refractive power of the lens of the eye
can be increased voluntarily from 20 diopters to about
34 diopters; this in an “accommodation” of 14 diopters.
To do this, the shape of the lens is changed from that
of a moderately convex lens to that of a very convex
lens. The mechanism is as follows.
In

 very convex
In a young person, the lens is composed of a strong
elastic capsule ﬁlled with viscous, proteinaceous, but
transparent ﬂuid. When the lens is in a relaxed state
with no tension on its capsule, it assumes an almost
spherical shape, owing mainly to the elastic retraction
of the lens capsule. However, as shown in Figure
49–10, about 70 suspensory ligaments attach radially
around the lens, pulling the lens edges toward the
Image
Total refractive power 

ens edges toward the
Image
Total refractive power = 59 diopters
Vitreous
humor
1.34
Lens
1.40
Aqueous
humor
1.33
Cornea
1.38
Air
1.00
Object
Figure 49–9
The eye as a camera. The numbers are the refractive indices.
Choroid
Circular
fibers
Sclerocorneal
junction
Meridional
fibers
Cornea
Ciliary muscle
Suspensory
ligaments
Lens
ligaments
Sclera
Figure 49–10
Mechanism of accommodation (focusing).
618
Unit X
outer circle of the eyeball. These ligam

al Senses
outer circle of the eyeball. These ligaments are con-
stantly tensed by their attachments at the anterior
border of the choroid and retina. The tension on the
ligaments causes the lens to remain relatively ﬂat
under normal conditions of the eye.
However, also located at the lateral attachments of
the lens ligaments to the eyeball is the ciliary muscle,
which itself has two separate sets of smooth muscle
ﬁbers—meridional ﬁbers and circular ﬁbers. The
meridional ﬁbers extend from the per

ar ﬁbers. The
meridional ﬁbers extend from the peripheral ends of
the suspensory ligaments to the corneoscleral junction.
When these muscle ﬁbers contract, the peripheral
insertions of the lens ligaments are pulled medially
toward the edges of the cornea, thereby releasing the
ligaments’ tension on the lens. The circular ﬁbers are
arranged circularly all the way around the ligament
attachments so that when they contract, a sphincter-
like action occurs, decreasing the diameter of the circle
of l

occurs, decreasing the diameter of the circle
of ligament attachments; this also allows the ligaments
to pull less on the lens capsule.
Thus, contraction of either set of smooth muscle
ﬁbers in the ciliary muscle relaxes the ligaments to the
lens capsule, and the lens assumes a more spherical
shape, like that of a balloon, because of the natural
elasticity of the lens capsule.
Accommodation Is Controlled by Parasympathetic Nerves.
The ciliary muscle is controlled almost entirely by
parasympathet

cle is controlled almost entirely by
parasympathetic nerve signals transmitted to the eye
through the third cranial nerve from the third nerve
nucleus in the brain stem, as explained in Chapter 51.
Stimulation of the parasympathetic nerves contracts
both sets of ciliary muscle ﬁbers, which relaxes the lens
ligaments, thus allowing the lens to become thicker
and increase its refractive power. With this increased
refractive power, the eye focuses on objects nearer
than when the eye has less refrac

n objects nearer
than when the eye has less refractive power. Conse-
quently, as a distant object moves toward the eye, the
number of parasympathetic impulses impinging on 
the ciliary muscle must be progressively increased for
the eye to keep the object constantly in focus. (Sym-
pathetic stimulation has an additional effect in relax-
ing the ciliary muscle, but this effect is so weak that it
plays almost no role in the normal accommodation
mechanism; the neurology of this is discussed in
Chapt

anism; the neurology of this is discussed in
Chapter 51.)
Presbyopia. As a person grows older, the lens grows
larger and thicker and becomes far less elastic, partly
because of progressive denaturation of the lens pro-
teins.The ability of the lens to change shape decreases
with age. The power of accommodation decreases
from about 14 diopters in a child to less than 2 diopters
by the time a person reaches 45 to 50 years; it then
decreases to essentially 0 diopters at age 70 years.
Thereafter, th

ntially 0 diopters at age 70 years.
Thereafter, the lens remains almost totally nonaccom-
modating, a condition known as “presbyopia.”
Once a person has reached the state of presbyopia,
each eye remains focused permanently at an almost
constant distance; this distance depends on the physi-
cal characteristics of each person’s eyes. The eyes can
no longer accommodate for both near and far vision.
To see clearly both in the distance and nearby, an 
older person must wear bifocal glasses with the u

older person must wear bifocal glasses with the upper
segment focused for far-seeing and the lower segment
focused for near-seeing (e.g., for reading).
The major function of the iris is to increase the amount
of light that enters the eye during darkness and to
decrease the amount of light that enters the eye in day-
light. The reﬂexes for controlling this mechanism are
considered in the discussion of the neurology of the
eye in Chapter 51.
The amount of light that enters the 

n Chapter 51.
The amount of light that enters the eye through the
pupil is proportional to the area of the pupil or to the
square of the diameter of the pupil. The pupil of the
human eye can become as small as about 1.5 millime-
ters and as large as 8 millimeters in diameter. The
quantity of light entering the eye can change about 30-
fold as a result of changes in pupillary aperture.
“Depth of Focus” of the Lens System Increases with Decreas-
ing Pupillary Diameter. Figure 49–11 shows two eyes 

g Pupillary Diameter. Figure 49–11 shows two eyes that
are exactly alike except for the diameters of the pupil-
lary apertures. In the upper eye, the pupillary aperture
is small, and in the lower eye, the aperture is large. In
front of each of these two eyes are two small point
sources of light; light from each passes through the
pupillary aperture and focuses on the retina. Conse-
quently, in both eyes, the retina sees two spots of light
in perfect focus. It is evident from the diagrams,
howeve

ect focus. It is evident from the diagrams,
however, that if the retina is moved forward or back-
ward to an out-of-focus position, the size of each spot
will not change much in the upper eye, but in the lower
eye the size of each spot will increase greatly, becom-
ing a “blur circle.” In other words, the upper lens
system has far greater depth of focus than the bottom
lens system. When a lens system has great depth of
focus, the retina can be displaced considerably from
the focal plane or the l

splaced considerably from
the focal plane or the lens strength can change con-
siderably from normal and the image will still remain
Lens
Lens
Focal point
Point sources of light
Figure 49–11
Effect of small (top) and large (bottom) pupillary apertures on
“depth of focus.”
Chapter 49
619
nearly in sharp focus, whereas when a lens system has
a “shallow” depth of focus, moving the retina only
slightly away from the focal plane causes extreme blur-

tly away from the focal plane causes extreme blur-
ring.
The greatest possible depth of focus occurs when the
pupil is extremely small. The reason for this is that,
with a very small aperture, almost all the rays pass
through the center of the lens, and the central most
rays are always in focus, as explained earlier.
Emmetropia (Normal Vision). As shown in Figure 49–12, the
eye is considered to be normal, or “emmetropic,” if par-
allel light rays from distant objects are in 

par-
allel light rays from distant objects are in sharp focus on
the retina when the ciliary muscle is completely relaxed.
This means that the emmetropic eye can see all distant
objects clearly with its ciliary muscle relaxed. However,
to focus objects at close range, the eye must contract its
ciliary muscle and thereby provide appropriate degrees
of accommodation.
Hyperopia (Farsightedness). Hyperopia,
known as “farsightedness,” is usually due to either an
eyeball that is too shor

 usually due to either an
eyeball that is too short or, occasionally, a lens system
that is too weak. In this condition, as seen in the middle
panel of Figure 49–12, parallel light rays are not bent
sufﬁciently by the relaxed lens system to come to focus
by the time they reach the retina. To overcome this
abnormality, the ciliary muscle must contract to increase
the strength of the lens. By using the mechanism of
accommodation, a farsighted person is capable of focus-
ing distant objects on the 

n is capable of focus-
ing distant objects on the retina. If the person has used
only a small amount of strength in the ciliary muscle to
accommodate for the distant objects, he or she still has
much accommodative power left, and objects closer 
and closer to the eye can also be focused sharply until
the ciliary muscle has contracted to its limit. In old age,
when the lens becomes “presbyopic,” a farsighted
person is often unable to accommodate the lens sufﬁ-
ciently to focus even distant object

he lens sufﬁ-
ciently to focus even distant objects, much less near
objects.
Myopia (Nearsightedness). In myopia, or “nearsighted-
ness,” when the ciliary muscle is completely relaxed, the
light rays coming from distant objects are focused in
front of the retina, as shown in the bottom panel of
Figure 49–12. This is usually due to too long an eyeball,
but it can result from too much refractive power in the
lens system of the eye.
No mechanism exists by which the eye can decrease
the strength of 

sts by which the eye can decrease
the strength of its lens to less than that which exists
when the ciliary muscle is completely relaxed.A myopic
person has no mechanism by which to focus distant
objects sharply on the retina. However, as an object
moves nearer to the person’s eye, it ﬁnally gets close
enough that its image can be focused. Then, when the
object comes still closer to the eye, the person can use
the mechanism of accommodation to keep the image
focused clearly. A myopic person has a

p the image
focused clearly. A myopic person has a deﬁnite limiting
“far point” for clear vision.
Correction of Myopia and Hyperopia by Use of Lenses.
It will be recalled that light rays passing through a
concave lens diverge. If the refractive surfaces of the eye
have too much refractive power, as in myopia, this
excessive refractive power can be neutralized by placing
in front of the eye a concave spherical lens, which will
diverge rays. Such correction is demonstrated in the
upper diagram of 

orrection is demonstrated in the
upper diagram of Figure 49–13.
Conversely, in a person who has hyperopia—that is,
someone who has too weak a lens system—the abnor-
mal vision can be corrected by adding refractive power
using a convex lens in front of the eye. This correction
is demonstrated in the lower diagram of Figure 49–13.
One usually determines the strength of the concave
or convex lens needed for clear vision by “trial and
error”—that is, by trying ﬁrst a strong lens and then a
Emmetropi

 by trying ﬁrst a strong lens and then a
Emmetropia
Hyperopia
Myopia
Figure 49–12
Parallel light rays focus on the retina in emmetropia, behind the
retina in hyperopia, and in front of the retina in myopia.
Figure 49–13
Correction of myopia with a concave lens, and correction of hyper-
opia with a convex lens.
620
Unit X
stronger or weaker lens until the one that gives the best
visual acuity is found.
Astigmatism
Astigmatism is a refractive error of the 

igmatism
Astigmatism is a refractive error of the eye that causes
the visual image in one plane to focus at a different dis-
tance from that of the plane at right angles. This most
often results from too great a curvature of the cornea
in one plane of the eye. An example of an astigmatic
lens would be a lens surface like that of an egg lying
sidewise to the incoming light. The degree of curvature
in the plane through the long axis of the egg is not
nearly as great as the degree of curvature in t

ot
nearly as great as the degree of curvature in the plane
through the short axis.
Because the curvature of the astigmatic lens along
one plane is less than the curvature along the other
plane, light rays striking the peripheral portions of the
lens in one plane are not bent nearly as much as the rays
striking the peripheral portions of the other plane. This
is demonstrated in Figure 49–14, which shows rays 
of light originating from a point source and passing
through an oblong, astigmatic lens.

ce and passing
through an oblong, astigmatic lens. The light rays in the
vertical plane, indicated by plane BD, are refracted
greatly by the astigmatic lens because of the greater cur-
vature in the vertical direction than in the horizontal
direction. By contrast, the light rays in the horizontal
plane, indicated by plane AC, are not bent nearly as
much as the light rays in vertical plane BD. It is obvious
that light rays passing through an astigmatic lens do not
all come to a common focal point

matic lens do not
all come to a common focal point, because the light rays
passing through one plane focus far in front of those
passing through the other plane.
The accommodative power of the eye can never 
compensate for astigmatism because, during accommo-
dation, the curvature of the eye lens changes approxi-
mately equally in both planes; therefore, in astigmatism,
each of the two planes requires a different degree of
accommodation. Thus, without the aid of glasses, a
person with astigmatis

thout the aid of glasses, a
person with astigmatism never sees in sharp focus.
Correction of Astigmatism with a Cylindrical Lens. One
may consider an astigmatic eye as having a lens system
made up of two cylindrical lenses of different strengths
and placed at right angles to each other. To correct for
astigmatism, the usual procedure is to ﬁnd a spherical
lens by trial and error that corrects the focus in one of
the two planes of the astigmatic lens.Then an additional
cylindrical lens is used to

ens.Then an additional
cylindrical lens is used to correct the remaining error 
in the remaining plane. To do this, both the axis and 
the strength of the required cylindrical lens must be
determined.
There are several methods for determining the axis
of the abnormal cylindrical component of the lens
system of an eye. One of these methods is based on the
use of parallel black bars of the type shown in Figure
49–15. Some of these parallel bars are vertical, some
horizontal, and some at various an

 vertical, some
horizontal, and some at various angles to the vertical
and horizontal axes. After placing various spherical
lenses in front of the astigmatic eye, a strength of lens
is usually found that causes sharp focus of one set of
parallel bars but does not correct the fuzziness of the
set of bars at right angles to the sharp bars. It can be
shown from the physical principles of optics discussed
earlier in this chapter that the axis of the out-of-focus
cylindrical component of the optical 

out-of-focus
cylindrical component of the optical system is parallel
to the bars that are fuzzy. Once this axis is found, the
examiner tries progressively stronger and weaker posi-
tive or negative cylindrical lenses, the axes of which 
are placed in line with the out-of-focus bars, until the
patient sees all the crossed bars with equal clarity.When
this has been accomplished, the examiner directs the
optician to grind a special lens combining both the
spherical correction and the cylindrical co

th the
spherical correction and the cylindrical correction at 
the appropriate axis.
Correction of Optical Abnormalities by Use of Contact Lenses
Glass or plastic contact lenses can be inserted that ﬁt
snugly against the anterior surface of the cornea. These
lenses are held in place by a thin layer of tear ﬂuid that
ﬁlls the space between the contact lens and the anterior
eye surface.
A special feature of the contact lens is that it nulliﬁes
almost entirely the refraction that normally occurs at

st entirely the refraction that normally occurs at
the anterior surface of the cornea.The reason for this is
A
B
C
D
Focal line
for plane BD
Focal line
for plane AC
Plane BD
(more refractive
power)
Plane AC
(less refractive
power)
of light
Figure 49–14
Astigmatism, demonstrating that light rays focus at one focal dis-
tance in one focal plane (plane AC) and at another focal distance
in the plane at a right angle (plane BD).
12
11
10
9
8
7
6
5
4
3
2
1
Figure 49–15
Chart composed of p

9
8
7
6
5
4
3
2
1
Chart composed of parallel black bars at different angular orien-
tations for determining the axis of astigmatism.
Chapter 49
621
that the tears between the contact lens and the cornea
have a refractive index almost equal to that of the
cornea, so that the anterior surface of the cornea no
longer plays a signiﬁcant role in the eye’s optical system.
Instead, the outer surface of the contact lens plays the
major role. Thus, the refraction

ct lens plays the
major role. Thus, the refraction of this surface of the
contact lens substitutes for the cornea’s usual refraction.
This is especially important in people whose eye refrac-
tive errors are caused by an abnormally shaped cornea,
such as those who have an odd-shaped, bulging
cornea—a condition called keratoconus. Without the
contact lens, the bulging cornea causes such severe
abnormality of vision that almost no glasses can correct
the vision satisfactorily; when a contact lens i

t
the vision satisfactorily; when a contact lens is used,
however, the corneal refraction is neutralized, and
normal refraction by the outer surface of the contact
lens is substituted.
The contact lens has several other advantages as well,
including (1) the lens turns with the eye and gives a
broader ﬁeld of clear vision than glasses do, and (2) the
contact lens has little effect on the size of the object the
person sees through the lens, whereas lenses placed 1
centimeter or so in front of the 

 lenses placed 1
centimeter or so in front of the eye do affect the size of
the image, in addition to correcting the focus.
Cataracts
“Cataracts” are an especially common eye abnormality
that occurs mainly in older people.A cataract is a cloudy
or opaque area or areas in the lens. In the early stage of
cataract formation, the proteins in some of the lens
ﬁbers become denatured. Later, these same proteins
coagulate to form opaque areas in place of the normal
transparent protein ﬁbers.
When a cata

 the normal
When a cataract has obscured light transmission so
greatly that it seriously impairs vision, the condition can
be corrected by surgical removal of the lens. When this
is done, the eye loses a large portion of its refractive
power, which must be replaced by a powerful convex
lens in front of the eye; usually, however, an artiﬁcial
plastic lens is implanted in the eye in place of the
removed lens.
Theoretically, light from a distant point source

y
Theoretically, light from a distant point source, when
focused on the retina, should be inﬁnitely small.
However, because the lens system of the eye is never
perfect, such a retinal spot ordinarily has a total diam-
eter of about 11 micrometers, even with maximal res-
olution of the normal eye optical system. The spot is
brightest in its center and shades off gradually toward
the edges, as shown by the two-point images in 
Figure 49–16.
The average diameter of the cones in the fovea of
the ret

rage diameter of the cones in the fovea of
the retina—the central part of the retina, where vision
is most highly developed—is about 1.5 micrometers,
which is one seventh the diameter of the spot of light.
Nevertheless, because the spot of light has a bright
center point and shaded edges, a person can normally
distinguish two separate points if their centers lie as
much as 2 micrometers apart on the retina, which is
slightly greater than the width of a foveal cone. This
discrimination between po

h of a foveal cone. This
discrimination between points is also shown in Figure
49–16.
The normal visual acuity of the human eye for dis-
criminating between point sources of light is about 25
seconds of arc. That is, when light rays from two sepa-
rate points strike the eye with an angle of at least 25
seconds between them, they can usually be recognized
as two points instead of one. This means that a person
with normal visual acuity looking at two bright pin-
point spots of light 10 meters away

wo bright pin-
point spots of light 10 meters away can barely distin-
guish the spots as separate entities when they are 1.5
to 2 millimeters apart.
The fovea is less than 0.5 millimeter (less than 500
micrometers) in diameter, which means that maximum
visual acuity occurs in less than 2 degrees of the visual
ﬁeld. Outside this foveal area, the visual acuity
becomes progressively poorer, decreasing more than
10-fold as the periphery is approached. This is caused
by the connection of more and mor

. This is caused
by the connection of more and more rods and cones
to each optic nerve ﬁber in the nonfoveal,
more peripheral parts of the retina, as discussed in
Chapter 51.
Clinical Method for Stating Visual Acuity. The chart for
testing eyes usually consists of letters of different sizes
placed 20 feet away from the person being tested. If
the person can see well the letters of a size that he or
she should be able to see at 20 feet, the person is said
to have 20/20 vision—that is, normal visi

 is said
to have 20/20 vision—that is, normal vision. If the
person can see only letters that he or she should be
able to see at 200 feet, the person is said to have 20/200
vision. In other words, the clinical method for express-
ing visual acuity is to use a mathematical fraction that
expresses the ratio of two distances, which is also the
ratio of one’s visual acuity to that of a person with
normal visual acuity.
Determination of Distance 
of an Object from the Eye—
“Depth Perception”
A person

n Object from the Eye—
A person normally perceives distance by three major
means: (1) the sizes of the images of known objects on
the retina, (2) the phenomenon of moving parallax,
2 µm
17 mm 
1 mm 
10 meters 
Figure 49–16
Maximum visual acuity for two point sources of light.
622
Unit X
and (3) the phenomenon of stereopsis. This ability to
determine distance is called depth perception.
Determination of Distance by Sizes of Retinal Imag

Determination of Distance by Sizes of Retinal Images of Known
Objects. If one knows that a person being viewed is 6
feet tall, one can determine how far away the person
is simply by the size of the person’s image on the
retina. One does not consciously think about the size,
but the brain has learned to calculate automatically
from image sizes the distances of objects when the
dimensions are known.
Determination of Distance by Moving Parallax. Another
important means by which the eyes determine d

ther
important means by which the eyes determine distance
is that of moving parallax. If an individual looks off
into the distance with the eyes completely still, he or
she perceives no moving parallax, but when the person
moves his or her head to one side or the other, the
images of close-by objects move rapidly across the
retinas, while the images of distant objects remain
almost completely stationary. For instance, by moving
the head 1 inch to the side when the object is only 1
inch in front 

 the side when the object is only 1
inch in front of the eye, the image moves almost all the
way across the retinas, whereas the image of an object
200 feet away from the eyes does not move percepti-
bly. Thus, by using this mechanism of moving parallax,
one can tell the relative distances of different objects
even though only one eye is used.
Determination of Distance by Stereopsis—Binocular Vision.
Another method by which one perceives parallax is
that of “binocular vision.” Because one eye is

 is
that of “binocular vision.” Because one eye is a little
more than 2 inches to one side of the other eye, the
images on the two retinas are different from each
other. For instance, an object 1 inch in front of the nose
forms an image on the left side of the retina of the left
eye but on the right side of the retina of the right eye,
whereas a small object 20 feet in front of the nose has
its image at closely corresponding points in the centers
of the two retinas. This type of parallax is demo

of the two retinas. This type of parallax is demon-
strated in Figure 49–17, which shows the images of a
red spot and a yellow square actually reversed on the
two retinas because they are at different distances in
front of the eyes. This gives a type of parallax that is
present all the time when both eyes are being used. It
is almost entirely this binocular parallax (or stereop-
sis) that gives a person with two eyes far greater ability
to judge relative distances when objects are nearby
than a

 relative distances when objects are nearby
than a person who has only one eye. However, stere-
opsis is virtually useless for depth perception at dis-
tances beyond 50 to 200 feet.
The ophthalmoscope is an instrument through which an
observer can look into another person’s eye and see the
retina with clarity. Although the ophthalmoscope
appears to be a relatively complicated instrument, its
principles are simple. The basic components are shown
in Figure 49–18 and can be explained

nts are shown
in Figure 49–18 and can be explained as follows.
If a bright spot of light is on the retina of an
emmetropic eye, light rays from this spot diverge toward
the lens system of the eye. After passing through the
lens system, they are parallel with one another because
the retina is located one focal length distance behind 
the lens system.Then, when these parallel rays pass into
an emmetropic eye of another person, they focus again
to a point focus on the retina of the second person,
b

 point focus on the retina of the second person,
because his or her retina is also one focal length dis-
tance behind the lens. Any spot of light on the retina of
the observed eye projects to a focal spot on the retina
of the observing eye. Thus, if the retina of one person is
made to emit light, the image of his or her retina will be
focused on the retina of the observer, provided the two
eyes are emmetropic and are simply looking into each
other.
To make an ophthalmoscope, one need only devise

r.
To make an ophthalmoscope, one need only devise a
means for illuminating the retina to be examined.Then,
the reﬂected light from that retina can be seen by the
observer simply by putting the two eyes close to each
other. To illuminate the retina of the observed eye, an
angulated mirror or a segment of a prism is placed in
front of the observed eye in such a manner, as shown in
Figure 49–18, that light from a bulb is reﬂected into the
observed eye.Thus, the retina is illuminated through the
pu

eye.Thus, the retina is illuminated through the
pupil, and the observer sees into the subject’s pupil by
looking over the edge of the mirror or prism or through
an appropriately designed prism.
1. Size of image
2. Stereopsis
Object of known
distance and size
Unknown
object
Unknown
object
Figure 49–17
Perception of distance (1) by the size of the image on the retina
and (2) as a result of stereopsis.
Mirror
Corrective lens in
turret

lt of stereopsis.
Mirror
turret  (–4 diopters
for normal eyes)
Illuminate retina
showing blood
vessel
Observer’s eye
Observed eye
Collimating lens
Figure 49–18
Optical system of the ophthalmoscope.
Chapter 49
623
It is clear that these principles apply only to people
with completely emmetropic eyes. If the refractive
power of either the observed eye or the observer’s eye
is abnormal, it is necessary to correct the refractive
power for the observer 

 to correct the refractive
power for the observer to see a sharp image of the
observed retina. The usual ophthalmoscope has a series
of very small lenses mounted on a turret so that the
turret can be rotated from one lens to another until the
correction for abnormal refraction is made by selecting
a lens of appropriate strength. In normal young adults,
natural accommodative reﬂexes occur that cause an
approximate +2-diopter increase in strength of the lens
of each eye. To correct for this, it is

f the lens
of each eye. To correct for this, it is necessary that 
the lens turret be rotated to approximately –4-diopter
correction.
Fluid System of the Eye—
Intraocular Fluid
The eye is ﬁlled with intraocular ﬂuid, which maintains
sufﬁcient pressure in the eyeball to keep it distended.
Figure 49–19 demonstrates that this ﬂuid can be divided
into two portions—aqueous humor, which lies in front
of the lens, and vitreous humor, which is between the
posterior surface of the lens and the retina.The

e
posterior surface of the lens and the retina.The aqueous
humor is a freely ﬂowing ﬂuid, whereas the vitreous
humor, sometimes called the vitreous body, is a gelati-
nous mass held together by a ﬁne ﬁbrillar network com-
posed primarily of greatly elongated proteoglycan
molecules. Both water and dissolved substances can
diffuse slowly in the vitreous humor, but there is little
ﬂow of ﬂuid.
Aqueous humor is continually being formed and
reabsorbed. The balance between formation and reab-
sorption

. The balance between formation and reab-
sorption of aqueous humor regulates the total volume
and pressure of the intraocular ﬂuid.
Formation of Aqueous Humor 
by the Ciliary Body
Aqueous humor is formed in the eye at an average rate
of 2 to 3 microliters each minute. Essentially all of it is
secreted by the ciliary processes, which are linear folds
projecting from the ciliary body into the space behind
the iris where the lens ligaments and ciliary muscle
attach to the eyeball. A cross section 

ary muscle
attach to the eyeball. A cross section of these ciliary
processes is shown in Figure 49–20, and their relation to
the ﬂuid chambers of the eye can be seen in Figure
49–19. Because of their folded architecture, the total
surface area of the ciliary processes is about 6 square
centimeters in each eye—a large area, considering the
small size of the ciliary body. The surfaces of these
processes are covered by highly secretory epithelial
cells, and immediately beneath them is a highly vasc

lls, and immediately beneath them is a highly vascu-
lar area.
Aqueous humor is formed almost entirely as an active
secretion by the epithelium of the ciliary processes.
Secretion begins with active transport of sodium ions
into the spaces between the epithelial cells. The sodium
ions pull chloride and bicarbonate ions along with them
to maintain electrical neutrality. Then all these ions
together cause osmosis of water from the blood capil-
laries lying below into the same epithelial intercellu

es lying below into the same epithelial intercellular
spaces, and the resulting solution washes from the
spaces of the ciliary processes into the anterior chamber
of the eye. In addition, several nutrients are transported
across the epithelium by active transport or facilitated
diffusion; they include amino acids, ascorbic acid, and
glucose.
Outﬂow of Aqueous Humor 
from the Eye
After aqueous humor is formed by the ciliary processes,
it ﬁrst ﬂows, as shown in Figure 49–19, through the pupil
into

, as shown in Figure 49–19, through the pupil
into the anterior chamber of the eye. From here, the ﬂuid
ﬂows anterior to the lens and into the angle between the
Vitreous
humor
Aqueous humor
Iris
Flow of fluid
Lens
Formation
of aqueous
humor
Spaces of Fontana
Canal of Schlemm
Ciliary body
Diffusion of
fluid and other
constituents
Filtration and
diffusion at
retinal vessels
Figure 49–19
Formation and ﬂow of ﬂuid in the eye.
Formation
of aqueous
humor
Vascular layer
Cilia

 aqueous
humor
Ciliary processes
Figure 49–20
Anatomy of the ciliary processes. Aqueous humor is formed on
surfaces.
624
Unit X
cornea and the iris, then through a meshwork of trabec-
ulae, ﬁnally entering the canal of Schlemm, which
empties into extraocular veins. Figure 49–21 demon-
strates the anatomical structures at this iridocorneal
angle, showing that the spaces between the trabeculae
extend all the way from the anter

n the trabeculae
extend all the way from the anterior chamber to the
canal of Schlemm. The canal of Schlemm is a thin-
walled vein that extends circumferentially all the way
around the eye. Its endothelial membrane is so porous
that even large protein molecules, as well as small par-
ticulate matter up to the size of red blood cells, can pass
from the anterior chamber into the canal of Schlemm.
Even though the canal of Schlemm is actually a venous
blood vessel, so much aqueous humor normally ﬂow

s
blood vessel, so much aqueous humor normally ﬂows
into it that it is ﬁlled only with aqueous humor rather
than with blood.The small veins that lead from the canal
of Schlemm to the larger veins of the eye usually contain
only aqueous humor, and they are called aqueous veins.
The average normal intraocular pressure is about 
15 mm Hg, with a range from 12 to 20 mm Hg.
Tonometry. Because it is impractical to pass a needle into
a patient’s eye to measure intraocular pressure,

o
a patient’s eye to measure intraocular pressure, this
pressure is measured clinically by using a “tonometer,”
the principle of which is shown in Figure 49–22. The
cornea of the eye is anesthetized with a local anesthetic,
and the footplate of the tonometer is placed on the
cornea. A small force is then applied to a central
plunger, causing the part of the cornea beneath the
plunger to be displaced inward. The amount of dis-
placement is recorded on the scale of the tonometer,
and this is calib

d on the scale of the tonometer,
and this is calibrated in terms of intraocular pressure.
Regulation of Intraocular Pressure. Intraocular pressure
remains constant in the normal eye, usually within 
±2 mm Hg of its normal level, which averages about 
15 mm Hg. The level of this pressure is determined
mainly by the resistance to outﬂow of aqueous humor
This outﬂow resistance results from the meshwork of
trabeculae through which the ﬂuid must pe

hwork of
trabeculae through which the ﬂuid must percolate on its
way from the lateral angles of the anterior chamber to
the wall of the canal of Schlemm.These trabeculae have
minute openings of only 2 to 3 micrometers.The rate of
ﬂuid ﬂow into the canal increases markedly as the pres-
sure rises. At about 15 mm Hg in the normal eye, the
amount of ﬂuid leaving the eye by way of the canal of
Schlemm usually averages 2.5 ml/min and equals the
inﬂow of ﬂuid from the ciliary body. The pressure nor-
m

of ﬂuid from the ciliary body. The pressure nor-
mally remains at about this level of 15 mm Hg.
Mechanism for Cleansing the Trabecular Spaces and Intraocular
Fluid. When large amounts of debris are present in the
aqueous humor, as occurs after hemorrhage into the eye
or during intraocular infection, the debris is likely to
accumulate in the trabecular spaces leading from the
anterior chamber to the canal of Schlemm; this debris
can prevent adequate reabsorption of ﬂuid from the
anterior chamber,

te reabsorption of ﬂuid from the
anterior chamber, sometimes causing “glaucoma,” as
explained subsequently. However, on the surfaces of the
trabecular plates are large numbers of phagocytic cells.
Immediately outside the canal of Schlemm is a layer of
interstitial gel that contains large numbers of reticu-
loendothelial cells that have an extremely high capacity
for engulﬁng debris and digesting it into small molecu-
lar substances that can then be absorbed. Thus, this
phagocytic system keeps th

be absorbed. Thus, this
phagocytic system keeps the trabecular spaces cleaned.
The surface of the iris and other surfaces of the eye
behind the iris are covered with an epithelium that is
capable of phagocytizing proteins and small particles
from the aqueous humor, thereby helping to maintain a
clear ﬂuid.
“Glaucoma,” a Principal Cause of Blindness. Glaucoma is one
of the most common causes of blindness. It is a disease
of the eye in which the intraocular pressure becomes
pathologically high, so

traocular pressure becomes
pathologically high, sometimes rising acutely to 60 to 
70 mm Hg. Pressures above 25 to 30 mm Hg can cause
loss of vision when maintained for long periods.
Extremely high pressures can cause blindness within
days or even hours. As the pressure rises, the axons of
Cornea
Sclera
Trabeculae
Iris
Blood veins
Aqueous veins
Figure 49–21
Anatomy of the iridocorneal angle, showing the system for outﬂow
of aqueous humor from the eyeball into the conjunctival ve

us humor from the eyeball into the conjunctival veins.
Intraocular pressure
Pressure applied
Central plunger
Footplate
Figure 49–22
Principles of the tonometer.
Chapter 49
625
the optic nerve are compressed where they leave the
eyeball at the optic disc.This compression is believed to
block axonal ﬂow of cytoplasm from the retinal neu-
ronal cell bodies into the optic nerve ﬁbers leading to
the brain. The result is lack of appropriate nutrition of
the ﬁbers, which ev

ck of appropriate nutrition of
the ﬁbers, which eventually causes death of the involved
ﬁbers. It is possible that compression of the retinal
artery, which enters the eyeball at the optic disc, also
adds to the neuronal damage by reducing nutrition to
In most cases of glaucoma, the abnormally high pres-
sure results from increased resistance to ﬂuid outﬂow
through the trabecular spaces into the canal of Schlemm
at the iridocorneal junction. For instance, in acute 
eye inﬂammation, wh

ction. For instance, in acute 
eye inﬂammation, white blood cells and tissue debris
can block these trabecular spaces and cause an acute
increase in intraocular pressure. In chronic conditions,
especially in older individuals, ﬁbrous occlusion of the
trabecular spaces appears to be the likely culprit.
Glaucoma can sometimes be treated by placing drops
in the eye that contain a drug that diffuses into the
eyeball and reduces the secretion or increases the
absorption of aqueous humor. When drug th

ases the
absorption of aqueous humor. When drug therapy fails,
operative techniques to open the spaces of the trabec-
ulae or to make channels to allow ﬂuid to ﬂow directly
from the ﬂuid space of the eyeball into the subconjunc-
tival space outside the eyeball can often effectively
reduce the pressure.
Buisseret P: Inﬂuence of extraocular muscle proprioception
on vision. Physiol Rev 75:323, 1995.
Congdon NG, Friedman DS, Lietman T: Important causes of
visual impairment in the world to

ortant causes of
visual impairment in the world today. JAMA 290:2057,
2003.
Corbett JJ: The bedside and ofﬁce neuro-ophthalmology
examination. Semin Neurol 23:63, 2003.
Doane JF: Accommodating intraocular lenses. Curr Opin
Ophthalmol 15:16, 2004.
Farr AK, Guyton DL: Strabismus after retinal detachment
surgery. Curr Opin Ophthalmol 11:207, 2000.
Gilmartin B: Myopia: precedents for research in the twenty-
ﬁrst century. Clin Exp Ophthalmol 32:305, 2004.
Guyton DL: Sights and Sounds in Ophthalmology

004.
Guyton DL: Sights and Sounds in Ophthalmology: Ocular
Motility and Binocular Vision. St Louis: CV Mosby,
1989.
Khaw PT, Shah P, Elkington AR: Glaucoma—1: diagnosis.
BMJ 328:97, 2004.
Krag S,Andreassen TT: Mechanical properties of the human
lens capsule. Prog Retin Eye Res 22:749, 2003.
Mathias RT, Rae JL, Baldo GJ: Physiological properties of
the normal lens. Physiol Rev 77:21, 1997.
McBrien NA, Gentle A: Role of the sclera in the develop-
ment and pathological complications of myopia. Prog

ent and pathological complications of myopia. Prog
Retin Eye Res 22:307, 2003.
O’Shea J, Walsh V: Visual awareness: the eye ﬁelds have it?
Curr Biol 14:R279, 2004.
Schaeffel F, Simon P, Feldkaemper M, et al: Molecular
biology of myopia. Clin Exp Optom 86:295, 2003.
Schwartz K, Budenz D: Current management of glaucoma.
Curr Opin Ophthalmol 15:119, 2004.
Smith G: The optical properties of the crystalline lens and
their signiﬁcance. Clin Exp Optom 86:3, 2003.
Smith G, Atchison DA: The Eye and Visua

6:3, 2003.
Smith G, Atchison DA: The Eye and Visual Optical Instru-
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Lancet 363:1711, 2004.
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626
The Eye: II. Receptor and 
The retina is the light-sensitive portion of the eye
that contains (1) the cones, which are responsible
for color vision, and (2) the rods, which are mainly
responsible for black and white vision and vision in
the dark. When e

ck and white vision and vision in
the dark. When either rods or cones are excited,
signals are transmitted ﬁrst through successive
layers of neurons in the retina itself and, ﬁnally, into
optic nerve ﬁbers and the cerebral cortex. The
purpose of this chapter is to explain the mechanisms by which the rods and
cones detect light and color and convert the visual image into optic nerve
signals.
Anatomy and Function of the Structural
Elements of the Retina
Layers of the Retina. Figure 50–1 shows the 

etina
Layers of the Retina. Figure 50–1 shows the functional components of the retina which
are arranged in layers from the outside to the inside as follows: (1) pigmented layer,
(2) layer of rods and cones projecting to the pigment, (3) outer nuclear layer con-
taining the cell bodies of the rods and cones, (4) outer plexiform layer, (5) inner
nuclear layer, (6) inner plexiform layer, (7) ganglionic layer, (8) layer of optic nerve
ﬁbers, and (9) inner limiting membrane.
After light passes throu

 inner limiting membrane.
After light passes through the lens system of the eye and then through the vitre-
ous humor, it enters the retina from the inside (see Figure 50–1); that is, it passes ﬁrst
through the ganglion cells and then through the plexiform and nuclear layers before
it ﬁnally reaches the layer of rods and cones located all the way on the outer edge
of the retina. This distance is a thickness of several hundred micrometers; visual
acuity is decreased by this passage through such n

acuity is decreased by this passage through such nonhomogeneous tissue. However,
in the central foveal region of the retina, as discussed subsequently, the inside layers
are pulled aside to decrease this loss of acuity.
Foveal Region of the Retina and Its Importance in Acute Vision. The fovea is a minute area
in the center of the retina, shown in Figure 50–2, occupying a total area a little more
than 1 square millimeter; it is especially capable of acute and detailed vision. The
central fovea, o

of acute and detailed vision. The
central fovea, only 0.3 millimeter in diameter, is composed almost entirely of cones;
these cones have a special structure that aids their detection of detail in the visual
image. That is, the foveal cones have especially long and slender bodies, in con-
tradistinction to the much fatter cones located more peripherally in the retina.Also,
in the foveal region, the blood vessels, ganglion cells, inner nuclear layer of cells,
and plexiform layers are all displaced

r of cells,
and plexiform layers are all displaced to one side rather than resting directly on top
of the cones. This allows light to pass unimpeded to the cones.
Rods and Cones. Figure 50–3 is a diagrammatic representation of the essential com-
ponents of a photoreceptor (either a rod or a cone). As shown in Figure 50–4, the
outer segment of the cone is conical in shape. In general, the rods are narrower and
longer than the cones, but this is not always the case. In the peripheral portions of
t

t always the case. In the peripheral portions of
the retina, the rods are 2 to 5 micrometers in diameter, whereas the cones are 5 to
8 micrometers in diameter; in the central part of the retina, in the fovea, there are
rods, and the cones are slender and have a diameter of only 1.5 micrometers.
To the right in Figure 50–3 are labeled the major functional segments of either a
rod or a cone: (1) the outer segment, (2) the inner segment, (3) the nucleus, and (4)
the synaptic body. The light-sensiti

leus, and (4)
the synaptic body. The light-sensitive photochemical is found in the outer segment.
In the case of the rods, this is rhodopsin; in the cones, it is one of three “color”
Chapter 50
The Eye: II. Receptor and Neural Function of the Retina
627
Pigmented layer
Cone
Cone
Vertical
pathway
Lateral pathway
Rod
Outer nuclear layer
Horizontal cell
Amacrine cell
Bipolar
cell
Bipolar
cell
Amacrine
cell
Distal
Proximal
Inner nuclear layer
Inner plexiform
layer
Ganglion cell
To optic nerve
Gangli

lexiform
layer
Ganglion cell layer
Stratum opticum
Inner limiting
membrane
Outer plexiform
layer
DIRECTION OF LIGHT
Figure 50–1
Layers of retina.
Figure 50–2
Photomicrograph of the macula and of the fovea in its center. Note that the inner layers of the retina are pulled to the side to decrease
interference with light transmission. (From Fawcett DW: Bloom and Fawcett: A Textbook of Histology, 11th ed. Philadelphia: WB Saunders,
1986; courtesy H. Mizoguchi.)
photochem

 Saunders,
photochemicals, usually called simply color pigments,
that function almost exactly the same as rhodopsin
except for differences in spectral sensitivity.
Note in the outer segments of the rods and cones in
Figures 50–3 and 50–4 the large numbers of discs. Each
of the discs is actually an infolded shelf of cell mem-
brane. There are as many as 1000 discs in each rod or
cone.
Both rhodopsin and the color pigments are conju-
gated proteins. They are incorpora

ents are conju-
gated proteins. They are incorporated into the mem-
branes of the discs in the form of transmembrane
proteins. The concentrations of these photosensitive
pigments in the discs are so great that the pigments
themselves constitute about 40 per cent of the entire
mass of the outer segment.
The inner segment of the rod or cone contains the
usual cytoplasm with cytoplasmic organelles. Particu-
larly important are the mitochondria; as explained later,
these mitochondria play the import

xplained later,
these mitochondria play the important role of provid-
ing energy for function of the photoreceptors.
The synaptic body is the portion of the rod or cone
that connects with subsequent neuronal cells, the 
628
Unit X
horizontal and bipolar cells, that represent the next
stages in the vision chain.
Pigment Layer of the Retina. The black pigment melanin in
the pigment layer prevents light reﬂection throughout
the globe of the eyeball; this is

ction throughout
the globe of the eyeball; this is extremely important for
clear vision. This pigment performs the same function
in the eye as the black coloring inside the bellows of a
camera. Without it, light rays would be reﬂected in all
directions within the eyeball and would cause diffuse
lighting of the retina rather than the normal contrast
between dark and light spots required for formation of
precise images.
The importance of melanin in the pigment layer is
well illustrated by its abse

 the pigment layer is
well illustrated by its absence in albinos, people who are
hereditarily lacking in melanin pigment in all parts of
their bodies. When an albino enters a bright room, light
that impinges on the retina is reﬂected in all directions
inside the eyeball by the unpigmented surfaces of 
the retina and by the underlying sclera, so that a 
single discrete spot of light that would normally 
excite only a few rods or cones is reﬂected everywhere
and excites many receptors. Therefore, 

everywhere
and excites many receptors. Therefore, the visual acuity
of albinos, even with the best optical correction, is
seldom better than 20/100 to 20/200 rather than the
normal 20/20 values.
The pigment layer also stores large quantities of
vitamin A. This vitamin A is exchanged back and forth
through the cell membranes of the outer segments of
the rods and cones, which themselves are embedded in
the pigment. We show later that vitamin A is an impor-
tant precursor of the photosensitive chem

n impor-
tant precursor of the photosensitive chemicals of the
rods and cones.
Blood Supply of the Retina—The Central Retinal Artery and the
Choroid. The nutrient blood supply for the internal
layers of the retina is derived from the central retinal
artery, which enters the eyeball through the center of
the optic nerve and then divides to supply the entire
inside retinal surface. Thus, the inner layers of the retina
have their own blood supply independent of the other
structures of the eye.
Howe

dependent of the other
However, the outermost layer of the retina is adher-
ent to the choroid, which is also a highly vascular tissue
lying between the retina and the sclera.The outer layers
of the retina, especially the outer segments of the rods
and cones, depend mainly on diffusion from the choroid
blood vessels for their nutrition, especially for their
oxygen.
Retinal Detachment. The neural retina occasionally
detaches from the pigment epithelium. In some instances,
t

 from the pigment epithelium. In some instances,
the cause of such detachment is injury to the eyeball
that allows ﬂuid or blood to collect between the neural
retina and the pigment epithelium. Detachment is occa-
sionally caused by contracture of ﬁne collagenous ﬁbrils
in the vitreous humor, which pull areas of the retina
toward the interior of the globe.
Partly because of diffusion across the detachment gap
and partly because of the independent blood supply 
to the neural retina through the re

 blood supply 
to the neural retina through the retinal artery, the
detached retina can resist degeneration for days and can
become functional again if it is surgically replaced in its
normal relation with the pigment epithelium. If it is not
replaced soon, however, the retina will be destroyed and
will be unable to function even after surgical repair.
Both rods and cones contain chemicals that decom-
pose on exposure to light and, in the process, excite the

light and, in the process, excite the
nerve ﬁbers leading from the eye. The light-sensitive
chemical in the rods is called rhodopsin; the light-
sensitive chemicals in the cones, called cone pigments
or color pigments, have compositions only slightly dif-
ferent from that of rhodopsin.
Outer segment
Membrane shelves
lined with rhodopsin
or color pigment
Inner segment
Nucleus
Outer limiting
membrane
Synaptic body
Figure 50–3
Schematic drawing of the functional parts of the rods and c

 drawing of the functional parts of the rods and cones.
Figure 50–4
Membranous structures of the outer segments of a rod (left) and
a cone (right). (Courtesy Dr. Richard Young.)
Chapter 50
629
In this section, we discuss principally the photo-
chemistry of rhodopsin, but the same principles can be
applied to the cone pigments.
Rhodopsin-Retinal Visual Cycle, and
Rhodopsin and Its Decomposition by Light Energy. The out

sin and Its Decomposition by Light Energy. The outer
segment of the rod that projects into the pigment layer
of the retina has a concentration of about 40 per cent
of the light-sensitive pigment called rhodopsin, or
visual purple. This substance is a combination of the
protein scotopsin and the carotenoid pigment retinal
(also called “retinene”). Furthermore, the retinal is a
particular type called 11-cis retinal. This cis form of
retinal is important because only this form can bind
with scotops

rtant because only this form can bind
with scotopsin to synthesize rhodopsin.
When light energy is absorbed by rhodopsin, the
rhodopsin begins to decompose within a very small
fraction of a second, as shown at the top of Figure
50–5. The cause of this is photoactivation of electrons
in the retinal portion of the rhodopsin, which leads to
instantaneous change of the cis form of retinal into an
all-trans form that still has the same chemical structure
as the cis form but has a different physical s

ure
as the cis form but has a different physical structure—
a straight molecule rather than an angulated molecule.
Because the three-dimensional orientation of the 
reactive sites of the all-trans retinal no longer ﬁts 
with the orientation of the reactive sites on the protein 
scotopsin,
the all-trans
retinal begins to pull 
away from the scotopsin. The immediate product is
bathorhodopsin, which is a partially split combination
of the all-trans retinal and scotopsin. Bathorhodopsin
is extremely

retinal and scotopsin. Bathorhodopsin
is extremely unstable and decays in nanoseconds to
lumirhodopsin. This then decays in microseconds 
to metarhodopsin I, then in about a millisecond to
metarhodopsin II, and ﬁnally, much more slowly (in
seconds), into the completely split products scotopsin
and all-trans retinal.
It is the metarhodopsin II, also called activated
rhodopsin, that excites electrical changes in the rods,
and the rods then transmit the visual image into the
central nervous system 

 the visual image into the
central nervous system in the form of optic nerve
action potential, as we discuss later.
Re-formation of Rhodopsin. The ﬁrst stage in re-formation
of rhodopsin, as shown in Figure 50–5, is to reconvert
the all-trans retinal into 11-cis retinal. This process
requires metabolic energy and is catalyzed by the
enzyme retinal isomerase. Once the 11-cis retinal is
formed,it automatically recombines with the scotopsin
to re-form rhodopsin, which then remains stable until
its 

rm rhodopsin, which then remains stable until
its decomposition is again triggered by absorption of
light energy.
Role of Vitamin A for Formation of Rhodopsin. Note in
Figure 50–5 that there is a second chemical route by
which all-trans retinal can be converted into 11-cis
retinal.This is by conversion of the all-trans retinal ﬁrst
into all-trans retinol, which is one form of vitamin A.
Then the all-trans retinol is converted into 11-cis
retinol under the inﬂuence of the enzyme isomerase.
Finall

under the inﬂuence of the enzyme isomerase.
Finally, the 11-cis retinol is converted into 11-cis
retinal, which combines with scotopsin to form new
rhodopsin.
Vitamin A is present both in the cytoplasm of the
rods and in the pigment layer of the retina. Therefore,
vitamin A is normally always available to form new
retinal when needed. Conversely, when there is excess
retinal in the retina, it is converted back into vitamin
A, thus reducing the amount of light-sensitive pigment
in the retina. We 

ount of light-sensitive pigment
in the retina. We shall see later that this interconver-
sion between retinal and vitamin A is especially impor-
tant in long-term adaptation of the retina to different
light intensities.
Night Blindness. Night blindness occurs in any person
with severe vitamin A deﬁciency. The simple reason for
this is that without vitamin A,the amounts of retinal and
rhodopsin that can be formed are severely depressed.
This condition is called night blindness because the
amount 

tion is called night blindness because the
amount of light available at night is too little to permit
adequate vision in vitamin A–deﬁcient persons.
For night blindness to occur, a person usually must
remain on a vitamin A–deﬁcient diet for months,
because large quantities of vitamin A are normally
stored in the liver and can be made available to the eyes.
Once night blindness develops, it can sometimes be
reversed in less than 1 hour by intravenous injection of
vitamin A.
Excitation of the Rod 

ous injection of
vitamin A.
Excitation of the Rod When Rhodopsin 
Is Activated by Light
The Rod Receptor Potential Is Hyperpolarizing, Not Depolariz-
ing. When the rod is exposed to light, the resulting
Rhodopsin
Scotopsin
11-cis retinal
11-cis retinol
all-trans retinal
all-trans retinol
(Vitamin A)
Bathorhodopsin
(nsec)
Lumirhodopsin
(µsec)
Metarhodopsin I
(msec)
Metarhodopsin II
(sec)
Light energy
Isomerase
Isomerase
(p sec)
(minutes)
Figure 50–5
Rhodopsin-retinal visual cycle in the rod, show

–5
Rhodopsin-retinal visual cycle in the rod, showing decomposition
of rhodopsin during exposure to light and subsequent slow re-
formation of rhodopsin by the chemical processes.
630
Unit X
receptor potential is different from the receptor poten-
tials in almost all other sensory receptors.That is, exci-
tation of the rod causes increased negativity of the
intrarod membrane potential, which is a state of hyper-
polarization, meaning that there is more n

 hyper-
polarization, meaning that there is more negativity
than normal inside the rod membrane. This is exactly
opposite to the decreased negativity (the process 
of “depolarization”) that occurs in almost all other
sensory receptors.
But how does activation of rhodopsin cause hyper-
polarization? The answer is that when rhodopsin
decomposes, it decreases the rod membrane conduc-
tance for sodium ions in the outer segment of the rod.
This causes hyperpolarization of the entire rod mem-
brane in

 hyperpolarization of the entire rod mem-
brane in the following way.
Figure 50–6 shows movement of sodium ions in a
complete electrical circuit through the inner and outer
segments of the rod. The inner segment continually
pumps sodium from inside the rod to the outside,
thereby creating a negative potential on the inside of
the entire cell. However, the outer segment of the rod,
where the photoreceptor discs are located, is entirely
different; here, the rod membrane, in the dark state,
is very

here, the rod membrane, in the dark state,
is very leaky to sodium ions. Therefore, positively
charged sodium ions continually leak back to the
inside of the rod and thereby neutralize much of the
negativity on the inside of the entire cell. Thus, under
normal dark conditions, when the rod is not excited,
there is reduced electronegativity inside the membrane
of the rod, measuring about –40 millivolts rather than
the usual –70 to –80 millivolts found in most sensory
Then, when the rho

ound in most sensory
Then, when the rhodopsin in the outer segment 
of the rod is exposed to light, the rhodopsin begins 
to decompose, and this decreases the outer segment
membrane conductance of sodium to the interior of
the rod, even though sodium ions continue to be
pumped outward through the membrane of the inner
segment. Thus, more sodium ions now leave the rod
than leak back in. Because they are positive ions, their
loss from inside the rod creates increased negativity
inside t

side the rod creates increased negativity
inside the membrane, and the greater the amount of
light energy striking the rod, the greater the elec-
tronegativity becomes—that is, the greater is the
degree of hyperpolarization. At maximum light inten-
sity, the membrane potential approaches –70 to –80
millivolts, which is near the equilibrium potential for
potassium ions across the membrane.
Duration of the Receptor Potential, and Logarithmic
Relation of the Receptor Potential to Light Intensity.
W

on of the Receptor Potential to Light Intensity.
When a sudden pulse of light strikes the retina, the
transient hyperpolarization that occurs in the rods—
that is, the receptor potential that occurs—reaches a
peak in about 0.3 second and lasts for more than a
second. In cones, the change occurs four times as fast
as in the rods. A visual image impinged on the rods 
of the retina for only one millionth of a second can
sometimes cause the sensation of seeing the image for
longer than a second.
Ano

 of seeing the image for
Another characteristic of the receptor potential is
that it is approximately proportional to the logarithm
of the light intensity. This is exceedingly important,
because it allows the eye to discriminate light intensi-
ties through a range many thousand times as great as
would be possible otherwise.
Mechanism by Which Rhodopsin Decomposition
Decreases
Membrane Sodium Conductance—The
Excitation “Cascade.” Under optimal conditions,
a single photon of 

de.” Under optimal conditions,
a single photon of light, the smallest possible quantal
unit of light energy, can cause a measurable receptor
potential in a rod of about 1 millivolt. Only 30 photons
of light will cause half saturation of the rod. How 
can such a small amount of light cause such great 
excitation? The answer is that the photoreceptors 
have an extremely sensitive chemical cascade that
ampliﬁes the stimulatory effects about a millionfold,
1. The photon activates an elec

nfold,
1. The photon activates an electron in the 11-cis
retinal portion of the rhodopsin; this leads to the
formation of metarhodopsin II, which is the active
form of rhodopsin, as already discussed and
shown in Figure 50–5.
2. The activated rhodopsin functions as an enzyme to
activate many molecules of transducin, a protein
present in an inactive form in the membranes of
the discs and cell membrane of the rod.
3. The activated transducin activates many more
molecules of phosphodies

ducin activates many more
molecules of phosphodiesterase.
-
-
-
-
-
-
-
-
-
-
-
-
-
- -
-
-
-
-
Leakage current
decreased by
decomposing
rhodopsin
Sodium
current
Sodium
pump
Na+
Na+
Figure 50–6
Theoretical basis for generation of a “hyperpolarization receptor
potential” caused by rhodopsin decomposition, which decreases
the ﬂow of positively charged sodium ions into the outer segment
of the rod.
Chapter 50
631
4. Activated phosphodiesterase

n of the Retina
631
4. Activated phosphodiesterase is another enzyme;
it immediately hydrolyzes many molecules of
cyclic guanosine monophosphate (cGMP), thus
destroying it. Before being destroyed, the cGMP
had been bound with the sodium channel protein
of the rod’s outer membrane in a way that
“splints” it in the open state. But in light, when
phosphodiesterase hydrolyzes the cGMP, this
removes the splinting and allows the sodium
channels to close. Several hundred channels 
close for each origin

e. Several hundred channels 
close for each originally activated molecule of
rhodopsin. Because the sodium ﬂux through 
each of these channels has been extremely rapid,
ﬂow of more than a million sodium ions is
blocked by the channel closure before the 
channel opens again. This diminution of sodium
ion ﬂow is what excites the rod, as already
discussed.
5. Within about a second, another enzyme,
rhodopsin kinase, which is always present in 
the rod, inactivates the activated rhodopsin 
(the metar

d, inactivates the activated rhodopsin 
(the metarhodopsin II), and the entire cascade
reverses back to the normal state with open
sodium channels.
Thus, the rods have developed an important chemi-
cal cascade that ampliﬁes the effect of a single photon
of light to cause movement of millions of sodium ions.
This explains the extreme sensitivity of the rods under
dark conditions.
The cones are about 30 to 300 times less sensitive
than the rods, but even this allows color vision at 
any intensity 

t even this allows color vision at 
any intensity of light greater than extremely dim 
twilight.
Photochemistry of Color Vision by the Cones
It was pointed out at the outset of this discussion that
the photochemicals in the cones have almost exactly
the same chemical composition as that of rhodopsin 
in the rods.The only difference is that the protein por-
tions, or the opsins—called photopsins in the cones—
are slightly different from the scotopsin of the rods.
The retinal portion of all the vi

sin of the rods.
The retinal portion of all the visual pigments is exactly
the same in the cones as in the rods. The color-
sensitive pigments of the cones, therefore, are combi-
nations of retinal and photopsins.
In the discussion of color vision later in the chapter,
it will become evident that only one of three types of
color pigments is present in each of the different cones,
thus making the cones selectively sensitive to different
colors: blue, green, or red. These color pigments 
are calle

ue, green, or red. These color pigments 
are called, respectively, blue-sensitive pigment, green-
sensitive pigment, and red-sensitive pigment. The
absorption characteristics of the pigments in the three
types of cones show peak absorbencies at light wave-
lengths of 445, 535, and 570 nanometers, respectively.
These are also the wavelengths for peak light sensitiv-
ity for each type of cone, which begins to explain how
the retina differentiates the colors. The approximate
absorption curves for t

he colors. The approximate
absorption curves for these three pigments are shown
in Figure 50–7. Also shown is the absorption curve 
for the rhodopsin of the rods, with a peak at 505
Automatic Regulation of 
Retinal Sensitivity—Light and 
Dark Adaptation
Light and Dark Adaptation. If a person has been in bright
light for hours, large portions of the photochemicals in
both the rods and the cones will have been reduced to
retinal and opsins. Furthermore, much of the retinal of
both the 

ins. Furthermore, much of the retinal of
both the rods and the cones will have been converted
into vitamin A. Because of these two effects, the con-
centrations of the photosensitive chemicals remaining
in the rods and cones are considerably reduced, and
the sensitivity of the eye to light is correspondingly
reduced. This is called light adaptation.
Conversely, if a person remains in darkness for a
long time, the retinal and opsins in the rods and cones
are converted back into the light-sensitiv

d cones
are converted back into the light-sensitive pigments.
Furthermore, vitamin A is converted back into retinal
to give still more light-sensitive pigments, the ﬁnal
limit being determined by the amount of opsins in the
rods and cones to combine with the retinal. This is
called dark adaptation.
Figure 50–8 shows the course of dark adaptation
when a person is exposed to total darkness after
having been exposed to bright light for several hours.
Note that the sensitivity of the retina is very 

.
Note that the sensitivity of the retina is very low on
ﬁrst entering the darkness, but within 1 minute, the
sensitivity has already increased 10-fold—that is, the
retina can respond to light of one tenth the previously
required intensity. At the end of 20 minutes, the sensi-
tivity has increased about 6000-fold, and at the end of
40 minutes, about 25,000-fold.
400
Blue
cone
Green
cone
Red
cone
Rods
500
Violet
Blue
Green
Yellow
Red
Orange
600
700
Wavelength (nanometers)
Light absorption
(per ce

0
(per cent of maximum)
0
25
50
75
100
Figure 50–7
Light absorption by the pigment of the rods and by the pigments
of the three color-receptive cones of the human retina. (Drawn
from curves recorded by Marks WB, Dobelle WH, MacNichol EF
Jr: Visual pigments of single primate cones. Science 143:1181,
1964, and by Brown PK, Wald G: Visual pigments in single rods
and cones of the human retina: direct measurements reveal mech-
anisms of human night and color v

nts reveal mech-
anisms of human night and color vision. Science 144:45, 1964. 
“ 1964 by the American Association for the Advancement of
Science.)
632
Unit X
The resulting curve of Figure 50–8 is called the dark
adaptation curve. Note, however, the inﬂection in the
curve.The early portion of the curve is caused by adap-
tation of the cones, because all the chemical events of
vision, including adaptation, occur about four times as
rapidly in cones as in 

 occur about four times as
rapidly in cones as in rods. However, the cones do not
achieve anywhere near the same degree of sensitivity
change in darkness as the rods do. Therefore, despite
rapid adaptation, the cones cease adapting after only
a few minutes, while the slowly adapting rods continue
to adapt for many minutes and even hours, their sen-
sitivity increasing tremendously. In addition, still more
sensitivity of the rods is caused by neuronal signal con-
vergence of 100 or more rods onto

onal signal con-
vergence of 100 or more rods onto a single ganglion
cell in the retina; these rods summate to increase their
sensitivity, as discussed later in the chapter.
Other Mechanisms of Light and Dark Adaptation. In addition 
to adaptation caused by changes in concentrations of
rhodopsin or color photochemicals, the eye has two
other mechanisms for light and dark adaptation. The
ﬁrst of these is a change in pupillary size, as discussed
in Chapter 49. This can cause adaptation of approxi-

 Chapter 49. This can cause adaptation of approxi-
mately 30-fold within a fraction of a second, because of
changes in the amount of light allowed through the
pupillary opening.
The other mechanism is neural adaptation, involving
the neurons in the successive stages of the visual chain
in the retina itself and in the brain. That is, when light
intensity ﬁrst increases, the signals transmitted by the
bipolar cells, horizontal cells, amacrine cells, and gan-
glion cells are all intense. However, m

, and gan-
glion cells are all intense. However, most of these signals
decrease rapidly at different stages of transmission in
the neural circuit. Although the degree of adaptation is
only a fewfold rather than the many thousandfold that
occurs during adaptation of the photochemical system,
neural adaptation occurs in a fraction of a second, in
contrast to the many minutes to hours required for full
adaptation by the photochemicals.
Value of Light and Dark Adaptation in Vision. Between the
limit

t and Dark Adaptation in Vision. Between the
limits of maximal dark adaptation and maximal light
adaptation, the eye can change its sensitivity to light
as much as 500,000 to 1 million times, the sensitivity
automatically adjusting to changes in illumination.
Because registration of images by the retina
requires detection of both dark and light spots in the
image, it is essential that the sensitivity of the retina
always be adjusted so that the receptors respond to the
lighter areas but not to t

eceptors respond to the
lighter areas but not to the darker areas. An example
of maladjustment of retinal adaptation occurs when a
person leaves a movie theater and enters the bright
sunlight.Then, even the dark spots in the images seem
exceedingly bright, and as a consequence, the entire
visual image is bleached, having little contrast among
its different parts. This is poor vision, and it remains
poor until the retina has adapted sufﬁciently so that
the darker areas of the image no longer stim

 that
the darker areas of the image no longer stimulate the
receptors excessively.
Conversely, when a person ﬁrst enters darkness, the
sensitivity of the retina is usually so slight that even
the light spots in the image cannot excite the retina.
After dark adaptation, the light spots begin to regis-
ter. As an example of the extremes of light and dark
adaptation, the intensity of sunlight is about 10 billion
times that of starlight, yet the eye can function both in
bright sunlight after light a

can function both in
bright sunlight after light adaptation and in starlight
after dark adaptation.
From the preceding sections, we have learned that dif-
ferent cones are sensitive to different colors of light.
This section is a discussion of the mechanisms by
which the retina detects the different gradations of
color in the visual spectrum.
Tricolor Mechanism of Color
Detection
All theories of color vision are based on the well-
known observation that the human eye can detect
almo

own observation that the human eye can detect
almost all gradations of colors when only red, green,
and blue monochromatic lights are appropriately
mixed in different combinations.
Spectral Sensitivities of the Three Types of Cones. On the
basis of color vision tests, the spectral sensitivities of
the three types of cones in humans have proved to be
essentially the same as the light absorption curves for
the three types of pigment found in the cones. These
curves are shown in Figure 50–7 and sli

nes. These
curves are shown in Figure 50–7 and slightly differ-
ently in Figure 50–9.They can explain most of the phe-
nomena of color vision.
Interpretation of Color in the Nervous System. Refer-
ring to Figure 50–9, one can see that an orange 
0
10
20
30
40
50
Minutes in dark
Retinal sensitivity
1
10
100
1000
10,000
100,000
Rod adaptation
Cone adaptation
Figure 50–8
Dark adaptation, demonstrating the relation of cone adaptation to
rod adaptation.
Chapter 50
The Eye: II. Receptor and Neural Fun

n.
Chapter 50
633
monochromatic light with a wavelength of 580
nanometers stimulates the red cones to a stimulus
value of about 99 (99 per cent of the peak stimulation
at optimum wavelength); it stimulates the green cones
to a stimulus value of about 42, but the blue cones not
at all. Thus, the ratios of stimulation of the three types
of cones in this instance are 99:42:0. The nervous
system interprets this set of ratios as the sensation of

 interprets this set of ratios as the sensation of
orange. Conversely, a monochromatic blue light with a
wavelength of 450 nanometers stimulates the red
cones to a stimulus value of 0, the green cones to a
value of 0, and the blue cones to a value of 97.This set
of ratios—0:0:97—is interpreted by the nervous
system as blue. Likewise, ratios of 83:83:0 are inter-
preted as yellow, and 31:67:36 as green.
Perception of White Light. About equal stimulation of
all the red, green, and blue cones gives

lation of
all the red, green, and blue cones gives one the sensa-
tion of seeing white. Yet there is no single wavelength
of light corresponding to white;instead, white is a com-
bination of all the wavelengths of the spectrum. Fur-
thermore, the perception of white can be achieved by
stimulating the retina with a proper combination of
only three chosen colors that stimulate the respective
types of cones about equally.
Red-Green Color Blindness. When a single group of color-
rece

olor Blindness. When a single group of color-
receptive cones is missing from the eye, the person is
unable to distinguish some colors from others. For
instance, one can see in Figure 50–9 that green, yellow,
orange, and red colors, which are the colors between the
wavelengths of 525 and 675 nanometers, are normally
distinguished from one another by the red and green
cones. If either of these two cones is missing, the person
cannot use this mechanism for distinguishing these four
colors; the per

nism for distinguishing these four
colors; the person is especially unable to distinguish red
from green and is therefore said to have red-green color
blindness.
A person with loss of red cones is called a protanope;
the overall visual spectrum is noticeably shortened at
the long wavelength end because of a lack of the red
cones. A color-blind person who lacks green cones is
called a deuteranope; this person has a perfectly normal
visual spectral width because red cones are available to
detect t

 width because red cones are available to
detect the long wavelength red color.
Red-green color blindness is a genetic disorder that
occurs almost exclusively in males. That is, genes in the
female X chromosome code for the respective cones.
Yet color blindness almost never occurs in females
because at least one of the two X chromosomes almost
always has a normal gene for each type of cone. Because
the male has only one X chromosome, a missing gene
can lead to color blindness.
Because the X chro

ne
Because the X chromosome in the male is always
inherited from the mother, never from the father, color
blindness is passed from mother to son, and the mother
is said to be a color blindness carrier; this is true in about
8 per cent of all women.
Blue Weakness. Only rarely are blue cones missing,
although sometimes they are underrepresented, which
is a genetically inherited state giving rise to the phe-
nomenon called blue weakness.
Color Test Charts. A rapid metho

ed blue weakness.
Color Test Charts. A rapid method for determining color
blindness is based on the use of spot charts such as those
shown in Figure 50–10. These charts are arranged with
a confusion of spots of several different colors. In the
top chart, the person with normal color vision reads
“74,” whereas the red-green color-blind person reads
“21.” In the bottom chart, the person with normal color
vision reads “42,” whereas the red-blind person reads
“2,” and the green-blind person reads “4

son reads
“2,” and the green-blind person reads “4.”
If one studies these charts while at the same time
observing the spectral sensitivity curves of the different
cones depicted in Figure 50–9, it can be readily under-
stood how excessive emphasis can be placed on spots of
certain colors by color-blind people.
Figure 50–1 shows the tremendous complexity of
neural organization in the retina. To simplify this,
Figure 50–11 presents the e

ina. To simplify this,
Figure 50–11 presents the essentials of the retina’s
neural connections, showing at the left the circuit in
the peripheral retina and at the right the circuit in the
foveal retina. The different neuronal cell types are as
follows:
1. The photoreceptors themselves—the rods and
cones—which transmit signals to the outer
plexiform layer, where they synapse with bipolar
cells and horizontal cells
2. The horizontal cells, which transmit signals
horizontally in the outer plexifor

ransmit signals
horizontally in the outer plexiform layer from the
rods and cones to bipolar cells
3. The bipolar cells, which transmit signals vertically
from the rods, cones, and horizontal cells to the
inner plexiform layer, where they synapse with
ganglion cells and amacrine cells
400
Blue
cone
Green
cone
Red
cone
500
Violet
Blue
Green
Yellow
Red
Orange
600
700
Blue
Green
Yellow
Orange
0
25
50
67
36
42
97
99
83
83
31
0
75
100
Fig

range
0
25
50
67
36
42
97
99
83
83
31
0
75
100
Figure 50–9
Demonstration of the degree of stimulation of the different color-
sensitive cones by monochromatic lights of four colors: blue,
green, yellow, and orange.
634
Unit X
4. The amacrine cells, which transmit signals in two
directions, either directly from bipolar cells to
ganglion cells or horizontally within the inner
plexiform layer from axons of the bipolar cells to
dendrites of the ganglion cell

he bipolar cells to
dendrites of the ganglion cells or to other
amacrine cells
5. The ganglion cells, which transmit output signals
from the retina through the optic nerve into the
brain
A sixth type of neuronal cell in the retina, not very
prominent and not shown in the ﬁgure, is the inter-
plexiform cell. This cell transmits signals in the retro-
grade direction from the inner plexiform layer to the
outer plexiform layer.These signals are inhibitory and
are believed to control lateral spread o

itory and
are believed to control lateral spread of visual signals
by the horizontal cells in the outer plexiform layer.
Their role may be to help control the degree of con-
trast in the visual image.
Figure 50–10
Two Ishihara charts. Upper: In this chart, the normal person
reads “74,” but the red-green color-blind person reads “21.”
Lower: In this chart, the red-blind person (protanope) reads
“2,” but the green-blind person (deuteranope) reads “4.” The
normal person reads “42.” (Reproduced from

4.” The
normal person reads “42.” (Reproduced from Ishihara’s Tests
for Colour Blindness. Tokyo: Kanehara & Co., but tests for
color blindness cannot be conducted with this material. For
accurate testing, the original plates should be used.)
Chapter 50
635
The Visual Pathway from the Cones to the Ganglion Cells Func-
tions Differently from the Rod Pathway. As is true for many
of our other sensory systems, the retina has both an
old type of 

nsory systems, the retina has both an
old type of vision based on rod vision and a new type
of vision based on cone vision. The neurons and nerve
ﬁbers that conduct the visual signals for cone vision
are considerably larger than those that conduct the
visual signals for rod vision, and the signals are con-
ducted to the brain two to ﬁve times as rapidly. Also,
the circuitry for the two systems is slightly different, as
follows.
To the right in Figure 50–11 is the visual pathway
from the foveal p

gure 50–11 is the visual pathway
from the foveal portion of the retina, representing the
new, fast cone system. This shows three neurons in 
the direct pathway: (1) cones, (2) bipolar cells, and (3)
ganglion cells. In addition, horizontal cells transmit
inhibitory signals laterally in the outer plexiform layer,
and amacrine cells transmit signals laterally in the
inner plexiform layer.
To the left in Figure 50–11 are the neural connec-
tions for the peripheral retina, where both rods and
cones a

the peripheral retina, where both rods and
cones are present. Three bipolar cells are shown; the
middle of these connects only to rods, representing the
type of visual system present in many lower animals.
The output from the bipolar cell passes only to
amacrine cells, which relay the signals to the ganglion
cells. Thus, for pure rod vision, there are four neurons
in the direct visual pathway: (1) rods, (2) bipolar 
cells, (3) amacrine cells, and (4) ganglion cells.
Also, horizontal and amacrine

 (4) ganglion cells.
Also, horizontal and amacrine cells provide lateral
connectivity.
The other two bipolar cells shown in the peripheral
retinal circuitry of Figure 50–11 connect with both rods
and cones; the outputs of these bipolar cells pass both
directly to ganglion cells and by way of amacrine cells.
Neurotransmitters Released by Retinal Neurons. Not all the
neurotransmitter chemical substances used for synap-
tic transmission in the retina have been entirely delin-
eated. However, both t

a have been entirely delin-
eated. However, both the rods and the cones release
glutamate at their synapses with the bipolar cells.
Histological and pharmacological studies have shown
there to be many types of amacrine cells secreting at
least eight types of transmitter substances, including
gamma-aminobutyric acid, glycine, dopamine, acetyl-
choline, and indolamine, all of which normally function
as inhibitory transmitters. The transmitters of the
bipolar,
horizontal,
and interplexiform cells a

he
bipolar,
and interplexiform cells are
unclear, but at least some of the horizontal cells
release inhibitory transmitters.
Transmission of Most Signals Occurs in the Retinal Neurons by
Electrotonic Conduction, Not by Action Potentials. The only
retinal neurons that always transmit visual signals by
means of action potentials are the ganglion cells, and
they send their signals all the way to the brain through
the optic nerve. Occasionally, action potentials have
also been recorded i

nally, action potentials have
also been recorded in amacrine cells, although the
importance of these action potentials is questionable.
Otherwise, all the retinal neurons conduct their visual
signals by electrotonic conduction, which can be
explained as follows.
Electrotonic conduction means direct ﬂow of elec-
tric current, not action potentials, in the neuronal cyto-
plasm and nerve axons from the point of excitation all
the way to the output synapses. Even in the rods and
cones, conduction fr

ynapses. Even in the rods and
cones, conduction from their outer segments, where
the visual signals are generated, to the synaptic bodies
is by electrotonic conduction. That is, when hyperpo-
larization occurs in response to light in the outer
segment of a rod or a cone, almost the same degree of
hyperpolarization is conducted by direct electric
current ﬂow in the cytoplasm all the way to the synap-
tic body, and no action potential is required. Then,
when the transmitter from a rod or cone stim

Then,
when the transmitter from a rod or cone stimulates a
bipolar cell or horizontal cell, once again the signal is
transmitted from the input to the output by direct elec-
tric current ﬂow, not by action potentials.
The importance of electrotonic conduction is that it
allows graded conduction of signal strength. Thus, for
the rods and cones, the strength of the hyperpolariz-
ing output signal is directly related to the intensity of
illumination; the signal is not all or none, as would be
the c

; the signal is not all or none, as would be
the case for each action potential.
Lateral Inhibition to Enhance Visual Contrast—
Function of the Horizontal Cells
The horizontal cells, shown in Figure 50–11, connect
laterally between the synaptic bodies of the rods and
cones, as well as connecting with the dendrites of the
bipolar cells. The outputs of the horizontal cells are
always inhibitory. Therefore, this lateral connection
provides the same phenomenon of lateral inhibition
that is important

phenomenon of lateral inhibition
that is important in all other sensory systems—that is,
helping to ensure transmission of visual patterns with
proper visual contrast. This phenomenon is demon-
strated in Figure 50–12, which shows a minute spot of
light focused on the retina. The visual pathway from
Rods
Rod nuclei
Bipolar
cells
cells
Amacrine
cells
Ganglion
cells
Cones
Pigment layer
Figure 50–11
Neural organization of the retina: peripheral area to the left, foveal
area to the right.

pheral area to the left, foveal
636
Unit X
the centralmost area where the light strikes is excited,
whereas an area to the side is inhibited. In other words,
instead of the excitatory signal spreading widely in the
retina because of spreading dendritic and axonal trees
in the plexiform layers, transmission through the hor-
izontal cells puts a stop to this by providing lateral
inhibition in the surrounding areas. This is essential to
a

n in the surrounding areas. This is essential to
allow high visual accuracy in transmitting contrast
borders in the visual image.
Some of the amacrine cells probably provide addi-
tional lateral inhibition and further enhancement of
visual contrast in the inner plexiform layer of the
retina as well.
Excitation of Some Bipolar Cells and 
Inhibition of Others—The Depolarizing 
and Hyperpolarizing Bipolar Cells
Two types of bipolar cells provide opposing excitatory
and inhibitory signals in the vis

osing excitatory
and inhibitory signals in the visual pathway: (1) the
depolarizing bipolar cell and (2) the hyperpolarizing
bipolar cell. That is, some bipolar cells depolarize 
when the rods and cones are excited, and others 
hyperpolarize.
There are two possible explanations for this differ-
ence. One explanation is that the two bipolar cells are
of entirely different types—one responding by depo-
larizing in response to the glutamate neurotransmitter
released by the rods and cones, and the o

nsmitter
released by the rods and cones, and the other respond-
ing by hyperpolarizing.The other possibility is that one
of the bipolar cells receives direct excitation from the
rods and cones, whereas the other receives its signal
indirectly through a horizontal cell. Because the hori-
zontal cell is an inhibitory cell, this would reverse the
polarity of the electrical response.
Regardless of the mechanism for the two types of
bipolar responses, the importance of this phenomenon
is that it allo

 the importance of this phenomenon
is that it allows half the bipolar cells to transmit 
positive signals and the other half to transmit negative
signals. We shall see later that both positive and nega-
tive signals are used in transmitting visual information
Another important aspect of this reciprocal relation
between depolarizing and hyperpolarizing bipolar
cells is that it provides a second mechanism for 
lateral inhibition, in addition to the horizontal cell
mechanism. Because 

ddition to the horizontal cell
mechanism. Because depolarizing and hyperpolarizing
bipolar cells lie immediately against each other, this
provides a mechanism for separating contrast borders
in the visual image, even when the border lies exactly
between two adjacent photoreceptors. In contrast, the
horizontal cell mechanism for lateral inhibition oper-
ates over a much greater distance.
Amacrine Cells and Their Functions
About 30 types of amacrine cells have been identiﬁed
by morphological or hi

e cells have been identiﬁed
by morphological or histochemical means. The func-
tions of about half a dozen types of amacrine cells have
been characterized, and all of them are different. One
type of amacrine cell is part of the direct pathway 
for rod vision—that is, from rod to bipolar cells to
amacrine cells to ganglion cells.
Another type of amacrine cell responds strongly at
the onset of a continuing visual signal,but the response
dies rapidly.
Other amacrine cells respond strongly at the of

y.
Other amacrine cells respond strongly at the offset
of visual signals, but again, the response dies quickly.
Still other amacrine cells respond when a light is
turned either on or off, signaling simply a change in
illumination, irrespective of direction.
Still another type of amacrine cell responds to
movement of a spot across the retina in a speciﬁc
direction; therefore, these amacrine cells are said to be
directional sensitive.
In a sense, then, many or most amacrine cells are
interneurons 

hen, many or most amacrine cells are
interneurons that help analyze visual signals before
they ever leave the retina.
Each retina contains about 100 million rods and 3
million cones; yet the number of ganglion cells is only
about 1.6 million. Thus, an average of 60 rods and 2
cones converge on each ganglion cell and the optic
nerve ﬁber leading from the ganglion cell to the brain.
However, major differences exist between the
peripheral retina and the central

xist between the
peripheral retina and the central retina. As one
approaches the fovea, fewer rods and cones converge
on each optic ﬁber, and the rods and cones also
become more slender. These effects progressively
increase the acuity of vision in the central retina. In
the center, in the central fovea, there are only slender
cones—about 35,000 of them—and no rods. Also, the
number of optic nerve ﬁbers leading from this part of
the retina is almost exactly equal to the number of
cones, as shown 

st exactly equal to the number of
cones, as shown to the right in Figure 50–11. This
explains the high degree of visual acuity in the central
retina in comparison with the much poorer acuity
peripherally.
Light beam
Neither excited
nor inhibited
Excited area
Inhibited area
Figure 50–12
Excitation and inhibition of a retinal area caused by a small beam
of light, demonstrating the principle of lateral inhibition.
Chapter 50
637
Another differ

d Neural Function of the Retina
637
Another difference between the peripheral and
central portions of the retina is the much greater sen-
sitivity of the peripheral retina to weak light. This
results partly from the fact that rods are 30 to 300
times more sensitive to light than cones are, but it is
further magniﬁed by the fact that as many as 200 rods
converge on a single optic nerve ﬁber in the more
peripheral portions of the retina, so that signals from
the rods summate to give even more inte

gnals from
the rods summate to give even more intense stimula-
tion of the peripheral ganglion cells and their optic
nerve ﬁbers.
Three Types of Retinal Ganglion Cells and
Their Respective Fields
There are three distinct types of ganglion cells, desig-
nated W, X, and Y cells. Each of these serves a differ-
ent function.
Transmission of Rod Vision by the W Cells. The W cells, con-
stituting about 40 per cent of all the ganglion cells, are
small, having a diameter less than 10 micrometers, and
th

having a diameter less than 10 micrometers, and
they transmit signals in their optic nerve ﬁbers at the
slow velocity of only 8 m/sec. These ganglion cells
receive most of their excitation from rods, transmitted
by way of small bipolar cells and amacrine cells. They
have broad ﬁelds in the peripheral retina because the
dendrites of the ganglion cells spread widely in the
inner plexiform layer, receiving signals from broad
areas.
On the basis of histology as well as physiologic
experiments, the W

istology as well as physiologic
experiments, the W cells seem to be especially sensi-
tive for detecting directional movement in the ﬁeld of
vision, and they are probably important for much of
our crude rod vision under dark conditions.
Transmission of the Visual Image and Color by the X Cells. The
most numerous of the ganglion cells are the X cells,
representing 55 per cent of the total. They are of
medium diameter, between 10 and 15 micrometers,
and transmit signals in their optic nerve ﬁbers 

,
and transmit signals in their optic nerve ﬁbers at about
14 m/sec.
The X cells have small ﬁelds because their dendrites
do not spread widely in the retina. Because of this,
their signals represent discrete retinal locations.There-
fore, it is mainly through the X cells that the ﬁne
details of the visual image are transmitted. Also,
because every X cell receives input from at least one
cone, X cell transmission is probably responsible for
all color vision.
Function of the Y Cells to Transmit In

lor vision.
Function of the Y Cells to Transmit Instantaneous Changes in
the Visual Image. The Y cells are the largest of all, up to
35 micrometers in diameter, and they transmit their
signals to the brain at 50 m/sec or faster. They are the
least numerous of all the ganglion cells, representing
only 5 per cent of the total.Also, they have broad den-
dritic ﬁelds, so that signals are picked up by these cells
from widespread retinal areas.
The Y ganglion cells respond, like many of the
amacrine c

anglion cells respond, like many of the
amacrine cells, to rapid changes in the visual image—
either rapid movement or rapid change in light inten-
sity—sending bursts of signals for only small fractions
of a second. These ganglion cells presumably apprise
the central nervous system almost instantaneously
when a new visual event occurs anywhere in the visual
ﬁeld, but without specifying with great accuracy the
location of the event, other than to give appropriate
clues that make the eyes move to

 give appropriate
clues that make the eyes move toward the exciting
vision.
Spontaneous, Continuous Action Potentials in the Ganglion
Cells. It is from the ganglion cells that the long ﬁbers
of the optic nerve lead into the brain. Because of the
distance involved, the electrotonic method of conduc-
tion employed in the rods, cones, and bipolar cells
within the retina is no longer appropriate; therefore,
ganglion cells transmit their signals by means of repet-
iti

ells transmit their signals by means of repet-
itive action potentials instead. Furthermore, even
when unstimulated, they still transmit continuous im-
pulses at rates varying between 5 and 40 per second.
The visual signals, in turn, are superimposed onto this
background ganglion cell ﬁring.
Transmission of Changes in Light Intensity—The On-Off
Response. As noted previously, many ganglion cells are
speciﬁcally excited by changes in light intensity.This is
demonstrated by the records of nerve imp

y.This is
demonstrated by the records of nerve impulses in
Figure 50–13. The upper panel shows rapid impulses
for a fraction of a second when a light is ﬁrst turned
on, but decreasing rapidity in the next fraction of a
second. The lower tracing is from a ganglion cell
located lateral to the spot of light; this cell is markedly
inhibited when the light is turned on because of lateral
inhibition. Then, when the light is turned off, opposite
effects occur. Thus, these records are called “on-off”
an

 occur. Thus, these records are called “on-off”
and “off-on” responses. The opposite directions of
these responses to light are caused, respectively, by the
depolarizing and hyperpolarizing bipolar cells, and the
transient nature of the responses is probably at least
partly generated by the amacrine cells, many of which
have similar transient responses themselves.
This capability of the eyes to detect change in light
intensity is strongly developed in both the peripheral
on
1
2
off
La

ed in both the peripheral
on
1
2
off
Lateral inhibition
Figure 50–13
Responses of a ganglion cell to light in (1) an area excited by a
spot of light and (2) an area adjacent to the excited spot; the gan-
glion cell in this area is inhibited by the mechanism of lateral 
inhibition. (Modiﬁed from Granit R: Receptors and Sensory 
Perception: A Discussion of Aims, Means, and Results of Electro-
physiological Research into the Process of Reception. New
Haven, Conn: Yale University Press, 1

ception. New
Haven, Conn: Yale University Press, 1955.)
638
Unit X
retina and the central retina. For instance, a minute
gnat ﬂying across the ﬁeld of vision is instantaneously
detected. Conversely, the same gnat sitting quietly
remains below the threshold of visual detection.
Transmission of Signals Depicting Contrasts 
in the Visual Scene—The Role of 
Lateral Inhibition
Many ganglion cells respond mainly to contrast
borders in the scene. Because this s

y to contrast
borders in the scene. Because this seems to be the
major means by which the pattern of a scene is trans-
mitted to the brain, let us explain how this process
occurs.
When ﬂat light is applied to the entire retina—that
is, when all the photoreceptors are stimulated equally
by the incident light—the contrast type of ganglion cell
is neither stimulated nor inhibited. The reason for this
is that signals transmitted directly from the photore-
ceptors through depolarizing bipolar cells a

tore-
ceptors through depolarizing bipolar cells are excita-
tory, while the signals transmitted laterally through
hyperpolarizing bipolar cells as well as through 
horizontal cells are mainly inhibitory. Thus, the direct
excitatory signal through one pathway is likely to be
neutralized by inhibitory signals through lateral path-
ways. One circuit for this is demonstrated in Figure
50–14, which shows at the top three photoreceptors.
The central receptor excites a depolarizing bipolar
cell. The t

eceptor excites a depolarizing bipolar
cell. The two receptors on each side are connected 
to the same bipolar cell through inhibitory horizontal
cells that neutralize the direct excitatory signal if 
all three receptors are stimulated simultaneously by
light.
Now, let us examine what happens when a contrast
border occurs in the visual scene. Referring again to
Figure 50–14, assume that the central photoreceptor is
stimulated by a bright spot of light while one of the
two lateral receptors is in

light while one of the
two lateral receptors is in the dark. The bright spot of
light excites the direct pathway through the bipolar
cell. The fact that one of the lateral photoreceptors is
in the dark causes one of the horizontal cells to remain
unstimulated. Therefore, this cell does not inhibit 
the bipolar cell, and this allows extra excitation of the
bipolar cell. Thus, where visual contrasts occur, the
signals through the direct and lateral pathways accen-
tuate one another.
In summary, th

 pathways accen-
In summary, the mechanism of lateral inhibition
functions in the eye in the same way that it functions
in most other sensory systems—to provide contrast
detection and enhancement.
Transmission of Color Signals by the 
Ganglion Cells
A single ganglion cell may be stimulated by several
cones or by only a few.When all three types of cones—
the red, blue, and green types—stimulate the same
ganglion cell, the signal transmitted through the gan-
glion cell is the sa

 transmitted through the gan-
glion cell is the same for any color of the spectrum.
Therefore, the signal from the ganglion cell plays no
role in the detection of different colors. Instead, it is a
“white” signal.
Conversely, some of the ganglion cells are excited by
only one color type of cone but inhibited by a second
type. For instance, this frequently occurs for the red
and green cones, with red causing excitation and green
causing inhibition, or vice versa.
The same type of reciprocal effec

, or vice versa.
The same type of reciprocal effect occurs between
blue cones on the one hand and a combination 
of red and green cones (both of which are excited 
by yellow) on the other hand, giving a reciprocal 
excitation-inhibition relation between the blue and
yellow colors.
The mechanism of this opposing effect of colors is
the following: One color type of cone excites the gan-
glion cell by the direct excitatory route through a
depolarizing bipolar cell, whereas the other color type
inhi

ng bipolar cell, whereas the other color type
inhibits the ganglion cell by the indirect inhibitory
route through a hyperpolarizing bipolar cell.
The importance of these color-contrast mechanisms
is that they represent a means by which the retina itself
begins to differentiate colors.Thus, each color-contrast
type of ganglion cell is excited by one color but inhib-
ited by the “opponent” color.Therefore, color analysis
begins in the retina and is not entirely a function of
the brain.

 not entirely a function of
the brain.
H
H
B
G
Figure 50–14
Typical arrangement of rods, horizontal cells (H), a bipolar cell (B),
and a ganglion cell (G) in the retina, showing excitation at the
synapses between the rods and the bipolar cell and horizontal
cells, but inhibition from the horizontal cells to the bipolar cell.
Chapter 50
639
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5
1
640
The Eye: III. Central
Figure 51–1 shows the principal visual pathways
from the two retinas to the visual cortex. The visual
nerve signals leave the retinas through the op

ual
nerve signals leave the retinas through the optic
nerves. At the optic chiasm, the optic nerve ﬁbers
from the nasal halves of the retinas cross to the
opposite sides, where they join the ﬁbers from the
opposite temporal retinas to form the optic tracts. The ﬁbers of each optic tract
then synapse in the dorsal lateral geniculate nucleus of the thalamus, and from
there, geniculocalcarine ﬁbers pass by way of the optic radiation (also called the
geniculocalcarine tract) to the primary visual co

geniculocalcarine tract) to the primary visual cortex in the calcarine ﬁssure area
of the medial occipital lobe.
Visual ﬁbers also pass to several older areas of the brain: (1) from the optic
tracts to the suprachiasmatic nucleus of the hypothalamus, presumably to
control circadian rhythms that synchronize various physiologic changes of the
body with night and day; (2) into the pretectal nuclei in the midbrain, to elicit
reﬂex movements of the eyes to focus on objects of importance and to activ

yes to focus on objects of importance and to activate
the pupillary light reﬂex; (3) into the superior colliculus, to control rapid direc-
tional movements of the two eyes; and (4) into the ventral lateral geniculate
nucleus of the thalamus and surrounding basal regions of the brain, presumably
to help control some of the body’s behavioral functions.
Thus, the visual pathways can be divided roughly into an old system
to the midbrain and base of the forebrain and a new system for direct trans-
mi

he forebrain and a new system for direct trans-
mission of visual signals into the visual cortex located in the occipital lobes. In
human beings, the new system is responsible for perception of virtually all
aspects of visual form, colors, and other conscious vision. Conversely, in many
primitive animals, even visual form is detected by the older system, using the
superior colliculus in the same manner that the visual cortex is used in
mammals.
Function of the Dorsal Lateral Geniculate Nucleus 

unction of the Dorsal Lateral Geniculate Nucleus 
of the Thalamus
The optic nerve ﬁbers of the new visual system terminate in the dorsal lateral
geniculate nucleus, located at the dorsal end of the thalamus and also called
simply the lateral geniculate body, as shown in Figure 51–1. The dorsal lateral
geniculate nucleus serves two principal functions: First, it relays visual infor-
mation from the optic tract to the visual cortex by way of the optic radiation
(also called the geniculocalcarine t

tic radiation
(also called the geniculocalcarine tract). This relay function is so accurate that
there is exact point-to-point transmission with a high degree of spatial ﬁdelity
all the way from the retina to the visual cortex.
It will be recalled that half the ﬁbers in each optic tract after passing the optic
chiasm are derived from one eye and half from the other eye, representing cor-
responding points on the two retinas. However, the signals from the two eyes
are kept apart in the dorsal lat

from the two eyes
are kept apart in the dorsal lateral geniculate nucleus.This nucleus is composed
of six nuclear layers. Layers II, III, and V (from ventral to dorsal) receive signals
from the lateral half of the ipsilateral retina, whereas layers I, IV, and VI receive
signals from the medial half of the retina of the opposite eye. The respective
retinal areas of the two eyes connect with neurons that are superimposed over
Chapter 51
The Eye: III. Central Neurophysiology of Vision
641
one anoth

I. Central Neurophysiology of Vision
641
one another in the paired layers, and similar parallel
transmission is preserved all the way to the visual
cortex.
The second major function of the dorsal lateral
geniculate nucleus is to “gate” the transmission of
signals to the visual cortex—that is, to control how
much of the signal is allowed to pass to the cortex.The
nucleus receives gating control signals from two major
sources: (1) corticofugal ﬁbers returning in a backward
direction from the prima

s returning in a backward
direction from the primary visual cortex to the lateral
geniculate nucleus, and (2) reticular areas of the mes-
encephalon. Both of these are inhibitory and, when
stimulated, can turn off transmission through selected
portions of the dorsal lateral geniculate nucleus. It is
assumed that both of these gating circuits help high-
light the visual information that is allowed to pass.
Finally, the dorsal lateral geniculate nucleus is
divided in another way: (1) Layers I and 

cleus is
divided in another way: (1) Layers I and II are called
magnocellular layers
because they contain large
neurons.These receive their input almost entirely from
the large type Y retinal ganglion cells. This magnocel-
lular system provides a rapidly conducting pathway to
the visual cortex. However, this system is color blind,
transmitting only black-and-white information. Also,
its point-to-point transmission is poor because there
are not many Y ganglion cells, and their dendrites
spread wi

ny Y ganglion cells, and their dendrites
spread widely in the retina. (2) Layers III through VI
are called parvocellular layers because they contain
large numbers of small to medium-sized neurons.
These neurons receive their input almost entirely from
the type X retinal ganglion cells that transmit color
and convey accurate point-to-point spatial informa-
tion, but at only a moderate velocity of conduction
rather than at high velocity.
Organization and Function 
of the Visual Cortex
Figures 51–2

on and Function 
Figures 51–2 and 51–3 show the visual cortex located
primarily on the medial aspect of the occipital lobes.
Like the cortical representations of the other sensory
systems, the visual cortex is divided into a primary
visual cortex and secondary visual areas.
Primary Visual Cortex. The primary visual cortex (see
Figure 51–2) lies in the calcarine ﬁssure area, extend-
ing forward from the occipital pole on the medial
Optic radiation
Opt

edial
Left eye
Right eye
Visual cortex
Lateral geniculate body
Superior
colliculus
Figure 51–1
Principal visual pathways from the eyes to the visual cortex. (Mod-
iﬁed from Polyak SL: The Retina. Chicago: University of Chicago,
1941.)
Macula
Secondary
visual areas
Calcarine fissure
Primary
visual cortex
20∞
60∞
90∞
Figure 51–2
Visual cortex in the calcarine ﬁssure area of the medial occipital
cortex.
Somatosensory area I
Form,
3-D

ortex.
Form,
3-D position,
motion
18
17
Primary
visual
cortex
Secondary
visual
cortex
Visual
detail,
color
Figure 51–3
Transmission of visual signals from the primary visual cortex into
secondary visual areas on the lateral surfaces of the occipital and
parietal cortices. Note that the signals representing form, third-
dimensional position, and motion are transmitted mainly into the
superior portions of the occipital lobe and posterior portions of the
parietal l

ital lobe and posterior portions of the
parietal lobe. By contrast, the signals for visual detail and color
are transmitted mainly into the anteroventral portion of the occip-
ital lobe and the ventral portion of the posterior temporal lobe.
642
Unit X
aspect of each occipital cortex. This area is the termi-
nus of direct visual signals from the eyes. Signals from
the macular area of the retina terminate near the
occipital pole, as shown in Figure 51–2, 

near the
occipital pole, as shown in Figure 51–2, while signals
from the more peripheral retina terminate at or in con-
centric half circles anterior to the pole but still along
the calcarine ﬁssure on the medial occipital lobe. The
upper portion of the retina is represented superiorly
and the lower portion inferiorly.
Note in the ﬁgure the especially large area that rep-
resents the macula. It is to this region that the retinal
fovea transmits its signals.The fovea is responsible for
the highes

ts signals.The fovea is responsible for
the highest degree of visual acuity. Based on retinal
area, the fovea has several hundred times as much rep-
resentation in the primary visual cortex as do the most
peripheral portions of the retina.
The primary visual cortex is also called visual area
I. Still another name is the striate cortex because this
area has a grossly striated appearance.
Secondary Visual Areas of the Cortex. The secondary visual
areas, also called visual association areas, lie la

reas, also called visual association areas, lie lateral,
anterior, superior, and inferior to the primary visual
cortex. Most of these areas also fold outward over the
lateral surfaces of the occipital and parietal cortex, as
shown in Figure 51–3. Secondary signals are transmit-
ted to these areas for analysis of visual meanings. For
instance, on all sides of the primary visual cortex is
Brodmann’s area 18 (see Figure 51–3), which is where
virtually all signals from the primary visual cortex pass

ly all signals from the primary visual cortex pass
next. Therefore, Brodmann’s area 18 is called visual
area II, or simply V-2. The other, more distant second-
ary visual areas have speciﬁc designations—V-3, V-4,
and so forth—up to more than a dozen areas. The
importance of all these areas is that various aspects 
of the visual image are progressively dissected and
analyzed.
Layered Structure of the Primary
Visual Cortex
Like almost all other portions of the cerebral cortex,
the primary visual c

tions of the cerebral cortex,
the primary visual cortex has six distinct layers, as
shown in Figure 51–4. Also, as is true for the other
sensory systems, the geniculocalcarine ﬁbers terminate
mainly in layer IV. But this layer, too, is organized into
subdivisions.The rapidly conducted signals from the Y
retinal ganglion cells terminate in layer IVca, and
from there they are relayed vertically both outward
toward the cortical surface and inward toward deeper
levels.
The visual signals from the me

ward deeper
levels.
The visual signals from the medium-sized optic
nerve ﬁbers, derived from the X ganglion cells in the
retina, also terminate in layer IV, but at points differ-
ent from the Y signals. They terminate in layers IVa
and IVcb, the shallowest and deepest portions of layer
IV, shown to the right in Figure 51–4. From there, these
signals are transmitted vertically both toward the
surface of the cortex and to deeper layers. It is these
X ganglion pathways that transmit the accurate po

X ganglion pathways that transmit the accurate point-
to-point type of vision as well as color vision.
Vertical Neuronal Columns in the Visual Cortex. The visual
cortex is organized structurally into several million
vertical columns of neuronal cells, each column having
a diameter of 30 to 50 micrometers. The same vertical
columnar organization is found throughout the cere-
bral cortex for the other senses as well (and also in the
motor and analytical cortical regions). Each column
represents a

ytical cortical regions). Each column
represents a functional unit. One can roughly calculate
that each of the visual vertical columns has perhaps
1000 or more neurons.
After the optic signals terminate in layer IV,they are
further processed as they spread both outward and
inward along each vertical column unit. This process-
ing is believed to decipher separate bits of visual infor-
mation at successive stations along the pathway. The
signals that pass outward to layers I, II, and III even-
tua

at pass outward to layers I, II, and III even-
tually transmit signals for short distances laterally in
the cortex. Conversely, the signals that pass inward to
layers V and VI excite neurons that transmit signals
much greater distances.
“Color Blobs” in the Visual Cortex. Interspersed among the
primary visual columns as well as among the columns
(cb)
I
II
III
IV
Color
“blobs”
LGN
(parvocellular)
Retinal
"X"
ganglion
Very Accurate, Color
LGN
(magnocellular)
Retinal
"Y"
ganglion
Fast, Black and Wh

ocellular)
Retinal
"Y"
ganglion
Fast, Black and White
(a)
(b)
(ca)
V
VI
Figure 51–4
Six layers of the primary visual cortex. The connections shown on
the left side of the ﬁgure originate in the magnocellular layers of
the lateral geniculate nucleus (LGN) and transmit rapidly chang-
ing black-and-white visual signals. The pathways to the right orig-
inate in the parvocellular layers (layers III through VI) of the LGN;
they transmit signals that depict accurate spatial detail as well as
color. Not

pict accurate spatial detail as well as
color. Note especially the areas of the visual cortex called “color
blobs,” which are necessary for detection of color.
Chapter 51
643
of some of the secondary visual areas are special
column-like areas called color blobs. They receive
lateral signals from adjacent visual columns and are
activated speciﬁcally by color signals. Therefore, it is
presumed that these blobs are the primary areas for
deciphering co

ese blobs are the primary areas for
deciphering color.
Interaction of Visual Signals from the Two Separate Eyes.
Recall that the visual signals from the two separate
eyes are relayed through separate neuronal layers in
the lateral geniculate nucleus. These signals still
remain separated from each other when they arrive in
layer IV of the primary visual cortex. In fact, layer IV
is interlaced with stripes of neuronal columns, each
stripe about 0.5 millimeter wide; the signals from one
eye enter t

 millimeter wide; the signals from one
eye enter the columns of every other stripe, alter-
nating with signals from the second eye. This cortical
area deciphers whether the respective areas of the 
two visual images from the two separate eyes are “in
register” with each other—that is, whether correspon-
ding points from the two retinas ﬁt with each other. In
turn, the deciphered information is used to adjust the
directional gaze of the separate eyes so that they will
fuse with each other (be bro

yes so that they will
fuse with each other (be brought into “register”). The
information observed about degree of register of
images from the two eyes also allows a person to dis-
tinguish the distance of objects by the mechanism of
stereopsis.
Two Major Pathways for Analysis of
Visual Information—(1) The Fast
“Position” and “Motion” Pathway; 
(2) The Accurate Color Pathway
Figure 51–3 shows that after leaving the primary visual
cortex, the visual information is analyzed in two major
pathways in

l information is analyzed in two major
pathways in the secondary visual areas.
1. Analysis of Third-Dimensional Position, Gross Form, and
Motion of Objects. One of the analytical pathways,
demonstrated in Figure 51–3 by the black arrows, ana-
lyzes the third-dimensional positions of visual objects
in the space around the body. This pathway also ana-
lyzes the gross physical form of the visual scene as well
as motion in the scene. In other words, this pathway
tells where every object is during ea

this pathway
tells where every object is during each instant and
whether it is moving. After leaving the primary visual
cortex, the signals ﬂow generally into the posterior
midtemporal area and upward into the broad occipi-
toparietal cortex. At the anterior border of the parietal
cortex, the signals overlap with signals from the pos-
terior somatic association areas that analyze three-
dimensional aspects of somatosensory signals. The
signals transmitted in this position-form-motion
pathway are

ansmitted in this position-form-motion
pathway are mainly from the large Y optic nerve ﬁbers
of the retinal Y ganglion cells, transmitting rapid
signals but depicting only black and white with no
color.
2. Analysis of Visual Detail and Color. The red arrows in
Figure 51–3, passing from the primary visual cortex
into secondary visual areas of the inferior, ventral, and
medial regions of the occipital and temporal cortex,
show the principal pathway for analysis of visual
detail. Separate portions 

 for analysis of visual
detail. Separate portions of this pathway speciﬁcally
dissect out color as well. Therefore, this pathway is
concerned with such visual feats as recognizing letters,
reading, determining the texture of surfaces, deter-
mining detailed colors of objects, and deciphering
from all this information what the object is and what
it means.
Neuronal Patterns of
Stimulation During Analysis 
of the Visual Image
Analysis of Contrasts in the Visual Image. If a person looks
at a blank w

n the Visual Image. If a person looks
at a blank wall, only a few neurons in the primary
visual cortex will be stimulated, regardless of whether
the illumination of the wall is bright or weak. There-
fore, the question must be asked, What does the
primary visual cortex detect? To answer this, let us now
place on the wall a large solid cross, as shown to the
left in Figure 51–5. To the right is shown the spatial
pattern of the most excited neurons in the visual
cortex. Note that the areas of maxi

 in the visual
cortex. Note that the areas of maximum excitation
occur along the sharp borders of the visual pattern.
Thus, the visual signal in the primary visual cortex is
concerned mainly with contrasts in the visual scene,
rather than with noncontrasting areas. We saw in
Chapter 50 that this is true of most of the retinal gan-
glion cells as well, because equally stimulated adjacent
retinal receptors mutually inhibit one another. But at
any border in the visual scene where there is a change

order in the visual scene where there is a change
from dark to light or light to dark, mutual inhibition
does not occur, and the intensity of stimulation of most
neurons is proportional to the gradient of contrast—
that is, the greater the sharpness of contrast and the
greater the intensity difference between light and dark
areas, the greater the degree of stimulation.
Visual Cortex Also Detects Orientation of Lines and Borders—
“Simple” Cells. The visual cortex detects not only the
existence of

he visual cortex detects not only the
existence of lines and borders in the different areas of
the retinal image but also the direction of orientation
Retinal image
Cortical stimulation
Figure 51–5
Pattern of excitation that occurs in the visual cortex in response
to a retinal image of a dark cross.
644
Unit X
of each line or border—that is, whether it is vertical or
horizontal or lies at some degree of inclination. This is
believed to result from linear

nclination. This is
believed to result from linear organizations of mutu-
ally inhibiting cells that excite second-order neurons
when inhibition occurs all along a line of cells where
there is a contrast edge. Thus, for each such orienta-
tion of a line, speciﬁc neuronal cells are stimulated. A
line oriented in a different direction excites a different
set of cells.These neuronal cells are called simple cells.
They are found mainly in layer IV of the primary
visual cortex.
Detection of Line Orie

 the primary
Detection of Line Orientation When a Line Is Displaced Later-
ally or Vertically in the Visual Field—“Complex” Cells. As
the visual signal progresses farther away from layer IV,
some neurons respond to lines that are oriented in the
same direction but are not position-speciﬁc. That is,
even if a line is displaced moderate distances laterally
or vertically in the ﬁeld, the same few neurons will still
be stimulated if the line has the same direction. These
cells are cal

e line has the same direction. These
cells are called complex cells.
Detection of Lines of Speciﬁc Lengths, Angles, or Other Shapes.
Some neurons in the outer layers of the primary visual
columns, as well as neurons in some secondary visual
areas, are stimulated only by lines or borders of spe-
ciﬁc lengths, by speciﬁc angulated shapes, or by images
that have other characteristics. That is, these neurons
detect still higher orders of information from the
visual scene. Thus, as one goes farther i

from the
visual scene. Thus, as one goes farther into the analyt-
ical pathway of the visual cortex, progressively more
characteristics of each visual scene are deciphered.
Color is detected in much the same way that lines are
detected: by means of color contrast. For instance, a
red area is often contrasted against a green area, a blue
area against a red area, or a green area against a yellow
area. All these colors can also be contrasted against a
white area within the visual

 contrasted against a
white area within the visual scene. In fact, this con-
trasting against white is believed to be mainly respon-
sible for the phenomenon called “color constancy”;
that is, when the color of an illuminating light changes,
the color of the “white” changes with the light, and
appropriate computation in the brain allows red to be
interpreted as red even though the illuminating light
has changed the color entering the eyes.
The mechanism of color contrast analysis depends
on the 

chanism of color contrast analysis depends
on the fact that contrasting colors, called “opponent
colors,” excite speciﬁc neuronal cells. It is presumed
that the initial details of color contrast are detected by
simple cells, whereas more complex contrasts are
detected by complex and hypercomplex cells.
Effect of Removing the Primary 
Removal of the primary visual cortex in the human
being causes loss of conscious vision, that is, blindness.
However, psychological studies demonstrat

indness.
However, psychological studies demonstrate that such
“blind” people can still, at times, react subconsciously
to changes in light intensity, to movement in the visual
scene, or, rarely, even to some gross patterns of vision.
These reactions include turning the eyes, turning the
head, and avoidance. This vision is believed to be sub-
served by neuronal pathways that pass from the optic
tracts mainly into the superior colliculi and other por-
tions of the older visual system.
Fields of Vi

or-
The ﬁeld of vision is the visual area seen by an eye at a
given instant. The area seen to the nasal side is called
the nasal ﬁeld of vision, and the area seen to the lateral
side is called the temporal ﬁeld of vision.
To diagnose blindness in speciﬁc portions of the
retina, one charts the ﬁeld of vision for each eye by a
process called perimetry. This is done by having the
subject look with one eye closed and the other eye
looking

look with one eye closed and the other eye
looking toward a central spot directly in front of the eye.
Then a small dot of light or a small object is moved back
and forth in all areas of the ﬁeld of vision, and the
subject indicates when the spot of light or object can be
seen and when it cannot.Thus, the ﬁeld of vision for the
left eye is plotted as shown in Figure 51–6. In all perime-
try charts, a blind spot caused by lack of rods and cones
in the retina over the optic disc is found about 15

 the retina over the optic disc is found about 15
degrees lateral to the central point of vision, as shown
in the ﬁgure.
Abnormalities in the Fields of Vision. Occasionally, blind
spots are found in portions of the ﬁeld of vision other
than the optic disc area. Such blind spots are called sco-
tomata; they frequently are caused by damage to the
optic nerve resulting from glaucoma (too much ﬂuid
pressure in the eyeball), from allergic reactions in the
retina, or from toxic conditions such as lead

 the
retina, or from toxic conditions such as lead poisoning
or excessive use of tobacco.
90
80
70
60
50
80
70
60
50
40
40
30
30
20
20
10
10
20
20
10
30
30
40
40
50
50
60
60
70 80
70
80
75
45
15
345
330
315
300
285
255
270
240
225
210
195
180
165
150
135
Left
Optic
disc
Right
120
105
60
30
0
Figure 51–6
Perimetry chart, showing the ﬁeld of vision for the left eye.
Chapter 51
645
Still another condition that can be diagnosed by
perimetry is retiniti

ion that can be diagnosed by
perimetry is retinitis pigmentosa. In this disease, por-
tions of the retina degenerate, and excessive melanin
pigment deposits in the degenerated areas. Retinitis pig-
mentosa usually causes blindness in the peripheral ﬁeld
of vision ﬁrst and then gradually encroaches on the
central areas.
Effect of Lesions in the Optic Pathway on the Fields of
Vision. Destruction of an entire optic nerve causes
blindness of the affected eye.
Destruction of the optic chiasm prevents

cted eye.
Destruction of the optic chiasm prevents the crossing
of impulses from the nasal half of each retina to the
opposite optic tract. Therefore, the nasal half of each
retina is blinded, which means that the person is blind
in the temporal ﬁeld of vision for each eye because the
image of the ﬁeld of vision is inverted on the retina by
the optical system of the eye; this condition is called
bitemporal hemianopsia. Such lesions frequently result
from tumors of the pituitary gland pressing up

ult
from tumors of the pituitary gland pressing upward
from the sella turcica on the bottom of the optic chiasm.
Interruption of an optic tract denervates the corre-
sponding half of each retina on the same side as the
lesion; as a result, neither eye can see objects to the
opposite side of the head. This condition is known as
homonymous hemianopsia.
Eye Movements and 
Their Control
To make full use of the visual abilities of the eyes,
almost equally as important as interpretation of the
visual 

ally as important as interpretation of the
visual signals from the eyes is the cerebral control
system for directing the eyes toward the object to be
viewed.
Muscular Control of Eye Movements. The eye movements
are controlled by three pairs of muscles, shown in
Figure 51–7: (1) the medial and lateral recti, (2) the
superior and inferior recti, and (3) the superior and
inferior obliques. The medial and lateral recti contract
to move the eyes from side to side. The superior and
inferior recti cont

side to side. The superior and
inferior recti contract to move the eyes upward or
downward. The oblique muscles function mainly to
rotate the eyeballs to keep the visual ﬁelds in the
upright position.
Neural Pathways for Control of Eye Movements. Figure 51–7
also shows brain stem nuclei for the third, fourth, and
sixth cranial nerves and their connections with the
peripheral nerves to the ocular muscles. Shown, too,
are interconnections among the brain stem nuclei by
way of the nerve tract calle

 brain stem nuclei by
way of the nerve tract called the medial longitudinal
fasciculus. Each of the three sets of muscles to each
eye is reciprocally innervated so that one muscle of the
pair relaxes while the other contracts.
Figure 51–8 demonstrates cortical control of the
oculomotor apparatus, showing spread of signals from
visual areas in the occipital cortex through occipito-
tectal and occipitocollicular tracts to the pretectal and
superior colliculus areas of the brain stem. From both
the

 colliculus areas of the brain stem. From both
the pretectal and the superior colliculus areas, the ocu-
lomotor control signals pass to the brain stem nuclei
of the oculomotor nerves. Strong signals are also trans-
mitted from the body’s equilibrium control centers in
the brain stem into the oculomotor system (from the
vestibular nuclei by way of the medial longitudinal 
fasciculus).
Perhaps the most important movements of the eyes
are those that cause the eyes to

ments of the eyes
are those that cause the eyes to “ﬁx” on a discrete
portion of the ﬁeld of vision. Fixation movements are
controlled by two neuronal mechanisms. The ﬁrst of
these allows a person to move the eyes voluntarily to
ﬁnd the object on which he or she wants to ﬁx the
vision; this is called the voluntary ﬁxation mechanism.
The second is an involuntary mechanism that holds the
eyes ﬁrmly on the object once it has been found; this
is called the involuntary ﬁxation mechanism.
The voluntar

ed the involuntary ﬁxation mechanism.
The voluntary ﬁxation movements are controlled by
a cortical ﬁeld located bilaterally in the premotor cor-
tical regions of the frontal lobes, as shown in Figure
51–8. Bilateral dysfunction or destruction of these
areas makes it difﬁcult or almost impossible for a
person to “unlock” the eyes from one point of ﬁxation
and move them to another point. It is usually neces-
sary to blink the eyes or put a hand over the eyes for
a short time, which then allows the

r the eyes for
a short time, which then allows the eyes to be moved.
Conversely, the ﬁxation mechanism that causes the
eyes to “lock” on the object of attention once it is
found is controlled by secondary visual areas in the
occipital cortex, located mainly anterior to the primary
visual cortex. When this ﬁxation area is destroyed
bilaterally in an animal, the animal has difﬁculty
keeping its eyes directed toward a given ﬁxation point
or may become totally unable to do so.
To summarize, posterio

me totally unable to do so.
To summarize, posterior “involuntary” occipital cor-
tical eye ﬁelds automatically “lock” the eyes on a given
Lateral
rectus
Inferior oblique
Medial
longitudinal
fasciculus
Superior oblique
Nuclei
N.III
N.IV
N.VI
Superior rectus
Inferior rectus
Medial rectus
Figure 51–7
Extraocular muscles of the eye and their innervation.
646
Unit X
spot of the visual ﬁeld and thereby prevent movement
of the image across the retinas. To unloc

movement
of the image across the retinas. To unlock this visual
ﬁxation, voluntary signals must be transmitted from
cortical “voluntary” eye ﬁelds located in the frontal
cortices.
Mechanism of Involuntary Locking Fixation—Role of the Supe-
rior Colliculi. The involuntary locking type of ﬁxation
discussed in the previous section results from a nega-
tive feedback mechanism that prevents the object of
attention from leaving the foveal portion of the retina.
The eyes normally have three types of co

e retina.
The eyes normally have three types of continuous but
almost imperceptible movements: (1) a continuous
tremor at a rate of 30 to 80 cycles per second caused
by successive contractions of the motor units in the
ocular muscles, (2) a slow drift of the eyeballs in one
direction or another, and (3) sudden ﬂicking move-
ments that are controlled by the involuntary ﬁxation
When a spot of light has become ﬁxed on the foveal
region of the retina, the tremulous movements cause
the spo

 the retina, the tremulous movements cause
the spot to move back and forth at a rapid rate across
the cones, and the drifting movements cause the spot
to drift slowly across the cones. Each time the spot
drifts as far as the edge of the fovea, a sudden reﬂex
reaction occurs, producing a ﬂicking movement that
moves the spot away from this edge back toward the
center of the fovea.Thus,an automatic response moves
the image back toward the central point of vision.
These drifting and ﬂicking motions 

int of vision.
These drifting and ﬂicking motions are demon-
strated in Figure 51–9, which shows by the dashed lines
the slow drifting across the fovea and by the solid lines
the ﬂicks that keep the image from leaving the foveal
region. This involuntary ﬁxation capability is mostly
lost when the superior colliculi are destroyed.
Saccadic Movement of the Eyes—A Mechanism of Successive
Fixation Points. When a visual scene is moving contin-
ually before the eyes, such as when a person is riding
in 

fore the eyes, such as when a person is riding
in a car, the eyes ﬁx on one highlight after another in
the visual ﬁeld, jumping from one to the next at a rate
of two to three jumps per second.The jumps are called
saccades, and the movements are called opticokinetic
movements. The saccades occur so rapidly that no
more than 10 per cent of the total time is spent in
moving the eyes, with 90 per cent of the time being
allocated to the ﬁxation sites. Also, the brain sup-
presses the visual image dur

 Also, the brain sup-
presses the visual image during saccades, so that the
person is not conscious of the movements from point
to point.
Saccadic Movements During Reading. During the
process of reading, a person usually makes several sac-
cadic movements of the eyes for each line. In this case,
the visual scene is not moving past the eyes, but the
eyes are trained to move by means of several succes-
sive saccades across the visual scene to extract the
important information. Similar saccades occ

ct the
important information. Similar saccades occur when a
Visual
association
areas
Primary
Occipitotectal and
occipitocollicular tracts
Pretectal nuclei
Visceral nucleus III nerve
Superior colliculus
Oculomotor nucleus 
Frontotectal tract
Voluntary fixation
area
Involuntary
fixation
area
III nerve
IV nerve
VI nerve
Medial longitudinal
Trochlear nucleus
Abducens nucleus
Vestibular nuclei
Figure 51–8
Neural pathways for control of
conjugate movement of the eyes.
Chapter 51
The Eye:

onjugate movement of the eyes.
Chapter 51
647
person observes a painting, except that the saccades
occur in upward, sideways, downward, and angulated
directions one after another from one highlight of the
painting to another, and so forth.
Fixation on Moving Objects—“Pursuit Movement.” The eyes
can also remain ﬁxed on a moving object, which is
called pursuit movement. A highly developed cortical
mechanism automatically detects the course of move-
m

hanism automatically detects the course of move-
ment of an object and then rapidly develops a similar
course of movement for the eyes. For instance, if an
object is moving up and down in a wavelike form at a
rate of several times per second, the eyes at ﬁrst may
be unable to ﬁxate on it. However, after a second or
so, the eyes begin to jump by means of saccades in
approximately the same wavelike pattern of move-
ment as that of the object. Then, after another few
seconds, the eyes develop progr

 after another few
seconds, the eyes develop progressively smoother
movements and ﬁnally follow the wave movement
almost exactly. This represents a high degree of auto-
matic subconscious computational ability by the
pursuit system for controlling eye movements.
Superior Colliculi Are Mainly Responsible 
for Turning the Eyes and Head Toward a
Visual Disturbance
Even after the visual cortex has been destroyed, a
sudden visual disturbance in a lateral area of the visual
ﬁeld often causes immediate

ral area of the visual
ﬁeld often causes immediate turning of the eyes in that
direction. This does not occur if the superior colliculi
have also been destroyed.To support this function, the
various points of the retina are represented topo-
graphically in the superior colliculi in the same way as
in the primary visual cortex, although with less accu-
racy. Even so, the principal direction of a ﬂash of light
in a peripheral retinal ﬁeld is mapped by the colliculi,
and secondary signals are trans

 by the colliculi,
and secondary signals are transmitted to the oculo-
motor nuclei to turn the eyes. To help in this 
directional movement of the eyes, the superior colli-
culi also have topological maps of somatic sensations
from the body and acoustic signals from the ears.
The optic nerve ﬁbers from the eyes to the colliculi
that are responsible for these rapid turning move-
ments are branches from the rapidly conducting Y
ﬁbers, with one branch going to the visual cortex and
the other going 

ch going to the visual cortex and
the other going to the superior colliculi. (The superior
colliculi and other regions of the brain stem are also
strongly supplied with visual signals transmitted in
type W optic nerve ﬁbers. These represent the oldest
visual pathway, but their function is unclear.)
In addition to causing the eyes to turn toward a
visual disturbance, signals are relayed from the supe-
rior colliculi through the medial longitudinal fascicu-
lus to other levels of the brain stem to

 fascicu-
lus to other levels of the brain stem to cause turning
of the whole head and even of the whole body toward
the direction of the disturbance. Other types of non-
visual disturbances, such as strong sounds or even
stroking of the side of the body, cause similar turning
of the eyes, head, and body, but only if the superior col-
liculi are intact. Therefore, the superior colliculi play a
global role in orienting the eyes, head, and body with
respect to external disturbances, whether they a

h
respect to external disturbances, whether they are
visual, auditory, or somatic.
“Fusion” of the Visual Images from
the Two Eyes
To make the visual perceptions more meaningful, the
visual images in the two eyes normally fuse with each
other on “corresponding points” of the two retinas.
The visual cortex plays an important role in fusion. It
was pointed out earlier in the chapter that correspon-
ding points of the two retinas transmit visual signals to
different neuronal layers of the lateral g

nals to
different neuronal layers of the lateral geniculate
body, and these signals in turn are relayed to parallel
neurons in the visual cortex. Interactions occur
between these cortical neurons to cause interference
excitation in speciﬁc neurons when the two visual
images are not “in register”—that is, are not precisely
“fused.” This excitation presumably provides the
signal that is transmitted to the oculomotor apparatus
to cause convergence or divergence or rotation of the
eyes so that fusio

r divergence or rotation of the
eyes so that fusion can be re-established. Once the cor-
responding points of the two retinas are in register,
excitation of the speciﬁc “interference” neurons in the
visual cortex disappears.
Neural Mechanism of Stereopsis for Judging
Distances of Visual Objects
In Chapter 49, it is pointed out that because the two
eyes are more than 2 inches apart, the images on the
two retinas are not exactly the same. That is, the right
eye sees a little more of the right-hand

the right
eye sees a little more of the right-hand side of the
object, and the left eye a little more of the left-hand
side, and the closer the object, the greater the dispar-
ity. Therefore, even when the two eyes are fused with
each other, it is still impossible for all corresponding
points in the two visual images to be exactly in regis-
ter at the same time. Furthermore, the nearer the
Voluntary
movement to
fixation site
Figure 51–9
Movements of a spot of light on the fovea, showing sudden “

 of a spot of light on the fovea, showing sudden “ﬂick-
ing” eye movements that move the spot back toward the center
of the fovea whenever it drifts to the foveal edge. (The dashed
lines represent slow drifting movements, and the solid lines rep-
resent sudden ﬂicking movements.) (Modiﬁed from Whitteridge D:
Central control of the eye movements. In Field J, Magoun HW, Hall
VE (eds): Handbook of Physiology. vol. 2, sec. 1. Washington, DC,
American Physiological Society, 1960.)
648
Unit X
The Nerv

 Physiological Society, 1960.)
648
Unit X
object is to the eyes, the less the degree of register.This
degree of nonregister provides the neural mechanism
for stereopsis, an important mechanism for judging 
the distances of visual objects up to about 200 feet 
(60 meters).
The neuronal cellular mechanism for stereopsis is
based on the fact that some of the ﬁber pathways from
the retinas to the visual cortex stray 1 to 2 degrees on
each side of the central

x stray 1 to 2 degrees on
each side of the central pathway.Therefore, some optic
pathways from the two eyes are exactly in register for
objects 2 meters away; still another set of pathways is
in register for objects 25 meters away. Thus, the dis-
tance is determined by which set or sets of pathways
are excited by nonregister or register. This phenome-
non is called depth perception, which is another name
for stereopsis.
Strabismus
Strabismus, also called squint or cross-eye, means lack
of fusion

o called squint or cross-eye, means lack
of fusion of the eyes in one or more of the visual coor-
dinates: horizontal, vertical, or rotational. The basic
types of strabismus are shown in Figure 51–10: (1) hor-
izontal strabismus, (2) torsional strabismus, and (3) ver-
tical strabismus. Combinations of two or even all three
of the different types of strabismus often occur.
Strabismus is often caused by abnormal “set” of the
fusion mechanism of the visual system. That is, in a
young child’s early 

 visual system. That is, in a
young child’s early efforts to ﬁxate the two eyes on the
same object, one of the eyes ﬁxates satisfactorily while
the other fails do so, or they both ﬁxate satisfactorily but
never simultaneously. Soon the patterns of conjugate
movements of the eyes become abnormally “set” in the
neuronal control pathways themselves, so that the eyes
never fuse.
Suppression of the Visual Image from a Repressed Eye. In a few
patients with strabismus, the eyes alternate in ﬁxing on
th

with strabismus, the eyes alternate in ﬁxing on
the object of attention. In other patients, one eye alone
is used all the time,and the other eye becomes repressed
and is never used for precise vision.The visual acuity of
the repressed eye develops only slightly, sometimes
remaining 20/400 or less. If the dominant eye then
becomes blinded, vision in the repressed eye can
develop only to a slight extent in adults but far more in
young children. This demonstrates that visual acuity is
highly depend

s demonstrates that visual acuity is
highly dependent on proper development of central
nervous system synaptic connections from the eyes. In
fact, even anatomically, the numbers of neuronal con-
nections diminish in the visual cortex areas that would
normally receive signals from the repressed eye.
Autonomic Control of
Accommodation and 
Pupillary Aperture
Autonomic Nerves to the Eyes. The eye is innervated by
both parasympathetic and sympathetic nerve ﬁbers, as
shown in Figure 51–11. The parasy

 nerve ﬁbers, as
shown in Figure 51–11. The parasympathetic pregan-
glionic ﬁbers arise in the Edinger-Westphal nucleus
(the visceral nucleus portion of the third cranial nerve)
and then pass in the third nerve to the ciliary ganglion,
which lies immediately behind the eye. There, the pre-
ganglionic ﬁbers synapse with postganglionic parasym-
pathetic neurons, which in turn send ﬁbers through
ciliary nerves into the eyeball. These nerves excite (1)
the ciliary muscle that controls focusing of th

1)
the ciliary muscle that controls focusing of the eye lens
and (2) the sphincter of the iris that constricts the
pupil.
The sympathetic innervation of the eye originates in
the intermediolateral horn cells of the ﬁrst thoracic
segment of the spinal cord. From there, sympathetic
ﬁbers enter the sympathetic chain and pass upward to
the superior cervical ganglion, where they synapse
with postganglionic neurons. Postganglionic sympa-
thetic ﬁbers from these then spread along the surfaces
of the ca

rom these then spread along the surfaces
of the carotid artery and successively smaller arteries
until they reach the eye. There, the sympathetic ﬁbers
strabismus
Torsional
Vertical
Figure 51–10
Basic types of strabismus.
Pretectal
region
Edinger-
Westphal
nucleus
Upper thoracic segments
of spinal cord
Ciliary
ganglion
N.III
N.II
Pons
Carotid plexus
Superior cervical
sympathetic
ganglion
Cervical
trunk
Figure 51–11
Autonomic innervation of the eye, sh

Autonomic innervation of the eye, showing also the reﬂex arc of
the light reﬂex. (Modiﬁed from Ranson SW, Clark SL: Anatomy of
the Nervous System: Its Development and Function, 10th ed.
Philadelphia: WB Saunders, 1959.)
Chapter 51
649
innervate the radial ﬁbers of the iris (which open the
pupil) as well as several extraocular muscles of the eye,
which are discussed subsequently in relation to
Horner’s syndrome.
Control of Accommodatio

tion to
Control of Accommodation 
(Focusing the Eyes)
The accommodation mechanism—that is, the mecha-
nism that focuses the lens system of the eye—is essen-
tial for a high degree of visual acuity.Accommodation
results from contraction or relaxation of the eye ciliary
muscle. Contraction causes increased refractive power
of the lens, as explained in Chapter 49, and relaxation
causes decreased power. How does a person adjust
accommodation to keep the eyes in focus all the time?

ommodation to keep the eyes in focus all the time?
Accommodation of the lens is regulated by a nega-
tive feedback mechanism that automatically adjusts
the refractive power of the lens to achieve the highest
degree of visual acuity. When the eyes have been
focused on some far object and must then suddenly
focus on a near object, the lens usually accommodates
for best acuity of vision within less than 1 second.
Although the precise control mechanism that causes
this rapid and accurate focusing of

sm that causes
this rapid and accurate focusing of the eye is unclear,
some of the known features are the following.
First, when the eyes suddenly change distance of the
ﬁxation point, the lens changes its strength in the
proper direction to achieve a new state of focus within
a fraction of a second. Second, different types of 
clues help to change the lens strength in the proper
direction:
1. Chromatic aberration appears to be important.
That is, red light rays focus slightly posteriorly to
blu

, red light rays focus slightly posteriorly to
blue light rays because the lens bends blue rays
more than red rays. The eyes appear to be able to
detect which of these two types of rays is in better
focus, and this clue relays information to the
accommodation mechanism whether to make the
lens stronger or weaker.
2. When the eyes ﬁxate on a near object, the eyes
must converge. The neural mechanisms for
convergence cause a simultaneous signal to
strengthen the lens of the eye.
3. Because the fove

trengthen the lens of the eye.
3. Because the fovea lies in a hollowed-out depression
that is slightly deeper than the remainder of the
retina, the clarity of focus in the depth of the fovea
is different from the clarity of focus on the edges. It
has been suggested that this also gives clues about
which way the strength of the lens needs to be
changed.
4. It has been found that the degree of
accommodation of the lens oscillates slightly all
the time at a frequency up to twice per second.
The vis

ime at a frequency up to twice per second.
The visual image becomes clearer when the
oscillation of the lens strength is changing in 
the appropriate direction and becomes poorer
when the lens strength is changing in the wrong
direction. This could give a rapid clue as to which
way the strength of the lens needs to change to
provide appropriate focus.
The brain cortical areas that control accommoda-
tion closely parallel those that control ﬁxation move-
ments of the eyes, with analysis of the vi

n move-
ments of the eyes, with analysis of the visual signals in
Brodmann’s cortical areas 18 and 19 and transmission
of motor signals to the ciliary muscle through the 
pretectal area in the brain stem, then through the
Edinger-Westphal nucleus, and ﬁnally by way of
parasympathetic nerve ﬁbers to the eyes.
Stimulation of the parasympathetic nerves also excites
the pupillary sphincter muscle, thereby decreasing the
pupillary aperture; this is called miosis. Convers

pupillary aperture; this is called miosis. Conversely,
stimulation of the sympathetic nerves excites the
radial ﬁbers of the iris and causes pupillary dilation,
called mydriasis.
Pupillary Light Reﬂex. When light is shone into the eyes,
the pupils constrict, a reaction called the pupillary light
reﬂex. The neuronal pathway for this reﬂex is demon-
strated by the upper two black traces in Figure 51–11.
When light impinges on the retina, a few of the result-
ing impulses pass from the optic nerves

he result-
ing impulses pass from the optic nerves to the pretec-
tal nuclei. From here, secondary impulses pass to the
Edinger-Westphal nucleus and, ﬁnally, back through
parasympathetic nerves to constrict the sphincter of the
iris. Conversely, in darkness, the reﬂex becomes inhib-
ited, which results in dilation of the pupil.
The function of the light reﬂex is to help the eye
adapt extremely rapidly to changing light conditions,
as explained in Chapter 50. The limits of pupillary
diameter are 

 Chapter 50. The limits of pupillary
diameter are about 1.5 millimeters on the small side
and 8 millimeters on the large side.Therefore, because
light brightness on the retina increases with the square
of pupillary diameter,the range of light and dark adap-
tation that can be brought about by the pupillary reﬂex
is about 30 to 1—that is, up to as much as 30 times
change in the amount of light entering the eye.
Pupillary Reﬂexes or Reactions in Central Nervous System
Disease. A few central nervou

ntral Nervous System
Disease. A few central nervous system diseases damage
nerve transmission of visual signals from the retinas to
the Edinger-Westphal nucleus, thus sometimes blocking
the pupillary reﬂexes. Such blocks frequently occur as a
result of central nervous system syphilis, alcoholism,
encephalitis, and so forth.The block usually occurs in the
pretectal region of the brain stem, although it can result
from destruction of some small ﬁbers in the optic
nerves.
The ﬁnal nerve ﬁbers in th

rs in the optic
nerves.
The ﬁnal nerve ﬁbers in the pathway through the pre-
tectal area to the Edinger-Westphal nucleus are mostly
of the inhibitory type. When their inhibitory effect is
lost, the nucleus becomes chronically active, causing the
pupils to remain mostly constricted, in addition to their
failure to respond to light.
Yet the pupils can constrict a little more if the
Edinger-Westphal nucleus is stimulated through some
other pathway. For instance, when the eyes ﬁxate on a
near object

For instance, when the eyes ﬁxate on a
near object, the signals that cause accommodation of
the lens and those that cause convergence of the two
eyes cause a mild degree of pupillary constriction at 
the same time. This is called the pupillary reaction to
650
Unit X
accommodation. A pupil that fails to respond to light
but does respond to accommodation and is also very
small (an Argyll Robertson pupil) is an important diag-
nostic sign of central nervous

 an important diag-
nostic sign of central nervous system disease—often
syphilis.
Horner’s Syndrome. The sympathetic nerves to the eye are
occasionally interrupted. Interruption frequently occurs
in the cervical sympathetic chain. This causes the clini-
cal condition called Horner’s syndrome, which consists
of the following effects: First, because of interruption of
sympathetic nerve ﬁbers to the pupillary dilator muscle,
the pupil remains persistently constricted to a smaller
diameter than the 

tently constricted to a smaller
diameter than the pupil of the opposite eye. Second, the
superior eyelid droops because it is normally main-
tained in an open position during waking hours partly
by contraction of smooth muscle ﬁbers embedded in the
superior eyelid and innervated by the sympathetics.
Therefore, destruction of the sympathetic nerves makes
it impossible to open the superior eyelid as widely as
normally. Third, the blood vessels on the corresponding
side of the face and head become 

he corresponding
side of the face and head become persistently dilated.
Fourth, sweating (which requires sympathetic nerve
signals) cannot occur on the side of the face and head
affected by Horner’s syndrome.
Becker W, Reubel H, Mergner T: Current Oculomotor
Research. New York: Plenum Press, 1999.
Burr D: Eye movements: keeping vision stable. Curr Biol
14:R195, 2004.
Buttner-Ennever JA, Eberhorn A, Horn A

R195, 2004.
Buttner-Ennever JA, Eberhorn A, Horn AK: Motor and
sensory innervation of extraocular eye muscles. Ann N Y
Acad Sci 1004:40, 2003.
Chalupa LM, Gunhan E: Development of on and off retinal
pathways and retinogeniculate projections. Prog Retin
Eye Res 23:31, 2004.
Crawford JD, Martinez-Trujillo JC, Klier EM: Neural control
of three-dimensional eye and head movements. Curr Opin
Neurobiol 13:655, 2003.
Dacey DM: Parallel pathways for spectral coding in primate
retina. Annu Rev Neurosci 23

ral coding in primate
retina. Annu Rev Neurosci 23:743, 2000.
Demer JL: The orbital pulley system: a revolution in con-
cepts of orbital anatomy. Ann N Y Acad Sci 956:17, 2002.
Derrington AM, Webb BS: Visual system: how is the retina
wired up to the cortex? Curr Biol 14:R14, 2004.
Ferster D, Miller KD: Neural mechanisms of orientation
selectivity in the visual cortex.Annu Rev Neurosci 23:441,
2000.
Hikosaka O

nforma-
Hikosaka O, Takikawa Y, Kawagoe R: Role of the basal
ganglia in the control of purposive saccadic eye move-
ments. Physiol Rev 80:953, 2000.
Kastner S, Ungerleider LG: Mechanisms of visual attention
in the human cortex. Annu Rev Neurosci 23:315, 2000.
Krauzlis RJ: Recasting the smooth pursuit eye movement
system. J Neurophysiol 91:591, 2004.
Leigh RJ, Lee DS: The Neurology of Eye Movements. New
York: Oxford University Press, 1999.
Mante V, Carandini M: Vis

 University Press, 1999.
Mante V, Carandini M: Visual cortex: seeing motion. Curr
Biol 13:R906, 2003.
Martinez-Conde S, Macknik SL, Hubel DH: The role of 
ﬁxational eye movements in visual perception. Nat Rev
Neurosci 5:229, 2004.
Munoz DP, Everling S: Look away: the anti-saccade task and
the voluntary control of eye movement. Nat Rev Neurosci
5:218, 2004.
Pierrot-Deseilligny C, Milea D, Muri RM: Eye movement
control by the cerebral cortex. Curr Opin Neurol 17:17,
2004.
Treue S: Visual attention

Opin Neurol 17:17,
2004.
Treue S: Visual attention: the where, what, how and why of
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5
2
651
This chapter describes the mechanisms by which the
ear receives sound waves, discriminates their fre-
quencies, and transmits auditory information into
the central nervous system, where its meaning is
deciphered.
Tympanic Membrane 

here its meaning is
Tympanic Membrane and the Ossicular System
Conduction of Sound from the Tympanic Membrane 
to the Cochlea
Figure 52–1 shows the tympanic membrane (commonly called the eardrum) and
the ossicles, which conduct sound from the tympanic membrane through the
middle ear to the cochlea (the inner ear). Attached to the tympanic membrane
is the handle of the malleus. The malleus is bound to the incus by minute liga-
ments, so that whenever the malleus moves, the incus moves

o that whenever the malleus moves, the incus moves with it. The oppo-
site end of the incus articulates with the stem of the stapes, and the faceplate of
the stapes lies against the membranous labyrinth of the cochlea in the opening
of the oval window.
The tip end of the handle of the malleus is attached to the center of the tym-
panic membrane, and this point of attachment is constantly pulled by the tensor
tympani muscle, which keeps the tympanic membrane tensed.This allows sound
vibrations on

ic membrane tensed.This allows sound
vibrations on any portion of the tympanic membrane to be transmitted to the
ossicles, which would not be true if the membrane were lax.
The ossicles of the middle ear are suspended by ligaments in such a way that
the combined malleus and incus act as a single lever, having its fulcrum approx-
imately at the border of the tympanic membrane.
The articulation of the incus with the stapes causes the stapes to push forward
on the oval window and on the cochlear ﬂu

 forward
on the oval window and on the cochlear ﬂuid on the other side of window every
time the tympanic membrane moves inward, and to pull backward on the ﬂuid
every time the malleus moves outward.
“Impedance Matching” by the Ossicular System. The amplitude of movement of the
stapes faceplate with each sound vibration is only three fourths as much as the
amplitude of the handle of the malleus. Therefore, the ossicular lever system
does not increase the movement distance of the stapes, as is com

ase the movement distance of the stapes, as is commonly believed.
Instead, the system actually reduces the distance but increases the force of
movement about 1.3 times. In addition, the surface area of the tympanic mem-
brane is about 55 square millimeters, whereas the surface area of the stapes
averages 3.2 square millimeters. This 17-fold difference times the 1.3-fold ratio
of the lever system causes about 22 times as much total force to be exerted on
the ﬂuid of the cochlea as is exerted by t

rted on
the ﬂuid of the cochlea as is exerted by the sound waves against the tympanic
membrane. Because ﬂuid has far greater inertia than air does, it is easily under-
stood that increased amounts of force are needed to cause vibration in the ﬂuid.
Therefore, the tympanic membrane and ossicular system provide impedance
matching between the sound waves in air and the sound vibrations in the ﬂuid
of the cochlea. Indeed, the impedance matching is about 50 to 75 per cent of
perfect for sound frequen

out 50 to 75 per cent of
perfect for sound frequencies between 300 and 3000 cycles per second, which
allows utilization of most of the energy in the incoming sound waves.
652
Unit X
In the absence of the ossicular system and tympanic
membrane, sound waves can still travel directly
through the air of the middle ear and enter the cochlea
at the oval window. However, the sensitivity for
hearing is then 15 to 20 decibels less than for ossicu-
lar transmissio

 20 decibels less than for ossicu-
lar transmission—equivalent to a decrease from a
medium to a barely perceptible voice level.
Attenuation of Sound by Contraction of the Tensor Tympani and
Stapedius Muscles. When loud sounds are transmitted
through the ossicular system and from there into the
central nervous system, a reﬂex occurs after a latent
period of only 40 to 80 milliseconds to cause contrac-
tion of the stapedius muscle and, to a lesser extent, the
tensor tympani muscle. The tensor tymp

extent, the
tensor tympani muscle. The tensor tympani muscle
pulls the handle of the malleus inward while the
stapedius muscle pulls the stapes outward. These two
forces oppose each other and thereby cause the entire
ossicular system to develop increased rigidity, thus
greatly reducing the ossicular conduction of low-
frequency sound, mainly frequencies below 1000
cycles per second.
This attenuation reﬂex can reduce the intensity of
lower-frequency sound transmission by 30 to 40 deci-
bels, whic

cy sound transmission by 30 to 40 deci-
bels, which is about the same difference as that
between a loud voice and a whisper. The function of
this mechanism is believed to be twofold:
1. To protect the cochlea from damaging vibrations
caused by excessively loud sound.
2. To mask low-frequency sounds in loud
environments. This usually removes a major share
of the background noise and allows a person to
concentrate on sounds above 1000 cycles per
second, where most of the pertinent information
in v

cond, where most of the pertinent information
in voice communication is transmitted.
Another function of the tensor tympani and
stapedius muscles is to decrease a person’s hearing
sensitivity to his or her own speech. This effect is acti-
vated by collateral nerve signals transmitted to these
muscles at the same time that the brain activates the
voice mechanism.
Because the inner ear, the cochlea, is embedded in a
bony cavity in the temporal bone, called the bo

 a
bony cavity in the temporal bone, called the bony
labyrinth, vibrations of the entire skull can cause ﬂuid
vibrations in the cochlea itself.Therefore,under appro-
priate conditions, a tuning fork or an electronic vibra-
tor placed on any bony protuberance of the skull, but
especially on the mastoid process near the ear, causes
the person to hear the sound. However, the energy
available even in loud sound in the air is not sufﬁcient
to cause hearing via bone conduction unless a special
electro

aring via bone conduction unless a special
electromechanical sound-amplifying device is applied
to the bone.
Cochlea
The cochlea is a system of coiled tubes, shown in
Figure 52–1 and in cross section in Figures 52–2 and
52–3. It consists of three tubes coiled side by side: (1)
the scala vestibuli, (2) the scala media, and (3) the scala
tympani. The scala vestibuli and scala media are sepa-
rated from each other by Reissner’s membrane (also
called the vestibular 

y Reissner’s membrane (also
called the vestibular membrane), shown in Figure 52–3;
the scala tympani and scala media are separated from
each other by the basilar membrane. On the surface of
the basilar membrane lies the organ of Corti, which
contains a series of electromechanically sensitive 
cells, the hair cells. They are the receptive end organs
that generate nerve impulses in response to sound
vibrations.
Figure 52–4 diagrams the functional parts of the
uncoiled cochlea for conduction of sou

arts of the
uncoiled cochlea for conduction of sound vibrations.
First, note that Reissner’s membrane is missing from
Cochlear
nerve
Spiral
ganglion
Cochlea
Oval window
Scala tympani
Stapes
Incus
Malleus
Scala vestibuli
Round
window
Auditory canal
Tympanic membrane
Figure 52–1
Tympanic membrane, ossicular system of the middle ear, and
inner ear.
Basilar
membrane
Spiral organ
of Corti
Vestibular membrane
Stria
vascularis
Scala media
Cochlear nerve
Spiral ganglion
Spi

a
Spiral
ligament
Figure 52–2
Cochlea. (Redrawn from Gray H, Goss CM [eds]: Gray’s Anatomy
of the Human Body. Philadelphia: Lea & Febiger, 1948.)
Chapter 52
653
this ﬁgure. This membrane is so thin and so easily
moved that it does not obstruct the passage of sound
vibrations from the scala vestibuli into the scala media.
Therefore, as far as ﬂuid conduction of sound is con-
cerned, the scala vestibuli and scala media are con-
side

 the scala vestibuli and scala media are con-
sidered to be a single chamber. (The importance of
Reissner’s membrane is to maintain a special kind of
ﬂuid in the scala media that is required for normal
function of the sound-receptive hair cells, as discussed
later in the chapter.)
Sound vibrations enter the scala vestibuli from the
faceplate of the stapes at the oval window. The face-
plate covers this window and is connected with the
window’s edges by a loose annular ligament so that it
can mov

ges by a loose annular ligament so that it
can move inward and outward with the sound vibra-
tions. Inward movement causes the ﬂuid to move
forward in the scala vestibuli and scala media, and
outward movement causes the ﬂuid to move back-
ward.
Basilar Membrane and Resonance in the Cochlea. The
basilar membrane is a ﬁbrous membrane that sepa-
rates the scala media from the scala tympani. It 
contains 20,000 to 30,000 basilar ﬁbers that project
from the bony center of the cochlea, the modiolus,
t

om the bony center of the cochlea, the modiolus,
toward the outer wall. These ﬁbers are stiff, elastic,
reedlike structures that are ﬁxed at their basal ends 
in the central bony structure of the cochlea (the modi-
olus) but are not ﬁxed at their distal ends, except 
that the distal ends are embedded in the loose basilar
membrane. Because the ﬁbers are stiff and free at 
one end, they can vibrate like the reeds of a 
harmonica.
The lengths of the basilar ﬁbers increase progres-
sively beginning 

 basilar ﬁbers increase progres-
sively beginning at the oval window and going from
the base of the cochlea to the apex, increasing from a
length of about 0.04 millimeter near the oval and
round windows to 0.5 millimeter at the tip of the
cochlea (the “helicotrema”), a 12-fold increase in
length.
The diameters of the ﬁbers, however, decrease from
the oval window to the helicotrema, so that their
overall stiffness decreases more than 100-fold. As a
result, the stiff, short ﬁbers near the oval win

a
result, the stiff, short ﬁbers near the oval window of the
cochlea vibrate best at a very high frequency, while the
long, limber ﬁbers near the tip of the cochlea vibrate
best at a low frequency.
Thus, high-frequency resonance of the basilar mem-
brane occurs near the base, where the sound waves
enter the cochlea through the oval window. But low-
frequency resonance occurs near the helicotrema,
mainly because of the less stiff ﬁbers but also because
of increased “loading” with extra masses of 

cause
of increased “loading” with extra masses of ﬂuid that
must vibrate along the cochlear tubules.
Tectorial membrane
Basilar membrane
Organ of Corti
Spiral
prominence
Spiral limbus
Reissner's membrane
Stria vascularis
Figure 52–3
Section through one of the turns
of the cochlea. (Drawn by Sylvia
Colard Keene. From Fawcett DW:
Bloom & Fawcett: A Textbook of
Histology, 11th ed. Philadelphia:
and scala m

a:
and scala media
Oval
window
Basilar
membrane
Scala
tympani
Round
window
Stapes
Helicotrema
Figure 52–4
Movement of ﬂuid in the cochlea after forward thrust of the stapes.
654
Unit X
Transmission of Sound Waves in the
When the foot of the stapes moves inward against the
oval window, the round window must bulge outward
because the cochlea is bounded on all sides by bony
walls.The initial effec

unded on all sides by bony
walls.The initial effect of a sound wave entering at the
oval window is to cause the basilar membrane at 
the base of the cochlea to bend in the direction of the
round window. However, the elastic tension that is
built up in the basilar ﬁbers as they bend toward the
round window initiates a ﬂuid wave that “travels”
along the basilar membrane toward the helicotrema,
as shown in Figure 52–5. Figure 52–5A shows move-
ment of a high-frequency wave down the basilar mem-
bra

of a high-frequency wave down the basilar mem-
brane; Figure 52–5B, a medium-frequency wave; and
Figure 52–5C, a very low frequency wave. Movement
of the wave along the basilar membrane is compara-
ble to the movement of a pressure wave along the arte-
rial walls, which is discussed in Chapter 15; it is also
comparable to a wave that travels along the surface of
a pond.
Pattern of Vibration of the Basilar Membrane for Different
Sound Frequencies. Note in Figure 52–5 the different
patterns of tra

 Note in Figure 52–5 the different
patterns of transmission for sound waves of different
frequencies. Each wave is relatively weak at the outset
but becomes strong when it reaches that portion of the
basilar membrane that has a natural resonant fre-
quency equal to the respective sound frequency. At
this point, the basilar membrane can vibrate back and
forth with such ease that the energy in the wave is dis-
sipated. Consequently, the wave dies at this point and
fails to travel the remaining dis

t this point and
fails to travel the remaining distance along the basilar
membrane. Thus, a high-frequency sound wave travels
only a short distance along the basilar membrane
before it reaches its resonant point and dies, a
medium-frequency sound wave travels about halfway
and then dies, and a very low frequency sound wave
travels the entire distance along the membrane.
Another feature of the traveling wave is that it
travels fast along the initial portion of the basilar
membrane but becomes pro

al portion of the basilar
membrane but becomes progressively slower as it goes
farther into the cochlea. The cause of this is the high
coefﬁcient of elasticity of the basilar ﬁbers near the
oval window and a progressively decreasing coefﬁcient
farther along the membrane. This rapid initial trans-
mission of the wave allows the high-frequency sounds
to travel far enough into the cochlea to spread out and
separate from one another on the basilar membrane.
Without this, all the high-frequency waves

mbrane.
Without this, all the high-frequency waves would be
bunched together within the ﬁrst millimeter or so of
the basilar membrane, and their frequencies could not
be discriminated from one another.
Amplitude Pattern of Vibration of the Basilar Mem-
brane. The dashed curves of Figure 52–6A show the
position of a sound wave on the basilar membrane
when the stapes (a) is all the way inward, (b) has
moved back to the neutral point, (c) is all the way
outward, and (d) has moved back again to the 

 way
outward, and (d) has moved back again to the neutral
point but is moving inward. The shaded area around
these different waves shows the extent of vibration of
the basilar membrane during a complete vibratory
cycle. This is the amplitude pattern of vibration of 
the basilar membrane for this particular sound 
Figure 52–6B shows the amplitude patterns of vibra-
tion for different frequencies, demonstrating that the
High frequency
A
B
C
Medium frequency
Low frequency
Figure 52–5
“Tr

B
C
“Traveling waves” along the basilar membrane for high-, 
medium-, and low-frequency sounds.
A
B
Distance from stapes (millimeters)
0
5
8000
a
b
d
c
4000
2000
1000
600
400
200
10
20
30
15
25
Frequency
35
Figure 52–6
A, Amplitude pattern of vibration of the basilar membrane for 
a medium-frequency sound. B, Amplitude patterns for sounds of
frequencies between 200 and 8000 cycles per second, showing
the points of maximum amplitude on the basilar membra

 points of maximum amplitude on the basilar membrane for the
different frequencies.
Chapter 52
655
maximum amplitude for sound at 8000 cycles per
second occurs near the base of the cochlea, whereas
that for frequencies less than 200 cycles per second is
all the way at the tip of the basilar membrane near the
helicotrema, where the scala vestibuli opens into the
scala tympani.
The principal method by which sound frequencies
are discriminated from one another is based on the
“

e discriminated from one another is based on the
“place” of maximum stimulation of the nerve ﬁbers
from the organ of Corti lying on the basilar membrane,
as explained in the next section.
The organ of Corti, shown in Figures 52–2, 52–3, and
52–7, is the receptor organ that generates nerve
impulses in response to vibration of the basilar mem-
brane. Note that the organ of Corti lies on the surface
of the basilar ﬁbers and basilar membrane. The actual
sensory recepto

s and basilar membrane. The actual
sensory receptors in the organ of Corti are two spe-
cialized types of nerve cells called hair cells—a single
row of internal (or “inner”) hair cells, numbering about
3500 and measuring about 12 micrometers in diame-
ter, and three or four rows of external (or “outer”) hair
cells, numbering about 12,000 and having diameters 
of only about 8 micrometers. The bases and sides of
the hair cells synapse with a network of cochlea 
nerve endings. Between 90 and 95 per

k of cochlea 
nerve endings. Between 90 and 95 per cent of these
endings terminate on the inner hair cells, which
emphasizes their special importance for the detection
of sound.
The nerve ﬁbers stimulated by the hair cells lead to
the spiral ganglion of Corti, which lies in the modiolus
(center) of the cochlea. The spiral ganglion neuronal
cells send axons—a total of about 30,000—into the
cochlear nerve and then into the central nervous
system at the level of the upper medulla. The relation
of t

 the level of the upper medulla. The relation
of the organ of Corti to the spiral ganglion and to the
cochlear nerve is shown in Figure 52–2.
Excitation of the Hair Cells. Note in Figure 52–7 that
minute hairs, or stereocilia, project upward from the
hair cells and either touch or are embedded in the
surface gel coating of the tectorial membrane, which
lies above the stereocilia in the scala media.These hair
cells are similar to the hair cells found in the macula
and cristae ampullaris of the ve

und in the macula
and cristae ampullaris of the vestibular apparatus,
which are discussed in Chapter 55. Bending of the
hairs in one direction depolarizes the hair cells, and
bending in the opposite direction hyperpolarizes them.
This in turn excites the auditory nerve ﬁbers synaps-
ing with their bases.
Figure 52–8 shows the mechanism by which vibra-
tion of the basilar membrane excites the hair endings.
The outer ends of the hair cells are ﬁxed tightly in a
rigid structure composed of a ﬂat pl

 tightly in a
rigid structure composed of a ﬂat plate, called the retic-
ular lamina, supported by triangular rods of Corti,
which are attached tightly to the basilar ﬁbers. The
basilar ﬁbers, the rods of Corti, and the reticular
lamina move as a rigid unit.
Upward movement of the basilar ﬁber rocks the
reticular lamina upward and inward toward the modi-
olus. Then, when the basilar membrane moves down-
ward, the reticular lamina rocks downward and
outward. The inward and outward motion causes t

nd
outward. The inward and outward motion causes the
hairs on the hair cells to shear back and forth against
the tectorial membrane.Thus, the hair cells are excited
whenever the basilar membrane vibrates.
Auditory Signals Are Transmitted Mainly by the Inner Hair Cells.
Even though there are three to four times as many
outer hair cells as inner hair cells, about 90 per cent of
the auditory nerve ﬁbers are stimulated by the inner
cells rather than by the outer cells. Yet, despite this, if
the oute

by the outer cells. Yet, despite this, if
the outer cells are damaged while the inner cells
remain fully functional, a large amount of hearing 
loss occurs. Therefore, it has been proposed that the
outer hair cells in some way control the sensitivity 
of the inner hair cells at different sound pitches, a 
Basilar fiber
Outer hair cells
Inner hair cells
Figure 52–7
Organ of Corti, showing especially the hair cells and the tectorial
membrane pressi

y the hair cells and the tectorial
membrane pressing against the projecting hairs.
Reticular lamina
Rods of Corti
Modiolus
Hairs
Figure 52–8
Stimulation of the hair cells by to-and-fro movement of the hairs
projecting into the gel coating of the tectorial membrane.
656
Unit X
phenomenon called “tuning” of the receptor system.
In support of this concept, a large number of retro-
grade nerve ﬁbers pass from the brain stem t

etro-
grade nerve ﬁbers pass from the brain stem to the
vicinity of the outer hair cells. Stimulating these nerve
ﬁbers can actually cause shortening of the outer hair
cells and possibly also change their degree of stiffness.
These effects suggest a retrograde nervous mechanism
for control of the ear’s sensitivity to different sound
pitches, activated through the outer hair cells.
Hair Cell Receptor Potentials and Excitation of Auditory Nerve
Fibers. The stereocilia (the “hairs”protruding from t

ers. The stereocilia (the “hairs”protruding from the
ends of the hair cells) are stiff structures because each
has a rigid protein framework. Each hair cell has about
100 stereocilia on its apical border.These become pro-
gressively longer on the side of the hair cell away from
the modiolus,and the tops of the shorter stereocilia are
attached by thin ﬁlaments to the back sides of their
adjacent longer stereocilia. Therefore, whenever the
cilia are bent in the direction of the longer ones, the
ti

e bent in the direction of the longer ones, the
tips of the smaller stereocilia are tugged outward from
the surface of the hair cell. This causes a mechanical
transduction that opens 200 to 300 cation-conducting
channels, allowing rapid movement of positively
charged potassium ions from the surrounding scala
media ﬂuid into the stereocilia, which causes depolar-
ization of the hair cell membrane.
Thus, when the basilar ﬁbers bend toward the scala
vestibuli, the hair cells depolarize, and in the 

vestibuli, the hair cells depolarize, and in the opposite
direction they hyperpolarize, thereby generating an
alternating hair cell receptor potential. This, in turn,
stimulates the cochlear nerve endings that synapse
with the bases of the hair cells. It is believed that a
rapidly acting neurotransmitter is released by the hair
cells at these synapses during depolarization. It is pos-
sible that the transmitter substance is glutamate, but
this is not certain.
Endocochlear Potential. To explain 

s not certain.
Endocochlear Potential. To explain even more fully the
electrical potentials generated by the hair cells, we need
to explain another electrical phenomenon called the
endocochlear potential: The scala media is ﬁlled with a
ﬂuid called endolymph, in contradistinction to the peri-
lymph present in the scala vestibuli and scala tympani.
The scala vestibuli and scala tympani communicate
directly with the subarachnoid space around the brain,
so that the perilymph is almost identical wit

ain,
so that the perilymph is almost identical with cere-
brospinal ﬂuid. Conversely, the endolymph that ﬁlls 
the scala media is an entirely different ﬂuid secreted by
the stria vascularis, a highly vascular area on the outer
wall of the scala media. Endolymph contains a high 
concentration of potassium and a low concentration 
of sodium, which is exactly opposite to the contents of
perilymph.
An electrical potential of about +80 millivolts exists
all the time between endolymph and perilymph, w

ts
all the time between endolymph and perilymph, with
positivity inside the scala media and negativity outside.
This is called the endocochlear potential, and it is gen-
erated by continual secretion of positive potassium ions
into the scala media by the stria vascularis.
The importance of the endocochlear potential is that
the tops of the hair cells project through the reticular
lamina and are bathed by the endolymph of the scala
media, whereas perilymph bathes the lower bodies of
the hair cell

perilymph bathes the lower bodies of
the hair cells. Furthermore, the hair cells have a nega-
tive intracellular potential of –70 millivolts with respect
to the perilymph but –150 millivolts with respect to the
endolymph at their upper surfaces where the hairs
project through the reticular lamina and into the
endolymph. It is believed that this high electrical poten-
tial at the tips of the stereocilia sensitizes the cell an
extra amount, thereby increasing its ability to respond
to the slightes

 increasing its ability to respond
to the slightest sound.
Determination of Sound Frequency—
The “Place” Principle
From earlier discussions in this chapter, it is apparent
that low-frequency sounds cause maximal activation
of the basilar membrane near the apex of the cochlea,
and high-frequency sounds activate the basilar mem-
brane near the base of the cochlea. Intermediate-
frequency sounds activate the membrane at inter-
mediate distances between the two extremes.
Furthermore, there is spatia

een the two extremes.
Furthermore, there is spatial organization of the nerve
ﬁbers in the cochlear pathway, all the way from the
cochlea to the cerebral cortex. Recording of signals in
the auditory tracts of the brain stem and in the audi-
tory receptive ﬁelds of the cerebral cortex shows that
speciﬁc brain neurons are activated by speciﬁc sound
frequencies. Therefore, the major method used by 
the nervous system to detect different sound frequen-
cies is to determine the positions along the ba

n-
cies is to determine the positions along the basilar
membrane that are most stimulated. This is called 
the place principle for the determination of sound 
Yet, referring again to Figure 52–6, one can see that
the distal end of the basilar membrane at the heli-
cotrema is stimulated by all sound frequencies below
200 cycles per second. Therefore, it has been difﬁcult
to understand from the place principle how one can
differentiate between low sound frequencies in the
range from 200

etween low sound frequencies in the
range from 200 down to 20. It is postulated that these
low frequencies are discriminated mainly by the so-
called volley or frequency principle. That is, low-
frequency sounds, from 20 to 1500 to 2000 cycles per
second, can cause volleys of nerve impulses synchro-
nized at the same frequencies, and these volleys are
transmitted by the cochlear nerve into the cochlear
nuclei of the brain. It is further suggested that the
cochlear nuclei can distinguish the diff

 that the
cochlear nuclei can distinguish the different frequen-
cies of the volleys. In fact, destruction of the entire
apical half of the cochlea, which destroys the basilar
membrane where all lower-frequency sounds are nor-
mally detected, does not totally eliminate discrimina-
tion of the lower-frequency sounds.
Loudness is determined by the auditory system in at
least three ways.
First, as the sound becomes louder, the amplitude of
vibration of the basilar membrane

the amplitude of
vibration of the basilar membrane and hair cells also
increases, so that the hair cells excite the nerve endings
at more rapid rates.
Chapter 52
657
Second, as the amplitude of vibration increases, it
causes more and more of the hair cells on the fringes
of the resonating portion of the basilar membrane to
become stimulated, thus causing spatial summation of
impulses—that is, transmission through many nerve
ﬁbers rather than through only a few.
Third, the ou

bers rather than through only a few.
Third, the outer hair cells do not become stimulated
signiﬁcantly until vibration of the basilar membrane
reaches high intensity, and stimulation of these cells
presumably apprises the nervous system that the
sound is loud.
Detection of Changes in Loudness—The Power Law. As
pointed out in Chapter 46, a person interprets changes
in intensity of sensory stimuli approximately in pro-
portion to an inverse power function of the actual
intensity. In the case of so

unction of the actual
intensity. In the case of sound, the interpreted sensa-
tion changes approximately in proportion to the cube
root of the actual sound intensity. To express this
another way, the ear can discriminate differences in
sound intensity from the softest whisper to the loudest
possible noise, representing an approximately 1 trillion
times increase in sound energy or 1 million times
increase in amplitude of movement of the basilar
membrane.Yet the ear interprets this much difference

mbrane.Yet the ear interprets this much difference
in sound level as approximately a 10,000-fold change.
Thus, the scale of intensity is greatly “compressed” by
the sound perception mechanisms of the auditory
system.This allows a person to interpret differences in
sound intensities over an extremely wide range—a far
wider range than would be possible were it not for
compression of the intensity scale.
Decibel Unit. Because of the extreme changes in sound
intensities that the ear can detect and d

in sound
intensities that the ear can detect and discriminate,
sound intensities are usually expressed in terms of the
logarithm of their actual intensities.A 10-fold increase
in sound energy is called 1 bel, and 0.1 bel is called 1
decibel. One decibel represents an actual increase in
sound energy of 1.26 times.
Another reason for using the decibel system to
express changes in loudness is that, in the usual sound
intensity range for communication, the ears can barely
distinguish an approximatel

n, the ears can barely
distinguish an approximately 1-decibel change in
sound intensity.
Threshold for Hearing Sound at Different Frequencies. Figure
52–9 shows the pressure thresholds at which sounds of
different frequencies can barely be heard by the ear.
This ﬁgure demonstrates that a 3000-cycle-per-second
sound can be heard even when its intensity is as low as
70 decibels below 1 dyne/cm2 sound pressure level,
which is one ten-millionth microwatt per square cen-
timeter. Conversely, a 100-cy

watt per square cen-
timeter. Conversely, a 100-cycle-per-second sound can
be detected only if its intensity is 10,000 times as great
as this.
Frequency Range of Hearing. The frequencies of sound that
a young person can hear are between 20 and 20,000
cycles per second. However, referring again to Figure
52–9, we see that the sound range depends to a great
extent on loudness. If the loudness is 60 decibels below
1 dyne/cm2 sound pressure level, the sound range is 500
to 5000 cycles per second; on

e sound range is 500
to 5000 cycles per second; only with intense sounds can
the complete range of 20 to 20,000 cycles be achieved.
In old age, this frequency range is usually shortened to
50 to 8000 cycles per second or less, as discussed later in
Figure 52–10 shows the major auditory pathways. It
shows that nerve ﬁbers from the spiral ganglion of
Corti enter the dorsal and ventral cochlear nuclei
located in the upper part of th

al cochlear nuclei
located in the upper part of the medulla.At this point,
all the ﬁbers synapse, and second-order neurons pass
mainly to the opposite side of the brain stem to ter-
minate in the superior olivary nucleus. A few second-
order ﬁbers also pass to the superior olivary nucleus
on the same side. From the superior olivary nucleus,
the auditory pathway passes upward through the
lateral lemniscus. Some of the ﬁbers terminate in the
nucleus of the lateral lemniscus, but many bypass this
n

s of the lateral lemniscus, but many bypass this
nucleus and travel on to the inferior colliculus, where
all or almost all the auditory ﬁbers synapse. From
there, the pathway passes to the medial geniculate
nucleus, where all the ﬁbers do synapse. Finally, the
pathway proceeds by way of the auditory radiation to
the auditory cortex, located mainly in the superior
gyrus of the temporal lobe.
Several important points should be noted. First,
signals from both ears are transmitted through the
pathwa

 from both ears are transmitted through the
pathways of both sides of the brain, with a prepon-
derance of transmission in the contralateral pathway.
In at least three places in the brain stem, crossing over
occurs between the two pathways: (1) in the trapezoid
body, (2) in the commissure between the two nuclei of
Pressure in db
(0 decibel = 1 dyne/cm2)
Frequency
1 2
Vibration
Sound
5 10 20
100
500
2000 10,000
–80
–60
–40
–20
0
20
40
60
80
100
Pricking
(in middle ear)
Tactual
threshold
Threshold

icking
Tactual
threshold
Threshold
of hearing
Reference
pressure = -73.8
Figure 52–9
Relation of the threshold of hearing and of somesthetic percep-
tion (pricking and tactual threshold) to the sound energy level at
each sound frequency.
658
Unit X
the lateral lemnisci, and (3) in the commissure con-
necting the two inferior colliculi.
Second, many collateral ﬁbers from the auditory
tracts pass directly into the reticular activating syste

 pass directly into the reticular activating system
of the brain stem. This system projects diffusely
upward in the brain stem and downward into the
spinal cord and activates the entire nervous system in
response to loud sounds. Other collaterals go to the
vermis of the cerebellum, which is also activated instan-
taneously in the event of a sudden noise.
Third, a high degree of spatial orientation is main-
tained in the ﬁber tracts from the cochlea all the way
to the cortex. In fact, there are t

ea all the way
to the cortex. In fact, there are three spatial patterns for
termination of the different sound frequencies in the
cochlear nuclei, two patterns in the inferior colliculi,
one precise pattern for discrete sound frequencies in
the auditory cortex, and at least ﬁve other less precise
patterns in the auditory cortex and auditory associa-
tion areas.
Firing Rates at Different Levels of the Auditory Pathways. Single
nerve ﬁbers entering the cochlear nuclei from the audi-
tory nerve can

 the cochlear nuclei from the audi-
tory nerve can ﬁre at rates up to at least 1000 per
second, the rate being determined mainly by the loud-
ness of the sound. At sound frequencies up to 2000 to
4000 cycles per second, the auditory nerve impulses are
often synchronized with the sound waves, but they do
not necessarily occur with every wave.
In the auditory tracts of the brain stem, the ﬁring is
usually no longer synchronized with the sound fre-
quency, except at sound frequencies below 200 cycl

quency, except at sound frequencies below 200 cycles
per second.Above the level of the inferior colliculi,even
this synchronization is mainly lost. These ﬁndings
demonstrate that the sound signals are not transmitted
unchanged directly from the ear to the higher levels of
the brain; instead, information from the sound signals
begins to be dissected from the impulse trafﬁc at levels
as low as the cochlear nuclei. We will have more to say
about this later, especially in relation to perception of
d

s later, especially in relation to perception of
direction from which sound comes.
Function of the Cerebral Cortex 
in Hearing
The projection area of auditory signals to the cerebral
cortex is shown in Figure 52–11, which demonstrates
that the auditory cortex lies principally on the
supratemporal plane of the superior temporal gyrus but
also extends onto the lateral side of the temporal lobe,
over much of the insular cortex, and even onto the
lateral portion of the parietal operculum.
Two separa

eral portion of the parietal operculum.
Two separate subdivisions are shown in Figure
52–11: the primary auditory cortex and the auditory
association cortex (also called the secondary auditory
cortex).The primary auditory cortex is directly excited
by projections from the medial geniculate body,
whereas the auditory association areas are excited
secondarily by impulses from the primary auditory
cortex as well as by some projections from thalamic
association areas adjacent to the medial geniculat

association areas adjacent to the medial geniculate
body.
Sound Frequency Perception in the Primary Auditory Cortex.
At least six tonotopic maps have been found in the
primary auditory cortex and auditory association
areas. In each of these maps, high-frequency sounds
excite neurons at one end of the map, whereas low-
frequency sounds excite neurons at the opposite end.
In most, the low-frequency sounds are located anteri-
orly, as shown in Figure 52–11, and the high-frequency
sounds are located

e 52–11, and the high-frequency
sounds are located posteriorly. This is not true for all
Primary
auditry
cortex
Medial
geniculate
nucleus
Inferior
Nucleus of
the lateral
lemniscus
Superior
olivary
nuclei
acoustic
site
Medulla
Trapezoid
body
Midbrain
Midbrain
Pons
Pons
Dorsal
acoustic
stria
Cochlear
nuclei
N. VIlI
Figure 52–10
Auditory nervous pathways. (Modiﬁed from Brodal A: The auditory
system. In Neurological Anatomy in Relation to Clinical Medicine,
3rd ed. New York: 

 Relation to Clinical Medicine,
3rd ed. New York: Oxford University Press, 1981.)
Chapter 52
659
the maps. The question one must ask is, Why does the
auditory cortex have so many different tonotopic
maps? The answer, presumably, is that each of the sep-
arate areas dissects out some speciﬁc feature of the
sounds. For instance, one of the large maps in the
primary auditory cortex almost certainly discriminates
the sound frequencies themselves and gives the person
the psychic 

ncies themselves and gives the person
the psychic sensation of sound pitches. Another map
is probably used to detect the direction from which the
sound comes. Other auditory cortex areas detect
special qualities, such as the sudden onset of sounds,
or perhaps special modulations, such as noise versus
pure frequency sounds.
The frequency range to which each individual
neuron in the auditory cortex responds is much 
narrower than that in the cochlear and brain stem
relay nuclei. Referring back to 

ar and brain stem
relay nuclei. Referring back to Figure 52–6B, note that
the basilar membrane near the base of the cochlea is
stimulated by sounds of all frequencies, and in the
cochlear nuclei, this same breadth of sound represen-
tation is found. Yet, by the time the excitation has
reached the cerebral cortex, most sound-responsive
neurons respond to only a narrow range of frequen-
cies rather than to a broad range. Therefore, some-
where along the pathway, processing mechanisms
“sharpen” the

g the pathway, processing mechanisms
“sharpen” the frequency response. It is believed that
this sharpening effect is caused mainly by the phe-
nomenon of lateral inhibition, which is discussed 
in Chapter 46 in relation to mechanisms for transmit-
ting information in nerves. That is, stimulation of the
cochlea at one frequency inhibits sound frequencies 
on both sides of this primary frequency; this is 
caused by collateral ﬁbers angling off the primary
signal pathway and exerting inhibitory inﬂ

primary
signal pathway and exerting inhibitory inﬂuences on 
adjacent pathways. The same effect has been demon-
strated to be important in sharpening patterns of
somesthetic images, visual images, and other types of
sensations.
Many of the neurons in the auditory cortex, espe-
cially in the auditory association cortex, do not respond
only to speciﬁc sound frequencies in the ear. It is
believed that these neurons “associate” different
sound frequencies with one another or associate sound
informat

ncies with one another or associate sound
information with information from other sensory
areas of the cortex. Indeed, the parietal portion of the
auditory association cortex partly overlaps somatosen-
sory area II, which could provide an easy opportunity
for the association of auditory information with
somatosensory information.
Discrimination of Sound “Patterns” by the Auditory Cortex.
Complete bilateral removal of the auditory cortex
does not prevent a cat or monkey from detecting
sounds or r

prevent a cat or monkey from detecting
sounds or reacting in a crude manner to sounds.
However, it does greatly reduce or sometimes 
even abolish the animal’s ability to discriminate dif-
ferent sound pitches and especially patterns of sound.
For instance, an animal that has been trained to rec-
ognize a combination or sequence of tones, one fol-
lowing the other in a particular pattern, loses this
ability when the auditory cortex is destroyed; further-
more, the animal cannot relearn this type 

urther-
more, the animal cannot relearn this type of response.
Therefore, the auditory cortex is especially important
in the discrimination of tonal and sequential sound 
patterns.
Destruction of both primary auditory cortices in 
the human being greatly reduces one’s sensitivity 
for hearing. Destruction of one side only slightly
reduces hearing in the opposite ear; it does not cause
deafness in the ear because of many crossover con-
nections from side to side in the auditory neural
pathway. Ho

om side to side in the auditory neural
pathway. However, it does affect one’s ability to 
localize the source of a sound, because comparative
signals in both cortices are required for the localiza-
tion function.
Lesions that affect the auditory association areas
but not the primary auditory cortex do not decrease a
person’s ability to hear and differentiate sound tones
or even to interpret at least simple patterns of sound.
However, he or she is often unable to interpret the
meaning of the soun

 often unable to interpret the
meaning of the sound heard. For instance, lesions in
the posterior portion of the superior temporal gyrus,
which is called Wernicke’s area and is part of the audi-
tory association cortex, often make it impossible for a
person to interpret the meanings of words even though
he or she hears them perfectly well and can even
repeat them. These functions of the auditory associa-
tion areas and their relation to the overall intellectual
functions of the brain are discuss

ll intellectual
functions of the brain are discussed in more detail in
Chapter 57.
Low
frequency
High
frequency
Primary
Primary
Association
Figure 52–11
Auditory cortex.
660
Unit X
Determination of the Direction from
Which Sound Comes
A person determines the horizontal direction from
which sound comes by two principal means: (1) the
time lag between the entry of sound into one ear and
its entry into the opposite ear, and (2) the difference
be

y into the opposite ear, and (2) the difference
between the intensities of the sounds in the two ears.
The ﬁrst mechanism functions best at frequencies
below 3000 cycles per second, and the second mecha-
nism operates best at higher frequencies because the
head is a greater sound barrier at these frequencies.
The time lag mechanism discriminates direction much
more exactly than the intensity mechanism because it
does not depend on extraneous factors but only on the
exact interval of time between

ors but only on the
exact interval of time between two acoustical signals.
If a person is looking straight toward the source of the
sound, the sound reaches both ears at exactly the same
instant, whereas if the right ear is closer to the sound
than the left ear is, the sound signals from the right ear
enter the brain ahead of those from the left ear.
The two aforementioned mechanisms cannot tell
whether the sound is emanating from in front of or
behind the person or from above or below. This dis

behind the person or from above or below. This dis-
crimination is achieved mainly by the pinnae of the
two ears. The shape of the pinna changes the quality
of the sound entering the ear, depending on the direc-
tion from which the sound comes. It does this by
emphasizing speciﬁc sound frequencies from the dif-
ferent directions.
Neural Mechanisms for Detecting Sound Direction. Destruc-
tion of the auditory cortex on both sides of the brain,
whether in human beings or in lower mammals, causes
lo

her in human beings or in lower mammals, causes
loss of almost all ability to detect the direction from
which sound comes. Yet the neural analyses for this
detection process begin in the superior olivary nuclei
in the brain stem, even though the neural pathways all
the way from these nuclei to the cortex are required
for interpretation of the signals. The mechanism is
believed to be the following.
The superior olivary nucleus is divided into two sec-
tions: (1) the medial superior olivary nucleu

sec-
tions: (1) the medial superior olivary nucleus and (2)
the lateral superior olivary nucleus. The lateral nucleus
is concerned with detecting the direction from which
the sound is coming, presumably by simply comparing
the difference in intensities of the sound reaching the
two ears and sending an appropriate signal to the audi-
tory cortex to estimate the direction.
The medial superior olivary nucleus, however, has a
speciﬁc mechanism for detecting the time lag between
acoustical signals en

tecting the time lag between
acoustical signals entering the two ears. This nucleus
contains large numbers of neurons that have two
major dendrites, one projecting to the right and the
other to the left. The acoustical signal from the right
ear impinges on the right dendrite, and the signal from
the left ear impinges on the left dendrite.The intensity
of excitation of each neuron is highly sensitive to a
speciﬁc time lag between the two acoustical signals
from the two ears.The neurons near one b

l signals
from the two ears.The neurons near one border of the
nucleus respond maximally to a short time lag, while
those near the opposite border respond to a long time
lag; those in between respond to intermediate time
lags. Thus, a spatial pattern of neuronal stimulation
develops in the medial superior olivary nucleus, with
sound from directly in front of the head stimulating
one set of olivary neurons maximally and sounds from
different side angles stimulating other sets of neurons
on opposi

angles stimulating other sets of neurons
on opposite sides. This spatial orientation of signals is
then transmitted to the auditory cortex, where sound
direction is determined by the locus of the maximally
stimulated neurons. It is believed that all these signals
for determining sound direction are transmitted
through a different pathway and excite a different
locus in the cerebral cortex from the transmission
pathway and termination locus for tonal patterns of
sound.
This mechanism for detectio

nal patterns of
sound.
This mechanism for detection of sound direction
indicates again how speciﬁc information in sensory
signals is dissected out as the signals pass through 
different levels of neuronal activity. In this case, the
“quality” of sound direction is separated from the
“quality” of sound tones at the level of the superior
olivary nuclei.
Centrifugal Signals from the 
Central Nervous System to Lower
Auditory Centers
Retrograde pathways have been demonstrated at each
level of the aud

ys have been demonstrated at each
level of the auditory nervous system from the cortex to
the cochlea in the ear itself. The ﬁnal pathway is mainly
from the superior olivary nucleus to the sound-receptor
hair cells in the organ of Corti.
These retrograde ﬁbers are inhibitory. Indeed, direct
stimulation of discrete points in the olivary nucleus has
been shown to inhibit speciﬁc areas of the organ of
Corti, reducing their sound sensitivities 15 to 20 deci-
bels. One can readily understand how this

20 deci-
bels. One can readily understand how this could allow
a person to direct his or her attention to sounds of par-
ticular qualities while rejecting sounds of other quali-
ties. This is readily demonstrated when one listens to a
single instrument in a symphony orchestra.
Deafness is usually divided into two types: (1) that
caused by impairment of the cochlea or impairment of
the auditory nerve, which is usually classiﬁed as “nerve
deafness,” and (2) 

is usually classiﬁed as “nerve
deafness,” and (2) that caused by impairment of the
physical structures of the ear that conduct sound 
itself to the cochlea, which is usually called “conduction
deafness.”
If either the cochlea or the auditory nerve is
destroyed, the person becomes permanently deaf.
However, if the cochlea and nerve are still intact but the
tympanum-ossicular system has been destroyed or
ankylosed (“frozen” in place by ﬁbrosis or calciﬁcation),
sound waves can still be conducted i

alciﬁcation),
sound waves can still be conducted into the cochlea by
means of bone conduction from a sound generator
applied to the skull over the ear.
Audiometer. To determine the nature of hearing dis-
abilities, the “audiometer” is used. Simply an earphone
Chapter 52
661
connected to an electronic oscillator capable of emitting
pure tones ranging from low frequencies to high fre-
quencies, the instrument is calibrated so that zero-
intensity-level sound at each frequency 

hat zero-
intensity-level sound at each frequency is the loudness
that can barely be heard by the normal ear.A calibrated
volume control can increase the loudness above the
zero level. If the loudness must be increased to 30 deci-
bels above normal before it can be heard, the person is
said to have a hearing loss of 30 decibels at that partic-
ular frequency.
In performing a hearing test using an audiometer, one
tests about 8 to 10 frequencies covering the auditory
spectrum, and the hearing loss

vering the auditory
spectrum, and the hearing loss is determined for each of
these frequencies. Then the so-called audiogram is
plotted, as shown in Figures 52–12 and 52–13, depicting
hearing loss at each of the frequencies in the auditory
spectrum. The audiometer,
in addition to being
equipped with an earphone for testing air conduction
by the ear, is equipped with a mechanical vibrator for
testing bone conduction from the mastoid process of the
skull into the cochlea.
Audiogram in Nerve Deafne

Audiogram in Nerve Deafness. In nerve deafness—
which includes damage to the cochlea, the auditory
nerve, or the central nervous system circuits from the
ear—the person has decreased or total loss of ability to
hear sound as tested by both air conduction and bone
conduction.An audiogram depicting partial nerve deaf-
ness is shown in Figure 52–12. In this ﬁgure, the deaf-
ness is mainly for high-frequency sound. Such deafness
could be caused by damage to the base of the c

ess
could be caused by damage to the base of the cochlea.
This type of deafness occurs to some extent in almost
all older people.
Other patterns of nerve deafness frequently occur as
follows: (1) deafness for low-frequency sounds caused
by excessive and prolonged exposure to very loud
sounds (a rock band or a jet airplane engine), because
low-frequency sounds are usually louder and more dam-
aging to the organ of Corti, and (2) deafness for all 
frequencies caused by drug sensitivity of the orga

frequencies caused by drug sensitivity of the organ of
Corti—in particular, sensitivity to some antibiotics such
as streptomycin, kanamycin, and chloramphenicol.
Audiogram for Middle Ear Conduction Deafness. A
common type of deafness is caused by ﬁbrosis in the
middle ear following repeated infection or by ﬁbrosis
that occurs in the hereditary disease called otosclerosis.
In either case, the sound waves cannot be transmitted
easily through the ossicles from the tympanic mem-
brane to the oval wi

sicles from the tympanic mem-
brane to the oval window. Figure 52–13 shows an audio-
gram from a person with “middle ear air conduction
deafness.” In this case, bone conduction is essentially
normal, but conduction through the ossicular system is
greatly depressed at all frequencies, but more so at low
frequencies. In some instances of conduction deafness,
the faceplate of the stapes becomes “ankylosed” by
bone overgrowth to the edges of the oval window. In
this case, the person becomes totally 

 window. In
this case, the person becomes totally deaf for ossicular
conduction but can regain almost normal hearing by the
surgical removal of the stapes and its replacement with
a minute Teﬂon or metal prosthesis that transmits the
sound from the incus to the oval window.
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Frolenkov GI, Belyantseva IA, Friedman TB, Grifﬁth AJ:
Genetic insights into the morphogenesis of inner ear hair
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Grifﬁths TD, Warren JD, Scott SK, et al: Cort

004.
Grifﬁths TD, Warren JD, Scott SK, et al: Cortical processing
of complex sound: a way forward? Trends Neurosci 27:181,
2004.
Loss in decibels
Frequency
1
250
500
1000 2000 4000 8000
100
90
80
70
60
50
40
30
20
-10
10
Normal
X  Air conduction
X
* Bone conduction
*
X
*
X
*
X
*
X*
X
X
*
Figure 52–12
Audiogram of the old-age type of nerve deafness.
Frequency
125
250
500
100
90
80
70
60
50
40
30
20
-10
10
Normal
X
*
X
*
X
*

l
X
*
X
*
X
*
X
*
X
*
X
X
*
Figure 52–13
Audiogram of air conduction deafness resulting from middle ear
sclerosis.
662
Unit X
Grothe B: New roles for synaptic inhibition in sound local-
ization. Nat Rev Neurosci 4:540, 2003.
Joris PX, Schreiner CE, Rees A: Neural processing of ampli-
tude-modulated sounds. Physiol Rev 84:541, 2004.
McDermott BM Jr, Lopez-Schier H: Inner ear: Ca2+ you feel
the noise? Curr Biol 14:R231, 

 ear: Ca2+ you feel
the noise? Curr Biol 14:R231, 2004.
Minor LB, Schessel DA, Carey JP: Meniere’s disease. Curr
Opin Neurol 17:9, 2004.
Rauschecker JP, Shannon RV: Sending sound to the brain.
Science 295:1025, 2002.
Read HL, Winer JA, Schreiner CE: Functional architecture
of auditory cortex. Curr Opin Neurobiol 12:433, 2002.
Robles L, Ruggero MA: Mechanics of the mammalian
cochlea. Physiol Rev 81:1305, 2001.
Santos-Sacchi J: New tunes from Corti’s organ: the outer hair
cell boogie rules. Curr O

’s organ: the outer hair
cell boogie rules. Curr Opin Neurobiol 13:459, 2003.
Schreiner CE, Read HL, Sutter ML: Modular organization
of frequency integration in primary auditory cortex.Annu
Rev Neurosci 23:501, 2000.
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100, 2003.
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ametric corticofugal modulation and
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H
A
P
T
E
R
 
tex. Nat Rev Neurosci 5:279, 2004.
C
H
A
P
T
E
R
 
5
3
663
The Chemical Senses—
Taste and Smell
The senses of taste and smell allow us to separate
undesirable or even lethal foods from those that are
pleasant to eat and nutritious. The sense of smell
also allows animals to recognize the proximity of
other animals or even individuals among animals.
Finally, both senses are strongly tied to primitive
emotional and behavioral functions of our nervous
systems.
Taste is mainly a functi

s
systems.
Taste is mainly a function of the taste buds in the mouth, but it is common expe-
rience that one’s sense of smell also contributes strongly to taste perception. In
addition, the texture of food, as detected by tactual senses of the mouth, and
the presence of substances in the food that stimulate pain endings, such as
pepper, greatly alter the taste experience. The importance of taste lies in the
fact that it allows a person to select food in accord with desires and oft

rson to select food in accord with desires and often in
accord with the body tissues’ metabolic need for speciﬁc substances.
The identities of the speciﬁc chemicals that excite different taste receptors are
not all known. Even so, psychophysiologic and neurophysiologic studies have
identiﬁed at least 13 possible or probable chemical receptors in the taste cells,
as follows: 2 sodium receptors, 2 potassium receptors, 1 chloride receptor,
1 adenosine receptor, 1 inosine

chloride receptor,
1 adenosine receptor, 1 inosine receptor, 2 sweet receptors, 2 bitter receptors,
1 glutamate receptor, and 1 hydrogen ion receptor.
For practical analysis of taste, the aforementioned receptor capabilities have
also been grouped into ﬁve general categories called the primary sensations of
taste. They are sour, salty, sweet, bitter, and “umami.”
A person can perceive hundreds of different tastes. They are all supposed to
be combinations of the elementary taste sensations, just 

inations of the elementary taste sensations, just as all the colors we can
see are combinations of the three primary colors, as described in Chapter 50.
Sour Taste. The sour taste is caused by acids, that is, by the hydrogen ion con-
centration, and the intensity of this taste sensation is approximately propor-
tional to the logarithm of the hydrogen ion concentration.That is,the more acidic
the food, the stronger the sour sensation becomes.
Salty Taste. The salty taste is elicited by ionized sa

y Taste. The salty taste is elicited by ionized salts, mainly by the sodium ion
concentration. The quality of the taste varies somewhat from one salt to
another, because some salts elicit other taste sensations in addition to saltiness.
The cations of the salts, especially sodium cations, are mainly responsible for
the salty taste, but the anions also contribute to a lesser extent.
Sweet Taste. The sweet taste is not caused by any single class of chemicals. Some
of the types of chemicals that ca

 chemicals. Some
of the types of chemicals that cause this taste include sugars, glycols, alcohols,
aldehydes, ketones, amides, esters, some amino acids, some small proteins,
664
Unit X
sulfonic acids, halogenated acids, and inorganic salts of
lead and beryllium. Note speciﬁcally that most of the
substances that cause a sweet taste are organic chem-
icals. It is especially interesting that slight changes in
the chemical structure, such as addition of a s

in
the chemical structure, such as addition of a simple
radical, can often change the substance from sweet to
bitter.
Bitter Taste. The bitter taste, like the sweet taste, is not
caused by any single type of chemical agent. Here
again, the substances that give the bitter taste are
almost entirely organic substances. Two particular
classes of substances are especially likely to cause
bitter taste sensations: (1) long-chain organic sub-
stances that contain nitrogen, and (2) alkaloids. The
alkaloi

t contain nitrogen, and (2) alkaloids. The
alkaloids include many of the drugs used in medicines,
such as quinine, caffeine, strychnine, and nicotine.
Some substances that at ﬁrst taste sweet have a
bitter aftertaste.This is true of saccharin, which makes
this substance objectionable to some people.
The bitter taste, when it occurs in high intensity,
usually causes the person or animal to reject the food.
This is undoubtedly an important function of the bitter
taste sensation, because many deadl

 of the bitter
taste sensation, because many deadly toxins found in
poisonous plants are alkaloids, and virtually all of
these cause intensely bitter taste, usually followed by
rejection of the food.
Umami Taste. Umami is a Japanese word (meaning
“delicious”) designating a pleasant taste sensation that
is qualitatively different from sour, salty, sweet, or
bitter. Umami is the dominant taste of food contain-
ing L-glutamate, such as meat extracts and aging
cheese, and some physiologists consider

 and aging
cheese, and some physiologists consider it to be a sep-
arate, ﬁfth category of primary taste stimuli.
A taste receptor for L-glutamate may be related to
one of the glutamate receptors expressed in neuronal
synapses of the brain. However, the precise molecular
mechanisms responsible for umami taste are still
unclear.
Threshold for Taste
The threshold for stimulation of the sour taste by
hydrochloric acid averages 0.0009 N; for stimulation of
the salty taste by sodium chloride,0.01 M;f

ion of
the salty taste by sodium chloride,0.01 M;for the sweet
taste by sucrose, 0.01 M; and for the bitter taste by
quinine, 0.000008 M. Note especially how much more
sensitive is the bitter taste sense than all the others,
which would be expected, because this sensation pro-
vides an important protective function against many
dangerous toxins in food.
Table 53–1 gives the relative taste indices (the recip-
rocals of the taste thresholds) of different substances.
In this table, the intensities 

ferent substances.
In this table, the intensities of four of the primary sen-
sations of taste are referred, respectively, to the inten-
sities of the taste of hydrochloric acid, quinine, sucrose,
and sodium chloride, each of which is arbitrarily
chosen to have a taste index of 1.
Taste Blindness. Some people are taste blind for certain
substances, especially for different types of thiourea
compounds.A substance used frequently by psycholo-
gists for demonstrating taste blindness is phenylthio-

 for demonstrating taste blindness is phenylthio-
carbamide, for which about 15 to 30 per cent of all
people exhibit taste blindness; the exact percentage
depends on the method of testing and the concentra-
tion of the substance.
Figure 53–1 shows a taste bud, which has a diameter
of about 1/30 millimeter and a length of about 1/16 mil-
limeter. The taste bud is composed of about 50 modi-
ﬁed epithelial cells, some of which are supporting cells
called sustentacular cel

hich are supporting cells
called sustentacular cells and others of which are taste
cells. The taste cells are continually being replaced by
mitotic division of surrounding epithelial cells, so that
some taste cells are young cells. Others are mature
cells that lie toward the center of the bud; these soon
break up and dissolve. The life span of each taste cell
Table 53–1
Relative Taste Indices of Different Substances
Sour Substances
Index
Bitter Substances
Index
Sweet Substances
Index
Salty Subst

ubstances
Index
Index
Salty Substances
Index
Hydrochloric acid
1
Quinine
1
Sucrose
1
NaCl
1
Formic acid
1.1
Brucine
11
1-Propoxy-2-amino-
5000
NaF
2
Chloracetic acid
0.9
Strychnine
3.1
4-nitrobenzene
CaCl2
1
Acetylacetic acid
0.85
Nicotine
1.3
Saccharin
675
NaBr
0.4
Lactic acid
0.85
Phenylthiourea
0.9
Chloroform
40
NaI
0.35
Tartaric acid
0.7
Caffeine
0.4
Fructose
1.7
LiCl
0.4
Malic acid
0.6
Veratrine
0.2
Alanine
1.3
NH4Cl
2.5
Potassium H tartrate
0.58
Pilocarpine
0.16
Glucose
0.

assium H tartrate
0.58
0.16
Glucose
0.8
KCl
0.6
Acetic acid
0.55
Atropine
0.13
Maltose
0.45
Citric acid
0.46
Cocaine
0.02
Galactose
0.32
Carbonic acid
0.06
Morphine
0.02
Lactose
0.3
From Pfaffman C: Handbook of Physiology. Sec I, Vol 1. Baltimore: Williams & Wilkins, 1959, p. 507.
Chapter 53
The Chemical Senses—Taste and Smell
665
is about 10 days in lower mammals but is unknown for
humans.
The outer tips of the taste cells are arranged around
a minute taste pore, shown in Figure 53–

ed around
a minute taste pore, shown in Figure 53–1. From the tip
of each taste cell, several microvilli, or taste hairs, pro-
trude outward into the taste pore to approach the
cavity of the mouth. These microvilli provide the
receptor surface for taste.
Interwoven around the bodies of the taste cells is a
branching terminal network of taste nerve ﬁbers that
are stimulated by the taste receptor cells. Some of
these ﬁbers invaginate into folds of the taste cell mem-
branes.Many vesicles form bene

the taste cell mem-
branes.Many vesicles form beneath the cell membrane
near the ﬁbers. It is believed that these vesicles contain
a neurotransmitter substance that is released through
the cell membrane to excite the nerve ﬁber endings in
response to taste stimulation.
Location of the Taste Buds. The taste buds are found on
three types of papillae of the tongue, as follows: (1) 
A large number of taste buds are on the walls of 
the troughs that surround the circumvallate papillae,
which form a V

urround the circumvallate papillae,
which form a V line on the surface of the posterior
tongue. (2) Moderate numbers of taste buds are on the
fungiform papillae over the ﬂat anterior surface of the
tongue. (3) Moderate numbers are on the foliate papil-
lae located in the folds along the lateral surfaces of 
the tongue. Additional taste buds are located on the
palate, and a few are found on the tonsillar pillars, on
the epiglottis, and even in the proximal esophagus.
Adults have 3000 to 10,000 ta

 proximal esophagus.
Adults have 3000 to 10,000 taste buds, and children
have a few more. Beyond the age of 45 years, many
taste buds degenerate, causing the taste sensation to
become progressively less critical in old age.
Speciﬁcity of Taste Buds for a Primary Taste Stimulus. Micro-
electrode studies from single taste buds show that each
taste bud usually responds mostly to one of the ﬁve
primary taste stimuli when the taste substance is in low
concentration. But at high concentration, most bu

concentration. But at high concentration, most buds
can be excited by two or more of the primary taste
stimuli, as well as by a few other taste stimuli that do
not ﬁt into the “primary” categories.
Mechanism of Stimulation of Taste Buds
Receptor Potential. The membrane of the taste cell,
like that of most other sensory receptor cells, is nega-
tively charged on the inside with respect to the outside.
Application of a taste substance to the taste hairs
causes partial loss of this negative potent

 hairs
causes partial loss of this negative potential—that is,
the taste cell becomes depolarized. In most instances,
the decrease in potential, within a wide range, is
approximately proportional to the logarithm of con-
centration of the stimulating substance. This change in
electrical potential in the taste cell is called the recep-
tor potential for taste.
The mechanism by which most stimulating sub-
stances react with the taste villi to initiate the recep-
tor potential is by binding of the 

ate the recep-
tor potential is by binding of the taste chemical to a
protein receptor molecule that lies on the outer
surface of the taste receptor cell near to or protruding
through a villus membrane. This, in turn, opens ion
channels, which allows positively charged sodium ions
or hydrogen ions to enter and depolarize the normal
negativity of the cell. Then the taste chemical itself is
gradually washed away from the taste villus by the
saliva, which removes the stimulus.
The type of receptor 

 which removes the stimulus.
The type of receptor protein in each taste villus
determines the type of taste that will be perceived.
For sodium ions and hydrogen ions, which elicit salty
and sour taste sensations, respectively, the receptor
proteins open speciﬁc ion channels in the apical mem-
branes of the taste cells, thereby activating the recep-
tors.However,for the sweet and bitter taste sensations,
the portions of the receptor protein molecules that
protrude through the apical membranes act

les that
protrude through the apical membranes activate
second-messenger transmitter substances inside the
taste cells, and these second messengers cause intra-
cellular chemical changes that elicit the taste signals.
Generation of Nerve Impulses by the Taste Bud. On
ﬁrst application of the taste stimulus, the rate of dis-
charge of the nerve ﬁbers from taste buds rises to a
peak in a small fraction of a second but then adapts
within the next few seconds back to a lower, steady
level as long as 

 seconds back to a lower, steady
level as long as the taste stimulus remains. Thus, a
strong immediate signal is transmitted by the taste
nerve, and a weaker continuous signal is transmitted
as long as the taste bud is exposed to the taste 
stimulus.
Transmission of Taste Signals into the
Figure 53–2 shows the neuronal pathways for trans-
mission of taste signals from the tongue and pharyn-
geal region into the central nervous system. Taste
impulses from the anterior two t

ous system. Taste
impulses from the anterior two thirds of the tongue
pass ﬁrst into the lingual nerve, then through the
chorda tympani into the facial nerve, and ﬁnally into
the tractus solitarius in the brain stem.Taste sensations
from the circumvallate papillae on the back of the
Stratified
squamous
Pore
Microvilli
Nerve fibers
Taste cells
Subepithelial
connective
tissue
Figure 53–1
Taste bud.
666
Unit X
tongue and from other posterior regi

pecial Senses
tongue and from other posterior regions of the mouth
and throat are transmitted through the glossopharyn-
geal nerve also into the tractus solitarius, but at a
slightly more posterior level. Finally, a few taste signals
are transmitted into the tractus solitarius from the base
of the tongue and other parts of the pharyngeal region
by way of the vagus nerve.
All taste ﬁbers synapse in the posterior brain stem
in the nuclei of the tractus solitarius. These nuclei send
second-order ne

ctus solitarius. These nuclei send
second-order neurons to a small area of the ventral
posterior medial nucleus of the thalamus, located
slightly medial to the thalamic terminations of the
facial regions of the dorsal column–medial lemniscal
system. From the thalamus, third-order neurons are
transmitted to the lower tip of the postcentral gyrus in
the parietal cerebral cortex, where it curls deep into the
sylvian ﬁssure, and into the adjacent opercular insular
area. This lies slightly lateral, v

rcular insular
area. This lies slightly lateral, ventral, and rostral to the
area for tongue tactile signals in cerebral somatic area
I. From this description of the taste pathways, it is
evident that they closely parallel the somatosensory
pathways from the tongue.
Taste Reﬂexes Integrated in the Brain Stem. From the
tractus solitarius, many taste signals are transmitted
within the brain stem itself directly into the superior
and inferior salivatory nuclei, and these areas transmit
signals to t

tory nuclei, and these areas transmit
signals to the submandibular, sublingual, and parotid
glands to help control the secretion of saliva during the
ingestion and digestion of food.
Adaptation of Taste. Everyone is familiar with the fact
that taste sensations adapt rapidly, often almost com-
pletely within a minute or so of continuous stimula-
tion.Yet, from electrophysiologic studies of taste nerve
ﬁbers, it is clear that adaptation of the taste buds them-
selves usually accounts for no more t

e buds them-
selves usually accounts for no more than about half of
this. Therefore, the ﬁnal extreme degree of adaptation
that occurs in the sensation of taste almost certainly
occurs in the central nervous system itself, although
the mechanism and site of this are not known. At any
rate, it is a mechanism different from that of most
other sensory systems, which adapt almost entirely at
the receptors.
Taste Preference and Control 
of the Diet
Taste preference simply means that an animal will
ch

ste preference simply means that an animal will
choose certain types of food in preference to others,
and the animal automatically uses this to help control
the type of diet it eats. Furthermore, its taste prefer-
ences often change in accord with the body’s need for
certain speciﬁc substances.
The following experiments demonstrate this ability
of animals to choose food in accord with the needs 
of their bodies. First, adrenalectomized, salt-depleted
animals automatically select drinking water w

eted
animals automatically select drinking water with a
high concentration of sodium chloride in preference to
pure water, and this is often sufﬁcient to supply the
needs of the body and prevent salt-depletion death.
Second, an animal given injections of excessive
amounts of insulin develops a depleted blood sugar,
and the animal automatically chooses the sweetest
food from among many samples. Third, calcium-
depleted parathyroidectomized animals automatically
choose drinking water with a high c

 automatically
choose drinking water with a high concentration of
calcium chloride.
The same phenomena are also observed in everyday
life. For instance, the “salt licks” of desert regions are
known to attract animals from far and wide. Also,
human beings reject any food that has an unpleasant
affective sensation, which in many instances protects
our bodies from undesirable substances.
The phenomenon of taste preference almost cer-
tainly results from some mechanism located in the
central nervous

from some mechanism located in the
central nervous system and not from a mechanism in
the taste receptors themselves, although it is true that
the receptors often become sensitized in favor of a
needed nutrient. An important reason for believing
that taste preference is mainly a central nervous
system phenomenon is that previous experience with
unpleasant or pleasant tastes plays a major role in
determining one’s taste preferences. For instance, if a
person becomes sick soon after eating a parti

if a
person becomes sick soon after eating a particular type
of food, the person generally develops a negative taste
preference, or taste aversion, for that particular food
thereafter; the same effect can be demonstrated in
lower animals.
Nucleus of
solitary tract
Gustatory
area
Gustatory cortex
(anterior insula-
frontal operculum)
Ventral posterior
medial nucleus of
thalamus
Geniculate
ganglion
N. VII
N. IX
N. X
Petrosal
ganglion
Nodose
ganglion
Pharynx
Glossopharyngeal
Tongue
Chorda
tympani
Fi

Pharynx
Tongue
Chorda
tympani
Figure 53–2
Transmission of taste signals into the central nervous system.
Chapter 53
667
Smell is the least understood of our senses.This results
partly from the fact that the sense of smell is a sub-
jective phenomenon that cannot be studied with ease
in lower animals. Another complicating problem is
that the sense of smell is poorly developed in human
beings in comparison with the sense of smell

human
beings in comparison with the sense of smell in many
The olfactory membrane, the histology of which is
shown in Figure 53–3, lies in the superior part of each
nostril. Medially, the olfactory membrane folds down-
ward along the surface of the superior septum; later-
ally, it folds over the superior turbinate and even over
a small portion of the upper surface of the middle
turbinate. In each nostril, the olfactory membrane has
a surface area of about 2.4 sq

actory membrane has
a surface area of about 2.4 square centimeters.
Olfactory Cells. The receptor cells for the smell sensa-
tion are the olfactory cells (see Figure 53–3), which are
actually bipolar nerve cells derived originally from the
central nervous system itself. There are about 100
million of these cells in the olfactory epithelium inter-
spersed among sustentacular cells, as shown in Figure
53–3. The mucosal end of the olfactory cell forms a
knob from which 4 to 25 olfactory hairs (also

ms a
knob from which 4 to 25 olfactory hairs (also called
olfactory cilia), measuring 0.3 micrometer in diameter
and up to 200 micrometers in length, project into the
mucus that coats the inner surface of the nasal cavity.
These projecting olfactory cilia form a dense mat in
the mucus, and it is these cilia that react to odors in
the air and stimulate the olfactory cells, as discussed
later. Spaced among the olfactory cells in the olfactory
membrane are many small Bowman’s glands that
secrete mu

ane are many small Bowman’s glands that
secrete mucus onto the surface of the olfactory 
membrane.
Mechanism of Excitation of the Olfactory Cells. The portion
of each olfactory cell that responds to the olfactory
chemical stimuli is the olfactory cilia.The odorant sub-
stance, on coming in contact with the olfactory mem-
brane surface, ﬁrst diffuses into the mucus that covers
the cilia. Then it binds with receptor proteins in the
membrane of each cilium. Each r

or proteins in the
membrane of each cilium. Each receptor protein is
actually a long molecule that threads its way through
the membrane about seven times, folding inward and
outward. The odorant binds with the portion of the
receptor protein that folds to the outside. The inside
of the folding protein, however, is coupled to a so-
called G-protein, itself a combination of three sub-
units. On excitation of the receptor protein, an alpha
subunit breaks away from the G-protein and immedi-
ately ac

reaks away from the G-protein and immedi-
ately activates adenylyl cyclase, which is attached to
the inside of the ciliary membrane near the receptor
cell body.The activated cyclase, in turn, converts many
molecules of intracellular adenosine triphosphate into
cyclic adenosine monophosphate (cAMP). Finally, this
cAMP activates another nearby membrane protein, a
gated sodium ion channel, that opens its “gate” and
allows large numbers of sodium ions to pour through
the membrane into the receptor c

s to pour through
the membrane into the receptor cell cytoplasm. The
sodium ions increase the electrical potential in the pos-
itive direction inside the cell membrane, thus exciting
the olfactory neuron and transmitting action poten-
tials into the central nervous system by way of the
olfactory nerve.
The importance of this mechanism for activating
olfactory nerves is that it greatly multiplies the excita-
tory effect of even the weakest odorant.To summarize:
(1) Activation of the receptor prot

.To summarize:
(1) Activation of the receptor protein by the odorant
substance activates the G-protein complex. (2) This, in
turn, activates multiple molecules of adenylyl cyclase
inside the olfactory cell membrane. (3) This causes the
formation of many times more molecules of cAMP. (4)
Finally, the cAMP opens still many times more sodium
ion channels. Therefore, even the most minute con-
centration of a speciﬁc odorant initiates a cascading
effect that opens extremely large numbers of sodium
ch

ct that opens extremely large numbers of sodium
channels. This accounts for the exquisite sensitivity of
the olfactory neurons to even the slightest amount of
odorant.
In addition to the basic chemical mechanism by
which the olfactory cells are stimulated, several phys-
ical factors affect the degree of stimulation. First, only
volatile substances that can be sniffed into the nostrils
can be smelled. Second, the stimulating substance
must be at least slightly water soluble so that it can
pass th

east slightly water soluble so that it can
pass through the mucus to reach the olfactory cilia.
Third, it is helpful for the substance to be at least
slightly lipid soluble, presumably because lipid con-
stituents of the cilium itself are a weak barrier to 
non-lipid-soluble odorants.
Bowman’s
gland
Sustentacular
cells
Olfactory cell
Olfactory cilia
Mucus layer
Olfactory tract
Olfactory bulb
Mitral cell
Figure 53–3
Organization of the olfactory membrane and olfactory bulb, and
connect

olfactory membrane and olfactory bulb, and
connections to the olfactory tract.
668
Unit X
Membrane Potentials and Action Potentials in Olfactory Cells.
The membrane potential inside unstimulated olfactory
cells, as measured by microelectrodes, averages about
–55 millivolts. At this potential, most of the cells gen-
erate continuous action potentials at a very slow rate,
varying from once every 20 seconds up to two or three
per second.
Most odorants cause

up to two or three
Most odorants cause depolarization of the olfactory
cell membrane, decreasing the negative potential in
the cell from the normal level of –55 millivolts to –30
millivolts or less—that is, changing the voltage in the
positive direction. Along with this, the number of
action potentials increases to 20 to 30 per second,
which is a high rate for the minute olfactory nerve
ﬁbers.
Over a wide range, the rate of olfactory nerve
impulses changes approximately in proportion

nerve
impulses changes approximately in proportion to the
logarithm of the stimulus strength, which demon-
strates that the olfactory receptors obey principles 
of transduction similar to those of other sensory 
Adaptation. The olfactory receptors adapt about 50 per
cent in the ﬁrst second or so after stimulation. There-
after, they adapt very little and very slowly. Yet we all
know from our own experience that smell sensations
adapt almost to extinction within a minute or so after
en

lmost to extinction within a minute or so after
entering a strongly odorous atmosphere. Because this
psychological adaptation is far greater than the degree
of adaptation of the receptors themselves, it is almost
certain that most of the additional adaptation occurs
within the central nervous system. This seems to be
true for the adaptation of taste sensations as well.
A postulated neuronal mechanism for the adapta-
tion is the following: Large numbers of centrifugal
nerve ﬁbers pass from the ol

umbers of centrifugal
nerve ﬁbers pass from the olfactory regions of the
brain backward along the olfactory tract and terminate
on special inhibitory cells in the olfactory bulb, the
granule cells. It has been postulated that after the onset
of an olfactory stimulus, the central nervous system
quickly develops strong feedback inhibition to sup-
press relay of the smell signals through the olfactory
bulb.
Search for the Primary Sensations of Smell
In the past, most physiologists were convinced th

In the past, most physiologists were convinced that the
many smell sensations are subserved by a few rather
discrete primary sensations, in the same way that
vision and taste are subserved by a few select primary
sensations. Based on psychological studies, one at-
tempt to classify these sensations is the following:
1. Camphoraceous
2. Musky
3. Floral
4. Pepperminty
5. Ethereal
6. Pungent
7. Putrid
It is certain that this list does not represent the true
primary sensations of smell. In recent y

 the true
primary sensations of smell. In recent years, multiple
clues, including speciﬁc studies of the genes that
encode for the receptor proteins, suggest the existence
of at least 100 primary sensations of smell—a marked
contrast to only three primary sensations of color
detected by the eyes and only four or ﬁve primary 
sensations of taste detected by the tongue. Further
support for the many primary sensations of smell is
that people have been found who have odor blindness
for single substa

en found who have odor blindness
for single substances; such discrete odor blindness has
been identiﬁed for more than 50 different substances.
It is presumed that odor blindness for each substance
represents lack of the appropriate receptor protein in
olfactory cells for that particular substance.
“Affective Nature of Smell.” Smell, even more so than
taste, has the affective quality of either pleasantness or
unpleasantness. Because of this, smell is probably even
more important than taste for th

 is probably even
more important than taste for the selection of food.
Indeed, a person who has previously eaten food that
disagreed with him or her is often nauseated by the
smell of that same food on a second occasion.
Conversely, perfume of the right quality can wreak
havoc with human emotions. In addition, in some 
lower animals, odors are the primary excitant of sexual
drive.
Threshold for Smell. One of the principal characteristics
of smell is the minute quantity of stimulating agent in
th

 is the minute quantity of stimulating agent in
the air that can elicit a smell sensation. For instance,
the substance methylmercaptan can be smelled when
only one 25 trillionth of a gram is present in each mil-
liliter of air. Because of this very low threshold, this
substance is mixed with natural gas to give the gas an
odor that can be detected when even small amounts
of gas leak from a pipeline.
Gradations of Smell Intensities. Although the threshold
concentrations of substances that evoke s

hreshold
concentrations of substances that evoke smell are
extremely slight, for many (if not most) odorants, con-
centrations only 10 to 50 times above the threshold
evoke maximum intensity of smell. This is in contrast
to most other sensory systems of the body, in which the
ranges of intensity discrimination are tremendous—
for example, 500,000 to 1 in the case of the eyes and 
1 trillion to 1 in the case of the ears. This difference
might be explained by the fact that smell is concerned
more 

xplained by the fact that smell is concerned
more with detecting the presence or absence of 
odors rather than with quantitative detection of their
intensities.
Transmission of Smell Signals into
The olfactory portions of the brain were among the
ﬁrst brain structures developed in primitive animals,
and much of the remainder of the brain developed
around these olfactory beginnings. In fact, part of 
the brain that originally subserved olfaction later
evolved into the b

nally subserved olfaction later
evolved into the basal brain structures that control
emotions and other aspects of human behavior; this is
the system we call the limbic system, discussed in
Chapter 58.
Chapter 53
669
Transmission of Olfactory Signals into the Olfactory Bulb. The
olfactory bulb is shown in Figure 53–4. The olfactory
nerve ﬁbers leading backward from the bulb are called
cranial nerve I, or the olfactory tract. However,
in reality, both the tract

factory tract. However,
in reality, both the tract and the bulb are an anterior
outgrowth of brain tissue from the base of the brain;
the bulbous enlargement at its end, the olfactory bulb,
lies over the cribriform plate, separating the brain
cavity from the upper reaches of the nasal cavity.
The cribriform plate has multiple small perforations
through which an equal number of small nerves 
pass upward from the olfactory membrane in the 
nasal cavity to enter the olfactory bulb in the cranial
ca

vity to enter the olfactory bulb in the cranial
cavity. Figure 53–3 demonstrates the close relation
between the olfactory cells in the olfactory membrane
and the olfactory bulb, showing short axons from 
the olfactory cells terminating in multiple globular 
structures within the olfactory bulb called glomeruli.
Each bulb has several thousand such glomeruli,
each of which is the terminus for about 25,000 axons
from olfactory cells. Each glomerulus also is the ter-
minus for dendrites from about 2

 also is the ter-
minus for dendrites from about 25 large mitral cells
and about 60 smaller tufted cells, the cell bodies of
which lie in the olfactory bulb superior to the
glomeruli. These dendrites receive synapses from the
olfactory cell neurons, and the mitral and tufted cells
send axons through the olfactory tract to transmit
olfactory signals to higher levels in the central nervous
system.
Some research has suggested that different
glomeruli respond to different odors. It is possible that

i respond to different odors. It is possible that
speciﬁc glomeruli are the real clue to the analysis of
different odor signals transmitted into the central
The Very Old, the Less Old, and the 
Newer Olfactory Pathways into the 
The olfactory tract enters the brain at the anterior
junction between the mesencephalon and cerebrum;
there, the tract divides into two pathways, as shown in
Figure 53–4, one passing medially into the medial
olfactory area of the br

 medially into the medial
olfactory area of the brain stem, and the other passing
laterally into the lateral olfactory area. The medial
olfactory area represents a very old olfactory system,
whereas the lateral olfactory area is the input to (1) a
less old olfactory system and (2) a newer system.
The Very Old Olfactory System—The Medial Olfactory Area.
The medial olfactory area consists of a group of nuclei
located in the midbasal portions of the brain immedi-
ately anterior to the hypothalamus.

 brain immedi-
ately anterior to the hypothalamus. Most conspicuous
are the septal nuclei, which are midline nuclei that feed
into the hypothalamus and other primitive portions 
of the brain’s limbic system. This is the brain area 
most concerned with basic behavior (described in
Chapter 58).
The importance of this medial olfactory area is best
understood by considering what happens in animals
when the lateral olfactory areas on both sides of the
brain are removed and only the medial system
rema

remains.The answer is that this hardly affects the more
primitive responses to olfaction, such as licking the
lips, salivation, and other feeding responses caused by
the smell of food or by primitive emotional drives
associated with smell. Conversely, removal of the
lateral areas abolishes the more complicated olfactory
conditioned reﬂexes.
The Less Old Olfactory System—The Lateral Olfactory Area.
The lateral olfactory area is composed mainly of the

 lateral olfactory area is composed mainly of the
prepyriform and pyriform cortex plus the cortical
portion of the amygdaloid nuclei. From these areas,
signal pathways pass into almost all portions of the
limbic system, especially into less primitive portions
such as the hippocampus, which seem to be most
important for learning to like or dislike certain foods
depending on one’s experiences with them. For
instance, it is believed that this lateral olfactory 
area and its many connections with th

l olfactory 
area and its many connections with the limbic behav-
ioral system cause a person to develop an absolute
aversion to foods that have caused nausea and 
vomiting.
An important feature of the lateral olfactory area is
that many signal pathways from this area also feed
directly into an older part of the cerebral cortex called
the paleocortex in the anteromedial portion of the tem-
poral lobe. This is the only area of the entire cerebral
cortex where sensory signals pass directly to the 

cortex where sensory signals pass directly to the cortex
without passing ﬁrst through the thalamus.
The Newer Pathway. A newer olfactory pathway has now
been found that passes through the thalamus, passing
to the dorsomedial thalamic nucleus and then to the
lateroposterior quadrant of the orbitofrontale cortex.
Based on studies in monkeys, this newer system prob-
ably helps in the conscious analysis of odor.
Olfactory
nerve
Olfactory
bulb
Olfactory
tract
Mitral
cell
Temporal
cortex
Orbito-
front

ry
tract
Mitral
cell
Temporal
cortex
Orbito-
frontal
cortex
Brain stem
cortex
Medial olfactory area
Lateral
olfactory
area
Figure 53–4
Neural connections of the olfactory system.
670
Unit X
Summary. Thus, there appear to be a very old olfactory
system that subserves the basic olfactory reﬂexes, a 
less old system that provides automatic but partially
learned control of food intake and aversion to toxic
and unhealthy fo

food intake and aversion to toxic
and unhealthy foods, and a newer system that is com-
parable to most of the other cortical sensory systems
and is used for conscious perception and analysis of
olfaction.
Centrifugal Control of Activity in the Olfactory Bulb by the
Central Nervous System. Many nerve ﬁbers that originate
in the olfactory portions of the brain pass from the
brain in the outward direction into the olfactory tract
to the olfactory bulb (i.e., “centrifugally” from the
brain to the pe

lb (i.e., “centrifugally” from the
brain to the periphery). These terminate on a large
number of small granule cells located among the mitral
and tufted cells in the olfactory bulb.The granule cells
send inhibitory signals to the mitral and tufted cells. It
is believed that this inhibitory feedback might be a
means for sharpening one’s speciﬁc ability to distin-
guish one odor from another.
Bermudez-Rattoni F: Molecular mechanisms of taste-recog-
nition memory. Nat Rev Neurosci 5:209,

aste-recog-
nition memory. Nat Rev Neurosci 5:209, 2004.
Dohlman HG: G proteins and pheromone signaling. Annu
Rev Physiol 64:129, 2002.
Gibson AD, Garbers DL: Guanylyl cyclases as a family of
putative odorant receptors. Annu Rev Neurosci 23:417,
2000.
Herness MS, Gilbertson TA: Cellular mechanisms of taste
transduction. Annu Rev Physiol 61:873, 1999.
Levine AS, Kotz CM, Gosnell BA: Sugars and fats: the neu-
robiology of preference. J Nutr 133:831S, 2003.
Lledo PM, Gheusi G: Olfactory processing 

S, 2003.
Lledo PM, Gheusi G: Olfactory processing in a changing
brain. Neuroreport 14:1655, 2003.
Lowe G: Electrical signaling in the olfactory bulb. Curr Opin
Neurobiol 13:476, 2003.
Margolskee RF: Molecular mechanisms of bitter and sweet
taste transduction. J Biol Chem 277:1, 2002.
Matthews HR, Reisert J: Calcium, the two-faced messenger
of olfactory transduction and adaptation. Curr Opin Neu-
robiol 13:469, 2003.
Menini A, Lagostena L, Boccaccio A: Olfaction: from
odorant molecules to the olf

io A: Olfaction: from
odorant molecules to the olfactory cortex. News Physiol
Sci 19:101, 2004.
Mombaerts P: Odorant receptor gene choice in olfactory
sensory neurons: the one receptor–one neuron hypothesis
revisited. Curr Opin Neurobiol 14:31, 2004.
Montmayeur JP, Matsunami H: Receptors for bitter and
sweet taste. Curr Opin Neurobiol 12:366, 2002.
Reed RR: The contribution of signaling pat

, 2002.
Reed RR: The contribution of signaling pathways to olfac-
tory organization and development. Curr Opin Neurobiol
13:482, 2003.
Ronnett GV, Moon C: G proteins and olfactory signal trans-
duction. Annu Rev Physiol 64:189, 2002.
Sewards TV: Dual separate pathways for sensory and
hedonic aspects of taste. Brain Res Bull 62:271, 2004.
Smith DV, Margolskee RF: Making sense of taste. Sci Am
284:32, 2001.
U
N
I
T
XI
System: C. Motor
and Integrative
Neurophysiology
54. Motor Function

54. Motor Functions of the Spinal Cord; the 
Cord Reﬂexes
55. Cortical and Brain Stem Control of 
56. Contributions of the Cerebellum and Basal 
Ganglia to Overall Motor Control
57. Cerebral Cortex, Intellectual Functions 
58. Behavioral and Motivational Mechanisms 
of the Brain—The Limbic System and 
the Hypothalamus
59. States of Brain Activity—Sleep, Brain Waves,
Epilepsy, Psychoses
60. Autonomic Nervous System a

60. Autonomic Nervous System and the 
Adrenal Medulla
61. Cerebral Blood Flow, Cerebrospinal Fluid, 
and Brain Metabolism
C
H
A
P
T
E
R
 
5
4
673
Motor Functions of the Spinal
Cord; the Cord Reﬂexes
We have already noted that sensory information is
integrated at all levels of the nervous system and
causes appropriate motor responses that begin in
the spinal cord with relatively simple muscle
reﬂexes, extend into the brain stem with more com-
plicated responses, and ﬁnally ext

 with more com-
plicated responses, and ﬁnally extend to the cere-
brum, where the most complicated muscle skills are
controlled.
In this chapter, we discuss the control of muscle function by the spinal cord.
Without the special neuronal circuits of the cord, even the most complex motor
control systems in the brain could not cause any purposeful muscle movement.
To give an example, there is no neuronal circuit anywhere in the brain that
causes the speciﬁc to-and-fro movement of the legs that is 

e speciﬁc to-and-fro movement of the legs that is required in walking.
Instead, the circuits for these movements are in the cord, and the brain simply
sends command signals to the spinal cord to set into motion the walking process.
Let us not belittle the role of the brain, however, because the brain gives
directions that control the sequential cord activities—to promote turning move-
ments when they are required, to lean the body forward during acceleration, to
change the movements from walking

acceleration, to
change the movements from walking to jumping as needed, and to monitor con-
tinuously and control equilibrium. All this is done through “analytical” and
“command” signals generated in the brain. But it also requires the many neu-
ronal circuits of the spinal cord that are the objects of the commands. These 
circuits provide all but a small fraction of the direct control of the muscles.
Spinal Animal and the Decerebrate Animal. Two types of experimental preparations have
been esp

o types of experimental preparations have
been especially useful in studying spinal cord function: (1) the spinal animal, in
which the spinal cord is transected, frequently in the neck so that most of the cord
still remains functional, and (2) the decerebrate animal, in which the brain stem is
transected in the middle to lower part of the mesencephalon.
Immediately after preparation of a spinal animal, most spinal cord function below
the level of transection is severely depressed. After a few ho

 transection is severely depressed. After a few hours in rats and cats or
a few days to weeks in monkeys, most of the intrinsic spinal cord functions return
to nearly normal and provide a suitable experimental preparation for research
study.
In the decerebrate animal, the brain stem is transected at the middle to lower mes-
encephalic level, which blocks normal inhibitory signals from the higher control
centers of the brain to the pontile and vestibular muscle control nuclei. This allows
these n

tibular muscle control nuclei. This allows
these nuclei to become tonically active, transmitting facilitatory signals to most of
the spinal cord motor control circuits. The result is that the spinal cord motor
reﬂexes become very excitable and, therefore, easy to activate by even the slightest
sensory input signals to the cord. Using this preparation, one can study the intrin-
sic excitatory motor functions of the cord itself.
Organization of the Spinal Cord 
for Motor Functions
The cord gray ma

 Spinal Cord 
The cord gray matter is the integrative area for the cord reﬂexes. Figure 54–1
shows the typical organization of the cord gray matter in a single cord segment.
Sensory signals enter the cord almost entirely through the sensory (posterior)
roots. After entering the cord, every sensory signal travels to two separate 
674
Unit XI
The Nervous System: C. Motor and Integrative Neurophysiology
destinations: (1) One branch of the sensory nerve ter-
minates almost immedi

ch of the sensory nerve ter-
minates almost immediately in the gray matter of the
cord and elicits local segmental cord reﬂexes and other
local effects. (2) Another branch transmits signals to
higher levels of the nervous system—to higher levels
in the cord itself, to the brain stem, or even to the cere-
bral cortex, as described in earlier chapters.
Each segment of the spinal cord (at the level of 
each spinal nerve) has several million neurons in its
gray matter. Aside from the sensory relay n

in its
gray matter. Aside from the sensory relay neurons 
discussed in Chapters 47 and 48, the other neurons 
are of two types: (1) anterior motor neurons and (2)
interneurons.
Anterior Motor Neurons. Located in each segment of the
anterior horns of the cord gray matter are several
thousand neurons that are 50 to 100 per cent larger
than most of the others and are called anterior motor
neurons. They give rise to the nerve ﬁbers that leave
the cord by way of the anterior roots and directly
innerv

d by way of the anterior roots and directly
innervate the skeletal muscle ﬁbers. The neurons are
of two types, alpha motor neurons and gamma motor
neurons.
Alpha Motor Neurons. The alpha motor neurons give
rise to large type A alpha (Aa) motor nerve ﬁbers,
averaging 14 micrometers in diameter; these ﬁbers
branch many times after they enter the muscle and
innervate the large skeletal muscle ﬁbers. Stimulation
of a single alpha nerve ﬁber excites anywhere from
three to several hundred skeletal mus

nywhere from
three to several hundred skeletal muscle ﬁbers, which
are collectively called the motor unit. Transmission of
nerve impulses into skeletal muscles and their stimu-
lation of the muscle motor units are discussed in 
Chapters 6 and 7.
Gamma Motor Neurons. Along with the alpha motor
neurons, which excite contraction of the skeletal
muscle ﬁbers, about one half as many much smaller
gamma motor neurons are located in the spinal cord
anterior horns. These gamma motor neurons transmit
impu

ior horns. These gamma motor neurons transmit
impulses through much smaller type A gamma (Ag)
motor nerve ﬁbers, averaging 5 micrometers in diam-
eter, which go to small, special skeletal muscle ﬁbers
called intrafusal ﬁbers, shown in Figure 54–2. These
ﬁbers constitute the middle of the muscle spindle,
which helps control basic muscle “tone,” as discussed
Interneurons. Interneurons are present in all areas of
the cord gray matter—in the dorsal horns, the anterior
horns, a

 matter—in the dorsal horns, the anterior
horns, and the intermediate areas between them, as
shown in Figure 54–1. These cells are about 30 times
as numerous as the anterior motor neurons. They are
small and highly excitable, often exhibiting sponta-
neous activity and capable of ﬁring as rapidly as 1500
times per second. They have many interconnections
with one another, and many of them also synapse
directly with the anterior motor neurons, as shown 
in Figure 54–1. The interconnections among t

hown 
in Figure 54–1. The interconnections among the
interneurons and anterior motor neurons are respon-
sible for most of the integrative functions of the spinal
cord that are discussed in the remainder of this
chapter.
Essentially all the different types of neuronal cir-
cuits described in Chapter 46 are found in the
interneuron pool of cells of the spinal cord, including
diverging, converging, repetitive-discharge, and other
types of circuits. In this chapter, we examine many
applications of 

 In this chapter, we examine many
applications of these different circuits in the perform-
ance of speciﬁc reﬂex acts by the spinal cord.
Only a few incoming sensory signals from the spinal
nerves or signals from the brain terminate directly on
the anterior motor neurons. Instead, almost all these
signals are transmitted ﬁrst through interneurons,
where they are appropriately processed. Thus, in
Sensory root
Solitary cell
External basal cells
Corticospinal tract
Interneurons
Anterior motor
neuro

ticospinal tract
neurons
Motor root
Figure 54–1
Connections of peripheral sensory ﬁbers and corticospinal ﬁbers
with the interneurons and anterior motor neurons of the spinal
cord.
Alpha motor
ending
 Gamma motor
ending
Fluid
cavity
Sheath
Primary
ending
1 cm
Motor
Sensory
Motor
Extrafusal
fibers
Secondary
ending
Intrafusal
fibers
14 mm
17 mm
5 mm
g
g
Ia
II
8 mm
a
5 mm
Figure 54–2
Muscle spindle, showing its relation to the large extrafusal skele-
tal muscle ﬁbers. No

o the large extrafusal skele-
tal muscle ﬁbers. Note also both motor and sensory innervation of
the muscle spindle.
Chapter 54
Motor Functions of the Spinal Cord; the Cord Reﬂexes
675
Figure 54–1, the corticospinal tract from the brain is
shown to terminate almost entirely on spinal interneu-
rons, where the signals from this tract are combined
with signals from other spinal tracts or spinal nerves
before ﬁnally converging on the anterior motor
Renshaw Cell In

eurons to control muscle function.
Renshaw Cell Inhibitory System. Also located in the anterior
horns of the spinal cord, in close association with the
motor neurons, are a large number of small neurons
called Renshaw cells. Almost immediately after the
anterior motor neuron axon leaves the body of the
neuron, collateral branches from the axon pass to adja-
cent Renshaw cells.These are inhibitory cells that trans-
mit inhibitory signals to the surrounding motor neurons.
Thus, stimulation of each

rrounding motor neurons.
Thus, stimulation of each motor neuron tends to inhibit
adjacent motor neurons, an effect called lateral inhibi-
tion. This effect is important for the following major
reason: The motor system uses this lateral inhibition to
focus, or sharpen, its signals in the same way that the
sensory system uses the same principle—that is, to allow
unabated transmission of the primary signal in the
desired direction while suppressing the tendency for
signals to spread laterally.
Mult

the tendency for
Multisegmental Connections from One Spinal Cord Level 
to Other Levels—Propriospinal Fibers
More than half of all the nerve ﬁbers that ascend and
descend in the spinal cord are propriospinal ﬁbers.
These ﬁbers run from one segment of the cord to
another. In addition, as the sensory ﬁbers enter the cord
from the posterior cord roots, they bifurcate and branch
both up and down the spinal cord; some of the branches
transmit signals to only a segment or 

he branches
transmit signals to only a segment or two, while others
transmit signals to many segments.These ascending and
descending propriospinal ﬁbers of the cord provide
pathways for the multisegmental reﬂexes described
later in this chapter, including reﬂexes that coordinate
simultaneous movements in the forelimbs and hind
limbs.
Muscle Sensory Receptors—
Muscle Spindles and Golgi
Tendon Organs—And Their
Roles in Muscle Control
Proper control of muscle function requires not only
excitation o

 of muscle function requires not only
excitation of the muscle by spinal cord anterior motor
neurons but also continuous feedback of sensory
information from each muscle to the spinal cord, indi-
cating the functional status of each muscle at each
instant. That is, what is the length of the muscle, what
is its instantaneous tension, and how rapidly is its
length or tension changing? To provide this informa-
tion, the muscles and their tendons are supplied abun-
dantly with two special types of s

 supplied abun-
dantly with two special types of sensory receptors: (1)
muscle spindles (see Figure 54–2), which are distrib-
uted throughout the belly of the muscle and send
information to the nervous system about muscle
length or rate of change of length, and (2) Golgi
tendon organs (see Figure 54–7), which are located in
the muscle tendons and transmit information about
tendon tension or rate of change of tension.
The signals from these two receptors are either
entirely or almost entirely for

ceptors are either
entirely or almost entirely for the purpose of intrinsic
muscle control. They operate almost completely at a
subconscious level. Even so, they transmit tremendous
amounts of information not only to the spinal cord but
also to the cerebellum and even to the cerebral cortex,
helping each of these portions of the nervous system
function to control muscle contraction.
Receptor Function of the 
Muscle Spindle
Structure and Motor Innervation of the Muscle Spindle. The
organization o

ervation of the Muscle Spindle. The
organization of the muscle spindle is shown in Figure
54–2. Each spindle is 3 to 10 millimeters long. It is built
around 3 to 12 very small intrafusal muscle ﬁbers that
are pointed at their ends and attached to the glycoca-
lyx of the surrounding large extrafusal skeletal muscle
ﬁbers.
Each intrafusal muscle ﬁber is a very small skeletal
muscle ﬁber. However, the central region of each 
of these ﬁbers—that is, the area midway between 
its two ends—has few or n

the area midway between 
its two ends—has few or no actin and myosin ﬁla-
ments.Therefore,this central portion does not contract
when the ends do. Instead, it functions as a sensory
receptor, as described later. The end portions that 
do contract are excited by small gamma motor nerve
ﬁbers that originate from small type A gamma motor
neurons in the anterior horns of the spinal cord,
as described earlier. These gamma motor nerve ﬁbers
are also called gamma efferent ﬁbers, in contradistinc-
tion 

lled gamma efferent ﬁbers, in contradistinc-
tion to the large alpha efferent ﬁbers (type A alpha
nerve ﬁbers) that innervate the extrafusal skeletal
muscle.
Sensory Innervation of the Muscle Spindle. The receptor
portion of the muscle spindle is its central portion. In
this area, the intrafusal muscle ﬁbers do not have
myosin and actin contractile elements. As shown in
Figure 54–2 and in more detail in Figure 54–3, sensory
ﬁbers originate in this area. They are stimulated by
stretching of this 

s area. They are stimulated by
stretching of this midportion of the spindle. One can
readily see that the muscle spindle receptor can be
excited in two ways:
1. Lengthening the whole muscle stretches the
midportion of the spindle and, therefore, excites
2. Even if the length of the entire muscle does not
change, contraction of the end portions of the
spindle’s intrafusal ﬁbers stretches the midportion
of the spindle and therefore excites the receptor.
Two types of sensory endings a

cites the receptor.
Two types of sensory endings are found in this
central receptor area of the muscle spindle. They are
the primary ending and the secondary ending.
Primary Ending. In the center of the receptor area, a
large sensory nerve ﬁber encircles the central portion
of each intrafusal ﬁber, forming the so-called primary
ending or annulospiral ending. This nerve ﬁber is a
type Ia ﬁber averaging 17 micrometers in diameter,
and it transmits sensory signals to the spinal cord at a
676
Unit X

sensory signals to the spinal cord at a
676
Unit XI
velocity of 70 to 120 m/sec, as rapidly as any type of
nerve ﬁber in the entire body.
Secondary Ending. Usually one but sometimes two
smaller sensory nerve ﬁbers—type II ﬁbers with an
average diameter of 8 micrometers—innervate the
receptor region on one or both sides of the primary
ending, as shown in Figures 54–2 and 54–3. This
sensory ending is called the secondary ending; some-
ti

ry ending is called the secondary ending; some-
times it encircles the intrafusal ﬁbers in the same way
that the type Ia ﬁber does, but often it spreads like
branches on a bush.
Division of the Intrafusal Fibers into Nuclear Bag and Nuclear
Chain Fibers—Dynamic and Static Responses of the Muscle
Spindle. There are also two types of muscle spindle
intrafusal ﬁbers: (1) nuclear bag muscle ﬁbers (one to
three in each spindle), in which several muscle ﬁber
nuclei are congregated in expanded “bags” i

e ﬁber
nuclei are congregated in expanded “bags” in the
central portion of the receptor area, as shown by the
top ﬁber in Figure 54–3, and (2) nuclear chain ﬁbers
(three to nine), which are about half as large in diam-
eter and half as long as the nuclear bag ﬁbers and have
nuclei aligned in a chain throughout the receptor area,
as shown by the bottom ﬁber in the ﬁgure.The primary
sensory nerve ending (the 17-micrometer sensory
ﬁber) is excited by both the nuclear bag intrafusal
ﬁbers and the nu

y both the nuclear bag intrafusal
ﬁbers and the nuclear chain ﬁbers. Conversely, the sec-
ondary ending (the 8-micrometer sensory ﬁber) is
usually excited only by nuclear chain ﬁbers.These rela-
tions are shown in Figure 54–3.
Response of Both the Primary and the Secondary
Endings to the Length of the Receptor—“Static”
Response. When the receptor portion of the muscle
spindle is stretched slowly, the number of impulses
transmitted from both the primary and the secondary
endings increases almost 

rimary and the secondary
endings increases almost directly in proportion to the
degree of stretching,and the endings continue to trans-
mit these impulses for several minutes. This effect is
called the static response of the spindle receptor,
meaning simply that both the primary and secondary
endings continue to transmit their signals for at least
several minutes if the muscle spindle itself remains
stretched.
Response of the Primary Ending (but Not the Sec-
ondary Ending) to Rate of Change of R

Not the Sec-
ondary Ending) to Rate of Change of Receptor
Length—“Dynamic” Response. When the length of
the spindle receptor increases suddenly, the primary
ending (but not the secondary ending) is stimulated
especially powerfully. This excess stimulus of the
primary ending is called the dynamic response, which
means that the primary ending responds extremely
actively to a rapid rate of change in spindle length.
Even when the length of a spindle receptor increases
only a fraction of a micrometer

receptor increases
only a fraction of a micrometer, if this increase occurs
in a fraction of a second, the primary receptor trans-
mits tremendous numbers of excess impulses to the
large 17-micrometer sensory nerve ﬁber, but only
while the length is actually increasing. As soon as the
length stops increasing, this extra rate of impulse dis-
charge returns to the level of the much smaller static
response that is still present in the signal.
Conversely, when the spindle receptor shortens,
exactly 

sely, when the spindle receptor shortens,
exactly opposite sensory signals occur. Thus, the
primary ending sends extremely strong, either positive
or negative, signals to the spinal cord to apprise it of
any change in length of the spindle receptor.
Control of Intensity of the Static and Dynamic Responses by the
Gamma Motor Nerves. The gamma motor nerves to the
muscle spindle can be divided into two types: gamma-
dynamic (gamma-d) and gamma-static (gamma-s).The
ﬁrst of these excites mainly the n

c (gamma-s).The
ﬁrst of these excites mainly the nuclear bag intrafusal
ﬁbers, and the second excites mainly the nuclear chain
intrafusal ﬁbers. When the gamma-d ﬁbers excite the
nuclear bag ﬁbers, the dynamic response of the muscle
spindle becomes tremendously enhanced, whereas the
static response is hardly affected. Conversely, stimula-
tion of the gamma-s ﬁbers, which excite the nuclear
chain ﬁbers, enhances the static response while having
little inﬂuence on the dynamic response. Subsequent

ttle inﬂuence on the dynamic response. Subsequent
paragraphs illustrate that these two types of muscle
spindle responses are important in different types of
muscle control.
Continuous Discharge of the Muscle Spindles Under Normal
Conditions. Normally, particularly when there is some
degree of gamma nerve excitation, the muscle spindles
emit sensory nerve impulses continuously. Stretching
the muscle spindles increases the rate of ﬁring,
whereas shortening the spindle decreases the rate of
ﬁring. 

ortening the spindle decreases the rate of
ﬁring. Thus, the spindles can send to the spinal cord
either positive signals—that is, increased numbers of
impulses to indicate stretch of a muscle—or negative
signals—below-normal numbers of impulses to indi-
cate that the muscle is unstretched.
The simplest manifestation of muscle spindle function
is the muscle stretch reﬂex. Whenever a muscle is
stretched suddenly, excitation of the spindles causes
Group II fiber
(secondary
effe

the spindles causes
efferent)
Nuclear bag fiber
(intrafusal muscle)
Nuclear chain fiber
Plate
ending
Dynamic g fiber
(efferent)
Static g fiber
Trail ending
Group Ia fiber
(primary afferent)
Figure 54–3
Details of nerve connections from the nuclear bag and nuclear
chain muscle spindle ﬁbers. (Modiﬁed from Stein RB: Peripheral
control of movement. Physiol Rev 54:225, 1974.)
Chapter 54
677

unctions of the Spinal Cord; the Cord Reﬂexes
677
reﬂex contraction of the large skeletal muscle ﬁbers of
the stretched muscle and also of closely allied syner-
gistic muscles.
Neuronal Circuitry of the Stretch Reﬂex. Figure 54–4
demonstrates the basic circuit of the muscle spindle
stretch reﬂex, showing a type Ia proprioceptor nerve
ﬁber originating in a muscle spindle and entering a
dorsal root of the spinal cord. A branch of this ﬁber
then goes directly to the anterior horn of the cord gray
m

s directly to the anterior horn of the cord gray
matter and synapses with anterior motor neurons that
send motor nerve ﬁbers back to the same muscle from
which the muscle spindle ﬁber originated. Thus, this is
a monosynaptic pathway that allows a reﬂex signal to
return with the shortest possible time delay back to the
muscle after excitation of the spindle. Most type II
ﬁbers from the muscle spindle terminate on multiple
interneurons in the cord gray matter, and these trans-
mit delayed signals 

gray matter, and these trans-
mit delayed signals to the anterior motor neurons or
serve other functions.
Dynamic Stretch Reﬂex and Static Stretch Reﬂexes. The
stretch reﬂex can be divided into two components: the
dynamic stretch reﬂex and the static stretch reﬂex.
The dynamic stretch reﬂex is elicited by the potent
dynamic signal transmitted from the primary sensory
endings of the muscle spindles, caused by rapid stretch
or unstretch. That is, when a muscle is suddenly
stretched or unstretched,

hen a muscle is suddenly
stretched or unstretched, a strong signal is transmitted
to the spinal cord; this causes an instantaneous strong
reﬂex contraction (or decrease in contraction) of the
same muscle from which the signal originated. Thus,
the reﬂex functions to oppose sudden changes in muscle
length.
The dynamic stretch reﬂex is over within a fraction
of a second after the muscle has been stretched (or
unstretched) to its new length, but then a weaker static
stretch reﬂex continues for a pr

n a weaker static
stretch reﬂex continues for a prolonged period there-
after. This reﬂex is elicited by the continuous static
receptor signals transmitted by both primary and 
secondary endings.The importance of the static stretch
reﬂex is that it causes the degree of muscle contraction
to remain reasonably constant, except when the
person’s nervous system speciﬁcally wills otherwise.
“Damping” Function of the Dynamic and Static
Stretch Reﬂexes
An especially important function of the stretch re

An especially important function of the stretch reﬂex
is its ability to prevent oscillation or jerkiness of body
movements.This is a damping, or smoothing, function,
as explained in the following paragraph.
Damping Mechanism in Smoothing Muscle Contraction.
Signals from the spinal cord are often transmitted to a
muscle in an unsmooth form, increasing in intensity for
a few milliseconds, then decreasing in intensity, then
changing to another intensity level, and so forth.When
the muscle spindle a

sity level, and so forth.When
the muscle spindle apparatus is not functioning satis-
factorily, the muscle contraction is jerky during the
course of such a signal. This effect is demonstrated in
Figure 54–5. In curve A, the muscle spindle reﬂex of
the excited muscle is intact. Note that the contraction
is relatively smooth, even though the motor nerve to
the muscle is excited at a slow frequency of only 8
signals per second. Curve B illustrates the same exper-
iment in an animal whose muscle spi

he same exper-
iment in an animal whose muscle spindle sensory
nerves had been sectioned 3 months earlier. Note the
unsmooth muscle contraction. Thus, curve A graphi-
cally demonstrates the damping mechanism’s ability to
smooth muscle contractions, even though the primary
input signals to the muscle motor system may them-
selves be jerky. This effect can also be called a signal
averaging function of the muscle spindle reﬂex.
Stretch reflex
Sensory nerve
Figure 54–4
Neu

eflex
Neuronal circuit of the stretch reﬂex.
Stimulus
(8 per second)
Seconds
0
1
2
B
A
3
Figure 54–5
Muscle contraction caused by a spinal cord signal under two con-
ditions: curve A, in a normal muscle, and curve B, in a muscle
whose muscle spindles were denervated by section of the poste-
rior roots of the cord 82 days previously. Note the smoothing effect
of the muscle spindle reﬂex in curve A. (Modiﬁed from Creed RS,
et al: Reﬂex 

 in curve A. (Modiﬁed from Creed RS,
et al: Reﬂex Activity of the Spinal Cord. New York: Oxford Univer-
sity Press, 1932.)
678
Unit XI
Role of the Muscle Spindle 
in Voluntary Motor Activity
To emphasize the importance of the gamma efferent
system, one needs to recognize that 31 per cent of all
the motor nerve ﬁbers to the muscle are the small type
A gamma efferent ﬁbers rather than large type A
alpha motor ﬁbers. Whenever signals are 

ge type A
alpha motor ﬁbers. Whenever signals are transmitted
from the motor cortex or from any other area of the
brain to the alpha motor neurons, in most instances the
gamma motor neurons are stimulated simultaneously,
an effect called coactivation of the alpha and gamma
motor neurons. This causes both the extrafusal skele-
tal muscle ﬁbers and the muscle spindle intrafusal
muscle ﬁbers to contract at the same time.
The purpose of contracting the muscle spindle intra-
fusal ﬁbers at the same t

he muscle spindle intra-
fusal ﬁbers at the same time that the large skeletal
muscle ﬁbers contract is twofold: First, it keeps the
length of the receptor portion of the muscle spindle
from changing during the course of the whole muscle
contraction. Therefore, coactivation keeps the muscle
spindle reﬂex from opposing the muscle contraction.
Second, it maintains the proper damping function of
the muscle spindle, regardless of any change in muscle
length. For instance, if the muscle spindle did no

length. For instance, if the muscle spindle did not con-
tract and relax along with the large muscle ﬁbers,
the receptor portion of the spindle would sometimes
be ﬂail and sometimes be overstretched, in neither
instance operating under optimal conditions for
spindle function.
Brain Areas for Control of the Gamma 
Motor System
The gamma efferent system is excited speciﬁcally by
signals from the bulboreticular facilitatory region of
the brain stem and, secondarily, by impulses transmit-
ted into t

and, secondarily, by impulses transmit-
ted into the bulboreticular area from (1) the cerebel-
lum, (2) the basal ganglia, and (3) the cerebral cortex.
Little is known about the precise mechanisms of
control of the gamma efferent system. However,
because the bulboreticular facilitatory area is par-
ticularly concerned with antigravity contractions, and
because the antigravity muscles have an especially
high density of muscle spindles, emphasis is given to
the importance of the gamma efferent mec

 given to
the importance of the gamma efferent mechanism for
damping the movements of the different body parts
during walking and running.
Muscle Spindle System Stabilizes Body
Position During Tense Action
One of the most important functions of the muscle
spindle system is to stabilize body position during
tense motor action.To do this, the bulboreticular facil-
itatory region and its allied areas of the brain stem
transmit excitatory signals through the gamma nerve
ﬁbers to the intrafusal muscl

ough the gamma nerve
ﬁbers to the intrafusal muscle ﬁbers of the muscle 
spindles. This shortens the ends of the spindles and
stretches the central receptor regions, thus increasing
their signal output. However, if the spindles on both
sides of each joint are activated at the same time, reﬂex
excitation of the skeletal muscles on both sides of the
joint also increases, producing tight, tense muscles
opposing each other at the joint. The net effect is that
the position of the joint becomes strong

t is that
the position of the joint becomes strongly stabilized,
and any force that tends to move the joint from its
current position is opposed by highly sensitized stretch
reﬂexes operating on both sides of the joint.
Any time a person must perform a muscle function
that requires a high degree of delicate and exact posi-
tioning, excitation of the appropriate muscle spindles
by signals from the bulboreticular facilitatory region
of the brain stem stabilizes the positions of the major
joints. T

em stabilizes the positions of the major
joints. This aids tremendously in performing the 
additional detailed voluntary movements (of ﬁngers 
or other body parts) required for intricate motor 
procedures.
Clinical Applications of the 
Stretch Reﬂex
Almost every time a clinician performs a physical exam-
ination on a patient, he or she elicits multiple stretch
reﬂexes. The purpose is to determine how much back-
ground excitation, or “tone,” the brain is sending to the
spinal cord. This reﬂex is 

rain is sending to the
spinal cord. This reﬂex is elicited as follows.
Knee Jerk and Other Muscle Jerks. Clinically, a method used
to determine the sensitivity of the stretch reﬂexes is to
elicit the knee jerk and other muscle jerks.The knee jerk
can be elicited by simply striking the patellar tendon
with a reﬂex hammer; this instantaneously stretches the
quadriceps muscle and excites a dynamic stretch reﬂex
that causes the lower leg to “jerk” forward. The upper
part of Figure 54–6 shows a myogr

rward. The upper
part of Figure 54–6 shows a myogram from the quadri-
ceps muscle recorded during a knee jerk.
Similar reﬂexes can be obtained from almost any
muscle of the body either by striking the tendon of the
muscle or by striking the belly of the muscle itself. In
other words, sudden stretch of muscle spindles is all that
is required to elicit a dynamic stretch reﬂex.
The muscle jerks are used by neurologists to assess
the degree of facilitation of spinal cord centers. When
M

tation of spinal cord centers. When
Muscle length
0
200
400
Patellar tendon struck
Knee jerk
Ankle clonus
600
800
Figure 54–6
Myograms recorded from the quadriceps muscle during elicita-
tion of the knee jerk (above) and from the gastrocnemius muscle
during ankle clonus (below).
Chapter 54
679
large numbers of facilitatory impulses are being trans-
mitted from the upper regions of the central nervous
system into the cord, the musc

the central nervous
system into the cord, the muscle jerks are greatly exag-
gerated. Conversely, if the facilitatory impulses are
depressed or abrogated, the muscle jerks are consider-
ably weakened or absent. These reﬂexes are used most
frequently in determining the presence or absence of
muscle spasticity caused by lesions in the motor areas
of the brain or diseases that excite the bulboreticular
facilitatory area of the brain stem. Ordinarily, large
lesions in the motor areas of the cerebral

, large
lesions in the motor areas of the cerebral cortex but not
in the lower motor control areas (especially lesions
caused by strokes or brain tumors) cause greatly exag-
gerated muscle jerks in the muscles on the opposite side
of the body.
Clonus—Oscillation of Muscle Jerks. Under some condi-
tions, the muscle jerks can oscillate, a phenomenon
called clonus (see lower myogram, Figure 54–6). Oscil-
lation can be explained particularly well in relation to
ankle clonus, as follows.
If a person 

relation to
If a person standing on the tip ends of the feet sud-
denly drops his or her body downward and stretches 
the gastrocnemius muscles, stretch reﬂex impulses are
transmitted from the muscle spindles into the spinal
cord. These impulses reﬂexively excite the stretched
muscle, which lifts the body up again. After a fraction
of a second, the reﬂex contraction of the muscle dies out
and the body falls again, thus stretching the spindles a
second time. Again, a dyn

tretching the spindles a
second time. Again, a dynamic stretch reﬂex lifts the
body, but this too dies out after a fraction of a second,
and the body falls once more to begin a new cycle. In
this way, the stretch reﬂex of the gastrocnemius muscle
continues to oscillate, often for long periods; this is
clonus.
Clonus ordinarily occurs only when the stretch reﬂex
is highly sensitized by facilitatory impulses from the
brain. For instance, in a decerebrate animal, in which the
stretch reﬂexes are hi

ebrate animal, in which the
stretch reﬂexes are highly facilitated, clonus develops
readily. To determine the degree of facilitation of the
spinal cord, neurologists test patients for clonus by sud-
denly stretching a muscle and applying a steady stretch-
ing force to it. If clonus occurs, the degree of facilitation
is certain to be high.
Golgi Tendon Organ Helps Control Muscle Tension. The Golgi
tendon organ, shown in Figure 54–7, is an encapsulated
sensory receptor through w

4–7, is an encapsulated
sensory receptor through which muscle tendon ﬁbers
pass. About 10 to 15 muscle ﬁbers are usually con-
nected to each Golgi tendon organ, and the organ is
stimulated when this small bundle of muscle ﬁbers is
“tensed” by contracting or stretching the muscle.Thus,
the major difference in excitation of the Golgi tendon
organ versus the muscle spindle is that the spindle
detects muscle length and changes in muscle length,
whereas the tendon organ detects muscle tension as
reﬂe

as the tendon organ detects muscle tension as
reﬂected by the tension in itself.
The tendon organ, like the primary receptor of the
muscle spindle, has both a dynamic response and a
static response, responding intensely when the muscle
tension suddenly increases (the dynamic response) but
settling down within a fraction of a second to a lower
level of steady-state ﬁring that is almost directly 
proportional to the muscle tension (the static
response). Thus, Golgi tendon organs provide the
nervou

nse). Thus, Golgi tendon organs provide the
nervous system with instantaneous information on the
degree of tension in each small segment of each
muscle.
Transmission of Impulses from the Tendon Organ into the
Central Nervous System. Signals from the tendon organ
are transmitted through large, rapidly conducting type
Ib nerve ﬁbers that average 16 micrometers in diame-
ter, only slightly smaller than those from the primary
endings of the muscle spindle. These ﬁbers, like those
from the primary sp

indle. These ﬁbers, like those
from the primary spindle endings, transmit signals
both into local areas of the cord and, after synapsing
in a dorsal horn of the cord, through long ﬁber path-
ways such as the spinocerebellar tracts into the cere-
bellum and through still other tracts to the cerebral
cortex. The local cord signal excites a single inhibitory
interneuron that inhibits the anterior motor neuron.
This local circuit directly inhibits the individual muscle
without affecting adjacent mus

e individual muscle
without affecting adjacent muscles. The relation
between signals to the brain and function of the cere-
bellum and other parts of the brain for muscle control
is discussed in Chapter 56.
Inhibitory Nature of the Tendon Reﬂex and 
Its Importance
When the Golgi tendon organs of a muscle tendon are
stimulated by increased tension in the connecting
muscle, signals are transmitted to the spinal cord to
cause reﬂex effects in the respective muscle.This reﬂex
is entirely inhibitory.

spective muscle.This reﬂex
is entirely inhibitory. Thus, this reﬂex provides a nega-
tive feedback mechanism that prevents the develop-
ment of too much tension on the muscle.
When tension on the muscle and, therefore, on the
tendon becomes extreme, the inhibitory effect from
the tendon organ can be so great that it leads to a
sudden reaction in the spinal cord that causes instan-
taneous relaxation of the entire muscle. This effect is
called the lengthening reaction; it is probably a pro-
tecti

 lengthening reaction; it is probably a pro-
tective mechanism to prevent tearing of the muscle or
avulsion of the tendon from its attachments to the
bone.We know, for instance, that direct electrical stim-
ulation of muscles in the laboratory, which cannot be
opposed by this negative reﬂex, can occasionally cause
such destructive effects.
Tendon
Muscle
Nerve fiber (16 mm)
Figure 54–7
Golgi tendon organ.
680
Unit XI
Possible Role of th

nd Integrative Neurophysiology
Possible Role of the Tendon Reﬂex to Equalize Contractile
Force Among the Muscle Fibers. Another likely function
of the Golgi tendon reﬂex is to equalize contractile
forces of the separate muscle ﬁbers.That is,those ﬁbers
that exert excess tension become inhibited by the
reﬂex, whereas those that exert too little tension
become more excited because of absence of reﬂex
inhibition. This spreads the muscle load over all the
ﬁbers and prevents damage in isolated areas 

l the
ﬁbers and prevents damage in isolated areas of 
a muscle where small numbers of ﬁbers might be 
overloaded.
Function of the Muscle Spindles and
Golgi Tendon Organs in Conjunction
with Motor Control from Higher Levels
of the Brain
Although we have emphasized the function of the
muscle spindles and Golgi tendon organs in spinal
cord control of motor function, these two sensory
organs also apprise the higher motor control centers
of instantaneous changes taking place in the muscles.
For insta

ous changes taking place in the muscles.
For instance, the dorsal spinocerebellar tracts carry
instantaneous information from both the muscle 
spindles and the Golgi tendon organs directly to the
cerebellum at conduction velocities approaching 
120 m/sec, the most rapid conduction anywhere in the
brain or spinal cord. Additional pathways transmit
similar information into the reticular regions of the
brain stem and, to a lesser extent, all the way to the
motor areas of the cerebral cortex. As dis

 to the
motor areas of the cerebral cortex. As discussed in
Chapters 55 and 56, the information from these recep-
tors is crucial for feedback control of motor signals
that originate in all these areas.
Flexor Reﬂex and the
Withdrawal Reﬂexes
In the spinal or decerebrate animal, almost any type
of cutaneous sensory stimulus from a limb is likely to
cause the ﬂexor muscles of the limb to contract,
thereby withdrawing the limb from the stimulating
object. This is called the ﬂexor reﬂex.
In its cla

In its classic form, the ﬂexor reﬂex is elicited most
powerfully by stimulation of pain endings, such as by
a pinprick, heat, or a wound, for which reason it is also
called a nociceptive reﬂex, or simply a pain reﬂex. Stim-
ulation of touch receptors can also elicit a weaker and
less prolonged ﬂexor reﬂex.
If some part of the body other than one of the limbs
is painfully stimulated, that part will similarly be with-
drawn from the stimulus, but the reﬂex m

 be with-
drawn from the stimulus, but the reﬂex may not be con-
ﬁned to ﬂexor muscles, even though it is basically the
same type of reﬂex. Therefore, the many patterns of
these reﬂexes in the different areas of the body are
called withdrawal reﬂexes.
Neuronal Mechanism of the Flexor Reﬂex. The left-hand
portion of Figure 54–8 shows the neuronal pathways
for the ﬂexor reﬂex. In this instance, a painful stimu-
lus is applied to the hand; as a result, the ﬂexor muscles
of the upper arm become exci

lt, the ﬂexor muscles
of the upper arm become excited, thus withdrawing
the hand from the painful stimulus.
The pathways for eliciting the ﬂexor reﬂex do not
pass directly to the anterior motor neurons but instead
pass ﬁrst into the spinal cord interneuron pool of
neurons and only secondarily to the motor neurons.
The shortest possible circuit is a three- or four-neuron
pathway; however, most of the signals of the reﬂex tra-
verse many more neurons and involve the following
basic types of circui

ns and involve the following
basic types of circuits: (1) diverging circuits to spread
the reﬂex to the necessary muscles for withdrawal; (2)
circuits to inhibit the antagonist muscles, called recip-
rocal inhibition circuits; and (3) circuits to cause after-
discharge lasting many fractions of a second after the
stimulus is over.
Figure 54–9 shows a typical myogram from a ﬂexor
muscle during a ﬂexor reﬂex.Within a few milliseconds
after a pain nerve begins to be stimulated, the ﬂexor
response a

erve begins to be stimulated, the ﬂexor
response appears. Then, in the next few seconds, the
reﬂex begins to fatigue, which is characteristic of essen-
tially all complex integrative reﬂexes of the spinal
cord. Finally, after the stimulus is over, the contraction
of the muscle returns toward the baseline, but because
of afterdischarge, it takes many milliseconds for this 
to occur. The duration of afterdischarge depends on
the intensity of the sensory stimulus that elicited the
reﬂex; a weak tac

nsory stimulus that elicited the
reﬂex; a weak tactile stimulus causes almost no after-
discharge, but after a strong pain stimulus, the after-
discharge may last for a second or more.
FLEXOR
REFLEX
CROSSED EXTENSOR
REFLEX
Painful
stimulus
from hand
Polysynaptic
circuit
Excited
RECIPROCAL INHIBITION
Inhibited
Excited
Inhibited
Figure 54–8
Flexor reﬂex, crossed extensor reﬂex, and reciprocal inhibition.
Chapter 54
681
The afterdischarge that oc

d; the Cord Reﬂexes
681
The afterdischarge that occurs in the ﬂexor reﬂex
almost certainly results from both types of repetitive
discharge circuits discussed in Chapter 46. Electro-
physiologic studies indicate that immediate afterdis-
charge, lasting for about 6 to 8 milliseconds, results
from repetitive ﬁring of the excited interneurons
themselves. Also, prolonged afterdischarge occurs
after strong pain stimuli, almost certainly resulting
from recurrent pathways that initiate oscillation in
re

recurrent pathways that initiate oscillation in
reverberating interneuron circuits. These, in turn,
transmit impulses to the anterior motor neurons,
sometimes for several seconds after the incoming
sensory signal is over.
Thus, the ﬂexor reﬂex is appropriately organized to
withdraw a pained or otherwise irritated part of the
body from a stimulus. Further, because of afterdis-
charge, the reﬂex can hold the irritated part away from
the stimulus for 0.1 to 3 seconds after the irritation is
over. D

r 0.1 to 3 seconds after the irritation is
over. During this time, other reﬂexes and actions of the
central nervous system can move the entire body away
from the painful stimulus.
Pattern of Withdrawal. The pattern of withdrawal that
results when the ﬂexor reﬂex is elicited depends on
which sensory nerve is stimulated. Thus, a pain stimu-
lus on the inward side of the arm elicits not only 
contraction of the ﬂexor muscles of the arm but also
contraction of abductor muscles to pull the arm
outwar

raction of abductor muscles to pull the arm
outward. In other words, the integrative centers of 
the cord cause those muscles to contract that can 
most effectively remove the pained part of the body
away from the object causing the pain. Although this
principle, called the principle of “local sign,” applies to
any part of the body, it is especially applicable to the
limbs because of their highly developed ﬂexor reﬂexes.
About 0.2 to 0.5 second after a stimulus elicits a ﬂ

out 0.2 to 0.5 second after a stimulus elicits a ﬂexor
reﬂex in one limb, the opposite limb begins to extend.
This is called the crossed extensor reﬂex. Extension of
the opposite limb can push the entire body away from
the object causing the painful stimulus in the with-
drawn limb.
Neuronal Mechanism of the Crossed Extensor Reﬂex. The
right-hand portion of Figure 54–8 shows the neuronal
circuit responsible for the crossed extensor reﬂex,
demonstrating that signals from sensory nerves cross
to t

rating that signals from sensory nerves cross
to the opposite side of the cord to excite extensor
muscles. Because the crossed extensor reﬂex usually
does not begin until 200 to 500 milliseconds after onset
of the initial pain stimulus, it is certain that many
interneurons are involved in the circuit between the
incoming sensory neuron and the motor neurons of
the opposite side of the cord responsible for the
crossed extension. After the painful stimulus is
removed, the crossed extensor reﬂex ha

stimulus is
removed, the crossed extensor reﬂex has an even
longer period of afterdischarge than does the ﬂexor
reﬂex. Again, it is presumed that this prolonged after-
discharge results from reverberating circuits among
the interneuronal cells.
Figure 54–10 shows a typical myogram recorded
from a muscle involved in a crossed extensor reﬂex.
This demonstrates the relatively long latency before
the reﬂex begins and the long afterdischarge at the end
of the stimulus. The prolonged afterdischarge is

d
of the stimulus. The prolonged afterdischarge is of
beneﬁt in holding the pained area of the body away
from the painful object until other nervous reactions
cause the entire body to move away.
Reciprocal Inhibition and
Reciprocal Innervation
In the previous paragraphs, we pointed out several
times that excitation of one group of muscles is often
associated with inhibition of another group. For
instance, when a stretch reﬂex excites one muscle, it
often simultaneously inhibits the antagonist mu

it
often simultaneously inhibits the antagonist muscles.
This is the phenomenon of reciprocal inhibition, and
the neuronal circuit that causes this reciprocal relation
Seconds
Flexor contraction
0
1
Duration of stimulus
Fatigue
Afterdischarge
2
3
Figure 54–9
Myogram of the ﬂexor reﬂex showing rapid onset of the reﬂex, 
an interval of fatigue, and, ﬁnally, afterdischarge after the input
Extensor contraction
Seconds
0
stimulus
1
2
3
4
Figure 54–10
Myogr

stimulus
1
2
3
4
Myogram of a crossed extensor reﬂex showing slow onset but pro-
longed afterdischarge.
682
Unit XI
is called reciprocal innervation. Likewise, reciprocal
relations often exist between the muscles on the two
sides of the body, as exempliﬁed by the ﬂexor and
extensor muscle reﬂexes described earlier.
Figure 54–11 shows a typical example of reciprocal
inhibition. In this instance, a moderate bu

procal
inhibition. In this instance, a moderate but prolonged
ﬂexor reﬂex is elicited from one limb of the body;
while this reﬂex is still being elicited, a stronger ﬂexor
reﬂex is elicited in the limb on the opposite side of the
body. This stronger reﬂex sends reciprocal inhibitory
signals to the ﬁrst limb and depresses its degree of
ﬂexion. Finally, removal of the stronger reﬂex allows
the original reﬂex to reassume its previous intensity.
Reﬂexes of Posture and
Locomotion
Postural and Locomot

xes of Posture and
Postural and Locomotive Reﬂexes 
of the Cord
Positive Supportive Reaction. Pressure on the footpad of
a decerebrate animal causes the limb to extend against
the pressure applied to the foot. Indeed, this reﬂex 
is so strong that if an animal whose spinal cord has
been transected for several months—that is, after the
reﬂexes have become exaggerated—is placed on its
feet, the reﬂex often stiffens the limbs sufﬁciently to
support the weight of the body.This reﬂex is ca

to
support the weight of the body.This reﬂex is called the
positive supportive reaction.
The positive supportive reaction involves a complex
circuit in the interneurons similar to the circuits
responsible for the ﬂexor and cross extensor reﬂexes.
The locus of the pressure on the pad of the foot deter-
mines the direction in which the limb will extend; pres-
sure on one side causes extension in that direction, an
effect called the magnet reaction. This helps keep an
animal from falling to that si

 This helps keep an
animal from falling to that side.
Cord “Righting” Reﬂexes. When a spinal animal is laid on
its side, it will make incoordinate movements trying to
raise itself to the standing position. This is called 
the cord righting reﬂex. Such a reﬂex demonstrates
that some relatively complex reﬂexes associated with
posture are integrated in the spinal cord. Indeed, an
animal with a well-healed transected thoracic cord
between the levels for forelimb and hindlimb innerva-
tion can right 

for forelimb and hindlimb innerva-
tion can right itself from the lying position and even
walk using its hindlimbs in addition to its forelimbs.
In the case of an opossum with a similar transection 
of the thoracic cord, the walking movements of 
the hindlimbs are hardly different from those in a
normal opossum—except that the hindlimb walking
movements are not synchronized with those of the
forelimbs.
Stepping and Walking Movements
Rhythmical Stepping Movements of a Single Limb. Rhythmi-
cal st

epping Movements of a Single Limb. Rhythmi-
cal stepping movements are frequently observed in the
limbs of spinal animals. Indeed, even when the lumbar
portion of the spinal cord is separated from the
remainder of the cord and a longitudinal section is
made down the center of the cord to block neuronal
connections between the two sides of the cord and
between the two limbs, each hindlimb can still perform
individual stepping functions. Forward ﬂexion of the
limb is followed a second or so later 

xion of the
limb is followed a second or so later by backward
extension. Then ﬂexion occurs again, and the cycle is
repeated over and over.
This oscillation back and forth between ﬂexor and
extensor muscles can occur even after the sensory
nerves have been cut, and it seems to result mainly
from mutually reciprocal inhibition circuits within the
matrix of the cord itself, oscillating between the
neurons controlling agonist and antagonist muscles.
The sensory signals from the footpads and from th

The sensory signals from the footpads and from the
position sensors around the joints play a strong role in
controlling foot pressure and frequency of stepping
when the foot is allowed to walk along a surface. In
fact, the cord mechanism for control of stepping can
be even more complex. For instance, if the top of the
foot encounters an obstruction during forward thrust,
the forward thrust will stop temporarily; then, in rapid
sequence, the foot will be lifted higher and proceed
forward to be p

 will be lifted higher and proceed
forward to be placed over the obstruction. This is the
stumble reﬂex. Thus, the cord is an intelligent walking
controller.
Reciprocal Stepping of Opposite Limbs. If the lumbar spinal
cord is not split down its center, every time stepping
occurs in the forward direction in one limb, the oppo-
site limb ordinarily moves backward.This effect results
from reciprocal innervation between the two limbs.
Diagonal Stepping of All Four Limbs—“Mark Time” Reﬂex. If
a well-

ng of All Four Limbs—“Mark Time” Reﬂex. If
a well-healed spinal animal (with spinal transection in
the neck above the forelimb area of the cord) is held
up from the ﬂoor and its legs are allowed to dangle, as
shown in Figure 54–12, the stretch on the limbs occa-
sionally elicits stepping reﬂexes that involve all four
limbs. In general, stepping occurs diagonally between
the forelimbs and hindlimbs. This diagonal response is
another manifestation of reciprocal innervation, this

of reciprocal innervation, this
0
1
Duration of inhibitory stimulus
Duration of flexor reflex stimulus
2
3
Seconds
Figure 54–11
Myogram of a ﬂexor reﬂex showing reciprocal inhibition caused
by an inhibitory stimulus from a stronger ﬂexor reﬂex on the oppo-
Chapter 54
683
time occurring the entire distance up and down the
cord between the forelimbs and hindlimbs. Such a
walking pattern is called a mark 

indlimbs. Such a
walking pattern is called a mark time reﬂex.
Galloping Reﬂex. Another type of reﬂex that occasion-
ally develops in a spinal animal is the galloping reﬂex,
in which both forelimbs move backward in unison
while both hindlimbs move forward. This often occurs
when almost equal stretch or pressure stimuli are
applied to the limbs on both sides of the body at the
same time; unequal stimulation elicits the diagonal
walking reﬂex. This is in keeping with the normal pat-
terns of walkin

is in keeping with the normal pat-
terns of walking and galloping, because in walking,
only one forelimb and one hindlimb at a time are stim-
ulated, which would predispose the animal to continue
walking. Conversely, when the animal strikes the
ground during galloping, both forelimbs and both
hindlimbs are stimulated about equally; this predis-
poses the animal to keep galloping and, therefore, con-
tinues this pattern of motion.
An especially important cord reﬂex in some animals i

 especially important cord reﬂex in some animals is
the scratch reﬂex, which is initiated by itch or tickle sen-
sation. It involves two functions: (1) a position sense that
allows the paw to ﬁnd the exact point of irritation on
the surface of the body, and (2) a to-and-fro scratching
movement.
The position sense of the scratch reﬂex is a highly
developed function. If a ﬂea is crawling as far forward
as the shoulder of a spinal animal, the hind paw can still
ﬁnd its position, even though 19 musc

aw can still
ﬁnd its position, even though 19 muscles in the limb
must be contracted simultaneously in a precise pattern
to bring the paw to the position of the crawling ﬂea. To
make the reﬂex even more complicated, when the ﬂea
crosses the midline, the ﬁrst paw stops scratching and
the opposite paw begins the to-and-fro motion and
eventually ﬁnds the ﬂea.
The to-and-fro movement, like the stepping move-
ments of locomotion, involves reciprocal innervation
circuits that cause oscillation.
Spinal

nnervation
Spinal Cord Reﬂexes That
Cause Muscle Spasm
In human beings, local muscle spasm is often observed.
In many, if not most, instances, localized pain is the
cause of the local spasm.
Muscle Spasm Resulting from a Broken Bone. One type of
clinically important spasm occurs in muscles that sur-
round a broken bone. This results from pain impulses
initiated from the broken edges of the bone, which
cause the muscles that surround the area to contract
tonically

scles that surround the area to contract
tonically. Pain relief obtained by injecting a local anes-
thetic at the broken edges of the bone relieves the
spasm; a deep general anesthetic of the entire body, such
as ether anesthesia, also relieves the spasm. One of
these two anesthetic procedures is often necessary
before the spasm can be overcome sufﬁciently for the
two ends of the bone to be set back into their appro-
priate positions.
Abdominal Muscle Spasm in Peritonitis. Another type of
local 

uscle Spasm in Peritonitis. Another type of
local spasm caused by cord reﬂexes is abdominal spasm
resulting from irritation of the parietal peritoneum by
peritonitis. Here again, relief of the pain caused by the
peritonitis allows the spastic muscle to relax. The same
type of spasm often occurs during surgical operations;
for instance, during abdominal operations, pain im-
pulses from the parietal peritoneum often cause the
abdominal muscles to contract extensively, sometimes
extruding the intes

ontract extensively, sometimes
extruding the intestines through the surgical wound. For
this reason,deep anesthesia is usually required for intra-
abdominal operations.
Muscle Cramps. Still another type of local spasm is the
typical muscle cramp. Electromyographic studies indi-
cate that the cause of at least some muscle cramps is as
follows: Any local irritating factor or metabolic abnor-
mality of a muscle, such as severe cold, lack of blood
ﬂow, or overexercise, can elicit pain or other senso

w, or overexercise, can elicit pain or other sensory
signals transmitted from the muscle to the spinal cord,
which in turn cause reﬂex feedback muscle contraction.
The contraction is believed to stimulate the same
sensory receptors even more, which causes the spinal
cord to increase the intensity of contraction. Thus, pos-
itive feedback develops, so that a small amount of initial
irritation causes more and more contraction until a full-
blown muscle cramp ensues.
Autonomic Reﬂexes in the
Spinal

scle cramp ensues.
Many types of segmental autonomic reﬂexes are inte-
grated in the spinal cord, most of which are discussed in
other chapters. Brieﬂy, these include (1) changes in vas-
cular tone resulting from changes in local skin heat (see
Chapter 73); (2) sweating, which results from localized
heat on the surface of the body (see Chapter 73); (3)
intestinointestinal reﬂexes that control some motor
functions of the gut (see Chapter 62); (4) peritoneoin-

ons of the gut (see Chapter 62); (4) peritoneoin-
testinal reﬂexes that inhibit gastrointestinal motility in
response to peritoneal irritation (see Chapter 66); and
(5) evacuation reﬂexes for emptying the full bladder
(see Chapter 31) or the colon (see Chapter 63). In addi-
tion, all the segmental reﬂexes can at times be elicited
simultaneously in the form of the so-called mass reﬂex,
described next.
Figure 54–12
Diagonal stepping movements exhibited by a spinal animal.
684
Unit XI
The Nervous S

ited by a spinal animal.
684
Unit XI
Mass Reﬂex. In a spinal animal or human being, some-
times the spinal cord suddenly becomes excessively
active, causing massive discharge in large portions of the
cord.The usual stimulus that causes this is a strong pain
stimulus to the skin or excessive ﬁlling of a viscus, such
as overdistention of the bladder or the gut. Regardless
of the type of stimulus, the resulting reﬂex, called the
mass reﬂe

timulus, the resulting reﬂex, called the
mass reﬂex, involves large portions or even all of the
cord. The effects are (1) a major portion of the body’s
skeletal muscles goes into strong ﬂexor spasm; (2) the
colon and bladder are likely to evacuate; (3) the arte-
rial pressure often rises to maximal values, sometimes
to a systolic pressure well over 200 mm Hg; and (4) large
areas of the body break out into profuse sweating.
Because the mass reﬂex can last for minutes, it pre-
sumably results from

can last for minutes, it pre-
sumably results from activation of great numbers of
reverberating circuits that excite large areas of the cord
at once. This is similar to the mechanism of epileptic
seizures, which involve reverberating circuits that occur
in the brain instead of in the cord.
Spinal Cord Transection and
Spinal Shock
When the spinal cord is suddenly transected in the
upper neck, at ﬁrst, essentially all cord functions, includ-
ing the cord reﬂexes, immediately become depressed to
th

e cord reﬂexes, immediately become depressed to
the point of total silence, a reaction called spinal shock.
The reason for this is that normal activity of the cord
neurons depends to a great extent on continual tonic
excitation by the discharge of nerve ﬁbers entering the
cord from higher centers, particularly discharge trans-
mitted through the reticulospinal tracts, vestibulospinal
tracts, and corticospinal tracts.
After a few hours to a few weeks, the spinal neurons
gradually regain their exc

eks, the spinal neurons
gradually regain their excitability. This seems to be a
natural characteristic of neurons everywhere in the
nervous system—that is, after they lose their source of
facilitatory impulses, they increase their own natural
degree of excitability to make up at least partially for
the loss. In most nonprimates, excitability of the cord
centers returns essentially to normal within a few hours
to a day or so, but in human beings, the return is often
delayed for several weeks and 

the return is often
delayed for several weeks and occasionally is never
complete; conversely, sometimes recovery is excessive,
with resultant hyperexcitability of some or all cord 
Some of the spinal functions speciﬁcally affected
during or after spinal shock are the following:
1. At onset of spinal shock, the arterial blood
pressure falls instantly and drastically—sometimes
to as low as 40 mm Hg—thus demonstrating that
sympathetic nervous system activity becomes
blocked almost to ext

vous system activity becomes
blocked almost to extinction. The pressure
ordinarily returns to normal within a few days, even
in human beings.
2. All skeletal muscle reﬂexes integrated in the spinal
cord are blocked during the initial stages of shock.
In lower animals, a few hours to a few days are
required for these reﬂexes to return to normal;
in human beings, 2 weeks to several months are
sometimes required. In both animals and 
humans, some reﬂexes may eventually become
hyperexcitable, partic

ﬂexes may eventually become
hyperexcitable, particularly if a few facilitatory
pathways remain intact between the brain and the
cord while the remainder of the spinal cord is
transected. The ﬁrst reﬂexes to return are the
stretch reﬂexes, followed in order by the
progressively more complex reﬂexes: ﬂexor reﬂexes,
postural antigravity reﬂexes, and remnants of
stepping reﬂexes.
3. The sacral reﬂexes for control of bladder and colon
evacuation are suppressed in human beings for the
ﬁrst few weeks a

uppressed in human beings for the
ﬁrst few weeks after cord transection, but in most
cases they eventually return. These effects are
discussed in Chapters 31 and 66.
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685
Cortical and Brain Stem Control 
of Motor Function
In this chapter, we discuss control of body move-
ments by the cerebral cortex and brain stem.
Most “voluntary” movements initiated by the
cerebral cortex are achieved when the cortex acti-
vates “patterns” of function stored in lower brain
areas—the cord, brain stem, basal ganglia, and cere-
bellum. These lower centers, in turn, send speciﬁc
control signals to the muscles.
For a few types

ﬁc
For a few types of movements, however, the cortex has almost a direct
pathway to the anterior motor neurons of the cord, bypassing some motor
centers on the way.This is especially true for control of the ﬁne dexterous move-
ments of the ﬁngers and hands. This chapter and Chapter 56 explain the inter-
play among the different motor areas of the brain and spinal cord to provide
overall synthesis of voluntary motor function.
MOTOR CORTEX AND CORTICOSPINAL TRACT
Fi

 function.
Figure 55–1 shows the functional areas of the cerebral cortex. Anterior to the
central cortical sulcus, occupying approximately the posterior one third of 
the frontal lobes, is the motor cortex. Posterior to the central sulcus is the
somatosensory cortex (an area discussed in detail in earlier chapters), which
feeds the motor cortex many of the signals that initiate motor activities.
The motor cortex itself is divided into three subareas, each of 

ex itself is divided into three subareas, each of which has its
own topographical representation of muscle groups and speciﬁc motor func-
tions: (1) the primary motor cortex, (2) the premotor area, and (3) the supple-
mentary motor area.
The primary motor cortex, shown in Figure 55–1, lies in the ﬁrst convolution of
the frontal lobes anterior to the central sulcus. It begins laterally in the sylvian
ﬁssure, spreads superiorly to the uppermost portion of the brain, and then d

 to the uppermost portion of the brain, and then dips
deep into the longitudinal ﬁssure. (This area is the same as area 4 in Brodmann’s
classiﬁcation of the brain cortical areas, shown in Figure 47–5.)
Figure 55–1 lists the approximate topographical representations of the dif-
ferent muscle areas of the body in the primary motor cortex, beginning with the
face and mouth region near the sylvian ﬁssure; the arm and hand area, in the
midportion of the primary motor cortex; the trunk, near the apex 

he primary motor cortex; the trunk, near the apex of the brain;
and the leg and foot areas, in the part of the primary motor cortex that dips into
the longitudinal ﬁssure. This topographical organization is demonstrated even
more graphically in Figure 55–2, which shows the degrees of representation of
the different muscle areas as mapped by Penﬁeld and Rasmussen.This mapping
was done by electrically stimulating the different areas of the motor cortex in
human beings who were undergoing neurosurg

rtex in
human beings who were undergoing neurosurgical operations. Note that more
than one half of the entire primary motor cortex is concerned with controlling
the muscles of the hands and the muscles of speech. Point stimulation in these
hand and speech motor areas on rare occasion causes contraction of a single
686
Unit XI
muscle; most often, stimulation contracts a group of
muscles instead. To express this in another way, exci-
tat

instead. To express this in another way, exci-
tation of a single motor cortex neuron usually excites
a speciﬁc movement rather than one speciﬁc muscle.
To do this, it excites a “pattern” of separate muscles,
each of which contributes its own direction and
strength of muscle movement.
The premotor area, also shown in Figure 55–1, lies 1
to 3 centimeters anterior to the primary motor cortex,
extending inferiorly into the sylvian ﬁssure and supe-
riorly into the longitudinal ﬁssure, 

re and supe-
riorly into the longitudinal ﬁssure, where it abuts 
the supplementary motor area, which has functions
similar to those of the premotor area. The topograph-
ical organization of the premotor cortex is roughly the
same as that of the primary motor cortex, with the
mouth and face areas located most laterally; as one
moves upward, the hand, arm, trunk, and leg areas are
encountered.
Nerve signals generated in the premotor area cause
much more complex “patterns” of movement than 
the di

h more complex “patterns” of movement than 
the discrete patterns generated in the primary motor
cortex. For instance, the pattern may be to position 
the shoulders and arms so that the hands are properly
oriented to perform speciﬁc tasks. To achieve these
results, the most anterior part of the premotor area
ﬁrst develops a “motor image” of the total muscle
movement that is to be performed. Then, in the poste-
rior premotor cortex, this image excites each succes-
sive pattern of muscle activity 

ites each succes-
sive pattern of muscle activity required to achieve the
image.This posterior part of the premotor cortex sends
its signals either directly to the primary motor cortex
to excite speciﬁc muscles or, often, by way of the basal
ganglia and thalamus back to the primary motor
cortex. Thus, the premotor cortex, basal ganglia, thal-
amus, and primary motor cortex constitute a complex
overall system for the control of complex patterns of
coordinated muscle activity.
Supplementary Motor 

The supplementary motor area has yet another topo-
graphical organization for the control of motor func-
tion. It lies mainly in the longitudinal ﬁssure but
extends a few centimeters onto the superior frontal
cortex. Contractions elicited by stimulating this area
are often bilateral rather than unilateral. For instance,
stimulation frequently leads to bilateral grasping
movements of both hands simultaneously; these
movements are perhaps rudi

s simultaneously; these
movements are perhaps rudiments of the hand func-
tions required for climbing. In general, this area func-
tions in concert with the premotor area to provide
body-wide attitudinal movements, ﬁxation movements
of the different segments of the body, positional
movements of the head and eyes, and so forth, as back-
ground for the ﬁner motor control of the arms and
hands by the premotor area and primary motor cortex.
Some Specialized Areas of Motor
Control Found in the Human 

ialized Areas of Motor
Motor Cortex
Neurosurgeons have found a few highly specialized
motor regions of the human cerebral cortex (shown in
Figure 55–3) that control speciﬁc motor functions.
Primary
motor
cortex
Supplementary
area
Premotor
area
Face
Hand
Arm
4
Trunk
Feet
Legs
7
6
5
3, 2, 1
Mouth
Somatic
area 1
Motor
Sensory
Somatic
area
Figure 55–1
Motor and somatosensory functional areas of the cerebral cortex.
The numbers 4, 5, 6, and 7 are Brodmann’s cor

tex.
The numbers 4, 5, 6, and 7 are Brodmann’s cortical areas, as
explained in Chapter 47.
Trunk
Shoulder
Elbow
Wrist
Hand
Thumb
Neck
Brow
Face
Lips
Mastication
Salivation
Jaw
Tongue
Swallowing
Eyelid and eyeball
Hip
Knee
Ankle
Toes
Figure 55–2
Degree of representation of the different muscles of the body in
the motor cortex. (Redrawn from Penﬁeld W, Rasmussen T: The
Cerebral Cortex of Man: A Clinical Study of Localization of Func

x of Man: A Clinical Study of Localization of Func-
tion. New York: Hafner, 1968.)
Chapter 55
Cortical and Brain Stem Control of Motor Function
687
These regions have been localized either by electrical
stimulation or by noting the loss of motor function
when destructive lesions occur in speciﬁc cortical
areas. Some of the more important regions are the 
Broca’s Area and Speech. Figure 55–3 shows a premotor
area labeled “word formation” lying immediately
anterior to the primary motor 

” lying immediately
anterior to the primary motor cortex and immediately
above the sylvian ﬁssure. This region is called Broca’s
area. Damage to it does not prevent a person from
vocalizing, but it does make it impossible for the
person to speak whole words rather than uncoordi-
nated utterances or an occasional simple word such as
“no” or “yes.” A closely associated cortical area also
causes appropriate respiratory function, so that respi-
ratory activation of the vocal cords can occur simulta-

y activation of the vocal cords can occur simulta-
neously with the movements of the mouth and tongue
during speech. Thus, the premotor neuronal activities
related to speech are highly complex.
“Voluntary” Eye Movement Field. In the premotor area
immediately above Broca’s area is a locus for control-
ling voluntary eye movements. Damage to this area
prevents a person from voluntarily moving the eyes
toward different objects. Instead, the eyes tend to lock
involuntarily onto speciﬁc objects, an e

d to lock
involuntarily onto speciﬁc objects, an effect controlled
by signals from the occipital visual cortex, as explained
in Chapter 51. This frontal area also controls eyelid
movements such as blinking.
Head Rotation Area. Slightly higher in the motor associ-
ation area, electrical stimulation elicits head rotation.
This area is closely associated with the eye movement
ﬁeld; it directs the head toward different objects.
Area for Hand Skills. In the premotor area immediately
anterior to the p

In the premotor area immediately
anterior to the primary motor cortex for the hands and
ﬁngers is a region neurosurgeons have identiﬁed as
important for “hand skills.” That is, when tumors or
other lesions cause destruction in this area, hand
movements become uncoordinated and nonpurpose-
ful, a condition called motor apraxia.
Transmission of Signals from the
Motor Cortex to the Muscles
Motor signals are transmitted directly from the cortex
to the spinal cord through the corticospinal tract and

e spinal cord through the corticospinal tract and
indirectly through multiple accessory pathways that
involve the basal ganglia, cerebellum, and various
nuclei of the brain stem. In general, the direct pathways
are concerned more with discrete and detailed move-
ments, especially of the distal segments of the limbs,
particularly the hands and ﬁngers.
Corticospinal (Pyramidal) Tract
The most important output pathway from the motor
cortex is the corticospinal tract, also called the pyram-
idal tra

rticospinal tract, also called the pyram-
idal tract, shown in Figure 55–4.The corticospinal tract
originates about 30 per cent from the primary motor
cortex, 30 per cent from the premotor and supple-
mentary motor areas, and 40 per cent from the
somatosensory areas posterior to the central sulcus.
After leaving the cortex, it passes through the pos-
terior limb of the internal capsule (between the
caudate nucleus and the putamen of the basal ganglia)
and then downward through the brain stem, fo

glia)
and then downward through the brain stem, forming
the pyramids of the medulla. The majority of the
pyramidal ﬁbers then cross in the lower medulla to the
opposite side and descend into the lateral corticospinal
tracts of the cord, ﬁnally terminating principally on the
interneurons in the intermediate regions of the cord
gray matter; a few terminate on sensory relay neurons
in the dorsal horn, and a very few terminate directly
on the anterior motor neurons that cause muscle 
A 

r motor neurons that cause muscle 
A few of the ﬁbers do not cross to the opposite side
in the medulla but pass ipsilaterally down the cord in
the ventral corticospinal tracts. Many if not most of
these ﬁbers eventually cross to the opposite side of the
cord either in the neck or in the upper thoracic region.
These ﬁbers may be concerned with control of bilat-
eral postural movements by the supplementary motor
cortex.
The most impressive ﬁbers in the pyramidal tract are
a population

sive ﬁbers in the pyramidal tract are
a population of large myelinated ﬁbers with a mean
diameter of 16 micrometers. These ﬁbers originate
from giant pyramidal cells, called Betz cells, that are
found only in the primary motor cortex.The Betz cells
are about 60 micrometers in diameter, and their ﬁbers
transmit nerve impulses to the spinal cord at a veloc-
ity of about 70 m/sec, the most rapid rate of transmis-
sion of any signals from the brain to the cord. There
are about 34,000 of these large 

o the cord. There
are about 34,000 of these large Betz cell ﬁbers in each
corticospinal tract. The total number of ﬁbers in each
corticospinal tract is more than 1 million,so these large
Chewing
Tongue
Jaw
Lips
Neck
Fingers
Hips
Legs
Feet
Thumb
Trunk
Arms
Supplemental
and premotor
areas
Primary
motor
cortex
Contralateral
eye movements
Head rotation
Choice
of words
Eye
fixation
Hand skills
Word formation
(Broca’s area)
Figure 55–3
Representation of the different muscles of

re 55–3
Representation of the different muscles of the body in the motor
cortex and location of other cortical areas responsible for speciﬁc
types of motor movements.
688
Unit XI
ﬁbers represent only 3 per cent of the total. The other
97 per cent are mainly ﬁbers smaller than 4 microme-
ters in diameter that conduct background tonic signals
to the motor areas of the cord.
Other Fiber Pathways from the Motor Cortex
The motor cortex give

thways from the Motor Cortex
The motor cortex gives rise to large numbers of addi-
tional, mainly small ﬁbers that go to deep regions in the
cerebrum and brain stem, including the following:
1. The axons from the giant Betz cells send short
collaterals back to the cortex itself. These
collaterals are believed to inhibit adjacent regions
of the cortex when the Betz cells discharge, thereby
“sharpening” the boundaries of the excitatory
signal.
2. A large number of ﬁbers pass from the motor
cortex 

 large number of ﬁbers pass from the motor
cortex into the caudate nucleus and putamen. From
there, additional pathways extend into the brain
stem and spinal cord, as discussed in the next
chapter, mainly to control body postural muscle
3. A moderate number of motor ﬁbers pass to red
nuclei of the midbrain. From these, additional ﬁbers
pass down the cord through the rubrospinal tract.
4. A moderate number of motor ﬁbers deviate into
the reticular substance and vestibular nuclei of 

the reticular substance and vestibular nuclei of the
brain stem; from there, signals go to the cord by
way of reticulospinal and vestibulospinal tracts,
and others go to the cerebellum by way of
reticulocerebellar and vestibulocerebellar tracts.
5. A tremendous number of motor ﬁbers synapse 
in the pontile nuclei, which give rise to the
pontocerebellar ﬁbers, carrying signals into the
cerebellar hemispheres.
6. Collaterals also terminate in the inferior olivary
nuclei, and from there, secondary

inferior olivary
nuclei, and from there, secondary olivocerebellar
ﬁbers transmit signals to multiple areas of the
cerebellum.
Thus, the basal ganglia, brain stem, and cerebellum all
receive strong motor signals from the corticospinal
system every time a signal is transmitted down the
spinal cord to cause a motor activity.
Incoming Fiber Pathways to the
The functions of the motor cortex are controlled mainly
by nerve signals from the somatosensory system but
also, to some degree, fr

 somatosensory system but
also, to some degree, from other sensory systems such
as hearing and vision. Once the sensory information is
received, the motor cortex operates in association with
the basal ganglia and cerebellum to excite an appropri-
ate course of motor action. The more important incom-
ing ﬁber pathways to the motor cortex are the following:
1. Subcortical ﬁbers from adjacent regions of 
the cerebral cortex, especially from (a) the
somatosensory areas of the parietal cortex, (b) th

somatosensory areas of the parietal cortex, (b) the
adjacent areas of the frontal cortex anterior to 
the motor cortex, and (c) the visual and auditory
cortices.
2. Subcortical ﬁbers that arrive through the corpus
callosum from the opposite cerebral hemisphere.
These ﬁbers connect corresponding areas of the
cortices in the two sides of the brain.
3. Somatosensory ﬁbers that arrive directly from the
ventrobasal complex of the thalamus. These relay
mainly cutaneous tactile signals and joint and
mu

muscle signals from the peripheral body.
4. Tracts from the ventrolateral and ventroanterior
nuclei of the thalamus, which in turn receive signals
from the cerebellum and basal ganglia. These tracts
provide signals that are necessary for coordination
among the motor control functions of the motor
cortex, basal ganglia, and cerebellum.
5. Fibers from the intralaminar nuclei of the thalamus.
These ﬁbers control the general level of excitability
of th

rs control the general level of excitability
of the motor cortex in the same way they control
the general level of excitability of most other
regions of the cerebral cortex.
Red Nucleus Serves as an Alternative
Pathway for Transmitting Cortical
Signals to the Spinal Cord
The red nucleus, located in the mesencephalon, func-
tions in close association with the corticospinal tract.
Posterior limb of internal
capsule
Genu of corpus callosum
Longitudinal fascicles
of pons
Basis pedunculi

nal fascicles
of pons
Basis pedunculi of
mesencephalon
Pyramid of medulla
oblongata
Lateral corticospinal tract
Ventral corticospinal tract
Figure 55–4
Pyramidal tract. (Modiﬁed from Ranson SW, Clark SL: Anatomy of
the Nervous System. Philadelphia: WB Saunders, 1959.)
Chapter 55
689
As shown in Figure 55–5, it receives a large number of
direct ﬁbers from the primary motor cortex through
the corticorubral tract, as well as branching ﬁ

gh
the corticorubral tract, as well as branching ﬁbers from
the corticospinal tract as it passes through the mesen-
cephalon. These ﬁbers synapse in the lower portion of
the red nucleus, the magnocellular portion, which con-
tains large neurons similar in size to the Betz cells in
the motor cortex.These large neurons then give rise to
the rubrospinal tract, which crosses to the opposite
side in the lower brain stem and follows a course
immediately adjacent and anterior to the corticospinal
tract

y adjacent and anterior to the corticospinal
tract into the lateral columns of the spinal cord.
The rubrospinal ﬁbers terminate mostly on the
interneurons of the intermediate areas of the cord gray
matter, along with the corticospinal ﬁbers, but some of
the rubrospinal ﬁbers terminate directly on anterior
motor neurons, along with some corticospinal ﬁbers.
The red nucleus also has close connections with the
cerebellum, similar to the connections between the
motor cortex and the cerebellum.
Funct

between the
Function of the Corticorubrospinal System. The magnocel-
lular portion of the red nucleus has a somatographic
representation of all the muscles of the body, as is true
of the motor cortex. Therefore, stimulation of a single
point in this portion of the red nucleus causes con-
traction of either a single muscle or a small group of
muscles. However, the ﬁneness of representation of
the different muscles is far less developed than in the
motor cortex. Th

is far less developed than in the
motor cortex. This is especially true in human beings,
who have relatively small red nuclei.
The corticorubrospinal pathway serves as an acces-
sory route for transmission of relatively discrete
signals from the motor cortex to the spinal cord.When
the corticospinal ﬁbers are destroyed but the corti-
corubrospinal pathway is intact, discrete movements
can still occur, except that the movements for ﬁne
control of the ﬁngers and hands are considerably
impaired. Wr

the ﬁngers and hands are considerably
impaired. Wrist movements are still functional, which
is not the case when the corticorubrospinal pathway is
also blocked.
Therefore, the pathway through the red nucleus to
the spinal cord is associated with the corticospinal
system. Further, the rubrospinal tract lies in the lateral
columns of the spinal cord, along with the corti-
cospinal tract, and terminates on the interneurons and
motor neurons that control the more distal muscles of
the limbs.Therefor

trol the more distal muscles of
the limbs.Therefore, the corticospinal and rubrospinal
tracts together are called the lateral motor system 
of the cord, in contradistinction to a vestibuloreticu-
lospinal system, which lies mainly medially in the cord
and is called the medial motor system of the cord, as
The term extrapyramidal motor system is widely used in
clinical circles to denote all those portions of the brain
and brain stem that con

hose portions of the brain
and brain stem that contribute to motor control but are
not part of the direct corticospinal-pyramidal system.
These include pathways through the basal ganglia, the
reticular formation of the brain stem, the vestibular
nuclei, and often the red nuclei. This is such an all-
inclusive and diverse group of motor control areas that
it is difﬁcult to ascribe speciﬁc neurophysiologic func-
tions to the so-called extrapyramidal system as a whole.
For this reason, the term “ex

l system as a whole.
For this reason, the term “extrapyramidal” is being used
less often both clinically and physiologically.
Excitation of the Spinal Cord Motor
Control Areas by the Primary Motor
Cortex and Red Nucleus
Vertical Columnar Arrangement of the Neurons in the Motor
Cortex. In Chapters 47 and 51, we pointed out that the
cells in the somatosensory cortex and visual cortex are
organized in vertical columns of cells. In like manner,
the cells of the motor cortex are organized in vertical

ells of the motor cortex are organized in vertical
columns a fraction of a millimeter in diameter, with
thousands of neurons in each column.
Each column of cells functions as a unit, usually
stimulating a group of synergistic muscles, but some-
times stimulating just a single muscle. Also, each
column has six distinct layers of cells, as is true
throughout nearly all the cerebral cortex. The pyram-
idal cells that give rise to the corticospinal ﬁbers all lie
in the ﬁfth layer of cells from the c

bers all lie
in the ﬁfth layer of cells from the cortical surface. Con-
versely, the input signals all enter by way of layers 2
through 4. And the sixth layer gives rise mainly to
ﬁbers that communicate with other regions of the cere-
bral cortex itself.
Function of Each Column of Neurons. The neurons of
each column operate as an integrative processing
system, using information from multiple input sources
Motor
cortex
Interpositus
nucleus
Dentate
nucleus
Red nucleus
Reticular formatio

nucleus
Reticular formation
Rubrospinal tract
Corticorubral
tract
Figure 55–5
Corticorubrospinal pathway for motor control, showing also the
relation of this pathway to the cerebellum.
690
Unit XI
to determine the output response from the column. In
addition, each column can function as an amplifying
system to stimulate large numbers of pyramidal ﬁbers
to the same muscle or to synergistic muscles simulta-
neousl

e muscle or to synergistic muscles simulta-
neously. This is important, because stimulation of a
single pyramidal cell can seldom excite a muscle.
Usually, 50 to 100 pyramidal cells need to be excited
simultaneously or in rapid succession to achieve deﬁn-
itive muscle contraction.
Dynamic and Static Signals Transmitted by the Py-
ramidal Neurons. If a strong signal is sent to a muscle
to cause initial rapid contraction, then a much weaker
continuing signal can maintain the contraction for long
p

ing signal can maintain the contraction for long
periods thereafter. This is the usual manner in which
excitation is provided to cause muscle contractions.To
do this, each column of cells excites two populations
of pyramidal cell neurons, one called dynamic neurons
and the other static neurons. The dynamic neurons are
excessively excited for a short period at the beginning
of a contraction, causing the initial rapid development
of force. Then the static neurons ﬁre at a much slower
rate, but the

 static neurons ﬁre at a much slower
rate, but they continue ﬁring at this slow rate to main-
tain the force of contraction as long as the contraction
is required.
The neurons of the red nucleus have similar
dynamic and static characteristics, except that a
greater percentage of dynamic neurons is in the red
nucleus and a greater percentage of static neurons is
in the primary motor cortex.This may be related to the
fact that the red nucleus is closely allied with the cere-
bellum, and the cerebe

osely allied with the cere-
bellum, and the cerebellum plays an important role in
rapid initiation of muscle contraction, as explained in
the next chapter.
Somatosensory Feedback to the Motor 
Cortex Helps Control the Precision of 
When nerve signals from the motor cortex cause a
muscle to contract, somatosensory signals return all
the way from the activated region of the body to the
neurons in the motor cortex that are initiating the
action. Most of these somatosensory signal

ing the
action. Most of these somatosensory signals arise in (1)
the muscle spindles, (2) the tendon organs of the
muscle tendons, or (3) the tactile receptors of the skin
overlying the muscles. These somatic signals often
cause positive feedback enhancement of the muscle
contraction in the following ways: In the case of the
muscle spindles, if the fusimotor muscle ﬁbers in the
spindles contract more than the large skeletal muscle
ﬁbers contract, the central portions of the spindles
become stret

 the central portions of the spindles
become stretched and, therefore, excited. Signals from
these spindles then return rapidly to the pyramidal
cells in the motor cortex to signal them that the large
muscle ﬁbers have not contracted enough.The pyram-
idal cells further excite the muscle, helping its con-
traction to catch up with the contraction of the muscle
spindles. In the case of the tactile receptors, if the
muscle contraction causes compression of the skin
against an object, such as compr

ssion of the skin
against an object, such as compression of the ﬁngers
around an object being grasped, the signals from 
the skin receptors can, if necessary, cause further 
excitation of the muscles and, therefore, increase the
tightness of the hand grasp.
Stimulation of the Spinal Motor Neurons
Figure 55–6 shows a cross section of a spinal cord
segment demonstrating (1) multiple motor and senso-
rimotor control tracts entering the cord segment and
(2) a representative anterior motor neuron in 

and
(2) a representative anterior motor neuron in the
middle of the anterior horn gray matter. The corti-
cospinal tract and the rubrospinal tract lie in the dorsal
portions of the lateral white columns. Their ﬁbers ter-
minate mainly on interneurons in the intermediate
area of the cord gray matter.
In the cervical enlargement of the cord where the
hands and ﬁngers are represented, large numbers of
both corticospinal and rubrospinal ﬁbers also termi-
nate directly on the anterior motor neurons, 

rmi-
nate directly on the anterior motor neurons, thus
allowing a direct route from the brain to activate
muscle contraction.This is in keeping with the fact that
the primary motor cortex has an extremely high
degree of representation for ﬁne control of hand,
ﬁnger, and thumb actions.
Patterns of Movement Elicited by Spinal Cord Centers. From
Chapter 54, recall that the spinal cord can provide
certain speciﬁc reﬂex patterns of movement in
response to sensory nerve stimulation. Many of these
same

e to sensory nerve stimulation. Many of these
same patterns are also important when the cord’s ante-
rior motor neurons are excited by signals from the
brain. For example, the stretch reﬂex is functional at
all times, helping to damp any oscillations of the motor
movements initiated from the brain, and probably also
providing at least part of the motive power required
to cause muscle contractions when the intrafusal ﬁbers
of the muscle spindles contract more than the large
skeletal muscle ﬁbers 

ontract more than the large
skeletal muscle ﬁbers do, thus eliciting reﬂex “servo-
assist” stimulation of the muscle, in addition to the
direct stimulation by the corticospinal ﬁbers.
Sensory neurons
Propriospinal tract
from pyramidal cells
of cortex
Reticulospinal tract
neuron
Tectospinal and
reticulospinal tracts
Vestibulospinal and
Figure 55–6
Convergence of different motor control pathways on 

onvergence of different motor control pathways on the anterior
motor neurons.
Chapter 55
691
Also, when a brain signal excites a muscle, it usually
is not necessary to transmit an inverse signal to relax
the antagonist muscle at the same time; this is achieved
by the reciprocal innervation circuit that is always
present in the cord for coordinating the function of
antagonistic pairs of muscles.
Finally, other cord reﬂex mechanisms, such as 
withd

nally, other cord reﬂex mechanisms, such as 
withdrawal, stepping and walking, scratching, and 
postural mechanisms, can each be activated by
“command” signals from the brain. Thus, simple
command signals from the brain can initiate many
normal motor activities, particularly for such functions
as walking and attaining different postural attitudes of
the body.
Effect of Lesions in the Motor Cortex or in the Corticospinal
Pathway—The “Stroke”
The motor control system can be damaged by the
common a

otor control system can be damaged by the
common abnormality called a “stroke.” This is caused
either by a ruptured blood vessel that hemorrhages into
the brain or by thrombosis of one of the major arteries
supplying the brain. In either case, the result is loss of
blood supply to the cortex or to the corticospinal tract
where it passes through the internal capsule between
the caudate nucleus and the putamen.Also,experiments
have been performed in animals to selectively remove
different parts of

n animals to selectively remove
different parts of the motor cortex.
Removal of the Primary Motor Cortex (Area Pyramidalis).
Removal of a portion of the primary motor cortex—the
area that contains the giant Betz pyramidal cells—
causes varying degrees of paralysis of the represented
muscles. If the sublying caudate nucleus and adjacent
premotor and supplementary motor areas are not
damaged, gross postural and limb “ﬁxation” movements
can still occur, but there is loss of voluntary control of
dis

cur, but there is loss of voluntary control of
discrete movements of the distal segments of the limbs,
especially of the hands and ﬁngers. This does not mean
that the hand and ﬁnger muscles themselves cannot con-
tract; rather, the ability to control the ﬁne movements is
gone. From these observations, one can conclude that
the area pyramidalis is essential for voluntary initiation
of ﬁnely controlled movements, especially of the hands
and ﬁngers.
Muscle Spasticity Caused by Lesions That Damage L

Muscle Spasticity Caused by Lesions That Damage Large Areas
Adjacent to the Motor Cortex. The primary motor cortex
normally exerts a continual tonic stimulatory effect on
the motor neurons of the spinal cord; when this stimu-
latory effect is removed, hypotonia results. Most lesions
of the motor cortex, especially those caused by a stroke,
involve not only the primary motor cortex but also adja-
cent parts of the brain such as the basal ganglia. In these
instances, muscle spasm almost invariabl

 In these
instances, muscle spasm almost invariably occurs in the
afﬂicted muscle areas on the opposite side of the body
(because the motor pathways cross to the opposite
side). This spasm results mainly from damage to acces-
sory pathways from the nonpyramidal portions of the
motor cortex. These pathways normally inhibit the
vestibular and reticular brain stem motor nuclei. When
these nuclei cease their state of inhibition (i.e., are “dis-
inhibited”), they become spontaneously active and
cause

ited”), they become spontaneously active and
cause excessive spastic tone in the involved muscles, as
we discuss more fully later in the chapter. This is the
spasticity that normally accompanies a “stroke” in a
human being.
Role of the Brain Stem in
Controlling Motor Function
The brain stem consists of the medulla, pons, and mes-
encephalon. In one sense, it is an extension of the
spinal cord upward into the cranial cavity, because it
contains motor and sensory nuclei that perform motor
and sens

tor and sensory nuclei that perform motor
and sensory functions for the face and head regions in
the same way that the spinal cord performs these func-
tions from the neck down. But in another sense, the
brain stem is its own master because it provides many
special control functions, such as the following:
1. Control of respiration
2. Control of the cardiovascular system
3. Partial control of gastrointestinal function
4. Control of many stereotyped movements of the
body
5. Control of equilibrium

ed movements of the
body
6. Control of eye movements
Finally, the brain stem serves as a way station for
“command signals” from higher neural centers. In the
following sections,we discuss the role of the brain stem
in controlling whole-body movement and equilibrium.
Especially important for these purposes are the brain
stem’s reticular nuclei and vestibular nuclei.
Roles of the Reticular and 
Vestibular Nuclei
Figure 55–7 shows the l

lar and 
Figure 55–7 shows the locations of the reticular and
vestibular nuclei in the brain stem.
Pontine reticular
nuclei
Medullary reticular
nuclei
Figure 55–7
Locations of the reticular and vestibular nuclei in the brain stem.
692
Unit XI
Excitatory-Inhibitory Antagonism Between
Pontine and Medullary Reticular Nuclei
The reticular nuclei are divided into two major groups:
(1) pontine reticular nu

ed into two major groups:
(1) pontine reticular nuclei, located slightly posteriorly
and laterally in the pons and extending into the mes-
encephalon, and (2) medullary reticular nuclei, which
extend through the entire medulla, lying ventrally and
medially near the midline. These two sets of nuclei
function mainly antagonistically to each other, with
the pontine exciting the antigravity muscles and the
medullary relaxing these same muscles.
Pontine Reticular System. The pontine reticular nuclei

ne Reticular System. The pontine reticular nuclei
transmit excitatory signals downward into the cord
through the pontine reticulospinal tract in the anterior
column of the cord, as shown in Figure 55–8.The ﬁbers
of this pathway terminate on the medial anterior
motor neurons that excite the axial muscles of the
body, which support the body against gravity—that is,
the muscles of the vertebral column and the extensor
muscles of the limbs.
The pontine reticular nuclei have a high degree of
natural 

ne reticular nuclei have a high degree of
natural excitability. In addition, they receive strong
excitatory signals from the vestibular nuclei, as well as
from deep nuclei of the cerebellum. Therefore, when
the pontine reticular excitatory system is unopposed
by the medullary reticular system, it causes powerful
excitation of antigravity muscles throughout the body,
so much so that four-legged animals can be placed in
a standing position, supporting the body against
gravity without any signals f

ing the body against
gravity without any signals from higher levels of the
brain.
Medullary Reticular System. The medullary reticular
nuclei transmit inhibitory signals to the same anti-
gravity anterior motor neurons by way of a different
tract, the medullary reticulospinal tract, located in the
lateral column of the cord, as also shown in Figure
55–8. The medullary reticular nuclei receive strong
input collaterals from (1) the corticospinal tract, (2)
the rubrospinal tract, and (3) other motor

ct, (2)
the rubrospinal tract, and (3) other motor pathways.
These normally activate the medullary reticular
inhibitory system to counterbalance the excitatory
signals from the pontine reticular system, so that under
normal conditions, the body muscles are not abnor-
mally tense.
Yet some signals from higher areas of the brain can
“disinhibit” the medullary system when the brain
wishes to excite the pontine system to cause standing.
At other times, excitation of the medullary reticular
system ca

s, excitation of the medullary reticular
system can inhibit antigravity muscles in certain por-
tions of the body to allow those portions to perform
special motor activities. The excitatory and inhibitory
reticular nuclei constitute a controllable system that is
manipulated by motor signals from the cerebral cortex
and elsewhere to provide necessary background
muscle contractions for standing against gravity and to
inhibit appropriate groups of muscles as needed so
that other functions can be pe

uscles as needed so
that other functions can be performed.
Role of the Vestibular Nuclei to Excite the
Antigravity Muscles
All the vestibular nuclei, shown in Figure 55–7, func-
tion in association with the pontine reticular nuclei to
control the antigravity muscles. The vestibular nuclei
transmit strong excitatory signals to the antigravity
muscles by way of the lateral and medial vestibu-
lospinal tracts in the anterior columns of the spinal
cord, as shown in Figure 55–8. Without this support 

rd, as shown in Figure 55–8. Without this support of
the vestibular nuclei, the pontine reticular system
would lose much of its excitation of the axial anti-
gravity muscles.
The speciﬁc role of the vestibular nuclei, however,
is to selectively control the excitatory signals to the dif-
ferent antigravity muscles to maintain equilibrium in
response to signals from the vestibular apparatus. We
discuss this more fully later in the chapter.
The Decerebrate Animal Develops Spastic Rigidity
When the 

rebrate Animal Develops Spastic Rigidity
When the brain stem of an animal is sectioned below
the midlevel of the mesencephalon, but the pontine and
medullary reticular systems as well as the vestibular
system are left intact, the animal develops a condition
called decerebrate rigidity. This rigidity does not occur
in all muscles of the body but does occur in the anti-
gravity muscles—the muscles of the neck and trunk and
the extensors of the legs.
The cause of decerebrate rigidity is blockage of

.
The cause of decerebrate rigidity is blockage of nor-
mally strong input to the medullary reticular nuclei
from the cerebral cortex, the red nuclei, and the basal
ganglia. Lacking this input, the medullary reticular
inhibitor system becomes nonfunctional; full overactiv-
ity of the pontine excitatory system occurs, and rigidity
develops. We shall see later that other causes of rigidity
occur in other neuromotor diseases, especially lesions of
the basal ganglia.
Vestibular Sensations and
Mainte

he basal ganglia.
Maintenance of Equilibrium
The vestibular apparatus, shown in Figure 55–9, is the
sensory organ for detecting sensations of equilibrium.
It is encased in a system of bony tubes and chambers
Medullary reticulospinal
tract
Lateral vestibulospinal
tract
Pontine reticulospinal tract
Medial vestibulospinal tract
Figure 55–8
Vestibulospinal and reticulospinal tracts descending in the spinal
cord to excite (solid lines) or inhibit (dashed

al
cord to excite (solid lines) or inhibit (dashed lines) the anterior
motor neurons that control the body’s axial musculature.
Chapter 55
693
located in the petrous portion of the temporal bone,
called the bony labyrinth. Within this system are mem-
branous tubes and chambers called the membranous
labyrinth. The membranous labyrinth is the functional
part of the vestibular apparatus.
The top of Figure 55–9 shows the membranous
labyrinth. It is c

igure 55–9 shows the membranous
labyrinth. It is composed mainly of the cochlea (ductus
cochlearis); three semicircular canals; and two large
chambers, the utricle and saccule. The cochlea is the
major sensory organ for hearing (see Chapter 52) and
has little to do with equilibrium. However, the semi-
circular canals, the utricle, and the saccule are all inte-
gral parts of the equilibrium mechanism.
“Maculae”—Sensory Organs of the Utricle and Saccule for
Detecting Orientation of the Head with R

ccule for
Detecting Orientation of the Head with Respect to Gravity.
Located on the inside surface of each utricle and
saccule, shown in the top diagram of Figure 55–9, is a
small sensory area slightly over 2 millimeters in diam-
eter called a macula. The macula of the utricle lies
mainly in the horizontal plane on the inferior surface
of the utricle and plays an important role in deter-
mining orientation of the head when the head is
upright. Conversely, the macula of the saccule is
located mai

nversely, the macula of the saccule is
located mainly in a vertical plane and signals head 
orientation when the person is lying down.
Each macula is covered by a gelatinous layer in
which many small calcium carbonate crystals called
statoconia are embedded.Also in the macula are thou-
sands of hair cells, one of which is shown in Figure
55–10; these project cilia up into the gelatinous layer.
The bases and sides of the hair cells synapse with
sensory endings of the vestibular nerve.
The calciﬁe

nsory endings of the vestibular nerve.
The calciﬁed statoconia have a speciﬁc gravity two
to three times the speciﬁc gravity of the surrounding
ﬂuid and tissues. The weight of the statoconia bends
the cilia in the direction of gravitational pull.
Directional Sensitivity of the Hair Cells—Kinocilium. Each
hair cell has 50 to 70 small cilia called stereocilia, plus
Gelatinous
layer
Hair tufts
Sustentacular cells
Hair cells
mass of
cupula
Hair tufts
Utric

r cells
mass of
cupula
Hair tufts
Utricle
Ampullae
Anterior
Semi-
circular
canals
Maculae and
statoconia
Crista ampullaris
Ductus
cochlearis
Saccule
Posterior
Ductus endolymphaticus
MEMBRANOUS LABYRINTH
Statoconia
CRISTA AMPULLARIS AND MACULA
Nerve
fibers
Hair
cells
Figure 55–9
Membranous labyrinth, and organization of the crista ampullaris
and the macula.
Stereocilia
Kinocilium
Filamentous
attachments
Figure 55–10
Hair cell of the equilibrium apparatus and its synapses wi

l of the equilibrium apparatus and its synapses with the
vestibular nerve.
694
Unit XI
one large cilium, the kinocilium, as shown in Figure
55–10. The kinocilium is always located to one side,
and the stereocilia become progressively shorter
toward the other side of the cell. Minute ﬁlamentous
attachments, almost invisible even to the electron
microscope, connect the tip of each stereocilium to the
next longer stereocilium and, ﬁnally,

ilium to the
next longer stereocilium and, ﬁnally, to the kinocilium.
Because of these attachments,when the stereocilia and
kinocilium bend in the direction of the kinocilium, the
ﬁlamentous attachments tug in sequence on the ster-
eocilia, pulling them outward from the cell body. This
opens several hundred ﬂuid channels in the neuronal
cell membrane around the bases of the stereocilia,
and these channels are capable of conducting large
numbers of positive ions.Therefore, positive ions pour
into

f positive ions.Therefore, positive ions pour
into the cell from the surrounding endolymphatic
ﬂuid, causing receptor membrane depolarization. Con-
versely, bending the pile of stereocilia in the opposite
direction (backward to the kinocilium) reduces the
tension on the attachments; this closes the ion chan-
nels, thus causing receptor hyperpolarization.
Under normal resting conditions, the nerve ﬁbers
leading from the hair cells transmit continuous nerve
impulses at a rate of about 100 per seco

ous nerve
impulses at a rate of about 100 per second. When the
stereocilia are bent toward the kinocilium, the impulse
trafﬁc increases, often to several hundred per second;
conversely, bending the cilia away from the kinocilium
decreases the impulse trafﬁc, often turning it off com-
pletely. Therefore, as the orientation of the head in
space changes and the weight of the statoconia bends
the cilia, appropriate signals are transmitted to the
brain to control equilibrium.
In each macula, each of 

n to control equilibrium.
In each macula, each of the hair cells is oriented in
a different direction so that some of the hair cells are
stimulated when the head bends forward, some are
stimulated when it bends backward, others are stimu-
lated when it bends to one side, and so forth. There-
fore, a different pattern of excitation occurs in the
macular nerve ﬁbers for each orientation of the head
in the gravitational ﬁeld. It is this “pattern” that
apprises the brain of the head’s orientation in

at
apprises the brain of the head’s orientation in space.
Semicircular Ducts. The three semicircular ducts in each
vestibular apparatus, known as the anterior, posterior,
and lateral (horizontal) semicircular ducts,
are
arranged at right angles to one another so that they
represent all three planes in space. When the head is
bent forward about 30 degrees, the lateral semicircu-
lar ducts are approximately horizontal with respect 
to the surface of the earth; the anterior ducts are in
vertical pl

f the earth; the anterior ducts are in
vertical planes that project forward and 45 degrees
outward,
whereas the posterior ducts are in 
vertical planes that project backward and 45 degrees
outward.
Each semicircular duct has an enlargement at one
of its ends called the ampulla, and the ducts and
ampulla are ﬁlled with a ﬂuid called endolymph. Flow
of this ﬂuid through one of the ducts and through its
ampulla excites the sensory organ of the ampulla in
the following manner: Figure 55–11 shows in 

la in
the following manner: Figure 55–11 shows in each
ampulla a small crest called a crista ampullaris. On top
of this crista is a loose gelatinous tissue mass, the
cupula. When a person’s head begins to rotate in any
direction, the inertia of the ﬂuid in one or more of the
semicircular ducts causes the ﬂuid to remain station-
ary while the semicircular duct rotates with the head.
This causes ﬂuid to ﬂow from the duct and through the
ampulla, bending the cupula to one side, as demon-
strated by

nding the cupula to one side, as demon-
strated by the position of the colored cupula in Figure
55–11. Rotation of the head in the opposite direction
causes the cupula to bend to the opposite side.
Into the cupula are projected hundreds of cilia from
hair cells located on the ampullary crest. The kinocilia
of these hair cells are all oriented in the same direc-
tion in the cupula, and bending the cupula in that
direction causes depolarization of the hair cells,
whereas bending it in the opposite

the hair cells,
whereas bending it in the opposite direction hyperpo-
larizes the cells. Then, from the hair cells, appropriate
signals are sent by way of the vestibular nerve to
apprise the central nervous system of a change in rota-
tion of the head and the rate of change in each of the
three planes of space.
Function of the Utricle and Saccule in
the Maintenance of Static Equilibrium
It is especially important that the hair cells are all ori-
ented in different directions in the maculae of th

ented in different directions in the maculae of the utri-
cles and saccules, so that with different positions of the
head, different hair cells become stimulated.The “pat-
terns” of stimulation of the different hair cells apprise
the brain of the position of the head with respect to
the pull of gravity. In turn, the vestibular, cerebellar,
and reticular motor nerve systems of the brain excite
appropriate postural muscles to maintain proper 
equilibrium.
Cupula
Ampulla
Cristae
ampullaris
Hair cel

ibrium.
Cupula
Ampulla
Cristae
Hair cells
Nerve
Figure 55–11
Movement of the cupula and its embedded hairs at the onset of
rotation.
Chapter 55
695
This utricle and saccule system functions extremely
effectively for maintaining equilibrium when the head
is in the near-vertical position. Indeed, a person 
can determine as little as half a degree of dysequilib-
rium when the body leans from the precise upright
position.
Detection of Line

om the precise upright
position.
Detection of Linear Acceleration by the Utricle and Saccule
Maculae. When the body is suddenly thrust forward—
that is, when the body accelerates—the statoconia,
which have greater mass inertia than the surrounding
ﬂuid, fall backward on the hair cell cilia, and informa-
tion of dysequilibrium is sent into the nervous centers,
causing the person to feel as though he or she were
falling backward.This automatically causes the person
to lean forward until the result

causes the person
to lean forward until the resulting anterior shift of the
statoconia exactly equals the tendency for the stato-
conia to fall backward because of the acceleration. At
this point, the nervous system senses a state of proper
equilibrium and leans the body forward no farther.
Thus, the maculae operate to maintain equilibrium
during linear acceleration in exactly the same manner
as they operate during static equilibrium.
The maculae do not operate for the detection of
linear veloci

 do not operate for the detection of
linear velocity. When runners ﬁrst begin to run, they
must lean far forward to keep from falling backward
because of initial acceleration, but once they have
achieved running speed, if they were running in a
vacuum, they would not have to lean forward. When
running in air, they lean forward to maintain equilib-
rium only because of air resistance against their
bodies; in this instance, it is not the maculae that make
them lean but air pressure acting on press

at make
them lean but air pressure acting on pressure end-
organs in the skin, which initiate appropriate equilib-
rium adjustments to prevent falling.
Detection of Head Rotation by the
When the head suddenly begins to rotate in any direc-
tion (called angular acceleration), the endolymph in
the semicircular ducts, because of its inertia, tends to
remain stationary while the semicircular ducts turn.
This causes relative ﬂuid ﬂow in the ducts in the direc-
tion opposite to head

w in the ducts in the direc-
tion opposite to head rotation.
Figure 55–12 shows a typical discharge signal from
a single hair cell in the crista ampullaris when an
animal is rotated for 40 seconds, demonstrating that
(1) even when the cupula is in its resting position, the
hair cell emits a tonic discharge of about 100 impulses
per second; (2) when the animal begins to rotate, the
hairs bend to one side and the rate of discharge
increases greatly; and (3) with continued rotation,
the excess disc

; and (3) with continued rotation,
the excess discharge of the hair cell gradually 
subsides back to the resting level during the next few
seconds.
The reason for this adaptation of the receptor is that
within the ﬁrst few seconds of rotation, back resistance
to the ﬂow of ﬂuid in the semicircular duct and past
the bent cupula causes the endolymph to begin rotat-
ing as rapidly as the semicircular canal itself; then, in
another 5 to 20 seconds, the cupula slowly returns to
its resting position i

he cupula slowly returns to
its resting position in the middle of the ampulla
because of its own elastic recoil.
When the rotation suddenly stops, exactly opposite
effects take place: The endolymph continues to rotate
while the semicircular duct stops.This time, the cupula
bends in the opposite direction, causing the hair cell to
stop discharging entirely. After another few seconds,
the endolymph stops moving and the cupula gradually
returns to its resting position, thus allowing hair cell
disch

ts resting position, thus allowing hair cell
discharge to return to its normal tonic level, as shown
to the right in Figure 55–12. Thus, the semicircular 
duct transmits a signal of one polarity when the head
begins to rotate and of opposite polarity when it stops
rotating.
“Predictive” Function of the Semicircular Duct System in the
Maintenance of Equilibrium. Because the semicircular
ducts do not detect that the body is off balance in the
forward direction, in the side direction, or in the bac

rd direction, in the side direction, or in the back-
ward direction, one might ask: What is the semicircu-
lar ducts’ function in the maintenance of equilibrium?
All they detect is that the person’s head is beginning
or stopping to rotate in one direction or another.
Therefore, the function of the semicircular ducts is not
to maintain static equilibrium or to maintain equilib-
rium during steady directional or rotational move-
ments. Yet loss of function of the semicircular ducts
does cause a pe

function of the semicircular ducts
does cause a person to have poor equilibrium when
attempting to perform rapid, intricate changing body
movements.
We can explain the function of the semicircular
ducts best by the following illustration: If a person is
running forward rapidly and then suddenly begins to
turn to one side, he or she will fall off balance a frac-
tion of a second later unless appropriate corrections
are made ahead of time. But the maculae of the utricle
and saccule cannot detect t

maculae of the utricle
and saccule cannot detect that he or she is off balance
until after this has occurred. The semicircular ducts,
0
10
20
30
40
50
60
70
80
90
0
100
200
300
400
Rotation
Stop rotation
Begin rotation
Tonic
level of
discharge
Seconds
Seconds
Figure 55–12
Response of a hair cell when a semicircular canal is stimulated
ﬁrst by the onset of head rotation and then by stopping rotation.
696
Unit XI
howe

tem: C. Motor and Integrative Neurophysiology
however, will have already detected that the person is
turning, and this information can easily apprise the
central nervous system of the fact that the person will
fall off balance within the next fraction of a second or
so unless some anticipatory correction is made.
In other words, the semicircular duct mechanism
predicts that dysequilibrium is going to occur and
thereby causes the equilibrium centers to make appro-
priate anticipatory preventive a

rs to make appro-
priate anticipatory preventive adjustments. In this way,
the person need not fall off balance at all before he or
she begins to correct the situation.
Removal of the ﬂocculonodular lobes of the cere-
bellum prevents normal detection of semicircular duct
signals but has less effect on detecting macular signals.
It is especially interesting that the cerebellum serves
as a “predictive” organ for most rapid movements of
the body, as well as for those having to do with equi-
librium

 well as for those having to do with equi-
librium. These other functions of the cerebellum are
discussed in the following chapter.
Vestibular Mechanisms for
Stabilizing the Eyes
When a person changes his or her direction of move-
ment rapidly or even leans the head sideways, forward,
or backward, it would be impossible to maintain a stable
image on the retinas unless the person had some auto-
matic control mechanism to stabilize the direction of the
eyes’ gaze. In addition, the eyes would be of

 the
eyes’ gaze. In addition, the eyes would be of little use in
detecting an image unless they remained “ﬁxed” on
each object long enough to gain a clear image. Fortu-
nately, each time the head is suddenly rotated, signals
from the semicircular ducts cause the eyes to rotate in
a direction equal and opposite to the rotation of the
head.This results from reﬂexes transmitted through the
vestibular nuclei and the medial longitudinal fasciculus
to the oculomotor nuclei. These reﬂexes are described

the oculomotor nuclei. These reﬂexes are described
in Chapter 51.
Other Factors Concerned 
with Equilibrium
Neck Proprioceptors. The vestibular apparatus detects the
orientation and movement only of the head. Therefore,
it is essential that the nervous centers also receive
appropriate information about the orientation of the
head with respect to the body.This information is trans-
mitted from the proprioceptors of the neck and body
directly to the vestibular and reticular nuclei in the
brain ste

e vestibular and reticular nuclei in the
brain stem and indirectly by way of the cerebellum.
Among the most important proprioceptive informa-
tion needed for the maintenance of equilibrium is that
transmitted by joint receptors of the neck. When the
head is leaned in one direction by bending the neck,
impulses from the neck proprioceptors keep the signals
originating in the vestibular apparatus from giving the
person a sense of dysequilibrium.They do this by trans-
mitting signals that exactly o

y do this by trans-
mitting signals that exactly oppose the signals transmit-
ted from the vestibular apparatus. However, when the
entire body leans in one direction, the impulses from the
vestibular apparatus are not opposed by signals from 
the neck proprioceptors; therefore, in this case, the
person does perceive a change in equilibrium status of
the entire body.
Proprioceptive and Exteroceptive Information from Other Parts 
of the Body. Proprioceptive information from parts of 
the body othe

ioceptive information from parts of 
the body other than the neck is also important in the
maintenance of equilibrium. For instance, pressure 
sensations from the footpads tell one (1) whether
weight is distributed equally between the two feet and
(2) whether weight on the feet is more forward or 
backward.
Exteroceptive information is especially necessary for
the maintenance of equilibrium when a person is
running. The air pressure against the front of the body
signals that a force is opposing 

ront of the body
signals that a force is opposing the body in a direction
different from that caused by gravitational pull; as a
result, the person leans forward to oppose this.
Importance of Visual Information in the Maintenance of Equilib-
rium. After destruction of the vestibular apparatus, and
even after loss of most proprioceptive information from
the body, a person can still use the visual mechanisms
reasonably effectively for maintaining equilibrium.
Even a slight linear or rotational mov

quilibrium.
Even a slight linear or rotational movement of the body
instantaneously shifts the visual images on the retina,
and this information is relayed to the equilibrium
centers. Some people with bilateral destruction of the
vestibular apparatus have almost normal equilibrium as
long as their eyes are open and all motions are per-
formed slowly. But when moving rapidly or when the
eyes are closed, equilibrium is immediately lost.
Neuronal Connections of the Vestibular Apparatus with the
Cen

nnections of the Vestibular Apparatus with the
Figure 55–13 shows the connections in the hindbrain of
the vestibular nerve. Most of the vestibular nerve ﬁbers
terminate in the brain stem in the vestibular nuclei,
which are located approximately at the junction of the
medulla and the pons. Some ﬁbers pass directly to the
brain stem reticular nuclei without synapsing and also
to the cerebellar fastigial, uvular, and ﬂocculonodular
lobe nuclei. The ﬁbers that end in the brain

dular
lobe nuclei. The ﬁbers that end in the brain stem
vestibular nuclei synapse with second-order neurons
that also send ﬁbers into the cerebellum, the vestibu-
lospinal tracts, the medial longitudinal fasciculus, and
Recticulospinal
tract
Vestibulospinal tract
Fastigioreticular
tract
Reticular
substance
Red
nucleus
Fastigial
nucleus
Dentate nucleus
Vestibular nucleus
Flocculonodular
lobe
Vestibular nerve
Figure 55–13
Connections of vestibular n

lar nerve
Connections of vestibular nerves through the vestibular nuclei (the
large oval white area) with other areas of the central nervous
system.
Chapter 55
697
other areas of the brain stem, particularly the reticular
nuclei.
The primary pathway for the equilibrium reﬂexes
begins in the vestibular nerves, where the nerves are
excited by the vestibular apparatus. The pathway then
passes to the vestibular nuclei and cerebellum. Nex

asses to the vestibular nuclei and cerebellum. Next,
signals are sent into the reticular nuclei of the brain
stem, as well as down the spinal cord by way of the
vestibulospinal and reticulospinal tracts. The signals to
the cord control the interplay between facilitation and
inhibition of the many antigravity muscles, thus auto-
matically controlling equilibrium.
The ﬂocculonodular lobes of the cerebellum are espe-
cially concerned with dynamic equilibrium signals from
the semicircular ducts. In 

uilibrium signals from
the semicircular ducts. In fact, destruction of these lobes
results in almost exactly the same clinical symptoms as
destruction of the semicircular ducts themselves.That is,
severe injury to either the lobes or the ducts causes loss
of dynamic equilibrium during rapid changes in direc-
tion of motion but does not seriously disturb equilib-
rium under static conditions. It is believed that the uvula
of the cerebellum plays a similar important role in static
Sig

 similar important role in static
Signals transmitted upward in the brain stem from
both the vestibular nuclei and the cerebellum by way of
the medial longitudinal fasciculus cause corrective
movements of the eyes every time the head rotates, so
that the eyes remain ﬁxed on a speciﬁc visual object.
Signals also pass upward (either through this same tract
or through reticular tracts) to the cerebral cortex, ter-
minating in a primary cortical center for equilibrium
located in the par

cortical center for equilibrium
located in the parietal lobe deep in the sylvian ﬁssure
on the opposite side of the ﬁssure from the auditory
area of the superior temporal gyrus. These signals
apprise the psyche of the equilibrium status of the body.
Functions of Brain Stem
Nuclei in Controlling
Subconscious, Stereotyped
Movements
Rarely, a baby is born without brain structures above
the mesencephalic region, a condition called anen-
cephaly. Some of these babies have been kept alive for
many mon

 of these babies have been kept alive for
many months. They are able to perform some stereo-
typed movements for feeding, such as suckling, extru-
sion of unpleasant food from the mouth, and moving the
hands to the mouth to suck the ﬁngers. In addition, they
can yawn and stretch. They can cry and can follow
objects with movements of the eyes and head. Also,
placing pressure on the upper anterior parts of their legs
causes them to pull to the sitting position. It is clear that
many of the stereot

ing position. It is clear that
many of the stereotyped motor functions of the human
being are integrated in the brain stem.
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processing. Curr Opin Neurobiol 13:440, 2003.
Angelaki DE, Dickman JD: Gravity or translation: central
processing of vestibular signals to detect motion or tilt. J
Vestib Res 13:245, 2003.
Barmack NH: Central vestibular system: vestibular nuclei
and posterior cerebellum. Brain Res Bull 60:511, 2003.
Blake 

or cerebellum. Brain Res Bull 60:511, 2003.
Blake DT, Byl NN, Merzenich MM: Representation of the
hand in the cerebral cortex. Behav Brain Res 135:179,
2002.
Boyle R: Vestibulospinal control of reﬂex and voluntary
head movement. Ann N Y Acad Sci 942:364, 2001.
Cullen KE, Roy JE: Signal processing in the vestibular
system during active versus passive head movements. J
Neurophysiol 91:1919, 2004.
cerebellar modules—relevance for motor coordi

s to
Johansen-Berg H: Motor physiology: a brain of two halves.
Curr Biol 13:R802, 2003.
Raineteau O, Schwab ME: Plasticity of motor systems after
incomplete spinal cord injury. Nat Rev Neurosci 2:263,
2001.
Raphael Y, Altschuler RA: Structure and innervation of the
cochlea. Brain Res Bull 60:397, 2003.
Salenius S, Hari R: 

a. Physiol Rev 81:1305, 2001.
Salenius S, Hari R: Synchronous cortical oscillatory activity
during motor action. Curr Opin Neurobiol 13:678, 2003.
Sanes JN: Neocortical mechanisms in motor learning. Curr
Opin Neurobiol 13:225, 2003.
Schieber MH: Motor control: basic units of cortical output?
Curr Biol 14:R353, 2004.
Scott SH: The role of primary motor cortex in goal-directed
movements: insights from neurophysiological studies on
non-human primates. Curr Opin Neurobiol 13:671, 2003.
Umilta MA: Fr

s. Curr Opin Neurobiol 13:671, 2003.
Umilta MA: Frontal cortex: goal-relatedness and the corti-
cal motor system. Curr Biol 14:R204, 2004.
Yates BJ, Miller AD, Lucot JB: Physiological basis and phar-
macology of motion sickness: an update. Brain Res Bull
47:395, 1998.
C
H
A
P
T
E
R
 
5
6
698
Contributions of the Cerebellum
and Basal Ganglia to Overall
Aside from the areas in the cerebral cortex that
stimulate muscle contraction, two other brain struc-
tures are also essential for n

 other brain struc-
tures are also essential for normal motor function.
They are the cerebellum and the basal ganglia. Yet
neither of these two can control muscle function by
themselves. Instead, they always function in associa-
tion with other systems of motor control.
Basically, the cerebellum plays major roles in the
timing of motor activities and in rapid, smooth progression from one muscle
movement to the next. It also helps to control intensity of muscle contraction
when the muscle load ch

sity of muscle contraction
when the muscle load changes, as well as controlling necessary instantaneous
interplay between agonist and antagonist muscle groups.
The basal ganglia help to plan and control complex patterns of muscle move-
ment, controlling relative intensities of the separate movements, directions of
movements, and sequencing of multiple successive and parallel movements for
achieving speciﬁc complicated motor goals. This chapter explains the basic
mechanisms of function of the cer

plains the basic
mechanisms of function of the cerebellum and basal ganglia and discusses the
overall brain mechanisms for achieving intricate coordination of total motor
activity.
The cerebellum, illustrated in Figures 56–1 and 56–2, has long been called a
silent area of the brain, principally because electrical excitation of the cerebel-
lum does not cause any conscious sensation and rarely causes any motor move-
ment. Removal of the cerebellum, however, does

ve-
ment. Removal of the cerebellum, however, does cause body movements to
become highly abnormal. The cerebellum is especially vital during rapid mus-
cular activities such as running, typing, playing the piano, and even talking. Loss
of this area of the brain can cause almost total incoordination of these activi-
ties even though its loss causes paralysis of no muscles.
But how is it that the cerebellum can be so important when it has no direct
ability to cause muscle contraction? The answer i

ability to cause muscle contraction? The answer is that it helps to sequence 
the motor activities and also monitors and makes corrective adjustments in the
body’s motor activities while they are being executed so that they will conform to
the motor signals directed by the cerebral motor cortex and other parts of the
brain.
The cerebellum receives continuously updated information about the desired
sequence of muscle contractions from the brain motor control areas; it also
receives continuous se

otor control areas; it also
receives continuous sensory information from the peripheral parts of the body,
giving sequential changes in the status of each part of the body—its position,
rate of movement, forces acting on it, and so forth. The cerebellum then com-
pares the actual movements as depicted by the peripheral sensory feedback
information with the movements intended by the motor system. If the two do
not compare favorably, then instantaneous subconscious corrective signals are
transmitt

eous subconscious corrective signals are
transmitted back into the motor system to increase or decrease the levels of
activation of speciﬁc muscles.
The cerebellum also aids the cerebral cortex in planning the next sequential
movement a fraction of a second in advance while the current movement is still
Chapter 56
Contributions of the Cerebellum and Basal Ganglia to Overall Motor Control
699
being executed, thus helping the person to progress
smoothly from one movement to the next. Also, it
lear

othly from one movement to the next. Also, it
learns by its mistakes—that is, if a movement does not
occur exactly as intended, the cerebellar circuit learns
to make a stronger or weaker movement the next time.
To do this, changes occur in the excitability of appro-
priate cerebellar neurons, thus bringing subsequent
muscle contractions into better correspondence with the
intended movements.
Anatomical Functional Areas 
of the Cerebellum
Anatomically, the cerebellum is divided into three lobes
b

ally, the cerebellum is divided into three lobes
by two deep ﬁssures, as shown in Figures 56–1 and 56–2:
(1) the anterior lobe, (2) the posterior lobe, and (3) the
ﬂocculonodular lobe. The ﬂocculonodular lobe is the
oldest of all portions of the cerebellum; it developed
along with (and functions with) the vestibular system in
controlling body equilibrium, as discussed in Chapter
55.
Longitudinal Functional Divisions of the Anterior and Posterior
Lobes. From a functional point of view, the anteri

Lobes. From a functional point of view, the anterior and
posterior lobes are organized not by lobes but along the
longitudinal axis, as demonstrated in Figure 56–2, which
shows a posterior view of the human cerebellum after
the lower end of the posterior cerebellum has been
rolled downward from its normally hidden position.
Note down the center of the cerebellum a narrow band
called the vermis, separated from the remainder of the
cerebellum by shallow grooves. In this area, most cere-
bellar con

allow grooves. In this area, most cere-
bellar control functions for muscle movements of the
axial body, neck, shoulders, and hips are located.
To each side of the vermis is a large, laterally pro-
truding cerebellar hemisphere, and each of these hemi-
spheres is divided into an intermediate zone and a lateral
zone.
The intermediate zone of the hemisphere is con-
cerned with controlling muscle contractions in the distal
portions of the upper and lower limbs, especially the
hands and ﬁngers and f

lower limbs, especially the
hands and ﬁngers and feet and toes.
The lateral zone of the hemisphere operates at a
much more remote level because this area joins with the
cerebral cortex in the overall planning of sequential
motor movements. Without this lateral zone, most dis-
crete motor activities of the body lose their appropriate
timing and sequencing and therefore become incoordi-
nate, as we discuss more fully later.
Topographical Representation of the Body in the Vermis and Inter-
mediate 

tion of the Body in the Vermis and Inter-
mediate Zones. In the same manner that the cerebral
sensory cortex, motor cortex, basal ganglia, red nuclei,
and reticular formation all have topographical repre-
sentations of the different parts of the body, so also is
this true for the vermis and intermediate zones of the
cerebellum. Figure 56–3 shows two such representa-
tions. Note that the axial portions of the body lie in the
vermis part of the cerebellum, whereas the limbs and
facial regions lie 

ebellum, whereas the limbs and
facial regions lie in the intermediate zones. These topo-
graphical representations receive afferent nerve signals
from all the respective parts of the body as well as from
corresponding topographical motor areas in the cere-
bral cortex and brain stem. In turn, they send motor
Pons
Anterior lobe
Posterior lobe
Medulla
lobe
Figure 56–1
Anatomical lobes of the cerebellum as seen from the lateral side.
Hemisphere
Vermis
Vermis
Intermediate zone
of hem

Vermis
Vermis
of hemisphere
Anterior
lobe
Posterior
lobe
Lateral zone
lobe
Figure 56–2
Functional parts of the cerebellum as seen from the posteroinfe-
rior view, with the inferiormost portion of the cerebellum rolled
outward to ﬂatten the surface.
Figure 56–3
Somatosensory projection areas in the cerebellar cortex.
700
Unit XI
signals back to the same respective topographical

signals back to the same respective topographical areas
of the cerebral motor cortex, as well as to topographi-
cal areas of the red nucleus and reticular formation in
the brain stem.
Note that the large lateral portions of the cerebellar
hemispheres do not have topographical representations
of the body. These areas of the cerebellum receive their
input signals almost exclusively from the cerebral
cortex, especially from the premotor areas of the frontal
cortex and from the somatosensory and ot

e frontal
cortex and from the somatosensory and other sensory
association areas of the parietal cortex. It is believed
that this connectivity with the cerebral cortex allows the
lateral portions of the cerebellar hemispheres to play
important roles in planning and coordinating the body’s
rapid sequential muscular activities that occur one after
another within fractions of a second.
The human cerebellar cortex is actually a large folded
sheet, about 17 centimete

 actually a large folded
sheet, about 17 centimeters wide by 120 centimeters
long, with the folds lying crosswise, as shown in Figures
56–2 and 56–3. Each fold is called a folium. Lying deep
beneath the folded mass of cerebellar cortex are deep
cerebellar nuclei.
Input Pathways to the Cerebellum
Afferent Pathways from Other Parts of the Brain. The basic
input pathways to the cerebellum are shown in Figure
56–4. An extensive and important afferent pathway is
the corticopontocerebellar pathway, wh

 pathway is
the corticopontocerebellar pathway, which originates in
the cerebral motor and premotor cortices and also in the
cerebral somatosensory cortex. It passes by way of the
pontile nuclei and pontocerebellar tracts mainly to 
the lateral divisions of the cerebellar hemispheres on
the opposite side of the brain from the cerebral areas.
In addition, important afferent tracts originate in
each side of the brain stem; they include (1) an exten-
sive olivocerebellar tract, which passes from th

-
sive olivocerebellar tract, which passes from the inferior
olive to all parts of the cerebellum and is excited in the
olive by ﬁbers from the cerebral motor cortex, basal
ganglia, widespread areas of the reticular formation, and
spinal cord; (2) vestibulocerebellar ﬁbers, some of which
originate in the vestibular apparatus itself and others
from the brain stem vestibular nuclei—almost all of
these terminate in the ﬂocculonodular lobe and fastigial
nucleus of the cerebellum; and (3) reticulocer

ial
nucleus of the cerebellum; and (3) reticulocerebellar
ﬁbers, which originate in different portions of the brain
stem reticular formation and terminate in the midline
cerebellar areas (mainly in the vermis).
Afferent Pathways from the Periphery. The cerebellum also
receives important sensory signals directly from the
peripheral parts of the body mainly through four tracts
on each side, two of which are located dorsally in the
cord and two ventrally.The two most important of these
tracts are s

rally.The two most important of these
tracts are shown in Figure 56–5: the dorsal spinocere-
bellar tract and the ventral spinocerebellar tract. The
dorsal tract enters the cerebellum through the inferior
cerebellar peduncle and terminates in the vermis and
intermediate zones of the cerebellum on the same side
as its origin. The ventral tract enters the cerebellum
through the superior cerebellar peduncle, but it termi-
nates in both sides of the cerebellum.
The signals transmitted in the dorsal 

The signals transmitted in the dorsal spinocerebellar
tracts come mainly from the muscle spindles and to a
lesser extent from other somatic receptors throughout
the body, such as Golgi tendon organs, large tactile
receptors of the skin, and joint receptors. All these
signals apprise the cerebellum of the momentary status
of (1) muscle contraction, (2) degree of tension on the
muscle tendons, (3) positions and rates of movement 
of the parts of the body, and (4) forces acting on the 

 parts of the body, and (4) forces acting on the 
surfaces of the body.
Conversely, the ventral spinocerebellar tracts receive
less information from the peripheral receptors. Instead,
they are excited mainly by motor signals arriving in the
anterior horns of the spinal cord from (1) the brain
through the corticospinal and rubrospinal tracts and (2)
the internal motor pattern generators in the cord itself.
Thus, this ventral ﬁber pathway tells the cerebellum
Superior cerebellar
peduncle
Ventral
s

 cerebellum
peduncle
Ventral
tract
Cerebropontile
tract
Pontocerebellar
tract
Middle cerebellar
peduncle
Vestibulocerebellar tract
Olivocerebellar and
reticulocerebellar tract
Inferior cerebellar peduncle
Ventral spinocerebellar tract
Dorsal spinocerebellar tract
lobe
Anterior
lobe
Posterior
lobe
Figure 56–4
Principal afferent tracts to the cerebellum.
Superior cerebellar peduncle
Clark’s cells
Ventral s

rebellar tract
Dorsal external arcuate fiber
Figure 56–5
Spinocerebellar tracts.
Chapter 56
701
which motor signals have arrived at the anterior horns;
this feedback is called the efference copy of the anterior
horn motor drive.
The spinocerebellar pathways can transmit impulses
a

e spinocerebellar pathways can transmit impulses
at velocities up to 120 m/sec, which is the most rapid
conduction in any pathway in the central nervous
system. This extremely rapid conduction is important
for instantaneous apprisal of the cerebellum of changes
in peripheral muscle actions.
In addition to signals from the spinocerebellar tracts,
signals are transmitted into the cerebellum from the
body periphery through the spinal dorsal columns to the
dorsal column nuclei of the medulla and the

to the
dorsal column nuclei of the medulla and then relayed
to the cerebellum. Likewise, signals are transmitted up
the spinal cord through the spinoreticular pathway to
the reticular formation of the brain stem and also
through the spino-olivary pathway to the inferior
olivary nucleus. Then signals are relayed from both of
these areas to the cerebellum.Thus, the cerebellum con-
tinually collects information about the movements and
positions of all parts of the body even though it is oper-
ating

ll parts of the body even though it is oper-
ating at a subconscious level.
Output Signals from the Cerebellum
Deep Cerebellar Nuclei and the Efferent Pathways. Located
deep in the cerebellar mass on each side are three deep
cerebellar nuclei—the dentate, interposed, and fastigial.
(The vestibular nuclei in the medulla also function in
some respects as if they were deep cerebellar nuclei
because of their direct connections with the cortex of
the ﬂocculonodular lobe.) All the deep cerebellar nucl

ﬂocculonodular lobe.) All the deep cerebellar nuclei
receive signals from two sources: (1) the cerebellar
cortex and (2) the deep sensory afferent tracts to the
Each time an input signal arrives in the cerebellum,
it divides and goes in two directions: (1) directly to one
of the cerebellar deep nuclei and (2) to a corresponding
area of the cerebellar cortex overlying the deep nucleus.
Then, a fraction of a second later, the cerebellar cortex
relays an inhibitory output signal to the 

 cortex
relays an inhibitory output signal to the deep nucleus.
Thus, all input signals that enter the cerebellum even-
tually end in the deep nuclei in the form of initial exci-
tatory signals followed a fraction of a second later by
inhibitory signals. From the deep nuclei, output signals
leave the cerebellum and are distributed to other parts
The general plan of the major efferent pathways
leading out of the cerebellum is shown in Figure 56–6
and consists of the following:
1. A 

n Figure 56–6
1. A pathway that originates in the midline structures
of the cerebellum (the vermis) and then passes
through the fastigial nuclei into the medullary and
pontile regions of the brain stem. This circuit
functions in close association with the equilibrium
apparatus and brain stem vestibular nuclei to
control equilibrium, and also in association with the
reticular formation of the brain stem to control the
postural attitudes of the body. It was discussed

e
postural attitudes of the body. It was discussed in
detail in Chapter 55 in relation to equilibrium.
2. A pathway that originates in (1) the intermediate
zone of the cerebellar hemisphere and then passes
through (2) the interposed nucleus to (3) the
ventrolateral and ventroanterior nuclei of the
thalamus and then to (4) the cerebral cortex, to (5)
several midline structures of the thalamus and then
to (6) the basal ganglia and (7) the red nucleus and
reticular formation of the upper portion of

us and
reticular formation of the upper portion of the
brain stem. This complex circuit helps to coordinate
mainly the reciprocal contractions of agonist and
antagonist muscles in the peripheral portions of the
limbs, especially in the hands, ﬁngers, and thumbs.
3. A pathway that begins in the cerebellar cortex of
the lateral zone of the cerebellar hemisphere and
then passes to the dentate nucleus, next to the
thalamus, and, ﬁnally, to the cerebral 

lei of the
thalamus, and, ﬁnally, to the cerebral cortex. This
pathway plays an important role in helping
coordinate sequential motor activities initiated by
the cerebral cortex.
Functional Unit of the Cerebellar Cortex—
The Purkinje Cell and the Deep Nuclear Cell
The cerebellum has about 30 million nearly identical
functional units, one of which is shown to the left in
Figure 56–7. This functional unit centers on a single,
very large Purkinje cell (30 million of which are in 
the cerebellar cor

ll (30 million of which are in 
the cerebellar cortex) and on a corresponding deep
nuclear cell.
To the top and right in Figure 56–7, the three major
layers of the cerebellar cortex are shown: the molecu-
lar layer, Purkinje cell layer, and granule cell layer.
Beneath these cortical layers, in the center of the cere-
bellar mass, are the deep cerebellar nuclei that send
output signals to other parts of the nervous system.
Neuronal Circuit of the Functional Unit. Also shown in the
left half of Fi

Functional Unit. Also shown in the
left half of Figure 56–7 is the neuronal circuit of the
functional unit, which is repeated with little variation
30 million times in the cerebellum. The output from
the functional unit is from a deep nuclear cell.This cell
is continually under both excitatory and inhibitory
inﬂuences. The excitatory inﬂuences arise from direct
connections with afferent ﬁbers that enter the cere-
bellum from the brain or the periphery.The inhibitory
inﬂuence arises entirely from

phery.The inhibitory
inﬂuence arises entirely from the Purkinje cell in the
cortex of the cerebellum.
The afferent inputs to the cerebellum are mainly of
two types, one called the climbing ﬁber type and the
other called the mossy ﬁber type.
The climbing ﬁbers all originate from the inferior
olives of the medulla. There is one climbing ﬁber for
Fastigioreticular tract
Cerebellothalamocortical
tract
Dentate
peduncle
Reticulum of
To thalamus
Fastigioret

Fastigial nucleus
Paleocerebellum
Figure 56–6
Principal efferent tracts from the cerebellum.
702
Unit XI
about 5 to 10 Purkinje cells.After sending branches to
several deep nuclear cells, the climbing ﬁber continues
all the way to the outer layers of the cerebellar cortex,
where it makes about 300 synapses with the soma 
and dendrites of each Purkinje cell. This climbing 
ﬁ

 dendrites of each Purkinje cell. This climbing 
ﬁber is distinguished by the fact that a single impulse
in it will always cause a single, prolonged (up to 1
second), peculiar type of action potential in each 
Purkinje cell with which it connects, beginning with 
a strong spike and followed by a trail of weakening
secondary spikes. This action potential is called the
complex spike.
The mossy ﬁbers are all the other ﬁbers that enter
the cerebellum from multiple sources: from the higher
brain, bra

 from multiple sources: from the higher
brain, brain stem, and spinal cord. These ﬁbers also
send collaterals to excite the deep nuclear cells. Then
they proceed to the granule cell layer of the cortex,
where they too synapse with hundreds to thousands of
granule cells. In turn, the granule cells send very, very
small axons, less than 1 micrometer in diameter, up to
the molecular layer on the outer surface of the cere-
bellar cortex. Here the axons divide into two branches
that extend 1 to 2 mil

ns divide into two branches
that extend 1 to 2 millimeters in each direction paral-
lel to the folia. There are many millions of these par-
allel nerve ﬁbers because there are some 500 to 1000
granule cells for every 1 Purkinje cell. It is into this
molecular layer that the dendrites of the Purkinje cells
project and 80,000 to 200,000 of the parallel ﬁbers
synapse with each Purkinje cell.
The mossy ﬁber input to the Purkinje cell is quite
different from the climbing ﬁber input because their
syna

nt from the climbing ﬁber input because their
synaptic connections are weak, so that large numbers
of mossy ﬁbers must be stimulated simultaneously to
excite the Purkinje cell. Furthermore, activation
usually takes the form of a much weaker short-
duration Purkinje cell action potential called a simple
spike, rather than the prolonged complex action
potential caused by climbing ﬁber input.
Purkinje Cells and Deep Nuclear Cells Fire Continuously Under
Normal Resting Conditions. One characteristic

nder
Normal Resting Conditions. One characteristic of both
Purkinje cells and deep nuclear cells is that normally
both of them ﬁre continuously; the Purkinje cell ﬁres
at about 50 to 100 action potentials per second, and
the deep nuclear cells at much higher rates. Further-
more, the output activity of both these cells can be
modulated upward or downward.
Balance Between Excitation and Inhibition at the Deep Cere-
bellar Nuclei. Referring again to the circuit of Figure
56–7, one should note that

o the circuit of Figure
56–7, one should note that direct stimulation of the
deep nuclear cells by both the climbing and the mossy
ﬁbers excites them. By contrast, signals arriving from
the Purkinje cells inhibit them. Normally, the balance
between these two effects is slightly in favor of exci-
tation, so that, under quiet conditions, output from the
deep nuclear cell remains relatively constant at a mod-
erate level of continuous stimulation.
In execution of a rapid motor movement, the 
initia

n execution of a rapid motor movement, the 
initiating signal from the cerebral motor cortex or
brain stem at ﬁrst greatly increases deep nuclear 
cell excitation. Then, another few milliseconds later,
feedback inhibitory signals from the Purkinje cell
circuit arrive. In this way, there is ﬁrst a rapid excita-
tory signal sent by the deep nuclear cells into the
motor output pathway to enhance the motor move-
ment, but this is followed within another small fraction
of a second by an inhibitory si

her small fraction
of a second by an inhibitory signal. This inhibitory
signal resembles a “delay-line” negative feedback
signal of the type that is effective in providing
damping. That is, when the motor system is excited,
a negative feedback signal occurs after a short delay
to stop the muscle movement from overshooting its
mark. Otherwise, oscillation of the movement would
occur.
Other Inhibitory Cells in the Cerebellum. In addition to the
deep nuclear cells, granule cells, and Purkinje cells

p nuclear cells, granule cells, and Purkinje cells, two
other types of neurons are located in the cerebellum:
basket cells and stellate cells. These are inhibitory cells
with short axons. Both the basket cells and the stellate
cells are located in the molecular layer of the cerebel-
lar cortex, lying among and stimulated by the small
parallel ﬁbers. These cells in turn send their axons at
right angles across the parallel ﬁbers and cause lateral
inhibition of adjacent Purkinje cells, thus sharpen

nhibition of adjacent Purkinje cells, thus sharpening
the signal in the same manner that lateral inhibition
sharpens contrast of signals in many other neuronal
circuits of the nervous system.
Turn-On/Turn-Off and Turn-Off/Turn-On Output
Signals from the Cerebellum 
The typical function of the cerebellum is to help
provide rapid turn-on signals for the agonist muscles
and simultaneous reciprocal turn-off signals for the
antagonist muscles at the onset of a movement. Then
on approaching terminatio

nset of a movement. Then
on approaching termination of the movement, the
cerebellum is mainly responsible for timing and exe-
cuting the turn-off signals to the agonists and turn-on
signals to the antagonists. Although the exact details
are not fully known, one can speculate from the basic
Purkinje
cell
Climbing
fiber
Deep
nuclear
cell
Input
(inferior olive)
Output
Granule
cells
Cortex
Input
(all other afferents)
Mossy
fiber
Molecular
layer
Purkinje
cell layer
Granule
cell 

Molecular
layer
Purkinje
cell layer
Granule
cell layer
Deep
nuclei
Figure 56–7
The left side of this ﬁgure shows the basic neuronal circuit of the
cerebellum, with excitatory neurons shown in red and the Purkinje
cell (an inhibitory neuron) shown in black. To the right is shown
the physical relationship of the deep cerebellar nuclei to the cere-
bellar cortex with its three layers.
Chapter 56
703
cerebellar circuit of Fi

Overall Motor Control
703
cerebellar circuit of Figure 56–7 how this might work,
Let us suppose that the turn-on/turn-off pattern of
agonist/antagonist contraction at the onset of move-
ment begins with signals from the cerebral cortex.
These signals pass through noncerebellar brain stem
and cord pathways directly to the agonist muscle to
begin the initial contraction.
At the same time, parallel signals are sent by way of
the pontile mossy ﬁbers into the cerebellum. One
branch of eac

mossy ﬁbers into the cerebellum. One
branch of each mossy ﬁber goes directly to deep
nuclear cells in the dentate or other deep cerebellar
nuclei; this instantly sends an excitatory signal back
into the cerebral corticospinal motor system, either by
way of return signals through the thalamus to the cere-
bral cortex or by way of neuronal circuitry in the brain
stem, to support the muscle contraction signal that had
already been begun by the cerebral cortex. As a con-
sequence, the turn-on signal

ral cortex. As a con-
sequence, the turn-on signal, after a few milliseconds,
becomes even more powerful than it was at the start
because it becomes the sum of both the cortical 
and the cerebellar signals. This is the normal effect
when the cerebellum is intact, but in the absence of the
cerebellum, the secondary extra supportive signal is
missing. This cerebellar support makes the turn-on
muscle contraction much stronger than it would be if
the cerebellum did not exist.
Now, what causes the tu

 cerebellum did not exist.
Now, what causes the turn-off signal for the agonist
muscles at the termination of the movement? Remem-
ber that all mossy ﬁbers have a second branch that
transmits signals by way of the granule cells to the
cerebellar cortex and eventually, by way of “parallel”
ﬁbers, to the Purkinje cells. The Purkinje cells in turn
inhibit the deep nuclear cells. This pathway passes
through some of the smallest, slowest-conducting
nerve ﬁbers in the nervous system: that is, the para

rve ﬁbers in the nervous system: that is, the parallel
ﬁbers of the cerebellar cortical molecular layer, which
have diameters of only a fraction of a millimeter.Also,
the signals from these ﬁbers are weak so that they
require a ﬁnite period of time to build up enough exci-
tation in the dendrites of the Purkinje cell to excite it.
But once the Purkinje cell is excited, it in turn sends a
strong inhibitory signal to the same deep nuclear cell
that had originally turned on the movement. There-
for

 had originally turned on the movement. There-
fore, this helps to turn off the movement after a short
time.
Thus, one can see how the complete cerebellar
circuit could cause a rapid turn-on agonist muscle con-
traction at the beginning of a movement and yet cause
also a precisely timed turn-off of the same agonist con-
traction after a given time period.
Now let us speculate on the circuit for the antago-
nist muscles. Most important, remember that through-
out the spinal cord there are recipro

hat through-
out the spinal cord there are reciprocal agonist/
antagonist circuits for virtually every movement that
the cord can initiate. Therefore, these circuits are part
of the basis for antagonist turn-off at the onset of
movement and then turn-on at termination of move-
ment, mirroring whatever occurs in the agonist
muscles. But we must remember, too, that the cere-
bellum contains several other types of inhibitory cells
besides Purkinje cells. The functions of some of these
are still to 

ells. The functions of some of these
are still to be determined; they, too, could play roles in
the initial inhibition of the antagonist muscles at onset
of a movement and subsequent excitation at the end
of a movement.
All these mechanisms are still partly speculation.
They are presented here especially to illustrate ways
by which the cerebellum could cause exaggerated turn-
on and turn-off signals, controlling the agonist and
antagonist muscles, and controlling the timing as well.
The Purkinje

, and controlling the timing as well.
The Purkinje Cells “Learn” to Correct Motor
Errors—Role of the Climbing Fibers
The degree to which the cerebellum supports onset
and offset of muscle contractions, as well as timing of
contractions, must be learned by the cerebellum. Typ-
ically, when a person ﬁrst performs a new motor act,
the degree of motor enhancement by the cerebellum
at the onset of contraction, the degree or inhibition at
the end of contraction, and the timing of these are
almost alwa

ntraction, and the timing of these are
almost always incorrect for precise performance of the
movement. But after the act has been performed many
times, the individual events become progressively
more precise, sometimes requiring only a few move-
ments before the desired result is achieved, but at
other times requiring hundreds of movements.
How do these adjustments come about? The exact
answer is not known, although it is known that sensi-
tivity levels of cerebellar circuits themselves progres

y levels of cerebellar circuits themselves progres-
sively adapt during the training process. Especially,
the sensitivity of the Purkinje cells to respond to the
granule cell excitation becomes altered. Furthermore,
this sensitivity change is brought about by signals from
the climbing ﬁbers entering the cerebellum from the
inferior olivary complex.
Under resting conditions, the climbing ﬁbers ﬁre
about once per second. But each time they do ﬁre, they
cause extreme depolarization of the entire de

they
cause extreme depolarization of the entire dendritic
tree of the Purkinje cell, lasting for up to a second.
During this time, the Purkinje cell ﬁres with one initial
strong output spike followed by a series of diminish-
ing spikes. When a person performs a new movement
for the ﬁrst time, feedback signals from the muscle and
joint proprioceptors will usually denote to the cere-
bellum how much the actual movement fails to match
the intended movement.And the climbing ﬁber signals
in some way 

ovement.And the climbing ﬁber signals
in some way alter long-term sensitivity of the Purkinje
cells. Over a period of time, this change in sensitivity,
along with other possible “learning” functions of the
cerebellum, is believed to make the timing and other
aspects of cerebellar control of movements approach
perfection. When this has been achieved, the climbing
ﬁbers no longer need to send “error” signals to the
cerebellum to cause further change.
Function of the Cerebellum in Overall
Motor Con

e.
The nervous system uses the cerebellum to coordinate
motor control functions at three levels, as follows:
1. The vestibulocerebellum. This consists principally
of the small ﬂocculonodular cerebellar lobes (that
704
Unit XI
lie under the posterior cerebellum) and adjacent
portions of the vermis. It provides neural circuits
for most of the body’s equilibrium movements.
2. The spinoce

f the body’s equilibrium movements.
2. The spinocerebellum. This consists of most of the
vermis of the posterior and anterior cerebellum
plus the adjacent intermediate zones on both
sides of the vermis. It provides the circuitry for
coordinating mainly movements of the distal
portions of the limbs, especially the hands and
ﬁngers.
3. The cerebrocerebellum. This consists of the large
lateral zones of the cerebellar hemispheres, lateral
to the intermediate zones. It receives virtually all
its inpu

rmediate zones. It receives virtually all
its input from the cerebral motor cortex and
adjacent premotor and somatosensory cortices of
the cerebrum. It transmits its output information
in the upward direction back to the brain,
functioning in a feedback manner with the
cerebral cortical sensorimotor system to plan
sequential voluntary body and limb movements,
planning these as much as tenths of a second in
advance of the actual movements. This is called
development of “motor imagery” of movement

 called
development of “motor imagery” of movements
Vestibulocerebellum—Its Function in
Association with the Brain Stem and 
Spinal Cord to Control Equilibrium and
Postural Movements
The vestibulocerebellum originated phylogenetically
at about the same time that the vestibular apparatus
in the inner ear developed. Furthermore, as discussed
in Chapter 55, loss of the ﬂocculonodular lobes and
adjacent portions of the vermis of the cerebellum,
which constitute the vestibulocerebell

which constitute the vestibulocerebellum,
causes
extreme disturbance of equilibrium and postural
We still must ask the question, what role does the
vestibulocerebellum play in equilibrium that cannot
be provided by other neuronal machinery of the brain
stem? A clue is the fact that in people with vestibulo-
cerebellar dysfunction, equilibrium is far more dis-
turbed during performance of rapid motions than
during stasis, especially so when these movements
involve changes i

pecially so when these movements
involve changes in direction of movement and stimu-
late the semicircular ducts. This suggests that the
vestibulocerebellum is especially important in con-
trolling balance between agonist and antagonist
muscle contractions of the spine, hips, and shoulders
during rapid changes in body positions as required by
the vestibular apparatus.
One of the major problems in controlling balance is
the amount of time required to transmit position
signals and velocity of move

 to transmit position
signals and velocity of movement signals from the dif-
ferent parts of the body to the brain. Even when the
most rapidly conducting sensory pathways are used,up
to 120 m/sec in the spinocerebellar afferent tracts, the
delay for transmission from the feet to the brain is still
15 to 20 milliseconds. The feet of a person running
rapidly can move as much as 10 inches during that
time. Therefore, it is never possible for return signals
from the peripheral parts of the body to r

signals
from the peripheral parts of the body to reach the
brain at the same time that the movements actually
occur. How, then, is it possible for the brain to know
when to stop a movement and to perform the next
sequential act, especially when the movements are
performed rapidly? The answer is that the signals from
the periphery tell the brain how rapidly and in which
directions the body parts are moving. It is then the
function of the vestibulocerebellum to calculate in
advance from these rate

cerebellum to calculate in
advance from these rates and directions where the 
different parts will be during the next few milliseconds.
The results of these calculations are the key to the
brain’s progression to the next sequential movement.
Thus, during control of equilibrium, it is presumed
that information from both the body periphery and the
vestibular apparatus is used in a typical feedback
control circuit to provide anticipatory correction of
postural motor signals necessary for maintainin

of
postural motor signals necessary for maintaining equi-
librium even during extremely rapid motion, including
rapidly changing directions of motion.
Spinocerebellum—Feedback Control of Distal
Limb Movements by Way of the Intermediate
Cerebellar Cortex and the Interposed Nucleus
As shown in Figure 56–8, the intermediate zone of
each cerebellar hemisphere receives two types of
information when a movement is performed: (1) infor-
mation from the cerebral motor cortex and from 
the midbrain red nu

erebral motor cortex and from 
the midbrain red nucleus, telling the cerebellum the
intended sequential plan of movement for the next few
fractions of a second, and (2) feedback information
Thalamus
Muscles
zone of
cerebellum
Spinocerebellar
tract
Mesencephalon,
pons, and medulla
Reticulospinal
and rubrospinal
tracts
Figure 56–8
Cerebral and cerebellar control of voluntary movements, involving
especially the intermediate zone of the cereb

ving
especially the intermediate zone of the cerebellum.
Chapter 56
705
from the peripheral parts of the body, especially from
the distal proprioceptors of the limbs, telling the cere-
bellum what actual movements result.
After the intermediate zone of the cerebellum has
compared the intended movements with the actual
movements, the deep nuclear cells of the interposed
nucleus send corrective output signals (1) back to th

leus send corrective output signals (1) back to the
cerebral motor cortex through relay nuclei in the thal-
amus and (2) to the magnocellular  portion (the lower
portion) of the red nucleus that gives rise to the
rubrospinal tract. The rubrospinal tract in turn joins
the corticospinal tract in innervating the lateral most
motor neurons in the anterior horns of the spinal cord
gray matter, the neurons that control the distal parts
of the limbs, particularly the hands and ﬁngers.
This part of the 

rticularly the hands and ﬁngers.
This part of the cerebellar motor control system pro-
vides smooth, coordinate movements of the agonist
and antagonist muscles of the distal limbs for per-
forming acute purposeful patterned movements. The
cerebellum seems to compare the “intentions” of the
higher levels of the motor control system, as transmit-
ted to the intermediate cerebellar zone through the
corticopontocerebellar tract, with the “performance”
by the respective parts of the body, as transmit

”
by the respective parts of the body, as transmitted 
back to the cerebellum from the periphery. In fact,
the ventral spinocerebellar tract even transmits back
to the cerebellum an “efference” copy of the actual
motor control signals that reach the anterior motor
neurons, and this, too, is integrated with the signals
arriving from the muscle spindles and other proprio-
ceptor sensory organs, transmitted principally in the
dorsal spinocerebellar tract. We learned earlier that
similar comparator 

tract. We learned earlier that
similar comparator signals also go to the inferior
olivary complex; if the signals do not compare favor-
ably, the olivary-Purkinje cell system along with pos-
sibly other cerebellar learning mechanisms eventually
corrects the motions until they perform the desired
function.
Function of the Cerebellum to Prevent Overshoot of Movements
and to “Damp” Movements. Almost all movements of the
body are “pendular.” For instance, when an arm is
moved, momentum develops, and

ance, when an arm is
moved, momentum develops, and the momentum
must be overcome before the movement can be
stopped. Because of momentum, all pendular move-
ments have a tendency to overshoot. If overshooting
does occur in a person whose cerebellum has been
destroyed,the conscious centers of the cerebrum even-
tually recognize this and initiate a movement in the
reverse direction attempting to bring the arm to its
intended position. But the arm,by virtue of its momen-
tum, overshoots once more i

y virtue of its momen-
tum, overshoots once more in the opposite direction,
and appropriate corrective signals must again be insti-
tuted. Thus, the arm oscillates back and forth past its
intended point for several cycles before it ﬁnally ﬁxes
on its mark. This effect is called an action tremor, or
intention tremor.
But, if the cerebellum is intact, appropriate learned,
subconscious signals stop the movement precisely at
the intended point, thereby preventing the overshoot
as well as the tremor.

by preventing the overshoot
as well as the tremor. This is the basic characteristic 
of a damping system. All control systems regulating
pendular elements that have inertia must have
damping circuits built into the mechanisms. For motor
control by the nervous system, the cerebellum pro-
vides most of this damping function.
Cerebellar Control of Ballistic Movements. Most rapid
movements of the body, such as the movements of the
ﬁngers in typing, occur so rapidly that it is not possi-
ble to recei

ccur so rapidly that it is not possi-
ble to receive feedback information either from the
periphery to the cerebellum or from the cerebellum
back to the motor cortex before the movements are
over.These movements are called ballistic movements,
meaning that the entire movement is preplanned and
set into motion to go a speciﬁc distance and then to
stop.Another important example is the saccadic move-
ments of the eyes, in which the eyes jump from one
position to the next when reading or when lookin

e
position to the next when reading or when looking at
successive points along a road as a person is moving
in a car.
Much can be understood about the function of the
cerebellum by studying the changes that occur in these
ballistic movements when the cerebellum is removed.
Three major changes occur: (1) The movements are
slow to develop and do not have the extra onset surge
that the cerebellum usually provides, (2) the force
developed is weak, and (3) the movements are slow to
turn off, usually 

d (3) the movements are slow to
turn off, usually allowing the movement to go well
beyond the intended mark. Therefore, in the absence
of the cerebellar circuit, the motor cortex has to think
extra hard to turn ballistic movements on and again
has to think hard and take extra time to turn the
movement off.Thus, the automatism of ballistic move-
ments is lost.
If one considers once again the circuitry of the cere-
bellum as described earlier, one sees that it is beauti-
fully organized to perform

sees that it is beauti-
fully organized to perform this biphasic, ﬁrst excitatory
and then delayed inhibitory function that is required
for preplanned rapid ballistic movements. One also
sees that the built-in timing circuits of the cerebellar
cortex are fundamental to this particular ability of the
Cerebrocerebellum—Function of the 
Large Lateral Zone of the Cerebellar
Hemisphere to Plan, Sequence, and Time
Complex Movements
In human beings, the lateral zones of the two cerebel-
lar

 beings, the lateral zones of the two cerebel-
lar hemispheres are highly developed and greatly
enlarged. This goes along with human abilities to plan
and perform intricate sequential patterns of move-
ment, especially with the hands and ﬁngers, and to
speak. Yet the large lateral zones of the cerebellar
hemispheres have no direct input of information from
the peripheral parts of the body. Also, almost all com-
munication between these lateral cerebellar areas and
the cerebral cortex is not with

rebellar areas and
the cerebral cortex is not with the primary cerebral
motor cortex itself but instead with the premotor area
and primary and association somatosensory areas.
Even so, destruction of the lateral zones of the cere-
bellar hemispheres along with their deep nuclei, the
dentate nuclei, can lead to extreme incoordination of
complex purposeful movements of the hands, ﬁngers,
706
Unit XI
and feet and of the speech apparatus. 

ophysiology
and feet and of the speech apparatus. This has been
difﬁcult to understand because of lack of direct com-
munication between this part of the cerebellum and
the primary motor cortex. However, experimental
studies suggest that these portions of the cerebellum
are concerned with two other important but indirect
aspects of motor control: (1) the planning of sequen-
tial movements and (2) the “timing” of the sequential
Planning of Sequential Movements. The planning of
sequenti

 of Sequential Movements. The planning of
sequential movements requires that the lateral zones
of the hemispheres communicate with both the pre-
motor and the sensory portions of the cerebral cortex,
and it requires two-way communication between these
cerebral cortex areas with corresponding areas of the
basal ganglia. It seems that the “plan” of sequential
movements actually begins in the sensory and premo-
tor areas of the cerebral cortex, and from there the
plan is transmitted to the lateral 

from there the
plan is transmitted to the lateral zones of the cerebel-
lar hemispheres. Then, amid much two-way trafﬁc
between cerebellum and cerebral cortex, appropriate
motor signals provide transition from one sequence of
movements to the next.
An exceedingly interesting observation that sup-
ports this view is that many neurons in the cerebellar
dentate nuclei display the activity pattern for the
sequential movement that is yet to come while the
present movement is still occurring. Thus, th

 the
present movement is still occurring. Thus, the lateral
cerebellar zones appear to be involved not with what
movement is happening at a given moment but with
what will be happening during the next sequential
movement a fraction of a second or perhaps even
seconds later.
To summarize, one of the most important features
of normal motor function is one’s ability to progress
smoothly from one movement to the next in orderly
succession. In the absence of the large lateral zones of
the cerebellar 

ence of the large lateral zones of
the cerebellar hemispheres, this capability is seriously
disturbed for rapid movements.
Timing Function. Another important function of the
lateral zones of the cerebellar hemispheres is to
provide appropriate timing for each succeeding move-
ment. In the absence of these cerebellar zones, one
loses the subconscious ability to predict ahead of time
how far the different parts of the body will move in a
given time. Without this timing capability, the person
becom

. Without this timing capability, the person
becomes unable to determine when the next sequen-
tial movement needs to begin. As a result, the suc-
ceeding movement may begin too early or, more likely,
too late. Therefore, lesions in the lateral zones of the
cerebellum cause complex movements (such as those
required for writing, running, or even talking) to
become incoordinate and lacking ability to progress in
orderly sequence from one movement to the next.
Such cerebellar lesions are said to ca

o the next.
Such cerebellar lesions are said to cause failure of
smooth progression of movements.
Extramotor Predictive Functions of the Cerebrocerebellum.
The cerebrocerebellum (the large lateral lobes) also
helps to “time” events other than movements of the
body. For instance, the rates of progression of both
auditory and visual phenomena can be predicted by
the brain, but both of these require cerebellar partici-
pation. As an example, a person can predict from 
the changing visual scene how 

n can predict from 
the changing visual scene how rapidly he or she is
approaching an object. A striking experiment that
demonstrates the importance of the cerebellum in this
ability is the effects of removing the large lateral por-
tions of the cerebellum in monkeys. Such a monkey
occasionally charges the wall of a corridor and literally
bashes its brains because it is unable to predict when
it will reach the wall.
We are only now beginning to learn about these
extramotor predictive functions o

earn about these
extramotor predictive functions of the cerebellum. It
is quite possible that the cerebellum provides a “time-
base,” perhaps using time-delay circuits, against which
signals from other parts of the central nervous system
can be compared; it is often stated that the cerebellum
is particularly helpful in interpreting rapidly changing
spatiotemporal relations in sensory information.
Clinical Abnormalities of 
the Cerebellum
An important feature of clinical cerebellar abnormali-
tie

tant feature of clinical cerebellar abnormali-
ties is that destruction of small portions of the lateral
cerebellar cortex seldom causes detectable abnormali-
ties in motor function. In fact, several months after as
much as one half of the lateral cerebellar cortex on one
side of the brain has been removed, if the deep cere-
bellar nuclei are not removed along with the cortex, the
motor functions of the animal appear to be almost
normal as long as the animal performs all movements
slowly. Thus, 

s the animal performs all movements
slowly. Thus, the remaining portions of the motor
control system are capable of compensating tremen-
dously for loss of parts of the cerebellum.
Therefore, to cause serious and continuing dysfunc-
tion of the cerebellum, the cerebellar lesion usually
must involve one or more of the deep cerebellar
nuclei—the dentate, interposed, or fastigial nuclei.
Dysmetria and Ataxia. Two of the most important symp-
toms of cerebellar disease are dysmetria and ataxia. As
po

cerebellar disease are dysmetria and ataxia. As
pointed out previously, in the absence of the cerebel-
lum, the subconscious motor control system cannot
predict how far movements will go. Therefore, the
movements ordinarily overshoot their intended mark;
then the conscious portion of the brain overcompen-
sates in the opposite direction for the succeeding com-
pensatory movement.This effect is called dysmetria, and
it results in uncoordinated movements that are called
ataxia. Dysmetria and ataxi

ements that are called
ataxia. Dysmetria and ataxia can also result from lesions
in the spinocerebellar tracts because feedback informa-
tion from the moving parts of the body to the cerebel-
lum is essential for cerebellar timing of movement
termination.
Past Pointing. Past pointing means that in the absence
of the cerebellum, a person ordinarily moves the hand
or some other moving part of the body considerably
beyond the point of intention. This results from the
fact that normally the cerebell

s results from the
fact that normally the cerebellum initiates most of the
motor signal that turns off a movement after it is
begun; if the cerebellum is not available to do this, the
movement ordinarily goes beyond the intended mark.
Chapter 56
707
Therefore, past pointing is actually a manifestation of
dysmetria.
Failure of Progression
Dysdiadochokinesia. When the motor control system
fails to predict where the differen

control system
fails to predict where the different parts of the body will
be at a given time, it “loses” perception of the parts
during rapid motor movements. As a result, the suc-
ceeding movement may begin much too early or much
too late, so that no orderly “progression of movement”
can occur. One can demonstrate this readily by having
a patient with cerebellar damage turn one hand upward
and downward at a rapid rate. The patient rapidly
“loses” all perception of the instantaneous position of

s” all perception of the instantaneous position of
the hand during any portion of the movement. As a
result, a series of stalled attempted but jumbled move-
ments occurs instead of the normal coordinate upward
and downward motions. This is called dysdiadochokine-
sia.
Dysarthria. Another example in which failure of pro-
gression occurs is in talking because the formation of
words depends on rapid and orderly succession of indi-
vidual muscle movements in the larynx, mouth, and res-
piratory syst

ments in the larynx, mouth, and res-
piratory system. Lack of coordination among these and
inability to adjust in advance either the intensity of
sound or duration of each successive sound causes
jumbled vocalization, with some syllables loud, some
weak, some held for long intervals, some held for short
intervals, and resultant speech that is often unintelligi-
ble. This is called dysarthria.
Intention Tremor. When a person who has lost the cere-
bellum performs a voluntary act, the movements te

bellum performs a voluntary act, the movements tend
to oscillate, especially when they approach the intended
mark, ﬁrst overshooting the mark and then vibrating
back and forth several times before settling on the
mark. This reaction is called an intention tremor or an
action tremor, and it results from cerebellar overshoot-
ing and failure of the cerebellar system to “damp” the
motor movements.
Cerebellar Nystagmus. Cerebellar nystagmus is tremor of
the eyeballs that occurs usually when one att

r of
the eyeballs that occurs usually when one attempts to
ﬁxate the eyes on a scene to one side of the head. This
off-center type of ﬁxation results in rapid, tremulous
movements of the eyes rather than steady ﬁxation, and
it is another manifestation of failure of damping by the
cerebellum. It occurs especially when the ﬂocculonodu-
lar lobes of the cerebellum are damaged; in this instance
it is also associated with loss of equilibrium because of
dysfunction of the pathways through the ﬂocculon

f
dysfunction of the pathways through the ﬂocculonodu-
lar cerebellum from the semicircular ducts.
Hypotonia. Loss of the deep cerebellar nuclei, particu-
larly of the dentate and interposed nuclei, causes
decreased tone of the peripheral body musculature on
the side of the cerebellar lesion. The hypotonia results
from loss of cerebellar facilitation of the motor cortex
and brain stem motor nuclei by tonic signals from the
deep cerebellar nuclei.
Basal Ganglia—Their Motor
Functions
The basal gan

Functions
The basal ganglia, like the cerebellum, constitute
another accessory motor system that functions usually
not by itself but in close association with the cerebral
cortex and corticospinal motor control system. In fact,
the basal ganglia receive most of their input signals
from the cerebral cortex itself and also return almost
all their output signals back to the cortex.
Figure 56–9 shows the anatomical relations of the
basal ganglia to other structures of the 

s of the
basal ganglia to other structures of the brain. On each
side of the brain, these ganglia consist of the caudate
nucleus, putamen, globus pallidus, substantia nigra,
and subthalamic nucleus. They are located mainly
lateral to and surrounding the thalamus, occupying a
large portion of the interior regions of both cerebral
hemispheres. Note also that almost all motor and
Longitudinal fissure
Caudate nucleus
Tail of caudate
LATERAL
Fibers to and from
spinal cord in
internal capsule
Putamen 

 and from
Putamen and
globus pallidus
POSTERIOR
ANTERIOR
Thalamus
Figure 56–9
Anatomical relations of the basal
ganglia to the cerebral cortex and
thalamus, shown in three-dimen-
sional view. (Redrawn from Guyton
AC: Basic Neuroscience: Anatomy
and Physiology. Philadelphia: WB
Saunders Co, 1992.)
708
Unit XI
sensory nerve ﬁbers connecting the cerebral cortex
and spinal cord pass through the space that li

tex
and spinal cord pass through the space that lies
between the major masses of the basal ganglia, the
caudate nucleus and the putamen. This space is called
the internal capsule of the brain. It is important for our
current discussion because of the intimate association
between the basal ganglia and the corticospinal system
for motor control.
Neuronal Circuitry of the Basal Ganglia. The anatomical
connections between the basal ganglia and the other
brain elements that provide motor control are

her
complex, as shown in Figure 56–10.To the left is shown
the motor cortex, thalamus, and associated brain stem
and cerebellar circuitry. To the right is the major cir-
cuitry of the basal ganglia system, showing the tremen-
dous interconnections among the basal ganglia
themselves plus extensive input and output pathways
between the other motor regions of the brain and the
basal ganglia.
In the next few sections we will concentrate espe-
cially on t

few sections we will concentrate espe-
cially on two major circuits,the putamen circuit and the
caudate circuit.
Function of the Basal Ganglia in
Executing Patterns of Motor Activity—
The Putamen Circuit
One of the principal roles of the basal ganglia in motor
control is to function in association with the corti-
cospinal system to control complex patterns of motor
activity. An example is the writing of letters of the
alphabet. When there is serious damage to the basal
ganglia, the cortical syst

ous damage to the basal
ganglia, the cortical system of motor control can no
longer provide these patterns. Instead, one’s writing
becomes crude, as if one were learning for the ﬁrst
time how to write.
Other patterns that require the basal ganglia are
cutting paper with scissors, hammering nails, shooting
a basketball through a hoop, passing a football, throw-
ing a baseball, the movements of shoveling dirt, most
aspects of vocalization, controlled movements of the
eyes, and virtually any other 

eyes, and virtually any other of our skilled movements,
most of them performed subconsciously.
Neural Pathways of the Putamen Circuit. Figure 56–11
shows the principal pathways through the basal
ganglia for executing learned patterns of movement.
They begin mainly in the premotor and supplementary
areas of the motor cortex and in the somatosensory
areas of the sensory cortex. Next they pass to the
putamen (mainly bypassing the caudate nucleus), then
to the internal portion of

 caudate nucleus), then
to the internal portion of the globus pallidus, next to
the ventroanterior and ventrolateral relay nuclei of
the thalamus, and ﬁnally return to the cerebral primary
motor cortex and to portions of the premotor and sup-
plementary cerebral areas closely associated with the
primary motor cortex. Thus, the putamen circuit has its
Thalamus
Muscles
Globus
pallidus
Inferior olive
Putamen
Caudate
nucleus
Premotor and
supplemental motor

en
Caudate
nucleus
association areas
Subthalamus
Substantia nigra
Figure 56–10
Relation of the basal ganglial circuitry to the corticospinal-
cerebellar system for movement control.
Caudate
supplemental Primary motor
Ventroanterior and
ventrolateral
nuclei of thalamus
Putamen
Globus
pallidus
internal/external
Figure 56–11
Putamen circuit through the basal ganglia for subconscious exe-
c

 through the basal ganglia for subconscious exe-
cution of learned patterns of movement.
Chapter 56
709
inputs mainly from those parts of the brain adjacent to
the primary motor cortex but not much from the
primary motor cortex itself. Then its outputs do go
mainly back to the primary motor cortex or closely
associated premotor and supplementary cortex. Func-
tioning in close association with this primary putamen
circuit 

ose association with this primary putamen
circuit are ancillary circuits that pass from the
putamen through the external globus pallidus, the sub-
thalamus, and the substantia nigra—ﬁnally returning
to the motor cortex by way of the thalamus.
Abnormal Function in the Putamen Circuit: Athetosis, Hemibal-
lismus, and Chorea. How does the putamen circuit func-
tion to help execute patterns of movement? The
answer is poorly known. However, when a portion of
the circuit is damaged or blocked, certain

tion of
the circuit is damaged or blocked, certain patterns of
movement become severely abnormal. For instance,
lesions in the globus pallidus frequently lead to spon-
taneous and often continuous writhing movements of
a hand, an arm, the neck, or the face—movements
called athetosis.
A lesion in the subthalamus often leads to sudden
ﬂailing movements of an entire limb, a condition called
hemiballismus.
Multiple small lesions in the putamen lead to ﬂick-
ing movements in the hands, face, and othe

o ﬂick-
ing movements in the hands, face, and other parts of
the body, called chorea.
Lesions of the substantia nigra lead to the common
and extremely severe disease of rigidity, akinesia, and
tremors known as Parkinson’s disease, which we
discuss in more detail later.
Role of the Basal Ganglia for
Cognitive Control of Sequences of
Motor Patterns—The Caudate Circuit
The term cognition means the thinking processes of
the brain, using both sensory input to the brain plus
information already stored

input to the brain plus
information already stored in memory. Most of our
motor actions occur as a consequence of thoughts gen-
erated in the mind, a process called cognitive control
of motor activity. The caudate nucleus plays a major
role in this cognitive control of motor activity.
The neural connections between the caudate
nucleus and the corticospinal motor control system,
shown in Figure 56–12, are somewhat different from
those of the putamen circuit. Part of the reason for this
is that th

en circuit. Part of the reason for this
is that the caudate nucleus, as shown in Figure 56–9,
extends into all lobes of the cerebrum, beginning ante-
riorly in the frontal lobes, then passing posteriorly
through the parietal and occipital lobes, and ﬁnally
curving forward again like the letter “C” into the tem-
poral lobes. Furthermore, the caudate nucleus receives
large amounts of its input from the association areas
of the cerebral cortex overlying the caudate nucleus,
mainly areas that also i

ying the caudate nucleus,
mainly areas that also integrate the different types of
sensory and motor information into usable thought
patterns.
After the signals pass from the cerebral cortex to 
the caudate nucleus, they are next transmitted to the
internal globus pallidus, then to the relay nuclei of the
ventroanterior and ventrolateral thalamus, and ﬁnally
back to the prefrontal, premotor, and supplementary
motor areas of the cerebral cortex, but with almost
none of the returning signals passin

t with almost
none of the returning signals passing directly to the
primary motor cortex. Instead, the returning signals go
to those accessory motor regions in the premotor and
supplementary motor areas that are concerned with
putting together sequential patterns of movement
lasting 5 or more seconds instead of exciting individ-
ual muscle movements.
A good example of this would be a person seeing a
lion approach and then responding instantaneously
and automatically by (1) turning away from the 

ly
and automatically by (1) turning away from the lion,
(2) beginning to run, and (3) even attempting to climb
a tree. Without the cognitive functions, the person
might not have the instinctive knowledge, without
thinking for too long a time, to respond quickly and
appropriately.Thus, cognitive control of motor activity
determines subconsciously, and within seconds, which
patterns of movement will be used together to achieve
a complex goal that might itself last for many seconds.
Function of the

ight itself last for many seconds.
Function of the Basal Ganglia to
Change the Timing and to Scale the
Intensity of Movements
Two important capabilities of the brain in controlling
movement are (1) to determine how rapidly the move-
ment is to be performed and (2) to control how large
the movement will be. For instance, a person may write
the letter “a” slowly or rapidly. Also, he or she may
write a small “a” on a piece of paper or a large  “a” on
Caudate
Premotor an

n
Caudate
Putamen
Globus
pallidus
Figure 56–12
Caudate circuit through the basal ganglia for cognitive planning
of sequential and parallel motor patterns to achieve speciﬁc con-
scious goals.
710
Unit XI
a chalkboard. Regardless of the choice, the propor-
tional characterist

ess of the choice, the propor-
tional characteristics of the letter remain nearly the
same.
In patients with severe lesions of the basal ganglia,
these timing and scaling functions are poor; in fact,
sometimes they are nonexistent. Here again, the basal
ganglia do not function alone; they function in close
association with the cerebral cortex. One especially
important cortical area is the posterior parietal cortex,
which is the locus of the spatial coordinates for motor
control of all parts of t

al coordinates for motor
control of all parts of the body as well as for the rela-
tion of the body and its parts to all its surroundings.
Figure 56–13 shows the way in which a person lacking
a left posterior parietal cortex might draw the face of
another human being, providing proper proportions
for the right side of the face but almost ignoring the
left side (which is in his or her right ﬁeld of vision).
Also, such a person will try always to avoid using his
or her right arm, right hand, or ot

void using his
or her right arm, right hand, or other portions of his or
her right body for the performance of tasks, almost not
knowing that these parts of his or her body exist.
Because the caudate circuit of the basal ganglial
system functions mainly with association areas of the
cerebral cortex such as the posterior parietal cortex,
presumably the timing and scaling of movements are
functions of this caudate cognitive motor control
circuit. However, our understanding of function in the
basal

owever, our understanding of function in the
basal ganglia is still so imprecise that much of what is
conjectured in the last few sections is analytical deduc-
tion rather than proven fact.
Functions of Speciﬁc
Neurotransmitter Substances in the
Basal Ganglial System
Figure 56–14 demonstrates the interplay of several
speciﬁc neurotransmitters that are known to function
within the basal ganglia, showing (1) dopamine path-
ways from the substantia nigra to the caudate nucleus
and putamen, (2) gamm

nigra to the caudate nucleus
and putamen, (2) gamma-aminobutyric acid (GABA)
pathways from the caudate nucleus and putamen to
the globus pallidus and substantia nigra, (3) acetyl-
choline pathways from the cortex to the caudate
nucleus and putamen, and (4) multiple general path-
ways from the brain stem that secrete norepinephrine,
serotonin, enkephalin, and several other neurotrans-
mitters in the basal ganglia as well as in other parts of
the cerebrum. In addition to all these are multiple glu

erebrum. In addition to all these are multiple glu-
tamate pathways that provide most of the excitatory
signals (not shown in the ﬁgure) that balance out 
the large numbers of inhibitory signals transmitted 
especially by the dopamine, GABA, and serotonin
inhibitory transmitters.We will have more to say about
some of these neurotransmitter and hormonal systems
in subsequent sections when we discuss diseases of the
basal ganglia, as well as in subsequent chapters when
we discuss behavior, sleep, 

sequent chapters when
we discuss behavior, sleep, wakefulness, and functions
of the autonomic nervous system.
For the present, it should be remembered that 
the neurotransmitter GABA always functions as an
inhibitory agent. Therefore, GABA neurons in the
feedback loops from the cortex through the basal
Figure 56–13
Typical drawing that might be made by a person who has severe
damage in his or her left parietal cortex where the spatial coordi-
nates of the right ﬁeld of vision are stored.
Caudate

es of the right ﬁeld of vision are stored.
From cortex
From brain stem
nigra
1. Norepinephrine
2. Serotonin
3. Enkephalin
Putamen
Globus
pallidus
Dopamine
GABA
Ach
Figure 56–14
Neuronal pathways that secrete different types of neurotransmit-
ter substances in the basal ganglia. Ach, acetylcholine; GABA,
gamma-aminobutyric acid.
Chapter 56
711
ganglia and then back to the cortex make virtually 
a

glia and then back to the cortex make virtually 
all these loops negative feedback loops, rather than
positive feedback loops, thus lending stability to the
motor control systems. Dopamine also functions as an
inhibitory neurotransmitter in most parts of the brain,
so that it, too, undoubtedly functions as a stabilizer
under some conditions.
Clinical Syndromes Resulting from
Damage to the Basal Ganglia
Aside from athetosis and hemiballismus, which have
already been mentioned in relation to lesio

h have
already been mentioned in relation to lesions in the
globus pallidus and subthalamus, two other major dis-
eases result from damage in the basal ganglia.These are
Parkinson’s disease and Huntington’s disease.
Parkinson’s Disease
Parkinson’s disease, known also as paralysis agitans,
results from widespread destruction of that portion of
the substantia nigra (the pars compacta) that sends
dopamine-secreting nerve ﬁbers to the caudate nucleus
and putamen. The disease is characterized by (1) 

and putamen. The disease is characterized by (1) rigid-
ity of much of the musculature of the body, (2) invol-
untary tremor of the involved areas even when the
person is resting at a ﬁxed rate of 3 to 6 cycles per
second, and (3) serious difﬁculty in initiating movement,
called akinesia.
The causes of these abnormal motor effects are
unknown. However, the dopamine secreted in the
caudate nucleus and putamen is an inhibitory transmit-
ter; therefore, destruction of the dopaminergic neurons
in t

fore, destruction of the dopaminergic neurons
in the substantia nigra of the parkinsonian patient the-
oretically would allow the caudate nucleus and putamen
to become overly active and possibly cause continuous
output of excitatory signals to the corticospinal motor
control system. These signals could overly excite 
many or all of the muscles of the body, thus leading to
rigidity.
Some of the feedback circuits might easily oscillate
because of high feedback gains after loss of their inhi-
bitio

igh feedback gains after loss of their inhi-
bition, leading to the tremor of Parkinson’s disease.This
tremor is quite different from that of cerebellar disease
because it occurs during all waking hours and therefore
is an involuntary tremor, in contradistinction to cere-
bellar tremor, which occurs only when the person per-
forms intentionally initiated movements and therefore
is called intention tremor.
The akinesia that occurs in Parkinson’s disease is often
much more distressing to the patie

isease is often
much more distressing to the patient than are the symp-
toms of muscle rigidity and tremor, because to perform
even the simplest movement in severe parkinsonism, the
person must exert the highest degree of concentration.
The mental effort, even mental anguish, that is necessary
to make the desired movements is often at the limit of
the patient’s willpower. Then, when the movements do
occur, they are usually stiff and staccato in character
instead of smooth.The cause of this akine

haracter
instead of smooth.The cause of this akinesia is still spec-
ulative. However, dopamine secretion in the limbic
system, especially in the nucleus accumbens, is often
decreased along with its decrease in the basal ganglia. It
has been suggested that this might reduce the psychic
drive for motor activity so greatly that akinesia results.
Treatment with l-Dopa. Administration of the drug L-
dopa to patients with Parkinson’s disease usually ame-
liorates many of the symptoms, especially the 

me-
liorates many of the symptoms, especially the rigidity
and akinesia. The reason for this is believed to be that
L-dopa is converted in the brain into dopamine, and the
dopamine then restores the normal balance between
inhibition and excitation in the caudate nucleus and
putamen. Administration of dopamine itself does not
have the same effect because dopamine has a chemical
structure that will not allow it to pass through the blood-
brain barrier, even though the slightly different struc-
tur

ier, even though the slightly different struc-
ture of L-dopa does allow it to pass.
Treatment with l-Deprenyl. Another treatment for
Parkinson’s disease is the drug L-deprenyl. This drug
inhibits monoamine oxidase, which is responsible for
destruction of most of the dopamine after it has been
secreted. Therefore, any dopamine that is released
remains in the basal ganglial tissues for a longer time.
In addition, for reasons not understood, this treatment
helps to slow destruction of the dopamine

reatment
helps to slow destruction of the dopamine-secreting
neurons in the substantia nigra. Therefore, appropriate
combinations of L-dopa therapy along with L-deprenyl
therapy usually provide much better treatment than use
of one of these drugs alone.
Treatment with Transplanted Fetal Dopamine Cells.
Transplantation of dopamine-secreting cells (cells
obtained from the brains of aborted fetuses) into the
caudate nuclei and putamen has been used with some
short-term success to treat Parkinson’s 

with some
short-term success to treat Parkinson’s disease.
However, the cells do not live for more than a few
months. If persistence could be achieved, perhaps this
would become the treatment of the future.
Treatment by Destroying Part of the Feedback Circuitry
in the Basal Ganglia. Because abnormal signals from
the basal ganglia to the motor cortex cause most of the
abnormalities in Parkinson’s disease, multiple attempts
have been made to treat these patients by blocking
these signals surgicall

these patients by blocking
these signals surgically. For a number of years, surgical
lesions were made in the ventrolateral and ventroante-
rior nuclei of the thalamus, which blocked part of the
feedback circuit from the basal ganglia to the cortex;
variable degrees of success were achieved—as well as
sometimes serious neurological damage. In monkeys
with Parkinson’s disease, lesions placed in the subthal-
amus have been used, sometimes with surprisingly good
results.
Huntington’s Disease (Hunti

risingly good
results.
Huntington’s Disease (Huntington’s Chorea)
Huntington’s disease is a hereditary disorder that
usually begins causing symptoms at age 30 to 40 years.
It is characterized at ﬁrst by ﬂicking movements in indi-
vidual muscles and then progressive severe distortional
movements of the entire body. In addition, severe
dementia develops along with the motor dysfunctions.
The abnormal movements of Huntington’s disease
are believed to be caused by loss of most of the cell
bodies of 

o be caused by loss of most of the cell
bodies of the GABA-secreting neurons in the caudate
nucleus and putamen and of acetylcholine-secreting
neurons in many parts of the brain. The axon terminals
of the GABA neurons normally inhibit portions of the
globus pallidus and substantia nigra. This loss of inhibi-
tion is believed to allow spontaneous outbursts of
globus pallidus and substantia nigra activity that cause
the distortional movements.
The dementia in Huntington’s disease probably does
not

dementia in Huntington’s disease probably does
not result from the loss of GABA neurons but from the
loss of acetylcholine-secreting neurons, perhaps espe-
cially in the thinking areas of the cerebral cortex.
712
Unit XI
The abnormal gene that causes Huntington’s disease
has been found; it has a many-times-repeating codon,
CAG, that codes for multiple extra glutamine amino
acids in the molecular structure of an abnormal neu-
ronal cell

molecular structure of an abnormal neu-
ronal cell protein called huntingtin that causes the symp-
toms. How this protein causes the disease effects is now
the question for major research effort.
Integration of the Many 
Parts of the Total Motor
Control System
Finally, we need to summarize as best we can what is
known about overall control of movement. To do this,
let us ﬁrst give a synopsis of the different levels of
control.
Programmed in the spinal cord are local patterns of
move

mmed in the spinal cord are local patterns of
movement for all muscle areas of the body—for
instance, programmed withdrawal reﬂexes that pull
any part of the body away from a source of pain. The
cord is the locus also of complex patterns of rhythmi-
cal motions such as to-and-fro movement of the limbs
for walking, plus reciprocal motions on opposite sides
of the body or of the hindlimbs versus the forelimbs
in four-legged animals.
All these programs of the cord can be commanded
into action by hi

ams of the cord can be commanded
into action by higher levels of motor control, or they
can be inhibited while the higher levels take over
control.
The hindbrain provides two major functions for
general motor control of the body: (1) maintenance of
axial tone of the body for the purpose of standing and
(2) continuous modiﬁcation of the degrees of tone in
the different muscles in response to information from
the vestibular apparatuses for the purpose of main-
taining body equilibr

ses for the purpose of main-
taining body equilibrium.
The motor cortex system provides most of the acti-
vating motor signals to the spinal cord. It functions
partly by issuing sequential and parallel commands
that set into motion various cord patterns of motor
action. It can also change the intensities of the differ-
ent patterns or modify their timing or other charac-
teristics. When needed, the corticospinal system can
bypass the cord patterns, replacing them with higher-

ss the cord patterns, replacing them with higher-
level patterns from the brain stem or cerebral cortex.
The cortical patterns usually are complex; also, they
can be “learned,” whereas cord patterns are mainly
determined by heredity and are said to be “hard
wired.”
Associated Functions of the Cerebellum. The cerebellum
functions with all levels of muscle control. It functions
with the spinal cord especially to enhance the stretch
reﬂex, so that when a contracting muscle encounters
an unexpectedl

hen a contracting muscle encounters
an unexpectedly heavy load, a long stretch reﬂex signal
transmitted all the way through the cerebellum and
back again to the cord strongly enhances the load-
resisting effect of the basic stretch reﬂex.
At the brain stem level, the cerebellum functions to
make the postural movements of the body—especially
the rapid movements required by the equilibrium
system—smooth and continuous and without abnor-
mal oscillations.
At the cerebral cortex level, the cerebellu

tions.
At the cerebral cortex level, the cerebellum oper-
ates in association with the cortex to provide many
accessory motor functions, especially to provide extra
motor force for turning on muscle contraction rapidly
at the start of a movement. Near the end of each move-
ment, the cerebellum turns on antagonist muscles at
exactly the right time and with proper force to stop the
movement at the intended point. Furthermore, there
is good physiologic evidence that all aspects of this
turn-on/turn

gic evidence that all aspects of this
turn-on/turn-off patterning by the cerebellum can be
learned with experience.
The cerebellum functions with the cerebral cortex at
still another level of motor control: it helps to program
in advance muscle contractions that are required for
smooth progression from a present rapid movement in
one direction to the next rapid movement in another
direction, all this occurring in a fraction of a second.
The neural circuit for this passes from the cerebral
cortex

l circuit for this passes from the cerebral
cortex to the large lateral zones of the cerebellar hemi-
spheres and then back to the cerebral cortex.
The cerebellum functions mainly when muscle
movements have to be rapid. Without the cerebellum,
slow and calculated movements can still occur, but it
is difﬁcult for the corticospinal system to achieve rapid
and changing intended movements to execute a par-
ticular goal or especially to progress smoothly from
one rapid movement to the next.
Associate

hly from
Associated Functions of the Basal Ganglia. The basal
ganglia are essential to motor control in ways entirely
different from those of the cerebellum. Their most
important functions are (1) to help the cortex execute
subconscious but learned patterns of movement and (2)
to help plan multiple parallel and sequential patterns
of movement that the mind must put together to
accomplish a purposeful task.
The types of motor patterns that require the basal
ganglia

s of motor patterns that require the basal
ganglia include those for writing all the different
letters of the alphabet, for throwing a ball, and for
typing. Also, the basal ganglia are required to modify
these patterns for writing small or writing very large,
thus controlling dimensions of the patterns.
At a still higher level of control is another combined
cerebral and basal ganglia circuit, beginning in the
thinking processes of the cerebrum to provide overall
sequential steps of action for re

 provide overall
sequential steps of action for responding to each new
situation, such as planning one’s immediate motor
response to an assailant who hits the person in the face
Chapter 56
713
or one’s sequential response to an unexpectedly fond
embrace.
What is it that arouses us from inactivity and sets into
play our trains of movement? We are beginning to
learn about the motivational systems o

eginning to
learn about the motivational systems of the brain.
Basically, the brain has an older core located beneath,
anterior, and lateral to the thalamus—including the
hypothalamus, amygdala, hippocampus, septal region
anterior to the hypothalamus and thalamus, and even
old regions of the thalamus and cerebral cortex them-
selves—all of which function together to initiate most
motor and other functional activities of the brain.
These areas are collectively called the limbic system
of the brai

 collectively called the limbic system
of the brain. We discuss this system in detail in 
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7
714
Cerebral Cortex, Intellectual
Functions of the Brain, Learning
and Memory
It is ironic that of all the parts of the brain, we know
least about the functions of the cerebral cortex,
even though it is by far the largest portion of the
nervous system. But we do know the effects of
damage or speciﬁc stimulation in various portions
of the cortex. In the ﬁrst part of this chapter, the
facts known about cortical function are discussed;
the

s known about cortical function are discussed;
then basic theories of neuronal mechanisms
involved in thought processes, memory, analysis of sensory information, and so
forth are presented brieﬂy.
Physiologic Anatomy of the Cerebral Cortex
The functional part of the cerebral cortex is a thin layer of neurons covering the
surface of all the convolutions of the cerebrum. This layer is only 2 to 5 millimeters
thick, with a total area of about one quarter of a square meter. The total cerebral
cortex

arter of a square meter. The total cerebral
cortex contains about 100 billion neurons.
Figure 57–1 shows the typical histological structure of the neuronal surface of the
cerebral cortex, with its successive layers of different types of neurons. Most of the
neurons are of three types: (1) granular (also called stellate), (2) fusiform, and (3)
pyramidal, the last named for their characteristic pyramidal shape.
The granular neurons generally have short axons and, therefore, function mainly
as inte

hort axons and, therefore, function mainly
as interneurons that transmit neural signals only short distances within the cortex
itself. Some are excitatory, releasing mainly the excitatory neurotransmitter gluta-
mate; others are inhibitory and release mainly the inhibitory neurotransmitter
gamma-aminobutyric acid (GABA). The sensory areas of the cortex as well as the
association areas between sensory and motor areas have large concentrations of
these granule cells, suggesting a high degree of in

hese granule cells, suggesting a high degree of intracortical processing of incoming
sensory signals within the sensory areas and association areas.
The pyramidal and fusiform cells give rise to almost all the output ﬁbers from the
cortex. The pyramidal cells are larger and more numerous than the fusiform cells.
They are the source of the long, large nerve ﬁbers that go all the way to the spinal
cord. They also give rise to most of the large subcortical association ﬁber bundles
that pass from on

ortical association ﬁber bundles
that pass from one major part of the brain to another.
To the right in Figure 57–1 is shown the typical organization of nerve ﬁbers within
the different layers of the cerebral cortex. Note particularly the large number of
horizontal ﬁbers that extend between adjacent areas of the cortex, but note also the
vertical ﬁbers that extend to and from the cortex to lower areas of the brain and
some all the way to the spinal cord or to distant regions of the cerebral cort

al cord or to distant regions of the cerebral cortex
through long association bundles.
The functions of the speciﬁc layers of the cerebral cortex are discussed in Chap-
ters 47 and 51. By way of review, let us recall that most incoming speciﬁc sensory
signals from the body terminate in cortical layer IV. Most of the output signals leave
the cortex through neurons located in layers V and VI; the very large ﬁbers to the
brain stem and cord arise generally in layer V; and the tremendous numbers of 

nerally in layer V; and the tremendous numbers of ﬁbers
to the thalamus arise in layer VI. Layers I, II, and III perform most of the intra-
cortical association functions, with especially large numbers of neurons in layers II
and III making short horizontal connections with adjacent cortical areas.
Anatomical and Functional Relations of the Cerebral Cortex to the Thalamus and Other Lower
Centers. All areas of the cerebral cortex have extensive to-and-fro efferent 
and afferent connections with d

-and-fro efferent 
and afferent connections with deeper structures of the brain. It is especially
Chapter 57
Cerebral Cortex, Intellectual Functions of the Brain, Learning and Memory
715
important to emphasize the relation between the cere-
bral cortex and the thalamus. When the thalamus is
damaged along with the cortex, the loss of cerebral
function is far greater than when the cortex alone is
damaged because thalamic excitation of the cortex is
necessary for almost all cortical activity.
Figur

Figure 57–2 shows the areas of the cerebral cortex
that connect with speciﬁc parts of the thalamus. These
connections act in two directions, both from the thala-
mus to the cortex and then from the cortex back to
essentially the same area of the thalamus. Further-
more, when the thalamic connections are cut, the func-
tions of the corresponding cortical area become almost
entirely lost. Therefore, the cortex operates in close
association with the thal

cortex operates in close
association with the thalamus and can almost be con-
sidered both anatomically and functionally a unit with
the thalamus: for this reason, the thalamus and the
cortex together are sometimes called the thalamocor-
tical system. Almost all pathways from the sensory
receptors and sensory organs to the cortex pass
through the thalamus, with the principal exception of
some sensory pathways of olfaction.
Functions of Speciﬁc 
Cortical Areas
Studies in human beings by neurosurg

ortical Areas
Studies in human beings by neurosurgeons, neurolo-
gists, and neuropathologists have shown that different
cerebral cortical areas have separate functions. Figure
57–3 is a map of some of these functions as determined
by Penﬁeld and Rasmussen from electrical stimulation
of the cortex in awake patients or during neurological
examination of patients after portions of the cortex
had been removed.The electrically stimulated patients
told their thoughts evoked by the stimulation, and
som

 their thoughts evoked by the stimulation, and
sometimes they experienced movements. Occasionally
they spontaneously emitted a sound or even a word or
gave some other evidence of the stimulation.
I
VIb
VIa
V
IV
III
II
Figure 57–1
Structure of the cerebral cortex, showing: I, molecular layer; II,
external granular layer; III, layer of pyramidal cells; IV, internal
granular layer;V, large pyramidal cell layer; and VI, layer of fusiform
or polymorphic cells. (Redrawn from Ranson SW, Clark SL [after

ic cells. (Redrawn from Ranson SW, Clark SL [after
Saunders Co, 1959.)
N. dorsalis
medialis
Med. geniculate
body indeterminate
Pulvinar
N. lateralis
posterior
Lat.
body
N. ventralis
posterolateralis
lateralis
Figure 57–2
Areas of the cerebral cortex that connect with speciﬁc portions of
vision
Bilateral
vision
Elaboration
of thought
motor synergies
Hand
skills
Eye
turning

lementary
Hand
skills
Eye
turning
Speech
Speech
Sens. II
Hearing
Voluntary motor
     Speech
Memory patterns
Figure 57–3
Functional areas of the human cerebral cortex as determined by
electrical stimulation of the cortex during neurosurgical operations
and by neurological examinations of patients with destroyed corti-
cal regions. (Redrawn from Penﬁeld W, Rasmussen T: The Cerebral
Cortex of Man: A Clinical Study of Localization of Function. New
York: Hafner Co, 1968

ocalization of Function. New
York: Hafner Co, 1968.)
716
Unit XI
Putting large amounts of information together from
many different sources gives a more general map, as
shown in Figure 57–4. This ﬁgure shows the major
primary and secondary premotor and supplementary
motor areas of the cortex as well as the major primary
and secondary sensory areas for somatic sensation,
vision, and hearing, all of which are discussed in earlier
chapters

ng, all of which are discussed in earlier
chapters. The primary motor areas have direct con-
nections with speciﬁc muscles for causing discrete
muscle movements. The primary sensory areas detect
speciﬁc sensations—visual, auditory, or somatic—
transmitted directly to the brain from peripheral
sensory organs.
The secondary areas make sense out of the signals
in the primary areas. For instance, the supplementary
and premotor areas function along with the primary
motor cortex and basal ganglia to p

th the primary
motor cortex and basal ganglia to provide “patterns”
of motor activity. On the sensory side, the secondary
sensory areas, located within a few centimeters of the
primary areas, begin to analyze the meanings of the
speciﬁc sensory signals, such as (1) interpretation of
the shape or texture of an object in one’s hand; (2)
interpretation of color, light intensity, directions of
lines and angles, and other aspects of vision; and (3)
interpretations of the meanings of sound tones and
s

terpretations of the meanings of sound tones and
sequence of tones in the auditory signals.
Figure 57–4 also shows several large areas of the cere-
bral cortex that do not ﬁt into the rigid categories of
primary and secondary motor and sensory areas.These
areas are called association areas because they receive
and analyze signals simultaneously from multiple
regions of both the motor and sensory cortices as well
as from subcortical structures.Yet even the association
areas have

cal structures.Yet even the association
areas have their specializations. The most important
association areas are (1) the parieto-occipitotemporal
association area, (2) the prefrontal association area,
and (3) the limbic association area. Following are
explanations of the functions of these areas.
Parieto-occipitotemporal Association Area. This association
area lies in the large parietal and occipital cortical
space bounded by the somatosensory cortex anteri-
orly, the visual cortex posteriorly

cortex anteri-
orly, the visual cortex posteriorly, and the auditory
cortex laterally. As would be expected, it provides a
high level of interpretative meaning for signals from
all the surrounding sensory areas. However, even the
parieto-occipitotemporal association area has its own
functional subareas, which are shown in Figure 57–5.
1. Analysis of the Spatial Coordinates of the Body. An
area beginning in the posterior parietal cortex and
extending into the superior occipital cortex provides
co

ing into the superior occipital cortex provides
continuous analysis of the spatial coordinates of all
parts of the body as well as of the surroundings 
of the body. This area receives visual sensory informa-
tion from the posterior occipital cortex and simulta-
neous somatosensory information from the anterior
parietal cortex. From all this information, it computes
the coordinates of the visual, auditory, and body 
surroundings.
2. Area for Language Comprehension. The major area
for language com

age Comprehension. The major area
for language comprehension, called Wernicke’s area,
lies behind the primary auditory cortex in the posterior
part of the superior gyrus of the temporal lobe. We
discuss this area much more fully later; it is the most
important region of the entire brain for higher intel-
lectual function because almost all such intellectual
functions are language based.
3. Area for Initial Processing of Visual Language
(Reading). Posterior to the language comprehension
area, lyi

 Posterior to the language comprehension
area, lying mainly in the anterolateral region of the
occipital lobe, is a visual association area that feeds
visual information conveyed by words read from a
book into Wernicke’s area, the language comprehen-
sion area. This so-called angular gyrus area is needed
to make meaning out of the visually perceived words.
In its absence, a person can still have excellent 
language comprehension through hearing but not
through reading.
4. Area for Naming Objects

ut not
4. Area for Naming Objects. In the most lateral por-
tions of the anterior occipital lobe and posterior tem-
poral lobe is an area for naming objects. The names are
learned mainly through auditory input, whereas the
physical natures of the objects are learned mainly
through visual input. In turn, the names are essential
for both auditory and visual language comprehension
(functions performed in Wernicke’s area located imme-
diately superior to the auditory “names” region 

-
diately superior to the auditory “names” region and
anterior to the visual word processing area).
Prefrontal Association Area. In Chapter 56, we learned
that the prefrontal association area functions in close
association with the motor cortex to plan complex 
Primary
auditory
Area
Parieto-
occipito-
temporal
Area
Secondary
auditory
Limbic
Area
Primary motor
Primary somatic
Secondary somatic
Secondary
visual
Primary
visual

Secondary
visual
Primary
visual
Figure 57–4
Locations of major association areas of the cerebral cortex, as well
as primary and secondary motor and sensory areas.
Chapter 57
717
patterns and sequences of motor movements.To aid in
this function, it receives strong input through a
massive subcortical bundle of nerve ﬁbers connecting
the parieto-occipitotemporal association area with the
prefrontal associat

oral association area with the
prefrontal association area. Through this bundle, the
prefrontal cortex receives much preanalyzed sensory
information, especially information on the spatial
coordinates of the body that is necessary for planning
effective movements. Much of the output from the
prefrontal area into the motor control system passes
through the caudate portion of the basal ganglia-
thalamic feedback circuit for motor planning, which
provides many of the sequential and parallel compo-
n

vides many of the sequential and parallel compo-
nents of movement stimulation.
The prefrontal association area is also essential to
carrying out “thought” processes in the mind.This pre-
sumably results from some of the same capabilities of
the prefrontal cortex that allow it to plan motor activ-
ities. It seems to be capable of processing nonmotor as
well as motor information from widespread areas of
the brain and therefore to achieve nonmotor types of
thinking as well as motor types. In fact,

types of
thinking as well as motor types. In fact, the prefrontal
association area is frequently described simply as
important for elaboration of thoughts, and it is said to
store on a short-term basis “working memories” that
are used to combine new thoughts while they are
entering the brain.
Broca’s Area. A special region in the frontal cortex,
called Broca’s area, provides the neural circuitry for
word formation. This area, shown in Figure 57–5, is
located partly in the posterior lateral prefr

, is
located partly in the posterior lateral prefrontal cortex
and partly in the premotor area. It is here that plans
and motor patterns for expressing individual words or
even short phrases are initiated and executed. This
area also works in close association with Wernicke’s
language comprehension center in the temporal 
association cortex, as we discuss more fully later in the
chapter.
An especially interesting discovery is the following:
When a person has already learned one language and
then

a person has already learned one language and
then learns a new language, the area in the brain where
the new language is stored is slightly removed from the
storage area for the ﬁrst language. If both languages
are learned simultaneously, they are stored together in
the same area of the brain.
Limbic Association Area. Figures 57–4 and 57–5 show still
another association area called the limbic association
area. This area is found in the anterior pole of the tem-
poral lobe, in the ventral portio

pole of the tem-
poral lobe, in the ventral portion of the frontal lobe,
and in the cingulate gyrus lying deep in the longitu-
dinal ﬁssure on the midsurface of each cerebral 
hemisphere. It is concerned primarily with behavior,
emotions, and motivation. We will learn in Chapter 58
that the limbic cortex is part of a much more extensive
system, the limbic system, that includes a complex set
of neuronal structures in the midbasal regions of the
brain. This limbic system provides most of the emo-

ain. This limbic system provides most of the emo-
tional drives for activating other areas of the brain and
even provides motivational drive for the process of
learning itself.
Area for Recognition of Faces
An interesting type of brain abnormality called
prosophenosia is inability to recognize faces. This
occurs in people who have extensive damage on the
medial undersides of both occipital lobes and along
the medioventral surfaces of the temporal lobes, as
shown in Figure 57–6. Loss of these fac

 lobes, as
shown in Figure 57–6. Loss of these face recognition
areas, strangely enough, results in little other abnor-
mality of brain function.
One wonders why so much of the cerebral cortex
should be reserved for the simple task of face recog-
nition. Most of our daily tasks involve associations
Naming of
objects
Vision
Visual
processing
of words
Spatial
coordinates
of body and
surroundings
Somato-
sensory
Word
formation
Word
formation
Broca‘s
Area
Broca‘s
Area
Limbic
Area
Limbic

Area
Broca‘s
Area
Limbic
Area
Limbic
Area
Wernicke‘s
Area
Area
Auditory
Behavior,
emotions,
motivation
Language
comprehension
intelligence
Language
Motor
Planning complex
movements and
elaboration of
thoughts
Figure 57–5
Map of speciﬁc functional areas in
the cerebral cortex, showing espe-
cially Wernicke’s and Broca’s
areas for language comprehension
and speech production, which in 95
per cent of all people are located
in the left h

5
in the left hemisphere.
718
Unit XI
with other people, and one can see the importance of
this intellectual function.
The occipital portion of this facial recognition area
is contiguous with the visual cortex, and the temporal
portion is closely associated with the limbic system
that has to do with emotions, brain activation, and
control of one’s behavioral response to the environ-
ment, as we see in

vioral response to the environ-
ment, as we see in Chapter 58.
Comprehensive Interpretative
Function of the Posterior Superior
Temporal Lobe—“Wernicke’s Area” 
(a General Interpretative Area)
The somatic, visual, and auditory association areas all
meet one another in the posterior part of the superior
temporal lobe, shown in Figure 57–7, where the tem-
poral, parietal, and occipital lobes all come together.
This area of conﬂuence of the different sensory inter-
pretative areas is especially high

 sensory inter-
pretative areas is especially highly developed in the
dominant side of the brain—the left side in almost all
right-handed people—and it plays the greatest single
role of any part of the cerebral cortex for the higher
comprehension levels of brain function that we call
intelligence. Therefore, this region has been called by
different names suggestive of an area that has almost
global importance: the general interpretative area, the
gnostic area, the knowing area, the tertiary asso

gnostic area, the knowing area, the tertiary association
area, and so forth. It is best known as Wernicke’s area
in honor of the neurologist who ﬁrst described its
special signiﬁcance in intellectual processes.
After severe damage in Wernicke’s area, a person
might hear perfectly well and even recognize different
words but still be unable to arrange these words into
a coherent thought. Likewise, the person may be able
to read words from the printed page but be unable to
recognize the thought th

ted page but be unable to
recognize the thought that is conveyed.
Electrical stimulation in Wernicke’s area of a con-
scious person occasionally causes a highly complex
thought. This is particularly true when the stimulation
electrode is passed deep enough into the brain to
approach the corresponding connecting areas of the
thalamus. The types of thoughts that might be experi-
enced include complicated visual scenes that one
might remember from childhood, auditory hallucina-
tions such as a spec

hildhood, auditory hallucina-
tions such as a speciﬁc musical piece, or even a state-
ment made by a speciﬁc person. For this reason, it is
believed that activation of Wernicke’s area can call
forth complicated memory patterns that involve more
than one sensory modality even though most of the
individual memories may be stored elsewhere. This
belief is in accord with the importance of Wernicke’s
area in interpreting the complicated meanings of dif-
ferent patterns of sensory experiences.
Angular

f-
Angular Gyrus—Interpretation of Visual Information. The
angular gyrus is the most inferior portion of the pos-
terior parietal lobe, lying immediately behind Wer-
nicke’s area and fusing posteriorly into the visual areas
of the occipital lobe as well. If this region is destroyed
while Wernicke’s area in the temporal lobe is still
intact, the person can still interpret auditory experi-
ences as usual, but the stream of visual experiences
passing into Wer

 the stream of visual experiences
passing into Wernicke’s area from the visual cortex is
mainly blocked. Therefore, the person may be able to
see words and even know that they are words but not
be able to interpret their meanings. This is the condi-
tion called dyslexia, or word blindness.
Let us again emphasize the global importance of
Wernicke’s area for processing most intellectual 
functions of the brain. Loss of this area in an 
adult usually leads thereafter to a lifetime of almost
demente

y leads thereafter to a lifetime of almost
demented existence.
Frontal
lobe
Temporal
lobe
Facial
recognition area
Figure 57–6
Facial recognition areas located on the underside of the brain in the
medial occipital and temporal lobes. (Redrawn from Geschwind N:
Specializations of the human brain. Sci Am 241:180, 1979. “ 1979 by
Scientiﬁc American, Inc. All rights reserved.)
Primary
auditory
area
Broca's
speech
area
Motor
Primary
Somatic
Somatic
Interpretative
areas
Visual
interpretative

Somatic
areas
Visual
areas
Auditory
areas
Wernicke‘s area
Primary
visual
Figure 57–7
Organization of the somatic auditory and visual association areas
into a general mechanism for interpretation of sensory experience.
All of these feed also into Wernicke’s area, located in the postero-
superior portion of the temporal lobe. Note also the prefrontal area
and Broca’s speech area in the frontal lobe.
Chapter 57
Cerebral Cortex, Intellectual Functions of 

ter 57
719
Concept of the Dominant Hemisphere
The general interpretative functions of Wernicke’s
area and the angular gyrus, as well as the functions 
of the speech and motor control areas, are usually
much more highly developed in one cerebral 
hemisphere than in the other. Therefore, this hemi-
sphere is called the dominant hemisphere. In about 95
per cent of all people, the left hemisphere is the dom-
inant one.
Even at

the left hemisphere is the dom-
inant one.
Even at birth, the area of the cortex that will even-
tually become Wernicke’s area is as much as 50 per
cent larger in the left hemisphere than in the right in
more than one half of neonates. Therefore, it is easy 
to understand why the left side of the brain might
become dominant over the right side. However, if for
some reason this left side area is damaged or removed
in very early childhood, the opposite side of the brain
will usually develop domina

site side of the brain
will usually develop dominant characteristics.
A theory that can explain the capability of one
hemisphere to dominate the other hemisphere is the
The attention of the “mind” seems to be directed 
to one principal thought at a time. Presumably,
because the left posterior temporal lobe at birth is
usually slightly larger than the right, the left side 
normally begins to be used to a greater extent 
than the right. Thereafter, because of the tendency to
direct one’

Thereafter, because of the tendency to
direct one’s attention to the better developed region,
the rate of learning in the cerebral hemisphere that
gains the ﬁrst start increases rapidly, whereas in the
opposite, less-used side, learning remains slight.
Therefore, the left side normally becomes dominant
over the right.
In about 95 per cent of all people, the left temporal
lobe and angular gyrus become dominant, and in the
remaining 5 per cent, either both sides develop simul-
taneously to have du

her both sides develop simul-
taneously to have dual function, or, more rarely, the
right side alone becomes highly developed, with full
dominance.
As discussed later in the chapter, the premotor
speech area (Broca’s area), located far laterally in the
intermediate frontal lobe, is also almost always domi-
nant on the left side of the brain. This speech area is
responsible for formation of words by exciting simul-
taneously the laryngeal muscles, respiratory muscles,
and muscles of the mouth.
Th

 respiratory muscles,
The motor areas for controlling hands are also 
dominant in the left side of the brain in about 9 of 
10 persons, thus causing right-handedness in most
people.
Although the interpretative areas of the temporal
lobe and angular gyrus, as well as many of the motor
areas, are usually highly developed in only the left
hemisphere, these areas receive sensory information
from both hemispheres and are capable also of con-
trolling motor activities in both

ble also of con-
trolling motor activities in both hemispheres. For this
purpose, they use mainly ﬁber pathways in the corpus
callosum for communication between the two hemi-
spheres. This unitary, cross-feeding organization pre-
vents interference between the two sides of the brain;
such interference could create havoc with both mental
thoughts and motor responses.
Role of Language in the Function of
Wernicke’s Area and in Intellectual Functions
A major share of our sensory experience is conver

A major share of our sensory experience is converted
into its language equivalent before being stored in the
memory areas of the brain and before being processed
for other intellectual purposes. For instance, when we
read a book, we do not store the visual images of 
the printed words but instead store the words them-
selves or their conveyed thoughts often in language
form.
The sensory area of the dominant hemisphere for
interpretation of language is Wernicke’s area, and this
is closely associ

age is Wernicke’s area, and this
is closely associated with both the primary and sec-
ondary hearing areas of the temporal lobe. This close
relation probably results from the fact that the ﬁrst
introduction to language is by way of hearing. Later 
in life, when visual perception of language through 
the medium of reading develops, the visual informa-
tion conveyed by written words is then presumably
channeled through the angular gyrus, a visual associa-
tion area, into the already developed Wern

ssocia-
tion area, into the already developed Wernicke’s lan-
guage interpretative area of the dominant temporal
lobe.
Functions of the Parieto-
occipitotemporal Cortex in the
Nondominant Hemisphere
When Wernicke’s area in the dominant hemisphere of
an adult person is destroyed, the person normally loses
almost all intellectual functions associated with lan-
guage or verbal symbolism, such as the ability to read,
the ability to perform mathematical operations, and
even the ability to think throu

al operations, and
even the ability to think through logical problems.
Many other types of interpretative capabilities, some
of which use the temporal lobe and angular gyrus
regions of the opposite hemisphere, are retained.
Psychological studies in patients with damage to the
nondominant hemisphere have suggested that this
hemisphere may be especially important for under-
standing and interpreting music, nonverbal visual
experiences (especially visual patterns), spatial rela-
tions between the p

isual patterns), spatial rela-
tions between the person and their surroundings, the
signiﬁcance of “body language” and intonations of
people’s voices, and probably many somatic experi-
ences related to use of the limbs and hands.Thus, even
though we speak of the “dominant” hemisphere, this
is primarily for language-based intellectual functions;
the so-called nondominant hemisphere might actually
be dominant for some other types of intelligence.
Higher Intellectual Functions of the
Prefrontal Ass

igher Intellectual Functions of the
For years, it has been taught that the prefrontal cortex
is the locus of “higher intellect” in the human being,
principally because the main difference between the
brains of monkeys and of human beings is the great
prominence of the human prefrontal areas.Yet, efforts
720
Unit XI
to show that the prefrontal cortex is more important
in higher intellectual functions than ot

important
in higher intellectual functions than other portions of
the brain have not been successful. Indeed, destruction
of the language comprehension area in the posterior
superior temporal lobe (Wernicke’s area) and the
adjacent angular gyrus region in the dominant hemi-
sphere causes much more harm to the intellect than
does destruction of the prefrontal areas.The prefrontal
areas do, however, have less deﬁnable but nevertheless
important intellectual functions of their own. These
functions 

ellectual functions of their own. These
functions can best be explained by describing what
happens to patients in whom the prefrontal areas have
become nonfunctional, as follows.
Several decades ago, before the advent of modern
drugs for treating psychiatric conditions, it was found
that some patients could receive signiﬁcant relief from
severe psychotic depression by severing the neuronal
connections between the prefrontal areas of the brain
and the remainder of the brain, that is, by a procedu

 the remainder of the brain, that is, by a procedure
called prefrontal lobotomy. This was done by inserting
a blunt, thin-bladed knife through a small opening in
the lateral frontal skull on each side of the head and
slicing the brain at the back edge of the prefrontal
lobes from top to bottom. Subsequent studies in these
patients showed the following mental changes:
1. The patients lost their ability to solve complex
problems.
2. They became unable to string together sequential
tasks to reach c

ble to string together sequential
tasks to reach complex goals.
3. They became unable to learn to do several
parallel tasks at the same time.
4. Their level of aggressiveness was decreased,
sometimes markedly, and, in general, they lost
ambition.
5. Their social responses were often inappropriate
for the occasion, often including loss of morals
and little reticence in relation to sex and
6. The patients could still talk and comprehend
language, but they were unable to carry through
an

any long trains of thought, and their moods
changed rapidly from sweetness to rage to
exhilaration to madness.
7. The patients could also still perform most of the
usual patterns of motor function that they had
performed throughout life, but often without
purpose.
From this information, let us try to piece together a
coherent understanding of the function of the pre-
frontal association areas.
Decreased Aggressiveness and Inappropriate Social Re-
s

ased Aggressiveness and Inappropriate Social Re-
sponses. These two characteristics probably result
from loss of the ventral parts of the frontal lobes on
the underside of the brain. As explained earlier and
shown in Figures 57–4 and 57–5, this area is part of the
limbic association cortex, rather than of the prefrontal
association cortex. This limbic area helps to control
behavior, which is discussed in detail in Chapter 58.
Inability to Progress Toward Goals or to Carry Through Sequen-
tial Th

s Toward Goals or to Carry Through Sequen-
tial Thoughts. We learned earlier in this chapter that the
prefrontal association areas have the capability of
calling forth information from widespread areas of the
brain and using this information to achieve deeper
thought patterns for attaining goals. If these goals
include motor action, so be it. If they do not, then 
the thought processes attain intellectual analytical
goals.
Although people without prefrontal cortices can
still think, they show li

 prefrontal cortices can
still think, they show little concerted thinking in logical
sequence for longer than a few seconds or a minute or
so at most. One of the results is that people without
prefrontal cortices are easily distracted from their
central theme of thought, whereas people with func-
tioning prefrontal cortices can drive themselves to
completion of their thought goals irrespective of 
distractions.
Elaboration of Thought, Prognostication, and Performance of
Higher Intellectual Funct

tion, and Performance of
Higher Intellectual Functions by the Prefrontal Areas—Concept
of a “Working Memory.” Another function that has been
ascribed to the prefrontal areas by psychologists and
neurologists is elaboration of thought. This means
simply an increase in depth and abstractness of the dif-
ferent thoughts put together from multiple sources of
information. Psychological tests have shown that pre-
frontal lobectomized lower animals presented with
successive bits of sensory information 

ented with
successive bits of sensory information fail to keep
track of these bits even in temporary memory, proba-
bly because they are distracted so easily that they
cannot hold thoughts long enough for memory storage
to take place.
This ability of the prefrontal areas to keep track of
many bits of information simultaneously and to cause
recall of this information instantaneously as it is
needed for subsequent thoughts is called the brain’s
“working memory.” This could well explain the many
fu

rking memory.” This could well explain the many
functions of the brain that we associate with higher
intelligence. In fact, studies have shown that the pre-
frontal areas are divided into separate segments for
storing different types of temporary memory, such as
one area for storing shape and form of an object or a
part of the body and another for storing movement.
By combining all these temporary bits of working
memory, we have the abilities to (1) prognosticate; (2)
plan for the future; (3) de

(1) prognosticate; (2)
plan for the future; (3) delay action in response to
incoming sensory signals so that the sensory informa-
tion can be weighed until the best course of response
is decided; (4) consider the consequences of motor
actions before they are performed; (5) solve compli-
cated mathematical, legal, or philosophical problems;
(6) correlate all avenues of information in diagnosing
rare diseases; and (7) control our activities in accord
with moral laws.
Function of the Brain in
Commu

rd
Communication—Language
Input and Language Output
One of the most important differences between human
beings and lower animals is the facility with which
human beings can communicate with one another. Fur-
thermore, because neurological tests can easily assess
Chapter 57
721
the ability of a person to communicate with others, we
know more about the sensory and motor systems
relat

now more about the sensory and motor systems
related to communication than about any other
segment of brain cortex function. Therefore, we will
review, with the help of anatomical maps of neural path-
ways in Figure 57–8, function of the cortex in commu-
nication. From this, one will see immediately how the
principles of sensory analysis and motor control apply
to this art.
There are two aspects to communication: ﬁrst, the
sensory aspect (language input), involving the ears and
eyes, and, second

e input), involving the ears and
eyes, and, second, the motor aspect (language output),
involving vocalization and its control.
Sensory Aspects of Communication. We noted earlier in the
chapter that destruction of portions of the auditory or
visual association areas of the cortex can result in inabil-
ity to understand the spoken word or the written word.
These effects are called, respectively, auditory receptive
aphasia and visual receptive aphasia or, more commonly,
word deafness and word blin

sia or, more commonly,
word deafness and word blindness (also called dyslexia).
Wernicke’s Aphasia and Global Aphasia. Some people
are capable of understanding either the spoken word or
the written word but are unable to interpret the thought
that is expressed. This results most frequently when
Wernicke’s area in the posterior superior temporal gyrus
in the dominant hemisphere is damaged or destroyed.
Therefore, this type of aphasia is called Wernicke’s
aphasia.
When the lesion in Wernicke’s are

nicke’s
aphasia.
When the lesion in Wernicke’s area is widespread and
extends (1) backward into the angular gyrus region, (2)
inferiorly into the lower areas of the temporal lobe, and
(3) superiorly into the superior border of the sylvian
ﬁssure, the person is likely to be almost totally
demented for language understanding or communica-
tion and therefore is said to have global aphasia.
Motor Aspects of Communication. The process of speech
involves two principal stages of mentation: (1) forma-
t

es two principal stages of mentation: (1) forma-
tion in the mind of thoughts to be expressed as well 
as choice of words to be used and then (2) motor 
control of vocalization and the actual act of vocalization
itself.
The formation of thoughts and even most choices of
words are the function of sensory association areas of
the brain. Again, it is Wernicke’s area in the posterior
part of the superior temporal gyrus that is most impor-
tant for this ability.Therefore, a person with either Wer-
ni

is ability.Therefore, a person with either Wer-
nicke’s aphasia or global aphasia is unable to formulate
the thoughts that are to be communicated. Or, if the
lesion is less severe, the person may be able to formu-
late the thoughts but unable to put together appropri-
ate sequences of words to express the thought. The
person sometimes is even ﬂuent with words but the
words are jumbled.
Loss of Broca’s Area Causes Motor Aphasia. Sometimes
a person is capable of deciding what he or she wants to
sa

 is capable of deciding what he or she wants to
say but cannot make the vocal system emit words
instead of noises. This effect, called motor aphasia,
results from damage to Broca’s speech area, which lies
in the prefrontal and premotor facial region of the cere-
bral cortex—about 95 per cent of the time in the left
hemisphere, as shown in Figures 57–5 and 57–8. There-
fore, the skilled motor patterns for control of the larynx,
lips, mouth, respiratory system, and other accessory
muscles of speec

atory system, and other accessory
muscles of speech are all initiated from this area.
Articulation. Finally, we have the act of articulation,
which means the muscular movements of the mouth,
tongue, larynx, vocal cords, and so forth that are respon-
sible for the intonations, timing, and rapid changes in
intensities of the sequential sounds.The facial and laryn-
geal regions of the motor cortex activate these muscles,
and the cerebellum, basal ganglia, and sensory cortex all
help to control the 

nglia, and sensory cortex all
help to control the sequences and intensities of muscle
contractions, making liberal use of basal ganglial and
cerebellar feedback mechanisms described in Chapters
55 and 56. Destruction of any of these regions can cause
either total or partial inability to speak distinctly.
Summary. Figure 57–8 shows two principal pathways for
communication. The upper half of the ﬁgure shows the
pathway involved in hearing and speaking. This
sequence is the following: (1) reception

ing. This
sequence is the following: (1) reception in the primary
Arcuate fasciculus
Broca’s area
Wernicke’s area
Primary auditory area
SPEAKING A HEARD WORD
Angular gyrus
Primary
visual area
SPEAKING A WRITTEN WORD
Figure 57–8
Brain pathways for (top) perceiving a heard word and then speak-
ing the same word, and (bottom) perceiving a written word and then
speaking the same word. (Redrawn from Geschwind N: Specializa-
tions of the human bra

om Geschwind N: Specializa-
tions of the human brain. Sci Am 241:180, 1979. “ 1979 by Scientiﬁc
American, Inc. All rights reserved.)
722
Unit XI
auditory area of the sound signals that encode the
words; (2) interpretation of the words in Wernicke’s
area; (3) determination, also in Wernicke’s area, of the
thoughts and the words to be spoken; (4) transmission
of signals from Wernicke’s area to Broca’s area by way
of the arcuate fasciculu

ea to Broca’s area by way
of the arcuate fasciculus; (5) activation of the skilled
motor programs in Broca’s area for control of word for-
mation; and (6) transmission of appropriate signals into
the motor cortex to control the speech muscles.
The lower ﬁgure illustrates the comparable steps in
reading and then speaking in response.The initial recep-
tive area for the words is in the primary visual area
rather than in the primary auditory area.Then the infor-
mation passes through early stages o

en the infor-
mation passes through early stages of interpretation in
the angular gyrus region and ﬁnally reaches its full level
of recognition in Wernicke’s area. From here, the
sequence is the same as for speaking in response to the
spoken word.
Function of the Corpus
Callosum and Anterior
Commissure to Transfer
Thoughts, Memories, Training,
and Other Information
Between the Two Cerebral
Hemispheres
Fibers in the corpus callosum provide abundant bi-
directional neural connections between most 

t bi-
directional neural connections between most of the
respective cortical areas of the two cerebral hemi-
spheres except for the anterior portions of the tempo-
ral lobes; these temporal areas, including especially 
the amygdala, are interconnected by ﬁbers that pass
through the anterior commissure.
Because of the tremendous number of ﬁbers in the
corpus callosum, it was assumed from the beginning
that this massive structure must have some important
function to correlate activities of the two

ortant
function to correlate activities of the two cerebral
hemispheres. However, when the corpus callosum was
destroyed in laboratory animals, it was at ﬁrst difﬁcult
to discern deﬁcits in brain function. Therefore, for a
long time, the function of the corpus callosum was a
mystery.
Properly designed psychological experiments have
now demonstrated extremely important functions for
the corpus callosum and anterior commissure. These
functions can best be explained by describing one of
the experim

best be explained by describing one of
the experiments:A monkey is ﬁrst prepared by cutting
the corpus callosum and splitting the optic chiasm lon-
gitudinally, so that signals from each eye can go only
to the cerebral hemisphere on the side of the eye.Then
the monkey is taught to recognize different objects
with its right eye while its left eye is covered. Next,
the right eye is covered and the monkey is tested 
to determine whether its left eye can recognize the 
same objects. The answer to th

 can recognize the 
same objects. The answer to this is that the left eye
cannot recognize the objects. However, on repeating
the same experiment in another monkey with the optic
chiasm split but the corpus callosum intact, it is found
invariably that recognition in one hemisphere of the
brain creates recognition in the opposite hemisphere.
Thus, one of the functions of the corpus callosum
and the anterior commissure is to make information
stored in the cortex of one hemisphere available to
corr

 in the cortex of one hemisphere available to
corresponding cortical areas of the opposite hemi-
sphere. Important examples of such cooperation
between the two hemispheres are the following.
1. Cutting the corpus callosum blocks transfer 
of information from Wernicke’s area of the
dominant hemisphere to the motor cortex on 
the opposite side of the brain. Therefore, the
intellectual functions of Wernicke’s area, located
in the left hemisphere, lose control over the right
motor cortex that initia

se control over the right
motor cortex that initiates voluntary motor
functions of the left hand and arm, even though
the usual subconscious movements of the left
hand and arm are normal.
2. Cutting the corpus callosum prevents transfer of
somatic and visual information from the right
hemisphere into Wernicke’s area in the left
dominant hemisphere. Therefore, somatic and
visual information from the left side of the body
frequently fails to reach this general interpretative
area of the brain and 

this general interpretative
area of the brain and therefore cannot be used for
decision making.
3. Finally, people whose corpus callosum is
completely sectioned have two entirely separate
conscious portions of the brain. For example, in a
teenage boy with a sectioned corpus callosum,
only the left half of his brain could understand
both the written word and the spoken word
because the left side was the dominant
hemisphere. Conversely, the right side of the brain
could understand the written word

ide of the brain
could understand the written word but not the
spoken word. Furthermore, the right cortex could
elicit a motor action response to the written word
without the left cortex ever knowing why the
response was performed.
The effect was quite different when an emotional
response was evoked in the right side of the brain:
In this case, a subconscious emotional response
occurred in the left side of the brain as well. This
undoubtedly occurred because the areas of the 
two sides of the br

rred because the areas of the 
two sides of the brain for emotions, the anterior
temporal cortices and adjacent areas, were still
communicating with each other through the
anterior commissure that was not sectioned. For
instance, when the command “kiss” was written 
for the right half of his brain to see, the boy
immediately and with full emotion said, “No way!”
This response required function of Wernicke’s 
area and the motor areas for speech in the left
hemisphere because these left-side areas

 the left
hemisphere because these left-side areas were
necessary to speak the words “No way!” But when
questioned why he said this, the boy could not
explain. Thus, the two halves of the brain have
independent capabilities for consciousness, memory
storage, communication, and control of motor
activities. The corpus callosum is required for the
two sides to operate cooperatively at the superﬁcial
subconscious level, and the anterior commissure
plays an important additional role in unifying the
e

ays an important additional role in unifying the
emotional responses of the two sides of the brain.
Chapter 57
723
Thoughts, Consciousness, 
and Memory
Our most difﬁcult problem in discussing conscious-
ness, thoughts, memory, and learning is that we do not
know the neural mechanisms of a thought and we
know little about the mechanisms of memory. We
know that destruction of large portions of the cerebral
cortex does not pr

 large portions of the cerebral
cortex does not prevent a person from having
thoughts, but it does reduce the depth of the thoughts
and also the degree of awareness of the surroundings.
Each thought certainly involves simultaneous
signals in many portions of the cerebral cortex, thala-
mus, limbic system, and reticular formation of the
brain stem. Some crude thoughts probably depend
almost entirely on lower centers; the thought of pain
is probably a good example because electrical stimu-
lation 

y a good example because electrical stimu-
lation of the human cortex seldom elicits anything
more than mild pain, whereas stimulation of certain
areas of the hypothalamus, amygdala, and mesen-
cephalon can cause excruciating pain. Conversely, a
type of thought pattern that does require large
involvement of the cerebral cortex is that of vision,
because loss of the visual cortex causes complete
inability to perceive visual form or color.
We might formulate a provisional deﬁnition of a
thought in

 formulate a provisional deﬁnition of a
thought in terms of neural activity as follows: A
thought results from a “pattern” of stimulation of
many parts of the nervous system at the same time,
probably involving most importantly the cerebral
cortex, thalamus, limbic system, and upper reticular
formation of the brain stem. This is called the holistic
theory of thoughts. The stimulated areas of the limbic
system, thalamus, and reticular formation are believed
to determine the general nature of the 

e believed
to determine the general nature of the thought, giving
it such qualities as pleasure, displeasure, pain, comfort,
crude modalities of sensation, localization to gross
areas of the body, and other general characteristics.
However, speciﬁc stimulated areas of the cerebral
cortex determine discrete characteristics of the
thought, such as (1) speciﬁc localization of sensations
on the surface of the body and of objects in the ﬁelds
of vision, (2) the feeling of the texture of silk, (3) vis

n, (2) the feeling of the texture of silk, (3) visual
recognition of the rectangular pattern of a concrete
block wall, and (4) other individual characteristics that
enter into one’s overall awareness of a particular
instant. Consciousness can perhaps be described as our
continuing stream of awareness of either our sur-
roundings or our sequential thoughts.
Memory—Roles of Synaptic
Facilitation and Synaptic Inhibition
Physiologically, memories are stored in the brain by
changing the basic sensiti

 stored in the brain by
changing the basic sensitivity of synaptic transmission
between neurons as a result of previous neural activ-
ity.The new or facilitated pathways are called memory
traces. They are important because once the traces are
established, they can be selectively activated by the
thinking mind to reproduce the memories.
Experiments in lower animals have demonstrated
that memory traces can occur at all levels of the
nervous system. Even spinal cord reﬂexes can change
at least slig

 Even spinal cord reﬂexes can change
at least slightly in response to repetitive cord activa-
tion, and these reﬂex changes are part of the memory
process.
Also,
long-term memories result from
changed synaptic conduction in lower brain centers.
However, most memory that we associate with intel-
lectual processes is based on memory traces in the
cerebral cortex.
Positive and Negative Memory—“Sensitization” or “Habitua-
tion” of Synaptic Transmission. Although we often think
of memories as being p

on. Although we often think
of memories as being positive recollections of previous
thoughts or experiences, probably the greater share 
of our memories are negative memories, not positive.
That is, our brain is inundated with sensory informa-
tion from all our senses. If our minds attempted 
to remember all this information, the memory capacity
of the brain would be exceeded within minutes. Fortu-
nately, the brain has the capability to learn to ignore
information that is of no consequence.This

 ignore
information that is of no consequence.This results from
inhibition of the synaptic pathways for this type of
information; the resulting effect is called habituation.
This is a type of negative memory.
Conversely, for incoming information that causes
important consequences such as pain or pleasure, the
brain has a different automatic capability of enhanc-
ing and storing the memory traces. This is positive
memory. It results from facilitation of the synaptic
pathways, and the process is c

ion of the synaptic
pathways, and the process is called memory sensitiza-
tion. We will learn later that special areas in the basal
limbic regions of the brain determine whether infor-
mation is important or unimportant and make the sub-
conscious decision whether to store the thought as a
sensitized memory trace or to suppress it.
Classiﬁcation of Memories. We know that some memo-
ries last for only a few seconds, whereas others last for
hours, days, months, or years. For the purpose of dis-
cu

days, months, or years. For the purpose of dis-
cussing these, let us use a common classiﬁcation of
memories that divides memories into (1) short-term
memory, which includes memories that last for
seconds or at most minutes unless they are converted
into longer-term memories; (2) intermediate long-term
memories, which last for days to weeks but then fade
away; and (3) long-term memory, which, once stored,
can be recalled up to years or even a lifetime later.
In addition to this general classiﬁca

etime later.
In addition to this general classiﬁcation of memo-
ries, we also discussed earlier (in connection with the
prefrontal lobes) another type of memory, called
“working memory,” that includes mainly short-term
memory that is used during the course of intellectual
reasoning but is terminated as each stage of the
problem is resolved.
Memories are frequently classiﬁed according to the
type of information that is stored. One of these classi-
ﬁcations divides memory into declarative memory a

ﬁcations divides memory into declarative memory and
skill memory, as follows:
1. Declarative memory basically means memory of
the various details of an integrated thought, such
as memory of an important experience that
724
Unit XI
includes (1) memory of the surroundings, (2)
memory of time relationships, (3) memory of
causes of the experience, (4) memory of the
meaning of the experience, and (5) memory of
one’s deductions that were le

e, and (5) memory of
one’s deductions that were left in the person’s
mind.
2. Skill memory is frequently associated with motor
activities of the person’s body, such as all the skills
developed for hitting a tennis ball, including
automatic memories to (1) sight the ball, (2)
calculate the relationship and speed of the ball to
the racquet, and (3) deduce rapidly the motions of
the body, the arms, and the racquet required to hit
the ball as desired—all of these activated instantly
based on previou

—all of these activated instantly
based on previous learning of the game of
tennis—then moving on to the next stroke of the
game while forgetting the details of the previous
stroke.
Short-term memory is typiﬁed by one’s memory of 7
to 10 numerals in a telephone number (or 7 to 10 other
discrete facts) for a few seconds to a few minutes at a
time but lasting only as long as the person continues
to think about the numbers or facts.
Many physiologists have suggested that this shor

.
Many physiologists have suggested that this short-
term memory is caused by continual neural activity
resulting from nerve signals that travel around and
around a temporary memory trace in a circuit of rever-
berating neurons. It has not yet been possible to prove
this theory. Another possible explanation of short-
term memory is presynaptic facilitation or inhibition.
This occurs at synapses that lie on terminal nerve
ﬁbrils immediately before these ﬁbrils synapse with a
subsequent neuron. Th

 these ﬁbrils synapse with a
subsequent neuron. The neurotransmitter chemicals
secreted at such terminals frequently cause facilitation
or inhibition lasting for seconds up to several minutes.
Circuits of this type could lead to short-term memory.
Intermediate long-term memories may last for many
minutes or even weeks. They will eventually be lost
unless the memory traces are activated enough to
become more permanent; then they are classiﬁed as
long-term memories. E

; then they are classiﬁed as
long-term memories. Experiments in primitive animals
have demonstrated that memories of the intermediate
long-term type can result from temporary chemical or
physical changes, or both, in either the synapse presy-
naptic terminals or the synapse postsynaptic mem-
brane, changes that can persist for a few minutes up to
several weeks. These mechanisms are so important
that they deserve special description.
Memory Based on Chemical Changes in 
the Presynaptic Terminal o

on Chemical Changes in 
the Presynaptic Terminal or Postsynaptic
Neuronal Membrane
Figure 57–9 shows a mechanism of memory studied
especially by Kandel and his colleagues that can cause
memories lasting from a few minutes up to 3 weeks in
the large snail Aplysia. In this ﬁgure, there are two
synaptic terminals. One terminal is from a sensory
input neuron and terminates directly on the surface of
the neuron that is to be stimulated; this is called the
sensory terminal. The other terminal is a pre

 the
sensory terminal. The other terminal is a presynaptic
ending that lies on the surface of the sensory terminal,
and it is called the facilitator terminal. When the
sensory terminal is stimulated repeatedly but without
stimulation of the facilitator terminal, signal transmis-
sion at ﬁrst is great, but it becomes less and less intense
with repeated stimulation until transmission almost
ceases. This phenomenon is habituation, as was
explained previously. It is a type of negative memory
that ca

reviously. It is a type of negative memory
that causes the neuronal circuit to lose its response to
repeated events that are insigniﬁcant.
Conversely, if a noxious stimulus excites the facili-
tator terminal at the same time that the sensory ter-
minal is stimulated, then instead of the transmitted
signal into the postsynaptic neuron becoming pro-
gressively weaker, the ease of transmission becomes
stronger and stronger; and it will remain strong for
minutes, hours, days, or, with more intense t

 for
minutes, hours, days, or, with more intense training, up
to about 3 weeks even without further stimulation of
the facilitator terminal. Thus, the noxious stimulus
causes the memory pathway through the sensory 
terminal to become facilitated for days or weeks 
thereafter. It is especially interesting that even after
habituation has occurred, this pathway can be con-
verted back to a facilitated pathway with only a few
noxious stimuli.
Molecular Mechanism of Intermediate Memory
Mechanism for 

ar Mechanism of Intermediate Memory
Mechanism for Habituation. At the molecular level,
the habituation effect in the sensory terminal results
from progressive closure of calcium channels through
the terminal membrane, though the cause of this
calcium channel closure is not fully known. Neverthe-
less, much smaller than normal amounts of calcium
ions can diffuse into the habituated terminal, and
much less sensory terminal transmitter is therefore
released because calcium entry is the principal st

released because calcium entry is the principal stim-
ulus for transmitter release (as was discussed in
Chapter 45).
Facilitator
terminal
terminal
Serotonin
cAMP
cAMP
Noxious
stimulus
Sensory
stimulus
Sensory
terminal
Sensory
terminal
Calcium
channels
Calcium
ions
Figure 57–9
Memory system that has been discovered in the snail Aplysia.
Chapter 57
725
Mechanism for Facilitation. In the case of facilitation,
at l

or Facilitation. In the case of facilitation,
at least part of the molecular mechanism is believed
to be the following:
1. Stimulation of the facilitator presynaptic terminal
at the same time that the sensory terminal is
stimulated causes serotonin release at the
facilitator synapse on the surface of the sensory
terminal.
2. The serotonin acts on serotonin receptors in the
sensory terminal membrane, and these receptors
activate the enzyme adenyl cyclase inside the
membrane. And, ﬁnally, the aden

cyclase inside the
membrane. And, ﬁnally, the adenyl cyclase causes
formation of cyclic adenosine monophosphate
(cAMP) also inside the sensory presynaptic
terminal.
3. The cyclic AMP activates a protein kinase that
causes phosphorylation of a protein that itself is
part of the potassium channels in the sensory
synaptic terminal membrane; this in turn blocks
the channels for potassium conductance. The
blockage can last for minutes up to several 
weeks.
4. Lack of potassium conductance causes a gr

eeks.
4. Lack of potassium conductance causes a greatly
prolonged action potential in the synaptic
terminal because ﬂow of potassium ions out of the
terminal is necessary for rapid recovery from the
5. The prolonged action potential causes prolonged
activation of the calcium channels, allowing
tremendous quantities of calcium ions to enter the
sensory synaptic terminal. These calcium ions
cause greatly increased transmitter release by the
synapse, thereby markedly facilitating 

ase by the
synapse, thereby markedly facilitating synaptic
transmission to the subsequent neuron.
Thus, in a very indirect way, the associative effect of
stimulating the facilitator terminal at the same time
that the sensory terminal is stimulated causes pro-
longed increase in excitatory sensitivity of the sensory
terminal,and this establishes the memory trace.Studies
by Byrne and colleagues, also in the snail Aplysia, have
suggested still another mechanism of synaptic memory.
Their studies hav

er mechanism of synaptic memory.
Their studies have shown that stimuli from separate
sources acting on a single neuron, under appropriate
conditions, can cause long-term changes in membrane
properties of the post-synaptic neuron instead of in the
presynaptic neuronal membrane, but leading to essen-
tially the same memory effects.
There is no obvious demarcation between the more
prolonged types of intermediate long-term memory
and true long-term memory. The distinction is one of

 true long-term memory. The distinction is one of
degree. However, long-term memory is generally
believed to result from actual structural changes,
instead of only chemical changes, at the synapses, and
these enhance or suppress signal conduction. Again,
let us recall experiments in primitive animals (where
the nervous systems are much easier to study) that
have aided immensely in understanding possible
mechanisms of long-term memory.
Structural Changes Occur in Synapses During
the Development o

Changes Occur in Synapses During
the Development of Long-Term Memory
Electron microscopic pictures taken from invertebrate
animals have demonstrated multiple physical struc-
tural changes in many synapses during development of
long-term memory traces. The structural changes will
not occur if a drug is given that blocks DNA stimula-
tion of protein replication in the presynaptic neuron;
nor will the permanent memory trace develop. There-
fore, it appears that development of true long-term
memory 

appears that development of true long-term
memory depends on physically restructuring the
synapses themselves in a way that changes their sensi-
tivity for transmitting nervous signals.
The most important of the physical structural
changes that occur are the following:
1. Increase in vesicle release sites for secretion of
transmitter substance.
2. Increase in number of transmitter vesicles
released.
3. Increase in number of presynaptic terminals.
4. Changes in structures of the dendritic spines 

4. Changes in structures of the dendritic spines that
permit transmission of stronger signals.
Thus, in several different ways, the structural capa-
bility of synapses to transmit signals appears to
increase during establishment of true long-term
memory traces.
Number of Neurons and Their Connectivities
Often Change Signiﬁcantly During Learning
During the ﬁrst few weeks, months, and perhaps even
year or so of life, many parts of the brain produce a
great excess of neurons, and the neurons send 

e a
great excess of neurons, and the neurons send out
numerous axon branches to make connections with
other neurons. If the new axons fail to connect with
appropriate subsequent neurons, muscle cells, or gland
cells, the new axons themselves will dissolute within a
few weeks. Thus, the number of neuronal connections
is determined by speciﬁc nerve growth factors released
retrogradely from the stimulated cells. Furthermore,
when insufﬁcient connectivity occurs, the entire
neuron that is sending ou

ivity occurs, the entire
neuron that is sending out the axon branches might
eventually disappear.
Therefore, soon after birth, there is a principle of  
“use it or lose it” that governs the ﬁnal number of
neurons and their connectivities in respective parts of
the human nervous system. This is a type of learning.
For example, if one eye of a newborn animal is covered
for many weeks after birth, neurons in alternate stripes
of the cerebral visual cortex—neurons normally con-
nected to the covered

cortex—neurons normally con-
nected to the covered eye—will degenerate, and the
covered eye will remain either partially or totally blind
for the remainder of life. Until recently, it was believed
that very little “learning” is achieved in adult human
beings and animals by modiﬁcation of numbers of
neurons in the memory circuits; however, recent
research suggests that even adults use this mechanism
to at least some extent.
For short-term memory to be converted into long-t

For short-term memory to be converted into long-term
memory that can be recalled weeks or years later, it
726
Unit XI
must become “consolidated.” That is, the short-term
memory if activated repeatedly will initiate chemical,
physical, and anatomical changes in the synapses that
are responsible for the long-term type of memory.This
process requires 5 to 10 minutes for minimal consoli-
dation and 1 hour or more for strong consolidation.

ation and 1 hour or more for strong consolidation.
For instance, if a strong sensory impression is made on
the brain but is then followed within a minute or so by
an electrically induced brain convulsion, the sensory
experience will not be remembered. Likewise, brain
concussion,
sudden application of deep general 
anesthesia, or any other effect that temporarily 
blocks the dynamic function of the brain can prevent
consolidation.
Consolidation and the time required for it to occur
can probably b

d the time required for it to occur
can probably be explained by the phenomenon of
rehearsal of the short-term memory as follows.
Rehearsal Enhances the Transference of Short-Term Memory
into Long-Term Memory. Psychological studies have
shown that rehearsal of the same information again
and again in the mind accelerates and potentiates the
degree of transfer of short-term memory into long-
term memory and therefore accelerates and enhances
consolidation. The brain has a natural tendency to
rehea

idation. The brain has a natural tendency to
rehearse newfound information, especially newfound
information that catches the mind’s attention. There-
fore, over a period of time, the important features of
sensory experiences become progressively more and
more ﬁxed in the memory stores. This explains why a
person can remember small amounts of information
studied in depth far better than large amounts of infor-
mation studied only superﬁcially. It also explains why
a person who is wide awake can c

also explains why
a person who is wide awake can consolidate memories
far better than a person who is in a state of mental
fatigue.
New Memories Are Codiﬁed During Consolidation. One of the
most important features of consolidation is that new
memories are codiﬁed into different classes of infor-
mation. During this process, similar types of informa-
tion are pulled from the memory storage bins and used
to help process the new information. The new and old
are compared for similarities and differe

 and old
are compared for similarities and differences, and part
of the storage process is to store the information about
these similarities and differences, rather than to store
the new information unprocessed. Thus, during con-
solidation, the new memories are not stored randomly
in the brain but are stored in direct association with
other memories of the same type. This is necessary if
one is to be able to “search” the memory store at a
later date to ﬁnd the required information.
Role of Spec

date to ﬁnd the required information.
Role of Speciﬁc Parts of the Brain in the
Memory Process
Hippocampus Promotes Storage of Memories—Anterograde
Amnesia After Hippocampal Lesions. The hippocampus is
the most medial portion of the temporal lobe cortex,
where it folds ﬁrst medially underneath the brain and
then upward into the lower, inside surface of the
lateral ventricle. The two hippocampi have been
removed for the treatment of epilepsy in a few
patients. This procedure does not seriously af

few
patients. This procedure does not seriously affect the
person’s memory for information stored in the brain
before removal of the hippocampi. However, after
removal, these people have virtually no capability
thereafter for storing verbal and symbolic types of
memories (declarative types of memory) in long-term
memory, or even in intermediate memory lasting
longer than a few minutes.Therefore, these people are
unable to establish new long-term memories of those
types of information that are th

memories of those
types of information that are the basis of intelligence.
This is called anterograde amnesia.
But why are the hippocampi so important in helping
the brain to store new memories? The probable
answer is that the hippocampi are among the most
important output pathways from the “reward” and
“punishment” areas of the limbic system, as explained
in Chapter 58. Sensory stimuli or thoughts that cause
pain or aversion excite the limbic punishment centers,
and stimuli that cause pleasure,

nishment centers,
and stimuli that cause pleasure, happiness, or sense of
reward excite the limbic reward centers. All these
together provide the background mood and motiva-
tions of the person. Among these motivations is the
drive in the brain to remember those experiences and
thoughts that are either pleasant or unpleasant. The
hippocampi especially and to a lesser degree the dorsal
medial nuclei of the thalamus, another limbic struc-
ture, have proved especially important in making the
decisi

e proved especially important in making the
decision about which of our thoughts are important
enough on a basis of reward or punishment to be
worthy of memory.
Retrograde Amnesia—Inability to Recall Memories from the
Past. When retrograde amnesia occurs, the degree of
amnesia for recent events is likely to be much greater
than for events of the distant past. The reason for this
difference is probably that the distant memories have
been rehearsed so many times that the memory traces
are deeply e

 so many times that the memory traces
are deeply engrained, and elements of these memories
are stored in widespread areas of the brain.
In some people who have hippocampal lesions,
some degree of retrograde amnesia occurs along with
anterograde amnesia, which suggests that these two
types of amnesia are at least partially related and 
that hippocampal lesions can cause both. However,
damage in some thalamic areas may lead speciﬁcally
to retrograde amnesia without causing signiﬁcant
anterograde a

e amnesia without causing signiﬁcant
anterograde amnesia. A possible explanation of this is
that the thalamus may play a role in helping the person
“search” the memory storehouses and thus “read out”
the memories. That is, the memory process not only
requires the storing of memories but also an ability to
search and ﬁnd the memory at a later date. The possi-
ble function of the thalamus in this process is discussed
further in Chapter 58.
Hippocampi Are Not Important in Reﬂexive Learning. People

pi Are Not Important in Reﬂexive Learning. People
with hippocampal lesions usually do not have difﬁculty
in learning physical skills that do not involve verbal-
ization or symbolic types of intelligence. For instance,
these people can still learn the rapid hand and 
physical skills required in many types of sports. This
Chapter 57
727
type of learning is called skill learning or reﬂexive
learning; it depends on physically 

ng or reﬂexive
learning; it depends on physically repeating the
required tasks over and over again, rather than on
symbolical rehearsing in the mind.
Baddeley A: Working memory: looking back and looking
forward. Nat Rev Neurosci 4:829, 2003.
Blank T, Nijholt I, Spiess J: Molecular determinants mediat-
ing effects of acute stress on hippocampus-dependent
synaptic plasticity and learning. Mol Neurobiol 29:131,
2004.
Bookheimer S:
Functional MRI of language:
new
approaches to understandi

nal MRI of language:
new
approaches to understanding the cortical organization of
semantic processing. Annu Rev Neurosci 25:151, 2002.
Conlon R, Hobson JA: Understanding the Human Mind.
New York: John Wiley, 1999.
Dash PK, Hebert AE, Runyan JD: A uniﬁed theory for
systems and cellular memory consolidation. Brain Res
Brain Res Rev 45:30, 2004.
Dick P, Katsuyuki S: The prefrontal cortex and working
memory: physiology and brain imaging. Curr Opin Neu-
robiol 14:163, 2004.
Dudai Y: The neurobiology 

u-
Dudai Y: The neurobiology of consolidations, or, how stable
is the engram? Annu Rev Psychol 55:51, 2004.
Ehrlich YM: Molecular and Cellular Mechanisms of Neu-
ronal Plasticity. New York: Plenum Press, 1998.
Guillery RW: Branching thalamic afferents link action and
perception. J Neurophysiol 90:539, 2003.
Haines DE:
New York:
Hamann S, Canli T: Individual differences in emotion pro-
cessing. Curr Opin Neurobiol 14:233,

 emotion pro-
cessing. Curr Opin Neurobiol 14:233, 2004.
Kandel ER: The molecular biology of memory storage: a 
dialogue between genes and synapses. Science 294:1030,
2001.
LeDoux JE: Emotion, memory and the brain: the neural
routes underlying the formation of memories about prim-
itive emotional experiences, such as fear, have been
traced. Sci Am June: 50, 1994.
Leuner B, Shors TJ: New spines,

Am June: 50, 1994.
Leuner B, Shors TJ: New spines, new memories. Mol Neuro-
biol 29:117, 2004.
Lynch MA: Long-term potentiation and memory. Physiol
Rev 84:87, 2004.
Miller BL, Cummings JL: The Human Frontal Lobes. New
York: Guilford Press, 1998.
Nakazawa K, McHugh TJ, Wilson MA, Tonegawa S: NMDA
receptors, place cells and hippocampal spatial memory.
Nat Rev Neurosci 5:361, 2004.
Phelps EA: Human emotion and memory: interactions of the
amygdala and hippocampal complex. Curr Opin Neuro-
biol 14:19

d hippocampal complex. Curr Opin Neuro-
biol 14:198, 2004.
Sah P, Faber ES, Lopez De Armentia M, Power J:The amyg-
daloid complex: anatomy and physiology. Physiol Rev
83:803, 2003.
Shors TJ: Memory traces of trace memories: neurogenesis,
synaptogenesis and awareness. Trends Neurosci 27:250,
2004.
Wixted JT: The psychology and neuroscience of forgetting.
Annu Rev Psychol 55:235, 2004.
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728
Behavioral and Motivational
Mechanisms of the Brain—The
Limbic System and the
Hypothalamu

of the Brain—The
Control of behavior is a function of the entire
nervous system. Even the wakefulness and sleep
cycle discussed in Chapter 59 is one of our most
important behavioral patterns.
In this chapter, we deal ﬁrst with those mecha-
nisms that control levels of activity in the different
parts of the brain. Then we discuss the causes of
motivational drives, especially motivational control
of the learning process and feelings of pleasure and punishment. Th

rocess and feelings of pleasure and punishment. These func-
tions of the nervous system are performed mainly by the basal regions of the
brain, which together are loosely called the limbic system, meaning the “border”
system.
Without continuous transmission of nerve signals from the lower brain into the
cerebrum, the cerebrum becomes useless. In fact, severe compression of the
brain stem at the juncture between the mesencephalon and cerebrum, as some-
time

ween the mesencephalon and cerebrum, as some-
times results from a pineal tumor, often causes the person to go into unremit-
ting coma lasting for the remainder of his or her life.
Nerve signals in the brain stem activate the cerebral part of the brain in two
ways: (1) by directly stimulating a background level of neuronal activity in wide
areas of the brain and (2) by activating neurohormonal systems that release 
speciﬁc facilitory or inhibitory hormone-like neurotransmitter substances into
se

y hormone-like neurotransmitter substances into
selected areas of the brain.
Control of Cerebral Activity by Continuous Excitatory
Signals from the Brain Stem
Reticular Excitatory Area of the Brain Stem
Figure 58–1 shows a general system for controlling the level of activity of the
brain.The central driving component of this system is an excitatory area located
in the reticular substance of the pons and mesencephalon.This area is also known
by the name bulboreticular facilitory area. We also dis

e name bulboreticular facilitory area. We also discuss this area in Chapter
55 because it is the same brain stem reticular area that transmits facilitory
signals downward to the spinal cord to maintain tone in the antigravity muscles
and to control levels of activity of the spinal cord reﬂexes. In addition to these
downward signals,this area also sends a profusion of signals in the upward direc-
tion. Most of these go ﬁrst to the thalamus, where they excite a different set of
neurons that transm

they excite a different set of
neurons that transmit nerve signals to all regions of the cerebral cortex as well
as to multiple subcortical areas.
The signals passing through the thalamus are of two types. One type is rapidly
transmitted action potentials that excite the cerebrum for only a few millisec-
onds. These originate from large neuronal cell bodies that lie throughout the
Chapter 58
Behavioral and Motivational Mechanisms of the Brain—The Limbic System and the Hypothalamus
729
brain stem

 Limbic System and the Hypothalamus
729
brain stem reticular area. Their nerve endings release
the neurotransmitter substance acetylcholine, which
serves as an excitatory agent, lasting for only a few mil-
liseconds before it is destroyed.
The second type of excitatory signal originates from
large numbers of small neurons spread throughout the
brain stem reticular excitatory area. Again, most of
these pass to the thalamus, but this time through small,
slowly conducting ﬁbers that synapse mainly 

mall,
slowly conducting ﬁbers that synapse mainly in the
intralaminar nuclei of the thalamus and in the reticu-
lar nuclei over the surface of the thalamus. From here,
additional small ﬁbers are distributed everywhere in
the cerebral cortex. The excitatory effect caused by
this system of ﬁbers can build up progressively for
many seconds to a minute or more, which suggests that
its signals are especially important for controlling
longer-term background excitability level of the brain.
Excitation 

round excitability level of the brain.
Excitation of the Excitatory Area by Peripheral Sensory Signals.
The level of activity of the excitatory area in the brain
stem, and therefore the level of activity of the entire
brain, is determined to a great extent by the number
and type of sensory signals that enter the brain from
the periphery. Pain signals in particular increase activ-
ity in this excitatory area and therefore strongly excite
the brain to attention.
The importance of sensory signals i

 to attention.
The importance of sensory signals in activating the
excitatory area is demonstrated by the effect of cutting
the brain stem above the point where the ﬁfth cerebral
nerves enter the pons. These nerves are the highest
nerves entering the brain that transmit signiﬁcant
numbers of somatosensory signals into the brain.
When all these input sensory signals are gone, the level
of activity in the brain excitatory area diminishes
abruptly, and the brain proceeds instantly to a state 
of gr

and the brain proceeds instantly to a state 
of greatly reduced activity, approaching a permanent
state of coma. But when the brain stem is transected
below the ﬁfth nerves, which leaves much input of
sensory signals from the facial and oral regions, the
coma is averted.
Increased Activity of the Excitatory Area Caused by Feedback
Signals Returning from the Cerebral Cortex. Not only do
excitatory signals pass to the cerebral cortex from 
the bulboreticular excitatory area of the brain stem,
but 

oreticular excitatory area of the brain stem,
but feedback signals also return from the cerebral
cortex back to this same area. Therefore, any time 
the cerebral cortex becomes activated by either 
brain thought processes or motor processes, signals 
are sent from the cortex to the brain stem excitatory
area, which in turn sends still more excitatory signals
to the cortex. This helps to maintain the level of 
excitation of the cerebral cortex or even to enhance it.
This is a general mechanism of

even to enhance it.
This is a general mechanism of positive feedback that
allows any beginning activity in the cerebral cortex to
support still more activity, thus leading to an “awake”
mind.
Thalamus Is a Distribution Center That Controls Activity in Spe-
ciﬁc Regions of the Cortex. As pointed out in Chapter 57
and shown in Figure 57–2, almost every area of the
cerebral cortex connects with its own highly speciﬁc
area in the thalamus. Therefore, electrical stimulation
of a speciﬁc point in the 

 electrical stimulation
of a speciﬁc point in the thalamus generally activates
its own speciﬁc small region of the cortex. Further-
more, signals regularly reverberate back and forth
between the thalamus and the cerebral cortex, the
thalamus exciting the cortex and the cortex then re-
exciting the thalamus by way of return ﬁbers. It has
been suggested that the thinking process establishes
long-term memories by activating such back-and-forth
reverberation of signals.
Can the thalamus also functio

beration of signals.
Can the thalamus also function to call forth speciﬁc
memories from the cortex or to activate speciﬁc
thought processes? Proof of this is still lacking, but the
thalamus does have appropriate neuronal circuitry for
these purposes.
A Reticular Inhibitory Area Located in the
Lower Brain Stem
Figure 58–1 shows still another area that is important
in controlling brain activity. This is the reticular
inhibitory area, located medially and ventrally in the
medulla. In Chapter 55, we

ly and ventrally in the
medulla. In Chapter 55, we learned that this area can
inhibit the reticular facilitory area of the upper brain
stem and thereby decrease activity in the superior por-
tions of the brain as well. One of the mechanisms for
this is to excite serotonergic neurons; these in turn
secrete the inhibitory neurohormone serotonin at
crucial points in the brain; we will discuss this in more
detail later.
Thalamus
Inhibitory area
Excitatory area
5th Cranial nerve
Figure 58–1
Excitator

atory area
Excitatory-activating system of the brain. Also shown is an
inhibitory area in the medulla that can inhibit or depress the acti-
vating system.
730
Unit XI
Neurohormonal Control of 
Brain Activity
Aside from direct control of brain activity by speciﬁc
transmission of nerve signals from the lower brain
areas to the cortical regions of the brain, still another
physiologic mechanism is very often u

till another
physiologic mechanism is very often used to control
brain activity. This is to secrete excitatory or inhibitory
neurotransmitter hormonal agents into the substance
of the brain. These neurohormones often persist for
minutes or hours and thereby provide long periods of
control, rather than just instantaneous activation or
Figure 58–2 shows three neurohormonal systems
that have been mapped in detail in the rat brain: (1) a
norepinephrine system, (2) a dopamine system, and 

norepinephrine system, (2) a dopamine system, and (3)
a serotonin system. Norepinephrine usually functions
as an excitatory hormone, whereas serotonin usually is
inhibitory, and dopamine is excitatory in some areas
but inhibitory in others. As would be expected, these
three systems have different effects on levels of
excitability in different parts of the brain. The norepi-
nephrine system spreads to virtually every area of the
brain, whereas the serotonin and dopamine systems
are directed much 

 serotonin and dopamine systems
are directed much more to speciﬁc brain regions—
the dopamine system mainly into the basal ganglial
regions and the serotonin system more into the
midline structures.
Neurohormonal Systems in the Human Brain. Figure 58–3
shows the brain stem areas in the human brain for acti-
vating four neurohormonal systems, the same three
discussed for the rat and one other, the acetylcholine
system. Some of the speciﬁc functions of these are as
follows:
1. The locus ceruleus a

s of these are as
follows:
1. The locus ceruleus and the norepinephrine system.
The locus ceruleus is a small area located
bilaterally and posteriorly at the juncture between
the pons and mesencephalon. Nerve ﬁbers 
from this area spread throughout the brain, the
same as shown for the rat in the top frame of
Figure 58–2, and they secrete norepinephrine. The
norepinephrine generally excites the brain to
increased activity. However, it has inhibitory
effects in a few brain areas because of inhibit

ry
effects in a few brain areas because of inhibitory
receptors at certain neuronal synapses. In Chapter
59, we will see that this system probably plays an
important role in causing dreaming, thus leading
to a type of sleep called rapid eye movement
sleep (REM sleep).
2. The substantia nigra and the dopamine system.
The substantia nigra is discussed in Chapter 56 in
Locus cerulus
Basal brain areas
Brain stem
Olfactory
region
Cerebral cortex
NOREPINEPHRI

egion
NOREPINEPHRINE
Frontal
cortex
Cingulate
cortex
DOPAMINE
Midline nuclei
SEROTONIN
Figure 58–2
Three neurohormonal systems that have been mapped in the rat
brain: a norepinephrine system, a dopamine system, and a sero-
tonin system. (Adapted from Kelly, after Cooper, Bloom, and Roth,
in Kandel ER, Schwartz JH: Principles of Neural Science, 2nd ed.
New York: Elsevier, 1985.)
To diencephalon
and cerebrum
To cord
Medulla
Pons
To cerebellum
Substanti

Medulla
Pons
(dopamine)
Gigantocellular
neurons of 
reticular formation
(acetylcholine)
Locus ceruleus
(norepinephrine)
Nuclei of the raphe
(serotonin)
Figure 58–3
Multiple centers in the brain stem, the neurons of which secrete
different transmitter substances (speciﬁed in parentheses). These
neurons send control signals upward into the diencephalon and
cerebrum and downward into the spinal cord.
Chapter 58
Behavioral and Motivational Mechanisms of t

ter 58
731
relation to the basal ganglia. It lies anteriorly in
the superior mesencephalon, and its neurons send
nerve endings mainly to the caudate nucleus and
putamen of the cerebrum, where they secrete
dopamine. Other neurons located in adjacent
regions also secrete dopamine, but they send their
endings into more ventral areas of the brain,
especially to the hypothalamus and the limbic
system. The dopam

 the hypothalamus and the limbic
system. The dopamine is believed to act as an
inhibitory transmitter in the basal ganglia, but 
in some other areas of the brain it is possibly
excitatory. Also, remember from Chapter 56 that
destruction of the dopaminergic neurons in the
substantia nigra is the basic cause of Parkinson’s
disease.
3. The raphe nuclei and the serotonin system. In 
the midline of the pons and medulla are several
thin nuclei called the raphe nuclei. Many of the
neurons in these nucl

he raphe nuclei. Many of the
neurons in these nuclei secrete serotonin. They
send ﬁbers into the diencephalon and a few ﬁbers
to the cerebral cortex; still other ﬁbers descend to
the spinal cord. The serotonin secreted at the cord
ﬁber endings has the ability to suppress pain,
which was discussed in Chapter 48. The serotonin
released in the diencephalon and cerebrum almost
certainly plays an essential inhibitory role to help
cause normal sleep, as we discuss in Chapter 59.
4. The gigantocellular

s we discuss in Chapter 59.
4. The gigantocellular neurons of the reticular
excitatory area and the acetylcholine system.
Earlier we discussed the gigantocellular neurons
(the giant cells) in the reticular excitatory area of
the pons and mesencephalon. The ﬁbers from
these large cells divide immediately into two
branches, one passing upward to the higher levels
of the brain and the other passing downward
through the reticulospinal tracts into the spinal
cord. The neurohormone secreted at their
t

 spinal
terminals is acetylcholine. In most places,
the acetylcholine functions as an excitatory
neurotransmitter. Activation of these
acetylcholine neurons leads to an acutely awake
and excited nervous system.
Other Neurotransmitters and Neurohormonal Substances
Secreted in the Brain. Without describing their function,
the following is a list of still other neurohormonal 
substances that function either at speciﬁc synapses or
by release into the ﬂuids of

t speciﬁc synapses or
by release into the ﬂuids of the brain: enkephalins,
gamma-aminobutyric acid, glutamate, vasopressin,
adrenocorticotropic hormone, epinephrine, histamine,
endorphins, angiotensin II, and neurotensin. Thus,
there are multiple neurohormonal systems in the
brain, the activation of each of which plays its own role
in controlling a different quality of brain function.
The word “limbic” means “border.” Originally, the
term “limbic” was used to describe the border st

e
term “limbic” was used to describe the border struc-
tures around the basal regions of the cerebrum, but as
we have learned more about the functions of the
limbic system, the term limbic system has been
expanded to mean the entire neuronal circuitry that
controls emotional behavior and motivational drives.
A major part of the limbic system is the hypothala-
mus, with its related structures. In addition to their
roles in behavioral control, these areas control many
internal conditions of the bo

e areas control many
internal conditions of the body, such as body temper-
ature, osmolality of the body ﬂuids, and the drives to
eat and drink and to control body weight. These inter-
nal functions are collectively called vegetative func-
tions of the brain, and their control is closely related
to behavior.
Functional Anatomy of the
Limbic System; Key Position
Figure 58–4 shows the anatomical structures of the
limbic system, demonstrating that they are an inter-
connected co

demonstrating that they are an inter-
connected complex of basal brain elements. Located
in the middle of all these is the extremely small hypo-
thalamus,which from a physiologic point of view is one
of the central elements of the limbic system. Figure
58–5 illustrates schematically this key position of the
hypothalamus in the limbic system and shows sur-
rounding it other subcortical structures of the limbic
system, including the septum, the paraolfactory area,
the anterior nucleus of the thala

aolfactory area,
the anterior nucleus of the thalamus, portions of the
basal ganglia, the hippocampus, and the amygdala.
And surrounding the subcortical limbic areas is the
limbic cortex, composed of a ring of cerebral cortex in
each side of the brain (1) beginning in the orbitofrontal
area on the ventral surface of the frontal lobes, (2)
extending upward into the subcallosal gyrus, (3) then
over the top of the corpus callosum onto the medial
aspect of the cerebral hemisphere in the cingulate
gy

ect of the cerebral hemisphere in the cingulate
gyrus, and ﬁnally (4) passing behind the corpus callo-
sum and downward onto the ventromedial surface of
the temporal lobe to the parahippocampal gyrus and
uncus.
Thus, on the medial and ventral surfaces of each
cerebral hemisphere is a ring of mostly paleocortex
that surrounds a group of deep structures intimately
associated with overall behavior and emotions. In turn,
this ring of limbic cortex functions as a two-way com-
munication and associati

nctions as a two-way com-
munication and association linkage between the neo-
cortex and the lower limbic structures.
Many of the behavioral functions elicited from the
hypothalamus and other limbic structures are also
mediated through the reticular nuclei in the brain stem
and their associated nuclei. It was pointed out in
Chapter 55 as well as earlier in this chapter that stim-
ulation of the excitatory portion of this reticular for-
mation can cause high degrees of cerebral excitability
while

 cause high degrees of cerebral excitability
while also increasing the excitability of much of the
spinal cord synapses. In Chapter 60, we will see that
most of the hypothalamic signals for controlling the
autonomic nervous system are also transmitted
through synaptic nuclei located in the brain stem.
An important route of communication between 
the limbic system and the brain stem is the medial 
732
Unit XI
forebrain bundle, which ext

rative Neurophysiology
forebrain bundle, which extends from the septal and
orbitofrontal regions of the cerebral cortex downward
through the middle of the hypothalamus to the brain
stem reticular formation. This bundle carries ﬁbers in
both directions, forming a trunk line communication
system. A second route of communication is through
short pathways among the reticular formation of the
brain stem, thalamus, hypothalamus, and most other
contiguous areas of the basal brain.
Hypothalamus, a Major

us areas of the basal brain.
Control Headquarters for the
The hypothalamus, despite its very small size of only a
few cubic centimeters, has two-way communicating
pathways with all levels of the limbic system. In turn,
it and its closely allied structures send output signals
in three directions: (1) backward and downward to the
brain stem, mainly into the reticular areas of the mes-
encephalon, pons, and medulla and from these areas
into the peripheral nerves 

a and from these areas
into the peripheral nerves of the autonomic nervous
system; (2) upward toward many higher areas of the
diencephalon and cerebrum, especially to the anterior
thalamus and limbic portions of the cerebral cortex;
and (3) into the hypothalamic infundibulum to control
or partially control most of the secretory functions of
both the posterior and the anterior pituitary glands.
Thus, the hypothalamus, which represents less than
1 per cent of the brain mass, is one of the most imp

per cent of the brain mass, is one of the most impor-
tant of the control pathways of the limbic system. It
controls most of the vegetative and endocrine func-
tions of the body as well as many aspects of emotional
Connecting spinal cord
Stria terminalis
Fimbria
of fornix
Gyrus
fasciolaris
Isthmus
Mamillotegmental
tract
Mamillothalamic
tract
Dorsal fornix
Body of fornix
Stria medullaris
thalami
Cingulate gyrus and cingulum
Indusium griseum
and longitudinal striae
Septum pellucidum
(supracommissu

ngitudinal striae
(supracommissural septum)
Anterior nuclear
group of thalamus
Anterior commissure
Subcallosal gyrus
Paraterminal gyrus
(precommissural
septum)
Orbitofrontal cortex
Prehippocampal rudiment
Paraolfactory area
HYPOTHALAMUS
Column of fornix
(postcommissural fornix)
Uncus
Amygdaloid body
Mamillary body
Parahippocampal gyrus
Dentate gyrus
Figure 58–4
Anatomy of the limbic system, shown in the dark pink area. (Redrawn from Warwick R, William

e dark pink area. (Redrawn from Warwick R, Williams PL: Gray’s Anatomy, 35th Br. ed. London:
Longman Group Ltd, 1973.)
Cingulate gyrus
Subcallosal
gyrus
Orbitofrontal
cortex
Uncus
Paraolfactory
area
Septum
area
Anterior
nuclei of
thalamus
Hippocampus 
Amygdala
Portions
of basal
ganglia
Figure 58–5
Limbic system, showing the key position of the hypothalamus.
Chapter 58
733

e Brain—The Limbic System and the Hypothalamus
733
behavior. Let us discuss ﬁrst the vegetative and
endocrine control functions and then return to the
behavioral functions of the hypothalamus to see how
these operate together.
Vegetative and Endocrine Control
The different hypothalamic mechanisms for control-
ling multiple functions of the body are so important
that they are discussed in multiple chapters through-
out this text. For instance, the role of the hypotha

t this text. For instance, the role of the hypothala-
mus to help regulate arterial pressure is discussed in
Chapter 18, thirst and water conservation in Chapter
29,
temperature regulation in Chapter 73,
and
endocrine control in Chapter 75. To illustrate the
organization of the hypothalamus as a functional unit,
let us summarize the more important of its vegetative
and endocrine functions here as well.
Figures 58–6 and 58–7 show enlarged sagittal and
coronal views of the hypothalamus, which repr

 and
coronal views of the hypothalamus, which represents
only a small area in Figure 58–4. Take a few minutes
to study these diagrams, especially to see in Figure
58–6 the multiple activities that are excited or inhib-
ited when respective hypothalamic nuclei are stimu-
lated. In addition to the centers shown in Figure 58–6,
a large lateral hypothalamic area (shown in Figure
58–7) is present on each side of the hypothalamus.The
lateral areas are especially important in controlling
thirst, hunger

especially important in controlling
thirst, hunger, and many of the emotional drives.
A word of caution must be issued for studying these
diagrams because the areas that cause speciﬁc activi-
ties are not nearly as accurately localized as suggested
in the ﬁgures.Also, it is not known whether the effects
noted in the ﬁgures result from stimulation of speciﬁc
control nuclei or whether they result merely from 
activation of ﬁber tracts leading from or to control
nuclei located elsewhere.With this c

or to control
nuclei located elsewhere.With this caution in mind, we
can give the following general description of the veg-
etative and control functions of the hypothalamus.
Cardiovascular Regulation. Stimulation of different areas
throughout the hypothalamus can cause every known
type of neurogenic effect on the cardiovascular system,
including increased arterial pressure, decreased arterial
pressure, increased heart rate, and decreased heart rate.
In general, stimulation in the posterior and 

ate.
In general, stimulation in the posterior and lateral hypo-
thalamus increases the arterial pressure and heart rate,
whereas stimulation in the preoptic area often has oppo-
site effects, causing a decrease in both heart rate and
arterial pressure. These effects are transmitted mainly
through speciﬁc cardiovascular control centers in the
reticular regions of the pons and medulla.
Regulation of Body Temperature. The anterior portion of the
hypothalamus, especially the preoptic area, is concer

othalamus, especially the preoptic area, is concerned
POSTERIOR
Posterior hypothalamus
       (Increased blood pressure)
       (Pupillary dilation)
       (Shivering)
Perifornical nucleus
       (Hunger)
       (Rage)
Ventromedial nucleus
       (Satiety)
       (Neuroendocrine control)
Dorsomedial nucleus
       (GI stimulation)
       (Feeding reflexes)
Arcuate nucleus and periventricular zone
Latera

icular zone
Lateral hypothalamic area (not shown)
       (Thirst and hunger)
ANTERIOR
Paraventricular nucleus
       (Oxytocin release)
       (Water conservation)
Supraoptic nucleus
       (Vasopressin release)
Optic chiasm (Optic nerve)
Medial preoptic area
       (Bladder contraction)
       (Decreased heart rate)
       (Decreased blood pressure)
Posterior preoptic and
anterior hypothalamic areas
       (Body temperature regulation)
       (Panting)
       (Sw

emperature regulation)
       (Panting)
       (Sweating)
       (Thyrotropin inhibition)
Infundibulum
Figure 58–6
Control centers of the hypothala-
mus (sagittal view).
Dorsomedial
Fornix
Lateral
Ventromedial
Arcuate
Thalamus
Optic
tract
Anterior
Figure 58–7
Coronal view of the hypothalamus, showing the mediolateral posi-
tions of the respective hypothalamic nuclei.
734
Unit XI
The Nervous System: C. Motor and Integrative Neur

with regulation of body temperature.An increase in the
temperature of the blood ﬂowing through this area
increases the activity of temperature-sensitive neurons,
while a decrease in temperature decreases their activ-
ity. In turn, these neurons control mechanisms for
increasing or decreasing body temperature, as discussed
in Chapter 73.
Regulation of Body Water. The hypothalamus regulates
body water in two ways: (1) by creating the se

tes
body water in two ways: (1) by creating the sensation of
thirst, which makes the animal or person drink water,
and (2) by controlling the excretion of water into the
urine. An area called the thirst center is located in the
lateral hypothalamus. When the ﬂuid electrolytes in
either this center or closely allied areas become too
concentrated, the animal develops an intense desire to
drink water; it will search out the nearest source of
water and drink enough to return the electrolyte con-
cen

nd drink enough to return the electrolyte con-
centration of the thirst center to normal.
Control of renal excretion of water is vested mainly
in the supraoptic nuclei. When the body ﬂuids become
too concentrated, the neurons of these areas become
stimulated. Nerve ﬁbers from these neurons project
downward through the infundibulum of the hypothala-
mus into the posterior pituitary gland, where the nerve
endings secrete the hormone antidiuretic hormone (also
called vasopressin). This hormone is t

rmone (also
called vasopressin). This hormone is then absorbed into
the blood and transported to the kidneys where it acts
on the collecting ducts of the kidneys to cause increased
reabsorption of water. This decreases loss of water into
the urine but allows continuing excretion of electrolytes,
thus decreasing the concentration of the body ﬂuids
back toward normal. These functions are presented in
Chapter 28.
Regulation of Uterine Contractility and of Milk Ejection from the
Breasts. Stimulation

and of Milk Ejection from the
Breasts. Stimulation of the paraventricular nuclei causes
their neuronal cells to secrete the hormone oxytocin.
This in turn causes increased contractility of the uterus
as well as contraction of the myoepithelial cells sur-
rounding the alveoli of the breasts, which then causes
the alveoli to empty their milk through the nipples.
At the end of pregnancy, especially large quantities of
oxytocin are secreted, and this secretion helps to
promote labor contractions tha

 secretion helps to
promote labor contractions that expel the baby. Then,
whenever the baby suckles the mother’s breast, a reﬂex
signal from the nipple to the posterior hypothalamus
also causes oxytocin release, and the oxytocin now per-
forms the necessary function of contracting the ductules
of the breast, thereby expelling milk through the nipples
so that the baby can nourish itself. These functions are
discussed in Chapter 82.
Gastrointestinal and Feeding Regulation. Stimulation of
several a

l and Feeding Regulation. Stimulation of
several areas of the hypothalamus causes an animal to
experience extreme hunger, a voracious appetite, and an
intense desire to search for food. One area associated
with hunger is the lateral hypothalamic area. Con-
versely, damage to this area on both sides of the hypo-
thalamus causes the animal to lose desire for food,
sometimes causing lethal starvation.
A center that opposes the desire for food, called the
satiety center, is located in the ventromedi

d the
satiety center, is located in the ventromedial nuclei.
When this center is stimulated electrically, an animal
that is eating food suddenly stops eating and shows
complete indifference to food. However, if this area is
destroyed bilaterally, the animal cannot be satiated;
instead, its hypothalamic hunger centers become 
overactive, so that it has a voracious appetite, resulting
eventually in tremendous obesity. Another area of the 
hypothalamus that enters into overall control of gas-
troin

mus that enters into overall control of gas-
trointestinal activity is the mamillary bodies; these
control at least partially the patterns of many feeding
reﬂexes, such as licking the lips and swallowing.
Hypothalamic Control of Endocrine Hormone Secretion by the
Anterior Pituitary Gland. Stimulation of certain areas of the
hypothalamus also causes the anterior pituitary gland to
secrete its endocrine hormones.This subject is discussed
in detail in Chapter 74 in relation to neural control of
the

in Chapter 74 in relation to neural control of
the endocrine glands. Brieﬂy, the basic mechanisms are
the following.
The anterior pituitary gland receives its blood supply
mainly from blood that ﬂows ﬁrst through the lower
part of the hypothalamus and then through the anterior
pituitary vascular sinuses.As the blood courses through
the hypothalamus before reaching the anterior pitu-
itary, speciﬁc releasing and inhibitory hormones are
secreted into the blood by various hypothalamic nuclei.
These

to the blood by various hypothalamic nuclei.
These hormones are then transported via the blood 
to the anterior pituitary gland, where they act on the
glandular cells to control release of speciﬁc anterior
pituitary hormones.
Summary. A number of areas of the hypothalamus
control speciﬁc vegetative and endocrine functions.
These areas are still poorly delimited, so much so that
the speciﬁcation given above of different areas for dif-
ferent hypothalamic functions is partially tentative.
Behavior

halamic functions is partially tentative.
Behavioral Functions of the
Hypothalamus and Associated 
Limbic Structures
Effects Caused by Stimulation. In addition to the vegeta-
tive and endocrine functions of the hypothalamus,
stimulation of or lesions in the hypothalamus often
have profound effects on emotional behavior of
animals and human beings.
In animals, some of the behavioral effects of stimu-
lation are the following:
1. Stimulation in the lateral hypothalamus not only
causes thirst and e

 lateral hypothalamus not only
causes thirst and eating, as discussed above, but
also increases the general level of activity of the
animal, sometimes leading to overt rage and
ﬁghting, as discussed subsequently.
2. Stimulation in the ventromedial nucleus
and surrounding areas mainly causes effects
opposite to those caused by lateral hypothalamic
stimulation—that is, a sense of satiety, decreased
eating, and tranquility.
3. Stimulation of a thin zone of periventricular
nuclei, located immediatel

zone of periventricular
nuclei, located immediately adjacent to the third
ventricle (or also stimulation of the central gray
area of the mesencephalon that is continuous with
this portion of the hypothalamus), usually leads to
fear and punishment reactions.
4. Sexual drive can be stimulated from several 
areas of the hypothalamus, especially the most
anterior and most posterior portions of the
hypothalamus.
Chapter 58
Behavioral and Motivational Mechanisms of the Brain—The Limbic System and the 

Mechanisms of the Brain—The Limbic System and the Hypothalamus
735
Effects Caused by Hypothalamic Lesions. Lesions in the
hypothalamus, in general, cause effects opposite to
those caused by stimulation. For instance:
1. Bilateral lesions in the lateral hypothalamus will
decrease drinking and eating almost to zero, often
leading to lethal starvation. These lesions cause
extreme passivity of the animal as well, with loss
of most of its overt drives.
2. Bilateral lesions of the ventromedial areas o

.
2. Bilateral lesions of the ventromedial areas of the
hypothalamus cause effects that are mainly
opposite to those caused by lesions of the lateral
hypothalamus: excessive drinking and eating as
well as hyperactivity and often continuous
savagery along with frequent bouts of extreme
rage on the slightest provocation.
Stimulation or lesions in other regions of the limbic
system, especially in the amygdala, the septal area,
and areas in the mesencephalon, often cause effects
similar to those eli

cephalon, often cause effects
similar to those elicited from the hypothalamus. We
will discuss some of these in more detail later.
“Reward” and “Punishment” Function
of the Limbic System
From the discussion thus far, it is already clear that
several limbic structures are particularly concerned
with the affective nature of sensory sensations—that is,
whether the sensations are pleasant or unpleasant.
These affective qualities are also called reward or pun-
ishment, or satisfaction or aversion. El

d or pun-
ishment, or satisfaction or aversion. Electrical stimula-
tion of certain limbic areas pleases or satisﬁes the
animal, whereas electrical stimulation of other regions
causes terror, pain, fear, defense, escape reactions,
and all the other elements of punishment.The degrees
of stimulation of these two oppositely responding
systems greatly affect the behavior of the animal.
Reward Centers
Figure 58–8 shows a technique that has been used for
localizing speciﬁc reward and punishment areas 

or
localizing speciﬁc reward and punishment areas of the
brain. In this ﬁgure, a lever is placed at the side of 
the cage and is arranged so that depressing the lever
makes electrical contact with a stimulator. Electrodes
are placed successively at different areas in the brain
so that the animal can stimulate the area by pressing
the lever. If stimulating the particular area gives the
animal a sense of reward, then it will press the lever
again and again, sometimes as much as hundreds or
even th

nd again, sometimes as much as hundreds or
even thousands of times per hour. Furthermore, when
offered the choice of eating some delectable food as
opposed to the opportunity to stimulate the reward
center,
the animal often chooses the electrical 
By using this procedure, the major reward centers
have been found to be located along the course of the
medial forebrain bundle, especially in the lateral and
ventromedial nuclei of the hypothalamus. It is strange
that the lateral nucleus 

othalamus. It is strange
that the lateral nucleus should be included among the
reward areas—indeed, it is one of the most potent of
all—because even stronger stimuli in this area can
cause rage. But this is true in many areas, with weaker
stimuli giving a sense of reward and stronger ones a
sense of punishment. Less potent reward centers,
which are perhaps secondary to the major ones in the
hypothalamus, are found in the septum, the amygdala,
certain areas of the thalamus and basal ganglia, and

tain areas of the thalamus and basal ganglia, and
extending downward into the basal tegmentum of the
mesencephalon.
Punishment Centers
The apparatus shown in Figure 58–8 can also be 
connected so that the stimulus to the brain continues
all the time except when the lever is pressed. In this
case, the animal will not press the lever to turn the
stimulus off when the electrode is in one of the reward
areas; but when it is in certain other areas, the animal
immediately learns to turn it off. Stimul

e animal
immediately learns to turn it off. Stimulation in these
areas causes the animal to show all the signs of 
displeasure, fear, terror, pain, punishment, and even
sickness.
By means of this technique, the most potent areas
for punishment and escape tendencies have been
found in the central gray area surrounding the aque-
duct of Sylvius in the mesencephalon and extending
upward into the periventricular zones of the hypo-
thalamus and thalamus. Less potent punishment areas
are found in some

us. Less potent punishment areas
are found in some locations in the amygdala and hip-
pocampus. It is particularly interesting that stimulation
in the punishment centers can frequently inhibit the
reward and pleasure centers completely, demonstrat-
ing that punishment and fear can take precedence over
pleasure and reward.
Rage—Its Association with 
An emotional pattern that involves the punish-
ment centers of the hypothalamus and other limbic
Figure 58–8
Technique for localiz

and other limbic
Technique for localizing reward and punishment centers in the
brain of a monkey. 
736
Unit XI
structures, and has also been well characterized, is the
rage pattern, described as follows.
Strong stimulation of the punishment centers of 
the brain, especially in the periventricular zone of the
hypothalamus and in the lateral hypothalamus, causes
the animal to (1) develop a defense posture, (2) extend
its claw

(1) develop a defense posture, (2) extend
its claws, (3) lift its tail, (4) hiss, (5) spit, (6) growl, and
(7) develop piloerection, wide-open eyes, and dilated
pupils. Furthermore, even the slightest provocation
causes an immediate savage attack. This is approxi-
mately the behavior that one would expect from an
animal being severely punished, and it is a pattern of
behavior that is called rage.
Fortunately, in the normal animal, the rage phe-
nomenon is held in check mainly by inhibitory signa

omenon is held in check mainly by inhibitory signals
from the ventromedial nuclei of the hypothalamus. In
addition, portions of the hippocampi and anterior
limbic cortex, especially in the anterior cingulate 
gyri and subcallosal gyri, help suppress the rage 
phenomenon.
Placidity and Tameness. Exactly the opposite emotional
behavior patterns occur when the reward centers are
stimulated: placidity and tameness.
Importance of Reward or Punishment
in Behavior
Almost everything that we do is relate

Almost everything that we do is related in some way
to reward and punishment. If we are doing something
that is rewarding, we continue to do it; if it is punish-
ing, we cease to do it. Therefore, the reward and pun-
ishment centers undoubtedly constitute one of the
most important of all the controllers of our bodily
activities, our drives, our aversions, our motivations.
Effect of Tranquilizers on the Reward or Punishment Centers.
Administration of a tranquilizer, such as chlorpro-

ministration of a tranquilizer, such as chlorpro-
mazine, usually inhibits both the reward and the pun-
ishment centers, thereby decreasing the affective
reactivity of the animal. Therefore, it is presumed that
tranquilizers function in psychotic states by suppress-
ing many of the important behavioral areas of the
hypothalamus and its associated regions of the limbic
brain.
Importance of Reward or Punishment 
in Learning and Memory—Habituation 
Versus Reinforcement
Animal experiments have shown

Animal experiments have shown that a sensory expe-
rience that causes neither reward nor punishment is
hardly remembered at all. Electrical recordings from
the brain show that a newly experienced sensory stim-
ulus almost always excites multiple areas in the cere-
bral cortex. But, if the sensory experience does not
elicit a sense of either reward or punishment, repeti-
tion of the stimulus over and over leads to almost com-
plete extinction of the cerebral cortical response

plete extinction of the cerebral cortical response. That
is, the animal becomes habituated to that speciﬁc
sensory stimulus and thereafter ignores it.
If the stimulus does cause either reward or punish-
ment rather than indifference, the cerebral cortical
response becomes progressively more and more
intense during repeated stimulation instead of fading
away, and the response is said to be reinforced. An
animal builds up strong memory traces for sensations
that are either rewarding or punishing b

ensations
that are either rewarding or punishing but, conversely,
develops complete habituation to indifferent sensory
stimuli.
It is evident that the reward and punishment centers
of the limbic system have much to do with selecting
the information that we learn, usually throwing away
more than 99 per cent of it and selecting less than 1
per cent for retention.
Speciﬁc Functions of Other
Parts of the Limbic System
The hippocampus is the elongated portion of the cere-

 hippocampus is the elongated portion of the cere-
bral cortex that folds inward to form the ventral
surface of much of the inside of the lateral ventricle.
One end of the hippocampus abuts the amygdaloid
nuclei, and along its lateral border it fuses with the
parahippocampal gyrus, which is the cerebral cortex
on the ventromedial outside surface of the temporal
lobe.
The hippocampus (and its adjacent temporal and
parietal lobe structures, all together called the hip-
pocampal formation) has nume

ether called the hip-
pocampal formation) has numerous but mainly indi-
rect connections with many portions of the cerebral
cortex as well as with the basal structures of the limbic
system—the amygdala, the hypothalamus, the septum,
and the mamillary bodies. Almost any type of sensory
experience causes activation of at least some part of
the hippocampus, and the hippocampus in turn dis-
tributes many outgoing signals to the anterior thala-
mus, hypothalamus, and other parts of the limbic
system,

ypothalamus, and other parts of the limbic
system, especially through the fornix, a major commu-
nicating pathway. Thus, the hippocampus is an addi-
tional channel through which incoming sensory signals
can initiate behavioral reactions for different pur-
poses. As in other limbic structures, stimulation of dif-
ferent areas in the hippocampus can cause almost any
of the different behavioral patterns such as pleasure,
rage, passivity, or excess sex drive.
Another feature of the hippocampus is th

ex drive.
Another feature of the hippocampus is that it can
become hyperexcitable. For instance, weak electrical
stimuli can cause focal epileptic seizures in small areas
of the hippocampi. These often persist for many
seconds after the stimulation is over, suggesting that
the hippocampi can perhaps give off prolonged output
signals even under normal functioning conditions.
During hippocampal seizures, the person experiences
various psychomotor effects,
including olfactory,
visual, auditory, tac

ffects,
visual, auditory, tactile, and other types of hallucina-
tions that cannot be suppressed as long as the seizure
persists even though the person has not lost con-
sciousness and knows these hallucinations to be
Chapter 58
737
unreal. Probably one of the reasons for this hyper-
excitability of the hippocampi is that they have a dif-
ferent type of cortex from that elsewhere in th

f-
ferent type of cortex from that elsewhere in the
cerebrum, having only three nerve cell layers in some
of its areas instead of the six layers found elsewhere.
Role of the Hippocampus in Learning
Effect of Bilateral Removal of the Hippocampi—Inability to
Learn. Portions of the hippocampi have been surgically
removed bilaterally in a few human beings for treat-
ment of epilepsy. These people can recall most previ-
ously learned memories satisfactorily. However, they
often can learn essentially 

torily. However, they
often can learn essentially no new information that is
based on verbal symbolism. In fact, they often cannot
even learn the names of people with whom they come
in contact every day. Yet they can remember for a
moment or so what transpires during the course of
their activities. Thus, they are capable of short-term
memory for seconds up to a minute or two, although
their ability to establish memories lasting longer than
a few minutes is either completely or almost com-
pletel

minutes is either completely or almost com-
pletely abolished. This is the phenomenon called
anterograde amnesia that was discussed in Chapter 57.
Theoretical Function of the Hippocampus in Learning. The
hippocampus originated as part of the olfactory
cortex. In many lower animals, this cortex plays essen-
tial roles in determining whether the animal will eat 
a particular food, whether the smell of a particular
object suggests danger, or whether the odor is sexually
inviting, thus making decisi

 the odor is sexually
inviting, thus making decisions that are of life-or-death
importance.Very early in evolutionary development of
the brain, the hippocampus presumably became a crit-
ical decision-making neuronal mechanism, determin-
ing the importance of the incoming sensory signals.
Once this critical decision-making capability had been
established, presumably the remainder of the brain
also began to call on the hippocampus for decision
making. Therefore, if the hippocampus signals that a
n

ng. Therefore, if the hippocampus signals that a
neuronal input is important, the information is likely
to be committed to memory.
Thus, a person rapidly becomes habituated to indif-
ferent stimuli but learns assiduously any sensory expe-
rience that causes either pleasure or pain. But what is
the mechanism by which this occurs? It has been sug-
gested that the hippocampus provides the drive that
causes translation of short-term memory into long-
term memory—that is, the hippocampus transmits
so

some signal or signals that seem to make the mind
rehearse over and over the new information until per-
manent storage takes place.Whatever the mechanism,
without the hippocampi, consolidation of long-term
memories of the verbal or symbolic thinking type is
poor or does not take place.
The amygdala is a complex of multiple small nuclei
located immediately beneath the cerebral cortex of the
medial anterior pole of each temp

al cortex of the
medial anterior pole of each temporal lobe. It has abun-
dant bidirectional connections with the hypothalamus
as well as with other areas of the limbic system.
In lower animals,the amygdala is concerned to a great
extent with olfactory stimuli and their interrelations
with the limbic brain. Indeed, it is pointed out in
Chapter 53 that one of the major divisions of the olfac-
tory tract terminates in a portion of the amygdala 
called the corticomedial nuclei, which lies immediate

led the corticomedial nuclei, which lies immediately
beneath the cerebral cortex in the olfactory pyriform
area of the temporal lobe. In the human being, another
portion of the amygdala, the basolateral nuclei, has
become much more highly developed than the olfactory
portion and plays important roles in many behavioral
activities not generally associated with olfactory stimuli.
The amygdala receives neuronal signals from all por-
tions of the limbic cortex, as well as from the neocortex
of the t

bic cortex, as well as from the neocortex
of the temporal, parietal, and occipital lobes—especially
from the auditory and visual association areas. Because
of these multiple connections, the amygdala has been
called the “window” through which the limbic system
sees the place of the person in the world. In turn, the
amygdala transmits signals (1) back into these same cor-
tical areas, (2) into the hippocampus, (3) into the
septum, (4) into the thalamus, and (5) especially into the
E

lamus, and (5) especially into the
Effects of Stimulating the Amygdala. In general, stimulation
in the amygdala can cause almost all the same effects as
those elicited by direct stimulation of the hypothalamus,
plus other effects. Effects initiated from the amygdala
and then sent through the hypothalamus include (1)
increases or decreases in arterial pressure, (2) increases
or decreases in heart rate, (3) increases or decreases in
gastrointestinal motility and secretion, (4) defeca

astrointestinal motility and secretion, (4) defecation or
micturition, (5) pupillary dilation or, rarely, constriction,
(6) piloerection, and (7) secretion of various anterior
pituitary hormones, especially the gonadotropins and
adrenocorticotropic hormone.
Aside from these effects mediated through the hypo-
thalamus, amygdala stimulation can also cause several
types of involuntary movement. These include (1) tonic
movements, such as raising the head or bending the
body; (2) circling movements; 

head or bending the
body; (2) circling movements; (3) occasionally clonic,
rhythmical movements; and (4) different types of move-
ments associated with olfaction and eating, such as
licking, chewing, and swallowing.
In addition, stimulation of certain amygdaloid nuclei
can cause a pattern of rage, escape, punishment, severe
pain, and fear similar to the rage pattern elicited from
the hypothalamus, as described earlier. Stimulation of
other amygdaloid nuclei can give reactions of reward
and pleas

loid nuclei can give reactions of reward
and pleasure.
Finally, excitation of still other portions of the amyg-
dala can cause sexual activities that include erection,
copulatory movements, ejaculation, ovulation, uterine
activity, and premature labor.
Effects of Bilateral Ablation of the Amygdala—The Klüver-Bucy
Syndrome. When the anterior parts of both temporal
lobes are destroyed in a monkey, this removes not only
portions of temporal cortex but also of the amygdalas
that lie inside these par

ut also of the amygdalas
that lie inside these parts of the temporal lobes. This
causes changes in behavior called the Klüver-Bucy syn-
drome, which is demonstrated by an animal that (1) is
not afraid of anything, (2) has extreme curiosity about
everything, (3) forgets rapidly, (4) has a tendency to
place everything in its mouth and sometimes even tries
to eat solid objects, and (5) often has a sex drive so
strong that it attempts to copulate with immature
animals, animals of the wrong sex, or e

h immature
animals, animals of the wrong sex, or even animals of a
738
Unit XI
different species. Although similar lesions in human
beings are rare,afﬂicted people respond in a manner not
too different from that of the monkey.
Overall Function of the Amygdalas. The amygdalas seem to
be behavioral awareness areas that operate at a semi-
conscious level. They also seem to project into the
limbic system one’s current status in relation to

limbic system one’s current status in relation to both
surroundings and thoughts. On the basis of this infor-
mation, the amygdala is believed to make the person’s
behavioral response appropriate for each occasion.
The most poorly understood portion of the limbic
system is the ring of cerebral cortex called the limbic
cortex that surrounds the subcortical limbic structures.
This cortex functions as a transitional zone through
which signals are transmitted from the 

ne through
which signals are transmitted from the remainder of the
brain cortex into the limbic system and also in the oppo-
site direction. Therefore, the limbic cortex in effect 
functions as a cerebral association area for control of
behavior.
Stimulation of the different regions of the limbic
cortex has failed to give any real idea of their functions.
However, as is true of so many other portions of the
limbic system, essentially all behavioral patterns can be
elicited by stimulation of spec

al patterns can be
elicited by stimulation of speciﬁc portions of the limbic
cortex. Likewise, ablation of some limbic cortical areas
can cause persistent changes in an animal’s behavior, as
follows.
Ablation of the Anterior Temporal Cortex. When the anterior
temporal cortex is ablated bilaterally, the amygdalas are
almost invariably damaged as well. This was discussed
earlier in this chapter; it was pointed out that the
Klüver-Bucy syndrome occurs. The animal especially
develops consummatory be

rs. The animal especially
develops consummatory behavior: it investigates any
and all objects, has intense sex drives toward inappro-
priate animals or even inanimate objects, and loses all
fear—and thus develops tameness as well.
Ablation of the Posterior Orbital Frontal Cortex. Bilateral
removal of the posterior portion of the orbital frontal
cortex often causes an animal to develop insomnia asso-
ciated with intense motor restlessness, becoming unable
to sit still and moving about continuousl

g unable
to sit still and moving about continuously.
Ablation of the Anterior Cingulate Gyri and Subcallosal Gyri. The
anterior cingulate gyri and the subcallosal gyri are the
portions of the limbic cortex that communicate between
the prefrontal cerebral cortex and the subcortical limbic
structures. Destruction of these gyri bilaterally releases
the rage centers of the septum and hypothalamus from
prefrontal inhibitory inﬂuence. Therefore, the animal
can become vicious and much more subject to ﬁ

imal
can become vicious and much more subject to ﬁts of
rage than normally.
Summary. Until further information is available, it is
perhaps best to state that the cortical regions of the
limbic system occupy intermediate associative positions
between the functions of the speciﬁc areas of the cere-
bral cortex and functions of the subcortical limbic struc-
tures for control of behavioral patterns. Thus, in the
anterior temporal cortex, one especially ﬁnds gustatory
and olfactory behavioral associa

ly ﬁnds gustatory
and olfactory behavioral associations. In the parahip-
pocampal gyri, there is a tendency for complex auditory
associations as well as complex thought associations
derived from Wernicke’s area of the posterior temporal
lobe. In the middle and posterior cingulate cortex, there
is reason to believe that sensorimotor behavioral asso-
ciations occur.
Adell A, Celada P, Abellan MT, Artigas F: Origin and func-
tional role of the extracellular serotonin in the midbrain
raph

f the extracellular serotonin in the midbrain
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Bechara A, Damasio H, Damasio AR: Role of the amygdala
in decision-making. Ann N Y Acad Sci 985:356, 2003.
2004.
Bouret SG, Simerly RB: Leptin and development of hypo-
thalamic feeding circuits. Endocrinology 145:2621, 2004.
Conlon 

ng circuits. Endocrinology 145:2621, 2004.
Denton DA, McKinley MJ, Weisinger RS: Hypothalamic
integration of body ﬂuid regulation. Proc Natl Acad Sci 
U S A 93:7397, 1996.
Drevets WC: Neuroimaging abnormalities in the amygdala
in mood disorders. Ann N Y Acad Sci 985:420, 2003
Gerashchenko D, Shiromani PJ: Different neuronal pheno-
types in the lateral hypothalamus and their role in sleep
and wakefulness. Mol Neurobiol

their role in sleep
and wakefulness. Mol Neurobiol 29:41, 2004.
Haines DE:
New York:
Holland PC, Gallagher M: Amygdala—frontal interactions
and reward expectancy. Curr Opin Neurobiol 14:148, 2004.
Joels M, Verkuyl JM, Van Riel E: Hippocampal and hypo-
thalamic function after chronic stress. Ann N Y Acad Sci
1007:367, 2003.
Kandel ER, Schwartz J

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Kelley AE: Ventral striatal control of appetitive motivation:
role in ingestive behavior and reward-related learning.
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LeDoux JE: Emotion circuits in the brain. Annu Rev Neu-
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inescapable pain. News
Morris JF, Ludwig M: Magnocellular dendrites: prototypic
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thalamic feeding
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5
9
739
States of Brain Activity—Sleep,
Brain Waves, Epilepsy, Psychoses
All of us are aware of the many different states 
of brain activity, including sleep, wakefulness,
extreme excitement, and even different levels of
mood s

me excitement, and even different levels of
mood such as exhilaration, depression, and fear. All
these states result from different activating 
or inhibiting forces generated usually within the
brain itself. In Chapter 58, we began a partial dis-
cussion of this subject when we described different
systems that are capable of activating large portions of the brain. In this chapter,
we present brief surveys of speciﬁc states of brain activity, beginning with 
sleep.
Sleep
Sleep is deﬁned as uncons

nning with 
sleep.
Sleep
Sleep is deﬁned as unconsciousness from which the person can be aroused by
sensory or other stimuli. It is to be distinguished from coma, which is uncon-
sciousness from which the person cannot be aroused. There are multiple stages
of sleep, from very light sleep to very deep sleep; sleep researchers also divide
sleep into two entirely different types of sleep that have different qualities, as
follows.
Two Types of Sleep. During each night, a person goes through stages o

 During each night, a person goes through stages of two types
of sleep that alternate with each other. They are called (1) slow-wave sleep,
because in this type of sleep the brain waves are very strong and very low fre-
quency, as we discuss later, and (2) rapid eye movement sleep (REM sleep),
because in this type of sleep the eyes undergo rapid movements despite the fact
that the person is still asleep.
Most sleep during each night is of the slow-wave variety; this is the deep,
restful sleep th

w-wave variety; this is the deep,
restful sleep that the person experiences during the ﬁrst hour of sleep after
having been awake for many hours. REM sleep, on the other hand, occurs in
episodes that occupy about 25 per cent of the sleep time in young adults; each
episode normally recurs about every 90 minutes. This type of sleep is not so
restful, and it is usually associated with vivid dreaming.
Most of us can understand the characteristics of deep slow-wave sleep by
rememberin

racteristics of deep slow-wave sleep by
remembering the last time we were kept awake for more than 24 hours and
then the deep sleep that occurred during the ﬁrst hour after going to sleep.This
sleep is exceedingly restful and is associated with decrease in both peripheral
vascular tone and many other vegetative functions of the body. For instance,
there are 10 to 30 per cent decreases in blood pressure, respiratory rate, and
basal metabolic rate.
Although slow-wave sleep is frequently called “dr

Although slow-wave sleep is frequently called “dreamless sleep,” dreams and
sometimes even nightmares do occur during slow-wave sleep. The difference
between the dreams that occur in slow-wave sleep and those that occur in REM
sleep is that those of REM sleep are associated with more bodily muscle 
activity, and the dreams of slow-wave sleep usually are not remembered. That
740
Unit XI
is, during slow-wave sleep, consolidation of the 

is, during slow-wave sleep, consolidation of the dreams
in memory does not occur.
REM Sleep (Paradoxical Sleep,
In a normal night of sleep, bouts of REM sleep lasting
5 to 30 minutes usually appear on the average every
90 minutes. When the person is extremely sleepy, each
bout of REM sleep is short, and it may even be absent.
Conversely, as the person becomes more rested
through the night, the durations of the REM bouts
increase.
There are several important characteristics

rease.
There are several important characteristics of REM
sleep:
1. It is usually associated with active dreaming and
active bodily muscle movements.
2. The person is even more difﬁcult to arouse by
sensory stimuli than during deep slow-wave sleep,
and yet people usually awaken spontaneously in
the morning during an episode of REM sleep.
3. Muscle tone throughout the body is exceedingly
depressed, indicating strong inhibition of the
spinal muscle control areas.
4. Heart rate and respiratory rate

 control areas.
4. Heart rate and respiratory rate usually become
irregular, which is characteristic of the dream
state.
5. Despite the extreme inhibition of the peripheral
muscles, irregular muscle movements do occur.
These are in addition to the rapid movements of
the eyes.
6. The brain is highly active in REM sleep, and
overall brain metabolism may be increased as
much as 20 per cent. The electroencephalogram
(EEG) shows a pattern of brain waves similar to
those that occur during wakefulness.

es similar to
those that occur during wakefulness. This type of
sleep is also called paradoxical sleep because it is
a paradox that a person can still be asleep despite
marked activity in the brain.
In summary, REM sleep is a type of sleep in which
the brain is quite active. However, the brain activity is
not channeled in the proper direction for the person
to be fully aware of his or her surroundings, and there-
fore the person is truly asleep.
Sleep Is Believed to Be Ca

Sleep Is Believed to Be Caused by an Active Inhibitory Process.
An earlier theory of sleep was that the excitatory areas
of the upper brain stem, the reticular activating system,
simply fatigued during the waking day and became
inactive as a result. This was called the passive theory
of sleep. An important experiment changed this view
to the current belief that sleep is caused by an active
inhibitory process: it was discovered that transecting
the brain stem at the level 

ered that transecting
the brain stem at the level of the midpons creates a
brain whose cortex never goes to sleep. In other words,
there seems to be some center located below the mid-
pontile level of the brain stem that is required to cause
sleep by inhibiting other parts of the brain.
Neuronal Centers, Neurohumoral Substances,
and Mechanisms That Can Cause Sleep—
A Possible Speciﬁc Role for Serotonin
Stimulation of several speciﬁc areas of the brain 
can produce sleep with characteristics near

brain 
can produce sleep with characteristics near those 
of natural sleep. Some of these areas are the 
1. The most conspicuous stimulation area for causing
almost natural sleep is the raphe nuclei in the
lower half of the pons and in the medulla. These
nuclei are a thin sheet of special neurons located
in the midline. Nerve ﬁbers from these nuclei
spread locally in the brain stem reticular
formation and also upward into the thalamus,
hypothalamus, most areas of the limbic system,
an

and even the neocortex of the cerebrum. In
addition, ﬁbers extend downward into the spinal
cord, terminating in the posterior horns where
they can inhibit incoming sensory signals,
including pain, as discussed in Chapter 48. It is
also known that many nerve endings of ﬁbers
from these raphe neurons secrete serotonin.
When a drug that blocks the formation of
serotonin is administered to an animal, the animal
often cannot sleep for the next several d

e animal
often cannot sleep for the next several days.
Therefore, it has been assumed that serotonin is a
transmitter substance associated with production
of sleep.
2. Stimulation of some areas in the nucleus of the
tractus solitarius can also cause sleep. This nucleus
is the termination in the medulla and pons for
visceral sensory signals entering by way of the
vagus and glossopharyngeal nerves.
3. Stimulation of several regions in the diencephalon
can also promote sleep, including (1) the rost

lon
can also promote sleep, including (1) the rostral
part of the hypothalamus, mainly in the
suprachiasmal area, and (2) an occasional area in
the diffuse nuclei of the thalamus.
Lesions in Sleep-Promoting Centers Can Cause Intense Wake-
fulness. Discrete lesions in the raphe nuclei lead to a
high state of wakefulness. This is also true of bilateral
lesions in the medial rostral suprachiasmal area in the
anterior hypothalamus. In both instances, the excita-
tory reticular nuclei of the mesencep

 the excita-
tory reticular nuclei of the mesencephalon and upper
pons seem to become released from inhibition, thus
causing the intense wakefulness. Indeed, sometimes
lesions of the anterior hypothalamus can cause such
intense wakefulness that the animal actually dies of
exhaustion.
Other Possible Transmitter Substances Related to Sleep.
Experiments have shown that the cerebrospinal ﬂuid
as well as the blood or urine of animals that have been
kept awake for several days contains a substance or

pt awake for several days contains a substance or
substances that will cause sleep when injected into the
brain ventricular system of another animal. One likely
substance has been identiﬁed as muramyl peptide, a
low-molecular-weight substance that accumulates in
the cerebrospinal ﬂuid and urine in animals kept
awake for several days. When only micrograms of this
Chapter 59
States of Brain Activity—Sleep, Brain Waves, Epilepsy, Psychoses
741
sleep-producing substance are injected into the third
v

-producing substance are injected into the third
ventricle, almost natural sleep occurs within a few
minutes, and the animal may stay asleep for several
hours. Another substance that has similar effects in
causing sleep is a nonapeptide isolated from the blood
of sleeping animals. And still a third sleep factor, not
yet identiﬁed molecularly, has been isolated from the
neuronal tissues of the brain stem of animals kept
awake for days. It is possible that prolonged wakeful-
ness causes progressiv

ble that prolonged wakeful-
ness causes progressive accumulation of a sleep factor
or factors in the brain stem or in the cerebrospinal
ﬂuid that lead to sleep.
Possible Cause of REM Sleep. Why slow-wave sleep is
broken periodically by REM sleep is not understood.
However, drugs that mimic the action of acetylcholine
increase the occurrence of REM sleep. Therefore, it
has been postulated that the large acetylcholine-
secreting neurons in the upper brain stem reticular for-
mation might, through 

r brain stem reticular for-
mation might, through their extensive efferent ﬁbers,
activate many portions of the brain. This theoretically
could cause the excess activity that occurs in certain
brain regions in REM sleep, even though the signals
are not channeled appropriately in the brain to cause
normal conscious awareness that is characteristic of
wakefulness.
Cycle Between Sleep and Wakefulness
The preceding discussions have merely identiﬁed 
neuronal areas, transmitters, and mechanisms that 

neuronal areas, transmitters, and mechanisms that are
related to sleep. They have not explained the cyclical,
reciprocal operation of the sleep-wakefulness cycle.
There is as yet no explanation. Therefore, we can let
our imaginations run wild and suggest the following
possible mechanism for causing the sleep-wakefulness
cycle.
When the sleep centers are not activated, the mes-
encephalic and upper pontile reticular activating
nuclei are released from inhibition, which allows the
reticular activa

from inhibition, which allows the
reticular activating nuclei to become spontaneously
active.This in turn excites both the cerebral cortex and
the peripheral nervous system, both of which send
numerous positive feedback signals back to the same
reticular activating nuclei to activate them still further.
Therefore, once wakefulness begins, it has a natural
tendency to sustain itself because of all this positive
feedback activity.
Then, after the brain remains activated for many
hours, even the ne

rain remains activated for many
hours, even the neurons themselves in the activating
system presumably become fatigued. Consequently,
the positive feedback cycle between the mesen-
cephalic reticular nuclei and the cerebral cortex fades,
and the sleep-promoting effects of the sleep centers
take over, leading to rapid transition from wakefulness
back to sleep.
This overall theory could explain the rapid transi-
tions from sleep to wakefulness and from wakefulness
to sleep. It could also explain a

from wakefulness
to sleep. It could also explain arousal, the insomnia
that occurs when a person’s mind becomes preoccu-
pied with a thought, and the wakefulness that is pro-
duced by bodily physical activity.
Sleep causes two major types of physiologic effects:
ﬁrst, effects on the nervous system itself, and second,
effects on other functional systems of the body. The
nervous system effects seem to be by far the more
important because any person who has a transected

spinal cord in the neck (and therefore has no sleep-
wakefulness cycle below the transection) shows no
harmful effects in the body beneath the level of 
transection that can be attributed directly to a sleep-
wakefulness cycle.
Lack of sleep certainly does, however, affect the
functions of the central nervous system. Prolonged
wakefulness is often associated with progressive mal-
function of the thought processes and sometimes even
causes abnorm

thought processes and sometimes even
causes abnormal behavioral activities.
We are all familiar with the increased sluggishness
of thought that occurs toward the end of a prolonged
wakeful period, but in addition, a person can become
irritable or even psychotic after forced wakefulness.
Therefore, we can assume that sleep in multiple ways
restores both normal levels of brain activity and
normal “balance” among the different functions of the
central nervous system. This might be likened to the
“r

al nervous system. This might be likened to the
“rezeroing” of electronic analog computers after pro-
longed use, because computers of this type gradually
lose their “baseline” of operation; it is reasonable to
assume that the same effect occurs in the central
nervous system because overuse of some brain areas
during wakefulness could easily throw these areas out
of balance with the remainder of the nervous system.
We might postulate that the principal value of sleep
is to restore natural balanc

ncipal value of sleep
is to restore natural balances among the neuronal
centers. The speciﬁc physiologic functions of sleep
remain a mystery, and they are the subject of much
research.
Electrical recordings from the surface of the brain or
even from the outer surface of the head demonstrate
that there is continuous electrical activity in the brain.
Both the intensity and the patterns of this electrical
activity are determined by the level of excitation of dif-
ferent parts of the bra

evel of excitation of dif-
ferent parts of the brain resulting from sleep, wakeful-
ness, or brain diseases such as epilepsy or even
psychoses. The undulations in the recorded electrical
potentials, shown in Figure 59–1, are called brain waves,
and the entire record is called an EEG (electroen-
cephalogram).
The intensities of brain waves recorded from the
surface of the scalp range from 0 to 200 microvolts, and
their frequencies range from once every few seconds to
50 or more per second. The ch

every few seconds to
50 or more per second. The character of the waves is
dependent on the degree of activity in respective parts
of the cerebral cortex, and the waves change markedly
between the states of wakefulness and sleep and coma.
Much of the time, the brain waves are irregular, and
no speciﬁc pattern can be discerned in the EEG. At
other times, distinct patterns do appear, some of which
are characteristic of speciﬁc abnormalities of the brain
such as epilepsy, which is discussed later.
7

rain
742
Unit XI
In normal healthy people, most waves in the EEG can
be classiﬁed as alpha, beta, theta, and delta waves, which
are shown in Figure 59–1.
Alpha waves are rhythmical waves that occur at fre-
quencies between 8 and 13 cycles per second and are
found in the EEGs of almost all normal adult people
when they are awake and in a quiet, resting state of 
cerebration.These waves occur m

 resting state of 
cerebration.These waves occur most intensely in the oc-
cipital region but can also be recorded from the parietal
and frontal regions of the scalp. Their voltage usually is
about 50 microvolts. During deep sleep, the alpha waves
disappear.
When the awake person’s attention is directed to
some speciﬁc type of mental activity, the alpha waves
are replaced by asynchronous, higher-frequency but
lower-voltage beta waves. Figure 59–2 shows the effect
on the alpha waves of simply ope

 shows the effect
on the alpha waves of simply opening the eyes in bright
light and then closing the eyes. Note that the visual sen-
sations cause immediate cessation of the alpha waves
and that these are replaced by low-voltage, asynchro-
nous beta waves.
Beta waves occur at frequencies greater than 14 cycles
per second and as high as 80 cycles per second.They are
recorded mainly from the parietal and frontal regions
during speciﬁc activation of these parts of the brain.
Theta waves have freque

 these parts of the brain.
Theta waves have frequencies between 4 and 7 cycles
per second. They occur normally in the parietal and
temporal regions in children, but they also occur during
emotional stress in some adults, particularly during dis-
appointment and frustration. Theta waves also occur in
many brain disorders, often in degenerative brain states.
Delta waves include all the waves of the EEG with
frequencies less than 3.5 cycles per second, and they
often have voltages two to four times

nd, and they
often have voltages two to four times greater than most
other types of brain waves. They occur in very deep
sleep, in infancy, and in serious organic brain disease.
They also occur in the cortex of animals that have had
subcortical transections separating the cerebral cortex
from the thalamus. Therefore, delta waves can occur
strictly in the cortex independent of activities in lower
regions of the brain.
The discharge of a single neuron or single nerve ﬁber 
in

charge of a single neuron or single nerve ﬁber 
in the brain can never be recorded from the surface of
the head. Instead, many thousands or even millions of
neurons or ﬁbers must ﬁre synchronously; only then will
the potentials from the individual neurons or ﬁbers
summate enough to be recorded all the way through the
skull. Thus, the intensity of the brain waves from the
scalp is determined mainly by the numbers of neurons
and ﬁbers that ﬁre in synchrony with one another, not
by the total level 

ynchrony with one another, not
by the total level of electrical activity in the brain. In
fact, strong nonsynchronous nerve signals often nullify
one another in the recorded brain waves because of
opposing polarities.This is demonstrated in Figure 59–2,
which shows, when the eyes were closed, synchronous
discharge of many neurons in the cerebral cortex at a
frequency of about 12 per second, thus causing alpha
waves. Then, when the eyes were opened, the activity of
the brain increased greatly, bu

d, the activity of
the brain increased greatly, but synchronization of the
signals became so little that the brain waves mainly nul-
liﬁed one another, and the resultant effect was very low
voltage waves of generally high but irregular frequency,
the beta waves.
Origin of Alpha Waves. Alpha waves will not occur in the
cerebral cortex without cortical connections with the
thalamus. Conversely, stimulation in the nonspeciﬁc
layer of reticular nuclei that surround the thalamus or
in “diffuse” nucle

i that surround the thalamus or
in “diffuse” nuclei deep inside the thalamus often sets
up electrical waves in the thalamocortical system at a
frequency between 8 and 13 per second, which is the
natural frequency of the alpha waves. Therefore, it is
believed that the alpha waves result from spontaneous
feedback oscillation in this diffuse thalamocortical
system, possibly including the reticular activating
system in the brain stem as well. This oscillation pre-
sumably causes both the periodicity

cillation pre-
sumably causes both the periodicity of the alpha waves
and the synchronous activation of literally millions of
cortical neurons during each wave.
Origin of Delta Waves. Transection of the ﬁber tracts from
the thalamus to the cerebral cortex, which blocks thal-
amic activation of the cortex and thereby eliminates the
alpha waves, nevertheless does not block delta waves in
the cortex.This indicates that some synchronizing mech-
anism can occur in the cortical neuronal system by
itse

 can occur in the cortical neuronal system by
itself—mainly independent of lower structures in the
brain—to cause the delta waves.
Delta waves also occur during deep slow-wave sleep;
this suggests that the cortex then is mainly released
Alpha
Beta
Theta
Delta
1 sec
50 mV
Figure 59–1
Different types of brain waves
in the normal electro-
encephalogram.
Eyes open
Eyes closed
Figure 59–2
Replacement of the alpha rhythm by an asynchronous, low-
voltage beta rhythm when the eyes are opened.
Chapter 59

e beta rhythm when the eyes are opened.
Chapter 59
743
from the activating inﬂuences of the thalamus and other
lower centers.
Effect of Varying Levels of 
Cerebral Activity on the Frequency
of the EEG
There is a general correlation between level of cerebral
activity and average frequency of the EEG rhythm, the
average frequency increasing progressively with higher
degrees of activity. This is demonstrated in Figure 59–3,
which show

y. This is demonstrated in Figure 59–3,
which shows the existence of delta waves in stupor,
surgical anesthesia, and deep sleep; theta waves in 
psychomotor states and in infants; alpha waves during
relaxed states; and beta waves during periods of intense
mental activity. During periods of mental activity, the
waves usually become asynchronous rather than syn-
chronous, so that the voltage falls considerably, despite
markedly increased cortical activity,
as shown in 
Figure 59–2.
Changes in the 

ctivity,
as shown in 
Changes in the EEG at Different
Stages of Wakefulness and Sleep
Figure 59–4 shows EEG patterns from a typical person
in different stages of wakefulness and sleep.Alert wake-
fulness is characterized by high-frequency beta waves,
whereas quiet wakefulness is usually associated with
alpha waves, as demonstrated by the ﬁrst two EEGs of
the ﬁgure.
Slow-wave sleep is divided into four stages. In the ﬁrst
stage, a stage of very light sleep, the voltage of the EEG
wav

ge of very light sleep, the voltage of the EEG
waves becomes very low; this is broken by “sleep spin-
dles,” that is, short spindle-shaped bursts of alpha waves
that occur periodically. In stages 2, 3, and 4 of slow-wave
sleep, the frequency of the EEG becomes progressively
slower until it reaches a frequency of only 1 to 3 waves
per second in stage 4; these are delta waves.
Finally, the bottom record in Figure 59–4 shows the
EEG during REM sleep. It is often difﬁcult to tell the
difference betw

. It is often difﬁcult to tell the
difference between this brain wave pattern and that of
an awake, active person. The waves are irregular and
high-frequency, which are normally suggestive of desyn-
chronized nervous activity as found in the awake state.
Therefore, REM sleep is frequently called desynchro-
nized sleep because there is lack of synchrony in the
ﬁring of the neurons, despite signiﬁcant brain activity.
Epilepsy
Epilepsy (also called “seizures”) is characterized by
uncontrolled exces

“seizures”) is characterized by
uncontrolled excessive activity of either part or all of the
central nervous system.A person who is predisposed to
epilepsy has attacks when the basal level of excitability
of the nervous system (or of the part that is susceptible
to the epileptic state) rises above a certain critical
threshold. As long as the degree of excitability is held
below this threshold, no attack occurs.
Epilepsy can be classiﬁed into three major types:
grand mal epilepsy, petit mal epile

e major types:
grand mal epilepsy, petit mal epilepsy, and focal epilepsy.
Grand mal epilepsy is characterized by extreme neu-
ronal discharges in all areas of the brain—in the cere-
bral cortex, in the deeper parts of the cerebrum, and
even in the brain stem. Also, discharges transmitted all
the way into the spinal cord sometimes cause general-
ized tonic seizures of the entire body, followed toward
the end of the attack by alternating tonic and spasmodic
muscle contractions 

ternating tonic and spasmodic
muscle contractions called tonic-clonic seizures. Often
the person bites or “swallows” his or her tongue and
may have difﬁculty breathing, sometimes to the extent
that cyanosis occurs. Also, signals transmitted from the
Stupor
Surgical
Sleep   Psychomotor
Slow component
of petit mal
Infants   Relaxation
Deteriorated epileptics
Attention
Fright
Grand mal
Fast component
Confusion
1 second
Figure 59–3
Effect of varying degrees of cere-
bral acti

 59–3
bral activity on the basic rhythm
of 
the 
electroencephalogram.
(Redrawn from Gibbs FA, Gibbs
EL: Atlas of Electroencephalogra-
phy, 2nd ed, Vol I: Methodology
and Controls. “ 1974. Reprinted
by permission of Prentice-Hall,
Inc., Upper Saddle River, NJ.)
Alert wakefulness (beta waves)
Quiet wakefulness (alpha waves)
Stage 1 sleep (low voltage and spindles)
Stages 2 and 3 sleep (theta waves)
Stage 4 slow wave sleep (delta waves)
REM sleep (beta waves)
1 s

ave sleep (delta waves)
1 sec
50 mV
Figure 59–4
Progressive change in the characteristics of the brain waves
during different stages of wakefulness and sleep.
744
Unit XI
brain to the viscera frequently cause urination and defe-
cation.
The usual grand mal seizure lasts from a few seconds
to 3 to 4 minutes. It is also characterized by postseizure
depression of the entire nervous system; the person
remains in stup

 entire nervous system; the person
remains in stupor for 1 to many minutes after the
seizure attack is over, and then often remains severely
fatigued and asleep for hours thereafter.
The top recording of Figure 59–5 shows a typical
EEG from almost any region of the cortex during the
tonic phase of a grand mal attack. This demonstrates
that high-voltage, high-frequency discharges occur over
the entire cortex. Furthermore, the same type of dis-
charge occurs on both sides of the brain at the same

rge occurs on both sides of the brain at the same
time, demonstrating that the abnormal neuronal cir-
cuitry responsible for the attack strongly involves the
basal regions of the brain that drive the two halves of
the cerebrum simultaneously.
In laboratory animals and even in human beings,
grand mal attacks can be initiated by administering a
neuronal stimulant such as the drug pentylenetetrazol,
or they can be caused by insulin hypoglycemia, or by
passage of alternating electrical current direc

by
passage of alternating electrical current directly through
the brain. Electrical recordings from the thalamus as
well as from the reticular formation of the brain stem
during the grand mal attack show typical high-voltage
activity in both of these areas similar to that recorded
from the cerebral cortex. Presumably, therefore, a grand
mal attack involves not only abnormal activation of the
thalamus and cerebral cortex but also abnormal activa-
tion in the subthalamic brain stem portions of the

tion in the subthalamic brain stem portions of the brain
activating system itself.
What Initiates a Grand Mal Attack? Most people who have
grand mal attacks have hereditary predisposition to
epilepsy, a predisposition that occurs in about 1 of every
50 to 100 persons. In such people, factors that can
increase the excitability of the abnormal “epilepto-
genic” circuitry enough to precipitate attacks include
(1) strong emotional stimuli, (2) alkalosis caused by
overbreathing, (3) drugs, (4) fever,

sis caused by
overbreathing, (3) drugs, (4) fever, and (5) loud noises
or ﬂashing lights.
Even in people who are not genetically predisposed,
certain types of traumatic lesions in almost any part of
the brain can cause excess excitability of local brain
areas, as we discuss shortly; these, too, sometimes trans-
mit signals into the activating systems of the brain to
elicit grand mal seizures.
What Stops the Grand Mal Attack? The cause of the extreme
neuronal overactivity during a grand mal attac

eme
neuronal overactivity during a grand mal attack is pre-
sumed to be massive simultaneous activation of many
reverberating neuronal pathways throughout the brain.
Presumably, the major factor that stops the attack after
a few minutes is neuronal fatigue. A second factor is
probably active inhibition by inhibitory neurons that
have been activated by the attack.
Petit mal epilepsy almost certainly involves the thala-
mocortical brain activating system. It is usually charac-
t

l brain activating system. It is usually charac-
terized by 3 to 30 seconds of unconsciousness (or
diminished consciousness) during which time the
person has twitch-like contractions of muscles usually in
the head region, especially blinking of the eyes; this is
followed by return of consciousness and resumption of
previous activities. This total sequence is called the
absence syndrome or absence epilepsy. The patient 
may have one such attack in many months or, in rare
instances, may have a rap

 many months or, in rare
instances, may have a rapid series of attacks, one after
the other. The usual course is for the petit mal attacks
to appear ﬁrst during late childhood and then to disap-
pear by the age of 30. On occasion, a petit mal epilep-
tic attack will initiate a grand mal attack.
The brain wave pattern in petit mal epilepsy is
demonstrated by the middle recording of Figure 59–5,
which is typiﬁed by a spike and dome pattern. The spike
and dome can be recorded over most or all of th

ke
and dome can be recorded over most or all of the cere-
bral cortex, showing that the seizure involves much 
or most of the thalamocortical activating system of 
the brain. In fact, animal studies suggest that it results
from oscillation of (1) inhibitory thalamic reticular
neurons (which are inhibitory gamma-aminobutyric
acid [GABA]–producing neurons) and (2) excitatory
thalamocortical and corticothalamic neurons.
Focal epilepsy can involve almost any local part of the
brain, e

 can involve almost any local part of the
brain, either localized regions of the cerebral cortex or
deeper structures of both the cerebrum and brain stem.
Most often, focal epilepsy results from some localized
organic lesion or functional abnormality, such as (1) scar
tissue in the brain that pulls on the adjacent neuronal
tissue, (2) a tumor that compresses an area of the brain,
(3) a destroyed area of brain tissue, or (4) congenitally
deranged local circuitry.
Lesions such as these can promote

local circuitry.
Lesions such as these can promote extremely rapid
discharges in the local neurons; when the discharge rate
rises above several hundred per second, synchronous
waves begin to spread over adjacent cortical regions.
These waves presumably result from localized reverber-
ating circuits that gradually recruit adjacent areas of the
cortex into the epileptic discharge zone. The process
spreads to adjacent areas at a rate as slow as a few mil-
limeters a minute to as fast as several cen

w mil-
limeters a minute to as fast as several centimeters per
second.When such a wave of excitation spreads over the
motor cortex, it causes progressive “march” of muscle
Grand mal
Petit mal
Psychomotor
100 mV
50 mV
50 mV
Figure 59–5
Electroencephalograms in different types of epilepsy.
Chapter 59
745
contractions throughout the opposite side of the body,
beginning most characteristically in the mouth region
and marching progressi

tically in the mouth region
and marching progressively downward to the legs but at
other times marching in the opposite direction. This is
called jacksonian epilepsy.
A focal epileptic attack may remain conﬁned to a
single area of the brain, but in many instances, the
strong signals from the convulsing cortex excite the
mesencephalic portion of the brain activating system so
greatly that a grand mal epileptic attack ensues as well.
Another type of focal epilepsy is the so-called psy-
chomotor se

f focal epilepsy is the so-called psy-
chomotor seizure, which may cause (1) a short period of
amnesia; (2) an attack of abnormal rage; (3) sudden
anxiety, discomfort, or fear; and/or (4) a moment of
incoherent speech or mumbling of some trite phrase.
Sometimes the person cannot remember his or her
activities during the attack, but at other times, he or she
is conscious of everything that he or she is doing but
unable to control it. Attacks of this type frequently
involve part of the limbic port

is type frequently
involve part of the limbic portion of the brain, such as
the hippocampus, the amygdala, the septum, and/or por-
tions of the temporal cortex.
The lowest tracing of Figure 59–5 demonstrates a
typical EEG during a psychomotor seizure, showing a
low-frequency rectangular wave with a frequency
between 2 and 4 per second and with occasional super-
imposed 14-per-second waves.
Surgical Excision of Epileptic Foci Can Often Prevent Seizures.
The EEG can be used to localize abnormal sp

zures.
The EEG can be used to localize abnormal spiking
waves originating in areas of organic brain disease that
predispose to focal epileptic attacks. Once such a focal
point is found, surgical excision of the focus frequently
prevents future attacks.
Psychotic Behavior and
Dementia—Roles of Speciﬁc
Clinical studies of patients with different psychoses or
different types of dementia have suggested that many
of these conditions result from diminished function of
neurons 

itions result from diminished function of
neurons that secrete a speciﬁc neurotransmitter. Use of
appropriate drugs to counteract loss of the respective
neurotransmitter has been successful in treating some
patients.
In Chapter 56, we discussed the cause of Parkinson’s
disease. This disease results from loss of neurons in the
substantia nigra whose nerve endings secrete dopamine
in the caudate nucleus and putamen. Also in Chapter
56, we pointed out that in Huntington’s disease, loss of
GABA-secr

ut that in Huntington’s disease, loss of
GABA-secreting neurons and acetylcholine-secreting
neurons is associated with speciﬁc abnormal motor pat-
terns plus dementia occurring in the same patient.
Depression and Manic-Depressive
Psychoses—Decreased Activity of
the Norepinephrine and Serotonin
Much evidence has accumulated suggesting that mental
depression psychosis, which occurs in about 8 million
people in the United States, might be caused by dimin-
ished formation in

ates, might be caused by dimin-
ished formation in the brain of norepinephrine or sero-
tonin, or both. (New evidence has implicated still other
neurotransmitters.) Depressed patients experience
symptoms of grief, unhappiness, despair, and misery.
In addition, they often lose their appetite and sex 
drive and have severe insomnia. Often associated with
these is a state of psychomotor agitation despite the
depression.
Moderate 
numbers 
of 
norepinephrine-secreting
neurons are located in the brai

nephrine-secreting
neurons are located in the brain stem, especially in 
the locus ceruleus. These neurons send ﬁbers upward 
to most parts of the brain limbic system, thalamus,
and cerebral cortex. Also, many serotonin-producing
neurons located in the midline raphe nuclei of the lower
pons and medulla send ﬁbers to many areas of the
limbic system and to some other areas of the brain.
A principal reason for believing that depres-
sion might be caused by diminished activity of 
norepinephrine- an

used by diminished activity of 
norepinephrine- and serotonin-secreting neurons is that
drugs that block secretion of norepinephrine and sero-
tonin, such as reserpine, frequently cause depression.
Conversely, about 70 per cent of depressive patients can
be treated effectively with drugs that increase the exci-
tatory effects of norepinephrine and serotonin at the
nerve endings—for instance, (1) monoamine oxidase
inhibitors, which block destruction of norepinephrine
and serotonin once they are f

on of norepinephrine
and serotonin once they are formed; and (2) tricyclic
antidepressants, such as imipramine and amitriptyline,
which block reuptake of norepinephrine and serotonin
by nerve endings so that these transmitters remain
active for longer periods after secretion.
Mental depression can be treated by electroconvul-
sive therapy—commonly called “shock therapy.” In this
therapy, electrical current is passed through the brain to
cause a generalized seizure similar to that of an epilep-
t

eneralized seizure similar to that of an epilep-
tic attack. This has been shown to enhance norepine-
phrine activity.
Some patients with mental depression alternate
between depression and mania, which is called either
bipolar disorder or manic-depressive psychosis, and a
few people exhibit only mania without the depressive
episodes. Drugs that diminish the formation or action of
norepinephrine and serotonin, such as lithium com-
pounds, can be effective in treating the manic phase of
the condit

ffective in treating the manic phase of
the condition.
It is presumed that the norepinephrine and serotonin
systems normally provide drive to the limbic areas of
the brain to increase a person’s sense of well-being, to
create happiness, contentment, good appetite, appropri-
ate sex drive, and psychomotor balance—although too
much of a good thing can cause mania. In support of this
concept is the fact that pleasure and reward centers of
the hypothalamus and surrounding areas receive large
numbers

alamus and surrounding areas receive large
numbers of nerve endings from the norepinephrine and
serotonin systems.
Schizophrenia—Possible
Exaggerated Function of Part of the
Dopamine System
Schizophrenia comes in many varieties. One of the most
common types is seen in the person who hears voices
and has delusions of grandeur, intense fear, or other
types of feelings that are unreal. Many schizophrenics
(1) are highly paranoid,with a sense of persecution from
outside sources; (2) may develop inco

ecution from
outside sources; (2) may develop incoherent speech, dis-
sociation of ideas, and abnormal sequences of thought;
and (3) are often withdrawn, sometimes with abnormal
posture and even rigidity.
746
Unit XI
There are reasons to believe that schizophrenia
results from one or more of three possibilities: (1) mul-
tiple areas in the cerebral cortex prefrontal lobes in
which neural signals have become blocked or where
processing 

l signals have become blocked or where
processing of the signals becomes dysfunctional because
many synapses normally excited by the neurotransmit-
ter glutamate lose their responsiveness to this transmit-
ter; (2) excessive excitement of a group of neurons that
secrete dopamine in the behavioral centers of the brain,
including in the frontal lobes; and/or (3) abnormal func-
tion of a crucial part of the brain’s limbic behavioral
control system centered around the hippocampus.
The reason for bel

entered around the hippocampus.
The reason for believing that the prefrontal lobes are
involved in schizophrenia is that a schizophrenic-like
pattern of mental activity can be induced in monkeys by
making multiple minute lesions in widespread areas of
the prefrontal lobes.
Dopamine has been implicated as a possible cause of
schizophrenia because many patients with Parkinson’s
disease develop schizophrenic-like symptoms when
they are treated with the drug called L-dopa. This drug
releases dopamin

the drug called L-dopa. This drug
releases dopamine in the brain, which is advantageous
for treating Parkinson’s disease, but at the same time it
depresses various portions of the prefrontal lobes and
other related areas.
It has been suggested that in schizophrenia excess
dopamine is secreted by a group of dopamine-secreting
neurons whose cell bodies lie in the ventral tegmentum
of the mesencephalon, medial and superior to the sub-
stantia nigra. These neurons give rise to the so-called
mesolimb

 These neurons give rise to the so-called
mesolimbic dopaminergic system that projects nerve
ﬁbers and dopamine secretion into the medial and ante-
rior portions of the limbic system, especially into the
hippocampus, amygdala, anterior caudate nucleus, and
portions of the prefrontal lobes. All of these are pow-
erful behavioral control centers.
An even more compelling reason for believing that
schizophrenia might be caused by excess production of
dopamine is that many drugs that are effective in

dopamine is that many drugs that are effective in treat-
ing schizophrenia—such as chlorpromazine, haloperi-
dol, and thiothixene—-all either decrease secretion of
dopamine at dopaminergic nerve endings or decrease
the effect of dopamine on subsequent neurons.
Finally, possible involvement of the hippocampus in
schizophrenia was discovered recently when it was
learned that in schizophrenia, the hippocampus is often
reduced in size, especially in the dominant hemisphere.
Alzheimer’s Disease—Amyl

 the dominant hemisphere.
Alzheimer’s Disease—Amyloid
Plaques and Depressed Memory
Alzheimer’s disease is deﬁned as premature aging of the
brain, usually beginning in mid-adult life and progress-
ing rapidly to extreme loss of mental powers—similar
to that seen in very, very old age. The clinical features
of Alzheimer’s disease include (1) an amnesic type of
memory impairment, (2) deterioration of language, and
(3) visuospatial deﬁcits. Motor and sensory abnormali-
ties, gait disturbances, and s

 sensory abnormali-
ties, gait disturbances, and seizures are uncommon until
the late phases of the disease. One consistent ﬁnding in
Alzheimer’s disease is loss of neurons in that part of the
limbic pathway that drives the memory process. Loss of
this memory function is devastating.
Alzheimer’s disease is a progressive and fatal neu-
rodegenerative disorder that results in impairment of
the person’s ability to perform activities of daily living
as well as a variety of neuropsychiatric symptoms 

as well as a variety of neuropsychiatric symptoms and
behavioral disturbances in the later stages of the
disease. Patients with Alzheimer’s disease usually
require continuous care within a few years after the
disease begins.
Alzheimer’s disease is the most common form of
dementia in the elderly and about 5 million people in
the United States are estimated to be afﬂicted by this
disorder. The percentage of persons with Alzheimer’s
disease approximately doubles with every ﬁve years of
age, with ab

ately doubles with every ﬁve years of
age, with about 1 percent of 60-year-olds and about 30
percent of 85-year-olds having the disease.
Alzheimer’s Disease Is Associated with Accumulation of Brain
Beta-Amyloid Peptide. Pathologically, one ﬁnds increased
amounts of beta-amyloid peptide in the brains of
patients with Alzheimer’s disease. The peptide accumu-
lates in amyloid plaques, which range in diameter from
10 micrometers to several hundred micrometers and are
found in widespread areas of the

crometers and are
found in widespread areas of the brain, including in the
cerebral cortex, hippocampus, basal ganglia, thalamus,
and even the cerebellum. Thus, Alzheimer’s disease
appears to be a metabolic degenerative disease.
A key role for excess accumulation of beta-amyloid
peptide in the pathogenesis of Alzheimer’s disease is
suggested by the following observations: (1) all cur-
rently known mutations associated with Alzheimer’s
disease increase the production of beta-amyloid
peptide; (2) 

rease the production of beta-amyloid
peptide; (2) patients with trisomy 21 (Down syndrome)
have three copies of the gene for amyloid precursor
protein and develop neurological characteristics of
Alzheimer’s disease by midlife; (3) patients who have
abnormality of a gene that controls apolipoprotein E, a
blood protein that transports cholesterol to the tissues,
have accelerated deposition of amyloid and greatly
increased risk for Alzheimer’s disease; (4) transgenic
mice that overproduce the human

se; (4) transgenic
mice that overproduce the human amyloid precursor
protein have learning and memory deﬁcits in associa-
tion with the accumulation of amyloid plaques; and (5)
generation of anti-amyloid antibodies in humans with
Alzheimer’s disease appears to attenuate the disease
process.
Vascular Disorders May Contribute to Progression of Alzheimer’s
Disease. There is also accumulating evidence that 
cerebrovascular disease caused by hypertension and 
atherosclerosis may play a role in Alzhei

ion and 
atherosclerosis may play a role in Alzheimer’s disease.
Cerebrovascular disease is the second most common
cause of acquired cognitive impairment and dementia
and 
likely 
contributes 
to 
cognitive 
decline 
in
Alzheimer’s disease. In fact, many of the common risk
factors for cerebrovascular disease, such as hyperten-
sion, diabetes, and hyperlipidemia, are also recognized
to greatly increase the risk for developing Alzheimer’s
disease.
Aldrich MS: Sleep Medicine: Normal Slee

Aldrich MS: Sleep Medicine: Normal Sleep and Its Disor-
ders. New York: Oxford University Press, 1999.
Beardsley T: Waking up. Sci Am Jul 18, 1996.
Bryant PA, Trinder J, Curtis N: Sick and tired: does sleep
have a vital role in the immune system? Nat Rev Immunol
4:457, 2004.
Casserly I, Topol E: Convergence of atherosclerosis and
Alzheimer’s disease: inﬂammation, cholesterol, and mis-
folded proteins. Lancet 363:1139, 2004.
Chapter 59
States of Brain Activity—Sleep, Brain Waves, Epile

747
Cummings JL: Alzheimer’s disease. N Engl J Med 351:56,
2004.
de la Torre JC: Is Alzheimer’s disease a neurodegenerative
or a vascular disorder? Data,dogma,and dialectics.Lancet
Neurol 3:184, 2004.
George AL Jr: Molecular basis of inherited epilepsy. Arch
Neurol 61:473, 2004.
and wakefulness. Mol Neurobiol 29:41, 200

in sleep
Golde TE: Alzheimer disease therapy: can the amyloid
cascade be halted? J Clin Invest 111:11, 2003.
Gourﬁnkel-An I, Baulac S, Nabbout R, et al: Monogenic
idiopathic epilepsies. Lancet Neurol 3:209, 2004.
Greene R, Siegel J: Sleep: a functional enigma. Neuromo-
lecular Med 5:59, 2004.
Heller HC, Ruby NF: Sleep and circadian rhythms in mam-
malian torpor. Annu Rev Physiol 66:275, 2004.
Iadecola C: Neurovascular regulation in the normal brain
and

: Neurovascular regulation in the normal brain
and in Alzheimer’s disease. Nat Rev Neurosci 5:347-360,
2004.
LaRoche SM, Helmers SL: The new antiepileptic drugs: sci-
entiﬁc review. JAMA 291:605, 2004.
McCormick DA, Contreras D: On the cellular and network
bases of epileptic seizures. Annu Rev Physiol 63:815,
2001.
Noebels JL: The biology of epilepsy genes. Annu Rev Neu-
rosci 26:599, 2003.
Selkoe DJ: Alzheimer disease: mechanistic understanding
predicts novel therapies. Ann Intern Med 140:627, 

predicts novel therapies. Ann Intern Med 140:627, 2004.
Selkoe DJ:Alzheimer’s disease: genes, proteins, and therapy.
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Steinlein OK: Genetic mechanisms that underlie epilepsy.
Nat Rev Neurosci 5:400-408, 2004.
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748
The Autonomic Nervous System
and the Adrenal Medulla
The portion of the nervous system that controls
most visceral functions of the body is called the
autonomic nervous system. This system helps to
control arterial pressure, gastrointestinal 

ps to
control arterial pressure, gastrointestinal motility,
gastrointestinal secretion, urinary bladder empty-
ing, sweating, body temperature, and many other
activities, some of which are controlled almost
entirely and some only partially by the autonomic
One of the most striking characteristics of the autonomic nervous system is
the rapidity and intensity with which it can change visceral functions. For
instance, within 3 to 5 seconds it can increase the heart rate to twice nor

econds it can increase the heart rate to twice normal,
and within 10 to 15 seconds the arterial pressure can be doubled; or, at the other
extreme, the arterial pressure can be decreased low enough within 10 to 15
seconds to cause fainting. Sweating can begin within seconds, and the urinary
bladder may empty involuntarily, also within seconds.
The autonomic nervous system is activated mainly by centers located in the
spinal cord, brain stem, an

centers located in the
spinal cord, brain stem, and hypothalamus.Also, portions of the cerebral cortex,
especially of the limbic cortex, can transmit signals to the lower centers and in
this way inﬂuence autonomic control.
The autonomic nervous system also often operates by means of visceral
reﬂexes. That is, subconscious sensory signals from a visceral organ can enter
the autonomic ganglia, the brain stem, or the hypothalamus and then return 
subconscious reﬂex responses directly back to the vi

bconscious reﬂex responses directly back to the visceral organ to control its
activities.
The efferent autonomic signals are transmitted to the various organs of the
body through two major subdivisions called the sympathetic nervous system and
the parasympathetic nervous system, the characteristics and functions of which
follow.
Physiologic Anatomy of the Sympathetic Nervous System
Figure 60–1 shows the general organization of the peripheral portions of the sym-
pathetic nervous system. Shown sp

ions of the sym-
pathetic nervous system. Shown speciﬁcally in the ﬁgure are (1) one of the two 
paravertebral sympathetic chains of ganglia that are interconnected with the spinal
nerves on the side of the vertebral column, (2) two prevertebral ganglia (the celiac
and hypogastric), and (3) nerves extending from the ganglia to the different inter-
nal organs.
The sympathetic nerve ﬁbers originate in the spinal cord along with spinal nerves
between cord segments T-1 and L-2 and pass ﬁrst into the

n cord segments T-1 and L-2 and pass ﬁrst into the sympathetic chain and
then to the tissues and organs that are stimulated by the sympathetic nerves.
Preganglionic and Postganglionic Sympathetic Neurons
The sympathetic nerves are different from skeletal motor nerves in the following
way: Each sympathetic pathway from the cord to the stimulated tissue is composed
Chapter 60
The Autonomic Nervous System and the Adrenal Medulla
749
of two neurons, a preganglionic neuron and a postgan-
glionic neur

a preganglionic neuron and a postgan-
glionic neuron, in contrast to only a single neuron in the
skeletal motor pathway. The cell body of each pregan-
glionic neuron lies in the intermediolateral horn of the
spinal cord; its ﬁber passes, as shown in Figure 60–2,
through an anterior root of the cord into the corre-
sponding spinal nerve.
Immediately after the spinal nerve leaves the spinal
canal, the preganglionic sympathetic ﬁbers leave the
spinal nerve and pass through a white ramus into one of

l nerve and pass through a white ramus into one of
the ganglia of the sympathetic chain. Then the course 
of the ﬁbers can be one of the following three: (1) It 
can synapse with postganglionic sympathetic neurons 
in the ganglion that it enters; (2) it can pass upward 
or downward in the chain and synapse in one of the 
other ganglia of the chain; or (3) it can pass for variable
distances through the chain and then through one 
of the sympathetic nerves radiating outward from the
chain, ﬁnally 

c nerves radiating outward from the
chain, ﬁnally synapsing in a peripheral sympathetic 
ganglion.
The postganglionic sympathetic neuron thus origi-
nates either in one of the sympathetic chain ganglia 
or in one of the peripheral sympathetic ganglia.
From either of these two sources, the postganglionic
ﬁbers then travel to their destinations in the various
organs.
Sympathetic Nerve Fibers in the Skeletal Nerves. Some of the
postganglionic ﬁbers pass back from the sympathetic
chain into the spin

pass back from the sympathetic
chain into the spinal nerves through gray rami at all
levels of the cord, as shown in Figure 60–2. These sym-
pathetic ﬁbers are all very small type C ﬁbers, and they
extend to all parts of the body by way of the skeletal
nerves.They control the blood vessels, sweat glands, and
piloerector muscles of the hairs.About 8 per cent of the
ﬁbers in the average skeletal nerve are sympathetic
ﬁbers, a fact that indicates their great importance.
Segmental Distribution of th

eir great importance.
Segmental Distribution of the Sympathetic Nerve Fibers. The
sympathetic pathways that originate in the different
segments of the spinal cord are not necessarily distrib-
uted to the same part of the body as the somatic spinal
nerve ﬁbers from the same segments. Instead, the sym-
pathetic ﬁbers from cord segment T-1 generally pass up
the sympathetic chain to terminate in the head; from T-2
to terminate in the neck; from T-3, T-4, T-5, and T-6 
into the thorax; from T-7, T-8,

T-4, T-5, and T-6 
into the thorax; from T-7, T-8, T-9, T-10, and T-11 into
the abdomen; and from T-12, L-1, and L-2 into the legs.
This distribution is only approximate and overlaps
greatly.
The distribution of sympathetic nerves to each organ
is determined partly by the locus in the embryo from
which the organ originated. For instance, the heart
receives many sympathetic nerve ﬁbers from the neck
portion of the sympathetic chain because the heart orig-
inated in the neck of the embryo before t

rt orig-
inated in the neck of the embryo before translocating
into the thorax. Likewise, the abdominal organs receive
most of their sympathetic innervation from the lower
thoracic spinal cord segments because most of the prim-
itive gut originated in this area.
Special Nature of the Sympathetic Nerve Endings in the Adrenal
Medullae. Preganglionic sympathetic nerve ﬁbers pass,
without synapsing, all the way from the intermedio-
lateral horn cells of the spinal cord, through the 
Bronchi
Heart
Ey

 of the spinal cord, through the 
Bronchi
Heart
Eye
Celiac
ganglion
Blood
vessel
Sweat
gland
Piloerector
muscle
12
T-1
5
5
L-1
8
Hypogastric plexus
Pylorus
Adrenal
medulla
Kidney
Ureter
Intestine
Ileocecal valve
Anal sphincter
Detrusor
Trigone
Figure 60–1
Sympathetic nervous system. The black dashed lines represent
postganglionic ﬁbers in the gray rami leading from the sympa-
thetic chains into spinal nerves for distribution to blood vessels,
sweat glands, and piloerector muscles.
Peripheral gan

at glands, and piloerector muscles.
Peripheral ganglion
Postganglionic nerve
fibers
Preganglionic nerve
fiber
Effector endings
Sensory endings
Anterior root
Gray ramus
White ramus
Posterior root
Gut
Intermedio-
lateral horn
Figure 60–2
Nerve connections between the spinal cord, spinal nerves, sym-
pathetic chain, and peripheral sympathetic nerves.
750
Unit XI
sympathetic chains, then through the splanchni

ogy
sympathetic chains, then through the splanchnic nerves,
and ﬁnally into the two adrenal medullae. There they
end directly on modiﬁed neuronal cells that secrete epi-
nephrine and norepinephrine into the blood stream.
These secretory cells embryologically are derived from
nervous tissue and are actually themselves postgan-
glionic neurons; indeed, they even have rudimentary
nerve ﬁbers, and it is the endings of these ﬁbers that
secrete 
the 
adrenal 
hormones 
and 

drenal 
hormones 
and 
Physiologic Anatomy of the
The parasympathetic nervous system is shown in Figure
60–3, demonstrating that parasympathetic ﬁbers leave
the central nervous system through cranial nerves III,
VII, IX, and X; additional parasympathetic ﬁbers leave
the lowermost part of the spinal cord through the
second and third sacral spinal nerves and occasionally
the ﬁrst and fourth sacral nerves. About 75 per cent of
all parasympa

 sacral nerves. About 75 per cent of
all parasympathetic nerve ﬁbers are in the vagus nerves
(cranial nerve X), passing to the entire thoracic and
abdominal regions of the body.Therefore, a physiologist
speaking of the parasympathetic nervous system often
thinks mainly of the two vagus nerves.The vagus nerves
supply parasympathetic nerves to the heart, lungs,
esophagus, stomach, entire small intestine, proximal half
of the colon, liver, gallbladder, pancreas, kidneys, and
upper portions of the u

er, pancreas, kidneys, and
upper portions of the ureters.
Parasympathetic ﬁbers in the third cranial nerve go to
the pupillary sphincter and ciliary muscle of the eye.
Fibers from the seventh cranial nerve pass to the
lacrimal, nasal, and submandibular glands. And ﬁbers
from the ninth cranial nerve go to the parotid gland.
The sacral parasympathetic ﬁbers are in the pelvic
nerves, which pass through the spinal nerve sacral
plexus on each side of the cord at the S-2 and S-3 levels.
These ﬁbers th

the cord at the S-2 and S-3 levels.
These ﬁbers then distribute to the descending colon,
rectum, urinary bladder, and lower portions of the
ureters.Also, this sacral group of parasympathetics sup-
plies nerve signals to the external genitalia to cause
erection.
Preganglionic and Postganglionic Parasympathetic Neurons.
The parasympathetic system, like the sympathetic, has
both 
preganglionic 
and 
postganglionic 
neurons.
However, except in the case of a few cranial parasym-
pathetic nerves, the 

se of a few cranial parasym-
pathetic nerves, the preganglionic ﬁbers pass uninter-
rupted all the way to the organ that is to be controlled.
In the wall of the organ are located the postganglionic
neurons. The preganglionic ﬁbers synapse with these,
and very, very short postganglionic ﬁbers, a fraction of
a millimeter to several centimeters in length, leave the
neurons to innervate the tissues of the organ. This loca-
tion of the parasympathetic postganglionic neurons in
the visceral organ itse

 postganglionic neurons in
the visceral organ itself is quite different from the
arrangement of the sympathetic ganglia, because the
cell bodies of the sympathetic postganglionic neurons
are almost always located in the ganglia of the sympa-
thetic chain or in various other discrete ganglia in the
abdomen, rather than in the excited organ itself.
Basic Characteristics 
of Sympathetic and
Parasympathetic Function
Secretion of Acetylcholine or
The 

The sympathetic and parasympathetic nerve ﬁbers
secrete mainly one or the other of two synaptic trans-
mitter substances, acetylcholine or norepinephrine.
Those ﬁbers that secrete acetylcholine are said to be
cholinergic.Those that secrete norepinephrine are said
to be adrenergic, a term derived from adrenalin, which
is an alternate name for epinephrine.
All preganglionic neurons are cholinergic in both 
the sympathetic and the parasympathetic nervou

th 
the sympathetic and the parasympathetic nervous
systems. Acetylcholine or acetylcholine-like sub-
stances, when applied to the ganglia, will excite both
sympathetic and parasympathetic postganglionic
neurons. Either all or almost all of the postganglionic
neurons of the parasympathetic system are also cholin-
ergic. Conversely, most of the postganglionic sym-
pathetic neurons are adrenergic.
However,
the
postganglionic sympathetic nerve ﬁbers to the sweat
glands, to the piloerector muscles o

 to the sweat
glands, to the piloerector muscles of the hairs, and to
a very few blood vessels are cholinergic.
Heart
Parotid gland
Otic ganglion
Submandibular gland
Submandibular ganglion
Nasal glands
Lacrimal glands
Sphenopalatine ganglion
Pupillary sphincter
Ciliary muscles of eye
Ciliary ganglion
Pylorus
Colon
Bladder
Detrusor
Trigone
Sacral
Stomach
1
X
IX
VII
V
III
2
3
4
Figure 60–3
Parasympathetic nervous system.
Chapter 60
The Autonomic Nervo

tic nervous system.
Chapter 60
751
Thus, the terminal nerve endings of the parasympa-
thetic system all or virtually all secrete acetylcholine.
Almost all of the sympathetic nerve endings secrete
norepinephrine, but a few secrete acetylcholine. These
hormones in turn act on the different organs to cause
respective parasympathetic or sympathetic effects.
Therefore, acetylcholine is called a parasympathetic
transmitter and norepinephrine is call

transmitter and norepinephrine is called a sympathetic
The molecular structures of acetylcholine and nor-
epinephrine are the following:
Synthesis of Acetylcholine, Its Destruction After Secretion, and
Its Duration of Action. Acetylcholine is synthesized in the
terminal endings and varicosities of the cholinergic
nerve ﬁbers where it is stored in vesicles in highly con-
centrated form until it is released. The basic chemical
reaction of this synthesis is the following:
O

cal
Once acetylcholine is secreted into a tissue by a
cholinergic nerve ending, it persists in the tissue for a
few seconds while it performs its nerve signal trans-
mitter function. Then it is split into an acetate ion and
choline, catalyzed by the enzyme acetylcholinesterase
that is bound with collagen and glycosaminoglycans in
the local connective tissue.This is the same mechanism
for acetylcholine signal transmission and subsequent
acetylcholine d

signal transmission and subsequent
acetylcholine destruction that occurs at the neuro-
muscular junctions of skeletal nerve ﬁbers.The choline
that is formed is then transported back into the ter-
minal nerve ending, where it is used again and again
for synthesis of new acetylcholine.
Synthesis of Norepinephrine, Its Removal, and Its Duration of
Action. Synthesis of norepinephrine begins in the axo-
plasm of the terminal nerve endings of adrenergic
nerve ﬁbers but is completed inside the secretor

c
nerve ﬁbers but is completed inside the secretory vesi-
cles. The basic steps are the following:
1.
2.
3. Transport of dopamine into the vesicles
4.
In the adrenal medulla, this reaction goes still one
step further to transform about 80 per cent of the nor-
epinephrine into epinephrine, as follows:
5.
After secretion of norepinephrine by the terminal
nerve endings, it is removed from the secretory site in
three ways: (1) reuptake into the adrenergic nerve
endings themselves by an active transp

ergic nerve
endings themselves by an active transport process—
accounting for removal of 50 to 80 per cent of the
secreted norepinephrine; (2) diffusion away from the
nerve endings into the surrounding body ﬂuids and
then into the blood—accounting for removal of most
of the remaining norepinephrine; and (3) destruction
of small amounts by tissue enzymes (one of these
enzymes is monoamine oxidase, which is found in the
nerve endings, and another is catechol-O-methyl trans-
ferase, which is presen

s catechol-O-methyl trans-
ferase, which is present diffusely in all tissues).
Ordinarily,the norepinephrine secreted directly into
a tissue remains active for only a few seconds, demon-
strating that its reuptake and diffusion away from the
tissue are rapid. However, the norepinephrine and 
epinephrine secreted into the blood by the adrenal
medullae remain active until they diffuse into some
tissue, where they can be destroyed by catechol-O-
methyl transferase; this occurs mainly in the liver.

hyl transferase; this occurs mainly in the liver.
Therefore, when secreted into the blood, both norepi-
nephrine and epinephrine remain very active for 10 to
methylation
æ
Æ
æææ
Dopamine
hydroxylation
æ
Æ
ææææ
Dopa
Dopamine
decarboxylation
æ
Æ
ææææ
Tyrosine
Dopa
æ
Æ
ææææ
Choline
choline acetyl-
+
æ
Æ
ææææ
O
CH2
CH2
C
N
O
CH3
NH2
CH2
CH
HO
OH
CH3
CH3
CH3
+
HO
Mechanisms of Tra

H2
CH2
CH
HO
OH
CH3
CH3
CH3
+
HO
Mechanisms of Transmitter Secretion and
Subsequent Removal of the Transmitter 
at the Postganglionic Endings
Secretion of Acetylcholine and Norepinephrine by Postgan-
glionic Nerve Endings. A few of the postganglionic 
autonomic nerve endings, especially those of the
parasympathetic nerves, are similar to but much
smaller than those of the skeletal neuromuscular junc-
tion. However, many of the parasympathetic nerve
ﬁbers and almost all the sympathetic ﬁbers mere

ve
ﬁbers and almost all the sympathetic ﬁbers merely
touch the effector cells of the organs that they inner-
vate as they pass by; or in some instances, they termi-
nate in connective tissue located adjacent to the cells
that are to be stimulated.Where these ﬁlaments touch
or pass over or near the cells to be stimulated, they
usually have bulbous enlargements called varicosities;
it is in these varicosities that the transmitter vesicles of
acetylcholine or norepinephrine are synthesized and
stor

choline or norepinephrine are synthesized and
stored. Also in the varicosities are large numbers of
mitochondria that supply adenosine triphosphate,
which is required to energize acetylcholine or norepi-
nephrine synthesis.
When an action potential spreads over the terminal
ﬁbers, the depolarization process increases the perme-
ability of the ﬁber membrane to calcium ions, allowing
these ions to diffuse into the nerve terminals or nerve
varicosities. The calcium ions in turn cause the termi-
nal

ies. The calcium ions in turn cause the termi-
nals or varicosities to empty their contents to the exte-
rior. Thus, the transmitter substance is secreted.
752
Unit XI
30 seconds; but their activity declines to extinction
over 1 to several minutes.
Before an acetylcholine, norepinephrine, or epineph-
rine secreted at an autonomic nerve ending can stim-
ulate an effector organ, it must ﬁrst bind with spe

late an effector organ, it must ﬁrst bind with speciﬁc
receptors on the effector cells. The receptor is on the
outside of the cell membrane, bound as a prosthetic
group to a protein molecule that penetrates all the way
through the cell membrane.When the transmitter sub-
stance binds with the receptor, this causes a confor-
mational change in the structure of the protein
molecule. In turn, the altered protein molecule excites
or inhibits the cell, most often by (1) causing a change
in cell membra

 most often by (1) causing a change
in cell membrane permeability to one or more ions or
(2) activating or inactivating an enzyme attached to
the other end of the receptor protein where it pro-
trudes into the interior of the cell.
Excitation or Inhibition of the Effector Cell by Changing Its
Membrane Permeability. Because the receptor protein 
is an integral part of the cell membrane, a conforma-
tional change in structure of the receptor protein 
often opens or closes an ion channel through th

n 
often opens or closes an ion channel through the 
interstices of the protein molecule, thus altering the
permeability of the cell membrane to various ions.
For instance, sodium and/or calcium ion channels 
frequently become opened and allow rapid inﬂux of
the respective ions into the cell, usually depolarizing
the cell membrane and exciting the cell. At other 
times, potassium channels are opened, allowing potas-
sium ions to diffuse out of the cell, and this usually
inhibits
the cell because

e cell, and this usually
inhibits
the cell because loss of electropositive 
potassium ions creates hypernegativity inside the cell.
In some cells, the changed intracellular ion environ-
ment will cause an internal cell action, such as a direct
effect of calcium ions to promote smooth muscle 
Receptor Action by Altering Intracellular “Second Messenger”
Enzymes. Another way a receptor often functions is to
activate or inactivate an enzyme (or other intracellu-
lar chemical) inside the

yme (or other intracellu-
lar chemical) inside the cell. The enzyme often is
attached to the receptor protein where the receptor
protrudes into the interior of the cell. For instance,
binding of norepinephrine with its receptor on the
outside of many cells increases the activity of the
enzyme adenylyl cyclase on the inside of the cell, and
this causes formation of cyclic adenosine monophos-
phate (cAMP). The cAMP in turn can initiate any one
of many different intracellular actions, the exact eff

any different intracellular actions, the exact effect
depending on the chemical machinery of the effector
cell.
It is easy to understand how an autonomic trans-
mitter substance can cause inhibition in some organs
or excitation in others. This is usually determined by
the nature of the receptor protein in the cell mem-
brane and the effect of receptor binding on its con-
formational state. In each organ, the resulting effects
are likely to be entirely different from those in other
organs.
Two Pr

irely different from those in other
organs.
Two Principal Types of Acetylcholine
Receptors—Muscarinic and Nicotinic
Receptors
Acetylcholine activates mainly two types of receptors.
They are called muscarinic and nicotinic receptors.The
reason for these names is that muscarine, a poison
from toadstools, activates only muscarinic receptors
and will not activate nicotinic receptors, whereas nico-
tine activates only nicotinic receptors; acetylcholine
activates both of them.
Muscarinic receptors are

e
Muscarinic receptors are found on all effector cells
that are stimulated by the postganglionic cholinergic
neurons of either the parasympathetic nervous system
or the sympathetic system.
Nicotinic receptors are found in the autonomic
ganglia at the synapses between the preganglionic and
postganglionic neurons of both the sympathetic and
parasympathetic systems. (Nicotinic receptors are also
present at many nonautonomic nerve endings—for
instance, at the neuromuscular ju

erve endings—for
instance, at the neuromuscular junctions in skeletal
muscle [discussed in Chapter 7].)
An understanding of the two types of receptors is
especially important because speciﬁc drugs are fre-
quently used as medicine to stimulate or block one or
the other of the two types of receptors.
Adrenergic Receptors—Alpha and 
Beta Receptors
There are also two major types of adrenergic recep-
tors, alpha receptors and beta receptors. (The beta
receptors in turn are divided into beta1 and bet

a
receptors in turn are divided into beta1 and beta2 recep-
tors because certain chemicals affect only certain beta
receptors. Also, there is a division of alpha receptors
into alpha1 and alpha2 receptors.)
Norepinephrine and epinephrine, both of which are
secreted into the blood by the adrenal medulla,
have slightly different effects in exciting the alpha and
beta receptors. Norepinephrine excites mainly alpha
receptors but excites the beta receptors to a lesser
extent as well. Conversely, epin

ptors to a lesser
extent as well. Conversely, epinephrine excites both
types of receptors approximately equally. Therefore,
the relative effects of norepinephrine and epinephrine
on different effector organs are determined by the
types of receptors in the organs. If they are all 
beta receptors, epinephrine will be the more effective
excitant.
Table 60–1 gives the distribution of alpha and beta
receptors in some of the organs and systems controlled
by the sympathetics. Note that certain alpha fu

ed
by the sympathetics. Note that certain alpha functions
are excitatory, whereas others are inhibitory. Likewise,
certain beta functions are excitatory and others are
inhibitory.Therefore, alpha and beta receptors are not
necessarily associated with excitation or inhibition but
simply with the afﬁnity of the hormone for the recep-
tors in the given effector organ.
A synthetic hormone chemically similar to epi-
nephrine and norepinephrine, isopropyl norepineph-
rine, has an extremely strong acti

pyl norepineph-
rine, has an extremely strong action on beta receptors
but essentially no action on alpha receptors.
Chapter 60
753
Excitatory and Inhibitory 
Actions of Sympathetic and 
Parasympathetic Stimulation
Table 60–2 lists the effects on different visceral func-
tions of the body caused by stimulating either the
parasympathetic nerves or the sympathetic nerves.
From this table, it can be seen again that sympathetic
stimulation causes 

be seen again that sympathetic
stimulation causes excitatory effects in some organs but
inhibitory effects in others. Likewise, parasympathetic
stimulation causes excitation in some but inhibition in
others. Also, when sympathetic stimulation excites a
particular organ, parasympathetic stimulation some-
times inhibits it, demonstrating that the two systems
occasionally act reciprocally to each other. But most
organs are dominantly controlled by one or the other
of the two systems.
There is no ge

ne or the other
There is no generalization one can use to explain
whether sympathetic or parasympathetic stimulation
will cause excitation or inhibition of a particular organ.
Therefore, to understand sympathetic and parasympa-
thetic function, one must learn all the separate func-
tions of these two nervous systems on each organ, as
listed in Table 60–2. Some of these functions need to
be clariﬁed in still greater detail, as follows.
Effects of Sympathetic and
Parasympatheti

follows.
Parasympathetic Stimulation 
on Speciﬁc Organs
Eyes. Two functions of the eyes are controlled by the
autonomic nervous system. They are (1) the pupillary
opening and (2) the focus of the lens.
Sympathetic stimulation contracts the meridional
ﬁbers of the iris that dilate the pupil, whereas parasym-
pathetic stimulation contracts the circular muscle of the
iris to constrict the pupil.
The parasympathetics that control the pupil are
reﬂexly stimulated when exces

ontrol the pupil are
reﬂexly stimulated when excess light enters the eyes,
which is explained in Chapter 51; this reﬂex reduces the
pupillary opening and decreases the amount of light
that strikes the retina. Conversely, the sympathetics
become stimulated during periods of excitement and
increase pupillary opening at these times.
Focusing of the lens is controlled almost entirely by
the parasympathetic nervous system. The lens is nor-
mally held in a ﬂattened state by intrinsic elastic tension
o

in a ﬂattened state by intrinsic elastic tension
of its radial ligaments. Parasympathetic excitation con-
tracts the ciliary muscle, which is a ringlike body of
smooth muscle ﬁbers that encircles the outside ends 
of the lens radial ligaments. This contraction releases 
the tension on the ligaments and allows the lens to
become more convex, causing the eye to focus on
objects near at hand. The detailed focusing mechanism
is discussed in Chapters 49 and 51 in relation to func-
tion of the eyes.
G

49 and 51 in relation to func-
Glands of the Body. The nasal, lacrimal, salivary, and many
gastrointestinal glands are strongly stimulated by the
parasympathetic nervous system, usually resulting in
copious quantities of watery secretion. The glands of 
the alimentary tract most strongly stimulated by the
parasympathetics are those of the upper tract, especially
those of the mouth and stomach. On the other hand, the
glands of the small and large intestines are controlled
princi

e small and large intestines are controlled
principally by local factors in the intestinal tract itself
and by the intestinal enteric nervous system and much
less by the autonomic nerves.
Sympathetic stimulation has a direct effect on most
alimentary gland cells to cause formation of a concen-
trated secretion that contains high percentages of
enzymes and mucus. But it also causes vasoconstriction
of the blood vessels that supply the glands and in this
way sometimes reduces their rates of secret

n this
way sometimes reduces their rates of secretion.
The sweat glands secrete large quantities of sweat
when the sympathetic nerves are stimulated, but no
effect is caused by stimulating the parasympathetic
nerves. However, the sympathetic ﬁbers to most sweat
glands are cholinergic (except for a few adrenergic
ﬁbers to the palms and soles), in contrast to almost all
other sympathetic ﬁbers, which are adrenergic. Further-
more, the sweat glands are stimulated primarily by
centers in the hypotha

are stimulated primarily by
centers in the hypothalamus that are usually considered
to be parasympathetic centers. Therefore, sweating
could be called a parasympathetic function,even though
it is controlled by nerve ﬁbers that anatomically are dis-
tributed through the sympathetic nervous system.
The apocrine glands in the axillae secrete a thick,
odoriferous secretion as a result of sympathetic stimu-
lation, but they do not respond to parasympathetic stim-
ulation. This secretion actually func

thetic stim-
ulation. This secretion actually functions as a lubricant
to allow easy sliding motion of the inside surfaces under
the shoulder joint. The apocrine glands, despite their
close embryological relation to sweat glands, are acti-
vated by adrenergic ﬁbers rather than by cholinergic
ﬁbers and are also controlled by the sympathetic centers
of the central nervous system rather than by the
parasympathetic centers.
Intramural Nerve Plexus of the Gastrointestinal System. The
gastrointestinal

 the Gastrointestinal System. The
gastrointestinal system has its own intrinsic set of nerves
known as the intramural plexus or the intestinal enteric
nervous system,located in the walls of the gut.Also,both
parasympathetic and sympathetic stimulation originat-
ing in the brain can affect gastrointestinal activity
mainly by increasing or decreasing speciﬁc actions in 
the gastrointestinal intramural plexus. Parasympathetic
stimulation, in general, increases overall degree of activ-
ity of the ga

, increases overall degree of activ-
ity of the gastrointestinal tract by promoting peristalsis
and relaxing the sphincters, thus allowing rapid propul-
sion of contents along the tract. This propulsive effect 
is associated with simultaneous increases in rates of
Table 60–1
Adrenergic Receptors and Function
Alpha Receptor
Beta Receptor
Vasodilation (b2)
Iris dilation
Cardioacceleration (b1)
Intestinal relaxation
Increased myocardial 
strength (b1)
Intestinal sphincter contracti

dial 
Intestinal sphincter contraction
Intestinal relaxation (b2)
Uterus relaxation (b2)
Pilomotor contraction
Bronchodilation (b2)
Bladder sphincter contraction
Calorigenesis (b2)
Glycogenolysis (b2)
Lipolysis (b1)
Bladder wall relaxation (b2)
754
Unit XI
secretion by many of the gastrointestinal glands,
Normal function of the gastrointestinal tract is not
very dependent on sympathetic stimulation. How

not
very dependent on sympathetic stimulation. However,
strong sympathetic stimulation inhibits peristalsis and
increases the tone of the sphincters. The net result is
greatly slowed propulsion of food through the tract and
sometimes decreased secretion as well—even to the
extent of sometimes causing constipation.
Heart. In general, sympathetic stimulation increases the
overall activity of the heart. This is accomplished by
increasing both the rate and force of heart contraction.

te and force of heart contraction.
Parasympathetic stimulation causes mainly opposite
effects—decreased heart rate and strength of contrac-
tion. To express these effects in another way, sympa-
thetic stimulation increases the effectiveness of the
heart as a pump, as required during heavy exercise,
whereas parasympathetic stimulation decreases heart
pumping, allowing the heart to rest between bouts of
strenuous activity.
Systemic Blood Vessels. Most systemic blood vessels, espe-
cially those of 

ost systemic blood vessels, espe-
cially those of the abdominal viscera and skin of the
limbs, are constricted by sympathetic stimulation.
Parasympathetic stimulation has almost no effects on
most blood vessels except to dilate vessels in certain
restricted areas, such as in the blush area of the face.
Under some conditions, the beta function of the sym-
pathetics causes vascular dilation instead of the usual
sympathetic vascular constriction, but this occurs rarely
except after drugs have paral

t this occurs rarely
except after drugs have paralyzed the sympathetic alpha
Table 60–2
Autonomic Effects on Various Organs of the Body
Organ
Effect of Sympathetic Stimulation
Effect of Parasympathetic Stimulation
Eye
Pupil
Dilated
Constricted
Slight relaxation (far vision)
Constricted (near vision)
Glands
Vasoconstriction and slight secretion
Stimulation of copious secretion (containing many enzymes for
Nasal
enzyme-secreting glands)
Lacrimal
Parotid
Gastric
Pancrea

ds)
Lacrimal
Parotid
Gastric
Sweat glands
Copious sweating (cholinergic)
Sweating on palms of hands
Apocrine glands
Thick, odoriferous secretion
None
Blood vessels
Most often constricted
Most often little or no effect
Heart
Muscle
Increased rate
Slowed rate
Increased force of contraction
Decreased force of contraction (especially of atria)
Coronaries
Dilated (b2); constricted (a)
Dilated
Lungs
Bronchi
Dilated
Mildly constricted
? Dilated
Gut
Lum

? Dilated
Gut
Lumen
Decreased peristalsis and tone
Increased peristalsis and tone
Sphincter
Increased tone (most times)
Relaxed (most times)
Liver
Glucose released
Slight glycogen synthesis
Gallbladder and bile ducts
Relaxed
Kidney
Decreased output and renin secretion
None
Bladder
Detrusor
Relaxed (slight)
Trigone
Relaxed
Penis
Erection
Systemic arterioles
Abdominal viscera
None
Muscle
Constricted (adrenerg

cera
None
Muscle
Constricted (adrenergic a)
None
Dilated (adrenergic b2)
Dilated (cholinergic)
Skin
None
Blood
Increased
None
Glucose
Increased
None
Lipids
Increased
None
Basal metabolism
Increased up to 100%
None
Adrenal medullary secretion
Increased
None
Mental activity
Increased
None
Piloerector muscles
None
None
Increased strength
Fat cells
Lipolysis
None
Chapter 60
The Autonomic Nervous System and the Ad

Chapter 60
755
vasoconstrictor effects, which, in blood vessels, are
usually far dominant over the beta effects.
Effect of Sympathetic and Parasympathetic Stimulation on Arter-
ial Pressure. The arterial pressure is determined by two
factors: propulsion of blood by the heart and resistance
to ﬂow of blood through the peripheral blood vessels.
Sympathetic stimulation increases both propulsion by
the heart and resistance to ﬂow, which usually ca

the heart and resistance to ﬂow, which usually causes a
marked acute increase in arterial pressure but often very
little change in long-term pressure unless the sympa-
thetics stimulate the kidneys to retain salt and water at
the same time.
Conversely, moderate parasympathetic stimulation
via the vagal nerves decreases pumping by the heart but
has virtually no effect on vascular peripheral resistance.
Therefore, the usual effect is a slight decrease in arte-
rial pressure. But very strong vagal

ease in arte-
rial pressure. But very strong vagal parasympathetic
stimulation can almost stop or occasionally actually stop
the heart entirely for a few seconds and cause tempo-
rary loss of all or most arterial pressure.
Effects of Sympathetic and Parasympathetic Stimulation on Other
Functions of the Body. Because of the great importance of
the sympathetic and parasympathetic control systems,
they are discussed many times in this text in relation 
to multiple body functions. In general, most o

on 
to multiple body functions. In general, most of the 
entodermal structures, such as the ducts of the liver,
gallbladder, ureter, urinary bladder, and bronchi, are
inhibited by sympathetic stimulation but excited by
parasympathetic stimulation. Sympathetic stimulation
also has multiple metabolic effects such as release of
glucose from the liver, increase in blood glucose con-
centration, increase in glycogenolysis in both liver and
muscle, increase in skeletal muscle strength, increase in
bas

rease in skeletal muscle strength, increase in
basal metabolic rate, and increase in mental activity.
Finally, the sympathetics and parasympathetics are
involved in execution of the male and female sexual
acts, as explained in Chapters 80 and 81.
Stimulation of the sympathetic nerves to the adrenal
medullae causes large quantities of epinephrine and
norepinephrine to be released into the circulating
blood, and these two hormones in turn are carried in
the blood t

se two hormones in turn are carried in
the blood to all tissues of the body. On the average,
about 80 per cent of the secretion is epinephrine and
20 per cent is norepinephrine, although the relative
proportions can change considerably under different
physiologic conditions.
The circulating epinephrine and norepinephrine
have almost the same effects on the different organs
as the effects caused by direct sympathetic stimulation,
except that the effects last 5 to 10 times as long because
both of 

ffects last 5 to 10 times as long because
both of these hormones are removed from the blood
slowly over a period of 2 to 4 minutes.
The circulating norepinephrine causes constriction
of essentially all the blood vessels of the body; it also
causes increased activity of the heart, inhibition of the
gastrointestinal tract, dilation of the pupils of the eyes,
and so forth.
Epinephrine causes almost the same effects as those
caused by norepinephrine, but the effects differ in the
following respects:

 but the effects differ in the
following respects: First, epinephrine, because of its
greater effect in stimulating the beta receptors, has a
greater effect on cardiac stimulation than does nor-
epinephrine. Second, epinephrine causes only weak 
constriction of the blood vessels in the muscles,in com-
parison with much stronger constriction caused by
norepinephrine. Because the muscle vessels represent
a major segment of the vessels of the body, this differ-
ence is of special importance because

this differ-
ence is of special importance because norepinephrine
greatly increases the total peripheral resistance and
elevates arterial pressure, whereas epinephrine raises
the arterial pressure to a lesser extent but increases the
cardiac output more.
A third difference between the actions of epineph-
rine and norepinephrine relates to their effects on
tissue metabolism. Epinephrine has 5 to 10 times as
great a metabolic effect as norepinephrine. Indeed, the
epinephrine secreted by the adrena

ne. Indeed, the
epinephrine secreted by the adrenal medullae can
increase the metabolic rate of the whole body often to
as much as 100 per cent above normal, in this way
increasing the activity and excitability of the body. It
also increases the rates of other metabolic activities,
such as glycogenolysis in the liver and muscle, and
glucose release into the blood.
In summary, stimulation of the adrenal medullae
causes release of the hormones epinephrine and nor-
epinephrine, which together have 

ephrine and nor-
epinephrine, which together have almost the same
effects throughout the body as direct sympathetic
stimulation, except that the effects are greatly pro-
longed, lasting 2 to 4 minutes after the stimulation is
over.
Value of the Adrenal Medullae to the Function of the Sympa-
thetic Nervous System. Epinephrine and norepinephrine
are almost always released by the adrenal medullae at
the same time that the different organs are stimulated
directly by generalized sympathetic activatio

ated
directly by generalized sympathetic activation. There-
fore, the organs are actually stimulated in two ways:
directly by the sympathetic nerves and indirectly by
the adrenal medullary hormones. The two means 
of stimulation support each other, and either can, in
most instances, substitute for the other. For instance,
destruction of the direct sympathetic pathways to 
the different body organs does not abrogate sympa-
thetic excitation of the organs because norepinephrine
and epinephrine are

 organs because norepinephrine
and epinephrine are still released into the circulating
blood and indirectly cause stimulation. Likewise,
loss of the two adrenal medullae usually has little
effect on the operation of the sympathetic nervous
system because the direct pathways can still perform
almost all the necessary duties. Thus, the dual mecha-
nism of sympathetic stimulation provides a safety
factor, one mechanism substituting for the other if it is
missing.
Another important value of the adre

it is
missing.
Another important value of the adrenal medullae 
is the capability of epinephrine and norepinephrine 
to stimulate structures of the body that are not 
innervated by direct sympathetic ﬁbers. For instance,
the metabolic rate of every cell of the body is increased
by these hormones,
especially by epinephrine,
even though only a small proportion of all the cells 
in the body are innervated directly by sympathetic
ﬁbers.
756
Unit XI
The Nervous System: C. Motor and Integrative Neurop

he Nervous System: C. Motor and Integrative Neurophysiology
Relation of Stimulus Rate 
to Degree of Sympathetic and
Parasympathetic Effect
A special difference between the autonomic nervous
system and the skeletal nervous system is that only a
low frequency of stimulation is required for full acti-
vation of autonomic effectors. In general, only one
nerve impulse every few seconds sufﬁces to maintain
normal sympathetic or parasympathetic effect, and full
activation occurs when the nerve ﬁbers di

and full
activation occurs when the nerve ﬁbers discharge 10 to
20 times per second.This compares with full activation
in the skeletal nervous system at 50 to 500 or more
impulses per second.
Sympathetic and 
Parasympathetic “Tone”
Normally,
the sympathetic and parasympathetic
systems are continually active, and the basal rates of
activity are known, respectively, as sympathetic tone
and parasympathetic tone.
The value of tone is that it allows a single nervous
system both to increase and to dec

 single nervous
system both to increase and to decrease the activity of
a stimulated organ. For instance, sympathetic tone nor-
mally keeps almost all the systemic arterioles con-
stricted to about one half their maximum diameter.
By increasing the degree of sympathetic stimulation
above normal, these vessels can be constricted even
more; conversely, by decreasing the stimulation below
normal, the arterioles can be dilated. If it were not for
the continual background sympathetic tone, the sym-
p

 continual background sympathetic tone, the sym-
pathetic system could cause only vasoconstriction,
never vasodilation.
Another interesting example of tone is the back-
ground “tone”
of the parasympathetics in the 
gastrointestinal tract.Surgical removal of the parasym-
pathetic supply to most of the gut by cutting the vagus
nerves can cause serious and prolonged gastric and
intestinal “atony” with resulting blockage of much of
the normal gastrointestinal propulsion and consequent
serious 
const

ntestinal propulsion and consequent
serious 
constipation,
thus 
demonstrating 
that
parasympathetic tone to the gut is normally very much
required. This tone can be decreased by the brain,
thereby inhibiting gastrointestinal motility, or it can be
increased, thereby promoting increased gastrointesti-
nal activity.
Tone Caused by Basal Secretion of Epinephrine and Norepi-
nephrine by the Adrenal Medullae. The normal resting 
rate of secretion by the adrenal medullae is about 
0.2 mg/kg/min of ep

the adrenal medullae is about 
0.2 mg/kg/min of epinephrine and about 0.05 mg/kg/min
of norepinephrine.These quantities are considerable—
indeed, enough to maintain the blood pressure almost
up to normal even if all direct sympathetic pathways to
the cardiovascular system are removed. Therefore, it is
obvious that much of the overall tone of the sympa-
thetic nervous system results from basal secretion of
epinephrine and norepinephrine in addition to the tone
resulting from direct sympathetic st

n to the tone
resulting from direct sympathetic stimulation.
Effect of Loss of Sympathetic or Parasympathetic Tone After
Denervation. Immediately after a sympathetic or
parasympathetic nerve is cut, the innervated organ
loses its sympathetic or parasympathetic tone. In the
case of the blood vessels, for instance, cutting the sym-
pathetic nerves results within 5 to 30 seconds in almost
maximal vasodilation. However, over minutes, hours,
days, or weeks, intrinsic tone in the smooth muscle of
the 

weeks, intrinsic tone in the smooth muscle of
the vessels increases—that is, increased tone caused by
increased smooth muscle contractile force that is not
the result of sympathetic stimulation but of chemical
adaptations in the smooth muscle ﬁbers themselves.
This intrinsic tone eventually restores almost normal
vasoconstriction.
Essentially the same effects occur in most other
effector organs whenever sympathetic or parasympa-
thetic tone is lost. That is, intrinsic compensation soon
develops 

st. That is, intrinsic compensation soon
develops to return the function of the organ almost to
its normal basal level. However, in the parasympa-
thetic system, the compensation sometimes requires
many months. For instance, loss of parasympathetic
tone to the heart after cardiac vagotomy increases the
heart rate to 160 beats per minute in a dog, and this
will still be partially elevated 6 months later.
Denervation Supersensitivity of
Sympathetic and Parasympathetic
Organs After Denervation
Duri

 and Parasympathetic
During the ﬁrst week or so after a sympathetic or
parasympathetic nerve is destroyed, the innervated
organ becomes more sensitive to injected norepineph-
rine or acetylcholine, respectively. This effect is demon-
strated in Figure 60–4, showing blood ﬂow in the
forearm before removal of the sympathetics to be about
200 ml/min; a test dose of norepinephrine causes only a
slight depression in ﬂow lasting a minute or so.Then the
stellate ganglion is rem

g a minute or so.Then the
stellate ganglion is removed, and normal sympathetic
tone is lost. At ﬁrst, the blood ﬂow rises markedly
because of the lost vascular tone, but over a period of
days to weeks the blood ﬂow returns much of the way
back toward normal because of progressive increase in
400
Normal
Effect of test dose
of norepinephrine
Stellate
ganglionectomy
Effect of same
test dose of
norepinephrine
Blood flow in arm (ml/min)
200
0
0
1
2
3
4
5
6
Weeks
Weeks
Figure 60–4
Effect of sympathect

4
5
6
Weeks
Weeks
Effect of sympathectomy on blood ﬂow in the arm, and effect of a
test dose of norepinephrine before and after sympathectomy,
showing supersensitization of the vasculature to norepinephrine.
Chapter 60
757
intrinsic tone of the vascular musculature itself, thus
partially compensating for the loss of sympathetic tone.
Then, another test dose of norepinephrine is adminis-
tered, and the blood ﬂow decreases much more 

nis-
tered, and the blood ﬂow decreases much more than
before, demonstrating that the blood vessels have
become about two to four times as responsive to nor-
epinephrine as previously. This phenomenon is called
denervation supersensitivity. It occurs in both sympa-
thetic and parasympathetic organs but to far greater
extent in some organs than in others, occasionally
increasing the response more than 10-fold.
Mechanism of Denervation Supersensitivity. The cause of
denervation supersensitivity is

vity. The cause of
denervation supersensitivity is only partially known.
Part of the answer is that the number of receptors in 
the postsynaptic membranes of the effector cells
increases—sometimes manyfold—when norepineph-
rine or acetylcholine is no longer released at the
synapses, a process called “up-regulation” of the recep-
tors. Therefore, when a dose of the hormone is now
injected into the circulating blood, the effector reaction
is vastly enhanced.
Many visceral functio

 enhanced.
Many visceral functions of the body are regulated by
autonomic reﬂexes. Throughout this text, the functions
of these reﬂexes are discussed in relation to individual
organ systems; to illustrate their importance, a few are
presented here brieﬂy.
Cardiovascular Autonomic Reﬂexes. Several reﬂexes in the
cardiovascular system help to control especially the
arterial blood pressure and the heart rate. One of these
is the baroreceptor reﬂex, which is described in Chapter
18

roreceptor reﬂex, which is described in Chapter
18 along with other cardiovascular reﬂexes. Brieﬂy,
stretch receptors called baroreceptors are located in the
walls of several major arteries, including especially the
internal carotid arteries and the arch of the aorta.When
these become stretched by high pressure, signals are
transmitted to the brain stem, where they inhibit the
sympathetic impulses to the heart and blood vessels and
excite the parasympathetics; this allows the arterial
pressure t

asympathetics; this allows the arterial
pressure to fall back toward normal.
Gastrointestinal Autonomic Reﬂexes. The uppermost part of
the gastrointestinal tract and the rectum are controlled
principally by autonomic reﬂexes. For instance, the
smell of appetizing food or the presence of food in 
the mouth initiates signals from the nose and mouth 
to the vagal, glossopharyngeal, and salivatory nuclei 
of the brain stem.These in turn transmit signals through
the parasympathetic nerves to the secr

als through
the parasympathetic nerves to the secretory glands of
the mouth and stomach, causing secretion of digestive
juices sometimes even before food enters the mouth.
When fecal matter ﬁlls the rectum at the other end 
of the alimentary canal, sensory impulses initiated by
stretching the rectum are sent to the sacral portion of
the spinal cord, and a reﬂex signal is transmitted back
through the sacral parasympathetics to the distal parts
of the colon; these result in strong peristaltic cont

the colon; these result in strong peristaltic contrac-
tions that cause defecation.
Other Autonomic Reﬂexes. Emptying of the urinary bladder
is controlled in the same way as emptying the rectum;
stretching of the bladder sends impulses to the sacral
cord, and this in turn causes reﬂex contraction of the
bladder and relaxation of the urinary sphincters,
thereby promoting micturition.
Also important are the sexual reﬂexes, which are ini-
tiated both by psychic stimuli from the brain and by
stimuli

h by psychic stimuli from the brain and by
stimuli from the sexual organs. Impulses from these
sources converge on the sacral cord and, in the male,
result ﬁrst in erection, mainly a parasympathetic func-
tion, and then ejaculation, partially a sympathetic 
function.
Other autonomic control functions include reﬂex
contributions to the regulation of pancreatic secretion,
gallbladder emptying, kidney excretion of urine, sweat-
ing, blood glucose concentration, and many other vis-
ceral functions, 

ncentration, and many other vis-
ceral functions, all of which are discussed in detail at
other points in this text.
Stimulation of Discrete
Organs in Some Instances and
Mass Stimulation in Other
Instances by the Sympathetic
Sympathetic System Often Responds by Mass Discharge. In
many instances, almost all portions of the sympathetic
nervous system discharge simultaneously as a com-
plete unit, a phenomenon called mass discharge. This
frequently occurs when the hypoth

 discharge. This
frequently occurs when the hypothalamus is activated
by fright or fear or severe pain. The result is a wide-
spread reaction throughout the body called the alarm
or stress response, which we shall discuss shortly.
At other times, activation occurs in isolated portions
of the sympathetic nervous system. The most impor-
tant of these are the following: (1) During the process
of heat regulation, the sympathetics control sweating
and blood ﬂow in the skin without affecting other
org

 blood ﬂow in the skin without affecting other
organs innervated by the sympathetics. (2) Many
“local reﬂexes” involving sensory afferent ﬁbers travel
centrally in the peripheral nerves to the sympathetic
ganglia and spinal cord and cause highly localized
reﬂex responses. For instance, heating a local skin area
causes local vasodilation and enhanced local sweating,
whereas cooling causes opposite effects. (3) Many of
the sympathetic reﬂexes that control gastrointestinal
functions operate by way 

control gastrointestinal
functions operate by way of nerve pathways that do
not even enter the spinal cord, merely passing from the
gut mainly to the paravertebral ganglia, and then back
to the gut through sympathetic nerves to control
motor or secretory activity.
Parasympathetic System Usually Causes Speciﬁc Localized
Responses. In contrast to the common mass discharge
response of the sympathetic system, control functions
by the parasympathetic system are much more likely
to be highly speciﬁc. 

system are much more likely
to be highly speciﬁc. For instance, parasympathetic
cardiovascular reﬂexes usually act only on the heart to
increase or decrease its rate of beating. Likewise, other
parasympathetic reﬂexes cause secretion mainly by
the mouth glands, or in other instances secretion is
mainly by the stomach glands. Finally, the rectal emp-
tying reﬂex does not affect other parts of the bowel to
a major extent.
758
Unit XI
Yet

stem: C. Motor and Integrative Neurophysiology
Yet there is often association between closely allied
parasympathetic functions. For instance, although sali-
vary secretion can occur independently of gastric
secretion, these two also often occur together, and
pancreatic secretion frequently occurs at the same
time. Also, the rectal emptying reﬂex often initiates a
urinary bladder emptying reﬂex, resulting in simulta-
neous emptying of both the bladder and the rectum.
Conversely, the bladder empty

dder and the rectum.
Conversely, the bladder emptying reﬂex can help ini-
tiate rectal emptying.
“Alarm” or “Stress” Response of the
When large portions of the sympathetic nervous
system discharge at the same time—that is, a mass dis-
charge—this increases in many ways the ability of the
body to perform vigorous muscle activity. Let us sum-
marize these ways:
1. Increased arterial pressure
2. Increased blood ﬂow to active muscles concurrent
with decreased blood ﬂow to 

ve muscles concurrent
with decreased blood ﬂow to organs such as the
gastrointestinal tract and the kidneys that are not
needed for rapid motor activity
3. Increased rates of cellular metabolism throughout
the body
4. Increased blood glucose concentration
5. Increased glycolysis in the liver and in muscle
6. Increased muscle strength
7. Increased mental activity
8. Increased rate of blood coagulation
The sum of these effects permits a person to
perform far more strenuous physical activity than
w

erform far more strenuous physical activity than
would otherwise be possible. Because either mental or
physical stress can excite the sympathetic system, it is
frequently said that the purpose of the sympathetic
system is to provide extra activation of the body in
states of stress: this is called the sympathetic stress
response.
The sympathetic system is especially strongly acti-
vated in many emotional states. For instance, in the
state of rage, which is elicited to a great extent by stim-
ulat

 which is elicited to a great extent by stim-
ulating the hypothalamus, signals are transmitted
downward through the reticular formation of the brain
stem and into the spinal cord to cause massive sym-
pathetic discharge; most aforementioned sympathetic
events ensue immediately. This is called the sympa-
thetic alarm reaction. It is also called the ﬁght or ﬂight
reaction because an animal in this state decides almost
instantly whether to stand and ﬁght or to run. In either
event, the sympathetic

d ﬁght or to run. In either
event, the sympathetic alarm reaction makes the
animal’s subsequent activities vigorous.
Medullary, Pontine, and
Mesencephalic Control of the
Many neuronal areas in the brain stem reticular sub-
stance and along the course of the tractus solitarius of
the medulla, pons, and mesencephalon, as well as in
many special nuclei (Figure 60–5), control different
autonomic functions such as arterial pressure, heart
rate, glandular secretion in the gast

ssure, heart
rate, glandular secretion in the gastrointestinal tract,
gastrointestinal peristalsis, and degree of contraction
of the urinary bladder. Control of each of these is dis-
cussed at appropriate points in this text. Sufﬁce it to
point out here that the most important factors con-
trolled in the brain stem are arterial pressure, heart rate,
and respiratory rate. Indeed, transection of the brain
stem above the midpontine level allows basal control
of arterial pressure to continue as befo

l control
of arterial pressure to continue as before but prevents
its modulation by higher nervous centers such as the
hypothalamus. Conversely, transection immediately
below the medulla causes the arterial pressure to fall
to less than one-half normal.
Closely associated with the cardiovascular regula-
tory centers in the brain stem are the medullary and
pontine centers for regulation of respiration, which are
discussed in Chapter 41. Although this is not consid-
ered to be an autonomic functio

his is not consid-
ered to be an autonomic function, it is one of the invol-
untary functions of the body.
Control of Brain Stem Autonomic Centers by Higher Areas.
Signals from the hypothalamus and even from the
cerebrum can affect the activities of almost all the
brain stem autonomic control centers. For instance,
stimulation in appropriate areas mainly of the poste-
rior hypothalamus can activate the medullary cardio-
vascular control centers strongly enough to increase
arterial pressure to mo

trongly enough to increase
arterial pressure to more than twice normal. Likewise,
other hypothalamic centers control body temperature,
increase or decrease salivation and gastrointes-
tinal activity, and cause bladder emptying. To some
extent, therefore, the autonomic centers in the brain
stem act as relay stations for control activities 
Heat control
Water
balance
Feeding
control
Respiratory center
Cardiac slowing
Cardiac acceleration

atory center
and vasoconstriction
Urinary bladder control
Pons
Medulla
Figure 60–5
Autonomic control areas in the brain stem and hypothalamus.
Chapter 60
759
initiated at higher levels of the brain, especially in the
In Chapters 58 and 59, it is pointed out also that
many of our behavioral responses are mediated
through (1) the hypothalamus, (2) the reticular are

hrough (1) the hypothalamus, (2) the reticular areas
of the brain stem, and (3) the autonomic nervous
system. Indeed, some higher areas of the brain can
alter function of the whole autonomic nervous system
or of portions of it strongly enough to cause severe
autonomic-induced disease such as peptic ulcer of the
stomach or duodenum, constipation, heart palpitation,
or even heart attack.
Pharmacology of the
Drugs That Act on Adrenergic
Effector Organs—
Sympathomimetic Drug

n Adrenergic
Sympathomimetic Drugs
From the foregoing discussion, it is obvious that intra-
venous injection of norepinephrine causes essentially
the same effects throughout the body as sympathetic
stimulation. Therefore, norepinephrine is called a 
sympathomimetic or adrenergic drug. Epinephrine and
methoxamine are also sympathomimetic drugs, and
there are many others. They differ from one another in
the degree to which they stimulate different sympa-
thetic effector organs and

mulate different sympa-
thetic effector organs and in their duration of action.
Norepinephrine and epinephrine have actions as short
as 1 to 2 minutes, whereas the actions of some other
commonly used sympathomimetic drugs last for 30
minutes to 2 hours.
Important drugs that stimulate speciﬁc adrenergic
receptors but not others are phenylephrine (alpha
receptors), isoproterenol (beta receptors), and albuterol
(only beta2 receptors).
Drugs That Cause Release of Norepinephrine from Nerve Endings.
C

se Release of Norepinephrine from Nerve Endings.
Certain drugs have an indirect sympathomimetic action
instead of directly exciting adrenergic effector organs.
These drugs include ephedrine, tyramine, and ampheta-
mine. Their effect is to cause release of norepinephrine
from its storage vesicles in the sympathetic nerve
endings.The released norepinephrine in turn causes the
sympathetic effects.
Drugs That Block Adrenergic Activity. Adrenergic activity
can be blocked at several points in the stim

ivity
can be blocked at several points in the stimulatory
process, as follows:
1. The synthesis and storage of norepinephrine in the
sympathetic nerve endings can be prevented. The
best known drug that causes this effect is reserpine.
2. Release of norepinephrine from the sympathetic
endings can be blocked. This is caused by
guanethidine.
3. The sympathetic alpha receptors can be blocked.
Two drugs that cause this effect are
phenoxybenzamine and phentolamine.
4. The sympathetic beta receptors ca

phentolamine.
4. The sympathetic beta receptors can be blocked.
A drug that blocks beta1 and beta2 receptors is
propranolol. One that blocks mainly beta1
receptors is metoprolol.
5. Sympathetic activity can be blocked by drugs that
block transmission of nerve impulses through the
autonomic ganglia. They are discussed in a later
section, but an important drug for blockade of both
sympathetic and parasympathetic transmission
through the ganglia is hexamethonium.
Drugs That Act on Cholinergic
Effec

hexamethonium.
Effector Organs
Parasympathomimetic 
Drugs 
(Cholinergic 
Drugs). Acetyl-
choline injected intravenously usually does not cause
exactly the same effects throughout the body as
parasympathetic stimulation because most of the acetyl-
choline is destroyed by cholinesterase in the blood and
body ﬂuids before it can reach all the effector organs.
Yet a number of other drugs that are not so rapidly
destroyed can produce typical widespread parasympa-
thetic 

 can produce typical widespread parasympa-
thetic effects, and they are called parasympathomimetic
drugs.
Two commonly used parasympathomimetic drugs are
pilocarpine and methacholine. They act directly on the
muscarinic type of cholinergic receptors.
Drugs That Have a Parasympathetic Potentiating Effect—Anti-
cholinesterase Drugs. Some drugs do not have a direct
effect on parasympathetic effector organs but do poten-
tiate the effects of the naturally secreted acetylcholine
at the parasympatheti

rally secreted acetylcholine
at the parasympathetic endings.They are the same drugs
as those discussed in Chapter 7 that potentiate the effect
of acetylcholine at the neuromuscular junction. They
include neostigmine, pyridostigmine, and ambenonium.
These drugs inhibit acetylcholinesterase, thus prevent-
ing rapid destruction of the acetylcholine liberated at
parasympathetic nerve endings. As a consequence, the
quantity of acetylcholine increases with successive
stimuli, and the degree of action 

with successive
stimuli, and the degree of action also increases.
Drugs That Block Cholinergic Activity at Effector Organs—
Antimuscarinic Drugs. Atropine and similar drugs, such as
homatropine and scopolamine, block the action of
acetylcholine on the muscarinic type of cholinergic effec-
tor organs.These drugs do not affect the nicotinic action
of acetylcholine on the postganglionic neurons or on
Drugs That Stimulate or Block
Postganglionic Neuro

mpathetic and Parasympathetic
Postganglionic Neurons
Drugs That Stimulate Autonomic Postganglionic Neurons. The
preganglionic neurons of both the parasympathetic and
the sympathetic nervous systems secrete acetylcholine
at their endings, and this acetylcholine in turn stimulates
the postganglionic neurons.
injected
acetylcholine can also stimulate the postganglionic
neurons of both systems, thereby causing at the same
time both sympathetic and parasympathetic effects
throughout the 

thetic and parasympathetic effects
throughout the body.
Nicotine is another drug that can stimulate postgan-
glionic neurons in the same manner as acetylcholine
because the membranes of these neurons all contain the
nicotinic type of acetylcholine receptor. Therefore, drugs
that cause autonomic effects by stimulating postgan-
glionic neurons are called nicotinic drugs. Some other
drugs, such as methacholine, have both nicotinic and
muscarinic actions, whereas pilocarpine has only mus-
carinic ac

ions, whereas pilocarpine has only mus-
carinic actions.
760
Unit XI
Nicotine excites both the sympathetic and parasym-
pathetic postganglionic neurons at the same time,
resulting in strong sympathetic vasoconstriction in the
abdominal organs and limbs but at the same time result-
ing in parasympathetic effects such as increased gas-
trointestinal activity and, sometimes, slowing of the
heart.
Ganglionic Blocking Drugs. Many important 

heart.
Ganglionic Blocking Drugs. Many important drugs block
impulse transmission from the autonomic preganglionic
neurons to the postganglionic neurons,
including
tetraethyl ammonium ion, hexamethonium ion, and pen-
tolinium. These drugs block acetylcholine stimulation of
the postganglionic neurons in both the sympathetic and
the parasympathetic systems simultaneously. They are
often used for blocking sympathetic activity but seldom
for blocking parasympathetic activity because their
effects o

g parasympathetic activity because their
effects of sympathetic blockade usually far overshadow
the effects of parasympathetic blockade.The ganglionic
blocking drugs especially can reduce the arterial pres-
sure in many patients with hypertension,but these drugs
are not very useful clinically because their effects are
difﬁcult to control.
Andresen MC, Doyle MW, Jin YH, Bailey TW: Cellular
mechanisms of baroreceptor integration at the nucleus
tractus solitarius. Ann N Y Acad Sci 940:13

ucleus
tractus solitarius. Ann N Y Acad Sci 940:132, 2001
Chang HY, Mashimo H, Goyal RK: Musings on the wan-
derer: what’s new in our understanding of vago-vagal
reﬂex? IV. Current concepts of vagal efferent projections
to the gut. Am J Physiol Gastrointest Liver Physiol
284:G357, 2003.
Dajas-Bailador F, Wonnacott S: Nicotinic acetylcholine
receptors and the regulation of neuronal signalling.Trends
Pharmacol Sci 25:317, 2004.
Dampney RA, Horiuchi J, Tagawa T, et al: Medullary and
supramedullary 

 J, Tagawa T, et al: Medullary and
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Physiol Rev 77:75, 1997.
Gibbins IL, Jobling P, Morris JL: Functional organization 
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Goldstein DS: Catecholamines and stress. Endocr Regul
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adrenergic receptor sig-
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2001.
Saper CB: The central autonomic nervous system: conscious
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Taylor EW, Jordan D, Coote JH: Central cont

 2002.
Wanamaker CP, Christianson JC, Green WN: Regulation of
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C
H
A
P
T
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6
1
761
Cerebral Blood Flow,
Cerebrospinal Flu

R
 
6
1
761
Cerebrospinal Fluid, and Brain
Thus far, we have discussed the function of the brain as
if it were independent of its blood ﬂow, its metabolism,
and its ﬂuids. However, this is far from true because
abnormalities of any of these can profoundly affect
brain function. For instance, total cessation of blood
ﬂow to the brain causes unconsciousness within 5 to 10
seconds. This occurs because lack of oxygen delivery to
the brain cells shuts down most metabol

elivery to
the brain cells shuts down most metabolism in these
cells. Also, on a longer time scale, abnormalities of the
cerebrospinal ﬂuid, either its composition or its ﬂuid pressure, can have equally
severe effects on brain function.
Normal blood ﬂow through the brain of the adult person averages 50 to 65 milli-
liters per 100 grams of brain tissue per minute. For the entire brain, this amounts to
750 to 900 ml/min, or 15 per cent of the 

ounts to
750 to 900 ml/min, or 15 per cent of the resting cardiac output.
As in most other vascular areas of the body, cerebral blood ﬂow is highly related to
metabolism of the tissue.At least three metabolic factors have potent effects in con-
trolling cerebral blood ﬂow: (1) carbon dioxide concentration, (2) hydrogen ion con-
centration, and (3) oxygen concentration.
Increase of Cerebral Blood Flow in Response to Excess Carbon Dioxide or Excess Hydrogen Ion
Co

to Excess Carbon Dioxide or Excess Hydrogen Ion
Concentration. An increase in carbon dioxide concentration in the arterial blood per-
fusing the brain greatly increases cerebral blood ﬂow.This is demonstrated in Figure
61–1, which shows that a 70 per cent increase in arterial PCO2 approximately doubles
cerebral blood ﬂow.
Carbon dioxide is believed to increase cerebral blood ﬂow by combining ﬁrst with
water in the body ﬂuids to form carbonic acid, with subsequent dissociation of this
acid to for

, with subsequent dissociation of this
acid to form hydrogen ions. The hydrogen ions then cause vasodilation of the cere-
bral vessels—the dilation being almost directly proportional to the increase in
hydrogen ion concentration up to a blood ﬂow limit of about twice normal.
Any other substance that increases the acidity of the brain tissue, and therefore
increases hydrogen ion concentration, will likewise increase cerebral blood ﬂow.
Such substances include lactic acid, pyruvic acid, and any ot

nces include lactic acid, pyruvic acid, and any other acidic material
formed during the course of tissue metabolism.
Importance of Cerebral Blood Flow Control by Carbon Dioxide and Hydrogen Ions. Increased
hydrogen ion concentration greatly depresses neuronal activity. Therefore, it is for-
tunate that an increase in hydrogen ion concentration also causes an increase in
blood ﬂow, which in turn carries hydrogen ions, carbon dioxide, and other acid-
forming substances away from the brain tissues.

d-
forming substances away from the brain tissues. Loss of carbon dioxide removes
carbonic acid from the tissues; this, along with removal of other acids, reduces the
hydrogen ion concentration back toward normal. Thus, this mechanism helps main-
tain a constant hydrogen ion concentration in the cerebral ﬂuids and thereby helps
to maintain a normal, constant level of neuronal activity.
Oxygen Deﬁciency as a Regulator of Cerebral Blood Flow. Except during periods of intense
brain activity, the ra

t during periods of intense
brain activity, the rate of utilization of oxygen by the brain tissue remains within
762
Unit XI
narrow limits—almost exactly 3.5 (± 0.2) milliliters of
oxygen per 100 grams of brain tissue per minute. If
blood ﬂow to the brain ever becomes insufﬁcient to
supply this needed amount of oxygen, the oxygen deﬁ-
ciency mechanism for causing vasodilation immediately
causes vasodilation, returning the brain blood ﬂ

y
causes vasodilation, returning the brain blood ﬂow and
transport of oxygen to the cerebral tissues to near
normal. Thus, this local blood ﬂow regulatory mecha-
nism is almost exactly the same in the brain as in coro-
nary blood vessels, in skeletal muscle, and in most other
circulatory areas of the body.
Experiments have shown that a decrease in cerebral
tissue PO2 below about 30 mm Hg (normal value is 35 to
40 mm Hg) immediately begins to increase cerebral
blood ﬂow. This is fortuitous becaus

ease cerebral
blood ﬂow. This is fortuitous because brain function
becomes deranged at not much lower values of PO2,
especially so at PO2 levels below 20 mm Hg. Even coma
can result at these low levels. Thus, the oxygen mecha-
nism for local regulation of cerebral blood ﬂow is a very
important protective response against diminished cere-
bral neuronal activity and, therefore, against derange-
ment of mental capability.
Measurement of Cerebral Blood Flow, and Effect of Brain Activ-
ity on the Flo

od Flow, and Effect of Brain Activ-
ity on the Flow. A method has been developed to record
blood ﬂow in as many as 256 isolated segments of the
human cerebral cortex simultaneously. To do this, a
radioactive substance, such as radioactive xenon, is
injected into the carotid artery; then the radioactivity of
each segment of the cortex is recorded as the radioac-
tive substance passes through the brain tissue. For this
purpose, 256 small radioactive scintillation detectors are
pressed against the 

e scintillation detectors are
pressed against the surface of the cortex. The rapidity
of rise and decay of radioactivity in each tissue segment
is a direct measure of the rate of blood ﬂow through that
segment.
Using this technique, it has become clear that blood
ﬂow in each individual segment of the brain changes as
much as 100 to 150 per cent within seconds in response
to changes in local neuronal activity. For instance,
simply making a ﬁst of the hand causes an immedi-
ate increase in blood ﬂ

the hand causes an immedi-
ate increase in blood ﬂow in the motor cortex of the
opposite side of the brain. Reading a book increases the
blood ﬂow, especially in the visual areas of the occipital
cortex and in the language perception areas of the tem-
poral cortex.This measuring procedure can also be used
for localizing the origin of epileptic attacks because
local brain blood ﬂow increases acutely and markedly
at the focal point of each attack.
Demonstrating the effect of local neuronal activit

Demonstrating the effect of local neuronal activity on
cerebral blood ﬂow, Figure 61–2 shows a typical increase
in occipital blood ﬂow recorded in a cat’s brain when
intense light is shined into its eyes for one-half minute.
Autoregulation of Cerebral Blood Flow When the Arterial Pressure
Changes. Cerebral 
blood 
ﬂow 
is “autoregulated”
extremely well between arterial pressure limits of 60
and 140 mm Hg. That is, mean arterial pressure can be
decreased acutely to as low as 60 mm Hg or increased

creased acutely to as low as 60 mm Hg or increased
to as high as 140 mm Hg without signiﬁcant change in
cerebral blood ﬂow. And, in people who have hyper-
tension, autoregulation of cerebral blood ﬂow occurs
even when the mean arterial pressure rises to as high as
160 to 180 mm Hg. This is demonstrated in Figure 61–3,
which shows cerebral blood ﬂow measured both in
persons with normal blood pressure and in hypertensive
and hypotensive patients. Note the extreme constancy
of cerebral blood ﬂow be

ote the extreme constancy
of cerebral blood ﬂow between the limits of 60 and 
180 mm Hg mean arterial pressure. But, if the arterial
pressure falls below 60 mm Hg, cerebral blood ﬂow then
does become severely decreased.
Role of the Sympathetic Nervous System in Controlling Cerebral
Blood Flow. The cerebral circulatory system has strong
sympathetic innervation that passes upward from the
superior cervical sympathetic ganglia in the neck and
then into the brain along with the cerebral arteries.Thi

nto the brain along with the cerebral arteries.This
innervation supplies both the large brain arteries and
the arteries that penetrate into the substance of the
brain. However, transection of the sympathetic nerves
or mild to moderate stimulation of them usually causes
very little change in cerebral blood ﬂow because the
blood ﬂow autoregulation mechanism can override the
nervous effects.
When mean arterial pressure rises acutely to an
exceptionally high level, such as during strenuous 
Normal
C

lly high level, such as during strenuous 
Normal
Cerebral blood flow (times normal)
0.4
0
20
40
60
80
100
2.0
1.6
1.2
0.8
Arterial Pco2
Figure 61–1
Relationship between arterial PCO2 and cerebral blood ﬂow.
100
110
120
130
0
0.5
Light shining
in eyes
1.0
1.5
140
Minutes
Minutes
Figure 61–2
Increase in blood ﬂow to the occipital regions of a cat’s brain when
light is shined into its eyes.
Chapter 61
Cerebral Blood Flow, Cerebrospinal Fluid, and Brain 

rebral Blood Flow, Cerebrospinal Fluid, and Brain Metabolism
763
exercise or during other states of excessive circulatory
activity, the sympathetic nervous system normally con-
stricts the large- and intermediate-sized brain arteries
enough to prevent the high pressure from reaching the
smaller brain blood vessels. This is important in pre-
venting vascular hemorrhages into the brain—that is,
for preventing the occurrence of “cerebral stroke.”
As is true for almost all 

rebral Microcirculation
As is true for almost all other tissues of the body, the
number of blood capillaries in the brain is greatest
where the metabolic needs are greatest. The overall
metabolic rate of the brain gray matter where the neu-
ronal cell bodies lie is about four times as great as that
of white matter; correspondingly, the number of capil-
laries and rate of blood ﬂow are also about four times
as great in the gray matter.
An important structural characteristic of the brain
capillari

t structural characteristic of the brain
capillaries is that they are much less “leaky” than the
blood capillaries in almost any other tissue of the body.
One reason for this is that the capillaries are supported
on all sides by “glial feet,” which are small projections
from the surrounding glial cells that abut against all sur-
faces of the capillaries and provide physical support to
prevent overstretching of the capillaries in case of high
capillary blood pressure.
The walls of the small arter

llary blood pressure.
The walls of the small arterioles leading to the brain
capillaries become greatly thickened in people who
develop high blood pressure, and these arterioles
remain signiﬁcantly constricted all the time to prevent
transmission of the high pressure to the capillaries. We
shall see later in the chapter that whenever these
systems for protecting against transudation of ﬂuid into
the brain break down, serious brain edema ensues,
which can lead rapidly to coma and death.
Cerebral 

hich can lead rapidly to coma and death.
Cerebral “Stroke” Occurs When
Cerebral Blood Vessels Are Blocked
Almost all elderly people have blockage of some small
arteries in the brain, and as many as 10 per cent even-
tually have enough blockage to cause serious distur-
bance of brain function, a condition called a “stroke.”
Most strokes are caused by arteriosclerotic plaques
that occur in one or more of the feeder arteries to the
brain. The plaques can activate the clotting mechanism
of the blood

s can activate the clotting mechanism
of the blood, causing a blood clot to occur and block
blood ﬂow in the artery, thereby leading to acute loss of
brain function in a localized area.
In about one quarter of people who develop strokes,
high blood pressure makes one of the blood vessels
burst; hemorrhage then occurs, compressing the local
brain tissue and further compromising its functions.The
neurological effects of a stroke are determined by the
brain area affected. One of the most common typ

he
brain area affected. One of the most common types of
stroke is blockage of the middle cerebral artery that sup-
plies the midportion of one brain hemisphere. For
instance, if the middle cerebral artery is blocked on the
left side of the brain, the person is likely to become
almost totally demented because of lost function in
Wernicke’s speech comprehension area in the left cere-
bral hemisphere, and he or she also becomes unable to
speak words because of loss of Broca’s motor area for
word fo

 because of loss of Broca’s motor area for
word formation. In addition, loss of function of neural
motor control areas of the left hemisphere can create
spastic paralysis of most muscles on the opposite side of
the body.
In a similar manner, blockage of a posterior cerebral
artery will cause infarction of the occipital pole of the
hemisphere on the same side as the blockage, which
causes loss of vision in both eyes in the half of the retina
on the same side as the stroke lesion. Especially devas

e same side as the stroke lesion. Especially devas-
tating are strokes that involve the blood supply to the
midbrain because this can block nerve conduction in
major pathways between the brain and spinal cord,
causing both sensory and motor abnormalities.
The entire cerebral cavity enclosing the brain and spinal
cord has a capacity of about 1600 to 1700 milliliters;
about 150 milliliters of this capacity is occupied by cere-
brospinal ﬂuid and the remainder by the brai

cere-
brospinal ﬂuid and the remainder by the brain and cord.
This ﬂuid, as shown in Figure 61–4, is present in the ven-
tricles of the brain, in the cisterns around the outside of
the brain, and in the subarachnoid space around both the
brain and the spinal cord. All these chambers are con-
nected with one another, and the pressure of the ﬂuid
is maintained at a surprisingly constant level.
Cushioning Function of the
A major function of the cerebrospinal ﬂuid is to cushion
t

function of the cerebrospinal ﬂuid is to cushion
the brain within its solid vault. The brain and the cere-
brospinal ﬂuid have about the same speciﬁc gravity
(only about 4 per cent different), so that the brain
simply ﬂoats in the ﬂuid. Therefore, a blow to the head,
if it is not too intense, moves the entire brain simulta-
neously with the skull, causing no one portion of the
brain to be momentarily contorted by the blow.
Cerebral blood flow (ml/100 g/min)
0
20
40
60
0
150
50
Hypert

/100 g/min)
0
20
40
60
0
150
50
100
Mean arterial blood pressure (mm Hg)
Figure 61–3
Effect of differences in mean arterial pressure, from hypotensive
to hypertensive level, on cerebral blood ﬂow in different human
beings. (Modiﬁed from Lassen NA: Cerebral blood ﬂow and
oxygen consumption in man. Physiol Rev 39:183, 1959.)
764
Unit XI
Contrecoup. When a blow to the head is e

hysiology
Contrecoup. When a blow to the head is extremely
severe, it may not damage the brain on the side of the
head where the blow is struck but on the opposite side.
This phenomenon is known as “contrecoup,” and the
reason for this effect is the following: When the blow is
struck, the ﬂuid on the struck side is so incompressible
that as the skull moves, the ﬂuid pushes the brain at the
same time in unison with the skull. On the side oppo-
site to the area that is struck, the sudden movement 

e to the area that is struck, the sudden movement of
the whole skull causes the skull to pull away from the
brain momentarily because of the brain’s inertia, creat-
ing for a split second a vacuum space in the cranial vault
in the area opposite to the blow.Then, when the skull is
no longer being accelerated by the blow, the vacuum
suddenly collapses and the brain strikes the inner
surface of the skull.
The poles and the inferior surfaces of the frontal and
temporal lobes, where the brain comes i

rontal and
temporal lobes, where the brain comes into contact with
bony protuberances in the base of the skull, are often
the sites of injury and contusions (bruises) after a severe
blow to the head, such as that experienced by a boxer.
If the contusion occurs on the same side as the impact
injury, it is a coup injury; if it occurs on the opposite side,
the contusion is a contrecoup injury.
Formation, Flow, and Absorption 
of Cerebrospinal Fluid
Cerebrospinal ﬂuid is formed at a rate of about 50

Cerebrospinal ﬂuid is formed at a rate of about 500 mil-
liliters each day, which is three to four times as much as
the total volume of ﬂuid in the entire cerebrospinal ﬂuid
system.About two thirds or more of this ﬂuid originates
as secretion from the choroid plexuses in the four ven-
tricles, mainly in the two lateral ventricles. Additional
small amounts of ﬂuid are secreted by the ependymal
surfaces of all the ventricles and by the arachnoidal
membranes; and a small amount comes from the brain

itself through the perivascular spaces that surround the
blood vessels passing through the brain.
The arrows in Figure 61–4 show that the main chan-
nels of ﬂuid ﬂow from the choroid plexuses and then
through the cerebrospinal ﬂuid system. The ﬂuid
secreted in the lateral ventricles passes ﬁrst into the third
ventricle; then, after addition of minute amounts of ﬂuid
from the third ventricle, it ﬂows downward along the
aqueduct of Sylvius into th

ows downward along the
aqueduct of Sylvius into the fourth ventricle, where still
another minute amount of ﬂuid is added. Finally, the
ﬂuid passes out of the fourth ventricle through three
small openings, two lateral foramina of Luschka and a
midline foramen of Magendie, entering the cisterna
magna, a ﬂuid space that lies behind the medulla and
beneath the cerebellum.
The cisterna magna is continuous with the subarach-
noid space that surrounds the entire brain and spinal
cord. Almost all the ce

he entire brain and spinal
cord. Almost all the cerebrospinal ﬂuid then ﬂows
upward from the cisterna magna through the subarach-
noid spaces surrounding the cerebrum. From here, the
ﬂuid ﬂows into and through multiple arachnoidal villi
that project into the large sagittal venous sinus and
other venous sinuses of the cerebrum. Thus, any extra
ﬂuid empties into the venous blood through pores of
these villi.
Secretion by the Choroid Plexus. The choroid plexus, a
section of which is shown in Figure

roid plexus, a
section of which is shown in Figure 61–5,is a cauliﬂower-
like growth of blood vessels covered by a thin layer of
epithelial cells. This plexus projects into (1 and 2) the
temporal horn of each lateral ventricle, (3) the posterior
portion of the third ventricle, and (4) the roof of the
fourth ventricle.
Secretion of ﬂuid into the ventricles by the choroid
plexus depends mainly on active transport of sodium
ions through the epithelial cells lining the outside of the
plexus.The sodi

al cells lining the outside of the
plexus.The sodium ions in turn pull along large amounts
of chloride ions as well because the positive charge of
the sodium ion attracts the chloride ion’s negative
charge. The two of these together increase the quantity
of osmotically active sodium chloride in the cere-
brospinal ﬂuid, which then causes almost immediate
osmosis of water through the membrane, thus providing
the ﬂuid of the secretion.
Choroid
plexuses
Tentorium
cerebelli
Fourth
ventricle
Foramen 

uses
Tentorium
cerebelli
Fourth
ventricle
Foramen of
Magendie
Arachnoidal
villi
Foramen of
Luschka
Aqueduct
of Sylvius
Third
ventricle
Foramen
of Monro
Lateral
ventricles
Figure 61–4
The arrows show the pathway of cerebrospinal ﬂuid ﬂow from the
choroid plexuses in the lateral ventricles to the arachnoidal villi
protruding into the dural sinuses.
Artery
Ependyma
Vein
Taenia
fornicis
Taenia
choroidea
Ependyma
Villus epithelium
Villus connective tissue
Tela
choroides
Figure 61–5
Choro

connective tissue
Tela
choroides
Choroid plexus in a lateral ventricle.
Chapter 61
Cerebral Blood Flow, Cerebrospinal Fluid, and Brain Metabolism
765
Less important transport processes move small
amounts of glucose into the cerebrospinal ﬂuid and both
potassium and bicarbonate ions out of the cerebrospinal
ﬂuid into the capillaries. Therefore, the resulting char-
acteristics of the cerebrospinal ﬂuid become the follow-
ing: osmotic pressure, approximately equal to that of
plasma; sod

essure, approximately equal to that of
plasma; sodium ion concentration, also approximately
equal to that of plasma; chloride ion, about 15 per cent
greater than in plasma; potassium ion, approximately 40
per cent less; and glucose, about 30 per cent less.
Absorption of Cerebrospinal Fluid Through the Arachnoidal Villi.
The arachnoidal villi are microscopic ﬁngerlike inward
projections of the arachnoidal membrane through the
walls and into the venous sinuses. Conglomerates of
these villi form ma

nous sinuses. Conglomerates of
these villi form macroscopic structures called arach-
noidal granulations that can be seen protruding into the
sinuses. The endothelial cells covering the villi have
been shown by electron microscopy to have vesicular
passages directly through the bodies of the cells large
enough to allow relatively free ﬂow of (1) cerebrospinal
ﬂuid, (2) dissolved protein molecules, and (3) even par-
ticles as large as red and white blood cells into the
Perivascular 

e blood cells into the
Perivascular Spaces and Cerebrospinal Fluid. The large arter-
ies and veins of the brain lie on the surface of the brain
but their ends penetrate inward, carrying with them a
layer of pia mater, the membrane that covers the brain,
as shown in Figure 61–6. The pia is only loosely adher-
ent to the vessels, so that a space, the perivascular space,
exists between it and each vessel. Therefore, perivascu-
lar spaces follow both the arteries and the veins into the

es follow both the arteries and the veins into the
brain as far as the arterioles and venules go.
Lymphatic Function of the Perivascular Spaces. As is true
elsewhere in the body, a small amount of protein leaks
out of the brain capillaries into the interstitial spaces of
the brain. Because no true lymphatics are present in
brain tissue, excess protein in the brain tissue leaves the
tissue ﬂowing with ﬂuid through the perivascular spaces
into the subarachnoid spaces. On reaching the sub-
arachnoi

subarachnoid spaces. On reaching the sub-
arachnoid spaces, the protein then ﬂows with the cere-
brospinal ﬂuid, to be absorbed through the arachnoidal
villi into the large cerebral veins. Therefore, perivascu-
lar spaces, in effect, are a specialized lymphatic system
for the brain.
In addition to transporting ﬂuid and proteins, the
perivascular spaces transport extraneous particulate
matter out of the brain. For instance, whenever infec-
tion occurs in the brain, dead white blood cells and
othe

curs in the brain, dead white blood cells and
other infectious debris are carried away through the
perivascular spaces.
The normal pressure in the cerebrospinal ﬂuid system
when one is lying in a horizontal position averages 130
millimeters of water (10 mm Hg), although this may be
as low as 65 millimeters of water or as high as 195 mil-
limeters of water even in the normal healthy person.
Regulation of Cerebrospinal Fluid Pressure by the Arachnoidal
Villi. The norma

Fluid Pressure by the Arachnoidal
Villi. The normal rate of cerebrospinal ﬂuid formation
remains very nearly constant, so that changes in ﬂuid
formation are seldom a factor in pressure control. Con-
versely, the arachnoidal villi function like “valves” that
allow cerebrospinal ﬂuid and its contents to ﬂow readily
into the blood of the venous sinuses while not allowing
blood to ﬂow backward in the opposite direction. Nor-
mally, this valve action of the villi allows cerebrospinal
ﬂuid to begin to

of the villi allows cerebrospinal
ﬂuid to begin to ﬂow into the blood when cerebrospinal
ﬂuid pressure is about 1.5 mm Hg greater than the pres-
sure of the blood in the venous sinuses.Then, if the cere-
brospinal ﬂuid pressure rises still higher, the valves open
more widely, so that under normal conditions, the cere-
brospinal ﬂuid pressure almost never rises more than a
few millimeters of mercury higher than the pressure in
the cerebral venous sinuses.
Conversely, in disease states, the villi 

sinuses.
Conversely, in disease states, the villi sometimes
become blocked by large particulate matter, by ﬁbrosis,
or by excesses of blood cells that have leaked into the
cerebrospinal ﬂuid in brain diseases. Such blockage can
cause high cerebrospinal ﬂuid pressure, as follows.
High Cerebrospinal Fluid Pressure in Pathological Conditions of
the Brain. Often a large brain tumor elevates the cere-
brospinal ﬂuid pressure by decreasing reabsorption of
the cerebrospinal ﬂuid back into the blood. As

 of
the cerebrospinal ﬂuid back into the blood. As a result,
the cerebrospinal ﬂuid pressure can rise to as much as
500 millimeters of water (37 mm Hg) or about four
The cerebrospinal ﬂuid pressure also rises consider-
ably when hemorrhage or infection occurs in the cranial
vault. In both these conditions, large numbers of red
and/or white blood cells suddenly appear in the cere-
brospinal ﬂuid, and they can cause serious blockage of
the small absorption channels through the arachn

f
the small absorption channels through the arachnoidal
villi. This also sometimes elevates the cerebrospinal
ﬂuid pressure to 400 to 600 millimeters of water (about
four times normal).
Some babies are born with high cerebrospinal ﬂuid
pressure.This is often caused by abnormally high resist-
ance to ﬂuid reabsorption through the arachnoidal villi,
resulting either from too few arachnoidal villi or from
villi with abnormal absorptive properties. This is dis-
cussed later in connection with hydroc

his is dis-
cussed later in connection with hydrocephalus.
Measurement of Cerebrospinal Fluid Pressure. The usual pro-
cedure for measuring cerebrospinal ﬂuid pressure is
very simple and is the following: First, the person lies
exactly horizontally on his or her side so that the ﬂuid
Arachnoid membrane
Arachnoid trabecula
Subarachnoid space
Pia mater
Perivascular space
Brain tissue
Figure 61–6
Drainage of a perivascular space into the subarachnoid space.
(Redrawn from Ranson SW, Cla

e subarachnoid space.
(Redrawn from Ranson SW, Clark SL: Anatomy of the Nervous
System. Philadelphia: WB Saunders Co, 1959.)
766
Unit XI
pressure in the spinal canal is equal to the pressure in
the cranial vault. A spinal needle is then inserted into
the lumbar spinal canal below the lower end of the cord,
and the needle is connected to a vertical glass tube that
is open to the air at its top. The spinal ﬂuid is allowed
to rise in the 

ts top. The spinal ﬂuid is allowed
to rise in the tube as high as it will. If it rises to a level
136 millimeters above the level of the needle, the pres-
sure is said to be 136 millimeters of water pressure or,
dividing this by 13.6, which is the speciﬁc gravity of
mercury, about 10 mm Hg pressure.
High Cerebrospinal Fluid Pressure Causes Edema of the Optic
Disc—Papilledema. Anatomically, the dura of the brain
extends as a sheath around the optic nerve and then
connects with the sclera of the e

c nerve and then
connects with the sclera of the eye. When the pressure
rises in the cerebrospinal ﬂuid system, it also rises inside
the optic nerve sheath. The retinal artery and vein
pierce this sheath a few millimeters behind the eye and
then pass along with the optic nerve ﬁbers into the eye
itself. Therefore, (1) high cerebrospinal ﬂuid pressure
pushes ﬂuid ﬁrst into the optic nerve sheath and then
along the spaces between the optic nerve ﬁbers to the
interior of the eyeball; (2) the high p

ers to the
interior of the eyeball; (2) the high pressure decreases
outward ﬂuid ﬂow in the optic nerves, causing accumu-
lation of excess ﬂuid in the optic disc at the center of
the retina; and (3) the pressure in the sheath also
impedes ﬂow of blood in the retinal vein, thereby
increasing the retinal capillary pressure throughout the
eye, which results in still more retinal edema.
The tissues of the optic disc are much more distensi-
ble than those of the remainder of the retina, so that the
d

hose of the remainder of the retina, so that the
disc becomes far more edematous than the remainder
of the retina and swells into the cavity of the eye. The
swelling of the disc can be observed with an ophthal-
moscope and is called papilledema. Neurologists can
estimate the cerebrospinal ﬂuid pressure by assessing
the extent to which the edematous optic disc protrudes
into the eyeball.
Obstruction to Flow of
Cerebrospinal Fluid Can 
Cause Hydrocephalus
“Hydrocephalus” means excess water in the 

ephalus
“Hydrocephalus” means excess water in the cranial
vault. This condition is frequently divided into commu-
nicating hydrocephalus and noncommunicating hydro-
cephalus. In communicating hydrocephalus ﬂuid ﬂows
readily from the ventricular system into the subarach-
noid space, whereas in noncommunicating hydro-
cephalus ﬂuid ﬂow out of one or more of the ventricles
is blocked.
Usually the noncommunicating
type of hydro-
cephalus is caused by a block in the aqueduct of Sylvius,
resulting fro

 a block in the aqueduct of Sylvius,
resulting from atresia (closure) before birth in many
babies or from blockage by a brain tumor at any age.
As ﬂuid is formed by the choroid plexuses in the two
lateral and the third ventricles, the volumes of these
three ventricles increase greatly. This ﬂattens the brain
into a thin shell against the skull. In neonates, the
increased pressure also causes the whole head to swell
because the skull bones have not yet fused.
The communicating type of hydrocephal

t yet fused.
The communicating type of hydrocephalus is usually
caused by blockage of ﬂuid ﬂow in the subarachnoid
spaces around the basal regions of the brain or by block-
age of the arachnoidal villi where the ﬂuid is normally
absorbed into the venous sinuses. Fluid therefore col-
lects both on the outside of the brain and to a lesser
extent inside the ventricles.This will also cause the head
to swell tremendously if it occurs in infancy when the
skull is still pliable and can be stretched, an

he
skull is still pliable and can be stretched, and it can
damage the brain at any age. A therapy for many types
of hydrocephalus is surgical placement of a silicone tube
shunt all the way from one of the brain ventricles to the
peritoneal cavity where the excess ﬂuid can be absorbed
Blood–Cerebrospinal Fluid and
Blood-Brain Barriers
It has already been pointed out that the concentrations
of several important constituents of cerebrospinal ﬂuid
are not the same as in extracellular

rospinal ﬂuid
are not the same as in extracellular ﬂuid elsewhere 
in the body. Furthermore, many large molecular sub-
stances hardly pass at all from the blood into the cere-
brospinal ﬂuid or into the interstitial ﬂuids of the brain,
even though these same substances pass readily into 
the usual interstitial ﬂuids of the body. Therefore, it is
said that barriers, called the blood–cerebrospinal ﬂuid
barrier and the blood-brain barrier, exist between the
blood and the cerebrospinal ﬂuid and brai

ween the
blood and the cerebrospinal ﬂuid and brain ﬂuid,
respectively.
Barriers exist both at the choroid plexus and at the
tissue capillary membranes in essentially all areas of the
brain parenchyma except in some areas of the hypothal-
amus, pineal gland,and area postrema,where substances
diffuse with greater ease into the tissue spaces.The ease
of diffusion in these areas is important because they
have sensory receptors that respond to speciﬁc changes
in the body ﬂuids, such as changes in os

c changes
in the body ﬂuids, such as changes in osmolality and in
glucose concentration, as well as receptors for peptide
hormones that regulate thirst, such as angiotensin II.
The blood-brain barrier also has speciﬁc carrier mole-
cules that facilitate transport of hormones, such as
leptin, from the blood into the hypothalamus where
they bind to speciﬁc receptors that control other func-
tions such as appetite and sympathetic nervous system
activity.
In general, the blood–cerebrospinal ﬂuid and

vity.
In general, the blood–cerebrospinal ﬂuid and blood-
brain barriers are highly permeable to water, carbon
dioxide, oxygen, and most lipid-soluble substances 
such as alcohol and anesthetics; slightly permeable to
electrolytes such as sodium, chloride, and potassium;
and almost totally impermeable to plasma proteins and
most non–lipid-soluble large organic molecules. There-
fore, the blood–cerebrospinal ﬂuid and blood-brain bar-
riers often make it impossible to achieve effective

ake it impossible to achieve effective
concentrations of therapeutic drugs, such as protein
antibodies and non–lipid-soluble drugs, in the cere-
brospinal ﬂuid or parenchyma of the brain.
The cause of the low permeability of the blood–
cerebrospinal ﬂuid and blood-brain barriers is the
manner in which the endothelial cells of the brain tissue
capillaries are joined to one another.They are joined by
so-called tight junctions. That is, the membranes of the
adjacent endothelial cells are tightly fu

s of the
adjacent endothelial cells are tightly fused rather than
having large slit-pores between them, as is the case for
most other capillaries of the body.
One of the most serious complications of abnormal
cerebral ﬂuid dynamics is the development of brain
edema. Because the brain is encased in a solid cranial
vault, accumulation of extra edema ﬂuid compresses the
Chapter 61
767
blood vessels, often causing seriously d

olism
767
blood vessels, often causing seriously decreased blood
ﬂow and destruction of brain tissue.
The usual cause of brain edema is either greatly
increased capillary pressure or damage to the capillary
wall that makes the wall leaky to ﬂuid. A very common
cause is a serious blow to the head, leading to brain con-
cussion, in which the brain tissues and capillaries are
traumatized so that capillary ﬂuid leaks into the trau-
matized tissues.
Once brain edema begins,it often initiates two vici

nce brain edema begins,it often initiates two vicious
circles because of the following positive feedbacks:
(1) Edema compresses the vasculature. This in turn
decreases blood ﬂow and causes brain ischemia. The
ischemia in turn causes arteriolar dilation with still
further increase in capillary pressure.The increased cap-
illary pressure then causes more edema ﬂuid, so that the
edema becomes progressively worse. (2) The decreased
cerebral blood ﬂow also decreases oxygen delivery.This
increases the

 also decreases oxygen delivery.This
increases the permeability of the capillaries, allowing
still more ﬂuid leakage. It also turns off the sodium
pumps of the neuronal tissue cells, thus allowing these
cells to swell in addition.
Once these two vicious circles have begun, heroic
measures must be used to prevent total destruction of
the brain. One such measure is to infuse intravenously
a concentrated osmotic substance, such as a very con-
centrated mannitol solution. This pulls ﬂuid by osmosis

ted mannitol solution. This pulls ﬂuid by osmosis
from the brain tissue and breaks up the vicious circles.
Another procedure is to remove ﬂuid quickly from the
lateral ventricles of the brain by means of ventricular
needle puncture, thereby relieving the intracerebral
pressure.
Like other tissues, the brain requires oxygen and food
nutrients to supply its metabolic needs. However, there
are special peculiarities of brain metabolism that
require mention.
Total Brain Metabolic Rat

sm that
Total Brain Metabolic Rate and Metabolic Rate of Neurons.
Under resting but awake conditions, the metabolism of
the brain accounts for about 15 per cent of the total
metabolism in the body, even though the mass of the
brain is only 2 per cent of the total body mass. There-
fore,under resting conditions,brain metabolism per unit
mass of tissue is about 7.5 times the average metabo-
lism in non–nervous system tissues.
Most of this excess metabolism of the brain occurs in
t

of this excess metabolism of the brain occurs in
the neurons, not in the glial supportive tissues. The
major need for metabolism in the neurons is to pump
ions through their membranes, mainly to transport
sodium and calcium ions to the outside of the neuronal
membrane and potassium ions to the interior. Each time
a neuron conducts an action potential, these ions move
through the membranes, increasing the need for addi-
tional membrane transport to restore proper ionic con-
centration differences

o restore proper ionic con-
centration differences across the neuron membranes.
Therefore, during excessive brain activity, neuronal
metabolism can increase as much as 100 to 150 per cent.
Special Requirement of the Brain for Oxygen—Lack of Signiﬁcant
Anaerobic Metabolism. Most tissues of the body can live
without oxygen for several minutes and some for as long
as 30 minutes. During this time, the tissue cells obtain
their energy through processes of anaerobic metab-
olism, which means release o

s of anaerobic metab-
olism, which means release of energy by partially 
breaking down glucose and glycogen but without com-
bining these with oxygen. This delivers energy only at
the expense of consuming tremendous amounts of
glucose and glycogen. However, it does keep the tissues
alive.
The brain is not capable of much anaerobic metabo-
lism. One of the reasons for this is the high metabolic
rate of the neurons, so that most neuronal activity
depends on second-by-second delivery of oxygen from

epends on second-by-second delivery of oxygen from
the blood. Putting these factors together, one can under-
stand why sudden cessation of blood ﬂow to the brain
or sudden total lack of oxygen in the blood can cause
unconsciousness within 5 to 10 seconds.
Under Normal Conditions Most Brain Energy Is Supplied by
Glucose. Under normal conditions, almost all the energy
used by the brain cells is supplied by glucose derived
from the blood. As is true for oxygen, most of this is
derived minute by min

 for oxygen, most of this is
derived minute by minute and second by second from
the capillary blood, with a total of only about a 2-minute
supply of glucose normally stored as glycogen in the
neurons at any given time.
A special feature of glucose delivery to the neurons
is that its transport into the neurons through the cell
membrane is not dependent on insulin, even though
insulin is required for glucose transport into most other
body cells. Therefore, in patients who have serious dia-
betes w

erefore, in patients who have serious dia-
betes with essentially zero secretion of insulin, glucose
still diffuses readily into the neurons—which is most
fortunate in preventing loss of mental function in 
diabetic patients. Yet, when a diabetic patient is
overtreated with insulin, the blood glucose concentra-
tion can fall extremely low because the excess insulin
causes almost all the glucose in the blood to be trans-
ported rapidly into the vast numbers of insulin-sensitive
non-neural cells t

st numbers of insulin-sensitive
non-neural cells throughout the body, especially into
muscle and liver cells. When this happens, not enough
glucose is left in the blood to supply the neurons prop-
erly, and mental function does then become seriously
deranged, leading sometimes to coma and even more
often to mental imbalances and psychotic distur-
bances—all caused by overtreatment with insulin.
Anderson CM, Nedergaard M: Astrocyte-mediated control
of cerebral microcirculation. Trends 

ated control
of cerebral microcirculation. Trends Neurosci 26:340,
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Burmester T, Hankeln T: Neuroglobin: a respiratory protein
of the nervous system. News Physiol Sci 19:110, 2004.
Chesler M: Regulation and modulation of pH in the brain.
Physiol Rev 83:1183, 2003.
Duelli R, Kuschinsky W: Brain glucose transporters: rela-
tionship to local energy demand. News Physiol 

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Faraci FM: Vascular protection. Stroke 34:327, 2003.
Faraci FM, Heistad DD: Regulation of the cerebral circula-
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Ganong WF: Circumventricular organs: deﬁnition and role
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al MRI of the
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port: a lymphatic perspective. News Physiol Sci 17:227,
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Logothetis NK, Wandell BA: Interpreting the BOLD signal.
Annu Rev Physiol 66:735, 2004.
Nadareishvili Z, Hallenbeck J: Neuronal

735, 2004.
Nadareishvili Z, Hallenbeck J: Neuronal regeneration after
stroke. N Engl J Med 348:2355, 2003.
Osborn JW, Collister JP, Carlson SH: Angiotensin and
osmoreceptor inputs to the area postrema: role in long-
term control of ﬂuid homeostasis and arterial pressure.
Clin Exp Pharmacol Physiol 27:443, 2000.
Pardridge WM:An Introduction to the Blood-Brain Barrier.
Cambridge: Cambridge University Press, 1998.
Paulson OB: Blood-brain barrier, brain metabolism and
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ain metabolism and
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U
N
I
T
XII
62. General Principles of Gastrointestinal Function—
Motility, Nervous Control, and Blood Circulation
63. Propulsion 

ous Control, and Blood Circulation
63. Propulsion and Mixing of Food in the 
64. Secretory Functions of the Alimentary Tract
65. Digestion and Absorption in the 
66. Physiology of Gastrointestinal Disorders
C
H
A
P
T
E
R
 
6
2
771
General Principles of
Gastrointestinal Function—
Motility, Nervous Control, and
Blood Circulation
The alimentary tract provides the body with a con-
tinual supply of water, electrolytes, and nutrients.To
achieve this requires (1)

olytes, and nutrients.To
achieve this requires (1) movement of food through
the alimentary tract; (2) secretion of digestive juices
and digestion of the food; (3) absorption of water,
various electrolytes, and digestive products; (4) 
circulation of blood through the gastrointestinal
organs to carry away the absorbed substances; and
(5) control of all these functions by local, nervous, and hormonal systems.
Figure 62–1 shows the entire alimentary tract. Each part is adapted to its spe-
ciﬁc func

 tract. Each part is adapted to its spe-
ciﬁc functions: some to simple passage of food, such as the esophagus; others to
temporary storage of food, such as the stomach; and others to digestion and
absorption, such as the small intestine. In this chapter, we discuss the basic prin-
ciples of function in the entire alimentary tract; in the following chapters, we
will discuss the speciﬁc functions of different segments of the tract.
General Principles of Gastrointestinal Motility
Physiologic Anato

les of Gastrointestinal Motility
Physiologic Anatomy of the Gastrointestinal Wall
Figure 62–2 shows a typical cross section of the intestinal wall, including the fol-
lowing layers from outer surface inward: (1) the serosa, (2) a longitudinal muscle
layer, (3) a circular muscle layer, (4) the submucosa, and (5) the mucosa. In addi-
tion, sparse bundles of smooth muscle ﬁbers, the mucosal muscle, lie in the
deeper layers of the mucosa. The motor functions of the gut are performed by
the different

unctions of the gut are performed by
the different layers of smooth muscle.
The general characteristics of smooth muscle and its function are discussed
in Chapter 8, which should be reviewed as a background for the following sec-
tions of this chapter.The speciﬁc characteristics of smooth muscle in the gut are
Gastrointestinal Smooth Muscle Functions as a Syncytium. The individual smooth
muscle ﬁbers in the gastrointestinal tract are 200 to 500 micrometers in length
and 2 to 10 mi

re 200 to 500 micrometers in length
and 2 to 10 micrometers in diameter, and they are arranged in bundles of as
many as 1000 parallel ﬁbers. In the longitudinal muscle layer, the bundles extend
longitudinally down the intestinal tract; in the circular muscle layer, they extend
around the gut.
Within each bundle, the muscle ﬁbers are electrically connected with one
another through large numbers of gap junctions that allow low-resistance move-
ment of ions from one muscle cell to the next. Therefo

 of ions from one muscle cell to the next. Therefore, electrical signals that
initiate muscle contractions can travel readily from one ﬁber to the next within
each bundle but more rapidly along the length of the bundle than sideways.
Each bundle of smooth muscle ﬁbers is partly separated from the next by
loose connective tissue, but the muscle bundles fuse with one another at many
772
Unit XII
points, so that in reality each muscle layer represents
a branching lattice

y each muscle layer represents
a branching latticework of smooth muscle bundles.
Therefore, each muscle layer functions as a syncytium;
that is, when an action potential is elicited anywhere
within the muscle mass, it generally travels in all direc-
tions in the muscle.The distance that it travels depends
on the excitability of the muscle; sometimes it stops
after only a few millimeters and at other times it
travels many centimeters or even the entire length and
breadth of the intestinal tract.

ntire length and
Also, a few connections exist between the longitu-
dinal and circular muscle layers, so that excitation of
one of these layers often excites the other as well.
Electrical Activity of Gastrointestinal 
The smooth muscle of the gastrointestinal tract is
excited by almost continual slow, intrinsic electrical
activity along the membranes of the muscle ﬁbers.
This activity has two basic types of electrical waves: (1)
slow waves and (2) s

ypes of electrical waves: (1)
slow waves and (2) spikes, both of which are shown 
in Figure 62–3. In addition, the voltage of the resting
membrane potential of the gastrointestinal smooth
muscle can be made to change to different levels, and
this too can have important effects in controlling
motor activity of the gastrointestinal tract.
Slow Waves. Most gastrointestinal contractions occur
rhythmically, and this rhythm is determined mainly by
the frequency of so-called “slow waves” of smooth
musc

frequency of so-called “slow waves” of smooth
muscle membrane potential. These waves, shown in
Figure 62–3, are not action potentials. Instead, they are
slow, undulating changes in the resting membrane
potential. Their intensity usually varies between 5 and
15 millivolts, and their frequency ranges in different
parts of the human gastrointestinal tract from 3 to 12
per minute: about 3 in the body of the stomach, as
much as 12 in the duodenum, and about 8 or 9 in the
terminal ileum. Therefore, th

 about 8 or 9 in the
terminal ileum. Therefore, the rhythm of contraction
of the body of the stomach usually is about 3 per
minute, of the duodenum about 12 per minute, and of
the ileum 8 to 9 per minute.
The precise cause of the slow waves is not com-
pletely understood, although they appear to be caused
Mouth
Salivary glands
Esophagus
Liver
Gallbladder
Duodenum
Ascending
colon
colon
Stomach
Pancreas
Jejunum
colon
Ileum
Anus
Figure 62–1
Alimentary tract.
Sero

olon
Ileum
Anus
Serosa
Circular muscle
Longitudinal
muscle
Submucosa
Mucosa
Meissner's
nerve plexus
lining
Mucosal
muscle
Mucosal gland
Submucosal gland
Mesentery
Myenteric nerve
plexus
Figure 62–2
Typical cross section of the gut.
-70
-60
-50
-40
-30
-20
-10
0
0
6
12
18
Spikes
Stimulation by
2. Sympathetics
1. Stretch
2. Acetylcholine
3. Parasympathetics
Resting
Hyperpolariza

lcholine
Resting
Slow
waves
24
30
36
42
48
54
Seconds
Seconds
Figure 62–3
Membrane potentials in intestinal smooth muscle. Note the slow
waves, the spike potentials, total depolarization, and hyperpolar-
ization, all of which occur under different physiologic conditions
of the intestine.
Chapter 62
General Principles of Gastrointestinal Function—Motility, Nervous Control, and Blood Circulation
773
by complex interactions among the smooth muscle
cells and spe

interactions among the smooth muscle
cells and specialized cells, called the interstitial cells of
Cajal, that are believed to act as electrical pacemakers
for smooth muscle cells. These interstitial cells form a
network with each other and are interposed between
the smooth muscle layers, with synaptic-like contacts
to smooth muscle cells. The interstitial cells of Cajal
undergo cyclic changes in membrane potential due 
to unique ion channels that periodically open and
produce inward (pacemaker)

t periodically open and
produce inward (pacemaker) currents that may gen-
erate slow wave activity.
The slow waves usually do not by themselves cause
muscle contraction in most parts of the gastrointesti-
nal tract, except perhaps in the stomach. Instead, they
mainly excite the appearance of intermittent spike
potentials, and the spike potentials in turn actually
excite the muscle contraction.
Spike Potentials. The spike potentials are true action
potentials. They occur automatically when the re

n
potentials. They occur automatically when the resting
muscle becomes more positive than about -40 milli-
volts (the normal resting membrane potential in the
smooth muscle ﬁbers of the gut is between -50 and 
-60 millivolts).Thus,note in Figure 62–3 that each time
the peaks of the slow waves temporarily become more
positive than -40 millivolts, spike potentials appear on
these peaks. The higher the slow wave potential rises,
the greater the freq

he slow wave potential rises,
the greater the frequency of the spike potentials,
usually ranging between 1 and 10 spikes per second.
The spike potentials last 10 to 40 times as long in gas-
trointestinal muscle as the action potentials in large
nerve ﬁbers, each gastrointestinal spike lasting as long
as 10 to 20 milliseconds.
Another important difference between the action
potentials of the gastrointestinal smooth muscle and
those of nerve ﬁbers is the manner in which they are
generated. In nerv

is the manner in which they are
generated. In nerve ﬁbers, the action potentials are
caused almost entirely by rapid entry of sodium ions
through sodium channels to the interior of the ﬁbers.
In gastrointestinal smooth muscle ﬁbers, the channels
responsible for the action potentials are somewhat dif-
ferent; they allow especially large numbers of calcium
ions to enter along with smaller numbers of sodium
ions and therefore are called calcium-sodium channels.
These channels are much slower to ope

um channels.
These channels are much slower to open and close
than are the rapid sodium channels of large nerve
ﬁbers. The slowness of opening and closing of the
calcium-sodium channels accounts for the long dura-
tion of the action potentials. Also, the movement of
large amounts of calcium ions to the interior of the
muscle ﬁber during the action potential plays a special
role in causing the intestinal muscle ﬁbers to contract,
as we discuss shortly.
Changes in Voltage of the Resting Membrane P

rtly.
Changes in Voltage of the Resting Membrane Potential. In
addition to the slow waves and spike potentials, the
baseline voltage level of the smooth muscle resting
membrane potential also can change. Under normal
conditions, the resting membrane potential averages
about -56 millivolts, but multiple factors can change
this level. When the potential becomes less negative,
which is called depolarization of the membrane, the
muscle ﬁbers become more excitable.When the poten-
tial becomes more ne

ore excitable.When the poten-
tial becomes more negative, which is called hyperpo-
larization, the ﬁbers become less excitable.
Factors that depolarize the membrane—that is,
make it more excitable—are (1) stretching of the
muscle, (2) stimulation by acetylcholine, (3) stimula-
tion by parasympathetic nerves that secrete acetyl-
choline at their endings, and (4) stimulation by several
speciﬁc gastrointestinal hormones.
Important factors that make the membrane poten-
tial more negative—that is, hy

the membrane poten-
tial more negative—that is, hyperpolarize the mem-
brane and make the muscle ﬁbers less excitable—are
(1) the effect of norepinephrine or epinephrine on the
ﬁber membrane and (2) stimulation of the sympathetic
nerves that secrete mainly norepinephrine at their
endings.
Calcium Ions and Muscle Contraction. Smooth muscle con-
traction occurs in response to entry of calcium ions
into the muscle ﬁber. As explained in Chapter 8,
calcium ions, acting through a calmodulin control
me

lcium ions, acting through a calmodulin control
mechanism, activate the myosin ﬁlaments in the ﬁber,
causing attractive forces to develop between the
myosin ﬁlaments and the actin ﬁlaments, thereby
causing the muscle to contract.
The slow waves do not cause calcium ions to enter
the smooth muscle ﬁber (only sodium ions).Therefore,
the slow waves by themselves usually cause no muscle
contraction. Instead, it is during the spike potentials,
generated at the peaks of the slow waves, that signiﬁ-
ca

ed at the peaks of the slow waves, that signiﬁ-
cant quantities of calcium ions do enter the ﬁbers and
cause most of the contraction.
Tonic Contraction of Some Gastrointestinal Smooth Muscle.
Some smooth muscle of the gastrointestinal tract
exhibits tonic contraction as well as or instead of rhyth-
mical contractions.Tonic contraction is continuous, not
associated with the basic electrical rhythm of the slow
waves but often lasting several minutes or even hours.
The tonic contraction often incre

s or even hours.
The tonic contraction often increases or decreases in
intensity but continues.
Tonic contraction is sometimes caused by con-
tinuous repetitive spike potentials—the greater the
frequency, the greater the degree of contraction. At
other times, tonic contraction is caused by hormones
or other factors that bring about continuous partial
depolarization of the smooth muscle membrane
without causing action potentials. A third cause of
tonic contraction is continuous entry of calcium i

tonic contraction is continuous entry of calcium ions
into the interior of the cell brought about in ways not
associated with changes in membrane potential. The
details of these mechanisms are still unclear.
Neural Control of
Enteric Nervous System
The gastrointestinal tract has a nervous system all its
own called the enteric nervous system. It lies entirely
774
Unit XII
in the wall of the gut, beginning in the esophagus and
extending all th

t, beginning in the esophagus and
extending all the way to the anus. The number of
neurons in this enteric system is about 100 million,
almost exactly equal to the number in the entire spinal
cord. This highly developed enteric nervous system is
especially important in controlling gastrointestinal
movements and secretion.
The enteric nervous system is composed mainly of
two plexuses, shown in Figure 62–4: (1) an outer plexus
lying between the longitudinal and circular muscle
layers, called the m

gitudinal and circular muscle
layers, called the myenteric plexus or Auerbach’s
plexus, and (2) an inner plexus, called the submucosal
plexus or Meissner’s plexus, that lies in the submucosa.
The nervous connections within and between these
two plexuses are also shown in Figure 62–4.
The myenteric plexus controls mainly the gastroin-
testinal movements, and the submucosal plexus con-
trols mainly gastrointestinal secretion and local blood
ﬂow.
Note especially in Figure 62–4 the extrinsic sympa-

te especially in Figure 62–4 the extrinsic sympa-
thetic and parasympathetic ﬁbers that connect to both
the myenteric and submucosal plexuses. Although the
enteric nervous system can function on its own, inde-
pendently of these extrinsic nerves, stimulation by the
parasympathetic and sympathetic systems can greatly
enhance or inhibit gastrointestinal functions, as we
Also shown in Figure 62–4 are sensory nerve
endings that originate in the gastrointestinal epithe-
lium or gut wal

te in the gastrointestinal epithe-
lium or gut wall and send afferent ﬁbers to both
plexuses of the enteric system, as well as (1) to the pre-
vertebral ganglia of the sympathetic nervous system,
(2) to the spinal cord, and (3) in the vagus nerves all
the way to the brain stem. These sensory nerves can
elicit local reﬂexes within the gut wall itself and still
other reﬂexes that are relayed to the gut from either
the prevertebral ganglia or the basal regions of the
brain.
Differences Between the 

sal regions of the
brain.
Differences Between the Myenteric
and Submucosal Plexuses
The myenteric plexus consists mostly of a linear chain
of many interconnecting neurons that extends the
entire length of the gastrointestinal tract. A section of
this chain is shown in Figure 62–4.
Because the myenteric plexus extends all the way
along the intestinal wall and because it lies between
the longitudinal and circular layers of intestinal
smooth muscle, it is concerned mainly with controlling
muscle ac

 it is concerned mainly with controlling
muscle activity along the length of the gut. When this
plexus is stimulated, its principal effects are (1)
increased tonic contraction, or “tone,” of the gut wall,
(2) increased intensity of the rhythmical contractions,
(3) slightly increased rate of the rhythm of contraction,
and (4) increased velocity of conduction of excitatory
waves along the gut wall, causing more rapid move-
ment of the gut peristaltic waves.
The myenteric plexus should not be consi

ic waves.
The myenteric plexus should not be considered
entirely excitatory because some of its neurons are
inhibitory; their ﬁber endings secrete an inhibitory
transmitter, possibly vasoactive intestinal polypeptide
or some other inhibitory peptide. The resulting
inhibitory signals are especially useful for inhibiting
some of the intestinal sphincter muscles that impede
movement of food along successive segments of the
gastrointestinal tract, such as the pyloric sphincter,
which controls emptyi

ch as the pyloric sphincter,
which controls emptying of the stomach into the duo-
denum, and the sphincter of the ileocecal valve, which
controls emptying from the small intestine into the
cecum.
The submucosal plexus, in contrast to the myenteric
plexus, is mainly concerned with controlling function
within the inner wall of each minute segment of the
intestine. For instance, many sensory signals originate
from the gastrointestinal epithelium and are then 
To prevertebral
ganglia, spinal
cord, a

 are then 
cord, and brain
stem
Sensory
neurons
Submucosal
plexus
Myenteric
plexus
(mainly postganglionic)
(preganglionic)
Figure 62–4
Neural control of the gut wall, showing (1)
the myenteric and submucosal plexuses
(black ﬁbers); (2) extrinsic control of these
plexuses 
by 
the 
and
parasympathetic nervous systems (red
ﬁbers); and (3) sensory ﬁbers passing
from the luminal epithelium and gut wall
to the enteric

the luminal epithelium and gut wall
to the enteric plexuses, then to the pre-
vertebral ganglia of the spinal cord and
directly to the spinal cord and brain stem
(dashed ﬁbers).
Chapter 62
775
integrated in the submucosal plexus to help control 
local intestinal secretion, local absorption, and local
contraction of the submucosal muscle that causes
various degrees of infolding of the gastrointestinal

rious degrees of infolding of the gastrointestinal
mucosa.
Types of Neurotransmitters Secreted
by Enteric Neurons
In an attempt to understand better the multiple func-
tions of the gastrointestinal enteric nervous system,
research workers the world over have identiﬁed a
dozen or more different neurotransmitter substances
that are released by the nerve endings of different
types of enteric neurons. Two of them with which we
are already familiar are (1) acetylcholine and (2) nor-
epinephrine. Othe

e (1) acetylcholine and (2) nor-
epinephrine. Others are (3) adenosine triphosphate,
(4) serotonin, (5) dopamine, (6) cholecystokinin, (7)
substance P, (8) vasoactive intestinal polypeptide,
(9) somatostatin,
(10) leu-enkephalin,
(11) met-
enkephalin, and (12) bombesin. The speciﬁc functions
of many of these are not known well enough to justify
discussion here, other than to point out the following.
Acetylcholine most often excites gastrointestinal
activity. Norepinephrine almost always inhibits

al
activity. Norepinephrine almost always inhibits gas-
trointestinal activity. This is also true of epinephrine,
which reaches the gastrointestinal tract mainly by way
of the blood after it is secreted by the adrenal medul-
lae into the circulation. The other aforementioned
transmitter substances are a mixture of excitatory and
inhibitory agents, some of which we will discuss in the
following chapter.
Autonomic Control of the 
Parasympathetic Innervation. The parasympathe

ract
Parasympathetic Innervation. The parasympathetic sup-
ply to the gut is divided into cranial and sacral divi-
sions, which were discussed in Chapter 60.
Except for a few parasympathetic ﬁbers to the
mouth and pharyngeal regions of the alimentary tract,
the cranial parasympathetic nerve ﬁbers are almost
entirely in the vagus nerves. These ﬁbers provide
extensive innervation to the esophagus, stomach, and
pancreas and somewhat less to the intestines down
through the ﬁrst half of the large int

stines down
through the ﬁrst half of the large intestine.
The sacral parasympathetics originate in the second,
third, and fourth sacral segments of the spinal cord and
pass through the pelvic nerves to the distal half of the
large intestine and all the way to the anus. The sig-
moidal, rectal, and anal regions are considerably better
supplied with parasympathetic ﬁbers than are the
other intestinal areas. These ﬁbers function especially
to execute the defecation reﬂexes, discussed in
Chapter 63.

e the defecation reﬂexes, discussed in
The postganglionic neurons of the gastrointestinal
parasympathetic system are located mainly in the
myenteric and submucosal plexuses. Stimulation of
these parasympathetic nerves causes general increase
in activity of the entire enteric nervous system. This 
in turn enhances activity of most gastrointestinal 
Sympathetic Innervation. The sympathetic ﬁbers to the
gastrointestinal tract originate in the spinal cord
between segments T-5 

originate in the spinal cord
between segments T-5 and L-2. Most of the pregan-
glionic ﬁbers that innervate the gut, after leaving the
cord, enter the sympathetic chains that lie lateral to the
spinal column, and many of these ﬁbers then pass on
through the chains to outlying ganglia such as to the
celiac ganglion and various mesenteric ganglia. Most of
the postganglionic sympathetic neuron bodies are in
these ganglia, and postganglionic ﬁbers then spread
through postganglionic sympathetic nerve

en spread
through postganglionic sympathetic nerves to all parts
of the gut.. The sympathetics innervate essentially all
of the gastrointestinal tract, rather than being more
extensive nearest the oral cavity and anus as is true of
the parasympathetics. The sympathetic nerve endings
secrete mainly norepinephrine but also small amounts
of epinephrine.
In general, stimulation of the sympathetic nervous
system inhibits activity of the gastrointestinal tract,
causing many effects opposite to those o

al tract,
causing many effects opposite to those of the
parasympathetic system. It exerts its effects in two
ways: (1) to a slight extent by direct effect of secreted
norepinephrine to inhibit intestinal tract smooth
muscle (except the mucosal muscle, which it excites)
and (2) to a major extent by an inhibitory effect of
norepinephrine on the neurons of the entire enteric
Strong stimulation of the sympathetic system can
inhibit motor movements of the gut so greatly that this
lite

tor movements of the gut so greatly that this
literally can block movement of food through the gas-
trointestinal tract.
Afferent Sensory Nerve Fibers from the Gut
Many afferent sensory nerve ﬁbers innervate the gut.
Some of them have their cell bodies in the enteric
nervous system itself and some in the dorsal root
ganglia of the spinal cord.These sensory nerves can be
stimulated by (1) irritation of the gut mucosa, (2)
excessive distention of the gut, or (3) presence of spe-
ciﬁc chemical subs

he gut, or (3) presence of spe-
ciﬁc chemical substances in the gut. Signals trans-
mitted through the ﬁbers can then cause excitation or,
under other conditions, inhibition of intestinal move-
ments or intestinal secretion.
In addition, other sensory signals from the gut go all
the way to multiple areas of the spinal cord and 
even the brain stem. For example, 80 per cent of the
nerve ﬁbers in the vagus nerves are afferent rather 
than efferent. These afferent ﬁbers transmit sensory
signals fro

 These afferent ﬁbers transmit sensory
signals from the gastrointestinal tract into the brain
medulla, which in turn initiates vagal reﬂex signals that
return to the gastrointestinal tract to control many of
its functions.
Gastrointestinal Reﬂexes
The anatomical arrangement of the enteric nervous
system and its connections with the sympathetic and
parasympathetic systems support three types of gas-
trointestinal reﬂexes that are essential to gastroin-
testinal control. They are the following:
1.

roin-
1. Reﬂexes that are integrated entirely within the gut
wall enteric nervous system. These include reﬂexes
that control much gastrointestinal secretion,
776
Unit XII
peristalsis, mixing contractions, local inhibitory
effects, and so forth.
2. Reﬂexes from the gut to the prevertebral
sympathetic ganglia and then back to the
gastrointestinal tract. These reﬂexes transmit
signals long distances to other areas of the
gastroin

nals long distances to other areas of the
gastrointestinal tract, such as signals from the
stomach to cause evacuation of the colon (the
gastrocolic reﬂex), signals from the colon and
small intestine to inhibit stomach motility and
stomach secretion (the enterogastric reﬂexes), and
reﬂexes from the colon to inhibit emptying of ileal
contents into the colon (the colonoileal reﬂex).
3. Reﬂexes from the gut to the spinal cord or brain
stem and then back to the gastrointestinal tract.
These include 

back to the gastrointestinal tract.
These include especially (1) reﬂexes from the
stomach and duodenum to the brain stem and
back to the stomach—by way of the vagus
nerves—to control gastric motor and secretory
activity; (2) pain reﬂexes that cause general
inhibition of the entire gastrointestinal tract; and
(3) defecation reﬂexes that travel from the colon
and rectum to the spinal cord and back again 
to produce the powerful colonic, rectal, and
abdominal contractions required for defecation
(t

(the defecation reﬂexes).
Hormonal Control of 
Gastrointestinal Motility
In Chapter 64, we discuss the extreme importance of
several hormones for controlling gastrointestinal
secretion. Most of these same hormones also affect
motility in some parts of the gastrointestinal 
tract. Although the motility effects are usually less
important than the secretory effects of the hormones,
some of the more important of them are the 
Gastrin is secr

ortant of them are the 
Gastrin is secreted by the “G” cells of the antrum of
the stomach in response to stimuli associated with
ingestion of a meal, such as distention of the stomach,
the products of proteins, and gastrin releasing peptide,
which is released by the nerves of the gastric mucosa
during vagal stimulation. The primary actions of
gastrin are (1) stimulation of gastric acid secretion and
(2) stimulation of growth of the gastric mucosa.
Cholecystokinin is secreted by “I” ce

tric mucosa.
Cholecystokinin is secreted by “I” cells in the
mucosa of the duodenum and jejunum mainly in
response to digestive products of fat, fatty acids,
and monoglycerides in the intestinal contents. This
hormone strongly contracts the gallbladder, expelling
bile into the small intestine where the bile in turn plays
important roles in emulsifying fatty substances, allow-
ing them to be digested and absorbed. Cholecystokinin
also inhibits stomach contraction moderately. There-
fore, at the s

mach contraction moderately. There-
fore, at the same time that this hormone causes emp-
tying of the gallbladder, it also slows the emptying of
food from the stomach to give adequate time for diges-
tion of the fats in the upper intestinal tract.
Secretin was the ﬁrst gastrointestinal hormone dis-
covered and is secreted by the “S” cells in the mucosa
of the duodenum in response to acidic gastric juice
emptying into the duodenum from the pylorus of the
stomach. Secretin has a mild effect on mot

 of the
stomach. Secretin has a mild effect on motility of the
gastrointestinal tract and acts to promote pancreatic
secretion of bicarbonate which in turn helps to neu-
tralize the acid in the small intestine.
Gastric inhibitory peptide is secreted by the mucosa
of the upper small intestine, mainly in response to fatty
acids and amino acids but to a lesser extent in response
to carbohydrate. It has a mild effect in decreasing
motor activity of the stomach and therefore slows
emptying of gastric

he stomach and therefore slows
emptying of gastric contents into the duodenum when
the upper small intestine is already overloaded with
food products.
Motilin is secreted by the upper duodenum during
fasting, and the only known function of this hormone
is to increase gastrointestinal motility. Motilin is
released cyclically and stimulates waves of gastroin-
testinal motility called interdigestive myoelectric com-
plexes that move through the stomach and small
intestine every 90 minutes in a fast

ach and small
intestine every 90 minutes in a fasted person. Motilin
secretion is inhibited after ingestion by mechanisms
that are not fully understood.
Functional Types of
Movements in the
Two types of movements occur in the gastrointestinal
tract: (1) propulsive movements, which cause food to
move forward along the tract at an appropriate rate 
to accommodate digestion and absorption, and (2)
mixing movements, which keep the intestinal contents
thoroughly mixed at all ti

the intestinal contents
thoroughly mixed at all times.
The basic propulsive movement of the gastrointestinal
tract is peristalsis, which is illustrated in Figure 62–5.
A contractile ring appears around the gut and then
moves forward; this is analogous to putting one’s
ﬁngers around a thin distended tube, then constricting
the ﬁngers and sliding them forward along the tube.
Any material in front of the contractile ring is moved
forward.
Peristalsis is an inherent 

ring is moved
forward.
Peristalsis is an inherent property of many syncytial
smooth muscle tubes; stimulation at any point in the
gut can cause a contractile ring to appear in the circu-
lar muscle, and this ring then spreads along the gut
tube. (Peristalsis also occurs in the bile ducts, glandu-
lar ducts, ureters, and many other smooth muscle tubes
of the body.)
The usual stimulus for intestinal peristalsis is disten-
tion of the gut. That is, if a large amount of food col-
lects at any point 

if a large amount of food col-
lects at any point in the gut, the stretching of the gut
wall stimulates the enteric nervous system to contract
the gut wall 2 to 3 centimeters behind this point, and
a contractile ring appears that initiates a peristaltic
movement. Other stimuli that can initiate peristalsis
include chemical or physical irritation of the epithelial
Chapter 62
777
lining in the gut.Also

, and Blood Circulation
777
lining in the gut.Also, strong parasympathetic nervous
signals to the gut will elicit strong peristalsis.
Function of the Myenteric Plexus in Peristalsis. Peristalsis
occurs only weakly or not at all in any portion of the
gastrointestinal tract that has congenital absence of
the myenteric plexus. Also, it is greatly depressed or
completely blocked in the entire gut when a person is
treated with atropine to paralyze the cholinergic nerve
endings of the myenteric plexus

 cholinergic nerve
endings of the myenteric plexus. Therefore, effectual
peristalsis requires an active myenteric plexus.
Directional Movement of Peristaltic Waves Toward the Anus.
Peristalsis, theoretically, can occur in either direction
from a stimulated point, but it normally dies out
rapidly in the orad direction while continuing for a
considerable distance toward the anus.The exact cause
of this directional transmission of peristalsis has never
been ascertained, although it probably results

ver
been ascertained, although it probably results mainly
from the fact that the myenteric plexus itself is “polar-
ized” in the anal direction, which can be explained as
follows.
Peristaltic Reﬂex and the “Law of the Gut.” When a
segment of the intestinal tract is excited by distention
and thereby initiates peristalsis, the contractile ring
causing the peristalsis normally begins on the orad side
of the distended segment and moves toward the dis-
tended segment, pushing the intestinal contents 

-
tended segment, pushing the intestinal contents in the
anal direction for 5 to 10 centimeters before dying out.
At the same time, the gut sometimes relaxes several
centimeters downstream toward the anus, which is
called “receptive relaxation,” thus allowing the food to
be propelled more easily anally than orad.
This complex pattern does not occur in the absence
of the myenteric plexus. Therefore, the complex is
called the myenteric reﬂex or the peristaltic reﬂex. The
peristaltic reﬂex plus the

 peristaltic reﬂex. The
peristaltic reﬂex plus the anal direction of movement
of the peristalsis is called the “law of the gut.”
Mixing movements differ in different parts of the ali-
mentary tract. In some areas, the peristaltic contrac-
tions themselves cause most of the mixing. This is
especially true when forward progression of the intes-
tinal contents is blocked by a sphincter, so that a 
peristaltic wave can then only churn the intestinal con-
tents, rather than propellin

n the intestinal con-
tents, rather than propelling them forward. At other
times, local intermittent constrictive contractions occur
every few centimeters in the gut wall. These constric-
tions usually last only 5 to 30 seconds; then new con-
strictions occur at other points in the gut, thus
“chopping” and “shearing” the contents ﬁrst here and
then there. These peristaltic and constrictive move-
ments are modiﬁed in different parts of the gastroin-
testinal tract for proper propulsion and mixing

n-
testinal tract for proper propulsion and mixing, as
discussed for each portion of the tract in Chapter 63.
The blood vessels of the gastrointestinal system are
part of a more extensive system called the splanchnic
circulation, shown in Figure 62–6. It includes the blood
ﬂow through the gut itself plus blood ﬂows through the
spleen, pancreas, and liver.The design of this system is
such that all the blood that courses through the gut,
spleen

all the blood that courses through the gut,
spleen, and pancreas then ﬂows immediately into the
liver by way of the portal vein. In the liver, the blood
passes through millions of minute liver sinusoids and
ﬁnally leaves the liver by way of hepatic veins that
empty into the vena cava of the general circulation.
This ﬂow of blood through the liver, before it empties
into the vena cava, allows the reticuloendothelial cells
that line the liver sinusoids to remove bacteria and
Leading wave of disten

oids to remove bacteria and
Leading wave of distention
Zero time
5 seconds later
Peristaltic contraction
Figure 62–5
Peristalsis.
Vena cava
Hepatic artery 
Aorta
Splenic
vein
Intestinal artery
Intestinal vein
Capillary
Portal
vein
Hepatic vein
Hepatic
sinuses
Figure 62–6
Splanchnic circulation.
778
Unit XII
other particulate matter that might enter the blood
from the gastrointestinal tract, thus preventing direct
transport of potentially harmful agents into the
remain

port of potentially harmful agents into the
remainder of the body.
The nonfat, water-soluble nutrients absorbed from
the gut (such as carbohydrates and proteins) are trans-
ported in the portal venous blood to the same liver
sinusoids. Here, both the reticuloendothelial cells and
the principal parenchymal cells of the liver, the hepatic
cells, absorb and store temporarily from one half to
three quarters of the nutrients. Also, much chemical
intermediary processing of these nutrients occurs in
th

mediary processing of these nutrients occurs in
the liver cells.We discuss these nutritional functions of
the liver in Chapters 67 through 71. Almost all of the
fats absorbed from the intestinal tract are not carried
in the portal blood but instead are absorbed into the
intestinal lymphatics and then conducted to the sys-
temic circulating blood by way of the thoracic duct,
bypassing the liver.
Anatomy of the Gastrointestinal 
Figure 62–7 shows the general plan of the arterial
blood

 62–7 shows the general plan of the arterial
blood supply to the gut, including the superior mesen-
teric and inferior mesenteric arteries supplying the
walls of the small and large intestines by way of an
arching arterial system. Not shown in the ﬁgure is the
celiac artery, which provides a similar blood supply to
the stomach.
On entering the wall of the gut, the arteries branch
and send smaller arteries circling in both directions
around the gut, with the tips of these arteries meeting
on the 

t, with the tips of these arteries meeting
on the side of the gut wall opposite the mesenteric
attachment. From the circling arteries, still much
smaller arteries penetrate into the intestinal wall and
spread (1) along the muscle bundles, (2) into the intes-
tinal villi, and (3) into submucosal vessels beneath the
epithelium to serve the secretory and absorptive func-
tions of the gut.
Figure 62–8 shows the special organization of the
blood ﬂow through an intestinal villus, including a
small art

hrough an intestinal villus, including a
small arteriole and venule that interconnect with a
system of multiple looping capillaries.The walls of the
arterioles are highly muscular and are highly active in
controlling villus blood ﬂow.
Effect of Gut Activity and Metabolic
Factors on Gastrointestinal 
Blood Flow
Under normal conditions, the blood ﬂow in each area
of the gastrointestinal tract, as well as in each layer of
the gut wall, is directly related to the level of local
activity. For instanc

elated to the level of local
activity. For instance, during active absorption of nutri-
ents, blood ﬂow in the villi and adjacent regions of the
submucosa is increased as much as eightfold. Likewise,
blood ﬂow in the muscle layers of the intestinal wall
increases with increased motor activity in the gut. For
instance, after a meal, the motor activity, secretory
colon
colon
Jejunum
Jejunal
 Ileal 
Ileum
Branch of
inferior
mesenteric
Superior
Right colic
Ascending

enteric
Superior
Ascending
colon
Middle colic
Aorta
Ileocolic
Figure 62–7
Arterial blood supply to the intes-
tines through the mesenteric
web.
Chapter 62
779
activity, and absorptive activity all increase; likewise,
the blood ﬂow increases greatly but then decreases
back to the resting level over another 2 to 4 hours.
Possible Causes of the Increased Blood Flow During Gastroin

Causes of the Increased Blood Flow During Gastroin-
testinal Activity. Although the precise cause or causes
of the increased blood ﬂow during increased gastroin-
testinal activity are still unclear, some facts are known.
First, several vasodilator substances are released
from the mucosa of the intestinal tract during the
digestive process. Most of these are peptide hormones,
including cholecystokinin, vasoactive intestinal peptide,
gastrin, and secretin. These same hormones control
speciﬁc motor

ecretin. These same hormones control
speciﬁc motor and secretory activities of the gut, as we
will see in Chapters 63 and 64.
Second, some of the gastrointestinal glands also
release into the gut wall two kinins, kallidin and
bradykinin, at the same time that they secrete their
secretions into the lumen. These kinins are powerful
vasodilators that are believed to cause much of the
increased mucosal vasodilation that occurs along with
Third, decreased oxygen concentration in the gut
wa

hird, decreased oxygen concentration in the gut
wall can increase intestinal blood ﬂow at least 50 to
100 per cent; therefore, the increased mucosal and gut
wall metabolic rate during gut activity probably lowers
the oxygen concentration enough to cause much of the
vasodilation. The decrease in oxygen can also lead to
as much as a fourfold increase of adenosine, a well-
known vasodilator that could be responsible for much
of the increased ﬂow.
Thus, the increased blood ﬂow during increased gas-

us, the increased blood ﬂow during increased gas-
trointestinal activity is probably a combination of
many of the aforementioned factors plus still others
yet undiscovered.
“Countercurrent” Blood Flow in the Villi. Note in Figure
62–8 that the arterial ﬂow into the villus and the
venous ﬂow out of the villus are in directions opposite
to each other, and that the vessels lie in close apposi-
tion to each other. Because of this vascular arrange-
ment, much of the blood oxygen diffuses out of the
a

nt, much of the blood oxygen diffuses out of the
arterioles directly into the adjacent venules without
ever being carried in the blood to the tips of the villi.
As much as 80 per cent of the oxygen may take this
short-circuit route and therefore not be available for
local metabolic functions of the villi. The reader will
recognize that this type of countercurrent mechanism
in the villi is analogous to the countercurrent mecha-
nism in the vasa recta of the kidney medulla, discussed
in detail in 

cta of the kidney medulla, discussed
in detail in Chapter 28.
Under normal conditions, this shunting of oxygen
from the arterioles to the venules is not harmful to the
villi, but in disease conditions in which blood ﬂow to
the gut becomes greatly curtailed, such as in circula-
tory shock, the oxygen deﬁcit in the tips of the villi can
become so great that the villus tip or even the whole
villus suffers ischemic death and can disintegrate.
Therefore, for this reason and others, in many gas-
troin

re, for this reason and others, in many gas-
trointestinal diseases the villi become seriously
blunted, leading to greatly diminished intestinal
absorptive capacity.
Nervous Control of Gastrointestinal
Blood Flow
Stimulation of the parasympathetic nerves going to the
stomach and lower colon increases local blood ﬂow at
the same time that it increases glandular secretion.
This increased ﬂow probably results secondarily from
the increased glandular activity and not as a direct
effect of the nervou

 activity and not as a direct
effect of the nervous stimulation.
Sympathetic stimulation, by contrast, has a direct
effect on essentially all the gastrointestinal tract to
cause intense vasoconstriction of the arterioles with
greatly decreased blood ﬂow. After a few minutes of
this vasoconstriction, the ﬂow often returns almost to
normal by means of a mechanism called “autoregula-
tory escape.” That is, the local metabolic vasodilator
mechanisms that are elicited by ischemia become pre-
potent o

that are elicited by ischemia become pre-
potent over the sympathetic vasoconstriction and,
therefore, redilate the arterioles, thus causing return of
necessary nutrient blood ﬂow to the gastrointestinal
glands and muscle.
Central lacteal
Vein
Artery
Blood capillaries
Figure 62–8
Microvasculature of the villus, showing a countercurrent arrange-
ment of blood ﬂow in the arterioles and venules.
780
Unit XII
Importance of Nervous Depression of Gastrointestinal Blood
Flow

 Nervous Depression of Gastrointestinal Blood
Flow When Other Parts of the Body Need Extra Blood Flow. A
major value of sympathetic vasoconstriction in the gut
is that it allows shut-off of gastrointestinal and other
splanchnic blood ﬂow for short periods of time during
heavy exercise, when increased ﬂow is needed by the
skeletal muscle and heart. Also, in circulatory shock,
when all the body’s vital tissues are in danger of cel-
lular death for lack of blood ﬂow—especially the brain
and the hea

lack of blood ﬂow—especially the brain
and the heart—sympathetic stimulation can decrease
splanchnic blood ﬂow to very little for many hours.
Sympathetic stimulation also causes strong vaso-
constriction of the large-volume intestinal and mesen-
teric veins. This decreases the volume of these veins,
thereby displacing large amounts of blood into other
parts of the circulation. In hemorrhagic shock or other
states of low blood volume, this mechanism can
provide as much as 200 to 400 milliliters o

sm can
provide as much as 200 to 400 milliliters of extra blood
to sustain the general circulation.
Adelson DW, Million M: Tracking the moveable feast:
sonomicrometry and gastrointestinal motility.
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Physiol Sci 19:27, 2004.
Daniel EE: Physiology and pathophysiology of the intersti-
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281:G1329, 2001.
Furness JB

rointest Liver Physiol
Furness JB, Jones C, Nurgali K, Clerc N: Intrinsic primary
afferent neurons and nerve circuits within the intestine.
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Hansen MB: The enteric nervous system II: gastrointestinal
functions. Pharmacol Toxicol 92:249-57, 2003.
Hobson AR, Aziz Q: Central nervous system processing of
human visceral pain in health and disease. News Physiol
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Hocker M: Molecular mechanisms of gastrin-dependent
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s of gastrin-dependent
gene regulation. Ann N Y Acad Sci 1014:97, 2004.
Huizinga JD: Physiology and pathophysiology of the inter-
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6
3
781
Propulsion and Mixing of 
Food in the Alimentary Tract
For

on and Mixing of 
For food to be processed optimally in the alimen-
tary tract, the time that it remains in each part of the
tract is critical. Also, appropriate mixing must be
provided. Yet because the requirements for mixing
and propulsion are quite different at each stage of
processing, multiple automatic nervous and hor-
monal feedback mechanisms control the timing of
each of these so that they will occur optimally, not
too rapidly, not too slowly.
The purpose of

y, not
The purpose of this chapter is to discuss these movements,especially the auto-
matic mechanisms of this control.
The amount of food that a person ingests is determined principally by intrinsic
desire for food called hunger.The type of food that a person preferentially seeks
is determined by appetite. These mechanisms in themselves are extremely
important automatic regulatory systems for maintaining an adequate nutri-
tional supply for the bod

ining an adequate nutri-
tional supply for the body; they are discussed in Chapter 71 in relation to nutri-
tion of the body. The current discussion of food ingestion is conﬁned to the
mechanics of ingestion, especially mastication and swallowing.
The teeth are admirably designed for chewing, the anterior teeth (incisors) pro-
viding a strong cutting action and the posterior teeth (molars), a grinding action.
All the jaw muscles working together can close the teeth with a f

cles working together can close the teeth with a force as great
as 55 pounds on the incisors and 200 pounds on the molars.
Most of the muscles of chewing are innervated by the motor branch of the
ﬁfth cranial nerve, and the chewing process is controlled by nuclei in the brain
stem. Stimulation of speciﬁc reticular areas in the brain stem taste centers will
cause rhythmical chewing movements. Also, stimulation of areas in the hypo-
thalamus, amygdala, and even the cerebral cortex near the sensory

ala, and even the cerebral cortex near the sensory areas for
taste and smell can often cause chewing.
Much of the chewing process is caused by a chewing reﬂex, which may be
explained as follows: The presence of a bolus of food in the mouth at ﬁrst ini-
tiates reﬂex inhibition of the muscles of mastication, which allows the lower jaw
to drop. The drop in turn initiates a stretch reﬂex of the jaw muscles that leads
to rebound contraction.This automatically raises the jaw to cause closure of the
te

atically raises the jaw to cause closure of the
teeth, but it also compresses the bolus again against the linings of the mouth,
which inhibits the jaw muscles once again, allowing the jaw to drop and rebound
another time; this is repeated again and again.
Chewing is important for digestion of all foods, but especially important for
most fruits and raw vegetables because these have indigestible cellulose mem-
branes around their nutrient portions that must be broken before the food can
be digeste

that must be broken before the food can
be digested. Also, chewing aids the digestion of food for still another simple
reason: Digestive enzymes act only on the surfaces of food particles; therefore,
782
Unit XII
the rate of digestion is absolutely dependent on the
total surface area exposed to the digestive secretions.
In addition, grinding the food to a very ﬁne particulate
consistency prevents excoriation of the gastrointesti-
nal tract and increases the ease with 

trointesti-
nal tract and increases the ease with which food is
emptied from the stomach into the small intestine,
then into all succeeding segments of the gut.
Swallowing is a complicated mechanism, principally
because the pharynx subserves respiration as well as
swallowing. The pharynx is converted for only a few
seconds at a time into a tract for propulsion of food.
It is especially important that respiration not be com-
promised because of swallowing.
In general, swa

m-
In general, swallowing can be divided into (1) a vol-
untary stage, which initiates the swallowing process; (2)
a pharyngeal stage, which is involuntary and consti-
tutes passage of food through the pharynx into the
esophagus; and (3) an esophageal stage, another invol-
untary phase that transports food from the pharynx to
Voluntary Stage of Swallowing. When the food is ready for
swallowing, it is “voluntarily” squeezed or rolled pos-
teriorly into

voluntarily” squeezed or rolled pos-
teriorly into the pharynx by pressure of the tongue
upward and backward against the palate, as shown in
Figure 63–1. From here on, swallowing becomes
entirely—or almost entirely—automatic and ordinar-
ily cannot be stopped.
Pharyngeal Stage of Swallowing. As the bolus of food
enters the posterior mouth and pharynx, it stimulates
epithelial swallowing receptor areas all around the
opening of the pharynx, especially on the tonsillar
pillars, and impulses from t

ally on the tonsillar
pillars, and impulses from these pass to the brain stem
to initiate a series of automatic pharyngeal muscle
contractions as follows:
1. The soft palate is pulled upward to close the
posterior nares, to prevent reﬂux of food into the
nasal cavities.
2. The palatopharyngeal folds on each side of the
pharynx are pulled medially to approximate each
other. In this way, these folds form a sagittal 
slit through which the food must pass into the
posterior pharynx. This slit perfor

 pass into the
posterior pharynx. This slit performs a selective
action, allowing food that has been masticated
sufﬁciently to pass with ease. Because this stage of
swallowing lasts less than 1 second, any large
object is usually impeded too much to pass into
the esophagus.
3. The vocal cords of the larynx are strongly
approximated, and the larynx is pulled upward
and anteriorly by the neck muscles. These actions,
combined with the presence of ligaments that
prevent upward movement of the epiglo

gaments that
prevent upward movement of the epiglottis, cause
the epiglottis to swing backward over the opening
of the larynx. All these effects acting together
prevent passage of food into the nose and
trachea. Most essential is the tight approximation
of the vocal cords, but the epiglottis helps to
prevent food from ever getting as far as the vocal
cords. Destruction of the vocal cords or of the
muscles that approximate them can cause
strangulation.
4. The upward movement of the larynx also pu

tion.
4. The upward movement of the larynx also pulls 
up and enlarges the opening to the esophagus.
At the same time, the upper 3 to 4 centimeters 
of the esophageal muscular wall, called the 
upper esophageal sphincter (also called the
pharyngoesophageal sphincter) relaxes, thus
allowing food to move easily and freely from the
posterior pharynx into the upper esophagus.
Between swallows, this sphincter remains strongly
contracted, thereby preventing air from going 
into the esophagus during re

nting air from going 
into the esophagus during respiration. The 
upward movement of the larynx also lifts the
glottis out of the main stream of food ﬂow, so 
that the food mainly passes on each side of the
epiglottis rather than over its surface; this adds
still another protection against entry of food into
the trachea.
5. Once the larynx is raised and the
pharyngoesophageal sphincter becomes relaxed,
the entire muscular wall of the pharynx contracts,
beginning in the superior part of the phary

racts,
beginning in the superior part of the pharynx,
then spreading downward over the middle and
inferior pharyngeal areas, which propels the food
by peristalsis into the esophagus.
To summarize the mechanics of the pharyngeal
stage of swallowing: The trachea is closed, the esoph-
agus is opened, and a fast peristaltic wave initiated by
the nervous system of the pharynx forces the bolus of
food into the upper esophagus, the entire process
occurring in less than 2 seconds.
Esophagus
Vagus
Glosso

ing in less than 2 seconds.
Esophagus
Vagus
nerve
Trigeminal nerve
Bolus of food
Uvula
Vocal cords
Peristalsis
Pharynx
Medulla
center
Figure 63–1
Swallowing mechanism.
Chapter 63
Propulsion and Mixing of Food in the Alimentary Tract
783
Nervous Initiation of the Pharyngeal Stage of Swallow-
ing. The most sensitive tactile areas of the posterior
mouth and pharynx for initiating the pharyngeal stage
of swallowing lie in a ring around the pharyngeal
opening, w

ing lie in a ring around the pharyngeal
opening, with greatest sensitivity on the tonsillar
pillars. Impulses are transmitted from these areas
through the sensory portions of the trigeminal and
glossopharyngeal nerves into the medulla oblongata,
either into or closely associated with the tractus soli-
tarius, which receives essentially all sensory impulses
from the mouth.
The successive stages of the swallowing process 
are then automatically initiated in orderly sequence 
by neuronal areas of t

tiated in orderly sequence 
by neuronal areas of the reticular substance of the
medulla and lower portion of the pons. The sequence
of the swallowing reﬂex is the same from one swallow
to the next, and the timing of the entire cycle also
remains constant from one swallow to the next. The
areas in the medulla and lower pons that control 
swallowing are collectively called the deglutition or
swallowing center.
The motor impulses from the swallowing center to
the pharynx and upper esophagus that ca

 center to
the pharynx and upper esophagus that cause swallow-
ing are transmitted successively by the 5th, 9th, 10th,
and 12th cranial nerves and even a few of the superior
cervical nerves.
In summary, the pharyngeal stage of swallowing is
principally a reﬂex act. It is almost always initiated by
voluntary movement of food into the back of the
mouth, which in turn excites involuntary pharyngeal
sensory receptors to elicit the swallowing reﬂex.
Effect of the Pharyngeal Stage of Swallowing on Res

ffect of the Pharyngeal Stage of Swallowing on Res-
piration. The entire pharyngeal stage of swallowing
usually occurs in less than 6 seconds, thereby inter-
rupting respiration for only a fraction of a usual res-
piratory cycle. The swallowing center speciﬁcally
inhibits the respiratory center of the medulla during
this time, halting respiration at any point in its cycle to
allow swallowing to proceed. Yet, even while a person
is talking, swallowing interrupts respiration for such a
short time 

wing interrupts respiration for such a
short time that it is hardly noticeable.
Esophageal Stage of Swallowing. The esophagus functions
primarily to conduct food rapidly from the pharynx to
the stomach, and its movements are organized specif-
ically for this function.
The esophagus normally exhibits two types of peri-
staltic movements: primary peristalsis and secondary
peristalsis. Primary peristalsis is simply continuation of
the peristaltic wave that begins in the pharynx and
spreads into the

ve that begins in the pharynx and
spreads into the esophagus during the pharyngeal
stage of swallowing.This wave passes all the way from
the pharynx to the stomach in about 8 to 10 seconds.
Food swallowed by a person who is in the upright posi-
tion is usually transmitted to the lower end of the
esophagus even more rapidly than the peristaltic wave
itself, in about 5 to 8 seconds, because of the additional
effect of gravity pulling the food downward.
If the primary peristaltic wave fails to move

ard.
If the primary peristaltic wave fails to move into the
stomach all the food that has entered the esophagus,
secondary peristaltic waves result from distention of
the esophagus itself by the retained food; these waves
continue until all the food has emptied into the
stomach. The secondary peristaltic waves are initiated
partly by intrinsic neural circuits in the myenteric
nervous system and partly by reﬂexes that begin in the
pharynx and are then transmitted upward through
vagal afferent ﬁbe

then transmitted upward through
vagal afferent ﬁbers to the medulla and back again to
the esophagus through glossopharyngeal and vagal
efferent nerve ﬁbers.
The musculature of the pharyngeal wall and upper
third of the esophagus is striated muscle.Therefore, the
peristaltic waves in these regions are controlled by
skeletal nerve impulses from the glossopharyngeal and
vagus nerves. In the lower two thirds of the esophagus,
the musculature is smooth muscle, but this portion of
the esophagus is als

h muscle, but this portion of
the esophagus is also strongly controlled by the vagus
nerves acting through connections with the esophageal
myenteric nervous system. When the vagus nerves to
the esophagus are cut, the myenteric nerve plexus of
the esophagus becomes excitable enough after several
days to cause strong secondary peristaltic waves even
without support from the vagal reﬂexes. Therefore,
even after paralysis of the brain stem swallowing
reﬂex, food fed by tube or in some other way into

reﬂex, food fed by tube or in some other way into the
esophagus still passes readily into the stomach.
Receptive Relaxation of the Stomach. When the
esophageal peristaltic wave approaches toward the
stomach, a wave of relaxation, transmitted through
myenteric inhibitory neurons, precedes the peristalsis.
Furthermore, the entire stomach and, to a lesser
extent, even the duodenum become relaxed as this
wave reaches the lower end of the esophagus and 
thus are prepared ahead of time to receive the

nd 
thus are prepared ahead of time to receive the food
propelled into the esophagus during the swallowing
act.
Function of the Lower Esophageal Sphincter (Gastro-
esophageal Sphincter). At the lower end of the esoph-
agus, extending upward about 3 centimeters above its
juncture with the stomach, the esophageal circular
muscle functions as a broad lower esophageal sphinc-
ter, also called the gastroesophageal sphincter. This
sphincter normally remains tonically constricted with
an intraluminal p

mains tonically constricted with
an intraluminal pressure at this point in the esophagus
of about 30 mm Hg, in contrast to the midportion of
the esophagus, which normally remains relaxed.When
a peristaltic swallowing wave passes down the esoph-
agus, there is “receptive relaxation” of the lower
esophageal sphincter ahead of the peristaltic wave,
which allows easy propulsion of the swallowed food
into the stomach. Rarely, the sphincter does not relax
satisfactorily, resulting in a condition calle

lax
satisfactorily, resulting in a condition called achalasia,
which is discussed in Chapter 66.
The stomach secretions are highly acidic and
contain many proteolytic enzymes. The esophageal
mucosa, except in the lower one eighth of the esoph-
agus, is not capable of resisting for long the digestive
action of gastric secretions. Fortunately, the tonic con-
striction of the lower esophageal sphincter helps to
prevent signiﬁcant reﬂux of stomach contents into the
esophagus except under very abnorm

ntents into the
esophagus except under very abnormal conditions.
784
Unit XII
Additional Prevention of Esophageal Reﬂux by Valve-
like Closure of the Distal End of the Esophagus.
Another factor that helps to prevent reﬂux is a valve-
like mechanism of a short portion of the esophagus
that extends slightly into the stomach. Increased intra-
abdominal pressure caves the esophagus inward at this
point. Thus, this valvelike closure of the lower esoph-
agus helps to preven

e closure of the lower esoph-
agus helps to prevent high intra-abdominal pressure
from forcing stomach contents backward into the
esophagus. Otherwise, every time we walked, coughed,
or breathed hard,we might expel stomach acid into the
esophagus.
Motor Functions of the
Stomach
The motor functions of the stomach are threefold: (1)
storage of large quantities of food until the food can
be processed in the stomach, duodenum, and lower
intestinal tract; (2) mixing of this food with gastric
secretio

act; (2) mixing of this food with gastric
secretions until it forms a semiﬂuid mixture called
chyme; and (3) slow emptying of the chyme from the
stomach into the small intestine at a rate suitable for
proper digestion and absorption by the small intestine.
Figure 63–2 shows the basic anatomy of the
stomach. Anatomically, the stomach is usually divided
into two major parts: (1) the body and (2) the antrum.
Physiologically, it is more appropriately divided into
(1) the “orad” portion, comprising a

 divided into
(1) the “orad” portion, comprising about the ﬁrst two
thirds of the body, and (2) the “caudad” portion, com-
prising the remainder of the body plus the antrum.
As food enters the stomach, it forms concentric circles
of the food in the orad portion of the stomach, the
newest food lying closest to the esophageal opening
and the oldest food lying nearest the outer wall of the
stomach. Normally, when food stretches the stomach,
a “vagovagal reﬂex” from t

 stretches the stomach,
a “vagovagal reﬂex” from the stomach to the brain
stem and then back to the stomach reduces the tone
in the muscular wall of the body of the stomach so that
the wall bulges progressively outward, accommodating
greater and greater quantities of food up to a limit in
the completely relaxed stomach of 0.8 to 1.5 liters.The
pressure in the stomach remains low until this limit is
approached.
Mixing and Propulsion of Food in the
Stomach—The Basic Electrical
Rhythm of the Stomac

Rhythm of the Stomach Wall
The digestive juices of the stomach are secreted by
gastric glands, which are present in almost the entire
wall of the body of the stomach except along a narrow
strip on the lesser curvature of the stomach. These
secretions come immediately into contact with that
portion of the stored food lying against the mucosal
surface of the stomach. As long as food is in the
stomach, weak peristaltic constrictor waves, called
mixing waves, begin in t

constrictor waves, called
mixing waves, begin in the mid- to upper portions of
the stomach wall and move toward the antrum about
once every 15 to 20 seconds. These waves are initiated
by the gut wall basic electrical rhythm, which was dis-
cussed in Chapter 62, consisting of electrical “slow
waves” that occur spontaneously in the stomach wall.
As the constrictor waves progress from the body of the
stomach into the antrum, they become more intense,
some becoming extremely intense and providing po

,
some becoming extremely intense and providing pow-
erful peristaltic action potential–driven constrictor
rings that force the antral contents under higher and
higher pressure toward the pylorus.
These constrictor rings also play an important role
in mixing the stomach contents in the following way:
Each time a peristaltic wave passes down the antral
wall toward the pylorus, it digs deeply into the food
contents in the antrum.Yet the opening of the pylorus
is still small enough that only a few 

the pylorus
is still small enough that only a few milliliters or less
of antral contents are expelled into the duodenum
with each peristaltic wave. Also, as each peristaltic
wave approaches the pylorus, the pyloric muscle itself
often contracts, which further impedes emptying
through the pylorus.Therefore, most of the antral con-
tents are squeezed upstream through the peristaltic
ring toward the body of the stomach, not through the
pylorus. Thus, the moving peristaltic constrictive ring,
combin

, the moving peristaltic constrictive ring,
combined with this upstream squeezing action, called
“retropulsion,” is an exceedingly important mixing
mechanism in the stomach.
Chyme. After food in the stomach has become thor-
oughly mixed with the stomach secretions, the result-
ing mixture that passes down the gut is called chyme.
The degree of ﬂuidity of the chyme leaving the
stomach depends on the relative amounts of food,
water, and stomach secretions and on the degree of
digestion that has oc

retions and on the degree of
digestion that has occurred. The appearance of chyme
is that of a murky semiﬂuid or paste.
Fundus
Esophagus
Cardia
Angular
notch
Pyloric
sphincter
Duodenum
Antrum
Rugae
Body
Pylorus
Figure 63–2
Physiologic anatomy of the stomach.
Chapter 63
785
Hunger Contractions. Besides the peristaltic contractions
that occur when food is present in the stomach,
another type of intense contractions, called hunger
contractions, 

intense contractions, called hunger
contractions, often occurs when the stomach has been
empty for several hours or more. They are rhythmical
peristaltic contractions in the body of the stomach.
When the successive contractions become extremely
strong, they often fuse to cause a continuing tetanic
contraction that sometimes lasts for 2 to 3 minutes.
Hunger contractions are most intense in young,
healthy people who have high degrees of gastroin-
testinal tonus; they are also greatly increased by 

estinal tonus; they are also greatly increased by the
person’s having lower than normal levels of blood
sugar.When hunger contractions occur in the stomach,
the person sometimes experiences mild pain in the pit
of the stomach, called hunger pangs. Hunger pangs
usually do not begin until 12 to 24 hours after the last
ingestion of food; in starvation, they reach their great-
est intensity in 3 to 4 days and gradually weaken in
succeeding days.
Stomach emptying is promoted by inten

ach Emptying
Stomach emptying is promoted by intense peristaltic
contractions in the stomach antrum.At the same time,
emptying is opposed by varying degrees of resistance
to passage of chyme at the pylorus.
Intense Antral Peristaltic Contractions During Stomach Empty-
ing—“Pyloric Pump.” Most of the time, the rhythmical
stomach contractions are weak and function mainly to
cause mixing of food and gastric secretions. However,
for about 20 per cent of the time while food is in the
stomach, the con

 of the time while food is in the
stomach, the contractions become intense, beginning
in midstomach and spreading through the caudad
stomach no longer as weak mixing contractions but as
strong peristaltic, very tight ringlike constrictions that
can cause stomach emptying.As the stomach becomes
progressively more and more empty, these constric-
tions begin farther and farther up the body of the
stomach, gradually pinching off the food in the body
of the stomach and adding this food to the chyme i

of the stomach and adding this food to the chyme in
the antrum. These intense peristaltic contractions
often create 50 to 70 centimeters of water pressure,
which is about six times as powerful as the usual
mixing type of peristaltic waves.
When pyloric tone is normal, each strong peristaltic
wave forces up to several milliliters of chyme into the
duodenum. Thus, the peristaltic waves, in addition to
causing mixing in the stomach, also provide a pumping
action called the “pyloric pump.”
Role of t

umping
Role of the Pylorus in Controlling Stomach Emptying. The
distal opening of the stomach is the pylorus. Here the
thickness of the circular wall muscle becomes 50 to 100
per cent greater than in the earlier portions of the
stomach antrum, and it remains slightly tonically con-
tracted almost all the time. Therefore, the pyloric cir-
cular muscle is called the pyloric sphincter.
Despite normal tonic contraction of the pyloric
sphincter, the pylorus usually i

on of the pyloric
sphincter, the pylorus usually is open enough for water
and other ﬂuids to empty from the stomach into the
duodenum with ease. Conversely, the constriction
usually prevents passage of food particles until they
have become mixed in the chyme to almost ﬂuid con-
sistency. The degree of constriction of the pylorus is
increased or decreased under the inﬂuence of nervous
and humoral reﬂex signals from both the stomach and
the duodenum, as discussed shortly.
Regulation of Stomach Emp

m, as discussed shortly.
The rate at which the stomach empties is regulated by
signals from both the stomach and the duodenum.
However, the duodenum provides by far the more
potent of the signals, controlling the emptying of
chyme into the duodenum at a rate no greater than the
rate at which the chyme can be digested and absorbed
in the small intestine.
Gastric Factors That Promote Emptying
Effect of Gastric Food Volume on Rate of Emptying. Increased
food volume in

lume on Rate of Emptying. Increased
food volume in the stomach promotes increased emp-
tying from the stomach. But this increased emptying
does not occur for the reasons that one would expect.
It is not increased storage pressure of the food in the
stomach that causes the increased emptying because,
in the usual normal range of volume, the increase 
in volume does not increase the pressure much.
However, stretching of the stomach wall does elicit
local myenteric reﬂexes in the wall that greatly 

local myenteric reﬂexes in the wall that greatly accen-
tuate activity of the pyloric pump and at the same time
inhibit the pylorus.
Effect of the Hormone Gastrin on Stomach Emptying. In
Chapter 64, we shall see that stomach wall stretch and
the presence of certain types of foods in the stomach—
particularly digestive products of meat—elicit release
of a hormone called gastrin from the antral mucosa.
This has potent effects to cause secretion of highly
acidic gastric juice by the stomach glands

 highly
acidic gastric juice by the stomach glands. Gastrin also
has mild to moderate stimulatory effects on motor
functions in the body of the stomach. Most important,
it seems to enhance the activity of the pyloric pump.
Thus, it, too, probably promotes stomach emptying.
Powerful Duodenal Factors That Inhibit
Inhibitory Effect of Enterogastric Nervous Reﬂexes from the
Duodenum. When food enters the duodenum, multiple
nervous reﬂexes are initiated from the duodenal wall
that pa

ﬂexes are initiated from the duodenal wall
that pass back to the stomach to slow or even stop
stomach emptying if the volume of chyme in the duo-
denum becomes too much. These reﬂexes are medi-
ated by three routes: (1) directly from the duodenum
to the stomach through the enteric nervous system 
in the gut wall, (2) through extrinsic nerves that go to
the prevertebral sympathetic ganglia and then back
through inhibitory sympathetic nerve ﬁbers to the
stomach, and (3) probably to a slight extent

o the
stomach, and (3) probably to a slight extent through
the vagus nerves all the way to the brain stem, where
they inhibit the normal excitatory signals transmitted
786
Unit XII
to the stomach through the vagi. All these parallel
reﬂexes have two effects on stomach emptying: ﬁrst,
they strongly inhibit the “pyloric pump” propulsive
contractions, and second, they increase the tone of the
pyloric sphincter.
The types of factors that are continually monitored
in the d

of factors that are continually monitored
in the duodenum and that can initiate enterogastric
inhibitory reﬂexes include the following:
1. The degree of distention of the duodenum
2. The presence of any degree of irritation of the
duodenal mucosa
3. The degree of acidity of the duodenal chyme
4. The degree of osmolality of the chyme
5. The presence of certain breakdown products in
the chyme, especially breakdown products of
proteins and perhaps to a lesser extent of fats
The enterogastric inhibi

o a lesser extent of fats
The enterogastric inhibitory reﬂexes are especially
sensitive to the presence of irritants and acids in the
duodenal chyme, and they often become strongly acti-
vated within as little as 30 seconds. For instance, when-
ever the pH of the chyme in the duodenum falls below
about 3.5 to 4, the reﬂexes frequently block further
release of acidic stomach contents into the duodenum
until the duodenal chyme can be neutralized by pan-
creatic and other secretions.
Breakdown prod

 pan-
Breakdown products of protein digestion also elicit
inhibitory enterogastric reﬂexes; by slowing the rate of
stomach emptying, sufﬁcient time is ensured for ade-
quate protein digestion in the duodenum and small
intestine.
Finally, either hypotonic or hypertonic ﬂuids (espe-
cially hypertonic) elicit the inhibitory reﬂexes. Thus,
too rapid ﬂow of nonisotonic ﬂuids into the small
intestine is prevented, thereby also preventing rapid
changes in electrolyte conce

also preventing rapid
changes in electrolyte concentrations in the whole-
body extracellular ﬂuid during absorption of the intes-
tinal contents.
Hormonal Feedback from the Duodenum Inhibits Gastric Emp-
tying—Role of Fats and the Hormone Cholecystokinin. Not
only do nervous reﬂexes from the duodenum to the
stomach inhibit stomach emptying, but hormones
released from the upper intestine do so as well. The
stimulus for releasing these inhibitory hormones is
mainly fats entering the duodenum, alth

ormones is
mainly fats entering the duodenum, although other
types of foods can increase the hormones to a lesser
degree.
On entering the duodenum, the fats extract several
different hormones from the duodenal and jejunal
epithelium, either by binding with “receptors” on 
the epithelial cells or in some other way. In turn, the 
hormones are carried by way of the blood to the
stomach, where they inhibit the pyloric pump and at
the same time increase the strength of contraction of
the pyloric sphi

se the strength of contraction of
the pyloric sphincter. These effects are important
because fats are much slower to be digested than most
other foods.
Precisely which hormones cause the hormonal 
feedback inhibition of the stomach is not fully clear.
The most potent appears to be cholecystokinin (CCK),
which is released from the mucosa of the jejunum in
response to fatty substances in the chyme. This
hormone acts as an inhibitor to block increased
stomach motility caused by gastrin.
Other possi

ed
Other possible inhibitors of stomach emptying are
the hormones secretin and gastric inhibitory peptide
(GIP). Secretin is released mainly from the duodenal
mucosa in response to gastric acid passed from the
stomach through the pylorus. GIP has a general but
weak effect of decreasing gastrointestinal motility.
GIP is released from the upper small intestine in
response mainly to fat in the chyme, but to a lesser
extent to carbohydrates as well. Although GIP d

er
extent to carbohydrates as well. Although GIP does
inhibit gastric motility under some conditions,its effect
at physiologic concentrations is probably mainly to
stimulate secretion of insulin by the pancreas.
These hormones are discussed at greater length
elsewhere in this text, especially in Chapter 64 in rela-
tion to control of gallbladder emptying and control of
rate of pancreatic secretion.
In summary, hormones, especially CCK, can inhibit
gastric emptying when excess quantities of chyme

t
gastric emptying when excess quantities of chyme,
especially acidic or fatty chyme, enter the duodenum
from the stomach.
Summary of the Control of Stomach Emptying
Emptying of the stomach is controlled only to a mod-
erate degree by stomach factors such as the degree 
of ﬁlling in the stomach and the excitatory effect of
gastrin on stomach peristalsis. Probably the more
important control of stomach emptying resides in
inhibitory feedback signals from the duodenum,
including both enterogastric 

s from the duodenum,
including both enterogastric inhibitory nervous feed-
back reﬂexes and hormonal feedback by CCK. These
feedback inhibitory mechanisms work together to
slow the rate of emptying when (1) too much chyme
is already in the small intestine or (2) the chyme is
excessively acidic, contains too much unprocessed
protein or fat, is hypotonic or hypertonic, or is irritat-
ing. In this way, the rate of stomach emptying is limited
to that amount of chyme that the small intestine can
proc

 amount of chyme that the small intestine can
process.
Movements of the 
Small Intestine
The movements of the small intestine, like those else-
where in the gastrointestinal tract, can be divided into
mixing contractions and propulsive contractions. To a
great extent, this separation is artiﬁcial because essen-
tially all movements of the small intestine cause at
least some degree of both mixing and propulsion. The
usual classiﬁcation of these processes is the following.
Mixing Contractions (Seg

ocesses is the following.
Mixing Contractions (Segmentation
When a portion of the small intestine becomes dis-
tended with chyme, stretching of the intestinal wall
elicits localized concentric contractions spaced at
intervals along the intestine and lasting a fraction of a
minute. The contractions cause “segmentation” of the
Chapter 63
787
small intestine, as shown in Figure 63–3. That is, they
divide the intestine into spaced s

. That is, they
divide the intestine into spaced segments that have the
appearance of a chain of sausages. As one set of seg-
mentation contractions relaxes, a new set often begins,
but the contractions this time occur mainly at new
points between the previous contractions. Therefore,
the segmentation contractions “chop” the chyme two
to three times per minute, in this way promoting pro-
gressive mixing of the food with secretions of the small
The maximum frequency of the segmentation

testine.
The maximum frequency of the segmentation con-
tractions in the small intestine is determined by the
frequency of electrical slow waves in the intestinal wall,
which is the basic electrical rhythm described in
Chapter 62. Because this frequency normally is not
over 12 per minute in the duodenum and proximal
jejunum, the maximum frequency of the segmentation
contractions in these areas is also about 12 per minute,
but this occurs only under extreme conditions of stim-
ulation. In the ter

er extreme conditions of stim-
ulation. In the terminal ileum, the maximum frequency
is usually 8 to 9 contractions per minute.
The segmentation contractions become exceedingly
weak when the excitatory activity of the enteric
nervous system is blocked by the drug atropine.There-
fore, even though it is the slow waves in the smooth
muscle itself that cause the segmentation contractions,
these contractions are not effective without back-
ground excitation mainly from the myenteric nerve
plexus.
Pr

itation mainly from the myenteric nerve
plexus.
Peristalsis in the Small Intestine. Chyme is propelled
through the small intestine by peristaltic waves. These
can occur in any part of the small intestine, and they
move toward the anus at a velocity of 0.5 to 2.0 cm/sec,
faster in the proximal intestine and slower in the ter-
minal intestine. They normally are very weak and
usually die out after traveling only 3 to 5 centimeters,
very rarely farther than 10 centimeters, so th

rs,
very rarely farther than 10 centimeters, so that forward
movement of the chyme is very slow, so slow in fact
that net movement along the small intestine normally
averages only 1 cm/min. This means that 3 to 5 hours
are required for passage of chyme from the pylorus to
the ileocecal valve.
Control of Peristalsis by Nervous and Hormonal Signals. Peri-
staltic activity of the small intestine is greatly increased
after a meal. This is caused partly by the beginning
entry of chyme into the duoden

ly by the beginning
entry of chyme into the duodenum causing stretch of
the duodenal wall, but also by a so-called gastroenteric
reﬂex that is initiated by distention of the stomach and
conducted principally through the myenteric plexus
from the stomach down along the wall of the small
In addition to the nervous signals that may affect
small intestinal peristalsis, several hormonal factors
also affect peristalsis. They include gastrin, CCK,
insulin, motilin, and serotonin, all of whic

 CCK,
insulin, motilin, and serotonin, all of which enhance
intestinal motility and are secreted during various
phases of food processing. Conversely, secretin and
glucagon inhibit small intestinal motility. The physio-
logic importance of each of these hormonal factors for
controlling motility is still questionable.
The function of the peristaltic waves in the small
intestine is not only to cause progression of chyme
toward the ileocecal valve but also to spread out the
chyme along the intestin

ut also to spread out the
chyme along the intestinal mucosa. As the chyme
enters the intestines from the stomach and elicits peri-
stalsis, this immediately spreads the chyme along the
intestine; and this process intensiﬁes as additional
chyme enters the duodenum. On reaching the ileoce-
cal valve, the chyme is sometimes blocked for several
hours until the person eats another meal; at that time,
a gastroileal reﬂex intensiﬁes peristalsis in the ileum
and forces the remaining chyme through the il

leum
and forces the remaining chyme through the ileocecal
valve into the cecum of the large intestine.
Propulsive Effect of the Segmentation Movements. The seg-
mentation movements, although lasting for only a few
seconds at a time, often also travel 1 centimeter or so
in the anal direction and during that time help propel
the food down the intestine. The difference between
the segmentation and the peristaltic movements is not
as great as might be implied by their separation into
these two class

e implied by their separation into
these two classiﬁcations.
Peristaltic Rush. Although peristalsis in the small 
intestine is normally weak, intense irritation of the
intestinal mucosa, as occurs in some severe cases of
infectious diarrhea, can cause both powerful and rapid
peristalsis, called the peristaltic rush. This is initiated
partly by nervous reﬂexes that involve the autonomic
nervous system and brain stem and partly by intrinsic
enhancement of the myenteric plexus reﬂexes within
the gu

ment of the myenteric plexus reﬂexes within
the gut wall itself.The powerful peristaltic contractions
travel long distances in the small intestine within
minutes, sweeping the contents of the intestine into the
colon and thereby relieving the small intestine of irri-
tative chyme and excessive distention.
Movements Caused by the Muscularis Mucosae and Muscle Fibers
of the Villi. The muscularis mucosae can cause short 
folds to appear in the intestinal mucosa. In addition,
Regularly spaced
Isolat

tinal mucosa. In addition,
Isolated
Irregularly spaced
Weak regularly spaced
Figure 63–3
Segmentation movements of the small intestine.
788
Unit XII
individual ﬁbers from this muscle extend into the intes-
tinal villi and cause them to contract intermittently.The
mucosal folds increase the surface area exposed to the
chyme, thereby increasing absorption. Also, contrac-
tions of the villi—shortening, elongating, and shorten-
ing again—“milk” the villi,

ongating, and shorten-
ing again—“milk” the villi, so that lymph ﬂows freely
from the central lacteals of the villi into the lymphatic
system. These mucosal and villous contractions are ini-
tiated mainly by local nervous reﬂexes in the submu-
cosal nerve plexus that occur in response to chyme in
the small intestine.
A principal function of the ileocecal valve is to prevent
backﬂow of fecal contents from the colon into the
small intestine. As shown in Figure 63–4,

into the
small intestine. As shown in Figure 63–4, the ileocecal
valve itself protrudes into the lumen of the cecum and
therefore is forcefully closed when excess pressure
builds up in the cecum and tries to push cecal contents
backward against the valve lips. The valve usually can
resist reverse pressure of at least 50 to 60 centimeters
of water.
In addition, the wall of the ileum for several cen-
timeters immediately upstream from the ileocecal
valve has a thickened circular muscle called the 

valve has a thickened circular muscle called the ileo-
cecal sphincter.This sphincter normally remains mildly
constricted and slows emptying of ileal contents into
the cecum. However, immediately after a meal, a gas-
troileal reﬂex (described earlier) intensiﬁes peristalsis
in the ileum, and emptying of ileal contents into the
cecum proceeds.
Resistance to emptying at the ileocecal valve pro-
longs the stay of chyme in the ileum and thereby 
facilitates absorption. Normally, only 1500 to 2000 m

ilitates absorption. Normally, only 1500 to 2000 mil-
liliters of chyme empty into the cecum each day.
Feedback Control of the Ileocecal Sphincter. The degree of
contraction of the ileocecal sphincter and the intensity
of peristalsis in the terminal ileum are controlled sig-
niﬁcantly by reﬂexes from the cecum.When the cecum
is distended, contraction of the ileocecal sphincter
becomes intensiﬁed and ileal peristalsis is inhibited,
both of which greatly delay emptying of additional
chyme into the

reatly delay emptying of additional
chyme into the cecum from the ileum. Also, any irri-
tant in the cecum delays emptying. For instance, when
a person has an inﬂamed appendix, the irritation of
this vestigial remnant of the cecum can cause such
intense spasm of the ileocecal sphincter and partial
paralysis of the ileum that these effects together block
emptying of the ileum into the cecum. The reﬂexes
from the cecum to the ileocecal sphincter and ileum
are mediated both by way of the myenteric 

d ileum
are mediated both by way of the myenteric plexus in
the gut wall itself and of the extrinsic autonomic
nerves, especially by way of the prevertebral sympa-
thetic ganglia.
The principal functions of the colon are (1) absorption
of water and electrolytes from the chyme to form solid
feces and (2) storage of fecal matter until it can be
expelled. The proximal half of the colon, shown in
Figure 63–5, is concerned principally with absorption,
and the distal half with s

ipally with absorption,
and the distal half with storage. Because intense colon
wall movements are not required for these functions,
the movements of the colon are normally very slug-
gish. Yet in a sluggish manner, the movements still
have characteristics similar to those of the small intes-
tine and can be divided once again into mixing move-
ments and propulsive movements.
Mixing Movements—“Haustrations.” In the same manner
that segmentation movements occur in the small 
intestine, large circ

ovements occur in the small 
intestine, large circular constrictions occur in the large
Pressure or chemical irritation
in cecum inhibits peristalsis
of ileum and excites sphincter
Ileocecal sphincter
Ileum
Valve
Colon
Pressure and chemical
irritation relax sphincter
and excite peristalsis
Fluidity of contents
promotes emptying
Figure 63–4
Emptying at the ileocecal valve.
Mush
Semifluid
Fluid
Ileocecal
valve
Solid
Poor motility causes
greater absorption, and
hard feces in transverse
colon causes

orption, and
constipation
Semi-
mush
Semi-
solid
Excess motility causes
less absorption and
diarrhea or loose feces
Figure 63–5
Absorptive and storage functions of the large intestine.
Chapter 63
789
intestine.At each of these constrictions, about 2.5 cen-
timeters of the circular muscle contracts, sometimes
constricting the lumen of the colon almost to occlu-
sion. At the same time, the longitudinal musc

clu-
sion. At the same time, the longitudinal muscle of the
colon, which is aggregated into three longitudinal
strips called the teniae coli, contracts. These combined
contractions of the circular and longitudinal strips of
muscle cause the unstimulated portion of the large
intestine to bulge outward into baglike sacs called
haustrations.
Each haustration usually reaches peak intensity in
about 30 seconds and then disappears during the next
60 seconds.They also at times move slowly toward the
an

conds.They also at times move slowly toward the
anus during contraction, especially in the cecum and
ascending colon, and thereby provide a minor amount
of forward propulsion of the colonic contents. After
another few minutes, new haustral contractions occur
in other areas nearby. Therefore, the fecal material in
the large intestine is slowly dug into and rolled over in
much the same manner that one spades the earth. In
this way, all the fecal material is gradually exposed to
the mucosal surface

terial is gradually exposed to
the mucosal surface of the large intestine, and ﬂuid and
dissolved substances are progressively absorbed until
only 80 to 200 milliliters of feces are expelled each day.
Propulsive Movements—“Mass Movements.” Much of the
propulsion in the cecum and ascending colon results
from the slow but persistent haustral contractions,
requiring as many as 8 to 15 hours to move the chyme
from the ileocecal valve through the colon, while the
chyme itself becomes fecal in quality

n, while the
chyme itself becomes fecal in quality, a semisolid slush
instead of semiﬂuid.
From the cecum to the sigmoid, mass movements
can, for many minutes at a time, take over the propul-
sive role. These movements usually occur only one to
three times each day, in many people especially for
about 15 minutes during the ﬁrst hour after eating
breakfast.
A mass movement is a modiﬁed type of peristalsis
characterized by the following sequence of events:
First, a constrictive ring occurs in resp

 events:
First, a constrictive ring occurs in response to a dis-
tended or irritated point in the colon, usually in the
transverse colon. Then, rapidly, the 20 or more cen-
timeters of colon distal to the constrictive ring lose their
haustrations and instead contract as a unit, propelling
the fecal material in this segment en masse further
down the colon. The contraction develops progres-
sively more force for about 30 seconds, and relaxation
occurs during the next 2 to 3 minutes. Then, another

urs during the next 2 to 3 minutes. Then, another
mass movement occurs,this time perhaps farther along
the colon.
A series of mass movements usually persists for 10
to 30 minutes. Then they cease but return perhaps a
half day later. When they have forced a mass of feces
into the rectum, the desire for defecation is felt.
Initiation of Mass Movements by Gastrocolic and Duo-
denocolic Reﬂexes. Appearance of mass movements
after meals is facilitated by gastrocolic and duodeno-
colic reﬂexes. These 

 by gastrocolic and duodeno-
colic reﬂexes. These reﬂexes result from distention of
the stomach and duodenum. They occur either not at
all or hardly at all when the extrinsic autonomic nerves
to the colon have been removed;therefore,the reﬂexes
almost certainly are transmitted by way of the auto-
nomic nervous system.
Irritation in the colon can also initiate intense mass
movements. For instance, a person who has an ulcer-
ated condition of the colon mucosa (ulcerative colitis)
frequently has ma

olon mucosa (ulcerative colitis)
frequently has mass movements that persist almost all
the time.
Most of the time, the rectum is empty of feces. This
results partly from the fact that a weak functional
sphincter exists about 20 centimeters from the anus 
at the juncture between the sigmoid colon and the
rectum.There is also a sharp angulation here that con-
tributes additional resistance to ﬁlling of the rectum.
When a mass movement forces feces into the
rectum, the desire for defecat

rces feces into the
rectum, the desire for defecation occurs immediately,
including reﬂex contraction of the rectum and relax-
ation of the anal sphincters.
Continual dribble of fecal matter through the anus
is prevented by tonic constriction of (1) an internal
anal sphincter, a several-centimeters-long thickening
of the circular smooth muscle that lies immediately
inside the anus, and (2) an external anal sphincter, com-
posed of striated voluntary muscle that both surrounds
the internal sphinc

ary muscle that both surrounds
the internal sphincter and extends distal to it. The
external sphincter is controlled by nerve ﬁbers in the
pudendal nerve, which is part of the somatic nervous
system and therefore is under voluntary, conscious
or at least subconscious control; subconsciously,
the external sphincter is usually kept continuously
constricted unless conscious signals inhibit the 
constriction.
Defecation Reﬂexes. Ordinarily, defecation is initiated
by defecation reﬂexes.One of these 

n is initiated
by defecation reﬂexes.One of these reﬂexes is an intrin-
sic reﬂex mediated by the local enteric nervous system
in the rectal wall. This can be described as follows:
When feces enter the rectum, distention of the rectal
wall initiates afferent signals that spread through the
myenteric plexus to initiate peristaltic waves in the
descending colon, sigmoid, and rectum, forcing feces
toward the anus. As the peristaltic wave approaches
the anus, the internal anal sphincter is relaxed b

the anus, the internal anal sphincter is relaxed by
inhibitory signals from the myenteric plexus; if the
external anal sphincter is also consciously, voluntarily
relaxed at the same time, defecation occurs.
The intrinsic myenteric defecation reﬂex function-
ing by itself normally is relatively weak. To be effec-
tive in causing defecation, it usually must be fortiﬁed
by another type of defecation reﬂex, a parasympathetic
defecation reﬂex that involves the sacral segments of
the spinal cord, show

olves the sacral segments of
the spinal cord, shown in Figure 63–6. When the nerve
endings in the rectum are stimulated, signals are trans-
mitted ﬁrst into the spinal cord and then reﬂexly back
to the descending colon, sigmoid, rectum, and anus by
way of parasympathetic nerve ﬁbers in the pelvic
nerves.These parasympathetic signals greatly intensify
the peristaltic waves as well as relax the internal anal
790
Unit XII
sphincter, thus converting the intrinsic myenteri

sphincter, thus converting the intrinsic myenteric defe-
cation reﬂex from a weak effort into a powerful
process of defecation that is sometimes effective in
emptying the large bowel all the way from the splenic
ﬂexure of the colon to the anus.
Defecation signals entering the spinal cord initiate
other effects, such as taking a deep breath, closure of
the glottis, and contraction of the abdominal wall
muscles to force the fecal contents of the colon down-
ward and at the same time cause the pel

olon down-
ward and at the same time cause the pelvic ﬂoor to
relax downward and pull outward on the anal ring to
evaginate the feces.
When it becomes convenient for the person to defe-
cate,the defecation reﬂexes can purposely be activated
by taking a deep breath to move the diaphragm down-
ward and then contracting the abdominal muscles to
increase the pressure in the abdomen, thus forcing
fecal contents into the rectum to cause new reﬂexes.
Reﬂexes initiated in this way are almost never as ef

ﬂexes initiated in this way are almost never as effec-
tive as those that arise naturally, for which reason
people who too often inhibit their natural reﬂexes are
likely to become severely constipated.
In newborn babies and in some people with tran-
sected spinal cords, the defecation reﬂexes cause auto-
matic emptying of the lower bowel at inconvenient
times during the day because of lack of conscious
control exercised through voluntary contraction or
relaxation of the external anal sphincter.

ion or
Other Autonomic Reﬂexes
That Affect Bowel Activity
Aside from the duodenocolic, gastrocolic, gastroileal,
enterogastric, and defecation reﬂexes that have been
discussed in this chapter, several other important
nervous reﬂexes also can affect the overall degree of
bowel activity. They are the peritoneointestinal reﬂex,
renointestinal reﬂex, and vesicointestinal reﬂex.
The peritoneointestinal reﬂex results from irritation
of the peritoneum; it stro

results from irritation
of the peritoneum; it strongly inhibits the excitatory
enteric nerves and thereby can cause intestinal paral-
ysis, especially in patients with peritonitis. The reno-
intestinal and vesicointestinal reﬂexes inhibit intestinal
activity as a result of kidney or bladder irritation.
News
Chelimsky G, Chelimsky TC: Evaluation and treatment of
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Cooke HJ, Wunderlich J, ChristoﬁFL: “The force be with
you”: ATP in gut mechanosensory transduction. News
Physiol Sci 18:43, 2003.
De Giorgio R, Guerrini S, Barbara G, et al: Inﬂammatory
neuropathies of the enteric nervous system. Gastroen-
terology 126:1872, 2004.
Gonella J, Bouvier M, Blanquet F: Extrinsic nervous control
of motility of small and large intestines and re

l
of motility of small and large intestines and related sphinc-
ters. Physiol Rev 67:902, 1987.
Hall KE:Aging and neural control of the GI tract. II. Neural
control of the aging gut: can an old dog learn new tricks?
Am J Physiol Gastrointest Liver Physiol 283:G827, 2002.
Hansen MB: Neurohumoral control of gastrointestinal
motility. Physiol Res 52:1, 2003.
Hatoum OA, Miura H, Binion DG: The vascular contribu-
tion in the pathogenesis of inﬂammatory bowel disease.
Am J Physiol Heart Circ Physiol 2

y bowel disease.
Am J Physiol Heart Circ Physiol 285:H1791, 2003.
Jensen RT: Involvement of cholecystokinin/gastrin-related
peptides and their receptors in clinical gastrointestinal dis-
orders. Pharmacol Toxicol 91:333, 2002.
Kirkup AJ, Brunsden AM, Grundy D: Receptors and trans-
mission in the brain-gut axis: potential for novel therapies.
I. Receptors on visceral afferents. Am J Physiol Gastroin-
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s. Am J Physiol Gastroin-
test Liver Physiol 280:G787, 2001.
Laroux FS, Pavlick KP, Wolf RE, Grisham MB: Dysregula-
tion of intestinal mucosal immunity: implications in
inﬂammatory bowel disease. News Physiol Sci 16:272,
2001.
Orr WC, Chen CL:Aging and neural control of the GI tract:
IV. Clinical and physiological aspects of gastrointestinal
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Physiol 283:G1226, 2002.
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ult fecal incontinence. Gas-
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Sanders KM, Ordog T, Koh SD, Ward SM:A Novel Pace-
maker Mechanism Drives Gastrointestinal Rhythmicity.
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Takahashi T: Pathophysiological signiﬁcance of neuronal
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rapeutic
From
conscious
cortex
Afferent
nerve
fibers
(pelvic nerves)
Sigmoid
colon
Rectum
External anal sphincter
Internal anal sphincter
colon
Skeletal
motor nerve
Figure 63–6
Afferent and efferent pathways of the parasympathetic mecha-
nism for enhancing the defecation reﬂex.
C
H
A
P
T
E
R
 
6
4
791
Secretory Functions of the
Throughout the gastrointestinal tract, secretory
glands subserve two pr

intestinal tract, secretory
glands subserve two primary functions: First, diges-
tive enzymes are secreted in most areas of the ali-
mentary tract, from the mouth to the distal end of
the ileum. Second, mucous glands, from the mouth
to the anus, provide mucus for lubrication and pro-
tection of all parts of the alimentary tract.
Most digestive secretions are formed only in
response to the presence of food in the alimentary tract, and the quantity
secreted in each segment of the tract is almost e

secreted in each segment of the tract is almost exactly the amount needed for
proper digestion. Furthermore, in some portions of the gastrointestinal tract,
even the types of enzymes and other constituents of the secretions are varied in
accordance with the types of food present. The purpose of this chapter, there-
fore, is to describe the different alimentary secretions, their functions, and reg-
ulation of their production.
General Principles of Alimentary 
Tract Secretion
Anatomical Types of

of Alimentary 
Several types of glands provide the different types of alimentary tract secretions.
First, on the surface of the epithelium in most parts of the gastrointestinal tract are
billions of single-cell mucous glands called simply mucous cells or sometimes goblet
cells because they look like goblets. They function mainly in response to local irri-
tation of the epithelium: they extrude mucus directly onto the epithelial surface to
act as a lubri

ctly onto the epithelial surface to
act as a lubricant that also protects the surfaces from excoriation and digestion.
Second, many surface areas of the gastrointestinal tract are lined by pits that rep-
resent invaginations of the epithelium into the submucosa. In the small intestine,
these pits, called crypts of Lieberkühn, are deep and contain specialized secretory
cells. One of these cells is shown in Figure 64–1.
Third, in the stomach and upper duodenum are large numbers of deep tubular
gla

per duodenum are large numbers of deep tubular
glands. A typical tubular gland can be seen in Figure 64–4, which shows an acid- and
pepsinogen-secreting gland of the stomach (oxyntic gland).
Fourth, also associated with the alimentary tract are several complex glands—the
salivary glands, pancreas, and liver—that provide secretions for digestion or emul-
siﬁcation of food. The liver has a highly specialized structure that is discussed in
Chapter 70. The salivary glands and the pancreas are compou

0. The salivary glands and the pancreas are compound acinous glands of
the type shown in Figure 64–2. These glands lie outside the walls of the alimentary
tract and, in this, differ from all other alimentary glands. They contain millions of
acini lined with secreting glandular cells; these acini feed into a system of ducts that
ﬁnally empty into the alimentary tract itself.
Basic Mechanisms of Stimulation of the Alimentary 
Tract Glands
Effect of Contact of Food with the Epithelium—Function of E

 Contact of Food with the Epithelium—Function of Enteric Nervous Stimuli. The
mechanical presence of food in a particular segment of the gastrointestinal tract
usually causes the glands of that region and often of adjacent regions to secrete
792
Unit XII
moderate to large quantities of juices. Part of this local
effect, especially the secretion of mucus by mucous
cells, results from direct contact stimulation of the
surface glandular cells by the food.
In addition, lo

rface glandular cells by the food.
In addition, local epithelial stimulation also acti-
vates the enteric nervous system of the gut wall. The
types of stimuli that do this are (1) tactile stimulation,
(2) chemical irritation, and (3) distention of the gut
wall. The resulting nervous reﬂexes stimulate both the
mucous cells on the gut epithelial surface and the deep
glands in the gut wall to increase their secretion.
Autonomic Stimulation of Secretion
Parasympathetic 
Stimulation. Stimulation 
of 

ion
of 
the
parasympathetic nerves to the alimentary tract almost
invariably increases the rates of alimentary glandular
secretion. This is especially true of the glands in the
upper portion of the tract (innervated by the glos-
sopharyngeal and vagus parasympathetic nerves) 
such as the salivary glands, esophageal glands, gastric
glands, pancreas, and Brunner’s glands in the duode-
num. It is also true of some glands in the distal portion
of the large 

of some glands in the distal portion
of the large intestine, innervated by pelvic parasym-
pathetic nerves. Secretion in the remainder of the
small intestine and in the ﬁrst two thirds of the large
intestine occurs mainly in response to local neural and
hormonal stimuli in each segment of the gut.
Sympathetic Stimulation. Stimulation of the sympa-
thetic nerves going to the gastrointestinal tract causes
a slight to moderate increase in secretion by some 
of the local glands. But sympathetic stim

by some 
of the local glands. But sympathetic stimulation also
results in constriction of the blood vessels that supply
the glands. Therefore, sympathetic stimulation can
have a dual effect: First, sympathetic stimulation alone
usually slightly increases secretion. But, second, if
parasympathetic or hormonal stimulation is already
causing copious secretion by the glands, superimposed
sympathetic stimulation usually reduces the secretion,
sometimes signiﬁcantly so, mainly because of vaso-
constri

s signiﬁcantly so, mainly because of vaso-
constrictive reduction of the blood supply.
Regulation of Glandular Secretion by Hormones. In the
stomach and intestine,several different gastrointestinal
hormones help regulate the volume and character of
the secretions. These hormones are liberated from the
gastrointestinal mucosa in response to the presence of
food in the lumen of the gut. The hormones then are
absorbed into the blood and carried to the glands,
where they stimulate secretion. This ty

he glands,
where they stimulate secretion. This type of stimula-
tion is particularly valuable to increase the output of
gastric juice and pancreatic juice when food enters the
stomach or duodenum.
Chemically,
the gastrointestinal hormones are
polypeptides or polypeptide derivatives.
Basic Mechanism of Secretion 
by Glandular Cells
Secretion of Organic Substances. Although all the 
basic mechanisms by which glandular cells function
are not known, experimental evidence points to the
following pri

 experimental evidence points to the
following principles of secretion, as shown in Figure
64–1.
1. The nutrient material needed for formation of 
the secretion must ﬁrst diffuse or be actively
transported by the blood in the capillaries into the
base of the glandular cell.
2. Many mitochondria located inside the glandular
cell near its base use oxidative energy to form
adenosine triphosphate (ATP).
3. Energy from the ATP, along with appropriate
substrates provided by the nutrients, is then used

to synthesize the organic secretory substances;
this synthesis occurs almost entirely in the
endoplasmic reticulum and Golgi complex of the
glandular cell. Ribosomes adherent to the
Zymogen
granules
Ribosomes
Nerve
fiber
Basement
membrane
reticulum
Golgi
apparatus
Secretion
Capillary
Figure 64–1
Typical function of a glandular cell for formation and secretion of
enzymes and other secretory substances.
Primary secretion:

nd other secretory substances.
  1. Ptyalin
  2. Mucus
  3. Extracellular fluid
Saliva
Na+ active absorption
Cl- passive absorption
K+ active secretion
HCO3
- secretion
Figure 64–2
Formation and secretion of saliva by a submandibular salivary
gland.
Chapter 64
Secretory Functions of the Alimentary Tract
793
reticulum are speciﬁcally responsible for
formation of the proteins that are secreted.
4. The secretory materials are transported through
the tubules of the endoplasmic ret

sported through
the tubules of the endoplasmic reticulum, passing
in about 20 minutes all the way to the vesicles of
the Golgi complex.
5. In the Golgi complex, the materials are modiﬁed,
added to, concentrated, and discharged into the
cytoplasm in the form of secretory vesicles, which
are stored in the apical ends of the secretory cells.
6. These vesicles remain stored until nervous or
hormonal control signals cause the cells to
extrude the vesicular contents through the cells’
surface. This pr

cular contents through the cells’
surface. This probably occurs in the following 
way: The control signal ﬁrst increases the cell
membrane permeability to calcium ions, and
calcium enters the cell. The calcium in turn causes
many of the vesicles to fuse with the apical cell
membrane. Then the apical cell membrane breaks
open, thus emptying the vesicles to the exterior;
this process is called exocytosis.
Water and Electrolyte Secretion. A second necessity for
glandular secretion is secretion of s

ecessity for
glandular secretion is secretion of sufﬁcient water and
electrolytes to go along with the organic substances.
The following is a postulated method by which
nervous stimulation causes water and salts to pass
through the glandular cells in great profusion, washing
the organic substances through the secretory border of
the cells at the same time:
1. Nerve stimulation has a speciﬁc effect on the
basal portion of the cell membrane to cause active
transport of chloride ions to the cell in

e active
transport of chloride ions to the cell interior.
2. The resulting increase in electronegativity induced
inside the cell by excess negatively charged
chloride ions then causes positive ions such as
sodium ions also to move through the cell
membrane to the interior of the cell.
3. Now, the new excess of both negative and 
positive ions inside the cell creates an osmotic
force that causes osmosis of water to the interior,
thereby increasing cell volume and hydrostatic
pressure inside the c

 cell volume and hydrostatic
pressure inside the cell, causing the cell itself to
swell.
4. The pressure in the cell then initiates minute
openings of the secretory border of the cell,
causing ﬂushing of water, electrolytes, and organic
materials out of the secretory end of the glandular
cell.
In support of these secretory processes have been
the following ﬁndings: First, the nerve endings on glan-
dular cells are principally on the bases of the cells.
Second, microelectrode studies show that th

cells.
Second, microelectrode studies show that the normal
electrical potential across the membrane at the base of
the cell is between 30 and 40 millivolts, with negativ-
ity on the interior and positivity on the exterior.
Parasympathetic stimulation increases this polariza-
tion voltage to values some 10 and 20 millivolts more
negative than normal. This increase in polarization
voltage lasts for 1 second or longer after the nerve
signal has arrived, indicating that it is caused by move-
ment of

ved, indicating that it is caused by move-
ment of negative ions (presumably chloride ions)
through the membrane to the interior of the cell, thus
leading to secretion.
Although this mechanism for secretion is still partly
theoretical, it does explain how it would be possible
for nerve impulses to regulate secretion. Hormones
acting on the cell membrane are believed also to cause
similar secretory results to those caused by nervous
Lubricating and Protective
Properties of Mucus, and

ubricating and Protective
Importance of Mucus in the
Mucus is a thick secretion composed mainly of water,
electrolytes, and a mixture of several glycoproteins,
which themselves are composed of large polysaccha-
rides bound with much smaller quantities of protein.
Mucus is slightly different in different parts of the gas-
trointestinal tract, but everywhere it has several im-
portant characteristics that make it both an excellent
lubricant and a pro

that make it both an excellent
lubricant and a protectant for the wall of the gut. First,
mucus has adherent qualities that make it adhere tightly
to the food or other particles and to spread as a thin ﬁlm
over the surfaces. Second, it has sufﬁcient body that 
it coats the wall of the gut and prevents actual contact
of most food particles with the mucosa. Third, mucus
has a low resistance for slippage, so that the particles
can slide along the epithelium with great ease. Fourth,
mucus causes fec

ithelium with great ease. Fourth,
mucus causes fecal particles to adhere to one another
to form the feces that are expelled during a bowel move-
ment. Fifth, mucus is strongly resistant to digestion by
the gastrointestinal enzymes. And sixth, the glycopro-
teins of mucus have amphoteric properties, which
means that they are capable of buffering small amounts
of either acids or alkalies; also, mucus often contains
moderate quantities of bicarbonate ions which speciﬁ-
cally neutralize acids.
In su

e ions which speciﬁ-
In summary, mucus has the ability to allow easy slip-
page of food along the gastrointestinal tract and to
prevent excoriative or chemical damage to the epithe-
lium. A person becomes acutely aware of the lubricat-
ing qualities of mucus when the salivary glands fail to
secrete saliva, because then it is difﬁcult to swallow solid
food even when it is eaten along with large amounts of
water.
Salivary Glands; Characteristics of Saliv

f Saliva
Salivary Glands; Characteristics of Saliva. The principal
glands of salivation are the parotid, submandibular,
and sublingual glands; in addition, there are many very
small buccal glands. Daily secretion of saliva normally
ranges between 800 and 1500 milliliters, as shown by
the average value of 1000 milliliters in Table 64–1.
Saliva contains two major types of protein secretion:
(1) a serous secretion that contains ptyalin (an 
a-amylase), which is an enzyme for digesting starches,
and

e), which is an enzyme for digesting starches,
and (2) mucus secretion that contains mucin for lubri-
cating and for surface protective purposes.
The parotid glands secrete almost entirely the
serous type of secretion, while the submandibular and
sublingual glands secrete both serous secretion and
794
Unit XII
mucus. The buccal glands secrete only mucus. Saliva
has a pH between 6.0 and 7.0, a favorable range for
the digestive action of ptyalin.
Secretion of Ions in Sa

gestive action of ptyalin.
Secretion of Ions in Saliva. Saliva contains especially
large quantities of potassium and bicarbonate ions.
Conversely, the concentrations of both sodium and
chloride ions are several times less in saliva than in
plasma. One can understand these special concentra-
tions of ions in the saliva from the following descrip-
tion of the mechanism for secretion of saliva.
Figure 64–2 shows secretion by the submandibular
gland, a typical compound gland that contains acini
and 

 a typical compound gland that contains acini
and salivary ducts. Salivary secretion is a two-stage
operation: the ﬁrst stage involves the acini, and the
second, the salivary ducts. The acini secrete a primary
secretion that contains ptyalin and/or mucin in a solu-
tion of ions in concentrations not greatly different
from those of typical extracellular ﬂuid. As the
primary secretion ﬂows through the ducts, two major
active transport processes take place that markedly
modify the ionic composition

e place that markedly
modify the ionic composition of the ﬂuid in the saliva.
First, sodium ions are actively reabsorbed from all
the salivary ducts and potassium ions are actively
secreted in exchange for the sodium. Therefore, the
sodium ion concentration of the saliva becomes
greatly reduced, whereas the potassium ion concen-
tration becomes increased. However, there is excess
sodium reabsorption over potassium secretion, and
this creates electrical negativity of about -70 millivolts
in the s

trical negativity of about -70 millivolts
in the salivary ducts; this in turn causes chloride ions
to be reabsorbed passively.Therefore, the chloride ion
concentration in the salivary ﬂuid falls to a very low
level, matching the ductal decrease in sodium ion 
Second, bicarbonate ions are secreted by the ductal
epithelium into the lumen of the duct. This is at least
partly caused by passive exchange of bicarbonate for
chloride ions, but it may also result partly from an
active secr

 but it may also result partly from an
active secretory process.
The net result of these transport processes is that
under resting conditions, the concentrations of sodium
and chloride ions in the saliva are only about 15 mEq/
L each, about one seventh to one tenth their concen-
trations in plasma. Conversely, the concentration of
potassium ions is about 30 mEq/L, seven times as great
as in plasma; and the concentration of bicarbonate
ions is 50 to 70 mEq/L, about two to three times that
of plas

to 70 mEq/L, about two to three times that
of plasma.
During maximal salivation, the salivary ionic con-
centrations change considerably because the rate of
formation of primary secretion by the acini can
increase as much as 20-fold. This acinar secretion 
then ﬂows through the ducts so rapidly that the ductal
reconditioning of the secretion is considerably
reduced. Therefore, when copious quantities of saliva
are being secreted, the sodium chloride concentration
rises only to one half or two th

ide concentration
rises only to one half or two thirds that of plasma, and
the potassium concentration rises to only four times
that of plasma.
Function of Saliva for Oral Hygiene. Under basal awake con-
ditions, about 0.5 milliliter of saliva, almost entirely of
the mucous type, is secreted each minute; but during
sleep, secretion becomes very little. This secretion plays
an exceedingly important role for maintaining healthy
oral tissues. The mouth is loaded with pathogenic bac-
teria that can 

uth is loaded with pathogenic bac-
teria that can easily destroy tissues and cause dental
caries. Saliva helps prevent the deteriorative processes
in several ways.
First, the ﬂow of saliva itself helps wash away patho-
genic bacteria as well as food particles that provide their
metabolic support.
Second, saliva contains several factors that destroy
bacteria. One of these is thiocyanate ions and another 
is 
several 
proteolytic 
enzymes—most 
important,
lysozyme—that (a) attack the bacteria, (b)

ortant,
lysozyme—that (a) attack the bacteria, (b) aid the thio-
cyanate ions in entering the bacteria where these ions
in turn become bactericidal, and (c) digest food parti-
cles, thus helping further to remove the bacterial meta-
bolic support.
Third, saliva often contains signiﬁcant amounts of
protein antibodies that can destroy oral bacteria, includ-
ing some that cause dental caries. In the absence of 
salivation, oral tissues often become ulcerated and oth-
erwise infected, and caries of 

ulcerated and oth-
erwise infected, and caries of the teeth can become
rampant.
Nervous Regulation of Salivary
Secretion 
Figure 64–3 shows the parasympathetic nervous path-
ways for regulating salivation, demonstrating that the
salivary glands are controlled mainly by parasympa-
thetic nervous signals all the way from the superior and
inferior salivatory nuclei in the brain stem.
The salivatory nuclei are located approximately at
the juncture of the medulla and pons and are excited
by both tast

 the medulla and pons and are excited
by both taste and tactile stimuli from the tongue and
other areas of the mouth and pharynx. Many taste
stimuli, especially the sour taste (caused by acids),
elicit copious secretion of saliva—often 8 to 20 times
the basal rate of secretion.Also, certain tactile stimuli,
such as the presence of smooth objects in the mouth
(e.g., a pebble), cause marked salivation, whereas
rough objects cause less salivation and occasionally
even inhibit salivation.

d occasionally
Salivation can also be stimulated or inhibited by
nervous signals arriving in the salivatory nuclei from
Table 64–1
Daily Secretion of Intestinal Juices
Daily Volume (ml)
pH
Saliva
1000
6.0–7.0
Gastric secretion
1500
1.0–3.5
Pancreatic secretion
1000
8.0–8.3
Bile
1000
7.8
Small intestine secretion
1800
7.5–8.0
Brunner’s gland secretion
200
8.0–8.9
Large intestinal secretion
200
7.5–8.0
Total
6700
Chapter 64
795
h

795
higher centers of the central nervous system. For
instance, when a person smells or eats favorite foods,
salivation is greater than when disliked food is smelled
or eaten.The appetite area of the brain, which partially
regulates these effects, is located in proximity to the
parasympathetic centers of the anterior hypothalamus,
and it functions to a great extent in response to signals
from the taste and smell areas of the cerebral cortex
or amygdal

 and smell areas of the cerebral cortex
or amygdala.
Salivation also occurs in response to reﬂexes origi-
nating in the stomach and upper small intestines—par-
ticularly when irritating foods are swallowed or when
a person is nauseated because of some gastrointestinal
abnormality. The saliva, when swallowed, helps to
remove the irritating factor in the gastrointestinal tract
by diluting or neutralizing the irritant substances.
Sympathetic stimulation can also increase salivation
a slight amount,

tion can also increase salivation
a slight amount, much less so than does parasympa-
thetic stimulation. The sympathetic nerves originate
from the superior cervical ganglia and travel along the
surfaces of the blood vessel walls to the salivary 
glands.
A secondary factor that also affects salivary secre-
tion is the blood supply to the glands because secretion
always requires adequate nutrients from the blood.
The parasympathetic nerve signals that induce copious
salivation also moderately dila

hat induce copious
salivation also moderately dilate the blood vessels. In
addition, salivation itself directly dilates the blood
vessels, thus providing increased salivatory gland
nutrition as needed by the secreting cells. Part of this
additional vasodilator effect is caused by kallikrein
secreted by the activated salivary cells, which in turn
acts as an enzyme to split one of the blood proteins,
an alpha2-globulin, to form bradykinin, a strong
vasodilator.
The esophageal 

The esophageal secretions are entirely mucous in char-
acter and principally provide lubrication for swallowing.
The main body of the esophagus is lined with many
simple mucous glands. At the gastric end and to a lesser
extent in the initial portion of the esophagus, there 
are also many compound mucous glands. The mucus
secreted by the compound glands in the upper esopha-
gus prevents mucosal excoriation by newly entering
food, whereas the compound glands loca

ly entering
food, whereas the compound glands located near the
esophagogastric junction protect the esophageal wall
from digestion by acidic gastric juices that often reﬂux
from the stomach back into the lower esophagus.
Despite this protection, a peptic ulcer at times can still
occur at the gastric end of the esophagus.
Characteristics of the Gastric
Secretions
In addition to mucus-secreting cells that line the entire
surface of the stomach, the stomach mucosa has two
importan

f the stomach, the stomach mucosa has two
important types of tubular glands: oxyntic glands (also
called gastric glands) and pyloric glands. The oxyntic
(acid-forming) glands secrete hydrochloric acid,
pepsinogen, intrinsic factor, and mucus. The pyloric
glands secrete mainly mucus for protection of the
pyloric mucosa from the stomach acid. They also
secrete the hormone gastrin.
The oxyntic glands are located on the inside surfaces
of the body and fundus of the stomach,constituting the
proximal 

d fundus of the stomach,constituting the
proximal 80 per cent of the stomach. The pyloric
glands are located in the antral portion of the stomach,
the distal 20 per cent of the stomach.
Secretions from the Oxyntic (Gastric) Glands
A typical stomach oxyntic gland is shown in Figure
64–4. It is composed of three types of cells: (1) mucous
neck cells, which secrete mainly mucus; (2) peptic
(or chief) cells, which secrete large quantities of
Superior and inferior
salivatory nuclei
Submandibular glan

 and inferior
ganglion
Sublingual gland
Chorda
tympani
Taste and
tactile stimuli
Tongue
nerve
Parotid
gland
Tractus
solitarius
Facial
nerve
Figure 64–3
Parasympathetic nervous regulation of salivary secretion.
Surface
Mucous neck
cells
Oxyntic
(or parietal)
cells
Peptic
(or chief)
cells
Figure 64–4
Oxyntic gland from the body of the stomach.
796
Unit XII
pepsinogen; and (3) p

pepsinogen; and (3) parietal (or oxyntic) cells, which
secrete hydrochloric acid and intrinsic factor.Secretion
of hydrochloric acid by the parietal cells involves
special mechanisms, as follows.
Basic Mechanism of Hydrochloric Acid Secretion. When
stimulated, the parietal cells secrete an acid solution
that contains about 160 millimoles of hydrochloric acid
per liter, which is almost exactly isotonic with the body
ﬂuids. The pH of this acid is about 0.8, demonstrati

ids. The pH of this acid is about 0.8, demonstrating
its extreme acidity. At this pH, the hydrogen ion con-
centration is about 3 million times that of the arterial
blood. To concentrate the hydrogen ions this tre-
mendous amount requires more than 1500 calories of
energy per liter of gastric juice.
Figure 64–5 shows schematically the functional
structure of a parietal cell (also called oxyntic cell),
demonstrating that it contains large branching intra-
cellular canaliculi. The hydrochloric aci

g intra-
cellular canaliculi. The hydrochloric acid is formed at
the villus-like projections inside these canaliculi and is
then conducted through the canaliculi to the secretory
end of the cell.
Different suggestions for the chemical mechanism
of hydrochloric acid formation have been offered.
One of these, shown in Figure 64–6, consists of the 
following steps:
1. Chloride ion is actively transported from the
cytoplasm of the parietal cell into the lumen of
the canaliculus, and sodium ions are 

the lumen of
the canaliculus, and sodium ions are actively
transported out of the canaliculus into the
cytoplasm of the parietal cell. These two effects
together create a negative potential of -40 to -70
millivolts in the canaliculus, which in turn causes
diffusion of positively charged potassium ions 
and a small number of sodium ions from the 
cell cytoplasm into the canaliculus. Thus, in 
effect, mainly potassium chloride and much
smaller amounts of sodium chloride enter the
canaliculus.
2. W

nts of sodium chloride enter the
2. Water becomes dissociated into hydrogen ions
and hydroxyl ions in the cell cytoplasm. The
hydrogen ions are then actively secreted into the
canaliculus in exchange for potassium ions: this
active exchange process is catalyzed by H+,K+-
ATPase. In addition, the sodium ions are actively
reabsorbed by a separate sodium pump. Thus,
most of the potassium and sodium ions that had
diffused into the canaliculus are reabsorbed into
the cell cytoplasm, and 

culus are reabsorbed into
the cell cytoplasm, and hydrogen ions take their
place in the canaliculus, giving a strong solution 
of hydrochloric acid in the canaliculus. The
hydrochloric acid is then secreted outward
through the open end of the canaliculus into the
lumen of the gland.
3. Water passes into the canaliculus by osmosis
because of extra ions secreted into the canaliculus.
Thus, the ﬁnal secretion from the canaliculus
contains water, hydrochloric acid at a
concentration of about 150 to 

rochloric acid at a
concentration of about 150 to 160 mEq/L,
Mucous
neck cells
Oxyntic
(parietal)
cell
Canaliculi
Secretion
Figure 64–5
Schematic anatomy of the canaliculi in a parietal (oxyntic) cell.
CO2
CO2
H2O
H2O
H2O
(Osmosis)
HCO3
-
HCO3
-
CO2 + OH- + H+
K+
K+
K+
Na+
Na+
Na+
Na+
Cl–
Cl–
(3 mEq/L)
Cl–
H+ (155 mEq/L)
K+ (15 mEq/L)
Cl- (173 mEq/L)
Parietal cell
Lumen of canaliculus
P
P
P
P
Figure 64–6
Postulated mechanism for secre-
tion of hydrochloric acid. (The
points l

or secre-
points labeled “P” indicate active
pumps, and the dashed lines
represent 
free 
diffusion 
and
osmosis.)
Chapter 64
797
potassium chloride at a concentration of 15 mEq/
L, and a small amount of sodium chloride.
4. Finally, carbon dioxide, either formed during
metabolism in the cell or entering the cell from
the blood, combines under the inﬂuence of
carbonic anhydrase with the hydroxyl ions (from
step 2) to form

drase with the hydroxyl ions (from
step 2) to form bicarbonate ions. These then
diffuse out of the cell cytoplasm into the
extracellular ﬂuid in exchange for chloride ions
that enter the cell from the extracellular ﬂuid and
are later secreted into the canaliculus.
Secretion and Activation of Pepsinogen. Several slightly
different types of pepsinogen are secreted by 
the peptic and mucous cells of the gastric glands.
Even so, all the pepsinogens perform the same 
When pepsinogen is ﬁrs

erform the same 
When pepsinogen is ﬁrst secreted, it has no diges-
tive activity. However, as soon as it comes in contact
with hydrochloric acid, it is activated to form active
pepsin. In this process, the pepsinogen molecule,
having a molecular weight of about 42,500, is split to
form a pepsin molecule, having a molecular weight of
about 35,000.
Pepsin functions as an active proteolytic enzyme in
a highly acid medium (optimum pH 1.8 to 3.5), but
above a pH of about 5 it has almost n

to 3.5), but
above a pH of about 5 it has almost no proteolytic
activity and becomes completely inactivated in a short
time. Hydrochloric acid is as necessary as pepsin for
protein digestion in the stomach; this is discussed
further in Chapter 65.
Secretion of Intrinsic Factor. The substance intrinsic factor,
essential for absorption of vitamin B12 in the ileum, is
secreted by the parietal cells along with the secretion of
hydrochloric acid. When the acid-producing parietal
cells of the stomach 

 the acid-producing parietal
cells of the stomach are destroyed, which frequently
occurs in chronic gastritis, the person develops not only
achlorhydria (lack of stomach acid secretion) but often
also pernicious anemia because of failure of maturation
of the red blood cells in the absence of vitamin B12 stim-
ulation of the bone marrow. This is discussed in detail
in Chapter 32.
Pyloric Glands—Secretion of Mucus
and Gastrin
The pyloric glands are structurally similar to the
oxyntic glands but co

 structurally similar to the
oxyntic glands but contain few peptic cells and almost
no parietal cells. Instead, they contain mostly mucous
cells that are identical with the mucous neck cells of
the oxyntic glands. These cells secrete a small amount
of pepsinogen, as discussed earlier, and an especially
large amount of thin mucus that helps to lubricate
food movement, as well as to protect the stomach wall
from digestion by the gastric enzymes. The pyloric
glands also secrete the hormone gastrin,

e pyloric
glands also secrete the hormone gastrin, which plays a
key role in controlling gastric secretion, as we discuss
shortly.
The entire surface of the stomach mucosa between
glands has a continuous layer of a special type of
mucous cells called simply “surface mucous cells.”
They secrete large quantities of a very viscid mucus
that coats the stomach mucosa with a gel layer of
mucus often more than 1 millimeter thick, thus 
providing a major shell of protection for the 

us 
providing a major shell of protection for the stomach
wall as well as contributing to lubrication of food
transport.
Another characteristic of this mucus is that it is alka-
line. Therefore, the normal underlying stomach wall is
not directly exposed to the highly acidic, proteolytic
stomach secretion. Even the slightest contact with
food or any irritation of the mucosa directly stimulates
the surface mucous cells to secrete additional quanti-
ties of this thick, alkaline, viscid mucus.
Stimu

Parietal Cells of the Oxyntic Glands Are the Only Cells That
Secrete Hydrochloric Acid. The parietal cells, located deep
in the oxyntic glands of the main body of the stomach,
are the only cells that secrete hydrochloric acid. As
noted earlier in the chapter, the acidity of the ﬂuid
secreted by these cells can be very great, with pH as
low as 0.8. However, secretion of this acid is under con-
tinuous control by bo

n of this acid is under con-
tinuous control by both endocrine and nervous signals.
Furthermore, the parietal cells operate in close associ-
ation with another type of cell called enterochromaf-
ﬁn-like cells (ECL cells), the primary function of which
is to secrete histamine.
The ECL cells lie in the deep recesses of the oxyntic
glands and therefore release histamine in direct
contact with the parietal cells of the glands. The rate
of formation and secretion of hydrochloric acid by the
parietal 

nd secretion of hydrochloric acid by the
parietal cells is directly related to the amount of his-
tamine secreted by the ECL cells. In turn, the ECL
cells can be stimulated to secrete histamine in several
different ways: (1) Probably the most potent mecha-
nism for stimulating histamine secretion is by the hor-
monal substance gastrin, which is formed almost
entirely in the antral portion of the stomach mucosa
in response to proteins in the foods being digested.
(2) In addition, the ECL cells ca

 being digested.
(2) In addition, the ECL cells can be stimulated by 
(a) acetylcholine released from stomach vagal nerve
endings and (b) probably also by hormonal substances
secreted by the enteric nervous system of the stomach
wall. Let us discuss ﬁrst the gastrin mechanism for
control of the ECL cells and their subsequent control
of parietal cell secretion of hydrochloric acid.
Stimulation of Acid Secretion by Gastrin. Gastrin is itself a
hormone secreted by gastrin cells, also called G cells

one secreted by gastrin cells, also called G cells.
These cells are located in the pyloric glands in the
distal end of the stomach. Gastrin is a large polypep-
tide secreted in two forms: a large form called G-34,
which contains 34 amino acids, and a smaller form,
G-17, which contains 17 amino acids.Although both of
these are important, the smaller is more abundant.
798
Unit XII
When meats or other protein-containing foods
reach the antral end of the stomach, some of 

oods
reach the antral end of the stomach, some of the pro-
teins from these foods have a special stimulatory effect
on the gastrin cells in the pyloric glands to cause
release of gastrin into the digestive juices of the
stomach. The vigorous mixing of the gastric juices
transports the gastrin rapidly to the ECL cells in the
body of the stomach, causing release of histamine
directly into the deep oxyntic glands. The histamine
then acts quickly to stimulate gastric hydrochloric acid
Reg

stimulate gastric hydrochloric acid
Regulation of pepsinogen secretion by the peptic cells
in the oxyntic glands is much less complex than regu-
lation of acid secretion; it occurs in response to two
types of signals: (1) stimulation of the peptic cells by
acetylcholine released from the vagus nerves or from
the gastric enteric nervous plexus, and (2) stimulation
of peptic cell secretion in response to acid in the
stomach. The acid probably does not 

o acid in the
stomach. The acid probably does not stimulate the
peptic cells directly but instead elicits additional
enteric nervous reﬂexes that support the original
nervous signals to the peptic cells. Therefore, the rate
of secretion of pepsinogen, the precursor of the
enzyme pepsin
that causes protein digestion, is
strongly inﬂuenced by the amount of acid in the
stomach. In people who have lost the ability to secrete
normal amounts of acid, secretion of pepsinogen is
also decreased, even tho

ecretion of pepsinogen is
also decreased, even though the peptic cells may
otherwise appear to be normal.
Phases of Gastric Secretion
Gastric secretion is said to occur in three “phases” (as
shown in Figure 64–7): a cephalic phase, a gastric phase,
and an intestinal phase.
Cephalic Phase. The cephalic phase of gastric secretion
occurs even before food enters the stomach, especially
while it is being eaten. It results from the sight, smell,
thought, or taste of food, and the greater the appetite,

t, or taste of food, and the greater the appetite,
the more intense is the stimulation. Neurogenic signals
that cause the cephalic phase of gastric secretion origi-
nate in the cerebral cortex and in the appetite centers
of the amygdala and hypothalamus. They are trans-
mitted through the dorsal motor nuclei of the vagi and
thence through the vagus nerves to the stomach. This
phase of secretion normally accounts for about 20 per
cent of the gastric secretion associated with eating a
meal.
Gastri

ic secretion associated with eating a
meal.
Gastric Phase. Once food enters the stomach, it excites
(1) long vagovagal reﬂexes from the stomach to the
brain and back to the stomach, (2) local enteric reﬂexes,
and (3) the gastrin mechanism, all of which in turn cause
secretion of gastric juice during several hours while food
remains in the stomach. The gastric phase of secretion
accounts for about 70 per cent of the total gastric secre-
tion associated with eating a meal and therefore
accounts fo

iated with eating a meal and therefore
accounts for most of the total daily gastric secretion of
about 1500 milliliters.
Intestinal Phase. The presence of food in the upper
portion of the small intestine, particularly in the duo-
denum, will continue to cause stomach secretion of
small amounts of gastric juice, probably partly because
of small amounts of gastrin released by the duodenal
mucosa.
Inhibition of Gastric Secretion 
by Other Post-Stomach 
Intestinal Factors
Although intestinal chyme s

ch 
Although intestinal chyme slightly stimulates gastric
secretion during the early intestinal phase of stomach
secretion, it paradoxically inhibits gastric secretion at
other times. This inhibition results from at least two
inﬂuences.
Vagal center
of medulla
Cephalic phase via vagus
Parasympathetics excite
pepsin and acid production
Intestinal phase:
1. Nervous mechanisms
2. Hormonal mechanisms
Circulatory system
Afferent
fibers
Vagus
trunk
Secretory
fiber
Small bowel
Gastri

ers
Vagus
trunk
Secretory
fiber
Gastrin
Food
Local nerve
plexus
Gastric phase:
1. Local nervous
    secretory reflexes
2. Vagal reflexes
3. Gastrin-histamine
    stimulation
Figure 64–7
Phases of gastric secretion and their regulation.
Chapter 64
799
1. The presence of food in the small intestine initiates
a reverse enterogastric reﬂex, transmitted through
the myenteric nervous system as well as through
extrinsic sympathetic and vagus nerve

l as through
extrinsic sympathetic and vagus nerves, that inhibits
stomach secretion. This reﬂex can be initiated by
distending the small bowel, by the presence of acid
in the upper intestine, by the presence of protein
breakdown products, or by irritation of the mucosa.
This is part of the complex mechanism discussed in
Chapter 63 for slowing stomach emptying when the
intestines are already ﬁlled.
2. The presence of acid, fat, protein breakdown
products, hyperosmotic or hypo-osmotic ﬂuids, or
a

any irritating factor in the upper small intestine
causes release of several intestinal hormones. One
of these is secretin, which is especially important 
for control of pancreatic secretion. However,
secretin opposes stomach secretion. Three other
hormones—gastric inhibitory peptide, vasoactive
intestinal polypeptide, and somatostatin—also have
slight to moderate effects in inhibiting gastric
The functional purpose of inhibitory gastri

etion.
The functional purpose of inhibitory gastric secretion
by intestinal factors is presumably to slow passage of
chyme from the stomach when the small intestine is
already ﬁlled or already overactive. In fact, the entero-
gastric inhibitory reﬂexes plus inhibitory hormones
usually also reduce stomach motility at the same time
that they reduce gastric secretion, as was discussed in
Gastric Secretion During the Interdigestive Period. The
stomach secretes a few milliliters of gastri

. The
stomach secretes a few milliliters of gastric juice each
hour during the “interdigestive period,” when little or
no digestion is occurring anywhere in the gut.The secre-
tion that does occur usually is almost entirely of the
nonoxyntic type, composed mainly of mucus but little
pepsin and almost no acid.
Unfortunately, emotional stimuli frequently increase
interdigestive gastric secretion (highly peptic and
acidic) to 50 milliliters or more  per hour, in very much
the same way that the ceph

 per hour, in very much
the same way that the cephalic phase of gastric secre-
tion excites secretion at the onset of a meal. This
increase of secretion in response to emotional stimuli is
believed to be one of the causative factors in develop-
ment of peptic ulcers, as discussed in Chapter 66.
Chemical Composition of Gastrin and
Other Gastrointestinal Hormones
Gastrin, cholecystokinin, and secretin are all large
polypeptides with approximate molecular weights,
respectively, of 2000, 4200, and 3

ecular weights,
respectively, of 2000, 4200, and 3400. The terminal ﬁve
amino acids in the gastrin and cholecystokinin molec-
ular chains are the same. The functional activity of
gastrin resides in the terminal four amino acids, and
the activity for cholecystokinin resides in the terminal
eight amino acids. All the amino acids in the secretin
molecule are essential.
A synthetic gastrin, composed of the terminal four
amino acids of natural gastrin plus the amino acid
alanine, has all the same phy

 plus the amino acid
alanine, has all the same physiologic properties as 
the natural gastrin. This synthetic product is called
pentagastrin.
The pancreas, which lies parallel to and beneath the
stomach (illustrated in Figure 64–10), is a large com-
pound gland with most of its internal structure similar
to that of the salivary glands shown in Figure 64–2.The
pancreatic digestive enzymes are secreted by pancre-
atic acini, and large volumes of sodium bicarbonate
solution are

d large volumes of sodium bicarbonate
solution are secreted by the small ductules and larger
ducts leading from the acini.The combined product of
enzymes and sodium bicarbonate then ﬂows through
a long pancreatic duct that normally joins the hepatic
duct immediately before it empties into the duodenum
through the papilla of Vater, surrounded by the sphinc-
ter of Oddi.
Pancreatic juice is secreted most abundantly in
response to the presence of chyme in the upper por-
tions of the small intestine

yme in the upper por-
tions of the small intestine, and the characteristics of
the pancreatic juice are determined to some extent by
the types of food in the chyme. (The pancreas also
secretes insulin, but this is not secreted by the same
pancreatic tissue that secretes intestinal pancreatic
juice. Instead, insulin is secreted directly into the
blood—not into the intestine—by the islets of Langer-
hans that occur in islet patches throughout the pan-
creas. These are discussed in detail in Chapte

an-
creas. These are discussed in detail in Chapter 78.)
Pancreatic secretion contains multiple enzymes for
digesting all of the three major types of food: proteins,
carbohydrates, and fats. It also contains large quanti-
ties of bicarbonate ions, which play an important role
in neutralizing the acidity of the chyme emptied from
the stomach into the duodenum.
The most important of the pancreatic enzymes for
digesting proteins are trypsin, chymotrypsin, and car-
boxyp

proteins are trypsin, chymotrypsin, and car-
boxypolypeptidase. By far the most abundant of these
is trypsin.
Trypsin and chymotrypsin split whole and partially
digested proteins into peptides of various sizes but do
not cause release of individual amino acids. However,
carboxypolypeptidase does split some peptides into
individual amino acids, thus completing digestion of
some proteins all the way to the amino acid state.
The pancreatic enzyme for digesting carbohydrates
is pancreatic amylase, w

r digesting carbohydrates
is pancreatic amylase, which hydrolyzes starches, glyco-
gen, and most other carbohydrates (except cellulose)
to form mostly disaccharides and a few trisaccharides.
The main enzymes for fat digestion are (1) pancre-
atic lipase, which is capable of hydrolyzing neutral fat
into fatty acids and monoglycerides; (2) cholesterol
esterase, which causes hydrolysis of cholesterol esters;
and (3) phospholipase, which splits fatty acids from
phospholipids.
When ﬁrst synthesized i

 acids from
When ﬁrst synthesized in the pancreatic cells, the
proteolytic digestive enzymes are in the inactive 
forms trypsinogen, chymotrypsinogen, and procar-
boxypolypeptidase, which are all inactive enzymati-
cally. They become activated only after they are
secreted into the intestinal tract. Trypsinogen is 
800
Unit XII
activated by an enzyme called enterokinase, which is
secreted by the intestinal mucosa when chyme comes
in contact with the muco

l mucosa when chyme comes
in contact with the mucosa. Also, trypsinogen can be
autocatalytically activated by trypsin that has already
been formed from previously secreted trypsinogen.
Chymotrypsinogen is activated by trypsin to form chy-
motrypsin, and procarboxypolypeptidase is activated
in a similar manner.
Secretion of Trypsin Inhibitor Prevents Digestion of the 
Pancreas Itself. It is important that the proteolytic
enzymes of the pancreatic juice not become activated
until after they have b

juice not become activated
until after they have been secreted into the intestine
because the trypsin and the other enzymes would
digest the pancreas itself. Fortunately, the same cells
that secrete proteolytic enzymes into the acini of the
pancreas secrete simultaneously another substance
called trypsin inhibitor.This substance is formed in the
cytoplasm of the glandular cells, and it prevents acti-
vation of trypsin both inside the secretory cells and in
the acini and ducts of the pancreas. An

lls and in
the acini and ducts of the pancreas. And, because it is
trypsin that activates the other pancreatic proteolytic
enzymes, trypsin inhibitor prevents activation of the
others as well.
When the pancreas becomes severely damaged or
when a duct becomes blocked, large quantities of pan-
creatic secretion sometimes become pooled in the
damaged areas of the pancreas. Under these condi-
tions, the effect of trypsin inhibitor is often over-
whelmed, in which case the pancreatic secretions
rapid

med, in which case the pancreatic secretions
rapidly become activated and can literally digest the
entire pancreas within a few hours, giving rise to the
condition called acute pancreatitis. This sometimes is
lethal because of accompanying circulatory shock;
even if not lethal, it usually leads to a subsequent life-
time of pancreatic insufﬁciency.
Although the enzymes of the pancreatic juice are
secreted entirely by the acini of the pancreatic glands,
the other two

 the acini of the pancreatic glands,
the other two important components of pancreatic
juice, bicarbonate ions and water, are secreted mainly
by the epithelial cells of the ductules and ducts that
lead from the acini. When the pancreas is stimulated
to secrete copious quantities of pancreatic juice, the
bicarbonate ion concentration can rise to as high as
145 mEq/L, a value about ﬁve times that of bicarbon-
ate ions in the plasma. This provides a large quantity
of alkali in the pancreatic juice t

large quantity
of alkali in the pancreatic juice that serves to neutral-
ize the hydrochloric acid emptied into the duodenum
The basic steps in the cellular mechanism for secret-
ing sodium bicarbonate solution into the pancreatic
ductules and ducts are shown in Figure 64–8. They are
1. Carbon dioxide diffuses to the interior of the 
cell from the blood and, under the inﬂuence of
carbonic anhydrase, combines with water to form
carbonic acid (H2CO3). The carbonic 

water to form
carbonic acid (H2CO3). The carbonic acid in turn
dissociates into bicarbonate ions and hydrogen
ions (HCO3
_ and H+). Then the bicarbonate ions
are actively transported in association with
sodium ions (Na+) through the luminal border of
the cell into the lumen of the duct.
2. The hydrogen ions formed by dissociation of
carbonic acid inside the cell are exchanged for
sodium ions through the blood border of the 
cell by a secondary active transport process.
This supplies the sodium i

tive transport process.
This supplies the sodium ions (Na+) that are
transported through the luminal border into the
pancreatic duct lumen to provide electrical
neutrality for the secreted bicarbonate ions.
3. The overall movement of sodium and bicarbonate
ions from the blood into the duct lumen creates
an osmotic pressure gradient that causes osmosis
of water also into the pancreatic duct, thus
forming an almost completely isosmotic
bicarbonate solution.
Basic

solution.
Basic Stimuli That Cause Pancreatic Secretion
Three basic stimuli are important in causing pancre-
atic secretion:
1. Acetylcholine, which is released from the
parasympathetic vagus nerve endings and from
other cholinergic nerves in the enteric nervous
system
2. Cholecystokinin, which is secreted by the
duodenal and upper jejunal mucosa when food
enters the small intestine
3. Secretin, which is also secreted by the duodenal
and jejunal mucosa when hig

creted by the duodenal
and jejunal mucosa when highly acid food enters
the small intestine
The ﬁrst two of these stimuli, acetylcholine and
cholecystokinin, stimulate the acinar cells of the 
Ductule cells
(Active
(Active
(Carbonic    anhydrase)
Blood
Lumen
Na+
Na+
H+
Na+
H+
HCO3
-
HCO3
H2CO3
H2O
+
CO2 
CO2 
H2O 
H2O 
Figure 64–8
Secretion of isosmotic sodium bicarbonate solution by the pan-
creatic ductules and ducts.
Chapter 64

er 64
801
pancreas, causing production of large quantities of
pancreatic digestive enzymes but relatively small
quantities of water and electrolytes to go with the
enzymes. Without the water, most of the enzymes
remain temporarily stored in the acini and ducts until
more ﬂuid secretion comes along to wash them into
the duodenum. Secretin, in contrast to the ﬁrst two
basic stimuli, stimulates secretion of large quantities of
water solution of sodium bic

f large quantities of
water solution of sodium bicarbonate by the pancre-
atic ductal epithelium.
Multiplicative Effects of Different Stimuli. When all the dif-
ferent stimuli of pancreatic secretion occur at once, the
total secretion is far greater than the sum of the secre-
tions caused by each one separately. Therefore, the
various stimuli are said to “multiply,” or “potentiate,”
one another. Thus, pancreatic secretion normally
results from the combined effects of the multiple basic
stimuli, 

e combined effects of the multiple basic
stimuli, not from one alone.
Phases of Pancreatic Secretion
Pancreatic secretion occurs in three phases, the same
as for gastric secretion: the cephalic phase, the gastric
phase, and the intestinal phase.Their characteristics are
Cephalic and Gastric Phases. During the cephalic phase
of pancreatic secretion, the same nervous signals from
the brain that cause secretion in the stomach also
cause acetylcholine release by the vagal nerve endings
i

acetylcholine release by the vagal nerve endings
in the pancreas. This causes moderate amounts of
enzymes to be secreted into the pancreatic acini,
accounting for about 20 per cent of the total secretion
of pancreatic enzymes after a meal. But little of the
secretion ﬂows immediately through the pancreatic
ducts into the intestine because only small amounts of
water and electrolytes are secreted along with the
enzymes.
During the gastric phase, the nervous stimulation of
enzyme secretion continu

he nervous stimulation of
enzyme secretion continues, accounting for another 5
to 10 per cent of pancreatic enzymes secreted after a
meal. But, again, only small amounts reach the duo-
denum because of continued lack of signiﬁcant ﬂuid
Intestinal Phase. After chyme leaves the stomach and
enters the small intestine,
pancreatic secretion
becomes copious, mainly in response to the hormone
secretin.
Secretin Stimulates Secretion of Copious Quantities of
Bicarbonate Ions—Neutralization of 

 Quantities of
Bicarbonate Ions—Neutralization of Acidic Stomach
Chyme. Secretin is a polypeptide, containing 27 amino
acids (molecular weight about 3400), present in an
inactive form, prosecretin, in so-called S cells in the
mucosa of the duodenum and jejunum. When acid
chyme with pH less than 4.5 to 5.0 enters the duode-
num from the stomach, it causes duodenal mucosal
release and activation of secretin, which is then
absorbed into the blood. The one truly potent con-
stituent of chyme that ca

he one truly potent con-
stituent of chyme that causes this secretin release is the
hydrochloric acid from the stomach.
Secretin in turn causes the pancreas to secrete large
quantities of ﬂuid containing a high concentration of
bicarbonate ion (up to 145 mEq/L) but a low concen-
tration of chloride ion. The secretin mechanism is
especially important for two reasons: First, secretin
begins to be released from the mucosa of the small
intestine when the pH of the duodenal contents falls
below 4.5 t

 the pH of the duodenal contents falls
below 4.5 to 5.0, and its release increases greatly as the
pH falls to 3.0.This immediately causes copious secre-
tion of pancreatic juice containing abundant amounts
of sodium bicarbonate. The net result is then the fol-
lowing reaction in the duodenum:
Then the carbonic acid immediately dissociates 
into carbon dioxide and water. The carbon dioxide 
is absorbed into the blood and expired through the
lungs, thus leaving a neutral solution of sodium chlo-
r

 thus leaving a neutral solution of sodium chlo-
ride in the duodenum. In this way, the acid contents
emptied into the duodenum from the stomach become
neutralized, so that further peptic digestive activity by
the gastric juices in the duodenum is immediately
blocked. Because the mucosa of the small intestine
cannot withstand the digestive action of acid gastric
juice, this is an essential protective mechanism to
prevent development of duodenal ulcers, as is dis-
cussed in further detail in Chap

lcers, as is dis-
cussed in further detail in Chapter 66.
Bicarbonate ion secretion by the pancreas provides
an appropriate pH for action of the pancreatic diges-
tive enzymes, which function optimally in a slightly
alkaline or neutral medium, at a pH of 7.0 to 8.0.
Fortunately, the pH of the sodium bicarbonate secre-
tion averages 8.0.
Cholecystokinin—Its Contribution to Control of Diges-
tive Enzyme Secretion by the Pancreas. The presence
of food in the upper small intestine also causes a
seco

od in the upper small intestine also causes a
second hormone, cholecystokinin, a polypeptide con-
taining 33 amino acids, to be released from yet another
group of cells, the I cells, in the mucosa of the duode-
num and upper jejunum. This release of cholecys-
tokinin results especially from the presence of
proteoses and peptones (products of partial protein
digestion) and long-chain fatty acids in the chyme
coming from the stomach.
Cholecystokinin, like secretin, passes by way of the
blood to th

n, like secretin, passes by way of the
blood to the pancreas but instead of causing sodium
bicarbonate secretion causes mainly secretion of still
much more pancreatic digestive enzymes by the acinar
cells.This effect is similar to that caused by vagal stim-
ulation but even more pronounced, accounting for 70
to 80 per cent of the total secretion of the pancreatic
digestive enzymes after a meal.
The differences between the pancreatic stimulatory
effects of secretin and cholecystokinin are shown i

ffects of secretin and cholecystokinin are shown in
Figure 64–9, which demonstrates (1) intense sodium
bicarbonate secretion in response to acid in the duo-
denum, stimulated by secretin, (2) a dual effect in
response to soap (a fat), and (3) intense digestive
enzyme secretion (when peptones enter the duode-
num) stimulated by cholecystokinin.
HCl
NaHCO
NaCl
H CO
3
2
3
+
Æ
+
802
Unit XII
Figure 64–10 summarizes the more important
factors in the regulation of pancreati

e important
factors in the regulation of pancreatic secretion. The
total amount secreted each day is about 1 liter.
Secretion of Bile by the Liver;
Functions of the Biliary Tree
One of the many functions of the liver is to secrete bile,
normally between 600 and 1000 ml/day.Bile serves two
important functions:
First, bile plays an important role in fat digestion
and absorption, not because of any enzymes in the bile
that cause fat digestion, but because bile acids in the
bile do two things: (1) t

ecause bile acids in the
bile do two things: (1) they help to emulsify the large
fat particles of the food into many minute particles,
the surface of which can then be attacked by lipase
enzymes secreted in pancreatic juice, and (2) they aid
in absorption of the digested fat end products through
the intestinal mucosal membrane.
Second, bile serves as a means for excretion of
several important waste products from the blood.
These include especially bilirubin, an end product of
hemoglobin destruct

y bilirubin, an end product of
hemoglobin destruction, and excesses of cholesterol.
Physiologic Anatomy of 
Biliary Secretion
Bile is secreted in two stages by the liver: (1) The initial
portion is secreted by the principal functional cells of
the liver, the hepatocytes; this initial secretion contains
large amounts of bile acids, cholesterol, and other
organic constituents. It is secreted into minute bile
canaliculi that originate between the hepatic cells.
(2) Next, the bile ﬂows in the canali

patic cells.
(2) Next, the bile ﬂows in the canaliculi toward the
interlobular septa, where the canaliculi empty into ter-
minal bile ducts and then into progressively larger
ducts, ﬁnally reaching the hepatic duct and common
bile duct. From these the bile either empties directly
into the duodenum or is diverted for minutes up to
several hours through the cystic duct into the gall-
bladder, shown in Figure 64–11.
In its course through the bile ducts, a second portion
of liver secretion is added 

cts, a second portion
of liver secretion is added to the initial bile. This addi-
tional secretion is a watery solution of sodium and
bicarbonate ions secreted by secretory epithelial cells
that line the ductules and ducts. This second secretion
sometimes increases the total quantity of bile by as
much as an additional 100 per cent. The second secre-
tion is stimulated especially by secretin, which causes
release of additional quantities of bicarbonate ions to
supplement the bicarbonate ions in 

bonate ions to
supplement the bicarbonate ions in pancreatic secre-
tion (for neutralizing acid that empties into the duo-
denum from the stomach).
Storing and Concentrating Bile in the Gallbladder. Bile is
secreted continually by the liver cells, but most of it is
normally stored in the gallbladder until needed in the
duodenum.The maximum volume that the gallbladder
can hold is only 30 to 60 milliliters. Nevertheless, as
much as 12 hours of bile secretion (usually about 450
milliliters) can be 

 secretion (usually about 450
milliliters) can be stored in the gallbladder because
water, sodium, chloride, and most other small elec-
trolytes are continually absorbed through the gall-
bladder mucosa, concentrating the remaining bile
constituents that contain the bile salts, cholesterol,
lecithin, and bilirubin.
Most of this gallbladder absorption is caused by
active transport of sodium through the gallbladder
epithelium, and this is followed by secondary absorp-
tion of chloride ions, water,

by secondary absorp-
tion of chloride ions, water, and most other diffusible
Water and
NaHCO3
Enzymes
HCI
Soap
Peptone
Rate of pancreatic secretion
Figure 64–9
Sodium bicarbonate (NaHCO3), water, and enzyme secretion by
the pancreas, caused by the presence of acid (HCl), fat (soap),
or peptone solutions in the duodenum.
Vagal
releases
enzymes
into acini
Secretin causes
copious secretion
of pancreatic fluid
and bicarbonate;
cholecystokinin
causes secretion
of enzymes
Acid from stomach

inin
of enzymes
releases secretin from
wall of duodenum;
fats and amino acids
cause release of
Common
bile duct
Secretin and
absorbed into
blood stream
Figure 64–10
Regulation of pancreatic secretion.
Chapter 64
803
constituents. Bile is normally concentrated in this way
about 5-fold, but it can be concentrated up to a
maximum of 20-fold.
Composition of Bile. Table 64–2 gives the compos

.
Composition of Bile. Table 64–2 gives the composition of
bile when it is ﬁrst secreted by the liver and then after
it has been concentrated in the gallbladder. This table
shows that by far the most abundant substances
secreted in the bile are bile salts, which account for
about one half of the total solutes also in the bile.Also
secreted or excreted in large concentrations are biliru-
bin, cholesterol, lecithin, and the usual electrolytes of
plasma.
In the concentrating process in the gallblad

asma.
In the concentrating process in the gallbladder,
water and large portions of the electrolytes (except
calcium ions) are reabsorbed by the gallbladder
mucosa; essentially all other constituents, especially
the bile salts and the lipid substances cholesterol and
lecithin, are not reabsorbed and, therefore, become
highly concentrated in the gallbladder bile.
Emptying of the Gallbladder—Stimulatory Role of Cholecys-
tokinin. When food begins to be digested in the upper
gastrointestinal tract, 

 be digested in the upper
gastrointestinal tract, the gallbladder begins to empty,
especially when fatty foods reach the duodenum about
30 minutes after a meal. The mechanism of gallblad-
der emptying is rhythmical contractions of the wall of
the gallbladder, but effective emptying also requires
simultaneous relaxation of the sphincter of Oddi,
which guards the exit of the common bile duct into the
duodenum.
By far the most potent stimulus for causing the gall-
bladder contractions is the hormon

using the gall-
bladder contractions is the hormone cholecystokinin.
This is the same cholecystokinin discussed earlier that
causes increased secretion of digestive enzymes by the
acinar cells of the pancreas. The stimulus for cholecys-
tokinin entry into the blood from the duodenal
mucosa is mainly the presence of fatty foods in the
duodenum.
In addition to cholecystokinin, the gallbladder is
stimulated less strongly by acetylcholine-secreting
nerve ﬁbers from both the vagi and the intestinal
e

erve ﬁbers from both the vagi and the intestinal
enteric nervous system. They are the same nerves that
promote motility and secretion in other parts of the
upper gastrointestinal tract.
In summary, the gallbladder empties its store of con-
centrated bile into the duodenum mainly in response
to the cholecystokinin stimulus that itself is initiated
Stomach
Acid
Liver
Bile acids via blood
stimulate parenchymal
secretion
Secretin via
stimulates
liver ductal
secretion
Bile stored and
con

mulates
secretion
concentrated up
to 15 times in
gallbladder 
Cholecystokinin via blood stream causes:
    1. Gallbladder contraction
    2. Relaxation of sphincter of Oddi 
Vagal stimulation
causes weak
contraction of
Pancreas
Sphincter of
Oddi
Duodenum
Figure 64–11
Liver secretion and gallbladder emptying.
Table 64–2
Composition of Bile
Liver Bile
Gallbladder Bile
Water
97.5 g/dl
92 g/dl
Bile salts
1.1 g/dl
6 g/dl
Bilirubin
0.04 g/dl
0.3 g/dl
Cholestero

/dl
6 g/dl
Bilirubin
0.04 g/dl
0.3 g/dl
0.1 g/dl
0.3 to 0.9 g/dl
0.12 g/dl
0.3 to 1.2 g/dl
Lecithin
0.04 g/dl
0.3 g/dl
Na+
145.04 mEq/L
130 mEq/L
K+
5 mEq/L
12 mEq/L
Ca++
5 mEq/L
23 mEq/L
Cl-
100 mEq/L
25 mEq/L
HCO3
-
28 mEq/L
10 mEq/L
804
Unit XII
mainly by fatty foods. When fat is not in the food,
the gallbladder empties poorly, but when signiﬁcant
quantities of fat are present, the gallbladder normally
empties completely in about 1 hour. Fig

r normally
empties completely in about 1 hour. Figure 64–11 sum-
marizes the secretion of bile, its storage in the gall-
bladder, and its ultimate release from the bladder to
the duodenum.
Function of Bile Salts in Fat Digestion
and Absorption
The liver cells synthesize about 6 grams of bile salts
daily. The precursor of the bile salts is cholesterol,
which is either present in the diet or synthesized in the
liver cells during the course of fat metabolism. The
cholesterol is ﬁrst converted to ch

etabolism. The
cholesterol is ﬁrst converted to cholic acid or che-
nodeoxycholic acid in about equal quantities. These
acids in turn combine principally with glycine and to a
lesser extent with taurine to form glyco- and tauro-
conjugated bile acids. The salts of these acids, mainly
sodium salts, are then secreted in the bile.
The bile salts have two important actions in the
intestinal tract:
First, they have a detergent action on the fat parti-
cles in the food. This decreases the surface tens

cles in the food. This decreases the surface tension of
the particles and allows agitation in the intestinal tract
to break the fat globules into minute sizes. This is
called the emulsifying or detergent function of bile
salts.
Second, and even more important than the emulsi-
fying function, bile salts help in the absorption of 
(1) fatty acids, (2) monoglycerides, (3) cholesterol, and
(4) other lipids from the intestinal tract. They do this
by forming very small physical complexes with these
l

forming very small physical complexes with these
lipids; the complexes are called micelles, and they are
semi-soluble in the chyme because of the electrical
charges of the bile salts. The intestinal lipids are
“ferried” in this form to the intestinal mucosa, where
they are then absorbed into the blood, as will be
described in detail in Chapter 65.Without the presence
of bile salts in the intestinal tract, up to 40 per cent of
the ingested fats are lost into the feces, and the person
often develo

e lost into the feces, and the person
often develops a metabolic deﬁcit because of this
nutrient loss.
Enterohepatic Circulation of Bile Salts. About 94 per cent of
the bile salts are reabsorbed into the blood from the
small intestine, about one half of this by diffusion
through the mucosa in the early portions of the small
intestine and the remainder by an active transport
process through the intestinal mucosa in the distal
ileum. They then enter the portal blood and pass back
to the liver. On 

r the portal blood and pass back
to the liver. On reaching the liver, on ﬁrst passage
through the venous sinusoids these salts are absorbed
almost entirely back into the hepatic cells and then are
resecreted into the bile.
In this way, about 94 per cent of all the bile salts are
recirculated into the bile, so that on the average these
salts make the entire circuit some 17 times before being
carried out in the feces.The small quantities of bile salts
lost into the feces are replaced by new amount

lts
lost into the feces are replaced by new amounts formed
continually by the liver cells. This recirculation of the
bile salts is called the enterohepatic circulation of bile
salts.
The quantity of bile secreted by the liver each day is
highly dependent on the availability of bile salts—the
greater the quantity of bile salts in the enterohepatic cir-
culation (usually a total of only about 2.5 grams), the
greater the rate of bile secretion. Indeed, ingestion of
supplemental bile salts can incre

ed, ingestion of
supplemental bile salts can increase bile secretion by
several hundred milliliters per day.
If a bile ﬁstula empties the bile salts to the exterior
for several days to several weeks so that they cannot be
reabsorbed from the ileum, the liver increases its pro-
duction of bile salts 6- to 10-fold, which increases the
rate of bile secretion most of the way back to normal.
This demonstrates that the daily rate of liver bile salt
secretion is actively controlled by the availability 

retion is actively controlled by the availability (or
lack of availability) of bile salts in the enterohepatic 
Role of Secretin in Helping to Control Bile Secretion. In addi-
tion to the strong stimulating effect of bile acids to cause
bile secretion, the hormone secretin that also stimulates
pancreatic secretion increases bile secretion, sometimes
more than doubling its secretion for several hours after
a meal. This increase in secretion is almost entirely
secretion of a sodium bi

retion is almost entirely
secretion of a sodium bicarbonate-rich watery solution
by the epithelial cells of the bile ductules and ducts, and
not increased secretion by the liver parenchymal cells
themselves. The bicarbonate in turn passes into the
small intestine and joins the bicarbonate from the pan-
creas in neutralizing the hydrochloric acid from the
stomach. Thus, the secretin feedback mechanism for
neutralizing duodenal acid operates not only through its
effects on pancreatic secretion but

ly through its
effects on pancreatic secretion but also to a lesser extent
through its effect on secretion by the liver ductules and
ducts.
Liver Secretion of Cholesterol and
Bile salts are formed in the hepatic cells from choles-
terol in the blood plasma. In the process of secreting the
bile salts, about 1 to 2 grams of cholesterol are removed
from the blood plasma and secreted into the bile each
day.
Cholesterol is almost completely insoluble in pure
water, but the bile sa

ompletely insoluble in pure
water, but the bile salts and lecithin in bile combine
physically with the cholesterol to form ultramicroscopic
micelles in the form of a colloidal solution, as explained
in more detail in Chapter 65. When the bile becomes
concentrated in the gallbladder, the bile salts and
lecithin become concentrated along with the choles-
terol, which keeps the cholesterol in solution.
Under abnormal conditions, the cholesterol may pre-
cipitate in the gallbladder, resulting in the

pre-
cipitate in the gallbladder, resulting in the formation of
cholesterol gallstones, as shown in Figure 64–12. The
amount of cholesterol in the bile is determined partly
by the quantity of fat that the person eats, because liver
cells synthesize cholesterol as one of the products of fat
metabolism in the body. For this reason, people on a
high-fat diet over a period of years are prone to the
development of gallstones.
Inﬂammation of the gallbladder epithelium, often
resulting from low-grade c

adder epithelium, often
resulting from low-grade chronic infection, may also
change the absorptive characteristics of the gallbladder
mucosa, sometimes allowing excessive absorption of
water and bile salts but leaving behind the cholesterol
Chapter 64
805
in the bladder in progressively greater concentrations.
Then the cholesterol begins to precipitate, ﬁrst forming
many small crystals of cholesterol on the surface of 
the inﬂamed mucosa, but then prog

 the surface of 
the inﬂamed mucosa, but then progressing to large 
gallstones.
Secretions of the 
Secretion of Mucus by Brunner’s
Glands in the Duodenum
An extensive array of compound mucous glands,
called Brunner’s glands, is located in the wall of the
ﬁrst few centimeters of the duodenum, mainly
between the pylorus of the stomach and the papilla of
Vater where pancreatic secretion and bile empty into
the duodenum. These glands secrete large amounts of
alkaline mucus in respons

secrete large amounts of
alkaline mucus in response to (1) tactile or irritating
stimuli on the duodenal mucosa; (2) vagal stimulation,
which causes increased Brunner’s glands secretion
concurrently with increase in stomach secretion; and
(3) gastrointestinal hormones, especially secretin.
The function of the mucus secreted by Brunner’s
glands is to protect the duodenal wall from digestion
by the highly acid gastric juice emptying from the
stomach. In addition, the mucus contains a large excess

h. In addition, the mucus contains a large excess
of bicarbonate ions, which add to the bicarbonate ions
from pancreatic secretion and liver bile in neutralizing
the hydrochloric acid entering the duodenum from the
stomach.
Brunner’s glands are inhibited by sympathetic stim-
ulation; therefore, such stimulation in very excitable
persons is likely to leave the duodenal bulb unpro-
tected and is perhaps one of the factors that cause this
area of the gastrointestinal tract to be the site of peptic

e gastrointestinal tract to be the site of peptic
ulcers in about 50 per cent of ulcer patients.
Secretion of Intestinal Digestive
Juices by the Crypts of Lieberkühn
Located over the entire surface of the small intestine
are small pits called crypts of Lieberkühn, one of which
is illustrated in Figure 64–13. These crypts lie between
the intestinal villi. The surfaces of both the crypts and
the villi are covered by an epithelium composed of two
types of cells: (1) a moderate number of goblet cell

pes of cells: (1) a moderate number of goblet cells,
which secrete mucus that lubricates and protects the
intestinal surfaces, and (2) a large number of entero-
cytes, which, in the crypts, secrete large quantities of
water and electrolytes and, over the surfaces of adja-
cent villi, reabsorb the water and electrolytes along
with end products of digestion.
The intestinal secretions are formed by the entero-
cytes of the crypts at a rate of about 1800 ml/day.These
secretions are almost pure extra

1800 ml/day.These
secretions are almost pure extracellular ﬂuid and have
a slightly alkaline pH in the range of 7.5 to 8.0. The
secretions also are rapidly reabsorbed by the villi.This
ﬂow of ﬂuid from the crypts into the villi supplies a
watery vehicle for absorption of substances from
chyme when it comes in contact with the villi.Thus, the
primary function of the small intestine is to absorb
nutrients and their digestive products into the blood.
Mechanism of Secretion of the Watery Fluid. The 

.
Mechanism of Secretion of the Watery Fluid. The exact
mechanism that controls the marked secretion of
watery ﬂuid by the crypts of Lieberkühn is not known.
It is believed to involve at least two active secretory
processes: (1) active secretion of chloride ions into the
crypts and (2) active secretion of bicarbonate ions.The
secretion of both of these ions causes electrical drag
as well of positively charged sodium ions through the
membrane and into the secreted ﬂuid. Finally, all these
ions to

into the secreted ﬂuid. Finally, all these
ions together cause osmotic movement of water.
Course followed by bile:
    1. During rest
    2. During digestion
Liver
Stones
Hepatic duct
Common bile duct
Sphincter of Oddi
Pancreatic duct
Duodenum
Papilla of Vater
Cystic duct
Stones
Causes of gallstones:
    1. Too much absorption of water
        from bile
    2. Too much absorption of bile
        acids from bile
    3. Too much cholesterol in bile
    4. Inflammation of epithelium
Fig

erol in bile
Figure 64–12
Formation of gallstones.
Paneth cell
Epithelial cell
Mucous goblet
cell
Figure 64–13
A crypt of Lieberkühn, found in all parts of the small intestine
between the villi, which secretes almost pure extracellular ﬂuid.
806
Unit XII
Digestive Enzymes in the Small Intestinal Secretion. When
secretions of the small intestine are collected without
cellular debris, they have almost no enzymes. The en-
terocytes of th

ey have almost no enzymes. The en-
terocytes of the mucosa, especially those that cover the
villi, do contain digestive enzymes that digest speciﬁc
food substances while they are being absorbed through
the epithelium. These enzymes are the following:
(1) several peptidases for splitting small peptides into
amino acids, (2) four enzymes—sucrase, maltase, iso-
maltase, and lactase—for splitting disaccharides into
monosaccharides, and (3) small amounts of intestinal
lipase for splitting neutral fat

nts of intestinal
lipase for splitting neutral fats into glycerol and fatty
acids.
The epithelial cells deep in the crypts of Lieberkühn
continually undergo mitosis, and new cells migrate
along the basement membrane upward out of the
crypts toward the tips of the villi, thus continually
replacing the villus epithelium and also forming new
digestive enzymes. As the villus cells age, they are
ﬁnally shed into the intestinal secretions.The life cycle
of an intestinal epithelial cell is about 5 days

e
of an intestinal epithelial cell is about 5 days.This rapid
growth of new cells also allows rapid repair of excori-
ations that occur in the mucosa.
Regulation of Small Intestine
Secretion—Local Stimuli
By far the most important means for regulating small
intestine secretion are local enteric nervous reﬂexes,
especially reﬂexes initiated by tactile or irritative
stimuli from the chyme in the intestines.
Large Intestine
Mucus Secretion. The mucosa of the large intestine, like

Secretion. The mucosa of the large intestine, like
that of the small intestine, has many crypts of
Lieberkühn; however, unlike the small intestine, there
are no villi. The epithelial cells contain almost no
enzymes. Instead, they consist mainly of mucous cells
that secrete only mucus. The great preponderance of
secretion in the large intestine is mucus. This mucus
contains moderate amounts of bicarbonate ions
secreted by a few non–mucus-secreting epithelial cells.
The rate of secretion of mucus 

 epithelial cells.
The rate of secretion of mucus is regulated principally
by direct, tactile stimulation of the epithelial cells
lining the large intestine and by local nervous reﬂexes
to the mucous cells in the crypts of Lieberkühn.
Stimulation of the pelvic nerves from the spinal
cord, which carry parasympathetic innervation to the
distal one half to two thirds of the large intestine, also
can cause marked increase in mucus secretion. This
occurs along with increase in peristaltic motility of

urs along with increase in peristaltic motility of the
colon, which was discussed in Chapter 63.
During extreme parasympathetic stimulation, often
caused by emotional disturbances, so much mucus can
occasionally be secreted into the large intestine that
the person has a bowel movement of ropy mucus as
often as every 30 minutes; this mucus often contains
little or no fecal material.
Mucus in the large intestine protects the intestinal
wall against excoriation, but in addition, it provides an
adhe

 excoriation, but in addition, it provides an
adherent medium for holding fecal matter together.
Furthermore, it protects the intestinal wall from the
great amount of bacterial activity that takes place
inside the feces, and, ﬁnally, the mucus plus the alka-
linity of the secretion (pH of 8.0 caused by large
amounts of sodium bicarbonate) provides a barrier to
keep acids formed in the feces from attacking the
intestinal wall.
Diarrhea Caused by Excess Secretion of Water and Electrolytes
in Respo

xcess Secretion of Water and Electrolytes
in Response to Irritation. Whenever a segment of the
large intestine becomes intensely irritated, as occurs
when bacterial infection becomes rampant during
enteritis, the mucosa secretes extra large quantities of
water and electrolytes in addition to the normal viscid
alkaline mucus.This acts to dilute the irritating factors
and to cause rapid movement of the feces toward the
anus. The result is diarrhea, with loss of large quanti-
ties of water and elec

 with loss of large quanti-
ties of water and electrolytes. But the diarrhea also
washes away irritant factors, which promotes earlier
recovery from the disease than might otherwise occur.
Barrett KE, Keely SJ: Chloride secretion by the intestinal
epithelium: molecular basis and regulatory aspects. Annu
Rev Physiol 62:535, 2000.
Chiang JY: Bile acid regulation of hepatic physiology: III.
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test Liver Physiol 284:G349, 2003.
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astroin-
Dockray GJ,Varro A,Dimaline R,Wang T:The gastrins:their
production and biological activities. Annu Rev Physiol
63:119, 2001.
Dockray GJ, Varro A, Dimaline R: Gastric endocrine cells:
gene expression, processing, and targeting of active prod-
ucts. Physiol Rev 76:767,1996.
Flemstrom G, Isenberg JI: Gastroduodenal mucosal alkaline
secretion and mucosal protection. News Physiol Sci 16:23,
2001.
Fuchs M: Bile acid regulation of hepatic physiology: III. Reg

le acid regulation of hepatic physiology: III. Reg-
ulation of bile acid synthesis: past progress and future
challenges. Am J Physiol Gastrointest Liver Physiol
284:G551, 2003.
Jass JR, Walsh MD: Altered mucin expression in the gas-
trointestinal tract: a review. J Cell Mol Med 5:327, 2001.
Kidd JF, Thorn P: Intracellular Ca

s:
Kidd JF, Thorn P: Intracellular Ca2+ and Cl- channel activa-
tion in secretory cells. Annu Rev Physiol 62:493, 2000.
Lee MG,Ahn W, Lee JA, Kim JY, et al: Coordination of pan-
creatic HCO3
_ secretion by protein-protein interaction
between membrane transporters. JOP 2(4 Suppl):203,
2001.
Marve GM: Nerves and hormones interact to control gall-
bladder function. News Physiol Sci 13:64, 1998.
Portincasa P, Di Ciaula A, vanBerge-Henegouwen GP:
Smooth muscle function and dysfunction in

uwen GP:
Smooth muscle function and dysfunction in gallbladder
disease. Curr Gastroenterol Rep 6:151, 2004.
Russell DW: The enzymes, regulation, and genetics of bile
acid synthesis. Annu Rev Biochem 72:137, 2003.
Chapter 64
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Tandler B, Gresik EW, Nagato T, Phillips CJ: Secretion by
striated ducts of mammalian major salivary glands: review
from an ultrastructural, functional, and evolutionary per-
spective. Anat Rec 264:121, 2001.
Trauner M, Boyer JL

ctive. Anat Rec 264:121, 2001.
Trauner M, Boyer JL: Bile salt transporters: molecular char-
acterization, function, and regulation. Physiol Rev 83:633,
2003.
Williams JA: Intracellular signaling mechanisms activated 
by cholecystokinin-regulating synthesis and secretion of
digestive enzymes in pancreatic acinar cells. Annu Rev
Physiol 63:77, 2001.
Zanner R, Gratzl M, Prinz C: Circle of life of secretory vesi-
cles in gastric enterochromafﬁn-like cells. Ann N Y Acad
Sci 971:389, 2002.
C
H
A
P
T
E

cells. Ann N Y Acad
C
H
A
P
T
E
R
 
6
5
808
The major foods on which the body lives (with the
exception of small quantities of substances such as
vitamins and minerals) can be classiﬁed as carbo-
hydrates, fats, and proteins.They generally cannot be
absorbed in their natural forms through the gas-
trointestinal mucosa and, for this reason, are useless
as nutrients without preliminary digestion. There-
fore, this chapter di

reliminary digestion. There-
fore, this chapter discusses, ﬁrst, the processes by
which carbohydrates, fats, and proteins are digested into small enough com-
pounds for absorption and, second, the mechanisms by which the digestive end
products as well as water, electrolytes, and other substances are absorbed.
Digestion of the Various Foods by Hydrolysis
Hydrolysis of Carbohydrates. Almost all the carbohydrates of the diet are either
large polysaccharides or disaccharides, which are combinations 

charides or disaccharides, which are combinations of monosaccha-
rides bound to one another by condensation. This means that a hydrogen ion
(H+) has been removed from one of the monosaccharides, and a hydroxyl ion
(-OH) has been removed from the next one. The two monosaccharides then
combine with each other at these sites of removal, and the hydrogen and
hydroxyl ions combine to form water (H2O).
When carbohydrates are digested, the above process is reversed and the car-
bohydrates are converted

 is reversed and the car-
bohydrates are converted into monosaccharides. Speciﬁc enzymes in the diges-
tive juices of the gastrointestinal tract return the hydrogen and hydroxyl ions
from water to the polysaccharides and thereby separate the monosaccharides
from each other. This process, called hydrolysis, is the following (in which 
R≤-R¢ is a disaccharide):
Hydrolysis of Fats. Almost the entire fat portion of the diet consists of triglyc-
erides (neutral fats), which are combinations of three 

s (neutral fats), which are combinations of three fatty acid molecules con-
densed with a single glycerol molecule. During condensation, three molecules
of water are removed.
Digestion of the triglycerides consists of the reverse process: the fat-
digesting enzymes return three molecules of water to the triglyceride molecule
and thereby split the fatty acid molecules away from the glycerol. Here again,
the digestive process is one of hydrolysis.
Hydrolysis of Proteins. Proteins are formed from m

Hydrolysis of Proteins. Proteins are formed from multiple amino acids that are
bound together by peptide linkages. At each linkage, a hydroxyl ion has been
removed from one amino acid and a hydrogen ion has been removed from the
succeeding one; thus, the successive amino acids in the protein chain are also
bound together by condensation, and digestion occurs by the reverse effect:
hydrolysis. That is, the proteolytic enzymes return hydrogen and hydroxyl ions
from water molecules to the protein m

ydroxyl ions
from water molecules to the protein molecules to split them into their con-
stituent amino acids.
¢¢
¢ +
æ
Æ
æææ
¢¢
+
¢
R -R
H O
R OH
R H
2
digestive
enzyme
Chapter 65
Digestion and Absorption in the Gastrointestinal Tract
809
Therefore, the chemistry of digestion is simple
because, in the case of all three major types of food,
the same basic process of hydrolysis is involved. The
only difference lies in the types of enzymes required
to promote the hydrolysis reactions for each type

to promote the hydrolysis reactions for each type of
food.
All the digestive enzymes are proteins. Their secre-
tion by the different gastrointestinal glands was dis-
cussed in Chapter 64.
Carbohydrate Foods of the Diet. Only three major sources
of carbohydrates exist in the normal human diet.They
are sucrose, which is the disaccharide known popularly
as cane sugar; lactose, which is a disaccharide found 
in milk; and starches, which are large polysaccharides
present 

starches, which are large polysaccharides
present in almost all nonanimal foods, particularly in
potatoes and the different types of grains. Other car-
bohydrates ingested to a slight extent are amylose,
glycogen, alcohol, lactic acid, pyruvic acid, pectins, dex-
trins, and minor quantities of carbohydrate derivatives
in meats.
The diet also contains a large amount of cellulose,
which is a carbohydrate. However, no enzymes
capable of hydrolyzing cellulose are secreted in the
human digestive trac

cellulose are secreted in the
human digestive tract. Consequently, cellulose cannot
be considered a food for humans.
Digestion of Carbohydrates in the Mouth and Stomach. When
food is chewed, it is mixed with saliva, which contains
the digestive enzyme ptyalin (an a-amylase) secreted
mainly by the parotid glands. This enzyme hydrolyzes
starch into the disaccharide maltose and other small
polymers of glucose that contain three to nine glucose
molecules, as shown in Figure 65–1. However, the food
r

ules, as shown in Figure 65–1. However, the food
remains in the mouth only a short time, so that prob-
ably not more than 5 per cent of all the starches will
have become hydrolyzed by the time the food is 
swallowed.
However, starch digestion sometimes continues in
the body and fundus of the stomach for as long as 1
hour before the food becomes mixed with the stomach
secretions. Then activity of the salivary amylase is
blocked by acid of the gastric secretions because the
amylase is essentially 

ric secretions because the
amylase is essentially nonactive as an enzyme once the
pH of the medium falls below about 4.0. Nevertheless,
on the average, before food and its accompanying
saliva do become completely mixed with the gastric
secretions, as much as 30 to 40 per cent of the starches
will have been hydrolyzed mainly to form maltose.
Digestion of Carbohydrates in the Small Intestine
Digestion by Pancreatic Amylase. Pancreatic secre-
tion, like saliva, contains a large quantity of a-amylas

like saliva, contains a large quantity of a-amylase
that is almost identical in its function with the 
a-amylase of saliva but is several times as powerful.
Therefore, within 15 to 30 minutes after the chyme
empties from the stomach into the duodenum and
mixes with pancreatic juice, virtually all the carbohy-
drates will have become digested.
In general, the carbohydrates are almost totally con-
verted into maltose and/or other very small glucose
polymers before passing beyond the duodenum or
up

upper jejunum.
Hydrolysis of Disaccharides and Small Glucose Polymers 
into Monosaccharides by Intestinal Epithelial Enzymes. The
enterocytes lining the villi of the small intestine
contain four enzymes (lactase, sucrase, maltase, and 
a-dextrinase), which are capable of splitting the disac-
charides lactose, sucrose, and maltose, plus other small
glucose polymers, into their constituent monosaccha-
rides.These enzymes are located in the enterocyte

rides.These enzymes are located in the enterocytes cov-
ering the intestinal microvilli brush border, so that the
disaccharides are digested as they come in contact with
these enterocytes.
Lactose splits into a molecule of galactose and a
molecule of glucose. Sucrose splits into a molecule of
fructose and a molecule of glucose. Maltose and other
small glucose polymers all split into multiple molecules
of glucose. Thus, the ﬁnal products of carbohydrate
digestion are all monosaccharides. They ar

hydrate
digestion are all monosaccharides. They are all water
soluble and are absorbed immediately into the portal
blood.
In the ordinary diet, which contains far more
starches than all other carbohydrates combined,
glucose represents more than 80 per cent of the ﬁnal
products of carbohydrate digestion, and galactose and
fructose each seldom more than 10 per cent.
The major steps in carbohydrate digestion are sum-
marized in Figure 65–1.
Ptyalin (saliva)–20–40%
Starches
Glucose
Maltose and 3 to 

(saliva)–20–40%
Starches
Glucose
Maltose and 3 to 9 glucose polymers
Lactose
Sucrose
Fructose
Galactose
Sucrase
(intestine)
Lactase
Pancreatic amylase–50–80%
Maltase and a-dextrinase
Figure 65–1
Digestion of carbohydrates.
810
Unit XII
Proteins of the Diet. The dietary proteins are chemically
long chains of amino acids bound together by peptide
linkages. A typical linkage is the following:
Digestion of Proteins by Pancreat

s the following:
Digestion of Proteins by Pancreatic Secretions. Most
protein digestion occurs in the upper small intestine,
in the duodenum and jejunum, under the inﬂuence 
of proteolytic enzymes from pancreatic secretion.
Immediately on entering the small intestine from the
stomach, the partial breakdown products of the
protein foods are attacked by major proteolytic 
pancreatic enzymes: trypsin, chymotrypsin, carboxy-
polypeptidase, and proelastase, as shown in Figure
65–2.
Both trypsin and c

stase, as shown in Figure
65–2.
Both trypsin and chymotrypsin split protein mole-
cules into small polypeptides; carboxypolypeptidase
then cleaves individual amino acids from the carboxyl
ends of the polypeptides. Proelastase, in turn, is con-
verted into elastase, which then digests elastin ﬁbers
that partially hold meats together.
Only a small percentage of the proteins are digested
all the way to their constituent amino acids by the 
pancreatic juices. Most remain as dipeptides and
tripeptide

c juices. Most remain as dipeptides and
tripeptides.
Digestion of Peptides by Peptidases in the Enterocytes That
Line the Small Intestinal Villi. The last digestive stage of
the proteins in the intestinal lumen is achieved by the
enterocytes that line the villi of the small intestine,
mainly in the duodenum and jejunum.These cells have
a brush border that consists of hundreds of microvilli
projecting from the surface of each cell. In the mem-
brane of each of these microvilli are multiple pepti-

ne of each of these microvilli are multiple pepti-
dases that protrude through the membranes to the
exterior,where they come in contact with the intestinal
ﬂuids.
Two types of peptidase enzymes are especially
important, aminopolypeptidase and several dipepti-
dases. They succeed in splitting the remaining larger
polypeptides into tripeptides and dipeptides and a few
into amino acids. Both the amino acids plus the dipep-
tides and tripeptides are easily transported through
the microvillar membran

easily transported through
the microvillar membrane to the interior of the 
enterocyte.
Finally, inside the cytosol of the enterocyte are mul-
tiple other peptidases that are speciﬁc for the remain-
ing types of linkages between amino acids. Within
minutes, virtually all the last dipeptides and tripeptides
Pepsin
Peptidases
Polypeptides
+
Proteoses
Peptones
Proteins
Trypsin, chymotrypsin, carboxypolypeptidase,
proelastase
Figure 65–2
Digestion of proteins.
C

C
N
O
CH
COOH
OH + H
NH2
CH
R
R
H
C
N
O
CH
COOH + H2O
NH2
CH
R
R
H
The characteristics of each protein are determined
by the types of amino acids in the protein molecule
and by the sequential arrangements of these amino
acids. The physical and chemical characteristics of dif-
ferent proteins important in human tissues are dis-
cussed in Chapter 69.
Digestion of Proteins in the Stomach. Pepsin, the important
peptic enzyme of the stomach, is most act

mportant
peptic enzyme of the stomach, is most active at a pH
of 2.0 to 3.0 and is inactive at a pH above about 5.0.
Consequently, for this enzyme to cause digestive
action on protein, the stomach juices must be acidic.
As explained in Chapter 64, the gastric glands secrete
a large quantity of hydrochloric acid. This hydrochlo-
ric acid is secreted by the parietal (oxyntic) cells in the
glands at a pH of about 0.8, but by the time it is mixed
with the stomach contents and with secretions from
th

h the stomach contents and with secretions from
the nonoxyntic glandular cells of the stomach, the pH
then averages around 2.0 to 3.0, a highly favorable
range of acidity for pepsin activity.
One of the important features of pepsin digestion is
its ability to digest the protein collagen, an albuminoid
type of protein that is affected little by other digestive
enzymes. Collagen is a major constituent of the inter-
cellular connective tissue of meats; therefore, for the
digestive enzymes of the di

ts; therefore, for the
digestive enzymes of the digestive tract to penetrate
meats and digest the other meat proteins, it is ﬁrst nec-
essary that the collagen ﬁbers be digested. Conse-
quently, in persons who lack pepsin in the stomach
juices, the ingested meats are less well penetrated by
the other digestive enzymes and, therefore, may be
poorly digested.
As shown in Figure 65–2, pepsin only initiates the
process of protein digestion, usually providing only 10
to 20 per cent of the total prote

roviding only 10
to 20 per cent of the total protein digestion to convert
the protein to proteoses, peptones, and a few polypep-
tides. This splitting of proteins occurs as a result of
hydrolysis at the peptide linkages between amino
acids.
Chapter 65
811
are digested to the ﬁnal stage to form single amino
acids; these then pass on through to the other side of
the enterocyte and thence into the blood.
More than 99 per cent of the ﬁnal protei

he blood.
More than 99 per cent of the ﬁnal protein digestive
products that are absorbed are individual amino acids,
with only rare absorption of peptides and very, very
rare absorption of whole protein molecules. Even
these very few absorbed molecules of whole protein
can sometimes cause serious allergic or immunologic
disturbances, as discussed in Chapter 34.
Fats of the Diet. By far the most abundant fats of the
diet are the neutral fats, also known as triglycerides,
each mo

neutral fats, also known as triglycerides,
each molecule of which is composed of a glycerol
nucleus and three fatty acid side chains, as shown in
Figure 65–3. Neutral fat is a major constituent in food
of animal origin but much, much less so in food of
plant origin.
In the usual diet are also small quantities of phos-
pholipids, cholesterol, and cholesterol esters.The phos-
pholipids and cholesterol esters contain fatty acid and
therefore can be considered fats themselves. Choles-
terol, however

considered fats themselves. Choles-
terol, however, is a sterol compound that contains no
fatty acid, but it does exhibit some of the physical and
chemical characteristics of fats; plus, it is derived from
fats and is metabolized similarly to fats. Therefore,
cholesterol is considered, from a dietary point of view,
a fat.
Digestion of Fats in the Intestine. A small amount of
triglycerides is digested in the stomach by lingual lipase
that is secreted by lingual glands in the mouth and
swallowed w

ted by lingual glands in the mouth and
swallowed with the saliva. This amount of digestion is
less than 10 per cent and generally unimportant.
Instead, essentially all fat digestion occurs in the small
intestine as follows.
Emulsiﬁcation of Fat by Bile Acids and Lecithin. The
ﬁrst step in fat digestion is physically to break the fat
globules into very small sizes so that the water-soluble
digestive enzymes can act on the globule surfaces.This
process is called emulsiﬁcation of the fat, and it be

cess is called emulsiﬁcation of the fat, and it begins
by agitation in the stomach to mix the fat with the
products of stomach digestion.
Then, most of the emulsiﬁcation occurs in the duo-
denum under the inﬂuence of bile, the secretion from
the liver that does not contain any digestive enzymes.
However,bile does contain a large quantity of bile salts
as well as the phospholipid lecithin. Both of these,
but especially the lecithin, are extremely important for
emulsiﬁcation of the fat. The polar 

important for
emulsiﬁcation of the fat. The polar parts (the points
where ionization occurs in water) of the bile salts and
lecithin molecules are highly soluble in water, whereas
most of the remaining portions of their molecules are
highly soluble in fat. Therefore, the fat-soluble por-
tions of these liver secretions dissolve in the surface
layer of the fat globules, with the polar portions pro-
jecting.The polar projections, in turn, are soluble in the
surrounding watery ﬂuids, which greatly 

le in the
surrounding watery ﬂuids, which greatly decreases the
interfacial tension of the fat and makes it soluble as
well.
When the interfacial tension of a globule of non-
miscible ﬂuid is low, this nonmiscible ﬂuid, on agita-
tion, can be broken up into many very minute particles
far more easily than it can when the interfacial tension
is great. Consequently, a major function of the bile
salts and lecithin, especially the lecithin, in the bile is
to make the fat globules readily fragmentable

e is
to make the fat globules readily fragmentable by agi-
tation with the water in the small bowel. This action is
the same as that of many detergents that are widely
used in household cleaners for removing grease.
Each time the diameters of the fat globules are sig-
niﬁcantly decreased as a result of agitation in the small
intestine, the total surface area of the fat increases
manyfold. Because the average diameter of the fat
particles in the intestine after emulsiﬁcation has
occurred is less 

ntestine after emulsiﬁcation has
occurred is less than 1 micrometer, this represents an
increase of as much as 1000-fold in total surface areas
of the fats caused by the emulsiﬁcation process.
The lipase enzymes are water-soluble compounds
and can attack the fat globules only on their surfaces.
Consequently,it can be readily understood how impor-
tant this detergent function of bile salts and lecithin is
for digestion of fats.
Digestion of Triglycerides by Pancreatic Lipase. By far the
most impo

cerides by Pancreatic Lipase. By far the
most important enzyme for digestion of the triglyc-
erides is pancreatic lipase, present in enormous quan-
tities in pancreatic juice, enough to digest within 1
minute all triglycerides that it can reach. In addition,
the enterocytes of the small intestine contain still more
lipase, known as enteric lipase, but this is usually not
needed.
End Products of Fat Digestion. Most of the triglyc-
erides of the diet are split by pancreatic lipase into 
CH3
(CH2)1

et are split by pancreatic lipase into 
CH3
(CH2)16
C
O
O
CH2
CH3
(CH2)16
C
O
O
CH2
CH3
(CH2)16
C
O
Lipase
(Tristearin)
CH3
(CH2)16
(CH2)16
C
O
O
C
O
OH
HO
CH2
HO
CH2
(2-Monoglyceride)
(Stearic acid)
O
CH + 2H2O
CH + 2CH3
Figure 65–3
Hydrolysis of neutral fat catalyzed by lipase.
812
Unit XII
free fatty acids and 2-monoglycerides, as shown in
Figure 65–4.
Role of Bile Salts to Accelerate Fat Digestion—Formation of
Micelles. The hydrolysis of triglycerides is a highly

lles. The hydrolysis of triglycerides is a highly
reversible process; therefore, accumulation of mono-
glycerides and free fatty acids in the vicinity of digest-
ing fats quickly blocks further digestion. But the bile
salts play the additional important role of removing
the monoglycerides and free fatty acids from the vicin-
ity of the digesting fat globules almost as rapidly as
these end products of digestion are formed. This
occurs in the following way.
Bile salts, when in high enough concentr

wing way.
Bile salts, when in high enough concentration in
water, have the propensity to form micelles, which are
small spherical, cylindrical globules 3 to 6 nanometers
in diameter composed of 20 to 40 molecules of bile
salt. These develop because each bile salt molecule is
composed of a sterol nucleus that is highly fat-soluble
and a polar group that is highly water-soluble. The
sterol nucleus encompasses the fat digestate, forming
a small fat globule in the middle of a resulting micelle,
with

globule in the middle of a resulting micelle,
with polar groups of bile salts projecting outward to
cover the surface of the micelle. Because these polar
groups are negatively charged, they allow the entire
micelle globule to dissolve in the water of the diges-
tive ﬂuids and to remain in stable solution until the fat
is absorbed into the blood.
The bile salt micelles also act as a transport medium
to carry the monoglycerides and free fatty acids, both
of which would otherwise be relatively inso

, both
of which would otherwise be relatively insoluble, to
the brush borders of the intestinal epithelial cells.
There the monoglycerides and free fatty acids are
absorbed into the blood, as discussed later, but the bile
salts themselves are released back into the chyme to
be used again and again for this “ferrying” process.
Digestion of Cholesterol Esters and Phospholipids. Most cho-
lesterol in the diet is in the form of cholesterol esters,
which are combinations of free cholesterol and one
m

ich are combinations of free cholesterol and one
molecule of fatty acid. Phospholipids also contain fatty
acid within their molecules. Both the cholesterol esters
and the phospholipids are hydrolyzed by two other
lipases in the pancreatic secretion that free the fatty
acids—the enzyme cholesterol ester hydrolase to
hydrolyze the cholesterol ester, and phospholipase A2
to hydrolyze the phospholipid.
The bile salt micelles play the same role in “ferry-
ing” free cholesterol and phospholipid molecu

rry-
ing” free cholesterol and phospholipid molecule diges-
tates that they play in “ferrying” monoglycerides and
free fatty acids. Indeed, essentially no cholesterol is
absorbed without this function of the micelles.
Basic Principles of
Gastrointestinal Absorption
It is suggested that the reader review the basic princi-
ples of transport of substances through cell mem-
branes discussed in detail in Chapter 4. The following
paragraphs present specialized applications of these
transport processes

cialized applications of these
transport processes during gastrointestinal absorption.
The total quantity of ﬂuid that must be absorbed each
day by the intestines is equal to the ingested ﬂuid
(about 1.5 liters) plus that secreted in the various gas-
trointestinal secretions (about 7 liters). This comes to
a total of 8 to 9 liters. All but about 1.5 liters of this is
absorbed in the small intestine, leaving only 1.5 liters
to pass through the ileocecal valve into t

 liters
to pass through the ileocecal valve into the colon each
day.
The stomach is a poor absorptive area of the gas-
trointestinal tract because it lacks the typical villus
type of absorptive membrane, and also because the
junctions between the epithelial cells are tight junc-
tions. Only a few highly lipid-soluble substances,
such as alcohol and some drugs like aspirin, can be
absorbed in small quantities.
Absorptive Surface of the Small Intestinal Mucosa Villi.
Figure 65–5 demonstrates the a

tinal Mucosa Villi.
Figure 65–5 demonstrates the absorptive surface of the
small intestinal mucosa, showing many folds called
(Bile + Agitation)
Pancreatic lipase
Emulsified fat
Fatty acids and
2-monoglycerides
Fat
Figure 65–4
Digestion of fats.
Villi
Food
movement
Food
movement
Valvulae
conniventes
Figure 65–5
Longitudinal section of the small intestine, showing the valvulae
conniventes covered by villi.
Chapter 65
813
valvul

bsorption in the Gastrointestinal Tract
813
valvulae conniventes (or folds of Kerckring), which
increase the surface area of the absorptive mucosa
about threefold. These folds extend circularly most of
the way around the intestine and are especially well
developed in the duodenum and jejunum, where they
often protrude as much as 8 millimeters into the
lumen.
Also located on the epithelial surface of the small
intestine all the way down to the ileocecal valve are
literally millions of small villi

ocecal valve are
literally millions of small villi. These project about 1
millimeter from the surface of the mucosa, as shown
on the surfaces of the valvulae conniventes in Figure
65–5 and in individual detail in Figure 65–6. The villi
lie so close to one another in the upper small intestine
that they touch in most areas, but their distribution is
less profuse in the distal small intestine. The presence
of villi on the mucosal surface enhances the total
absorptive area another 10-fold.
Finally, 

e total
Finally, each intestinal epithelial cell on each villus
is characterized by a brush border, consisting of as
many as 1000 microvilli 1 micrometer in length and 0.1
micrometer in diameter protruding into the intestinal
chyme; these microvilli are shown in the electron
micrograph in Figure 65–7. This increases the surface
area exposed to the intestinal materials at least
another 20-fold.
Thus, the combination of the folds of Kerckring, the
villi, and the mi

n of the folds of Kerckring, the
villi, and the microvilli increases the total absorptive
area of the mucosa perhaps 1000-fold, making a
tremendous total area of 250 or more square meters
for the entire small intestine—about the surface area
of a tennis court.
Figure 65–6A shows in longitudinal section the
general organization of the villus, emphasizing (1) the
advantageous arrangement of the vascular system for
absorption of ﬂuid and dissolved material into the
portal blood and (2) the arrangem

aterial into the
portal blood and (2) the arrangement of the “central
lacteal” lymph vessel for absorption into the lymph.
Figure 65–6B shows a cross section of the villus, and
Figure 65–7 shows many small pinocytic vesicles, which
are pinched-off portions of infolded enterocyte mem-
brane forming vesicles of absorbed ﬂuids that have
been entrapped. Small amounts of substances are
absorbed by this physical process of pinocytosis.
Extending from the epithelial cell body into each
microvillus of t

he epithelial cell body into each
microvillus of the brush border are multiple actin ﬁla-
ments that contract rhythmically to cause continual
movement of the microvilli, keeping them constantly
exposed to new quantities of intestinal ﬂuid.
Absorption in the 
Absorption from the small intestine each day consists
of several hundred grams of carbohydrates, 100 or
Central
lacteal
Central
lacteal
Venules
Basement
membrane
A
B
Brush
border
Arteriole
Vein
Artery
Blood
capill

der
Arteriole
Vein
Artery
Blood
Figure 65–6
Functional organization of the
villus. A,
Longitudinal section. 
B, Cross section showing a base-
ment membrane beneath the
epithelial cells and a brush
border at the other ends of these
cells.
Figure 65–7
Brush border of a gastrointestinal epithelial cell, showing also
absorbed pinocytic vesicles, mitochondria, and endoplasmic
reticulum lying immediately beneath the brush border. (Courtesy
Dr. William Lockwood.)
814
Unit XII
Ga

. (Courtesy
814
Unit XII
more grams of fat, 50 to 100 grams of amino acids, 50
to 100 grams of ions, and 7 to 8 liters of water. The
absorptive capacity of the normal small intestine is far
greater than this: as much as several kilograms of car-
bohydrates per day, 500 grams of fat per day, 500 to
700 grams of proteins per day, and 20 or more liters of
water per day.The large intestine can absorb still addi-
tional water and ions, although very 

 still addi-
tional water and ions, although very few nutrients.
Isosmotic Absorption. Water is transported through the
intestinal membrane entirely by diffusion. Further-
more, this diffusion obeys the usual laws of osmosis.
Therefore, when the chyme is dilute enough, water is
absorbed through the intestinal mucosa into the blood
of the villi almost entirely by osmosis.
Conversely, water can also be transported in the
opposite direction—from plasma into the chyme. This
occur

e direction—from plasma into the chyme. This
occurs especially when hyperosmotic solutions are dis-
charged from the stomach into the duodenum. Within
minutes, sufﬁcient water usually will be transferred by
osmosis to make the chyme isosmotic with the plasma.
Active Transport of Sodium. Twenty to 30 grams of sodium
are secreted in the intestinal secretions each day.
In addition, the average person eats 5 to 8 grams of
sodium each day. Therefore, to prevent net loss of
sodium i

ch day. Therefore, to prevent net loss of
sodium into the feces, the intestines must absorb 25 to
35 grams of sodium each day, which is equal to about
one seventh of all the sodium present in the body.
Whenever signiﬁcant amounts of intestinal secre-
tions are lost to the exterior, as in extreme diarrhea,
the sodium reserves of the body can sometimes be
depleted to lethal levels within hours. Normally,
however, less than 0.5 per cent of the intestinal sodium
is lost in the feces each day because

tinal sodium
is lost in the feces each day because it is rapidly
absorbed through the intestinal mucosa. Sodium also
plays an important role in helping to absorb sugars 
and amino acids, as we shall see in subsequent 
discussions.
The basic mechanism of sodium absorption from the
intestine is shown in Figure 65–8.The principles of this
mechanism, discussed in Chapter 4, are also essentially
the same as for absorption of sodium from the 
gallbladder and renal tubules as discussed in Chapter
27.
T

er and renal tubules as discussed in Chapter
27.
The motive power for sodium absorption is pro-
vided by active transport of sodium from inside the
epithelial cells through the basal and side walls of
these cells into paracellular spaces. This is demon-
strated by the heavy red arrows in Figure 65–8. This
active transport obeys the usual laws of active trans-
port: it requires energy, and the energy process is cat-
alyzed by appropriate adenosine triphosphatase
enzymes in the cell membrane (see 

 triphosphatase
enzymes in the cell membrane (see Chapter 4). Part of
the sodium is absorbed along with chloride ions; in
fact, the negatively charged chloride ions are mainly
passively “dragged” by the positive electrical charges
of the sodium ions.
Active transport of sodium through the basolateral
membranes of the cell reduces the sodium concentra-
tion inside the cell to a low value (about 50 mEq/L),
as also shown in Figure 65–8. Because the sodium con-
centration in the chyme is normally ab

sodium con-
centration in the chyme is normally about 142 mEq/L
(that is, about equal to that in plasma), sodium moves
down this steep electrochemical gradient from the
chyme through the brush border of the epithelial cell
into the epithelial cell cytoplasm. This provides still
more sodium ions to be transported by the epithelial
cells into the paracellular spaces.
Osmosis of the Water. The next step in the transport
process is osmosis of water into the paracellular
spaces. This occurs because a

nto the paracellular
spaces. This occurs because a large osmotic gradient
has been created by the elevated concentration of ions
in the paracellular space. Much of this osmosis occurs
through the tight junctions between the apical borders
of the epithelial cells, but much also occurs through the
cells themselves. And osmotic movement of water
creates ﬂow of ﬂuid into and through the paracellular
spaces and, ﬁnally, into the circulating blood of the
villus.
Aldosterone Greatly Enhances Sodium Abs

he
villus.
Aldosterone Greatly Enhances Sodium Absorption. When a
person becomes dehydrated, large amounts of aldos-
terone almost always are secreted by the cortices of
the adrenal glands.Within 1 to 3 hours this aldosterone
causes increased activation of the enzyme and trans-
port mechanisms for all aspects of sodium absorption
by the intestinal epithelium.And the increased sodium
absorption in turn causes secondary increases in
absorption of chloride ions, water, and some other 
T

hloride ions, water, and some other 
This effect of aldosterone is especially important in
the colon because it allows virtually no loss of sodium
chloride in the feces and also little water loss.Thus, the
H2O
H2O
H2O
H2O
H2O
H2O
Na
Diffusion
Osmosis
Na
(50 mEq/L)
Na (142 mEq/L)
H2O
Na
Figure 65–8
Absorption of sodium through the intestinal epithelium. Note also
osmotic absorption of water—that is, water “follows” sodium
through the epithelial membrane.
Chapter 65
Di

ium
Chapter 65
815
function of aldosterone in the intestinal tract is the
same as that achieved by aldosterone in the renal
tubules, which also serves to conserve sodium chloride
and water in the body when a person becomes 
dehydrated.
Absorption of Chloride Ions in the Duodenum and Jejunum. In
the upper part of the small intestine, chloride ion
absorption is rapid and occurs mainly by diffusion—
that is, abs

rapid and occurs mainly by diffusion—
that is, absorption of sodium ions through the epithe-
lium creates electronegativity in the chyme and
electropositivity in the paracellular spaces between
the epithelial cells.Then chloride ions move along this
electrical gradient to “follow” the sodium ions.
Absorption of Bicarbonate Ions in the Duodenum and Jejunum.
Often large quantities of bicarbonate ions must be
reabsorbed from the upper small intestine because
large amounts of bicarbonate ions have b

e because
large amounts of bicarbonate ions have been secreted
into the duodenum in both pancreatic secretion and
bile.The bicarbonate ion is absorbed in an indirect way
as follows: When sodium ions are absorbed, moderate
amounts of hydrogen ions are secreted into the lumen
of the gut in exchange for some of the sodium. These
hydrogen ions in turn combine with the bicarbonate
ions to form carbonic acid (H2CO3), which then disso-
ciates to form water and carbon dioxide. The water
remains as part 

ter and carbon dioxide. The water
remains as part of the chyme in the intestines, but the
carbon dioxide is readily absorbed into the blood and
subsequently expired through the lungs. Thus, this is
so-called “active absorption of bicarbonate ions.” It is
the same mechanism that occurs in the tubules of the
kidneys.
Secretion of Bicarbonate Ions in the Ileum 
and Large Intestine—Simultaneous Absorption
of Chloride Ions
The epithelial cells on the surfaces of the villi in the
ileum as well as on a

surfaces of the villi in the
ileum as well as on all surfaces of the large intestine
have a special capability of secreting bicarbonate ions
in exchange for absorption of chloride ions. This is
important because it provides alkaline bicarbonate
ions that neutralize acid products formed by bacteria
in the large intestine.
Extreme Secretion of Chloride Ions, Sodium Ions, and Water from
the Large Intestine Epithelium in Some Types of Diarrhea. Deep
in the spaces between the intestinal epithelial fo

in the spaces between the intestinal epithelial folds are
immature epithelial cells that continually divide to form
new epithelial cells. These in turn spread outward over
the luminal surfaces of the intestines. While still in the
deep folds, the epithelial cells secrete sodium chloride
and water into the intestinal lumen. This secretion in
turn is reabsorbed by the older epithelial cells outside
the folds, thus providing ﬂow of water for absorbing
intestinal digestates.
The toxins of cholera an

ng
The toxins of cholera and of some other types of diar-
rheal bacteria can stimulate the fold secretion so greatly
that this secretion often becomes much greater than can
be reabsorbed, thus sometimes causing loss of 5 to 10
liters of water and sodium chloride as diarrhea each day.
Within 1 to 5 days, many severely affected patients die
from this loss of ﬂuid alone.
Extreme diarrheal secretion is initiated by entry of 
a subunit of cholera toxin into the epithelial cells

subunit of cholera toxin into the epithelial cells.
This stimulates formation of excess cyclic adenosine
monophosphate, which opens tremendous numbers of
chloride channels, allowing chloride ions to ﬂow rapidly
from inside the cell into the intestinal crypts. In turn, this
is believed to activate a sodium pump that pumps
sodium ions into the crypts to go along with the chlo-
ride ions. Finally, all this extra sodium chloride causes
extreme osmosis of water from the blood, thus provid-
ing rapid 

s of water from the blood, thus provid-
ing rapid ﬂow of ﬂuid along with the salt.All this excess
ﬂuid washes away most of the bacteria and is of value
in combating the disease, but too much of a good thing
can be lethal because of serious dehydration of the
whole body that might ensue. In most instances, the life
of a cholera victim can be saved by administration of
tremendous amounts of sodium chloride solution to
make up for the loss.
Absorption of Other Ions. Calcium ions are actively
absorb

on of Other Ions. Calcium ions are actively
absorbed into the blood especially from the duode-
num, and the amount of calcium ion absorption is very
exactly controlled to supply exactly the daily need of
the body for calcium. One important factor controlling
calcium absorption is parathyroid hormone secreted
by the parathyroid glands, and another is vitamin D.
Parathyroid hormone activates vitamin D, and the
activated vitamin D in turn greatly enhances 
calcium absorption. These effects are disc

hances 
calcium absorption. These effects are discussed in
Iron ions are also actively absorbed from the small
intestine. The principles of iron absorption and regu-
lation of its absorption in proportion to the body’s
need for iron, especially for the formation of hemo-
globin, are discussed in Chapter 32.
Potassium, magnesium, phosphate, and probably still
other ions can also be actively absorbed through the
intestinal mucosa. In general, the monovalent ions are
absorbed with ease 

neral, the monovalent ions are
absorbed with ease and in great quantities. Conversely,
bivalent ions are normally absorbed in only small
amounts;
for example,
maximum absorption of
calcium ions is only 1/50 as great as the normal absorp-
tion of sodium ions. Fortunately, only small quantities
of the bivalent ions are normally required daily by the
body.
Absorption of Carbohydrates
Essentially all the carbohydrates in the food are
absorbed in the form of monosaccharides; o

ood are
absorbed in the form of monosaccharides; only a small
fraction are absorbed as disaccharides and almost
none as larger carbohydrate compounds. By far 
the most abundant of the absorbed monosaccharides
is glucose,usually accounting for more than 80 per cent
of carbohydrate calories absorbed. The reason for 
this is that glucose is the ﬁnal digestion product of our
most abundant carbohydrate food, the starches. The
remaining 20 per cent of absorbed monosaccharides
are composed almost entir

absorbed monosaccharides
are composed almost entirely of galactose and fruc-
tose, the galactose derived from milk and the fructose
as one of the monosaccharides digested from cane
sugar.
816
Unit XII
Virtually all the monosaccharides are absorbed by
an active transport process. Let us ﬁrst discuss the
absorption of glucose.
Glucose Is Transported by a Sodium Co-Transport Mechanism.
In the absence of sodium transport through the intes-
tinal membrane, virtually no glu

hrough the intes-
tinal membrane, virtually no glucose can be absorbed.
The reason is that glucose absorption occurs in a co-
transport mode with active transport of sodium.
There are two stages in the transport of sodium
through the intestinal membrane. First is active trans-
port of sodium ions through the basolateral mem-
branes of the intestinal epithelial cells into the blood,
thereby depleting sodium inside the epithelial cells.
Second, decrease of sodium inside the cells causes
sodium fro

rease of sodium inside the cells causes
sodium from the intestinal lumen to move through the
brush border of the epithelial cells to the cell interiors
by a process of facilitated diffusion. That is, a sodium
ion combines with a transport protein, but the trans-
port protein will not transport the sodium to the inte-
rior of the cell until the protein itself also combines
with some other appropriate substance such as
glucose. Fortunately, intestinal glucose also combines
simultaneously with the 

nal glucose also combines
simultaneously with the same transport protein, and
then both the sodium ion and glucose molecule are
transported together to the interior of the cell. Thus,
the low concentration of sodium inside the cell liter-
ally “drags” sodium to the interior of the cell and along
with it the glucose at the same time. Once inside the
epithelial cell, other transport proteins and enzymes
cause facilitated diffusion of the glucose through the
cell’s basolateral membrane into the par

rough the
cell’s basolateral membrane into the paracellular
space and from there into the blood.
To summarize, it is the initial active transport of
sodium through the basolateral membranes of the
intestinal epithelial cells that provides the eventual
motive force for moving glucose also through the
membranes.
Absorption of Other Monosaccharides. Galactose is trans-
ported by almost exactly the same mechanism as
glucose. Conversely, fructose transport does not occur
by the sodium co-transport me

sport does not occur
by the sodium co-transport mechanism. Instead, fruc-
tose is transported by facilitated diffusion all the way
through the intestinal epithelium but not coupled with
sodium transport.
Much of the fructose, on entering the cell, becomes
phosphorylated, then converted to glucose, and ﬁnally
transported in the form of glucose the rest of the way
into the blood. Because fructose is not co-transported
with sodium, its overall rate of transport is only about
one half that of glucos

of transport is only about
one half that of glucose or galactose.
Absorption of Proteins
As explained earlier in the chapter, most proteins,
after digestion, are absorbed through the luminal
membranes of the intestinal epithelial cells in the form
of dipeptides, tripeptides, and a few free amino acids.
The energy for most of this transport is supplied by a
sodium co-transport mechanism in the same way that
sodium co-transport of glucose occurs. That is, most
peptide or amino acid molecules bind 

hat is, most
peptide or amino acid molecules bind in the cell’s
microvillus membrane with a speciﬁc transport protein
that requires sodium binding before transport can
occur.After binding, the sodium ion then moves down
its electrochemical gradient to the interior of the cell
and pulls the amino acid or peptide along with it. This
is called co-transport (or secondary active transport)
of the amino acids and peptides. A few amino acids do
not require this sodium co-transport mechanism but
instead

ire this sodium co-transport mechanism but
instead are transported by special membrane trans-
port proteins in the same way that fructose is trans-
ported, by facilitated diffusion.
At least ﬁve types of transport proteins for trans-
porting amino acids and peptides have been found in
the luminal membranes of intestinal epithelial cells.
This multiplicity of transport proteins is required
because of the diverse binding properties of different
amino acids and peptides.
Absorption of Fats
Earlier 

no acids and peptides.
Earlier in this chapter, it was pointed out that when
fats are digested to form monoglycerides and free fatty
acids, both of these digestive end products ﬁrst become
dissolved in the central lipid portions of bile micelles.
Because the molecular dimensions of these micelles
are only 3 to 6 nanometers in diameter, and because
of their highly charged exterior, they are soluble in
chyme. In this form, the monoglycerides and free fatty
acids are carried to t

noglycerides and free fatty
acids are carried to the surfaces of the microvilli of the
intestinal cell brush border and then penetrate into the
recesses among the moving, agitating microvilli. Here,
both the monoglycerides and fatty acids diffuse imme-
diately out of the micelles and into the interior of 
the epithelial cells, which is possible because the lipids
are also soluble in the epithelial cell membrane. This
leaves the bile micelles still in the chyme, where they
function again and agai

l in the chyme, where they
function again and again to help absorb still more
monoglycerides and fatty acids.
Thus, the micelles perform a “ferrying” function that
is highly important for fat absorption. In the presence
of an abundance of bile micelles, about 97 per cent of
the fat is absorbed; in the absence of the bile micelles,
only 40 to 50 per cent can be absorbed.
After entering the epithelial cell, the fatty acids and
monoglycerides are taken up by the cell’s smooth
endoplasmic reticulum;

ken up by the cell’s smooth
endoplasmic reticulum; here, they are mainly used to
form new triglycerides that are subsequently released
in the form of chylomicrons through the base of the
epithelial cell, to ﬂow upward through the thoracic
lymph duct and empty into the circulating blood.
Direct Absorption of Fatty Acids into the Portal Blood. Small
quantities of short- and medium-chain fatty acids, such
as those from butterfat, are absorbed directly into the
portal blood rather than being convert

ly into the
portal blood rather than being converted into triglyc-
erides and absorbed by way of the lymphatics. The
cause of this difference between short- and long-chain
fatty acid absorption is that the short-chain fatty acids
are more water-soluble and mostly are not recon-
verted into triglycerides by the endoplasmic reticulum.
This allows direct diffusion of these short-chain fatty
acids from the intestinal epithelial cells directly into
the capillary blood of the intestinal villi.
Chapter

e capillary blood of the intestinal villi.
Chapter 65
817
Absorption in the Large
Intestine: Formation of Feces
About 1500 milliliters of chyme normally pass through
the ileocecal valve into the large intestine each day.
Most of the water and electrolytes in this chyme are
absorbed in the colon, usually leaving less than 100
milliliters of ﬂuid to be excreted in the feces. Also,
essentially all the ions are absorbed, leaving only 1 to
5 mill

ll the ions are absorbed, leaving only 1 to
5 milliequivalents each of sodium and chloride ions to
be lost in the feces.
Most of the absorption in the large intestine occurs
in the proximal one half of the colon, giving this
portion the name absorbing colon, whereas the distal
colon functions principally for feces storage until a
propitious time for feces excretion and is therefore
called the storage colon.
Absorption and Secretion of Electrolytes and Water. The
mucosa of the large intestine, li

s and Water. The
mucosa of the large intestine, like that of the small
intestine, has a high capability for active absorption of
sodium, and the electrical potential gradient created
by absorption of the sodium causes chloride absorp-
tion as well. The tight junctions between the epithelial
cells of the large intestinal epithelium are much tighter
than those of the small intestine. This prevents signif-
icant amounts of back-diffusion of ions through these
junctions, thus allowing the large inte

ough these
junctions, thus allowing the large intestinal mucosa to
absorb sodium ions far more completely—that is,
against a much higher concentration gradient—than
can occur in the small intestine. This is especially true
when large quantities of aldosterone are available
because aldosterone greatly enhances sodium trans-
port capability.
In addition, as occurs in the distal portion of the
small intestine, the mucosa of the large intestine
secretes bicarbonate ions while it simultaneously
absor

tes bicarbonate ions while it simultaneously
absorbs an equal number of chloride ions in an
exchange transport process that has already been
described. The bicarbonate helps neutralize the acidic
end products of bacterial action in the large intestine.
Absorption of sodium and chloride ions creates an
osmotic gradient across the large intestinal mucosa,
which in turn causes absorption of water.
Maximum Absorption Capacity of the Large Intestine. The
large intestine can absorb a maximum of 5 to 8

The
large intestine can absorb a maximum of 5 to 8 liters
of ﬂuid and electrolytes each day.When the total quan-
tity entering the large intestine through the ileocecal
valve or by way of large intestine secretion exceeds
this amount, the excess appears in the feces as diar-
rhea. As noted earlier in the chapter, toxins from
cholera or certain other bacterial infections often
cause the crypts in the terminal ileum and in the large
intestine to secrete 10 or more liters of ﬂuid each day,
leading 

crete 10 or more liters of ﬂuid each day,
leading to severe and sometimes lethal diarrhea.
Bacterial Action in the Colon. Numerous bacteria, especially
colon bacilli, are present even normally in the absorbing
colon. They are capable of digesting small amounts of
cellulose, in this way providing a few calories of extra
nutrition for the body. In herbivorous animals, this
source of energy is signiﬁcant, although it is of negligi-
ble importance in human beings.
Other substances formed as a result

 human beings.
Other substances formed as a result of bacterial activ-
ity are vitamin K, vitamin B12, thiamine, riboﬂavin, and
various gases that contribute to ﬂatus in the colon, espe-
cially carbon dioxide, hydrogen gas, and methane.
The bacteria-formed vitamin K is especially important
because the amount of this vitamin in the daily ingested
foods is normally insufﬁcient to maintain adequate
blood coagulation.
Composition of the Feces. The feces normally are about
three-fourths water and one

ces normally are about
three-fourths water and one-fourth solid matter that
itself is composed of about 30 per cent dead bacteria, 10
to 20 per cent fat, 10 to 20 per cent inorganic matter, 2
to 3 per cent protein, and 30 per cent undigested
roughage from the food and dried constituents of diges-
tive juices, such as bile pigment and sloughed epithelial
cells. The brown color of feces is caused by stercobilin
and urobilin, derivatives of bilirubin.The odor is caused
principally by products of ba

n.The odor is caused
principally by products of bacterial action; these prod-
ucts vary from one person to another, depending on
each person’s colonic bacterial ﬂora and on the type of
food eaten. The actual odoriferous products include
indole, skatole, mercaptans, and hydrogen sulﬁde.
Adibi SA: Regulation of expression of the intestinal
oligopeptide transporter (Pept-1) in health and disease.
Am J Physiol Gastrointest Liver Physiol 285:G779, 2003.
Barrett KE, Keely SJ: Chloride secre

5:G779, 2003.
Daniel H: Molecular and integrative physiology of intestinal
peptide transport. Annu Rev Physiol 66:361, 2004.
Farhadi A, Banan A, Fields J, Keshavarzian A: Intestinal
barrier: an interface between health and disease. J Gas-
troenterol Hepatol 18:479, 2003.
Ferraris RP, Diamond J: Regulation of intestinal sugar trans-
port. Physiol Rev 77:257

f intestinal sugar trans-
port. Physiol Rev 77:257, 1997.
Field M: Intestinal ion transport and the pathophysiology of
diarrhea. J Clin Invest 111:931, 2003.
Greig E, Sandle GI: Diarrhea in ulcerative colitis. The role
of altered colonic sodium transport. Ann N Y Acad Sci
915:327, 2000.
Hershberg RM: The epithelial cell cytoskeleton and intra-
cellular trafﬁcking. V. Polarized compartmentalization of
antigen processing and Toll-like receptor signaling in
intestinal epithelial cells. Am J Physiol

aling in
intestinal epithelial cells. Am J Physiol Gastrointest Liver
Physiol 283:G833, 2002.
Kullak-Ublick GA, Stieger B, Meier PJ: Enterohepatic bile
salt transporters in normal physiology and liver disease.
Gastroenterology 126:322, 2004.
Kunzelmann K, Mall M: Electrolyte transport in the mam-
malian colon: mechanisms and implications for disease.
Physiol Rev 82:245, 2002.
Meier PJ, Stieger B: Bile salt transporters.Annu

.
Meier PJ, Stieger B: Bile salt transporters.Annu Rev Physiol
64:635, 2002.
Pacha J: Development of intestinal transport function in
mammals. Physiol Rev 80:1633, 2000.
Pappenheimer JR: Intestinal absorption of hexoses and
amino acids: from apical cytosol to villus capillaries. J
Membr Biol 184:233, 2001.
818
Unit XII
Peng JB, Brown EM, Hediger MA: Apical entry channels in
calcium-transporting epithelia. News Physiol Sci 18:158,
2003.
Reuss L: One-hundred years of in

Sci 18:158,
2003.
Reuss L: One-hundred years of inquiry: the mechanism of
glucose absorption in the intestine. Annu Rev Physiol
62:939, 2000.
Rothman S, Liebow C, Isenman L: Conservation of digestive
enzymes. Physiol Rev 82:1, 2002.
Said HM: Recent advances in carrier-mediated intestinal
absorption of w

ces in carrier-mediated intestinal
absorption of water-soluble vitamins. Annu Rev Physiol
2004;66:419-46.
Stevens CE, Hume ID: Contributions of microbes in verte-
brate gastrointestinal tract to production and conservation
of nutrients. Physiol Rev 78:393, 1998.
Topping DL, Clifton PM: Short-chain fatty acids and human
colonic function: roles of resistant starch and nonstarch
polysaccharides. Physiol Rev 81:1031, 2001.
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6
6
819
Physiology of Gastrointestinal
Disorders
Effective th

siology of Gastrointestinal
Disorders
Effective therapy for most gastrointestinal disorders
depends on a basic knowledge of gastrointestinal phys-
iology. The purpose of this chapter, therefore, is to
discuss a few representative types of gastrointestinal
malfunction that have special physiologic bases or 
consequences.
Disorders of Swallowing and of the Esophagus
Paralysis of the Swallowing Mechanism. Damage to the 5th, 9th, or 10th cerebral nerve
can cause paralysis of signiﬁcant portions of t

ve
can cause paralysis of signiﬁcant portions of the swallowing mechanism.Also, a few
diseases, such as poliomyelitis or encephalitis, can prevent normal swallowing by
damaging the swallowing center in the brain stem. Finally, paralysis of the swal-
lowing muscles, as occurs in muscle dystrophy or in failure of neuromuscular trans-
mission in myasthenia gravis or botulism, can also prevent normal swallowing.
When the swallowing mechanism is partially or totally paralyzed, the abnormal-
ities tha

ally or totally paralyzed, the abnormal-
ities that can occur include (1) complete abrogation of the swallowing act so 
that swallowing cannot occur, (2) failure of the glottis to close so that food passes
into the lungs instead of the esophagus, and (3) failure of the soft palate and 
uvula to close the posterior nares so that food reﬂuxes into the nose during 
swallowing.
One of the most serious instances of paralysis of the swallowing mechanism
occurs when patients are under deep anesthesia. 

m
occurs when patients are under deep anesthesia. Often, while on the operating table,
they vomit large quantities of materials from the stomach into the pharynx; then,
instead of swallowing the materials again, they simply suck them into the trachea
because the anesthetic has blocked the reﬂex mechanism of swallowing.As a result,
such patients occasionally choke to death on their own vomitus.
Achalasia and Megaesophagus. Achalasia is a condition in which the lower esophageal
sphincter fails to 

 in which the lower esophageal
sphincter fails to relax during swallowing. As a result, food swallowed into the
esophagus then fails to pass from the esophagus into the stomach. Pathological
studies have shown damage in the neural network of the myenteric plexus in the
lower two thirds of the esophagus. As a result, the musculature of the lower esoph-
agus remains spastically contracted, and the myenteric plexus has lost its ability to
transmit a signal to cause “receptive relaxation” of the gas

 signal to cause “receptive relaxation” of the gastroesophageal sphincter
as food approaches this sphincter during swallowing.
When achalasia becomes severe,the esophagus often cannot empty the swallowed
food into the stomach for many hours, instead of the few seconds that is the normal
time. Over months and years, the esophagus becomes tremendously enlarged until
it often can hold as much as 1 liter of food, which often becomes putridly infected
during the long periods of esophageal stasis. The

during the long periods of esophageal stasis. The infection may also cause ulcera-
tion of the esophageal mucosa, sometimes leading to severe substernal pain or even
rupture and death. Considerable beneﬁt can be achieved by stretching the lower
end of the esophagus by means of a balloon inﬂated on the end of a swallowed
esophageal tube. Antispasmotic drugs (drugs that relax smooth muscle) can also be
helpful.
Gastritis—Inﬂammation of the Gastric Mucosa. Mild to moderate

nﬂammation of the Gastric Mucosa. Mild to moderate chronic gastritis is
exceedingly common in the population as a whole, especially in the middle to later
years of adult life.
820
Unit XII
The inﬂammation of gastritis may be only superﬁcial
and therefore not very harmful, or it can penetrate
deeply into the gastric mucosa, in many long-standing
cases causing almost complete atrophy of the gastric
mucosa. In a few cases, gastritis can be acute and severe,
with ulcerati

, gastritis can be acute and severe,
with ulcerative excoriation of the stomach mucosa by
the stomach’s own peptic secretions.
Research suggests that much gastritis is caused 
by chronic bacterial infection of the gastric mucosa.
This often can be treated successfully by an intensive
regimen of antibacterial therapy.
In addition,certain ingested irritant substances can be
especially damaging to the protective gastric mucosal
barrier—that is, to the mucous glands and to the tight
epithelial junct

he mucous glands and to the tight
epithelial junctions between the gastric lining cells—
often leading to severe acute or chronic gastritis.Two of
the most common of these substances are excesses of
alcohol or aspirin.
Gastric Barrier and Its Penetration in Gastritis. Absorp-
tion of food from the stomach directly into the blood is
normally slight. This low level of absorption is mainly
caused by two speciﬁc features of the gastric mucosa:
(1) it is lined with highly resistant mucous cells that

 is lined with highly resistant mucous cells that
secrete a viscid and adherent mucus and (2) it has tight
junctions between the adjacent epithelial cells. These
two together plus other impediments to gastric absorp-
tion are called the “gastric barrier.”
The gastric barrier normally is resistant enough to dif-
fusion so that even the highly concentrated hydrogen
ions of the gastric juice, averaging about 100,000 times
the concentration of hydrogen ions in plasma, seldom
diffuse even to the slig

en ions in plasma, seldom
diffuse even to the slightest extent through the lining
mucus as far as the epithelial membrane itself. In gas-
tritis, the permeability of the barrier is greatly increased.
The hydrogen ions do then diffuse into the stomach
epithelium, creating additional havoc and leading to a
vicious circle of progressive stomach mucosal damage
and atrophy. It also makes the mucosa susceptible to
digestion by the peptic digestive enzymes, thus fre-
quently resulting in gastric ulcer.

mes, thus fre-
Gastric Atrophy. In many people who have chronic gas-
tritis, the mucosa gradually becomes more and more
atrophic until little or no gastric gland digestive secre-
tion remains. It is also believed that some people
develop autoimmunity against the gastric mucosa, this
also leading eventually to gastric atrophy. Loss of the
stomach secretions in gastric atrophy leads to achlorhy-
dria and, occasionally, to pernicious anemia.
Achlorhydria 
(and 
H

nally, to pernicious anemia.
(and 
Hypochlorhydria). Achlorhydria
means simply that the stomach fails to secrete
hydrochloric acid; it is diagnosed when the pH of the
gastric secretions fails to decrease below 6.5 after
maximal stimulation. Hypochlorhydria means dimin-
ished acid secretion. When acid is not secreted, pepsin
also usually is not secreted; even when it is, the lack of
acid prevents it from functioning because pepsin
requires an acid medium for activity.
Pernicious Ane

quires an acid medium for activity.
Pernicious Anemia in Gastric Atrophy. Pernicious
anemia is a common accompaniment of gastric atrophy
and achlorhydria. Normal gastric secretions contain a
glycoprotein called intrinsic factor, secreted by the same
parietal cells that secrete hydrochloric acid. Intrinsic
factor must be present for adequate absorption of
vitamin B12 from the ileum.That is, intrinsic factor com-
bines with vitamin B12 in the stomach and protects it
from being digested and destroy

ch and protects it
from being digested and destroyed as it passes into the
small intestine. Then, when the intrinsic factor–vitamin
B12 complex reaches the terminal ileum, the intrinsic
factor binds with receptors on the ileal epithelial
surface. This in turn makes it possible for the vitamin
B12 to be absorbed.
In the absence of intrinsic factor, only about 1/50 of
the vitamin B12 is absorbed. And, without intrinsic
factor, an adequate amount of vitamin B12 is not made
available from the foods 

 vitamin B12 is not made
available from the foods to cause young, newly forming
red blood cells to mature in the bone marrow.The result
is pernicious anemia. This is discussed in more detail in
Chapter 32.
A peptic ulcer is an excoriated area of stomach or intes-
tinal mucosa caused principally by the digestive action
of gastric juice or upper small intestinal secretions.
Figure 66–1 shows the points in the gastrointestinal tract
at which peptic ulcers most frequently occur, demon-

which peptic ulcers most frequently occur, demon-
strating that the most frequent site is within a few 
centimeters of the pylorus. In addition, peptic ulcers fre-
quently occur along the lesser curvature of the antral
end of the stomach or, more rarely, in the lower end of
the esophagus where stomach juices frequently reﬂux.
A type of peptic ulcer called a marginal ulcer also often
occurs wherever a surgical opening such as a gastro-
jejunostomy has been made between the stomach and
the jejunum

 has been made between the stomach and
the jejunum of the small intestine.
Basic Cause of Peptic Ulceration. The usual cause of peptic
ulceration is an imbalance between the rate of secretion
of gastric juice and the degree of protection afforded by
(1) the gastroduodenal mucosal barrier and (2) the neu-
tralization of the gastric acid by duodenal juices. It will
be recalled that all areas normally exposed to gastric
juice are well supplied with mucous glands, beginning
with compound mucous glan

mucous glands, beginning
with compound mucous glands in the lower esophagus
plus the mucous cell coating of the stomach mucosa, the
mucous neck cells of the gastric glands, the deep pyloric
glands that secrete mainly mucus, and, ﬁnally, the glands
of Brunner of the upper duodenum, which secrete a
highly alkaline mucus.
In addition to the mucus protection of the mucosa,
the duodenum is protected by the alkalinity of the small
Causes:
   1. High acid and peptic content
   2. Irritation
   3. Poor 

id and peptic content
   3. Poor blood supply
   4. Poor secretion of mucus
   5. Infection, H. pylori
Cardia
Marginal
ulcer
Pylorus
Ulcer
sites
Figure 66–1
Peptic ulcer. H. pylori, Helicobacter pylori.
Chapter 66
Physiology of Gastrointestinal Disorders
821
intestinal secretions. Especially important is pancreatic
secretion, which contains large quantities of sodium
bicarbonate that neutralize the hydrochloric acid of the
gastric juice, thus also inactivating pepsin and prevent

c juice, thus also inactivating pepsin and prevent-
ing digestion of the mucosa. In addition, large amounts
of bicarbonate ions are provided in (1) the secretions of
the large Brunner’s glands in the ﬁrst few centimeters
of the duodenal wall and (2) in bile coming from the
liver.
Finally, two feedback control mechanisms normally
ensure that this neutralization of gastric juices is com-
plete, as follows:
1. When excess acid enters the duodenum, it reﬂexly
inhibits gastric secretion and peristals

t reﬂexly
inhibits gastric secretion and peristalsis in the
stomach, both by nervous reﬂexes and by hormonal
feedback from the duodenum, thereby decreasing
the rate of gastric emptying.
2. The presence of acid in the small intestine liberates
secretin from the intestinal mucosa, which then
passes by way of the blood to the pancreas to
promote rapid secretion of pancreatic juice. This
juice also contains a high concentration of sodium
bicarbonate, thus making still more sodium
bicarbonate availab

 thus making still more sodium
bicarbonate available for neutralization of the acid.
Therefore, a peptic ulcer can be caused in either of
two ways: (1) excess secretion of acid and pepsin by the
gastric mucosa or (2) diminished ability of the gastro-
duodenal mucosal barrier to protect against the diges-
tive properties of the stomach acid–pepsin secretion.
Speciﬁc Causes of Peptic Ulcer in
the Human Being
Bacterial Infection by Helicobacter pylori Breaks Down the Gas-
troduodenal Mucosal Barrie

ri Breaks Down the Gas-
troduodenal Mucosal Barrier. Many peptic ulcer patients
have been found to have chronic infection of the ter-
minal portions of the gastric mucosa and initial portions
of the duodenal mucosa, infection most often caused by
the bacterium Helicobacter pylori. Once this infection
begins, it can last a lifetime unless it is eradicated by
antibacterial therapy. Furthermore, the bacterium is
capable of penetrating the mucosal barrier both by
virtue of its physical capability to

rrier both by
virtue of its physical capability to burrow through the
barrier and by releasing bacterial digestive enzymes
that liquefy the barrier. As a result, the strong acidic
digestive juices of the stomach secretions can then pen-
etrate into the underlying epithelium and literally digest
the gastrointestinal wall,
thus leading to peptic 
ulceration.
Other Causes of Ulceration. In many people who have
peptic ulcer in the initial portion of the duodenum, the
rate of gastric acid secretion i

the duodenum, the
rate of gastric acid secretion is greater than normal,
sometimes as much as twice normal. Although part of
this increased secretion may be stimulated by bacterial
infection, studies in both animals and human beings
have shown that excess secretion of gastric juices for
any reason (for instance, even in psychic disturbances)
may cause peptic ulceration.
Other factors that predispose to ulcers include (1)
smoking, presumably because of increased nervous
stimulation of the stomach

se of increased nervous
stimulation of the stomach secretory glands; (2) alcohol,
because it tends to break down the mucosal barrier; and
(3) aspirin and other non-steroidal anti-inﬂammatory
drugs that also have a strong propensity for breaking
down this barrier.
Physiology of Treatment. Since discovery of the bacterial
infectious basis for much peptic ulceration, therapy has
changed immensely. Initial reports are that almost all
patients with peptic ulceration can be treated effectively
by two 

ptic ulceration can be treated effectively
by two measures: (1) use of antibiotics along with other
agents to kill infectious bacteria and (2) administration
of an acid-suppressant drug, especially ranitidine, an
antihistaminic that blocks the stimulatory effect of his-
tamine on gastric gland histamine2 receptors, thus
reducing gastric acid secretion by 70 to 80 per cent.
In the past, before these approaches to peptic ulcer
therapy were developed, it was often necessary to
remove as much as fou

d, it was often necessary to
remove as much as four ﬁfths of the stomach, thus
reducing stomach acid–peptic juices enough to cure
most patients.Another therapy was to cut the two vagus
nerves that supply parasympathetic stimulation to the
gastric glands. This blocked almost all secretion of acid
and pepsin and often cured the ulcer or ulcers within 1
week after the operation was performed. However,
much of the basal stomach secretion returned after a
few months, and in many patients the ulcer al

er a
few months, and in many patients the ulcer also
returned.
The newer physiologic approaches to therapy may
prove to be miraculous. Even so, in a few instances, the
patient’s condition is so severe—including massive
bleeding from the ulcer—that heroic operative proce-
dures often must still be used.
Disorders of the 
Abnormal Digestion of Food in the
Small Intestine—Pancreatic Failure
A serious cause of abnormal digestion is failure of the
pancreas to secrete pancreatic juice 

ilure of the
pancreas to secrete pancreatic juice into the small intes-
tine. Lack of pancreatic secretion frequently occurs (1)
in pancreatitis (which is discussed later), (2) when the
pancreatic duct is blocked by a gallstone at the papilla
of Vater, or (3) after the head of the pancreas has been
removed because of malignancy.
Loss of pancreatic juice means loss of trypsin, chy-
motrypsin, carboxypolypeptidase, pancreatic amylase,
pancreatic lipase, and still a few other digestive
enzymes.With

pase, and still a few other digestive
enzymes.Without these enzymes, as much as 60 per cent
of the fat entering the small intestine may be unab-
sorbed, as well as one third to one half of the proteins
and carbohydrates. As a result, large portions of the
ingested food cannot be used for nutrition, and copious,
fatty feces are excreted.
Pancreatitis. Pancreatitis means inﬂammation of the
pancreas, and this can occur in the form of either acute
pancreatitis or chronic pancreatitis.
The most commo

ncreatitis or chronic pancreatitis.
The most common cause of pancreatitis is drinking
excess alcohol, and the second most common cause is
blockage of the papilla of Vater by a gallstone; the two
together account for more than 90 per cent of all cases.
When a gallstone blocks the papilla of Vater, this blocks
the main secretory duct from the pancreas as well as the
common bile duct. The pancreatic enzymes are then
dammed up in the ducts and acini of the pancreas. Even-
tually, so much trypsinogen

of the pancreas. Even-
tually, so much trypsinogen accumulates that it over-
comes the trypsin inhibitor in the secretions, and a small
quantity of trypsinogen becomes activated to form
trypsin. Once this happens, the trypsin activates still
more trypsinogen as well as chymotrypsinogen and car-
boxypolypeptidase, resulting in a vicious circle until
most of the proteolytic enzymes in the pancreatic ducts
822
Unit XII
and acini become activated. These enzymes rapidly
di

d acini become activated. These enzymes rapidly
digest large portions of the pancreas itself, sometimes
completely and permanently destroying the ability of
the pancreas to secrete digestive enzymes.
Malabsorption by the Small
Intestinal Mucosa—Sprue
Occasionally, nutrients are not adequately absorbed
from the small intestine even though the food has
become well digested. Several diseases can cause
decreased absorption by the mucosa; they are often
classiﬁed together under the general term “spru

en
classiﬁed together under the general term “sprue.” Mal-
absorption also can occur when large portions of the
small intestine have been removed.
Nontropical Sprue. One type of sprue, called variously
idiopathic sprue, celiac disease (in children), or gluten
enteropathy, results from the toxic effects of gluten
present in certain types of grains, especially wheat and
rye. Only some people are susceptible to this effect, but
in those who are susceptible, gluten has a direct destruc-
tive effect 

eptible, gluten has a direct destruc-
tive effect on intestinal enterocytes. In milder forms of
the disease, only the microvilli of the absorbing entero-
cytes on the villi are destroyed, thus decreasing the
absorptive surface area as much as twofold. In the more
severe forms, the villi themselves become blunted or
disappear altogether, thus still further reducing the
absorptive area of the gut. Removal of wheat and rye
ﬂour from the diet frequently results in cure within
weeks, especially in ch

tly results in cure within
weeks, especially in children with this disease.
Tropical Sprue. A different type of sprue called tropical
sprue frequently occurs in the tropics and can often be
treated with antibacterial agents. Even though no spe-
ciﬁc bacterium has been implicated as the cause, it is
believed that this variety of sprue is usually caused by
inﬂammation of the intestinal mucosa resulting from
unidentiﬁed infectious agents.
Malabsorption in Sprue. In the early stages of sprue, intes-

ion in Sprue. In the early stages of sprue, intes-
tinal absorption of fat is more impaired than absorption
of other digestive products. The fat that appears in the
stools is almost entirely in the form of salts of fatty acids
rather than undigested fat, demonstrating that the
problem is one of absorption, not of digestion. In fact,
the condition is frequently called steatorrhea, which
means simply excess fats in the stools.
In very severe cases of sprue, in addition to malab-
sorption of fats t

of sprue, in addition to malab-
sorption of fats there is also impaired absorption of pro-
teins, carbohydrates, calcium, vitamin K, folic acid, and
vitamin B12. As a result, the person suffers (1) severe
nutritional deﬁciency, often developing wasting of the
body; (2) osteomalacia (demineralization of the bones
because of lack of calcium); (3) inadequate blood coag-
ulation caused by lack of vitamin K; and (4) macrocytic
anemia of the pernicious anemia type, owing to dimin-
ished vitamin B12 an

 anemia type, owing to dimin-
ished vitamin B12 and folic acid absorption.
Constipation means slow movement of feces through the
large intestine; it is often associated with large quantities
of dry, hard feces in the descending colon that accumu-
late because of over-absorption of ﬂuid. Any pathology
of the intestines that obstructs movement of intestinal
contents, such as tumors, adhesions that constrict the
intestines, or ulcers, can cause constip

trict the
intestines, or ulcers, can cause constipation. A frequent
functional cause of constipation is irregular bowel
habits that have developed through a lifetime of inhibi-
tion of the normal defecation reﬂexes.
Infants are seldom constipated, but part of their train-
ing in the early years of life requires that they learn to
control defecation; this control is effected by inhibiting
the natural defecation reﬂexes. Clinical experience
shows that if one does not allow defecation to occur
when

hat if one does not allow defecation to occur
when the defecation reﬂexes are excited or if one over-
uses laxatives to take the place of natural bowel func-
tion, the reﬂexes themselves become progressively less
strong over months or years, and the colon becomes
atonic. For this reason, if a person establishes regular
bowel habits early in life, usually defecating in the
morning after breakfast when the gastrocolic and duo-
denocolic reﬂexes cause mass movements in the large
intestine, the deve

se mass movements in the large
intestine, the development of constipation in later life is
much less likely.
Constipation can also result from spasm of a small
segment of the sigmoid colon. It should be recalled that
motility even normally is weak in the large intestine, so
that even a slight degree of spasm is often capable of
causing serious constipation. After the constipation has
continued for several days and excess feces have accu-
mulated above a spastic sigmoid colon, excessive
colonic s

above a spastic sigmoid colon, excessive
colonic secretions often then lead to a day or so of diar-
rhea. After this, the cycle begins again, with repeated
bouts of alternating constipation and diarrhea.
Megacolon. Occasionally, constipation is so severe that
bowel movements occur only once every several days
or sometimes only once a week.This allows tremendous
quantities of fecal matter to accumulate in the colon,
causing the colon sometimes to distend to a diameter of
3 to 4 inches. The condit

distend to a diameter of
3 to 4 inches. The condition is called megacolon, or
Hirschsprung’s disease.
A frequent cause of megacolon is lack of or deﬁciency
of ganglion cells in the myenteric plexus in a segment
of the sigmoid colon. As a consequence, neither defeca-
tion reﬂexes nor strong peristaltic motility can occur in
this area of the large intestine. The sigmoid itself
becomes small and almost spastic while feces accumu-
late proximal to this area, causing megacolon in the
ascending, trans

is area, causing megacolon in the
ascending, transverse, and descending colons.
Diarrhea
Diarrhea results from rapid movement of fecal matter
through the large intestine. Several causes of diarrhea
with important physiologic sequelae are the following.
Enteritis. Enteritis means inﬂammation usually caused
either by a virus or by bacteria in the intestinal tract. In
usual infectious diarrhea, the infection is most extensive
in the large intestine and the distal end of the ileum.
Everywhere the in

and the distal end of the ileum.
Everywhere the infection is present, the mucosa
becomes extensively irritated, and its rate of secretion
becomes greatly enhanced. In addition, motility of the
intestinal wall usually increases manyfold. As a result,
large quantities of ﬂuid are made available for washing
the infectious agent toward the anus, and at the same
time strong propulsive movements propel this ﬂuid
Chapter 66
823
forward.This is an important mecha

l Disorders
823
forward.This is an important mechanism for ridding the
intestinal tract of a debilitating infection.
Of special interest is diarrhea caused by cholera (and
less often by other bacteria such as some pathogenic
colon bacilli).As explained in Chapter 65, cholera toxin
directly stimulates excessive secretion of electrolytes
and ﬂuid from the crypts of Lieberkühn in the distal
ileum and colon. The amount can be 10 to 12 liters 
per day, although the colon can usually reabsorb a
maximu

, although the colon can usually reabsorb a
maximum of only 6 to 8 liters per day.Therefore, loss of
ﬂuid and electrolytes can be so debilitating within
several days that death can ensue.
The most important physiologic basis of therapy in
cholera is to replace the ﬂuid and electrolytes as rapidly
as they are lost, mainly by giving the patient intravenous
solutions. With proper therapy, along with the use of
antibiotics, almost no cholera patients die, but without
therapy, as many as 50 per cent 

 die, but without
therapy, as many as 50 per cent do.
Psychogenic Diarrhea. Everyone is familiar with the diar-
rhea that accompanies periods of nervous tension, such
as during examination time or when a soldier is about
to go into battle. This type of diarrhea, called psy-
chogenic emotional diarrhea, is caused by excessive
stimulation of the parasympathetic nervous system,
which greatly excites both (1) motility and (2) excess
secretion of mucus in the distal colon.These two effects
added toge

s in the distal colon.These two effects
added together can cause marked diarrhea.
Ulcerative Colitis. Ulcerative colitis is a disease in which
extensive areas of the walls of the large intestine
become inﬂamed and ulcerated. The motility of the
ulcerated colon is often so great that mass movements
occur much of the day rather than for the usual 10 to 
30 minutes. Also, the colon’s secretions are greatly
enhanced.As a result, the patient has repeated diarrheal
bowel movements.
The cause of ulcera

ted diarrheal
The cause of ulcerative colitis is unknown. Some cli-
nicians believe that it results from an allergic or immune
destructive effect, but it also could result from chronic
bacterial infection not yet understood. Whatever the
cause, there is a strong hereditary tendency for suscep-
tibility to ulcerative colitis. Once the condition has pro-
gressed very far, the ulcers seldom will heal until an
ileostomy is performed to allow the small intestinal con-
tents to drain 

to allow the small intestinal con-
tents to drain to the exterior rather than to pass through
the colon. Even then the ulcers sometimes fail to heal,
and the only solution might be surgical removal of the
entire colon.
Paralysis of Defecation in Spinal
Cord Injuries
From Chapter 63 it will be recalled that defecation is
normally initiated by accumulating feces in the rectum,
which causes a spinal cord–mediated defecation reﬂex
passing from the rectum to the conus medullaris of the
spinal cord an

ctum to the conus medullaris of the
spinal cord and then back to the descending colon,
sigmoid, rectum, and anus.
When the spinal cord is injured somewhere between
the conus medullaris and the brain, the voluntary
portion of the defecation act is blocked while the basic
cord reﬂex for defecation is still intact. Nevertheless,
loss of the voluntary aid to defecation—that is, loss of
the increased abdominal pressure and relaxation of the
voluntary anal sphincter—often makes defecation a dif-
ﬁcult

anal sphincter—often makes defecation a dif-
ﬁcult process in the person with this type of upper cord
injury. But, because the cord defecation reﬂex can still
occur, a small enema to excite action of this cord reﬂex,
usually given in the morning shortly after a meal, can
often cause adequate defecation. In this way, people
with spinal cord injuries that do not destroy the conus
medullaris of the spinal cord can usually control their
bowel movements each day.
General Disorders of the
Gastrointest

ts each day.
Vomiting
Vomiting is the means by which the upper gastroin-
testinal tract rids itself of its contents when almost any
part of the upper tract becomes excessively irritated,
overdistended, or even overexcitable. Excessive disten-
tion or irritation of the duodenum provides an espe-
cially strong stimulus for vomiting.
The sensory signals that initiate vomiting originate
mainly from the pharynx, esophagus, stomach, and
upper portions of

pharynx, esophagus, stomach, and
upper portions of the small intestines. And the nerve
impulses are transmitted, as shown in Figure 66–2, by
both vagal and sympathetic afferent nerve ﬁbers to mul-
Chemoreceptor
trigger zone
“Vomiting center”
Vagal
afferents
Vagal
afferents
Apomorphine, morphine
Sympathetic afferents
Figure 66–2
Neutral connections of the “vomiting center.” This so-called vom-
iting center includes multiple sensory, motor, and control nuclei
mainly in the medullary and pontile re

trol nuclei
mainly in the medullary and pontile reticular formation but also
extending into the spinal cord.
824
Unit XII
tiple distributed nuclei in the brain stem that all
together are called the “vomiting center.” From here,
motor impulses that cause the actual vomiting are trans-
mitted from the vomiting center by way of the 5th, 7th,
9th, 10th, and 12th cranial nerves to the upper gastroin-
testinal tract, through vagal and sympathetic nerves to
the lower tract, 

 vagal and sympathetic nerves to
the lower tract, and through spinal nerves to the
diaphragm and abdominal muscles.
Antiperistalsis, the Prelude to Vomiting. In the early stages
of excessive gastrointestinal irritation or overdistention,
antiperistalsis begins to occur often many minutes
before vomiting appears.Antiperistalsis means peristal-
sis up the digestive tract rather than downward. This
may begin as far down in the intestinal tract as the
ileum, and the antiperistaltic wave travels back

e
ileum, and the antiperistaltic wave travels backward up
the intestine at a rate of 2 to 3 cm/sec; this process 
can actually push a large share of the lower small intes-
tine contents all the way back to the duodenum 
and stomach within 3 to 5 minutes.Then, as these upper
portions of the gastrointestinal tract, especially the duo-
denum,
become overly distended,
this distention
becomes the exciting factor that initiates the actual
vomiting act.
At the onset of vomiting, strong intrinsic contra

At the onset of vomiting, strong intrinsic contractions
occur in both the duodenum and the stomach, along
with partial relaxation of the esophageal-stomach
sphincter, thus allowing vomitus to begin moving from
the stomach into the esophagus. From here, a speciﬁc
vomiting act involving the abdominal muscles takes
over and expels the vomitus to the exterior, as explained
in the next paragraph.
Vomiting Act. Once the vomiting center has been sufﬁ-
ciently stimulated and the vomiting act instituted

ciently stimulated and the vomiting act instituted, the
ﬁrst effects are (1) a deep breath, (2) raising of the hyoid
bone and larynx to pull the upper esophageal sphincter
open, (3) closing of the glottis to prevent vomitus ﬂow
into the lungs, and (4) lifting of the soft palate to close
the posterior nares. Next comes a strong downward con-
traction of the diaphragm along with simultaneous con-
traction of all the abdominal wall muscles.This squeezes
the stomach between the diaphragm and the abd

ezes
the stomach between the diaphragm and the abdominal
muscles, building the intragastric pressure to a high
level. Finally, the lower esophageal sphincter relaxes
completely, allowing expulsion of the gastric contents
upward through the esophagus.
Thus, the vomiting act results from a squeezing action
of the muscles of the abdomen associated with simulta-
neous contraction of the stomach wall and opening of
the esophageal sphincters so that the gastric contents
can be expelled.
“Chemoreceptor

e gastric contents
“Chemoreceptor Trigger Zone” in the Brain Medulla for Initiation
of Vomiting by Drugs or by Motion Sickness. Aside from the
vomiting initiated by irritative stimuli in the gastroin-
testinal tract itself, vomiting can also be caused by
nervous signals arising in areas of the brain. This is par-
ticularly true for a small area located bilaterally on the
ﬂoor of the fourth ventricle called the chemoreceptor
trigger zone for vomiting. Electrical stimulation of th

er zone for vomiting. Electrical stimulation of this
area can initiate vomiting; but, more important, admin-
istration of certain drugs, including apomorphine, mor-
phine, and some digitalis derivatives, can directly
stimulate this chemoreceptor trigger zone and initiate
vomiting. Destruction of this area blocks this type of
vomiting but does not block vomiting resulting from
irritative stimuli in the gastrointestinal tract itself.
Also, it is well known that rapidly changing direction
or rhythm

ll known that rapidly changing direction
or rhythm of motion of the body can cause certain
people to vomit. The mechanism for this is the follow-
ing: The motion stimulates receptors in the vestibular
labyrinth of the inner ear, and from here impulses are
transmitted mainly by way of the brain stem vestibular
nuclei into the cerebellum, then to the chemoreceptor
trigger zone, and ﬁnally to the vomiting center to cause
vomiting.
Nausea
Everyone has experienced the sensation of nausea and
knows th

s experienced the sensation of nausea and
knows that it is often a prodrome of vomiting. Nausea
is the conscious recognition of subconscious excitation
in an area of the medulla closely associated with or part
of the vomiting center, and it can be caused by (1) irri-
tative impulses coming from the gastrointestinal tract,
(2) impulses that originate in the lower brain associated
with motion sickness, or (3) impulses from the cerebral
cortex to initiate vomiting.Vomiting occasionally occurs
witho

itiate vomiting.Vomiting occasionally occurs
without the prodromal sensation of nausea, indicating
that only certain portions of the vomiting center are
associated with the sensation of nausea.
The gastrointestinal tract can become obstructed at
almost any point along its course, as shown in Figure
66–3. Some common causes of obstruction are (1)
cancer, (2) ﬁbrotic constriction resulting from ulceration
or from peritoneal adhesions, (3) spasm of a segment of
the gut,

neal adhesions, (3) spasm of a segment of
the gut, and (4) paralysis of a segment of the gut.
The abnormal consequences of obstruction de-
pend on the point in the gastrointestinal tract that
becomes obstructed. If the obstruction occurs at the
pylorus, which results often from ﬁbrotic constriction
after peptic ulceration, persistent vomiting of stomach
contents occurs. This depresses bodily nutrition; it 
also causes excessive loss of hydrogen ions from the
stomach and can result in various deg

ons from the
stomach and can result in various degrees of whole-
body alkalosis.
If the obstruction is beyond the stomach, antiperi-
staltic reﬂux from the small intestine causes intestinal
juices to ﬂow backward into the stomach, and these
Causes
   1. Cancer
   2. Ulcer
   3. Spasm
   4. Paralytic ileus
   5. Adhesions
High obstruction
causes extreme
vomiting
Obstruction at pylorus
causes acid vomitus
Obstruction below
duodenum causes
neutral or basic
vomitus
Low obstruction
con

r basic
vomitus
constipation with
less vomiting
Figure 66–3
Obstruction in different parts of the gastrointestinal tract.
Chapter 66
825
juices are vomited along with the stomach secretions. In
this instance, the person loses large amounts of water
and electrolytes, so that he or she becomes severely
dehydrated, but the loss of acid from the stomach and
base from the small intestine may be approximately
equal, so that little

testine may be approximately
equal, so that little change in acid-base balance occurs.
If the obstruction is near the distal end of the large
intestine, feces can accumulate in the colon for a week
or more.The patient develops an intense feeling of con-
stipation, but at ﬁrst vomiting is not severe. After the
large intestine has become completely ﬁlled and it
ﬁnally becomes impossible for additional chyme to
move from the small intestine into the large intestine,
severe vomiting does then occur.

 large intestine,
severe vomiting does then occur. Prolonged obstruction
of the large intestine can ﬁnally causes rupture of the
intestine itself or dehydration and circulatory shock
resulting from the severe vomiting.
Gases in the Gastrointestinal 
Tract; “Flatus”
Gases, called ﬂatus, can enter the gastrointestinal tract
from three sources: (1) swallowed air, (2) gases formed
in the gut as a result of bacterial action, or (3) gases that
diffuse from the blood into the gastrointestinal tract.
Mo

from the blood into the gastrointestinal tract.
Most gases in the stomach are mixtures of nitrogen and
oxygen derived from swallowed air. In the typical
person these gases are expelled by belching. Only small
amounts of gas normally occur in the small intestine,
and much of this gas is air that passes from the stomach
into the intestinal tract.
In the large intestine, most of the gases are derived
from bacterial action, including especially carbon
dioxide, methane, and hydrogen. When methane and

n
hydrogen become suitably mixed with oxygen, an actual
explosive mixture is sometimes formed. Use of the elec-
tric cautery during sigmoidoscopy has been known to
cause a mild explosion.
Certain foods are known to cause greater expulsion
of ﬂatus through the anus than others—beans, cabbage,
onion, cauliﬂower, corn, and certain irritant foods 
such as vinegar. Some of these foods serve as a 
suitable medium for gas-forming bacteria, especially
una

le medium for gas-forming bacteria, especially
unabsorbed fermentable types of carbohydrates. For
instance, beans contain an indigestible carbohydrate
that passes into the colon and becomes a superior food
for colonic bacteria.But in other instances,excess expul-
sion of gas results from irritation of the large intestine,
which promotes rapid peristaltic expulsion of gases
through the anus before they can be absorbed.
The amount of gases entering or forming in the large
intestine each day averag

 or forming in the large
intestine each day averages 7 to 10 liters, whereas the
average amount expelled through the anus is usually
only about 0.6 liter.The remainder is normally absorbed
into the blood through the intestinal mucosa and
expelled through the lungs.
Berkes J,Viswanathan VK, Savkovic SD, Hecht G: Intestinal
epithelial responses to enteric pathogens: effects on the
tight junction barrier, ion transport, and inﬂammation.
Gut 52:439, 2003.
Blaser MJ, Atherton JC: Helicobac

ut 52:439, 2003.
Blaser MJ, Atherton JC: Helicobacter pylori persistence:
biology and disease. J Clin Invest 113:321, 2004.
Dierkes J, Ebert M, Malfertheiner P, Luley C: Helicobacter
pylori infection, vitamin B12 and homocysteine. A review.
Dig Dis 21:237, 2003.
Egan LJ, Sandborn WJ: Advances in the treatment of
Crohn’s disease. Gastroenterology 126:1574, 2004.
Elson CO: Genes, microbes, and T cells—new therapeutic
targets in Crohn’s disease. N Engl J Med 346(8):614, 2002.
Itzkowitz SH, Yio X: I

ngl J Med 346(8):614, 2002.
Itzkowitz SH, Yio X: Inﬂammation and cancer. IV. Colorec-
tal cancer in inﬂammatory bowel disease: the role of
inﬂammation. Am J Physiol Gastrointest Liver Physiol
287:G7, 2004
Kapadia CR: Gastric atrophy, metaplasia, and dysplasia: a
clinical perspective. J Clin Gastroenterol 36(5 Suppl):S29,
2003
malian colon: mechanisms and implications f

e mam-
2001.
Larson DW, Pemberton JH: Current concepts and contro-
versies in surgery for IBD. Gastroenterology 126:1611,
2004.
Ming SC, Goldman H: Pathology of the Gastrointestinal
Tract. Baltimore: Williams and Wilkins, 1998.
Podolsky DK: Inﬂammatory bowel 

and Wilkins, 1998.
Podolsky DK: Inﬂammatory bowel disease. N Engl J Med
347:417, 2002.
Puri P, Shinkai T: Pathogenesis of Hirschsprung’s disease and
its variants: recent progress. Semin Pediatr Surg 13:18,
2004.
Spirt MJ: Stress-related mucosal disease: risk factors and
prophylactic therapy. Clin Ther 26:197, 2004.
Suerbaum S, Michetti P: Helicobacter pylori infection.
N Engl J Med 347(15):1175, 2002.
Wolfe MM, Lichtenstein DR, Singh G: Gastrointestinal tox-
icity of nonsteroidal antiinﬂammatory

estinal tox-
icity of nonsteroidal antiinﬂammatory drugs. N Engl J
Med 340(24):1888, 1999.
Wood JD: Neuropathophysiology of irritable bowel syn-
drome. J Clin Gastroenterol 35(1 Suppl):S11, 2002.
Wright EM, Martin MG, Turk E: Intestinal absorption in
health and disease—sugars. Best Pract Res Clin Gas-
troenterol 17:943, 2003.
U
N
I
T
XIII
Metabolism and
67. Metabolism of Carbohydrates, and Formation 
of Adenosine Triphosphate
68. Lipid Metabolism
69. Protein Metabolism
70.

te
70. The Liver as an Organ
71. Dietary Balances; Regulation of Feeding; 
Obesity and Starvation; Vitamins and Minerals
72. Energetics and Metabolic Rate
73. Body Temperature, Temperature Regulation, 
and Fever
C
H
A
P
T
E
R
 
6
7
829
The next few chapters deal with metabolism in the
body, which means the chemical processes that make it
possible for the cells to continue living. It i

it
possible for the cells to continue living. It is not the
purpose of this textbook to present the chemical details
of all the various cellular reactions, because this lies in
the discipline of biochemistry. Instead, these chapters
are devoted to (1) a review of the principal chemical
processes of the cell and (2) an analysis of their physio-
logic implications, especially the manner in which they
ﬁt into the overall concept of homeostasis.
Release of Energy from Foods, and the Concept of 
“Fre

ase of Energy from Foods, and the Concept of 
“Free Energy”
A great proportion of the chemical reactions in the cells is concerned with making
the energy in foods available to the various physiologic systems of the cell. For
instance, energy is required for muscle activity, secretion by the glands, maintenance
of membrane potentials by the nerve and muscle ﬁbers, synthesis of substances 
in the cells, absorption of foods from the gastrointestinal tract, and many other 
Coupled Reactio

 tract, and many other 
Coupled Reactions. All the energy foods—carbohydrates, fats, and proteins—can be
oxidized in the cells, and during this process, large amounts of energy are released.
These same foods can also be burned with pure oxygen outside the body in an actual
ﬁre, also releasing large amounts of energy; in this case, however, the energy is
released suddenly, all in the form of heat. The energy needed by the physiologic
processes of the cells is not heat but energy to cau

ocesses of the cells is not heat but energy to cause mechanical movement in the
case of muscle function, to concentrate solutes in the case of glandular secretion,
and to effect other functions.To provide this energy, the chemical reactions must be
“coupled” with the systems responsible for these physiologic functions. This cou-
pling is accomplished by special cellular enzyme and energy transfer systems, some
of which are explained in this and subsequent chapters.
“Free Energy.” The amount of e

ubsequent chapters.
“Free Energy.” The amount of energy liberated by complete oxidation of a food is
called the free energy of oxidation of the food, and this is generally represented by
the symbol DG. Free energy is usually expressed in terms of calories per mole of
substance. For instance, the amount of free energy liberated by complete oxidation
of 1 mole (180 grams) of glucose is 686,000 calories.
Role of Adenosine Triphosphate in Metabolism
Adenosine triphosphate (ATP) is an essential link 

Adenosine triphosphate (ATP) is an essential link between energy-utilizing 
and energy-producing functions of the body (Figure 67–1). For this reason, ATP 
has been called the energy currency of the body, and it can be gained and spent
repeatedly.
Energy derived from the oxidation of carbohydrates, proteins, and fats is used to
convert adenosine diphosphate (ADP) to ATP, which is then consumed by the
various reactions of the body that are necessary for (1) active transport of mole-
cules across 

ry for (1) active transport of mole-
cules across cell membranes; (2) contraction of muscles and performance of
mechanical work; (3) various synthetic reactions that create hormones, cell mem-
branes, and many other essential molecules of the body; (4) conduction of nerve
impulses; (5) cell division and growth; and (6) many other physiologic functions that
are necessary to maintain and propagate life.
830
Unit XIII
Metabolism and Temperature Regulation
ATP is a labile chemical compound that is p

lation
ATP is a labile chemical compound that is present in
all cells. It has the chemical structure shown in Figure
67–2. From this formula, it can be seen that ATP is a
combination of adenine, ribose, and three phosphate
radicals. The last two phosphate radicals are connected
with the remainder of the molecule by high-energy
bonds, which are indicated by the symbol ~.
The amount of free energy in each of these high-
energy bonds per mole of ATP is about 7300 calories
under standard conditions 

 is about 7300 calories
under standard conditions and about 12,000 calories
under the usual conditions of temperature and concen-
trations of the reactants in the body. Therefore, in the
body, removal of each of the last two phosphate radicals
liberates about 12,000 calories of energy. After loss of
one phosphate radical from ATP, the compound
becomes ADP, and after loss of the second phosphate
radical, it becomes adenosine monophosphate (AMP).
The interconversions among ATP, ADP, and AMP are
th

he interconversions among ATP, ADP, and AMP are
ATP is present everywhere in the cytoplasm and
nucleoplasm of all cells, and essentially all the physio-
logic mechanisms that require energy for operation
obtain it directly from ATP (or another similar high-
energy compound—guanosine triphosphate [GTP]). In
turn, the food in the cells is gradually oxidized, and the
released energy is used to form new ATP, thus always
maintaining a supply of this substance. All these energy
transfer

upply of this substance. All these energy
transfers take place by means of coupled reactions.
The principal purpose of this chapter is to explain
how the energy from carbohydrates can be used to form
ATP in the cells. Normally, 90 per cent or more of all
the carbohydrates utilized by the body are used for this
purpose.
Central Role of Glucose in
As explained in Chapter 65, the ﬁnal products of car-
bohydrate digestion in the alimentary tract are almost
entirely glucose, f

he alimentary tract are almost
entirely glucose, fructose, and galactose—with glucose
representing, on average, about 80 per cent of these.
After absorption from the intestinal tract, much of the
fructose and almost all the galactose are rapidly con-
verted into glucose in the liver.Therefore, little fructose
and galactose are present in the circulating blood.
Glucose thus becomes the ﬁnal common pathway for the
transport of almost all carbohydrates to the tissue cells.
In liver cells, appropria

tes to the tissue cells.
In liver cells, appropriate enzymes are available to
promote interconversions among the monosaccha-
rides—glucose, fructose, and galactose—as shown in
Figure 67–3. Furthermore, the dynamics of the reactions
are such that when the liver releases the monosaccha-
rides back into the blood, the ﬁnal product is almost
entirely glucose.The reason for this is that the liver cells
contain large amounts of glucose phosphatase. There-
fore, glucose-6-phosphate can be degraded to g

re-
fore, glucose-6-phosphate can be degraded to glucose
ADP + Pi
ATP
Oxidation
Energy production
•Proteins
•Carbohydrates
•Fats
Energy utilization
•Active ion transport
•Muscle contraction
•Synthesis of molecules
•Cell division and growth
Figure 67–1
Adenosine triphosphate (ATP) as the central link between energy-
producing and energy-utilizing systems of the body. ADP, adeno-
sine diphosphate; Pi, inorganic phosphate.
P
O
O
O-
O-
O-
OH
OH
H
NH2
H
N
N
N
C
C
C
N
C
C
C
O
C
H
H
O-
O
O
P
O ~
~
P
O

H
N
N
N
C
C
C
N
C
C
C
O
C
H
H
O-
O
O
P
O ~
~
P
O
Adenine
Ribose
CH2
CH
HC
Figure 67–2
Chemical structure of adenosine
triphosphate (ATP).
-12,000 cal
+12,000 cal
ADP
PO3
+
ADP
2PO3
+
ATP
Chapter 67
Metabolism of Carbohydrates, and Formation of Adenosine Triphosphate
831
and phosphate, and the glucose can then be transported
through the liver cell membrane back into the blood.
Once again, it should be emphasized that usually
more than 95 per cent of all the m

ed that usually
more than 95 per cent of all the monosaccharides that
circulate in the blood are the ﬁnal conversion product,
glucose.
Transport of Glucose
Through the Cell Membrane
Before glucose can be used by the body’s tissue cells, it
must be transported through the tissue cell membrane
into the cellular cytoplasm. However, glucose cannot
easily diffuse through the pores of the cell membrane
because the maximum molecular weight of particles
that can diffuse readily is about 100, and glucose

that can diffuse readily is about 100, and glucose has a
molecular weight of 180. Yet glucose does pass to the
interior of the cells with a reasonable degree of freedom
by the mechanism of facilitated diffusion. The principles
of this type of transport are discussed in Chapter 4.Basi-
cally, they are the following. Penetrating through the
lipid matrix of the cell membrane are large numbers of
protein carrier molecules that can bind with glucose. In
this bound form, the glucose can be transported

In
this bound form, the glucose can be transported by the
carrier from one side of the membrane to the other side
and then released. Therefore, if the concentration of
glucose is greater on one side of the membrane than on
the other side, more glucose will be transported from
the high-concentration area to the low-concentration
area than in the opposite direction.
The transport of glucose through the membranes of
most tissue cells is quite different from that which
occurs through the gastrointes

ent from that which
occurs through the gastrointestinal membrane or
through the epithelium of the renal tubules. In both
these cases, the glucose is transported by the mechanism
of active sodium-glucose co-transport, in which active
transport of sodium provides energy for absorbing
glucose against a concentration difference. This sodium
co-transport mechanism functions only in certain
special epithelial cells that are speciﬁcally adapted for
active absorption of glucose. At other cell membranes,

ve absorption of glucose. At other cell membranes,
glucose is transported only from higher concentration
toward lower concentration by facilitated diffusion,
made possible by the special binding properties of mem-
brane glucose carrier protein. The details of facilitated
diffusion for cell membrane transport are presented in
Chapter 4.
Insulin Increases Facilitated
Diffusion of Glucose
The rate of glucose transport as well as transport of
some other monosaccharides is greatly increased by
insuli

her monosaccharides is greatly increased by
insulin. When large amounts of insulin are secreted by
the pancreas, the rate of glucose transport into most
cells increases to 10 or more times the rate of transport
when no insulin is secreted. Conversely, the amounts of
glucose that can diffuse to the insides of most cells of
the body in the absence of insulin, with the exception 
of liver and brain cells, are far too little to supply the
amount of glucose normally required for energy metab-
olism.

lucose normally required for energy metab-
olism.
In effect, the rate of carbohydrate utilization by most
cells is controlled by the rate of insulin secretion from
the pancreas. The functions of insulin and its control of
carbohydrate metabolism are discussed in detail in
Chapter 78.
Immediately on entry into the cells, glucose combines
with a phosphate radical in accordance with the follow-
ing reaction:
This phosphorylation is promoted mainly by the
enzyme glucokinas

lation is promoted mainly by the
enzyme glucokinase in the liver and by hexokinase in
most other cells. The phosphorylation of glucose is
almost completely irreversible except in the liver cells,
the renal tubular epithelial cells, and the intestinal
epithelial cells; in these cells, another enzyme, glucose
phosphatase, is also available, and when this is activated,
it can reverse the reaction. In most tissues of the body,
phosphorylation serves to capture the glucose in the
cell.That is, becaus

to capture the glucose in the
cell.That is, because of its almost instantaneous binding
with phosphate, the glucose will not diffuse back out,
except from those special cells, especially liver cells, that
have phosphatase.
Glycogen Is Stored in Liver
and Muscle
After absorption into a cell, glucose can be used imme-
diately for release of energy to the cell, or it can be
Glucose
Glucose-6-phosphate
glucokinase or hexokinase
ATP
æ
Æ
ææææææææ
+
ATP
ATP
ATP
Galactose
Galactose-1-phosp

ATP
ATP
Galactose
Galactose-1-phosphate
Uridine diphosphate galactose
Uridine diphosphate glucose
Glucose-1-phosphate
Fructose-6-phosphate
Glucose
Fructose
Glycogen
Figure 67–3
Interconversions of the three major monosaccharides—glucose,
fructose, and galactose—in liver cells.
832
Unit XIII
stored in the form of glycogen, which is a large polymer
of glucose.
All cells of the body are capable of storing at least
s

ells of the body are capable of storing at least
some glycogen,but certain cells can store large amounts,
especially liver cells, which can store up to 5 to 8 per
cent of their weight as glycogen, and muscle cells, which
can store up to 1 to 3 per cent glycogen. The glycogen
molecules can be polymerized to almost any molecular
weight, with the average molecular weight being 5
million or greater; most of the glycogen precipitates in
the form of solid granules.
This conversion of the monosaccharid

lid granules.
This conversion of the monosaccharides into a high-
molecular-weight precipitated compound (glycogen)
makes it possible to store large quantities of carbohy-
drates without signiﬁcantly altering the osmotic pres-
sure of the intracellular ﬂuids. High concentrations of
low-molecular-weight soluble monosaccharides would
play havoc with the osmotic relations between intracel-
lular and extracellular ﬂuids.
Glycogenesis—The Process 
of Glycogen Formation
The chemical reactions for glyc

Glycogen Formation
The chemical reactions for glycogenesis are shown in
Figure 67–4. From this ﬁgure, it can be seen that glucose-
6-phosphate can become glucose-1-phosphate; this is
converted to uridine diphosphate glucose, which is
ﬁnally converted into glycogen.
Several speciﬁc
enzymes are required to cause these conversions, and
any monosaccharide that can be converted into glucose
can enter into the reactions. Certain smaller com-
pounds, including lactic acid, glycerol, pyruvic acid, and
s

cluding lactic acid, glycerol, pyruvic acid, and
some deaminated amino acids, can also be converted
into glucose or closely allied compounds and then con-
verted into glycogen.
Glycogenolysis means the breakdown of the cell’s
stored glycogen to re-form glucose in the cells. The
glucose can then be used to provide energy. Glycogenol-
ysis does not occur by reversal of the same chemical
reactions that form glycogen; instead, each succeeding
glucose molecu

 glycogen; instead, each succeeding
glucose molecule on each branch of the glycogen
polymer is split away by phosphorylation, catalyzed by
the enzyme phosphorylase.
Under resting conditions, the phosphorylase is in an
inactive form, so that glycogen will remain stored.When
it is necessary to re-form glucose from glycogen, the
phosphorylase must ﬁrst be activated. This can be
accomplished in several ways, including the following
two.
Activation of Phosphorylase by Epinephrine or by Glucagon. Two

 Phosphorylase by Epinephrine or by Glucagon. Two
hormones, epinephrine and glucagon, can activate phos-
phorylase and thereby cause rapid glycogenolysis. The
initial effect of each of these hormones is to promote
the formation of cyclic AMP in the cells, which then 
initiates a cascade of chemical reactions that activates
the phosphorylase.This is discussed in detail in Chapter
78.
Epinephrine is released by the adrenal medullae
when the sympathetic nervous system is stimulated.
Therefore, one 

etic nervous system is stimulated.
Therefore, one of the functions of the sympathetic
nervous system is to increase the availability of glucose
for rapid energy metabolism. This function of epineph-
rine occurs markedly in both liver cells and muscle,
thereby contributing, along with other effects of sym-
pathetic stimulation, to preparing the body for action,
as discussed fully in Chapter 60.
Glucagon is a hormone secreted by the alpha cells
of the pancreas when the blood glucose concentration

the pancreas when the blood glucose concentration
falls too low. It stimulates formation of cyclic AMP
mainly in the liver cells, and this in turn promotes 
conversion of liver glycogen into glucose and its 
release into the blood, thereby elevating the blood
glucose concentration. The function of glucagon in
blood glucose regulation is discussed more fully in
Release of Energy from the
Glucose Molecule by the
Because complete oxidation of 1 gram-molecule of
glucos

se complete oxidation of 1 gram-molecule of
glucose releases 686,000 calories of energy and 
only 12,000 calories of energy are required to form 1
gram-molecule of ATP, energy would be wasted if
glucose were decomposed all at once into water and
carbon dioxide while forming only a single ATP mole-
cule. Fortunately, all cells of the body contain special
protein enzymes that cause the glucose molecule to split
a little at a time in many successive steps, so that its
energy is released in small pa

 steps, so that its
energy is released in small packets to form one mole-
cule of ATP at a time, forming a total of 38 moles of
ATP for each mole of glucose metabolized by the cells.
The next sections describe the basic principles of 
the processes by which the glucose molecule is pro-
gressively dissected and its energy released to form
ATP.
Glycogen
Glucose
Blood
glucose
(glucokinase)
(phosphatase)
(ph

cose
Blood
glucose
(phosphorylase)
Figure 67–4
Chemical reactions of glycogenesis and glycogenolysis, showing
also interconversions between blood glucose and liver glycogen.
(The phosphatase required for the release of glucose from the cell
is present in liver cells but not in most other cells.)
Chapter 67
833
Glycolysis and the Formation 
of Pyruvic Acid
By far the most important means of releasing 

Acid
By far the most important means of releasing energy
from the glucose molecule is initiated by glycolysis. The
end products of glycolysis are then oxidized to provide
energy. Glycolysis means splitting of the glucose mole-
cule to form two molecules of pyruvic acid.
Glycolysis occurs by 10 successive chemical reactions,
shown in Figure 67–5. Each step is catalyzed by at least
one speciﬁc protein enzyme. Note that glucose is ﬁrst
converted into fructose-1,6-diphosphate and then split
into two

o fructose-1,6-diphosphate and then split
into two three-carbon-atom molecules, glyceraldehyde-
3-phosphate, each of which is then converted through
ﬁve additional steps into pyruvic acid.
Formation of ATP During Glycolysis. Despite the many chem-
ical reactions in the glycolytic series, only a small
portion of the free energy in the glucose molecule is
released at most steps. However, between the 1,3-
diphosphoglyceric acid and the 3-phosphoglyceric acid
stages, and again between the phosphoeno

eric acid
stages, and again between the phosphoenolpyruvic acid
and the pyruvic acid stages, the packets of energy
released are greater than 12,000 calories per mole, the
amount required to form ATP, and the reactions are
coupled in such a way that ATP is formed. Thus, a total
of 4 moles of ATP were formed for each mole of fruc-
tose-1,6-diphosphate that is split into pyruvic acid.
Yet 2 moles of ATP were required to phosphorylate
the original glucose to form fructose-1,6-diphosphate
before glyc

ucose to form fructose-1,6-diphosphate
before glycolysis could begin. Therefore, the net gain in
ATP molecules by the entire glycolytic process is only 2
moles for each mole of glucose utilized. This amounts to
24,000 calories of energy that becomes transferred to
ATP, but during glycolysis, a total of 56,000 calories of
energy were lost from the original glucose, giving an
overall efﬁciency for ATP formation of only 43 per cent.
The remaining 57 per cent of the energy is lost in the
form of hea

 per cent of the energy is lost in the
form of heat.
Conversion of Pyruvic Acid 
to Acetyl Coenzyme A
The next stage in the degradation of glucose is a two-
step conversion of the two pyruvic acid molecules from
Figure 67–5 into two molecules of acetyl coenzyme 
A (acetyl-CoA), in accordance with the following 
reaction:
From this reaction, it can be seen that two carbon
dioxide molecules and four hydrogen atoms are
released, while the remaining portions of the two
pyruvic acid molecules combine

portions of the two
pyruvic acid molecules combine with coenzyme A, a
derivative of the vitamin pantothenic acid, to form two
molecules of acetyl-CoA. In this conversion, no ATP is
formed, but up to six molecules of ATP are formed
when the four released hydrogen atoms are later oxi-
dized, as discussed later.
Citric Acid Cycle (Krebs Cycle)
The next stage in the degradation of the glucose mole-
cule is called the citric acid cycle (also called the tricar-
boxylic acid cycle or Krebs cycle). This

e tricar-
boxylic acid cycle or Krebs cycle). This is a sequence of
chemical reactions in which the acetyl portion of acetyl-
CoA is degraded to carbon dioxide and hydrogen
atoms. These reactions all occur in the matrix of the
mitochondrion. The released hydrogen atoms add to 
the number of these atoms that will subsequently be
oxidized (as discussed later), releasing tremendous
amounts of energy to form ATP.
Figure 67–6 shows the different stages of the chemi-
cal reactions in the citric acid c

s of the chemi-
cal reactions in the citric acid cycle. The substances to
the left are added during the chemical reactions, and the
products of the chemical reactions are shown to the
right. Note at the top of the column that the cycle begins
with oxaloacetic acid, and at the bottom of the chain of
reactions, oxaloacetic acid is formed again. Thus, the
cycle can continue over and over.
In the initial stage of the citric acid cycle, acetyl-CoA
combines with oxaloacetic acid to form citric acid. T

bines with oxaloacetic acid to form citric acid. The
coenzyme A portion of the acetyl-CoA is released and
can be used again and again for the formation of still
more quantities of acetyl-CoA from pyruvic acid. The
+
2CH3
C
SH
COOH
2CoA
O
(Pyruvic acid)
(Coenzyme A)
2CH3
C
S
CoA + 2CO2 + 4H
O
(Acetyl-CoA)
ATP
ADP
ATP
ADP
2ADP
+2ATP
2ADP
2ATP
Net reaction per molecule of glucose:
Glucose + 2ADP + 2PO4
-       2 Pyruvic acid + 2ATP + 4H
4H
Glucose
Fructose-1

lucose-6-phosphate
Fructose-1,6-diphosphate
2 (1,3-Diphosphoglyceric acid)
2 (3-Phosphoglyceric acid)
2 (2-Phosphoglyceric acid)
Dihydroxyacetone phosphate
2 (Glyceraldehyde-3-phosphate)
2 (Phosphoenolpyruvic acid)
2 (Pyruvic acid)
-- 
Figure 67–5
Sequence of chemical reactions responsible for glycolysis.
834
Unit XIII
acetyl portion, however, becomes an integral part of the
citric acid molecule. During the successive stages of the
citri

olecule. During the successive stages of the
citric acid cycle, several molecules of water are added,
as shown on the left in the ﬁgure, and carbon dioxide
and hydrogen atoms are released at other stages in the
cycle, as shown on the right in the ﬁgure.
The net results of the entire citric acid cycle are given
in the explanation at the bottom of Figure 67–6, demon-
strating that for each molecule of glucose originally
metabolized, 2 acetyl-CoA molecules enter into the
citric acid cycle, along wi

lecules enter into the
citric acid cycle, along with 6 molecules of water. These
are then degraded into 4 carbon dioxide molecules, 16
hydrogen atoms, and 2 molecules of coenzyme A. Two
molecules of ATP are formed, as follows.
Formation of ATP in the Citric Acid Cycle. Not a great amount
of energy is released during the citric acid cycle itself; in
only one of the chemical reactions—during the change
from a-ketoglutaric acid to succinic acid—is a molecule
of ATP formed. Thus, for each molecule o

 molecule
of ATP formed. Thus, for each molecule of glucose
metabolized, two acetyl-CoA molecules pass through
the citric acid cycle, each forming a molecule of ATP, or
a total of two molecules of ATP formed.
Function of Dehydrogenases and Nicotinamide Adenine Dinu-
cleotide in Causing Release of Hydrogen Atoms in the Citric Acid
Cycle. As already noted at several points in this discus-
sion, hydrogen atoms are released during different
chemical reactions of the citric acid cycle—4 hydrogen
atom

reactions of the citric acid cycle—4 hydrogen
atoms during glycolysis, 4 during formation of acetyl-
CoA from pyruvic acid, and 16 in the citric acid cycle;
this makes a total of 24 hydrogen atoms released for each
original molecule of glucose. However, the hydrogen
atoms are not simply turned loose in the intracellular
ﬂuid. Instead, they are released in packets of two, and
in each instance, the release is catalyzed by a speciﬁc
protein enzyme called a dehydrogenase. Twenty of the
24 hydrogen a

alled a dehydrogenase. Twenty of the
24 hydrogen atoms immediately combine with nicoti-
namide adenine dinucleotide (NAD+), a derivative of
the vitamin niacin, in accordance with the following
reaction:
This reaction will not occur without intermediation of
the speciﬁc dehydrogenase or without the availability of
NAD+ to act as a hydrogen carrier. Both the free hydro-
gen ion and the hydrogen bound with NAD+ subse-
quently enter into multiple oxidative chemical reactions
that form tremendous qua

dative chemical reactions
that form tremendous quantities of ATP, as discussed
later.
The remaining four hydrogen atoms released during
the breakdown of glucose—the four released during the
citric acid cycle between the succinic and fumaric acid
stages—combine with a speciﬁc dehydrogenase but are
not subsequently released to NAD+. Instead, they pass
directly from the dehydrogenase into the oxidative
process.
Function of Decarboxylases in Causing Release of Carbon Dioxide.
Referring again to the 

Release of Carbon Dioxide.
Referring again to the chemical reactions of the 
citric acid cycle, as well as to those for the formation of
H
H
NADH  +  H+  +  Substrate
+  NAD+  dehydrogenase
Substrate
H2O
H2O
H2O
H2O
H2O
ADP
ATP
H2C
COOH
H2C
COOH
H2C
COOH
C
H
COOH
HC
COOH
H2C
COOH
HC
COOH
(Citric acid)
HOC
COOH
HOC
COOH
H2C
COOH
H2C
CO2
CO2
H2C
COOH
H2C
COOH
H2C
COOH
HC
COOH
HC
COOH
CH
H
HOOC
C
O
COOH
H2C
COOH
H2C
COOH
2H
2H
2H
2H
C
O
COOH
C
O
COOH
H2C
COOH
(Oxaloacetic acid)
(cis-Aconitic acid)

H
H2C
COOH
(Isocitric acid)
(Oxalosuccinic acid)
C
HO
COOH
(Malic acid)
(a-Ketoglutaric acid)
(Succinic acid)
(Fumaric acid)
(Acetyl coenzyme A)
C
O
COOH
CH3
CO
CoA
CoA
2 Acetyl-CoA + 6H2O + 2ADP
4CO2 + 16H + 2CoA + 2ATP
Figure 67–6
Chemical reactions of the citric acid cycle, showing the release of
carbon dioxide and a number of hydrogen atoms during the cycle.
Chapter 67

the cycle.
Chapter 67
835
acetyl-CoA from pyruvic acid, we ﬁnd that there are
three stages in which carbon dioxide is released. To
cause the release of carbon dioxide, other speciﬁc
protein enzymes, called decarboxylases, split the carbon
dioxide away from the substrate. The carbon dioxide is
then dissolved in the body ﬂuids and transported to the
lungs, where it is expired from the body (see Chapter
40).
Formation of Large Qua

 the body (see Chapter
40).
Formation of Large Quantities 
of ATP by Oxidation of Hydrogen
(the Process of Oxidative
Phosphorylation)
Despite all the complexities of (1) glycolysis, (2) the
citric acid cycle, (3) dehydrogenation, and (4) decar-
boxylation, pitifully small amounts of ATP are formed
during all these processes—only two ATP molecules in
the glycolysis scheme and another two in the citric acid
cycle for each molecule of glucose metabolized. Instead,
almost 90 per cent of the total AT

lized. Instead,
almost 90 per cent of the total ATP created through
glucose metabolism is formed during subsequent oxi-
dation of the hydrogen atoms that were released at
early stages of glucose degradation. Indeed, the princi-
pal function of all these earlier stages is to make the
hydrogen of the glucose molecule available in forms
that can be oxidized.
Oxidation of hydrogen is accomplished, as illustrated
in Figure 67–7, by a series of enzymatically catalyzed
reactions in the mitochondria. Th

ically catalyzed
reactions in the mitochondria. These reactions (1) split
each hydrogen atom into a hydrogen ion and an elec-
tron and (2) use the electrons eventually to combine dis-
solved oxygen of the ﬂuids with water molecules to form
hydroxyl ions. Then the hydrogen and hydroxyl ions
combine with each other to form water. During 
this sequence of oxidative reactions,
tremendous 
quantities of energy are released to form ATP. Forma-
tion of ATP in this manner is called oxidative phospho-
ry

ATP in this manner is called oxidative phospho-
rylation. This occurs entirely in the mitochondria by a
highly specialized process called the chemiosmotic
Chemiosmotic Mechanism of the
Mitochondria to Form ATP
Ionization of Hydrogen, the Electron Transport Chain, and Forma-
tion of Water. The ﬁrst step in oxidative phosphorylation
in the mitochondria is to ionize the hydrogen atoms that
have been removed from the food substrates. As
described earlier, these hydrogen atoms are removed 

scribed earlier, these hydrogen atoms are removed in
pairs: one immediately becomes a hydrogen ion, H+; the
other combines with NAD+ to form NADH. The upper
portion of Figure 67–7 shows the subsequent fate of the
NADH and H+. The initial effect is to release the other
hydrogen atom from the NADH to form another hydro-
gen ion, H+; this process also reconstitutes NAD+ that
will be reused again and again.
The electrons that are removed from the hydrogen
atoms to cause the hydrogen ionization immed

rogen
atoms to cause the hydrogen ionization immediately
enter an electron transport chain of electron acceptors
that are an integral part of the inner membrane (the
shelf membrane) of the mitochondrion. The electron
acceptors can be reversibly reduced or oxidized by
accepting or giving up electrons. The important
members of this electron transport chain include ﬂavo-
protein, several iron sulﬁde proteins, ubiquinone, and
cytochromes B, C1, C, A, and A3. Each electron is shut-
tled from one of t

 and A3. Each electron is shut-
tled from one of these acceptors to the next until it
ﬁnally reaches cytochrome A3,
which is called
cytochrome oxidase because it is capable of giving up
two electrons and thus reducing elemental oxygen to
form ionic oxygen, which then combines with hydrogen
ions to form water.
Thus, Figure 67–7 shows the transport of electrons
through the electron chain and then their ultimate 
use by cytochrome oxidase to cause the formation 
of water molecules. During the trans

he formation 
of water molecules. During the transport of these elec-
trons through the electron transport chain, energy is
released that is used to cause the synthesis of ATP, as
follows.
Pumping of Hydrogen Ions into the Outer Chamber of the Mito-
chondrion, Caused by the Electron Transport Chain. As the
electrons pass through the electron transport chain,
large amounts of energy are released. This energy is
used to pump hydrogen ions from the inner matrix of
the mitochondrion (to the right in

inner matrix of
the mitochondrion (to the right in Figure 67–7) into the
outer chamber between the inner and outer mitochon-
drial membranes (to the left). This creates a high con-
centration of positively charged hydrogen ions in this
chamber; it also creates a strong negative electrical
potential in the inner matrix.
Formation of ATP. The next step in oxidative phosphory-
lation is to convert ADP into ATP. This occurs in con-
junction with a large protein molecule that protrudes all
the way th

rge protein molecule that protrudes all
the way through the inner mitochondrial membrane
and projects with a knoblike head into the inner mito-
chondrial matrix. This molecule is an ATPase, the phys-
ical nature of which is shown in Figure 67–7. It is called
ATP synthetase.
3 ADP
3 ADP
ATP
ATP
Food substrate
6H+
6H+
6H+
6H+
2H+
2H+
2H+
2H+
2H+
2H+
3 ATP
3 ATP
Diffusion
Diffusion
Outer
membrane
Outer
membrane
Inner
membrane
Inner
membrane
diffusion
diffusion

mbrane
diffusion
diffusion
ATPase
ATPase
FMN
FMN
FeS
FeS
FeS
FeS
Q
b
C1
C1
C1
C1
a3
a3
a
-2e
-2e
NAD+
NAD+
H+
H+
H+
H+
NADH  +
NADH  +
2e + 1/2 O2
2e + 1/2 O2
H2O
H2O
ADP
ADP
Figure 67–7
Mitochondrial chemiosmotic mechanism of oxidative phosphory-
lation for forming large quantities of ATP. This ﬁgure shows the
relationship of the oxidative and phosphorylation steps at the outer
and inner membranes of the mitochondrion.
836
Unit XIII

6
Unit XIII
The high concentration of positively charged hydro-
gen ions in the outer chamber and the large electrical
potential difference across the inner membrane cause
the hydrogen ions to ﬂow into the inner mitochondrial
matrix through the substance of the ATPase molecule. In
doing so, energy derived from this hydrogen ion ﬂow is
used by ATPase to convert ADP into ATP by combin-
ing ADP with a free ionic phosphate radical (Pi), thus
adding another high-

 phosphate radical (Pi), thus
adding another high-energy phosphate bond to the 
molecule.
The ﬁnal step in the process is transfer of ATP from
the inside of the mitochondrion back to the cell cyto-
plasm. This occurs by facilitated diffusion outward
through the inner membrane and then by simple dif-
fusion through the permeable outer mitochondrial
membrane. In turn, ADP is continually transferred in
the other direction for continual conversion into ATP.
For each two electrons that pass through t

to ATP.
For each two electrons that pass through the entire elec-
tron transport chain (representing the ionization of 
two hydrogen atoms), up to three ATP molecules are
synthesized.
Summary of ATP Formation During
the Breakdown of Glucose
We can now determine the total number of ATP mole-
cules that, under optimal conditions, can be formed by
the energy from one molecule of glucose.
1. During glycolysis, four molecules of ATP are
formed, and two are expended to cause the initial
phosphorylatio

o are expended to cause the initial
phosphorylation of glucose to get the process
going. This gives a net gain of two molecules of
ATP.
2. During each revolution of the citric acid cycle,
one molecule of ATP is formed. However, because
each glucose molecule splits into two pyruvic acid
molecules, there are two revolutions of the cycle 
for each molecule of glucose metabolized, giving 
a net production of two more molecules of 
ATP.
3. During the entire schema of glucose breakdown, a
total of 24 

entire schema of glucose breakdown, a
total of 24 hydrogen atoms are released during
glycolysis and during the citric acid cycle. Twenty of
these atoms are oxidized in conjunction with the
chemiosmotic mechanism shown in Figure 67–7,
with the release of three ATP molecules per two
atoms of hydrogen metabolized. This gives an
additional 30 ATP molecules.
4. The remaining four hydrogen atoms are released
by their dehydrogenase into the chemiosmotic
oxidative schema in the mitochondrion beyond the

ﬁrst stage of Figure 67–7. Two ATP molecules are
usually released for every two hydrogen atoms
oxidized, thus giving a total of four more ATP
Now, adding all the ATP molecules formed, we 
ﬁnd a maximum of 38 ATP molecules formed for each
molecule of glucose degraded to carbon dioxide 
and water.Thus, 456,000 calories of energy can be stored
in the form of ATP, whereas 686,000 calories are
released during the complete oxidation of each 

re
released during the complete oxidation of each gram-
molecule of glucose. This represents an overall
maximum efﬁciency of energy transfer of 66 per cent.
The remaining 34 per cent of the energy becomes heat
and, therefore, cannot be used by the cells to perform
speciﬁc functions.
Control of Energy Release from
Stored Glycogen When the Body
Needs Additional Energy: Effect 
of ATP and ADP Cell Concentrations
in Controlling the Rate of Glycolysis
Continual release of energy from glucose when ene

Continual release of energy from glucose when energy
is not needed by the cells would be an extremely waste-
ful process. Instead, glycolysis and the subsequent oxi-
dation of hydrogen atoms are continually controlled in
accordance with the cells’ need for ATP. This control is
accomplished by multiple feedback control mechanisms
within the chemical schemata. Among the more impor-
tant of these are the effects of cell concentrations of
both ADP and ATP in controlling the rates of chemical
reacti

nd ATP in controlling the rates of chemical
reactions in the energy metabolism sequence.
One important way in which ATP helps control
energy metabolism is to inhibit the enzyme phospho-
fructokinase. Because this enzyme promotes the forma-
tion of fructose-1,6-diphosphate, one of the initial steps
in the glycolytic series of reactions, the net effect of
excess cellular ATP is to slow or even stop glycolysis,
which in turn stops most carbohydrate metabolism.
Conversely, ADP (and AMP as well) caus

Conversely, ADP (and AMP as well) causes the oppo-
site change in this enzyme, greatly increasing its activ-
ity. Whenever ATP is used by the tissues for energizing
a major fraction of almost all intracellular chemical
reactions, this reduces the ATP inhibition of the enzyme
phosphofructokinase and at the same time increases its
activity as a result of the excess ADP formed. Thus, the
glycolytic process is set in motion, and the total cellular
store of ATP is replenished.
Another con

 cellular
Another control linkage is the citrate ion formed in
the citric acid cycle. An excess of this ion also strongly
inhibits phosphofructokinase,
thus preventing the 
glycolytic process from getting ahead of the citric 
acid cycle’s ability to use the pyruvic acid formed during
glycolysis.
A third way by which the ATP-ADP-AMP system
controls carbohydrate metabolism, as well as control-
ling energy release from fats and proteins, is the fol-
lowing:Referring bac

ats and proteins, is the fol-
lowing:Referring back to the various chemical reactions
for energy release, we see that if all the ADP in the 
cell has already been converted into ATP, additional
ATP simply cannot be formed. As a result, the entire
sequence involved in the use of foodstuffs—glucose,
fats, and proteins—to form ATP is stopped. Then, when
ATP is used by the cell to energize the different phy-
siologic functions in the cell, the newly formed ADP and
AMP turn on the energy processes ag

formed ADP and
AMP turn on the energy processes again, and ADP and
AMP are almost instantly returned to the ATP state. In
this way, essentially a full store of ATP is automatically
maintained, except during extreme cellular activity, such
as very strenuous exercise.
Anaerobic Release of Energy—
“Anaerobic Glycolysis”
Occasionally, oxygen becomes either unavailable or
insufﬁcient, so that oxidative phosphorylation cannot
take place. Yet even under these conditions, a small
amount of energy can st

 these conditions, a small
amount of energy can still be released to the cells by the
glycolysis stage of carbohydrate degradation, because
the chemical reactions for the breakdown of glucose to
pyruvic acid do not require oxygen.
Chapter 67
837
This process is extremely wasteful of glucose, because
only 24,000 calories of energy are used to form ATP for
each molecule of glucose metabolized, which represents
only a little over 

 metabolized, which represents
only a little over 3 per cent of the total energy in the
glucose molecule. Nevertheless, this release of glycolytic
energy to the cells, which is called anaerobic energy, can
be a lifesaving measure for up to a few minutes when
oxygen becomes unavailable.
Formation of Lactic Acid During Anaerobic Glycolysis Allows
Release of Extra Anaerobic Energy. The law of mass action
states that as the end products of a chemical reaction
build up in a reacting medium, the rate 

 reaction
build up in a reacting medium, the rate of the reaction
decreases, approaching zero. The two end products of
the glycolytic reactions (see Figure 67–5) are (1) pyruvic
acid and (2) hydrogen atoms combined with NAD+ to
form NADH and H+. The buildup of either or both of
these would stop the glycolytic process and prevent
further formation of ATP. When their quantities begin
to be excessive, these two end products react with each
other to form lactic acid, in accordance with the fol-
lowi

form lactic acid, in accordance with the fol-
lowing equation:
Thus, under anaerobic conditions, the major portion
of the pyruvic acid is converted into lactic acid, which
diffuses readily out of the cells into the extracellular
ﬂuids and even into the intracellular ﬂuids of other 
less active cells. Therefore, lactic acid represents a type
of “sinkhole” into which the glycolytic end products 
can disappear, thus allowing glycolysis to proceed far
longer than would otherwise be possible. Indeed,

r
longer than would otherwise be possible. Indeed, gly-
colysis could proceed for only a few seconds without
this conversion. Instead, it can proceed for several
minutes, supplying the body with considerable extra
quantities of ATP, even in the absence of respiratory
oxygen.
Reconversion of Lactic Acid to Pyruvic Acid When Oxygen
Becomes Available Again. When a person begins to breathe
oxygen again after a period of anaerobic metabolism,
the lactic acid is rapidly reconverted to pyruvic acid and

ic acid is rapidly reconverted to pyruvic acid and
NADH plus H+. Large portions of these are immedi-
ately oxidized to form large quantities of ATP. This
excess ATP then causes as much as three fourths of the
remaining excess pyruvic acid to be converted back into
glucose.
Thus, the large amount of lactic acid that forms during
anaerobic glycolysis is not lost from the body because,
when oxygen is available again, the lactic acid can be
either reconverted to glucose or used directly for
energy. 

converted to glucose or used directly for
energy. By far the greatest portion of this reconversion
 lactic
+
+
CH3
C
COOH
NADH
H+
OH
(Lactic acid)
+
CH3
C
COOH
NAD+
OH
H
occurs in the liver, but a small amount can also occur in
Use of Lactic Acid by the Heart for Energy. Heart muscle
is especially capable of converting lactic acid to pyruvic
acid and then using the pyruvic acid for energy. This
occurs to a great extent during heavy exercise, when
large

o a great extent during heavy exercise, when
large amounts of lactic acid are released into the blood
from the skeletal muscles and consumed as an extra
energy source by the heart.
Release of Energy from
Glucose by the Pentose
Phosphate Pathway
In almost all the body’s muscles, essentially all the car-
bohydrates utilized for energy are degraded to pyruvic
acid by glycolysis and then oxidized. However, this gly-
colytic scheme is not the only means by which glucose
can be degraded and used to pr

ns by which glucose
can be degraded and used to provide energy. A second
important mechanism for the breakdown and oxidation
of glucose is called the pentose phosphate pathway (or
phosphogluconate pathway), which is responsible for as
much as 30 per cent of the glucose breakdown in the
liver and even more than this in fat cells.
This pathway is especially important because it can
provide energy independently of all the enzymes of the
citric acid cycle and therefore is an alternative pathway
for 

cycle and therefore is an alternative pathway
for energy metabolism when certain enzymatic abnor-
malities occur in cells. It has a special capacity for pro-
viding energy to multiple cellular synthetic processes.
Release of Carbon Dioxide and Hydrogen by Means of the Pentose
Phosphate Pathway. Figure 67–8 shows most of the basic
chemical reactions in the pentose phosphate pathway. It
demonstrates that glucose, during several stages of con-
version, can release one molecule of carbon dioxide and

on, can release one molecule of carbon dioxide and
four atoms of hydrogen, with the resultant formation 
of a ﬁve-carbon sugar, D-ribulose. This substance can
change progressively into several other ﬁve-, four-,
seven-, and three-carbon sugars. Finally, various combi-
nations of these sugars can resynthesize glucose.
However, only ﬁve molecules of glucose are resynthe-
sized for every six molecules of glucose that initially enter
into the reactions. That is, the pentose phosphate
pathway is a cy

ns. That is, the pentose phosphate
pathway is a cyclical process in which one molecule of
glucose is metabolized for each revolution of the cycle.
Thus, by repeating the cycle again and again, all the
glucose can eventually be converted into carbon dioxide
and hydrogen, and the hydrogen can enter the oxidative
phosphorylation pathway to form ATP; more often,
however, it is used for the synthesis of fat or other sub-
stances, as follows.
Use of Hydrogen to Synthesize Fat; the Function of Nicotina

drogen to Synthesize Fat; the Function of Nicotinamide
Adenine Dinucleotide Phosphate. The hydrogen released
during the pentose phosphate cycle does not combine
with NAD+ as in the glycolytic pathway but combines
with nicotinamide adenine dinucleotide phosphate
(NADP+), which is almost identical to NAD+ except 
for an extra phosphate radical, P. This difference is
extremely signiﬁcant, because only hydrogen bound
with NADP+ in the form of NADPH can be used for 
the synthesis of fats from carbohy

an be used for 
the synthesis of fats from carbohydrates (as discussed 
in Chapter 68) and for the synthesis of some other 
838
Unit XIII
When the glycolytic pathway for using glucose
becomes slowed because of cellular inactivity, the
pentose phosphate pathway remains operative (mainly
in the liver) to break down any excess glucose that con-
tinues to be transported into the cells, and NADPH
becomes abundant to help convert acetyl-CoA, also
deriv

es abundant to help convert acetyl-CoA, also
derived from glucose, into long fatty acid chains. This is
another way in which energy in the glucose molecule is
used other than for the formation of ATP—in this
instance, for the formation and storage of fat in the body.
Glucose Conversion to Glycogen 
or Fat
When glucose is not immediately required for energy,
the extra glucose that continually enters the cells is
either stored as glycogen or converted into fat. Glucose
is preferentially stored as 

ted into fat. Glucose
is preferentially stored as glycogen until the cells have
stored as much glycogen as they can—an amount sufﬁ-
cient to supply the energy needs of the body for only 12
to 24 hours.
When the glycogen-storing cells (primarily liver and
muscle cells) approach saturation with glycogen, the
additional glucose is converted into fat in liver and fat
cells and is stored as fat in the fat cells. Other steps 
in the chemistry of this conversion are discussed in
Chapter 68.
Formation o

onversion are discussed in
Formation of Carbohydrates
from Proteins and Fats—
“Gluconeogenesis”
When the body’s stores of carbohydrates decrease
below normal, moderate quantities of glucose can be
formed from amino acids and the glycerol portion of fat.
This process is called gluconeogenesis.
Gluconeogenesis is especially important in prevent-
ing an excessive reduction in the blood glucose concen-
tration during fasting. Glucose is the primary substrate
for energy in tissues such as

he primary substrate
for energy in tissues such as the brain and the red blood
cells, and adequate amounts of glucose must be present
in the blood for several hours between meals. The liver
plays a key role in maintaining blood glucose levels
during fasting by converting its stored glycogen to
glucose (glycogenolysis) and by synthesizing glucose,
mainly from lactate and amino acids (gluconeogenesis).
Approximately 25 per cent of the liver’s glucose pro-
duction during fasting is from gluconeogen

se pro-
duction during fasting is from gluconeogenesis, helping
to provide a steady supply of glucose to the brain.
During prolonged fasting, the kidneys also synthesize
considerable amounts of glucose from amino acids and
other precursors.
About 60 per cent of the amino acids in the body pro-
teins can be converted easily into carbohydrates; the
remaining 40 per cent have chemical conﬁgurations that
make this difﬁcult or impossible. Each amino acid is
converted into glucose by a slightly differ

cid is
converted into glucose by a slightly different chemical
process. For instance, alanine can be converted directly
into pyruvic acid simply by deamination; the pyruvic
acid is then converted into glucose or stored glycogen.
Several of the more complicated amino acids can be
converted into different sugars that contain three-,
four-, ﬁve-, or seven-carbon atoms; they can then enter
the phosphogluconate pathway and eventually form
glucose. Thus, by means of deamination plus several
simple int

s, by means of deamination plus several
simple interconversions, many of the amino acids can
become glucose. Similar interconversions can change
glycerol into glucose or glycogen.
Regulation of Gluconeogenesis. Diminished carbohydrates
in the cells and decreased blood sugar are the basic
stimuli that increase the rate of gluconeogenesis. Dimin-
ished carbohydrates can directly reverse many of the
glycolytic and phosphogluconate reactions, thus allow-
ing the conversion of deaminated amino acids 

low-
ing the conversion of deaminated amino acids and 
glycerol into carbohydrates. In addition, the hormone
cortisol is especially important in this regulation, as
follows.
Effect of Corticotropin and Glucocorticoids on Gluco-
neogenesis. When normal quantities of carbohydrates
are not available to the cells, the adenohypophysis,
for reasons not completely understood, begins to
secrete increased quantities of the hormone corti-
cotropin. This stimulates the adrenal cortex to produce
large quant

imulates the adrenal cortex to produce
large quantities of glucocorticoid hormones, especially
cortisol. In turn, cortisol mobilizes proteins from 
essentially all cells of the body, making these available
in the form of amino acids in the body ﬂuids. A high 
proportion of these immediately become deaminated 
in the liver and provide ideal substrates for conversion
into glucose. Thus, one of the most important means 
by which gluconeogenesis is promoted is through 
the release of glucocorticoids

romoted is through 
the release of glucocorticoids from the adrenal 
cortex.
6-Phosphoglucono-d-lactone
6-Phosphogluconic acid
3-Keto-6-phosphogluconic acid
D-Ribulose-5-phosphate
D-Xylulose-5-phosphate
D-Ribose-5-phosphate
D-Sedoheptulose-7-phosphate
+
+
+
D-Glyceraldehyde-3-phosphate
Erythrose-4-phosphate
Net reaction:
Glucose + 12NADP+ + 6H2O
6CO2 + 12H + 12NADPH
2H
2H
CO2
H2O
Figure 67–8
Pentose phosphate pathway for glucose metabolism.
Chapter 67
Met

ate pathway for glucose metabolism.
Chapter 67
839
The normal blood glucose concentration in a person
who has not eaten a meal within the past 3 to 4 hours
is about 90 mg/dl.After a meal containing large amounts
of carbohydrates, this level seldom rises above 140 mg/
dl unless the person has diabetes mellitus, which is dis-
The regulation of blood glucose concentration is inti-
mately related

lood glucose concentration is inti-
mately related to the pancreatic hormones insulin and
glucagon; this subject is discussed in detail in Chapter
78 in relation to the functions of these hormones.
Barrett EJ: Insulin’s effect on glucose production: direct or
indirect? J Clin Invest 111:434, 2003.
Barthel A, Schmoll D: Novel concepts in insulin regulation
of hepatic gluconeogenesis. Am J Physiol Endocrinol
Metab 285:E685, 2003.
Ceulemans H, Bollen M: Functional diversity of protein
ph

ns H, Bollen M: Functional diversity of protein
phosphatase-1, a cellular economizer and reset button.
Physiol Rev 84:1, 2004.
Ferrer JC, Favre C, Gomis RR, et al: Control of glycogen
deposition. FEBS Lett 546:127, 2003.
Gleeson TT: Post-exercise lactate metabolism: a comparative
review of sites, pathways, and regulation. Annu Rev
Physiol 58:565, 1996.
Gunter TE, Yule DI, Gunter KK, et al: Calcium and mito-
c

TE, Yule DI, Gunter KK, et al: Calcium and mito-
chondria. FEBS Lett 567:96, 2004.
Jackson JB: Proton translocation by transhydrogenase.
FEBS Lett 545:18, 2003.
Jiang G, Zhang BB: Glucagon and regulation of glucose
metabolism. Am J Physiol Endocrinol Metab 284:E671,
2003.
Jungas RL, Halperin ML, Brosnan JT: Quantitative analysis
of amino acid oxidation and related gluconeogenesis in
humans. Physiol Rev 72:419, 1992.
Krebs HA: The tricarboxylic acid cycle. Harvey Lect 44:165,
1948-1949.
Kunji ER:

id cycle. Harvey Lect 44:165,
Kunji ER: The role and structure of mitochondrial carriers.
FEBS Lett 564:239, 2004.
Lam TK, Carpentier A, Lewis GF, et al: Mechanisms of 
the free fatty acid-induced increase in hepatic glucose 
production. Am J Physiol Endocrinol Metab 284:E863,
2003.
Mills DA, Ferguson-Miller S: Understanding the mechanism
of proton movement linked to oxygen reduction in
cytochrome c oxidase: lessons from other proteins. FEBS
Lett 545:47, 2003.
Pilkis SJ, Granner DK: M

. FEBS
Pilkis SJ, Granner DK: Molecular physiology of the regula-
tion of hepatic gluconeogenesis and glycolysis. Annu Rev
Physiol 54:885, 1992.
Roden M, Bernroider E: Hepatic glucose metabolism in
humans—its role in health and disease. Best Pract Res
Clin Endocrinol Metab 17:365, 2003.
Ronquist G, Waldenstrom A: Imbalance of plasma mem-
brane ion leak and pump relationship as a new aetiologi-
cal basis of certain disease states. J Intern Med 254:517,
2003.
Spriet LL, Watt MJ:

s. J Intern Med 254:517,
2003.
Spriet LL, Watt MJ: Regulatory mechanisms in the interac-
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C
H
A
P
T
E
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6
8
840
Several chemical compounds in food and in the 
body are classiﬁed as lipids. They include (1) neutral fat,
also known as triglycerides;
(2) phospholipids;
(3) cholesterol; and (4) a 

es;
(3) cholesterol; and (4) a few others of less importance.
Chemically, the basic lipid moiety of the triglycerides
and the phospholipids is fatty acids, which are simply
long-chain hydrocarbon organic acids. A typical fatty
acid, palmitic acid, is the following: CH3(CH2)14COOH.
Although cholesterol does not contain fatty acid, its
sterol nucleus is synthesized from portions of fatty acid molecules, thus giving it
many of the physical and chemical properties of other lipid s

 physical and chemical properties of other lipid substances.
The triglycerides are used in the body mainly to provide energy for the different
metabolic processes, a function they share almost equally with the carbohydrates.
However, some lipids, especially cholesterol, the phospholipids, and small amounts
of triglycerides, are used to form the membranes of all cells of the body and to
perform other cellular functions.
Basic Chemical Structure of Triglycerides (Neutral Fat). Because most of this

 Triglycerides (Neutral Fat). Because most of this chapter deals
with the utilization of triglycerides for energy, the following typical structure of the
triglyceride molecule must be understood.
CH3—(CH2)16—COO—CH2
|
CH3—(CH2)16—COO—CH
|
Note that three long-chain fatty acid molecules are bound with one molecule of
glycerol. The three fatty acids most commonly present in the triglycerides of the
human body are (1) stearic acid (shown in the tristearin example abov

stearic acid (shown in the tristearin example above), which has
an 18-carbon chain and is fully saturated with hydrogen atoms; (2) oleic acid, which
also has an 18-carbon chain but has one double bond in the middle of the chain;
and (3) palmitic acid, which has 16 carbon atoms and is fully saturated.
Transport of Triglycerides and Other Lipids from the
Gastrointestinal Tract by Lymph—The Chylomicrons
As explained in Chapter 65, almost all the fats in the di

ained in Chapter 65, almost all the fats in the diet, with the principal excep-
tion of a few short-chain fatty acids, are absorbed from the intestines into the intes-
tinal lymph. During digestion, most triglycerides are split into monoglycerides and
fatty acids. Then, while passing through the intestinal epithelial cells, the mono-
glycerides and fatty acids are resynthesized into new molecules of triglycerides that
enter the lymph as minute, dispersed droplets called chylomicrons, whose diame

ispersed droplets called chylomicrons, whose diame-
ters are between 0.08 and 0.6 micron. A small amount of apoprotein B is adsorbed
to the outer surfaces of the chylomicrons. This leaves the remainder of the protein
molecules projecting into the surrounding water and thereby increases the suspen-
sion stability of the chylomicrons in the lymph ﬂuid and prevents their adherence
to the lymphatic vessel walls.
Most of the cholesterol and phospholipids absorbed from the gastrointestinal
tract enter

ids absorbed from the gastrointestinal
tract enter the chylomicrons. Thus, although the chylomicrons are composed prin-
cipally of triglycerides, they also contain about 9 per cent phospholipids, 3 per cent
cholesterol, and 1 per cent apoprotein B. The chylomicrons are then transported
upward through the thoracic duct and emptied into the circulating venous blood at
the juncture of the jugular and subclavian veins.
Chapter 68
841
Removal of the Chylomicrons 
from the Blood
About

About 1 hour after a meal that contains large quantities
of fat, the chylomicron concentration in the plasma may
rise to 1 to 2 per cent of the total plasma, and because
of the large size of the chylomicrons, the plasma appears
turbid and sometimes yellow. However, the chylomi-
crons have a half-life of less than 1 hour, so the plasma
becomes clear again within a few hours. The fat of the
chylomicrons is removed mainly in the following way.
Chylomicro

is removed mainly in the following way.
Chylomicron Triglycerides Are Hydrolyzed by Lipoprotein Lipase,
and Fat Is Stored in Adipose Tissue and Liver Cells. Most of the
chylomicrons are removed from the circulating blood as
they pass through the capillaries of adipose tissue or the
liver. Both adipose tissue and the liver contain large
quantities of the enzyme lipoprotein lipase. This enzyme
is especially active in the capillary endothelium, where
it hydrolyzes the triglycerides of chylomicrons 

e
it hydrolyzes the triglycerides of chylomicrons as they
come in contact with the endothelial wall, thus releas-
ing fatty acids and glycerol.
The fatty acids, being highly miscible with the mem-
branes of the cells, immediately diffuse into the fat cells
of the adipose tissue and into the liver cells. Once inside
these cells, the fatty acids are again synthesized into
triglycerides, with new glycerol being supplied by the
metabolic processes of the storage cells, as discussed
later in the chap

 the storage cells, as discussed
later in the chapter. The lipase also causes hydrolysis of
phospholipids; this, too, releases fatty acids to be stored
in the cells in the same way.
“Free Fatty Acids” Are Transported
in the Blood in Combination 
with Albumin
When fat that has been stored in the adipose tissue is to
be used elsewhere in the body to provide energy, it must
ﬁrst be transported from the adipose tissue to the other
tissue. It is transported mainly in the form of free fatty
acids. Thi

ported mainly in the form of free fatty
acids. This is achieved by hydrolysis of the triglycerides
back into fatty acids and glycerol.
At least two classes of stimuli play important roles 
in promoting this hydrolysis. First, when the amount 
of glucose available to the fat cell is inadequate, one of
the glucose breakdown products, a-glycerophosphate, is
also available in insufﬁcient quantities. Because this
substance is required to maintain the glycerol portion
of triglycerides, the result is h

glycerol portion
of triglycerides, the result is hydrolysis of triglycerides.
Second, a hormone-sensitive cellular lipase can be acti-
vated by several hormones from the endocrine glands,
and this also promotes rapid hydrolysis of triglycerides.
This is discussed later in the chapter.
On leaving fat cells, fatty acids ionize strongly in the
plasma, and the ionic portion combines immediately
with albumin molecules of the plasma proteins. Fatty
acids bound in this manner are called free fatty acid

ds bound in this manner are called free fatty acids
or nonesteriﬁed fatty acids, to distinguish them from
other fatty acids in the plasma that exist in the form 
of (1) esters of glycerol, (2) cholesterol, or (3) other 
The concentration of free fatty acids in the plasma
under resting conditions is about 15 mg/dl, which is a
total of only 0.45 gram of fatty acids in the entire circu-
latory system. Strangely enough, even this small amount
accounts for almost all the transport of fatt

ount
accounts for almost all the transport of fatty acids from
one part of the body to another for the following
reasons:
1. Despite the minute amount of free fatty acid in 
the blood, its rate of “turnover” is extremely 
rapid: half the plasma fatty acid is replaced by 
new fatty acid every 2 to 3 minutes. One can
calculate that at this rate, almost all the normal
energy requirements of the body can be provided
by the oxidation of transported free fatty acids,
without using any carbohydrates or

ee fatty acids,
without using any carbohydrates or proteins for
energy.
2. Conditions that increase the rate of utilization 
of fat for cellular energy also increase the free 
fatty acid concentration in the blood; in fact, the
concentration sometimes increases ﬁvefold to
eightfold. Such a large increase occurs especially in
cases of starvation and in diabetes; in both these
conditions, the person derives little or no metabolic
energy from carbohydrates.
Under normal conditions, only about 3 mol

ydrates.
Under normal conditions, only about 3 molecules of
fatty acid combine with each molecule of albumin, but
as many as 30 fatty acid molecules can combine with a
single albumin molecule when the need for fatty acid
transport is extreme. This shows how variable the rate
of lipid transport can be under different physiologic
Lipoproteins—Their Special
Function in Transporting
Cholesterol and Phospholipids
In the postabsorptive state, after all the chylomicrons
have been removed fr

e, after all the chylomicrons
have been removed from the blood, more than 95 
per cent of all the lipids in the plasma are in the 
form of lipoprotein. These are small particles—much
smaller than chylomicrons, but qualitatively similar 
in composition—containing triglycerides, cholesterol,
phospholipids, and protein. The total concentration 
of lipoproteins in the plasma averages about 700 mg 
per 100 ml of plasma—that is, 700 mg/dl. This can be
broken down into the following individual lipoprot

broken down into the following individual lipoprotein
constituents:
mg/dl of plasma
180
160
160
Protein
200
Types of Lipoproteins. Aside from the chylomicrons, which
are themselves very large lipoproteins, there are four
major types of lipoproteins, classiﬁed by their densities
as measured in the ultracentrifuge: (1) very low density
lipoproteins, which contain high concentrations of
triglycerides and moderate concentrations of both cho-
lesterol and phosp

ate concentrations of both cho-
lesterol and phospholipids; (2) intermediate-density
lipoproteins, which are very low density lipoproteins
from which a share of the triglycerides has been
removed, so that the concentrations of cholesterol and
phospholipids are increased; (3) low-density lipopro-
teins, which are derived from intermediate-density
lipoproteins by the removal of almost all the triglyc-
erides, leaving an especially high concentration of 
cholesterol and a moderately high concentrat

n of 
cholesterol and a moderately high concentration of
phospholipids; and (4) high-density lipoproteins, which
contain a high concentration of protein (about 50 per
cent) but much smaller concentrations of cholesterol
and phospholipids.
842
Unit XIII
Formation and Function of Lipoproteins. Almost all the
lipoproteins are formed in the liver, which is also where
most of the plasma cholesterol, phospholipids, and
triglycerides are synthesized. In addition, s

 and
triglycerides are synthesized. In addition, small quanti-
ties of high-density lipoproteins are synthesized in the
intestinal epithelium during the absorption of fatty
acids from the intestines.
The primary function of the lipoproteins is to trans-
port their lipid components in the blood. The very low
density lipoproteins transport triglycerides synthesized
in the liver mainly to the adipose tissue, whereas the
other lipoproteins are especially important in the dif-
ferent stages of phosph

ally important in the dif-
ferent stages of phospholipid and cholesterol transport
from the liver to the peripheral tissues or from the
periphery back to the liver. Later in the chapter, we
discuss in more detail special problems of cholesterol
transport in relation to the disease atherosclerosis,which
is associated with the development of fatty lesions on
the insides of arterial walls.
Large quantities of fat are stored in two major tissues
of the body, the adipose t

ed in two major tissues
of the body, the adipose tissue and the liver. The adipose
tissue is usually called fat deposits, or simply tissue fat.
The major function of adipose tissue is storage of
triglycerides until they are needed to provide energy
elsewhere in the body. A subsidiary function is to
provide heat insulation for the body, as discussed in
Chapter 73.
Fat Cells (Adipocytes). The fat cells (adipocytes) of
adipose tissue are modiﬁed ﬁbroblasts that store almost
pure triglycerides in qu

oblasts that store almost
pure triglycerides in quantities as great as 80 to 95 per
cent of the entire cell volume. Triglycerides inside the
fat cells are generally in a liquid form. When the tissues
are exposed to prolonged cold, the fatty acid chains of
the cell triglycerides, over a period of weeks, become
either shorter or more unsaturated to decrease their
melting point, thereby always allowing the fat to remain
in a liquid state. This is particularly important, because
only liquid fat can 

rticularly important, because
only liquid fat can be hydrolyzed and transported from
the cells.
Fat cells can synthesize very small amounts of fatty
acids and triglycerides from carbohydrates; this function
supplements the synthesis of fat in the liver, as discussed
later in the chapter.
Exchange of Fat Between the Adipose Tissue and the Blood—
Tissue Lipases. As discussed earlier, large quantities of
lipases are present in adipose tissue. Some of these
enzymes catalyze the deposition of cell tr

f these
enzymes catalyze the deposition of cell triglycerides
from the chylomicrons and lipoproteins. Others, when
activated by hormones, cause splitting of the triglyc-
erides of the fat cells to release free fatty acids. Because
of the rapid exchange of fatty acids, the triglycerides in
fat cells are renewed about once every 2 to 3 weeks,
which means that the fat stored in the tissues today is
not the same fat that was stored last month, thus empha-
sizing the dynamic state of storage fat.
Liv

mpha-
The principal functions of the liver in lipid metabolism
are to (1) degrade fatty acids into small compounds that
can be used for energy; (2) synthesize triglycerides,
mainly from carbohydrates, but to a lesser extent from
proteins as well; and (3) synthesize other lipids from
fatty acids, especially cholesterol and phospholipids.
Large quantities of triglycerides appear in the liver
(1) during the early stages of starvation, (2) in dia

 during the early stages of starvation, (2) in diabetes
mellitus, and (3) in any other condition in which fat
instead of carbohydrates is being used for energy. In
these conditions, large quantities of triglycerides are
mobilized from the adipose tissue, transported as free
fatty acids in the blood, and redeposited as triglycerides
in the liver, where the initial stages of much of fat degra-
dation begin. Thus, under normal physiologic condi-
tions, the total amount of triglycerides in the liver

ns, the total amount of triglycerides in the liver is
determined to a great extent by the overall rate at which
lipids are being used for energy.
The liver cells, in addition to containing triglycerides,
contain large quantities of phospholipids and choles-
terol, which are continually synthesized by the liver.
Also, the liver cells are much more capable than other
tissues of desaturating fatty acids, so that liver triglyc-
erides normally are much more unsaturated than the
triglycerides of adip

ch more unsaturated than the
triglycerides of adipose tissue.This capability of the liver
to desaturate fatty acids is functionally important to all
tissues of the body, because many structural elements of
all cells contain reasonable quantities of unsaturated
fats, and their principal source is the liver. This desatu-
ration is accomplished by a dehydrogenase in the liver
cells.
Use of Triglycerides 
for Energy: Formation 
About 40 per cent of the calories in a typical

ate
About 40 per cent of the calories in a typical American
diet are derived from fats, which is almost equal to the
calories derived from carbohydrates. Therefore, the use
of fats by the body for energy is as important as the use
of carbohydrates is. In addition, many of the carbohy-
drates ingested with each meal are converted into
triglycerides, then stored, and used later in the form of
fatty acids released from the triglycerides for energy.
Hydrolysis of Triglycerides. The ﬁrst stage in usi

Hydrolysis of Triglycerides. The ﬁrst stage in using triglyc-
erides for energy is their hydrolysis into fatty acids and
glycerol. Then, both the fatty acids and the glycerol are
transported in the blood to the active tissues, where they
will be oxidized to give energy. Almost all cells—with
some exceptions, such as brain tissue and red blood
cells—can use fatty acids for energy.
Glycerol, on entering the active tissue, is immediately
changed by intracellular enzymes into glycerol-3-
phosphate, 

intracellular enzymes into glycerol-3-
phosphate, which enters the glycolytic pathway for
glucose breakdown and is thus used for energy. Before
the fatty acids can be used for energy, they must be
processed further in the following way.
Entry of Fatty Acids into Mitochondria. Degradation and oxi-
dation of fatty acids occur only in the mitochondria.
Therefore, the ﬁrst step for the use of fatty acids is 
their transport into the mitochondria. This is a carrier-
mediated process that uses carniti

s is a carrier-
mediated process that uses carnitine as the carrier sub-
stance. Once inside the mitochondria, fatty acids split
away from carnitine and are degraded and oxidized.
Degradation of Fatty Acids to Acetyl Coenzyme A by Beta-
Oxidation. The fatty acid molecule is degraded in the
Chapter 68
843
mitochondria by progressive release of two-carbon seg-
ments in the form of acetyl coenzyme A (acetyl-CoA).
This process, which is shown in Figure 68–1, is called the
beta-oxida

 is shown in Figure 68–1, is called the
beta-oxidation process for degradation of fatty acids.
To understand the essential steps in the beta-
oxidation process, note that in equation 1 the ﬁrst step
is combination of the fatty acid molecule with coenzyme
A (CoA) to form fatty acyl-CoA. In equations 2, 3, and
4, the beta carbon (the second carbon from the right) of
the fatty acyl-CoA binds with an oxygen molecule—that
is, the beta carbon becomes oxidized.
Then, in equation 5, the right-hand two-c

xidized.
Then, in equation 5, the right-hand two-carbon
portion of the molecule is split off to release acetyl-CoA
into the cell ﬂuid. At the same time, another CoA mol-
ecule binds at the end of the remaining portion of the
fatty acid molecule, and this forms a new fatty acyl-CoA
molecule; this time, however, the molecule is two carbon
atoms shorter because of the loss of the ﬁrst acetyl-CoA
from its terminal end.
Next, this shorter fatty acyl-CoA enters into equation
2 and progresses through e

oA enters into equation
2 and progresses through equations 3, 4, and 5 to release
still another acetyl-CoA molecule, thus shortening the
original fatty acid molecule by another two carbons. In
addition to the released acetyl-CoA molecules, four
atoms of hydrogen are released from the fatty acid mol-
ecule at the same time, entirely separate from the
acetyl-CoA.
Oxidation of Acetyl-CoA. The acetyl-CoA molecules
formed by beta-oxidation of fatty acids in the mito-
chondria enter immediately into t

ids in the mito-
chondria enter immediately into the citric acid cycle (see
Chapter 67), combining ﬁrst with oxaloacetic acid to
form citric acid, which then is degraded into carbon
dioxide and hydrogen atoms. The hydrogen is subse-
quently oxidized by the chemiosmotic oxidative system
of the mitochondria, which was also explained in
Chapter 67. The net reaction in the citric acid cycle for
each molecule of acetyl-CoA is the following:
CH COCo-A +Oxaloacetic acid
H O+ ADP
CO
H+ HCo-A + ATP+Oxalo

+Oxaloacetic acid
H O+ ADP
CO
H+ HCo-A + ATP+Oxaloacetic acid
3
2
2
 
+
æ
Æ
ææææææ
+
3
2
8
Citric acid cycle
Thus, after initial degradation of fatty acids to acetyl-
CoA, their ﬁnal breakdown is precisely the same as that
of the acetyl-CoA formed from pyruvic acid during the
metabolism of glucose. And the extra hydrogen atoms
are also oxidized by the same chemiosmotic oxidative
system of the mitochondria that is used in carbohydrate
oxidation,
liberating large amounts of adenosine
triphosphat

Tremendous Amounts of ATP Are Formed by Oxidation of Fatty
Acids. In Figure 68–1, note that the 4 separate hydrogen
atoms released each time a molecule of acetyl-CoA is
split from the fatty acid chain are released in the forms
FADH2, NADH, and H+. Therefore, for every stearic
fatty acid molecule that is split to form 9 acetyl-CoA
molecules, 32 extra hydrogen atoms are removed. In
addition, for each of the 9 molecules of acetyl-CoA that
a

, for each of the 9 molecules of acetyl-CoA that
are subsequently degraded by the citric acid cycle, 8
more hydrogen atoms are removed, making another 72
hydrogens. This makes a total of 104 hydrogen atoms
eventually released by the degradation of each stearic
acid molecule. Of this group, 34 are removed from 
the degrading fatty acids by ﬂavoproteins, and 70 are
removed by nicotinamide adenine dinucleotide (NAD+)
as NADH and H+.
These two groups of hydrogen atoms are oxidized in
the mitochondri

 of hydrogen atoms are oxidized in
the mitochondria, as discussed in Chapter 67, but they
enter the oxidative system at different points, so that 
1 molecule of ATP is synthesized for each of the 34
ﬂavoprotein hydrogens, and 1.5 molecules of ATP are
synthesized for each of the 70 NADH and H+ hydro-
gens.This makes 34 plus 105, or a total of 139 molecules
of ATP formed by the oxidation of hydrogen derived
from each molecule of stearic acid.Another 9 molecules
of ATP are formed in the citric acid

r 9 molecules
of ATP are formed in the citric acid cycle itself (sepa-
rate from the ATP released by the oxidation of hydro-
gen), one for each of the 9 acetyl-CoA molecules
metabolized. Thus, a total of 148 molecules of ATP are
formed during the complete oxidation of 1 molecule of
stearic acid. However, two high-energy bonds are con-
sumed in the initial combination of CoA with the stearic
acid molecule, making a net gain of 146 molecules of
ATP.
 (1) RCH2CH2CH2COOH + CoA + ATP
(Fatty acid)
Thi

.
Thiokinase
Acyl dehydrogenase
Enoyl hydrase
Thiolase
b-Hydroxyacyl
 (2) RCH2CH2CH2COCoA + FAD 
 (4) RCH2CHOHCH2COCoA + NAD+ 
RCH2COCH2COCoA + NADH + H+ 
 (5) RCH2COCH2COCoA + CoA 
RCH2COCoA + CH3COCoA
RCH2CHOHCH2COCoA
RCH2CH=CHCOCoA + FADH2
RCH2CH2CH2COCoA + AMP + Pyrophosphate
(Fatty acyl-CoA)
(Fatty acyl-CoA) (Acetyl-CoA)
 (3) RCH2CH=CHCOCoA + H2O
Figure 68–1
Beta-oxidation of fatty acids to yield acetyl coenzyme A.

dation of fatty acids to yield acetyl coenzyme A.
844
Unit XIII
Formation of Acetoacetic Acid 
in the Liver and Its Transport 
A large share of the initial degradation of fatty acids
occurs in the liver, especially when excessive amounts
of lipids are being used for energy. However, the liver
uses only a small proportion of the fatty acids for its own
intrinsic metabolic processes. Instead, when the fatty
acid chains have been split into acetyl-

the fatty
acid chains have been split into acetyl-CoA, two mole-
cules of acetyl-CoA condense to form one molecule of
acetoacetic acid, which is then transported in the blood
to the other cells throughout the body, where it is used
for energy. The chemical processes are the following:
Part of the acetoacetic acid is also converted into 
b-hydroxybutyric acid,
and minute quantities are 
converted into acetone in accord with the following
reactions:
The acetoacetic acid, b-hydroxybutyric acid, and

:
acetone diffuse freely through the liver cell membranes
and are transported by the blood to the peripheral
tissues. Here they again diffuse into the cells, where
reverse reactions occur and acetyl-CoA molecules are
formed. These in turn enter the citric acid cycle and are
oxidized for energy, as already explained.
Normally, the acetoacetic acid and b-hydroxybutyric
acid that enter the blood are transported so rapidly to
the tissues that their co

ransported so rapidly to
the tissues that their combined concentration in the
plasma seldom rises above 3 mg/dl. Yet despite this
small concentration in the blood, large quantities are
actually transported, as is also true for free fatty acid
transport. The rapid transport of both these substances
results from their high solubility in the membranes of
the target cells, which allows almost instantaneous dif-
fusion into the cells.
Ketosis in Starvation, Diabetes, and Other Diseases. The con-
cent

ation, Diabetes, and Other Diseases. The con-
centrations of acetoacetic acid, b-hydroxybutyric acid,
and acetone occasionally rise to levels many times
normal in the blood and interstitial ﬂuids; this condition
is called ketosis, because acetoacetic acid is a keto acid.
The three compounds are called ketone bodies. Ketosis
occurs especially in starvation, in diabetes mellitus, and
sometimes even when a person’s diet is composed
almost entirely of fat. In all these states, essentially no
CH3
C
C

f fat. In all these states, essentially no
CH3
C
CH2
O
C
OH
O
Acetoacetic acid
Acetone
CH3
CH
CH2
OH
CH3
C
CH3
O
+  2H
-CO2
C
OH
O
b-Hydroxybutyric acid
2
2
CH COCo-A + H O
liver cells 
other cells
CH COCH COOH
HCo-A
3
2
3
2
-
æ
Æ
ææææ
æ
¨
æ
æææææ
+
Acetyl CoA
carbohydrates are metabolized—in starvation and with
a high-fat diet because carbohydrates are not available,
and in diabetes because insulin is not available to cause
glucose transport into the cells.
When carbohydrates a

ose transport into the cells.
When carbohydrates are not used for energy, almost
all the energy of the body must come from metabolism
of fats.We shall see later in the chapter that the unavail-
ability of carbohydrates automatically increases the rate
of removal of fatty acids from adipose tissues; in addi-
tion, several hormonal factors—such as increased secre-
tion of glucocorticoids by the adrenal cortex, increased
secretion of glucagon by the pancreas, and decreased
secretion of insulin by t

 pancreas, and decreased
secretion of insulin by the pancreas—further enhance
the removal of fatty acids from the fat tissues. As a
result, tremendous quantities of fatty acids become
available (1) to the peripheral tissue cells to be used for
energy and (2) to the liver cells, where much of the fatty
acids is converted to ketone bodies.
The ketone bodies pour out of the liver to be carried
to the cells. For several reasons, the cells are limited in
the amount of ketone bodies that can be oxidiz

 in
the amount of ketone bodies that can be oxidized; the
most important reason is the following:One of the prod-
ucts of carbohydrate metabolism is the oxaloacetate that
is required to bind with acetyl-CoA before it can be
processed in the citric acid cycle. Therefore, deﬁciency
of oxaloacetate derived from carbohydrates limits the
entry of acetyl-CoA into the citric acid cycle, and when
there is a simultaneous outpouring of large quantities 
of acetoacetic acid and other ketone bodies from the

 acetoacetic acid and other ketone bodies from the
liver, the blood concentrations of acetoacetic acid and
b-hydroxybutyric acid sometimes rise to as high as 
20 times normal, thus leading to extreme acidosis, as
explained in Chapter 30.
The acetone that is formed during ketosis is a volatile
substance, some of which is blown off in small quanti-
ties in the expired air of the lungs. This gives the breath
an acetone smell that is frequently used as a diagnostic
criterion of ketosis.
Adaptation t

as a diagnostic
Adaptation to a High-Fat Diet. When changing slowly from
a carbohydrate diet to an almost completely fat diet, a
person’s body adapts to use far more acetoacetic acid
than usual, and in this instance, ketosis normally does
not occur. For instance, the Inuit (Eskimos), who some-
times live almost entirely on a fat diet, do not develop
ketosis. Undoubtedly, several factors, none of which is
clear, enhance the rate of acetoacetic acid metabolism
by the cells. A

ate of acetoacetic acid metabolism
by the cells. After a few weeks, even the brain cells,
which normally derive almost all their energy from
glucose,can derive 50 to 75 per cent of their energy from
fats.
Synthesis of Triglycerides 
from Carbohydrates
Whenever a greater quantity of carbohydrates enters
the body than can be used immediately for energy or
can be stored in the form of glycogen, the excess is
rapidly converted into triglycerides and stored in this
form in the adipose tissue.
In huma

stored in this
In human beings, most triglyceride synthesis occurs in
the liver, but minute quantities are also synthesized in
the adipose tissue itself. The triglycerides formed in the
liver are transported mainly in very low density lipopro-
teins to the adipose tissue, where they are stored.
Conversion of Acetyl-CoA into Fatty Acids. The ﬁrst step 
in the synthesis of triglycerides is conversion of 
Chapter 68
845
carbohydrates into acetyl-CoA.
As 

 Metabolism
845
As explained in
Chapter 67, this occurs during the normal degradation
of glucose by the glycolytic system. Because fatty acids
are actually large polymers of acetic acid, it is easy to
understand how acetyl-CoA can be converted into fatty
acids. However, the synthesis of fatty acids from acetyl-
CoA is not achieved by simply reversing the oxidative
degradation described earlier.Instead,this occurs by the
two-step process shown in Figure 68–2, using 

 the
two-step process shown in Figure 68–2, using malonyl-
CoA and NADPH as the principal intermediates in the
polymerization process.
Combination of Fatty Acids with a-Glycerophosphate to Form
Triglycerides. Once the synthesized fatty acid chains have
grown to contain 14 to 18 carbon atoms, they bind with
glycerol to form triglycerides. The enzymes that cause
this conversion are highly speciﬁc for fatty acids with
chain lengths of 14 carbon atoms or greater,a factor that
controls the physical q

s or greater,a factor that
controls the physical quality of the triglycerides stored
in the body.
As shown in Figure 68–3, the glycerol portion of
triglycerides is furnished by a-glycerophosphate, which
is another product derived from the glycolytic scheme
of glucose degradation. This mechanism is discussed in
Efﬁciency of Carbohydrate Conversion into Fat. During triglyc-
eride synthesis, only about 15 per cent of the original
energy in the glucose is lost in the form of heat; the
re

in the glucose is lost in the form of heat; the
remaining 85 per cent is transferred to the stored
triglycerides.
Importance of Fat Synthesis and Storage. Fat synthesis from
carbohydrates is especially important for two reasons:
1. The ability of the different cells of the body to
store carbohydrates in the form of glycogen is
generally slight; a maximum of only a few hundred
grams of glycogen can be stored in the liver, the
skeletal muscles, and all other tissues of the body
put together. In co

 all other tissues of the body
put together. In contrast, many kilograms of fat 
can be stored. Therefore, fat synthesis provides a
means by which the energy of excess ingested
carbohydrates (and proteins) can be stored for
later use. Indeed, the average person has almost
150 times as much energy stored in the form of fat
as stored in the form of carbohydrate.
2. Each gram of fat contains almost two and a half
times the calories of energy contained by each
gram of glycogen. Therefore, for a give

ed by each
gram of glycogen. Therefore, for a given weight
gain, a person can store several times as much
energy in the form of fat as in the form of
carbohydrate, which is exceedingly important when
an animal must be highly motile to survive.
Failure to Synthesize Fats from Carbohydrates in the Absence of
Insulin. When no insulin is available, as occurs in serious
diabetes mellitus, fats are poorly synthesized, if at all,
for the following reasons: First, when insulin is not avail-
able, glucos

ns: First, when insulin is not avail-
able, glucose does not enter the fat and liver cells satis-
factorily, so that little of the acetyl-CoA and NADPH
needed for fat synthesis can be derived from glucose.
Second, lack of glucose in the fat cells greatly reduces
the availability of a-glycerophosphate, which also
makes it difﬁcult for the tissues to form triglycerides.
from Proteins
Many amino acids can be converted into acetyl-CoA, as
discussed in Chapter 69. The acet

o acetyl-CoA, as
discussed in Chapter 69. The acetyl-CoA can then be
synthesized into triglycerides. Therefore, when people
have more proteins in their diets than their tissues can
use as proteins, a large share of the excess is stored as
fat.
COOH
(Acetyl-CoA carboxylase)
CH3COCoA + CO2 + ATP
+ ADP + PO4
–3 
CH2
C
O
CoA
1 Acetyl-CoA + Malonyl-CoA + 16NADPH + 16H+
1 Steric acid + 8CO2 + 9CoA + 16NADP+ + 7H2O
Step 1:
Step 2:
Figure 68–2
Synthesis of fatty acids.
Glucose
Tr

2
Glucose
a-Glycerophosphate + Acetyl-CoA + NADH + H+
NADPH + H+
Glycolytic
pathway
Pentose
phosphate
pathway
Figure 68–3
Overall schema for synthesis of
triglycerides from glucose.
846
Unit XIII
Regulation of Energy
Release from Triglycerides
Carbohydrates Are Preferred over Fats for Energy When Excess
Carbohydrates Are Available. When excess quantities of
carbohydrates are available in the body, carbohydr

carbohydrates are available in the body, carbohydrates
are used preferentially over triglycerides for energy.
There are several reasons for this “fat-sparing” effect of
carbohydrates. One of the most important is the fol-
lowing:The fats in adipose tissue cells are present in two
forms: stored triglycerides and small quantities of free
fatty acids. They are in constant equilibrium with each
other.When excess quantities of a-glycerophosphate are
present (which occurs when excess carbohydrates are

resent (which occurs when excess carbohydrates are
available), the excess a-glycerophosphate binds the 
free fatty acids in the form of stored triglycerides. As a
result, the equilibrium between free fatty acids and
triglycerides shifts toward the stored triglycerides;
consequently, only minute quantities of fatty acids 
are available to be used for energy. Because a-
glycerophosphate is an important product of glucose
metabolism, the availability of large amounts of glucose
automatically inhibi

y of large amounts of glucose
automatically inhibits the use of fatty acids for energy.
Second, when carbohydrates are available in excess,
fatty acids are synthesized more rapidly than they are
degraded. This effect is caused partially by the large
quantities of acetyl-CoA formed from the carbohy-
drates and by the low concentration of free fatty acids
in the adipose tissue, thus creating conditions appropri-
ate for the conversion of acetyl-CoA into fatty acids.
An even more important effect t

 into fatty acids.
An even more important effect that promotes the
conversion of carbohydrates to fats is the following:The
ﬁrst step, which is the rate-limiting step, in the synthesis
of fatty acids is carboxylation of acetyl-CoA to form
malonyl-CoA. The rate of this reaction is controlled 
primarily by the enzyme acetyl-CoA carboxylase, the
activity of which is accelerated in the presence of inter-
mediates of the citric acid cycle. When excess carbohy-
drates are being used, these intermediat

 carbohy-
drates are being used, these intermediates increase,
automatically causing increased synthesis of fatty acids.
Thus, an excess of carbohydrates in the diet not only
acts as a fat-sparer but also increases fat stores. In fact,
all the excess carbohydrates not used for energy or
stored in the small glycogen deposits of the body are
converted to fat for storage.
Acceleration of Fat Utilization for Energy in the Absence of Car-
bohydrates. All the fat-sparing effects of carbohydrates
are l

All the fat-sparing effects of carbohydrates
are lost and actually reversed when carbohydrates are
not available. The equilibrium shifts in the opposite
direction, and fat is mobilized from the adipose cells and
used for energy in place of carbohydrates.
Also important are several hormonal changes that
take place to promote rapid fatty acid mobilization from
adipose tissue. Among the most important of these is 
a marked decrease in pancreatic secretion of insulin
caused by the absence of carbohy

retion of insulin
caused by the absence of carbohydrates. This not only
reduces the rate of glucose utilization by the tissues 
but also decreases fat storage, which further shifts the
equilibrium in favor of fat metabolism in place of 
carbohydrates.
Hormonal Regulation of Fat Utilization. At least seven of 
the hormones secreted by the endocrine glands have
signiﬁcant effects on fat utilization. Some important
hormonal effects on fat metabolism—in addition to
insulin lack, discussed in the pre

—in addition to
insulin lack, discussed in the previous paragraph—are
noted here.
Probably the most dramatic increase that occurs in fat
utilization is that observed during heavy exercise. This
results almost entirely from release of epinephrine and
norepinephrine by the adrenal medullae during exer-
cise, as a result of sympathetic stimulation. These two
hormones directly activate hormone-sensitive triglyc-
eride lipase, which is present in abundance in the fat
cells, and this causes rapid brea

dance in the fat
cells, and this causes rapid breakdown of triglycerides
and mobilization of fatty acids. Sometimes the free fatty
acid concentration in the blood of an exercising person
rises as much as eightfold, and the use of these fatty
acids by the muscles for energy is correspondingly
increased. Other types of stress that activate the sym-
pathetic nervous system can also increase fatty acid
mobilization and utilization in a similar manner.
Stress also causes large quantities of corticotr

.
Stress also causes large quantities of corticotropin to
be released by the anterior pituitary gland, and this
causes the adrenal cortex to secrete extra quantities of
glucocorticoids. Both corticotropin and glucocorticoids
activate either the same hormone-sensitive triglyceride
lipase as that activated by epinephrine and norepi-
nephrine or a similar lipase. When corticotropin and
glucocorticoids are secreted in excessive amounts for
long periods, as occurs in the endocrine condition called
Cu

ds, as occurs in the endocrine condition called
Cushing’s disease, fats are frequently mobilized to such
a great extent that ketosis results. Corticotropin and 
glucocorticoids are then said to have a ketogenic effect.
Growth hormone has an effect similar to but weaker
than that of corticotropin and glucocorticoids in 
activating hormone-sensitive lipase. Therefore, growth
hormone can also have a mild ketogenic effect.
Finally, thyroid hormone causes rapid mobilization 
of fat, which is believed

uses rapid mobilization 
of fat, which is believed to result indirectly from an
increased overall rate of energy metabolism in all cells
of the body under the inﬂuence of this hormone. The
resulting reduction in acetyl-CoA and other intermedi-
ates of both fat and carbohydrate metabolism in the
cells is a stimulus to fat mobilization.
The effects of the different hormones on metabolism
are discussed further in the chapters dealing with each
hormone.
Obesity
Obesity means deposition of excess fat

ne.
Obesity
Obesity means deposition of excess fat in the body.This
subject is discussed in Chapter 71 in relation to dietary
balances, but brieﬂy, it is caused by the ingestion of
greater amounts of food than can be used by the body
for energy.The excess food, whether fats, carbohydrates,
or proteins, is then stored almost entirely as fat in the
adipose tissue, to be used later for energy.
Strains of rats have been found in which hereditary
obesity occurs. In at least one of these, the obesity 

ity occurs. In at least one of these, the obesity is
caused by ineffective mobilization of fat from the
adipose tissue by tissue lipase, while synthesis and
storage of fat continue normally. Such a one-way
process causes progressive enhancement of the fat
stores, resulting in severe obesity.
Phospholipids and
The major types of body phospholipids are lecithins,
cephalins, and sphingomyelin; their typical chemical 
Chapter 68
847
formulas are shown in Fi

r 68
847
formulas are shown in Figure 68–4. Phospholipids
always contain one or more fatty acid molecules and
one phosphoric acid radical, and they usually contain a
nitrogenous base. Although the chemical structures of
phospholipids are somewhat variant, their physical
properties are similar because they are all lipid soluble,
transported in lipoproteins, and used throughout the
body for various structural purposes, such as in cell
membranes and intracellular membranes.
Formati

ell
Formation of Phospholipids. Phospholipids are synthesized
in essentially all cells of the body, although certain cells
have a special ability to form great quantities of them.
Probably 90 per cent are formed in the liver cells; sub-
stantial quantities are also formed by the intestinal
epithelial cells during lipid absorption from the gut.
The rate of phospholipid formation is governed to
some extent by the usual factors that control the overall
rate of

the usual factors that control the overall
rate of fat metabolism because, when triglycerides are
deposited in the liver, the rate of phospholipid forma-
tion increases.Also, certain speciﬁc chemical substances
are needed for the formation of some phospholipids.
For instance, choline, either obtained in the diet or syn-
thesized in the body, is needed for the formation of
lecithin, because choline is the nitrogenous base of the
lecithin molecule. Also, inositol is needed for the for-
mation of s

 Also, inositol is needed for the for-
mation of some cephalins.
Speciﬁc Uses of Phospholipids. Several functions of the
phospholipids are the following: (1) Phospholipids 
are an important constituent of lipoproteins in the
blood and are essential for the formation and function
of most of these; in their absence, serious abnormalities
of transport of cholesterol and other lipids can occur.
(2) Thromboplastin, which is needed to initiate the clot-
ting process, is composed mainly of one of the c

-
ting process, is composed mainly of one of the cephalins.
(3) Large quantities of sphingomyelin are present in the
nervous system; this substance acts as an electrical insu-
lator in the myelin sheath around nerve ﬁbers. (4) Phos-
pholipids are donors of phosphate radicals when these
radicals are needed for different chemical reactions in
the tissues. (5) Perhaps the most important of all the
functions of phospholipids is participation in the for-
mation of structural elements—mainly membranes

or-
mation of structural elements—mainly membranes—in
cells throughout the body, as discussed in the next
section of this chapter in connection with a similar func-
tion for cholesterol.
Cholesterol, the formula of which is shown in Figure
68–5, is present in the diets of all people, and it can be
absorbed slowly from the gastrointestinal tract into the
intestinal lymph. It is highly fat soluble but only slightly
soluble in water. It is speciﬁcally capable of forming
esters with fatt

is speciﬁcally capable of forming
esters with fatty acids. Indeed, about 70 per cent of the
cholesterol in the lipoproteins of the plasma is in the
form of cholesterol esters.
Formation of Cholesterol. Besides the cholesterol absorbed
each day from the gastrointestinal tract, which is called
exogenous cholesterol, an even greater quantity is
formed in the cells of the body, called endogenous cho-
lesterol. Essentially all the endogenous cholesterol that
circulates in the lipoproteins of the plas

ol that
circulates in the lipoproteins of the plasma is formed by
the liver, but all other cells of the body form at least
C
O
O
(CH2)7
(CH2)7
CH3
CH
CH
H2C
P
O
O
N+
O
CH2
OH
CH2
CH3
CH3
CH3
H2C
C
O
O
(CH2)16
CH3
HC
C
O
O
(CH2)7
(CH2)7
CH3
CH
CH
H2C
P
O
O
N+H3
O
CH2
OH
CH2
H2C
C
O
O
(CH2)16
CH3
HC
P
O
O
O
CH2
CH
CH
OH
CH2
HC
H
C
NH
O
(CH2)16
CH3
HC
N+
CH
CH3
CH3
H
C
HO
(CH2)12
CH3
A lecithin
A cephalin
Sphingomyelin
Figure 68–4
Typical phospholipids.
CH
HO
CH
CH3
(CH2)3
CH3
CH3
CH3
CH3
Figure 68

ids.
CH
HO
CH
CH3
(CH2)3
CH3
CH3
CH3
CH3
Figure 68–5
Cholesterol.
848
Unit XIII
some cholesterol, which is consistent with the fact that
many of the membranous structures of all cells are par-
tially composed of this substance.
The basic structure of cholesterol is a sterol nucleus.
This is synthesized entirely from multiple molecules of
acetyl-CoA. In turn, the sterol nucleus can be modiﬁed
by means of various side chains to form (1) cholesterol;
(2) cholic

us side chains to form (1) cholesterol;
(2) cholic acid,which is the basis of the bile acids formed
in the liver; and (3) many important steroid hormones
secreted by the adrenal cortex, the ovaries, and the
testes (these hormones are discussed in later chapters).
Factors That Affect Plasma Cholesterol Concentration—Feed-
back Control of Body Cholesterol. Among the important
factors that affect plasma cholesterol concentration are
1. An increase in the amount of cholesterol ingeste

. An increase in the amount of cholesterol ingested
each day increases the plasma concentration
slightly. However, when cholesterol is ingested,
the rising concentration of cholesterol inhibits the
most essential enzyme for endogenous synthesis 
of cholesterol, 3-hydroxy-3-methylglutaryl CoA
reductase, thus providing an intrinsic feedback
control system to prevent an excessive increase 
in plasma cholesterol concentration. As a result,
plasma cholesterol concentration usually is not
changed upwa

lesterol concentration usually is not
changed upward or downward more than ±15 per
cent by altering the amount of cholesterol in the
diet, although the response of individuals differs
markedly.
2. A highly saturated fat diet increases blood
cholesterol concentration 15 to 25 per cent. This
results from increased fat deposition in the liver,
which then provides increased quantities of 
acetyl-CoA in the liver cells for the production 
of cholesterol. Therefore, to decrease the blood
cholesterol c

ol. Therefore, to decrease the blood
cholesterol concentration, it is usually just as
important, if not more important, to maintain a diet
low in saturated fat as to maintain a diet low in
cholesterol.
3. Ingestion of fat containing highly unsaturated fatty
acids usually depresses the blood cholesterol
concentration a slight to moderate amount. The
mechanism of this effect is unknown, despite the
fact that this observation is the basis of much
present-day dietary strategy.
4. Lack of insulin or 

esent-day dietary strategy.
4. Lack of insulin or thyroid hormone increases the
blood cholesterol concentration, whereas excess
thyroid hormone decreases the concentration.
These effects are probably caused mainly by
changes in the degree of activation of speciﬁc
enzymes responsible for the metabolism of lipid
Speciﬁc Uses of Cholesterol in the Body. By far the most
abundant nonmembranous use of cholesterol in the
body is to form cholic acid in the liver. As much as 80
per cent of ch

ic acid in the liver. As much as 80
per cent of cholesterol is converted into cholic acid. As
explained in Chapter 70, this is conjugated with other
substances to form bile salts, which promote digestion
and absorption of fats.
A small quantity of cholesterol is used by (1) the
adrenal glands to form adrenocortical hormones, (2) 
the ovaries to form progesterone and estrogen, and (3)
the testes to form testosterone. These glands can 
also synthesize their own sterols and then form hor-
mones fro

ize their own sterols and then form hor-
mones from them, as discussed in the chapters on
A large amount of cholesterol is precipitated in the
corneum of the skin.This, along with other lipids, makes
the skin highly resistant to the absorption of water-
soluble substances and to the action of many chemical
agents, because cholesterol and the other skin lipids are
highly inert to acids and to many solvents that might
otherwise easily penetrate the body. Also, these lipid
substances

y penetrate the body. Also, these lipid
substances help prevent water evaporation from the
skin; without this protection, the amount of evaporation
can be 5 to 10 liters per day (as occurs in burn patients
who have lost their skin) instead of the usual 300 to 
400 milliliters.
Cellular Structural Functions of
The previously mentioned uses of phospholipids and
cholesterol are of only minor importance in comparison
with their function of form

portance in comparison
with their function of forming specialized structures,
mainly membranes, in all cells of the body. In 
Chapter 2, it was pointed out that large quantities of
phospholipids and cholesterol are present in both the
cell membrane and the membranes of the internal
organelles of all cells. It is also known that the ratio
of membrane cholesterol to phospholipids is especially
important in determining the ﬂuidity of the cell 
For membranes to be formed, substances that 

anes.
For membranes to be formed, substances that are not
soluble in water must be available. In general, the only
substances in the body that are not soluble in water
(besides the inorganic substances of bone) are the lipids
and some proteins. Thus, the physical integrity of cells
everywhere in the body is based mainly on phospho-
lipids, cholesterol, and certain insoluble proteins. The
polar charges on the phospholipids also reduce the
interfacial tension between the cell membranes and 
the su

ial tension between the cell membranes and 
the surrounding ﬂuids.
Another fact that indicates the importance of phos-
pholipids and cholesterol for the formation of structural
elements of the cells is the slow turnover rates of 
these substances in most nonhepatic tissues—turnover
rates measured in months or years. For instance, their
function in brain cells to provide memory processes 
is related mainly to their indestructible physical 
Atherosclerosis is a disease 

ies.
Atherosclerosis is a disease of the large and intermedi-
ate-sized arteries in which fatty lesions called athero-
matous plaques develop on the inside surfaces of the
arterial walls. Arteriosclerosis, in contrast, is a general
term that refers to thickened and stiffened blood vessels
of all sizes.
One abnormality that can be measured very early 
in blood vessels that later become atherosclerotic is
damage to the vascular endothelium. This, in turn,
increases the expression o

thelium. This, in turn,
increases the expression of adhesion molecules on
endothelial cells and decreases their ability to release
nitric oxide and other substances that help prevent
adhesion of macromolecules, platelets, and monocytes
to the endothelium. After damage to the vascular
endothelium occurs, circulating monocytes and lipids
(mostly low-density lipoproteins) begin to accumulate
at the site of injury (Figure 68–6A).The monocytes cross
the endothelium, enter the intima of the vessel wal

he endothelium, enter the intima of the vessel wall, and
Chapter 68
849
Adhesion
molecule
Damaged
particle
Normal
artery
Lipid
droplets
foam cell
Growth/
inflammatory
factors
Receptor
Arterial
intima
Arterial
lumen
Blood monocyte
Monocyte
adhered to
Monocyte
migrating
into intima
Intima
Media
Smooth
muscle
cells
Thrombosis
of a ruptured
plaque
Large
plaque
Small
plaque
A
B
Figure 68–6
Development of atheroscler

plaque
A
B
Development of atherosclerotic plaque. A, Attachment of a monocyte to an adhesion molecule on a damaged endothelial cell of an artery.
The monocyte then migrates through the endothelium into the intimal layer of the arterial wall and is transformed into a macrophage. The
macrophage then ingests and oxidizes lipoprotein molecules, becoming a macrophage foam cell. The foam cells release substances that
cause inﬂammation and growth of the intimal layer. B, Additional accumul

growth of the intimal layer. B, Additional accumulation of macrophages and growth of the intima cause the plaque
to grow larger and accumulate lipids. Eventually, the plaque could occlude the vessel or rupture, causing the blood in the artery to coag-
ulate and form a thrombus. (Modiﬁed from Libby P: Inﬂammation in atherosclerosis. Nature 420:868, 2002.)
differentiate to become macrophages, which then 
ingest and oxidize the accumulated lipoproteins,
giving the macrophages a foamlike appearance.

ins,
giving the macrophages a foamlike appearance. These
macrophage foam cells then aggregate on the blood
vessel and form a visible fatty streak.
With time, the fatty streaks grow larger and coalesce,
and the surrounding ﬁbrous and smooth muscle tissues
proliferate to form larger and larger plaques (see Figure
68–6B). Also, the macrophages release substances that
cause inﬂammation and further proliferation of smooth
muscle and ﬁbrous tissue on the inside surfaces of the
arterial wall. The lipid

he inside surfaces of the
arterial wall. The lipid deposits plus the cellular prolif-
eration can become so large that the plaque bulges into
the lumen of the artery and greatly reduces blood 
ﬂow, sometimes completely occluding the vessel. Even
without occlusion, the ﬁbroblasts of the plaque eventu-
ally deposit extensive amounts of dense connective
tissue; sclerosis (ﬁbrosis) becomes so great that the
arteries become stiff and unyielding. Still later, calcium
salts often precipitate with the c

 later, calcium
salts often precipitate with the cholesterol and other
lipids of the plaques, leading to bony-hard calciﬁcations
that can make the arteries rigid tubes. Both of these
850
Unit XIII
later stages of the disease are called “hardening of the
arteries.”
Atherosclerotic arteries lose most of their distensi-
bility, and because of the degenerative areas in their
walls, they are easily ruptured. Also, where the plaques
protrude into the ﬂowing blood,

 where the plaques
protrude into the ﬂowing blood, their rough surfaces
can cause blood clots to develop, with resultant throm-
bus or embolus formation (see Chapter 36), leading to
a sudden blockage of all blood ﬂow in the artery.
Almost half of all deaths in the United States and
Europe are due to vascular disease.About two thirds of
these deaths are caused by thrombosis of one or more
coronary arteries. The remaining one third are caused
by thrombosis or hemorrhage of vessels in other organs

rombosis or hemorrhage of vessels in other organs
of the body, especially the brain (causing strokes), but
also the kidneys, liver, gastrointestinal tract, limbs, and
so forth.
Basic Causes of Atherosclerosis—
The Roles of Cholesterol 
and Lipoproteins
Increased Low-Density Lipoproteins. An important factor 
in causing atherosclerosis is a high blood plasma con-
centration of cholesterol in the form of low-density
lipoproteins. The plasma concentration of these high-
cholesterol low-density lipo

ration of these high-
cholesterol low-density lipoproteins is increased by
several factors, including eating highly saturated fat in
the daily diet, obesity, and physical inactivity.To a lesser
extent, eating excess cholesterol may also raise plasma
levels of low-density lipoproteins.
An interesting example occurs in rabbits, which 
normally have low plasma cholesterol concentrations
because of their vegetarian diet. Simply feeding these
animals large quantities of cholesterol as part of their
d

large quantities of cholesterol as part of their
daily diet leads to serious atherosclerotic plaques
throughout their arterial systems.
Familial Hypercholesterolemia. This is a disease in which
the person inherits defective genes for the formation of
low-density lipoprotein receptors on the membrane sur-
faces of the body’s cells. In the absence of these recep-
tors, the liver cannot absorb either intermediate-density
or low-density lipoproteins.Without this absorption, the
cholesterol machinery

Without this absorption, the
cholesterol machinery of the liver cells goes on a
rampage, producing new cholesterol; it is no longer
responsive to the feedback inhibition of too much
plasma cholesterol. As a result, the number of very low
density lipoproteins released by the liver into the
plasma increases immensely.
Patients 
with 
full-blown 
familial 
hypercholes-
terolemia have blood cholesterol concentrations of 600
to 1000 mg/dl, levels that are four to six times normal.
Many of these peopl

 are four to six times normal.
Many of these people die before age 20 because of
myocardial infarction or other sequelae of atheroscle-
rotic blockage of blood vessels throughout the body.
Role of High-Density Lipoproteins in Preventing Atherosclerosis.
Much less is known about the function of high-density
lipoproteins compared with that of low-density lipopro-
teins. It is believed that high-density lipoproteins can
actually absorb cholesterol crystals that are beginning
to be deposited in arte

rystals that are beginning
to be deposited in arterial walls. Whether this mecha-
nism is true or not, high-density lipoproteins do help
protect against the development of atherosclerosis.
Consequently, when a person has a high ratio of 
high-density to low-density lipoproteins, the likelihood
of developing atherosclerosis is greatly reduced.
Other Major Risk Factors 
for Atherosclerosis
In some people with perfectly normal levels of choles-
terol and lipoproteins, atherosclerosis still develops

l and lipoproteins, atherosclerosis still develops.
Some of the factors that are known to predispose to ath-
erosclerosis are (1) physical inactivity and obesity, (2)
diabetes mellitus, (3) hypertension, (4) hyperlipidemia,
and (5) cigarette smoking.
Hypertension, for example, increases the risk for 
atherosclerotic coronary artery disease by at least
twofold. Likewise, a person with diabetes mellitus has,
on average, more than a twofold increased risk of devel-
oping coronary artery disease. Wh

d risk of devel-
oping coronary artery disease. When hypertension and
diabetes mellitus occur together, the risk for coronary
artery disease is increased by more than eightfold. And
when hypertension, diabetes mellitus, and hyperlipemia
are all present, the risk for atherosclerotic coronary
artery disease is increased almost 20-fold, suggesting
that these factors interact in a synergistic manner to
increase the risk of developing atherosclerosis. In many
overweight and obese patients, these thre

 In many
overweight and obese patients, these three risk factors
do occur together, greatly increasing their risk for 
atherosclerosis, which in turn may lead to heart attack,
stroke, and kidney disease.
In early and middle adulthood, men are more likely
to develop atherosclerosis than are women of compa-
rable age, suggesting that male sex hormones might be
atherogenic or, conversely, that female sex hormones
might be protective.
Some of these factors cause atherosclerosis by
increasing the con

actors cause atherosclerosis by
increasing the concentration of low-density lipoproteins
in the plasma. Others, such as hypertension, lead to 
atherosclerosis by causing damage to the vascular en-
dothelium and other changes in the vascular tissues that
predispose to cholesterol deposition.
To add to the complexity of atherosclerosis, experi-
mental studies suggest that excess blood levels of iron
can lead to atherosclerosis, perhaps by forming free rad-
icals in the blood that damage the vessel

ree rad-
icals in the blood that damage the vessel walls. About
one quarter of all people have a special type of low-
density lipoprotein called lipoprotein(a), containing an
additional protein, apoprotein(a), that almost doubles
the incidence of atherosclerosis. The precise mecha-
nisms of these atherogenic effects have yet to be 
discovered.
The most important measures to protect against the
development of atherosclerosis and its progression to
serious vascular di

lerosis and its progression to
serious vascular disease are (1) maintaining a healthy
weight, being physically active, and eating a diet that
contains mainly unsaturated fat with a low cholesterol
content; (2) preventing hypertension by maintaining a
healthy diet and being physically active, or effectively
controlling blood pressure with antihypertensive drugs
if hypertension does develop; (3) effectively controlling
blood glucose with insulin treatment or other drugs if
diabetes develops; and (

eatment or other drugs if
diabetes develops; and (4) avoiding cigarette smoking.
Several types of drugs that lower plasma lipids and
cholesterol have proved to be valuable in preventing
atherosclerosis. Most of the cholesterol formed in the
liver is converted into bile acids and secreted in this
Chapter 68
851
form into the duodenum; then, more than 90 per cent of
these same bile acids is reabsorbed in the terminal ileum
and used over and over again in the bile.Therefore, any
ag

 over and over again in the bile.Therefore, any
agent that combines with the bile acids in the gastroin-
testinal tract and prevents their reabsorption into the
circulation can decrease the total bile acid pool in the
circulating blood. This causes far more of the liver cho-
lesterol to be converted into new bile acids.Thus, simply
eating oat bran, which binds bile acids and is a con-
stituent of many breakfast cereals, increases the pro-
portion of liver cholesterol that forms new bile acids
ra

 of liver cholesterol that forms new bile acids
rather than forming new low-density lipoproteins and
atherogenic plaques. Resin agents can also be used to
bind bile acids in the gut and increase their fecal excre-
tion, thereby reducing cholesterol synthesis by the liver.
Another group of drugs called statins competitively
inhibits hydroxymethylglutaryl-coenzyme A (HMG-
CoA) reductase, a rate-limiting enzyme in the synthesis
of cholesterol.This inhibition decreases cholesterol syn-
thesis and in

nhibition decreases cholesterol syn-
thesis and increases low-density lipoprotein receptors in
the liver, usually causing a 25 to 50 per cent reduction
in plasma levels of low-density lipoproteins. The statins
may also have other beneﬁcial effects that help prevent
atherosclerosis, such as attenuating vascular inﬂamma-
tion. These drugs are now widely used to treat patients
who have increased plasma cholesterol levels.
In general, studies show that for each 1 mg/dl
decrease in low-density lipopr

at for each 1 mg/dl
decrease in low-density lipoprotein cholesterol in the
plasma, there is about a 2 per cent decrease in mortal-
ity from atherosclerotic heart disease.Therefore, appro-
priate preventive measures are valuable in decreasing
heart attacks.
Assmann G, Nofer JR: Atheroprotective effects of high-
density lipoproteins. Annu Rev Med 54:321, 2003.
Brown MS, Goldstein JL: A proteolytic pathway that con-
trols the cholesterol content of membranes, cells, and
blood. Proc Natl 

content of membranes, cells, and
blood. Proc Natl Acad Sci U S A 96:11041, 1999.
Fielding BA, Frayn KN: Lipid metabolism. Curr Opin
Lipidol 14:389, 2003.
Fromenty B, Robin MA, Igoudjil A, et al: The ins and outs
of mitochondrial dysfunction in NASH. Diabetes Metab
30:121, 2004.
Gotto AM Jr, Brinton EA: Assessing low levels of high-
density lipopr

n EA: Assessing low levels of high-
density lipoprotein cholesterol as a risk factor in coronary
heart disease: a working group report and update. J Am
Coll Cardiol 43:717, 2004.
Grundy SM, Brewer HB Jr, Cleeman JI, et al: Deﬁnition of
metabolic syndrome: report of the National Heart, Lung,
and Blood Institute/American Heart Association confer-
ence on scientiﬁc issues related to deﬁnition. Circulation
109:433, 2004.
Haemmerle G, Zimmermann R, Zechner R: Letting lipids
go: hormone-sensitive lipa

chner R: Letting lipids
go: hormone-sensitive lipase. Curr Opin Lipidol 14:289,
2003.
Havel RJ, Hamilton RL: Hepatic catabolism of remnant
lipoproteins:where the action is.Arterioscler Thromb Vasc
Biol 24:213, 2004.
Hilgemann DW: Getting ready for the decade of the lipids.
Annu Rev Physiol 65:697, 2003.
Horton JD, Goldstein JL, Brown MS: SREBPs: activators of
the complete program of cholesterol and fatty acid syn-
thesis in the liver. J Clin Invest 109:1125, 2002.
LaRosa JC, Gotto AM Jr: Past, p

st 109:1125, 2002.
LaRosa JC, Gotto AM Jr: Past, present, and future standards
for management of dyslipidemia. Am J Med 116(Suppl
6A):3S, 2004.
2002.
Osterud B, Bjorklid E: Role of monocytes in atherogenesis.
Physiol Rev 83:1069, 2003.
Rinaldo P, Matern D, Bennett MJ: Fatty acid oxidation dis-
orders. Annu Rev Physiol 64:477, 2002.
Roden M: How free fatty acids inhibit glucose utilization in
human skeletal muscle. News Physiol Sci 19:92, 2

n
human skeletal muscle. News Physiol Sci 19:92, 2004.
Ruderman NB, Saha AK, Kraegen EW: Malonyl CoA,AMP-
activated protein kinase, and adiposity. Endocrinology
144:5166, 2003.
Schonbeck U, Libby P: Inﬂammation, immunity, and HMG-
CoA reductase inhibitors: statins as antiinﬂammatory
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Rev Physiol 65:333, 2003.
C
H
A
P
T
E
R
 
6
9
852
About three quarters of the body solids are proteins.
These include structural proteins, enzymes, nucleopro-
teins, proteins that transport oxygen, proteins of the
muscle that cause muscle contraction, and many other
types that perform speciﬁc intracellular and extracellu-
lar functions throughout the body.
The basic chemical properti

s throughout the body.
The basic chemical properties that explain proteins’
diverse 
functions 
are 
so 
extensive 
that 
they 
constitute a major portion of the entire discipline of
biochemistry. For this reason, the current discussion is conﬁned to a few speciﬁc
aspects of protein metabolism that are important as background for other discus-
sions in this text.
The principal constituents of proteins are amino acids, 20 of which are present in
the body proteins in s

 20 of which are present in
the body proteins in signiﬁcant quantities. Figure 69–1 shows the chemical formu-
las of these 20 amino acids, demonstrating that they all have two features in
common: each amino acid has an acidic group (—COOH) and a nitrogen atom
attached to the molecule, usually represented by the amino group (—NH2).
Peptide Linkages and Peptide Chains. In proteins, the amino acids are aggregated into
long chains by means of peptide linkages. The chemical nature of this linkage is

 linkages. The chemical nature of this linkage is
demonstrated by the following reaction:
CO
CH
COOH
OH + R'
NH2
CH
R
H NH
CO
NH2
NH + H2O
CH
R
COOH
CH
R'
Note in this reaction that the nitrogen of the amino radical of one amino acid
bonds with the carbon of the carboxyl radical of the other amino acid. A hydrogen
ion is released from the amino radical, and a hydroxyl ion is released from the car-
boxyl radical; these two combine to form a molecule of water. After the peptide
linkage has been fo

le of water. After the peptide
linkage has been formed, an amino radical and a carboxyl radical are still at oppo-
site ends of the new, longer molecule. Each of these radicals is capable of combin-
ing with additional amino acids to form a peptide chain. Some complicated protein
molecules have many thousand amino acids combined by peptide linkages, and even
the smallest protein molecule usually has more than 20 amino acids combined by
peptide linkages. The average is about 400 amino acids.
Othe

nkages. The average is about 400 amino acids.
Other Linkages in Protein Molecules. Some protein molecules are composed of several
peptide chains rather than a single chain, and these chains are bound to one another
Chapter 69
853
C
H
H
COOH
NH2
C
H
H
N
COOH
CH2
H2C
H2C
C
C
H
C
O
COOH
H
NH2
NH2
C
H
H
COOH
H
C
H
NH2
C
H
H
COOH
OH
C
H
NH2
C
H
H
COOH
SH
C
H
NH2
H
C
H
COOH
COOH
C
NH2
H
H
C
H
COOH
COOH
C
NH2
H
H
C
H
H
C
HC
N
COOH
CH
C
N
H
H
H
C
H
C
C
C
H
H
C
O
COOH
H
NH2
H
NH2
H
H
H

C
N
H
H
H
C
H
C
C
C
H
H
C
O
COOH
H
NH2
H
NH2
H
H
H
H
H
H
C
C
H
COOH
H
HO
H
NH2
C
C
H
COOH
H
H
NH2
H
C
C
C
C
CH3
COOH
H
H
H
H
H
H
C
C
H
H
C
H
H
C
C
NH2
COOH
H
H
H
H
C
C
H
H
C
H
H
C
COOH
NH2
COOH
CH3
S
C
C
H
H
H
C
H
H
COOH
H
C
C
H
H
OH
C
H
H
NH2
COOH
H
C
C
H
H
C
H
H
H
NH2
NH2
NH2
C
C
H
H
H
N
C
NH H
C
H
H
C
C
H
H
COOH
NH2
NH2
C
H
H
H
H2N
H
C
C
H
COOH
H
NH2
NH2
H
C
CH
N
H
Alanine
Glycine
Proline
Glutamine
Serine
Cysteine
Aspartic Acid
Glutamic Acid
HISTIDINE
ISOLEUCINE
Tyrosine
LEUCINE
TR

amic Acid
HISTIDINE
Tyrosine
LEUCINE
TRYPTOPHAN
VALINE
PHENYLALANINE
METHIONINE
THREONINE
AMINO ACIDS
LYSINE
ESSENTIAL AMINO ACIDS
ARGININE
Figure 69–1
Amino acids. The 10 essential amino acids cannot be synthesized in sufﬁcient quantities in the body; these essential amino acids must
be obtained, already formed, from food.
854
Unit XIII
by other linkages, often by hydrogen bonding between
the CO and NH radicals of the peptides, as follows:
Many p

nd NH radicals of the peptides, as follows:
Many peptide chains are coiled or folded, and the suc-
cessive coils or folds are held in a tight spiral or in other
shapes by similar hydrogen bonding and other forces.
Transport and Storage 
of Amino Acids
The normal concentration of amino acids in the blood
is between 35 and 65 mg/dl. This is an average of about
2 mg/dl for each of the 20 amino acids, although some
are present in far greater amounts than others. Because
the amino a

r greater amounts than others. Because
the amino acids are relatively strong acids, they exist 
in the blood principally in the ionized state, resulting
from the removal of one hydrogen atom from the NH2
radical.They actually account for 2 to 3 milliequivalents
of the negative ions in the blood. The precise distribu-
tion of the different amino acids in the blood depends
to some extent on the types of proteins eaten, but 
the concentrations of at least some individual amino
acids are regulated b

 least some individual amino
acids are regulated by selective synthesis in the differ-
ent cells.
Fate of Amino Acids Absorbed from the Gastrointestinal Tract.
The products of protein digestion and absorption in the
gastrointestinal tract are almost entirely amino acids;
only rarely are polypeptides or whole protein molecules
absorbed from the digestive tract into the blood. Imme-
diately after a meal, the amino acid concentration in a
person’s blood rises, but the increase is usually only a
few

lood rises, but the increase is usually only a
few milligrams per deciliter, for two reasons: First,
protein digestion and absorption are usually extended
over 2 to 3 hours, which allows only small quantities of
amino acids to be absorbed at a time. Second, after
entering the blood, the excess amino acids are absorbed
within 5 to 10 minutes by cells throughout the body,
especially by the liver. Therefore, almost never do large
concentrations of amino acids accumulate in the blood
and tissue ﬂuid

mino acids accumulate in the blood
and tissue ﬂuids. Nevertheless, the turnover rate of the
amino acids is so rapid that many grams of proteins can
be carried from one part of the body to another in the
form of amino acids each hour.
Active Transport of Amino Acids into the Cells. The molecules
of all the amino acids are much too large to diffuse
readily through the pores of the cell membranes.There-
fore, signiﬁcant quantities of amino acids can move
either inward or outward through the membran

 move
either inward or outward through the membranes only
by facilitated transport or active transport using carrier
mechanisms. The nature of some of the carrier mecha-
nisms is still poorly understood, but a few are discussed
in Chapter 4.
R
C
N
H
O
C
O
H
N
HC
R'
CH
Renal Threshold for Amino Acids. In the kidneys, the
different amino acids can be actively reabsorbed
through the proximal tubular epithelium,
which
removes them from the glomerular ﬁltrate and returns
them to the blood if they sho

 ﬁltrate and returns
them to the blood if they should ﬁlter into the renal
tubules through the glomerular membranes. However,
as is true of other active transport mechanisms in the
renal tubules, there is an upper limit to the rate at which
each type of amino acid can be transported. For this
reason, when the concentration of a particular type 
of amino acid becomes too high in the plasma and
glomerular ﬁltrate, the excess that cannot be actively
reabsorbed is lost into the urine.
Storage of Ami

Storage of Amino Acids as Proteins
in the Cells
Almost immediately after entry into tissue cells, amino
acids combine with one another by peptide linkages,
under the direction of the cell’s messenger RNA and
ribosomal system, to form cellular proteins. Therefore,
the concentration of free amino acids inside the cells
usually remains low. Thus, storage of large quantities 
of free amino acids does not occur in the cells; instead,
they are stored mainly in the f

he cells; instead,
they are stored mainly in the form of actual proteins.
But many of these intracellular proteins can be rapidly
decomposed again into amino acids under the inﬂuence
of intracellular lysosomal digestive enzymes; these
amino acids can then be transported back out of the 
cell into the blood. Special exceptions to this reversal
process are the proteins in the chromosomes of the
nucleus and the structural proteins such as collagen 
and muscle contractile proteins; these proteins do

and muscle contractile proteins; these proteins do not
participate signiﬁcantly in this reverse digestion and
transport back out of the cells.
Some tissues of the body participate in the storage 
of amino acids to a greater extent than others. For
instance, the liver, which is a large organ and has special
systems for processing amino acids, can store large
quantities of rapidly exchangeable proteins; this is also
true to a lesser extent of the kidneys and the intestinal
mucosa.
Release of Amino

idneys and the intestinal
mucosa.
Release of Amino Acids from the Cells as a Means of Regulating
Plasma Amino Acid Concentration. Whenever plasma amino
acid concentrations fall below normal levels, the
required amino acids are transported out of the cells to
replenish their supply in the plasma. In this way, the
plasma concentration of each type of amino acid is main-
tained at a reasonably constant value. Later, it is noted
that some of the hormones secreted by the endocrine
glands are able to 

ones secreted by the endocrine
glands are able to alter the balance between tissue pro-
teins and circulating amino acids. For instance, growth
hormone and insulin increase the formation of tissue
proteins, whereas adrenocortical glucocorticoid hor-
mones increase the concentration of plasma amino acids.
Reversible Equilibrium Between the Proteins in Different Parts of
the Body. Because cellular proteins in the liver (and, to
a much less extent, in other tissues) can be synthesized
rapidly from 

in other tissues) can be synthesized
rapidly from plasma amino acids, and because many of
these proteins can be degraded and returned to the
plasma almost as rapidly, there is constant interchange
and equilibrium between the plasma amino acids and
labile proteins in virtually all cells of the body. For
instance, if any particular tissue requires proteins, it can
synthesize new proteins from the amino acids of the
Chapter 69
855
blood; in turn, the blood amino acids are repleni

blood; in turn, the blood amino acids are replenished by
degradation of proteins from other cells of the body,
especially from the liver cells. These effects are particu-
larly noticeable in relation to protein synthesis in cancer
cells. Cancer cells are often proliﬁc users of amino acids;
therefore, the proteins of the other cells can become
markedly depleted.
Upper Limit for the Storage of Proteins. Each particular type
of cell has an upper limit with regard to the amount of
proteins it can s

mit with regard to the amount of
proteins it can store. After all the cells have reached
their limits, the excess amino acids still in the circulation
are degraded into other products and used for energy,
as discussed subsequently, or they are converted to fat
or glycogen and stored in these forms.
Functional Roles of the
Plasma Proteins
The major types of protein present in the plasma are
albumin, globulin, and ﬁbrinogen.
A major function of albumin is to provide colloid
osmotic pressure in the

umin is to provide colloid
osmotic pressure in the plasma, which prevents plasma
loss from the capillaries, as discussed in Chapter 16.
The globulins perform a number of enzymatic func-
tions in the plasma, but equally important, they are prin-
cipally responsible for the body’s both natural and
acquired immunity against invading organisms, dis-
cussed in Chapter 34.
Fibrinogen polymerizes into long ﬁbrin threads
during blood coagulation, thereby forming blood clots
that help repair leaks in the

 forming blood clots
that help repair leaks in the circulatory system, dis-
cussed in Chapter 36.
Formation of the Plasma Proteins. Essentially all the
albumin and ﬁbrinogen of the plasma proteins, as well
as 50 to 80 per cent of the globulins, are formed in the
liver. The remainder of the globulins are formed almost
entirely in the lymphoid tissues. They are mainly the
gamma globulins that constitute the antibodies used in
the immune system.
The rate of plasma protein formation by the liver can

 rate of plasma protein formation by the liver can
be extremely high, as much as 30 g/day. Certain disease
conditions cause rapid loss of plasma proteins; severe
burns that denude large surface areas of the skin can
cause the loss of several liters of plasma through the
denuded areas each day.The rapid production of plasma
proteins by the liver is valuable in preventing death in
such states. Occasionally, a person with severe renal
disease loses as much as 20 grams of plasma protein in
the urine

as much as 20 grams of plasma protein in
the urine each day for months, and it is continually
replaced mainly by liver production of the required 
proteins.
In cirrhosis of the liver, large amounts of ﬁbrous tissue
develop among the liver parenchymal cells, causing a
reduction in their ability to synthesize plasma proteins.
As discussed in Chapter 25, this leads to decreased
plasma colloid osmotic pressure, which causes general-
ized edema.
Plasma Proteins as a Source of Amino Acids for the Tiss

a Proteins as a Source of Amino Acids for the Tissues.
When the tissues become depleted of proteins, the
plasma proteins can act as a source of rapid replace-
ment. Indeed, whole plasma proteins can be imbibed in
toto by tissue macrophages through the process of
pinocytosis; once in these cells, they are split into amino
acids that are transported back into the blood and used
throughout the body to build cellular proteins wherever
needed. In this way, the plasma proteins function as a
labile pro

 way, the plasma proteins function as a
labile protein storage medium and represent a readily
available source of amino acids whenever a particular
tissue requires them.
Reversible Equilibrium Between the Plasma Proteins and the
Tissue Proteins. There is a constant state of equilibrium,
as shown in Figure 69–2, among the plasma proteins, the
amino acids of the plasma, and the tissue proteins. It has
been estimated from radioactive tracer studies that
normally about 400 grams of body protein are 

that
normally about 400 grams of body protein are synthe-
sized and degraded each day as part of the continual
state of ﬂux of amino acids. This demonstrates the
general principle of reversible exchange of amino acids
among the different proteins of the body. Even during
starvation or severe debilitating diseases, the ratio of
total tissue proteins to total plasma proteins in the body
remains relatively constant at about 33:1.
Because of this reversible equilibrium between
plasma proteins and th

ersible equilibrium between
plasma proteins and the other proteins of the body, one
of the most effective therapies for severe, acute whole-
body protein deﬁciency is intravenous transfusion of
plasma protein. Within a few days, or sometimes within
hours, the amino acids of the administered protein are
distributed throughout the cells of the body to form new
proteins where they are needed.
Essential and Nonessential 
Ten of the amino acids normally present in animal pro-
teins can be

acids normally present in animal pro-
teins can be synthesized in the cells, whereas the other
10 either cannot be synthesized or are synthesized in
quantities too small to supply the body’s needs. This
second group of amino acids that cannot be synthesized
is called the essential amino acids. Use of the word
“essential” does not mean that the other 10 “nonessen-
tial” amino acids are not required for the formation of
proteins, but only that the others are not essential in the
diet because they 

others are not essential in the
diet because they can be synthesized in the body.
Synthesis of the nonessential amino acids depends
mainly on the formation of appropriate a-keto acids,
Tissue cells
Liver cells
Proteins
Proteins
Amino
acids
Amino
acids
Proteins
Proteins
Plasma proteins
Imbibed plasma protein
Reticuloendothelial cell
Blood
Blood
Figure 69–2
Rever

no acids
Blood
Blood
Reversible equilibrium among the tissue proteins, plasma pro-
teins, and plasma amino acids.
856
Unit XIII
which are the precursors of the respective amino acids.
For instance, pyruvic acid, which is formed in large
quantities during the glycolytic breakdown of glucose,
is the keto acid precursor of the amino acid alanine.
Then, by the process of transamination, an amino
radical is transferred to the a-keto acid, 

 amino
radical is transferred to the a-keto acid, and the keto
oxygen is transferred to the donor of the amino radical.
This reaction is shown in Figure 69–3. Note in this ﬁgure
that the amino radical is transferred to the pyruvic acid
from another chemical that is closely allied to the amino
acids, glutamine. Glutamine is present in the tissues in
large quantities, and one of its principal functions is to
serve as an amino radical storehouse. In addition, amino
radicals can be transferred from 

 addition, amino
radicals can be transferred from asparagine, glutamic
acid, and aspartic acid.
Transamination is promoted by several enzymes,
among which are the aminotransferases, which are deriv-
atives of pyridoxine, one of the B vitamins (B6).Without
this vitamin,the amino acids are synthesized only poorly,
and protein formation cannot proceed normally.
Use of Proteins for Energy
Once the cells are ﬁlled to their limits with stored
protein, any additional amino acids in the body ﬂuids are
d

any additional amino acids in the body ﬂuids are
degraded and used for energy or are stored mainly as fat
or secondarily as glycogen. This degradation occurs
almost entirely in the liver, and it begins with deamina-
tion, which is explained in the following section.
Deamination. Deamination means removal of the amino
groups from the amino acids. This occurs mainly by
transamination, which means transfer of the amino
group to some acceptor substance, which is the reverse
of the transamination exp

ce, which is the reverse
of the transamination explained earlier in relation to the
synthesis of amino acids.
The greatest amount of deamination occurs by the
following transamination schema:
Note from this schema that the amino group from the
amino acid is transferred to a-ketoglutaric acid, which
then becomes glutamic acid.The glutamic acid can then
transfer the amino group to still other substances or
release it in the form of ammonia (NH3). In the process
of losing the amino group, the gluta

n the process
of losing the amino group, the glutamic acid once again
becomes a-ketoglutaric acid, so that the cycle can be
repeated again and again. To initiate this process, the
excess amino acids in the cells, especially in the liver,
induce the activation of large quantities of aminotrans-
ferases, the enzymes responsible for initiating most
deamination.
Urea Formation by the Liver. The ammonia released
during deamination of amino acids is removed from the
blood almost entirely by conversion

moved from the
blood almost entirely by conversion into urea; two mol-
ecules of ammonia and one molecule of carbon dioxide
combine in accordance with the following net reaction:
2 NH3 + CO2 Æ H2N—C—NH2 + H2O
||
O
Essentially all urea formed in the human body is syn-
thesized in the liver. In the absence of the liver or in
serious liver disease, ammonia accumulates in the
blood. This is extremely toxic, especially to the brain,
often leading to a state called hepatic coma.
The stages in the form

 state called hepatic coma.
The stages in the formation of urea are essentially the
After its formation, the urea diffuses from the liver cells
into the body ﬂuids and is excreted by the kidneys.
Oxidation of Deaminated Amino Acids. Once amino acids
have been deaminated, the resulting keto acids can, in
most instances, be oxidized to release energy for meta-
bolic purposes. This usually involves two successive
processes: (1) the keto acid is changed into an appro-
priate chemical subs

cid is changed into an appro-
priate chemical substance that can enter the citric acid
cycle, and (2) this substance is degraded by the cycle and
Citrulline
Arginine
Urea
Ornithine + CO2 + NH3
NH3
-H2O
(Arginase)
+ H2O
-H2O
(Glutamine)
(a-Ketoglutamic acid)
(Alanine)
+
+
O
O
NH2
Transaminase
NH2
CH2
CH2
CH
C
COOH
CH3
C
COOH
O
O
NH2
CH2
CH2
C
C
COOH
NH
CH3
C
COOH
Figure 69–3
Synthesis of alanine from pyruvic
acid by transamination.
a-Ketoglutaric acid + Amino acid
Glutamic acid + a

Glutamic acid + a-Keto acid
+ NAD+ + H2O
NADH + H+ + NH3
Chapter 69
857
used for energy in the same manner that acetyl coen-
zyme A (acetyl-CoA) derived from carbohydrate and
lipid metabolism is used, as explained in Chapters 67
and 68. In general, the amount of adenosine triphos-
phate (ATP) formed for each gram of protein that is
oxidized is slightly less than that formed for each gram
of glucose oxidized.
Gluconeogenesis and Ketogenesis. Cer

ose oxidized.
Gluconeogenesis and Ketogenesis. Certain deaminated
amino acids are similar to the substrates normally used
by the cells, mainly the liver cells, to synthesize glucose
or fatty acids. For instance, deaminated alanine is
pyruvic acid. This can be converted into either glucose
or glycogen. Alternatively, it can be converted into
acetyl-CoA, which can then be polymerized into fatty
acids. Also, two molecules of acetyl-CoA can condense
to form acetoacetic acid, which is one of the keto

to form acetoacetic acid, which is one of the ketone
bodies, as explained in Chapter 68.
The conversion of amino acids into glucose or glyco-
gen is called gluconeogenesis, and the conversion of
amino acids into keto acids or fatty acids is called keto-
genesis. Of the 20 deaminated amino acids, 18 have
chemical structures that allow them to be converted into
glucose, and 19 of them can be converted into fatty
acids.
When a person eats no proteins, a certain pr

teins
When a person eats no proteins, a certain proportion of
body proteins is degraded into amino acids and then
deaminated and oxidized. This involves 20 to 30 grams
of protein each day, which is called the obligatory 
loss of proteins. Therefore, to prevent net loss of 
protein from the body, one must ingest a minimum 
of 20 to 30 grams of protein each day; to be on the 
safe side, a minimum of 60 to 75 grams is usually 
recommended.
The ratios of the different amino acids in the dietary
prot

s of the different amino acids in the dietary
protein must be about the same as the ratios in the body
tissues if the entire dietary protein is to be fully usable
to form new proteins in the tissues.If one particular type
of essential amino acid is low in concentration, the
others become unusable because cells synthesize either
whole proteins or none at all, as explained in Chapter 3
in relation to protein synthesis. The unusable amino
acids are deaminated and oxidized. A protein that has
a rati

deaminated and oxidized. A protein that has
a ratio of amino acids different from that of the average
body protein is called a partial protein or incomplete
protein, and such a protein is less valuable for nutrition
than is a complete protein.
Effect of Starvation on Protein Degradation. Except for the 20
to 30 grams of obligatory protein degradation each day,
the body uses almost entirely carbohydrates or fats 
for energy, as long as they are available. However,
after several weeks of starvatio

ailable. However,
after several weeks of starvation, when the quantities 
of stored carbohydrates and fats begin to run out, the
amino acids of the blood are rapidly deaminated and
oxidized for energy. From this point on, the proteins 
of the tissues degrade rapidly—as much as 125 grams
daily—and, as a result, cellular functions deteriorate
precipitously. Because carbohydrate and fat utilization
for energy normally occurs in preference to protein 
utilization, carbohydrates and fats are called p

utilization, carbohydrates and fats are called protein
sparers.
Hormonal Regulation 
of Protein Metabolism
Growth Hormone Increases the Synthesis of Cellular Proteins.
Growth hormone causes the tissue proteins to increase.
The precise mechanism by which this occurs is not
known, but it is believed to result mainly from increased
transport of amino acids through the cell membranes or
acceleration of the DNA and RNA transcription and
translation processes for protein synthesis.
Insulin Is Necess

processes for protein synthesis.
Insulin Is Necessary for Protein Synthesis. Total lack of
insulin reduces protein synthesis to almost zero. The
mechanism by which this occurs is also unknown, but
insulin does accelerate the transport of some amino
acids into cells, which could be the stimulus to protein
synthesis. Also, insulin increases the availability of
glucose to the cells, so that the need for amino acids for
energy is correspondingly reduced.
Glucocorticoids Increase Breakdown of Most Ti

ced.
Glucocorticoids Increase Breakdown of Most Tissue Proteins.
The glucocorticoids secreted by the adrenal cortex
decrease the quantity of protein in most tissues while
increasing the amino acid concentration in the plasma,
as well as increasing both liver proteins and plasma pro-
teins. It is believed that the glucocorticoids act by
increasing the rate of breakdown of extrahepatic pro-
teins, thereby making increased quantities of amino
acids available in the body ﬂuids.This supposedly allows

available in the body ﬂuids.This supposedly allows
the liver to synthesize increased quantities of hepatic
cellular proteins and plasma proteins.
Testosterone Increases Protein Deposition in Tissues. Testos-
terone, the male sex hormone, causes increased deposi-
tion of protein in tissues throughout the body, especially
the contractile proteins of the muscles (30 to 50 per cent
increase). The mechanism of this effect is unknown, but
it is deﬁnitely different from the effect of growth
hormone, in

ly different from the effect of growth
hormone, in the following way: Growth hormone causes
tissues to continue growing almost indeﬁnitely, whereas
testosterone causes the muscles and, to a much lesser
extent, some other protein tissues to enlarge for only
several months. Once the muscles and other protein
tissues have reached a maximum, despite continued
administration of testosterone, further protein deposi-
tion ceases.
Estrogen. Estrogen, the principal female sex hormone,
also causes some de

 principal female sex hormone,
also causes some deposition of protein, but its effect 
is relatively insigniﬁcant in comparison with that of
Thyroxine. Thyroxine increases the rate of metabolism of
all cells and, as a result, indirectly affects protein metab-
olism. If insufﬁcient carbohydrates and fats are avail-
able for energy, thyroxine causes rapid degradation of
proteins and uses them for energy. Conversely, if ade-
quate quantities of carbohydrates and fats are available
and

tities of carbohydrates and fats are available
and excess amino acids are also available in the extra-
cellular ﬂuid, thyroxine can actually increase the rate of
protein synthesis. In growing animals or human beings,
deﬁciency of thyroxine causes growth to be greatly
inhibited because of lack of protein synthesis. In
essence, it is believed that thyroxine has little speciﬁc
effect on protein metabolism but does have an 
important general effect by increasing the rates of 
both normal anabolic an

y increasing the rates of 
both normal anabolic and normal catabolic protein
reactions.
858
Unit XIII
Altenberg GA: The engine of ABC proteins. News Physiol
Sci 18:191, 2003.
Caldwell J: Pharmacogenetics and individual variation in 
the range of amino acid adequacy: the biological aspects.
J Nutr 134(6 Suppl):1600S, 2004.
Deves R, Boyd CA

t. Annu Rev Physiol 66:361, 2004.
Deves R, Boyd CA: Transporters for cationic amino acids in
animal cells: discovery, structure, and function. Physiol
Rev 78:487, 1998.
Fukagawa NK, Galbraith RA: Advancing age and other
factors inﬂuencing the balance between amino acid
requirements and toxicity. J Nutr 134(6 Suppl):1569S,
2004.
Jans DA, Hubner S: Regulation of protein transport to the
nucleus: central role of phosphorylation. Physiol Rev
76:651, 1996.
Kadowaki M, Kanazawa T:Amino acids as regula

1996.
Kadowaki M, Kanazawa T:Amino acids as regulators of pro-
teolysis. J Nutr 133(6 Suppl 1):2052S, 2003.
Kimball SR, Jefferson LS: Regulation of global and speciﬁc
mRNA translation by oral administration of branched-
chain amino acids. Biochem Biophys Res Commun
313:423, 2004.
Kuhn CM:Anabolic steroids. Recent Prog Horm Res 57:411,
2002.
Layman DK, Baum JI: Dietary protein impact on glycemic
control during weight loss. J Nutr 134:968S, 2004.
Mann GE, Yudilevich DL, Sobrevia L: Regulation of a

ann GE, Yudilevich DL, Sobrevia L: Regulation of amino
acid and glucose transporters in endothelial and smooth
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Meijer AJ: Amino acids as regulators and components of
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2003.
Moriwaki H, Miwa Y,Tajika M, et al: Branched-chain amino
acids as a protein- and energy-source in liver cirrhosis.
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no acid needs for early growth
and development. J Nutr 134(6 Suppl):1566S, 2004.
Prod’homme M, Rieu I, Balage M, et al: Insulin and amino
acids both strongly participate to the regulation of protein
metabolism. Curr Opin Clin Nutr Metab Care 7:71, 2004.
Tessari P:Protein metabolism in liver cirrhosis:from albumin
to muscle myoﬁbrils. Curr Opin Clin Nutr Metab Care
6:79, 2003.
van de Poll MC, Soeters PB, Deutz NE, et al: Renal metab-
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859
The liver performs many different functions yet is also
a discrete organ, and many of its functions interrelate
with one another. This becomes especially evident in
abnormalities of the liver, because many of its functions
are disturbed simultaneously. The purpose of this
chapter is to summarize the liver’s different functions,
including (1) ﬁltration and storage of blo

ctions,
including (1) ﬁltration and storage of blood; (2)
metabolism of carbohydrates, proteins, fats, hormones,
and foreign chemicals; (3) formation of bile; (4) storage
of vitamins and iron; and (5) formation of coagulation factors.
The liver is the largest organ in the body, contributing about 2 per cent of the total
body weight, or about 1.5 kg in the average adult human. The basic functional unit
of the liver is the liver lobule, which is a cylindrical struc

 is the liver lobule, which is a cylindrical structure several millimeters in
length and 0.8 to 2 millimeters in diameter. The human liver contains 50,000 to
100,000 individual lobules.
The liver lobule, shown in cut-away format in Figure 70–1, is constructed around
a central vein that empties into the hepatic veins and then into the vena cava. The
lobule itself is composed principally of many liver cellular plates (two of which are
shown in Figure 70–1) that radiate from the central vein like s

re 70–1) that radiate from the central vein like spokes in a wheel. Each
hepatic plate is usually two cells thick, and between the adjacent cells lie small bile
canaliculi that empty into bile ducts in the ﬁbrous septa separating the adjacent liver
lobules.
In the septa are small portal venules that receive their blood mainly from the
venous outﬂow of the gastrointestinal tract by way of the portal vein. From these
venules blood ﬂows into ﬂat, branching hepatic sinusoids that lie between the
hep

anching hepatic sinusoids that lie between the
hepatic plates and then into the central vein.Thus, the hepatic cells are exposed con-
tinuously to portal venous blood.
Hepatic arterioles are also present in the interlobular septa. These arterioles
supply arterial blood to the septal tissues between the adjacent lobules, and many
of the small arterioles also empty directly into the hepatic sinusoids, most frequently
emptying into those located about one third the distance from the interlobular
se

ut one third the distance from the interlobular
septa, as shown in Figure 70–1.
In addition to the hepatic cells, the venous sinusoids are lined by two other types
of cell: (1) typical endothelial cells and (2) large Kupffer cells (also called reticu-
loendothelial cells), which are resident macrophages that line the sinusoids and are
capable of phagocytizing bacteria and other foreign matter in the hepatic sinus
blood.
The endothelial lining of the sinusoids has extremely large pores, some of w

the sinusoids has extremely large pores, some of which
are almost 1 micrometer in diameter. Beneath this lining, lying between the
endothelial cells and the hepatic cells, are narrow tissue spaces called the spaces of
Disse, also known as the perisinusoidal spaces. The millions of spaces of Disse
connect with lymphatic vessels in the interlobular septa. Therefore, excess ﬂuid in
these spaces is removed through the lymphatics. Because of the large pores in the
endothelium, substances in the plasm

 pores in the
endothelium, substances in the plasma move freely into the spaces of Disse. Even
large portions of the plasma proteins diffuse freely into these spaces.
Hepatic Vascular and Lymph Systems
The function of the hepatic vascular system is discussed in Chapter 15 in connec-
tion with the portal veins and can be summarized as follows.
860
Unit XIII
Blood Flows Through the Liver from
the Portal Vein and Hepatic Artery
The Liver Has High Blood Flow and

d Hepatic Artery
The Liver Has High Blood Flow and Low Vascular Resistance.
About 1050 milliliters of blood ﬂows from the portal
vein into the liver sinusoids each minute, and an addi-
tional 300 milliliters ﬂows into the sinusoids from the
hepatic artery, the total averaging about 1350 ml/min.
This amounts to 27 per cent of the resting cardiac
output.
The pressure in the portal vein leading into the liver
averages about 9 mm Hg, and the pressure in the
hepatic vein leading from the liver into t

 in the
hepatic vein leading from the liver into the vena cava
normally averages almost exactly 0 mm Hg. This small
pressure difference,only 9 mm Hg, shows that the resist-
ance to blood ﬂow through the hepatic sinusoids is nor-
mally very low,especially when one considers that about
1350 milliliters of blood ﬂows by this route each minute.
Cirrhosis of the Liver Greatly Increases Resistance to Blood Flow.
When liver parenchymal cells are destroyed, they are
replaced with ﬁbrous tissue that even

ed, they are
replaced with ﬁbrous tissue that eventually contracts
around the blood vessels, thereby greatly impeding the
ﬂow of portal blood through the liver. This disease
process is known as cirrhosis of the liver. It results most
commonly from alcoholism, but it can also follow inges-
tion of poisons such as carbon tetrachloride, viral dis-
eases such as infectious hepatitis, obstruction of the bile
ducts, and infectious processes in the bile ducts.
The portal system is also occasionally blo

 ducts.
The portal system is also occasionally blocked by a
large clot that develops in the portal vein or its major
branches. When the portal system is suddenly blocked,
the return of blood from the intestines and spleen
through the liver portal blood ﬂow system to the sys-
temic circulation is tremendously impeded, resulting in
portal hypertension and increasing the capillary pres-
sure in the intestinal wall to 15 to 20 mm Hg above
normal. The patient often dies within a few hours
because of 

 patient often dies within a few hours
because of excessive loss of ﬂuid from the capillaries
into the lumens and walls of the intestines.
The Liver Functions as 
a Blood Reservoir
Because the liver is an expandable organ, large quanti-
ties of blood can be stored in its blood vessels. Its
normal blood volume, including both that in the hepatic
veins and that in the hepatic sinuses, is about 450 milli-
liters, or almost 10 per cent of the body’s total blood
volume. When high pressure in the righ

total blood
volume. When high pressure in the right atrium causes
backpressure in the liver, the liver expands, and 0.5 to 1
liter of extra blood is occasionally stored in the hepatic
veins and sinuses. This occurs especially in cardiac
failure with peripheral congestion, which is discussed in
Chapter 22. Thus, in effect, the liver is a large, expand-
able, venous organ capable of acting as a valuable blood
reservoir in times of excess blood volume and capable
of supplying extra blood in times o

me and capable
of supplying extra blood in times of diminished blood
volume.
The Liver Has Very High 
Lymph Flow
Because the pores in the hepatic sinusoids are very
permeable and allow ready passage of both ﬂuid and
proteins into the spaces of Disse, the lymph draining
from the liver usually has a protein concentration of
about 6 g/dl, which is only slightly less than the protein
concentration of plasma. Also, the extreme permeabil-
ity of the liver sinusoid epithelium allows large quanti-
ties 

ver sinusoid epithelium allows large quanti-
ties of lymph to form. Therefore, about half of all the
lymph formed in the body under resting conditions
arises in the liver.
High Hepatic Vascular Pressures Can Cause Fluid Transudation
into the Abdominal Cavity from the Liver and Portal Capillaries—
Ascites. When the pressure in the hepatic veins rises
only 3 to 7 mm Hg above normal, excessive amounts of
ﬂuid begin to transude into the lymph and leak through
the outer surface of the liver capsule d

k through
the outer surface of the liver capsule directly into the
abdominal cavity. This ﬂuid is almost pure plasma, con-
taining 80 to 90 per cent as much protein as normal
plasma. At vena caval pressures of 10 to 15 mm Hg,
hepatic lymph ﬂow increases to as much as 20 times
normal, and the “sweating” from the surface of the liver
can be so great that it causes large amounts of free ﬂuid
in the abdominal cavity, which is called ascites. Block-
age of portal ﬂow through the liver also causes hig

ge of portal ﬂow through the liver also causes high 
capillary pressures in the entire portal vascular system
of the gastrointestinal tract, resulting in edema of 
the gut wall and transudation of ﬂuid through the serosa
of the gut into the abdominal cavity.This, too, can cause
ascites.
Regulation of Liver Mass—
Regeneration
The liver possesses a remarkable ability to restore itself
after signiﬁcant hepatic tissue loss from either partial
hepatectomy or acute liver injury, as long as the injury

tomy or acute liver injury, as long as the injury
is uncomplicated by viral infection or inﬂammation.
Partial hepatectomy, in which up to 70 per cent of the
liver is removed, causes the remaining lobes to enlarge
and restore the liver to its original size. This regenera-
tion is remarkably rapid and requires only 5 to 7 days
in rats. During liver regeneration, hepatocytes are esti-
mated to replicate once or twice, and after the original
Liver cell plate
Sinusoids
Terminal
lymphat

ll plate
Sinusoids
Terminal
lymphatics
Portal
vein
Hepatic
artery
Bile duct
Central
vein
Kupffer cell
Bile canaliculi
Lymphatic
duct
Figure 70–1
Basic structure of a liver lobule, showing the liver cellular plates,
the blood vessels, the bile-collecting system, and the lymph ﬂow
system composed of the spaces of Disse and the interlobular 
lymphatics. (Modiﬁed from Guyton AC, Taylor AE, Granger HJ: 
Circulatory Physiology. Vol 2: Dynamics and Control of the Body
Fluids. Philadelphi

namics and Control of the Body
Chapter 70
861
size and volume of the liver are achieved, the hepato-
cytes revert to their usual quiescent state.
Control of this rapid regeneration of the liver is still
poorly understood, but hepatocyte growth factor (HGF)
appears to be an important factor causing liver cell divi-
sion and growth. HGF is produced by mesenchymal
cells in the liver and in other tissues, but not by hepa-
tocytes. Blood

 in other tissues, but not by hepa-
tocytes. Blood levels of HGF rise more than 20-fold
after partial hepatectomy, but mitogenic responses are
usually found only in the liver after these operations,
suggesting that HGF may be activated only in the
affected organ. Other growth factors, especially epider-
mal growth factor, and cytokines such as tumor necro-
sis factor and interleukin-6 may also be involved in
stimulating regeneration of liver cells.
After the liver has returned to its original si

s.
After the liver has returned to its original size, the
process of hepatic cell division is terminated. Again,
the factors involved are not well understood, although
transforming growth factor-b, a cytokine secreted by
hepatic cells, is a potent inhibitor of liver cell prolifera-
tion and has been suggested as the main terminator of
liver regeneration.
Physiologic experiments indicate that liver growth is
closely regulated by some unknown signal related to
body size, so that an optimal liver-t

l related to
body size, so that an optimal liver-to-body weight ratio
is maintained for optimal metabolic function. In liver
diseases associated with ﬁbrosis, inﬂammation, or viral
infections, however, the regenerative process of the liver
is severely impaired, and liver function deteriorates.
Hepatic Macrophage System Serves
a Blood-Cleansing Function
Blood ﬂowing through the intestinal capillaries picks up
many bacteria from the intestines. Indeed, a sample of
blood taken from the portal veins

eed, a sample of
blood taken from the portal veins before it enters the
liver almost always grows colon bacilli when cultured,
whereas growth of colon bacilli from blood in the sys-
temic circulation is extremely rare.
Special high-speed motion pictures of the action of
Kupffer cells, the large phagocytic macrophages that
line the hepatic venous sinuses, have demonstrated that
these cells efﬁciently cleanse blood as it passes through
the sinuses; when a bacterium comes into momentary
contact wit

 when a bacterium comes into momentary
contact with a Kupffer cell, in less than 0.01 second the
bacterium passes inward through the wall of the Kupffer
cell to become permanently lodged therein until it is
digested. Probably less than 1 per cent of the bacteria
entering the portal blood from the intestines succeeds
in passing through the liver into the systemic 
Metabolic Functions 
of the Liver
The liver is a large, chemically reactant pool of cells that
have a high rate of metabo

tant pool of cells that
have a high rate of metabolism, sharing substrates and
energy from one metabolic system to another, process-
ing and synthesizing multiple substances that are trans-
ported to other areas of the body, and performing
myriad other metabolic functions. For these reasons, a
major share of the entire discipline of biochemistry is
devoted to the metabolic reactions in the liver. But here,
let us summarize those metabolic functions that are
especially important in understanding 

ns that are
especially important in understanding the integrated
physiology of the body.
In carbohydrate metabolism, the liver performs the fol-
lowing functions, as summarized from Chapter 67:
1. Storage of large amounts of glycogen
2. Conversion of galactose and fructose to glucose
3. Gluconeogenesis
4. Formation of many chemical compounds from
intermediate products of carbohydrate metabolism
The liver is especially important for maintaining a
normal blood glucose conce

rtant for maintaining a
normal blood glucose concentration. Storage of glyco-
gen allows the liver to remove excess glucose from the
blood, store it, and then return it to the blood when the
blood glucose concentration begins to fall too low. This
is called the glucose buffer function of the liver. In a
person with poor liver function, blood glucose concen-
tration after a meal rich in carbohydrates may rise two
to three times as much as in a person with normal liver
function.
Gluconeogenesis in

son with normal liver
function.
Gluconeogenesis in the liver is also important in
maintaining a normal blood glucose concentration,
because gluconeogenesis occurs to a signiﬁcant extent
only when the glucose concentration falls below normal.
In such a case, large amounts of amino acids and glyc-
erol from triglycerides are converted into glucose,
thereby helping to maintain a relatively normal blood
Although most cells of the body metabolize fat, certain
asp

most cells of the body metabolize fat, certain
aspects of fat metabolism occur mainly in the liver.
Speciﬁc functions of the liver in fat metabolism, as 
summarized from Chapter 68, are the following:
1. Oxidation of fatty acids to supply energy for other
body functions
2. Synthesis of large quantities of cholesterol,
phospholipids, and most lipoproteins
3. Synthesis of fat from proteins and carbohydrates
To derive energy from neutral fats, the fat is ﬁrst split
into glycerol and fatty acids; th

at is ﬁrst split
into glycerol and fatty acids; then the fatty acids are split
by beta-oxidation into two-carbon acetyl radicals that
form acetyl coenzyme A (acetyl-CoA). This can enter
the citric acid cycle and be oxidized to liberate tremen-
dous amounts of energy. Beta-oxidation can take place
in all cells of the body, but it occurs especially rapidly 
in the hepatic cells. The liver itself cannot use all the
acetyl-CoA that is formed; instead, it is converted by the
condensation of two molec

, it is converted by the
condensation of two molecules of acetyl-CoA into 
acetoacetic acid, a highly soluble acid that passes from
the hepatic cells into the extracellular ﬂuid and is then
transported throughout the body to be absorbed by
other tissues. These tissues reconvert the acetoacetic
acid into acetyl-CoA and then oxidize it in the usual
manner. Thus, the liver is responsible for a major part
of the metabolism of fats.
About 80 per cent of the cholesterol synthesized in
the liver is con

of the cholesterol synthesized in
the liver is converted into bile salts, which are secreted
into the bile; the remainder is transported in the
lipoproteins and carried by the blood to the tissue cells
everywhere in the body. Phospholipids are likewise syn-
thesized in the liver and transported principally in the
lipoproteins. Both cholesterol and phospholipids are
used by the cells to form membranes, intracellular 
structures, and multiple chemical substances that are
important to cellular func

cal substances that are
important to cellular function.
Almost all the fat synthesis in the body from carbo-
hydrates and proteins also occurs in the liver. After fat
862
Unit XIII
is synthesized in the liver, it is transported in the
lipoproteins to the adipose tissue to be stored.
The body cannot dispense with the liver’s contribution
to protein metabolism for more than a few days without
death ensuing.The most important functions of the

death ensuing.The most important functions of the liver
in protein metabolism, as summarized from Chapter 69,
are the following:
1. Deamination of amino acids
2. Formation of urea for removal of ammonia from
the body ﬂuids
3. Formation of plasma proteins
4. Interconversions of the various amino acids and
synthesis of other compounds from amino acids
Deamination of amino acids is required before they
can be used for energy or converted into carbohydrates
or fats.A small amount of deamination can

hydrates
or fats.A small amount of deamination can occur in the
other tissues of the body, especially in the kidneys, but
this is much less important than the deamination of
amino acids by the liver.
Formation of urea by the liver removes ammonia
from the body ﬂuids. Large amounts of ammonia are
formed by the deamination process, and additional
amounts are continually formed in the gut by bacteria
and then absorbed into the blood.Therefore, if the liver
does not form urea, the plasma ammonia con

e liver
does not form urea, the plasma ammonia concentration
rises rapidly and results in hepatic coma and death.
Indeed, even greatly decreased blood ﬂow through the
liver—as occurs occasionally when a shunt develops
between the portal vein and the vena cava—can cause
excessive ammonia in the blood, an extremely toxic 
Essentially all the plasma proteins, with the exception
of part of the gamma globulins, are formed by the
hepatic cells. This accounts for about 90 per cent of all
the

ls. This accounts for about 90 per cent of all
the plasma proteins. The remaining gamma globulins
are the antibodies formed mainly by plasma cells in the
lymph tissue of the body.The liver can form plasma pro-
teins at a maximum rate of 15 to 50 g/day. Therefore,
even if as much as half the plasma proteins are lost from
the body, they can be replenished in 1 or 2 weeks.
It is particularly interesting that plasma protein
depletion causes rapid mitosis of the hepatic cells and
growth of the liver 

osis of the hepatic cells and
growth of the liver to a larger size; these effects are
coupled with rapid output of plasma proteins until the
plasma concentration returns to normal. With chronic
liver disease (e.g., cirrhosis), plasma proteins, such as
albumin, may fall to very low levels, causing generalized
edema and ascites, as explained in Chapter 29.
Among the most important functions of the liver is
its ability to synthesize certain amino acids and to 
synthesize other important chemical co

ids and to 
synthesize other important chemical compounds from
amino acids. For instance, the so-called nonessential
amino acids can all be synthesized in the liver. To do
this, a keto acid having the same chemical composition
(except at the keto oxygen) as that of the amino acid to
be formed is synthesized.Then an amino radical is trans-
ferred through several stages of transamination from an
available amino acid to the keto acid to take the place
of the keto oxygen.
Other Metabolic Functions 

ce
The Liver Is a Storage Site for Vitamins. The liver has a par-
ticular propensity for storing vitamins and has long been
known as an excellent source of certain vitamins in the
treatment of patients. The vitamin stored in greatest
quantity in the liver is vitamin A, but large quantities of
vitamin D and vitamin B12 are normally stored as well.
Sufﬁcient quantities of vitamin A can be stored to
prevent vitamin A deﬁciency for as long 

stored to
prevent vitamin A deﬁciency for as long as 10 months.
Sufﬁcient vitamin D can be stored to prevent deﬁciency
for 3 to 4 months, and enough vitamin B12 can be stored
to last for at least 1 year and maybe several years.
The Liver Stores Iron as Ferritin. Except for the iron in the
hemoglobin of the blood, by far the greatest proportion
of iron in the body is stored in the liver in the form of
ferritin. The hepatic cells contain large amounts of a
protein called apoferritin, which is capa

nts of a
protein called apoferritin, which is capable of combin-
ing reversibly with iron. Therefore, when iron is avail-
able in the body ﬂuids in extra quantities, it combines
with apoferritin to form ferritin and is stored in this
form in the hepatic cells until needed elsewhere. When
the iron in the circulating body ﬂuids reaches a low
level, the ferritin releases the iron.Thus, the apoferritin-
ferritin system of the liver acts as a blood iron buffer, as
well as an iron storage medium. Othe

on buffer, as
well as an iron storage medium. Other functions of the
liver in relation to iron metabolism and red blood cell
formation are considered in Chapter 32.
The Liver Forms a Large Proportion of the Blood Substances Used
in Coagulation. Substances formed in the liver that are
used in the coagulation process include ﬁbrinogen, pro-
thrombin, accelerator globulin, Factor VII, and several
other important factors. Vitamin K is required by the
metabolic processes of the liver for the formatio

metabolic processes of the liver for the formation of
several of these substances, especially prothrombin and
Factors VII, IX, and X. In the absence of vitamin K, the
concentrations of all these decrease markedly, and this
almost prevents blood coagulation.
The Liver Removes or Excretes Drugs, Hormones, and Other Sub-
stances. The active chemical medium of the liver is well
known for its ability to detoxify or excrete into the bile
many drugs, including sulfonamides, penicillin, ampi-
cillin, a

ncluding sulfonamides, penicillin, ampi-
cillin, and erythromycin.
In a similar manner, several of the hormones secreted
by the endocrine glands are either chemically altered or
excreted by the liver,including thyroxine and essentially
all the steroid hormones, such as estrogen, cortisol, and
aldosterone. Liver damage can lead to excess accumu-
lation of one or more of these hormones in the body
ﬂuids and therefore cause overactivity of the hormonal
systems.
Finally, one of the major routes for 

nal
systems.
Finally, one of the major routes for excreting calcium
from the body is secretion by the liver into the bile,
which then passes into the gut and is lost in the feces.
Measurement of Bilirubin 
in the Bile as a Clinical
Diagnostic Tool
The formation of bile by the liver and the function of
the bile salts in the digestive and absorptive processes
of the intestinal tract are discussed in Chapters 64 and
65. In addition, many substances are excreted in the bile
and then eliminated in th

are excreted in the bile
and then eliminated in the feces. One of these is the
greenish yellow pigment bilirubin. This is a major end
product of hemoglobin degradation, as pointed out in
Chapter 32. However, it also provides an exceedingly
valuable tool for diagnosing both hemolytic blood 
Chapter 70
863
diseases and various types of liver diseases. Therefore,
while referring to Figure 70–2, let us explain this.
Brieﬂy, when the red blood cells have lived out their
life spa

 the red blood cells have lived out their
life span (on average, 120 days) and have become too
fragile to exist in the circulatory system, their cell 
membranes rupture, and the released hemoglobin is
phagocytized by tissue macrophages (also called the
reticuloendothelial system) throughout the body. The
hemoglobin is ﬁrst split into globin and heme, and 
the heme ring is opened to give (1) free iron, which is
transported in the blood by transferrin, and (2) a
straight chain of four pyrrole nucl

rin, and (2) a
straight chain of four pyrrole nuclei, which is the sub-
strate from which bilirubin will eventually be formed.
The ﬁrst substance formed is biliverdin, but this is
rapidly reduced to free bilirubin, which is gradually
released from the macrophages into the plasma. The
free bilirubin immediately combines strongly with
plasma albumin and is transported in this combination
throughout the blood and interstitial ﬂuids. Even when
bound with plasma protein, this bilirubin is still calle

with plasma protein, this bilirubin is still called
“free bilirubin” to distinguish it from “conjugated biliru-
bin,” which is discussed later.
Within hours, the free bilirubin is absorbed through
the hepatic cell membrane. In passing to the inside of
the liver cells,it is released from the plasma albumin and
soon thereafter conjugated about 80 per cent with glu-
curonic acid to form bilirubin glucuronide, about 10 per
cent with sulfate to form bilirubin sulfate, and about 10
per cent with a mul

ilirubin sulfate, and about 10
per cent with a multitude of other substances. In these
forms, the bilirubin is excreted from the hepatocytes by
an active transport process into the bile canaliculi and
then into the intestines.
Formation and Fate of Urobilinogen. Once in the intestine,
about half of the “conjugated” bilirubin is converted by
bacterial action into the substance urobilinogen, which
is highly soluble. Some of the urobilinogen is reab-
sorbed through the intestinal mucosa back into t

-
sorbed through the intestinal mucosa back into the
blood. Most of this is re-excreted by the liver back into
the gut, but about 5 per cent is excreted by the kidneys
into the urine.After exposure to air in the urine,the uro-
bilinogen becomes oxidized to urobilin; alternatively, in
the feces, it becomes altered and oxidized to form ster-
cobilin. These interrelations of bilirubin and the other
bilirubin products are shown in Figure 70–2.
Jaundice—Excess Bilirubin in the
Jau

ce—Excess Bilirubin in the
Jaundice refers to a yellowish tint to the body tissues,
including a yellowness of the skin as well as the deep
tissues.The usual cause of jaundice is large quantities of
bilirubin in the extracellular ﬂuids, either free bilirubin
or conjugated bilirubin. The normal plasma concentra-
tion of bilirubin, which is almost entirely the free form,
averages 0.5 mg/dl of plasma. In certain abnormal con-
ditions, this can rise to as high as 40 mg/dl, and muc

ons, this can rise to as high as 40 mg/dl, and much of
Fragile red blood cells
Reticuloendothelial
system
Liver
Liver
Absorbed
Conjugated bilirubin
Urobilinogen
Urobilin
Stercobilinogen
Stercobilin
Oxidation
Oxidation
Bacterial
action
Kidneys
Free bilirubin (protein-bound)
Plasma
Intestinal Contents
Urine
Figure 70–2
Bilirubin formation and excretion.
864
Unit XIII
it can become the conjugated type. The skin usually
begins to appear

conjugated type. The skin usually
begins to appear jaundiced when the concentration rises
to about three times normal—that is, above 1.5 mg/dl.
The common causes of jaundice are (1) increased
destruction of red blood cells, with rapid release of
bilirubin into the blood, and (2) obstruction of the bile
ducts or damage to the liver cells so that even the usual
amounts of bilirubin cannot be excreted into the gas-
trointestinal tract. These two types of jaundice are
called, respectively, hemolytic

es of jaundice are
called, respectively, hemolytic jaundice and obstructive
jaundice. They differ from each other in the following
ways.
Hemolytic Jaundice Is Caused by Hemolysis of Red Blood Cells.
In hemolytic jaundice,the excretory function of the liver
is not impaired, but red blood cells are hemolyzed so
rapidly that the hepatic cells simply cannot excrete the
bilirubin as quickly as it is formed. Therefore, the
plasma concentration of free bilirubin rises to above-
normal levels. Likewise,

bilirubin rises to above-
normal levels. Likewise, the rate of formation of uro-
bilinogen in the intestine is greatly increased, and much
of this is absorbed into the blood and later excreted in
the urine.
Obstructive Jaundice Is Caused by Obstruction of Bile Ducts or
Liver Disease. In obstructive jaundice, caused either by
obstruction of the bile ducts (which most often occurs
when a gallstone or cancer blocks the common bile
duct) or by damage to the hepatic cells (which occurs in
hepatitis),

 to the hepatic cells (which occurs in
hepatitis), the rate of bilirubin formation is normal, but
the bilirubin formed cannot pass from the blood into the
intestines. The free bilirubin still enters the liver cells
and becomes conjugated in the usual way. This conju-
gated bilirubin is then returned to the blood, probably
by rupture of the congested bile canaliculi and direct
emptying of the bile into the lymph leaving the liver.
Thus, most of the bilirubin in the plasma becomes the
conjugated t

e bilirubin in the plasma becomes the
conjugated type rather than the free type.
Diagnostic Differences Between Hemolytic and Obstructive 
Jaundice. Chemical laboratory tests can be used to dif-
ferentiate between free and conjugated bilirubin in the
plasma. In hemolytic jaundice, almost all the bilirubin is
in the “free” form; in obstructive jaundice, it is mainly
in the “conjugated” form. A test called the van den
Bergh reaction can be used to differentiate between the
two.
When there is total

differentiate between the
two.
When there is total obstruction of bile ﬂow, no biliru-
bin can reach the intestines to be converted into uro-
bilinogen by bacteria. Therefore, no urobilinogen is
reabsorbed into the blood, and none can be excreted by
the kidneys into the urine. Consequently, in total
obstructive jaundice, tests for urobilinogen in the urine
are completely negative. Also, the stools become clay
colored owing to a lack of stercobilin and other bile 
pigments.
Another major differen

n and other bile 
pigments.
Another major difference between free and conju-
gated bilirubin is that the kidneys can excrete small
quantities of the highly soluble conjugated bilirubin 
but not the albumin-bound free bilirubin. Therefore,
in severe obstructive jaundice, signiﬁcant quantities of
conjugated bilirubin appear in the urine. This can be
demonstrated simply by shaking the urine and observ-
ing the foam, which turns an intense yellow. Thus,
by understanding the physiology of bilirubin e

us,
by understanding the physiology of bilirubin excretion
by the liver and by the use of a few simple tests, it is
often possible to differentiate among multiple types of
hemolytic diseases and liver diseases, as well as to deter-
mine the severity of the disease.
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inside and outside the liver? Cell Prolif 37:1, 2004.
Angulo P: Nonalcoholic fatty liver disease. N Engl J Med
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Ankoma-Sey V: Hepatic regeneration—re

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Ankoma-Sey V: Hepatic regeneration—revisiting the myth
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Bauer M: Heme oxygenase in liver transplantation: heme
catabolism and metabolites in the search of function.
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Black D, Lyman S, Heider TR, Behrns KE: Molecular and
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es of hepatic regeneration. J Surg Res
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Bonder CS, Kubes P: The future of GI and liver research:
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fatty 
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trointest Liver Physiol 282:G1, 2002.
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C
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7
1
865
Dietary Balances; Regulation of
Energy Intake and Out

ation;
Energy Intake and Output
Are Balanced Under Steady-
State Conditions
Intake of carbohydrates, fats, and proteins provides
energy that can be used to perform various body
functions or stored for later use. Stability of body
weight and composition over long periods requires that a person’s energy intake
and energy expenditure be balanced. When a person is overfed and energy
intake persistently exceeds expenditure, most of the excess energy is stored as
fat, and body we

of the excess energy is stored as
fat, and body weight increases; conversely, loss of body mass and starvation
occur when energy intake is insufﬁcient to meet the body’s metabolic needs.
Because different foods contain different proportions of proteins, carbohy-
drates, fats, minerals, and vitamins, appropriate balances must also be main-
tained among these constituents so that all segments of the body’s metabolic
systems can be supplied with the requisite materials. This chapter discusses the
m

 requisite materials. This chapter discusses the
mechanisms by which food intake is regulated in accordance with the body’s
metabolic needs and some of the problems of maintaining balance among the
different types of foods.
The energy liberated from each gram of carbohydrate as it is oxidized to carbon
dioxide and water is 4.1 Calories (1 Calorie equals 1 kilocalorie), and that liberated
from fat is 9.3 Calories.The energy liberated from metabolism of t

Calories.The energy liberated from metabolism of the average dietary
protein as each gram is oxidized to carbon dioxide, water, and urea is 4.35 Calories.
Also, these substances vary in the average percentages that are absorbed from the
gastrointestinal tract: about 98 per cent of carbohydrate, 95 per cent of fat, and 92
per cent of protein. Therefore, the average physiologically available energy in each
gram of these three foodstuffs is as follows:
Calories
4
Fat
9
Protein
4
Averag

ws:
Calories
4
Fat
9
Protein
4
Average Americans receive about 15 per cent of their energy from protein, 40
per cent from fat, and 45 per cent from carbohydrate. In most non-Western coun-
tries, the quantity of energy derived from carbohydrates far exceeds that derived
from both proteins and fats. Indeed, in some parts of the world where meat is scarce,
the energy received from fats and proteins combined may be no greater than 15 to
20 per cent.
Table 71–1 gives the compositions of 

Table 71–1 gives the compositions of selected foods, demonstrating especially the
high proportions of fat and protein in meat products and the high proportion of car-
bohydrate in most vegetable and grain products. Fat is deceptive in the diet because
it usually exists as 100 per cent fat, whereas both proteins and carbohydrates are
mixed in watery media so that each of these normally represents less than 25 per
866
Unit XIII
cent of the weight.

ism and Temperature Regulation
cent of the weight.Therefore, the fat of one pat of butter
mixed with an entire helping of potato sometimes con-
tains as much energy as the potato itself.
Average Daily Requirement for Protein Is 30 to 50 Grams.
Twenty to 30 grams of the body proteins are degraded
and used to produce other body chemicals daily. There-
fore, all cells must continue to form new proteins to take
the place of those that are being destroyed, and a supply
of protein is needed in the die

oyed, and a supply
of protein is needed in the diet for this purpose. An
average person can maintain normal stores of protein,
provided the daily intake is above 30 to 50 grams.
Some proteins have inadequate quantities of certain
essential amino acids and therefore cannot be used to
replace the degraded proteins. Such proteins are called
partial proteins, and when they are present in large
quantities in the diet, the daily protein requirement is
much greater than normal. In general, proteins der

much greater than normal. In general, proteins derived
from animal foodstuffs are more complete than are pro-
teins derived from vegetable and grain sources. For
example, the protein of corn has almost no tryptophan,
one of the essential amino acids. Therefore, individuals
in economically disadvantaged countries who consume
cornmeal as the principal source of protein sometimes
develop the protein-deﬁciency syndrome called kwashi-
orkor, which consists of failure to grow, lethargy,
depressed ment

sists of failure to grow, lethargy,
depressed mentality, and edema caused by low plasma
protein concentration.
Carbohydrates and Fats Act as “Protein Sparers.” When the
diet contains an abundance of carbohydrates and 
fats, almost all the body’s energy is derived from these
two substances, and little is derived from proteins.
Therefore, both carbohydrates and fats are said to 
be protein sparers. Conversely, in starvation, after the 
carbohydrates and fats have been depleted,
the 
body’s protein

s and fats have been depleted,
the 
body’s protein stores are consumed rapidly for energy,
sometimes at rates approaching several hundred 
grams per day rather than the normal daily rate of 30
to 50 grams.
Methods for Determining Metabolic
Utilization of Proteins,
Carbohydrates, and Fats
Nitrogen Excretion Can Be Used to Assess Protein Metabolism.
The average protein contains about 16 per cent nitro-
gen. During metabolism of the protein, about 90 per
cent of this nitrogen is excreted in the uri

0 per
cent of this nitrogen is excreted in the urine in the form
of urea, uric acid, creatinine, and other less important
nitrogen products.The remaining 10 per cent is excreted
in the feces.Therefore, the rate of protein breakdown in
the body can be estimated by measuring the amount of
nitrogen in the urine, then adding 10 per cent for the
nitrogen excreted in the feces, and multiplying by 6.25
(i.e., 100/16) to determine the total amount of protein
metabolism in grams per day.Thus, excretion o

tein
metabolism in grams per day.Thus, excretion of 8 grams
of nitrogen in the urine each day means that there has
been about 55 grams of protein breakdown. If the daily
intake of protein is less than the daily breakdown of
protein, the person is said to have a negative nitrogen
balance, which means that his or her body stores of
protein are decreasing daily.
Table 71–1
Protein, Fat, and Carbohydrate Content of Different Foods
Food
% Protein
% Fat
% Carbohydrate
Fuel Value per 100 Grams (Calorie

t
Fuel Value per 100 Grams (Calories)
Apples
0.3
0.4
14.9
64
Asparagus
2.2
0.2
3.9
26
Bacon, fat
6.2
76.0
0.7
712
broiled
25.0
55.0
1.0
599
Beef (average)
17.5
22.0
1.0
268
Beets, fresh
1.6
0.1
9.6
46
Bread, white
9.0
3.6
49.8
268
Butter
0.6
81.0
0.4
733
Cabbage
1.4
0.2
5.3
29
Carrots
1.2
0.3
9.3
45
Cashew nuts
19.6
47.2
26.4
609
Cheese, cheddar, American
23.9
32.3
1.7
393
Chicken, total edible
21.6
2.7
1.0
111
Chocolate
5.5
52.9
18.0
570
Corn (maize)
10.0
4.3
73.4
372
Haddock
17.

8.0
570
10.0
4.3
73.4
372
Haddock
17.2
0.3
0.5
72
Lamb, leg (average)
18.0
17.5
1.0
230
Milk, fresh whole
3.5
3.9
4.9
69
Molasses
0.0
0.0
60.0
240
Oatmeal, dry, uncooked
14.2
7.4
68.2
396
Oranges
0.9
0.2
11.2
50
Peanuts
26.9
44.2
23.6
600
Peas, fresh
6.7
0.4
17.7
101
Pork, ham
15.2
31.0
1.0
340
Potatoes
2.0
0.1
19.1
85
Spinach
2.3
0.3
3.2
25
Strawberries
0.8
0.6
8.1
41
Tomatoes
1.0
0.3
4.0
23
Tuna, canned
24.2
10.8
0.5
194
Walnuts, English
15.0
64.4
15.6
702
Chapter 71
Dietary Balan

nglish
15.0
64.4
15.6
702
Chapter 71
Dietary Balances; Regulation of Feeding; Obesity and Starvation; Vitamins and Minerals
867
“Respiratory Quotient” Is the Ratio of CO2 Production to O2
Utilization and Can Be Used to Estimate Fat and Carbohydrate 
Utilization. When carbohydrates are metabolized with
oxygen, exactly one carbon dioxide molecule is formed
for each molecule of oxygen consumed. This ratio of
carbon dioxide output to oxygen usage is called the 
respiratory quotient, so the respirato

called the 
respiratory quotient, so the respiratory quotient for 
carbohydrates is 1.0.
When fat is oxidized in the body’s cells, an average of
70 carbon dioxide molecules are formed for each 100
molecules of oxygen consumed. The respiratory quo-
tient for the metabolism of fat averages 0.70.When pro-
teins are oxidized by the cells, the average respiratory
quotient is 0.80. The reason that the respiratory quo-
tients for fats and proteins are lower than that for 
carbohydrates is that a large 

ower than that for 
carbohydrates is that a large share of the oxygen metab-
olized with these foods is required to combine with the
excess hydrogen atoms present in their molecules, so
that less carbon dioxide is formed in relation to the
oxygen used.
Now let us see how one can make use of the respira-
tory quotient to determine the relative utilization of dif-
ferent foods by the body. First, it will be recalled from
Chapter 39 that the output of carbon dioxide by the
lungs divided by the upta

of carbon dioxide by the
lungs divided by the uptake of oxygen during the same
period is called the respiratory exchange ratio. Over a
period of 1 hour or more, the respiratory exchange ratio
exactly equals the average respiratory quotient of the
metabolic reactions throughout the body.If a person has
a respiratory quotient of 1.0, he or she is metabolizing
almost entirely carbohydrates, because the respiratory
quotients for both fat and protein metabolism are con-
siderably less than 1.0. Likew

metabolism are con-
siderably less than 1.0. Likewise, when the respiratory
quotient is about 0.70, the body is metabolizing almost
entirely fats, to the exclusion of carbohydrates and 
proteins. And, ﬁnally, if we ignore the normally small
amount of protein metabolism, respiratory quotients
between 0.70 and 1.0 describe the approximate ratios of
carbohydrate to fat metabolism. To be more exact, one
can ﬁrst determine the protein utilization by measuring
nitrogen excretion and then, using the ap

easuring
nitrogen excretion and then, using the appropriate
mathematical formula, calculate almost exactly the uti-
lization of the three foodstuffs.
Some of the important ﬁndings from studies of respi-
ratory quotients are the following:
1. Immediately after a meal, almost all the food 
that is metabolized is carbohydrates, so that the
respiratory quotient at that time approaches 1.0.
2. About 8 to 10 hours after a meal, the body has
already used up most of its readily available
carbohydrates, 

d up most of its readily available
carbohydrates, and the respiratory quotient
approaches that for fat metabolism, about 0.70.
3. In untreated diabetes mellitus, little carbohydrate
can be used by the body’s cells under any
conditions, because insulin is required for this.
Therefore, when diabetes is severe, most of the
time the respiratory quotient remains near that for
fat metabolism, 0.70.
Regulation of Food Intake and
Energy Storage
Stability of the body’s total mass and composition over
lon

 of the body’s total mass and composition over
long periods requires that energy intake match energy
expenditure.As discussed in Chapter 72, only about 27
per cent of the energy ingested normally reaches the
functional systems of the cells, and much of this is
eventually converted to heat, which is generated as a
result of protein metabolism, muscle activity, and
activities of the various organs and tissues of the body.
Excess energy intake is stored mainly as fat, whereas
a deﬁcit of energy int

ored mainly as fat, whereas
a deﬁcit of energy intake causes loss of total body 
mass until energy expenditure eventually equals
energy intake or death occurs.
Although there is considerable variability in the
amount of energy storage (i.e., fat mass) in different
individuals, maintenance of an adequate energy supply
is necessary for survival. Therefore, the body is
endowed with powerful physiologic control systems
that help maintain adequate energy intake. Deﬁcits of
energy stores, for example,

rgy intake. Deﬁcits of
energy stores, for example, rapidly activate multiple
mechanisms that cause hunger and drive a person to
seek food. In athletes and laborers, energy expenditure
for the high level of muscle activity may be as high as
6000 to 7000 Calories per day, compared with only
about 2000 Calories per day for sedentary individuals.
Thus, a large energy expenditure associated with phys-
ical work usually stimulates equally large increases in
caloric intake.
What are the physiologic mec

es in
What are the physiologic mechanisms that sense
changes in energy balance and inﬂuence the quest 
for food? Maintenance of adequate energy supply in
the body is so critical that there are multiple short-
term and long-term control systems that regulate not 
only food intake but also energy expenditure and
energy stores. In the next few sections we describe
some of these control systems and their operation 
in physiologic conditions, as well as in obesity and 
starvation.
Neu

itions, as well as in obesity and 
The sensation of hunger is associated with a craving
for food and several other several physiologic effects,
such as rhythmical contractions of the stomach and
restlessness, which cause the person to search for an
adequate food supply. A person’s appetite is a desire
for food, often of a particular type, and is useful in
helping to choose the quality of the food to be eaten.
If the quest for food is successful, th

 be eaten.
If the quest for food is successful, the feeling of satiety
occurs. Each of these feelings is inﬂuenced by envi-
ronmental and cultural factors, as well as by physio-
logic controls that inﬂuence speciﬁc centers of the
brain, especially the hypothalamus.
The Hypothalamus Contains Hunger and Satiety Centers.
Several neuronal centers of the hypothalamus partici-
pate in the control of food intake. The lateral nuclei of
the hypothalamus serve as a feeding center, and stimu-
lation of thi

erve as a feeding center, and stimu-
lation of this area causes an animal to eat voraciously
(hyperphagia). Conversely, destruction of the lateral
hypothalamus causes lack of desire for food and 
progressive inanition, a condition characterized by
marked weight loss, muscle weakness, and decreased
metabolism. The lateral hypothalamic feeding center
operates by exciting the motor drives to search for
food.
868
Unit XIII
The ventromedial nuclei of the hypothal

The ventromedial nuclei of the hypothalamus serve
as the satiety center. This center is believed to give a
sense of nutritional satisfaction that inhibits the
feeding center. Electrical stimulation of this region can
cause complete satiety, and even in the presence 
of highly appetizing food, the animal refuses to eat
(aphagia). Conversely, destruction of the ventromedial
nuclei causes voracious and continued eating until the
animal becomes extremely obese, sometimes as large
as four 

comes extremely obese, sometimes as large
as four times normal.
The paraventricular, dorsomedial, and arcuate nuclei
of the hypothalamus also play a major role in regulat-
ing food intake. For example, lesions of the paraven-
tricular nuclei often cause excessive eating, whereas
lesions of the dorsomedial nuclei usually depress
eating behavior. As discussed later, the arcuate nuclei
are the sites in the hypothalamus where multiple hor-
mones released from the gastrointestinal tract and
adipose t

ased from the gastrointestinal tract and
adipose tissue converge to regulate food intake as well
as energy expenditure.
There is much chemical cross-talk among the
neurons on the hypothalamus, and together, these
centers coordinate the processes that control eating
behavior and the perception of satiety. These nuclei 
of the hypothalamus also inﬂuence the secretion of
several hormones that are important in regulating
energy balance and metabolism, including those from
the thyroid and adrenal gla

, including those from
the thyroid and adrenal glands, as well as the pancre-
atic islet cells.
The hypothalamus receives neural signals from the
gastrointestinal tract that provide sensory information
about stomach ﬁlling, chemical signals from nutrients
in the blood (glucose, amino acids, and fatty acids) that
signify satiety, signals from gastrointestinal hormones,
signals from hormones released by adipose tissue,
and signals from the cerebral cortex (sight, smell,
and taste) that inﬂuence fe

 cortex (sight, smell,
and taste) that inﬂuence feeding behavior. Some of
these inputs to the hypothalamus are shown in Figure
71–1.
The hypothalamic feeding and satiety centers have
a high density of receptors for neurotransmitters and
hormones that inﬂuence feeding behavior. A few of
the many substances that have been shown to alter
appetite and feeding behavior in experimental studies
are listed in Table 71–2 and are generally categorized
as (1) orexigenic substances that stimulate feeding, o

1) orexigenic substances that stimulate feeding, or
(2) anorexigenic substances that inhibit feeding.
Neurons and Neurotransmitters in the Hypothalamus That Stim-
ulate or Inhibit Feeding. There are two distinct types of
neurons in the arcuate nuclei of the hypothalamus that
are especially important as controllers of both appetite
and energy expenditure (Figure 71–2): (1) pro-
opiomelanocortin (POMC) neurons that produce a-
melanocyte-stimulating hormone (a-MSH) together
with cocaine- and amphet

 hormone (a-MSH) together
with cocaine- and amphetamine-related transcript
(CART), and (2) neurons that produce the orexigenic
substances neuropeptide Y (NPY) and agouti-related
protein (AGRP). Activation of the POMC neurons
decreases food intake and increases energy expendi-
ture, whereas activation of the NPY-AGRP neurons
increases food intake and reduces energy expenditure.
As discussed later, these neurons appear to be the
major targets for the actions of several hormones that
regulate appet

he actions of several hormones that
regulate appetite, including leptin, insulin, cholecys-
tokinin (CCK), and ghrelin. In fact, the neurons of the
arcuate nuclei appear to be a site of convergence of
many of thenervous and peripheral signals that regu-
late energy stores.
The POMC neurons release a-MSH, which then
acts on melanocortin receptors found especially in
neurons of the paraventricular nuclei. Although there
are at least ﬁve subtypes of melanocortin receptor
(MCR), MCR-3 and MCR-4 are 

 melanocortin receptor
(MCR), MCR-3 and MCR-4 are especially important
in regulating food intake and energy balance. Activa-
tion of these receptors reduces food intake while
increasing energy expenditure. Conversely, inhibition
of MCR-3 and MCR-4 greatly increases food intake
and decreases energy expenditure. The effect of MCR
activation to increase energy expenditure appears to
Stomach
Leptin
PYY
Insulin
CCK
Ghrelin
Large intestine
+
-
-
Figure 71–1
Fee

ll intestine
+
-
-
Feedback mechanisms for control of food intake. Stretch recep-
tors in the stomach activate sensory afferent pathways in the
vagus nerve and inhibit food intake. Peptide YY (PYY), cholecys-
tokinin (CCK), and insulin are gastrointestinal hormones that are
released by the ingestion of food and suppress further feeding.
Ghrelin is released by the stomach, especially during fasting, and
stimulates appetite. Leptin is a hormone produced in increasing
am

ite. Leptin is a hormone produced in increasing
amounts by fat cells as they increase in size; it inhibits food intake.
Chapter 71
869
Table 71–2
Neurotransmitters and Hormones That Inﬂuence Feeding and Satiety Centers in the Hypothalamus
Decrease Feeding (Anorexigenic)
Increase Feeding (Orexigenic)
a-Melanocyte-stimulating hormone (a-MSH)
Leptin
Agouti-related protein (AGRP)
Serotonin
Mel

Leptin
Serotonin
Melanin-concentrating hormone (MCH)
Orexins A and B
Insulin
Galanin (GAL)
Amino acids (glutamate and g-aminobutyric acid)
Glucagon-like peptide (GLP)
Cortisol
Cocaine- and amphetamine-regulated transcript (CART)
Ghrelin
To nucleus
tractus solitarius
(NTS)
   •Sympathetic activity
   •Energy expenditure
Neurons
of PVN
Neuron
Y1r
Y1r
Third
ventricle
MCR-4

urons
of PVN
Neuron
Y1r
Y1r
Third
ventricle
MCR-4
MCR-3
MCR-3
Arcuate
nucleus
Food
intake
Food
intake
POMC/
CART
AGRP/
NPY
LepR
LepR
a-MSH
a-MSH
Insulin,
leptin,
CCK
Ghrelin
+
-
+
 Food intake
Figure 71–2
Control of energy balance by two types of neurons of the arcuate nuclei: (1) pro-opiomelanocortin (POMC) neurons that release a-
melanocyte-stimulating hormone (a-MSH) and cocaine- and amphetamine-regulated transcript (CART), decreasing food intake and
increasing energy expenditure; and (2) neu

ake and
increasing energy expenditure; and (2) neurons that produce agouti-related protein (AGRP) and neuropeptide Y (NPY), increasing food
intake and reducing energy expenditure. a-MSH released by POMC neurons stimulates melanocortin receptors (MCR-3 and MCR-4) in
the paraventricular nuclei (PVN), which then activate neuronal pathways that project to the nucleus tractus solitarius (NTS) and increase
sympathetic activity and energy expenditure. AGRP acts as an antagonist of MCR-4. Insulin, lepti

GRP acts as an antagonist of MCR-4. Insulin, leptin, and cholecystokinin (CCK) are hor-
mones that inhibit AGRP-NPY neurons and stimulate adjacent POMC-CART neurons, thereby reducing food intake. Ghrelin, a hormone
secreted from the stomach, activates AGRP-NPY neurons and stimulates food intake. LepR, leptin receptor; Y1R, neuropeptide Y1 
receptor. (Redrawn from Barsh GS, Schwartz MW: Nature Rev Genetics 3:589, 2002).
be mediated, at least in part, by activation of neuronal
pathways that projec

rt, by activation of neuronal
pathways that project from the paraventricular nuclei
to the nucleus tractus solitarius and stimulate sympa-
thetic nervous system activity.
The hypothalamic melanocortin system plays a pow-
erful role in regulating energy stores of the body, and
defective signaling of the melanocortin pathway is
associated with extreme obesity. In fact, mutations of
MCR-4 represent the most common known mono-
genic (single-gene) cause of human obesity, and some
studies suggest that

se of human obesity, and some
studies suggest that MCR-4 mutations may account for
as much as 5 to 6 percentof early-onset severe obesity
in children. In contrast, excessive activation of the
melanocortin system reduces appetite. Some studies
870
Unit XIII
suggest that this activation may play a role in causing
the anorexia associated with severe infections or
cancer tumors.
AGRP released from the orexigenic neurons of the
hypothalamus is a natural antagonis

neurons of the
hypothalamus is a natural antagonist of MCR-3 and
MCR-4 and probably increases feeding by inhibiting
the effects of a-MSH to stimulate melanocortin recep-
tors (see Figure 71–2). Although the role of AGRP in
normal physiologic control of food intake is unclear,
excessive formation of AGRP in mice and humans,
due to gene mutations, is associated with excessive
feeding and obesity.
NPY is also released from orexigenic neurons of the
arcuate nuclei. When energy stores of the body are

low, orexigenic neurons are activated to release NPY,
which stimulates appetite. At the same time, ﬁring of
the POMC neurons is reduced, thereby decreasing the
activity of the melanocortin pathway and further stim-
ulating appetite.
Neural Centers That Inﬂuence the Mechanical Process of
Feeding. Another aspect of feeding is the mechanical
act of the feeding process itself. If the brain is sec-
tioned below the hypothalamus but above the mesen-
c

oned below the hypothalamus but above the mesen-
cephalon, the animal can still perform the basic
mechanical features of the feeding process. It can sali-
vate, lick its lips, chew food, and swallow. Therefore,
the actual mechanics of feeding are controlled by
centers in the brain stem. The function of the other
centers in feeding, then, is to control the quantity of
food intake and to excite these centers of feeding
mechanics to activity.
Neural centers higher than the hypothalamus also
play im

 centers higher than the hypothalamus also
play important roles in the control of feeding, partic-
ularly in the control of appetite. These centers include
the amygdala and the prefrontal cortex, which are
closely coupled with the hypothalamus. It will be
recalled from the discussion of the sense of smell in
Chapter 53 that portions of the amygdala are a major
part of the olfactory nervous system. Destructive
lesions in the amygdala have demonstrated that some
of its areas increase feeding, wher

ated that some
of its areas increase feeding, whereas others inhibit
feeding. In addition, stimulation of some areas of the
amygdala elicits the mechanical act of feeding. An
important effect of destruction of the amygdala on
both sides of the brain is a “psychic blindness” in the
choice of foods. In other words, the animal (and pre-
sumably the human being as well) loses or at least par-
tially loses the appetite control that determines the
type and quality of food it eats.
Factors That Regulat

 and quality of food it eats.
Factors That Regulate Quantity 
of Food Intake
Regulation of the quantity of food intake can be
divided into short-term regulation, which is concerned
primarily with preventing overeating at each meal, and
long-term regulation, which is concerned primarily
with maintenance of normal quantities of energy
stores in the body.
Short-Term Regulation of Food Intake
When a person is driven by hunger to eat voraciously
and rapidly, what turns off the eating when he or she
h

apidly, what turns off the eating when he or she
has eaten enough? There has not been enough time for
changes in the body’s energy stores to occur, and 
it takes hours for enough nutritional factors to be
absorbed into the blood to cause the necessary inhibi-
tion of eating. Yet it is important that the person not
overeat and that he or she eat an amount of food that
approximates nutritional needs. The following are
several types of rapid feedback signals that are impor-
tant for these purposes.

k signals that are impor-
Gastrointestinal Filling Inhibits Feeding. When the gas-
trointestinal tract becomes distended, especially the
stomach and the duodenum, stretch inhibitory signals
are transmitted mainly by way of the vagi to suppress
the feeding center, thereby reducing the desire for
food (Fig. 71–1).
Gastrointestinal Hormonal Factors Suppress Feeding. Chole-
cystokinin is released mainly in response to fat enter-
ing the duodenum and has a direct effect on th

er-
ing the duodenum and has a direct effect on the
feeding centers to reduce subsequent eating. Studies in
experimental animals suggest that CCK may decrease
feeding mainly by activation of the melanocortin
pathway in the hypothalamus.
Peptide YY (PYY) is secreted from the entire gas-
trointestinal tract, but especially from the ileum and
colon. Food intake stimulates release of PYY, with
blood concentrations rising to peak levels 1 to 2 hours
after ingesting a meal. These peak levels of PYY ar

fter ingesting a meal. These peak levels of PYY are
inﬂuenced by the number of calories ingestedand the
composition of the food, with higher levels of PYY
observed after meals with a high fat content.Although
injections of PYY into mice have been shown to
decrease food intake for 12 hours or more, the impor-
tance of this gastrointestinal hormone in regulating
appetite in humans is still unclear.
For reasons that are not entirely understood, the
presence of food in the intestines stimulates them

presence of food in the intestines stimulates them to
secrete glucagon-like peptide, which in turn enhances
glucose-dependent insulin production and secretion
from the pancreas. Glucagon-like peptide and insulin
both tend to suppress appetite. Thus, eating a meal
stimulates the release of several gastrointestinal hor-
mones that may induce satiety and reduce further
intake of food (see Fig. 71–1).
Ghrelin—a Gastrointestinal Hormone—Increases Feeding.
Ghrelin is a hormone released mainly by the o

ing.
Ghrelin is a hormone released mainly by the oxyntic
cells of the stomach but also, to a much less extent, by
the intestine. Blood levels of ghrelin rise during
fasting, peak just before eating, and then fall rapidly
after a meal, suggesting a possible role in stimulating
feeding. Also, administration of ghrelin increases food
intake in experimental animals, further supporting 
the possibility that it may be an orexigenic hormone.
However, its physiologic role in humans is still 
uncertain.

s physiologic role in humans is still 
Chapter 71
871
Oral Receptors Meter Food Intake. When an animal with
an esophageal ﬁstula is fed large quantities of food,
even though this food is immediately lost again to the
exterior, the degree of hunger is decreased after a 
reasonable quantity of food has passed through the
mouth. This effect occurs despite the fact that the gas-
trointestinal tract does

te the fact that the gas-
trointestinal tract does not become the least bit ﬁlled.
Therefore, it is postulated that various “oral factors”
related to feeding, such as chewing, salivation, swal-
lowing, and tasting, “meter” the food as it passes
through the mouth, and after a certain amount has
passed, the hypothalamic feeding center becomes
inhibited. However, the inhibition caused by this
metering mechanism is considerably less intense and
of shorter duration, usually lasting for only 20 to 40

orter duration, usually lasting for only 20 to 40
minutes, than is the inhibition caused by gastrointesti-
nal ﬁlling.
Intermediate and Long-Term Regulation 
An animal that has been starved for a long time and
is then presented with unlimited food eats a far greater
quantity than does an animal that has been on a
regular diet.Conversely,an animal that has been force-
fed for several weeks eats very little when allowed to
eat according to its own desires. Thus, the feeding
control 

ing to its own desires. Thus, the feeding
control mechanism of the body is geared to the nutri-
tional status of the body.
Effect of Blood Concentrations of Glucose, Amino Acids, and
Lipids on Hunger and Feeding. It has long been known that
a decrease in blood glucose concentration causes
hunger, which has led to the so-called glucostatic
theory of hunger and feeding regulation. Similar
studies have demonstrated the same effect for blood
amino acid concentration and blood concentration of
breakd

id concentration and blood concentration of
breakdown products of lipids such as the keto acids
and some fatty acids, leading to the aminostatic and
lipostatic theories of regulation.That is, when the avail-
ability of any of the three major types of food
decreases, the desire for feeding is increased, eventu-
ally returning the blood metabolite concentrations
Neurophysiologic studies of function in speciﬁc
areas of the brain also support the glucostatic, amino-
static, and l

also support the glucostatic, amino-
static, and lipostatic theories, by the following obser-
vations: (1) A rise in blood glucose level increases the
rate of ﬁring of glucoreceptor neurons in the satiety
center in the ventromedial and paraventricular nuclei of
the hypothalamus. (2) The same increase in blood
glucose level simultaneously decreases the ﬁring of glu-
cosensitive neurons in the hunger center of the lateral
hypothalamus. In addition, some amino acids and lipid
substances affect the 

 some amino acids and lipid
substances affect the rates of ﬁring of these same
neurons or other closely associated neurons.
Temperature Regulation and Food Intake. When an animal
is exposed to cold, it tends to increase feeding; when
it is exposed to heat, it tends to decrease its caloric
intake. This is caused by interaction within the hypo-
thalamus between the temperature-regulating system
(see Chapter 73) and the food intake–regulating
system. This is important, because increased food
intake

. This is important, because increased food
intake in a cold animal (1) increases its metabolic rate
and (2) provides increased fat for insulation, both of
which tend to correct the cold state.
Feedback Signals from Adipose Tissue Regulate Food Intake.
Most of the stored energy in the body consists of fat,
the amount of which can vary considerably in differ-
ent individuals.What regulates this energy reserve, and
why is there so much variability among individuals?
Recent studies suggest that the

among individuals?
Recent studies suggest that the hypothalamus
senses energy storage through the actions of leptin, a
peptide hormone released from adipocytes. When the
amount of adipose tissue increases (signaling excess
energy storage), the adipocytes produce increased
amounts of leptin, which is released into the blood.
Leptin then circulates to the brain, where it moves
across the blood-brain barrier by facilitated diffusion
and occupies leptin receptors at multiple sites in the
hypothalamu

tin receptors at multiple sites in the
hypothalamus, especially the POMC neurons of the
arcuate nuclei and neurons of the paraventricular
nuclei.
Stimulation of leptin receptors in these hypothala-
mic nuclei initiates multiple actions that decrease fat
storage, including (1) decreased production in the
hypothalamus of appetite stimulators, such as NPY
and AGRP; (2) activation of POMC neurons, causing
release of a-MSH and activation of melanocortin
receptors; (3) increased production in the hypo

in
receptors; (3) increased production in the hypothala-
mus of substances, such as corticotropin-releasing
hormone, that decrease food intake; (4) increased 
sympathetic nerve activity (through neural projections
from the hypothalamus to the vasomotor centers),
which increases metabolic rate and energy expendi-
ture; and (5) decreased insulin secretion by the pan-
creatic beta cells, which decreases energy storage.
Thus, leptin may be an important means by which 
the adipose tissue signals the 

nt means by which 
the adipose tissue signals the brain that enough energy
has been stored and that intake of food is no longer
In mice or humans with mutations that render their
fat cells unable to produce leptin or mutations that
cause defective leptin receptors in the hypothalamus,
marked hyperphagia and morbid obesity occur. In
most obese humans, however, there does not appear
to be a deﬁciency of leptin production, because plasma
leptin levels increase in proportion with increasi

leptin levels increase in proportion with increasing
adiposity. Therefore, some physiologists believe that
obesity may be associated with leptin resistance; that
is, leptin receptors or post-receptor signaling pathways
normally activated by leptin may be defective in obese
people, who continue to eat despite very high levels of
leptin.
Another explanation for the failure of leptin to
prevent increasing adiposity in obese individuals is
that there are many redundant systems that control
feeding b

 are many redundant systems that control
feeding behavior, as well as social and cultural factors
that can cause continued excess food intake even in
the presence of high levels of leptin.
Summary of Long-Term Regulation. Even though our
information on the different feedback factors in 
872
Unit XIII
long-term feeding regulation is imprecise, we can
make the following general statement: When the
energy stores of the body fall below normal, the
feeding center

 of the body fall below normal, the
feeding centers of the hypothalamus and other areas
of the brain become highly active, and the person
exhibits increased hunger as well as searching for food;
conversely, when the energy stores (mainly the fat
stores) are already abundant, the person usually loses
the sensation of hunger and develops a state of satiety.
Importance of Having Both Long- and Short-
Term Regulatory Systems for Feeding
The long-term regulatory system for feeding, which
includes all

 regulatory system for feeding, which
includes all the nutritional energy feedback mecha-
nisms, helps maintain constant stores of nutrients in
the tissues, preventing them from becoming too low or
too high. The short-term regulatory stimuli serve two
other purposes. First, they tend to make the person eat
smaller quantities at each eating session, thus allowing
food to pass through the gastrointestinal tract at a
steadier pace, so that its digestive and absorptive
mechanisms can work at optimal

tive and absorptive
mechanisms can work at optimal rates rather than
becoming periodically overburdened. Second, they
help prevent the person from eating amounts at each
meal that would be too much for the metabolic storage
systems once all the food has been absorbed.
Obesity
Obesity can be deﬁned as an excess of body fat. A sur-
rogate marker for body fat content is the body mass
index (BMI), which is calculated as:
BMI = Weight in kg/Height m2
In clinical terms, a BMI between 25 and 29.9 kg/m2

In clinical terms, a BMI between 25 and 29.9 kg/m2 is
called overweight, and a BMI greater than 30 kg/m2 is
called obese. BMI is not a direct estimate of adiposity
and does not take into account the fact that some indi-
viduals have a high BMI due to a large muscle mass. A
better way to deﬁne obesity is to actually measure the
percentage of total body fat. Obesity is usually deﬁned
as 25 per cent or greater total body fat in men and 35
per cent or greater in women. Although percentage of
body fa

r greater in women. Although percentage of
body fat can be estimated with various methods, such as
measuring skin-fold thickness, bioelectrical impedance,
or underwater weighing, these methods are rarely used
in clinical practice, where BMI is commonly used to
assess obesity.
The prevalence of obesity in children and adults in
the United States and in many other industrialized
countries is rapidly increasing, rising by more than 30
per cent over the past decade. Approximately 64 per
cent of adul

the past decade. Approximately 64 per
cent of adults in the United States are overweight, and
nearly 33 per cent of adults are obese.
Obesity Results from Greater Energy Intake than Energy Expendi-
ture. When greater quantities of energy (in the form of
food) enter the body than are expended, the body
weight increases, and most of the excess energy is stored
as fat. Therefore, excessive adiposity (obesity) is caused
by energy intake in excess of energy output. For each
9.3 Calories of excess ene

energy output. For each
9.3 Calories of excess energy that enter the body,
approximately 1 gram of fat is stored.
Fat is stored mainly in adipocytes in subcutaneous
tissue and in the intraperitoneal cavity, although the
liver and other tissues of the body often accumulate sig-
niﬁcant amounts of lipids in obese persons. The meta-
bolic processes involved in fat storage were discussed in
It was previously believed that the number of
adipocytes could increase substantially only during

ipocytes could increase substantially only during
infancy and childhood and that excess energy intake 
in children led to hyperplastic obesity, associated with
increased numbers of adipocytes and only small
increases in adipocyte size. In contrast, obesity devel-
oping in adults was thought to increase only adipocyte
size, resulting in hypertrophic obesity. Recent studies,
however, have shown that new adipocytes can differen-
tiate from ﬁbroblast-like preadipocytes at any period of
life and that

-like preadipocytes at any period of
life and that the development of obesity in adults is
accompanied by increased numbers, as well as increased
size,of adipocytes.An extremely obese person may have
as many as four times as many adipocytes, each con-
taining twice as much lipid, as a lean person.
Once a person has become obese and a stable weight
is obtained, energy intake once again equals energy
output. For a person to lose weight, energy intake must
be less than energy expenditure.
Decreased

ke must
Decreased Physical Activity and
Abnormal Feeding Regulation as
The causes of obesity are complex. Although genes 
play an important role in determining food intake and
energy metabolism, lifestyle and environmental factors
may play the dominant role in many obese people. The
rapid increase in the prevalence of obesity in the past
20 to 30 years emphasizes the important role of lifestyle
and environmental factors, because genetic changes

nd environmental factors, because genetic changes
could not have occurred so rapidly.
Sedentary Lifestyle Is a Major Cause of Obesity. Regular phys-
ical activity and physical training are known to increase
muscle mass and decrease body fat mass, whereas 
inadequate physical activity is typically associated 
with decreased muscle mass and increased adiposity.
For example, studies have shown a close association
between sedentary behaviors, such as prolonged televi-
sion watching, and obesity.
Abo

 prolonged televi-
About 25 to 30 per cent of the energy used each day
by the average person goes into muscular activity, and
in a laborer, as much as 60 to 70 per cent is used in this
way. In obese people, increased physical activity usually
increases energy expenditure more than food intake,
resulting in signiﬁcant weight loss. Even a single episode
of strenuous exercise may increase basal energy expen-
diture for several hours after the physical activity is
stopped

veral hours after the physical activity is
stopped. Because muscular activity is by far the most
important means by which energy is expended in the
body, increased physical activity is often an effective
means of reducing fat stores.
Abnormal Feeding Behavior Is an Important Cause of Obesity.
Although powerful physiologic mechanisms regulate
food intake, there are also important environmental and
psychological factors that can cause abnormal feeding
behavior, excessive energy intake, and obesity

ing
behavior, excessive energy intake, and obesity.
Environmental, Social, and Psychological Factors Con-
tribute to Abnormal Feeding. As discussed previously,
the importance of environmental factors is evident from
Chapter 71
873
the rapid increase in the prevalence of obesity in most
industrialized countries, which has coincided with an
abundance of high-energy foods (especially fatty foods)
and sedentary li

gy foods (especially fatty foods)
and sedentary lifestyles.
Psychological factors may contribute to obesity in
some people. For example, people often gain large
amounts of weight during or after stressful situations,
such as the death of a parent, a severe illness, or even
mental depression. It seems that eating can be a means
of releasing tension.
Childhood Overnutrition as a Possible Cause of Obesity.
One factor that may contribute to obesity is the preva-
lent idea that healthy eating habits 

s the preva-
lent idea that healthy eating habits require three meals
a day and that each meal must be ﬁlling. Many young
children are forced into this habit by overly solicitous
parents, and the children continue to practice it
The rate of formation of new fat cells is especially
rapid in the ﬁrst few years of life, and the greater the
rate of fat storage, the greater the number of fat cells.
The number of fat cells in obese children is often as
much as three times that in norm

ldren is often as
much as three times that in normal children. Therefore,
it has been suggested that overnutrition of children—
especially in infancy and, to a lesser extent, during the
later years of childhood—can lead to a lifetime of
obesity.
Neurogenic Abnormalities as a Cause of Obesity. We pre-
viously pointed out that lesions in the ventromedial
nuclei of the hypothalamus cause an animal to eat
excessively and become obese. People with hypophysial
tumors that encroach on the hypothalamus 

ophysial
tumors that encroach on the hypothalamus often
develop progressive obesity, demonstrating that obesity
in human beings, too, can result from damage to the 
Although hypothalamic damage is almost never
found in obese people, it is possible that the functional
organization of the hypothalamic or other neurogenic
feeding centers in obese individuals is different from
that in nonobese persons. Also, there may be abnor-
malities of neurotransmitters or receptor mechanisms in
th

 of neurotransmitters or receptor mechanisms in
the neural pathways of the hypothalamus that control
feeding. In support of this theory, an obese person who
has reduced to normal weight by strict dietary measures
usually develops intense hunger that is demonstrably
far greater than that of a normal person. This indicates
that the “set-point” of an obese person’s feeding control
system is at a much higher level of nutrient storage than
that of a nonobese person.
Studies in experimental animals al

onobese person.
Studies in experimental animals also indicate that
when food intake is restricted in obese animals, there
are marked neurotransmitter changes in the hypothala-
mus that greatly increase hunger and oppose weight
loss. Some of these changes include increased formation
of orexigenic neurotransmitters such as NPY and
decreased formation of anorexic substances such as
leptin and a-MSH.
Genetic Factors as a Cause of Obesity. Obesity deﬁnitely
runs in families. Yet it has been difﬁcult 

nitely
runs in families. Yet it has been difﬁcult to determine
the precise role of genetics in contributing to obesity,
because family members generally share many of the
same eating habits and physical activity patterns.
Current evidence, however, suggests that 20 to 25 per
cent of cases of obesity may be caused by genetic
factors.
Genes can contribute to obesity by causing abnor-
malities of (1) one or more of the pathways that regu-
late the feeding centers and (2) energy expenditure and
fat 

eeding centers and (2) energy expenditure and
fat storage.Three of the monogenic (single-gene) causes
of obesity are (1) mutations of MCR-4, the most
common monogenic form of obesity discovered thus
far; (2) congenital leptin deﬁciency caused by mutations
of the leptin gene, which are very rare; and (3) muta-
tions of the leptin receptor, also very rare. All these
monogenic forms of obesity account for only a very
small percentage of obesity. It is likely that many gene
variations interact with 

is likely that many gene
variations interact with environmental factors to inﬂu-
ence the amount and distribution of body fat.
Treatment of obesity depends on decreasing energy
input below energy expenditure and creating a sus-
tained negative energy balance until the desired weight
loss is achieved. In other words, this means either reduc-
ing energy intake or increasing energy expenditure.The
current National Institutes of Health (NIH) guidelines
recommend a decrease in ca

Health (NIH) guidelines
recommend a decrease in caloric intake of 500 kilo-
calories per day for overweight and moderately obese
persons (BMI greater than 25 but less than 35 kg/m2) to
achieve a weight loss of approximately 1 pound each
week. A more aggressive energy deﬁcit of 500 to 1000
kilocalories per day is recommended for persons with
BMIs greater than 35 kg/m2. Typically, such an energy
deﬁcit, if it can be achieved and sustained, will cause a
weight loss of about 1 to 2 pounds per week, 

se a
weight loss of about 1 to 2 pounds per week, or about a
10 per cent weight loss after 6 months. For most people
attempting to lose weight, increasing physical activity is
also an important component of successful long-term
To decrease energy intake, most reducing diets are
designed to contain large quantities of “bulk,” which is
generally made up of non-nutritive cellulose substances.
This bulk distends the stomach and thereby partially
appeases hunger. In most lower animals, s

artially
appeases hunger. In most lower animals, such a proce-
dure simply makes the animal increase its food intake
even more, but human beings can often fool themselves
because their food intake is sometimes controlled as
much by habit as by hunger.As pointed out later in con-
nection with starvation, it is important to prevent
vitamin deﬁciencies during the dieting period.
Various drugs for decreasing the degree of hunger
have been used in the treatment of obesity. The most
widely used drugs 

 treatment of obesity. The most
widely used drugs are amphetamines (or amphetamine
derivatives), which directly inhibit the feeding centers
in the brain. One drug for treating obesity is sibu-
tramine, a sympathomimetic that reduces food intake
and increases energy expenditure. The danger in using
these drugs is that they simultaneously overexcite the
central nervous system, making the person nervous and
elevating the blood pressure.Also, a person soon adapts
to the drug, so that weight reductio

n soon adapts
to the drug, so that weight reduction is usually no
greater than 5 to 10 per cent.
Another group of drugs works by altering lipid
metabolism. For example, orlistat, a lipase inhibitor,
reduces the intestinal digestion of fat. This causes a
portion of the ingested fat to be lost in the feces and
therefore reduces energy absorption. However, fecal fat
loss may cause unpleasant gastrointestinal side effects,
as well as loss of fat-soluble vitamins in the feces.
Signiﬁcant weight loss 

ble vitamins in the feces.
Signiﬁcant weight loss can be achieved in many obese
persons with increased physical activity. The more 
874
Unit XIII
exercise one gets, the greater the daily energy expendi-
ture and the more rapidly the obesity disappears.
Therefore, forced exercise is often an essential part of
treatment. The current clinical guidelines for the treat-
ment of obesity recommend that the ﬁrst step be
lifestyle modiﬁcations that include increased 

 be
lifestyle modiﬁcations that include increased physical
activity combined with a reduction in caloric intake. For
morbidly obese patients with BMIs greater than 40, or
for patients with BMIs greater than 35 and conditions
such as hypertension or type II diabetes that predispose
them to other serious diseases, various surgical proce-
dures can be used to decrease the fat mass of the body
or to decrease the amount of food that can be eaten at
each meal.
Two of the most common surgical procedure

ch meal.
Two of the most common surgical procedures used in
the United States to treat morbid obesity are gastric
bypass surgery and gastric banding surgery. Gastric
bypass surgery involves construction of a small pouch in
the proximal part of the stomach that is then connected
to the jejunum with a section of small bowel of varying
lengths; the pouch is separated from the remaining part
of the stomach with staples. Gastric banding surgery
involves placing an adjustable band aroundthe stomach
ne

es placing an adjustable band aroundthe stomach
near its upper end; this also creates a small stomach
pouch that restricts the amount of food that can be
eaten at each meal. Although these surgical procedures
generally produce substantial weight loss in obese
patients, they are major operations, and their long-
term effects on overall health and mortality are still
Inanition, Anorexia, and
Cachexia
Inanition is the opposite of obesity and is characterized
by extreme weight loss. It ca

and is characterized
by extreme weight loss. It can be caused by inadequate
availability of food or by pathophysiologic conditions
that greatly decrease the desire for food, including psy-
chogenic disturbances, hypothalamic abnormalities,
and factors released from peripheral tissues. In many
instances, especially in those with serious diseases such
as cancer, the reduced desire for food may be associated
with increased energy expenditure, causing serious
Anorexia can be deﬁned as a

g serious
Anorexia can be deﬁned as a reduction in food intake
caused primarily by diminished appetite, as opposed to
the literal deﬁnition of “not eating.” This deﬁnition
emphasizes the important role of central neural mech-
anisms in the pathophysiology of anorexia in diseases
such as cancer, when other common problems, such as
pain and nausea, may also cause a person to consume
lessfood. Anorexia nervosa is an abnormal psychic state
in which a person loses all desire for food and

te
in which a person loses all desire for food and even
becomes nauseated by food; as a result, severe inanition
occurs.
Cachexia is a metabolic disorder of increased energy
expenditure leading to weight loss greater than that
caused by reduced food intake alone. Anorexia and
cachexia often occur together in many types of cancer
or in the “wasting syndrome” observed in patients 
with acquired immunodeﬁciency syndrome (AIDS) and
chronic inﬂammatory disorders. Almost all types of
cancer cause both

y disorders. Almost all types of
cancer cause both anorexiaand cachexia, and more than
half of cancer patients develop anorexia-cachexia syn-
drome during the course of their disease.
Central neural and peripheral factors are believed to
contribute to cancer-induced anorexia and cachexia.
Several inﬂammatory cytokines,
including tumor 
necrosis factor-a, interleukin-6, interleukin-1b, and a
proteolysis-inducing factor, have been shown to cause
anorexia and cachexia. Most of these inﬂammatory
cyt

orexia and cachexia. Most of these inﬂammatory
cytokines appear to mediate anorexia by activation of
the melanocortin system in the hypothalamus. The
precise mechanisms by which cytokines or tumor prod-
ucts interact with the melanocortinpathway to decrease
food intake are still unclear, but blockade of the hypo-
thalamic melanocortin receptors appears to almost
completely prevent their anorexic and cachectic effects
in experimental animals. Additional research, however,
is needed to better unde

tional research, however,
is needed to better understand the pathophysiologic
mechanisms of anorexia and cachexia in cancer patients
and to develop therapeutic agents to improve their
nutritional status and survival.
Depletion of Food Stores in the Body Tissues During Starvation.
Even though the tissues preferentially use carbohydrate
for energy over both fat and protein, the quantity of car-
bohydrate normally stored in the entire body is only a
few hundred grams (mainly glycogen in 

y is only a
few hundred grams (mainly glycogen in the liver and
muscles), and it can supply the energy required for body
functions for perhaps half a day. Therefore, except for
the ﬁrst few hours of starvation, the major effects are
progressive depletion of tissue fat and protein. Because
fat is the prime source of energy (100 times as much fat
energy is stored in the normal person as carbohydrate
energy), the rate of fat depletion continues unabated, as
shown in Figure 71–3, until most of the f

ated, as
shown in Figure 71–3, until most of the fat stores in the
body are gone.
Protein undergoes three phases of depletion: rapid
depletion at ﬁrst, then greatly slowed depletion, and,
ﬁnally, rapid depletion again shortly before death. The
initial rapid depletion is caused by the use of easily
mobilized protein for direct metabolism or for conver-
sion to glucose and then metabolism of glucose mainly
by the brain. After the readily mobilized protein stores
have been depleted during the early

protein stores
have been depleted during the early phase of starvation,
0
Fat
Protein
1
2
3
4
5
6
7
8
Quantities of stored foodstuffs
(kilograms)
Weeks of starvation
12
10
8
6
4
2
0
Figure 71–3
Effect of starvation on the food stores of the body.
Chapter 71
875
the remaining protein is not so easily removed. At this
time, the rate of gluconeogenesis decreases to one third
to one ﬁfth its previous 

s decreases to one third
to one ﬁfth its previous rate, and the rate of depletion
of protein becomes greatly decreased. The lessened
availability of glucose then initiates a series of events
that leads to excessive fat utilization and conversion of
some of the fat breakdown products to ketone bodies,
producing the state of ketosis, which is discussed in
Chapter 68. The ketone bodies, like glucose, can cross
the blood-brain barrier and can be used by the brain
cells for energy. Therefore, about t

 by the brain
cells for energy. Therefore, about two thirds of the
brain’s energy is now derived from these ketone bodies,
principally from beta-hydroxybutyrate. This sequence
of events leads to at least partial preservation of the
protein stores of the body.
There ﬁnally comes a time when the fat stores are
almost depleted, and the only remaining source of
energy is protein. At that time, the protein stores once
again enter a stage of rapid depletion. Because proteins
are also essential for the

etion. Because proteins
are also essential for the maintenance of cellular func-
tion, death ordinarily ensues when the proteins of the
body have been depleted to about half their normal
level.
Vitamin Deﬁciencies in Starvation. The stores of some of the
vitamins, especially the water-soluble vitamins—the
vitamin B group and vitamin C—do not last long during
starvation. Consequently, after a week or more of 
starvation, mild vitamin deﬁciencies usually begin to
appear, and after several weeks, s

sually begin to
appear, and after several weeks, severe vitamin deﬁ-
ciencies can occur. These deﬁciencies can add to the
debility that leads to death.
Vitamins
Daily Requirements of Vitamins. A vitamin is an organic
compound needed in small quantities for normal
metabolism that cannot be manufactured in the cells of
the body. Lack of vitamins in the diet can cause impor-
tant metabolic deﬁcits. Table 71–3 lists the amounts 
of important vitamins required daily by the average
person. These requi

 required daily by the average
person. These requirements vary considerably, depend-
ing on such factors as body size, rate of growth, amount
of exercise, and pregnancy.
Storage of Vitamins in the Body. Vitamins are stored to a
slight extent in all cells. Some vitamins are stored to a
major extent in the liver. For instance, the quantity of
vitamin A stored in the liver may be sufﬁcient to main-
tain a person for 5 to 10 months without any intake of
vitamin A. The quantity of vitamin D stored in

 of
vitamin A. The quantity of vitamin D stored in the liver
is usually sufﬁcient to maintain a person for 2 to 4
months without any additional intake of vitamin D.
The storage of most water-soluble vitamins is rela-
tively slight. This applies especially to most vitamin B
compounds.When a person’s diet is deﬁcient in vitamin
B compounds, clinical symptoms of the deﬁciency can
sometimes be recognized within a few days (except for
vitamin B12, which can last in the liver in a bound form
for a yea

ch can last in the liver in a bound form
for a year or longer). Absence of vitamin C, another
water-soluble vitamin, can cause symptoms within a few
weeks and can cause death from scurvy in 20 to 30
weeks.
Vitamin A
Vitamin A occurs in animal tissues as retinol. This
vitamin does not occur in foods of vegetable origin, but
provitamins for the formation of vitamin A do occur in
abundance in many vegetable foods. These are the
yellow and red carotenoid pigments, which, because
their chemical struc

noid pigments, which, because
their chemical structures are similar to that of vitamin
A, can be changed into vitamin A in the liver.
Vitamin A Deﬁciency Causes “Night Blindness” and Abnormal
Epithelial Cell Growth. One basic function of vitamin A is
its use in the formation of the retinal pigments of the
eye, which is discussed in Chapter 50. Vitamin A is
needed to form the visual pigments and, therefore, to
prevent night blindness.
Vitamin A is also necessary for normal growth of
most cells of

 also necessary for normal growth of
most cells of the body and especially for normal growth
and proliferation of the different types of epithelial
cells.When vitamin A is lacking, the epithelial structures
of the body tend to become stratiﬁed and keratinized.
Vitamin A deﬁciency manifests itself by (1) scaliness of
the skin and sometimes acne; (2) failure of growth of
young animals, including cessation of skeletal growth;
(3) failure of reproduction, associated especially with
atrophy of the ge

tion, associated especially with
atrophy of the germinal epithelium of the testes and
sometimes with interruption of the female sexual cycle;
and (4) keratinization of the cornea, with resultant
corneal opacity and blindness.
In vitamin A deﬁciency, the damaged epithelial struc-
tures often become infected, for example, the conjunc-
tivae of the eyes, the linings of the urinary tract, and the
respiratory passages. Vitamin A has been called an
“anti-infection” vitamin.
Thiam

ti-infection” vitamin.
Thiamine operates in the metabolic systems of the body
principally as thiamine pyrophosphate; this compound
functions as a cocarboxylase, operating mainly in con-
junction with a protein decarboxylase for decarboxyla-
tion of pyruvic acid and other a-keto acids, as discussed
in Chapter 67.
Thiamine deﬁciency (beriberi) causes decreased uti-
lization of pyruvic acid and some amino acids by the
tissues, but increased utilization of fats. Thus, thiamine

but increased utilization of fats. Thus, thiamine
is speciﬁcally needed for the ﬁnal metabolism of carbo-
hydrates and many amino acids. The decreased utiliza-
tion of these nutrients is responsible for many debilities
associated with thiamine deﬁciency.
Table 71–3
Required Daily Amounts of Vitamins
Vitamin
Amount
A
5000 IU
Thiamine
1.5 mg
Riboﬂavin
1.8 mg
Niacin
20 mg
Ascorbic acid
45 mg
D
400 IU
E
15 IU
K
70 mg
Folic acid
0.4 mg
B12
3 mg
Pyridoxine
2 mg
Pantothenic acid
Unknown
876
Unit XIII
M

xine
2 mg
Unknown
876
Unit XIII
Thiamine Deﬁciency Causes Lesions of the Central and Peripheral
Nervous Systems. The central nervous system normally
depends almost entirely on the metabolism of carbohy-
drates for its energy. In thiamine deﬁciency, the utiliza-
tion of glucose by nervous tissue may be decreased 50
to 60 per cent and is replaced by the utilization of
ketone bodies derived from fat metabolism. The neu-
ronal cells of the centr

 fat metabolism. The neu-
ronal cells of the central nervous system frequently
show chromatolysis and swelling during thiamine deﬁ-
ciency, changes that are characteristic of neuronal 
cells with poor nutrition. These changes can disrupt
communication in many portions of the central nervous
system.
Thiamine deﬁciency can cause degeneration of myelin
sheaths of nerve ﬁbers in both the peripheral nerves and
the central nervous system. Lesions in the peripheral
nerves frequently cause them to becom

e peripheral
nerves frequently cause them to become extremely irri-
table, resulting in “polyneuritis,” characterized by pain
radiating along the course of one or many peripheral
nerves. Also, ﬁber tracts in the cord can degenerate to
such an extent that paralysis occasionally results; even
in the absence of paralysis, the muscles atrophy, result-
ing in severe weakness.
Thiamine Deﬁciency Weakens the Heart and Causes Peripheral
Vasodilation. A person with severe thiamine deﬁciency
eventually de

erson with severe thiamine deﬁciency
eventually develops cardiac failure because of weak-
ened cardiac muscle. Further, the venous return of
blood to the heart may be increased to as much as two
times normal. This occurs because thiamine deﬁciency
causes peripheral vasodilation throughout the circula-
tory system, presumably as a result of decreased release
of metabolic energy in the tissues, leading to local vas-
cular dilation.The cardiac effects of thiamine deﬁciency
are due partly to high bl

ts of thiamine deﬁciency
are due partly to high blood ﬂow into the heart and
partly to primary weakness of the cardiac muscle.
Peripheral edema and ascites also occur to a major
extent in some people with thiamine deﬁciency, mainly
because of cardiac failure.
Thiamine Deﬁciency Causes Gastrointestinal Tract Disturbances.
Among the gastrointestinal symptoms of thiamine deﬁ-
ciency are indigestion, severe constipation, anorexia,
gastric atony,and hypochlorhydria.All these effects pre-
sumably resu

ypochlorhydria.All these effects pre-
sumably result from failure of the smooth muscle and
glands of the gastrointestinal tract to derive sufﬁcient
energy from carbohydrate metabolism.
The overall picture of thiamine deﬁciency, including
polyneuritis, cardiovascular symptoms, and gastro-
intestinal disorders,
is frequently referred to as 
beriberi—especially when the cardiovascular symptoms
predominate.
Niacin
Niacin, also called nicotinic acid, functions in the body
as coenzymes in the form of 

functions in the body
as coenzymes in the form of nicotinamide adenine 
dinucleotide (NAD) and nicotinamide adenine dinu-
cleotide phosphate (NADP). These coenzymes are
hydrogen acceptors; they combine with hydrogen atoms
as they are removed from food substrates by many types
of dehydrogenases. The typical operation of both these
coenzymes is presented in Chapter 67. When a deﬁ-
ciency of niacin exists, the normal rate of dehydrogena-
tion cannot be maintained; therefore, oxidative delivery
of e

 be maintained; therefore, oxidative delivery
of energy from the foodstuffs to the functioning ele-
ments of all cells cannot occur at normal rates.
In the early stages of niacin deﬁciency, simple phys-
iologic changes such as muscle weakness and poor glan-
dular secretion may occur, but in severe niacin
deﬁciency, actual tissue death ensues. Pathological
lesions appear in many parts of the central nervous
system, and permanent dementia or many types of psy-
choses may result. Also, the skin dev

ypes of psy-
choses may result. Also, the skin develops a cracked,
pigmented scaliness in areas that are exposed to
mechanical irritation or sun irradiation; thus, it appears
that in persons with niacin deﬁciency, the skin is unable
to repair irritative damage.
Niacin deﬁciency causes intense irritation and
inﬂammation of the mucous membranes of the mouth
and other portions of the gastrointestinal tract, result-
ing in many digestive abnormalities that can lead to
widespread gastrointestinal hem

s that can lead to
widespread gastrointestinal hemorrhage in severe cases.
It is possible that this results from generalized depres-
sion of metabolism in the gastrointestinal epithelium
and failure of appropriate epithelial repair.
The clinical entity called pellagra and the canine
disease called black tongue are caused mainly by niacin
deﬁciency. Pellagra is greatly exacerbated in people on
a corn diet, because corn is deﬁcient in the amino acid
tryptophan, which can be converted in limited qu

d
tryptophan, which can be converted in limited quanti-
ties to niacin in the body.
Riboﬂavin normally combines in the tissues with phos-
phoric acid to form two coenzymes, ﬂavin mononu-
cleotide (FMN) and ﬂavin adenine dinucleotide (FAD).
They operate as hydrogen carriers in important oxida-
tive systems of the mitochondria. NAD, operating 
in association with speciﬁc dehydrogenases, usually
accepts hydrogen removed from various food substrates
and then passes the hydroge

arious food substrates
and then passes the hydrogen to FMN or FAD; ﬁnally,
the hydrogen is released as an ion into the mitochon-
drial matrix to become oxidized by oxygen (described
in Chapter 67).
Deﬁciency of riboﬂavin in experimental animals
causes severe dermatitis, vomiting, diarrhea, muscle
spasticity that ﬁnally becomes muscle weakness, coma
and decline in body temperature, and then death. Thus,
severe riboﬂavin deﬁciency can cause many of the same
effects as a lack of niacin in the diet;

 the same
effects as a lack of niacin in the diet; presumably, the
debilities that result in each instance are due to gener-
ally depressed oxidative processes within the cells.
In the human being, there are no known cases of
riboﬂavin deﬁciency severe enough to cause the marked
debilities noted in experimental animals, but mild
riboﬂavin deﬁciency is probably common. Such deﬁ-
ciency causes digestive disturbances, burning sensations
of the skin and eyes, cracking at the corners of the
mouth, he

and eyes, cracking at the corners of the
mouth, headaches, mental depression, forgetfulness, and
so on.
Although the manifestations of riboﬂavin deﬁciency
are usually relatively mild, this deﬁciency frequently
occurs in association with deﬁciency of thiamine, niacin,
or both. Many deﬁciency syndromes, including pellagra,
beriberi, sprue, and kwashiorkor, are probably due to a
combined deﬁciency of a number of vitamins, as well as
other aspects of malnutrition.
Several cobalamin compo

 malnutrition.
Several cobalamin compounds that possess the common
prosthetic group shown next exhibit so-called vitamin
B12 activity.
Chapter 71
877
Note that this prosthetic group contains cobalt, which
has bonds similar to those of iron in the hemoglobin
molecule. It is likely that the cobalt atom functions in
much the same way that the iron atom functions to
combine reversibly with other substa

 functions to
combine reversibly with other substances.
Vitamin B12 Deﬁciency Causes Pernicious Anemia. Vitamin B12
performs several metabolic functions, acting as a hydro-
gen acceptor coenzyme. Its most important function is
to act as a coenzyme for reducing ribonucleotides to
deoxyribonucleotides, a step that is necessary in the
replication of genes. This could explain the major func-
tions of vitamin B12: (1) promotion of growth and (2)
promotion of red blood cell formation and maturation.
T

tion of red blood cell formation and maturation.
This red cell function is described in detail in Chapter
32 in relation to pernicious anemia, a type of anemia
caused by failure of red blood cell maturation when
vitamin B12 is deﬁcient.
Vitamin B12 Deﬁciency Causes Demyelination of the Large Nerve
Fibers of the Spinal Cord. The demyelination of nerve
ﬁbers in people with vitamin B12 deﬁciency occurs espe-
cially in the posterior columns, and occasionally the
lateral columns, of the spinal cord. 

sionally the
lateral columns, of the spinal cord. As a result, many
people with pernicious anemia have loss of peripheral
sensation and, in severe cases, even become paralyzed.
The usual cause of vitamin B12 deﬁciency is not lack
of this vitamin in the food but deﬁciency of formation
of intrinsic factor, which is normally secreted by the
parietal cells of the gastric glands and is essential for
absorption of vitamin B12 by the ileal mucosa.This is dis-
cussed in Chapters 32 and 66.
Folic Acid (P

s dis-
Several pteroylglutamic acids exhibit the “folic acid
effect.” Folic acid functions as a carrier of hydroxy-
methyl and formyl groups. Perhaps its most important
use in the body is in the synthesis of purines and thymine,
which are required for formation of DNA. Therefore,
folic acid, like vitamin B12, is required for replication of
the cellular genes. This may explain one of the most
important functions of folic acid—to prom

the most
important functions of folic acid—to promote growth.
Indeed, when it is absent from the diet, an animal grows
very little.
Folic acid is an even more potent growth promoter
than vitamin B12 and, like vitamin B12, is important 
for the maturation of red blood cells, as discussed 
in Chapter 32. However, vitamin B12 and folic acid 
each perform speciﬁc and different chemical functions
in promoting growth and maturation of red blood 
cells. One of the signiﬁcant effects of folic acid 
deﬁc

 One of the signiﬁcant effects of folic acid 
deﬁciency is the development of macrocytic anemia,
almost identical to that which occurs in pernicious
anemia. This often can be treated effectively with folic
acid alone.
Pyridoxine exists in the form of pyridoxal phosphate in
the cells and functions as a coenzyme for many chemi-
cal reactions related to amino acid and protein metab-
olism. Its most important role is that of coenzyme in the
transamination process for the synt

oenzyme in the
transamination process for the synthesis of amino acids.
As a result, pyridoxine plays many key roles in metab-
olism, especially protein metabolism.Also, it is believed
to act in the transport of some amino acids across cell
Co
N
CH2
N
N
N
N
H2C
OH
OH
OH
CH2
CH3
N
Dietary lack of pyridoxine in lower animals can cause
dermatitis, decreased rate of growth, development of
fatty liver, anemia, and evidence of mental deteriora-
tion. Rarely, in children, pyridoxi

tal deteriora-
tion. Rarely, in children, pyridoxine deﬁciency has been
known to cause seizures, dermatitis, and gastrointestinal
disturbances such as nausea and vomiting.
Pantothenic acid mainly is incorporated in the body into
coenzyme A (CoA), which has many metabolic roles in
the cells. Two of these discussed at length in Chapters
67 and 68 are (1) conversion of decarboxylated pyruvic
acid into acetyl-CoA before its entry into the citric acid
cycle, and (2) degradation of fa

o the citric acid
cycle, and (2) degradation of fatty acid molecules into
multiple molecules of acetyl-CoA. Thus, lack of pan-
tothenic acid can lead to depressed metabolism of both
carbohydrates and fats.
Deﬁciency of pantothenic acid in lower animals can
cause retarded growth, failure of reproduction, graying
of the hair, dermatitis, fatty liver, and hemorrhagic
adrenocortical necrosis. In the human being, no deﬁnite
deﬁciency syndrome has been proved, presumably
because of the wide occurrence

 proved, presumably
because of the wide occurrence of this vitamin in almost
all foods and because small amounts can probably be
synthesized in the body. This does not mean that pan-
tothenic acid is not of value in the metabolic systems of
the body; indeed, it is perhaps as necessary as any other
vitamin.
Ascorbic Acid Deﬁciency Weakens Collagen Fibers Throughout
the Body. Ascorbic acid is essential for activating the
enzyme prolyl hydroxylase, which promotes the hydro

nzyme prolyl hydroxylase, which promotes the hydrox-
ylation step in the formation of hydroxyproline, an inte-
gral constituent of collagen. Without ascorbic acid, the
collagen ﬁbers that are formed in virtually all tissues of
the body are defective and weak.Therefore, this vitamin
is essential for the growth and strength of the ﬁbers in
subcutaneous tissue, cartilage, bone, and teeth.
878
Unit XIII
Ascorbic Acid Deﬁciency Causes Scurvy. Deﬁciency of ascor-

Acid Deﬁciency Causes Scurvy. Deﬁciency of ascor-
bic acid for 20 to 30 weeks, which occurred frequently
during long ship voyages in the past, causes scurvy. One
of the most important effects of scurvy is failure of
wounds to heal. This is caused by failure of the cells to
deposit collagen ﬁbrils and intercellular cement sub-
stances. As a result, healing of a wound may require
several months instead of the several days ordinarily
Lack of ascorbic acid also causes cessation of bone
gr

 of ascorbic acid also causes cessation of bone
growth. The cells of the growing epiphyses continue to
proliferate, but no new collagen is laid down between
the cells, and the bones fracture easily at the point of
growth because of failure to ossify. Also, when an
already ossiﬁed bone fractures in a person with ascor-
bic acid deﬁciency, the osteoblasts cannot form new
bone matrix. Consequently, the fractured bone does not
heal.
The blood vessel walls become extremely fragile in
scurvy because o

walls become extremely fragile in
scurvy because of (1) failure of the endothelial cells to
be cemented together properly and (2) failure to form
the collagen ﬁbrils normally present in vessel walls.
The capillaries are especially likely to rupture, and as 
a result, many small petechial hemorrhages occur
throughout the body. The hemorrhages beneath the 
skin cause purpuric blotches, sometimes over the entire
body. To test for ascorbic acid deﬁciency, one can
produce such petechial hemorrhages b

ency, one can
produce such petechial hemorrhages by inﬂating a
blood pressure cuff over the upper arm; this occludes
the venous return of blood, the capillary pressure rises,
and red blotches occur on the forearm if the ascorbic
acid deﬁciency is sufﬁciently severe.
In extreme scurvy, the muscle cells sometimes frag-
ment; lesions of the gums occur, with loosening of the
teeth; infections of the mouth develop; and vomiting of
blood, bloody stools, and cerebral hemorrhage can all
occur. Finally, 

, and cerebral hemorrhage can all
occur. Finally, high fever often develops before death.
Vitamin D
Vitamin D increases calcium absorption from the gas-
trointestinal tract and helps control calcium deposition in
the bone. The mechanism by which vitamin D increases
calcium absorption is mainly to promote active trans-
port of calcium through the epithelium of the ileum. In
particular, it increases the formation of a calcium-
binding protein in the intestinal epithelial cells that aids
in calcium

e intestinal epithelial cells that aids
in calcium absorption. The speciﬁc functions of vitamin
D in relation to overall body calcium metabolism and
bone formation are presented in Chapter 79.
Vitamin E
Several related compounds exhibit so-called vitamin E
activity. Only rare instances of proved vitamin E deﬁ-
ciency have occurred in human beings. In experimental
animals, lack of vitamin E can cause degeneration of the
germinal epithelium in the testis and, therefore, can
cause male sterility. L

testis and, therefore, can
cause male sterility. Lack of vitamin E can also cause
resorption of a fetus after conception in the female.
Because of these effects of vitamin E deﬁciency, vitamin
E is sometimes called the “antisterility vitamin.” Deﬁ-
ciency of vitamin E prevents normal growth and some-
times causes degeneration of the renal tubular cells and
the muscle cells.
Vitamin E is believed to play a protective role in the
prevention of oxidation of unsaturated fats. In the
absence of vitam

ation of unsaturated fats. In the
absence of vitamin E, the quantity of unsaturated fats
in the cells becomes diminished, causing abnormal
structure and function of such cellular organelles as 
the mitochondria, the lysosomes, and even the cell
membrane.
Vitamin K
Vitamin K is necessary for the formation by the liver of
prothrombin, Factor VII (proconvertin), Factor IX, and
Factor X, all of which are important in blood coagula-
tion.Therefore, when vitamin K deﬁciency occurs, blood
clotting is r

en vitamin K deﬁciency occurs, blood
clotting is retarded. The function of this vitamin and 
its relation to some of the anticoagulants, such as
dicumarol, are presented in greater detail in Chapter 35.
Several compounds, both natural and synthetic,
exhibit vitamin K activity. Because vitamin K is synthe-
sized by bacteria in the colon, it is rare for a person to
have a bleeding tendency because of vitamin K deﬁ-
ciency in the diet. However, when the bacteria of the
colon are destroyed by the ad

 the bacteria of the
colon are destroyed by the administration of large quan-
tities of antibiotic drugs, vitamin K deﬁciency occurs
rapidly because of the paucity of this compound in the
normal diet.
The functions of many of the minerals, such as sodium,
potassium, and chloride, are presented at appropriate
points in the text. Only speciﬁc functions of minerals 
not covered elsewhere are mentioned here. The body
content of the most important minerals is listed in Table
71–4, 

 most important minerals is listed in Table
71–4, and the daily requirements of these are given in
Table 71–5.
Magnesium. Magnesium is about one sixth as plentiful in
cells as potassium. Magnesium is required as a catalyst
for many intracellular enzymatic reactions, particularly
those related to carbohydrate metabolism.
The extracellular ﬂuid magnesium concentration is
slight, only 1.8 to 2.5 mEq/L. Increased extracellular
concentration of magnesium depresses nervous system
activity as well as s

ium depresses nervous system
activity as well as skeletal muscle contraction.This latter
effect can be blocked by the administration of calcium.
Low magnesium concentration causes increased irri-
tability of the nervous system, peripheral vasodilation,
Table 71–4
Average Content of a 70-Kilogram Man
Amount (grams)
Water
41,400
Fat
12,600
Protein
12,600
300
Sodium
63
Potassium
150
Calcium
1,160
Magnesium
21
Chloride
85
670
Sulfur
112
Iron
3
Iodine
0.014
Chapter

sphorus
670
Sulfur
112
Iron
3
Iodine
0.014
Chapter 71
879
and cardiac arrhythmias, especially after acute myocar-
dial infarction.
Calcium. Calcium is present in the body mainly in the
form of calcium phosphate in the bone. This subject is
discussed in detail in Chapter 79, as is the calcium
content of extracellular ﬂuid. Excess quantities of
calcium ions in extracellular ﬂuid can cause the heart to
stop in sy

tracellular ﬂuid can cause the heart to
stop in systole and can act as a mental depressant. At
the other extreme, low levels of calcium can cause spon-
taneous discharge of nerve ﬁbers, resulting in tetany, as
discussed in Chapter 79.
Phosphorus. Phosphate is the major anion of intracellular
ﬂuid. Phosphates have the ability to combine reversibly
with many coenzyme systems and with multiple other
compounds that are necessary for the operation of
metabolic processes. Many important reactions of p

metabolic processes. Many important reactions of phos-
phates have been catalogued at other points in this text,
especially in relation to the functions of adenosine
triphosphate, adenosine diphosphate, phosphocreatine,
and so forth. Also, bone contains a tremendous amount
of calcium phosphate, which is discussed in Chapter 79.
Iron. The function of iron in the body, especially in rela-
tion to the formation of hemoglobin, is discussed in
Chapter 32. Two thirds of the iron in the body is in the

 32. Two thirds of the iron in the body is in the
form of hemoglobin, although smaller quantities are
present in other forms, especially in the liver and the
bone marrow. Electron carriers containing iron (espe-
cially the cytochromes) are present in the mitochondria
of all cells of the body and are essential for most of the
oxidation that occurs in the cells. Therefore, iron is
absolutely essential for both the transport of oxygen to
the tissues and the operation of oxidative systems within
the

 and the operation of oxidative systems within
the tissue cells, without which life would cease within a
few seconds.
Important Trace Elements in the Body. A few elements are
present in the body in such small quantities that they
are called trace elements. The amounts of these elements
in foods are also usually minute.Yet without any one of
them, a speciﬁc deﬁciency syndrome is likely to develop.
Three of the most important are iodine, zinc, and 
ﬂuorine.
Iodine. The best known of the trace elem

ﬂuorine.
Iodine. The best known of the trace elements is iodine.
This element is discussed in Chapter 76 in connection
with the formation and function of thyroid hormone; as
shown in Table 71–4, the entire body contains an
average of only 14 milligrams. Iodine is essential for the
formation of thyroxine and triiodothyronine, the two
thyroid hormones that are essential for maintenance of
normal metabolic rates in all cells of the body.
Zinc. Zinc is an integral part of many enzymes, one of
the m

 is an integral part of many enzymes, one of
the most important of which is carbonic anhydrase,
present in especially high concentration in the red blood
cells. This enzyme is responsible for rapid combination
of carbon dioxide with water in the red blood cells of
the peripheral capillary blood and for rapid release of
carbon dioxide from the pulmonary capillary blood into
the alveoli. Carbonic anhydrase is also present to a
major extent in the gastrointestinal mucosa, the tubules
of the kidney,

astrointestinal mucosa, the tubules
of the kidney, and the epithelial cells of many glands of
the body. Consequently, zinc in small quantities is essen-
tial for the performance of many reactions related to
carbon dioxide metabolism.
Zinc is also a component of lactic dehydrogenase and
is therefore important for the interconversions between
pyruvic acid and lactic acid. Finally, zinc is a component
of some peptidases and is important for the digestion of
proteins in the gastrointestinal tract.
F

stion of
Fluorine. Fluorine does not seem to be a necessary
element for metabolism, but the presence of a small
quantity of ﬂuorine in the body during the period of life
when the teeth are being formed subsequently protects
against caries. Fluorine does not make the teeth them-
selves stronger but has a poorly understood effect in
suppressing the cariogenic process. It has been sug-
gested that ﬂuorine is deposited in the hydroxyapatite
crystals of the too

eposited in the hydroxyapatite
crystals of the tooth enamel and combines with and
therefore blocks the functions of various trace metals
that are necessary for activation of the bacterial
enzymes that cause caries. Therefore, when ﬂuorine is
present, the enzymes remain inactive and cause no
caries.
Excessive intake of ﬂuorine causes ﬂuorosis, which
manifests in its mild state by mottled teeth and in its
more severe state by enlarged bones. It has been pos-
tulated that in this condition, ﬂuorine

 been pos-
tulated that in this condition, ﬂuorine combines with
trace metals in some of the metabolic enzymes, includ-
ing the phosphatases, so that various metabolic systems
become partially inactivated. According to this theory,
the mottled teeth and enlarged bones are due to ab-
normal enzyme systems in the odontoblasts and
osteoblasts. Even though the mottled teeth are highly
resistant to the development of caries, the structural
strength of these teeth may be considerably lessened by
the m

 these teeth may be considerably lessened by
the mottling process.
Barsh GS, Schwartz MW: Genetic approaches to studying
energy balance: perception and integration. Nat Rev
Genet 3:589, 2002.
Coll AP, Farooqi IS, Challis BG, et al: Proopiomelanocortin
and energy balance: insights from human and murine
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Ann N Y Acad Sci 994:1

es on melanocortin neurons.
Cowley MA, Grove KL: Ghrelin—satisfying a hunger for the
mechanism. Endocrinology 145:2604, 2004.
Table 71–5
Average Required Daily Amounts of Minerals for Adults
Mineral
Amount
Sodium
3.0 g
Potassium
1.0 g
Chloride
3.5 g
Calcium
1.2 g
1.2 g
Iron
18.0 mg
Iodine
150.0 mg
Magnesium
0.4 g
Cobalt
Unknown
Copper
Unknown
Manganese
Unknown
Zinc
15 mg
880
Unit XIII
da Silva AA, Kuo JJ, Hall JE: R

erature Regulation
da Silva AA, Kuo JJ, Hall JE: Role of hypothalamic
melanocortin 3/4-receptors in mediating chronic cardio-
vascular, renal, and metabolic actions of leptin. Hyperten-
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Davy KP, Hall JE: Obesity and hypertension: two epidemics
or one? Am J Physiol Regul Integr Comp Physiol
286:R803, 2004.
Druce MR, Small CJ, Bloom SR: Gut peptides regulating
satiety. Endocrinology 145:2660, 2004.
Dutta A, Dutta SK: Vitamin E and its role in the prevention
of atherosclerosis a

nd its role in the prevention
of atherosclerosis and carcinogenesis: a review. J Am Coll
Nutr 22:258, 2003.
Flier JS: Obesity wars: molecular progress confronts an
expanding epidemic. Cell 116:337, 2004.
Fraser PD, Bramley PM: The biosynthesis and nutritional
uses of carotenoids. Prog Lipid Res 43:228, 2004.
Friedman JM, Halaas JL: Leptin and the regulation of body
weight in mammals. Nature 395:763, 1998.
Grundy SM: Obesity, metabolic syndrome, and cardiovascu-
lar disease. J Clin Endocrinol Met

nd cardiovascu-
lar disease. J Clin Endocrinol Metab 89:2595, 2004.
cause of chronic kidney disease? Adv Ren Replace Ther
Hall JE, Jones DW: What can we do about the “epidemic”
of obesity. Am J Hypertens 15:657, 2002.
Jequier E, Tappy L: Regulation of body weight in humans.
Physiol Rev 79:451, 1

on of body weight in humans.
Physiol Rev 79:451, 1999.
Jones G, Strugnell SA, DeLuca HF: Current understanding
of the molecular actions of vitamin D. Physiol Rev
78:1193, 1998.
Kershaw EE, Flier JS:Adipose tissue as an endocrine organ.
J Clin Endocrinol Metab 89:2548, 2004.
Korner J,Aronne LJ: Pharmacological approaches to weight
reduction: therapeutic targets. J Clin Endocrinol Metab
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Korner J, Leibel RL: To eat or not to eat—how the gut talks
to the brain. N Engl J Med 349:926, 2

he gut talks
to the brain. N Engl J Med 349:926, 2003.
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ogy 144:3757, 2003.
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Nutr 77:1352, 2003.
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feast and famine. J Clin
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peptide YY, and pancreatic polypeptide. Am J Physiol
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C
H
A
P
T
E
R
 
7
2
881
Adenosine Triphosphate
(ATP) Functions as 
an “Energy Currency” 
In the past few chapters, we have pointed out that car-
bohydrates, fats, and proteins can all be used by cells to
synthesiz

and proteins can all be used by cells to
synthesize large quantities of adenosine triphosphate
(ATP), which can be used as an energy source for almost all other cellular func-
tions. For this reason,ATP has been called an energy “currency” in cell metabolism.
Indeed, the transfer of energy from foodstuffs to most functional systems of the cells
can be done only through this medium of ATP (or the similar nucleotide guanosine
triphosphate, GTP). Many of the attributes of ATP are presented in Chapt

ny of the attributes of ATP are presented in Chapter 2.
An attribute of ATP that makes it highly valuable as an energy currency is the
large quantity of free energy (about 7300 calories, or 7.3 Calories [kilocalories], per
mole under standard conditions, but as much as 12,000 calories under physiologic
conditions) vested in each of its two high-energy phosphate bonds. The amount of
energy in each bond, when liberated by decomposition of ATP, is enough to cause
almost any step of any chemical rea

nough to cause
almost any step of any chemical reaction in the body to take place if appropriate
energy transfer is achieved. Some chemical reactions that require ATP energy use
only a few hundred of the available 12,000 calories, and the remainder of this energy
is lost in the form of heat.
ATP Is Generated by Combustion of Carbohydrates, Fats, and Proteins. In previous chapters,
we discussed the transfer of energy from various foods to ATP. To summarize, ATP
is produced from
1. Combustion of c

summarize, ATP
1. Combustion of carbohydrates—mainly glucose, but also smaller amounts of
other sugars such as fructose; this occurs in the cytoplasm of the cell through
the anaerobic process of glycolysis and in the cell mitochondria through the
aerobic citric acid (Krebs) cycle.
2. Combustion of fatty acids in the cell mitochondria by beta-oxidation.
3. Combustion of proteins, which requires hydrolysis to their component amino
acids and degradation of the amino acids to interm

acids and degradation of the amino acids to intermediate compounds of the
citric acid cycle and then to acetyl coenzyme A and carbon dioxide.
ATP Energizes the Synthesis of Most Important Cellular Components. Among the most impor-
tant intracellular processes that require ATP energy is the formation of peptide link-
ages between amino acids during the synthesis of proteins. The different peptide
linkages, depending on which types of amino acids are linked, require from 500 to
5000 calories of en

re linked, require from 500 to
5000 calories of energy per mole. It will be recalled from the discussion of protein
synthesis in Chapter 3 that four high-energy phosphate bonds are expended during
the cascade of reactions required to form each peptide linkage.This provides a total
of 48,000 calories of energy, which is far more than the 500 to 5000 calories even-
tually stored in each of the peptide linkages.
ATP energy is also used in the synthesis of glucose from lactic acid and in the syn-
th

sis of glucose from lactic acid and in the syn-
thesis of fatty acids from acetyl coenzyme A. In addition,ATP energy is used for the
synthesis of cholesterol,phospholipids,the hormones,and almost all other substances
of the body. Even the urea excreted by the kidneys requires ATP to cause its 
formation from ammonia. One might wonder about the advisability of expending
energy to form urea, which is simply discarded by the body. However, remembering
the extreme toxicity of ammonia in the body ﬂui

ng
the extreme toxicity of ammonia in the body ﬂuids, one can see the value of this reac-
tion, which keeps the ammonia concentration of the body ﬂuids at a low level.
ATP Energizes Muscle Contraction. Muscle contraction will not occur without energy
from ATP. Myosin, one of the important contractile proteins of the muscle ﬁber, acts
as an enzyme to cause breakdown of ATP into adenosine diphosphate (ADP), thus
882
Unit XIII
releasing the energy required to c

ture Regulation
releasing the energy required to cause contraction. Only
a small amount of ATP is normally degraded in muscles
when muscle contraction is not occurring, but this rate
of ATP usage can rise to at least 150 times the resting
level during short bursts of maximal contraction. The
postulated mechanism by which ATP energy is used to
cause muscle contraction is discussed in Chapter 6.
ATP 
Energizes 
Active 
Transport 
Across 
Membranes. In 
Chapters 4, 27, and 65, active transport of e

 In 
Chapters 4, 27, and 65, active transport of electrolytes
and various nutrients across cell membranes and from
the renal tubules and gastrointestinal tract into the
blood is discussed. In each instance, we noted that active
transport of most electrolytes and substances such as
glucose, amino acids, and acetoacetate can occur against
an electrochemical gradient, even though the natural
diffusion of the substances would be in the opposite
direction. To oppose the electrochemical gradient
requi

tion. To oppose the electrochemical gradient
requires energy, which is provided by ATP.
ATP Energizes Glandular Secretion. The same principles
apply to glandular secretion as to the absorption of 
substances against concentration gradients, because
energy is required to concentrate substances as they are
secreted by the glandular cells. In addition, energy is
required to synthesize the organic compounds to be
secreted.
ATP Energizes Nerve Conduction. The energy used during
propagation of a nerve

ion. The energy used during
propagation of a nerve impulse is derived from 
the potential energy stored in the form of concentration
differences of ions across the membranes. That is, a 
high concentration of potassium inside the ﬁber and a
low concentration outside the ﬁber constitute a type 
of energy storage. Likewise, a high concentration 
of sodium on the outside of the membrane and a low
concentration on the inside represent another store 
of energy. The energy needed to pass each action 

potential along the ﬁber membrane is derived from 
this energy storage, with small amounts of potassium 
transferring out of the cell and sodium into the cell
during each of the action potentials. However, active
transport systems energized by ATP then retransport
the ions back through the membrane to their former
positions.
Phosphocreatine Functions as an
Accessory Storage Depot for
Energy and as an “ATP Buffer”
Despite the paramount importance 

 an “ATP Buffer”
Despite the paramount importance of ATP as a cou-
pling agent for energy transfer, this substance is not the
most abundant store of high-energy phosphate bonds in
the cells. Phosphocreatine, which also contains high-
energy phosphate bonds, is three to eight times as 
abundant. Also, the high-energy bond (~) of phospho-
creatine contains about 8500 calories per mole under
standard conditions and as much as 13,000 calories per
mole under conditions in the body (37°C and low con-

e under conditions in the body (37°C and low con-
centrations of the reactants).This is slightly greater than
the 12,000 calories per mole in each of the two high-
energy phosphate bonds of ATP. The formula for crea-
tinine phosphate is the following:
Unlike ATP, phosphocreatine cannot act as a direct
coupling agent for energy transfer between the foods
and the functional cellular systems, but it can transfer
energy interchangeably with ATP. When extra amounts
of ATP are available in the cell, m

 extra amounts
of ATP are available in the cell, much of its energy is
used to synthesize phosphocreatine, thus building up
this storehouse of energy. Then, when the ATP begins
to be used up, the energy in the phosphocreatine is
transferred rapidly back to ATP and then to the func-
tional systems of the cells. This reversible interrelation
between ATP and phosphocreatine is demonstrated by
Phosphocreatine + ADP
Ø≠
ATP + Creatine
Note particularly that the higher energy le

eatine
Note particularly that the higher energy level of the
high-energy phosphate bond in phosphocreatine (1000
to 1500 calories per mole greater than that in ATP)
causes the reaction between phosphocreatine and ADP
to proceed rapidly toward the formation of new ATP
every time even the slightest amount of ATP expends
its energy elsewhere. Therefore, the slightest usage of
ATP by the cells calls forth the energy from the phos-
phocreatine to synthesize new ATP. This effect keeps
the concentratio

hesize new ATP. This effect keeps
the concentration of ATP at an almost constant high
level as long as any phosphocreatine remains. For this
reason, we can call the ATP-phosphocreatine system an
ATP “buffer” system. One can readily understand the
importance of keeping the concentration of ATP nearly
constant, because the rates of almost all the metabolic
reactions in the body depend on this constancy.
Anaerobic energy means energy that can be derived
from foods wi

rgy means energy that can be derived
from foods without the simultaneous utilization of
oxygen; aerobic energy means energy that can be
derived from foods only by oxidative metabolism. In the
discussions in Chapters 67 through 69, it is noted that
carbohydrates, fats, and proteins can all be oxidized to
cause synthesis of ATP. However, carbohydrates are the
only signiﬁcant foods that can be used to provide energy
without the utilization of oxygen; this energy release
occurs during glycolytic bre

; this energy release
occurs during glycolytic breakdown of glucose or glyco-
gen to pyruvic acid. For each mole of glucose that is split
into pyruvic acid, 2 moles of ATP are formed. However,
when stored glycogen in a cell is split to pyruvic acid,
each mole of glucose in the glycogen gives rise to 3
moles of ATP. The reason for this difference is that free
glucose entering the cell must be phosphorylated by
using 1 mole of ATP before it can begin to be split; this
is not true of glucose derive

in to be split; this
is not true of glucose derived from glycogen because it
comes from the glycogen already in the phosphorylated
state, without the additional expenditure of ATP. Thus,
the best source of energy under anaerobic conditions is
the stored glycogen of the cells.
Anaerobic Energy Utilization During Hypoxia. One of the prime
examples of anaerobic energy utilization occurs in acute
hypoxia.When a person stops breathing, there is already
a small amount of oxygen stored in the lungs and

y
a small amount of oxygen stored in the lungs and an
O
O
H
N
H
N
C
NH
~
HOOC
CH2
CH3
P
OH
Chapter 72
883
additional amount stored in the hemoglobin of the
blood. This oxygen is sufﬁcient to keep the metabolic
processes functioning for only about 2 minutes. Contin-
ued life beyond this time requires an additional source
of energy. This can be derived for another minute or so
from glycolysis—that is, the glycogen of the cells split-
ting into pyruvic acid, and the py

he cells split-
ting into pyruvic acid, and the pyruvic acid becoming
lactic acid, which diffuses out of the cells, as described
Anaerobic Energy Utilization During Strenuous Bursts of Activity
Is Derived Mainly from Glycolysis. Skeletal muscles can
perform extreme feats of strength for a few seconds but
are much less capable during prolonged activity. Most
of the extra energy required during these bursts of activ-
ity cannot come from the oxidative processes because
they are too 

from the oxidative processes because
they are too slow to respond. Instead, the extra energy
comes from anaerobic sources: (1) ATP already present
in the muscle cells, (2) phosphocreatine in the cells, and
(3) anaerobic energy released by glycolytic breakdown
of glycogen to lactic acid.
The maximum amount of ATP in muscle is only about
5 mmol/L of intracellular ﬂuid, and this amount can
maintain maximum muscle contraction for no more
than a second or so. The amount of phosphocreatine in
the cell

d or so. The amount of phosphocreatine in
the cells is three to eight times this amount, but even by
using all the phosphocreatine, maximum contraction
can be maintained for only 5 to 10 seconds.
Release of energy by glycolysis can occur much more
rapidly than can oxidative release of energy. Conse-
quently, most of the extra energy required during stren-
uous activity that lasts for more than 5 to 10 seconds but
less than 1 to 2 minutes is derived from anaerobic gly-
colysis. As a result, the g

ed from anaerobic gly-
colysis. As a result, the glycogen content of muscles
during strenuous bouts of exercise is reduced, whereas
the lactic acid concentration of the blood rises.After the
exercise is over, oxidative metabolism is used to recon-
vert about four ﬁfths of the lactic acid into glucose;
the remainder becomes pyruvic acid and is degraded
and oxidized in the citric acid cycle. The reconversion 
to glucose occurs principally in the liver cells, and 
the glucose is then transported in

ver cells, and 
the glucose is then transported in the blood back to the
muscles, where it is stored once more in the form of
glycogen.
Oxygen Debt Is the Extra Consumption of Oxygen After Completion
of Strenuous Exercise. After a period of strenuous exer-
cise, a person continues to breathe hard and to consume
large amounts of oxygen for at least a few minutes and
sometimes for as long as 1 hour thereafter. This addi-
tional oxygen is used (1) to reconvert the lactic acid that
has accumulated d

o reconvert the lactic acid that
has accumulated during exercise back into glucose, (2)
to reconvert adenosine monophosphate and ADP to
ATP, (3) to reconvert creatine and phosphate to phos-
phocreatine, (4) to re-establish normal concentrations
of oxygen bound with hemoglobin and myoglobin, and
(5) to raise the concentration of oxygen in the lungs to
its normal level.This extra consumption of oxygen after
exercise is over is called repaying the oxygen debt.
The principle of oxygen debt is discus

xygen debt.
The principle of oxygen debt is discussed further in
Chapter 84 in relation to sports physiology; the ability
of a person to build up an oxygen debt is especially
important in many types of athletics.
Summary of Energy Utilization 
by the Cells
With the background of the past few chapters and of the
preceding discussion, we can now synthesize a compos-
ite picture of overall energy utilization by the cells, as
shown in Figure 72–1. This ﬁgure demonstrates the
anaerobic utilization of

is ﬁgure demonstrates the
anaerobic utilization of glycogen and glucose to form
ATP and the aerobic utilization of compounds derived
from carbohydrates, fats, proteins, and other substances
to form additional ATP. In turn, ATP is in reversible
equilibrium with phosphocreatine in the cells, and
because larger quantities of phosphocreatine are
present in the cells than ATP, much of the cells’ stored
energy is in this energy storehouse.
Energy from ATP can be used by the different func-
tioning sys

ATP can be used by the different func-
tioning systems of the cells to provide for synthesis 
and growth, muscle contraction, glandular secretion,
nerve impulse conduction, active absorption, and other
cellular activities. If greater amounts of energy are
demanded for cellular activities than can be provided
by oxidative metabolism, the phosphocreatine store-
house is used ﬁrst, and then anaerobic breakdown of
glycogen follows rapidly. Thus, oxidative metabolism
Deaminated

ative metabolism
 Other substrates
Glucose
Glycogen
ATP
AMP
(Anaerobic)
Energy for
1. Synthesis and growth
2. Muscular contraction
3. Glandular secretion
4. Nerve conduction
5. Active absorption
6. Etc.
CO2 + H2O
Creatine + PO4
Lactic acid         Pyruvic acid
Figure 72–1
Overall schema of energy trans-
fer from foods to the adenylic
acid system and then to the 
functional elements of the cells.
(Modiﬁed from Soskin S, Levine
R: 
Carbohydrate 
Me

Modiﬁed from Soskin S, Levine
R: 
Metabolism.
Chicago: University of Chicago
Press, 1946, 1952.)
884
Unit XIII
cannot deliver bursts of extreme energy to the cells
nearly as rapidly as the anaerobic processes can, but at
slower rates of usage, the oxidative processes can con-
tinue as long as energy stores (mainly fat) exist.
Control of Energy Release 
in the Cell
Rate Control of Enzyme-Catalyzed Reactions. Before dis-
cussing the control of en

d Reactions. Before dis-
cussing the control of energy release in the cell, it is nec-
essary to consider the basic principles of rate control of
enzymatically catalyzed chemical reactions, which are
the types of reactions that occur almost universally
The mechanism by which an enzyme catalyzes a
chemical reaction is for the enzyme ﬁrst to combine
loosely with one of the substrates of the reaction. This
alters the bonding forces on the substrate sufﬁciently so
that it can re

ces on the substrate sufﬁciently so
that it can react with other substances. Therefore, the
rate of the overall chemical reaction is determined by
both the concentration of the enzyme and the concen-
tration of the substrate that binds with the enzyme. The
basic equation expressing this concept is as follows:
This is called the Michaelis-Menten equation. Figure
72–2 shows the application of this equation.
Role of Enzyme Concentration in Regulation of Metabolic
Reactions. Figure 72–2 shows that w

n of Metabolic
Reactions. Figure 72–2 shows that when the substrate is
present in high concentration, as shown in the right half
of the ﬁgure, the rate of a chemical reaction is deter-
mined almost entirely by the concentration of the
enzyme. Thus, as the enzyme concentration increases
from an arbitrary value of 1 up to 2, 4, or 8, the rate of
the reaction increases proportionately, as demonstrated
by the rising levels of the curves. As an example, when
large quantities of glucose enter the rena

e, when
large quantities of glucose enter the renal tubules in a
person with diabetes mellitus—that is, the substrate
glucose is in great excess in the tubules—further
increases in tubular glucose have little effect on glucose
Rate of reaction
K
Enzyme
Substrate
K
Substrate
1
2
=
¥ [
] ¥ [
]
+[
]
reabsorption, because the transport enzymes are satu-
rated. Under these conditions, the rate of reabsorption
of the glucose is limited by the concentration of the
transport enzymes in the proximal tubu

tion of the
transport enzymes in the proximal tubular cells, not by
the concentration of the glucose itself.
Role of Substrate Concentration in Regulation of Meta-
bolic Reactions. Note also in Figure 72–2 that when the
substrate concentration becomes low enough that only
a small portion of the enzyme is required in the reac-
tion, the rate of the reaction becomes directly propor-
tional to the substrate concentration as well as the
enzyme concentration. This is the relationship seen in
the abso

tration. This is the relationship seen in
the absorption of substances from the intestinal tract
and renal tubules when their concentrations are low.
Rate Limitation in a Series of Reactions. Almost all chemical
reactions of the body occur in series, with the product
of one reaction acting as a substrate for the next reac-
tion, and so on. Therefore, the overall rate of a complex
series of chemical reactions is determined mainly by the
rate of reaction of the slowest step in the series. This is

action of the slowest step in the series. This is
called the rate-limiting step in the entire series.
ADP Concentration as a Rate-Controlling Factor in Energy
Release. Under resting conditions, the concentration of
ADP in the cells is extremely slight, so that the chemi-
cal reactions that depend on ADP as one of the sub-
strates are quite slow. They include all the oxidative
metabolic pathways that release energy from food, as
well as essentially all other pathways for the release of
energy in 

y all other pathways for the release of
energy in the body. Thus, ADP is a major rate-limiting
factor for almost all energy metabolism of the body.
When the cells become active, regardless of the type
of activity, ATP is converted into ADP, increasing the
concentration of ADP in direct proportion to the degree
of activity of the cell. This ADP then automatically
increases the rates of all the reactions for the metabolic
release of energy from food. Thus, by this simple
process, the amount of ene

d. Thus, by this simple
process, the amount of energy released in the cell is con-
trolled by the degree of activity of the cell. In the
absence of cellular activity, the release of energy stops
because all the ADP soon becomes ATP.
The metabolism of the body simply means all the chem-
ical reactions in all the cells of the body, and the meta-
bolic rate is normally expressed in terms of the rate of
heat liberation during chemical reactions.
Heat Is the End Product of Almost All t

Heat Is the End Product of Almost All the Energy Released in the
Body. In discussing many of the metabolic reactions in
the preceding chapters, we noted that not all the energy
in foods is transferred to ATP; instead, a large portion
of this energy becomes heat. On average, 35 per cent of
the energy in foods becomes heat during ATP forma-
tion.Then, still more energy becomes heat as it is trans-
ferred from ATP to the functional systems of the cells,
so that even under optimal conditi

s of the cells,
so that even under optimal conditions, no more than 27
per cent of all the energy from food is ﬁnally used by
the functional systems.
Even when 27 per cent of the energy reaches the func-
tional systems of the cells, most of this eventually
becomes heat. For example, when proteins are syn-
thesized, large portions of ATP are used to form the
peptide linkages, and this stores energy in these link-
8
4
Enzyme concentration
2
1
Substrate concentration
Figure 72–2
Ef

of reaction
Effect of substrate and enzyme concentrations on the rate of
enzyme-catalyzed reaction.
Chapter 72
885
ages. But there is also continuous turnover of proteins—
some being degraded while others are being formed.
When proteins are degraded, the energy stored in the
peptide linkages is released in the form of heat into the
body.
Another example is the energy used for muscle activ-
ity. Much of this energy simply overcomes

e activ-
ity. Much of this energy simply overcomes the viscosity
of the muscles themselves or of the tissues so that the
limbs can move. This viscous movement causes friction
within the tissues, which generates heat.
Consider also the energy expended by the heart in
pumping blood. The blood distends the arterial system,
and this distention itself represents a reservoir of poten-
tial energy. As the blood ﬂows through the peripheral
vessels, the friction of the different layers of blood
ﬂowing ov

riction of the different layers of blood
ﬂowing over one another and the friction of the blood
against the walls of the vessels turn all this energy into
heat.
Essentially all the energy expended by the body is
eventually converted into heat. The only signiﬁcant
exception occurs when the muscles are used to perform
some form of work outside the body. For instance, when
the muscles elevate an object to a height or propel the
body up steps, a type of potential energy is created by
raising a mass a

of potential energy is created by
raising a mass against gravity. But when external expen-
diture of energy is not taking place, all the energy
released by the metabolic processes eventually becomes
body heat.
The Calorie. To discuss the metabolic rate of the body
and related subjects intelligently, it is necessary to use
some unit for expressing the quantity of energy released
from the different foods or expended by the different
functional processes of the body. Most often, the Calorie
is the 

esses of the body. Most often, the Calorie
is the unit used for this purpose. It will be recalled that
1 calorie—spelled with a small “c” and often called a
gram calorie—is the quantity of heat required to raise
the temperature of 1 gram of water 1°C. The calorie is
much too small a unit when referring to energy in the
body. Consequently, the Calorie—sometimes spelled
with a capital “C” and often called a kilocalorie, which
is equivalent to 1000 calories—is the unit ordinarily
used in discussing

calories—is the unit ordinarily
used in discussing energy metabolism.
Measurement of the Whole-Body
Direct Calorimetry Measures Heat Liberated from the Body.
Because a person ordinarily is not performing any
external work, the whole-body metabolic rate can be
determined by simply measuring the total quantity of
heat liberated from the body in a given time.
In determining the metabolic rate by direct calorime-
try,one measures the quantity of heat liberated from the
body in a large

uantity of heat liberated from the
body in a large, specially constructed calorimeter. The
subject is placed in an air chamber that is so well insu-
lated that no heat can leak through the walls of the
chamber. Heat formed by the subject’s body warms the
air of the chamber. However, the air temperature within
the chamber is maintained at a constant level by forcing
the air through pipes in a cool water bath. The rate of
heat gain by the water bath,which can be measured with
an accurate thermomet

h,which can be measured with
an accurate thermometer, is equal to the rate at which
heat is liberated by the subject’s body.
Direct calorimetry is physically difﬁcult to perform
and is used only for research purposes.
Indirect Calorimetry—The “Energy Equivalent” of Oxygen.
Because more than 95 per cent of the energy expended
in the body is derived from reactions of oxygen with the
different foods, the whole-body metabolic rate can also
be calculated with a high degree of accuracy from the
rate o

ted with a high degree of accuracy from the
rate of oxygen utilization. When 1 liter of oxygen is
metabolized with glucose, 5.01 Calories of energy are
released; when metabolized with starches, 5.06 Calories
are released; with fat, 4.70 Calories; and with protein,
4.60 Calories.
Using these ﬁgures, it is striking how nearly equiva-
lent are the quantities of energy liberated per liter of
oxygen, regardless of the type of food being metabo-
lized. For the average diet, the quantity of energy libe

 For the average diet, the quantity of energy liber-
ated per liter of oxygen used in the body averages about
4.825 Calories. This is called the energy equivalent of
oxygen; using this energy equivalent, one can calculate
with a high degree of precision the rate of heat libera-
tion in the body from the quantity of oxygen used in a
given period of time.
If a person metabolizes only carbohydrates during
the period of the metabolic rate determination, the cal-
culated quantity of energy liberated,

on, the cal-
culated quantity of energy liberated, based on the value
for the average energy equivalent of oxygen (4.825
Calories/L), would be about 4 per cent too little. Con-
versely, if the person obtains most energy from fat, the
calculated value would be about 4 per cent too great.
Energy Metabolism—
Factors That Inﬂuence
Energy Output
As discussed in Chapter 71, energy intake is balanced
with energy output in healthy adults who maintain a
stable body weight. About 45 per cent of daily ener

table body weight. About 45 per cent of daily energy
intake is derived from carbohydrates, 40 per cent 
from fats, and 15 per cent from proteins in the average
American diet. Energy output can also be partitioned
into several measurable components, including energy
used for (1) performing essential metabolic functions of
the body (the “basal” metabolic rate); (2) performing
various physical activities; (3) digesting, absorbing, and
processing food; and (4) maintaining body temperature.
Overall E

d; and (4) maintaining body temperature.
Overall Energy Requirements 
for Daily Activities
An average man who weighs 70 kilograms and lies in
bed all day uses about 1650 Calories of energy. The
process of eating and digesting food increases the
amount of energy used each day by an additional 200
or more Calories, so that the same man lying in bed and
eating a reasonable diet requires a dietary intake of
about 1850 Calories per day. If he sits in a chair all day
without exercising, his total ener

a chair all day
without exercising, his total energy requirement reaches
2000 to 2250 Calories. Therefore, the approximate daily
energy requirement for a very sedentary man perform-
ing only essential functions is 2000 Calories.
The amount of energy used to perform daily physical
activities is normally about 25 per cent of the total
energy expenditure, but it can vary markedly in differ-
ent individuals, depending on the type and amount of
physical activity. For example, walking up stairs requir

al activity. For example, walking up stairs requires
about 17 times as much energy as lying in bed asleep.
In general, over a 24-hour period, a person performing
heavy labor can achieve a maximal rate of energy uti-
886
Unit XIII
lization as great as 6000 to 7000 Calories, or as much as
3.5 times the energy used under conditions of no phys-
ical activity.
Basal Metabolic Rate (BMR)—
The Minimum Energy Expenditure
for the Body to Exist
Even when a person is a

ture
Even when a person is at complete rest, considerable
energy is required to perform all the chemical reactions
of the body. This minimum level of energy required 
to exist is called the basal metabolic rate (BMR) and
accounts for about 50 to 70 per cent of the daily energy
expenditure in most sedentary individuals (Figure 72–3).
Because the level of physical activity is highly vari-
able among different individuals, measurement of 
the BMR provides a useful means of com

urement of 
the BMR provides a useful means of comparing one
person’s metabolic rate with that of another. The usual
method for determining BMR is to measure the rate of
oxygen utilization over a given period of time under the
following conditions:
1. The person must not have eaten food for at least 12
hours.
2. The BMR is determined after a night of restful
sleep.
3. No strenuous activity is performed for at least 1
hour before the test.
4. All psychic and physical factors that cause
excitement

psychic and physical factors that cause
excitement must be eliminated.
5. The temperature of the air must be comfortable
and between 68° and 80°F.
6. No physical activity is permitted during the test.
The BMR normally averages about 65 to 70 Calories
per hour in an average 70-kilogram man. Although
much of the BMR is accounted for by essential activi-
ties of the central nervous system, heart, kidneys, and
other organs, the variations in BMR among different
individuals are related mainly to diff

g different
individuals are related mainly to differences in the
amount of skeletal muscle and body size.
Skeletal muscle, even under resting conditions,
accounts for 20 to 30 per cent of the BMR. For this
reason, BMR is usually corrected for differences in
body size by expressing it as Calories per hour per
square meter of body surface area, calculated from
height and weight. The average values for males and
females of different ages are shown in Figure 72–4.
Much of the decline in BMR with inc

n Figure 72–4.
Much of the decline in BMR with increasing age is
probably related to loss of muscle mass and replace-
ment of muscle with adipose tissue, which has a lower
rate of metabolism. Likewise, slightly lower BMRs in
women, compared with men, are due partly to their
lower percentage of muscle mass and higher percentage
of adipose tissue. However, there are other factors that
can inﬂuence the BMR, as discussed next.
Thyroid Hormone Increases Metabolic Rate. When the thyroid
gland secretes

es Metabolic Rate. When the thyroid
gland secretes maximal amounts of thyroxine, the meta-
bolic rate sometimes rises 50 to 100 per cent above
normal. Conversely, total loss of thyroid secretion
decreases the metabolic rate to 40 to 60 per cent of
normal.As discussed in Chapter 76, thyroxine increases
the rates of the chemical reactions of many cells in the
body and therefore increases metabolic rate. Adapta-
tion of the thyroid gland—with increased secretion in
cold climates and decreased secre

sed secretion in
cold climates and decreased secretion in hot climates—
contributes to the differences in BMRs among people
living in different geographical zones; for example,
people living in arctic regions have BMRs 10 to 20 per
cent higher than those of persons living in tropical
regions.
Male Sex Hormone Increases Metabolic Rate. The male sex
hormone testosterone can increase the metabolic rate
about 10 to 15 per cent. The female sex hormones may
increase the BMR a small amount, but usually

s may
increase the BMR a small amount, but usually not
enough to be signiﬁcant. Much of this effect of the male
sex hormone is related to its anabolic effect to increase
skeletal muscle mass.
Growth Hormone Increases Metabolic Rate. Growth hormone
can increase the metabolic rate 15 to 20 per cent as a
result of direct stimulation of cellular metabolism.
100
Purposeful physical activity (25%)
Nonexercise activity (7%)
Thermic effect of food (8%)
Arousal
Sleeping
metabolic
rate
Basal
metabolic
rat

rousal
Sleeping
metabolic
rate
Basal
metabolic
rate (60%)
75
50
25
0
% Daily energy usage
Figure 72–3
Components of energy expenditure.
54
52
50
48
46
44
42
40
38
Males
Females
36
34
32
30
0
10
30
20
40
60
50
70
80
Basal metabolism (Calories/m2/hour)
Figure 72–4
Normal basal metabolic rates at different ages for each sex.
Chapter 72
887
Fever Increases Metabolic Rate. Fever, regardless of its
cause, increases the chemical reactions of the body by
an aver

ases the chemical reactions of the body by
an average of about 120 per cent for every 10°C rise 
in temperature. This is discussed in more detail in
Sleep Decreases Metabolic Rate. The metabolic rate
decreases 10 to 15 per cent below normal during sleep.
This fall is due to two principal factors: (1) decreased
tone of the skeletal musculature during sleep and (2)
decreased activity of the central nervous system.
Malnutrition Decreases Metabolic Rate. Prolonged malnutri-
tion can decr

 Metabolic Rate. Prolonged malnutri-
tion can decrease the metabolic rate 20 to 30 per cent,
presumably due to the paucity of food substances in the
cells. In the ﬁnal stages of many disease conditions, the
inanition that accompanies the disease causes a marked
decrease in metabolic rate, to the extent that the body
temperature may fall several degrees shortly before
death.
The factor that most dramatically increases metabolic
rate is strenuous exercise. Short

reases metabolic
rate is strenuous exercise. Short bursts of maximal
muscle contraction in a single muscle can liberate as
much as 100 times its normal resting amount of heat for
a few seconds. For the entire body, maximal muscle
exercise can increase the overall heat production of the
body for a few seconds to about 50 times normal, or to
about 20 times normal for more sustained exercise in a
well-trained individual.
Table 72–1 shows the energy expenditure during dif-
ferent types of physical a

expenditure during dif-
ferent types of physical activity for a 70-kilogram man.
Because of the great variation in the amount of physi-
cal activity among individuals, this component of energy
expenditure is the most important reason for the dif-
ferences in caloric intake required to maintain energy
balance. However, in industrialized countries where
food supplies are plentiful, such as the United States,
caloric intake periodically exceeds energy expenditure,
and the excess energy is stored ma

gy expenditure,
and the excess energy is stored mainly as fat.This under-
scores the importance of maintaining a proper level of
physical activity to prevent excess fat stores and obesity.
Even in sedentary individuals who perform little or
no daily exercise or physical work, signiﬁcant energy 
is spent on spontaneous physical activity required 
to maintain muscle tone and body posture and on other
nonexercise activities such as “ﬁdgeting.” Together,
these nonexercise activities account for abou

her,
these nonexercise activities account for about 7 per cent
of a person’s daily energy usage.
After a meal is ingested, the metabolic rate increases as
a result of the different chemical reactions associated
with digestion, absorption, and storage of food in the
body. This is called the thermogenic effect of food,
because these processes require energy and generate
heat.
After a meal that contains a large quantity of carbo-
hydrates 

that contains a large quantity of carbo-
hydrates or fats, the metabolic rate usually increases
about 4 per cent. However, after a high-protein meal,
the metabolic rate usually begins rising within an hour,
reaching a maximum of about 30 per cent above normal,
and this lasts for 3 to 12 hours. This effect of protein on
the metabolic rate is called the speciﬁc dynamic action
of protein. The thermogenic effect of food accounts for
about 8 per cent of the total daily energy expenditure
in many pers

of the total daily energy expenditure
in many persons.
Energy Used for Nonshivering
Thermogenesis—Role of
Although physical work and the thermogenic effect of
food cause liberation of heat, these mechanisms are not
aimed primarily at regulation of body temperature.
Shivering provides a regulated means of producing heat
by increasing muscle activity in response to cold stress,
as discussed in Chapter 73. Another mechanism,
nonshivering thermogenesis, can also produce heat 

nonshivering thermogenesis, can also produce heat in
response to cold stress. This type of thermogenesis is
stimulated by sympathetic nervous system activation,
which releases norepinephrine and epinephrine, which
in turn increase metabolic activity and heat generation.
In certain types of fat tissue, called brown fat, sym-
pathetic nervous stimulation causes liberation of large
amounts of heat.This type of fat contains large numbers
of mitochondria and many small globules of fat instead
of one 

ria and many small globules of fat instead
of one large fat globule. In these cells, the process of
oxidative phosphorylation in the mitochondria is mainly
“uncoupled.” That is, when the cells are stimulated by
the sympathetic nerves, the mitochondria produce a
large amount of heat but almost no ATP, so that almost
all the released oxidative energy immediately becomes
heat.
A neonate has a considerable number of brown fat
cells, and maximal sympathetic stimulation can increase
the child’s metabo

thetic stimulation can increase
the child’s metabolism more than 100 per cent.The mag-
nitude of this type of thermogenesis in an adult human,
who has virtually no brown fat, is probably less than 15
per cent, although this might increase signiﬁcantly after
cold adaptation.
Nonshivering thermogenesis may also serve as a
buffer against obesity. Recent studies indicate that 
sympathetic nervous system activity is increased in
obese persons who have a persistent excess caloric
intake.The mechanism 

 a persistent excess caloric
intake.The mechanism responsible for sympathetic acti-
Table 72–1
Energy Expenditure During Different Types of Activity for a
70-Kilogram Man
Form of Activity
Calories per Hour
Sleeping
65
Awake lying still
77
Sitting at rest
100
Standing relaxed
105
Dressing and undressing
118
Typewriting rapidly
140
Walking slowly (2.6 miles per hour)
200
Carpentry, metalworking, industrial painting
240
Sawing wood
480
Swimming
500
Running (5.3 miles per hour)
570
Walking up stairs

570
Walking up stairs rapidly
1100
Extracted from data compiled by Professor M. S. Rose.
888
Unit XIII
vation in obese persons is uncertain, but it may be medi-
ated partly through the effects of increased leptin, which 
activates pro-opiomelanocortin neurons in the hypo-
thalamus. Sympathetic stimulation, by increasing ther-
mogenesis, helps to limit excess weight gain.
Argyropoulos G, Harper ME: Uncoupling proteins a

s
Argyropoulos G, Harper ME: Uncoupling proteins and ther-
moregulation. J Appl Physiol 92:2187, 2002.
Cannon B, Nedergaard J: Brown adipose tissue: function and
physiological signiﬁcance. Physiol Rev 84:277, 2004.
Chakravarthy MV, Booth FW: Eating, exercise, and “thrifty”
genotypes: connecting the dots toward an evolutionary
understanding of modern chronic diseases. J Appl Physiol
96:3, 2004.
Evans RM, Barish GD, Wang YX: PPARs and the complex
journey to obesity. Nat Med 10:355, 2004.
Levine JA

ourney to obesity. Nat Med 10:355, 2004.
Levine JA: Nonexercise activity thermogenesis (NEAT):
environment and biology.Am J Physiol Endocrinol Metab
286:E675, 2004.
Livingstone MB, Black AE: Markers of the validity of
reported energy intake. J Nutr 133(Suppl 3):895S, 2003.
Lowell BB, Bachman ES: Beta-adrenergic receptors, diet-
induced thermogenesis,
and obesity.
J Biol Chem
278:29385, 2003.
Morrison SF: Central pathways controlling brown adipose
tissue thermogenesis. News Physiol Sci 19:67, 200

tissue thermogenesis. News Physiol Sci 19:67, 2004.
and Obesity in Adults: The Evidence Report. Bethesda,
Diseases. 1998. Available at: http://www.nhlbi.nih.gov/
guidelines/index.htm
Robidoux J, Martin TL, Collins S: Beta-adrenergic receptors
and regulation of energy expenditure:

ic receptors
and regulation of energy expenditure: a family affair.
Annu Rev Pharmacol Toxicol 44:297, 2004.
Rousset S, Alves-Guerra MC, Mozo J, et al: The biology of
mitochondrial uncoupling proteins. Diabetes 53(Suppl
1):S130, 2004.
Seals DR, Bell C: Chronic sympathetic activation: conse-
quence and cause of age-associated obesity? Diabetes
53:276, 2004.
Silva JE: The thermogenic effect of thyroid hormone and its
clinical implications. Ann Intern Med 139:205, 2003.
van Marken Lichtenbelt WD, D

rn Med 139:205, 2003.
van Marken Lichtenbelt WD, Daanen HA: Cold-induced
metabolism. Curr Opin Clin Nutr Metab Care 6:469, 2003.
Westerterp KR: Limits to sustainable human metabolic rate.
J Exp Biol 204:3183, 2001.
Westerterp KR: Impacts of vigorous and non-vigorous activ-
ity on daily energy expenditure. Proc Nutr Soc 62:645,
2003.
Wilson MM, Morley JE: Aging and energy balance. J Appl
Physiol 95:1728, 2003.
Winder WW: Energy-sensing and signaling by AMP-acti-
vated protein kinase in skeletal m

ng by AMP-acti-
vated protein kinase in skeletal muscle. J Appl Physiol
91:1017, 2001.
Yen PM: Physiological and molecular basis of thyroid
hormone action. Physiol Rev 81:1097, 2001.
C
H
A
P
T
E
R
 
7
3
889
Core Temperature and Skin Temperature. The tempera-
ture of the deep tissues of the body—the “core” of
the body—remains very constant, within ±1°F
(±0.6°C), day in and day out, except when a person
develops a febrile

d day out, except when a person
develops a febrile illness. Indeed, a nude person can
be exposed to temperatures as low as 55°F or as
high as 130°F in dry air and still maintain an almost constant core temperature.
The mechanisms for regulating body temperature represent a beautifully
designed control system.The purpose of this chapter is to discuss this system as
it operates in health and in disease.
The skin temperature, in contrast to the core temperature, rises and falls 
with the temperatur

 temperature, rises and falls 
with the temperature of the surroundings. The skin temperature is the im-
portant temperature when we refer to the skin’s ability to lose heat to the 
Normal Core Temperature. No single core temperature can be considered normal,
because measurements in many healthy people have shown a range of normal
temperatures measured orally, as shown in Figure 73–1, from less than 97°F
(36°C) to over 99.5°F (37.5°C). The average normal core temperature is gener-

C). The average normal core temperature is gener-
ally considered to be between 98.0° and 98.6°F when measured orally and about
1°F higher when measured rectally.
The body temperature increases during exercise and varies with temperature
extremes of the surroundings, because the temperature regulatory mechanisms
are not perfect. When excessive heat is produced in the body by strenuous 
exercise, the temperature can rise temporarily to as high as 101° to 104°F. Con-
versely, when the body is expo

101° to 104°F. Con-
versely, when the body is exposed to extreme cold, the temperature can often
fall to values below 96°F.
Body Temperature Is Controlled by Balancing
Heat Production Against Heat Loss
When the rate of heat production in the body is greater than the rate at which
heat is being lost, heat builds up in the body and the body temperature rises.
Conversely, when heat loss is greater, both body heat and body temperature
decrease. Most of the remainder of this chapter is concerned with

of the remainder of this chapter is concerned with this balance
between heat production and heat loss and the mechanisms by which the body
controls each of these.
Heat production is a principal by-product of metabolism. In Chapter 72, which
summarizes body energetics, we discuss the different factors that determine the
rate of heat production, called the metabolic rate of the body. The most impor-
tant of these factors are listed again here: (1) basal rate of metabolism of all 
t

again here: (1) basal rate of metabolism of all 
the cells of the body; (2) extra rate of metabolism caused by muscle activity,
890
Unit XIII
including muscle contractions caused by shivering; (3)
extra metabolism caused by the effect of thyroxine
(and, to a less extent, other hormones, such as growth
hormone and testosterone) on the cells; (4) extra
metabolism caused by the effect of epinephrine, nor-
epinephrine, and sympathetic stimulation on the cells;
(

hrine, and sympathetic stimulation on the cells;
(5) extra metabolism caused by increased chemical
activity in the cells themselves, especially when the cell
temperature increases; and (6) extra metabolism
needed for digestion, absorption, and storage of food
(thermogenic effect of food).
Heat Loss
Most of the heat produced in the body is generated in
the deep organs, especially in the liver, brain, and
heart, and in the skeletal muscles during exercise.Then
this heat is transferred from the dee

xercise.Then
this heat is transferred from the deeper organs and
tissues to the skin, where it is lost to the air and other
surroundings. Therefore, the rate at which heat is lost
is determined almost entirely by two factors: (1) how
rapidly heat can be conducted from where it is pro-
duced in the body core to the skin and (2) how rapidly
heat can then be transferred from the skin to the sur-
roundings. Let us begin by discussing the system that
insulates the core from the skin surface.
Insulato

Insulator System of the Body
The skin, the subcutaneous tissues, and especially the
fat of the subcutaneous tissues act together as a heat
insulator for the body. The fat is important because it
conducts heat only one third as readily as other tissues.
When no blood is ﬂowing from the heated internal
organs to the skin, the insulating properties of the
normal male body are about equal to three quarters
the insulating properties of a usual suit of clothes

e insulating properties of a usual suit of clothes. In
women, this insulation is even better.
The insulation beneath the skin is an effective
means of maintaining normal internal core tempera-
ture, even though it allows the temperature of the skin
to approach the temperature of the surroundings.
Blood Flow to the Skin from the Body Core
Provides Heat Transfer
Blood vessels are distributed profusely beneath the
skin. Especially important is a continuous venous
plexus that is supplied by inﬂow of

tinuous venous
plexus that is supplied by inﬂow of blood from the skin
capillaries, shown in Figure 73–2. In the most exposed
areas of the body—the hands, feet, and ears—blood is
also supplied to the plexus directly from the small
arteries 
through 
highly 
muscular 
arteriovenous 
anastomoses.
The rate of blood ﬂow into the skin venous plexus
can vary tremendously—from barely above zero to as
great as 30 per cent of the total cardiac output. A high
rate of skin ﬂow causes heat to be conducted f

igh
rate of skin ﬂow causes heat to be conducted from the
core of the body to the skin with great efﬁciency,
whereas reduction in the rate of skin ﬂow can decrease
the heat conduction from the core to very little.
Figure 73–3 shows quantitatively the effect of envi-
ronmental air temperature on conductance of heat
°F
-
°C
-
104
102
100
98
96
40
39
38
37
36
Oral
Rectal
Hard exercise
Hard work, emotion
A few normal adults
Many active children
Usual range
of normal
Early morning
Cold weather, etc.

 range
of normal
Usual range 
of normal
Emotion or
moderate exercise
Figure 73–1
Estimated range of body “core” temperature in normal people.
(Redrawn from DuBois EF: Fever. Springﬁeld, IL: Charles C
Thomas, 1948.)
Epidermis
Arteries
Veins
Venous plexus
Arteriovenous
anastomosis
Artery
Dermis
Subcutaneous tissue
Figure 73–2
Skin circulation.
Chapter 73
Body Temperature, Temperat

Chapter 73
Body Temperature, Temperature Regulation, and Fever
891
from the core to the skin surface and then conductance
into the air, demonstrating an approximate eightfold
increase in heat conductance between the fully vaso-
constricted state and the fully vasodilated state.
Therefore, the skin is an effective controlled “heat
radiator” system, and the ﬂow of blood to the skin is a
most effective mechanism for heat transfer from the
body core to the skin.
Control of Heat Conducti

he
Control of Heat Conduction to the Skin by the Sympathetic
Nervous System. Heat conduction to the skin by the
blood is controlled by the degree of vasoconstriction
of the arterioles and the arteriovenous anastomoses
that supply blood to the venous plexus of the skin.This
vasoconstriction is controlled almost entirely by the
sympathetic nervous system in response to changes in
body core temperature and changes in environmental
temperature. This is discussed later in the c

ntal
temperature. This is discussed later in the chapter in
connection with control of body temperature by the
Basic Physics of How Heat Is Lost from the
Skin Surface
The various methods by which heat is lost from the
skin to the surroundings are shown in Figure 73–4.
They include radiation, conduction, and evaporation,
which are explained next.
Radiation. As shown in Figure 73–4, in a nude person
sitting inside at normal room temperature, about 60
per cent of total heat loss is by

rature, about 60
per cent of total heat loss is by radiation.
Loss of heat by radiation means loss in the form of
infrared heat rays, a type of electromagnetic wave.
Most infrared heat rays that radiate from the body
have wavelengths of 5 to 20 micrometers, 10 to 30
times the wavelengths of light rays.All objects that are
not at absolute zero temperature radiate such rays.The
human body radiates heat rays in all directions. Heat
rays are also being radiated from the walls of rooms
and other obje

ng radiated from the walls of rooms
and other objects toward the body. If the temperature
of the body is greater than the temperature of the sur-
roundings, a greater quantity of heat is radiated from
the body than is radiated to the body.
Conduction. As shown in Figure 73–4, only minute
quantities of heat, about 3 per cent, are normally lost
from the body by direct conduction from the surface
of the body to solid objects, such as a chair or a bed.
Loss of heat by conduction to air, however, rep

d.
Loss of heat by conduction to air, however, represents
a sizable proportion of the body’s heat loss (about 15
per cent) even under normal conditions.
It will be recalled that heat is actually the kinetic
energy of molecular motion, and the molecules of 
the skin are continually undergoing vibratory motion.
Much of the energy of this motion can be transferred
to the air if the air is colder than the skin, thus increas-
ing the velocity of the air molecules’ motion. Once the
temperature of the 

ir molecules’ motion. Once the
temperature of the air adjacent to the skin equals the
temperature of the skin, no further loss of heat occurs
in this way, because now an equal amount of heat is
conducted from the air to the body. Therefore, con-
duction of heat from the body to the air is self-limited
unless the heated air moves away from the skin, so that
new, unheated air is continually brought in contact
with the skin, a phenomenon called air convection.
Convection. The removal of heat from t

convection.
Convection. The removal of heat from the body by con-
vection air currents is commonly called heat loss by
convection. Actually, the heat must ﬁrst be conducted
to the air and then carried away by the convection air
currents.
A small amount of convection almost always occurs
around the body because of the tendency for air adja-
cent to the skin to rise as it becomes heated. There-
fore, in a nude person seated in a comfortable room
without gross air movement, about 15 per cent of his

thout gross air movement, about 15 per cent of his
or her total heat loss occurs by conduction to the air
and then by air convection away from the body.
8
7
6
5
4
3
2
1
0
50
Vasoconstricted
Vasodilated
60
70
80
90
100 110
120
Heat conductance through skin
(times the vasoconstricted rate)
Environmental temperature (°F)
Figure 73–3
Effect of changes in the environmental temperature on heat con-
ductance from the body core to the skin surface. (Modiﬁed from
Benzinger TH: Heat and Temperature Fundam

ﬁed from
Benzinger TH: Heat and Temperature Fundamentals of Medical
Physiology. New York: Dowden, Hutchinson & Ross, 1980.)
Walls
Conduction to
objects (3%)
Evaporation (22%)
Radiation (60%)
heat waves
Conduction to air (15%)
Air currents
(convection)
Figure 73–4
Mechanisms of heat loss from the body.
892
Unit XIII
Cooling Effect of Wind. When the body is exposed to
wind, the layer of air immediately adjacent to the skin
is replaced by new air much more rapi

 to the skin
is replaced by new air much more rapidly than normally,
and heat loss by convection increases accordingly. The
cooling effect of wind at low velocities is about propor-
tional to the square root of the wind velocity. For
instance, a wind of 4 miles per hour is about twice as
effective for cooling as a wind of 1 mile per hour.
Conduction and Convection of Heat from a Person Sus-
pended in Water. Water has a speciﬁc heat several thou-
sand times as great as that of air, so that each u

sand times as great as that of air, so that each unit
portion of water adjacent to the skin can absorb far
greater quantities of heat than air can. Also, heat con-
ductivity in water is very great in comparison with that
in air. Consequently, it is impossible for the body to heat
a thin layer of water next to the body to form an “insu-
lator zone” as occurs in air. Therefore, the rate of heat
loss to water is usually many times greater than the rate
of heat loss to air.
Evaporation. When water e

ate
Evaporation. When water evaporates from the body
surface, 0.58 Calorie (kilocalorie) of heat is lost for
each gram of water that evaporates. Even when a
person is not sweating, water still evaporates insensi-
bly from the skin and lungs at a rate of about 600 to
700 ml/day. This causes continual heat loss at a rate of
16 to 19 Calories per hour.This insensible evaporation
through the skin and lungs cannot be controlled for
purposes of temperature regulation because it re

r
purposes of temperature regulation because it results
from continual diffusion of water molecules through
the skin and respiratory surfaces.However,loss of heat
by evaporation of sweat can be controlled by regulat-
ing the rate of sweating, which is discussed later in the
chapter.
Evaporation Is a Necessary Cooling Mechanism at Very High
Air Temperatures. As long as skin temperature is greater
than the temperature of the surroundings, heat can be
lost by radiation and conduction. But when the 

be
lost by radiation and conduction. But when the tem-
perature of the surroundings becomes greater than that
of the skin, instead of losing heat, the body gains heat
by both radiation and conduction. Under these condi-
tions, the only means by which the body can rid itself of
heat is by evaporation.
Therefore, anything that prevents adequate evapo-
ration when the surrounding temperature is higher
than the skin temperature will cause the internal body
temperature to rise.This occurs occasionall

l body
temperature to rise.This occurs occasionally in human
beings who are born with congenital absence of sweat
glands. These people can stand cold temperatures as
well as normal people can, but they are likely to die of
heatstroke in tropical zones because without the evap-
orative refrigeration system, they cannot prevent a rise
in body temperature when the air temperature is
above that of the body.
Effect of Clothing on Conductive Heat Loss. Clothing
entraps air next to the skin in the weav

 Clothing
entraps air next to the skin in the weave of the cloth,
thereby increasing the thickness of the so-called
private zone of air adjacent to the skin and also
decreasing the ﬂow of convection air currents. Conse-
quently, the rate of heat loss from the body by con-
duction and convection is greatly depressed. A usual
suit of clothes decreases the rate of heat loss to about
half that from the nude body, but arctic-type clothing
can decrease this heat loss to as little as one sixth.
About h

 this heat loss to as little as one sixth.
About half the heat transmitted from the skin to the
clothing is radiated to the clothing instead of being
conducted across the small intervening space. There-
fore, coating the inside of clothing with a thin layer of
gold, which reﬂects radiant heat back to the body,
makes the insulating properties of clothing far more
effective than otherwise. Using this technique, clothing
for use in the arctic can be decreased in weight by
about half.
The effectiven

 decreased in weight by
The effectiveness of clothing in maintaining body
temperature is almost completely lost when the cloth-
ing becomes wet, because the high conductivity of
water increases the rate of heat transmission through
cloth 20-fold or more. Therefore, one of the most
important factors for protecting the body against cold
in arctic regions is extreme caution against allowing
the clothing to become wet. Indeed, one must be
careful not to become overheated even temporarily

careful not to become overheated even temporarily,
because sweating in one’s clothes makes them much
less effective thereafter as an insulator.
Sweating and Its Regulation by the Autonomic
Stimulation of the anterior hypothalamus-preoptic
area in the brain either electrically or by excess heat
causes sweating. The nerve impulses from this area
that cause sweating are transmitted in the autonomic
pathways to the spinal cord and then through sympa-
thetic outﬂow to the skin everywh

n through sympa-
thetic outﬂow to the skin everywhere in the body.
It should be recalled from the discussion of the 
autonomic nervous system in Chapter 60 that the
sweat glands are innervated by cholinergic nerve 
ﬁbers (ﬁbers that secrete acetylcholine but that run 
in the sympathetic nerves along with the adrenergic
ﬁbers). These glands can also be stimulated to 
some extent by epinephrine or norepinephrine circu-
lating in the blood, even though the glands themselves
do not have adrenergic i

ugh the glands themselves
do not have adrenergic innervation. This is important
during exercise, when these hormones are secreted by
the adrenal medullae and the body needs to lose
excessive amounts of heat produced by the active
muscles.
Mechanism of Sweat Secretion. In Figure 73–5, the sweat
gland is shown to be a tubular structure consisting of
two parts: (1) a deep subdermal coiled portion that
secretes the sweat, and (2) a duct portion that passes
outward through the dermis and epidermis of

passes
outward through the dermis and epidermis of the skin.
As is true of so many other glands, the secretory
portion of the sweat gland secretes a ﬂuid called the
primary secretion or precursor secretion; the concen-
trations of constituents in the ﬂuid are then modiﬁed
as the ﬂuid ﬂows through the duct.
The precursor secretion is an active secretory
product of the epithelial cells lining the coiled portion
of the sweat gland. Cholinergic sympathetic nerve
ﬁbers ending on or near the glandular

thetic nerve
ﬁbers ending on or near the glandular cells elicit the
Chapter 73
893
The composition of the precursor secretion is
similar to that of plasma, except that it does not
contain plasma proteins. The concentration of sodium
is about 142 mEq/L and that of chloride is about 
104 mEq/L, with much smaller concentrations of the
other solutes of plasma. As this precursor solution
ﬂows through the duct portion of the gland, it is m

ows through the duct portion of the gland, it is mod-
iﬁed by reabsorption of most of the sodium and chlo-
ride ions. The degree of this reabsorption depends on
the rate of sweating, as follows.
When the sweat glands are stimulated only slightly,
the precursor ﬂuid passes through the duct slowly. In
this instance, essentially all the sodium and chloride
ions are reabsorbed, and the concentration of each
falls to as low as 5 mEq/L. This reduces the osmotic
pressure of the sweat ﬂuid to such a low

e osmotic
pressure of the sweat ﬂuid to such a low level that most
of the water is also reabsorbed, which concentrates
most of the other constituents. Therefore, at low rates
of sweating, such constituents as urea, lactic acid, and
potassium ions are usually very concentrated.
Conversely, when the sweat glands are strongly 
stimulated by the sympathetic nervous system, large
amounts of precursor secretion are formed, and the
duct may reabsorb only slightly more than half the
sodium chloride; the

y slightly more than half the
sodium chloride; the concentrations of sodium and
chloride ions are then (in an unacclimatized person) a
maximum of about 50 to 60 mEq/L, slightly less than
half the concentrations in plasma. Furthermore, the
sweat ﬂows through the glandular tubules so rapidly
that little of the water is reabsorbed. Therefore, the
other dissolved constituents of sweat are only moder-
ately increased in concentration—urea is about twice
that in the plasma, lactic acid about 4 times, 

ce
that in the plasma, lactic acid about 4 times, and potas-
sium about 1.2 times.
There is a signiﬁcant loss of sodium chloride in the
sweat when a person is unacclimatized to heat. There
is much less electrolyte loss, despite increased sweat-
ing capacity, once a person has become acclimatized,
Acclimatization of the Sweating Mechanism to Heat—Role 
of Aldosterone. Although a normal, unacclimatized
person seldom produces more than about 1 liter of
sweat per hour, when this person i

bout 1 liter of
sweat per hour, when this person is exposed to hot
weather for 1 to 6 weeks, he or she begins to sweat
more profusely, often increasing maximum sweat pro-
duction to as much as 2 to 3 L/hour. Evaporation of
this much sweat can remove heat from the body at a
rate more than 10 times the normal basal rate of heat
production. This increased effectiveness of the sweat-
ing mechanism is caused by a change in the internal
sweat gland cells themselves to increase their sweating
capabilit

ls themselves to increase their sweating
capability.
Also associated with acclimatization is a further
decrease in the concentration of sodium chloride in
the sweat, which allows progressively better conserva-
tion of body salt. Most of this effect is caused by
increased secretion of aldosterone by the adrenocor-
tical glands, which results from a slight decrease in
sodium chloride concentration in the extracellular
ﬂuid and plasma. An unacclimatized person who
sweats profusely often loses 15 to

ized person who
sweats profusely often loses 15 to 30 grams of salt each
day for the ﬁrst few days.After 4 to 6 weeks of acclima-
tization, the loss is usually 3 to 5 g/day.
Loss of Heat by Panting
Many lower animals have little ability to lose heat from
the surfaces of their bodies, for two reasons: (1) the sur-
faces are usually covered with fur, and (2) the skin of
most lower animals is not supplied with sweat glands,
which prevents most of the evaporative loss of heat
from the skin. A substi

e evaporative loss of heat
from the skin. A substitute mechanism, the panting
mechanism, is used by many lower animals as a means
of dissipating heat.
The phenomenon of panting is “turned on” by the
thermoregulator centers of the brain. That is, when the
blood becomes overheated, the hypothalamus initiates
neurogenic signals to decrease the body temperature.
One of these signals initiates panting.The actual panting
process is controlled by a panting center that is associ-
ated with the pneumotax

ing center that is associ-
ated with the pneumotaxic respiratory center located in
the pons.
When an animal pants, it breathes in and out rapidly,
so that large quantities of new air from the exterior
come in contact with the upper portions of the respira-
tory passages; this cools the blood in the respiratory
Gland
Duct
Pore
Epidermis
Dermis
nerve
Primary
secretion,
mainly
protein-
free
filtrate
Absorption, mainly
sodium and chloride ions
Figure 73–5
Sweat gland innervated by an ace

 ions
Sweat gland innervated by an acetylcholine-secreting sympa-
thetic nerve. A primary protein-free secretion is formed by the
glandular portion, but most of the electrolytes are reabsorbed in
the duct, leaving a dilute, watery secretion.
894
Unit XIII
passage mucosa as a result of water evaporation from
the mucosal surfaces, especially evaporation of saliva
from the tongue.Yet panting does not increase the alve-
olar ventilation more than is 

 increase the alve-
olar ventilation more than is required for proper control
of the blood gases, because each breath is extremely
shallow; therefore, most of the air that enters the alveoli
is dead-space air mainly from the trachea and not from
the atmosphere.
Regulation of Body
Temperature—Role of the
Figure 73–6 shows what happens to the body “core”
temperature of a nude person after a few hours’ expo-
sure to dry air ranging from 30° to 160°F. The precise
dimensions of this curv

 30° to 160°F. The precise
dimensions of this curve depend on the wind move-
ment of the air, the amount of moisture in the air, and
even the nature of the surroundings. In general, a nude
person in dry air between 55° and 130°F is capable of
maintaining a normal body core temperature some-
where between 97° and 100°F.
The temperature of the body is regulated almost
entirely by nervous feedback mechanisms, and almost
all these operate through temperature-regulating
centers located in the hypotha

perature-regulating
centers located in the hypothalamus. For these feed-
back mechanisms to operate, there must also be 
temperature detectors to determine when the body
temperature becomes either too high or too low.
Role of the Anterior Hypothalamic-Preoptic
Area in Thermostatic Detection 
of Temperature
Experiments have been performed in which minute
areas in the brain of an animal have been either heated
or cooled by use of a thermode. This small, needle-like
device is heated by electrical m

mall, needle-like
device is heated by electrical means or by passing hot
water through it, or it is cooled by cold water. The 
principal areas in the brain where heat or cold from 
a thermode affects body temperature control are the
preoptic and anterior hypothalamic nuclei of the
Using the thermode, the anterior hypothalamic-
preoptic area has been found to contain large numbers
of heat-sensitive neurons as well as about one third 
as many cold-sensitive neurons. These neurons are

believed to function as temperature sensors for con-
trolling body temperature. The heat-sensitive neurons
increase their ﬁring rate 2- to 10-fold in response to a
10°C increase in body temperature. The cold-sensitive
neurons, by contrast, increase their ﬁring rate when the
body temperature falls.
When the preoptic area is heated, the skin all over
the body immediately breaks out in a profuse sweat,
while the skin blood vessels over the entire b

at,
while the skin blood vessels over the entire body
become greatly dilated. This is an immediate reaction
to cause the body to lose heat, thereby helping to
return the body temperature toward the normal level.
In addition, any excess body heat production is inhib-
ited. Therefore, it is clear that the hypothalamic-
preoptic area has the capability to serve as a ther-
mostatic body temperature control center.
Detection of Temperature by Receptors in the
Skin and Deep Body Tissues
Although the s

s in the
Although the signals generated by the temperature
receptors of the hypothalamus are extremely power-
ful in controlling body temperature, receptors in other
parts of the body play additional roles in temperature
regulation. This is especially true of temperature
receptors in the skin and in a few speciﬁc deep tissues
It will be recalled from the discussion of sensory
receptors in Chapter 48 that the skin is endowed 
with both cold and warmth recep

e skin is endowed 
with both cold and warmth receptors. There are far
more cold receptors than warmth receptors—in fact,
10 times as many in many parts of the skin. Therefore,
peripheral detection of temperature mainly concerns
detecting cool and cold instead of warm temperatures.
When the skin is chilled over the entire body, imme-
diate reﬂex effects are invoked and begin to increase
the temperature of the body in several ways: (1) by
providing a strong stimulus to cause shivering, with a
resu

 a strong stimulus to cause shivering, with a
resultant increase in the rate of body heat production;
(2) by inhibiting the process of sweating, if this is
already occurring; and (3) by promoting skin vaso-
constriction to diminish loss of body heat from the
skin.
Deep body temperature receptors are found mainly
in the spinal cord, in the abdominal viscera, and in or
around the great veins in the upper abdomen and
thorax.These deep receptors function differently from
Body temperature (°F)
Atmosp

tion differently from
Atmospheric temperature (°F)
30
50
70
90
110
130
150
170
120
110
100
90
80
70
60
Figure 73–6
Effect of high and low atmospheric temperatures of several hours’
duration on the internal body “core” temperature. Note that the
internal body temperature remains stable despite wide changes
in atmospheric temperature.
Chapter 73
895
the skin receptors because they are exposed to the
body core temperature rat

 they are exposed to the
body core temperature rather than the body surface
temperature. Yet, like the skin temperature receptors,
they detect mainly cold rather than warmth. It is prob-
able that both the skin and the deep body receptors
are concerned with preventing hypothermia—that is,
preventing low body temperature.
Posterior Hypothalamus Integrates the Central
and Peripheral Temperature Sensory Signals
Even though many temperature sensory signals arise
in peripheral receptors, these signal

ignals arise
in peripheral receptors, these signals contribute to
body temperature control mainly through the hypo-
thalamus. The area of the hypothalamus that they
stimulate is located bilaterally in the posterior hypo-
thalamus approximately at the level of the mammil-
lary bodies. The temperature sensory signals from the
anterior hypothalamic-preoptic area are also trans-
mitted into this posterior hypothalamic area. Here the
signals from the preoptic area and the signals from
elsewhere in th

preoptic area and the signals from
elsewhere in the body are combined and integrated to
control the heat-producing and heat-conserving reac-
tions of the body.
Neuronal Effector Mechanisms 
That Decrease or Increase Body
When the hypothalamic temperature centers detect
that the body temperature is either too high or too 
low, they institute appropriate temperature-decreasing
or temperature-increasing procedures. The reader is
probably familiar with most of these from personal
experie

 familiar with most of these from personal
experience, but special features are the following.
Temperature-Decreasing Mechanisms 
When the Body Is Too Hot
The temperature control system uses three important
mechanisms to reduce body heat when the body tem-
perature becomes too great:
1. Vasodilation of skin blood vessels. In almost all
areas of the body, the skin blood vessels become
intensely dilated. This is caused by inhibition 
of the sympathetic centers in the posterior
hypothalamus that ca

etic centers in the posterior
hypothalamus that cause vasoconstriction. Full
vasodilation can increase the rate of heat transfer
to the skin as much as eightfold.
2. Sweating. The effect of increased body
temperature to cause sweating is demonstrated by
the blue curve in Figure 73–7, which shows a
sharp increase in the rate of evaporative heat 
loss resulting from sweating when the body core
temperature rises above the critical level of 37°C
(98.6°F). An additional 1°C increase in body
temperatu

6°F). An additional 1°C increase in body
temperature causes enough sweating to remove 10
times the basal rate of body heat production.
3. Decrease in heat production. The mechanisms that
cause excess heat production, such as shivering
and chemical thermogenesis, are strongly
inhibited.
Temperature-Increasing Mechanisms When the
Body Is Too Cold
When the body is too cold, the temperature control
system institutes exactly opposite procedures. They
are:
1. Skin vasoconstriction throughout the body.

are:
This is caused by stimulation of the posterior
hypothalamic sympathetic centers.
2. Piloerection. Piloerection means hairs “standing
on end.” Sympathetic stimulation causes the
arrector pili muscles attached to the hair follicles
to contract, which brings the hairs to an upright
stance. This is not important in human beings, but
in lower animals, upright projection of the hairs
allows them to entrap a thick layer of “insulator
air” next to the s

rap a thick layer of “insulator
air” next to the skin, so that transfer of heat to
the surroundings is greatly depressed.
3. Increase in thermogenesis (heat production). Heat
production by the metabolic systems is increased
by promoting shivering, sympathetic excitation of
heat production, and thyroxine secretion. These
methods of increasing heat require additional
explanation, which follows.
Hypothalamic Stimulation of Shivering. Located in the 
dorsomedial portion of the posterior hypothalamus

near the wall of the third ventricle is an area called 
the primary motor center for shivering. This area is
Calories per second
Head temperature (°C)
36.4
36.6
36.8
37.0
37.2
37.4
37.6
90
80
70
60
50
40
30
20
10
Evaporative heat loss
0
Figure 73–7
Effect of hypothalamic temperature on evaporative heat loss from
the body and on heat production caused primarily by muscle
activity and shivering. This ﬁgure demonstrates the extremel

nd shivering. This ﬁgure demonstrates the extremely crit-
ical temperature level at which increased heat loss begins and
heat production reaches a minimum stable level.
896
Unit XIII
normally inhibited by signals from the heat center in
the anterior hypothalamic-preoptic area but is excited
by cold signals from the skin and spinal cord. There-
fore, as shown by the sudden increase in “heat pro-
duction” (see the red curve in Figure 73–7), this center
becomes

the red curve in Figure 73–7), this center
becomes activated when the body temperature falls
even a fraction of a degree below a critical tempera-
ture level. It then transmits signals that cause shiver-
ing through bilateral tracts down the brain stem, into
the lateral columns of the spinal cord, and ﬁnally to
the anterior motor neurons. These signals are non-
rhythmical and do not cause the actual muscle shaking.
Instead, they increase the tone of the skeletal muscles
throughout the body by fa

 of the skeletal muscles
throughout the body by facilitating the activity of the
anterior motor neurons. When the tone rises above a
certain critical level, shivering begins. This probably
results from feedback oscillation of the muscle spindle
stretch reﬂex mechanism, which is discussed in
Chapter 54. During maximum shivering, body heat
production can rise to four to ﬁve times normal.
Sympathetic “Chemical” Excitation of Heat Production. As
pointed out in Chapter 72, an increase in either sym-

ted out in Chapter 72, an increase in either sym-
pathetic stimulation or circulating norepinephrine and
epinephrine in the blood can cause an immediate
increase in the rate of cellular metabolism. This effect
is called chemical thermogenesis. It results at least par-
tially from the ability of norepinephrine and epineph-
rine to uncouple oxidative phosphorylation, which
means that excess foodstuffs are oxidized and thereby
release energy in the form of heat but do not cause
adenosine triphospha

form of heat but do not cause
adenosine triphosphate to be formed.
The degree of chemical thermogenesis that occurs
in an animal is almost directly proportional to the
amount of brown fat in the animal’s tissues. This is a
type of fat that contains large numbers of special 
mitochondria where uncoupled oxidation occurs, as
described in Chapter 72; these cells are supplied by
strong sympathetic innervation.
Acclimatization greatly affects the intensity of
chemical thermogenesis; some animals, suc

nsity of
chemical thermogenesis; some animals, such as rats,
that have been exposed to a cold environment for
several weeks exhibit a 100 to 500 per cent increase in
heat production when acutely exposed to cold, in con-
trast to the unacclimatized animal, which responds
with an increase of perhaps one third as much. This
increased thermogenesis also leads to a corresponding
increase in food intake.
In adult human beings, who have almost no brown
fat, it is rare for chemical thermogenesis to incr

fat, it is rare for chemical thermogenesis to increase
the rate of heat production more than 10 to 15 per
cent. However, in infants, who do have a small amount
of brown fat in the interscapular space, chemical ther-
mogenesis can increase the rate of heat production 100
per cent, which is probably an important factor in
maintaining normal body temperature in neonates.
Increased Thyroxine Output as a Long-Term Cause of Increased
Heat Production. Cooling the anterior hypothalamic-
preoptic area al

ooling the anterior hypothalamic-
preoptic area also increases production of the neu-
rosecretory hormone thyrotropin-releasing hormone
by the hypothalamus. This hormone is carried by way
of the hypothalamic portal veins to the anterior pitu-
itary gland, where it stimulates secretion of thyroid-
Thyroid-stimulating hormone in turn stimulates
increased output of thyroxine by the thyroid gland, as
explained in Chapter 76. The increased thyroxine
increases the rate of cellular

increased thyroxine
increases the rate of cellular metabolism throughout
the body, which is yet another mechanism of chemical
thermogenesis. This increase in metabolism does not
occur immediately but requires several weeks’ expo-
sure to cold to make the thyroid gland hypertrophy
and reach its new level of thyroxine secretion.
Exposure of animals to extreme cold for several
weeks can cause their thyroid glands to increase in size
20 to 40 per cent. However, human beings seldom
allow themselves t

t. However, human beings seldom
allow themselves to be exposed to the same degree of
cold as that to which animals are often subjected.
Therefore, we still do not know, quantitatively, how
important the thyroid mechanism of adaptation to
cold is in the human being.
Isolated measurements have shown that military
personnel residing for several months in the arctic
develop increased metabolic rates;
some Inuit
(Eskimos) also have abnormally high basal metabolic
rates. Further, the continuous stimul

al metabolic
rates. Further, the continuous stimulatory effect of
cold on the thyroid gland may explain the much higher
incidence of toxic thyroid goiters in people who live in
cold climates than in those who live in warm climates.
Concept of a “Set-Point” 
for Temperature Control
In the example of Figure 73–7, it is clear that at a crit-
ical body core temperature of about 37.1°C (98.8∞F),
drastic changes occur in the rates of both heat loss and
heat production. At temperatures above this level

heat production. At temperatures above this level, the
rate of heat loss is greater than that of heat produc-
tion, so the body temperature falls and approaches the
37.1°C level.At temperatures below this level, the rate
of heat production is greater than that of heat loss, so
the body temperature rises and again approaches the
37.1°C level. This crucial temperature level is called
the “set-point” of the temperature control mechanism.
That is, all the temperature control mechanisms con-
tinuall

ll the temperature control mechanisms con-
tinually attempt to bring the body temperature back
to this set-point level.
Feedback Gain for Body Temperature Control. Let us recall
the discussion of feedback gain of control systems pre-
sented in Chapter 1. Feedback gain is a measure of the
effectiveness of a control system. In the case of body
temperature control, it is important for the internal
core temperature to change as little as possible, even
though the environmental temperature might chan

en
though the environmental temperature might change
greatly from day to day or even hour to hour.The feed-
back gain of the temperature control system is equal
to the ratio of the change in environmental tempera-
ture to the change in body core temperature minus 
1.0 (see Chapter 1 for this formula). Experiments 
have shown that the body temperature of humans
changes about 1°C for each 25° to 30°C change in 
Chapter 73
897
environmental temper

ure Regulation, and Fever
897
environmental temperature. Therefore, the feedback
gain of the total mechanism for body temperature
control averages about 27 (28/1.0 – 1.0 = 27), which is
an extremely high gain for a biological control system
(the baroreceptor arterial pressure control system, for
instance, has a feedback gain of less than 2).
Skin Temperature Can Slightly Alter the 
Set-Point for Core Temperature Control
The critical temperature set-point in the hypothalamus
above which sweating 

et-point in the hypothalamus
above which sweating begins and below which shiver-
ing begins is determined mainly by the degree of activ-
ity of the heat temperature receptors in the anterior
hypothalamic-preoptic area. However, temperature
signals from the peripheral areas of the body, espe-
cially from the skin and certain deep body tissues
(spinal cord and abdominal viscera), also contribute
slightly to body temperature regulation. But how do
they contribute? The answer is that they alter the 

hey contribute? The answer is that they alter the set-
point of the hypothalamic temperature control center.
This effect is shown in Figures 73–8 and 73–9.
Figure 73–8 demonstrates the effect of different skin
temperatures on the set-point for sweating, showing
that the set-point increases as the skin temperature
decreases. Thus, for the person represented in this
ﬁgure, the hypothalamic set-point increased from
36.7°C when the skin temperature was higher than
33°C to a set-point of 37.4°C when 

as higher than
33°C to a set-point of 37.4°C when the skin tempera-
ture had fallen to 29°C. Therefore, when the skin 
temperature was high, sweating began at a lower 
hypothalamic temperature than when the skin tem-
perature was low. One can readily understand the
value of such a system, because it is important that
sweating be inhibited when the skin temperature is
low; otherwise, the combined effect of low skin 
temperature and sweating could cause far too much
loss of body heat.
A similar ef

cause far too much
A similar effect occurs in shivering, as shown in
Figure 73–9. That is, when the skin becomes cold,
it drives the hypothalamic centers to the shivering
threshold even when the hypothalamic temperature
itself is still on the hot side of normal. Here again, one
can understand the value of the control system,
because a cold skin temperature would soon lead to a
deeply depressed body temperature unless heat pro-
duction were increased. Thus, a cold skin temperat

duction were increased. Thus, a cold skin temperature
actually “anticipates” a fall in internal body tempera-
ture and prevents this.
Behavioral Control of Body
Aside from the subconscious mechanisms for 
body temperature control, the body has another 
temperature-control mechanism that is even more
potent.This is behavioral control of temperature, which
Evaporative heat loss (calories/second)
Internal head temperature (°C)
36.0 36.2 36.4 36.6
Insensible
evaporation
Skin temperature

36.4 36.6
32°C
31°C
30°C
29°C
36.8 37.0 37.2 37.4
0
10
20
30
40
90
80
70
60
50
33°C
to
39°C
Set-point
Sweating
Figure 73–8
Effect of changes in the internal head temperature on the rate of
evaporative heat loss from the body. Note that the skin tempera-
ture determines the set-point level at which sweating begins.
(Courtesy Dr. T. H. Benzinger.)
Basal heat
31°
30°
28°
26°
24°
22°
20°
Skin temperature (20°)
10
20
30
40
Heat pr

ture (20°)
10
20
30
40
Heat production (calories/second)
36.6
37.6
0
90
80
70
60
50
Set-point
Shivering
Figure 73–9
heat production by the body. Note that the skin temperature
determines the set-point level at which shivering begins. 
898
Unit XIII
can be explained as follows: Whenever the internal
body temperat

ed as follows: Whenever the internal
body temperature becomes too high, signals from the 
temperature-controlling areas in the brain give the
person a psychic sensation of being overheated. Con-
versely, whenever the body becomes too cold, signals
from the skin and probably also from some deep body
receptors elicit the feeling of cold discomfort. There-
fore, the person makes appropriate environmental
adjustments to re-establish comfort, such as moving
into a heated room or wearing well-insulate

moving
into a heated room or wearing well-insulated clothing
in freezing weather. This is a much more powerful
system of body temperature control than most 
physiologists have acknowledged in the past. Indeed,
this is the only really effective mechanism to prevent
body heat control breakdown in severely cold 
environments.
Local Skin Temperature Reﬂexes
When a person places a foot under a hot lamp and
leaves it there for a short time, local vasodilation and
mild local sweating occur. Conversely,

ilation and
mild local sweating occur. Conversely, placing the foot
in cold water causes local vasoconstriction and local ces-
sation of sweating. These reactions are caused by local
effects of temperature directly on the blood vessels and
also by local cord reﬂexes conducted from skin recep-
tors to the spinal cord and back to the same skin area
and the sweat glands. The intensity of these local effects
is, in addition, controlled by the central brain tempera-
ture controller, so that their ove

 brain tempera-
ture controller, so that their overall effect is propor-
tional to the hypothalamic heat control signal times the
local signal. Such reﬂexes can help prevent excessive
heat exchange from locally cooled or heated portions of
the body.
Regulation of Internal Body Temperature Is Impaired by Cutting
the Spinal Cord. After cutting the spinal cord in the neck
above the sympathetic outﬂow from the cord,regulation
of body temperature becomes extremely poor because
the hypothalamus can no

mes extremely poor because
the hypothalamus can no longer control either skin
blood ﬂow or the degree of sweating anywhere in the
body. This is true even though the local temperature
reﬂexes originating in the skin, spinal cord, and intra-
abdominal receptors still exist. These reﬂexes are
extremely weak in comparison with hypothalamic
control of body temperature.
In people with this condition, body temperature 
must be regulated principally by the patient’s psychic
response to cold and hot sens

he patient’s psychic
response to cold and hot sensations in the head region—
that is, by behavioral control of clothing and by moving
into an appropriate warm or cold environment.
Abnormalities of Body
Temperature Regulation
Fever
Fever, which means a body temperature above the
usual range of normal, can be caused by abnormalities
in the brain itself or by toxic substances that affect the
temperature-regulating centers. Some causes of fever
(and also of subnormal body temperatures) are pre-
sent

also of subnormal body temperatures) are pre-
sented in Figure 73–10.They include bacterial diseases,
brain tumors, and environmental conditions that may
terminate in heatstroke.
Resetting the Hypothalamic Temperature-
Regulating Center in Febrile Diseases—
Effect of Pyrogens
Many proteins, breakdown products of proteins, and
certain other substances, especially lipopolysaccharide
toxins released from bacterial cell membranes, can
cause the set-point of the hypothalamic thermostat to
rise. Subst

oint of the hypothalamic thermostat to
rise. Substances that cause this effect are called pyro-
gens. Pyrogens released from toxic bacteria or those
released from degenerating body tissues cause fever
during disease conditions. When the set-point of the
hypothalamic temperature-regulating center becomes
higher than normal, all the mechanisms for raising the
body temperature are brought into play, including heat
conservation and increased heat production. Within a
few hours after the set-point ha

duction. Within a
few hours after the set-point has been increased, the
body temperature also approaches this level, as shown
in Figure 73–11.
Mechanism of Action of Pyrogens in Causing Fever—Role of
Interleukin-1. Experiments in animals have shown that
some pyrogens, when injected into the hypothalamus,
can act directly and immediately on the hypothalamic
temperature-regulating center to increase its set-point.
Other pyrogens function indirectly and may require
several hours of latency before c

 and may require
several hours of latency before causing their effects.
This is true of many of the bacterial pyrogens, espe-
cially the endotoxins from gram-negative bacteria.
When bacteria or breakdown products of bacteria
are present in the tissues or in the blood, they are
phagocytized by the blood leukocytes, by tissue
macrophages, and by large granular killer lymphocytes.
All these cells digest the bacterial products and then
°F
-
°C
-
114
110
106
102
90
94
98
82
86
78
74
44
42
40
36
38
34

110
106
102
90
94
98
82
86
78
74
44
42
40
36
38
34
32
30
28
26
24
Heatstroke
Brain lesions
  Fever therapy
Upper limit
of survival?
seriously
impaired
efficient in
febrile disease,
health, and work
impaired
lost
Febrile disease
and hard exercise
Lower limit
of normal
Figure 73–10
Body temperatures under different conditions. (Redrawn from
DuBois EF: Fever. Springﬁeld, IL: Charles 

wn from
DuBois EF: Fever. Springﬁeld, IL: Charles C Thomas, 1948.)
Chapter 73
899
release the substance interleukin-1—also called leuko-
cyte pyrogen or endogenous pyrogen—into the body
ﬂuids. The interleukin-1, on reaching the hypothala-
mus, immediately activates the processes to produce
fever, sometimes increasing the body temperature a
noticeable amount in only 8 to 10 minutes. As little as
one ten-millionth of a gram of endotoxin lipopolys

one ten-millionth of a gram of endotoxin lipopolysac-
charide from bacteria, acting in concert with the blood
leukocytes, tissue macrophages, and killer lympho-
cytes, can cause fever.The amount of interleukin-1 that
is formed in response to lipopolysaccharide to cause
fever is only a few nanograms.
Several experiments have suggested that inter-
leukin-1 causes fever by ﬁrst inducing the formation of
one of the prostaglandins, mainly prostaglandin E2, or
a similar substance, which acts in the hy

n E2, or
a similar substance, which acts in the hypothalamus to
elicit the fever reaction. When prostaglandin forma-
tion is blocked by drugs, the fever is either completely
abrogated or at least reduced. In fact, this may be the
explanation for the manner in which aspirin reduces
fever, because aspirin impedes the formation of
prostaglandins from arachidonic acid. Drugs such as
aspirin that reduce fever are called antipyretics.
Fever Caused by Brain Lesions. When a brain surgeon
operates in the

rain Lesions. When a brain surgeon
operates in the region of the hypothalamus, severe
fever almost always occurs; rarely, the opposite effect,
hypothermia, occurs, demonstrating both the potency
of the hypothalamic mechanisms for body tempera-
ture control and the ease with which abnormalities of
the hypothalamus can alter the set-point of tempera-
ture control.Another condition that frequently causes
prolonged high temperature is compression of the
hypothalamus by a brain tumor.

the
Characteristics of Febrile Conditions
Chills. When the set-point of the hypothalamic 
temperature-control center is suddenly changed from
the normal level to higher than normal (as a result of
tissue destruction, pyrogenic substances, or dehydra-
tion), the body temperature usually takes several hours
to reach the new temperature set-point.
Figure 73–11 demonstrates the effect of suddenly
increasing the temperature set-point to a level of 103°F.
Because the blo

ure set-point to a level of 103°F.
Because the blood temperature is now less than the set-
point of the hypothalamic temperature controller, the
usual responses that cause elevation of body tempera-
ture occur. During this period, the person experiences
chills and feels extremely cold, even though his or her
body temperature may already be above normal. Also,
the skin becomes cold because of vasoconstriction,
and the person shivers. Chills can continue until the
body temperature reaches the hypo

ntinue until the
body temperature reaches the hypothalamic set-point
of 103°F. Then the person no longer experiences chills
but instead feels neither cold nor hot. As long as the
factor that is causing the higher set-point of the hypo-
thalamic temperature controller is present, the body
temperature is regulated more or less in the normal
manner, but at the high temperature set-point level.
Crisis, or “Flush.” If the factor that is causing the high
temperature is removed, the set-point of the hy

gh
temperature is removed, the set-point of the hypothal-
amic temperature controller will be reduced to a lower
value—perhaps even back to the normal level, as shown
in Figure 73–11. In this instance, the body temperature
is still 103°F, but the hypothalamus is attempting 
to regulate the temperature to 98.6°F. This situation 
is analogous to excessive heating of the anterior 
hypothalamic-preoptic area,
which causes intense
sweating and the sudden development of hot skin
because of vasodilatio

den development of hot skin
because of vasodilation everywhere.This sudden change
of events in a febrile state is known as the “crisis” or,
more appropriately, the “ﬂush.” In the days before the
advent of antibiotics, the crisis was always anxiously
awaited, because once this occurred, the doctor
assumed that the patient’s temperature would soon
begin falling.
The upper limit of air temperature that one can stand
depends almost entirely on whether the air is dry or wet.
If the air is 

y on whether the air is dry or wet.
If the air is dry and sufﬁcient convection air currents are
ﬂowing to promote rapid evaporation from the body, a
person can withstand several hours of air temperature
at 130°F. Conversely, if the air is 100 per cent humidi-
ﬁed or if the body is in water, the body temperature
begins to rise whenever the environmental temperature
rises above about 94°F. If the person is performing
heavy work, the critical environmental temperature
above which heatstroke is like

nmental temperature
above which heatstroke is likely to occur may be as low
as 85° to 90°F.
When the body temperature rises beyond a critical
temperature, into the range of 105° to 108°F, the person
is likely to develop heatstroke. The symptoms include
dizziness, abdominal distress sometimes accompanied
by vomiting, sometimes delirium, and eventually loss of
consciousness if the body temperature is not soon
decreased. These symptoms are often exacerbated by a
degree of circulatory shock brought 

cerbated by a
degree of circulatory shock brought on by excessive loss
of ﬂuid and electrolytes in the sweat.
The hyperpyrexia itself is also exceedingly damaging
to the body tissues, especially the brain, and is respon-
sible for many of the effects. In fact, even a few minutes
of very high body temperature can sometimes be 
fatal. For this reason, many authorities recommend
immediate treatment of heatstroke by placing the
Sweating
1. Vasoconstriction
2. Piloerection
3. Epinephrine

      secretion
4. Shivering
0
1
2
Chills:
Crisis
Setting of the thermostat
Actual body temperature
3
4
5
98
105
104
103
102
101
100
99
Set-point
suddenly
raised to
high value
Set-point
suddenly
reduced to
low value
Figure 73–11
Effects of changing the set-point of the hypothalamic temperature
900
Unit XIII
person in a cold water bath. Because this often induce

on in a cold water bath. Because this often induces
uncontrollable shivering, with a considerable increase in
the rate of heat production, others have suggested that
sponge or spray cooling of the skin is likely to be 
more effective for rapidly decreasing the body core 
temperature.
Harmful Effects of High Temperature. The pathological ﬁnd-
ings in a person who dies of hyperpyrexia are local
hemorrhages and parenchymatous degeneration of cells
throughout the entire body, but especially in the b

hroughout the entire body, but especially in the brain.
Once neuronal cells are destroyed, they can never be
replaced. Also, damage to the liver, kidneys, and other
organs can often be severe enough that failure of one
or more of these organs eventually causes death, but
sometimes not until several days after the heatstroke.
Acclimatization to Heat. It can be extremely important 
to acclimatize people to extreme heat. Examples of
people requiring acclimatization are soldiers on duty 
in the trop

 acclimatization are soldiers on duty 
in the tropics and miners working in the 2-mile-deep 
gold mines of South Africa, where the temperature
approaches body temperature and the humidity
approaches 100 per cent. A person exposed to heat for
several hours each day while performing a reasonably
heavy workload will develop increased tolerance to hot
and humid conditions in 1 to 3 weeks.
Among the most important physiological changes
that occur during this acclimatization process are an
approximate

ng this acclimatization process are an
approximately twofold increase in the maximum rate of
sweating, an increase in plasma volume, and diminished
loss of salt in the sweat and urine to almost none; the
last two effects result from increased secretion of aldos-
terone by the adrenal glands.
Exposure of the Body to 
Extreme Cold
Unless treated immediately, a person exposed to ice
water for 20 to 30 minutes ordinarily dies because of
heart standstill or heart ﬁbrillation. By that time, the
intern

ill or heart ﬁbrillation. By that time, the
internal body temperature will have fallen to about
77°F. If warmed rapidly by the application of external
heat, the person’s life can often be saved.
Loss of Temperature Regulation at Low Temperatures. As noted
in Figure 73–10, once the body temperature has fallen
below about 85°F, the ability of the hypothalamus to
regulate temperature is lost; it is greatly impaired even
when the body temperature falls below about 94°F. Part
of the reason for this d

ls below about 94°F. Part
of the reason for this diminished temperature regula-
tion is that the rate of chemical heat production in each
cell is depressed almost twofold for each 10°F decrease
in body temperature. Also, sleepiness develops (later
followed by coma), which depresses the activity of the
central nervous system heat control mechanisms and
prevents shivering.
Frostbite. When the body is exposed to extremely low
temperatures, surface areas can freeze; the freezing is
called frostbite.

reas can freeze; the freezing is
called frostbite. This occurs especially in the lobes of the
ears and in the digits of the hands and feet. If the freeze
has been sufﬁcient to cause extensive formation of ice
crystals in the cells, permanent damage usually results,
such as permanent circulatory impairment as well as
local tissue damage. Often gangrene follows thawing,
and the frostbitten areas must be removed surgically.
Cold-Induced Vasodilation Is a Final Protection Against
Frostbite at Almost

 Is a Final Protection Against
Frostbite at Almost Freezing Temperatures. When the
temperature of tissues falls almost to freezing, the
smooth muscle in the vascular wall becomes paralyzed
because of the cold itself, and sudden vasodilation
occurs, often manifested by a ﬂush of the skin. This
mechanism helps prevent frostbite by delivering warm
blood to the skin. This mechanism is far less developed
in humans than in most lower animals that live in the
cold all the time.
Artiﬁcial Hypothermia. I

in the
Artiﬁcial Hypothermia. It is easy to decrease the tempera-
ture of a person by ﬁrst administering a strong sedative
to depress the reactivity of the hypothalamic tempera-
ture controller and then cooling the person with ice or
cooling blankets until the temperature falls. The tem-
perature can then be maintained below 90°F for several
days to a week or more by continual sprinkling of cool
water or alcohol on the body. Such artiﬁcial cooling has
been used during heart su

ch artiﬁcial cooling has
been used during heart surgery so that the heart can be
stopped artiﬁcially for many minutes at a time. Cooling
to this extent does not cause tissue damage, but it does
slow the heart and greatly depresses cell metabolism,
so that the body’s cells can survive 30 minutes to more
than 1 hour without blood ﬂow during the surgical 
procedure.
Aronoff DM, Neilson EG: Antipyretics: mechanisms of
action and clinical use in fever suppression. Am J Med
111:304, 2001.
B

se in fever suppression. Am J Med
Blatteis CM, Li S, Li Z, et al: Signaling the brain in systemic
inﬂammation: the role of complement. Front Biosci 9:915,
2004.
Boulant JA: Hypothalamic neurons. Mechanisms of sensi-
tivity to temperature. Ann N Y Acad Sci 856:108, 1998.
Conti B, Tabarean I, Andrei C, Bartfai T: Cytokines and
fever. Front Biosci 9:1433, 2004.
Florez-Duquet M, McDonald RB: Cold-induced thermoreg-
ulation and biological aging. Physiol Rev 78:339, 1998.
Gourine AV, Da

al aging. Physiol Rev 78:339, 1998.
Gourine AV, Dale N, Gourine VN, Spyer KM: Fever in sys-
temic inﬂammation: roles of purines. Front Biosci 9:1011,
2004.
Hildebrand F, Giannoudis PV, van Griensven M, et al:
Pathophysiologic changes and effects of hypothermia on
outcome in elective surgery and trauma patients. Am J
Surg 187:363, 2004.
Ivanov AI, Romanovsky AA: Prostaglandin E2 as a media-
tor of fever: synthesis and catabolism. Front Biosci 9:1977,
2004.
Katschinski DM: On heat and cells and pr

77,
2004.
Katschinski DM: On heat and cells and proteins. News
Physiol Sci 19:11, 2004.
Kenney WL, Munce TA:Aging and human temperature reg-
ulation. J Appl Physiol 95:2598, 2003.
Kozak W, Kluger MJ,Tesfaigzi J, et al: Molecular mechanisms
of fever and endogenous antipyresis. Ann N Y Acad Sci
917:121, 2000.
Leon LR: Cytokine regulation of fever: studies using gene
knockout mice. J Appl Physiol 92:2648, 2002.
McDermott MF: Genetic clues to understanding periodic
fevers, and possible therapies. Tr

anding periodic
fevers, and possible therapies. Trends Mol Med 8:550,
2002.
Chapter 73
901
Olsen TS, Weber UJ, Kammersgaard LP: Therapeutic
hypothermia for acute stroke. Lancet Neurol 2:410, 2003.
Patapoutian A, Peier AM, Story GM, Viswanath V: Ther-
moTRP channels and beyond: mechanisms of tempera-
ture sensation. Nat Rev Neurosci 4:529,

f tempera-
ture sensation. Nat Rev Neurosci 4:529, 2003.
Roth J, Zeisberger E, Vybiral S, Jansky L: Endogenous
antipyretics: neuropeptides and glucocorticoids. Front
Biosci 9:816, 2004.
Saper CB: Neurobiological basis of fever.Ann N Y Acad Sci
856:90, 1998.
Tatro JB, Sinha PS: The central melanocortin system and
fever. Ann N Y Acad Sci 994:246, 2003.
U
N
I
T
XIV
Endocrinology and
Reproducti

69, 2003.
U
N
I
T
XIV
74. Introduction to Endocrinology
75. Pituitary Hormones and Their Control 
76. Thyroid Metabolic Hormones
77. Adrenocortical Hormones
78. Insulin, Glucagon, and Diabetes Mellitus
79. Parathyroid Hormone, Calcitonin, Calcium and
Phosphate Metabolism, Vitamin D, Bone, and Teeth
80. Reproductive and Hormonal Functions of the Male
81. Female Physiology Before Pregnancy
and the Female Hormones
82. Pregnancy and Lactation
83. Fe

Female Hormones
83. Fetal and Neonatal Physiology
C
H
A
P
T
E
R
 
7
4
905
Coordination of Body
Functions by Chemical
Messengers
The multiple activities of the cells, tissues, and
organs of the body are coordinated by the interplay
of several types of chemical messenger systems:
1. Neurotransmitters are released by axon terminals of neurons into the
synaptic junctions and act locally to control nerve cell functions.
2. Endocrine hormones a

trol nerve cell functions.
2. Endocrine hormones are released by glands or specialized cells into the
circulating blood and inﬂuence the function of cells at another location in
the body.
3. Neuroendocrine hormones are secreted by neurons into the circulating
blood and inﬂuence the function of cells at another location in the body.
4. Paracrines are secreted by cells into the extracellular ﬂuid and affect
neighboring cells of a different type.
5. Autocrines are secreted by cells into the extrace

 Autocrines are secreted by cells into the extracellular ﬂuid and affect the
function of the same cells that produced them by binding to cell surface
6. Cytokines are peptides secreted by cells into the extracellular ﬂuid and can
function as autocrines, paracrines, or endocrine hormones. Examples of
cytokines include the interleukins and other lymphokines that are secreted
by helper cells and act on other cells of the immune system (see Chapter
34). Cytokine hormones (e.g., leptin) pr

e Chapter
34). Cytokine hormones (e.g., leptin) produced by adipocytes are
sometimes called adipokines.
In the next few chapters,we discuss mainly the endocrine and neuroendocrine
hormone systems, keeping in mind that many of the body’s chemical messen-
ger systems interact with one another to maintain homeostasis. For example,
the adrenal medullae and the pituitary gland secrete their hormones primarily
in response to neural stimuli. The neuroendocrine cells, located in the hypo-
thalamus, have

docrine cells, located in the hypo-
thalamus, have axons that terminate in the posterior pituitary gland and median
eminence and secrete several neurohormones, including antidiuretic hormone
(ADH), oxytocin, and hypophysiotropic hormones, which control the secretion
of anterior pituitary hormones.
The endocrine hormones are carried by the circulatory system to cells
throughout the body, including the nervous system in some cases, where they
bind with receptors and initiate many reactions. Some e

with receptors and initiate many reactions. Some endocrine hormones
affect many different types of cells of the body; for example, growth hormone
(from the anterior pituitary gland) causes growth in most parts of the body, and
thyroxine (from the thyroid gland) increases the rate of many chemical reac-
tions in almost all the body’s cells.
Other hormones affect only speciﬁc target tissues, because only these tissues
have receptors for the hormone. For example, adrenocorticotropic hormone
(ACTH) 

. For example, adrenocorticotropic hormone
(ACTH) from the anterior pituitary gland speciﬁcally stimulates the adrenal
cortex, causing it to secrete adrenocortical hormones, and the ovarian hormones
have speciﬁc effects on the female sex organs as well as on the secondary sexual
characteristics of the female body.
Figure 74–1 shows the anatomical loci of the major endocrine glands and
endocrine tissues of the body, except for the placenta, which is an additional
source of the sex hormones. Table

is an additional
source of the sex hormones. Table 74–1 provides an overview of the different
hormone systems and their most important actions.
906
Unit XIV
The multiple hormone systems play a key role in
regulating almost all body functions, including metab-
olism, growth and development, water and electrolyte
balance, reproduction, and behavior. For instance,
without growth hormone, a person would be a dwarf.
Without thyroxine and triiodothyronine from the
thyroi

out thyroxine and triiodothyronine from the
thyroid gland, almost all the chemical reactions of the
body would become sluggish, and the person would
become sluggish as well.Without insulin from the pan-
creas, the body’s cells could use little of the food car-
bohydrates for energy.And without the sex hormones,
sexual development and sexual functions would be
absent.
Chemical Structure and
Synthesis of Hormones
There are three general classes of hormones:
1. Proteins and polypeptides, including 

hormones:
1. Proteins and polypeptides, including hormones
secreted by the anterior and posterior pituitary
gland, the pancreas (insulin and glucagon), the
parathyroid gland (parathyroid hormone), and
many others (see Table 74–1).
2. Steroids secreted by the adrenal cortex (cortisol
and aldosterone), the ovaries (estrogen and
progesterone), the testes (testosterone), and the
placenta (estrogen and progesterone).
3. Derivatives of the amino acid tyrosine, secreted 
by the thyroid (thyroxine and t

yrosine, secreted 
by the thyroid (thyroxine and triiodothyronine)
and the adrenal medullae (epinephrine and
norepinephrine). There are no known
polysaccharides or nucleic acid hormones.
Polypeptide and Protein Hormones Are Stored in Secretory Vesi-
cles Until Needed. Most of the hormones in the body 
are polypeptides and proteins. These hormones range
in size from small peptides with as few as 3 amino 
acids (thyrotropin-releasing hormone) to proteins
with almost 200 amino acids (growth hormone

oteins
with almost 200 amino acids (growth hormone and
prolactin). In general, polypeptides with 100 or more
amino acids are called proteins, and those with fewer
than 100 amino acids are referred to as peptides.
Protein and peptide hormones are synthesized on
the rough end of the endoplasmic reticulum of the dif-
ferent endocrine cells, in the same fashion as most
other proteins (Figure 74–2). They are usually synthe-
sized ﬁrst as larger proteins that are not biologically
active (preprohormone

ns that are not biologically
active (preprohormones) and are cleaved to form
smaller prohormones in the endoplasmic reticulum.
These are then transferred to the Golgi apparatus for
packaging into secretory vesicles. In this process,
enzymes in the vesicles cleave the prohormones to
produce smaller, biologically active hormones and
inactive fragments. The vesicles are stored within the
cytoplasm, and many are bound to the cell membrane
until their secretion is needed. Secretion of the hor-
mones 

 secretion is needed. Secretion of the hor-
mones (as well as the inactive fragments) occurs when
the secretory vesicles fuse with the cell membrane and
the granular contents are extruded into the interstitial
ﬂuid or directly into the blood stream by exocytosis.
In many cases, the stimulus for exocytosis is an
increase in cytosolic calcium concentration caused by
depolarization of the plasma membrane. In other
instances, stimulation of an endocrine cell surface
receptor causes increased cyclic 

ine cell surface
receptor causes increased cyclic adenosine monophos-
phate (cAMP) and subsequently activation of protein
kinases that initiate secretion of the hormone. The
peptide hormones are water soluble, allowing them to
enter the circulatory system easily, where they are
carried to their target tissues.
Steroid Hormones Are Usually Synthesized from Cholesterol and
Are Not Stored. The chemical structure of steroid hor-
mones is similar to that of cholesterol, and in most
instances they are

hat of cholesterol, and in most
instances they are synthesized from cholesterol itself.
Adrenal
glands
Adipose
tissue
Thymus gland
Pineal gland
Parathyroid glands
(behind thyroid gland)
Stomach
Kidney
Pancreas
Ovaries
(female)
Small
intestine
Testes (male)
Figure 74–1
Anatomical loci of the principal endocrine glands and tissues of
the body.
Chapter 74
907
Table 74–1
Endocrine Glands, Hormones, and Their Functions and Struc

ne Glands, Hormones, and Their Functions and Structure
Chemical
Gland/Tissue
Hormones
Major Functions
Structure
Stimulates secretion of TSH and prolactin
Peptide
(Chapter 75)
Causes release of ACTH
Peptide
Causes release of growth hormone
Peptide
Growth hormone inhibitory hormone (GHIH)
Inhibits release of growth hormone
Peptide
(somatostatin)
Gonadotropin-releasing horm

Peptide
Causes release of LH and FSH
Dopamine or prolactin-inhibiting factor (PIF)
Inhibits release of prolactin
Amine
Stimulates protein synthesis and overall growth 
Peptide
of most cells and tissues
Thyroid-stimulating hormone (TSH)
Stimulates synthesis and secretion of thyroid 
Peptide
hormones (thyroxine and triiodothyronine)
Stimulates synthesis and secreti

Stimulates synthesis and secretion of adrenocortical
Peptide
hormones (cortisol, androgens, and aldosterone)
Prolactin
Promotes development of the female breasts and 
Peptide
secretion of milk
Causes growth of follicles in the ovaries and sperm
Peptide
maturation in Sertoli cells of testes
Stimulates testosterone synthesis in Leydig cells of 
Peptide
testes; stimulates ovulation, formation of corpus 
luteum, and estrog

ovulation, formation of corpus 
luteum, and estrogen and progesterone synthesis 
in ovaries
Posterior pituitary
Antidiuretic hormone (ADH) (also called
Increases water reabsorption by the kidneys and 
Peptide
vasopressin)
causes vasoconstriction and increased blood 
pressure
Oxytocin
Stimulates milk ejection from breasts and uterine
Peptide
Thyroid
Thyroxine (T4) and triiodothyronine (T3)
Increases the rates of chemical reactions in most 
Amine
(Chapter 76)
cells, thus 

reactions in most 
Amine
cells, thus increasing body metabolic rate
Promotes deposition of calcium in the bones and
Peptide
decreases extracellular ﬂuid calcium ion 
Cortisol
Has multiple metabolic functions for controlling
Steroid
(Chapter 77)
metabolism of proteins, carbohydrates, and fats;
also has anti-inﬂammatory effects
Increases renal sodium reabsorption, potassium 
Steroid
secretion, and hydrogen ion secretion
Adrenal medul

ecretion, and hydrogen ion secretion
Norepinephrine, epinephrine
Same effects as sympathetic stimulation
Amine
(Chapter 60)
Pancreas
Insulin (b cells)
Promotes glucose entry in many cells, and in 
Peptide
(Chapter 78)
this way controls carbohydrate metabolism
Glucagon (a cells)
Increases synthesis and release of glucose from 
Peptide
the liver into the body ﬂuids
Controls serum calcium ion concentration by 
Peptide
(Chapter 79)
increasing cal

centration by 
Peptide
increasing calcium absorption by the gut and 
kidneys and releasing calcium from bones
Testes
Promotes development of male reproductive 
Steroid
(Chapter 80)
system and male secondary sexual characteristics
Ovaries
Estrogens
Promotes growth and development of female 
Steroid
(Chapter 81)
reproductive system, female breasts, and female 
secondary sexual characteristics
Stimulates secretion of “uterine milk” by the uterine
Steroid
endom

ion of “uterine milk” by the uterine
Steroid
endometrial glands and promotes development of
secretory apparatus of breasts
Placenta
Human chorionic gonadotropin (HCG)
Promotes growth of corpus luteum and secretion of
Peptide
(Chapter 82)
estrogens and progesterone by corpus luteum
Human somatomammotropin
Probably helps promote development of some 
Peptide
fetal tissues as well as the mother’s breasts
Estrogens
See actions of estrogens from ovaries
Steroid
See actions of progesterone

s
Steroid
See actions of progesterone from ovaries
Steroid
Kidney
Renin
Catalyzes conversion of angiotensinogen to 
Peptide
(Chapter 26)
angiotensin I (acts as an enzyme)
1,25-Dihydroxycholecalciferol
Increases intestinal absorption of calcium and bone
Steroid
Increases erythrocyte production
Peptide
Heart
Increases sodium excretion by kidneys, reduces 
Peptide
(Chapter 22)
Stomach
Gastrin
Stimulates HCl s

2)
Stomach
Gastrin
Stimulates HCl secretion by parietal cells
Peptide
(Chapter 64)
Secretin
Stimulates pancreatic acinar cells to release 
Peptide
bicarbonate and water
Stimulates gallbladder contraction and release of
Peptide
pancreatic enzymes
Leptin
Inhibits appetite, stimulates thermogenesis
Peptide
(Chapter 71)
908
Unit XIV
They are lipid soluble and consist of three cyclohexyl
rings 

pid soluble and consist of three cyclohexyl
rings and one cyclopentyl ring combined into a single
structure (Figure 74–3).
Although there is usually very little hormone
storage in steroid-producing endocrine cells, large
stores of cholesterol esters in cytoplasm vacuoles can
be rapidly mobilized for steroid synthesis after a stim-
ulus. Much of the cholesterol in steroid-producing cells
comes from the plasma, but there is also de novo 
synthesis of cholesterol in steroid-producing cells.
Because

of cholesterol in steroid-producing cells.
Because the steroids are highly lipid soluble, once they
are synthesized, they simply diffuse across the cell
membrane and enter the interstitial ﬂuid and then the
blood.
Amine Hormones Are Derived from Tyrosine. The two groups
of hormones derived from tyrosine, the thyroid and
the adrenal medullary hormones, are formed by the
actions of enzymes in the cytoplasmic compartments
of the glandular cells. The thyroid hormones are 
synthesized and stored in t

 thyroid hormones are 
synthesized and stored in the thyroid gland and incor-
porated into macromolecules of the protein thy-
roglobulin, which is stored in large follicles within 
the thyroid gland. Hormone secretion occurs when 
the amines are split from thyroglobulin, and the free
hormones are then released into the blood stream.
After entering the blood, most of the thyroid hor-
mones combine with plasma proteins, especially thy-
roxine-binding globulin, which slowly releases the
hormones to

ng globulin, which slowly releases the
hormones to the target tissues.
Epinephrine and norepinephrine are formed in the
adrenal medulla, which normally secretes about four
times more epinephrine than norepinephrine. Cate-
cholamines are taken up into preformed vesicles and
stored until secreted. Similar to the protein hormones
stored in secretory granules, catecholamines are also
released from adrenal medullary cells by exocytosis.
Once the catecholamines enter the circulation, they
can exist in

holamines enter the circulation, they
can exist in the plasma in free form or in conjugation
with other substances.
Hormone Secretion, Transport,
and Clearance from the Blood
Onset of Hormone Secretion After a Stimulus, and Duration of
Action of Different Hormones. Some hormones, such as
norepinephrine and epinephrine, are secreted within
seconds after the gland is stimulated, and they may
develop full action within another few seconds to
minutes; the actions of other hormones, such as thy-
roxi

; the actions of other hormones, such as thy-
roxine or growth hormone, may require months for
full effect.Thus, each of the different hormones has its
own characteristic onset and duration of action—each
tailored to perform its speciﬁc control function.
Concentrations of Hormones in the Circulating Blood, and Hor-
monal Secretion Rates. The concentrations of hormones
required to control most metabolic and endocrine
functions are incredibly small. Their concentrations in
the blood range from as 

. Their concentrations in
the blood range from as little as 1 picogram (which is
one millionth of one millionth of a gram) in each mil-
liliter of blood up to at most a few micrograms (a few
Golgi
apparatus
Storage
Storage
Secretion
Secretion
Stimulus
fluid
Ca++
cAMP
Secretory
vesicles
reticulum
Packaging
Packaging
Synthesis
Synthesis
Translation
Nucleus
DNA
mRNA
Figure 74–2
Synthesis and secretion of peptide hormones. The stimulus for
hormone 

ion of peptide hormones. The stimulus for
hormone secretion often involves changes in intracellular calcium
or changes in cyclic adenosine monophosphate (cAMP) in the
cell.
O
HO
OH
C
O
CH2OH
O
HO
HC C
O
O CH2OH
O
OH
HO
OH
Cortisol
Estradiol
Figure 74–3
Chemical structures of several steroid hormones.
Chapter 74
909
millionths of a gram) per milliliter of blood. Similarly,
the rates of secretion of the various hormones are
extremely small, us

on of the various hormones are
extremely small, usually measured in micrograms or
milligrams per day. We shall see later in this chapter
that highly specialized mechanisms are available in the
target tissues that allow even these minute quantities
of hormones to exert powerful control over the phys-
iological systems.
Feedback Control of 
Negative Feedback Prevents Overactivity of Hormone Systems.
Although the plasma concentrations of many hor-
mones ﬂuctuate in response to var

ons of many hor-
mones ﬂuctuate in response to various stimuli that
occur throughout the day, all hormones studied thus
far appear to be closely controlled. In most instances,
this control is exerted through negative feedback mech-
anisms that ensure a proper level of hormone activity
at the target tissue. After a stimulus causes release of
the hormone,conditions or products resulting from the
action of the hormone tend to suppress its further
release. In other words, the hormone (or one of its

lease. In other words, the hormone (or one of its
products) has a negative feedback effect to prevent
oversecretion of the hormone or overactivity at the
target tissue.
The controlled variable is often not the secretory
rate of the hormone itself but the degree of activity of
the target tissue.Therefore, only when the target tissue
activity rises to an appropriate level will feedback
signals to the endocrine gland become powerful
enough to slow further secretion of the hormone.
Feedback regulati

urther secretion of the hormone.
Feedback regulation of hormones can occur at all
levels, including gene transcription and translation
steps involved in the synthesis of hormones and steps
involved in processing hormones or releasing stored
hormones.
Surges of Hormones Can Occur with Positive Feedback. In a
few instances, positive feedback occurs when the bio-
logical action of the hormone causes additional secre-
tion of the hormone. One example of this is the surge
of luteinizing hormone (LH) 

 of this is the surge
of luteinizing hormone (LH) that occurs as a result of
the stimulatory effect of estrogen on the anterior pitu-
itary before ovulation. The secreted LH then acts on
the ovaries to stimulate additional secretion of estro-
gen, which in turn causes more secretion of LH. Even-
tually, LH reaches an appropriate concentration, and
typical negative feedback control of hormone secre-
tion is then exerted.
Cyclical Variations Occur in Hormone Release. Superim-
posed on the negative

in Hormone Release. Superim-
posed on the negative and positive feedback control
of hormone secretion are periodic variations in
hormone release that are inﬂuenced by seasonal
changes, various stages of development and aging, the
diurnal (daily) cycle, and sleep. For example, the secre-
tion of growth hormone is markedly increased during
the early period of sleep but is reduced during the later
stages of sleep. In many cases, these cyclical variations
in hormone secretion are due to changes in a

tions
in hormone secretion are due to changes in activity 
of neural pathways involved in controlling hormone
release.
Water-soluble 
hormones
(peptides 
and 
cate-
cholamines) are dissolved in the plasma and trans-
ported from their sites of synthesis to target tissues,
where they diffuse out of the capillaries, into the inter-
stitial ﬂuid, and ultimately to target cells.
Steroid and thyroid hormones, in contrast, circulate
in the blood mainly bound to plasma

ast, circulate
in the blood mainly bound to plasma proteins. Usually
less than 10 per cent of steroid or thyroid hormones in
the plasma exist free in solution. For example, more
than 99 per cent of the thyroxine in the blood is bound
to plasma proteins. However, protein-bound hor-
mones cannot easily diffuse across the capillaries and
gain access to their target cells and are therefore bio-
logically inactive until they dissociate from plasma
proteins.
The relatively large amounts of hormones bo

teins.
The relatively large amounts of hormones bound to
proteins serve as reservoirs, replenishing the concen-
tration of free hormones when they are bound to
target receptors or lost from the circulation. Binding
of hormones to plasma proteins greatly slows their
clearance from the plasma.
“Clearance” of Hormones 
Two factors can increase or decrease the concentration
of a hormone in the blood. One of these is the rate of
hormone secretion into the blood. The second is 
the rate

 secretion into the blood. The second is 
the rate of removal of the hormone from the blood,
which is called the metabolic clearance rate. This 
is usually expressed in terms of the number of milli-
liters of plasma cleared of the hormone per minute.
To calculate this clearance rate, one measures (1) 
the rate of disappearance of the hormone from the
plasma per minute and (2) the concentration of the
hormone in each milliliter of plasma. Then, the meta-
bolic clearance rate is calculated by the 

e meta-
bolic clearance rate is calculated by the following
formula:
Metabolic clearance rate = Rate of disappearance
of hormone from the plasma/Concentration of 
hormone in each milliliter of plasma
The usual procedure for making this measurement
is the following:A puriﬁed solution of the hormone to
be measured is tagged with a radioactive substance.
Then the radioactive hormone is infused at a constant
rate into the blood stream until the radioactive con-
centration in the plasma becomes stead

active con-
centration in the plasma becomes steady. At this time,
the rate of disappearance of the radioactive hormone
from the plasma equals the rate at which it is infused,
which gives one the rate of disappearance.At the same
time, the plasma concentration of the radioactive
hormone is measured using a standard radioactive
counting procedure.Then, using the formula just cited,
the metabolic clearance rate is calculated.
910
Unit XIV
Hormones are “cleared” from 

logy and Reproduction
Hormones are “cleared” from the plasma in several
ways, including (1) metabolic destruction by the
tissues,(2) binding with the tissues,(3) excretion by the
liver into the bile, and (4) excretion by the kidneys into
the urine. For certain hormones, a decreased metabolic
clearance rate may cause an excessively high concen-
tration of the hormone in the circulating body ﬂuids.
For instance, this occurs for several of the steroid hor-
mones when the liver is diseased, because 

id hor-
mones when the liver is diseased, because these hor-
mones are conjugated mainly in the liver and then
“cleared” into the bile.
Hormones are sometimes degraded at their target
cells by enzymatic processes that cause endocytosis of
the cell membrane hormone-receptor complex; the
hormone is then metabolized in the cell, and the recep-
tors are usually recycled back to the cell membrane.
Most of the peptide hormones and catecholamines
are water soluble and circulate freely in the blood.
The

ter soluble and circulate freely in the blood.
They are usually degraded by enzymes in the blood
and tissues and rapidly excreted by the kidneys and
liver, thus remaining in the blood for only a short time.
For example, the half-life of angiotensin II circulating
in the blood is less than a minute.
Hormones that are bound to plasma proteins are
cleared from the blood at much slower rates and may
remain in the circulation for several hours or even
days.The half-life of adrenal steroids in the cir

days.The half-life of adrenal steroids in the circulation,
for example, ranges between 20 and 100 minutes,
whereas the half-life of the protein-bound thyroid hor-
mones may be as long as 1 to 6 days.
Mechanisms of Action 
of Hormones
Hormone Receptors and Their
The ﬁrst step of a hormone’s action is to bind to spe-
ciﬁc receptors at the target cell. Cells that lack recep-
tors for the hormones do not respond. Receptors 
for some hormones are located on the target cell 
membrane, wher

nes are located on the target cell 
membrane, whereas other hormone receptors are
located in the cytoplasm or the nucleus. When the
hormone combines with its receptor, this usually initi-
ates a cascade of reactions in the cell, with each stage
becoming more powerfully activated so that even
small concentrations of the hormone can have a large
effect.
Hormonal receptors are large proteins, and each cell
that is to be stimulated usually has some 2000 to
100,000 receptors.Also, each receptor is us

000 to
100,000 receptors.Also, each receptor is usually highly
speciﬁc for a single hormone; this determines the type
of hormone that will act on a particular tissue. The
target tissues that are affected by a hormone are those
that contain its speciﬁc receptors.
The locations for the different types of hormone
receptors are generally the following:
1. In or on the surface of the cell membrane. The
membrane receptors are speciﬁc mostly for the
protein, peptide, and catecholamine hormones.
2. In t

tein, peptide, and catecholamine hormones.
2. In the cell cytoplasm. The primary receptors for
the different steroid hormones are found mainly
in the cytoplasm.
3. In the cell nucleus. The receptors for the thyroid
hormones are found in the nucleus and are
believed to be located in direct association with
one or more of the chromosomes.
The Number and Sensitivity of Hormone Receptors Are Regu-
lated. The number of receptors in a target cell usually
does not remain constant from day to day, or ev

ly
does not remain constant from day to day, or even
from minute to minute. The receptor proteins them-
selves are often inactivated or destroyed during the
course of their function, and at other times they 
are reactivated or new ones are manufactured by 
the protein-manufacturing mechanism of the cell.
For instance, increased hormone concentration and
increased binding with its target cell receptors some-
times cause the number of active receptors to
decrease. This down-regulation of the recep

ors to
decrease. This down-regulation of the receptors can
occur as a result of (1) inactivation of some of the
receptor molecules, (2) inactivation of some of the
intracellular protein signaling molecules, (3) tempo-
rary sequestration of the receptor to the inside of the
cell, away from the site of action of hormones that
interact with cell membrane receptors, (4) destruction
of the receptors by lysosomes after they are internal-
ized, or (5) decreased production of the receptors. In
each case

ecreased production of the receptors. In
each case, receptor down-regulation decreases the
target tissue’s responsiveness to the hormone.
Some hormones cause up-regulation of receptors
and intracellular signaling proteins; that is, the stimu-
lating hormone induces greater than normal formation
of receptor or intracellular signaling molecules by the
protein-manufacturing machinery of the target cell, or
greater availability of the receptor for interaction with
the hormone. When this occurs, the 

teraction with
the hormone. When this occurs, the target tissue
becomes progressively more sensitive to the stimulat-
ing effects of the hormone.
Intracellular Signaling After Hormone
Almost without exception, a hormone affects its target
tissues by ﬁrst forming a hormone-receptor complex.
This alters the function of the receptor itself, and the
activated receptor initiates the hormonal effects. To
explain this, let us give a few examples of the different
types of interaction

few examples of the different
types of interactions.
Ion Channel–Linked Receptors. Virtually all the neuro-
transmitter substances, such as acetylcholine and 
norepinephrine, combine with receptors in the postsy-
naptic membrane. This almost always causes a change
in the structure of the receptor, usually opening or
closing a channel for one or more ions. Some of these
ion channel–linked receptors open (or close) channels
for sodium ions, others for potassium ions, others for
calcium ions, and s

for potassium ions, others for
calcium ions, and so forth. The altered movement of
these ions through the channels causes the subsequent
effects on the postsynaptic cells. Although a few 
Chapter 74
911
hormones may exert some of their actions through
activation of ion channel receptors, most hormones
that open or close ions channels do this indirectly by
coupling with G protein–linked or enzyme-linked
receptors, as discussed next.
G Protein–Linked Hormone Receptors

discussed next.
G Protein–Linked Hormone Receptors. Many hormones
activate receptors that indirectly regulate the activity
of target proteins (e.g., enzymes or ion channels) by
coupling with groups of cell membrane proteins called
heterotrimeric GTP-binding proteins (G proteins)
(Figure 74–4). There are more than 1000 known G
protein–coupled receptors, all of which have seven
transmembrane segments that loop in and out of the
cell membrane. Some parts of the receptor that pro-
trude into the cel

parts of the receptor that pro-
trude into the cell cytoplasm (especially the cytoplas-
mic tail of the receptor) are coupled to G proteins that
include three (i.e., trimeric) parts—the a, b, and g sub-
units. When the ligand (hormone) binds to the extra-
cellular part of the receptor, a conformational change
occurs in the receptor that activates the G proteins and
induces intracellular signals that either (1) open or
close cell membrane ion channels or (2) change the
activity of an enzyme in th

nels or (2) change the
activity of an enzyme in the cytoplasm of the cell.
The trimeric G proteins are named for their ability
to bind guanosine nucleotides. In their inactive state,
the a, b, and g subunits of G proteins form a complex
that binds guanosine diphosphate (GDP) on the a
subunit. When the receptor is activated, it undergoes
a conformational change that causes the GDP-bound
trimeric G protein to associate with the cytoplasmic
part of the receptor and to exchange GDP for guano-
sine t

the receptor and to exchange GDP for guano-
sine triphosphate (GTP). Displacement of GDP by
GTP causes the a subunit to dissociate from the
trimeric complex and to associate with other intracel-
lular signaling proteins; these proteins, in turn, alter
the activity of ion channels or intracellular enzymes
such as adenylyl cyclase or phospholipase C, which
alters cell function.
The signaling event is rapidly terminated when the
hormone is removed and the a subunit inactivates
itself by converting 

nd the a subunit inactivates
itself by converting its bound GTP to GDP; then the
a subunit once again combines with the b and g sub-
units to form an inactive, membrane-bound trimeric G
protein.
Some hormones are coupled to inhibitory G pro-
teins (denoted Gi 
proteins), whereas others are
coupled to stimulatory G proteins (denoted Gs pro-
teins). Thus, depending on the coupling of a hormone
receptor to an inhibitory or stimulatory G protein, a
hormone can either increase or decrease the activit

ormone can either increase or decrease the activity
of intracellular enzymes. This complex system of cell
membrane G proteins provides a vast array of poten-
tial cell responses to different hormones in the various
target tissues of the body.
Enzyme-Linked 
Hormone 
Receptors. Some 
receptors,
when activated, function directly as enzymes or are
closely associated with enzymes that they activate.
These enzyme-linked receptors are proteins that pass
through the membrane only once, in contrast to t

s
through the membrane only once, in contrast to the
seven-transmembrane G protein–coupled receptors.
Enzyme-linked receptors have their hormone-binding
site on the outside of the cell membrane and their cat-
alytic or enzyme-binding site on the inside. When the
hormone binds to the extracellular part of the recep-
tor, an enzyme immediately inside the cell membrane
is activated (or occasionally inactivated). Although
many enzyme-linked receptors have intrinsic enzyme
activity, others rely on en

 have intrinsic enzyme
activity, others rely on enzymes that are closely asso-
ciated with the receptor to produce changes in cell
function.
One example of an enzyme-linked receptor is the
leptin receptor (Figure 74–5). Leptin is a hormone
Receptor
Hormone
Cytoplasm
fluid
GTP activated
(enzyme)
GTP
G-protein
(active)
a
b
g
a
b
g
b
g
a
GDP
G-protein
Figure 74–4
Mechanism of activation of a G protein–coupled receptor. When the hormone activates the receptor,

receptor. When the hormone activates the receptor, the inactive a, b, and g G protein
complex associates with the receptor and is activated, with an exchange of guanosine triphosphate (GTP) for guanosine diphosphate
(GDP). This causes the a subunit (to which the GTP is bound) to dissociate from the b and g subunits of the G protein and to interact with
membrane-bound target proteins (enzymes) that initiate intracellular signals.
912
Unit XIV
secreted by fat cells a

crinology and Reproduction
secreted by fat cells and has many physiological
effects, but it is especially important in regulating
appetite and energy balance, as discussed in Chapter
71. The leptin receptor is a member of a large family
of cytokine receptors that do not themselves contain
enzymatic activity but signal through associated
enzymes. In the case of the leptin receptor, one of the
signaling pathways occurs through a tyrosine kinase of
the janus kinase (JAK) family, JAK2. The leptin re

the janus kinase (JAK) family, JAK2. The leptin recep-
tor exists as a dimer (i.e., in two parts), and binding of
leptin to the extracellular part of the receptor alters its
conformation, enabling phosphorylation and activa-
tion of the intracellular associated JAK2 molecules.
The activated JAK2 molecules then phosphorylate
other tyrosine residues within the leptin receptor–
JAK2 complex to mediate intracellular signaling. The
intracellular signals include phosphorylation of signal
transducer an

ls include phosphorylation of signal
transducer and activator of transcription (STAT) pro-
teins, which activates transcription by leptin target
genes to initiate protein synthesis. Phosphorylation of
JAK2 also leads to activation of other intracellular
enzyme pathways such as mitogen-activated protein
kinases (MAPK) and phosphatidylinositol 3-kinase
(PI3K). Some of the effects of leptin occur rapidly as
a result of activation of these intracellular enzymes,
whereas other actions occur more slow

lar enzymes,
whereas other actions occur more slowly and require
synthesis of new proteins.
Another example, one widely used in hormonal
control of cell function, is for the hormone to bind 
with a special transmembrane receptor, which then
becomes the activated enzyme adenylyl cyclase at the
end that protrudes to the interior of the cell. This
cyclase catalyzes the formation of cAMP, which has a
multitude of effects inside the cell to control cell activ-
ity, as discussed in greater detail late

ll activ-
ity, as discussed in greater detail later. cAMP is called
a second messenger because it is not the hormone itself
that directly institutes the intracellular changes;
instead, the cAMP serves as a second messenger to
cause these effects.
For a few peptide hormones, such as atrial natri-
uretic peptide (ANP), cyclic guanosine monophos-
phate (cGMP), which is only slightly different from
cAMP, serves in a similar manner as a second 
messenger.
Intracellular Hormone Receptors and Activatio

ger.
Intracellular Hormone Receptors and Activation of Genes.
Several hormones, including adrenal and gonadal
steroid hormones, thyroid hormones, retinoid hor-
mones, and vitamin D, bind with protein receptors
inside the cell rather than in the cell membrane.
Because these hormones are lipid soluble, they readily
cross the cell membrane and interact with receptors in
the cytoplasm or nucleus. The activated hormone-
receptor complex then binds with a speciﬁc regulatory
(promoter) sequence of the 

h a speciﬁc regulatory
(promoter) sequence of the DNA called the hormone
response element, and in this manner either activates
or represses transcription of speciﬁc genes and for-
mation of messenger RNA (mRNA) (Figure 74–6).
Therefore, minutes, hours, or even days after the
hormone has entered the cell, newly formed proteins
appear in the cell and become the controllers of new
or altered cellular functions.
Many different tissues have identical intracellular
hormone receptors, but the genes tha

hormone receptors, but the genes that the receptors
regulate are different in the various tissues. An intra-
cellular receptor can activate a gene response only if
the appropriate combination of gene regulatory pro-
teins is present, and many of these regulatory proteins
are tissue speciﬁc. Thus, the responses of different
tissues to a hormone are determined not only by the
speciﬁcity of the receptors but also by the genes that
the receptor regulates.
Second Messenger Mechanisms 
f

receptor regulates.
for Mediating Intracellular 
Hormonal Functions
We noted earlier that one of the means by which hor-
mones exert intracellular actions is to stimulate for-
mation of the second messenger cAMP inside the cell
membrane. The cAMP then causes subsequent intra-
cellular effects of the hormone. Thus, the only direct
effect that the hormone has on the cell is to activate a
single type of membrane receptor. The second mes-
senger does the rest.

. The second mes-
of enzymes
Leptin receptor
Physiological
effects
Translation 
mRNA 
Stat3
Stat3
P
P
Stat3
Jak2
Leptin
Jak2
Y
Y
Y
Y
Y
Y
P
P
P
P
P
P
Stat3
Stat3
Stat3
P
P
Figure 74–5
An enzyme-linked receptor—the leptin receptor. The receptor
exists as a homodimer (two identical parts), and leptin binds to
the extracellular part of the receptor, causing phosphorylation and
activation of the intracellular associated janus kinase 2 (JAK2).
This causes p

ar associated janus kinase 2 (JAK2).
This causes phosphorylation of signal transducer and activator of
transcription (STAT) proteins, which then activates the transcrip-
tion of target genes and the synthesis of proteins. JAK2 phos-
phorylation also activates several other enzyme systems that
mediate some of the more rapid effects of leptin.
Chapter 74
913
cAMP is not the only second messenger used by the
different hormones. Two other especially important
ones are (

ormones. Two other especially important
ones are (1) calcium ions and associated calmodulin
and 
(2) 
products 
of 
membrane 
phospholipid 
breakdown.
Adenylyl Cyclase–cAMP Second 
Messenger System
Table 74–2 shows a few of the many hormones that use
the adenylyl cyclase–cAMP mechanism to stimulate
their target tissues, and Figure 74–7 shows the adeny-
lyl 
cyclase–cAMP 
second 
messenger 
system 
itself. Binding of the hormones with the receptor
allows coupling of the receptor to a G protein. I

allows coupling of the receptor to a G protein. If the
G protein stimulates the adenylyl cyclase–cAMP
system, it is called a Gs protein, denoting a stimulatory
G protein. Stimulation of adenylyl cyclase, a mem-
brane-bound enzyme, by the Gs protein then catalyzes
the conversion of a small amount of cytoplasmic
adenosine triphosphate (ATP) into cAMP inside the
cell. This then activates cAMP-dependent protein
kinase, which phosphorylates speciﬁc proteins in the
cell, triggering biochemical reacti

roteins in the
cell, triggering biochemical reactions that ultimately
lead to the cell’s response to the hormone.
Table 74–2
Some Hormones That Use the Adenylyl Cyclase–cAMP
Second Messenger System
Angiotensin II (epithelial cells)
Catecholamines (b receptors)
Glucagon
Secretin
Thy

arathyroid hormone (PTH)
Secretin
Thyroid-stimulating hormone  (TSH)
Vasopressin (V2 receptor, epithelial cells)
Diffusion
Diffusion
Lipophilic hormone
Proteins
Proteins
Ribosome
Ribosome
Nuclear envelope
Nuclear pore
mRNA
mRNA
Hormone
response
element
Hormone
response
element
Hormone
receptor
complex
Hormone
receptor
complex
receptor
receptor
Nucl

lex
receptor
receptor
Nucleus
Nucleus
Nuclear
receptor
Nuclear
receptor
DNA
DNA
mRNA
mRNA
Figure 74–6
Mechanisms of interaction of
lipophilic hormones, such as
steroids, with intracellular recep-
tors in target cells. After the
hormone binds to the receptor in
the cytoplasm or in the nucleus,
the hormone-receptor complex
binds to the hormone response
element (promoter) on the DNA.
This either activates or inhibits
gene transcription, formation of
messenger RNA (mRNA), and

nscription, formation of
protein synthesis.
b
g
a
Active
cAMP-
dependent
protein
kinase
Inactive
cAMP-
dependent
protein
kinase
cAMP
Adenylyl
cyclase
Protein – PO4 + ADP             Protein + ATP
Cell’s response
ATP
Cytoplasm
fluid
Hormone
GTP
Figure 74–7
Cyclic adenosine monophosphate (cAMP) mechanism by which
many hormones exert their control of cell function. ADP, adeno-
sine diphosphate; ATP, adenosine triphosphate.
914
Unit XIV
Endocrinology and Repro

914
Unit XIV
Once cAMP is formed inside the cell, it usually acti-
vates a cascade of enzymes. That is, ﬁrst one enzyme is
activated, which activates a second enzyme, which acti-
vates a third, and so forth. The importance of this
mechanism is that only a few molecules of activated
adenylyl cyclase immediately inside the cell mem-
brane can cause many more molecules of the next
enzyme to be activated, which can cause still more
molecules of the third 

which can cause still more
molecules of the third enzyme to be activated, and 
so forth. In this way, even the slightest amount of
hormone acting on the cell surface can initiate a pow-
erful cascading activating force for the entire cell.
If binding of the hormone to its receptors is coupled
to an inhibitory G protein (denoted Gi protein),
adenylyl cyclase will be inhibited, reducing the forma-
tion of cAMP and ultimately leading to an inhibitory
action in the cell. Thus, depending on the coupl

y
action in the cell. Thus, depending on the coupling of
the hormone receptor to an inhibitory or a stimulatory
G protein, a hormone can either increase or decrease
the concentration of cAMP and phosphorylation of
key proteins inside the cell.
The speciﬁc action that occurs in response to
increases or decreases of cAMP in each type of target
cell depends on the nature of the intracellular machin-
ery—some cells have one set of enzymes, and other
cells have other enzymes. Therefore, different fun

cells have other enzymes. Therefore, different func-
tions are elicited in different target cells, such as initi-
ating synthesis of speciﬁc intracellular chemicals,
causing muscle contraction or relaxation, initiating
secretion by the cells, and altering cell permeability.
Thus, a thyroid cell stimulated by cAMP forms the
metabolic hormones thyroxine and triiodothyronine,
whereas the same cAMP in an adrenocortical cell
causes secretion of the adrenocortical steroid hor-
mones. In epithelial cel

renocortical steroid hor-
mones. In epithelial cells of the renal tubules, cAMP
increases their permeability to water.
The Cell Membrane Phospholipid Second
Some hormones activate transmembrane receptors
that activate the enzyme phospholipase C attached to
the inside projections of the receptors (Table 74–3).
This enzyme catalyzes the breakdown of some phos-
pholipids in the cell membrane, especially phos-
phatidylinositol biphosphate (PIP2), into two different
second messenger 

phate (PIP2), into two different
second messenger products: inositol triphosphate 
(IP3) and diacylglycerol (DAG). The IP3 mobilizes
calcium ions from mitochondria and the endoplasmic
reticulum, and the calcium ions then have their own
second messenger effects, such as smooth muscle con-
traction and changes in cell secretion.
DAG, the other lipid second messenger, activates
the enzyme protein kinase C (PKC), which then phos-
phorylates a large number of proteins, leading to the
cell’s response 

umber of proteins, leading to the
cell’s response (Figure 74–8). In addition to these
effects, the lipid portion of DAG is arachidonic acid,
which is the precursor for the prostaglandins and other
local hormones that cause multiple effects in tissues
Calcium-Calmodulin Second 
Another second messenger system operates in
response to the entry of calcium into the cells. Calcium
entry may be initiated by (1) changes in membrane
potential that open calcium chann

nges in membrane
potential that open calcium channels or (2) a hormone
interacting with membrane receptors that open
calcium channels.
On entering a cell, calcium ions bind with the
protein calmodulin. This protein has four calcium sites,
and when three or four of these sites have bound with
calcium, the calmodulin changes its shape and initiates
multiple effects inside the cell, including activation 
or inhibition of protein kinases. Activation of 
Table 74–3
Some Hormones That Use the Phosphol

of 
Table 74–3
Some Hormones That Use the Phospholipase C Second
Angiotensin II (vascular smooth muscle)
Catecholamines (a receptors)
Oxytocin
Thyroid-releasing hormone (TRH)
Vasopressin (V1 receptor, vascular smooth muscle)
Active
protein
kinase C
Inactive
protein
kinase C
Protein – PO4             Protein 
Ca++
Cytoplasm
Receptor
Peptide
hormone

Peptide
hormone
G protein
DAG + IP3                            PIP2
Figure 74–8
The cell membrane phospholipid second messenger system by
which some hormones exert their control of cell function. DAG,
diacylglycerol; IP3, inositol triphosphate; PIP2, phosphatidylinosi-
tol biphosphate.
Chapter 74
915
calmodulin-dependent protein kinases causes, via
phosphorylation, activation or inhib

s causes, via
phosphorylation, activation or inhibition of proteins
involved in the cell’s response to the hormone. For
example, one speciﬁc function of calmodulin is to acti-
vate myosin kinase, which acts directly on the myosin
of smooth muscle to cause smooth muscle contraction.
The normal calcium ion concentration in most cells
of the body is 10-8 to 10-7 mol/L, which is not enough
to activate the calmodulin system. But when the
calcium ion concentration rises to 10-6 to 10-5 mol/L,
enough b

oncentration rises to 10-6 to 10-5 mol/L,
enough binding occurs to cause all the intracellular
actions of calmodulin. This is almost exactly the same
amount of calcium ion change that is required in skele-
tal muscle to activate troponin C, which causes skele-
tal muscle contraction, as explained in Chapter 7. It is
interesting that troponin C is similar to calmodulin in
both function and protein structure.
Hormones That Act Mainly on the
Genetic Machinery of the Cell
Steroid Hormones Increase P

 Machinery of the Cell
Steroid Hormones Increase Protein Synthesis
Another means by which hormones act—speciﬁcally,
the steroid hormones secreted by the adrenal cortex,
ovaries, and testes—is to cause synthesis of proteins in
the target cells. These proteins then function as
enzymes, transport proteins, or structural proteins,
which in turn provide other functions of the cells.
The sequence of events in steroid function is essen-
tially the following:
1. The steroid hormone diffuses across the c

wing:
1. The steroid hormone diffuses across the cell
membrane and enters the cytoplasm of the cell,
where it binds with a speciﬁc receptor protein.
2. The combined receptor protein–hormone then
diffuses into or is transported into the nucleus.
3. The combination binds at speciﬁc points on the
DNA strands in the chromosomes, which activates
the transcription process of speciﬁc genes to form
mRNA.
4. The mRNA diffuses into the cytoplasm, where it
promotes the translation process at the ribosomes

to form new proteins.
To give an example, aldosterone, one of the hor-
mones secreted by the adrenal cortex, enters the cyto-
plasm of renal tubular cells, which contain a speciﬁc
aldosterone receptor protein. Therefore, in these cells,
the sequence of events cited earlier ensues. After
about 45 minutes, proteins begin to appear in the renal
tubular cells and promote sodium reabsorption from
the tubules and potassium secretion into the tubules.
T

ubules and potassium secretion into the tubules.
Thus, the full action of the steroid hormone is charac-
teristically delayed for at least 45 minutes—up to
several hours or even days. This is in marked contrast
to the almost instantaneous action of some of the
peptide and amino acid–derived hormones, such as
vasopressin and norepinephrine.
Thyroid Hormones Increase Gene
Transcription in the Cell Nucleus
The thyroid hormones thyroxine and triiodothyronine
cause increased transcription by speciﬁc 

hyronine
cause increased transcription by speciﬁc genes in the
nucleus. To accomplish this, these hormones ﬁrst bind
directly with receptor proteins in the nucleus itself;
these receptors are probably protein molecules
located within the chromosomal complex, and they
likely control the function of the genetic promoters or
operators, as explained in Chapter 3.
Two important features of thyroid hormone func-
tion in the nucleus are the following:
1. They activate the genetic mechanisms for the
for

. They activate the genetic mechanisms for the
formation of many types of intracellular
proteins—probably 100 or more. Many of these
are enzymes that promote enhanced intracellular
metabolic activity in virtually all cells of the body.
2. Once bound to the intranuclear receptors, the
thyroid hormones can continue to express their
control functions for days or even weeks.
Measurement of Hormone
Most hormones are present in the blood in extremely
minute quantities; some

 in the blood in extremely
minute quantities; some concentrations are as low as
one billionth of a milligram (1 picogram) per milliliter.
Therefore, it was very difﬁcult to measure these con-
centrations by the usual chemical means.An extremely
sensitive method, however, was developed about 40
years ago that revolutionized the measurement of hor-
mones, their precursors, and their metabolic end prod-
ucts. This method is called radioimmunoassay.
The method of performing radioimm

The method of performing radioimmunoassay is as
follows. First, an antibody that is highly speciﬁc for the
hormone to be measured is produced.
Second, a small quantity of this antibody is (1)
mixed with a quantity of ﬂuid from the animal con-
taining the hormone to be measured and (2) mixed
simultaneously with an appropriate amount of puriﬁed
standard hormone that has been tagged with a
radioactive isotope. However, one speciﬁc condition
must be met:There must be too little anti

ondition
must be met:There must be too little antibody to bind
completely both the radioactively tagged hormone
and the hormone in the ﬂuid to be assayed.Therefore,
the natural hormone in the assay ﬂuid and the radio-
active standard hormone compete for the binding sites
of the antibody. In the process of competing, the quan-
tity of each of the two hormones, the natural and the
radioactive, that binds is proportional to its concen-
tration in the assay ﬂuid.
Third, after binding has reached equ

e assay ﬂuid.
Third, after binding has reached equilibrium, the
antibody-hormone complex is separated from the
remainder of the solution, and the quantity of radio-
active hormone bound in this complex is measured by
radioactive counting techniques. If a large amount of
radioactive hormone has bound with the antibody, it is
clear that there was only a small amount of natural
hormone to compete with the radioactive hormone,
and therefore the concentration of the natural
hormone in the assayed ﬂui

ntration of the natural
hormone in the assayed ﬂuid was small. Conversely, if
916
Unit XIV
only a small amount of radioactive hormone has
bound, it is clear that there was a large amount of
natural hormone to compete for the binding sites.
Fourth, to make the assay highly quantitative, the
radioimmunoassay procedure is also performed for
“standard” solutions of untagged hormone at several
concentration levels. Then a “standard curve” is
plotted, as shown in Figure 

a “standard curve” is
plotted, as shown in Figure 74–9. By comparing the
radioactive counts recorded from the “unknown”
assay procedures with the standard curve, one can
determine within an error of 10 to 15 per cent the con-
centration of the hormone in the “unknown” assayed
ﬂuid.As little as billionths or even trillionths of a gram
of hormone can often be assayed in this way.
Enzyme-Linked Immunosorbent 
Assay (ELISA)
Enzyme-linked immunosorbent assays (ELISAs) can
be used to measure almost an

t assays (ELISAs) can
be used to measure almost any protein, including hor-
mones. This test combines the speciﬁcity of antibodies
with the sensitivity of simple enzyme assays. Figure
74–10 shows the basic elements of this method, which
is often performed on plastic plates that each have 96
small wells. Each well is coated with an antibody (AB1)
that is speciﬁc for the hormone being assayed. Samples
or standards are added to each of the wells, followed
by a second antibody (AB2) that is also spe

llowed
by a second antibody (AB2) that is also speciﬁc for the
hormone but binds to a different site of the hormone
molecule. A third antibody (AB3) is added that recog-
nizes AB2 and is coupled to an enzyme that converts 
a suitable substrate to a product that can be easily
detected by colorimetric or ﬂuorescent optical
methods.
Because each molecule of enzyme catalyzes the
formation of many thousands of product molecules,
even very small amounts of hormone molecules can be
detected. In contras

s of hormone molecules can be
detected. In contrast to competitive radioimmuno-
assay methods, ELISA methods use excess antib-
odies so that all hormone molecules are captured in
antibody-hormone complexes. Therefore, the amount
of hormone present in the sample or in the standard
is proportional to the amount of product formed.
The ELISA method has become widely used in clin-
ical laboratories because (1) it does not employ
radioactive isotopes,(2) much of the assay can be auto-
mated using 96-w

2) much of the assay can be auto-
mated using 96-well plates, and (3) it has proved to be
a cost-effective and accurate method for assessing
hormone levels.
Aranda A, Pascual A: Nuclear hormone receptors and gene
Clark AJ, Baig AH, Noon L, et al: Expression, desensitiza-
tion, and internalization of the ACTH receptor (MC2R).
Ann N Y Acad Sci 994:

of the ACTH receptor (MC2R).
Ann N Y Acad Sci 994:111, 2003.
Katzmann DJ, Odorizzi G, Emr SD: Receptor downregula-
tion and multivesicular-body sorting. Nat Rev Mol Cell
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100
90
80
70
60
50
40
30
20
10
0
2
4
8
16
32
64
128
Percent of antibody bound with
radioactive aldosterone
Aldosterone concentration i

adioactive aldosterone
Aldosterone concentration in
test sample (ng/d)
Figure 74–9
“Standard curve” for radioimmunoassay of aldosterone. (Courtesy
Dr. Manis Smith.)
P
P
S
S
S
S
S
S
P
P
P
P
E
AB3
AB2
AB1
H
Figure 74–10
Basic principles of the enzyme-linked immunosorbent assay
(ELISA) for measuring the concentration of a hormone (H). AB1
and AB2 are antibodies that recognize the hormone at different
binding sites, and AB3 is an antibody that recognizes AB2. E is an
enzyme linked to AB3 that cataly

izes AB2. E is an
enzyme linked to AB3 that catalyzes the formation of a colored 
ﬂuorescent product (P) from a substrate (S). The amount of the
product is measured using optical methods and is proportional to
the amount of hormone in the well if there are excess antibodies
in the well.
Chapter 74
917
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918
Pituitary Hormones and Their
Control by the Hypothalamus
Pituitary Gland and 
Its Relation to the
Pituitary Gland: Two Distinct Parts–The Anterior and Poste-
rior Lobes. The pituitary gland (Figure 75–1), also
called the hypophysis, is a small gland—about 1 cen-
timeter in diameter and 0.5 to 1 gram in weight—
that lies in the sella turcica, a bony cavity at the base of the brain, and is
connected to the hypothalamus by the pitui

 and is
connected to the hypothalamus by the pituitary (or hypophysial) stalk. Physio-
logically, the pituitary gland is divisible into two distinct portions: the anterior
pituitary, also known as the adenohypophysis, and the posterior pituitary, also
known as the neurohypophysis. Between these is a small, relatively avascular
zone called the pars intermedia, which is almost absent in the human being but
is much larger and much more functional in some lower animals.
Embryologically, the two port

 some lower animals.
Embryologically, the two portions of the pituitary originate from different
sources—the anterior pituitary from Rathke’s pouch, which is an embryonic
invagination of the pharyngeal epithelium, and the posterior pituitary from a
neural tissue outgrowth from the hypothalamus.The origin of the anterior pitu-
itary from the pharyngeal epithelium explains the epithelioid nature of its cells,
and the origin of the posterior pituitary from neural tissue explains the pres-
ence of l

ry from neural tissue explains the pres-
ence of large numbers of glial-type cells in this gland.
Six important peptide hormones plus several less important ones are secreted
by the anterior pituitary, and two important peptide hormones are secreted 
by the posterior pituitary. The hormones of the anterior pituitary play major
roles in the control of metabolic functions throughout the body, as shown in
Figure 75–2.
• Growth hormone promotes growth of the entire body by affecting protein
formatio

h of the entire body by affecting protein
formation, cell multiplication, and cell differentiation.
• Adrenocorticotropin (corticotropin) controls the secretion of some of the
adrenocortical hormones, which affect the metabolism of glucose, proteins,
and fats.
• Thyroid-stimulating hormone (thyrotropin) controls the rate of secretion of
thyroxine and triiodothyronine by the thyroid gland, and these hormones
control the rates of most intracellular chemical reactions in the body.
• Prolactin promo

 chemical reactions in the body.
• Prolactin promotes mammary gland development and milk production.
• Two separate gonadotropic hormones, follicle-stimulating hormone and
luteinizing hormone, control growth of the ovaries and testes, as well as
their hormonal and reproductive activities.
The two hormones secreted by the posterior pituitary play other roles.
• Antidiuretic hormone (also called vasopressin) controls the rate of water
excretion into the urine, thus helping to control the concentra

o the urine, thus helping to control the concentration of water
in the body ﬂuids.
• Oxytocin helps express milk from the glands of the breast to the nipples
during suckling and possibly helps in the delivery of the baby at the end of
gestation.
Anterior Pituitary Gland Contains Several Different Cell Types That Synthesize and Secrete 
Hormones. Usually, there is one cell type for each major hormone formed in the 
Chapter 75
Pituitary Hormones and Their Control by the Hypothalamus
919
anterior p

d Their Control by the Hypothalamus
919
anterior pituitary gland. With special stains attached to
high-afﬁnity antibodies that bind with the distinctive
hormones, at least ﬁve cell types can be differentiated
(Figure 75–3). Table 75–1 provides a summary of these
cell types, the hormones they produce, and their physio-
logical actions. These ﬁve cell types are:
1. Somatotropes—human growth hormone (hGH)
2. Corticotropes—adrenocorticotropin (ACTH)
3. Thyrotropes—thyroid-stimulating hormone (TSH)
4

4. Gonadotropes—gonadotropic hormones, which
include both luteinizing hormone (LH) and follicle-
stimulating hormone (FSH)
5. Lactotropes—prolactin (PRL)
About 30 to 40 per cent of the anterior pituitary cells
are somatotropes that secrete growth hormone, and
about 20 per cent are corticotropes that secrete ACTH.
Each of the other cell types accounts for only 3 to 5 per
cent of the total; nevertheless, they secrete powerful
hormones for controllin

ess, they secrete powerful
hormones for controlling thyroid function, sexual func-
tions, and milk secretion by the breasts.
Somatotropes stain strongly with acid dyes and are
therefore called acidophils. Thus, pituitary tumors that
secrete large quantities of human growth hormone are
called acidophilic tumors.
Posterior Pituitary Hormones Are Synthesized by Cell Bodies in 
the Hypothalamus. The bodies of the cells that secrete 
the posterior pituitary hormones are not located in the 
pituitary 

uitary hormones are not located in the 
pituitary gland itself but are large neurons, called mag-
nocellular neurons, located in the supraoptic and par-
aventricular nuclei of the hypothalamus. The hormones
are then transported in the axoplasm of the neurons’
nerve ﬁbers passing from the hypothalamus to the pos-
terior pituitary gland. This is discussed more fully later
Hypothalamus Controls
Pituitary Secretion
Almost all secretion by the pituitary is controlled by
either hormona

n by the pituitary is controlled by
either hormonal or nervous signals from the hypo-
thalamus. Indeed, when the pituitary gland is removed
from its normal position beneath the hypothalamus
and transplanted to some other part of the body, its
Pars intermedia
Hypophysial stalk
Figure 75–1
Pituitary gland.
Thyroid
gland
Mammary
gland
Increases blood
glucose level
Pancreas
Ovary
ACH
Promotes secretion
of insulin
Anterior
pituitary
g

of insulin
Anterior
pituitary
gland
Growth
Follicle
Prolactin
Figure 75–2
Metabolic functions of the anterior pituitary hormones. ACH,
adrenal corticosteroid hormones.
Sinusoid
Gamma
(g) cell
Alpha
(a) cell
Epsilon (e)
acidophil cell
Delta (d)
basophil cell
Beta (b) cell
Figure 75–3
Cellular structure of the anterior pituitary gland. (Redrawn from
Guyton AC: Physiology of the Human Body, 6th ed. Philadelphia:
Saunders College P

man Body, 6th ed. Philadelphia:
Saunders College Publishing, 1984.)
920
Unit XIV
rates of secretion of the different hormones (except
for prolactin) fall to very low levels.
Secretion from the posterior pituitary is controlled
by nerve signals that originate in the hypothalamus
and terminate in the posterior pituitary. In contrast,
secretion by the anterior pituitary is controlled by hor-
mones called hypothalamic releasing and hypothala-
mic inhibitory hormones (o

eleasing and hypothala-
mic inhibitory hormones (or factors) secreted within
the hypothalamus itself and then conducted, as shown
in Figure 75–4, to the anterior pituitary through
minute blood vessels called hypothalamic-hypophysial
portal vessels. In the anterior pituitary, these releasing
and inhibitory hormones act on the glandular cells to
control their secretion. This system of control is dis-
cussed in the next section of this chapter.
The hypothalamus receives signals from many
sources in

hypothalamus receives signals from many
sources in the nervous system. Thus, when a person is
exposed to pain, a portion of the pain signal is trans-
mitted into the hypothalamus. Likewise, when a
person experiences some powerful depressing or excit-
ing thought, a portion of the signal is transmitted into
the hypothalamus. Olfactory stimuli denoting pleasant
or unpleasant smells transmit strong signal compo-
nents directly and through the amygdaloid nuclei into
the hypothalamus. Even the concen

loid nuclei into
the hypothalamus. Even the concentrations of nutri-
ents, electrolytes, water, and various hormones in the
blood excite or inhibit various portions of the hypo-
thalamus.Thus, the hypothalamus is a collecting center
for information concerning the internal well-being 
of the body, and much of this information is used 
to control secretions of the many globally important
Hypothalamic-Hypophysial Portal
Blood Vessels of the Anterior 
The anterior

sels of the Anterior 
The anterior pituitary is a highly vascular gland with
extensive capillary sinuses among the glandular cells.
Almost all the blood that enters these sinuses passes
ﬁrst through another capillary bed in the lower 
hypothalamus. The blood then ﬂows through small
Table 75–1
Cells and Hormones of the Anterior Pituitary Gland and Their Physiological Functions
Cell
Hormone
Chemistry
Physiological Actions
Somatotropes
Growth hormone (GH;
Single chain of 191 amino a

es
Single chain of 191 amino acids
Stimulates body growth; stimulates
somatotropin)
secretion of IGF-1; stimulates lipolysis;
inhibits actions of insulin on 
carbohydrate and lipid metabolism
Corticotropes
Adrenocorticotropic hormone
Single chain of 39 amino acids
Stimulates production of glucocorticoids 
(ACTH; corticotropin)
and androgens by the adrenal cortex;
maintains size of zona fasciculata and 
zona reticularis of cortex
Thyrotropes
Thyroid-stimulating hormone 
Glycop

ex
Glycoprotein of two subunits,
Stimulates production of thyroid 
(TSH; thyrotropin)
a (89 amino acids) and 
hormones by thyroid follicular cells;
b (112 amino acids)
maintains size of follicular cells
Follicle-stimulating hormone 
Stimulates development of ovarian 
(FSH)
follicles; regulates spermatogenesis in 
the testis
Glycoprotein of two s

tis
Causes ovulation and formation of the 
corpus luteum in the ovary; stimulates 
b (115 amino acids)
production of estrogen and progesterone
by the ovary; stimulates testosterone 
production by the testis
Lactotropes,
Single chain of 198 amino 
Stimulates milk secretion and production
Mammotropes
acids
IGF, insulin-like
Artery
plexus
Hypoth

ptic chiasm
Artery
plexus
hypophysial
Sinuses
gland
Vein
gland
Figure 75–4
Hypothalamic-hypophysial portal system.
Chapter 75
921
hypothalamic-hypophysial portal blood vessels into the
anterior pituitary sinuses. Figure 75–4 shows the low-
ermost portion of the hypothalamus, called the median
eminence, which connects inferiorly

led the median
eminence, which connects inferiorly with the pituitary
stalk. Small arteries penetrate into the substance 
of the median eminence and then additional small
vessels return to its surface, coalescing to form the
hypothalamic-hypophysial portal blood vessels. These
pass downward along the pituitary stalk to supply
blood to the anterior pituitary sinuses.
Hypothalamic Releasing and Inhibitory Hormones Are Secreted
into the Median Eminence. Special neurons in the hypo-
thalamus synthes

nce. Special neurons in the hypo-
thalamus synthesize and secrete the hypothalamic
releasing and inhibitory hormones that control secre-
tion of the anterior pituitary hormones.These neurons
originate in various parts of the hypothalamus and
send their nerve ﬁbers to the median eminence and
tuber cinereum, an extension of hypothalamic tissue
into the pituitary stalk.
The endings of these ﬁbers are different from most
endings in the central nervous system, in that their
function is not to transmi

s system, in that their
function is not to transmit signals from one neuron to
another but rather to secrete the hypothalamic releas-
ing and inhibitory hormones into the tissue ﬂuids.
These hormones are immediately absorbed into the
hypothalamic-hypophysial portal system and carried
directly to the sinuses of the anterior pituitary gland.
Hypothalamic Releasing and Inhibitory Hormones Control Ante-
rior Pituitary Secretion. The function of the releasing and
inhibitory hormones is to control sec

eleasing and
inhibitory hormones is to control secretion of the ante-
rior pituitary hormones. For most of the anterior pitu-
itary hormones, it is the releasing hormones that are
important, but for prolactin, a hypothalamic inhibitory
hormone probably exerts more control. The major
hypothalamic releasing and inhibitory hormones are
summarized in Table 75–2 and are the following:
1. Thyrotropin-releasing hormone (TRH), which
causes release of thyroid-stimulating hormone
2. Corticotropin-releasin

roid-stimulating hormone
2. Corticotropin-releasing hormone (CRH), which
causes release of adrenocorticotropin
3. Growth hormone–releasing hormone (GHRH),
which causes release of growth hormone, and
growth hormone inhibitory hormone (GHIH), also
called somatostatin, which inhibits release of
growth hormone
4. Gonadotropin-releasing hormone (GnRH), which
causes release of the two gonadotropic hormones,
luteinizing hormone and follicle-stimulating
hormone
5. Prolactin inhibitory hormone (PIH), whi

hormone
5. Prolactin inhibitory hormone (PIH), which causes
inhibition of prolactin secretion
There are some additional hypothalamic hormones,
including one that stimulates prolactin secretion and
perhaps others that inhibit release of the anterior pitu-
itary hormones. Each of the more important hypo-
thalamic hormones is discussed in detail as the speciﬁc
hormonal systems controlled by them are presented in
this and subsequent chapters.
Speciﬁc Areas in the Hypothalamus Control Secretion of Sp

 Areas in the Hypothalamus Control Secretion of Spe-
ciﬁc Hypothalamic Releasing and Inhibitory Hormones. All or
most of the hypothalamic hormones are secreted at
nerve endings in the median eminence before being
transported to the anterior pituitary gland. Electrical
stimulation of this region excites these nerve endings
and, therefore, causes release of essentially all the
hypothalamic hormones. However, the neuronal cell
bodies that give rise to these median eminence nerve
endings are located

to these median eminence nerve
endings are located in other discrete areas of the hypo-
thalamus or in closely related areas of the basal brain.
The speciﬁc loci of the neuronal cell bodies that form
the different hypothalamic releasing or inhibitory 
hormones are still poorly known, so that it would be
misleading to attempt delineation here.
Physiological Functions 
of Growth Hormone
All the major anterior pituitary hormones, except for
growth hormone, exert their principal effects by stim-
ula

ormone, exert their principal effects by stim-
ulating target glands, including thyroid gland, adrenal
cortex, ovaries, testicles, and mammary glands. The
functions of each of these pituitary hormones are so
intimately concerned with the functions of the respec-
tive target glands that, except for growth hormone,
their functions are discussed in subsequent chapters
along with the target glands. Growth hormone, in 
Table 75–2
Hypothalamic Releasing and Inhibitory Hormones That Control Secretion o

g and Inhibitory Hormones That Control Secretion of the Anterior Pituitary Gland
Hormone
Structure
Primary Action on Anterior Pituitary
Peptide of 3 amino acids
Stimulates secretion of TSH by thyrotropes
Single chain of 10 amino acids
Stimulates secretion of FSH and LH by gonadotropes
Single chain of 41 amino acids
Stimulates secretion of ACTH by corticotropes
Growth hormone–releasing 

of ACTH by corticotropes
Growth hormone–releasing hormone 
Single chain of 44 amino acids
Stimulates secretion of growth hormone by somatotropes
(GHRH)
Growth hormone inhibitory hormone 
Single chain of 14 amino acids
Inhibits secretion of growth hormone by somatotropes
Prolactin-inhibiting hormone (PIH)
Dopamine (a catecholamine)
Inhibits secretion of prolactin by lactotropes
ACTH, adrenocorticotropic hormone; FSH, follicle-stimulating hormone; LH, luteinizing hormone; TSH, thyro

ating hormone; LH, luteinizing hormone; TSH, thyroid-stimulating hormone.
922
Unit XIV
contrast to other hormones, does not function through
a target gland but exerts its effects directly on all or
almost all tissues of the body.
Growth Hormone Promotes Growth 
of Many Body Tissues
Growth hormone, also called somatotropic hormone
or somatotropin, is a small protein molecule that con-
tains 191 amino acids in a single chain and has a
molecular weight of 22,005. It c

e chain and has a
molecular weight of 22,005. It causes growth of almost
all tissues of the body that are capable of growing. It 
promotes increased sizes of the cells and increased
mitosis, with development of greater numbers of cells
and speciﬁc differentiation of certain types of cells
such as bone growth cells and early muscle cells.
Figure 75–5 shows typical weight charts of two
growing littermate rats, one of which received daily
injections of growth hormone and the other of which
did not 

 of growth hormone and the other of which
did not receive growth hormone. This ﬁgure shows
marked enhancement of growth in the rat given
growth hormone—in the early days of life and even
after the two rats reached adulthood. In the early
stages of development, all organs of the treated rat
increased proportionately in size; after adulthood was
reached, most of the bones stopped lengthening, but
many of the soft tissues continued to grow.This results
from the fact that once the epiphyses of the l

lts
from the fact that once the epiphyses of the long bones
have united with the shafts, further lengthening of
bone cannot occur, even though most other tissues of
the body can continue to grow throughout life.
Growth Hormone Has Several
Metabolic Effects
Aside from its general effect in causing growth, growth
hormone has multiple speciﬁc metabolic effects,
including (1) increased rate of protein synthesis in
most cells of the body; (2) increased mobilization of
fatty acids from adipose tissue,

d mobilization of
fatty acids from adipose tissue, increased free fatty
acids in the blood, and increased use of fatty acids for
energy; and (3) decreased rate of glucose utilization
throughout the body. Thus, in effect, growth hormone
enhances body protein, uses up fat stores, and con-
serves carbohydrates.
Growth Hormone Promotes Protein Deposition
in Tissues
Although the precise mechanisms by which growth
hormone increases protein deposition are not known,
a series of different effects are kn

re not known,
a series of different effects are known, all of which
could lead to enhanced protein deposition.
Enhancement of Amino Acid Transport Through the Cell Mem-
branes. Growth hormone directly enhances transport
of at least some and perhaps most amino acids through
the cell membranes to the interior of the cells. This
increases the amino acid concentrations in the cells
and is presumed to be at least partly responsible for
the increased protein synthesis. This control of amino
acid trans

rotein synthesis. This control of amino
acid transport is similar to the effect of insulin in con-
trolling glucose transport through the membrane, as
discussed in Chapters 67 and 78.
Enhancement of RNA Translation to Cause Protein Synthesis by
the Ribosomes. Even when the amino acid concentra-
tions are not increased in the cells, growth hormone
still increases RNA translation, causing protein to be
synthesized in greater amounts by the ribosomes in the
Increased Nuclear Transcriptio

s in the
Increased Nuclear Transcription of DNA to Form RNA. Over
more prolonged periods (24 to 48 hours), growth
hormone also stimulates the transcription of DNA in
the nucleus, causing the formation of increased quan-
tities of RNA. This promotes more protein synthesis
and promotes growth if sufﬁcient energy, amino acids,
vitamins, and other requisites for growth are available.
In the long run, this may be the most important func-
tion of growth hormone.
Decreased Catabolism of Prot

on of growth hormone.
Decreased Catabolism of Protein and Amino Acids. In addi-
tion to the increase in protein synthesis, there is a
decrease in the breakdown of cell protein. A probable
reason for this is that growth hormone also mobilizes
large quantities of free fatty acids from the adipose
tissue, and these are used to supply most of the energy
for the body’s cells, thus acting as a potent “protein
sparer.”
Summary. Growth hormone enhances almost all facets
of amino acid uptake and protein 

lmost all facets
of amino acid uptake and protein synthesis by cells,
while at the same time reducing the breakdown of 
proteins.
Growth Hormone Enhances Fat Utilization 
for Energy
Growth hormone has a speciﬁc effect in causing 
the release of fatty acids from adipose tissue and,
Control
Injected daily with
500
400
300
200
200
0
0
600
500
400
300
200
100
Body weight (grams)
Days
Figure 75–5
Comparison of weight gain of a rat injected daily with growth
hormone with that of a norma

ted daily with growth
hormone with that of a normal littermate.
Chapter 75
923
therefore, increasing the concentration of fatty acids in
the body ﬂuids. In addition, in tissues throughout the
body, growth hormone enhances the conversion of
fatty acids to acetyl coenzyme A (acetyl-CoA) and its 
subsequent utilization for energy. Therefore, under 
the inﬂuence of growth hormone, fat is used for 
energy in preference to the use of carbohydrat

or 
energy in preference to the use of carbohydrates and
proteins.
Growth hormone’s ability to promote fat utilization,
together with its protein anabolic effect, causes an
increase in lean body mass. However, mobilization of
fat by growth hormone requires several hours to occur,
whereas enhancement of protein synthesis can begin
in minutes under the inﬂuence of growth hormone.
“Ketogenic” Effect of Growth Hormone. Under the inﬂu-
ence of excessive amounts of growth hormone, fat
mobilization fro

ve amounts of growth hormone, fat
mobilization from adipose tissue sometimes becomes
so great that large quantities of acetoacetic acid are
formed by the liver and released into the body ﬂuids,
thus causing ketosis. This excessive mobilization of fat
from the adipose tissue also frequently causes a fatty
liver.
Growth Hormone Decreases 
Carbohydrate Utilization
Growth hormone causes multiple effects that 
inﬂuence carbohydrate metabolism, including (1)
decreased glucose uptake in tissues such as

ng (1)
decreased glucose uptake in tissues such as skeletal
muscle and fat, (2) increased glucose production by
the liver, and (3) increased insulin secretion.
Each of these changes results from growth
hormone–induced “insulin resistance,” which attenu-
ates insulin’s actions to stimulate the uptake and uti-
lization of glucose in skeletal muscle and fat and to
inhibit gluconeogenesis (glucose production) by the
liver; this leads to increased blood glucose concentra-
tion and a compensatory incr

od glucose concentra-
tion and a compensatory increase in insulin secretion.
For these reasons, growth hormone’s effects are called
diabetogenic, and excess secretion of growth hormone
can produce metabolic disturbances very similar to
those found in patients with type II (non-insulin-
dependent) diabetes, who are also very resistant to the
metabolic effects of insulin.
We do not know the precise mechanism by which
growth hormone causes insulin resistance and
decreased glucose utilization by the

esistance and
decreased glucose utilization by the cells. However,
growth hormone–induced increases in blood concen-
trations of fatty acids may impair insulin’s actions on
tissue glucose utilization. Experimental studies indi-
cate that raising blood levels of fatty acids above
normal rapidly decreases the sensitivity of the liver
and skeletal muscle to insulin’s effects on carbohy-
drate metabolism.
Necessity of Insulin and Carbohydrate for the Growth-
Promoting Action of Growth Hormone. Growt

 Growth-
Promoting Action of Growth Hormone. Growth hormone
fails to cause growth in an animal that lacks a pan-
creas; it also fails to cause growth if carbohydrates are
excluded from the diet. This shows that adequate
insulin activity and adequate availability of carbohy-
drates are necessary for growth hormone to be effec-
tive. Part of this requirement for carbohydrates and
insulin is to provide the energy needed for the metab-
olism of growth, but there seem to be other effects as
well. Esp

h, but there seem to be other effects as
well. Especially important is insulin’s ability to
enhance the transport of some amino acids into cells,
in the same way that it stimulates glucose transport.
Growth Hormone Stimulates Cartilage
and Bone Growth
Although growth hormone stimulates increased de-
position of protein and increased growth in almost 
all tissues of the body, its most obvious effect is to
increase growth of the skeletal frame.This results from
multiple effects of growth hormone o

 results from
multiple effects of growth hormone on bone, including
(1) increased deposition of protein by the chondro-
cytic and osteogenic cells that cause bone growth,
(2) increased rate of reproduction of these cells,
and (3) a speciﬁc effect of converting chondrocytes 
into osteogenic cells, thus causing deposition of new
bone.
There are two principal mechanisms of bone
growth. First, in response to growth hormone stimula-
tion, the long bones grow in length at the epiphyseal
cartilages, wh

es grow in length at the epiphyseal
cartilages, where the epiphyses at the ends of the bone
are separated from the shaft. This growth ﬁrst causes
deposition of new cartilage, followed by its conversion
into new bone, thus elongating the shaft and pushing
the epiphyses farther and farther apart. At the same
time, the epiphyseal cartilage itself is progressively
used up, so that by late adolescence, no additional epi-
physeal cartilage remains to provide for further long
bone growth.At this time, 

rovide for further long
bone growth.At this time, bony fusion occurs between
the shaft and the epiphysis at each end, so that no
further lengthening of the long bone can occur.
Second, osteoblasts in the bone periosteum and in
some bone cavities deposit new bone on the surfaces
of older bone. Simultaneously, osteoclasts in the bone
(discussed in detail in Chapter 79) remove old bone.
When the rate of deposition is greater than that of
resorption, the thickness of the bone increases. Growth
hormo

 the thickness of the bone increases. Growth
hormone strongly stimulates osteoblasts. Therefore, the
bones can continue to become thicker throughout life
under the inﬂuence of growth hormone; this is espe-
cially true for the membranous bones. For instance, the
jaw bones can be stimulated to grow even after ado-
lescence, causing forward protrusion of the chin and
lower teeth. Likewise, the bones of the skull can grow
in thickness and give rise to bony protrusions over the
eyes.
Growth Hormone E

o bony protrusions over the
eyes.
Growth Hormone Exerts Much 
of Its Effect Through Intermediate
Substances Called “Somatomedins”
(Also Called “Insulin-Like 
When growth hormone is supplied directly to cartilage
chondrocytes cultured outside the body, proliferation
924
Unit XIV
or enlargement of the chondrocytes usually fails to
occur. Yet growth hormone injected into the intact
animal does cause proliferation and growth of the
same cells.
In brief

oliferation and growth of the
In brief, it has been found that growth hormone
causes the liver (and, to a much less extent,
other tissues) to form several small proteins 
called somatomedins that have the potent effect of
increasing all aspects of bone growth. Many of the
somatomedin effects on growth are similar to the
effects of insulin on growth. Therefore, the somato-
medins are also called insulin-like growth factors
(IGFs).
At least four somatomedins have been isolated,
but by 

east four somatomedins have been isolated,
but by far the most important of these is somatomedin
C (also called IGF-I). The molecular weight of
somatomedin C is about 7500, and its concentration in
the plasma closely follows the rate of growth hormone
The pygmies of Africa have a congenital inability to
synthesize signiﬁcant amounts of somatomedin C.
Therefore, even though their plasma concentration of
growth hormone is either normal or high, they have
diminished amounts of somatomedi

r high, they have
diminished amounts of somatomedin C in the plasma;
this apparently accounts for the small stature of these
people. Some other dwarfs (e.g., the Lévi-Lorain
dwarf) also have this problem.
It has been postulated that most, if not all, of the
growth effects of growth hormone result from
somatomedin C and other somatomedins, rather 
than from direct effects of growth hormone on the
bones and other peripheral tissues. Even so, experi-
ments have demonstrated that injection of growth

-
hormone directly into the epiphyseal cartilages of
bones of living animals causes the speciﬁc growth 
of these cartilage areas, and the amount of growth
hormone required for this is minute. Some aspects 
of the somatomedin hypothesis are still questionable.
One possibility is that growth hormone can cause 
the formation of enough somatomedin C in the 
local tissue to cause local growth. It is also possible
that growth hormone itself is directly 

o possible
that growth hormone itself is directly responsible 
for increased growth in some tissues and that the
somatomedin mechanism is an alternative means of
increasing growth but not always a necessary 
one.
Short Duration of Action of Growth Hormone but Prolonged
Action of Somatomedin C. Growth hormone attaches only
weakly to the plasma proteins in the blood.Therefore,
it is released from the blood into the tissues rapidly,
having a half-time in the blood of less than 20 minutes.
By contra

me in the blood of less than 20 minutes.
By contrast, somatomedin C attaches strongly to a
carrier protein in the blood that, like somatomedin C,
is produced in response to growth hormone. As a
result, somatomedin C is released only slowly from the
blood to the tissues, with a half-time of about 20 hours.
This greatly prolongs the growth-promoting effects of
the bursts of growth hormone secretion shown in
Figure 75–6.
Regulation of Growth 
For many years it was believed that gr

e Secretion
For many years it was believed that growth hormone
was secreted primarily during the period of growth but
then disappeared from the blood at adolescence. This
has proved to be untrue. After adolescence, secretion
decreases slowly with aging, ﬁnally falling to about 25
per cent of the adolescent level in very old age.
Growth hormone is secreted in a pulsatile pattern,
increasing and decreasing. The precise mechanisms
that control secretion of growth hormone are not fully
understood, b

tion of growth hormone are not fully
understood, but several factors related to a person’s
state of nutrition or stress are known to stimulate
secretion: (1) starvation, especially with severe protein
deﬁciency; (2) hypoglycemia or low concentration of
fatty acids in the blood; (3) exercise; (4) excitement; and
(5) trauma. Growth hormone also characteristically
increases during the ﬁrst 2 hours of deep sleep, as
shown in Figure 75–6. Table 75–3 summarizes some of
the factors that are known to in

ummarizes some of
the factors that are known to inﬂuence growth
hormone secretion.
The normal concentration of growth hormone in the
plasma of an adult is between 1.6 and 3 ng/ml; in a
child or adolescent, it is about 6 ng/ml. These values
often increase to as high as 50 ng/ml after depletion of
the body stores of proteins or carbohydrates during
prolonged starvation.
Under acute conditions, hypoglycemia is a far more
potent stimulator of growth hormone secretion than is
an acute decrease in pro

hormone secretion than is
an acute decrease in protein intake. Conversely, in
chronic conditions, growth hormone secretion seems
to correlate more with the degree of cellular protein
depletion than with the degree of glucose insufﬁciency.
For instance, the extremely high levels of growth
hormone that occur during starvation are closely
related to the amount of protein depletion.
30
20
10
0
8 am
4 am
8 pm
4 am
12
Noon
12
Midnight
8 am
(ng/ml plasma)
Strenuous
exercise
Sleep
Figure 

ne
Strenuous
exercise
Sleep
Figure 75–6
Typical variations in growth hormone secretion throughout the day,
demonstrating the especially powerful effect of strenuous exercise
and also the high rate of growth hormone secretion that occurs
during the ﬁrst few hours of deep sleep.
Chapter 75
925
Figure 75–7 demonstrates the effect of protein deﬁ-
ciency on plasma growth hormone and then the effect
of adding protein to the diet. 

nd then the effect
of adding protein to the diet. The ﬁrst column shows
very high levels of growth hormone in children with
extreme protein deﬁciency during the protein malnu-
trition condition called kwashiorkor; the second
column shows the levels in the same children after 3
days of treatment with more than adequate quantities
of carbohydrates in their diets, demonstrating that 
the carbohydrates did not lower the plasma growth
hormone concentration. The third and fourth columns
show the level

ation. The third and fourth columns
show the levels after treatment with protein supple-
ments for 3 and 25 days, respectively, with a concomi-
tant decrease in the hormone.
These results demonstrate that under severe condi-
tions of protein malnutrition, adequate calories alone
are not sufﬁcient to correct the excess production of
growth hormone. The protein deﬁciency must also be
corrected before the growth hormone concentration
will return to normal.
Role of the Hypothalamus, Growth
Hormone–R

normal.
Hormone–Releasing Hormone, and
Somatostatin in the Control of Growth
From the preceding description of the many factors
that can affect growth hormone secretion, one can
readily understand the perplexity of physiologists as
they attempt to unravel the mysteries of regulation of
growth hormone secretion. It is known that growth
hormone secretion is controlled by two factors
secreted in the hypothalamus and then transported to
the anterior

 hypothalamus and then transported to
the anterior pituitary gland through the hypothalamic-
hypophysial 
portal 
vessels.
They 
are 
growth
hormone–releasing hormone and growth hormone
inhibitory hormone (also called somatostatin). Both of
these are polypeptides; GHRH is composed of 44
amino acids, and somatostatin is composed of 14
amino acids.
The part of the hypothalamus that causes secretion
of GHRH is the ventromedial nucleus; this is the 
same area of the hypothalamus that is sensitive to

blood glucose concentration, causing satiety in hyper-
glycemic states and hunger in hypoglycemic states.The
secretion of somatostatin is controlled by other nearby
areas of the hypothalamus. Therefore, it is reasonable
to believe that some of the same signals that modify a
person’s behavioral feeding instincts also alter the rate
of growth hormone secretion.
In a similar manner, hypothalamic signals depicting
emotions, stress, and trauma can al

nals depicting
emotions, stress, and trauma can all affect hypothala-
mic control of growth hormone secretion. In fact,
experiments 
have 
shown 
that 
catecholamines,
dopamine, and serotonin, each of which is released by
a different neuronal system in the hypothalamus, all
increase the rate of growth hormone secretion.
Most of the control of growth hormone secretion 
is probably mediated through GHRH rather than
through the inhibitory hormone somatostatin. GHRH
stimulates growth hormone secreti

matostatin. GHRH
stimulates growth hormone secretion by attaching to
speciﬁc cell membrane receptors on the outer surfaces
of the growth hormone cells in the pituitary gland.The
receptors activate the adenylyl cyclase system inside
the cell membrane, increasing the intracellular level of
cyclic adenosine monophosphate (cAMP). This has
both a short-term and a long-term effect. The short-
term effect is to increase calcium ion transport into the
cell; within minutes, this causes fusion of the grow

ll; within minutes, this causes fusion of the growth
hormone secretory vesicles with the cell membrane
and release of the hormone into the blood. The long-
term effect is to increase transcription in the nucleus
Table 75–3
Factors That Stimulate or Inhibit Secretion of 
Stimulate Growth Hormone 
Inhibit Growth Hormone 
Secretion
Secretion
Decreased blood glucose
Increased blood glucose
Decreased blood free fatty 
Increased blood free fatty 
acids
acids
Starvation or fasting, prote

ee fatty 
acids
acids
Starvation or fasting, protein 
Aging
deﬁciency
Obesity
Trauma, stress, excitement
Growth hormone inhibitory 
Exercise
hormone (somatostatin)
Testosterone, estrogen
Growth hormone (exogenous)
Deep sleep ( stages II and IV)
Somatomedins (insulin-like
Growth hormone–releasing
growth factors)
hormone
40
30
20
10
0
Protein
(kwashiorkor)
Protein
treatment
(25 days)
Protein
treatment
(3 days)
treatment
(3 days)
Plasma growth hormone (ng/ml)
Figure 75–7
Eff

ays)
Effect of extreme protein deﬁciency on the plasma concentration
of growth hormone in the disease kwashiorkor. Also shown is the
failure of carbohydrate treatment but the effectiveness of protein
treatment in lowering growth hormone concentration. (Drawn from
data in Pimstone BL, Barbezat G, Hansen JD, Murray P: Studies
on growth hormone secretion in protein-calorie malnutrition. Am J
Clin Nutr 21:482, 1968.)
926
Unit XIV
Endocrinology and Reproducti

, 1968.)
926
Unit XIV
by the genes to stimulate the synthesis of new growth
hormone.
When growth hormone is administered directly into
the blood of an animal over a period of hours, the rate
of endogenous growth hormone secretion decreases.
This demonstrates that growth hormone secretion is
subject to typical negative feedback control, as is true
for essentially all hormones. The nature of this feed-
back mechanism and whether it is mediated mainly
through inhibiti

and whether it is mediated mainly
through inhibition of GHRH or enhancement of
somatostatin, which inhibits growth hormone secre-
tion, are uncertain.
In summary, our knowledge of the regulation of
growth hormone secretion is not sufﬁcient to describe
a composite picture.Yet, because of the extreme secre-
tion of growth hormone during starvation and its
important long-term effect to promote protein syn-
thesis and tissue growth, we can propose the follow-
ing: the major long-term controller of g

e follow-
ing: the major long-term controller of growth hormone
secretion is the long-term state of nutrition of the
tissues themselves, especially their level of protein
nutrition.That is, nutritional deﬁciency or excess tissue
need for cellular proteins—for instance, after a severe
bout of exercise when the muscles’ nutritional status
has been taxed—in some way increases the rate of
growth hormone secretion. Growth hormone, in turn,
promotes synthesis of new proteins while at the same
time con

nthesis of new proteins while at the same
time conserving the proteins already present in the
cells.
Abnormalities of Growth 
Panhypopituitarism. This term means decreased secretion
of all the anterior pituitary hormones. The decrease in
secretion may be congenital (present from birth), or it
may occur suddenly or slowly at any time during life,
most often resulting from a pituitary tumor that
destroys the pituitary gland.
Dwarﬁsm. Most instances of dwarﬁsm result from gener-
a

sm. Most instances of dwarﬁsm result from gener-
alized deﬁciency of anterior pituitary secretion (panhy-
popituitarism) during childhood. In general, all the
physical parts of the body develop in appropriate pro-
portion to one another, but the rate of development is
greatly decreased. A child who has reached the age of
10 years may have the bodily development of a child
aged 4 to 5 years, and the same person at age 20 years
may have the bodily development of a child aged 7 to
10 years.
A perso

development of a child aged 7 to
10 years.
A person with panhypopituitary dwarﬁsm does not
pass through puberty and never secretes sufﬁcient
quantities of gonadotropic hormones to develop adult
sexual functions. In one third of such dwarfs, however,
only growth hormone is deﬁcient; these persons do
mature sexually and occasionally reproduce. In one type
of dwarﬁsm (the African pygmy and the Lévi-Lorain
dwarf), the rate of growth hormone secretion is normal
or high, but there is a hereditary inab

 is normal
or high, but there is a hereditary inability to form
somatomedin C, which is a key step for the promotion
of growth by growth hormone.
Treatment with Human Growth Hormone. Growth hor-
mones from different species of animals are sufﬁciently
different from one another that they will cause growth
only in the one species or, at most, closely related
species. For this reason, growth hormone prepared from
lower animals (except, to some extent, from primates)
is not effective in human beings

t, from primates)
is not effective in human beings. Therefore, the growth
hormone of the human being is called human growth
hormone to distinguish it from the others.
In the past, because growth hormone had to be pre-
pared from human pituitary glands, it was difﬁcult to
obtain sufﬁcient quantities to treat patients with growth
hormone deﬁciency, except on an experimental basis.
However, human growth hormone can now be synthe-
sized by Escherichia coli bacteria as a result of success-
ful applic

a coli bacteria as a result of success-
ful application of recombinant DNA technology.
Therefore, this hormone is now available in sufﬁcient
quantities for treatment purposes. Dwarfs who have
pure growth hormone deﬁciency can be completely
cured if treated early in life. Human growth hormone
may prove to be beneﬁcial in other metabolic disorders
because of its widespread metabolic functions.
Panhypopituitarism in the Adult. Panhypopituitarism ﬁrst
occurring in adulthood frequently results from o

t
occurring in adulthood frequently results from one of
three common abnormalities.Two tumorous conditions,
craniopharyngiomas or chromophobe tumors, may
compress the pituitary gland until the functioning 
anterior pituitary cells are totally or almost totally
destroyed.The third cause is thrombosis of the pituitary
blood vessels. This abnormality occasionally occurs
when a new mother develops circulatory shock after the
birth of her baby.
The general effects of adult panhypopituitarism are
(1) 

neral effects of adult panhypopituitarism are
(1) hypothyroidism, (2) depressed production of gluco-
corticoids by the adrenal glands, and (3) suppressed
secretion of the gonadotropic hormones so that sexual
functions are lost. Thus, the picture is that of a lethargic
person (from lack of thyroid hormones) who is gaining
weight (because of lack of fat mobilization by growth,
adrenocorticotropic, adrenocortical, and thyroid hor-
mones) and has lost all sexual functions. Except for the
abnormal se

t all sexual functions. Except for the
abnormal sexual functions, the patient can usually be
treated satisfactorily by administering adrenocortical
and thyroid hormones.
Gigantism. Occasionally,
the 
acidophilic,
growth
hormone–producing cells of the anterior pituitary gland
become excessively active, and sometimes even aci-
dophilic tumors occur in the gland. As a result, large
quantities of growth hormone are produced. All body
tissues grow rapidly, including the bones. If the condi-
tion occu

idly, including the bones. If the condi-
tion occurs before adolescence, before the epiphyses of
the long bones have become fused with the shafts,
height increases so that the person becomes a giant—
up to 8 feet tall.
The giant ordinarily has hyperglycemia, and the beta
cells of the islets of Langerhans in the pancreas are
prone to degenerate because they become overactive
owing to the hyperglycemia. Consequently, in about 10
per cent of giants, full-blown diabetes mellitus eventu-
ally develop

 full-blown diabetes mellitus eventu-
ally develops.
In most giants, panhypopituitarism eventually devel-
ops if they remain untreated, because the gigantism is
usually caused by a tumor of the pituitary gland that
grows until the gland itself is destroyed. This eventual
general deﬁciency of pituitary hormones usually causes
death in early adulthood. However, once gigantism is
diagnosed, further effects can often be blocked by
microsurgical removal of the tumor or by irradiation of
the pituitary

al of the tumor or by irradiation of
the pituitary gland.
Chapter 75
927
Acromegaly. If an acidophilic tumor occurs after adoles-
cence—that is, after the epiphyses of the long bones
have fused with the shafts—the person cannot grow
taller, but the bones can become thicker and the soft
tissues can continue to grow. This condition, shown in
Figure 75–8, is known as acromegaly. Enlargement is
especially marked in the bones of the hands and f

especially marked in the bones of the hands and feet
and in the membranous bones, including the cranium,
nose, bosses on the forehead, supraorbital ridges, lower
jawbone, and portions of the vertebrae, because their
growth does not cease at adolescence. Consequently,
the lower jaw protrudes forward, sometimes as much as
half an inch, the forehead slants forward because of
excess development of the supraorbital ridges, the 
nose increases to as much as twice normal size, the feet
require size 14

uch as twice normal size, the feet
require size 14 or larger shoes, and the ﬁngers become
extremely thickened so that the hands are almost twice
normal size. In addition to these effects, changes in the
vertebrae ordinarily cause a hunched back, which is
known clinically as kyphosis. Finally, many soft tissue
organs, such as the tongue, the liver, and especially the
kidneys, become greatly enlarged.
Possible Role of Decreased Growth Hormone Secretion 
in Causing Changes Associated with Aging
In 

tion 
In people who have lost the ability to secrete growth
hormone, some features of the aging process accelerate.
For instance, a 50-year-old person who has been without
growth hormone for many years may have the appear-
ance of a person aged 65. The aged appearance seems
to result mainly from decreased protein deposition in
most tissues of the body and increased fat deposition in
its place. The physical and physiological effects are
increased wrinkling

 and physiological effects are
increased wrinkling of the skin, diminished rates of
function of some of the organs, and diminished muscle
mass and strength.
As one ages, the average plasma concentration of
growth hormone in an otherwise normal person
changes approximately as follows:
Thus, it is highly possible that some of the normal
aging effects result from diminished growth hormone
secretion. In fact, multiple tests of growth hormone
therapy in older people have demonstrated three
important 

in older people have demonstrated three
important effects that suggest antiaging actions: (1)
increased protein deposition in the body, especially in
the muscles; (2) decreased fat deposits; and (3) a feeling
of increased energy.
Posterior Pituitary Gland 
and Its Relation to the
The posterior pituitary gland, also called the neurohy-
pophysis, is composed mainly of glial-like cells called
pituicytes.The pituicytes do not secrete hormones;they
act simply as a supporting structure fo

mones;they
act simply as a supporting structure for large numbers
of terminal nerve ﬁbers and terminal nerve endings
from nerve tracts that originate in the supraoptic and
paraventricular nuclei of the hypothalamus, as shown
in Figure 75–9. These tracts pass to the neurohypoph-
ysis through the pituitary stalk (hypophysial stalk).The
nerve endings are bulbous knobs that contain many
secretory granules.These endings lie on the surfaces of
capillaries, where they secrete two posterior pituitary
ho

ies, where they secrete two posterior pituitary
hormones: (1) antidiuretic hormone (ADH), also
called vasopressin, and (2) oxytocin.
Figure 75–8
Acromegalic patient.
ng/ml
5 to 20 years
6
20 to 40 years
3
40 to 70 years
1.6
928
Unit XIV
If the pituitary stalk is cut above the pituitary gland
but the entire hypothalamus is left intact, the posterior
pituitary hormones continue to be secreted normally,
after a transient decrease for a few days; they are then
secreted

nt decrease for a few days; they are then
secreted by the cut ends of the ﬁbers within the hypo-
thalamus and not by the nerve endings in the poste-
rior pituitary. The reason for this is that the hormones
are initially synthesized in the cell bodies of the
supraoptic and paraventricular nuclei and are then
transported in combination with “carrier” proteins
called neurophysins down to the nerve endings in the
posterior pituitary gland, requiring several days to
reach the gland.
ADH is formed pri

several days to
ADH is formed primarily in the supraoptic nuclei,
whereas oxytocin is formed primarily in the paraven-
tricular nuclei. Each of these nuclei can synthesize
about one sixth as much of the second hormone as of
its primary hormone.
When nerve impulses are transmitted downward
along the ﬁbers from the supraoptic or paraventri-
cular nuclei, the hormone is immediately released
from the secretory granules in the nerve endings by
the usual secretory mechanism of exocyto

ndings by
the usual secretory mechanism of exocytosis and is
absorbed into adjacent capillaries. Both the neuro-
physin and the hormone are secreted together, but
because they are only loosely bound to each other, the
hormone separates almost immediately. The neuro-
physin has no known function after leaving the nerve
terminals.
Chemical Structures of ADH 
and Oxytocin
Both oxytocin and ADH (vasopressin) are polypep-
tides, each containing nine amino acids. Their amino
acid sequences are the fol

mino acids. Their amino
acid sequences are the following:
Vasopressin: Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Arg-
GlyNH2
Oxytocin: Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-GlyNH2
Note that these two hormones are almost identical
except that in vasopressin, phenylalanine and arginine
replace isoleucine and leucine of the oxytocin molecule.
The similarity of the molecules explains their partial
functional similarities.
The injection of extremely minute quantities of
ADH—as small as 2 n

extremely minute quantities of
ADH—as small as 2 nanograms—can cause decreased
excretion of water by the kidneys (antidiuresis). This
antidiuretic effect is discussed in detail in Chapter 28.
Brieﬂy, in the absence of ADH, the collecting tubules
and ducts become almost impermeable to water, which
prevents signiﬁcant reabsorption of water and there-
fore allows extreme loss of water into the urine, also
causing extreme dilution of the urine. Conversely, in
the presence of ADH, the permeability of

rsely, in
the presence of ADH, the permeability of the collect-
ing ducts and tubules to water increases greatly and
allows most of the water to be reabsorbed as the
tubular ﬂuid passes through these ducts, thereby con-
serving water in the body and producing very concen-
trated urine.
The precise mechanism by which ADH acts on the
collecting ducts to increase their permeability is only
partially known. Without ADH, the luminal mem-
branes of the tubular epithelial cells of the collecting
ducts 

 tubular epithelial cells of the collecting
ducts are almost impermeable to water. However,
immediately inside the cell membrane are a large
number of special vesicles that have highly water-
permeable pores called aquaporins. When ADH acts
on the cell, it ﬁrst combines with membrane receptors
that activate adenylyl cyclase and cause the formation
of cAMP inside the tubular cell cytoplasm.This causes
phosphorylation of elements in the special vesicles,
which then causes the vesicles to insert in

icles,
which then causes the vesicles to insert into the apical
cell membranes, thus providing many areas of high
water permeability. All this occurs within 5 to 10
minutes. Then, in the absence of ADH, the entire
process reverses in another 5 to 10 minutes. Thus, this
process temporarily provides many new pores that
allow free diffusion of water from the tubular ﬂuid
through the tubular epithelial cells and into the renal
interstitial ﬂuid. Water is then absorbed from the col-
lecting tubules a

r is then absorbed from the col-
lecting tubules and ducts by osmosis, as explained in
Chapter 28 in relation to the urine-concentrating
mechanism of the kidneys.
Regulation of ADH Production
Osmotic Regulation. When a concentrated electrolyte
solution is injected into the artery that supplies the
hypothalamus,the ADH neurons in the supraoptic and
paraventricular nuclei immediately transmit impulses
into the posterior pituitary to release large quantities
of ADH into the circulating blood, somet

uantities
of ADH into the circulating blood, sometimes increas-
ing the ADH secretion to as high as 20 times normal.
Conversely, injection of a dilute solution into this
artery causes cessation of the impulses and therefore
almost total cessation of ADH secretion.Thus,the con-
centration of ADH in the body ﬂuids can change from
small amounts to large amounts, or vice versa, in only
a few minutes.
tract
nucleus
Posterior pituita

optic
nucleus
nucleus
Figure 75–9
Hypothalamic control of the posterior pituitary.
Chapter 75
929
The precise way that the osmotic concentration of
the extracellular ﬂuid controls ADH secretion is not
clear. Yet somewhere in or near the hypothalamus 
are modiﬁed neuron receptors called osmoreceptors.
When the extracellular ﬂuid becomes too concen-
trated, ﬂuid is pulled 

r ﬂuid becomes too concen-
trated, ﬂuid is pulled by osmosis out of the osmore-
ceptor cell,
decreasing its size and initiating
appropriate nerve signals in the hypothalamus to
cause additional ADH secretion. Conversely, when the
extracellular ﬂuid becomes too dilute, water moves by
osmosis in the opposite direction, into the cell, and this
decreases the signal for ADH secretion. Although
some researchers place these osmoreceptors in the
hypothalamus itself (possibly even in the supraoptic
nucle

amus itself (possibly even in the supraoptic
nuclei), others believe that they are located in the
organum vasculosum, a highly vascular structure in the
anteroventral wall of the third ventricle.
Regardless of the mechanism, concentrated body
ﬂuids stimulate the supraoptic nuclei, whereas dilute
body ﬂuids inhibit them. A feedback control system is
available to control the total osmotic pressure of the
Further details on the role of ADH in controlling
renal function and body ﬂuid osm

DH in controlling
renal function and body ﬂuid osmolality are presented
in Chapter 28.
Vasoconstrictor and Pressor Effects of ADH,
and Increased ADH Secretion Caused by Low
Whereas minute concentrations of ADH cause
increased water conservation by the kidneys, higher
concentrations of ADH have a potent effect of con-
stricting the arterioles throughout the body and there-
fore increasing the arterial pressure. For this reason,
ADH has another name, vasopressin.
One of the stimuli fo

s another name, vasopressin.
One of the stimuli for causing intense ADH secre-
tion is decreased blood volume. This occurs especially
strongly when the blood volume decreases 15 to 25 per
cent or more; the secretory rate then sometimes rises
to as high as 50 times normal. The cause of this is the
The atria have stretch receptors that are excited by
overﬁlling.When excited, they send signals to the brain
to inhibit ADH secretion. Conversely, when the recep-
tors are unexcited as a resu

sely, when the recep-
tors are unexcited as a result of underﬁlling, the oppo-
site occurs, with greatly increased ADH secretion.
Decreased stretch of the baroreceptors of the carotid,
aortic, and pulmonary regions also stimulates ADH
secretion. For further details about this blood volume-
pressure feedback mechanism, refer to Chapter 28.
Oxytocin Causes Contraction of the Pregnant Uterus. The
hormone oxytocin, in accordance with its name, pow-
erfully stimulates contraction of 

 its name, pow-
erfully stimulates contraction of the pregnant uterus,
especially toward the end of gestation. Therefore,
many obstetricians believe that this hormone is at least
partially responsible for causing birth of the baby.This
is supported by the following facts: (1) In a hypophy-
sectomized animal, the duration of labor is prolonged,
indicating a possible effect of oxytocin during delivery.
(2) The amount of oxytocin in the plasma increases
during labor, especially during the last stag

ases
during labor, especially during the last stage. (3) Stim-
ulation of the cervix in a pregnant animal elicits
nervous signals that pass to the hypothalamus and
cause increased secretion of oxytocin. These effects
and this possible mechanism for aiding in the birth
process are discussed in much more detail in Chapter
82.
Oxytocin Aids in Milk Ejection by the Breasts. Oxytocin also
plays an especially important role in lactation—a role
that is far better understood than its role in delivery.
I

far better understood than its role in delivery.
In lactation, oxytocin causes milk to be expressed from
the alveoli into the ducts of the breast so that the baby
can obtain it by suckling.
This mechanism works as follows:The suckling stim-
ulus on the nipple of the breast causes signals to be
transmitted through sensory nerves to the oxytocin
neurons in the paraventricular and supraoptic nuclei
in the hypothalamus, which causes release of oxytocin
by the posterior pituitary gland. The oxytocin 

in
by the posterior pituitary gland. The oxytocin is then
carried by the blood to the breasts, where it causes
contraction of myoepithelial cells that lie outside of
and form a latticework surrounding the alveoli of the
mammary glands. In less than a minute after the begin-
ning of suckling, milk begins to ﬂow. This mechanism
is called milk letdown or milk ejection. It is discussed
further in Chapter 82 in relation to the physiology of
lactation.
Antunes-Rodrigues J, de Castro M, Elia

Besser 
GM,
Thorner 
MO:
Comprehensive 
Clinical
Endocrinology, 3rd ed. Philadelphia: Mosby, Elsevier
Science Limited, 2002.
Burbach JP, Luckman SM, Murphy D, Gainer H: Gene 
regulation 
in 
the 
magnocellular 
hypothalamo-
neurohypophysial system. Physiol Rev 81:1197, 2001.
Butler AA, Le Roith D: Control of growth by the somatropic
axis: growth hormone and th

owth by the somatropic
axis: growth hormone and the insulin-like growth factors
have related and independent roles. Annu Rev Physiol
63:141, 2001.
Cummings DE, Merriam GR: Growth hormone therapy in
adults. Annu Rev Med 54:513, 2003.
Dattani M, Preece M: Growth hormone deﬁciency and
related disorders: insights into causation, diagnosis, and
treatment. Lancet 363:1977, 2004.
Dunn AJ, Swiergiel AH, Palamarchouk V: Brain circuits
involved in corticotropin-releasing factor–norepinephrine

ropin-releasing factor–norepinephrine
interactions during stress.Ann N Y Acad Sci 1018:25, 2004.
Edmondson SR,Thumiger SP,Werther GA,Wraight CJ: Epi-
dermal homeostasis: the role of the growth hormone and
insulin-like growth factor systems. Endocr Rev 24:737,
2003.
Eugster EA, Pescovitz OH: Gigantism. J Clin Endocrinol
Metab 84:4379, 1999.
Freeman ME, Kanyicska B, Lerant A, Nagy G: Prolactin:
structure, function, and regulation of secretion. Physiol
Rev 80:1523, 2000.
Gimpl G, Fahrenholz F:The o

iol
Gimpl G, Fahrenholz F:The oxytocin receptor system: struc-
ture, function, and regulation. Physiol Rev 81:629, 2001.
930
Unit XIV
Goldspink G: Age-related muscle loss and progressive dys-
function in mechanosensitive growth factor signaling.Ann
N Y Acad Sci 1019:294, 2004.
Isley WL: Growth hormone therapy for adults: not ready for
prime time? Ann Intern Med 137:190, 2002.
Williams Textbook of E

 HM, Melmed S, Polonsky KS:
Williams Textbook of Endocrinology, 10th ed. Philadel-
phia: WB Saunders, 2003.
Lohmeier TE: Neurohypophysial hormones. Am J Physiol
Regul Integr Comp Physiol 285:R715, 2003.
Longo VD, Finch CE: Evolutionary medicine: from dwarf
model systems to healthy centenarians? Science 299:1342,
2003.
Moenter SM, Defazio RA, Straume M, Nunemaker CS:
Steroid regulation of GnRH neurons. Ann N Y Acad Sci
1007:143, 2003.
Murray RD: Adult growth hormone replacement: current
understan

dult growth hormone replacement: current
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Curr Opin Pharmacol 3:672, 2003.
C
H
A
P
T
E
R
 
7
6
931
The thyr

E
R
 
7
6
931
The thyroid gland, located immediately below the
larynx on each side of and anterior to the trachea,
is one of the largest of the endocrine glands,
normally weighing 15 to 20 grams in adults. The
thyroid secretes two major hormones, thyroxine and
triiodothyronine, commonly called T4 and T3, respec-
tively. Both of these hormones profoundly increase
the metabolic rate of the body. Complete lack of
thyroid secretion usually causes the basal metabolic rate 

secretion usually causes the basal metabolic rate to fall 40 to 50 per cent
below normal, and extreme excesses of thyroid secretion can increase the basal
metabolic rate to 60 to 100 per cent above normal. Thyroid secretion is con-
trolled primarily by thyroid-stimulating hormone (TSH) secreted by the ante-
rior pituitary gland.
The thyroid gland also secretes calcitonin, an important hormone for calcium
metabolism that is considered in detail in Chapter 79.
The purpose of this chapter is to dis

 Chapter 79.
The purpose of this chapter is to discuss the formation and secretion of the
thyroid hormones, their metabolic functions, and regulation of their secretion.
About 93 per cent of the metabolically active hormones secreted by the thyroid
gland is thyroxine, and 7 per cent triiodothyronine. However, almost all the thy-
roxine is eventually converted to triiodothyronine in the tissues, so that both
are functionally important. The

sues, so that both
are functionally important. The functions of these two hormones are qualita-
tively the same, but they differ in rapidity and intensity of action. Triiodothy-
ronine is about four times as potent as thyroxine, but it is present in the blood
in much smaller quantities and persists for a much shorter time than does 
thyroxine.
Physiologic Anatomy of the Thyroid Gland. The thyroid gland is composed, as shown
in Figure 76–1, of large numbers of closed follicles (100 to 300 microme

ge numbers of closed follicles (100 to 300 micrometers in
diameter) ﬁlled with a secretory substance called colloid and lined with cuboidal
epithelial cells that secrete into the interior of the follicles.The major constituent
of colloid is the large glycoprotein thyroglobulin, which contains the thyroid
hormones within its molecule. Once the secretion has entered the follicles, it
must be absorbed back through the follicular epithelium into the blood before
it can function in the body. The thyr

blood before
it can function in the body. The thyroid gland has a blood ﬂow about ﬁve times
the weight of the gland each minute, which is a blood supply as great as that of
any other area of the body, with the possible exception of the adrenal cortex.
Iodine Is Required for Formation of Thyroxine
To form normal quantities of thyroxine, about 50 milligrams of ingested iodine
in the form of iodides are required each year, or about 1 mg/week. To prevent
iodine deﬁciency, common table salt is iodize

vent
iodine deﬁciency, common table salt is iodized with about 1 part sodium iodide
to every 100,000 parts sodium chloride.
Fate of Ingested Iodides. Iodides ingested orally are absorbed from the gastro-
intestinal tract into the blood in about the same manner as chlorides. Normally,
932
Unit XIV
most of the iodides are rapidly excreted by the
kidneys, but only after about one ﬁfth are selectively
removed from the circulating blood by the cells of the
thyroid gland

irculating blood by the cells of the
thyroid gland and used for synthesis of the thyroid
hormones.
The ﬁrst stage in the formation of thyroid hormones,
shown in Figure 76–2, is transport of iodides from the
blood into the thyroid glandular cells and follicles.The
basal membrane of the thyroid cell has the speciﬁc
ability to pump the iodide actively to the interior of
the cell. This is called iodide trapping. In a normal
gland, the iodide pump concentrates the iodide

mal
gland, the iodide pump concentrates the iodide to
about 30 times its concentration in the blood. When
the thyroid gland becomes maximally active, this con-
centration ratio can rise to as high as 250 times. The
rate of iodide trapping by the thyroid is inﬂuenced 
by several factors, the most important being the 
concentration of TSH; TSH stimulates and hypophy-
sectomy greatly diminishes the activity of the iodide
pump in thyroid cells.
Thyroglobulin, and Chemistry 
of Thyroxine and 
Triiodo

globulin, and Chemistry 
Triiodothyronine Formation
Formation and Secretion of Thyroglobulin by the Thyroid Cells.
The thyroid cells are typical protein-secreting glandu-
lar cells, as shown in Figure 76–2. The endoplasmic
reticulum and Golgi apparatus synthesize and secrete
into the follicles a large glycoprotein molecule called
thyroglobulin, with a molecular weight of about
335,000.
Each molecule of thyroglobulin contains about 70
tyrosine amino acids, and they are the major

ut 70
tyrosine amino acids, and they are the major substrates
that combine with iodine to form the thyroid hor-
mones. Thus, the thyroid hormones form within the
thyroglobulin molecule. That is, the thyroxine and 
triiodothyronine hormones formed from the tyrosine
amino acids remain part of the thyroglobulin molecule
during synthesis of the thyroid hormones and even
afterward as stored hormones in the follicular colloid.
Oxidation of the Iodide Ion. The ﬁrst essential step in the
formation of th

on. The ﬁrst essential step in the
formation of the thyroid hormones is conversion of 
the iodide ions to an oxidized form of iodine, either
nascent iodine (I0) or I3
_, that is then capable of com-
bining directly with the amino acid tyrosine. This oxi-
dation of iodine is promoted by the enzyme peroxidase
and its accompanying hydrogen peroxide, which
provide a potent system capable of oxidizing iodides.
The peroxidase is either located in the apical mem-
brane of the cell or attached to it, th

pical mem-
brane of the cell or attached to it, thus providing the
oxidized iodine at exactly the point in the cell where
the thyroglobulin molecule issues forth from the Golgi
apparatus and through the cell membrane into the
stored thyroid gland colloid. When the peroxidase
system is blocked or when it is hereditarily absent
from the cells, the rate of formation of thyroid hor-
mones falls to zero.
Follicle
Cuboidal epithelial cells
Colloid
Red blood cells
Figure 76–1
Microscopic appearance of 

Microscopic appearance of the thyroid gland, showing secretion
of thyroglobulin into the follicles.
Iodination
and
coupling
+
I2
I-
I-
H2O2
Peroxidase
Pinocytosis
Secretion
MIT
DIT 
MIT, DIT 
ER
Golgi
Proteases
Colloid
droplet
Deiodination
Tyrosine
Thyroglobulin
precursor (TG)
TG
TG
T3
T4
T3
T4
T3
T4
Figure 76–2
Thyroid cellular mechanisms for iodine transport,
thyroxine and triiodothyronine formation, and thy-
roxine and triiodothyronine release into the blood

roxine and triiodothyronine release into the blood.
MIT, monoiodotyrosine; DIT, diiodotyrosine; T3, tri-
iodothyronine; T4, thyroxine; TG, thyroglobulin.
Chapter 76
933
Iodination of Tyrosine and Formation of the Thyroid Hormones—
“Organiﬁcation” of Thyroglobulin. The binding of iodine
with the thyroglobulin molecule is called organiﬁca-
tion of the thyroglobulin. Oxidized iodine even in the
molecular form will bind directly but very slowly with
the amino acid tyrosine

ectly but very slowly with
the amino acid tyrosine. In the thyroid cells, however,
the oxidized iodine is associated with an iodinase
enzyme (Figure 76–2) that causes the process to occur
within seconds or minutes.Therefore, almost as rapidly
as the thyroglobulin molecule is released from the
Golgi apparatus or as it is secreted through the apical
cell membrane into the follicle, iodine binds with
about one sixth of the tyrosine amino acids within the
thyroglobulin molecule.
Figure 76–3 shows th

n the
Figure 76–3 shows the successive stages of 
iodination of tyrosine and ﬁnal formation of the two
important thyroid hormones,
thyroxine and tri-
iodothyronine.Tyrosine is ﬁrst iodized to monoiodoty-
rosine and then to diiodotyrosine. Then, during the
next few minutes, hours, and even days, more and more
of the iodotyrosine residues become coupled with one
another.
The major hormonal product of the coupling reac-
tion is the molecule thyroxine that remains part of 
th

is the molecule thyroxine that remains part of 
the thyroglobulin molecule. Or one molecule of
monoiodotyrosine couples with one molecule of
diiodotyrosine to form triiodothyronine, which repre-
sents about one ﬁfteenth of the ﬁnal hormones.
Storage of Thyroglobulin. The thyroid gland is unusual
among the endocrine glands in its ability to store large
amounts of hormone. After synthesis of the thyroid
hormones has run its course, each thyroglobulin mol-
ecule contains up to 30 thyroxine molecule

in mol-
ecule contains up to 30 thyroxine molecules and a few
triiodothyronine molecules. In this form, the thyroid
hormones are stored in the follicles in an amount suf-
ﬁcient to supply the body with its normal requirements
of thyroid hormones for 2 to 3 months. Therefore,
when synthesis of thyroid hormone ceases, the physi-
ologic effects of deﬁciency are not observed for several
months.
Release of Thyroxine and
Triiodothyronine from the 
Thyroglobulin itself is not released int

oid Gland
Thyroglobulin itself is not released into the circulating
blood in measurable amounts; instead, thyroxine and
triiodothyronine must ﬁrst be cleaved from the thy-
roglobulin molecule, and then these free hormones are
released. This process occurs as follows: The apical
surface of the thyroid cells sends out pseudopod exten-
sions that close around small portions of the colloid to
form pinocytic vesicles that enter the apex of the
thyroid cell. Then lysosomes in the cell cytoplasm
immedi

 cell. Then lysosomes in the cell cytoplasm
immediately fuse with these vesicles to form digestive
vesicles containing digestive enzymes from the lyso-
somes mixed with the colloid. Multiple proteases
among the enzymes digest the thyroglobulin mole-
cules and release thyroxine and triiodothyronine in
free form. These then diffuse through the base of the
thyroid cell into the surrounding capillaries. Thus, the
thyroid hormones are released into the blood.
About three quarters of the iodinated tyr

e blood.
About three quarters of the iodinated tyrosine in
the thyroglobulin never becomes thyroid hormones
but remains monoiodotyrosine and diiodotyrosine.
During the digestion of the thyroglobulin molecule to
cause release of thyroxine and triiodothyronine, these
iodinated tyrosines also are freed from the thyroglob-
ulin molecules.However,they are not secreted into the
blood. Instead, their iodine is cleaved from them by a
deiodinase enzyme that makes virtually all this iodine
available again

at makes virtually all this iodine
available again for recycling within the gland for
forming additional thyroid hormones. In the congeni-
tal absence of this deiodinase enzyme, many persons
become iodine-deﬁcient because of failure of this recy-
cling process.
Daily Rate of Secretion of Thyroxine and Triiodothyronine.
About 93 per cent of the thyroid hormone released
from the thyroid gland is normally thyroxine and only
7 per cent is triiodothyronine. However, during the
ensuing few days, about

onine. However, during the
ensuing few days, about one half of the thyroxine is
slowly deiodinated to form additional triiodothyro-
nine. Therefore, the hormone ﬁnally delivered to and
HO
CH2
CHNH2
COOH
O
Diiodotyrosine + Diiodotyrosine
Monoiodotyrosine + Diiodotyrosine
CH2
CHNH2
COOH
HO
O
CH2
CHNH2
COOH
HO
HO
CH2
CHNH2
COOH +
I2 + HO
CH2
CHNH2
COOH
Iodinase
Tyrosine
Monoiodotyrosine
Diiodotyrosine
3,5,3'-Triiodothyronine
Thyroxine
Figure 76–3
Chemistry of thyroxine and triiodothyronine formatio

emistry of thyroxine and triiodothyronine formation.
934
Unit XIV
used by the tissues is mainly triiodothyronine, a total
of about 35 micrograms of triiodothyronine per day.
Transport of Thyroxine and
Triiodothyronine to Tissues
Thyroxine and Triiodothyronine Are Bound to Plasma Proteins.
On entering the blood, over 99 per cent of the thy-
roxine and triiodothyronine combines immediately
with several of the plasma proteins, all of which are
synthesized by the liver

roteins, all of which are
synthesized by the liver. They combine mainly with
thyroxine-binding globulin and much less so with 
thyroxine-binding prealbumin and albumin.
Thyroxine and Triiodothyronine Are Released Slowly to Tissue
Cells. Because of high afﬁnity of the plasma-binding
proteins for the thyroid hormones, these substances—
in particular, thyroxine—are released to the tissue
cells slowly. Half the thyroxine in the blood is released
to the tissue cells about every 6 days, whereas half t

he tissue cells about every 6 days, whereas half the
triiodothyronine—because of its lower afﬁnity—is
released to the cells in about 1 day.
On entering the tissue cells, both thyroxine and tri-
iodothyronine again bind with intracellular proteins,
the thyroxine binding more strongly than the tri-
iodothyronine. Therefore, they are again stored, but
this time in the target cells themselves, and they are
used slowly over a period of days or weeks.
Thyroid Hormones Have Slow Onset and Long Duration

Thyroid Hormones Have Slow Onset and Long Duration of Action.
After injection of a large quantity of thyroxine into a
human being, essentially no effect on the metabolic
rate can be discerned for 2 to 3 days, thereby demon-
strating that there is a long latent period before thy-
roxine activity begins. Once activity does begin, it
increases progressively and reaches a maximum in 10
to 12 days, as shown in Figure 76–4. Thereafter, it
decreases with a half-life of about 15 days. Some of the
activi

h a half-life of about 15 days. Some of the
activity persists for as long as 6 weeks to 2 months.
The actions of triiodothyronine occur about four
times as rapidly as those of thyroxine, with a latent
period as short as 6 to 12 hours and maximal cellular
activity occurring within 2 to 3 days.
Most of the latency and prolonged period of action
of these hormones are probably caused by their
binding with proteins both in the plasma and in the
tissue cells, followed by their slow release. However,
w

 cells, followed by their slow release. However,
we shall see in subsequent discussions that part of the
latent period also results from the manner in which
these hormones perform their functions in the cells
themselves.
Physiologic Functions of the
Thyroid Hormones Increase the
Transcription of Large Numbers 
of Genes
The general effect of thyroid hormone is to activate
nuclear transcription of large numbers of genes
(Figure 76–5). Therefore, in virtually all cells of the
body,

5). Therefore, in virtually all cells of the
body, great numbers of protein enzymes, structural
proteins, transport proteins, and other substances are
synthesized. The net result is generalized increase in
functional activity throughout the body.
Most of the Thyroxine Secreted by the Thyroid Is Converted to
Triiodothyronine. Before acting on the genes to increase
genetic transcription, one iodide is removed from
almost all the thyroxine, thus forming triiodothyro-
nine. Intracellular thyroid hor

ming triiodothyro-
nine. Intracellular thyroid hormone receptors have a
very high afﬁnity for triiodothyronine. Consequently,
more than 90 per cent of the thyroid hormone mole-
cules that bind with the receptors is triiodothyronine.
Thyroid Hormones Activate Nuclear Receptors. The thyroid
hormone receptors are either attached to the DNA
genetic strands or located in proximity to them.
The thyroid hormone receptor usually forms a het-
erodimer with retinoid X receptor (RXR) at speciﬁc
thyroid hor

h retinoid X receptor (RXR) at speciﬁc
thyroid hormone response elements on the DNA. On
binding with thyroid hormone, the receptors become
activated and initiate the transcription process. Then
large numbers of different types of messenger RNA
are formed, followed within another few minutes or
hours by RNA translation on the cytoplasmic ribo-
somes to form hundreds of new intracellular proteins.
However, not all the proteins are increased by similar
percentages—some only slightly, and others at 

lar
percentages—some only slightly, and others at least as
much as sixfold. It is believed that most, if not all, of
the actions of thyroid hormone result from the subse-
quent enzymatic and other functions of these new 
proteins.
Thyroid Hormones Increase Cellular
The thyroid hormones increase the metabolic activi-
ties of almost all the tissues of the body. The basal
+10
+5
0
0
10
20
30
40
Basal metabolic rate
Days
Days
Thyroxine injected
Figure 76–4
Approximate prolonged ef

xine injected
Approximate prolonged effect on the basal metabolic rate caused
by administering a single large dose of thyroxine.
Chapter 76
935
metabolic rate can increase to 60 to 100 per cent above
normal when large quantities of the hormones are
secreted. The rate of utilization of foods for energy is
greatly accelerated. Although the rate of protein syn-
thesis is increased, at the same time the rate of protein
catabolism is also increased.The growth ra

protein
catabolism is also increased.The growth rate of young
people is greatly accelerated. The mental processes 
are excited, and the activities of most of the other
endocrine glands are increased.
Thyroid Hormones Increase the Number and Activity of Mito-
chondria. When thyroxine or triiodothyronine is given
to an animal, the mitochondria in most cells of the
animal’s body increase in size as well as number.
Furthermore, the total membrane surface area of the
mitochondria increases almost dir

face area of the
mitochondria increases almost directly in proportion
to the increased metabolic rate of the whole animal.
Therefore, one of the principal functions of thyroxine
might be simply to increase the number and activity 
of mitochondria, which in turn increases the rate of
formation of adenosine triphosphate (ATP) to ener-
gize cellular function. However, the increase in the
number and activity of mitochondria could be the
result of increased activity of the cells as well as 
the cause

reased activity of the cells as well as 
the cause of the increase.
Thyroid Hormones Increase Active Transport of Ions Through
Cell Membranes. One of the enzymes that increases its
activity in response to thyroid hormone is Na+-K+-
ATPase.
This 
in 
turn 
increases 
the 
rate 
of 
transport of both sodium and potassium ions through
the cell membranes of some tissues. Because this
process uses energy and increases the amount of heat
produced in the body, it has been suggested that this
might be o

e body, it has been suggested that this
might be one of the mechanisms by which thyroid
hormone increases the body’s metabolic rate. In fact,
thyroid hormone also causes the cell membranes of
most cells to become leaky to sodium ions, which
further activates the sodium pump and further
increases heat production.
Iodinase
Iodinase
Cytoplasm
Cytoplasm
Nuclear
membrane
Nuclear
membrane
Nucleus
Nucleus
Thyroid
hormone
receptor
Thyroid
hormone
receptor
Thyroid
hormone
resp

ptor
Thyroid
hormone
receptor
Thyroid
hormone
response
Thyroid
hormone
response
Gene
Gene
mRNA
mRNA
Synthesis of
new proteins
Tissue blood flow
Heart rate
Heart rate
Na+-K+-ATPase
O2 consumption
Glucose absorption
Lipolysis
BMR
O2 co

ein synthesis
BMR
Lipolysis
BMR
Many other
systems
Many other
systems
Gene
Retinoid X
receptor
T3
T4
T3
T3
CNS
CNS
Growth
Growth
Figure 76–5
Thyroid hormone activation of
target cells. Thyroxine (T4) and
triiodothyronine 
(T3) 
readily
diffuse through the cell mem-
brane. Much of the T4 is deiodi-
nated to form T3, which interacts
wit

4 is deiodi-
with the thyroid hormone re-
ceptor, bound as a heterodimer
with a retinoid X receptor, of 
the thyroid hormone response
element of the gene. This causes
either increases or decreases in
transcription of genes that lead 
to formation of proteins, thus
producing the thyroid hormone
response of the cell. The actions
of thyroid hormone on cells 
of 
several 
different 
systems 
are shown. mRNA, messenger
ribonucleic acid.
936
Unit XIV

r
936
Unit XIV
Thyroid hormone has both general and speciﬁc effects
on growth. For instance, it has long been known that
thyroid hormone is essential for the metamorphic
change of the tadpole into the frog.
In humans, the effect of thyroid hormone on growth
is manifest mainly in growing children. In those who
are hypothyroid, the rate of growth is greatly retarded.
In those who are hyperthyroid, excessive skelet

d.
In those who are hyperthyroid, excessive skeletal
growth often occurs, causing the child to become con-
siderably taller at an earlier age. However, the bones
also mature more rapidly and the epiphyses close 
at an early age, so that the duration of growth and 
the eventual height of the adult may actually be 
shortened.
An important effect of thyroid hormone is to
promote growth and development of the brain during
fetal life and for the ﬁrst few years of postnatal life.
If the fetus does not

few years of postnatal life.
If the fetus does not secrete sufﬁcient quantities of
thyroid hormone, growth and maturation of the brain
both before birth and afterward are greatly retarded,
and the brain remains smaller than normal. Without
speciﬁc thyroid therapy within days or weeks after
birth, the child without a thyroid gland will remain
mentally deﬁcient throughout life. This is discussed
more fully later in the chapter.
Effects of Thyroid Hormone 
on Speciﬁc Bodily Mechanisms
Stimulation o

ormone 
Stimulation of Carbohydrate Metabolism. Thyroid hormone
stimulates almost all aspects of carbohydrate meta-
bolism, including rapid uptake of glucose by the 
cells, enhanced glycolysis, enhanced gluconeogenesis,
increased rate of absorption from the gastrointestinal
tract, and even increased insulin secretion with its
resultant secondary effects on carbohydrate metabo-
lism. All these effects probably result from the overall
increase in cellular metabolic enz

rom the overall
increase in cellular metabolic enzymes caused by
Stimulation of Fat Metabolism. Essentially all aspects of
fat metabolism are also enhanced under the inﬂuence
of thyroid hormone. In particular, lipids are mobilized
rapidly from the fat tissue, which decreases the fat
stores of the body to a greater extent than almost any
other tissue element. This also increases the free fatty
acid concentration in the plasma and greatly acceler-
ates the oxidation of free fatty 

greatly acceler-
ates the oxidation of free fatty acids by the cells.
Effect on Plasma and Liver Fats. Increased thyroid
hormone decreases the concentrations of cholesterol,
phospholipids, and triglycerides in the plasma, even
though it increases the free fatty acids. Conversely,
decreased thyroid secretion greatly increases the
plasma concentrations of cholesterol, phospholipids,
and triglycerides and almost always causes excessive
deposition of fat in the liver as well.The large increase
in ci

 fat in the liver as well.The large increase
in circulating plasma cholesterol in prolonged
hypothyroidism is often associated with severe ather-
osclerosis, discussed in Chapter 68.
One of the mechanisms by which thyroid hormone
decreases the plasma cholesterol concentration is to
increase signiﬁcantly the rate of cholesterol secretion
in the bile and consequent loss in the feces.A possible
mechanism for the increased cholesterol secretion is
that thyroid hormone induces increased numbers of
lo

at thyroid hormone induces increased numbers of
low-density lipoprotein receptors on the liver cells,
leading to rapid removal of low-density lipoproteins
from the plasma by the liver and subsequent secretion
of cholesterol in these lipoproteins by the liver cells.
Increased Requirement for Vitamins. Because thyroid
hormone increases the quantities of many bodily
enzymes and because vitamins are essential parts of
some of the enzymes or coenzymes, thyroid hormone
causes increased need for vitami

, thyroid hormone
causes increased need for vitamins. Therefore, a rela-
tive vitamin deﬁciency can occur when excess thyroid
hormone is secreted, unless at the same time increased
quantities of vitamins are made available.
Increased 
Basal 
Metabolic 
Rate. Because 
thyroid
hormone increases metabolism in almost all cells of
the body, excessive quantities of the hormone can
occasionally increase the basal metabolic rate 60 to
100 per cent above normal. Conversely, when no
thyroid hormone is pro

normal. Conversely, when no
thyroid hormone is produced, the basal metabolic 
rate falls almost to one-half normal. Figure 76–6 shows
the approximate relation between the daily supply 
of thyroid hormones and the basal metabolic rate.
Extreme amounts of the hormones are required to
cause very high basal metabolic rates.
Decreased Body Weight. Greatly increased thyroid
hormone almost always decreases the body weight,
+30
+20
+10
0
-40
-30
-20
-10
-45
0
300
200
100
Thyroid hor

-45
0
300
200
100
Thyroid hormones (mg/day)
Figure 76–6
Approximate relation of daily rate of thyroid hormone (T4 and T3)
secretion to the basal metabolic rate.
Chapter 76
937
and greatly decreased hormone almost always
increases the body weight; these effects do not always
occur, because thyroid hormone also increases the
appetite, and this may counterbalance the change in
the metabolic rate.
Effect of Thyroid Hormones on

Effect of Thyroid Hormones on the Cardiovascular System
Increased Blood Flow and Cardiac Output. Increased
metabolism in the tissues causes more rapid utilization
of oxygen than normal and release of greater than
normal quantities of metabolic end products from the
tissues. These effects cause vasodilation in most body
tissues, thus increasing blood ﬂow. The rate of blood
ﬂow in the skin especially increases because of the
increased need for heat elimination from the body. A

creased need for heat elimination from the body. As
a consequence of the increased blood ﬂow, cardiac
output also increases, sometimes rising to 60 per cent
or more above normal when excessive thyroid
hormone is present and falling to only 50 per cent of
normal in very severe hypothyroidism.
Increased Heart Rate. The heart rate increases con-
siderably more under the inﬂuence of thyroid
hormone than would be expected from the increase in
cardiac output. Therefore, thyroid hormone seems to
have a

output. Therefore, thyroid hormone seems to
have a direct effect on the excitability of the heart,
which in turn increases the heart rate. This effect is of
particular importance because the heart rate is one of
the sensitive physical signs that the clinician uses in
determining whether a patient has excessive or dimin-
ished thyroid hormone production.
Increased Heart Strength. The increased enzymatic
activity caused by increased thyroid hormone produc-
tion apparently increases the strength of

 produc-
tion apparently increases the strength of the heart
when only a slight excess of thyroid hormone is
secreted. This is analogous to the increase in heart
strength that occurs in mild fevers and during exercise.
However,
when thyroid hormone is increased
markedly,
the heart muscle strength becomes
depressed because of long-term excessive protein
catabolism. Indeed, some severely thyrotoxic patients
die of cardiac decompensation secondary to myocar-
dial failure and to increased cardiac lo

o myocar-
dial failure and to increased cardiac load imposed by
the increase in cardiac output.
Normal Arterial Pressure. The mean arterial pressure
usually remains about normal after administration of
thyroid hormone. Because of increased blood ﬂow
through the tissues between heartbeats, the pulse pres-
sure is often increased, with the systolic pressure ele-
vated in hyperthyroidism 10 to 15 mm Hg and the
diastolic pressure reduced a corresponding amount.
Increased Respiration. The increased r

ing amount.
Increased Respiration. The increased rate of metabolism
increases the utilization of oxygen and formation of
carbon dioxide; these effects activate all the mecha-
nisms that increase the rate and depth of respiration.
Increased 
Gastrointestinal 
Motility. In 
addition 
to
increased appetite and food intake, which has been
discussed, thyroid hormone increases both the rates of
secretion of the digestive juices and the motility of the
gastrointestinal tract. Hyperthyroidism often resu

gastrointestinal tract. Hyperthyroidism often results in
diarrhea. Lack of thyroid hormone can cause consti-
pation.
Excitatory Effects on the Central Nervous System. In general,
thyroid hormone increases the rapidity of cerebration
but also often dissociates this; conversely, lack of
thyroid hormone decreases this function. The hyper-
thyroid individual is likely to have extreme nervous-
ness and many psychoneurotic tendencies, such as
anxiety complexes, extreme worry, and paranoia.
Effect on t

omplexes, extreme worry, and paranoia.
Effect on the Function of the Muscles. Slight increase in
thyroid hormone usually makes the muscles react with
vigor, but when the quantity of hormone becomes
excessive, the muscles become weakened because of
excess protein catabolism. Conversely, lack of thyroid
hormone causes the muscles to become sluggish, and
they relax slowly after a contraction.
Muscle Tremor. One of the most characteristic signs of
hyperthyroidism is a ﬁne muscle tremor.This is not t

perthyroidism is a ﬁne muscle tremor.This is not the
coarse tremor that occurs in Parkinson’s disease or in
shivering, because it occurs at the rapid frequency of
10 to 15 times per second.The tremor can be observed
easily by placing a sheet of paper on the extended
ﬁngers and noting the degree of vibration of the paper.
This tremor is believed to be caused by increased reac-
tivity of the neuronal synapses in the areas of the
spinal cord that control muscle tone. The tremor is an
important mean

ntrol muscle tone. The tremor is an
important means for assessing the degree of thyroid
hormone effect on the central nervous system.
Effect on Sleep. Because of the exhausting effect of
thyroid hormone on the musculature and on the
central nervous system, the hyperthyroid subject often
has a feeling of constant tiredness, but because of the
excitable effects of thyroid hormone on the synapses,
it is difﬁcult to sleep. Conversely, extreme somnolence
is characteristic of hypothyroidism, with slee

nce
is characteristic of hypothyroidism, with sleep some-
times lasting 12 to 14 hours a day.
Effect on Other Endocrine Glands. Increased thyroid
hormone increases the rates of secretion of most other
endocrine glands, but it also increases the need of the
tissues for the hormones. For instance, increased thy-
roxine secretion increases the rate of glucose metabo-
lism everywhere in the body and therefore causes a
corresponding need for increased insulin secretion by
the pancreas. Also, thyroid 

 insulin secretion by
the pancreas. Also, thyroid hormone increases many
metabolic activities related to bone formation and, as
a consequence, increases the need for parathyroid
hormone. Thyroid hormone also increases the rate at
which adrenal glucocorticoids are inactivated by the
liver. This leads to feedback increase in adrenocorti-
cotropic hormone production by the anterior pituitary
and, therefore, increased rate of glucocorticoid secre-
tion by the adrenal glands.
Effect of Thyroid Hormon

on by the adrenal glands.
Effect of Thyroid Hormone on Sexual Function. For normal
sexual function,
thyroid secretion needs to be 
938
Unit XIV
approximately normal. In men, lack of thyroid
hormone is likely to cause loss of libido; great excesses
of the hormone, however, sometimes cause impotence.
In women, lack of thyroid hormone often causes
menorrhagia and polymenorrhea— that is, respec-
tively, excessive and frequent menstrual bleeding.
Yet, strangely enough, 

equent menstrual bleeding.
Yet, strangely enough, in other women thyroid lack
may cause irregular periods and occasionally even
amenorrhea.
A hypothyroid woman, like a man, is likely to have
greatly decreased libido.To make the picture still more
confusing, in the hyperthyroid woman, oligomenor-
rhea, which means greatly reduced bleeding, is
common, and occasionally amenorrhea results.
The action of thyroid hormone on the gonads
cannot be pinpointed to a speciﬁc function but prob-
ably results f

ted to a speciﬁc function but prob-
ably results from a combination of direct metabolic
effects on the gonads as well as excitatory and
inhibitory feedback effects operating through the
anterior pituitary hormones that control the sexual
Regulation of Thyroid
To maintain normal levels of metabolic activity in the
body, precisely the right amount of thyroid hormone
must be secreted at all times; to achieve this, speciﬁc
feedback mechanisms operate through the hypothal

c
feedback mechanisms operate through the hypothala-
mus and anterior pituitary gland to control the rate of
thyroid secretion. These mechanisms are as follows.
TSH (from the Anterior Pituitary Gland) Increases Thyroid Secre-
tion. TSH, also known as thyrotropin, is an anterior
pituitary hormone, a glycoprotein with a molecular
weight of about 28,000. This hormone, also discussed
in Chapter 74, increases the secretion of thyroxine and
triiodothyronine by the thyroid gland. Its speciﬁc
effects on

onine by the thyroid gland. Its speciﬁc
effects on the thyroid gland are as follows:
1. Increased proteolysis of the thyroglobulin that 
has already been stored in the follicles, with
resultant release of the thyroid hormones into 
the circulating blood and diminishment of the
follicular substance itself
2. Increased activity of the iodide pump, which
increases the rate of “iodide trapping” in the
glandular cells, sometimes increasing the ratio of
intracellular to extracellular iodide concentrat

f
intracellular to extracellular iodide concentration
in the glandular substance to as much as eight
times normal
3. Increased iodination of tyrosine to form the
thyroid hormones
4. Increased size and increased secretory activity of
the thyroid cells
5. Increased number of thyroid cells plus a change
from cuboidal to columnar cells and much
infolding of the thyroid epithelium into the
follicles
In summary, TSH increases all the known secretory
activities of the thyroid glandular cells.
The most 

tivities of the thyroid glandular cells.
The most important early effect after administration
of TSH is to initiate proteolysis of the thyroglobulin,
which causes release of thyroxine and triiodothyro-
nine into the blood within 30 minutes. The other
effects require hours or even days and weeks to
develop fully.
Cyclic Adenosine Monophosphate Mediates the Stimulatory
Effect of TSH. In the past, it was difﬁcult to explain the
many and varied effects of TSH on the thyroid cell. It
is now clear tha

ts of TSH on the thyroid cell. It
is now clear that most, if not all, of these effects result
from activation of the “second messenger” cyclic
adenosine monophosphate (cAMP) system of the cell.
The ﬁrst event in this activation is binding of TSH
with speciﬁc TSH receptors on the basal membrane
surfaces of the thyroid cell. This then activates adeny-
lyl cyclase in the membrane, which increases the for-
mation of cAMP inside the cell. Finally, the cAMP acts
as a second messenger to activate prote

 cAMP acts
as a second messenger to activate protein kinase, which
causes multiple phosphorylations throughout the cell.
The result is both an immediate increase in secretion
of thyroid hormones and prolonged growth of the
thyroid glandular tissue itself.
This method for control of thyroid cell activity is
similar to the function of cAMP as a “second messen-
ger” in many other target tissues of the body, as 
discussed in Chapter 74.
Anterior Pituitary Secretion of TSH Is
Regulated by Thyrotropin

itary Secretion of TSH Is
Regulated by Thyrotropin-Releasing
Anterior pituitary secretion of TSH is controlled 
by a hypothalamic hormone, thyrotropin-releasing
hormone (TRH), which is secreted by nerve endings
in the median eminence of the hypothalamus. From
the median eminence, the TRH is then transported to
the anterior pituitary by way of the hypothalamic-
hypophysial portal blood, as explained in Chapter 74.
TRH has been obtained in pure form. It is a simple
su

 has been obtained in pure form. It is a simple
substance, a tripeptide amide—pyroglutamyl-histidyl-
proline-amide. TRH directly affects the anterior pitu-
itary gland cells to increase their output of TSH.When
the blood portal system from the hypothalamus to the
anterior pituitary gland becomes blocked, the rate of
secretion of TSH by the anterior pituitary decreases
greatly but is not reduced to zero.
The molecular mechanism by which TRH causes 
the TSH-secreting cells of the anterior pituitar

the TSH-secreting cells of the anterior pituitary to
produce TSH is ﬁrst to bind with TRH receptors in the
pituitary cell membrane.This in turn activates the phos-
pholipase second messenger system inside the pituitary
cells to produce large amounts of phospholipase C, fol-
lowed by a cascade of other second messengers,
including calcium ions and diacyl glycerol, which even-
tually leads to TSH release.
Effects of Cold and Other Neurogenic Stimuli on TRH and TSH
Secretion. One of the best-know

uli on TRH and TSH
Secretion. One of the best-known stimuli for increas-
ing the rate of TRH secretion by the hypothalamus,
Chapter 76
939
and therefore TSH secretion by the anterior pituitary
gland, is exposure of an animal to cold. This effect
almost certainly results from excitation of the hypo-
thalamic centers for body temperature control.
Exposure of rats for several weeks to severe cold
increases the output of thyroid hormones sometimes
to more than 100 per cent

yroid hormones sometimes
to more than 100 per cent of normal and can increase
the basal metabolic rate as much as 50 per cent.
Indeed, persons moving to arctic regions have been
known to develop basal metabolic rates 15 to 20 per
cent above normal.
Various emotional reactions can also affect the
output of TRH and TSH and therefore indirectly
affect the secretion of thyroid hormones. Excitement
and anxiety—conditions that greatly stimulate the
sympathetic nervous system—cause an acute decrease
in

pathetic nervous system—cause an acute decrease
in secretion of TSH, perhaps because these states
increase the metabolic rate and body heat and there-
fore exert an inverse effect on the heat control center.
Neither these emotional effects nor the effect of
cold is observed after the hypophysial stalk has been
cut, demonstrating that both of these effects are medi-
ated by way of the hypothalamus.
Feedback Effect of Thyroid Hormone
to Decrease Anterior Pituitary
Secretion of TSH
Increased thyroi

terior Pituitary
Increased thyroid hormone in the body ﬂuids
decreases secretion of TSH by the anterior pituitary.
When the rate of thyroid hormone secretion rises to
about 1.75 times normal, the rate of TSH secretion
falls essentially to zero. Almost all this feedback
depressant effect occurs even when the anterior pitu-
itary has been separated from the hypothalamus.
Therefore, as shown in Figure 76–7, it is probable that
increased thyroid hormone inhibits anterior pituitary
s

ased thyroid hormone inhibits anterior pituitary
secretion of TSH mainly by a direct effect on the 
anterior pituitary gland itself. Regardless of the mech-
anism of the feedback, its effect is to maintain an
almost constant concentration of free thyroid hor-
mones in the circulating body ﬂuids.
Antithyroid Substances
Drugs that suppress thyroid secretion are called antithy-
roid substances.The best known of these substances are
thiocyanate, propylthiouracil, and high concentrations
of inorganic

ylthiouracil, and high concentrations
of inorganic iodides. The mechanism by which each of
these blocks thyroid secretion is different from the
others, and they can be explained as follows.
Thiocyanate Ions Decrease Iodide Trapping. The same active
pump that transports iodide ions into the thyroid cells
can also pump thiocyanate ions, perchlorate ions, and
nitrate ions. Therefore, the administration of thio-
cyanate (or one of the other ions as well) in high enough
concentration can cause compet

ell) in high enough
concentration can cause competitive inhibition of
iodide transport into the cell— that is, inhibition of the
iodide-trapping mechanism.
The decreased availability of iodide in the glandular
cells does not stop the formation of thyroglobulin; it
merely prevents the thyroglobulin that is formed from
becoming iodinated and therefore from forming the
thyroid hormones. This deﬁciency of the thyroid hor-
mones in turn leads to increased secretion of TSH by
the anterior pituitary gl

ased secretion of TSH by
the anterior pituitary gland, which causes overgrowth of
the thyroid gland even though the gland still does not
form adequate quantities of thyroid hormones. There-
fore, the use of thiocyanates and some other ions to
block thyroid secretion can lead to development of a
greatly enlarged thyroid gland, which is called a goiter.
Propylthiouracil Decreases Thyroid Hormone Formation. Propy-
lthiouracil (and other, similar compounds, such as
methimazole and carbimazole) preve

pounds, such as
methimazole and carbimazole) prevents formation of
thyroid hormone from iodides and tyrosine. The mech-
anism of this is partly to block the peroxidase enzyme
that is required for iodination of tyrosine and partly to
block the coupling of two iodinated tyrosines to form
thyroxine or triiodothyronine.
Propylthiouracil, like thiocyanate, does not prevent
formation of thyroglobulin. The absence of thyroxine
and triiodothyronine in the thyroglobulin can lead to
tremendous feedback en

e thyroglobulin can lead to
tremendous feedback enhancement of TSH secretion
by the anterior pituitary gland, thus promoting growth
of the glandular tissue and forming a goiter.
Iodides in High Concentrations Decrease Thyroid Activity and
Thyroid Gland Size. When iodides are present in the blood
in high concentration (100 times the normal plasma
level),most activities of the thyroid gland are decreased,
but often they remain decreased for only a few weeks.
The effect is to reduce the rate of iod

few weeks.
The effect is to reduce the rate of iodide trapping, so
that the rate of iodination of tyrosine to form thyroid
hormones is also decreased. Even more important, the
normal endocytosis of colloid from the follicles by the
thyroid glandular cells is paralyzed by the high iodide
concentrations. Because this is the ﬁrst step in release
of the thyroid hormones from the storage colloid, there
is almost immediate shutdown of thyroid hormone
secretion into the blood.
Because iodides in high c

ecretion into the blood.
Because iodides in high concentrations decrease all
phases of thyroid activity, they slightly decrease the size
of the thyroid gland and especially decrease its blood
(? increased temperature)
(Thyrotropin-releasing hormone)
Iodine
Thyroxine
Thyroid
Increased
secretion
Thyroid-
hormone
Cells
Increased
Inhibits
Inhibits
?
?
Figure 76–7
Regulation of thyroid secretion.
940
Unit XIV
Endocrinology and Reprodu

 secretion.
940
Unit XIV
supply, in contradistinction to the opposite effects
caused by most of the other antithyroid agents. For this
reason, iodides are frequently administered to patients
for 2 to 3 weeks before surgical removal of the thyroid
gland to decrease the necessary amount of surgery,
especially to decrease the amount of bleeding.
Most effects of hyperthyroidism are obvious from the
preceding discussion of the var

e obvious from the
preceding discussion of the various physiologic effects
of thyroid hormone. However, some speciﬁc effects
should be mentioned in connection especially with the
development, diagnosis, and treatment of hyperthy-
roidism.
Causes of Hyperthyroidism (Toxic Goiter, Thyrotoxicosis, Graves’
Disease). In most patients with hyperthyroidism, the
thyroid gland is increased to two to three times 
normal size, with tremendous hyperplasia and infolding
of the follicular cell lining into the

d infolding
of the follicular cell lining into the follicles, so that the
number of cells is increased greatly. Also, each cell
increases its rate of secretion severalfold; radioactive
iodine uptake studies indicate that some of these hyper-
plastic glands secrete thyroid hormone at rates 5 to 15
The changes in the thyroid gland in most instances 
are similar to those caused by excessive TSH. However,
plasma TSH concentrations are less than normal 
rather than enhanced in almost al

ess than normal 
rather than enhanced in almost all patients and often
are essentially zero. However, other substances that
have actions similar to those of TSH are found in 
the blood of almost all these patients. These substances
are immunoglobulin antibodies that bind with the 
same membrane receptors that bind TSH. They induce
continual activation of the cAMP system of the cells,
with resultant development of hyperthyroidism. These
antibodies are called thyroid-stimulating immunoglobu-
lin a

 are called thyroid-stimulating immunoglobu-
lin and are designated TSI. They have a prolonged 
stimulating effect on the thyroid gland, lasting for as
long as 12 hours, in contrast to a little over 1 hour for
TSH. The high level of thyroid hormone secretion
caused by TSI in turn suppresses anterior pituitary for-
mation of TSH.
The antibodies that cause hyperthyroidism almost
certainly occur as the result of autoimmunity that has
developed against thyroid tissue. Presumably, at some
time in the

st thyroid tissue. Presumably, at some
time in the history of the person, an excess of thyroid
cell antigens was released from the thyroid cells,and this
has resulted in the formation of antibodies against the
thyroid gland itself.
Thyroid Adenoma. Hyperthyroidism occasionally results
from a localized adenoma (a tumor) that develops in the
thyroid tissue and secretes large quantities of thyroid
hormone. This is different from the more usual type of
hyperthyroidism, in that it usually is not asso

of
hyperthyroidism, in that it usually is not associated with
evidence of any autoimmune disease. An interesting
effect of the adenoma is that as long as it continues to
secrete large quantities of thyroid hormone, secretory
function in the remainder of the thyroid gland is almost
totally inhibited because the thyroid hormone from the
adenoma depresses the production of TSH by the pitu-
itary gland.
The symptoms of hyperthyroidism are obvious from the
preceding discus

erthyroidism are obvious from the
preceding discussion of the physiology of the thyroid
hormones: (1) a high state of excitability, (2) intolerance
to heat, (3) increased sweating, (4) mild to extreme
weight loss (sometimes as much as 100 pounds), (5)
varying degrees of diarrhea, (6) muscle weakness, (7)
nervousness or other psychic disorders, (8) extreme
fatigue but inability to sleep, and (9) tremor of the
hands.
Exophthalmos. Most 
people 
with 
hyperthyroidism
develop some degree of protrusi

h 
develop some degree of protrusion of the eyeballs, as
shown in Figure 76–8. This condition is called exoph-
thalmos. A major degree of exophthalmos occurs in
about one third of hyperthyroid patients, and the con-
dition sometimes becomes so severe that the eyeball
protrusion stretches the optic nerve enough to damage
vision. Much more often, the eyes are damaged because
the eyelids do not close completely when the person
blinks or is asleep. As a result, the epithelial surface

 or is asleep. As a result, the epithelial surfaces of
the eyes become dry and irritated and often infected,
resulting in ulceration of the cornea.
The cause of the protruding eyes is edematous
swelling of the retro-orbital tissues and degenerative
changes in the extraocular muscles. In most patients,
immunoglobulins can be found in the blood that react
with the eye muscles. Furthermore, the concentration of
these immunoglobulins is usually highest in patients
who have high concentrations of TSI

st in patients
who have high concentrations of TSIs. Therefore, there
is much reason to believe that exophthalmos,like hyper-
thyroidism itself, is an autoimmune process. The exoph-
thalmos usually is greatly ameliorated with treatment of
the hyperthyroidism.
Figure 76–8
Patient with exophthalmic hyperthyroidism. Note protrusion of the
eyes and retraction of the superior eyelids. The basal metabolic
rate was +40. (Courtesy Dr. Leonard Posey.)
Chapter 76
941

pter 76
941
Diagnostic Tests for Hyperthyroidism. For the usual case of
hyperthyroidism, the most accurate diagnostic test is
direct measurement of the concentration of “free” thy-
roxine (and sometimes triiodothyronine) in the plasma,
using appropriate radioimmunoassay procedures.
Other tests that are sometimes used are as follows:
1. The basal metabolic rate is usually increased to +30
to +60 in severe hyperthyroidism.
2. The concentration of TSH in the plasma is
mea

.
measured by radioimmunoassay. In the usual 
type of thyrotoxicosis, anterior pituitary secretion
of TSH is so completely suppressed by the 
large amounts of circulating thyroxine and
triiodothyronine that there is almost no plasma
TSH.
3. The concentration of TSI is measured by
radioimmunoassay. This is usually high in
thyrotoxicosis but low in thyroid adenoma.
Physiology of Treatment in Hyperthyroidism. The most direct
treatment for hyperthyroidism

ism. The most direct
treatment for hyperthyroidism is surgical removal of
most of the thyroid gland. In general, it is desirable to
prepare the patient for surgical removal of the gland
before the operation. This is done by administering
propylthiouracil, usually for several weeks, until the
basal metabolic rate of the patient has returned to
normal. Then, administration of high concentrations of
iodides for 1 to 2 weeks immediately before operation
causes the gland itself to recede in size and 

ion
causes the gland itself to recede in size and its blood
supply to diminish. By using these preoperative proce-
dures, the operative mortality is less than 1 in 1000 in
the better hospitals, whereas before development of
modern procedures, operative mortality was 1 in 25.
Treatment of the Hyperplastic Thyroid Gland with 
Eighty to 90 per cent of an injected dose of iodide is
absorbed by the hyperplastic, toxic thyroid gland within
1 day after injection. If this injected iod

within
1 day after injection. If this injected iodine is radio-
active, it can destroy most of the secretory cells of the
thyroid gland. Usually 5 millicuries of radioactive iodine
is given to the patient, whose condition is reassessed
several weeks later. If the patient is still hyperthyroid,
additional doses are administered until normal thyroid
status is reached.
The effects of hypothyroidism, in general, are opposite
to those of hyperthyroidism, but there are a few 
physiologi

f hyperthyroidism, but there are a few 
physiologic mechanisms peculiar to hypothyroidism.
Hypothyroidism, like hyperthyroidism, probably is ini-
tiated by autoimmunity against the thyroid gland, but
immunity that destroys the gland rather than stimulates
it.The thyroid glands of most of these patients ﬁrst have
autoimmune “thyroiditis,” which means thyroid inﬂam-
mation.This causes progressive deterioration and ﬁnally
ﬁbrosis of the gland, with resultant diminished or absent
secretion of thyroi

resultant diminished or absent
secretion of thyroid hormone. Several other types of
hypothyroidism also occur, often associated with devel-
opment of enlarged thyroid glands, called thyroid goiter,
Endemic Colloid Goiter Caused by Dietary Iodide Deﬁciency. The
term “goiter” means a greatly enlarged thyroid gland.
As pointed out in the discussion of iodine metabolism,
about 50 milligrams of iodine are required each year
for the formation of adequate quantities of thyroid
hormone. In c

on of adequate quantities of thyroid
hormone. In certain areas of the world, notably in the
Swiss Alps, the Andes, and the Great Lakes region of
the United States, insufﬁcient iodine is present in the
soil for the foodstuffs to contain even this minute quan-
tity. Therefore, in the days before iodized table salt,
many people who lived in these areas developed
extremely large thyroid glands, called endemic goiters.
The mechanism for development of large endemic
goiters is the following: Lack of i

 large endemic
goiters is the following: Lack of iodine prevents pro-
duction of both thyroxine and triiodothyronine. As a
result, no hormone is available to inhibit production of
TSH by the anterior pituitary; this causes the pituitary
to secrete excessively large quantities of TSH. The TSH
then stimulates the thyroid cells to secrete tremendous
amounts of thyroglobulin colloid into the follicles, and
the gland grows larger and larger. But because of lack
of iodine, thyroxine and triiodothyroni

se of lack
of iodine, thyroxine and triiodothyronine production
does not occur in the thyroglobulin molecule and there-
fore does not cause the normal suppression of TSH pro-
duction by the anterior pituitary. The follicles become
tremendous in size, and the thyroid gland may increase
to 10 to 20 times normal size.
Idiopathic Nontoxic Colloid Goiter. Enlarged thyroid glands
similar to those of endemic colloid goiter can also occur
in people who do not have iodine deﬁciency. These
goitrous glands

o not have iodine deﬁciency. These
goitrous glands may secrete normal quantities of
thyroid hormones, but more frequently, the secretion of
hormone is depressed, as in endemic colloid goiter.
The exact cause of the enlarged thyroid gland in
patients with idiopathic colloid goiter is not known, but
most of these patients show signs of mild thyroiditis;
therefore, it has been suggested that the thyroiditis
causes slight hypothyroidism, which then leads to
increased TSH secretion and progressive gr

eads to
increased TSH secretion and progressive growth of the
noninﬂamed portions of the gland. This could explain
why these glands usually are nodular, with some por-
tions of the gland growing while other portions are
being destroyed by thyroiditis.
In some persons with colloid goiter, the thyroid gland
has an abnormality of the enzyme system required for
formation of the thyroid hormones. Among the abnor-
malities often encountered are the following:
1. Deﬁcient iodide-trapping mechanism, in 

lowing:
1. Deﬁcient iodide-trapping mechanism, in which
iodine is not pumped adequately into the thyroid
cells
2. Deﬁcient peroxidase system, in which the iodides
are not oxidized to the iodine state
3. Deﬁcient coupling of iodinated tyrosines in the
thyroglobulin molecule, so that the ﬁnal thyroid
hormones cannot be formed
4. Deﬁciency of the deiodinase enzyme, which
prevents recovery of iodine from the iodinated
tyrosines that are not coupled to form the thyroid
hormones (this is about two thi

o form the thyroid
hormones (this is about two thirds of the iodine),
thus leading to iodine deﬁciency
Finally, some foods contain goitrogenic substances
that have a propylthiouracil-type of antithyroid activity,
thus also leading to TSH-stimulated enlargement of the
thyroid gland. Such goitrogenic substances are found
especially in some varieties of turnips and cabbages.
Physiologic 
Characteristics 
of 
Hypothyroidism. Whether
hypothyroidism is due to thyroiditis, endemic colloid
goiter, idiop

 due to thyroiditis, endemic colloid
goiter, idiopathic colloid goiter, destruction of the
thyroid gland by irradiation, or surgical removal of the
thyroid gland, the physiologic effects are the same.They
942
Unit XIV
include fatigue and extreme somnolence with sleeping
up to 12 to 14 hours a day, extreme muscular sluggish-
ness, slowed heart rate, decreased cardiac output,
decreased blood volume, sometimes increased body
weight, constipation, mental sluggishness, 

d body
weight, constipation, mental sluggishness, failure of
many trophic functions in the body evidenced by
depressed growth of hair and scaliness of the skin,
development of a froglike husky voice, and, in severe
cases,
development of an edematous appearance
throughout the body called myxedema.
Myxedema. Myxedema develops in the patient with
almost total lack of thyroid hormone function. Figure
76–9 shows such a patient, demonstrating bagginess
under the eyes and swelling of the face. In this

condition, for reasons not explained, greatly increased
quantities of hyaluronic acid and chondroitin sulfate
bound with protein form excessive tissue gel in the
interstitial spaces, and this causes the total quantity of
interstitial ﬂuid to increase. Because of the gel nature of
the excess ﬂuid, it is mainly immobile, and the edema is
the nonpitting type.
Atherosclerosis in Hypothyroidism. As pointed out earlier,
lack of thyroid hormone increase

nted out earlier,
lack of thyroid hormone increases the quantity of blood
cholesterol because of altered fat and cholesterol
metabolism 
and 
diminished 
liver 
excretion 
of
cholesterol in the bile.The increase in blood cholesterol
is usually associated with increased atherosclerosis.
Therefore, many hypothyroid patients, particularly
those with myxedema, develop atherosclerosis, which 
in turn results in peripheral vascular disease, deafness,
and coronary artery disease with consequent early
d

nd coronary artery disease with consequent early
death.
Diagnostic Tests in Hypothyroidism. The tests already
described for diagnosis of hyperthyroidism give oppo-
site results in hypothyroidism. The free thyroxine in 
the blood is low. The basal metabolic rate in myxedema
ranges between -30 and -50. And the secretion of 
TSH by the anterior pituitary when a test dose of 
TRH is administered is usually greatly increased
(except in those rare instances of hypothyroidism
caused by depressed respon

ances of hypothyroidism
caused by depressed response of the pituitary gland to
TRH).
Treatment of Hypothyroidism. Figure 76–4 shows the effect
of thyroxine on the basal metabolic rate, demonstrating
that the hormone normally has a duration of action of
more than 1 month. Consequently, it is easy to maintain
a steady level of thyroid hormone activity in the body
by daily oral ingestion of a tablet or more containing
proper treatment of the
hypothyroid patient results in su

treatment of the
hypothyroid patient results in such complete normality
that formerly myxedematous patients have lived into
their 90s after treatment for more than 50 years.
Cretinism
Cretinism is caused by extreme hypothyroidism during
fetal life, infancy, or childhood. This condition is char-
acterized especially by failure of body growth and by
mental retardation. It results from congenital lack of a
thyroid gland (congenital cretinism), from failure of the
thyroid gland to produce thyroid ho

failure of the
thyroid gland to produce thyroid hormone because of a
genetic defect of the gland, or from iodine lack in the
diet (endemic cretinism). The severity of endemic cre-
tinism varies greatly, depending on the amount of iodine
in the diet, and whole populaces of an endemic geo-
graphic iodine-deﬁcient soil area have been known to
have cretinoid tendencies.
A neonate without a thyroid gland may have normal
appearance and function because it was supplied with
some (but usually not enough

 it was supplied with
some (but usually not enough) thyroid hormone by the
mother while in utero, but a few weeks after birth, the
neonate’s movements become sluggish and both physi-
cal and mental growth begin to be greatly retarded.
Treatment of the neonate with cretinism at any time
with adequate iodine or thyroxine usually causes
normal return of physical growth, but unless the cre-
tinism is treated within a few weeks after birth, mental
growth remains permanently retarded.This results from

wth remains permanently retarded.This results from
retardation of the growth, branching, and myelination
of the neuronal cells of the central nervous system at
this critical time in the normal development of the
mental powers.
Skeletal growth in the child with cretinism is charac-
teristically more inhibited than is soft tissue growth. As
a result of this disproportionate rate of growth, the soft
tissues are likely to enlarge excessively, giving the child
with cretinism an obese, stocky, and sho

the child
with cretinism an obese, stocky, and short appearance.
Occasionally the tongue becomes so large in relation to
the skeletal growth that it obstructs swallowing and
breathing, inducing a characteristic guttural breathing
that sometimes chokes the child.
Figure 76–9
Patient with myxedema. (Courtesy Dr. Herbert Langford.)
Chapter 76
943
Besser 
GM,
Thorner 
MO:
Clinical
Science, 2002.

d ed. Philadelphia: Mosby, Elsevier
Burger AG: Environment and thyroid function. J Clin
Endocrinol Metab 89:1526, 2004.
Cooper DS: Hyperthyroidism. Lancet 362:459, 2003.
De La Vieja A, Dohan O, et al: Molecular analysis of the
sodium/iodide symporter: impact on thyroid and extrathy-
roid pathophysiology. Physiol Rev 80:1083, 2000.
Dayan CM: Interpretation of thyroid function tests. Lancet
357:619, 2001.
Dohan O, De La Vieja A, Paroder V, et al:The sodium/iodide
Symporter 
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der V, et al:The sodium/iodide
Symporter 
(NIS):
characterization,
regulation,
and
medical signiﬁcance. Endocr Rev 24:48, 2003.
phia: WB Saunders Co, 2003.
Marino M, McCluskey RT: Role of thyroglobulin endocytic
pathways in the control of thyroid hormone release.Am J
Physiol Cell Physiol 279:C1295, 2000
O’Reilly DS: Thyroid function tests—time for a reassess-
ment. BMJ 320:1332, 2000.
Pearce E

for a reassess-
Pearce EN, Farwell AP, Braverman LE: Thyroiditis. N Engl
J Med 348:2646, 2003.
Roberts CG, Ladenson PW: Hypothyroidism. Lancet 363:
793, 2004.
Stassi G, De Maria R: Autoimmune thyroid disease: new
models of cell death in autoimmunity. Nat Rev Immunol
2:195, 2002.
Szkudlinski MW, Fremont V, Ronin C, Weintraub BD:
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n C, Weintraub BD:
Thyroid-stimulating hormone and thyroid-stimulating
hormone 
receptor 
structure-function 
relationships.
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Tomer Y, Davies TF: Searching for the autoimmune thyroid
disease susceptibility genes: from gene mapping to gene
function. Endocr Rev 24:694, 2003
Vaidya B, Kendall-Taylor P, Pearce SH: The genetics of
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hormone receptor interaction

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Zhang J, Lazar MA:The mechanism of action of thyroid hor-
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C
H
A
P
T
E
R
 
7
7
944
The two adrenal glands, each of which weighs about
4 grams, lie at the superior poles of the two kidneys.
As shown in Figure 77–1, e

les of the two kidneys.
As shown in Figure 77–1, each gland is composed of
two distinct parts, the adrenal medulla and the
adrenal cortex. The adrenal medulla, the central 20
per cent of the gland, is functionally related to the
sympathetic nervous system; it secretes the hor-
mones epinephrine and norepinephrine in response
to sympathetic stimulation. In turn, these hormones cause almost the same
effects as direct stimulation of the sympathetic nerves in all parts of the body.
These hormones an

nerves in all parts of the body.
These hormones and their effects are discussed in detail in Chapter 60 in rela-
tion to the sympathetic nervous system.
The adrenal cortex secretes an entirely different group of hormones, called
corticosteroids. These hormones are all synthesized from the steroid cholesterol,
and they all have similar chemical formulas. However, slight differences in their
molecular structures give them several different but very important functions.
Corticosteroids Mineralocort

 important functions.
Corticosteroids Mineralocorticoids, Glucocorticoids, and Androgens. Two major types of
adrenocortical hormones, the mineralocorticoids and the glucocorticoids, are
secreted by the adrenal cortex. In addition to these, small amounts of sex hor-
mones are secreted, especially androgenic hormones, which exhibit about the
same effects in the body as the male sex hormone testosterone. They are nor-
mally of only slight importance, although in certain abnormalities of the adrenal

, although in certain abnormalities of the adrenal
cortices, extreme quantities can be secreted (which is discussed later in the
chapter) and can result in masculinizing effects.
The mineralocorticoids have gained this name because they especially affect
the electrolytes (the “minerals”) of the extracellular ﬂuids-sodium and potas-
sium, in particular. The glucocorticoids have gained their name because they
exhibit important effects that increase blood glucose concentration. They have
additional

 blood glucose concentration. They have
additional effects on both protein and fat metabolism that are equally as impor-
tant to body function as their effects on carbohydrate metabolism.
More than 30 steroids have been isolated from the adrenal cortex, but two
are of exceptional importance to the normal endocrine function of the human
body: aldosterone, which is the principal mineralocorticoid, and cortisol, which
is the principal glucocorticoid.
Adrenocortical Hormo

.
The Adrenal Cortex Has Three Distinct Layers. Figure 77–1 shows that the adrenal
cortex is composed of three relatively distinct layers:
1. The zona glomerulosa, a thin layer of cells that lies just underneath the
capsule, constitutes about 15 per cent of the adrenal cortex. These cells are
the only ones in the adrenal gland capable of secreting signiﬁcant amounts
of aldosterone because they contain the enzyme aldosterone synthase,
which is n

ontain the enzyme aldosterone synthase,
which is necessary for synthesis of aldosterone. The secretion of these cells
is controlled mainly by the extracellular ﬂuid concentrations of angiotensin
II and potassium, both of which stimulate aldosterone secretion.
2. The zona fasciculata, the middle and widest layer, constitutes about 75 per
cent of the adrenal cortex and secretes the glucocorticoids cortisol and
Chapter 77
945
corticosterone, as well as small amounts of 
adre

adrenal androgens and estrogens. The secretion 
of these cells is controlled in large part by 
the hypothalamic-pituitary axis via
adrenocorticotropic hormone (ACTH).
3. The zona reticularis, the deep layer of the 
cortex, secretes the adrenal androgens
dehydroepiandrosterone (DHEA) and
androstenedione, as well as small amounts of
estrogens and some glucocorticoids. ACTH also
regulates secretion of these cells, although other
factors such as cortical

ese cells, although other
factors such as cortical androgen-stimulating
hormone, released from the pituitary, may also be
involved. The mechanisms for controlling adrenal
androgen production, however, are not nearly as
well understood as those for glucocorticoids and
mineralocorticoids.
Aldosterone and cortisol secretion are regulated by
independent mechanisms. Factors such as angiotensin
II that speciﬁcally increase the output of aldosterone
and cause hypertrophy of the zona glomerulosa have
no

 cause hypertrophy of the zona glomerulosa have
no effect on the other two zones. Similarly, factors such
as ACTH that increase secretion of cortisol and
adrenal androgens and cause hypertrophy of the zona
fasciculata and zona reticularis have little or no effect
on the zona glomerulosa.
Adrenocortical Hormones Are Steroids Derived from Cholesterol.
All human steroid hormones, including those produced
by the adrenal cortex, are synthesized from cholesterol.
Although the cells of the adrenal cort

holesterol.
Although the cells of the adrenal cortex can synthesize
de novo small amounts of cholesterol from acetate,
approximately 80 per cent of the cholesterol used for
steroid synthesis is provided by low-density lipoproteins
(LDL) in the circulating plasma. The LDLs, which have
high concentrations of cholesterol, diffuse from the
plasma into the interstitial ﬂuid and attach to speciﬁc
receptors contained in structures called coated pits on
the adrenocortical cell membranes. The coated pits

the adrenocortical cell membranes. The coated pits are
then internalized by endocytosis, forming vesicles that
eventually fuse with cell lysosomes and release choles-
terol that can be used to synthesize adrenal steroid 
hormones.
Transport of cholesterol into the adrenal cells is reg-
ulated by feedback mechanisms that can markedly alter
the amount available for steroid synthesis. For example,
ACTH, which stimulates adrenal steroid synthesis,
increases the number of adrenocortical cell receptor

creases the number of adrenocortical cell receptors
for LDL, as well as the activity of enzymes that liberate
cholesterol from LDL.
Once the cholesterol enters the cell, it is delivered to
the mitochondria, where it is cleaved by the enzyme
cholesterol desmolase to form pregnenolone; this is the
rate-limiting step in the eventual formation of adrenal
steroids (Figure 77–2). In all three zones of the adrenal
cortex, this initial step in steroid synthesis is stimulated
by the different factors tha

nthesis is stimulated
by the different factors that control secretion of the
major hormone products aldosterone and cortisol. For
example, both ACTH, which stimulates cortisol secre-
tion, and angiotensin II, which stimulates aldosterone
secretion, increase the conversion of cholesterol to
pregnenolone.
Synthetic Pathways for Adrenal Steroids. Figure 77–2 gives
the principal steps in the formation of the important
steroid products of the adrenal cortex: aldosterone, cor-
tisol, and the androgens

cortex: aldosterone, cor-
tisol, and the androgens. Essentially all these steps occur
in two of the organelles of the cell, the mitochondria and
the endoplasmic reticulum, some steps occurring in one
of these organelles and some in the other. Each step is
catalyzed by a speciﬁc enzyme system.A change in even
a single enzyme in the schema can cause vastly differ-
ent types and relative proportions of hormones to be
formed. For example, very large quantities of masculin-
izing sex hormones or othe

quantities of masculin-
izing sex hormones or other steroid compounds not 
normally present in the blood can occur with altered
activity of only one of the enzymes in this pathway.
The chemical formulas of aldosterone and cortisol,
which are the most important mineralocorticoid and
glucocorticoid hormones, respectively, are shown in
Figure 77–2. Cortisol has a keto-oxygen on carbon
number 3 and is hydroxylated at carbon numbers 11 and
21. The mineralocorticoid aldosterone has an oxygen
atom boun

eralocorticoid aldosterone has an oxygen
atom bound at the number 18 carbon.
In addition to aldosterone and cortisol, other steroids
having glucocorticoid or mineralocorticoid activities, or
both, are normally secreted in small amounts by the
adrenal cortex. And several additional potent steroid
hormones not normally formed in the adrenal glands
have been synthesized and are used in various forms of
therapy. Some of the more important of the corticos-
teroid hormones, including the synthetic one

icos-
teroid hormones, including the synthetic ones, are the
following as summarized in Table 77–1:
• Aldosterone (very potent, accounts for about 90 per
cent of all mineralocorticoid activity)
• Desoxycorticosterone (1/30 as potent as
aldosterone, but very small quantities secreted)
• Corticosterone (slight mineralocorticoid activity)
• 9a-Fluorocortisol (synthetic, slightly more potent
than aldosterone)
Cortisol
and
androgens
Magnified section
aldosterone
Zo

Cortex
Medulla
(catecholamines)
Figure 77–1
Secretion of adrenocortical hormones by the different zones of the
adrenal cortex and secretion of catecholamines by the adrenal
medulla.
946
Unit XIV
desmolase
(P450 scc)
17a-Hydroxylase
(P450 c17)
3b-Hydroxysteroid
HO
C
O
CH3
C
D
A
HO
3
2
1
10
14
15
19
11
12
13
18
17
16
21 20 22
23
24
26
27
25
4
6
9
7
8
5
B
17a-Hy

17
16
21 20 22
23
24
26
27
25
4
6
9
7
8
5
B
(P450 c17)
21b-Hydroxylase
(P450 c21)
O
C
O
CH3
11b-Hydroxylase
(P450 c11)
O
C
O
CH2OH
synthase
(P450 c11AS)
O
HO
C
O
CH2OH
17, 20 Lyase
(P450 c17)
(P450 c17)
HO
C
O
OH
OH
OH
OH
CH3
O
C
O
CH3
O
C
O
CH2OH
HO
O
O
O
O
HO
C
O
CH2OH
O
HO
HC C
O
O CH2OH
Dehydroepl-

epl-
Cortisol
Figure 77–2
Pathways for synthesis of steroid hormones by the adrenal cortex. The enzymes are shown in italics.
Chapter 77
947
• Cortisol (very slight mineralocorticoid activity, but
large quantity secreted)
• Cortisone (synthetic, slight mineralocorticoid
activity)
• Cortisol (very potent, accounts for about 95 per
cent of all glucocorticoid activity)
• Corticosterone (pro

all glucocorticoid activity)
• Corticosterone (provides about 4 per cent of total
glucocorticoid activity, but much less potent than
cortisol)
• Cortisone (synthetic, almost as potent as cortisol)
• Prednisone (synthetic, four times as potent as
cortisol)
• Methylprednisone (synthetic, ﬁve times as potent as
cortisol)
• Dexamethasone (synthetic, 30 times as potent as
cortisol)
It is clear from this list that some of these hormones
have both glucocorticoid and mineralocorticoid activi-
ties. It i

corticoid and mineralocorticoid activi-
ties. It is especially signiﬁcant that cortisol has a small
amount of mineralocorticoid activity, because some syn-
dromes of excess cortisol secretion can cause signiﬁcant
mineralocorticoid effects, along with its much more
potent glucocorticoid effects.
The intense glucocorticoid activity of the synthetic
hormone dexamethasone, which has almost zero min-
eralocorticoid activity, makes this an especially impor-
tant drug for stimulating speciﬁc glucocorti

mpor-
tant drug for stimulating speciﬁc glucocorticoid activity.
Adrenocortical Hormones Are Bound to Plasma Proteins.
Approximately 90 to 95 per cent of the cortisol in the
plasma binds to plasma proteins, especially a globulin
called cortisol-binding globulin or transcortin and, to a
lesser extent, to albumin.This high degree of binding to
plasma proteins slows the elimination of cortisol from
the plasma; therefore, cortisol has a relatively long half-
life of 60 to 90 minutes. Only about 60 p

ng half-
life of 60 to 90 minutes. Only about 60 per cent of cir-
culating aldosterone combines with the plasma proteins,
so that about 40 per cent is in the free form; as a result,
aldosterone has a relatively short half-life of about 20
minutes. In both the combined and free forms, the hor-
mones are transported throughout the extracellular
ﬂuid compartment.
Binding of adrenal steroids to the plasma proteins
may serve as a reservoir to lessen rapid ﬂuctuations 
in free hormone concentrations, 

apid ﬂuctuations 
in free hormone concentrations, as would occur, for
example, with cortisol during brief periods of stress and
episodic secretion of ACTH. This reservoir function
may also help to ensure a relatively uniform distribu-
tion of the adrenal hormones to the tissues.
Adrenocortical Hormones Are Metabolized in the Liver. The
adrenal steroids are degraded mainly in the liver and
conjugated especially to glucuronic acid and, to a lesser
extent, sulfates. These substances are inactive an

extent, sulfates. These substances are inactive and do
not have mineralocorticoid or glucocorticoid activity.
About 25 per cent of these conjugates are excreted in
the bile and then in the feces.The remaining conjugates
formed by the liver enter the circulation but are not
bound to plasma proteins, are highly soluble in the
plasma, and are therefore ﬁltered readily by the kidneys
and excreted in the urine. Diseases of the liver markedly
depress the rate of inactivation of adrenocortical hor-
mon

he rate of inactivation of adrenocortical hor-
mones, and kidney diseases reduce the excretion of the
inactive conjugates.
The normal concentration of aldosterone in blood 
is about 6 nanograms (6 billionths of a gram) per 
100 ml, and the average secretory rate is approximately
150 µg/day (0.15 mg/day).
The concentration of cortisol in the blood averages
12 µg/100 ml, and the secretory rate averages 15 to 
20 mg/day.
Functions of the
Mineralocorticoids-
Mineralocorticoid Deﬁciency C

ticoids-
Mineralocorticoid Deﬁciency Causes Severe Renal Sodium
Chloride Wasting and Hyperkalemia. Total loss of adreno-
cortical secretion usually causes death within 3 days to
2 weeks unless the person receives extensive salt
therapy or injection of mineralocorticoids.
Without mineralocorticoids, potassium ion concen-
tration of the extracellular ﬂuid rises markedly, sodium
Table 77–1
Adrenal Steroid Hormones in Adults; Synthetic Steroids and Their Relative Glucocorticoid and Miner

eroids and Their Relative Glucocorticoid and Mineralocorticoid Activities
Average Plasma
(free and bound,
Average Amount 
Steroids
mg/100 ml)
Secreted (mg/24 hr)
Activity
Activity
Adrenal Steroids
Cortisol
12
15
1
1
0.4
3
0.3
15.0
0.006
0.15
0.3
3000
0.006
0.2
0.2
100
175
20
—
—
Synthetic Steroids
Cortisone
—
—
1.0
1.0
—
—
4
0.8
Methylprednisone
—
—
5
—

e
—
—
4
0.8
—
—
5
—
—
—
30
—
9a-ﬂuorocortisol
—
—
10
125
Glucocorticoid and mineralocorticoid activities of the steroids are relative to cortisol, with cortisol being 1.0.
948
Unit XIV
and chloride are rapidly lost from the body, and the
total extracellular ﬂuid volume and blood volume
become greatly reduced. The person soon develops
diminished cardiac output, which progresses to a
shocklike state,
followed by death. This entire
seque

cklike state,
sequence can be prevented by the administration of
aldosterone or some other mineralocorticoid. There-
fore, the mineralocorticoids are said to be the acute
“lifesaving” portion of the adrenocortical hormones.
The glucocorticoids are equally necessary, however,
allowing the person to resist the destructive effects of
life’s intermittent physical and mental “stresses,” as
Aldosterone Is the Major Mineralocorticoid Secret

Aldosterone Is the Major Mineralocorticoid Secreted by the
Adrenals. Aldosterone exerts nearly 90 per cent of the
mineralocorticoid activity of the adrenocortical secre-
tions, but cortisol, the major glucocorticoid secreted by
the adrenal cortex, also provides a signiﬁcant amount
of mineralocorticoid activity. Aldosterone’s mineralo-
corticoid activity is about 3000 times greater than that
of cortisol, but the plasma concentration of cortisol is
nearly 2000 times that of aldosterone.
Renal and

s
Renal and Circulatory Effects 
of Aldosterone
Aldosterone Increases Renal Tubular Reabsorption of Sodium
and Secretion of Potassium. It will be recalled from
Chapter 27 that aldosterone increases absorption 
of sodium and simultaneously increases secretion 
of potassium by the renal tubular epithelial cells-
especially in the principal cells of the collecting tubules
and, to a lesser extent, in the distal tubules and col-
lecting ducts. Therefore, aldoste

tubules and col-
lecting ducts. Therefore, aldosterone causes sodium to
be conserved in the extracellular ﬂuid while increasing
potassium excretion in the urine.
A high concentration of aldosterone in the plasma
can transiently decrease the sodium loss into the urine
to as little as a few milliequivalents a day. At the same
time, potassium loss into the urine increases several-
fold. Therefore, the net effect of excess aldosterone 
in the plasma is to increase the total quantity of
sodium in the

is to increase the total quantity of
sodium in the extracellular ﬂuid while decreasing the
Conversely, total lack of aldosterone secretion can
cause transient loss of 10 to 20 grams of sodium in the
urine a day, an amount equal to one tenth to one ﬁfth
of all the sodium in the body. At the same time, potas-
sium is conserved tenaciously in the extracellular ﬂuid.
Excess Aldosterone Increases Extracellular Fluid Volume and
Arterial Pressure but Has Only a Small Effect on Plasma Sodium

sure but Has Only a Small Effect on Plasma Sodium
Concentration. Although aldosterone has a potent
effect in decreasing the rate of sodium ion excretion
by the kidneys, the concentration of sodium in the
extracellular ﬂuid often rises only a few milliequiva-
lents. The reason for this is that when sodium is reab-
sorbed by the tubules, there is simultaneous osmotic
absorption of almost equivalent amounts of water.
Also, small increases in extracellular ﬂuid sodium 
concentration stimulate thirst

llular ﬂuid sodium 
concentration stimulate thirst and increased water
intake, if water is available. Therefore, the extracellu-
lar ﬂuid volume increases almost as much as the
retained sodium, but without much change in sodium
Even though aldosterone is one of the body’s most
powerful sodium-retaining hormones, only transient
sodium retention occurs when excess amounts are
secreted. An aldosterone-mediated increase in extra-
cellular ﬂuid volume lasting more than 1 to 2 days 
als

lar ﬂuid volume lasting more than 1 to 2 days 
also leads to an increase in arterial pressure, as
explained in Chapter 19. The rise in arterial pressure
then increases kidney excretion of both salt and 
water, called pressure natriuresis and pressure diuresis,
respectively. Thus, after the extracellular ﬂuid volume
increases 5 to 15 per cent above normal, arterial pres-
sure also increases 15 to 25 mm Hg, and this elevated
blood pressure returns the renal output of salt and
water to normal despi

the renal output of salt and
water to normal despite the excess aldosterone (Figure
77–3).
This return to normal of salt and water excretion by
the kidneys as a result of pressure natriuresis and
100
400
300
200
-4
14
-2
2
4
6
8
10
12
0
Urinary sodium
excretion (mEq/day)
100
90
120
110
volume  (% Normal)
100
80
120
Mean arterial
pressure  (mm Hg)
Figure 77–3
Effect of aldosterone infusion on arterial pressure, extracellular
ﬂuid volume, and sodium excr

essure, extracellular
ﬂuid volume, and sodium excretion in dogs. Although aldosterone
was infused at a rate that raised plasma concentrations to about
20 times normal, note the “escape” from sodium retention on the
second day of infusion as arterial pressure increased and urinary
sodium excretion returned to normal. (Drawn from data in Hall JE,
Granger JP, Smith MJ Jr, Premen N: Role of hemodynamics and
arterial pressure in aldosterone “escape.” Hypertension 6 (suppl
I):I-183–I-192, 1984.)
Chapt

Hypertension 6 (suppl
Chapter 77
949
diuresis is called aldosterone escape. Thereafter, the
rate of gain of salt and water by the body is zero, and
balance is maintained between salt and water intake
and output by the kidneys despite continued excess
aldosterone. In the meantime, however, the person has
developed hypertension, which lasts as long as the
person remains exposed to high levels of aldosterone.
Conversely, when aldosterone secretion beco

erone.
Conversely, when aldosterone secretion becomes
zero, large amounts of salt are lost in the urine, not
only diminishing the amount of sodium chloride in the
extracellular ﬂuid but also decreasing the extracellular
ﬂuid volume. The result is severe extracellular ﬂuid
dehydration and low blood volume, leading to circula-
tory shock. Without therapy, this usually causes death
within a few days after the adrenal glands suddenly
stop secreting aldosterone.
Excess Aldosterone Causes Hypokalemia 

ldosterone.
Excess Aldosterone Causes Hypokalemia and Muscle Weak-
ness; Too Little Aldosterone Causes Hyperkalemia and Cardiac
Toxicity. Excess aldosterone not only causes loss of
potassium ions from the extracellular ﬂuid into the
urine but also stimulates transport of potassium 
from the extracellular ﬂuid into most cells of the 
body. Therefore, excessive secretion of aldosterone,
as occurs with some types of adrenal tumors, may 
cause a serious decrease in the plasma potassium 
concentratio

ous decrease in the plasma potassium 
concentration, sometimes from the normal value of 
4.5 mEq/L to as low as 2 mEq/L. This condition is
called hypokalemia. When the potassium ion concen-
tration falls below about one-half normal, severe
muscle weakness often develops. This is caused by
alteration of the electrical excitability of the nerve and
muscle ﬁber membranes (see Chapter 5), which pre-
vents transmission of normal action potentials.
Conversely, when aldosterone is deﬁcient, the extra-

versely, when aldosterone is deﬁcient, the extra-
cellular ﬂuid potassium ion concentration can rise far
above normal.When it rises to 60 to 100 per cent above
normal, serious cardiac toxicity, including weakness of
heart contraction and development of arrhythmia,
becomes evident; progressively higher concentrations
of potassium lead inevitably to heart failure.
Excess Aldosterone Increases Tubular Hydrogen Ion Secretion,
and Causes Mild Alkalosis. Aldosterone not only causes
potassium to be sec

s. Aldosterone not only causes
potassium to be secreted into the tubules in exchange
for sodium reabsorption in the principal cells of the
renal collecting tubules but also causes secretion of
hydrogen ions in exchange for sodium in the interca-
lated cells of the cortical collecting tubules. This
decreases the hydrogen ion concentration in the extra-
cellular ﬂuid, causing a mild degree of alkalosis.
Aldosterone Stimulates Sodium and
Potassium Transport in Sweat Glands,
Salivary Glands, and Int

ransport in Sweat Glands,
Salivary Glands, and Intestinal
Epithelial Cells
Aldosterone has almost the same effects on sweat
glands and salivary glands as it has on the renal
tubules. Both these glands form a primary secretion
that contains large quantities of sodium chloride, but
much of the sodium chloride, on passing through the
excretory ducts, is reabsorbed, whereas potassium and
bicarbonate ions are secreted. Aldosterone greatly
increases the reabsorption of sodium chloride and the
secretio

e reabsorption of sodium chloride and the
secretion of potassium by the ducts. The effect on the
sweat glands is important to conserve body salt in hot
environments, and the effect on the salivary glands is
necessary to conserve salt when excessive quantities of
saliva are lost.
Aldosterone also greatly enhances sodium absorp-
tion by the intestines, especially in the colon, which
prevents loss of sodium in the stools. Conversely, in the
absence of aldosterone, sodium absorption can be
poor, lea

of aldosterone, sodium absorption can be
poor, leading to failure to absorb chloride and other
anions and water as well. The unabsorbed sodium
chloride and water then lead to diarrhea, with further
loss of salt from the body.
Cellular Mechanism of 
Aldosterone Action
Although for many years we have known the overall
effects of mineralocorticoids on the body, the basic
action of aldosterone on the tubular cells to increase
transport of sodium is still not fully understood.
However, the cellular s

till not fully understood.
However, the cellular sequence of events that leads 
to increased sodium reabsorption seems to be the 
First, because of its lipid solubility in the cellular
membranes,aldosterone diffuses readily to the interior
of the tubular epithelial cells.
Second, in the cytoplasm of the tubular cells, aldos-
terone combines with a highly speciﬁc cytoplasmic
receptor protein, a protein that has a stereomolecular
conﬁguration that allows only aldosterone or very
similar

ation that allows only aldosterone or very
similar compounds to combine with it.
Third, the aldosterone-receptor complex or a
product of this complex diffuses into the nucleus,
where it may undergo further alterations, ﬁnally induc-
ing one or more speciﬁc portions of the DNA to form
one or more types of messenger RNA related to the
process of sodium and potassium transport.
Fourth, the messenger RNA diffuses back into the
cytoplasm, where, operating in conjunction with the
ribosomes, it causes 

ting in conjunction with the
ribosomes, it causes protein formation. The proteins
formed are a mixture of (1) one or more enzymes 
and (2) membrane transport proteins that,
all 
acting together, are required for sodium, potassium,
and hydrogen transport through the cell membrane.
One of the enzymes especially increased is sodium-
potassium adenosine triphosphatase, which serves as
the principal part of the pump for sodium and potas-
sium exchange at the basolateral membranes of the
renal tubular

 at the basolateral membranes of the
renal tubular cells. Additional proteins, perhaps
equally important, are epithelial sodium channel pro-
teins inserted into the luminal membrane of the same
tubular cells that allows rapid diffusion of sodium ions
from the tubular lumen into the cell; then the sodium
950
Unit XIV
is pumped the rest of the way by the sodium-potassium
pump located in the basolateral membranes of the cell.
Thus,aldosterone does not have an immediat

e cell.
Thus,aldosterone does not have an immediate effect
on sodium transport; rather, this effect must await the
sequence of events that leads to the formation of the
speciﬁc intracellular substances required for sodium
transport. About 30 minutes is required before new
RNA appears in the cells, and about 45 minutes is
required before the rate of sodium transport begins to
increase; the effect reaches maximum only after
several hours.
Possible Nongenomic Actions 
of Aldosterone and Other 
Ster

Nongenomic Actions 
Recent studies suggest that many steroids, including
aldosterone, elicit not only slowly developing genomic
effects that have a latency of 60 to 90 minutes and
require gene transcription and synthesis of new pro-
teins, but also rapid nongenomic effects that take place
in a few seconds or minutes.
These nongenomic actions are believed to be medi-
ated by binding of steroids to cell membrane receptors
that are coupled to second messen

mbrane receptors
that are coupled to second messenger systems, similar
to those used for peptide hormone signal transduction.
For example, aldosterone has been shown to increase
formation of cAMP in vascular smooth muscle cells
and in epithelial cells of the renal collecting tubules in
less than two minutes, a time period that is far too
short for gene transcription and synthesis of new pro-
teins. In other cell types, aldosterone has been shown
to rapidly stimulate the phosphatidylinositol seco

to rapidly stimulate the phosphatidylinositol second
messenger system. However, the precise structure of
receptors responsible for the rapid effects of aldos-
terone has not been determined, nor is the physiolog-
ical signiﬁcance of these nongenomic actions of
steroids well understood.
The regulation of aldosterone secretion is so deeply
intertwined with the regulation of extracellular ﬂuid
electrolyte concentrations, extracellular ﬂuid volume,
blood volume, a

ations, extracellular ﬂuid volume,
blood volume, arterial pressure, and many special
aspects of renal function that it is difﬁcult to discuss
the regulation of aldosterone secretion independently
of all these other factors. This subject is presented in
detail in Chapters 28 and 29, to which the reader is
referred. However, it is important to list here some of
the more important points of aldosterone secretion
control.
The regulation of aldosterone secretion by the zona
glomerulosa cells is almos

e secretion by the zona
glomerulosa cells is almost entirely independent of the
regulation of cortisol and androgens by the zona fas-
ciculata and zona reticularis.
Four factors are known to play essential roles in the
regulation of aldosterone. In the probable order of
their importance, they are as follows:
1. Increased potassium ion concentration in the
extracellular ﬂuid greatly increases aldosterone
2. Increased activity of the renin-angiotensin system
(increased levels of angiote

in-angiotensin system
(increased levels of angiotensin II) also greatly
increases aldosterone secretion.
3. Increased sodium ion concentration in the
extracellular ﬂuid very slightly decreases
4. ACTH from the anterior pituitary gland is
necessary for aldosterone secretion but has little
effect in controlling the rate of secretion.
Of these factors,
potassium ion concentration
and the renin-angiotensin system are by far the most
potent in regulating aldosterone secretion. 

 most
potent in regulating aldosterone secretion. A small
percentage increase in potassium concentration can
cause a severalfold increase in aldosterone secretion.
Likewise, activation of the renin-angiotensin system,
usually in response to diminished blood ﬂow to the
kidneys or to sodium loss, can cause a severalfold
increase in aldosterone secretion. In turn, the aldos-
terone acts on the kidneys (1) to help them excrete the
excess potassium ions and (2) to increase the blood
volume and arteri

ns and (2) to increase the blood
volume and arterial pressure, thus returning the renin-
angiotensin system toward its normal level of activity.
These feedback control mechanisms are essential for
maintaining life, and the reader is referred again to
Chapters 27 and 29 for a full understanding of their
Figure 77–4 shows the effects on plasma aldosterone
concentration caused by blocking the formation of
angiotensin II with an angiotensin-converting enzyme
inhibitor after several weeks 

n-converting enzyme
inhibitor after several weeks of a low-sodium diet that
increases plasma aldosterone concentration several-
fold. Note that blocking angiotensin II formation
markedly decreases plasma aldosterone concentration
without signiﬁcantly changing cortisol concentration;
this indicates the important role of angiotensin II in
stimulating aldosterone secretion when sodium intake
and extracellular ﬂuid volume are reduced.
By contrast, the effects of sodium ion concentration
per se and o

e effects of sodium ion concentration
per se and of ACTH in controlling aldosterone secre-
tion are usually minor.Nevertheless,a 10 to 20 per cent
decrease in extracellular ﬂuid sodium ion concentra-
tion, which occurs on rare occasions, can perhaps
double aldosterone secretion. In the case of ACTH, if
there is even a small amount of ACTH secreted by the
anterior pituitary gland, it is usually enough to permit
the adrenal glands to secrete whatever amount of
aldosterone is required, but total ab

er amount of
aldosterone is required, but total absence of ACTH
can signiﬁcantly reduce aldosterone secretion.
Even though mineralocorticoids can save the life of 
an acutely adrenalectomized animal, the animal still 
is far from normal. Instead, its metabolic systems 
for utilization of proteins, carbohydrates, and fats
remain considerably deranged.
the
Chapter 77
951
animal cannot resist different types of physical or even
m

annot resist different types of physical or even
mental stress, and minor illnesses such as respiratory
tract infections can lead to death. Therefore, the glu-
cocorticoids have functions just as important to the
long-continued life of the animal as those of the min-
eralocorticoids. They are explained in the following 
sections.
At least 95 per cent of the glucocorticoid activity of
the adrenocortical secretions results from the secre-
tion of cortisol, known also as hydrocortisone. In 
additio

ortisol, known also as hydrocortisone. In 
addition to this, a small but signiﬁcant amount of glu-
cocorticoid activity is provided by corticosterone.
Effects of Cortisol on 
Stimulation of Gluconeogenesis. By far the best-known
metabolic effect of cortisol and other glucocorticoids
on metabolism is their ability to stimulate gluconeo-
genesis (formation of carbohydrate from proteins and
some other substances) by the liver, often increasing
the rate of gluconeogenesis as 

, often increasing
the rate of gluconeogenesis as much as 6- to 10-fold.
This results mainly from two effects of cortisol.
1. Cortisol increases the enzymes required to convert
amino acids into glucose in the liver cells. This
results from the effect of the glucocorticoids to
activate DNA transcription in the liver cell nuclei
in the same way that aldosterone functions in the
renal tubular cells, with formation of messenger
RNAs that in turn lead to the array of enzymes
required for gluconeogene

 to the array of enzymes
required for gluconeogenesis.
2. Cortisol causes mobilization of amino acids from
the extrahepatic tissues mainly from muscle. As a
result, more amino acids become available in the
plasma to enter into the gluconeogenesis process
of the liver and thereby to promote the formation
One of the effects of increased gluconeogenesis is a
marked increase in glycogen storage in the liver cells.
This effect of cortisol allows other glycolytic hor-
mones, such as epinep

allows other glycolytic hor-
mones, such as epinephrine and glucagon, to mobilize
glucose in times of need, such as between meals.
Decreased Glucose Utilization by Cells. Cortisol also causes
a moderate decrease in the rate of glucose utilization
by most cells in the body. Although the cause of this
decrease is unknown, most physiologists believe that
somewhere between the point of entry of glucose 
into the cells and its ﬁnal degradation, cortisol directly
delays the rate of glucose utilization

ol directly
delays the rate of glucose utilization. A suggested
mechanism is based on the observation that glucocor-
ticoids depress the oxidation of nicotinamide-adenine
dinucleotide (NADH) to form NAD+.
Because
NADH must be oxidized to allow glycolysis, this effect
could account for the diminished utilization of glucose
by the cells.
Elevated Blood Glucose Concentration and “Adrenal Diabetes.”
Both the increased rate of gluconeogenesis and the
moderate reduction in the rate of glucose utilizat

moderate reduction in the rate of glucose utilization
by the cells cause the blood glucose concentrations to
rise. The rise in blood glucose in turn stimulates 
secretion of insulin. The increased plasma levels of
insulin, however, are not as effective in maintaining
plasma glucose as they are under normal conditions.
For reasons that are not entirely clear, high levels 
of glucocorticoid reduce the sensitivity of many
tissues, especially skeletal muscle and adipose tissue,
to the stimulatory ef

l muscle and adipose tissue,
to the stimulatory effects of insulin on glucose uptake
and utilization. One possible explanation is that high
levels of fatty acids, caused by the effect of glucocor-
ticoids to mobilize lipids from fat depots, may impair
insulin’s actions on the tissues. In this way, excess
secretion of glucocorticoids may produce disturbances
of carbohydrate metabolism very similar to those
found in patients with excess levels of growth
hormone.
The increase in blood glucose conce

rowth
hormone.
The increase in blood glucose concentration is occa-
sionally great enough (50 per cent or more above
normal) that the condition is called adrenal diabetes.
Administration of insulin lowers the blood glucose
concentration only a moderate amount in adrenal 
diabetes-not nearly as much as it does in pancreatic
0.0
3.0
2.0
1.0
Control
ACE inhibitor
+
Ang II infusion
ACE
inhibitor
Plasma cortisol
20
50
40
30
Plasma aldosterone
(ng/100 ml)
Figure 77–4
Effects of treating so

one
Effects of treating sodium-depleted dogs with an angiotensin-
converting enzyme (ACE) inhibitor for 7 days to block formation
of angiotensin II (Ang II) and of infusing exogenous Ang II to
restore plasma Ang II levels after ACE inhibition. Note that block-
ing Ang II formation reduced plasma aldosterone concentration
with little effect on cortisol, demonstrating the important role of
Ang II in stimulating aldosterone secretion during sodium deple-
tion. (Drawn from da

ecretion during sodium deple-
tion. (Drawn from data in Hall JE, Guyton AC, Smith MJ Jr,
Coleman TG: Chronic blockade of angiotensin II formation during
sodium deprivation. Am J Physiol 237:F424, 1979.)
952
Unit XIV
diabetes-because the tissues are resistant to the effects
of insulin.
Reduction in Cellular Protein. One of the principal effects
of cortisol on the metabolic systems of the body is
reduction of the protein sto

ystems of the body is
reduction of the protein stores in essentially all body
cells except those of the liver. This is caused by both
decreased protein synthesis and increased catabolism
of protein already in the cells. Both these effects may
result from decreased amino acid transport into extra-
hepatic tissues, as discussed later; this probably is not
the major cause, because cortisol also depresses the
formation of RNA and subsequent protein synthesis
in many extrahepatic tissues, especially 

ynthesis
in many extrahepatic tissues, especially in muscle and
lymphoid tissue.
In the presence of great excesses of cortisol, the
muscles can become so weak that the person cannot
rise from the squatting position. And the immunity
functions of the lymphoid tissue can be decreased to a
small fraction of normal.
Cortisol Increases Liver and Plasma Proteins. Coinciden-
tally with the reduced proteins elsewhere in the body,
the liver proteins become enhanced. Furthermore, the
plasma proteins (whic

e enhanced. Furthermore, the
plasma proteins (which are produced by the liver and
then released into the blood) are also increased.These
increases are exceptions to the protein depletion that
occurs elsewhere in the body. It is believed that this
difference results from a possible effect of cortisol to
enhance amino acid transport into liver cells (but 
not into most other cells) and to enhance the 
liver enzymes required for protein synthesis.
Increased Blood Amino Acids, Diminished Transport o

ncreased Blood Amino Acids, Diminished Transport of Amino
Acids into Extrahepatic Cells, and Enhanced Transport into
Hepatic Cells. Studies in isolated tissues have demon-
strated that cortisol depresses amino acid transport
into muscle cells and perhaps into other extrahepatic
cells.
The decreased transport of amino acids into extra-
hepatic cells decreases their intracellular amino acid
concentrations and consequently decreases the syn-
thesis of protein.Yet,catabolism of proteins in the cells

of protein.Yet,catabolism of proteins in the cells
continues to release amino acids from the already
existing proteins, and these diffuse out of the cells to
increase the plasma amino acid concentration. There-
fore, cortisol mobilizes amino acids from the nonhep-
atic tissues and in doing so diminishes the tissue stores
of protein.
The increased plasma concentration of amino acids
and enhanced transport of amino acids into the
hepatic cells by cortisol could also account for
enhanced utilizatio

ortisol could also account for
enhanced utilization of amino acids by the liver to
cause such effects as (1) increased rate of deamination
of amino acids by the liver, (2) increased protein 
synthesis in the liver, (3) increased formation of
plasma proteins by the liver, and (4) increased con-
version of amino acids to glucose-that is, enhanced 
gluconeogenesis. Thus, it is possible that many of the
effects of cortisol on the metabolic systems of the body
result mainly from this ability of corti

 the body
result mainly from this ability of cortisol to mobilize
amino acids from the peripheral tissues while at the
same time increasing the liver enzymes required for
the hepatic effects.
Mobilization of Fatty Acids. In much the same manner
that cortisol promotes amino acid mobilization from
muscle, it promotes mobilization of fatty acids from
adipose tissue.This increases the concentration of free
fatty acids in the plasma, which also increases their ut

acids in the plasma, which also increases their uti-
lization for energy. Cortisol also seems to have a direct
effect to enhance the oxidation of fatty acids in the
cells.
The mechanism by which cortisol promotes fatty
acid mobilization is not completely understood.
However, part of the effect probably results from
diminished transport of glucose into the fat cells.
Recall that a-glycerophosphate, which is derived from
glucose, is required for both deposition and mainte-
nance of triglycerides i

th deposition and mainte-
nance of triglycerides in these cells, and in its absence
the fat cells begin to release fatty acids.
The increased mobilization of fats by cortisol, com-
bined with increased oxidation of fatty acids in the
cells, helps shift the metabolic systems of the cells from
utilization of glucose for energy to utilization of fatty
acids in times of starvation or other stresses. This cor-
tisol mechanism, however, requires several hours to
become fully developed-not nearly so ra

l hours to
become fully developed-not nearly so rapid or so pow-
erful an effect as a similar shift elicited by a decrease
in insulin, as we discuss in Chapter 78. Nevertheless,
the increased use of fatty acids for metabolic energy is
an important factor for long-term conservation of
body glucose and glycogen.
Obesity Caused by Excess Cortisol. Despite the fact that
cortisol can cause a moderate degree of fatty acid
mobilization from adipose tissue, many people with
excess cortisol secretion dev

ue, many people with
excess cortisol secretion develop a peculiar type of
obesity, with excess deposition of fat in the chest and
head regions of the body, giving a buffalo-like torso
and a rounded “moon face.” Although the cause is
unknown, it has been suggested that this obesity
results from excess stimulation of food intake, with fat
being generated in some tissues of the body more
rapidly than it is mobilized and oxidized.
Cortisol is Important in Resisting
Stress and Inﬂammation
Almost any 

nt in Resisting
Almost any type of stress, whether physical or neuro-
genic, causes an immediate and marked increase 
in ACTH secretion by the anterior pituitary gland,
followed within minutes by greatly increased adreno-
cortical secretion of cortisol. This is demonstrated 
Chapter 77
953
dramatically by the experiment shown in Figure 77–5,
in which corticosteroid formation and secretion
increased sixfold in a rat within 4 to 20 minutes after
fract

ixfold in a rat within 4 to 20 minutes after
fracture of two leg bones.
Some of the different types of stress that increase
cortisol release are the following:
1. Trauma of almost any type
2. Infection
3. Intense heat or cold
4. Injection of norepinephrine and other
sympathomimetic drugs
5. Surgery
6. Injection of necrotizing substances beneath the
skin
7. Restraining an animal so that it cannot move
8. Almost any debilitating disease
Even though we know that cortisol secretion often
increases g

 we know that cortisol secretion often
increases greatly in stressful situations, we are not sure
why this is of signiﬁcant beneﬁt to the animal. One
possibility is that the glucocorticoids cause rapid mobi-
lization of amino acids and fats from their cellular
stores, making them immediately available both for
energy and for synthesis of other compounds, includ-
ing glucose, needed by the different tissues of the body.
Indeed, it has been shown in a few instances that
damaged tissues that are mo

n a few instances that
damaged tissues that are momentarily depleted of pro-
teins can use the newly available amino acids to form
new proteins that are essential to the lives of the cells.
Also, the amino acids are perhaps used to synthesize
other essential intracellular substances such as purines,
pyrimidines, and creatine phosphate, which are neces-
sary for maintenance of cellular life and reproduction
of new cells.
But all this is mainly supposition. It is supported
only by the fact that co

position. It is supported
only by the fact that cortisol usually does not mobilize
the basic functional proteins of the cells, such as 
the muscle contractile proteins and the proteins of
neurons, until almost all other proteins have been
released. This preferential effect of cortisol in 
mobilizing labile proteins could make amino acids
available to needy cells to synthesize substances essen-
tial to life.
Anti-inﬂammatory Effects of High Levels 
of Cortisol
When tissues are damaged by trauma, 

When tissues are damaged by trauma, by infection
with bacteria, or in other ways, they almost always
become “inﬂamed.” In some conditions, such as in
rheumatoid arthritis, the inﬂammation is more dam-
aging than the trauma or disease itself. The adminis-
tration of large amounts of cortisol can usually block
this inﬂammation or even reverse many of its effects
once it has begun. Before attempting to explain the
way in which cortisol functions to block inﬂammation,
let us review the

 functions to block inﬂammation,
let us review the basic steps in the inﬂammation
process, discussed in more detail in Chapter 33.
There are ﬁve main stages of inﬂammation: (1)
release from the damaged tissue cells of chemical 
substances that activate the inﬂammation process-
chemicals such as histamine, bradykinin, proteolytic
enzymes, prostaglandins, and leukotrienes; (2) an
increase in blood ﬂow in the inﬂamed area caused by
some of the released products from the tissues, an
effect called er

sed products from the tissues, an
effect called erythema; (3) leakage of large quantities
of almost pure plasma out of the capillaries into the
damaged areas because of increased capillary perme-
ability, followed by clotting of the tissue ﬂuid, thus
causing a nonpitting type of edema; (4) inﬁltration of
the area by leukocytes; and (5) after days or weeks,
ingrowth of ﬁbrous tissue that often helps in the
healing process.
When large amounts of cortisol are secreted or
injected into a person, the

rtisol are secreted or
injected into a person, the cortisol has two basic anti-
inﬂammatory effects: (1) it can block the early stages
of the inﬂammation process before inﬂammation 
even begins, or (2) if inﬂammation has already begun,
it causes rapid resolution of the inﬂammation and
increased rapidity of healing. These effects are
explained further as follows.
Cortisol Prevents the Development of Inﬂammation by Stabiliz-
ing Lysosomes and by Other Effects. Cortisol has the fol-
lowing effects 

her Effects. Cortisol has the fol-
lowing effects in preventing inﬂammation:
1. Cortisol stabilizes the lysosomal membranes. This
is one of its most important anti-inﬂammatory
effects, because it is much more difﬁcult than
normal for the membranes of the intracellular
lysosomes to rupture. Therefore, most of the
proteolytic enzymes that are released by damaged
cells to cause inﬂammation, which are mainly
stored in the lysosomes, are released in greatly
decreased quantity.
-0
45
40
35
30
25
20
15

reatly
-0
45
40
35
30
25
20
15
10
5
55
50
45
40
35
30
25
20
15
10
5
Seconds
Minutes
15
30
45 60
90 2
3 4 5 6 8 101215 2025 30
Plasma corticosterone
Adrenal cortisone
(mg/g)
Figure 77–5
Rapid reaction of the adrenal cortex of a rat to stress caused by
fracture of the tibia and ﬁbula at time zero. (In the rat, corticos-
terone is secreted in place of cortisol.) (Courtesy Drs. Guillemin,
Dear, and Lipscomb.)
954
Unit XIV
Endocrinology and 

ar, and Lipscomb.)
954
Unit XIV
2. Cortisol decreases the permeability of the
capillaries, probably as a secondary effect of the
reduced release of proteolytic enzymes. This
prevents loss of plasma into the tissues.
3. Cortisol decreases both migration of white blood
cells into the inﬂamed area and phagocytosis of the
damaged cells. These effects probably result from
the fact that cortisol diminishes the formation 
of prostaglandins and leukotrienes that 
otherwise

otherwise would increase vasodilation, capillary
permeability, and mobility of white blood cells.
4. Cortisol suppresses the immune system, causing
lymphocyte reproduction to decrease markedly.
The T lymphocytes are especially suppressed. In
turn, reduced amounts of T cells and antibodies 
in the inﬂamed area lessen the tissue reactions
that would otherwise promote the inﬂammation
process.
5. Cortisol attenuates fever mainly because it reduces
the release

nuates fever mainly because it reduces
the release of interleukin-1 from the white blood
cells, which is one of the principal excitants to the
hypothalamic temperature control system. The
decreased temperature in turn reduces the degree
of vasodilation.
Thus, cortisol has an almost global effect in reduc-
ing all aspects of the inﬂammatory process. How much
of this results from the simple effect of cortisol in 
stabilizing lysosomal and cell membranes versus its
effect to reduce the formation of

ranes versus its
effect to reduce the formation of prostaglandins and
leukotrienes from arachidonic acid in damaged cell
membranes and other effects of cortisol is unclear.
Cortisol Causes Resolution of Inﬂammation. Even after
inﬂammation has become well established, the admin-
istration of cortisol can often reduce inﬂammation
within hours to a few days. The immediate effect is to
block most of the factors that are promoting the
inﬂammation. But in addition, the rate of healing is
enhanced.This

 in addition, the rate of healing is
enhanced.This probably results from the same, mainly
undeﬁned, factors that allow the body to resist many
other types of physical stress when large quantities of
cortisol are secreted. Perhaps this results from the
mobilization of amino acids and use of these to repair
the damaged tissues; perhaps it results from the
increased glucogenesis that makes extra glucose avail-
able in critical metabolic systems; perhaps it results
from increased amounts of fatty ac

haps it results
from increased amounts of fatty acids available for cel-
lular energy; or perhaps it depends on some effect of
cortisol for inactivating or removing inﬂammatory
products.
Regardless of the precise mechanisms by which the
anti-inﬂammatory effect occurs, this effect of cortisol
plays a major role in combating certain types of dis-
eases, such as rheumatoid arthritis, rheumatic fever,
and acute glomerulonephritis. All these diseases are
characterized by severe local inﬂammation, and

are
characterized by severe local inﬂammation, and the
harmful effects on the body are caused mainly by 
the inﬂammation itself and not by other aspects of the
disease.
When cortisol or other glucocorticoids are adminis-
tered to patients with these diseases, almost invariably
the inﬂammation begins to subside within 24 hours.
And even though the cortisol does not correct the
basic disease condition, merely preventing the damag-
ing effects of the inﬂammatory response, this alone can
often be a 

e inﬂammatory response, this alone can
often be a lifesaving measure.
Cortisol Blocks the Inﬂammatory Response to Allergic Reactions.
The basic allergic reaction between antigen and anti-
body is not affected by cortisol, and even some of the
secondary effects of the allergic reaction still occur.
However, because the inﬂammatory response is respon-
sible for many of the serious and sometimes lethal
effects of allergic reactions, administration of cortisol,
followed by 

eactions, administration of cortisol,
followed by its effect in reducing inﬂammation and the
release of inﬂammatory products, can be lifesaving. For
instance, cortisol effectively prevents shock or death in
anaphylaxis, which otherwise kills many people, as
explained in Chapter 34.
Effect on Blood Cells and on Immunity in Infectious Diseases.
Cortisol decreases the number of eosinophils and lym-
phocytes in the blood; this effect begins within a few
minutes after the injection of cortisol and be

few
minutes after the injection of cortisol and becomes
marked within a few hours. Indeed, a ﬁnding of lym-
phocytopenia or eosinopenia is an important diagnostic
criterion for overproduction of cortisol by the adrenal
gland.
Likewise, the administration of large doses of cortisol
causes signiﬁcant atrophy of all the lymphoid tissue
throughout the body, which in turn decreases the output
of both T cells and antibodies from the lymphoid tissue.
As a result, the level of immunity for almost all fo

 a result, the level of immunity for almost all foreign
invaders of the body is decreased. This occasionally can
lead to fulminating infection and death from diseases
that would otherwise not be lethal, such as fulminating
tuberculosis in a person whose disease had previously
been arrested. Conversely, this ability of cortisol and
other glucocorticoids to suppress immunity makes them
useful drugs in preventing immunological rejection of
transplanted hearts, kidneys, and other tissues.
Cortisol i

ted hearts, kidneys, and other tissues.
Cortisol increases the production of red blood cells
by mechanisms that are unclear.When excess cortisol is
secreted by the adrenal glands, polycythemia often
results, and conversely, when the adrenal glands secrete
no cortisol, anemia often results.
Cortisol Action
Cortisol, like other steroid hormones, exerts its effects
by ﬁrst interacting with intracellular receptors in target
cells. Because cortisol is lipid soluble, it can easi

ls. Because cortisol is lipid soluble, it can easily
diffuse through the cell membrane. Once inside the cell,
cortisol binds with its protein receptor in the cytoplasm,
and the hormone-receptor complex then interacts with
speciﬁc regulatory DNA sequences, called glucocorti-
coid response elements, to induce or repress gene tran-
scription. Other proteins in the cell, called transcription
factors, are also necessary for the hormone-receptor
complex to interact appropriately with the glucocorti-
c

x to interact appropriately with the glucocorti-
coid response elements.
Glucocorticoids increase or decrease transcription of
many genes to alter synthesis of mRNA for the proteins
that mediate their multiple physiologic effects. Thus,
most of the metabolic effects of cortisol are not 
Chapter 77
955
immediate but require 45 to 60 minutes for proteins to
be synthesized, and up to several hours or days to fully
develop. Recent evidence suggests that glucocorticoids,
espec

cent evidence suggests that glucocorticoids,
especially at high concentrations, may also have some
rapid nongenomic effects on cell membrane ion trans-
port that may contribute to their therapeutic beneﬁts.
Regulation of Cortisol Secretion 
by Adrenocorticotropic Hormone 
from the Pituitary Gland
ACTH Stimulates Cortisol Secretion. Unlike aldosterone
secretion by the zona glomerulosa, which is controlled
mainly by potassium and angiotensin acting directly
on the adrenocortical cells, almost no s

 directly
on the adrenocortical cells, almost no stimuli have
direct control effects on the adrenal cells that secrete
cortisol. Instead, secretion of cortisol is controlled
almost entirely by ACTH secreted by the anterior
pituitary gland.This hormone, also called corticotropin
or adrenocorticotropin, also enhances the production
of adrenal androgens.
Chemistry of ACTH. ACTH has been isolated in pure
form from the anterior pituitary. It is a large polypep-
tide, having a chain length of 39 amino

e polypep-
tide, having a chain length of 39 amino acids.A smaller
polypeptide, a digested product of ACTH having a
chain length of 24 amino acids, has all the effects of the
total molecule.
ACTH Secretion Is Controlled by Corticotropin-Releasing Factor
from the Hypothalamus. In the same way that other pitu-
itary hormones are controlled by releasing factors
from the hypothalamus, an important releasing factor
also controls ACTH secretion. This is called corti-
cotropin-releasing factor (CRF). I

s called corti-
cotropin-releasing factor (CRF). It is secreted into the
primary capillary plexus of the hypophysial portal
system in the median eminence of the hypothalamus
and then carried to the anterior pituitary gland, where
it induces ACTH secretion. CRF is a peptide com-
posed of 41 amino acids.The cell bodies of the neurons
that secrete CRF are located mainly in the paraven-
tricular nucleus of the hypothalamus. This nucleus in
turn receives many nervous connections from the
limbic syste

ves many nervous connections from the
limbic system and lower brain stem.
The anterior pituitary gland can secrete only minute
quantities of ACTH in the absence of CRF. Instead,
most conditions that cause high ACTH secretory rates
initiate this secretion by signals that begin in the basal
regions of the brain, including the hypothalamus, and
are then transmitted by CRF to the anterior pituitary
gland.
ACTH Activates Adrenocortical Cells to Produce Steroids by
Increasing Cyclic Adenosine Monophos

e Steroids by
Increasing Cyclic Adenosine Monophosphate (cAMP). The
principal effect of ACTH on the adrenocortical cells is
to activate adenylyl cyclase in the cell membrane. This
then induces the formation of cAMP in the cell cyto-
plasm, reaching its maximal effect in about 3 minutes.
The cAMP in turn activates the intracellular enzymes
that cause formation of the adrenocortical hormones.
This is another example of cAMP as a second messen-
ger signal system.
The most important of all the ACTH-

signal system.
The most important of all the ACTH-stimulated
steps for controlling adrenocortical secretion is acti-
vation of the enzyme protein kinase A, which causes
initial conversion of cholesterol to pregnenolone. This
initial conversion is the “rate-limiting” step for all the
adrenocortical hormones, which explains why ACTH
normally is necessary for any adrenocortical hormones
to be formed. Long-term stimulation of the adrenal
cortex by ACTH not only increases secretory activity
but also 

TH not only increases secretory activity
but also causes hypertrophy and proliferation of the
adrenocortical cells, especially in the zona fasciculata
and zona reticularis, where cortisol and the androgens
are secreted.
Physiologic Stress Increases ACTH and
Adrenocortical Secretion
As pointed out earlier in the chapter, almost any type
of physical or mental stress can lead within minutes 
to greatly enhanced secretion of ACTH and conse-
quently cortisol as well, often increasing cortisol secre-

ortisol as well, often increasing cortisol secre-
tion as much as 20-fold. This effect was demonstrated
by the rapid and strong adrenocortical secretory
responses after trauma shown in Figure 77–5.
Pain stimuli caused by physical stress or tissue
damage are transmitted ﬁrst upward through the brain
stem and eventually to the median eminence of the
hypothalamus, as shown in Figure 77–6. Here CRF is
secreted into the hypophysial portal system. Within
minutes the entire control sequence leads to la

in
minutes the entire control sequence leads to large
quantities of cortisol in the blood.
Mental stress can cause an equally rapid increase in
ACTH secretion. This is believed to result from
increased activity in the limbic system, especially in the
region of the amygdala and hippocampus, both of
which then transmit signals to the posterior medial
Inhibitory Effect of Cortisol on the Hypothalamus and on the
Anterior Pituitary to Decrease ACTH Secretion. Cortisol has
direct negativ

crease ACTH Secretion. Cortisol has
direct negative feedback effects on (1) the hypothala-
mus to decrease the formation of CRF and (2) the
anterior pituitary gland to decrease the formation 
of ACTH. Both of these feedbacks help regulate 
the plasma concentration of cortisol.That is, whenever
the cortisol concentration becomes too great, the 
feedbacks automatically reduce the ACTH toward 
a normal control level.
Summary of the Cortisol Control System
Figure 77–6 shows the overall system for co

System
Figure 77–6 shows the overall system for control of
cortisol secretion. The key to this control is the exci-
tation of the hypothalamus by different types of stress.
Stress stimuli activate the entire system to cause rapid
release of cortisol, and the cortisol in turn initiates a
series of metabolic effects directed toward relieving
the damaging nature of the stressful state.
There is also direct feedback of the cortisol to both
the hypothalamus and the anterior pituitary gland to
decreas

alamus and the anterior pituitary gland to
decrease the concentration of cortisol in the plasma 
at times when the body is not experiencing stress.
956
Unit XIV
However, the stress stimuli are the prepotent ones;
they can always break through this direct inhibitory
feedback of cortisol, causing either periodic exacerba-
tions of cortisol secretion at multiple times during the
day (Figure 77–7) or prolonged cortisol secretion in
times of chronic stress.
Circadian Rh

secretion in
Circadian Rhythm of Glucocorticoid Secretion. The secretory
rates of CRF, ACTH, and cortisol are high in the early
morning but low in the late evening, as shown in Figure
77–7; the plasma cortisol level ranges between a high of
about 20 mg/dl an hour before arising in the morning
and a low of about 5 mg/dl around midnight. This effect
results from a 24-hour cyclical alteration in the signals
from the hypothalamus that cause cortisol secretion.
When a person 

amus that cause cortisol secretion.
When a person changes daily sleeping habits, the cycle
changes correspondingly. Therefore, measurements of
blood cortisol levels are meaningful only when
expressed in terms of the time in the cycle at which the
measurements are made.
Synthesis and Secretion of ACTH in
Association with Melanocyte-Stimulating
Hormone, Lipotropin, and Endorphin
When ACTH is secreted by the anterior pituitary
gland, several other hormones that have similar chem-
ical structures ar

ormones that have similar chem-
ical structures are secreted simultaneously.The reason
for this is that the gene that is transcribed to form the
RNA molecule that causes ACTH synthesis initially
causes the formation of a considerably larger protein,
a 
preprohormone 
called 
proopiomelanocortin
(POMC), which is the precursor of ACTH as well 
as several other peptides, including melanocyte-
stimulating hormone (MSH), b-lipotropin, b-endor-
phin, and a few others (Figure 77–8). Under normal
condit

nd a few others (Figure 77–8). Under normal
conditions, none of these hormones is secreted in
enough quantity by the pituitary to have a signiﬁcant
effect on the human body, but when the rate of secre-
tion of ACTH is high, as may occur in Addison’s
disease, formation of some of the other POMC-
derived hormones may also be increased.
The POMC gene is actively transcribed in several
tissues, including the corticotroph cells of the anterior
pituitary, POMC neurons in the arcuate nucleus of 
the hy

ry, POMC neurons in the arcuate nucleus of 
the hypothalamus, cells of the dermis, and lymphoid
tissue. In all of these cell types, POMC is processed to
form a series of smaller peptides. The precise type 
of POMC-derived products from a particular tissue
depends on the type of processing enzymes present in
the tissue. Thus, pituitary corticotroph cells express
prohormone convertase 1 (PC1), but not PC2, result-
ing in the production of N-terminal peptide, joining
peptide, ACTH, b-endorphin, and

l peptide, joining
peptide, ACTH, b-endorphin, and b-lipotropin. In the
hypothalamus, the expression of PC2 leads to the 
production of a-, b-, and g-MSH, but not ACTH. As
discussed in Chapter 71, a-MSH formed by neurons of
the hypothalamus plays a major role in appetite 
In melanocytes located in abundance between the
dermis and epidermis of the skin, MSH stimulates
formation of the black pigment melanin and disperses
it to the epidermis. Injection of MSH into a person
over 8 to 10 

rmis. Injection of MSH into a person
over 8 to 10 days can greatly increase darkening of the
skin. The effect is much greater in people who have
genetically dark skins than in light-skinned people.
In some lower animals, an intermediate “lobe” of
the pituitary gland, called the pars intermedia, is highly
developed, lying between the anterior and posterior
pituitary lobes. This lobe secretes an especially large
Portal
vessel
(CRF)
1 Gluconeogenesis
2 Protein mobilization
3 Fat mobilization
4 Stab

s
4 Stabilizes lysosomes
Inhibits
Cortisol
ACTH
Adrenal
cortex
Relieves
Stress
Excites
Median
eminence
Figure 77–6
Mechanism for regulation of glucocorticoid secretion. ACTH,
adrenocorticotropic hormone; CRF, corticotropin-releasing factor.
0
20
15
10
5
12:00 4:00
8:00 12:00 4:00
8:00 12:00
Cortisol concentration
(mg/dl)
AM
PM
Noon
Figure 77–7
Typical pattern of cortisol concentration during the day. Note the
oscillations in secretion as wel

the day. Note the
oscillations in secretion as well as a daily secretory surge an hour
or so after awaking in the morning.
Chapter 77
957
amount of MSH. Furthermore, this secretion is inde-
pendently controlled by the hypothalamus in response
to the amount of light to which the animal is exposed
or in response to other environmental factors. For
instance, some arctic animals develop darkened fur in
the summer and yet have entirely white fur in the
winter.
ACTH, because it

entirely white fur in the
winter.
ACTH, because it contains an MSH sequence, has
about 1/30 as much melanocyte-stimulating effect as
MSH. Furthermore, because the quantities of pure
MSH secreted in the human being are extremely
small, whereas those of ACTH are large, it is likely that
ACTH normally is more important than MSH in
determining the amount of melanin in the skin.
Several moderately active male sex hormones called
adrenal androgens (the most important of which is
dehy

nal androgens (the most important of which is
dehydroepiandrosterone) are continually secreted by
the adrenal cortex, especially during fetal life, as dis-
cussed more fully in Chapter 83.Also, progesterone and
estrogens, which are female sex hormones, are secreted
in minute quantities.
Normally, the adrenal androgens have only weak
effects in humans. It is possible that part of the early
development of the male sex organs results from child-
hood secretion of adrenal androgens. The adrenal
andr

d secretion of adrenal androgens. The adrenal
androgens also exert mild effects in the female, not only
before puberty but also throughout life. Much of the
growth of the pubic and axillary hair in the female
results from the action of these hormones.
In extra-adrenal tissues, some of the adrenal andro-
gens are converted to testosterone,the primary male sex
hormone, which probably accounts for much of their
androgenic activity. The physiologic effects of andro-
gens are discussed in Chapter 80 

ffects of andro-
gens are discussed in Chapter 80 in relation to male
sexual function.
Abnormalities of
Addison’s disease results from failure of the adrenal
cortices to produce adrenocortical hormones, and this
in turn is most frequently caused by primary atrophy of
the adrenal cortices. In about 80 per cent of the cases,
the atrophy is caused by autoimmunity against the cor-
tices. Adrenal gland hypofunction is also frequently
caused by

al gland hypofunction is also frequently
caused by tuberculous destruction of the adrenal glands
or invasion of the adrenal cortices by cancer. The dis-
turbances in Addison’s disease are as follows.
Mineralocorticoid Deﬁciency. Lack of aldosterone secre-
tion greatly decreases renal tubular sodium reabsorp-
tion and consequently allows sodium ions, chloride ions,
and water to be lost into urine in great profusion. The
net result is a greatly decreased extracellular ﬂuid
volume. Furthermore, hyp

reased extracellular ﬂuid
volume. Furthermore, hyponatremia, hyperkalemia, and
mild acidosis develop because of failure of potassium
and hydrogen ions to be secreted in exchange for
sodium reabsorption.
As the extracellular ﬂuid becomes depleted, plasma
volume falls,
red blood cell concentration rises
markedly, cardiac output decreases, and the patient dies
in shock, death usually occurring in the untreated
patient 4 days to 2 weeks after cessation of mineralo-
corticoid secretion.
Glucocorticoi

on of mineralo-
Glucocorticoid Deﬁciency. Loss of cortisol secretion makes
it impossible for a person with Addison’s disease to
maintain normal blood glucose concentration between
meals because he or she cannot synthesize signiﬁcant
quantities of glucose by gluconeogenesis. Furthermore,
lack of cortisol reduces the mobilization of both 
proteins and fats from the tissues, thereby depressing
many other metabolic functions of the body. This 
Proopiomelanocortin
COOH
NH2
ACTH
J

he body. This 
COOH
NH2
ACTH
Joining
protein
N-Terminal protein
g-MSH
a-MSH
b-MSH
g-Lipotropin
CLIP
PCI
PC2
b-Lipotropin
Figure 77–8
Proopiomelanocortin 
(POMC)
processing by prohormone con-
vertase 1 (PC1, red arrows) and
PC 2 (blue arrows). Tissue spe-
ciﬁc expression of these two
enzymes results in different pep-
tides produced in various tissues.
The anterior pituitary expresses
PC1, resulting in formation of N-
terminal peptide, joining peptide,
ACTH, and b-l

-
ACTH, and b-lipotropin. Expres-
sion of PC2 within the hypothala-
mus leads to the production of a-,
b-, and g-melanocyte stimulating
hormone (MSH), but not ACTH.
CLIP, corticotropin-like intermedi-
ate peptide.
958
Unit XIV
sluggishness of energy mobilization when cortisol is not
available is one of the major detrimental effects of glu-
cocorticoid lack. Even when excess quantities of glucose
and other nutrients are available, 

ies of glucose
and other nutrients are available, the person’s muscles
are weak, indicating that glucocorticoids are needed to
maintain other metabolic functions of the tissues in
addition to energy metabolism.
Lack of adequate glucocorticoid secretion also makes
a person with Addison’s disease highly susceptible to
the deteriorating effects of different types of stress, and
even a mild respiratory infection can cause death.
Melanin Pigmentation. Another characteristic of most
people with Addiso

 Another characteristic of most
people with Addison’s disease is melanin pigmentation
of the mucous membranes and skin. This melanin is not
always deposited evenly but occasionally is deposited in
blotches, and it is deposited especially in the thin skin
areas, such as the mucous membranes of the lips and the
thin skin of the nipples.
The cause of the melanin deposition is believed to be
the following: When cortisol secretion is depressed, the
normal negative feedback to the hypothalamus and
ant

rmal negative feedback to the hypothalamus and
anterior pituitary gland is also depressed, therefore
allowing tremendous rates of ACTH secretion as well
as simultaneous secretion of increased amounts of
MSH. Probably the tremendous amounts of ACTH
cause most of the pigmenting effect because they can
stimulate formation of melanin by the melanocytes in
the same way that MSH does.
Treatment of People with Addison’s Disease. An untreated
person with total adrenal destruction dies within a few
days 

 total adrenal destruction dies within a few
days to a few weeks because of weakness and usually
circulatory shock.Yet such a person can live for years if
small quantities of mineralocorticoids and glucocorti-
coids are administered daily.
Addisonian Crisis. As noted earlier in the chapter, great
quantities of glucocorticoids are occasionally secreted
in response to different types of physical or mental
stress. In a person with Addison’s disease, the output of
glucocorticoids does not increase d

 the output of
glucocorticoids does not increase during stress. Yet
whenever different types of trauma, disease, or other
stresses, such as surgical operations, supervene, a person
is likely to have an acute need for excessive amounts of
glucocorticoids and often must be given 10 or more
times the normal quantities of glucocorticoids to
prevent death.
This critical need for extra glucocorticoids and the
associated severe debility in times of stress is called an
addisonian crisis.
Hyperadrenalism

ss is called an
Hyperadrenalism-Cushing’s
Syndrome
Hypersecretion by the adrenal cortex causes a complex
cascade of hormone effects called Cushing’s syndrome.
Most of the abnormalities of Cushing’s syndrome are
ascribable to abnormal amounts of cortisol, but excess
secretion of androgens may also cause important
effects. Hypercortisolism can occur from multiple
causes, including (1) adenomas of the anterior pituitary
that secrete large amounts of ACTH, which then causes
adrena

te large amounts of ACTH, which then causes
adrenal hyperplasia and excess cortisol secretion; (2)
abnormal function of the hypothalamus that causes high
levels of corticotropin-releasing hormone (CRH),
which stimulates excess ACTH release; (3) “ectopic
secretion” of ACTH by a tumor elsewhere in the body,
such as an abdominal carcinoma; and (4) adenomas of
the adrenal cortex. When Cushing’s syndrome is sec-
ondary to excess secretion of ACTH by the anterior
pituitary, this is referred to as Cush

he anterior
pituitary, this is referred to as Cushing’s disease.
Excess ACTH secretion is the most common cause of
Cushing’s syndrome and is characterized by high plasma
levels of ACTH as well as cortisol. Primary overpro-
duction of cortisol by the adrenal glands accounts for
about 20 to 25 per cent of clinical cases of Cushing’s syn-
drome and is usually associated with reduced ACTH
levels due to cortisol feedback inhibition of ACTH
secretion by the anterior pituitary gland.
Administration of 

y the anterior pituitary gland.
Administration of large doses of dexamethasone,
a synthetic glucocorticoid, can be used to distinguish
between ACTH-dependent
and ACTH-independent
Cushing’s syndrome. In patients who have overproduc-
tion of ACTH due to an ACTH-secreting pituitary
adenoma or to hypothalamic-pituitary dysfunction,
even large doses of dexamethasone usually do not 
suppress ACTH secretion. In contrast, patients with
primary adrenal overproduction of cortisol (ACTH-
independent) usual

erproduction of cortisol (ACTH-
independent) usually have low or undetectable levels of
ACTH. The dexamethasone test, although widely used,
can sometimes give an incorrect diagnosis, because
some ACTH-secreting pituitary tumors respond to 
dexamethasone with suppressed ACTH secretion.
Therefore, it is usually considered to be a ﬁrst step in
the differential diagnosis of Cushing’s syndrome.
Cushing’s syndrome can also occur when large
amounts of glucocorticoids are administered over pro-
longed p

lucocorticoids are administered over pro-
longed periods for therapeutic purposes. For example,
patients with chronic inﬂammation associated with dis-
eases such as rheumatoid arthritis are often treated with
glucocorticoids and may develop some of the clinical
symptoms of Cushing’s syndrome.
A special characteristic of Cushing’s syndrome is
mobilization of fat from the lower part of the body, with
concomitant extra deposition of fat in the thoracic and
upper abdominal regions, giving rise to a 

cic and
upper abdominal regions, giving rise to a buffalo torso.
The excess secretion of steroids also leads to an ede-
matous appearance of the face, and the androgenic
potency of some of the hormones sometimes causes
acne and hirsutism (excess growth of facial hair). The
appearance of the face is frequently described as a
“moon face,” as demonstrated in the untreated patient
with Cushing’s syndrome to the left in Figure 77–8.
About 80 per cent of patients have hypertension, pre-
sumably becaus

of patients have hypertension, pre-
sumably because of the slight mineralocorticoid effects
of cortisol.
Effects on Carbohydrate and Protein Metabolism. The abun-
dance of cortisol secreted in Cushing’s syndrome can
cause increased blood glucose concentration, some-
times to values as high as 200 mg/dl after meals-as much
as twice normal.This results mainly from enhanced glu-
coneogenesis and decreased glucose utilization by the
tissues.
The effects of glucocorticoids on protein catabolism
are o

cts of glucocorticoids on protein catabolism
are often profound in Cushing’s syndrome, causing
greatly decreased tissue proteins almost everywhere in
the body with the exception of the liver; the plasma pro-
teins also remain unaffected. The loss of protein from
the muscles in particular causes severe weakness. The
loss of protein synthesis in the lymphoid tissues leads to
a suppressed immune system, so that many of these
patients die of infections. Even the protein collagen
ﬁbers in the subcuta

ns. Even the protein collagen
ﬁbers in the subcutaneous tissue are diminished so that
Chapter 77
959
the subcutaneous tissues tear easily, resulting in devel-
opment of large purplish striae where they have torn
apart. In addition, severely diminished protein deposi-
tion in the bones often causes severe osteoporosis with
consequent weakness of the bones.
Treatment of Cushing’s Syndrome. Treatment of Cushing’s
syndrome consists of removing an adrenal tumor if this
is the 

sists of removing an adrenal tumor if this
is the cause or decreasing the secretion of ACTH, if 
this is possible. Hypertrophied pituitary glands or even
small tumors in the pituitary that oversecrete ACTH
can sometimes be surgically removed or destroyed 
by radiation. Drugs that block steroidogenesis, such as
metyrapone, ketoconazole, and aminoglutethimide, or
that inhibit ACTH secretion, such as serotonin antago-
nists and GABA-transaminase inhibitors, can also be
used when surgery is not feas

ibitors, can also be
used when surgery is not feasible. If ACTH secretion
cannot easily be decreased, the only satisfactory treat-
ment is usually bilateral partial (or even total) adrena-
lectomy, followed by administration of adrenal steroids
to make up for any insufﬁciency that develops.
Primary Aldosteronism 
(Conn’s Syndrome)
Occasionally a small tumor of the zona glomerulosa
cells occurs and secretes large amounts of aldosterone;
the resulting condition is called “primary aldostero-
nism” 

ing condition is called “primary aldostero-
nism” or “Conn’s syndrome.” Also, in a few instances,
hyperplastic adrenal cortices secrete aldosterone rather
than cortisol. The effects of the excess aldosterone are
discussed in detail earlier in the chapter. The most
important effects are hypokalemia, slight increase 
in extracellular ﬂuid volume and blood volume, very
slight increase in plasma sodium concentration (usually
not more than a 4 to 6 mEq/L increase), and,
almost always, hypertension. E

q/L increase), and,
almost always, hypertension. Especially interesting in
primary aldosteronism are occasional periods of muscle
paralysis caused by the hypokalemia. The paralysis is
caused by a depressant effect of low extracellular potas-
sium concentration on action potential transmission by
the nerve ﬁbers, as explained in Chapter 5.
One of the diagnostic criteria of primary aldostero-
nism is a decreased plasma renin concentration. This
results from feedback suppression of renin secretion

ults from feedback suppression of renin secretion
caused by the excess aldosterone or by the excess extra-
cellular ﬂuid volume and arterial pressure resulting
from the aldosteronism. Treatment of primary aldos-
teronism is usually surgical removal of the tumor or of
most of the adrenal tissue when hyperplasia is the cause.
An occasional adrenocortical tumor secretes excessive
quantities of androgens that cause intense masculiniz-
ing effects throughout the body. If this o

uliniz-
ing effects throughout the body. If this occurs in a
female, she develops virile characteristics, including
growth of a beard, a much deeper voice, occasionally
baldness if she also has the genetic trait for baldness,
masculine distribution of hair on the body and the pubis,
growth of the clitoris to resemble a penis, and deposi-
tion of proteins in the skin and especially in the muscles
to give typical masculine characteristics.
In the prepubertal male, a virilizing adrenal tumor
causes

repubertal male, a virilizing adrenal tumor
causes the same characteristics as in the female plus
rapid development of the male sexual organs, as shown
in Figure 77–9, which depicts a 4-year-old boy with
adrenogenital syndrome. In the adult male, the viriliz-
ing characteristics of adrenogenital syndrome are
usually obscured by the normal virilizing characteristics
of the testosterone secreted by the testes. It is often dif-
ﬁcult to make a diagnosis of adrenogenital syndrome in
the adult male. 

osis of adrenogenital syndrome in
the adult male. In adrenogenital syndrome, the excre-
tion of 17-ketosteroids (which are derived from andro-
gens) in the urine may be 10 to 15 times normal. This
ﬁnding can be used in diagnosing the disease.
Figure 77–9
A person with Cushing’s syn-
drome before (left) and after
(right) subtotal adrenalectomy.
(Courtesy Dr. Leonard Posey.)
960
Unit XIV
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Figure 77–10
Adrenogenital syndrome in a 4-year-old boy. (Courtesy Dr.
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A
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7
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961
Insulin, Glucagon, and 
The pancreas, in addition to its digestive functions,
secretes two important hormones, insulin and
glucagon, that are crucial for normal regulation of
glucose, lipid, an

rucial for normal regulation of
glucose, lipid, and protein metabolism.Although the
pancreas secretes other hormones, such as amylin,
somatostatin, and pancreatic polypeptide, their func-
tions are not as well established. The main purpose
of this chapter is to discuss the physiologic roles of
insulin and glucagon and the pathophysiology of diseases, especially diabetes
mellitus, caused by abnormal secretion or activity of these hormones.
Physiologic Anatomy of the Pancreas. The pancreas is comp

ogic Anatomy of the Pancreas. The pancreas is composed of two major types of
tissues, as shown in Figure 78–1: (1) the acini, which secrete digestive juices into the
duodenum, and (2) the islets of Langerhans, which secrete insulin and glucagon
directly into the blood. The digestive secretions of the pancreas are discussed in
Chapter 64.
The human pancreas has 1 to 2 million islets of Langerhans, each only about 0.3
millimeter in diameter and organized around small capillaries into which its cel

anized around small capillaries into which its cells
secrete their hormones.The islets contain three major types of cells, alpha, beta, and
delta cells, which are distinguished from one another by their morphological and
staining characteristics.
The beta cells, constituting about 60 per cent of all the cells of the islets, lie mainly
in the middle of each islet and secrete insulin and amylin, a hormone that is often
secreted in parallel with insulin, although its function is unclear. The alpha 

ulin, although its function is unclear. The alpha cells,
about 25 per cent of the total, secrete glucagon. And the delta cells, about 10 per
cent of the total, secrete somatostatin. In addition, at least one other type of cell,
the PP cell, is present in small numbers in the islets and secretes a hormone of uncer-
tain function called pancreatic polypeptide.
The close interrelations among these cell types in the islets of Langerhans allow
cell-to-cell communication and direct control of secretio

-cell communication and direct control of secretion of some of the hormones
by the other hormones. For instance, insulin inhibits glucagon secretion, amylin
inhibits insulin secretion, and somatostatin inhibits the secretion of both insulin and
glucagon.
Insulin was ﬁrst isolated from the pancreas in 1922 by Banting and Best, and
almost overnight the outlook for the severely diabetic patient changed from one
of rapid decline and death to that of a nearly normal 

apid decline and death to that of a nearly normal person. Historically, insulin
has been associated with “blood sugar,” and true enough, insulin has profound
effects on carbohydrate metabolism. Yet it is abnormalities of fat metabolism,
causing such conditions as acidosis and arteriosclerosis, that are the usual causes
of death in diabetic patients. Also, in patients with prolonged diabetes, dimin-
ished ability to synthesize proteins leads to wasting of the tissues as well as many
cellular func

sting of the tissues as well as many
cellular functional disorders. Therefore, it is clear that insulin affects fat and
protein metabolism almost as much as it does carbohydrate metabolism.
Insulin Is a Hormone Associated with Energy Abundance
As we discuss insulin in the next few pages, it will become apparent that insulin
secretion is associated with energy abundance. That is, when there is great 
962
Unit XIV
abundance of energy-giving foods in the diet, espe-
c

ndance of energy-giving foods in the diet, espe-
cially excess amounts of carbohydrates, insulin is
secreted in great quantity. In turn, the insulin plays an
important role in storing the excess energy. In the case
of excess carbohydrates, it causes them to be stored as
glycogen mainly in the liver and muscles. Also, all the
excess carbohydrates that cannot be stored as glyco-
gen are converted under the stimulus of insulin into
fats and stored in the adipose tissue. In the case of pro-
teins, i

n the adipose tissue. In the case of pro-
teins, insulin has a direct effect in promoting amino
acid uptake by cells and conversion of these amino
acids into protein. In addition, it inhibits the break-
down of the proteins that are already in the cells.
Insulin Chemistry and Synthesis
Insulin is a small protein; human insulin has a molec-
ular weight of 5808. It is composed of two amino acid
chains, shown in Figure 78–2, connected to each other
by disulﬁde linkages. When the two amino acid chai

by disulﬁde linkages. When the two amino acid chains
are split apart, the functional activity of the insulin
molecule is lost.
Insulin is synthesized in the beta cells by the usual
cell machinery for protein synthesis, as explained in
Chapter 3, beginning with translation of the insulin
RNA by ribosomes attached to the endoplasmic re-
ticulum to form an insulin preprohormone. This 
initial preprohormone has a molecular weight of about
11,500, but it is then cleaved in the endoplasmic retic-
ulum

 it is then cleaved in the endoplasmic retic-
ulum to form a proinsulin with a molecular weight 
of about 9000; most of this is further cleaved in the
Golgi apparatus to form insulin and peptide fragments
before being packaged in the secretory granules.
However, about one sixth of the ﬁnal secreted product
is still in the form of proinsulin.The proinsulin has vir-
tually no insulin activity.
When insulin is secreted into the blood, it circulates
almost entirely in an unbound form; it has a plasm

almost entirely in an unbound form; it has a plasma
half-life that averages only about 6 minutes, so that it
is mainly cleared from the circulation within 10 to 15
minutes. Except for that portion of the insulin that
combines with receptors in the target cells, the remain-
der is degraded by the enzyme insulinase mainly in the
liver, to a lesser extent in the kidneys and muscles, and
slightly in most other tissues. This rapid removal from
the plasma is important, because, at times, it is as
impo

sma is important, because, at times, it is as
important to turn off rapidly as to turn on the control
functions of insulin.
Activation of Target Cell Receptors by Insulin
and the Resulting Cellular Effects
To initiate its effects on target cells, insulin ﬁrst binds
with and activates a membrane receptor protein that
has a molecular weight of about 300,000 (Figure 78–3).
It is the activated receptor, not the insulin, that causes
the subsequent effects.
The insulin receptor is a combination of fou

ects.
The insulin receptor is a combination of four sub-
units held together by disulﬁde linkages: two alpha
subunits that lie entirely outside the cell membrane
and two beta subunits that penetrate through the mem-
brane, protruding into the cell cytoplasm. The insulin
binds with the alpha subunits on the outside of the cell,
but because of the linkages with the beta subunits, the
portions of the beta subunits protruding into the cell
become autophosphorylated. Thus, the insulin recep-
tor is a

ophosphorylated. Thus, the insulin recep-
tor is an example of an enzyme-linked receptor, dis-
cussed in Chapter 74.Autophosphorylation of the beta
subunits of the receptor activates a local tyrosine
Islet of
Langerhans
acini
Delta cell
Alpha cell
Beta cell
Figure 78–1
Physiologic anatomy of an islet of Langerhans in the pancreas.
Gly•Ileu•Val•Glu•Glu•Cy•Cy•Thr•Ser•Ileu•Cy•Ser•Leu•Tyr•Glu•Leu•Glu•Asp•Tyr•Cy•Asp
Phe•Val•Asp•Glu•His•Leu•Cy•Gly•Ser•His•Leu•Val•Glu•Ala•Leu

Asp•Glu•His•Leu•Cy•Gly•Ser•His•Leu•Val•Glu•Ala•Leu•Tyr•Leu•Val•Cy•Gly•Glu•Arg•Gly•Phe•Phe•Tyr•Thr•Pro•Lys•Thr
NH2
NH2
NH2
NH2
NH2
NH2
S
S
S
S
S
Figure 78–2
Human insulin molecule.
Chapter 78
Insulin, Glucagon, and Diabetes Mellitus
963
kinase, which in turn causes phosphorylation of multi-
ple other intracellular enzymes including a group
called insulin-receptor substrates (IRS). Different
types of IRS (e.g. IRS-1, IRS-2, IRS-3) are expressed
in different tissues. The net effect is to activate s

different tissues. The net effect is to activate some
of these enzymes while inactivating others. In this way,
insulin directs the intracellular metabolic machinery to
produce the desired effects on carbohydrate, fat, and
protein metabolism. The end effects of insulin stimu-
1. Within seconds after insulin binds with its
membrane receptors, the membranes of about 
80 per cent of the body’s cells markedly increase
their uptake of glucose. This is especially true of
muscl

ptake of glucose. This is especially true of
muscle cells and adipose cells but is not true of
most neurons in the brain. The increased glucose
transported into the cells is immediately
phosphorylated and becomes a substrate for all
the usual carbohydrate metabolic functions. The
increased glucose transport is believed to result
from translocation of multiple intracellular
vesicles to the cell membranes; these vesicles
carry in their own membranes multiple molecules
of glucose transport proteins

s multiple molecules
of glucose transport proteins, which bind with the
cell membrane and facilitate glucose uptake into
the cells. When insulin is no longer available, these
vesicles separate from the cell membrane within
about 3 to 5 minutes and move back to the cell
interior to be used again and again as needed.
2. The cell membrane becomes more permeable to
many of the amino acids, potassium ions, and
phosphate ions, causing increased transport of
these substances into the cell.
3. Slower ef

rt of
3. Slower effects occur during the next 10 to 15
minutes to change the activity levels of many
more intracellular metabolic enzymes. These
effects result mainly from the changed states of
phosphorylation of the enzymes.
4. Much slower effects continue to occur for hours
and even several days. They result from changed
rates of translation of messenger RNAs at the
ribosomes to form new proteins and still slower
effects from changed rates of transcription of
DN

DNA in the cell nucleus. In this way, insulin
remolds much of the cellular enzymatic machinery
to achieve its metabolic goals.
Effect of Insulin on Carbohydrate
Immediately after a high-carbohydrate meal, the
glucose that is absorbed into the blood causes rapid
secretion of insulin, which is discussed in detail later
in the chapter.The insulin in turn causes rapid uptake,
storage, and use of glucose by almost all tissues of the
body, but

 of glucose by almost all tissues of the
body, but especially by the muscles, adipose tissue, and
liver.
Insulin Promotes Muscle Glucose Uptake 
During much of the day, muscle tissue depends not on
glucose for its energy but on fatty acids. The principal
reason for this is that the normal resting muscle mem-
brane is only slightly permeable to glucose, except
when the muscle ﬁber is stimulated by insulin; between
meals, the amount of insulin that is secreted is too
small to promot

of insulin that is secreted is too
small to promote signiﬁcant amounts of glucose entry
into the muscle cells.
However, under two conditions the muscles do use
large amounts of glucose. One of these is during mod-
erate or heavy exercise.This usage of glucose does not
require large amounts of insulin, because exercising
muscle ﬁbers become more permeable to glucose even
in the absence of insulin because of the contraction
process itself.
The second condition for muscle usage of large
amounts of 

nd condition for muscle usage of large
amounts of glucose is during the few hours after a
meal. At this time the blood glucose concentration is
high and the pancreas is secreting large quantities of
insulin. The extra insulin causes rapid transport of
glucose into the muscle cells. This causes the muscle
cell during this period to use glucose preferentially
over fatty acids, as we discuss later.
Storage of Glycogen in Muscle. If the muscles are not
exercising after a meal and yet glucose is tran

ot
exercising after a meal and yet glucose is transported
into the muscle cells in abundance, then most of the
glucose is stored in the form of muscle glycogen
S S
S
b
a
a
b
S
S
Glucose
Insulin
receptor
Insulin
Tyrosine
kinase
Tyrosine
kinase
S
Insulin receptor substrates (IRS)
Phosphorylation of enzymes
Glucose
transport
Fat
synthesis
Protein
synthesis
Glucose
synthesis
Growth
and gene
Figure 78–3
Schematic of the insulin receptor. Insulin binds to the a-subunit of
its 

n receptor. Insulin binds to the a-subunit of
its receptor, which causes autophosphorylation of the b-subunit
receptor, which in turn induces tyrosine kinase activity. The recep-
tor tyrosine kinase activity begins a cascade of cell phosphoryla-
tion that increases or decreases the activity of enzymes, including
insulin receptor substrates, that mediate the effects of glucose on
glucose, fat, and protein metabolism. For example, glucose trans-
porters are moved to the cell membrane to facilitate

rters are moved to the cell membrane to facilitate glucose entry
into the cell.
964
Unit XIV
instead of being used for energy, up to a limit of 2 to
3 per cent concentration. The glycogen can later be
used for energy by the muscle. It is especially useful
for short periods of extreme energy use by the muscles
and even to provide spurts of anaerobic energy for a
few minutes at a time by glycolytic breakdown of the
glycogen to lactic acid, which can occur even in the

ycogen to lactic acid, which can occur even in the
absence of oxygen.
Quantitative Effect of Insulin to Facilitate Glucose Transport
Through the Muscle Cell Membrane. The quantitative
effect of insulin to facilitate glucose transport through
the muscle cell membrane is demonstrated by the
experimental results shown in Figure 78–4. The lower
curve labeled “control” shows the concentration of
free glucose measured inside the cell, demonstrating
that the glucose concentration remained almost zero
d

t the glucose concentration remained almost zero
despite increased extracellular glucose concentration
up to as high as 750 mg/100 ml. In contrast, the curve
labeled “insulin” demonstrates that the intracellular
glucose concentration rose to as high as 400 mg/100 ml
when insulin was added. Thus, it is clear that insulin
can increase the rate of transport of glucose into the
resting muscle cell by at least 15-fold.
Insulin Promotes Liver Uptake, Storage, and
Use of Glucose
One of the most importa

torage, and
One of the most important of all the effects of insulin
is to cause most of the glucose absorbed after a meal
to be stored almost immediately in the liver in the
form of glycogen. Then, between meals, when food is
not available and the blood glucose concentration
begins to fall, insulin secretion decreases rapidly and
the liver glycogen is split back into glucose, which is
released back into the blood to keep the glucose con-
centration from falling too low.
The mechan

e con-
The mechanism by which insulin causes glucose
uptake and storage in the liver includes several almost
simultaneous steps:
1. Insulin inactivates liver phosphorylase, the
principal enzyme that causes liver glycogen to
split into glucose. This prevents breakdown of the
glycogen that has been stored in the liver cells.
2. Insulin causes enhanced uptake of glucose from
the blood by the liver cells. It does this by
increasing the activity of the enzyme glucokin

 by
increasing the activity of the enzyme glucokinase,
which is one of the enzymes that causes the initial
phosphorylation of glucose after it diffuses into
the liver cells. Once phosphorylated, the glucose is
temporarily trapped inside the liver cells because
phosphorylated glucose cannot diffuse back
through the cell membrane.
3. Insulin also increases the activities of the enzymes
that promote glycogen synthesis, including
especially glycogen synthase, which is responsible
for polymerization 

synthase, which is responsible
for polymerization of the monosaccharide units to
form the glycogen molecules.
The net effect of all these actions is to increase the
amount of glycogen in the liver. The glycogen can
increase to a total of about 5 to 6 per cent of the liver
mass, which is equivalent to almost 100 grams of stored
glycogen in the whole liver.
Glucose Is Released from the Liver Between Meals. When the
blood glucose level begins to fall to a low level
between meals, several events tra

l to a low level
between meals, several events transpire that cause the
liver to release glucose back into the circulating blood:
1. The decreasing blood glucose causes the pancreas
to decrease its insulin secretion.
2. The lack of insulin then reverses all the effects
listed earlier for glycogen storage, essentially
stopping further synthesis of glycogen in the liver
and preventing further uptake of glucose by the
liver from the blood.
3. The lack of insulin (along with increase of
glucagon, wh

ck of insulin (along with increase of
glucagon, which is discussed later) activates the
enzyme phosphorylase, which causes the splitting
of glycogen into glucose phosphate.
4. The enzyme glucose phosphatase, which had been
inhibited by insulin, now becomes activated by the
insulin lack and causes the phosphate radical to
split away from the glucose; this allows the free
glucose to diffuse back into the blood.
Thus, the liver removes glucose from the blood
when it is present in excess after a mea

the blood
when it is present in excess after a meal and returns it
to the blood when the blood glucose concentration
falls between meals. Ordinarily, about 60 per cent of
the glucose in the meal is stored in this way in the liver
and then returned later.
Insulin Promotes Conversion of Excess Glucose into Fatty Acids
and Inhibits Gluconeogenesis in the Liver. When the quan-
tity of glucose entering the liver cells is more than can
be stored as glycogen or can be used for local hepato-
cyte metabo

cogen or can be used for local hepato-
cyte metabolism, insulin promotes the conversion 
of all this excess glucose into fatty acids. These fatty
acids are subsequently packaged as triglycerides in
0
400
Insulin
Control
300
200
100
0
900
600
300
Intracellular glucose
Extracellular glucose
Figure 78–4
Effect of insulin in enhancing the concentration of glucose inside
muscle cells. Note that in the absence of insulin (control), the 
intracellular glucose concentration remai

l), the 
intracellular glucose concentration remains near zero, despite
high extracellular glucose concentrations. (Data from Eisenstein
AB: The Biochemical Aspects of Hormone Action, Boston, Little,
Brown, 1964.)
Chapter 78
965
very-low-density lipoproteins and transported in this
form by way of the blood to the adipose tissue and
deposited as fat.
Insulin also inhibits gluconeogenesis. It does this
mainly by decreasing the quantities and activities of
t

y by decreasing the quantities and activities of
the liver enzymes required for gluconeogenesis.
However, part of the effect is caused by an action 
of insulin that decreases the release of amino acids
from muscle and other extrahepatic tissues and in 
turn the availability of these necessary precursors
This is discussed 
further in relation to the effect of insulin on protein
Lack of Effect of Insulin on Glucose Uptake
and Usage by the Brain
The brain i

 Glucose Uptake
The brain is quite different from most other tissues of
the body in that insulin has little effect on uptake or
use of glucose. Instead, the brain cells are permeable to
glucose and can use glucose without the intermediation
The brain cells are also quite different from most
other cells of the body in that they normally use only
glucose for energy and can use other energy sub-
strates, such as fats, only with difﬁculty. Therefore, it is
essentia

ts, only with difﬁculty. Therefore, it is
essential that the blood glucose level always be main-
tained above a critical level, which is one of the most
important functions of the blood glucose control
system. When the blood glucose falls too low, into 
the range of 20 to 50 mg/100 ml, symptoms of hypo-
glycemic shock develop, characterized by progressive
nervous irritability that leads to fainting, seizures, and
even coma.
Effect of Insulin on Carbohydrate Metabolism
in Other Cells
Insulin incr

arbohydrate Metabolism
Insulin increases glucose transport into and glucose
usage by most other cells of the body (with the excep-
tion of the brain cells, as noted) in the same way that
it affects glucose transport and usage in muscle cells.
The transport of glucose into adipose cells mainly pro-
vides substrate for the glycerol portion of the fat mol-
ecule. Therefore, in this indirect way, insulin promotes
deposition of fat in these cells.
Al

hese cells.
Although not quite as visible as the acute effects of
insulin on carbohydrate metabolism, insulin’s effects
on fat metabolism are, in the long run, equally impor-
tant. Especially dramatic is the long-term effect of
insulin lack in causing extreme atherosclerosis, often
leading to heart attacks, cerebral strokes, and other
vascular accidents. But ﬁrst, let us discuss the acute
effects of insulin on fat metabolism.
Insulin Promotes Fat Synthesis and

fat metabolism.
Insulin Promotes Fat Synthesis and Storage
Insulin has several effects that lead to fat storage in
adipose tissue. First, insulin increases the utilization 
of glucose by most of the body’s tissues, which 
automatically decreases the utilization of fat, thus
functioning as a fat sparer. However, insulin also 
promotes fatty acid synthesis. This is especially true
when more carbohydrates are ingested than can be
used for immediate energy, thus providing the sub-
strate for fat syn

energy, thus providing the sub-
strate for fat synthesis.Almost all this synthesis occurs
in the liver cells, and the fatty acids are then trans-
ported from the liver by way of the blood lipoproteins
to the adipose cells to be stored. The different factors
that lead to increased fatty acid synthesis in the liver
include the following:
1. Insulin increases the transport of glucose into the
liver cells. After the liver glucogen concentration
reaches 5 to 6 per cent, this in itself inhibits
furthe

es 5 to 6 per cent, this in itself inhibits
further glycogen synthesis. Then all the additional
glucose entering the liver cells becomes available
to form fat. The glucose is ﬁrst split to pyruvate 
in the glycolytic pathway, and the pyruvate
subsequently is converted to acetyl coenzyme A
(acetyl-CoA), the substrate from which fatty acids
are synthesized.
2. An excess of citrate and isocitrate ions is formed
by the citric acid cycle when excess amounts of
glucose are being used for energy. These

mounts of
glucose are being used for energy. These ions then
have a direct effect in activating acetyl-CoA
carboxylase, the enzyme required to carboxylate
acetyl-CoA to form malonyl-CoA, the ﬁrst stage
of fatty acid synthesis.
3. Most of the fatty acids are then synthesized within
the liver itself and used to form triglycerides, the
usual form of storage fat. They are released from
the liver cells to the blood in the lipoproteins.
Insulin activates lipoprotein lipase in the capillary
walls of th

es lipoprotein lipase in the capillary
walls of the adipose tissue, which splits the
triglycerides again into fatty acids, a requirement
for them to be absorbed into the adipose cells,
where they are again converted to triglycerides
and stored.
Role of Insulin in Storage of Fat in the Adipose Cells. Insulin
has two other essential effects that are required for fat
storage in adipose cells:
1. Insulin inhibits the action of hormone-sensitive
lipase. This is the enzyme that causes hydrolysis of
th

e. This is the enzyme that causes hydrolysis of
the triglycerides already stored in the fat cells.
Therefore, the release of fatty acids from the
adipose tissue into the circulating blood is
2. Insulin promotes glucose transport through the cell
membrane into the fat cells in exactly the same
ways that it promotes glucose transport into
muscle cells. Some of this glucose is then used to
synthesize minute amounts of fatty acids, but
more important, it also forms large quantities of
a-g

e important, it also forms large quantities of
a-glycerol phosphate. This substance supplies the
glycerol that combines with fatty acids to form 
the triglycerides that are the storage form of fat 
in adipose cells. Therefore, when insulin is not
available, even storage of the large amounts of
fatty acids transported from the liver in the
lipoproteins is almost blocked.
966
Unit XIV
Insulin Deﬁciency Increases Use of Fat 
for Energy
All aspects of fat breakdown and

f Fat 
for Energy
All aspects of fat breakdown and use for providing
energy are greatly enhanced in the absence of insulin.
This occurs even normally between meals when secre-
tion of insulin is minimal, but it becomes extreme in
diabetes mellitus when secretion of insulin is almost
zero. The resulting effects are as follows.
Insulin Deﬁciency Causes Lipolysis of Storage Fat and Release
of Free Fatty Acids. In the absence of insulin, all the
effects of insulin noted earlier that cause storage of

cts of insulin noted earlier that cause storage of 
fat are reversed. The most important effect is that 
the enzyme hormone-sensitive lipase in the fat cells
becomes strongly activated. This causes hydrolysis of
the stored triglycerides, releasing large quantities of
fatty acids and glycerol into the circulating blood.
Consequently, the plasma concentration of free fatty
acids begins to rise within minutes. This free fatty acid
then becomes the main energy substrate used by
essentially all tissu

ain energy substrate used by
essentially all tissues of the body besides the brain.
Figure 78–5 shows the effect of insulin lack on the
plasma concentrations of free fatty acids, glucose, and
acetoacetic acid. Note that almost immediately after
removal of the pancreas, the free fatty acid concentra-
tion in the plasma begins to rise, more rapidly even
than the concentration of glucose.
Insulin Deﬁciency Increases Plasma Cholesterol and Phospho-
lipid Concentrations. The excess of fatty acids in 

ipid Concentrations. The excess of fatty acids in the
plasma associated with insulin deﬁciency also pro-
motes liver conversion of some of the fatty acids into
phospholipids and cholesterol, two of the major prod-
ucts of fat metabolism. These two substances, along
with excess triglycerides formed at the same time in
the liver, are then discharged into the blood in the
lipoproteins. Occasionally the plasma lipoproteins
increase as much as threefold in the absence of insulin,
giving a total conce

ld in the absence of insulin,
giving a total concentration of plasma lipids of several
per cent rather than the normal 0.6 per cent. This high
lipid concentration—especially the high concentration
of cholesterol—promotes the development of athero-
sclerosis in people with serious diabetes.
Excess Usage of Fats During Insulin Lack Causes Ketosis and
Acidosis. Insulin lack also causes excessive amounts of
acetoacetic acid to be formed in the liver cells. This
results from the following effect: In 

cells. This
results from the following effect: In the absence of
insulin but in the presence of excess fatty acids in the
liver cells, the carnitine transport mechanism for trans-
porting fatty acids into the mitochondria becomes
increasingly activated. In the mitochondria, beta oxi-
dation of the fatty acids then proceeds very rapidly,
releasing extreme amounts of acetyl-CoA.A large part
of this excess acetyl-CoA is then condensed to form
acetoacetic acid, which in turn is released into the cir

cetic acid, which in turn is released into the cir-
culating blood. Most of this passes to the peripheral
cells, where it is again converted into acetyl-CoA and
used for energy in the usual manner.
At the same time, the absence of insulin also
depresses the utilization of acetoacetic acid in the
peripheral tissues. Thus, so much acetoacetic acid is
released from the liver that it cannot all be metabo-
lized by the tissues.Therefore, as shown in Figure 78–5,
its concentration rises during the day

igure 78–5,
its concentration rises during the days after cessation
of insulin secretion, sometimes reaching concentra-
tions of 10 mEq/L or more, which is a severe state of
body ﬂuid acidosis.
As explained in Chapter 68, some of the acetoacetic
acid is also converted into b-hydroxybutyric acid and
acetone. These two substances, along with the ace-
toacetic acid, are called ketone bodies, and their pres-
ence in large quantities in the body ﬂuids is called
ketosis. We see later that in severe di

 is called
ketosis. We see later that in severe diabetes the ace-
toacetic acid and the b-hydroxybutyric acid can cause
severe acidosis and coma, which often leads to death.
Effect of Insulin on Protein
Metabolism and on Growth
Insulin Promotes Protein Synthesis and Storage. During the
few hours after a meal when excess quantities of nutri-
ents are available in the circulating blood, not only car-
bohydrates and fats but proteins as well are stored in
the tissues; insulin is required for this t

red in
the tissues; insulin is required for this to occur. The
manner in which insulin causes protein storage is not
as well understood as the mechanisms for both glucose
and fat storage. Some of the facts follow.
1. Insulin stimulates transport of many of the amino
acids into the cells. Among the amino acids most
strongly transported are valine, leucine, isoleucine,
tyrosine, and phenylalanine. Thus, insulin shares
with growth hormone the capability of increasing
the uptake of amino acids into 

lity of increasing
the uptake of amino acids into cells. However, the
amino acids affected are not necessarily the same
ones.
0
Free fatty acids
Depancreatized
Control
Removal of pancreas
2
3
1
4
Days
0
Control
2
3
1
4
Days
Figure 78–5
Effect of removing the pancreas on the approximate concentra-
tions of blood glucose, plasma free fatty acids, and acetoacetic
acid.
Chapter 78
Insuli

tty acids, and acetoacetic
acid.
Chapter 78
967
2. Insulin increases the translation of messenger
RNA, thus forming new proteins. In some
unexplained way, insulin “turns on” the ribosomal
machinery. In the absence of insulin, the
ribosomes simply stop working, almost as if
insulin operates an “on-off” mechanism.
3. Over a longer period of time, insulin also
increases the rate of transcription of selected DNA
genetic sequences in the cell nuclei, thus form

NA
genetic sequences in the cell nuclei, thus forming
increased quantities of RNA and still more
protein synthesis—especially promoting a vast
array of enzymes for storage of carbohydrates,
fats, and proteins.
4. Insulin inhibits the catabolism of proteins, thus
decreasing the rate of amino acid release from the
cells, especially from the muscle cells. Presumably
this results from the ability of insulin to diminish
the normal degradation of proteins by the cellular
lysosomes.
5. In the liver, in

ins by the cellular
5. In the liver, insulin depresses the rate of
gluconeogenesis. It does this by decreasing 
the activity of the enzymes that promote
gluconeogenesis. Because the substrates most used
for synthesis of glucose by gluconeogenesis are
the plasma amino acids, this suppression of
gluconeogenesis conserves the amino acids in the
In summary, insulin promotes protein formation and
prevents the degradation of proteins.
Insulin Lack Causes Protein 

radation of proteins.
Insulin Lack Causes Protein Depletion and Increased Plasma
Amino Acids. Virtually all protein storage comes to a
halt when insulin is not available. The catabolism of
proteins increases, protein synthesis stops, and large
quantities of amino acids are dumped into the plasma.
The plasma amino acid concentration rises consider-
ably, and most of the excess amino acids are used
either directly for energy or as substrates for gluco-
neogenesis. This degradation of the amino aci

uco-
neogenesis. This degradation of the amino acids also
leads to enhanced urea excretion in the urine. The
resulting protein wasting is one of the most serious of
all the effects of severe diabetes mellitus. It can lead
to extreme weakness as well as many deranged func-
tions of the organs.
Insulin and Growth Hormone Interact Synergistically to Promote
Growth. Because insulin is required for the synthesis of
proteins, it is as essential for growth of an animal as
growth hormone is. This is dem

wth of an animal as
growth hormone is. This is demonstrated in Figure
78–6, which shows that a depancreatized, hypophysec-
tomized rat without therapy hardly grows at all.
Furthermore, the administration of either growth
hormone or insulin one at a time causes almost no
growth. Yet a combination of these hormones causes
dramatic growth. Thus, it appears that the two hor-
mones function synergistically to promote growth,
each performing a speciﬁc function that is separate
from that of the other. 

function that is separate
from that of the other. Perhaps a small part of this
necessity for both hormones results from the fact that
each promotes cellular uptake of a different selection
of amino acids, all of which are required if growth is
to be achieved.
Figure 78–7 shows the basic cellular mechanisms for
insulin secretion by the pancreatic beta cells in
response to increased blood glucose concentration, the
primary controller of insulin secretion. The beta c

rimary controller of insulin secretion. The beta cells
have a large number of glucose transporters (GLUT-
2) that permit a rate of glucose inﬂux that is propor-
tional to the blood concentration in the physiologic
range. Once inside the cells, glucose is phosphorylated
to glucose-6-phosphate by glucokinase. This step
appears to be the rate limiting for glucose metabolism
in the beta cell and is considered the major mechanism
for glucose sensing and adjustment of the amount of
secreted insulin to

nd adjustment of the amount of
secreted insulin to the blood glucose levels.
The glucose-6-phosphate is subsequently oxidized to
form adenosine triphosphate (ATP),which inhibits the
ATP-sensitive potassium channels of the cell. Closure
of the potassium channels depolarizes the cell mem-
brane, thereby opening voltage-gated calcium chan-
nels, which are sensitive to changes in membrane
voltage. This produces an inﬂux of calcium that stimu-
lates fusion of the docked insulin-containing vesicles
wi

usion of the docked insulin-containing vesicles
with the cell membrane and secretion of insulin into
the extracellular ﬂuid by exocytosis.
Other nutrients, such as certain amino acids, can also
be metabolized by the beta cells to increase intracel-
lular ATP levels and stimulate insulin secretion. Some
hormones, such as glucagon and gastric inhibitory
peptide, as well as acetylcholine increase intracellular
calcium levels through other signaling pathways and
enhance the effect of glucose, althou

pathways and
enhance the effect of glucose, although they do not
have major effects on insulin secretion in the absence
of glucose. Other hormones, including somatostatin
and norepinephrine (by activating a-adrenergic recep-
tors), inhibit exocytosis of insulin.
Sulfonylurea drugs stimulate insulin secretion by
binding to the ATP-sensitive potassium channels and
blocking their activity. This results in a depolarizing
0
50
Depancreatized and
hypophysectomized
Growth
hormone
Insulin

hysectomized
Growth
hormone
Insulin
and insulin
100
150
200
0
250
200
150
50
100
250
Weight (grams)
Days
Figure 78–6
Effect of growth hormone, insulin, and growth hormone plus insulin
on growth in a depancreatized and hypophysectomized rat.
968
Unit XIV
effect that triggers insulin secretion, making these
drugs very useful in stimulating insulin secretion in
patients with type II diabetes, as we will discuss later.
Table 78–1 summarizes some of the f

Table 78–1 summarizes some of the factors that can
increase or decrease insulin secretion.
Formerly, it was believed that insulin secretion was
controlled almost entirely by the blood glucose con-
centration. However, as more has been learned about
the metabolic functions of insulin for protein and fat
metabolism, it has become apparent that blood amino
acids and other factors also play important roles in
controlling insulin secretion (see Table 78–1).

in
Increased Blood Glucose Stimulates Insulin Secretion. At 
the normal fasting level of blood glucose of 80 to 
90 mg/100 ml, the rate of insulin secretion is minimal—
on the order of 25 ng/min/kg of body weight, a level
that has only slight physiologic activity. If the blood
glucose concentration is suddenly increased to a level
two to three times normal and kept at this high level
thereafter, insulin secretion increases markedly in two
stages, a

ulin secretion increases markedly in two
stages, as shown by the changes in plasma insulin con-
centration seen in Figure 78–8.
1. Plasma insulin concentration increases almost 
10-fold within 3 to 5 minutes after the acute
elevation of the blood glucose; this results from
immediate dumping of preformed insulin from the
beta cells of the islets of Langerhans. However,
the initial high rate of secretion is not maintained;
instead, the insulin concentration decreases about
halfway back toward norm

entration decreases about
halfway back toward normal in another 5 to 10
minutes.
2. Beginning at about 15 minutes, insulin secretion
rises a second time and reaches a new plateau in 
2 to 3 hours, this time usually at a rate of
secretion even greater than that in the initial
phase. This secretion results both from additional
release of preformed insulin and from activation
of the enzyme system that synthesizes and
releases new insulin from the cells.
Feedback Relation Between Blood Glucose Conce

lls.
Feedback Relation Between Blood Glucose Concentration and
Insulin Secretion Rate. As the concentration of blood
glucose rises above 100 mg/100 ml of blood, the rate
of insulin secretion rises rapidly, reaching a peak some
10 to 25 times the basal level at blood glucose con-
centrations between 400 and 600 mg/100 ml, as shown
Glucose
ATP
Ca++
K+
Oxidation
GLUT 2
Ca++ channel
(open)
ATP + K+ channel
(closed)
Insulin
Glucose
Figure 78–7
Basic mech

el
(closed)
Insulin
Glucose
Basic mechanisms of glucose stimulation of insulin secretion by
beta cells of the pancreas. GLUT, glucose transporter.
Table 78–1
Factors and Conditions That Increase or Decrease 
Increase Insulin Secretion
Decrease Insulin Secretion
• Increased blood glucose
• Decreased blood glucose
• Increased blood free fatty acids
• Fasting
• Increased blood amino acids
• Somatostatin
• Gastrointestinal hormones 
• a-Adrenergic activity
(gastrin, cho

al hormones 
(gastrin, cholecystokinin, secretin,
• Leptin
gastric inhibitory peptide)
• Glucagon, growth hormone,
cortisol
• Parasympathetic stimulation;
acetylcholine
• b-Adrenergic stimulation
• Insulin resistance; obesity
• Sulfonylurea drugs (glyburide,
tolbutamide)
-10
0
10
20
30
40
50
60
70
0
250
80
60
40
20
80
Plasma insulin (mU/ml)
Minutes
Figure 78–8
Increase in plasma insulin concentration after a sudden increase
in blood glucose to two to three times the norma

e
in blood glucose to two to three times the normal range. Note an
initial rapid surge in insulin concentration and then a delayed but
higher and continuing increase in concentration beginning 15 to
20 minutes later.
Chapter 78
969
in Figure 78–9. Thus, the increase in insulin secretion
under a glucose stimulus is dramatic both in its rapid-
ity and in the tremendous level of secretion achieved.
Furthermore, the turn-off of insulin secretion is almost
equ

e, the turn-off of insulin secretion is almost
equally as rapid, occurring within 3 to 5 minutes after
reduction in blood glucose concentration back to the
fasting level.
This response of insulin secretion to an elevated
blood glucose concentration provides an extremely
important feedback mechanism for regulating blood
glucose concentration. That is, any rise in blood
glucose increases insulin secretion, and the insulin in
turn increases transport of glucose into liver, muscle,
and other cells, 

t of glucose into liver, muscle,
and other cells, thereby reducing the blood glucose
concentration back toward the normal value.
Other Factors That Stimulate 
Amino Acids. In addition to the stimulation of insulin
secretion by excess blood glucose, some of the amino
acids have a similar effect.The most potent of these are
arginine and lysine. This effect differs from glucose stim-
ulation of insulin secretion in the following way:Amino
acids administered in the absence of a ris

y:Amino
acids administered in the absence of a rise in blood
glucose cause only a small increase in insulin secretion.
However, when administered at the same time that the
blood glucose concentration is elevated, the glucose-
induced secretion of insulin may be as much as doubled
in the presence of the excess amino acids. Thus, the
amino acids strongly potentiate the glucose stimulus for
insulin secretion.
The stimulation of insulin secretion by amino acids 
is important, because the insulin in 

is important, because the insulin in turn promotes 
transport of amino acids into the tissue cells as well as
intracellular formation of protein. That is, insulin is
important for proper utilization of excess amino acids
in the same way that it is important for the utilization
of carbohydrates.
Gastrointestinal Hormones. A mixture of several impor-
tant 
hormones—gastrin,
secretin,
cholecystokinin, and gastric inhibitory peptide (which
seems to be the most potent)—

itory peptide (which
seems to be the most potent)—causes a moderate
increase in insulin secretion. These hormones are
released in the gastrointestinal tract after a person eats
a meal. They then cause an “anticipatory” increase in
blood insulin in preparation for the glucose and amino
acids to be absorbed from the meal. These gastroin-
testinal hormones generally act the same way as amino
acids to increase the sensitivity of insulin response to
increased blood glucose, almost doubling the rate o

ncreased blood glucose, almost doubling the rate of
insulin secretion as the blood glucose level rises.
Other Hormones and the Autonomic Nervous System. Other hor-
mones that either directly increase insulin secretion or
potentiate the glucose stimulus for insulin secretion
include glucagon, growth hormone, cortisol, and, to a
lesser extent, progesterone and estrogen. The impor-
tance of the stimulatory effects of these hormones is
that prolonged secretion of any one of them in large
quantities 

 secretion of any one of them in large
quantities can occasionally lead to exhaustion of the
beta cells of the islets of Langerhans and thereby
increase the risk for developing diabetes mellitus.
Indeed, diabetes often occurs in people who are main-
tained on high pharmacological doses of some of these
hormones. Diabetes is particularly common in giants or
acromegalic people with growth hormone-secreting
tumors, or in people whose adrenal glands secrete
excess glucocorticoids.
Under some conditi

secrete
Under some conditions, stimulation of the parasym-
pathetic nerves to the pancreas can increase insulin
secretion. However, it is doubtful that this effect is of
physiologic signiﬁcance for regulating insulin secretion.
Role of Insulin (and Other Hormones)
in “Switching” Between Carbohydrate
and Lipid Metabolism
From the preceding discussions, it should be clear that
insulin promotes the utilization of carbohydrates for
energy, whereas it depresses the utilization

s for
energy, whereas it depresses the utilization of fats.
Conversely, lack of insulin causes fat utilization mainly
to the exclusion of glucose utilization, except by brain
tissue. Furthermore, the signal that controls this
switching mechanism is principally the blood glucose
concentration. When the glucose concentration is low,
insulin secretion is suppressed and fat is used almost
exclusively for energy everywhere except in the brain.
When the glucose concentration is high, insulin secre-
ti

e glucose concentration is high, insulin secre-
tion is stimulated and carbohydrate is used instead of
fat, and the excess blood glucose is stored in the form
of liver glycogen, liver fat, and muscle glycogen.There-
fore, one of the most important functional roles of
insulin in the body is to control which of these two
foods from moment to moment will be used by the
cells for energy.
At least four other known hormones also play
important roles in this switching mechanism: growth
hormone from the

 this switching mechanism: growth
hormone from the anterior pituitary gland, cortisol
from the adrenal cortex, epinephrine from the adrenal
medulla, and glucagon from the alpha cells of the islets
of Langerhans in the pancreas. Glucagon is discussed
in the next section of this chapter. Both growth
hormone and cortisol are secreted in response to
X
0
100
200
300
400
500
0
20
15
10
5
600
Figure 78–9
Approximate insulin secre

Approximate insulin secretion at different plasma glucose levels.
970
Unit XIV
hypoglycemia, and both inhibit cellular utilization of
glucose while promoting fat utilization. However,
the effects of both of these hormones develop 
slowly, usually requiring many hours for maximal
expression.
Epinephrine is especially important in increasing
plasma glucose concentration during periods of stress
when the sympathetic nervous system is excited.

s
However, epinephrine acts differently from the other
hormones in that it increases the plasma fatty acid
concentration at the same time. The reasons for these
effects are as follows: (1) epinephrine has the potent
effect of causing glycogenolysis in the liver, thus
releasing within minutes large quantities of glucose
into the blood; (2) it also has a direct lipolytic effect
on the adipose cells because it activates adipose tissue
hormone-sensitiv

cause it activates adipose tissue
hormone-sensitive lipase, thus greatly enhancing the
blood concentration of fatty acids as well. Quantita-
tively, the enhancement of fatty acids is far greater
than the enhancement of blood glucose. Therefore,
epinephrine especially enhances the utilization of fat
in such stressful states as exercise, circulatory shock,
and anxiety.
Glucagon, a hormone secreted by the alpha cells of the
islets of Langerhans when the blood glucose conc

e
islets of Langerhans when the blood glucose concen-
tration falls, has several functions that are diametri-
cally opposed to those of insulin. Most important of
these functions is to increase the blood glucose con-
centration, an effect that is exactly the opposite that of
insulin.
Like insulin, glucagon is a large polypeptide. It has
a molecular weight of 3485 and is composed of a chain
of 29 amino acids. On injection of puriﬁed glucagon
into an animal, a profound hyperglycemic effect
occurs.

an animal, a profound hyperglycemic effect
occurs. Only 1 mg/kg of glucagon can elevate the blood
glucose concentration about 20 mg/100 ml of blood (a
25 per cent increase) in about 20 minutes. For this
reason, glucagon is also called the hyperglycemic
hormone.
The major effects of glucagon on glucose metabolism
are (1) breakdown of liver glycogen (glycogenolysis)
and (2) increased gluconeogenesis in the liver. Both of
these effects greatly enhance the availability 

of
these effects greatly enhance the availability of glucose
to the other organs of the body.
Glucagon Causes Glycogenolysis and Increased Blood Glucose
Concentration. The most dramatic effect of glucagon is
its ability to cause glycogenolysis in the liver, which in
turn increases the blood glucose concentration within
minutes.
It does this by the following complex cascade of
events:
1. Glucagon activates adenylyl cyclase in the hepatic
cell membrane,
2. Which causes the formation of cyclic aden

rane,
2. Which causes the formation of cyclic adenosine
monophosphate,
3. Which activates protein kinase regulator protein,
4. Which activates protein kinase,
5. Which activates phosphorylase b kinase,
6. Which converts phosphorylase b into
phosphorylase a,
7. Which promotes the degradation of glycogen into
glucose-1-phosphate,
8. Which then is dephosphorylated; and the glucose
is released from the liver cells.
This sequence of events is exceedingly important for
several reasons. First, it is on

gly important for
several reasons. First, it is one of the most thoroughly
studied of all the second messenger functions of cyclic
adenosine monophosphate. Second, it demonstrates 
a cascade system in which each succeeding product 
is produced in greater quantity than the preceding
product. Therefore, it represents a potent amplifying
mechanism; this type of amplifying mechanism is
widely used throughout the body for controlling many,
if not most, cellular metabolic systems, often causing
as muc

, cellular metabolic systems, often causing
as much as a millionfold ampliﬁcation in response.This
explains how only a few micrograms of glucagon can
cause the blood glucose level to double or increase even
more within a few minutes.
Infusion of glucagon for about 4 hours can cause such
intensive liver glycogenolysis that all the liver stores of
glycogen become depleted.
Glucagon Increases Gluconeogenesis. Even after all the
glycogen in the liver has been exhausted under the
inﬂuence of glucagon

 has been exhausted under the
inﬂuence of glucagon, continued infusion of this
hormone still causes continued hyperglycemia. This
results from the effect of glucagon to increase the rate
of amino acid uptake by the liver cells and then the
conversion of many of the amino acids to glucose by
gluconeogenesis. This is achieved by activating multi-
ple enzymes that are required for amino acid transport
and gluconeogenesis, especially activation of the
enzyme system for converting pyruvate to phospho

e
enzyme system for converting pyruvate to phospho-
enolpyruvate, a rate-limiting step in gluconeogenesis.
Other Effects of Glucagon
Most other effects of glucagon occur only when its
concentration rises well above the maximum normally
found in the blood. Perhaps the most important effect
is that glucagon activates adipose cell lipase, making
increased quantities of fatty acids available to the
energy systems of the body. Glucagon also inhibits 
the storage of triglycerides in the liver, which p

the storage of triglycerides in the liver, which prevents
the liver from removing fatty acids from the blood; this
also helps make additional amounts of fatty acids
available for the other tissues of the body.
Glucagon in very high concentrations also (1)
enhances the strength of the heart; (2) increases blood
ﬂow in some tissues, especially the kidneys; (3)
enhances bile secretion; and (4) inhibits gastric acid
secretion. All these effects are probably of minimal
importance in the normal functi

robably of minimal
importance in the normal function of the body.
Chapter 78
971
Increased Blood Glucose Inhibits Glucagon Secretion. The
blood glucose concentration is by far the most potent
factor that controls glucagon secretion. Note speciﬁ-
cally, however, that the effect of blood glucose concen-
tration on glucagon secretion is in exactly the opposite
direction from the effect of glucose on insulin secretion.
This is

he effect of glucose on insulin secretion.
This is demonstrated in Figure 78–10, showing that
a decrease in the blood glucose concentration from its
normal fasting level of about 90 mg/100 ml of blood
down to hypoglycemic levels can increase the plasma
concentration of glucagon severalfold. Conversely,
increasing the blood glucose to hyperglycemic levels
decreases plasma glucagon. Thus, in hypoglycemia,
glucagon is secreted in large amounts; it then greatly
increases the output of glucose from t

hen greatly
increases the output of glucose from the liver and
thereby serves the important function of correcting the
hypoglycemia.
Increased Blood Amino Acids Stimulate Glucagon Secretion.
High concentrations of amino acids, as occur in the
blood after a protein meal (especially the amino acids
alanine and arginine), stimulate the secretion of
glucagon.This is the same effect that amino acids have
in stimulating insulin secretion. Thus, in this instance,
the glucagon and insulin responses are 

 instance,
the glucagon and insulin responses are not opposites.
The importance of amino acid stimulation of glucagon
secretion is that the glucagon then promotes rapid
conversion of the amino acids to glucose, thus making
even more glucose available to the tissues.
Exercise Stimulates Glucagon Secretion. In exhaustive
exercise, the blood concentration of glucagon often
increases fourfold to ﬁvefold. What causes this is not
understood, because the blood glucose concentration
does not necessarily

e blood glucose concentration
does not necessarily fall. A beneﬁcial effect of the
glucagon is that it prevents a decrease in blood glucose.
One of the factors that might increase glucagon
secretion in exercise is increased circulating amino
acids. Other factors, such as b-adrenergic stimulation
of the islets of Langerhans, may also play a role.
Somatostatin Inhibits
Glucagon and Insulin
Secretion
The delta cells of the islets of Langerhans secrete the
hormone somatostatin, a polypeptide contain

te the
hormone somatostatin, a polypeptide containing only 14
amino acids that has an extremely short half-life of only
3 minutes in the circulating blood. Almost all factors
related to the ingestion of food stimulate somatostatin
secretion. They include (1) increased blood glucose, (2)
increased amino acids, (3) increased fatty acids, and (4)
increased concentrations of several of the gastrointesti-
nal hormones released from the upper gastrointestinal
tract in response to food intake.
In turn,

estinal
In turn, somatostatin has multiple inhibitory effects
1. Somatostatin acts locally within the islets of
Langerhans themselves to depress the secretion of
both insulin and glucagon.
2. Somatostatin decreases the motility of the stomach,
duodenum, and gallbladder.
3. Somatostatin decreases both secretion and
absorption in the gastrointestinal tract.
Putting all this information together, it has been sug-
gested that the principal role of somato

been sug-
gested that the principal role of somatostatin is to
extend the period of time over which the food nutrients
are assimilated into the blood. At the same time, the
effect of somatostatin to depress insulin and glucagon
secretion decreases the utilization of the absorbed
nutrients by the tissues, thus preventing rapid exhaus-
tion of the food and therefore making it available over
a longer period of time.
It should also be recalled that somatostatin is the
same chemical substance as grow

omatostatin is the
same chemical substance as growth hormone inhibitory
hormone, which is secreted in the hypothalamus and
suppresses anterior pituitary gland growth hormone
Summary of Blood 
Glucose Regulation
In a normal person, the blood glucose concentration is
narrowly controlled, usually between 80 and 90 mg/
100 ml of blood in the fasting person each morning
before breakfast. This concentration increases to 120
to 140 mg/100 ml during the ﬁrst hour or so after a
meal, but the f

during the ﬁrst hour or so after a
meal, but the feedback systems for control of blood
glucose return the glucose concentration rapidly back
to the control level, usually within 2 hours after the last
absorption of carbohydrates. Conversely, in starvation,
the gluconeogenesis function of the liver provides the
glucose that is required to maintain the fasting blood
glucose level.
The mechanisms for achieving this high degree of
control have been presented in this chapter. Let us
summarize them.
6

resented in this chapter. Let us
60
80
100
120
0
4
3
2
1
Plasma glucagon
Figure 78–10
Approximate plasma glucagon concentration at different blood
glucose levels.
972
Unit XIV
1. The liver functions as an important blood glucose
buffer system. That is, when the blood glucose
rises to a high concentration after a meal and the
rate of insulin secretion also increases, as much as
two thirds of the glucose absorb

eases, as much as
two thirds of the glucose absorbed from the gut is
almost immediately stored in the liver in the form
of glycogen. Then, during the succeeding hours,
when both the blood glucose concentration and
the rate of insulin secretion fall, the liver releases
the glucose back into the blood. In this way, the
liver decreases the ﬂuctuations in blood glucose
concentration to about one third of what they
would otherwise be. In fact, in patients with
severe liver disease, it becomes almost 

ents with
severe liver disease, it becomes almost impossible
to maintain a narrow range of blood glucose
2. Both insulin and glucagon function as important
feedback control systems for maintaining a normal
blood glucose concentration. When the glucose
concentration rises too high, insulin is secreted;
the insulin in turn causes the blood glucose
concentration to decrease toward normal.
Conversely, a decrease in blood glucose stimulates
glucagon secretion; the glucagon then functio

ates
glucagon secretion; the glucagon then functions in
the opposite direction to increase the glucose
toward normal. Under most normal conditions,
the insulin feedback mechanism is much more
important than the glucagon mechanism, but in
instances of starvation or excessive utilization of
glucose during exercise and other stressful
situations, the glucagon mechanism also becomes
valuable.
3. Also, in severe hypoglycemia, a direct effect of
low blood glucose on the hypothalamus stimulates
the sym

ood glucose on the hypothalamus stimulates
the sympathetic nervous system. In turn, the
epinephrine secreted by the adrenal glands causes
still further release of glucose from the liver. This,
too, helps protect against severe hypoglycemia.
4. And ﬁnally, over a period of hours and days, both
growth hormone and cortisol are secreted in
response to prolonged hypoglycemia, and they
both decrease the rate of glucose utilization by
most cells of the body, converting instead to
greater amounts of fat

body, converting instead to
greater amounts of fat utilization. This, too, helps
return the blood glucose concentration toward
normal.
Importance of Blood Glucose Regulation. One might ask the
question:Why is it so important to maintain a constant
blood glucose concentration, particularly because
most tissues can shift to utilization of fats and proteins
for energy in the absence of glucose? The answer is
that glucose is the only nutrient that normally can be
used by the brain, retina, and germi

rmally can be
used by the brain, retina, and germinal epithelium 
of the gonads in sufﬁcient quantities to supply them
optimally with their required energy. Therefore, it is
important to maintain the blood glucose concentration
at a sufﬁciently high level to provide this necessary
nutrition.
Most of the glucose formed by gluconeogenesis
during the interdigestive period is used for metabolism
in the brain. Indeed, it is important that the pancreas
not secrete any insulin during this time; otherwi

not secrete any insulin during this time; otherwise, the
scant supplies of glucose that are available would all
go into the muscles and other peripheral tissues,
leaving the brain without a nutritive source.
It is also important that the blood glucose concen-
tration not rise too high for four reasons: (1) Glucose
can exert a large amount of osmotic pressure in the
extracellular ﬂuid, and if the glucose concentration
rises to excessive values, this can cause considerable
cellular dehydration.(2

his can cause considerable
cellular dehydration.(2) An excessively high level of
blood glucose concentration causes loss of glucose in
the urine. (3) Loss of glucose in the urine also causes
osmotic diuresis by the kidneys, which can deplete 
the body of its ﬂuids and electrolytes.(4) Long-term
increases in blood glucose may cause damage to many
tissues, especially to blood vessels. Vascular injury,
associated with uncontrolled diabetes mellitus,leads to
increased risk for heart attack, stroke, 

leads to
increased risk for heart attack, stroke, end-stage renal
disease, and blindness.
Diabetes mellitus is a syndrome of impaired carbohy-
drate, fat, and protein metabolism caused by either lack
of insulin secretion or decreased sensitivity of the
tissues to insulin. There are two general types of dia-
betes mellitus:
1. Type I diabetes, also called insulin-dependent
diabetes mellitus (IDDM), is caused by lack of
2. Type II diabetes, also called non–insu

cretion.
2. Type II diabetes, also called non–insulin-dependent
diabetes mellitus (NIDDM), is caused by decreased
sensitivity of target tissues to the metabolic effect
of insulin. This reduced sensitivity to insulin is often
called insulin resistance.
In both types of diabetes mellitus, metabolism of all
the main foodstuffs is altered.The basic effect of insulin
lack or insulin resistance on glucose metabolism is to
prevent the efﬁcient uptake and utilization of glucose
by most cells of the body

d utilization of glucose
by most cells of the body, except those of the brain. As
a result, blood glucose concentration increases, cell 
utilization of glucose falls increasingly lower, and 
utilization of fats and proteins increases.
Type I Diabetes—Lack of Insulin
Production by Beta Cells 
of the Pancreas
Injury to the beta cells of the pancreas or diseases that
impair insulin production can lead to type I diabetes.
Viral infections or autoimmune disorders may be
involved in the destruction of

ne disorders may be
involved in the destruction of beta cells in many patients
with type I diabetes, although heredity also plays a
major role in determining the susceptibility of the beta
cells to destruction by these insults. In some instances,
there may be a hereditary tendency for beta cell degen-
eration even without viral infections or autoimmune
disorders.
The usual onset of type I diabetes occurs at about 14
years of age in the United States, and for this reason it
is often called juveni

tes, and for this reason it
is often called juvenile diabetes mellitus. Type I diabetes
may develop very abruptly, over a period of a few days
or weeks, with three principal sequelae: (1) increased
blood glucose, (2) increased utilization of fats for energy
Chapter 78
973
and for formation of cholesterol by the liver, and (3)
depletion of the body’s proteins.
Blood Glucose Concentration Rises to Very High Levels in Diabetes
Mellitus. The lack of insulin d

Levels in Diabetes
Mellitus. The lack of insulin decreases the efﬁciency 
of peripheral glucose utilization and augments 
glucose production, raising plasma glucose to 300 to
1200 mg/100 ml. The increased plasma glucose then has
multiple effects throughout the body.
Increased Blood Glucose Causes Loss of Glucose in the Urine
The high blood glucose causes more glucose to ﬁlter
into the renal tubules than can be reabsorbed, and the
excess glucose spills into the urine.This normally occurs
when the

pills into the urine.This normally occurs
when the blood glucose concentration rises above 
180 mg/100 ml, a level that is called the blood “thresh-
old” for the appearance of glucose in the urine. When
the blood glucose level rises to 300 to 500 mg/100 ml—
common values in people with severe untreated dia-
betes—100 or more grams of glucose can be lost into
the urine each day.
Increased Blood Glucose Causes Dehydration The very high
levels of blood glucose (sometimes as high as 8 to 10
times no

ood glucose (sometimes as high as 8 to 10
times normal in severe untreated diabetes) can cause
severe cell dehydration throughout the body. This
occurs partly because glucose does not diffuse easily
through the pores of the cell membrane, and the
increased osmotic pressure in the extracellular ﬂuids
causes osmotic transfer of water out of the cells.
In addition to the direct cellular dehydrating effect of
excessive glucose, the loss of glucose in the urine causes
osmotic diuresis. That is, the o

 the urine causes
osmotic diuresis. That is, the osmotic effect of glucose in
the renal tubules greatly decreases tubular reabsorption
of ﬂuid. The overall effect is massive loss of ﬂuid in the
urine, causing dehydration of the extracellular ﬂuid,
which in turn causes compensatory dehydration of the
intracellular ﬂuid, for reasons discussed in Chapter 26.
Thus, polyuria (excessive urine excretion), intracellular
and extracellular dehydration, and increased thirst are
classic symptoms of diabetes

 increased thirst are
classic symptoms of diabetes.
Chronic High Glucose Concentration Causes Tissue Injury When
blood glucose is poorly controlled over long periods in
diabetes mellitus, blood vessels in multiple tissues
throughout the body begin to function abnormally and
undergo structural changes that result in inadequate
blood supply to the tissues. This in turn leads to
increased risk for heart attack, stroke, end-stage kidney
disease, retinopathy and blindness, and ischemia and
gangrene o

nopathy and blindness, and ischemia and
gangrene of the limbs.
Chronic high glucose concentration also causes
damage to many other tissues. For example, peripheral
neuropathy, which is abnormal function of peripheral
nerves, and autonomic nervous system dysfunction are
frequent complications of chronic, uncontrolled dia-
betes mellitus. These abnormalities can result in
impaired cardiovascular reﬂexes, impaired bladder
control, decreased sensation in the extremities, and
other symptoms of periph

n in the extremities, and
other symptoms of peripheral nerve damage.
The precise mechanisms that cause tissue injury in
diabetes are not well understood but probably involve
multiple effects of high glucose concentrations and
other metabolic abnormalities on proteins of endothe-
lial and vascular smooth muscle cells, as well as other
tissues. In addition, hypertension, secondary to renal
injury, and atherosclerosis, secondary to abnormal lipid
metabolism, often develop in patients with diabetes 

tabolism, often develop in patients with diabetes and
amplify the tissue damage caused by the elevated
glucose.
Diabetes Mellitus Causes Increased Utilization of Fats and Meta-
bolic Acidosis. The shift from carbohydrate to fat metab-
olism in diabetes increases the release of keto acids,
such as acetoacetic acid and b-hydroxybutyric acid, into
the plasma more rapidly than they can be taken up and
oxidized by the tissue cells. As a result, the patient
develops severe metabolic acidosis from the 

tient
develops severe metabolic acidosis from the excess keto
acids, which, in association with dehydration due to the
excessive urine formation, can cause severe acidosis.
This leads rapidly to diabetic coma and death unless the
condition is treated immediately with large amounts of
insulin.
All the usual physiologic compensations that occur 
in metabolic acidosis take place in diabetic acidosis.
They include rapid and deep breathing,
which 
causes increased expiration of carbon dioxide; this
b

ses increased expiration of carbon dioxide; this
buffers the acidosis but also depletes extracellular ﬂuid
bicarbonate stores.The kidneys compensate by decreas-
ing bicarbonate excretion and generating new bicar-
bonate that is added back to the extracellular ﬂuid.
Although extreme acidosis occurs only in the most
severe instances of uncontrolled diabetes, when the pH
of the blood falls below about 7.0, acidotic coma and
death can occur within hours.The overall changes in the
electrolytes of the

urs.The overall changes in the
electrolytes of the blood as a result of severe diabetic
acidosis are shown in Figure 78–11.
Excess fat utilization in the liver occurring over a long
time causes large amounts of cholesterol in the circu-
lating blood and increased deposition of cholesterol in
Glucose
Keto acids
Total cations
pH
HCO3
Cl-
180 mg/dL
360 mg/dL
100 mg/dL
400+ mg/dL
7.4
6.9
103 mEq
90 mEq
155 mEq
130 mEq
155 mEq
30 mEq
27 mEq
5 mEq
Figure 78–11
Changes in blood constituents

q
5 mEq
Changes in blood constituents in diabetic coma, showing normal
values (lavender bars) and diabetic coma values (red bars). 
974
Unit XIV
the arterial walls. This leads to severe arteriosclerosis
and other vascular lesions, as discussed earlier.
Diabetes Causes Depletion of the Body’s Proteins. Failure to
use glucose for energy leads to increased utilization and
decreased storage of proteins as well as fat. Therefore,
a person with severe untrea

ell as fat. Therefore,
a person with severe untreated diabetes mellitus suffers
rapid weight loss and asthenia (lack of energy) despite
eating large amounts of food (polyphagia). Without
treatment, these metabolic abnormalities can cause
severe wasting of the body tissues and death within a
few weeks.
Type II Diabetes—Resistance to the
Type II diabetes is far more common than type I,
accounting for about 90 per cent of all cases of diabetes
mellitus. In most cases, t

f all cases of diabetes
mellitus. In most cases, the onset of type II diabetes
occurs after age 30, often between the ages of 50 and 60
years, and the disease develops gradually.Therefore, this
syndrome is often referred to as adult-onset diabetes. In
recent years, however, there has been a steady increase
in the number of younger individuals, some less than 20
years old, with type II diabetes.This trend appears to be
related mainly to the increasing prevalence of obesity,
the most important ris

sing prevalence of obesity,
the most important risk factor for type II diabetes in chil-
dren as well as in adults.
Obesity, Insulin Resistance, and “Metabolic Syndrome” Usually
Precede Development of Type II Diabetes. Type II diabetes, in
contrast to type I, is associated with increased plasma
insulin concentration (hyperinsulinemia).This occurs as
a compensatory response by the pancreatic beta cells
for diminished sensitivity of target tissues to the meta-
bolic effects of insulin, a condition

to the meta-
bolic effects of insulin, a condition referred to as insulin
resistance. The decrease in insulin sensitivity impairs
carbohydrate utilization and storage, raising blood
glucose and stimulating a compensatory increase in
Development of insulin resistance and impaired
glucose metabolism is usually a gradual process, begin-
ning with excess weight gain and obesity. The mecha-
nisms that link obesity with insulin resistance, however,
are still uncertain. Some studies 

tance, however,
are still uncertain. Some studies suggest that there are
fewer insulin receptors, especially in the skeletal muscle,
liver, and adipose tissue, in obese than in lean subjects.
However, most of the insulin resistance appears to be
caused by abnormalities of the signaling pathways that
link receptor activation with multiple cellular effects.
Impaired insulin signaling appears to be closely related
to toxic effects of lipid accumulation in tissues such as
skeletal muscle and liver s

ion in tissues such as
skeletal muscle and liver secondary to excess weight
gain.
Insulin resistance is part of a cascade of disorders that
is often called the “metabolic syndrome.” Some of the
features of the metabolic syndrome include: (1) obesity,
especially accumulation of abdominal fat; (2) insulin
resistance; (3) fasting hyperglycemia; (4) lipid abnor-
malities such as increased blood triglycerides and
decreased blood high-density lipoprotein-cholesterol;
and (5) hypertension. All of the f

in-cholesterol;
and (5) hypertension. All of the features of the meta-
bolic syndrome are closely related to excess weight gain,
especially when it is associated with accumulation of
adipose tissue in the abdominal cavity around the vis-
ceral organs.
The role of insulin resistance in contributing to some
of the components of the metabolic syndrome is
unclear, although it is clear that insulin resistance is the
primary cause of increased blood glucose concentra-
tion. The major adverse consequen

ucose concentra-
tion. The major adverse consequence of the metabolic
syndrome is cardiovascular disease, including athero-
sclerosis and injury to various organs throughout the
body. Several of the metabolic abnormalities associated
with the syndrome are risk factors for cardiovascular
disease, and insulin resistance predisposes to the devel-
opment of type II diabetes mellitus, also a major cause
of cardiovascular disease.
Other Factors That Can Cause Insulin Resistance and Type II Dia-
betes.

n Cause Insulin Resistance and Type II Dia-
betes. Although most patients with type II diabetes are
overweight or have substantial accumulation of visceral
fat, severe insulin resistance and type II diabetes can
also occur as a result of other acquired or genetic con-
ditions that impair insulin signaling in peripheral tissues
(Table 78–2).
Polycystic ovary syndrome (PCOS), for example, is
associated with marked increases in ovarian androgen
production and insulin resistance and is one of the mo

uction and insulin resistance and is one of the most
common endocrine disorders in women, affecting ap-
proximately 6 per cent of all women during their re-
productive life. Although the pathogenesis of PCOS
remains uncertain, insulin resistance and hyperinsuline-
mia are found in approximately 80 per cent of affected
women. The long-term consequences include increased
risk for diabetes mellitus, increased blood lipids, and
Excess formation of glucocorticoids (Cushing’s s

.
Excess formation of glucocorticoids (Cushing’s syn-
drome) or growth hormone (acromegaly) also decreases
the sensitivity of various tissues to the metabolic effects
of insulin and can lead to development of diabetes mel-
litus. Genetic causes of obesity and insulin resistance, if
severe enough, also can lead to type II diabetes as well
as many other features of the metabolic syndrome,
including cardiovascular disease.
Development of Type II Diabetes During Prolonged Insulin Resis-
tance. With 

betes During Prolonged Insulin Resis-
tance. With prolonged and severe insulin resistance,
even the increased levels of insulin are not sufﬁcient to
maintain normal glucose regulation. As a result, mod-
erate hyperglycemia occurs after ingestion of carbohy-
drates in the early stages of the disease.
Table 78–2
Some Causes of Insulin Resistance
• Obesity/overweight (especially excess visceral adiposity)
• Excess glucocorticoids (Cushing’s syndrome or steroid therapy)
• Excess growth hormone (acro

 or steroid therapy)
• Excess growth hormone (acromegaly)
• Pregnancy, gestational diabetes
• Polycystic ovary disease
• Lipodystrophy (acquired or genetic; associated with lipid 
accumulation in liver)
• Autoantibodies to the insulin receptor
• Mutations of insulin receptor
• Mutations of the peroxisome proliferators’ activator receptor g
(PPARg)
• Mutations that cause genetic obesity (e.g., melanocortin 
receptor mutations)
• Hemochromatosis (a hereditary disease that causes tissue iron 
accum

 hereditary disease that causes tissue iron 
accumulation)
Chapter 78
975
In the later stages of type II diabetes, the pancreatic
beta cells become “exhausted” and are unable to
produce enough insulin to prevent more severe 
hyperglycemia, especially after the person ingests a 
carbohydrate-rich meal.
Some obese people, although having marked insulin
resistance and greater than normal increases in blood
glucose after a meal, never develop clinically signi

ucose after a meal, never develop clinically signiﬁcant
diabetes mellitus; apparently, the pancreas in these
people produces enough insulin to prevent severe
abnormalities of glucose metabolism.
In others,
however, the pancreas gradually becomes exhausted
from secreting large amounts of insulin, and full-blown
diabetes mellitus occurs. Some studies suggest that
genetic factors play an important role in determining
whether an individual’s pancreas can sustain the high
output of insulin over many 

 can sustain the high
output of insulin over many years that is necessary to
avoid the severe abnormalities of glucose metabolism in
type II diabetes.
In many instances, type II diabetes can be effectively
treated, at least in the early stages, with exercise, caloric
restriction, and weight reduction, and no exogenous
insulin administration is required. Drugs that increase
insulin sensitivity, such as thiazolidinediones and met-
formin, or drugs that cause additional release of insulin
by the pa

that cause additional release of insulin
by the pancreas, such as sulfonylureas, may also be used.
However, in the later stages of type II diabetes, insulin
administration is usually required to control plasma
glucose.
Physiology of Diagnosis 
of Diabetes Mellitus
Table 78–3 compares some of clinical features of type I
and type II diabetes mellitus. The usual methods for
diagnosing diabetes are based on various chemical tests
of the urine and the blood.
Urinary Glucose. Simple ofﬁce tests or mor

 blood.
Urinary Glucose. Simple ofﬁce tests or more complicated
quantitative laboratory tests may be used to determine
the quantity of glucose lost in the urine. In general, a
normal person loses undetectable amounts of glucose,
whereas a person with diabetes loses glucose in small to
large amounts, in proportion to the severity of disease
and the intake of carbohydrates.
Fasting Blood Glucose and Insulin Levels. The fasting blood
glucose level in the early morning is normally 80 to 
90 mg/100 m

n the early morning is normally 80 to 
90 mg/100 ml, and 110 mg/100 ml is considered to be the
upper limit of normal. A fasting blood glucose level
above this value often indicates diabetes mellitus or a
least marked insulin resistance.
In type I diabetes, plasma insulin levels are very low
or undetectable during fasting and even after a meal. In
type II diabetes, plasma insulin concentration may be
severalfold higher than normal and usually increases to
a greater extent after ingestion of a sta

eases to
a greater extent after ingestion of a standard glucose
load during a glucose tolerance test (see the next 
paragraph).
Glucose Tolerance Test. As demonstrated by the bottom
curve in Figure 78–12, called a “glucose tolerance
curve,” when a normal, fasting person ingests 1 gram 
of glucose per kilogram of body weight, the blood
glucose level rises from about 90 mg/100 ml to 120 to
140 mg/100 ml and falls back to below normal in 
about 2 hours.
In a person with diabetes, the fasting blood 

urs.
In a person with diabetes, the fasting blood glucose
concentration is almost always above 110 mg/100 ml
and often above 140 mg/100 ml. Also, the glucose toler-
ance test is almost always abnormal. On ingestion of
glucose, these people exhibit a much greater than
normal rise in blood glucose level, as demonstrated by
the upper curve in Figure 78–12, and the glucose level
falls back to the control value only after 4 to 6 hours;
furthermore, it fails to fall below the control level. The
slow f

 fails to fall below the control level. The
slow fall of this curve and its failure to fall below the
control level demonstrate that either (1) the normal
increase in insulin secretion after glucose ingestion does
not occur or (2) there is decreased sensitivity to insulin.
A diagnosis of diabetes mellitus can usually be estab-
lished on the basis of such a curve, and type I and type
II diabetes can be distinguished from each other by
measurements of plasma insulin, with plasma insulin
being low 

 of plasma insulin, with plasma insulin
being low or undetectable in type I diabetes and
increased in type II diabetes.
Table 78–3
Clinical Characteristics of Patients with Type I and Type II
Feature
Type I
Type II
Usually <20 years
Usually >30 years
Body mass
Low (wasted) to 
Obese
normal
Low or absent
Normal to high 
initially
High, can be 
High, resistant to 
suppressed
Increased
Increased
Insulin sensiti

Increased
Increased
Normal
Reduced
Therapy
Insulin
Weight loss,
thiazolidinediones,
metformin,
sulfonylureas,
insulin
0
1
Normal
Diabetes
2
3
4
80
200
180
160
140
120
100
5
Blood glucose level
Hours
Figure 78–12
Glucose tolerance curve in a normal person and in a person with
diabetes.
976
Unit XIV
Acetone Breath. As pointed out in Chapter 68, small
quantities of acetoacetic acid in the blood, which
increase greatly in 

etic acid in the blood, which
increase greatly in severe diabetes, are converted to
acetone. This is volatile and vaporized into the expired
air. Consequently, one can frequently make a diagnosis
of type I diabetes mellitus simply by smelling acetone
on the breath of a patient. Also, keto acids can be
detected by chemical means in the urine, and their
quantitation aids in determining the severity of the dia-
betes. In the early stages of type II diabetes, however,
keto acids are usually not prod

diabetes, however,
keto acids are usually not produced in excess amounts.
However, when insulin resistance becomes very severe
and there is greatly increased utilization of fats for
energy, keto acids are then produced in persons with
The theory of treatment of type I diabetes mellitus is to
administer enough insulin so that the patient will have
carbohydrate, fat, and protein metabolism that is as
normal as possible. Insulin is available in several forms.

s possible. Insulin is available in several forms.
“Regular” insulin has a duration of action that lasts
from 3 to 8 hours, whereas other forms of insulin (pre-
cipitated with zinc or with various protein derivatives)
are absorbed slowly from the injection site and there-
fore have effects that last as long as 10 to 48 hours. Ordi-
narily, a patient with severe type I diabetes is given a
single dose of one of the longer-acting insulins each day
to increase overall carbohydrate metabolism through

o increase overall carbohydrate metabolism through-
out the day.Then additional quantities of regular insulin
are given during the day at those times when the blood
glucose level tends to rise too high, such as at mealtimes.
Thus, each patient is provided with an individualized
pattern of treatment.
In persons with type II diabetes, dieting and exercise
are usually recommended in an attempt to induce
weight loss and to reverse the insulin resistance. If this
fails, drugs may be administered to i

nce. If this
fails, drugs may be administered to increase insulin sen-
sitivity or to stimulate increased production of insulin
by the pancreas. In many persons, however, exogenous
insulin must be used to regulate blood glucose.
In the past, the insulin used for treatment was derived
from animal pancreata. However, human insulin pro-
duced by the recombinant DNA process has become
more widely used because some patients develop immu-
nity and sensitization against animal insulin, thus limit-
ing 

itization against animal insulin, thus limit-
ing its effectiveness.
Relation of Treatment to Arteriosclerosis. Diabetic patients,
mainly because of their high levels of circulating cho-
lesterol and other lipids, develop atherosclerosis, arte-
riosclerosis, severe coronary heart disease, and multiple
microcirculatory lesions far more easily than do normal
people. Indeed, those who have poorly controlled dia-
betes throughout childhood are likely to die of heart
disease in early adulthood.
In th

 to die of heart
In the early days of treating diabetes, the tendency
was to severely reduce the carbohydrates in the diet so
that the insulin requirements would be minimized. This
procedure kept the blood glucose from increasing too
high and attenuated loss of glucose in the urine, but it
did not prevent many of the abnormalities of fat metab-
olism.Consequently,the current tendency is to allow the
patient an almost normal carbohydrate diet and to give
large enough i

ormal carbohydrate diet and to give
large enough insulin to metabolize the carbohydrates.
This decreases the rate of fat metabolism and depresses
the high level of blood cholesterol.
Because the complications of diabetes—such as ath-
erosclerosis, greatly increased susceptibility to infection,
diabetic retinopathy,
cataracts,
hypertension,
and
chronic renal disease—are closely associated with the
level of blood lipids as well as the level of blood glucose,
most physicians also use lipid-lowering

d glucose,
most physicians also use lipid-lowering drugs to help
prevent these disturbances.
Although much rarer than diabetes, excessive insulin
production occasionally occurs from an adenoma of an
islet of Langerhans. About 10 to 15 per cent of these
adenomas are malignant, and occasionally metastases
from the islets of Langerhans spread throughout the
body, causing tremendous production of insulin by both
the primary and the metastatic cancers. Indeed, more
than 100

 and the metastatic cancers. Indeed, more
than 1000 grams of glucose have had to be administered
every 24 hours to prevent hypoglycemia in some of
these patients.
Insulin Shock and Hypoglycemia. As already emphasized,
the central nervous system normally derives essentially
all its energy from glucose metabolism,and insulin is not
necessary for this use of glucose. However, if high levels
of insulin cause blood glucose to fall to low values, the
metabolism of the central nervous system becomes
de

etabolism of the central nervous system becomes
depressed. Consequently, in patients with insulin-secret-
ing tumors or in patients with diabetes who administer
too much insulin to themselves, the syndrome called
insulin shock may occur as follows.
As the blood glucose level falls into the range 
of 50 to 70 mg/100 ml, the central nervous system
usually becomes quite excitable, because this degree of 
hypoglycemia sensitizes neuronal activity. Sometimes
various forms of hallucinations result, bu

metimes
various forms of hallucinations result, but more often
the patient simply experiences extreme nervousness,
trembles all over, and breaks out in a sweat. As the
blood glucose level falls to 20 to 50 mg/100 ml, clonic
seizures and loss of consciousness are likely to occur.As
the glucose level falls still lower, the seizures cease and
only a state of coma remains. Indeed, at times it is dif-
ﬁcult by simple clinical observation to distinguish
between diabetic coma as a result of insulin-lac

h
between diabetic coma as a result of insulin-lack acido-
sis and coma due to hypoglycemia caused by excess
insulin. The acetone breath and the rapid, deep breath-
ing of diabetic coma are not present in hypoglycemic
coma.
Proper treatment for a patient who has hypoglycemic
shock or coma is immediate intravenous administration
of large quantities of glucose. This usually brings 
the patient out of shock within a minute or more. Also,
the administration of glucagon (or, less effectively,
epineph

tration of glucagon (or, less effectively,
epinephrine) can cause glycogenolysis in the liver and
thereby increase the blood glucose level extremely
rapidly. If treatment is not effected immediately, per-
manent damage to the neuronal cells of the central
nervous system often occurs.
of hepatic gluconeogenesis. Am J Physiol End

ation
Chapter 78
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Besser GM, Thorner MO: Comprehensive Clinical Endo-
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Bryant NJ, Govers R, James DE: Regulated transport of the
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Caumo A, Luzi L: First-phase insulin secretion: does it exist
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life? Considerations on shape and function. Am J
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DeWitt DE, Hirsch IB: Outpatient insulin therapy in type 1
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Dunne MJ, Cosgrove KE, Shepherd RM, et al: Hyperin-
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C
H
A
P
T
E
R
 
7
9
978
and Teeth
The physiology of calcium and phosphate metabo-
lism, formation of bone and teeth, and regulation of
vitamin D, parathyroid hormone (PTH), and calci-
tonin are all closely intertwined. Ex

, and calci-
tonin are all closely intertwined. Extracellular
calcium ion concentration, for example, is deter-
mined by the interplay of calcium absorption from
the intestine, renal excretion of calcium, and bone
uptake and release of calcium, each of which is reg-
ulated by the hormones just noted. Because phosphate homeostasis and calcium
homeostasis are closely associated, they are discussed together in this chapter.
an

hosphate
and Plasma
Extracellular ﬂuid calcium concentration normally is regulated very precisely,
seldom rising or falling more than a few per cent from the normal value of about
9.4 mg/dl, which is equivalent to 2.4 mmol calcium per liter.This precise control
is essential, because calcium plays a key role in many physiologic processes,
including contraction of skeletal, cardiac, and smooth muscles; blood clotting;
and transmission of nerve impulses, to na

lotting;
and transmission of nerve impulses, to name just a few. Excitable cells, such as
neurons, are very sensitive to changes in calcium ion concentrations, and
increases in calcium ion concentration above normal (hypercalcemia) cause pro-
gressive depression of the nervous system; conversely, decreases in calcium con-
centration (hypocalcemia) cause the nervous system to become more excited.
An important feature of extracellular calcium regulation is that only about
0.1 per cent of the total

ation is that only about
0.1 per cent of the total body calcium is in the extracellular ﬂuid, about 1 per
cent is in the cells, and the rest is stored in bones.Therefore, the bones can serve
as large reservoirs, releasing calcium when extracellular ﬂuid concentration
decreases and storing excess calcium.
Approximately 85 per cent of the body’s phosphate is stored in bones, 14 to
15 per cent is in the cells, and less than 1 per cent is in the extracellular ﬂuid.
Although extracellular ﬂuid phosph

acellular ﬂuid.
Although extracellular ﬂuid phosphate concentration is not nearly as well reg-
ulated as calcium concentration, phosphate serves several important functions
and is controlled by many of the same factors that regulate calcium.
Calcium in the Plasma and Interstitial Fluid
The calcium in the plasma is present in three forms, as shown in Figure 79–1.
(1) About 41 per cent (1 mmol/L) of the calcium is combined with the plasma
proteins and in this form is nondiffusible through the capi

and in this form is nondiffusible through the capillary membrane.
(2) About 9 per cent of the calcium (0.2 mmol/L) is diffusible through the cap-
illary membrane but is combined with anionic substances of the plasma and
interstitial ﬂuids (citrate and phosphate, for instance) in such a manner that it
Chapter 79
Parathyroid Hormone, Calcitonin, Calcium and Phosphate Metabolism, Vitamin D, Bone, and Teeth
979
is not ionized. (3) The remaining 50 per cent of the
calcium in the plasma is both diffus

r cent of the
calcium in the plasma is both diffusible through the
capillary membrane and ionized.
Thus,the plasma and interstitial ﬂuids have a normal
calcium ion concentration of about 1.2 mmol/L (or 
2.4 mEq/L, because it is a divalent ion), a level only
one half the total plasma calcium concentration.
This ionic calcium is the form that is important for
most functions of calcium in the body, including the
effect of calcium on the heart, the nervous system, and
bone formation.
Inorganic Phosp

ervous system, and
Inorganic Phosphate in the
Inorganic phosphate in the plasma is mainly in two
forms: HPO4
- and H2PO4
-. The concentration of
HPO4
- is about 1.05 mmol/L, and the concentration of
H2PO4
- is about 0.26 mmol/L. When the total quantity
of phosphate in the extracellular ﬂuid rises, so does the
quantity of each of these two types of phosphate ions.
Furthermore, when the pH of the extracellular ﬂuid
becomes more acidic, there is a relative incre

uid
becomes more acidic, there is a relative increase in
H2PO4
_ and a decrease in HPO4
-, whereas the oppo-
site occurs when the extracellular ﬂuid becomes alka-
line. These relations were presented in the discussion
of acid-base balance in Chapter 30.
Because it is difﬁcult to determine chemically the
exact quantities of HPO4
- and H2PO4
- in the blood,
ordinarily the total quantity of phosphate is expressed
in terms of milligrams of phosphorus per deciliter 
(100 ml) of blood. The average tot

 per deciliter 
(100 ml) of blood. The average total quantity of inor-
ganic phosphorus represented by both phosphate ions
is about 4 mg/dl, varying between normal limits of 3 to 
4 mg/dl in adults and 4 to 5 mg/dl in children.
Non-Bone Physiologic Effects 
of Altered Calcium and Phosphate
Concentrations in the Body Fluids
Changing the level of phosphate in the extracellular
ﬂuid from far below normal to two to three times
normal does not cause major immediate effects on the
body. In contrast, e

ajor immediate effects on the
body. In contrast, even slight increases or decreases of
calcium ion in the extracellular ﬂuid can cause extreme
immediate physiologic effects. In addition, chronic
hypocalcemia or hypophosphatemia greatly decreases
bone mineralization, as explained later in the chapter.
Hypocalcemia Causes Nervous System Excitement and Tetany. When
the extracellular ﬂuid concentration of calcium ions
falls below normal, the nervous system becomes 
progressively more excitable, beca

system becomes 
progressively more excitable, because this causes
increased neuronal membrane permeability to sodium
ions, allowing easy initiation of action potentials. At
plasma calcium ion concentrations about 50 per cent
below normal, the peripheral nerve ﬁbers become so
excitable that they begin to discharge spontaneously,
initiating trains of nerve impulses that pass to the
peripheral skeletal muscles to elicit tetanic muscle
hypocalcemia 
causes
tetany. It also 

onsequently,
causes
tetany. It also occasionally causes seizures because of
its action of increasing excitability in the brain.
Figure 79–2 shows tetany in the hand, which usually
occurs before tetany develops in most other parts of
the body. This is called “carpopedal spasm.”
Tetany ordinarily occurs when the blood concentra-
tion of calcium falls from its normal level of 9.4 mg/dl
to about 6 mg/dl, which is only 35 per cent below the
normal calcium concentration, and it is usuall

the
normal calcium concentration, and it is usually lethal
at about 4 mg/dl.
In laboratory animals, in which calcium can gradu-
ally be reduced beyond the usual lethal levels, very
extreme hypocalcemia can cause other effects that are
seldom evident in patients, such as marked dilatation
Protein-bound calcium
41%
(1.0 mmol/L)
Ionized calcium
50%
(1.2 mmol/L)
Calcium complexed to
anions 9% (0.2 mmol/L)
Figure 79–1
Distribution of ionized calcium (Ca++), diffusible but un-ionized
calcium complexed

Ca++), diffusible but un-ionized
calcium complexed to anions, and nondiffusible protein-bound
calcium in blood plasma.
Figure 79–2
Hypocalcemic tetany in the hand, called carpopedal spasm.
980
Unit XIV
of the heart, changes in cellular enzyme activities,
increased membrane permeability in some cells (in
addition to nerve cells), and impaired blood clotting.
Hypercalcemia Depresses Nervous System and Muscle Activity.
When the level of calcium in the body ﬂuids rises

above normal, the nervous system becomes depressed
and reﬂex activities of the central nervous system are
sluggish. Also, increased calcium ion concentration
decreases the QT interval of the heart and causes 
lack of appetite and constipation, probably because of
depressed contractility of the muscle walls of the gas-
These depressive effects begin to appear when the
blood level of calcium rises above about 12 mg/dl, and
the

vel of calcium rises above about 12 mg/dl, and
they can become marked as the calcium level rises
above 15 mg/dl. When the level of calcium rises above
about 17 mg/dl in the blood, calcium phosphate crys-
tals are likely to precipitate throughout the body; this
condition is discussed later in connection with
parathyroid poisoning.
Absorption and Excretion of Calcium
and Phosphate
Intestinal Absorption and Fecal Excretion of Calcium and Phos-
phate. The usual rates of intake are about 1000 mg/day

. The usual rates of intake are about 1000 mg/day
each for calcium and phosphorus, about the amounts
in 1 liter of milk. Normally, divalent cations such as
calcium ions are poorly absorbed from the intestines.
However, as discussed later, vitamin D promotes
calcium absorption by the intestines, and about 35 per
cent (350 mg/day) of the ingested calcium is usually
absorbed; the calcium remaining in the intestine is
excreted in the feces. An additional 250 mg/day of
calcium enters the intestines v

onal 250 mg/day of
calcium enters the intestines via secreted gastroin-
testinal juices and sloughed mucosal cells. Thus, about
90 per cent (900 mg/day) of the daily intake of calcium
is excreted in the feces (Figure 79–3).
Intestinal absorption of phosphate occurs very
easily. Except for the portion of phosphate that is
excreted in the feces in combination with nonabsorbed
calcium, almost all the dietary phosphate is absorbed
into the blood from the gut and later excreted in the
urine.
Renal Ex

 the gut and later excreted in the
urine.
Renal Excretion of Calcium and Phosphate. Approximately
10 per cent (100 mg/day) of the ingested calcium is
excreted in the urine. About 41 per cent of the plasma
calcium is bound to plasma proteins and is therefore
not ﬁltered by the glomerular capillaries. The rest is
combined with anions such as phosphate (9 per cent)
or ionized (50 per cent) and is ﬁltered through the
glomeruli into the renal tubules.
Normally, the renal tubules reabsorb 99 per cent 

Normally, the renal tubules reabsorb 99 per cent of
the ﬁltered calcium, and about 100 mg/day is excreted
in the urine.Approximately 90 per cent of the calcium
in the glomerular ﬁltrate is reabsorbed in the proximal
tubules, loops of Henle, and early distal tubules.
Then in the late distal tubules and early collecting
ducts, reabsorption of the remaining 10 per cent is very
selective, depending on the calcium ion concentration
in the blood.
When calcium concentration is low, this reabsorp-
tion

 calcium concentration is low, this reabsorp-
tion is great, so that almost no calcium is lost in the
urine. Conversely, even a minute increase in blood
calcium ion concentration above normal increases
calcium excretion markedly. We shall see later in the
chapter that the most important factor controlling this
reabsorption of calcium in the distal portions of the
nephron, and therefore controlling the rate of calcium
excretion, is PTH.
Renal phosphate excretion is controlled by an over-
ﬂow mech

phate excretion is controlled by an over-
ﬂow mechanism, as explained in Chapter 29. That is,
when phosphate concentration in the plasma is below
the critical value of about 1 mmol/L, all the phosphate
in the glomerular ﬁltrate is reabsorbed and no phos-
phate is lost in the urine. But above this critical con-
centration, the rate of phosphate loss is directly
proportional to the additional increase. Thus, the
kidneys regulate the phosphate concentration in the
extracellular ﬂuid by altering the

ntration in the
extracellular ﬂuid by altering the rate of phosphate
excretion in accordance with the plasma phosphate
concentration and the rate of phosphate ﬁltration by
the kidneys.
However, as discussed later in the chapter, PTH can
greatly increase phosphate excretion by the kidneys,
thereby playing an important role in the control of
plasma phosphate concentration as well as calcium
Bone and Its Relation 
to Extracellular Calcium 
Bone is composed of a tough or

cium 
Bone is composed of a tough organic matrix that is
greatly strengthened by deposits of calcium salts.
Cells
(13,000 mg)
Bone
(1,000,000 mg)
Deposition
(500 mg/day)
(9880 mg/day)
(9980 mg/day)
(350 mg/day)
Calcium
intake
Feces
(900 mg/day)
Urine
(100 mg/day)
Kidneys
Secretion
(250 mg/day)
fluid
(1300 mg)
Figure 79–3
Overview of calcium exchange between different tissue compart-
ments in a per

e between different tissue compart-
ments in a person ingesting 1000 mg of calcium per day. Note
that most of the ingested calcium is normally eliminated in the
feces, although the kidneys have the capacity to excrete large
amounts by reducing tubular reabsorption of calcium.
Chapter 79
981
Average compact bone contains by weight about 30
per cent matrix and 70 per cent salts. Newly formed
bone may have

 and 70 per cent salts. Newly formed
bone may have a considerably higher percentage of
matrix in relation to salts.
Organic Matrix of Bone. The organic matrix of bone is 90
to 95 per cent collagen ﬁbers, and the remainder is a
homogeneous gelatinous medium called ground sub-
stance. The collagen ﬁbers extend primarily along the
lines of tensional force and give bone its powerful
tensile strength.
The ground substance is composed of extracellular
ﬂuid plus proteoglycans, especially chondroitin su

ﬂuid plus proteoglycans, especially chondroitin sulfate
and hyaluronic acid. The precise function of each of
these is not known, although they do help to control
the deposition of calcium salts.
Bone Salts. The crystalline salts deposited in the
organic matrix of bone are composed principally of
calcium and phosphate. The formula for the major
crystalline salt, known as hydroxyapatite, is the 
Ca10 (PO4)6(OH)2
Each crystal—about 400 angstroms long, 10 to 30
angstroms thick, and 100 an

gstroms long, 10 to 30
angstroms thick, and 100 angstroms wide—is shaped
like a long, ﬂat plate. The relative ratio of calcium to
phosphorus can vary markedly under different nutri-
tional conditions, the Ca/P ratio on a weight basis
varying between 1.3 and 2.0.
Magnesium, sodium, potassium, and carbonate ions
are also present among the bone salts, although x-ray
diffraction studies fail to show deﬁnite crystals formed
by them.Therefore, they are believed to be conjugated
to the hydroxyapatite c

 believed to be conjugated
to the hydroxyapatite crystals rather than organized
into distinct crystals of their own. This ability of many
types of ions to conjugate to bone crystals extends to
many ions normally foreign to bone, such as strontium,
uranium, plutonium, the other transuranic elements,
lead, gold, other heavy metals, and at least 9 of 14 of
the major radioactive products released by explosion of
the hydrogen bomb. Deposition of radioactive sub-
stances in the bone can cause prolonge

active sub-
stances in the bone can cause prolonged irradiation 
of the bone tissues, and if a sufﬁcient amount is
deposited, an osteogenic sarcoma (bone cancer) even-
tually develops in most cases.
Tensile and Compressional Strength of Bone. Each collagen
ﬁber of compact bone is composed of repeating peri-
odic segments every 640 angstroms along its length;
hydroxyapatite crystals lie adjacent to each segment of
the ﬁber,bound tightly to it.This intimate bonding pre-
vents “shear” in the bone; 

 intimate bonding pre-
vents “shear” in the bone; that is, it prevents the crys-
tals and collagen ﬁbers from slipping out of place,
which is essential in providing strength to the bone. In
addition, the segments of adjacent collagen ﬁbers
overlap one another, also causing hydroxyapatite crys-
tals to be overlapped like bricks keyed to one another
in a brick wall.
The collagen ﬁbers of bone, like those of tendons,
have great tensile strength, whereas the calcium salts
have great compressional st

reas the calcium salts
have great compressional strength. These combined
properties plus the degree of bondage between the
collagen ﬁbers and the crystals provide a bony struc-
ture that has both extreme tensile strength and
extreme compressional strength.
Precipitation and Absorption 
of Calcium and Phosphate 
in Bone—Equilibrium with the
Hydroxyapatite Does Not Precipitate in Extracellular Fluid
Despite Supersaturation of Calcium and Phosphate Ions. The
concentrations of c

alcium and Phosphate Ions. The
concentrations of calcium and phosphate ions in extra-
cellular ﬂuid are considerably greater than those
required to cause precipitation of hydroxyapatite.
However, inhibitors are present in almost all tissues of
the body as well as in plasma to prevent such precipi-
tation; one such inhibitor is pyrophosphate. Therefore,
hydroxyapatite crystals fail to precipitate in normal
tissues except in bone despite the state of supersatu-
ration of the ions.
Mechanism of Bon

of supersatu-
Mechanism of Bone Calciﬁcation. The initial stage in bone
production is the secretion of collagen molecules
(called collagen monomers) and ground substance
(mainly proteoglycans) by osteoblasts. The collagen
monomers polymerize rapidly to form collagen ﬁbers;
the resultant tissue becomes osteoid, a cartilage-like
material differing from cartilage in that calcium salts
readily precipitate in it.As the osteoid is formed, some
of the osteoblasts become entrapped in

ormed, some
of the osteoblasts become entrapped in the osteoid
and become quiescent. At this stage they are called
osteocytes.
Within a few days after the osteoid is formed,
calcium salts begin to precipitate on the surfaces of the
collagen ﬁbers. The precipitates ﬁrst appear at inter-
vals along each collagen ﬁber, forming minute nidi 
that rapidly multiply and grow over a period of days
and weeks into the ﬁnished product, hydroxyapatite
crystals.
The initial calcium salts to be deposited are n

s.
The initial calcium salts to be deposited are not
hydroxyapatite crystals but amorphous compounds
(noncrystalline), a mixture of salts such as CaHPO4 ·
2H2O, Ca3(PO4)2 · 3H2O, and others.Then by a process
of substitution and addition of atoms, or reabsorption
and reprecipitation, these salts are converted into 
the hydroxyapatite crystals over a period of weeks or
months. A few per cent may remain permanently in 
the amorphous form. This is important because 
these amorphous salts can be abso

portant because 
these amorphous salts can be absorbed rapidly when
there is need for extra calcium in the extracellular
ﬂuid.
The mechanism that causes calcium salts to be
deposited in osteoid is not fully understood. One
theory holds that at the time of formation, the colla-
gen ﬁbers are specially constituted in advance for
causing precipitation of calcium salts. The osteoblasts
supposedly also secrete a substance into the osteoid to
neutralize an inhibitor (believed to be pyrophosphate)
that

e an inhibitor (believed to be pyrophosphate)
that normally prevents hydroxyapatite crystallization.
Once the pyrophosphate has been neutralized, the
982
Unit XIV
natural afﬁnity of the collagen ﬁbers for calcium salts
causes the precipitation.
Precipitation of Calcium in Nonosseous Tissues Under
Abnormal Conditions. Although calcium salts almost
never precipitate in normal tissues besides bone, under
abnormal conditions, they do precipitate. For instance,
they pre

tions, they do precipitate. For instance,
they precipitate in arterial walls in the condition called
arteriosclerosis and cause the arteries to become bone-
like tubes. Likewise, calcium salts frequently deposit in
degenerating tissues or in old blood clots. Presumably,
in these instances, the inhibitor factors that normally
prevent deposition of calcium salts disappear from the
tissues, thereby allowing precipitation.
Calcium Exchange Between Bone and
If soluble calcium salt

ne and
If soluble calcium salts are injected intravenously, the
calcium ion concentration may increase immediately
to high levels. However, within 30 minutes to 1 hour
or more, the calcium ion concentration returns to
normal.Likewise,if large quantities of calcium ions are
removed from the circulating body ﬂuids, the calcium
ion concentration again returns to normal within 30
minutes to about 1 hour. These effects result in great
part from the fact that the bone contains a ty

eat
part from the fact that the bone contains a type of
exchangeable calcium that is always in equilibrium
with the calcium ions in the extracellular ﬂuids.
A small portion of this exchangeable calcium is also
the calcium found in all tissue cells, especially in highly
permeable types of cells such as those of the liver 
and the gastrointestinal tract. However, most of the
exchangeable calcium is in the bone. It normally
amounts to about 0.4 to 1 per cent of the total bone
calcium. This calcium 

 per cent of the total bone
calcium. This calcium is deposited in the bones in a
form of readily mobilizable salt such as CaHPO4 and
other amorphous calcium salts.
The importance of exchangeable calcium is that it
provides a rapid buffering mechanism to keep the
calcium ion concentration in the extracellular ﬂuids
from rising to excessive levels or falling to very low
levels under transient conditions of excess or
decreased availability of calcium.
Deposition and Absorption of
Bone—Remodeling of

m.
Bone—Remodeling of Bone
Deposition of Bone by the Osteoblasts. Bone is continually
being deposited by osteoblasts, and it is continually
being absorbed where osteoclasts are active (Figure
79–4). Osteoblasts are found on the outer surfaces of
the bones and in the bone cavities. A small amount of
osteoblastic activity occurs continually in all living
bones (on about 4 per cent of all surfaces at any given
time in an adult), so that at least some new bone is being
f

 adult), so that at least some new bone is being
formed constantly.
Absorption of Bone—Function of the Osteoclasts. Bone is also
being continually absorbed in the presence of osteo-
clasts, which are large phagocytic, multinucleated cells
(as many as 50 nuclei), derivatives of monocytes or
monocyte-like cells formed in the bone marrow. The
osteoclasts are normally active on less than 1 per cent
of the bone surfaces of an adult. Later in the chapter we
see that PTH controls the bone absorptive ac

er we
see that PTH controls the bone absorptive activity of
osteoclasts.
Histologically, bone absorption occurs immediately
adjacent to the osteoclasts. The mechanism of this
absorption is believed to be the following: The osteo-
clasts send out villus-like projections toward the bone,
forming a so-called rufﬂed border adjacent to the bone.
The villi secrete two types of substances: (1) proteolytic
enzymes, released from the lysosomes of the osteoclasts,
and (2) several acids, including citric a

oclasts,
and (2) several acids, including citric acid and lactic acid,
released from the mitochondria and secretory vesicles.
The enzymes digest or dissolve the organic matrix of the
bone, and the acids cause solution of the bone salts.The
osteoclastic cells also imbibe by phagocytosis minute
particles of bone matrix and crystals, eventually also dis-
soluting these and releasing the products into the blood.
Bone Deposition and Absorption Are Normally in Equilibrium.
Normally, except in growing 

mally in Equilibrium.
Normally, except in growing bones, the rates of bone
deposition and absorption are equal to each other, so
that the total mass of bone remains constant. Osteo-
clasts usually exist in small but concentrated masses,and
once a mass of osteoclasts begins to develop, it usually
eats away at the bone for about 3 weeks, creating a
tunnel that ranges in diameter from 0.2 to 1 millimeter
and is several millimeters long. At the end of this time,
the osteoclasts disappear and the tun

f this time,
the osteoclasts disappear and the tunnel is invaded by
osteoblasts instead; then new bone begins to develop.
Bone deposition then continues for several months, the
new bone being laid down in successive layers of con-
centric circles (lamellae) on the inner surfaces of the
cavity until the tunnel is ﬁlled. Deposition of new bone
ceases when the bone begins to encroach on the blood
vessels supplying the area. The canal through which
these vessels run, called the haversian canal, is a

hese vessels run, called the haversian canal, is all that
remains of the original cavity. Each new area of bone
deposited in this way is called an osteon, as shown in
Figure 79–5.
Vein
Bone
Fibrous periosteum
Figure 79–4
Osteoblastic and osteoclastic activity in the same bone.
Chapter 79
983
Value of Continual Bone Remodeling The continual deposi-
tion and absorption of bone have

continual deposi-
tion and absorption of bone have several physiologi-
cally important functions. First, bone ordinarily adjusts
its strength in proportion to the degree of bone stress.
Consequently, bones thicken when subjected to heavy
loads. Second, even the shape of the bone can be
rearranged for proper support of mechanical forces by
deposition and absorption of bone in accordance with
stress patterns. Third, because old bone becomes rela-
tively brittle and weak, new organic matrix is need

ively brittle and weak, new organic matrix is needed as
the old organic matrix degenerates. In this manner, the
normal toughness of bone is maintained. Indeed, the
bones of children, in whom the rates of deposition and
absorption are rapid, show little brittleness in compari-
son with the bones of the elderly, in whom the rates of
deposition and absorption are slow.
Control of the Rate of Bone Deposition by Bone “Stress.” Bone
is deposited in proportion to the compressional load
that the bone mu

portion to the compressional load
that the bone must carry. For instance, the bones of 
athletes become considerably heavier than those of
nonathletes. Also, if a person has one leg in a cast but
continues to walk on the opposite leg, the bone of the
leg in the cast becomes thin and as much as 30 per cent
decalciﬁed within a few weeks, whereas the opposite
bone remains thick and normally calciﬁed. Therefore,
continual physical stress stimulates osteoblastic deposi-
tion and calciﬁcation of bone.

steoblastic deposi-
Bone stress also determines the shape of bones under
certain circumstances. For instance, if a long bone of the
leg breaks in its center and then heals at an angle, the
compression stress on the inside of the angle causes
increased deposition of bone, and increased absorption
occurs on the outer side of the angle where the bone is
not compressed. After many years of increased deposi-
tion on the inner side of the angulated bone and absorp-
tion 

inner side of the angulated bone and absorp-
tion on the outer side, the bone can become almost
straight, especially in children because of the rapid
remodeling of bone at younger ages.
Repair of a Fracture Activates Osteoblasts. Fracture of a bone
in some way maximally activates all the periosteal and
intraosseous osteoblasts involved in the break. Also,
immense numbers of new osteoblasts are formed almost
immediately from osteoprogenitor cells, which are bone
stem cells in the surface tissue l

 which are bone
stem cells in the surface tissue lining bone, called the
“bone membrane.” Therefore, within a short time, a
large bulge of osteoblastic tissue and new organic bone
matrix, followed shortly by the deposition of calcium
salts, develops between the two broken ends of the
bone. This is called a callus.
Many bone surgeons use the phenomenon of bone
stress to accelerate the rate of fracture healing. This is
done by use of special mechanical ﬁxation apparatuses
for holding the ends of t

ical ﬁxation apparatuses
for holding the ends of the broken bone together so that
the patient can continue to use the bone immediately.
This causes stress on the opposed ends of the broken
bones, which accelerates osteoblastic activity at the
break and often shortens convalescence.
Vitamin D
Vitamin D has a potent effect to increase calcium
absorption from the intestinal tract; it also has impor-
tant effects on both bone deposition and bone absorp-
tion, as discussed later. However, vitamin D i

rp-
tion, as discussed later. However, vitamin D itself is not
the active substance that actually causes these effects.
Instead, vitamin D must ﬁrst be converted through a
succession of reactions in the liver and the kidneys to
the ﬁnal active product, 1,25-dihydroxycholecalciferol,
also called 1,25(OH)2D3. Figure 79–6 shows the suc-
cession of steps that lead to the formation of this sub-
stance from vitamin D. Let us discuss these steps.
Cholecalciferol (Vitamin D3) Is Formed in the Skin. Seve

alciferol (Vitamin D3) Is Formed in the Skin. Several
compounds derived from sterols belong to the vitamin
D family, and they all perform more or less the same
functions. Vitamin D3 (also called cholecalciferol) is
the most important of these and is formed in the skin
as a result of irradiation of 7-dehydrocholesterol, a sub-
stance normally in the skin, by ultraviolet rays from
the sun. Consequently, appropriate exposure to the
sun prevents vitamin D deﬁciency. The additional
vitamin D compound

min D deﬁciency. The additional
vitamin D compounds that we ingest in food are iden-
tical to the cholecalciferol formed in the skin, except
for the substitution of one or more atoms that do not
affect their function.
Cholecalciferol Is Converted to 25-Hydroxycholecalciferol in
the Liver. The ﬁrst step in the activation of cholecal-
ciferol is to convert it to 25-hydroxycholecalciferol;
this occurs in the liver. The process is a limited one,
because the 25-hydroxycholecalciferol has a feedback
i

use the 25-hydroxycholecalciferol has a feedback
inhibitory effect on the conversion reactions. This
feedback effect is extremely important for two
reasons.
First, the feedback mechanism precisely regulates
the concentration of 25-hydroxycholecalciferol in the
plasma, an effect that is shown in Figure 79–7. Note
that the intake of vitamin D3 can increase many times,
Epiphyseal line
Osteon
Lacunae
Haversian
canal
Magnified
section
Figure 79–5
Structure of bone.
984
Unit

yseal line
984
Unit XIV
and yet the concentration of 25-hydroxycholecalcif-
erol remains nearly normal. This high degree of feed-
back control prevents excessive action of vitamin D
when intake of vitamin D3 is altered over a wide range.
Second, this controlled conversion of vitamin D3 to
25-hydroxycholecalciferol conserves the vitamin D
stored in the liver for future use. Once it is converted,
it persists in the body for only a few w

onverted,
it persists in the body for only a few weeks, whereas in
the vitamin D form, it can be stored in the liver for
many months.
Formation of 1,25-Dihydroxycholecalciferol in the Kidneys 
and Its Control by Parathyroid Hormone. Figure 79–6 
also shows the conversion in the proximal tubules 
of the kidneys of 25-hydroxycholecalciferol to 1,25-
dihydroxycholecalciferol.This latter substance is by far
the most active form of vitamin D, because the previ-
ous products in the scheme of Figure 79

the previ-
ous products in the scheme of Figure 79–6 have less
than 1/1000 of the vitamin D effect. Therefore, in the
absence of the kidneys, vitamin D loses almost all its
effectiveness.
Note also in Figure 79–6 that the conversion of 25-
hydroxycholecalciferol to 1,25-dihydroxycholecalcif-
erol requires PTH. In the absence of PTH, almost
none of the 1,25-dihydroxycholecalciferol is formed.
Therefore, PTH exerts a potent inﬂuence in determin-
ing the functional effects of vitamin D in the body.

g the functional effects of vitamin D in the body.
Calcium Ion Concentration Controls the Formation of 1,25-
Dihydroxycholecalciferol. Figure 79–8 demonstrates that
the plasma concentration of 1,25-dihydroxycholecal-
ciferol is inversely affected by the concentration of
calcium in the plasma. There are two reasons for this.
First, the calcium ion itself has a slight effect in pre-
venting the conversion of 25-hydroxycholecalciferol to
1,25-dihydroxycholecalciferol. Second, and even more
1,25-Dih

oxycholecalciferol. Second, and even more
Intestinal absorption of calcium
Plasma calcium ion concentration
25-Hydroxycholecalciferol
Cholecalciferol (vitamin D3)
Calcium-
stimulated
ATPase
Alkaline
Intestinal
Kidney
Liver
Skin
Calcium-
binding
protein
hormone
Figure 79–6
Activation of vitamin D3 to form 1,25-dihydroxycholecalciferol 
and the role of vitamin D in controlling the plasma calcium 
conc

 vitamin D in controlling the plasma calcium 
0
0.5
1.0
1.5
2.0
0
1.2
1.0
0.8
0.6
0.4
0.2
2.5
Intake of vitamin D3 (times normal)
Plasma 25-
hydroxycholecalciferol
Figure 79–7
Effect of increasing vitamin D3 intake on the plasma concentration
of 25-hydroxycholecalciferol. This ﬁgure shows that tremendous
changes in vitamin D intake have little effect on the ﬁnal quantity
of activated vitamin D that is formed.
X
0
2
4
6
8
10
12
14
0
1
2
3
4
5
6
16
Plasma

rmed.
X
0
2
4
6
8
10
12
14
0
1
2
3
4
5
6
16
Plasma calcium (mg/100 mL)
Normal
Plasma 1,25-
Figure 79–8
Effect of plasma calcium concentration on the plasma concentra-
tion of 1,25-dihydroxycholecalciferol. This ﬁgure shows that a
slight decrease in calcium concentration below normal causes
increased formation of activated vitamin D, which in turn leads to
greatly increased absorption of calcium from the intestine.
Chapter 79
Parathyroid Hormone, Calcitonin, 

tine.
Chapter 79
985
important, as we shall see later in the chapter, the rate
of secretion of PTH is greatly suppressed when the
plasma calcium ion concentration rises above 9 to 
10 mg/100 ml. Therefore, at calcium concentrations
below this level, PTH promotes the conversion of 25-
erol in the kidneys. At higher calcium concentrations,
when PTH is s

s. At higher calcium concentrations,
when PTH is suppressed, the 25-hydroxycholecalcif-
erol is converted to a different compound—24,25-
dihydroxycholecalciferol—that has almost no vitamin
D effect.
When the plasma calcium concentration is already
too high, the formation of 1,25-dihydroxycholecalcif-
erol is greatly depressed. Lack of this in turn decreases
the absorption of calcium from the intestines, the
bones, and the renal tubules, thus causing the calcium
ion concentration to fall back tow

ing the calcium
ion concentration to fall back toward its normal level.
The active form of vitamin D, 1,25-dihydroxycholecal-
ciferol, has several effects on the intestines, kidneys,
and bones that increase absorption of calcium and
phosphate into the extracellular ﬂuid and contribute
to feedback regulation of these substances.
“Hormonal” Effect of Vitamin D to Promote Intestinal Calcium
Absorption. 1,25-Dihydroxycholecalciferol itself func-
tions as a type of “hormone” to p

rol itself func-
tions as a type of “hormone” to promote intestinal
absorption of calcium. It does this principally by
increasing, over a period of about 2 days, formation of
a calcium-binding protein in the intestinal epithelial
cells. This protein functions in the brush border of
these cells to transport calcium into the cell cytoplasm,
and the calcium then moves through the basolateral
membrane of the cell by facilitated diffusion. The rate
of calcium absorption is directly proportional to th

 calcium absorption is directly proportional to the
quantity of this calcium-binding protein. Furthermore,
this protein remains in the cells for several weeks after
the 1,25-dihydroxycholecalciferol has been removed
from the body, thus causing a prolonged effect on
calcium absorption.
Other effects of 1,25-dihydroxycholecalciferol that
might play a role in promoting calcium absorption are
the formation of (1) a calcium-stimulated ATPase in
the brush border of the epithelial cells and (2) an alka

ush border of the epithelial cells and (2) an alka-
line phosphatase in the epithelial cells. The precise
details of all these effects are unclear.
Vitamin D Promotes Phosphate Absorption by the Intestines.
Although phosphate is usually absorbed easily, phos-
phate ﬂux through the gastrointestinal epithelium is
enhanced by vitamin D. It is believed that this results
from a direct effect of 1,25-dihydroxycholecalciferol,
but it is possible that it results secondarily from this
hormone’s action on

 results secondarily from this
hormone’s action on calcium absorption, the cal-
cium in turn acting as a transport mediator for the
phosphate.
Vitamin D Decreases Renal Calcium and Phosphate Excretion.
Vitamin D also increases calcium and phosphate
absorption by the epithelial cells of the renal tubules,
thereby tending to decrease excretion of these sub-
stances in the urine. However, this is a weak effect 
and probably not of major importance in regulating
the 
extracellular 
ﬂuid 

n regulating
the 
ﬂuid 
of 
these 
Effect of Vitamin D on Bone and Its Relation to Parathyroid
Hormone Activity. Vitamin D plays important roles in
both bone absorption and bone deposition. The
administration of extreme quantities of vitamin D
causes absorption of bone. In the absence of vitamin
D, the effect of PTH in causing bone absorption (dis-
cussed in the next section) is greatly reduced or even
prevented. The mechanism of this action of vitamin D

evented. The mechanism of this action of vitamin D
is not known, but it is believed to result from the effect
of 1,25-dihydroxycholecalciferol to increase calcium
transport through cellular membranes.
Vitamin D in smaller quantities promotes bone cal-
ciﬁcation. One of the ways in which it does this is to
increase calcium and phosphate absorption from the
intestines. However, even in the absence of such
increase, it enhances the mineralization of bone. Here
again, the mechanism of the effect is 

 bone. Here
again, the mechanism of the effect is unknown, but it
probably also results from the ability of 1,25-dihy-
droxycholecalciferol to cause transport of calcium ions
through cell membranes—but in this instance, perhaps
in the opposite direction through the osteoblastic or
osteocytic cell membranes.
Parathyroid hormone provides a powerful mechanism
for controlling extracellular calcium and phosphate
concentrations by regulating intestinal reabsorption,
renal excretion

egulating intestinal reabsorption,
renal excretion, and exchange between the extracellu-
lar ﬂuid and bone of these ions. Excess activity of the
parathyroid gland causes rapid absorption of calcium
salts from the bones, with resultant hypercalcemia in
the extracellular ﬂuid; conversely, hypofunction of the
parathyroid glands causes hypocalcemia, often with
resultant tetany.
Physiologic Anatomy of the Parathyroid Glands. Normally
there are four parathyroid glands in humans; they are
located immed

rathyroid glands in humans; they are
located immediately behind the thyroid gland—one
behind each of the upper and each of the lower poles
of the thyroid. Each parathyroid gland is about 6 mil-
limeters long, 3 millimeters wide, and 2 millimeters
thick and has a macroscopic appearance of dark brown
fat. The parathyroid glands are difﬁcult to locate
during thyroid operations because they often look like
just another lobule of the thyroid gland. For this
reason, before the importance of these glan

r this
reason, before the importance of these glands was gen-
erally recognized, total or subtotal thyroidectomy fre-
quently resulted in removal of the parathyroid glands
as well.
Removal of half the parathyroid glands usually
causes no major physiologic abnormalities. However,
removal of three of the four normal glands causes
transient hypoparathyroidism. But even a small quan-
tity of remaining parathyroid tissue is usually capable
986
Unit XIV
of hypertrophying

V
of hypertrophying satisfactorily to perform the func-
tion of all the glands.
The parathyroid gland of the adult human being,
shown in Figure 79–9, contains mainly chief cells and
a small to moderate number of oxyphil cells, but
oxyphil cells are absent in many animals and in 
young humans. The chief cells are believed to secrete
most, if not all, of the PTH.The function of the oxyphil
cells is not certain, but they are believed to be modi-
ﬁed or depleted chief 

ey are believed to be modi-
ﬁed or depleted chief cells that no longer secrete
hormone.
Chemistry of Parathyroid Hormone. PTH has been isolated
in a pure form. It is ﬁrst synthesized on the ribosomes
in the form of a preprohormone, a polypeptide chain
of 110 amino acids. This is cleaved ﬁrst to a prohor-
mone with 90 amino acids, then to the hormone itself
with 84 amino acids by the endoplasmic reticulum and
Golgi apparatus, and ﬁnally is packaged in secretory
granules in the cytoplasm of the ce

d in secretory
granules in the cytoplasm of the cells. The ﬁnal
hormone has a molecular weight of about 9500.
Smaller compounds with as few as 34 amino acids
adjacent to the N terminus of the molecule have also
been isolated from the parathyroid glands that exhibit
full PTH activity. In fact, because the kidneys rapidly
remove the whole 84-amino acid hormone within
minutes but fail to remove many of the fragments for
hours, a large share of the hormonal activity is caused
by the fragments.
Effec

ormonal activity is caused
Effect of Parathyroid Hormone 
on Calcium and Phosphate
Concentrations in the 
Figure 79–10 shows the approximate effects on the
blood calcium and phosphate concentrations caused
by suddenly infusing PTH into an animal and contin-
uing this for several hours. Note that at the onset of
infusion the calcium ion concentration begins to rise
and reaches a plateau in about 4 hours.The phosphate
concentration, however, falls more rapidly

osphate
concentration, however, falls more rapidly than the
calcium rises and reaches a depressed level within 1 or
2 hours. The rise in calcium concentration is caused
principally by two effects: (1) an effect of PTH to
bone and (2) a rapid effect of PTH to decrease the
excretion of calcium by the kidneys. The decline in
phosphate concentration is caused by a strong effect
of PTH to increase renal phosphate excretion, an
effect that is usually 

al phosphate excretion, an
effect that is usually great enough to override
increased phosphate absorption from the bone.
Parathyroid Hormone Increases Calcium and
Phosphate Absorption from the Bone
PTH has two effects on bone in causing absorption of
calcium and phosphate. One is a rapid phase that
begins in minutes and increases progressively for
several hours.This phase results from activation of the
already existing bone cells (mainly the osteocytes) to
promote calcium and phosphate absorptio

ocytes) to
promote calcium and phosphate absorption. The
second phase is a much slower one, requiring several
days or even weeks to become fully developed; it
results from proliferation of the osteoclasts, followed
by greatly increased osteoclastic reabsorption of the
bone itself, not merely absorption of the calcium phos-
phate salts from the bone.
Rapid Phase of Calcium and Phosphate Absorption—Osteoly-
sis. When large quantities of PTH are injected, the
calcium ion concentration in the blood 

ected, the
calcium ion concentration in the blood begins to rise
Oxyphil cell
Chief cell
Parathyroid gland
(located on posterior
side of the thyroid
gland)
Figure 79–9
The four parathyroid glands lie immediately behind the thyroid
gland. Almost all of the parathyroid hormone (PTH) is synthesized
and secreted by the chief cells. The function of the oxyphil cells
is uncertain, but they may be modiﬁed or depleted chief cells that
no longer secrete PTH.
0
1
2
3
4
Phospha

ells that
0
1
2
3
4
Phosphate
Calcium
5
2.40
2.35
2.30
0.8
1.2
1.0
6
Hours
Calcium (mmol/L)
Phosphate (mmol/L)
Begin parathyroid hormone
Figure 79–10
Approximate changes in calcium and phosphate concentrations
during the ﬁrst 5 hours of parathyroid hormone infusion at a mod-
erate rate.
Chapter 79
987
within minutes, long before any new bone cells can be
developed. Histological an

y new bone cells can be
developed. Histological and physiologic studies have
shown that PTH causes removal of bone salts from
two areas in the bone: (1) from the bone matrix in the
vicinity of the osteocytes lying within the bone itself
and (2) in the vicinity of the osteoblasts along the bone
surface.
One does not usually think of either osteoblasts or
osteocytes functioning to cause bone salt absorption,
because both these types of cells are osteoblastic in
nature and normally associated with 

teoblastic in
nature and normally associated with bone deposition
and its calciﬁcation. However, studies have shown that
the osteoblasts and osteocytes form a system of inter-
connected cells that spreads all through the bone and
over all the bone surfaces except the small surface
areas adjacent to the osteoclasts. In fact, long, ﬁlmy
processes extend from osteocyte to osteocyte through-
out the bone structure, and these processes also
connect with the surface osteocytes and osteoblasts.
This ex

th the surface osteocytes and osteoblasts.
This extensive system is called the osteocytic mem-
brane system, and it is believed to provide a membrane
that separates the bone itself from the extracellular
ﬂuid.
Between the osteocytic membrane and the bone is
a small amount of bone ﬂuid. Experiments suggest that
the osteocytic membrane pumps calcium ions from the
bone ﬂuid into the extracellular ﬂuid, creating a
calcium ion concentration in the bone ﬂuid only one
third that in the extracellular ﬂu

e ﬂuid only one
third that in the extracellular ﬂuid. When the osteo-
cytic pump becomes excessively activated, the bone
ﬂuid calcium concentration falls even lower, and
calcium phosphate salts are then absorbed from the
bone. This effect is called osteolysis, and it occurs
without absorption of the bone’s ﬁbrous and gel
matrix. When the pump is inactivated, the bone 
ﬂuid calcium concentration rises to a higher level,
and calcium phosphate salts are redeposited in the
matrix.
But where does PTH

 are redeposited in the
matrix.
But where does PTH ﬁt into this picture? First,
the cell membranes of both the osteoblasts and the
osteocytes have receptor proteins for binding PTH.
PTH can activate the calcium pump strongly, thereby
causing rapid removal of calcium phosphate salts from
those amorphous bone crystals that lie near the cells.
PTH is believed to stimulate this pump by increasing
the calcium permeability of the bone ﬂuid side of the
osteocytic membrane, thus allowing calcium ions to

diffuse into the membrane cells from the bone ﬂuid.
Then the calcium pump on the other side of the cell
membrane transfers the calcium ions the rest of the
way into the extracellular ﬂuid.
Slow Phase of Bone Absorption and Calcium Phosphate
Release—Activation of the Osteoclasts. A much better
known effect of PTH and one for which the evidence
is much clearer is its activation of the osteoclasts. Yet
the osteoclasts do not themselves have membran

Yet
the osteoclasts do not themselves have membrane
receptor proteins for PTH. Instead, it is believed that
the activated osteoblasts and osteocytes send a sec-
ondary but unknown “signal” to the osteoclasts,
causing them to set about their usual task of gobbling
up the bone over a period of weeks or months.
Activation of the osteoclastic system occurs in two
stages: (1) immediate activation of the osteoclasts that
are already formed and (2) formation of new osteo-
clasts. Several days of excess

ation of new osteo-
clasts. Several days of excess PTH usually cause the
osteoclastic system to become well developed, but it
can continue to grow for months under the inﬂuence
of strong PTH stimulation.
After a few months of excess PTH, osteoclastic
resorption of bone can lead to weakened bones and
secondary stimulation of the osteoblasts that attempt
to correct the weakened state. Therefore, the late
effect is actually to enhance both osteoblastic and
osteoclastic activity. Still, even in the 

tic and
osteoclastic activity. Still, even in the late stages, there
is more bone absorption than bone deposition in the
presence of continued excess PTH.
Bone contains such great amounts of calcium in
comparison with the total amount in all the extracel-
lular ﬂuids (about 1000 times as much) that even when
PTH causes a great rise in calcium concentration in the
ﬂuids, it is impossible to discern any immediate effect
on the bones. Prolonged administration or secretion of
PTH—over a period of ma

nistration or secretion of
PTH—over a period of many months or years—ﬁnally
results in very evident absorption in all the bones and
even development of large cavities ﬁlled with large,
multinucleated osteoclasts.
Parathyroid Hormone Decreases Calcium
Excretion and Increases Phosphate Excretion
Administration of PTH causes rapid loss of phosphate
in the urine owing to the effect of the hormone to
diminish proximal tubular reabsorption of phosphate
ions.
PTH also increases renal tub

on of phosphate
ions.
PTH also increases renal tubular reabsorption of
calcium at the same time that it diminishes phosphate
reabsorption. Moreover, it increases the rate of reab-
sorption of magnesium ions and hydrogen ions while
it decreases the reabsorption of sodium, potassium,
and amino acid ions in much the same way that it
affects phosphate. The increased calcium absorption
occurs mainly in the late distal tubules, the collecting
tubules, the early collecting ducts, and possibly the
ascen

the early collecting ducts, and possibly the
ascending loop of Henle to a lesser extent.
Were it not for the effect of PTH on the kidneys 
to increase calcium reabsorption, continual loss of
calcium into the urine would eventually deplete both
the extracellular ﬂuid and the bones of this mineral.
Parathyroid Hormone Increases Intestinal
Absorption of Calcium and Phosphate
At this point, we should be reminded again that PTH
greatly enhances both calcium and phosphate absorp-
tion from the intesti

alcium and phosphate absorp-
tion from the intestines by increasing the formation in
the kidneys of 1,25-dihydroxycholecalciferol from
vitamin D, as discussed earlier in the chapter.
Cyclic Adenosine Monophosphate Mediates the Effects of
Parathyroid Hormone. A large share of the effect of PTH
on its target organs is mediated by the cyclic adeno-
sine monophosphate (cAMP) second messenger
mechanism. Within a few minutes after PTH adminis-
tration, the concentration of cAMP increases in the
988
Un

 the concentration of cAMP increases in the
988
Unit XIV
osteocytes, osteoclasts, and other target cells. This
cAMP in turn is probably responsible for such func-
tions as osteoclastic secretion of enzymes and acids to
cause bone reabsorption and formation of 1,25-
dihydroxycholecalciferol in the kidneys. There are
probably other direct effects of PTH that function
independently of the second messenger mechanism.
Control of Parathyroid Secretion 
by Calcium Ion Con

ntrol of Parathyroid Secretion 
by Calcium Ion Concentration
Even the slightest decrease in calcium ion concentra-
tion in the extracellular ﬂuid causes the parathyroid
glands to increase their rate of secretion within
minutes; if the decreased calcium concentration per-
sists,the glands will hypertrophy,sometimes ﬁvefold or
more. For instance, the parathyroid glands become
greatly enlarged in rickets, in which the level of
calcium is usually depressed only a small amount; also,
they become grea

ressed only a small amount; also,
they become greatly enlarged in pregnancy, even
though the decrease in calcium ion concentration in
the mother’s extracellular ﬂuid is hardly measurable;
and they are greatly enlarged during lactation because
calcium is used for milk formation.
Conversely, conditions that increase the calcium ion
concentration above normal cause decreased activity
and reduced size of the parathyroid glands. Such con-
ditions include (1) excess quantities of calcium in the
diet, 

ude (1) excess quantities of calcium in the
diet, (2) increased vitamin D in the diet, and (3) bone
absorption caused by factors other than PTH (for
example, bone absorption caused by disuse of the
bones).
Figure 79–11 shows the approximate relation
between plasma calcium concentration and plasma
PTH concentration. The solid red curve shows the
acute effect when the calcium concentration is
changed over a period of a few hours. This shows that
even small decreases in calcium concentration from
t

en small decreases in calcium concentration from
the normal value can double or triple the plasma PTH.
The approximate chronic effect that one ﬁnds when
the calcium ion concentration changes over a period
of many weeks, thus allowing time for the glands to
hypertrophy greatly, is shown by the dashed red line;
this demonstrates that a decrease of only a fraction of
a milligram per deciliter in plasma calcium concentra-
tion can double PTH secretion. This is the basis of the
body’s extremely poten

n. This is the basis of the
body’s extremely potent feedback system for long-
term control of plasma calcium ion concentration.
Calcitonin, a peptide hormone secreted by the thyroid
gland, tends to decrease plasma calcium concentra-
tion and, in general, has effects opposite to those of
PTH. However, the quantitative role of calcitonin is
far less than that of PTH in regulating calcium ion 
Synthesis and secretion of calcitonin occur in the
parafollicular cells, or C ce

citonin occur in the
parafollicular cells, or C cells, lying in the interstitial
ﬂuid between the follicles of the thyroid gland. These
cells constitute only about 0.1 per cent of the human
thyroid gland and are the remnants of the ultimo-
brachial glands of lower animals such as ﬁsh, amphib-
ians, reptiles, and birds. Calcitonin is a 32-amino acid
peptide with a molecular weight of about 3400.
Increased Plasma Calcium Concentration Stimulates Calcitonin
Secretion. The primary stimulus for calci

lcitonin
Secretion. The primary stimulus for calcitonin secre-
tion is increased plasma calcium ion concentration.
This contrasts with PTH secretion, which is stimulated
by decreased calcium concentration.
In young animals, but much less so in older animals
and in humans, an increase in plasma calcium concen-
tration of about 10 per cent causes an immediate
twofold or more increase in the rate of secretion of cal-
citonin, which is shown by the blue line in Figure
79–11. This provides a second h

lue line in Figure
79–11. This provides a second hormonal feedback
mechanism for controlling the plasma calcium ion
concentration, but one that is relatively weak and
works in a way opposite that of the PTH system.
Calcitonin Decreases Plasma Calcium Concentration. In
some young animals, calcitonin decreases blood
calcium ion concentration rapidly, beginning within
minutes after injection of the calcitonin, in at least two
ways.
1. The immediate effect is to decrease the absorptive
activities of

effect is to decrease the absorptive
activities of the osteoclasts and possibly the
osteolytic effect of the osteocytic membrane
throughout the bone, thus shifting the balance 
in favor of deposition of calcium in the
exchangeable bone calcium salts. This effect is
especially signiﬁcant in young animals because of
the rapid interchange of absorbed and deposited
calcium.
0
2
4
Normal levels
6
8
10 12 14
3
2
1
0
0
1000
800
600
400
200
16
(ng/mL)
Plasma ca

cium (mg/dL)
(ng/mL)
Plasma calcitonin
(pg/mL)
Acute
effect
Chronic
effect
Figure 79–11
Approximate effect of plasma calcium concentration on the
plasma concentrations of parathyroid hormone and calcitonin.
Note especially that long-term, chronic changes in calcium con-
centration of only a few percentage points can cause as much as
100 per cent change in parathyroid hormone concentration.
Chapter 79
Parathyroid Hormone, Calcitonin, Calcium and 

r 79
989
2. The second and more prolonged effect of
calcitonin is to decrease the formation of new
osteoclasts. Also, because osteoclastic resorption
of bone leads secondarily to osteoblastic activity,
decreased numbers of osteoclasts are followed 
by decreased numbers of osteoblasts. Therefore,
over a long period, the net result is reduced
osteoclastic and osteoblastic activity and,
consequently, very 

and osteoblastic activity and,
consequently, very little prolonged effect on
plasma calcium ion concentration. That is, the
effect on plasma calcium is mainly a transient one,
lasting for a few hours to a few days at most.
Calcitonin also has minor effects on calcium han-
dling in the kidney tubules and the intestines. Again,
the effects are opposite those of PTH, but they appear
to be of such little import that they are seldom 
considered.
Calcitonin Has a Weak Effect on Plasma Calcium Concentr

tonin Has a Weak Effect on Plasma Calcium Concentration
in the Adult Human. The reason for the weak effect of
calcitonin on plasma calcium is twofold. First, any
initial reduction of the calcium ion concentration
caused by calcitonin leads within hours to a powerful
stimulation of PTH secretion, which almost overrides
the calcitonin effect. When the thyroid gland is
removed and calcitonin is no longer secreted, the long-
term blood calcium ion concentration is not measura-
bly altered, which aga

ncentration is not measura-
bly altered, which again demonstrates the overriding
effect of the PTH system of control.
Second, in the adult, the daily rates of absorption
and deposition of calcium are small, and even after the
rate of absorption is slowed by calcitonin, this still has
only a small effect on plasma calcium ion concentra-
tion. The effect of calcitonin in children is much
greater because bone remodeling occurs rapidly in
children, with absorption and deposition of calcium as
great 

ith absorption and deposition of calcium as
great as 5 grams or more per day—equal to 5 to 10
times the total calcium in all the extracellular ﬂuid.
Also, in certain bone diseases, such as Paget’s disease,
in which osteoclastic activity is greatly accelerated, cal-
citonin has a much more potent effect of reducing the
Summary of Control of
At times, the amount of calcium absorbed into or lost
from the body ﬂuids is as much as 0.3 gram in 1 hour.
For 

 body ﬂuids is as much as 0.3 gram in 1 hour.
For instance, in cases of diarrhea, several grams of
calcium can be secreted in the intestinal juices, passed
into the intestinal tract, and lost into the feces each
day.
Conversely, after ingestion of large quantities of
calcium, particularly when there is also an excess of
vitamin D activity, a person may absorb as much as 0.3
gram in 1 hour.This ﬁgure compares with a total quan-
tity of calcium in all the extracellular ﬂuid of about 
1 gram. The a

all the extracellular ﬂuid of about 
1 gram. The addition or subtraction of 0.3 gram to 
or from such a small amount of calcium in the extra-
cellular ﬂuid would cause serious hypercalcemia or
hypocalcemia. However, there is a ﬁrst line of defense
to prevent this from occurring even before the
parathyroid 
and 
calcitonin 
hormonal 
feedback
systems have a chance to act.
Buffer Function of the Exchangeable Calcium in Bones—the
First Line of Defense. The exchangeable calcium salts 
in the bones, 

se. The exchangeable calcium salts 
in the bones, discussed earlier in this chapter, are
amorphous calcium phosphate compounds, probably
mainly CaHPO4 or some similar compound loosely
bound in the bone and in reversible equilibrium with
the calcium and phosphate ions in the extracellular
ﬂuid.
The quantity of these salts that is available for
exchange is about 0.5 to 1 per cent of the total calcium
salts of the bone, a total of 5 to 10 grams of calcium.
Because of the ease of deposition of these

alcium.
Because of the ease of deposition of these exchange-
able salts and their ease of resolubility, an increase in
the concentrations of extracellular ﬂuid calcium and
phosphate ions above normal causes immediate dep-
osition of exchangeable salt. Conversely, a decrease in
these concentrations causes immediate absorption of
exchangeable salt. This reaction is rapid because the
amorphous bone crystals are extremely small and their
total surface area exposed to the ﬂuids of the bone is
perhaps

e area exposed to the ﬂuids of the bone is
perhaps 1 acre or more.
Also, about 5 per cent of all the blood ﬂows through
the bones each minute—that is, about 1 per cent of all
the extracellular ﬂuid each minute. Therefore, about
one half of any excess calcium that appears in the
extracellular ﬂuid is removed by this buffer function
of the bones in about 70 minutes.
In addition to the buffer function of the bones, the
mitochondria of many of the tissues of the body, espe-
cially of the liver and i

ssues of the body, espe-
cially of the liver and intestine, contain a reasonable
amount of exchangeable calcium (a total of about 10
grams in the whole body) that provides an additional
buffer system for helping to maintain constancy of the
extracellular ﬂuid calcium ion concentration.
Hormonal Control of Calcium Ion Concentration—the Second
Line of Defense. At the same time that the exchange-
able calcium mechanism in the bones is “buffering”the
calcium in the extracellular ﬂuid, both the parat

calcium in the extracellular ﬂuid, both the parathyroid
and the calcitonin hormonal systems are beginning to
act.Within 3 to 5 minutes after an acute increase in the
calcium ion concentration, the rate of PTH secretion
decreases. As already explained, this sets into play
multiple mechanisms for reducing the calcium ion con-
centration back toward normal.
At the same time that PTH decreases, calcitonin
increases. In young animals and possibly in young chil-
dren (but probably to a smaller extent

young chil-
dren (but probably to a smaller extent in adults), the
calcitonin causes rapid deposition of calcium in the
bones, and perhaps in some cells of other tissues.
Therefore, in very young animals, excess calcitonin can
cause a high calcium ion concentration to return to
normal perhaps considerably more rapidly than can 
be achieved by the exchangeable calcium-buffering
mechanism alone.
In prolonged calcium excess or prolonged calcium
deﬁciency, only the PTH mechanism seems to be really
9

iency, only the PTH mechanism seems to be really
990
Unit XIV
important in maintaining a normal plasma calcium ion
concentration. When a person has a continuing deﬁ-
ciency of calcium in the diet, PTH often can stimulate
enough calcium absorption from the bones to main-
tain a normal plasma calcium ion concentration for 1
year or more, but eventually, even the bones will run
out of calcium. Thus, in effect, the bones are a large
buffer-reservoir of calcium that can

s are a large
buffer-reservoir of calcium that can be manipulated 
by PTH. Yet, when the bone reservoir either runs out
of calcium or, oppositely, becomes saturated with
calcium, the long-term control of extracellular calcium
ion concentration resides almost entirely in the roles
of PTH and vitamin D in controlling calcium absorp-
tion from the gut and calcium excretion in the urine.
Pathophysiology of
Parathyroid Hormone,
Vitamin D, and 
Bone Disease
When the parathyroid glan

sease
When the parathyroid glands do not secrete sufﬁcient
PTH, the osteocytic reabsorption of exchangeable
calcium decreases and the osteoclasts become almost
totally inactive. As a result, calcium reabsorption from
the bones is so depressed that the level of calcium in the
body ﬂuids decreases. Yet, because calcium and phos-
phates are not being absorbed from the bone, the bone
usually remains strong.
When the parathyroid glands are suddenly removed,
the calcium level in the

nds are suddenly removed,
the calcium level in the blood falls from the normal of
9.4 mg/dl to 6 to 7 mg/dl within 2 to 3 days, and the
blood phosphate concentration may double. When this
low calcium level is reached, the usual signs of tetany
develop.Among the muscles of the body especially sen-
sitive to tetanic spasm are the laryngeal muscles. Spasm
of these muscles obstructs respiration, which is the usual
cause of death in tetany unless appropriate treatment is
applied.
Treatment of Hypopar

opriate treatment is
applied.
Treatment of Hypoparathyroidism with PTH and Vitamin D. PTH
is occasionally used for treating hypoparathyroidism.
However, because of the expense of this hormone,
because its effect lasts for a few hours at most, and
because the tendency of the body to develop anti-
bodies against it makes it progressively less and less
effective, hypoparathyroidism is usually not treated
with PTH administration.
In most patients with hypothyroidism, the administra-
tion of extremel

h hypothyroidism, the administra-
tion of extremely large quantities of vitamin D,to as high
as 100,000 units per day,along with intake of 1 to 2 grams
of calcium, keeps the calcium ion concentration in a
normal range. At times, it might be necessary to admin-
ister 1,25-dihydroxycholecalciferol instead of the nonac-
tivated form of vitamin D because of its much more
potent and much more rapid action. This can also cause
unwanted effects, because it is sometimes difﬁcult to
prevent overactivity 

 it is sometimes difﬁcult to
prevent overactivity by this activated form of vitamin D.
In primary hyperparathyroidism, an abnormality of the
parathyroid glands causes inappropriate, excess PTH
secretion. The cause of primary hyperparathyroidism
ordinarily is a tumor of one of the parathyroid glands;
such tumors occur much more frequently in women
than in men or children, mainly because pregnancy and
lactation stimulate the parathyroid glands and therefore
predispose t

 the parathyroid glands and therefore
predispose to the development of such a tumor.
Hyperparathyroidism causes extreme osteoclastic
activity in the bones. This elevates the calcium ion con-
centration in the extracellular ﬂuid while usually
depressing the concentration of phosphate ions because
of increased renal excretion of phosphate.
Bone Disease in Hyperparathyroidism. Although in mild
hyperparathyroidism new bone can be deposited
rapidly enough to compensate for the increased osteo-
clasti

ough to compensate for the increased osteo-
clastic reabsorption of bone, in severe hyperparathy-
roidism the osteoclastic absorption soon far outstrips
osteoblastic deposition, and the bone may be eaten
away almost entirely. Indeed, the reason a hyper-
parathyroid person seeks medical attention is often a
broken bone. Radiographs of the bone show extensive
decalciﬁcation and, occasionally, large punched-out
cystic areas of the bone that are ﬁlled with osteoclasts
in the form of so-called giant 

d with osteoclasts
in the form of so-called giant cell osteoclast “tumors.”
Multiple fractures of the weakened bones can result
from only slight trauma, especially where cysts develop.
The cystic bone disease of hyperparathyroidism is called
osteitis ﬁbrosa cystica.
Osteoblastic activity in the bones also increases
greatly in a vain attempt to form enough new bone to
make up for the old bone absorbed by the osteoclastic
activity. When the osteoblasts become active, they
secrete large quantities 

asts become active, they
secrete large quantities of alkaline phosphatase. There-
fore, one of the important diagnostic ﬁndings in hyper-
parathyroidism is a high level of plasma alkaline
Effects of Hypercalcemia in Hyperparathyroidism. Hyper-
parathyroidism can at times cause the plasma calcium
level to rise to 12 to 15 mg/dl and, rarely, even higher.
The effects of such elevated calcium levels, as detailed
earlier in the chapter, are depression of the central and
peripheral nervou

re depression of the central and
peripheral nervous systems, muscle weakness, constipa-
tion, abdominal pain, peptic ulcer, lack of appetite, and
depressed relaxation of the heart during diastole.
Parathyroid Poisoning and Metastatic Calciﬁcation When, on
rare occasions, extreme quantities of PTH are secreted,
the level of calcium in the body ﬂuids rises rapidly to
high values. Even the extracellular ﬂuid phosphate con-
centration often rises markedly instead of falling, as is
usually the case, 

kedly instead of falling, as is
usually the case, probably because the kidneys cannot
excrete rapidly enough all the phosphate being
absorbed from the bone. Therefore, the calcium and
phosphate in the body ﬂuids become greatly supersatu-
rated, so that calcium phosphate (CaHPO4) crystals
begin to deposit in the alveoli of the lungs, the tubules
of the kidneys, the thyroid gland, the acid-producing
area of the stomach mucosa, and the walls of the arter-
ies throughout the body. This extensive met

arter-
ies throughout the body. This extensive metastatic dep-
osition of calcium phosphate can develop within a few
days.
Ordinarily, the level of calcium in the blood must rise
above 17 mg/dl before there is danger of parathyroid
poisoning, but once such elevation develops along with
concurrent elevation of phosphate, death can occur in
only a few days.
Chapter 79
991
Formation of Kidney Stones in Hyp

e, and Teeth
991
Formation of Kidney Stones in Hyperparathyroidism Most
patients with mild hyperparathyroidism show few signs
of bone disease and few general abnormalities as a
result of elevated calcium, but they do have an extreme
tendency to form kidney stones. The reason is that the
excess calcium and phosphate absorbed from the intes-
tines or mobilized from the bones in hyperparathy-
roidism must eventually be excreted by the kidneys,
causing a proportionate increase in the concentrations

ng a proportionate increase in the concentrations
of these substances in the urine. As a result, crystals of
calcium phosphate tend to precipitate in the kidney,
forming calcium phosphate stones.Also,calcium oxalate
stones develop because even normal levels of oxalate
cause calcium precipitation at high calcium levels.
Because the solubility of most renal stones is slight in
alkaline media, the tendency for formation of renal
calculi is considerably greater in alkaline urine than in
acid urine. 

bly greater in alkaline urine than in
acid urine. For this reason, acidotic diets and acidic
drugs are frequently used for treating renal calculi.
Secondary Hyperparathyroidism
In secondary hyperparathyroidism, high levels of PTH
occur as a compensation for hypocalcemia rather than
as a primary abnormality of the parathyroid glands.This
contrasts with primary hyperparathyroidism, which is
associated with hypercalcemia.
Secondary hyperparathyroidism can be caused by
vitamin D deﬁciency or chronic

sm can be caused by
vitamin D deﬁciency or chronic renal disease in which
the damaged kidneys are unable to produce sufﬁcient
amounts of the active form of vitamin D, 1,25-
dihydroxycholecalciferol.As discussed in more detail in
the next section, the vitamin D deﬁciency leads to osteo-
malacia (inadequate mineralization of the bones), and
high levels of PTH cause absorption of the bones.
Rickets occurs mainly in children. It results from
calcium or phosphate deﬁciency

en. It results from
calcium or phosphate deﬁciency in the extracellular
ﬂuid, usually caused by lack of vitamin D. If the child is
adequately exposed to sunlight, the 7-dehydrocholes-
terol in the skin becomes activated by the ultraviolet
rays and forms vitamin D3, which prevents rickets by
promoting calcium and phosphate absorption from the
intestines, as discussed earlier in the chapter.
Children who remain indoors through the winter in
general do not receive adequate quantities of vitamin D
w

 do not receive adequate quantities of vitamin D
without some supplementation in the diet.Rickets tends
to occur especially in the spring months because vitamin
D formed during the preceding summer is stored in the
liver and available for use during the early winter
months. Also, calcium and phosphate absorption from
the bones can prevent clinical signs of rickets for the
ﬁrst few months of vitamin D deﬁciency.
Plasma Concentrations of Calcium and Phosphate Decrease 
in Rickets
The plasma calciu

d Phosphate Decrease 
in Rickets
The plasma calcium concentration in rickets
is only slightly depressed, but the level of phosphate is
greatly depressed. This is because the parathyroid
glands prevent the calcium level from falling by pro-
moting bone absorption every time the calcium level
begins to fall. However, there is no good regulatory
system for preventing a falling level of phosphate, and
the increased parathyroid activity actually increases the
excretion of phosphates in the urine.
Ric

ases the
Rickets Weakens the Bones During prolonged rickets, the
marked compensatory increase in PTH secretion causes
extreme osteoclastic absorption of the bone; this in turn
causes the bone to become progressively weaker and
imposes marked physical stress on the bone, resulting in
rapid osteoblastic activity as well. The osteoblasts lay
down large quantities of osteoid, which does not
become calciﬁed because of insufﬁcient calcium and
phosphate ions. Conse

e of insufﬁcient calcium and
phosphate ions. Consequently, the newly formed, uncal-
ciﬁed, and weak osteoid gradually takes the place of the
older bone that is being reabsorbed.
Tetany in Rickets In the early stages of rickets, tetany
almost never occurs because the parathyroid glands
continually stimulate osteoclastic absorption of bone
and, therefore, maintain an almost normal level of
calcium in the extracellular ﬂuid. However, when the
bones ﬁnally become exhausted of calcium, the level of
c

ﬁnally become exhausted of calcium, the level of
calcium may fall rapidly. As the blood level of calcium
falls below 7 mg/dl, the usual signs of tetany develop,
and the child may die of tetanic respiratory spasm
unless intravenous calcium is administered, which
relieves the tetany immediately.
Treatment of Rickets. The treatment of rickets depends on
supplying adequate calcium and phosphate in the diet
and, equally important, on administering large amounts
of vitamin D. If vitamin D is not admin

ge amounts
of vitamin D. If vitamin D is not administered, little
calcium and phosphate are absorbed from the gut.
Osteomalacia—“Adult Rickets”. Adults seldom have a
serious dietary deﬁciency of vitamin D or calcium
because large quantities of calcium are not needed for
bone growth as in children. However, serious deﬁcien-
cies of both vitamin D and calcium occasionally 
occur as a result of steatorrhea (failure to absorb fat)
because vitamin D is fat-soluble and calcium tends to
form insoluble 

s fat-soluble and calcium tends to
form insoluble soaps with fat; consequently, in steator-
rhea, both vitamin D and calcium tend to pass into the
feces. Under these conditions, an adult occasionally has
such poor calcium and phosphate absorption that adult
rickets can occur, although this almost never proceeds
to the stage of tetany but often is a cause of severe bone
disability.
Osteomalacia and Rickets Caused by Renal Disease. “Renal
rickets” is a type of osteomalacia that results from 
prolo

is a type of osteomalacia that results from 
prolonged 
kidney 
damage.
The 
cause 
of 
this 
condition is mainly failure of the damaged kidneys to
form 1,25-dihydroxycholecalciferol, the active form of
vitamin D. In patients whose kidneys have been
removed or destroyed and who are being treated by
hemodialysis, the problem of renal rickets is often a
severe one.
Another type of renal disease that leads to rickets 
and osteomalacia is congenital hypophosphatemia,
resulting from congenitally redu

hypophosphatemia,
resulting from congenitally reduced reabsorption of
phosphates by the renal tubules. This type of rickets
must be treated with phosphate compounds instead 
of calcium and vitamin D, and it is called vitamin
D–resistant rickets.
Osteoporosis—Decreased 
Bone Matrix
Osteoporosis is the most common of all bone diseases
in adults, especially in old age. It is different from osteo-
992
Unit XIV
malacia and rickets because it results from diminished
orga

nd rickets because it results from diminished
organic bone matrix rather than from poor bone calci-
ﬁcation. In osteoporosis the osteoblastic activity in the
bone usually is less than normal, and consequently the
rate of bone osteoid deposition is depressed. But occa-
sionally, as in hyperparathyroidism, the cause of the
diminished bone is excess osteoclastic activity.
The many common causes of osteoporosis are (1) lack
of physical stress on the bones because of inactivity; (2)
malnutrition to t

bones because of inactivity; (2)
malnutrition to the extent that sufﬁcient protein matrix
cannot be formed; (3) lack of vitamin C, which is nec-
essary for the secretion of intercellular substances by all
cells, including formation of osteoid by the osteoblasts;
(4) postmenopausal lack of estrogen secretion because
estrogens decrease the number and activity of osteo-
clasts; (5) old age, in which growth hormone and other
growth factors diminish greatly, plus the fact that many
of the protein ana

reatly, plus the fact that many
of the protein anabolic functions also deteriorate with
age, so that bone matrix cannot be deposited satisfac-
torily; and (6) Cushing’s syndrome, because massive
quantities of glucocorticoids secreted in this disease
cause decreased deposition of protein throughout the
body and increased catabolism of protein and have the
speciﬁc effect of depressing osteoblastic activity. Thus,
many diseases of deﬁciency of protein metabolism can
cause osteoporosis.
Physiology o

in metabolism can
The teeth cut, grind, and mix the food eaten. To
perform these functions, the jaws have powerful
muscles capable of providing an occlusive force
between the front teeth of 50 to 100 pounds and for
the jaw teeth, 150 to 200 pounds. Also, the upper and
lower teeth are provided with projections and facets
that interdigitate, so that the upper set of teeth ﬁts with
the lower. This ﬁtting is called occlusion, and it allows
even small parti

s called occlusion, and it allows
even small particles of food to be caught and ground
between the tooth surfaces.
Function of the Different Parts 
of the Teeth
Figure 79–12 shows a sagittal section of a tooth,
demonstrating its major functional parts: the enamel,
dentin, cementum, and pulp. The tooth can also be
divided into the crown, which is the portion that pro-
trudes out from the gum into the mouth, and the root,
which is the portion within the bony socket of the jaw.
The collar between t

n the bony socket of the jaw.
The collar between the crown and the root where the
tooth is surrounded by the gum is called the neck.
Enamel. The outer surface of the tooth is covered by a
layer of enamel that is formed before eruption of the
tooth by special epithelial cells called ameloblasts.
Once the tooth has erupted, no more enamel is
formed. Enamel is composed of very large and very
dense crystals of hydroxyapatite with adsorbed 
carbonate, magnesium, sodium, potassium, and other
ions imbe

 magnesium, sodium, potassium, and other
ions imbedded in a ﬁne meshwork of strong and
almost insoluble protein ﬁbers that are similar in phys-
ical characteristics (but not chemically identical) to the
keratin of hair.
The crystalline structure of the salts makes the
enamel extremely hard-much harder than the dentin.
Also, the special protein ﬁber meshwork, although
constituting only about 1 per cent of the enamel mass,
makes enamel resistant to acids, enzymes, and other
corrosive agents becaus

 acids, enzymes, and other
corrosive agents because this protein is one of the
most insoluble and resistant proteins known.
Dentin. The main body of the tooth is composed of
dentin, which has a strong, bony structure. Dentin is
made up principally of hydroxyapatite crystals similar
to those in bone but much more dense. These crystals
are imbedded in a strong meshwork of collagen ﬁbers.
In other words, the principal constituents of dentin are
very much the same as those of bone. The major dif-
fe

 much the same as those of bone. The major dif-
ference is its histological organization, because dentin
does not contain any osteoblasts, osteocytes, osteo-
clasts, or spaces for blood vessels or nerves. Instead, it
is deposited and nourished by a layer of cells called
odontoblasts, which line its inner surface along the wall
of the pulp cavity.
The calcium salts in dentin make it extremely resist-
ant to compressional forces, and the collagen ﬁbers
make it tough and resistant to tensional forc

bers
make it tough and resistant to tensional forces that
might result when the teeth are struck by solid objects.
Cementum. Cementum is a bony substance secreted 
by cells of the periodontal membrane, which lines 
the tooth socket. Many collagen ﬁbers pass directly
from the bone of the jaw, through the periodontal
membrane, and then into the cementum. These colla-
gen ﬁbers and the cementum hold the tooth in place.
When the teeth are exposed to excessive strain, the
layer of cementum becomes th

excessive strain, the
layer of cementum becomes thicker and stronger.
Also, it increases in thickness and strength with age,
Cementum
Dentin
Pulp chamber
Enamel
Crown
Root
Neck
Figure 79–12
Functional parts of a tooth.
Chapter 79
993
causing the teeth to become more ﬁrmly seated in the
jaws by adulthood and later.
Pulp. The pulp cavity of each tooth is ﬁlled with pulp,
which is composed of connective ti

lled with pulp,
which is composed of connective tissue with an abun-
dant supply of nerve ﬁbers, blood vessels, and lym-
phatics. The cells lining the surface of the pulp cavity
are the odontoblasts, which, during the formative years
of the tooth, lay down the dentin but at the same time
encroach more and more on the pulp cavity, making 
it smaller. In later life, the dentin stops growing and
the pulp cavity remains essentially constant in size.
However, the odontoblasts are still viable and sen

However, the odontoblasts are still viable and send
projections into small dentinal tubules that penetrate
all the way through the dentin; they are of importance
for exchange of calcium, phosphate, and other miner-
als with the dentin.
Dentition
Humans and most other mammals develop two sets of
teeth during a lifetime. The ﬁrst teeth are called the
deciduous teeth, or milk teeth, and they number 20 in
humans. They erupt between the 7th month and the
2nd year of life, and they last until the 6th 

the
2nd year of life, and they last until the 6th to the 13th
year. After each deciduous tooth is lost, a permanent
tooth replaces it, and an additional 8 to 12 molars
appear posteriorly in the jaws, making the total
number of permanent teeth 28 to 32, depending on
whether the four wisdom teeth ﬁnally appear, which
does not occur in everyone.
Formation of the Teeth. Figure 79–13 shows the forma-
tion and eruption of teeth. Figure 79–13A shows
invagination of the oral epithelium into the dental
l

agination of the oral epithelium into the dental
lamina; this is followed by the development of a tooth-
producing organ. The epithelial cells above form
ameloblasts, which form the enamel on the outside of
the tooth.The epithelial cells below invaginate upward
into the middle of the tooth to form the pulp cavity
and the odontoblasts that secrete dentin.Thus, enamel
is formed on the outside of the tooth, and dentin is
formed on the inside, giving rise to an early tooth, as
shown in Figure 79–13B

 rise to an early tooth, as
shown in Figure 79–13B.
Eruption of Teeth. During early childhood, the teeth
begin to protrude outward from the bone through the
oral epithelium into the mouth. The cause of “erup-
tion” is unknown, although several theories have been
offered in an attempt to explain this phenomenon.The
most likely theory is that growth of the tooth root as
well as of the bone underneath the tooth progressively
shoves the tooth forward.
Development of the Permanent Teeth. During embry

.
Development of the Permanent Teeth. During embry-
onic life, a tooth-forming organ also develops in the
deeper dental lamina for each permanent tooth that
will be needed after the deciduous teeth are gone.
These tooth-producing organs slowly form the perma-
nent teeth throughout the ﬁrst 6 to 20 years of life.
When each permanent tooth becomes fully formed, it,
like the deciduous tooth, pushes outward through the
bone. In so doing, it erodes the root of the deciduous
tooth and eventually cause

e root of the deciduous
tooth and eventually causes it to loosen and fall out.
Soon thereafter, the permanent tooth erupts to take
the place of the original one.
Metabolic Factors Inﬂuence Development of the Teeth.
The rate of development and the speed of eruption of
teeth can be accelerated by both thyroid and growth
hormones. Also, the deposition of salts in the early-
forming teeth is affected considerably by various
factors of metabolism, such as the availability of
calcium and phosphate in 

h as the availability of
calcium and phosphate in the diet, the amount of
vitamin D present,and the rate of PTH secretion.When
all these factors are normal, the dentin and enamel will
be correspondingly healthy, but when they are deﬁ-
cient, the calciﬁcation of the teeth also may be defec-
tive, so that the teeth will be abnormal throughout life.
The salts of teeth, like those of bone, are composed of
hydroxyapatite with adsorbed carbonates and various
cations bound tog

 adsorbed carbonates and various
cations bound together in a hard crystalline substance.
Also, new salts are constantly being deposited while
old salts are being reabsorbed from the teeth, as occurs
in bone. Deposition and reabsorption occur mainly in
the dentin and cementum and to a very limited extent
Dentin
Enamel
Primordium of
enamel organ of
permanent tooth
Enamel organ
of milk tooth
Mesenchymal primordium of pulp
Oral epithelium
Alveolar bone
A
B
C
Figure 79–13
A, Primordial tooth organ. B

ne
A
B
C
A, Primordial tooth organ. B, Developing tooth. C, Erupting tooth.
994
Unit XIV
in the enamel. In the enamel, these processes occur
mostly by diffusional exchange of minerals with the
saliva instead of with the ﬂuids of the pulp cavity.
The rate of absorption and deposition of minerals in
the cementum is about equal to that in the surround-
ing bone of the jaw, whereas the rate of deposition and
absorption of minerals in the dentin is only one

d
absorption of minerals in the dentin is only one third
that of bone.The cementum has characteristics almost
identical to those of usual bone, including the presence
of osteoblasts and osteoclasts, whereas dentin does not
have these characteristics, as explained earlier. This
difference undoubtedly explains the different rates of
mineral exchange.
In summary, continual mineral exchange occurs in
the dentin and cementum of teeth, although the mech-
anism of this exchange in dentin is unclear. Ho

h-
anism of this exchange in dentin is unclear. However,
enamel exhibits extremely slow mineral exchange, so
that it maintains most of its original mineral comple-
ment throughout life.
The two most common dental abnormalities are caries
and malocclusion. Caries refers to erosion of the teeth,
whereas malocclusion is failure of the projections of
the upper and lower teeth to interdigitate properly.
Caries and the Role of Bacteria and Ingested Carbohydrates.
It is generally a

eria and Ingested Carbohydrates.
It is generally agreed that caries result from the action
of bacteria on the teeth, the most common of which is
Streptococcus mutans. The ﬁrst event in the develop-
ment of caries is the deposit of plaque, a ﬁlm of pre-
cipitated products of saliva and food, on the teeth.
Large numbers of bacteria inhabit this plaque and 
are readily available to cause caries. These bacteria
depend to a great extent on carbohydrates for their
food. When carbohydrates are availabl

es for their
food. When carbohydrates are available, their meta-
bolic systems are strongly activated and they multiply.
In addition, they form acids (particularly lactic acid)
and proteolytic enzymes. The acids are the major
culprit in causing caries because the calcium salts of
teeth are slowly dissolved in a highly acidic medium.
And once the salts have become absorbed, the remain-
ing organic matrix is rapidly digested by the prote-
olytic enzymes.
The enamel of the tooth is the primary barr

zymes.
The enamel of the tooth is the primary barrier to the
development of caries. Enamel is far more resistant to
demineralization by acids than is dentin, primarily
because the crystals of enamel are dense, but also
because each enamel crystal is about 200 times as large
in volume as each dentin crystal. Once the carious
process has penetrated through the enamel to the
dentin, it proceeds many times as rapidly because of
the high degree of solubility of the dentin salts.
Because of the depend

ubility of the dentin salts.
Because of the dependence of the caries bacteria on
carbohydrates for their nutrition, it has frequently
been taught that eating a diet high in carbohydrate
content will lead to excessive development of caries.
However, it is not the quantity of carbohydrate
ingested but the frequency with which it is eaten that
is important. If carbohydrates are eaten in many small
parcels throughout the day, such as in the form of
candy, the bacteria are supplied with their prefere

andy, the bacteria are supplied with their preferential
metabolic substrate for many hours of the day, and the
development of caries is greatly increased.
Role of Fluorine in Preventing Caries. Teeth formed in chil-
dren who drink water that contains small amounts of
ﬂuorine develop enamel that is more resistant to caries
than the enamel in children who drink water that does
not contain ﬂuorine. Fluorine does not make the
enamel harder than usual, but ﬂuorine ions replace
many of the hydroxyl io

, but ﬂuorine ions replace
many of the hydroxyl ions in the hydroxyapatite crys-
tals, which in turn makes the enamel several times less
soluble. Fluorine may also be toxic to the bacteria.
Finally, when small pits do develop in the enamel, ﬂu-
orine is believed to promote deposition of calcium
phosphate to “heal” the enamel surface. Regardless of
the precise means by which ﬂuorine protects the teeth,
it is known that small amounts of ﬂuorine deposited in
enamel make teeth about three times as r

osited in
enamel make teeth about three times as resistant to
caries as teeth without ﬂuorine.
Malocclusion. Malocclusion is usually caused by a
hereditary abnormality that causes the teeth of one
jaw to grow to abnormal positions. In malocclusion,
the teeth do not interdigitate properly and therefore
cannot perform their normal grinding or cutting action
adequately. Malocclusion occasionally also results in
abnormal displacement of the lower jaw in relation to
the upper jaw, causing such undesi

 in relation to
the upper jaw, causing such undesirable effects as pain
in the mandibular joint and deterioration of the teeth.
The orthodontist can usually correct malocclusion
by applying prolonged gentle pressure against the
teeth with appropriate braces. The gentle pressure
causes absorption of alveolar jaw bone on the com-
pressed side of the tooth and deposition of new bone
on the tensional side of the tooth. In this way, the tooth
gradually moves to a new position as directed by the
appli

y moves to a new position as directed by the
applied pressure.
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Reproductive and Hormonal

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Functions of the Male (and
Function of the Pineal Gland)
The reproductive functions of the male can be
divided into three major subdivisions: (1) sper-
matogenesis, which means simply the formation of
sperm; (2) performance of the male sexual act; and
(3) regulation of male reproductive functions 
by the various hormones. Associated with these
reproductive functions are the effects of the male
sex hormones on the accessory sexual organs, cellu-

x hormones on the accessory sexual organs, cellu-
lar metabolism, growth, and other functions of the body.
Male Sexual Organs
Figure 80–1A shows the various portions of the male reproductive system, and
Figure 80–1B gives a more detailed structure of the testis and epididymis.The testis
is composed of up to 900 coiled seminiferous tubules, each averaging more than one
half meter long, in which the sperm are formed. The sperm then empty into the 
epididymis, another co

 sperm then empty into the 
epididymis, another coiled tube about 6 meters long. The epididymis leads into the
vas deferens, which enlarges into the ampulla of the vas deferens immediately before
the vas enters the body of the prostate gland.
Two seminal vesicles, one located on each side of the prostate, empty into the pro-
static end of the ampulla, and the contents from both the ampulla and the seminal
vesicles pass into an ejaculatory duct leading through the body of the prostate gland
and t

ading through the body of the prostate gland
and then emptying into the internal urethra. Prostatic ducts, too, empty from the
prostate gland into the ejaculatory duct and from there into the prostatic urethra.
Finally, the urethra is the last connecting link from the testis to the exterior. The
urethra is supplied with mucus derived from a large number of minute urethral
glands located along its entire extent and even more so from bilateral bulbourethral
glands (Cowper’s glands) located near th

ourethral
glands (Cowper’s glands) located near the origin of the urethra.
During formation of the embryo, the primordial germ cells migrate into the
testes and become immature germ cells called spermatogonia which lie in two
or three layers of the inner surfaces of the seminiferous tubules (a cross section
of one is shown in Figure 80–2A). The spermatogonia begin to undergo mitotic
division, beginning at puberty, and continually proliferate and differentiate
through deﬁnite stag

proliferate and differentiate
through deﬁnite stages of development to form sperm, as shown in Figure
80–2B.
Spermatogenesis occurs in the seminiferous tubules during active sexual life as
the result of stimulation by anterior pituitary gonadotropic hormones, begin-
ning at an average age of 13 years and continuing throughout most of the
remainder of life but decreasing markedly in old age.
In the ﬁrst stage of spermatogenesis, the spermatogonia migrate among
Sertoli cel

nesis, the spermatogonia migrate among
Sertoli cells toward the central lumen of the seminiferous tubule. The Sertoli
Chapter 80
Reproductive and Hormonal Functions of the Male (and Function of the Pineal Gland)
997
cells are very large, with overﬂowing cytoplasmic
envelopes that surround the developing spermatogo-
nia all the way to the central lumen of the tubule.
Meiosis. Spermatogonia that cross the barrier into the
Sertoli cell layer become progressively modiﬁed and
enlarged to form large p

progressively modiﬁed and
enlarged to form large primary spermatocytes (Figure
80–3). Each of these, in turn, undergoes meiotic 
division to form two secondary spermatocytes. After
another few days, these too divide to form spermatids
that are eventually modiﬁed to become spermatozoa
(sperm).
During the change from the spermatocyte stage to
the spermatid stage, the 46 chromosomes (23 pairs of
chromosomes) of the spermatocyte are divided, so that
23 chromosomes go to one spermatid and the other 2

23 chromosomes go to one spermatid and the other 23
to the second spermatid. This also divides the chro-
mosomal genes so that only one half of the genetic
characteristics of the eventual fetus are provided by
the father, while the other half are derived from the
oocyte provided by the mother.
The entire period of spermatogenesis, from sper-
matogonia to spermatozoa, takes about 74 days.
Sex Chromosomes. In each spermatogonium, one of the
23 pairs of chromosomes carries the genetic informa-
tion

s of chromosomes carries the genetic informa-
tion that determines the sex of each eventual offspring.
This pair is composed of one X chromosome, which is
called the female chromosome, and one Y chromo-
some, the male chromosome. During meiotic division,
the male Y chromosome goes to one spermatid that
then becomes a male sperm, and the female X chro-
mosome goes to another spermatid that becomes a
female sperm. The sex of the eventual offspring is
determined by which of these two types of sperm

is
determined by which of these two types of sperm 
fertilizes the ovum. This is discussed further in 
Chapter 82.
Formation of Sperm. When the spermatids are ﬁrst
formed, they still have the usual characteristics of
epithelioid cells, but soon they begin to differentiate
and elongate into spermatozoa. As shown in Figure
80–4, each spermatozoon is composed of a head and a
Prepuce
Glans
penis
tubules
Testis
A
B
Scrotum
Head of
epididymis
Erectile
tissue
Prostate
gland
Urinary
bladder

mis
Erectile
tissue
Prostate
gland
Urinary
bladder
Ampulla
Seminal
vesicle
Ejaculatory
duct
Bulbourethral
gland
Efferent
ductules
Body of
Rete testis
Tail of epididymis
tubules
Testicular artery
Figure 80–1
A, Male reproduction system. (Modiﬁed from Bloom V, Fawcett
DW: Textbook of Histology, 10th ed. Philadelphia: WB Saunders
Co, 1975.) B, Internal structure of the testis and relation of the
testis to the epididymis. (Redrawn from Guy

of the
testis to the epididymis. (Redrawn from Guyton AC: Anatomy and
Physiology. Philadelphia: Saunders College Publishing, 1985.)
tubules
B
A
Interstitial cells
Secondary
Primary
Spermatids
Figure 80–2
A, Cross section of a seminiferous tubule. B, Stages in the devel-
opment of sperm from spermatogonia.
998
Unit XIV
tail. The head comprises the condensed nucleus of the
cell with only a

ises the condensed nucleus of the
cell with only a thin cytoplasmic and cell membrane
layer around its surface. On the outside of the anterior
two thirds of the head is a thick cap called the acro-
some that is formed mainly from the Golgi apparatus.
This contains a number of enzymes similar to those
found in lysosomes of the typical cell, including
hyaluronidase (which can digest proteoglycan ﬁla-
ments of tissues) and powerful proteolytic enzymes
(which can digest proteins). These enzymes play

es
important roles in allowing the sperm to enter the
ovum and fertilize it.
The tail of the sperm, called the ﬂagellum, has three
major components: (1) a central skeleton constructed
of 11 microtubules, collectively called the axoneme—
the structure of this is similar to that of cilia found 
on the surfaces of other types of cells described in
Chapter 2; (2) a thin cell membrane covering the
axoneme; and (3) a collection of mitochondria sur-
round

e; and (3) a collection of mitochondria sur-
rounding the axoneme in the proximal portion of the
tail (called the body of the tail).
Back-and-forth movement of the tail (ﬂagellar
movement) provides motility for the sperm. This
movement results from a rhythmical longitudinal
sliding motion between the anterior and posterior
tubules that make up the axoneme.The energy for this
process is supplied in the form of adenosine triphos-
phate that is synthesized by the mitochondria in the
body of the tai

hesized by the mitochondria in the
body of the tail.
Normal sperm move in a ﬂuid medium at a velocity
of 1 to 4 mm/min. This allows them to move through
the female genital tract in quest of the ovum.
Hormonal Factors That Stimulate
We shall discuss the role of hormones in reproduction
later, but at this point, let us note that several hor-
mones play essential roles in spermatogenesis. Some
of these are as follows:
1. Testosterone, secreted by the Leydig cells located
in the inte

, secreted by the Leydig cells located
in the interstitium of the testis, is essential for
Primordial germ cell
Enters
testis
Primary
Secondary
spermatocytes
Meiotic division II
Meiotic division I
Mature
sperm
proliferate by mitotic
cell division inside
testis
Birth
Puberty
25 days
12-14
years
9 days
19 days
21 days
Figure 80–3
Cell divisions during spermatogenesis. During embryonic 
development the primordial germ cells migrate

nic 
development the primordial germ cells migrate to the testis where
they become spermatogonia. At puberty (usually 12 to14 years
after birth), the spermatogonia proliferate rapidly by mitosis. Some
begin meiosis to become primary spermatocytes and continue
through meiotic division I to become secondary spermato-
cytes. After completion of meiotic division II, the secondary 
spermatocytes produce spermatids, which differentiate to form
spermatozoa.
Acrosome
Surface membrane
Vacuole
Anterior he

zoa.
Acrosome
Vacuole
Anterior head cap
Posterior head cap
Neck
Body
Chief piece of tail
End piece of tail
Figure 80–4
Structure of the human spermatozoon.
Chapter 80
999
growth and division of the testicular germinal
cells, which is the ﬁrst stage in forming sperm.
2. Luteinizing hormone, secreted by the anterior
pituitary gland, stimulates the Leydig cells to
secrete testosterone.
3

ulates the Leydig cells to
3. Follicle-stimulating hormone, also secreted by the
anterior pituitary gland, stimulates the Sertoli
cells; without this stimulation, the conversion 
of the spermatids to sperm (the process of
spermiogenesis) will not occur.
4. Estrogens, formed from testosterone by the
Sertoli cells when they are stimulated by follicle-
stimulating hormone, are probably also essential
for spermiogenesis.
5. Growth hormone (as well as most of the other
body horm

th hormone (as well as most of the other
body hormones) is necessary for controlling
background metabolic functions of the testes.
Growth hormone speciﬁcally promotes early
division of the spermatogonia themselves; in its
absence, as in pituitary dwarfs, spermatogenesis 
is severely deﬁcient or absent, thus causing
Maturation of Sperm in the Epididymis
After formation in the seminiferous tubules, the sperm
require several days to pass through the 6-meter-long
tubule of the epididymi

s through the 6-meter-long
tubule of the epididymis. Sperm removed from the
seminiferous tubules and from the early portions of
the epididymis are nonmotile, and they cannot fertil-
ize an ovum. However, after the sperm have been 
in the epididymis for some 18 to 24 hours, they deve-
lop the capability of motility, even though several
inhibitory proteins in the epididymal ﬂuid still prevent
ﬁnal motility until after ejaculation.
Storage of Sperm. The two testes of the human adult
form up to 120 

 The two testes of the human adult
form up to 120 million sperm each day. A small quan-
tity of these can be stored in the epididymis, but most
are stored in the vas deferens.They can remain stored,
maintaining their fertility, for at least a month. During
this time, they are kept in a deeply suppressed inactive
state by multiple inhibitory substances in the secre-
tions of the ducts. Conversely, with a high level of
sexual activity and ejaculations, storage may be no
longer than a few days.
Aft

ons, storage may be no
After ejaculation, the sperm become motile, and
they also become capable of fertilizing the ovum, a
process called maturation. The Sertoli cells and the
epithelium of the epididymis secrete a special nutrient
ﬂuid that is ejaculated along with the sperm. This ﬂuid
contains hormones (including both testosterone and
estrogens), enzymes, and special nutrients that are
essential for sperm maturation.
Physiology of the Mature Sperm. The normal motile, fe

siology of the Mature Sperm. The normal motile, fertile
sperm are capable of ﬂagellated movement though the
ﬂuid medium at velocities of 1 to 4 mm/min. The 
activity of sperm is greatly enhanced in a neutral 
and slightly alkaline medium, as exists in the ejacu-
lated semen, but it is greatly depressed in a mildly
acidic medium. A strong acidic medium can cause
rapid death of sperm.
The activity of sperm increases markedly with in-
creasing temperature, but so does the rate of metabo-
lism, caus

rature, but so does the rate of metabo-
lism, causing the life of the sperm to be considerably
shortened.Although sperm can live for many weeks in
the suppressed state in the genital ducts of the testes,
life expectancy of ejaculated sperm in the female
genital tract is only 1 to 2 days.
Each seminal vesicle is a tortuous, loculated tube lined
with a secretory epithelium that secretes a mucoid
material containing an abundance of fructose,
citric acid, and other n

an abundance of fructose,
citric acid, and other nutrient substances, as well 
as large quantities of prostaglandins and ﬁbrinogen.
During the process of emission and ejaculation, each
seminal vesicle empties its contents into the ejacula-
tory duct shortly after the vas deferens empties the
sperm. This adds greatly to the bulk of the ejaculated
semen, and the fructose and other substances in the
seminal ﬂuid are of considerable nutrient value for 
the ejaculated sperm until one of the sperm fer

r 
the ejaculated sperm until one of the sperm fertilizes
the ovum.
Prostaglandins are believed to aid fertilization in
two ways: (1) by reacting with the female cervical
mucus to make it more receptive to sperm movement
and (2) by possibly causing backward, reverse peri-
staltic contractions in the uterus and fallopian tubes to
move the ejaculated sperm toward the ovaries (a few
sperm reach the upper ends of the fallopian tubes
within 5 minutes).
The prostate glan

.
The prostate gland secretes a thin, milky ﬂuid that con-
tains calcium, citrate ion, phosphate ion, a clotting
enzyme, and a proﬁbrinolysin. During emission, the
capsule of the prostate gland contracts simultaneously
with the contractions of the vas deferens so that the
thin, milky ﬂuid of the prostate gland adds further to
the bulk of the semen. A slightly alkaline characteris-
tic of the prostatic ﬂuid may be quite important for
successful fertilization of the 

ite important for
successful fertilization of the ovum, because the ﬂuid
of the vas deferens is relatively acidic owing to the
presence of citric acid and metabolic end products of
the sperm and, consequently, helps to inhibit sperm
fertility. Also, the vaginal secretions of the female are
acidic (pH of 3.5 to 4.0). Sperm do not become opti-
mally motile until the pH of the surrounding ﬂuids
rises to about 6.0 to 6.5. Consequently, it is probable
that the slightly alkaline prostatic ﬂuid helps t

that the slightly alkaline prostatic ﬂuid helps to neu-
tralize the acidity of the other seminal ﬂuids during
ejaculation, and thus enhances the motility and fertil-
ity of the sperm.
Semen
Semen, which is ejaculated during the male sexual 
act, is composed of the ﬂuid and sperm from the vas
1000
Unit XIV
deferens (about 10 per cent of the total), ﬂuid from the
seminal vesicles (almost 60 per cent), ﬂuid from the
prostate gland (about 30 per cent), and small amoun

rostate gland (about 30 per cent), and small amounts
from the mucous glands, especially the bulbourethral
glands. Thus, the bulk of the semen is seminal vesicle
ﬂuid, which is the last to be ejaculated and serves 
to wash the sperm through the ejaculatory duct and
urethra.
The average pH of the combined semen is about 7.5,
the alkaline prostatic ﬂuid having more than neutral-
ized the mild acidity of the other portions of the
semen. The prostatic ﬂuid gives the semen a milky
appearance, and ﬂuid

 ﬂuid gives the semen a milky
appearance, and ﬂuid from the seminal vesicles and
mucous glands gives the semen a mucoid consistency.
Also, a clotting enzyme from the prostatic ﬂuid causes
the ﬁbrinogen of the seminal vesicle ﬂuid to form a
weak ﬁbrin coagulum that holds the semen in the
deeper regions of the vagina where the uterine cervix
lies.The coagulum then dissolves during the next 15 to
30 minutes because of lysis by ﬁbrinolysin formed
from the prostatic proﬁbrinolysin. In the early minut

m the prostatic proﬁbrinolysin. In the early minutes
after ejaculation, the sperm remain relatively immo-
bile, possibly because of the viscosity of the coagulum.
As the coagulum dissolves, the sperm simultaneously
become highly motile.
Although sperm can live for many weeks in the male
genital ducts, once they are ejaculated in the semen,
their maximal life span is only 24 to 48 hours at 
body temperature.At lowered temperatures, however,
semen can be stored for several weeks, and when
frozen a

can be stored for several weeks, and when
frozen at temperatures below -100°C, sperm have
been preserved for years.
“Capacitation” of the Spermatozoa—Making It
Possible for Them to Penetrate the Ovum
Although spermatozoa are said to be “mature” when
they leave the epididymis, their activity is held in check
by multiple inhibitory factors secreted by the genital
duct epithelia. Therefore, when they are ﬁrst expelled
in the semen, they are unable to perform their duties
in fertilizing the ovum. Ho

o perform their duties
in fertilizing the ovum. However, on coming in contact
with the ﬂuids of the female genital tract, multiple
changes occur that activate the sperm for the ﬁnal
processes of fertilization. These collective changes are
called capacitation of the spermatozoa. This normally
requires from 1 to 10 hours. Some changes that are
believed to occur are the following:
1. The uterine and fallopian tube ﬂuids wash away
the various inhibitory factors that suppress sperm
activity in the ma

ory factors that suppress sperm
activity in the male genital ducts.
2. While the spermatozoa remain in the ﬂuid of the
male genital ducts, they are continually exposed 
to many ﬂoating vesicles from the seminiferous
tubules containing large amounts of cholesterol.
This cholesterol is continually added to the
cellular membrane covering the sperm acrosome,
toughening this membrane and preventing release
of its enzymes. After ejaculation, the sperm
deposited in the vagina swim away from the
cholest

cholesterol vesicles upward into the uterine cavity,
and they gradually lose much of their other excess
cholesterol over the next few hours. In so doing,
the membrane at the head of the sperm (the
acrosome) becomes much weaker.
3. The membrane of the sperm also becomes much
more permeable to calcium ions, so that calcium
now enters the sperm in abundance and changes
the activity of the ﬂagellum, giving it a powerful
whiplash motion in contrast to its pr

t a powerful
whiplash motion in contrast to its previously weak
undulating motion. In addition, the calcium ions
cause changes in the cellular membrane that
covers the leading edge of the acrosome, making
it possible for the acrosome to release its enzymes
rapidly and easily as the sperm penetrates the
granulosa cell mass surrounding the ovum, and
even more so as it attempts to penetrate the zona
pellucida of the ovum itself.
Thus, multiple changes occur during the process of
capacitation.Withou

es occur during the process of
capacitation.Without these, the sperm cannot make its
way to the interior of the ovum to cause fertilization.
Acrosome Enzymes, the “Acrosome Reaction,”
and Penetration of the Ovum
Stored in the acrosome of the sperm are large quanti-
ties of hyaluronidase
and proteolytic enzymes.
Hyaluronidase depolymerizes the hyaluronic acid
polymers in the intercellular cement that hold the
ovarian granulosa cells together. The proteolytic
enzymes digest proteins in the structu

proteolytic
enzymes digest proteins in the structural elements of
tissue cells that still adhere to the ovum.
When the ovum is expelled from the ovarian fol-
licle into the fallopian tube, it still carries with it mul-
tiple layers of granulosa cells. Before a sperm can
fertilize the ovum, it must dissolute these granulosa
cell layers, and then it must penetrate though the thick
covering of the ovum itself, the zona pellucida. To
achieve this, the stored enzymes in the acrosome begin
to be relea

e stored enzymes in the acrosome begin
to be released. It is believed that the hyaluronidase
among these enzymes is especially important in open-
ing pathways between the granulosa cells so that the
sperm can reach the ovum.
When the sperm reaches the zona pellucida of the
ovum, the anterior membrane of the sperm itself binds
speciﬁcally with receptor proteins in the zona pellu-
cida. Then, rapidly, the entire acrosome dissolves,
and all the acrosomal enzymes are released. Within
minutes, these 

somal enzymes are released. Within
minutes, these enzymes open a penetrating pathway
for passage of the sperm head through the zona pel-
lucida to the inside of the ovum. Within another 30
minutes, the cell membranes of the sperm head and of
the oocyte fuse with each other to form a single cell.
At the same time, the genetic material of the sperm
and the oocyte combine to form a completely new cell
genome, containing equal numbers of chromosomes
and genes from mother and father. This is the proc

and genes from mother and father. This is the process
of fertilization; then the embryo begins to develop, as
Why Does Only One Sperm Enter the Oocyte? With as many
sperm as there are, why does only one enter the
oocyte? The reason is not entirely known, but within
a few minutes after the ﬁrst sperm penetrates the zona
pellucida of the ovum, calcium ions diffuse inward
through the oocyte membrane and cause multiple 
Chapter 80
Reproductive and Hormonal Functions of the M

er 80
1001
cortical granules to be released by exocytosis from 
the oocyte into the perivitelline space. These granules
contain substances that permeate all portions of the
zona pellucida and prevent binding of additional
sperm, and they even cause any sperm that have
already begun to bind to fall off. Thus, almost never
does more than one sperm enter the oocyte during 
fertilization.
Abnormal Spermatogenesis and

ring 
Male Fertility
The seminiferous tubular epithelium can be destroyed
by a number of diseases. For instance, bilateral orchitis
of the testes resulting from mumps causes sterility in
some affected males. Also, some male infants are born
with degenerate tubular epithelia as a result of strictures
in the genital ducts or other abnormalities. Finally,
another cause of sterility, usually temporary, is excessive
temperature of the testes.
Effect of Temp

xcessive
Effect of Temperature on Spermatogenesis. Increasing the
temperature of the testes can prevent spermatogenesis
by causing degeneration of most cells of the semini-
ferous tubules besides the spermatogonia. It has often
been stated that the reason the testes are located in 
the dangling scrotum is to maintain the temperature 
of these glands below the internal temperature of the
body, although usually only about 2°C below the inter-
nal temperature. On cold day

 2°C below the inter-
nal temperature. On cold days, scrotal reﬂexes cause 
the musculature of the scrotum to contract, pulling the
testes close to the body to maintain this 2° differential.
Thus, the scrotum theoretically acts as a cooling mech-
anism for the testes (but a controlled cooling), without
which spermatogenesis might be deﬁcient during hot
weather.
Cryptorchidism means failure of a testis to descend
from the abdomen into the scrotum at or near the time
of birth of a f

to the scrotum at or near the time
of birth of a fetus. During development of the male
fetus, the testes are derived from the genital ridges in
the abdomen. However, at about 3 weeks to 1 month
before birth of the baby, the testes normally descend
through the inguinal canals into the scrotum. Occasion-
ally this descent does not occur or occurs incompletely,
so that one or both testes remain in the abdomen, in the
inguinal canal, or elsewhere along the route of descent.
A testis that remains thr

ng the route of descent.
A testis that remains throughout life in the abdomi-
nal cavity is incapable of forming sperm. The tubular
epithelium becomes degenerate, leaving only the 
interstitial structures of the testis. It has been claimed
that even the few degrees’ higher temperature in the
abdomen than in the scrotum is sufﬁcient to cause 
this degeneration of the tubular epithelium and, conse-
quently, to cause sterility, although this is not certain.
Nevertheless, for this reason, operations

certain.
Nevertheless, for this reason, operations to relocate the
cryptorchid testes from the abdominal cavity into the
scrotum before the beginning of adult sexual life are 
frequently performed on boys who have undescended
testes.
Testosterone secretion by the fetal testes themselves
is the normal stimulus that causes the testes to descend
into the scrotum from the abdomen. Therefore, many, if
not most, instances of cryptorchidism are caused by
abnormally formed testes that are unable to secr

y
abnormally formed testes that are unable to secrete
enough testosterone. The surgical operation for cryp-
torchidism in these patients is unlikely to be successful.
Effect of Sperm Count on Fertility. The usual quantity of
semen ejaculated during each coitus averages about 3.5
milliliters, and in each milliliter of semen is an average
of about 120 million sperm, although even in “normal”
males this can vary from 35 million to 200 million. This
means an average total of 400 million sperm are us

means an average total of 400 million sperm are usually
present in the several milliliters of each ejaculate.When
the number of sperm in each milliliter falls below about
20 million, the person is likely to be infertile.Thus, even
though only a single sperm is necessary to fertilize the
ovum, for reasons not understood, the ejaculate usually
must contain a tremendous number of sperm for only
one sperm to fertilize the ovum.
Effect of Sperm Morphology and Motility on Fertility. Occa-
sionally a m

logy and Motility on Fertility. Occa-
sionally a man has a normal number of sperm but is still
infertile. When this occurs, sometimes as many as one
half the sperm are found to be abnormal physically,
having two heads, abnormally shaped heads, or abnor-
mal tails, as shown in Figure 80–5. At other times, the
sperm appear to be structurally normal, but for reasons
not understood, they are either entirely nonmotile 
or relatively nonmotile. Whenever the majority of the
sperm are morphologically ab

r the majority of the
sperm are morphologically abnormal or are nonmotile,
the person is likely to be infertile, even though the
remainder of the sperm appear to be normal.
Neuronal Stimulus for Performance 
of the Male Sexual Act
The most important source of sensory nerve signals 
for initiating the male sexual act is the glans penis.
The glans contains an especially sensitive sensory end-
organ system that transmits into the central nervous
system that special modality of sensa

tral nervous
system that special modality of sensation called sexual
sensation.The slippery massaging action of intercourse
on the glans stimulates the sensory end-organs, and the
Figure 80–5
Abnormal infertile sperm, compared with a normal sperm on the
right.
1002
Unit XIV
sexual signals in turn pass through the pudendal nerve,
then through the sacral plexus into the sacral portion
of the spinal cord, and ﬁnally up the cord to undeﬁned
areas of the brain.
Impulses

 the cord to undeﬁned
Impulses may also enter the spinal cord from 
areas adjacent to the penis to aid in stimulating the
sexual act. For instance, stimulation of the anal epithe-
lium, the scrotum, and perineal structures in general
can send signals into the cord that add to the sexual
sensation. Sexual sensations can even originate in
internal structures, such as in areas of the urethra,
bladder, prostate, seminal vesicles, testes, and vas 
deferens. Indeed, one of the caus

testes, and vas 
deferens. Indeed, one of the causes of “sexual drive”
is ﬁlling of the sexual organs with secretions. Mild
infection and inﬂammation of these sexual organs
sometimes cause almost continual sexual desire,
and some “aphrodisiac” drugs, such as cantharidin,
increase sexual desire by irritating the bladder and
urethral mucosa, inducing inﬂammation and vascular
congestion.
Psychic Element of Male Sexual Stimulation. Appropriate
psychic stimuli can greatly enhance the ability of a
per

c stimuli can greatly enhance the ability of a
person to perform the sexual act. Simply thinking
sexual thoughts or even dreaming that the act of inter-
course is being performed can initiate the male act,
culminating in ejaculation.Indeed,nocturnal emissions
during dreams occur in many males during some
stages of sexual life, especially during the teens.
Integration of the Male Sexual Act in the Spinal Cord.
Although psychic factors usually play an important
part in the male sexual act and can 

 an important
part in the male sexual act and can initiate or inhibit
it, brain function is probably not necessary for its per-
formance because appropriate genital stimulation can
cause ejaculation in some animals and occasionally in
humans after their spinal cords have been cut above
the lumbar region. The male sexual act results from
inherent reﬂex mechanisms integrated in the sacral
and lumbar spinal cord, and these mechanisms can be
initiated by either psychic stimulation from the brain
or 

d by either psychic stimulation from the brain
or actual sexual stimulation from the sex organs, but
usually it is a combination of both.
Penile Erection—Role of the Parasympathetic Nerves. Penile
erection is the ﬁrst effect of male sexual stimulation,
and the degree of erection is proportional to the
degree of stimulation, whether psychic or physical.
Erection is caused by parasympathetic impulses that
pass from the sacral portion of the spinal cord through
the pel

 sacral portion of the spinal cord through
the pelvic nerves to the penis. These parasympathetic
nerve ﬁbers, in contrast to most other parasympathetic
ﬁbers, are believed to release nitric oxide and/or
vasoactive intestinal peptide in addition to acetyl-
choline. The nitric oxide especially relaxes the arteries
of the penis, as well as relaxes the trabecular mesh-
work of smooth muscle ﬁbers in the erectile tissue of
the corpora cavernosa and corpus spongiosum in the
shaft of the penis, shown i

rpus spongiosum in the
shaft of the penis, shown in Figure 80–6.
This erectile tissue consists of large cavernous sinu-
soids, which are normally relatively empty of blood 
but become dilated tremendously when arterial 
blood ﬂows rapidly into them under pressure while the
venous outﬂow is partially occluded. Also, the erectile
bodies, especially the two corpora cavernosa, are 
surrounded by strong ﬁbrous coats; therefore, high
pressure within the sinusoids causes ballooning of the
erectile tiss

e sinusoids causes ballooning of the
erectile tissue to such an extent that the penis becomes
hard and elongated. This is the phenomenon of 
erection.
Lubrication, a Parasympathetic Function. During sexual
stimulation, the parasympathetic impulses, in addition
to promoting erection, cause the urethral glands and
the bulbourethral glands to secrete mucus.This mucus
ﬂows through the urethra during intercourse to aid in
the lubrication during coitus. However, most of the
lubrication of coitus is pr

. However, most of the
lubrication of coitus is provided by the female sexual
organs rather than by the male. Without satisfactory
lubrication, the male sexual act is seldom successful
because unlubricated intercourse causes grating,
painful sensations that inhibit rather than excite sexual
Emission and Ejaculation—Function of the Sympathetic
Nerves. Emission and ejaculation are the culmination
of the male sexual act. When the sexual stimulus
becomes extremely intense, the reﬂex cent

stimulus
becomes extremely intense, the reﬂex centers of the
spinal cord begin to emit sympathetic impulses that
leave the cord at T-12 to L-2 and pass to the genital
organs through the hypogastric and pelvic sympathetic
nerve plexuses to initiate emission, the forerunner of
Emission begins with contraction of the vas defer-
ens and the ampulla to cause expulsion of sperm into
the internal urethra. Then, contractions of the muscu-
lar coat of the prostate gland followed by contracti

r coat of the prostate gland followed by contraction
of the seminal vesicles expel prostatic and seminal
ﬂuid also into the urethra, forcing the sperm forward.
All these ﬂuids mix in the internal urethra with mucus
already secreted by the bulbourethral glands to form
the semen. The process to this point is emission.
The ﬁlling of the internal urethra with semen 
elicits sensory signals that are transmitted through the
pudendal nerves to the sacral regions of the cord,
giving the feeling of sudde

l regions of the cord,
giving the feeling of sudden fullness in the internal
Deep penile
fascia
Corpus
cavernosum
Corpus
spongiosum
Central artery
Urethra
Figure 80–6
Erectile tissue of the penis.
Chapter 80
1003
genital organs. Also, these sensory signals further
excite rhythmical contraction of the internal genital
organs and cause contraction of the ischiocavernosus
and bulbocavernosus muscles that compress the

osus
and bulbocavernosus muscles that compress the bases
of the penile erectile tissue. These effects together
cause rhythmical, wavelike increases in pressure in
both the erectile tissue of the penis and the genital
ducts and urethra, which “ejaculate” the semen from
the urethra to the exterior. This ﬁnal process is called
ejaculation. At the same time, rhythmical contractions
of the pelvic muscles and even of some of the muscles
of the body trunk cause thrusting movements of the
pelvis and pen

nk cause thrusting movements of the
pelvis and penis, which also help propel the semen into
the deepest recesses of the vagina and perhaps even
slightly into the cervix of the uterus.
This entire period of emission and ejaculation is
called the male orgasm. At its termination, the male
sexual excitement disappears almost entirely within 
1 to 2 minutes and erection ceases, a process called 
resolution.
Testosterone and Other Male
Sex Hormones
Secretion, Metabolism, and Chemistry
of the Male Sex 

retion, Metabolism, and Chemistry
of the Male Sex Hormone
Secretion of Testosterone by the Interstitial Cells of Leydig 
in the Testes. The testes secrete several male sex 
hormones, which are collectively called androgens,
including 
testosterone,
dihydrotestosterone,
and
androstenedione. Testosterone is so much more abun-
dant than the others that one can consider it to be the
signiﬁcant testicular hormone, although as we shall
see, much, if not most, of the testosterone is eventually
converte

t most, of the testosterone is eventually
converted into the more active hormone dihy-
drotestosterone in the target tissues.
Testosterone is formed by the interstitial cells of
Leydig, which lie in the interstices between the semi-
niferous tubules and constitute about 20 per cent of
the mass of the adult testes, as shown in Figure 80–7.
Leydig cells are almost nonexistent in the testes during
childhood when the testes secrete almost no testos-
terone, but they are numerous in the newborn male

infant for the ﬁrst few months of life and in the adult
male any time after puberty; at both these times the
testes secrete large quantities of testosterone. Fur-
thermore, when tumors develop from the interstitial
cells of Leydig, great quantities of testosterone are
secreted. Finally, when the germinal epithelium of the
testes is destroyed by x-ray treatment or excessive
heat, the Leydig cells, which are less easily destroyed,
often continue to

which are less easily destroyed,
often continue to produce testosterone.
Secretion of Androgens Elsewhere in the Body. The term
“androgen” means any steroid hormone that has mas-
culinizing effects, including testosterone itself; it also
includes male sex hormones produced elsewhere in the
body besides the testes. For instance, the adrenal glands
secrete at least ﬁve androgens, although the total mas-
culinizing activity of all these is normally so slight (less
than 5 per cent of the total in th

so slight (less
than 5 per cent of the total in the adult male) that even
in women they do not cause signiﬁcant masculine char-
acteristics, except for causing growth of pubic and axil-
lary hair. But when an adrenal tumor of the adrenal
androgen-producing cells occurs,
the quantity of 
androgenic hormones may then become great enough
to cause all the usual male secondary sexual charac-
teristics to occur even in the female. These effects 
are described in connection with the adrenogenital 
synd

scribed in connection with the adrenogenital 
syndrome in Chapter 77.
Rarely, embryonic rest cells in the ovary can deve-
lop into a tumor that produces excessive quantities 
of androgens in women; one such tumor is the
arrhenoblastoma. The normal ovary also produces
minute quantities of androgens, but they are not 
signiﬁcant.
Chemistry of the Androgens. All androgens are steroid
compounds, as shown by the formulas in Figure 80–8 for
testosterone and dihydrotestosterone. Both in the testes
and 

e and dihydrotestosterone. Both in the testes
and in the adrenals, the androgens can be synthesized
either from cholesterol or directly from acetyl coen-
zyme A.
Metabolism of Testosterone. After secretion by the testes,
about 97 per cent of the testosterone becomes either
loosely bound with plasma albumin or more tightly
bound with a beta globulin called sex hormone–binding
globulin and circulates in the blood in these states 
for 30 minutes to several hours. By that time, the 
testosterone eit

several hours. By that time, the 
testosterone either is transferred to the tissues or is
degraded into inactive products that are subsequently
excreted.
Much of the testosterone that becomes ﬁxed to 
the tissues is converted within the tissue cells to dihy-
drotestosterone, especially in certain target organs such
as the prostate gland in the adult and the external gen-
italia of the male fetus. Some actions of testosterone are
dependent on this conversion, whereas other actions are
not.The int

 conversion, whereas other actions are
not.The intracellular functions are discussed later in the
chapter.
of Leydig
Germinal
Fibroblasts
Figure 80–7
Interstitial cells of Leydig, the cells that secrete testosterone,
located in the interstices between the seminiferous tubules.
1004
Unit XIV
Degradation and Excretion of Testosterone. The testosterone
that does not become ﬁxed to the tissues is rapidly con-
verted, mainly by

d to the tissues is rapidly con-
verted, mainly by the liver, into androsterone and dehy-
droepiandrosterone and simultaneously conjugated as
either glucuronides or sulfates (glucuronides, particu-
larly). These are excreted either into the gut by way of
the liver bile or into the urine through the kidneys.
Production of Estrogen in the Male. In addition to testos-
terone, small amounts of estrogens are formed in the
male (about one ﬁfth the amount in the nonpregnant
female), and a reasonable qu

nt in the nonpregnant
female), and a reasonable quantity of estrogens can be
recovered from a man’s urine.The exact source of estro-
gens in the male is unclear, but the following are known:
(1) the concentration of estrogens in the ﬂuid of the
seminiferous tubules is quite high and probably plays an
important role in spermiogenesis. This estrogen is
believed to be formed by the Sertoli cells by converting
testosterone to estradiol. (2) Much larger amounts 
of estrogens are formed from testoster

er amounts 
of estrogens are formed from testosterone and
androstanediol in other tissues of the body, especially
the liver, probably accounting for as much as 80 per cent
of the total male estrogen production.
In general, testosterone is responsible for the distin-
guishing characteristics of the masculine body. Even
during fetal life, the testes are stimulated by chorionic
gonadotropin from the placenta to produce moderate
quantities of testosterone throughout the ent

rate
quantities of testosterone throughout the entire period
of fetal development and for 10 or more weeks after
birth;
thereafter,
essentially no testosterone is 
produced during childhood until about the ages of 10
to 13 years. Then testosterone production increases
rapidly under the stimulus of anterior pituitary
gonadotropic hormones at the onset of puberty and
lasts throughout most of the remainder of life, as
shown in Figure 80–9, dwindling rapidly beyond age 50
to become 20 to 50 per cent

 rapidly beyond age 50
to become 20 to 50 per cent of the peak value by age
80.
Functions of Testosterone During 
Testosterone begins to be elaborated by the male fetal
testes at about the seventh week of embryonic life.
Indeed, one of the major functional differences
between the female and the male sex chromosome is
that the male chromosome causes the newly develop-
ing genital ridge to secrete testosterone, whereas the
female chromosome causes this ridge to secrete estro-
gen

chromosome causes this ridge to secrete estro-
gens. Injection of large quantities of male sex hormone
into pregnant animals causes development of male
sexual organs even though the fetus is female. Also,
removal of the testes in the early male fetus causes
development of female sexual organs.
Thus, testosterone secreted ﬁrst by the genital ridges
and later by the fetal testes is responsible for the devel-
opment of the male body characteristics, including 
the formation of a penis and a scrotum

 including 
the formation of a penis and a scrotum rather than 
formation of a clitoris and a vagina. Also, it causes 
formation of the prostate gland, seminal vesicles, and
male genital ducts, while at the same time suppressing
the formation of female genital organs.
Effect of Testosterone to Cause Descent of the Testes. The
testes usually descend into the scrotum during the last
2 to 3 months of gestation when the testes begin secret-
ing reasonable quantities of testosterone. If a male
child 

nable quantities of testosterone. If a male
child is born with undescended but otherwise normal
testes, the administration of testosterone usually
causes the testes to descend in the usual manner if the
inguinal canals are large enough to allow the testes to
pass.
Administration of gonadotropic hormones, which
stimulate the Leydig cells of the newborn child’s testes
to produce testosterone, can also cause the testes to
descend. Thus, the stimulus for descent of the testes is
testosterone, indica

 for descent of the testes is
testosterone, indicating again that testosterone is an
important hormone for male sexual development
during fetal life.
Effect of Testosterone on Development 
of Adult Primary and Secondary 
Sexual Characteristics
After puberty, the increasing amounts of testosterone
secretion cause the penis, scrotum, and testes to
enlarge about eightfold before the age of 20 years. In
addition, testosterone causes the secondary sexual
characteristics of the male to develop, beginn

ual
characteristics of the male to develop, beginning at
puberty and ending at maturity. These secondary
sexual characteristics, in addition to the sexual organs
themselves, distinguish the male from the female as
follows.
Effect on the Distribution of Body Hair. Testosterone causes
growth of hair (1) over the pubis, (2) upward along the
linea alba of the abdomen sometimes to the umbilicus
and above, (3) on the face, (4) usually on the chest, and
(5) less often on other regions of the body, such

(5) less often on other regions of the body, such as the
back. It also causes the hair on most other portions of
the body to become more proliﬁc.
Baldness. Testosterone decreases the growth of hair on
the top of the head; a man who does not have func-
O
O
OH
CH3
CH3
CH3
CH3
H
H
OH
Figure 80–8
Testosterone and dihydrotestosterone.
Chapter 80
1005
tional testes does not become bald.

al Gland)
1005
tional testes does not become bald. However, many
virile men never become bald because baldness is a
result of two factors: ﬁrst, a genetic background for the
development of baldness and, second, superimposed
on this genetic background, large quantities of andro-
genic hormones. A woman who has the appropriate
genetic background and who develops a long-
sustained androgenic tumor becomes bald in the same
manner as does a man.
Effect on the Voice. Testosterone secreted by the teste

t on the Voice. Testosterone secreted by the testes
or injected into the body causes hypertrophy of the
laryngeal mucosa and enlargement of the larynx. The
effects cause at ﬁrst a relatively discordant,“cracking”
voice, but this gradually changes into the typical adult
masculine voice.
Testosterone Increases Thickness of the Skin and Can Con-
tribute to Development of Acne. Testosterone increases
the thickness of the skin over the entire body and
increases the ruggedness of the subcutaneous tiss

increases the ruggedness of the subcutaneous tissues.
Testosterone also increases the rate of secretion by
some or perhaps all the body’s sebaceous glands.
Especially important is excessive secretion by the
sebaceous glands of the face, because this can result 
in acne. Therefore, acne is one of the most common
features of male adolescence when the body is ﬁrst
becoming introduced to increased testosterone. After
several years of testosterone secretion, the skin nor-
mally adapts to the testost

cretion, the skin nor-
mally adapts to the testosterone in a way that allows
it to overcome the acne.
Testosterone Increases Protein Formation and Muscle Devel-
opment. One of the most important male characteris-
tics is development of increasing musculature after
puberty, averaging about a 50 per cent increase in
muscle mass over that in the female. This is associated
with increased protein in the nonmuscle parts of the
body as well. Many of the changes in the skin are due
to deposition of prot

 changes in the skin are due
to deposition of proteins in the skin, and the changes
in the voice also result partly from this protein ana-
bolic function of testosterone.
Because of the great effect that testosterone and
other androgens have on the body musculature,
synthetic androgens are widely used by athletes to
improve their muscular performance.This practice is to
be severely deprecated because of prolonged harmful
effects of excess androgens, as we discuss in Chapter 84
in relation to spo

ns, as we discuss in Chapter 84
in relation to sports physiology. Testosterone or syn-
thetic androgens are also occasionally used in old age
as a “youth hormone” to improve muscle strength and
vigor, but with questionable results.
Testosterone Increases Bone Matrix and Causes Calcium Reten-
tion. After the great increase in circulating testos-
terone that occurs at puberty (or after prolonged
injection of testosterone),the bones grow considerably
thicker and deposit considerable additional calc

y
thicker and deposit considerable additional calcium
salts. Thus, testosterone increases the total quantity 
of bone matrix and causes calcium retention. The
increase in bone matrix is believed to result from the
general protein anabolic function of testosterone plus
deposition of calcium salts in response to the increased
protein.
Testosterone has a speciﬁc effect on the pelvis to (1)
narrow the pelvic outlet, (2) lengthen it, (3) cause a
funnel-like shape instead of the broad ovoid shape of
t

l-like shape instead of the broad ovoid shape of
the female pelvis, and (4) greatly increase the strength
of the entire pelvis for load-bearing. In the absence of
testosterone, the male pelvis develops into a pelvis that
is similar to that of the female.
Because of the ability of testosterone to increase the
size and strength of bones, it is often used in older men
to treat osteoporosis.
When great quantities of testosterone (or any other
androgen) are secreted abnormally in the still-growing
ch

n) are secreted abnormally in the still-growing
child, the rate of bone growth increases markedly,
causing a spurt in total body height. However, the
1st 2nd
3rd
1
10
17
80
60
40
2.5
5.0
50
100
Fetal
Neonatal
Pubertal
Adult
Year
Trimester
gestation Birth
Old age
(ng/ml)
Figure 80–9
The different stages of male
sexual function as reﬂected by
average 
plasma 
concentrations (red line) and
sperm production (blue line) at
diff

red line) and
different ages. (Modiﬁed from
Grifﬁn JF, Wilson JD: The testis. In:
Bondy PK, Rosenberg LE [eds]:
Metabolic Control and Disease,
8th ed. Philadelphia: WB Saun-
ders Co, 1980.)
1006
Unit XIV
testosterone also causes the epiphyses of the long
bones to unite with the shafts of the bones at an early
age. Therefore, despite the rapidity of growth, this
early uniting of the epiphyses prevents the person
from growing as tall a

iphyses prevents the person
from growing as tall as he would have grown had
testosterone not been secreted at all. Even in normal
men, the ﬁnal adult height is slightly less than that
which occurs in males castrated before puberty.
Testosterone Increases Basal Metabolism. Injection of
large quantities of testosterone can increase the basal
metabolic rate by as much as 15 per cent. Also, even
the usual quantity of testosterone secreted by the
testes during adolescence and early adult life increas

es during adolescence and early adult life increases
the rate of metabolism some 5 to 10 per cent above
the value that it would be were the testes not active.
This increased rate of metabolism is possibly an 
indirect result of the effect of testosterone on protein
anabolism, the increased quantity of proteins—the
enzymes especially—increasing the activities of all
cells.
Effect on Red Blood Cells. When normal quantities of
testosterone are injected into a castrated adult, the
number of red bloo

ted into a castrated adult, the
number of red blood cells per cubic millimeter of
blood increases 15 to 20 per cent. Also, the average
man has about 700,000 more red blood cells per cubic
millimeter than the average woman. This difference
may be due partly to the increased metabolic rate that
occurs after testosterone administration rather than 
to a direct effect of testosterone on red blood cell 
Effect on Electrolyte and Water Balance. As pointed out in
Chapter 77, many steroid ho

nce. As pointed out in
Chapter 77, many steroid hormones can increase the
reabsorption of sodium in the distal tubules of the
kidneys. Testosterone also has such an effect, but only
to a minor degree in comparison with the adrenal min-
eralocorticoids. Nevertheless, after puberty, the blood
and extracellular ﬂuid volumes of the male in relation
to body weight increase as much as 5 to 10 per cent.
Basic Intracellular Mechanism 
of Action of Testosterone
Most of the effects of testosterone result 

terone
Most of the effects of testosterone result basically
from increased rate of protein formation in the target
cells. This has been studied extensively in the prostate
gland, one of the organs that is most affected by testos-
terone. In this gland, testosterone enters the prostatic
cells within a few minutes after secretion. Then it 
is most often converted, under the inﬂuence of the
intracellular enzyme 5a-reductase, to dihydrotestos-
terone, and this in turn binds with a cytoplasmic
“recep

, and this in turn binds with a cytoplasmic
“receptor protein.” This combination migrates to the
cell nucleus, where it binds with a nuclear protein and
induces DNA-RNA transcription. Within 30 minutes,
RNA polymerase has become activated and the con-
centration of RNA begins to increase in the prostatic
cells; this is followed by progressive increase in cellu-
lar protein. After several days, the quantity of DNA 
in the prostate gland has also increased and there 
has been a simultaneous increa

ncreased and there 
has been a simultaneous increase in the number of
prostatic cells.
Testosterone stimulates production of proteins 
virtually everywhere in the body, although more
speciﬁcally affecting those proteins in “target” organs
or tissues responsible for the development of both
primary and secondary male sexual characteristics.
Recent studies suggest that testosterone, like other
steroidal hormones, may also exert some rapid, non-
genomic effects that do not require synthesis of new 

mic effects that do not require synthesis of new 
proteins. The physiological role of these nongenomic
actions of testosterone, however, has yet to be 
Control of Male Sexual Functions 
by Hormones from the Hypothalamus
and Anterior Pituitary Gland
A major share of the control of sexual functions in
both the male and the female begins with secretion 
of gonadotropin-releasing hormone (GnRH) by the
hypothalamus (see Figure 80–10). This hormone in
turn stimulates the anterior pituitary

 hormone in
turn stimulates the anterior pituitary gland to secrete
two other hormones called gonadotropic hormones:
(1) luteinizing hormone (LH) and (2) follicle-stimulat-
ing hormone (FSH). In turn, LH is the primary stimu-
lus for the secretion of testosterone by the testes, and
FSH mainly stimulates spermatogenesis.
GnRH and Its Effect in Increasing the
Secretion of LH and FSH
GnRH is a 10-amino acid peptide secreted by neurons
whose cell bodies are located in the arcuate nuclei of
the hypot

ies are located in the arcuate nuclei of
the hypothalamus. The endings of these neurons 
terminate mainly in the median eminence of the 
hypothalamus, where they release GnRH into the
hypothalamic-hypophysial portal vascular system.
Then the GnRH is transported to the anterior 
pituitary gland in the hypophysial portal blood and
stimulates the release of the two gonadotropins, LH
and FSH.
GnRH is secreted intermittently a few minutes at a
time once every 1 to 3 hours. The intensity of this
hormo

ce every 1 to 3 hours. The intensity of this
hormone’s stimulus is determined in two ways: (1) by
the frequency of these cycles of secretion and (2) by
the quantity of GnRH released with each cycle.
The secretion of LH by the anterior pituitary gland
is also cyclical, with LH following fairly faithfully the
pulsatile release of GnRH. Conversely, FSH secretion
increases and decreases only slightly with each 
ﬂuctuation of GnRH secretion; instead, it changes
more slowly over a period of many hours

it changes
more slowly over a period of many hours in response
to longer-term changes in GnRH. Because of the 
much closer relation between GnRH secretion and
LH secretion, GnRH is also widely known as LH-
releasing hormone.
Gonadotropic Hormones: LH and FSH
Both of the gonadotropic hormones, LH and FSH, are
secreted by the same cells, called gonadotropes, in the
Chapter 80
1007
anterior pituitary gland. In the ab

al Gland)
1007
anterior pituitary gland. In the absence of GnRH
secretion from the hypothalamus, the gonadotropes in
the pituitary gland secrete almost no LH or FSH.
LH and FSH are glycoproteins. They exert their
effects on their target tissues in the testes mainly by
activating the cyclic adenosine monophosphate second
messenger system, which in turn activates speciﬁc
enzyme systems in the respective target cells.
Testosterone—Regulation of Its Production by LH. Testos-
terone is secreted by th

Production by LH. Testos-
terone is secreted by the interstitial cells of Leydig in
the testes, but only when they are stimulated by LH
from the anterior pituitary gland. Furthermore, the
quantity of testosterone secreted increases approxi-
mately in direct proportion to the amount of LH 
Mature Leydig cells are normally found in a child’s
testes for a few weeks after birth but then disappear
until after the age of about 10 years. However, either
injection of puriﬁed LH into a child a

ver, either
injection of puriﬁed LH into a child at any age or
secretion of LH at puberty causes testicular interstitial
cells that look like ﬁbroblasts to evolve into function-
ing Leydig cells.
Inhibition of Anterior Pituitary Secretion of LH and FSH by
Testosterone—Negative Feedback Control of Testosterone
Secretion. The testosterone secreted by the testes in
response to LH has the reciprocal effect of inhibiting
anterior pituitary secretion of LH (see Figure 80–10).
Most of this inhibition p

f LH (see Figure 80–10).
Most of this inhibition probably results from a direct
effect of testosterone on the hypothalamus to decrease
the secretion of GnRH. This in turn causes a corre-
sponding decrease in secretion of both LH and FSH
by the anterior pituitary, and the decrease in LH
reduces the secretion of testosterone by the testes.
Thus, whenever secretion of testosterone becomes too
great, this automatic negative feedback effect, operat-
ing through the hypothalamus and anterior pituitary

ng through the hypothalamus and anterior pituitary
gland, reduces the testosterone secretion back toward
the desired operating level. Conversely, too little
testosterone allows the hypothalamus to secrete large
amounts of GnRH, with a corresponding increase in
anterior pituitary LH and FSH secretion and conse-
quent increase in testicular testosterone secretion.
Regulation of Spermatogenesis by FSH 
and Testosterone
FSH binds with speciﬁc FSH receptors attached to the
Sertoli cells in the semini

eptors attached to the
Sertoli cells in the seminiferous tubules. This causes
these cells to grow and secrete various spermatogenic
substances. Simultaneously, testosterone (and dihy-
drotestosterone) diffusing into the seminiferous
tubules from the Leydig cells in the interstitial spaces
also has a strong tropic effect on spermatogenesis.
Thus, to initiate spermatogenesis, both FSH and testos-
terone are necessary.
Negative Feedback Control of Seminiferous Tubule Activity—
Role of the Hormone I

eminiferous Tubule Activity—
Role of the Hormone Inhibin. When the seminiferous
tubules fail to produce sperm, secretion of FSH by 
the anterior pituitary gland increases markedly. Con-
versely, when spermatogenesis proceeds too rapidly,
pituitary secretion of FSH diminishes.The cause of this
negative feedback effect on the anterior pituitary is
believed to be secretion by the Sertoli cells of still
another hormone called inhibin (see Figure 80–10).
This hormone has a strong direct effect on the

0).
This hormone has a strong direct effect on the ante-
rior pituitary gland to inhibit the secretion of FSH and
possibly a slight effect on the hypothalamus to inhibit
secretion of GnRH.
Inhibin is a glycoprotein, like both LH and FSH,
having a molecular weight between 10,000 and 30,000.
It has been isolated from cultured Sertoli cells. Its
potent inhibitory feedback effect on the anterior pitu-
Virilizing effects
Sertoli
cell
FSH
+
Testis
LH
Behavorial
effects
CNS

eydig cell
FSH
+
Testis
LH
effects
CNS
GnRH
Anterior
pituitary
Inhibin
+
+
+
+
-
-
-
-
+
Figure 80–10
Feedback regulation of the hypothalamic-pituitary-testicular 
axis in males. Stimulatory effects are shown by  and negative 
feedbacks 
inhibitory 
effects 
are 
shown 
by 
. 
GnRH,
gonadotropin-releasing hormone; LH, luteinizing hormone; FSH,
follicle-stimulating hormone.
1008
Unit XIV
itary gland provides an important negati

roduction
itary gland provides an important negative feedback
mechanism for control of spermatogenesis, operating
simultaneously with and in parallel to the negative
feedback mechanism for control of testosterone 
Psychic Factors That Affect Gonadotropin
Secretion and Sexual Activity
Many psychic factors, feeding especially from the
limbic system of the brain into the hypothalamus, can
affect the rate of secretion of GnRH by the hypothal-
amus and therefore can also affect most other 

al-
amus and therefore can also affect most other aspects
of sexual and reproductive functions in both the male
and the female. For instance, transporting a prize bull
in a rough truck is said to inhibit the bull’s fertility—
and the human male is hardly different.
Human Chorionic Gonadotropin Secreted by
the Placenta During Pregnancy Stimulates
Testosterone Secretion by the Fetal Testes
During pregnancy, the hormone human chorionic
gonadotropin (hCG) is secreted by the placenta, and
it circulat

(hCG) is secreted by the placenta, and
it circulates both in the mother and in the fetus. This
hormone has almost the same effects on the sexual
organs as LH.
During pregnancy, if the fetus is a male, hCG from
the placenta causes the testes of the fetus to secrete
testosterone.This testosterone is critical for promoting
formation of the male sexual organs, as pointed out
earlier. We discuss hCG and its functions during preg-
nancy in greater detail in Chapter 82.
Puberty and Regulation of Its On

il in Chapter 82.
Puberty and Regulation of Its Onset
Initiation of the onset of puberty has long been a
mystery. But it has now been determined that during
childhood the hypothalamus simply does not secrete
signiﬁcant amounts of GnRH. One of the reasons 
for this is that, during childhood, the slightest secretion
of any sex steroid hormones exerts a strong inhibitory
effect on hypothalamic secretion of GnRH. Yet, for
reasons still not understood, at the time of puberty, the
secretion of hypotha

, at the time of puberty, the
secretion of hypothalamic GnRH breaks through the
childhood inhibition, and adult sexual life begins.
Male Adult Sexual Life and Male Climacteric. After puberty,
gonadotropic hormones are produced by the male pitu-
itary gland for the remainder of life, and at least some
spermatogenesis usually continues until death. Most
men, however, begin to exhibit slowly decreasing sexual
functions in their late 40s or 50s, and one study showed
that the average age for terminat

one study showed
that the average age for terminating intersexual rela-
tions was 68, although the variation was great. This
decline in sexual function is related to decrease in
testosterone secretion, as shown in Figure 80–9. The
decrease in male sexual function is called the male cli-
macteric. Occasionally the male climacteric is associated
with symptoms of hot ﬂashes, suffocation, and psychic
disorders similar to the menopausal symptoms of the
female. These symptoms can be abrogated by admin

e
female. These symptoms can be abrogated by adminis-
tration of testosterone, synthetic androgens, or even
estrogens that are used for treatment of menopausal
symptoms in the female.
Abnormalities of Male
Sexual Function
Prostate Gland and 
Its Abnormalities
The prostate gland remains relatively small throughout
childhood and begins to grow at puberty under the 
stimulus of testosterone. This gland reaches an almost
stationary size by the age of 20 years and remains at 
this size up to the age 

 20 years and remains at 
this size up to the age of about 50 years.At that time, in
some men it begins to involute, along with decreased
production of testosterone by the testes.
A benign prostatic ﬁbroadenoma frequently develops
in the prostate in many older men and can cause urinary
obstruction. This hypertrophy is caused not by testos-
terone but instead by abnormal overgrowth of prostate
tissue itself.
Cancer of the prostate gland is a different problem
and is a common cause of death, accou

rent problem
and is a common cause of death, accounting for about
2 to 3 per cent of all male deaths. Once cancer of the
prostate gland does occur, the cancerous cells are
usually stimulated to more rapid growth by testosterone
and are inhibited by removal of both testes so that
testosterone cannot be formed. Prostatic cancer usually
can be inhibited by administration of estrogens.
Even some patients who have prostatic cancer that has
already metastasized to almost all the bones of the body
can 

astasized to almost all the bones of the body
can be successfully treated for a few months to years by
removal of the testes, by estrogen therapy, or by both;
after this therapy the metastases usually diminish in size
and the bones partially heal. This treatment does not
stop the cancer but does slow it and sometimes greatly
diminishes the severe bone pain.
When the testes of a male fetus are nonfunctional
during fetal life, none of the male sexual characteristics
develo

fe, none of the male sexual characteristics
develop in the fetus. Instead, female organs are formed.
The reason for this is that the basic genetic characteris-
tic of the fetus, whether male or female, is to form
female sexual organs if there are no sex hormones. But
in the presence of testosterone, formation of female
sexual organs is suppressed, and instead, male organs
are induced.
When a boy loses his testes before puberty, a state of
eunuchism ensues in which he continues to have infan-
til

sm ensues in which he continues to have infan-
tile sex organs and other infantile sexual characteristics
throughout life.The height of an adult eunuch is slightly
greater than that of a normal man because the bone 
epiphyses are slow to unite, although the bones are
quite thin and the muscles are considerably weaker 
than those of a normal man.The voice is childlike, there
is no loss of hair on the head, and the normal adult 
masculine hair distribution on the face and elsewhere
does not occur.

ribution on the face and elsewhere
When a man is castrated after puberty, some of his
male secondary sexual characteristics revert to those of
a child and others remain of adult masculine character.
The sexual organs regress slightly in size but not to a
childlike state, and the voice regresses from the bass
quality only slightly. Conversely, there is loss of mascu-
line hair production, loss of the thick masculine bones,
and loss of the musculature of the virile male.
Also in a 

 of the musculature of the virile male.
Also in a castrated adult male, sexual desires are
decreased but not lost, provided sexual activities have
Chapter 80
1009
been practiced previously. Erection can still occur as
before, although with less ease, but it is rare that ejacu-
lation can take place, primarily because the semen-
forming organs degenerate and there has been a loss of
the testosterone-driven psychic 

as been a loss of
the testosterone-driven psychic desire.
Some instances of hypogonadism are caused by a
genetic inability of the hypothalamus to secrete normal
amounts of GnRH. This often is associated with a 
simultaneous abnormality of the feeding center of the
hypothalamus, causing the person to greatly overeat.
Consequently, obesity occurs along with eunuchism. A
patient with this condition is shown in Figure 80–11; the
condition is called adiposogenital syndrome, Fröhlich’s
syndrome, or hy

diposogenital syndrome, Fröhlich’s
syndrome, or hypothalamic eunuchism.
Testicular Tumors and
Hypergonadism in the Male
Interstitial Leydig cell tumors develop in rare instances
in the testes, but when they do develop, they sometimes
produce as much as 100 times the normal quantities of
testosterone. When such tumors develop in young chil-
dren, they cause rapid growth of the musculature and
bones but also cause early uniting of the epiphyses, so
that the eventual adult height actually is consid

that the eventual adult height actually is considerably
less than that which would have been achieved other-
wise. Such interstitial cell tumors also cause excessive
development of the male sexual organs, all skeletal
muscles, and other male sexual characteristics. In the
adult male, small interstitial cell tumors are difﬁcult 
to diagnose because masculine features are already
present.
Much more common than the interstitial Leydig cell
tumors are tumors of the germinal epithelium. Because
germ

re tumors of the germinal epithelium. Because
germinal cells are capable of differentiating into almost
any type of cell, many of these tumors contain multiple
tissues, such as placental tissue, hair, teeth, bone, skin,
and so forth, all found together in the same tumorous
mass called a teratoma. These tumors often secrete few
hormones, but if a signiﬁcant quantity of placental tissue
develops in the tumor, it may secrete large quantities 
of hCG with functions similar to those of LH. Also,
estr

 with functions similar to those of LH. Also,
estrogenic hormones are sometimes secreted by these
tumors and cause the condition called gynecomastia
(overgrowth of the breasts).
Pineal Gland—Its Function
in Controlling Seasonal
Fertility in Some Animals
For as long as the pineal gland has been known to exist,
myriad functions have been ascribed to it, including its
(1) being the seat of the soul, (2) enhancing sex, (3)
staving off infection, (4) promoting sleep, (5) enhancing
mood, and (6) incre

promoting sleep, (5) enhancing
mood, and (6) increasing longevity (as much as 10 to 25
per cent). It is known from comparative anatomy that
the pineal gland is a vestigial remnant of what was a
third eye located high in the back of the head in some
lower animals. Many physiologists have been content
with the idea that this gland is a nonfunctional remnant,
but others have claimed for many years that it plays
important roles in the control of sexual activities 
and reproduction, functions that st

al activities 
and reproduction, functions that still others said were
nothing more than the fanciful imaginings of physiolo-
gists preoccupied with sexual delusions.
But now, after years of dispute, it looks as though the
sex advocates have won and that the pineal gland does
indeed play a regulatory role in sexual and reproductive
function. In lower animals that bear their young at
certain seasons of the year and in which the pineal gland
has been removed or the nervous circuits to the pineal
g

en removed or the nervous circuits to the pineal
gland have been sectioned, the normal periods of sea-
sonal fertility are lost. To these animals, such seasonal
fertility is important because it allows birth of the off-
spring at the time of year, usually springtime or early
summer, when survival is most likely.The mechanism of
this effect is not entirely clear, but it seems to be the 
First, the pineal gland is controlled by the amount of
light or “time pattern” of light seen by the 

t of
light or “time pattern” of light seen by the eyes each day.
For instance, in the hamster, greater than 13 hours of
darkness each day activates the pineal gland, whereas
less than that amount of darkness fails to activate it,
with a critical balance between activation and nonacti-
vation. The nervous pathway involves the passage of
light signals from the eyes to the suprachiasmal nucleus
of the hypothalamus and then to the pineal gland,
activating pineal secretion.
Second, the pineal gland s

ating pineal secretion.
Second, the pineal gland secretes melatonin and
several other, similar substances. Either melatonin or
one of the other substances is believed to pass either by
Figure 80–11
Adiposogenital syndrome in an adolescent male. Note the obesity
and childlike sexual organs. (Courtesy Dr. Leonard Posey.)
1010
Unit XIV
way of the blood or through the ﬂuid of the third 
ventricle to the anterior pituitary gland to decrease
gonadotropic hormone secretio

ry gland to decrease
gonadotropic hormone secretion.
Thus, in the presence of pineal gland secretion,
gonadotropic hormone secretion is suppressed in some
species of animals,and the gonads become inhibited and
even partly involuted. This is what presumably occurs
during the early winter months when there is increasing
darkness. But after about 4 months of dysfunction,
gonadotropic hormone secretion breaks through the
inhibitory effect of the pineal gland and the gonads
become functional once mor

al gland and the gonads
become functional once more, ready for a full spring-
time of activity.
But does the pineal gland have a similar function for
control of reproduction in humans? The answer to this
question is unknown. However, tumors often occur in
the region of the pineal gland. Some of these secrete
excessive quantities of pineal hormones, whereas others
are tumors of surrounding tissue and press on the pineal
gland to destroy it. Both types of tumors are often asso-
ciated with hypogon

ypes of tumors are often asso-
ciated with hypogonadal or hypergonadal function. So
perhaps the pineal gland does play at least some role in
controlling sexual drive and reproduction in humans.
Anderson RA, Baird DT: Male contraception. Endocr Rev
23:735, 2002.
Barry MJ, Roehrborn CG: Benign prostatic hyperplasia.
BMJ 323:1042, 2001.
Brennan J, Capel B: One tissue, two fates: molecular genetic
events that underlie testis versus ovary development. Nat
Rev Genet 5:509, 2004.
Cajochen C,

evelopment. Nat
Cajochen C, Krauchi K, Wirz-Justice A: Role of melatonin
in the regulation of human circadian rhythms and sleep.
J Neuroendocrinol 15:432, 2003.
Cheng CY, Mruk DD: Cell junction dynamics in the testis:
Sertoli-germ cell interactions and male contraceptive
development. Physiol Rev 82:825, 2002.
Cooke HJ, Saunders PT: Mouse models of male infertility.
Nat Rev Genet 3:790, 2002.
de Kretser DM: Is spermatogenic damage associated with
Leydig cell dysfunction? J 

damage associated with
Leydig cell dysfunction? J Clin Endocrinol Metab 89:3158,
2004.
DeMarzo AM, Nelson WG, Isaacs WB, Epstein JI: Patho-
logical and molecular aspects of prostate cancer. Lancet
361:955, 2003.
Foresta C, Moro E, Ferlin A: Y chromosome microdeletions
and alterations of spermatogenesis. Endocr Rev 22:226,
2001.
Heinlein CA, Chang C: Androgen receptor (AR) coregula-
tors: an overview. Endocr Rev 23:175, 2002.
Jobling MA, Tyler-Smith C: The human Y chromosome: an
evolutionary mark

th C: The human Y chromosome: an
evolutionary marker comes of age. Nat Rev Genet 4:598,
2003.
Kandeel FR, Koussa VK, Swerdloff RS: Male sexual func-
tion and its disorders: physiology, pathophysiology, clinical
investigation, and treatment. Endocr Rev 22:342, 2001.
Lahn BT, Pearson NM, Jegalian K: The human Y chromo-
some, in the light of evolution. Nat Rev Genet 2:207, 2001.
Lanfranco F, Kamischke A, Zitzmann M, Nieschlag E:
Klinefelter’s syndrome. Lancet 364:273, 2004.
Liu PY, Death AK, Handel

me. Lancet 364:273, 2004.
Liu PY, Death AK, Handelsman DJ: Androgens and 
cardiovascular disease. Endocr Rev 24:313, 2003.
Nelson WG, De Marzo AM, Isaacs WB: Prostate cancer.
N Engl J Med 349:366, 2003.
Nelson PS, Montgomery B: Unconventional therapy for
prostate cancer: good, bad or questionable? Nat Rev
Cancer 3:845, 2003.
O’Donnell L, Robertson KM, Jones ME, Simpson ER:
Estrogen and spermatogenesis. Endocr Rev 22:289, 2001.
Plant TM, Marshall GR: The functional signiﬁcance of FSH
in spermatog

GR: The functional signiﬁcance of FSH
in spermatogenesis and the control of its secretion in male
primates. Endocr Rev 22:764, 2001.
Rhoden EL,
Morgentaler A:
Risks of testosterone-
replacement therapy and recommendations for monitor-
ing. N Engl J Med 350:482, 2004.
Riggs BL, Khosla S, Melton LJ 3rd: Sex steroids and the con-
struction and conservation of the adult skeleton. Endocr
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tion of the adult skeleton. Endocr
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Shabsigh R, Anastasiadis AG: Erectile dysfunction. Annu
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C
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A
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8
1
1011
Female Physiology Before
Pregnancy and Female Hormones
Female reproductive functions can be divided into
two majo

eproductive functions can be divided into
two major phases: (1) preparation of the female
body for conception and pregnancy, and (2) the
period of pregnancy itself.This chapter is concerned
with preparation of the female body for pregnancy,
and Chapter 82 presents the physiology of preg-
nancy and childbirth.
Figures 81–1 and 81–2 show the principal organs of the human female reproductive
tract, the most important of which are the ovaries, fallopi

e most important of which are the ovaries, fallopian tubes, uterus, and vagina.
Reproduction begins with the development of ova in the ovaries. In the middle of
each monthly sexual cycle, a single ovum is expelled from an ovarian follicle into
the abdominal cavity near the open ﬁmbriated ends of the two fallopian tubes. This
ovum then passes through one of the fallopian tubes into the uterus; if it has been
fertilized by a sperm, it implants in the uterus, where it develops into a fetus, a 
plac

he uterus, where it develops into a fetus, a 
placenta, and fetal membranes—and eventually into a baby.
During fetal life, the outer surface of the ovary is covered by a germinal epithe-
lium, which embryologically is derived from the epithelium of the germinal ridges.
As the female fetus develops, primordial ova differentiate from this germinal epithe-
lium and migrate into the substance of the ovarian cortex. Each ovum then collects
around it a layer of spindle cells from the ovarian stroma (t

 layer of spindle cells from the ovarian stroma (the supporting tissue of
the ovary) and causes them to take on epithelioid characteristics; they are then
called granulosa cells. The ovum surrounded by a single layer of granulosa cells is
called a primordial follicle. The ovum itself at this stage is still immature, requiring
two more cell divisions before it can be fertilized by a sperm.At this time, the ovum
is called a primary oocyte.
During all the reproductive years of adult life, between a

ll the reproductive years of adult life, between about 13 and 46 years of
age, 400 to 500 of the primordial follicles develop enough to expel their ova—one
each month; the remainder degenerate (become atretic).At the end of reproductive
capability (at menopause), only a few primordial follicles remain in the ovaries, and
even these degenerate soon thereafter.
The female hormonal system, like that of the male, consists of three hierarchies
of hormones, as follows:
1. A hypo

ree hierarchies
1. A hypothalamic releasing hormone, gonadotropin-releasing hormone
(GnRH)
2. The anterior pituitary sex hormones, follicle-stimulating hormone (FSH)
and luteinizing hormone (LH), both of which are secreted in response to
the release of GnRH from the hypothalamus
3. The ovarian hormones, estrogen and progesterone, which are secreted by
the ovaries in response to the two female sex hormones from the anterior
These various hormones are not s

r
These various hormones are not secreted in constant amounts throughout the
female monthly sexual cycle; they are secreted at drastically differing rates
during different parts of the cycle. Figure 81–3 shows the approximate 
1012
Unit XIV
during the monthly sexual cycle. It is secreted in short
pulses averaging once every 90 minutes, as occurs in
the male.
Monthly Ovarian Cycle;
Function of the Gonadotropic
Hormones
The normal reproductive years o

adotropic
Hormones
The normal reproductive years of the female are char-
acterized by monthly rhythmical changes in the rates
of secretion of the female hormones and correspon-
ding physical changes in the ovaries and other sexual
organs. This rhythmical pattern is called the female
monthly sexual cycle (or, less accurately, the menstrual
cycle). The duration of the cycle averages 28 days. It
may be as short as 20 days or as long as 45 days in some
women, although abnormal cycle length is freque

me
women, although abnormal cycle length is frequently
associated with decreased fertility.
There are two signiﬁcant results of the female sexual
cycle. First, only a single ovum is normally released
from the ovaries each month, so that normally only a
single fetus will begin to grow at a time. Second, the
uterine endometrium is prepared in advance for
implantation of the fertilized ovum at the required
time of the month.
Gonadotropic Hormones and 
Their Effects on the Ovaries
The ovarian change

d 
The ovarian changes that occur during the sexual cycle
depend completely on the gonadotropic hormones
FSH and LH, secreted by the anterior pituitary gland.
In the absence of these hormones, the ovaries remain
inactive, which is the case throughout childhood,
when almost no pituitary gonadotropic hormones are
secreted. At age 9 to 12 years, the pituitary begins to
secrete progressively more FSH and LH, which leads
to onset of normal monthly sexual cycles beginning

o onset of normal monthly sexual cycles beginning
between the ages of 11 and 15 years. This period of
change is called puberty, and the time of the ﬁrst men-
strual cycle is called menarche. Both FSH and LH are
small glycoproteins having molecular weights of about
30,000.
Cervix
Entrance to
uterine tube
Uterine tube
Uterine cavity
Uterine tube (sectioned)
Vagina
Fimbriae
Uterus
Ovary
Ovary
Uterus
Urethra
Clitoris
Labium
minora
Labium
majora
Vagina
Cervix
Urinary
bladder
Ovary
Rectum

majora
Vagina
Cervix
Urinary
bladder
Ovary
Rectum
Anus
Figure 81–1
Female reproductive organs.
Fimbriae
Ovarian vessels
Ovarian follicles
Broad ligament of uterus
Perimetrium
Isthmus of uterine tube
Ovarian ligament
Ovarian stroma
Ampulla of
Mucosal folds
of uterine tube
Uterosacral
ligament
Cervical canal
Cervix
Vagina
Vaginal rugae
Isthmus of uterus
Figure 81–2
Internal structures of the uterus,
ovary, 
and 
a 
u

ternal structures of the uterus,
ovary, 
and 
a 
uterine 
tube.
(Redrawn 
from 
Guyton 
AC: 
Physiology of the Human Body,
6th ed. Philadelphia: Saunders
College Publishing, 1984.)
changing concentrations of the anterior pituitary
gonadotropic hormones FSH and LH (bottom two
curves) and of the ovarian hormones estradiol (estro-
gen) and progesterone (top two curves).
The amount of GnRH released from the hypothal-
amus increases and decreases much less drastically
Chapter 81
Female Physiology Bef

 less drastically
Chapter 81
Female Physiology Before Pregnancy and Female Hormones
1013
During each month of the female sexual cycle, there
is a cyclical increase and decrease of both FSH and
LH, as shown in the bottom of Figure 81–3. These
cyclical variations cause cyclical ovarian changes,
which are explained in the following sections.
Both FSH and LH stimulate their ovarian target
cells by combining with highly speciﬁc FSH and LH
receptors in the ovarian target cell membranes. In
turn, the a

 the ovarian target cell membranes. In
turn, the activated receptors increase the cells’ rates 
of secretion and usually the growth and proliferation
of the cells as well. Almost all these stimulatory 
effects result from activation of the cyclic adenosine
monophosphate second messenger system in the cell
cytoplasm, which causes the formation of protein
kinase and multiple phosphorylations of key enzymes
that stimulate sex hormone synthesis, as explained in
Chapter 74.
Ovarian Follicle Growth—
“

xplained in
“Follicular” Phase of the 
Figure 81–4 shows the progressive stages of follicular
growth in the ovaries. When a female child is born,
each ovum is surrounded by a single layer of granu-
losa cells; the ovum, with this granulosa cell sheath, is
called a primordial follicle, as shown in the ﬁgure.
Throughout childhood,
the granulosa cells are
believed to provide nourishment for the ovum and to
secrete an oocyte maturation-inhibiting fa

 and to
secrete an oocyte maturation-inhibiting factor that
keeps the ovum suspended in its primordial state in
the prophase stage of meiotic division. Then, after
puberty, when FSH and LH from the anterior pituitary
gland begin to be secreted in signiﬁcant quantities, the
ovaries, together with some of the follicles within
them, begin to grow.
The ﬁrst stage of follicular growth is moderate
enlargement of the ovum itself, which increases in
diameter twofold to threefold. Then follows growth of

eter twofold to threefold. Then follows growth of
additional layers of granulosa cells in some of the fol-
licles; these follicles are known as primary follicles.
Development of Antral and Vesicular Follicles. During the
ﬁrst few days of each monthly female sexual cycle,
the concentrations of both FSH and LH secreted by
the anterior pituitary gland increase slightly to mod-
erately, with the increase in FSH slightly greater than
that of LH and preceding it by a few days. These hor-
mones, especi

eceding it by a few days. These hor-
mones, especially FSH, cause accelerated growth of 6
to 12 primary follicles each month. The initial effect is
rapid proliferation of the granulosa cells, giving rise to
many more layers of these cells. In addition, spindle
cells derived from the ovary interstitium collect in
several layers outside the granulosa cells, giving rise to
a second mass of cells called the theca. This is divided
into two layers. In the theca interna, the cells take on
epithelioid c

the theca interna, the cells take on
epithelioid characteristics similar to those of the gran-
ulosa cells and develop the ability to secrete additional
steroid sex hormones (estrogen and progesterone).
The outer layer, the theca externa, develops into a
highly vascular connective tissue capsule that becomes
the capsule of the developing follicle.
After the early proliferative phase of growth, lasting
for a few days, the mass of granulosa cells secretes a
follicular ﬂuid that contains a high con

ecretes a
follicular ﬂuid that contains a high concentration of
estrogen, one of the important female sex hormones
(discussed later). Accumulation of this ﬂuid causes an
0 2 4 6 8 10 12 14 16 18 20 22 24 26
FSH
LH
28
0
800
0
800
600
Ovulation
Estradiol
Ovulation
400
200
600
400
200
0
8
Days of female sexual cycle
FSH and LH (ng/mL) Estradiol (pg/mL)
Figure 81–3
Approximate plasma concentrations of the gonadotropins and
ovarian hormones during the no

e gonadotropins and
ovarian hormones during the normal female sexual cycle. FSH,
follicle-stimulating hormone; LH, luteinizing hormone.
Ovum
Ovum
Antrum
Corona radiata
Ovulation
Degenerating
corpus luteum
follicle
Preantral
follicle
Antral
follicle
(mature) follicle
Theca
Figure 81–4
Stages of follicular growth in the ovary, also showing formation of
the corpus luteum.
1014
Unit XIV

teum.
1014
Unit XIV
antrum to appear within the mass of granulosa cells, as
shown in Figure 81–4.
The early growth of the primary follicle up to 
the antral stage is stimulated mainly by FSH alone.
Then greatly accelerated growth occurs, leading to still
larger follicles called vesicular follicles. This acceler-
ated growth is caused by the following: (1) Estrogen is
secreted into the follicle and causes the granulosa cells
to form increasing numbers of FSH recepto

sa cells
to form increasing numbers of FSH receptors; this
causes a positive feedback effect, because it makes the
granulosa cells even more sensitive to FSH. (2) The
pituitary FSH and the estrogens combine to promote
LH receptors on the original granulosa cells, thus
allowing LH stimulation to occur in addition to FSH
stimulation and creating an even more rapid increase
in follicular secretion. (3) The increasing estrogens
from the follicle plus the increasing LH from the ante-
rior pituitary g

 the increasing LH from the ante-
rior pituitary gland act together to cause proliferation
of the follicular thecal cells and increase their secre-
tion as well.
Once the antral follicles begin to grow, their 
growth occurs almost explosively. The ovum itself 
also enlarges in diameter another threefold to four-
fold, giving a total ovum diameter increase up to 10-
fold, or a mass increase of 1000-fold. As the follicle
enlarges, the ovum itself remains embedded in a mass
of granulosa cells locat

emains embedded in a mass
of granulosa cells located at one pole of the follicle.
Only One Follicle Fully Matures Each Month, and the Remain-
der Undergo Atresia. After a week or more of growth—
but before ovulation occurs—one of the follicles
begins to outgrow all the others; the remaining 5 to 11
developing follicles involute (a process called atresia),
and these follicles are said to become atretic.
The cause of the atresia is unknown, but it has been
postulated to be the following: The large

has been
postulated to be the following: The large amounts of
estrogen from the most rapidly growing follicle act on
the hypothalamus to depress further enhancement of
FSH secretion by the anterior pituitary gland, in this
way blocking further growth of the less well developed
follicles. Therefore, the largest follicle continues to
grow because of its intrinsic positive feedback effects,
while all the other follicles stop growing and actually
involute.
This process of atresia is important, becau

olute.
This process of atresia is important, because it nor-
mally allows only one of the follicles to grow large
enough each month to ovulate; this usually prevents
more than one child from developing with each preg-
nancy.The single follicle reaches a diameter of 1 to 1.5
centimeters at the time of ovulation and is called the
mature follicle.
Ovulation
Ovulation in a woman who has a normal 28-day
female sexual cycle occurs 14 days after the onset of
menstruation. Shortly before ovulation, the 

et of
menstruation. Shortly before ovulation, the protruding
outer wall of the follicle swells rapidly, and a small area
in the center of the follicular capsule, called the stigma,
protrudes like a nipple. In another 30 minutes or 
so, ﬂuid begins to ooze from the follicle through the
stigma, and about 2 minutes later, the stigma ruptures
widely, allowing a more viscous ﬂuid, which has 
occupied the central portion of the follicle, to evagi-
nate outward. This viscous ﬂuid carries with it the
ov

 outward. This viscous ﬂuid carries with it the
ovum surrounded by a mass of several thousand small
granulosa cells, called the corona radiata.
Surge of LH Is Necessary for Ovulation. LH is necessary 
for ﬁnal follicular growth and ovulation. Without this
hormone, even when large quantities of FSH are 
available, the follicle will not progress to the stage 
of ovulation.
About 2 days before ovulation (for reasons that are
not completely understood but are discussed in more
detail later in the ch

d but are discussed in more
detail later in the chapter), the rate of secretion of LH
by the anterior pituitary gland increases markedly,
rising 6- to 10-fold and peaking about 16 hours before
ovulation. FSH also increases about 2-fold to 3-fold at
the same time, and the FSH and LH act synergistically
to cause rapid swelling of the follicle during the last
few days before ovulation. The LH also has a speciﬁc
effect on the granulosa and theca cells, converting
them mainly to progesterone-secretin

s, converting
them mainly to progesterone-secreting cells. There-
fore, the rate of secretion of estrogen begins to 
fall about 1 day before ovulation, while increasing
amounts of progesterone begin to be secreted.
It is in this environment of (1) rapid growth of the
follicle, (2) diminishing estrogen secretion after a pro-
longed phase of excessive estrogen secretion, and (3)
initiation of secretion of progesterone that ovulation
occurs. Without the initial preovulatory surge of LH,
ovulation w

 the initial preovulatory surge of LH,
ovulation will not take place.
Initiation of Ovulation. Figure 81–5 gives a schema for the
initiation of ovulation, showing the role of the large
quantity of LH secreted by the anterior pituitary
gland. This LH causes rapid secretion of follicular
steroid hormones that contain progesterone. Within a
few hours, two events occur, both of which are neces-
sary for ovulation: (1) The theca externa (the capsule
of the follicle) begins to release proteolytic enzy

f the follicle) begins to release proteolytic enzymes
from lysosomes, and these cause dissolution of the fol-
licular capsular wall and consequent weakening of the
wall, resulting in further swelling of the entire follicle
and degeneration of the stigma. (2) Simultaneously,
there is rapid growth of new blood vessels into the fol-
licle wall, and at the same time, prostaglandins (local
hormones that cause vasodilation) are secreted into
the follicular tissues. These two effects cause plasma
trans

ular tissues. These two effects cause plasma
transudation into the follicle, which contributes to 
follicle swelling. Finally, the combination of follicle
swelling and simultaneous degeneration of the stigma
causes follicle rupture, with discharge of the ovum.
Corpus Luteum—“Luteal” Phase 
of the Ovarian Cycle
During the ﬁrst few hours after expulsion of the ovum
from the follicle, the remaining granulosa and theca
interna cells change rapidly into lutein cells. They
enlarge in diameter two or m

to lutein cells. They
enlarge in diameter two or more times and become
ﬁlled with lipid inclusions that give them a yellowish
appearance.This process is called luteinization, and the
Chapter 81
1015
total mass of cells together is called the corpus luteum,
which is shown in Figure 81–4. A well-developed 
vascular supply also grows into the corpus luteum.
The granulosa cells in the corpus luteum develop
extensive intracellular smooth endoplas

um develop
extensive intracellular smooth endoplasmic reticula
that form large amounts of the female sex hormones
progesterone and estrogen (more progesterone than
estrogen). The theca cells form mainly the androgens
androstenedione and testosterone rather than female
sex hormones. However, most of these hormones are
also converted by the granulosa cells into the female
hormones.
In the normal female, the corpus luteum grows to
about 1.5 centimeters in diameter, reaching this stage
of developmen

ers in diameter, reaching this stage
of development 7 to 8 days after ovulation. Then it
begins to involute and eventually loses its secretory
function as well as its yellowish, lipid characteristic
about 12 days after ovulation, becoming the corpus
albicans; during the ensuing few weeks, this is replaced
by connective tissue and over months is absorbed.
Luteinizing Function of LH. The change of granulosa and
theca interna cells into lutein cells is dependent mainly
on LH secreted by the anterio

 is dependent mainly
on LH secreted by the anterior pituitary gland. In fact,
this function gives LH its name—“luteinizing,” for
“yellowing.” Luteinization also depends on extrusion
of the ovum from the follicle. A yet uncharacterized
local hormone in the follicular ﬂuid, called luteiniza-
tion-inhibiting factor, seems to hold the luteinization
process in check until after ovulation.
Secretion by the Corpus Luteum: An Additional Function of LH.
The corpus luteum is a highly secretory organ, secr

he corpus luteum is a highly secretory organ, secret-
ing large amounts of both progesterone and estrogen.
Once LH (mainly that secreted during the ovulatory
surge) has acted on the granulosa and theca cells 
to cause luteinization, the newly formed lutein cells
seem to be programmed to go through a preordained
sequence of (1) proliferation, (2) enlargement, and (3)
secretion, followed by (4) degeneration.All this occurs
in about 12 days. We shall see in the discussion of 
pregnancy in Chapter 8

l see in the discussion of 
pregnancy in Chapter 82 that another hormone with
almost exactly the same properties as LH, chorionic
gonadotropin, which is secreted by the placenta, can
act on the corpus luteum to prolong its life—usually
maintaining it for at least the ﬁrst 2 to 4 months of
pregnancy.
Involution of the Corpus Luteum and Onset of the Next Ovarian
Cycle. Estrogen in particular and progesterone to a
lesser extent, secreted by the corpus luteum during the
luteal phase of the ovarian c

us luteum during the
luteal phase of the ovarian cycle, have strong feedback
effects on the anterior pituitary gland to maintain low
secretory rates of both FSH and LH.
In addition, the lutein cells secrete small amounts of
the hormone inhibin, the same as the inhibin secreted
by the Sertoli cells of the male testes. This hormone
inhibits secretion by the anterior pituitary gland, espe-
cially FSH secretion. Low blood concentrations of both
FSH and LH result, and loss of these hormones ﬁnally
ca

nd LH result, and loss of these hormones ﬁnally
causes the corpus luteum to degenerate completely, a
process called involution of the corpus luteum.
Final involution normally occurs at the end of
almost exactly 12 days of corpus luteum life, which is
around the 26th day of the normal female sexual cycle,
2 days before menstruation begins. At this time, the
sudden cessation of secretion of estrogen, proges-
terone, and inhibin by the corpus luteum removes the
feedback inhibition of the anterior p

 removes the
feedback inhibition of the anterior pituitary gland,
allowing it to begin secreting increasing amounts of
FSH and LH again. FSH and LH initiate the growth
of new follicles, beginning a new ovarian cycle. The
paucity of secretion of progesterone and estrogen at
this time also leads to menstruation by the uterus, as
explained later.
Summary
About every 28 days, gonadotropic hormones from the
anterior pituitary gland cause about 8 to 12 new folli-
cles to begin to grow in the ovaries. 

 new folli-
cles to begin to grow in the ovaries. One of these fol-
licles ﬁnally becomes “mature” and ovulates on the
14th day of the cycle. During growth of the follicles,
mainly estrogen is secreted.
After ovulation, the secretory cells of the ovulating
follicle develop into a corpus luteum that secretes
large quantities of both the major female hormones,
progesterone and estrogen.After another 2 weeks, the
corpus luteum degenerates, whereupon the ovarian
hormones estrogen and progesterone de

 the ovarian
hormones estrogen and progesterone decrease greatly,
and menstruation begins. A new ovarian cycle then
follows.
Follicle rupture
Evagination of ovum 
Follicular steroid hormones
(progesterone)
Proteolytic enzymes
(collagenase)
Weakened follicle wall
Follicular hyperemia
and
prostaglandin secretion
Degeneration
of stigma
Plasma transudation
into follicle
Follicle swelling
Figure 81–5
Postulated mechanism of ovulation.
1016
Unit XIV
F

on.
1016
Unit XIV
Functions of the Ovarian
Hormones—Estradiol 
and Progesterone
The two types of ovarian sex hormones are the estro-
gens and the progestins. By far the most important of
the estrogens is the hormone estradiol, and by far the
most important progestin is progesterone. The estro-
gens mainly promote proliferation and growth of spe-
ciﬁc cells in the body that are responsible for the
development of most secondary sexual characteristics
of the female. T

 secondary sexual characteristics
of the female. The progestins function mainly to
prepare the uterus for pregnancy and the breasts for
Estrogens. In the normal nonpregnant female, estro-
gens are secreted in signiﬁcant quantities only by the
ovaries, although minute amounts are also secreted by
the adrenal cortices. During pregnancy, tremendous
quantities of estrogens are also secreted by the 
placenta, as discussed in Chapter 82.
Only three estrogens ar

s discussed in Chapter 82.
Only three estrogens are present in signiﬁcant quan-
tities in the plasma of the human female: b-estradiol,
estrone, and estriol, the formulas for which are shown
in Figure 81–6. The principal estrogen secreted by 
the ovaries is b-estradiol. Small amounts of estrone 
are also secreted, but most of this is formed in the
peripheral tissues from androgens secreted by the
adrenal cortices and by ovarian thecal cells. Estriol is
a weak estrogen; it is an oxidative product 

ol is
a weak estrogen; it is an oxidative product derived
from both estradiol and estrone, with the conversion
occurring mainly in the liver.
The estrogenic potency of b-estradiol is 12 times
that of estrone and 80 times that of estriol. Consider-
ing these relative potencies, one can see that the total
estrogenic effect of b-estradiol is usually many times
that of the other two together. For this reason, b-
estradiol is considered the major estrogen, although
the estrogenic effects of estrone a

ogen, although
the estrogenic effects of estrone are not negligible.
Progestins. By far the most important of the progestins
is progesterone. However, small amounts of another
progestin, 17-a-hydroxyprogesterone, are secreted
along with progesterone and have essentially the same
effects. Yet, for practical purposes, it is usually reason-
able to consider progesterone the only important
progestin.
In the normal nonpregnant female, progesterone is
secreted in signiﬁcant amounts only during the lat

secreted in signiﬁcant amounts only during the latter
half of each ovarian cycle, when it is secreted by the
corpus luteum.
As we shall see in Chapter 82, large amounts of
progesterone are also secreted by the placenta during
pregnancy, especially after the fourth month of 
Synthesis of the Estrogens and Progestins. Note from the
chemical formulas of the estrogens and progesterone
in Figure 81–6 that they are all steroids. They are 
synthesized in the ovaries mainly from cholesterol
d

nthesized in the ovaries mainly from cholesterol
derived from the blood but also to a slight extent from
acetyl coenzyme A, multiple molecules of which can
combine to form the appropriate steroid nucleus.
During synthesis, mainly progesterone and the male
sex hormone testosterone are synthesized ﬁrst; then,
during the follicular phase of the ovarian cycle, before
these two initial hormones can leave the ovaries,
almost all the testosterone and much of the proges-
terone are converted into estrog

ch of the proges-
terone are converted into estrogens by the granulosa
cells. During the luteal phase of the cycle, far too much
progesterone is formed for all of it to be converted,
which accounts for the large secretion of progesterone
into the circulating blood at this time. Also, about one
ﬁfteenth as much testosterone is secreted into the
plasma of the female by the ovaries as is secreted into
the plasma of the male by the testes.
Estrogens and Progesterone Are Transported in the Blood Boun

and Progesterone Are Transported in the Blood Bound
to Plasma Proteins. Both estrogens and progesterone
are transported in the blood bound mainly with
plasma albumin and with speciﬁc estrogen- and prog-
esterone-binding globulins.The binding between these
hormones and the plasma proteins is loose enough
that they are rapidly released to the tissues over a
period of 30 minutes or so.
Functions of the Liver in Estrogen Degradation. The liver
conjugates the estrogens to form glucuronides and sul-
f

ates the estrogens to form glucuronides and sul-
fates, and about one ﬁfth of these conjugated products
HO
b-Estradiol
Estrone
H
OH
CH3
HO
O
CH3
HO
H
OH
H
OH
CH3
O
C
CH3
CH3
O
CH3
Estriol
Figure 81–6
Chemical formulas of the principal female hormones.
Chapter 81
1017
is excreted in the bile; most of the remainder is
excreted in the urine. Also, the liver converts the
potent estrogens estradiol and estrone into the almost
totally

gens estradiol and estrone into the almost
totally impotent estrogen estriol. Therefore, dimin-
ished 
liver 
function 
actually 
increases
the 
activity of estrogens in the body, sometimes causing
hyperestrinism.
Fate of Progesterone. Within a few minutes after secre-
tion, almost all the progesterone is degraded to other
steroids that have no progestational effect.As with the
estrogens, the liver is especially important for this
metabolic degradation.
The major end product of progesterone degr

dation.
The major end product of progesterone degradation
is pregnanediol. About 10 per cent of the original prog-
esterone is excreted in the urine in this form. There-
fore, one can estimate the rate of progesterone
formation in the body from the rate of this excretion.
Functions of the Estrogens—
Their Effects on the Primary and
Secondary Female Sex Characteristics
A primary function of the estrogens is to cause cellu-
lar proliferation and growth of the tissues of the sex
organs and other ti

owth of the tissues of the sex
organs and other tissues related to reproduction.
Effect of Estrogens on the Uterus and External Female Sex
Organs. During childhood, estrogens are secreted only
in minute quantities, but at puberty, the quantity
secreted in the female under the inﬂuence of the 
pituitary gonadotropic hormones increases 20-fold or
more. At this time, the female sex organs change from
those of a child to those of an adult. The ovaries, fal-
lopian tubes, uterus, and vagina all incre

s, fal-
lopian tubes, uterus, and vagina all increase several
times in size. Also, the external genitalia enlarge, with
deposition of fat in the mons pubis and labia majora
and enlargement of the labia minora.
In addition, estrogens change the vaginal epithelium
from a cuboidal into a stratiﬁed type, which is consid-
erably more resistant to trauma and infection than 
is the prepubertal cuboidal cell epithelium. Vaginal
infections in children can often be cured by the admin-
istration of estroge

 often be cured by the admin-
istration of estrogens simply because of the resulting
increased resistance of the vaginal epithelium.
During the ﬁrst few years after puberty, the size of
the uterus increases twofold to threefold, but more
important than the increase in uterus size are the
changes that take place in the uterine endometrium
under the inﬂuence of estrogens. Estrogens cause
marked proliferation of the endometrial stroma 
and greatly increased development of the endometrial
glands, wh

ncreased development of the endometrial
glands, which will later aid in providing nutrition to 
the implanted ovum. These effects are discussed later
in the chapter in connection with the endometrial
cycle.
Effect of Estrogens on the Fallopian Tubes. The estrogens’
effect on the mucosal lining of the fallopian tubes is
similar to that on the uterine endometrium.They cause
the glandular tissues of this lining to proliferate;
especially important, they cause the number of ciliated

they cause the number of ciliated
epithelial cells that line the fallopian tubes to increase.
Also, activity of the cilia is considerably enhanced.
These cilia always beat toward the uterus, which helps
propel the fertilized ovum in that direction.
Effect of Estrogens on the Breasts. The primordial breasts
of females and males are exactly alike. In fact, under
the inﬂuence of appropriate hormones, the masculine
breast during the ﬁrst 2 decades of life can develop 
sufﬁciently to produce milk in 

 life can develop 
sufﬁciently to produce milk in the same manner as the
female breast.
Estrogens cause (1) development of the stromal
tissues of the breasts,(2) growth of an extensive ductile
system, and (3) deposition of fat in the breasts. The
lobules and alveoli of the breast develop to a slight
extent under the inﬂuence of estrogens alone, but it 
is progesterone and prolactin that cause the ultimate
determinative growth and function of these structures.
In summary, the estrogens initiate g

e structures.
In summary, the estrogens initiate growth of the
breasts and of the milk-producing apparatus. They 
are also responsible for the characteristic growth 
and external appearance of the mature female breast.
However, they do not complete the job of converting
the breasts into milk-producing organs.
Effect of Estrogens on the Skeleton. Estrogens inhibit
osteoclastic activity in the bones and therefore stimu-
late bone growth. At puberty, when the female enters
her reproductive years, h

, when the female enters
her reproductive years, her growth in height becomes
rapid for several years. However, estrogens have
another potent effect on skeletal growth: They cause
uniting of the epiphyses with the shafts of the long
bones. This effect of estrogen in the female is much
stronger than the similar effect of testosterone in the
male. As a result, growth of the female usually ceases
several years earlier than growth of the male.A female
eunuch who is devoid of estrogen production usua

e
eunuch who is devoid of estrogen production usually
grows several inches taller than a normal mature
female because her epiphyses do not unite at the
normal time.
Osteoporosis of the Bones Caused by Estrogen 
Deﬁciency in Old Age. After menopause, almost no
estrogens are secreted by the ovaries. This estrogen
deﬁciency leads to (1) increased osteoclastic activity in
the bones, (2) decreased bone matrix, and (3)
decreased deposition of bone calcium and phosphate.
In some women, this effect is e

ium and phosphate.
In some women, this effect is extremely severe, and the
resulting condition is osteoporosis,
described in
Chapter 79. Because this can greatly weaken the bones
and lead to bone fracture, especially fracture of the
vertebrae, a large share of postmenopausal women are
treated prophylactically with estrogen replacement to
prevent the osteoporotic effects.
Effect of Estrogens on Protein Deposition. Estrogens cause
a slight increase in total body protein, which is evi-
denced by a 

 in total body protein, which is evi-
denced by a slight positive nitrogen balance when
estrogens are administered. This mainly results from
the growth-promoting effect of estrogen on the sexual
1018
Unit XIV
organs, the bones, and a few other tissues of the body.
The enhanced protein deposition caused by testos-
terone is much more general and many times as pow-
erful as that caused by estrogens.
Effect of Estrogens on Body Metabolism and Fat Deposition.
Estrogens

s on Body Metabolism and Fat Deposition.
Estrogens increase the whole-body metabolic rate
slightly, but only about one third as much as the
increase caused by the male sex hormone testosterone.
They also cause deposition of increased quantities of
fat in the subcutaneous tissues. As a result, the per-
centage of body fat in the female body is considerably
greater than that in the male body, which contains
more protein. In addition to deposition of fat in the
breasts and subcutaneous tissues, est

f fat in the
breasts and subcutaneous tissues, estrogens cause the
deposition of fat in the buttocks and thighs, which is
characteristic of the feminine ﬁgure.
Effect of Estrogens on Hair Distribution. Estrogens do not
greatly affect hair distribution. However, hair does
develop in the pubic region and in the axillae after
puberty. Androgens formed in increased quantities by
the female adrenal glands after puberty are mainly
responsible for this.
Effect of Estrogens on the Skin. Estrogens cause 

Effect of Estrogens on the Skin. Estrogens cause the skin
to develop a texture that is soft and usually smooth,
but even so, the skin of a woman is thicker than that
of a child or a castrated female. Also, estrogens cause
the skin to become more vascular; this is often associ-
ated with increased warmth of the skin and also pro-
motes greater bleeding of cut surfaces than is observed
in men.
Effect of Estrogens on Electrolyte Balance. The chemical
similarity of estrogenic hormones to adrenocort

al
similarity of estrogenic hormones to adrenocortical
hormones has been pointed out. Estrogens, like aldos-
terone and some other adrenocortical hormones,cause
sodium and water retention by the kidney tubules.This
effect of estrogens is normally slight and rarely of sig-
niﬁcance, but during pregnancy, the tremendous for-
mation of estrogens by the placenta may contribute to
body ﬂuid retention, as discussed in Chapter 82.
Effect of Progesterone on the Uterus. By far t

one
Effect of Progesterone on the Uterus. By far the most
important function of progesterone is to promote
secretory changes in the uterine endometrium during
the latter half of the monthly female sexual cycle, thus
preparing the uterus for implantation of the fertilized
ovum. This function is discussed later in connection
with the endometrial cycle of the uterus.
In addition to this effect on the endometrium,
progesterone decreases the frequency and intensity 
of uterine contractions, thereby h

 and intensity 
of uterine contractions, thereby helping to prevent
expulsion of the implanted ovum.
Effect of Progesterone on the Fallopian Tubes. Progesterone
also promotes increased secretion by the mucosal
lining of the fallopian tubes. These secretions are 
necessary for nutrition of the fertilized, dividing ovum
as it traverses the fallopian tube before implantation.
Effect of Progesterone on the Breasts. Progesterone 
promotes development of the lobules and alveoli of
the breasts, causing

of the lobules and alveoli of
the breasts, causing the alveolar cells to proliferate,
enlarge, and become secretory in nature. However,
progesterone does not cause the alveoli to secrete
milk; as discussed in Chapter 82, milk is secreted only
after the prepared breast is further stimulated by pro-
lactin from the anterior pituitary gland.
Progesterone also causes the breasts to swell. Part
of this swelling is due to the secretory development in
the lobules and alveoli, but part also results from

he lobules and alveoli, but part also results from
increased ﬂuid in the subcutaneous tissue.
Monthly Endometrial Cycle 
and Menstruation
Associated with the monthly cyclical production 
of estrogens and progesterone by the ovaries is an
endometrial cycle in the lining of the uterus that oper-
ates through the following stages: (1) proliferation of
the uterine endometrium; (2) development of secre-
tory changes in the endometrium; and (3) desqua-
mation of the endometrium, which is known as
mens

menstruation. The various phases of this endometrial
cycle are shown in Figure 81–7.
Proliferative Phase (Estrogen Phase) of the Endometrial Cycle,
Occurring Before Ovulation. At the beginning of each
monthly cycle, most of the endometrium has been
desquamated by menstruation. After menstruation,
only a thin layer of endometrial stroma remains, and
the only epithelial cells that are left are those located
in the remaining deeper portions of the gland

ated
in the remaining deeper portions of the glands and
crypts of the endometrium. Under the inﬂuence of
estrogens, secreted in increasing quantities by the
ovary during the ﬁrst part of the monthly ovarian
cycle, the stromal cells and the epithelial cells 
Endometrial
thickness
phase
(11 days)
Secretory
phase
(12 days)
Menstrual
phase
(5 days)
Figure 81–7
Phases of endometrial growth and menstruation during each
monthly female sexual cycle.
Chapter 81
Female Physiology Before Preg

al cycle.
Chapter 81
1019
proliferate rapidly. The endometrial surface is re-
epithelialized within 4 to 7 days after the beginning of
menstruation.
Then, during the next week and a half—that is,
before ovulation occurs—the endometrium increases
greatly in thickness, owing to increasing numbers of
stromal cells and to progressive growth of the endome-
trial glands and new blood vessels into the endo-
metrium.At the time of ovulation, the end

he endo-
metrium.At the time of ovulation, the endometrium is
3 to 5 millimeters thick.
The endometrial glands, especially those of the cer-
vical region, secrete a thin, stringy mucus. The mucus
strings actually align themselves along the length of
the cervical canal, forming channels that help guide
sperm in the proper direction from the vagina into the
uterus.
Secretory Phase (Progestational Phase) of the Endometrial
Cycle, Occurring After Ovulation. During most of the latter
half of the mont

lation. During most of the latter
half of the monthly cycle, after ovulation has occurred,
progesterone and estrogen together are secreted in
large quantities by the corpus luteum. The estrogens
cause slight additional cellular proliferation in the
endometrium during this phase of the cycle, whereas
progesterone causes marked swelling and secretory
development of the endometrium.The glands increase
in tortuosity; an excess of secretory substances accu-
mulates in the glandular epithelial cells.A

 accu-
mulates in the glandular epithelial cells.Also, the cyto-
plasm of the stromal cells increases; lipid and glycogen
deposits increase greatly in the stromal cells; and the
blood supply to the endometrium further increases 
in proportion to the developing secretory activity,
with the blood vessels becoming highly tortuous. At
the peak of the secretory phase, about 1 week after
ovulation, the endometrium has a thickness of 5 to 
6 millimeters.
The whole purpose of all these endometrial chang

.
The whole purpose of all these endometrial changes
is to produce a highly secretory endometrium that con-
tains large amounts of stored nutrients to provide
appropriate conditions for implantation of a fertilized
ovum during the latter half of the monthly cycle. From
the time a fertilized ovum enters the uterine cavity
from the fallopian tube (which occurs 3 to 4 days 
after ovulation) until the time the ovum implants (7 
to 9 days after ovulation), the uterine secretions,
called “uterine milk

ion), the uterine secretions,
called “uterine milk,” provide nutrition for the early
dividing ovum. Then, once the ovum implants in the
endometrium, the trophoblastic cells on the surface of
the implanting ovum (in the blastocyst stage) begin to
digest the endometrium and absorb the endometrial
stored substances, thus making great quantities of
nutrients available to the early implanting embryo.
Menstruation. If the ovum is not fertilized, about 2 days
before the end of the monthly cycle, the co

2 days
before the end of the monthly cycle, the corpus luteum
in the ovary suddenly involutes, and the ovarian 
hormones (estrogens and progesterone) decrease to
low levels of secretion, as shown in Figure 81–3.
Menstruation follows.
Menstruation is caused by the reduction of estro-
gens and progesterone, especially progesterone, at the
end of the monthly ovarian cycle. The ﬁrst effect is
decreased stimulation of the endometrial cells by these
two hormones, followed rapidly by involution of the

o hormones, followed rapidly by involution of the
endometrium itself to about 65 per cent of its previous
thickness. Then, during the 24 hours preceding the
onset of menstruation, the tortuous blood vessels
leading to the mucosal layers of the endometrium
become vasospastic, presumably because of some
effect of involution, such as release of a vasoconstric-
tor material—possibly one of the vasoconstrictor types
of prostaglandins that are present in abundance at this
time.
The vasospasm, the decr

in abundance at this
time.
The vasospasm, the decrease in nutrients to the
endometrium, and the loss of hormonal stimulation
initiate necrosis in the endometrium, especially of the
blood vessels. As a result, blood at ﬁrst seeps into the
vascular layer of the endometrium, and the hemor-
rhagic areas grow rapidly over a period of 24 to 36
hours. Gradually, the necrotic outer layers of the
endometrium separate from the uterus at the sites 
of the hemorrhages until, about 48 hours after the 
onset 

emorrhages until, about 48 hours after the 
onset of menstruation, all the superﬁcial layers of 
the endometrium have desquamated. The mass of
desquamated tissue and blood in the uterine cavity,
plus contractile effects of prostaglandins or other 
substances in the decaying desquamate, all acting
together, initiate uterine contractions that expel the
uterine contents.
During normal menstruation, approximately 40 
milliliters of blood and an additional 35 milliliters of
serous ﬂuid are lost. The 

ional 35 milliliters of
serous ﬂuid are lost. The menstrual ﬂuid is normally
nonclotting because a ﬁbrinolysin is released along
with the necrotic endometrial material. If excessive
bleeding occurs from the uterine surface, the quantity
of ﬁbrinolysin may not be sufﬁcient to prevent clot-
ting.The presence of clots during menstruation is often
clinical evidence of uterine pathology.
Within 4 to 7 days after menstruation starts, the loss
of blood ceases because, by this time, the endometrium
has 

ceases because, by this time, the endometrium
has become re-epithelialized.
Leukorrhea During Menstruation. During menstrua-
tion, tremendous numbers of leukocytes are released
along with the necrotic material and blood. It is prob-
able that some substance liberated by the endometrial
necrosis causes this outﬂow of leukocytes. As a result
of these leukocytes and possibly other factors, the
uterus is highly resistant to infection during men-
struation, even though the endometrial surfaces are
de

ation, even though the endometrial surfaces are
denuded. This is of extreme protective value.
Regulation of the Female
Monthly Rhythm—Interplay
Between the Ovarian and
Hypothalamic-Pituitary
Hormones
Now that we have presented the major cyclical
changes that occur during the monthly female sexual
cycle, we can attempt to explain the basic rhythmical
mechanism that causes the cyclical variations.
1020
Unit XIV
The Hypothalamus Secretes GnRH, Which
Causes the Anterio

pothalamus Secretes GnRH, Which
Causes the Anterior Pituitary Gland to Secrete
LH and FSH
As pointed out in Chapter 74, secretion of most of the
anterior pituitary hormones is controlled by “releasing
hormones” formed in the hypothalamus and then
transported to the anterior pituitary gland by way of
the hypothalamic-hypophysial portal system. In the
case of the gonadotropins, one releasing hormone,
GnRH, is important. This hormone has been puriﬁed
and has been found to be a decapeptide with the 

d
following formula:
Glu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2
Intermittent, Pulsatile Secretion of GnRH by the Hypothalamus
Stimulates Pulsatile Release of LH from the Anterior Pituitary
Gland. Experiments have demonstrated that the hypo-
thalamus does not secrete GnRH continuously but
instead secretes it in pulses lasting 5 to 25 minutes that
occur every 1 to 2 hours. The lower curve in Figure
81–8 shows the electrical pulsatile signals in the

81–8 shows the electrical pulsatile signals in the hypo-
thalamus that cause the hypothalamic pulsatile output
of GnRH.
It is intriguing that when GnRH is infused continu-
ously so that it is available all the time rather than in
pulses, its ability to cause the release of LH and FSH
by the anterior pituitary gland is lost. Therefore, for
reasons unknown, the pulsatile nature of GnRH
release is essential to its function.
The pulsatile release of GnRH also causes intermit-
tent output of LH secre

GnRH also causes intermit-
tent output of LH secretion about every 90 minutes.
This is shown by the upper curve in Figure 81–8.
Hypothalamic Centers for Release of GnRH. The neuronal
activity that causes pulsatile release of GnRH occurs
primarily in the mediobasal hypothalamus, especially
in the arcuate nuclei of this area. Therefore, it is
believed that these arcuate nuclei control most 
female sexual activity, although neurons located in the
preoptic area of the anterior hypothalamus also
secr

eoptic area of the anterior hypothalamus also
secrete GnRH in moderate amounts. Multiple neu-
ronal centers in the higher brain’s “limbic” system (the
system for psychic control) transmit signals into the
arcuate nuclei to modify both the intensity of GnRH
release and the frequency of the pulses, thus providing
a partial explanation of why psychic factors often
modify female sexual function.
Negative Feedback Effects of Estrogen 
and Progesterone in Decreasing Both LH 
and FSH Secretion
Estrogen

 in Decreasing Both LH 
Estrogen in small amounts has a strong effect to
inhibit the production of both LH and FSH. Also,
when progesterone is available, the inhibitory effect of
estrogen is multiplied, even though progesterone by
itself has little effect.
These feedback effects seem to operate mainly on
the anterior pituitary gland directly, but they also
operate to a lesser extent on the hypothalamus to
decrease secretion of GnRH, especially by altering the
frequency of the G

nRH, especially by altering the
frequency of the GnRH pulses.
Hormone Inhibin from the Corpus Luteum Inhibits FSH and LH
Secretion. In addition to the feedback effects of estro-
gen and progesterone, other hormones seem to be
involved, especially inhibin, which is secreted along
with the steroid sex hormones by the granulosa cells
of the ovarian corpus luteum in the same way that
Sertoli cells secrete inhibin in the male testes. This
hormone has the same effect in the female as in the
male—inhib

the same effect in the female as in the
male—inhibiting the secretion of FSH and, to a lesser
extent, LH by the anterior pituitary gland. Therefore,
it is believed that inhibin might be especially impor-
tant in causing the decrease in secretion of FSH and
LH at the end of the monthly female sexual cycle.
0
120
240
360
480
MUA
LH
1000
2000
0
40
60
80
100
Minutes
Multi-unit electrical activity (MUA)
(spikes/min)
(ng/mL)
Figure 81–8
Upper curve: Pulsatile change in
luteini

gure 81–8
luteinizing hormone (LH) in the
peripheral 
circulation 
of 
a 
pentobarbital-anesthetized
ovariectomized rhesus monkey.
Lower curve:
Minute-by-minute
recording of multi-unit electrical
activity (MUA) in the mediobasal
hypothalamus. (Data from Wilson
RC, Kesner JS, Kaufman JM, et
al: 
Central 
electrophysiologic 
correlates of pulsatile luteinizing
hormone 
secretion. 
Neuroen-
docrinology 39:256, 1984.)
Chapter 81
Female Physiology Before Pregnancy and

Chapter 81
1021
Positive Feedback Effect of Estrogen Before
Ovulation—The Preovulatory LH Surge
For reasons not completely understood, the anterior
pituitary gland secretes greatly increased amounts of
LH for 1 to 2 days beginning 24 to 48 hours before
ovulation. This effect is demonstrated in Figure 81–3.
The ﬁgure shows a much smaller preovulatory surge
of FSH as well.
Experiments have shown that infusion of estrogen
into a female above a

wn that infusion of estrogen
into a female above a critical rate for 2 to 3 days during
the latter part of the ﬁrst half of the ovarian cycle will
cause rapidly accelerating growth of the ovarian folli-
cles, as well as rapidly accelerating secretion of ovarian
estrogens. During this period, secretions of both FSH
and LH by the anterior pituitary gland are at ﬁrst
slightly suppressed. Then secretion of LH increases
abruptly sixfold to eightfold, and secretion of FSH
increases about twofold. The 

and secretion of FSH
increases about twofold. The greatly increased secre-
tion of LH causes ovulation to occur.
The cause of this abrupt surge in LH secretion is not
known. However, several possible explanations are as
follows: (1) It has been suggested that estrogen at this
point in the cycle has a peculiar positive feedback effect
of stimulating pituitary secretion of LH and, to a lesser
extent, FSH; this is in sharp contrast to its normal neg-
ative feedback effect that occurs during the rem

-
ative feedback effect that occurs during the remainder
of the female monthly cycle. (2) The granulosa cells of
the follicles begin to secrete small but increasing quan-
tities of progesterone a day or so before the preovu-
latory LH surge, and it has been suggested that this
might be the factor that stimulates the excess LH
Without this normal preovulatory surge of LH,
ovulation will not occur.
Feedback Oscillation of the
Hypothalamic-Pituitary-Ovarian
System
Now, after discussing m

c-Pituitary-Ovarian
System
Now, after discussing much of the known information
about the interrelations of the different components
of the female hormonal system, we can attempt to
explain the feedback oscillation that controls the
rhythm of the female sexual cycle. It seems to operate
in approximately the following sequence of three
events.
1. Postovulatory Secretion of the Ovarian Hormones, and
Depression of the Pituitary Gonadotropins. The easiest part
of the cycle to explain is the events th

iest part
of the cycle to explain is the events that occur during
the postovulatory phase—between ovulation and 
the beginning of menstruation. During this time, the
corpus luteum secretes large quantities of both prog-
esterone and estrogen, as well as the hormone inhibin.
All these hormones together have a combined nega-
tive feedback effect on the anterior pituitary gland and
hypothalamus, causing the suppression of both FSH
and LH secretion and decreasing them to their lowest
levels about 3 

nd decreasing them to their lowest
levels about 3 to 4 days before the onset of menstrua-
tion. These effects are shown in Figure 81–3.
2. Follicular Growth Phase. Two to 3 days before men-
struation, the corpus luteum has regressed to almost
total involution, and the secretion of estrogen, proges-
terone, and inhibin from the corpus luteum decreases
to a low ebb. This releases the hypothalamus and 
anterior pituitary from the negative feedback effect of
these hormones. Therefore, a day or so la

ffect of
these hormones. Therefore, a day or so later, at about
the time that menstruation begins, pituitary secretion
of FSH begins to increase again, as much as twofold;
then, several days after menstruation begins, LH secre-
tion increases slightly as well. These hormones initiate
new ovarian follicle growth and a progressive increase
in the secretion of estrogen, reaching a peak estrogen
secretion at about 12.5 to 13 days after the onset of the
new female monthly sexual cycle.
During the ﬁrs

he
During the ﬁrst 11 to 12 days of this follicle growth,
the rates of pituitary secretion of the gonadotropins
FSH and LH decrease slightly because of the negative
feedback effect, mainly of estrogen, on the anterior
pituitary gland. Then there is a sudden, marked
increase in the secretion of LH and, to a lesser extent,
FSH. This is the preovulatory surge of LH and FSH,
which is followed by ovulation.
3. Preovulatory Surge of LH and FSH Causes Ovulation. At
abou

tory Surge of LH and FSH Causes Ovulation. At
about 11.5 to 12 days after the onset of the monthly
cycle, the decline in secretion of FSH and LH comes
to an abrupt halt. It is believed that the high level of
estrogens at this time (or the beginning of proges-
terone secretion by the follicles) causes a positive
feedback stimulatory effect on the anterior pituitary,
as explained earlier, which leads to a terriﬁc surge in
the secretion of LH and, to a lesser extent, FSH.What-
ever the cause of thi

o a lesser extent, FSH.What-
ever the cause of this preovulatory LH and FSH surge,
the great excess of LH leads to both ovulation and sub-
sequent development of and secretion by the corpus
luteum. Thus, the hormonal system begins its new
round of secretions until the next ovulation.
Anovulatory Cycles—Sexual Cycles at Puberty
If the preovulatory surge of LH is not of sufﬁcient
magnitude, ovulation will not occur, and the cycle is
said to be “anovulatory.”The phases of the sexual cycle
continue,

vulatory.”The phases of the sexual cycle
continue, but they are altered in the following ways:
First, lack of ovulation causes failure of development
of the corpus luteum, so there is almost no secretion
of progesterone during the latter portion of the cycle.
Second, the cycle is shortened by several days, but the
rhythm continues. Therefore, it is likely that proges-
terone is not required for maintenance of the cycle
itself, although it can alter its rhythm.
The ﬁrst few cycles after the onset

er its rhythm.
The ﬁrst few cycles after the onset of puberty are
usually anovulatory, as are the cycles occurring several
months to years before menopause, presumably
because the LH surge is not potent enough at these
times to cause ovulation.
Puberty means the onset of adult sexual life, and
menarche means the beginning of the cycle of 
1022
Unit XIV
menstruation. The period of puberty is caused by a
gradual increase in gonadotropic hormone s

ed by a
gradual increase in gonadotropic hormone secretion
by the pituitary, beginning in about the eighth year of
life, as shown in Figure 81–9, and usually culminating
in the onset of puberty and menstruation between ages
11 and 16 years in girls (average, 13 years).
In the female, as in the male, the infantile pituitary
gland and ovaries are capable of full function if appro-
priately stimulated. However, as is also true in the
male, and for reasons not understood, the hypothala-
mus does not

easons not understood, the hypothala-
mus does not secrete signiﬁcant quantities of GnRH
during childhood. Experiments have shown that 
the hypothalamus itself is capable of secreting this
hormone, but the appropriate signal from some other
area of brain to cause the secretion is lacking. There-
fore, it is now believed that the onset of puberty is 
initiated by some maturation process that occurs else-
where in the brain, perhaps somewhere in the limbic
system.
Figure 81–10 shows (1) the increa

e limbic
system.
Figure 81–10 shows (1) the increasing levels of estro-
gen secretion at puberty, (2) the cyclical variation
during the monthly sexual cycle, (3) the further
increase in estrogen secretion during the ﬁrst few years
of reproductive life, (4) the progressive decrease in
estrogen secretion toward the end of reproductive life,
and, ﬁnally, (5) almost no estrogen or progesterone
secretion beyond menopause.
Menopause
At age 40 to 50 years, the sexual cycle usually becomes
irregular,and

rs, the sexual cycle usually becomes
irregular,and ovulation often fails to occur.After a few
months to a few years, the cycle ceases altogether, as
shown in Figure 81–10. The period during which the
cycle ceases and the female sex hormones diminish to
almost none is called menopause.
The cause of menopause is “burning out” of the
ovaries. Throughout a woman’s reproductive life,
about 400 of the primordial follicles grow into mature
follicles and ovulate, and hundreds of thousands of ova
degener

 ovulate, and hundreds of thousands of ova
degenerate. At about age 45 years, only a few primor-
dial follicles remain to be stimulated by FSH and LH,
and, as shown in Figure 81–10, the production of 
estrogens by the ovaries decreases as the number of
primordial follicles approaches zero. When estrogen
production falls below a critical value, the estrogens
can no longer inhibit the production of the
gonadotropins FSH and LH. Instead, as shown in
Figure 81–9, the gonadotropins FSH and LH (mainly

FSH) are produced after menopause in large and con-
tinuous quantities, but as the remaining primordial fol-
licles become atretic, the production of estrogens by
the ovaries falls virtually to zero.
At the time of menopause, a woman must readjust
her life from one that has been physiologically stimu-
lated by estrogen and progesterone production to one
devoid of these hormones. The loss of estrogens often
causes marked physiological changes in 

gens often
causes marked physiological changes in the function
of the body, including (1) “hot ﬂushes” characterized
by extreme ﬂushing of the skin, (2) psychic sensations
of dyspnea, (3) irritability, (4) fatigue, (5) anxiety,
(6) occasionally various psychotic states, and (7)
decreased strength and calciﬁcation of bones through-
out the body. These symptoms are of sufﬁcient magni-
tude in about 15 per cent of women to warrant
treatment. If counseling fails, daily administration of
estrogen in 

seling fails, daily administration of
estrogen in small quantities usually reverses the symp-
toms, and by gradually decreasing the dose, post-
menopausal women can likely avoid severe symptoms.
0
80
70
60
50
40
30
20
Female
Male
Puberty
Menopause
10
0
60
50
40
30
20
10
Age (yr)
Total urinary gonadotropins
(rat units/24 hr)
Figure 81–9
Total rates of secretion of gonadotropic hormones throughout the
sexual lives of female and male human beings, showing an espe-
cially abrupt increase in gonadotr

howing an espe-
cially abrupt increase in gonadotropic hormones at menopause in
the female.
0
12
13
40
50
60
0
400
300
300
100
Age (yr)
Estrogens excreted in urine
(mg/24 hr)
Puberty
Menopause
Figure 81–10
Estrogen secretion throughout the sexual life of the female human
being.
Chapter 81
1023
Abnormalities of Secretion
by the Ovaries
Hypogonadism. Less than normal secretion by the ovaries
can result from poorly formed ovaries, lack of ovari

n result from poorly formed ovaries, lack of ovaries,
or genetically abnormal ovaries that secrete the wrong
hormones because of missing enzymes in the secretory
cells. When ovaries are absent from birth or when 
they become nonfunctional before puberty, female
eunuchism occurs. In this condition, the usual secondary
sexual characteristics do not appear, and the sexual
organs remain infantile. Especially characteristic of 
this condition is prolonged growth of the long bones
because the epiphyse

nged growth of the long bones
because the epiphyses do not unite with the shafts as
early as they do in a normal woman. Consequently, the
female eunuch is essentially as tall as or perhaps even
slightly taller than her male counterpart of similar
genetic background.
When the ovaries of a fully developed woman are
removed, the sexual organs regress to some extent so
that the uterus becomes almost infantile in size, the
vagina becomes smaller, and the vaginal epithelium
becomes thin and easily dam

the vaginal epithelium
becomes thin and easily damaged. The breasts atrophy
and become pendulous, and the pubic hair becomes
thinner. The same changes occur in women after
Irregularity of Menses, and Amenorrhea Caused by
Hypogonadism. As pointed out in the preceding dis-
cussion of menopause, the quantity of estrogens pro-
duced by the ovaries must rise above a critical value in
order to cause rhythmical sexual cycles. Consequently,
in hypogonadism or when the gonads are secreting
sma

 hypogonadism or when the gonads are secreting
small quantities of estrogens as a result of other factors,
such as hypothyroidism, the ovarian cycle often does
not occur normally. Instead, several months may elapse
between menstrual periods, or menstruation may cease
altogether (amenorrhea). Prolonged ovarian cycles are
frequently associated with failure of ovulation, presum-
ably because of insufﬁcient secretion of LH at the time
of the preovulatory surge of LH, which is necessary for
ovulation

tory surge of LH, which is necessary for
ovulation.
Hypersecretion by the Ovaries. Extreme hypersecretion of
ovarian hormones by the ovaries is a rare clinical entity,
because excessive secretion of estrogens automatically
decreases the production of gonadotropins by the 
pituitary, and this limits the production of ovarian 
hormones. Consequently, hypersecretion of feminizing
hormones is usually recognized clinically only when a
feminizing tumor develops.
A rare granulosa cell tumor can develop

 develops.
A rare granulosa cell tumor can develop in an ovary,
occurring more often after menopause than before.
These tumors secrete large quantities of estrogens,
which exert the usual estrogenic effects, including
hypertrophy of the uterine endometrium and irregular
bleeding from this endometrium. In fact, bleeding is
often the ﬁrst and only indication that such a tumor
exists.
Stimulation of the Female Sexual Act. As is true in the male
sexual act,successful performance of

e in the male
sexual act,successful performance of the female sexual
act depends on both psychic stimulation and local
sexual stimulation.
Thinking sexual thoughts can lead to female sexual
desire, and this aids greatly in the performance of the
female sexual act. Such desire is based largely on a
woman’s background training as well as on her physi-
ological drive, although sexual desire does increase in
proportion to the level of sex hormones secreted.
Desire also changes during the monthly sex

creted.
Desire also changes during the monthly sexual cycle,
reaching a peak near the time of ovulation, probably
because of the high levels of estrogen secretion during
the preovulatory period.
Local sexual stimulation in women occurs in more
or less the same manner as in men because massage
and other types of stimulation of the vulva, vagina, and
other perineal regions can create sexual sensations.
The glans of the clitoris is especially sensitive for ini-
tiating sexual sensations.
As in the 

ive for ini-
As in the male, the sexual sensory signals are trans-
mitted to the sacral segments of the spinal cord
through the pudendal nerve and sacral plexus. Once
these signals have entered the spinal cord, they are
transmitted to the cerebrum. Also, local reﬂexes inte-
grated in the sacral and lumbar spinal cord are at least
partly responsible for some of the reactions in the
female sexual organs.
Female Erection and Lubrication. Located around the
introitus and e

nd Lubrication. Located around the
introitus and extending into the clitoris is erectile
tissue almost identical to the erectile tissue of the
penis. This erectile tissue, like that of the penis, is 
controlled by the parasympathetic nerves that pass
through the nervi erigentes from the sacral plexus to
the external genitalia. In the early phases of sexual
stimulation, parasympathetic signals dilate the arteries
of the erectile tissue, probably resulting from release
of acetylcholine, nitric oxi

esulting from release
of acetylcholine, nitric oxide, and vasoactive intestinal
polypeptide (VIP) at the nerve endings. This allows
rapid accumulation of blood in the erectile tissue so
that the introitus tightens around the penis; this aids
the male greatly in his attainment of sufﬁcient sexual
stimulation for ejaculation to occur.
Parasympathetic signals also pass to the bilateral
Bartholin’s glands located beneath the labia minora
and cause them to secrete mucus immediately inside
the introit

em to secrete mucus immediately inside
the introitus.This mucus is responsible for much of the
lubrication during sexual intercourse, although much
is also provided by mucus secreted by the vaginal
epithelium and a small amount from the male urethral
glands.This lubrication is necessary during intercourse
to establish a satisfactory massaging sensation rather
than an irritative sensation, which may be provoked 
by a dry vagina. A massaging sensation constitutes 
the optimal stimulus for evoking 

ion constitutes 
the optimal stimulus for evoking the appropriate
reﬂexes that culminate in both the male and female 
climaxes.
Female Orgasm. When local sexual stimulation reaches
maximum intensity, and especially when the local 
sensations are supported by appropriate psychic 
conditioning signals from the cerebrum, reﬂexes are
initiated that cause the female orgasm, also called 
the female climax. The female orgasm is analogous to
emission and ejaculation in the male, and it may help
1024
Uni

 ejaculation in the male, and it may help
1024
Unit XIV
promote fertilization of the ovum. Indeed, the human
female is known to be somewhat more fertile when
inseminated by normal sexual intercourse rather than
by artiﬁcial methods, thus indicating an important
function of the female orgasm. Possible reasons for
this are as follows.
First, during the orgasm, the perineal muscles of the
female contract rhythmically, which results from spinal
cord reﬂexes similar to 

which results from spinal
cord reﬂexes similar to those that cause ejaculation in
the male. It is possible that these reﬂexes increase
uterine and fallopian tube motility during the orgasm,
thus helping to transport the sperm upward through
the uterus toward the ovum; information on this
subject is scanty, however. Also, the orgasm seems to
cause dilation of the cervical canal for up to 30
minutes, thus allowing easy transport of the sperm.
Second, in many lower animals, copulation causes
the po

d, in many lower animals, copulation causes
the posterior pituitary gland to secrete oxytocin; this
effect is probably mediated through the brain amyg-
daloid nuclei and then through the hypothalamus to
the pituitary. The oxytocin causes increased rhythmi-
cal contractions of the uterus, which have been postu-
lated to cause increased transport of the sperm. A few
sperm have been shown to traverse the entire length
of the fallopian tube in the cow in about 5 minutes, a
rate at least 10 times as 

w in about 5 minutes, a
rate at least 10 times as fast as that which the swim-
ming motions of the sperm themselves could possibly
achieve. Whether this occurs in the human female is
unknown.
In addition to the possible effects of the orgasm on
fertilization, the intense sexual sensations that develop
during the orgasm also pass to the cerebrum and cause
intense muscle tension throughout the body. But after
culmination of the sexual act, this gives way during the
succeeding minutes to a sense of

es way during the
succeeding minutes to a sense of satisfaction charac-
terized by relaxed peacefulness, an effect called 
Fertile Period of Each Sexual Cycle. The ovum remains
viable and capable of being fertilized after it is expelled
from the ovary probably no longer than 24 hours.There-
fore, sperm must be available soon after ovulation if fer-
tilization is to take place.A few sperm can remain fertile
in the female reproductive tract for up to 5 days.There-
fore

le reproductive tract for up to 5 days.There-
fore, for fertilization to take place, intercourse must
occur sometime between 4 and 5 days before ovulation
up to a few hours after ovulation. Thus, the period of
female fertility during each month is short, about 4 to 5
days.
Rhythm Method of Contraception. One of the commonly
practiced methods of contraception is to avoid inter-
course near the time of ovulation. The difﬁculty with
this method of contraception is predicting the exact
time of ovula

ontraception is predicting the exact
time of ovulation. Yet the interval from ovulation until
the next succeeding onset of menstruation is almost
always between 13 and 15 days. Therefore, if the men-
strual cycle is regular, with an exact periodicity of 28
days, ovulation usually occurs within 1 day of the 14th
day of the cycle. If, in contrast, the periodicity of the
cycle is 40 days, ovulation usually occurs within 1 day of
the 26th day of the cycle. Finally, if the periodicity of the
cycle is

cycle. Finally, if the periodicity of the
cycle is 21 days, ovulation usually occurs within 1 day of
the 7th day of the cycle. Therefore, it is usually stated
that avoidance of intercourse for 4 days before the cal-
culated day of ovulation and 3 days afterward prevents
conception. But such a method of contraception can be
used only when the periodicity of the menstrual cycle is
regular.
Hormonal Suppression of Fertility—“The Pill.” It has long
been known that administration of either estrogen o

een known that administration of either estrogen or
progesterone, if given in appropriate quantities during
the ﬁrst half of the monthly cycle, can inhibit ovulation.
The reason for this is that appropriate administration of
either of these hormones can prevent the preovulatory
surge of LH secretion by the pituitary gland, which is
essential in causing ovulation.
Why the administration of estrogen or progesterone
prevents the preovulatory surge of LH secretion is not
fully understood. However, e

f LH secretion is not
fully understood. However, experimental work has sug-
gested that immediately before the surge occurs, there
is probably a sudden depression of estrogen secretion
by the ovarian follicles, and this might be the necessary
signal that causes the subsequent feedback effect on the
anterior pituitary that leads to the LH surge.The admin-
istration of sex hormones (estrogens or progesterone)
could prevent the initial ovarian hormonal depression
that might be the initiating signal

nal depression
that might be the initiating signal for ovulation.
The problem in devising methods for the hormonal
suppression of ovulation has been in developing appro-
priate combinations of estrogens and progestins that
suppress ovulation but do not cause other, unwanted
effects. For instance, too much of either hormone can
cause abnormal menstrual bleeding patterns. However,
use of certain synthetic progestins in place of proges-
terone, especially the 19-norsteroids, along with small
amount

cially the 19-norsteroids, along with small
amounts of estrogens usually prevents ovulation yet
allows an almost normal pattern of menstruation.
Therefore, almost all “pills” used for the control of fer-
tility consist of some combination of synthetic estrogens
and synthetic progestins.The main reason for using syn-
thetic estrogens and progestins is that the natural hor-
mones are almost entirely destroyed by the liver within
a short time after they are absorbed from the gastroin-
testinal trac

they are absorbed from the gastroin-
testinal tract into the portal circulation. However,
many of the synthetic
hormones can resist this 
destructive propensity of the liver, thus allowing oral
Two of the most commonly used synthetic estrogens
are ethinyl estradiol and mestranol. Among the most
commonly used progestins are norethindrone, norethyn-
odrel, ethynodiol, and norgestrel. The drug is usually
begun in the early stages of the monthly cycle and con-
tinued beyond the time 

the monthly cycle and con-
tinued beyond the time that ovulation would normally
occur. Then the drug is stopped, allowing menstruation
to occur and a new cycle to begin.
Abnormal Conditions That Cause Female Sterility. About 5 to
10 per cent of women are infertile. Occasionally, no
abnormality can be discovered in the female genital
organs, in which case it must be assumed that the infer-
tility is due to either abnormal physiological function of
the genital system or abnormal genetic developmen

the genital system or abnormal genetic development of
the ova themselves.
Probably by far the most common cause of female
sterility is failure to ovulate. This can result from
hyposecretion of gonadotropic hormones, in which 
case the intensity of the hormonal stimuli is simply
Chapter 81
1025
insufﬁcient to cause ovulation, or it can result from
abnormal ovaries that do not allow ovulation. For
instance, thick ovarian capsules occasionally

 For
instance, thick ovarian capsules occasionally exist on the
outsides of the ovaries, making ovulation difﬁcult.
Because of the high incidence of anovulation in
sterile women, special methods are often used to 
determine whether ovulation occurs.These methods are
based mainly on the effects of progesterone on the
body, because the normal increase in progesterone
secretion usually does not occur during the latter half
of anovulatory cycles. In the absence of progestational
effects, the cycle c

the absence of progestational
effects, the cycle can be assumed to be anovulatory.
One of these tests is simply to analyze the urine for a
surge in pregnanediol, the end product of progesterone
metabolism, during the latter half of the sexual cycle;
the lack of this substance indicates failure of ovulation.
Another common test is for the woman to chart her
body temperature throughout the cycle. Secretion of
progesterone during the latter half of the cycle raises
the body temperature about 0.5°F,

the cycle raises
the body temperature about 0.5°F, with the temperature
rise coming abruptly at the time of ovulation. Such a
temperature chart, showing the point of ovulation, is
illustrated in Figure 81–11.
Lack of ovulation caused by hyposecretion of the
pituitary gonadotropic hormones can sometimes be
treated by appropriately timed administration of hu-
man chorionic gonadotropin, a hormone (discussed in
Chapter 82) that is extracted from the human placenta.
This hormone, although secreted b

 human placenta.
This hormone, although secreted by the placenta, has
almost the same effects as LH and is therefore a pow-
erful stimulator of ovulation. However, excess use of
this hormone can cause ovulation from many follicles
simultaneously; this results in multiple births, an effect
that has caused as many as eight babies (mostly 
stillborn) to be born to mothers treated for infertility
with this hormone.
One of the most common causes of female sterility is
endometriosis, a common conditio

male sterility is
endometriosis, a common condition in which endome-
trial tissue almost identical to that of the normal uterine
endometrium grows and even menstruates in the pelvic
cavity surrounding the uterus, fallopian tubes, and
ovaries. Endometriosis causes ﬁbrosis throughout the
pelvis, and this ﬁbrosis sometimes so enshrouds 
the ovaries that an ovum cannot be released into the
abdominal cavity. Often, endometriosis occludes the fal-
lopian tubes, either at the ﬁmbriated ends or elsewher

an tubes, either at the ﬁmbriated ends or elsewhere
along their extent.
Another common cause of female infertility is salp-
ingitis, that is, inﬂammation of the fallopian tubes; this
causes ﬁbrosis in the tubes, thereby occluding them. In
the past, such inﬂammation occurred mainly as a result
of gonococcal infection, but with modern therapy,
this is becoming a less prevalent cause of female 
Still another cause of infertility is secretion of abnor-
mal mucus by the uterine cervix. O

etion of abnor-
mal mucus by the uterine cervix. Ordinarily, at the time
of ovulation, the hormonal environment of estrogen
causes the secretion of mucus with special charac-
teristics that allow rapid mobility of sperm into the
uterus and actually guide the sperm up along mucous
“threads.” Abnormalities of the cervix itself, such as
low-grade infection or inﬂammation, or abnormal hor-
monal stimulation of the cervix can lead to a viscous
mucous plug that prevents fertilization.
Altko

plug that prevents fertilization.
Bastian LA, Smith CM, Nanda K: Is this woman peri-
menopausal? JAMA 289:895, 2003.
Beral V, Banks E, Reeves G: Evidence from randomised
trials on the long-term effects of hormone replacement
therapy. Lancet 360:942, 2002.
2001.
Gruber CJ, Tschugguel W, Schneeberger C, Huber JC: Pro-
duction and actions of estroge

r C, Huber JC: Pro-
duction and actions of estrogens. N Engl J Med 346:340,
2002.
Hamilton-Fairley D, Taylor A: Anovulation. BMJ 327:546,
2003.
Lorenzo J: A new hypothesis for how sex steroid hormones
regulate bone mass. J Clin Invest 111:1641, 2003.
Manson JE,
Martin KA:
Postmenopausal hormone-
replacement therapy. N Engl J Med 345:34, 2001.
Nadal A, Diaz M, Valverde MA: The estrogen trinity:
membrane, cytosolic, and nuclear effects. News Physiol Sci
Nelson HD: Commonly used types

l Sci
Nelson HD: Commonly used types of postmenopausal estro-
gen for treatment of hot ﬂashes: scientiﬁc review. JAMA
291:1610, 2004.
Nilsson S, Makela S, Treuter E, et al: Mechanisms of estro-
gen action. Physiol Rev 81:1535, 2001.
Niswender GD, Juengel JL, Silva PJ, et al: Mechanisms con-
trolling the function and life span of the corpus luteum.
Physiol Rev 80:1, 2000.
Petitti DB: Combination estrogen-progestin oral contracep-
tives. N Engl J Med 349:1443, 2003.
Pettersson K, Gus

es. N Engl J Med 349:1443, 2003.
Pettersson K, Gustafsson JA: Role of estrogen receptor beta
in estrogen action. Annu Rev Physiol 63:165, 2001.
Riggs BL: The mechanisms of estrogen regulation of bone
resorption. J Clin Invest 106:1203, 2000.
Riggs BL, Hartmann LC: Selective estrogen-receptor mod-
ulators—mechanisms of action and application to clinical
practice. N Engl J Med 348:618, 2003.
Smith S, Pfeifer SM, Collins JA: Diagnosis and management
of female infertility. JAMA 290:1767, 2003.
0
2
4

0
2
4
6
8 10 12 14 16 18 20 22 24 26 28
0
99°
98°
97°
Day of cycle
Ovulation
Figure 81–11
Elevation in body temperature shortly after ovulation.
1026
Unit XIV
Terasawa E, Fernandez DL: Neurobiological mechanisms 
of the onset of puberty in primates. Endocr Rev 22:111,
2001.
Toran-Allerand CD: A plethora of estrogen receptors in 
the brain: where will it end? Endocrinology 145:1069,
2004.
van der Eer

 it end? Endocrinology 145:1069,
2004.
van der Eerden BC, Karperien M, Wit JM: Systemic and
local regulation of the growth plate. Endocr Rev 24:782,
2003.
C
H
A
P
T
E
R
 
8
2
1027
In Chapters 80 and 81, the sexual functions of the
male and female are described to the point of 
fertilization of the ovum. If the ovum 

 point of 
fertilization of the ovum. If the ovum becomes 
fertilized, a new sequence of events called gestation,
or pregnancy, takes place, and the fertilized ovum
eventually develops into a full-term fetus. The
purpose of this chapter is to discuss the early stages
of ovum development after fertilization and then to
discuss the physiology of pregnancy. In Chapter 83, some special aspects of fetal
and early childhood physiology are discussed.
While still

turation and Fertilization of the Ovum
While still in the ovary, the ovum is in the primary oocyte stage. Shortly before
it is released from the ovarian follicle, its nucleus divides by meiosis and a ﬁrst
polar body is expelled from the nucleus of the oocyte. The primary oocyte then
becomes the secondary oocyte. In this process, each of the 23 pairs of chromo-
somes loses one of its partners, which becomes incorporated in a polar body
that is expelled. This leaves 23 unpaired chromosomes in the 

elled. This leaves 23 unpaired chromosomes in the secondary oocyte.
It is at this time that the ovum, still in the secondary oocyte stage, is ovulated
into the abdominal cavity. Then, almost immediately, it enters the ﬁmbriated
end of one of the fallopian tubes.
Entry of the Ovum into the Fallopian Tube (Oviduct). When ovulation occurs, the ovum,
along with a hundred or more attached granulosa cells that constitute the
corona radiata, is expelled directly into the peritoneal cavity and must then

 directly into the peritoneal cavity and must then
enter one of the fallopian tubes to reach the cavity of the uterus. The ﬁmbri-
ated ends of each fallopian tube fall naturally around the ovaries.The inner sur-
faces of the ﬁmbriated tentacles are lined with ciliated epithelium, and the cilia
are activated by estrogen from the ovaries, which causes the cilia to beat toward
the opening, or ostium, of the involved fallopian tube. One can actually see a
slow ﬂuid current ﬂowing toward the ostium. 

see a
slow ﬂuid current ﬂowing toward the ostium. By this means, the ovum enters
one of the fallopian tubes.
It seems likely that many ova might fail to enter the fallopian tubes. However,
on the basis of conception studies, it is probable that as many as 98 per cent
succeed in this task. Indeed, in some recorded cases, women with one ovary
removed and the opposite fallopian tube removed have had several children
with relative ease of conception, thus demonstrating that ova can even enter
the op

 thus demonstrating that ova can even enter
the opposite fallopian tube.
Fertilization of the Ovum. After the male ejaculates semen into the vagina during
intercourse, a few sperm are transported within 5 to 10 minutes upward from
the vagina and through the uterus and fallopian tubes to the ampullae of the
fallopian tubes near the ovarian ends of the tubes. This transport of the sperm
is aided by contractions of the uterus and fallopian tubes stimulated by
prostaglandins in the male seminal ﬂuid

mulated by
prostaglandins in the male seminal ﬂuid and also by oxytocin released from the
posterior pituitary gland of the female during her orgasm. Of the almost half a
billion sperm deposited in the vagina, a few thousand succeed in reaching each
ampulla.
Fertilization of the ovum normally takes place in the ampulla of one of the
fallopian tubes soon after both the sperm and the ovum enter the ampulla. But
1028
Unit XIV
before a sperm can enter the ovum, it must 

uction
before a sperm can enter the ovum, it must ﬁrst pene-
trate the multiple layers of granulosa cells attached to
the outside of the ovum (the corona radiata) and then
bind to and penetrate the zona pellucida surrounding
the ovum itself.The mechanisms used by the sperm for
these purposes are presented in Chapter 80.
Once a sperm has entered the ovum (which is still
in the secondary oocyte stage of development), the
oocyte divides again to form the mature ovum plus 
a second polar body that i

m the mature ovum plus 
a second polar body that is expelled. The mature 
ovum still carries in its nucleus (now called the female
pronucleus) 23 chromosomes. One of these chromo-
somes is the female chromosome, known as the X
In the meantime, the fertilizing sperm has also
changed. On entering the ovum, its head swells to form
a male pronucleus, shown in Figure 82–1D. Later, the
23 unpaired chromosomes of the male pronucleus and
the 23 unpaired chromosomes of the female pronu-
cleus

23 unpaired chromosomes of the female pronu-
cleus align themselves to re-form a complete comple-
ment of 46 chromosomes (23 pairs) in the fertilized
ovum (see Figure 82–1E).
What Determines the Sex of the Fetus That Is Created? After
formation of the mature sperm, half of these carry in
their genome an X chromosome (the female chromo-
some) and half carry a Y chromosome (the male 
chromosome). Therefore, if an X chromosome from 
a sperm combines with an X chromosome from an
ovum, giving an XX c

 with an X chromosome from an
ovum, giving an XX combination, a female child will
be born, as explained in Chapter 80. But if a Y chro-
mosome from a sperm is paired with an X chromo-
some from an ovum,giving an XY combination,a male
child will be born.
Transport of the Fertilized Ovum 
in the Fallopian Tube
After fertilization has occurred, an additional 3 to 5
days is normally required for transport of the fertilized
ovum through the remainder of the fallopian tube into
the cavity of the uteru

of the fallopian tube into
the cavity of the uterus (Figure 82–2).This transport is
effected mainly by a feeble ﬂuid current in the tube
resulting from epithelial secretion plus action of the
ciliated epithelium that lines the tube; the cilia always
beat toward the uterus. Weak contractions of the 
fallopian tube may also aid the ovum passage.
The fallopian tubes are lined with a rugged, cryptoid
surface that impedes passage of the ovum despite the
ﬂuid current. Also, the isthmus of the fallopia

he
ﬂuid current. Also, the isthmus of the fallopian tube
(the last 2 centimeters before the tube enters the
uterus) remains spastically contracted for about the
ﬁrst 3 days after ovulation. After this time, the rapidly
increasing progesterone secreted by the ovarian
corpus luteum ﬁrst promotes increasing progesterone
receptors on the fallopian tube smooth muscle cells;
then the progesterone activates the receptors, exerting
a tubular relaxing effect that allows entry of the ovum
into the uterus.

ect that allows entry of the ovum
This delayed transport of the fertilized ovum
through the fallopian tube allows several stages of cell
division to occur before the dividing ovum—now
called a blastocyst, with about 100 cells—enters the
Corona
radiata
Sperm
A
B
C
D
E
Female
pronucleus
Centrosome
Sperm
Male
Dispersed corona radiata
Figure 82–1
Fertilization of the ovum. A, The mature ovum surrounded by the
corona radiata. B, Dispersal of the corona radiata. C, Entry of

a. B, Dispersal of the corona radiata. C, Entry of the
sperm. D, Formation of the male and female pronuclei. E, Reor-
ganization of a full complement of chromosomes and beginning
division of the ovum. (Modiﬁed from Arey LB: Developmental
Anatomy: A Textbook and Laboratory Manual of Embryology, 7th
ed. Philadelphia: WB Saunders, 1974.)
(day 1)
Cell division
Fallopian
tube
Ovary
Ovum
Ovulation
Zygote
Blastocyst
reaches
uterus
(days 4-5)
implants
(days 5-7)
Uteru

us
(days 4-5)
implants
(days 5-7)
Uterus
A
B
Amniotic
cavity
Trophoblastic
cells invading
endometrium
Figure 82–2
A, Ovulation, fertilization of the ovum in the fallopian tube, and
implantation of the blastocyst in the uterus. B, Action of tro-
phoblast cells in implantation of the blastocyst in the uterine
endometrium.
Chapter 82
1029
uterus. During this time, the fallopian tube secretory
cells produce large quantities of secretions used for the
nutrition of t

antities of secretions used for the
nutrition of the developing blastocyst.
Implantation of the Blastocyst 
in the Uterus
After reaching the uterus, the developing blastocyst
usually remains in the uterine cavity an additional 1 to
3 days before it implants in the endometrium; thus,
implantation ordinarily occurs on about the ﬁfth to
seventh day after ovulation. Before implantation,
the blastocyst obtains its nutrition from the uterine
endometrial secretions, called “uterine milk.”
Implantation 

l secretions, called “uterine milk.”
Implantation results from the action of trophoblast
cells that develop over the surface of the blastocyst.
These cells secrete proteolytic enzymes that digest and
liquefy the adjacent cells of the uterine endometrium.
Some of the ﬂuid and nutrients released are actively
transported by the same trophoblast cells into the blas-
tocyst, adding more sustenance for growth. Figure
82–3 shows an early implanted human blastocyst, with
a small embryo. Once implantatio

 blastocyst, with
a small embryo. Once implantation has taken place,
the trophoblast cells and other adjacent cells (from the
blastocyst and the uterine endometrium) proliferate
rapidly, forming the placenta and the various mem-
branes of pregnancy.
In Chapter 81, we pointed out that the progesterone
secreted by the ovarian corpus luteum during the
latter half of each monthly sexual cycle has an 
effect on the uterine endometrium, converting the
endometrial stromal 

e endometrium, converting the
endometrial stromal cells into large swollen cells con-
taining extra quantities of glycogen, proteins, lipids,
and even some minerals necessary for development of
the conceptus. Then, when the conceptus implants in
the endometrium, the continued secretion of proges-
terone causes the endometrial cells to swell further
and to store even more nutrients. These cells are now
called decidual cells, and the total mass of cells is called
the decidua.
As the trophoblast ce

cells is called
As the trophoblast cells invade the decidua, digest-
ing and imbibing it, the stored nutrients in the decidua
are used by the embryo for growth and development.
During the ﬁrst week after implantation, this is the
only means by which the embryo can obtain nutrients;
the embryo continues to obtain at least some of its
nutrition in this way for up to 8 weeks, although the
placenta also begins to provide nutrition after about
the 16th day beyond fertilization (a little 

about
the 16th day beyond fertilization (a little more than 1
week after implantation). Figure 82–4 shows this tro-
phoblastic period of nutrition, which gradually gives
way to placental nutrition.
Developmental and Physiologic
Anatomy of the Placenta
While the trophoblastic cords from the blastocyst are
attaching to the uterus, blood capillaries grow into the
cords from the vascular system of the newly forming
embryo. By the 16th day after fertilization, blood also
begi

 the 16th day after fertilization, blood also
begins to be pumped by the heart of the embryo itself.
Simultaneously, blood sinuses supplied with blood from
the mother develop around the outsides of the tro-
phoblastic cords. The trophoblast cells send out more
and more projections, which become placental villi into
Trophoblasts
Ovum
Figure 82–3
Implantation of the early human embryo, showing trophoblastic
digestion and invasion of the endometrium. (Courtesy Dr. Arthur
Hertig.)
0
4
8

 endometrium. (Courtesy Dr. Arthur
Hertig.)
0
4
8
12
16
20
24
28
32
36
Ovulation
nutrition
Placental
diffusion
40
0
25
50
75
100
Duration of pregnancy (weeks)
Placental membrane conductivity
Figure 82–4
Nutrition of the fetus. Most of the early nutrition is due to tro-
phoblastic digestion and absorption of nutrients from the endome-
trial decidua, and essentially all the later nutrition results from
diffusion through the placental membrane.
1030
U

m
1030
Unit XIV
which fetal capillaries grow.Thus, the villi, carrying fetal
blood, are surrounded by sinuses that contain maternal
blood.
The ﬁnal structure of the placenta is shown in Figure
82–5. Note that the fetus’s blood ﬂows through two
umbilical arteries, then into the capillaries of the villi,
and ﬁnally back through a single umbilical vein into the
fetus. At the same time, the mother’s blood ﬂows from
her uterine 

me time, the mother’s blood ﬂows from
her uterine arteries into large maternal sinuses that sur-
round the villi and then back into the uterine veins of
the mother. The lower part of Figure 82–5 shows the
relation between the fetal blood of each fetal placental
villus and the blood of the mother surrounding the out-
sides of the villus in the fully developed placenta.
The total surface area of all the villi of the mature pla-
centa is only a few square meters—many times less than
the area of the

square meters—many times less than
the area of the pulmonary membrane in the lungs. Nev-
ertheless, nutrients and other substances pass through
this placental membrane mainly by diffusion in much
the same manner that diffusion occurs through the 
alveolar membranes of the lungs and the capillary 
membranes elsewhere in the body.
Placental Permeability and Membrane
Diffusion Conductance
The major function of the placenta is to provide for
diffusion of foodstuffs and oxygen from the mother’s
blood

n of foodstuffs and oxygen from the mother’s
blood into the fetus’s blood and diffusion of excretory
products from the fetus back into the mother.
In the early months of pregnancy, the placental
membrane is still thick because it is not fully devel-
oped. Therefore, its permeability is low. Further, the
surface area is small because the placenta has not
grown signiﬁcantly. Therefore, the total diffusion con-
ductance is minuscule at ﬁrst. Conversely, in later preg-
nancy, the permeability increa

ely, in later preg-
nancy, the permeability increases because of thinning
of the membrane diffusion layers and because the
surface area expands many times over, thus giving the
tremendous increase in placental diffusion shown in
Figure 82–4.
Rarely, “breaks” occur in the placental membrane,
which allows fetal blood cells to pass into the mother
or, even less commonly, the mother’s cells to pass into
the fetus. Fortunately, it is rare for the fetus to bleed
severely into the mother’s circulation 

s to bleed
severely into the mother’s circulation because of a rup-
tured placental membrane.
Diffusion of Oxygen Through the Placental Membrane.
Almost the same principles for diffusion of oxygen
through the pulmonary membrane (discussed in detail
in Chapter 39) are applicable for diffusion of oxy-
gen through the placental membrane. The dissolved
oxygen in the blood of the large maternal sinuses
passes into the fetal blood by simple diffusion, driven
by an oxygen pressure gradient from the mot

driven
by an oxygen pressure gradient from the mother’s
blood to the fetus’s blood. Near the end of pregnancy,
the mean PO2 of the mother’s blood in the placental
sinuses is about 50 mm Hg, and the mean PO2 in the
fetal blood after it becomes oxygenated in the placenta
is about 30 mm Hg. Therefore, the mean pressure gra-
dient for diffusion of oxygen through the placental
membrane is about 20 mm Hg.
One might wonder how it is possible for a fetus to
obtain sufﬁcient oxygen when the fetal blood l

 to
obtain sufﬁcient oxygen when the fetal blood leaving
the placenta has a PO2 of only 30 mm Hg. There are
three reasons why even this low PO2 is capable of
allowing the fetal blood to transport almost as much
oxygen to the fetal tissues as is transported by the
mother’s blood to her tissues.
First, the hemoglobin of the fetus is mainly fetal
hemoglobin, a type of hemoglobin synthesized in the
fetus before birth. Figure 82–6 shows the comparative
oxygen dissociation curves for maternal hemoglob

e
oxygen dissociation curves for maternal hemoglobin
and fetal hemoglobin,demonstrating that the curve for
fetal hemoglobin is shifted to the left of that for mater-
nal hemoglobin. This means that at the low PO2 levels
in fetal blood, the fetal hemoglobin can carry 20 to 50
per cent more oxygen than maternal hemoglobin can.
Second, the hemoglobin concentration of fetal blood
is about 50 per cent greater than that of the mother; this
is an even more important factor in enhancing the
amount of ox

ore important factor in enhancing the
amount of oxygen transported to the fetal tissues.
Third, the Bohr effect, which is explained in relation
to the exchange of carbon dioxide and oxygen in the
lung in Chapter 40, provides another mechanism to
Villus
Maternal
vessels
Marginal
sinus
VILLUS
PLACENTA
Chorion
Amnion
Umbilical arteries
Umbilical vein
Umbilical cord
Fetal capillaries
Intervillous space
Chorionic epithelium
Intravillus
space
Limiting layer
Stratum spongiosum
Placental sep

ce
Placental septum
To the mother
From the mother
Figure 82–5
Above, Organization of the mature placenta. Below, Relation of the
fetal blood in the villus capillaries to the mother’s blood in the inter-
villous spaces. (Modiﬁed from Gray H, Goss CM: Anatomy of the
Human Body, 25th ed. Philadelphia: Lea & Febiger, 1948; and
from Arey LB: Developmental Anatomy: A Textbook and Labora-
tory Manual of Embryology, 7th ed. Philadelphia: WB Saunders,
1974.)
Chapter 82
P

1974.)
Chapter 82
1031
enhance the transport of oxygen by fetal blood. That
is, hemoglobin can carry more oxygen at a low PCO2
than it can at a high PCO2.The fetal blood entering the
placenta carries large amounts of carbon dioxide, but
much of this carbon dioxide diffuses from the fetal
blood into the maternal blood. Loss of the carbon
dioxide makes the fetal blood more alkaline, whereas
the increased carbon dioxide in the maternal blood
ma

 increased carbon dioxide in the maternal blood
makes it more acidic.
These changes cause the capacity of fetal blood to
combine with oxygen to increase and that of maternal
blood to decrease. This forces still more oxygen from
the maternal blood, while enhancing oxygen uptake by
the fetal blood. Thus, the Bohr shift operates in one
direction in the maternal blood and in the other direc-
tion in the fetal blood. These two effects make the
Bohr shift twice as important here as it is for oxygen
ex

ift twice as important here as it is for oxygen
exchange in the lungs; therefore, it is called the double
Bohr effect.
By these three means, the fetus is capable of receiv-
ing more than adequate oxygen through the placental
membrane, despite the fact that the fetal blood leaving
the placenta has a PO2 of only 30 mm Hg.
The total diffusing capacity of the entire placenta for
oxygen at term is about 1.2 milliliters of oxygen per
minute per millimeter of mercury oxygen pressure dif-
ference across

ter of mercury oxygen pressure dif-
ference across the membrane.This compares favorably
with that of the lungs of the newborn baby.
Diffusion of Carbon Dioxide Through the Placental Membrane.
Carbon dioxide is continually formed in the tissues of
the fetus in the same way that it is formed in maternal
tissues, and the only means for excreting the carbon
dioxide from the fetus is through the placenta into the
mother’s blood. The PCO2 of the fetal blood is 2 to 3
mm Hg higher than that of the mate

blood is 2 to 3
mm Hg higher than that of the maternal blood. This
small pressure gradient for carbon dioxide across the
membrane is more than sufﬁcient to allow adequate
diffusion of carbon dioxide, because the extreme sol-
ubility of carbon dioxide in the placental membrane
allows carbon dioxide to diffuse about 20 times as
rapidly as oxygen.
Diffusion of Foodstuffs Through the Placental Membrane.
Other metabolic substrates needed by the fetus 
diffuse into the fetal blood in the same manner a

diffuse into the fetal blood in the same manner as
oxygen does. For instance, in the late stages of 
pregnancy, the fetus often uses as much glucose as 
the entire body of the mother uses. To provide this
much glucose, the trophoblast cells lining the placen-
tal villi provide for facilitated diffusion of glucose
through the placental membrane. That is, the glucose
is transported by carrier molecules in the trophoblast
cells of the membrane. Even so, the glucose level in
fetal blood is 20 to 30

n so, the glucose level in
fetal blood is 20 to 30 per cent lower than that in
maternal blood.
Because of the high solubility of fatty acids in cell
membranes, these also diffuse from the maternal
blood into the fetal blood, but more slowly than
glucose, so that glucose is used more easily by the fetus
for nutrition. Also, such substances as ketone bodies
and potassium, sodium, and chloride ions diffuse with
relative ease from the maternal blood into the fetal
blood.
Excretion of Waste Products 

into the fetal
blood.
Excretion of Waste Products Through the Placental Membrane.
In the same manner that carbon dioxide diffuses 
from the fetal blood into the maternal blood, other
excretory products formed in the fetus also diffuse
through the placental membrane into the maternal
blood and are then excreted along with the excretory
products of the mother. These include especially the
nonprotein nitrogens such as urea, uric acid, and crea-
tinine. The level of urea in fetal blood is only sligh

ne. The level of urea in fetal blood is only slightly
greater than that in maternal blood, because urea dif-
fuses through the placental membrane with great ease.
However, creatinine, which does not diffuse as easily,
has a fetal blood concentration considerably higher
than that in the mother’s blood. Therefore, excretion
from the fetus occurs mainly, if not entirely, as a result
of diffusion gradients across the placental membrane,
because there are higher concentrations of the excre-
tory prod

 are higher concentrations of the excre-
tory products in the fetal blood than in the maternal
blood.
Hormonal Factors 
in Pregnancy
In pregnancy, the placenta forms especially large 
quantities of human chorionic gonadotropin,estrogens,
progesterone,
and human chorionic somatomam-
motropin, the ﬁrst three of which, and probably 
the fourth as well, are all essential to a normal 
0
20
40
Fetal
Maternal
Human
60
80
100
0
100
80
60
40
20
PO2 (mm Hg)
Oxyhemoglobin (per cent)
Figure 82–6

PO2 (mm Hg)
Oxygen-hemoglobin dissociation curves for maternal and fetal
blood, showing that fetal blood can carry a greater quantity of
oxygen than can maternal blood for a given blood PO2. (Data from
Metcalfe J, Moll W, Bartels H: Gas exchange across the placenta.
Fed Proc 23:775, 1964.)
1032
Unit XIV
Human Chorionic Gonadotropin 
and Its Effect to Cause Persistence 
of the Corpus Luteum and 
to Prevent Menstruation
Menstruat

rpus Luteum and 
Menstruation normally occurs in a nonpregnant
woman about 14 days after ovulation, at which time
most of the endometrium of the uterus sloughs away
from the uterine wall and is expelled to the exterior.
If this should happen after an ovum has implanted,
the pregnancy would terminate. However, this is 
prevented by the secretion of human chorionic
gonadotropin by the newly developing embryonic
tissues in the following manner.
Coincidental with the developm

e following manner.
Coincidental with the development of the tro-
phoblast cells from the early fertilized ovum, the
hormone human chorionic gonadotropin is secreted by
the syncytial trophoblast cells into the ﬂuids of the
mother, as shown in Figure 82–7. The secretion of this
hormone can ﬁrst be measured in the blood 8 to 9 days
after ovulation, shortly after the blastocyst implants in
the endometrium. Then the rate of secretion rises
rapidly to reach a maximum at about 10 to 12 weeks
of pregna

 reach a maximum at about 10 to 12 weeks
of pregnancy and decreases back to a lower value by
16 to 20 weeks. It continues at this level for the remain-
der of pregnancy.
Function 
of 
Human 
Chorionic 
Gonadotropin. Human 
chorionic gonadotropin is a glycoprotein having a
molecular weight of about 39,000 and much the 
same molecular structure and function as luteinizing
hormone secreted by the pituitary gland. By far, its
most important function is to prevent involution of the
corpus luteum at t

is to prevent involution of the
corpus luteum at the end of the monthly female sexual
cycle. Instead, it causes the corpus luteum to secrete
even larger quantities of its sex hormones—proges-
terone and estrogens—for the next few months. These
sex hormones prevent menstruation and cause the
endometrium to continue to grow and store large
amounts of nutrients rather than being shed in the
menstruum. As a result, the decidua-like cells that
develop in the endometrium during the normal female
sexua

 in the endometrium during the normal female
sexual cycle become actual decidual cells—greatly
swollen and nutritious—at about the time that the
blastocyst implants.
Under 
the 
inﬂuence 
of 
human 
chorionic
gonadotropin, the corpus luteum in the mother’s ovary
grows to about twice its initial size by a month or so
after pregnancy begins, and its continued secretion of
estrogens and progesterone maintains the decidual
nature of the uterine endometrium, which is necessary
for the early developme

etrium, which is necessary
for the early development of the fetus.
If the corpus luteum is removed before approxi-
mately the 7th week of pregnancy, spontaneous 
abortion almost always occurs, sometimes even up to
the 12th week. After that time, the placenta secretes
sufﬁcient quantities of progesterone and estrogens to
maintain pregnancy for the remainder of the gestation
period. The corpus luteum involutes slowly after the
13th to 17th week of gestation.
Effect of Human Chorionic Gonadotropin 

Effect of Human Chorionic Gonadotropin on the Fetal
Testes. Human chorionic gonadotropin also exerts an
interstitial cell–stimulating effect on the testes of the
male fetus, resulting in the production of testosterone
in male fetuses until the time of birth.This small secre-
tion of testosterone during gestation is what causes the
fetus to grow male sex organs instead of female
organs. Near the end of pregnancy, the testosterone
secreted by the fetal testes also causes the testes to
d

ed by the fetal testes also causes the testes to
descend into the scrotum.
Secretion of Estrogens 
by the Placenta
The placenta, like the corpus luteum, secretes both
estrogens and progesterone.
Histochemical and 
0
4
8
12
16
20
Human chorionic
Estrogens
24
28
32
36
40
Ovulation
0
0
24
22
20
18
16
14
12
10
8
6
4
2
0
300
200
100
120
100
80
60
40
20
Human chorionic gonadotropin (IU/mL)
Estrogens (mg/24 hr estradiol equivalent)
Progesterone (m

ns (mg/24 hr estradiol equivalent)
Progesterone (mg/24 hr)
Figure 82–7
Rates of secretion of estrogens
and 
and 
con-
centration of human chorionic
gonadotropin at different stages
Chapter 82
1033
physiological studies show that these two hormones,
like most other placental hormones, are secreted by
the syncytial trophoblast cells of the placenta.
Figure 82–7 shows that toward the end of pregnancy,
the daily production of placental

nd of pregnancy,
the daily production of placental estrogens in-
creases to about 30 times the mother’s normal level 
of production. However, the secretion of estrogens 
by the placenta is quite different from secretion by 
the ovaries. Most important, the estrogens secreted 
by the placenta are not synthesized de novo from 
basic substrates in the placenta. Instead, they are
formed almost entirely from androgenic steroid 
compounds,
dehydroepiandrosterone 
and 
16-
hydroxydehydroepiandrosterone

drosterone 
and 
16-
hydroxydehydroepiandrosterone, which are formed
both in the mother’s adrenal glands and in the adrenal
glands of the fetus. These weak androgens are trans-
ported by the blood to the placenta and converted by
the trophoblast cells into estradiol, estrone, and estriol.
(The cortices of the fetal adrenal glands are extremely
large, and about 80 per cent consists of a so-called 
fetal zone, the primary function of which seems to 
be 
to 
secrete 
during 

be 
to 
secrete 
during 
pregnancy.)
Function of Estrogen in Pregnancy. In the discussions of
estrogens in Chapter 81, we pointed out that these hor-
mones exert mainly a proliferative function on most
reproductive and associated organs of the mother.
During pregnancy, the extreme quantities of estrogens
cause (1) enlargement of the mother’s uterus, (2)
enlargement of the mother’s breasts and growth of the
breast ductal structure, and (3) enlargement of the
mother’s fem

structure, and (3) enlargement of the
mother’s female external genitalia.
The estrogens also relax the pelvic ligaments 
of the mother, so that the sacroiliac joints become 
relatively limber and the symphysis pubis becomes
elastic.These changes allow easier passage of the fetus
through the birth canal. There is much reason to
believe that estrogens also affect many general aspects
of fetal development during pregnancy, for example,
by affecting the rate of cell reproduction in the early
embryo.

the rate of cell reproduction in the early
embryo.
Secretion of Progesterone 
Progesterone is also essential for a successful 
pregnancy—in fact, it is just as important as estrogen.
In addition to being secreted in moderate quantities
by the corpus luteum at the beginning of pregnancy, it
is secreted later in tremendous quantities by the 
placenta, averaging about a 10-fold increase during the
course of pregnancy, as shown in Figure 82–7.
The special effects of progesterone that

ure 82–7.
The special effects of progesterone that are essen-
tial for the normal progression of pregnancy are as
follows:
1. Progesterone causes decidual cells to develop 
in the uterine endometrium, and these cells play
an important role in the nutrition of the early
embryo.
2. Progesterone decreases the contractility 
of the pregnant uterus, thus preventing uterine
contractions from causing spontaneous 
abortion.
3. Progesterone contributes to the development of
the conceptus even before impl

 the development of
the conceptus even before implantation, because
it speciﬁcally increases the secretions of the
mother’s fallopian tubes and uterus to provide
appropriate nutritive matter for the developing
morula and blastocyst. There is also reason to
believe that progesterone affects cell cleavage in
the early developing embryo.
4. The progesterone secreted during pregnancy helps
the estrogen prepare the mother’s breasts for
lactation, which is discussed later in this chapter.
Human Chorio

h is discussed later in this chapter.
Human Chorionic
Somatomammotropin
A more recently discovered placental hormone is
called human chorionic somatomammotropin. It is a
protein with a molecular weight of about 38,000, and
it begins to be secreted by the placenta at about the
ﬁfth week of pregnancy. Secretion of this hormone
increases progressively throughout the remainder 
of pregnancy in direct proportion to the weight of 
the placenta. Although the functions of chorionic
somatomammotropin are

h the functions of chorionic
somatomammotropin are uncertain, it is secreted in
quantities several times greater than all the other preg-
nancy hormones combined. It has several possible
important effects.
First, when administered to several types of lower
animals,
human 
chorionic 
somatomammotropin
causes at least partial development of the animal’s
breasts and in some instances causes lactation.
Because this was the ﬁrst function of the hormone dis-
covered, it was ﬁrst named human placental 

e dis-
covered, it was ﬁrst named human placental lactogen
and was believed to have functions similar to those of
prolactin. However, attempts to promote lactation in
humans with its use have not been successful.
Second, this hormone has weak actions similar to
those of growth hormone, causing the formation of
protein tissues in the same way that growth hormone
does. It also has a chemical structure similar to that of
growth hormone, but 100 times as much human 
chorionic somatomammotropin as gr

s as much human 
chorionic somatomammotropin as growth hormone 
is required to promote growth.
Third,
human 
chorionic 
causes decreased insulin sensitivity and decreased uti-
lization of glucose in the mother, thereby making
larger quantities of glucose available to the fetus.
Because glucose is the major substrate used by the
fetus to energize its growth, the possible importance
of such a hormonal effect is obvious. Further, the
hormone promotes the release of free fatty acid

he
hormone promotes the release of free fatty acids from
the fat stores of the mother, thus providing this alter-
native source of energy for the mother’s metabolism
during pregnancy. Therefore, it appears that human
chorionic somatomammotropin is a general metabolic
hormone that has speciﬁc nutritional implications for
both the mother and the fetus.
1034
Unit XIV
Other Hormonal Factors 
Almost all the nonsexual endocrine glands of the
mother also reac

nonsexual endocrine glands of the
mother also react markedly to pregnancy. This results
mainly from the increased metabolic load on the mother
but also, to some extent, from the effects of placental
hormones on the pituitary and other glands. Some of the
most notable effects are the following.
Pituitary Secretion. The anterior pituitary gland of the
mother enlarges at least 50 per cent during pregnancy
and increases its production of corticotropin, thy-
rotropin, and prolactin. Conversely, pitui

n, thy-
rotropin, and prolactin. Conversely, pituitary secretion
of 
follicle-stimulating 
hormone 
and 
luteinizing
hormone is almost totally suppressed as a result of the
inhibitory effects of estrogens and progesterone from
the placenta.
Corticosteroid Secretion. The rate of adrenocortical secre-
tion of the glucocorticoids is moderately increased
throughout pregnancy. It is possible that these gluco-
corticoids help mobilize amino acids from the mother’s
tissues so that these can be used for

the mother’s
tissues so that these can be used for synthesis of tissues
in the fetus.
Pregnant women usually have about a twofold
increase in the secretion of aldosterone, reaching a peak
at the end of gestation. This, along with the actions of
estrogens, causes a tendency for even a normal pregnant
woman to reabsorb excess sodium from her renal
tubules and, therefore, to retain ﬂuid, occasionally
leading to pregnancy-induced hypertension.
Secretion by the Thyroid Gland. The mother’s thyroid gla

ion by the Thyroid Gland. The mother’s thyroid gland
ordinarily enlarges up to 50 per cent during pregnancy
and increases its production of thyroxine a correspon-
ding amount. The increased thyroxine production is
caused at least partly by a thyrotropic effect of human
chorionic gonadotropin secreted by the placenta and 
by small quantities of a speciﬁc thyroid-stimulating
hormone, human chorionic thyrotropin, also secreted by
Secretion by the Parathyroid Glands. The mother’s parat

tion by the Parathyroid Glands. The mother’s parathy-
roid glands usually enlarge during pregnancy; this is
especially true if the mother is on a calcium-deﬁcient
diet. Enlargement of these glands causes calcium
absorption from the mother’s bones, thereby maintain-
ing normal calcium ion concentration in the mother’s
extracellular ﬂuid even while the fetus removes calcium
to ossify its own bones. This secretion of parathyroid
hormone is even more intensiﬁed during lactation after
the baby’s birt

 intensiﬁed during lactation after
the baby’s birth, because the growing baby requires
many times more calcium than the fetus does.
Secretion of “Relaxin” by the Ovaries and Placenta. Another
substance besides the estrogens and progesterone, a
hormone called relaxin,is secreted by the corpus luteum
of the ovary and by placental tissues. Its secretion is
increased by a stimulating effect of human chorionic
gonadotropin at the same time that the corpus luteum
and the placenta secrete large quantit

rpus luteum
and the placenta secrete large quantities of estrogens
and progesterone.
Relaxin is a polypeptide having a molecular weight of
about 9000. This hormone, when injected, causes relax-
ation of the ligaments of the symphysis pubis in the
estrous rat and guinea pig. This effect is weak or possi-
bly even absent in pregnant women. Instead, this role is
probably played mainly by the estrogens, which also
cause relaxation of the pelvic ligaments. It has also been
claimed that relaxin soften

ents. It has also been
claimed that relaxin softens the cervix of the pregnant
woman at the time of delivery.
Response of the Mother’s
Body to Pregnancy
Most apparent among the many reactions of the mother
to the fetus and to the excessive hormones of pregnancy
is the increased size of the various sexual organs. For
instance, the uterus increases from about 50 grams to
1100 grams, and the breasts approximately double in
size. At the same time, the vagina enlarges and the
introitus opens more wid

e vagina enlarges and the
introitus opens more widely.Also, the various hormones
can cause marked changes in a pregnant woman’s
appearance, sometimes resulting in the development of
edema, acne, and masculine or acromegalic features.
Weight Gain in the Pregnant Woman
The average weight gain during pregnancy is about 24
pounds, with most of this gain occurring during the last
two trimesters. Of this, about 7 pounds is fetus and 4
pounds is amniotic ﬂuid, placenta, and fetal membranes.
The uterus 

c ﬂuid, placenta, and fetal membranes.
The uterus increases about 2 pounds and the breasts
another 2 pounds, still leaving an average weight
increase of 9 pounds. About 6 pounds of this is extra
ﬂuid in the blood and extracellular ﬂuid, and the
remaining 3 pounds is generally fat accumulation. The
extra ﬂuid is excreted in the urine during the ﬁrst few
days after birth, that is, after loss of the ﬂuid-retaining
hormones from the placenta.
During pregnancy, a woman often has a greatly
increased d

pregnancy, a woman often has a greatly
increased desire for food, partly as a result of removal
of food substrates from the mother’s blood by the fetus
and partly because of hormonal factors.Without appro-
priate prenatal control of diet, the mother’s weight gain
can be as great as 75 pounds instead of the usual 24
pounds.
Metabolism During Pregnancy
As a consequence of the increased secretion of many
hormones during pregnancy,
including thyroxine,
adrenocortical hormones, and the sex hormones, 

e,
adrenocortical hormones, and the sex hormones, the
basal metabolic rate of the pregnant woman increases
about 15 per cent during the latter half of pregnancy.As
a result,she frequently has sensations of becoming over-
heated. Also, owing to the extra load that she is carry-
ing, greater amounts of energy than normal must be
expended for muscle activity.
Nutrition During Pregnancy. By far the greatest growth of
the fetus occurs during the last trimester of pregnancy;
its weight almost doubles 

trimester of pregnancy;
its weight almost doubles during the last 2 months of
pregnancy. Ordinarily, the mother does not absorb suf-
ﬁcient protein, calcium, phosphates, and iron from her
diet during the last months of pregnancy to supply these
extra needs of the fetus. However, anticipating these
extra needs, the mother’s body has already been storing
these substances—some in the placenta, but most in the
normal storage depots of the mother.
If appropriate nutritional elements are not present i

appropriate nutritional elements are not present in
a pregnant woman’s diet, a number of maternal deﬁ-
ciencies can occur, especially in calcium, phosphates,
iron, and the vitamins. For example, about 375 mil-
ligrams of iron is needed by the fetus to form its blood,
and an additional 600 milligrams is needed by the
Chapter 82
1035
mother to form her own extra blood. The normal store
of nonhemoglobin iron in the mother at the outset of
pregnancy is often only 100 milligra

the outset of
pregnancy is often only 100 milligrams and almost never
more than 700 milligrams. Therefore, without sufﬁcient
iron in her food, a pregnant woman usually develops
hypochromic anemia. Also, it is especially important
that she receive vitamin D, because although the total
quantity of calcium used by the fetus is small, calcium
is normally poorly absorbed by the mother’s gastroin-
testinal tract without vitamin D. Finally, shortly before
birth of the baby, vitamin K is often added to 

re
birth of the baby, vitamin K is often added to the
mother’s diet so that the baby will have sufﬁcient pro-
thrombin to prevent hemorrhage, particularly brain
hemorrhage, caused by the birth process.
Changes in the Maternal Circulatory
System During Pregnancy
Blood Flow Through the Placenta, and Cardiac Output During
Pregnancy. About 625 milliliters of blood ﬂows through
the maternal circulation of the placenta each minute
during the last month of pregnancy. This, plus the
general increase in 

 of pregnancy. This, plus the
general increase in the mother’s metabolism, increases
the mother’s cardiac output to 30 to 40 per cent above
normal by the 27th week of pregnancy; then, for reasons
unexplained, the cardiac output falls to only a little
above normal during the last 8 weeks of pregnancy,
despite the high uterine blood ﬂow.
Blood Volume During Pregnancy. The maternal blood
volume shortly before term is about 30 per cent above
normal. This increase occurs mainly during the latter
half

This increase occurs mainly during the latter
half of pregnancy, as shown by the curve of Figure 82–8.
The cause of the increased volume is likely due, at least
in part, to aldosterone and estrogens, which are greatly
increased in pregnancy, and to increased ﬂuid retention
by the kidneys.Also, the bone marrow becomes increas-
ingly active and produces extra red blood cells to go
with the excess ﬂuid volume. Therefore, at the time of
birth of the baby, the mother has about 1 to 2 liters of
extra 

baby, the mother has about 1 to 2 liters of
extra blood in her circulatory system. Only about one
fourth of this amount is normally lost through bleeding
during delivery of the baby, thereby allowing a consid-
erable safety factor for the mother.
Maternal Respiration During Pregnancy
Because of the increased basal metabolic rate of a preg-
nant woman and because of her greater size, the total
amount of oxygen used by the mother shortly before
birth of the baby is about 20 per cent above normal, 

th of the baby is about 20 per cent above normal, and
a commensurate amount of carbon dioxide is formed.
These effects cause the mother’s minute ventilation to
increase. It is also believed that the high levels of prog-
esterone during pregnancy increase the minute ventila-
tion even more, because progesterone increases the
respiratory center’s sensitivity to carbon dioxide. The
net result is an increase in minute ventilation of about
50 per cent and a decrease in arterial PCO2 to several
millim

 and a decrease in arterial PCO2 to several
millimeters of mercury below that in a nonpregnant
woman. Simultaneously, the growing uterus presses
upward against the abdominal contents, and these press
upward against the diaphragm, so that the total excur-
sion of the diaphragm is decreased. Consequently, the
respiratory rate is increased to maintain the extra 
Function of the Maternal Urinary System During Pregnancy
The rate of urine formation by a pregnant woman is
usually slightly 

formation by a pregnant woman is
usually slightly increased because of increased ﬂuid
intake and increased load or excretory products. But in
addition, several special alterations of urinary function
occur.
First, the renal tubules’ reabsorptive capacity for
sodium, chloride, and water is increased as much as 
50 per cent as a consequence of increased production 
of steroid hormones by the placenta and adrenal 
cortex.
Second, the glomerular ﬁltration rate increases as
much as 50 per cent during

ation rate increases as
much as 50 per cent during pregnancy, which tends to
increase the rate of water and electrolyte excretion in
the urine. When all these effects are considered, the
normal pregnant woman ordinarily accumulates only
about 6 pounds of extra water and salt.
Amniotic Fluid and Its Formation
Normally, the volume of amniotic ﬂuid (the ﬂuid inside
the uterus in which the fetus ﬂoats) is between 500 mil-
liliters and 1 liter, but it can be only a few milliliters or
as much as sever

 can be only a few milliliters or
as much as several liters. Isotope studies of the rate of
formation of amniotic ﬂuid show that, on average, the
water in amniotic ﬂuid is replaced once every 3 hours,
and the electrolytes sodium and potassium are replaced
an average of once every 15 hours. A large portion of
the ﬂuid is derived from renal excretion by the fetus.
Likewise, a certain amount of absorption occurs by way
of the gastrointestinal tract and lungs of the fetus.
However, even after in ute

and lungs of the fetus.
However, even after in utero death of a fetus, some
turnover of the amniotic ﬂuid is still present, which indi-
cates that some of the ﬂuid is formed and absorbed
directly through the amniotic membranes.
Preeclampsia and Eclampsia
About 5 per cent of all pregnant women experience a
rapid rise in arterial blood pressure to hypertensive
levels during the last few months of pregnancy. This is
also associated with leakage of large amounts of protein
into the urine. This condi

arge amounts of protein
into the urine. This condition is called preeclampsia or
toxemia of pregnancy. It is often characterized by excess
salt and water retention by the mother’s kidneys and by
weight gain and development of edema and hyperten-
sion in the mother. In addition, there is impaired func-
tion of the vascular endothelium, and arterial spasm
occurs in many parts of the mother’s body, most sig-
niﬁcantly in the kidneys, brain, and liver. Both the renal
blood ﬂow and the glomerular ﬁlt

r. Both the renal
blood ﬂow and the glomerular ﬁltration rate are
decreased, which is exactly opposite to the changes that
occur in the normal pregnant woman. The renal effects
0
4
8
12 16
20
24 28
32 36
40
44
0
6
5
4
(liters)
Figure 82–8
Effect of pregnancy to increase the mother’s blood volume.
1036
Unit XIV
also include thickened glomerular tufts that contain a
protein deposit in the basement membranes.
Vari

 a
Various attempts have been made to prove that
preeclampsia is caused by excessive secretion of pla-
cental or adrenal hormones, but proof of a hormonal
basis is still lacking. Another theory is that preeclamp-
sia results from some type of autoimmunity or allergy
in the mother caused by the presence of the fetus. In
support of this, the acute symptoms usually disappear
within a few days after birth of the baby. There is also
evidence that preeclampsi

 the baby. There is also
evidence that preeclampsia is initiated by insufﬁcient
blood supply to the placenta, resulting in the placenta’s
release of substances that cause widespread dysfunction
of the maternal vascular endothelium. During normal
placental development, the trophoblasts invade the
arterioles of the uterine endometrium and completely
remodel the maternal arterioles into large blood vessels
with low resistance to blood ﬂow. In patients with
preeclampsia, the maternal arterioles fail

ts with
preeclampsia, the maternal arterioles fail to undergo
these adaptive changes, for reasons that are still unclear,
and there is insufﬁcient blood supply to the placenta.
This, in turn, causes the placenta to release various sub-
stances that enter the mother’s circulation and cause
impaired vascular endothelial function, decreased blood
ﬂow to the kidneys, excess salt and water retention, and
increased blood pressure.Although the factors that link
reduced placental blood supply with mater

hat link
reduced placental blood supply with maternal endothe-
lial dysfunction are still uncertain, some experimental
studies suggest a role for increased levels of inﬂamma-
tory cytokines such as tumor necrosis factor-a and 
interleukin-6.
Eclampsia is an extreme degree of preeclampsia,
characterized by vascular spasm throughout the body;
clonic seizures in the mother, sometimes followed by
coma; greatly decreased kidney output; malfunction of
the liver; often extreme hypertension; and a gener

the liver; often extreme hypertension; and a generalized
toxic condition of the body. It usually occurs shortly
before birth of the baby. Without treatment, a high per-
centage of eclamptic mothers die.However,with optimal
and immediate use of rapidly acting vasodilating drugs
to reduce the arterial pressure to normal, followed by
immediate termination of pregnancy—by cesarean
section if necessary—the mortality even in eclamptic
mothers has been reduced to 1 per cent or less.
Increas

reduced to 1 per cent or less.
Increased Uterine Excitability 
Near Term
Parturition means birth of the baby.Toward the end of
pregnancy, the uterus becomes progressively more
excitable, until ﬁnally it develops such strong rhythmi-
cal contractions that the baby is expelled. The exact
cause of the increased activity of the uterus is not
known, but at least two major categories of effects lead
up to the intense contractions responsible for parturi-
tion: (1) progressive hormonal chan

e for parturi-
tion: (1) progressive hormonal changes that cause
increased excitability of the uterine musculature, and
(2) progressive mechanical changes.
Hormonal Factors That Increase 
Uterine Contractility
Increased Ratio of Estrogens to Progesterone. Progesterone
inhibits uterine contractility during pregnancy, thereby
helping to prevent expulsion of the fetus. Conversely,
estrogens have a deﬁnite tendency to increase the
degree of uterine contractility,
partly because 
estrogens increase t

ontractility,
estrogens increase the number of gap junctions
between the adjacent uterine smooth muscle cells, but
also because of other poorly understood effects. Both
progesterone and estrogen are secreted in progres-
sively greater quantities throughout most of preg-
nancy, but from the seventh month onward, estrogen
secretion continues to increase while progesterone
secretion remains constant or perhaps even decreases
slightly. Therefore, it has been postulated that the
estro

. Therefore, it has been postulated that the
estrogen-to-progesterone ratio increases sufﬁciently
toward the end of pregnancy to be at least partly
responsible for the increased contractility of the
uterus.
Effect of Oxytocin on the Uterus. Oxytocin is a hormone
secreted by the neurohypophysis that speciﬁcally
causes uterine contraction (see Chapter 75).There are
four reasons to believe that oxytocin might be impor-
tant in increasing the contractility of the uterus near
term: (1) The uterine mu

tility of the uterus near
term: (1) The uterine muscle increases its oxytocin
receptors and, therefore, increases its responsiveness
to a given dose of oxytocin during the latter few
months of pregnancy. (2) The rate of oxytocin secre-
tion by the neurohypophysis is considerably increased
at the time of labor. (3) Although hypophysectomized
animals can still deliver their young at term, labor is
prolonged.
(4) Experiments in animals indicate 
that irritation or stretching of the uterine cervix, 

t irritation or stretching of the uterine cervix, as
occurs during labor, can cause a neurogenic reﬂex
through the paraventricular and supraoptic nuclei of
the hypothalamus that causes the posterior pituitary
gland (the neurohypophysis) to increase its secretion
of oxytocin.
Effect of Fetal Hormones on the Uterus. The fetus’s pitu-
itary gland secretes increasing quantities of oxytocin,
which might play a role in exciting the uterus.Also, the
fetus’s adrenal glands secrete large quantities of co

us’s adrenal glands secrete large quantities of corti-
sol, another possible uterine stimulant. In addition, the
fetal membranes release prostaglandins in high con-
centration at the time of labor.These, too, can increase
the intensity of uterine contractions.
Mechanical Factors That Increase 
Stretch of the Uterine Musculature. Simply stretching
smooth muscle organs usually increases their contrac-
tility. Further, intermittent stretch, as occurs repeat-
edly in the uterus

tent stretch, as occurs repeat-
edly in the uterus because of fetal movements, can also
elicit smooth muscle contraction. Note especially that
twins are born, on average, 19 days earlier than a single
child, which emphasizes the importance of mechanical
stretch in eliciting uterine contractions.
Stretch or Irritation of the Cervix. There is reason to
believe that stretching or irritating the uterine cervix
is particularly important in eliciting uterine contrac-
tions. For instance, the obstetric

terine contrac-
tions. For instance, the obstetrician frequently induces
labor by rupturing the membranes so that the head of
Chapter 82
1037
the baby stretches the cervix more forcefully than
usual or irritates it in other ways.
The mechanism by which cervical irritation excites
the body of the uterus is not known. It has been 
suggested that stretching or irritation of nerves in the
cervix initiates reﬂexes to the body of the uterus, but
the effect could also result sim

f the uterus, but
the effect could also result simply from myogenic
transmission of signals from the cervix to the body of
the uterus.
Onset of Labor—A Positive Feedback
During most of the months of pregnancy, the uterus
undergoes periodic episodes of weak and slow rhyth-
mical contractions called Braxton Hicks contractions.
These contractions become progressively stronger
toward the end of pregnancy; then they change sud-
denly, within hours, to become exceptionally

 sud-
denly, within hours, to become exceptionally strong
contractions that start stretching the cervix and later
force the baby through the birth canal, thereby causing
parturition.This process is called labor, and the strong
contractions that result in ﬁnal parturition are called
labor contractions.
We do not know what suddenly changes the slow,
weak rhythmicity of the uterus into strong labor con-
tractions. However, based on experience with other
types of physiological control systems, a the

ther
types of physiological control systems, a theory has
been proposed for explaining the onset of labor. The
positive feedback theory suggests that stretching of 
the cervix by the fetus’s head ﬁnally becomes great
enough to elicit a strong reﬂex increase in contractil-
ity of the uterine body. This pushes the baby forward,
which stretches the cervix more and initiates more
positive feedback to the uterine body. Thus, the
process repeats until the baby is expelled. This theory
is shown in Figu

the baby is expelled. This theory
is shown in Figure 82–9, and the observations sup-
porting it are the following.
First, labor contractions obey all the principles of
positive feedback. That is, once the strength of uterine
contraction becomes greater than a critical value, each
contraction leads to subsequent contractions that
become stronger and stronger until maximum effect is
achieved. Referring to the discussion in Chapter 1 of
positive feedback in control systems, one can see that
this is

dback in control systems, one can see that
this is the precise nature of all positive feedback mech-
anisms when the feedback gain becomes greater than
a critical value.
Second, two known types of positive feedback
increase uterine contractions during labor: (1) Stretch-
ing of the cervix causes the entire body of the uterus
to contract, and this contraction stretches the cervix
even more because of the downward thrust of the
baby’s head. (2) Cervical stretching also causes the
pituitary gland t

vical stretching also causes the
pituitary gland to secrete oxytocin, which is another
means for increasing uterine contractility.
To summarize, we can assume that multiple factors
increase the contractility of the uterus toward the 
end of pregnancy. Eventually a uterine contraction
becomes strong enough to irritate the uterus, espe-
cially at the cervix, and this increases uterine 
contractility still more because of positive feedback,
resulting in a second uterine contraction stronger than
th

g in a second uterine contraction stronger than
the ﬁrst, a third stronger than the second, and so forth.
Once these contractions become strong enough to
cause this type of feedback, with each succeeding con-
traction greater than the preceding one, the process
proceeds to completion—all because positive feedback
initiates a vicious circle when the gain of the feedback
is greater than a critical level.
One might ask about the many instances of false
labor, in which the contractions become strong

lse
labor, in which the contractions become stronger and
stronger and then fade away. Remember that for a
vicious circle to continue, each new cycle of the posi-
tive feedback must be stronger than the previous one.
If at any time after labor starts some contractions fail
to re-excite the uterus sufﬁciently, the positive feed-
back could go into a retrograde decline, and the labor
contractions would fade away.
Abdominal Muscle Contractions
During Labor
Once uterine contractions become strong dur

 Labor
Once uterine contractions become strong during
labor, pain signals originate both from the uterus itself
and from the birth canal. These signals, in addition to
causing suffering, elicit neurogenic reﬂexes in the
spinal cord to the abdominal muscles, causing intense
contractions of these muscles. The abdominal contrac-
tions add greatly to the force that causes expulsion of
the baby.
The uterine contractions during labor begin mainly at
the top of the uterine fund

 labor begin mainly at
the top of the uterine fundus and spread downward over
1. Baby's head stretches cervix
2. Cervical stretch excites fundic contraction
3. Fundic contraction pushes baby down and stretches
    cervix some more
4. Cycle repeats over and over again
Figure 82–9
Theory for the onset of intensely strong contractions during labor.
1038
Unit XIV
the body of the uterus.Also, the intensity of contraction
is great in the top and body of the uterus but we

is great in the top and body of the uterus but weak in
the lower segment of the uterus adjacent to the cervix.
Therefore, each uterine contraction tends to force the
baby downward toward the cervix.
In the early part of labor, the contractions might
occur only once every 30 minutes. As labor progresses,
the contractions ﬁnally appear as often as once every 1
to 3 minutes, and the intensity of contraction increases
greatly, with only a short period of relaxation between
contractions. The combine

od of relaxation between
contractions. The combined contractions of the uterine
and abdominal musculature during delivery of the baby
cause a downward force on the fetus of about 25 pounds
during each strong contraction.
It is fortunate that the contractions of labor occur
intermittently, because strong contractions impede or
sometimes even stop blood ﬂow through the placenta
and would cause death of the fetus if the contractions
were continuous. Indeed, overuse of various uterine
stimulants, su

 Indeed, overuse of various uterine
stimulants, such as oxytocin, can cause uterine spasm
rather than rhythmical contractions and can lead to
death of the fetus.
In about 95 per cent of births, the head is the ﬁrst part
of the baby to be expelled, and in most of the remain-
ing instances, the buttocks are presented ﬁrst. The head
acts as a wedge to open the structures of the birth canal
as the fetus is forced downward.
The ﬁrst major obstruction to expulsion of the fetus
is the uterine cervix. T

to expulsion of the fetus
is the uterine cervix. Toward the end of pregnancy, the
cervix becomes soft, which allows it to stretch when
labor contractions begin in the uterus.The so-called ﬁrst
stage of labor is a period of progressive cervical dilation,
lasting until the cervical opening is as large as the head
of the fetus. This stage usually lasts for 8 to 24 hours in
the ﬁrst pregnancy but often only a few minutes after
many pregnancies.
Once the cervix has dilated fully, the fetal membranes

the cervix has dilated fully, the fetal membranes
usually rupture and the amniotic ﬂuid is lost suddenly
through the vagina.Then the fetus’s head moves rapidly
into the birth canal, and with additional force from
above, it continues to wedge its way through the canal
until delivery is effected. This is called the second stage
of labor, and it may last from as little as 1 minute after
many pregnancies to 30 minutes or more in the ﬁrst
Separation and Delivery 
For 10 to 

eparation and Delivery 
For 10 to 45 minutes after birth of the baby, the 
uterus continues to contract to a smaller and smaller
size, which causes a shearing effect between the walls of
the uterus and the placenta,thus separating the placenta
from its implantation site. Separation of the placenta
opens the placental sinuses and causes bleeding. The
amount of bleeding is limited to an average of 350 mil-
liliters by the following mechanism: The smooth muscle
ﬁbers of the uterine 

mechanism: The smooth muscle
ﬁbers of the uterine musculature are arranged in ﬁgures
of eight around the blood vessels as the vessels pass
through the uterine wall. Therefore, contraction of 
the uterus after delivery of the baby constricts the
vessels that had previously supplied blood to the pla-
centa. In addition, it is believed that vasoconstrictor
prostaglandins formed at the placental separation site
cause additional blood vessel spasm.
With each uterine contraction, the mothe

bor Pains
With each uterine contraction, the mother experiences
considerable pain. The cramping pain in early labor 
is probably caused mainly by hypoxia of the uterine
muscle resulting from compression of the blood vessels
in the uterus. This pain is not felt when the visceral
sensory hypogastric nerves, which carry the visceral
sensory ﬁbers leading from the uterus, have been 
sectioned.
However, during the second stage of labor, when the
fetus is being expelled through the birth canal, much
m

 is being expelled through the birth canal, much
more severe pain is caused by cervical stretching, per-
ineal stretching, and stretching or tearing of structures
in the vaginal canal itself. This pain is conducted to the
mother’s spinal cord and brain by somatic nerves
instead of by the visceral sensory nerves.
Involution of the Uterus 
After Parturition
During the ﬁrst 4 to 5 weeks after parturition, the uterus
involutes. Its weight becomes less than half its immedi-
ate postpartum weight with

s than half its immedi-
ate postpartum weight within 1 week, and in 4 weeks, if
the mother lactates, the uterus may become as small as
it was before pregnancy. This effect of lactation results
from the suppression of pituitary gonadotropin and
ovarian hormone secretion during the ﬁrst few months
of lactation, as discussed later. During early involution
of the uterus, the placental site on the endometrial
surface autolyzes, causing a vaginal discharge known as
“lochia,” which is ﬁrst bloody and t

arge known as
“lochia,” which is ﬁrst bloody and then serous in nature,
continuing for a total of about 10 days. After this time,
the endometrial surface becomes re-epithelialized and
ready for normal, nongravid sex life again.
Lactation
The breasts, shown in Figure 82–10, begin to develop
at puberty. This development is stimulated by the
estrogens of the monthly female sexual cycle; estro-
gens stimulate growth of the breasts’ mammary glands
plus the deposition of fat

breasts’ mammary glands
plus the deposition of fat to give the breasts mass. In
addition, far greater growth occurs during the high-
estrogen state of pregnancy, and only then does the
glandular tissue become completely developed for the
production of milk.
Growth of the Ductal System—Role of the Estrogens. All
through pregnancy, the large quantities of estrogens
secreted by the placenta cause the ductal system of the
breasts to grow and branch. Simultaneously, the
stroma of the breasts increase

Simultaneously, the
stroma of the breasts increases in quantity, and large
quantities of fat are laid down in the stroma.
Also important for growth of the ductal system are
at least four other hormones: growth hormone, pro-
lactin, the adrenal glucocorticoids, and insulin. Each of
these is known to play at least some role in protein
metabolism, which presumably explains their function
in the development of the breasts.
Chapter 82
1039
Development of the Lobule-Alveolar Sy

ctation
1039
Development of the Lobule-Alveolar System—Role of Proges-
terone. Final development of the breasts into milk-
secreting organs also requires progesterone. Once the
ductal system has developed, progesterone—acting
synergistically with estrogen, as well as with the other
hormones just mentioned—causes additional growth
of the breast lobules, with budding of alveoli and
development of secretory characteristics in the cells of
the alveoli. These changes are analogous to the 
secretory e

i. These changes are analogous to the 
secretory effects of progesterone on the endometrium
of the uterus during the latter half of the female 
menstrual cycle.
Initiation of Lactation—Function 
of Prolactin
Although estrogen and progesterone are essential for
the physical development of the breasts during preg-
nancy, a speciﬁc effect of both these hormones is to
inhibit the actual secretion of milk. Conversely, the
hormone prolactin has exactly the opposite effect 
on milk secretion—promoting 

 the opposite effect 
on milk secretion—promoting it. This hormone is
secreted by the mother’s anterior pituitary gland, and
its concentration in her blood rises steadily from the
ﬁfth week of pregnancy until birth of the baby, at
which time it has risen to 10 to 20 times the normal
nonpregnant level. This high level of prolactin at the
end of pregnancy is shown in Figure 82–11.
In addition, the placenta secretes large quantities of
human chorionic somatomammotropin, which proba-
bly has lactoge

ic somatomammotropin, which proba-
bly has lactogenic properties, thus supporting the pro-
lactin from the mother’s pituitary during pregnancy.
Even so, because of the suppressive effects of estrogen
and progesterone, no more than a few milliliters of
ﬂuid are secreted each day until after the baby is born.
The ﬂuid secreted during the last few days before and
the ﬁrst few days after parturition is called colostrum;
it contains essentially the same concentrations of pro-
teins and lactose as mil

me concentrations of pro-
teins and lactose as milk, but it has almost no fat, and
its maximum rate of production is about 1/100 the 
subsequent rate of milk production.
Immediately after the baby is born, the sudden loss
of both estrogen and progesterone secretion from the
placenta allows the lactogenic effect of prolactin from
the mother’s pituitary gland to assume its natural milk-
promoting role, and over the next 1 to 7 days, the
breasts begin to secrete copious quantities of milk
instead o

in to secrete copious quantities of milk
instead of colostrum. This secretion of milk requires 
an adequate background secretion of most of the
mother’s other hormones as well, but most important
are growth hormone, cortisol, parathyroid hormone,
and insulin. These hormones are necessary to provide
the amino acids, fatty acids, glucose, and calcium
required for milk formation.
After birth of the baby, the basal level of prolactin
secretion returns to the nonpregnant level over the
next few weeks

s to the nonpregnant level over the
next few weeks, as shown in Figure 82–11. However,
each time the mother nurses her baby, nervous signals
from the nipples to the hypothalamus cause a 10- to
20-fold surge in prolactin secretion that lasts for about
1 hour, which is also shown in Figure 82–11. This 
prolactin acts on the mother’s breasts to keep the
mammary glands secreting milk into the alveoli for the
subsequent nursing periods. If this prolactin surge is
absent or blocked as a result of hypo

tin surge is
absent or blocked as a result of hypothalamic or pitu-
itary damage or if nursing does not continue, the
breasts lose their ability to produce milk within 1 week
or so. However, milk production can continue for
several years if the child continues to suckle, although
the rate of milk formation normally decreases consid-
erably after 7 to 9 months.
Hypothalamic Control of Prolactin Secretion. The hypothal-
amus plays an essential role in controlling prolactin
Pectoralis major
Adipose

 in controlling prolactin
Lobules and
alveoli
Lactiferous
sinus (ampulla)
Lactiferous duct
Nipple
Areola
Ductule
Alveoli
A
B
C
Milk
Milk
secreting
epithelial
cells
Lobule
Myoepithelial cells
Figure 82–10
The breast and its secretory lobules, alveoli, and lactiferous ducts
(milk ducts) that constitute its mammary gland (A). The enlarge-
ments show a lobule (B) and milk-secreting cells of an alveolus
(C).
1040
Unit XIV
secretion, as it

IV
secretion, as it does for almost all the other anterior
pituitary hormones. However, this control is different
in one aspect: The hypothalamus mainly stimulates
production of all the other hormones, but it mainly
inhibits prolactin production. Consequently, damage
to the hypothalamus or blockage of the hypothalamic-
hypophysial portal system often increases prolactin
secretion while it depresses secretion of the other
anterior pituitary hormones.
Therefore, it i

other
Therefore, it is believed that anterior pituitary secre-
tion of prolactin is controlled either entirely or almost
entirely by an inhibitory factor formed in the hypo-
thalamus and transported through the hypothalamic-
hypophysial portal system to the anterior pituitary
gland. This factor is called prolactin inhibitory
hormone. It is almost certainly the same as the cate-
cholamine dopamine, which is known to be secreted
by the arcuate nuclei of the hypothalamu

 secreted
by the arcuate nuclei of the hypothalamus and can
decrease prolactin secretion as much as 10-fold.
Suppression of the Female Ovarian Cycles in Nursing Mothers
for Many Months After Delivery. In most nursing mothers,
the ovarian cycle (and ovulation) does not resume
until a few weeks after cessation of nursing.The reason
seems to be that the same nervous signals from 
the breasts to the hypothalamus that cause prolactin
secretion during suckling—either because of the
nervous signals the

suckling—either because of the
nervous signals themselves or because of a subsequent
effect of increased prolactin—inhibit secretion of
gonadotropin-releasing hormone by the hypothala-
mus.This, in turn, suppresses formation of the pituitary
gonadotropic hormones—luteinizing hormone and
follicle-stimulating hormone. However, after several
months of lactation, in some mothers, especially in
those who nurse their babies only some of the time,
the pituitary begins to secrete sufﬁcient gonadotropic

ituitary begins to secrete sufﬁcient gonadotropic
hormones to reinstate the monthly sexual cycle, even
though nursing continues.
Ejection (or “Let-Down”) Process in
Milk Secretion—Function of Oxytocin
Milk is secreted continuously into the alveoli of the
breasts, but milk does not ﬂow easily from the alveoli
into the ductal system and, therefore, does not contin-
ually leak from the breast nipples. Instead, the milk
must be ejected from the alveoli into the ducts before
the baby can obtain it. T

li into the ducts before
the baby can obtain it. This is caused by a combined
neurogenic and hormonal reﬂex that involves the 
posterior pituitary hormone oxytocin, as follows.
When the baby suckles, it receives virtually no milk
for the ﬁrst half minute or so. Sensory impulses must
ﬁrst be transmitted through somatic nerves from the
nipples to the mother’s spinal cord and then to her
hypothalamus, where they cause nerve signals that
promote oxytocin secretion at the same time that they
cause pr

ocin secretion at the same time that they
cause prolactin secretion.The oxytocin is carried in the
blood to the breasts, where it causes myoepithelial cells
(which surround the outer walls of the alveoli) to con-
tract, thereby expressing the milk from the alveoli into
the ducts at a pressure of +10 to 20 mm Hg. Then the
baby’s suckling becomes effective in removing the
milk. Thus, within 30 seconds to 1 minute after a baby
begins to suckle, milk begins to ﬂow. This process is
called milk ejecti

 begins to ﬂow. This process is
called milk ejection or milk let-down.
Suckling on one breast causes milk ﬂow not only in
that breast but also in the opposite breast. It is espe-
cially interesting that fondling of the baby by the
mother or hearing the baby crying often gives enough
of an emotional signal to the hypothalamus to cause
milk ejection.
Inhibition of Milk Ejection. A particular problem in
nursing a baby comes from the fact that many psy-
chogenic factors or even generalized sympathet

sy-
chogenic factors or even generalized sympathetic
nervous system stimulation throughout the mother’s
-8
-4
0
4
8
12
16
20
Intermittent secretion
of prolactin during
nursing
24
28
32
36
Prolactin
Estrogens
0
300
2.0
200
100
0
1.5
1.0
0.5
0
200
100
Weeks after parturition
Figure 82–11
Changes in rates of secretion 
of estrogens, progesterone, and
prolactin for 8 weeks before par-
turitio

one, and
turition and 36 weeks thereafter.
Note especially the decrease of
prolactin secretion back to basal
levels within a few weeks after
parturition, but also the intermit-
tent periods of marked prolactin
secretion (for about 1 hour at a
time) during and after periods of
nursing.
Chapter 82
1041
body can inhibit oxytocin secretion and consequently
depress milk ejection. For this reason, many mothers
must have an undisturbed puerperiu

n, many mothers
must have an undisturbed puerperium if they are to be
successful in nursing their babies.
Milk Composition and the 
Metabolic Drain on the Mother 
Caused by Lactation
Table 82–1 lists the contents of human milk and cow’s
milk. The concentration of lactose in human milk is
about 50 per cent greater than in cow’s milk, but the
concentration of protein in cow’s milk is ordinarily two
or more times greater than in human milk. Finally, only
one third as much ash, which contains calciu

 only
one third as much ash, which contains calcium and
other minerals, is found in human milk compared with
cow’s milk.
At the height of lactation in the human mother, 1.5
liters of milk may be formed each day (and even more
if the mother has twins). With this degree of lactation,
great quantities of metabolic substrates are drained
from the mother. For instance, about 50 grams of fat
enter the milk each day, and about 100 grams of
lactose, which must be derived by conversion from the
mother’s 

h must be derived by conversion from the
mother’s glucose. Also, 2 to 3 grams of calcium phos-
phate may be lost each day; unless the mother is drink-
ing large quantities of milk and has an adequate intake
of vitamin D, the output of calcium and phosphate by
the lactating mammae will often be much greater than
the intake of these substances. To supply the needed
calcium and phosphate, the parathyroid glands enlarge
greatly, and the bones become progressively decalci-
ﬁed. The mother’s bone deca

progressively decalci-
ﬁed. The mother’s bone decalciﬁcation is usually not a
big problem during pregnancy, but it can become more
important during lactation.
Antibodies and Other Anti-infectious Agents in Milk. Not only
does milk provide the newborn baby with needed
nutrients, but it also provides important protection
against infection. For instance, multiple types of anti-
bodies and other anti-infectious agents are secreted in
milk along with the nutrients. Also, several different
types of wh

the nutrients. Also, several different
types of white blood cells are secreted, including both
neutrophils and macrophages, some of which are 
especially lethal to bacteria that could cause deadly
infections in newborn babies. Particularly important
are antibodies and macrophages that destroy
Escherichia coli bacteria, which often cause lethal diar-
rhea in newborns.
When cow’s milk is used to supply nutrition for the
baby in place of mother’s milk, the protective agents
in it are usually of lit

lk, the protective agents
in it are usually of little value because they are 
normally destroyed within minutes in the internal
environment of the human being.
Alexander BT, Bennett WA, Khalil RA, Granger JP:
Preeclampsia: linking placental ischemia with cardiovas-
cular-renal dysfunction. News Physiol Sci 16:282, 2001.
Ben-Jonathan N, Hnasko R: Dopamine as a prolactin (PRL)
inhibitor. Endocr Rev 22:724, 2001.
Casey ML, MacDonald PC: The endocrinology of human
parturition. Ann N Y Aca

he endocrinology of human
parturition. Ann N Y Acad Sci 828:273, 1997.
Challis JR, Lye SJ, Gibb W, et al: Understanding preterm
labor. Ann N Y Acad Sci 943:225, 2001.
Cross JC, Simmons DG, Watson ED: Chorioallantoic mor-
phogenesis and formation of the placental villous tree.
Ann N Y Acad Sci 995:84, 2003.
Davison JM, Homuth V, Jeyabalan A, et al: New aspects in
the pathophysiology of preeclampsia. J Am Soc Nephrol
15:2440, 2004.
Dekker G, Sibai B: Primary, secondary, and tertiary preven-
tion o

B: Primary, secondary, and tertiary preven-
tion of pre-eclampsia. Lancet 357:209, 2001.
Gofﬁn V, Binart N, Touraine P, Kelly PA: Prolactin: the new
biology of an old hormone.Annu Rev Physiol 64:47, 2002.
Hall JG: Twinning. Lancet 362:735, 2003.
Kanaka

2.
Kanaka-Gantenbein C,
Mastorakos G,
Chrousos GP:
Endocrine-related causes and consequences of intrauter-
ine growth retardation. Ann N Y Acad Sci 997:150, 2003.
Khalaf Y: ABC of subfertility: tubal subfertility. BMJ
327:610, 2003.
Khalil RA, Granger JP: Vascular mechanisms of increased
arterial pressure in preeclampsia: lessons from animal
models.Am J Physiol Regul Integr Comp Physiol 283:R29,
2002.
Labbok MH, Clark D, Goldman AS: Breastfeeding: main-
t

ok MH, Clark D, Goldman AS: Breastfeeding: main-
taining an irreplaceable immunological resource. Nat Rev
Immunol 4:565, 2004.
MacLaughlin DT, Donahoe PK: Sex determination and 
differentiation. N Engl J Med 350:367, 2004.
Mastorakos G, Ilias I: Maternal and fetal hypothalamic-
pituitary-adrenal axes during pregnancy and postpartum.
Ann N Y Acad Sci 997:136, 2003.
Moffett-King A: Natural killer cells and pregnancy. Nat Rev
Immunol 2:656, 2002.
Roberts JM, Cooper DW: Pathogenesis and genetics of 

berts JM, Cooper DW: Pathogenesis and genetics of 
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Roberts JM, Pearson G, Cutler J, Lindheimer M: Summary
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tension During Pregnancy. Hypertension 41:437, 2003.
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fetal development. J Nutr 134:2169, 2004.
Table 82–1
Composition of Milk
Human Milk (%)
Cow’s Milk (%)
Water
88.5
87.0
Fat
3.3
3.5
Lactose
6.8
4.8
Casein
0.9
2.7
Lactalbumin and other 
0.4

6.8
4.8
Casein
0.9
2.7
0.4
0.7
proteins
Ash
0.2
0.7
C
H
A
P
T
E
R
 
8
3
1042
A complete discussion of fetal development, function-
ing of the child immediately after birth, and growth and
development through the early years of life lies within
the province of formal courses in obstetrics and pedi-
atrics. However, many physiologic principles are pecu-
liar to the infant itself. This chapter discusses the more
important of these.
Growth and Fun

cusses the more
Initial development of the placenta and fetal membranes occurs far more rapidly
than development of the fetus itself. In fact, during the ﬁrst 2 to 3 weeks after
implantation of the blastocyst, the fetus remains almost microscopic, but thereafter,
as shown in Figure 83–1, the length of the fetus increases almost in proportion to
age. At 12 weeks, the length is about 10 centimeters; at 20 weeks, 25 centimeters;
an

ut 10 centimeters; at 20 weeks, 25 centimeters;
and at term (40 weeks), 53 centimeters (about 21 inches). Because the weight of the
fetus is approximately proportional to the cube of length, the weight increases
almost in proportion to the cube of the age of the fetus.
Note in Figure 83–1 that the weight remains minuscule during the ﬁrst 12 weeks
and reaches 1 pound only at 23 weeks (5 1/2 months) of gestation. Then, during the
last trimester of pregnancy, the fetus gains tremendously, so that 2

pregnancy, the fetus gains tremendously, so that 2 months before
birth, the weight averages 3 pounds, 1 month before birth 4.5 pounds, and at birth
7 pounds—the ﬁnal birth weight varying from as low as 4.5 pounds to as high as 11
pounds in normal infants with normal gestational periods.
Within 1 month after fertilization of the ovum, the gross characteristics of all the
different organs of the fetus have already begun to develop, and during the next 2
to 3 months

egun to develop, and during the next 2
to 3 months, most of the details of the different organs are established. Beyond
month 4, the organs of the fetus are grossly the same as those of the neonate.
However, cellular development in each organ is usually far from complete and
requires the full remaining 5 months of pregnancy for complete development. Even
at birth, certain structures, particularly in the nervous system, the kidneys, and the
liver, lack full development, as discussed in more detai

 lack full development, as discussed in more detail later in the chapter.
Circulatory System. The human heart begins beating during the fourth week after fer-
tilization, contracting at a rate of about 65 beats/min.This increases steadily to about
140 beats/min immediately before birth.
Formation of Blood Cells. Nucleated red blood cells begin to be formed in the yolk sac
and mesothelial layers of the placenta at about the third week of fetal development.
This is followed 1 week later (at 4 to 5

elopment.
This is followed 1 week later (at 4 to 5 weeks) by formation of non-nucleated red
blood cells by the fetal mesenchyme and also by the endothelium of the fetal blood
vessels.Then, at 6 weeks, the liver begins to form blood cells, and in the third month,
the spleen and other lymphoid tissues of the body begin forming blood cells. Finally,
from the third month on, the bone marrow gradually becomes the principal source
of the red blood cells as well as most of the white blood cells, except

s as well as most of the white blood cells, except for continued
lymphocyte and plasma cell production in lymphoid tissue.
Respiratory System. Respiration cannot occur during fetal life because there is no air
to breathe in the amniotic cavity. However, attempted respiratory movements do
take place beginning at the end of the ﬁrst trimester of pregnancy. Tactile stimuli
and fetal asphyxia especially cause these attempted respiratory movements.
Chapter 83
1043
During

apter 83
1043
During the last 3 to 4 months of pregnancy, the res-
piratory movements of the fetus are mainly inhibited,
for reasons unknown, and the lungs remain almost com-
pletely deﬂated.The inhibition of respiration during the
later months of fetal life prevents ﬁlling of the lungs
with ﬂuid and debris from the meconium excreted by
the fetus’s gastrointestinal tract into the amniotic ﬂuid.
Also, small amounts of ﬂuid are secreted into the lungs
by the alveolar 

 ﬂuid are secreted into the lungs
by the alveolar epithelium up until the moment of birth,
thus keeping only clean ﬂuid in the lungs.
Nervous System. Most of the reﬂexes of the fetus that
involve the spinal cord and even the brain stem are
present by the third to fourth months of pregnancy.
However, those nervous system functions that involve
the cerebral cortex are still only in the early stages of
development even at birth. Indeed, myelinization of
some major tracts of the brain itself becomes

n of
some major tracts of the brain itself becomes complete
only after about 1 year of postnatal life.
Gastrointestinal. Tract By midpregnancy, the fetus begins
to ingest and absorb large quantities of amniotic ﬂuid,
and during the last 2 to 3 months, gastrointestinal 
function approaches that of the normal neonate. By 
that time, small quantities of meconium are continually
formed in the gastrointestinal tract and excreted from
the anus into the amniotic ﬂuid. Meconium is composed
partly of res

 amniotic ﬂuid. Meconium is composed
partly of residue from swallowed amniotic ﬂuid and
partly of mucus and other residues of excretory prod-
ucts from the gastrointestinal mucosa and glands.
Kidneys. The fetal kidneys begin to excrete urine during
the second trimester pregnancy, and fetal urine accounts
for about 70 to 80 per cent of the amniotic ﬂuid.Abnor-
mal kidney development or severe impairment of
kidney function in the fetus greatly reduce the forma-
tion of amniotic ﬂuid (oligohydramni

ce the forma-
tion of amniotic ﬂuid (oligohydramnios) and can lead to
fetal death.
Although the fetal kidneys form urine, the renal
control systems for regulating fetal extracellular ﬂuid
volume and electrolyte balances, and especially acid-
base balance, are almost nonexistent until late fetal life
and do not reach full development until a few months
Fetal Metabolism. The fetus uses mainly glucose for
energy, and it has a high capability to store fat and
protein, much if not most o

ility to store fat and
protein, much if not most of the fat being synthesized
from glucose rather than being absorbed directly from
the mother’s blood. In addition to these generalities,
there are special problems of fetal metabolism in rela-
tion to calcium, phosphate, iron, and some vitamins.
Metabolism of Calcium and Phosphate. Figure 83–2 shows
the rates of calcium and phosphate accumulation in the
fetus, demonstrating that about 22.5 grams of calcium
and 13.5 grams of phosphorus are accumul

f calcium
and 13.5 grams of phosphorus are accumulated in the
average fetus during gestation. About one half of these
accumulate during the last 4 weeks of gestation, which
is coincident with the period of rapid ossiﬁcation of the
fetal bones and with the period of rapid weight gain of
the fetus.
During the earlier part of fetal life, the bones are 
relatively unossiﬁed and have mainly a cartilaginous
matrix. Indeed x-ray ﬁlms ordinarily do not show any
ossiﬁcation until after the fourth month o

how any
ossiﬁcation until after the fourth month of pregnancy.
Note especially that the total amounts of calcium and
phosphate needed by the fetus during gestation repre-
sent only about 2 per cent of the quantities of these 
substances in the mother’s bones. Therefore, this is a
minimal drain from the mother. Much greater drain
occurs after birth during lactation.
Accumulation of Iron. Figure 83–2 also shows that iron
accumulates in the fetus even more rapidly than calcium
and phosphate. Most o

en more rapidly than calcium
and phosphate. Most of the iron is in the form of
hemoglobin, which begins to be formed as early as the
third week after fertilization of the ovum.
Small amounts of iron are concentrated in the
mother’s uterine progestational endometrium even
before implantation of the ovum; this iron is ingested
into the embryo by the trophoblastic cells and is used
0
36
32
28
24
20
16
12
8
4
Ovulation
40
50
Length
Weight
40
30
20
10
0
2
1
3
0
Age of fetus (weeks after l

40
30
20
10
0
2
1
3
0
Age of fetus (weeks after last menstruation)
Length (centimeters)
Weight (Kilograms)
Figure 83–1
Growth of the fetus.
0
4
8
12 16
20 24 28 32
36 40
Ovulation
25
20
15
10
5
0
Iron
Calcium
0
250
200
150
50
100
Grams of calcium or phosphorus stored
Milligrams of iron stored
Figure 83–2
Iron, calcium, and phosphorus storage in the fetus at different
stages of gestation.
1044
Unit XIV
Endocrinology and Reproduc

estation.
1044
Unit XIV
to form the very early red blood cells. About one third
of the iron in a fully developed fetus is normally stored
in the liver. This iron can then be used for several
months after birth by the neonate for formation of addi-
tional hemoglobin.
Utilization and Storage of Vitamins. The fetus needs vitamins
equally as much as the adult and in some instances to a
far greater extent. In general, the vitamins function the
same in the fetus as in th

e vitamins function the
same in the fetus as in the adult, as discussed in Chapter
71. Special functions of several vitamins should be
mentioned, however.
The B vitamins, especially vitamin B12 and folic acid,
are necessary for formation of red blood cells and
nervous tissue, as well as for overall growth of the fetus.
Vitamin C is necessary for appropriate formation of
intercellular substances, especially the bone matrix and
ﬁbers of connective tissue.
Vitamin D is needed for normal bone growth

tissue.
Vitamin D is needed for normal bone growth in the
fetus, but even more important, the mother needs it for
adequate absorption of calcium from her gastrointesti-
nal tract. If the mother has plenty of this vitamin in her
body ﬂuids, large quantities of the vitamin will be stored
by the fetal liver to be used by the neonate for several
months after birth.
Vitamin E, although the mechanisms of its functions
are not clear, is necessary for normal development of
the early embryo. In its absen

rmal development of
the early embryo. In its absence in laboratory animals,
spontaneous abortion usually occurs at an early stage of
Vitamin K is used by the fetal liver for formation of
Factor VII, prothrombin, and several other blood coag-
ulation factors. When vitamin K is insufﬁcient in the
mother, Factor VII and prothrombin become deﬁcient
in the fetus as well as in the mother. Because most
vitamin K is formed by bacterial action in the mother’s
colon, the neonate has no adequate

in the mother’s
colon, the neonate has no adequate source of vitamin K
for the ﬁrst week or so of life after birth until normal
colonic bacterial ﬂora become established in the
newborn infant. Therefore, prenatal storage in the fetal
liver of at least small amounts of vitamin K derived from
the mother is helpful in preventing fetal hemorrhage,
particularly hemorrhage in the brain when the head is
traumatized by squeezing through the birth canal.
Adjustments of the Infant 
to Extrauterine Life
On

The most obvious effect of birth on the baby is loss of
the placental connection with the mother and, there-
fore, loss of this means of metabolic support. One of the
most important immediate adjustments required of the
infant is to begin breathing.
Cause of Breathing at Birth. After normal delivery from a
mother who has not been depressed by anesthetics, the
child ordinarily begins to breathe within seconds and
has a normal resp

ns to breathe within seconds and
has a normal respiratory rhythm within less than 1
minute after birth.The promptness with which the fetus
begins to breathe indicates that breathing is initiated by
sudden exposure to the exterior world, probably result-
ing from (1) a slightly asphyxiated state incident to the
birth process, but also from (2) sensory impulses that
originate in the suddenly cooled skin. In an infant who
does not breathe immediately, the body becomes 
progressively more hypoxic an

y, the body becomes 
progressively more hypoxic and hypercapnic, which
provides additional stimulus to the respiratory center
and usually causes breathing within an additional
minute after birth.
Delayed or Abnormal Breathing at Birth—Danger of Hypoxia. If
the mother has been depressed by a general anesthetic
during delivery, which at least partially anesthetizes 
the fetus as well, the onset of respiration is likely to be
delayed for several minutes, thus demonstrating the
importance of using a

utes, thus demonstrating the
importance of using as little anesthesia as feasible.Also,
many infants who have had head trauma during deliv-
ery or who undergo prolonged delivery are slow to
breathe or sometimes do not breathe at all. This can
result from two possible effects: First, in a few infants,
intracranial hemorrhage or brain contusion causes a
concussion syndrome with a greatly depressed respira-
tory center. Second, and probably much more impor-
tant, prolonged fetal hypoxia during deli

e impor-
tant, prolonged fetal hypoxia during delivery can cause
serious depression of the respiratory center.
Hypoxia frequently occurs during delivery because of
(1) compression of the umbilical cord; (2) premature
separation of the placenta; (3) excessive contraction of
the uterus, which can cut off the mother’s blood ﬂow to
the placenta; or (4) excessive anesthesia of the mother,
which depresses oxygenation even of her blood.
Degree of Hypoxia That an Infant Can Tolerate. In an adult,
failur

a That an Infant Can Tolerate. In an adult,
failure to breathe for only 4 minutes often causes death,
but a neonate often survives as long as 10 minutes of
failure to breathe after birth. Permanent and very
serious brain impairment often ensues if breathing is
delayed more than 8 to 10 minutes. Indeed, actual
lesions develop mainly in the thalamus, in the inferior
colliculi, and in other brain stem areas, thus permanently
affecting many of the motor functions of the body.
Expansion of the Lungs 

tor functions of the body.
Expansion of the Lungs at Birth. At birth, the walls of the
alveoli are at ﬁrst collapsed because of the surface
tension of the viscid ﬂuid that ﬁlls them. More than 
25 mm Hg of negative inspiratory pressure in the lungs
is usually required to oppose the effects of this surface
tension and to open the alveoli for the ﬁrst time. But
once the alveoli do open, further respiration can be
effected with relatively weak respiratory movements.
Fortunately, the ﬁrst inspiratio

ratory movements.
Fortunately, the ﬁrst inspirations of the normal neonate
are extremely powerful, usually capable of creating as
much as 60 mm Hg negative pressure in the intrapleural
space.
Figure 
83–3 
shows 
the 
negative
intrapleural pressures required to open the lungs at 
the onset of breathing. At the top is shown the pres-
sure-volume curve (“compliance” curve) for the ﬁrst 
breath after birth. Observe, ﬁrst, the lower part of 
the curve beginning at the zero pressure point

of 
the curve beginning at the zero pressure point and
moving to the right.The curve shows that the volume of
air in the lungs remains almost exactly zero until the
negative pressure has reached -40 centimeters water 
(-30 mm Hg). Then, as the negative pressure increases
to -60 centimeters of water, about 40 milliliters of air
enters the lungs. To deﬂate the lungs, considerable pos-
itive pressure, about +40 centimeters of water, is
required because of viscous resistance offered by the
ﬂuid in t

use of viscous resistance offered by the
ﬂuid in the bronchioles.
Note that the second breath is much easier, with far
less negative and positive pressures required. Breathing
does not become completely normal until about 40
minutes after birth, as shown by the third compliance
Chapter 83
1045
curve, the shape of which compares favorably with that
for the normal adult, as shown in Chapter 38.
Respiratory Distress Syndrome Caused When Surfactant Secretion
Is Deﬁcient

drome Caused When Surfactant Secretion
Is Deﬁcient. A small number of infants, especially pre-
mature infants and infants born of diabetic mothers,
develop severe respiratory distress in the early hours to
the ﬁrst several days after birth, and some die within the
next day or so. The alveoli of these infants at death
contain large quantities of proteinaceous ﬂuid, almost
as if pure plasma had leaked out of the capillaries into
the alveoli. The ﬂuid also contains desquamated alveo-
lar epithelial

id also contains desquamated alveo-
lar epithelial cells.This condition is called hyaline mem-
brane disease because microscopic slides of the lung
show the material ﬁlling the alveoli to look like a
hyaline membrane.
One of the most characteristic ﬁndings in respiratory
distress syndrome is failure of the respiratory epithe-
lium to secrete adequate quantities of surfactant, a sub-
stance normally secreted into the alveoli that decreases
the surface tension of the alveolar ﬂuid, therefore allow

face tension of the alveolar ﬂuid, therefore allow-
ing the alveoli to open easily during inspiration.The sur-
factant-secreting cells (type II alveolar epithelial cells)
do not begin to secrete surfactant until the last 1 to 3
months of gestation. Therefore, many premature babies
and a few full-term babies are born without the capa-
bility to secrete sufﬁcient surfactant, which causes both
a collapse tendency of the alveoli and development of
pulmonary edema. The role of surfactant in preventin

lmonary edema. The role of surfactant in preventing
these effects is discussed in Chapter 37.
Equally as essential as the onset of breathing at birth
are immediate circulatory adjustments that allow ade-
quate blood ﬂow through the lungs. Also, circulatory
adjustments during the ﬁrst few hours of life cause more
and more blood ﬂow through the baby’s liver, which up
to this point has had very little blood ﬂow. To describe
these readjustments, we must ﬁrst consid

 describe
these readjustments, we must ﬁrst consider the anatom-
ical structure of the fetal circulation.
Speciﬁc Anatomical Structure of the Fetal Circulation. Because
the lungs are mainly nonfunctional during fetal life and
because the liver is only partially functional, it is not
necessary for the fetal heart to pump much blood
through either the lungs or the liver. However, the fetal
heart must pump large quantities of blood through the
placenta. Therefore, special anatomical arrangements
ca

nta. Therefore, special anatomical arrangements
cause the fetal circulatory system to operate much 
differently from that of the newborn baby.
First, as shown in Figure 83–4, blood returning from
the placenta through the umbilical vein passes through
the ductus venosus, mainly bypassing the liver. Then
most of the blood entering the right atrium from the
inferior vena cava is directed in a straight pathway
across the posterior aspect of the right atrium and
through the foramen ovale directly int

 atrium and
through the foramen ovale directly into the left atrium.
Thus, the well-oxygenated blood from the placenta
enters mainly the left side of the heart, rather than the
right side, and is pumped by the left ventricle mainly
into the arteries of the head and forelimbs.
The blood entering the right atrium from the supe-
rior vena cava is directed downward through the tri-
cuspid valve into the right ventricle.This blood is mainly
deoxygenated blood from the head region of the fetus,
and it

ed blood from the head region of the fetus,
and it is pumped by the right ventricle into the pul-
monary artery and then mainly through the ductus arte-
riosus into the descending aorta, then through the two
umbilical arteries into the placenta, where the deoxy-
genated blood becomes oxygenated.
Figure 83–5 gives the relative percentages of the total
blood pumped by the heart that pass through the dif-
ferent vascular circuits of the fetus. This ﬁgure shows
that 55 per cent of all the blood goes

ﬁgure shows
that 55 per cent of all the blood goes through the pla-
centa, leaving only 45 per cent to pass through all the
tissues of the fetus. Furthermore, during fetal life, only
12 per cent of the blood ﬂows through the lungs; imme-
diately after birth, virtually all the blood ﬂows through
the lungs.
Changes in the Fetal Circulation at Birth. The basic changes
in the fetal circulation at birth are discussed in 
+40
+20
0
-60
-20
-40
Pressure
Second Breath
60
40
20
0
+40
+20
0
-6

60
40
20
0
+40
+20
0
-60
-20
-40
Pressure
First Breath
60
40
20
0
+40
+20
0
-60
-20
-40
Pressure
40 Minutes
60
40
20
0
Figure 83–3
Pressure-volume curves of the lungs (“compliance” curves) of a
neonate immediately after birth, showing the extreme forces
required for breathing during the ﬁrst two breaths of life and devel-
opment of a nearly normal compliance curve within 40 minutes
after birth. (Redrawn from Smith CA: The ﬁrst breath. Sci Am
209

Redrawn from Smith CA: The ﬁrst breath. Sci Am
209:32, 1963, © 1963 by Scientiﬁc American, Inc. All rights
reserved.)
1046
Unit XIV
Chapter 23 in relation to congenital anomalies of the
ductus arteriosus and foramen ovale that persist
throughout life in a few persons. Brieﬂy, these changes
are the following.
Primary Changes in Pulmonary and Systemic Vascular
Resistances at Birth 
The primary changes in the circulation at birth are, ﬁrst,
loss of the tremendous bloo

on at birth are, ﬁrst,
loss of the tremendous blood ﬂow through the placenta,
which approximately doubles the systemic vascular
resistance at birth. This increases the aortic pressure as
well as the pressures in the left ventricle and left atrium.
Second, the pulmonary vascular resistance greatly
decreases as a result of expansion of the lungs. In the
unexpanded fetal lungs, the blood vessels are com-
pressed because of the small volume of the lungs. Imme-
diately on expansion, these vessels are

ngs. Imme-
diately on expansion, these vessels are no longer
compressed and the resistance to blood ﬂow decreases
severalfold. Also, in fetal life, the hypoxia of the lungs
causes considerable tonic vasoconstriction of the lung
blood vessels, but vasodilation takes place when aera-
tion of the lungs eliminates the hypoxia. All these
changes together reduce the resistance to blood ﬂow
through the lungs as much as ﬁvefold, which reduces the
pulmonary arterial pressure, right ventricular pressure,

ry arterial pressure, right ventricular pressure,
and right atrial pressure.
Closure of the Foramen Ovale 
The low right atrial pressure and the high left atrial pres-
sure that occur secondarily to the changes in pulmonary
and systemic resistances at birth cause blood now to
attempt to ﬂow backward through the foramen ovale;
that is, from the left atrium into the right atrium, rather
than in the other direction, as occurred during fetal life.
Consequently, the small valve that lies over the for

nsequently, the small valve that lies over the foramen
ovale on the left side of the atrial septum closes over
this opening, thereby preventing further ﬂow through
the foramen ovale.
In two thirds of all people, the valve becomes adher-
ent over the foramen ovale within a few months to a few
years and forms a permanent closure. But even if per-
manent closure does not occur, the left atrial pressure
throughout life normally remains 2 to 4 mm Hg greater
than the right atrial pressure, and the bac

reater
than the right atrial pressure, and the backpressure
keeps the valve closed.
Closure of the Ductus Arteriosus 
The ductus arteriosus also closes, but for different
reasons. First, the increased systemic resistance elevates
the aortic pressure while the decreased pulmonary
resistance reduces the pulmonary arterial pressure. As a
consequence, after birth, blood begins to ﬂow backward
from the aorta into the pulmonary artery through the
ductus arteriosus, rather than in the other direction a

s arteriosus, rather than in the other direction as
in fetal life. However, after only a few hours, the muscle
wall of the ductus arteriosus constricts markedly, and
within 1 to 8 days, the constriction is usually sufﬁcient
to stop all blood ﬂow. This is called functional closure
of the ductus arteriosus. Then, during the next 1 to 
Aorta
Ductus arteriosus
Umbilical
arteries
Umbilical
vein
Foramen ovale
Lung
Lung
Gut
Liver
Superior
vena cava
Figure 83–4
Organization of the feta

ior
vena cava
Organization of the fetal circulation. (Modiﬁed from Arey LB:
Developmental Anatomy: A Textbook and Laboratory Manual of
Embryology. 7th ed. Philadelphia: WB Saunders Co, 1974.)
Forequarters
Hindquarters
Placenta
Placenta
Right
atrium
Right
atrium
Right
ventricle
Right
ventricle
Ductus
Ductus
Foramen
ovale
Foramen
ovale
Left
atrium
Left
atrium
Left
ventricle
Left
ventricle
Lungs
Lungs
58
58
46
46
27
27
15
15
15
15
43
43
42

Lungs
Lungs
58
58
46
46
27
27
15
15
15
15
43
43
42
42
30
30
73
73
73
73
12
12
18
18
55
55
Figure 83–5
Diagram of the fetal circulatory system, showing relative distribu-
tion of blood ﬂow to the different vascular areas. The numerals
represent the percentage of the total output from both sides of the
heart ﬂowing through each particular area.
Chapter 83
1047
4 months, the ductus arteriosus ordinarily becomes
anatomically occluded by growth of ﬁbrous tissue into
its 

ally occluded by growth of ﬁbrous tissue into
its lumen.
The cause of ductus arteriosus closure relates to the
increased oxygenation of the blood ﬂowing through the
ductus. In fetal life the PO2 of the ductus blood is only
15 to 20 mm Hg, but it increases to about 100 mm Hg
within a few hours after birth. Furthermore, many
experiments have shown that the degree of contraction
of the smooth muscle in the ductus wall is highly related
to this availability of oxygen.
In one of several thousand infa

ability of oxygen.
In one of several thousand infants, the ductus fails to
close, resulting in a patent ductus arteriosus, the conse-
quences of which are discussed in Chapter 23. The
failure of closure has been postulated to result from
excessive ductus dilation caused by vasodilating
prostaglandins in the ductus wall. In fact, administration
of the drug indomethacin, which blocks synthesis of
prostaglandins, often leads to closure.
Closure of the Ductus Venosus In fetal life, the portal blood

he Ductus Venosus In fetal life, the portal blood
from the fetus’s abdomen joins the blood from the
umbilical vein, and these together pass by way of the
ductus venosus directly into the vena cava immediately 
below the heart but above the liver, thus bypassing the
liver.
Immediately after birth, blood ﬂow through the
umbilical vein ceases, but most of the portal blood 
still ﬂows through the ductus venosus, with only a small
amount passing through the channels of the liver.
However, within 1 to

gh the channels of the liver.
However, within 1 to 3 hours the muscle wall of the
ductus venosus contracts strongly and closes this avenue
of ﬂow. As a consequence, the portal venous pressure
rises from near 0 to 6 to 10 mm Hg, which is enough to
force portal venous blood ﬂow through the liver sinuses.
Although the ductus venosus rarely fails to close, we
know almost nothing about what causes the closure.
Before birth, the fetus derives almost all its energy from
glucose

e fetus derives almost all its energy from
glucose obtained from the mother’s blood. After birth,
the amount of glucose stored in the infant’s body in the
form of liver and muscle glycogen is sufﬁcient to supply
the infant’s needs for only a few hours. The liver of the
neonate is still far from functionally adequate at birth,
which prevents signiﬁcant gluconeogenesis. Therefore,
the infant’s blood glucose concentration frequently falls
the ﬁrst day to as low as 30 to 40 mg/dl of plasma, less
tha

ay to as low as 30 to 40 mg/dl of plasma, less
than one half the normal value. Fortunately, however,
appropriate mechanisms are available for the infant to
use its stored fats and proteins for metabolism until
mother’s milk can be provided 2 to 3 days later.
Special problems are also frequently associated with
getting an adequate ﬂuid supply to the neonate because
the infant’s rate of body ﬂuid turnover averages seven
times that of an adult, and the mother’s milk supply
requires several days to 

the mother’s milk supply
requires several days to develop. Ordinarily, the infant’s
weight decreases 5 to 10 per cent and sometimes as
much as 20 per cent within the ﬁrst 2 to 3 days of life.
Most of this weight loss is loss of ﬂuid rather than of
body solids.
Special Functional Problems
in the Neonate
An important characteristic of the neonate is instability
of the various hormonal and neurogenic control
systems.This results partly from immature development
of the different organs of the body a

 development
of the different organs of the body and partly from the
fact that the control systems simply have not become
adjusted to the new way of life.
The normal rate of respiration in a neonate is about 40
breaths per minute, and tidal air with each breath aver-
ages 16 milliliters. This gives a total minute respiratory
volume of 640 ml/min, which is about twice as great in
relation to the body weight as that of an adult. The func-
tional residual capacity of the infant’s

The func-
tional residual capacity of the infant’s lungs is only one
half that of an adult in relation to body weight. This 
difference causes excessive cyclical increases and
decreases in the newborn baby’s blood gas concentra-
tions if the respiratory rate becomes slowed because it
is the residual air in the lungs that smooths out the
blood gas variations.
Blood Volume. The blood volume of a neonate immedi-
ately after birth averages about 300 milliliters, but if the
infant is left

s about 300 milliliters, but if the
infant is left attached to the placenta for a few minutes
after birth or if the umbilical cord is stripped to force
blood out of its vessels into the baby, an additional 75
milliliters of blood enters the infant, to make a total of
375 milliliters. Then, during the ensuing few hours, ﬂuid
is lost into the neonate’s tissue spaces from this blood,
which increases the hematocrit but returns the blood
volume once again to the normal value of about 300 mil-
liliter

gain to the normal value of about 300 mil-
liliters. Some pediatricians believe that this extra blood
volume caused by stripping the umbilical cord can lead
to mild pulmonary edema with some degree of respira-
tory distress, but the extra red blood cells are often very
valuable to the infant.
Cardiac Output. The cardiac output of the neonate 
averages 500 ml/min, which, like respiration and body
metabolism, is about twice as much in relation to body
weight as in the adult. Occasionally a child i

ody
weight as in the adult. Occasionally a child is born with
an especially low cardiac output caused by hemorrhage
of much of its blood volume from the placenta at birth.
Arterial Pressure. The arterial pressure during the ﬁrst
day after birth averages about 70 mm Hg systolic and 
50 mm Hg diastolic; this increases slowly during the
next several months to about 90/60. Then there is a
much slower rise during the subsequent years until the
adult pressure of 115/70 is attained at adolescence.
Bloo

ressure of 115/70 is attained at adolescence.
Blood Characteristics. The red blood cell count in the
neonate averages about 4 million per cubic millimeter.
If blood is stripped from the cord into the infant, the red
blood cell count rises an additional 0.5 to 0.75 million
during the ﬁrst few hours of life, giving a red blood cell
count of about 4.75 million per cubic millimeter, as
shown in Figure 83–6. Subsequent to this, however, few
new red blood cells are formed in the infant during the
ﬁrst

ood cells are formed in the infant during the
ﬁrst few weeks of life, presumably because the hypoxic
stimulus of fetal life is no longer present to stimulate
red cell production. Thus, as shown in Figure 83–6, the
average red blood cell count falls to less than 4 million
per cubic millimeter by about 6 to 8 weeks of age. From
that time on, increasing activity by the baby provides
the appropriate stimulus for returning the red blood cell
1048
Unit XIV
count to norma

 XIV
count to normal within another 2 to 3 months. Immedi-
ately after birth, the white blood cell count of the
neonate is about 45,000 per cubic millimeter, which is
about ﬁve times as great as that of the normal adult.
Neonatal Jaundice and Erythroblastosis Fetalis. Biliru-
bin formed in the fetus can cross the placenta into 
the mother and be excreted through the liver of the
mother, but immediately after birth, the only means 
for ridding the neonate of bilirub

the only means 
for ridding the neonate of bilirubin is through the
neonate’s own liver, which for the ﬁrst week or so of life
functions poorly and is incapable of conjugating signi-
ﬁcant quantities of bilirubin with glucuronic acid for
excretion into the bile. Consequently, the plasma biliru-
bin concentration rises from a normal value of less than
1 mg/dl to an average of 5 mg/dl during the ﬁrst 3 days
of life and then gradually falls back to normal as the
liver becomes functional. This effec

normal as the
liver becomes functional. This effect, called physiologic
hyperbilirubinemia, is shown in Figure 83–6, and it 
is associated with mild jaundice (yellowness) of the
infant’s skin and especially of the sclerae of its eyes for
a week or two.
However, by far the most important abnormal cause
of serious neonatal jaundice is erythroblastosis fetalis,
which is discussed in detail in Chapter 32 in relation 
to Rh factor incompatibility between the fetus and
mother. Brieﬂy, the erythroblast

een the fetus and
mother. Brieﬂy, the erythroblastotic baby inherits Rh-
positive red cells from the father, while the mother is
Rh negative. The mother then becomes immunized
against the Rh-positive factor (a protein) in the fetus’s
blood cells, and her antibodies destroy fetal red cells,
releasing extreme quantities of bilirubin into the fetus’s
plasma and often causing fetal death for lack of ade-
quate red cells. Before the advent of modern obstetri-
cal therapeutics, this condition occurred

bstetri-
cal therapeutics, this condition occurred either mildly or
seriously in 1 of every 50 to 100 neonates.
Fluid Balance, Acid-Base Balance,
and Renal Function
The rate of ﬂuid intake and ﬂuid excretion in the
newborn infant is seven times as great in relation to
weight as in the adult, which means that even a slight
percentage alteration of ﬂuid intake or ﬂuid output can
cause rapidly developing abnormalities.
The rate of metabolism in the infant is also twice as
great in relation to body 

infant is also twice as
great in relation to body mass as in the adult, which
means that twice as much acid is normally formed,
which gives a tendency toward acidosis in the infant.
Functional development of the kidneys is not complete
until the end of about the ﬁrst month of life. For
instance, the kidneys of the neonate can concentrate
urine to only 1.5 times the osmolality of the plasma
instead of the adult three to four times. Therefore, con-
sidering the immaturity of the kidneys, together 

sidering the immaturity of the kidneys, together with the
marked ﬂuid turnover in the infant and rapid formation
of acid, one can readily understand that among the most
important problems of infancy are acidosis, dehydra-
tion, and, more rarely, overhydration.
During the ﬁrst few days of life, liver function in the
neonate may be quite deﬁcient, as evidenced by the 
following effects:
1. The liver of the neonate conjugates bilirubin with
glucuronic acid poorly and therefore excre

in with
glucuronic acid poorly and therefore excretes
bilirubin only slightly during the ﬁrst few days of
life.
2. The liver of the neonate is deﬁcient in forming
plasma proteins, so that the plasma protein
concentration falls during the ﬁrst weeks of life to
15 to 20 per cent less than that for older children.
Occasionally the protein concentration falls so low
that the infant develops hypoproteinemic edema.
3. The gluconeogenesis function of the liver is
particularly deﬁcient. As a result, the

e liver is
particularly deﬁcient. As a result, the blood glucose
level of the unfed neonate falls to about 30 to 40
mg/dl (about 40 per cent of normal), and the infant
must depend mainly on its stored fats for energy
until sufﬁcient feeding can occur.
4. The liver of the neonate usually also forms too
little of the blood factors needed for normal blood
Digestion, Absorption, and
Metabolism of Energy Foods; 
and Nutrition
In general, the ability of the neonate to digest, absorb,
and 

the ability of the neonate to digest, absorb,
and metabolize foods is no different from that of the
older child, with the following three exceptions.
First, secretion of pancreatic amylase in the neonate
is deﬁcient, so that the neonate uses starches less ade-
quately than do older children.
Second, absorption of fats from the gastrointestinal
tract is somewhat less than that in the older child. Con-
sequently, milk with a high fat content, such as cow’s
milk, is frequently inadequately absorbed

as cow’s
milk, is frequently inadequately absorbed.
Third, because the liver functions imperfectly during
at least the ﬁrst week of life, the glucose concentration
in the blood is unstable and low.
The neonate is especially capable of synthesizing and
storing proteins. Indeed, with an adequate diet, as much
0
2
4
6
8
10
12
14
16
0
1
2
3
4
5
Age in weeks
Bilirubin
Red blood count
Serum bilirubin (mg/dl)
0
1
2
3
4
5
Red blood cell count (millions/mm3)
Figure 83–6
Changes in the red blood cell coun

m3)
Changes in the red blood cell count and in serum bilirubin con-
centration during the ﬁrst 16 weeks of life, showing physiologic
anemia at 6 to 12 weeks of life and physiologic hyperbilirubine-
mia during the ﬁrst 2 weeks of life.
Chapter 83
1049
as 90 per cent of the ingested amino acids are used for
formation of body proteins. This is a much higher per-
centage than in adults.
Metabolic Rate and Body Temperature. The normal meta-
bolic rate of the 

y Temperature. The normal meta-
bolic rate of the neonate in relation to body weight is
about twice that of the adult, which accounts also for
the twice as great cardiac output and twice as great
minute respiratory volume in relation to body weight in
the infant.
Because the body surface area is large in relation to
body mass, heat is readily lost from the body.As a result,
the body temperature of the neonate, particularly of
premature infants, falls easily. Figure 83–7 shows that
the body tempe

alls easily. Figure 83–7 shows that
the body temperature of even a normal infant often falls
several degrees during the ﬁrst few hours after birth but
returns to normal in 7 to 10 hours. Still, the body tem-
perature regulatory mechanisms remain poor during
the early days of life, allowing marked deviations in
temperature, which are also shown in Figure 83–7.
Nutritional Needs During the Early Weeks of Life. At birth, a
neonate is usually in complete nutritional balance, pro-
vided the mother ha

lete nutritional balance, pro-
vided the mother has had an adequate diet. Further-
more, function of the gastrointestinal system is usually
more than adequate to digest and assimilate all the
nutritional needs of the infant if appropriate nutrients
are provided in the diet. However, three speciﬁc prob-
lems do occur in the early nutrition of the infant.
Need for Calcium and Vitamin D 
The neonate is in a stage of rapid ossiﬁcation of its
bones at birth, so that a ready supply of calcium
througho

 birth, so that a ready supply of calcium
throughout infancy is needed.This is ordinarily supplied
adequately by the usual diet of milk. Yet absorption of
calcium by the gastrointestinal tract is poor in the
absence of vitamin D. Therefore, the vitamin D–
deﬁcient infant can develop severe rickets in only a few
weeks. This is particularly true in premature babies
because their gastrointestinal tracts absorb calcium
even less effectively than those of normal infants.
Necessity for Iron in the Die

e of normal infants.
Necessity for Iron in the Diet
If the mother has had adequate amounts of iron in her
diet, the liver of the infant usually has stored enough
iron to keep forming blood cells for 4 to 6 months after
birth. But if the mother has had insufﬁcient iron in her
diet, severe anemia is likely to occur in the infant after
about 3 months of life. To prevent this possibility, early
feeding of the infant with egg yolk, which contains rea-
sonably large quantities of iron, or the administ

sonably large quantities of iron, or the administration of
iron in some other form is desirable by the second or
third month of life.
Vitamin C Deﬁciency in Infants 
Ascorbic acid (vitamin C) is not stored in signiﬁcant
quantities in the fetal tissues; yet it is required for
proper formation of cartilage, bone, and other intercel-
lular structures of the infant. Furthermore, milk has
poor supplies of ascorbic acid, especially cow’s milk,
which has only one fourth as much as human milk. For
this

as only one fourth as much as human milk. For
this reason, orange juice or other sources of ascorbic
acid are often prescribed by the third week of life.
Immunity
The neonate inherits much immunity from the mother
because many protein antibodies diffuse from the
mother’s blood through the placenta into the fetus.
However, the neonate does not form antibodies of its
own to a signiﬁcant extent.By the end of the ﬁrst month,
the baby’s gamma globulins, which contain the anti-
bodies, have decreased 

s, which contain the anti-
bodies, have decreased to less than one half the original
level,with a corresponding decrease in immunity.There-
after, the baby’s own immunity system begins to form
antibodies, and the gamma globulin concentration
returns essentially to normal by the age of 12 to 20
months.
Despite the decrease in gamma globulins soon after
birth, the antibodies inherited from the mother protect
the infant for about 6 months against most major child-
hood infectious diseases, includin

st major child-
hood infectious diseases, including diphtheria, measles,
and polio. Therefore, immunization against these 
diseases before 6 months is usually unnecessary.
Conversely, the inherited antibodies against whooping
cough are normally insufﬁcient to protect the neonate;
therefore, for full safety, the infant requires immuniza-
tion against this disease within the ﬁrst month or so of
life.
Allergy. The newborn infant is seldom subject to allergy.
Several months later, however, when the 

 allergy.
Several months later, however, when the infant’s own
antibodies ﬁrst begin to form, extreme allergic states
can develop, often resulting in serious eczema, gas-
trointestinal abnormalities, and even anaphylaxis. As
the child grows older and still higher degrees of immu-
nity develop, these allergic manifestations usually dis-
appear.This relation of immunity to allergy is discussed
in Chapter 34.
Ordinarily, the endocrine system of the infant is highly
developed at b

rine system of the infant is highly
developed at birth, and the infant seldom exhibits any
immediate endocrine abnormalities. However, there are
special instances in which the endocrinology of infancy
is important:
1. If a pregnant mother bearing a female child is
treated with an androgenic hormone or if an
0 2 4 6 8 10 12
4 6
2
20
93
99
98
97
96
95
94
Hours after birth
Days after birth
Birth
Figure 83–7
Fall in body temperature of the neonate immediately a

l in body temperature of the neonate immediately after birth,
and instability of body temperature during the ﬁrst few days of life.
1050
Unit XIV
androgenic tumor develops during pregnancy,
the child will be born with a high degree of
masculinization of her sexual organs, thus resulting
in a type of hermaphroditism.
2. The sex hormones secreted by the placenta and by
the mother’s glands during pregnancy occasionally
cause the neonate’s breasts to form milk 
during 

during the ﬁrst days of life. Sometimes the breasts
then become inﬂamed, or infectious mastitis
develops.
3. An infant born of an untreated diabetic mother will
have considerable hypertrophy and hyperfunction
of the islets of Langerhans in the pancreas.
As a consequence, the infant’s blood glucose
concentration may fall to lower than 20 mg/dl
shortly after birth. Fortunately, however, in the
neonate, unlike in the adult, insulin shock or coma
from this 

ike in the adult, insulin shock or coma
from this low level of blood glucose concentration
only rarely develops.
Maternal type II diabetes is the most common
cause of large babies. Type II diabetes in the
mother is associated with resistance to the
metabolic effects of insulin and compensatory
increases in plasma insulin concentration. The high
levels of insulin are believed to stimulate fetal
growth factor and contribute to increased birth
weight. Increased supply of glucose and other
nutrients

t. Increased supply of glucose and other
nutrients to the fetus may also contribute to
increased fetal growth. However, most of the
increased fetal weight is due to increased body fat;
there is usually little increase in body length
although the size of some organs may be increased
(organomegaly).
In the mother with uncontrolled type I diabetes
(caused by lack of insulin secretion), fetal growth
may be stunted because of metabolic deﬁcits in the
mother, and growth and tissue maturation of the
ne

neonate are often impaired. Also, there is a high
rate of intrauterine mortality, and among those
fetuses that do come to term, there is still a high
mortality rate. Two thirds of the infants who die
succumb to respiratory distress syndrome, described
4. Occasionally a child is born with hypofunctional
adrenal cortices, often resulting from agenesis of
the adrenal glands or exhaustion atrophy, which can
occur when the adrena

xhaustion atrophy, which can
occur when the adrenal glands have been vastly
overstimulated.
5. If a pregnant woman has hyperthyroidism or is
treated with excess thyroid hormone, the infant is
likely to be born with a temporarily hyposecreting
thyroid gland. Conversely, if before pregnancy a
woman had had her thyroid gland removed, her
pituitary gland may secrete great quantities of
thyrotropin during gestation, and the child might be
born with temporary hyperthyroidism.
6. In a fetus lacking thy

mporary hyperthyroidism.
6. In a fetus lacking thyroid hormone secretion, the
bones grow poorly and there is mental retardation.
This causes the condition called cretin dwarﬁsm,
discussed in Chapter 76.
Special Problems 
of Prematurity
All the problems in neonatal life just noted are severely
exacerbated in prematurity. They can be categorized
under the following two headings: (1) immaturity of
certain organ systems and (2) instability of the different
homeostatic control systems. Because of the

ferent
homeostatic control systems. Because of these effects, a
premature baby seldom lives if it is born more than 3
months before term.
Immature Development of the
Premature Infant
Almost all the organ systems of the body are immature
in the premature infant, but some require particular
attention if the life of the premature baby is to be saved.
Respiration. The respiratory system is especially likely to
be underdeveloped in the premature infant. The vital
capacity and the functional residual 

t. The vital
capacity and the functional residual capacity of the
lungs are especially small in relation to the size of the
infant.Also, surfactant secretion is depressed or absent.
As a consequence, respiratory distress syndrome is a
common cause of death. Also, the low functional resid-
ual capacity in the premature infant is often associated
with periodic breathing of the Cheyne-Stokes type.
Gastrointestinal Function. Another major problem of the
premature infant is to ingest and absorb adequ

the
premature infant is to ingest and absorb adequate food.
If the infant is more than 2 months premature, the 
digestive and absorptive systems are almost always
inadequate.The absorption of fat is so poor that the pre-
mature infant must have a low-fat diet. Furthermore,
the premature infant has unusual difﬁculty in absorbing
calcium and, therefore, can develop severe rickets
before the difﬁculty is recognized. For this reason,
special attention must be paid to adequate calcium and
vitamin D i

n must be paid to adequate calcium and
vitamin D intake.
Function of Other Organs. Immaturity of other organ
systems that frequently causes serious difﬁculties in the
premature infant includes (1) immaturity of the liver,
which results in poor intermediary metabolism and
often a bleeding tendency as a result of poor formation
of coagulation factors; (2) immaturity of the kidneys,
which are particularly deﬁcient in their ability to rid the
body of acids, thereby predisposing to acidosis as well
a

 acids, thereby predisposing to acidosis as well
as to serious ﬂuid balance abnormalities; (3) immaturity
of the blood-forming mechanism of the bone marrow,
which allows rapid development of anemia; and (4)
depressed formation of gamma globulin by the lym-
phoid system, which often leads to serious infection.
Instability of the Homeostatic
Control Systems in the 
Immaturity of the different organ systems in the pre-
mature infant creates a high degree of instability in the
homeo

 creates a high degree of instability in the
homeostatic mechanisms of the body. For instance, the
acid-base balance can vary tremendously, particularly
when the rate of food intake varies from time to time.
Likewise, the blood protein concentration is usually low
because of immature liver development, often leading
to hypoproteinemic edema. And inability of the infant
to regulate its calcium ion concentration frequently
brings on hypocalcemic tetany. Also, the blood glucose
concentration can va

tany. Also, the blood glucose
concentration can vary between the extremely wide
limits of 20 to more than 100 mg/dl, depending princi-
Chapter 83
1051
pally on the regularity of feeding. It is no wonder, then,
with these extreme variations in the internal environ-
ment of the premature infant, that mortality is high.
Instability of Body Temperature. One of the particular prob-
lems of the premature infant is inability to maintain
normal body temperature. Its tempera

y to maintain
normal body temperature. Its temperature tends to
approach that of its surroundings. At normal room 
temperature, the infant’s temperature may stabilize in
the low 90°s or even in the 80°s F. Statistical studies
show that a body temperature maintained below 96°F
(35.5°C) is associated with a particularly high incidence
of death, which explains the almost mandatory use of
the incubator in treatment of prematurity.
Danger of Blindness Caused 
by Excess Oxygen Therapy 
in the Prematur

 Caused 
in the Premature Infant
Because premature infants frequently develop respira-
tory distress, oxygen therapy has often been used in
treating prematurity. However, it has been discovered
that use of excess oxygen in treating premature infants,
especially in early prematurity, can lead to blindness.
The reason is that too much oxygen stops the growth of
new blood vessels in the retina. Then when oxygen
therapy is stopped, the blood vessels try to make up for
lost 

topped, the blood vessels try to make up for
lost time and burst forth with a great mass of vessels
growing all through the vitreous humor, blocking light
from the pupil to the retina. And still later, the vessels
are replaced with a mass of ﬁbrous tissue where the
eye’s clear vitreous humor should be.
This condition, known as retrolental ﬁbroplasia,
causes permanent blindness. For this reason, it is par-
ticularly important to avoid treatment of premature
infants with high concentrations of res

 premature
infants with high concentrations of respiratory oxygen.
Physiologic studies indicate that the premature infant is
usually safe with up to 40 per cent oxygen in the air
breathed, but some child physiologists believe that com-
plete safety can be achieved only at normal oxygen con-
centration in the air that is breathed.
Growth and Development 
of the Child
The major physiologic problems of the child beyond the
neonatal period are related to special metabolic needs
for growth, which hav

d to special metabolic needs
for growth, which have been fully covered in the sec-
tions of this book on metabolism and endocrinology.
Figure 83–8 shows the changes in heights of boys and
girls from the time of birth until the age of 20 years.
Note especially that these parallel each other almost
exactly until the end of the ﬁrst decade of life. Between
the ages of 11 and 13 years, the female estrogens begin
to be formed and cause rapid growth in height but early
uniting of the epiphyses of the 

 height but early
uniting of the epiphyses of the long bones at about the
14th to 16th year of life, so that growth in height then
ceases. This contrasts with the effect of testosterone in
the male, which causes extra growth at a slightly later
age—mainly between ages 13 and 17 years. The male,
however, undergoes much more prolonged growth
because of much delayed uniting of the epiphyses, so
that his ﬁnal height is considerably greater than that of
0
20
4
4
8 12 16
8
12
16
20 24
20
7

of
0
20
4
4
8 12 16
8
12
16
20 24
20
70
60
50
40
30
Boys
Height (inches)
Girls
Figure 83–8
Average height of boys and girls from infancy to 20 years of age.
12
12
11
11
10
10
9
8
7
6
5
4
3
2
1
0
Birth
Birth
Walks alone
Stands alone
Walks with support
Pulls up
Pulls up
Grasps
Grasps
Crawls
Crawls
Sits briefly
Rolls over
Rolls over
Hand control
Head control
Vocali

trol
Vocalizes
Vocalizes
Smiles
Smiles
Suckles
Suckles
Figure 83–9
Behavioral development of the infant during the ﬁrst year of life.
1052
Unit XIV
Behavioral growth is principally a problem of maturity
of the nervous system. It is extremely difﬁcult to 
dissociate maturity of the anatomical structures of the
nervous system from maturity caused by training.
Anatomical studies show that certain major tracts in

atomical studies show that certain major tracts in the
central nervous system are not completely myelinated
until the end of the ﬁrst year of life. For this reason,
it is frequently stated that the nervous system is not 
fully functional at birth. The brain cortex and its associ-
ated functions, such as vision, seem to require several
months after birth for ﬁnal functional development to
occur.
At birth, the infant brain mass is only 26 per cent of
the adult brain mass and 55 per cent at 1 year,

of
the adult brain mass and 55 per cent at 1 year, but it
reaches almost adult proportions by the end of the
second year. This is also associated with closure of the
fontanels and sutures of the skull, which allows only 20
per cent additional growth of the brain beyond the ﬁrst
2 years of life. Figure 83–9 shows a normal progress
chart for the infant during the ﬁrst year of life. Com-
parison of this chart with the baby’s actual development
is used for clinical assessment of mental and behaviora

ed for clinical assessment of mental and behavioral
growth.
Bissonnette JM: Mechanisms regulating hypoxic respiratory
depression during fetal and postnatal life. Am J Physiol
Regul Integr Comp Physiol 278:R1391, 2000.
Chen Y, Lasaitiene D, Friberg P: The renin-angiotensin
system in kidney development. Acta Physiol Scand
181:529, 2004.
Challis JRG, Matthews SG, Gibb W, Lye SJ: Endocrine and
paracrine regulation of birth at term and preterm. Endocr
Rev 21:514, 2000.
de Castro F: Chemotr

erm. Endocr
de Castro F: Chemotropic molecules: guides for axonal
pathﬁnding and cell migration during CNS development.
News Physiol Sci 18:130, 2003.
Holemans K, Aerts L, Van Assche FA: Lifetime conse-
quences of abnormal fetal pancreatic develop

onse-
quences of abnormal fetal pancreatic development. J
Physiol 547:11, 2003.
Jensen BL, Stubbe J, Madsen K, et al: The renin-angiotensin
system in kidney development: role of COX-2 and adrenal
steroids. Acta Physiol Scand 181:549, 2004.
Johnson MH: Functional brain development in humans. Nat
Rev Neurosci 2:475, 2001.
Kanwar YS,Wada J, Lin S, Danesh FR, et al: Update of extra-
cellular matrix, its receptors, and cell adhesion molecules
in mammalian nephrogenesis.Am J Physiol Renal Physiol
286:

lian nephrogenesis.Am J Physiol Renal Physiol
286:F202, 2004.
Kovacs CS, Kronenberg HM: Maternal-fetal calcium and
bone metabolism during pregnancy, puerperium, and 
lactation. Endocr Rev 18:832, 1997.
MacLellan WR, Schneider MD: Genetic dissection of
cardiac growth control pathways. Annu Rev Physiol
62:289, 2000.
Matias A, Montenegro N, Areias JC, Leite LP: Haemody-

ias A, Montenegro N, Areias JC, Leite LP: Haemody-
namic evaluation of the ﬁrst trimester fetus with special
emphasis on venous return. Hum Reprod Update 6:177,
2000.
McMurtry IF: Pre- and postnatal lung development, matu-
ration, and plasticity. Am J Physiol Lung Cell Mol Physiol
282:L341, 2002.
Olver RE, Walters DV, M Wilson S: Developmental regula-
tion of lung liquid transport. Annu Rev Physiol 66:77,
2004.
Ross MG, Nijland MJ: Development of ingestive behavior.
Am J Physiol 274:R879, 1998.

 ingestive behavior.
Shannon JM, Hyatt BA: Epithelial-mesenchymal interac-
tions in the developing lung. Annu Rev Physiol 66:625,
2004.
Srivastava D: Genetic assembly of the heart: implications for
congenital heart disease. Annu Rev Physiol 63:451, 2001.
Wood CE, Tong H: Central nervous system regulation of
reﬂex responses to hypotension during fetal life. Am J
Physiol 277:R1541, 1999.
U
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T
XV
84. Sports Physiology
C
H
A
P
T
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8
4
1055
Sports

 Sports Physiology
C
H
A
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4
1055
There are few stresses to which the body is exposed that
even nearly approach the extreme stresses of heavy
exercise. In fact, if some of the extremes of exercise
were continued for even moderately prolonged periods,
they might be lethal. Therefore, in the main, sports
physiology is a discussion of the ultimate limits to which
several of the bodily mechanisms can be stressed. To
give one simple example: In a person who has
extremely h

ne simple example: In a person who has
extremely high fever approaching the level of lethality,
the body metabolism increases to about 100 per cent above normal. By compari-
son, the metabolism of the body during a marathon race may increase to 2000 per
Female and Male Athletes Most of the quantitative data that are given in this chapter
are for the young male athlete, not because it is desirable to know only these values
but because it is only in male athletes that relatively

ecause it is only in male athletes that relatively complete measurements have
been made. However, for those measurements that have been made in the female
athlete, almost identical basic physiologic principles apply, except for quantitative
differences caused by differences in body size, body composition, and the presence
or absence of the male sex hormone testosterone.
In general, most quantitative values for women—such as muscle strength, pul-
monary ventilation, and cardiac output, all of whi

monary ventilation, and cardiac output, all of which are related mainly to the muscle
mass—vary between two thirds and three quarters of the values recorded in men.
When measured in terms of strength per square centimeter of cross-sectional area,
the female muscle can achieve almost exactly the same maximal force of contrac-
tion as that of the male-between 3 and 4 kg/cm2. Therefore, most of the difference
in total muscle performance lies in the extra percentage of the male body that is
muscle, 

extra percentage of the male body that is
muscle, caused by endocrine differences that we discuss later.
The performance capabilities of the female versus the male athlete are illustrated
by the relative running speeds for a marathon race. In a recent comparison, the top
female performer had a running speed that was 11 per cent less than that of the top
male performer. For other events, however, women have at times held records faster
than men—for instance, for the two-way swim across the Englis

r instance, for the two-way swim across the English Channel, where
the availability of extra fat seems to be an advantage for heat insulation, buoyancy,
and extra long-term energy.
Testosterone secreted by the male testes has a powerful anabolic effect in causing
greatly increased deposition of protein everywhere in the body, but especially in the
muscles. In fact, even a male who participates in very little sports activity but who
nevertheless is well endowed with testosterone will have muscles

s well endowed with testosterone will have muscles that grow about
40 per cent larger than those of a comparable female without the testosterone.
The female sex hormone estrogen probably also accounts for some of the differ-
ence between female and male performance, although not nearly so much as testos-
terone. Estrogen is known to increase the deposition of fat in the female, especially
in the breasts, hips, and subcutaneous tissue. At least partly for this reason, the
average nonathletic fema

rtly for this reason, the
average nonathletic female has about 27 per cent body fat composition, in contrast
to the nonathletic male, who has about 15 per cent.This is a detriment to the highest
levels of athletic performance in those events in which performance depends on
speed or on ratio of total body muscle strength to body weight.
The ﬁnal common determinant of success in athletic events is what the muscles can
do for you—what st

 events is what the muscles can
do for you—what strength they can give when it is needed, what power they can
achieve in the performance of work, and how long they can continue their activity.
1056
Unit XV
The strength of a muscle is determined mainly by its
size, with a maximal contractile force between 3 and 
4 kg/cm2 of muscle cross-sectional area.Thus, a man who
is well supplied with testosterone or who has enlarged
his muscles through an exercise training program will
have

les through an exercise training program will
have correspondingly increased muscle strength.
To give an example of muscle strength, a world-class
weight lifter might have a quadriceps muscle with a
cross-sectional area as great as 150 square centimeters.
This would translate into a maximal contractile strength
of 525 kilograms (or 1155 pounds), with all this force
applied to the patellar tendon. Therefore, one can
readily understand how it is possible for this tendon at
times to be ruptured or 

ssible for this tendon at
times to be ruptured or actually to be avulsed from its
insertion into the tibia below the knee. Also, when such
forces occur in tendons that span a joint, similar forces
are applied to the surfaces of the joint or sometimes to
ligaments spanning the joints, thus accounting for such
happenings as displaced cartilages, compression frac-
tures about the joint, and torn ligaments.
The holding strength of muscles is about 40 per cent
greater than the contractile strength.Th

 per cent
greater than the contractile strength.That is, if a muscle
is already contracted and a force then attempts to
stretch out the muscle, as occurs when landing after a
jump, this requires about 40 per cent more force than
can be achieved by a shortening contraction.Therefore,
the force of 525 kilograms calculated above for the
patellar tendon during muscle contraction becomes 735
kilograms (1617 pounds) during holding contractions.
This further compounds the problems of the tendons,
joint

rther compounds the problems of the tendons,
joints, and ligaments. It can also lead to internal tearing
in the muscle itself. In fact, forceful stretching of a max-
imally contracted muscle is one of the surest ways to
create the highest degree of muscle soreness.
Mechanical work performed by a muscle is the
amount of force applied by the muscle multiplied by the
distance over which the force is applied. The power of
muscle contraction is different from muscle strength,
because power is a measu

ent from muscle strength,
because power is a measure of the total amount of 
work that the muscle performs in a unit period of time.
Power is therefore determined not only by the strength
of muscle contraction but also by its distance of 
contraction and the number of times that it contracts
each minute. Muscle power is generally measured in
kilogram meters (kg-m) per minute. That is, a muscle
that can lift 1 kilogram weight to a height of 1 meter or
that can move some object laterally against a

r or
that can move some object laterally against a force of 1
kilogram for a distance of 1 meter in 1 minute is said to
have a power of 1 kg-m/min. The maximal power
achievable by all the muscles in the body of a highly
trained athlete with all the muscles working together is
is for the next 30 minutes, because the efﬁciency for
translation of muscle power output into athletic per-
formance is often much less during rapid activity than
during less rapid but sustained

apid activity than
during less rapid but sustained activity. Thus, the veloc-
ity of the 100-meter dash is only 1.75 times as great as
the velocity of a 30-minute race, despite the fourfold dif-
ference in short-term versus long-term muscle power
Another 
measure 
of 
muscle 
performance 
is
endurance. This, to a great extent, depends on the nutri-
tive support for the muscle—more than anything else
on the amount of glycogen that has been stored in the
muscle before the period of exe

been stored in the
muscle before the period of exercise. A person on a
high-carbohydrate diet stores far more glycogen in
muscles than a person on either a mixed diet or a high-
fat diet. Therefore, endurance is greatly enhanced by a
high-carbohydrate diet. When athletes run at speeds
typical for the marathon race, their endurance (as meas-
ured by the time that they can sustain the race until
complete exhaustion) is approximately the following:
kg-m/min
First 8 to 10 seconds
7000
Next 1 minute

kg-m/min
7000
4000
Next 30 minutes
1700
Thus, it is clear that a person has the capability of
extreme power surges for short periods of time, such as
during a 100-meter dash that is completed entirely
within 10 seconds, whereas for long-term endurance
events, the power output of the muscles is only one
fourth as great as during the initial power surge.
This does not mean that one’s athletic performance is
four times as great during the initial power surge as i

times as great during the initial power surge as it
Minutes
High-carbohydrate diet
240
Mixed diet
120
High-fat diet
85
The corresponding amounts of glycogen stored in the
muscle before the race started explain these differences.
The amounts stored are approximately the following:
g/kg Muscle
40
Mixed diet
20
6
Muscle Metabolic Systems 
in Exercise
The same basic metabolic systems are present in muscle
as in other parts of the body; these are discussed in
deta

her parts of the body; these are discussed in
detail in Chapters 67 through 73. However, special
quantitative measures of the activities of three meta-
bolic systems are exceedingly important in understand-
ing the limits of physical activity. These systems are (1)
the phosphocreatine-creatine system, (2) the glycogen-
lactic acid system, and (3) the aerobic system.
Adenosine Triphosphate. The source of energy actually
used to cause muscle contraction is adenosine triphos-
phate (ATP), which has

ction is adenosine triphos-
phate (ATP), which has the following basic formula:
Adenosine-PO3 ~ PO3 ~ PO3
-
The bonds attaching the last two phosphate radicals
to the molecule, designated by the symbol ~, are high-
energy phosphate bonds. Each of these bonds stores
7300 calories of energy per mole of ATP under standard
conditions (and even slightly more than this under the
physical conditions in the body, which is discussed in
detail in Chapter 67). Therefore, when one phosphate
radical is remov

7). Therefore, when one phosphate
radical is removed, more than 7300 calories of energy
are released to energize the muscle contractile process.
Then, when the second phosphate radical is removed,
Chapter 84
1057
still another 7300 calories become available. Removal
of the ﬁrst phosphate converts the ATP into adenosine
diphosphate (ADP),and removal of the second converts
this ADP into adenosine monophosphate (AMP).
The amount of ATP present in the muscles, even in a
well-traine

 ATP present in the muscles, even in a
well-trained athlete, is sufﬁcient to sustain maximal
muscle power for only about 3 seconds, maybe enough
for one half of a 50-meter dash. Therefore, except for a
few seconds at a time, it is essential that new ATP be
formed continuously, even during the performance of
short athletic events. Figure 84–1 shows the overall
metabolic system, demonstrating the breakdown of
ATP ﬁrst to ADP and then to AMP, with the release of
energy to the muscles for contractio

he release of
energy to the muscles for contraction.The left-hand side
of the ﬁgure shows the three metabolic systems that
provide a continuous supply of ATP in the muscle ﬁbers.
Phosphocreatine (also called creatine phosphate) is
another chemical compound that has a high-energy
phosphate bond, with the following formula:
Creatine ~ PO3
-
This can decompose to creatine and phosphate ion, as
shown to the left in Figure 84–1, and in doing so release
large amounts of

ure 84–1, and in doing so release
large amounts of energy. In fact, the high-energy phos-
phate bond of phosphocreatine has more energy than
the bond of ATP, 10,300 calories per mole in compari-
son with 7300. Therefore, phosphocreatine can easily
provide enough energy to reconstitute the high-energy
bond of ATP. Furthermore, most muscle cells have two
to four times as much phosphocreatine as ATP.
A special characteristic of energy transfer from phos-
phocreatine to ATP is that it occurs within 

phos-
phocreatine to ATP is that it occurs within a small frac-
tion of a second. Therefore, all the energy stored in the
muscle phosphocreatine is almost instantaneously avail-
able for muscle contraction, just as is the energy stored
in ATP.
The combined amounts of cell ATP and cell phos-
phocreatine are called the phosphagen energy system.
These together can provide maximal muscle power for
8 to 10 seconds, almost enough for the 100-meter run.
Thus, the energy from the phosphagen system is us

Thus, the energy from the phosphagen system is used for
maximal short bursts of muscle power.
Glycogen-Lactic Acid System. The stored glycogen in
muscle can be split into glucose and the glucose then
used for energy. The initial stage of this process, called
glycolysis, occurs without use of oxygen and, therefore,
is said to be anaerobic metabolism (see Chapter 67).
During glycolysis,each glucose molecule is split into two
pyruvic acid molecules, and energy is released to form
four ATP molecule

, and energy is released to form
four ATP molecules for each original glucose molecule,
as explained in Chapter 67. Ordinarily, the pyruvic 
acid then enters the mitochondria of the muscle cells 
and reacts with oxygen to form still many more 
ATP molecules. However, when there is insufﬁcient
oxygen for this second stage (the oxidative stage) of
glucose metabolism to occur, most of the pyruvic acid
then is converted into lactic acid, which diffuses out of
the muscle cells into the interstitial ﬂ

es out of
the muscle cells into the interstitial ﬂuid and blood.
Therefore, much of the muscle glycogen is transformed
to lactic acid, but in doing so, considerable amounts of
ATP are formed entirely without the consumption of
oxygen.
Another characteristic of the glycogen-lactic acid
system is that it can form ATP molecules about 2.5 
times as rapidly as can the oxidative mechanism of the
mitochondria. Therefore, when large amounts of ATP
are required for short to moderate periods of muscle
con

quired for short to moderate periods of muscle
contraction, this anaerobic glycolysis mechanism can be
used as a rapid source of energy. It is, however, only
about one half as rapid as the phosphagen system.
Under optimal conditions, the glycogen-lactic acid
system can provide 1.3 to 1.6 minutes of maximal
muscle activity in addition to the 8 to 10 seconds pro-
vided by the phosphagen system, although at somewhat
reduced muscle power.
Aerobic System. The aerobic system is the oxidation of
foodst

tem. The aerobic system is the oxidation of
foodstuffs in the mitochondria to provide energy. That
is,as shown to the left in Figure 84–1,glucose,fatty acids,
and amino acids from the foodstuffs—after some inter-
mediate processing—combine with oxygen to release
tremendous amounts of energy that are used to convert
AMP and ADP into ATP, as discussed in Chapter 67.
In comparing this aerobic mechanism of energy
supply with the glycogen-lactic acid system and the
phosphagen system, the relative max

system and the
phosphagen system, the relative maximal rates of power
generation in terms of moles of ATP generation per
minute are the following:
I. Phosphocreatine
Creatine + PO3
-
II. Glycogen
ATP
Energy
for muscle
ADP
AMP
+
Urea
III. Glucose
     Fatty acids
     Amino acids
CO2 + H2O
+ O2
Figure 84–1
Important 
metabolic 
systems 
that supply energy for muscle
Moles of ATP/min
Phosphagen system
4
Glycogen–lactic acid system
2.5
Aerobic system
1
1058
Unit

–lactic acid system
2.5
1
1058
Unit XV
oxidative metabolism of the aerobic system can be used
to reconstitute all the other systems-the ATP, the phos-
phocreatine, and the glycogen-lactic acid system.
Reconstitution of the lactic acid system means mainly
the removal of the excess lactic acid that has accumu-
lated in all the ﬂuids of the body. This is especially
important because lactic acid causes extreme fatigue.
When adequate amounts of energy are available fr

.
When adequate amounts of energy are available from
oxidative metabolism, removal of lactic acid is achieved
in two ways: (1) A small portion of it is converted back
into pyruvic acid and then metabolized oxidatively by
all the body tissues. (2) The remaining lactic acid is
reconverted into glucose mainly in the liver, and the
glucose in turn is used to replenish the glycogen stores
of the muscles.
Recovery of the Aerobic System After Exercise. Even during
the early stages of heavy exercise, a 

Even during
the early stages of heavy exercise, a portion of one’s
aerobic energy capability is depleted. This results from
two effects: (1) the so-called oxygen debt and (2) deple-
tion of the glycogen stores of the muscles.
Oxygen Debt. The body normally contains about 2 liters
of stored oxygen that can be used for aerobic metabo-
lism even without breathing any new oxygen. This
stored oxygen consists of the following: (1) 0.5 liter in
the air of the lungs, (2) 0.25 liter dissolved in the body

of the lungs, (2) 0.25 liter dissolved in the body
ﬂuids, (3) 1 liter combined with the hemoglobin of the
blood, and (4) 0.3 liter stored in the muscle ﬁbers them-
selves, combined mainly with myoglobin, an oxygen-
binding chemical similar to hemoglobin.
In heavy exercise,almost all this stored oxygen is used
within a minute or so for aerobic metabolism. Then,
after the exercise is over, this stored oxygen must be
replenished by breathing extra amounts of oxygen over
and above the normal require

mounts of oxygen over
and above the normal requirements. In addition, about
9 liters more oxygen must be consumed to provide for
reconstituting both the phosphagen system and the
lactic acid system. All this extra oxygen that must be
“repaid,” about 11.5 liters, is called the oxygen debt.
Figure 84–2 shows this principle of oxygen debt.
During the ﬁrst 4 minutes of the ﬁgure, the person exer-
cises heavily, and the rate of oxygen uptake increases
When comparing the same systems for endurance, th

When comparing the same systems for endurance, the
relative values are the following:
Time
8 to 10 seconds
1.3 to 1.6 minutes
Unlimited time (as long as
nutrients last)
Thus, one can readily see that the phosphagen system
is the one used by the muscle for power surges of a few
seconds, and the aerobic system is required for pro-
longed athletic activity. In between is the glycogen-lactic
acid system, which is especially important for 

ic
acid system, which is especially important for giving
extra power during such intermediate races as the 200-
to 800-meter runs.
What Types of Sports Use Which Energy Systems? By consid-
ering the vigor of a sports activity and its duration, one
can estimate closely which of the energy systems is used
for each activity. Various approximations are presented
in Table 84–1.
Recovery of the Muscle Metabolic Systems After Exercise. In
the same way that the energy from phosphocreatine can
be used to

hat the energy from phosphocreatine can
be used to reconstitute ATP, energy from the glycogen-
lactic acid system can be used to reconstitute both phos-
phocreatine and ATP. And then energy from the
Table 84–1
Energy Systems Used in Various Sports
Phosphagen system, almost entirely
100-meter dash
Jumping
Weight lifting
Diving
Football dashes
Phosphagen and glycogen-lactic acid systems
200-meter dash
Basketball
Baseball home run
Ice hockey dashes
Glycogen-lactic acid system, mainly
400-meter dash

100-meter swim
Tennis
Soccer
Glycogen-lactic acid and aerobic systems
800-meter dash
200-meter swim
1500-meter skating
Boxing
2000-meter rowing
1500-meter run
1-mile run
400-meter swim
10,000-meter skating
Cross-country skiing
Marathon run (26.2 miles, 42.2 km)
Jogging
0
4
8
12
16
20
24
28
32
36
40
44
0
5
4
3
2
1
Rate of oxygen uptake (L/min)
Exercise
Minutes
Alactacid oxygen debt = 3.5 liters
Lactic acid oxygen debt = 8 liters
Fi

= 3.5 liters
Figure 84–2
Rate of oxygen uptake by the lungs during maximal exercise for
4 minutes and then for about 40 minutes after the exercise is over.
This ﬁgure demonstrates the principle of oxygen debt.
Chapter 84
1059
more than 15-fold.Then, even after the exercise is over,
the oxygen uptake still remains above normal, at ﬁrst
very high while the body is reconstituting the phospha-
gen system and repaying the stored oxygen portion of
t

system and repaying the stored oxygen portion of
the oxygen debt, and then for another 40 minutes at a
lower level while the lactic acid is removed. The early
portion of the oxygen debt is called the alactacid oxygen
debt and amounts to about 3.5 liters. The latter portion
is called the lactic acid oxygen debt and amounts to
about 8 liters.
Recovery of Muscle Glycogen. Recovery from exhaustive
muscle glycogen depletion is not a simple matter. This
often requires days, rather than the seconds, mi

s
often requires days, rather than the seconds, minutes, or
hours required for recovery of the phosphagen and
lactic acid metabolic systems. Figure 84–3 shows this
recovery process under three conditions: ﬁrst, in people
on a high-carbohydrate diet; second, in people on a
high-fat, high-protein diet; and third, in people with no
food. Note that on a high-carbohydrate diet, full recov-
ery occurs in about 2 days. Conversely, people on a high-
fat, high-protein diet or on no food at all show very 

 high-protein diet or on no food at all show very little
recovery even after as long as 5 days. The messages of
this comparison are (1) that it is important for an athlete
to have a high-carbohydrate diet before a grueling ath-
letic event and (2) not to participate in exhaustive exer-
cise during the 48 hours preceding the event.
Nutrients Used During 
Muscle Activity
In addition to the large usage of carbohydrates by the
muscles during exercise, especially during the early
stages of exercise, 

, especially during the early
stages of exercise, muscles use large amounts of fat for
energy in the form of fatty acids and acetoacetic acid (see
Chapter 68), and they use to a much less extent proteins
in the form of amino acids. In fact, even under the best
conditions, in those endurance athletic events that last
longer than 4 to 5 hours, the glycogen stores of the
muscle become almost totally depleted and are of little
further use for energizing muscle contraction. Instead,
the muscle now de

ing muscle contraction. Instead,
the muscle now depends on energy from other sources,
mainly from fats.
Figure 84–4 shows the approximate relative usage of
carbohydrates and fat for energy during prolonged
exhaustive exercise under three dietary conditions:
0
10
20
30
40
No food
Fat and protein diet
0
24
20
16
12
8
4
50
2 hours of
exercise
5 days
Muscle glycogen content (g/kg muscle)
Hours of recovery
Figure 84–3
Effect of diet on the rate of muscle
glycogen replenishment 

diet on the rate of muscle
glycogen replenishment after pro-
longed exercise. (Redrawn from
Fox 
EL: 
Sports 
Physiology.
Philadelphia: Saunders College
Publishing, 1979.)
0 10
40
Mixed diet
20
4
1
2
3
2
4
0
0
25
50
75
100
100
75
50
25
Per cent carbohydrate usage
Per cent fat usage
Minutes
Duration of exercise
Seconds
Hours
Exhaustion
Figure 84–4
Effect of duration of exercise as well as type of diet on relative
percentages of carbohydrate or fat used for ene

ve
percentages of carbohydrate or fat used for energy by muscles.
(Based partly on data in Fox EL: Sports Physiology. Philadelphia:
Saunders College Publishing, 1979.)
1060
Unit XV
high-carbohydrate diet, mixed diet, and high-fat diet.
Note that most of the energy is derived from carbohy-
drates during the ﬁrst few seconds or minutes of the
exercise, but at the time of exhaustion, as much as 60 to
85 per cent of the energy is being derived from fats,
rather than carbohydrates.

ing derived from fats,
Not all the energy from carbohydrates comes from
the stored muscle glycogen. In fact, almost as much
glycogen is stored in the liver as in the muscles, and this
can be released into the blood in the form of glucose
and then taken up by the muscles as an energy source.
In addition, glucose solutions given to an athlete to
drink during the course of an athletic event can provide
as much as 30 to 40 per cent of the energy required
during prolonged e

per cent of the energy required
during prolonged events such as marathon races.
Therefore, if muscle glycogen and blood glucose are
available, they are the energy nutrients of choice for
intense muscle activity. Even so, for a long-term
endurance event,one can expect fat to supply more than
50 per cent of the required energy after about the ﬁrst
3 to 4 hours.
Effect of Athletic Training on
Muscles and Muscle Performance
Importance of Maximal Resistance Training. One of the car-
dinal principles 

stance Training. One of the car-
dinal principles of muscle development during athletic
training is the following: Muscles that function under no
load, even if they are exercised for hours on end,
increase little in strength.At the other extreme, muscles
that contract at more than 50 per cent maximal force of
contraction will develop strength rapidly even if the
contractions are performed only a few times each day.
Using this principle, experiments on muscle building
have shown that six nearly m

ts on muscle building
have shown that six nearly maximal muscle contractions
performed in three sets 3 days a week give approxi-
mately optimal increase in muscle strength, without 
producing chronic muscle fatigue.
The upper curve in Figure 84–5 shows the approxi-
mate percentage increase in strength that can be
achieved in a previously untrained young person by this
resistive training program, demonstrating that the
muscle strength increases about 30 per cent during the
ﬁrst 6 to 8 weeks but a

out 30 per cent during the
ﬁrst 6 to 8 weeks but almost plateaus after that time.
Along with this increase in strength is an approximately
equal percentage increase in muscle mass, which is
called muscle hypertrophy.
In old age, many people become so sedentary that
their muscles atrophy tremendously. In these instances,
muscle training often increases muscle strength more
than 100 per cent.
Muscle Hypertrophy. The average size of a person’s
muscles is determined to a great extent by heredity plu

es is determined to a great extent by heredity plus
the level of testosterone secretion, which, in men, causes
considerably larger muscles than in women. With train-
ing, however, the muscles can become hypertrophied
perhaps an additional 30 to 60 per cent. Most of this
hypertrophy results from increased diameter of the
muscle ﬁbers rather than increased numbers of ﬁbers,
but this probably is not entirely true, because a very few
greatly enlarged muscle ﬁbers are believed to split down
the middl

 muscle ﬁbers are believed to split down
the middle along their entire length to form entirely
new ﬁbers, thus increasing the number of ﬁbers slightly.
The changes that occur inside the hypertrophied
muscle ﬁbers themselves include (1) increased numbers
of myoﬁbrils, proportionate to the degree of hypertro-
phy; (2) up to 120 per cent increase in mitochondrial
enzymes; (3) as much as 60 to 80 per cent increase in
the components of the phosphagen metabolic system,
including both ATP and phosphocr

metabolic system,
including both ATP and phosphocreatine; (4) as much
as 50 per cent increase in stored glycogen; and (5) as
much as 75 to 100 per cent increase in stored triglyc-
eride (fat). Because of all these changes, the capabilities
of both the anaerobic and the aerobic metabolic systems
are increased, increasing especially the maximum oxi-
dation rate and efﬁciency of the oxidative metabolic
system as much as 45 per cent.
Fast-Twitch and Slow-Twitch Muscle Fibers. In the human
being, all

Slow-Twitch Muscle Fibers. In the human
being, all muscles have varying percentages of fast-
twitch and slow-twitch muscle ﬁbers. For instance, the
gastrocnemius muscle has a higher preponderance of
fast-twitch ﬁbers, which gives it the capability of force-
ful and rapid contraction of the type used in jumping.
In contrast, the soleus muscle has a higher preponder-
ance of slow-twitch muscle ﬁbers and therefore is used
to a greater extent for prolonged lower leg muscle 
activity.
The basic diffe

longed lower leg muscle 
activity.
The basic differences between the fast-twitch and the
slow-twitch ﬁbers are the following:
1. Fast-twitch ﬁbers are about twice as large in
diameter.
2. The enzymes that promote rapid release of energy
from the phosphagen and glycogen-lactic acid
energy systems are two to three times as active in
fast-twitch ﬁbers as in slow-twitch ﬁbers, thus
making the maximal power that can be achieved
for very short periods of time by fast-twitch ﬁbers
about twice as great 

of time by fast-twitch ﬁbers
about twice as great as that of slow-twitch ﬁbers.
3. Slow-twitch ﬁbers are mainly organized for
endurance, especially for generation of aerobic
energy. They have far more mitochondria than 
the fast-twitch ﬁbers. In addition, they contain
considerably more myoglobin, a hemoglobin-like
protein that combines with oxygen within the
muscle ﬁber; the extra myoglobin increases the rate
of diffusion of oxygen throughout the ﬁber by
shuttling oxygen from one molecule of myo

 ﬁber by
shuttling oxygen from one molecule of myoglobin
to the next. In addition, the enzymes of the aerobic
0
2
4
6
8
10
0
30
25
20
15
10
5
Per cent increase in strength
Weeks of training
Resistive training
No-load training
Figure 84–5
Approximate effect of optimal resistive exercise training on
increase in muscle strength over a training period of 10 weeks.
Chapter 84
1061
metabolic system are considerably more active in
slow-twitch ﬁbers than in fast-twitch ﬁbers.
4. The nu

-twitch ﬁbers than in fast-twitch ﬁbers.
4. The number of capillaries is greater in the vicinity
of slow-twitch ﬁbers than in the vicinity of fast-
twitch ﬁbers.
In summary, fast-twitch ﬁbers can deliver extreme
amounts of power for a few seconds to a minute or 
so. Conversely, slow-twitch ﬁbers provide endurance,
delivering prolonged strength of contraction over many
minutes to hours.
Hereditary Differences Among Athletes for Fast-Twitch Versus
Slow-Twitch Muscle Fibers. Some people have consid

Slow-Twitch Muscle Fibers. Some people have considerably
more fast-twitch than slow-twitch ﬁbers, and others have
more slow-twitch ﬁbers; this could determine to some
extent the athletic capabilities of different individuals.
Athletic training has not been shown to change the rel-
ative proportions of fast-twitch and slow-twitch ﬁbers
however much an athlete might want to develop one
type of athletic prowess over another.Instead,this seems
to be determined almost entirely by genetic inheritance,

determined almost entirely by genetic inheritance,
and this in turn helps determine which area of athletics
is most suited to each person: some people appear to be
born to be marathoners; others are born to be sprinters
and jumpers. For example, the following are recorded
percentages of fast-twitch versus slow-twitch ﬁber in the
quadriceps muscles of different types of athletes:
Limits of Pulmonary Ventilation. How severely do we stress
our respiratory systems during exercise? This can be
answer

ratory systems during exercise? This can be
answered by the following comparison for a normal
young man:
0
1.0
Moderate
exercise
Severe
exercise
2.0
3.0
4.0
0
120
110
100
80
60
40
20
Figure 84–6
(Redrawn from Gray JS: Pulmonary Ventilation and Its Physiolog-
ical Regulation. Springﬁeld, IL: Charles C Thomas, 1950.)
Fast-Twitch
Slow-Twitch
Marathoners
18
82
Swimmers
26
74
Average male
5

ch
18
82
Swimmers
26
74
55
45
Weight lifters
55
45
Sprinters
63
37
Jumpers
63
37
Although one’s respiratory ability is of relatively little
concern in the performance of sprint types of athletics,
it is critical for maximal performance in endurance 
athletics.
Oxygen Consumption and Pulmonary Ventilation in Exercise.
Normal oxygen consumption for a young man at rest is
about 250 ml/min. However, under maximal conditions,
this can be increased to a

der maximal conditions,
this can be increased to approximately the following
average levels:
ml/min
Untrained average male
3600
Athletically trained average male
4000
Male marathon runner
5100
Figure 84–6 shows the relation between oxygen con-
sumption and total pulmonary ventilation at different
levels of exercise. It is clear from this ﬁgure, as would
be expected, that there is a linear relation. Both oxygen
consumption and total pulmonary ventilation increase
about 20-fold between the resting

ilation increase
about 20-fold between the resting state and maximal
intensity of exercise in the well-trained athlete.
L/min
Pulmonary ventilation at maximal exercise
100 to 110
Maximal breathing capacity
150 to 170
Thus, the maximal breathing capacity is about 50 per
cent greater than the actual pulmonary ventilation
during maximal exercise. This provides an element of
safety for athletes, giving them extra ventilation that can
be called on in such conditions as (1) exercise at high
altitudes,

such conditions as (1) exercise at high
altitudes, (2) exercise under very hot conditions, and (3)
abnormalities in the respiratory system.
The important point is that the respiratory system is
not normally the most limiting factor in the delivery of
oxygen to the muscles during maximal muscle aerobic
metabolism. We shall see shortly that the ability of the
heart to pump blood to the muscles is usually a greater
limiting factor.
Effect of Training on V
.
O2 Max. The abbreviation for the rate
of 

g on V
.
of oxygen usage under maximal aerobic metabolism is
V
.
O2 Max. Figure 84–7 shows the progressive effect of
athletic training on V
.
O2 Max recorded in a group of sub-
jects beginning at the level of no training and then pur-
suing the training program for 7 to 13 weeks. In this
study, it is surprising that the V
.
O2 Max increased only
about 10 per cent. Furthermore, the frequency of train-
ing, whether two times or ﬁve times per week, had little
e

ther two times or ﬁve times per week, had little
effect on the increase in V
.
O2 Max. Yet, as pointed out
earlier, the V
.
O2 Max of a marathoner is about 45 per
cent greater than that of an untrained person. Part of
this greater V
.
O2 Max of the marathoner probably is
genetically determined; that is, those people who have
greater chest sizes in relation to body size and stronger
respiratory muscles select themselves to become
1062
Unit XV
marathoners. However, it is also lik

ts Physiology
marathoners. However, it is also likely that many years
of training increase the marathoner’s V
.
O2 Max by
values considerably greater than the 10 per cent that has
been recorded in short-term experiments such as that in
Figure 84–7.
Oxygen Diffusing Capacity of Athletes. The oxygen diffusing
capacity is a measure of the rate at which oxygen can
diffuse from the pulmonary alveoli into the blood. This
is expressed in terms of milliliters of oxygen that will
diffuse each minute for 

iters of oxygen that will
diffuse each minute for each millimeter of mercury dif-
ference between alveolar partial pressure of oxygen and
pulmonary blood oxygen pressure. That is, if the partial
pressure of oxygen in the alveoli is 91 mm Hg and the
oxygen pressure in the blood is 90 mm Hg, the amount
of oxygen that diffuses through the respiratory mem-
brane each minute is equal to the diffusing capacity.The
following are measured values for different diffusing
capacities:
naturally greater diff

erent diffusing
naturally greater diffusing capacities choose these types
of sports, or is it because something about the training
procedures increases the diffusing capacity? The answer
is not known, but it is very likely that training, particu-
larly endurance training, does play an important role.
Blood Gases During Exercise. Because of the great usage of
oxygen by the muscles in exercise, one would expect 
the oxygen pressure of the arterial blood to decrease
markedly during stre

he arterial blood to decrease
markedly during strenuous athletics and the carbon
dioxide pressure of the venous blood to increase far
above normal. However, this normally is not the case.
Both of these values remain nearly normal, demon-
strating the extreme ability of the respiratory system to
provide adequate aeration of the blood even during
heavy exercise.
This demonstrates another important point: The
blood gases do not always have to become abnormal for
respiration to be stimulated in exer

 abnormal for
respiration to be stimulated in exercise. Instead, respira-
tion is stimulated mainly by neurogenic mechanisms
during exercise, as discussed in Chapter 41. Part of this
stimulation results from direct stimulation of the respi-
ratory center by the same nervous signals that are trans-
mitted from the brain to the muscles to cause the
exercise. An additional part is believed to result from
sensory signals transmitted into the respiratory center
from the contracting muscles and moving

ory center
from the contracting muscles and moving joints.All this
extra nervous stimulation of respiration is normally suf-
ﬁcient to provide almost exactly the necessary increase
in pulmonary ventilation required to keep the blood
respiratory gases—the oxygen and the carbon dioxide—
very near to normal.
Effect of Smoking on Pulmonary Ventilation in Exercise. It is
widely known that smoking can decrease an athlete’s
“wind.” This is true for many reasons. First, one effect
of nicotine is constri

 reasons. First, one effect
of nicotine is constriction of the terminal bronchioles of
the lungs, which increases the resistance of airﬂow into
and out of the lungs. Second, the irritating effects of the
smoke itself cause increased ﬂuid secretion into the
bronchial tree, as well as some swelling of the epithelial
linings.Third, nicotine paralyzes the cilia on the surfaces
of the respiratory epithelial cells that normally beat con-
tinuously to remove excess ﬂuids and foreign particles
from the 

emove excess ﬂuids and foreign particles
from the respiratory passageways. As a result, much
debris accumulates in the passageways and adds further
to the difﬁculty of breathing. Putting all these factors
together, even a light smoker often feels respiratory
strain during maximal exercise, and the level of per-
formance may be reduced.
Much more severe are the effects of chronic smoking.
There are few chronic smokers in whom some degree
of emphysema does not develop. In this disease, the fol-
lo

ema does not develop. In this disease, the fol-
lowing occur: (1) chronic bronchitis, (2) obstruction of
many of the terminal bronchioles, and (3) destruction 
of many alveolar walls. In severe emphysema, as much
as four ﬁfths of the respiratory membrane can be
destroyed; then even the slightest exercise can cause
respiratory distress. In fact, many such patients cannot
even perform the simple feat of walking across the ﬂoor
of a single room without gasping for breath.
Cardiovascular System 
in 

out gasping for breath.
Muscle Blood Flow. A key requirement of cardiovascular
function in exercise is to deliver the required oxygen
0
Training frequency
= 5 days/wk
= 4 days/wk
= 2 days/wk
2
4
6
8
10
12
14
2.8
3.8
3.6
3.4
3.2
3.0
Vo2 Max (L/min)
Figure 84–7
Increase in V
.
O2 Max over a period of 7 to 13 weeks of athletic
training. (Redrawn from Fox EL: Sports Physiology. Philadelphia:
ml/min
Nonathlete at

rs College Publishing, 1979.)
ml/min
Nonathlete at rest
23
Nonathlete during maximal exercise
48
Speed skaters during maximal exercise
64
Swimmers during maximal exercise
71
Oarsman during maximal exercise
80
The most startling fact about these results is the sev-
eralfold increase in diffusing capacity between the
resting state and the state of maximal exercise. This
results mainly from the fact that blood ﬂow through
many of the pulmonary capillaries is sluggish or even
dormant in the resting 

laries is sluggish or even
dormant in the resting state, whereas in maximal exer-
cise, increased blood ﬂow through the lungs causes 
all the pulmonary capillaries to be perfused at their
maximal rates, thus providing a far greater surface area
through which oxygen can diffuse into the pulmonary
capillary blood.
It is also clear from these values that those athletes
who require greater amounts of oxygen per minute have
higher diffusing capacities. Is this because people with
Chapter 84
Sports Ph

. Is this because people with
Chapter 84
1063
and other nutrients to the exercising muscles. For this
purpose, the muscle blood ﬂow increases drastically
during exercise. Figure 84–8 shows a recording of
muscle blood ﬂow in the calf of a person for a period of
6 minutes during moderately strong intermittent con-
tractions. Note not only the great increase in ﬂow—
about 13-fold—but also the ﬂow decrease during each
muscle contraction. Two points can be made from 
this study: (1)

tion. Two points can be made from 
this study: (1) The actual contractile process itself 
temporarily decreases muscle blood ﬂow because the
contracting skeletal muscle compresses the intramuscu-
lar blood vessels; therefore, strong tonic muscle con-
tractions can cause rapid muscle fatigue because of lack
of delivery of enough oxygen and other nutrients during
the continuous contraction. (2) The blood ﬂow to
muscles during exercise increases markedly. The fol-
lowing comparison shows the maxima

kedly. The fol-
lowing comparison shows the maximal increase in blood
ﬂow that can occur in a well-trained athlete.
Therefore, a 30 per cent increase in blood pressure can
often more than double the blood ﬂow; this multiplies
the great increase in ﬂow already caused by the meta-
bolic vasodilation at least another twofold.
Work Output, Oxygen Consumption, and Cardiac Output During
Exercise. Figure 84–9 shows the interrelations among
work output, oxygen consumption, and cardiac output
during exer

oxygen consumption, and cardiac output
during exercise. It is not surprising that all these are
directly related to one another, as shown by the linear
functions, because the muscle work output increases
oxygen consumption, and oxygen consumption in turn
dilates the muscle blood vessels, thus increasing venous
return and cardiac output. Typical cardiac outputs at
several levels of exercise are the following:
0
10
18
16
Calf
flow
20
40
Blood flow (100 mL/min)
Minutes
Figure 84–8

20
40
Minutes
Effects of muscle exercise on blood ﬂow in the calf of a leg dur-
ing strong rhythmical contraction. The blood ﬂow was much less
during contraction than between contractions. (Redrawn from 
Barcroft H, Dornhors AC: Blood ﬂow through human calf during
rhythmic exercise. J Physiol 109:402, 1949.)
ml/100 g Muscle/min
Resting blood ﬂow
3.6
Blood ﬂow during maximal exercise
90
Thus, muscle blood ﬂow can increase a maximum of
about 25-fold during the 

an increase a maximum of
about 25-fold during the most strenuous exercise.
Almost one half this increase in ﬂow results from intra-
muscular vasodilation caused by the direct effects of
increased muscle metabolism, as explained in Chapter
21.The remaining increase results from multiple factors,
the most important of which is probably the moderate
increase in arterial blood pressure that occurs in exer-
cise, usually about a 30 per cent increase. The increase
in pressure not only forces more bloo

The increase
in pressure not only forces more blood through the
blood vessels but also stretches the walls of the arteri-
oles and further reduces the vascular resistance.
L/min
Cardiac output in young man at rest
5.5
Maximal cardiac output during exercise in young
23
untrained man
Maximal cardiac output during exercise in 
30
average male marathoner
Thus, the normal untrained person can increase
cardiac output a little over fourfold, and the well-
trained athlete can increase output about sixfo

l-
trained athlete can increase output about sixfold.
(Individual marathoners have been clocked at cardiac
outputs as great as 35 to 40 L/min, seven to eight times
normal resting output.)
Effect of Training on Heart Hypertrophy and on Cardiac Output.
From the foregoing data, it is clear that marathoners can
achieve maximal cardiac outputs about 40 per cent
greater than those achieved by untrained persons. This
results mainly from the fact that the heart chambers of
marathoners enlarge about 40 p

e heart chambers of
marathoners enlarge about 40 per cent; along with this
enlargement of the chambers, the heart mass also
increases 40 per cent or more. Therefore, not only do
the skeletal muscles hypertrophy during athletic train-
ing but the heart does also. However, heart enlargement
and increased pumping capacity occur almost entirely
in the endurance types, not in the sprint types, of ath-
letic training.
Even though the heart of the marathoner is consid-
erably larger than that of the no

honer is consid-
erably larger than that of the normal person, resting
cardiac output is almost exactly the same as that in the
normal person. However, this normal cardiac output is
achieved by a large stroke volume at a reduced heart
rate.Table 84–2 compares stroke volume and heart rate
in the untrained person and the marathoner.
Thus, the heart-pumping effectiveness of each heart-
beat is 40 to 50 per cent greater in the highly trained
athlete than in the untrained person, but there is a cor-

than in the untrained person, but there is a cor-
responding decrease in heart rate at rest.
Role of Stroke Volume and Heart Rate in Increasing the Cardiac
Output. Figure 84–10 shows the approximate changes in
stroke volume and heart rate as the cardiac output
increases from its resting level of about 5.5 L/min to 
30 L/min in the marathon runner. The stroke volume
increases from 105 to 162 milliliters,an increase of about
50 per cent, whereas the heart rate increases from 50 to
185 beats/min, a

e heart rate increases from 50 to
185 beats/min, an increase of 270 per cent. Therefore,
the heart rate increase accounts by far for a greater 
1064
Unit XV
proportion of the increase in cardiac output than does
the increase in stroke volume during strenuous exercise.
The stroke volume normally reaches its maximum by
the time the cardiac output has increased only halfway
to its maximum. Any further increase in cardiac output
must occur by increasing the heart rate.
Relation of 

t occur by increasing the heart rate.
Relation of Cardiovascular Performance to V
.
O2 Max. During
maximal exercise, both the heart rate and the stroke
volume are increased to about 95 per cent of their
maximal levels. Because the cardiac output is equal 
to stroke volume times heart rate, one ﬁnds that the
cardiac output is about 90 per cent of the maximum that
the person can achieve. This is in contrast to about 65
per cent of maximum for pulmonary ventilation.There-
fore, one can readily see 

nary ventilation.There-
fore, one can readily see that the cardiovascular system
is normally much more limiting on V
.
O2 Max than is the
respiratory system, because oxygen utilization by the
body can never be more than the rate at which the car-
diovascular system can transport oxygen to the tissues.
For this reason, it is frequently stated that the level 
of athletic performance that can be achieved by the
marathoner mainly depends on the performance capa-
bility of his or her heart, because t

rmance capa-
bility of his or her heart, because this is the most limit-
ing link in the delivery of adequate oxygen to the
exercising muscles. Therefore, the 40 per cent greater
cardiac output that the marathoner can achieve over the
average untrained male is probably the single most
important physiologic beneﬁt of the marathoner’s train-
ing program.
Effect of Heart Disease and Old Age on Athletic Performance.
Because of the critical limitation that the cardiovascu-
lar system places on maxima

 that the cardiovascu-
lar system places on maximal performance in endurance
athletics, one can readily understand that any type of
heart disease that reduces maximal cardiac output will
cause an almost corresponding decrease in achievable
total body muscle power. Therefore, a person with con-
gestive heart failure frequently has difﬁculty achieving
even the muscle power required to climb out of bed,
much less to walk across the ﬂoor.
The maximal cardiac output of older people also
decreases con

 cardiac output of older people also
decreases considerably—there is as much as a 50 per
cent decrease between age 18 and age 80. Also, there is
even more decrease in maximal breathing capacity. For
these reasons, as well as reduced skeletal muscle mass,
0
200
400
600
800
1000 1200 1400
Dexter 1951
Douglas 1922
Christensen 1931
Donald 1956
1600
0
0
4
3
2
1
0
15
10
5
35
30
25
20
15
10
5
Work output during exercise (kg-mete

ex (L/min/m2)
Work output during exercise (kg-meters/min)
Cardiac output and cardiac index
Figure 84–9
Relation between cardiac output
and work output (solid line) and
between oxygen consumption
and work output (dashed line)
during different levels of exercise.
(Redrawn from Guyton AC, Jones
CE, Coleman TB: Circulatory
Physiology: Cardiac Output and
Its Regulation. Philadelphia: WB
Table 84–2
Comparison of Cardiac Function Between Marathoner 
and Nonathlete

ardiac Function Between Marathoner 
Stroke Volume (ml)
Heart Rate (beats/min)
Resting
Nonathlete
75
75
Marathoner
105
50
Maximum
110
195
162
185
0
10
15
20
25
30
105
50
Stroke volume
Heart rate
190
170
150
130
110
90
70
165
150
135
120
Heart rate (beats/min)
Stroke volume (ml/beat)
Figure 84–10
Approximate stroke volume output and heart rate at different levels
of cardiac output in a marathon athlete.
Chapter 84
1065
t

athon athlete.
Chapter 84
1065
the maximal achievable muscle power is greatly
reduced in old age.
Almost all the energy released by the body’s metabo-
lism of nutrients is eventually converted into body heat.
This applies even to the energy that causes muscle con-
traction for the following reasons: First, the maximal
efﬁciency for conversion of nutrient energy into muscle
work, even under the best of conditions, is only 20 to 25
per cent; the remainder of

tions, is only 20 to 25
per cent; the remainder of the nutrient energy is con-
verted into heat during the course of the intracellular
chemical reactions. Second, almost all the energy that
does go into creating muscle work still becomes body
heat because all but a small portion of this energy is
used for (1) overcoming viscous resistance to the move-
ment of the muscles and joints, (2) overcoming the 
friction of the blood ﬂowing through the blood vessels,
and (3) other, similar effects—all of 

od vessels,
and (3) other, similar effects—all of which convert 
the muscle contractile energy into heat.
Now, recognizing that the oxygen consumption by the
body can increase as much as 20-fold in the well-trained
athlete and that the amount of heat liberated in the
body is almost exactly proportional to the oxygen con-
sumption (as discussed in Chapter 72), one quickly real-
izes that tremendous amounts of heat are injected into
the internal body tissues when performing endurance
athletic even

dy tissues when performing endurance
athletic events. Next, with a vast rate of heat ﬂow into
the body, on a very hot and humid day so that the sweat-
ing mechanism cannot eliminate the heat, an intolera-
ble and even lethal condition called heatstroke can
easily develop in the athlete.
Heatstroke. During endurance athletics, even under
normal environmental conditions, the body tempera-
ture often rises from its normal level of 98.6° to 102° or
103°F (37° to 40°C). With very hot and humid condi-

03°F (37° to 40°C). With very hot and humid condi-
tions or excess clothing, the body temperature can easily
rise to 106° to 108°F (41° to 42°C).At this level, the ele-
vated temperature itself becomes destructive to tissue
cells, especially the brain cells. When this happens, mul-
tiple symptoms begin to appear, including extreme
weakness, exhaustion, headache, dizziness, nausea,
profuse sweating, confusion, staggering gait, collapse,
and unconsciousness.
This whole complex is called heatstroke

ciousness.
This whole complex is called heatstroke, and failure
to treat it immediately can lead to death. In fact, even
though the person has stopped exercising, the tempera-
ture does not easily decrease by itself. One of the
reasons for this is that at these high temperatures, the
temperature-regulating mechanism itself often fails (see
Chapter 73). A second reason is that in heatstroke, the
very high body temperature itself approximately
doubles the rates of all intracellular chemical reacti

les the rates of all intracellular chemical reactions,
thus liberating still more heat.
The treatment of heatstroke is to reduce the body
temperature as rapidly as possible. The most practical
way to do this is to remove all clothing, maintain a spray
of cool water on all surfaces of the body or continually
sponge the body, and blow air over the body with a fan.
Experiments have shown that this treatment can reduce
the temperature either as rapidly or almost as rapidly
as any other procedure, al

ly or almost as rapidly
as any other procedure, although some physicians
prefer total immersion of the body in water containing
a mush of crushed ice if available.
Body Fluids and Salt 
As much as a 5- to 10-pound weight loss has been
recorded in athletes in a period of 1 hour during
endurance athletic events under hot and humid condi-
tions. Essentially all this weight loss results from loss of
sweat. Loss of enough sweat to decrease body weight
only 3 per cent can signiﬁcantly dimi

 body weight
only 3 per cent can signiﬁcantly diminish a person’s per-
formance, and a 5 to 10 per cent rapid decrease in
weight can often be serious, leading to muscle cramps,
nausea, and other effects. Therefore, it is essential to
replace ﬂuid as it is lost.
Replacement of Sodium Chloride and Potassium. Sweat con-
tains a large amount of sodium chloride, for which
reason it has long been stated that all athletes should
take salt (sodium chloride) tablets when performing
exercise on hot and hu

de) tablets when performing
exercise on hot and humid days. However, overuse of
salt tablets has often done as much harm as good. Fur-
thermore, if an athlete becomes acclimatized to the heat
by progressive increase in athletic exposure over a
period of 1 to 2 weeks rather than performing maximal
athletic feats on the ﬁrst day, the sweat glands also
become acclimatized, so that the amount of salt lost in
the sweat becomes only a small fraction of that lost
before acclimatization.This sweat gland

 that lost
before acclimatization.This sweat gland acclimatization
results mainly from increased aldosterone secretion by
the adrenal cortex. The aldosterone in turn has a direct
effect on the sweat glands, increasing reabsorption of
sodium chloride from the sweat before the sweat itself
issues forth from the sweat gland tubules onto the
surface of the skin. Once the athlete is acclimatized,
only rarely do salt supplements need to be considered
during athletic events.
Experience by military unit

uring athletic events.
Experience by military units exposed to heavy exer-
cise in the desert has demonstrated still another elec-
trolyte problem—the loss of potassium. Potassium loss
results partly from the increased secretion of aldos-
terone during heat acclimatization, which increases the
loss of potassium in the urine as well as in the sweat.As
a consequence of these ﬁndings, some of the supple-
mental ﬂuids for athletics contain properly propor-
tioned amounts of potassium along with sodi

ropor-
tioned amounts of potassium along with sodium, usually
in the form of fruit juices.
Without belaboring this issue, let us list some of the
effects of drugs in athletics.
First, caffeine is believed by some to increase athletic
performance. In one experiment on a marathon runner,
running time for the marathon was reduced by 7 per
cent by judicious use of caffeine in amounts similar to
those found in one to three cups of coffee. Yet experi-
ments by others have failed to 

offee. Yet experi-
ments by others have failed to conﬁrm any advantage,
thus leaving this issue in doubt.
Second, use of male sex hormones (androgens) or
other anabolic steroids to increase muscle strength
undoubtedly can increase athletic performance under
some conditions, especially in women and even in men.
However, anabolic steroids also greatly increase the risk
of cardiovascular damage because they often cause
hypertension, decreased high-density blood lipopro-
teins, and increased low-den

ensity blood lipopro-
teins, and increased low-density lipoproteins, all of
which promote heart attacks and strokes.
1066
Unit XV
In men, any type of male sex hormone preparation
also leads to decreased testicular function, including
both decreased formation of sperm and decreased
secretion of the person’s own natural testosterone, with
residual effects sometimes lasting at least for many
months and perhaps indeﬁnitely. In a woman, even
more dire effects can occur because she i

an, even
more dire effects can occur because she is not normally
adapted to the male sex hormone—hair on the face, a
bass voice, ruddy skin, and cessation of menses.
Other drugs, such as amphetamines and cocaine, have
been reputed to increase one’s athletic performance. It
is equally true that overuse of these drugs can lead to
deterioration of performance. Furthermore, experi-
ments have failed to prove the value of these drugs
except as a psychic stimulant. Some athletes have been
known to die

ic stimulant. Some athletes have been
known to die during athletic events because of interac-
tion between such drugs and the norepinephrine and
epinephrine released by the sympathetic nervous
system during exercise. One of the possible causes of
death under these conditions is overexcitability of the
heart, leading to ventricular ﬁbrillation, which is lethal
Multiple studies have now shown that people who main-
tain appropriate body ﬁtness, using judic

ho main-
tain appropriate body ﬁtness, using judicious regimens
of exercise and weight control, have the additional
beneﬁt of prolonged life. Especially between the ages of
50 and 70, studies have shown mortality to be three
times less in the most ﬁt people than in the least ﬁt.
But why does body ﬁtness prolong life? The follow-
ing are the two most evident reasons.
First, body ﬁtness and weight control greatly reduce
cardiovascular disease. This results from (1) mainte-
nance of moderately lowe

 results from (1) mainte-
nance of moderately lower blood pressure and (2)
reduced blood cholesterol and low-density lipoprotein
along with increased high-density lipoprotein. As
pointed out earlier, these changes all work together to
reduce the number of heart attacks and brain strokes.
Second, and perhaps equally important, the athleti-
cally ﬁt person has more bodily reserves to call on when
he or she does become sick. For instance, an 80-year-old
nonﬁt person may have a respiratory system th

-old
nonﬁt person may have a respiratory system that limits
oxygen delivery to the tissues to no more than 1 L/min;
this means a respiratory reserve of no more than three-
fold to fourfold. However, an athletically ﬁt old person
may have twice as much reserve. This is especially
important in preserving life when the older person
develops conditions such as pneumonia that can rapidly
require all available respiratory reserve. In addition, the
ability to increase cardiac output in times of need (t

ity to increase cardiac output in times of need (the
“cardiac reserve”) is often 50 per cent greater in the ath-
letically ﬁt old person than in the nonﬁt person.
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A
A bands, 72, 73f, 90f
A wave, 107f, 108
Abdominal cavity
ﬂuid accumulation in, 177, 

, 108
ﬂuid accumulation in, 177, 306, 860
pressure in, 177
Abdominal compression reﬂex, 213
Abdominal muscles
during parturition, 1037
in defecation, 790
in respiration, 471, 472f
Abdominal veins, reservoir function of, 179
ABO blood group, 451–453, 452f, 452t
Absence syndrome, 744
Acceleration
angular, 695–696, 695f
linear, 695
Acceleratory forces
centrifugal, 541–542, 542f
linear, 542–543, 543f
Acclimatization, 519
cold, 896
heat, 893–894, 900, 1065
high altitude. See High alt

at, 893–894, 900, 1065
high altitude. See High altitude.
natural, 540, 540f
work capacity and, 540
ocular, 617–618, 617f, 649–650
of pacinian corpuscle, 576
Acetate, blood ﬂow and, 203
Acetazolamide, 403t, 404
Acetoacetic acid, 844, 861, 966, 966f
Acetone, 844, 966
Acetone breath, 844, 976
Acetyl coenzyme A, 22f, 23
degradation of, 833–835, 834f
fatty acid conversion of, 844–845, 845f
fatty acid degradation to, 842–843, 843f
oxidation of, 843, 861
pyruvic acid conversion to, 23, 83

on of, 843, 861
pyruvic acid conversion to, 23, 833, 833f
Acetylcholine, 562–563, 562t, 730f, 731
botulinum toxin effect on, 88, 88f
bronchial effects of, 479
coronary blood ﬂow and, 251
curare effect on, 88, 88f
destruction of, 87
drug effects on, 88–89
in cardiac function, 121, 148
in gallbladder emptying, 803
in gastrointestinal function, 775
in pancreatic secretion, 800–801
molecular biology of, 88
pepsinogen secretion and, 798
receptors for, 86–87, 86f, 87f, 96, 752
REM sleep and, 741
secre

 86–87, 86f, 87f, 96, 752
secretion of, 750–751
in bronchiolar function, 479
in skeletal muscle function, 85–88,
86f–88f
in smooth muscle function, 96
structure of, 751
synthesis of, 751
Acetylcholine channel, 48, 85–88, 86f–88f
Acetylcholine pathway, 710, 710f
Acetylcholinesterase, 87
drug inactivation of, 89
Acetyl-CoA. See Acetyl coenzyme A.
Acetyl-CoA carboxylase, 846
Achalasia, 783, 819
Achlorhydria, 797, 820
Acid(s). See also Acid-base balance;
Acidosis; Hydrogen ions.
d

also Acid-base balance;
deﬁnition of, 383
ingestion of, 398
ionization of, 385–386
nonvolatile, 390
strong, 384
titratable, 394–395
weak, 384
Acid-base balance, 383–400
buffer systems in, 385–388, 387f. See also
Buffer systems.
disorders of, 396–397, 396t. See also
Acidosis; Alkalosis.
acid-base nomogram in, 399–400, 400f
anion gap in, 400, 400t
diagnosis of, 398–400, 399f, 400f, 400t
gastrointestinal tract obstruction and,
824–825, 824f
mixed, 399
potassium concentratio

d,
mixed, 399
potassium concentration in, 366
isohydric principle of, 388
neonatal, 1048
nomogram for, 399–400, 400f
normal values for, 399–400, 400f
renal regulation of, 308, 390–400
ammonia buffer system in, 393–394,
394f
bicarbonate reabsorption in, 390–392,
390f–392f, 395, 395t
hydrogen ion secretion in, 390–392,
391f, 392f, 395, 395t
in acidosis, 396, 396t
in alkalosis, 396–397, 396t
phosphate buffer system in, 392–393,
393f
quantiﬁcation of, 394–395, 395t
respiratory regulati

antiﬁcation of, 394–395, 395t
respiratory regulation of, 388–389, 388f,
389f
Acid-base nomogram, 399–400, 400f
Acidophils, 919, 919f, 920t
Acidosis, 384, 390
ammonium ion excretion in, 394
hydrogen ion secretion in, 395
in hemorrhagic shock, 283–284
metabolic, 386, 399f, 400f
calcium reabsorption in, 372
hyperchloremic, 400, 400t
in chronic renal failure, 411f, 412
in diabetes mellitus type I, 973–974, 973f
potassium concentration i

us type I, 973–974, 973f
renal correction of, 396, 396t
oxygen-hemoglobin dissociation curve in,
508, 508f
plasma calcium in, 371
potassium secretion in, 371
renal tubular, 397, 413
respiratory, 386, 389, 399f, 400f
diagnosis of, 398–400, 399f, 400f
in diver, 547
Acidosis (Continued)
synaptic transmission and, 570
treatment of, 398
Acini, pancreatic, 799
salivary, 793, 961–962
Acne, testosterone and, 10

Acne, testosterone and, 1005
Acquired immunodeﬁciency syndrome
(AIDS), T-cell destruction in, 447
Acromegaly, 927, 927f, 974
Acrosome reaction, 1000
Actin, 72–74, 73f, 74f
in pinocytosis, 19, 19f
Actin ﬁlaments
in ameboid movement, 24
of cardiac muscle, 103
of skeletal muscle, 72–73, 76–78, 76f
of smooth muscle, 93–94, 93f
Action potential. See also Membrane
cardiac muscle, 104–106, 104f, 105f. See
also Cardiac muscle, contraction of.
at sinus node, 116–118, 11

muscle, contraction of.
at sinus node, 116–118, 117f, 120f
atrial transmission of, 117f, 118, 118f,
120f
duration of, 105
excitation-contraction coupling and, 106
hyperpolarization of, 67–68
plateau in, 67–68, 104f, 105
refractory period of, 105–106, 105f
relative refractory period of, 105–106,
105f
rhythmicity of, 67–68, 67f, 117–118, 117f
through atrioventricular node, 117f,
118–119, 118f, 120f
through internodal pathways, 117f, 118,
118f, 120f
through Purkinje system, 117f, 119, 120f
velocity

velocity of, 105
ventricular transmission of, 117f,
119–120, 120f
gastrointestinal smooth muscle, 772–773,
772f
inspiratory, 514
neural, 9, 61–65, 61f–64f, 564–568, 564f,
566f, 567f. See also
Neurotransmitters; Synapse(s).
absolute refractory period of, 70
all-or-nothing principle of, 66
anesthetic effect on, 70
anions in, 64
calcium in, 64–65, 86, 86f, 560
cathode ray recording of, 70–71, 70f
conduction of, 68–69, 69f
depolarization stage of, 61–62, 61f

of, 68–69, 69f
electrode excitation of, 69
elicitation of, 69–70, 69f
energy metabolism and, 66, 66f
frequency of, 575
generation of, 566, 566f
hyperpolarization of, 67f, 68
inhibition of, 70
initiation of, 65, 69
local, 69, 69f
plateau in, 66–67, 67f
point of origin of, 566, 566f
positive feedback in, 9
potassium in, 61–65, 61f–64f, 66
I
N
D
E
X
Note: Page numbers followed by f refer to ﬁgures; page numbers followed by t refer to tables.
1067
1068
Index
Actio

llowed by t refer to tables.
1067
1068
Index
Action potential (Continued)
propagation of, 65–66, 65f
receptor potential and, 574, 574f
recording of, 70–71, 70f
refractory period of, 70
repolarization stage of, 61f, 62
resting, 61, 61f, 564–565, 564f
rhythmicity of, 67–68, 67f
safety factor for, 66, 70
saltatory conduction of, 68–69, 69f
sodium in, 61–65, 61f–64f, 66, 87,
87f
subthreshold, 69, 69f
threshold for, 65, 69–70, 69f
voltage-gated potassium channel in,
62–63, 62f, 64, 64f
voltage-gated 

ium channel in,
voltage-gated sodium channel in, 62,
62f, 64, 64f
vs. stimulus artifact, 70f, 71
olfactory, 667–668
retinal, 637–638, 637f
skeletal muscle, 89–91, 90f, 91f. See also
Muscle (skeletal), contraction of.
excitation-contraction coupling in,
89–91, 91f
initiation of, 87–88, 88f
safety factor for, 88
T tubules in, 89, 90f
smooth muscle, 96–98, 97f
calcium in, 97
of gastrointestinal tract, 772–773
plateau of, 96–97, 97f
spontaneous generation of, 97
stretch excitatio

7f
stretch excitation of, 97
Action tremor, 705, 707
Activator operator, 35f, 36
Active transport, 19, 46, 46f, 52–56
adenosine triphosphate in, 882
co-transport, 54–55, 55f
renal, 329–330, 330f, 333–334
sodium–amino acid, 55, 816
sodium–glucose, 55, 55f, 816, 831
counter-transport, 54–55
renal, 330, 330f, 333
in tubular reabsorption, 328–332,
328f–331f
primary, 53–54, 53f
renal, 328f, 329
secondary, 54–55
renal, 329–330, 330f
through cellular sheets, 55–56, 56f
Act

–330, 330f
Actomyosin ﬁlaments, of lymphatic
capillary, 193
Acupuncture, 603
of olfactory receptors, 668
of pacinian corpuscle, 575–576, 575f
of retinal receptors, 631–632, 632f
of sensory receptors, 575–576, 575f
of taste receptors, 666
of thermal receptors, 608
pain receptors and, 599
to cold, 896
to dark, 631–632, 632f
to heat, 893–894, 900, 1065
to high altitude. See High altitude.
to light, 631–632, 632f
Adaptive control, 9
Addisonian crisis, 9

632, 632f
Addisonian crisis, 958
Addison’s disease, 342, 957–958
hyperkalemia in, 366, 370
sodium excretion in, 363, 379
Adenine, 27, 28f, 29, 29f
Adenohypophysis. See Pituitary gland,
anterior.
Adenoma, thyroid, 940
Adenosine
in blood ﬂow, 196–197, 247, 251–252
in irreversible shock, 284
Adenosine diphosphate (ADP)
formation of, 22, 22f
in glycolysis regulation, 836
in platelet plug formation, 458
oxygen utilization and, 508–509, 509f
rate-limiting activity of, 884
Adenosine

509, 509f
Adenosine monophosphate (AMP), 830
cyclic (cAMP)
in adrenal steroid synthesis, 955
second messenger activity of, 912,
913–914, 913f, 913t
thyroid-stimulating hormone function
and, 938
Adenosine triphosphatase (ATPase), 23,
835–836, 835f
in muscle contraction, 76
in renal potassium secretion, 368, 368f
in sodium-potassium pump, 53, 53f
Adenosine triphosphate (ATP), 829–830,
830f, 881–884
cellular utilization of, 23–24, 23f, 829–830,
830f, 883–884, 883f
for

n of, 23–24, 23f, 829–830,
formation of, 17, 22–24, 22f, 23f, 829–830,
830f, 881
anaerobic, 836–837
in citric acid cycle, 834, 834f, 836
in fatty acid oxidation, 842–844, 843f
in glycolysis, 832–837, 833f
in mitochondria, 17, 22–24, 22f, 23f,
in oxidative phosphorylation, 835–836,
835f
summary of, 836
in action potential, 66
in active transport, 882
in cardiac muscle, 251–252
in glandular secretion, 882
in

ysis regulation, 836
in metabolism, 829–830, 830f
in muscle contraction, 77, 79–80, 881–882
in nerve conduction, 882
phosphocreatine interrelation with, 882
phosphofructokinase inhibition by, 836
structure of, 22
Adenosine triphosphate (ATP) synthetase,
23
Adenyl cyclase–cAMP second messenger
system, 913–914, 913f, 913t
ADH. See Antidiuretic hormone (ADH).
Adipocytes (fat cells), 842
Adipokines, 905
Adipose cell lipase, 970
Adipose tissue, 842, 906f
brown, 887–888, 896

 970
fat storage in, 965
glucose transport to, 965
in athlete, 1055
in feeding regulation, 871
lipoprotein lipase of, 841, 842
triglycerides of, 12f, 842
Adiposogenital syndrome, 1009, 1009f
ADP. See Adenosine diphosphate (ADP).
Adrenal cortex, 907t, 944, 945f
disorders of, 957–960, 959f, 960f
hormones of, 944–960. See also
Aldosterone; Androgens, adrenal;
Cortisol.
layers of, 944–945, 945f
tumor of, 959, 960f
zona fasciculata of, 944–945, 945f
zona 

959, 960f
zona glomerulosa of, 944, 945f
zona reticularis of, 945, 945f
Adrenal diabetes, 951–952
Adrenal gland, 906f, 944, 945f. See also
Adrenal cortex; Adrenal medulla.
Adrenal medulla, 907t, 944, 945f
epinephrine secretion by, 207–208, 755,
756
function of, 755
norepinephrine secretion by, 207–208, 755,
756
sympathetic nerve endings in, 749–750,
755
Adrenalin. See Epinephrine.
Adrenergic receptors, 752, 753t
Adrenocorticotropin/adrenocorticotropic
hormone (

hormone (ACTH), 907t, 918, 919f,
920t
aldosterone secretion and, 950
corticotropin-releasing factor regulation
of, 955
cortisol secretion and, 955–957, 956f
excess of, 958–959, 959f
inhibition of, 955
structure of, 955
thyroid hormone effects on, 937
Adrenogenital syndrome, 959, 960f
Afterdischarge, 581–582, 581f
Afterload, 110f, 111
Age
cardiac output and, 232, 233f, 237
growth hormone and, 927
pulse pressure and, 173, 173f
vision and, 618
Agglutination,

sure and, 173, 173f
Agglutination, antigen, 444, 445
in transfusion reactions, 452–453
anti-A, 452, 452t, 453, 453t
anti-B, 452, 452t, 453, 453t
anti-Rh, 453
Agglutinogens, 451–452, 452t
Aggressiveness, 720
Agouti-related protein, 868–870, 869t
Air. See also Nitrogen; Oxygen.
alveolar, 493–496
carbon dioxide concentration in,
495–496, 495f, 496f
composition of, 493–494, 493t
humidiﬁcation of, 494
oxygen concentration in, 494–495, 495f,
496f
renewal rate of, 494, 494f

n, 494–495, 495f,
496f
atmospheric, composition of, 493–494,
493t
dead space, 495–496, 496f
expired, 495–496, 496f
Air embolism
brain surgery and, 178
submarine escape and, 550
Air hunger (dyspnea), 527, 531, 532
Air travel. See Aviation.
Airway obstruction, 526, 526f
in asthma, 529
in atelectasis, 528–529, 529f
in emphysema, 526–527, 527f, 528f
Airway resistance, in emphysema,
527
Akinesia, 711
Alanine, 853f
glucose formation from, 857
synthesis of, 856, 856f
Alarm re

rmation from, 857
Alarm response, 208, 757, 758
Albinism, 628
Albino, 628
Albumin, 855–857, 855f. See also Plasma
proteins.
capillary diffusion of, 184, 184t
colloid osmotic pressure and, 188
Index
1069
Albumin (Continued)
fatty acid binding to, 841
thyroxine binding to, 934
urinary, 317
Albuminuria, 317
Albuterol, 759
Alcohol ingestion
headache and, 607
peptic ulcers and, 821
Aldosterone, 907t, 946f, 947t
ACTH effects on, 950
angiotensin and, 225
arterial pressure and, 2

 950
arterial pressure and, 225, 230f, 231,
948–949, 948f
cellular effects of, 949–950
deﬁciency of, 947–948, 957
potassium balance and, 342
sodium balance and, 342, 379
excess of, 948–949, 948f, 959
alkalosis and, 398
hypertension and, 223
extracellular ﬂuid osmolarity and,
362–363, 363f
functions of, 947–950, 948f
hepatic metabolism of, 947
hydrogen ion secretion and, 395
in cardiac failure, 263–264
in extracellular s

395
in extracellular sodium regulation,
in heat acclimatization, 893, 1065
in pregnancy, 1034
inhibitors of, 403t, 404
intestinal sodium absorption and,
814–815, 814f, 949
intracellular potassium and, 366, 366t
nongenomic actions of, 950
plasma protein binding of, 947
potassium effects on, 950
potassium secretion and, 366, 369–370,
369f, 370f, 378, 948
protein synthesis and, 915, 949–950
radioimmunoassay of, 916, 916f
receptor for, 949–950
regulation of,

of, 916, 916f
regulation of, 950, 951f
renin-angiotensin system effects on, 225,
950, 951f
salivary gland effects of, 949
sodium effects on, 950
sodium reabsorption and, 225, 342, 342t,
378–379, 948
structure of, 908f
sweat gland effects of, 949
synthesis of, 945–947, 946f, 947t
tumor-related secretion of, 223, 959
Aldosterone antagonists, 403t, 404
Aldosterone escape, 379, 948–949
Aldosteronism. See Aldosterone, excess of.
Alimentary tract, 771, 772f. See also

Gastrointestinal tract and speciﬁc
components.
Alkali
deﬁnition of, 383–384
Alkaline phosphatase, 990
Alkalosis, 384, 390
aldosterone excess and, 949
causes of, 397–398
renal correction of, 396–397, 396t
Alkalosis (Continued)
respiratory, 386, 399f, 400f
altitude and,

tinued)
altitude and, 539
causes of, 397
renal correction of, 396–397, 396t, 539
whole-body, 824
Allergen, 449
Allergy, 449–450
anaphylaxis in, 285–286, 287, 449–450
atopic, 449
basophils in, 436
cortisol in, 954
delayed-reaction, 449
eosinophils in, 436
mast cells in, 436
neonatal, 1049
Allograft, 455
All-or-nothing principle, 66
Alpha bra

ograft, 455
Alpha brain waves, 742, 742f, 743, 743f
Alpha receptors, 251, 752, 753t
Altitude. See High altitude.
Alveolar macrophages, 433, 481
Alveolar membrane, 5.
Alveolar pressure, 472–473, 472f
Alveolar ventilation, 477–478
alveolar Pco2 and, 494–495, 495f
arterial Pco2 and, 521, 521f
arterial Pco2 and, 519–520, 519f
dead space and, 477–478, 478f
Pco2 and, 495–496, 495f, 517, 517f
pH and, 388–389, 388f, 517, 517f
rate of, 478, 494, 494f
ventilation-perfusion rat

ventilation-perfusion ratio and, 499–501,
500f
wasted, 500–501
Alveolus (alveoli), 479, 479f. See also
Lungs.
air in, 493–496
oxygen concentration in, 485, 494–495,
495f, 496f
carbon dioxide diffusion into, 504–505,
505f
collapse of (atelectasis), 528–529, 529f
ﬂuid in, 487
gas exchange in. See Gas exchange.
interstitial pressure effect on, 

ee Gas exchange.
interstitial pressure effect on, 488, 488f
J receptors of, 521
occlusion of, 474
oxygen diffusion from, 502–503, 503f
pressures in, 472–473, 472f, 492
structure of, 496, 496f, 497f
surface tension of, 473–474
type II epithelial cells of, 474
walls of, 496, 497f
Alzheimer’s disease, 746
Amacrine cells, 634, 635, 635f, 636
Ambenonium, 759
Ameboid movement, 24, 24f, 431
Ameloblasts, 992
Amenorrhea, 938, 1023
Amiloride, 403t, 404
Amino acids, 33–35, 34f, 810, 810f, 852–854,
853f. Se

no acids, 33–35, 34f, 810, 810f, 852–854,
853f. See also Protein(s).
active transport of, 854
blood concentration of, 854
glucagon secretion and, 971
Amino acids (Continued)
cortisol effects on, 952
co-transport of, 55, 854
deamination of, 856, 862
essential, 853f, 855
extracellular, 4, 46f, 854–855
facilitated diffusion of, 50, 966, 969
gastrointestinal absorption of, 854
hydrogen bonds of, 854
insulin secretion and, 969
interstitial, 294t
intracellular, 22, 22f, 46f

969
intracellular, 22, 22f, 46f, 294t. See also
Adenosine triphosphate (ATP).
nonessential, 855–856, 856f, 862
oxidation of, 856–857
peptide linkages of, 34–35, 808, 810, 810f,
852, 881
plasma concentration of, 854–855
insulin deﬁciency and, 967
proximal tubular transport of, 330, 330f
renal threshold for, 854
RNA codons for, 31–32, 32t
storage of, 854–855
transamination of, 856, 856f, 862, 877
urinary, 413
Aminoaciduria, 413
Aminopolypeptidase, 810, 8

 413
Aminopolypeptidase, 810, 810f
Aminotransferases, 856
Amitriptyline, 745
Ammonia, 862. See also Urea.
Ammonia buffer system, 393–394, 394f
Ammonium chloride, in alkalosis, 398
Ammonium citrate, 467
Ammonium ion, 393–394, 394f
Amnesia
anterograde, 737
retrograde, 725, 726
Amniotic ﬂuid, 1035
Amorphosynthesis, 592
Amphetamines, 759
exercise and, 1066
in obesity, 873
Ampulla, 693f, 694
Amygdala, 737–738, 870
Amylase, pancreatic, 799, 809, 809f
Amylin, 961
Amyloid plaques, 746

c, 799, 809, 809f
Amyloidosis, 409
Anaerobic energy, 836–837, 882–883
Anal sphincters, 789
Analgesia system, of central nervous
system, 602–603, 602f
Anaphase, 37f, 39
Anaphylaxis, 285–286, 287, 449–450
slow-reacting substance of, 450, 480,
529
Anatomic dead space, 478
Anchoring ﬁlaments, of lymphatic capillary,
191, 192f
Androgens. See also Testosterone.
adrenal, 944, 957, 1003, 1018
tumor-related excess of, 959, 960f
chemistry of, 1003, 1004f
exercise and, 1065

 960f
exercise and, 1065–1066
ovarian, 1003
Androstenedione, 946f
Anemia, 422, 426–427
aplastic, 426
blood loss, 426
bone marrow response to, 422
cardiac effects of, 236–237, 236f, 427
exercise and, 427
hematocrit in, 293
hemolytic, 427
1070
Index
Anemia (Continued)
hypochromic, 421f, 425, 426
in erythroblastosis fetalis, 454
in kidney disease, 412, 423
megaloblastic, 421f, 427
microcytic hypochromic, 421f, 426
pernicious, 423, 427, 797, 820, 877
sickle cell, 421f, 424,

s, 423, 427, 797, 820, 877
sickle cell, 421f, 424, 427
Anencephaly, 697
cardiac arrest and, 156
cardiac output and, 242, 242f
local, 70
neurogenic shock and, 285
respiratory effects of, 522
Angina pectoris, 145, 255–256. See also
Myocardial infarction.
growth factors in, 41, 200–201
high altitude effects on, 539–540
Angiogenic factors, 41, 200–201
Angiogenin, 200
Angioplasty, coronary, 256
Angiotensin I, 223f, 224
Angiotensin II. See also Re

giotensin I, 223f, 224
Angiotensin II. See also Renin-angiotensin
system.
arteriolar effects of, 201–202
362–363
formation of, 223f, 224
in cardiac failure, 263
glomerular ﬁltration rate and, 322, 322t,
325
in arterial pressure regulation, 201–202,
224–226, 225f, 226f, 378
in extracellular volume control, 377–378
in hemorrhagic shock, 281
in pressure natriuresis, 378, 378f
in renal blood ﬂow, 322, 322t
in renal ischemia, 413
in sodium excretion, 377–378
in sodi

ischemia, 413
in sodium reabsorption, 342–343, 342t
in tubular reabsorption, 342, 342t
in water reabsorption, 342–343, 342t
thirst and, 361, 361t
urine output and, 342
Angiotensin II antagonists, glomerular
ﬁltration rate and, 325
Angiotensin-converting enzyme (ACE)
glomerular ﬁltration rate and, 325
pressure natriuresis effects of, 378, 378f
Angiotensinase, 223f, 224
Angiotensinogen, 223f, 224
Angular acceleration, 695–696, 695f
Angular gyrus, 716, 716f, 

leration, 695–696, 695f
Angular gyrus, 716, 716f, 718
Anion gap, 400, 400t
Anions
action potential and, 64
blood ﬂow and, 203
Annulospiral ending, 675–676
Anorexia, 874
Anorexia nervosa, 874
Anorexigenic substances, 868, 869t
Anovulation, 1021, 1024–1025
Anterior commissure, 722
Anterograde amnesia, 737
Anterolateral sensory pathway, 587,
595–597, 596f
in peptic ulcers, 821
renal toxicity of, 406
Antibody (antibodies), 432, 441, 441f. See
also at Immunoglobulin.
afﬁnity constant of, 

 See
afﬁnity constant of, 444
Antibody (antibodies) (Continued)
agglutination by, 444, 445
agglutinin, 452, 452t, 453, 453t
bivalent, 444, 444f
classes of, 444
complement interaction with, 445–446,
445f
direct action of, 444
for passive immunity, 449
formation of, 442–446, 443f–445f
in human milk, 1041
lysis by, 444, 445, 445f
mechanisms of action of, 444–446,
445f
neutralization by, 444, 445
precipitation by, 444
salivary, 794
sensitizing, 449
speciﬁcity of, 444, 444f
st

, 794
structure of, 443–444, 444f
Anticholinesterase drugs, 759
clinical, 466–468
endogenous, 459, 463–464
Anticodons, 32–33, 32f
Antidepressants, 745
Antidiuretic hormone (ADH), 907t, 918,
919f
blood ﬂow and, 202
functions of, 928–929
in extracellular ﬂuid sodium regulation,
358–359, 358f, 362–363, 362f
in urine concentration, 337, 350–351, 353,
356, 379
in urine dilution, 337, 348–350, 349f
secretion of, 220
art

dilution, 337, 348–350, 349f
arterial pressure and, 360, 360t
blood volume and, 360, 360f, 360t
cardiopulmonary reﬂexes and, 360
drug effects on, 360t, 361
hypothalamus and, 734
in cardiac failure, 264
inappropriate, 357, 379
nausea and, 360–361
plasma osmolarity and, 358–360, 358f,
360t
regulation of, 358–361, 358f–360f, 360t,
928–929
thirst system integration with, 362–363,
362f
structure of, 928
synthesis of, 359–360, 359f
vasoconstrictor effects of, 929
water reabsorption a

soconstrictor effects of, 929
water reabsorption and, 337, 338, 342t,
343, 348–349, 379, 734
Antigen(s), 440, 441f. See also Antibody
(antibodies).
antibody interaction with, 442, 444–446,
445f
HLA, 455–456
red blood cell, 451–454, 452f, 452t, 453t
self-, 448
Antigen-presenting cells, 446, 446f
Antihemophilic factor (factor VIII), 459t,
465
Antimuscarinic drugs, 759
Antioncogenes, 41
Antiperistalsis, 824
Antipyrine, for total body water
measurement, 296
Antithrombin III, 463–464
Antithyroid drug

nt, 296
Antithyroid drugs, 939–940
Anuria
in cardiac failure, 259, 263–264
in kidney failure, 405, 406
Aorta
baroreceptors of, 209–212, 209f
chemoreceptors of, 211–212, 518–519,
518f
coarctation of, 227, 274
pressure in, 109, 173, 173f
Aortic body, 211–212, 518, 518f
mechanism of action of, 518–519
Aortic-coronary bypass surgery, 256
Aortic pressure curve, 109
circulatory dynamics in, 272–273
heart sounds in, 271f, 272
pulse pressure in, 173, 173f, 

ounds in, 271f, 272
pulse pressure in, 173, 173f, 174
congenital, 274
Aortic valve, 109, 109f
Aphagia, 868
Aphasia, 721
Aplysia, 724, 724f
Apnea, sleep, 522–523
Apocrine glands. See Sweating.
Apoferritin, 425
Apolipoprotein E, 746
Apoptosis, 40
Apotransferrin, 425, 426
Appendix, inﬂammation of, 605, 605f
Appetite, 734, 781, 867, 870. See also
Feeding; Food, intake of.
brain area of, 795

also
salt, 363
Apraxia, motor, 687
Aquaporins, 928
Aqueous humor, 623–624, 623f, 624f
Arachnoidal villi, 765
Arcuate arteries, 309, 309f
Arcuate veins, 309f, 310
Arginine, 853f
Argyll Robertson pupil, 650
Arm
lever system of, 82, 82f
veins of, 178
Arrhenoblastoma, 1003
Arrhythmias, 147–156. See also at speciﬁc
arrhythmias.
arrest, 156
atrial ﬁbrillation, 155–156, 155f
atrial ﬂutter, 156, 156f
atrioventricular block, 148–150, 149f
intraventricular bloc

ricular block, 148–150, 149f
intraventricular block, 150, 150f
paroxysmal tachycardia, 151–152, 153f
premature contraction, 150–151, 150f, 151f
sinoatrial block, 148, 148f
sinus, 148, 148f
ventricular ﬁbrillation, 152–155, 153f–155f
Arterial blood pressure, 166–167, 166f
abdominal compression reﬂex and, 213
aldosterone effects on, 225, 948–949, 948f
angiotensin effect on, 201–202, 224–226,
225f, 226f, 378
antidiuretic hormone effect on, 360, 379
auscultation of, 175, 175f
baroreceptor response t

baroreceptor response to, 6–7, 209–212,
209f–211f, 230, 230f
blood ﬂow and, 164, 164f. See also Blood
ﬂow.
blood volume and, 172, 172f, 219–220,
220f
body ﬂuid volume and, 219–220, 220f,
374–376, 375f
capillary ﬂuid shift and, 230f, 231
cardiac output and, 114, 114f, 219–220,
220f
Index
1071
Arterial blood pressure (Continued)
centrifugal acceleratory force effects on,
542, 542f
cerebral blood ﬂow and, 762–763, 763f
chemoreceptor response to, 211–212, 230,
230f
control

moreceptor response to, 211–212, 230,
230f
control of, 163, 230–231, 230f
capillary ﬂuid shift mechanism in, 230f,
231
hypothalamus in, 207, 733, 733f
neural. See Arterial blood pressure,
nervous system control of.
renal. See Arterial blood pressure, renal
control of.
stress-relaxation mechanisms in,
230–231, 230f
diastolic, 162, 163f, 173, 175–176, 175f
diuresis and, 216, 217–220, 217f–220f
during exercise, 208, 213–214, 235–236,
248–249, 248f
equilibrium point of, 217–218
frequency distributio

quilibrium point of, 217–218
frequency distribution of, 210, 211f
hemorrhage effect on, 224, 224f
high. See Hypertension.
high-ﬁdelity measurement of, 166–167,
167f
in cardiogenic shock, 262
in cerebral ischemia, 212–213
in circulatory shock, 279, 279f
in extracellular ﬂuid balance, 374–376,
375f, 376f
in hemorrhage, 224, 224f
in hemorrhagic shock, 279–280, 279f, 280f
in polycythemia, 428
in sodium balance, 374–376, 375f
integrated regulation of, 230–231, 230f
low-pressure receptor response to, 

 230–231, 230f
low-pressure receptor response to, 212
Mayer waves of, 214, 214f
mean, 175–176, 176f, 220
measurement of, 166–167, 166f, 167f,
175–176, 175f, 178
neonatal, 1047
nervous system control of, 208–213, 230,
230f, 235–236, 235f, 755
abdominal compression reﬂex in, 213
adrenal medullae in, 207–208
atrial reﬂex in, 212
Bainbridge reﬂex in, 212
baroreceptors in, 209–212, 209f–211f,
214, 214f
Cushing reaction in, 213, 213f
during exercise, 208, 213–214
ischemic response in, 212–213, 213f,
2

08, 213–214
214, 214f
norepinephrine in, 207
parasympathetic system in, 205–206
rapidity of, 208–213, 209f–211f
reﬂex mechanisms in, 209–212,
209f–211f, 214, 214f
respiratory waves and, 214, 214f
stress and, 208
sympathetic system in, 204–205, 205f,
206–208, 206f
vasoconstriction and, 204–205, 205f,
vasodilation and, 208
vasomotor center in, 206–207, 206f,
212–213, 213f, 214, 214f
volume reﬂex in, 212
normal, 175, 176f
peripheral vascular resist

, 212
peripheral vascular resistance and, 170,
170f, 219, 219f, 220, 220f
pressure diuresis and, 375–376, 375f,
376f
pressure natriuresis and, 375–376, 375f
renal, 320–321, 321t
renal control of, 216–231, 230f, 308, 323,
341–342, 374–376, 375f, 376f. See also
Kidneys; Renal tubules.
aldosterone escape and, 948–949, 948f
angiotensin in, 223–226, 224f–226f
atrial reﬂexes and, 212
baroreceptor reﬂexes and, 211
experimental demonstration of, 217,

ﬂexes and, 211
217f
graphical analysis of, 217–218, 218f
peripheral resistance and, 219–220,
219f, 220f
quantitation of, 217–220, 217f–220f
renal resistance and, 219
renin-angiotensin system in, 223–226,
224f–226f
salt intake/output and, 218–219, 218f,
220, 224–226, 226f
water intake/output and, 218–219, 218f,
224–226, 226f
renin-angiotensin system effects on,
224–226, 224f–226f
respiratory waves in, 214
salt sensitivity of, 229, 229f
sodium excretion and, 374

ensitivity of, 229, 229f
sodium excretion and, 374–375, 375f
stress relaxation and, 230–231, 230f
systemic (greater, peripheral), 162, 163f
systolic, 162, 163f, 173, 175–176, 175f, 220
units of, 166–167
urine output and, 216, 323, 341–342,
374–376, 375f, 376f
aldosterone escape and, 948–949,
948f
vascular resistance and, 170, 170f, 219,
219f, 220, 220f
vasoconstrictor tone and, 206, 207f
vasomotor waves of,

onstrictor tone and, 206, 207f
vasomotor waves of, 214, 214f
vasopressin effect on, 202
Arteriole(s), 181, 182f
angiotensin effect on, 201–202
antidiuretic hormone effects on, 929
autonomic nervous system control of, 204,
205f, 754t
blood distribution to, 162f
bradykinin effect on, 202
cross-sectional area of, 162
diameter of, 168, 181
during exercise, 247–248
function of, 161
hepatic, 859
hydrogen ion effect on, 202–203
intermittent contraction of (vasomotion),
182–183, 187
nitric oxide synthes

of (vasomotion),
nitric oxide synthesis in, 199–200
pressure pulse in, 174–175, 174f
renal, 309, 309f, 311f
resistance of, 168
Arteriosclerosis, 982. See also
Atherosclerosis.
diabetes mellitus and, 976
pulse pressure in, 173, 173f
Arteriovenous ﬁstula, cardiac output in, 236,
236f, 242–243, 243f, 267, 267f
Artery (arteries). See also Arteriole(s);
Capillary (capillaries).
autonomic nervous system control of,
754–755, 754t
baroreceptors of. See Baroreceptor(s).
blood ﬂow in. See Blo

ors of. See Baroreceptor(s).
blood ﬂow in. See Blood ﬂow.
collateral, 201
compliance of, 171–172, 173, 173f
Artery (arteries) (Continued)
diameter of, 167–168, 168f
distensibility of, 171, 173–176, 173f–176f
hardening of, 848–851, 849f. See also
Arteriosclerosis; Atherosclerosis;
mean pressure in, 175–176, 176f
parallel circuits of, 168–169, 169f
pressure in. See Arterial blood 
pressure.
pres

9f
pressure.
sympathetic innervation of, 172, 173f, 204,
205f
volume-pressure curves of, 172, 172f
Articulation, 721–722, 721f. See also
Language; Speech.
Artiﬁcial cooling, 900
Artiﬁcial kidney (dialysis), 414–415, 414f,
415t
Artiﬁcial respiration, 532, 532f
Arytenoid cartilage, 481, 481f
Ascites, 177, 306, 381, 860
in thiamine deﬁciency, 876
Ascorbic acid. See Vitamin C (ascorbic
acid).
Asparagine, 853f
Aspartic acid, 853f
Aspir

acid).
Aspirin, peptic ulcers and, 821
Astereognosis, 591
Asthenia, 974
Asthma, 436, 450, 480, 529
Astigmatism, 620, 620f
Astronauts. See Space travel.
Ataxia, 706
Atelectasis, 528–529, 529f
Atherosclerosis, 842, 848–851, 849f. See also
Arteriosclerosis.
arterial pressure in, 175
blood pressure in, 175
cholesterol in, 850
coronary, 252, 256. See also Myocardial
infarction; Myocardial ischemia.
in Alzheimer’s disease, 746
in hypothyroidism, 942
lipoproteins i

disease, 746
lipoproteins in, 850
prevention of, 850–851
renal, 408
risk factors for, 850
Athetosis, 709
Athlete, 1055. See also Exercise.
bradycardia in, 147
Atmospheric pressure, under water, 545,
546f
Atony
bladder, 313–314
colonic, 822
ATP. See Adenosine triphosphate (ATP).
Atrial ﬁbrillation, 155–156, 155f, 156f, 273
Atrial ﬂutter, 156, 156f
Atrial natriuretic peptide, 907t
extracellular ﬂuid volume  and, 379–380
tubular reabsorption and, 342t,

rdiac failure, 264
tubular reabsorption and, 342t, 343
Atrial paroxysmal tachycardia, 152, 152f
heart rate and, 212
left, 107f, 108, 484, 487
edema and, 489, 489f
right, 176
cardiac output and, 240–243, 240f–243f.
See also Cardiac output.
measurement of, 176, 179
peripheral venous pressure and, 177
Atrioventricular block, 121
1072
Index
Atrioventricular bundle, 104
action potential transmission in, 117f,
118–119, 118f
left and right branches of, 117f, 118f, 119
pacemaker function

ht branches of, 117f, 118f, 119
pacemaker function in, 120–121
Atrioventricular node
acetylcholine effects on, 121
118–119, 118f, 119f
block of, 148–150, 149f
hyperpolarization of, 121
in atrial ﬁbrillation, 155–156
intrinsic rhythmical rate of, 120
norepinephrine effects on, 122
premature contractions of, 150–151,
151f
safety factor for, 121
Atrioventricular valves, 107f, 108, 109, 109f.
See also Mitral valve; Tricuspid valv

, 109, 109f.
See also Mitral valve; Tricuspid valve.
sounds at, 269, 270
Atrium (atria), 104
action potential transmission in, 117f, 118,
118f, 119f, 120f
in mitral valve lesions, 273
pressure in, 107f, 108, 484, 487
low-pressure receptors of, 212
pressure waves of, 107f, 108
primer pump function of, 107–108, 107f
pressure in, 176
cardiac output and, 240–243, 240f–
243f. See also Cardiac output.
stretch of, 112
vol

heral venous pressure and, 177
volume reﬂex of, 212
Atrophy
gastric, 820
muscle, 83
Atropine, 759
Audiogram, 661, 661f
Audiometry, 660–661, 661f
Auditory association cortex, 658, 659, 659f
lesions of, 659
Auditory cortex, 657–659
primary, 658–659, 659f
lesions of, 660
pathway to, 657–658, 658f
sound direction and, 660
sound frequency response of, 659
sound pattern discrimination by, 659
secondary, 658, 659, 659f
Auditory nerve
disorders of, 660–661
stimulation of,

ditory nerve
stimulation of, 655–656
Auditory receptive aphasia, 721
Auerbach’s (myenteric) plexus, 774–775,
774f, 777
Auscultation
of blood pressure, 175, 175f
of heart sounds, 270–271, 270f
Autacoids, in renal blood ﬂow, 322–323, 322t
Autocrines, 905
Autograft, 455
Autoimmune disease, 448
Autolysis, 20
Autonomic nervous system, 6, 556, 748–760.
See also Parasympathetic nervous
system; Sympathetic nervous system.
anatomy of, 748–750, 749f, 750f
control of, 758–759, 758f
fu

, 748–750, 749f, 750f
functions of, 750–757, 753t, 754t
in blood ﬂow regulation, 204–208,
205f–207f
Autonomic nervous system (Continued)
in ocular accommodation, 618, 648–649,
648f
pharmacology of, 759–760
vasomotor center of, 205f, 206, 206f
in coarctation of aorta, 227
of blood ﬂow, 198–199, 198f, 219–220,
220f
A-V node. See Atrioventricular node.
Aviation. See also Space travel.
acceleratory forces in, 541–543, 542f,
543f
in unpressurized airplane, 538

541–543, 542f,
543f
in unpressurized airplane, 538, 538f
mental proﬁciency in, 539
Axis. See Mean electrical axis.
Axon, 68, 68f, 555, 556f, 559, 559f. See also
Action potential, neural.
Axonal streaming, 564
Axoneme, of cilia, 25
Axoplasm, 68, 68f
Azotemia, 411f, 412. See also Kidneys,
failure of.
B
Bacteria, 18, 18f. See also Infection.
colonic, 817
Bactericidal agents
of lysosomes, 20
of saliva, 794
of white blood cells, 432
Bainbridge reﬂex, 212, 233
Baldness, 1005
Baroreceptor(s), 6–7, 209–

212, 233
Baroreceptor(s), 6–7, 209–212, 757
activation of, 209–210, 209f, 210f
anatomy of, 209, 209f
buffer function of, 210, 211f, 230, 230f
denervation of, 210, 211f, 228
in antidiuretic hormone secretion, 360
in cardiac failure, 258–259
in extracellular volume control, 377
oscillation of, 214, 214f
postural effects on, 210
resetting of, 210–211
Barrel chest, 529
Basal ganglia, 558, 707–712, 708f
caudate circuit of, 709, 709f, 710
in movement control, 7

 circuit of, 709, 709f, 710
in movement control, 709–710, 710f,
712–713
lesions of, 709, 710, 710f, 711–712
neurotransmitters of, 710–711, 710f
putamen circuit of, 708–709, 708f
Basal metabolic rate. See Metabolic rate,
basal.
Base, 383–384
weak, 384
Basement membrane, of glomerular
capillary, 316, 316f
Basilar membrane, 652, 652f, 653, 653f
vibration of, 654–655, 654f
Basket cells, 702
Basophil(s), 421f, 429–430, 430f, 436
activation of, 445–446
concentration of, 430
heparin product

 of, 445–446
heparin production by, 464
Basophil erythroblast, 421f, 422
Bathorhodopsin, 629, 629f
B-cell growth factors, 447
B-cell stimulating factors, 447
Bellini, duct of, 311f
Bends (decompression sickness), 548–549,
548f
Beriberi, 875
blood ﬂow in, 198
cardiac output in, 236, 236f, 267, 267f
oxygen uptake in, 530
Beta-aminoisobutyricaciduria, 413
Beta-amyloid peptide, 746
Beta blockers, 256
Beta brain waves, 742, 742f, 743, 743f
Beta receptors, 251, 752, 753t
Beta-oxi

 743, 743f
Beta-oxidation, 842–843, 843f, 861
Betz cells, 687–688
Bicarbonate. See also Acid-base balance.
blood, 394
extracellular, 4, 7t, 46f, 294f, 294t
from carbonic acid dissociation, 511
ileal secretion of, 815
intestinal absorption of, 815
intestinal secretion of, 815, 817
intracellular, 294f, 294t
loop of Henle transport of, 334–336,
335f
pancreatic secretion of, 800, 800f, 801
reabsorption of, 328t, 338f, 390–392,
re

n of, 328t, 338f, 390–392,
renal formation of, 392–394, 393f, 394f
394f
phosphate buffer system in, 393, 393f
renal titration of, 392
urinary, 394
Bicarbonate buffer system, 385–387, 386f
dissociation constant in, 385–386
Henderson-Hasselbalch equation in,
386
titration curve for, 386–387, 386f
Bile, 802–805, 803f, 803t. See also Bile salts.
composition of, 803, 803t
deﬁciency of, 464–465
gallbladder storage of, 802–803
in fat

of, 464–465
in fat digestion, 811
secretion of, 802, 803f, 804
Bile acids, 802, 804
atherosclerosis and, 850–851
Bile ducts, 802
obstruction of, 864
Bile salts, 803, 803t, 861
emulsifying function of, 804, 811
enterohepatic circulation of, 804
extracellular, 294f
function of, 804–805
in fat digestion, 804–805, 811, 812, 816
plasma, 294f
tubular secretion of, 334
Bilirubin, 426, 862–864, 863f
excretion of, 307, 803, 803t
free vs. conjugated, 863

 803t
free vs. conjugated, 863, 864
Bilirubin glucuronide, 863
Bilirubin sulfate, 863
Bipolar cells, 633, 635, 635f, 636, 638f
Bipolar disorder, 745
Birth, 929, 1036–1038, 1037f
breathing at, 1044–1045, 1045f
2,3-Bisphosphoglycerate, oxygen-
hemoglobin dissociation curve effects
of, 508, 508f
Bitemporal hemianopsia, 645
Black tongue, 876
Bladder. See also Kidneys; Urine.
anatomy of, 309, 309f, 311–312, 311f
atonic, 313–314
autonomic control of, 754t, 757
cystomet

, 313–314
cystometrography of, 312–313, 313f
Index
1073
Bladder (Continued)
ﬁlling of, 312–313, 313f
incontinent, 313–314
infection of, 409
innervation of, 311f, 312, 313
micturition reﬂex of, 311, 313–314
neurogenic, 314
sensory stretch receptors of, 313
tabetic, 314
urine transport to, 312–313, 313f
voluntary control of, 313
Blastocyst, 1029, 1029f
Bleeding. See also Coagulation;
Hemorrhage.
in coagulation factor deﬁciency, 464–465
in hemophilia, 465
in thromboc

 deﬁciency, 464–465
in thrombocytopenia, 465
in vitamin C deﬁciency, 878
Bleeding time, 467
Blind spot, 644
Blindness
color, 633, 634f
glaucoma and, 624–625
in premature infant, 1051
night, 629, 875
visual cortex removal and, 644
Blood. See also Plasma.
acetoacetic acid transport in, 844
amino acid concentration of, 854
754t
bicarbonate in, 394–395
carbon dioxide in, 385–386
circulation of, 4–5, 4f. See also Circ

in, 385–386
circulation of, 4–5, 4f. See also Circulation
(blood).
citrate treatment of, 467
clotting of, 458–459, 458f, 460, 464. See
also Coagulation.
collection of, 466–467
extracorporeal circulation of, 276
fatty acid concentration of, 841
ﬂow of. See Blood ﬂow; Circulation
(blood).
glucose concentration of, 839, 975, 975f
feeding and, 871
glucagon secretion and, 971, 971f
insulin secretion and, 968–969, 968f,
969f
regulation of, 971–972
hematocrit of, 169–170, 169f, 170f
hepatic cleansing o

tocrit of, 169–170, 169f, 170f
hepatic cleansing of, 861
hepatic storage of, 860
hormone clearance from, 909–910
hormone transport in, 909
O-A-B, 451–453, 452f, 452t
osmolar clearance of, 357
Pco2 of, 386, 399, 399f, 400f
pH of, 384, 384t
alveolar ventilation and, 389, 389f
measurement of, 398–400, 399f, 400f
phosphorus in, 979
physiologic shunt of, 500, 501, 503,
503f
reservoirs for, 179–180, 180f
Rh, 453–454
shunted, 500, 501, 503, 503f
silicone containers for, 466–467
sludged, 281–282
transfu

e containers for, 466–467
transfusion of
blood typing for, 453, 453t
in shock, 286
reactions to, 452–453, 454
typing of, 451–454, 452f, 452t, 453t
Blood (Continued)
venous admixture of, 503, 503f
viscosity of
in anemia, 427
vascular resistance and, 169–170, 170f
volume of. See Blood volume.
Blood clot, 458–459, 458f, 460. See also
Coagulation.
intravascular, 465–466
lysis of, 464
retraction of, 458f, 

cular, 465–466
retraction of, 458f, 460
Blood ﬂow, 164–170. See also Cardiac
output; Circulation (blood).
acute control of, 196–200, 196f–198f
after vascular occlusion, 198
anion effect on, 203
arrest of, 287–288
autonomic nervous system regulation of,
204–208, 205f–207f
autoregulation of, 198–199, 198f, 219–220,
220f
blood pressure and, 164, 164f, 166–167,
166f, 170, 170f
calcium ion effect on, 202
capillary, 4–5, 4f, 5f, 18

cium ion effect on, 202
capillary, 4–5, 4f, 5f, 182–183
average rate of, 183
during exercise, 246
carbon dioxide effect on, 203
cerebral, 199, 761–763, 762f, 763f. See also
Cerebral blood ﬂow.
conductance in, 167–168, 168f, 169
control of
acute, 196–200, 196f–198f
angiotensin in, 201–202
bradykinin in, 202
endothelin in, 202
epinephrine in, 201
histamine in, 202
humoral, 201–203
ions in, 202–203
kidney in, 216–231. See also Arterial
blood pressure, renal control of.
local, 195–203, 196f, 196t. S

e, renal control of.
local, 195–203, 196f, 196t. See also
Blood ﬂow, local control of.
long-term, 196, 200–201
nervous system in, 204–214. See also
Arterial blood pressure, nervous
system control of.
nitric oxide in, 199–200
norepinephrine in, 201
vasopressin in, 202
coronary, 249–256. See also Coronary
blood ﬂow.
deﬁnition of, 164
during exercise, 195, 198, 201, 246–249,
247f, 248f
eddy currents of, 165f, 166
electromagnetic ﬂowmeter measurement
of, 164, 165f, 220
endothelin effect on, 202
epin

epinephrine effect on, 201
gastrointestinal, 777–780, 778f–780f
glucose effect on, 198
hepatic, 860
histamine effect on, 202
humoral control of, 201–203
in collateral circulation, 201
laminar, 165–166, 168
local control of, 195–203
adenosine in, 196–197
angiogenesis and, 200–201
Blood ﬂow (Continued)
autoregulatory mechanisms in, 198–199,
198f, 219–220, 220f
collateral circulation and, 201
during e

220, 220f
during exercise, 198, 201
glucose in, 198
hyperemia and, 198
in brain, 199
in kidneys, 199. See also Kidneys, blood
ﬂow of.
long-term, 198f, 200–201
metabolic theory of, 199
myogenic theory of, 199
oxygen availability and, 196–198, 196f,
197f, 200
oxygen lack theory of, 197–198, 197f
tissue metabolism and, 196, 196f
vascularity and, 200–201
vasodilator theory of, 196–197
vitamin deﬁciency and, 198
magnesium effect on, 202
maximum need for, 201
measuremen

um effect on, 202
measurement of, 164–165, 165f
metabolic rate and, 196, 196f
norepinephrine and, 201, 247, 251, 322,
322t
of kidneys. See Kidneys, blood ﬂow of.
Ohm’s law and, 164
oxygen deﬁciency and, 196–198, 197f
oxygen effect on, 200
oxygen utilization and, 509
parabolic velocity proﬁle of, 165–166, 165f
placental, 1035
potassium ion effect on, 202
pulmonary, 244, 244f
in atelectasis,

ffect on, 202
in atelectasis, 529, 529f
rate of, 163
renal. See Kidneys, blood ﬂow of.
resistance to. See Vascular resistance.
Reynolds’ number for, 166
right atrial pressure and, 176
shunt, 500, 503
skeletal muscle, 246–249, 247f, 248f
splanchnic, 777–780, 778f–780f
sympathetic nervous system regulation of,
170, 170f, 204–205, 205f, 206–208,
206f, 207f
tissue vascularity and, 200
tissue-speciﬁc, 195–196, 196t, 199, 233–234,
233f
turbulent, 16

c, 195–196, 196t, 199, 233–234,
233f
turbulent, 165, 165f, 166
ultrasonic Doppler ﬂowmeter
units of, 164
vasa recta, 321, 354–355, 355f
vascular endothelial growth factor and,
200–201
vascular resistance and, 164, 164f,
167–169, 168f–170f
vasoconstrictor control of, 201–202
vasodilators and, 196–197, 202
velocity of, 162, 165–166, 166f
vessel diameter and, 167–168, 168f
Blood iron buffer, 862
Blood pressure. 

–168, 168f
Blood pressure. See Arterial blood pressure.
Blood transfusion
citrate in, 467
1074
Index
Blood transfusion (Continued)
Blood types
determination of, 453, 453t
Blood vessels. See also Arteriole(s); Artery
(arteries); Capillary (capillaries);
Vein(s).
blood clotting in, 458–459, 458f. See also
Coagul

compliance of, 171–172
delayed compliance of, 172, 173f
distensibility of, 171–180
arterial pressure and, 173–176,
173f–176f
units of, 171
venous pressure and, 176–179,
177f–179f
platelet plug repair of, 458
stress-relaxation of, 172, 173f
sympathetic innervation of, 204, 205f,
thromboembolism of, 465–466
trauma-induced constriction of, 457
Blood volume, 293
antidiuretic hormone secretion and, 360,
360f, 360t
arterial pressur

ne secretion and, 360,
360f, 360t
arterial pressure and, 172, 172f, 219–220,
220f
atrial natriuretic peptide and, 379–380
atrial reﬂex effect on, 212
cardiac output and, 237, 241–242, 242f
sympathetic nervous system response
to, 377
distribution of, 161–162, 162f
in aortic valve lesions, 273
in cardiac failure, 259–260, 380
in liver cirrhosis, 381
in nephrotic syndrome, 381
in pregnancy, 381, 1035, 1035f
in valvular disease, 273
in varicose veins, 381
increases in, 3

isease, 273
increases in, 380–381
mean circulatory ﬁlling pressure and, 239,
239f
measurement of, 296
regulation of, 375–377, 375f–377f
Blood-brain barrier, 766
Blood-cerebrospinal ﬂuid barrier, 766
Blue weakness, 633
Body ﬂuids. See also Blood; Cerebrospinal
ﬂuid (CSF); Extracellular ﬂuid;
Interstitial ﬂuid; Intracellular ﬂuid;
Urine; Water.
compartments of, 292–293, 292f
volume measurement of, 295–296, 295f,
296t
corrected osmolar act

ment of, 295–296, 295f,
296t
corrected osmolar activity of, 294t, 298,
351
distribution of, 296–299, 298f
in neonate, 1048
in potential spaces, 305–306
indicator-dilution measurement of,
295–296, 295f
measurement of, 295–296, 295f, 296t
osmolarity of, 294t, 298
osmotic equilibrium of, 298–299, 298f
osmotic pressure of, 52, 294t, 298
retention of. See Edema.
transcellular, 292. See also Cerebrospinal
ﬂuid (CSF).
Body mass index, 872
Bohr effect, 508, 508f
Bone(s), 980–983
absorption of, 982–983, 

08, 508f
absorption of, 982–983, 982f, 985
calciﬁcation of, 981–982, 985
calcium of, 371–372, 981, 989
compact, 981
cystic disease of, 990
deconditioning of, in space travel, 544
deposition of, 982–983, 982f, 985
estrogen effects on, 1017
fracture of, 983
growth hormone effects on, 923
in rickets, 991
organic matrix of, 981
osteoporosis of, 992
parathyroid hormone effects on,
986–987
phosphate of, 981
remodeling of, 982–983, 982f
sound conduction thro

sound conduction through, 652
stress on, 983
structure of, 983f
tensile strength of, 981
testosterone effects on, 1005–1006
vitamin D effects on, 985
Bone ﬂuid, 987
aplasia of, 426, 437
B lymphocyte preprocessing in, 441,
448
lymphocytes in, 440
macrophages in, 434
red blood cell production in, 420–423,
420f, 421f
white blood cell production in, 430–431,
430f, 435, 437
Botulinum toxin, end plate potential effects
of, 88, 88f
Botulism, swallowing in, 819
B

effects
of, 88, 88f
Bowel. See Colon.
Bowman’s capsule
anatomy of, 309f, 310, 310f
hydrostatic pressure of, 318, 318f
Boyle’s law, 545, 546f
Bradycardia, 147–148, 148f
gastrointestinal release of, 779
in pain, 599
in renal blood ﬂow, 322
in salivary gland secretion, 795
mast cell release of, 436
Brain. See also Brain stem; Cerebellum;
Cerebral cortex; Motor cortex;
Somatosensory cortex.
activating-driving systems of, 728–7

nsory cortex.
activating-driving systems of, 728–731,
729f, 730f
brain stem in, 728–729, 729f
cerebral cortex in, 729, 729f
neurohormonal, 730–731, 730f
sensory input to, 729
arterioles of, 763
blood ﬂow in. See Cerebral blood ﬂow.
capillaries of, 182, 763
cognitive activity of. See Learning;
Memory; Thinking.
during sleep, 739–741. See also Sleep.
edema of, 521, 766–767
glucose for, 767, 965
hypothalamus of. See Hypothalamus.
in circulatory arrest, 287–288
in epilepsy, 743–745, 744f

st, 287–288
inhibitory circuits of, 583
Brain (Continued)
injury to, 764
Cheyne-Stokes breathing in, 522
concussive, 767
shock and, 285
interstitial ﬂuid pressure of, 187
limbic system of, 731–738, 732f. See also
Limbic system.
lymphatic system of, 765, 765f
metabolism of, 767
negative pressure in, 178
neurohormonal systems of, 730–731, 730f
oxygen requirement for, 767
thyroid hormone effects on, 936, 937
tumor of, 765, 899
vasomotor center of, 205f, 206–207, 206f,
212

 899
212–213
Brain stem, 728, 729f
autonomic control areas of, 758–759, 758f
bulboreticular facilitatory area of, 678,
728–729, 729f
cerebral cortex feedback to, 729
in feeding behavior, 870
in hearing, 657–658
in motor function, 678, 688, 691–692, 691f,
697
in pain sensation, 600–601, 600f
in taste, 665–666, 666f
neospinothalamic tract to, 600–601, 600f
paleospinothalamic tract to, 600f, 601
reticular excitatory area of, 601, 728–729,
729f
reticular inh

citatory area of, 601, 728–729,
729f
reticular inhibitory area of, 729, 729f
reticular nuclei of, 691–692, 691f
transection of, 673
arterial pressure and, 758
positive supportive reaction and, 682
spastic rigidity and, 692
vestibular nuclei of, 691–692, 691f
Brain tumor, 765, 899
Brain waves, 740, 741–743, 742f
Braxton Hicks contractions, 1037
Breast(s)
anatomy of, 1039f
development of, 1038–1039
oxytocin effects on, 929
progesterone effects on, 1018
Breast milk, 1041, 

Breast milk, 1041, 1041t
Breathing, 471–472, 472f. See also
Respiration.
Cheyne-Stokes, 522, 522f
in sleep apnea, 523
periodic, 522, 522f
work of, 475
Broca’s area, 687, 687f, 717, 717f, 719
loss of, 721
Brodmann’s areas, 589, 589f, 642
Bronchial arteries, 483
Bronchioles, 479
acetylcholine effect on, 479
blood supply of, 483
constriction of, 479–480, 529
dilation of, 479
epinephrine effect on, 479
norepinephrine effect on, 479
terminal, 479
Brown fat, 887–888, 896

fect on, 479
Brown-Séquard syndrome, 606
Brunner’s glands, 805
Buccal glands, 793–795
Buffer nerves, 210, 211f
Buffer systems, 384, 385–388
ammonia, 393–394, 394f
baroreceptor, 210, 211f
bicarbonate, 385–387, 386f
Index
1075
Buffer systems (Continued)
intracellular, 387–388
phosphate, 387, 393, 393f
protein, 387–388
respiratory, 389, 389f
Bulboreticular facilitatory area, 678,
Bulbourethral glands, 996, 997f
Bumetani

729, 729f
Bumetanide, 403, 403t
Bundle branch block, 139–140, 139f, 140f,
141, 145
Burns
water loss and, 291–292
Bursa of Fabricius, 440. See also
Lymphocyte(s), B.
C
C wave, 107f, 108
Cachexia, 874
Caffeine
Caisson disease, 548–549, 548f
Cajal, interstitial cells of, 773
Calciﬁcation, metastatic, 990
Calcitonin, 907t, 988–989
Calcitriol (1,25-dihydroxy vitamin D3), 308
Calcium, 878t, 879, 879t
activ

xy vitamin D3), 308
active transport of, 54, 64–65
arterial deposition of, 982
bone, 371–372, 981–982, 989
deﬁciency of, 371, 978, 979–980, 979f
deionization of, 467
dietary intake of, 371
excess of, 371, 978, 980, 990
excretion of, 371–372, 372f, 373t, 980, 985
parathyroid hormone and, 987
extracellular, 46f, 294f, 294t, 978
in smooth muscle contraction, 99
normal value for, 7t, 371
regulation of, 371–373, 372f, 373t
fecal excretion of, 980
fetal, 104

373, 372f, 373t
fetal, 1043, 1043f
hepatic excretion of, 862
in action potential, 64–65
in cardiac function, 105, 106, 114, 117–118
in coagulation, 463, 467
in 1,25-dihydroxycholecalciferol
formation, 984–985, 984f
in gastrointestinal smooth muscle
contraction, 773
in memory formation, 724, 724f
in parathyroid hormone secretion, 988,
988f
in sinus nodal rhythmicity, 117–118
in skeletal muscle function, 76, 90–91, 91f
in smooth muscle function, 97, 99, 99f
in vasoconstrict

ooth muscle function, 97, 99, 99f
in vasoconstriction, 202
intake of, 371
interstitial, 294t, 978–979, 979f
intestinal absorption of, 815, 878, 980,
980f, 985
intracellular, 46f, 294f, 294t
335f
parathyroid hormone effects on, 986–988,
986f, 989–990
plasma, 371–372, 372f, 978–979, 979f
calcitonin secretion and, 989
Calcium (Continued)
reabsorption of, 342t, 343, 372, 372f
regulation of, 989–990
sarcoplasmic reticulu

 372f
sarcoplasmic reticulum release of, 90–91,
91f, 99, 99f
urinary, 372
Calcium-binding protein, 985
Calcium-calmodulin second messenger
system, 914–915
Calcium channel
slow, of cardiac muscle, 105, 117
voltage-gated, 64–65
of neuromuscular junction, 86, 86f
of presynaptic membrane, 560
Calcium pump, 54, 64, 67
in skeletal muscle contraction, 90
Calcium-sodium channel, 64–65, 67, 773
Callus, 983
Calmodul

odium channel, 64–65, 67, 773
Calmodulin, 95
Calorie, 885
Calorimetry, 885
Calsequestrin, 90
Camosine, 294t
Canal of Schlemm, 624, 624f
Cancer, 40–42
invasiveness of, 41
prostatic, 1008
Capillary (capillaries). See also Artery
(arteries); Vein(s).
average function of, 183
blood ﬂow in, 4–5, 4f, 5f, 182–183
carbon dioxide diffusion from, 183,
504–505, 505f
colloid osmotic pressure of, 185–186, 185f
fragility of, 87

185f
fragility of, 878
function of, 161, 162
glomerular, 309–310, 309f, 316–317, 316f,
317f, 317t
intercellular cleft of, 182, 182f
lymphatic, 190–191, 192f
membrane of. See Capillary membrane.
micropipette cannulation of, 186
neutrophil adherence to, 430f, 435
oxygen diffusion from, 183, 503–504,
504f
peritubular, 309–310, 309f, 340, 340f,
341t
plasmalemmal vesicles of, 182, 182f
potential space ﬂuid exchange with,
305–306
pressure 

ential space ﬂuid exchange with,
305–306
pressures in. See Capillary pressure.
pulmonary. See Pulmonary capillaries.
renal, 309–310, 309f
structure of, 181–182, 182f
surface area of, 182
vasomotion of, 182–183, 197–198, 197f
wall of, 181–182, 182f
Capillary ﬁltration coefﬁcient, 185–186, 190
Capillary ﬂuid shift, arterial pressure and,
230f, 231
carbon dioxide diffusion across, 183,
504–505, 505f. See also Carbon
dioxide, transport of.
concentr

f. See also Carbon
concentration gradients across, 184
diffusion through, 4–5, 4f, 183–184, 183f,
184t, 503–505, 504f, 505f
concentration difference and, 184
net, 184
Capillary membrane (Continued)
ﬁltration across, 185–190, 302
abnormalities in, 190
at arterial end, 189
at venous end, 189
capillary pressure in, 185, 185f, 186–187,
186f
ﬁltration coefﬁcient in, 185–186, 190
interstitial ﬂuid colloid osmotic
pressure in, 185, 185f, 188–189
interstitial ﬂuid pressure in, 185

, 185f, 188–189
interstitial ﬂuid pressure in, 185, 185f,
187–188
net ﬁltration pressure in, 185–186, 189
net reabsorption pressure and, 189
plasma colloid osmotic pressure in, 185,
185f, 188
rate of, 186, 302
Starling equilibrium for, 189–190
glomerular, 182
oxygen diffusion across, 183, 503–504,
504f. See also Oxygen, transport of.
permeability of, 183–184, 184t
in hemorrhagic shock, 282f
pores of, 182, 182f
diffusion throug

of, 182, 182f
diffusion through, 183–184, 184t
rate of diffusion across, 184
Starling equilibrium at, 189–190
vesicular channels of, 182, 182f
Capillary pressure, 185–187, 185f, 186f
abnormal, 190
functional, 186–187, 189
isogravimetric measurement of, 186–187,
186f
measurement of, 186–187, 186f
micropipette measurement of, 186–187
pulmonary, 162–163, 163f, 484, 484f, 487,
488, 488f
Captopril, 225
Carbachol, neuromuscular junction effec

opril, 225
Carbachol, neuromuscular junction effects
of, 88–89
Carbaminohemoglobin, 511
cellular, 12f
depletion of, 874–875, 874f
digestion of, 809, 809f. See also
Carbohydrates, metabolism of.
energy content of, 865–866, 866t
fat synthesis from, 837–838, 844–845,
845f
fat-sparing effects of, 846
formation of, 838
Golgi apparatus formation of, 21
hydrolysis of, 808
intestinal absorption of, 815–816
metabolism of, 829–839. See also
Glucose.
adenosine triphosphate in, 829–830,
830f
a

lucose.
830f
anaerobic, 836–837, 882–883
citric acid cycle in, 833–835, 834f
control of, 836
cortisol effects on, 951–952
glucose in, 830–831, 831f
glycogen storage in, 831–832
glycogenesis in, 832, 832f
glycogenolysis in, 832
glycolysis in, 832–837, 833f–835f
growth hormone and, 923
in Cushing’s syndrome, 958–959
insulin and, 831, 963–965, 964f, 969–970
liver in, 861
1076
Index
Carbohydrates (Continued)
measurement of, 867
oxidativ

ohydrates (Continued)
oxidative phosphorylation in, 835–836,
835f
pentose phosphate pathway in,
837–838, 838f
respiratory quotient for, 867
thyroid hormone and, 936
vs. fat metabolism, 846
of cell membrane, 14
protein-sparing activity of, 866
storage of, 831–832
triglyceride synthesis from, 844–845, 845f
alveolar, 495–496, 495f, 496f
partial pressure of, 500, 500f
altitude effects on, 537, 538t
blood, 385–386, 399, 399f, 400f
alveolar ventilation and, 517, 517f

99, 399f, 400f
in respiratory insufﬁciency, 524–525
chemoreceptor response to, 518
chemosensitive area stimulation by,
516–517, 516f
diffusing capacity of, 498–499, 499f
diffusion coefﬁcient of, 493
diffusion of. See Carbon dioxide, transport
of.
dissociation curve of, 511, 511f
excess of, 528, 528f, 531–532
extracellular, 4, 6, 7t, 388
Haldane effect on, 511–512, 512f
hemoglobin reaction with, 511
in hydrochloric acid formation, 796f, 797
i

h, 511
in respiration regulation, 516–517, 516f,
517f
in vasodilation, 203
lipid solubility of, 47
metabolic formation of, 388, 833–835,
834f, 837, 838f, 867
partial pressure of, 492, 493t
alveolar, 495–496, 495f, 500, 500f, 537,
538t
alveolar ventilation and, 519–520, 519f
cerebral blood ﬂow and, 761, 762f
during exercise, 521, 521f
high altitude and, 438t, 537
in blood, 386, 399, 399f, 400f
in Cheyne-Stokes breathing, 522, 522f
in extrace

okes breathing, 522, 522f
in extracellular ﬂuid, 46f, 388
in intracellular ﬂuid, 46f
interstitial, 505, 505f
respiration and, 517, 517f
placental, 1031
plasma protein combination with, 511
pulmonary expiration of, 388
release of
Haldane effect and, 511–512, 512f
in citric acid cycle, 834–835, 834f
in pentose phosphate pathway, 837, 838f
solubility coefﬁcient of, 492
transport of, 5, 183, 184, 510–512, 510f
from peripheral tissue cells to
capillaries, 504–505, 505f
from pulmonary ca

ls to
from pulmonary capillaries to alveoli,
hemoglobin in, 511
in bicarbonate form, 510–511
in dissolved state, 510
pH and, 512
respiratory exchange ratio in, 512
water reaction with, 510–511
Carbon dioxide dissociation curve, 511–512,
511f
diffusing capacity of, 499, 499f
hemoglobin binding of, 509–510, 510f
Carbon monoxide poisoning, 196, 510
Ca

ent of, 492
Carbon monoxide–hemoglobin dissociation
curve, 509–510, 510f
Carbon tetrachloride, renal toxicity of, 406
Carbonate, in bone, 981
Carbonic acid, 391, 391f, 510–511
Carbonic anhydrase, 385, 419, 510–511, 879
in hydrogen ion secretion, 391, 391f
Carbonic anhydrase inhibitors, 403t, 404
Carboxypeptidase, 202
Carboxypolypeptidase, 799–800, 810, 810f
Carcinogens, 41
Cardiac arrest, 156, 287–288
Cardiac cycle, 106–109, 107f. See also
Cardiac muscle, cont

ycle, 106–109, 107f. See also
Cardiac muscle, contraction of.
current ﬂow during, 126–127, 126f, 127f
electrocardiogram of, 107, 107f
volume-pressure diagram of, 110–111,
110f
Cardiac failure, 258–267
acute effects of, 258–259, 259f
aldosterone in, 263–264
anemia and, 427
angiotensin in, 263
arteriovenous ﬁstula and, 267, 267f
atrial natriuretic factor in, 264
beriberi and, 267, 267f
blood volume in, 380, 485
cardiac output curve in, 258, 259f,
265–267, 265f–267f
cardiac reserve in, 260, 264–265

65–267, 265f–267f
cardiac reserve in, 260, 264–265, 265f
chronic effects of, 259–260, 259f
compensated, 259f, 260, 265–266, 265f
decompensated, 260–262, 261f, 266–267,
266f, 267f
deﬁnition of, 258
digitalis in, 262, 266–267
edema in, 259–260, 263–264, 263f, 303
glomerular ﬁltration rate in, 263
graphical analysis of, 261, 261f, 265–267,
265f–267f
high-output, 267, 267f
left-sided, 262, 487, 488–489
low-output, 262–263
peripheral venous p

7, 488–489
peripheral venous pressure in, 177
pulmonary edema in, 264, 488–489
quantitative analysis of, 265–267, 265f,
266f
red blood cell production in, 422
renin-angiotensin system in, 263
sympathetic nervous system in, 258–259,
259f, 261, 261f, 265
treatment of, 261–262, 266–267, 267f
unilateral, 262
Cardiac impulse, 116–120, 117f, 118f, 120f.
See also Heart, conductive system of.
Cardiac index, 232, 233f
Cardiac muscle, 103–114. See also A

dex, 232, 233f
Cardiac muscle, 103–114. See also Atrium
(atria); Heart; Ventricle(s), cardiac.
actin ﬁlaments of, 103
action potential of, 104–106, 104f, 105f.
See also Action potential, cardiac
muscle.
anatomy of, 103–104, 104f
as syncytium, 103–104, 104f
blood supply of, 249, 249f
conductive system of. See Heart,
conductive system of.
Cardiac muscle (Continued)
contraction of, 106–109, 107f. See also
Cardiac output.
afterload and, 111
arterial pressure and, 114, 114f
calcium in, 114
efﬁciency 

pressure and, 114, 114f
efﬁciency of, 111
ejection period in, 108
energy for, 111
excitation-contraction coupling in, 106
Frank-Starling mechanism in, 111–113,
112f, 113f, 233
isovolumic (isometric) contraction in,
107f, 108, 110f, 111
isovolumic (isometric) relaxation in,
parasympathetic regulation of, 113, 113f
potassium in, 113–114
preload and, 111
regulation of, 111–114, 112f–114f
rhythmical excitation in, 116–122, 117f,
118f, 120f
strength of, 106, 114
s

 116–122, 117f,
118f, 120f
stretch and, 112
sympathetic regulation of, 112–113, 113f
T tubules in, 106
temperature effect on, 114
venous return and, 111–113, 112f, 113f
ventricular function curves for, 112, 112f
histology of, 103–104, 104f
hypertrophy of, 235, 255, 272–273, 276
cardiac output and, 235
in athlete, 1063
intercalated discs of, 103–104, 104f
lactic acid use by, 837
metabolism of, 251–252
myosin ﬁlaments of, 103
oxygen con

ism of, 251–252
oxygen consumption of, 253
oxygen requirement of, 253
parasympathetic innervation of, 205–206,
205f
physiology of, 103–106, 104f, 105f
post-infarction recovery of, 260
potassium channel of, 67–68, 67f, 117
rupture of, 254
scarring of, 255, 255f
sympathetic innervation of, 205, 205f
transverse tubules of, 106
work output of, 110–111, 110f, 249–250
Cardiac output. See also Cardiac output
curve.
abdominal compression reﬂex effect on,
213
abnormalities of, 236

ression reﬂex effect on,
213
abnormalities of, 236–243, 236f, 238f–243f
after myocardial infarction, 253–254, 253f,
255, 259f, 260–262, 261f
age-related, 232, 233f, 237
arterial pressure and, 114, 114f, 219–220,
220f
at rest, 232
autonomic nervous system effects on,
235–236, 235f
blood volume and, 237, 241–243, 242f
cardiac hypertrophy and, 235
decrease in, 234, 234f, 235
deﬁnition of, 164, 232
dinitrophenol effect on, 235, 235f
during exercise, 213–214, 232–233, 233f,
235–236, 248–250, 248f, 10

213–214, 232–233, 233f,
235–236, 248–250, 248f, 1063–1064,
1064f, 1064t
during pregnancy, 1035
of, 243, 243f
heat production and, 885
high, 236–237, 236f
Index
1077
Cardiac outpu (Continued)
hypereffective, 234–235, 234f
hypoeffective, 234, 234f, 235
in anemia, 236–237, 236f, 427
in arteriovenous ﬁstula, 236, 236f,
242–243, 243f, 267, 267f
in beriberi, 236, 236f, 267, 267f
in cardiac failure, 258–260, 259f
in hemorrhage, 237
in hemorrhagic shock, 279–280, 279

hemorrhage, 237
in hemorrhagic shock, 279–280, 279f, 281,
283f
in hypertension, 228, 236f
in hyperthyroidism, 236, 236f
in septic shock, 286
in venous dilation, 237
indicator dilution method measurement
of, 244–245, 244f
intrapleural pressure and, 237–238, 238f
limits on, 234, 234f
low, 236f, 237. See also Circulatory shock.
measurement of, 243–245, 243f, 244f
normal, 232, 233f, 234, 234f, 242, 242f
oxygen Fick principle for, 244, 244f
parasympathetic nervous system effects
on, 11

244f
on, 113, 113f, 234–235
pathologic alterations in, 236–237, 236f
peripheral resistance and, 234, 234f,
236–237, 236f
quantitative analysis of, 237–245
cardiac output curve in, 234, 234f,
237–238, 238f, 241–243, 242f, 243f
venous return curve in, 238–243,
239f–243f
regulation of, 163, 375f, 376
resting, 232
right atrial pressure and, 238, 239,
241–243, 242f, 243f
spinal anesthesia and, 242, 242f
sympathetic nervous system effects on,
112–113, 113f, 234–2

ic nervous system effects on,
112–113, 113f, 234–235, 242, 242f
venous return and, 232–236, 234f, 235f. See
also Venous return.
volume-loading hypertension and,
221–223, 221f, 222f
Cardiac output curve, 234, 234f, 237–238,
238f, 241–243, 242f, 243f
cardiac failure and, 258, 259f, 265–267,
265f–267f
external pressure effects on, 237–238, 238f
in hemorrhagic shock, 281, 283f
normal, 241, 242f, 265, 265f
parasympathetic nervous system effect
on, 113, 113f
post-infarc

ic nervous system effect
post-infarction, 259f, 260
sympathetic nervous system effect on,
113, 113f
Cardiac reserve
after myocardial infarction, 255
deﬁnition of, 264
exercise test of, 164–165, 165f
in cardiac failure, 260, 264–265, 265f
in patent ductus arteriosus, 275
in valvular disease, 273–274
Cardiac (cardiogenic) shock, 237, 254,
262–263
Cardiac tamponade, 238, 238f, 254
Cardiac veins, 249
Cardiopulmonary resuscitation, 155
Cardiotachometer, 148, 148f
Cardiotonic drugs, 261–

ardiotachometer, 148, 148f
Cardiotonic drugs, 261–262, 266–267, 267f
Caries, 994
Carotid body, 209, 209f, 211–212, 518, 518f
Carotid sinus syndrome, 148
Carpopedal spasm, 979, 979f
Carrier proteins, 46f, 47, 49–50, 53–54, 53f
Cartilage, growth hormone effects on, 923
Caspases, 40
Castration, 1008–1009
Catalase, 16
Cataracts, 621
Catecholamines. See Epinephrine;
Norepinephrine.
Cathode ray oscilloscope, 70–71, 70f
Caudate circuit, 709, 709f
Caudate nucleus, 688, 70

audate circuit, 709, 709f
Caudate nucleus, 688, 708, 708f
Caveolae, of smooth muscle, 99, 99f
Celiac disease, 822
Cell(s), 3, 11–25. See also speciﬁc cells.
ameboid movement of, 24, 24f
apoptosis of, 40
carbohydrates of, 12
cilia of, 24–25, 25f
cytoplasm of, 13f, 14–17, 15f, 16f. See also
Cytoplasm.
diameter of, 18
differentiation of, 40
endocytosis by, 19–20, 19f. See also
Phagocytosis.
enzyme regulation of, 36–37
ﬁlaments of, 17, 17f
genetic regulation of, 35–36, 35f
growth factor effects on, 

gulation of, 35–36, 35f
growth factor effects on, 39
growth of, 39–40
high altitude effects on, 540
ingestion by, 19–20, 19f
ions of, 11, 46f, 294f, 294t. See also speciﬁc
ions.
life cycle of, 37–40, 37f
lipids of, 12
locomotion of, 24–25, 24f, 25f
lysis of, 366
membrane of, 12–14, 12f, 14f. See also Cell
membrane.
nuclear membrane of, 17–18, 18f
nucleolus of, 12f, 13f, 18, 18f
nucleus of, 17, 18, 18f, 27–30, 28f–30f
organelles of, 13f, 14–17, 15f, 16f. See also
at Cytoplasm.
origin of, 18
prote

f, 16f. See also
proteins of, 11–12. See also Cell
membrane, proteins of.
red. See Red blood cell(s).
reproduction of, 37–40, 37f
ribosomes of, 18
size of, 40
structure of, 12–18, 12f, 13f
volume of, 53–54, 53f
water of, 11
white. See White blood cell(s).
Cell membrane, 12–14, 12f, 13f
active transport across, 46, 52–56. See also
Active transport.
carbohydrates of, 14
co-transport across, 55, 55f
counter-transport across, 55
diffusion across, 46–52. See also Diffusion

ss, 55
diffusion across, 46–52. See also Diffusion.
electrical potential of, 50, 50f. See also
Membrane potential.
lipid bilayer of, 12–13, 14f, 45, 46f, 848
permeability of, 51, 58
pressure difference across, 50–51
proteins of, 13–14, 14f, 45
carrier, 13, 46f, 47, 49–50, 53–54, 53f
channel, 13, 46f, 47–49, 47f, 48f
integral, 13, 14
peripheral, 13, 14, 14f
replenishment of, 22
Cellular sheet, active transport through,
55–56, 56f
Cellulose, 809, 817
Cementum, 992–993, 992f
Central fovea, 626, 636

17
Central nervous system, 6. See also Brain;
Spine; Spinal cord.
analgesia system of, 602–603, 602f
levels of, 557–558
neuronal pools of, 578–583, 579f–583f. See
also Neuronal pools.
neurons of, 555, 556f. See also Neuron(s).
synapses of, 559–569. See also Synapse(s).
Central nervous system ischemic 
response, 212–213, 213f, 214, 214f, 230,
230f
in heart failure, 258–259
Central retinal artery, 628
Central sleep apnea, 52

entral retinal artery, 628
Central sleep apnea, 523
Central venous catheter, 179
Central venous pressure, 176
Centrifugal acceleratory forces, 541–542,
542f
Centrioles, 13f, 17, 37f, 38–39
Centromere, 37f, 38
Centrosome, 39
Cephalins, 846–847, 847f. See also
Phospholipids.
Cerebellar hemispheres, 699, 699f, 704
intermediate zone of, 699, 699f, 704–705,
704f
lateral zones of, 699, 699f, 705–706
Cerebellar nystagmus, 707
Cerebellum, 558, 698–707, 699f
afferent pathways to, 700–701, 700f
anatomy of

99f
anatomy of, 699–700, 699f
anterior lobe of, 699, 699f
basket cells of, 702
climbing ﬁbers to, 701–702, 702f, 703
corrective function of, 698–699, 703,
704–705, 704f
damping in, 702, 705
deep nuclear cells of, 701–702, 702f
disorders of, 706–707
efferent pathways from, 701, 701f
extramotor predictive function of,
706
ﬂocculonodular lobes of, 696, 697, 699,
699f, 703–704
functional unit of, 701–702, 702f
granule cells of, 702, 702f
in motor function, 702–706

ule cells of, 702, 702f
in motor function, 702–706, 704f, 712
lateral inhibition in, 702
mossy ﬁbers to, 702, 702f
neuronal circuit of, 699f, 700–702, 701f,
702f
peripheral afferent pathways to, 700–701,
700f
planning function of, 706
posterior lobe of, 699, 699f
Purkinje cells of, 701–702, 702f, 703
stellate cells of, 702
timing function of, 706
topographic representation in, 699–700,
699f
turn-on/turn-off output from, 702–703
Cerebral blood ﬂow, 199, 761–763
arterial pressure effects on, 762–7

, 199, 761–763
arterial pressure effects on, 762–763,
763f
autoregulation of, 762–763, 763f
blockage of, 763
carbon dioxide effect on, 761, 762f
circulatory arrest and, 287–288
hydrogen ion effect on, 761
in hemorrhagic shock, 280
neuronal activity effect on, 762, 762f
normal, 761
1078
Index
Cerebral blood ﬂow (Continued)
oxygen effect on, 761–762
regulation of, 761–763, 762f, 763f
vasomotor center response to, 212–213
Cerebral blood pressure, negative
centrifugal acceleratory force effects
on, 

gative
on, 542
Cerebral cortex, 558. See also Motor cortex;
anatomy of, 714–715, 715f
association areas of, 716–718, 716f
Broca’s area of, 687, 717, 717f, 719, 721
Brodmann’s areas of, 589, 589f, 642
corticofugal signals of, 597
dominant hemisphere of, 718, 719
face recognition area of, 717–718, 717f,
718f
functional areas of, 715–720, 715f–716f
horizontal ﬁbers of, 714, 715f
in pain sensation, 601
in vasomotor control, 207
information

nsation, 601
information storage in, 557
limbic association area of, 716f, 717, 717f
neurons of, 714, 715f, 725
nondominant hemisphere of, 719
parieto-occipitotemporal association area
of, 716, 716f
prefrontal association area of, 716–717,
716f, 719–720
thalamus relation to, 714–715, 715f, 729,
729f
vasomotor center control by, 206f, 207
vertical ﬁbers of, 714, 715f
Wernicke’s area of, 716, 718–719, 718f,
721, 721f
Cerebral edema, 766–767
high-altitude, 541
respiratory 

ral edema, 766–767
respiratory effects of, 521
Cerebral ischemia, 212–213, 220–221, 691,
763. See also Cerebral blood ﬂow.
Cerebrocerebellum, 705–706
Cerebrospinal ﬂuid (CSF), 292, 763–767
arachnoidal absorption of, 765
circulation of, 764, 764f
composition of, 765
cushioning function of, 763–764
obstruction of, 766
osmolarity of, 361
perivascular spaces and, 765, 765f
pressure of, 765–766
Cushing reaction and, 213, 213f
removal of, headache and,

 reaction and, 213, 213f
removal of, headache and, 607
secretion of, 764–765, 764f
Cervix, stretch of, 1036–1037
CGS units, 167
Chain-terminating sequence, 31
Channel proteins, 45, 46f, 47–49, 47f, 48f
Chemicals. See also Drugs;
Neurotransmitters and speciﬁc
mutation and, 41
pain elicitation by, 599, 604
Chemiosmotic mechanism, 23, 835, 835f, 843
Chemoreceptor(s), 572, 573t
aortic, 211–212, 518–519, 518f
arterial pressure and, 211–212, 230, 230f
carotid, 209, 209f, 211–212, 518–519,

, 230, 230f
518f
in acclimatization, 539
in respiration, 516, 518–520, 518f, 519f
olfactory, 667–668, 667f
oscillation of, 214
Po2 stimulation of, 518–519, 518f, 519f
taste, 665–665, 666f
Chemoreceptor trigger zone, in vomiting,
823f, 824
Chemosensitive area, of respiratory center,
Chemotaxis, 24, 430f, 431, 435, 445
Chenodeoxycholic acid, 804
Chest
barrel, 529
current ﬂow in, 126–127, 127f
Chewing, 781–782
Cheyne-St

t ﬂow in, 126–127, 127f
Cheyne-Stokes breathing, 522, 522f
Chief cells, 795–796, 797
Childbirth, 8–9, 929, 1036–1038, 1037f
Chills, 899, 899f
Chloride
capillary diffusion of, 183–184, 184t
excretion of, 328t
in hemorrhagic shock, 283
in neuronal inhibition, 566, 566f, 567
intestinal absorption of, 815, 817
intestinal secretion of, 815
loop of Henle transport of, 334–336, 335f
Nernst potential

Henle transport of, 334–336, 335f
Nernst potential for, 565
neuronal concentration of, 564f, 565
plasma, in chronic renal failure, 410, 410f
reabsorption of, 328t, 332–333, 332f, 338f
Chloride channel, 562, 566, 566f
Chloride shift, 511
Chlorothiazide, 403t, 404
Cholecalciferol (vitamin D3), 983–984, 984f
Cholecystokinin, 907t
composition of, 799
in feeding, 870
in gallbladder emptying, 776, 803–804
in insulin secretion, 969
in pancreatic enzyme secretion, 800–801
in stomach emptyi

reatic enzyme secretion, 800–801
in stomach emptying, 786
Cholera, 815, 823
Cholesterol, 840, 847–848, 847f. See also
Lipid(s).
cellular, 12, 13, 46f
dietary, 848
digestion of, 812
extracellular, 46f, 294f
formation of, 847–848
functions of, 848
hepatic secretion of, 804–805
in adrenal steroid synthesis, 945, 946f,
955
in bile salt formation, 804
in familial hypercholesterolemia, 850
in gallstone formation, 804–805, 805f
plasma, 294f, 848
insulin deﬁciency and, 966
thyroid hormone effects on, 84

 deﬁciency and, 966
thyroid hormone effects on, 848, 936
transport of, 841–842
Cholesterol esterase, 799
Cholic acid, 804
Choline acetyltransferase, 563
Cholinesterase, 563
Chondroitin sulfate, 21
Chordae tendineae, 109, 109f
Chorea, 709, 711–712
Choroid, 628
Choroid plexus, 764–765, 764f
Chromatids, 38
Chromatin, 17, 18f
Chromosomes, 17, 36
daughter, 37f, 39
replication of, 38
sex, 997, 1028
Chronic obstructive lung disease, 501
Chylomicrons, 816, 840–841
Chyme, 784
intestinal movement of, 786–

6, 840–841
Chyme, 784
intestinal movement of, 786–788, 787f
neutralization of, 801
Chymotrypsin, 799–800, 810, 810f
Chymotrypsinogen, 799, 800
Cigarette smoking
in submarine, 550
respiratory effects of, 1062
Cilia
movement of, 24–25, 25f
of hair cells, 693–694, 693f
of olfactory cells, 667, 667f
of respiratory system, 480
structure of, 25, 25f
Ciliary body, aqueous humor formation by,
623, 623f
Ciliary muscle, 617f, 618
autonomic nervous system control of, 753,
754t
excess

onomic nervous system control of, 753,
754t
excessive contraction of, 607
Cingulate gyri, ablation of, 738
Circulation (blood), 4–5, 4f. See also Blood
ﬂow.
blood volumes in, 161–162, 162f, 172,
172f
541–542, 542f
collapse of. See Circulatory shock.
compliance in. See Vascular compliance.
kidney in, 216–231, 308. See also
Arterial blood pressure, renal
nervous system in, 204–214. See

 renal
dynamics of. See Blood ﬂow.
exercise-induced adjustments in, 247–249,
248f
extracorporeal, 276
fetal, 1042, 1045, 1046f
function of, 163
hemodynamics of. See Blood ﬂow.
in cardiac failure, 258–259, 259f
in hypertension, 221–223, 222f
in persistent patent ductus arteriosus,
274–275, 274f
in tetralogy of Fallot, 275–276, 275f
micro-. See Arteriole(s); Capi

llot, 275–276, 275f
micro-. See Arteriole(s); Capillary
(capillaries).
neonatal, 1045–1048, 1046f
physical characteristics of, 161–163, 162f,
162t
pressures in, 162–163, 163f. See also
Arterial blood pressure.
pulmonary, 161, 162f, 483–490. See also
Pulmonary circulation.
splanchnic, 777–780, 778f
systemic (greater, peripheral), 161, 162f
theory of, 163
vascular distensibility in, 171–172
volume-pressure curve and, 172, 172f
Circulation (lymph), 192–193, 192f, 193f
Circulatory arrest, 287–288
Ci

192–193, 192f, 193f
Circulatory shock, 236f, 237, 278–288
anaphylactic, 285–286, 287
arterial pressure in, 279, 279f
cardiogenic, 254, 262–263
decreased cardiac output and, 278
decreased venous return and, 278
Index
1079
Circulatory shock (Continued)
hemorrhagic, 279–284. See also
Hemorrhagic shock.
irreversible stage of, 279
neurogenic, 285, 287
nonprogressive stage of, 279
plasma loss and, 284–285
progressive stage of, 279
septic, 286
stages of, 279
tissue deteriora

f, 279
tissue deterioration in, 279
traumatic, 285
treatment of, 286–287
without decreased cardiac output, 278
Circulatory system, 4–5, 4f. See also
Arteriole(s); Artery (arteries); Capillary
(capillaries); Cardiac muscle; Heart;
Vein(s).
Circus movements, 153, 153f, 273
Cirrhosis, 381, 855, 860
bleeding in, 464
edema in, 303–304
Cisterna magna, 764
Citrate
anticoagulant action of, 467
phosphofructokinase inhibition by, 467
Citri

, 203
Citric acid cycle (Krebs cycle), 23, 833–835,
834f, 843
Clathrin, 19, 19f, 88
Clearance
creatinine, 344–345, 344t
free-water, 357
inulin, 344, 344t, 345f, 347
Climacteric
female, 1011, 1022, 1022f
male, 1007–1008, 1008f
Clonus, 678f, 679
Clot. See Blood clot; Coagulation;
Coagulation factors.
Clothing, body temperature and, 892
Clotting time, 467
Coagulation, 459–464
abnormal, 465–466
calcium ions in, 463, 467
endothelial cell surface factors in, 463

463, 467
extrinsic pathway of, 461–462, 461f, 463
ﬁbrin formation in, 460
intrinsic pathway of, 461f, 462–463
positive feedback in, 8, 460–461
prevention of, 463–464
prothrombin activator in, 461–463, 461f,
462f
tests of, 467–468, 467f
thrombin formation in, 459–460, 459f
vicious circle in, 460–461
Coagulation factors, 8, 459t, 461–463, 461f,
462f
hepatic formation of, 862
vitamin K–related, 460, 464–465, 878
C

ion of, 862
Coarctation of aorta, 227, 274
Coated pits, 19, 19f, 88
Cobalamin. See Vitamin B12 (cobalamin).
Cocaine, exercise and, 1066
Cochlea, 652–657
anatomy of, 652–653, 652f, 653f, 693, 693f
basilar membrane in, 653, 653f
fluid movement in, 652–653, 653f, 654, 654f
impairment of, 660–661
organ of Corti of, 655–656, 655f
resonance in, 653
sound transmission in, 594, 654–656, 654f,
655f
Cochlear nerve, 652f, 655
Cochlear nuclei, 657, 658, 658f
Codons, 30, 

, 655
Codons, 30, 31–32, 32t
Coenzyme A, 877. See also Acetyl coenzyme
A.
Cognition. See Memory; Thinking.
Cold exposure, 900
acclimatization to, 896
spermatogenesis and, 1001
thyrotropin-releasing hormone secretion
and, 896, 938–939
Cold receptors, 607–609, 894–895
Cold sensation, 572, 573t, 607–609, 608f
transmission of, 587, 595–597, 596f, 609,
895
Colitis, ulcerative, 823
Collagen
digestion of, 810
in ascorbic acid deﬁciency, 877
of interstiti

 810
of interstitium, 184–185, 184f
of lung tissue, 473
Collateral vessels, 201
Colloid osmotic pressure. See also Osmotic
pressure.
interstitial, 185–186, 185f, 188–189,
193–194
normal, 188
of peritubular capillaries, 309, 309f, 310t,
340, 340f
plasma, 185–186, 185f, 188
Colon. See also Gastrointestinal tract.
absorption in, 817
atonic, 822
bacteria of, 817
bicarbonate secretion by, 815, 817
disorders of, 822–823
distension of, 822
feces formation in, 817
haustra

haustrations of, 788–789, 788f
mass movements of, 789
mixing movements of, 788–789, 788f
mucus secretion by, 806
propulsive movements of, 789
sodium absorption in, 817
water absorption in, 817
Colonoileal reﬂex, 776
Colony-forming unit–erythrocyte, 420, 421f
Color blindness, 633, 634f
Color pigments, 627, 631, 631f
in dark/light adaptation, 631–632, 632f
Color vision, 632–633, 633f, 638. See also
Cones.
Ishihara chart tests of, 633, 634f
Coma
acidotic, 

Coma
acidotic, 973–974, 973f
hepatic, 862
Commissure, anterior, 722
Communication, 720–722, 721f. See also
Complement, 432, 445–446, 445f
Complement complex, 443
Compliance
lung, 473–475, 473f
thoracic, 474–475
vascular, 171–172
delayed (stress-relaxation), 94–95, 172,
173f
pulmonary, 483
pulse pressure and, 173, 174
Compressed air sickness, 548–549, 548f
Computer, 558, 558f
Concentration gradient, in gas diffusion,
4

, 558f
491, 492f
Concussion, 767
Conductance, in blood ﬂow, 167–168, 168f,
169
Conduction (heat), 891, 891f, 892
Conductive system. See Heart, conductive
system of.
Cones, 626–628, 627f, 628f, 632–633, 633f,
635f. See also Rods.
absence of, 633, 634f
dark/light adaptation of, 631–632, 632f
discs of, 627, 628f
inner segment of, 627, 628f
outer segment of, 627, 628f
photochemistry of, 631, 631f
spectral sensitivities of, 632–633, 633f
synaptic body of, 627

sitivities of, 632–633, 633f
synaptic body of, 627–628, 628f
Conn’s syndrome, 342, 379, 959
hypokalemia in, 366
sodium retention in, 379
Consciousness, 723. See also Thinking.
Constipation, 607, 822
Contact junctions, 96
Contact lenses, 620–621
Continuous positive airway pressure, 523
Contraception, 1024
Contrecoup head injury, 764
Control systems, 6–9, 8f
feed-forward, 9
gain of, 7–8
negative feedback of, 7, 8f
positive feedback of, 8–9, 8f
Convection (heat), 891–

itive feedback of, 8–9, 8f
Convection (heat), 891–892, 891f
Convergence
of light rays, 613–614, 614f, 615, 615f
of nerve ﬁbers, 580, 580f
Cooling, 891–892, 891f
artiﬁcial, 900
Cord righting reﬂex, 682
Cordotomy, for pain interruption, 602
Cornea. See also Eye(s); Lens; Vision.
bulging of, 621
Coronary arteries, 249, 249f
blood ﬂow in, 249–256, 250f. See also
Coronary blood ﬂow.
collateral, 201, 252–253, 252f, 260
embolus of, 252
epicardial, 250, 250f
innervation of, 251
intramuscular, 250, 250f

250f
occlusion of, 252–254, 253f. See also
spasm of, 252
subendocardial, 250, 250f
thrombus of, 252
vasodilation of, 250–251, 251
Coronary artery angioplasty, 256
Coronary artery disease, 252–255, 253f. See
also Myocardial infarction; Myocardial
ischemia.
acute occlusion in, 252–253
atherosclerosis in, 252, 256. See also
collateral vessel formation in, 201,
252–253, 252f
pain in, 255–256
treatment of, 256
Corona

53, 252f
Coronary blood ﬂow, 249–256
adenosine and, 251–252
anatomy of, 249, 249f
autonomic nervous system effect on, 251
control of, 250–251
normal, 249–250, 250f
oxygen demand and, 250–251
Coronary sinus, 249
1080
Index
Coronary steal syndrome, 255
Corpus callosum, 591, 719, 722
transection of, 722
Corpus luteum, 1015
Corti
organ of, 652, 652f, 653f, 655–656, 655f
rods of, 655, 655f
spiral ganglion of, 655, 655f

f
Cortical collecting duct. See Renal tubules,
cortical collecting.
Corticofugal signals, 597
Corticopontocerebellar pathway, 700,
700f
Corticorubral tract, 688–689, 689f
Corticorubrospinal system, 688–689, 689f
Corticospinal (pyramidal) tract, 687–688,
688f
lesions of, 691
Corticosteroids, 944–960. See also
Aldosterone; Cortisol.
Corticosterone, 945, 947, 947t, 951
Corticotropes, 919, 920t
in fat me

1
in fat metabolism, 838
in gluconeogenesis, 838
Corticotropin-releasing hormone, 907t, 921,
921t, 955
Cortisol, 907t, 946f, 947, 947t
ACTH effects on, 955–957, 956f
amino acid transport and, 952
anterior pituitary effects of, 955
carbohydrate metabolism and, 838,
951–952
cellular action of, 954–955
cellular protein and, 952
circadian rhythm of, 956, 956f
deﬁciency of, 957–958
fat metabolism and, 846, 952
function

58–959, 959f
functions of, 950–957, 953f, 956f
hypothalamic effects of, 955
in allergic reaction, 954
inﬂammation-related secretion of,
952–954
liver proteins and, 952
obesity and, 952
plasma proteins and, 947, 952
protein metabolism and, 952
secretion of, 955–957, 956f
stress-related, 952–954
synthesis of, 946f, 947, 947t
Cortisol-binding globulin, 947
Cortisone, 947, 94

Cortisone, 947, 947t
Co-transport, 54–55, 55f
in renal reabsorption, 329–330, 330f,
333–334
sodium-glucose, 55, 55f, 816, 831
Cotton wick method, for interstitial ﬂuid
pressure, 187
Cough reﬂex, 480
Coumarins, 466
Countercurrent mechanism, of loop of
Henle, 351–352, 352f, 357–358
Counter-transport, 54–55
Coupled reactions, 829
Cramps, 604, 683
Cranial nerves, 750, 750f
Creatine, 294t
excretion of, 307, 32

0f
excretion of, 307, 328t, 333, 338f
in chronic renal failure, 410, 410f
plasma, 294f, 344–345, 346f
transport maximum and, 331
Creatinine clearance, 344–345, 344t
Cretinism, 942
Cricoid cartilage, 481, 481f
Crisis, febrile, 899, 899f
Crista ampullaris, 693f, 694
of skeletal muscle, 72, 75, 75f, 76–77, 77f
of smooth muscle, 94
Crossed extensor reﬂex, 680f, 681, 681f
Cryptorchidism, 1001
Crypts of Lieberkühn, 791, 792f
secretions of, 805–806, 805f
Cupula, 

n, 791, 792f
Cupula, 693f, 694
Curare, end plate potential effects of, 88, 88f
Current of injury, cardiac, 141–145,
142f–145f, 254
Cushing reaction, 213, 213f
Cushing’s disease, 846
Cushing’s syndrome, 958–959, 959f, 974
osteoporosis in, 992
Cyanide poisoning, 196, 530
Cyanocobalamin. See Vitamin B12
(cobalamin).
Cyanosis, 531
in carbon monoxide poisoning, 510
in tetralogy of Fallot, 276
Cyclosporine, in organ transplantation, 456
Cysteine, 853f

ine, in organ transplantation, 456
Cystinuria, 413
Cystitis, 409
Cystometrography, 312–313, 313f
Cytochrome oxidase, 835, 835f
cyanide blockade of, 530
Cytokines, 29, 29f, 905
cachexia and, 874
Cytoplasm, 12f, 14–17, 15f, 16f
endoplasmic reticulum of, 15, 15f, 20–21,
21f, 33, 34f
ﬁlaments of, 17
Golgi apparatus of, 15, 15f, 21–22
lysosomes of, 13f, 16, 20, 20f
microtubules of, 11, 13f, 17, 17f
mitochondria of, 12, 13f, 16–17, 16f. See
also Mitochondrion (mitochondria).
peroxisomes

See
peroxisomes of, 16
secretory vesicles of, 13f, 16, 16f, 21, 21f
transcription in, 30–35, 32f, 32t, 34f
Cytosine, 27, 28f
Cytoskeleton, 11, 17
Cytosol, 14
D
Dark adaptation, 631–632, 632f
Dead space
air in, 478, 495–496, 496f
alveolar ventilation and, 477–478, 478f,
500–501
anatomic, 478
measurement of, 477–478, 478f
physiologic, 478, 500–501
Deafness, 660–661, 661f
Deamination, 856, 862
Decarboxylases, in citric acid cycle, 834–835
Deceleratory forces, in p

tric acid cycle, 834–835
Deceleratory forces, in parachute jumps, 543
Decerebrate animal, 673, 682, 692, 758
Decibel, 657, 657f
Decidua, 1029
Decompression, of diver, 547–549, 548f
Decompression sickness, 548–549, 548f
Deconditioning, in space travel, 544
Deep nuclear cells, of cerebellum, 701–702,
702f
Deep-sea diving, 545–549
atmospheric pressures during, 545,
546f
carbon dioxide pressures during, 547
decompression after, 547–549, 548f
decompression sickness with, 548–549,
548f
gas compression

ssion sickness with, 548–549,
548f
gas compression during, 545, 546f
helium-oxygen mixture in, 549
nitrogen pressures during, 545–546
oxygen pressures during, 546–547, 546f
Defecation, 757, 789–790, 790f, 822
in spinal cord injury, 823
Deﬁbrillation, 154–155, 155f
Deglutition, 782–784, 782f
Deglutition center, 783
Dehydration
hyperosmotic, 301t, 302
hypo-osmotic, 301, 301t
in diabetes mellitus type I, 973
low sodium intake and, 356
seawater consumption and, 350
threshold of drinki

mption and, 350
threshold of drinking and, 362
Dehydroepiandrosterone, 946f
Dehydrogenases, in citric acid cycle, 834
Delta brain waves, 742–743, 742f, 743f
Dementia, 746
Dendrites, 555, 556f, 559, 559f
decremental conduction in, 569
electrotonic conduction in, 568–569,
568f
spatial ﬁeld of, 568
summation in, 569
Dendritic cells, 446, 446f
Denervation supersensitivity, 757
Dense bars, 86, 86f
Dense bodies, 93, 93f
Dentate nucleus, 701, 701f, 706
Dentin, 992, 992f
Dentition. See Te

701, 701f, 706
Dentition. See Teeth.
Deoxyadenylic acid, 27, 29f
Deoxycorticosterone, 947t
Deoxycytidylic acid, 27, 29f
Deoxyguanylic acid, 27, 29f
Deoxyribonucleic acid (DNA), 27–30
activator operator of, 35f, 36
bases of, 27, 28f, 29–30, 29f, 30f
deoxyribose of, 27, 28f, 29f
mitochondrial, 17
nucleotides of, 27, 29, 29f
operon of, 35, 35f, 36
phosphoric acid of, 27, 28f
promoter of, 35, 35f
proofreading of, 38
replication of, 37–38
repressor operator of, 35–36, 35f
Deoxyribon

37–38
Deoxyribonucleic acid (DNA) ligase, 38
Deoxyribonucleic acid (DNA) polymerase,
38
Deoxyribose, 27, 28f
Deoxythymidylic acid, 27, 29f
Depression, 745
Depth of focus, 618–619, 618f
Depth perception, 621–622, 622f, 647–648
Dermatomes, 596f, 597
Desmopressin, in central diabetes insipidus,
357
Desoxycorticosterone, 945
Detrusor muscle, 311, 311f
Deuteranope, 633, 634f
Dexamethasone, 947, 947t
Index
1081
Dextran
glomerular ﬁltration of, 317, 317f
in shock, 286–

in shock, 286–287
a-Dextrinase, 809
Diabetes, adrenal, 951–952
Diabetes insipidus, 302
central, 357
nephrogenic, 357–358, 414
Diabetes mellitus, 972–976
acidosis in, 398
arteriosclerosis and, 976
atherosclerosis and, 850
complications of, 976
diagnosis of, 975–976, 975f, 975t
hyperkalemia in, 366
in gigantism, 926
insulin overtreatment in, 767
ketosis in, 844
obesity and, 974
plasma glucose in, 331
potassium distribution in, 366, 366t
pregnancy and, 1050
prote

stribution in, 366, 366t
protein wasting in, 967
renal failure in, 407, 408, 408t
respiratory quotient in, 867
type I, 972–974, 973f
treatment of, 976
type II, 972, 974–975, 974t
Diacylglycerol, 914, 914f
Dialysis, 414–415, 414f, 415t
Dialyzing ﬂuid, 415, 415t
neutrophil, 430f, 431
reticulocyte, 422
Diarrhea, 822–823
acidosis and, 397
infectious, 806, 815, 817
ion secretion in, 81

397
ion secretion in, 815
peristaltic rush in, 788
psychogenic, 823
water loss with, 292
Diastole, 106–107, 107f, 108, 110–111, 110f
blood ﬂow during, 250, 250f
Diet. See also Food.
carbohydrates in, 866, 866t, 867. See also
Carbohydrates.
energy in, 865–866, 866t
fats in, 866, 866t, 867. See also Fat(s).
high-fat, 844, 848
in atherosclerosis prevention, 851
minerals in, 878–879, 878t, 879t
protein in, 866, 866t. See also Protein(s).
vitamins in, 875–878, 875t. See also

o Protein(s).
vitamins in, 875–878, 875t. See also 
speciﬁc vitamins.
Dieting, 873
Differentiation inducers, in stem cell
development, 420
Diffusing capacity
of carbon dioxide, 498–499, 499f
of carbon monoxide, 499, 499f
of oxygen, 498, 499f, 539, 1062
Diffusion, 19, 46–52, 46f
capillary, 4–5, 5f, 183–184, 183f, 184t
electrical potential and, 50, 50f
facilitated (carrier-mediated), 46f, 47,
49–50, 49f, 831
in renal reabsorption, 332–333, 332f
interstitial, 185
membrane potential and, 57–58, 58f

erstitial, 185
net rate of, 50–51, 50f
of gases, 491–493, 492f. See also Carbon
dioxide, transport of; Gas exchange;
Oxygen, transport of.
Diffusion (Continued)
of lipid-insoluble substances, 47
of lipid-soluble substances, 47
of sodium, 60–61, 60f
of water, 47, 51–52, 51f
pressure difference and, 50–51, 50f
protein-channel, 47–49, 47f, 48f
simple, 46–47, 46f
Diffusion coefﬁcient, of gases, 493, 498
Diffusion potential, 57–58, 58f. See also

tential, 57–58, 58f. See also
calculation of, 58
potassium, 60, 60f
Digestion, 808–817
carbohydrate, 808, 809, 809f
fat, 808, 811–812, 811f, 812f
hydrolysis in, 808–809
intracellular, 16, 20, 20f
protein, 808–809, 810–811, 810f
Digitalis
in cardiac failure, 261–262, 266–267,
267f
toxicity of, 146, 146f
Dihydroepiandrosterone, 947t
Dihydrotestosterone, 1003
1,25-Dihydroxycholecalciferol, 308, 907t,
984–985, 984f
Diisopropyl ﬂuorophosphate, 89
Dinitrophenol, cardiac output effe

rophosphate, 89
Dinitrophenol, cardiac output effects of,
235, 235f
Diopter, 616–617, 616f
Dipalmitoylphosphatidylcholine, 474
Dipeptidases, 810–811
Disse, spaces of, 859
Disseminated intravascular coagulation, 286,
466
Dissociation constant (K¢), 385–386
Diuresis, pressure, 216, 323, 341–342,
Diuretics, 402–404, 403f, 403t
loop, 403, 403t
osmotic, 402–403, 403t
potassium-sparing, 337

403t
potassium-sparing, 337, 403t, 404
thiazide, 403t, 404
Divergence
of light rays, 614, 614f, 615, 615f
Diving. See Deep-sea diving.
DNA. See Deoxyribonucleic acid (DNA).
Donnan effect, 188, 293
Dopamine, 563, 730–731, 730f, 907t
in Parkinson’s disease, 711
in schizophrenia, 745–746
Dopamine pathway, 710–711, 710f
Doppler ﬂowmeter, 164–165, 165f
Dorsal column–medial lemniscal system,
587, 588–589, 592–594
anatomy of, 588, 588f
ﬁber orientation i

, 592–594
ﬁber orientation in, 589
in position sensation, 595, 595f
in two-point discrimination, 592–593,
593f
in vibratory sensation, 593
lateral inhibition in, 593
neuronal circuit of, 592, 592f
Dorsal lateral geniculate nucleus, 640–641,
641f
Dorsal spinocerebellar tract, 700, 700f
Double helix, 28f, 29
Dreaming, 739–740
Drinking, 361–362, 361t. See also Thirst.
seawater, 350
threshold for, 362
Drugs. See also speciﬁc drugs.
antidiuretic hormone secretion and, 360t,
361

gs.
361
diabetes insipidus and, 357
ganglionic blocking, 760
hepatic processing of, 862
immunosuppressive, 456
in obesity treatment, 873–874
leukopenia and, 437
nicotinic, 759–760
parasympathomimetic, 759
sympathomimetic, 759
D-tubocurarine, neuromuscular junction
effects of, 89
Duct of Bellini, 311f
Ductus arteriosus, 1045, 1046f
closure of, 274, 1046–1047
patent, 274–275, 274f
Ductus venosus, 1045, 1046f
closure of, 1047

274f
Duodenocolic reﬂex, 789, 822
Duodenum. See also Gastrointestinal tract.
bicarbonate ion absorption in, 815
Brunner’s glands of, 805
calcium absorption in, 815
chloride absorption in, 815
gastric inhibition by, 785–786
hormone secretion by, 776, 786. See also
Cholecystokinin; Secretin.
mucus secretion by, 805
tubular glands of, 791, 795f
Dwarﬁsm, 924, 926
Dynein, 25
Dynorphin, 603
Dysarthria, 707
Dysbarism, 548–549, 548f
Dysdiadochokinesia, 707
Dy

ysbarism, 548–549, 548f
Dysequilibrium, 695–696. See also
Equilibrium.
Dyslexia, 718, 721
Dysmetria, 706
Dyspnea, 527, 531, 532
E
Ear
cochlea of. See Cochlea.
disorders of, 660–661, 661f
ossicular system of, 651–652, 652f
sound conduction in. See also Hearing.
from tympanic membrane to cochlea,
651–652, 652f
within cochlea, 654–655, 654f
Eardrum, 651–652, 652f
Eclampsia, 1035–1036
Ectoplasm, 17
Eddy currents, 165f, 166
Edema, 185, 190, 302–305
cerebral, 766–767
high-altit

ma, 185, 190, 302–305
respiration and, 521
ﬂuid dynamics of, 376–377, 377f
hypoproteinemic, in premature infant,
1050
in cardiac failure, 259–260, 261, 263–264,
263f, 303
in cirrhosis, 303–304, 381
in kidney disease, 303, 381, 406
intracellular, 302
optic disc, 766
pitting, 305
preventive mechanisms in, 304–305, 340f
1082
Index
Edema (Continued)
pulmonary, 488–489
high altitude, 540–541
hypoxia in, 530–531

488–489
hypoxia in, 530–531, 531f
in cardiac failure, 262, 264, 488–489
in unilateral left-sided cardiac failure,
262
Edinger-Westphal nucleus, 648, 648f, 649
Effusion, 306
pericardial, 140
pleural, 140–141, 490
Einthoven’s law, 128
Einthoven’s triangle, 127f, 128
Ejaculation, 757
Ejaculatory duct, 996, 997f
Ejection fraction, 108
Elastase, 810, 810f
Elastin, 473
Electrical alternans, 150, 150f
Ele

estion of, 810
Electrical potential, 50, 50f, 564f, 565. See
also Membrane potential.
endocochlear, 656
Electrical stimulation, in pain treatment,
603
Electrocardiogram, 107, 107f, 123–130, 124f
after myocardial infarction, 140, 140f, 145,
145f
augmented unipolar limb leads for, 129f,
130–131
bipolar limb leads for, 127–129, 127f
axes of, 132, 132f
vectorial analysis of, 133–134, 133f
chest leads for, 130, 130f
current ﬂow and, 126–127, 126f, 127f
current of injur

rent ﬂow and, 126–127, 126f, 127f
current of injury effect on, 141–145,
142f–145f
depolarization wave on, 123–125, 124f,
125f
Einthoven’s law and, 128
Einthoven’s triangle in, 127f, 128
electrical axis from, 137–138, 137f
heart rate on, 126
in atrial ﬁbrillation, 155–156, 155f
in atrial ﬂutter, 156, 156f
in atrioventricular block, 148–150, 149f
in bradycardia, 147–148, 148f
in bundle branch block, 139–140, 139f,
140f
in cardiac myopathy, 140, 140f
in current of injury, 141–145, 142f–146f
in digi

f
in digitalis toxicity, 146, 146f
in electrical alternans, 150, 150f
in intraventricular block, 150, 150f
in mild ischemia, 145–146, 146f
in myocardial infarction, 143–145, 143f,
144f
in paroxysmal tachycardia, 151–152
in pericardial effusion, 140
in pleural effusion, 140–141
in premature atrial contractions, 150, 150f
in premature atrioventricular
contractions, 150–151, 151f
in premature contractions, 150–151, 150f,
151f
in premature ventricular contrac

0–151, 150f,
151f
in premature ventricular contractions,
151, 151f
in pulmonary emphysema, 141
in sinoatrial nodal block, 148, 148f
in T wave abnormalities, 145–146, 146f
in tachycardia, 147, 148f
in ventricular ﬁbrillation, 154, 154f
in ventricular hypertrophy, 138–139, 138f,
139f
Electrocardiogram (Continued)
leads for, 127–130, 127f–129f
limb leads for, 127–129, 127f
mean electrical axis from, 137–140,
137f–140f. See a

an electrical axis from, 137–140,
137f–140f. See also Mean electrical
axis.
monophasic ventricular action potential
on, 124–125, 125f
normal, 107, 107f, 128–129, 128f, 129f,
134–137, 135f–137f
P wave of, 107, 107f, 123, 124f, 136, 136f
pen recording system for, 126
precordial leads for, 130, 130f
Q wave of, 107, 107f, 123, 124f
QRS complex of, 107, 107f, 124–125, 125f.
See also QRS complex.
R wave of, 107, 107f, 123, 124f, 134, 135f
repolarization wave on, 123–125, 124f,
125f
S wave of, 107, 107

n wave on, 123–125, 124f,
125f
S wave of, 107, 107f, 123, 124f, 134, 135f
T wave of, 107, 107f, 123, 124–125, 124f,
125f
abnormalities of, 145–146, 146f
vectorial analysis of, 134–136, 136f
time calibration in, 125–126
vectorial analysis of, 131–146
during atrial depolarization, 136, 136f
during depolarization, 134, 135f
during repolarization, 134–136, 136f
in bipolar limb leads, 133–134, 133f
140f
in current of injury, 141–14

c myopathy, 140, 140f
144f
151, 151f
in ventricular hypertrophy, 138–139,
138f, 139f
lead axes in, 132, 132f
mean electrical axis and, 137–140,
137f–140f
of abnormal electrocardiogram,
140–146, 140f, 142f–146f
of normal electrocardiogram, 134–137,
135f–137f
principles of, 131–134, 132f, 133f
projected vector in, 133, 133f
vector direction

3f
vector direction in, 131–132, 132f
vectorcardiogram for, 136–137, 137f
voltages in, 125
Electroconvulsive therapy, 745
Electroencephalography
during seizures, 744, 744f
during sleep, 741–743, 742f, 743f
during wakefulness, 743, 743f
Electrogenic pump, 59, 60f
Electrolytes. See Chloride; Magnesium;
Potassium; Sodium.
Electromagnetic ﬂowmeter, 164, 165f
Electromagnetic receptors, 572, 573t
Electromotive force, 50, 58
Electron transport chain, 835, 835f
Electrotoni

58
Electrotonic conduction, 568–569, 568f, 635
Embolus
air, 178, 550
coronary, 252
pulmonary, 466
Embryo. See also Fetus.
cell differentiation in, 40
early nutrition of, 1029–1031, 1029f–1031f
Embryonic cells, ameboid movement of,
24
Emmetropia, 619, 619f
Emotion
fainting and, 208
limbic system in, 731–732, 732f
and, 939
Emotional dyspnea, 532
Emphysema, 526–527, 527f, 528f
acidosis in, 389
electrocardiogram in, 141
gas 

8f
gas exchange in, 498
Enamel, 992, 992f
Encephalitis, swallowing in, 819
End-diastolic pressure, 110, 111
End-diastolic volume, 108
Endocochlear potential, 656
Endocrine system, 6, 905–916, 906f, 907t. See
also speciﬁc glands and hormones.
Endocytosis, 19–20, 19f
Endolymph, 656
Endometriosis, 1025
blastocyst implantation in, 1029, 1029f
cyclic changes of, 1018–1019, 1018f
Endoplasmic matrix, 15, 20
Endoplasmic reticulum, 15, 15f
agranular,

, 15, 20
agranular, 13f, 15, 15f
functions of, 21, 21f
functions of, 20–21, 21f
granular, 13f, 15, 15f
functions of, 20, 21f
lipid formation by, 21, 21f
protein formation by, 20, 21f
ribosome attachment to, 33, 34f
Endoplasmic reticulum vesicles, 15, 15f
Endorphins, 602–603, 956–957, 957f
Endothelial glycocalyx, 463
Endothelin, in blood ﬂow, 202, 322, 322t
Endothelium-derived relaxing factor,
199–200
Endotoxin, in hemorrhagic shock, 282
End-plate potential, 87–88, 

emorrhagic shock, 282
End-plate potential, 87–88, 87f, 88f
End-stage renal disease, 407–408, 407f, 408t.
See also Kidneys, failure of.
End-systolic volume, 108
Energy. See also Adenosine triphosphate
(ATP).
aerobic, 882
daily requirement for, 885–886
excess intake of, 872–874. See also
Obesity.
fats for, 966, 966f
for food processing, 887
for physical activity, 872, 887, 887t
for respiration, 475
for thermogenesis, 887–888
free, 829
from food, 865–866, 866t
in action 

–888
free, 829
in action potential, 66, 66f
in active transport, 54
kinetic, 46, 46f, 51–52
minimum expenditure of, 886–887,
887f
Enkephalins, 602–603, 602f
Enteric nervous system, 753–754, 773–776,
791–792
myenteric (Auerbach’s) plexus of,
774–775, 774f
reﬂexes of, 775–776
submucosal (Meissner’s) plexus of,
Enteritis, 806, 822–823
Enterochromafﬁn-like cells, 797
Index
1083
Enterocytes, 805–806, 805f
enzymes of, 809, 810–811
Enterogastric reﬂexes, 776, 785–

 of, 809, 810–811
Enterogastric reﬂexes, 776, 785–786, 799
Enterokinase, 800
Enzyme(s), 13. See also speciﬁc enzymes.
activation of, 36–37
catalytic action of, 884, 884f
digestive, 782
hydrolytic, 16
in phagocytosis, 432
inhibition of, 36
intestinal, 806, 809, 809f
intracellular, 36–37
of complement system, 445–446, 445f
of sperm, 1000
pancreatic, 799–800, 801, 809, 809f
peptidase, 810–811, 810f
proteolytic, 432, 1000
Enzyme-linked immunosorbent assay, for
hormone measurement, 916, 916f
Eosinoph

assay, for
Eosinophil(s), 421f, 429–430, 430f
cortisol effects on, 954
in parasitic infection, 436
Eosinophil chemotactic factor, 436, 529
Ephedrine, 759
Epididymis, 996, 997f, 999
Epilepsy, 743–745
focal, 744–745
grand mal, 743–744, 743f, 744f
petit mal, 743f, 744, 744f
psychomotor, 744f, 745
surgery in, 745
Epinephrine, 907t
basal secretion of, 756
blood ﬂow and, 201, 251, 322, 322t
in br

 effects of, 479
in exercise, 247, 846
in glycogenolysis, 832
intracellular potassium and, 366
plasma fatty acids and, 970
plasma glucose and, 970
potassium effects of, 366
receptors for, 752
secretion of, 207–208, 750, 751–752, 755,
756
active transport through, 55–56, 56f
in vitamin A deﬁciency, 875
of glomerular capilla

of glomerular capillary, 316, 316f
Eplerenone, 403t, 404
Equilibrium, 692–697
central nervous system in, 696–697,
696f
exteroceptive information in, 696
ﬂocculonodular lobes in, 696, 697
neck proprioceptors in, 696
static, 694–695
vestibular apparatus in, 692–696,
693f–695f
vestibulocerebellum in, 703–704
visual information in, 696
Equilibrium point, of arterial pressure,
217–218
Erection, 757
Erythremia, 428
Erythroblast, 421f, 422
Erythroblastosis fetalis, 421f, 42

last, 421f, 422
Erythroblastosis fetalis, 421f, 427, 454,
1048
Erythrocytes. See Red blood cell(s).
Erythropoietin, 907t
in chronic renal failure, 412
in red blood cell production, 422–423,
422f
secretion of, 308
Esophageal reﬂux, 783, 784
Esophagus
achalasia of, 819
disorders of, 819
dryness of, 361, 361t
enlargement of, 819
in swallowing, 782f, 783–784
mucus secretion by, 795
peristalsis of, 783
smooth muscle of, 783
striated muscle of, 783
Estradiol. See also Estrogen.
in female reproduction,

radiol. See also Estrogen.
in female reproduction, 1016–1018, 1016f
Estrogen, 907t
blood transport of, 1016
bone effects of, 1017
breast effects of, 1017
deﬁciency of, 1017
electrolyte effects of, 1018
fallopian tube effects of, 1017
in female reproduction, 1011, 1013f,
1016–1018, 1016f
in spermatogenesis, 999
liver metabolism of, 1016–1017
metabolic rate and, 1018
placental secretion of, 1032–1033
protein deposition and, 857, 1017–1018
skin effects 

otein deposition and, 857, 1017–1018
skin effects of, 1018
synthesis of
in female, 1016
in male, 1004
uterine effects of, 1017
vaginal effects of, 1017
Ethacrynic acid, 403, 403t
Ethylene glycol, renal toxicity of, 406
Evans blue dye, for plasma volume, 296,
296t
Evaporation, 891f, 892
Excitation-contraction coupling
in cardiac muscle, 106
in skeletal muscle, 89–91, 90f, 91f
Exercise, 1055–1066
age-related changes and, 1064–1065
amphetamines and, 1066
androgens and, 1065–1066
art

 1066
arterial pressure during, 208, 213–214,
235–236, 248–249, 248f
blood ﬂow during, 246–249, 247f, 248f
blood gases during, 1062
body heat during, 1065
caffeine and, 1065
cardiac output during, 213–214, 232–233,
233f, 235–236, 248–250, 248f,
1063–1064, 1064f, 1064t
cardiovascular system and, 1062–1065,
1063f
cigarette smoking and, 1062
cocaine and, 1066
cramps during, 683
energy expenditure during, 887, 887t
epinephrine secretion during, 247, 846
fat u

 887t
fat utilization during, 846
growth hormone secretion and, 924, 924f
heart disease and, 1064–1065
heart rate during, 1063–1064, 1064f, 1064t
heatstroke and, 1065
hyperkalemia and, 366
Exercise (Continued)
in obesity, 873–874
life-prolonging effect of, 1066
mean systemic ﬁlling pressure during, 248,
249
muscles in, 1055–1061. See also at Muscle
(skeletal).
adenosine triphosphate system of,
1056–10

eletal).
1056–1057
aerobic system of, 1057–1058
anaerobic system of, 883
blood ﬂow of, 246, 247f, 1062–1063,
1063f
endurance of, 1056
fast-twitch, 1060–1061
glycogen recovery of, 1059, 1059f
glycogen-lactic acid system of, 1057
hypertrophy of, 1060–1061, 1060f
metabolic systems of, 883, 1056–1059,
1057f, 1058t
nutrients for, 1059–1060, 1059f
phosphocreatine-creatine system of,
1057–1059, 1058t
power of, 1056
recovery of, 1058, 1059, 1059

1058t
recovery of, 1058, 1059, 1059f
resistance training for, 1060, 1060f
slow-twitch, 1060–1061
strength of, 1056, 1060, 1060f
norepinephrine secretion during, 247, 846
oxygen consumption during, 233, 233f,
247, 506–507, 506f, 520, 520f,
1061–1062, 1061f, 1062f
oxygen debt in, 883, 1058–1059, 1059f
oxygen diffusing capacity in, 498, 499f,
1062
oxygen transport during, 507–508, 509
oxygen-hemoglobin dissociation curve
during, 508, 508f
potassium replacement in, 1065
pulmonary bloo

508f
pulmonary blood ﬂow during, 485f,
486–487, 487f
pulmonary oxygen uptake during, 503,
503f
respiration during, 516, 520–521, 520f,
sodium chloride replacement in, 1065
sympathetic nervous system in, 247,
total body circulation during, 247–249,
248f
venous return curve in, 248f, 249
ventilation-perfusion ratio during, 501
ventilatory rate during, 1061–1062, 1062f,
1064
Exercise testing, in heart disease, 264–265
Exhaustion a

se testing, in heart disease, 264–265
Exhaustion atrophy, 1050
Exocytosis, 20, 21, 21f, 906
Exophthalmos, 940, 940f
Expiration, 471–472, 472f, 476f. See also
Breathing; Inspiration; Respiration.
maximum ﬂow of, 525–526, 525f, 526f
regulation of, 515
Expiratory compliance curve, 473, 473f
Expiratory ﬂow, maximum, in respiratory
insufﬁciency, 525–526, 525f, 526f
Expiratory reserve volume, 475, 476f
Extracellular ﬂuid, 3–4, 292f, 293
calcium in, 7t, 46f, 294f, 294t, 371–373,
372f, 373t, 978
carbon 

 46f, 294f, 294t, 371–373,
carbon dioxide in, 6, 7t
composition of, 293–295, 294f, 294t
corrected osmolar activity of, 294t, 298
hypertonic, 298f, 299–301, 300f
hypertonic saline effect on, 298f, 299–301
1084
Index
Extracellular ﬂuid (Continued)
hypotonic, 298, 298f, 299, 300f
in cardiac failure, 380
in pregnancy, 381
increase in, 380–381
ions of, 7, 7t, 46f
magnesium in, 373
measurement of, 296, 296t
osmolarity of
potassium distribution and, 366–367
re

molarity of
regulation of, 358–363
aldosterone in, 362–363, 363f
angiotensin II in, 362–363
antidiuretic hormone in, 358–361,
359f, 360f, 360t, 362, 362f
thirst in, 361–363, 361t, 362f, 363f
osmotic pressure of, 294t, 298
oxygen in, 6, 7t
pH of, 46f
abnormalities of, 396–397, 396t. See also
Acid-base balance; Acidosis;
Alkalosis.
potassium in, 365–371, 366f, 366t, 369f,
370f
aldosterone blockade and, 370, 370f
aldosterone secretion and, 369–370,
369f, 370f
nor

369f, 370f
normal, 370, 370f
renal potassium secretion and, 368, 369f
sodium in
aldosterone effect on, 362–363, 363f
angiotensin II effect on, 362–363
antidiuretic hormone effect on, 379
deﬁciency of, 363
osmolarity from, 358
regulation of, 358–363, 358f, 373–374
salt-appetite mechanism and, 363
transport of, 4–5, 5f
volume of
abnormalities of, 301–302, 301t
angiotensin II effects on, 377–378
antidiuretic hormones and, 379
arterial pressure and, 219–220, 220f

nes and, 379
regulation of, 373–381, 375f–378f
sodium excretion and, 373–374
377
Extracorporeal circulation, 276
Extrapyramidal motor system, 689
Eye(s). See also Lens; Retina; Vision.
accommodation of, 617–618, 617f, 649
aging of, 618
astigmatic, 620, 620f
autonomic innervation of, 648–649, 648f
cataracts of, 621
cerebellar nystagmus of, 707
dar

dark adaptation of, 631–632, 632f
depth perception of, 647–648
emmetropic, 619, 619f
examination of
for color vision, 633, 634f
for depth perception, 621–622, 622f
for ﬁeld of vision, 644–645, 644f
for visual acuity, 621, 621f
ophthalmoscopic, 622–623, 622f
extraocular muscles of, 645–648, 645f–647f
ﬁxation movements of, 627f, 645–648, 646f
ﬂicking movements of, 646, 647f
ﬂuid system of, 623–625, 623f, 624f
glaucoma of, 624–625
Horner’s syndrome of,

f, 624f
Horner’s syndrome of, 650
hyperopic, 619, 619f
image formation in, 617
Eye(s) (Continued)
innervation of, 648–649, 648f
intraocular ﬂuid of, 623–625, 623f, 624f
lens of, 617–618, 617f. See also Lens.
light adaptation of, 631–632, 632f
ﬁxation, 627f, 645–648, 646f
ﬂicking, 646, 647f
occipital cortex in, 645–646, 646f
premotor area in, 687, 687f, 696
pursuit, 647
saccadic, 646–647
semicircular duct effect on, 696, 697
turning, 647
myopic, 619–620, 619f
negative centrif

urning, 647
negative centrifugal acceleratory force
effects on, 542
nystagmus of, 707
oblique muscles of, 645, 645f
optics of, 617–622, 617f, 618f
parasympathetic innervation of, 648, 648f
pressure of, 624–625, 624f
pupil of, 618–619, 618f, 649–650
pursuit movements of, 647
reading-related saccadic movements of,
646–647
rectus muscles of, 645, 645f
reduced, 617
refractive errors of, 619–621, 619f, 620f
refractive power of, 617, 617f
repressed, 648
saccadic movements of, 646

17, 617f
saccadic movements of, 646–647
slow drift of, 646
stereopsis of, 622, 622f, 643, 647–648
sympathetic innervation of, 648–649, 648f
tremor of, 646
turning movements of, 647
ultraviolet light exposure of, 607
visual acuity of, 621, 621f
F
Fabricius, bursa of, 440. See also
Face, trigeminal neuralgia of, 605–606
Face recognition, 717–718, 717f, 718f
neuronal, 568, 579, 579f
synaptic, 557
memory and, 724–725, 724f
Factor I (ﬁbrinogen), 188, 459f

and, 724–725, 724f
Factor I (ﬁbrinogen), 188, 459f, 459t, 460
Factor II (prothrombin), 459–460, 459f, 459t
Factor III (tissue factor), 459t, 461, 461f
Factor IV. See Calcium.
Factor V (proaccelerin), 459t
Factor VII (serum prothrombin conversion
accelerator), 459t
Factor VIII (antihemophilic factor), 459t,
465
Factor IX (plasma thromboplastin
component), 459t, 462f, 463
Factor X (Stuart factor), 459t, 461f, 462,
462f, 463
Factor XI (plasma thromboplastin
antecedent), 459t, 462f, 463
Factor XII (

oplastin
Factor XII (Hageman factor), 459t
Factor XIII (ﬁbrin-stabilizing factor), 459t,
460
Fainting, 208
Fallopian tubes, 1011, 1012f
inﬂammation of, 1025
ovum in, 1027, 1028–1029, 1028f
Fallot, tetralogy of, 139, 275–276, 275f
Familial hypercholesterolemia, 850
Fanconi’s syndrome, 414
Farsightedness, 619, 619f
Fastigial nucleus, 701, 701f
Fat(s). See also Lipid(s); Triglycerides.
carbohydr

d(s); Triglycerides.
carbohydrate formation from, 838
deposition of, estrogen and, 1018
digestion of, 811–812, 811f, 812f
emulsiﬁcation of, 811
energy content of, 865–866, 866t, 966, 966f
formation of, 837–838, 842, 844–845, 845f
in gallbladder emptying, 803–804
intestinal absorption of, 816
intracellular, 46f
metabolism of. See also at Triglycerides.
bile salts and, 804–805
carbohydrate lack and, 846
cor

 salts and, 804–805
cortisol and, 846, 952
growth hormone and, 846, 922–923
in diabetes mellitus type I, 973–974
insulin and, 965–966, 969–970
liver in, 842, 844, 861–862
regulation of, 846
thyroid hormone and, 846, 936
neutral, 12f, 46f, 294f, 850. See also
Lipid(s).
storage of, 842, 845. See also Adipose
tissue.
insulin and, 965
tissue. See Adipose tissue.
Fat cells (adipocytes), 12, 842. See also
Adipose tissue.
Fatigue
neuromuscular

42. See also
Fatigue
neuromuscular junction, 88
skeletal muscle, 82
synaptic, 570
Fatty acids, 840. See also Triglycerides.
acetyl-CoA conversion to, 844–845, 845f
beta-oxidation of, 842–843, 843f
blood transport of, 841
direct absorption of, 816
epinephrine effect on, 970
extracellular, 4
free, 841
glucose conversion to, 964–965
a-glycerophosphate combination with,
845, 845f
intracellular, 22, 22f. See also Adenosine
plasma, 841
transport of,

sine
transport of, 841
Fatty streak, 849
Feces
colonic movements of, 788–789, 788f
composition of, 817
expulsion of, 757, 789–790, 790f, 822, 823
formation of, 817
water in, 292, 292t
Feedback
negative, 7, 8f
positive, 8–9, 8f
Feedback gain, in arterial pressure
regulation, 217–218
Feed-forward control, 9
Feeding. See also Food.
abnormal, 872–873. See also Obesity.
adipose tissue feedback in, 871
aminostatic theory of, 871
brain stem function in, 870

 of, 871
cholecystokinin and, 870
environmental temperature and, 871
Index
1085
Feeding (Continued)
excess, 872–874
gastrointestinal factors in, 870
ghrelin and, 870
glucostatic theory of, 871
hypothalamic regulation of, 733f, 734,
867–870, 868f, 869f, 869t, 871–872
inhibition of, 868–870, 869f, 870
insulin and, 870
lipostatic theory of, 871
long-term regulation of, 871–872
mechanics of, 781–784, 782f, 870
neural regulation of, 733f, 734, 867–870,
868f, 869f, 869t
neu

ation of, 733f, 734, 867–870,
neurotransmitters in, 868–870, 869f, 869t
oral receptors in, 871
peptide YY and, 870
quantity of, 870–872
short-term regulation of, 870–871
stimulation of, 868–870, 869f
thermogenic effect of, 887
Femoral vein, thrombosis of, 466
Fenestrae, of glomerular capillary
membrane, 316, 316f
Ferritin, 425, 425f, 862
Fertility, 1024–1025
Fertilization, 1000, 1027–1029, 1028f
Fetal cells, in Parkinson’s disease, 711
Fetus, 1042–1044. See also Neonate;
Newborn

e, 711
Newborn.
calcium metabolism in, 1043, 1043f
circulation of, 1042, 1045, 1046f
erythroblastosis of, 421f, 427, 454, 1048
gastrointestinal tract of, 1043
growth of, 1042, 1043f
iron metabolism in, 1043–1044, 1043f
kidneys of, 1043
metabolism in, 1043
nervous system of, 1043
phosphate metabolism in, 1043, 1043f
red blood cells of, 1042
respiratory system of, 1042–1043
sex of, 1028
vitamins in, 1044
Fever, 898–900, 898f
brain lesions and, 899
interleukin-1 

98–900, 898f
interleukin-1 in, 898–899
metabolic rate and, 887
rheumatic, 271
Fibrin, 459f, 460
antithrombin action of, 463–464
formation of, 460
Fibrinogen (factor I), 459f, 459t, 460,
855–857, 855f. See also Plasma proteins.
osmotic pressure and, 188
Fibrinolysin, 1019
Fibrin-stabilizing factor (factor XIII), 458,
459t, 460
Fibroblast growth factor, 200
Fight or ﬂight reaction, 758
Filaments
actin
of skeletal muscle,

s
actin
cellular, 17, 17f
myosin
of skeletal muscle, 72–73, 73f, 75–78, 75f
proteoglycan, 184–185, 184f, 305
Filtration coefﬁcient, 185–186, 190, 340, 341t
Filtration fraction, 316, 319, 319f, 340, 346
Fitness. See Exercise.
Flagellum, of sperm, 17, 17f, 25
Flatus, 817, 825
Flexor reﬂex, 680–681, 680f, 681f
Flowmeter
Doppler, 164–165, 165f
electromagnetic,

Flowmeter
electromagnetic, 164, 165f, 243, 243f
Fluid(s)
body. See Blood; Body ﬂuids;
Cerebrospinal ﬂuid (CSF);
Extracellular ﬂuid; Interstitial ﬂuid;
Intracellular ﬂuid; Pleural ﬂuid;
dialyzing, 415, 415t
transcellular, 293. See also Cerebrospinal
ﬂuid(CSF).
Fluorine, 879, 994
9a-Fluorocortisol, 945, 947t
Fluorosis, 879
Flush, 899, 899f
Foam cells, 849
Focal epilepsy, 744–745
Focal length, 614f, 615
Focal line, 614, 615
Focal point, 614–615, 614f
Focus, dep

ne, 614, 615
Focus, depth of, 618–619, 618f
Folic acid, 875t, 877
deﬁciency of, 423
in red blood cell maturation, 423
Follicle-stimulating hormone, 907t, 918, 919f,
920t
in female reproduction, 1011, 1012–1013,
1013f
in male reproduction, 999, 1007
testosterone inhibition of, 1007
Food. See also Carbohydrates; Fat(s);
Protein(s), dietary.
appetite for, 781
chewing of, 781–782
colonic mixing of, 788–789
digestion of, 808–817. See also Digestion;
Gastrointestinal tract.

–817. See also Digestion;
free energy of oxidation of, 829
gas-forming, 825
gastric mixing of, 784
gastric peristalsis of, 785–786
hunger for, 733f, 734, 781, 785, 867
intake of, 781–784, 782f, 867–872. See also
Feeding.
intestinal mixing of, 786–787, 787f
peristalsis of, 787–788
storage of
in colon, 788–789, 788f
in stomach, 784, 784f
swallowing of, 782–784, 782f
taste of. See Taste sensation.
volume of, stomach emptyin

rmogenic effect of, 887
volume of, stomach emptying and, 785
Foramen ovale, 1045, 1046f
closure of, 1046
Forced expiratory vital capacity, 526, 526f
Forced expiratory volume during ﬁrst
second (FEV1), 526, 526f
Fourth power law, 168
Fourth ventricle, 763, 764, 764f
Fovea, 626, 627f, 635, 635f, 636
Fracture, 683, 983
Frank-Starling mechanism, 111–113, 112f,
113f, 233
Free energy, 829
Free-water clearance, 357
Freon gas, in submarine, 550
Fröhlich’s syndrome, 1009, 1009f
Frostbite, 900
Fructose, i

s syndrome, 1009, 1009f
Fructose, intestinal absorption of, 816
Functional residual capacity, 475, 476–477,
476f, 494, 529
Functional residual capacity (Continued)
Furosemide, 403, 403t
G
G cells, 797–798. See also Gastrin.
G (GTP-binding) proteins, in second
messenger system, 561–562, 561f, 911,
911f, 913–914, 913f
GABA (gamma-aminobutyric acid), 563
GABA (gamma-aminobutyric acid)
pathway, 710–711, 710f
Gain, of control system, 7–8
Galactose, intestinal absorption 

trol system, 7–8
Galactose, intestinal absorption of, 816
bile storage in, 802–803, 803f
emptying of, 776, 803–804
Galloping reﬂex, 683
Gallstones, 804–805, 805f
of papilla of Vater, 821–822
Gamma-aminobutyric acid (GABA), 563
Ganglia, basal. See Basal ganglia.
Ganglion cells, 634, 635, 635f, 636–638
action potential in, 637–638, 637f
color signals from, 638
excitation of, 637–638, 637f, 638f
lateral inhibition and, 638, 638f
on-o

 637f, 638f
on-off response of, 637–638, 637f
types of, 637
W, 637
X, 637
Y, 637
Ganglionic blocking drugs, 760
Gangrene, hyperbaric oxygen therapy in, 550
Gap junctions, 92–93, 559, 771
Gas(es). See also Carbon dioxide; Nitrogen;
Oxygen.
Boyle’s law of, 545, 546f
compression of, under water, 545, 546f
diffusion coefﬁcient of, 493, 498
diffusion of, 491–493, 492f. See also Gas
exchange.
concentration gradient in, 491, 492f
ne

exchange.
net, 491, 492, 493
through respiratory membrane,
496–499, 496f, 497f, 499f
exchange of. See Gas exchange.
gastrointestinal, 817, 825
in ﬂuids, 492, 493
in tissues, 493
partial pressures of, 492. See also Carbon
dioxide, partial pressure of; Oxygen,
partial pressure of.
pressures of, 491–492
solubility of, 493, 498
Gas bubbles, in decompression sickness,
548–549
Gas exchange, 491–501
alveolar vs. atmospheric air composi

ange, 491–501
alveolar vs. atmospheric air composition
in, 493–496, 493t, 494f–496f
carbon dioxide transport in, 5, 510–512,
510f–512f. See also Carbon dioxide,
transport of.
diffusion in, 491–492, 496–499, 496f, 497f,
499f
in pneumonia, 528, 528f
net diffusion in, 491, 492, 493
oxygen transport in, 502–510, 503f–506f,
508f, 509f. See also Oxygen, transport
of.
1086
Index
Gas exchange (Continued)
physics of, 491–493, 492f
physiologic shunt in, 500
respiratory m

0
respiratory membrane in, 496–499, 496f,
497f. See also Respiratory membrane.
respiratory unit in, 496, 496f
500f
Gas gangrene, hyperbaric oxygen therapy
in, 550
Gastric banding surgery, 874
Gastric bypass surgery, 874
Gastric glands, 784
secretions of, 795–799, 795f, 796f, 798f
inhibition of, 798–799
interdigestive, 799
phases of, 798, 798f
Gastric inhibitory peptide, 776, 786, 969
Gastric juice. See also Hydrochloric

776, 786, 969
Gastric juice. See also Hydrochloric acid.
neutralization of, 820–821
visceral pain and, 604
Gastrin, 907t
secretion of, 776, 797–798
stomach emptying and, 785
Gastritis, 819–820
Gastrocolic reﬂex, 776, 789, 822
Gastroenteric reﬂex, 787
Gastroesophageal sphincter, 783
Gastroileal reﬂex, 787
Gastrointestinal tract, 771, 772f. See also
Colon; Small intestine; Stomach.
absorption in
anatomical basis of, 812–813, 812f, 813f
calcium, 980, 9

ical basis of, 812–813, 812f, 813f
calcium, 980, 980f, 985, 987
carbohydrate, 815–816
electrolyte, 817
fat, 816
ﬂuid, 812
ion, 814–815, 814f
iron, 426
large intestine in, 817
lymphatic system in, 192
nutrient, 815–816
phosphate, 980, 985, 987
protein, 816
small intestine in, 813–816, 814f
sodium absorption in, 949
vitamin B12, 423
water, 814, 817
afferent sensory ﬁbers from, 775
amino acid absorption from, 854
antiperistalsis of, 824
atony of, 756
autonomic reﬂexes of, 757
blood ﬂow of, 777–780,

6
blood ﬂow of, 777–780, 777f–780f
blood supply to, 778, 778f
calcium absorption from, 878
capillaries of, 182
complex glands of, 791, 792f
crypts of Lieberkühn of, 791, 792f
digestion in, 808–817. See also Digestion.
disorders of, 819–825
esophageal, 819
gastric, 819–821, 820f
large intestine, 822–823
obstructive, 824–825, 824f
small intestine, 821–822
vomiting in, 823–824, 823f
distention of
peristalsis and, 776
electro

electrolytes in, 793
Gastrointestinal tract (Continued)
enteric nervous system of, 773–776, 774f,
792
hormones in, 776
neurotransmitters of, 775–776
fetal, 1043
ﬁlling of, 870
food intake regulation by, 870
glands of, 791–793, 792f
gastric, 795–799, 796f, 798f
secretions of, 792–793, 792f. See also
Gastrointestinal tract, secretions
of.
stimulation of, 791–792
hormones of, 776, 779, 792. See also
Cholecystokinin; Gastrin; Secretin.
hypothalamic input fr

stokinin; Gastrin; Secretin.
hypothalamic input from, 868, 868f
in premature infant, 1050
iron absorption in, 426
layers of, 771–772, 772f
lipid absorption in, 840
local intermittent constrictive
contractions of, 777
mixing movements of, 777
motility of, 771–773, 772f
hormone effects on, 776, 937
mucous glands of, 791, 795
mucus of, 791, 793, 797
myenteric plexus of, 777
nervous system of, 753–754, 791–792
nutrient absorption in, 5
obstruction of, 285, 824–825, 824f
pain in, 

in, 5
pain in, 603–605, 603f, 605f
on, 775, 779
peristalsis of, 776–777, 777f
pyloric glands of, 797
salivary glands of, 793–795, 794t, 795f
secretions of, 792–793, 792f
cellular mechanism of, 792–793
cephalic phase of, 798, 798f
exocytosis in, 793
gastric, 794t, 795–799, 796f, 798f
gastric phase of, 798, 798f
hepatic, 794t, 802–805, 803f, 803t,
805f
hormonal stimulation of, 792
inh

 803f, 803t,
805f
intestinal phase of, 798, 798f
large intestine, 794t, 806
pancreatic, 794t, 799–802, 800f, 802f
parasympathetic stimulation of, 792
pyloric, 797
saliva, 793–795, 794t
small intestine, 794t, 805–806, 805f
sympathetic stimulation of, 792
vesicles for, 793
water in, 793
sensory nerves of, 775
smooth muscle of, 771–773, 772f
contraction of, 772–773, 772f
intermittent contraction of, 777
resting membrane potent

ittent contraction of, 777
resting membrane potential of, 773
slow waves of, 772–773, 772f
spike potentials of, 772f, 773
tonic contraction of, 773
splanchnic circulation of, 777–780,
778f
775, 779–780
syncytium of, 771–772
villi of, 812–813, 812f, 813f
blood ﬂow of, 779, 779f
vitamin B12 absorption from, 423
Gene(s), 27–30. See also Deoxyribonucleic
acid (DNA).
agglutinogen, 451–452, 452t
in o

cleic
mutation in, 38, 41
regulation of, 35–37, 35f
regulatory, 36
structural, 35, 35f
Gene segments, in lymphocyte processing,
442
Genetic code, 29–30, 29f, 30f
Ghrelin, 870
Gigantism, 926
Gigantocellular neurons, 730f, 731
Glaucoma, 624–625
Global aphasia, 721
Globulins, 855–857. See also at
Immunoglobulin; Plasma proteins.
Globus pallidus, lesions of, 709
Glomerular ﬁltration, 316–320
capillary membrane in, 316–

lar ﬁltration, 316–320
capillary membrane in, 316–317, 316f,
317f, 317t
solute electrical charge in, 317, 317f
solute size in, 317, 317t
Glomerular ﬁltration coefﬁcient, 318, 320t
Glomerular ﬁltration rate (GFR), 315, 316,
317–320, 318f
angiotensin II effect on, 322, 322t, 325
autacoid effect on, 322–323, 322t
autoregulation of, 323–325, 323f, 324f
creatinine clearance for, 344–345, 344t
decrease in, 318–320, 319f, 320t, 321–322
endothelin effect on, 322, 322t
epinephrine effect on, 322, 322t
es

 on, 322, 322t
estimation of, 343–347, 344t, 345f, 346f
ﬁltration coefﬁcient in, 318, 320t
formula for, 317–318
hormonal control of, 322–323, 322t
in acute renal failure, 405
in chronic renal failure, 409–411, 410f
in hyperkalemia, 367
increase in, 318
inulin clearance for, 344, 344t, 345f, 347
macula densa feedback and, 374
measurement of, 343–347, 344t, 345f,
346f
nitric oxide effect on, 322, 322t
norepinephrine effect on, 322, 322t
norm

322, 322t
normal, 323
plasma creatinine for, 344–345, 346f
prostaglandin effect on, 322–323, 322t
quantiﬁcation of, 343–347, 344t, 345f,
346f
321–322
tubular reabsorption and, 327–328, 328t,
339
tubuloglomerular feedback and, 323–325,
324f
Glomerular tufts, 182
Glomerulonephritis, 405–406, 405t, 407,
408–409, 408t
autoimmune, 448
chronic, 409
edema in, 303
Glomerulosclerosis, 408, 408f
Glomerulotu

 in, 303
Glomerulotubular balance, 323, 339, 374
olfactory bulb, 669
Index
1087
Glomerulus (Continued)
renal, 309f, 310
capillaries of, 309, 309f, 316–317, 316f,
317f, 317t
colloid osmotic pressure of, 318–319,
319f
hydrostatic pressure of, 319–320, 319f
Glomus cells, 519
Glossopharyngeal neuralgia, 605–606
Glucagon, 907t, 970–971
regulation of, 971
secretion of, 832, 961, 970–971
somatostatin inhibition o

on of, 832, 961, 970–971
somatostatin inhibition of, 971
Glucagon-like peptide, 870
Glucocorticoids, 944. See also Cortisol.
in fat metabolism, 838, 846
in organ transplantation, 456
in protein metabolism, 857
in shock, 287
Glucokinase, 831, 967, 968f
Gluconeogenesis, 838, 857
cortisol effects on, 838, 951
glucagon in, 970
hepatic, 832, 832f, 861
insulin effects on, 965
renal, 3

c, 832, 832f, 861
renal, 308
Glucose, 830–831, 831f
blood, 839, 975, 975f
969f
renal blood ﬂow and, 325
blood ﬂow and, 198
capillary permeability to, 183–184, 184t
cortisol effects on, 951
co-transport of, 55, 55f, 816, 831
energy release from, 832–838, 833f–835f,
838f. See also Adenosine

, 833f–835f,
triphosphate (ATP), formation of.
extracellular, 4, 7t, 46f, 294f
facilitated diffusion of, 50, 55, 55f, 831
fatty acid conversion of, 964–965
formation of. See Gluconeogenesis.
glucagon effect on, 970
in brain metabolism, 767, 965
insulin effects on, 831
liver uptake of, 964–965
muscle storage of, 963–964
musc

uptake of, 964–965
muscle transport of, 964, 964f
muscle uptake of, 963
phosphorylation of, 831
reabsorption of, 328t, 330, 330f, 331, 331f,
338f
urinary, 413, 975
Glucose phosphatase, 830–831, 831f, 964
Glucose solution, body ﬂuid effects of,
301
Glucose tolerance test, 975, 975f
Glucose transporters, 967, 968f
Glucose-1-phosphate, 832, 832f
Glutamate, 564
Glutamic acid, 853f
Glutamin

n pain sensation, 601
Glutamine, 853f, 856, 856f
renal metabolism of, 393, 394, 394f
Gluten enteropathy, 822
Glycerol-3-phosphate, 842
a-Glycerophosphate, 841, 845, 845f, 846
Glycine, 563, 853f
Glycinuria, 413
Glycocalyx, 463
Glycogen, 12, 13f
breakdown of, 832, 970
formation of, 832, 832f
storage of, 831–832, 836, 838, 861
Glycogen synthase, 964
Glycogenesis, 832, 832f
Glycogenolysis, 832, 970
Glycolipids, 14
Glycolysis, 22–23, 832–837, 833f–835f, 882
acetyl coenzyme A forma

3, 832–837, 833f–835f, 882
acetyl coenzyme A formation in, 833,
833f
in muscle, 79, 883
835f
pyruvic acid formation and, 833, 833f
rate of, 836
Glycoproteins, 13, 14
Glycosuria, 413
Goblet cells, 805, 805f
Goiter, 939, 940, 941
Goldblatt hypertension, 226–227, 226f
Goldman equation (Goldman-Hodgkin-
Katz equation), 58
Golgi apparatus, 13f, 15, 15f
functions of, 21–22, 21f
vesicle formation by, 21, 21f
G

ons of, 21–22, 21f
Golgi tendon organ, 675, 679–680, 679f
Golgi tendon reﬂex, 679–680
Gonadotropes, 919, 920t
Gonadotropin-releasing hormone, 907t, 921,
921t
in female reproduction, 1011, 1020
in male reproduction, 1006
Grand mal epilepsy, 743–744, 743f, 744f
Granule cells
cerebellar, 702, 702f
olfactory, 668, 670
Granules, secretory, 13f, 16, 16f, 21, 21f
Granulocyte(s), 429–430, 430f. See also
Basophil(s); Eosinophil(s);
Neutrophil(s).
in inﬂammation, 435
life spa

il(s);
life span of, 431
Granulocyte colony-stimulating factor, 435,
436f
Granulocyte-monocyte colony-stimulating
factor, 435, 436f
Granulosa cell tumor, 1023
Graves’ disease, 940
Gravitational pressure. See Hydrostatic
pressure.
Ground substance, 21
Growth, 1051–1052, 1051f
fetal, 1042, 1043f
growth hormone effects on, 922, 922f,
923–924
insulin and, 967, 967f
neonatal, 1051f, 1052
thyroid hormone effects on, 936
Growth facto

–1006
Growth factors, 39
Growth hormone, 907t, 918, 919f, 920t,
921–927
abnormalities of, 926–927, 927f
aging-related changes in, 927
amino acid transport and, 922
bone growth and, 923
carbohydrate utilization and, 923
cartilage growth and, 923
daily variation in, 924, 924f
diabetogenic effects of, 923
fat utilization and, 846, 922–923
growth effects of, 922, 922f, 923–924
growth hormone–releasing hormone
effect on, 925–926
in dwarﬁsm, 926

, 925–926
insulin interaction with, 923, 967, 967f,
969
ketogenic effect of, 923
metabolic effects of, 922–923
protein synthesis and, 857, 922
protein-sparing effect of, 922
regulation of, 924–926, 924f, 925f, 925t
RNA formation and, 922
somatostatin effect on, 925–926
tissue protein deposition and, 922
Growth hormone inhibitory hormone, 907t,
921, 921t
glucagon inhibition and, 971
insulin inhibition and, 971
secretion of, 961
Growt

nsulin inhibition and, 971
Growth hormone–releasing hormone, 907t,
921, 921t, 925–926
Growth inducers, in stem cell development,
420
Guanethidine, 759
Guanine, 27, 28f, 29, 29f
Guanosine monophosphate, cyclic (cGMP)
in rod function, 631
second messenger activity of, 912
H
H band, 73f
Habituation, memory and, 723, 724, 736
Hageman factor (factor XII), 459t
Hagﬁsh, kidney of, 216
Hair
estrogen effects on, 1018
testosterone effects on, 1005
Hair cells
of macula, 693–694, 693f
of o

 on, 1005
Hair cells
of organ of Corti, 652, 655–656, 655f
Hair end-organ, 586
Haldane effect, 511–512, 512f
Hand skills, motor cortex and, 687, 687f,
691
Haustrations, 788–789, 788f
Hay fever, 450
Head. See also Brain.
contrecoup injury to, 764
gravity-related orientation of, 693, 693f
rotation of, 687, 687f, 694, 694f, 695–696,
695f
visual orientation of, 647
Headache, 606–607, 606f
Head-down position, in shock, 287
Hearing, 651–661
abnormalities of, 660–661, 661f
audi

ring, 651–661
auditory cortex in, 657–659, 659f. See also
auditory nerve stimulation in, 655–656
centrifugal signals in, 660
decibel threshold of, 657, 657f
1088
Index
Hearing (Continued)
endocochlear potential in, 655
frequency range of, 657, 657f
hair cells in, 655–656, 655f
impedance matching in, 651–652
loss of, 660–661, 661f
loudness determination in, 656–657, 657f
organ of Corti in, 655–656, 655f
place principle in, 656
sound conduction in
f

655f
sound direction in, 660
sound frequency in, 654–655, 654f, 656,
657, 657f
stimulus intensity and, 594
tests of, 660–661, 661f
Heart. See also at Cardiac.
atria of. See Atrium (atria).
atrioventricular bundle of, 104, 118–119,
118f, 119f
atrioventricular valves of, 107f, 108, 109,
109f
205–206, 205f, 754, 754t
blood return to. See Ven

blood return to. See Venous return.
collateral circulation in, 252–253, 253f
conductive system of, 116–120, 117f. See
also Action potential, cardiac 
muscle.
abnormalities of, 147–156. See also
Arrhythmias.
after myocardial infarction, 254
atrioventricular node in, 117f, 118–119,
118f, 120f
control of, 120–122
electrocardiography of, 123–130. See
also Electrocardiogram.
internodal pathway in, 117f, 118, 118f,
120f
norepinephrine effects on,

, 117f, 118, 118f,
120f
on, 121
parasympathetic stimulation of, 121
Purkinje ﬁbers in, 117f, 119, 120f, 121
sinus node of, 116–118, 117f, 120–121,
120f. See also Sinus (sinoatrial)
node.
121–122
sympathetic stimulation of, 121–122
ventricular transmission in, 119–120,
120f
electrocardiography of, 123–130. See also
Electrocardiogram.
failure of. See Cardiac failure.
ﬂaccidity of, 114
hyper

e of. See Cardiac failure.
hypereffective, 234–235, 234f, 238, 238f
hypertrophy of, 235, 272–273, 276
hypoeffective, 234, 234f, 235, 238, 238f
ischemic disease of, 252–255, 253f. See
also Myocardial infarction.
recovery from, 254–255, 255f
T wave in, 145–146, 146f
output of. See Cardiac output.
papillary muscles of, 1

ut of. See Cardiac output.
papillary muscles of, 109
205f
Heart (Continued)
pressures in, 107f, 108. See also at Atrium
(atria); Ventricle(s), cardiac.
reservoir function of, 179
sinus node of, 116–118, 117f, 120–121. See
also Sinus (sinoatrial) node.
sounds of. See Heart sound(s).
systolic stretch of, 253–254, 253f, 254
valves of. See Heart valves.
vasomotor center control of, 206–207

eart valves.
ventricles of. See Ventricle(s), cardiac.
Heart attack. See Cardiac failure;
Heart beat, 106–109, 107f
Heart block, 148–150
ﬁrst degree, 149, 149f
second degree, 149, 149f
third degree, 149, 149f
Heart failure. See Cardiac failure.
Heart murmurs, 271f, 272
Heart rate
abnormal, 147–148, 148f
atrial pressure and, 212
during exercise, 1063–1064, 1064f, 106

re and, 212
during exercise, 1063–1064, 1064f, 1064t
hypothalamic regulation of, 733, 733f
on electrocardiogram, 126
temperature effects on, 114, 147, 148f
Heart sound(s), 109–110, 269–272
auscultation of, 270–271, 270f
ﬁrst, 110, 269–270, 270f, 271f
fourth, 270, 271f
phonocardiogram of, 271, 271f, 272
second, 110, 269–270, 270f, 271f
third, 270, 271f
Heart valves
congenital lesions of, 271–272
circulatory dynamics of, 274–276, 274f,
275f
lesions of, 271–272, 271

s of, 274–276, 274f,
275f
lesions of, 271–272, 271f
cardiac reserve and, 273–274
circulatory dynamics in, 272–274
exercise effects on, 273–274
murmurs of, 271f, 272
rheumatic fever–related lesions of, 271
sounds of, 269–272, 270f, 271f. See also
Heart sound(s).
Heart-lung machines, 276
Heat, 46. See also Temperature;
Thermogenesis.
loss of, 890–894, 890f, 891f
production of, 884–885, 889–890
Heat acclimatization, 893–894, 900, 1065
Heat receptors, 607–609, 608f
Heat sensation, 572, 573t, 607–609

, 607–609, 608f
Heat sensation, 572, 573t, 607–609, 608f
tissue damage and, 599, 599f
transmission of, 609
Heatstroke, 899–900
Heavy metals, renal toxicity of, 406
Helicobacter pylori, peptic ulcer and, 821
Helium
Helium-oxygen mixture, in diving, 549
Hematocrit, 169, 169f, 293
high altitude effects on, 539
in polycythemia vera, 428
Hematopoietic stem cells, 420–422, 421f
committe

f
committed, 420, 421f
pluripotential, 420, 421f
Heme, 424, 424f, 425f
Hemianopsia
bitemporal, 645
homonymous, 645
Hemiballismus, 709
Hemodialysis, 414–415, 414f, 415t
Hemodynamics. See Blood ﬂow; Circulation
(blood).
Hemoglobin, 419–420, 879
buffer action of, 6, 419, 506f, 507
carbon dioxide transport with, 511
carbon monoxide combination with,
509–510, 510f
concentration of, 420
deﬁciency of, 426–427. See also Anemia.
destruction of, 426
excessive deoxyg

 also Anemia.
excessive deoxygenation of, 531
fetal, 1030–1031, 1031f
formation of, 424–425, 424f
high altitude effects on, 538f, 539
iron in, 424–425, 424f. See also Iron.
oxygen combination with, 420, 424–425,
505–510. See also Oxygen-
hemoglobin dissociation curve.
altitude effects on, 538, 538f
during exercise, 507–508
reversibility of, 505–507, 507f
saturation of, 507, 507f
structure of, 424, 424f
subunits of, 424, 424f
tubular, 455
Hemoglobin S, 424, 427
Hemolysins, 453

ubular, 455
Hemolysis
in transfusion reactions, 453, 454, 455
jaundice in, 454, 455, 864
Hemolytic disease of the newborn, 421f,
427, 454
Hemophilia, 465
Hemorrhage. See also Bleeding;
arterial pressure and, 224, 224f
brain, 765
renal response to, 376
renin-angiotensin system response to, 224,
224f
sympathetic nervous system in, 172
Hemorrhagic shock, 279–284, 279f
acidosis in, 283–284
arterial pressure in, 279–280, 279f, 280f
capillary p

erial pressure in, 279–280, 279f, 280f
capillary permeability in, 282
cardiac output in, 279–280, 279f, 281,
283f
compensated, 280–281, 280f
endotoxin in, 282
irreversible, 284, 284f
liver damage in, 283, 283f
negative feedback mechanisms in, 281
nonprogressive, 280–281, 280f
positive feedback mechanisms in,
281–284, 282f
progressive, 280f, 281–284, 282f, 283f
sludged blood in, 281–282
sympathetic nervous system in, 279–280,
279f
tissue necrosis in, 283, 283f
vasomotor fail

in, 283, 283f
vasomotor failure in, 281
Hemosiderin, 425, 425f
Hemostasis, 457–459
clot formation in, 458–459, 458f, 459t. See
platelet plug formation in, 457–458
vascular constriction in, 457
Henderson-Hasselbalch equation, 386
Henle, loop of. See Loop of Henle.
Index
1089
Heparin
anticoagulant action of, 464, 466,
467
mast cell release of, 436, 464
Hepatic artery, 860, 860f
Hepatic coma, 862
Hepatic vein, 859, 860, 860f
Hepatitis, bleeding in, 464
Hepato

 859, 860, 860f
Hepatocyte growth factor, 861
Hereditary factors, in cancer, 41
Hering-Breuer inﬂation reﬂex, 515–516
Hering’s nerve, 209, 209f
Hermaphroditism, 1049
Herpes zoster infection, 605
Hexamethonium, 242, 759, 760
Hexokinase, 831
Hexose monophosphate, 294t
High altitude
acclimatization at, 519
angiogenesis and, 539–540
cellular effects of, 540
cerebral edema and, 541
hematocrit and, 539
hemoglobin and, 539
hemoglobin-oxygen combi

d, 539
hemoglobin-oxygen combination and,
538, 538f
hypoxia and, 537–539, 538f, 538t
mental proﬁciency at, 539
mountain sickness and, 540–541
native populations at, 540, 540f
oxygen diffusing capacity and, 539
oxygen-hemoglobin dissociation curve at,
507, 540, 540f
Po2 and, 537–538, 538f, 538t
polycythemia and, 427–428
pulmonary circulation and, 539–540
pulmonary edema and, 540–541
red blood cell effects of, 422, 539
respiratory alkalosis and, 397, 539
High-molecular-weight k

ry alkalosis and, 397, 539
High-molecular-weight kininogen
(Fitzgerald factor), 459t
Hippocampal formation, 736
functions of, 736–737
hyperexcitable, 736–737
in learning, 737
in memory, 725–726
in schizophrenia, 746
lesions of, 725
removal of, 737
Hirschsprung’s disease, 822
Histamine
bronchial effects of, 480
enterochromafﬁn-like cell secretion of,
797, 798
in anaphylaxis, 450
in hay fever, 450
in urticaria, 450
Histamine shock, 285–286
Histidine, 8

ticaria, 450
Histidine, 853f
Histiocytes, 433
Histones, 36, 38
Hives, 450
HLA antigens, 455–456
Hollow capsule method, for interstitial ﬂuid
pressure, 187, 188
Homatropine, 759
Homeostasis, 4–6
circulatory system in, 4–5, 4f, 5f
control systems in, 6–9, 8f
deﬁnition of, 4
endocrine system in, 6
gastrointestinal tract in, 5
Homeostasis (Continued)
kidneys in, 5
liver in, 5
musculoskeletal system in, 5
nervous system in, 5–6
reproductive system in, 6
respiratory system in,

6
respiratory system in, 4f, 5
Homonymous hemianopsia, 645
Horizontal cells, 633, 635f, 638f
Hormone(s), 6, 905–916, 906f, 907t. See also
speciﬁc glands and hormones.
amine, 906, 907t, 908
blood concentration of, 908–909
measurement of, 915–916, 916f
blood transport of, 909
classes of, 906, 907t
cyclical variations in, 909
enzyme-linked immunosorbent assay for,
916, 916f
feedback regulation of, 909
gastric inhibition by, 786
gastrointestinal, 776, 779
in renal blood ﬂow

 786
in renal blood ﬂow, 322–323, 322t
in smooth muscle contraction, 98
mechanism of actions of, 910–914
receptors in, 910–912, 911f–913f
second messengers in, 912–915, 913f,
913t, 914f, 914t
metabolic clearance rate of, 909–910
negative feedback regulation of, 909
peptide, 906, 907t
positive feedback regulation of, 909
radioimmunoassay for, 915–916, 916f
receptors for, 14, 910–912
down-regulation of, 910
enzyme-linked, 911–912, 912f
G protein–linked, 911, 911f
gene ac

 911–912, 912f
gene activation and, 912, 913f, 915
ion channel–linked, 910–911
sensitivity of, 910
up-regulation of, 910
secretion of, 906, 908–909, 908f
steroid, 906, 907t, 908
protein synthesis and, 915
storage of, 906, 908f
synthesis of, 906, 908
Hormone response element, 912, 913f
Hormone-sensitive lipase, 841, 846, 966
Horner’s syndrome, 650
Human chorionic gonadotropin, 907t, 1007,
1032, 1032f
Human chorionic somatomammotropin,
1033
Human chorionic thyrotropin, 

atomammotropin,
1033
Human chorionic thyrotropin, 1034
Human physiology, 3
Human somatomammotropin, 907t
Hunger, 733f, 734, 781, 785, 867, 871. See
also Feeding; Food.
Hunger center, 867, 868f, 871
Hunger pangs, 785
Huntington’s disease, 711–712
Hyaline membrane disease, 474, 529, 1045
Hyaluronic acid, 21, 184–185
Hyaluronidase, of sperm, 1000
Hydrocephalus, 765, 766
Hydrochloric acid, 810
absence of, 820
formation of, 796–797, 796f
secretion of, 54, 7

n of, 820–821
secretion of, 54, 796–797, 796f
Hydrocortisone. See Cortisol.
Hydrogen atoms. See also Hydrogen ions.
oxidation of, 835–836, 835f, 843
in citric acid cycle, 23, 834, 834f
Hydrogen bonds, 29, 29f, 854
Hydrogen ions
active transport of, 54
alveolar ventilation and, 389, 389f,
519–520, 519f
arteriolar effects of, 202–203
blood ﬂow and, 202–203
chemosensitive area e

emoreceptor response to, 518
chemosensitive area excitation by, 516
counter-transport of, 55
deﬁcit of. See Alkalosis.
excess of. See Acidosis.
formation of, 511, 835, 835f
in hydrochloric acid formation, 796–797,
796f
in pancreatic juice secretion, 800, 800f
517f
intercalated cell secretion of, 337, 392,
392f
mitochondrial, 835–836, 835f
net renal excretion of, 394–395
normal level of, 383, 384, 384t
regulation of, 383–400. See also Acid-base
balance.
s

ulation of, 383–400. See also Acid-base
balance.
secretion of, 390–392, 390f–392f
disorders of, 413
intercalated cells in, 337, 392, 392f
medullary collecting duct in, 338, 338f
regulation of, 395, 395t
sodium counter-transport and, 330,
330f, 333
Hydrogen peroxide, 432
Hydrogen-potassium ATPase transporter,
368
Hydrolases, 16, 20
fat, 808, 811–812, 811f
Hydrostatic press

Hydrostatic pressure
in interstitial ﬂuid, 185, 185f, 304–305, 304f
in pulmonary circulation, 485–487, 485f,
486f, 488, 488f
of capillaries, 185–187, 185f, 186
340, 340f
of renal interstitium, 340–341, 341f
venous pressure and, 177–178, 177f
Hydroxyapatite, 981
ß-Hydroxybutyric acid, 844
25-Hydroxycholecalciferol, 983–984, 984f
Hydroxyl ions, 432
Hydroxymethylglutaryl-coenzyme A
reductase inhibitors (statins), 851
Hype

oenzyme A
Hyperadrenalism, 958–959, 959f
Hyperalgesia, 599, 603f, 605
Hyperbaric oxygen therapy, 550–551
Hyperbarism, 545
Hyperbilirubinemia, 1048, 1048f
Hypercalcemia, 371, 978, 980, 990
Hypercapnia, 531–532
Hypercholesterolemia, 850
Hyperemia, 198
Hyperestrinism, 1017
Hyperglycemia, in gigantism, 926
Hyperinsulinism, 976
Hyperkalemia, 365
aldosterone deﬁciency and, 947–948, 949
cell lysis and, 366
exercise-induced, 366
in acute renal 

sis and, 366
in acute renal failure, 406
1090
Index
Hyperkalemia (Continued)
in Addison’s disease, 366, 370
in cell lysis, 366
in diabetes mellitus, 366, 366t
in hypertension, 366
Hypernatremia, 301t, 302
Hyperopia, 619, 619f
Hyperparathyroidism, 990–991
Hyperphagia, 867
Hyperpyrexia, 899–900
Hypersensitivity, 449–450. See also Allergy.
Hypertension, 220–223
aldosteronism and, 223
angiotensin II in, 378, 378f
cardiac ou

 in, 378, 378f
cardiac output in, 228, 236f
circulatory function in, 221–223, 222f
diuretics in, 229–230, 402–404, 403f, 403t
drug treatment of, 229–230
angiotensin II blockade in, 378, 378f
Goldblatt, 226–227, 226f
in baroreceptor nerve sectioning, 228
in lower animals, 228
in pregnancy, 227–228, 1034
in renal ischemia, 227
left ventricular hypertrophy and, 1

 ischemia, 227
left ventricular hypertrophy and, 138,
138f
life expectancy and, 220–221
neurogenic, 228
nonsalt-sensitive, 229, 229f
obesity in, 228–229
one-kidney, 226–227, 226f
portal, 860
pressure natriuresis in, 228–229
primary (essential), 228–230, 229f
pulmonary, 527
renal failure and, 407, 408, 408t, 412–413
renin-angiotensin system in, 223–227,
223f–227f
salt-sensitive, 229, 229f
sodium intake and, 378, 378f
sympathetic nervous system in, 228
vasodilator drugs in, 229–230
volume-loading,

 228
volume-loading, 221–223, 221f, 222f
Hyperthyroidism, 940–941, 940f
cardiac output in, 236, 236f
muscle tremor in, 937
Hypoadrenalism, 957–958
Hypocalcemia, 371, 978, 979–980, 979f
Hypochlorhydria, 820
Hypochlorite, 432
Hypogastric nerves, 311f, 312
female, 1023
male, 1008–1009
Hypokalemia, 365
exercise-related, 1065
in aldosteronism, 370
in Conn’s syndrome, 366
Hyponatremia, 301–302, 301t
Hypoparathyroidism, 990
Hypophos

ia, 301–302, 301t
Hypophosphatemia, 413, 991
Hypophysis. See Pituitary gland.
Hypothalamic-hypophysial portal vessels,
920–921, 920f
Hypothalamus, 558, 731, 732–736, 732f, 906f,
907t. See also Limbic system.
anterior, 733–734, 733f
antidiuretic hormone synthesis in,
359–360, 359f
Hypothalamus (Continued)
control centers of, 733, 733f, 758–759,
758f
cortisol effects on, 955
hunger center of, 867, 868f
in anorexia, 874
in behavior, 733, 733f, 734–736, 758f, 759
in cardiovas

havior, 733, 733f, 734–736, 758f, 759
in cardiovascular function, 733. See also
in endocrine function, 734. See also
in fever, 899
in gastrointestinal function, 734. See also
Feeding.
in suckling, 734
in temperature regulation, 733–734,
894–898, 894f, 895f, 897f
in uterine contractility, 734
in vasodilation, 206f, 207–208
in water regulation, 734
lateral areas of, 733, 733f, 734
lesions of, 735
magnocellular neu

, 733, 733f, 734
magnocellular neurons of, 919
paraventricular nucleus of, 919
pituitary gland regulation by, 919–921,
920f, 921t
posterior, 733–734, 733f
preoptic area of, 894
satiety center of, 867–868, 868f. See also
Feeding.
supraoptic nucleus of, 919
thyrotropin-releasing hormone of,
938–939
tumor of, 873
Hypothermia, 114, 895, 900
Hypothyroidism, 938, 941–942, 942f,
1023
Hypotonia, 691, 707
Hypovolemia, shock and, 279–28

Hypovolemia, shock and, 279–285. See also
Hypoxemia, in pneumonia, 528, 528f
Hypoxia
anaerobic energy utilization during,
882–883
at birth, 1044
at high altitude, 427–428, 537–539, 538f,
538t
atmospheric, 530
erythropoietin secretion and, 308
hypoventilation, 530, 531
in anemia, 531
in pulmonary edema, 530–531, 531f
oxygen therapy in, 530–531, 531f
red blood cell production and, 422–423,
422f, 427–428
t

23,
tissue oxygen deﬁciency and, 530–531,
531f
I
I bands, 72, 73f, 90f
I cells, 801
Idiopathic nontoxic colloid goiter, 941
Idiopathic thrombocytopenia, 465
Iggo dome receptors, 586, 586f
Ileocecal sphincter, 774, 788, 788f
Ileocecal valve, 788, 788f
Ileum, bicarbonate secretion in, 815
Imipramine, 745
Immune complexes, in glomerulonephritis,
405–406
Immune hypersensitivity, 449–450. See also
Allergy.
Immune tolerance, 448
Immun

450. See also
Allergy.
Immunity
acquired (adaptive), 439–449
antigens in, 440, 441f
cell-mediated (T-cell), 440, 441f,
446–448, 446f–448f
humoral (B-cell), 440, 441f, 443–446,
443f–445f. See also Antibody
primary, 443, 443f
secondary, 443, 443f
lymphocytes in, 440–442, 441f. See also
Lymphocyte(s), B; Lymphocyte(s),
T.
regulation of, 446–448, 447f
tolerance and, 448
types of, 440
cortisol effects in, 954
disorders of, 448, 449–450. See also
Allergy.
innate, 43

ers of, 448, 449–450. See also
Allergy.
innate, 439. See also Phagocytosis.
passive, 449
Immunization, 439–440, 448–449
Immunoglobulin(s). See also Antibody
mechanism of action of, 444–446, 444f,
445f
speciﬁcity of, 444
Immunoglobulin E, 449–450
Immunoglobulin G, 444
Immunoglobulin M, 444
Immunosuppression, in transplantation,
456
Impedance matching, 651
Implanted electronic cardiac pacemaker,
150, 156
Inanition, 867, 874
I

 cardiac pacemaker,
150, 156
Incontinence, 313–314
Incus, 651, 652f
Indicator-dilution method, 244–245, 244f,
Infection
brain, 765
diarrhea and, 806, 815, 817
inﬂammation in, 434–436, 436f. See also
Inﬂammation.
resistance to. See Immunity; Phagocytosis;
White blood cell(s).
streptococcal, 434, 448
Inﬂammation, 434–436, 436f. See also
Macrophage(s); Neutrophil(s).
appendiceal, 605, 605f
complement in, 445–446
cortisol effects in, 952–954
edema and, 302
gastric, 

tisol effects in, 952–954
gastric, 819–820
in niacin deﬁciency, 876
intestinal, 822–823
pancreatic, 821–822
stages of, 953
walling-off effect of, 434
Inhibin, 1007, 1015, 1020
Inositol triphosphate, 914, 914f
Insecticides, renal toxicity of, 406
Inspiration, 471–472, 472f, 476f. See also
Breathing; Expiration; Respiration.
regulation of, 514–516, 515f
Index
1091
Inspiratory capacity, 475, 476f
Inspiratory compliance curve, 473, 473f
Inspiratory reserve volume, 475, 47

rve, 473, 473f
Inspiratory reserve volume, 475, 476f
Instantaneous mean vector, 131, 132f
Insulin, 907t, 961–970
amino acid effects on, 969
appetite and, 870
blood glucose effect on, 968–969, 968f,
969f
brain glucose and, 965
carbohydrate metabolism and, 963–965,
964f, 969–970
cholecystokinin effects on, 969
cortisol effects on, 969
deﬁciency of, 845, 966, 966f, 967. See also
Diabetes mellitus.
energy storage and, 961–962
estrogen effects on, 969
excess of, 976
fat metabolism and, 965–966, 969–9

fat metabolism and, 965–966, 969–970
gastric inhibitory peptide effects on,
969
gastrin effects on, 969
glucagon effects on, 969
gluconeogenesis inhibition by, 965
glucose transport and, 831
growth hormone interaction with, 923,
967, 967f, 969
plasma, 975
progesterone effects on, 969
protein metabolism and, 857, 966–967
receptors for, 962–963, 963f
resistance to, 972, 974–975, 974t
secretin effects on, 969
secretion of, 961, 967–969, 968t
re

ffects on, 969
regulation of, 968–969, 968f, 969f
structure of, 962, 962f
synthesis of, 962
Insulinase, 962
Insulin-like growth factors, 923–924
Insulinoma, 976
Insulin-receptor substrates, 963
Intention tremor, 705, 707
Intercalated cells, 336–337, 336f
hydrogen ion secretion by, 54, 336, 392,
392f
potassium reabsorption by, 337, 368
Intercalated discs, 103–104, 104f
Intercellular cleft, 182, 182f
I

cs, 103–104, 104f
Intercostal muscles, 471, 472f
Interdigestive myoelectric complexes, 776
in fever, 898–899
in inﬂammation, 435, 436f
in lymphocyte activation, 442
Interleukin-2, 447
Interleukin-3, 420
Interleukin-4, 447
Interleukin-5, 447
Interleukin-6, 447
Interleukin(s), 905
T-cell production of, 447, 447f
Internal capsule, 708
Internal environment, 4
Interneurons, 674–675, 674f
Internodal pathways, action potential
transmission in, 117f, 118, 119

, action potential
transmission in, 117f, 118, 119f
Interplexiform cells, 634
Interposed nucleus, 701
Interstitial ﬂuid, 4–5, 5f, 184–185, 184f, 292f,
293
accumulation of, 302–305, 376–377, 377f
calculation of, 296, 296t
Interstitial ﬂuid (Continued)
colloid osmotic pressure of, 185–186, 185f,
193–194, 294t, 298
351
free, 185
gel form of, 185, 304–305, 304f
Po2 of, 503–504, 504f
pressure in. Se

f, 294t, 298
pressure in. See Interstitial ﬂuid pressure.
protein in, 188–189, 193–194
pulmonary, 487–489, 488f
renal, 340–341, 341f, 351–352, 352f
Interstitial ﬂuid colloid osmotic pressure,
185–186, 185f, 188–189
Interstitial ﬂuid pressure, 185, 185f, 187–188,
304–305, 340f
cotton wick measurement of, 187
hollow capsule measurement of, 187, 188
in lung, 487–488, 488f
lymphatic system and, 188, 192, 192f,
193–194
micropipette measurement of, 187
negative, 187, 188–189, 194

te measurement of, 187
of brain, 187
of kidney, 187
subatmospheric, 187–188
Interstitium, 184–185, 184f. See also
Interstitial ﬂuid.
Interventricular septal defect, 139
Intestinal lipase, 806
Intestine. See Colon; Gastrointestinal tract;
Small intestine.
Intra-abdominal pressure, venous pressure
and, 177
Intracellular ﬂuid, 4, 46f, 292f, 293
composition of, 294f, 294t, 295
hypertonic saline effect on, 

tivity of, 294t, 298
pH of, 384, 384t, 387–388
phosphate buffer system of, 387
protein buffers of, 387–388
Intrafusal ﬁbers, 674, 674f, 675–676, 676f
Intramyocardial pressure, 250, 250f
Intraocular ﬂuid, 623–625, 623f, 624f
Intraocular pressure, 624–625, 624f
Intraventricular block, 150, 150f
Intrinsic factor, 423, 797, 820, 877
Inulin, in water reabsorption evaluation, 339
Inulin clearance, 344, 344t, 345f, 347
Inulin spa

Inulin space, 296
Iodide
deﬁciency of, 941
oxidation of, 932, 932f
thyroid suppression with, 939–940
Iodide pump (iodide trapping), 932, 932f
Iodine, 878t, 879, 879t, 931–932
radioactive, 941
Ions, 11. See also speciﬁc ions.
extracellular, 4, 7, 7t, 46f, 294f, 294t
interstitial, 293–295, 294t
intracellular, 4, 46f, 294f, 294t
plasma, 293–295, 294t
Iridocorneal angle, 623–624, 624f
Iris, 97–98, 618–619, 618f
Iron, 878t, 879, 879t. See also Hemoglobin.
ather

coordination bonds of, 425
daily loss of, 426
fetal, 1043–1044, 1043f
hepatic storage of, 425, 862
intestinal absorption of, 426, 815
metabolism of, 425–426, 425f
Iron (Continued)
oxygen combination with, 424–425, 424f
regulation of, 426
storage of, 425, 862
transport of, 425–426, 425f
Iron lung, 532, 532f
Ischemia
cerebral, 212–213, 220–221, 763. See also
myoca

al blood ﬂow.
myocardial, 252–255, 253f. See also
pain in, 599, 604
renal, 227
Ishihara charts, 633, 634f
Islets of Langerhans, 961, 962f
Isograft, 455
Isogravimetric method, for capillary
pressure, 186, 186f
Isohydric principle, 388
Isoleucine, 853f
Isomaltase, 806
Isoproterenol, 759
Itching, 587
J
J point, in current of injury, 142–143, 143f
J receptors, 521
Jacksonian epilepsy, 744–745
Jaundice, 863–864
hemolytic vs.

 epilepsy, 744–745
hemolytic vs. obstructive, 864
in transfusion reactions, 455
Jejunum. See also Gastrointestinal tract.
hormone secretion by, 776. See also
Cholecystokinin.
Joints, position receptors of, 594–595
Junctional potential, 98
Juxtaglomerular cells, 223–224, 324, 324f
tumor of, 226
Juxtaglomerular complex, 324, 324f
K
Kallidin, 202, 779
Kallikrein, 202
sali

24, 324f
K
salivary, 795
Keratoconus, 621
Kerckring, folds of (valvulae conniventes),
812–813, 812f
Kernicterus, 454
Ketogenesis, 857
Ketone bodies, 844, 966
Ketosis, 844, 966
Kidneys. See also at Renal.
anatomy of, 308–312, 309f–311f
arterial pressure control by, 216–231, 308,
323, 341–342, 374–376, 375f, 376f
948f
exp

reﬂexes and, 212
217f
peripheral resistance and, 219, 219f
1092
Index
Kidneys (Continued)
arterioles of, reﬂex dilation of, 212
artiﬁcial (dialysis), 223, 414–415, 414f
autonomic nervous system control of, 754t
autoregulati

nomic nervous system control of, 754t
autoregulation of, 321
blood ﬂow of, 5, 199, 309–310, 309f,
320–323, 321f, 321t, 323f, 405
angiotensin II and, 322, 322t
arterial pressure and, 355
autacoid control of, 322–323, 322t
blood glucose and, 325
calculation of, 346
determinants of, 320–321, 321t
endothelin and, 322, 322t
epinephrine and, 322, 322t
in cardiac failure, 259–260
medullary, 354–355, 355f
myogenic autoregulation o

myogenic autoregulation of, 325
nitric oxide and, 322, 322t
norepinephrine and, 322, 322t
oxygen consumption and, 320, 321f
prostaglandins and, 322–323, 322t
protein intake and, 325
vasa recta in, 354–355, 355f
vasodilator effects on, 355
calcitriol production by, 308
calices of, 309, 309f
capillaries of, 184, 309–310, 309f
capsule of, 308, 309f
cortex of, 309, 309f, 310f
1,25-dihydroxycholecalciferol formation
in, 308, 984–985, 984f
disease of, 404–415. See also Kidneys

84–985, 984f
disease of, 404–415. See also Kidneys,
end-stage, 407–408, 407f, 408t
rickets in, 991
end-stage disease of, 407–408, 407f, 408t
erythropoietin of, 308, 422–423, 422f
failure of
acute, 404–406
dialysis for, 414–415, 414f, 415t
in transfusion reaction, 455
intrarenal, 405–406, 405t
physiologic effects of, 406
postrenal, 405t, 406
prerenal, 405, 405t
chronic, 406–413
anemia and, 412
causes of, 406–407, 407t
ed

406–407, 407t
edema in, 411f, 412
glomerular disease and, 408–409
hypertension and, 412–413
interstitial disease and, 409
nephron function in, 409–412, 410f,
411f, 411t
osteomalacia and, 412
progression of, 407–408, 407f, 408t
protein excretion in, 409
uremia in, 411–412, 411f
vascular lesions and, 408, 408f
ﬁltration fraction of, 316, 319, 319f, 340,
346
function curve of, 217, 217f
function of, 5, 3

ed)
function of, 5, 307. See also at Renal
tubules.
346f
glomerular tufts of, 182
glucose synthesis by, 308
hilum of, 308
in acid-base balance, 308, 390–400. See
also Acid-base balance, renal
regulation of.
in cardiac failure, 259–260, 263–264
in homeostasis, 5
in volume regulation, 212
intercalated cells of, 54, 336–337, 336f,
368, 392

368, 392, 392f
ischemia of, 405–406, 405t, 413
medulla of, 309, 309f, 310f
blood ﬂow of, 354–355, 355f
countercurrent exchange in, 354–355,
355f
hyperosmotic ﬂuid of, 350–355, 351t,
352f–355f
nephrons of, 310, 310f. See also Renal
tubules.
of hagﬁsh, 216
oxygen consumption by, 320, 321f
plasma ﬂow of, 345–346, 346f
pressure diuresis of, 216, 217–220,
217f–220f
pyramids of, 

diuresis of, 216, 217–220,
217f–220f
pyramids of, 309, 309f
size of, 308
sodium-loading function curves of, 229,
229f
stones of, 990
toxin injury to, 406
urine formation by. See Urine, formation
of.
vascular resistance of, 219, 291f
weight of, 308
Killer cells, 447–448, 447f, 448f
Kilocalorie, 885
Kinetic energy, 46, 46f, 51–52
Kinins, 202. See also Bradykinin.
Kinocilium, 693–694, 693f
Klüver-Bucy syndrome, 737–738
Knee jerk, 678–679, 678f
Korotkoff sounds, 175
Krause’s corpuscle, 573f
Krebs cy

koff sounds, 175
Krebs cycle, 23, 833–835, 834f, 843
Kupffer cells, 433, 433f, 859
Kwashiorkor, 866, 925, 925f
Kyphosis, 927
L
Labeled line principle, 572–573
Labor, 8–9, 929, 1036–1038, 1037f
Labyrinth, 652, 692–693, 693f
Lacrimal glands, 753, 754t
Lactase, 806, 809, 809f
Lactate, 294t
Lactation, 1038–1041, 1039f
milk composition in, 1041, 1041t
oxytocin in, 929, 1040–1041
prolactin in, 1039–1040
Lactic acid, 294f
formation of, 837
Lactose,

ion of, 837
Lactose, 809. See also Carbohydrates.
Lactotropes, 919, 920t
Language, 720–722, 721f. See also Cerebral
cortex.
angular gyrus in, 718
articulation of, 721–722, 721f
Broca’s area in, 687, 687f, 717, 717f, 719,
721
comprehension of, 716, 716f, 719, 721
visual, 716
Wernicke’s area in, 716, 718–719, 718f,
721, 721f
Large intestine. See Colon.
Larynx, 481, 481f
in swallowing, 782, 782f
Lateral inhibition, 593, 593f
Lateral olfactory area, 667f, 669
Lateral ve

 593f
Lateral ventricle, 763, 764, 764f
Law of mass action, 837
L-Deprenyl, 711
L-Dopa, 711
Learning. See also Memory; Thinking.
dominant hemisphere in, 719
exercise-related ventilation and, 521,
521f
habituation in, 736
hippocampus in, 737
reinforcement of, 736
skill, 725–726
Lecithins, 846–847, 847f. See also
Left bundle branch, 117f, 118f, 119
Left bundle branch block, 139, 139f, 145
Left coronary artery, 249, 249f
Left ve

 139f, 145
Left ventricular hypertrophy, 272–273
Left ventricular volume curve, 107f, 108
Left-to-right cardiac shunt, 274–275, 274f
Leg, pressures in, 177, 177f, 178–179, 178f
Lengthening reaction, 679
Lens. See also Eye(s); Lenses.
754t
depth of focus of, 618–619, 618f
Lenses
concave, 614, 614f
contact, 620–621

, 620f
Lenses
convex, 613–614, 614f, 616, 616f
cylindrical, 614–615, 614f, 615f, 620
focal length of, 615, 615f
image formation by, 616, 616f
in astigmatism, 620, 620f
in hyperopia, 619, 619f
in myopia, 619, 619f
refractive power of, 616–617, 616f
spherical, 614–615, 614f
Leptin
obesity and, 871, 873
receptor for, 871, 911–912, 912f
Leucine, 853f
Leukemia, 437
Leukocytes. See White blood cell(s).
Leukopenia, 436–437
Leukorrhea, during menstruation, 1019
Lever 

6–437
Lever system, 82, 82f
Leydig cell tumor, 1009
Lieberkühn, crypts of, 805–806, 805f
Ligands, 14
Index
1093
Light
adaptation to, 631–632, 632f
cone response to, 632–633, 633f
refraction of, 613–615, 614f, 615f
rod response to, 629–631, 630f
white, 633
Limbic association area, 716f, 717, 717f
Limbic cortex, 738
Limbic system, 731–738, 732f
amygdala of, 737–738
cortex of, 738
functional anatomy of, 731–732, 732f
hypothalamus of, 732–736, 733f. See also
Hyp

 732f
Hypothalamus.
in behavior, 731–732, 733f, 734–736, 735f
in olfaction, 669
lesions of, 735, 738
memory and, 736
punishment centers of, 725, 735–736
reward centers of, 725, 735, 735f
tranquilizer effect on, 736
Linear acceleration, 695
Linear acceleratory forces, in space travel,
542–543, 543f
Lingual lipase, 811
Lipase(s), 811
enteric, 811
intestinal, 806
lingual, 811
pancreatic, 799, 811–812, 812f
tissue, 841, 842
Lipid(s), 12f

799, 811–812, 812f
bile salt combination with, 804, 812, 816
formation of
by endoplasmic reticulum, 21, 21f
from carbohydrates, 844–845, 845f
from proteins, 845
metabolism of, 840–851
ATP formation in, 842–844, 843f
chylomicrons in, 840–841
during exercise, 846
fatty acid transport in, 841
hormonal regulation of, 846
in liver, 842, 844, 861–862
lipoproteins in, 841–842
vs. carbohydrate metabolism,

ory quotient for, 867
vs. carbohydrate metabolism, 846
solubility of, 47
transport of, 840–842
Lipid bilayer, 12–13, 14f, 45, 46f, 848
diffusion through, 47
Lipoprotein(a), 850
Lipoprotein lipase, 841, 965
Lipoproteins, 841–842, 850. See also
Cholesterol; Lipid(s).
b-Lipotropin, 956–957, 957f
Lissauer, tract of, 609
Liver, 791, 859–864
acetoacetic acid formation in, 844, 966,
966f
amino acids of, 854, 952
anatomy of, 859, 860f
arterioles of, 859
B lymphocyte preprocessing in, 441
bile secretion 

bile secretion by, 802–805, 802f, 803t. See
also Bile; Bile salts.
bleeding in, 464–465
blood ﬂow of, 860
blood storage in, 179, 860
blood-cleansing function of, 861
calcium excretion by, 862
capillaries of, 182, 184
Liver (Continued)
cells of, 859
cholecalciferol–25-hydroxycholecalciferol
conversion in, 983–984, 984f
cholesterol secretion by, 804–805
cirrhosis of, 381, 464, 855, 860
coagulation factor formation by, 862
detoxiﬁcation function 

n factor formation by, 862
detoxiﬁcation function of, 862
endothelial cells of, 859
estrogen metabolism of, 1016–1017
fat storage in, 842
ferritin storage in, 862
gluconeogenesis in, 832, 832f, 861
glucose uptake by, 964–965
glycogen storage in, 831–832, 861
glycogenolysis in, 832, 861, 970
in carbohydrate metabolism, 861
in hemorrhagic shock, 283, 283f
in lipid metabolism, 842, 844, 861–862
in protein metabolism, 862. See als

e infant, 1050
in protein metabolism, 862. See also
Amino acids; Protein(s).
iron storage in, 862
Kupffer cells of, 433, 433f, 861
lipoprotein formation in, 842
lobules of, 859, 860f
lymph ﬂow of, 860
macrophages of, 433, 433f, 861
mast cells of, 464
metabolic functions of, 861–862
pain in, 604
plasma protein formation by, 855, 862
regeneration of, 39, 860–861
sinusoids of, 859, 860f
triglycerides in, 842
urea formation in, 856, 862
vascular system 

, 842
vascular system of, 179, 859–861, 860f
vitamin storage in, 862, 875
Liver phosphorylase, 964
Lobotomy, prefrontal, 720
Locomotion. See Movement.
Locus ceruleus, 730, 730f
Loop diuretics, 403, 403t
Loop of Henle, 310, 310f, 311f, 327
ascending limb of, 310, 310f, 351–352,
351t, 352f
in urine concentration, 351–352, 352f,
355, 356f
in urine dilution, 349–350, 349f
osmolarity of, 355, 356f
countercurrent mechanism of, 351–352,
352f, 357

72
352f, 357–358
descending limb of, 310, 310f, 351–352,
351t, 352f
in urine concentration, 352, 352f, 355,
356f
in urine concentration, 351–352, 352f, 355,
356f
osmolarity in, 349–352, 349f, 352f
solute transport in, 334–336, 335f, 351,
351t
water transport in, 334–336, 335f, 351,
351t
Low cardiac output failure. See Cardiac
shock.
Lumbar puncture, 765–766
Lumirhodopsin, 629, 629f
Lung volumes, 475–477, 475f, 476f, 477t
Lungs. See a

ng volumes, 475–477, 475f, 476f, 477t
Lungs. See also Alveolus (alveoli);
Pulmonary ventilation; Respiration.
at birth, 1044–1045, 1045f
blood ﬂow of. See Pulmonary circulation.
blood volume of, 484–485
cilia in, 480
collagen ﬁbers of, 473
collapse of, 528–529, 529f
compliance of, 473–475, 473f
constriction of, 525–526, 526f. See also
Respiratory insufﬁciency.
contraction of, 471–472, 472f
dead space of, 478, 500–501
edema of, 488–489
in mitra

alve lesions, 273
262
elastin of, 473
expansion of, 471–472, 472f
expiratory reserve volume of, 475, 476f
ﬁbrosis of, 530
forced expiratory vital capacity of, 526,
526f
forced expiratory volume of, 526, 526f
functional residual capacity of, 475,
476–477, 476f, 529
infection of, 527–528, 528f, 530
inspiratory capacity of, 475, 476f
inspiratory reserve volume of, 475,
4

f, 475, 476f
476f
interstitial ﬂuid pressure of, 487, 488,
488f
irritant receptors of, 480, 521
lymph vessels of, 483
macrophages in, 433
mast cells of, 464, 480
mucus in, 480
oxygen poisoning of, 547
residual volume of, 475, 476f, 477, 529
shock, 283
stretch receptors of, 515–516
surface tension of, 474
surfactant of, 474, 529, 1045
tidal volume of, 475, 476f, 477
total capacity of, 476, 476f, 477
tuberculosis of, 530
v

capacity of, 476, 476f, 477
vital capacity of, 475–476, 476f
Lupus erythematosus, 448
Luschka, foramen of, 764, 764f
Luteinizing hormone, 907t, 918, 919f, 920t
1013f, 1020–1021, 1020f
Lymph
ﬂow of, 192–193, 192f, 193f, 194
in edema prevention, 305
in node, 433, 433f
formation of, 191–192
Lymph nodes, 190, 191f

ion of, 191–192
lymphocytes in, 440, 442
macrophages in, 433, 433f
structure of, 433, 433f
Lymphatic capillaries, 190–191, 192f, 193f
pump of, 193
Lymphatic system, 185, 190–194, 191f, 440
blockage of, 302–304
contraction of, 193
1094
Index
Lymphatic system (Continued)
interstitial ﬂuid control and, 188, 192,
192f, 193–194
of brain, 765, 765f
of lung, 483
pumping of, 193
valves of, 192–193, 193f
Lymphoblasts, 443
Lymphocyte(s), 429–430, 430f
B, 440

phoblasts, 443
B, 440–441, 441f
activation of, 442
antigen interaction with, 442–446,
antigen presentation by, 446, 446f
bone marrow preprocessing of, 441
clone of, 442, 448
interleukin effects on, 447
liver preprocessing of, 441
memory, 443, 443f
preprocessing of, 440–441, 441f
production of, 421f, 430–431, 430f, 440,
447, 447f
T, 440–441, 441f
activation of, 

activation of, 442, 446, 446f
cytotoxic, 447–448, 447f, 448f
helper, 442, 446–447, 447f
lymphokine production by, 447, 447f
memory, 446
production of, 430–431, 430f, 440
regulatory, 448
suppressor, 447, 448
surface receptor proteins of, 442, 446,
446f
thymus gland preprocessing of, 441
types of, 446–448, 447f
tissue distribution of, 440
Lymphocytic leukemia, 437
Lymphokines, 442, 447, 447f, 905
Lysine, 853f
Lys

Lysine monohydrochloride, in alkalosis, 398
Lysis
antigen, 444, 445, 445f
clot, 464
red blood cell, 453, 454, 455, 864
Lysoferrin, 20
Lysosomes, 13f, 16, 20, 20f
cortisol effects on, 953
Lysozyme, 20
M
Machinery murmur, 275
Macrocytes, 423
Macrophages, 431
alveolar, 433, 481
ameboid movement of, 24, 431
bactericidal agents of, 432
enzymes of, 432
in bone marrow, 434
in inﬂamm

 432
in inﬂammation, 434–436, 436f
in liver, 433, 433f, 859, 861
in lungs, 433
in lymph nodes, 433, 433f
in skin, 433
Macrophages (Continued)
in spleen, 434, 434f
lymphokine effects on, 447
phagocytosis by, 432, 435, 447
pinocytosis by, 19
plasma protein processing by, 855
tissue, 431, 433, 433f, 435
Macula densa, 310, 310f
sodium chloride concentration in, 324,
324f, 374
Maculae, 693, 693f
Magendie, foramen of, 764, 764f
Magnesiu

693, 693f
Magnesium, 878–879, 878t, 879t
excretion of, 373
extracellular, 46f, 294f, 294t, 373
in bone, 981
in vasodilation, 202
Magnet reaction, 682
Major basic protein, 436
Major histocompatibility complex, 446, 446f
Malabsorption, in sprue, 822
Malignant nephrosclerosis, 408
Malleus, 651, 652f
Malnutrition. See also Fee

 408
Malnutrition. See also Feeding; Food;
Starvation.
growth and, 925, 925f
urine concentration in, 353
Malocclusion, 994
Malonyl-CoA, 845, 845f
Maltase, 806, 809, 809f
Maltose, 809, 809f
Mamillary bodies, 734
Mania, 745
Manic-depressive psychosis, 745
Marginal ulcer, 820
Mark time reﬂex, 682–683
Mass action, law of, 837
Mass discharge, 757, 758
Mass reﬂex, 684
Mast cells, 436
heparin production by, 436, 464
in asthma, 480, 529
o

arin production by, 436, 464
of lungs, 480
Mastectomy, lymphatic blockage after,
302–303
Mastication, 781–782
Maximum expiratory ﬂow, in respiratory
Mayer waves, 214, 214f
Mean arterial pressure, 175–176, 176f
Mean circulatory ﬁlling pressure, 239–240,
239f
mean systemic ﬁlling pressure and, 240
Mean electrical axis, 137–140
bundle branch block effect on, 139–140,
139f
from electrocardiogram, 137–138, 137f
left bundle branch block effect on, 

137–138, 137f
left bundle branch block effect on, 139,
139f
left heart shift effect on, 138
left ventricular hypertrophy and, 138, 138f
right bundle branch block effect on,
139–140, 140f
right heart shift effect on, 138
right ventricular hypertrophy and,
138–139, 139f
Mean pulmonary ﬁlling pressure, in
unilateral left-sided cardiac failure, 262
Mean QRS vector, 132, 132f
Mean systemic ﬁlling pressure
during exercise, 248, 249
in cardiac failure, 259
mean circulatory ﬁlling pressure and, 240
spin

259
venous return curve and, 238, 239, 239f,
240, 240f
Mechanoreceptors, 572, 573f, 573t
adaptation of, 575–576, 575f
signal transmission from, 588–589, 588f,
595–596, 596f
Meconium, 1043
Medial forebrain bundle, 731–732
Medial olfactory area, 667f, 669
Median eminence, hormonal release at, 921,
921t
Medulla, 558. See also Brain stem.
pyramids of, 687, 688f
respiratory center of, 514–516, 515f
reticular nuclei of, 691f, 6

ter of, 514–516, 515f
reticular nuclei of, 691f, 692
vestibular nuclei of, 691f, 692, 701
vomiting center of, 824
Medullary collecting duct, 310, 310f. See also
Renal tubules.
Medullary reticulospinal tract, 692, 692f
Megacolon, 822
Megaesophagus, 819
Megakaryocytes, 429, 430f, 431
Megaloblasts, 427
Meiosis, 997, 998f
Meissner’s corpuscles, 573f, 586, 693
Meissner’s plexus, 774–775, 774f
Melanin, 956
in Addison’s disease, 958
retinal, 628
Melanocortin, 874
Melanocytes, 956
Melanocyte-stimulating

ortin, 874
Melanocyte-stimulating hormone, 956–957,
957f
Melatonin, 1009–1010
Membrane. See Capillary membrane; Cell
membrane; Respiratory membrane.
Membrane potential, 50, 50f, 54. See also
Action potential.
diffusion and, 57–58, 58f
neural, 57–58, 58f, 65. See also Action
potential, neural.
measurement of, 58–59, 59f
physics of, 57–58, 58f
resting, 59–61, 60f
of gastrointestinal smooth muscle, 773
of neuronal 

f gastrointestinal smooth muscle, 773
of neuronal soma, 564–565, 564f
of sinus nodal ﬁber, 117, 117f
potassium diffusion in, 60, 60f
sodium diffusion in, 60f, 61
sodium-potassium pump in, 60f, 61
smooth muscle, 96
slow wave rhythm of, 97, 97f, 772–773,
772f
Membrane proteins, 13–14, 14f
carrier, 46f, 47, 49–50, 53–54, 53f
channel, 46f, 47–49, 47f, 48f
Membrane-stabilizing factors, 70
Memory, 557, 723–727
classiﬁcation of, 723–724
codiﬁcation of, 725
consolidation of, 725–727
declarative, 723–724

725
failure of, 726
habituation and, 723, 724
hippocampus in, 726–727
in Aplysia, 724, 724f
intermediate long-term, 723, 724–725, 724f
Index
1095
Memory (Continued)
long-term, 723, 725, 726
negative, 723
positive, 723
rehearsal and, 726, 737
short-term, 723, 724
skill, 724
synaptic facilitation and, 723, 724–725, 724f
synaptic inhibition and, 723, 724
synaptic transmission in, 723, 724
Wernicke’s area in, 718, 718f
working, 720, 723
Memory cells, 44

a in, 718, 718f
Memory cells, 443, 443f, 446
Memory sensitization, 723
Memory traces, 723, 736
Menarche, 1021–1022, 1022f
Meningitis, 607
Menopause, 1011, 1022, 1022f
Menorrhagia, 938
Menstrual cycle, 1012–1015, 1013f
anovulatory, 1021, 1024–1025
endometrial changes in, 1018–1019, 1018f
follicular phase of, 1013–1014, 1021
luteal phase of, 1014–1015
ovulation and, 1014, 1015f, 1021
postpartum suppression of, 1040
regulation of, 1019–1021, 1020f, 1022f
Menstruation, 1019
Merkel’

1019–1021, 1020f, 1022f
Merkel’s discs, 586, 586f
Mesencephalon, 558. See also Brain stem.
Metabolic rate, 884–885
after meals, 887
basal, 886–887, 887f
cold effects on, 938–939
exercise effects on, 887, 887t
fever effects on, 887
growth hormone effects on, 887
malnutrition effects on, 887
sleep effects on, 887
testosterone effects on, 1006
thyroid hormone effects on, 886, 936,
936f
blood ﬂow and, 196, 196f
epinephrine effect on, 755
estrogen and, 1018
in Inuit, 896

n, 755
measurement of, 885
neonatal, 1048–1049
protein effect on, 887
Metabolic syndrome, 974
Metabolic theory, of blood ﬂow regulation,
199
Metabolism. See also Adenosine
triphosphate (ATP); Energy.
brain, 767
carbohydrate, 829–839. See also
coupled reactions in, 829
during pregnancy, 1034–1035
enzymes in, 884, 884f
in leukemia, 437
lipid, 840–851. See also Lipid(s),
metabolism of.
protein, 852–856. See also Protein(s),
metabolism o

rotein, 852–856. See also Protein(s),
rate of, 884–885, 884f
basal, 886–887, 887f, 887t. See also
Metabolic rate, basal.
Metaphase, 37f, 39
Metarhodopsin, 629, 629f
Metarterioles, 181, 182f
182–183, 187, 197–198, 197f
Metastatic calciﬁcation, 990
Methacholine, 759
neuromuscular junction effects of, 88–89
Methane, 825
Methionine, 853f
Methylmercaptan, 668
Methylprednisone, 947, 947t
Metoprolol, 759
Micelles, 804, 812, 816
Michaelis-Menten e

ol, 759
Michaelis-Menten equation, 884
Microcirculation, 181–182, 182f. See also
Arteriole(s); Capillary (capillaries).
Microﬁlaments, 13f
Microgravity, 543–544
Micropipette method
for capillary pressure, 186
for interstitial pressure, 187
Microtubules, 11, 13f, 17, 17f
Microvilli, intestinal, 813, 813f
Micturition, 309, 311–314, 311f, 313f
abnormalities of, 313–314
Micturition reﬂex, 311, 313–314
Micturition waves, 313, 313f
Migraine headache, 607
Milieu intérieur, 4, 9

13f
Milk ejection (milk let-down), 1040–1041
Milliosmole, 297
Mineralocorticoids, 944. See also
Aldosterone.
Minerals, metabolism of, 878–879, 878t
Minimal change nephropathy, 317, 409
Minimal change nephrotic syndrome,
409
Minute respiratory volume, 477
Miosis, 649
Mitochondrion (mitochondria), 12, 13f,
16–17, 16f
ATP formation in, 17, 22–24, 22f, 23f,
chemiosmotic mechanism

24, 22f, 23f,
chemiosmotic mechanism of, 835, 835f,
843
DNA of, 17
fatty acid metabolism in, 842–843, 843f
function of, 22–24, 22f, 23f
matrix of, 16f, 17
citric acid cycle of, 833–835, 834f
of muscle, 74
835f
structure of, 16f, 17
Mitosis, 17, 37–39, 37f
colchicine prevention of, 40
Mitotic apparatus, 37f, 38–39
Mitotic spindle, 17, 39
Mitral cells, of olfactory bulb, 667f, 669, 670
circulatory dynamics in, 273
heart soun

urgitation
Mitral valve, 109, 109f
stenosis of
pulmonary capillary pressure in, 489
pulmonary edema in, 488–489
Modiolus, 653, 655f
Monoamine oxidase inhibitors, 745
Monocyte(s), 421f, 429–430, 430f
diapedesis of, 431
Monocyte colony-stimulating factor, 435,
436f
Monocyte-macrophage cell system
(reticuloendothelial system), 18

phage cell system
(reticuloendothelial system), 180,
432–434, 433f, 434f
Motilin, 776
Motion sickness, 824
in space travel, 544
Motor aphasia, 721
Motor apraxia, 687
Motor cortex, 589–590, 685, 686f, 712
Betz cells of, 687–688
body representation in, 685–686, 686f
Broca’s area of, 687, 687f, 717, 717f,
719
corticorubral tract from, 688–689, 689f
corticospinal (pyramidal) tract from,
687–688, 688f
eye movement area of, 687, 687f
hand skill area of, 687, 687f
head rotation area of, 687, 687f
incom

, 687, 687f
incoming ﬁber pathways to, 688
muscle representation in, 685–686, 686f
premotor area of, 686, 686f
primary, 685, 686f
removal of, 691
signal transmission from, 687–688, 688f
signal transmission to, 688, 690
specialized areas of, 686–687, 687f
supplementary motor area of, 686, 686f
topographical representation of, 685–686,
686f
vertical columns of, 689–690
Motor end plate, 85, 86f
Motor neurons, 559–560, 559f, 690, 690f
alpha, 674, 674f

neurons, 559–560, 559f, 690, 690f
alpha, 674, 674f, 675
anterior, 674, 674f
dendrites of, 568–569, 568f
dynamic, 690
excitation of, 564–566, 566f
gamma, 674, 674f, 675, 676, 678
inhibition of, 566–568, 567f
pyramidal, 690
rate of ﬁring of, 569, 569f
resting membrane potential of, 564–565,
564f
static, 690
stimulation of, 690–691, 690f
Motor unit, 674
Mountain sickness
acute, 540–541
chronic, 541
Mouth, dryness of, thirst and, 361, 361t
Movement(s)
ameboid, 24, 24f, 4

rst and, 361, 361t
ameboid, 24, 24f, 431
amygdala in, 737
ballistic, 705
cellular, 24–25, 24f, 25f
ciliary, 24–25, 25f
control of. See also Muscle (skeletal),
contraction of.
basal ganglia in. See Basal ganglia.
brain stem in, 678, 691–692, 691f, 692f,
697
cerebellum in. See Cerebellum.
cognitive, 709, 709f
intermediate cerebellar cortex in,
motor cortex in. See Motor cortex.
posterior parietal cortex in, 710,
710f
prefrontal association area in, 716–
717

ontal association area in, 716–
717
spinal cord in. See Spinal cord, motor
system of; Spinal reﬂexes.
spinocerebellum in, 704–705, 704f
summary of, 712–713
693f–695f
1096
Index
Movement(s) (Continued)
disorders of
basal ganglia lesions and, 709, 710, 710f,
711–712
brain stem section and, 692
cerebellar lesions and, 706–707
motor cortex lesions and, 691
spinal cord transection and, 684
ﬁnger, 678
involuntary, 737
ocular, 645–648, 6

684
ﬁnger, 678
ocular, 645–648, 646f, 647f
pendular nature of, 705
sequential, 706
timing of, 706, 709–710
Mucin, 793
Mucous glands, 791, 795. See also Mucus.
Mucous neck cells, 795, 796f, 797
Mucus
duodenal, 805
esophageal, 795
gastric, 797, 799
gastrointestinal, 791, 793
large intestine, 806
respiratory tract, 480
small intestine, 805
Muramyl peptide, 740–741
Murmurs, 271f, 272
Mus

rs, 271f, 272
Muscle. See Cardiac muscle; Muscle
(skeletal); Muscle (smooth).
Muscle (skeletal), 72–83. See also Exercise,
muscles in.
action potential  of, 89–91, 90f, 91f
agonist, 82
anatomy of, 72–74, 73f, 74f
antagonist, 82
antigravity, 678, 691f, 692
atrophy of, 83
blood ﬂow in, 195, 196t
during exercise, 246, 247–248, 247f,
1062–1063, 1063f
nervous control of, 247
oxygen effect on, 247
capillaries of, 184, 

ociation curve
capillaries of, 184, 184t, 246
clonus of, 679
contraction of, 74–83. See also
Movement(s).
actin ﬁlament in, 75f–78f, 76–78
adenosine triphosphatase in, 76
adenosine triphosphate in, 77, 79–80
atrophy and, 83
blood ﬂow and, 246, 247f
calcium in, 74, 76, 90–91, 91f
cardiac output and, 213–214
coactivation in, 82
efﬁciency of, 79–80
energetics of, 78–80
energy sources for, 79–80
equalization of, 680
excitation of, 690
fast ﬁbers in, 80–81
fatigue of, 82
force of, 78, 7

ast ﬁbers in, 80–81
force of, 78, 78f
force summation in, 81, 81f
frequency summation in, 81, 81f
general mechanism of, 74
glycogen in, 79
Golgi tendon organ and, 679–680, 679f
hyperplasia and, 83
hypertrophy and, 83
in fast ﬁbers, 80–81
Muscle (skeletal) (Continued)
in slow ﬁbers, 81
isometric, 80, 80f
isotonic, 80, 80f
load and, 78, 79f
maximum strength of, 81
mechanics of, 81–82, 81f
molecular mechanism of, 74–78, 75f–77f
motor unit in, 81
multiple ﬁber summation in, 81
muscle 

unit in, 81
muscle length and, 78, 78f
muscle spindles and, 675–679, 676f–678f
myosin ﬁlament in, 75–78, 75f–78f
oxidative metabolism in, 79
phosphocreatine in, 79
ratchet theory of, 76–77, 77f
recovery of, 83
sliding ﬁlament mechanism of, 74–75, 75f
slow ﬁbers in, 81
somatosensory feedback in, 690
staircase effect (treppe) in, 81–82
strength of, 81–82, 81f
tension with, 77–78, 78f
tetanic, 246
tetanization of, 81, 81f
tropomyosin in, 76
troponin in, 76
velocity of

81f
velocity of, 78, 79f
voluntary, 678
walk-along theory of, 76–77, 77f
work output during, 78–79
contracture of, 83
denervation of, 83
energy metabolism in, 883, 1056–1059,
fast, 80–81
glucose storage in, 963–964
glucose transport in, 964, 964f
glucose uptake by, 963
glycolysis in, 883
Golgi tendon organ of, 675, 679–680, 679f
hyperplasia of, 83
hypertrophy of, 83

79–680, 679f
in exercise, 1055–1061. See also Exercise,
in poliomyelitis, 83
innervation of, 556, 557f
length of, 78, 78f, 83
muscle spindles of, 674, 674f, 675–679,
676f, 677f. See also Muscle spindles.
myoﬁbrils of, 72–74, 73f, 74f, 75–77, 76f,
77f, 90f
oxygen consumption by, 247
potassium release from, 366
reciprocal inhibition of, 681–682, 682f
red, 81
remodeling of, 82–83
sarcolemma of, 72, 90f
sarcoplasm of, 74, 74f

2–83
sarcoplasmic reticulum of, 74, 74f, 89–91,
90f
slow, 81
spasm of, 683, 691
pain and, 599
stretch reﬂexes of, 676–678, 677f
tone of, 82
tonic disorders of, 691
transverse tubules of, 89–90, 90f
white, 81
Muscle (smooth), 92–99
actin ﬁlaments of, 93–94, 93f
action potential of, 96–98, 97f, 772–773
calmodulin of, 95
contact junctions of, 96
contraction of, 93–99
action 

act junctions of, 96
action potential in, 96–98, 97f
calcium in, 95, 99, 99f
calmodulin in, 95
cessation of, 95
chemical factors in, 98
duration of, 94
energy for, 94
force of, 94
hormonal factors in, 98
in blood ﬂow autoregulation, 199
inhibition of, 98
latch mechanism of, 94, 95
myosin cross-bridges in, 94
myosin kinase in, 95
myosin phosphatase in, 95
sarcoplasmic reticulum in, 99, 99f
stress-relaxation of, 94–95
tissue factors in, 98
without action potentials, 97–98
cra

actors in, 98
cramps of, 604
dense bodies of, 93, 93f
diffuse junction of, 96, 96f
hormone receptors of, 98
hyperpolarization of, 98
innervation of, 95–96, 96f
junctional potential of, 97–98
membrane potentials in, 96
multi-unit, 92, 93f, 97–98
myosin ﬁlaments of, 93–94, 93f
neuromuscular junction of, 95–96, 96f
sarcoplasmic reticulum of, 99, 99f
stress-relaxation of, 94–95, 172, 173f,
281
stretch of, 97
structure of, 93–94, 93f
syncytial, 92–93, 93f
types of, 92

 of, 93–94, 93f
types of, 92–93, 93f
unitary (single-unit), 92–93, 93f, 96–97,
97f
visceral, 92–93, 93f
Muscle impulse, 65–66, 65f. See also Action
Muscle spindles, 595, 674, 674f, 675–679
continuous discharge of, 676
dynamic response of, 676
gamma motor nerves to, 676
in muscle stretch reﬂex, 676–677, 677f
in position stabilization, 678
in voluntary motor activity, 678
intrafusal ﬁbers of, 674, 674f, 675, 676,
676f
nuclear bag ﬁbers of, 676, 676f
nuclear chain ﬁ

6f
nuclear chain ﬁbers of, 676, 676f
receptor function of, 675–676
sensory innervation of, 674f, 675–676,
676f
signal averaging function of, 677, 677f
static response of, 676
Muscular dystrophy, swallowing in, 819
Muscularis mucosae, 787–788
Musculoskeletal system, 5. See also Bone(s);
Muscle (skeletal).
Mutation, 38, 41
Myasthenia gravis, 89, 448, 819
Mydriasis, 649
Myelin sheath, 68, 68f
degeneration of, 876
Myelogenous leukemia, 437
Myenteric (Auerbach’s) plexu

ogenous leukemia, 437
Myenteric (Auerbach’s) plexus, 774–775,
774f, 777
Index
1097
Myocardial infarction, 253–255, 253f. See
also Cardiac failure.
adenosine loss with, 252
anterior wall, 143–144, 143f, 145, 145f
cardiac output after, 255, 259f, 260–262,
261f
cardiac output in, 236f, 237
conduction pathways after, 254
congestive symptoms after, 254
electrocardiogram after, 140, 140f, 145,
145f
electrocardiogram in, 143–145, 143f, 144f
injury current in, 141–145, 142f–145f, 254
posterior wall, 144

nt in, 141–145, 142f–145f, 254
posterior wall, 144–145, 144f, 145f
recovery from, 254–255, 255f, 260
rupture after, 254
shock in, 262–263
subendocardial, 253
ventricular ﬁbrillation after, 254
Myocardial ischemia, 252–255, 253f. See also
Myoﬁbrils, 72–74, 73f, 74f, 75–77, 76f, 77f,
90f
Myogenic theory, of blood ﬂow regulation,
199
Myoglobin, 81
Myopia, 619–620, 619f
Myosin, 72–74, 73f, 74f
in pinocytosi

19–620, 619f
Myosin ﬁlaments
Myosin kinase, 95, 915
Myosin phosphatase, 95
Myxedema, 942, 942f
N
Naming, 716, 716f
Narcosis, nitrogen, 545–546
Natriuresis, pressure, 216, 323, 341–342,
in hypertension, 228–229
Nausea, 824
antidiuretic hormone secretion and,
360–361
Nearsighte

diuretic hormone secretion and,
360–361
Nearsightedness, 619–620, 619f
Neck
proprioceptors of, 696
veins of, 179
Necrosis, 40
hepatic, 283, 283f
renal, 406
tissue, 283, 283f
Negative feedback, 7, 8f
Neonate. See also Fetus.
acid-base balance in, 1048
allergy in, 1049
arterial pressure in, 1047
blood volume in, 1047
body temperature in, 1048–1049, 1049f
breathing onset in, 1044–1045, 1045f
calcium in, 1049
cardiac output in, 1047
circulation in, 1045–1048, 1046f
endocrine problems in, 1049–1050
ﬂ

045–1048, 1046f
ﬂuid balance in, 1048
growth of, 1051f, 1052
immunity in, 1049
Neonate (Continued)
iron for, 1049
jaundice in, 1048
liver function in, 1048
metabolic rate in, 1048–1049
nutrition for, 1047, 1049
of diabetic mother, 1050
premature, 1050–1051
red blood cell count in, 1047–1048,
1048f
renal function in, 1048
respiration in, 1047
thyroid disorders in, 1050
vitamin C deﬁciency in, 1049
vitamin D in, 1049
Neospinothalamic tract, 600, 600f
Neostigmine, 8

9
Neostigmine, 89, 759
Nephritis, 409
Nephrogenic diabetes insipidus, 302,
357–358, 414
Nephrons. See also Renal tubules.
anatomy of, 310, 310f
cortical, 310, 311f
in chronic renal failure, 409–412, 410f,
411f, 411t
intercalated cells of, 336–337, 336f
juxtamedullary, 310, 311f
principal cells of, 336–337, 336f
Nephrosclerosis, 408, 408f
Nephrotic syndrome, 303, 381, 409
Nernst equation, 50, 58
Nernst potential, 50, 50f, 58, 565
Nerve(s). See Nerve ﬁbers; Neurom

50, 50f, 58, 565
Nerve(s). See Nerve ﬁbers; Neuromuscular
junction; Neuron(s); Synapse(s).
Nerve ﬁbers. See also Neuron(s);
Synapse(s).
action potential of, 61–65, 61f–64f. See
also Action potential, neural.
anesthetic effects on, 70
classiﬁcation of, 576–577, 577f
conduction velocity in, 69
convergence of, 580, 580f
diameter of, 576–577, 577f
discharge zone of, 579, 579f
divergence of, 580, 580f
energy metabolism of, 66, 66f
facilitated zone of, 579, 579f
from sensory receptors, 576–578, 577f,

 579, 579f
578f
labeled line principle of, 572–573
membrane potential of, 57–58, 58f, 65
myelinated, 68, 68f, 577f
resting membrane potential of, 59–61, 60f
spatial summation in, 578, 578f
speciﬁcity of, 572–573
stimulatory ﬁeld of, 579, 579f
structure of, 68, 68f
temporal summation in, 578, 578f
type A, 576, 577, 577f
type C, 576, 577, 577f
unmyelinate

 576, 577, 577f
unmyelinated, 68, 68f, 577f
varicosities of, 96, 96f
Nerve gas, 89
Nerve impulse, 65–66, 65f. See also Action
Nervous system, 5–6
action potential of. See Action potential,
neural.
autonomic. See Autonomic nervous
system.
central. See Central nervous system.
computer analogy to, 558, 558f
Nervous system (Continued)
enteric. See Enteric nervous system.
integrative function of, 556–557
motor axis of, 556, 557f. See also
Movement(s), control

 axis of, 556, 557f. See also
Movement(s), control of; Muscle
(skeletal), contraction of.
somatosensory axis of, 555, 556f. See also
Sensory receptors; Somatic sensation.
Net ﬁltration pressure, 185–186, 185f,
189
Net reabsorption pressure, 189
Neuroendocrine system, 905. See also
Adrenal medulla; Hypothalamus;
Neurogenic dyspnea, 532
Neurogenic hypertension, 228
Neurogenic shock, 285, 287
Neurohypophysis. See Pituitary gland,
posterior.
Neuromuscular junction. See also
Synapse(

sterior.
skeletal muscle, 85–88
acetylcholine secretion at, 85–88,
86f–88f
anatomy of, 85, 86f
drug effects at, 88–89
fatigue of, 88
smooth muscle, 95–96, 96f
acetylcholine secretion at, 96
norepinephrine secretion at, 96
Neuron(s), 555, 556f. See also Action
potential, neural; Synapse(s).
chloride channels of, 566, 566f
chloride ion concentration of, 564f,
565
concentration gradients of, 564f, 565
continuous discharge from, 581,

ents of, 564f, 565
continuous discharge from, 581, 582
electrical potential of, 564f, 565
excitation of, 565–566, 566f
excitatory postsynaptic potential of,
565–566, 566f, 567, 567f, 568
excitatory state of, 569, 569f
excitatory stimulus of, 579, 579f
facilitation of, 568, 579, 579f
fusiform, 714, 715f
gigantocellular, 730f, 731
granular, 714, 715f
heat-sensitive, 894
hyperpolarization of, 566, 566f
inhibition of, 566, 566f, 567
inhibitory, 580, 581f
inhibitory postsynaptic potential of, 566,
56

581f
566f, 567, 568
inhibitory state of, 569
intracellular ﬂuid of, 565
intrinsic excitability of, 582
motor, 559–560, 559f. See also Motor
neurons.
NPY-AGRP, 868–870, 869f
of motor cortex, 689–690
of red nucleus, 690
of somatosensory cortex, 590–591, 591f
of vasomotor center, 206
parasympathetic, 750
pools of, 578–583, 579f–583f. See also
Neuronal pools.
postganglionic, 748–749, 749f, 750,
759–760
postsynaptic, 557, 559
excitation of, 562
inhibition of

ynaptic, 557, 559
inhibition of, 562
ion channels of, 561, 561f, 910–911
receptor proteins of, 560–562, 561f
second messenger system of, 561–562,
561f
1098
Index
Neuron(s) (Continued)
potassium channels of, 566, 566f
potassium ion concentration of, 564f,
565
preganglionic, 748–749, 749f, 750, 760
presynaptic, 559, 560
presynaptic inhibition of, 567
pro-opiomelanocortin, 868–870, 869f
pyramidal, 714, 715f
564f

569f
564f
reverberatory circuit of, 581–582,
581f–583f
rhythmical signal output from, 582–583,
583f
serotonergic, 729, 730, 730f
sodium channels of, 566, 566f
sodium ion concentration of, 564f, 565
spatial summation in, 567–568, 567f
subthreshold, 579, 579f
sympathetic, 748–749, 749f
temporal summation in, 568
Neuronal pools, 579–580
afterdischarge of, 581–583, 581f, 582f
continuous discharge from, 582, 582f
excitation of, 579, 579f
inhibition of, 579
inhi

f
organization of, 579, 579f
reciprocal inhibition circuit of, 580, 581f
rhythmical discharge from, 582–583, 583f
signal convergence in, 580, 580f
signal divergence in, 580, 580f
Neuropeptide Y (NPY), 868–870, 869t
Neuropeptide Y (NPY)–agouti-related
protein (AGRP) neuron, 868–870,
869f
Neuropeptides, 563t, 564
Neurotransmitters, 559, 562–564, 562t, 563t,
905. See also speciﬁc neurotransmitters

62t, 563t,
905. See also speciﬁc neurotransmitters.
excitatory, 561
in feeding, 868–870, 869f, 869t
in retina, 635
inhibitory, 561
neuropeptide, 563t, 564
of basal ganglial system, 710–711, 710f
of enteric nervous system, 775
receptor binding of, 560–562, 561f
release of, 560
sleep and, 740–741
small-molecule, 562–564, 562t
vesicles of, 560, 560f
Neutralization, antigen, 444, 445
Neutrophil(s), 421f, 429–430, 430f
ameboid movement of, 431
bactericidal

29–430, 430f
diapedesis of, 430f, 431
in inﬂammation, 435–436, 436f
margination of, 430f, 435
phagocytosis by, 432
Neutrophilia, 435
Newborn. See also Neonate.
hemolytic disease of, 421f, 427, 454
respiratory distress syndrome of, 474, 529,
1045
Niacin, 875t, 876
deﬁciency of, 198, 876
Nicotinamide adenine dinucleotide, 834
Nicotinamide adenine dinucleotide
phosphate, 837
Nicotine, 759
neuromuscular juncti

Nicotinic acid, 876
Nicotinic drugs, 759–760
Night blindness, 629, 875
Nitrates, 256
Nitric oxide, 564
blood ﬂow and, 199–200, 322, 322t
Nitrogen
excretion of, 866
pressures of, during deep-sea diving,
545–546, 548–549, 548f
Nitroglycerin, 256
Nociceptive reﬂex, 680–681, 680f
Nociceptors, 572, 573t. See also Pain.

0–681, 680f
Nontropical sprue, 822
Norepinephrine, 563, 907t
blood ﬂow and, 201, 247, 251, 322, 322t
in brain, 730–731, 730f
in cardiac function, 122
in depression, 745
in skeletal muscle blood ﬂow, 247
in vasoconstriction, 207
nerve secretion of, 96
receptors for, 96

tion, 207
secretion of, 207–208, 750, 755, 756
Nose
ﬁltration function of, 479f, 480–481
in vocalization, 481
Nuclear membrane (nuclear envelope), 12f,
13f, 17–18, 18f
Nuclear pores, 18, 18f
Nucleic acid, of virus, 18
Nucleolus, 12f, 13f, 18, 18f, 33
Nucleoplasm, 12f
Nucleotides. See Deoxyribonucleic acid
(DNA); Ribonucleic acid (RNA).
Nucleus, 12f, 17, 18f. See also Cell(s);
genes in, 27–30, 28f–30f

also Cell(s);
Nucleus accumbens, 711
Nucleus ambiguus, 515
Nucleus retroambiguus, 515
Nutrient lack theory, 197–198, 197f
Nutrients. See also Carbohydrates; Fat(s);
Food; Protein(s).
absorption of, 5
diffusion of, 183–183, 183f
Nutrition. See also Digestion; Feeding;
Food.
neonatal, 1047, 1049
Nystagmus, 707
O
O-A-B blood types, 451–453, 452f, 452t
Oat bran, 851
Obesity, 846, 872–874
cortisol and, 952
diabetes mellitus and, 974, 975

hyperplastic, 872
Obesity (Continued)
hypertrophic, 872
leptin and, 871
renal failure in, 407–408
sympathetic nervous system in, 887–888
treatment of, 873–874
Obligatory urine volume, 350, 356–357
Oblique muscles, of eye, 645, 645f
Obstructive sleep apnea, 523
Occipital cortex, in eye movement, 645–646,
646f
Oddi, sphincter of, 803, 803f
Odontoblasts, 992
Odor blindness, 668. See also Olfactory cells;
Smell sensation.
Ohm’s law, 164, 234
Oleic a

ells;
Oleic acid, 840. See also Triglycerides.
Olfactory area
lateral, 669
medial, 669
Olfactory bulb, 667f, 669–670
glomeruli of, 669
Olfactory cells, 667–669. See also Smell
sensation.
action potential of, 668
adaptation of, 668
excitation of, 667–668
glomerular termination of, 667f, 669
Olfactory cilia, 667, 667f
Olfactory membrane, 667, 667f
Olfactory nerve, 667–668, 669
Olfactory tract, 667f, 669–670
Oligohydramnios, 1043
Oligomenorrhea, 938
Oliguria, 40

ohydramnios, 1043
Oliguria, 405
Olivary nucleus, 657, 658f, 660, 688
Olivocerebellar ﬁbers, 688
Olivocerebellar tract, 700, 700f
Oncogenes, 41
Oncotic pressure. See Colloid osmotic
pressure.
Oocyte maturation-inhibiting factor,
1013
Oocytes, 1011
Operon, 35–36, 35f
Ophthalmoscopy, 622–623, 622f
Opiates, endogenous, 602–603, 602f
Opsonization, 19, 432, 445, 445f
Optic chiasm, 640, 641f
lesions of, 645
Optic disc, 644
edema of, 766
Optic nerve, 636–638, 640, 641f
lesions of, 64

766
Optics
ocular, 617–622, 617f–621f. See also
Eye(s); Vision.
physical principles of, 613–617, 614f–616f
Optokinetic movements, 646–647
Oral contraception, 1024
Orbital frontal cortex, ablation of, 738
Orexigenic substances, 868, 869t
Organ of Corti, 652, 652f, 653f, 655–656, 655f
Organ transplantation, 455–456
rejection prevention in, 456
tissue typing for, 455–456
Organelles, 13f, 14–17, 15f, 16f
Organomegaly, 1050
Organum vasculosum, 929
Orgas

f
Orgasm
female, 1023–1024
male, 1002–1003
Orlistat, 873
Osmolality, 52, 297
intracellular, 294t
Index
1099
Osmolar clearance, 357
Osmolarity, 52, 297–298
body ﬂuid, 294t, 298
calculation of, 297–298
cerebrospinal ﬂuid, 361
extracellular ﬂuid. See also Urine,
osmolarity of.
359f, 360f, 3

63
interstitial ﬂuid, 294t, 298
plasma, 294t, 298
348–349, 358–360, 358f, 360t
estimation of, 358
regulation of, 358–363, 358f
Osmole, 52, 297
Osmoreceptors, 358–361, 358f, 359f, 929. See
also Antidiuretic hormone (ADH).
thirst integration with, 362–363, 362f
Osmosis, 51–52, 51f, 296–299, 298f
of water, 332, 814
rate of, 297
Osmotic coefﬁcient, 297–298
Osmotic di

ate of, 297
Osmotic diuretics, 402–403, 403t
Osmotic pressure, 51–52, 52f, 297–298. See
also Colloid osmotic pressure.
interstitial, 294t, 298
renal, 309, 309f, 310t
diuretic effects on, 402–403, 403t
Ossicles, 651–652, 652f
ankylosis of, 661
Osteitis ﬁbrosa cystica, 990
Osteoblasts, 982, 982f
Osteoclasts, 982, 982f
activation of, 987
parathyroid hormone effect on, 987
Osteocytic membrane syste

d hormone effect on, 987
Osteocytic membrane system, 987
Osteolysis, 986–987
Osteomalacia, 412, 991
Osteon, 982, 983f
Osteoporosis, 992, 1017
Osteoprogenitor cells, 983
Otosclerosis, 661, 661f
Ovarian cycle, 1012–1015, 1013f
Ovary (ovaries), 906f, 907t, 1011, 10

1020f, 1022f
Ovary (ovaries), 906f, 907t, 1011, 1012f. See
also Reproduction, female.
abnormalities of, 1023
androgen secretion by, 1003
follicular phase of, 1013–1014, 1013f
monthly cycle of, 1012–1015, 1013f. See
also Ovarian cycle.
Overhydration
hypo-osmotic, 301–302, 301t
Overnutrition, 873. See also Obesity.
Ovulation, 1014, 1015f, 1021
temperature and, 1025, 1025f
Ovum, 1011, 1013, 1013f
fallopian tube transport of, 1028–1029,
1028f
Ovum (

llopian tube transport of, 1028–1029,
1028f
Ovum (Continued)
fertilization of, 1000–1001, 1027–1029,
1028f
Oxalate, renal tubule secretion of, 334
Oxaloacetic acid, 833–834, 834f
Oxidative phosphorylation, 835–836, 835f
Oxygen
concentration of, 485, 494–495, 495f,
496f
altitude effects on, 537–538, 538f, 538t
dissolved oxygen and, 546, 546f
free radical production and, 547
pure-oxygen breathing and, 538, 538f
tissue Po2 and, 546–547, 

breathing and, 538, 538f
tissue Po2 and, 546–547, 546f
pulmonary blood ﬂow and, 485
blood, in respiratory insufﬁciency, 525
brain requirement for, 767
chemoreceptor response to, 518–520, 518f,
519f
consumption of
blood ﬂow rate and, 504, 504f
cardiac, 250–251, 253
exercise and, 233, 233f, 247, 506–507,
506f, 520, 520f, 1061–1062, 1061f,
1062f
renal, 320, 321f
skeletal muscle, 247
deﬁciency of
blood ﬂow and, 196–198, 197f
cerebral blood ﬂow and, 761–762
diffusing capacity of, 498, 499f
exercise a

61–762
exercise and, 498, 1062
high altitude and, 539
diffusion of. See Oxygen, transport of.
energy equivalent of, 885
extracellular, 4, 6, 7t
gastrointestinal concentration of, 779
hemoglobin combination with, 420,
424–425, 505–510. See also Oxygen-
altitude and, 538, 538f
carbon dioxide displacement and,
511–512, 512f
exercise and, 507–508
reversibility of, 505–507, 5

ct and, 511–512, 512f
in space travel, 543
iron combination with, 424–425, 424f
alveolar, 494–495, 495f, 500, 500f
arterial, 518, 518f
chemoreceptor stimulation by, 518, 518f
extracellular, 46f
hemoglobin and, 507
in cerebral tissue, 761–762
interstitial, 503–504, 504f
intracellular, 46f, 504, 508–509, 509f
low, acclimatization to, 539–540
respiration and, 518–520, 518f, 519f
tissue, 5

540
tissue, 503–504, 504f, 506f, 507
alveolar Po2 and, 546–547, 546f
venous admixture and, 503, 503f
placental, 1030–1031, 1031f
blood ﬂow and, 196–198, 196f, 197f,
200
Bohr effect and, 508, 508f
disorders of, 530
transport of, 183, 184, 502–510
during exercise, 506–507, 508, 509
Oxygen (Continued)
from alveoli to pulmonary capillaries,
502–503, 503f
from peripheral capillaries to tissue
cells, 504
from

m peripheral capillaries to tissue
cells, 504
ﬂuid, 503–504, 504f
hemoglobin in, 505–508, 506f, 508f. See
also Hemoglobin; Oxygen-
in arterial blood, 503, 503f
in dissolved state, 509
utilization coefﬁcient and, 507
uptake of, during exercise, 503, 503f
utilization coefﬁcient of, 507, 508
utilization of, 508–509, 509f
adenosine diphosphate and, 508–509,
509f
blood ﬂow and, 509
diffusion distan

 508–509,
509f
diffusion distance and, 509
inadequate, 530–531, 531f
intracellular Po2 and, 508–509, 509f
tissue Po2 and, 503–504, 504f
Oxygen debt, 883, 1058–1059, 1059f
Oxygen diffusing capacity, 498, 499f
Oxygen Fick principle, for cardiac output,
244, 244f
Oxygen free radicals, in deep-sea diver, 547
Oxygen poisoning, 547
Oxygen therapy, 530–531, 531f
hyperbaric, 550–551
in premature infant, 1

bon monoxide poisoning, 510
Oxygen-hemoglobin dissociation curve, 507,
507f
at high altitude, 507, 540, 540f
during exercise, 508, 508f
shifts in, 507–508, 508f
Oxyntic glands, 795–797, 796f
Oxytocin, 907t, 918, 919f, 929
in lactation, 1040–1041
secretion of, 733f, 734
uterine effects of, 1036
P
P wave, 107, 107f, 123, 124f
vectorial analysis of, 136, 136f
Pacemaker. See also Sinus (sinoatrial) node.
artiﬁcial, 150, 156
ectopic, 120–121
P

rial) node.
Pacemaker waves, 97, 97f, 772–773
Pacinian corpuscles, 573f, 587
accommodation of, 576
adaptation of, 575, 575f
receptor potential of, 574–575, 574f, 575f
vibratory signals from, 693–694
Paget’s disease, 989
PAH. See Para-aminohippuric acid (PAH).
Pain, 598–606
brain stem areas of, 601
cerebral cortex in, 601
chemical stimuli in, 599
electrical suppression of, 603
elicitation of, 599
fast, 598, 600, 600f, 601
ﬂexor reﬂex in,

 of, 599
ﬂexor reﬂex in, 680–681, 680f, 681f
glutamate in, 601
headache, 606–607, 606f
1100
Index
Pain (Continued)
hypersensitivity to, 603f, 605
in coronary heart disease, 255–256
in decompression sickness, 549
in gastrointestinal spasm, 604
in ischemia, 604
in muscle spasm, 599
in shingles, 605
in tic douloureux, 605–606
intracranial, 606–607, 606f
ischemic stimuli in, 599
labor, 1038
localization of, 601
neospinothalamic tract for, 600, 600f
paleospinothalamic tract 

mic tract for, 600, 600f
paleospinothalamic tract for, 600–601,
600f
parietal, 604–605, 605f
receptors for, 598–599
referred, 603–605, 603f, 605f
reticular formation in, 601
slow, 598, 600, 600f, 601
spasm in, 599
substance P in, 601
suppression of, 602–603, 602f
surgical interruption of, 602
thalamus in, 601
thermal stimuli in, 599, 608, 608f
tissue damage in, 599
transmission of, 600–602, 600f
ureteral, 312
visceral, 603–605, 605f
Pain reﬂex, 680–681, 680f, 681f
Paleocortex, 669
Paleospinothal

80–681, 680f, 681f
Paleospinothalamic tract, 600–601, 600f
Palmitic acid, 840. See also Triglycerides.
Pancreas, 791, 792f, 906f, 907t
alpha cells of, 832
bicarbonate secretion by, 800, 800f, 801
cells of, 961, 962f
enzymes of, 799–800, 801, 809, 809f
glucagon secretion by, 832, 961, 970–971
insulin secretion by, 961–970. See also
Insulin.
secretions of, 799–802, 800f, 802f
cephalic phase of, 801
gastric phase of, 801
in protein digestion, 810, 810f
intestinal phase of, 801
phas

digestion, 810, 810f
phases of, 801, 802f
stimuli of, 800–801
somatostatin secretion by, 971
Pancreatic amylase, 799, 809, 809f
Pancreatic lipase, 799, 811–812, 812f
Pancreatic polypeptide, 961
Pancreatitis, 800, 821–822
Panhypopituitarism, 926
Panting, 893–894
Pantothenic acid, 875t, 877
Papilla of Vater, gallstone of, 821–822
Papillary muscles, 109, 109f
Papilledema, 766
Para-aminohippuric acid (PAH)
clearance of, 345–346, 346f
transport maximum in, 331
tubular secreti

46, 346f
Parachute jumps, 543
Paracrines, 905
Paralysis
diver’s, 548–549, 548f
of swallowing mechanism, 819
Paralysis agitans, 711
Parasitic infection, eosinophils in, 436
Parasympathetic nervous system. See also
Sympathetic nervous system.
acetylcholine secretion by, 750–751
anatomy of, 750, 750f
Parasympathetic nervous system
(Continued)
denervation effects on, 756–757, 756f
functions of, 750–757, 754t
in bronchiol

757, 756f
in cardiac function, 113, 113f, 121,
205–206, 205f, 234–235, 754, 754t, 755
in gastrointestinal blood ﬂow, 779
in gastrointestinal function, 753–754, 775,
792
in large intestine mucus secretion, 806
648f, 753, 754t
in respiratory function, 479–480
in salivary secretion, 794–795
localized responses of, 757–578
of eye, 648, 648f
postganglionic neurons of, 750
preganglionic neurons of, 750
supe

neurons of, 750
supersensitivity of, 756–757, 756f
tone of, 756–757, 756f
Parasympathomimetic drugs, 759
Parathyroid glands, 906f, 907t, 985–986,
986f
Parathyroid hormone, 907t, 985–988
bone effects of, 986–987
calcium regulation of, 988, 988f
cAMP effects on, 987–988
deﬁciency of, 990
disorders of, 990–992
excess of, 990–991
extracellular calcium and, 986–988, 986f
extracellular phosphate and, 986–988, 986f
in calcium homeostasis, 342t, 343,
371–372, 815, 989–990
i

um homeostasis, 342t, 343,
in phosphate homeostasis, 373
synthesis of, 986
Parathyroid poisoning, 990
Paraventricular nuclei, antidiuretic hormone
synthesis in, 359, 359f
Paraympathectomy, 756–757
Parietal (oxyntic) cells, 54, 796–797, 796f
hydrochloric acid secretion by, 54,
796–797, 796f, 810, 820
intrinsic factor secretion by, 797, 820
structure of, 796, 796f
Parietal cortex, in motor control, 710, 710f
Parietal lobe
ang

tex, in motor control, 710, 710f
angular gyrus of, 718
somatosensory areas of, 589, 590, 590f
Parietal pain, 604–605, 605f
Parieto-occipitotemporal association area,
716, 716f
Parkinson’s disease, 711
Parotid glands, 793–795, 795f
Paroxysmal tachycardia, 151–152
atrial, 152, 152f
ventricular, 152, 152f
Pars intermedia, 918, 956–957
Partial pressures, 491–492. See also Carbon
Parturition, 8–9, 929, 1036–1038, 1037f
Past poin

Past pointing, 706–707
Patch clamp testing, 48–49, 48f
Patent ductus arteriosus, 274–275, 274f,
1046–1047
heart sounds in, 271f
Pco2. See Carbon dioxide, partial pressure
of.
Pellagra, 876
Pelvic nerves, 311f, 312, 313, 775
Penis, 754t, 1002, 1002f. See also Sexual
function, male.
Pentagastrin, 799
Pentolinium, 760
Pentose phosphate pathway, 837–838, 838f
Pepsin, 797, 810, 810f
Pepsinogen, 797, 798
Peptic (chief) cells, 7

 810f
Peptic (chief) cells, 795–796, 797
Peptic ulcers, 820–821, 820f
Peptidases, 806, 810–811, 810f
Peptide chain, 852. See also Amino acids;
Protein(s).
Peptide linkages, 34–35, 808, 810, 810f, 852,
881
Peptide YY, in feeding, 870
Perforins, 447
Pericardial effusion, 140
Pericardial ﬂuid, 292
Pericardium, pain in, 604
Perilymph, 656
Perimetry, 644–645, 644f
Periodic breathing, 522, 522f
Periodontal membrane, 992–993
Peripheral neuropathy, in diabetes mellitus
type I, 973
P

ral neuropathy, in diabetes mellitus
type I, 973
Peripheral resistance unit (PRU), 167
Peripheral vascular resistance, 167. See also
Vascular resistance.
arterial pressure and, 219, 219f, 220,
220f
cardiac output and, 234, 234f, 236–237,
236f
Peripheral venous pressure, 176–177, 177f
Perisinusoidal spaces, 859
colonic, 789
esophageal, 783
gastric, 784, 785–786
gastrointestinal, 776–777, 777f
in vomiting, 824
intestinal, 787–788
Peristaltic rush, 788
Peritoneal ﬂuid, 292
Peritoneointe

altic rush, 788
Peritoneointestinal reﬂex, 790
Peritoneum, pain in, 604–605, 605f
Peritonitis, muscle spasm in, 683
Peritubular capillaries, 309–310, 309f
colloid osmotic pressure of, 309, 309f,
310t, 340, 340f
ﬁltration coefﬁcient of, 340, 341t
hydrostatic pressure of, 309, 309f, 310t,
340, 340f
Perivascular spaces, 765, 765f
Pernicious anemia, 423, 427, 797, 820, 877
Peroxisomes, 16, 432
Petit mal epilepsy, 743f, 744, 744f
pH, 384. See also Acid-base balance.
alveolar vent

alveolar ventilation and, 388–389, 388f,
389f, 517, 517f, 519–520, 519f
carbon dioxide transport and, 512
in respiratory insufﬁciency, 524
respiration and, 516–517, 516f, 517f
Henderson-Hasselbalch equation for,
386
intracellular, 46f, 384, 384t, 387–388
normal, 7t, 384, 384t
of chyme, 7

4, 384t, 387–388
of chyme, 786
shifts and, 507–508, 508f
urinary, 384, 384t, 392
Index
1101
Phagocytosis, 19–20, 430, 431–432
macrophages in, 432, 435
neutrophils in, 432
Phagosome, 432
Pharyngoesophageal sphincter, 782, 782f
Pharynx, in swallowing, 782–783, 782f
Phenoxybenzamine, 759
Phentolamine, 759
Phenylalanine, 853f
Phenylephrine, 759
Phenylthiocarbamide, taste blindness for,
664
Phonation, 481, 481f
Phonocardiogram, 271, 271f, 272

onation, 481, 481f
Phosphate
excretion of, 372–373, 980, 985
extracellular, 46f, 978, 979
intestinal absorption of, 815, 980, 985, 987
intracellular, 4, 46f
986f
reabsorption of, 372–373
failure of, 413
serum, in chronic renal failure, 412
urinary, 372–373
Phosphate buffer system, 387, 393, 393f
Phosphatidylinositol biph

r system, 387, 393, 393f
Phosphatidylinositol biphosphate, 914, 914f
Phosphocreatine, 882
in skeletal muscle contraction, 79
Phosphodiesterase, in rod function, 631
Phosphofructokinase, inhibition of, 836
Phosphogluconate pathway, 837–838, 838f
Phospholipase, 799
Phospholipase C second messenger system,
914, 914f, 914t
Phospholipids, 12f, 846–847, 847f. See also
Lipid(s).
formation of, 847
functions of, 847
hydrolysis of, 841
intrace

, 847
Phosphoric acid, 27, 28f
Phosphorus, 878t, 879, 879t
blood, 979
Phosphorylase, 832, 964
Photopsins, 631
Physical activity. See also Exercise.
energy for, 887, 887t
obesity and, 872, 873–874
Physiologic dead space, 478, 500–501
Physiologic shunt, 500, 501, 503, 503f
Physiology, 3
Physostigmine, 89
Pilocarpine, 759
Piloerection, 97–98, 895
Pineal gland, 906f, 1009–1010
Pinna,

n, 97–98, 895
Pinna, 660
Pinocytosis, 19, 19f, 330, 813
Pituicytes, 927
Pituitary gland, 906f
anterior, 918–919, 919f
ACTH secretion by, 955–957, 956f
blood supply to, 734
cells of, 919, 919f, 920t
feedback regulation of, 939, 939f
Pituitary gland (Continued)
gonadotropins of. See Follicle-
stimulating hormone; Luteinizing
hormone.
growth hormone of, 921–927. See also
Growth hormone.
hormones of, 907t, 918, 919f
hypothalamic regulation of, 9

 of, 907t, 918, 919f
hypothalamic regulation of, 919–921,
920f, 921t
hypothalamic-hypophysial portal
vessels of, 920–921, 920f
inhibin effects on, 1007, 1015, 1020
neural control of, 733f, 734
thyroid-stimulating hormone of,
938–939
antidiuretic hormone release by, 359–360,
359f
corticotropes of, 919, 920t
fetal, 1036
gonadotropes of, 919, 920t
lactotropes of, 919, 920t
posterior, 919, 919f, 927–929, 928f
antidiuretic hormone of, 927–929. See
cells of, 927
hormon

o Antidiuretic hormone (ADH).
hormone secretion by, 907t
920f, 921t, 927–928, 928f
oxytocin of, 927, 929. See also Oxytocin.
somatotropes of, 919, 920t
thyrotropes of, 919, 920t
Pituitary stalk, 927
Placenta, 1029–1031, 1029f
anatomy of, 1029–1030, 1030f
blood ﬂow in, 1035
carbon dioxide diffusion through, 1031
delivery of, 1038
estrogen secretion by, 1032–1033
hormones of, 907t
human chorionic somatomammotropin
secretion by, 1033
membrane of, 1

omatomammotropin
membrane of, 1030
nutrient diffusion through, 1031
oxygen diffusion through, 1030–1031,
1031f
permeability of, 1030
progesterone secretion by, 1033
waste excretion through, 1031
Plaque, atherosclerotic, 848, 849f
Plasma, 293. See also Blood.
agglutinins in, 452
amino acids of, 854–855. See also Plasma
proteins.
anion gap of, 400, 400t
calcium in, 371–372, 372f
cholesterol of, 848
chylomicrons of, 841
corrected osmolar activi

n of, 293–295, 294f, 294t
creatinine concentration of, 344–345, 346f
fatty acids in, 841
lipoproteins of, 841
loss of, shock and, 284–285
nonelectrolytes of, 294f
358–360, 358f, 360t
plasma sodium and, 358
osmotic pressure of, 294t, 298. See also
Colloid osmotic pressure.
proteins of. See Plasma proteins.
renal ﬂow of, 345–346, 346f
transfusion

 proteins.
transfusion of, in shock, 286
vs. serum, 460
Plasma cells, 429, 443
Plasma colloid osmotic pressure, 185, 185f,
188
blood viscosity and, 169–170
carbon dioxide reaction with, 511
colloid osmotic pressure and, 185–186,
185f, 188–189
decrease in, 303–304
formation of, 855, 862
functions of, 855–857, 855f, 856f
macrophage processing of, 855
tissue protein equilibrium wi

e processing of, 855
tissue protein equilibrium with, 855, 855f
Plasmablasts, 443
Plasmalemmal vesicles, 182, 182f
Plasmin, 464
Plasminogen, 464
Platelet(s), 421f, 429
deﬁciency of, 465
formation of, 430f, 431, 457
functions of, 457–458
in clot retraction, 460
life span of, 431, 458
membrane of, 458
Platelet plug, 457–458
cis-Platinum, renal toxicity of, 406
Pleural cavity
ﬂuid in. See Pleural ﬂuid.
Pleural ﬂuid, 472, 489–490, 490f
effusion of, 140–141, 490
Ple

, 472, 489–490, 490f
Pleural pressures, 489–490
cardiac output and, 237–238, 238f
in respiration, 472, 472f
Pleural space, 489
Pluripotential hematopoietic stem cells, 420,
421f, 430, 440
Pneumonia, 527–528, 528f
Pneumotaxic center, 514, 515, 515f
Po2. See Oxygen, partial pressure of.
Podocytes, 316, 316f
Poiseuille’s law, 168, 168f
Poison ivy, 449
Polarography, 525
Poliomyelitis, swallowing in, 819
Polycystic ovary syndrome, 974
blood viscosity and, 169, 1

rome, 974
blood viscosity and, 169, 169f
hematocrit in, 169f, 293
physiologic, 427–428
secondary, 427–428
Polycythemia vera, 428, 531
Polymenorrhea, 938
Polyneuritis, 876
Polyphagia, 974
Polyribosomes, 33
Pons, 558. See also Brain stem.
Pontine reticulospinal tract, 692, 692f
Portal hypertension, 860
Portal vein, 860
Position sensation, 585, 594–595, 595f
baroreceptors and, 210
muscle spindles in, 595, 678. See also
Muscle spindles.
transmission of, 59

678. See also
transmission of, 595, 595f
Positive feedback, 8–9, 8f
Positive supportive reaction, 682
Postural reﬂexes, 682
Potassium. See also Hyperkalemia;
Hypokalemia.
active transport of, 53–54, 53f, 59–60, 60f
1102
Index
Potassium (Continued)
channel leakage of, 60, 60f
depletion of, 368
excretion of, 328t, 365–367, 366f, 366t, 367f
sodium intake and, 370–371, 371f
exercise-related loss of, 1065
extracellular, 46f, 294f, 294t, 365–371
aldosterone blockade

lar, 46f, 294f, 294t, 365–371
369f, 370f, 950
disorders of. See Hyperkalemia;
distribution of, 366–367, 366f, 366t
exercise and, 366
myocardial infarction and, 254
normal value for, 7t, 365, 370, 370f
osmolarity and, 366–367
renal potassium secretion and, 368,
369–370, 369f, 370f
in acidosis, 366, 371
in action potential, 61–65, 61f–64f, 66
in cardiac 

 bone, 981
in cardiac muscle function, 105, 113–114
in diffusion potential, 57, 58f
796f
in membrane potential, 57, 58f, 59–60, 60f
in resting membrane potential, 60, 60f
intake of, 367
intracellular, 4, 46f, 294f, 294t, 365,
366–367, 366f, 366t
neuronal concentratio

5f
reabsorption of, 328t, 338f, 367, 367f
potassium depletion and, 368
renal ﬁltration of, 328t, 367, 367f
secretion of, 338f, 367–371, 367f
acidosis and, 371
aldosterone and, 342, 342t, 366,
369–370, 369f, 370f, 948
distal tubular ﬂow rate and, 370–371,
371f
extracellular potassium concentration
and, 368, 369–370, 369f, 370f
principal cells in, 336–337, 336f, 337f,
367–368, 368f
urinary, 367
Potassium channel, 47–48, 4

368, 368f
Potassium channel, 47–48, 47f
of cardiac muscle, 67–68, 67f, 117
of neuron, 566, 566f
of postsynaptic neuron, 561, 561f, 562
voltage clamp testing of, 63, 63f
voltage-gated, 62–63, 62f, 64, 64f, 66–67
activation of, 62–63, 62f
Potassium citrate, 467
Potassium-sodium leak channel, 60, 60f
Potassium-sparing diuretics, 337, 403t, 404
Potential spaces
capillary ﬂuid exchange with, 305–306
ﬂuid in, 305–306
hydrostatic pressure of, 306
protein of, 306
Power law, 594, 594f, 657
P

f, 306
P-Q interval, 125
P-R interval, 125
in atrioventricular heart block, 149, 149f
Prealbumin, 934
Precapillary sphincter, 181, 182f, 197, 197f
Precellular life forms, 18
Precipitation, antigen-antibody, 444
Prednisolone, 947t
Prednisone, 947
Preeclampsia, 227–228, 1035–1036
Prefrontal association area, 716–717, 716f,
719–720
injury to, 720
Prefrontal cortex, in feeding behavior, 87

to, 720
Prefrontal cortex, in feeding behavior, 870
Prefrontal lobes, in schizophrenia, 746
Prefrontal lobotomy, 720
Pregnancy. See also Fetus.
aldosterone secretion in, 1034
anterior pituitary gland in, 1034
blood volume during, 1035, 1035f
blood volume in, 381
cardiac output during, 1035
eclampsia in, 1035–1036
human chorionic gonadotropin in, 1007,
human chorionic somatomammotropin in,
1033
metabolism during, 1034
nutrition during, 1034–1035
oxytocin in, 929
parathyroid hormone in

 1034–1035
parathyroid hormone in, 1034
placenta in. See Placenta.
placental estrogen in, 1032–1033
placental progesterone in, 1033
preeclampsia in, 227–228, 1035–1036
relaxin in, 1034
respiration during, 1035
thyroid hormones in, 1034
toxemia of, 227–228, 1035–1036
urine formation during, 1035
weight gain in, 1034
Prekallikrein (Fletcher factor), 459t
Preload, 111
Prelymphatics, 190
Premature atrial contractions, 150, 150f
Premature atrioventricular contractions,
150–151, 151f

ture atrioventricular contractions,
Premature contractions, 150–151, 150f,
151f
atrial, 150, 150f
atrioventricular, 150–151, 151f
ventricular, 151, 151f
Prematurity, 1050–1051
Premotor area, 686, 686f
Preprohormones, 906
Presbyopia, 618
Pressoreceptors. See Baroreceptor(s).
Pressure(s)
alveolar, 472–473, 472f
aortic, 109, 173, 173f
arterial. See Arterial blood pressure.
capillary, 185–187, 185f, 186f. See also
Capillary pressure.
cardiac, 110–111, 110f, 162. See also at
Atrium (atr

rdiac, 110–111, 110f, 162. See also at
Atrium (atria); Ventricle(s),
cardiac.
cerebrospinal ﬂuid, 213, 213f, 765–766
circulatory, 162–163, 163f
diastolic, 110–111, 110f
difference in, 164, 164f
end-diastolic, 110, 111
ﬁltration, 185–186, 185f, 189
hydrostatic. See Hydrostatic pressure.
in dural sinuses, 178
interstitial. See Interstitial ﬂuid pressure.
intra-abdominal, 177
intramyocardial, 250, 250f
intraocular, 624–625, 624f
Pressure(s) (Continued)
leg, 177, 177f, 178–179, 178f
osmotic, 51–52, 

ed)
osmotic, 51–52, 52f. See also Osmotic
pressure.
partial, 491–492. See also Carbon dioxide,
partial pressure of; Oxygen, partial
pressure of.
pleural, 237–238, 238f, 472, 472f, 489–490
portal vein, 860
pulmonary artery, 109, 162–163, 163f, 484,
484f
reabsorption, 189
receptors for. See Baroreceptor(s).
recoil, 472f, 473
right atrial, 176, 177, 179, 240–243,
240f–243f. See also Cardiac output.
sensation of, 585–587
systolic, 110–111, 110f
transpulmonary, 472f, 473

tricuspid valve, 179, 179f
under water, 545, 546f
vapor, 492
venous. See Venous pressure.
ventricular, 107f, 108, 110–111, 110f,
483–484, 484f
Pressure difference
in blood ﬂow, 164, 164f
in diffusion, 493
Pressure diuresis, 216, 323, 341–342,
Pressure natriuresis, 216, 323, 341–342,
angiotensin II in, 

rone escape and, 948–949, 948f
Pressure pulse, 173, 173f
abnormal, 173f, 174
aortic, 173, 173f
damping of, 174–175, 174f
peripheral transmission of, 174–175,
174f
Presynaptic membrane, 560
Presynaptic terminals, 559–560, 559f, 560f
Primordial follicle, 1013, 1013f
Principal cells, 336–337, 336f
potassium secretion by, 367–368, 368f
Procaine, 70
Procarboxypolypeptidase, 799
Procaspases, 40
Procoagulants, 459
Proelastase, 810, 810f
Proerythrobl

oagulants, 459
Proerythroblasts, 421f, 423
Progesterone, 907t
breast effects of, 1018
degradation of, 1017
fallopian tube effects of, 1018
in female reproduction, 1011, 1013f, 1016,
1016f, 1018, 1019
placental secretion of, 1033
synthesis of, 1016
uterine effects of, 1018
Prognostication, 720
Prohormones, 906
Proinsulin, 962
Prolactin, 907t, 918, 919f, 920t
in lactation, 1039–1040, 1040f
Prolactin inhib

039–1040, 1040f
Prolactin inhibitory hormone, 921, 921t,
1040
Prolactin-inhibiting hormone, 907t
Prolyl hydroxylase, 877
Prometaphase, 37f, 39
Index
1103
Promoter, 31, 35, 35f
Proopiomelanocortin, 956–957, 957f
Pro-opiomelanocortin neurons, 868–870,
869f
Prophase, 37f, 39
Propranolol, 759
Proprioceptive receptors, 594–595, 696. See
also Muscle spindles; Pacinian
corpuscles.
Propriospinal ﬁbers, 675
Propylthiouracil, 939
Prorenin, 223
Prosecretin, 801
Prosophenosia, 717
Prostag

n, 223
in male reproduction, 999
Prostate gland, 996, 997f
function of, 999
tumors of, 1008
Prostatic ﬂuid, 1000
Protanope, 633, 634f
Protein(s)
activator, 35f, 36
amino acids of, 852–854, 853f. See also
Amino acids.
carrier, 13, 45, 46f, 47, 49–50, 53–54, 53f,
831
channel, 45, 46f, 47–49, 47f, 48f
chromosomal, 38
complete, 857
daily requirement for, 857, 866
deﬁciency of, 925, 9

ily requirement for, 857, 866
deﬁciency of, 925, 925f
dietary, 866, 866t, 867
digestion of, 808–809, 810–811, 810f
pancreatic digestion of, 810, 810f
pepsin digestion of, 810, 810f
urine concentration and, 353
differentiation, 420
endoplasmic reticulum formation of, 20,
21f
extracellular, 46f, 294f, 294t
functional, 11–12
hydrolysis of, 808–809, 810–811, 810f
incomplete, 857
inte

is of, 808–809, 810–811, 810f
integral, 13, 14, 14f
interstitial, 188–189, 193–194, 294t
intracellular, 11–12, 46f, 294f, 294t,
387–388
lymph, 192
membrane, 13–14, 14f
metabolism of, 852–856
cortisol effects on, 857, 952
deamination in, 856
hormonal regulation of, 857
insulin and, 

, 857
insulin and, 857, 966–967
liver in, 862, 952
measurement of, 866
oxidation in, 856–857
thyroid hormones and, 857, 934, 935f
obligatory degradation of, 857
partial, 857, 866
peripheral, 13, 14f
plasma. See Plasma proteins.
Protein(s) (Continued)
potential space, 306
reabsorption of, 330
repressor, 35–36, 35f
speciﬁc dynamic action of, 887
starvation effects on, 857
structural, 11
synthesis of, 20, 33–35, 34f, 854–855
aldosterone and, 949–95

f, 20, 33–35, 34f, 854–855
aldosterone and, 949–950
chemical steps in, 33–34, 34f
estrogen and, 857, 1017–1018
growth hormone and, 857, 922
steroid hormones and, 915
testosterone and, 857, 1005, 1006
tissue, 855, 855f, 857
transport, 45, 816. See also Protein(s),
membrane.
triglyceride formation from, 845
urinary, 317, 409
Protein C, 463
Protein channels, 47–49, 47f
gating of, 48–49, 48f
ligand-gated, 48
patch-clamp recording of, 48f

48f
patch-clamp recording of, 48f, 49
selective permeability of, 47–48, 47f
voltage-gated, 48–49, 48f
Protein kinase C, 914, 914f
Proteinuria, 317, 409
Proteoglycan, 14
Proteoglycan ﬁlaments, of interstitium,
184–185, 184f, 305
Proteolytic enzymes, 432, 1000
Prothrombin (factor II), 459–460, 459f,
459t
Prothrombin activator, 459, 459f, 461–464,
461f, 462f
Prothrombin time, 467–468, 467f
Protoplasm, 11–12
Psychic blindness, 870
Psychogenic diarrhea, 823
Psychomotor epilepsy, 744f

ychogenic diarrhea, 823
Psychomotor epilepsy, 744f, 745
Psychosis, 745–746
Pteroylglutamic acid. See Folic acid.
Ptyalin, 793, 809, 809f
Puberty
female, 1021–1022, 1022f
Pudendal nerve, 311f, 312, 313, 789
Pulmonary artery, 483
distensibility of, 171
Pulmonary artery pressure, 109, 162–163,
163f, 484, 484f
Pulmonary capacities, 475–476, 475f, 476f,
477t
Pulmonary capillaries
blood ﬂow through, 485–486, 487
dia

ry capillaries
diameter of, 497
dynamics of, 487–489, 488f, 489f
ﬂuid exchange in, 487–488, 488f
oxygen diffusion into, 502–503, 503f
pressures in, 162–163, 163f, 484, 484f, 487,
488, 488f
size of, 496, 497f
Pulmonary capillary pressure, 162–163, 163f,
484, 484f, 487, 488, 488f
Pulmonary circulation, 483–490
alveolar oxygen and, 485
cardiac output and, 486–487, 487f
distribution of, 162f, 485–487, 485f–487f
exercise effect on, 485f, 486–487, 487f
gas exchange in. 

e effect on, 485f, 486–487, 487f
Pulmonary circulation (Continued)
gaseous interface with. See Respiratory
membrane.
hydrostatic pressure gradients in, 485–487,
485f, 486f, 488, 488f
in left-sided heart failure, 487
left atrial pressure and, 487
oxygen absorption and, 244, 244f
physiologic anatomy of, 483
pressures in, 162–163, 163f, 483–484, 484f
shifts in, 485
vascular resistance in, 167
zones of, 485–486, 486f
Pulmonary edem

nce in, 167
Pulmonary edema, 488–489
in cardiac failure, 262, 264
in unilateral left-sided cardiac failure, 262
Pulmonary embolism, 466
Pulmonary function studies, 475–477, 475f,
476f, 477t
Pulmonary membrane. See Respiratory
membrane.
Pulmonary valve, 109
congenital stenosis of, 138–139, 139f
Pulmonary vascular resistance, 167. See also
Pulmonary vein, 483
dist

also
pressure in, 484
Pulmonary ventilation, 471–481. See also
Alveolar ventilation.
alveolar pressure in, 472–473, 472f
lung compliance in, 473–474, 473f
mechanics of, 471–475, 472f
pleural pressure in, 472, 472f
spirometric evaluation of, 475–477, 475f,
476f, 477t
surface tension in, 474
surfactant in, 474
thoracic cage in, 474–475
transpulmonary pressure in, 472f, 473
Pulmonary volumes, 475, 475f, 476f, 477t
Pulmonary wedge press

lumes, 475, 475f, 476f, 477t
Pulmonary wedge pressure, 484
Pulp cavity, 992f, 993
Pulse deﬁcit, 150
Pulse pressure, 173
abnormal, 174, 174f
age-related changes in, 173, 173f
Punishment centers, 725, 735–736
Pupil. See also Eye(s).
Argyll Robertson, 650
649–650, 753, 754t
diameter of, 618–619, 618f, 632, 649–650
in dark/light adaptation, 632
in Horner’s syndrome, 650
light reﬂex of, 649
Pupillary light reﬂex, 649
Purines, 27, 28f, 29, 29f, 37
Purkinje cells, o

49
Purkinje cells, of cerebellum, 701–702, 702f,
703
Purkinje ﬁbers
action potential transmission in, 117f, 119,
120f, 121
blockade of, 141
in atrioventricular (A-V) block, 121
Purpura, thrombocytopenic, 465
Pus, 435–436
Putamen, 688, 708, 708f
lesions of, 709
Putamen circuit, 708–709, 708f
Pyelonephritis, 409
1104
Index
Pyloric gland, 797–798. See also Gastrin.
Pyloric pump, 785
Pyloric sphincter, 774, 785
Pylorus, in stomach empty

oric sphincter, 774, 785
Pylorus, in stomach emptying, 785
Pyramidal (corticospinal) tract, 687–688,
688f
Pyridostigmine, 759
Pyridoxine, 875t, 877
Pyrimidines, 27, 28f, 29, 29f, 37
Pyrogens, 898–899
Pyrophosphate, 981
Pyruvic acid, 22, 22f
acetyl coenzyme A conversion from, 23,
833, 833f
formation of, 22–23, 833, 833f
lactic acid conversion from, 837
Q
Q wave, 107, 107f, 123, 124f
QRS complex, 107, 107f, 123, 124–125, 124f,
125f
bizarre patterns of, 139f, 141
current of injury e

current of injury effect on, 141–143, 142f
high-voltage, 138f, 139f, 140
151, 151f
low-voltage, 140–141, 140f
mean electrical axis of, 137–140, 137f–140f
prolongation of, 139f, 140f, 141
vectorial analysis of, 134, 135f
QRS vectorcardiogram, 137, 137f
Q-T interval, 125–126
R
R wave, 107, 107f, 123, 124f, 134, 135f
Radiation (heat), 891, 891f
Radiation exposure
Radiation therapy, leukopenia an

Radiation therapy, leukopenia and, 437
Radioimmunoassay, for hormone
measurement, 915–916, 916f
Rage, 735–736, 737, 738, 758
Ramp signal, 514–515
Ranitidine, in peptic ulcers, 821
Ranvier, node of, 68, 68f
Raphe nuclei, 730f, 731, 740
lesions in, 740
Rathke’s ouch, 918
Reading, 716, 716f, 719, 721f, 722
Reagins, 449
Receptor(s), 14
acetylcholine, 86–87, 86f, 87f, 752
adrenergic, 752, 753t
aldosterone, 949–950
enzyme-linked, 911–912, 912f, 913–914,
913f, 913t, 914f, 914t
G prote

912, 912f, 913–914,
hormone, 14, 910–912, 911f–913f. See also
Hormone(s), receptors for.
insulin, 962–963, 963f
intracellular, 912, 913f
ion channel, 910–911
leptin, 911–912, 912f
low-pressure, 212
muscarinic, 752
nicotinic, 752
sensory. See Sensory receptors.
stretch. See Baroreceptor(s).
thyroid hormone, 934, 935f
Receptor potential, 573–576, 574f, 575f
action potential and, 574, 574f. See also

and, 574, 574f. See also
maximum amplitude of, 574
mechanisms of, 574
of hair cells, 656
of pacinian corpuscle, 574–575, 574f,
575f
of rods, 629–630, 630f
of taste buds, 665
stimulus intensity and, 574–575, 575f
Reciprocal inhibition circuits, 580, 581f, 680f,
681–682, 682f, 691
Recoil pressure, 472f, 473
Rectum
emptying of, 789–790
empyting of, 757, 790f, 822, 823
Rectus muscles, of eye, 645, 645f
Red blood cell(s), 419–426
antigens of, 451–453, 452f, 452t, 453t
carbon

–426
carbonic anhydrase of, 419
concentration of, 419
destruction of, 180, 426
diameter of, 419
differentiation of, 420–422, 421f
fetal, 1042
functions of, 419
hemoglobin of, 419–420, 424–425, 424f.
See also Hemoglobin.
high altitude effects on, 422, 539
in pulmonary capillaries, 496, 497f
life span of, 426
maturation of, 423
neonatal, 1047–1048, 1048f
packed volume of (hematocrit), 293
production of, 420–423, 420f, 421f
cyanocobala

293
cyanocobalamin in, 423
erythropoietin in, 422–423, 422f
folic acid in, 423
hypoxia and, 422–423, 422f, 427–428
lung disease and, 422
regulation of, 422–423, 422f
sites of, 420, 420f
tissue oxygenation and, 422
vitamin B12 in, 423
shape of, 419, 421f
size of, 419
splenic storage of, 179–180, 180f
transfusion of, 451–455, 452f, 452t, 453t
volume of, 419
Red nucleus, 688–689, 689f, 690, 705
Red pulp, 180, 180f, 434
Red-green color

 690, 705
Red-green color blindness, 633
Referred pain, 603–605, 603f, 605f
Reﬂex(es)
abdominal compression, 213
atrial, 212
attenuation, 652
autonomic, 757
Bainbridge, 212, 233
baroreceptor, 209–212, 757. See also
Baroreceptor(s).
bladder emptying, 757, 758
chemoreceptor, 211–212. See also
Chemoreceptor(s).
chewing, 781
colonoileal, 776
cough, 480
crossed extensor, 680f, 681, 681f
defecation, 757, 758, 789–790, 790f, 822
duodenocolic, 789
enterogastric, 776, 785–786, 79

enterogastric, 776, 785–786, 799
reverse, 799
ﬂexor, 680–681, 680f, 681f
Reﬂex(es) (Continued)
galloping, 683
gastrocolic, 776, 789
gastroenteric, 787
gastroileal, 787
gastrointestinal, 775–776
Hering-Breuer, 515–516
light, 649
mark time, 682–683
mass, 684
micturition, 313–314
myenteric, 776–777
peristaltic, 776–777
peritoneointestinal, 790
postural, 682
renointestinal, 790
scratch, 587, 683
sneeze, 480
stretch, 676–677, 677f, 678–679, 678f
swallowing, 783
ur

h, 676–677, 677f, 678–679, 678f
ureterorenal, 312
vesicointestinal, 790
visceral, 748
volume, 212
Refraction, 613–615, 614f, 615f
errors of, 619–621, 619f, 620f
Refractive index, 613, 617, 617f
Refractive power, 616–617, 616f
Regression, tissue, 20
Regulatory genes, 36
Regulon, 36
Rehearsal, memory and, 726, 737
Reissner’s membrane, 652–653, 652f, 653f
Relaxin, 1034
Renal arteries, 310
constriction of, renin secretion and,
226–227, 226f
disease of, 408, 413
ﬁbromuscular hyperplas

, 226f
ﬁbromuscular hyperplasia of, 408
pressure in, 320–321, 321t
Renal failure. See Kidneys, failure of.
Renal function curve, 217, 217f
shift in, 218–219, 218f
sodium-loading, 229, 229f
Renal papillae, 309, 309f, 310
Renal pelvis, 309, 309f
Renal plasma ﬂow, 345–346, 346f
Renal pyramids, 309, 309f
Renal tubular acidosis, 397
Renal tubules, 309f, 310, 310f. See also
Glomerulus; Kidneys; Loop of Henle.
394f
failure of, 357–358, 414
in urin

in, 393–394,
394f
in urine concentration, 348–349,
350–351, 355–357, 928
osmoreceptor interaction with, 358–361,
358f–360f
bicarbonate titration by, 392
connecting, 310, 310f
cortical collecting, 310, 310f, 327
antidiuretic effects on, 928
bicarbonate formation in, 393–394,
394f
chloride reabsorption in, 336–337, 336f,
337f
hydrogen ion secretion by, 392, 392f
in urine concentration, 353, 353f, 356,
356f, 928
in urine dilution, 349f,

353, 353f, 356,
356f, 928
in urine dilution, 349f, 350
osmolarity in, 349f, 350, 356, 356f
potassium secretion by, 336–337, 336f,
337f, 367–371, 367f–371f
resorption by, 336–337, 336f, 337f
Index
1105
Renal tubules (Continued)
sodium reabsorption by, 336–337, 336f,
337f
disorders of, 413–414
distal, 309f, 310, 310f, 327
cells of, 54, 336–337, 336f
ﬂow rate of, potassium secretion and,
370–371
hydrogen ion secretion by, 54, 391, 391f,
392, 392f
in urine concentration,

, 54, 391, 391f,
392, 392f
in urine concentration, 352–353, 353f,
356, 356f
osmolarity in, 350, 356, 356f
potassium secretion by, 367–371,
367f–371f
solute resorption in, 336–337, 336f, 337f
excretion by, 314–315, 314f
acid-base, 390–400. See also Acid-base
balance, renal regulation of;
calcium, 371–372, 372f, 373t, 980, 985,
987
clearance method quantiﬁcation of,
343–347, 344t, 345f, 346f
magnesium, 373
phospha

,
phosphate, 372–373, 980, 985, 987
potassium, 328t, 365–367, 366f, 366t,
367, 367f
rates of, 357
sodium, 307–308, 308f. See also Sodium,
excretion of; Urine, concentration
of.
toxin, 307
urea, 307, 353–354
water. See at Urine.
inulin concentrations in, 339
medullary collecting duct, 309f, 310, 310f
bicarbonate formation in, 393–394, 394f
cells of, 337–338, 338f
in urine concentration, 353, 353f,
356–357, 356f
osmolari

e concentration, 353, 353f,
osmolarity of, 349f, 350, 356–357, 356f
reabsorption by, 337–338, 338f
necrosis of, 406
net acid excretion by, 394–395
potassium processing by, 367–371,
367f–371f. See also at Potassium.
proximal, 309f, 310, 310f, 327
amino acid transport in, 330, 330f
bicarbonate formation in, 393, 394f
390f–392f
bile salt secretion by, 334
catecholamine secretion by, 334
cells of, 333, 333f
chloride rea

secretion by, 334
chloride reabsorption in, 333–334, 334f
glucose transport in, 330, 330f
hydrogen ion secretion by, 390–392,
391f, 392f
organic acid secretion by, 334
osmolarity in, 349, 349f, 355, 356f
oxalate secretion by, 334
PAH secretion by, 334
protein reabsorption by, 330
sodium reabsorption in, 333–334, 333f,
334f
solute concentrations in, 334, 334f
toxin secretion by, 334
urate secretion by, 334
water reabsorption in, 332, 334
reabsorption by, 327–346, 328t
Renal tu

, 332, 334
active transport in, 328–332, 328f–331f
aldosterone effect on, 342, 342t
amino acid, 330, 330f, 413, 854
angiotensin II effect on, 342–343, 342t
antidiuretic hormone effect on, 342t,
343
arterial pressure in, 341–342
atrial natriuretic peptide effect on, 342t,
343
bicarbonate, 337, 390–392, 390f–392f,
404
calcium, 343, 372, 372f, 373t
calculation of, 339–340, 346–347
carbonic anhydrase inhibitor effect on,
404
chloride, 332–333,

ydrase inhibitor effect on,
404
chloride, 332–333, 332f, 336, 336f
cortical collecting tubule in, 336–337,
337f, 338–339, 338f
co-transport in, 329–330, 330f, 333–334
counter-transport in, 330, 330f, 333
diffusion in, 332–333, 332f
distal tubule in, 336–337, 336f, 338–339,
338f
ﬁltration coefﬁcient in, 340
glomerular ﬁltration rate and, 327–328,
328t
glomerulotubular balance in, 339
glucose, 330, 330f, 331, 331f, 413
gradient-time transport in, 331–332
hormonal control of, 

t-time transport in, 331–332
hormonal control of, 342–343, 342t
increase in, 339
interstitial pressures in, 340–341
loop of Henle in, 334–336, 335f,
338–339, 338f
medullary collecting duct in, 337–338,
338f
net, 346–347
osmosis in, 332
paracellular pathway in, 329, 332, 335
parathyroid hormone effect on, 342t,
343
passive mechanisms of, 332–333,
332f
peritubular capillary hydrostatic
pressure in, 340, 341f, 341t
phosphate, 372–373
physical forces in, 339–341, 340f
pinocytosis i

13
pinocytosis in, 330
potassium, 337, 337f, 367, 367f
protein, 330
proximal tubule in, 333–334, 333f, 334f,
346f
rate of, 339–340
regulation of, 339–343, 340f, 341f, 341t,
342t
selectivity of, 327–328, 328t
sodium, 333–334, 333f, 334f. See also
Sodium, reabsorption of.
solvent drag in, 332
summary of, 338, 338f
343
transcellular pathway in, 329
transport maximum for, 330–

llular pathway in, 329
transport maximum for, 330–331, 331f,
332
tubular load in, 330–331, 331f, 339
urea, 333, 338, 351t, 353–354, 354f
water, 224–225, 332, 334. See also Water,
reabsorption of.
secretion into, 314–315, 314f, 315f, 327
active transport in, 330
hydrogen ion, 390–392, 390f–392f. See
also Hydrogen ions, secretion of.
potassium, 367–371, 368f, 369f. See also
Potassium, secretion of.
toxin effects on, 334, 406
Renal urinary output curve (renal function
curv

06
curve), 217, 217f
Renal vein, 310
Renin, 223–226, 223f, 224f, 907t
chronic, 227
renal artery constriction and, 226–227,
226f
tumor-related, 226
Renin-angiotensin system, 223–230, 223f,
230f, 231. See also Angiotensin II.
aldosterone secretion and, 950, 951f
salt retention and, 224–226, 225f, 226f
vasoconstrictor p

tention and, 224–226, 225f, 226f
vasoconstrictor pressure response to, 224,
224f
water retention and, 224–226, 226f
Renointestinal reﬂex, 790
Renshaw cells, 675
Repressor operator, 35–36, 35f
Reproduction, 6
cell, 37–40, 37f
female, 1011–1025. See also Sexual
function, female.
anatomy of, 1011, 1012f
estradiol in, 1016–1018, 1016f
follicle-stimulating hormone in,
1012–1013, 1013f, 1020
gonadotropin-releasing hormone in,
1020
hormones in, 1011–1012
hypothalamic-pituitary-ov

0
hypothalamic-pituitary-ovarian
feedback system in, 1019–1021,
1020f
inhibin in, 1020
luteinizing hormone in, 1012–1013,
menstrual cycle in, 1012–1015, 1013f
progesterone in, 1016, 1016f, 1019
homeostasis and, 6
male, 996–1010. See also Sexual function,
male.
abnormalities of, 1001, 1001f
anatomy of, 996, 997f
hormones in, 1003–1008, 1003f, 1004f,
1008f
pineal gland in, 1009–1001
prostate gland in, 999
semen i

gland in, 1009–1001
semen in, 999–1001
seminal vesicles in, 999
spermatogenesis in, 996–1001, 997f, 998f
Reserpine, 759
Residual body, 20, 20f
Residual volume, 475, 476f, 477, 529
Resin agents, 851
Respiration, 471–481. See also Alveolar
ventilation; Pulmonary ventilation.
arterial pressure and, 214
artiﬁcial, 532, 532f
bronchi in, 479, 479f
bronchioles in, 479–480, 479f
disorders of. See Respiratory distress
syndrome; Respiratory insufﬁciency.
1106
Index
Re

1106
Index
Respiration (Continued)
energy for, 475
fetal, 1042–1043
in acid-base balance, 388–389, 388f, 389f,
396, 397
in exercise, 516, 520–521, 520f, 521f,
in neonate, 1047
in pregnancy, 1035
lung compliance and, 473–475, 473f
muscles of, 471–472, 472f
nose in, 479f, 480–481
pressure of, 472–473, 472f
rate of, 477
regulation of, 514–523
anesthesia and, 522
brain edema and,

n of, 514–523
brain edema and, 521
carbon dioxide in, 516–517, 516f, 517f,
518
chemical, 516–517, 516f, 517f
chemosensitive area in, 516–517,
516f
during exercise, 520–521, 520f, 521f
hydrogen ions in, 516–517, 516f, 517f,
518
irritant receptors in, 521
J receptors in, 521
oxygen in, 517, 518–520, 518f, 519f
peripheral chemoreceptors in, 518–520,
518f, 519f
respiratory center in, 514–516, 515f
voluntary, 521
reserve of, 1066
speech and, 481, 481f
spirometric evaluation of, 47

peech and, 481, 481f
476f, 477t
surfactant and, 474
swallowing and, 783
thoracic compliance in, 474–475
trachea in, 478–479
voluntary control of, 521
Respiratory acidosis. See Acidosis,
respiratory.
Respiratory alkalosis. See Alkalosis,
Respiratory center, 514–516, 515f
acclimatization effects on, 519, 539
chemosensitive area of, 516–517, 516f
disorders of, 523
dorsal neurons of, 514–515, 515f
exercise effects

exercise effects on, 520–521, 520f
in acid-base balance, 384
pneumotaxic center of, 515, 515f
ventral neurons of, 515, 515f
Respiratory distress syndrome, 1045, 1050
of newborn, 474, 529
Respiratory exchange ratio, 512, 867
Respiratory insufﬁciency, 524–532
artiﬁcial respiration in, 532, 532f
blood CO2 in, 524–525
blood pH in, 524
blood Po2 in, 525
cyanosis in, 531
dyspnea in, 532
forced expiratory vital capacity in, 526,
526f
forced expiratory volume in, 526, 5

 in, 526,
526f
forced expiratory volume in, 526, 526f
hypercapnia in, 531–532
in airway obstruction, 526–527, 526f, 527f,
528f, 529
Respiratory insufﬁciency (Continued)
in chronic pulmonary emphysema,
526–527, 527f, 528f
in pneumonia, 527–528, 528f
in tuberculosis, 530
maximum expiratory ﬂow in, 525–526,
525f, 526f
Respiratory membrane
gas exchange through,

g capacity of, 498–499, 499f
gas exchange through, 496–499, 496f, 497f.
See also Carbon dioxide, transport of;
diffusing capacity and, 498–499, 499f
rate of, 498
pressure difference across, 498
structure of, 497, 497f
surface area of, 498
thickness of, 498
Respiratory quotient, 867
Respiratory rate, 477
Respiratory system, 5, 478–481, 479f, 481f.
See also Respiration.
water loss through, 292
Respiratory unit (lobule), 486f, 496, 496f
Reticular excitatory area, 728–729, 729f

496, 496f
Reticular excitatory area, 728–729, 729f,
730f, 731
Reticular formation, in pain sensation, 601
Reticular inhibitory area, 729, 729f
Reticular lamina, 655, 655f
Reticular nuclei, 691–692, 691f
Reticular substance, 206f, 207
Reticulocyte, 421f, 422
Reticuloendothelial system, 180, 432–434,
433f, 434f
Retina. See also Eye(s); Vision.
action potential of, 637–638, 637f
amacrine cells of, 627f, 634, 635, 635f, 636
anatomy of, 626–628, 627f, 628f
bipolar cells of, 627f, 633, 635, 635f, 636,

 628f
638f
blood supply of, 628
central fovea of, 626, 636
cones of, 626–628, 627f, 628f, 631, 631f,
632–633, 633f, 635f
detachment of, 628
electrotonic conduction in, 635
fovea of, 626, 627f, 635, 635f, 636
ganglion cells of, 627f, 634, 635, 635f,
636–638. See also Ganglion cells.
glutamate in, 635
horizontal cells of, 627f, 633, 635f, 638f
image formation on, 617
interplexiform cells of, 634
lateral inhibition in, 6

erplexiform cells of, 634
lateral inhibition in, 635–636, 636f
layers of, 626, 627f
neural circuitry of, 633–636, 635f, 636f
neurotransmitters of, 635
ophthalmoscopic examination of, 622–623,
622f
peripheral area of, 635, 635f, 637
pigment layer of, 628
retrolental ﬁbroplasia of, 200
rods of, 626–628, 627f, 628f, 629–631,
629f–631f, 633, 635f, 638f
vitamin A in, 629
W cells of, 637
X cells of, 637
Y cells of, 637
Retinal, 629
vitamin A interaction with, 629, 62

7
vitamin A interaction with, 629, 629f
Retinal artery, 628
Retinal isomerase, 629, 629f
Retinitis pigmentosa, 645
Retinol, 875. See also Vitamin A (retinol).
Retrograde amnesia, 725
Retrolental ﬁbroplasia, 200, 1051
Reverberatory circuit, 581–582, 581f–583f
Reverse enterogastric reﬂex, 799
Reward centers, 725, 735, 735f, 736
Reynolds’ number, 166
Rh blood types, 453–454
Rh immunoglobulin globulin, in
erythroblastosis fetalis, 454
Rheumatic fever, 271, 448
Rhodopsin, 627, 629–631
ac

eumatic fever, 271, 448
activated, 629, 630
decomposition of, 629, 629f
in rod hyperpolarization, 629–631, 630f
re-formation of, 629, 629f
vitamin A and, 629, 629f
Rhodopsin kinase, 631
Riboﬂavin, 875t, 876
Ribonucleic acid (RNA), 27
growth hormone effects on, 922
messenger, 31–32, 32f
nucleotides of, 30–31
ribose of, 30
ribosomal, 33–35, 34f
synthesis of, 30–31, 30f
transfer, 32–33
uracil of, 30
Ribonucleic a

1, 30f
Ribonucleic acid (RNA) polymerase, 31,
35
Ribose, 30
Ribosomes, 13f, 15, 18, 18f
endoplasmic reticulum attachment of, 33,
34f
RNA of, 33–35, 34f
d-Ribulose, 837
Rickets, 988, 991
Right atrial pressure, 176
Right bundle branch, 117f, 118f, 119
Right bundle branch block, 139–140, 140f
Right coronary artery, 249, 249f
Right ventricular hy

ht coronary artery, 249, 249f
Right ventricular hypertrophy, 273
Righting reﬂex, 682
Right-to-left cardiac shunt, 275–276, 275f
Rigidity, decerebrate, 692
Rigor mortis, 83
RNA. See Ribonucleic acid (RNA).
Rod of Corti, 655, 655f
Rods, 626–628, 627f, 628f, 629–631. See also
Cones.
excitation of, 629–631, 630f
potential of, 629–631, 630f
rhodopsin of, 629, 629f
synaptic body of, 627–

30f
Rubrospinal tract, 688–689, 689f, 705
Rufﬁni’s end-organ, 573f, 586, 595
Running, 695, 696. See also Exercise.
S
S wave, 107, 107f, 123, 124f, 134, 135f
Saccades, 646–647
Saccule, 693, 693f
macula of, 693, 693f, 695
Index
1107
Saliva, 792f, 793–795, 794t
in carbohydrate digestion, 809, 809f
Salivary glands, 791, 792f, 793–795
aldosterone effects on, 949
754t
blood supply to, 79

s system control of, 753,
754t
blood supply to, 795
Salivatory nuclei, 666
Salpingitis, 1025
Salt. See Sodium; Sodium chloride.
Saltatory conduction, 68–69, 69f
Sarcolemma, 72, 90f
Sarcomere, 73, 73f
contraction of, 74–78, 75f–78f
relaxed state of, 74–75, 75f
Sarcoplasm, 74, 74f
Sarcoplasmic reticulum, 74, 74f, 89–91, 90f
calcium release by, 90–91, 91f, 99, 99f
Satiety, 867
Satiety center, 734, 867–868, 868f, 871
Saturation diving, 549
Scala media, 652, 652f, 656
Scala tympani, 652, 652f, 656
Sc

a, 652, 652f, 656
Scala vestibuli, 652, 652f, 656
Scar, cardiac, 255, 255f
Schistosomiasis, 436
Schizophrenia, 745–746
Schlemm, canal of, 624, 624f
Schwann cell, 68, 68f
Scopolamine, 759
Scotomata, 644–645
Scotopsin, 629
Scratch reﬂex, 683
Scratching, 587
Scuba diving, 549–550, 550f
Scurvy, 878
Seawater, 350
diving in. See Deep-sea diving.
pressure-depth relationship of, 545,
546f
Second messenger systems, 560, 561–562,
561f
adenyl cyclase–cAMP, 913–914, 913f,
913t

62,
561f
913t
calcium-calmodulin, 914–915
in hormonal function, 912–915, 913f, 913t,
914f, 914t
in postsynaptic neuron, 560, 561–562,
561f
phospholipase C, 914, 914f, 914t
Secretin, 907t
in bile secretion, 804
in gastrointestinal motility, 776
Secretory vesicles (granules), 13f, 16, 16f, 21,
21f
Seizures, 583, 743–745, 743f, 744f
hippocampal, 736–737
in deep

, 743–745, 743f, 744f
in deep-sea diver, 547
Self-antigen, 441
Semen, 999–1001
Semicircular canals (ducts), 693, 693f, 694,
694f
function of, 695–696, 695f
receptors of, 576
Semilunar valves, 109, 109f. See also Aortic
valve; Pulmonary valve.
Seminal ﬂuid, 999
Seminal vesicles, 996, 997f, 999
Seminiferous tubules, 996, 997f
Sensation. See Sensory receptors; Somatic
Sensory receptors, 5–6, 555, 556f, 572–584
adaptation of, 

56f, 572–584
auditory, 652, 655–656, 655f
differential sensitivity of, 572–573
downregulation of, 583–584
nerve ﬁbers from, 576–578, 577f, 578f
pain, 598–599
phasic, 576
potentials of, 573–576, 574f, 575f. See also
Receptor potential.
predictive function of, 576
proprioceptive, 594–595, 696. See also
Muscle spindles; Pacinian corpuscles.
rapidly adapting, 575f, 576
rate, 576
signal transduction in, 573–576, 574f,
575f
sl

576
575f
slowly adapting, 575f, 576
tactile, 585–586, 586f
taste, 664–665, 665f
thermal, 607–609, 608f
tonic, 576
types of, 572, 573f, 573t
upregulation of, 583–584
visual, 626–628, 627f, 628f. See also Cones;
Rods.
Sensory stimulus. See also Sensory
intensity of, 593–594, 594f
interpreted strength of, 594, 594f
modality of, 572–573
receptor potential and, 574–575, 574f,
575f
Septal nuclei, 669
Septic shock, 282, 286
Serine, 853f
Serotonin, 564, 

ptic shock, 282, 286
Serotonin, 564, 602, 602f, 730–731, 730f
in memory formation, 724f, 725
in sleep, 740
Sertoli cells, 996–997
Serum, 460
Sex hormone–binding globulin, 1003
Sexual function. See also Reproduction.
female, 1023–1025
clitoris in, 1023
erection in, 1023
lubrication in, 1023
orgasm in, 1023–1024
stimulation of, 1023
hypothalamus in, 734
male, 1001–1003
abnormalities in, 1008–1009, 1009f
ejaculation in, 1002

malities in, 1008–1009, 1009f
ejaculation in, 1002–1003
follicle-stimulating hormone and,
1007
gonadotropin-releasing hormone and,
1006
1008f
inhibin and, 1007
lubrication in, 1002
luteinizing hormone and, 1007
neuronal stimulus for, 1001–1002
penile erection in, 1002, 1002f
stages of, 1002–1003, 1002f
testosterone and, 1003–1006, 1007. See
also Testosterone.
thyroid hormone effects on, 937–938
Shingles, 605
Shivering, 895–896, 896

 on, 937–938
Shivering, 895–896, 896f
Shock
anaphylactic, 285–286, 287, 449–450
cardiac (cardiogenic), 237, 254,
262–263
circulatory, 236f, 237, 278–288, 899, 949.
See also Circulatory shock;
gastrointestinal blood ﬂow in, 780
glucocorticoids in, 287
head-down position in, 287
histamine, 285–286
hypovolemic, 279–285. See also
insulin, 976
oxygen 

c shock.
oxygen therapy in, 287
plasma loss–related, 284–285
septic, 282, 286
septicemic, 466
spinal, 684
Shock lung syndrome, 283
Shunt
left-to-right, 274–275, 274f
right-to-left, 275–276, 275f
physiologic, 500, 501, 503, 503f
Sibutramine, 873
Sickle cell anemia, 421f, 424, 427
Sigmoid colon, spasm of, 822
Sinus arrhythmia, 148, 148f
Sinus bradycardia, 147, 148f
Sinus infection, 606f, 607
Sinus (sinoatrial) nodal block, 148,

on, 606f, 607
Sinus (sinoatrial) nodal block, 148, 148f
Sinus (sinoatrial) node, 106, 116–118, 117f,
120f
action potential of, 117, 117f
automaticity of, 116–118, 117f
hyperpolarization of, 118, 121
ion channels of, 117
on, 121, 148
resting potential of, 117, 117f
rhythmicity of, 117–118, 117f, 120
self-excitation of, 117–118, 117f
stretch of, 212, 233
121, 1

 233
121, 148
Sinus tachycardia, 147, 148f
Skin
blood ﬂow to, 890–891, 890f
cholecalciferol (vitamin D3) formation in,
983, 984f
cyanosis of, 276, 428, 510, 531
dermatomes of, 596f, 597
free nerve endings in, 586, 587
heat conduction to, 890–891, 890f, 891f
heat loss from, 891–892, 891f
insulating property of, 890
tactile receptors in, 585–586, 586f
urticaria of,

, 586f
urticaria of, 450
yellow, 863–864
Sleep, 739–741
alpha waves during, 742, 742f, 743, 743f
beta waves during, 742, 742f, 743, 743f
brain waves during, 740, 741–743, 742f
delta waves during, 742–743, 742f
1108
Index
Sleep (Continued)
electroencephalography during, 741–743,
742f, 743f
neurotransmitters and, 740–741
passive theory of, 740
physiologic effects of, 741
raphe nuclei in, 740
REM, 739, 740, 741
slow-wave, 739–740
theories of, 74

, 739, 740, 741
theories of, 740–741
theta waves during, 742, 742f
tractus solitarius in, 740
Sleep apnea, 522–523
Sleep spindles, 743
Sleep-wakefulness cycle, 741, 743, 743f
Slit pores, of glomerular capillary, 316,
316f
Slow wave rhythm, of smooth muscle
membrane potential, 97, 97f, 772–773,
772f
Slow-reacting substance of anaphylaxis, 450,
480, 529
Small intestine. See also Gastrointestinal
tract.
absorption in, 813–816, 814f
bicarbonate abso

act.
bicarbonate absorption in, 815
carbohydrate absorption in, 815–816
carbohydrate digestion in, 809
crypts of Lieberkühn of, 791, 792f,
805–806, 805f
disorders of, 821–822
enzymes of, 806
fat absorption in, 816
iron absorption in, 815
magnesium absorption in, 815
mixing (segmentation) contractions of,
786–787, 787f
nutrient absorption in, 815–816
phosphate absorption in, 815
potassium absorption in, 815
propul

rption in, 815
propulsive movements of, 787–788
protein absorption in, 816
secretions of, 776, 805–806, 805f
slow waves of, 772–773, 772f, 787
sodium absorption in, 814–815, 814f,
949
blood ﬂow in, 779, 779f
brush border of, 813, 813f
contractions of, 788
renewal of, 806
water absorption in, 814
Smell sensation, 667–670. See also Olfactory
cells.
affective nature of, 668
centrifugal signals in, 670
gradations of, 668–669
olfactory cell s

ls in, 670
olfactory cell stimulation in, 667–669
olfactory membrane in, 667, 667f
pathways of, 667f, 669–670
threshold for, 668
types of, 668
Sneeze reﬂex, 480
Snoring, 523
Sodium. See also Sodium chloride.
active transport of, 53–54, 53f, 59–60, 60f,
814, 814f
co-transport with, 54–55, 55f
epithelial transport of, 55–56, 56f
excretion of, 307–308, 308f, 328t
Sodium (Continued)
aldosterone disorders and,

328t
aldosterone disorders and, 379
angiotensin II in, 377–378, 378f
antidiuretic hormone and, 379
arterial pressure and, 374–376, 375f, 379
glomerular ﬁltration rate and, 374
in Addison’s disease, 363, 379
pressure natriuresis and, 374–375, 375f
sodium intake and, 307–308, 308f,
373–374, 380
sympathetic nervous system in, 377
volume regulation and, 373–374
extracellular, 4, 46f, 294f, 294t, 358
aldosterone and, 362–363, 363f, 379
angiotensin II 

ldosterone and, 362–363, 363f, 379
angiotensin II and, 362–363
antidiuretic hormone and, 358–359,
358f, 362–363, 362f, 379
deﬁciency of, 301–302, 301t, 363
excess of, 301t, 302
normal value for, 7t, 358
renal regulation of, 373–374
salt appetite and, 363
thirst and, 361–363, 361t, 362f, 363f
glomerulotubular balance of, 374
glucose co-transport and, 330, 330f
hydrogen ion counter-transport and, 330,
330f
in bon

drogen ion counter-transport and, 330,
330f
in cardiac muscle action potential, 105
in end-plate potential, 87, 87f
796f
in membrane potential, 57, 58f, 59–61, 60f
in neural action potential, 61–65, 61f–64f,
66, 87, 87f
in neuronal excitation, 564–566, 564f, 566f,
567, 567f
in resting membrane potential, 60–61, 60f
in sinus nodal rhythmicity, 11

tential, 60–61, 60f
aldosterone secretion and, 370–371,
371f
arterial pressure and, 218–219, 218f,
integrated responses to, 380–381
potassium excretion and, 370–371, 371f
potassium secretion and, 370–371, 371f
renal function curve for, 229, 229f
renal response to, 307–308, 308f, 356,
variability in, 292
intestinal absorption of, 814–815, 814f, 817
aldosterone effects on, 814–815, 949
intestinal secretion of, 8

ffects on, 814–815, 949
plasma. See Sodium, extracellular.
reabsorption of, 328t, 332f, 333–334, 333f,
334f, 338f, 351t
abnormalities of, 374
active transport in, 329, 329f
aldosterone and, 225, 342, 342t,
angiotensin II and, 224–225, 225f,
342–343, 342t
antidiuretic hormone and, 343
arterial pressure and, 341–342
Sodium

one and, 343
carbonic anhydrase inhibitor and, 404
countercurrent mechanism in, 351–352,
352f
diuretics and, 402–404, 403f, 403t
in cardiac failure, 264, 380
in cirrhosis, 381
principal cells in, 336–337, 336f, 337f
rate of, 374
sodium intake and, 307–308, 308f
sympathetic nervous system in, 343
water reabsorption and, 332
Sodium bicarbonate. See also Bicarbonate;
Bicarb

4
Bicarbonate buffer system.
in acidosis, 398
Sodium-calcium channel, of cardiac muscle,
105, 117
Sodium channel, 47, 47f
all or none nature of, 48–49, 48f
calcium deﬁcit and, 65
neuronal, 562, 566, 566f
voltage-gated, 48, 48f, 62, 62f, 63f, 64, 64f,
66–67
activation of, 62, 62f, 65
calcium effect on, 65
inactivation of, 62, 62f
Sodium–amino acid co-transport, 55, 816
Sodium channel blockers, 403

co-transport, 55, 816
Sodium channel blockers, 403t, 404
Sodium chloride. See also Sodium.
appetite for, 363
in seawater, 350
macula densa concentration of, 324, 324f
Sodium chloride wasting, aldosterone
deﬁciency and, 947–948
Sodium citrate, 467
Sodium lactate, in acidosis, 398
Sodium-loading renal function curves, 229,
229f
Sodium-potassium pump, 53–54, 53f, 59–60,
60f
in resting membrane potential, 60f, 61
Sodium space, 296
Sodium-calcium

otential, 60f, 61
Sodium-calcium counter-transport, 55
Sodium-glucose co-transport, 55, 55f, 816,
831
Sodium-hydrogen counter-transport, 55
Solubility coefﬁcient, of gas, 492
Solution(s)
concentration of, 51–52
glucose, 301
hyperosmotic, 299
hypertonic, 298f, 299–301
hypo-osmotic, 299
isosmotic, 299
isotonic, 298, 298f, 299, 300f
osmolality of, 52, 297
osmolarity of, 297–298
osmotic pressure of, 297–298
Solvent drag, 332
Soma, 559, 559f
Somatic s

97–298
Somatic sensation, 585–597
classiﬁcation of, 585
corticofugal signals in, 597
deep, 585
dermatomes in, 597, 598f
exteroreceptive, 585
mechanoreceptive. See Position sensation;
Tactile sensation.
Index
1109
Somatic sensation (Continued)
nociceptive. See Pain.
position. See Position sensation.
range of, 594
receptors for. See Sensory receptors.
sensory pathways for, 587–595
anterolateral system of, 587, 595–597,
596f
dorsal column–medial lemniscal system
of

597,
596f
of, 587, 588–589, 588f, 592–594,
592f, 593f
somatosensory cortex in, 589–592,
589f–591f
spinal cord transection and, 606, 606f
stimulus intensity and, 594f
tactile. See Tactile sensation.
thalamus in, 596
thermoreceptive. See Cold sensation; Heat
visceral, 585
Somatomedins, 923–924
Somatosensory area I, 590, 590f
functions of, 591–592
Somatosensory area II, 590, 590f
Somatosensory association areas, 589f, 591,
592
Somatosensory cortex, 5

tion areas, 589f, 591,
592
Somatosensory cortex, 589–592
area I of, 590, 590f, 591–592
area II of, 590, 590f
body representation of, 590, 590f
layers of, 590–591, 591f
loss of, 596
vertical columns of, 591
Somatostatin, 907t, 921, 921t, 925–926
Somatotropes, 919, 920t
Somatotropic hormone. See Growth
hormone.
Somatotropin. See Growth hormone.
Sound
attenuation of, 652
conduction of. See also Hearing.
from tympanic membran

uction of. See also Hearing.
through bone, 652
within cochlea, 594, 654–655, 654f
direction of, 660
frequency of, 654–655, 654f, 656, 657,
657f
impedance matching of, 651–652
loudness of, 656–657, 657f
resonance of, 653
Space travel
artiﬁcial climates in, 543
deconditioning during, 544
linear acceleratory forces in, 542–543, 543f
motion sickness during, 544
weightlessness in, 543–544
Spaces of Disse, 859
Spasm
carpopedal, 979, 979f
g

e, 859
Spasm
gastrointestinal, 604
muscle, 683
drug-induced, 89
motor cortex lesion and, 691
sigmoidal, 822
vascular, 457
Spatial summation
in nerve ﬁbers, 578, 578f
in neurons, 567–568, 567f
in thermal sensation, 609
Speech, 721–722, 721f. See also Cerebral
cortex; Language.
Broca’s area in, 687, 687f, 717,
717f
dysarthria of, 707
721, 721f
Sperm, 997–998, 998f
abnormalities of, 1001, 100

f
acrosome of, 1000
capacitation of, 1000
count of, 1001
enzymes of, 1000
ﬂagellum of, 17, 17f, 25
motility of, 1001
physiology of, 999
storage of, 999
Spermatocytes, 997, 998f
Spermatogenesis, 996–1001
follicle-stimulating hormone effect on,
1007
hormonal stimulation of, 998–999
meiosis in, 997, 998f
regulation of, 1007
sperm formation in, 997–998, 998f,
999
temperature effects on, 1001
testosterone effect on, 1007

ture effects on, 1001
Spermatogonia, 996, 997f
Spherocytosis, hereditary, 427
Sphincter(s)
anal, 789
esophageal, 782f, 783, 819
gastroesophageal, 783
ileocecal, 774, 788, 788f
pharyngoesophageal, 782, 782f
precapillary, 181–183, 182f, 187, 197–198,
197f
pyloric, 774, 785
urinary, 312, 313
Sphincter of Oddi, 803, 803f
Sphingomyelin, 68, 846–847, 847f. See also
Spinal anesthesia, neurogenic shock and,
285
Spinal animal, 673. See also Spinal cord,
transec

transection of.
Spinal cord, 558
alpha motor neurons of, 674
anterior motor neurons of, 674
anterolateral pathway of, 587, 595–597,
596f
corticorubral tract to, 688–689, 689f
demyelination of, 877
of, 587, 588–589, 588f, 592–594, 592f,
593f
gamma motor neurons of, 674
gray matter of, 673–675, 674f
injury to. See also Spinal cord, transection
of.
defecation and, 823
urination and, 314
interneurons of, 674–675, 674f
l

rination and, 314
lateral motor system of, 687–689, 688f,
689f, 692, 692f
medial motor system of, 692, 692f,
696–697, 696f
motor neurons of, 690, 690f
motor system of, 673–675, 674f, 712. See
also Spinal reﬂexes.
experimental study of, 673, 682–683,
683f
Golgi tendon organ interaction with,
679–680, 679f
Spinal cord (Continued)
muscle spindle interaction with,
675–679, 676f–678f. See also Muscle
spindles.
neurons in, 674–675, 674f. See also
Motor neurons.
neospinot

, 674–675, 674f. See also
neospinothalamic tract of, 600–601, 600f
paleospinothalamic tract of, 600f, 601
propriospinal ﬁbers of, 675
Renshaw cells of, 675
surgical interruption of, for pain
treatment, 602
tract of Lissauer of, 609
body temperature and, 898
Brown-Séquard syndrome in, 606, 606f
ﬂexor reﬂex and, 680, 680f
galloping reﬂex and, 683
shock and, 684
stepping movements and, 682–683, 683f
Spinal nerves, 204, 748–749, 749f
Spinal reﬂexes, 690–691
autonom

204, 748–749, 749f
autonomic, 683–684
depression of, 684
Golgi tendon, 679–680, 679f
mark time, 682–683, 683f
mass, 683–684
pain (nociceptive), 680–681, 680f, 681f
reciprocal relations in, 681–682, 682f
righting, 682
scratch, 683
spasm and, 683
stepping, 682–683, 683f
stretch, 676–677, 677f, 678–679, 679f
withdrawal, 680–681, 680f, 681f
Spinal shock, 684
Spine. See also Spinal cord.

1f
542
Spinocerebellar pathways, 700–701, 700f
Spinocerebellum, 704–705, 704f
Spiral ganglion of Corti, 652f, 655
Spirometry, 475–477, 475f, 476f, 477t
abbreviations for, 476, 477t
pulmonary capacities on, 475–476,
476f
pulmonary volumes on, 475, 476f
Spironolactone, 403t, 404
Splanchnic circulation, 777–780, 778f
Spleen
blood-cleansing function of, 180
macrophages in, 434, 434f
reservoi

hocytes in, 440
reservoir function of, 179–180, 180f
reticuloendothelial cells of, 180
structure of, 180f, 434, 434f
Splenectomy, in thrombocytopenia, 465
Sports physiology, 1055–1066. See also
Exercise.
Sprue, 423, 427, 822
Standing, baroreceptor response to, 210
Stapedius muscle, 652
Stapes, 651, 652f
Staphylococcal infection, 434
Starches, 809. See also Carbohydrates.
Starling equilibrium, 189–190
Starvation, 874–875, 874f. See also Feeding;
Food.
h

arvation, 874–875, 874f. See also Feeding;
Food.
hypothalamic lesions and, 735
protein effects of, 857
1110
Index
Statins, 851
Statoconia, 693, 693f
Stearic acid, 840. See also Triglycerides.
Steatorrhea, 822, 991
Stellate cells, 702
Stem cells, 420–422, 421f, 430, 440
Stepping movements, 682–683, 683f
Stercobilin, 817
Stereocilia, 655, 693–694, 693f
Stereopsis, 622, 622f, 643
neuronal mechanism of, 647–648
Sterility, female, 1024–1025
Steroids, adrenocortical, 944–960. See also

–1025
Stimulus. See Sensory stimulus.
Stokes-Adams syndrome, 121, 149–150
Stomach. See also at Gastric;
anatomy of, 784, 784f
antrum of, 784, 784f
atrophy of, 820
body of, 784, 784f
carbohydrate digestion in, 809, 809f
chief cells of, 795–796, 797
constrictor waves of, 784
disorders of, 819–821
emptying of, 785–786
enterogastric inhibition of, 78

ptying of, 785–786
enterogastric inhibition of, 785–786
fat digestion in, 811
food mixing in, 784
food volume in, 785
gastrin secretion by, 776, 785, 797–798
hormonal inhibition of, 786
hunger contractions of, 785
796–797, 796f, 810
mixing function of, 784
mixing waves of, 784
motor functions of, 784–786
mucosa of, 820
mucous neck cells of, 795, 796f, 797
oxyntic glands of, 795–797, 796f
pain in, 604, 605f
parietal cells of, 54

797, 796f
parietal cells of, 54, 796–797, 796f
propulsive function of, 784
protein digestion in, 810
receptive relaxation of, 783
slow waves of, 772–773, 772f, 784
storage function of, 784
surface mucous cells of, 797
triglyceride digestion in, 811
ulcers of, 820–821, 820f
Stones
cholesterol, 804–805, 805f
kidney, 991
Strabismus, 648
Streptococc

–805, 805f
Streptococcal infection
inﬂammation in, 434
rheumatic fever and, 448
Streptokinase, in cardiogenic shock,
263
Stress
ACTH secretion and, 955
bone, 983
cortisol secretion and, 952–954
Stress response, 757, 758
Stress-relaxation, smooth muscle, 94–95, 172,
173f
Stretch
of right atrium, 112
of sinus node, 212, 233
of smooth muscle, 97
of viscus, 604
Stretch receptors. See Baroreceptor(s).
Stretch reﬂex, 676

receptors. See Baroreceptor(s).
Stretch reﬂex, 676–677, 677f
clinical application of, 678–679, 678f
damping mechanism of, 677, 677f
dynamic, 677
neuronal circuitry of, 677, 677f
static, 677
Stria vascularis, 656
Stroke, 212–213, 220–221, 691, 763. See also
Stroke volume, 108
pulse pressure and, 173
Stroke work output curve, 112, 112f
Strychnine, synaptic transmission and, 570
Stuart factor (factor X), 459t, 461f, 462, 462f,
463
Stumble

tor (factor X), 459t, 461f, 462, 462f,
463
Stumble reﬂex, 682
Subarachnoid space, 764
Subcallosal gyri, ablation of, 738
Subconscious, 558
interstitial ﬂuid pressure in, 187–188
Sublingual glands, 793–795
Submandibular glands, 792f, 793–795,
795f
Submarines, 550
Submucosal (Meissner’s) plexus, 774–775,
774f
Subneural cleft, 85, 86f
Substance P, in pain sensation, 601
Substantia gelatinosa, 600f, 601
Substantia nigra, 730–731, 730f
lesions of, 709, 

lesions of, 709, 711
Subthalamic surgery, in Parkinson’s disease,
711
Subthalamus, lesions of, 709
Suckling, 733f, 734
Sucrase, 806, 809, 809f
Sucrose, 809. See also Carbohydrates.
Sulfate, 46f, 294t
Sulfonylurea drugs, 967–968
Summation
in dendrites, 569
Sunburn, 605
Superior colliculi, 647
Superoxide, 432
Supplementary motor area, 686
Supraoptic nuclei, antidiuretic hormone
synthesis 

Surfactant, 474
deﬁciency of, 529, 1045
Suspensory ligaments, of lens, 617–618,
617f
Sustentacular cells, 664–665, 665f, 667,
667f
Swallowing, 782–784, 782f
esophageal stage of, 783–784
pharyngeal stage of, 782–783
respiration and, 783
voluntary stage of, 782, 782f
Swallowing center, 783
Sweat, evaporation of, 892
Sweating, 292, 292t, 895, 895f
absence of, 892
acclimatization of, 893–894
aldosterone effects on, 893

matization of, 893–894
aldosterone effects on, 893, 949
754t, 892–894, 893f
mechanism of, 892–893, 893f
Sympathectomy, 756–757, 756f
Sympathetic nervous system. See also
alarm response of, 578
anatomy of, 204, 205f, 748–750, 749f
functions of, 753–755, 754t
in adrenal medullae function, 749–750,
755
in arterial function, 170, 170f, 172, 173f,
754t, 755
in bl

al function, 170, 170f, 172, 173f,
754t, 755
in blood ﬂow regulation, 170, 170f,
204–205, 205f, 206–208, 206f, 207f
in cardiac failure, 258–259, 259f, 261, 261f,
265
in cardiac function, 112–113, 113f, 121,
205, 205f, 234–235, 242, 242f, 754, 754t
in cardiac output, 234–235, 242, 242f
in exercise, 247, 248–249, 248f
in gastrointestinal blood ﬂow, 779–780
in gastrointestinal function, 775, 792
in glome

80
in glomerular ﬁltration, 321–3322
in heat conduction, 891, 895–896
in hemorrhage, 172
in hemorrhagic shock, 279–280, 279f, 281
in hypertension, 228
in kidney function, 377
in nonshivering thermogenesis, 887–888
in ocular function, 753, 754t
in renal sodium reabsorption, 343
in respiration, 479
in salivary secretion, 795
in sweating, 753, 892–894, 893f
in venous function, 172, 173f
mass discharge of

3f
mass discharge of, 757, 758
mean circulatory ﬁlling pressure and,
239–240, 239f
norepinephrine secretion by, 751
of eye, 648–649, 648f
postganglionic neurons of, 749, 749f
preganglionic neurons of, 748–749,
749f
stress response of, 757, 758
vasoconstrictor nerve ﬁbers of, 206
vasoconstrictor tone of, 206, 207f
Sympathomimetic drugs, 287, 759
Synapse(s), 555, 556f, 559–5

metic drugs, 287, 759
Synapse(s), 555, 556f, 559–569. See also
Neuromuscular junction; Neuron(s).
acidosis effect on, 570
afterdischarge of, 581
alkalosis effect on, 570
anatomy of, 559–562, 559f, 560f
chemical, 559, 562–564, 562t, 563t. See also
speciﬁc neurotransmitters.
delay at, 570
downregulation at, 583–584
drug effect on, 570
Index
1111
Synapse(s) (Continued)
electrical, 559
facilitation of, 557, 723, 724, 725f
fatigue of, 570, 583–584, 583f
hypoxia effect on, 570
neurotransmitters at, 56

3f
neurotransmitters at, 562–564, 562t, 563t.
See also speciﬁc neurotransmitters.
one-way conduction at, 559
signal transmission by, 557, 570
acidosis and, 570
alkalosis and, 570
delay of, 570
drugs and, 570
fatigue of, 570
hypoxia and, 570
in memory formation, 723, 724–725,
724f
ion channels in, 561, 561f
neuronal excitation in, 564–566, 564f,
566f
neuronal inhibition in, 566–568, 566f
neurotransmitters in, 562–564, 562t,
563t
one-way, 559
postsynaptic function in, 560–56

563t
postsynaptic function in, 560–562, 561f
presynaptic terminals in, 560, 560f
second messenger systems in, 561–562,
561f
summation in, 567–568, 567f
upregulation at, 583–584
Synaptic cleft, 560, 560f
Synaptic delay, 570
Synaptic fatigue, 583–584, 583f
Synaptic gutter (trough), 85, 86f
Synaptic knobs, 560
Synaptic space (cleft), 85, 86f, 560, 560f
Synaptic vesicles, 85, 86f, 560, 560f
Syncope, vasovagal, 208
Syncytium
cardiac, 103–104, 104f
gastrointestinal, 771–772
Synovial ﬂuid,

104, 104f
Synovial ﬂuid, 292
Systole, 106–107, 107f, 108, 110–111, 110f
coronary blood ﬂow during, 250, 250f
Systolic stretch, 253–254, 253f, 254
T
T (transverse) tubules
of cardiac muscle, 106
of skeletal muscles, 89–90, 90f
T wave, 107, 107f, 123, 124–125, 124f, 125f
atrial, 125
151, 151f
Tabes dorsalis, 314
electroca

36, 136f
electrocardiogram in, 147, 148f
paroxysmal, 151–152
Tactile sensation, 585–587
free nerve endings in, 573f, 586
hair end-organ in, 586
Iggo dome receptors in, 586, 586f
in Brown-Séquard syndrome, 606
Meissner’s corpuscles in, 573f, 586, 699
Merkel’s discs in, 586, 586f
Pacinian corpuscles in, 573f–575f,
574–575, 576, 587, 693–694
receptors for, 572, 573f, 585–586, 586f
Tactile sensation (Continued)
Rufﬁni’s end-org

86f
Rufﬁni’s end-organ in, 573f, 586, 595
signal transmission in
anterolateral pathway in, 595–596, 596f
dorsal column-medial lemniscal
pathway in, 588–589, 588f, 592–594,
593f
peripheral nerve ﬁbers in, 586–587
two-point, 592–593, 593f
Tank decompression, 549
Tank respirator, 532, 532f
Taste aversion, 666
Taste blindness, 664
Taste buds, 665–666, 666f
stimulus speciﬁcity of, 665
Taste cells, 664–665, 665f
Taste pore, 665, 665f
Taste preference, 666
Taste sensation,

, 665, 665f
Taste sensation, 663–666
adaptation of, 666
bitter, 664, 664t
blindness for, 664
neuronal pathways of, 665–666, 666f
salty, 663, 664t
sour, 663, 664t
sweet, 663–664, 664t
threshold for, 664, 664t
umami, 664
Tectorial membrane, 655, 655f
Teeth, 992–994
caries of, 994
cementum of, 992–993, 992f
dentin of, 992, 992f
enamel of, 992, 992f
eruption of, 993
formation of, 993, 993f
functional parts of, 992–993, 992f
malocclusion of, 994
mineral exchange in, 993–994
pulp

cclusion of, 994
pulp cavity of, 992f, 993
Telophase, 37f, 39
Temperature. See also at Cold; Heat.
abnormalities of, 898–900, 898f
after ovulation, 1025, 1025f
behavioral control of, 892, 897–898
blood ﬂow and, 890–891, 890f, 891f
cardiac function and, 114
clothing and, 892
core, 889, 890f
decreasing mechanisms of, 895, 895f
evaporation and, 892
fat insulator system and, 890
feedback gain in, 896–897
heart rate and, 147, 148f
heat conduction and, 891, 891f
heat conve

47, 148f
heat convection and, 891, 891f
heat loss and, 890–894, 890f, 891f
heat production and, 889–890
heat radiation and, 891, 891f
high, 898–900, 898f
hypothalamic regulation of, 733–734,
733f, 894–898, 894f, 895f, 897f
in premature infant, 1050–1051
increasing mechanisms of, 895–896, 895f
local effects on, 898
low, 898f, 900
neonatal, 1048–1049, 1049f
neuronal regulation of, 895–896, 895f
normal, 7t, 889, 890f
effects of, 50

xygen-hemoglobin dissociation curve
effects of, 508, 508f
physics of, 891–892, 891f
regulation of, 894–898, 894f, 895f, 897f
hypothalamus in, 733–734, 733f,
Temperature (Continued)
set-point in, 896–897, 897f, 899, 899f
skin temperature and, 889, 897, 897f
spinal cord transection and, 898
sweating and, 892–894, 893f
sympathetic nervous system and, 891
water effect on, 892
wind effect on, 892
sensation of, 607–609, 608f. See also
Cold sensation; Heat sensation.
pain thre

See also
pain threshold for, 599, 599f, 608, 608f
vapor pressure and, 492
Temporal cortex, anterior, ablation of,
738
Temporal summation
in neurons, 568
Tensor tympani muscle, 651, 652
Teratoma, 1009
Terminal cisternae, 90f
Testis (testes), 906f, 907t, 996, 997f. See also
Reproduction, male.
descent of, 1005
fetal, 1032
nondescent of, 1001
tumors of, 1009
Testosterone, 907t, 1003–1008
after puberty, 1005–1006
bon

rone, 907t, 1003–1008
bone effects of, 1005–1006
degradation of, 1004
electrolyte effects of, 1006
excretion of, 1004
fetal, 1004–1005, 1007
functions of, 1004–1006, 1004f
hair distribution and, 1005
in spermatogenesis, 998–999
in testicular descent, 1005
intracellular action of, 1006
luteinizing hormone regulation of,
1007
metabolic rate and, 886, 1006
metabolism of, 1003–1004
muscle development and, 1005
protein synthesis and, 1005
protein tis

 and, 1005
protein tissue deposition and, 857
red blood cell effects of, 1006
secretion of, 1003, 1003f
skin effects of, 1005
voice and, 1005
water balance and, 1006
Tetanization, 81, 81f
Tetany, 7, 979–980, 979f
blood ﬂow and, 246
hypocalcemia and, 371
Tetracaine, 70
Tetraethyl ammonium ion, 760
Tetralogy of Fallot, 139, 275–276, 275f
Thalamocortical system, 714–715, 715f
Thalamus, 558
c

15, 715f
cauterization of, 602
cerebral cortex relationship to, 729, 729f
dorsal lateral geniculate nucleus of,
640–641, 641f
electrical stimulation of, 603
in motor function, 588
in somatic sensation, 596
1112
Index
Theobromine, synaptic transmission and,
570
Theophylline, synaptic transmission and,
570
Thermal receptors, 572, 573t, 607–609, 60

and,
570
Thermal receptors, 572, 573t, 607–609, 608f,
894–895. See also Temperature, body.
adaptation of, 608
distribution of, 607–608
stimulation of, 608, 608f, 609
Thermal sensation, 572, 573t, 607–609, 608f
Thermode, 894
chemical, 896
nonshivering, 887–888
shivering, 895–896
Theta brain waves, 742, 742f
Thiamine. See Vitamin B1 (thiamine).
Thiazide diuretics, 403t, 404
Thinking, 717, 723–727. See also Cerebral
cortex; Memory.
cor

17, 723–727. See also Cerebral
corpus callosum in, 722
deﬁnition of, 723
holistic theory of, 723
prefrontal association area in, 717,
719–720
Wernicke’s area in, 718–719, 718f
Thiocyanate, 794, 939
Third ventricle, 763, 764, 764f
Thirst, 361–363
central nervous system centers for, 359f,
361
osmoreceptor integration with, 362–363,
362f
stimuli for, 361–362, 361t
Thirst center, 361
Thoracic duct, 190, 191f, 192
Thorax, compliance of, 474–475
Threonine, 853f
Thrill, in aortic stenos

, 474–475
Thrill, in aortic stenosis, 272
Thrombin, 459–460, 459f, 461
Thrombocytes. See Platelet(s).
Thrombocytopenia, 465
Thrombomodulin, 463
Thrombosthenin, 457
Thromboxane A2, 457, 458
Thrombus, 252, 465–466
Thymine, 27, 28f, 29, 29f
Thymus gland, 906f
T lymphocyte preprocessing in, 440, 441,
448
Thyroglobulin, 908, 931
formation of, 932, 932f
organiﬁcation of, 933, 933f
storage of, 933
Thyroid cartilage, 481, 481f
Thyroid gland, 906f, 907t
adenoma of, 940
anatomy of, 931, 93

nd, 906f, 907t
anatomy of, 931, 932f
calcitonin secretion by, 988–989
cells of, 988
cold effects on, 896
disorders of, 940–942, 940f, 942f
drug suppression of, 939–940
hormones of. See Thyroxine;
Triiodothyronine.
neonatal, 1050
radioactive iodine destruction of, 941
surgical removal of, 941
Thyroid hormones. See Thyroxine;
Thyroid-stimulating hormone, 896, 907t, 918,
919f, 920t, 938
in thermogenesis, 896
Thyroid-stimulating hormone (Continued

enesis, 896
Thyroid-stimulating hormone (Continued)
measurement of, 941
regulation of, 938–939, 939f
Thyroid-stimulating immunoglobulin, 940
Thyrotoxicosis, 940
Thyrotropes, 919, 920t
Thyrotropin. See Thyroid-stimulating
hormone.
Thyrotropin-releasing hormone, 907t, 921,
921t
cold effects on, 896, 938–939
Thyroxine, 879, 907t
arterial pressure and, 937
basal metabolic rate and, 886, 936,
936f
body weight and, 936–937
carbohydrate metabolism and, 936
cardiac output and, 937
cardiovascular effects

936
cardiovascular effects of, 935f, 937
cellular active transport and, 935
cholesterol levels and, 848, 936
excitatory effects of, 937
fat metabolism and, 846, 936
formation of, 931–933, 932f
gastrointestinal motility and, 937
growth effects of, 935f, 936
heart rate and, 937
in chemical thermogenesis, 896
in fat utilization, 846
metabolic effects of, 934–935, 935f,
936–937, 936f
mitochondrial effects of, 935
muscle effects of, 937
N

chondrial effects of, 935
Na,K-ATPase effects of, 935
period of action of, 934, 934f
protein metabolism and, 857, 915
protein synthesis and, 934, 935f
receptor for, 934, 935f
respiration and, 937
secretion of, 933–934
sexual function and, 937–938
sleep effects of, 937
therapeutic, 942
tissue transport of, 934
vitamin requirements and, 936
Thyroxine-binding globulin, 908, 934
Tic douloureux, 605–606
Tickling, 587
Tidal volume, 475, 476f
Tissue factor (factor III), 459t, 461

e, 475, 476f
Tissue factor (factor III), 459t, 461, 461f
Tissue gel, 185
Tissue plasminogen activator, 464, 466
Tissue-type plasminogen activator, 464
in cardiogenic shock, 263
in thrombus, 466
Titin, 73–74
Tolerance, immune, 448
Tongue
black, 876
taste buds of, 664–665, 665f. See also Taste
Tonic-clonic seizures, 743–744, 743f, 744f
Tonometry, 624, 624f
Total lung capacity, 476, 476f
Touch sensation. See Tactile sensation.
Toxemia of pregnancy, 227–228, 1035–1036
Toxins, renal, 307, 

pregnancy, 227–228, 1035–1036
Toxins, renal, 307, 406
Trace elements, 878t, 879, 879t
Trachea, 478–479, 479f
Tractus solitarius, 666, 666f
in swallowing, 783
Tranquilizers, 736
Transamination, 856, 856f, 862
Transcellular ﬂuids, 293. See also
Cerebrospinal ﬂuid (CSF).
Transcortin, 947
Transcription, 30–35, 32f, 32t, 34f
partial, 36
Transferrin, 425, 425f, 426
Transforming growth factor-ß, 861
Transfusion reactions, 454–455
agglutination in, 452–453
he

on reactions, 454–455
hemolysis in, 453
Translation, 33–35, 34f. See also Protein(s),
synthesis of.
Transmitters. See Neurotransmitters.
Transplantation, 455–456
Transport
acetoacetic acid, 844
active. See Active transport.
carbon dioxide. See Carbon dioxide,
glucose, 55, 55f, 816, 831, 964, 964f
hormone, 909, 1016
iron, 425–426, 425f
lipid, 840–842
oxygen. See Oxygen, transport of.
passive. See Diffu

ygen. See Oxygen, transport of.
passive. See Diffusion.
Transport vesicles, 21, 21f
Transpulmonary pressure, 472f, 473
Transverse tubules
Trauma, shock and, 285
Tremor
action (intention), 705, 707
muscle, 937
Parkinson’s, 711
Treppe, 81–82
Triamterene, 403t, 404
Tricarboxylic acid cycle, 23, 833–835, 834f,
843
Trichinosis, 436
Tricuspid valve, 109
pressure measurement at, 179, 179f
Tricyclic antidepressants, 745
Trigeminal neuralgia, 605–606

antidepressants, 745
Triglycerides. See also Lipid(s);
cellular, 12, 842
energy release from, 842–844, 843f,
846
formation of, 842, 844–845, 845f
hydrolysis of, 841, 842
lipoprotein transport of, 841–842
storage of, 842. See also Adipose tissue.
structure of, 840
Trigone, 311, 311f
Triiodothyronine, 879, 907t
basal metabolic rate and, 936, 936f
carbohydrate me

936, 936f
cholesterol levels and, 936
fat metabolism and, 936
formation of, 932–933, 932f
Index
1113
Triiodothyronine (Continued)
metabolic rate and, 886

bolic rate and, 886
period of action of, 934
protein metabolism and, 857
protein synthesis and, 915, 934, 935f
receptor for, 934
thyroxine conversion to, 934
Trophoblasts, 1029, 1029f
Tropical sprue, 822
Tropomyosin, 76
Troponin, 76
Trypsin, 799–800, 810, 810f
Trypsin inhibitor, 800
Trypsinogen, 7

0, 810, 810f
Trypsinogen, 799–800
Tryptophan, 853f
Tuberculosis, 432, 530
D-Tubocurarine, neuromuscular junction
Tubular glands, 791, 795–797, 795f, 796f
Tubulin, 17
Tufted cells, of olfactory bulb, 669, 670
Tumor necrosis factor, in inﬂammation, 435,
436f
Two-point discrimination, 592–593, 593f
Tympanic membrane, 651–652, 652f
Tyramine, 759
Tyrosine, 853f
iodination of, 933, 933f
U
Ulcerative colitis, 823
Ulcers
intestinal, 823
marginal, 820
peptic, 820–821

cers
peptic, 820–821, 820f
Ultimobranchial glands, 988
Ultrasound Doppler ﬂowmeter, 164–165,
165f
Ultraviolet light, headache and, 607
Unconsciousness, 767
Uracil, 30
Urate, renal secretion of, 334
Urea
excretion of, 307, 328t, 353–354
formation of, 856, 862
in urine concentration, 351t, 353–354, 354f
in chronic renal failure, 410, 410f, 411f,
412
reabsorpti

nic renal failure, 410, 410f, 411f,
412
reabsorption of, 328t, 332–333, 332f, 338f,
351t, 353–354, 354f
Urea transporter AI, 353
Uremia, 411–412, 411f
Ureterorenal reﬂex, 312
Ureters, 312, 312f
Urethra, 996
posterior, 311, 311f, 312
Uric acid
excretion of, 307
Uridine diphosphate glucose, 832, 832f
Urinary tract obstruction, renal failure and,
406
Urination, 313–314, 313f
voluntary, 313
Urine. See also Bladder; Kidneys; Ren

luntary, 313
Urine. See also Bladder; Kidneys; Renal
tubules.
amino acids in, 413
bicarbonate in, 394
calcium in, 372
concentration of, 350–357, 351t, 352f,
353f–356f
angiotensin II in, 377–378
antidiuretic hormone in, 348–349,
350–351, 356
arterial pressure and, 374–375, 375f
cortical collecting tubule in, 353, 353f,
356, 356f
countercurrent mechanisms in, 351–352,
disorders of, 357–358
distal tubule in, 352–353, 353f, 356,
356f
diuretic effects on, 403
in chronic renal failure, 4

uretic effects on, 403
in chronic renal failure, 411, 411f
loop of Henle in, 351–352, 352f, 355,
356f
medullary blood ﬂow and, 354–355,
355f
medullary collecting duct in, 353, 353f,
obligatory urine volume and, 350
osmolarity in, 350–352, 352f
quantiﬁcation of, 357
summary of, 355–357, 356f
urea in, 351t, 353–354, 354f
vasodilator effect on, 355
dilution of, 348–350, 349f, 356
after excess water ingestion, 349–350,
349f
collecting tubules in, 349f, 35

ion, 349–350,
349f
collecting tubules in, 349f, 350
distal tubule in, 349f, 350
loop of Henle in, 349–350, 349f
diuretic effect on, 402–404, 403f, 403t
elimination of. See Micturition; Urination.
formation of, 292, 314–325, 314f, 315f. See
also Urine, concentration of; Urine,
dilution of.
antidiuretic hormone in, 348–349, 349f
arterial pressure and, 216, 323, 341–342,
374–376, 375f, 376f, 948–949, 948f
diuretic ef

42,
glomerular ﬁltration in, 316–323,
316f–320f, 320t, 323f, 323t, 342f. See
also Glomerular ﬁltration;
Glomerular ﬁltration rate (GFR).
obligatory volume for, 350, 356–357
renal blood ﬂow in, 320–323, 321f, 321t,
323f. See also Kidneys, blood ﬂow
of.
tubular processing in, 314–315, 314f,
315f. See also at Renal tubules.
water deﬁciency and, 350–357, 351t,
352f, 353f–356f, 379
w

ﬁciency and, 350–357, 351t,
water excess and, 348–350, 349f, 379
glucose in, 413, 975
keto acids in, 976
obligatory volume of, 350, 356–357
osmolarity of, 348
free-water clearance and, 357
Urine (Continued)
352f–356f
water excess and, 348–350, 349f
pH of, 384, 384t, 392
phosphate in, 372–373
potassium in, 367
protein in, 409
titratable acid of, 394–395
transport of, 312–313, 313f
volume of, 292, 292t, 327, 328t, 350
Urobilin, 817, 863, 863

 292, 292t, 327, 328t, 350
Urobilin, 817, 863, 863f
Urobilinogen, 863, 863f, 864
Urogenital diaphragm, 312
Urticaria, 450
Uterus, 1011, 1012f
contractions of, 8–9, 733f, 734, 1036–1038
involution of, 1038
oxytocin effects on, 1036
stretch of, 1036
Utilization coefﬁcient, of oxygen, 507, 508
Utricle, 693, 693f
Uvulopalatopharyngoplasty, 523
V
V wave, 107f, 108
Vagal reﬂex, in paroxysmal tachycardia, 152
Vagina, 1011

reﬂex, in paroxysmal tachycardia, 152
Vagina, 1011, 1012f
Vagus nerve, 750, 750f. See also
in cardiac function, 113, 113f, 121, 148,
152, 205, 205f
Valine, 853f
Valves
heart. See Heart valves.
lymph, 191, 192–193, 192f, 193f
venous, 178–179, 178f
Valvulae conniventes (folds of Kerckring),
Van den Bergh reaction, 864
Vapor pressure, 492
axonal, 96, 96f
venous, 178–179, 381
Vas deferens, 996, 997f
Vasa recta, 310,

–179, 381
Vasa recta, 310, 311f
blood ﬂow in, 321, 354–355, 355f
countercurrent exchange in, 354–355, 355f
Vascular compliance, 171–172
173f
reverse, in hemorrhagic shock, 281
Vascular endothelial growth factor, 200–201
Vascular resistance. See also Blood ﬂow;
Circulation (blood).
arteriolar, 168
blood ﬂow and, 164, 164f, 167–169,
168f–170f
blood pressure and, 170, 170f, 219, 2

9,
168f–170f
blood pressure and, 170, 170f, 219, 219f,
220, 220f
blood viscosity and, 169–170, 170f
CGS units for, 167
conductance and, 167–168, 168f, 169
fourth power law and, 168
hematocrit and, 169–170, 169f, 170f
in parallel circuits, 168–169, 169f
in series circuits, 168–169, 169f
intrarenal, 219, 219f
1114
Index
Vascular resistance (Continued)
peripheral, 167, 170, 170f, 219, 219f, 220,
220f
Poiseuille’s law and, 168
pressure pulse damping and, 175
pulmonary, 167
total, 167
units of, 167
v

and, 175
total, 167
venous, 176–177, 177f, 240–241, 240f
vessel diameter and, 167–168, 168, 168f
Vasoactive intestinal polypeptide, 774
Vasoconstriction. See also Vasodilation.
adrenal medullae in, 207
angiotensin II in, 201–202, 223f, 224, 224f
calcium in, 202
cerebral cortex in, 207
cerebral ischemia and, 212–213
gastrointestinal, 779–780
hypothalamus in, 207
in exercise, 247
in heating, 895
in vessel trauma, 457
nor

ise, 247
norepinephrine in, 207, 247
sympathetic nervous system in, 170, 170f,
204–205, 205f, 206–208, 206f, 207f,
Vasoconstrictor tone, 206, 207f
Vasodilation. See also Vasoconstriction.
anions in, 203
baroreceptors in, 210, 210f
bradykinin in, 202, 779
carbon dioxide in, 203
cardiac output and, 237
cold-induced, 895, 900
during exercise, 247
emotion-related, 208
hypothalamus in, 206f, 207–208
in thiamine 

n, 202
magnesium in, 202
potassium in, 202
206f, 207–208, 754–755, 754t
Vasodilators
in angina pectoris, 256
in hypertension, 229–230
urine concentration and, 355
Vasomotion, 182–183, 187, 197–198, 197f
anesthesia effect on, 285
baroreceptor effect on, 6–7, 210, 210f
chemoreceptor effect on, 212
control of, 206f, 207
in arterial pressure control, 6–7, 212–213
in hemorrhagic shock,

essure control, 6–7, 212–213
Vasomotor tone, sudden loss of, 285
Vasomotor waves, 214, 214f
Vasopressin. See Antidiuretic hormone
(ADH).
Vasovagal syncope, 208
Vater, papilla of, gallstone of, 821–822
Vector, 131, 132f
direction of, 131–132, 132f
Vectorcardiogram, 136–137, 137f
Vectorial analysis, 131–146. See also
Electrocardiogram, vectorial analysis
of.
Vein(s)
arm, 178–179
blood distribution to, 162f, 254
cross-sectional area o

 201
Vein(s) (Continued)
distensibility of, 171, 176–179, 177f–179f
functions of, 161, 176–180, 177f, 178f
neck, 179
obstruction of, cardiac output and, 237
pressures in, 176–180, 177f–179f
reservoir function of, 171, 179–180
172, 173f, 204, 205f
valves of, 178, 178f
varicose, 178–179, 381
Venae cavae, cross-sectional area of, 162
Venous pressure, 176–180
central, 176
g

ea of, 162
gravitational pressure and, 177–178, 177f
intra-abdominal pressure and, 177
measurement of, 179, 179f
peripheral, 176–177, 177f
pulmonary, 484
resistance and, 176–177, 177f
right atrial pressure and, 177, 179, 179f
Venous pump, 176, 178–179, 178f. See also
Venous return.
Venous resistance, 176–177, 177f, 240–241,
240f
Venous return, 111, 232–236, 234f, 235f. See
also Venous return curve.
artiﬁcial respirator and, 532
calculation of, 240
decrease i

respirator and, 532
decrease in, 236f, 237
deﬁnition of, 232
local blood ﬂow and, 233–234
mean systemic ﬁlling pressure and, 238,
239, 240, 240f
pressure gradient for, 240–241, 241f
resistance to, 238, 240–241, 241f
vasodilation-induced reduction in, 285
Venous return curve, 238–243, 239f–243f
during exercise, 248f, 249
in cardiac failure, 265–2

ring exercise, 248f, 249
in cardiac failure, 265–267, 265f, 266f
239, 239f, 240, 240f
normal, 238–239, 239f, 241, 242f, 265,
265f
plateau in, 239, 239f
right atrial pressure and, 238–239, 239f
venous resistance and, 241, 241f
Venous thrombosis, 466
Ventilation. See Alveolar ventilation;
Ventilation-perfusion ratio, 498, 499–501,
500f
abnormalities of, 501
above normal, 500–501
below normal, 500
during exercise, 501
in ch

0–501
in chronic obstructive lung disease, 501
in emphysema, 527
normal, 500, 500f
Ventral spinocerebellar tract, 700–701, 700f
Ventricle(s)
brain, 763, 764, 764f
cardiac, 104
current ﬂow in, 126–127, 126f, 127f
emptying of, 107f, 108
ﬁlling of, 107f, 108
function curves of, 112, 112f
Ventricle(s) (Continued)
function of, 107f, 108, 110–111, 110f, 121
hypertrophy of, 272–273
axis 

, 110–111, 110f, 121
axis deviation with, 138, 138f
in aortic valve lesions, 272–273
pressures in, 107f, 108, 110–111, 110f
stroke work output curve of, 112,
112f
volume output curve of, 112, 112f
work output of, 110–111, 110f
mean electrical axis of, 137–140,
137f–140f
coronary blood ﬂow in, 250
hypertrophy of, axis deviation with,
138, 138f
pressures in, 483–484, 484f
112f
w

 of, 112,
112f
work output of, 110
Ventricular escape, 121, 149–150
Ventricular ﬁbrillation, 152–155, 287–288
cardiopulmonary resuscitation in, 155
chain reaction mechanism of, 153–154,
153f
circus movements in, 153, 153f, 154
60-cycle alternating current in, 153–154,
153f
electrocardiogram in, 154, 154f
electroshock deﬁbrillation in, 154–155,
155f
impulse division in, 153f, 154
re-entry in, 153, 153f
Ventricular function curves,

e-entry in, 153, 153f
Ventricular function curves, 112, 112f
Ventricular paroxysmal tachycardia, 152,
152f
Ventricular volume output curve, 112,
112f
Ventrobasal complex, 600
Venule(s)
sympathetic innervation of, 204, 205f
Vermis, 699, 699f, 704
699f
Vesicointestinal reﬂex, 790
Vesicoureteral reﬂux, 312, 409
Vestibular apparatus, 692–694, 693f
in angular acceleration detection,
695–6

694, 693f
695–696, 695f
in linear acceleration detection, 695
saccule of, 693, 693f, 694–695
semicircular canals of, 694, 694f, 695–696,
695f
utricle of, 693, 693f, 694–695
Vestibular nerve, 694, 694f
Vestibular nuclei, 688, 691–692, 691f,
Vestibulocerebellum, 703–704
Vibration sensation, 585–587
transmission of, 588–589, 588f, 593
Villi
arachnoidal, 765
absorptive surface of, 812–813, 812f,
813f
brush bord

813, 812f,
813f
Viruses, 18, 18f
in cancer, 41
Index
1115
Visceral pain, 603–605, 605f
localization of, 604, 605f
transmission of, 604–605
Visceral reﬂexes, 748
Vision, 613–625. See also Eye(s); Retina.
acuity of, 621, 621f
autonomic nervous system in, 648–650,
648f
binocular, 622, 622f
color, 632–633, 633f, 638, 641f, 643, 644
cones in, 631, 631f, 632–633, 633f. See also
Cones.
contrast

631, 631f, 632–633, 633f. See also
Cones.
contrast, 638, 638f, 643, 643f
contrast in, 643, 643f, 644
depth perception in, 621–622, 622f,
647–648
detail in, 641f, 643
equilibrium and, 696
farsighted (hyperopia), 619, 619f
ﬁelds of, 644–645, 644f
form in, 641f, 643
image fusion in, 647–648
in albinism, 628
lateral inhibition in, 638, 638f
lines and borders in, 643–644
motion in, 641f, 643
nearsighted (myopia), 619–620, 619f
neurophysiology of, 640–650
accurate color pathway in, 642f, 643
color det

650
color detection in, 644
dorsal lateral geniculate nucleus in,
fast position and motion pathway in,
642f, 643
ﬁeld of vision in, 644–645, 644f
image analysis in, 643–645, 643f, 644f
visual cortex in, 640, 641–643, 641f, 642f.
See also Visual cortex.
normal (emmetropia), 619, 619f
ocular movements in, 645–648, 645f–648f
orientation in, 643–644
photochemistry of, 628–632
color pigments in, 631, 631f, 632–633,
633f
regulation of, 631–632, 632f
r

631f, 632–633,
633f
rhodopsin in, 629–631, 629f–631f
rods in, 629–631, 629f–631f. See also
Rods.
stereopsis in, 622, 622f, 643, 647–648
third-dimensional position in, 641f,
643
Visual acuity, 621, 621f
Visual area II, 641f, 642
Visual cortex, 640, 641–643, 641f
color blobs of, 642–643, 642f
complex cells of, 644
hypercomplex cells of, 644
in image fusion, 647–648
layer IV of, 642f, 643
layers of, 642–643, 642f
primary, 641–642, 641f
removal of, 644
secondary, 641f, 6

, 641–642, 641f
secondary, 641f, 642
separate signals to, 640–641, 642f, 643
simple cells of, 643–644
vertical columns of, 642, 642f
Visual purple. See Rhodopsin.
Visual receptive aphasia, 721
Vital capacity, 475–476, 476f
Vitamin(s), 875–878, 875t. See also speciﬁc
vitamins.
calcium absorption and, 815
deﬁciency of, 875
fetal, 1044
hepatic storage of, 862
Vitamin(s) (Continued)
requirements for, 875, 875t
storage of, 862, 875
Vitamin A (retinol), 

on, 936
Vitamin A (retinol), 875, 875t
deﬁciency of, 629, 875
retinal, 628, 629, 629f
Vitamin B1 (thiamine), 875, 875t
deﬁciency of, 875–876
blood ﬂow and, 198, 267, 267f, 876
cardiac output and, 236, 236f, 267, 267f,
876
gastrointestinal effects of, 876
nervous system effects of, 876
Vitamin B2 (riboﬂavin), 875t, 876
Vitamin B6 (pyridoxine), 875t, 877
Vitamin B12 (cobalamin), 875t, 876–877
absorption of, 8

in B12 (cobalamin), 875t, 876–877
absorption of, 820
in red blood cell production, 423
Vitamin C (ascorbic acid), 875t, 877–878
deﬁciency of, 878
Vitamin D, 875t, 878, 983–985
deﬁciency of, 991
formation of, 983–985, 984f
functions of, 985
in hypoparathyroidism, 990
in intestinal calcium absorption, 985
in intestinal phosphate absorption, 985
Vitamin D–r

osphate absorption, 985
Vitamin D–resistant rickets, 991
Vitamin E, 875t, 878
Vitamin K, 875t, 878
in coagulation, 460, 464–465
Vitreous body, 623
Vitreous humor, 623
Vocal folds (cords), 481, 481f
Vocal ligament, 481, 481f
Vocalization, 481, 481f. See also Language;
Speech.
Voice, testosterone effects on, 1005
Voltage clamp testing, 63, 63f
Volume reﬂex, 212
Volume-pressure relationship
of ar

lume reﬂex, 212
of arteries, 172, 172f
of cardiac cycle, 110–111, 110f
of veins, 172, 172f
of ventricles, 107f, 108
Vomiting, 823–824, 823f
acidosis and, 397–398
Vomiting center, 823–824, 823f
Von Willebrand factor, 458
Von Willebrand’s disease, 465
W
W ganglion cells, 637
Wakefulness, 558, 741. See also Sleep.
electroencephalography during, 743, 743f
Walking movements, 682–683, 683f
Warfarin, 466
Warmth receptors, 607–609, 608f, 894–895.
See also 

armth receptors, 607–609, 608f, 894–895.
See also Heat sensation.
Wasting syndrome, 874
Water
carbon dioxide reaction with, 510–511
cellular, 11
deﬁciency of, 301t, 302
renal response to, 350–357, 351t,
352f–356f
diffusion of, 47, 51–52, 51f, 183–184, 183f,
814
evaporation of, 891f, 892
excess of, 301–302, 301t
renal response to, 349–350, 349f
formation of, 835, 835f
free, 357
gases in, 492
gastrointestinal, 793
hypothalamic regulation of, 733f, 734
insensible loss of, 291–292
intake/output of, 

734
intake/output of, 291–292, 292t
angiotensin II and, 361, 361t
224–226, 226f, 361
dry mouth and, 361, 361t
gastrointestinal factors in, 361, 361t
thirst and, 361–362, 361t
intestinal osmosis of, 814
large intestine absorption of, 817
large intestine secretion of, 815
loss of, 291–292, 292t, 350, 362
osmosis of, 332, 814
reabsorption of, 339, 350–357, 351t,

 of, 332, 814
352f–356f, 734. See also Urine,
concentration of; Urine, dilution of.
angiotensin II and, 342–343, 342t,
377–378
antidiuretic hormone and, 337, 338,
342t, 343, 348–349, 379, 734
osmosis in, 332, 332f
tubular/plasma inulin evaluation of,
339
salt, 350
total, 293, 295–296, 296t
vapor pressure of, 492
Water vapor, partial pressure of, 4

essure of, 492
Water vapor, partial pressure of, 493t, 494
Weber-Fechner principle, 594
Weight, body
during pregnancy, 1034
excess, 872–874. See also Obesity.
loss of, 867, 874
thyroid hormone effects on, 936–937
Weightlessness, in space travel, 543–544
Wernicke’s aphasia, 721
Wernicke’s area, 716, 718–719, 718f, 721,
721f
White blood cell(s), 429–431. See also
speciﬁc cell types.
abnormal excess of, 437
chemotaxis of, 430f, 431

agents of, 432
deﬁciency of, 436–437
lymphocytic lineage of, 430, 430f
myelocytic lineage of, 430, 430f
phagocytosis by, 431–432
1116
Index
White blood cell(s) (Continued)
production of, 430–431, 430f
types of, 429–430, 430f
White pulp, 180, 180f
Withdrawal reﬂexes, 680–681, 680f, 681f
Word blindness, 718, 721
Word deafness, 721
Work, skeletal muscle contraction and,
78–79
Work capacity, at high altitude, 5

tion and,
78–79
Work capacity, at high altitude, 540
X
X ganglion cells, 637
Xenograft, 455
Y
Y ganglion cells, 637
Z
Z disc, 72–73, 73f, 74–75, 75f, 90f
Zero pressure reference level, 179
Zero reference level, in current of injury,
142–143, 143f
Zinc, 879, 879t